


HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                            J. Reschke, Ed.
Updates: 2817 (if approved)                                   greenbytes
Intended status: Standards Track                           July 15, 2013
Expires: January 16, 2014


     Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
                   draft-ietf-httpbis-p2-semantics-23

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypertext information
   systems.  This document defines the semantics of HTTP/1.1 messages,
   as expressed by request methods, request header fields, response
   status codes, and response header fields, along with the payload of
   messages (metadata and body content) and mechanisms for content
   negotiation.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft takes place on the HTTPBIS working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

   The current issues list is at
   <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
   documents (including fancy diffs) can be found at
   <http://tools.ietf.org/wg/httpbis/>.

   The changes in this draft are summarized in Appendix E.3.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on January 16, 2014.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Conformance and Error Handling . . . . . . . . . . . . . .  6
     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  6
   2.  Resources  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Representations  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Representation Metadata  . . . . . . . . . . . . . . . . .  8
       3.1.1.  Processing the Data  . . . . . . . . . . . . . . . . .  8
       3.1.2.  Encoding for Compression or Integrity  . . . . . . . . 11
       3.1.3.  Audience Language  . . . . . . . . . . . . . . . . . . 13
       3.1.4.  Identification . . . . . . . . . . . . . . . . . . . . 14
     3.2.  Representation Data  . . . . . . . . . . . . . . . . . . . 17
     3.3.  Payload Semantics  . . . . . . . . . . . . . . . . . . . . 17
     3.4.  Content Negotiation  . . . . . . . . . . . . . . . . . . . 18
       3.4.1.  Proactive Negotiation  . . . . . . . . . . . . . . . . 18
       3.4.2.  Reactive Negotiation . . . . . . . . . . . . . . . . . 19
   4.  Request Methods  . . . . . . . . . . . . . . . . . . . . . . . 20
     4.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 20



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     4.2.  Common Method Properties . . . . . . . . . . . . . . . . . 22
       4.2.1.  Safe Methods . . . . . . . . . . . . . . . . . . . . . 22
       4.2.2.  Idempotent Methods . . . . . . . . . . . . . . . . . . 23
       4.2.3.  Cacheable Methods  . . . . . . . . . . . . . . . . . . 23
     4.3.  Method Definitions . . . . . . . . . . . . . . . . . . . . 23
       4.3.1.  GET  . . . . . . . . . . . . . . . . . . . . . . . . . 24
       4.3.2.  HEAD . . . . . . . . . . . . . . . . . . . . . . . . . 24
       4.3.3.  POST . . . . . . . . . . . . . . . . . . . . . . . . . 25
       4.3.4.  PUT  . . . . . . . . . . . . . . . . . . . . . . . . . 26
       4.3.5.  DELETE . . . . . . . . . . . . . . . . . . . . . . . . 28
       4.3.6.  CONNECT  . . . . . . . . . . . . . . . . . . . . . . . 29
       4.3.7.  OPTIONS  . . . . . . . . . . . . . . . . . . . . . . . 31
       4.3.8.  TRACE  . . . . . . . . . . . . . . . . . . . . . . . . 32
   5.  Request Header Fields  . . . . . . . . . . . . . . . . . . . . 32
     5.1.  Controls . . . . . . . . . . . . . . . . . . . . . . . . . 32
       5.1.1.  Expect . . . . . . . . . . . . . . . . . . . . . . . . 33
       5.1.2.  Max-Forwards . . . . . . . . . . . . . . . . . . . . . 36
     5.2.  Conditionals . . . . . . . . . . . . . . . . . . . . . . . 36
     5.3.  Content Negotiation  . . . . . . . . . . . . . . . . . . . 37
       5.3.1.  Quality Values . . . . . . . . . . . . . . . . . . . . 37
       5.3.2.  Accept . . . . . . . . . . . . . . . . . . . . . . . . 38
       5.3.3.  Accept-Charset . . . . . . . . . . . . . . . . . . . . 40
       5.3.4.  Accept-Encoding  . . . . . . . . . . . . . . . . . . . 41
       5.3.5.  Accept-Language  . . . . . . . . . . . . . . . . . . . 42
     5.4.  Authentication Credentials . . . . . . . . . . . . . . . . 43
     5.5.  Request Context  . . . . . . . . . . . . . . . . . . . . . 44
       5.5.1.  From . . . . . . . . . . . . . . . . . . . . . . . . . 44
       5.5.2.  Referer  . . . . . . . . . . . . . . . . . . . . . . . 44
       5.5.3.  User-Agent . . . . . . . . . . . . . . . . . . . . . . 45
   6.  Response Status Codes  . . . . . . . . . . . . . . . . . . . . 47
     6.1.  Overview of Status Codes . . . . . . . . . . . . . . . . . 47
     6.2.  Informational 1xx  . . . . . . . . . . . . . . . . . . . . 49
       6.2.1.  100 Continue . . . . . . . . . . . . . . . . . . . . . 49
       6.2.2.  101 Switching Protocols  . . . . . . . . . . . . . . . 50
     6.3.  Successful 2xx . . . . . . . . . . . . . . . . . . . . . . 50
       6.3.1.  200 OK . . . . . . . . . . . . . . . . . . . . . . . . 50
       6.3.2.  201 Created  . . . . . . . . . . . . . . . . . . . . . 51
       6.3.3.  202 Accepted . . . . . . . . . . . . . . . . . . . . . 51
       6.3.4.  203 Non-Authoritative Information  . . . . . . . . . . 51
       6.3.5.  204 No Content . . . . . . . . . . . . . . . . . . . . 52
       6.3.6.  205 Reset Content  . . . . . . . . . . . . . . . . . . 52
     6.4.  Redirection 3xx  . . . . . . . . . . . . . . . . . . . . . 53
       6.4.1.  300 Multiple Choices . . . . . . . . . . . . . . . . . 54
       6.4.2.  301 Moved Permanently  . . . . . . . . . . . . . . . . 55
       6.4.3.  302 Found  . . . . . . . . . . . . . . . . . . . . . . 55
       6.4.4.  303 See Other  . . . . . . . . . . . . . . . . . . . . 56
       6.4.5.  305 Use Proxy  . . . . . . . . . . . . . . . . . . . . 56
       6.4.6.  306 (Unused) . . . . . . . . . . . . . . . . . . . . . 56



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       6.4.7.  307 Temporary Redirect . . . . . . . . . . . . . . . . 57
     6.5.  Client Error 4xx . . . . . . . . . . . . . . . . . . . . . 57
       6.5.1.  400 Bad Request  . . . . . . . . . . . . . . . . . . . 57
       6.5.2.  402 Payment Required . . . . . . . . . . . . . . . . . 57
       6.5.3.  403 Forbidden  . . . . . . . . . . . . . . . . . . . . 57
       6.5.4.  404 Not Found  . . . . . . . . . . . . . . . . . . . . 58
       6.5.5.  405 Method Not Allowed . . . . . . . . . . . . . . . . 58
       6.5.6.  406 Not Acceptable . . . . . . . . . . . . . . . . . . 58
       6.5.7.  408 Request Timeout  . . . . . . . . . . . . . . . . . 59
       6.5.8.  409 Conflict . . . . . . . . . . . . . . . . . . . . . 59
       6.5.9.  410 Gone . . . . . . . . . . . . . . . . . . . . . . . 59
       6.5.10. 411 Length Required  . . . . . . . . . . . . . . . . . 60
       6.5.11. 413 Payload Too Large  . . . . . . . . . . . . . . . . 60
       6.5.12. 414 URI Too Long . . . . . . . . . . . . . . . . . . . 60
       6.5.13. 415 Unsupported Media Type . . . . . . . . . . . . . . 60
       6.5.14. 417 Expectation Failed . . . . . . . . . . . . . . . . 61
       6.5.15. 426 Upgrade Required . . . . . . . . . . . . . . . . . 61
     6.6.  Server Error 5xx . . . . . . . . . . . . . . . . . . . . . 61
       6.6.1.  500 Internal Server Error  . . . . . . . . . . . . . . 61
       6.6.2.  501 Not Implemented  . . . . . . . . . . . . . . . . . 61
       6.6.3.  502 Bad Gateway  . . . . . . . . . . . . . . . . . . . 62
       6.6.4.  503 Service Unavailable  . . . . . . . . . . . . . . . 62
       6.6.5.  504 Gateway Timeout  . . . . . . . . . . . . . . . . . 62
       6.6.6.  505 HTTP Version Not Supported . . . . . . . . . . . . 62
   7.  Response Header Fields . . . . . . . . . . . . . . . . . . . . 62
     7.1.  Control Data . . . . . . . . . . . . . . . . . . . . . . . 63
       7.1.1.  Origination Date . . . . . . . . . . . . . . . . . . . 63
       7.1.2.  Location . . . . . . . . . . . . . . . . . . . . . . . 66
       7.1.3.  Retry-After  . . . . . . . . . . . . . . . . . . . . . 68
       7.1.4.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . 68
     7.2.  Validator Header Fields  . . . . . . . . . . . . . . . . . 69
     7.3.  Authentication Challenges  . . . . . . . . . . . . . . . . 70
     7.4.  Response Context . . . . . . . . . . . . . . . . . . . . . 70
       7.4.1.  Allow  . . . . . . . . . . . . . . . . . . . . . . . . 71
       7.4.2.  Server . . . . . . . . . . . . . . . . . . . . . . . . 71
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 72
     8.1.  Method Registry  . . . . . . . . . . . . . . . . . . . . . 72
       8.1.1.  Procedure  . . . . . . . . . . . . . . . . . . . . . . 72
       8.1.2.  Considerations for New Methods . . . . . . . . . . . . 72
       8.1.3.  Registrations  . . . . . . . . . . . . . . . . . . . . 73
     8.2.  Status Code Registry . . . . . . . . . . . . . . . . . . . 73
       8.2.1.  Procedure  . . . . . . . . . . . . . . . . . . . . . . 73
       8.2.2.  Considerations for New Status Codes  . . . . . . . . . 74
       8.2.3.  Registrations  . . . . . . . . . . . . . . . . . . . . 75
     8.3.  Header Field Registry  . . . . . . . . . . . . . . . . . . 76
       8.3.1.  Considerations for New Header Fields . . . . . . . . . 77
       8.3.2.  Registrations  . . . . . . . . . . . . . . . . . . . . 79
     8.4.  Content Coding Registry  . . . . . . . . . . . . . . . . . 79



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       8.4.1.  Procedure  . . . . . . . . . . . . . . . . . . . . . . 79
       8.4.2.  Registrations  . . . . . . . . . . . . . . . . . . . . 80
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 80
     9.1.  Attacks Based On File and Path Names . . . . . . . . . . . 80
     9.2.  Personal Information . . . . . . . . . . . . . . . . . . . 81
     9.3.  Sensitive Information in URIs  . . . . . . . . . . . . . . 81
     9.4.  Product Information  . . . . . . . . . . . . . . . . . . . 82
     9.5.  Fragment after Redirects . . . . . . . . . . . . . . . . . 82
     9.6.  Browser Fingerprinting . . . . . . . . . . . . . . . . . . 82
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 83
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 83
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 83
     11.2. Informative References . . . . . . . . . . . . . . . . . . 85
   Appendix A.  Differences between HTTP and MIME . . . . . . . . . . 87
     A.1.  MIME-Version . . . . . . . . . . . . . . . . . . . . . . . 87
     A.2.  Conversion to Canonical Form . . . . . . . . . . . . . . . 87
     A.3.  Conversion of Date Formats . . . . . . . . . . . . . . . . 88
     A.4.  Conversion of Content-Encoding . . . . . . . . . . . . . . 88
     A.5.  Conversion of Content-Transfer-Encoding  . . . . . . . . . 88
     A.6.  MHTML and Line Length Limitations  . . . . . . . . . . . . 88
   Appendix B.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 89
   Appendix C.  Imported ABNF . . . . . . . . . . . . . . . . . . . . 91
   Appendix D.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 92
   Appendix E.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 95
     E.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 95
     E.2.  Since draft-ietf-httpbis-p2-semantics-21 . . . . . . . . . 95
     E.3.  Since draft-ietf-httpbis-p2-semantics-22 . . . . . . . . . 96
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96






















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1.  Introduction

   Each Hypertext Transfer Protocol (HTTP) message is either a request
   or a response.  A server listens on a connection for a request,
   parses each message received, interprets the message semantics in
   relation to the identified request target, and responds to that
   request with one or more response messages.  A client constructs
   request messages to communicate specific intentions, and examines
   received responses to see if the intentions were carried out and
   determine how to interpret the results.  This document defines
   HTTP/1.1 request and response semantics in terms of the architecture
   defined in [Part1].

   HTTP provides a uniform interface for interacting with a resource
   (Section 2), regardless of its type, nature, or implementation, via
   the manipulation and transfer of representations (Section 3).

   HTTP semantics include the intentions defined by each request method
   (Section 4), extensions to those semantics that might be described in
   request header fields (Section 5), the meaning of status codes to
   indicate a machine-readable response (Section 6), and the meaning of
   other control data and resource metadata that might be given in
   response header fields (Section 7).

   This document also defines representation metadata that describe how
   a payload is intended to be interpreted by a recipient, the request
   header fields that might influence content selection, and the various
   selection algorithms that are collectively referred to as "content
   negotiation" (Section 3.4).

1.1.  Conformance and Error Handling

   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 [RFC2119].

   Conformance criteria and considerations regarding error handling are
   defined in Section 2.5 of [Part1].

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with the list rule extension defined in Section
   1.2 of [Part1].  Appendix C describes rules imported from other
   documents.  Appendix D shows the collected ABNF with the list rule
   expanded.

   This specification uses the terms "character", "character encoding



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   scheme", "charset", and "protocol element" as they are defined in
   [RFC6365].

2.  Resources

   The target of each HTTP request is called a resource.  HTTP does not
   limit the nature of a resource; it merely defines an interface that
   might be used to interact with resources.  Each resource is
   identified by a Uniform Resource Identifier (URI), as described in
   Section 2.7 of [Part1].

   When a client constructs an HTTP/1.1 request message, it sends the
   target URI in one of various forms, as defined in (Section 5.3 of
   [Part1]).  When a request is received, the server reconstructs an
   effective request URI for the target resource (Section 5.5 of
   [Part1]).

   One design goal of HTTP is to separate resource identification from
   request semantics, which is made possible by vesting the request
   semantics in the request method (Section 4) and a few request-
   modifying header fields (Section 5).  Resource owners SHOULD NOT
   include request semantics within a URI, such as by specifying an
   action to invoke within the path or query components of the effective
   request URI, unless those semantics are disabled when they are
   inconsistent with the request method.

3.  Representations

   If we consider that a resource could be anything, and that the
   uniform interface provided by HTTP is similar to a window through
   which one can observe and act upon such a thing only through the
   communication of messages to some independent actor on the other
   side, then we need an abstraction to represent ("take the place of")
   the current or desired state of that thing in our communications.  We
   call that abstraction a representation [REST].

   For the purposes of HTTP, a "representation" is information that is
   intended to reflect a past, current, or desired state of a given
   resource, in a format that can be readily communicated via the
   protocol, and that consists of a set of representation metadata and a
   potentially unbounded stream of representation data.

   An origin server might be provided with, or capable of generating,
   multiple representations that are each intended to reflect the
   current state of a target resource.  In such cases, some algorithm is
   used by the origin server to select one of those representations as
   most applicable to a given request, usually based on content
   negotiation.  We refer to that one representation as the "selected



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   representation" and use its particular data and metadata for
   evaluating conditional requests [Part4] and constructing the payload
   for 200 (OK) and 304 (Not Modified) responses to GET (Section 4.3.1).

3.1.  Representation Metadata

   Representation header fields provide metadata about the
   representation.  When a message includes a payload body, the
   representation header fields describe how to interpret the
   representation data enclosed in the payload body.  In a response to a
   HEAD request, the representation header fields describe the
   representation data that would have been enclosed in the payload body
   if the same request had been a GET.

   The following header fields are defined to convey representation
   metadata:

   +-------------------+-----------------+
   | Header Field Name | Defined in...   |
   +-------------------+-----------------+
   | Content-Type      | Section 3.1.1.5 |
   | Content-Encoding  | Section 3.1.2.2 |
   | Content-Language  | Section 3.1.3.2 |
   | Content-Location  | Section 3.1.4.2 |
   +-------------------+-----------------+

3.1.1.  Processing the Data

3.1.1.1.  Media Type

   HTTP uses Internet Media Types [RFC2046] in the Content-Type
   (Section 3.1.1.5) and Accept (Section 5.3.2) header fields in order
   to provide open and extensible data typing and type negotiation.
   Media types define both a data format and various processing models:
   how to process that data in accordance with each context in which it
   is received.

     media-type = type "/" subtype *( OWS ";" OWS parameter )
     type       = token
     subtype    = token

   The type/subtype MAY be followed by parameters in the form of
   attribute/value pairs.

     parameter      = attribute "=" value
     attribute      = token
     value          = word




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   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.  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.

   A parameter value that matches the token production can be
   transmitted as either a token or within a quoted-string.  The quoted
   and unquoted values are equivalent.  For example, the following
   examples are all equivalent, but the first is preferred for
   consistency:

     text/html;charset=utf-8
     text/html;charset=UTF-8
     Text/HTML;Charset="utf-8"
     text/html; charset="utf-8"

   Internet media types ought to be registered with IANA according to
   the procedures defined in [BCP13].

      Note: Unlike some similar constructs in other header fields, media
      type parameters do not allow whitespace (even "bad" whitespace)
      around the "=" character.

3.1.1.2.  Charset

   HTTP uses charset names to indicate or negotiate the character
   encoding scheme of a textual representation [RFC6365].  A charset is
   identified by a case-insensitive token.

     charset = token

   Charset names ought to be registered in IANA Character Set registry
   (<http://www.iana.org/assignments/character-sets>) according to the
   procedures defined in [RFC2978].

3.1.1.3.  Canonicalization and Text Defaults

   Internet media types are registered with a canonical form in order to
   be interoperable among systems with varying native encoding formats.
   Representations selected or transferred via HTTP ought to be in
   canonical form, for many of the same reasons described by the
   Multipurpose Internet Mail Extensions (MIME) [RFC2045].  However, the
   performance characteristics of email deployments (i.e., store and
   forward messages to peers) are significantly different from those
   common to HTTP and the Web (server-based information services).
   Furthermore, MIME's constraints for the sake of compatibility with



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   older mail transfer protocols do not apply to HTTP (see Appendix A).

   MIME's canonical form requires that media subtypes of the "text" type
   use CRLF as the text line break.  HTTP allows the transfer of text
   media with plain CR or LF alone representing a line break, when such
   line breaks are consistent for an entire representation.  HTTP
   senders MAY generate, and recipients MUST be able to parse, line
   breaks in text media that consist of CRLF, bare CR, or bare LF.  In
   addition, text media in HTTP is not limited to charsets that use
   octets 13 and 10 for CR and LF, respectively.  This flexibility
   regarding line breaks applies only to text within a representation
   that has been assigned a "text" media type; it does not apply to
   "multipart" types or HTTP elements outside the payload body (e.g.,
   header fields).

   If a representation is encoded with a content-coding, the underlying
   data ought to be in a form defined above prior to being encoded.

3.1.1.4.  Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more representations within a single message body.  All
   multipart types share a common syntax, as defined in Section 5.1.1 of
   [RFC2046], and include a boundary parameter as part of the media type
   value.  The message body is itself a protocol element; a sender MUST
   generate only CRLF to represent line breaks between body parts.

   HTTP message framing does not use the multipart boundary as an
   indicator of message body length, though it might be used by
   implementations that generate or process the payload.  For example,
   the "multipart/form-data" type is often used for carrying form data
   in a request, as described in [RFC2388], and the "multipart/
   byteranges" type is defined by this specification for use in some 206
   (Partial Content) responses [Part5].

3.1.1.5.  Content-Type

   The "Content-Type" header field indicates the media type of the
   associated representation: either the representation enclosed in the
   message payload or the selected representation, as determined by the
   message semantics.  The indicated media type defines both the data
   format and how that data is intended to be processed by a recipient,
   within the scope of the received message semantics, after any content
   codings indicated by Content-Encoding are decoded.

     Content-Type = media-type

   Media types are defined in Section 3.1.1.1.  An example of the field



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   is

     Content-Type: text/html; charset=ISO-8859-4

   A sender that generates a message containing a payload body SHOULD
   generate a Content-Type header field in that message unless the
   intended media type of the enclosed representation is unknown to the
   sender.  If a Content-Type header field is not present, recipients
   MAY either assume a media type of "application/octet-stream"
   ([RFC2046], Section 4.5.1) or examine the data to determine its type.

   In practice, resource owners do not always properly configure their
   origin server to provide the correct Content-Type for a given
   representation, with the result that some clients will examine a
   payload's content and override the specified type.  Clients that do
   so risk drawing incorrect conclusions, which might expose additional
   security risks (e.g., "privilege escalation").  Furthermore, it is
   impossible to determine the sender's intent by examining the data
   format: many data formats match multiple media types that differ only
   in processing semantics.  Implementers are encouraged to provide a
   means of disabling such "content sniffing" when it is used.

3.1.2.  Encoding for Compression or Integrity

3.1.2.1.  Content Codings

   Content coding values indicate an encoding transformation that has
   been or can be applied to a representation.  Content codings are
   primarily used to allow a representation to be compressed or
   otherwise usefully transformed without losing the identity of its
   underlying media type and without loss of information.  Frequently,
   the representation is stored in coded form, transmitted directly, and
   only decoded by the recipient.

     content-coding   = token

   All content-coding values are case-insensitive and ought to be
   registered within the HTTP Content Coding registry, as defined in
   Section 8.4.  They are used in the Accept-Encoding (Section 5.3.4)
   and Content-Encoding (Section 3.1.2.2) header fields.

   The following content-coding values are defined by this
   specification:

      compress (and x-compress): See Section 4.2.1 of [Part1].

      deflate: See Section 4.2.2 of [Part1].




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      gzip (and x-gzip): See Section 4.2.3 of [Part1].

3.1.2.2.  Content-Encoding

   The "Content-Encoding" header field indicates what content codings
   have been applied to the representation, beyond those inherent in the
   media type, and thus what decoding mechanisms have to be applied in
   order to obtain data in the media type referenced by the Content-Type
   header field.  Content-Encoding is primarily used to allow a
   representation's data to be compressed without losing the identity of
   its underlying media type.

     Content-Encoding = 1#content-coding

   An example of its use is

     Content-Encoding: gzip

   If multiple encodings have been applied to a representation, 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 header fields not defined by this specification.

   Unlike Transfer-Encoding (Section 3.3.1 of [Part1]), the codings
   listed in Content-Encoding are a characteristic of the
   representation; the representation is defined in terms of the coded
   form, and all other metadata about the representation is about the
   coded form unless otherwise noted in the metadata definition.
   Typically, the representation is only decoded just prior to rendering
   or analogous usage.

   If the media type includes an inherent encoding, such as a data
   format that is always compressed, then that encoding would not be
   restated in Content-Encoding even if it happens to be the same
   algorithm as one of the content codings.  Such a content coding would
   only be listed if, for some bizarre reason, it is applied a second
   time to form the representation.  Likewise, an origin server might
   choose to publish the same data as multiple representations that
   differ only in whether the coding is defined as part of Content-Type
   or Content-Encoding, since some user agents will behave differently
   in their handling of each response (e.g., open a "Save as ..." dialog
   instead of automatic decompression and rendering of content).

   An origin server MAY respond with a status code of 415 (Unsupported
   Media Type) if a representation in the request message has a content
   coding that is not acceptable.





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3.1.3.  Audience Language

3.1.3.1.  Language Tags

   A language tag, as defined in [RFC5646], 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 header fields.

   Accept-Language uses the looser language-range production defined in
   Section 5.3.5, whereas Content-Language uses the stricter language-
   tag production defined below.

     language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

   A language tag is composed of one or more parts: a primary language
   subtag followed by a possibly empty series of subtags.  White space
   is not allowed within the tag and all tags are case-insensitive.
   Example tags include:

     en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN

   See [RFC5646] for further information.

3.1.3.2.  Content-Language

   The "Content-Language" header field describes the natural language(s)
   of the intended audience for the representation.  Note that this
   might not be equivalent to all the languages used within the
   representation.

     Content-Language = 1#language-tag

   Language tags are defined in Section 3.1.3.1.  The primary purpose of
   Content-Language is to allow a user to identify and differentiate
   representations according to the users' own preferred language.
   Thus, if the 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



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   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 a
   representation 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.

3.1.4.  Identification

3.1.4.1.  Identifying a Representation

   When a complete or partial representation is transferred in a message
   payload, it is often desirable for the sender to supply, or the
   recipient to determine, an identifier for a resource corresponding to
   that representation.

   For a request message:

   o  If the request has a Content-Location header field, then the
      sender asserts that the payload is a representation of the
      resource identified by the Content-Location field-value.  However,
      such an assertion cannot be trusted unless it can be verified by
      other means (not defined by HTTP).  The information might still be
      useful for revision history links.

   o  Otherwise, the payload is unidentified.

   For a response message, the following rules are applied in order
   until a match is found:

   1.  If the request is GET or HEAD and the response status code is 200
       (OK), 204 (No Content), 206 (Partial Content), or 304 (Not
       Modified), the payload is a representation of the resource
       identified by the effective request URI (Section 5.5 of [Part1]).

   2.  If the request is GET or HEAD and the response status code is 203
       (Non-Authoritative Information), the payload is a potentially
       modified or enhanced representation of the target resource as
       provided by an intermediary.



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   3.  If the response has a Content-Location header field and its
       field-value is a reference to the same URI as the effective
       request URI, the payload is a representation of the resource
       identified by the effective request URI.

   4.  If the response has a Content-Location header field and its
       field-value is a reference to a URI different from the effective
       request URI, then the sender asserts that the payload is a
       representation of the resource identified by the Content-Location
       field-value.  However, such an assertion cannot be trusted unless
       it can be verified by other means (not defined by HTTP).

   5.  Otherwise, the payload is unidentified.

3.1.4.2.  Content-Location

   The "Content-Location" header field references a URI that can be used
   as an identifier for a specific resource corresponding to the
   representation in this message's payload.  In other words, if one
   were to perform a GET request on this URI at the time of this
   message's generation, then a 200 (OK) response would contain the same
   representation that is enclosed as payload in this message.

     Content-Location = absolute-URI / partial-URI

   The Content-Location value is not a replacement for the effective
   Request URI (Section 5.5 of [Part1]).  It is representation metadata.
   It has the same syntax and semantics as the header field of the same
   name defined for MIME body parts in Section 4 of [RFC2557].  However,
   its appearance in an HTTP message has some special implications for
   HTTP recipients.

   If Content-Location is included in a 2xx (Successful) response
   message and its value refers (after conversion to absolute form) to a
   URI that is the same as the effective request URI, then the recipient
   MAY consider the payload to be a current representation of that
   resource at the time indicated by the message origination date.  For
   a GET or HEAD request, this is the same as the default semantics when
   no Content-Location is provided by the server.  For a state-changing
   request like PUT or POST, it implies that the server's response
   contains the new representation of that resource, thereby
   distinguishing it from representations that might only report about
   the action (e.g., "It worked!").  This allows authoring applications
   to update their local copies without the need for a subsequent GET
   request.

   If Content-Location is included in a 2xx (Successful) response
   message and its field-value refers to a URI that differs from the



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   effective request URI, then the origin server claims that the URI is
   an identifier for a different resource corresponding to the enclosed
   representation.  Such a claim can only be trusted if both identifiers
   share the same resource owner, which cannot be programmatically
   determined via HTTP.

   o  For a response to a GET or HEAD request, this is an indication
      that the effective request URI refers to a resource that is
      subject to content negotiation and the Content-Location field-
      value is a more specific identifier for the selected
      representation.

   o  For a 201 (Created) response to a state-changing method, a
      Content-Location field-value that is identical to the Location
      field-value indicates that this payload is a current
      representation of the newly created resource.

   o  Otherwise, such a Content-Location indicates that this payload is
      a representation reporting on the requested action's status and
      that the same report is available (for future access with GET) at
      the given URI.  For example, a purchase transaction made via a
      POST request might include a receipt document as the payload of
      the 200 (OK) response; the Content-Location field-value provides
      an identifier for retrieving a copy of that same receipt in the
      future.

   A user agent that sends Content-Location in a request message is
   stating that its value refers to where the user agent originally
   obtained the content of the enclosed representation (prior to any
   modifications made by that user agent).  In other words, the user
   agent is providing a back link to the source of the original
   representation.

   An origin server that receives a Content-Location field in a request
   message MUST treat the information as transitory request context
   rather than as metadata to be saved verbatim as part of the
   representation.  An origin server MAY use that context to guide in
   processing the request or to save it for other uses, such as within
   source links or versioning metadata.  However, an origin server MUST
   NOT use such context information to alter the request semantics.

   For example, if a client makes a PUT request on a negotiated resource
   and the origin server accepts that PUT (without redirection), then
   the new state of that resource is expected to be consistent with the
   one representation supplied in that PUT; the Content-Location cannot
   be used as a form of reverse content selection identifier to update
   only one of the negotiated representations.  If the user agent had
   wanted the latter semantics, it would have applied the PUT directly



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   to the Content-Location URI.

3.2.  Representation Data

   The representation data associated with an HTTP message is either
   provided as the payload body of the message or referred to by the
   message semantics and the effective request URI.  The representation
   data is in a format and encoding defined by the representation
   metadata header fields.

   The data type of the representation data is determined via the header
   fields Content-Type and Content-Encoding.  These define a two-layer,
   ordered encoding model:

     representation-data := Content-Encoding( Content-Type( bits ) )

3.3.  Payload Semantics

   Some HTTP messages transfer a complete or partial representation as
   the message "payload".  In some cases, a payload might contain only
   the associated representation's header fields (e.g., responses to
   HEAD) or only some part(s) of the representation data (e.g., the 206
   (Partial Content) status code).

   The purpose of a payload in a request is defined by the method
   semantics.  For example, a representation in the payload of a PUT
   request (Section 4.3.4) represents the desired state of the target
   resource if the request is successfully applied, whereas a
   representation in the payload of a POST request (Section 4.3.3)
   represents an anonymous resource for providing data to be processed,
   such as the information that a user entered within an HTML form.

   In a response, the payload's purpose is defined by both the request
   method and the response status code.  For example, the payload of a
   200 (OK) response to GET (Section 4.3.1) represents the current state
   of the target resource, as observed at the time of the message
   origination date (Section 7.1.1.2), whereas the payload of the same
   status code in a response to POST might represent either the
   processing result or the new state of the target resource after
   applying the processing.  Response messages with an error status code
   usually contain a payload that represents the error condition, such
   that it describes the error state and what next steps are suggested
   for resolving it.

   Header fields that specifically describe the payload, rather than the
   associated representation, are referred to as "payload header
   fields".  Payload header fields are defined in other parts of this
   specification, due to their impact on message parsing.



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   +-------------------+--------------------------+
   | Header Field Name | Defined in...            |
   +-------------------+--------------------------+
   | Content-Length    | Section 3.3.2 of [Part1] |
   | Content-Range     | Section 4.2 of [Part5]   |
   | Transfer-Encoding | Section 3.3.1 of [Part1] |
   +-------------------+--------------------------+

3.4.  Content Negotiation

   When responses convey payload information, whether indicating a
   success or an error, the origin server often has different ways of
   representing that information; for example, in different formats,
   languages, or encodings.  Likewise, different users or user agents
   might have differing capabilities, characteristics, or preferences
   that could influence which representation, among those available,
   would be best to deliver.  For this reason, HTTP provides mechanisms
   for content negotiation.

   This specification defines two patterns of content negotiation that
   can be made visible within the protocol: "proactive", where the
   server selects the representation based upon the user agent's stated
   preferences, and "reactive" negotiation, where the server provides a
   list of representations for the user agent to choose from.  Other
   patterns of content negotiation include "conditional content", where
   the representation consists of multiple parts that are selectively
   rendered based on user agent parameters, "active content", where the
   representation contains a script that makes additional (more
   specific) requests based on the user agent characteristics, and
   "Transparent Content Negotiation" ([RFC2295]), where content
   selection is performed by an intermediary.  These patterns are not
   mutually exclusive, and each has trade-offs in applicability and
   practicality.

   Note that, in all cases, the supplier of representations to the
   origin server determines which representations might be considered to
   be the "same information".

3.4.1.  Proactive Negotiation

   When content negotiation preferences are sent by the user agent in a
   request in order to encourage an algorithm located at the server to
   select the preferred representation, it is called proactive
   negotiation (a.k.a., server-driven negotiation).  Selection is based
   on the available representations for a response (the dimensions over
   which it might vary, such as language, content-coding, etc.) compared
   to various information supplied in the request, including both the
   explicit negotiation fields of Section 5.3 and implicit



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   characteristics, such as the client's network address or parts of the
   User-Agent field.

   Proactive negotiation is advantageous when the algorithm for
   selecting from among the available representations is difficult to
   describe to a user agent, or when the server desires to send its
   "best guess" to the user agent 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, a user agent MAY send request header fields that
   describe its preferences.

   Proactive negotiation has serious disadvantages:

   o  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?);

   o  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 risk
      to the user's privacy;

   o  It complicates the implementation of an origin server and the
      algorithms for generating responses to a request; and,

   o  It limits the reusability of responses for shared caching.

   A user agent cannot rely on proactive negotiation preferences being
   consistently honored, since the origin server might not implement
   proactive negotiation for the requested resource or might decide that
   sending a response that doesn't conform to the user agent's
   preferences is better than sending a 406 (Not Acceptable) response.

   An origin server MAY generate a Vary header field (Section 7.1.4) in
   responses that are subject to proactive negotiation to indicate what
   parameters of request information might be used in its selection
   algorithm, thereby providing a means for recipients to determine the
   reusability of that same response for user agents with differing
   request information.

3.4.2.  Reactive Negotiation

   With reactive negotiation (a.k.a., 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



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   server with a list of alternative resources.  If the user agent is
   not satisfied by the initial response, it can perform a GET request
   on one or more of the alternative resources, selected based on
   metadata included in the list, to obtain a different form of
   representation.  Selection of alternatives might be performed
   automatically by the user agent or manually by the user selecting
   from a generated (possibly hypertext) menu.

   A server can send a 300 (Multiple Choices) response to indicate that
   reactive negotiation by the user agent is desired, or a 406 (Not
   Acceptable) status code to indicate that proactive negotiation has
   failed.  In both cases, the response ought to include information
   about the available representations so that the user or user agent
   can react by making a selection.

   Reactive 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.

   Reactive negotiation suffers from the disadvantages of transmitting a
   list of alternatives to the user agent, which degrades user-perceived
   latency if transmitted in the header section, and needing a second
   request to obtain an alternate representation.  Furthermore, this
   specification does not define a mechanism for supporting automatic
   selection, though it does not prevent such a mechanism from being
   developed as an extension.

4.  Request Methods

4.1.  Overview

   The request method token is the primary source of request semantics;
   it indicates the purpose for which the client has made this request
   and what is expected by the client as a successful result.  The
   request semantics might be further specialized by the semantics of
   some header fields when present in a request (Section 5) if those
   additional semantics do not conflict with the method.

     method = token

   HTTP was originally designed to be usable as an interface to
   distributed object systems.  The request method was envisioned as
   applying semantics to a target resource in much the same way as
   invoking a defined method on an identified object would apply
   semantics.  The method token is case-sensitive because it might be
   used as a gateway to object-based systems with case-sensitive method



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   names.

   Unlike distributed objects, the standardized request methods in HTTP
   are not resource-specific, since uniform interfaces provide for
   better visibility and reuse in network-based systems [REST].  Once
   defined, a standardized method ought to have the same semantics when
   applied to any resource, though each resource determines for itself
   whether those semantics are implemented or allowed.

   This specification defines a number of standardized methods that are
   commonly used in HTTP, as outlined by the following table.  By
   convention, standardized methods are defined in all-uppercase ASCII
   letters.

   +---------+-------------------------------------------------+-------+
   | Method  | Description                                     | Sec.  |
   +---------+-------------------------------------------------+-------+
   | GET     | Transfer a current representation of the target | 4.3.1 |
   |         | resource.                                       |       |
   | HEAD    | Same as GET, but only transfer the status line  | 4.3.2 |
   |         | and header block.                               |       |
   | POST    | Perform resource-specific processing on the     | 4.3.3 |
   |         | request payload.                                |       |
   | PUT     | Replace all current representations of the      | 4.3.4 |
   |         | target resource with the request payload.       |       |
   | DELETE  | Remove all current representations of the       | 4.3.5 |
   |         | target resource.                                |       |
   | CONNECT | Establish a tunnel to the server identified by  | 4.3.6 |
   |         | the target resource.                            |       |
   | OPTIONS | Describe the communication options for the      | 4.3.7 |
   |         | target resource.                                |       |
   | TRACE   | Perform a message loop-back test along the path | 4.3.8 |
   |         | to the target resource.                         |       |
   +---------+-------------------------------------------------+-------+

   All general-purpose servers MUST support the methods GET and HEAD.
   All other methods are OPTIONAL; when implemented, a server MUST
   implement the above methods according to the semantics defined for
   them in Section 4.3.

   Additional methods, outside the scope of this specification, have
   been standardized for use in HTTP.  All such methods ought to be
   registered within the HTTP Method Registry maintained by IANA, as
   defined in Section 8.1.

   The set of methods allowed by a target resource can be listed in an
   Allow header field (Section 7.4.1).  However, the set of allowed
   methods can change dynamically.  When a request method is received



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   that is unrecognized or not implemented by an origin server, the
   origin server SHOULD respond with the 501 (Not Implemented) status
   code.  When a request method is received that is known by an origin
   server but not allowed for the target resource, the origin server
   SHOULD respond with the 405 (Method Not Allowed) status code.

   A client can send conditional request header fields (Section 5.2) to
   make the requested action conditional on the current state of the
   target resource ([Part4]).

4.2.  Common Method Properties

4.2.1.  Safe Methods

   Request methods are considered "safe" if their defined semantics are
   essentially read-only; i.e., the client does not request, and does
   not expect, any state change on the origin server as a result of
   applying a safe method to a target resource.  Likewise, reasonable
   use of a safe method is not expected to cause any harm, loss of
   property, or unusual burden on the origin server.

   This definition of safe methods does not prevent an implementation
   from including behavior that is potentially harmful, not entirely
   read-only, or which causes side-effects while invoking a safe method.
   What is important, however, is that the client did not request that
   additional behavior and cannot be held accountable for it.  For
   example, most servers append request information to access log files
   at the completion of every response, regardless of the method, and
   that is considered safe even though the log storage might become full
   and crash the server.  Likewise, a safe request initiated by
   selecting an advertisement on the Web will often have the side-effect
   of charging an advertising account.

   Of the request methods defined by this specification, the GET, HEAD,
   OPTIONS, and TRACE methods are defined to be safe.

   The purpose of distinguishing between safe and unsafe methods is to
   allow automated retrieval processes (spiders) and cache performance
   optimization (pre-fetching) to work without fear of causing harm.  In
   addition, it allows a user agent to apply appropriate constraints on
   the automated use of unsafe methods when processing potentially
   untrusted content.

   A user agent SHOULD distinguish between safe and unsafe methods when
   presenting potential actions to a user, such that the user can be
   made aware of an unsafe action before it is requested.

   When a resource is constructed such that parameters within the



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   effective request URI have the effect of selecting an action, it is
   the resource owner's responsibility to ensure that the action is
   consistent with the request method semantics.  For example, it is
   common for Web-based content editing software to use actions within
   query parameters, such as "page?do=delete".  If the purpose of such a
   resource is to perform an unsafe action, then the resource MUST
   disable or disallow that action when it is accessed using a safe
   request method.  Failure to do so will result in unfortunate side-
   effects when automated processes perform a GET on every URI reference
   for the sake of link maintenance, pre-fetching, building a search
   index, etc.

4.2.2.  Idempotent Methods

   Request methods are considered "idempotent" if the intended effect of
   multiple identical requests is the same as for a single request.  Of
   the request methods defined by this specification, the PUT, DELETE,
   and safe request methods are idempotent.

   Like the definition of safe, the idempotent property only applies to
   what has been requested by the user; a server is free to log each
   request separately, retain a revision control history, or implement
   other non-idempotent side-effects for each idempotent request.

   Idempotent methods are distinguished because the request can be
   repeated automatically if a communication failure occurs before the
   client is able to read the server's response.  For example, if a
   client sends a PUT request and the underlying connection is closed
   before any response is received, then it can establish a new
   connection and retry the idempotent request because it knows that
   repeating the request will have the same effect even if the original
   request succeeded.  Note, however, that repeated failures would
   indicate a problem within the server.

4.2.3.  Cacheable Methods

   Request methods are considered "cacheable" if it is possible and
   useful to answer a current client request with a stored response from
   a prior request.  GET and HEAD are defined to be cacheable.  In
   general, safe methods that do not depend on a current or
   authoritative response are cacheable, though the overwhelming
   majority of caches only support GET and HEAD.  HTTP requirements for
   cache behavior and cacheable responses are defined in [Part6].

4.3.  Method Definitions






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4.3.1.  GET

   The GET method requests transfer of a current selected representation
   for the target resource.  GET is the primary mechanism of information
   retrieval and the focus of almost all performance optimizations.
   Hence, when people speak of retrieving some identifiable information
   via HTTP, they are generally referring to making a GET request.

   It is tempting to think of resource identifiers as remote filesystem
   pathnames, and of representations as being a copy of the contents of
   such files.  In fact, that is how many resources are implemented (see
   Section 9.1 for related security considerations).  However, there are
   no such limitations in practice.  The HTTP interface for a resource
   is just as likely to be implemented as a tree of content objects, a
   programmatic view on various database records, or a gateway to other
   information systems.  Even when the URI mapping mechanism is tied to
   a filesystem, an origin server might be configured to execute the
   files with the request as input and send the output as the
   representation, rather than transfer the files directly.  Regardless,
   only the origin server needs to know how each of its resource
   identifiers corresponds to an implementation, and how each
   implementation manages to select and send a current representation of
   the target resource in a response to GET.

   A client can alter the semantics of GET to be a "range request",
   requesting transfer of only some part(s) of the selected
   representation, by sending a Range header field in the request
   ([Part5]).

   A payload within a GET request message has no defined semantics;
   sending a payload body on a GET request might cause some existing
   implementations to reject the request.

   The response to a GET request is cacheable; a cache MAY use it to
   satisfy subsequent GET and HEAD requests unless otherwise indicated
   by the Cache-Control header field (Section 7.2 of [Part6]).

4.3.2.  HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   send a message body in the response (i.e., the response terminates at
   the end of the header block).  Aside from the payload header fields
   (Section 3.3), the server SHOULD send the same header fields in
   response to a HEAD request as it would have sent if the request had
   been a GET.  This method can be used for obtaining metadata about the
   selected representation without transferring the representation data.
   This method is often used for testing hypertext links for validity,
   accessibility, and recent modification.



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   A payload within a HEAD request message has no defined semantics;
   sending a payload body on a HEAD request might cause some existing
   implementations to reject the request.

   The response to a HEAD request is cacheable; a cache MAY use it to
   satisfy subsequent HEAD requests unless otherwise indicated by the
   Cache-Control header field (Section 7.2 of [Part6]).  A HEAD response
   might also have an effect on previously cached responses to GET; see
   Section 5 of [Part6].

4.3.3.  POST

   The POST method requests that the target resource process the
   representation enclosed in the request according to the resource's
   own specific semantics.  For example, POST is used for the following
   functions (among others):

   o  Providing a block of data, such as the fields entered into an HTML
      form, to a data-handling process;

   o  Posting a message to a bulletin board, newsgroup, mailing list,
      blog, or similar group of articles;

   o  Creating a new resource that has yet to be identified by the
      origin server; and

   o  Appending data to a resource's existing representation(s).

   An origin server indicates response semantics by choosing an
   appropriate status code depending on the result of processing the
   POST request; almost all of the status codes defined by this
   specification might be received in a response to POST (the exceptions
   being 206, 304, and 416).

   If one or more resources has been created on the origin server as a
   result of successfully processing a POST request, the origin server
   SHOULD send a 201 (Created) response containing a Location header
   field that provides an identifier for the primary resource created
   (Section 7.1.2) and a representation that describes the status of the
   request while referring to the new resource(s).

   Responses to POST requests are only cacheable when they include
   explicit freshness information (see Section 4.1.1 of [Part6]).
   However, POST caching is not widely implemented.  For cases where an
   origin server wishes the client to be able to cache the result of a
   POST in a way that can be reused by a later GET, the origin server
   MAY send a 200 (OK) response containing the result and a Content-
   Location header field that has the same value as the POST's effective



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   request URI (Section 3.1.4.2).

   If the result of processing a POST would be equivalent to a
   representation of an existing resource, an origin server MAY redirect
   the user agent to that resource by sending a 303 (See Other) response
   with the existing resource's identifier in the Location field.  This
   has the benefits of providing the user agent a resource identifier
   and transferring the representation via a method more amenable to
   shared caching, though at the cost of an extra request if the user
   agent does not already have the representation cached.

4.3.4.  PUT

   The PUT method requests that the state of the target resource be
   created or replaced with the state defined by the representation
   enclosed in the request message payload.  A successful PUT of a given
   representation would suggest that a subsequent GET on that same
   target resource will result in an equivalent representation being
   sent in a 200 (OK) response.  However, there is no guarantee that
   such a state change will be observable, since the target resource
   might be acted upon by other user agents in parallel, or might be
   subject to dynamic processing by the origin server, before any
   subsequent GET is received.  A successful response only implies that
   the user agent's intent was achieved at the time of its processing by
   the origin server.

   If the target resource does not have a current representation and the
   PUT successfully creates one, then the origin server MUST inform the
   user agent by sending a 201 (Created) response.  If the target
   resource does have a current representation and that representation
   is successfully modified in accordance with the state of the enclosed
   representation, then either a 200 (OK) or 204 (No Content) response
   SHOULD be sent to indicate successful completion of the request.

   An origin server SHOULD ignore unrecognized header fields received in
   a PUT request (i.e., do not save them as part of the resource state).

   An origin server SHOULD verify that the PUT representation is
   consistent with any constraints the server has for the target
   resource that cannot or will not be changed by the PUT.  This is
   particularly important when the origin server uses internal
   configuration information related to the URI in order to set the
   values for representation metadata on GET responses.  When a PUT
   representation is inconsistent with the target resource, the origin
   server SHOULD either make them consistent, by transforming the
   representation or changing the resource configuration, or respond
   with an appropriate error message containing sufficient information
   to explain why the representation is unsuitable.  The 409 (Conflict)



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   or 415 (Unsupported Media Type) status codes are suggested, with the
   latter being specific to constraints on Content-Type values.

   For example, if the target resource is configured to always have a
   Content-Type of "text/html" and the representation being PUT has a
   Content-Type of "image/jpeg", then the origin server SHOULD do one
   of:

   a.  reconfigure the target resource to reflect the new media type;

   b.  transform the PUT representation to a format consistent with that
       of the resource before saving it as the new resource state; or,

   c.  reject the request with a 415 (Unsupported Media Type) response
       indicating that the target resource is limited to "text/html",
       perhaps including a link to a different resource that would be a
       suitable target for the new representation.

   HTTP does not define exactly how a PUT method affects the state of an
   origin server beyond what can be expressed by the intent of the user
   agent request and the semantics of the origin server response.  It
   does not define what a resource might be, in any sense of that word,
   beyond the interface provided via HTTP.  It does not define how
   resource state is "stored", nor how such storage might change as a
   result of a change in resource state, nor how the origin server
   translates resource state into representations.  Generally speaking,
   all implementation details behind the resource interface are
   intentionally hidden by the server.

   An origin server MUST NOT send a validator header field
   (Section 7.2), such as an ETag or Last-Modified field, in a
   successful response to PUT unless the request's representation data
   was saved without any transformation applied to the body (i.e., the
   resource's new representation data is identical to the representation
   data received in the PUT request) and the validator field value
   reflects the new representation.  This requirement allows a user
   agent to know when the representation body it has in memory remains
   current as a result of the PUT, thus not in need of retrieving again
   from the origin server, and that the new validator(s) received in the
   response can be used for future conditional requests in order to
   prevent accidental overwrites (Section 5.2).

   The fundamental difference between the POST and PUT methods is
   highlighted by the different intent for the enclosed representation.
   The target resource in a POST request is intended to handle the
   enclosed representation according to the resource's own semantics,
   whereas the enclosed representation in a PUT request is defined as
   replacing the state of the target resource.  Hence, the intent of PUT



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   is idempotent and visible to intermediaries, even though the exact
   effect is only known by the origin server.

   Proper interpretation of a PUT request presumes that the user agent
   knows which target resource is desired.  A service that selects a
   proper URI on behalf of the client, after receiving a state-changing
   request, SHOULD be implemented using the POST method rather than PUT.
   If the origin server will not make the requested PUT state change to
   the target resource and instead wishes to have it applied to a
   different resource, such as when the resource has been moved to a
   different URI, then the origin server MUST send an appropriate 3xx
   (Redirection) response; the user agent MAY then make its own decision
   regarding whether or not to redirect the request.

   A PUT request applied to the target resource MAY have side-effects on
   other resources.  For example, an article might have a URI for
   identifying "the current version" (a resource) that is separate from
   the URIs identifying each particular version (different resources
   that at one point shared the same state as the current version
   resource).  A successful PUT request on "the current version" URI
   might therefore create a new version resource in addition to changing
   the state of the target resource, and might also cause links to be
   added between the related resources.

   An origin server SHOULD reject any PUT request that contains a
   Content-Range header field (Section 4.2 of [Part5]), since it might
   be misinterpreted as partial content (or might be partial content
   that is being mistakenly PUT as a full representation).  Partial
   content updates are possible by targeting a separately identified
   resource with state that overlaps a portion of the larger resource,
   or by using a different method that has been specifically defined for
   partial updates (for example, the PATCH method defined in [RFC5789]).

   Responses to the PUT method are not cacheable.  If a PUT request
   passes through a cache that has one or more stored responses for the
   effective request URI, those stored responses will be invalidated
   (see Section 6 of [Part6]).

4.3.5.  DELETE

   The DELETE method requests that the origin server remove the
   association between the target resource and its current
   functionality.  In effect, this method is similar to the rm command
   in UNIX: it expresses a deletion operation on the URI mapping of the
   origin server, rather than an expectation that the previously
   associated information be deleted.

   If the target resource has one or more current representations, they



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   might or might not be destroyed by the origin server, and the
   associated storage might or might not be reclaimed, depending
   entirely on the nature of the resource and its implementation by the
   origin server (which are beyond the scope of this specification).
   Likewise, other implementation aspects of a resource might need to be
   deactivated or archived as a result of a DELETE, such as database or
   gateway connections.  In general, it is assumed that the origin
   server will only allow DELETE on resources for which it has a
   prescribed mechanism for accomplishing the deletion.

   Relatively few resources allow the DELETE method -- its primary use
   is for remote authoring environments, where the user has some
   direction regarding its effect.  For example, a resource that was
   previously created using a PUT request, or identified via the
   Location header field after a 201 (Created) response to a POST
   request, might allow a corresponding DELETE request to undo those
   actions.  Similarly, custom user agent implementations that implement
   an authoring function, such as revision control clients using HTTP
   for remote operations, might use DELETE based on an assumption that
   the server's URI space has been crafted to correspond to a version
   repository.

   If a DELETE method is successfully applied, the origin server SHOULD
   send a 202 (Accepted) status code if the action seems okay but has
   not yet been enacted, a 204 (No Content) status code if the action
   has been enacted and no further information is to be supplied, or a
   200 (OK) status code if the action has been enacted and the response
   message includes a representation describing the status.

   A payload within a DELETE request message has no defined semantics;
   sending a payload body on a DELETE request might cause some existing
   implementations to reject the request.

   Responses to the DELETE method are not cacheable.  If a DELETE
   request passes through a cache that has one or more stored responses
   for the effective request URI, those stored responses will be
   invalidated (see Section 6 of [Part6]).

4.3.6.  CONNECT

   The CONNECT method requests that the recipient establish a tunnel to
   the destination origin server identified by the request-target and,
   if successful, thereafter restrict its behavior to blind forwarding
   of packets, in both directions, until the connection is closed.

   CONNECT is intended only for use in requests to a proxy.  An origin
   server that receives a CONNECT request for itself MAY respond with a
   2xx status code to indicate that a connection is established.



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   However, most origin servers do not implement CONNECT.

   A client sending a CONNECT request MUST send the authority form of
   request-target (Section 5.3 of [Part1]); i.e., the request-target
   consists of only the host name and port number of the tunnel
   destination, separated by a colon.  For example,

     CONNECT server.example.com:80 HTTP/1.1
     Host: server.example.com:80


   The recipient proxy can establish a tunnel either by directly
   connecting to the request-target or, if configured to use another
   proxy, by forwarding the CONNECT request to the next inbound proxy.
   Any 2xx (Successful) response indicates that the sender (and all
   inbound proxies) will switch to tunnel mode immediately after the
   blank line that concludes the successful response's header block;
   data received after that blank line is from the server identified by
   the request-target.  Any response other than a successful response
   indicates that the tunnel has not yet been formed and that the
   connection remains governed by HTTP.

   A server SHOULD NOT send any Transfer-Encoding or Content-Length
   header fields in a successful response.  A client MUST ignore any
   Content-Length or Transfer-Encoding header fields received in a
   successful response.

   There are significant risks in establishing a tunnel to arbitrary
   servers, particularly when the destination is a well-known or
   reserved TCP port that is not intended for Web traffic.  For example,
   a CONNECT to a request-target of "example.com:25" would suggest that
   the proxy connect to the reserved port for SMTP traffic; if allowed,
   that could trick the proxy into relaying spam email.  Proxies that
   support CONNECT SHOULD restrict its use to a limited set of known
   ports or a configurable whitelist of safe request targets.

   Proxy authentication might be used to establish the authority to
   create a tunnel.  For example,

     CONNECT server.example.com:80 HTTP/1.1
     Host: server.example.com:80
     Proxy-Authorization: basic aGVsbG86d29ybGQ=


   When a tunnel intermediary detects that either side has closed its
   connection, any outstanding data that came from that side will first
   be sent to the other side and then the intermediary will close both
   connections.  If there is outstanding data left undelivered, that



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   data will be discarded.

   A payload within a CONNECT request message has no defined semantics;
   sending a payload body on a CONNECT request might cause some existing
   implementations to reject the request.

   Responses to the CONNECT method are not cacheable.

4.3.7.  OPTIONS

   The OPTIONS method requests information about the communication
   options available on the request/response chain identified by the
   effective request URI.  This method allows a client to determine the
   options and/or requirements associated with a resource, or the
   capabilities of a server, without implying a resource action.

   An OPTIONS request with an asterisk ("*") as the request-target
   (Section 5.3 of [Part1]) applies 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 conformance (or lack thereof).

   If the request-target is not an asterisk, the OPTIONS request applies
   to the options that are available when communicating with the target
   resource.

   A server generating a successful response to OPTIONS SHOULD send any
   header fields that might indicate optional features implemented by
   the server and applicable to the target resource (e.g., Allow),
   including potential extensions not defined by this specification.
   The response payload, if any, might also describe the communication
   options in a machine or human-readable representation.  A standard
   format for such a representation is not defined by this
   specification, but might be defined by future extensions to HTTP.  A
   server MUST generate a Content-Length field with a value of "0" if no
   payload body is to be sent in the response.

   A client MAY send a Max-Forwards header field in an OPTIONS request
   to target a specific recipient in the request chain (see
   Section 5.1.2).  A proxy MUST NOT generate a Max-Forwards header
   field while forwarding a request unless that request was received
   with a Max-Forwards field.

   A client that generates an OPTIONS request containing a payload body
   MUST send a valid Content-Type header field describing the
   representation media type.  Although this specification does not



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   define any use for such a payload, future extensions to HTTP might
   use the OPTIONS body to make more detailed queries about the target
   resource.

   Responses to the OPTIONS method are not cacheable.

4.3.8.  TRACE

   The TRACE method requests a remote, application-level loop-back of
   the request message.  The final recipient of the request SHOULD
   reflect the message received, excluding some fields described below,
   back to the client as the message body of a 200 (OK) response with a
   Content-Type of "message/http" (Section 7.3.1 of [Part1]).  The final
   recipient is either the origin server or the first server to receive
   a Max-Forwards value of zero (0) in the request (Section 5.1.2).

   A client MUST NOT send header fields in a TRACE request containing
   sensitive data that might be disclosed by the response.  For example,
   it would be foolish for a user agent to send stored user credentials
   [Part7] or cookies [RFC6265] in a TRACE request.  The final recipient
   SHOULD exclude any request header fields from the response body that
   are likely to contain sensitive data.

   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 5.7.1 of
   [Part1]) 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.

   A client MUST NOT send a message body in a TRACE request.

   Responses to the TRACE method are not cacheable.

5.  Request Header Fields

   A client sends request header fields to provide more information
   about the request context, make the request conditional based on the
   target resource state, suggest preferred formats for the response,
   supply authentication credentials, or modify the expected request
   processing.  These fields act as request modifiers, similar to the
   parameters on a programming language method invocation.

5.1.  Controls

   Controls are request header fields that direct specific handling of
   the request.



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   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Cache-Control     | Section 7.2 of [Part6] |
   | Expect            | Section 5.1.1          |
   | Host              | Section 5.4 of [Part1] |
   | Max-Forwards      | Section 5.1.2          |
   | Pragma            | Section 7.4 of [Part6] |
   | Range             | Section 3.1 of [Part5] |
   | TE                | Section 4.3 of [Part1] |
   +-------------------+------------------------+

5.1.1.  Expect

   The "Expect" header field is used to indicate that particular server
   behaviors are required by the client.

     Expect       = 1#expectation

     expectation  = expect-name [ BWS "=" BWS expect-value ]
                                *( OWS ";" [ OWS expect-param ] )
     expect-param = expect-name [ BWS "=" BWS expect-value ]

     expect-name  = token
     expect-value = token / quoted-string

   If all received Expect header field(s) are syntactically valid but
   contain an expectation that the recipient does not understand or
   cannot comply with, the recipient MUST respond with a 417
   (Expectation Failed) status code.  A recipient of a syntactically
   invalid Expectation header field MUST respond with a 4xx status code
   other than 417.

   Comparison is case-insensitive for names (expect-name), and case-
   sensitive for values (expect-value).

   The Expect header field MUST be forwarded if the request is
   forwarded.

   Many older HTTP/1.0 and HTTP/1.1 servers do not understand the Expect
   header field.

5.1.1.1.  Use of the 100 (Continue) Status

   The only expectation defined by this specification is:






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   100-continue

      The request includes a payload body and the client will wait for a
      100 (Continue) response after sending the request header section
      but before sending the payload body.  The 100-continue expectation
      does not use any expect-params.

   The primary purpose of the 100 (Continue) status code (Section 6.2.1)
   is to allow a client that is sending a request message with a payload
   to determine if the origin server is willing to accept the request
   (based on the request header fields) before the client sends the
   payload body.  In some cases, it might either be inappropriate or
   highly inefficient for the client to send the payload 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 payload body, it MUST send an Expect header field with the
      "100-continue" expectation.

   o  A client MUST NOT send an Expect header field with the "100-
      continue" expectation if it does not intend to send a payload
      body.

   Because of the presence of older implementations, the protocol allows
   ambiguous situations in which a client might send "Expect: 100-
   continue" without receiving either a 417 (Expectation Failed) or a
   100 (Continue) status code.  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 code, the client SHOULD NOT
   wait for an indefinite period before sending the payload body.

   Requirements for HTTP/1.1 origin servers:

   o  Upon receiving a request that includes an Expect header field with
      the "100-continue" expectation, an origin server MUST either
      respond with 100 (Continue) status code and continue to read from
      the input stream, or respond with a final status code.  The origin
      server MUST NOT wait for the payload body before sending the 100
      (Continue) response.  If the origin server responds with a final
      status code, it MUST NOT have performed the request method and MAY
      either close the connection or continue to read and discard the
      rest of the request.

   o  An origin server SHOULD NOT send a 100 (Continue) response if the
      request message does not include an Expect header field with the
      "100-continue" expectation, and MUST NOT send a 100 (Continue)



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      response if such a request comes from an HTTP/1.0 (or earlier)
      client.  There is an exception to this rule: for compatibility
      with [RFC2068], a server MAY send a 100 (Continue) status code in
      response to an HTTP/1.1 PUT or POST request that does not include
      an Expect 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 code, 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 payload body for the
      corresponding request.

   o  An origin server that sends a 100 (Continue) response MUST
      ultimately send a final status code, once the payload 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 header field with the "100-continue" expectation, the
      request includes a payload body, and the server responds with a
      final status code before reading the entire payload 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 ought 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 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 code.

   o  Proxies SHOULD maintain a record of 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



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      not include an Expect header field with the "100-continue"
      expectation.  This requirement overrides the general rule for
      forwarding of 1xx responses (see Section 6.2.1).

5.1.2.  Max-Forwards

   The "Max-Forwards" header field provides a mechanism with the TRACE
   (Section 4.3.8) and OPTIONS (Section 4.3.7) methods to limit the
   number of times that the request is forwarded by proxies.  This can
   be useful when the client is attempting to trace a request that
   appears to be failing or looping mid-chain.

     Max-Forwards = 1*DIGIT

   The Max-Forwards value is a decimal integer indicating the remaining
   number of times this request message can be forwarded.

   Each 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 request
   methods.

5.2.  Conditionals

   The HTTP conditional request header fields [Part4] allow a client to
   place a precondition on the state of the target resource, so that the
   action corresponding to the method semantics will not be applied if
   the precondition evaluates to false.  Each precondition defined by
   this specification consists of a comparison between a set of
   validators obtained from prior representations of the target resource
   to the current state of validators for the selected representation
   (Section 7.2).  Hence, these preconditions evaluate whether the state
   of the target resource has changed since a given state known by the
   client.  The effect of such an evaluation depends on the method
   semantics and choice of conditional, as defined in Section 5 of
   [Part4].









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   +---------------------+------------------------+
   | Header Field Name   | Defined in...          |
   +---------------------+------------------------+
   | If-Match            | Section 3.1 of [Part4] |
   | If-None-Match       | Section 3.2 of [Part4] |
   | If-Modified-Since   | Section 3.3 of [Part4] |
   | If-Unmodified-Since | Section 3.4 of [Part4] |
   | If-Range            | Section 3.2 of [Part5] |
   +---------------------+------------------------+

5.3.  Content Negotiation

   The following request header fields are sent by a user agent to
   engage in proactive negotiation of the response content, as defined
   in Section 3.4.1.  The preferences sent in these fields apply to any
   content in the response, including representations of the target
   resource, representations of error or processing status, and
   potentially even the miscellaneous text strings that might appear
   within the protocol.

   +-------------------+---------------+
   | Header Field Name | Defined in... |
   +-------------------+---------------+
   | Accept            | Section 5.3.2 |
   | Accept-Charset    | Section 5.3.3 |
   | Accept-Encoding   | Section 5.3.4 |
   | Accept-Language   | Section 5.3.5 |
   +-------------------+---------------+

5.3.1.  Quality Values

   Many of the request header fields for proactive negotiation use a
   common parameter, named "q" (case-insensitive), to assign a relative
   "weight" to the preference for that associated kind of content.  This
   weight is referred to as a "quality value" (or "qvalue") because the
   same parameter name is often used within server configurations to
   assign a weight to the relative quality of the various
   representations that can be selected for a resource.

   The weight is normalized to a real number in the range 0 through 1,
   where 0.001 is the least preferred and 1 is the most preferred; a
   value of 0 means "not acceptable".  If no "q" parameter is present,
   the default weight is 1.

     weight = OWS ";" OWS "q=" qvalue
     qvalue = ( "0" [ "." 0*3DIGIT ] )
            / ( "1" [ "." 0*3("0") ] )




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   A sender of qvalue MUST NOT generate more than three digits after the
   decimal point.  User configuration of these values ought to be
   limited in the same fashion.

5.3.2.  Accept

   The "Accept" header field can be used by user agents to specify
   response media types that are acceptable.  Accept header fields 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 = #( media-range [ accept-params ] )

     media-range    = ( "*/*"
                      / ( type "/" "*" )
                      / ( type "/" subtype )
                      ) *( OWS ";" OWS parameter )
     accept-params  = weight *( accept-ext )
     accept-ext     = OWS ";" OWS token [ "=" word ]

   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 can include media type
   parameters that are applicable to that range.

   Each media-range might be followed by zero or more applicable media
   type parameters (e.g., charset), an optional "q" parameter for
   indicating a relative weight (Section 5.3.1), and then zero or more
   extension parameters.  The "q" parameter is necessary if any
   extensions (accept-ext) are present, since it acts as a separator
   between the two parameter sets.

      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".



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   A request without any Accept header field implies that the user agent
   will accept any media type in response.  If the header field is
   present in a request and none of the available representations for
   the response have a media type that is listed as acceptable, the
   origin server can either honor the header field by sending a 406 (Not
   Acceptable) response or disregard the header field by treating the
   response as if it is not subject to content negotiation.

   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 equally preferred media types, but if they do not exist, then
   send the text/x-dvi representation, and if that does not exist, send
   the text/plain representation".

   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/plain, text/plain;format=flowed, */*

   have the following precedence:

   1.  text/plain;format=flowed

   2.  text/plain

   3.  text/*

   4.  */*

   The media type quality factor associated with a given type is
   determined by finding the media range with the highest precedence
   that matches the 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:









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   +-------------------+---------------+
   | Media Type        | Quality Value |
   +-------------------+---------------+
   | 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 that cannot interact with other rendering agents, this
   default set ought to be configurable by the user.

5.3.3.  Accept-Charset

   The "Accept-Charset" header field can be sent by a user agent to
   indicate what charsets are acceptable in textual response content.
   This field allows user agents capable of understanding more
   comprehensive or special-purpose charsets to signal that capability
   to an origin server that is capable of representing information in
   those charsets.

     Accept-Charset = 1#( ( charset / "*" ) [ weight ] )

   Charset names are defined in Section 3.1.1.2.  A user agent MAY
   associate a quality value with each charset to indicate the user's
   relative preference for that charset, as defined in Section 5.3.1.
   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 charset that is not mentioned elsewhere in the Accept-
   Charset field.  If no "*" is present in an Accept-Charset field, then
   any charsets not explicitly mentioned in the field are considered
   "not acceptable" to the client.

   A request without any Accept-Charset header field implies that the
   user agent will accept any charset in response.  Most general-purpose
   user agents do not send Accept-Charset, unless specifically
   configured to do so, because a detailed list of supported charsets
   makes it easier for a server to identify an individual by virtue of
   the user agent's request characteristics (Section 9.6).

   If an Accept-Charset header field is present in a request and none of



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   the available representations for the response has a charset that is
   listed as acceptable, the origin server can either honor the header
   field, by sending a 406 (Not Acceptable) response, or disregard the
   header field by treating the resource as if it is not subject to
   content negotiation.

5.3.4.  Accept-Encoding

   The "Accept-Encoding" header field can be used by user agents to
   indicate what response content-codings (Section 3.1.2.1) are
   acceptable in the response.  An "identity" token is used as a synonym
   for "no encoding" in order to communicate when no encoding is
   preferred.

     Accept-Encoding  = #( codings [ weight ] )
     codings          = content-coding / "identity" / "*"

   Each codings value MAY be given an associated quality value
   representing the preference for that encoding, as defined in
   Section 5.3.1.  The asterisk "*" symbol in an Accept-Encoding field
   matches any available content-coding not explicitly listed in the
   header field.

   For example,

     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 request without an Accept-Encoding header field implies that the
   user agent has no preferences regarding content-codings.  Although
   this allows the server to use any content-coding in a response, it
   does not imply that the user agent will be able to correctly process
   all encodings.

   A server tests whether a content-coding for a given representation is
   acceptable using these rules:

   1.  If no Accept-Encoding field is in the request, any content-coding
       is considered acceptable by the user agent.

   2.  If the representation has no content-coding, then it is
       acceptable by default unless specifically excluded by the Accept-
       Encoding field stating either "identity;q=0" or "*;q=0" without a
       more specific entry for "identity".




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   3.  If the representation's 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 5.3.1, a qvalue of 0 means "not acceptable".)

   4.  If multiple content-codings are acceptable, then the acceptable
       content-coding with the highest non-zero qvalue is preferred.

   An Accept-Encoding header field with a combined field-value that is
   empty implies that the user agent does not want any content-coding in
   response.  If an Accept-Encoding header field is present in a request
   and none of the available representations for the response have a
   content-coding that is listed as acceptable, the origin server SHOULD
   send a response without any content-coding.

      Note: Most HTTP/1.0 applications do not recognize or obey qvalues
      associated with content-codings.  This means that qvalues might
      not work and are not permitted with x-gzip or x-compress.

5.3.5.  Accept-Language

   The "Accept-Language" header field can be used by user agents to
   indicate the set of natural languages that are preferred in the
   response.  Language tags are defined in Section 3.1.3.1.

     Accept-Language = 1#( language-range [ weight ] )
     language-range  =
               <language-range, defined in [RFC4647], Section 2.1>

   Each language-range can be given an associated quality value
   representing an estimate of the user's preference for the languages
   specified by that range, as defined in Section 5.3.1.  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 request without any Accept-Language header field implies that the
   user agent will accept any language in response.  If the header field
   is present in a request and none of the available representations for
   the response have a matching language tag, the origin server can
   either disregard the header field by treating the response as if it
   is not subject to content negotiation, or honor the header field by
   sending a 406 (Not Acceptable) response.  However, the latter is not
   encouraged, as doing so can prevent users from accessing content that
   they might be able to use (with translation software, for example).




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   Note that some recipients treat the order in which language tags are
   listed as an indication of descending priority, particularly for tags
   that are assigned equal quality values (no value is the same as q=1).
   However, this behavior cannot be relied upon.  For consistency and to
   maximize interoperability, many user agents assign each language tag
   a unique quality value while also listing them in order of decreasing
   quality.  Additional discussion of language priority lists can be
   found in Section 2.3 of [RFC4647].

   For matching, Section 3 of [RFC4647] defines several matching
   schemes.  Implementations can offer the most appropriate matching
   scheme for their requirements.  The "Basic Filtering" scheme
   ([RFC4647], Section 3.3.1) is identical to the matching scheme that
   was previously defined for HTTP in Section 14.4 of [RFC2616].

   It might be contrary to the privacy expectations of the user to send
   an Accept-Language header field with the complete linguistic
   preferences of the user in every request (Section 9.6).

   Since intelligibility is highly dependent on the individual user,
   user agents need to allow user control over the linguistic
   preference.  A user agent that does not provide such control to the
   user MUST NOT send an Accept-Language header field.

      Note: User agents ought to provide guidance to users when setting
      a preference, since users are rarely familiar with the details of
      language matching as described above.  For 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 the list for
      better matching behavior.

5.4.  Authentication Credentials

   Two header fields are used for carrying authentication credentials,
   as defined in [Part7].  Note that various custom mechanisms for user
   authentication use the Cookie header field for this purpose, as
   defined in [RFC6265].

   +---------------------+------------------------+
   | Header Field Name   | Defined in...          |
   +---------------------+------------------------+
   | Authorization       | Section 4.1 of [Part7] |
   | Proxy-Authorization | Section 4.3 of [Part7] |
   +---------------------+------------------------+






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5.5.  Request Context

   The following request header fields provide additional information
   about the request context, including information about the user, user
   agent, and resource behind the request.

   +-------------------+---------------+
   | Header Field Name | Defined in... |
   +-------------------+---------------+
   | From              | Section 5.5.1 |
   | Referer           | Section 5.5.2 |
   | User-Agent        | Section 5.5.3 |
   +-------------------+---------------+

5.5.1.  From

   The "From" header field contains an Internet email address for a
   human user who controls the requesting user agent.  The address ought
   to be machine-usable, as defined by "mailbox" in Section 3.4 of
   [RFC5322]:

     From    = mailbox

     mailbox = <mailbox, defined in [RFC5322], Section 3.4>

   An example is:

     From: webmaster@example.org

   The From header field is rarely sent by non-robotic user agents.  A
   user agent SHOULD NOT send a From header field without explicit
   configuration by the user, since that might conflict with the user's
   privacy interests or their site's security policy.

   Robotic user agents SHOULD send a valid From header field so that the
   person responsible for running the robot can be contacted if problems
   occur on servers, such as if the robot is sending excessive,
   unwanted, or invalid requests.

   Servers SHOULD NOT use the From header field for access control or
   authentication, since most recipients will assume that the field
   value is public information.

5.5.2.  Referer

   The "Referer" [sic] header field allows the user agent to specify a
   URI reference for the resource from which the target URI was obtained
   (i.e., the "referrer", though the field name is misspelled).  A user



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   agent MUST exclude any fragment or userinfo components [RFC3986] when
   generating the Referer field value.

     Referer = absolute-URI / partial-URI

   Referer allows servers to generate back-links to other resources for
   simple analytics, logging, optimized caching, etc.  It also allows
   obsolete or mistyped links to be found for maintenance.  Some servers
   use Referer as a means of denying links from other sites (so-called
   "deep linking") or restricting cross-site request forgery (CSRF), but
   not all requests contain a Referer header field.

   Example:

     Referer: http://www.example.org/hypertext/Overview.html

   If the target URI was obtained from a source that does not have its
   own URI (e.g., input from the user keyboard, or an entry within the
   user's bookmarks/favorites), the user agent MUST either exclude
   Referer or send it with a value of "about:blank".

   The Referer field has the potential to reveal information about the
   request context or browsing history of the user, which is a privacy
   concern if the referring resource's identifier reveals personal
   information (such as an account name) or a resource that is supposed
   to be confidential (such as behind a firewall or internal to a
   secured service).  Most general-purpose user agents do not send the
   Referer header field when the referring resource is a local "file" or
   "data" URI.  A user agent SHOULD NOT send a Referer header field in
   an unsecured HTTP request if the referring page was received with a
   secure protocol.  See Section 9.3 for additional security
   considerations.

   Some intermediaries have been known to indiscriminately remove
   Referer header fields from outgoing requests.  This has the
   unfortunate side-effect of interfering with protection against CSRF
   attacks, which can be far more harmful to their users.
   Intermediaries and user agent extensions that wish to limit
   information disclosure in Referer ought to restrict their changes to
   specific edits, such as replacing internal domain names with
   pseudonyms or truncating the query and/or path components.
   Intermediaries SHOULD NOT modify or delete the Referer field when the
   field value shares the same scheme and host as the request target.

5.5.3.  User-Agent

   The "User-Agent" header field contains information about the user
   agent originating the request, which is often used by servers to help



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   identify the scope of reported interoperability problems, to work
   around or tailor responses to avoid particular user agent
   limitations, and for analytics regarding browser or operating system
   use.  A user agent SHOULD send a User-Agent field in each request
   unless specifically configured not to do so.

     User-Agent = product *( RWS ( product / comment ) )

   The User-Agent field-value consists of one or more product
   identifiers, each followed by zero or more comments (Section 3.2 of
   [Part1]), which together identify the user agent software and its
   significant subproducts.  By convention, the product identifiers are
   listed in decreasing order of their significance for identifying the
   user agent software.  Each product identifier consists of a name and
   optional version.

     product         = token ["/" product-version]
     product-version = token

   Senders SHOULD limit generated product identifiers to what is
   necessary to identify the product; senders MUST NOT generate
   advertising or other non-essential information within the product
   identifier.  Senders SHOULD NOT generate information in product-
   version that is not a version identifier (i.e., successive versions
   of the same product name ought to only differ in the product-version
   portion of the product identifier).

   Example:

     User-Agent: CERN-LineMode/2.15 libwww/2.17b3

   A user agent SHOULD NOT generate a User-Agent field containing
   needlessly fine-grained detail and SHOULD limit the addition of
   subproducts by third parties.  Overly long and detailed User-Agent
   field values increase request latency and the risk of a user being
   identified against their wishes ("fingerprinting").

   Likewise, implementations are encouraged not to use the product
   tokens of other implementations in order to declare compatibility
   with them, as this circumvents the purpose of the field.  If a user
   agent masquerades as a different user agent, recipients can assume
   that the user intentionally desires to see responses tailored for
   that identified user agent, even if they might not work as well for
   the actual user agent being used.







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6.  Response Status Codes

   The status-code element is a 3-digit integer code giving the result
   of the attempt to understand and satisfy the request.

   HTTP status codes are extensible.  HTTP clients are not required to
   understand the meaning of all registered status codes, though such
   understanding is obviously desirable.  However, clients MUST
   understand the class of any status code, as indicated by the first
   digit, and treat an unrecognized status code as being equivalent to
   the x00 status code of that class, with the exception that a response
   with an unrecognized status code MUST NOT be cached.

   For example, if an unrecognized status code of 471 is received by a
   client, the client can assume that there was something wrong with its
   request and treat the response as if it had received a 400 status
   code.  The response message will usually contain a representation
   that explains the status.

   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): The request was received, continuing process

   o  2xx (Successful): The request was successfully received,
      understood, and accepted

   o  3xx (Redirection): Further action needs to 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

6.1.  Overview of Status Codes

   The status codes listed below are defined in this specification,
   Section 4 of [Part4], Section 4 of [Part5], and Section 3 of [Part7].
   The reason phrases listed here are only recommendations -- they can
   be replaced by local equivalents without affecting the protocol.








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   +------+-------------------------------+------------------------+
   | code | reason-phrase                 | Defined in...          |
   +------+-------------------------------+------------------------+
   | 100  | Continue                      | Section 6.2.1          |
   | 101  | Switching Protocols           | Section 6.2.2          |
   | 200  | OK                            | Section 6.3.1          |
   | 201  | Created                       | Section 6.3.2          |
   | 202  | Accepted                      | Section 6.3.3          |
   | 203  | Non-Authoritative Information | Section 6.3.4          |
   | 204  | No Content                    | Section 6.3.5          |
   | 205  | Reset Content                 | Section 6.3.6          |
   | 206  | Partial Content               | Section 4.1 of [Part5] |
   | 300  | Multiple Choices              | Section 6.4.1          |
   | 301  | Moved Permanently             | Section 6.4.2          |
   | 302  | Found                         | Section 6.4.3          |
   | 303  | See Other                     | Section 6.4.4          |
   | 304  | Not Modified                  | Section 4.1 of [Part4] |
   | 305  | Use Proxy                     | Section 6.4.5          |
   | 307  | Temporary Redirect            | Section 6.4.7          |
   | 400  | Bad Request                   | Section 6.5.1          |
   | 401  | Unauthorized                  | Section 3.1 of [Part7] |
   | 402  | Payment Required              | Section 6.5.2          |
   | 403  | Forbidden                     | Section 6.5.3          |
   | 404  | Not Found                     | Section 6.5.4          |
   | 405  | Method Not Allowed            | Section 6.5.5          |
   | 406  | Not Acceptable                | Section 6.5.6          |
   | 407  | Proxy Authentication Required | Section 3.2 of [Part7] |
   | 408  | Request Time-out              | Section 6.5.7          |
   | 409  | Conflict                      | Section 6.5.8          |
   | 410  | Gone                          | Section 6.5.9          |
   | 411  | Length Required               | Section 6.5.10         |
   | 412  | Precondition Failed           | Section 4.2 of [Part4] |
   | 413  | Payload Too Large             | Section 6.5.11         |
   | 414  | URI Too Long                  | Section 6.5.12         |
   | 415  | Unsupported Media Type        | Section 6.5.13         |
   | 416  | Range Not Satisfiable         | Section 4.4 of [Part5] |
   | 417  | Expectation Failed            | Section 6.5.14         |
   | 426  | Upgrade Required              | Section 6.5.15         |
   | 500  | Internal Server Error         | Section 6.6.1          |
   | 501  | Not Implemented               | Section 6.6.2          |
   | 502  | Bad Gateway                   | Section 6.6.3          |
   | 503  | Service Unavailable           | Section 6.6.4          |
   | 504  | Gateway Time-out              | Section 6.6.5          |
   | 505  | HTTP Version Not Supported    | Section 6.6.6          |
   +------+-------------------------------+------------------------+

   Note that this list is not exhaustive -- it does not include
   extension status codes defined in other specifications.



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   Responses with status codes that are defined as cacheable by default
   (e.g., 200, 203, 206, 300, 301, and 410 in this specification) can be
   reused by a cache with heuristic expiration unless otherwise
   indicated by the method definition or explicit cache controls
   [Part6]; all other status codes are not cacheable by default.

6.2.  Informational 1xx

   The 1xx (Informational) class of status code indicates an interim
   response for communicating connection status or request progress
   prior to completing the requested action and sending a final
   response.  All 1xx responses consist of only the status-line and
   optional header fields, and thus are terminated by the empty line at
   the end of the header block.  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 final response, even if the client does not expect one.  A
   user agent MAY ignore unexpected 1xx status responses.

   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 an "Expect: 100-continue" field when it forwards a
   request, then it need not forward the corresponding 100 (Continue)
   response(s).

6.2.1.  100 Continue

   The 100 (Continue) status code indicates that the initial part of a
   request has been received and has not yet been rejected by the
   server.  The server intends to send a final response after the
   request has been fully received and acted upon.

   When the request contains an Expect header field that includes a 100-
   continue expectation, the 100 response indicates that the server
   wishes to receive the request payload body, as described in
   Section 5.1.1.1.  The client ought to continue sending the request
   and discard the 100 response.

   If the request did not contain an Expect header field containing the
   100-continue expectation, the client can simply discard this interim
   response.







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6.2.2.  101 Switching Protocols

   The 101 (Switching Protocols) status code indicates that the server
   understands and is willing to comply with the client's request, via
   the Upgrade header field (Section 6.7 of [Part1]), for a change in
   the application protocol being used on this connection.  The server
   MUST generate an Upgrade header field in the response that indicates
   which protocol(s) will be switched to immediately after the empty
   line that terminates the 101 response.

   It is assumed that the server will only agree to switch protocols
   when it is advantageous to do so.  For example, switching to a newer
   version of HTTP might be advantageous over older versions, and
   switching to a real-time, synchronous protocol might be advantageous
   when delivering resources that use such features.

6.3.  Successful 2xx

   The 2xx (Successful) class of status code indicates that the client's
   request was successfully received, understood, and accepted.

6.3.1.  200 OK

   The
   200 (OK) status code indicates that the request has succeeded.  The
   payload sent in a 200 response depends on the request method.  For
   the methods defined by this specification, the intended meaning of
   the payload can be summarized as:

   GET  a representation of the target resource;

   HEAD  the same representation as GET, but without the representation
      data;

   POST  a representation of the status of, or results obtained from,
      the action;

   PUT, DELETE  a representation of the status of the action;

   OPTIONS  a representation of the communications options;

   TRACE  a representation of the request message as received by the end
      server.

   Aside from responses to CONNECT, a 200 response always has a payload,
   though an origin server MAY generate a payload body of zero length.
   If no payload is desired, an origin server ought to send 204 (No
   Content) instead.  For CONNECT, no payload is allowed because the



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   successful result is a tunnel, which begins immediately after the 200
   response header block.

   A 200 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.3.2.  201 Created

   The 201 (Created) status code indicates that the request has been
   fulfilled and has resulted in one or more new resources being
   created.  The primary resource created by the request is identified
   by either a Location header field in the response or, if no Location
   field is received, by the effective request URI.

   The 201 response payload typically describes and links to the
   resource(s) created.  See Section 7.2 for a discussion of the meaning
   and purpose of validator header fields, such as ETag and Last-
   Modified, in a 201 response.

6.3.3.  202 Accepted

   The 202 (Accepted) status code indicates that 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 in HTTP for re-sending a status code from an asynchronous
   operation.

   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 representation sent with this
   response ought to describe the request's current status and point to
   (or embed) a status monitor that can provide the user with an
   estimate of when the request will be fulfilled.

6.3.4.  203 Non-Authoritative Information

   The 203 (Non-Authoritative Information) status code indicates that
   the request was successful but the enclosed payload has been modified
   from that of the origin server's 200 (OK) response by a transforming
   proxy (Section 5.7.2 of [Part1]).  This status code allows the proxy
   to notify recipients when a transformation has been applied, since
   that knowledge might impact later decisions regarding the content.
   For example, future cache validation requests for the content might
   only be applicable along the same request path (through the same
   proxies).



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   The 203 response is similar to the Warning code of 214 Transformation
   Applied (Section 7.5 of [Part6]), which has the advantage of being
   applicable to responses with any status code.

   A 203 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.3.5.  204 No Content

   The 204 (No Content) status code indicates that the server has
   successfully fulfilled the request and that there is no additional
   content to send in the response payload body.  Metadata in the
   response header fields refer to the target resource and its selected
   representation after the requested action was applied.

   For example, if a 204 status code is received in response to a PUT
   request and the response contains an ETag header field, then the PUT
   was successful and the ETag field-value contains the entity-tag for
   the new representation of that target resource.

   The 204 response allows a server to indicate that the action has been
   successfully applied to the target resource, while implying that the
   user agent does not need to traverse away from its current "document
   view" (if any).  The server assumes that the user agent will provide
   some indication of the success to its user, in accord with its own
   interface, and apply any new or updated metadata in the response to
   its active representation.

   For example, a 204 status code is commonly used with document editing
   interfaces corresponding to a "save" action, such that the document
   being saved remains available to the user for editing.  It is also
   frequently used with interfaces that expect automated data transfers
   to be prevalent, such as within distributed version control systems.

   A 204 response is terminated by the first empty line after the header
   fields because it cannot contain a message body.

   A 204 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.3.6.  205 Reset Content

   The 205 (Reset Content) status code indicates that the server has
   fulfilled the request and desires that the user agent reset the
   "document view", which caused the request to be sent, to its original
   state as received from the origin server.

   This response is intended to support a common data entry use case



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   where the user receives content that supports data entry (a form,
   notepad, canvas, etc.), enters or manipulates data in that space,
   causes the entered data to be submitted in a request, and then the
   data entry mechanism is reset for the next entry so that the user can
   easily initiate another input action.

   Since the 205 status code implies that no additional content will be
   provided in the payload, the server MUST send a message body of zero
   length.  In other words, the server MUST send a "Content-Length: 0"
   field in a 205 response or close the connection immediately after
   sending the blank line terminating the header section.

6.4.  Redirection 3xx

   The 3xx (Redirection) class of status code indicates that further
   action needs to be taken by the user agent in order to fulfill the
   request.  If a Location header field (Section 7.1.2) is provided, the
   user agent MAY automatically redirect its request to the URI
   referenced by the Location field value, even if the specific status
   code is not understood.  Automatic redirection needs to done with
   care for methods not known to be safe, as defined in Section 4.2.1,
   since the user might not wish to redirect an unsafe request.

   There are several types of redirects:

   1.  Redirects that indicate the resource might be available at a
       different URI, as provided by the Location field, as in the
       status codes 301 (Moved Permanently), 302 (Found), and 307
       (Temporary Redirect).

   2.  Redirection that offers a choice of matching resources, each
       capable of representing the original request target, as in the
       300 (Multiple Choices) status code.

   3.  Redirection to a different resource, identified by the Location
       field, that can represent an indirect response to the request, as
       in the 303 (See Other) status code.

   4.  Redirection to a previously cached result, as in the 304 (Not
       Modified) status code.

      Note: In HTTP/1.0, the status codes 301 (Moved Permanently) and
      302 (Found) were defined for the first type of redirect
      ([RFC1945], Section 9.3).  Early user agents split on whether the
      method applied to the redirect target would be the same as the
      original request or would be rewritten as GET.  Although HTTP
      originally defined the former semantics for 301 and 302 (to match
      its original implementation at CERN), and defined 303 (See Other)



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      to match the latter semantics, prevailing practice gradually
      converged on the latter semantics for 301 and 302 as well.  The
      first revision of HTTP/1.1 added 307 (Temporary Redirect) to
      indicate the former semantics without being impacted by divergent
      practice.  Over 10 years later, most user agents still do method
      rewriting for 301 and 302; therefore, this specification makes
      that behavior conformant when the original request is POST.

   Clients SHOULD detect and intervene in cyclical redirections (i.e.,
   "infinite" redirection loops).

      Note: An earlier version of this specification recommended a
      maximum of five redirections ([RFC2068], Section 10.3).  Content
      developers need to be aware that some clients might implement such
      a fixed limitation.

6.4.1.  300 Multiple Choices

   The 300 (Multiple Choices) status code indicates that the target
   resource has more than one representation, each with its own more
   specific identifier, and information about the alternatives is being
   provided so that the user (or user agent) can select a preferred
   representation by redirecting its request to one or more of those
   identifiers.  In other words, the server desires that the user agent
   engage in reactive negotiation to select the most appropriate
   representation(s) for its needs (Section 3.4).

   If the server has a preferred choice, the server SHOULD generate a
   Location header field containing a preferred choice's URI reference.
   The user agent MAY use the Location field value for automatic
   redirection.

   For request methods other than HEAD, the server SHOULD generate a
   payload in the 300 response containing a list of representation
   metadata and URI reference(s) from which the user or user agent can
   choose the one most preferred.  The user agent MAY make a selection
   from that list automatically, depending upon the list format, but
   this specification does not define a standard for such automatic
   selection.

   A 300 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

      Note: The original proposal for 300 defined the URI header field
      as providing a list of alternative representations, such that it
      would be usable for 200, 300, and 406 responses and be transferred
      in responses to the HEAD method.  However, lack of deployment and
      disagreement over syntax led to both URI and Alternates (a



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      subsequent proposal) being dropped from this specification.  It is
      possible to communicate the list using a set of Link header fields
      [RFC5988], each with a relationship of "alternate", though
      deployment is a chicken-and-egg problem.

6.4.2.  301 Moved Permanently

   The 301 (Moved Permanently) status code indicates that the target
   resource has been assigned a new permanent URI and any future
   references to this resource ought to use one of the enclosed URIs.
   Clients with link editing capabilities ought to automatically re-link
   references to the effective request URI to one or more of the new
   references sent by the server, where possible.

   The server SHOULD generate a Location header field in the response
   containing a preferred URI reference for the new permanent URI.  The
   user agent MAY use the Location field value for automatic
   redirection.  The server's response payload usually contains a short
   hypertext note with a hyperlink to the new URI(s).

      Note: For historic reasons, user agents MAY change the request
      method from POST to GET for the subsequent request.  If this
      behavior is undesired, status code 307 (Temporary Redirect) can be
      used instead.

   A 301 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.4.3.  302 Found

   The 302 (Found) status code indicates that the target resource
   resides temporarily under a different URI.  Since the redirection
   might be altered on occasion, the client ought to continue to use the
   effective request URI for future requests.

   The server SHOULD generate a Location header field in the response
   containing a URI reference for the different URI.  The user agent MAY
   use the Location field value for automatic redirection.  The server's
   response payload usually contains a short hypertext note with a
   hyperlink to the different URI(s).

      Note: For historic reasons, user agents MAY change the request
      method from POST to GET for the subsequent request.  If this
      behavior is undesired, status code 307 (Temporary Redirect) can be
      used instead.






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6.4.4.  303 See Other

   The 303 (See Other) status code indicates that the server is
   redirecting the user agent to a different resource, as indicated by a
   URI in the Location header field, that is intended to provide an
   indirect response to the original request.  In order to satisfy the
   original request, a user agent ought to perform a retrieval request
   using the Location URI (a GET or HEAD request if using HTTP), which
   can itself be redirected further, and present the eventual result as
   an answer to the original request.  Note that the new URI in the
   Location header field is not considered equivalent to the effective
   request URI.

   This status code is applicable to any HTTP method.  It is primarily
   used to allow the output of a POST action to redirect the user agent
   to a selected resource, since doing so provides the information
   corresponding to the POST response in a form that can be separately
   identified, bookmarked, and cached independent of the original
   request.

   A 303 response to a GET request indicates that the origin server does
   not have a representation of the target resource that can be
   transferred by the server over HTTP.  However, the Location field
   value refers to a resource that is descriptive of the target
   resource, such that making a retrieval request on that other resource
   might result in a representation that is useful to recipients without
   implying that it represents the original target resource.  Note that
   answers to the questions of what can be represented, what
   representations are adequate, and what might be a useful description
   are outside the scope of HTTP.

   Except for responses to a HEAD request, the representation of a 303
   response ought to contain a short hypertext note with a hyperlink to
   the same URI reference provided in the Location header field.

6.4.5.  305 Use Proxy

   The 305 (Use Proxy) status code was defined in a previous version of
   this specification and is now deprecated (Appendix B).

6.4.6.  306 (Unused)

   The 306 status code was defined in a previous version of this
   specification, is no longer used, and the code is reserved.







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6.4.7.  307 Temporary Redirect

   The 307 (Temporary Redirect) status code indicates that the target
   resource resides temporarily under a different URI and the user agent
   MUST NOT change the request method if it performs an automatic
   redirection to that URI.  Since the redirection can change over time,
   the client ought to continue using the original effective request URI
   for future requests.

   The server SHOULD generate a Location header field in the response
   containing a URI reference for the different URI.  The user agent MAY
   use the Location field value for automatic redirection.  The server's
   response payload usually contains a short hypertext note with a
   hyperlink to the different URI(s).

      Note: This status code is similar to 302 (Found), except that it
      does not allow changing the request method from POST to GET.  This
      specification defines no equivalent counterpart for 301 (Moved
      Permanently) ([status-308], however, defines the status code 308
      (Permanent Redirect) for this purpose).

6.5.  Client Error 4xx

   The 4xx (Client Error) class of status code indicates that the client
   seems to have erred.  Except when responding to a HEAD request, the
   server SHOULD send a representation 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 representation to the user.

6.5.1.  400 Bad Request

   The 400 (Bad Request) status code indicates that the server cannot or
   will not process the request because the received syntax is invalid,
   nonsensical, or exceeds some limitation on what the server is willing
   to process.

6.5.2.  402 Payment Required

   The 402 (Payment Required) status code is reserved for future use.

6.5.3.  403 Forbidden

   The 403 (Forbidden) status code indicates that the server understood
   the request but refuses to authorize it.  A server that wishes to
   make public why the request has been forbidden can describe that
   reason in the response payload (if any).




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   If authentication credentials were provided in the request, the
   server considers them insufficient to grant access.  The client
   SHOULD NOT repeat the request with the same credentials.  The client
   MAY repeat the request with new or different credentials.  However, a
   request might be forbidden for reasons unrelated to the credentials.

   An origin server that wishes to "hide" the current existence of a
   forbidden target resource MAY instead respond with a status code of
   404 (Not Found).

6.5.4.  404 Not Found

   The 404 (Not Found) status code indicates that the origin server did
   not find a current representation for the target resource or is not
   willing to disclose that one exists.  A 404 status does not indicate
   whether this lack of representation is temporary or permanent; the
   410 (Gone) status code is preferred over 404 if the origin server
   knows, presumably through some configurable means, that the condition
   is likely to be permanent.

   A 404 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.5.  405 Method Not Allowed

   The 405 (Method Not Allowed) status code indicates that the method
   specified in the request-line is known by the origin server but not
   supported by the target resource.  The origin server MUST generate an
   Allow header field in a 405 response containing a list of the target
   resource's currently supported methods.

   A 405 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.6.  406 Not Acceptable

   The 406 (Not Acceptable) status code indicates that the target
   resource does not have a current representation that would be
   acceptable to the user agent, according to the proactive negotiation
   header fields received in the request (Section 5.3), and the server
   is unwilling to supply a default representation.

   The server SHOULD generate a payload containing a list of available
   representation characteristics and corresponding resource identifiers
   from which the user or user agent can choose the one most
   appropriate.  A user agent MAY automatically select the most
   appropriate choice from that list.  However, this specification does
   not define any standard for such automatic selection, as described in



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   Section 6.4.1.

6.5.7.  408 Request Timeout

   The 408 (Request Timeout) status code indicates that the server did
   not receive a complete request message within the time that it was
   prepared to wait.  A server SHOULD send the close connection option
   (Section 6.1 of [Part1]) in the response, since 408 implies that the
   server has decided to close the connection rather than continue
   waiting.  If the client has an outstanding request in transit, the
   client MAY repeat that request on a new connection.

6.5.8.  409 Conflict

   The 409 (Conflict) status code indicates that the request could not
   be completed due to a conflict with the current state of the
   resource.  This code is used in situations where the user might be
   able to resolve the conflict and resubmit the request.  The server
   SHOULD generate a payload that includes enough information for a user
   to recognize the source of the conflict.

   Conflicts are most likely to occur in response to a PUT request.  For
   example, if versioning were being used and the representation being
   PUT included changes to a resource that conflict with those made by
   an earlier (third-party) request, the origin server might use a 409
   response to indicate that it can't complete the request.  In this
   case, the response representation would likely contain information
   useful for merging the differences based on the revision history.

6.5.9.  410 Gone

   The 410 (Gone) status code indicates that access to the target
   resource is no longer available at the origin server and that this
   condition is likely to be permanent.  If the origin server does not
   know, or has no facility to determine, whether or not the condition
   is permanent, the status code 404 (Not Found) ought to be used
   instead.

   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 associated with the origin 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.




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   A 410 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.10.  411 Length Required

   The 411 (Length Required) status code indicates that the server
   refuses to accept the request without a defined Content-Length
   (Section 3.3.2 of [Part1]).  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.

6.5.11.  413 Payload Too Large

   The 413 (Payload Too Large) status code indicates that the server is
   refusing to process a request because the request payload 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 generate a Retry-
   After header field to indicate that it is temporary and after what
   time the client MAY try again.

6.5.12.  414 URI Too Long

   The 414 (URI Too Long) status code indicates that the server is
   refusing to service the request because the request-target (Section
   5.3 of [Part1]) 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 "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 potential security holes.

   A 414 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.13.  415 Unsupported Media Type

   The 415 (Unsupported Media Type) status code indicates that the
   origin server is refusing to service the request because the payload
   is in a format not supported by the target resource for this method.
   The format problem might be due to the request's indicated Content-
   Type or Content-Encoding, or as a result of inspecting the data
   directly.






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6.5.14.  417 Expectation Failed

   The 417 (Expectation Failed) status code indicates that the
   expectation given in the request's Expect header field
   (Section 5.1.1) could not be met by at least one of the inbound
   servers.

6.5.15.  426 Upgrade Required

   The 426 (Upgrade Required) status code indicates that the server
   refuses to perform the request using the current protocol but might
   be willing to do so after the client upgrades to a different
   protocol.  The server MUST send an Upgrade header field in a 426
   response to indicate the required protocol(s) (Section 6.7 of
   [Part1]).

   Example:

     HTTP/1.1 426 Upgrade Required
     Upgrade: HTTP/3.0
     Connection: Upgrade
     Content-Length: 53
     Content-Type: text/plain

     This service requires use of the HTTP/3.0 protocol.

6.6.  Server Error 5xx

   The 5xx (Server Error) class of status code indicates that the server
   is aware that it has erred or is incapable of performing the
   requested method.  Except when responding to a HEAD request, the
   server SHOULD send a representation containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition.  User agents SHOULD display any included representation to
   the user.  These response codes are applicable to any request method.

6.6.1.  500 Internal Server Error

   The 500 (Internal Server Error) status code indicates that the server
   encountered an unexpected condition that prevented it from fulfilling
   the request.

6.6.2.  501 Not Implemented

   The 501 (Not Implemented) status code indicates that 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.



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   A 501 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.6.3.  502 Bad Gateway

   The 502 (Bad Gateway) status code indicates that the server, while
   acting as a gateway or proxy, received an invalid response from an
   inbound server it accessed while attempting to fulfill the request.

6.6.4.  503 Service Unavailable

   The 503 (Service Unavailable) status code indicates that the server
   is currently unable to handle the request due to a temporary overload
   or scheduled maintenance, which will likely be alleviated after some
   delay.  The server MAY send a Retry-After header field
   (Section 7.1.3) to suggest an appropriate amount of time for the
   client to wait before retrying the request.

      Note: The existence of the 503 status code does not imply that a
      server has to use it when becoming overloaded.  Some servers might
      simply refuse the connection.

6.6.5.  504 Gateway Timeout

   The 504 (Gateway Timeout) status code indicates that the server,
   while acting as a gateway or proxy, did not receive a timely response
   from an upstream server it needed to access in order to complete the
   request.

6.6.6.  505 HTTP Version Not Supported

   The 505 (HTTP Version Not Supported) status code indicates that the
   server does not support, or refuses to support, the major version of
   HTTP 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 2.6 of [Part1],
   other than with this error message.  The server SHOULD generate a
   representation for the 505 response that describes why that version
   is not supported and what other protocols are supported by that
   server.

7.  Response Header Fields

   The response header fields allow the server to pass additional
   information about the response beyond what is placed in the status-
   line.  These header fields give information about the server, about
   further access to the target resource, or about related resources.




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   Although each response header field has a defined meaning, in
   general, the precise semantics might be further refined by the
   semantics of the request method and/or response status code.

7.1.  Control Data

   Response header fields can supply control data that supplements the
   status code, directs caching, or instructs the client where to go
   next.

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Age               | Section 7.1 of [Part6] |
   | Cache-Control     | Section 7.2 of [Part6] |
   | Expires           | Section 7.3 of [Part6] |
   | Date              | Section 7.1.1.2        |
   | Location          | Section 7.1.2          |
   | Retry-After       | Section 7.1.3          |
   | Vary              | Section 7.1.4          |
   | Warning           | Section 7.5 of [Part6] |
   +-------------------+------------------------+

7.1.1.  Origination Date

7.1.1.1.  Date/Time Formats

   Prior to 1995, there were three different formats commonly used by
   servers to communicate timestamps.  For compatibility with old
   implementations, all three are defined here.  The preferred format is
   a fixed-length and single-zone subset of the date and time
   specification used by the Internet Message Format [RFC5322].

     HTTP-date    = IMF-fixdate / obs-date

   An example of the preferred format is

     Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate

   Examples of the two obsolete formats are

     Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format
     Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format

   A recipient that parses a timestamp value in an HTTP header field
   MUST accept all three formats.  A sender MUST generate the IMF-
   fixdate format when sending an HTTP-date value in a header field.




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   An HTTP-date value represents time as an instance of Coordinated
   Universal Time (UTC).  The first two formats indicate UTC by the
   three-letter abbreviation for Greenwich Mean Time, "GMT", a
   predecessor of the UTC name; values in the asctime format are assumed
   to be in UTC.  A sender that generates HTTP-date values from a local
   clock ought to use NTP ([RFC1305]) or some similar protocol to
   synchronize its clock to UTC.

   Preferred format:

     IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT
     ; fixed length/zone subset of the format defined in
     ; Section 3.3 of [RFC5322]

     day-name     = %x4D.6F.6E ; "Mon", case-sensitive
                  / %x54.75.65 ; "Tue", case-sensitive
                  / %x57.65.64 ; "Wed", case-sensitive
                  / %x54.68.75 ; "Thu", case-sensitive
                  / %x46.72.69 ; "Fri", case-sensitive
                  / %x53.61.74 ; "Sat", case-sensitive
                  / %x53.75.6E ; "Sun", case-sensitive

     date1        = day SP month SP year
                  ; e.g., 02 Jun 1982

     day          = 2DIGIT
     month        = %x4A.61.6E ; "Jan", case-sensitive
                  / %x46.65.62 ; "Feb", case-sensitive
                  / %x4D.61.72 ; "Mar", case-sensitive
                  / %x41.70.72 ; "Apr", case-sensitive
                  / %x4D.61.79 ; "May", case-sensitive
                  / %x4A.75.6E ; "Jun", case-sensitive
                  / %x4A.75.6C ; "Jul", case-sensitive
                  / %x41.75.67 ; "Aug", case-sensitive
                  / %x53.65.70 ; "Sep", case-sensitive
                  / %x4F.63.74 ; "Oct", case-sensitive
                  / %x4E.6F.76 ; "Nov", case-sensitive
                  / %x44.65.63 ; "Dec", case-sensitive
     year         = 4DIGIT

     GMT          = %x47.4D.54 ; "GMT", case-sensitive

     time-of-day  = hour ":" minute ":" second
                  ; 00:00:00 - 23:59:60 (leap second)

     hour         = 2DIGIT
     minute       = 2DIGIT
     second       = 2DIGIT



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   Obsolete formats:

     obs-date     = rfc850-date / asctime-date


     rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
     date2        = day "-" month "-" 2DIGIT
                  ; e.g., 02-Jun-82

     day-name-l   = %x4D.6F.6E.64.61.79    ; "Monday", case-sensitive
            / %x54.75.65.73.64.61.79       ; "Tuesday", case-sensitive
            / %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive
            / %x54.68.75.72.73.64.61.79    ; "Thursday", case-sensitive
            / %x46.72.69.64.61.79          ; "Friday", case-sensitive
            / %x53.61.74.75.72.64.61.79    ; "Saturday", case-sensitive
            / %x53.75.6E.64.61.79          ; "Sunday", case-sensitive


     asctime-date = day-name SP date3 SP time-of-day SP year
     date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))
                  ; e.g., Jun  2

   HTTP-date is case sensitive.  A sender MUST NOT generate additional
   whitespace in an HTTP-date beyond that specifically included as SP in
   the grammar.  The semantics of day-name, day, month, year, and time-
   of-day are the same as those defined for the Internet Message Format
   constructs with the corresponding name ([RFC5322], Section 3.3).

   Recipients of a timestamp value in rfc850-date format, which uses a
   two-digit year, SHOULD interpret a timestamp that appears to be more
   than 50 years in the future as representing the most recent year in
   the past that had the same last two digits.

   Recipients of timestamp values are encouraged to be robust in parsing
   timestamps unless otherwise restricted by the field definition.  For
   example, messages are occasionally forwarded over HTTP from a non-
   HTTP source that might generate any of the date and time
   specifications defined by the Internet Message Format.

      Note: HTTP requirements for the date/time stamp format apply only
      to their usage within the protocol stream.  Implementations are
      not required to use these formats for user presentation, request
      logging, etc.








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7.1.1.2.  Date

   The "Date" header field represents the date and time at which the
   message was originated, having the same semantics as the Origination
   Date Field (orig-date) defined in Section 3.6.1 of [RFC5322].  The
   field value is an HTTP-date, as defined in Section 7.1.1.1.

     Date = HTTP-date

   An example is

     Date: Tue, 15 Nov 1994 08:12:31 GMT

   When a Date header field is generated, the sender SHOULD generate its
   field value as the best available approximation of the date and time
   of message generation.  In theory, the date ought to represent the
   moment just before the payload is generated.  In practice, the date
   can be generated at any time during message origination.

   An origin server MUST NOT send a Date header field if it does not
   have a clock capable of providing a reasonable approximation of the
   current instance in Coordinated Universal Time.  An origin server MAY
   send a Date header field if the response is in the 1xx
   (Informational) or 5xx (Server Error) class of status codes.  An
   origin server MUST send a Date header field in all other cases.

   A recipient with a clock that receives a response message without a
   Date header field MUST record the time it was received and append a
   corresponding Date header field to the message's header block if it
   is cached or forwarded downstream.

   A user agent MAY send a Date header field in a request, though
   generally will not do so unless it is believed to convey useful
   information to the server.  For example, custom applications of HTTP
   might convey a Date if the server is expected to adjust its
   interpretation of the user's request based on differences between the
   user agent and server clocks.

7.1.2.  Location

   The "Location" header field is used in some responses to refer to a
   specific resource in relation to the response.  The type of
   relationship is defined by the combination of request method and
   status code semantics.

     Location = URI-reference

   The field value consists of a single URI-reference.  When it has the



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   form of a relative reference ([RFC3986], Section 4.2), the final
   value is computed by resolving it against the effective request URI
   ([RFC3986], Section 5).

   For 201 (Created) responses, the Location value refers to the primary
   resource created by the request.  For 3xx (Redirection) responses,
   the Location value refers to the preferred target resource for
   automatically redirecting the request.

   When Location is provided in a 3xx (Redirection) response and the URI
   reference that the user agent used to generate the request target
   contains a fragment identifier, the user agent SHOULD process the
   redirection as if the Location field value inherits the original
   fragment.  In other words, if the Location does not have a fragment
   component, the user agent SHOULD interpret the Location reference as
   if it had the original reference's fragment.

   For example, a GET request generated for the URI reference
   "http://www.example.org/~tim" might result in a 303 (See Other)
   response containing the header field:

     Location: /People.html#tim

   which suggests that the user agent redirect to
   "http://www.example.org/People.html#tim"

   Likewise, a GET request generated for the URI reference
   "http://www.example.org/index.html#larry" might result in a 301
   (Moved Permanently) response containing the header field:

     Location: http://www.example.net/index.html

   which suggests that the user agent redirect to
   "http://www.example.net/index.html#larry", preserving the original
   fragment identifier.

   There are circumstances in which a fragment identifier in a Location
   value would not be appropriate.  For example, the Location header
   field in a 201 (Created) response is supposed to provide a URI that
   is specific to the created resource.

      Note: Some recipients attempt to recover from Location fields that
      are not valid URI references.  This specification does not mandate
      or define such processing, but does allow it for the sake of
      robustness.






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      Note: The Content-Location header field (Section 3.1.4.2) differs
      from Location in that the Content-Location refers to the most
      specific resource corresponding to the enclosed representation.
      It is therefore possible for a response to contain both the
      Location and Content-Location header fields.

7.1.3.  Retry-After

   Servers send the "Retry-After" header field to indicate how long the
   user agent ought to wait before making a follow-up request.  When
   sent with a 503 (Service Unavailable) response, Retry-After indicates
   how long the service is expected to be unavailable to the client.
   When sent with any 3xx (Redirection) response, Retry-After indicates
   the minimum time that the user agent is asked to 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 = HTTP-date / delta-seconds

   Time spans are non-negative decimal integers, representing time in
   seconds.

     delta-seconds  = 1*DIGIT

   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.

7.1.4.  Vary

   The "Vary" header field describes what parts of a request message,
   aside from the method, the Host header field and the request target,
   might influence the origin server's process for selecting and
   representing the response.  The value consists of either a single
   asterisk ("*") or a list of header field names (case-insensitive).

     Vary = "*" / 1#field-name

   A Vary field value of "*" signals that anything about the request
   might play a role in selecting the response representation, possibly
   including elements outside the message syntax (e.g., the client's
   network address), and thus a recipient will not be able to determine
   whether this response is appropriate for a later request without



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   forwarding the request to the origin server.  A proxy MUST NOT
   generate a Vary field with a "*" value.

   A Vary field value consisting of a comma-separated list of names
   indicates that the named request header fields, known as the
   selecting header fields, might have a role in selecting the
   representation.  The potential selecting header fields are not
   limited to those defined by this specification.

   For example, a response that contains

     Vary: accept-encoding, accept-language

   indicates that the origin server might have used the request's
   Accept-Encoding and Accept-Language fields (or lack thereof) as
   determining factors while choosing the content for this response.

   An origin server might send Vary with a list of fields for two
   purposes:

   1.  To inform cache recipients that they MUST NOT use this response
       to satisfy a later request unless the later request has the same
       values for the listed fields as the original request (Section 4.3
       of [Part6]).  In other words, Vary expands the cache key required
       to match a new request to the stored cache entry.

   2.  To inform user agent recipients that this response is subject to
       content negotiation (Section 5.3) and that a different
       representation might be sent in a subsequent request if
       additional parameters are provided in the listed header fields
       (proactive negotiation).

   An origin server SHOULD send a Vary header field when its algorithm
   for selecting a representation varies based on aspects of the request
   message other than the method and request target, unless the variance
   cannot be crossed or the origin server has been deliberately
   configured to prevent cache transparency.  For example, there is no
   need to send the Authorization field name in Vary because reuse
   across users is constrained by the field definition (Section 4.1 of
   [Part7]).  Likewise, an origin server might use Cache-Control
   directives (Section 7.2 of [Part6]) to supplant Vary if it considers
   the variance less significant than the performance cost of Vary's
   impact on caching.

7.2.  Validator Header Fields

   Validator header fields convey metadata about the selected
   representation (Section 3).  In responses to safe requests, validator



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   fields describe the selected representation chosen by the origin
   server while handling the response.  Note that, depending on the
   status code semantics, the selected representation for a given
   response is not necessarily the same as the representation enclosed
   as response payload.

   In a successful response to a state-changing request, validator
   fields describe the new representation that has replaced the prior
   selected representation as a result of processing the request.

   For example, an ETag header field in a 201 response communicates the
   entity-tag of the newly created resource's representation, so that it
   can be used in later conditional requests to prevent the "lost
   update" problem [Part4].

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | ETag              | Section 2.3 of [Part4] |
   | Last-Modified     | Section 2.2 of [Part4] |
   +-------------------+------------------------+

7.3.  Authentication Challenges

   Authentication challenges indicate what mechanisms are available for
   the client to provide authentication credentials in future requests.

   +--------------------+------------------------+
   | Header Field Name  | Defined in...          |
   +--------------------+------------------------+
   | WWW-Authenticate   | Section 4.4 of [Part7] |
   | Proxy-Authenticate | Section 4.2 of [Part7] |
   +--------------------+------------------------+

7.4.  Response Context

   The remaining response header fields provide more information about
   the target resource for potential use in later requests.

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Accept-Ranges     | Section 2.3 of [Part5] |
   | Allow             | Section 7.4.1          |
   | Server            | Section 7.4.2          |
   +-------------------+------------------------+





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7.4.1.  Allow

   The "Allow" header field lists the set of methods advertised as
   supported by the target resource.  The purpose of this field is
   strictly to inform the recipient of valid request methods associated
   with the resource.

     Allow = #method

   Example of use:

     Allow: GET, HEAD, PUT

   The actual set of allowed methods is defined by the origin server at
   the time of each request.  An origin server MUST generate an Allow
   field in a 405 (Method Not Allowed) response and MAY do so in any
   other response.  An empty Allow field value indicates that the
   resource allows no methods, which might occur in a 405 response if
   the resource has been temporarily disabled by configuration.

   A proxy MUST NOT modify the Allow header field -- it does not need to
   understand all of the indicated methods in order to handle them
   according to the generic message handling rules.

7.4.2.  Server

   The "Server" header field contains information about the software
   used by the origin server to handle the request, which is often used
   by clients to help identify the scope of reported interoperability
   problems, to work around or tailor requests to avoid particular
   server limitations, and for analytics regarding server or operating
   system use.  An origin server MAY generate a Server field in its
   responses.

     Server = product *( RWS ( product / comment ) )

   The Server field-value consists of one or more product identifiers,
   each followed by zero or more comments (Section 3.2 of [Part1]),
   which together identify the origin server software and its
   significant subproducts.  By convention, the product identifiers are
   listed in decreasing order of their significance for identifying the
   origin server software.  Each product identifier consists of a name
   and optional version, as defined in Section 5.5.3.

   Example:

     Server: CERN/3.0 libwww/2.17




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   An origin server SHOULD NOT generate a Server field containing
   needlessly fine-grained detail and SHOULD limit the addition of
   subproducts by third parties.  Overly long and detailed Server field
   values increase response latency and potentially reveal internal
   implementation details that might make it (slightly) easier for
   attackers to find and exploit known security holes.

8.  IANA Considerations

8.1.  Method Registry

   The HTTP Method Registry defines the name space for the request
   method token (Section 4).  The method registry will be created and
   maintained at <http://www.iana.org/assignments/http-methods>.

8.1.1.  Procedure

   HTTP method registrations MUST include the following fields:

   o  Method Name (see Section 4)

   o  Safe ("yes" or "no", see Section 4.2.1)

   o  Idempotent ("yes" or "no", see Section 4.2.2)

   o  Pointer to specification text

   Values to be added to this name space require IETF Review (see
   [RFC5226], Section 4.1).

8.1.2.  Considerations for New Methods

   Standardized methods are generic; that is, they are potentially
   applicable to any resource, not just one particular media type, kind
   of resource, or application.  As such, it is preferred that new
   methods be registered in a document that isn't specific to a single
   application or data format, since orthogonal technologies deserve
   orthogonal specification.

   Since message parsing (Section 3.3 of [Part1]) needs to be
   independent of method semantics (aside from responses to HEAD),
   definitions of new methods cannot change the parsing algorithm or
   prohibit the presence of a message body on either the request or the
   response message.  Definitions of new methods can specify that only a
   zero-length message body is allowed by requiring a Content-Length
   header field with a value of "0".

   A new method definition needs to indicate whether it is safe



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   (Section 4.2.1), idempotent (Section 4.2.2), cacheable
   (Section 4.2.3), what semantics are to be associated with the payload
   body if any is present in the request, and what refinements the
   method makes to header field or status code semantics.  If the new
   method is cacheable, its definition ought to describe how, and under
   what conditions, a cache can store a response and use it to satisfy a
   subsequent request.  The new method ought to describe whether it can
   be made conditional (Section 5.2) and, if so, how a server responds
   when the condition is false.  Likewise, if the new method might have
   some use for partial response semantics ([Part5]), it ought to
   document this too.

      Note: Avoid defining a method name that starts with "M-", since
      that prefix might be misinterpreted as having the semantics
      assigned to it by [RFC2774].

8.1.3.  Registrations

   The HTTP Method Registry shall be populated with the registrations
   below:

   +---------+------+------------+---------------+
   | Method  | Safe | Idempotent | Reference     |
   +---------+------+------------+---------------+
   | CONNECT | no   | no         | Section 4.3.6 |
   | DELETE  | no   | yes        | Section 4.3.5 |
   | GET     | yes  | yes        | Section 4.3.1 |
   | HEAD    | yes  | yes        | Section 4.3.2 |
   | OPTIONS | yes  | yes        | Section 4.3.7 |
   | POST    | no   | no         | Section 4.3.3 |
   | PUT     | no   | yes        | Section 4.3.4 |
   | TRACE   | yes  | yes        | Section 4.3.8 |
   +---------+------+------------+---------------+

8.2.  Status Code Registry

   The HTTP Status Code Registry defines the name space for the response
   status-code token (Section 6).  The status code registry is
   maintained at <http://www.iana.org/assignments/http-status-codes>.

   This Section replaces the registration procedure for HTTP Status
   Codes previously defined in Section 7.1 of [RFC2817].

8.2.1.  Procedure

   A registration MUST include the following fields:





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   o  Status Code (3 digits)

   o  Short Description

   o  Pointer to specification text

   Values to be added to the HTTP status code name space require IETF
   Review (see [RFC5226], Section 4.1).

8.2.2.  Considerations for New Status Codes

   When it is necessary to express semantics for a response that are not
   defined by current status codes, a new status code can be registered.
   Status codes are generic; they are potentially applicable to any
   resource, not just one particular media type, kind of resource, or
   application of HTTP.  As such, it is preferred that new status codes
   be registered in a document that isn't specific to a single
   application.

   New status codes are required to fall under one of the categories
   defined in Section 6.  To allow existing parsers to process the
   response message, new status codes cannot disallow a payload,
   although they can mandate a zero-length payload body.

   Proposals for new status codes that are not yet widely deployed ought
   to avoid allocating a specific number for the code until there is
   clear consensus that it will be registered; instead, early drafts can
   use a notation such as "4NN", or "3N0" .. "3N9", to indicate the
   class of the proposed status code(s) without consuming a number
   prematurely.

   The definition of a new status code ought to explain the request
   conditions that would cause a response containing that status code
   (e.g., combinations of request header fields and/or method(s)) along
   with any dependencies on response header fields (e.g., what fields
   are required, what fields can modify the semantics, and what header
   field semantics are further refined when used with the new status
   code).

   The definition of a new status code ought to specify whether or not
   it is cacheable.  Note that all status codes can be cached if the
   response they occur in has explicit freshness information; however,
   status codes that are defined as being cacheable are allowed to be
   cached without explicit freshness information.  Likewise, the
   definition of a status code can place constraints upon cache
   behavior.  See [Part6] for more information.

   Finally, the definition of a new status code ought to indicate



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   whether the payload has any implied association with an identified
   resource (Section 3.1.4.1).

8.2.3.  Registrations

   The HTTP Status Code Registry shall be updated with the registrations
   below:












































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   +-------+-------------------------------+----------------+
   | Value | Description                   | Reference      |
   +-------+-------------------------------+----------------+
   | 100   | Continue                      | Section 6.2.1  |
   | 101   | Switching Protocols           | Section 6.2.2  |
   | 200   | OK                            | Section 6.3.1  |
   | 201   | Created                       | Section 6.3.2  |
   | 202   | Accepted                      | Section 6.3.3  |
   | 203   | Non-Authoritative Information | Section 6.3.4  |
   | 204   | No Content                    | Section 6.3.5  |
   | 205   | Reset Content                 | Section 6.3.6  |
   | 300   | Multiple Choices              | Section 6.4.1  |
   | 301   | Moved Permanently             | Section 6.4.2  |
   | 302   | Found                         | Section 6.4.3  |
   | 303   | See Other                     | Section 6.4.4  |
   | 305   | Use Proxy                     | Section 6.4.5  |
   | 306   | (Unused)                      | Section 6.4.6  |
   | 307   | Temporary Redirect            | Section 6.4.7  |
   | 400   | Bad Request                   | Section 6.5.1  |
   | 402   | Payment Required              | Section 6.5.2  |
   | 403   | Forbidden                     | Section 6.5.3  |
   | 404   | Not Found                     | Section 6.5.4  |
   | 405   | Method Not Allowed            | Section 6.5.5  |
   | 406   | Not Acceptable                | Section 6.5.6  |
   | 408   | Request Timeout               | Section 6.5.7  |
   | 409   | Conflict                      | Section 6.5.8  |
   | 410   | Gone                          | Section 6.5.9  |
   | 411   | Length Required               | Section 6.5.10 |
   | 413   | Payload Too Large             | Section 6.5.11 |
   | 414   | URI Too Long                  | Section 6.5.12 |
   | 415   | Unsupported Media Type        | Section 6.5.13 |
   | 417   | Expectation Failed            | Section 6.5.14 |
   | 426   | Upgrade Required              | Section 6.5.15 |
   | 500   | Internal Server Error         | Section 6.6.1  |
   | 501   | Not Implemented               | Section 6.6.2  |
   | 502   | Bad Gateway                   | Section 6.6.3  |
   | 503   | Service Unavailable           | Section 6.6.4  |
   | 504   | Gateway Timeout               | Section 6.6.5  |
   | 505   | HTTP Version Not Supported    | Section 6.6.6  |
   +-------+-------------------------------+----------------+

8.3.  Header Field Registry

   HTTP header fields are registered within the Message Header Field
   Registry located at <http://www.iana.org/assignments/message-headers/
   message-header-index.html>, as defined by [BCP90].





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8.3.1.  Considerations for New Header Fields

   Header fields are key:value pairs that can be used to communicate
   data about the message, its payload, the target resource, or the
   connection (i.e., control data).  See Section 3.2 of [Part1] for a
   general definition of header field syntax in HTTP messages.

   The requirements for header field names are defined in [BCP90].
   Authors of specifications defining new fields are advised to keep the
   name as short as practical and to not prefix the name with "X-"
   unless the header field will never be used on the Internet.  (The
   "x-" prefix idiom has been extensively misused in practice; it was
   intended to only be used as a mechanism for avoiding name collisions
   inside proprietary software or intranet processing, since the prefix
   would ensure that private names never collide with a newly registered
   Internet name.)

   New header field values typically have their syntax defined using
   ABNF ([RFC5234]), using the extension defined in Appendix B of
   [Part1] as necessary, and are usually constrained to the range of
   ASCII characters.  Header fields needing a greater range of
   characters can use an encoding such as the one defined in [RFC5987].

   Leading and trailing whitespace in raw field values is removed upon
   field parsing (Section 3.2.4 of [Part1]).  Field definitions where
   leading or trailing whitespace in values is significant will have to
   use a container syntax such as quoted-string.

   Because commas (",") are used as a generic delimiter between field-
   values, they need to be treated with care if they are allowed in the
   field-value.  Typically, components that might contain a comma are
   protected with double-quotes using the quoted-string ABNF production
   (Section 3.2.6 of [Part1]).

   For example, a textual date and a URI (either of which might contain
   a comma) could be safely carried in field-values like these:

     Example-URI-Field: "http://example.com/a.html,foo",
                        "http://without-a-comma.example.com/"
     Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"

   Note that double-quote delimiters almost always are used with the
   quoted-string production; using a different syntax inside double-
   quotes will likely cause unnecessary confusion.

   Many header fields use a format including (case-insensitively) named
   parameters (for instance, Content-Type, defined in Section 3.1.1.5).
   Allowing both unquoted (token) and quoted (quoted-string) syntax for



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   the parameter value enables recipients to use existing parser
   components.  When allowing both forms, the meaning of a parameter
   value ought to be independent of the syntax used for it (for an
   example, see the notes on parameter handling for media types in
   Section 3.1.1.1).

   Authors of specifications defining new header fields are advised to
   consider documenting:

   o  Whether the field is a single value, or whether it can be a list
      (delimited by commas; see Section 3.2 of [Part1]).

      If it does not use the list syntax, document how to treat messages
      where the field occurs multiple times (a sensible default would be
      to ignore the field, but this might not always be the right
      choice).

      Note that intermediaries and software libraries might combine
      multiple header field instances into a single one, despite the
      field's definition not allowing the list syntax.  A robust format
      enables recipients to discover these situations (good example:
      "Content-Type", as the comma can only appear inside quoted
      strings; bad example: "Location", as a comma can occur inside a
      URI).

   o  Under what conditions the header field can be used; e.g., only in
      responses or requests, in all messages, only on responses to a
      particular request method, etc.

   o  Whether the field should be stored by origin servers that
      understand it upon a PUT request.

   o  Whether the field semantics are further refined by the context,
      such as by existing request methods or status codes.

   o  Whether it is appropriate to list the field-name in the Connection
      header field (i.e., if the header field is to be hop-by-hop; see
      Section 6.1 of [Part1]).

   o  Under what conditions intermediaries are allowed to insert,
      delete, or modify the field's value.

   o  Whether it is appropriate to list the field-name in a Vary
      response header field (e.g., when the request header field is used
      by an origin server's content selection algorithm; see
      Section 7.1.4).





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   o  Whether the header field is useful or allowable in trailers (see
      Section 4.1 of [Part1]).

   o  Whether the header field ought to be preserved across redirects.

8.3.2.  Registrations

   The Message Header Field Registry shall be updated with the following
   permanent registrations:

   +-------------------+----------+----------+-----------------+
   | Header Field Name | Protocol | Status   | Reference       |
   +-------------------+----------+----------+-----------------+
   | Accept            | http     | standard | Section 5.3.2   |
   | Accept-Charset    | http     | standard | Section 5.3.3   |
   | Accept-Encoding   | http     | standard | Section 5.3.4   |
   | Accept-Language   | http     | standard | Section 5.3.5   |
   | Allow             | http     | standard | Section 7.4.1   |
   | Content-Encoding  | http     | standard | Section 3.1.2.2 |
   | Content-Language  | http     | standard | Section 3.1.3.2 |
   | Content-Location  | http     | standard | Section 3.1.4.2 |
   | Content-Type      | http     | standard | Section 3.1.1.5 |
   | Date              | http     | standard | Section 7.1.1.2 |
   | Expect            | http     | standard | Section 5.1.1   |
   | From              | http     | standard | Section 5.5.1   |
   | Location          | http     | standard | Section 7.1.2   |
   | MIME-Version      | http     | standard | Appendix A.1    |
   | Max-Forwards      | http     | standard | Section 5.1.2   |
   | Referer           | http     | standard | Section 5.5.2   |
   | Retry-After       | http     | standard | Section 7.1.3   |
   | Server            | http     | standard | Section 7.4.2   |
   | User-Agent        | http     | standard | Section 5.5.3   |
   | Vary              | http     | standard | Section 7.1.4   |
   +-------------------+----------+----------+-----------------+

   The change controller for the above registrations is: "IETF
   (iesg@ietf.org) - Internet Engineering Task Force".

8.4.  Content Coding Registry

   The HTTP Content Coding Registry defines the name space for content
   coding names (Section 4.2 of [Part1]).  The content coding registry
   is maintained at <http://www.iana.org/assignments/http-parameters>.

8.4.1.  Procedure

   Content Coding registrations MUST include the following fields:




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   o  Name

   o  Description

   o  Pointer to specification text

   Names of content codings MUST NOT overlap with names of transfer
   codings (Section 4 of [Part1]), unless the encoding transformation is
   identical (as is the case for the compression codings defined in
   Section 4.2 of [Part1]).

   Values to be added to this name space require IETF Review (see
   Section 4.1 of [RFC5226]), and MUST conform to the purpose of content
   coding defined in this section.

8.4.2.  Registrations

   The HTTP Content Codings Registry shall be updated with the
   registrations below:

   +------------+--------------------------------------+---------------+
   | Name       | Description                          | Reference     |
   +------------+--------------------------------------+---------------+
   | compress   | UNIX "compress" data format [Welch]  | Section 4.2.1 |
   |            |                                      | of [Part1]    |
   | deflate    | "deflate" compressed data            | Section 4.2.2 |
   |            | ([RFC1951]) inside the "zlib" data   | of [Part1]    |
   |            | format ([RFC1950])                   |               |
   | gzip       | GZIP file format [RFC1952]           | Section 4.2.3 |
   |            |                                      | of [Part1]    |
   | identity   | Reserved (synonym for "no encoding"  | Section 5.3.4 |
   |            | in Accept-Encoding)                  |               |
   | x-compress | Deprecated (alias for compress)      | Section 4.2.1 |
   |            |                                      | of [Part1]    |
   | x-gzip     | Deprecated (alias for gzip)          | Section 4.2.3 |
   |            |                                      | of [Part1]    |
   +------------+--------------------------------------+---------------+

9.  Security Considerations

   This section is meant to inform developers, information providers,
   and users of known security concerns relevant to HTTP/1.1 semantics
   and its use for transferring information over the Internet.

9.1.  Attacks Based On File and Path Names

   Origin servers frequently make use of their local file system to
   manage the mapping from effective request URI to resource



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   representations.  Implementers need to be aware that most file
   systems are not designed to protect against malicious file or path
   names, and thus depend on the origin server to avoid mapping to file
   names, folders, or directories that have special significance to the
   system.

   For example, UNIX, Microsoft Windows, and other operating systems use
   ".." as a path component to indicate a directory level above the
   current one, and use specially named paths or file names to send data
   to system devices.  Similar naming conventions might exist within
   other types of storage systems.  Likewise, local storage systems have
   an annoying tendency to prefer user-friendliness over security when
   handling invalid or unexpected characters, recomposition of
   decomposed characters, and case-normalization of case-insensitive
   names.

   Attacks based on such special names tend to focus on either denial of
   service (e.g., telling the server to read from a COM port) or
   disclosure of configuration and source files that are not meant to be
   served.

9.2.  Personal Information

   Clients are often privy to large amounts of personal information,
   including both information provided by the user to interact with
   resources (e.g., the user's name, location, mail address, passwords,
   encryption keys, etc.) and information about the user's browsing
   activity over time (e.g., history, bookmarks, etc.).  Implementations
   need to prevent unintentional leakage of personal information.

9.3.  Sensitive Information in URIs

   URIs are intended to be shared, not secured, even when they identify
   secure resources.  URIs are often shown on displays, added to
   templates when a page is printed, and stored in a variety of
   unprotected bookmark lists.  It is therefore unwise to include
   information within a URI that is sensitive, personally identifiable,
   or a risk to disclose.

   Authors of services ought to avoid GET-based forms for the submission
   of sensitive data because that data will be placed in the request-
   target.  Many existing servers, proxies, and user agents log or
   display the request-target in places where it might be visible to
   third parties.  Such services ought to use POST-based form submission
   instead.

   Since the Referer header field tells a target site about the context
   that resulted in a request, it has the potential to reveal



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   information about the user's immediate browsing history and any
   personal information that might be found in the referring resource's
   URI.  Limitations on Referer are described in Section 5.5.2 to
   address some of its security considerations.

9.4.  Product Information

   The User-Agent (Section 5.5.3), Via (Section 5.7.1 of [Part1]), and
   Server (Section 7.4.2) header fields often reveal information about
   the respective sender's software systems.  In theory, this can make
   it easier for an attacker to exploit known security holes; in
   practice, attackers tend to try all potential holes regardless of the
   apparent software versions being used.

   Proxies that serve as a portal through a network firewall ought to
   take special precautions regarding the transfer of header information
   that might identify hosts behind the firewall.  The Via header field
   allows intermediaries to replace sensitive machine names with
   pseudonyms.

9.5.  Fragment after Redirects

   Although fragment identifiers used within URI references are not sent
   in requests, implementers ought to be aware that they will be visible
   to the user agent and any extensions or scripts running as a result
   of the response.  In particular, when a redirect occurs and the
   original request's fragment identifier is inherited by the new
   reference in Location (Section 7.1.2), this might have the effect of
   leaking one site's fragment to another site.  If the first site uses
   personal information in fragments, it ought to ensure that redirects
   to other sites include a (possibly empty) fragment component in order
   to block that inheritance.

9.6.  Browser Fingerprinting

   Browser fingerprinting is a set of techniques for identifying a
   specific user agent over time through its unique set of
   characteristics.  These characteristics might include information
   related to its TCP behavior, feature capabilities, and scripting
   environment, though of particular interest here is the set of unique
   characteristics that might be communicated via HTTP.  Fingerprinting
   is considered a privacy concern because it enables tracking of a user
   agent's behavior over time without the corresponding controls that
   the user might have over other forms of data collection (e.g.,
   cookies).  Many general-purpose user agents (i.e., Web browsers) have
   taken steps to reduce their fingerprints.

   There are a number of request header fields that might reveal



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   information to servers that is sufficiently unique to enable
   fingerprinting.  The From header field is the most obvious, though it
   is expected that From will only be sent when self-identification is
   desired by the user.  Likewise, Cookie header fields are deliberately
   designed to enable re-identification, so we can assume that
   fingerprinting concerns only apply to situations where cookies are
   disabled or restricted by browser configuration.

   The User-Agent header field might contain enough information to
   uniquely identify a specific device, usually when combined with other
   characteristics, particularly if the user agent sends excessive
   details about the user's system or extensions.  However, the source
   of unique information that is least expected by users is proactive
   negotiation (Section 5.3), including the Accept, Accept-Charset,
   Accept-Encoding, and Accept-Language header fields.

   In addition to the fingerprinting concern, detailed use of the
   Accept-Language header field can reveal information the user might
   consider to be of a private nature, because the understanding of
   particular languages is often strongly correlated to membership in a
   particular ethnic group.  An approach that limits such loss of
   privacy would be for a user agent to omit the sending of Accept-
   Language except for sites that have been whitelisted, perhaps via
   interaction after detecting a Vary header field that would indicate
   language negotiation might be useful.

   In environments where proxies are used to enhance privacy, user
   agents ought to be conservative in sending proactive negotiation
   header fields.  General-purpose user agents that provide a high
   degree of header field configurability ought to inform users about
   the loss of privacy that might result if too much detail is provided.
   As an extreme privacy measure, proxies could filter the proactive
   negotiation header fields in relayed requests.

10.  Acknowledgments

   See Section 9 of [Part1].

11.  References

11.1.  Normative References

   [Part1]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Message Syntax and
                 Routing", draft-ietf-httpbis-p1-messaging-23 (work in
                 progress), July 2013.

   [Part4]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext



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                 Transfer Protocol (HTTP/1.1): Conditional Requests",
                 draft-ietf-httpbis-p4-conditional-23 (work in
                 progress), July 2013.

   [Part5]       Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
                 "Hypertext Transfer Protocol (HTTP/1.1): Range
                 Requests", draft-ietf-httpbis-p5-range-23 (work in
                 progress), July 2013.

   [Part6]       Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
                 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                 draft-ietf-httpbis-p6-cache-23 (work in progress),
                 July 2013.

   [Part7]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Authentication",
                 draft-ietf-httpbis-p7-auth-23 (work in progress),
                 July 2013.

   [RFC1950]     Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data
                 Format Specification version 3.3", RFC 1950, May 1996.

   [RFC1951]     Deutsch, P., "DEFLATE Compressed Data Format
                 Specification version 1.3", RFC 1951, May 1996.

   [RFC1952]     Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and
                 G. Randers-Pehrson, "GZIP file format specification
                 version 4.3", RFC 1952, May 1996.

   [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of Internet
                 Message Bodies", RFC 2045, November 1996.

   [RFC2046]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Two: Media Types",
                 RFC 2046, November 1996.

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]     Berners-Lee, T., Fielding, R., and L. Masinter,
                 "Uniform Resource Identifier (URI): Generic Syntax",
                 STD 66, RFC 3986, January 2005.

   [RFC4647]     Phillips, A., Ed. and M. Davis, Ed., "Matching of
                 Language Tags", BCP 47, RFC 4647, September 2006.

   [RFC5234]     Crocker, D., Ed. and P. Overell, "Augmented BNF for



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                 Syntax Specifications: ABNF", STD 68, RFC 5234,
                 January 2008.

   [RFC5646]     Phillips, A., Ed. and M. Davis, Ed., "Tags for
                 Identifying Languages", BCP 47, RFC 5646,
                 September 2009.

   [RFC6365]     Hoffman, P. and J. Klensin, "Terminology Used in
                 Internationalization in the IETF", BCP 166, RFC 6365,
                 September 2011.

   [Welch]       Welch, T., "A Technique for High Performance Data
                 Compression", IEEE Computer 17(6), June 1984.

11.2.  Informative References

   [BCP13]       Freed, N., Klensin, J., and T. Hansen, "Media Type
                 Specifications and Registration Procedures", BCP 13,
                 RFC 6838, January 2013.

   [BCP90]       Klyne, G., Nottingham, M., and J. Mogul, "Registration
                 Procedures for Message Header Fields", BCP 90,
                 RFC 3864, September 2004.

   [REST]        Fielding, R., "Architectural Styles and the Design of
                 Network-based Software Architectures", Doctoral
                 Dissertation, University of California, Irvine ,
                 September 2000,
                 <http://roy.gbiv.com/pubs/dissertation/top.htm>.

   [RFC1305]     Mills, D., "Network Time Protocol (Version 3)
                 Specification, Implementation", RFC 1305, March 1992.

   [RFC1945]     Berners-Lee, T., Fielding, R., and H. Nielsen,
                 "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945,
                 May 1996.

   [RFC2049]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Five: Conformance Criteria
                 and Examples", RFC 2049, November 1996.

   [RFC2068]     Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and
                 T. Berners-Lee, "Hypertext Transfer Protocol --
                 HTTP/1.1", RFC 2068, January 1997.

   [RFC2295]     Holtman, K. and A. Mutz, "Transparent Content
                 Negotiation in HTTP", RFC 2295, March 1998.




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   [RFC2388]     Masinter, L., "Returning Values from Forms:  multipart/
                 form-data", RFC 2388, August 1998.

   [RFC2557]     Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
                 "MIME Encapsulation of Aggregate Documents, such as
                 HTML (MHTML)", RFC 2557, March 1999.

   [RFC2616]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2774]     Frystyk, H., Leach, P., and S. Lawrence, "An HTTP
                 Extension Framework", RFC 2774, February 2000.

   [RFC2817]     Khare, R. and S. Lawrence, "Upgrading to TLS Within
                 HTTP/1.1", RFC 2817, May 2000.

   [RFC2978]     Freed, N. and J. Postel, "IANA Charset Registration
                 Procedures", BCP 19, RFC 2978, October 2000.

   [RFC5226]     Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section in RFCs", BCP 26,
                 RFC 5226, May 2008.

   [RFC5322]     Resnick, P., "Internet Message Format", RFC 5322,
                 October 2008.

   [RFC5789]     Dusseault, L. and J. Snell, "PATCH Method for HTTP",
                 RFC 5789, March 2010.

   [RFC5987]     Reschke, J., "Character Set and Language Encoding for
                 Hypertext Transfer Protocol (HTTP) Header Field
                 Parameters", RFC 5987, August 2010.

   [RFC5988]     Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [RFC6265]     Barth, A., "HTTP State Management Mechanism", RFC 6265,
                 April 2011.

   [RFC6266]     Reschke, J., "Use of the Content-Disposition Header
                 Field in the Hypertext Transfer Protocol (HTTP)",
                 RFC 6266, June 2011.

   [status-308]  Reschke, J., "The Hypertext Transfer Protocol (HTTP)
                 Status Code 308 (Permanent Redirect)",
                 draft-reschke-http-status-308-07 (work in progress),
                 March 2012.




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Appendix A.  Differences between HTTP and MIME

   HTTP/1.1 uses many of the constructs defined for the Internet Message
   Format [RFC5322] and the Multipurpose Internet Mail Extensions (MIME)
   [RFC2045] to allow a message body to be transmitted in an open
   variety of representations and with extensible header fields.
   However, RFC 2045 is focused only on email; applications of HTTP have
   many characteristics that differ from email, and hence HTTP has
   features that differ from MIME.  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 MIME.
   Proxies and gateways to and from strict MIME environments need to be
   aware of these differences and provide the appropriate conversions
   where necessary.

A.1.  MIME-Version

   HTTP is not a MIME-compliant protocol.  However, messages can include
   a single MIME-Version 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
   conformance with the MIME protocol (as defined in [RFC2045]).
   Senders are responsible for ensuring full conformance (where
   possible) when exporting HTTP messages to strict MIME environments.

A.2.  Conversion to Canonical Form

   MIME requires that an Internet mail body part be converted to
   canonical form prior to being transferred, as described in Section 4
   of [RFC2049].  Section 3.1.1.3 of this document describes the forms
   allowed for subtypes of the "text" media type when transmitted over
   HTTP.  [RFC2046] 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.

   A proxy or gateway from HTTP to a strict MIME environment ought to
   translate all line breaks within the text media types described in
   Section 3.1.1.3 of this document to the RFC 2049 canonical form of
   CRLF.  Note, however, this might be complicated by the presence of a
   Content-Encoding and by the fact that HTTP allows the use of some
   charsets that do not use octets 13 and 10 to represent CR and LF,
   respectively.




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   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.3.  Conversion of Date Formats

   HTTP/1.1 uses a restricted set of date formats (Section 7.1.1.1) to
   simplify the process of date comparison.  Proxies and gateways from
   other protocols ought to 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.  Conversion of Content-Encoding

   MIME 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
   ought to either change the value of the Content-Type header field or
   decode the representation 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 the MIME standards).

A.5.  Conversion of Content-Transfer-Encoding

   HTTP does not use the Content-Transfer-Encoding field of MIME.
   Proxies and gateways from MIME-compliant protocols to HTTP need to
   remove any Content-Transfer-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 ought to transform and label the data with an
   appropriate Content-Transfer-Encoding if doing so will improve the
   likelihood of safe transport over the destination protocol.

A.6.  MHTML and Line Length Limitations

   HTTP implementations that share code with MHTML [RFC2557]
   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 transfers message-bodies as



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   payload and, aside from the "multipart/byteranges" type (Appendix A
   of [Part5]), does not interpret the content or any MIME header lines
   that might be contained therein.

Appendix B.  Changes from RFC 2616

   The primary changes in this revision have been editorial in nature:
   extracting the messaging syntax and partitioning HTTP semantics into
   separate documents for the core features, conditional requests,
   partial requests, caching, and authentication.  The conformance
   language has been revised to clearly target requirements and the
   terminology has been improved to distinguish payload from
   representations and representations from resources.  An algorithm has
   been added for determining if a payload is associated with a specific
   identifier (Section 3.1.4.1).

   A new requirement has been added that semantics embedded in a URI
   should be disabled when those semantics are inconsistent with the
   request method, since this is a common cause of interoperability
   failure.

   The default charset of ISO-8859-1 for text media types has been
   removed; the default is now whatever the media type definition says
   (Section 3.1.1.3).  Likewise, special treatment of ISO-8859-1 has
   been removed from the Accept-Charset header field (Section 5.3.3).

   The Content-Disposition header field has been removed since it is now
   defined by [RFC6266].

   The definition of Content-Location has been changed to no longer
   affect the base URI for resolving relative URI references, due to
   poor implementation support and the undesirable effect of potentially
   breaking relative links in content-negotiated resources
   (Section 3.1.4.2).

   The Content-MD5 header field has been removed because it was
   inconsistently implemented with respect to partial responses.

   To be consistent with the method-neutral parsing algorithm of
   [Part1], the definition of GET has been relaxed so that requests can
   have a body, even though a body has no meaning for GET
   (Section 4.3.1).

   Servers are no longer required to handle all Content-* header fields
   and use of Content-Range has been explicitly banned in PUT requests
   (Section 4.3.4).

   Definition of the CONNECT method has been moved from [RFC2817] to



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   this specification (Section 4.3.6).

   The OPTIONS (Section 4.3.7) and TRACE (Section 4.3.8) request methods
   have been defined as being safe.

   The definition of Expect has been both fixed to allow parameters for
   value-less expectations and simplified to allow trailing semicolons
   after "100-continue" (Section 5.1.1).

   The Max-Forwards header field (Section 5.1.2) has been restricted to
   the OPTIONS and TRACE methods; previously, extension methods could
   have used it as well.

   The "about:blank" URI has been suggested as a value for the Referer
   header field when no referring URI is applicable, which distinguishes
   that case from others where the Referer field is not sent or has been
   removed (Section 5.5.2).

   The 201 (Created) status description has been changed to allow for
   the possibility that more than one resource has been created
   (Section 6.3.2).

   The definition of 203 (Non-Authoritative Information) has been
   broadened to include cases of payload transformations as well
   (Section 6.3.4).

   The redirect status codes 301, 302, and 307 no longer have normative
   requirements on response payloads and user interaction (Section 6.4).

   The request methods that are safe to automatically redirect is no
   longer a closed set; user agents are able to make that determination
   based upon the request method semantics (Section 6.4).

   The description of 303 (See Other) status code has been changed to
   allow it to be cached if explicit freshness information is given, and
   a specific definition has been added for a 303 response to GET
   (Section 6.4.4).

   The status codes 301 and 302 (sections 6.4.2 and 6.4.3) have been
   changed to allow user agents to rewrite the method from POST to GET.

   The 305 (Use Proxy) status code has been deprecated due to security
   concerns regarding in-band configuration of a proxy (Section 6.4.5).

   The 400 (Bad Request) status code has been relaxed so that it isn't
   limited to syntax errors (Section 6.5.1).

   The 426 (Upgrade Required) status code has been incorporated from



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   [RFC2817] (Section 6.5.15).

   The following status codes are now cacheable (that is, they can be
   stored and reused by a cache without explicit freshness information
   present): 204, 404, 405, 414, 501.

   Allow has been reclassified as a response header field, removing the
   option to specify it in a PUT request.  Requirements relating to the
   content of Allow have been relaxed; correspondingly, clients are not
   required to always trust its value (Section 7.4.1).

   The target of requirements on HTTP-date and the Date header field
   have been reduced to those systems generating the date, rather than
   all systems sending a date (Section 7.1.1).

   The syntax of the Location header field has been changed to allow all
   URI references, including relative references and fragments, along
   with some clarifications as to when use of fragments would not be
   appropriate (Section 7.1.2).

   A Method Registry has been defined (Section 8.1).

   The Status Code Registry (Section 8.2) has been redefined by this
   specification; previously, it was defined in Section 7.1 of
   [RFC2817].

   Registration of Content Codings has been changed to require IETF
   Review (Section 8.4).

Appendix C.  Imported ABNF

   The following core rules are included by reference, as defined in
   Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
   CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
   (line feed), OCTET (any 8-bit sequence of data), SP (space), and
   VCHAR (any visible US-ASCII character).

   The rules below are defined in [Part1]:












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     BWS           = <BWS, defined in [Part1], Section 3.2.3>
     OWS           = <OWS, defined in [Part1], Section 3.2.3>
     RWS           = <RWS, defined in [Part1], Section 3.2.3>
     URI-reference = <URI-reference, defined in [Part1], Section 2.7>
     absolute-URI  = <absolute-URI, defined in [Part1], Section 2.7>
     comment       = <comment, defined in [Part1], Section 3.2.6>
     field-name    = <comment, defined in [Part1], Section 3.2>
     partial-URI   = <partial-URI, defined in [Part1], Section 2.7>
     quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
     token         = <token, defined in [Part1], Section 3.2.6>
     word          = <word, defined in [Part1], Section 3.2.6>

Appendix D.  Collected ABNF

   In the collected ABNF below, list rules are expanded as per Section
   1.2 of [Part1].

   Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
    OWS ( media-range [ accept-params ] ) ] ) ]
   Accept-Charset = *( "," OWS ) ( ( charset / "*" ) [ weight ] ) *( OWS
    "," [ OWS ( ( charset / "*" ) [ weight ] ) ] )
   Accept-Encoding = [ ( "," / ( codings [ weight ] ) ) *( OWS "," [ OWS
    ( codings [ weight ] ) ] ) ]
   Accept-Language = *( "," OWS ) ( language-range [ weight ] ) *( OWS
    "," [ OWS ( language-range [ weight ] ) ] )
   Allow = [ ( "," / method ) *( OWS "," [ OWS method ] ) ]

   BWS = <BWS, defined in [Part1], Section 3.2.3>

   Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
    content-coding ] )
   Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
    language-tag ] )
   Content-Location = absolute-URI / partial-URI
   Content-Type = media-type

   Date = HTTP-date

   Expect = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )

   From = mailbox

   GMT = %x47.4D.54 ; GMT

   HTTP-date = IMF-fixdate / obs-date

   IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT




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   Location = URI-reference

   Max-Forwards = 1*DIGIT

   OWS = <OWS, defined in [Part1], Section 3.2.3>

   RWS = <RWS, defined in [Part1], Section 3.2.3>
   Referer = absolute-URI / partial-URI
   Retry-After = HTTP-date / delta-seconds

   Server = product *( RWS ( product / comment ) )

   URI-reference = <URI-reference, defined in [Part1], Section 2.7>
   User-Agent = product *( RWS ( product / comment ) )

   Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
    ) )

   absolute-URI = <absolute-URI, defined in [Part1], Section 2.7>
   accept-ext = OWS ";" OWS token [ "=" word ]
   accept-params = weight *accept-ext
   asctime-date = day-name SP date3 SP time-of-day SP year
   attribute = token

   charset = token
   codings = content-coding / "identity" / "*"
   comment = <comment, defined in [Part1], Section 3.2.6>
   content-coding = token

   date1 = day SP month SP year
   date2 = day "-" month "-" 2DIGIT
   date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
   day = 2DIGIT
   day-name = %x4D.6F.6E ; Mon
    / %x54.75.65 ; Tue
    / %x57.65.64 ; Wed
    / %x54.68.75 ; Thu
    / %x46.72.69 ; Fri
    / %x53.61.74 ; Sat
    / %x53.75.6E ; Sun
   day-name-l = %x4D.6F.6E.64.61.79 ; Monday
    / %x54.75.65.73.64.61.79 ; Tuesday
    / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
    / %x54.68.75.72.73.64.61.79 ; Thursday
    / %x46.72.69.64.61.79 ; Friday
    / %x53.61.74.75.72.64.61.79 ; Saturday
    / %x53.75.6E.64.61.79 ; Sunday
   delta-seconds = 1*DIGIT



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   expect-name = token
   expect-param = expect-name [ BWS "=" BWS expect-value ]
   expect-value = token / quoted-string
   expectation = expect-name [ BWS "=" BWS expect-value ] *( OWS ";" [
    OWS expect-param ] )

   field-name = <comment, defined in [Part1], Section 3.2>

   hour = 2DIGIT

   language-range = <language-range, defined in [RFC4647], Section 2.1>
   language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

   mailbox = <mailbox, defined in [RFC5322], Section 3.4>
   media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
    ";" OWS parameter )
   media-type = type "/" subtype *( OWS ";" OWS parameter )
   method = token
   minute = 2DIGIT
   month = %x4A.61.6E ; Jan
    / %x46.65.62 ; Feb
    / %x4D.61.72 ; Mar
    / %x41.70.72 ; Apr
    / %x4D.61.79 ; May
    / %x4A.75.6E ; Jun
    / %x4A.75.6C ; Jul
    / %x41.75.67 ; Aug
    / %x53.65.70 ; Sep
    / %x4F.63.74 ; Oct
    / %x4E.6F.76 ; Nov
    / %x44.65.63 ; Dec

   obs-date = rfc850-date / asctime-date

   parameter = attribute "=" value
   partial-URI = <partial-URI, defined in [Part1], Section 2.7>
   product = token [ "/" product-version ]
   product-version = token

   quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
   qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )

   rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT

   second = 2DIGIT
   subtype = token

   time-of-day = hour ":" minute ":" second



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   token = <token, defined in [Part1], Section 3.2.6>
   type = token

   value = word

   weight = OWS ";" OWS "q=" qvalue
   word = <word, defined in [Part1], Section 3.2.6>

   year = 4DIGIT

Appendix E.  Change Log (to be removed by RFC Editor before publication)

E.1.  Since RFC 2616

   Changes up to the first Working Group Last Call draft are summarized
   in <http://trac.tools.ietf.org/html/
   draft-ietf-httpbis-p2-semantics-21#appendix-F>.

E.2.  Since draft-ietf-httpbis-p2-semantics-21

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/22>: "ETag (and
      other metadata) in status messages"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/96>: "Conditional
      GET text"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/146>: "Clarify
      description of 405 (Not Allowed)"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
      heuristic caching for new status codes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/315>: "method
      semantics: retrieval/representation"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/388>: "User
      confirmation for unsafe methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/404>: "Tentative
      Status Codes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/418>: "No-Transform"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/419>: "p2 editorial
      feedback"




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/424>: "Absence of
      Accept-Encoding"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/428>: "Accept-
      Language ordering for identical qvalues"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/432>: "Identify
      additional status codes as cacheable by default"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/434>: "mention in
      header field considerations that leading/trailing WS is lossy"

E.3.  Since draft-ietf-httpbis-p2-semantics-22

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/436>: "explain list
      expansion in ABNF appendices"

   o  <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/448>: "Fallback
      for Accept-Language"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/449>: "Receiving a
      higher minor HTTP version number"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/456>: "Language-tag
      vs. language-range"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/457>: "Registering
      x-gzip and x-deflate"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/459>: "RFC2774 and
      method registrations"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/488>: "Selection
      based upon request target"

Index

   1
      1xx Informational (status code class)  49

   2
      2xx Successful (status code class)  50

   3
      3xx Redirection (status code class)  53




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   4
      4xx Client Error (status code class)  57

   5
      5xx Server Error (status code class)  61

   1
      100 Continue (status code)  49
      100-continue (expect value)  33
      101 Switching Protocols (status code)  50

   2
      200 OK (status code)  50
      201 Created (status code)  51
      202 Accepted (status code)  51
      203 Non-Authoritative Information (status code)  51
      204 No Content (status code)  52
      205 Reset Content (status code)  52

   3
      300 Multiple Choices (status code)  54
      301 Moved Permanently (status code)  55
      302 Found (status code)  55
      303 See Other (status code)  56
      305 Use Proxy (status code)  56
      306 (Unused) (status code)  56
      307 Temporary Redirect (status code)  57

   4
      400 Bad Request (status code)  57
      402 Payment Required (status code)  57
      403 Forbidden (status code)  57
      404 Not Found (status code)  58
      405 Method Not Allowed (status code)  58
      406 Not Acceptable (status code)  58
      408 Request Timeout (status code)  59
      409 Conflict (status code)  59
      410 Gone (status code)  59
      411 Length Required (status code)  60
      413 Payload Too Large (status code)  60
      414 URI Too Long (status code)  60
      415 Unsupported Media Type (status code)  60
      417 Expectation Failed (status code)  61
      426 Upgrade Required (status code)  61

   5
      500 Internal Server Error (status code)  61
      501 Not Implemented (status code)  61



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      502 Bad Gateway (status code)  62
      503 Service Unavailable (status code)  62
      504 Gateway Timeout (status code)  62
      505 HTTP Version Not Supported (status code)  62

   A
      Accept header field  38
      Accept-Charset header field  40
      Accept-Encoding header field  41
      Accept-Language header field  42
      Allow header field  71

   C
      cacheable  23
      compress (content coding)  11
      conditional request  36
      CONNECT method  29
      content coding  11
      content negotiation  6
      Content-Encoding header field  12
      Content-Language header field  13
      Content-Location header field  15
      Content-Transfer-Encoding header field  88
      Content-Type header field  10

   D
      Date header field  66
      deflate (content coding)  11
      DELETE method  28

   E
      Expect header field  33
      Expect Values
         100-continue  33

   F
      From header field  44

   G
      GET method  24
      Grammar
         Accept  38
         Accept-Charset  40
         Accept-Encoding  41
         accept-ext  38
         Accept-Language  42
         accept-params  38
         Allow  71



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         asctime-date  65
         attribute  8
         charset  9
         codings  41
         content-coding  11
         Content-Encoding  12
         Content-Language  13
         Content-Location  15
         Content-Type  10
         Date  66
         date1  64
         day  64
         day-name  64
         day-name-l  64
         delta-seconds  68
         Expect  33
         expect-name  33
         expect-param  33
         expect-value  33
         expectation  33
         From  44
         GMT  64
         hour  64
         HTTP-date  63
         IMF-fixdate  64
         language-range  42
         language-tag  13
         Location  66
         Max-Forwards  36
         media-range  38
         media-type  8
         method  20
         minute  64
         month  64
         obs-date  65
         parameter  8
         product  46
         product-version  46
         qvalue  37
         Referer  45
         Retry-After  68
         rfc850-date  65
         second  64
         Server  71
         subtype  8
         time-of-day  64
         type  8
         User-Agent  46



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         value  8
         Vary  68
         weight  37
         year  64
      gzip (content coding)  11

   H
      HEAD method  24

   I
      idempotent  23

   L
      Location header field  66

   M
      Max-Forwards header field  36
      MIME-Version header field  87

   O
      OPTIONS method  31

   P
      payload  17
      POST method  25
      PUT method  26

   R
      Referer header field  44
      representation  7
      Retry-After header field  68

   S
      safe  22
      selected representation  7, 69
      Server header field  71
      Status Codes Classes
         1xx Informational  49
         2xx Successful  50
         3xx Redirection  53
         4xx Client Error  57
         5xx Server Error  61

   T
      TRACE method  32

   U
      User-Agent header field  45



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   V
      Vary header field  68

   X
      x-compress (content coding)  11
      x-gzip (content coding)  11

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/
























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