| rfc2616-symrefs.txt |  | draft-ietf-httpbis-p4-conditional-00.txt |
--- | --- | --- | --- | ---
| Network Working Group R. Fielding |  | Network Working Group R. Fielding, Ed. |
| Internet-Draft UC Irvine |  | Internet-Draft Day Software |
| Obsoletes: 2068 (if approved) J. Gettys |  | Obsoletes: 2068, 2616 J. Gettys |
| Intended status: Standards Track Compaq/W3C |  | (if approved) One Laptop per Child |
| Expires: December 3, 1999 J. Mogul |  | Intended status: Standards Track J. Mogul |
| Compaq |  | Expires: June 22, 2008 HP |
| H. Frystyk |  | H. Frystyk |
| W3C/MIT |  | Microsoft |
| L. Masinter |  | L. Masinter |
| Xerox |  | Adobe Systems |
| P. Leach |  | P. Leach |
| Microsoft |  | Microsoft |
| T. Berners-Lee |  | T. Berners-Lee |
| W3C/MIT |  | W3C/MIT |
| Hypertext Transfer Protocol -- HTTP/1.1 |  | December 20, 2007 |
| rfc2616-symrefs |  |  |
|  |  | HTTP/1.1, part 4: Conditional Requests |
|  |  | draft-ietf-httpbis-p4-conditional-00 |
| Status of this Memo |  | Status of this Memo |
| By submitting this Internet-Draft, each author represents that any |  | By submitting this Internet-Draft, each author represents that any |
| applicable patent or other IPR claims of which he or she is aware |  | applicable patent or other IPR claims of which he or she is aware |
| have been or will be disclosed, and any of which he or she becomes |  | have been or will be disclosed, and any of which he or she becomes |
| aware will be disclosed, in accordance with Section 6 of BCP 79. |  | aware will be disclosed, in accordance with Section 6 of BCP 79. |
| Internet-Drafts are working documents of the Internet Engineering |  | Internet-Drafts are working documents of the Internet Engineering |
| Task Force (IETF), its areas, and its working groups. Note that |  | Task Force (IETF), its areas, and its working groups. Note that |
| skipping to change at page 1, line 43 |  | skipping to change at page 1, line 45 |
| and may be updated, replaced, or obsoleted by other documents at any |  | and may be updated, replaced, or obsoleted by other documents at any |
| time. It is inappropriate to use Internet-Drafts as reference |  | time. It is inappropriate to use Internet-Drafts as reference |
| material or to cite them other than as "work in progress." |  | material or to cite them other than as "work in progress." |
| The list of current Internet-Drafts can be accessed at |  | The list of current Internet-Drafts can be accessed at |
| http://www.ietf.org/ietf/1id-abstracts.txt. |  | http://www.ietf.org/ietf/1id-abstracts.txt. |
| The list of Internet-Draft Shadow Directories can be accessed at |  | The list of Internet-Draft Shadow Directories can be accessed at |
| http://www.ietf.org/shadow.html. |  | http://www.ietf.org/shadow.html. |
| This Internet-Draft will expire on December 3, 1999. |  | This Internet-Draft will expire on June 22, 2008. |
| Copyright Notice |  | Copyright Notice |
| Copyright (C) The IETF Trust (1999). |  | Copyright (C) The IETF Trust (2007). |
| Abstract |  | Abstract |
| The Hypertext Transfer Protocol (HTTP) is an application-level |  | The Hypertext Transfer Protocol (HTTP) is an application-level |
| protocol for distributed, collaborative, hypermedia information |  | protocol for distributed, collaborative, hypermedia information |
| systems. It is a generic, stateless, protocol which can be used for |  | systems. HTTP has been in use by the World Wide Web global |
| many tasks beyond its use for hypertext, such as name servers and |  | information initiative since 1990. This document is Part 4 of the |
| distributed object management systems, through extension of its |  | seven-part specification that defines the protocol referred to as |
| request methods, error codes and headers [RFC2324]. A feature of |  | "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 4 defines |
| HTTP is the typing and negotiation of data representation, allowing |  | request header fields for indicating conditional requests and the |
| systems to be built independently of the data being transferred. |  | rules for constructing responses to those requests. |
| HTTP has been in use by the World-Wide Web global information |  |  |
| initiative since 1990. This specification defines the protocol |  |  |
| referred to as "HTTP/1.1", and is an update to RFC 2068 [RFC2068]. |  |  |
| Editorial Note (To be removed by RFC Editor) |  | Editorial Note (To be removed by RFC Editor) |
| This version of the HTTP specification contains only XML processing |  | This version of the HTTP specification contains only minimal |
| changes from [RFC2616] in internet-draft form for use in creating |  | editorial changes from [RFC2616] (abstract, introductory paragraph, |
| diffs. |  | and authors' addresses). All other changes are due to partitioning |
|  |  | the original into seven mostly independent parts. The intent is for |
|  |  | readers of future drafts to able to use draft 00 as the basis for |
|  |  | comparison when the WG makes later changes to the specification text. |
|  |  | This draft will shortly be followed by draft 01 (containing the first |
|  |  | round of changes that have already been agreed to on the mailing |
|  |  | list). There is no point in reviewing this draft other than to |
|  |  | verify that the partitioning has been done correctly. Roy T. |
|  |  | Fielding, Yves Lafon, and Julian Reschke will be the editors after |
|  |  | draft 00 is submitted. |
|  |  | Discussion of this draft should take place on the HTTPBIS working |
|  |  | group mailing list (ietf-http-wg@w3.org). The current issues list is |
|  |  | at <http://www3.tools.ietf.org/wg/httpbis/trac/report/11> and related |
|  |  | documents (including fancy diffs) can be found at |
|  |  | <http://www3.tools.ietf.org/wg/httpbis/>. |
| Table of Contents |  | Table of Contents |
| 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9 |  | 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 |
| 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . 9 |  | 2. Entity Tags . . . . . . . . . . . . . . . . . . . . . . . . . 4 |
| 1.2. Requirements . . . . . . . . . . . . . . . . . . . . . . 9 |  | 3. Status Code Definitions . . . . . . . . . . . . . . . . . . . 4 |
| 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . 10 |  | 3.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 4 |
| 1.4. Overall Operation . . . . . . . . . . . . . . . . . . . 14 |  | 3.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 5 |
| 2. Notational Conventions and Generic Grammar . . . . . . . . . 16 |  | 4. Weak and Strong Validators . . . . . . . . . . . . . . . . . . 5 |
| 2.1. Augmented BNF . . . . . . . . . . . . . . . . . . . . . 16 |  | 5. Rules for When to Use Entity Tags and Last-Modified Dates . . 8 |
| 2.2. Basic Rules . . . . . . . . . . . . . . . . . . . . . . 18 |  | 6. Header Field Definitions . . . . . . . . . . . . . . . . . . . 10 |
| 3. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 20 |  | 6.1. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 |
| 3.1. HTTP Version . . . . . . . . . . . . . . . . . . . . . . 20 |  | 6.2. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 10 |
| 3.2. Uniform Resource Identifiers . . . . . . . . . . . . . . 21 |  | 6.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 11 |
| 3.2.1. General Syntax . . . . . . . . . . . . . . . . . . . 21 |  | 6.4. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 13 |
| 3.2.2. http URL . . . . . . . . . . . . . . . . . . . . . . 22 |  | 6.5. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 14 |
| 3.2.3. URI Comparison . . . . . . . . . . . . . . . . . . . 22 |  | 6.6. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 15 |
| 3.3. Date/Time Formats . . . . . . . . . . . . . . . . . . . 23 |  | 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 |
| 3.3.1. Full Date . . . . . . . . . . . . . . . . . . . . . 23 |  | 8. Security Considerations . . . . . . . . . . . . . . . . . . . 16 |
| 3.3.2. Delta Seconds . . . . . . . . . . . . . . . . . . . 24 |  | 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 |
| 3.4. Character Sets . . . . . . . . . . . . . . . . . . . . . 24 |  | 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 |
| 3.4.1. Missing Charset . . . . . . . . . . . . . . . . . . 25 |  | Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 |
| 3.5. Content Codings . . . . . . . . . . . . . . . . . . . . 25 |  | Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 |
| 3.6. Transfer Codings . . . . . . . . . . . . . . . . . . . . 27 |  | Intellectual Property and Copyright Statements . . . . . . . . . . 20 |
| 3.6.1. Chunked Transfer Coding . . . . . . . . . . . . . . 28 |  |  |
| 3.7. Media Types . . . . . . . . . . . . . . . . . . . . . . 29 |  |  |
| 3.7.1. Canonicalization and Text Defaults . . . . . . . . . 30 |  |  |
| 3.7.2. Multipart Types . . . . . . . . . . . . . . . . . . 30 |  |  |
| 3.8. Product Tokens . . . . . . . . . . . . . . . . . . . . . 31 |  |  |
| 3.9. Quality Values . . . . . . . . . . . . . . . . . . . . . 32 |  |  |
| 3.10. Language Tags . . . . . . . . . . . . . . . . . . . . . 32 |  |  |
| 3.11. Entity Tags . . . . . . . . . . . . . . . . . . . . . . 33 |  |  |
| 3.12. Range Units . . . . . . . . . . . . . . . . . . . . . . 33 |  |  |
| 4. HTTP Message . . . . . . . . . . . . . . . . . . . . . . . . 34 |  |  |
| 4.1. Message Types . . . . . . . . . . . . . . . . . . . . . 34 |  |  |
| 4.2. Message Headers . . . . . . . . . . . . . . . . . . . . 34 |  |  |
| 4.3. Message Body . . . . . . . . . . . . . . . . . . . . . . 35 |  |  |
| 4.4. Message Length . . . . . . . . . . . . . . . . . . . . . 36 |  |  |
| 4.5. General Header Fields . . . . . . . . . . . . . . . . . 37 |  |  |
| 5. Request . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 |  |  |
| 5.1. Request-Line . . . . . . . . . . . . . . . . . . . . . . 38 |  |  |
| 5.1.1. Method . . . . . . . . . . . . . . . . . . . . . . . 38 |  |  |
| 5.1.2. Request-URI . . . . . . . . . . . . . . . . . . . . 39 |  |  |
| 5.2. The Resource Identified by a Request . . . . . . . . . . 40 |  |  |
| 5.3. Request Header Fields . . . . . . . . . . . . . . . . . 41 |  |  |
| 6. Response . . . . . . . . . . . . . . . . . . . . . . . . . . 42 |  |  |
| 6.1. Status-Line . . . . . . . . . . . . . . . . . . . . . . 42 |  |  |
| 6.1.1. Status Code and Reason Phrase . . . . . . . . . . . 43 |  |  |
| 6.2. Response Header Fields . . . . . . . . . . . . . . . . . 45 |  |  |
| 7. Entity . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 |  |  |
| 7.1. Entity Header Fields . . . . . . . . . . . . . . . . . . 46 |  |  |
| 7.2. Entity Body . . . . . . . . . . . . . . . . . . . . . . 46 |  |  |
| 7.2.1. Type . . . . . . . . . . . . . . . . . . . . . . . . 46 |  |  |
| 7.2.2. Entity Length . . . . . . . . . . . . . . . . . . . 47 |  |  |
| 8. Connections . . . . . . . . . . . . . . . . . . . . . . . . . 47 |  |  |
| 8.1. Persistent Connections . . . . . . . . . . . . . . . . . 47 |  |  |
| 8.1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . 47 |  |  |
| 8.1.2. Overall Operation . . . . . . . . . . . . . . . . . 48 |  |  |
| 8.1.3. Proxy Servers . . . . . . . . . . . . . . . . . . . 49 |  |  |
| 8.1.4. Practical Considerations . . . . . . . . . . . . . . 50 |  |  |
| 8.2. Message Transmission Requirements . . . . . . . . . . . 51 |  |  |
| 8.2.1. Persistent Connections and Flow Control . . . . . . 51 |  |  |
| 8.2.2. Monitoring Connections for Error Status Messages . . 51 |  |  |
| 8.2.3. Use of the 100 (Continue) Status . . . . . . . . . . 51 |  |  |
| 8.2.4. Client Behavior if Server Prematurely Closes |  |  |
| Connection . . . . . . . . . . . . . . . . . . . . . 53 |  |  |
| 9. Method Definitions . . . . . . . . . . . . . . . . . . . . . 54 |  |  |
| 9.1. Safe and Idempotent Methods . . . . . . . . . . . . . . 54 |  |  |
| 9.1.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 54 |  |  |
| 9.1.2. Idempotent Methods . . . . . . . . . . . . . . . . . 54 |  |  |
| 9.2. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . 55 |  |  |
| 9.3. GET . . . . . . . . . . . . . . . . . . . . . . . . . . 56 |  |  |
| 9.4. HEAD . . . . . . . . . . . . . . . . . . . . . . . . . . 57 |  |  |
| 9.5. POST . . . . . . . . . . . . . . . . . . . . . . . . . . 57 |  |  |
| 9.6. PUT . . . . . . . . . . . . . . . . . . . . . . . . . . 58 |  |  |
| 9.7. DELETE . . . . . . . . . . . . . . . . . . . . . . . . . 59 |  |  |
| 9.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . . 59 |  |  |
| 9.9. CONNECT . . . . . . . . . . . . . . . . . . . . . . . . 60 |  |  |
| 10. Status Code Definitions . . . . . . . . . . . . . . . . . . . 60 |  |  |
| 10.1. Informational 1xx . . . . . . . . . . . . . . . . . . . 60 |  |  |
| 10.1.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 61 |  |  |
| 10.1.2. 101 Switching Protocols . . . . . . . . . . . . . . 61 |  |  |
| 10.2. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 61 |  |  |
| 10.2.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 61 |  |  |
| 10.2.2. 201 Created . . . . . . . . . . . . . . . . . . . . 62 |  |  |
| 10.2.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 62 |  |  |
| 10.2.4. 203 Non-Authoritative Information . . . . . . . . . 62 |  |  |
| 10.2.5. 204 No Content . . . . . . . . . . . . . . . . . . . 63 |  |  |
| 10.2.6. 205 Reset Content . . . . . . . . . . . . . . . . . 63 |  |  |
| 10.2.7. 206 Partial Content . . . . . . . . . . . . . . . . 63 |  |  |
| 10.3. Redirection 3xx . . . . . . . . . . . . . . . . . . . . 64 |  |  |
| 10.3.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 64 |  |  |
| 10.3.2. 301 Moved Permanently . . . . . . . . . . . . . . . 65 |  |  |
| 10.3.3. 302 Found . . . . . . . . . . . . . . . . . . . . . 65 |  |  |
| 10.3.4. 303 See Other . . . . . . . . . . . . . . . . . . . 66 |  |  |
| 10.3.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 66 |  |  |
| 10.3.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . 67 |  |  |
| 10.3.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 67 |  |  |
| 10.3.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 67 |  |  |
| 10.4. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 68 |  |  |
| 10.4.1. 400 Bad Request . . . . . . . . . . . . . . . . . . 68 |  |  |
| 10.4.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 68 |  |  |
| 10.4.3. 402 Payment Required . . . . . . . . . . . . . . . . 69 |  |  |
| 10.4.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . 69 |  |  |
| 10.4.5. 404 Not Found . . . . . . . . . . . . . . . . . . . 69 |  |  |
| 10.4.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 69 |  |  |
| 10.4.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 69 |  |  |
| 10.4.8. 407 Proxy Authentication Required . . . . . . . . . 70 |  |  |
| 10.4.9. 408 Request Timeout . . . . . . . . . . . . . . . . 70 |  |  |
| 10.4.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 70 |  |  |
| 10.4.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 71 |  |  |
| 10.4.12. 411 Length Required . . . . . . . . . . . . . . . . 71 |  |  |
| 10.4.13. 412 Precondition Failed . . . . . . . . . . . . . . 71 |  |  |
| 10.4.14. 413 Request Entity Too Large . . . . . . . . . . . . 71 |  |  |
| 10.4.15. 414 Request-URI Too Long . . . . . . . . . . . . . . 72 |  |  |
| 10.4.16. 415 Unsupported Media Type . . . . . . . . . . . . . 72 |  |  |
| 10.4.17. 416 Requested Range Not Satisfiable . . . . . . . . 72 |  |  |
| 10.4.18. 417 Expectation Failed . . . . . . . . . . . . . . . 72 |  |  |
| 10.5. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 72 |  |  |
| 10.5.1. 500 Internal Server Error . . . . . . . . . . . . . 73 |  |  |
| 10.5.2. 501 Not Implemented . . . . . . . . . . . . . . . . 73 |  |  |
| 10.5.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . 73 |  |  |
| 10.5.4. 503 Service Unavailable . . . . . . . . . . . . . . 73 |  |  |
| 10.5.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . 73 |  |  |
| 10.5.6. 505 HTTP Version Not Supported . . . . . . . . . . . 74 |  |  |
| 11. Access Authentication . . . . . . . . . . . . . . . . . . . . 74 |  |  |
| 12. Content Negotiation . . . . . . . . . . . . . . . . . . . . . 74 |  |  |
| 12.1. Server-driven Negotiation . . . . . . . . . . . . . . . 75 |  |  |
| 12.2. Agent-driven Negotiation . . . . . . . . . . . . . . . . 76 |  |  |
| 12.3. Transparent Negotiation . . . . . . . . . . . . . . . . 76 |  |  |
| 13. Caching in HTTP . . . . . . . . . . . . . . . . . . . . . . . 77 |  |  |
| 13.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 |  |  |
| 13.1.1. Cache Correctness . . . . . . . . . . . . . . . . . 78 |  |  |
| 13.1.2. Warnings . . . . . . . . . . . . . . . . . . . . . . 79 |  |  |
| 13.1.3. Cache-control Mechanisms . . . . . . . . . . . . . . 80 |  |  |
| 13.1.4. Explicit User Agent Warnings . . . . . . . . . . . . 80 |  |  |
| 13.1.5. Exceptions to the Rules and Warnings . . . . . . . . 81 |  |  |
| 13.1.6. Client-controlled Behavior . . . . . . . . . . . . . 81 |  |  |
| 13.2. Expiration Model . . . . . . . . . . . . . . . . . . . . 82 |  |  |
| 13.2.1. Server-Specified Expiration . . . . . . . . . . . . 82 |  |  |
| 13.2.2. Heuristic Expiration . . . . . . . . . . . . . . . . 83 |  |  |
| 13.2.3. Age Calculations . . . . . . . . . . . . . . . . . . 83 |  |  |
| 13.2.4. Expiration Calculations . . . . . . . . . . . . . . 85 |  |  |
| 13.2.5. Disambiguating Expiration Values . . . . . . . . . . 86 |  |  |
| 13.2.6. Disambiguating Multiple Responses . . . . . . . . . 87 |  |  |
| 13.3. Validation Model . . . . . . . . . . . . . . . . . . . . 87 |  |  |
| 13.3.1. Last-Modified Dates . . . . . . . . . . . . . . . . 88 |  |  |
| 13.3.2. Entity Tag Cache Validators . . . . . . . . . . . . 88 |  |  |
| 13.3.3. Weak and Strong Validators . . . . . . . . . . . . . 89 |  |  |
| 13.3.4. Rules for When to Use Entity Tags and |  |  |
| Last-Modified Dates . . . . . . . . . . . . . . . . 91 |  |  |
| 13.3.5. Non-validating Conditionals . . . . . . . . . . . . 93 |  |  |
| 13.4. Response Cacheability . . . . . . . . . . . . . . . . . 93 |  |  |
| 13.5. Constructing Responses From Caches . . . . . . . . . . . 94 |  |  |
| 13.5.1. End-to-end and Hop-by-hop Headers . . . . . . . . . 94 |  |  |
| 13.5.2. Non-modifiable Headers . . . . . . . . . . . . . . . 95 |  |  |
| 13.5.3. Combining Headers . . . . . . . . . . . . . . . . . 96 |  |  |
| 13.5.4. Combining Byte Ranges . . . . . . . . . . . . . . . 97 |  |  |
| 13.6. Caching Negotiated Responses . . . . . . . . . . . . . . 98 |  |  |
| 13.7. Shared and Non-Shared Caches . . . . . . . . . . . . . . 99 |  |  |
| 13.8. Errors or Incomplete Response Cache Behavior . . . . . . 99 |  |  |
| 13.9. Side Effects of GET and HEAD . . . . . . . . . . . . . . 100 |  |  |
| 13.10. Invalidation After Updates or Deletions . . . . . . . . 100 |  |  |
| 13.11. Write-Through Mandatory . . . . . . . . . . . . . . . . 101 |  |  |
| 13.12. Cache Replacement . . . . . . . . . . . . . . . . . . . 101 |  |  |
| 13.13. History Lists . . . . . . . . . . . . . . . . . . . . . 102 |  |  |
| 14. Header Field Definitions . . . . . . . . . . . . . . . . . . 102 |  |  |
| 14.1. Accept . . . . . . . . . . . . . . . . . . . . . . . . . 102 |  |  |
| 14.2. Accept-Charset . . . . . . . . . . . . . . . . . . . . . 104 |  |  |
| 14.3. Accept-Encoding . . . . . . . . . . . . . . . . . . . . 105 |  |  |
| 14.4. Accept-Language . . . . . . . . . . . . . . . . . . . . 106 |  |  |
| 14.5. Accept-Ranges . . . . . . . . . . . . . . . . . . . . . 108 |  |  |
| 14.6. Age . . . . . . . . . . . . . . . . . . . . . . . . . . 108 |  |  |
| 14.7. Allow . . . . . . . . . . . . . . . . . . . . . . . . . 109 |  |  |
| 14.8. Authorization . . . . . . . . . . . . . . . . . . . . . 109 |  |  |
| 14.9. Cache-Control . . . . . . . . . . . . . . . . . . . . . 110 |  |  |
| 14.9.1. What is Cacheable . . . . . . . . . . . . . . . . . 112 |  |  |
| 14.9.2. What May be Stored by Caches . . . . . . . . . . . . 113 |  |  |
| 14.9.3. Modifications of the Basic Expiration Mechanism . . 113 |  |  |
| 14.9.4. Cache Revalidation and Reload Controls . . . . . . . 115 |  |  |
| 14.9.5. No-Transform Directive . . . . . . . . . . . . . . . 118 |  |  |
| 14.9.6. Cache Control Extensions . . . . . . . . . . . . . . 119 |  |  |
| 14.10. Connection . . . . . . . . . . . . . . . . . . . . . . . 119 |  |  |
| 14.11. Content-Encoding . . . . . . . . . . . . . . . . . . . . 120 |  |  |
| 14.12. Content-Language . . . . . . . . . . . . . . . . . . . . 121 |  |  |
| 14.13. Content-Length . . . . . . . . . . . . . . . . . . . . . 122 |  |  |
| 14.14. Content-Location . . . . . . . . . . . . . . . . . . . . 123 |  |  |
| 14.15. Content-MD5 . . . . . . . . . . . . . . . . . . . . . . 123 |  |  |
| 14.16. Content-Range . . . . . . . . . . . . . . . . . . . . . 125 |  |  |
| 14.17. Content-Type . . . . . . . . . . . . . . . . . . . . . . 127 |  |  |
| 14.18. Date . . . . . . . . . . . . . . . . . . . . . . . . . . 127 |  |  |
| 14.18.1. Clockless Origin Server Operation . . . . . . . . . 128 |  |  |
| 14.19. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . 128 |  |  |
| 14.20. Expect . . . . . . . . . . . . . . . . . . . . . . . . . 129 |  |  |
| 14.21. Expires . . . . . . . . . . . . . . . . . . . . . . . . 130 |  |  |
| 14.22. From . . . . . . . . . . . . . . . . . . . . . . . . . . 131 |  |  |
| 14.23. Host . . . . . . . . . . . . . . . . . . . . . . . . . . 131 |  |  |
| 14.24. If-Match . . . . . . . . . . . . . . . . . . . . . . . . 132 |  |  |
| 14.25. If-Modified-Since . . . . . . . . . . . . . . . . . . . 133 |  |  |
| 14.26. If-None-Match . . . . . . . . . . . . . . . . . . . . . 135 |  |  |
| 14.27. If-Range . . . . . . . . . . . . . . . . . . . . . . . . 136 |  |  |
| 14.28. If-Unmodified-Since . . . . . . . . . . . . . . . . . . 137 |  |  |
| 14.29. Last-Modified . . . . . . . . . . . . . . . . . . . . . 137 |  |  |
| 14.30. Location . . . . . . . . . . . . . . . . . . . . . . . . 138 |  |  |
| 14.31. Max-Forwards . . . . . . . . . . . . . . . . . . . . . . 138 |  |  |
| 14.32. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 139 |  |  |
| 14.33. Proxy-Authenticate . . . . . . . . . . . . . . . . . . . 140 |  |  |
| 14.34. Proxy-Authorization . . . . . . . . . . . . . . . . . . 140 |  |  |
| 14.35. Range . . . . . . . . . . . . . . . . . . . . . . . . . 140 |  |  |
| 14.35.1. Byte Ranges . . . . . . . . . . . . . . . . . . . . 140 |  |  |
| 14.35.2. Range Retrieval Requests . . . . . . . . . . . . . . 142 |  |  |
| 14.36. Referer . . . . . . . . . . . . . . . . . . . . . . . . 143 |  |  |
| 14.37. Retry-After . . . . . . . . . . . . . . . . . . . . . . 143 |  |  |
| 14.38. Server . . . . . . . . . . . . . . . . . . . . . . . . . 144 |  |  |
| 14.39. TE . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 |  |  |
| 14.40. Trailer . . . . . . . . . . . . . . . . . . . . . . . . 145 |  |  |
| 14.41. Transfer-Encoding . . . . . . . . . . . . . . . . . . . 146 |  |  |
| 14.42. Upgrade . . . . . . . . . . . . . . . . . . . . . . . . 146 |  |  |
| 14.43. User-Agent . . . . . . . . . . . . . . . . . . . . . . . 148 |  |  |
| 14.44. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . 148 |  |  |
| 14.45. Via . . . . . . . . . . . . . . . . . . . . . . . . . . 149 |  |  |
| 14.46. Warning . . . . . . . . . . . . . . . . . . . . . . . . 150 |  |  |
| 14.47. WWW-Authenticate . . . . . . . . . . . . . . . . . . . . 153 |  |  |
| 15. Security Considerations . . . . . . . . . . . . . . . . . . . 153 |  |  |
| 15.1. Personal Information . . . . . . . . . . . . . . . . . . 153 |  |  |
| 15.1.1. Abuse of Server Log Information . . . . . . . . . . 154 |  |  |
| 15.1.2. Transfer of Sensitive Information . . . . . . . . . 154 |  |  |
| 15.1.3. Encoding Sensitive Information in URI's . . . . . . 155 |  |  |
| 15.1.4. Privacy Issues Connected to Accept Headers . . . . . 155 |  |  |
| 15.2. Attacks Based On File and Path Names . . . . . . . . . . 156 |  |  |
| 15.3. DNS Spoofing . . . . . . . . . . . . . . . . . . . . . . 156 |  |  |
| 15.4. Location Headers and Spoofing . . . . . . . . . . . . . 157 |  |  |
| 15.5. Content-Disposition Issues . . . . . . . . . . . . . . . 157 |  |  |
| 15.6. Authentication Credentials and Idle Clients . . . . . . 157 |  |  |
| 15.7. Proxies and Caching . . . . . . . . . . . . . . . . . . 158 |  |  |
| 15.7.1. Denial of Service Attacks on Proxies . . . . . . . . 159 |  |  |
| 16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 159 |  |  |
| 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 161 |  |  |
| Appendix A. Appendices . . . . . . . . . . . . . . . . . . . . . 165 |  |  |
| A.1. Internet Media Type message/http and application/http . 165 |  |  |
| A.2. Internet Media Type multipart/byteranges . . . . . . . . 166 |  |  |
| A.3. Tolerant Applications . . . . . . . . . . . . . . . . . 167 |  |  |
| A.4. Differences Between HTTP Entities and RFC 2045 |  |  |
| Entities . . . . . . . . . . . . . . . . . . . . . . . . 168 |  |  |
| A.4.1. MIME-Version . . . . . . . . . . . . . . . . . . . . 169 |  |  |
| A.4.2. Conversion to Canonical Form . . . . . . . . . . . . 169 |  |  |
| A.4.3. Conversion of Date Formats . . . . . . . . . . . . . 169 |  |  |
| A.4.4. Introduction of Content-Encoding . . . . . . . . . . 170 |  |  |
| A.4.5. No Content-Transfer-Encoding . . . . . . . . . . . . 170 |  |  |
| A.4.6. Introduction of Transfer-Encoding . . . . . . . . . 170 |  |  |
| A.4.7. MHTML and Line Length Limitations . . . . . . . . . 171 |  |  |
| A.5. Additional Features . . . . . . . . . . . . . . . . . . 171 |  |  |
| A.5.1. Content-Disposition . . . . . . . . . . . . . . . . 171 |  |  |
| A.6. Compatibility with Previous Versions . . . . . . . . . . 172 |  |  |
| A.6.1. Changes from HTTP/1.0 . . . . . . . . . . . . . . . 173 |  |  |
| A.6.2. Compatibility with HTTP/1.0 Persistent Connections . 174 |  |  |
| A.6.3. Changes from RFC 2068 . . . . . . . . . . . . . . . 174 |  |  |
| Appendix B. Index . . . . . . . . . . . . . . . . . . . . . . . 177 |  |  |
| Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 |  |  |
| Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 188 |  |  |
| Intellectual Property and Copyright Statements . . . . . . . . . 190 |  |  |
| 1. Introduction |  | 1. Introduction |
| 1.1. Purpose |  | This document will define aspects of HTTP related to conditional |
|  |  | request messages based on time stamps and entity-tags. Right now it |
| The Hypertext Transfer Protocol (HTTP) is an application-level |  | only includes the extracted relevant sections of RFC 2616 [RFC2616] |
| protocol for distributed, collaborative, hypermedia information |  | without edit. |
| systems. HTTP has been in use by the World-Wide Web global |  |  |
| information initiative since 1990. The first version of HTTP, |  |  |
| referred to as HTTP/0.9, was a simple protocol for raw data transfer |  |  |
| across the Internet. HTTP/1.0, as defined by RFC 1945 [RFC1945], |  |  |
| improved the protocol by allowing messages to be in the format of |  |  |
| MIME-like messages, containing metainformation about the data |  |  |
| transferred and modifiers on the request/response semantics. |  |  |
| However, HTTP/1.0 does not sufficiently take into consideration the |  |  |
| effects of hierarchical proxies, caching, the need for persistent |  |  |
| connections, or virtual hosts. In addition, the proliferation of |  |  |
| incompletely-implemented applications calling themselves "HTTP/1.0" |  |  |
| has necessitated a protocol version change in order for two |  |  |
| communicating applications to determine each other's true |  |  |
| capabilities. |  |  |
| This specification defines the protocol referred to as "HTTP/1.1". |  |  |
| This protocol includes more stringent requirements than HTTP/1.0 in |  |  |
| order to ensure reliable implementation of its features. |  |  |
| Practical information systems require more functionality than simple |  |  |
| retrieval, including search, front-end update, and annotation. HTTP |  |  |
| allows an open-ended set of methods and headers that indicate the |  |  |
| purpose of a request [RFC2324]. It builds on the discipline of |  |  |
| reference provided by the Uniform Resource Identifier (URI) |  |  |
| [RFC1630], as a location (URL) [RFC1738] or name (URN) [RFC1737], for |  |  |
| indicating the resource to which a method is to be applied. Messages |  |  |
| are passed in a format similar to that used by Internet mail [RFC822] |  |  |
| as defined by the Multipurpose Internet Mail Extensions (MIME) |  |  |
| [RFC2045]. |  |  |
| HTTP is also used as a generic protocol for communication between |  |  |
| user agents and proxies/gateways to other Internet systems, including |  |  |
| those supported by the SMTP [RFC821], NNTP [RFC977], FTP [RFC959], |  |  |
| Gopher [RFC1436], and WAIS [WAIS] protocols. In this way, HTTP |  |  |
| allows basic hypermedia access to resources available from diverse |  |  |
| applications. |  |  |
| 1.2. Requirements |  |  |
| The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", |  |  |
| "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this |  |  |
| document are to be interpreted as described in RFC 2119 [RFC2119]. |  |  |
| An implementation is not compliant if it fails to satisfy one or more |  |  |
| of the MUST or REQUIRED level requirements for the protocols it |  |  |
| implements. An implementation that satisfies all the MUST or |  |  |
| REQUIRED level and all the SHOULD level requirements for its |  |  |
| protocols is said to be "unconditionally compliant"; one that |  |  |
| satisfies all the MUST level requirements but not all the SHOULD |  |  |
| level requirements for its protocols is said to be "conditionally |  |  |
| compliant." |  |  |
| 1.3. Terminology |  |  |
| This specification uses a number of terms to refer to the roles |  |  |
| played by participants in, and objects of, the HTTP communication. |  |  |
| connection |  |  |
| A transport layer virtual circuit established between two programs |  |  |
| for the purpose of communication. |  |  |
| message |  |  |
| The basic unit of HTTP communication, consisting of a structured |  |  |
| sequence of octets matching the syntax defined in Section 4 and |  |  |
| transmitted via the connection. |  |  |
| request |  |  |
| An HTTP request message, as defined in Section 5. |  |  |
| response |  |  |
| An HTTP response message, as defined in Section 6. |  |  |
| resource |  |  |
| A network data object or service that can be identified by a URI, |  |  |
| as defined in Section 3.2. Resources may be available in multiple |  |  |
| representations (e.g. multiple languages, data formats, size, and |  |  |
| resolutions) or vary in other ways. |  |  |
| entity |  |  |
| The information transferred as the payload of a request or |  |  |
| response. An entity consists of metainformation in the form of |  |  |
| entity-header fields and content in the form of an entity-body, as |  |  |
| described in Section 7. |  |  |
| representation |  |  |
| An entity included with a response that is subject to content |  |  |
| negotiation, as described in Section 12. There may exist multiple |  |  |
| representations associated with a particular response status. |  |  |
| content negotiation |  |  |
| The mechanism for selecting the appropriate representation when |  |  |
| servicing a request, as described in Section 12. The |  |  |
| representation of entities in any response can be negotiated |  |  |
| (including error responses). |  |  |
| variant |  |  |
| A resource may have one, or more than one, representation(s) |  |  |
| associated with it at any given instant. Each of these |  |  |
| representations is termed a `varriant'. Use of the term `variant' |  |  |
| does not necessarily imply that the resource is subject to content |  |  |
| negotiation. |  |  |
| client |  |  |
| A program that establishes connections for the purpose of sending |  |  |
| requests. |  |  |
| user agent |  |  |
| The client which initiates a request. These are often browsers, |  |  |
| editors, spiders (web-traversing robots), or other end user tools. |  |  |
| server |  |  |
| An application program that accepts connections in order to |  |  |
| service requests by sending back responses. Any given program may |  |  |
| be capable of being both a client and a server; our use of these |  |  |
| terms refers only to the role being performed by the program for a |  |  |
| particular connection, rather than to the program's capabilities |  |  |
| in general. Likewise, any server may act as an origin server, |  |  |
| proxy, gateway, or tunnel, switching behavior based on the nature |  |  |
| of each request. |  |  |
| origin server |  |  |
| The server on which a given resource resides or is to be created. |  |  |
| proxy |  |  |
| An intermediary program which acts as both a server and a client |  |  |
| for the purpose of making requests on behalf of other clients. |  |  |
| Requests are serviced internally or by passing them on, with |  |  |
| possible translation, to other servers. A proxy MUST implement |  |  |
| both the client and server requirements of this specification. A |  |  |
| "transparent proxy" is a proxy that does not modify the request or |  |  |
| response beyond what is required for proxy authentication and |  |  |
| identification. A "non-transparent proxy" is a proxy that |  |  |
| modifies the request or response in order to provide some added |  |  |
| service to the user agent, such as group annotation services, |  |  |
| media type transformation, protocol reduction, or anonymity |  |  |
| filtering. Except where either transparent or non-transparent |  |  |
| behavior is explicitly stated, the HTTP proxy requirements apply |  |  |
| to both types of proxies. |  |  |
| gateway |  |  |
| A server which acts as an intermediary for some other server. |  |  |
| Unlike a proxy, a gateway receives requests as if it were the |  |  |
| origin server for the requested resource; the requesting client |  |  |
| may not be aware that it is communicating with a gateway. |  |  |
| tunnel |  |  |
| An intermediary program which is acting as a blind relay between |  |  |
| two connections. Once active, a tunnel is not considered a party |  |  |
| to the HTTP communication, though the tunnel may have been |  |  |
| initiated by an HTTP request. The tunnel ceases to exist when |  |  |
| both ends of the relayed connections are closed. |  |  |
| cache |  |  |
| A program's local store of response messages and the subsystem |  |  |
| that controls its message storage, retrieval, and deletion. A |  |  |
| cache stores cacheable responses in order to reduce the response |  |  |
| time and network bandwidth consumption on future, equivalent |  |  |
| requests. Any client or server may include a cache, though a |  |  |
| cache cannot be used by a server that is acting as a tunnel. |  |  |
| cacheable |  |  |
| A response is cacheable if a cache is allowed to store a copy of |  |  |
| the response message for use in answering subsequent requests. |  |  |
| The rules for determining the cacheability of HTTP responses are |  |  |
| defined in Section 13. Even if a resource is cacheable, there may |  |  |
| be additional constraints on whether a cache can use the cached |  |  |
| copy for a particular request. |  |  |
| first-hand |  |  |
| A response is first-hand if it comes directly and without |  |  |
| unnecessary delay from the origin server, perhaps via one or more |  |  |
| proxies. A response is also first-hand if its validity has just |  |  |
| been checked directly with the origin server. |  |  |
| explicit expiration time |  |  |
| The time at which the origin server intends that an entity should |  |  |
| no longer be returned by a cache without further validation. |  |  |
| heuristic expiration time |  |  |
| An expiration time assigned by a cache when no explicit expiration |  |  |
| time is available. |  |  |
| age |  |  |
| The age of a response is the time since it was sent by, or |  |  |
| successfully validated with, the origin server. |  |  |
| freshness lifetime |  |  |
| The length of time between the generation of a response and its |  |  |
| expiration time. |  |  |
| fresh |  |  |
| A response is fresh if its age has not yet exceeded its freshness |  |  |
| lifetime. |  |  |
| stale |  |  |
| A response is stale if its age has passed its freshness lifetime. |  |  |
| semantically transparent |  |  |
| A cache behaves in a "semantically transparent" manner, with |  |  |
| respect to a particular response, when its use affects neither the |  |  |
| requesting client nor the origin server, except to improve |  |  |
| performance. When a cache is semantically transparent, the client |  |  |
| receives exactly the same response (except for hop-by-hop headers) |  |  |
| that it would have received had its request been handled directly |  |  |
| by the origin server. |  |  |
| validator |  |  |
| A protocol element (e.g., an entity tag or a Last-Modified time) |  |  |
| that is used to find out whether a cache entry is an equivalent |  |  |
| copy of an entity. |  |  |
| upstream/downstream |  |  |
| Upstream and downstream describe the flow of a message: all |  |  |
| messages flow from upstream to downstream. |  |  |
| inbound/outbound |  |  |
| Inbound and outbound refer to the request and response paths for |  |  |
| messages: "inbound" means "traveling toward the origin server", |  |  |
| and "outbound" means "traveling toward the user agent" |  |  |
| 1.4. Overall Operation |  |  |
| The HTTP protocol is a request/response protocol. A client sends a |  |  |
| request to the server in the form of a request method, URI, and |  |  |
| protocol version, followed by a MIME-like message containing request |  |  |
| modifiers, client information, and possible body content over a |  |  |
| connection with a server. The server responds with a status line, |  |  |
| including the message's protocol version and a success or error code, |  |  |
| followed by a MIME-like message containing server information, entity |  |  |
| metainformation, and possible entity-body content. The relationship |  |  |
| between HTTP and MIME is described in Appendix A.4. |  |  |
| Most HTTP communication is initiated by a user agent and consists of |  |  |
| a request to be applied to a resource on some origin server. In the |  |  |
| simplest case, this may be accomplished via a single connection (v) |  |  |
| between the user agent (UA) and the origin server (O). |  |  |
| request chain ------------------------> |  |  |
| UA -------------------v------------------- O |  |  |
| <----------------------- response chain |  |  |
| A more complicated situation occurs when one or more intermediaries |  |  |
| are present in the request/response chain. There are three common |  |  |
| forms of intermediary: proxy, gateway, and tunnel. A proxy is a |  |  |
| forwarding agent, receiving requests for a URI in its absolute form, |  |  |
| rewriting all or part of the message, and forwarding the reformatted |  |  |
| request toward the server identified by the URI. A gateway is a |  |  |
| receiving agent, acting as a layer above some other server(s) and, if |  |  |
| necessary, translating the requests to the underlying server's |  |  |
| protocol. A tunnel acts as a relay point between two connections |  |  |
| without changing the messages; tunnels are used when the |  |  |
| communication needs to pass through an intermediary (such as a |  |  |
| firewall) even when the intermediary cannot understand the contents |  |  |
| of the messages. |  |  |
| request chain --------------------------------------> |  |  |
| UA -----v----- A -----v----- B -----v----- C -----v----- O |  |  |
| <------------------------------------- response chain |  |  |
| The figure above shows three intermediaries (A, B, and C) between the |  |  |
| user agent and origin server. A request or response message that |  |  |
| travels the whole chain will pass through four separate connections. |  |  |
| This distinction is important because some HTTP communication options |  |  |
| may apply only to the connection with the nearest, non-tunnel |  |  |
| neighbor, only to the end-points of the chain, or to all connections |  |  |
| along the chain. Although the diagram is linear, each participant |  |  |
| may be engaged in multiple, simultaneous communications. For |  |  |
| example, B may be receiving requests from many clients other than A, |  |  |
| and/or forwarding requests to servers other than C, at the same time |  |  |
| that it is handling A's request. |  |  |
| Any party to the communication which is not acting as a tunnel may |  |  |
| employ an internal cache for handling requests. The effect of a |  |  |
| cache is that the request/response chain is shortened if one of the |  |  |
| participants along the chain has a cached response applicable to that |  |  |
| request. The following illustrates the resulting chain if B has a |  |  |
| cached copy of an earlier response from O (via C) for a request which |  |  |
| has not been cached by UA or A. |  |  |
| request chain ----------> |  |  |
| UA -----v----- A -----v----- B - - - - - - C - - - - - - O |  |  |
| <--------- response chain |  |  |
| Not all responses are usefully cacheable, and some requests may |  |  |
| contain modifiers which place special requirements on cache behavior. |  |  |
| HTTP requirements for cache behavior and cacheable responses are |  |  |
| defined in Section 13. |  |  |
| In fact, there are a wide variety of architectures and configurations |  |  |
| of caches and proxies currently being experimented with or deployed |  |  |
| across the World Wide Web. These systems include national hierarchies |  |  |
| of proxy caches to save transoceanic bandwidth, systems that |  |  |
| broadcast or multicast cache entries, organizations that distribute |  |  |
| subsets of cached data via CD-ROM, and so on. HTTP systems are used |  |  |
| in corporate intranets over high-bandwidth links, and for access via |  |  |
| PDAs with low-power radio links and intermittent connectivity. The |  |  |
| goal of HTTP/1.1 is to support the wide diversity of configurations |  |  |
| already deployed while introducing protocol constructs that meet the |  |  |
| needs of those who build web applications that require high |  |  |
| reliability and, failing that, at least reliable indications of |  |  |
| failure. |  |  |
| HTTP communication usually takes place over TCP/IP connections. The |  |  |
| default port is TCP 80 [RFC1700], but other ports can be used. This |  |  |
| does not preclude HTTP from being implemented on top of any other |  |  |
| protocol on the Internet, or on other networks. HTTP only presumes a |  |  |
| reliable transport; any protocol that provides such guarantees can be |  |  |
| used; the mapping of the HTTP/1.1 request and response structures |  |  |
| onto the transport data units of the protocol in question is outside |  |  |
| the scope of this specification. |  |  |
| In HTTP/1.0, most implementations used a new connection for each |  |  |
| request/response exchange. In HTTP/1.1, a connection may be used for |  |  |
| one or more request/response exchanges, although connections may be |  |  |
| closed for a variety of reasons (see Section 8.1). |  |  |
| 2. Notational Conventions and Generic Grammar |  |  |
| 2.1. Augmented BNF |  |  |
| All of the mechanisms specified in this document are described in |  |  |
| both prose and an augmented Backus-Naur Form (BNF) similar to that |  |  |
| used by RFC 822 [RFC822]. Implementors will need to be familiar with |  |  |
| the notation in order to understand this specification. The |  |  |
| augmented BNF includes the following constructs: |  |  |
| name = definition |  |  |
| The name of a rule is simply the name itself (without any |  |  |
| enclosing "<" and ">") and is separated from its definition by the |  |  |
| equal "=" character. White space is only significant in that |  |  |
| indentation of continuation lines is used to indicate a rule |  |  |
| definition that spans more than one line. Certain basic rules are |  |  |
| in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle |  |  |
| brackets are used within definitions whenever their presence will |  |  |
| facilitate discerning the use of rule names. |  |  |
| "literal" |  |  |
| Quotation marks surround literal text. Unless stated otherwise, |  |  |
| the text is case-insensitive. |  |  |
| rule1 | rule2 |  |  |
| Elements separated by a bar ("|") are alternatives, e.g., "yes | |  |  |
| no" will accept yes or no. |  |  |
| (rule1 rule2) |  |  |
| Elements enclosed in parentheses are treated as a single element. |  |  |
| Thus, "(elem (foo | bar) elem)" allows the token sequences "elem |  |  |
| foo elem" and "elem bar elem". |  |  |
| *rule |  |  |
| The character "*" preceding an element indicates repetition. The |  |  |
| full form is "<n>*<m>element" indicating at least <n> and at most |  |  |
| <m> occurrences of element. Default values are 0 and infinity so |  |  |
| that "*(element)" allows any number, including zero; "1*element" |  |  |
| requires at least one; and "1*2element" allows one or two. |  |  |
| [rule] |  |  |
| Square brackets enclose optional elements; "[foo bar]" is |  |  |
| equivalent to "*1(foo bar)". |  |  |
| N rule |  |  |
| Specific repetition: "<n>(element)" is equivalent to |  |  |
| "<n>*<n>(element)"; that is, exactly <n> occurrences of (element). |  |  |
| Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three |  |  |
| alphabetic characters. |  |  |
| #rule |  |  |
| A construct "#" is defined, similar to "*", for defining lists of |  |  |
| elements. The full form is "<n>#<m>element" indicating at least |  |  |
| <n> and at most <m> elements, each separated by one or more commas |  |  |
| (",") and OPTIONAL linear white space (LWS). This makes the usual |  |  |
| form of lists very easy; a rule such as |  |  |
| ( *LWS element *( *LWS "," *LWS element )) |  |  |
| can be shown as |  |  |
| 1#element |  |  |
| Wherever this construct is used, null elements are allowed, but do |  |  |
| not contribute to the count of elements present. That is, |  |  |
| "(element), , (element) " is permitted, but counts as only two |  |  |
| elements. Therefore, where at least one element is required, at |  |  |
| least one non-null element MUST be present. Default values are 0 |  |  |
| and infinity so that "#element" allows any number, including zero; |  |  |
| "1#element" requires at least one; and "1#2element" allows one or |  |  |
| two. |  |  |
| ; comment |  |  |
| A semi-colon, set off some distance to the right of rule text, |  |  |
| starts a comment that continues to the end of line. This is a |  |  |
| simple way of including useful notes in parallel with the |  |  |
| specifications. |  |  |
| implied *LWS |  |  |
| The grammar described by this specification is word-based. Except |  |  |
| where noted otherwise, linear white space (LWS) can be included |  |  |
| between any two adjacent words (token or quoted-string), and |  |  |
| between adjacent words and separators, without changing the |  |  |
| interpretation of a field. At least one delimiter (LWS and/or |  |  |
| separators) MUST exist between any two tokens (for the definition |  |  |
| of "token" below), since they would otherwise be interpreted as a |  |  |
| single token. |  |  |
| 2.2. Basic Rules |  |  |
| The following rules are used throughout this specification to |  |  |
| describe basic parsing constructs. The US-ASCII coded character set |  |  |
| is defined by ANSI X3.4-1986 [USASCII]. |  |  |
| OCTET = <any 8-bit sequence of data> |  |  |
| CHAR = <any US-ASCII character (octets 0 - 127)> |  |  |
| UPALPHA = <any US-ASCII uppercase letter "A".."Z"> |  |  |
| LOALPHA = <any US-ASCII lowercase letter "a".."z"> |  |  |
| ALPHA = UPALPHA | LOALPHA |  |  |
| DIGIT = <any US-ASCII digit "0".."9"> |  |  |
| CTL = <any US-ASCII control character |  |  |
| (octets 0 - 31) and DEL (127)> |  |  |
| CR = <US-ASCII CR, carriage return (13)> |  |  |
| LF = <US-ASCII LF, linefeed (10)> |  |  |
| SP = <US-ASCII SP, space (32)> |  |  |
| HT = <US-ASCII HT, horizontal-tab (9)> |  |  |
| <"> = <US-ASCII double-quote mark (34)> |  |  |
| HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all |  |  |
| protocol elements except the entity-body (see Appendix A.3 for |  |  |
| tolerant applications). The end-of-line marker within an entity-body |  |  |
| is defined by its associated media type, as described in Section 3.7. |  |  |
| CRLF = CR LF |  |  |
| HTTP/1.1 header field values can be folded onto multiple lines if the |  |  |
| continuation line begins with a space or horizontal tab. All linear |  |  |
| white space, including folding, has the same semantics as SP. A |  |  |
| recipient MAY replace any linear white space with a single SP before |  |  |
| interpreting the field value or forwarding the message downstream. |  |  |
| LWS = [CRLF] 1*( SP | HT ) |  |  |
| The TEXT rule is only used for descriptive field contents and values |  |  |
| that are not intended to be interpreted by the message parser. Words |  |  |
| of *TEXT MAY contain characters from character sets other than ISO- |  |  |
| 8859-1 [ISO-8859] only when encoded according to the rules of RFC |  |  |
| 2047 [RFC2047]. |  |  |
| TEXT = <any OCTET except CTLs, |  |  |
| but including LWS> |  |  |
| A CRLF is allowed in the definition of TEXT only as part of a header |  |  |
| field continuation. It is expected that the folding LWS will be |  |  |
| replaced with a single SP before interpretation of the TEXT value. |  |  |
| Hexadecimal numeric characters are used in several protocol elements. |  |  |
| HEX = "A" | "B" | "C" | "D" | "E" | "F" |  |  |
| | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT |  |  |
| Many HTTP/1.1 header field values consist of words separated by LWS |  |  |
| or special characters. These special characters MUST be in a quoted |  |  |
| string to be used within a parameter value (as defined in |  |  |
| Section 3.6). |  |  |
| token = 1*<any CHAR except CTLs or separators> |  |  |
| separators = "(" | ")" | "<" | ">" | "@" |  |  |
| | "," | ";" | ":" | "\" | <"> |  |  |
| | "/" | "[" | "]" | "?" | "=" |  |  |
| | "{" | "}" | SP | HT |  |  |
| Comments can be included in some HTTP header fields by surrounding |  |  |
| the comment text with parentheses. Comments are only allowed in |  |  |
| fields containing "comment" as part of their field value definition. |  |  |
| In all other fields, parentheses are considered part of the field |  |  |
| value. |  |  |
| comment = "(" *( ctext | quoted-pair | comment ) ")" |  |  |
| ctext = <any TEXT excluding "(" and ")"> |  |  |
| A string of text is parsed as a single word if it is quoted using |  |  |
| double-quote marks. |  |  |
| quoted-string = ( <"> *(qdtext | quoted-pair ) <"> ) |  |  |
| qdtext = <any TEXT except <">> |  |  |
| The backslash character ("\") MAY be used as a single-character |  |  |
| quoting mechanism only within quoted-string and comment constructs. |  |  |
| quoted-pair = "\" CHAR |  |  |
| 3. Protocol Parameters |  |  |
| 3.1. HTTP Version |  |  |
| HTTP uses a "<major>.<minor>" numbering scheme to indicate versions |  |  |
| of the protocol. The protocol versioning policy is intended to allow |  |  |
| the sender to indicate the format of a message and its capacity for |  |  |
| understanding further HTTP communication, rather than the features |  |  |
| obtained via that communication. No change is made to the version |  |  |
| number for the addition of message components which do not affect |  |  |
| communication behavior or which only add to extensible field values. |  |  |
| The <minor> number is incremented when the changes made to the |  |  |
| protocol add features which do not change the general message parsing |  |  |
| algorithm, but which may add to the message semantics and imply |  |  |
| additional capabilities of the sender. The <major> number is |  |  |
| incremented when the format of a message within the protocol is |  |  |
| changed. See RFC 2145 [RFC2145] for a fuller explanation. |  |  |
| The version of an HTTP message is indicated by an HTTP-Version field |  |  |
| in the first line of the message. |  |  |
| HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT |  |  |
| Note that the major and minor numbers MUST be treated as separate |  |  |
| integers and that each MAY be incremented higher than a single digit. |  |  |
| Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is |  |  |
| lower than HTTP/12.3. Leading zeros MUST be ignored by recipients |  |  |
| and MUST NOT be sent. |  |  |
| An application that sends a request or response message that includes |  |  |
| HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant |  |  |
| with this specification. Applications that are at least |  |  |
| conditionally compliant with this specification SHOULD use an HTTP- |  |  |
| Version of "HTTP/1.1" in their messages, and MUST do so for any |  |  |
| message that is not compatible with HTTP/1.0. For more details on |  |  |
| when to send specific HTTP-Version values, see RFC 2145 [RFC2145]. |  |  |
| The HTTP version of an application is the highest HTTP version for |  |  |
| which the application is at least conditionally compliant. |  |  |
| Proxy and gateway applications need to be careful when forwarding |  |  |
| messages in protocol versions different from that of the application. |  |  |
| Since the protocol version indicates the protocol capability of the |  |  |
| sender, a proxy/gateway MUST NOT send a message with a version |  |  |
| indicator which is greater than its actual version. If a higher |  |  |
| version request is received, the proxy/gateway MUST either downgrade |  |  |
| the request version, or respond with an error, or switch to tunnel |  |  |
| behavior. |  |  |
| Due to interoperability problems with HTTP/1.0 proxies discovered |  |  |
| since the publication of RFC 2068 [RFC2068], caching proxies MUST, |  |  |
| gateways MAY, and tunnels MUST NOT upgrade the request to the highest |  |  |
| version they support. The proxy/gateway's response to that request |  |  |
| MUST be in the same major version as the request. |  |  |
| Note: Converting between versions of HTTP may involve modification |  |  |
| of header fields required or forbidden by the versions involved. |  |  |
| 3.2. Uniform Resource Identifiers |  |  |
| URIs have been known by many names: WWW addresses, Universal Document |  |  |
| Identifiers, Universal Resource Identifiers [RFC1630], and finally |  |  |
| the combination of Uniform Resource Locators (URL) [RFC1738] and |  |  |
| Names (URN) [RFC1737]. As far as HTTP is concerned, Uniform Resource |  |  |
| Identifiers are simply formatted strings which identify--via name, |  |  |
| location, or any other characteristic--a resource. |  |  |
| 3.2.1. General Syntax |  |  |
| URIs in HTTP can be represented in absolute form or relative to some |  |  |
| known base URI [RFC1808], depending upon the context of their use. |  |  |
| The two forms are differentiated by the fact that absolute URIs |  |  |
| always begin with a scheme name followed by a colon. For definitive |  |  |
| information on URL syntax and semantics, see "Uniform Resource |  |  |
| Identifiers (URI): Generic Syntax and Semantics," RFC 2396 [RFC2396] |  |  |
| (which replaces RFCs 1738 [RFC1738] and RFC 1808 [RFC1808]). This |  |  |
| specification adopts the definitions of "URI-reference", |  |  |
| "absoluteURI", "relativeURI", "port", "host","abs_path", "rel_path", |  |  |
| and "authority" from that specification. |  |  |
| The HTTP protocol does not place any a priori limit on the length of |  |  |
| a URI. Servers MUST be able to handle the URI of any resource they |  |  |
| serve, and SHOULD be able to handle URIs of unbounded length if they |  |  |
| provide GET-based forms that could generate such URIs. A server |  |  |
| SHOULD return 414 (Request-URI Too Long) status if a URI is longer |  |  |
| than the server can handle (see Section 10.4.15). |  |  |
| Note: Servers ought to be cautious about depending on URI lengths |  |  |
| above 255 bytes, because some older client or proxy |  |  |
| implementations might not properly support these lengths. |  |  |
| 3.2.2. http URL |  |  |
| The "http" scheme is used to locate network resources via the HTTP |  |  |
| protocol. This section defines the scheme-specific syntax and |  |  |
| semantics for http URLs. |  |  |
| http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]] |  |  |
| If the port is empty or not given, port 80 is assumed. The semantics |  |  |
| are that the identified resource is located at the server listening |  |  |
| for TCP connections on that port of that host, and the Request-URI |  |  |
| for the resource is abs_path (Section 5.1.2). The use of IP |  |  |
| addresses in URLs SHOULD be avoided whenever possible (see RFC 1900 |  |  |
| [RFC1900]). If the abs_path is not present in the URL, it MUST be |  |  |
| given as "/" when used as a Request-URI for a resource |  |  |
| (Section 5.1.2). If a proxy receives a host name which is not a |  |  |
| fully qualified domain name, it MAY add its domain to the host name |  |  |
| it received. If a proxy receives a fully qualified domain name, the |  |  |
| proxy MUST NOT change the host name. |  |  |
| 3.2.3. URI Comparison |  |  |
| When comparing two URIs to decide if they match or not, a client |  |  |
| SHOULD use a case-sensitive octet-by-octet comparison of the entire |  |  |
| URIs, with these exceptions: |  |  |
| o A port that is empty or not given is equivalent to the default |  |  |
| port for that URI-reference; |  |  |
| o Comparisons of host names MUST be case-insensitive; |  |  |
| o Comparisons of scheme names MUST be case-insensitive; |  |  |
| o An empty abs_path is equivalent to an abs_path of "/". |  |  |
| Characters other than those in the "reserved" and "unsafe" sets (see |  |  |
| RFC 2396 [RFC2396]) are equivalent to their ""%" HEX HEX" encoding. |  |  |
| For example, the following three URIs are equivalent: |  |  |
| http://abc.com:80/~smith/home.html |  |  |
| http://ABC.com/%7Esmith/home.html |  |  |
| http://ABC.com:/%7esmith/home.html |  |  |
| 3.3. Date/Time Formats |  |  |
| 3.3.1. Full Date |  |  |
| HTTP applications have historically allowed three different formats |  |  |
| for the representation of date/time stamps: |  |  |
| Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123 |  |  |
| Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036 |  |  |
| Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format |  |  |
| The first format is preferred as an Internet standard and represents |  |  |
| a fixed-length subset of that defined by RFC 1123 [RFC1123] (an |  |  |
| update to RFC 822 [RFC822]). The second format is in common use, but |  |  |
| is based on the obsolete RFC 850 [RFC1036] date format and lacks a |  |  |
| four-digit year. HTTP/1.1 clients and servers that parse the date |  |  |
| value MUST accept all three formats (for compatibility with |  |  |
| HTTP/1.0), though they MUST only generate the RFC 1123 format for |  |  |
| representing HTTP-date values in header fields. See Appendix A.3 for |  |  |
| further information. |  |  |
| Note: Recipients of date values are encouraged to be robust in |  |  |
| accepting date values that may have been sent by non-HTTP |  |  |
| applications, as is sometimes the case when retrieving or posting |  |  |
| messages via proxies/gateways to SMTP or NNTP. |  |  |
| All HTTP date/time stamps MUST be represented in Greenwich Mean Time |  |  |
| (GMT), without exception. For the purposes of HTTP, GMT is exactly |  |  |
| equal to UTC (Coordinated Universal Time). This is indicated in the |  |  |
| first two formats by the inclusion of "GMT" as the three-letter |  |  |
| abbreviation for time zone, and MUST be assumed when reading the |  |  |
| asctime format. HTTP-date is case sensitive and MUST NOT include |  |  |
| additional LWS beyond that specifically included as SP in the |  |  |
| grammar. |  |  |
| HTTP-date = rfc1123-date | rfc850-date | asctime-date |  |  |
| rfc1123-date = wkday "," SP date1 SP time SP "GMT" |  |  |
| rfc850-date = weekday "," SP date2 SP time SP "GMT" |  |  |
| asctime-date = wkday SP date3 SP time SP 4DIGIT |  |  |
| date1 = 2DIGIT SP month SP 4DIGIT |  |  |
| ; day month year (e.g., 02 Jun 1982) |  |  |
| date2 = 2DIGIT "-" month "-" 2DIGIT |  |  |
| ; day-month-year (e.g., 02-Jun-82) |  |  |
| date3 = month SP ( 2DIGIT | ( SP 1DIGIT )) |  |  |
| ; month day (e.g., Jun 2) |  |  |
| time = 2DIGIT ":" 2DIGIT ":" 2DIGIT |  |  |
| ; 00:00:00 - 23:59:59 |  |  |
| wkday = "Mon" | "Tue" | "Wed" |  |  |
| | "Thu" | "Fri" | "Sat" | "Sun" |  |  |
| weekday = "Monday" | "Tuesday" | "Wednesday" |  |  |
| | "Thursday" | "Friday" | "Saturday" | "Sunday" |  |  |
| month = "Jan" | "Feb" | "Mar" | "Apr" |  |  |
| | "May" | "Jun" | "Jul" | "Aug" |  |  |
| | "Sep" | "Oct" | "Nov" | "Dec" |  |  |
| Note: HTTP requirements for the date/time stamp format apply only to |  |  |
| their usage within the protocol stream. Clients and servers are not |  |  |
| required to use these formats for user presentation, request logging, |  |  |
| etc. |  |  |
| 3.3.2. Delta Seconds |  |  |
| Some HTTP header fields allow a time value to be specified as an |  |  |
| integer number of seconds, represented in decimal, after the time |  |  |
| that the message was received. |  |  |
| delta-seconds = 1*DIGIT |  |  |
| 3.4. Character Sets |  |  |
| HTTP uses the same definition of the term "character set" as that |  |  |
| described for MIME: |  |  |
| The term "character set" is used in this document to refer to a |  |  |
| method used with one or more tables to convert a sequence of octets |  |  |
| into a sequence of characters. Note that unconditional conversion in |  |  |
| the other direction is not required, in that not all characters may |  |  |
| be available in a given character set and a character set may provide |  |  |
| more than one sequence of octets to represent a particular character. |  |  |
| This definition is intended to allow various kinds of character |  |  |
| encoding, from simple single-table mappings such as US-ASCII to |  |  |
| complex table switching methods such as those that use ISO-2022's |  |  |
| techniques. However, the definition associated with a MIME character |  |  |
| set name MUST fully specify the mapping to be performed from octets |  |  |
| to characters. In particular, use of external profiling information |  |  |
| to determine the exact mapping is not permitted. |  |  |
| Note: This use of the term "character set" is more commonly |  |  |
| referred to as a "character encoding." However, since HTTP and |  |  |
| MIME share the same registry, it is important that the terminology |  |  |
| also be shared. |  |  |
| HTTP character sets are identified by case-insensitive tokens. The |  |  |
| complete set of tokens is defined by the IANA Character Set registry |  |  |
| [RFC1700]. |  |  |
| charset = token |  |  |
| Although HTTP allows an arbitrary token to be used as a charset |  |  |
| value, any token that has a predefined value within the IANA |  |  |
| Character Set registry [RFC1700] MUST represent the character set |  |  |
| defined by that registry. Applications SHOULD limit their use of |  |  |
| character sets to those defined by the IANA registry. |  |  |
| Implementors should be aware of IETF character set requirements |  |  |
| [RFC2279] [RFC2277]. |  |  |
| 3.4.1. Missing Charset |  |  |
| Some HTTP/1.0 software has interpreted a Content-Type header without |  |  |
| charset parameter incorrectly to mean "recipient should guess." |  |  |
| Senders wishing to defeat this behavior MAY include a charset |  |  |
| parameter even when the charset is ISO-8859-1 and SHOULD do so when |  |  |
| it is known that it will not confuse the recipient. |  |  |
| Unfortunately, some older HTTP/1.0 clients did not deal properly with |  |  |
| an explicit charset parameter. HTTP/1.1 recipients MUST respect the |  |  |
| charset label provided by the sender; and those user agents that have |  |  |
| a provision to "guess" a charset MUST use the charset from the |  |  |
| content-type field if they support that charset, rather than the |  |  |
| recipient's preference, when initially displaying a document. See |  |  |
| Section 3.7.1. |  |  |
| 3.5. Content Codings |  |  |
| Content coding values indicate an encoding transformation that has |  |  |
| been or can be applied to an entity. Content codings are primarily |  |  |
| used to allow a document to be compressed or otherwise usefully |  |  |
| transformed without losing the identity of its underlying media type |  |  |
| and without loss of information. Frequently, the entity is stored in |  |  |
| coded form, transmitted directly, and only decoded by the recipient. |  |  |
| content-coding = token |  |  |
| All content-coding values are case-insensitive. HTTP/1.1 uses |  |  |
| content-coding values in the Accept-Encoding (Section 14.3) and |  |  |
| Content-Encoding (Section 14.11) header fields. Although the value |  |  |
| describes the content-coding, what is more important is that it |  |  |
| indicates what decoding mechanism will be required to remove the |  |  |
| encoding. |  |  |
| The Internet Assigned Numbers Authority (IANA) acts as a registry for |  |  |
| content-coding value tokens. Initially, the registry contains the |  |  |
| following tokens: |  |  |
| gzip |  |  |
| An encoding format produced by the file compression program "gzip" |  |  |
| (GNU zip) as described in RFC 1952 [RFC1952]. This format is a |  |  |
| Lempel-Ziv coding (LZ77) with a 32 bit CRC. |  |  |
| compress |  |  |
| The encoding format produced by the common UNIX file compression |  |  |
| program "compress". This format is an adaptive Lempel-Ziv-Welch |  |  |
| coding (LZW). |  |  |
| Use of program names for the identification of encoding formats is |  |  |
| not desirable and is discouraged for future encodings. Their use |  |  |
| here is representative of historical practice, not good design. |  |  |
| For compatibility with previous implementations of HTTP, |  |  |
| applications SHOULD consider "x-gzip" and "x-compress" to be |  |  |
| equivalent to "gzip" and "compress" respectively. |  |  |
| deflate |  |  |
| The "zlib" format defined in RFC 1950 [RFC1950] in combination |  |  |
| with the "deflate" compression mechanism described in RFC 1951 |  |  |
| [RFC1951]. |  |  |
| identity |  |  |
| The default (identity) encoding; the use of no transformation |  |  |
| whatsoever. This content-coding is used only in the Accept- |  |  |
| Encoding header, and SHOULD NOT be used in the Content-Encoding |  |  |
| header. |  |  |
| New content-coding value tokens SHOULD be registered; to allow |  |  |
| interoperability between clients and servers, specifications of the |  |  |
| content coding algorithms needed to implement a new value SHOULD be |  |  |
| publicly available and adequate for independent implementation, and |  |  |
| conform to the purpose of content coding defined in this section. |  |  |
| 3.6. Transfer Codings |  |  |
| Transfer-coding values are used to indicate an encoding |  |  |
| transformation that has been, can be, or may need to be applied to an |  |  |
| entity-body in order to ensure "safe transport" through the network. |  |  |
| This differs from a content coding in that the transfer-coding is a |  |  |
| property of the message, not of the original entity. |  |  |
| transfer-coding = "chunked" | transfer-extension |  |  |
| transfer-extension = token *( ";" parameter ) |  |  |
| Parameters are in the form of attribute/value pairs. |  |  |
| parameter = attribute "=" value |  |  |
| attribute = token |  |  |
| value = token | quoted-string |  |  |
| All transfer-coding values are case-insensitive. HTTP/1.1 uses |  |  |
| transfer-coding values in the TE header field (Section 14.39) and in |  |  |
| the Transfer-Encoding header field (Section 14.41). |  |  |
| Whenever a transfer-coding is applied to a message-body, the set of |  |  |
| transfer-codings MUST include "chunked", unless the message is |  |  |
| terminated by closing the connection. When the "chunked" transfer- |  |  |
| coding is used, it MUST be the last transfer-coding applied to the |  |  |
| message-body. The "chunked" transfer-coding MUST NOT be applied more |  |  |
| than once to a message-body. These rules allow the recipient to |  |  |
| determine the transfer-length of the message (Section 4.4). |  |  |
| Transfer-codings are analogous to the Content-Transfer-Encoding |  |  |
| values of MIME [RFC2045], which were designed to enable safe |  |  |
| transport of binary data over a 7-bit transport service. However, |  |  |
| safe transport has a different focus for an 8bit-clean transfer |  |  |
| protocol. In HTTP, the only unsafe characteristic of message-bodies |  |  |
| is the difficulty in determining the exact body length |  |  |
| (Section 7.2.2), or the desire to encrypt data over a shared |  |  |
| transport. |  |  |
| The Internet Assigned Numbers Authority (IANA) acts as a registry for |  |  |
| transfer-coding value tokens. Initially, the registry contains the |  |  |
| following tokens: "chunked" (Section 3.6.1), "identity" (section |  |  |
| 3.6.2), "gzip" (Section 3.5), "compress" (Section 3.5), and "deflate" |  |  |
| (Section 3.5). |  |  |
| New transfer-coding value tokens SHOULD be registered in the same way |  |  |
| as new content-coding value tokens (Section 3.5). |  |  |
| A server which receives an entity-body with a transfer-coding it does |  |  |
| not understand SHOULD return 501 (Unimplemented), and close the |  |  |
| connection. A server MUST NOT send transfer-codings to an HTTP/1.0 |  |  |
| client. |  |  |
| 3.6.1. Chunked Transfer Coding |  |  |
| The chunked encoding modifies the body of a message in order to |  |  |
| transfer it as a series of chunks, each with its own size indicator, |  |  |
| followed by an OPTIONAL trailer containing entity-header fields. |  |  |
| This allows dynamically produced content to be transferred along with |  |  |
| the information necessary for the recipient to verify that it has |  |  |
| received the full message. |  |  |
| Chunked-Body = *chunk |  |  |
| last-chunk |  |  |
| trailer |  |  |
| CRLF |  |  |
| chunk = chunk-size [ chunk-extension ] CRLF |  |  |
| chunk-data CRLF |  |  |
| chunk-size = 1*HEX |  |  |
| last-chunk = 1*("0") [ chunk-extension ] CRLF |  |  |
| chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] ) |  |  |
| chunk-ext-name = token |  |  |
| chunk-ext-val = token | quoted-string |  |  |
| chunk-data = chunk-size(OCTET) |  |  |
| trailer = *(entity-header CRLF) |  |  |
| The chunk-size field is a string of hex digits indicating the size of |  |  |
| the chunk. The chunked encoding is ended by any chunk whose size is |  |  |
| zero, followed by the trailer, which is terminated by an empty line. |  |  |
| The trailer allows the sender to include additional HTTP header |  |  |
| fields at the end of the message. The Trailer header field can be |  |  |
| used to indicate which header fields are included in a trailer (see |  |  |
| Section 14.40). |  |  |
| A server using chunked transfer-coding in a response MUST NOT use the |  |  |
| trailer for any header fields unless at least one of the following is |  |  |
| true: |  |  |
| 1. the request included a TE header field that indicates "trailers" |  |  |
| is acceptable in the transfer-coding of the response, as |  |  |
| described in Section 14.39; or, |  |  |
| 2. the server is the origin server for the response, the trailer |  |  |
| fields consist entirely of optional metadata, and the recipient |  |  |
| could use the message (in a manner acceptable to the origin |  |  |
| server) without receiving this metadata. In other words, the |  |  |
| origin server is willing to accept the possibility that the |  |  |
| trailer fields might be silently discarded along the path to the |  |  |
| client. |  |  |
| This requirement prevents an interoperability failure when the |  |  |
| message is being received by an HTTP/1.1 (or later) proxy and |  |  |
| forwarded to an HTTP/1.0 recipient. It avoids a situation where |  |  |
| compliance with the protocol would have necessitated a possibly |  |  |
| infinite buffer on the proxy. |  |  |
| An example process for decoding a Chunked-Body is presented in |  |  |
| Appendix A.4.6. |  |  |
| All HTTP/1.1 applications MUST be able to receive and decode the |  |  |
| "chunked" transfer-coding, and MUST ignore chunk-extension extensions |  |  |
| they do not understand. |  |  |
| 3.7. Media Types |  |  |
| HTTP uses Internet Media Types [RFC1590] in the Content-Type |  |  |
| (Section 14.17) and Accept (Section 14.1) header fields in order to |  |  |
| provide open and extensible data typing and type negotiation. |  |  |
| media-type = type "/" subtype *( ";" parameter ) |  |  |
| type = token |  |  |
| subtype = token |  |  |
| Parameters MAY follow the type/subtype in the form of attribute/value |  |  |
| pairs (as defined in Section 3.6). |  |  |
| The type, subtype, and parameter attribute names are case- |  |  |
| insensitive. Parameter values might or might not be case-sensitive, |  |  |
| depending on the semantics of the parameter name. Linear white space |  |  |
| (LWS) MUST NOT be used between the type and subtype, nor between an |  |  |
| attribute and its value. The presence or absence of a parameter |  |  |
| might be significant to the processing of a media-type, depending on |  |  |
| its definition within the media type registry. |  |  |
| Note that some older HTTP applications do not recognize media type |  |  |
| parameters. When sending data to older HTTP applications, |  |  |
| implementations SHOULD only use media type parameters when they are |  |  |
| required by that type/subtype definition. |  |  |
| Media-type values are registered with the Internet Assigned Number |  |  |
| Authority (IANA [RFC1700]). The media type registration process is |  |  |
| outlined in RFC 1590 [RFC1590]. Use of non-registered media types is |  |  |
| discouraged. |  |  |
| 3.7.1. Canonicalization and Text Defaults |  |  |
| Internet media types are registered with a canonical form. An |  |  |
| entity-body transferred via HTTP messages MUST be represented in the |  |  |
| appropriate canonical form prior to its transmission except for |  |  |
| "text" types, as defined in the next paragraph. |  |  |
| When in canonical form, media subtypes of the "text" type use CRLF as |  |  |
| the text line break. HTTP relaxes this requirement and allows the |  |  |
| transport of text media with plain CR or LF alone representing a line |  |  |
| break when it is done consistently for an entire entity-body. HTTP |  |  |
| applications MUST accept CRLF, bare CR, and bare LF as being |  |  |
| representative of a line break in text media received via HTTP. In |  |  |
| addition, if the text is represented in a character set that does not |  |  |
| use octets 13 and 10 for CR and LF respectively, as is the case for |  |  |
| some multi-byte character sets, HTTP allows the use of whatever octet |  |  |
| sequences are defined by that character set to represent the |  |  |
| equivalent of CR and LF for line breaks. This flexibility regarding |  |  |
| line breaks applies only to text media in the entity-body; a bare CR |  |  |
| or LF MUST NOT be substituted for CRLF within any of the HTTP control |  |  |
| structures (such as header fields and multipart boundaries). |  |  |
| If an entity-body is encoded with a content-coding, the underlying |  |  |
| data MUST be in a form defined above prior to being encoded. |  |  |
| The "charset" parameter is used with some media types to define the |  |  |
| character set (Section 3.4) of the data. When no explicit charset |  |  |
| parameter is provided by the sender, media subtypes of the "text" |  |  |
| type are defined to have a default charset value of "ISO-8859-1" when |  |  |
| received via HTTP. Data in character sets other than "ISO-8859-1" or |  |  |
| its subsets MUST be labeled with an appropriate charset value. See |  |  |
| Section 3.4.1 for compatibility problems. |  |  |
| 3.7.2. Multipart Types |  |  |
| MIME provides for a number of "multipart" types -- encapsulations of |  |  |
| one or more entities within a single message-body. All multipart |  |  |
| types share a common syntax, as defined in section 5.1.1 of RFC 2046 |  |  |
| [RFC2046], and MUST include a boundary parameter as part of the media |  |  |
| type value. The message body is itself a protocol element and MUST |  |  |
| therefore use only CRLF to represent line breaks between body-parts. |  |  |
| Unlike in RFC 2046, the epilogue of any multipart message MUST be |  |  |
| empty; HTTP applications MUST NOT transmit the epilogue (even if the |  |  |
| original multipart contains an epilogue). These restrictions exist |  |  |
| in order to preserve the self-delimiting nature of a multipart |  |  |
| message-body, wherein the "end" of the message-body is indicated by |  |  |
| the ending multipart boundary. |  |  |
| In general, HTTP treats a multipart message-body no differently than |  |  |
| any other media type: strictly as payload. The one exception is the |  |  |
| "multipart/byteranges" type (Appendix A.2) when it appears in a 206 |  |  |
| (Partial Content) response, which will be interpreted by some HTTP |  |  |
| caching mechanisms as described in sections 13.5.4 and 14.16. In all |  |  |
| other cases, an HTTP user agent SHOULD follow the same or similar |  |  |
| behavior as a MIME user agent would upon receipt of a multipart type. |  |  |
| The MIME header fields within each body-part of a multipart message- |  |  |
| body do not have any significance to HTTP beyond that defined by |  |  |
| their MIME semantics. |  |  |
| In general, an HTTP user agent SHOULD follow the same or similar |  |  |
| behavior as a MIME user agent would upon receipt of a multipart type. |  |  |
| If an application receives an unrecognized multipart subtype, the |  |  |
| application MUST treat it as being equivalent to "multipart/mixed". |  |  |
| Note: The "multipart/form-data" type has been specifically defined |  |  |
| for carrying form data suitable for processing via the POST |  |  |
| request method, as described in RFC 1867 [RFC1867]. |  |  |
| 3.8. Product Tokens |  |  |
| Product tokens are used to allow communicating applications to |  |  |
| identify themselves by software name and version. Most fields using |  |  |
| product tokens also allow sub-products which form a significant part |  |  |
| of the application to be listed, separated by white space. By |  |  |
| convention, the products are listed in order of their significance |  |  |
| for identifying the application. |  |  |
| product = token ["/" product-version] |  |  |
| product-version = token |  |  |
| Examples: |  |  |
| User-Agent: CERN-LineMode/2.15 libwww/2.17b3 |  |  |
| Server: Apache/0.8.4 |  |  |
| Product tokens SHOULD be short and to the point. They MUST NOT be |  |  |
| used for advertising or other non-essential information. Although |  |  |
| any token character MAY appear in a product-version, this token |  |  |
| SHOULD only be used for a version identifier (i.e., successive |  |  |
| versions of the same product SHOULD only differ in the product- |  |  |
| version portion of the product value). |  |  |
| 3.9. Quality Values |  |  |
| HTTP content negotiation (Section 12) uses short "floating point" |  |  |
| numbers to indicate the relative importance ("weight") of various |  |  |
| negotiable parameters. A weight is normalized to a real number in |  |  |
| the range 0 through 1, where 0 is the minimum and 1 the maximum |  |  |
| value. If a parameter has a quality value of 0, then content with |  |  |
| this parameter is `not acceptable' for the client. HTTP/1.1 |  |  |
| applications MUST NOT generate more than three digits after the |  |  |
| decimal point. User configuration of these values SHOULD also be |  |  |
| limited in this fashion. |  |  |
| qvalue = ( "0" [ "." 0*3DIGIT ] ) |  |  |
| | ( "1" [ "." 0*3("0") ] ) |  |  |
| "Quality values" is a misnomer, since these values merely represent |  |  |
| relative degradation in desired quality. |  |  |
| 3.10. Language Tags |  |  |
| A language tag identifies a natural language spoken, written, or |  |  |
| otherwise conveyed by human beings for communication of information |  |  |
| to other human beings. Computer languages are explicitly excluded. |  |  |
| HTTP uses language tags within the Accept-Language and Content- |  |  |
| Language fields. |  |  |
| The syntax and registry of HTTP language tags is the same as that |  |  |
| defined by RFC 1766 [RFC1766]. In summary, a language tag is |  |  |
| composed of 1 or more parts: A primary language tag and a possibly |  |  |
| empty series of subtags: |  |  |
| language-tag = primary-tag *( "-" subtag ) |  |  |
| primary-tag = 1*8ALPHA |  |  |
| subtag = 1*8ALPHA |  |  |
| White space is not allowed within the tag and all tags are case- |  |  |
| insensitive. The name space of language tags is administered by the |  |  |
| IANA. Example tags include: |  |  |
| en, en-US, en-cockney, i-cherokee, x-pig-latin |  |  |
| where any two-letter primary-tag is an ISO-639 language abbreviation |  |  |
| and any two-letter initial subtag is an ISO-3166 country code. (The |  |  |
| last three tags above are not registered tags; all but the last are |  |  |
| examples of tags which could be registered in future.) |  |  |
| 3.11. Entity Tags |  | 2. Entity Tags |
| Entity tags are used for comparing two or more entities from the same |  | Entity tags are used for comparing two or more entities from the same |
| requested resource. HTTP/1.1 uses entity tags in the ETag |  | requested resource. HTTP/1.1 uses entity tags in the ETag |
| (Section 14.19), If-Match (Section 14.24), If-None-Match |  | (Section 6.1), If-Match (Section 6.2), If-None-Match (Section 6.4), |
| (Section 14.26), and If-Range (Section 14.27) header fields. The |  | and If-Range (Section 5.3 of [Part5]) header fields. The definition |
| definition of how they are used and compared as cache validators is |  | of how they are used and compared as cache validators is in |
| in Section 13.3.3. An entity tag consists of an opaque quoted |  | Section 4. An entity tag consists of an opaque quoted string, |
| string, possibly prefixed by a weakness indicator. |  | possibly prefixed by a weakness indicator. |
| entity-tag = [ weak ] opaque-tag |  | entity-tag = [ weak ] opaque-tag |
| weak = "W/" |  | weak = "W/" |
| opaque-tag = quoted-string |  | opaque-tag = quoted-string |
| A "strong entity tag" MAY be shared by two entities of a resource |  | A "strong entity tag" MAY be shared by two entities of a resource |
| only if they are equivalent by octet equality. |  | only if they are equivalent by octet equality. |
| A "weak entity tag," indicated by the "W/" prefix, MAY be shared by |  | A "weak entity tag," indicated by the "W/" prefix, MAY be shared by |
| two entities of a resource only if the entities are equivalent and |  | two entities of a resource only if the entities are equivalent and |
| could be substituted for each other with no significant change in |  | could be substituted for each other with no significant change in |
| semantics. A weak entity tag can only be used for weak comparison. |  | semantics. A weak entity tag can only be used for weak comparison. |
| An entity tag MUST be unique across all versions of all entities |  | An entity tag MUST be unique across all versions of all entities |
| associated with a particular resource. A given entity tag value MAY |  | associated with a particular resource. A given entity tag value MAY |
| be used for entities obtained by requests on different URIs. The use |  | be used for entities obtained by requests on different URIs. The use |
| of the same entity tag value in conjunction with entities obtained by |  | of the same entity tag value in conjunction with entities obtained by |
| requests on different URIs does not imply the equivalence of those |  | requests on different URIs does not imply the equivalence of those |
| entities. |  | entities. |
| 3.12. Range Units |  | 3. Status Code Definitions |
| HTTP/1.1 allows a client to request that only part (a range of) the |  |  |
| response entity be included within the response. HTTP/1.1 uses range |  |  |
| units in the Range (Section 14.35) and Content-Range (Section 14.16) |  |  |
| header fields. An entity can be broken down into subranges according |  |  |
| to various structural units. |  |  |
| range-unit = bytes-unit | other-range-unit |  |  |
| bytes-unit = "bytes" |  |  |
| other-range-unit = token |  |  |
| The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 |  |  |
| implementations MAY ignore ranges specified using other units. |  |  |
| HTTP/1.1 has been designed to allow implementations of applications |  |  |
| that do not depend on knowledge of ranges. |  |  |
| 4. HTTP Message |  |  |
| 4.1. Message Types |  |  |
| HTTP messages consist of requests from client to server and responses |  |  |
| from server to client. |  |  |
| HTTP-message = Request | Response ; HTTP/1.1 messages |  |  |
| Request (Section 5) and Response (Section 6) messages use the generic |  |  |
| message format of RFC 822 [RFC822] for transferring entities (the |  |  |
| payload of the message). Both types of message consist of a start- |  |  |
| line, zero or more header fields (also known as "headers"), an empty |  |  |
| line (i.e., a line with nothing preceding the CRLF) indicating the |  |  |
| end of the header fields, and possibly a message-body. |  |  |
| generic-message = start-line |  |  |
| *(message-header CRLF) |  |  |
| CRLF |  |  |
| [ message-body ] |  |  |
| start-line = Request-Line | Status-Line |  |  |
| In the interest of robustness, servers SHOULD ignore any empty |  |  |
| line(s) received where a Request-Line is expected. In other words, |  |  |
| if the server is reading the protocol stream at the beginning of a |  |  |
| message and receives a CRLF first, it should ignore the CRLF. |  |  |
| Certain buggy HTTP/1.0 client implementations generate extra CRLF's |  |  |
| after a POST request. To restate what is explicitly forbidden by the |  |  |
| BNF, an HTTP/1.1 client MUST NOT preface or follow a request with an |  |  |
| extra CRLF. |  |  |
| 4.2. Message Headers |  |  |
| HTTP header fields, which include general-header (Section 4.5), |  |  |
| request-header (Section 5.3), response-header (Section 6.2), and |  |  |
| entity-header (Section 7.1) fields, follow the same generic format as |  |  |
| that given in Section 3.1 of RFC 822 [RFC822]. Each header field |  |  |
| consists of a name followed by a colon (":") and the field value. |  |  |
| Field names are case-insensitive. The field value MAY be preceded by |  |  |
| any amount of LWS, though a single SP is preferred. Header fields |  |  |
| can be extended over multiple lines by preceding each extra line with |  |  |
| at least one SP or HT. Applications ought to follow "common form", |  |  |
| where one is known or indicated, when generating HTTP constructs, |  |  |
| since there might exist some implementations that fail to accept |  |  |
| anything beyond the common forms. |  |  |
| message-header = field-name ":" [ field-value ] |  |  |
| field-name = token |  |  |
| field-value = *( field-content | LWS ) |  |  |
| field-content = <the OCTETs making up the field-value |  |  |
| and consisting of either *TEXT or combinations |  |  |
| of token, separators, and quoted-string> |  |  |
| The field-content does not include any leading or trailing LWS: |  |  |
| linear white space occurring before the first non-whitespace |  |  |
| character of the field-value or after the last non-whitespace |  |  |
| character of the field-value. Such leading or trailing LWS MAY be |  |  |
| removed without changing the semantics of the field value. Any LWS |  |  |
| that occurs between field-content MAY be replaced with a single SP |  |  |
| before interpreting the field value or forwarding the message |  |  |
| downstream. |  |  |
| The order in which header fields with differing field names are |  |  |
| received is not significant. However, it is "good practice" to send |  |  |
| general-header fields first, followed by request-header or response- |  |  |
| header fields, and ending with the entity-header fields. |  |  |
| Multiple message-header fields with the same field-name MAY be |  |  |
| present in a message if and only if the entire field-value for that |  |  |
| header field is defined as a comma-separated list [i.e., #(values)]. |  |  |
| It MUST be possible to combine the multiple header fields into one |  |  |
| "field-name: field-value" pair, without changing the semantics of the |  |  |
| message, by appending each subsequent field-value to the first, each |  |  |
| separated by a comma. The order in which header fields with the same |  |  |
| field-name are received is therefore significant to the |  |  |
| interpretation of the combined field value, and thus a proxy MUST NOT |  |  |
| change the order of these field values when a message is forwarded. |  |  |
| 4.3. Message Body |  |  |
| The message-body (if any) of an HTTP message is used to carry the |  |  |
| entity-body associated with the request or response. The message- |  |  |
| body differs from the entity-body only when a transfer-coding has |  |  |
| been applied, as indicated by the Transfer-Encoding header field |  |  |
| (Section 14.41). |  |  |
| message-body = entity-body |  |  |
| | <entity-body encoded as per Transfer-Encoding> |  |  |
| Transfer-Encoding MUST be used to indicate any transfer-codings |  |  |
| applied by an application to ensure safe and proper transfer of the |  |  |
| message. Transfer-Encoding is a property of the message, not of the |  |  |
| entity, and thus MAY be added or removed by any application along the |  |  |
| request/response chain. (However, Section 3.6 places restrictions on |  |  |
| when certain transfer-codings may be used.) |  |  |
| The rules for when a message-body is allowed in a message differ for |  |  |
| requests and responses. |  |  |
| The presence of a message-body in a request is signaled by the |  |  |
| inclusion of a Content-Length or Transfer-Encoding header field in |  |  |
| the request's message-headers. A message-body MUST NOT be included |  |  |
| in a request if the specification of the request method |  |  |
| (Section 5.1.1) does not allow sending an entity-body in requests. A |  |  |
| server SHOULD read and forward a message-body on any request; if the |  |  |
| request method does not include defined semantics for an entity-body, |  |  |
| then the message-body SHOULD be ignored when handling the request. |  |  |
| For response messages, whether or not a message-body is included with |  |  |
| a message is dependent on both the request method and the response |  |  |
| status code (Section 6.1.1). All responses to the HEAD request |  |  |
| method MUST NOT include a message-body, even though the presence of |  |  |
| entity-header fields might lead one to believe they do. All 1xx |  |  |
| (informational), 204 (no content), and 304 (not modified) responses |  |  |
| MUST NOT include a message-body. All other responses do include a |  |  |
| message-body, although it MAY be of zero length. |  |  |
| 4.4. Message Length |  |  |
| The transfer-length of a message is the length of the message-body as |  |  |
| it appears in the message; that is, after any transfer-codings have |  |  |
| been applied. When a message-body is included with a message, the |  |  |
| transfer-length of that body is determined by one of the following |  |  |
| (in order of precedence): |  |  |
| 1. Any response message which "MUST NOT" include a message-body |  |  |
| (such as the 1xx, 204, and 304 responses and any response to a |  |  |
| HEAD request) is always terminated by the first empty line after |  |  |
| the header fields, regardless of the entity-header fields present |  |  |
| in the message. |  |  |
| 2. If a Transfer-Encoding header field (Section 14.41) is present |  |  |
| and has any value other than "identity", then the transfer-length |  |  |
| is defined by use of the "chunked" transfer-coding (Section 3.6), |  |  |
| unless the message is terminated by closing the connection. |  |  |
| 3. If a Content-Length header field (Section 14.13) is present, its |  |  |
| decimal value in OCTETs represents both the entity-length and the |  |  |
| transfer-length. The Content-Length header field MUST NOT be |  |  |
| sent if these two lengths are different (i.e., if a Transfer- |  |  |
| Encoding header field is present). If a message is received with |  |  |
| both a Transfer-Encoding header field and a Content-Length header |  |  |
| field, the latter MUST be ignored. |  |  |
| 4. If the message uses the media type "multipart/byteranges", and |  |  |
| the ransfer-length is not otherwise specified, then this self- |  |  |
| elimiting media type defines the transfer-length. This media |  |  |
| type UST NOT be used unless the sender knows that the recipient |  |  |
| can arse it; the presence in a request of a Range header with |  |  |
| ultiple byte-range specifiers from a 1.1 client implies that the |  |  |
| lient can parse multipart/byteranges responses. |  |  |
| A range header might be forwarded by a 1.0 proxy that does not |  |  |
| understand multipart/byteranges; in this case the server MUST |  |  |
| delimit the message using methods defined in items 1, 3 or 5 |  |  |
| of this section. |  |  |
| 5. By the server closing the connection. (Closing the connection |  |  |
| cannot be used to indicate the end of a request body, since that |  |  |
| would leave no possibility for the server to send back a |  |  |
| response.) |  |  |
| For compatibility with HTTP/1.0 applications, HTTP/1.1 requests |  |  |
| containing a message-body MUST include a valid Content-Length header |  |  |
| field unless the server is known to be HTTP/1.1 compliant. If a |  |  |
| request contains a message-body and a Content-Length is not given, |  |  |
| the server SHOULD respond with 400 (bad request) if it cannot |  |  |
| determine the length of the message, or with 411 (length required) if |  |  |
| it wishes to insist on receiving a valid Content-Length. |  |  |
| All HTTP/1.1 applications that receive entities MUST accept the |  |  |
| "chunked" transfer-coding (Section 3.6), thus allowing this mechanism |  |  |
| to be used for messages when the message length cannot be determined |  |  |
| in advance. |  |  |
| Messages MUST NOT include both a Content-Length header field and a |  |  |
| non-identity transfer-coding. If the message does include a non- |  |  |
| identity transfer-coding, the Content-Length MUST be ignored. |  |  |
| When a Content-Length is given in a message where a message-body is |  |  |
| allowed, its field value MUST exactly match the number of OCTETs in |  |  |
| the message-body. HTTP/1.1 user agents MUST notify the user when an |  |  |
| invalid length is received and detected. |  |  |
| 4.5. General Header Fields |  |  |
| There are a few header fields which have general applicability for |  |  |
| both request and response messages, but which do not apply to the |  |  |
| entity being transferred. These header fields apply only to the |  |  |
| message being transmitted. |  |  |
| general-header = Cache-Control ; Section 14.9 |  |  |
| | Connection ; Section 14.10 |  |  |
| | Date ; Section 14.18 |  |  |
| | Pragma ; Section 14.32 |  |  |
| | Trailer ; Section 14.40 |  |  |
| | Transfer-Encoding ; Section 14.41 |  |  |
| | Upgrade ; Section 14.42 |  |  |
| | Via ; Section 14.45 |  |  |
| | Warning ; Section 14.46 |  |  |
| General-header field names can be extended reliably only in |  |  |
| combination with a change in the protocol version. However, new or |  |  |
| experimental header fields may be given the semantics of general |  |  |
| header fields if all parties in the communication recognize them to |  |  |
| be general-header fields. Unrecognized header fields are treated as |  |  |
| entity-header fields. |  |  |
| 5. Request |  |  |
| A request message from a client to a server includes, within the |  |  |
| first line of that message, the method to be applied to the resource, |  |  |
| the identifier of the resource, and the protocol version in use. |  |  |
| Request = Request-Line ; Section 5.1 |  |  |
| *(( general-header ; Section 4.5 |  |  |
| | request-header ; Section 5.3 |  |  |
| | entity-header ) CRLF) ; Section 7.1 |  |  |
| CRLF |  |  |
| [ message-body ] ; Section 4.3 |  |  |
| 5.1. Request-Line |  |  |
| The Request-Line begins with a method token, followed by the Request- |  |  |
| URI and the protocol version, and ending with CRLF. The elements are |  |  |
| separated by SP characters. No CR or LF is allowed except in the |  |  |
| final CRLF sequence. |  |  |
| Request-Line = Method SP Request-URI SP HTTP-Version CRLF |  |  |
| 5.1.1. Method |  |  |
| The Method token indicates the method to be performed on the resource |  |  |
| identified by the Request-URI. The method is case-sensitive. |  |  |
| Method = "OPTIONS" ; Section 9.2 |  |  |
| | "GET" ; Section 9.3 |  |  |
| | "HEAD" ; Section 9.4 |  |  |
| | "POST" ; Section 9.5 |  |  |
| | "PUT" ; Section 9.6 |  |  |
| | "DELETE" ; Section 9.7 |  |  |
| | "TRACE" ; Section 9.8 |  |  |
| | "CONNECT" ; Section 9.9 |  |  |
| | extension-method |  |  |
| extension-method = token |  |  |
| The list of methods allowed by a resource can be specified in an |  |  |
| Allow header field (Section 14.7). The return code of the response |  |  |
| always notifies the client whether a method is currently allowed on a |  |  |
| resource, since the set of allowed methods can change dynamically. |  |  |
| An origin server SHOULD return the status code 405 (Method Not |  |  |
| Allowed) if the method is known by the origin server but not allowed |  |  |
| for the requested resource, and 501 (Not Implemented) if the method |  |  |
| is unrecognized or not implemented by the origin server. The methods |  |  |
| GET and HEAD MUST be supported by all general-purpose servers. All |  |  |
| other methods are OPTIONAL; however, if the above methods are |  |  |
| implemented, they MUST be implemented with the same semantics as |  |  |
| those specified in Section 9. |  |  |
| 5.1.2. Request-URI |  |  |
| The Request-URI is a Uniform Resource Identifier (Section 3.2) and |  |  |
| identifies the resource upon which to apply the request. |  |  |
| Request-URI = "*" | absoluteURI | abs_path | authority |  |  |
| The four options for Request-URI are dependent on the nature of the |  |  |
| request. The asterisk "*" means that the request does not apply to a |  |  |
| particular resource, but to the server itself, and is only allowed |  |  |
| when the method used does not necessarily apply to a resource. One |  |  |
| example would be |  |  |
| OPTIONS * HTTP/1.1 |  |  |
| The absoluteURI form is REQUIRED when the request is being made to a |  |  |
| proxy. The proxy is requested to forward the request or service it |  |  |
| from a valid cache, and return the response. Note that the proxy MAY |  |  |
| forward the request on to another proxy or directly to the server |  |  |
| specified by the absoluteURI. In order to avoid request loops, a |  |  |
| proxy MUST be able to recognize all of its server names, including |  |  |
| any aliases, local variations, and the numeric IP address. An |  |  |
| example Request-Line would be: |  |  |
| GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1 |  |  |
| To allow for transition to absoluteURIs in all requests in future |  |  |
| versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI |  |  |
| form in requests, even though HTTP/1.1 clients will only generate |  |  |
| them in requests to proxies. |  |  |
| The authority form is only used by the CONNECT method (Section 9.9). |  |  |
| The most common form of Request-URI is that used to identify a |  |  |
| resource on an origin server or gateway. In this case the absolute |  |  |
| path of the URI MUST be transmitted (see Section 3.2.1, abs_path) as |  |  |
| the Request-URI, and the network location of the URI (authority) MUST |  |  |
| be transmitted in a Host header field. For example, a client wishing |  |  |
| to retrieve the resource above directly from the origin server would |  |  |
| create a TCP connection to port 80 of the host "www.w3.org" and send |  |  |
| the lines: |  |  |
| GET /pub/WWW/TheProject.html HTTP/1.1 |  |  |
| Host: www.w3.org |  |  |
| followed by the remainder of the Request. Note that the absolute |  |  |
| path cannot be empty; if none is present in the original URI, it MUST |  |  |
| be given as "/" (the server root). |  |  |
| The Request-URI is transmitted in the format specified in |  |  |
| Section 3.2.1. If the Request-URI is encoded using the "% HEX HEX" |  |  |
| encoding [RFC2396], the origin server MUST decode the Request-URI in |  |  |
| order to properly interpret the request. Servers SHOULD respond to |  |  |
| invalid Request-URIs with an appropriate status code. |  |  |
| A transparent proxy MUST NOT rewrite the "abs_path" part of the |  |  |
| received Request-URI when forwarding it to the next inbound server, |  |  |
| except as noted above to replace a null abs_path with "/". |  |  |
| Note: The "no rewrite" rule prevents the proxy from changing the |  |  |
| meaning of the request when the origin server is improperly using |  |  |
| a non-reserved URI character for a reserved purpose. Implementors |  |  |
| should be aware that some pre-HTTP/1.1 proxies have been known to |  |  |
| rewrite the Request-URI. |  |  |
| 5.2. The Resource Identified by a Request |  |  |
| The exact resource identified by an Internet request is determined by |  |  |
| examining both the Request-URI and the Host header field. |  |  |
| An origin server that does not allow resources to differ by the |  |  |
| requested host MAY ignore the Host header field value when |  |  |
| determining the resource identified by an HTTP/1.1 request. (But see |  |  |
| Appendix A.6.1.1 for other requirements on Host support in HTTP/1.1.) |  |  |
| An origin server that does differentiate resources based on the host |  |  |
| requested (sometimes referred to as virtual hosts or vanity host |  |  |
| names) MUST use the following rules for determining the requested |  |  |
| resource on an HTTP/1.1 request: |  |  |
| 1. If Request-URI is an absoluteURI, the host is part of the |  |  |
| Request-URI. Any Host header field value in the request MUST be |  |  |
| ignored. |  |  |
| 2. If the Request-URI is not an absoluteURI, and the request |  |  |
| includes a Host header field, the host is determined by the Host |  |  |
| header field value. |  |  |
| 3. If the host as determined by rule 1 or 2 is not a valid host on |  |  |
| the server, the response MUST be a 400 (Bad Request) error |  |  |
| message. |  |  |
| Recipients of an HTTP/1.0 request that lacks a Host header field MAY |  |  |
| attempt to use heuristics (e.g., examination of the URI path for |  |  |
| something unique to a particular host) in order to determine what |  |  |
| exact resource is being requested. |  |  |
| 5.3. Request Header Fields |  |  |
| The request-header fields allow the client to pass additional |  |  |
| information about the request, and about the client itself, to the |  |  |
| server. These fields act as request modifiers, with semantics |  |  |
| equivalent to the parameters on a programming language method |  |  |
| invocation. |  |  |
| request-header = Accept ; Section 14.1 |  |  |
| | Accept-Charset ; Section 14.2 |  |  |
| | Accept-Encoding ; Section 14.3 |  |  |
| | Accept-Language ; Section 14.4 |  |  |
| | Authorization ; Section 14.8 |  |  |
| | Expect ; Section 14.20 |  |  |
| | From ; Section 14.22 |  |  |
| | Host ; Section 14.23 |  |  |
| | If-Match ; Section 14.24 |  |  |
| | If-Modified-Since ; Section 14.25 |  |  |
| | If-None-Match ; Section 14.26 |  |  |
| | If-Range ; Section 14.27 |  |  |
| | If-Unmodified-Since ; Section 14.28 |  |  |
| | Max-Forwards ; Section 14.31 |  |  |
| | Proxy-Authorization ; Section 14.34 |  |  |
| | Range ; Section 14.35 |  |  |
| | Referer ; Section 14.36 |  |  |
| | TE ; Section 14.39 |  |  |
| | User-Agent ; Section 14.43 |  |  |
| Request-header field names can be extended reliably only in |  |  |
| combination with a change in the protocol version. However, new or |  |  |
| experimental header fields MAY be given the semantics of request- |  |  |
| header fields if all parties in the communication recognize them to |  |  |
| be request-header fields. Unrecognized header fields are treated as |  |  |
| entity-header fields. |  |  |
| 6. Response |  |  |
| After receiving and interpreting a request message, a server responds |  |  |
| with an HTTP response message. |  |  |
| Response = Status-Line ; Section 6.1 |  |  |
| *(( general-header ; Section 4.5 |  |  |
| | response-header ; Section 6.2 |  |  |
| | entity-header ) CRLF) ; Section 7.1 |  |  |
| CRLF |  |  |
| [ message-body ] ; Section 7.2 |  |  |
| 6.1. Status-Line |  |  |
| The first line of a Response message is the Status-Line, consisting |  |  |
| of the protocol version followed by a numeric status code and its |  |  |
| associated textual phrase, with each element separated by SP |  |  |
| characters. No CR or LF is allowed except in the final CRLF |  |  |
| sequence. |  |  |
| Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF |  |  |
| 6.1.1. Status Code and Reason Phrase |  |  |
| The Status-Code element is a 3-digit integer result code of the |  |  |
| attempt to understand and satisfy the request. These codes are fully |  |  |
| defined in Section 10. The Reason-Phrase is intended to give a short |  |  |
| textual description of the Status-Code. The Status-Code is intended |  |  |
| for use by automata and the Reason-Phrase is intended for the human |  |  |
| user. The client is not required to examine or display the Reason- |  |  |
| Phrase. |  |  |
| The first digit of the Status-Code defines the class of response. |  |  |
| The last two digits do not have any categorization role. There are 5 |  |  |
| values for the first digit: |  |  |
| o 1xx: Informational - Request received, continuing process |  |  |
| o 2xx: Success - The action was successfully received, understood, |  |  |
| and accepted |  |  |
| o 3xx: Redirection - Further action must be taken in order to |  |  |
| complete the request |  |  |
| o 4xx: Client Error - The request contains bad syntax or cannot be |  |  |
| fulfilled |  |  |
| o 5xx: Server Error - The server failed to fulfill an apparently |  |  |
| valid request |  |  |
| The individual values of the numeric status codes defined for |  |  |
| HTTP/1.1, and an example set of corresponding Reason-Phrase's, are |  |  |
| presented below. The reason phrases listed here are only |  |  |
| recommendations -- they MAY be replaced by local equivalents without |  |  |
| affecting the protocol. |  |  |
| Status-Code = |  |  |
| "100" ; Section 10.1.1: Continue |  |  |
| | "101" ; Section 10.1.2: Switching Protocols |  |  |
| | "200" ; Section 10.2.1: OK |  |  |
| | "201" ; Section 10.2.2: Created |  |  |
| | "202" ; Section 10.2.3: Accepted |  |  |
| | "203" ; Section 10.2.4: Non-Authoritative Information |  |  |
| | "204" ; Section 10.2.5: No Content |  |  |
| | "205" ; Section 10.2.6: Reset Content |  |  |
| | "206" ; Section 10.2.7: Partial Content |  |  |
| | "300" ; Section 10.3.1: Multiple Choices |  |  |
| | "301" ; Section 10.3.2: Moved Permanently |  |  |
| | "302" ; Section 10.3.3: Found |  |  |
| | "303" ; Section 10.3.4: See Other |  |  |
| | "304" ; Section 10.3.5: Not Modified |  |  |
| | "305" ; Section 10.3.6: Use Proxy |  |  |
| | "307" ; Section 10.3.8: Temporary Redirect |  |  |
| | "400" ; Section 10.4.1: Bad Request |  |  |
| | "401" ; Section 10.4.2: Unauthorized |  |  |
| | "402" ; Section 10.4.3: Payment Required |  |  |
| | "403" ; Section 10.4.4: Forbidden |  |  |
| | "404" ; Section 10.4.5: Not Found |  |  |
| | "405" ; Section 10.4.6: Method Not Allowed |  |  |
| | "406" ; Section 10.4.7: Not Acceptable |  |  |
| | "407" ; Section 10.4.8: Proxy Authentication Required |  |  |
| | "408" ; Section 10.4.9: Request Time-out |  |  |
| | "409" ; Section 10.4.10: Conflict |  |  |
| | "410" ; Section 10.4.11: Gone |  |  |
| | "411" ; Section 10.4.12: Length Required |  |  |
| | "412" ; Section 10.4.13: Precondition Failed |  |  |
| | "413" ; Section 10.4.14: Request Entity Too Large |  |  |
| | "414" ; Section 10.4.15: Request-URI Too Large |  |  |
| | "415" ; Section 10.4.16: Unsupported Media Type |  |  |
| | "416" ; Section 10.4.17: Requested range not satisfiable |  |  |
| | "417" ; Section 10.4.18: Expectation Failed |  |  |
| | "500" ; Section 10.5.1: Internal Server Error |  |  |
| | "501" ; Section 10.5.2: Not Implemented |  |  |
| | "502" ; Section 10.5.3: Bad Gateway |  |  |
| | "503" ; Section 10.5.4: Service Unavailable |  |  |
| | "504" ; Section 10.5.5: Gateway Time-out |  |  |
| | "505" ; Section 10.5.6: HTTP Version not supported |  |  |
| | extension-code |  |  |
| extension-code = 3DIGIT |  |  |
| Reason-Phrase = *<TEXT, excluding CR, LF> |  |  |
| HTTP status codes are extensible. HTTP applications are not required |  |  |
| to understand the meaning of all registered status codes, though such |  |  |
| understanding is obviously desirable. However, applications MUST |  |  |
| understand the class of any status code, as indicated by the first |  |  |
| digit, and treat any unrecognized response as being equivalent to the |  |  |
| x00 status code of that class, with the exception that an |  |  |
| unrecognized response MUST NOT be cached. For example, if an |  |  |
| unrecognized status code of 431 is received by the client, it can |  |  |
| safely assume that there was something wrong with its request and |  |  |
| treat the response as if it had received a 400 status code. In such |  |  |
| cases, user agents SHOULD present to the user the entity returned |  |  |
| with the response, since that entity is likely to include human- |  |  |
| readable information which will explain the unusual status. |  |  |
| 6.2. Response Header Fields |  |  |
| The response-header fields allow the server to pass additional |  |  |
| information about the response which cannot be placed in the Status- |  |  |
| Line. These header fields give information about the server and |  |  |
| about further access to the resource identified by the Request-URI. |  |  |
| response-header = Accept-Ranges ; Section 14.5 |  |  |
| | Age ; Section 14.6 |  |  |
| | ETag ; Section 14.19 |  |  |
| | Location ; Section 14.30 |  |  |
| | Proxy-Authenticate ; Section 14.33 |  |  |
| | Retry-After ; Section 14.37 |  |  |
| | Server ; Section 14.38 |  |  |
| | Vary ; Section 14.44 |  |  |
| | WWW-Authenticate ; Section 14.47 |  |  |
| Response-header field names can be extended reliably only in |  |  |
| combination with a change in the protocol version. However, new or |  |  |
| experimental header fields MAY be given the semantics of response- |  |  |
| header fields if all parties in the communication recognize them to |  |  |
| be response-header fields. Unrecognized header fields are treated as |  |  |
| entity-header fields. |  |  |
| 7. Entity |  |  |
| Request and Response messages MAY transfer an entity if not otherwise |  |  |
| restricted by the request method or response status code. An entity |  |  |
| consists of entity-header fields and an entity-body, although some |  |  |
| responses will only include the entity-headers. |  |  |
| In this section, both sender and recipient refer to either the client |  |  |
| or the server, depending on who sends and who receives the entity. |  |  |
| 7.1. Entity Header Fields |  |  |
| Entity-header fields define metainformation about the entity-body or, |  |  |
| if no body is present, about the resource identified by the request. |  |  |
| Some of this metainformation is OPTIONAL; some might be REQUIRED by |  |  |
| portions of this specification. |  |  |
| entity-header = Allow ; Section 14.7 |  |  |
| | Content-Encoding ; Section 14.11 |  |  |
| | Content-Language ; Section 14.12 |  |  |
| | Content-Length ; Section 14.13 |  |  |
| | Content-Location ; Section 14.14 |  |  |
| | Content-MD5 ; Section 14.15 |  |  |
| | Content-Range ; Section 14.16 |  |  |
| | Content-Type ; Section 14.17 |  |  |
| | Expires ; Section 14.21 |  |  |
| | Last-Modified ; Section 14.29 |  |  |
| | extension-header |  |  |
| extension-header = message-header |  |  |
| The extension-header mechanism allows additional entity-header fields |  |  |
| to be defined without changing the protocol, but these fields cannot |  |  |
| be assumed to be recognizable by the recipient. Unrecognized header |  |  |
| fields SHOULD be ignored by the recipient and MUST be forwarded by |  |  |
| transparent proxies. |  |  |
| 7.2. Entity Body |  |  |
| The entity-body (if any) sent with an HTTP request or response is in |  |  |
| a format and encoding defined by the entity-header fields. |  |  |
| entity-body = *OCTET |  |  |
| An entity-body is only present in a message when a message-body is |  |  |
| present, as described in Section 4.3. The entity-body is obtained |  |  |
| from the message-body by decoding any Transfer-Encoding that might |  |  |
| have been applied to ensure safe and proper transfer of the message. |  |  |
| 7.2.1. Type |  |  |
| When an entity-body is included with a message, the data type of that |  |  |
| body is determined via the header fields Content-Type and Content- |  |  |
| Encoding. These define a two-layer, ordered encoding model: |  |  |
| entity-body := Content-Encoding( Content-Type( data ) ) |  |  |
| Content-Type specifies the media type of the underlying data. |  |  |
| Content-Encoding may be used to indicate any additional content |  |  |
| codings applied to the data, usually for the purpose of data |  |  |
| compression, that are a property of the requested resource. There is |  |  |
| no default encoding. |  |  |
| Any HTTP/1.1 message containing an entity-body SHOULD include a |  |  |
| Content-Type header field defining the media type of that body. If |  |  |
| and only if the media type is not given by a Content-Type field, the |  |  |
| recipient MAY attempt to guess the media type via inspection of its |  |  |
| content and/or the name extension(s) of the URI used to identify the |  |  |
| resource. If the media type remains unknown, the recipient SHOULD |  |  |
| treat it as type "application/octet-stream". |  |  |
| 7.2.2. Entity Length |  |  |
| The entity-length of a message is the length of the message-body |  |  |
| before any transfer-codings have been applied. Section 4.4 defines |  |  |
| how the transfer-length of a message-body is determined. |  |  |
| 8. Connections |  |  |
| 8.1. Persistent Connections |  |  |
| 8.1.1. Purpose |  |  |
| Prior to persistent connections, a separate TCP connection was |  |  |
| established to fetch each URL, increasing the load on HTTP servers |  |  |
| and causing congestion on the Internet. The use of inline images and |  |  |
| other associated data often require a client to make multiple |  |  |
| requests of the same server in a short amount of time. Analysis of |  |  |
| these performance problems and results from a prototype |  |  |
| implementation are available [Pad1995] [Spe]. Implementation |  |  |
| experience and measurements of actual HTTP/1.1 (RFC 2068) |  |  |
| implementations show good results [Nie1997]. Alternatives have also |  |  |
| been explored, for example, T/TCP [Tou1998]. |  |  |
| Persistent HTTP connections have a number of advantages: |  |  |
| o By opening and closing fewer TCP connections, CPU time is saved in |  |  |
| routers and hosts (clients, servers, proxies, gateways, tunnels, |  |  |
| or caches), and memory used for TCP protocol control blocks can be |  |  |
| saved in hosts. |  |  |
| o HTTP requests and responses can be pipelined on a connection. |  |  |
| Pipelining allows a client to make multiple requests without |  |  |
| waiting for each response, allowing a single TCP connection to be |  |  |
| used much more efficiently, with much lower elapsed time. |  |  |
| o Network congestion is reduced by reducing the number of packets |  |  |
| caused by TCP opens, and by allowing TCP sufficient time to |  |  |
| determine the congestion state of the network. |  |  |
| o Latency on subsequent requests is reduced since there is no time |  |  |
| spent in TCP's connection opening handshake. |  |  |
| o HTTP can evolve more gracefully, since errors can be reported |  |  |
| without the penalty of closing the TCP connection. Clients using |  |  |
| future versions of HTTP might optimistically try a new feature, |  |  |
| but if communicating with an older server, retry with old |  |  |
| semantics after an error is reported. |  |  |
| HTTP implementations SHOULD implement persistent connections. |  |  |
| 8.1.2. Overall Operation |  |  |
| A significant difference between HTTP/1.1 and earlier versions of |  |  |
| HTTP is that persistent connections are the default behavior of any |  |  |
| HTTP connection. That is, unless otherwise indicated, the client |  |  |
| SHOULD assume that the server will maintain a persistent connection, |  |  |
| even after error responses from the server. |  |  |
| Persistent connections provide a mechanism by which a client and a |  |  |
| server can signal the close of a TCP connection. This signaling |  |  |
| takes place using the Connection header field (Section 14.10). Once |  |  |
| a close has been signaled, the client MUST NOT send any more requests |  |  |
| on that connection. |  |  |
| 8.1.2.1. Negotiation |  |  |
| An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to |  |  |
| maintain a persistent connection unless a Connection header including |  |  |
| the connection-token "close" was sent in the request. If the server |  |  |
| chooses to close the connection immediately after sending the |  |  |
| response, it SHOULD send a Connection header including the |  |  |
| connection-token close. |  |  |
| An HTTP/1.1 client MAY expect a connection to remain open, but would |  |  |
| decide to keep it open based on whether the response from a server |  |  |
| contains a Connection header with the connection-token close. In |  |  |
| case the client does not want to maintain a connection for more than |  |  |
| that request, it SHOULD send a Connection header including the |  |  |
| connection-token close. |  |  |
| If either the client or the server sends the close token in the |  |  |
| Connection header, that request becomes the last one for the |  |  |
| connection. |  |  |
| Clients and servers SHOULD NOT assume that a persistent connection is |  |  |
| maintained for HTTP versions less than 1.1 unless it is explicitly |  |  |
| signaled. See Appendix A.6.2 for more information on backward |  |  |
| compatibility with HTTP/1.0 clients. |  |  |
| In order to remain persistent, all messages on the connection MUST |  |  |
| have a self-defined message length (i.e., one not defined by closure |  |  |
| of the connection), as described in Section 4.4. |  |  |
| 8.1.2.2. Pipelining |  |  |
| A client that supports persistent connections MAY "pipeline" its |  |  |
| requests (i.e., send multiple requests without waiting for each |  |  |
| response). A server MUST send its responses to those requests in the |  |  |
| same order that the requests were received. |  |  |
| Clients which assume persistent connections and pipeline immediately |  |  |
| after connection establishment SHOULD be prepared to retry their |  |  |
| connection if the first pipelined attempt fails. If a client does |  |  |
| such a retry, it MUST NOT pipeline before it knows the connection is |  |  |
| persistent. Clients MUST also be prepared to resend their requests |  |  |
| if the server closes the connection before sending all of the |  |  |
| corresponding responses. |  |  |
| Clients SHOULD NOT pipeline requests using non-idempotent methods or |  |  |
| non-idempotent sequences of methods (see Section 9.1.2). Otherwise, |  |  |
| a premature termination of the transport connection could lead to |  |  |
| indeterminate results. A client wishing to send a non-idempotent |  |  |
| request SHOULD wait to send that request until it has received the |  |  |
| response status for the previous request. |  |  |
| 8.1.3. Proxy Servers |  |  |
| It is especially important that proxies correctly implement the |  |  |
| properties of the Connection header field as specified in |  |  |
| Section 14.10. |  |  |
| The proxy server MUST signal persistent connections separately with |  |  |
| its clients and the origin servers (or other proxy servers) that it |  |  |
| connects to. Each persistent connection applies to only one |  |  |
| transport link. |  |  |
| A proxy server MUST NOT establish a HTTP/1.1 persistent connection |  |  |
| with an HTTP/1.0 client (but see RFC 2068 [RFC2068] for information |  |  |
| and discussion of the problems with the Keep-Alive header implemented |  |  |
| by many HTTP/1.0 clients). |  |  |
| 8.1.4. Practical Considerations |  |  |
| Servers will usually have some time-out value beyond which they will |  |  |
| no longer maintain an inactive connection. Proxy servers might make |  |  |
| this a higher value since it is likely that the client will be making |  |  |
| more connections through the same server. The use of persistent |  |  |
| connections places no requirements on the length (or existence) of |  |  |
| this time-out for either the client or the server. |  |  |
| When a client or server wishes to time-out it SHOULD issue a graceful |  |  |
| close on the transport connection. Clients and servers SHOULD both |  |  |
| constantly watch for the other side of the transport close, and |  |  |
| respond to it as appropriate. If a client or server does not detect |  |  |
| the other side's close promptly it could cause unnecessary resource |  |  |
| drain on the network. |  |  |
| A client, server, or proxy MAY close the transport connection at any |  |  |
| time. For example, a client might have started to send a new request |  |  |
| at the same time that the server has decided to close the "idle" |  |  |
| connection. From the server's point of view, the connection is being |  |  |
| closed while it was idle, but from the client's point of view, a |  |  |
| request is in progress. |  |  |
| This means that clients, servers, and proxies MUST be able to recover |  |  |
| from asynchronous close events. Client software SHOULD reopen the |  |  |
| transport connection and retransmit the aborted sequence of requests |  |  |
| without user interaction so long as the request sequence is |  |  |
| idempotent (see Section 9.1.2). Non-idempotent methods or sequences |  |  |
| MUST NOT be automatically retried, although user agents MAY offer a |  |  |
| human operator the choice of retrying the request(s). Confirmation |  |  |
| by user-agent software with semantic understanding of the application |  |  |
| MAY substitute for user confirmation. The automatic retry SHOULD NOT |  |  |
| be repeated if the second sequence of requests fails. |  |  |
| Servers SHOULD always respond to at least one request per connection, |  |  |
| if at all possible. Servers SHOULD NOT close a connection in the |  |  |
| middle of transmitting a response, unless a network or client failure |  |  |
| is suspected. |  |  |
| Clients that use persistent connections SHOULD limit the number of |  |  |
| simultaneous connections that they maintain to a given server. A |  |  |
| single-user client SHOULD NOT maintain more than 2 connections with |  |  |
| any server or proxy. A proxy SHOULD use up to 2*N connections to |  |  |
| another server or proxy, where N is the number of simultaneously |  |  |
| active users. These guidelines are intended to improve HTTP response |  |  |
| times and avoid congestion. |  |  |
| 8.2. Message Transmission Requirements |  |  |
| 8.2.1. Persistent Connections and Flow Control |  |  |
| HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's |  |  |
| flow control mechanisms to resolve temporary overloads, rather than |  |  |
| terminating connections with the expectation that clients will retry. |  |  |
| The latter technique can exacerbate network congestion. |  |  |
| 8.2.2. Monitoring Connections for Error Status Messages |  |  |
| An HTTP/1.1 (or later) client sending a message-body SHOULD monitor |  |  |
| the network connection for an error status while it is transmitting |  |  |
| the request. If the client sees an error status, it SHOULD |  |  |
| immediately cease transmitting the body. If the body is being sent |  |  |
| using a "chunked" encoding (Section 3.6), a zero length chunk and |  |  |
| empty trailer MAY be used to prematurely mark the end of the message. |  |  |
| If the body was preceded by a Content-Length header, the client MUST |  |  |
| close the connection. |  |  |
| 8.2.3. Use of the 100 (Continue) Status |  |  |
| The purpose of the 100 (Continue) status (see Section 10.1.1) is to |  |  |
| allow a client that is sending a request message with a request body |  |  |
| to determine if the origin server is willing to accept the request |  |  |
| (based on the request headers) before the client sends the request |  |  |
| body. In some cases, it might either be inappropriate or highly |  |  |
| inefficient for the client to send the body if the server will reject |  |  |
| the message without looking at the body. |  |  |
| Requirements for HTTP/1.1 clients: |  |  |
| o If a client will wait for a 100 (Continue) response before sending |  |  |
| the request body, it MUST send an Expect request-header field |  |  |
| (Section 14.20) with the "100-continue" expectation. |  |  |
| o A client MUST NOT send an Expect request-header field |  |  |
| (Section 14.20) with the "100-continue" expectation if it does not |  |  |
| intend to send a request body. |  |  |
| Because of the presence of older implementations, the protocol allows |  |  |
| ambiguous situations in which a client may send "Expect: 100- |  |  |
| continue" without receiving either a 417 (Expectation Failed) status |  |  |
| or a 100 (Continue) status. Therefore, when a client sends this |  |  |
| header field to an origin server (possibly via a proxy) from which it |  |  |
| has never seen a 100 (Continue) status, the client SHOULD NOT wait |  |  |
| for an indefinite period before sending the request body. |  |  |
| Requirements for HTTP/1.1 origin servers: |  |  |
| o Upon receiving a request which includes an Expect request-header |  |  |
| field with the "100-continue" expectation, an origin server MUST |  |  |
| either respond with 100 (Continue) status and continue to read |  |  |
| from the input stream, or respond with a final status code. The |  |  |
| origin server MUST NOT wait for the request body before sending |  |  |
| the 100 (Continue) response. If it responds with a final status |  |  |
| code, it MAY close the transport connection or it MAY continue to |  |  |
| read and discard the rest of the request. It MUST NOT perform the |  |  |
| requested method if it returns a final status code. |  |  |
| o An origin server SHOULD NOT send a 100 (Continue) response if the |  |  |
| request message does not include an Expect request-header field |  |  |
| with the "100-continue" expectation, and MUST NOT send a 100 |  |  |
| (Continue) response if such a request comes from an HTTP/1.0 (or |  |  |
| earlier) client. There is an exception to this rule: for |  |  |
| compatibility with RFC 2068, a server MAY send a 100 (Continue) |  |  |
| status in response to an HTTP/1.1 PUT or POST request that does |  |  |
| not include an Expect request-header field with the "100-continue" |  |  |
| expectation. This exception, the purpose of which is to minimize |  |  |
| any client processing delays associated with an undeclared wait |  |  |
| for 100 (Continue) status, applies only to HTTP/1.1 requests, and |  |  |
| not to requests with any other HTTP-version value. |  |  |
| o An origin server MAY omit a 100 (Continue) response if it has |  |  |
| already received some or all of the request body for the |  |  |
| corresponding request. |  |  |
| o An origin server that sends a 100 (Continue) response MUST |  |  |
| ultimately send a final status code, once the request body is |  |  |
| received and processed, unless it terminates the transport |  |  |
| connection prematurely. |  |  |
| o If an origin server receives a request that does not include an |  |  |
| Expect request-header field with the "100-continue" expectation, |  |  |
| the request includes a request body, and the server responds with |  |  |
| a final status code before reading the entire request body from |  |  |
| the transport connection, then the server SHOULD NOT close the |  |  |
| transport connection until it has read the entire request, or |  |  |
| until the client closes the connection. Otherwise, the client |  |  |
| might not reliably receive the response message. However, this |  |  |
| requirement is not be construed as preventing a server from |  |  |
| defending itself against denial-of-service attacks, or from badly |  |  |
| broken client implementations. |  |  |
| Requirements for HTTP/1.1 proxies: |  |  |
| o If a proxy receives a request that includes an Expect request- |  |  |
| header field with the "100-continue" expectation, and the proxy |  |  |
| either knows that the next-hop server complies with HTTP/1.1 or |  |  |
| higher, or does not know the HTTP version of the next-hop server, |  |  |
| it MUST forward the request, including the Expect header field. |  |  |
| o If the proxy knows that the version of the next-hop server is |  |  |
| HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST |  |  |
| respond with a 417 (Expectation Failed) status. |  |  |
| o Proxies SHOULD maintain a cache recording the HTTP version numbers |  |  |
| received from recently-referenced next-hop servers. |  |  |
| o A proxy MUST NOT forward a 100 (Continue) response if the request |  |  |
| message was received from an HTTP/1.0 (or earlier) client and did |  |  |
| not include an Expect request-header field with the "100-continue" |  |  |
| expectation. This requirement overrides the general rule for |  |  |
| forwarding of 1xx responses (see Section 10.1). |  |  |
| 8.2.4. Client Behavior if Server Prematurely Closes Connection |  |  |
| If an HTTP/1.1 client sends a request which includes a request body, |  |  |
| but which does not include an Expect request-header field with the |  |  |
| "100-continue" expectation, and if the client is not directly |  |  |
| connected to an HTTP/1.1 origin server, and if the client sees the |  |  |
| connection close before receiving any status from the server, the |  |  |
| client SHOULD retry the request. If the client does retry this |  |  |
| request, it MAY use the following "binary exponential backoff" |  |  |
| algorithm to be assured of obtaining a reliable response: |  |  |
| 1. Initiate a new connection to the server |  |  |
| 2. Transmit the request-headers |  |  |
| 3. Initialize a variable R to the estimated round-trip time to the |  |  |
| server (e.g., based on the time it took to establish the |  |  |
| connection), or to a constant value of 5 seconds if the round- |  |  |
| trip time is not available. |  |  |
| 4. Compute T = R * (2**N), where N is the number of previous retries |  |  |
| of this request. |  |  |
| 5. Wait either for an error response from the server, or for T |  |  |
| seconds (whichever comes first) |  |  |
| 6. If no error response is received, after T seconds transmit the |  |  |
| body of the request. |  |  |
| 7. If client sees that the connection is closed prematurely, repeat |  |  |
| from step 1 until the request is accepted, an error response is |  |  |
| received, or the user becomes impatient and terminates the retry |  |  |
| process. |  |  |
| If at any point an error status is received, the client |  |  |
| o SHOULD NOT continue and |  |  |
| o SHOULD close the connection if it has not completed sending the |  |  |
| request message. |  |  |
| 9. Method Definitions |  |  |
| The set of common methods for HTTP/1.1 is defined below. Although |  |  |
| this set can be expanded, additional methods cannot be assumed to |  |  |
| share the same semantics for separately extended clients and servers. |  |  |
| The Host request-header field (Section 14.23) MUST accompany all |  |  |
| HTTP/1.1 requests. |  |  |
| 9.1. Safe and Idempotent Methods |  |  |
| 9.1.1. Safe Methods |  |  |
| Implementors should be aware that the software represents the user in |  |  |
| their interactions over the Internet, and should be careful to allow |  |  |
| the user to be aware of any actions they might take which may have an |  |  |
| unexpected significance to themselves or others. |  |  |
| In particular, the convention has been established that the GET and |  |  |
| HEAD methods SHOULD NOT have the significance of taking an action |  |  |
| other than retrieval. These methods ought to be considered "safe". |  |  |
| This allows user agents to represent other methods, such as POST, PUT |  |  |
| and DELETE, in a special way, so that the user is made aware of the |  |  |
| fact that a possibly unsafe action is being requested. |  |  |
| Naturally, it is not possible to ensure that the server does not |  |  |
| generate side-effects as a result of performing a GET request; in |  |  |
| fact, some dynamic resources consider that a feature. The important |  |  |
| distinction here is that the user did not request the side-effects, |  |  |
| so therefore cannot be held accountable for them. |  |  |
| 9.1.2. Idempotent Methods |  |  |
| Methods can also have the property of "idempotence" in that (aside |  |  |
| from error or expiration issues) the side-effects of N > 0 identical |  |  |
| requests is the same as for a single request. The methods GET, HEAD, |  |  |
| PUT and DELETE share this property. Also, the methods OPTIONS and |  |  |
| TRACE SHOULD NOT have side effects, and so are inherently idempotent. |  |  |
| However, it is possible that a sequence of several requests is non- |  |  |
| idempotent, even if all of the methods executed in that sequence are |  |  |
| idempotent. (A sequence is idempotent if a single execution of the |  |  |
| entire sequence always yields a result that is not changed by a |  |  |
| reexecution of all, or part, of that sequence.) For example, a |  |  |
| sequence is non-idempotent if its result depends on a value that is |  |  |
| later modified in the same sequence. |  |  |
| A sequence that never has side effects is idempotent, by definition |  |  |
| (provided that no concurrent operations are being executed on the |  |  |
| same set of resources). |  |  |
| 9.2. OPTIONS |  |  |
| The OPTIONS method represents a request for information about the |  |  |
| communication options available on the request/response chain |  |  |
| identified by the Request-URI. This method allows the client to |  |  |
| determine the options and/or requirements associated with a resource, |  |  |
| or the capabilities of a server, without implying a resource action |  |  |
| or initiating a resource retrieval. |  |  |
| Responses to this method are not cacheable. |  |  |
| If the OPTIONS request includes an entity-body (as indicated by the |  |  |
| presence of Content-Length or Transfer-Encoding), then the media type |  |  |
| MUST be indicated by a Content-Type field. Although this |  |  |
| specification does not define any use for such a body, future |  |  |
| extensions to HTTP might use the OPTIONS body to make more detailed |  |  |
| queries on the server. A server that does not support such an |  |  |
| extension MAY discard the request body. |  |  |
| If the Request-URI is an asterisk ("*"), the OPTIONS request is |  |  |
| intended to apply to the server in general rather than to a specific |  |  |
| resource. Since a server's communication options typically depend on |  |  |
| the resource, the "*" request is only useful as a "ping" or "no-op" |  |  |
| type of method; it does nothing beyond allowing the client to test |  |  |
| the capabilities of the server. For example, this can be used to |  |  |
| test a proxy for HTTP/1.1 compliance (or lack thereof). |  |  |
| If the Request-URI is not an asterisk, the OPTIONS request applies |  |  |
| only to the options that are available when communicating with that |  |  |
| resource. |  |  |
| A 200 response SHOULD include any header fields that indicate |  |  |
| optional features implemented by the server and applicable to that |  |  |
| resource (e.g., Allow), possibly including extensions not defined by |  |  |
| this specification. The response body, if any, SHOULD also include |  |  |
| information about the communication options. The format for such a |  |  |
| body is not defined by this specification, but might be defined by |  |  |
| future extensions to HTTP. Content negotiation MAY be used to select |  |  |
| the appropriate response format. If no response body is included, |  |  |
| the response MUST include a Content-Length field with a field-value |  |  |
| of "0". |  |  |
| The Max-Forwards request-header field MAY be used to target a |  |  |
| specific proxy in the request chain. When a proxy receives an |  |  |
| OPTIONS request on an absoluteURI for which request forwarding is |  |  |
| permitted, the proxy MUST check for a Max-Forwards field. If the |  |  |
| Max-Forwards field-value is zero ("0"), the proxy MUST NOT forward |  |  |
| the message; instead, the proxy SHOULD respond with its own |  |  |
| communication options. If the Max-Forwards field-value is an integer |  |  |
| greater than zero, the proxy MUST decrement the field-value when it |  |  |
| forwards the request. If no Max-Forwards field is present in the |  |  |
| request, then the forwarded request MUST NOT include a Max-Forwards |  |  |
| field. |  |  |
| 9.3. GET |  |  |
| The GET method means retrieve whatever information (in the form of an |  |  |
| entity) is identified by the Request-URI. If the Request-URI refers |  |  |
| to a data-producing process, it is the produced data which shall be |  |  |
| returned as the entity in the response and not the source text of the |  |  |
| process, unless that text happens to be the output of the process. |  |  |
| The semantics of the GET method change to a "conditional GET" if the |  |  |
| request message includes an If-Modified-Since, If-Unmodified-Since, |  |  |
| If-Match, If-None-Match, or If-Range header field. A conditional GET |  |  |
| method requests that the entity be transferred only under the |  |  |
| circumstances described by the conditional header field(s). The |  |  |
| conditional GET method is intended to reduce unnecessary network |  |  |
| usage by allowing cached entities to be refreshed without requiring |  |  |
| multiple requests or transferring data already held by the client. |  |  |
| The semantics of the GET method change to a "partial GET" if the |  |  |
| request message includes a Range header field. A partial GET |  |  |
| requests that only part of the entity be transferred, as described in |  |  |
| Section 14.35. The partial GET method is intended to reduce |  |  |
| unnecessary network usage by allowing partially-retrieved entities to |  |  |
| be completed without transferring data already held by the client. |  |  |
| The response to a GET request is cacheable if and only if it meets |  |  |
| the requirements for HTTP caching described in Section 13. |  |  |
| See Section 15.1.3 for security considerations when used for forms. |  |  |
| 9.4. HEAD |  |  |
| The HEAD method is identical to GET except that the server MUST NOT |  |  |
| return a message-body in the response. The metainformation contained |  |  |
| in the HTTP headers in response to a HEAD request SHOULD be identical |  |  |
| to the information sent in response to a GET request. This method |  |  |
| can be used for obtaining metainformation about the entity implied by |  |  |
| the request without transferring the entity-body itself. This method |  |  |
| is often used for testing hypertext links for validity, |  |  |
| accessibility, and recent modification. |  |  |
| The response to a HEAD request MAY be cacheable in the sense that the |  |  |
| information contained in the response MAY be used to update a |  |  |
| previously cached entity from that resource. If the new field values |  |  |
| indicate that the cached entity differs from the current entity (as |  |  |
| would be indicated by a change in Content-Length, Content-MD5, ETag |  |  |
| or Last-Modified), then the cache MUST treat the cache entry as |  |  |
| stale. |  |  |
| 9.5. POST |  |  |
| The POST method is used to request that the origin server accept the |  |  |
| entity enclosed in the request as a new subordinate of the resource |  |  |
| identified by the Request-URI in the Request-Line. POST is designed |  |  |
| to allow a uniform method to cover the following functions: |  |  |
| o Annotation of existing resources; |  |  |
| o Posting a message to a bulletin board, newsgroup, mailing list, or |  |  |
| similar group of articles; |  |  |
| o Providing a block of data, such as the result of submitting a |  |  |
| form, to a data-handling process; |  |  |
| o Extending a database through an append operation. |  |  |
| The actual function performed by the POST method is determined by the |  |  |
| server and is usually dependent on the Request-URI. The posted |  |  |
| entity is subordinate to that URI in the same way that a file is |  |  |
| subordinate to a directory containing it, a news article is |  |  |
| subordinate to a newsgroup to which it is posted, or a record is |  |  |
| subordinate to a database. |  |  |
| The action performed by the POST method might not result in a |  |  |
| resource that can be identified by a URI. In this case, either 200 |  |  |
| (OK) or 204 (No Content) is the appropriate response status, |  |  |
| depending on whether or not the response includes an entity that |  |  |
| describes the result. |  |  |
| If a resource has been created on the origin server, the response |  |  |
| SHOULD be 201 (Created) and contain an entity which describes the |  |  |
| status of the request and refers to the new resource, and a Location |  |  |
| header (see Section 14.30). |  |  |
| Responses to this method are not cacheable, unless the response |  |  |
| includes appropriate Cache-Control or Expires header fields. |  |  |
| However, the 303 (See Other) response can be used to direct the user |  |  |
| agent to retrieve a cacheable resource. |  |  |
| POST requests MUST obey the message transmission requirements set out |  |  |
| in Section 8.2. |  |  |
| See Section 15.1.3 for security considerations. |  |  |
| 9.6. PUT |  |  |
| The PUT method requests that the enclosed entity be stored under the |  |  |
| supplied Request-URI. If the Request-URI refers to an already |  |  |
| existing resource, the enclosed entity SHOULD be considered as a |  |  |
| modified version of the one residing on the origin server. If the |  |  |
| Request-URI does not point to an existing resource, and that URI is |  |  |
| capable of being defined as a new resource by the requesting user |  |  |
| agent, the origin server can create the resource with that URI. If a |  |  |
| new resource is created, the origin server MUST inform the user agent |  |  |
| via the 201 (Created) response. If an existing resource is modified, |  |  |
| either the 200 (OK) or 204 (No Content) response codes SHOULD be sent |  |  |
| to indicate successful completion of the request. If the resource |  |  |
| could not be created or modified with the Request-URI, an appropriate |  |  |
| error response SHOULD be given that reflects the nature of the |  |  |
| problem. The recipient of the entity MUST NOT ignore any Content-* |  |  |
| (e.g. Content-Range) headers that it does not understand or |  |  |
| implement and MUST return a 501 (Not Implemented) response in such |  |  |
| cases. |  |  |
| If the request passes through a cache and the Request-URI identifies |  |  |
| one or more currently cached entities, those entries SHOULD be |  |  |
| treated as stale. Responses to this method are not cacheable. |  |  |
| The fundamental difference between the POST and PUT requests is |  |  |
| reflected in the different meaning of the Request-URI. The URI in a |  |  |
| POST request identifies the resource that will handle the enclosed |  |  |
| entity. That resource might be a data-accepting process, a gateway |  |  |
| to some other protocol, or a separate entity that accepts |  |  |
| annotations. In contrast, the URI in a PUT request identifies the |  |  |
| entity enclosed with the request -- the user agent knows what URI is |  |  |
| intended and the server MUST NOT attempt to apply the request to some |  |  |
| other resource. If the server desires that the request be applied to |  |  |
| a different URI, it MUST send a 301 (Moved Permanently) response; the |  |  |
| user agent MAY then make its own decision regarding whether or not to |  |  |
| redirect the request. |  |  |
| A single resource MAY be identified by many different URIs. For |  |  |
| example, an article might have a URI for identifying "the current |  |  |
| version" which is separate from the URI identifying each particular |  |  |
| version. In this case, a PUT request on a general URI might result |  |  |
| in several other URIs being defined by the origin server. |  |  |
| HTTP/1.1 does not define how a PUT method affects the state of an |  |  |
| origin server. |  |  |
| PUT requests MUST obey the message transmission requirements set out |  |  |
| in Section 8.2. |  |  |
| Unless otherwise specified for a particular entity-header, the |  |  |
| entity-headers in the PUT request SHOULD be applied to the resource |  |  |
| created or modified by the PUT. |  |  |
| 9.7. DELETE |  |  |
| The DELETE method requests that the origin server delete the resource |  |  |
| identified by the Request-URI. This method MAY be overridden by |  |  |
| human intervention (or other means) on the origin server. The client |  |  |
| cannot be guaranteed that the operation has been carried out, even if |  |  |
| the status code returned from the origin server indicates that the |  |  |
| action has been completed successfully. However, the server SHOULD |  |  |
| NOT indicate success unless, at the time the response is given, it |  |  |
| intends to delete the resource or move it to an inaccessible |  |  |
| location. |  |  |
| A successful response SHOULD be 200 (OK) if the response includes an |  |  |
| entity describing the status, 202 (Accepted) if the action has not |  |  |
| yet been enacted, or 204 (No Content) if the action has been enacted |  |  |
| but the response does not include an entity. |  |  |
| If the request passes through a cache and the Request-URI identifies |  |  |
| one or more currently cached entities, those entries SHOULD be |  |  |
| treated as stale. Responses to this method are not cacheable. |  |  |
| 9.8. TRACE |  |  |
| The TRACE method is used to invoke a remote, application-layer loop- |  |  |
| back of the request message. The final recipient of the request |  |  |
| SHOULD reflect the message received back to the client as the entity- |  |  |
| body of a 200 (OK) response. The final recipient is either the |  |  |
| origin server or the first proxy or gateway to receive a Max-Forwards |  |  |
| value of zero (0) in the request (see Section 14.31). A TRACE |  |  |
| request MUST NOT include an entity. |  |  |
| TRACE allows the client to see what is being received at the other |  |  |
| end of the request chain and use that data for testing or diagnostic |  |  |
| information. The value of the Via header field (Section 14.45) is of |  |  |
| particular interest, since it acts as a trace of the request chain. |  |  |
| Use of the Max-Forwards header field allows the client to limit the |  |  |
| length of the request chain, which is useful for testing a chain of |  |  |
| proxies forwarding messages in an infinite loop. |  |  |
| If the request is valid, the response SHOULD contain the entire |  |  |
| request message in the entity-body, with a Content-Type of "message/ |  |  |
| http". Responses to this method MUST NOT be cached. |  |  |
| 9.9. CONNECT |  |  |
| This specification reserves the method name CONNECT for use with a |  |  |
| proxy that can dynamically switch to being a tunnel (e.g. SSL |  |  |
| tunneling [Luo1998]). |  |  |
| 10. Status Code Definitions |  |  |
| Each Status-Code is described below, including a description of which |  |  |
| method(s) it can follow and any metainformation required in the |  |  |
| response. |  |  |
| 10.1. Informational 1xx |  |  |
| This class of status code indicates a provisional response, |  |  |
| consisting only of the Status-Line and optional headers, and is |  |  |
| terminated by an empty line. There are no required headers for this |  |  |
| class of status code. Since HTTP/1.0 did not define any 1xx status |  |  |
| codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client |  |  |
| except under experimental conditions. |  |  |
| A client MUST be prepared to accept one or more 1xx status responses |  |  |
| prior to a regular response, even if the client does not expect a 100 |  |  |
| (Continue) status message. Unexpected 1xx status responses MAY be |  |  |
| ignored by a user agent. |  |  |
| Proxies MUST forward 1xx responses, unless the connection between the |  |  |
| proxy and its client has been closed, or unless the proxy itself |  |  |
| requested the generation of the 1xx response. (For example, if a |  |  |
| proxy adds a "Expect: 100-continue" field when it forwards a request, |  |  |
| then it need not forward the corresponding 100 (Continue) |  |  |
| response(s).) |  |  |
| 10.1.1. 100 Continue |  |  |
| The client SHOULD continue with its request. This interim response |  |  |
| is used to inform the client that the initial part of the request has |  |  |
| been received and has not yet been rejected by the server. The |  |  |
| client SHOULD continue by sending the remainder of the request or, if |  |  |
| the request has already been completed, ignore this response. The |  |  |
| server MUST send a final response after the request has been |  |  |
| completed. See Section 8.2.3 for detailed discussion of the use and |  |  |
| handling of this status code. |  |  |
| 10.1.2. 101 Switching Protocols |  |  |
| The server understands and is willing to comply with the client's |  |  |
| request, via the Upgrade message header field (Section 14.42), for a |  |  |
| change in the application protocol being used on this connection. |  |  |
| The server will switch protocols to those defined by the response's |  |  |
| Upgrade header field immediately after the empty line which |  |  |
| terminates the 101 response. |  |  |
| The protocol SHOULD be switched only when it is advantageous to do |  |  |
| so. For example, switching to a newer version of HTTP is |  |  |
| advantageous over older versions, and switching to a real-time, |  |  |
| synchronous protocol might be advantageous when delivering resources |  |  |
| that use such features. |  |  |
| 10.2. Successful 2xx |  |  |
| This class of status code indicates that the client's request was |  |  |
| successfully received, understood, and accepted. |  |  |
| 10.2.1. 200 OK |  |  |
| The request has succeeded. The information returned with the |  |  |
| response is dependent on the method used in the request, for example: |  |  |
| GET an entity corresponding to the requested resource is sent in the |  |  |
| response; |  |  |
| HEAD the entity-header fields corresponding to the requested |  |  |
| resource are sent in the response without any message-body; |  |  |
| POST an entity describing or containing the result of the action; |  |  |
| TRACE an entity containing the request message as received by the |  |  |
| end server. |  |  |
| 10.2.2. 201 Created |  |  |
| The request has been fulfilled and resulted in a new resource being |  |  |
| created. The newly created resource can be referenced by the URI(s) |  |  |
| returned in the entity of the response, with the most specific URI |  |  |
| for the resource given by a Location header field. The response |  |  |
| SHOULD include an entity containing a list of resource |  |  |
| characteristics and location(s) from which the user or user agent can |  |  |
| choose the one most appropriate. The entity format is specified by |  |  |
| the media type given in the Content-Type header field. The origin |  |  |
| server MUST create the resource before returning the 201 status code. |  |  |
| If the action cannot be carried out immediately, the server SHOULD |  |  |
| respond with 202 (Accepted) response instead. |  |  |
| A 201 response MAY contain an ETag response header field indicating |  |  |
| the current value of the entity tag for the requested variant just |  |  |
| created, see Section 14.19. |  |  |
| 10.2.3. 202 Accepted |  |  |
| The request has been accepted for processing, but the processing has |  |  |
| not been completed. The request might or might not eventually be |  |  |
| acted upon, as it might be disallowed when processing actually takes |  |  |
| place. There is no facility for re-sending a status code from an |  |  |
| asynchronous operation such as this. |  |  |
| The 202 response is intentionally non-committal. Its purpose is to |  |  |
| allow a server to accept a request for some other process (perhaps a |  |  |
| batch-oriented process that is only run once per day) without |  |  |
| requiring that the user agent's connection to the server persist |  |  |
| until the process is completed. The entity returned with this |  |  |
| response SHOULD include an indication of the request's current status |  |  |
| and either a pointer to a status monitor or some estimate of when the |  |  |
| user can expect the request to be fulfilled. |  |  |
| 10.2.4. 203 Non-Authoritative Information |  |  |
| The returned metainformation in the entity-header is not the |  |  |
| definitive set as available from the origin server, but is gathered |  |  |
| from a local or a third-party copy. The set presented MAY be a |  |  |
| subset or superset of the original version. For example, including |  |  |
| local annotation information about the resource might result in a |  |  |
| superset of the metainformation known by the origin server. Use of |  |  |
| this response code is not required and is only appropriate when the |  |  |
| response would otherwise be 200 (OK). |  |  |
| 10.2.5. 204 No Content |  |  |
| The server has fulfilled the request but does not need to return an |  |  |
| entity-body, and might want to return updated metainformation. The |  |  |
| response MAY include new or updated metainformation in the form of |  |  |
| entity-headers, which if present SHOULD be associated with the |  |  |
| requested variant. |  |  |
| If the client is a user agent, it SHOULD NOT change its document view |  |  |
| from that which caused the request to be sent. This response is |  |  |
| primarily intended to allow input for actions to take place without |  |  |
| causing a change to the user agent's active document view, although |  |  |
| any new or updated metainformation SHOULD be applied to the document |  |  |
| currently in the user agent's active view. |  |  |
| The 204 response MUST NOT include a message-body, and thus is always |  |  |
| terminated by the first empty line after the header fields. |  |  |
| 10.2.6. 205 Reset Content |  |  |
| The server has fulfilled the request and the user agent SHOULD reset |  |  |
| the document view which caused the request to be sent. This response |  |  |
| is primarily intended to allow input for actions to take place via |  |  |
| user input, followed by a clearing of the form in which the input is |  |  |
| given so that the user can easily initiate another input action. The |  |  |
| response MUST NOT include an entity. |  |  |
| 10.2.7. 206 Partial Content |  |  |
| The server has fulfilled the partial GET request for the resource. |  |  |
| The request MUST have included a Range header field (Section 14.35) |  |  |
| indicating the desired range, and MAY have included an If-Range |  |  |
| header field (Section 14.27) to make the request conditional. |  |  |
| The response MUST include the following header fields: |  |  |
| o Either a Content-Range header field (Section 14.16) indicating the |  |  |
| range included with this response, or a multipart/byteranges |  |  |
| Content-Type including Content-Range fields for each part. If a |  |  |
| Content-Length header field is present in the response, its value |  |  |
| MUST match the actual number of OCTETs transmitted in the message- |  |  |
| body. |  |  |
| o Date |  |  |
| o ETag and/or Content-Location, if the header would have been sent |  |  |
| in a 200 response to the same request |  |  |
| o Expires, Cache-Control, and/or Vary, if the field-value might |  |  |
| differ from that sent in any previous response for the same |  |  |
| variant |  |  |
| If the 206 response is the result of an If-Range request that used a |  |  |
| strong cache validator (see Section 13.3.3), the response SHOULD NOT |  |  |
| include other entity-headers. If the response is the result of an |  |  |
| If-Range request that used a weak validator, the response MUST NOT |  |  |
| include other entity-headers; this prevents inconsistencies between |  |  |
| cached entity-bodies and updated headers. Otherwise, the response |  |  |
| MUST include all of the entity-headers that would have been returned |  |  |
| with a 200 (OK) response to the same request. |  |  |
| A cache MUST NOT combine a 206 response with other previously cached |  |  |
| content if the ETag or Last-Modified headers do not match exactly, |  |  |
| see 13.5.4. |  |  |
| A cache that does not support the Range and Content-Range headers |  |  |
| MUST NOT cache 206 (Partial) responses. |  |  |
| 10.3. Redirection 3xx |  |  |
| This class of status code indicates that further action needs to be |  |  |
| taken by the user agent in order to fulfill the request. The action |  |  |
| required MAY be carried out by the user agent without interaction |  |  |
| with the user if and only if the method used in the second request is |  |  |
| GET or HEAD. A client SHOULD detect infinite redirection loops, |  |  |
| since such loops generate network traffic for each redirection. |  |  |
| Note: previous versions of this specification recommended a |  |  |
| maximum of five redirections. Content developers should be aware |  |  |
| that there might be clients that implement such a fixed |  |  |
| limitation. |  |  |
| 10.3.1. 300 Multiple Choices |  |  |
| The requested resource corresponds to any one of a set of |  |  |
| representations, each with its own specific location, and agent- |  |  |
| driven negotiation information (Section 12) is being provided so that |  |  |
| the user (or user agent) can select a preferred representation and |  |  |
| redirect its request to that location. |  |  |
| Unless it was a HEAD request, the response SHOULD include an entity |  |  |
| containing a list of resource characteristics and location(s) from |  |  |
| which the user or user agent can choose the one most appropriate. |  |  |
| The entity format is specified by the media type given in the |  |  |
| Content-Type header field. Depending upon the format and the |  |  |
| capabilities of the user agent, selection of the most appropriate |  |  |
| choice MAY be performed automatically. However, this specification |  |  |
| does not define any standard for such automatic selection. |  |  |
| If the server has a preferred choice of representation, it SHOULD |  |  |
| include the specific URI for that representation in the Location |  |  |
| field; user agents MAY use the Location field value for automatic |  |  |
| redirection. This response is cacheable unless indicated otherwise. |  |  |
| 10.3.2. 301 Moved Permanently |  |  |
| The requested resource has been assigned a new permanent URI and any |  |  |
| future references to this resource SHOULD use one of the returned |  |  |
| URIs. Clients with link editing capabilities ought to automatically |  |  |
| re-link references to the Request-URI to one or more of the new |  |  |
| references returned by the server, where possible. This response is |  |  |
| cacheable unless indicated otherwise. |  |  |
| The new permanent URI SHOULD be given by the Location field in the |  |  |
| response. Unless the request method was HEAD, the entity of the |  |  |
| response SHOULD contain a short hypertext note with a hyperlink to |  |  |
| the new URI(s). |  |  |
| If the 301 status code is received in response to a request other |  |  |
| than GET or HEAD, the user agent MUST NOT automatically redirect the |  |  |
| request unless it can be confirmed by the user, since this might |  |  |
| change the conditions under which the request was issued. |  |  |
| Note: When automatically redirecting a POST request after |  |  |
| receiving a 301 status code, some existing HTTP/1.0 user agents |  |  |
| will erroneously change it into a GET request. |  |  |
| 10.3.3. 302 Found |  |  |
| The requested resource resides temporarily under a different URI. |  |  |
| Since the redirection might be altered on occasion, the client SHOULD |  |  |
| continue to use the Request-URI for future requests. This response |  |  |
| is only cacheable if indicated by a Cache-Control or Expires header |  |  |
| field. |  |  |
| The temporary URI SHOULD be given by the Location field in the |  |  |
| response. Unless the request method was HEAD, the entity of the |  |  |
| response SHOULD contain a short hypertext note with a hyperlink to |  |  |
| the new URI(s). |  |  |
| If the 302 status code is received in response to a request other |  |  |
| than GET or HEAD, the user agent MUST NOT automatically redirect the |  |  |
| request unless it can be confirmed by the user, since this might |  |  |
| change the conditions under which the request was issued. |  |  |
| Note: RFC 1945 and RFC 2068 specify that the client is not allowed |  |  |
| to change the method on the redirected request. However, most |  |  |
| existing user agent implementations treat 302 as if it were a 303 |  |  |
| response, performing a GET on the Location field-value regardless |  |  |
| of the original request method. The status codes 303 and 307 have |  |  |
| been added for servers that wish to make unambiguously clear which |  |  |
| kind of reaction is expected of the client. |  |  |
| 10.3.4. 303 See Other |  |  |
| The response to the request can be found under a different URI and |  |  |
| SHOULD be retrieved using a GET method on that resource. This method |  |  |
| exists primarily to allow the output of a POST-activated script to |  |  |
| redirect the user agent to a selected resource. The new URI is not a |  |  |
| substitute reference for the originally requested resource. The 303 |  |  |
| response MUST NOT be cached, but the response to the second |  |  |
| (redirected) request might be cacheable. |  |  |
| The different URI SHOULD be given by the Location field in the |  |  |
| response. Unless the request method was HEAD, the entity of the |  |  |
| response SHOULD contain a short hypertext note with a hyperlink to |  |  |
| the new URI(s). |  |  |
| Note: Many pre-HTTP/1.1 user agents do not understand the 303 |  |  |
| status. When interoperability with such clients is a concern, the |  |  |
| 302 status code may be used instead, since most user agents react |  |  |
| to a 302 response as described here for 303. |  |  |
| 10.3.5. 304 Not Modified |  | 3.1. 304 Not Modified |
| If the client has performed a conditional GET request and access is |  | If the client has performed a conditional GET request and access is |
| allowed, but the document has not been modified, the server SHOULD |  | allowed, but the document has not been modified, the server SHOULD |
| respond with this status code. The 304 response MUST NOT contain a |  | respond with this status code. The 304 response MUST NOT contain a |
| message-body, and thus is always terminated by the first empty line |  | message-body, and thus is always terminated by the first empty line |
| after the header fields. |  | after the header fields. |
| The response MUST include the following header fields: |  | The response MUST include the following header fields: |
| o Date, unless its omission is required by Section 14.18.1 |  | o Date, unless its omission is required by Section 8.3.1 of [Part1] |
| If a clockless origin server obeys these rules, and proxies and |  | If a clockless origin server obeys these rules, and proxies and |
| clients add their own Date to any response received without one (as |  | clients add their own Date to any response received without one (as |
| already specified by [RFC 2068], section 14.19), caches will operate |  | already specified by [RFC 2068], section 14.19), caches will operate |
| correctly. |  | correctly. |
| o ETag and/or Content-Location, if the header would have been sent |  | o ETag and/or Content-Location, if the header would have been sent |
| in a 200 response to the same request |  | in a 200 response to the same request |
| o Expires, Cache-Control, and/or Vary, if the field-value might |  | o Expires, Cache-Control, and/or Vary, if the field-value might |
| differ from that sent in any previous response for the same |  | differ from that sent in any previous response for the same |
| variant |  | variant |
| If the conditional GET used a strong cache validator (see |  | If the conditional GET used a strong cache validator (see [Part6]), |
| Section 13.3.3), the response SHOULD NOT include other entity- |  | the response SHOULD NOT include other entity-headers. Otherwise |
| headers. Otherwise (i.e., the conditional GET used a weak |  | (i.e., the conditional GET used a weak validator), the response MUST |
| validator), the response MUST NOT include other entity-headers; this |  | NOT include other entity-headers; this prevents inconsistencies |
| prevents inconsistencies between cached entity-bodies and updated |  | between cached entity-bodies and updated headers. |
| headers. |  |  |
| If a 304 response indicates an entity not currently cached, then the |  | If a 304 response indicates an entity not currently cached, then the |
| cache MUST disregard the response and repeat the request without the |  | cache MUST disregard the response and repeat the request without the |
| conditional. |  | conditional. |
| If a cache uses a received 304 response to update a cache entry, the |  | If a cache uses a received 304 response to update a cache entry, the |
| cache MUST update the entry to reflect any new field values given in |  | cache MUST update the entry to reflect any new field values given in |
| the response. |  | the response. |
| 10.3.6. 305 Use Proxy |  | 3.2. 412 Precondition Failed |
| The requested resource MUST be accessed through the proxy given by |  |  |
| the Location field. The Location field gives the URI of the proxy. |  |  |
| The recipient is expected to repeat this single request via the |  |  |
| proxy. 305 responses MUST only be generated by origin servers. |  |  |
| Note: RFC 2068 was not clear that 305 was intended to redirect a |  |  |
| single request, and to be generated by origin servers only. Not |  |  |
| observing these limitations has significant security consequences. |  |  |
| 10.3.7. 306 (Unused) |  |  |
| The 306 status code was used in a previous version of the |  |  |
| specification, is no longer used, and the code is reserved. |  |  |
| 10.3.8. 307 Temporary Redirect |  |  |
| The requested resource resides temporarily under a different URI. |  |  |
| Since the redirection MAY be altered on occasion, the client SHOULD |  |  |
| continue to use the Request-URI for future requests. This response |  |  |
| is only cacheable if indicated by a Cache-Control or Expires header |  |  |
| field. |  |  |
| The temporary URI SHOULD be given by the Location field in the |  |  |
| response. Unless the request method was HEAD, the entity of the |  |  |
| response SHOULD contain a short hypertext note with a hyperlink to |  |  |
| the new URI(s) , since many pre-HTTP/1.1 user agents do not |  |  |
| understand the 307 status. Therefore, the note SHOULD contain the |  |  |
| information necessary for a user to repeat the original request on |  |  |
| the new URI. |  |  |
| If the 307 status code is received in response to a request other |  |  |
| than GET or HEAD, the user agent MUST NOT automatically redirect the |  |  |
| request unless it can be confirmed by the user, since this might |  |  |
| change the conditions under which the request was issued. |  |  |
| 10.4. Client Error 4xx |  |  |
| The 4xx class of status code is intended for cases in which the |  |  |
| client seems to have erred. Except when responding to a HEAD |  |  |
| request, the server SHOULD include an entity containing an |  |  |
| explanation of the error situation, and whether it is a temporary or |  |  |
| permanent condition. These status codes are applicable to any |  |  |
| request method. User agents SHOULD display any included entity to |  |  |
| the user. |  |  |
| If the client is sending data, a server implementation using TCP |  |  |
| SHOULD be careful to ensure that the client acknowledges receipt of |  |  |
| the packet(s) containing the response, before the server closes the |  |  |
| input connection. If the client continues sending data to the server |  |  |
| after the close, the server's TCP stack will send a reset packet to |  |  |
| the client, which may erase the client's unacknowledged input buffers |  |  |
| before they can be read and interpreted by the HTTP application. |  |  |
| 10.4.1. 400 Bad Request |  |  |
| The request could not be understood by the server due to malformed |  |  |
| syntax. The client SHOULD NOT repeat the request without |  |  |
| modifications. |  |  |
| 10.4.2. 401 Unauthorized |  |  |
| The request requires user authentication. The response MUST include |  |  |
| a WWW-Authenticate header field (Section 14.47) containing a |  |  |
| challenge applicable to the requested resource. The client MAY |  |  |
| repeat the request with a suitable Authorization header field |  |  |
| (Section 14.8). If the request already included Authorization |  |  |
| credentials, then the 401 response indicates that authorization has |  |  |
| been refused for those credentials. If the 401 response contains the |  |  |
| same challenge as the prior response, and the user agent has already |  |  |
| attempted authentication at least once, then the user SHOULD be |  |  |
| presented the entity that was given in the response, since that |  |  |
| entity might include relevant diagnostic information. HTTP access |  |  |
| authentication is explained in "HTTP Authentication: Basic and Digest |  |  |
| Access Authentication" [RFC2617]. |  |  |
| 10.4.3. 402 Payment Required |  |  |
| This code is reserved for future use. |  |  |
| 10.4.4. 403 Forbidden |  |  |
| The server understood the request, but is refusing to fulfill it. |  |  |
| Authorization will not help and the request SHOULD NOT be repeated. |  |  |
| If the request method was not HEAD and the server wishes to make |  |  |
| public why the request has not been fulfilled, it SHOULD describe the |  |  |
| reason for the refusal in the entity. If the server does not wish to |  |  |
| make this information available to the client, the status code 404 |  |  |
| (Not Found) can be used instead. |  |  |
| 10.4.5. 404 Not Found |  |  |
| The server has not found anything matching the Request-URI. No |  |  |
| indication is given of whether the condition is temporary or |  |  |
| permanent. The 410 (Gone) status code SHOULD be used if the server |  |  |
| knows, through some internally configurable mechanism, that an old |  |  |
| resource is permanently unavailable and has no forwarding address. |  |  |
| This status code is commonly used when the server does not wish to |  |  |
| reveal exactly why the request has been refused, or when no other |  |  |
| response is applicable. |  |  |
| 10.4.6. 405 Method Not Allowed |  |  |
| The method specified in the Request-Line is not allowed for the |  |  |
| resource identified by the Request-URI. The response MUST include an |  |  |
| Allow header containing a list of valid methods for the requested |  |  |
| resource. |  |  |
| 10.4.7. 406 Not Acceptable |  |  |
| The resource identified by the request is only capable of generating |  |  |
| response entities which have content characteristics not acceptable |  |  |
| according to the accept headers sent in the request. |  |  |
| Unless it was a HEAD request, the response SHOULD include an entity |  |  |
| containing a list of available entity characteristics and location(s) |  |  |
| from which the user or user agent can choose the one most |  |  |
| appropriate. The entity format is specified by the media type given |  |  |
| in the Content-Type header field. Depending upon the format and the |  |  |
| capabilities of the user agent, selection of the most appropriate |  |  |
| choice MAY be performed automatically. However, this specification |  |  |
| does not define any standard for such automatic selection. |  |  |
| Note: HTTP/1.1 servers are allowed to return responses which are |  |  |
| not acceptable according to the accept headers sent in the |  |  |
| request. In some cases, this may even be preferable to sending a |  |  |
| 406 response. User agents are encouraged to inspect the headers |  |  |
| of an incoming response to determine if it is acceptable. |  |  |
| If the response could be unacceptable, a user agent SHOULD |  |  |
| temporarily stop receipt of more data and query the user for a |  |  |
| decision on further actions. |  |  |
| 10.4.8. 407 Proxy Authentication Required |  |  |
| This code is similar to 401 (Unauthorized), but indicates that the |  |  |
| client must first authenticate itself with the proxy. The proxy MUST |  |  |
| return a Proxy-Authenticate header field (Section 14.33) containing a |  |  |
| challenge applicable to the proxy for the requested resource. The |  |  |
| client MAY repeat the request with a suitable Proxy-Authorization |  |  |
| header field (Section 14.34). HTTP access authentication is |  |  |
| explained in "HTTP Authentication: Basic and Digest Access |  |  |
| Authentication" [RFC2617]. |  |  |
| 10.4.9. 408 Request Timeout |  |  |
| The client did not produce a request within the time that the server |  |  |
| was prepared to wait. The client MAY repeat the request without |  |  |
| modifications at any later time. |  |  |
| 10.4.10. 409 Conflict |  |  |
| The request could not be completed due to a conflict with the current |  |  |
| state of the resource. This code is only allowed in situations where |  |  |
| it is expected that the user might be able to resolve the conflict |  |  |
| and resubmit the request. The response body SHOULD include enough |  |  |
| information for the user to recognize the source of the conflict. |  |  |
| Ideally, the response entity would include enough information for the |  |  |
| user or user agent to fix the problem; however, that might not be |  |  |
| possible and is not required. |  |  |
| Conflicts are most likely to occur in response to a PUT request. For |  |  |
| example, if versioning were being used and the entity being PUT |  |  |
| included changes to a resource which conflict with those made by an |  |  |
| earlier (third-party) request, the server might use the 409 response |  |  |
| to indicate that it can't complete the request. In this case, the |  |  |
| response entity would likely contain a list of the differences |  |  |
| between the two versions in a format defined by the response Content- |  |  |
| Type. |  |  |
| 10.4.11. 410 Gone |  |  |
| The requested resource is no longer available at the server and no |  |  |
| forwarding address is known. This condition is expected to be |  |  |
| considered permanent. Clients with link editing capabilities SHOULD |  |  |
| delete references to the Request-URI after user approval. If the |  |  |
| server does not know, or has no facility to determine, whether or not |  |  |
| the condition is permanent, the status code 404 (Not Found) SHOULD be |  |  |
| used instead. This response is cacheable unless indicated otherwise. |  |  |
| The 410 response is primarily intended to assist the task of web |  |  |
| maintenance by notifying the recipient that the resource is |  |  |
| intentionally unavailable and that the server owners desire that |  |  |
| remote links to that resource be removed. Such an event is common |  |  |
| for limited-time, promotional services and for resources belonging to |  |  |
| individuals no longer working at the server's site. It is not |  |  |
| necessary to mark all permanently unavailable resources as "gone" or |  |  |
| to keep the mark for any length of time -- that is left to the |  |  |
| discretion of the server owner. |  |  |
| 10.4.12. 411 Length Required |  |  |
| The server refuses to accept the request without a defined Content- |  |  |
| Length. The client MAY repeat the request if it adds a valid |  |  |
| Content-Length header field containing the length of the message-body |  |  |
| in the request message. |  |  |
| 10.4.13. 412 Precondition Failed |  |  |
| The precondition given in one or more of the request-header fields |  | The precondition given in one or more of the request-header fields |
| evaluated to false when it was tested on the server. This response |  | evaluated to false when it was tested on the server. This response |
| code allows the client to place preconditions on the current resource |  | code allows the client to place preconditions on the current resource |
| metainformation (header field data) and thus prevent the requested |  | metainformation (header field data) and thus prevent the requested |
| method from being applied to a resource other than the one intended. |  | method from being applied to a resource other than the one intended. |
| 10.4.14. 413 Request Entity Too Large |  | 4. Weak and Strong Validators |
| The server is refusing to process a request because the request |  |  |
| entity is larger than the server is willing or able to process. The |  |  |
| server MAY close the connection to prevent the client from continuing |  |  |
| the request. |  |  |
| If the condition is temporary, the server SHOULD include a Retry- |  |  |
| After header field to indicate that it is temporary and after what |  |  |
| time the client MAY try again. |  |  |
| 10.4.15. 414 Request-URI Too Long |  |  |
| The server is refusing to service the request because the Request-URI |  |  |
| is longer than the server is willing to interpret. This rare |  |  |
| condition is only likely to occur when a client has improperly |  |  |
| converted a POST request to a GET request with long query |  |  |
| information, when the client has descended into a URI "black hole" of |  |  |
| redirection (e.g., a redirected URI prefix that points to a suffix of |  |  |
| itself), or when the server is under attack by a client attempting to |  |  |
| exploit security holes present in some servers using fixed-length |  |  |
| buffers for reading or manipulating the Request-URI. |  |  |
| 10.4.16. 415 Unsupported Media Type |  |  |
| The server is refusing to service the request because the entity of |  |  |
| the request is in a format not supported by the requested resource |  |  |
| for the requested method. |  |  |
| 10.4.17. 416 Requested Range Not Satisfiable |  |  |
| A server SHOULD return a response with this status code if a request |  |  |
| included a Range request-header field (Section 14.35), and none of |  |  |
| the range-specifier values in this field overlap the current extent |  |  |
| of the selected resource, and the request did not include an If-Range |  |  |
| request-header field. (For byte-ranges, this means that the first- |  |  |
| byte-pos of all of the byte-range-spec values were greater than the |  |  |
| current length of the selected resource.) |  |  |
| When this status code is returned for a byte-range request, the |  |  |
| response SHOULD include a Content-Range entity-header field |  |  |
| specifying the current length of the selected resource (see |  |  |
| Section 14.16). This response MUST NOT use the multipart/byteranges |  |  |
| content-type. |  |  |
| 10.4.18. 417 Expectation Failed |  |  |
| The expectation given in an Expect request-header field (see |  |  |
| Section 14.20) could not be met by this server, or, if the server is |  |  |
| a proxy, the server has unambiguous evidence that the request could |  |  |
| not be met by the next-hop server. |  |  |
| 10.5. Server Error 5xx |  |  |
| Response status codes beginning with the digit "5" indicate cases in |  |  |
| which the server is aware that it has erred or is incapable of |  |  |
| performing the request. Except when responding to a HEAD request, |  |  |
| the server SHOULD include an entity containing an explanation of the |  |  |
| error situation, and whether it is a temporary or permanent |  |  |
| condition. User agents SHOULD display any included entity to the |  |  |
| user. These response codes are applicable to any request method. |  |  |
| 10.5.1. 500 Internal Server Error |  |  |
| The server encountered an unexpected condition which prevented it |  |  |
| from fulfilling the request. |  |  |
| 10.5.2. 501 Not Implemented |  |  |
| The server does not support the functionality required to fulfill the |  |  |
| request. This is the appropriate response when the server does not |  |  |
| recognize the request method and is not capable of supporting it for |  |  |
| any resource. |  |  |
| 10.5.3. 502 Bad Gateway |  |  |
| The server, while acting as a gateway or proxy, received an invalid |  |  |
| response from the upstream server it accessed in attempting to |  |  |
| fulfill the request. |  |  |
| 10.5.4. 503 Service Unavailable |  |  |
| The server is currently unable to handle the request due to a |  |  |
| temporary overloading or maintenance of the server. The implication |  |  |
| is that this is a temporary condition which will be alleviated after |  |  |
| some delay. If known, the length of the delay MAY be indicated in a |  |  |
| Retry-After header. If no Retry-After is given, the client SHOULD |  |  |
| handle the response as it would for a 500 response. |  |  |
| Note: The existence of the 503 status code does not imply that a |  |  |
| server must use it when becoming overloaded. Some servers may |  |  |
| wish to simply refuse the connection. |  |  |
| 10.5.5. 504 Gateway Timeout |  |  |
| The server, while acting as a gateway or proxy, did not receive a |  |  |
| timely response from the upstream server specified by the URI (e.g. |  |  |
| HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed |  |  |
| to access in attempting to complete the request. |  |  |
| Note: Note to implementors: some deployed proxies are known to |  |  |
| return 400 or 500 when DNS lookups time out. |  |  |
| 10.5.6. 505 HTTP Version Not Supported |  |  |
| The server does not support, or refuses to support, the HTTP protocol |  |  |
| version that was used in the request message. The server is |  |  |
| indicating that it is unable or unwilling to complete the request |  |  |
| using the same major version as the client, as described in |  |  |
| Section 3.1, other than with this error message. The response SHOULD |  |  |
| contain an entity describing why that version is not supported and |  |  |
| what other protocols are supported by that server. |  |  |
| 11. Access Authentication |  |  |
| HTTP provides several OPTIONAL challenge-response authentication |  |  |
| mechanisms which can be used by a server to challenge a client |  |  |
| request and by a client to provide authentication information. The |  |  |
| general framework for access authentication, and the specification of |  |  |
| "basic" and "digest" authentication, are specified in "HTTP |  |  |
| Authentication: Basic and Digest Access Authentication" [RFC2617]. |  |  |
| This specification adopts the definitions of "challenge" and |  |  |
| "credentials" from that specification. |  |  |
| 12. Content Negotiation |  |  |
| Most HTTP responses include an entity which contains information for |  |  |
| interpretation by a human user. Naturally, it is desirable to supply |  |  |
| the user with the "best available" entity corresponding to the |  |  |
| request. Unfortunately for servers and caches, not all users have |  |  |
| the same preferences for what is "best," and not all user agents are |  |  |
| equally capable of rendering all entity types. For that reason, HTTP |  |  |
| has provisions for several mechanisms for "content negotiation" -- |  |  |
| the process of selecting the best representation for a given response |  |  |
| when there are multiple representations available. |  |  |
| Note: This is not called "format negotiation" because the |  |  |
| alternate representations may be of the same media type, but use |  |  |
| different capabilities of that type, be in different languages, |  |  |
| etc. |  |  |
| Any response containing an entity-body MAY be subject to negotiation, |  |  |
| including error responses. |  |  |
| There are two kinds of content negotiation which are possible in |  |  |
| HTTP: server-driven and agent-driven negotiation. These two kinds of |  |  |
| negotiation are orthogonal and thus may be used separately or in |  |  |
| combination. One method of combination, referred to as transparent |  |  |
| negotiation, occurs when a cache uses the agent-driven negotiation |  |  |
| information provided by the origin server in order to provide server- |  |  |
| driven negotiation for subsequent requests. |  |  |
| 12.1. Server-driven Negotiation |  |  |
| If the selection of the best representation for a response is made by |  |  |
| an algorithm located at the server, it is called server-driven |  |  |
| negotiation. Selection is based on the available representations of |  |  |
| the response (the dimensions over which it can vary; e.g. language, |  |  |
| content-coding, etc.) and the contents of particular header fields in |  |  |
| the request message or on other information pertaining to the request |  |  |
| (such as the network address of the client). |  |  |
| Server-driven negotiation is advantageous when the algorithm for |  |  |
| selecting from among the available representations is difficult to |  |  |
| describe to the user agent, or when the server desires to send its |  |  |
| "best guess" to the client along with the first response (hoping to |  |  |
| avoid the round-trip delay of a subsequent request if the "best |  |  |
| guess" is good enough for the user). In order to improve the |  |  |
| server's guess, the user agent MAY include request header fields |  |  |
| (Accept, Accept-Language, Accept-Encoding, etc.) which describe its |  |  |
| preferences for such a response. |  |  |
| Server-driven negotiation has disadvantages: |  |  |
| 1. It is impossible for the server to accurately determine what |  |  |
| might be "best" for any given user, since that would require |  |  |
| complete knowledge of both the capabilities of the user agent and |  |  |
| the intended use for the response (e.g., does the user want to |  |  |
| view it on screen or print it on paper?). |  |  |
| 2. Having the user agent describe its capabilities in every request |  |  |
| can be both very inefficient (given that only a small percentage |  |  |
| of responses have multiple representations) and a potential |  |  |
| violation of the user's privacy. |  |  |
| 3. It complicates the implementation of an origin server and the |  |  |
| algorithms for generating responses to a request. |  |  |
| 4. It may limit a public cache's ability to use the same response |  |  |
| for multiple user's requests. |  |  |
| HTTP/1.1 includes the following request-header fields for enabling |  |  |
| server-driven negotiation through description of user agent |  |  |
| capabilities and user preferences: Accept (Section 14.1), Accept- |  |  |
| Charset (Section 14.2), Accept-Encoding (Section 14.3), Accept- |  |  |
| Language (Section 14.4), and User-Agent (Section 14.43). However, an |  |  |
| origin server is not limited to these dimensions and MAY vary the |  |  |
| response based on any aspect of the request, including information |  |  |
| outside the request-header fields or within extension header fields |  |  |
| not defined by this specification. |  |  |
| The Vary header field can be used to express the parameters the |  |  |
| server uses to select a representation that is subject to server- |  |  |
| driven negotiation. See Section 13.6 for use of the Vary header |  |  |
| field by caches and Section 14.44 for use of the Vary header field by |  |  |
| servers. |  |  |
| 12.2. Agent-driven Negotiation |  |  |
| With agent-driven negotiation, selection of the best representation |  |  |
| for a response is performed by the user agent after receiving an |  |  |
| initial response from the origin server. Selection is based on a |  |  |
| list of the available representations of the response included within |  |  |
| the header fields or entity-body of the initial response, with each |  |  |
| representation identified by its own URI. Selection from among the |  |  |
| representations may be performed automatically (if the user agent is |  |  |
| capable of doing so) or manually by the user selecting from a |  |  |
| generated (possibly hypertext) menu. |  |  |
| Agent-driven negotiation is advantageous when the response would vary |  |  |
| over commonly-used dimensions (such as type, language, or encoding), |  |  |
| when the origin server is unable to determine a user agent's |  |  |
| capabilities from examining the request, and generally when public |  |  |
| caches are used to distribute server load and reduce network usage. |  |  |
| Agent-driven negotiation suffers from the disadvantage of needing a |  |  |
| second request to obtain the best alternate representation. This |  |  |
| second request is only efficient when caching is used. In addition, |  |  |
| this specification does not define any mechanism for supporting |  |  |
| automatic selection, though it also does not prevent any such |  |  |
| mechanism from being developed as an extension and used within |  |  |
| HTTP/1.1. |  |  |
| HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable) |  |  |
| status codes for enabling agent-driven negotiation when the server is |  |  |
| unwilling or unable to provide a varying response using server-driven |  |  |
| negotiation. |  |  |
| 12.3. Transparent Negotiation |  |  |
| Transparent negotiation is a combination of both server-driven and |  |  |
| agent-driven negotiation. When a cache is supplied with a form of |  |  |
| the list of available representations of the response (as in agent- |  |  |
| driven negotiation) and the dimensions of variance are completely |  |  |
| understood by the cache, then the cache becomes capable of performing |  |  |
| server-driven negotiation on behalf of the origin server for |  |  |
| subsequent requests on that resource. |  |  |
| Transparent negotiation has the advantage of distributing the |  |  |
| negotiation work that would otherwise be required of the origin |  |  |
| server and also removing the second request delay of agent-driven |  |  |
| negotiation when the cache is able to correctly guess the right |  |  |
| response. |  |  |
| This specification does not define any mechanism for transparent |  |  |
| negotiation, though it also does not prevent any such mechanism from |  |  |
| being developed as an extension that could be used within HTTP/1.1. |  |  |
| 13. Caching in HTTP |  |  |
| HTTP is typically used for distributed information systems, where |  |  |
| performance can be improved by the use of response caches. The |  |  |
| HTTP/1.1 protocol includes a number of elements intended to make |  |  |
| caching work as well as possible. Because these elements are |  |  |
| inextricable from other aspects of the protocol, and because they |  |  |
| interact with each other, it is useful to describe the basic caching |  |  |
| design of HTTP separately from the detailed descriptions of methods, |  |  |
| headers, response codes, etc. |  |  |
| Caching would be useless if it did not significantly improve |  |  |
| performance. The goal of caching in HTTP/1.1 is to eliminate the |  |  |
| need to send requests in many cases, and to eliminate the need to |  |  |
| send full responses in many other cases. The former reduces the |  |  |
| number of network round-trips required for many operations; we use an |  |  |
| "expiration" mechanism for this purpose (see Section 13.2). The |  |  |
| latter reduces network bandwidth requirements; we use a "validation" |  |  |
| mechanism for this purpose (see Section 13.3). |  |  |
| Requirements for performance, availability, and disconnected |  |  |
| operation require us to be able to relax the goal of semantic |  |  |
| transparency. The HTTP/1.1 protocol allows origin servers, caches, |  |  |
| and clients to explicitly reduce transparency when necessary. |  |  |
| However, because non-transparent operation may confuse non-expert |  |  |
| users, and might be incompatible with certain server applications |  |  |
| (such as those for ordering merchandise), the protocol requires that |  |  |
| transparency be relaxed |  |  |
| o only by an explicit protocol-level request when relaxed by client |  |  |
| or origin server |  |  |
| o only with an explicit warning to the end user when relaxed by |  |  |
| cache or client |  |  |
| Therefore, the HTTP/1.1 protocol provides these important elements: |  |  |
| 1. Protocol features that provide full semantic transparency when |  |  |
| this is required by all parties. |  |  |
| 2. Protocol features that allow an origin server or user agent to |  |  |
| explicitly request and control non-transparent operation. |  |  |
| 3. Protocol features that allow a cache to attach warnings to |  |  |
| responses that do not preserve the requested approximation of |  |  |
| semantic transparency. |  |  |
| A basic principle is that it must be possible for the clients to |  |  |
| detect any potential relaxation of semantic transparency. |  |  |
| Note: The server, cache, or client implementor might be faced with |  |  |
| design decisions not explicitly discussed in this specification. |  |  |
| If a decision might affect semantic transparency, the implementor |  |  |
| ought to err on the side of maintaining transparency unless a |  |  |
| careful and complete analysis shows significant benefits in |  |  |
| breaking transparency. |  |  |
| 13.1. |  |  |
| 13.1.1. Cache Correctness |  |  |
| A correct cache MUST respond to a request with the most up-to-date |  |  |
| response held by the cache that is appropriate to the request (see |  |  |
| sections 13.2.5, 13.2.6, and 13.12) which meets one of the following |  |  |
| conditions: |  |  |
| 1. It has been checked for equivalence with what the origin server |  |  |
| would have returned by revalidating the response with the origin |  |  |
| server (Section 13.3); |  |  |
| 2. It is "fresh enough" (see Section 13.2). In the default case, |  |  |
| this means it meets the least restrictive freshness requirement |  |  |
| of the client, origin server, and cache (see Section 14.9); if |  |  |
| the origin server so specifies, it is the freshness requirement |  |  |
| of the origin server alone. If a stored response is not "fresh |  |  |
| enough" by the most restrictive freshness requirement of both the |  |  |
| client and the origin server, in carefully considered |  |  |
| circumstances the cache MAY still return the response with the |  |  |
| appropriate Warning header (see section 13.1.5 and 14.46), unless |  |  |
| such a response is prohibited (e.g., by a "no-store" cache- |  |  |
| directive, or by a "no-cache" cache-request-directive; see |  |  |
| Section 14.9). |  |  |
| 3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), or |  |  |
| error (4xx or 5xx) response message. |  |  |
| If the cache can not communicate with the origin server, then a |  |  |
| correct cache SHOULD respond as above if the response can be |  |  |
| correctly served from the cache; if not it MUST return an error or |  |  |
| warning indicating that there was a communication failure. |  |  |
| If a cache receives a response (either an entire response, or a 304 |  |  |
| (Not Modified) response) that it would normally forward to the |  |  |
| requesting client, and the received response is no longer fresh, the |  |  |
| cache SHOULD forward it to the requesting client without adding a new |  |  |
| Warning (but without removing any existing Warning headers). A cache |  |  |
| SHOULD NOT attempt to revalidate a response simply because that |  |  |
| response became stale in transit; this might lead to an infinite |  |  |
| loop. A user agent that receives a stale response without a Warning |  |  |
| MAY display a warning indication to the user. |  |  |
| 13.1.2. Warnings |  |  |
| Whenever a cache returns a response that is neither first-hand nor |  |  |
| "fresh enough" (in the sense of condition 2 in Section 13.1.1), it |  |  |
| MUST attach a warning to that effect, using a Warning general-header. |  |  |
| The Warning header and the currently defined warnings are described |  |  |
| in Section 14.46. The warning allows clients to take appropriate |  |  |
| action. |  |  |
| Warnings MAY be used for other purposes, both cache-related and |  |  |
| otherwise. The use of a warning, rather than an error status code, |  |  |
| distinguish these responses from true failures. |  |  |
| Warnings are assigned three digit warn-codes. The first digit |  |  |
| indicates whether the Warning MUST or MUST NOT be deleted from a |  |  |
| stored cache entry after a successful revalidation: |  |  |
| 1xx Warnings that describe the freshness or revalidation status of |  |  |
| the response, and so MUST be deleted after a successful |  |  |
| revalidation. 1XX warn-codes MAY be generated by a cache only when |  |  |
| validating a cached entry. It MUST NOT be generated by clients. |  |  |
| 2xx Warnings that describe some aspect of the entity body or entity |  |  |
| headers that is not rectified by a revalidation (for example, a |  |  |
| lossy compression of the entity bodies) and which MUST NOT be |  |  |
| deleted after a successful revalidation. |  |  |
| See Section 14.46 for the definitions of the codes themselves. |  |  |
| HTTP/1.0 caches will cache all Warnings in responses, without |  |  |
| deleting the ones in the first category. Warnings in responses that |  |  |
| are passed to HTTP/1.0 caches carry an extra warning-date field, |  |  |
| which prevents a future HTTP/1.1 recipient from believing an |  |  |
| erroneously cached Warning. |  |  |
| Warnings also carry a warning text. The text MAY be in any |  |  |
| appropriate natural language (perhaps based on the client's Accept |  |  |
| headers), and include an OPTIONAL indication of what character set is |  |  |
| used. |  |  |
| Multiple warnings MAY be attached to a response (either by the origin |  |  |
| server or by a cache), including multiple warnings with the same code |  |  |
| number. For example, a server might provide the same warning with |  |  |
| texts in both English and Basque. |  |  |
| When multiple warnings are attached to a response, it might not be |  |  |
| practical or reasonable to display all of them to the user. This |  |  |
| version of HTTP does not specify strict priority rules for deciding |  |  |
| which warnings to display and in what order, but does suggest some |  |  |
| heuristics. |  |  |
| 13.1.3. Cache-control Mechanisms |  |  |
| The basic cache mechanisms in HTTP/1.1 (server-specified expiration |  |  |
| times and validators) are implicit directives to caches. In some |  |  |
| cases, a server or client might need to provide explicit directives |  |  |
| to the HTTP caches. We use the Cache-Control header for this |  |  |
| purpose. |  |  |
| The Cache-Control header allows a client or server to transmit a |  |  |
| variety of directives in either requests or responses. These |  |  |
| directives typically override the default caching algorithms. As a |  |  |
| general rule, if there is any apparent conflict between header |  |  |
| values, the most restrictive interpretation is applied (that is, the |  |  |
| one that is most likely to preserve semantic transparency). However, |  |  |
| in some cases, cache-control directives are explicitly specified as |  |  |
| weakening the approximation of semantic transparency (for example, |  |  |
| "max-stale" or "public"). |  |  |
| The cache-control directives are described in detail in Section 14.9. |  |  |
| 13.1.4. Explicit User Agent Warnings |  |  |
| Many user agents make it possible for users to override the basic |  |  |
| caching mechanisms. For example, the user agent might allow the user |  |  |
| to specify that cached entities (even explicitly stale ones) are |  |  |
| never validated. Or the user agent might habitually add "Cache- |  |  |
| Control: max-stale=3600" to every request. The user agent SHOULD NOT |  |  |
| default to either non-transparent behavior, or behavior that results |  |  |
| in abnormally ineffective caching, but MAY be explicitly configured |  |  |
| to do so by an explicit action of the user. |  |  |
| If the user has overridden the basic caching mechanisms, the user |  |  |
| agent SHOULD explicitly indicate to the user whenever this results in |  |  |
| the display of information that might not meet the server's |  |  |
| transparency requirements (in particular, if the displayed entity is |  |  |
| known to be stale). Since the protocol normally allows the user |  |  |
| agent to determine if responses are stale or not, this indication |  |  |
| need only be displayed when this actually happens. The indication |  |  |
| need not be a dialog box; it could be an icon (for example, a picture |  |  |
| of a rotting fish) or some other indicator. |  |  |
| If the user has overridden the caching mechanisms in a way that would |  |  |
| abnormally reduce the effectiveness of caches, the user agent SHOULD |  |  |
| continually indicate this state to the user (for example, by a |  |  |
| display of a picture of currency in flames) so that the user does not |  |  |
| inadvertently consume excess resources or suffer from excessive |  |  |
| latency. |  |  |
| 13.1.5. Exceptions to the Rules and Warnings |  |  |
| In some cases, the operator of a cache MAY choose to configure it to |  |  |
| return stale responses even when not requested by clients. This |  |  |
| decision ought not be made lightly, but may be necessary for reasons |  |  |
| of availability or performance, especially when the cache is poorly |  |  |
| connected to the origin server. Whenever a cache returns a stale |  |  |
| response, it MUST mark it as such (using a Warning header) enabling |  |  |
| the client software to alert the user that there might be a potential |  |  |
| problem. |  |  |
| It also allows the user agent to take steps to obtain a first-hand or |  |  |
| fresh response. For this reason, a cache SHOULD NOT return a stale |  |  |
| response if the client explicitly requests a first-hand or fresh one, |  |  |
| unless it is impossible to comply for technical or policy reasons. |  |  |
| 13.1.6. Client-controlled Behavior |  |  |
| While the origin server (and to a lesser extent, intermediate caches, |  |  |
| by their contribution to the age of a response) are the primary |  |  |
| source of expiration information, in some cases the client might need |  |  |
| to control a cache's decision about whether to return a cached |  |  |
| response without validating it. Clients do this using several |  |  |
| directives of the Cache-Control header. |  |  |
| A client's request MAY specify the maximum age it is willing to |  |  |
| accept of an unvalidated response; specifying a value of zero forces |  |  |
| the cache(s) to revalidate all responses. A client MAY also specify |  |  |
| the minimum time remaining before a response expires. Both of these |  |  |
| options increase constraints on the behavior of caches, and so cannot |  |  |
| further relax the cache's approximation of semantic transparency. |  |  |
| A client MAY also specify that it will accept stale responses, up to |  |  |
| some maximum amount of staleness. This loosens the constraints on |  |  |
| the caches, and so might violate the origin server's specified |  |  |
| constraints on semantic transparency, but might be necessary to |  |  |
| support disconnected operation, or high availability in the face of |  |  |
| poor connectivity. |  |  |
| 13.2. Expiration Model |  |  |
| 13.2.1. Server-Specified Expiration |  |  |
| HTTP caching works best when caches can entirely avoid making |  |  |
| requests to the origin server. The primary mechanism for avoiding |  |  |
| requests is for an origin server to provide an explicit expiration |  |  |
| time in the future, indicating that a response MAY be used to satisfy |  |  |
| subsequent requests. In other words, a cache can return a fresh |  |  |
| response without first contacting the server. |  |  |
| Our expectation is that servers will assign future explicit |  |  |
| expiration times to responses in the belief that the entity is not |  |  |
| likely to change, in a semantically significant way, before the |  |  |
| expiration time is reached. This normally preserves semantic |  |  |
| transparency, as long as the server's expiration times are carefully |  |  |
| chosen. |  |  |
| The expiration mechanism applies only to responses taken from a cache |  |  |
| and not to first-hand responses forwarded immediately to the |  |  |
| requesting client. |  |  |
| If an origin server wishes to force a semantically transparent cache |  |  |
| to validate every request, it MAY assign an explicit expiration time |  |  |
| in the past. This means that the response is always stale, and so |  |  |
| the cache SHOULD validate it before using it for subsequent requests. |  |  |
| See Section 14.9.4 for a more restrictive way to force revalidation. |  |  |
| If an origin server wishes to force any HTTP/1.1 cache, no matter how |  |  |
| it is configured, to validate every request, it SHOULD use the "must- |  |  |
| revalidate" cache-control directive (see Section 14.9). |  |  |
| Servers specify explicit expiration times using either the Expires |  |  |
| header, or the max-age directive of the Cache-Control header. |  |  |
| An expiration time cannot be used to force a user agent to refresh |  |  |
| its display or reload a resource; its semantics apply only to caching |  |  |
| mechanisms, and such mechanisms need only check a resource's |  |  |
| expiration status when a new request for that resource is initiated. |  |  |
| See Section 13.13 for an explanation of the difference between caches |  |  |
| and history mechanisms. |  |  |
| 13.2.2. Heuristic Expiration |  |  |
| Since origin servers do not always provide explicit expiration times, |  |  |
| HTTP caches typically assign heuristic expiration times, employing |  |  |
| algorithms that use other header values (such as the Last-Modified |  |  |
| time) to estimate a plausible expiration time. The HTTP/1.1 |  |  |
| specification does not provide specific algorithms, but does impose |  |  |
| worst-case constraints on their results. Since heuristic expiration |  |  |
| times might compromise semantic transparency, they ought to used |  |  |
| cautiously, and we encourage origin servers to provide explicit |  |  |
| expiration times as much as possible. |  |  |
| 13.2.3. Age Calculations |  |  |
| In order to know if a cached entry is fresh, a cache needs to know if |  |  |
| its age exceeds its freshness lifetime. We discuss how to calculate |  |  |
| the latter in Section 13.2.4; this section describes how to calculate |  |  |
| the age of a response or cache entry. |  |  |
| In this discussion, we use the term "now" to mean "the current value |  |  |
| of the clock at the host performing the calculation." Hosts that use |  |  |
| HTTP, but especially hosts running origin servers and caches, SHOULD |  |  |
| use NTP [RFC1305] or some similar protocol to synchronize their |  |  |
| clocks to a globally accurate time standard. |  |  |
| HTTP/1.1 requires origin servers to send a Date header, if possible, |  |  |
| with every response, giving the time at which the response was |  |  |
| generated (see Section 14.18). We use the term "date_value" to |  |  |
| denote the value of the Date header, in a form appropriate for |  |  |
| arithmetic operations. |  |  |
| HTTP/1.1 uses the Age response-header to convey the estimated age of |  |  |
| the response message when obtained from a cache. The Age field value |  |  |
| is the cache's estimate of the amount of time since the response was |  |  |
| generated or revalidated by the origin server. |  |  |
| In essence, the Age value is the sum of the time that the response |  |  |
| has been resident in each of the caches along the path from the |  |  |
| origin server, plus the amount of time it has been in transit along |  |  |
| network paths. |  |  |
| We use the term "age_value" to denote the value of the Age header, in |  |  |
| a form appropriate for arithmetic operations. |  |  |
| A response's age can be calculated in two entirely independent ways: |  |  |
| 1. now minus date_value, if the local clock is reasonably well |  |  |
| synchronized to the origin server's clock. If the result is |  |  |
| negative, the result is replaced by zero. |  |  |
| 2. age_value, if all of the caches along the response path implement |  |  |
| HTTP/1.1. |  |  |
| Given that we have two independent ways to compute the age of a |  |  |
| response when it is received, we can combine these as |  |  |
| corrected_received_age = max(now - date_value, age_value) |  |  |
| and as long as we have either nearly synchronized clocks or all- |  |  |
| HTTP/1.1 paths, one gets a reliable (conservative) result. |  |  |
| Because of network-imposed delays, some significant interval might |  |  |
| pass between the time that a server generates a response and the time |  |  |
| it is received at the next outbound cache or client. If uncorrected, |  |  |
| this delay could result in improperly low ages. |  |  |
| Because the request that resulted in the returned Age value must have |  |  |
| been initiated prior to that Age value's generation, we can correct |  |  |
| for delays imposed by the network by recording the time at which the |  |  |
| request was initiated. Then, when an Age value is received, it MUST |  |  |
| be interpreted relative to the time the request was initiated, not |  |  |
| the time that the response was received. This algorithm results in |  |  |
| conservative behavior no matter how much delay is experienced. So, |  |  |
| we compute: |  |  |
| corrected_initial_age = corrected_received_age |  |  |
| + (now - request_time) |  |  |
| where "request_time" is the time (according to the local clock) when |  |  |
| the request that elicited this response was sent. |  |  |
| Summary of age calculation algorithm, when a cache receives a |  |  |
| response: |  |  |
| /* |  |  |
| * age_value |  |  |
| * is the value of Age: header received by the cache with |  |  |
| * this response. |  |  |
| * date_value |  |  |
| * is the value of the origin server's Date: header |  |  |
| * request_time |  |  |
| * is the (local) time when the cache made the request |  |  |
| * that resulted in this cached response |  |  |
| * response_time |  |  |
| * is the (local) time when the cache received the |  |  |
| * response |  |  |
| * now |  |  |
| * is the current (local) time |  |  |
| */ |  |  |
| apparent_age = max(0, response_time - date_value); |  |  |
| corrected_received_age = max(apparent_age, age_value); |  |  |
| response_delay = response_time - request_time; |  |  |
| corrected_initial_age = corrected_received_age + response_delay; |  |  |
| resident_time = now - response_time; |  |  |
| current_age = corrected_initial_age + resident_time; |  |  |
| The current_age of a cache entry is calculated by adding the amount |  |  |
| of time (in seconds) since the cache entry was last validated by the |  |  |
| origin server to the corrected_initial_age. When a response is |  |  |
| generated from a cache entry, the cache MUST include a single Age |  |  |
| header field in the response with a value equal to the cache entry's |  |  |
| current_age. |  |  |
| The presence of an Age header field in a response implies that a |  |  |
| response is not first-hand. However, the converse is not true, since |  |  |
| the lack of an Age header field in a response does not imply that the |  |  |
| response is first-hand unless all caches along the request path are |  |  |
| compliant with HTTP/1.1 (i.e., older HTTP caches did not implement |  |  |
| the Age header field). |  |  |
| 13.2.4. Expiration Calculations |  |  |
| In order to decide whether a response is fresh or stale, we need to |  |  |
| compare its freshness lifetime to its age. The age is calculated as |  |  |
| described in Section 13.2.3; this section describes how to calculate |  |  |
| the freshness lifetime, and to determine if a response has expired. |  |  |
| In the discussion below, the values can be represented in any form |  |  |
| appropriate for arithmetic operations. |  |  |
| We use the term "expires_value" to denote the value of the Expires |  |  |
| header. We use the term "max_age_value" to denote an appropriate |  |  |
| value of the number of seconds carried by the "max-age" directive of |  |  |
| the Cache-Control header in a response (see Section 14.9.3). |  |  |
| The max-age directive takes priority over Expires, so if max-age is |  |  |
| present in a response, the calculation is simply: |  |  |
| freshness_lifetime = max_age_value |  |  |
| Otherwise, if Expires is present in the response, the calculation is: |  |  |
| freshness_lifetime = expires_value - date_value |  |  |
| Note that neither of these calculations is vulnerable to clock skew, |  |  |
| since all of the information comes from the origin server. |  |  |
| If none of Expires, Cache-Control: max-age, or Cache-Control: |  |  |
| s-maxage (see Section 14.9.3) appears in the response, and the |  |  |
| response does not include other restrictions on caching, the cache |  |  |
| MAY compute a freshness lifetime using a heuristic. The cache MUST |  |  |
| attach Warning 113 to any response whose age is more than 24 hours if |  |  |
| such warning has not already been added. |  |  |
| Also, if the response does have a Last-Modified time, the heuristic |  |  |
| expiration value SHOULD be no more than some fraction of the interval |  |  |
| since that time. A typical setting of this fraction might be 10%. |  |  |
| The calculation to determine if a response has expired is quite |  |  |
| simple: |  |  |
| response_is_fresh = (freshness_lifetime > current_age) |  |  |
| 13.2.5. Disambiguating Expiration Values |  |  |
| Because expiration values are assigned optimistically, it is possible |  |  |
| for two caches to contain fresh values for the same resource that are |  |  |
| different. |  |  |
| If a client performing a retrieval receives a non-first-hand response |  |  |
| for a request that was already fresh in its own cache, and the Date |  |  |
| header in its existing cache entry is newer than the Date on the new |  |  |
| response, then the client MAY ignore the response. If so, it MAY |  |  |
| retry the request with a "Cache-Control: max-age=0" directive (see |  |  |
| Section 14.9), to force a check with the origin server. |  |  |
| If a cache has two fresh responses for the same representation with |  |  |
| different validators, it MUST use the one with the more recent Date |  |  |
| header. This situation might arise because the cache is pooling |  |  |
| responses from other caches, or because a client has asked for a |  |  |
| reload or a revalidation of an apparently fresh cache entry. |  |  |
| 13.2.6. Disambiguating Multiple Responses |  |  |
| Because a client might be receiving responses via multiple paths, so |  |  |
| that some responses flow through one set of caches and other |  |  |
| responses flow through a different set of caches, a client might |  |  |
| receive responses in an order different from that in which the origin |  |  |
| server sent them. We would like the client to use the most recently |  |  |
| generated response, even if older responses are still apparently |  |  |
| fresh. |  |  |
| Neither the entity tag nor the expiration value can impose an |  |  |
| ordering on responses, since it is possible that a later response |  |  |
| intentionally carries an earlier expiration time. The Date values |  |  |
| are ordered to a granularity of one second. |  |  |
| When a client tries to revalidate a cache entry, and the response it |  |  |
| receives contains a Date header that appears to be older than the one |  |  |
| for the existing entry, then the client SHOULD repeat the request |  |  |
| unconditionally, and include |  |  |
| Cache-Control: max-age=0 |  |  |
| to force any intermediate caches to validate their copies directly |  |  |
| with the origin server, or |  |  |
| Cache-Control: no-cache |  |  |
| to force any intermediate caches to obtain a new copy from the origin |  |  |
| server. |  |  |
| If the Date values are equal, then the client MAY use either response |  |  |
| (or MAY, if it is being extremely prudent, request a new response). |  |  |
| Servers MUST NOT depend on clients being able to choose |  |  |
| deterministically between responses generated during the same second, |  |  |
| if their expiration times overlap. |  |  |
| 13.3. Validation Model |  |  |
| When a cache has a stale entry that it would like to use as a |  |  |
| response to a client's request, it first has to check with the origin |  |  |
| server (or possibly an intermediate cache with a fresh response) to |  |  |
| see if its cached entry is still usable. We call this "validating" |  |  |
| the cache entry. Since we do not want to have to pay the overhead of |  |  |
| retransmitting the full response if the cached entry is good, and we |  |  |
| do not want to pay the overhead of an extra round trip if the cached |  |  |
| entry is invalid, the HTTP/1.1 protocol supports the use of |  |  |
| conditional methods. |  |  |
| The key protocol features for supporting conditional methods are |  |  |
| those concerned with "cache validators." When an origin server |  |  |
| generates a full response, it attaches some sort of validator to it, |  |  |
| which is kept with the cache entry. When a client (user agent or |  |  |
| proxy cache) makes a conditional request for a resource for which it |  |  |
| has a cache entry, it includes the associated validator in the |  |  |
| request. |  |  |
| The server then checks that validator against the current validator |  |  |
| for the entity, and, if they match (see Section 13.3.3), it responds |  |  |
| with a special status code (usually, 304 (Not Modified)) and no |  |  |
| entity-body. Otherwise, it returns a full response (including |  |  |
| entity-body). Thus, we avoid transmitting the full response if the |  |  |
| validator matches, and we avoid an extra round trip if it does not |  |  |
| match. |  |  |
| In HTTP/1.1, a conditional request looks exactly the same as a normal |  |  |
| request for the same resource, except that it carries a special |  |  |
| header (which includes the validator) that implicitly turns the |  |  |
| method (usually, GET) into a conditional. |  |  |
| The protocol includes both positive and negative senses of cache- |  |  |
| validating conditions. That is, it is possible to request either |  |  |
| that a method be performed if and only if a validator matches or if |  |  |
| and only if no validators match. |  |  |
| Note: a response that lacks a validator may still be cached, and |  |  |
| served from cache until it expires, unless this is explicitly |  |  |
| prohibited by a cache-control directive. However, a cache cannot |  |  |
| do a conditional retrieval if it does not have a validator for the |  |  |
| entity, which means it will not be refreshable after it expires. |  |  |
| 13.3.1. Last-Modified Dates |  |  |
| The Last-Modified entity-header field value is often used as a cache |  |  |
| validator. In simple terms, a cache entry is considered to be valid |  |  |
| if the entity has not been modified since the Last-Modified value. |  |  |
| 13.3.2. Entity Tag Cache Validators |  |  |
| The ETag response-header field value, an entity tag, provides for an |  |  |
| "opaque" cache validator. This might allow more reliable validation |  |  |
| in situations where it is inconvenient to store modification dates, |  |  |
| where the one-second resolution of HTTP date values is not |  |  |
| sufficient, or where the origin server wishes to avoid certain |  |  |
| paradoxes that might arise from the use of modification dates. |  |  |
| Entity Tags are described in Section 3.11. The headers used with |  |  |
| entity tags are described in sections 14.19, 14.24, 14.26 and 14.44. |  |  |
| 13.3.3. Weak and Strong Validators |  |  |
| Since both origin servers and caches will compare two validators to |  | Since both origin servers and caches will compare two validators to |
| decide if they represent the same or different entities, one normally |  | decide if they represent the same or different entities, one normally |
| would expect that if the entity (the entity-body or any entity- |  | would expect that if the entity (the entity-body or any entity- |
| headers) changes in any way, then the associated validator would |  | headers) changes in any way, then the associated validator would |
| change as well. If this is true, then we call this validator a |  | change as well. If this is true, then we call this validator a |
| "strong validator." |  | "strong validator." |
| However, there might be cases when a server prefers to change the |  | However, there might be cases when a server prefers to change the |
| validator only on semantically significant changes, and not when |  | validator only on semantically significant changes, and not when |
| insignificant aspects of the entity change. A validator that does |  | insignificant aspects of the entity change. A validator that does |
| not always change when the resource changes is a "weak validator." |  | not always change when the resource changes is a "weak validator." |
| Entity tags are normally "strong validators," but the protocol |  | Entity tags are normally "strong validators," but the protocol |
| provides a mechanism to tag an entity tag as "weak." One can think |  | provides a mechanism to tag an entity tag as "weak." One can think |
| of a strong validator as one that changes whenever the bits of an |  | of a strong validator as one that changes whenever the bits of an |
| entity changes, while a weak value changes whenever the meaning of an |  | entity changes, while a weak value changes whenever the meaning of an |
| entity changes. Alternatively, one can think of a strong validator |  | entity changes. Alternatively, one can think of a strong validator |
| skipping to change at page 90, line 24 |  | skipping to change at page 7, line 14 |
| o The strong comparison function: in order to be considered equal, |  | o The strong comparison function: in order to be considered equal, |
| both validators MUST be identical in every way, and both MUST NOT |  | both validators MUST be identical in every way, and both MUST NOT |
| be weak. |  | be weak. |
| o The weak comparison function: in order to be considered equal, |  | o The weak comparison function: in order to be considered equal, |
| both validators MUST be identical in every way, but either or both |  | both validators MUST be identical in every way, but either or both |
| of them MAY be tagged as "weak" without affecting the result. |  | of them MAY be tagged as "weak" without affecting the result. |
| An entity tag is strong unless it is explicitly tagged as weak. |  | An entity tag is strong unless it is explicitly tagged as weak. |
| Section 3.11 gives the syntax for entity tags. |  | Section 2 gives the syntax for entity tags. |
| A Last-Modified time, when used as a validator in a request, is |  | A Last-Modified time, when used as a validator in a request, is |
| implicitly weak unless it is possible to deduce that it is strong, |  | implicitly weak unless it is possible to deduce that it is strong, |
| using the following rules: |  | using the following rules: |
| o The validator is being compared by an origin server to the actual |  | o The validator is being compared by an origin server to the actual |
| current validator for the entity and, |  | current validator for the entity and, |
| o That origin server reliably knows that the associated entity did |  | o That origin server reliably knows that the associated entity did |
| not change twice during the second covered by the presented |  | not change twice during the second covered by the presented |
| skipping to change at page 91, line 36 |  | skipping to change at page 8, line 24 |
| described here. |  | described here. |
| A cache or origin server receiving a conditional request, other than |  | A cache or origin server receiving a conditional request, other than |
| a full-body GET request, MUST use the strong comparison function to |  | a full-body GET request, MUST use the strong comparison function to |
| evaluate the condition. |  | evaluate the condition. |
| These rules allow HTTP/1.1 caches and clients to safely perform sub- |  | These rules allow HTTP/1.1 caches and clients to safely perform sub- |
| range retrievals on values that have been obtained from HTTP/1.0 |  | range retrievals on values that have been obtained from HTTP/1.0 |
| servers. |  | servers. |
| 13.3.4. Rules for When to Use Entity Tags and Last-Modified Dates |  | 5. Rules for When to Use Entity Tags and Last-Modified Dates |
| We adopt a set of rules and recommendations for origin servers, |  | We adopt a set of rules and recommendations for origin servers, |
| clients, and caches regarding when various validator types ought to |  | clients, and caches regarding when various validator types ought to |
| be used, and for what purposes. |  | be used, and for what purposes. |
| HTTP/1.1 origin servers: |  | HTTP/1.1 origin servers: |
| o SHOULD send an entity tag validator unless it is not feasible to |  | o SHOULD send an entity tag validator unless it is not feasible to |
| generate one. |  | generate one. |
| skipping to change at page 93, line 26 |  | skipping to change at page 10, line 13 |
| conservative assumptions about the validators they receive. |  | conservative assumptions about the validators they receive. |
| HTTP/1.0 clients and caches will ignore entity tags. Generally, |  | HTTP/1.0 clients and caches will ignore entity tags. Generally, |
| last-modified values received or used by these systems will |  | last-modified values received or used by these systems will |
| support transparent and efficient caching, and so HTTP/1.1 origin |  | support transparent and efficient caching, and so HTTP/1.1 origin |
| servers should provide Last-Modified values. In those rare cases |  | servers should provide Last-Modified values. In those rare cases |
| where the use of a Last-Modified value as a validator by an |  | where the use of a Last-Modified value as a validator by an |
| HTTP/1.0 system could result in a serious problem, then HTTP/1.1 |  | HTTP/1.0 system could result in a serious problem, then HTTP/1.1 |
| origin servers should not provide one. |  | origin servers should not provide one. |
| 13.3.5. Non-validating Conditionals |  | 6. Header Field Definitions |
| The principle behind entity tags is that only the service author |  |  |
| knows the semantics of a resource well enough to select an |  |  |
| appropriate cache validation mechanism, and the specification of any |  |  |
| validator comparison function more complex than byte-equality would |  |  |
| open up a can of worms. Thus, comparisons of any other headers |  |  |
| (except Last-Modified, for compatibility with HTTP/1.0) are never |  |  |
| used for purposes of validating a cache entry. |  |  |
| 13.4. Response Cacheability |  |  |
| Unless specifically constrained by a cache-control (Section 14.9) |  |  |
| directive, a caching system MAY always store a successful response |  |  |
| (see Section 13.8) as a cache entry, MAY return it without validation |  |  |
| if it is fresh, and MAY return it after successful validation. If |  |  |
| there is neither a cache validator nor an explicit expiration time |  |  |
| associated with a response, we do not expect it to be cached, but |  |  |
| certain caches MAY violate this expectation (for example, when little |  |  |
| or no network connectivity is available). A client can usually |  |  |
| detect that such a response was taken from a cache by comparing the |  |  |
| Date header to the current time. |  |  |
| Note: some HTTP/1.0 caches are known to violate this expectation |  |  |
| without providing any Warning. |  |  |
| However, in some cases it might be inappropriate for a cache to |  |  |
| retain an entity, or to return it in response to a subsequent |  |  |
| request. This might be because absolute semantic transparency is |  |  |
| deemed necessary by the service author, or because of security or |  |  |
| privacy considerations. Certain cache-control directives are |  |  |
| therefore provided so that the server can indicate that certain |  |  |
| resource entities, or portions thereof, are not to be cached |  |  |
| regardless of other considerations. |  |  |
| Note that Section 14.8 normally prevents a shared cache from saving |  |  |
| and returning a response to a previous request if that request |  |  |
| included an Authorization header. |  |  |
| A response received with a status code of 200, 203, 206, 300, 301 or |  |  |
| 410 MAY be stored by a cache and used in reply to a subsequent |  |  |
| request, subject to the expiration mechanism, unless a cache-control |  |  |
| directive prohibits caching. However, a cache that does not support |  |  |
| the Range and Content-Range headers MUST NOT cache 206 (Partial |  |  |
| Content) responses. |  |  |
| A response received with any other status code (e.g. status codes 302 |  |  |
| and 307) MUST NOT be returned in a reply to a subsequent request |  |  |
| unless there are cache-control directives or another header(s) that |  |  |
| explicitly allow it. For example, these include the following: an |  |  |
| Expires header (Section 14.21); a "max-age", "s-maxage", "must- |  |  |
| revalidate", "proxy-revalidate", "public" or "private" cache-control |  |  |
| directive (Section 14.9). |  |  |
| 13.5. Constructing Responses From Caches |  |  |
| The purpose of an HTTP cache is to store information received in |  |  |
| response to requests for use in responding to future requests. In |  |  |
| many cases, a cache simply returns the appropriate parts of a |  |  |
| response to the requester. However, if the cache holds a cache entry |  |  |
| based on a previous response, it might have to combine parts of a new |  |  |
| response with what is held in the cache entry. |  |  |
| 13.5.1. End-to-end and Hop-by-hop Headers |  |  |
| For the purpose of defining the behavior of caches and non-caching |  |  |
| proxies, we divide HTTP headers into two categories: |  |  |
| o End-to-end headers, which are transmitted to the ultimate |  |  |
| recipient of a request or response. End-to-end headers in |  |  |
| responses MUST be stored as part of a cache entry and MUST be |  |  |
| transmitted in any response formed from a cache entry. |  |  |
| o Hop-by-hop headers, which are meaningful only for a single |  |  |
| transport-level connection, and are not stored by caches or |  |  |
| forwarded by proxies. |  |  |
| The following HTTP/1.1 headers are hop-by-hop headers: |  |  |
| o Connection |  |  |
| o Keep-Alive |  |  |
| o Proxy-Authenticate |  |  |
| o Proxy-Authorization |  |  |
| o TE |  |  |
| o Trailers |  |  |
| o Transfer-Encoding |  |  |
| o Upgrade |  |  |
| All other headers defined by HTTP/1.1 are end-to-end headers. |  |  |
| Other hop-by-hop headers MUST be listed in a Connection header, |  |  |
| (Section 14.10) to be introduced into HTTP/1.1 (or later). |  |  |
| 13.5.2. Non-modifiable Headers |  |  |
| Some features of the HTTP/1.1 protocol, such as Digest |  |  |
| Authentication, depend on the value of certain end-to-end headers. A |  |  |
| transparent proxy SHOULD NOT modify an end-to-end header unless the |  |  |
| definition of that header requires or specifically allows that. |  |  |
| A transparent proxy MUST NOT modify any of the following fields in a |  |  |
| request or response, and it MUST NOT add any of these fields if not |  |  |
| already present: |  |  |
| o Content-Location |  |  |
| o Content-MD5 |  |  |
| o ETag |  |  |
| o Last-Modified |  |  |
| A transparent proxy MUST NOT modify any of the following fields in a |  |  |
| response: |  |  |
| o Expires |  |  |
| but it MAY add any of these fields if not already present. If an |  |  |
| Expires header is added, it MUST be given a field-value identical to |  |  |
| that of the Date header in that response. |  |  |
| A proxy MUST NOT modify or add any of the following fields in a |  |  |
| message that contains the no-transform cache-control directive, or in |  |  |
| any request: |  |  |
| o Content-Encoding |  |  |
| o Content-Range |  |  |
| o Content-Type |  |  |
| A non-transparent proxy MAY modify or add these fields to a message |  |  |
| that does not include no-transform, but if it does so, it MUST add a |  |  |
| Warning 214 (Transformation applied) if one does not already appear |  |  |
| in the message (see Section 14.46). |  |  |
| Warning: unnecessary modification of end-to-end headers might |  |  |
| cause authentication failures if stronger authentication |  |  |
| mechanisms are introduced in later versions of HTTP. Such |  |  |
| authentication mechanisms MAY rely on the values of header fields |  |  |
| not listed here. |  |  |
| The Content-Length field of a request or response is added or deleted |  |  |
| according to the rules in Section 4.4. A transparent proxy MUST |  |  |
| preserve the entity-length (Section 7.2.2) of the entity-body, |  |  |
| although it MAY change the transfer-length (Section 4.4). |  |  |
| 13.5.3. Combining Headers |  |  |
| When a cache makes a validating request to a server, and the server |  |  |
| provides a 304 (Not Modified) response or a 206 (Partial Content) |  |  |
| response, the cache then constructs a response to send to the |  |  |
| requesting client. |  |  |
| If the status code is 304 (Not Modified), the cache uses the entity- |  |  |
| body stored in the cache entry as the entity-body of this outgoing |  |  |
| response. If the status code is 206 (Partial Content) and the ETag |  |  |
| or Last-Modified headers match exactly, the cache MAY combine the |  |  |
| contents stored in the cache entry with the new contents received in |  |  |
| the response and use the result as the entity-body of this outgoing |  |  |
| response, (see 13.5.4). |  |  |
| The end-to-end headers stored in the cache entry are used for the |  |  |
| constructed response, except that |  |  |
| o any stored Warning headers with warn-code 1xx (see Section 14.46) |  |  |
| MUST be deleted from the cache entry and the forwarded response. |  |  |
| o any stored Warning headers with warn-code 2xx MUST be retained in |  |  |
| the cache entry and the forwarded response. |  |  |
| o any end-to-end headers provided in the 304 or 206 response MUST |  |  |
| replace the corresponding headers from the cache entry. |  |  |
| Unless the cache decides to remove the cache entry, it MUST also |  |  |
| replace the end-to-end headers stored with the cache entry with |  |  |
| corresponding headers received in the incoming response, except for |  |  |
| Warning headers as described immediately above. If a header field- |  |  |
| name in the incoming response matches more than one header in the |  |  |
| cache entry, all such old headers MUST be replaced. |  |  |
| In other words, the set of end-to-end headers received in the |  |  |
| incoming response overrides all corresponding end-to-end headers |  |  |
| stored with the cache entry (except for stored Warning headers with |  |  |
| warn-code 1xx, which are deleted even if not overridden). |  |  |
| Note: this rule allows an origin server to use a 304 (Not |  |  |
| Modified) or a 206 (Partial Content) response to update any header |  |  |
| associated with a previous response for the same entity or sub- |  |  |
| ranges thereof, although it might not always be meaningful or |  |  |
| correct to do so. This rule does not allow an origin server to |  |  |
| use a 304 (Not Modified) or a 206 (Partial Content) response to |  |  |
| entirely delete a header that it had provided with a previous |  |  |
| response. |  |  |
| 13.5.4. Combining Byte Ranges |  |  |
| A response might transfer only a subrange of the bytes of an entity- |  |  |
| body, either because the request included one or more Range |  |  |
| specifications, or because a connection was broken prematurely. |  |  |
| After several such transfers, a cache might have received several |  |  |
| ranges of the same entity-body. |  |  |
| If a cache has a stored non-empty set of subranges for an entity, and |  |  |
| an incoming response transfers another subrange, the cache MAY |  |  |
| combine the new subrange with the existing set if both the following |  |  |
| conditions are met: |  |  |
| o Both the incoming response and the cache entry have a cache |  |  |
| validator. |  |  |
| o The two cache validators match using the strong comparison |  |  |
| function (see Section 13.3.3). |  |  |
| If either requirement is not met, the cache MUST use only the most |  |  |
| recent partial response (based on the Date values transmitted with |  |  |
| every response, and using the incoming response if these values are |  |  |
| equal or missing), and MUST discard the other partial information. |  |  |
| 13.6. Caching Negotiated Responses |  |  |
| Use of server-driven content negotiation (Section 12.1), as indicated |  |  |
| by the presence of a Vary header field in a response, alters the |  |  |
| conditions and procedure by which a cache can use the response for |  |  |
| subsequent requests. See Section 14.44 for use of the Vary header |  |  |
| field by servers. |  |  |
| A server SHOULD use the Vary header field to inform a cache of what |  |  |
| request-header fields were used to select among multiple |  |  |
| representations of a cacheable response subject to server-driven |  |  |
| negotiation. The set of header fields named by the Vary field value |  |  |
| is known as the "selecting" request-headers. |  |  |
| When the cache receives a subsequent request whose Request-URI |  |  |
| specifies one or more cache entries including a Vary header field, |  |  |
| the cache MUST NOT use such a cache entry to construct a response to |  |  |
| the new request unless all of the selecting request-headers present |  |  |
| in the new request match the corresponding stored request-headers in |  |  |
| the original request. |  |  |
| The selecting request-headers from two requests are defined to match |  |  |
| if and only if the selecting request-headers in the first request can |  |  |
| be transformed to the selecting request-headers in the second request |  |  |
| by adding or removing linear white space (LWS) at places where this |  |  |
| is allowed by the corresponding BNF, and/or combining multiple |  |  |
| message-header fields with the same field name following the rules |  |  |
| about message headers in Section 4.2. |  |  |
| A Vary header field-value of "*" always fails to match and subsequent |  |  |
| requests on that resource can only be properly interpreted by the |  |  |
| origin server. |  |  |
| If the selecting request header fields for the cached entry do not |  |  |
| match the selecting request header fields of the new request, then |  |  |
| the cache MUST NOT use a cached entry to satisfy the request unless |  |  |
| it first relays the new request to the origin server in a conditional |  |  |
| request and the server responds with 304 (Not Modified), including an |  |  |
| entity tag or Content-Location that indicates the entity to be used. |  |  |
| If an entity tag was assigned to a cached representation, the |  |  |
| forwarded request SHOULD be conditional and include the entity tags |  |  |
| in an If-None-Match header field from all its cache entries for the |  |  |
| resource. This conveys to the server the set of entities currently |  |  |
| held by the cache, so that if any one of these entities matches the |  |  |
| requested entity, the server can use the ETag header field in its 304 |  |  |
| (Not Modified) response to tell the cache which entry is appropriate. |  |  |
| If the entity-tag of the new response matches that of an existing |  |  |
| entry, the new response SHOULD be used to update the header fields of |  |  |
| the existing entry, and the result MUST be returned to the client. |  |  |
| If any of the existing cache entries contains only partial content |  |  |
| for the associated entity, its entity-tag SHOULD NOT be included in |  |  |
| the If-None-Match header field unless the request is for a range that |  |  |
| would be fully satisfied by that entry. |  |  |
| If a cache receives a successful response whose Content-Location |  |  |
| field matches that of an existing cache entry for the same Request- |  |  |
| URI, whose entity-tag differs from that of the existing entry, and |  |  |
| whose Date is more recent than that of the existing entry, the |  |  |
| existing entry SHOULD NOT be returned in response to future requests |  |  |
| and SHOULD be deleted from the cache. |  |  |
| 13.7. Shared and Non-Shared Caches |  |  |
| For reasons of security and privacy, it is necessary to make a |  |  |
| distinction between "shared" and "non-shared" caches. A non-shared |  |  |
| cache is one that is accessible only to a single user. Accessibility |  |  |
| in this case SHOULD be enforced by appropriate security mechanisms. |  |  |
| All other caches are considered to be "shared." Other sections of |  |  |
| this specification place certain constraints on the operation of |  |  |
| shared caches in order to prevent loss of privacy or failure of |  |  |
| access controls. |  |  |
| 13.8. Errors or Incomplete Response Cache Behavior |  |  |
| A cache that receives an incomplete response (for example, with fewer |  |  |
| bytes of data than specified in a Content-Length header) MAY store |  |  |
| the response. However, the cache MUST treat this as a partial |  |  |
| response. Partial responses MAY be combined as described in |  |  |
| Section 13.5.4; the result might be a full response or might still be |  |  |
| partial. A cache MUST NOT return a partial response to a client |  |  |
| without explicitly marking it as such, using the 206 (Partial |  |  |
| Content) status code. A cache MUST NOT return a partial response |  |  |
| using a status code of 200 (OK). |  |  |
| If a cache receives a 5xx response while attempting to revalidate an |  |  |
| entry, it MAY either forward this response to the requesting client, |  |  |
| or act as if the server failed to respond. In the latter case, it |  |  |
| MAY return a previously received response unless the cached entry |  |  |
| includes the "must-revalidate" cache-control directive (see |  |  |
| Section 14.9). |  |  |
| 13.9. Side Effects of GET and HEAD |  |  |
| Unless the origin server explicitly prohibits the caching of their |  |  |
| responses, the application of GET and HEAD methods to any resources |  |  |
| SHOULD NOT have side effects that would lead to erroneous behavior if |  |  |
| these responses are taken from a cache. They MAY still have side |  |  |
| effects, but a cache is not required to consider such side effects in |  |  |
| its caching decisions. Caches are always expected to observe an |  |  |
| origin server's explicit restrictions on caching. |  |  |
| We note one exception to this rule: since some applications have |  |  |
| traditionally used GETs and HEADs with query URLs (those containing a |  |  |
| "?" in the rel_path part) to perform operations with significant side |  |  |
| effects, caches MUST NOT treat responses to such URIs as fresh unless |  |  |
| the server provides an explicit expiration time. This specifically |  |  |
| means that responses from HTTP/1.0 servers for such URIs SHOULD NOT |  |  |
| be taken from a cache. See Section 9.1.1 for related information. |  |  |
| 13.10. Invalidation After Updates or Deletions |  |  |
| The effect of certain methods performed on a resource at the origin |  |  |
| server might cause one or more existing cache entries to become non- |  |  |
| transparently invalid. That is, although they might continue to be |  |  |
| "fresh," they do not accurately reflect what the origin server would |  |  |
| return for a new request on that resource. |  |  |
| There is no way for the HTTP protocol to guarantee that all such |  |  |
| cache entries are marked invalid. For example, the request that |  |  |
| caused the change at the origin server might not have gone through |  |  |
| the proxy where a cache entry is stored. However, several rules help |  |  |
| reduce the likelihood of erroneous behavior. |  |  |
| In this section, the phrase "invalidate an entity" means that the |  |  |
| cache will either remove all instances of that entity from its |  |  |
| storage, or will mark these as "invalid" and in need of a mandatory |  |  |
| revalidation before they can be returned in response to a subsequent |  |  |
| request. |  |  |
| Some HTTP methods MUST cause a cache to invalidate an entity. This |  |  |
| is either the entity referred to by the Request-URI, or by the |  |  |
| Location or Content-Location headers (if present). These methods |  |  |
| are: |  |  |
| o PUT |  |  |
| o DELETE |  |  |
| o POST |  |  |
| In order to prevent denial of service attacks, an invalidation based |  |  |
| on the URI in a Location or Content-Location header MUST only be |  |  |
| performed if the host part is the same as in the Request-URI. |  |  |
| A cache that passes through requests for methods it does not |  |  |
| understand SHOULD invalidate any entities referred to by the Request- |  |  |
| URI. |  |  |
| 13.11. Write-Through Mandatory |  |  |
| All methods that might be expected to cause modifications to the |  |  |
| origin server's resources MUST be written through to the origin |  |  |
| server. This currently includes all methods except for GET and HEAD. |  |  |
| A cache MUST NOT reply to such a request from a client before having |  |  |
| transmitted the request to the inbound server, and having received a |  |  |
| corresponding response from the inbound server. This does not |  |  |
| prevent a proxy cache from sending a 100 (Continue) response before |  |  |
| the inbound server has sent its final reply. |  |  |
| The alternative (known as "write-back" or "copy-back" caching) is not |  |  |
| allowed in HTTP/1.1, due to the difficulty of providing consistent |  |  |
| updates and the problems arising from server, cache, or network |  |  |
| failure prior to write-back. |  |  |
| 13.12. Cache Replacement |  |  |
| If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8) |  |  |
| response is received from a resource while any existing responses for |  |  |
| the same resource are cached, the cache SHOULD use the new response |  |  |
| to reply to the current request. It MAY insert it into cache storage |  |  |
| and MAY, if it meets all other requirements, use it to respond to any |  |  |
| future requests that would previously have caused the old response to |  |  |
| be returned. If it inserts the new response into cache storage the |  |  |
| rules in Section 13.5.3 apply. |  |  |
| Note: a new response that has an older Date header value than |  |  |
| existing cached responses is not cacheable. |  |  |
| 13.13. History Lists |  |  |
| User agents often have history mechanisms, such as "Back" buttons and |  |  |
| history lists, which can be used to redisplay an entity retrieved |  |  |
| earlier in a session. |  |  |
| History mechanisms and caches are different. In particular history |  |  |
| mechanisms SHOULD NOT try to show a semantically transparent view of |  |  |
| the current state of a resource. Rather, a history mechanism is |  |  |
| meant to show exactly what the user saw at the time when the resource |  |  |
| was retrieved. |  |  |
| By default, an expiration time does not apply to history mechanisms. |  |  |
| If the entity is still in storage, a history mechanism SHOULD display |  |  |
| it even if the entity has expired, unless the user has specifically |  |  |
| configured the agent to refresh expired history documents. |  |  |
| This is not to be construed to prohibit the history mechanism from |  |  |
| telling the user that a view might be stale. |  |  |
| Note: if history list mechanisms unnecessarily prevent users from |  |  |
| viewing stale resources, this will tend to force service authors |  |  |
| to avoid using HTTP expiration controls and cache controls when |  |  |
| they would otherwise like to. Service authors may consider it |  |  |
| important that users not be presented with error messages or |  |  |
| warning messages when they use navigation controls (such as BACK) |  |  |
| to view previously fetched resources. Even though sometimes such |  |  |
| resources ought not to cached, or ought to expire quickly, user |  |  |
| interface considerations may force service authors to resort to |  |  |
| other means of preventing caching (e.g. "once-only" URLs) in order |  |  |
| not to suffer the effects of improperly functioning history |  |  |
| mechanisms. |  |  |
| 14. Header Field Definitions |  |  |
| This section defines the syntax and semantics of all standard |  | This section defines the syntax and semantics of all standard |
| HTTP/1.1 header fields. For entity-header fields, both sender and |  | HTTP/1.1 header fields. For entity-header fields, both sender and |
| recipient refer to either the client or the server, depending on who |  | recipient refer to either the client or the server, depending on who |
| sends and who receives the entity. |  | sends and who receives the entity. |
| 14.1. Accept |  | 6.1. ETag |
| The Accept request-header field can be used to specify certain media |  |  |
| types which are acceptable for the response. Accept headers can be |  |  |
| used to indicate that the request is specifically limited to a small |  |  |
| set of desired types, as in the case of a request for an in-line |  |  |
| image. |  |  |
| Accept = "Accept" ":" |  |  |
| #( media-range [ accept-params ] ) |  |  |
| media-range = ( "*/*" |  |  |
| | ( type "/" "*" ) |  |  |
| | ( type "/" subtype ) |  |  |
| ) *( ";" parameter ) |  |  |
| accept-params = ";" "q" "=" qvalue *( accept-extension ) |  |  |
| accept-extension = ";" token [ "=" ( token | quoted-string ) ] |  |  |
| The asterisk "*" character is used to group media types into ranges, |  |  |
| with "*/*" indicating all media types and "type/*" indicating all |  |  |
| subtypes of that type. The media-range MAY include media type |  |  |
| parameters that are applicable to that range. |  |  |
| Each media-range MAY be followed by one or more accept-params, |  |  |
| beginning with the "q" parameter for indicating a relative quality |  |  |
| factor. The first "q" parameter (if any) separates the media-range |  |  |
| parameter(s) from the accept-params. Quality factors allow the user |  |  |
| or user agent to indicate the relative degree of preference for that |  |  |
| media-range, using the qvalue scale from 0 to 1 (Section 3.9). The |  |  |
| default value is q=1. |  |  |
| Note: Use of the "q" parameter name to separate media type |  |  |
| parameters from Accept extension parameters is due to historical |  |  |
| practice. Although this prevents any media type parameter named |  |  |
| "q" from being used with a media range, such an event is believed |  |  |
| to be unlikely given the lack of any "q" parameters in the IANA |  |  |
| media type registry and the rare usage of any media type |  |  |
| parameters in Accept. Future media types are discouraged from |  |  |
| registering any parameter named "q". |  |  |
| The example |  |  |
| Accept: audio/*; q=0.2, audio/basic |  |  |
| SHOULD be interpreted as "I prefer audio/basic, but send me any audio |  |  |
| type if it is the best available after an 80% mark-down in quality." |  |  |
| If no Accept header field is present, then it is assumed that the |  |  |
| client accepts all media types. If an Accept header field is |  |  |
| present, and if the server cannot send a response which is acceptable |  |  |
| according to the combined Accept field value, then the server SHOULD |  |  |
| send a 406 (not acceptable) response. |  |  |
| A more elaborate example is |  |  |
| Accept: text/plain; q=0.5, text/html, |  |  |
| text/x-dvi; q=0.8, text/x-c |  |  |
| Verbally, this would be interpreted as "text/html and text/x-c are |  |  |
| the preferred media types, but if they do not exist, then send the |  |  |
| text/x-dvi entity, and if that does not exist, send the text/plain |  |  |
| entity." |  |  |
| Media ranges can be overridden by more specific media ranges or |  |  |
| specific media types. If more than one media range applies to a |  |  |
| given type, the most specific reference has precedence. For example, |  |  |
| Accept: text/*, text/html, text/html;level=1, */* |  |  |
| have the following precedence: |  |  |
| 1) text/html;level=1 |  |  |
| 2) text/html |  |  |
| 3) text/* |  |  |
| 4) */* |  |  |
| The media type quality factor associated with a given type is |  |  |
| determined by finding the media range with the highest precedence |  |  |
| which matches that type. For example, |  |  |
| Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1, |  |  |
| text/html;level=2;q=0.4, */*;q=0.5 |  |  |
| would cause the following values to be associated: |  |  |
| text/html;level=1 = 1 |  |  |
| text/html = 0.7 |  |  |
| text/plain = 0.3 |  |  |
| image/jpeg = 0.5 |  |  |
| text/html;level=2 = 0.4 |  |  |
| text/html;level=3 = 0.7 |  |  |
| Note: A user agent might be provided with a default set of quality |  |  |
| values for certain media ranges. However, unless the user agent is a |  |  |
| closed system which cannot interact with other rendering agents, this |  |  |
| default set ought to be configurable by the user. |  |  |
| 14.2. Accept-Charset |  |  |
| The Accept-Charset request-header field can be used to indicate what |  |  |
| character sets are acceptable for the response. This field allows |  |  |
| clients capable of understanding more comprehensive or special- |  |  |
| purpose character sets to signal that capability to a server which is |  |  |
| capable of representing documents in those character sets. |  |  |
| Accept-Charset = "Accept-Charset" ":" |  |  |
| 1#( ( charset | "*" )[ ";" "q" "=" qvalue ] ) |  |  |
| Character set values are described in Section 3.4. Each charset MAY |  |  |
| be given an associated quality value which represents the user's |  |  |
| preference for that charset. The default value is q=1. An example |  |  |
| is |  |  |
| Accept-Charset: iso-8859-5, unicode-1-1;q=0.8 |  |  |
| The special value "*", if present in the Accept-Charset field, |  |  |
| matches every character set (including ISO-8859-1) which is not |  |  |
| mentioned elsewhere in the Accept-Charset field. If no "*" is |  |  |
| present in an Accept-Charset field, then all character sets not |  |  |
| explicitly mentioned get a quality value of 0, except for ISO-8859-1, |  |  |
| which gets a quality value of 1 if not explicitly mentioned. |  |  |
| If no Accept-Charset header is present, the default is that any |  |  |
| character set is acceptable. If an Accept-Charset header is present, |  |  |
| and if the server cannot send a response which is acceptable |  |  |
| according to the Accept-Charset header, then the server SHOULD send |  |  |
| an error response with the 406 (not acceptable) status code, though |  |  |
| the sending of an unacceptable response is also allowed. |  |  |
| 14.3. Accept-Encoding |  |  |
| The Accept-Encoding request-header field is similar to Accept, but |  |  |
| restricts the content-codings (Section 3.5) that are acceptable in |  |  |
| the response. |  |  |
| Accept-Encoding = "Accept-Encoding" ":" |  |  |
| 1#( codings [ ";" "q" "=" qvalue ] ) |  |  |
| codings = ( content-coding | "*" ) |  |  |
| Examples of its use are: |  |  |
| Accept-Encoding: compress, gzip |  |  |
| Accept-Encoding: |  |  |
| Accept-Encoding: * |  |  |
| Accept-Encoding: compress;q=0.5, gzip;q=1.0 |  |  |
| Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0 |  |  |
| A server tests whether a content-coding is acceptable, according to |  |  |
| an Accept-Encoding field, using these rules: |  |  |
| 1. If the content-coding is one of the content-codings listed in the |  |  |
| Accept-Encoding field, then it is acceptable, unless it is |  |  |
| accompanied by a qvalue of 0. (As defined in Section 3.9, a |  |  |
| qvalue of 0 means "not acceptable.") |  |  |
| 2. The special "*" symbol in an Accept-Encoding field matches any |  |  |
| available content-coding not explicitly listed in the header |  |  |
| field. |  |  |
| 3. If multiple content-codings are acceptable, then the acceptable |  |  |
| content-coding with the highest non-zero qvalue is preferred. |  |  |
| 4. The "identity" content-coding is always acceptable, unless |  |  |
| specifically refused because the Accept-Encoding field includes |  |  |
| "identity;q=0", or because the field includes "*;q=0" and does |  |  |
| not explicitly include the "identity" content-coding. If the |  |  |
| Accept-Encoding field-value is empty, then only the "identity" |  |  |
| encoding is acceptable. |  |  |
| If an Accept-Encoding field is present in a request, and if the |  |  |
| server cannot send a response which is acceptable according to the |  |  |
| Accept-Encoding header, then the server SHOULD send an error response |  |  |
| with the 406 (Not Acceptable) status code. |  |  |
| If no Accept-Encoding field is present in a request, the server MAY |  |  |
| assume that the client will accept any content coding. In this case, |  |  |
| if "identity" is one of the available content-codings, then the |  |  |
| server SHOULD use the "identity" content-coding, unless it has |  |  |
| additional information that a different content-coding is meaningful |  |  |
| to the client. |  |  |
| Note: If the request does not include an Accept-Encoding field, |  |  |
| and if the "identity" content-coding is unavailable, then content- |  |  |
| codings commonly understood by HTTP/1.0 clients (i.e., "gzip" and |  |  |
| "compress") are preferred; some older clients improperly display |  |  |
| messages sent with other content-codings. The server might also |  |  |
| make this decision based on information about the particular user- |  |  |
| agent or client. |  |  |
| Note: Most HTTP/1.0 applications do not recognize or obey qvalues |  |  |
| associated with content-codings. This means that qvalues will not |  |  |
| work and are not permitted with x-gzip or x-compress. |  |  |
| 14.4. Accept-Language |  |  |
| The Accept-Language request-header field is similar to Accept, but |  |  |
| restricts the set of natural languages that are preferred as a |  |  |
| response to the request. Language tags are defined in Section 3.10. |  |  |
| Accept-Language = "Accept-Language" ":" |  |  |
| 1#( language-range [ ";" "q" "=" qvalue ] ) |  |  |
| language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" ) |  |  |
| Each language-range MAY be given an associated quality value which |  |  |
| represents an estimate of the user's preference for the languages |  |  |
| specified by that range. The quality value defaults to "q=1". For |  |  |
| example, |  |  |
| Accept-Language: da, en-gb;q=0.8, en;q=0.7 |  |  |
| would mean: "I prefer Danish, but will accept British English and |  |  |
| other types of English." A language-range matches a language-tag if |  |  |
| it exactly equals the tag, or if it exactly equals a prefix of the |  |  |
| tag such that the first tag character following the prefix is "-". |  |  |
| The special range "*", if present in the Accept-Language field, |  |  |
| matches every tag not matched by any other range present in the |  |  |
| Accept-Language field. |  |  |
| Note: This use of a prefix matching rule does not imply that |  |  |
| language tags are assigned to languages in such a way that it is |  |  |
| always true that if a user understands a language with a certain |  |  |
| tag, then this user will also understand all languages with tags |  |  |
| for which this tag is a prefix. The prefix rule simply allows the |  |  |
| use of prefix tags if this is the case. |  |  |
| The language quality factor assigned to a language-tag by the Accept- |  |  |
| Language field is the quality value of the longest language-range in |  |  |
| the field that matches the language-tag. If no language-range in the |  |  |
| field matches the tag, the language quality factor assigned is 0. If |  |  |
| no Accept-Language header is present in the request, the server |  |  |
| SHOULD assume that all languages are equally acceptable. If an |  |  |
| Accept-Language header is present, then all languages which are |  |  |
| assigned a quality factor greater than 0 are acceptable. |  |  |
| It might be contrary to the privacy expectations of the user to send |  |  |
| an Accept-Language header with the complete linguistic preferences of |  |  |
| the user in every request. For a discussion of this issue, see |  |  |
| Section 15.1.4. |  |  |
| As intelligibility is highly dependent on the individual user, it is |  |  |
| recommended that client applications make the choice of linguistic |  |  |
| preference available to the user. If the choice is not made |  |  |
| available, then the Accept-Language header field MUST NOT be given in |  |  |
| the request. |  |  |
| Note: When making the choice of linguistic preference available to |  |  |
| the user, we remind implementors of the fact that users are not |  |  |
| familiar with the details of language matching as described above, |  |  |
| and should provide appropriate guidance. As an example, users |  |  |
| might assume that on selecting "en-gb", they will be served any |  |  |
| kind of English document if British English is not available. A |  |  |
| user agent might suggest in such a case to add "en" to get the |  |  |
| best matching behavior. |  |  |
| 14.5. Accept-Ranges |  |  |
| The Accept-Ranges response-header field allows the server to indicate |  |  |
| its acceptance of range requests for a resource: |  |  |
| Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges |  |  |
| acceptable-ranges = 1#range-unit | "none" |  |  |
| Origin servers that accept byte-range requests MAY send |  |  |
| Accept-Ranges: bytes |  |  |
| but are not required to do so. Clients MAY generate byte-range |  |  |
| requests without having received this header for the resource |  |  |
| involved. Range units are defined in Section 3.12. |  |  |
| Servers that do not accept any kind of range request for a resource |  |  |
| MAY send |  |  |
| Accept-Ranges: none |  |  |
| to advise the client not to attempt a range request. |  |  |
| 14.6. Age |  |  |
| The Age response-header field conveys the sender's estimate of the |  |  |
| amount of time since the response (or its revalidation) was generated |  |  |
| at the origin server. A cached response is "fresh" if its age does |  |  |
| not exceed its freshness lifetime. Age values are calculated as |  |  |
| specified in Section 13.2.3. |  |  |
| Age = "Age" ":" age-value |  |  |
| age-value = delta-seconds |  |  |
| Age values are non-negative decimal integers, representing time in |  |  |
| seconds. |  |  |
| If a cache receives a value larger than the largest positive integer |  |  |
| it can represent, or if any of its age calculations overflows, it |  |  |
| MUST transmit an Age header with a value of 2147483648 (2^31). An |  |  |
| HTTP/1.1 server that includes a cache MUST include an Age header |  |  |
| field in every response generated from its own cache. Caches SHOULD |  |  |
| use an arithmetic type of at least 31 bits of range. |  |  |
| 14.7. Allow |  |  |
| The Allow entity-header field lists the set of methods supported by |  |  |
| the resource identified by the Request-URI. The purpose of this |  |  |
| field is strictly to inform the recipient of valid methods associated |  |  |
| with the resource. An Allow header field MUST be present in a 405 |  |  |
| (Method Not Allowed) response. |  |  |
| Allow = "Allow" ":" #Method |  |  |
| Example of use: |  |  |
| Allow: GET, HEAD, PUT |  |  |
| This field cannot prevent a client from trying other methods. |  |  |
| However, the indications given by the Allow header field value SHOULD |  |  |
| be followed. The actual set of allowed methods is defined by the |  |  |
| origin server at the time of each request. |  |  |
| The Allow header field MAY be provided with a PUT request to |  |  |
| recommend the methods to be supported by the new or modified |  |  |
| resource. The server is not required to support these methods and |  |  |
| SHOULD include an Allow header in the response giving the actual |  |  |
| supported methods. |  |  |
| A proxy MUST NOT modify the Allow header field even if it does not |  |  |
| understand all the methods specified, since the user agent might have |  |  |
| other means of communicating with the origin server. |  |  |
| 14.8. Authorization |  |  |
| A user agent that wishes to authenticate itself with a server-- |  |  |
| usually, but not necessarily, after receiving a 401 response--does so |  |  |
| by including an Authorization request-header field with the request. |  |  |
| The Authorization field value consists of credentials containing the |  |  |
| authentication information of the user agent for the realm of the |  |  |
| resource being requested. |  |  |
| Authorization = "Authorization" ":" credentials |  |  |
| HTTP access authentication is described in "HTTP Authentication: |  |  |
| Basic and Digest Access Authentication" [RFC2617]. If a request is |  |  |
| authenticated and a realm specified, the same credentials SHOULD be |  |  |
| valid for all other requests within this realm (assuming that the |  |  |
| authentication scheme itself does not require otherwise, such as |  |  |
| credentials that vary according to a challenge value or using |  |  |
| synchronized clocks). |  |  |
| When a shared cache (see Section 13.7) receives a request containing |  |  |
| an Authorization field, it MUST NOT return the corresponding response |  |  |
| as a reply to any other request, unless one of the following specific |  |  |
| exceptions holds: |  |  |
| 1. If the response includes the "s-maxage" cache-control directive, |  |  |
| the cache MAY use that response in replying to a subsequent |  |  |
| request. But (if the specified maximum age has passed) a proxy |  |  |
| cache MUST first revalidate it with the origin server, using the |  |  |
| request-headers from the new request to allow the origin server |  |  |
| to authenticate the new request. (This is the defined behavior |  |  |
| for s-maxage.) If the response includes "s-maxage=0", the proxy |  |  |
| MUST always revalidate it before re-using it. |  |  |
| 2. If the response includes the "must-revalidate" cache-control |  |  |
| directive, the cache MAY use that response in replying to a |  |  |
| subsequent request. But if the response is stale, all caches |  |  |
| MUST first revalidate it with the origin server, using the |  |  |
| request-headers from the new request to allow the origin server |  |  |
| to authenticate the new request. |  |  |
| 3. If the response includes the "public" cache-control directive, it |  |  |
| MAY be returned in reply to any subsequent request. |  |  |
| 14.9. Cache-Control |  |  |
| The Cache-Control general-header field is used to specify directives |  |  |
| that MUST be obeyed by all caching mechanisms along the request/ |  |  |
| response chain. The directives specify behavior intended to prevent |  |  |
| caches from adversely interfering with the request or response. |  |  |
| These directives typically override the default caching algorithms. |  |  |
| Cache directives are unidirectional in that the presence of a |  |  |
| directive in a request does not imply that the same directive is to |  |  |
| be given in the response. |  |  |
| Note that HTTP/1.0 caches might not implement Cache-Control and |  |  |
| might only implement Pragma: no-cache (see Section 14.32). |  |  |
| Cache directives MUST be passed through by a proxy or gateway |  |  |
| application, regardless of their significance to that application, |  |  |
| since the directives might be applicable to all recipients along the |  |  |
| request/response chain. It is not possible to specify a cache- |  |  |
| directive for a specific cache. |  |  |
| Cache-Control = "Cache-Control" ":" 1#cache-directive |  |  |
| cache-directive = cache-request-directive |  |  |
| | cache-response-directive |  |  |
| cache-request-directive = |  |  |
| "no-cache" ; Section 14.9.1 |  |  |
| | "no-store" ; Section 14.9.2 |  |  |
| | "max-age" "=" delta-seconds ; Section 14.9.3, 14.9.4 |  |  |
| | "max-stale" [ "=" delta-seconds ] ; Section 14.9.3 |  |  |
| | "min-fresh" "=" delta-seconds ; Section 14.9.3 |  |  |
| | "no-transform" ; Section 14.9.5 |  |  |
| | "only-if-cached" ; Section 14.9.4 |  |  |
| | cache-extension ; Section 14.9.6 |  |  |
| cache-response-directive = |  |  |
| "public" ; Section 14.9.1 |  |  |
| | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1 |  |  |
| | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1 |  |  |
| | "no-store" ; Section 14.9.2 |  |  |
| | "no-transform" ; Section 14.9.5 |  |  |
| | "must-revalidate" ; Section 14.9.4 |  |  |
| | "proxy-revalidate" ; Section 14.9.4 |  |  |
| | "max-age" "=" delta-seconds ; Section 14.9.3 |  |  |
| | "s-maxage" "=" delta-seconds ; Section 14.9.3 |  |  |
| | cache-extension ; Section 14.9.6 |  |  |
| cache-extension = token [ "=" ( token | quoted-string ) ] |  |  |
| When a directive appears without any 1#field-name parameter, the |  |  |
| directive applies to the entire request or response. When such a |  |  |
| directive appears with a 1#field-name parameter, it applies only to |  |  |
| the named field or fields, and not to the rest of the request or |  |  |
| response. This mechanism supports extensibility; implementations of |  |  |
| future versions of the HTTP protocol might apply these directives to |  |  |
| header fields not defined in HTTP/1.1. |  |  |
| The cache-control directives can be broken down into these general |  |  |
| categories: |  |  |
| o Restrictions on what are cacheable; these may only be imposed by |  |  |
| the origin server. |  |  |
| o Restrictions on what may be stored by a cache; these may be |  |  |
| imposed by either the origin server or the user agent. |  |  |
| o Modifications of the basic expiration mechanism; these may be |  |  |
| imposed by either the origin server or the user agent. |  |  |
| o Controls over cache revalidation and reload; these may only be |  |  |
| imposed by a user agent. |  |  |
| o Control over transformation of entities. |  |  |
| o Extensions to the caching system. |  |  |
| 14.9.1. What is Cacheable |  |  |
| By default, a response is cacheable if the requirements of the |  |  |
| request method, request header fields, and the response status |  |  |
| indicate that it is cacheable. Section 13.4 summarizes these |  |  |
| defaults for cacheability. The following Cache-Control response |  |  |
| directives allow an origin server to override the default |  |  |
| cacheability of a response: |  |  |
| public |  |  |
| Indicates that the response MAY be cached by any cache, even if it |  |  |
| would normally be non-cacheable or cacheable only within a non- |  |  |
| shared cache. (See also Authorization, Section 14.8, for |  |  |
| additional details.) |  |  |
| private |  |  |
| Indicates that all or part of the response message is intended for |  |  |
| a single user and MUST NOT be cached by a shared cache. This |  |  |
| allows an origin server to state that the specified parts of the |  |  |
| response are intended for only one user and are not a valid |  |  |
| response for requests by other users. A private (non-shared) |  |  |
| cache MAY cache the response. |  |  |
| Note: This usage of the word private only controls where the |  |  |
| response may be cached, and cannot ensure the privacy of the |  |  |
| message content. |  |  |
| no-cache |  |  |
| If the no-cache directive does not specify a field-name, then a |  |  |
| cache MUST NOT use the response to satisfy a subsequent request |  |  |
| without successful revalidation with the origin server. This |  |  |
| allows an origin server to prevent caching even by caches that |  |  |
| have been configured to return stale responses to client requests. |  |  |
| If the no-cache directive does specify one or more field-names, |  |  |
| then a cache MAY use the response to satisfy a subsequent request, |  |  |
| subject to any other restrictions on caching. However, the |  |  |
| specified field-name(s) MUST NOT be sent in the response to a |  |  |
| subsequent request without successful revalidation with the origin |  |  |
| server. This allows an origin server to prevent the re-use of |  |  |
| certain header fields in a response, while still allowing caching |  |  |
| of the rest of the response. |  |  |
| Note: Most HTTP/1.0 caches will not recognize or obey this |  |  |
| directive. |  |  |
| 14.9.2. What May be Stored by Caches |  |  |
| no-store |  |  |
| The purpose of the no-store directive is to prevent the |  |  |
| inadvertent release or retention of sensitive information (for |  |  |
| example, on backup tapes). The no-store directive applies to the |  |  |
| entire message, and MAY be sent either in a response or in a |  |  |
| request. If sent in a request, a cache MUST NOT store any part of |  |  |
| either this request or any response to it. If sent in a response, |  |  |
| a cache MUST NOT store any part of either this response or the |  |  |
| request that elicited it. This directive applies to both non- |  |  |
| shared and shared caches. "MUST NOT store" in this context means |  |  |
| that the cache MUST NOT intentionally store the information in |  |  |
| non-volatile storage, and MUST make a best-effort attempt to |  |  |
| remove the information from volatile storage as promptly as |  |  |
| possible after forwarding it. |  |  |
| Even when this directive is associated with a response, users |  |  |
| might explicitly store such a response outside of the caching |  |  |
| system (e.g., with a "Save As" dialog). History buffers MAY store |  |  |
| such responses as part of their normal operation. |  |  |
| The purpose of this directive is to meet the stated requirements |  |  |
| of certain users and service authors who are concerned about |  |  |
| accidental releases of information via unanticipated accesses to |  |  |
| cache data structures. While the use of this directive might |  |  |
| improve privacy in some cases, we caution that it is NOT in any |  |  |
| way a reliable or sufficient mechanism for ensuring privacy. In |  |  |
| particular, malicious or compromised caches might not recognize or |  |  |
| obey this directive, and communications networks might be |  |  |
| vulnerable to eavesdropping. |  |  |
| 14.9.3. Modifications of the Basic Expiration Mechanism |  |  |
| The expiration time of an entity MAY be specified by the origin |  |  |
| server using the Expires header (see Section 14.21). Alternatively, |  |  |
| it MAY be specified using the max-age directive in a response. When |  |  |
| the max-age cache-control directive is present in a cached response, |  |  |
| the response is stale if its current age is greater than the age |  |  |
| value given (in seconds) at the time of a new request for that |  |  |
| resource. The max-age directive on a response implies that the |  |  |
| response is cacheable (i.e., "public") unless some other, more |  |  |
| restrictive cache directive is also present. |  |  |
| If a response includes both an Expires header and a max-age |  |  |
| directive, the max-age directive overrides the Expires header, even |  |  |
| if the Expires header is more restrictive. This rule allows an |  |  |
| origin server to provide, for a given response, a longer expiration |  |  |
| time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This |  |  |
| might be useful if certain HTTP/1.0 caches improperly calculate ages |  |  |
| or expiration times, perhaps due to desynchronized clocks. |  |  |
| Many HTTP/1.0 cache implementations will treat an Expires value that |  |  |
| is less than or equal to the response Date value as being equivalent |  |  |
| to the Cache-Control response directive "no-cache". If an HTTP/1.1 |  |  |
| cache receives such a response, and the response does not include a |  |  |
| Cache-Control header field, it SHOULD consider the response to be |  |  |
| non-cacheable in order to retain compatibility with HTTP/1.0 servers. |  |  |
| Note: An origin server might wish to use a relatively new HTTP |  |  |
| cache control feature, such as the "private" directive, on a |  |  |
| network including older caches that do not understand that |  |  |
| feature. The origin server will need to combine the new feature |  |  |
| with an Expires field whose value is less than or equal to the |  |  |
| Date value. This will prevent older caches from improperly |  |  |
| caching the response. |  |  |
| s-maxage |  |  |
| If a response includes an s-maxage directive, then for a shared |  |  |
| cache (but not for a private cache), the maximum age specified by |  |  |
| this directive overrides the maximum age specified by either the |  |  |
| max-age directive or the Expires header. The s-maxage directive |  |  |
| also implies the semantics of the proxy-revalidate directive (see |  |  |
| Section 14.9.4), i.e., that the shared cache must not use the |  |  |
| entry after it becomes stale to respond to a subsequent request |  |  |
| without first revalidating it with the origin server. The |  |  |
| s-maxage directive is always ignored by a private cache. |  |  |
| Note that most older caches, not compliant with this specification, |  |  |
| do not implement any cache-control directives. An origin server |  |  |
| wishing to use a cache-control directive that restricts, but does not |  |  |
| prevent, caching by an HTTP/1.1-compliant cache MAY exploit the |  |  |
| requirement that the max-age directive overrides the Expires header, |  |  |
| and the fact that pre-HTTP/1.1-compliant caches do not observe the |  |  |
| max-age directive. |  |  |
| Other directives allow a user agent to modify the basic expiration |  |  |
| mechanism. These directives MAY be specified on a request: |  |  |
| max-age |  |  |
| Indicates that the client is willing to accept a response whose |  |  |
| age is no greater than the specified time in seconds. Unless max- |  |  |
| stale directive is also included, the client is not willing to |  |  |
| accept a stale response. |  |  |
| min-fresh |  |  |
| Indicates that the client is willing to accept a response whose |  |  |
| freshness lifetime is no less than its current age plus the |  |  |
| specified time in seconds. That is, the client wants a response |  |  |
| that will still be fresh for at least the specified number of |  |  |
| seconds. |  |  |
| max-stale |  |  |
| Indicates that the client is willing to accept a response that has |  |  |
| exceeded its expiration time. If max-stale is assigned a value, |  |  |
| then the client is willing to accept a response that has exceeded |  |  |
| its expiration time by no more than the specified number of |  |  |
| seconds. If no value is assigned to max-stale, then the client is |  |  |
| willing to accept a stale response of any age. |  |  |
| If a cache returns a stale response, either because of a max-stale |  |  |
| directive on a request, or because the cache is configured to |  |  |
| override the expiration time of a response, the cache MUST attach a |  |  |
| Warning header to the stale response, using Warning 110 (Response is |  |  |
| stale). |  |  |
| A cache MAY be configured to return stale responses without |  |  |
| validation, but only if this does not conflict with any "MUST"-level |  |  |
| requirements concerning cache validation (e.g., a "must-revalidate" |  |  |
| cache-control directive). |  |  |
| If both the new request and the cached entry include "max-age" |  |  |
| directives, then the lesser of the two values is used for determining |  |  |
| the freshness of the cached entry for that request. |  |  |
| 14.9.4. Cache Revalidation and Reload Controls |  |  |
| Sometimes a user agent might want or need to insist that a cache |  |  |
| revalidate its cache entry with the origin server (and not just with |  |  |
| the next cache along the path to the origin server), or to reload its |  |  |
| cache entry from the origin server. End-to-end revalidation might be |  |  |
| necessary if either the cache or the origin server has overestimated |  |  |
| the expiration time of the cached response. End-to-end reload may be |  |  |
| necessary if the cache entry has become corrupted for some reason. |  |  |
| End-to-end revalidation may be requested either when the client does |  |  |
| not have its own local cached copy, in which case we call it |  |  |
| "unspecified end-to-end revalidation", or when the client does have a |  |  |
| local cached copy, in which case we call it "specific end-to-end |  |  |
| revalidation." |  |  |
| The client can specify these three kinds of action using Cache- |  |  |
| Control request directives: |  |  |
| End-to-end reload |  |  |
| The request includes a "no-cache" cache-control directive or, for |  |  |
| compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field |  |  |
| names MUST NOT be included with the no-cache directive in a |  |  |
| request. The server MUST NOT use a cached copy when responding to |  |  |
| such a request. |  |  |
| Specific end-to-end revalidation |  |  |
| The request includes a "max-age=0" cache-control directive, which |  |  |
| forces each cache along the path to the origin server to |  |  |
| revalidate its own entry, if any, with the next cache or server. |  |  |
| The initial request includes a cache-validating conditional with |  |  |
| the client's current validator. |  |  |
| Unspecified end-to-end revalidation |  |  |
| The request includes "max-age=0" cache-control directive, which |  |  |
| forces each cache along the path to the origin server to |  |  |
| revalidate its own entry, if any, with the next cache or server. |  |  |
| The initial request does not include a cache-validating |  |  |
| conditional; the first cache along the path (if any) that holds a |  |  |
| cache entry for this resource includes a cache-validating |  |  |
| conditional with its current validator. |  |  |
| max-age |  |  |
| When an intermediate cache is forced, by means of a max-age=0 |  |  |
| directive, to revalidate its own cache entry, and the client has |  |  |
| supplied its own validator in the request, the supplied validator |  |  |
| might differ from the validator currently stored with the cache |  |  |
| entry. In this case, the cache MAY use either validator in making |  |  |
| its own request without affecting semantic transparency. |  |  |
| However, the choice of validator might affect performance. The |  |  |
| best approach is for the intermediate cache to use its own |  |  |
| validator when making its request. If the server replies with 304 |  |  |
| (Not Modified), then the cache can return its now validated copy |  |  |
| to the client with a 200 (OK) response. If the server replies |  |  |
| with a new entity and cache validator, however, the intermediate |  |  |
| cache can compare the returned validator with the one provided in |  |  |
| the client's request, using the strong comparison function. If |  |  |
| the client's validator is equal to the origin server's, then the |  |  |
| intermediate cache simply returns 304 (Not Modified). Otherwise, |  |  |
| it returns the new entity with a 200 (OK) response. |  |  |
| If a request includes the no-cache directive, it SHOULD NOT |  |  |
| include min-fresh, max-stale, or max-age. |  |  |
| only-if-cached |  |  |
| In some cases, such as times of extremely poor network |  |  |
| connectivity, a client may want a cache to return only those |  |  |
| responses that it currently has stored, and not to reload or |  |  |
| revalidate with the origin server. To do this, the client may |  |  |
| include the only-if-cached directive in a request. If it receives |  |  |
| this directive, a cache SHOULD either respond using a cached entry |  |  |
| that is consistent with the other constraints of the request, or |  |  |
| respond with a 504 (Gateway Timeout) status. However, if a group |  |  |
| of caches is being operated as a unified system with good internal |  |  |
| connectivity, such a request MAY be forwarded within that group of |  |  |
| caches. |  |  |
| must-revalidate |  |  |
| Because a cache MAY be configured to ignore a server's specified |  |  |
| expiration time, and because a client request MAY include a max- |  |  |
| stale directive (which has a similar effect), the protocol also |  |  |
| includes a mechanism for the origin server to require revalidation |  |  |
| of a cache entry on any subsequent use. When the must-revalidate |  |  |
| directive is present in a response received by a cache, that cache |  |  |
| MUST NOT use the entry after it becomes stale to respond to a |  |  |
| subsequent request without first revalidating it with the origin |  |  |
| server. (I.e., the cache MUST do an end-to-end revalidation every |  |  |
| time, if, based solely on the origin server's Expires or max-age |  |  |
| value, the cached response is stale.) |  |  |
| The must-revalidate directive is necessary to support reliable |  |  |
| operation for certain protocol features. In all circumstances an |  |  |
| HTTP/1.1 cache MUST obey the must-revalidate directive; in |  |  |
| particular, if the cache cannot reach the origin server for any |  |  |
| reason, it MUST generate a 504 (Gateway Timeout) response. |  |  |
| Servers SHOULD send the must-revalidate directive if and only if |  |  |
| failure to revalidate a request on the entity could result in |  |  |
| incorrect operation, such as a silently unexecuted financial |  |  |
| transaction. Recipients MUST NOT take any automated action that |  |  |
| violates this directive, and MUST NOT automatically provide an |  |  |
| unvalidated copy of the entity if revalidation fails. |  |  |
| Although this is not recommended, user agents operating under |  |  |
| severe connectivity constraints MAY violate this directive but, if |  |  |
| so, MUST explicitly warn the user that an unvalidated response has |  |  |
| been provided. The warning MUST be provided on each unvalidated |  |  |
| access, and SHOULD require explicit user confirmation. |  |  |
| proxy-revalidate |  |  |
| The proxy-revalidate directive has the same meaning as the must- |  |  |
| revalidate directive, except that it does not apply to non-shared |  |  |
| user agent caches. It can be used on a response to an |  |  |
| authenticated request to permit the user's cache to store and |  |  |
| later return the response without needing to revalidate it (since |  |  |
| it has already been authenticated once by that user), while still |  |  |
| requiring proxies that service many users to revalidate each time |  |  |
| (in order to make sure that each user has been authenticated). |  |  |
| Note that such authenticated responses also need the public cache |  |  |
| control directive in order to allow them to be cached at all. |  |  |
| 14.9.5. No-Transform Directive |  |  |
| no-transform |  |  |
| Implementors of intermediate caches (proxies) have found it useful |  |  |
| to convert the media type of certain entity bodies. A non- |  |  |
| transparent proxy might, for example, convert between image |  |  |
| formats in order to save cache space or to reduce the amount of |  |  |
| traffic on a slow link. |  |  |
| Serious operational problems occur, however, when these |  |  |
| transformations are applied to entity bodies intended for certain |  |  |
| kinds of applications. For example, applications for medical |  |  |
| imaging, scientific data analysis and those using end-to-end |  |  |
| authentication, all depend on receiving an entity body that is bit |  |  |
| for bit identical to the original entity-body. |  |  |
| Therefore, if a message includes the no-transform directive, an |  |  |
| intermediate cache or proxy MUST NOT change those headers that are |  |  |
| listed in Section 13.5.2 as being subject to the no-transform |  |  |
| directive. This implies that the cache or proxy MUST NOT change |  |  |
| any aspect of the entity-body that is specified by these headers, |  |  |
| including the value of the entity-body itself. |  |  |
| 14.9.6. Cache Control Extensions |  |  |
| The Cache-Control header field can be extended through the use of one |  |  |
| or more cache-extension tokens, each with an optional assigned value. |  |  |
| Informational extensions (those which do not require a change in |  |  |
| cache behavior) MAY be added without changing the semantics of other |  |  |
| directives. Behavioral extensions are designed to work by acting as |  |  |
| modifiers to the existing base of cache directives. Both the new |  |  |
| directive and the standard directive are supplied, such that |  |  |
| applications which do not understand the new directive will default |  |  |
| to the behavior specified by the standard directive, and those that |  |  |
| understand the new directive will recognize it as modifying the |  |  |
| requirements associated with the standard directive. In this way, |  |  |
| extensions to the cache-control directives can be made without |  |  |
| requiring changes to the base protocol. |  |  |
| This extension mechanism depends on an HTTP cache obeying all of the |  |  |
| cache-control directives defined for its native HTTP-version, obeying |  |  |
| certain extensions, and ignoring all directives that it does not |  |  |
| understand. |  |  |
| For example, consider a hypothetical new response directive called |  |  |
| community which acts as a modifier to the private directive. We |  |  |
| define this new directive to mean that, in addition to any non-shared |  |  |
| cache, any cache which is shared only by members of the community |  |  |
| named within its value may cache the response. An origin server |  |  |
| wishing to allow the UCI community to use an otherwise private |  |  |
| response in their shared cache(s) could do so by including |  |  |
| Cache-Control: private, community="UCI" |  |  |
| A cache seeing this header field will act correctly even if the cache |  |  |
| does not understand the community cache-extension, since it will also |  |  |
| see and understand the private directive and thus default to the safe |  |  |
| behavior. |  |  |
| Unrecognized cache-directives MUST be ignored; it is assumed that any |  |  |
| cache-directive likely to be unrecognized by an HTTP/1.1 cache will |  |  |
| be combined with standard directives (or the response's default |  |  |
| cacheability) such that the cache behavior will remain minimally |  |  |
| correct even if the cache does not understand the extension(s). |  |  |
| 14.10. Connection |  |  |
| The Connection general-header field allows the sender to specify |  |  |
| options that are desired for that particular connection and MUST NOT |  |  |
| be communicated by proxies over further connections. |  |  |
| The Connection header has the following grammar: |  |  |
| Connection = "Connection" ":" 1#(connection-token) |  |  |
| connection-token = token |  |  |
| HTTP/1.1 proxies MUST parse the Connection header field before a |  |  |
| message is forwarded and, for each connection-token in this field, |  |  |
| remove any header field(s) from the message with the same name as the |  |  |
| connection-token. Connection options are signaled by the presence of |  |  |
| a connection-token in the Connection header field, not by any |  |  |
| corresponding additional header field(s), since the additional header |  |  |
| field may not be sent if there are no parameters associated with that |  |  |
| connection option. |  |  |
| Message headers listed in the Connection header MUST NOT include end- |  |  |
| to-end headers, such as Cache-Control. |  |  |
| HTTP/1.1 defines the "close" connection option for the sender to |  |  |
| signal that the connection will be closed after completion of the |  |  |
| response. For example, |  |  |
| Connection: close |  |  |
| in either the request or the response header fields indicates that |  |  |
| the connection SHOULD NOT be considered `persistent' (Section 8.1) |  |  |
| after the current request/response is complete. |  |  |
| HTTP/1.1 applications that do not support persistent connections MUST |  |  |
| include the "close" connection option in every message. |  |  |
| A system receiving an HTTP/1.0 (or lower-version) message that |  |  |
| includes a Connection header MUST, for each connection-token in this |  |  |
| field, remove and ignore any header field(s) from the message with |  |  |
| the same name as the connection-token. This protects against |  |  |
| mistaken forwarding of such header fields by pre-HTTP/1.1 proxies. |  |  |
| See Appendix A.6.2. |  |  |
| 14.11. Content-Encoding |  |  |
| The Content-Encoding entity-header field is used as a modifier to the |  |  |
| media-type. When present, its value indicates what additional |  |  |
| content codings have been applied to the entity-body, and thus what |  |  |
| decoding mechanisms must be applied in order to obtain the media-type |  |  |
| referenced by the Content-Type header field. Content-Encoding is |  |  |
| primarily used to allow a document to be compressed without losing |  |  |
| the identity of its underlying media type. |  |  |
| Content-Encoding = "Content-Encoding" ":" 1#content-coding |  |  |
| Content codings are defined in Section 3.5. An example of its use is |  |  |
| Content-Encoding: gzip |  |  |
| The content-coding is a characteristic of the entity identified by |  |  |
| the Request-URI. Typically, the entity-body is stored with this |  |  |
| encoding and is only decoded before rendering or analogous usage. |  |  |
| However, a non-transparent proxy MAY modify the content-coding if the |  |  |
| new coding is known to be acceptable to the recipient, unless the |  |  |
| "no-transform" cache-control directive is present in the message. |  |  |
| If the content-coding of an entity is not "identity", then the |  |  |
| response MUST include a Content-Encoding entity-header |  |  |
| (Section 14.11) that lists the non-identity content-coding(s) used. |  |  |
| If the content-coding of an entity in a request message is not |  |  |
| acceptable to the origin server, the server SHOULD respond with a |  |  |
| status code of 415 (Unsupported Media Type). |  |  |
| If multiple encodings have been applied to an entity, the content |  |  |
| codings MUST be listed in the order in which they were applied. |  |  |
| Additional information about the encoding parameters MAY be provided |  |  |
| by other entity-header fields not defined by this specification. |  |  |
| 14.12. Content-Language |  |  |
| The Content-Language entity-header field describes the natural |  |  |
| language(s) of the intended audience for the enclosed entity. Note |  |  |
| that this might not be equivalent to all the languages used within |  |  |
| the entity-body. |  |  |
| Content-Language = "Content-Language" ":" 1#language-tag |  |  |
| Language tags are defined in Section 3.10. The primary purpose of |  |  |
| Content-Language is to allow a user to identify and differentiate |  |  |
| entities according to the user's own preferred language. Thus, if |  |  |
| the body content is intended only for a Danish-literate audience, the |  |  |
| appropriate field is |  |  |
| Content-Language: da |  |  |
| If no Content-Language is specified, the default is that the content |  |  |
| is intended for all language audiences. This might mean that the |  |  |
| sender does not consider it to be specific to any natural language, |  |  |
| or that the sender does not know for which language it is intended. |  |  |
| Multiple languages MAY be listed for content that is intended for |  |  |
| multiple audiences. For example, a rendition of the "Treaty of |  |  |
| Waitangi," presented simultaneously in the original Maori and English |  |  |
| versions, would call for |  |  |
| Content-Language: mi, en |  |  |
| However, just because multiple languages are present within an entity |  |  |
| does not mean that it is intended for multiple linguistic audiences. |  |  |
| An example would be a beginner's language primer, such as "A First |  |  |
| Lesson in Latin," which is clearly intended to be used by an English- |  |  |
| literate audience. In this case, the Content-Language would properly |  |  |
| only include "en". |  |  |
| Content-Language MAY be applied to any media type -- it is not |  |  |
| limited to textual documents. |  |  |
| 14.13. Content-Length |  |  |
| The Content-Length entity-header field indicates the size of the |  |  |
| entity-body, in decimal number of OCTETs, sent to the recipient or, |  |  |
| in the case of the HEAD method, the size of the entity-body that |  |  |
| would have been sent had the request been a GET. |  |  |
| Content-Length = "Content-Length" ":" 1*DIGIT |  |  |
| An example is |  |  |
| Content-Length: 3495 |  |  |
| Applications SHOULD use this field to indicate the transfer-length of |  |  |
| the message-body, unless this is prohibited by the rules in |  |  |
| Section 4.4. |  |  |
| Any Content-Length greater than or equal to zero is a valid value. |  |  |
| Section 4.4 describes how to determine the length of a message-body |  |  |
| if a Content-Length is not given. |  |  |
| Note that the meaning of this field is significantly different from |  |  |
| the corresponding definition in MIME, where it is an optional field |  |  |
| used within the "message/external-body" content-type. In HTTP, it |  |  |
| SHOULD be sent whenever the message's length can be determined prior |  |  |
| to being transferred, unless this is prohibited by the rules in |  |  |
| Section 4.4. |  |  |
| 14.14. Content-Location |  |  |
| The Content-Location entity-header field MAY be used to supply the |  |  |
| resource location for the entity enclosed in the message when that |  |  |
| entity is accessible from a location separate from the requested |  |  |
| resource's URI. A server SHOULD provide a Content-Location for the |  |  |
| variant corresponding to the response entity; especially in the case |  |  |
| where a resource has multiple entities associated with it, and those |  |  |
| entities actually have separate locations by which they might be |  |  |
| individually accessed, the server SHOULD provide a Content-Location |  |  |
| for the particular variant which is returned. |  |  |
| Content-Location = "Content-Location" ":" |  |  |
| ( absoluteURI | relativeURI ) |  |  |
| The value of Content-Location also defines the base URI for the |  |  |
| entity. |  |  |
| The Content-Location value is not a replacement for the original |  |  |
| requested URI; it is only a statement of the location of the resource |  |  |
| corresponding to this particular entity at the time of the request. |  |  |
| Future requests MAY specify the Content-Location URI as the request- |  |  |
| URI if the desire is to identify the source of that particular |  |  |
| entity. |  |  |
| A cache cannot assume that an entity with a Content-Location |  |  |
| different from the URI used to retrieve it can be used to respond to |  |  |
| later requests on that Content-Location URI. However, the Content- |  |  |
| Location can be used to differentiate between multiple entities |  |  |
| retrieved from a single requested resource, as described in |  |  |
| Section 13.6. |  |  |
| If the Content-Location is a relative URI, the relative URI is |  |  |
| interpreted relative to the Request-URI. |  |  |
| The meaning of the Content-Location header in PUT or POST requests is |  |  |
| undefined; servers are free to ignore it in those cases. |  |  |
| 14.15. Content-MD5 |  |  |
| The Content-MD5 entity-header field, as defined in RFC 1864 |  |  |
| [RFC1864], is an MD5 digest of the entity-body for the purpose of |  |  |
| providing an end-to-end message integrity check (MIC) of the entity- |  |  |
| body. (Note: a MIC is good for detecting accidental modification of |  |  |
| the entity-body in transit, but is not proof against malicious |  |  |
| attacks.) |  |  |
| Content-MD5 = "Content-MD5" ":" md5-digest |  |  |
| md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864> |  |  |
| The Content-MD5 header field MAY be generated by an origin server or |  |  |
| client to function as an integrity check of the entity-body. Only |  |  |
| origin servers or clients MAY generate the Content-MD5 header field; |  |  |
| proxies and gateways MUST NOT generate it, as this would defeat its |  |  |
| value as an end-to-end integrity check. Any recipient of the entity- |  |  |
| body, including gateways and proxies, MAY check that the digest value |  |  |
| in this header field matches that of the entity-body as received. |  |  |
| The MD5 digest is computed based on the content of the entity-body, |  |  |
| including any content-coding that has been applied, but not including |  |  |
| any transfer-encoding applied to the message-body. If the message is |  |  |
| received with a transfer-encoding, that encoding MUST be removed |  |  |
| prior to checking the Content-MD5 value against the received entity. |  |  |
| This has the result that the digest is computed on the octets of the |  |  |
| entity-body exactly as, and in the order that, they would be sent if |  |  |
| no transfer-encoding were being applied. |  |  |
| HTTP extends RFC 1864 to permit the digest to be computed for MIME |  |  |
| composite media-types (e.g., multipart/* and message/rfc822), but |  |  |
| this does not change how the digest is computed as defined in the |  |  |
| preceding paragraph. |  |  |
| There are several consequences of this. The entity-body for |  |  |
| composite types MAY contain many body-parts, each with its own MIME |  |  |
| and HTTP headers (including Content-MD5, Content-Transfer-Encoding, |  |  |
| and Content-Encoding headers). If a body-part has a Content- |  |  |
| Transfer-Encoding or Content-Encoding header, it is assumed that the |  |  |
| content of the body-part has had the encoding applied, and the body- |  |  |
| part is included in the Content-MD5 digest as is -- i.e., after the |  |  |
| application. The Transfer-Encoding header field is not allowed |  |  |
| within body-parts. |  |  |
| Conversion of all line breaks to CRLF MUST NOT be done before |  |  |
| computing or checking the digest: the line break convention used in |  |  |
| the text actually transmitted MUST be left unaltered when computing |  |  |
| the digest. |  |  |
| Note: while the definition of Content-MD5 is exactly the same for |  |  |
| HTTP as in RFC 1864 for MIME entity-bodies, there are several ways |  |  |
| in which the application of Content-MD5 to HTTP entity-bodies |  |  |
| differs from its application to MIME entity-bodies. One is that |  |  |
| HTTP, unlike MIME, does not use Content-Transfer-Encoding, and |  |  |
| does use Transfer-Encoding and Content-Encoding. Another is that |  |  |
| HTTP more frequently uses binary content types than MIME, so it is |  |  |
| worth noting that, in such cases, the byte order used to compute |  |  |
| the digest is the transmission byte order defined for the type. |  |  |
| Lastly, HTTP allows transmission of text types with any of several |  |  |
| line break conventions and not just the canonical form using CRLF. |  |  |
| 14.16. Content-Range |  |  |
| The Content-Range entity-header is sent with a partial entity-body to |  |  |
| specify where in the full entity-body the partial body should be |  |  |
| applied. Range units are defined in Section 3.12. |  |  |
| Content-Range = "Content-Range" ":" content-range-spec |  |  |
| content-range-spec = byte-content-range-spec |  |  |
| byte-content-range-spec = bytes-unit SP |  |  |
| byte-range-resp-spec "/" |  |  |
| ( instance-length | "*" ) |  |  |
| byte-range-resp-spec = (first-byte-pos "-" last-byte-pos) |  |  |
| | "*" |  |  |
| instance-length = 1*DIGIT |  |  |
| The header SHOULD indicate the total length of the full entity-body, |  |  |
| unless this length is unknown or difficult to determine. The |  |  |
| asterisk "*" character means that the instance-length is unknown at |  |  |
| the time when the response was generated. |  |  |
| Unlike byte-ranges-specifier values (see Section 14.35.1), a byte- |  |  |
| range-resp-spec MUST only specify one range, and MUST contain |  |  |
| absolute byte positions for both the first and last byte of the |  |  |
| range. |  |  |
| A byte-content-range-spec with a byte-range-resp-spec whose last- |  |  |
| byte-pos value is less than its first-byte-pos value, or whose |  |  |
| instance-length value is less than or equal to its last-byte-pos |  |  |
| value, is invalid. The recipient of an invalid byte-content-range- |  |  |
| spec MUST ignore it and any content transferred along with it. |  |  |
| A server sending a response with status code 416 (Requested range not |  |  |
| satisfiable) SHOULD include a Content-Range field with a byte-range- |  |  |
| resp-spec of "*". The instance-length specifies the current length |  |  |
| of the selected resource. A response with status code 206 (Partial |  |  |
| Content) MUST NOT include a Content-Range field with a byte-range- |  |  |
| resp-spec of "*". |  |  |
| Examples of byte-content-range-spec values, assuming that the entity |  |  |
| contains a total of 1234 bytes: |  |  |
| o The first 500 bytes: |  |  |
| bytes 0-499/1234 |  |  |
| o The second 500 bytes: |  |  |
| bytes 500-999/1234 |  |  |
| o All except for the first 500 bytes: |  |  |
| bytes 500-1233/1234 |  |  |
| o The last 500 bytes: |  |  |
| bytes 734-1233/1234 |  |  |
| When an HTTP message includes the content of a single range (for |  |  |
| example, a response to a request for a single range, or to a request |  |  |
| for a set of ranges that overlap without any holes), this content is |  |  |
| transmitted with a Content-Range header, and a Content-Length header |  |  |
| showing the number of bytes actually transferred. For example, |  |  |
| HTTP/1.1 206 Partial content |  |  |
| Date: Wed, 15 Nov 1995 06:25:24 GMT |  |  |
| Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT |  |  |
| Content-Range: bytes 21010-47021/47022 |  |  |
| Content-Length: 26012 |  |  |
| Content-Type: image/gif |  |  |
| When an HTTP message includes the content of multiple ranges (for |  |  |
| example, a response to a request for multiple non-overlapping |  |  |
| ranges), these are transmitted as a multipart message. The multipart |  |  |
| media type used for this purpose is "multipart/byteranges" as defined |  |  |
| in Appendix A.2. See Appendix A.6.3 for a compatibility issue. |  |  |
| A response to a request for a single range MUST NOT be sent using the |  |  |
| multipart/byteranges media type. A response to a request for |  |  |
| multiple ranges, whose result is a single range, MAY be sent as a |  |  |
| multipart/byteranges media type with one part. A client that cannot |  |  |
| decode a multipart/byteranges message MUST NOT ask for multiple byte- |  |  |
| ranges in a single request. |  |  |
| When a client requests multiple byte-ranges in one request, the |  |  |
| server SHOULD return them in the order that they appeared in the |  |  |
| request. |  |  |
| If the server ignores a byte-range-spec because it is syntactically |  |  |
| invalid, the server SHOULD treat the request as if the invalid Range |  |  |
| header field did not exist. (Normally, this means return a 200 |  |  |
| response containing the full entity). |  |  |
| If the server receives a request (other than one including an If- |  |  |
| Range request-header field) with an unsatisfiable Range request- |  |  |
| header field (that is, all of whose byte-range-spec values have a |  |  |
| first-byte-pos value greater than the current length of the selected |  |  |
| resource), it SHOULD return a response code of 416 (Requested range |  |  |
| not satisfiable) (Section 10.4.17). |  |  |
| Note: clients cannot depend on servers to send a 416 (Requested |  |  |
| range not satisfiable) response instead of a 200 (OK) response for |  |  |
| an unsatisfiable Range request-header, since not all servers |  |  |
| implement this request-header. |  |  |
| 14.17. Content-Type |  |  |
| The Content-Type entity-header field indicates the media type of the |  |  |
| entity-body sent to the recipient or, in the case of the HEAD method, |  |  |
| the media type that would have been sent had the request been a GET. |  |  |
| Content-Type = "Content-Type" ":" media-type |  |  |
| Media types are defined in Section 3.7. An example of the field is |  |  |
| Content-Type: text/html; charset=ISO-8859-4 |  |  |
| Further discussion of methods for identifying the media type of an |  |  |
| entity is provided in Section 7.2.1. |  |  |
| 14.18. Date |  |  |
| The Date general-header field represents the date and time at which |  |  |
| the message was originated, having the same semantics as orig-date in |  |  |
| RFC 822. The field value is an HTTP-date, as described in |  |  |
| Section 3.3.1; it MUST be sent in RFC 1123 [RFC1123]-date format. |  |  |
| Date = "Date" ":" HTTP-date |  |  |
| An example is |  |  |
| Date: Tue, 15 Nov 1994 08:12:31 GMT |  |  |
| Origin servers MUST include a Date header field in all responses, |  |  |
| except in these cases: |  |  |
| 1. If the response status code is 100 (Continue) or 101 (Switching |  |  |
| Protocols), the response MAY include a Date header field, at the |  |  |
| server's option. |  |  |
| 2. If the response status code conveys a server error, e.g. 500 |  |  |
| (Internal Server Error) or 503 (Service Unavailable), and it is |  |  |
| inconvenient or impossible to generate a valid Date. |  |  |
| 3. If the server does not have a clock that can provide a reasonable |  |  |
| approximation of the current time, its responses MUST NOT include |  |  |
| a Date header field. In this case, the rules in Section 14.18.1 |  |  |
| MUST be followed. |  |  |
| A received message that does not have a Date header field MUST be |  |  |
| assigned one by the recipient if the message will be cached by that |  |  |
| recipient or gatewayed via a protocol which requires a Date. An HTTP |  |  |
| implementation without a clock MUST NOT cache responses without |  |  |
| revalidating them on every use. An HTTP cache, especially a shared |  |  |
| cache, SHOULD use a mechanism, such as NTP [RFC1305], to synchronize |  |  |
| its clock with a reliable external standard. |  |  |
| Clients SHOULD only send a Date header field in messages that include |  |  |
| an entity-body, as in the case of the PUT and POST requests, and even |  |  |
| then it is optional. A client without a clock MUST NOT send a Date |  |  |
| header field in a request. |  |  |
| The HTTP-date sent in a Date header SHOULD NOT represent a date and |  |  |
| time subsequent to the generation of the message. It SHOULD |  |  |
| represent the best available approximation of the date and time of |  |  |
| message generation, unless the implementation has no means of |  |  |
| generating a reasonably accurate date and time. In theory, the date |  |  |
| ought to represent the moment just before the entity is generated. |  |  |
| In practice, the date can be generated at any time during the message |  |  |
| origination without affecting its semantic value. |  |  |
| 14.18.1. Clockless Origin Server Operation |  |  |
| Some origin server implementations might not have a clock available. |  |  |
| An origin server without a clock MUST NOT assign Expires or Last- |  |  |
| Modified values to a response, unless these values were associated |  |  |
| with the resource by a system or user with a reliable clock. It MAY |  |  |
| assign an Expires value that is known, at or before server |  |  |
| configuration time, to be in the past (this allows "pre-expiration" |  |  |
| of responses without storing separate Expires values for each |  |  |
| resource). |  |  |
| 14.19. ETag |  |  |
| The ETag response-header field provides the current value of the |  | The ETag response-header field provides the current value of the |
| entity tag for the requested variant. The headers used with entity |  | entity tag for the requested variant. The headers used with entity |
| tags are described in sections 14.24, 14.26 and 14.44. The entity |  | tags are described in sections 6.2, 6.4 and Section 5.3 of [Part5]. |
| tag MAY be used for comparison with other entities from the same |  | The entity tag MAY be used for comparison with other entities from |
| resource (see Section 13.3.3). |  | the same resource (see Section 4). |
| ETag = "ETag" ":" entity-tag |  | ETag = "ETag" ":" entity-tag |
| Examples: |  | Examples: |
| ETag: "xyzzy" |  | ETag: "xyzzy" |
| ETag: W/"xyzzy" |  | ETag: W/"xyzzy" |
| ETag: "" |  | ETag: "" |
| 14.20. Expect |  | 6.2. If-Match |
| The Expect request-header field is used to indicate that particular |  |  |
| server behaviors are required by the client. |  |  |
| Expect = "Expect" ":" 1#expectation |  |  |
| expectation = "100-continue" | expectation-extension |  |  |
| expectation-extension = token [ "=" ( token | quoted-string ) |  |  |
| *expect-params ] |  |  |
| expect-params = ";" token [ "=" ( token | quoted-string ) ] |  |  |
| A server that does not understand or is unable to comply with any of |  |  |
| the expectation values in the Expect field of a request MUST respond |  |  |
| with appropriate error status. The server MUST respond with a 417 |  |  |
| (Expectation Failed) status if any of the expectations cannot be met |  |  |
| or, if there are other problems with the request, some other 4xx |  |  |
| status. |  |  |
| This header field is defined with extensible syntax to allow for |  |  |
| future extensions. If a server receives a request containing an |  |  |
| Expect field that includes an expectation-extension that it does not |  |  |
| support, it MUST respond with a 417 (Expectation Failed) status. |  |  |
| Comparison of expectation values is case-insensitive for unquoted |  |  |
| tokens (including the 100-continue token), and is case-sensitive for |  |  |
| quoted-string expectation-extensions. |  |  |
| The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST |  |  |
| return a 417 (Expectation Failed) status if it receives a request |  |  |
| with an expectation that it cannot meet. However, the Expect |  |  |
| request-header itself is end-to-end; it MUST be forwarded if the |  |  |
| request is forwarded. |  |  |
| Many older HTTP/1.0 and HTTP/1.1 applications do not understand the |  |  |
| Expect header. |  |  |
| See Section 8.2.3 for the use of the 100 (continue) status. |  |  |
| 14.21. Expires |  |  |
| The Expires entity-header field gives the date/time after which the |  |  |
| response is considered stale. A stale cache entry may not normally |  |  |
| be returned by a cache (either a proxy cache or a user agent cache) |  |  |
| unless it is first validated with the origin server (or with an |  |  |
| intermediate cache that has a fresh copy of the entity). See |  |  |
| Section 13.2 for further discussion of the expiration model. |  |  |
| The presence of an Expires field does not imply that the original |  |  |
| resource will change or cease to exist at, before, or after that |  |  |
| time. |  |  |
| The format is an absolute date and time as defined by HTTP-date in |  |  |
| Section 3.3.1; it MUST be in RFC 1123 date format: |  |  |
| Expires = "Expires" ":" HTTP-date |  |  |
| An example of its use is |  |  |
| Expires: Thu, 01 Dec 1994 16:00:00 GMT |  |  |
| Note: if a response includes a Cache-Control field with the max- |  |  |
| age directive (see Section 14.9.3), that directive overrides the |  |  |
| Expires field. |  |  |
| HTTP/1.1 clients and caches MUST treat other invalid date formats, |  |  |
| especially including the value "0", as in the past (i.e., "already |  |  |
| expired"). |  |  |
| To mark a response as "already expired," an origin server sends an |  |  |
| Expires date that is equal to the Date header value. (See the rules |  |  |
| for expiration calculations in Section 13.2.4.) |  |  |
| To mark a response as "never expires," an origin server sends an |  |  |
| Expires date approximately one year from the time the response is |  |  |
| sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one |  |  |
| year in the future. |  |  |
| The presence of an Expires header field with a date value of some |  |  |
| time in the future on a response that otherwise would by default be |  |  |
| non-cacheable indicates that the response is cacheable, unless |  |  |
| indicated otherwise by a Cache-Control header field (Section 14.9). |  |  |
| 14.22. From |  |  |
| The From request-header field, if given, SHOULD contain an Internet |  |  |
| e-mail address for the human user who controls the requesting user |  |  |
| agent. The address SHOULD be machine-usable, as defined by "mailbox" |  |  |
| in RFC 822 [RFC822] as updated by RFC 1123 [RFC1123]: |  |  |
| From = "From" ":" mailbox |  |  |
| An example is: |  |  |
| From: webmaster@w3.org |  |  |
| This header field MAY be used for logging purposes and as a means for |  |  |
| identifying the source of invalid or unwanted requests. It SHOULD |  |  |
| NOT be used as an insecure form of access protection. The |  |  |
| interpretation of this field is that the request is being performed |  |  |
| on behalf of the person given, who accepts responsibility for the |  |  |
| method performed. In particular, robot agents SHOULD include this |  |  |
| header so that the person responsible for running the robot can be |  |  |
| contacted if problems occur on the receiving end. |  |  |
| The Internet e-mail address in this field MAY be separate from the |  |  |
| Internet host which issued the request. For example, when a request |  |  |
| is passed through a proxy the original issuer's address SHOULD be |  |  |
| used. |  |  |
| The client SHOULD NOT send the From header field without the user's |  |  |
| approval, as it might conflict with the user's privacy interests or |  |  |
| their site's security policy. It is strongly recommended that the |  |  |
| user be able to disable, enable, and modify the value of this field |  |  |
| at any time prior to a request. |  |  |
| 14.23. Host |  |  |
| The Host request-header field specifies the Internet host and port |  |  |
| number of the resource being requested, as obtained from the original |  |  |
| URI given by the user or referring resource (generally an HTTP URL, |  |  |
| as described in Section 3.2.2). The Host field value MUST represent |  |  |
| the naming authority of the origin server or gateway given by the |  |  |
| original URL. This allows the origin server or gateway to |  |  |
| differentiate between internally-ambiguous URLs, such as the root "/" |  |  |
| URL of a server for multiple host names on a single IP address. |  |  |
| Host = "Host" ":" host [ ":" port ] ; Section 3.2.2 |  |  |
| A "host" without any trailing port information implies the default |  |  |
| port for the service requested (e.g., "80" for an HTTP URL). For |  |  |
| example, a request on the origin server for |  |  |
| <http://www.w3.org/pub/WWW/> would properly include: |  |  |
| GET /pub/WWW/ HTTP/1.1 |  |  |
| Host: www.w3.org |  |  |
| A client MUST include a Host header field in all HTTP/1.1 request |  |  |
| messages . If the requested URI does not include an Internet host |  |  |
| name for the service being requested, then the Host header field MUST |  |  |
| be given with an empty value. An HTTP/1.1 proxy MUST ensure that any |  |  |
| request message it forwards does contain an appropriate Host header |  |  |
| field that identifies the service being requested by the proxy. All |  |  |
| Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request) |  |  |
| status code to any HTTP/1.1 request message which lacks a Host header |  |  |
| field. |  |  |
| See sections 5.2 and A.6.1.1 for other requirements relating to Host. |  |  |
| 14.24. If-Match |  |  |
| The If-Match request-header field is used with a method to make it |  | The If-Match request-header field is used with a method to make it |
| conditional. A client that has one or more entities previously |  | conditional. A client that has one or more entities previously |
| obtained from the resource can verify that one of those entities is |  | obtained from the resource can verify that one of those entities is |
| current by including a list of their associated entity tags in the |  | current by including a list of their associated entity tags in the |
| If-Match header field. Entity tags are defined in Section 3.11. The |  | If-Match header field. Entity tags are defined in Section 2. The |
| purpose of this feature is to allow efficient updates of cached |  | purpose of this feature is to allow efficient updates of cached |
| information with a minimum amount of transaction overhead. It is |  | information with a minimum amount of transaction overhead. It is |
| also used, on updating requests, to prevent inadvertent modification |  | also used, on updating requests, to prevent inadvertent modification |
| of the wrong version of a resource. As a special case, the value "*" |  | of the wrong version of a resource. As a special case, the value "*" |
| matches any current entity of the resource. |  | matches any current entity of the resource. |
| If-Match = "If-Match" ":" ( "*" | 1#entity-tag ) |  | If-Match = "If-Match" ":" ( "*" | 1#entity-tag ) |
| If any of the entity tags match the entity tag of the entity that |  | If any of the entity tags match the entity tag of the entity that |
| would have been returned in the response to a similar GET request |  | would have been returned in the response to a similar GET request |
| (without the If-Match header) on that resource, or if "*" is given |  | (without the If-Match header) on that resource, or if "*" is given |
| and any current entity exists for that resource, then the server MAY |  | and any current entity exists for that resource, then the server MAY |
| perform the requested method as if the If-Match header field did not |  | perform the requested method as if the If-Match header field did not |
| exist. |  | exist. |
| A server MUST use the strong comparison function (see Section 13.3.3) |  | A server MUST use the strong comparison function (see Section 4) to |
| to compare the entity tags in If-Match. |  | compare the entity tags in If-Match. |
| If none of the entity tags match, or if "*" is given and no current |  | If none of the entity tags match, or if "*" is given and no current |
| entity exists, the server MUST NOT perform the requested method, and |  | entity exists, the server MUST NOT perform the requested method, and |
| MUST return a 412 (Precondition Failed) response. This behavior is |  | MUST return a 412 (Precondition Failed) response. This behavior is |
| most useful when the client wants to prevent an updating method, such |  | most useful when the client wants to prevent an updating method, such |
| as PUT, from modifying a resource that has changed since the client |  | as PUT, from modifying a resource that has changed since the client |
| last retrieved it. |  | last retrieved it. |
| If the request would, without the If-Match header field, result in |  | If the request would, without the If-Match header field, result in |
| anything other than a 2xx or 412 status, then the If-Match header |  | anything other than a 2xx or 412 status, then the If-Match header |
| MUST be ignored. |  | MUST be ignored. |
| The meaning of "If-Match: *" is that the method SHOULD be performed |  | The meaning of "If-Match: *" is that the method SHOULD be performed |
| if the representation selected by the origin server (or by a cache, |  | if the representation selected by the origin server (or by a cache, |
| possibly using the Vary mechanism, see Section 14.44) exists, and |  | possibly using the Vary mechanism, see Section 3.5 of [Part6]) |
| MUST NOT be performed if the representation does not exist. |  | exists, and MUST NOT be performed if the representation does not |
|  |  | exist. |
| A request intended to update a resource (e.g., a PUT) MAY include an |  | A request intended to update a resource (e.g., a PUT) MAY include an |
| If-Match header field to signal that the request method MUST NOT be |  | If-Match header field to signal that the request method MUST NOT be |
| applied if the entity corresponding to the If-Match value (a single |  | applied if the entity corresponding to the If-Match value (a single |
| entity tag) is no longer a representation of that resource. This |  | entity tag) is no longer a representation of that resource. This |
| allows the user to indicate that they do not wish the request to be |  | allows the user to indicate that they do not wish the request to be |
| successful if the resource has been changed without their knowledge. |  | successful if the resource has been changed without their knowledge. |
| Examples: |  | Examples: |
| If-Match: "xyzzy" |  | If-Match: "xyzzy" |
| If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |  | If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |
| If-Match: * |  | If-Match: * |
| The result of a request having both an If-Match header field and |  | The result of a request having both an If-Match header field and |
| either an If-None-Match or an If-Modified-Since header fields is |  | either an If-None-Match or an If-Modified-Since header fields is |
| undefined by this specification. |  | undefined by this specification. |
| 14.25. If-Modified-Since |  | 6.3. If-Modified-Since |
| The If-Modified-Since request-header field is used with a method to |  | The If-Modified-Since request-header field is used with a method to |
| make it conditional: if the requested variant has not been modified |  | make it conditional: if the requested variant has not been modified |
| since the time specified in this field, an entity will not be |  | since the time specified in this field, an entity will not be |
| returned from the server; instead, a 304 (not modified) response will |  | returned from the server; instead, a 304 (not modified) response will |
| be returned without any message-body. |  | be returned without any message-body. |
| If-Modified-Since = "If-Modified-Since" ":" HTTP-date |  | If-Modified-Since = "If-Modified-Since" ":" HTTP-date |
| An example of the field is: |  | An example of the field is: |
| skipping to change at page 134, line 18 |  | skipping to change at page 12, line 33 |
| date, the response is exactly the same as for a normal GET. |  | date, the response is exactly the same as for a normal GET. |
| 3. If the variant has not been modified since a valid If-Modified- |  | 3. If the variant has not been modified since a valid If-Modified- |
| Since date, the server SHOULD return a 304 (Not Modified) |  | Since date, the server SHOULD return a 304 (Not Modified) |
| response. |  | response. |
| The purpose of this feature is to allow efficient updates of cached |  | The purpose of this feature is to allow efficient updates of cached |
| information with a minimum amount of transaction overhead. |  | information with a minimum amount of transaction overhead. |
| Note: The Range request-header field modifies the meaning of If- |  | Note: The Range request-header field modifies the meaning of If- |
| Modified-Since; see Section 14.35 for full details. |  | Modified-Since; see Section 5.4 of [Part5] for full details. |
| Note: If-Modified-Since times are interpreted by the server, whose |  | Note: If-Modified-Since times are interpreted by the server, whose |
| clock might not be synchronized with the client. |  | clock might not be synchronized with the client. |
| Note: When handling an If-Modified-Since header field, some |  | Note: When handling an If-Modified-Since header field, some |
| servers will use an exact date comparison function, rather than a |  | servers will use an exact date comparison function, rather than a |
| less-than function, for deciding whether to send a 304 (Not |  | less-than function, for deciding whether to send a 304 (Not |
| Modified) response. To get best results when sending an If- |  | Modified) response. To get best results when sending an If- |
| Modified-Since header field for cache validation, clients are |  | Modified-Since header field for cache validation, clients are |
| advised to use the exact date string received in a previous Last- |  | advised to use the exact date string received in a previous Last- |
| skipping to change at page 135, line 5 |  | skipping to change at page 13, line 17 |
| possibility of clock-skew-related problems if the If-Modified- |  | possibility of clock-skew-related problems if the If-Modified- |
| Since date is derived from the client's clock without correction |  | Since date is derived from the client's clock without correction |
| to the server's clock. Corrections for different time bases |  | to the server's clock. Corrections for different time bases |
| between client and server are at best approximate due to network |  | between client and server are at best approximate due to network |
| latency. |  | latency. |
| The result of a request having both an If-Modified-Since header field |  | The result of a request having both an If-Modified-Since header field |
| and either an If-Match or an If-Unmodified-Since header fields is |  | and either an If-Match or an If-Unmodified-Since header fields is |
| undefined by this specification. |  | undefined by this specification. |
| 14.26. If-None-Match |  | 6.4. If-None-Match |
| The If-None-Match request-header field is used with a method to make |  | The If-None-Match request-header field is used with a method to make |
| it conditional. A client that has one or more entities previously |  | it conditional. A client that has one or more entities previously |
| obtained from the resource can verify that none of those entities is |  | obtained from the resource can verify that none of those entities is |
| current by including a list of their associated entity tags in the |  | current by including a list of their associated entity tags in the |
| If-None-Match header field. The purpose of this feature is to allow |  | If-None-Match header field. The purpose of this feature is to allow |
| efficient updates of cached information with a minimum amount of |  | efficient updates of cached information with a minimum amount of |
| transaction overhead. It is also used to prevent a method (e.g. |  | transaction overhead. It is also used to prevent a method (e.g. |
| PUT) from inadvertently modifying an existing resource when the |  | PUT) from inadvertently modifying an existing resource when the |
| client believes that the resource does not exist. |  | client believes that the resource does not exist. |
| skipping to change at page 135, line 35 |  | skipping to change at page 13, line 47 |
| given and any current entity exists for that resource, then the |  | given and any current entity exists for that resource, then the |
| server MUST NOT perform the requested method, unless required to do |  | server MUST NOT perform the requested method, unless required to do |
| so because the resource's modification date fails to match that |  | so because the resource's modification date fails to match that |
| supplied in an If-Modified-Since header field in the request. |  | supplied in an If-Modified-Since header field in the request. |
| Instead, if the request method was GET or HEAD, the server SHOULD |  | Instead, if the request method was GET or HEAD, the server SHOULD |
| respond with a 304 (Not Modified) response, including the cache- |  | respond with a 304 (Not Modified) response, including the cache- |
| related header fields (particularly ETag) of one of the entities that |  | related header fields (particularly ETag) of one of the entities that |
| matched. For all other request methods, the server MUST respond with |  | matched. For all other request methods, the server MUST respond with |
| a status of 412 (Precondition Failed). |  | a status of 412 (Precondition Failed). |
| See Section 13.3.3 for rules on how to determine if two entities tags |  | See Section 4 for rules on how to determine if two entities tags |
| match. The weak comparison function can only be used with GET or |  | match. The weak comparison function can only be used with GET or |
| HEAD requests. |  | HEAD requests. |
| If none of the entity tags match, then the server MAY perform the |  | If none of the entity tags match, then the server MAY perform the |
| requested method as if the If-None-Match header field did not exist, |  | requested method as if the If-None-Match header field did not exist, |
| but MUST also ignore any If-Modified-Since header field(s) in the |  | but MUST also ignore any If-Modified-Since header field(s) in the |
| request. That is, if no entity tags match, then the server MUST NOT |  | request. That is, if no entity tags match, then the server MUST NOT |
| return a 304 (Not Modified) response. |  | return a 304 (Not Modified) response. |
| If the request would, without the If-None-Match header field, result |  | If the request would, without the If-None-Match header field, result |
| in anything other than a 2xx or 304 status, then the If-None-Match |  | in anything other than a 2xx or 304 status, then the If-None-Match |
| header MUST be ignored. (See Section 13.3.4 for a discussion of |  | header MUST be ignored. (See Section 5 for a discussion of server |
| server behavior when both If-Modified-Since and If-None-Match appear |  | behavior when both If-Modified-Since and If-None-Match appear in the |
| in the same request.) |  | same request.) |
| The meaning of "If-None-Match: *" is that the method MUST NOT be |  | The meaning of "If-None-Match: *" is that the method MUST NOT be |
| performed if the representation selected by the origin server (or by |  | performed if the representation selected by the origin server (or by |
| a cache, possibly using the Vary mechanism, see Section 14.44) |  | a cache, possibly using the Vary mechanism, see Section 3.5 of |
| exists, and SHOULD be performed if the representation does not exist. |  | [Part6]) exists, and SHOULD be performed if the representation does |
| This feature is intended to be useful in preventing races between PUT |  | not exist. This feature is intended to be useful in preventing races |
| operations. |  | between PUT operations. |
| Examples: |  | Examples: |
| If-None-Match: "xyzzy" |  | If-None-Match: "xyzzy" |
| If-None-Match: W/"xyzzy" |  | If-None-Match: W/"xyzzy" |
| If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |  | If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |
| If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" |  | If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" |
| If-None-Match: * |  | If-None-Match: * |
| The result of a request having both an If-None-Match header field and |  | The result of a request having both an If-None-Match header field and |
| either an If-Match or an If-Unmodified-Since header fields is |  | either an If-Match or an If-Unmodified-Since header fields is |
| undefined by this specification. |  | undefined by this specification. |
| 14.27. If-Range |  | 6.5. If-Unmodified-Since |
| If a client has a partial copy of an entity in its cache, and wishes |  |  |
| to have an up-to-date copy of the entire entity in its cache, it |  |  |
| could use the Range request-header with a conditional GET (using |  |  |
| either or both of If-Unmodified-Since and If-Match.) However, if the |  |  |
| condition fails because the entity has been modified, the client |  |  |
| would then have to make a second request to obtain the entire current |  |  |
| entity-body. |  |  |
| The If-Range header allows a client to "short-circuit" the second |  |  |
| request. Informally, its meaning is `if the entity is unchanged, |  |  |
| send me the part(s) that I am missing; otherwise, send me the entire |  |  |
| new entity'. |  |  |
| If-Range = "If-Range" ":" ( entity-tag | HTTP-date ) |  |  |
| If the client has no entity tag for an entity, but does have a Last- |  |  |
| Modified date, it MAY use that date in an If-Range header. (The |  |  |
| server can distinguish between a valid HTTP-date and any form of |  |  |
| entity-tag by examining no more than two characters.) The If-Range |  |  |
| header SHOULD only be used together with a Range header, and MUST be |  |  |
| ignored if the request does not include a Range header, or if the |  |  |
| server does not support the sub-range operation. |  |  |
| If the entity tag given in the If-Range header matches the current |  |  |
| entity tag for the entity, then the server SHOULD provide the |  |  |
| specified sub-range of the entity using a 206 (Partial content) |  |  |
| response. If the entity tag does not match, then the server SHOULD |  |  |
| return the entire entity using a 200 (OK) response. |  |  |
| 14.28. If-Unmodified-Since |  |  |
| The If-Unmodified-Since request-header field is used with a method to |  | The If-Unmodified-Since request-header field is used with a method to |
| make it conditional. If the requested resource has not been modified |  | make it conditional. If the requested resource has not been modified |
| since the time specified in this field, the server SHOULD perform the |  | since the time specified in this field, the server SHOULD perform the |
| requested operation as if the If-Unmodified-Since header were not |  | requested operation as if the If-Unmodified-Since header were not |
| present. |  | present. |
| If the requested variant has been modified since the specified time, |  | If the requested variant has been modified since the specified time, |
| the server MUST NOT perform the requested operation, and MUST return |  | the server MUST NOT perform the requested operation, and MUST return |
| a 412 (Precondition Failed). |  | a 412 (Precondition Failed). |
| skipping to change at page 137, line 33 |  | skipping to change at page 15, line 14 |
| If the request normally (i.e., without the If-Unmodified-Since |  | If the request normally (i.e., without the If-Unmodified-Since |
| header) would result in anything other than a 2xx or 412 status, the |  | header) would result in anything other than a 2xx or 412 status, the |
| If-Unmodified-Since header SHOULD be ignored. |  | If-Unmodified-Since header SHOULD be ignored. |
| If the specified date is invalid, the header is ignored. |  | If the specified date is invalid, the header is ignored. |
| The result of a request having both an If-Unmodified-Since header |  | The result of a request having both an If-Unmodified-Since header |
| field and either an If-None-Match or an If-Modified-Since header |  | field and either an If-None-Match or an If-Modified-Since header |
| fields is undefined by this specification. |  | fields is undefined by this specification. |
| 14.29. Last-Modified |  | 6.6. Last-Modified |
| The Last-Modified entity-header field indicates the date and time at |  | The Last-Modified entity-header field indicates the date and time at |
| which the origin server believes the variant was last modified. |  | which the origin server believes the variant was last modified. |
| Last-Modified = "Last-Modified" ":" HTTP-date |  | Last-Modified = "Last-Modified" ":" HTTP-date |
| An example of its use is |  | An example of its use is |
| Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT |  | Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT |
| skipping to change at page 138, line 19 |  | skipping to change at page 15, line 48 |
| origination date. |  | origination date. |
| An origin server SHOULD obtain the Last-Modified value of the entity |  | An origin server SHOULD obtain the Last-Modified value of the entity |
| as close as possible to the time that it generates the Date value of |  | as close as possible to the time that it generates the Date value of |
| its response. This allows a recipient to make an accurate assessment |  | its response. This allows a recipient to make an accurate assessment |
| of the entity's modification time, especially if the entity changes |  | of the entity's modification time, especially if the entity changes |
| near the time that the response is generated. |  | near the time that the response is generated. |
| HTTP/1.1 servers SHOULD send Last-Modified whenever feasible. |  | HTTP/1.1 servers SHOULD send Last-Modified whenever feasible. |
| 14.30. Location |  | 7. IANA Considerations |
| The Location response-header field is used to redirect the recipient |  |  |
| to a location other than the Request-URI for completion of the |  |  |
| request or identification of a new resource. For 201 (Created) |  |  |
| responses, the Location is that of the new resource which was created |  |  |
| by the request. For 3xx responses, the location SHOULD indicate the |  |  |
| server's preferred URI for automatic redirection to the resource. |  |  |
| The field value consists of a single absolute URI. |  |  |
| Location = "Location" ":" absoluteURI |  |  |
| An example is: |  |  |
| Location: http://www.w3.org/pub/WWW/People.html |  |  |
| Note: The Content-Location header field (Section 14.14) differs |  |  |
| from Location in that the Content-Location identifies the original |  |  |
| location of the entity enclosed in the request. It is therefore |  |  |
| possible for a response to contain header fields for both Location |  |  |
| and Content-Location. Also see Section 13.10 for cache |  |  |
| requirements of some methods. |  |  |
| 14.31. Max-Forwards |  |  |
| The Max-Forwards request-header field provides a mechanism with the |  |  |
| TRACE (Section 9.8) and OPTIONS (Section 9.2) methods to limit the |  |  |
| number of proxies or gateways that can forward the request to the |  |  |
| next inbound server. This can be useful when the client is |  |  |
| attempting to trace a request chain which appears to be failing or |  |  |
| looping in mid-chain. |  |  |
| Max-Forwards = "Max-Forwards" ":" 1*DIGIT |  |  |
| The Max-Forwards value is a decimal integer indicating the remaining |  |  |
| number of times this request message may be forwarded. |  |  |
| Each proxy or gateway recipient of a TRACE or OPTIONS request |  |  |
| containing a Max-Forwards header field MUST check and update its |  |  |
| value prior to forwarding the request. If the received value is zero |  |  |
| (0), the recipient MUST NOT forward the request; instead, it MUST |  |  |
| respond as the final recipient. If the received Max-Forwards value |  |  |
| is greater than zero, then the forwarded message MUST contain an |  |  |
| updated Max-Forwards field with a value decremented by one (1). |  |  |
| The Max-Forwards header field MAY be ignored for all other methods |  |  |
| defined by this specification and for any extension methods for which |  |  |
| it is not explicitly referred to as part of that method definition. |  |  |
| 14.32. Pragma |  |  |
| The Pragma general-header field is used to include implementation- |  |  |
| specific directives that might apply to any recipient along the |  |  |
| request/response chain. All pragma directives specify optional |  |  |
| behavior from the viewpoint of the protocol; however, some systems |  |  |
| MAY require that behavior be consistent with the directives. |  |  |
| Pragma = "Pragma" ":" 1#pragma-directive |  |  |
| pragma-directive = "no-cache" | extension-pragma |  |  |
| extension-pragma = token [ "=" ( token | quoted-string ) ] |  |  |
| When the no-cache directive is present in a request message, an |  |  |
| application SHOULD forward the request toward the origin server even |  |  |
| if it has a cached copy of what is being requested. This pragma |  |  |
| directive has the same semantics as the no-cache cache-directive (see |  |  |
| Section 14.9) and is defined here for backward compatibility with |  |  |
| HTTP/1.0. Clients SHOULD include both header fields when a no-cache |  |  |
| request is sent to a server not known to be HTTP/1.1 compliant. |  |  |
| Pragma directives MUST be passed through by a proxy or gateway |  |  |
| application, regardless of their significance to that application, |  |  |
| since the directives might be applicable to all recipients along the |  |  |
| request/response chain. It is not possible to specify a pragma for a |  |  |
| specific recipient; however, any pragma directive not relevant to a |  |  |
| recipient SHOULD be ignored by that recipient. |  |  |
| HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had |  |  |
| sent "Cache-Control: no-cache". No new Pragma directives will be |  |  |
| defined in HTTP. |  |  |
| Note: because the meaning of "Pragma: no-cache as a response |  |  |
| header field is not actually specified, it does not provide a |  |  |
| reliable replacement for "Cache-Control: no-cache" in a response |  |  |
| 14.33. Proxy-Authenticate |  |  |
| The Proxy-Authenticate response-header field MUST be included as part |  |  |
| of a 407 (Proxy Authentication Required) response. The field value |  |  |
| consists of a challenge that indicates the authentication scheme and |  |  |
| parameters applicable to the proxy for this Request-URI. |  |  |
| Proxy-Authenticate = "Proxy-Authenticate" ":" 1#challenge |  |  |
| The HTTP access authentication process is described in "HTTP |  |  |
| Authentication: Basic and Digest Access Authentication" [RFC2617]. |  |  |
| Unlike WWW-Authenticate, the Proxy-Authenticate header field applies |  |  |
| only to the current connection and SHOULD NOT be passed on to |  |  |
| downstream clients. However, an intermediate proxy might need to |  |  |
| obtain its own credentials by requesting them from the downstream |  |  |
| client, which in some circumstances will appear as if the proxy is |  |  |
| forwarding the Proxy-Authenticate header field. |  |  |
| 14.34. Proxy-Authorization |  |  |
| The Proxy-Authorization request-header field allows the client to |  |  |
| identify itself (or its user) to a proxy which requires |  |  |
| authentication. The Proxy-Authorization field value consists of |  |  |
| credentials containing the authentication information of the user |  |  |
| agent for the proxy and/or realm of the resource being requested. |  |  |
| Proxy-Authorization = "Proxy-Authorization" ":" credentials |  |  |
| The HTTP access authentication process is described in "HTTP |  |  |
| Authentication: Basic and Digest Access Authentication" [RFC2617]. |  |  |
| Unlike Authorization, the Proxy-Authorization header field applies |  |  |
| only to the next outbound proxy that demanded authentication using |  |  |
| the Proxy-Authenticate field. When multiple proxies are used in a |  |  |
| chain, the Proxy-Authorization header field is consumed by the first |  |  |
| outbound proxy that was expecting to receive credentials. A proxy |  |  |
| MAY relay the credentials from the client request to the next proxy |  |  |
| if that is the mechanism by which the proxies cooperatively |  |  |
| authenticate a given request. |  |  |
| 14.35. Range |  |  |
| 14.35.1. Byte Ranges |  |  |
| Since all HTTP entities are represented in HTTP messages as sequences |  |  |
| of bytes, the concept of a byte range is meaningful for any HTTP |  |  |
| entity. (However, not all clients and servers need to support byte- |  |  |
| range operations.) |  |  |
| Byte range specifications in HTTP apply to the sequence of bytes in |  |  |
| the entity-body (not necessarily the same as the message-body). |  |  |
| A byte range operation MAY specify a single range of bytes, or a set |  |  |
| of ranges within a single entity. |  |  |
| ranges-specifier = byte-ranges-specifier |  |  |
| byte-ranges-specifier = bytes-unit "=" byte-range-set |  |  |
| byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec ) |  |  |
| byte-range-spec = first-byte-pos "-" [last-byte-pos] |  |  |
| first-byte-pos = 1*DIGIT |  |  |
| last-byte-pos = 1*DIGIT |  |  |
| The first-byte-pos value in a byte-range-spec gives the byte-offset |  |  |
| of the first byte in a range. The last-byte-pos value gives the |  |  |
| byte-offset of the last byte in the range; that is, the byte |  |  |
| positions specified are inclusive. Byte offsets start at zero. |  |  |
| If the last-byte-pos value is present, it MUST be greater than or |  |  |
| equal to the first-byte-pos in that byte-range-spec, or the byte- |  |  |
| range-spec is syntactically invalid. The recipient of a byte-range- |  |  |
| set that includes one or more syntactically invalid byte-range-spec |  |  |
| values MUST ignore the header field that includes that byte-range- |  |  |
| set. |  |  |
| If the last-byte-pos value is absent, or if the value is greater than |  |  |
| or equal to the current length of the entity-body, last-byte-pos is |  |  |
| taken to be equal to one less than the current length of the entity- |  |  |
| body in bytes. |  |  |
| By its choice of last-byte-pos, a client can limit the number of |  |  |
| bytes retrieved without knowing the size of the entity. |  |  |
| suffix-byte-range-spec = "-" suffix-length |  |  |
| suffix-length = 1*DIGIT |  |  |
| A suffix-byte-range-spec is used to specify the suffix of the entity- |  |  |
| body, of a length given by the suffix-length value. (That is, this |  |  |
| form specifies the last N bytes of an entity-body.) If the entity is |  |  |
| shorter than the specified suffix-length, the entire entity-body is |  |  |
| used. |  |  |
| If a syntactically valid byte-range-set includes at least one byte- |  |  |
| range-spec whose first-byte-pos is less than the current length of |  |  |
| the entity-body, or at least one suffix-byte-range-spec with a non- |  |  |
| zero suffix-length, then the byte-range-set is satisfiable. |  |  |
| Otherwise, the byte-range-set is unsatisfiable. If the byte-range- |  |  |
| set is unsatisfiable, the server SHOULD return a response with a |  |  |
| status of 416 (Requested range not satisfiable). Otherwise, the |  |  |
| server SHOULD return a response with a status of 206 (Partial |  |  |
| Content) containing the satisfiable ranges of the entity-body. |  |  |
| Examples of byte-ranges-specifier values (assuming an entity-body of |  |  |
| length 10000): |  |  |
| o The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-499 |  |  |
| o The second 500 bytes (byte offsets 500-999, inclusive): bytes=500- |  |  |
| 999 |  |  |
| o The final 500 bytes (byte offsets 9500-9999, inclusive): bytes=- |  |  |
| 500 |  |  |
| o Or bytes=9500- |  |  |
| o The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1 |  |  |
| o Several legal but not canonical specifications of the second 500 |  |  |
| bytes (byte offsets 500-999, inclusive): |  |  |
| bytes=500-600,601-999 |  |  |
| bytes=500-700,601-999 |  |  |
| 14.35.2. Range Retrieval Requests |  |  |
| HTTP retrieval requests using conditional or unconditional GET |  |  |
| methods MAY request one or more sub-ranges of the entity, instead of |  |  |
| the entire entity, using the Range request header, which applies to |  |  |
| the entity returned as the result of the request: |  |  |
| Range = "Range" ":" ranges-specifier |  |  |
| A server MAY ignore the Range header. However, HTTP/1.1 origin |  |  |
| servers and intermediate caches ought to support byte ranges when |  |  |
| possible, since Range supports efficient recovery from partially |  |  |
| failed transfers, and supports efficient partial retrieval of large |  |  |
| entities. |  |  |
| If the server supports the Range header and the specified range or |  |  |
| ranges are appropriate for the entity: |  |  |
| o The presence of a Range header in an unconditional GET modifies |  |  |
| what is returned if the GET is otherwise successful. In other |  |  |
| words, the response carries a status code of 206 (Partial Content) |  |  |
| instead of 200 (OK). |  |  |
| o The presence of a Range header in a conditional GET (a request |  |  |
| using one or both of If-Modified-Since and If-None-Match, or one |  |  |
| or both of If-Unmodified-Since and If-Match) modifies what is |  |  |
| returned if the GET is otherwise successful and the condition is |  |  |
| true. It does not affect the 304 (Not Modified) response returned |  |  |
| if the conditional is false. |  |  |
| In some cases, it might be more appropriate to use the If-Range |  |  |
| header (see Section 14.27) in addition to the Range header. |  |  |
| If a proxy that supports ranges receives a Range request, forwards |  |  |
| the request to an inbound server, and receives an entire entity in |  |  |
| reply, it SHOULD only return the requested range to its client. It |  |  |
| SHOULD store the entire received response in its cache if that is |  |  |
| consistent with its cache allocation policies. |  |  |
| 14.36. Referer |  |  |
| The Referer[sic] request-header field allows the client to specify, |  |  |
| for the server's benefit, the address (URI) of the resource from |  |  |
| which the Request-URI was obtained (the "referrer", although the |  |  |
| header field is misspelled.) The Referer request-header allows a |  |  |
| server to generate lists of back-links to resources for interest, |  |  |
| logging, optimized caching, etc. It also allows obsolete or mistyped |  |  |
| links to be traced for maintenance. The Referer field MUST NOT be |  |  |
| sent if the Request-URI was obtained from a source that does not have |  |  |
| its own URI, such as input from the user keyboard. |  |  |
| Referer = "Referer" ":" ( absoluteURI | relativeURI ) |  |  |
| Example: |  |  |
| Referer: http://www.w3.org/hypertext/DataSources/Overview.html |  |  |
| If the field value is a relative URI, it SHOULD be interpreted |  |  |
| relative to the Request-URI. The URI MUST NOT include a fragment. |  |  |
| See Section 15.1.3 for security considerations. |  |  |
| 14.37. Retry-After |  |  |
| The Retry-After response-header field can be used with a 503 (Service |  |  |
| Unavailable) response to indicate how long the service is expected to |  |  |
| be unavailable to the requesting client. This field MAY also be used |  |  |
| with any 3xx (Redirection) response to indicate the minimum time the |  |  |
| user-agent is asked wait before issuing the redirected request. The |  |  |
| value of this field can be either an HTTP-date or an integer number |  |  |
| of seconds (in decimal) after the time of the response. |  |  |
| Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds ) |  |  |
| Two examples of its use are |  |  |
| Retry-After: Fri, 31 Dec 1999 23:59:59 GMT |  |  |
| Retry-After: 120 |  |  |
| In the latter example, the delay is 2 minutes. |  |  |
| 14.38. Server |  |  |
| The Server response-header field contains information about the |  |  |
| software used by the origin server to handle the request. The field |  |  |
| can contain multiple product tokens (Section 3.8) and comments |  |  |
| identifying the server and any significant subproducts. The product |  |  |
| tokens are listed in order of their significance for identifying the |  |  |
| application. |  |  |
| Server = "Server" ":" 1*( product | comment ) |  |  |
| Example: |  |  |
| Server: CERN/3.0 libwww/2.17 |  |  |
| If the response is being forwarded through a proxy, the proxy |  |  |
| application MUST NOT modify the Server response-header. Instead, it |  |  |
| SHOULD include a Via field (as described in Section 14.45). |  |  |
| Note: Revealing the specific software version of the server might |  |  |
| allow the server machine to become more vulnerable to attacks |  |  |
| against software that is known to contain security holes. Server |  |  |
| implementors are encouraged to make this field a configurable |  |  |
| option. |  |  |
| 14.39. TE |  |  |
| The TE request-header field indicates what extension transfer-codings |  |  |
| it is willing to accept in the response and whether or not it is |  |  |
| willing to accept trailer fields in a chunked transfer-coding. Its |  |  |
| value may consist of the keyword "trailers" and/or a comma-separated |  |  |
| list of extension transfer-coding names with optional accept |  |  |
| parameters (as described in Section 3.6). |  |  |
| TE = "TE" ":" #( t-codings ) |  |  |
| t-codings = "trailers" | ( transfer-extension [ accept-params ] ) |  |  |
| The presence of the keyword "trailers" indicates that the client is |  |  |
| willing to accept trailer fields in a chunked transfer-coding, as |  |  |
| defined in Section 3.6.1. This keyword is reserved for use with |  |  |
| transfer-coding values even though it does not itself represent a |  |  |
| transfer-coding. |  |  |
| Examples of its use are: |  |  |
| TE: deflate |  |  |
| TE: |  |  |
| TE: trailers, deflate;q=0.5 |  |  |
| The TE header field only applies to the immediate connection. |  |  |
| Therefore, the keyword MUST be supplied within a Connection header |  |  |
| field (Section 14.10) whenever TE is present in an HTTP/1.1 message. |  |  |
| A server tests whether a transfer-coding is acceptable, according to |  |  |
| a TE field, using these rules: |  |  |
| 1. The "chunked" transfer-coding is always acceptable. If the |  |  |
| keyword "trailers" is listed, the client indicates that it is |  |  |
| willing to accept trailer fields in the chunked response on |  |  |
| behalf of itself and any downstream clients. The implication is |  |  |
| that, if given, the client is stating that either all downstream |  |  |
| clients are willing to accept trailer fields in the forwarded |  |  |
| response, or that it will attempt to buffer the response on |  |  |
| behalf of downstream recipients. |  |  |
| Note: HTTP/1.1 does not define any means to limit the size of a |  |  |
| chunked response such that a client can be assured of buffering |  |  |
| the entire response. |  |  |
| 2. If the transfer-coding being tested is one of the transfer- |  |  |
| codings listed in the TE field, then it is acceptable unless it |  |  |
| is accompanied by a qvalue of 0. (As defined in Section 3.9, a |  |  |
| qvalue of 0 means "not acceptable.") |  |  |
| 3. If multiple transfer-codings are acceptable, then the acceptable |  |  |
| transfer-coding with the highest non-zero qvalue is preferred. |  |  |
| The "chunked" transfer-coding always has a qvalue of 1. |  |  |
| If the TE field-value is empty or if no TE field is present, the only |  |  |
| transfer-coding is "chunked". A message with no transfer-coding is |  |  |
| always acceptable. |  |  |
| 14.40. Trailer |  |  |
| The Trailer general field value indicates that the given set of |  |  |
| header fields is present in the trailer of a message encoded with |  |  |
| chunked transfer-coding. |  |  |
| Trailer = "Trailer" ":" 1#field-name |  |  |
| An HTTP/1.1 message SHOULD include a Trailer header field in a |  |  |
| message using chunked transfer-coding with a non-empty trailer. |  |  |
| Doing so allows the recipient to know which header fields to expect |  |  |
| in the trailer. |  |  |
| If no Trailer header field is present, the trailer SHOULD NOT include |  |  |
| any header fields. See Section 3.6.1 for restrictions on the use of |  |  |
| trailer fields in a "chunked" transfer-coding. |  |  |
| Message header fields listed in the Trailer header field MUST NOT |  |  |
| include the following header fields: |  |  |
| o Transfer-Encoding |  |  |
| o Content-Length |  |  |
| o Trailer |  |  |
| 14.41. Transfer-Encoding |  |  |
| The Transfer-Encoding general-header field indicates what (if any) |  |  |
| type of transformation has been applied to the message body in order |  |  |
| to safely transfer it between the sender and the recipient. This |  |  |
| differs from the content-coding in that the transfer-coding is a |  |  |
| property of the message, not of the entity. |  |  |
| Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-coding |  |  |
| Transfer-codings are defined in Section 3.6. An example is: |  |  |
| Transfer-Encoding: chunked |  |  |
| If multiple encodings have been applied to an entity, the transfer- |  |  |
| codings MUST be listed in the order in which they were applied. |  |  |
| Additional information about the encoding parameters MAY be provided |  |  |
| by other entity-header fields not defined by this specification. |  |  |
| Many older HTTP/1.0 applications do not understand the Transfer- |  |  |
| Encoding header. |  |  |
| 14.42. Upgrade |  |  |
| The Upgrade general-header allows the client to specify what |  |  |
| additional communication protocols it supports and would like to use |  |  |
| if the server finds it appropriate to switch protocols. The server |  |  |
| MUST use the Upgrade header field within a 101 (Switching Protocols) |  |  |
| response to indicate which protocol(s) are being switched. |  |  |
| Upgrade = "Upgrade" ":" 1#product |  |  |
| For example, |  |  |
| Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11 |  |  |
| The Upgrade header field is intended to provide a simple mechanism |  |  |
| for transition from HTTP/1.1 to some other, incompatible protocol. |  |  |
| It does so by allowing the client to advertise its desire to use |  |  |
| another protocol, such as a later version of HTTP with a higher major |  |  |
| version number, even though the current request has been made using |  |  |
| HTTP/1.1. This eases the difficult transition between incompatible |  |  |
| protocols by allowing the client to initiate a request in the more |  |  |
| commonly supported protocol while indicating to the server that it |  |  |
| would like to use a "better" protocol if available (where "better" is |  |  |
| determined by the server, possibly according to the nature of the |  |  |
| method and/or resource being requested). |  |  |
| The Upgrade header field only applies to switching application-layer |  |  |
| protocols upon the existing transport-layer connection. Upgrade |  |  |
| cannot be used to insist on a protocol change; its acceptance and use |  |  |
| by the server is optional. The capabilities and nature of the |  |  |
| application-layer communication after the protocol change is entirely |  |  |
| dependent upon the new protocol chosen, although the first action |  |  |
| after changing the protocol MUST be a response to the initial HTTP |  |  |
| request containing the Upgrade header field. |  |  |
| The Upgrade header field only applies to the immediate connection. |  |  |
| Therefore, the upgrade keyword MUST be supplied within a Connection |  |  |
| header field (Section 14.10) whenever Upgrade is present in an |  |  |
| HTTP/1.1 message. |  |  |
| The Upgrade header field cannot be used to indicate a switch to a |  |  |
| protocol on a different connection. For that purpose, it is more |  |  |
| appropriate to use a 301, 302, 303, or 305 redirection response. |  |  |
| This specification only defines the protocol name "HTTP" for use by |  |  |
| the family of Hypertext Transfer Protocols, as defined by the HTTP |  |  |
| version rules of Section 3.1 and future updates to this |  |  |
| specification. Any token can be used as a protocol name; however, it |  |  |
| will only be useful if both the client and server associate the name |  |  |
| with the same protocol. |  |  |
| 14.43. User-Agent |  |  |
| The User-Agent request-header field contains information about the |  |  |
| user agent originating the request. This is for statistical |  |  |
| purposes, the tracing of protocol violations, and automated |  |  |
| recognition of user agents for the sake of tailoring responses to |  |  |
| avoid particular user agent limitations. User agents SHOULD include |  |  |
| this field with requests. The field can contain multiple product |  |  |
| tokens (Section 3.8) and comments identifying the agent and any |  |  |
| subproducts which form a significant part of the user agent. By |  |  |
| convention, the product tokens are listed in order of their |  |  |
| significance for identifying the application. |  |  |
| User-Agent = "User-Agent" ":" 1*( product | comment ) |  |  |
| Example: |  |  |
| User-Agent: CERN-LineMode/2.15 libwww/2.17b3 |  |  |
| 14.44. Vary |  |  |
| The Vary field value indicates the set of request-header fields that |  |  |
| fully determines, while the response is fresh, whether a cache is |  |  |
| permitted to use the response to reply to a subsequent request |  |  |
| without revalidation. For uncacheable or stale responses, the Vary |  |  |
| field value advises the user agent about the criteria that were used |  |  |
| to select the representation. A Vary field value of "*" implies that |  |  |
| a cache cannot determine from the request headers of a subsequent |  |  |
| request whether this response is the appropriate representation. See |  |  |
| Section 13.6 for use of the Vary header field by caches. |  |  |
| Vary = "Vary" ":" ( "*" | 1#field-name ) |  |  |
| An HTTP/1.1 server SHOULD include a Vary header field with any |  |  |
| cacheable response that is subject to server-driven negotiation. |  |  |
| Doing so allows a cache to properly interpret future requests on that |  |  |
| resource and informs the user agent about the presence of negotiation |  |  |
| on that resource. A server MAY include a Vary header field with a |  |  |
| non-cacheable response that is subject to server-driven negotiation, |  |  |
| since this might provide the user agent with useful information about |  |  |
| the dimensions over which the response varies at the time of the |  |  |
| response. |  |  |
| A Vary field value consisting of a list of field-names signals that |  |  |
| the representation selected for the response is based on a selection |  |  |
| algorithm which considers ONLY the listed request-header field values |  |  |
| in selecting the most appropriate representation. A cache MAY assume |  |  |
| that the same selection will be made for future requests with the |  |  |
| same values for the listed field names, for the duration of time for |  |  |
| which the response is fresh. |  |  |
| The field-names given are not limited to the set of standard request- |  |  |
| header fields defined by this specification. Field names are case- |  |  |
| insensitive. |  |  |
| A Vary field value of "*" signals that unspecified parameters not |  |  |
| limited to the request-headers (e.g., the network address of the |  |  |
| client), play a role in the selection of the response representation. |  |  |
| The "*" value MUST NOT be generated by a proxy server; it may only be |  |  |
| generated by an origin server. |  |  |
| 14.45. Via |  |  |
| The Via general-header field MUST be used by gateways and proxies to |  |  |
| indicate the intermediate protocols and recipients between the user |  |  |
| agent and the server on requests, and between the origin server and |  |  |
| the client on responses. It is analogous to the "Received" field of |  |  |
| RFC 822 [RFC822] and is intended to be used for tracking message |  |  |
| forwards, avoiding request loops, and identifying the protocol |  |  |
| capabilities of all senders along the request/response chain. |  |  |
| Via = "Via" ":" 1#( received-protocol received-by [ comment ] ) |  |  |
| received-protocol = [ protocol-name "/" ] protocol-version |  |  |
| protocol-name = token |  |  |
| protocol-version = token |  |  |
| received-by = ( host [ ":" port ] ) | pseudonym |  |  |
| pseudonym = token |  |  |
| The received-protocol indicates the protocol version of the message |  |  |
| received by the server or client along each segment of the request/ |  |  |
| response chain. The received-protocol version is appended to the Via |  |  |
| field value when the message is forwarded so that information about |  |  |
| the protocol capabilities of upstream applications remains visible to |  |  |
| all recipients. |  |  |
| The protocol-name is optional if and only if it would be "HTTP". The |  |  |
| received-by field is normally the host and optional port number of a |  |  |
| recipient server or client that subsequently forwarded the message. |  |  |
| However, if the real host is considered to be sensitive information, |  |  |
| it MAY be replaced by a pseudonym. If the port is not given, it MAY |  |  |
| be assumed to be the default port of the received-protocol. |  |  |
| Multiple Via field values represents each proxy or gateway that has |  |  |
| forwarded the message. Each recipient MUST append its information |  |  |
| such that the end result is ordered according to the sequence of |  |  |
| forwarding applications. |  |  |
| Comments MAY be used in the Via header field to identify the software |  |  |
| of the recipient proxy or gateway, analogous to the User-Agent and |  |  |
| Server header fields. However, all comments in the Via field are |  |  |
| optional and MAY be removed by any recipient prior to forwarding the |  |  |
| message. |  |  |
| For example, a request message could be sent from an HTTP/1.0 user |  |  |
| agent to an internal proxy code-named "fred", which uses HTTP/1.1 to |  |  |
| forward the request to a public proxy at nowhere.com, which completes |  |  |
| the request by forwarding it to the origin server at www.ics.uci.edu. |  |  |
| The request received by www.ics.uci.edu would then have the following |  |  |
| Via header field: |  |  |
| Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1) |  |  |
| Proxies and gateways used as a portal through a network firewall |  |  |
| SHOULD NOT, by default, forward the names and ports of hosts within |  |  |
| the firewall region. This information SHOULD only be propagated if |  |  |
| explicitly enabled. If not enabled, the received-by host of any host |  |  |
| behind the firewall SHOULD be replaced by an appropriate pseudonym |  |  |
| for that host. |  |  |
| For organizations that have strong privacy requirements for hiding |  |  |
| internal structures, a proxy MAY combine an ordered subsequence of |  |  |
| Via header field entries with identical received-protocol values into |  |  |
| a single such entry. For example, |  |  |
| Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy |  |  |
| could be collapsed to |  |  |
| Via: 1.0 ricky, 1.1 mertz, 1.0 lucy |  |  |
| Applications SHOULD NOT combine multiple entries unless they are all |  |  |
| under the same organizational control and the hosts have already been |  |  |
| replaced by pseudonyms. Applications MUST NOT combine entries which |  |  |
| have different received-protocol values. |  |  |
| 14.46. Warning |  |  |
| The Warning general-header field is used to carry additional |  |  |
| information about the status or transformation of a message which |  |  |
| might not be reflected in the message. This information is typically |  |  |
| used to warn about a possible lack of semantic transparency from |  |  |
| caching operations or transformations applied to the entity body of |  |  |
| the message. |  |  |
| Warning headers are sent with responses using: |  |  |
| Warning = "Warning" ":" 1#warning-value |  |  |
| warning-value = warn-code SP warn-agent SP warn-text |  |  |
| [SP warn-date] |  |  |
| warn-code = 3DIGIT |  |  |
| warn-agent = ( host [ ":" port ] ) | pseudonym |  |  |
| ; the name or pseudonym of the server adding |  |  |
| ; the Warning header, for use in debugging |  |  |
| warn-text = quoted-string |  |  |
| warn-date = <"> HTTP-date <"> |  |  |
| A response MAY carry more than one Warning header. |  |  |
| The warn-text SHOULD be in a natural language and character set that |  |  |
| is most likely to be intelligible to the human user receiving the |  |  |
| response. This decision MAY be based on any available knowledge, |  |  |
| such as the location of the cache or user, the Accept-Language field |  |  |
| in a request, the Content-Language field in a response, etc. The |  |  |
| default language is English and the default character set is ISO- |  |  |
| 8859-1. |  |  |
| If a character set other than ISO-8859-1 is used, it MUST be encoded |  |  |
| in the warn-text using the method described in RFC 2047 [RFC2047]. |  |  |
| Warning headers can in general be applied to any message, however |  |  |
| some specific warn-codes are specific to caches and can only be |  |  |
| applied to response messages. New Warning headers SHOULD be added |  |  |
| after any existing Warning headers. A cache MUST NOT delete any |  |  |
| Warning header that it received with a message. However, if a cache |  |  |
| successfully validates a cache entry, it SHOULD remove any Warning |  |  |
| headers previously attached to that entry except as specified for |  |  |
| specific Warning codes. It MUST then add any Warning headers |  |  |
| received in the validating response. In other words, Warning headers |  |  |
| are those that would be attached to the most recent relevant |  |  |
| response. |  |  |
| When multiple Warning headers are attached to a response, the user |  |  |
| agent ought to inform the user of as many of them as possible, in the |  |  |
| order that they appear in the response. If it is not possible to |  |  |
| inform the user of all of the warnings, the user agent SHOULD follow |  |  |
| these heuristics: |  |  |
| o Warnings that appear early in the response take priority over |  |  |
| those appearing later in the response. |  |  |
| o Warnings in the user's preferred character set take priority over |  |  |
| warnings in other character sets but with identical warn-codes and |  |  |
| warn-agents. |  |  |
| Systems that generate multiple Warning headers SHOULD order them with |  |  |
| this user agent behavior in mind. |  |  |
| Requirements for the behavior of caches with respect to Warnings are |  |  |
| stated in Section 13.1.2. |  |  |
| This is a list of the currently-defined warn-codes, each with a |  |  |
| recommended warn-text in English, and a description of its meaning. |  |  |
| 110 Response is stale |  |  |
| MUST be included whenever the returned response is stale. |  |  |
| 111 Revalidation failed |  |  |
| MUST be included if a cache returns a stale response because an |  |  |
| attempt to revalidate the response failed, due to an inability to |  |  |
| reach the server. |  |  |
| 112 Disconnected operation |  |  |
| SHOULD be included if the cache is intentionally disconnected from |  |  |
| the rest of the network for a period of time. |  |  |
| 113 Heuristic expiration |  |  |
| MUST be included if the cache heuristically chose a freshness |  |  |
| lifetime greater than 24 hours and the response's age is greater |  |  |
| than 24 hours. |  |  |
| 199 Miscellaneous warning |  |  |
| The warning text MAY include arbitrary information to be presented |  |  |
| to a human user, or logged. A system receiving this warning MUST |  |  |
| NOT take any automated action, besides presenting the warning to |  |  |
| the user. |  |  |
| 214 Transformation applied |  |  |
| MUST be added by an intermediate cache or proxy if it applies any |  |  |
| transformation changing the content-coding (as specified in the |  |  |
| Content-Encoding header) or media-type (as specified in the |  |  |
| Content-Type header) of the response, or the entity-body of the |  |  |
| response, unless this Warning code already appears in the |  |  |
| response. |  |  |
| 299 Miscellaneous persistent warning |  |  |
| The warning text MAY include arbitrary information to be presented |  |  |
| to a human user, or logged. A system receiving this warning MUST |  |  |
| NOT take any automated action. |  |  |
| If an implementation sends a message with one or more Warning headers |  |  |
| whose version is HTTP/1.0 or lower, then the sender MUST include in |  |  |
| each warning-value a warn-date that matches the date in the response. |  |  |
| If an implementation receives a message with a warning-value that |  |  |
| includes a warn-date, and that warn-date is different from the Date |  |  |
| value in the response, then that warning-value MUST be deleted from |  |  |
| the message before storing, forwarding, or using it. (This prevents |  |  |
| bad consequences of naive caching of Warning header fields.) If all |  |  |
| of the warning-values are deleted for this reason, the Warning header |  |  |
| MUST be deleted as well. |  |  |
| 14.47. WWW-Authenticate |  |  |
| The WWW-Authenticate response-header field MUST be included in 401 |  |  |
| (Unauthorized) response messages. The field value consists of at |  |  |
| least one challenge that indicates the authentication scheme(s) and |  |  |
| parameters applicable to the Request-URI. |  |  |
| WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge |  |  |
| The HTTP access authentication process is described in "HTTP |  |  |
| Authentication: Basic and Digest Access Authentication" [RFC2617]. |  |  |
| User agents are advised to take special care in parsing the WWW- |  |  |
| Authenticate field value as it might contain more than one challenge, |  |  |
| or if more than one WWW-Authenticate header field is provided, the |  |  |
| contents of a challenge itself can contain a comma-separated list of |  |  |
| authentication parameters. |  |  |
| 15. Security Considerations |  |  |
| This section is meant to inform application developers, information |  |  |
| providers, and users of the security limitations in HTTP/1.1 as |  |  |
| described by this document. The discussion does not include |  |  |
| definitive solutions to the problems revealed, though it does make |  |  |
| some suggestions for reducing security risks. |  |  |
| 15.1. Personal Information |  |  |
| HTTP clients are often privy to large amounts of personal information |  |  |
| (e.g. the user's name, location, mail address, passwords, encryption |  |  |
| keys, etc.), and SHOULD be very careful to prevent unintentional |  |  |
| leakage of this information via the HTTP protocol to other sources. |  |  |
| We very strongly recommend that a convenient interface be provided |  |  |
| for the user to control dissemination of such information, and that |  |  |
| designers and implementors be particularly careful in this area. |  |  |
| History shows that errors in this area often create serious security |  |  |
| and/or privacy problems and generate highly adverse publicity for the |  |  |
| implementor's company. |  |  |
| 15.1.1. Abuse of Server Log Information |  |  |
| A server is in the position to save personal data about a user's |  |  |
| requests which might identify their reading patterns or subjects of |  |  |
| interest. This information is clearly confidential in nature and its |  |  |
| handling can be constrained by law in certain countries. People |  |  |
| using the HTTP protocol to provide data are responsible for ensuring |  |  |
| that such material is not distributed without the permission of any |  |  |
| individuals that are identifiable by the published results. |  |  |
| 15.1.2. Transfer of Sensitive Information |  |  |
| Like any generic data transfer protocol, HTTP cannot regulate the |  |  |
| content of the data that is transferred, nor is there any a priori |  |  |
| method of determining the sensitivity of any particular piece of |  |  |
| information within the context of any given request. Therefore, |  |  |
| applications SHOULD supply as much control over this information as |  |  |
| possible to the provider of that information. Four header fields are |  |  |
| worth special mention in this context: Server, Via, Referer and From. |  |  |
| Revealing the specific software version of the server might allow the |  |  |
| server machine to become more vulnerable to attacks against software |  |  |
| that is known to contain security holes. Implementors SHOULD make |  |  |
| the Server header field a configurable option. |  |  |
| Proxies which serve as a portal through a network firewall SHOULD |  |  |
| take special precautions regarding the transfer of header information |  |  |
| that identifies the hosts behind the firewall. In particular, they |  |  |
| SHOULD remove, or replace with sanitized versions, any Via fields |  |  |
| generated behind the firewall. |  |  |
| The Referer header allows reading patterns to be studied and reverse |  |  |
| links drawn. Although it can be very useful, its power can be abused |  |  |
| if user details are not separated from the information contained in |  |  |
| the Referer. Even when the personal information has been removed, |  |  |
| the Referer header might indicate a private document's URI whose |  |  |
| publication would be inappropriate. |  |  |
| The information sent in the From field might conflict with the user's |  |  |
| privacy interests or their site's security policy, and hence it |  |  |
| SHOULD NOT be transmitted without the user being able to disable, |  |  |
| enable, and modify the contents of the field. The user MUST be able |  |  |
| to set the contents of this field within a user preference or |  |  |
| application defaults configuration. |  |  |
| We suggest, though do not require, that a convenient toggle interface |  |  |
| be provided for the user to enable or disable the sending of From and |  |  |
| Referer information. |  |  |
| The User-Agent (Section 14.43) or Server (Section 14.38) header |  |  |
| fields can sometimes be used to determine that a specific client or |  |  |
| server have a particular security hole which might be exploited. |  |  |
| Unfortunately, this same information is often used for other valuable |  |  |
| purposes for which HTTP currently has no better mechanism. |  |  |
| 15.1.3. Encoding Sensitive Information in URI's |  |  |
| Because the source of a link might be private information or might |  |  |
| reveal an otherwise private information source, it is strongly |  |  |
| recommended that the user be able to select whether or not the |  |  |
| Referer field is sent. For example, a browser client could have a |  |  |
| toggle switch for browsing openly/anonymously, which would |  |  |
| respectively enable/disable the sending of Referer and From |  |  |
| information. |  |  |
| Clients SHOULD NOT include a Referer header field in a (non-secure) |  |  |
| HTTP request if the referring page was transferred with a secure |  |  |
| protocol. |  |  |
| Authors of services which use the HTTP protocol SHOULD NOT use GET |  |  |
| based forms for the submission of sensitive data, because this will |  |  |
| cause this data to be encoded in the Request-URI. Many existing |  |  |
| servers, proxies, and user agents will log the request URI in some |  |  |
| place where it might be visible to third parties. Servers can use |  |  |
| POST-based form submission instead |  |  |
| 15.1.4. Privacy Issues Connected to Accept Headers |  |  |
| Accept request-headers can reveal information about the user to all |  |  |
| servers which are accessed. The Accept-Language header in particular |  |  |
| can reveal information the user would consider to be of a private |  |  |
| nature, because the understanding of particular languages is often |  |  |
| strongly correlated to the membership of a particular ethnic group. |  |  |
| User agents which offer the option to configure the contents of an |  |  |
| Accept-Language header to be sent in every request are strongly |  |  |
| encouraged to let the configuration process include a message which |  |  |
| makes the user aware of the loss of privacy involved. |  |  |
| An approach that limits the loss of privacy would be for a user agent |  |  |
| to omit the sending of Accept-Language headers by default, and to ask |  |  |
| the user whether or not to start sending Accept-Language headers to a |  |  |
| server if it detects, by looking for any Vary response-header fields |  |  |
| generated by the server, that such sending could improve the quality |  |  |
| of service. |  |  |
| Elaborate user-customized accept header fields sent in every request, |  |  |
| in particular if these include quality values, can be used by servers |  |  |
| as relatively reliable and long-lived user identifiers. Such user |  |  |
| identifiers would allow content providers to do click-trail tracking, |  |  |
| and would allow collaborating content providers to match cross-server |  |  |
| click-trails or form submissions of individual users. Note that for |  |  |
| many users not behind a proxy, the network address of the host |  |  |
| running the user agent will also serve as a long-lived user |  |  |
| identifier. In environments where proxies are used to enhance |  |  |
| privacy, user agents ought to be conservative in offering accept |  |  |
| header configuration options to end users. As an extreme privacy |  |  |
| measure, proxies could filter the accept headers in relayed requests. |  |  |
| General purpose user agents which provide a high degree of header |  |  |
| configurability SHOULD warn users about the loss of privacy which can |  |  |
| be involved. |  |  |
| 15.2. Attacks Based On File and Path Names |  |  |
| Implementations of HTTP origin servers SHOULD be careful to restrict |  |  |
| the documents returned by HTTP requests to be only those that were |  |  |
| intended by the server administrators. If an HTTP server translates |  |  |
| HTTP URIs directly into file system calls, the server MUST take |  |  |
| special care not to serve files that were not intended to be |  |  |
| delivered to HTTP clients. For example, UNIX, Microsoft Windows, and |  |  |
| other operating systems use ".." as a path component to indicate a |  |  |
| directory level above the current one. On such a system, an HTTP |  |  |
| server MUST disallow any such construct in the Request-URI if it |  |  |
| would otherwise allow access to a resource outside those intended to |  |  |
| be accessible via the HTTP server. Similarly, files intended for |  |  |
| reference only internally to the server (such as access control |  |  |
| files, configuration files, and script code) MUST be protected from |  |  |
| inappropriate retrieval, since they might contain sensitive |  |  |
| information. Experience has shown that minor bugs in such HTTP |  |  |
| server implementations have turned into security risks. |  |  |
| 15.3. DNS Spoofing |  |  |
| Clients using HTTP rely heavily on the Domain Name Service, and are |  |  |
| thus generally prone to security attacks based on the deliberate mis- |  |  |
| association of IP addresses and DNS names. Clients need to be |  |  |
| cautious in assuming the continuing validity of an IP number/DNS name |  |  |
| association. |  |  |
| In particular, HTTP clients SHOULD rely on their name resolver for |  |  |
| confirmation of an IP number/DNS name association, rather than |  |  |
| caching the result of previous host name lookups. Many platforms |  |  |
| already can cache host name lookups locally when appropriate, and |  |  |
| they SHOULD be configured to do so. It is proper for these lookups |  |  |
| to be cached, however, only when the TTL (Time To Live) information |  |  |
| reported by the name server makes it likely that the cached |  |  |
| information will remain useful. |  |  |
| If HTTP clients cache the results of host name lookups in order to |  |  |
| achieve a performance improvement, they MUST observe the TTL |  |  |
| information reported by DNS. |  |  |
| If HTTP clients do not observe this rule, they could be spoofed when |  |  |
| a previously-accessed server's IP address changes. As network |  |  |
| renumbering is expected to become increasingly common [RFC1900], the |  |  |
| possibility of this form of attack will grow. Observing this |  |  |
| requirement thus reduces this potential security vulnerability. |  |  |
| This requirement also improves the load-balancing behavior of clients |  |  |
| for replicated servers using the same DNS name and reduces the |  |  |
| likelihood of a user's experiencing failure in accessing sites which |  |  |
| use that strategy. |  |  |
| 15.4. Location Headers and Spoofing |  |  |
| If a single server supports multiple organizations that do not trust |  |  |
| one another, then it MUST check the values of Location and Content- |  |  |
| Location headers in responses that are generated under control of |  |  |
| said organizations to make sure that they do not attempt to |  |  |
| invalidate resources over which they have no authority. |  |  |
| 15.5. Content-Disposition Issues |  |  |
| RFC 1806 [RFC1806], from which the often implemented Content- |  |  |
| Disposition (see Appendix A.5.1) header in HTTP is derived, has a |  |  |
| number of very serious security considerations. Content-Disposition |  |  |
| is not part of the HTTP standard, but since it is widely implemented, |  |  |
| we are documenting its use and risks for implementors. See RFC 2183 |  |  |
| [RFC2183] (which updates RFC 1806) for details. |  |  |
| 15.6. Authentication Credentials and Idle Clients |  |  |
| Existing HTTP clients and user agents typically retain authentication |  |  |
| information indefinitely. HTTP/1.1. does not provide a method for a |  |  |
| server to direct clients to discard these cached credentials. This |  |  |
| is a significant defect that requires further extensions to HTTP. |  |  |
| Circumstances under which credential caching can interfere with the |  |  |
| application's security model include but are not limited to: |  |  |
| o Clients which have been idle for an extended period following |  |  |
| which the server might wish to cause the client to reprompt the |  |  |
| user for credentials. |  |  |
| o Applications which include a session termination indication (such |  |  |
| as a `logout' or `commit' button on a page) after which the server |  |  |
| side of the application `knows' that there is no further reason |  |  |
| for the client to retain the credentials. |  |  |
| This is currently under separate study. There are a number of work- |  |  |
| arounds to parts of this problem, and we encourage the use of |  |  |
| password protection in screen savers, idle time-outs, and other |  |  |
| methods which mitigate the security problems inherent in this |  |  |
| problem. In particular, user agents which cache credentials are |  |  |
| encouraged to provide a readily accessible mechanism for discarding |  |  |
| cached credentials under user control. |  |  |
| 15.7. Proxies and Caching |  |  |
| By their very nature, HTTP proxies are men-in-the-middle, and |  |  |
| represent an opportunity for man-in-the-middle attacks. Compromise |  |  |
| of the systems on which the proxies run can result in serious |  |  |
| security and privacy problems. Proxies have access to security- |  |  |
| related information, personal information about individual users and |  |  |
| organizations, and proprietary information belonging to users and |  |  |
| content providers. A compromised proxy, or a proxy implemented or |  |  |
| configured without regard to security and privacy considerations, |  |  |
| might be used in the commission of a wide range of potential attacks. |  |  |
| Proxy operators should protect the systems on which proxies run as |  |  |
| they would protect any system that contains or transports sensitive |  |  |
| information. In particular, log information gathered at proxies |  |  |
| often contains highly sensitive personal information, and/or |  |  |
| information about organizations. Log information should be carefully |  |  |
| guarded, and appropriate guidelines for use developed and followed. |  |  |
| (Section 15.1.1). |  |  |
| Caching proxies provide additional potential vulnerabilities, since |  |  |
| the contents of the cache represent an attractive target for |  |  |
| malicious exploitation. Because cache contents persist after an HTTP |  |  |
| request is complete, an attack on the cache can reveal information |  |  |
| long after a user believes that the information has been removed from |  |  |
| the network. Therefore, cache contents should be protected as |  |  |
| sensitive information. |  |  |
| Proxy implementors should consider the privacy and security |  |  |
| implications of their design and coding decisions, and of the |  |  |
| configuration options they provide to proxy operators (especially the |  |  |
| default configuration). |  |  |
| Users of a proxy need to be aware that they are no trustworthier than |  |  |
| the people who run the proxy; HTTP itself cannot solve this problem. |  |  |
| The judicious use of cryptography, when appropriate, may suffice to |  |  |
| protect against a broad range of security and privacy attacks. Such |  |  |
| cryptography is beyond the scope of the HTTP/1.1 specification. |  |  |
| 15.7.1. Denial of Service Attacks on Proxies |  |  |
| They exist. They are hard to defend against. Research continues. |  |  |
| Beware. |  |  |
| 16. Acknowledgments |  |  |
| This specification makes heavy use of the augmented BNF and generic |  |  |
| constructs defined by David H. Crocker for RFC 822 [RFC822]. |  |  |
| Similarly, it reuses many of the definitions provided by Nathaniel |  |  |
| Borenstein and Ned Freed for MIME [RFC2045]. We hope that their |  |  |
| inclusion in this specification will help reduce past confusion over |  |  |
| the relationship between HTTP and Internet mail message formats. |  |  |
| The HTTP protocol has evolved considerably over the years. It has |  |  |
| benefited from a large and active developer community--the many |  |  |
| people who have participated on the www-talk mailing list--and it is |  |  |
| that community which has been most responsible for the success of |  |  |
| HTTP and of the World-Wide Web in general. Marc Andreessen, Robert |  |  |
| Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois |  |  |
| Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob |  |  |
| McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc |  |  |
| VanHeyningen deserve special recognition for their efforts in |  |  |
| defining early aspects of the protocol. |  |  |
| This document has benefited greatly from the comments of all those |  |  |
| participating in the HTTP-WG. In addition to those already |  |  |
| mentioned, the following individuals have contributed to this |  |  |
| specification: |  |  |
| Gary Adams Ross Patterson |  |  |
| Harald Tveit Alvestrand Albert Lunde |  |  |
| Keith Ball John C. Mallery |  |  |
| Brian Behlendorf Jean-Philippe Martin-Flatin |  |  |
| Paul Burchard Mitra |  |  |
| Maurizio Codogno David Morris |  |  |
| Mike Cowlishaw Gavin Nicol |  |  |
| Roman Czyborra Bill Perry |  |  |
| Michael A. Dolan Jeffrey Perry |  |  |
| David J. Fiander Scott Powers |  |  |
| Alan Freier Owen Rees |  |  |
| Marc Hedlund Luigi Rizzo |  |  |
| Greg Herlihy David Robinson |  |  |
| Koen Holtman Marc Salomon |  |  |
| Alex Hopmann Rich Salz |  |  |
| Bob Jernigan Allan M. Schiffman |  |  |
| Shel Kaphan Jim Seidman |  |  |
| Rohit Khare Chuck Shotton |  |  |
| John Klensin Eric W. Sink |  |  |
| Martijn Koster Simon E. Spero |  |  |
| Alexei Kosut Richard N. Taylor |  |  |
| David M. Kristol Robert S. Thau |  |  |
| Daniel LaLiberte Bill (BearHeart) Weinman |  |  |
| Ben Laurie Francois Yergeau |  |  |
| Paul J. Leach Mary Ellen Zurko |  |  |
| Daniel DuBois Josh Cohen |  |  |
| Much of the content and presentation of the caching design is due to |  |  |
| suggestions and comments from individuals including: Shel Kaphan, |  |  |
| Paul Leach, Koen Holtman, David Morris, and Larry Masinter. |  |  |
| Most of the specification of ranges is based on work originally done |  |  |
| by Ari Luotonen and John Franks, with additional input from Steve |  |  |
| Zilles. |  |  |
| Thanks to the "cave men" of Palo Alto. You know who you are. |  |  |
| Jim Gettys (the current editor of this document) wishes particularly |  |  |
| to thank Roy Fielding, the previous editor of this document, along |  |  |
| with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen |  |  |
| Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and |  |  |
| Larry Masinter for their help. And thanks go particularly to Jeff |  |  |
| Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit. |  |  |
| The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik |  |  |
| Frystyk implemented RFC 2068 early, and we wish to thank them for the |  |  |
| discovery of many of the problems that this document attempts to |  |  |
| rectify. |  |  |
| 17. References |  |  |
| [ISO-8859] |  |  |
| International Organization for Standardization, |  |  |
| "Information technology - 8-bit single byte coded graphic |  |  |
| - character sets", 1987-1990. |  |  |
| Part 1: Latin alphabet No. 1, ISO-8859-1:1987. Part 2: |  |  |
| Latin alphabet No. 2, ISO-8859-2, 1987. Part 3: Latin |  |  |
| alphabet No. 3, ISO-8859-3, 1988. Part 4: Latin alphabet |  |  |
| No. 4, ISO-8859-4, 1988. Part 5: Latin/Cyrillic alphabet, |  |  |
| ISO-8859-5, 1988. Part 6: Latin/Arabic alphabet, ISO- |  |  |
| 8859-6, 1987. Part 7: Latin/Greek alphabet, ISO-8859-7, |  |  |
| 1987. Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988. |  |  |
| Part 9: Latin alphabet No. 5, ISO-8859-9, 1990. |  |  |
| [Luo1998] Luotonen, A., "Tunneling TCP based protocols through Web |  |  |
| proxy servers", Work in Progress. |  |  |
| [Nie1997] Nielsen, H., Gettys, J., Prud'hommeaux, E., Lie, H., and |  |  |
| C. Lilley, "Network Performance Effects of HTTP/1.1, CSS1, |  |  |
| and PNG", Proceedings of ACM SIGCOMM '97, Cannes France , |  |  |
| Sep 1997. |  |  |
| [Pad1995] Padmanabhan, V. and J. Mogul, "Improving HTTP Latency", |  |  |
| Computer Networks and ISDN Systems v. 28, pp. 25-35, |  |  |
| Dec 1995. |  |  |
| Slightly revised version of paper in Proc. 2nd |  |  |
| International WWW Conference '94: Mosaic and the Web, Oct. |  |  |
| 1994, which is available at <http://www.ncsa.uiuc.edu/SDG/ |  |  |
| IT94/Proceedings/DDay/mogul/HTTPLatency.html>. |  |  |
| [RFC1036] Horton, M. and R. Adams, "Standard for interchange of |  |  |
| USENET messages", RFC 1036, December 1987. |  |  |
| [RFC1123] Braden, R., "Requirements for Internet Hosts - Application |  |  |
| and Support", STD 3, RFC 1123, October 1989. |  |  |
| [RFC1305] Mills, D., "Network Time Protocol (Version 3) |  |  |
| Specification, Implementation", RFC 1305, March 1992. |  |  |
| [RFC1436] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., |  |  |
| Torrey, D., and B. Alberti, "The Internet Gopher Protocol |  |  |
| (a distributed document search and retrieval protocol)", |  |  |
| RFC 1436, March 1993. |  |  |
| [RFC1590] Postel, J., "Media Type Registration Procedure", RFC 1590, |  |  |
| November 1996. |  |  |
| [RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A |  |  |
| Unifying Syntax for the Expression of Names and Addresses |  |  |
| of Objects on the Network as used in the World-Wide Web", |  |  |
| RFC 1630, June 1994. |  |  |
| [RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, |  |  |
| RFC 1700, October 1994. |  |  |
| [RFC1737] Masinter, L. and K. Sollins, "Functional Requirements for |  |  |
| Uniform Resource Names", RFC 1737, December 1994. |  |  |
| [RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform |  |  |
| Resource Locators (URL)", RFC 1738, December 1994. |  |  |
| [RFC1766] Alvestrand, H., "Tags for the Identification of |  |  |
| Languages", RFC 1766, March 1995. |  |  |
| [RFC1806] Troost, R. and S. Dorner, "Communicating Presentation |  |  |
| Information in Internet Messages: The Content-Disposition |  |  |
| Header", RFC 1806, June 1995. |  |  |
| [RFC1808] Fielding, R., "Relative Uniform Resource Locators", |  |  |
| RFC 1808, June 1995. |  |  |
| [RFC1864] Myers, J. and M. Rose, "The Content-MD5 Header Field", |  |  |
| RFC 1864, October 1995. |  |  |
| [RFC1866] Berners-Lee, T. and D. Connolly, "Hypertext Markup |  |  |
| Language - 2.0", RFC 1866, November 1995. |  |  |
| [RFC1867] Masinter, L. and E. Nebel, "Form-based File Upload in |  |  |
| HTML", RFC 1867, November 1995. |  |  |
| [RFC1900] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", |  |  |
| RFC 1900, February 1996. |  |  |
| [RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext |  |  |
| Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996. |  |  |
| [RFC1950] Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format |  | TBD. |
| Specification version 3.3", RFC 1950, May 1996. |  |  |
| [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification |  | 8. Security Considerations |
| version 1.3", RFC 1951, May 1996. |  |  |
| [RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. |  | No additional security considerations have been identified beyond |
|  |  | those applicable to HTTP in general [Part1]. |
| Randers-Pehrson, "GZIP file format specification version |  | 9. Acknowledgments |
| 4.3", RFC 1952, May 1996. |  |  |
| [RFC2026] Bradner, S., "The Internet Standards Process -- Revision |  | Based on an XML translation of RFC 2616 by Julian Reschke. |
| 3", BCP 9, RFC 2026, October 1996. |  |  |
| [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail |  | 10. References |
| Extensions (MIME) Part One: Format of Internet Message |  |  |
| Bodies", RFC 2045, November 1996. |  |  |
| [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail |  | [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., |
| Extensions (MIME) Part Two: Media Types", RFC 2046, |  | Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1, |
| November 1996. |  | part 1: URIs, Connections, and Message Parsing", |
|  |  | draft-ietf-httpbis-p1-messaging-00 (work in progress), |
|  |  | December 2007. |
| [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) |  | [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., |
| Part Three: Message Header Extensions for Non-ASCII Text", |  | Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1, |
| RFC 2047, November 1996. |  | part 5: Range Requests and Partial Responses", |
|  |  | draft-ietf-httpbis-p5-range-00 (work in progress), |
|  |  | December 2007. |
| [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail |  | [Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., |
| Extensions (MIME) Part Five: Conformance Criteria and |  | Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1, |
| Examples", RFC 2049, November 1996. |  | part 6: Caching", draft-ietf-httpbis-p6-cache-00 (work in |
|  |  | progress), December 2007. |
| [RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. |  | [RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. |
| Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", |  | Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", |
| RFC 2068, January 1997. |  | RFC 2068, January 1997. |
| [RFC2069] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., |  | [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., |
| Luotonen, A., Sink, E., and L. Stewart, "An Extension to |  | Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext |
| HTTP : Digest Access Authentication", RFC 2069, |  | Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. |
| January 1997. |  |  |
| [RFC2076] Palme, J., "Common Internet Message Headers", RFC 2076, |  |  |
| February 1997. |  |  |
| [RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of |  |  |
| Aggregate Documents, such as HTML (MHTML)", RFC 2110, |  |  |
| March 1997. |  |  |
| [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate |  |  |
| Requirement Levels", BCP 14, RFC 2119, March 1997. |  |  |
| [RFC2145] Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, "Use |  |  |
| and Interpretation of HTTP Version Numbers", RFC 2145, |  |  |
| May 1997. |  |  |
| [RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating |  |  |
| Presentation Information in Internet Messages: The |  |  |
| Content-Disposition Header Field", RFC 2183, August 1997. |  |  |
| [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and |  |  |
| Languages", BCP 18, RFC 2277, January 1998. |  |  |
| [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO |  |  |
| 10646", RFC 2279, January 1998. |  |  |
| [RFC2324] Masinter, L., "Hyper Text Coffee Pot Control Protocol |  |  |
| (HTCPCP/1.0)", RFC 2324, April 1998. |  |  |
| [RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform |  |  |
| Resource Identifiers (URI): Generic Syntax", RFC 2396, |  |  |
| August 1998. |  |  |
| [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., |  |  |
| Leach, P., Luotonen, A., and L. Stewart, "HTTP |  |  |
| Authentication: Basic and Digest Access Authentication", |  |  |
| RFC 2617, June 1999. |  |  |
| [RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, |  |  |
| RFC 821, August 1982. |  |  |
| [RFC822] Crocker, D., "Standard for the format of ARPA Internet |  |  |
| text messages", STD 11, RFC 822, August 1982. |  |  |
| [RFC959] Postel, J. and J. Reynolds, "File Transfer Protocol", |  |  |
| STD 9, RFC 959, October 1985. |  |  |
| [RFC977] Kantor, B. and P. Lapsley, "Network News Transfer |  |  |
| Protocol", RFC 977, February 1986. |  |  |
| [Spe] Spero, S., "Analysis of HTTP Performance Problems", |  |  |
| <http://sunsite.unc.edu/mdma-release/http-prob.html>. |  |  |
| [Tou1998] Touch, J., Heidemann, J., and K. Obraczka, "Analysis of |  |  |
| HTTP Performance", ISI Research Report ISI/RR-98-463 |  |  |
| (original report dated Aug.1996), Aug 1998, |  |  |
| <http://www.isi.edu/touch/pubs/http-perf96/>. |  |  |
| [USASCII] American National Standards Institute, "Coded Character |  |  |
| Set -- 7-bit American Standard Code for Information |  |  |
| Interchange", ANSI X3.4, 1986. |  |  |
| [WAIS] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., |  |  |
| Wang, R., Sui, J., and M. Grinbaum, "WAIS Interface |  |  |
| Protocol Prototype Functional Specification (v1.5)", |  |  |
| Thinking Machines Corporation , April 1990. |  |  |
| Appendix A. Appendices |  |  |
| A.1. Internet Media Type message/http and application/http |  |  |
| In addition to defining the HTTP/1.1 protocol, this document serves |  |  |
| as the specification for the Internet media type "message/http" and |  |  |
| "application/http". The message/http type can be used to enclose a |  |  |
| single HTTP request or response message, provided that it obeys the |  |  |
| MIME restrictions for all "message" types regarding line length and |  |  |
| encodings. The application/http type can be used to enclose a |  |  |
| pipeline of one or more HTTP request or response messages (not |  |  |
| intermixed). The following is to be registered with IANA [RFC1590]. |  |  |
| Media Type name: message |  |  |
| Media subtype name: http |  |  |
| Required parameters: none |  |  |
| Optional parameters: version, msgtype |  |  |
| version: The HTTP-Version number of the enclosed message (e.g., |  |  |
| "1.1"). If not present, the version can be determined from the |  |  |
| first line of the body. |  |  |
| msgtype: The message type -- "request" or "response". If not |  |  |
| present, the type can be determined from the first line of the |  |  |
| body. |  |  |
| Encoding considerations: only "7bit", "8bit", or "binary" are |  |  |
| permitted |  |  |
| Security considerations: none |  |  |
| Media Type name: application |  |  |
| Media subtype name: http |  |  |
| Required parameters: none |  |  |
| Optional parameters: version, msgtype |  |  |
| version: The HTTP-Version number of the enclosed messages (e.g., |  |  |
| "1.1"). If not present, the version can be determined from the |  |  |
| first line of the body. |  |  |
| msgtype: The message type -- "request" or "response". If not |  |  |
| present, the type can be determined from the first line of the |  |  |
| body. |  |  |
| Encoding considerations: HTTP messages enclosed by this type are in |  |  |
| "binary" format; use of an appropriate Content-Transfer-Encoding |  |  |
| is required when transmitted via E-mail. |  |  |
| Security considerations: none |  |  |
| A.2. Internet Media Type multipart/byteranges |  |  |
| When an HTTP 206 (Partial Content) response message includes the |  |  |
| content of multiple ranges (a response to a request for multiple non- |  |  |
| overlapping ranges), these are transmitted as a multipart message- |  |  |
| body. The media type for this purpose is called "multipart/ |  |  |
| byteranges". |  |  |
| The multipart/byteranges media type includes two or more parts, each |  |  |
| with its own Content-Type and Content-Range fields. The required |  |  |
| boundary parameter specifies the boundary string used to separate |  |  |
| each body-part. |  |  |
| Media Type name: multipart |  |  |
| Media subtype name: byteranges |  |  |
| Required parameters: boundary |  |  |
| Optional parameters: none |  |  |
| Encoding considerations: only "7bit", "8bit", or "binary" are |  |  |
| permitted |  |  |
| Security considerations: none |  |  |
| For example: |  |  |
| HTTP/1.1 206 Partial Content |  |  |
| Date: Wed, 15 Nov 1995 06:25:24 GMT |  |  |
| Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT |  |  |
| Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES |  |  |
| --THIS_STRING_SEPARATES |  |  |
| Content-type: application/pdf |  |  |
| Content-range: bytes 500-999/8000 |  |  |
| ...the first range... |  |  |
| --THIS_STRING_SEPARATES |  |  |
| Content-type: application/pdf |  |  |
| Content-range: bytes 7000-7999/8000 |  |  |
| ...the second range |  |  |
| --THIS_STRING_SEPARATES-- |  |  |
| Notes: |  |  |
| 1. Additional CRLFs may precede the first boundary string in the |  |  |
| entity. |  |  |
| 2. Although RFC 2046 [RFC2046] permits the boundary string to be |  |  |
| quoted, some existing implementations handle a quoted boundary |  |  |
| string incorrectly. |  |  |
| 3. A number of browsers and servers were coded to an early draft of |  |  |
| the byteranges specification to use a media type of multipart/ |  |  |
| x-byteranges, which is almost, but not quite compatible with the |  |  |
| version documented in HTTP/1.1. |  |  |
| A.3. Tolerant Applications |  |  |
| Although this document specifies the requirements for the generation |  |  |
| of HTTP/1.1 messages, not all applications will be correct in their |  |  |
| implementation. We therefore recommend that operational applications |  |  |
| be tolerant of deviations whenever those deviations can be |  |  |
| interpreted unambiguously. |  |  |
| Clients SHOULD be tolerant in parsing the Status-Line and servers |  |  |
| tolerant when parsing the Request-Line. In particular, they SHOULD |  |  |
| accept any amount of SP or HT characters between fields, even though |  |  |
| only a single SP is required. |  |  |
| The line terminator for message-header fields is the sequence CRLF. |  |  |
| However, we recommend that applications, when parsing such headers, |  |  |
| recognize a single LF as a line terminator and ignore the leading CR. |  |  |
| The character set of an entity-body SHOULD be labeled as the lowest |  |  |
| common denominator of the character codes used within that body, with |  |  |
| the exception that not labeling the entity is preferred over labeling |  |  |
| the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1 |  |  |
| and 3.4.1. |  |  |
| Additional rules for requirements on parsing and encoding of dates |  |  |
| and other potential problems with date encodings include: |  |  |
| o HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date |  |  |
| which appears to be more than 50 years in the future is in fact in |  |  |
| the past (this helps solve the "year 2000" problem). |  |  |
| o An HTTP/1.1 implementation MAY internally represent a parsed |  |  |
| Expires date as earlier than the proper value, but MUST NOT |  |  |
| internally represent a parsed Expires date as later than the |  |  |
| proper value. |  |  |
| o All expiration-related calculations MUST be done in GMT. The |  |  |
| local time zone MUST NOT influence the calculation or comparison |  |  |
| of an age or expiration time. |  |  |
| o If an HTTP header incorrectly carries a date value with a time |  |  |
| zone other than GMT, it MUST be converted into GMT using the most |  |  |
| conservative possible conversion. |  |  |
| A.4. Differences Between HTTP Entities and RFC 2045 Entities |  |  |
| HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC |  |  |
| 822 [RFC822]) and the Multipurpose Internet Mail Extensions (MIME |  |  |
| [RFC2045]) to allow entities to be transmitted in an open variety of |  |  |
| representations and with extensible mechanisms. However, RFC 2045 |  |  |
| discusses mail, and HTTP has a few features that are different from |  |  |
| those described in RFC 2045. These differences were carefully chosen |  |  |
| to optimize performance over binary connections, to allow greater |  |  |
| freedom in the use of new media types, to make date comparisons |  |  |
| easier, and to acknowledge the practice of some early HTTP servers |  |  |
| and clients. |  |  |
| This appendix describes specific areas where HTTP differs from RFC |  |  |
| 2045. Proxies and gateways to strict MIME environments SHOULD be |  |  |
| aware of these differences and provide the appropriate conversions |  |  |
| where necessary. Proxies and gateways from MIME environments to HTTP |  |  |
| also need to be aware of the differences because some conversions |  |  |
| might be required. |  |  |
| A.4.1. MIME-Version |  |  |
| HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages |  |  |
| MAY include a single MIME-Version general-header field to indicate |  |  |
| what version of the MIME protocol was used to construct the message. |  |  |
| Use of the MIME-Version header field indicates that the message is in |  |  |
| full compliance with the MIME protocol (as defined in RFC |  |  |
| 2045[RFC2045]). Proxies/gateways are responsible for ensuring full |  |  |
| compliance (where possible) when exporting HTTP messages to strict |  |  |
| MIME environments. |  |  |
| MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT |  |  |
| MIME version "1.0" is the default for use in HTTP/1.1. However, |  |  |
| HTTP/1.1 message parsing and semantics are defined by this document |  |  |
| and not the MIME specification. |  |  |
| A.4.2. Conversion to Canonical Form |  |  |
| RFC 2045 [RFC2045] requires that an Internet mail entity be converted |  |  |
| to canonical form prior to being transferred, as described in section |  |  |
| 4 of RFC 2049 [RFC2049]. Section 3.7.1 of this document describes |  |  |
| the forms allowed for subtypes of the "text" media type when |  |  |
| transmitted over HTTP. RFC 2046 requires that content with a type of |  |  |
| "text" represent line breaks as CRLF and forbids the use of CR or LF |  |  |
| outside of line break sequences. HTTP allows CRLF, bare CR, and bare |  |  |
| LF to indicate a line break within text content when a message is |  |  |
| transmitted over HTTP. |  |  |
| Where it is possible, a proxy or gateway from HTTP to a strict MIME |  |  |
| environment SHOULD translate all line breaks within the text media |  |  |
| types described in Section 3.7.1 of this document to the RFC 2049 |  |  |
| canonical form of CRLF. Note, however, that this might be |  |  |
| complicated by the presence of a Content-Encoding and by the fact |  |  |
| that HTTP allows the use of some character sets which do not use |  |  |
| octets 13 and 10 to represent CR and LF, as is the case for some |  |  |
| multi-byte character sets. |  |  |
| Implementors should note that conversion will break any cryptographic |  |  |
| checksums applied to the original content unless the original content |  |  |
| is already in canonical form. Therefore, the canonical form is |  |  |
| recommended for any content that uses such checksums in HTTP. |  |  |
| A.4.3. Conversion of Date Formats |  |  |
| HTTP/1.1 uses a restricted set of date formats (Section 3.3.1) to |  |  |
| simplify the process of date comparison. Proxies and gateways from |  |  |
| other protocols SHOULD ensure that any Date header field present in a |  |  |
| message conforms to one of the HTTP/1.1 formats and rewrite the date |  |  |
| if necessary. |  |  |
| A.4.4. Introduction of Content-Encoding |  |  |
| RFC 2045 does not include any concept equivalent to HTTP/1.1's |  |  |
| Content-Encoding header field. Since this acts as a modifier on the |  |  |
| media type, proxies and gateways from HTTP to MIME-compliant |  |  |
| protocols MUST either change the value of the Content-Type header |  |  |
| field or decode the entity-body before forwarding the message. (Some |  |  |
| experimental applications of Content-Type for Internet mail have used |  |  |
| a media-type parameter of ";conversions=<content-coding>" to perform |  |  |
| a function equivalent to Content-Encoding. However, this parameter |  |  |
| is not part of RFC 2045). |  |  |
| A.4.5. No Content-Transfer-Encoding |  |  |
| HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC |  |  |
| 2045. Proxies and gateways from MIME-compliant protocols to HTTP |  |  |
| MUST remove any non-identity CTE ("quoted-printable" or "base64") |  |  |
| encoding prior to delivering the response message to an HTTP client. |  |  |
| Proxies and gateways from HTTP to MIME-compliant protocols are |  |  |
| responsible for ensuring that the message is in the correct format |  |  |
| and encoding for safe transport on that protocol, where "safe |  |  |
| transport" is defined by the limitations of the protocol being used. |  |  |
| Such a proxy or gateway SHOULD label the data with an appropriate |  |  |
| Content-Transfer-Encoding if doing so will improve the likelihood of |  |  |
| safe transport over the destination protocol. |  |  |
| A.4.6. Introduction of Transfer-Encoding |  |  |
| HTTP/1.1 introduces the Transfer-Encoding header field |  |  |
| (Section 14.41). Proxies/gateways MUST remove any transfer-coding |  |  |
| prior to forwarding a message via a MIME-compliant protocol. |  |  |
| A process for decoding the "chunked" transfer-coding (Section 3.6) |  |  |
| can be represented in pseudo-code as: |  |  |
| length := 0 |  |  |
| read chunk-size, chunk-extension (if any) and CRLF |  |  |
| while (chunk-size > 0) { |  |  |
| read chunk-data and CRLF |  |  |
| append chunk-data to entity-body |  |  |
| length := length + chunk-size |  |  |
| read chunk-size and CRLF |  |  |
| } |  |  |
| read entity-header |  |  |
| while (entity-header not empty) { |  |  |
| append entity-header to existing header fields |  |  |
| read entity-header |  |  |
| } |  |  |
| Content-Length := length |  |  |
| Remove "chunked" from Transfer-Encoding |  |  |
| A.4.7. MHTML and Line Length Limitations |  |  |
| HTTP implementations which share code with MHTML [RFC2110] |  |  |
| implementations need to be aware of MIME line length limitations. |  |  |
| Since HTTP does not have this limitation, HTTP does not fold long |  |  |
| lines. MHTML messages being transported by HTTP follow all |  |  |
| conventions of MHTML, including line length limitations and folding, |  |  |
| canonicalization, etc., since HTTP transports all message-bodies as |  |  |
| payload (see Section 3.7.2) and does not interpret the content or any |  |  |
| MIME header lines that might be contained therein. |  |  |
| A.5. Additional Features |  |  |
| RFC 1945 and RFC 2068 document protocol elements used by some |  |  |
| existing HTTP implementations, but not consistently and correctly |  |  |
| across most HTTP/1.1 applications. Implementors are advised to be |  |  |
| aware of these features, but cannot rely upon their presence in, or |  |  |
| interoperability with, other HTTP/1.1 applications. Some of these |  |  |
| describe proposed experimental features, and some describe features |  |  |
| that experimental deployment found lacking that are now addressed in |  |  |
| the base HTTP/1.1 specification. |  |  |
| A number of other headers, such as Content-Disposition and Title, |  |  |
| from SMTP and MIME are also often implemented (see RFC 2076 |  |  |
| [RFC2076]). |  |  |
| A.5.1. Content-Disposition |  |  |
| The Content-Disposition response-header field has been proposed as a |  |  |
| means for the origin server to suggest a default filename if the user |  |  |
| requests that the content is saved to a file. This usage is derived |  |  |
| from the definition of Content-Disposition in RFC 1806 [RFC1806]. |  |  |
| content-disposition = "Content-Disposition" ":" |  |  |
| disposition-type *( ";" disposition-parm ) |  |  |
| disposition-type = "attachment" | disp-extension-token |  |  |
| disposition-parm = filename-parm | disp-extension-parm |  |  |
| filename-parm = "filename" "=" quoted-string |  |  |
| disp-extension-token = token |  |  |
| disp-extension-parm = token "=" ( token | quoted-string ) |  |  |
| An example is |  |  |
| Content-Disposition: attachment; filename="fname.ext" |  |  |
| The receiving user agent SHOULD NOT respect any directory path |  |  |
| information present in the filename-parm parameter, which is the only |  |  |
| parameter believed to apply to HTTP implementations at this time. |  |  |
| The filename SHOULD be treated as a terminal component only. |  |  |
| If this header is used in a response with the application/ |  |  |
| octet-stream content-type, the implied suggestion is that the user |  |  |
| agent should not display the response, but directly enter a `save |  |  |
| response as...' dialog. |  |  |
| See Section 15.5 for Content-Disposition security issues. |  |  |
| A.6. Compatibility with Previous Versions |  |  |
| It is beyond the scope of a protocol specification to mandate |  |  |
| compliance with previous versions. HTTP/1.1 was deliberately |  |  |
| designed, however, to make supporting previous versions easy. It is |  |  |
| worth noting that, at the time of composing this specification |  |  |
| (1996), we would expect commercial HTTP/1.1 servers to: |  |  |
| o recognize the format of the Request-Line for HTTP/0.9, 1.0, and |  |  |
| 1.1 requests; |  |  |
| o understand any valid request in the format of HTTP/0.9, 1.0, or |  |  |
| 1.1; |  |  |
| o respond appropriately with a message in the same major version |  |  |
| used by the client. |  |  |
| And we would expect HTTP/1.1 clients to: |  |  |
| o recognize the format of the Status-Line for HTTP/1.0 and 1.1 |  |  |
| responses; |  |  |
| o understand any valid response in the format of HTTP/0.9, 1.0, or |  |  |
| 1.1. |  |  |
| For most implementations of HTTP/1.0, each connection is established |  |  |
| by the client prior to the request and closed by the server after |  |  |
| sending the response. Some implementations implement the Keep-Alive |  |  |
| version of persistent connections described in Section 19.7.1 of RFC |  |  |
| 2068 [RFC2068]. |  |  |
| A.6.1. Changes from HTTP/1.0 |  |  |
| This section summarizes major differences between versions HTTP/1.0 |  |  |
| and HTTP/1.1. |  |  |
| A.6.1.1. Changes to Simplify Multi-homed Web Servers and Conserve IP |  |  |
| Addresses |  |  |
| The requirements that clients and servers support the Host request- |  |  |
| header, report an error if the Host request-header (Section 14.23) is |  |  |
| missing from an HTTP/1.1 request, and accept absolute URIs |  |  |
| (Section 5.1.2) are among the most important changes defined by this |  |  |
| specification. |  |  |
| Older HTTP/1.0 clients assumed a one-to-one relationship of IP |  |  |
| addresses and servers; there was no other established mechanism for |  |  |
| distinguishing the intended server of a request than the IP address |  |  |
| to which that request was directed. The changes outlined above will |  |  |
| allow the Internet, once older HTTP clients are no longer common, to |  |  |
| support multiple Web sites from a single IP address, greatly |  |  |
| simplifying large operational Web servers, where allocation of many |  |  |
| IP addresses to a single host has created serious problems. The |  |  |
| Internet will also be able to recover the IP addresses that have been |  |  |
| allocated for the sole purpose of allowing special-purpose domain |  |  |
| names to be used in root-level HTTP URLs. Given the rate of growth |  |  |
| of the Web, and the number of servers already deployed, it is |  |  |
| extremely important that all implementations of HTTP (including |  |  |
| updates to existing HTTP/1.0 applications) correctly implement these |  |  |
| requirements: |  |  |
| o Both clients and servers MUST support the Host request-header. |  |  |
| o A client that sends an HTTP/1.1 request MUST send a Host header. |  |  |
| o Servers MUST report a 400 (Bad Request) error if an HTTP/1.1 |  |  |
| request does not include a Host request-header. |  |  |
| o Servers MUST accept absolute URIs. |  |  |
| A.6.2. Compatibility with HTTP/1.0 Persistent Connections |  |  |
| Some clients and servers might wish to be compatible with some |  |  |
| previous implementations of persistent connections in HTTP/1.0 |  |  |
| clients and servers. Persistent connections in HTTP/1.0 are |  |  |
| explicitly negotiated as they are not the default behavior. HTTP/1.0 |  |  |
| experimental implementations of persistent connections are faulty, |  |  |
| and the new facilities in HTTP/1.1 are designed to rectify these |  |  |
| problems. The problem was that some existing 1.0 clients may be |  |  |
| sending Keep-Alive to a proxy server that doesn't understand |  |  |
| Connection, which would then erroneously forward it to the next |  |  |
| inbound server, which would establish the Keep-Alive connection and |  |  |
| result in a hung HTTP/1.0 proxy waiting for the close on the |  |  |
| response. The result is that HTTP/1.0 clients must be prevented from |  |  |
| using Keep-Alive when talking to proxies. |  |  |
| However, talking to proxies is the most important use of persistent |  |  |
| connections, so that prohibition is clearly unacceptable. Therefore, |  |  |
| we need some other mechanism for indicating a persistent connection |  |  |
| is desired, which is safe to use even when talking to an old proxy |  |  |
| that ignores Connection. Persistent connections are the default for |  |  |
| HTTP/1.1 messages; we introduce a new keyword (Connection: close) for |  |  |
| declaring non-persistence. See Section 14.10. |  |  |
| The original HTTP/1.0 form of persistent connections (the Connection: |  |  |
| Keep-Alive and Keep-Alive header) is documented in RFC 2068. |  |  |
| [RFC2068] |  |  |
| A.6.3. Changes from RFC 2068 |  |  |
| This specification has been carefully audited to correct and |  |  |
| disambiguate key word usage; RFC 2068 had many problems in respect to |  |  |
| the conventions laid out in RFC 2119 [RFC2119]. |  |  |
| Clarified which error code should be used for inbound server failures |  |  |
| (e.g. DNS failures). (Section 10.5.5). |  |  |
| CREATE had a race that required an Etag be sent when a resource is |  |  |
| first created. (Section 10.2.2). |  |  |
| Content-Base was deleted from the specification: it was not |  |  |
| implemented widely, and there is no simple, safe way to introduce it |  |  |
| without a robust extension mechanism. In addition, it is used in a |  |  |
| similar, but not identical fashion in MHTML [RFC2110]. |  |  |
| Transfer-coding and message lengths all interact in ways that |  |  |
| required fixing exactly when chunked encoding is used (to allow for |  |  |
| transfer encoding that may not be self delimiting); it was important |  |  |
| to straighten out exactly how message lengths are computed. |  |  |
| (Sections 3.6, 4.4, 7.2.2, 13.5.2, 14.13, 14.16) |  |  |
| A content-coding of "identity" was introduced, to solve problems |  |  |
| discovered in caching. (Section 3.5) |  |  |
| Quality Values of zero should indicate that "I don't want something" |  |  |
| to allow clients to refuse a representation. (Section 3.9) |  |  |
| The use and interpretation of HTTP version numbers has been clarified |  |  |
| by RFC 2145. Require proxies to upgrade requests to highest protocol |  |  |
| version they support to deal with problems discovered in HTTP/1.0 |  |  |
| implementations (Section 3.1) |  |  |
| Charset wildcarding is introduced to avoid explosion of character set |  |  |
| names in accept headers. (Section 14.2) |  |  |
| A case was missed in the Cache-Control model of HTTP/1.1; s-maxage |  |  |
| was introduced to add this missing case. (Sections 13.4, 14.8, 14.9, |  |  |
| 14.9.3) |  |  |
| The Cache-Control: max-age directive was not properly defined for |  |  |
| responses. (Section 14.9.3) |  |  |
| There are situations where a server (especially a proxy) does not |  |  |
| know the full length of a response but is capable of serving a |  |  |
| byterange request. We therefore need a mechanism to allow byteranges |  |  |
| with a content-range not indicating the full length of the message. |  |  |
| (Section 14.16) |  |  |
| Range request responses would become very verbose if all meta-data |  |  |
| were always returned; by allowing the server to only send needed |  |  |
| headers in a 206 response, this problem can be avoided. (Section |  |  |
| 10.2.7, 13.5.3, and 14.27) |  |  |
| Fix problem with unsatisfiable range requests; there are two cases: |  |  |
| syntactic problems, and range doesn't exist in the document. The 416 |  |  |
| status code was needed to resolve this ambiguity needed to indicate |  |  |
| an error for a byte range request that falls outside of the actual |  |  |
| contents of a document. (Section 10.4.17, 14.16) |  |  |
| Rewrite of message transmission requirements to make it much harder |  |  |
| for implementors to get it wrong, as the consequences of errors here |  |  |
| can have significant impact on the Internet, and to deal with the |  |  |
| following problems: |  |  |
| 1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where |  |  |
| this was incorrectly placing a requirement on the behavior of an |  |  |
| implementation of a future version of HTTP/1.x |  |  |
| 2. Made it clear that user-agents should retry requests, not |  |  |
| "clients" in general. |  |  |
| 3. Converted requirements for clients to ignore unexpected 100 |  |  |
| (Continue) responses, and for proxies to forward 100 responses, |  |  |
| into a general requirement for 1xx responses. |  |  |
| 4. Modified some TCP-specific language, to make it clearer that non- |  |  |
| TCP transports are possible for HTTP. |  |  |
| 5. Require that the origin server MUST NOT wait for the request body |  |  |
| before it sends a required 100 (Continue) response. |  |  |
| 6. Allow, rather than require, a server to omit 100 (Continue) if it |  |  |
| has already seen some of the request body. |  |  |
| 7. Allow servers to defend against denial-of-service attacks and |  |  |
| broken clients. |  |  |
| This change adds the Expect header and 417 status code. The message |  |  |
| transmission requirements fixes are in sections 8.2, 10.4.18, |  |  |
| 8.1.2.2, 13.11, and 14.20. |  |  |
| Proxies should be able to add Content-Length when appropriate. |  |  |
| (Section 13.5.2) |  |  |
| Clean up confusion between 403 and 404 responses. (Section 10.4.4, |  |  |
| 10.4.5, and 10.4.11) |  |  |
| Warnings could be cached incorrectly, or not updated appropriately. |  |  |
| (Section 13.1.2, 13.2.4, 13.5.2, 13.5.3, 14.9.3, and 14.46) Warning |  |  |
| also needed to be a general header, as PUT or other methods may have |  |  |
| need for it in requests. |  |  |
| Transfer-coding had significant problems, particularly with |  |  |
| interactions with chunked encoding. The solution is that transfer- |  |  |
| codings become as full fledged as content-codings. This involves |  |  |
| adding an IANA registry for transfer-codings (separate from content |  |  |
| codings), a new header field (TE) and enabling trailer headers in the |  |  |
| future. Transfer encoding is a major performance benefit, so it was |  |  |
| worth fixing [Nie1997]. TE also solves another, obscure, downward |  |  |
| interoperability problem that could have occurred due to interactions |  |  |
| between authentication trailers, chunked encoding and HTTP/1.0 |  |  |
| clients.(Section 3.6, 3.6.1, and 14.39) |  |  |
| The PATCH, LINK, UNLINK methods were defined but not commonly |  |  |
| implemented in previous versions of this specification. See RFC 2068 |  |  |
| [RFC2068]. |  |  |
| The Alternates, Content-Version, Derived-From, Link, URI, Public and |  |  |
| Content-Base header fields were defined in previous versions of this |  |  |
| specification, but not commonly implemented. See RFC 2068 [RFC2068]. |  |  |
| Appendix B. Index |  |  |
| Please see the PostScript version of this RFC for the INDEX. |  |  |
| Index |  | Index |
| 1 |  |  |
| 100 Continue (status code) 61 |  |  |
| 101 Switching Protocols (status code) 61 |  |  |
| 2 |  |  |
| 200 OK (status code) 61 |  |  |
| 201 Created (status code) 62 |  |  |
| 202 Accepted (status code) 62 |  |  |
| 203 Non-Authoritative Information (status code) 62 |  |  |
| 204 No Content (status code) 63 |  |  |
| 205 Reset Content (status code) 63 |  |  |
| 206 Partial Content (status code) 63 |  |  |
| 3 |  | 3 |
| 300 Multiple Choices (status code) 64 |  | 304 Not Modified (status code) 4 |
| 301 Moved Permanently (status code) 65 |  |  |
| 302 Found (status code) 65 |  |  |
| 303 See Other (status code) 66 |  |  |
| 304 Not Modified (status code) 66 |  |  |
| 305 Use Proxy (status code) 67 |  |  |
| 306 (Unused) (status code) 67 |  |  |
| 307 Temporary Redirect (status code) 67 |  |  |
| 4 |  | 4 |
| 400 Bad Request (status code) 68 |  | 412 Precondition Failed (status code) 5 |
| 401 Unauthorized (status code) 68 |  |  |
| 402 Payment Required (status code) 69 |  |  |
| 403 Forbidden (status code) 69 |  |  |
| 404 Not Found (status code) 69 |  |  |
| 405 Method Not Allowed (status code) 69 |  |  |
| 406 Not Acceptable (status code) 69 |  |  |
| 407 Proxy Authentication Required (status code) 70 |  |  |
| 408 Request Timeout (status code) 70 |  |  |
| 409 Conflict (status code) 70 |  |  |
| 410 Gone (status code) 71 |  |  |
| 411 Length Required (status code) 71 |  |  |
| 412 Precondition Failed (status code) 71 |  |  |
| 413 Request Entity Too Large (status code) 71 |  |  |
| 414 Request-URI Too Long (status code) 72 |  |  |
| 415 Unsupported Media Type (status code) 72 |  |  |
| 416 Requested Range Not Satisfiable (status code) 72 |  |  |
| 417 Expectation Failed (status code) 72 |  |  |
| 5 |  |  |
| 500 Internal Server Error (status code) 73 |  |  |
| 501 Not Implemented (status code) 73 |  |  |
| 502 Bad Gateway (status code) 73 |  |  |
| 503 Service Unavailable (status code) 73 |  |  |
| 504 Gateway Timeout (status code) 73 |  |  |
| 505 HTTP Version Not Supported (status code) 74 |  |  |
| A |  |  |
| Accept header 102 |  |  |
| Accept-Charset header 104 |  |  |
| Accept-Encoding header 105 |  |  |
| Accept-Language header 106 |  |  |
| Accept-Ranges header 108 |  |  |
| age 13 |  |  |
| Age header 108 |  |  |
| Allow header 109 |  |  |
| Alternates header 177 |  |  |
| application/http Media Type 165 |  |  |
| Authorization header 109 |  |  |
| C |  |  |
| cache 12 |  |  |
| Cache Directives |  |  |
| max-age 115 |  |  |
| max-age 116 |  |  |
| max-stale 115 |  |  |
| min-fresh 115 |  |  |
| must-revalidate 117 |  |  |
| no-cache 112 |  |  |
| no-store 113 |  |  |
| no-transform 118 |  |  |
| only-if-cached 117 |  |  |
| private 112 |  |  |
| proxy-revalidate 118 |  |  |
| public 112 |  |  |
| s-maxage 114 |  |  |
| Cache-Control header 110 |  |  |
| cacheable 12 |  |  |
| client 11 |  |  |
| compress 26 |  |  |
| CONNECT method 60 |  |  |
| connection 10 |  |  |
| Connection header 119 |  |  |
| content negotiation 11 |  |  |
| Content-Base header 177 |  |  |
| Content-Disposition header 171 |  |  |
| Content-Encoding header 120 |  |  |
| Content-Language header 121 |  |  |
| Content-Length header 122 |  |  |
| Content-Location header 123 |  |  |
| Content-MD5 header 123 |  |  |
| Content-Range header 125 |  |  |
| Content-Type header 127 |  |  |
| Content-Version header 177 |  |  |
| D |  |  |
| Date header 127 |  |  |
| deflate 26 |  |  |
| DELETE method 59 |  |  |
| Derived-From header 177 |  |  |
| downstream 14 |  |  |
| E |  | E |
| entity 10 |  | ETag header 10 |
| ETag header 128 |  |  |
| Expect header 129 |  |  |
| Expires header 130 |  |  |
| explicit expiration time 13 |  |  |
| F |  |  |
| first-hand 12 |  |  |
| fresh 13 |  |  |
| freshness lifetime 13 |  |  |
| From header 131 |  |  |
| G |  | G |
| gateway 12 |  |  |
| GET method 56 |  |  |
| Grammar |  | Grammar |
| Accept 103 |  | entity-tag 4 |
| Accept-Charset 105 |  | ETag 10 |
| Accept-Encoding 105 |  | If-Match 10 |
| accept-extension 103 |  | If-Modified-Since 12 |
| Accept-Language 106 |  | If-None-Match 13 |
| accept-params 103 |  | If-Unmodified-Since 14 |
| Accept-Ranges 108 |  | Last-Modified 15 |
| acceptable-ranges 108 |  | opaque-tag 4 |
| Age 108 |  | weak 4 |
| age-value 108 |  |  |
| Allow 109 |  |  |
| ALPHA 18 |  |  |
| asctime-date 24 |  |  |
| attribute 27 |  |  |
| Authorization 109 |  |  |
| byte-content-range-spec 125 |  |  |
| byte-range-resp-spec 125 |  |  |
| byte-range-set 141 |  |  |
| byte-range-spec 141 |  |  |
| byte-ranges-specifier 141 |  |  |
| bytes-unit 33 |  |  |
| Cache-Control 111 |  |  |
| cache-directive 111 |  |  |
| cache-extension 111 |  |  |
| cache-request-directive 111 |  |  |
| cache-response-directive 111 |  |  |
| CHAR 18 |  |  |
| charset 25 |  |  |
| chunk 28 |  |  |
| chunk-data 28 |  |  |
| chunk-ext-name 28 |  |  |
| chunk-ext-val 28 |  |  |
| chunk-extension 28 |  |  |
| chunk-size 28 |  |  |
| Chunked-Body 28 |  |  |
| codings 105 |  |  |
| comment 19 |  |  |
| Connection 120 |  |  |
| connection-token 120 |  |  |
| content-coding 26 |  |  |
| content-disposition 172 |  |  |
| Content-Encoding 121 |  |  |
| Content-Language 121 |  |  |
| Content-Length 122 |  |  |
| Content-Location 123 |  |  |
| Content-MD5 123 |  |  |
| Content-Range 125 |  |  |
| content-range-spec 125 |  |  |
| Content-Type 127 |  |  |
| CR 18 |  |  |
| CRLF 18 |  |  |
| ctext 19 |  |  |
| CTL 18 |  |  |
| Date 127 |  |  |
| date1 24 |  |  |
| date2 24 |  |  |
| date3 24 |  |  |
| delta-seconds 24 |  |  |
| DIGIT 18 |  |  |
| disp-extension-parm 172 |  |  |
| disp-extension-token 172 |  |  |
| disposition-parm 172 |  |  |
| disposition-type 172 |  |  |
| entity-body 46 |  |  |
| entity-header 46 |  |  |
| entity-tag 33 |  |  |
| ETag 129 |  |  |
| Expect 129 |  |  |
| expect-params 129 |  |  |
| expectation 129 |  |  |
| expectation-extension 129 |  |  |
| Expires 130 |  |  |
| extension-code 44 |  |  |
| extension-header 46 |  |  |
| extension-method 39 |  |  |
| extension-pragma 139 |  |  |
| field-content 35 |  |  |
| field-name 35 |  |  |
| field-value 35 |  |  |
| filename-parm 172 |  |  |
| first-byte-pos 141 |  |  |
| From 131 |  |  |
| general-header 38 |  |  |
| generic-message 34 |  |  |
| HEX 19 |  |  |
| Host 131 |  |  |
| HT 18 |  |  |
| HTTP-date 24 |  |  |
| HTTP-message 34 |  |  |
| HTTP-Version 20 |  |  |
| http_URL 22 |  |  |
| If-Match 132 |  |  |
| If-Modified-Since 133 |  |  |
| If-None-Match 135 |  |  |
| If-Range 136 |  |  |
| If-Unmodified-Since 137 |  |  |
| instance-length 125 |  |  |
| language-range 106 |  |  |
| language-tag 32 |  |  |
| last-byte-pos 141 |  |  |
| last-chunk 28 |  |  |
| Last-Modified 137 |  |  |
| LF 18 |  |  |
| LOALPHA 18 |  |  |
| Location 138 |  |  |
| LWS 19 |  |  |
| Max-Forwards 138 |  |  |
| md5-digest 123 |  |  |
| media-range 103 |  |  |
| media-type 29 |  |  |
| message-body 35 |  |  |
| message-header 35 |  |  |
| Method 39 |  |  |
| MIME-Version 169 |  |  |
| month 24 |  |  |
| OCTET 18 |  |  |
| opaque-tag 33 |  |  |
| other-range-unit 33 |  |  |
| parameter 27 |  |  |
| Pragma 139 |  |  |
| pragma-directive 139 |  |  |
| primary-tag 32 |  |  |
| product 31 |  |  |
| product-version 31 |  |  |
| protocol-name 149 |  |  |
| protocol-version 149 |  |  |
| Proxy-Authenticate 140 |  |  |
| Proxy-Authorization 140 |  |  |
| pseudonym 149 |  |  |
| qdtext 19 |  |  |
| quoted-pair 20 |  |  |
| quoted-string 19 |  |  |
| qvalue 32 |  |  |
| Range 142 |  |  |
| range-unit 33 |  |  |
| ranges-specifier 141 |  |  |
| Reason-Phrase 44 |  |  |
| received-by 149 |  |  |
| received-protocol 149 |  |  |
| Referer 143 |  |  |
| Request 38 |  |  |
| request-header 42 |  |  |
| Request-Line 38 |  |  |
| Request-URI 39 |  |  |
| Response 42 |  |  |
| response-header 45 |  |  |
| Retry-After 143 |  |  |
| rfc850-date 24 |  |  |
| rfc1123-date 24 |  |  |
| separators 19 |  |  |
| Server 144 |  |  |
| SP 18 |  |  |
| start-line 34 |  |  |
| Status-Code 44 |  |  |
| Status-Line 42 |  |  |
| subtag 32 |  |  |
| subtype 29 |  |  |
| suffix-byte-range-spec 141 |  |  |
| suffix-length 141 |  |  |
| t-codings 144 |  |  |
| TE 144 |  |  |
| TEXT 19 |  |  |
| time 24 |  |  |
| token 19 |  |  |
| Trailer 146 |  |  |
| trailer 28 |  |  |
| transfer-coding 27 |  |  |
| Transfer-Encoding 146 |  |  |
| transfer-extension 27 |  |  |
| type 29 |  |  |
| UPALPHA 18 |  |  |
| Upgrade 147 |  |  |
| User-Agent 148 |  |  |
| value 27 |  |  |
| Vary 148 |  |  |
| Via 149 |  |  |
| warn-agent 151 |  |  |
| warn-code 151 |  |  |
| warn-date 151 |  |  |
| warn-text 151 |  |  |
| Warning 151 |  |  |
| warning-value 151 |  |  |
| weak 33 |  |  |
| weekday 24 |  |  |
| wkday 24 |  |  |
| WWW-Authenticate 153 |  |  |
| gzip 26 |  |  |
| H |  | H |
| HEAD method 57 |  |  |
| Headers |  | Headers |
| Accept 102 |  | ETag 10 |
| Accept-Charset 104 |  | If-Match 10 |
| Accept-Encoding 105 |  | If-Modified-Since 11 |
| Accept-Language 106 |  | If-None-Match 13 |
| Accept-Ranges 108 |  | If-Unmodified-Since 14 |
| Age 108 |  | Last-Modified 15 |
| Allow 109 |  |  |
| Alternate 177 |  |  |
| Authorization 109 |  |  |
| Cache-Control 110 |  |  |
| Connection 119 |  |  |
| Content-Base 177 |  |  |
| Content-Disposition 171 |  |  |
| Content-Encoding 120 |  |  |
| Content-Language 121 |  |  |
| Content-Length 122 |  |  |
| Content-Location 123 |  |  |
| Content-MD5 123 |  |  |
| Content-Range 125 |  |  |
| Content-Type 127 |  |  |
| Content-Version 177 |  |  |
| Date 127 |  |  |
| Derived-From 177 |  |  |
| ETag 128 |  |  |
| Expect 129 |  |  |
| Expires 130 |  |  |
| From 131 |  |  |
| Host 131 |  |  |
| If-Match 132 |  |  |
| If-Modified-Since 133 |  |  |
| If-None-Match 135 |  |  |
| If-Range 136 |  |  |
| If-Unmodified-Since 137 |  |  |
| Last-Modified 137 |  |  |
| Link 177 |  |  |
| Location 138 |  |  |
| Max-Forwards 138 |  |  |
| Pragma 139 |  |  |
| Proxy-Authenticate 140 |  |  |
| Proxy-Authorization 140 |  |  |
| Public 177 |  |  |
| Range 140 |  |  |
| Referer 143 |  |  |
| Retry-After 143 |  |  |
| Server 144 |  |  |
| TE 144 |  |  |
| Trailer 145 |  |  |
| Transfer-Encoding 146 |  |  |
| Upgrade 146 |  |  |
| URI 177 |  |  |
| User-Agent 148 |  |  |
| Vary 148 |  |  |
| Via 149 |  |  |
| Warning 150 |  |  |
| WWW-Authenticate 153 |  |  |
| heuristic expiration time 13 |  |  |
| Host header 131 |  |  |
| I |  | I |
| identity 26 |  | If-Match header 10 |
| If-Match header 132 |  | If-Modified-Since header 11 |
| If-Modified-Since header 133 |  | If-None-Match header 13 |
| If-None-Match header 135 |  | If-Unmodified-Since header 14 |
| If-Range header 136 |  |  |
| If-Unmodified-Since header 137 |  |  |
| inbound 14 |  |  |
| L |  | L |
| Last-Modified header 137 |  | Last-Modified header 15 |
| Link header 177 |  |  |
| LINK method 176 |  |  |
| Location header 138 |  |  |
| M |  |  |
| max-age |  |  |
| Cache Directive 115 |  |  |
| Cache Directive 116 |  |  |
| Max-Forwards header 138 |  |  |
| max-stale |  |  |
| Cache Directive 115 |  |  |
| Media Type |  |  |
| application/http 165 |  |  |
| message/http 165 |  |  |
| multipart/byteranges 166 |  |  |
| multipart/x-byteranges 167 |  |  |
| message 10 |  |  |
| message/http Media Type 165 |  |  |
| Methods |  |  |
| CONNECT 60 |  |  |
| DELETE 59 |  |  |
| GET 56 |  |  |
| HEAD 57 |  |  |
| LINK 176 |  |  |
| OPTIONS 55 |  |  |
| PATCH 176 |  |  |
| POST 57 |  |  |
| PUT 58 |  |  |
| TRACE 59 |  |  |
| UNLINK 176 |  |  |
| min-fresh |  |  |
| Cache Directive 115 |  |  |
| multipart/byteranges Media Type 166 |  |  |
| multipart/x-byteranges Media Type 167 |  |  |
| must-revalidate |  |  |
| Cache Directive 117 |  |  |
| N |  |  |
| no-cache |  |  |
| Cache Directive 112 |  |  |
| no-store |  |  |
| Cache Directive 113 |  |  |
| no-transform |  |  |
| Cache Directive 118 |  |  |
| O |  |  |
| only-if-cached |  |  |
| Cache Directive 117 |  |  |
| OPTIONS method 55 |  |  |
| origin server 11 |  |  |
| outbound 14 |  |  |
| P |  |  |
| PATCH method 176 |  |  |
| POST method 57 |  |  |
| Pragma header 139 |  |  |
| private |  |  |
| Cache Directive 112 |  |  |
| proxy 11 |  |  |
| Proxy-Authenticate header 140 |  |  |
| Proxy-Authorization header 140 |  |  |
| proxy-revalidate |  |  |
| Cache Directive 118 |  |  |
| public |  |  |
| Cache Directive 112 |  |  |
| Public header 177 |  |  |
| PUT method 58 |  |  |
| R |  |  |
| Range header 140 |  |  |
| Referer header 143 |  |  |
| representation 10 |  |  |
| request 10 |  |  |
| resource 10 |  |  |
| response 10 |  |  |
| Retry-After header 143 |  |  |
| S |  | S |
| s-maxage |  |  |
| Cache Directive 114 |  |  |
| semantically transparent 13 |  |  |
| server 11 |  |  |
| Server header 144 |  |  |
| stale 13 |  |  |
| Status Codes |  | Status Codes |
| 100 Continue 61 |  | 304 Not Modified 4 |
| 101 Switching Protocols 61 |  | 412 Precondition Failed 5 |
| 200 OK 61 |  |  |
| 201 Created 62 |  |  |
| 202 Accepted 62 |  |  |
| 203 Non-Authoritative Information 62 |  |  |
| 204 No Content 63 |  |  |
| 205 Reset Content 63 |  |  |
| 206 Partial Content 63 |  |  |
| 300 Multiple Choices 64 |  |  |
| 301 Moved Permanently 65 |  |  |
| 302 Found 65 |  |  |
| 303 See Other 66 |  |  |
| 304 Not Modified 66 |  |  |
| 305 Use Proxy 67 |  |  |
| 306 (Unused) 67 |  |  |
| 307 Temporary Redirect 67 |  |  |
| 400 Bad Request 68 |  |  |
| 401 Unauthorized 68 |  |  |
| 402 Payment Required 69 |  |  |
| 403 Forbidden 69 |  |  |
| 404 Not Found 69 |  |  |
| 405 Method Not Allowed 69 |  |  |
| 406 Not Acceptable 69 |  |  |
| 407 Proxy Authentication Required 70 |  |  |
| 408 Request Timeout 70 |  |  |
| 409 Conflict 70 |  |  |
| 410 Gone 71 |  |  |
| 411 Length Required 71 |  |  |
| 412 Precondition Failed 71 |  |  |
| 413 Request Entity Too Large 71 |  |  |
| 414 Request-URI Too Long 72 |  |  |
| 415 Unsupported Media Type 72 |  |  |
| 416 Requested Range Not Satisfiable 72 |  |  |
| 417 Expectation Failed 72 |  |  |
| 500 Internal Server Error 73 |  |  |
| 501 Not Implemented 73 |  |  |
| 502 Bad Gateway 73 |  |  |
| 503 Service Unavailable 73 |  |  |
| 504 Gateway Timeout 73 |  |  |
| 505 HTTP Version Not Supported 74 |  |  |
| T |  |  |
| TE header 144 |  |  |
| TRACE method 59 |  |  |
| Trailer header 145 |  |  |
| Transfer-Encoding header 146 |  |  |
| tunnel 12 |  |  |
| U |  |  |
| UNLINK method 176 |  |  |
| Upgrade header 146 |  |  |
| upstream 14 |  |  |
| URI header 177 |  |  |
| user agent 11 |  |  |
| User-Agent header 148 |  |  |
| V |  |  |
| validator 13 |  |  |
| variant 11 |  |  |
| Vary header 148 |  |  |
| Via header 149 |  |  |
| W |  |  |
| Warning header 150 |  |  |
| WWW-Authenticate header 153 |  |  |
| Authors' Addresses |  | Authors' Addresses |
| Roy T. Fielding |  | Roy T. Fielding (editor) |
| Department of Information and Computer Science |  | Day Software |
| University of California, Irvine |  | 23 Corporate Plaza DR, Suite 280 |
| Irvine, CA 92697-3425 |  | Newport Beach, CA 92660 |
|  |  | USA |
| Fax: +1(949)824-1715 |  | Phone: +1-949-706-5300 |
| Email: fielding@ics.uci.edu |  | Fax: +1-949-706-5305 |
|  |  | Email: fielding@gbiv.com |
|  |  | URI: http://roy.gbiv.com/ |
| James Gettys |  | Jim Gettys |
| World Wide Web Consortium |  | One Laptop per Child |
| MIT Laboratory for Computer Science, NE43-356 |  | 21 Oak Knoll Road |
| 545 Technology Square |  | Carlisle, MA 01741 |
| Cambridge, MA 02139 |  | USA |
|  |  | Email: jg@laptop.org |
|  |  | URI: http://www.laptop.org/ |
| Fax: +1(617)258-8682 |  |  |
| Email: jg@w3.org |  |  |
| Jeffrey C. Mogul |  | Jeffrey C. Mogul |
| Compaq Computer Corporation |  | Hewlett-Packard Company |
| Western Research Laboratory |  | HP Labs, Large Scale Systems Group |
| 250 University Avenue |  | 1501 Page Mill Road, MS 1177 |
| Palo Alto, CA 94305 |  | Palo Alto, CA 94304 |
|  |  | USA |
| Email: mogul@wrl.dec.com |  | Email: JeffMogul@acm.org |
| Henrik Frystyk Nielsen |  | Henrik Frystyk Nielsen |
| World Wide Web Consortium |  | Microsoft Corporation |
| MIT Laboratory for Computer Science, NE43-356 |  | 1 Microsoft Way |
| 545 Technology Square |  | Redmond, WA 98052 |
| Cambridge, MA 02139 |  | USA |
| Fax: +1(617)258-8682 |  |  |
| Email: frystyk@w3.org |  |  |
|  |  | Email: henrikn@microsoft.com |
| Larry Masinter |  | Larry Masinter |
| Xerox Corporation |  | Adobe Systems, Incorporated |
| MIT Laboratory for Computer Science, NE43-356 |  | 345 Park Ave |
| 3333 Coyote Hill Road |  | San Jose, CA 95110 |
| Palo Alto, CA 94034 |  | USA |
| Email: masinter@parc.xerox.com |  | Email: LMM@acm.org |
|  |  | URI: http://larry.masinter.net/ |
| Paul J. Leach |  | Paul J. Leach |
| Microsoft Corporation |  | Microsoft Corporation |
| 1 Microsoft Way |  | 1 Microsoft Way |
| Redmond, WA 98052 |  | Redmond, WA 98052 |
| Email: paulle@microsoft.com |  | Email: paulle@microsoft.com |
| Tim Berners-Lee |  | Tim Berners-Lee |
| World Wide Web Consortium |  | World Wide Web Consortium |
| MIT Laboratory for Computer Science, NE43-356 |  | MIT Computer Science and Artificial Intelligence Laboratory |
| 545 Technology Square |  | The Stata Center, Building 32 |
|  |  | 32 Vassar Street |
| Cambridge, MA 02139 |  | Cambridge, MA 02139 |
|  |  | USA |
| Fax: +1(617)258-8682 |  |  |
| Email: timbl@w3.org |  | Email: timbl@w3.org |
|  |  | URI: http://www.w3.org/People/Berners-Lee/ |
| Full Copyright Statement |  | Full Copyright Statement |
| Copyright (C) The IETF Trust (1999). |  | Copyright (C) The IETF Trust (2007). |
| This document is subject to the rights, licenses and restrictions |  | This document is subject to the rights, licenses and restrictions |
| contained in BCP 78, and except as set forth therein, the authors |  | contained in BCP 78, and except as set forth therein, the authors |
| retain all their rights. |  | retain all their rights. |
| This document and the information contained herein are provided on an |  | This document and the information contained herein are provided on an |
| "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS |  | "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS |
| OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND |  | OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND |
| THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS |  | THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS |
| OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF |  | OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF |
End of changes. 74 change blocks.
| 8261 lines changed or deleted |  | 196 lines changed or added |
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