draft-ietf-httpbis-p6-cache-10.txt   draft-ietf-httpbis-p6-cache-11.txt 
HTTPbis Working Group R. Fielding, Ed. HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Day Software Internet-Draft Day Software
Obsoletes: 2616 (if approved) J. Gettys Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track Alcatel-Lucent Intended status: Standards Track Alcatel-Lucent
Expires: January 13, 2011 J. Mogul Expires: February 5, 2011 J. Mogul
HP HP
H. Frystyk H. Frystyk
Microsoft Microsoft
L. Masinter L. Masinter
Adobe Systems Adobe Systems
P. Leach P. Leach
Microsoft Microsoft
T. Berners-Lee T. Berners-Lee
W3C/MIT W3C/MIT
Y. Lafon, Ed. Y. Lafon, Ed.
W3C W3C
M. Nottingham, Ed. M. Nottingham, Ed.
J. Reschke, Ed. J. Reschke, Ed.
greenbytes greenbytes
July 12, 2010 August 4, 2010
HTTP/1.1, part 6: Caching HTTP/1.1, part 6: Caching
draft-ietf-httpbis-p6-cache-10 draft-ietf-httpbis-p6-cache-11
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information protocol for distributed, collaborative, hypermedia information
systems. This document is Part 6 of the seven-part specification systems. This document is Part 6 of the seven-part specification
that defines the protocol referred to as "HTTP/1.1" and, taken that defines the protocol referred to as "HTTP/1.1" and, taken
together, obsoletes RFC 2616. Part 6 defines requirements on HTTP together, obsoletes RFC 2616. Part 6 defines requirements on HTTP
caches and the associated header fields that control cache behavior caches and the associated header fields that control cache behavior
or indicate cacheable response messages. or indicate cacheable response messages.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the HTTPBIS working Discussion of this draft should take place on the HTTPBIS working
group mailing list (ietf-http-wg@w3.org). The current issues list is group mailing list (ietf-http-wg@w3.org). The current issues list is
at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>. <http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix C.11. The changes in this draft are summarized in Appendix C.12.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2011. This Internet-Draft will expire on February 5, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 3, line 24 skipping to change at page 3, line 24
2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 12 2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 12
2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 13 2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 13
2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 14 2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 14
2.5. Request Methods that Invalidate . . . . . . . . . . . . . 14 2.5. Request Methods that Invalidate . . . . . . . . . . . . . 14
2.6. Shared Caching of Authenticated Responses . . . . . . . . 15 2.6. Shared Caching of Authenticated Responses . . . . . . . . 15
2.7. Caching Negotiated Responses . . . . . . . . . . . . . . . 16 2.7. Caching Negotiated Responses . . . . . . . . . . . . . . . 16
2.8. Combining Responses . . . . . . . . . . . . . . . . . . . 16 2.8. Combining Responses . . . . . . . . . . . . . . . . . . . 16
3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 17 3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 17
3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 18 3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 18
3.2.1. Request Cache-Control Directives . . . . . . . . . . . 19 3.2.1. Request Cache-Control Directives . . . . . . . . . . . 18
3.2.2. Response Cache-Control Directives . . . . . . . . . . 20 3.2.2. Response Cache-Control Directives . . . . . . . . . . 20
3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 23 3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 22
3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 26
4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 29 4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 28
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
5.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 29 5.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 28
5.2. Message Header Registration . . . . . . . . . . . . . . . 30 5.2. Header Field Registration . . . . . . . . . . . . . . . . 29
6. Security Considerations . . . . . . . . . . . . . . . . . . . 30 6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1. Normative References . . . . . . . . . . . . . . . . . . . 30 8.1. Normative References . . . . . . . . . . . . . . . . . . . 30
8.2. Informative References . . . . . . . . . . . . . . . . . . 31 8.2. Informative References . . . . . . . . . . . . . . . . . . 31
Appendix A. Compatibility with Previous Versions . . . . . . . . 32 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 31
A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 32 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 31
A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 32
Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 32
Appendix C. Change Log (to be removed by RFC Editor before Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 34 publication) . . . . . . . . . . . . . . . . . . . . 33
C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 34 C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 33
C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 34 C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33
C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 35 C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 35 C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34
C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 35 C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35 C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 34
C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 36 C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 36 C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35
C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36 C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 35
C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 37 C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 36
C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 37 C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 36
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 37
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction 1. Introduction
HTTP is typically used for distributed information systems, where HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing document defines aspects of HTTP/1.1 related to caching and reusing
response messages. response messages.
1.1. Purpose 1.1. Purpose
An HTTP cache is a local store of response messages and the subsystem An HTTP cache is a local store of response messages and the subsystem
that controls its message storage, retrieval, and deletion. A cache that controls its message storage, retrieval, and deletion. A cache
stores cacheable responses in order to reduce the response time and stores cacheable responses in order to reduce the response time and
network bandwidth consumption on future, equivalent requests. Any network bandwidth consumption on future, equivalent requests. Any
client or server may include a cache, though a cache cannot be used client or server MAY employ a cache, though a cache cannot be used by
by a server that is acting as a tunnel. a server that is acting as a tunnel.
Caching would be useless if it did not significantly improve Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to reuse a prior performance. The goal of caching in HTTP/1.1 is to reuse a prior
response message to satisfy a current request. In some cases, a response message to satisfy a current request. In some cases, a
stored response can be reused without the need for a network request, stored response can be reused without the need for a network request,
reducing latency and network round-trips; a "freshness" mechanism is reducing latency and network round-trips; a "freshness" mechanism is
used for this purpose (see Section 2.3). Even when a new request is used for this purpose (see Section 2.3). Even when a new request is
required, it is often possible to reuse all or parts of the payload required, it is often possible to reuse all or parts of the payload
of a prior response to satisfy the request, thereby reducing network of a prior response to satisfy the request, thereby reducing network
bandwidth usage; a "validation" mechanism is used for this purpose bandwidth usage; a "validation" mechanism is used for this purpose
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1.2. Terminology 1.2. Terminology
This specification uses a number of terms to refer to the roles This specification uses a number of terms to refer to the roles
played by participants in, and objects of, HTTP caching. played by participants in, and objects of, HTTP caching.
cacheable cacheable
A response is cacheable if a cache is allowed to store a copy of A response is cacheable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests. the response message for use in answering subsequent requests.
Even when a response is cacheable, there may be additional Even when a response is cacheable, there might be additional
constraints on whether a cache can use the cached copy to satisfy constraints on whether a cache can use the cached copy to satisfy
a particular request. a particular request.
explicit expiration time explicit expiration time
The time at which the origin server intends that an entity should The time at which the origin server intends that a representation
no longer be returned by a cache without further validation. no longer be returned by a cache without further validation.
heuristic expiration time heuristic expiration time
An expiration time assigned by a cache when no explicit expiration An expiration time assigned by a cache when no explicit expiration
time is available. time is available.
age age
The age of a response is the time since it was sent by, or The age of a response is the time since it was sent by, or
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A response is fresh if its age has not yet exceeded its freshness A response is fresh if its age has not yet exceeded its freshness
lifetime. lifetime.
stale stale
A response is stale if its age has passed its freshness lifetime A response is stale if its age has passed its freshness lifetime
(either explicit or heuristic). (either explicit or heuristic).
validator validator
A protocol element (e.g., an entity tag or a Last-Modified time) A protocol element (e.g., an entity-tag or a Last-Modified time)
that is used to find out whether a stored response is an that is used to find out whether a stored response has an
equivalent copy of an entity. equivalent copy of a representation.
shared cache shared cache
A cache that is accessible to more than one user. A non-shared A cache that is accessible to more than one user. A non-shared
cache is dedicated to a single user. cache is dedicated to a single user.
1.3. Requirements 1.3. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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status code. status code.
A cache that does not support the Range and Content-Range headers A cache that does not support the Range and Content-Range headers
MUST NOT store incomplete or partial responses. MUST NOT store incomplete or partial responses.
2.2. Constructing Responses from Caches 2.2. Constructing Responses from Caches
For a presented request, a cache MUST NOT return a stored response, For a presented request, a cache MUST NOT return a stored response,
unless: unless:
o The presented Effective Request URI (Section 4.3 of [Part1]) and o The presented effective request URI (Section 4.3 of [Part1]) and
that of the stored response match, and that of the stored response match, and
o the request method associated with the stored response allows it o the request method associated with the stored response allows it
to be used for the presented request, and to be used for the presented request, and
o selecting request-headers nominated by the stored response (if o selecting request-headers nominated by the stored response (if
any) match those presented (see Section 2.7), and any) match those presented (see Section 2.7), and
o the presented request and stored response are free from directives o the presented request and stored response are free from directives
that would prevent its use (see Section 3.2 and Section 3.4), and that would prevent its use (see Section 3.2 and Section 3.4), and
o the stored response is either: o the stored response is either:
* fresh (see Section 2.3), or * fresh (see Section 2.3), or
* allowed to be served stale (see Section 2.3.3), or * allowed to be served stale (see Section 2.3.3), or
* successfully validated (see Section 2.4). * successfully validated (see Section 2.4).
[[TODO-method-cacheability: define method cacheability for GET, HEAD When a stored response is used to satisfy a request without
and POST in p2-semantics.]] validation, caches MUST include a single Age header field
(Section 3.1) in the response with a value equal to the stored
When a stored response is used to satisfy a request, caches MUST response's current_age; see Section 2.3.2.
include a single Age header field (Section 3.1) in the response with
a value equal to the stored response's current_age; see
Section 2.3.2. [[DISCUSS-includes-validated: this currently includes
successfully validated responses.]]
Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
be written through the cache to the origin server; i.e., a cache must be written through the cache to the origin server; i.e., a cache must
not reply to such a request before having forwarded the request and not reply to such a request before having forwarded the request and
having received a corresponding response. having received a corresponding response.
Also, note that unsafe requests might invalidate already stored Also, note that unsafe requests might invalidate already stored
responses; see Section 2.5. responses; see Section 2.5.
Caches MUST use the most recent response (as determined by the Date Caches MUST use the most recent response (as determined by the Date
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When a response is "fresh" in the cache, it can be used to satisfy When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby subsequent requests without contacting the origin server, thereby
improving efficiency. improving efficiency.
The primary mechanism for determining freshness is for an origin The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using server to provide an explicit expiration time in the future, using
either the Expires header (Section 3.3) or the max-age response cache either the Expires header (Section 3.3) or the max-age response cache
directive (Section 3.2.2). Generally, origin servers will assign directive (Section 3.2.2). Generally, origin servers will assign
future explicit expiration times to responses in the belief that the future explicit expiration times to responses in the belief that the
entity is not likely to change in a semantically significant way representation is not likely to change in a semantically significant
before the expiration time is reached. way before the expiration time is reached.
If an origin server wishes to force a cache to validate every If an origin server wishes to force a cache to validate every
request, it can assign an explicit expiration time in the past. This request, it can assign an explicit expiration time in the past to
means that the response is always stale, so that caches should indicate that the response is already stale. Compliant caches will
validate it before using it for subsequent requests. [[TODO- validate the cached response before reusing it for subsequent
response-stale: This wording may cause confusion, because the requests.
response may still be served stale.]]
Since origin servers do not always provide explicit expiration times, Since origin servers do not always provide explicit expiration times,
HTTP caches may also assign heuristic expiration times when they are HTTP caches MAY assign heuristic expiration times when explicit times
not specified, employing algorithms that use other header values are not specified, employing algorithms that use other header values
(such as the Last-Modified time) to estimate a plausible expiration (such as the Last-Modified time) to estimate a plausible expiration
time. The HTTP/1.1 specification does not provide specific time. The HTTP/1.1 specification does not provide specific
algorithms, but does impose worst-case constraints on their results. algorithms, but does impose worst-case constraints on their results.
The calculation to determine if a response is fresh is: The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age) response_is_fresh = (freshness_lifetime > current_age)
The freshness_lifetime is defined in Section 2.3.1; the current_age The freshness_lifetime is defined in Section 2.3.1; the current_age
is defined in Section 2.3.2. is defined in Section 2.3.2.
Additionally, clients may need to influence freshness calculation. Additionally, clients might need to influence freshness calculation.
They can do this using several request cache directives, with the They can do this using several request cache directives, with the
effect of either increasing or loosening constraints on freshness. effect of either increasing or loosening constraints on freshness.
See Section 3.2.1. See Section 3.2.1.
[[ISSUE-no-req-for-directives: there are not requirements directly [[ISSUE-no-req-for-directives: there are not requirements directly
applying to cache-request-directives and freshness.]] applying to cache-request-directives and freshness.]]
Note that freshness applies only to cache operation; it cannot be Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a used to force a user agent to refresh its display or reload a
resource. See Section 4 for an explanation of the difference between resource. See Section 4 for an explanation of the difference between
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o Otherwise, no explicit expiration time is present in the response. o Otherwise, no explicit expiration time is present in the response.
A heuristic freshness lifetime might be applicable; see A heuristic freshness lifetime might be applicable; see
Section 2.3.1.1. Section 2.3.1.1.
Note that this calculation is not vulnerable to clock skew, since all Note that this calculation is not vulnerable to clock skew, since all
of the information comes from the origin server. of the information comes from the origin server.
2.3.1.1. Calculating Heuristic Freshness 2.3.1.1. Calculating Heuristic Freshness
If no explicit expiration time is present in a stored response that If no explicit expiration time is present in a stored response that
has a status code of 200, 203, 206, 300, 301 or 410, a heuristic has a status code whose definition allows heuristic freshness to be
expiration time can be calculated. Heuristics MUST NOT be used for used (including the following in Section 8 of [Part2]: 200, 203, 206,
other response status codes. 300, 301 and 410), a heuristic expiration time MAY be calculated.
Heuristics MUST NOT be used for response status codes that do not
explicitly allow it.
When a heuristic is used to calculate freshness lifetime, the cache When a heuristic is used to calculate freshness lifetime, the cache
SHOULD attach a Warning header with a 113 warn-code to the response SHOULD attach a Warning header with a 113 warn-code to the response
if its current_age is more than 24 hours and such a warning is not if its current_age is more than 24 hours and such a warning is not
already present. already present.
Also, if the response has a Last-Modified header (Section 6.6 of Also, if the response has a Last-Modified header (Section 6.6 of
[Part4]), the heuristic expiration value SHOULD be no more than some [Part4]), the heuristic expiration value SHOULD be no more than some
fraction of the interval since that time. A typical setting of this fraction of the interval since that time. A typical setting of this
fraction might be 10%. fraction might be 10%.
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requested URI, if present. However, if any of the stored responses requested URI, if present. However, if any of the stored responses
contains only partial content, its entity-tag SHOULD NOT be included contains only partial content, its entity-tag SHOULD NOT be included
in the If-None-Match header field unless the request is for a range in the If-None-Match header field unless the request is for a range
that would be fully satisfied by that stored response. that would be fully satisfied by that stored response.
A 304 (Not Modified) response status code indicates that the stored A 304 (Not Modified) response status code indicates that the stored
response can be updated and reused; see Section 2.8. response can be updated and reused; see Section 2.8.
A full response (i.e., one with a response body) indicates that none A full response (i.e., one with a response body) indicates that none
of the stored responses nominated in the conditional request is of the stored responses nominated in the conditional request is
suitable. Instead, the full response is used both to satisfy the suitable. Instead, the full response SHOULD be used to satisfy the
request and replace the stored response. [[TODO-req-missing: Should request and MAY replace the stored response.
there be a requirement here?]]
If a cache receives a 5xx response while attempting to validate a If a cache receives a 5xx response while attempting to validate a
response, it MAY either forward this response to the requesting response, it MAY either forward this response to the requesting
client, or act as if the server failed to respond. In the latter client, or act as if the server failed to respond. In the latter
case, it MAY return a previously stored response (see Section 2.3.3). case, it MAY return a previously stored response (see Section 2.3.3).
2.5. Request Methods that Invalidate 2.5. Request Methods that Invalidate
Because unsafe methods (Section 7.1.1 of [Part2]) have the potential Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
for changing state on the origin server, intervening caches can use for changing state on the origin server, intervening caches can use
them to keep their contents up-to-date. them to keep their contents up-to-date.
The following HTTP methods MUST cause a cache to invalidate the The following HTTP methods MUST cause a cache to invalidate the
Effective Request URI (Section 4.3 of [Part1]) as well as the URI(s) effective Request URI (Section 4.3 of [Part1]) as well as the URI(s)
in the Location and Content-Location headers (if present): in the Location and Content-Location headers (if present):
o PUT o PUT
o DELETE o DELETE
o POST o POST
An invalidation based on a URI from a Location or Content-Location An invalidation based on a URI from a Location or Content-Location
header MUST NOT be performed if the host part of that URI differs header MUST NOT be performed if the host part of that URI differs
from the host part in the Effective Request URI (Section 4.3 of from the host part in the effective request URI (Section 4.3 of
[Part1]). This helps prevent denial of service attacks. [Part1]). This helps prevent denial of service attacks.
[[TODO-def-host-part: "host part" needs to be specified better.]]
A cache that passes through requests for methods it does not A cache that passes through requests for methods it does not
understand SHOULD invalidate the Effective Request URI (Section 4.3 understand SHOULD invalidate the effective request URI (Section 4.3
of [Part1]). of [Part1]).
Here, "invalidate" means that the cache will either remove all stored Here, "invalidate" means that the cache will either remove all stored
responses related to the Effective Request URI, or will mark these as responses related to the effective request URI, or will mark these as
"invalid" and in need of a mandatory validation before they can be "invalid" and in need of a mandatory validation before they can be
returned in response to a subsequent request. returned in response to a subsequent request.
Note that this does not guarantee that all appropriate responses are Note that this does not guarantee that all appropriate responses are
invalidated. For example, the request that caused the change at the invalidated. For example, the request that caused the change at the
origin server might not have gone through the cache where a response origin server might not have gone through the cache where a response
is stored. is stored.
[[TODO-spec-success-invalidate: specify that only successful (2xx,
3xx?) responses invalidate.]]
2.6. Shared Caching of Authenticated Responses 2.6. Shared Caching of Authenticated Responses
Shared caches MUST NOT use a cached response to a request with an Shared caches MUST NOT use a cached response to a request with an
Authorization header (Section 3.1 of [Part7]) to satisfy any Authorization header (Section 3.1 of [Part7]) to satisfy any
subsequent request unless a cache directive that allows such subsequent request unless a cache directive that allows such
responses to be stored is present in the response. responses to be stored is present in the response.
In this specification, the following Cache-Control response In this specification, the following Cache-Control response
directives (Section 3.2.2) have such an effect: must-revalidate, directives (Section 3.2.2) have such an effect: must-revalidate,
public, s-maxage. public, s-maxage.
skipping to change at page 16, line 32 skipping to change at page 16, line 29
o combining multiple message-header fields with the same field name o combining multiple message-header fields with the same field name
(see Section 3.2 of [Part1]) (see Section 3.2 of [Part1])
o normalizing both header values in a way that is known to have o normalizing both header values in a way that is known to have
identical semantics, according to the header's specification identical semantics, according to the header's specification
(e.g., re-ordering field values when order is not significant; (e.g., re-ordering field values when order is not significant;
case-normalization, where values are defined to be case- case-normalization, where values are defined to be case-
insensitive) insensitive)
If (after any normalisation that may take place) a header field is If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is absent from a request, it can only match another request if it is
also absent there. also absent there.
A Vary header field-value of "*" always fails to match, and A Vary header field-value of "*" always fails to match, and
subsequent requests to that resource can only be properly interpreted subsequent requests to that resource can only be properly interpreted
by the origin server. by the origin server.
The stored response with matching selecting request-headers is known The stored response with matching selecting request-headers is known
as the selected response. as the selected response.
If no selected response is available, the cache MAY forward the If no selected response is available, the cache MAY forward the
presented request to the origin server in a conditional request; see presented request to the origin server in a conditional request; see
Section 2.4. Section 2.4.
2.8. Combining Responses 2.8. Combining Responses
When a cache receives a 304 (Not Modified) response or a 206 (Partial When a cache receives a 304 (Not Modified) response or a 206 (Partial
Content) response (in this section, the "new" response"), it needs to Content) response (in this section, the "new" response"), it needs to
created an updated response by combining the stored response with the created an updated response by combining the stored response with the
new one, so that the updated response can be used to satisfy the new one, so that the updated response can be used to satisfy the
request. request, and potentially update the cached response.
If the new response contains an ETag, it identifies the stored If the new response contains an ETag, it identifies the stored
response to use. [[TODO-mention-CL: may need language about Content- response to use. [[TODO-mention-CL: might need language about
Location here]][[TODO-inm-mult-etags: cover case where INM with Content-Location here]][[TODO-select-for-combine: Shouldn't this be
multiple etags was sent]] the selected response?]]
If the status code is 206 (partial content), both the stored and new If the new response's status code is 206 (partial content), both the
responses MUST have validators, and those validators MUST match using stored and new responses MUST have validators, and those validators
the strong comparison function (see Section 4 of [Part4]). MUST match using the strong comparison function (see Section 4 of
Otherwise, the responses MUST NOT be combined. [Part4]). Otherwise, the responses MUST NOT be combined.
The stored response headers are used as those of the updated The stored response headers are used as those of the updated
response, except that response, except that
o any stored Warning headers with warn-code 1xx (see Section 3.6) o any stored Warning headers with warn-code 1xx (see Section 3.6)
MUST be deleted from the stored response and the updated response. MUST be deleted.
o any stored Warning headers with warn-code 2xx MUST be retained in
the stored response and the updated response.
o any headers provided in the new response MUST replace the
corresponding headers from the stored response.
If a header field-name in the new response matches more than one o any stored Warning headers with warn-code 2xx MUST be retained.
header in the stored response, all such stored headers MUST be
replaced.
The updated response can [[TODO-is-req: requirement?]] be used to o any other headers provided in the new response MUST replace all
replace the stored response in cache. In the case of a 206 response, instances of the corresponding headers from the stored response.
the combined entity-body MAY be stored.
[[ISSUE-how-head: discuss how to handle HEAD updates]] The updated response headers MUST be used to replace those of the
stored response in cache (unless the stored response is removed from
cache). In the case of a 206 response, the combined representation
MAY be stored.
3. Header Field Definitions 3. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header This section defines the syntax and semantics of HTTP/1.1 header
fields related to caching. fields related to caching.
For entity-header fields, both sender and recipient refer to either
the client or the server, depending on who sends and who receives the
entity.
3.1. Age 3.1. Age
The "Age" response-header field conveys the sender's estimate of the The "Age" response-header field conveys the sender's estimate of the
amount of time since the response was generated or successfully amount of time since the response was generated or successfully
validated at the origin server. Age values are calculated as validated at the origin server. Age values are calculated as
specified in Section 2.3.2. specified in Section 2.3.2.
Age = "Age" ":" OWS Age-v Age = "Age" ":" OWS Age-v
Age-v = delta-seconds Age-v = delta-seconds
skipping to change at page 18, line 18 skipping to change at page 18, line 4
Age-v = delta-seconds Age-v = delta-seconds
Age field-values are non-negative integers, representing time in Age field-values are non-negative integers, representing time in
seconds. seconds.
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
If a cache receives a value larger than the largest positive integer If a cache receives a value larger than the largest positive integer
it can represent, or if any of its age calculations overflows, it it can represent, or if any of its age calculations overflows, it
MUST transmit an Age header with a field-value of 2147483648 (2^31). MUST transmit an Age header with a field-value of 2147483648 (2^31).
Caches SHOULD use an arithmetic type of at least 31 bits of range. Caches SHOULD use an arithmetic type of at least 31 bits of range.
The presence of an Age header field in a response implies that a The presence of an Age header field in a response implies that a
response is not first-hand. However, the converse is not true, since response is not first-hand. However, the converse is not true, since
HTTP/1.0 caches may not implement the Age header field. HTTP/1.0 caches might not implement the Age header field.
3.2. Cache-Control 3.2. Cache-Control
The "Cache-Control" general-header field is used to specify The "Cache-Control" general-header field is used to specify
directives for caches along the request/response chain. Such cache directives for caches along the request/response chain. Such cache
directives are unidirectional in that the presence of a directive in directives are unidirectional in that the presence of a directive in
a request does not imply that the same directive is to be given in a request does not imply that the same directive is to be given in
the response. the response.
HTTP/1.1 caches MUST obey the requirements of the Cache-Control HTTP/1.1 caches MUST obey the requirements of the Cache-Control
skipping to change at page 19, line 38 skipping to change at page 19, line 24
store any part of either this request or any response to it. This store any part of either this request or any response to it. This
directive applies to both non-shared and shared caches. "MUST NOT directive applies to both non-shared and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile best-effort attempt to remove the information from volatile
storage as promptly as possible after forwarding it. storage as promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for This directive is NOT a reliable or sufficient mechanism for
ensuring privacy. In particular, malicious or compromised caches ensuring privacy. In particular, malicious or compromised caches
might not recognize or obey this directive, and communications might not recognize or obey this directive, and communications
networks may be vulnerable to eavesdropping. networks might be vulnerable to eavesdropping.
max-age max-age
The max-age request directive indicates that the client is willing The max-age request directive indicates that the client is willing
to accept a response whose age is no greater than the specified to accept a response whose age is no greater than the specified
time in seconds. Unless the max-stale request directive is also time in seconds. Unless the max-stale request directive is also
present, the client is not willing to accept a stale response. present, the client is not willing to accept a stale response.
max-stale max-stale
The max-stale request directive indicates that the client is The max-stale request directive indicates that the client is
willing to accept a response that has exceeded its expiration willing to accept a response that has exceeded its expiration
time. If max-stale is assigned a value, then the client is time. If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its expiration time willing to accept a response that has exceeded its expiration time
by no more than the specified number of seconds. If no value is by no more than the specified number of seconds. If no value is
assigned to max-stale, then the client is willing to accept a assigned to max-stale, then the client is willing to accept a
stale response of any age. [[TODO-staleness: of any staleness? stale response of any age.
--mnot]]
min-fresh min-fresh
The min-fresh request directive indicates that the client is The min-fresh request directive indicates that the client is
willing to accept a response whose freshness lifetime is no less willing to accept a response whose freshness lifetime is no less
than its current age plus the specified time in seconds. That is, than its current age plus the specified time in seconds. That is,
the client wants a response that will still be fresh for at least the client wants a response that will still be fresh for at least
the specified number of seconds. the specified number of seconds.
no-transform no-transform
The no-transform request directive indicates that an intermediate The no-transform request directive indicates that an intermediate
cache or proxy MUST NOT change the Content-Encoding, Content-Range cache or proxy MUST NOT change the Content-Encoding, Content-Range
or Content-Type request headers, nor the request entity-body. or Content-Type request headers, nor the request representation.
only-if-cached only-if-cached
The only-if-cached request directive indicates that the client The only-if-cached request directive indicates that the client
only wishes to return a stored response. If it receives this only wishes to return a stored response. If it receives this
directive, a cache SHOULD either respond using a stored response directive, a cache SHOULD either respond using a stored response
that is consistent with the other constraints of the request, or that is consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status. If a group of caches respond with a 504 (Gateway Timeout) status code. If a group of
is being operated as a unified system with good internal caches is being operated as a unified system with good internal
connectivity, such a request MAY be forwarded within that group of connectivity, such a request MAY be forwarded within that group of
caches. caches.
3.2.2. Response Cache-Control Directives 3.2.2. Response Cache-Control Directives
cache-response-directive = cache-response-directive =
"public" "public"
/ "private" [ "=" DQUOTE 1#field-name DQUOTE ] / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
/ "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
/ "no-store" / "no-store"
skipping to change at page 21, line 20 skipping to change at page 21, line 7
is intended for a single user and MUST NOT be stored by a shared is intended for a single user and MUST NOT be stored by a shared
cache. A private (non-shared) cache MAY store the response. cache. A private (non-shared) cache MAY store the response.
If the private response directive specifies one or more field- If the private response directive specifies one or more field-
names, this requirement is limited to the field-values associated names, this requirement is limited to the field-values associated
with the listed response headers. That is, the specified field- with the listed response headers. That is, the specified field-
names(s) MUST NOT be stored by a shared cache, whereas the names(s) MUST NOT be stored by a shared cache, whereas the
remainder of the response message MAY be. remainder of the response message MAY be.
Note: This usage of the word private only controls where the Note: This usage of the word private only controls where the
response may be stored, and cannot ensure the privacy of the response can be stored; it cannot ensure the privacy of the
message content. Also, private response directives with field- message content. Also, private response directives with field-
names are often handled by implementations as if an unqualified names are often handled by implementations as if an unqualified
private directive was received; i.e., the special handling for the private directive was received; i.e., the special handling for the
qualified form is not widely implemented. qualified form is not widely implemented.
no-cache no-cache
The no-cache response directive indicates that the response MUST The no-cache response directive indicates that the response MUST
NOT be used to satisfy a subsequent request without successful NOT be used to satisfy a subsequent request without successful
validation on the origin server. This allows an origin server to validation on the origin server. This allows an origin server to
prevent caching even by caches that have been configured to return prevent a cache from using it to satisfy a request without
contacting it, even by caches that have been configured to return
stale responses. stale responses.
If the no-cache response directive specifies one or more field- If the no-cache response directive specifies one or more field-
names, this requirement is limited to the field-values associated names, this requirement is limited to the field-values associated
with the listed response headers. That is, the specified field- with the listed response headers. That is, the specified field-
name(s) MUST NOT be sent in the response to a subsequent request name(s) MUST NOT be sent in the response to a subsequent request
without successful validation on the origin server. This allows without successful validation on the origin server. This allows
an origin server to prevent the re-use of certain header fields in an origin server to prevent the re-use of certain header fields in
a response, while still allowing caching of the rest of the a response, while still allowing caching of the rest of the
response. response.
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store any part of either the immediate request or response. This store any part of either the immediate request or response. This
directive applies to both non-shared and shared caches. "MUST NOT directive applies to both non-shared and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile best-effort attempt to remove the information from volatile
storage as promptly as possible after forwarding it. storage as promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for This directive is NOT a reliable or sufficient mechanism for
ensuring privacy. In particular, malicious or compromised caches ensuring privacy. In particular, malicious or compromised caches
might not recognize or obey this directive, and communications might not recognize or obey this directive, and communications
networks may be vulnerable to eavesdropping. networks might be vulnerable to eavesdropping.
must-revalidate must-revalidate
The must-revalidate response directive indicates that once it has The must-revalidate response directive indicates that once it has
become stale, the response MUST NOT be used to satisfy subsequent become stale, the response MUST NOT be used to satisfy subsequent
requests without successful validation on the origin server. requests without successful validation on the origin server.
The must-revalidate directive is necessary to support reliable The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances an operation for certain protocol features. In all circumstances an
HTTP/1.1 cache MUST obey the must-revalidate directive; in HTTP/1.1 cache MUST obey the must-revalidate directive; in
particular, if the cache cannot reach the origin server for any particular, if the cache cannot reach the origin server for any
reason, it MUST generate a 504 (Gateway Timeout) response. reason, it MUST generate a 504 (Gateway Timeout) response.
Servers SHOULD send the must-revalidate directive if and only if Servers SHOULD send the must-revalidate directive if and only if
failure to validate a request on the entity could result in failure to validate a request on the representation could result
incorrect operation, such as a silently unexecuted financial in incorrect operation, such as a silently unexecuted financial
transaction. transaction.
proxy-revalidate proxy-revalidate
The proxy-revalidate response directive has the same meaning as The proxy-revalidate response directive has the same meaning as
the must-revalidate response directive, except that it does not the must-revalidate response directive, except that it does not
apply to non-shared caches. apply to non-shared caches.
max-age max-age
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The s-maxage response directive indicates that, in shared caches, The s-maxage response directive indicates that, in shared caches,
the maximum age specified by this directive overrides the maximum the maximum age specified by this directive overrides the maximum
age specified by either the max-age directive or the Expires age specified by either the max-age directive or the Expires
header. The s-maxage directive also implies the semantics of the header. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive. proxy-revalidate response directive.
no-transform no-transform
The no-transform response directive indicates that an intermediate The no-transform response directive indicates that an intermediate
cache or proxy MUST NOT change the Content-Encoding, Content-Range cache or proxy MUST NOT change the Content-Encoding, Content-Range
or Content-Type response headers, nor the response entity-body. or Content-Type response headers, nor the response representation.
3.2.3. Cache Control Extensions 3.2.3. Cache Control Extensions
The Cache-Control header field can be extended through the use of one The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional value. or more cache-extension tokens, each with an optional value.
Informational extensions (those that do not require a change in cache Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other behavior) can be added without changing the semantics of other
directives. Behavioral extensions are designed to work by acting as directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the new modifiers to the existing base of cache directives. Both the new
directive and the standard directive are supplied, such that directive and the standard directive are supplied, such that
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o Pointer to specification text o Pointer to specification text
Values to be added to this name space are subject to IETF review Values to be added to this name space are subject to IETF review
([RFC5226], Section 4.1). ([RFC5226], Section 4.1).
The registry itself is maintained at The registry itself is maintained at
<http://www.iana.org/assignments/http-cache-directives>. <http://www.iana.org/assignments/http-cache-directives>.
3.3. Expires 3.3. Expires
The "Expires" entity-header field gives the date/time after which the The "Expires" header field gives the date/time after which the
response is considered stale. See Section 2.3 for further discussion response is considered stale. See Section 2.3 for further discussion
of the freshness model. of the freshness model.
The presence of an Expires field does not imply that the original The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that resource will change or cease to exist at, before, or after that
time. time.
The field-value is an absolute date and time as defined by HTTP-date The field-value is an absolute date and time as defined by HTTP-date
in Section 6.1 of [Part1]; it MUST be sent in rfc1123-date format. in Section 6.1 of [Part1]; it MUST be sent in rfc1123-date format.
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resource. A server MAY include a Vary header field with a non- resource. A server MAY include a Vary header field with a non-
cacheable response that is subject to server-driven negotiation, cacheable response that is subject to server-driven negotiation,
since this might provide the user agent with useful information about since this might provide the user agent with useful information about
the dimensions over which the response varies at the time of the the dimensions over which the response varies at the time of the
response. response.
A Vary field value of "*" signals that unspecified parameters not A Vary field value of "*" signals that unspecified parameters not
limited to the request-headers (e.g., the network address of the limited to the request-headers (e.g., the network address of the
client), play a role in the selection of the response representation; client), play a role in the selection of the response representation;
therefore, a cache cannot determine whether this response is therefore, a cache cannot determine whether this response is
appropriate. The "*" value MUST NOT be generated by a proxy server; appropriate. The "*" value MUST NOT be generated by a proxy server.
it may only be generated by an origin server.
The field-names given are not limited to the set of standard request- The field-names given are not limited to the set of standard request-
header fields defined by this specification. Field names are case- header fields defined by this specification. Field names are case-
insensitive. insensitive.
3.6. Warning 3.6. Warning
The "Warning" general-header field is used to carry additional The "Warning" general-header field is used to carry additional
information about the status or transformation of a message that information about the status or transformation of a message that
might not be reflected in the message. This information is typically might not be reflected in the message. This information is typically
used to warn about possible incorrectness introduced by caching used to warn about possible incorrectness introduced by caching
operations or transformations applied to the entity body of the operations or transformations applied to the payload of the message.
message.
Warnings can be used for other purposes, both cache-related and Warnings can be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code, otherwise. The use of a warning, rather than an error status code,
distinguishes these responses from true failures. distinguishes these responses from true failures.
Warning headers can in general be applied to any message, however Warning headers can in general be applied to any message, however
some warn-codes are specific to caches and can only be applied to some warn-codes are specific to caches and can only be applied to
response messages. response messages.
Warning = "Warning" ":" OWS Warning-v Warning = "Warning" ":" OWS Warning-v
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Warnings are assigned three digit warn-codes. The first digit Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning is required to be deleted from a stored indicates whether the Warning is required to be deleted from a stored
response after validation: response after validation:
o 1xx Warnings describe the freshness or validation status of the o 1xx Warnings describe the freshness or validation status of the
response, and so MUST be deleted by caches after validation. They response, and so MUST be deleted by caches after validation. They
can only be generated by a cache when validating a cached entry, can only be generated by a cache when validating a cached entry,
and MUST NOT be generated in any other situation. and MUST NOT be generated in any other situation.
o 2xx Warnings describe some aspect of the entity body or entity o 2xx Warnings describe some aspect of the representation that is
headers that is not rectified by a validation (for example, a not rectified by a validation (for example, a lossy compression of
lossy compression of the entity bodies) and MUST NOT be deleted by the representation) and MUST NOT be deleted by caches after
caches after validation, unless a full response is returned, in validation, unless a full response is returned, in which case they
which case they MUST be. MUST be.
If an implementation sends a message with one or more Warning headers If an implementation sends a message with one or more Warning headers
to a receiver whose version is HTTP/1.0 or lower, then the sender to a receiver whose version is HTTP/1.0 or lower, then the sender
MUST include in each warning-value a warn-date that matches the Date MUST include in each warning-value a warn-date that matches the Date
header in the message. header in the message.
If an implementation receives a message with a warning-value that If an implementation receives a message with a warning-value that
includes a warn-date, and that warn-date is different from the Date includes a warn-date, and that warn-date is different from the Date
value in the response, then that warning-value MUST be deleted from value in the response, then that warning-value MUST be deleted from
the message before storing, forwarding, or using it. (preventing the the message before storing, forwarding, or using it. (preventing the
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199 Miscellaneous warning 199 Miscellaneous warning
The warning text can include arbitrary information to be presented The warning text can include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST to a human user, or logged. A system receiving this warning MUST
NOT take any automated action, besides presenting the warning to NOT take any automated action, besides presenting the warning to
the user. the user.
214 Transformation applied 214 Transformation applied
MUST be added by an intermediate cache or proxy if it applies any MUST be added by an intermediate proxy if it applies any
transformation changing the content-coding (as specified in the transformation to the representation, such as changing the
Content-Encoding header) or media-type (as specified in the content-coding, media-type, or modifying the representation data,
Content-Type header) of the response, or the entity-body of the unless this Warning code already appears in the response.
response, unless this Warning code already appears in the
response.
299 Miscellaneous persistent warning 299 Miscellaneous persistent warning
The warning text can include arbitrary information to be presented The warning text can include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST to a human user, or logged. A system receiving this warning MUST
NOT take any automated action. NOT take any automated action.
4. History Lists 4. History Lists
User agents often have history mechanisms, such as "Back" buttons and User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay an entity retrieved history lists, that can be used to redisplay a representation
earlier in a session. retrieved earlier in a session.
The freshness model (Section 2.3) does not necessarily apply to The freshness model (Section 2.3) does not necessarily apply to
history mechanisms. I.e., a history mechanism can display a previous history mechanisms. I.e., a history mechanism can display a previous
representation even if it has expired. representation even if it has expired.
This does not prohibit the history mechanism from telling the user This does not prohibit the history mechanism from telling the user
that a view might be stale, or from honoring cache directives (e.g., that a view might be stale, or from honoring cache directives (e.g.,
Cache-Control: no-store). Cache-Control: no-store).
5. IANA Considerations 5. IANA Considerations
5.1. Cache Directive Registry 5.1. Cache Directive Registry
The registration procedure for HTTP Cache Directives is defined by The registration procedure for HTTP Cache Directives is defined by
Section 3.2.3 of this document. Section 3.2.3 of this document.
The HTTP Cache Directive Registry should be created at The HTTP Cache Directive Registry shall be created at
<http://www.iana.org/assignments/http-cache-directives> and be <http://www.iana.org/assignments/http-cache-directives> and be
populated with the registrations below: populated with the registrations below:
+------------------------+------------------------------+ +------------------------+------------------------------+
| Cache Directive | Reference | | Cache Directive | Reference |
+------------------------+------------------------------+ +------------------------+------------------------------+
| max-age | Section 3.2.1, Section 3.2.2 | | max-age | Section 3.2.1, Section 3.2.2 |
| max-stale | Section 3.2.1 | | max-stale | Section 3.2.1 |
| min-fresh | Section 3.2.1 | | min-fresh | Section 3.2.1 |
| must-revalidate | Section 3.2.2 | | must-revalidate | Section 3.2.2 |
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| no-transform | Section 3.2.1, Section 3.2.2 | | no-transform | Section 3.2.1, Section 3.2.2 |
| only-if-cached | Section 3.2.1 | | only-if-cached | Section 3.2.1 |
| private | Section 3.2.2 | | private | Section 3.2.2 |
| proxy-revalidate | Section 3.2.2 | | proxy-revalidate | Section 3.2.2 |
| public | Section 3.2.2 | | public | Section 3.2.2 |
| s-maxage | Section 3.2.2 | | s-maxage | Section 3.2.2 |
| stale-if-error | [RFC5861], Section 4 | | stale-if-error | [RFC5861], Section 4 |
| stale-while-revalidate | [RFC5861], Section 3 | | stale-while-revalidate | [RFC5861], Section 3 |
+------------------------+------------------------------+ +------------------------+------------------------------+
5.2. Message Header Registration 5.2. Header Field Registration
The Message Header Registry located at <http://www.iana.org/ The Message Header Field Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> should be assignments/message-headers/message-header-index.html> shall be
updated with the permanent registrations below (see [RFC3864]): updated with the permanent registrations below (see [RFC3864]):
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference | | Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
| Age | http | standard | Section 3.1 | | Age | http | standard | Section 3.1 |
| Cache-Control | http | standard | Section 3.2 | | Cache-Control | http | standard | Section 3.2 |
| Expires | http | standard | Section 3.3 | | Expires | http | standard | Section 3.3 |
| Pragma | http | standard | Section 3.4 | | Pragma | http | standard | Section 3.4 |
| Vary | http | standard | Section 3.5 | | Vary | http | standard | Section 3.5 |
skipping to change at page 30, line 32 skipping to change at page 30, line 5
The change controller is: "IETF (iesg@ietf.org) - Internet The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force". Engineering Task Force".
6. Security Considerations 6. Security Considerations
Caches expose additional potential vulnerabilities, since the Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the a user believes that the information has been removed from the
network. Therefore, cache contents should be protected as sensitive network. Therefore, cache contents need to be protected as sensitive
information. information.
7. Acknowledgments 7. Acknowledgments
Much of the content and presentation of the caching design is due to Much of the content and presentation of the caching design is due to
suggestions and comments from individuals including: Shel Kaphan, suggestions and comments from individuals including: Shel Kaphan,
Paul Leach, Koen Holtman, David Morris, and Larry Masinter. Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
8. References 8. References
8.1. Normative References 8.1. Normative References
[Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections, and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
and Message Parsing", draft-ietf-httpbis-p1-messaging-10 and Message Parsing", draft-ietf-httpbis-p1-messaging-11
(work in progress), July 2010. (work in progress), August 2010.
[Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 2: Message and J. Reschke, Ed., "HTTP/1.1, part 2: Message
Semantics", draft-ietf-httpbis-p2-semantics-10 (work in Semantics", draft-ietf-httpbis-p2-semantics-11 (work in
progress), July 2010. progress), August 2010.
[Part4] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part4] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
Requests", draft-ietf-httpbis-p4-conditional-10 (work in Requests", draft-ietf-httpbis-p4-conditional-11 (work in
progress), July 2010. progress), August 2010.
[Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
Partial Responses", draft-ietf-httpbis-p5-range-10 (work Partial Responses", draft-ietf-httpbis-p5-range-11 (work
in progress), July 2010. in progress), August 2010.
[Part7] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part7] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication", and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
draft-ietf-httpbis-p7-auth-10 (work in progress), draft-ietf-httpbis-p7-auth-11 (work in progress),
July 2010. August 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
8.2. Informative References 8.2. Informative References
[RFC1305] Mills, D., "Network Time Protocol (Version 3) [RFC1305] Mills, D., "Network Time Protocol (Version 3)
skipping to change at page 32, line 5 skipping to change at page 31, line 26
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004. September 2004.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, April 2010. Content", RFC 5861, April 2010.
Appendix A. Compatibility with Previous Versions Appendix A. Changes from RFC 2616
A.1. Changes from RFC 2068
A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
was introduced to add this missing case. (Sections 2.1, 3.2).
Range request responses would become very verbose if all meta-data
were always returned; by allowing the server to only send needed
headers in a 206 response, this problem can be avoided.
(Section 2.8)
The Cache-Control: max-age directive was not properly defined for
responses. (Section 3.2.2)
Warnings could be cached incorrectly, or not updated appropriately.
(Section 2.3, 2.8, 3.2, and 3.6) Warning also needed to be a general
header, as PUT or other methods may have need for it in requests.
A.2. Changes from RFC 2616
Make the specified age calculation algorithm less conservative. Make the specified age calculation algorithm less conservative.
(Section 2.3.2) (Section 2.3.2)
Remove requirement to consider Content-Location in successful Remove requirement to consider Content-Location in successful
responses in order to determine the appropriate response to use. responses in order to determine the appropriate response to use.
(Section 2.4) (Section 2.4)
Clarify denial of service attack avoidance requirement. Clarify denial of service attack avoidance requirement.
(Section 2.5) (Section 2.5)
skipping to change at page 38, line 8 skipping to change at page 37, line 10
for cache-control directives" for cache-control directives"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic o <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic
caching of URLs with query components" caching of URLs with query components"
Partly resolved issues: Partly resolved issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the o <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
requested resource's URI" requested resource's URI"
C.12. Since draft-ietf-httpbis-p6-cache-10
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
entity / representation / variant terminology"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
removing the 'changes from 2068' sections"
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/223>: "Allowing
heuristic caching for new status codes"
o Clean up TODOs and prose in "Combining Responses."
Index Index
A A
age 6 age 6
Age header 17 Age header 17
C C
cache 5 cache 5
Cache Directives Cache Directives
max-age 19, 22 max-age 19, 22
max-stale 19 max-stale 19
min-fresh 20 min-fresh 19
must-revalidate 22 must-revalidate 22
no-cache 19, 21 no-cache 19, 21
no-store 19, 22 no-store 19, 21
no-transform 20, 23 no-transform 20, 22
only-if-cached 20 only-if-cached 20
private 21 private 20
proxy-revalidate 22 proxy-revalidate 22
public 20 public 20
s-maxage 22 s-maxage 22
Cache-Control header 18 Cache-Control header 18
cacheable 5 cacheable 5
E E
Expires header 24 Expires header 24
explicit expiration time 5 explicit expiration time 5
F F
first-hand 6 first-hand 6
fresh 6 fresh 6
freshness lifetime 6 freshness lifetime 6
G G
Grammar Grammar
Age 18 Age 17
Age-v 18 Age-v 17
Cache-Control 18 Cache-Control 18
Cache-Control-v 18 Cache-Control-v 18
cache-extension 18 cache-extension 18
cache-request-directive 19 cache-request-directive 18
cache-response-directive 20 cache-response-directive 20
delta-seconds 18 delta-seconds 17
Expires 24 Expires 24
Expires-v 24 Expires-v 24
extension-pragma 25 extension-pragma 24
Pragma 25 Pragma 24
pragma-directive 25 pragma-directive 24
Pragma-v 25 Pragma-v 24
Vary 26 Vary 25
Vary-v 26 Vary-v 25
warn-agent 27 warn-agent 26
warn-code 27 warn-code 26
warn-date 27 warn-date 26
warn-text 27 warn-text 26
Warning 27 Warning 26
Warning-v 27 Warning-v 26
warning-value 27 warning-value 26
H H
Headers Headers
Age 17 Age 17
Cache-Control 18 Cache-Control 18
Expires 24 Expires 24
Pragma 25 Pragma 24
Vary 25 Vary 25
Warning 26 Warning 26
heuristic expiration time 5 heuristic expiration time 5
M M
max-age max-age
Cache Directive 19, 22 Cache Directive 19, 22
max-stale max-stale
Cache Directive 19 Cache Directive 19
min-fresh min-fresh
Cache Directive 20 Cache Directive 19
must-revalidate must-revalidate
Cache Directive 22 Cache Directive 22
N N
no-cache no-cache
Cache Directive 19, 21 Cache Directive 19, 21
no-store no-store
Cache Directive 19, 22 Cache Directive 19, 21
no-transform no-transform
Cache Directive 20, 23 Cache Directive 20, 22
O O
only-if-cached only-if-cached
Cache Directive 20 Cache Directive 20
P P
Pragma header 25 Pragma header 24
private private
Cache Directive 21 Cache Directive 20
proxy-revalidate proxy-revalidate
Cache Directive 22 Cache Directive 22
public public
Cache Directive 20 Cache Directive 20
S S
s-maxage s-maxage
Cache Directive 22 Cache Directive 22
stale 6 stale 6
 End of changes. 79 change blocks. 
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