draft-ietf-httpbis-p6-cache-05.txt   draft-ietf-httpbis-p6-cache-06.txt 
Network 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 One Laptop per Child Intended status: Standards Track One Laptop per Child
Expires: May 20, 2009 J. Mogul Expires: September 10, 2009 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
J. Reschke, Ed. J. Reschke, Ed.
greenbytes greenbytes
November 16, 2008 March 9, 2009
HTTP/1.1, part 6: Caching HTTP/1.1, part 6: Caching
draft-ietf-httpbis-p6-cache-05 draft-ietf-httpbis-p6-cache-06
Status of this Memo Status of this Memo
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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. HTTP has been in use by the World Wide Web global systems. This document is Part 6 of the seven-part specification
information initiative since 1990. This document is Part 6 of the that defines the protocol referred to as "HTTP/1.1" and, taken
seven-part specification that defines the protocol referred to as together, obsoletes RFC 2616. Part 6 defines requirements on HTTP
"HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6 defines caches and the associated header fields that control cache behavior
requirements on HTTP caches and the associated header fields that or indicate cacheable response messages.
control cache behavior 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/11> and related at <http://tools.ietf.org/wg/httpbis/trac/report/11> 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 B.6. The changes in this draft are summarized in Appendix C.7.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 1.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
2. Notational Conventions and Generic Grammar . . . . . . . . . . 8 1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 7
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Cache Correctness . . . . . . . . . . . . . . . . . . . . 8 1.4.2. ABNF Rules defined in other Parts of the
3.2. Warnings . . . . . . . . . . . . . . . . . . . . . . . . . 9 Specification . . . . . . . . . . . . . . . . . . . . 7
3.3. Cache-control Mechanisms . . . . . . . . . . . . . . . . . 10 2. Cache Operation . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Explicit User Agent Warnings . . . . . . . . . . . . . . . 10 2.1. Response Cacheability . . . . . . . . . . . . . . . . . . 7
3.5. Exceptions to the Rules and Warnings . . . . . . . . . . . 11 2.1.1. Storing Partial and Incomplete Responses . . . . . . . 8
3.6. Client-controlled Behavior . . . . . . . . . . . . . . . . 11 2.2. Constructing Responses from Caches . . . . . . . . . . . . 8
4. Expiration Model . . . . . . . . . . . . . . . . . . . . . . . 12 2.3. Freshness Model . . . . . . . . . . . . . . . . . . . . . 9
4.1. Server-Specified Expiration . . . . . . . . . . . . . . . 12 2.3.1. Calculating Freshness Lifetime . . . . . . . . . . . . 10
4.2. Heuristic Expiration . . . . . . . . . . . . . . . . . . . 13 2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 11
4.3. Age Calculations . . . . . . . . . . . . . . . . . . . . . 13 2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 13
4.4. Expiration Calculations . . . . . . . . . . . . . . . . . 15 2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 13
4.5. Disambiguating Expiration Values . . . . . . . . . . . . . 16 2.5. Request Methods that Invalidate . . . . . . . . . . . . . 14
4.6. Disambiguating Multiple Responses . . . . . . . . . . . . 17 2.6. Caching Negotiated Responses . . . . . . . . . . . . . . . 15
5. Validation Model . . . . . . . . . . . . . . . . . . . . . . . 17 2.7. Combining Responses . . . . . . . . . . . . . . . . . . . 16
6. Response Cacheability . . . . . . . . . . . . . . . . . . . . 18 3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 16
7. Constructing Responses From Caches . . . . . . . . . . . . . . 19 3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. End-to-end and Hop-by-hop Headers . . . . . . . . . . . . 19 3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 17
7.2. Non-modifiable Headers . . . . . . . . . . . . . . . . . . 20 3.2.1. Request Cache-Control Directives . . . . . . . . . . . 18
7.3. Combining Headers . . . . . . . . . . . . . . . . . . . . 21 3.2.2. Response Cache-Control Directives . . . . . . . . . . 20
8. Caching Negotiated Responses . . . . . . . . . . . . . . . . . 22 3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 22
9. Shared and Non-Shared Caches . . . . . . . . . . . . . . . . . 23 3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. Errors or Incomplete Response Cache Behavior . . . . . . . . . 24 3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 23
11. Side Effects of GET and HEAD . . . . . . . . . . . . . . . . . 24 3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
12. Invalidation After Updates or Deletions . . . . . . . . . . . 24 3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 25
13. Write-Through Mandatory . . . . . . . . . . . . . . . . . . . 25 4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 28
14. Cache Replacement . . . . . . . . . . . . . . . . . . . . . . 26 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
15. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 26 5.1. Message Header Registration . . . . . . . . . . . . . . . 28
16. Header Field Definitions . . . . . . . . . . . . . . . . . . . 27 6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
16.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
16.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 27 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
16.2.1. What is Cacheable . . . . . . . . . . . . . . . . . . 29 8.1. Normative References . . . . . . . . . . . . . . . . . . . 29
16.2.2. What May be Stored by Caches . . . . . . . . . . . . 30 8.2. Informative References . . . . . . . . . . . . . . . . . . 30
16.2.3. Modifications of the Basic Expiration Mechanism . . . 31 Appendix A. Compatibility with Previous Versions . . . . . . . . 31
16.2.4. Cache Revalidation and Reload Controls . . . . . . . 33 A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 31
16.2.5. No-Transform Directive . . . . . . . . . . . . . . . 35 A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 31
16.2.6. Cache Control Extensions . . . . . . . . . . . . . . 36 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 31
16.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 37 Appendix C. Change Log (to be removed by RFC Editor before
16.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 38 publication) . . . . . . . . . . . . . . . . . . . . 33
16.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 33
16.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 39 C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
17.1. Message Header Registration . . . . . . . . . . . . . . . 42 C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34
18. Security Considerations . . . . . . . . . . . . . . . . . . . 42 C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
19. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 43 C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35
20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43 C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
20.1. Normative References . . . . . . . . . . . . . . . . . . . 43 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
20.2. Informative References . . . . . . . . . . . . . . . . . . 44 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
Appendix A. Compatibility with Previous Versions . . . . . . . . 44
A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 44
A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 45
Appendix B. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 45
B.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 45
B.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 45
B.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 46
B.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 46
B.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 46
B.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 47
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
Intellectual Property and Copyright Statements . . . . . . . . . . 52
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, and performance can be improved by the use of response caches. This
includes a number of elements intended to make caching work as well
as possible. Because these elements interact with each other, it is
useful to describe the caching design of HTTP separately. 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 include a cache, though a cache cannot be used
by a server that is acting as a tunnel. by 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, the response message to satisfy a current request. In some cases, a
existing response can be reused without the need for a network stored response can be reused without the need for a network request,
request, reducing latency and network round-trips; we use an reducing latency and network round-trips; a "freshness" mechanism is
"expiration" mechanism for this purpose (see Section 4). Even when a used for this purpose (see Section 2.3). Even when a new request is
new request is required, it is often possible to reuse all or parts required, it is often possible to reuse all or parts of the payload
of the payload of a prior response to satisfy the request, thereby of a prior response to satisfy the request, thereby reducing network
reducing network bandwidth usage; we use a "validation" mechanism for bandwidth usage; a "validation" mechanism is used for this purpose
this purpose (see Section 5). (see Section 2.4).
A cache behaves in a "semantically transparent" manner, with respect
to a particular response, when its use affects neither the requesting
client nor the origin server, except to improve performance. When a
cache is semantically transparent, the client receives exactly the
same response status and payload that it would have received had its
request been handled directly by the origin server.
In an ideal world, all interactions with an HTTP cache would be
semantically transparent. However, for some resources, semantic
transparency is not always necessary and can be effectively traded
for the sake of bandwidth scaling, disconnected operation, and high
availability. HTTP/1.1 allows origin servers, caches, and clients to
explicitly reduce transparency when necessary. However, because non-
transparent operation may confuse non-expert users and might be
incompatible with certain server applications (such as those for
ordering merchandise), the protocol requires that transparency be
relaxed
o only by an explicit protocol-level request when relaxed by client
or origin server
o only with an explicit warning to the end user when relaxed by
cache or client
Therefore, HTTP/1.1 provides these important elements:
1. Protocol features that provide full semantic transparency when
this is required by all parties.
2. Protocol features that allow an origin server or user agent to
explicitly request and control non-transparent operation.
3. Protocol features that allow a cache to attach warnings to
responses that do not preserve the requested approximation of
semantic transparency.
A basic principle is that it must be possible for the clients to
detect any potential relaxation of semantic transparency.
Note: The server, cache, or client implementor might be faced with
design decisions not explicitly discussed in this specification.
If a decision might affect semantic transparency, the implementor
ought to err on the side of maintaining transparency unless a
careful and complete analysis shows significant benefits in
breaking transparency.
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 may be additional
constraints on whether a cache can use the cached copy for a constraints on whether a cache can use the cached copy to satisfy
particular request. a particular request.
first-hand
A response is first-hand if it comes directly and without
unnecessary delay from the origin server, perhaps via one or more
proxies. A response is also first-hand if its validity has just
been checked directly with the origin server.
explicit expiration time explicit expiration time
The time at which the origin server intends that an entity should The time at which the origin server intends that an entity should
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
successfully validated with, the origin server. successfully validated with, the origin server.
first-hand
A response is first-hand if the freshness model is not in use;
i.e., its age is 0.
freshness lifetime freshness lifetime
The length of time between the generation of a response and its The length of time between the generation of a response and its
expiration time. expiration time.
fresh fresh
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).
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 cache entry is an equivalent that is used to find out whether a stored response is an
copy of an entity. equivalent copy of an entity.
shared cache
A cache that is accessible to more than one user. A non-shared
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
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
An implementation is not compliant if it fails to satisfy one or more An implementation is not compliant if it fails to satisfy one or more
of the MUST or REQUIRED level requirements for the protocols it of the MUST or REQUIRED level requirements for the protocols it
implements. An implementation that satisfies all the MUST or implements. An implementation that satisfies all the MUST or
REQUIRED level and all the SHOULD level requirements for its REQUIRED level and all the SHOULD level requirements for its
protocols is said to be "unconditionally compliant"; one that protocols is said to be "unconditionally compliant"; one that
satisfies all the MUST level requirements but not all the SHOULD satisfies all the MUST level requirements but not all the SHOULD
level requirements for its protocols is said to be "conditionally level requirements for its protocols is said to be "conditionally
compliant." compliant."
2. Notational Conventions and Generic Grammar 1.4. Syntax Notation
This specification uses the ABNF syntax defined in Section 2.1 of This specification uses the ABNF syntax defined in Section 1.2 of
[Part1] and the core rules defined in Section 2.2 of [Part1]: [Part1] (which extends the syntax defined in [RFC5234] with a list
rule). Appendix B shows the collected ABNF, with the list rule
expanded.
DIGIT = <DIGIT, defined in [Part1], Section 2.2> The following core rules are included by reference, as defined in
DQUOTE = <DQUOTE, defined in [Part1], Section 2.2> [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
SP = <SP, defined in [Part1], Section 2.2> (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), VCHAR (any visible USASCII character),
and WSP (whitespace).
quoted-string = <quoted-string, defined in [Part1], Section 2.2> 1.4.1. Core Rules
token = <token, defined in [Part1], Section 2.2>
OWS = <OWS, defined in [Part1], Section 2.2> The core rules below are defined in Section 1.2.2 of [Part1]:
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
token = <token, defined in [Part1], Section 1.2.2>
OWS = <OWS, defined in [Part1], Section 1.2.2>
1.4.2. ABNF Rules defined in other Parts of the Specification
The ABNF rules below are defined in other parts: The ABNF rules below are defined in other parts:
field-name = <field-name, defined in [Part1], Section 4.2> field-name = <field-name, defined in [Part1], Section 4.2>
HTTP-date = <HTTP-date, defined in [Part1], Section 3.3.1> HTTP-date = <HTTP-date, defined in [Part1], Section 3.2.1>
port = <port, defined in [Part1], Section 3.2> port = <port, defined in [Part1], Section 2.1>
pseudonym = <pseudonym, defined in [Part1], Section 8.9> pseudonym = <pseudonym, defined in [Part1], Section 8.9>
uri-host = <uri-host, defined in [Part1], Section 3.2> uri-host = <uri-host, defined in [Part1], Section 2.1>
3. Overview 2. Cache Operation
3.1. Cache Correctness 2.1. Response Cacheability
A correct cache MUST respond to a request with the most up-to-date A cache MUST NOT store a response to any request, unless:
response held by the cache that is appropriate to the request (see
Sections 4.5, 4.6, and 14) which meets one of the following
conditions:
1. It has been checked for equivalence with what the origin server o The request method is defined as being cacheable, and
would have returned by revalidating the response with the origin
server (Section 5);
2. It is "fresh enough" (see Section 4). In the default case, this o the "no-store" cache directive (see Section 3.2) does not appear
means it meets the least restrictive freshness requirement of the in request or response headers, and
client, origin server, and cache (see Section 16.2); if the
origin server so specifies, it is the freshness requirement of
the origin server alone. If a stored response is not "fresh
enough" by the most restrictive freshness requirement of both the
client and the origin server, in carefully considered
circumstances the cache MAY still return the response with the
appropriate Warning header (see Sections 3.5 and 16.6), unless
such a response is prohibited (e.g., by a "no-store" cache-
directive, or by a "no-cache" cache-request-directive; see
Section 16.2).
3. It is an appropriate 304 (Not Modified), 305 (Use Proxy), or o the "private" cache response directive (see Section 3.2 does not
error (4xx or 5xx) response message. appear in the response, if the cache is shared, and
If the cache can not communicate with the origin server, then a o the "Authorization" header (see Section 3.1 of [Part7]) does not
correct cache SHOULD respond as above if the response can be appear in the request, if the cache is shared (unless the "public"
correctly served from the cache; if not it MUST return an error or directive is present; see Section 3.2), and
warning indicating that there was a communication failure.
If a cache receives a response (either an entire response, or a 304 o the cache understands partial responses, if the response is
(Not Modified) response) that it would normally forward to the partial or incomplete (see Section 2.1.1).
requesting client, and the received response is no longer fresh, the
cache SHOULD forward it to the requesting client without adding a new
Warning (but without removing any existing Warning headers). A cache
SHOULD NOT attempt to revalidate a response simply because that
response became stale in transit; this might lead to an infinite
loop. A user agent that receives a stale response without a Warning
MAY display a warning indication to the user.
3.2. Warnings Note that in normal operation, most caches will not store a response
that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses.
Whenever a cache returns a response that is neither first-hand nor 2.1.1. Storing Partial and Incomplete Responses
"fresh enough" (in the sense of condition 2 in Section 3.1), it MUST
attach a warning to that effect, using a Warning general-header. The
Warning header and the currently defined warnings are described in
Section 16.6. The warning allows clients to take appropriate action.
Warnings MAY be used for other purposes, both cache-related and A cache that receives an incomplete response (for example, with fewer
otherwise. The use of a warning, rather than an error status code, bytes of data than specified in a Content-Length header) can store
distinguish these responses from true failures. the response, but MUST treat it as a partial response [Part5].
Partial responses can be combined as described in Section 4 of
[Part5]; the result might be a full response or might still be
partial. A cache MUST NOT return a partial response to a client
without explicitly marking it as such using the 206 (Partial Content)
status code.
Warnings are assigned three digit warn-codes. The first digit A cache that does not support the Range and Content-Range headers
indicates whether the Warning MUST or MUST NOT be deleted from a MUST NOT store incomplete or partial responses.
stored cache entry after a successful revalidation:
1xx Warnings that describe the freshness or revalidation status of 2.2. Constructing Responses from Caches
the response, and so MUST be deleted after a successful
revalidation. 1xx warn-codes MAY be generated by a cache only when
validating a cached entry. It MUST NOT be generated by clients.
2xx Warnings that describe some aspect of the entity body or entity For a presented request, a cache MUST NOT return a stored response,
headers that is not rectified by a revalidation (for example, a unless:
lossy compression of the entity bodies) and which MUST NOT be
deleted after a successful revalidation.
See Section 16.6 for the definitions of the codes themselves. o The presented Request-URI and that of the stored response match
(see [[anchor1: TBD]]), and
HTTP/1.0 caches will cache all Warnings in responses, without o the request method associated with the stored response allows it
deleting the ones in the first category. Warnings in responses that to be used for the presented request, and
are passed to HTTP/1.0 caches carry an extra warning-date field,
which prevents a future HTTP/1.1 recipient from believing an
erroneously cached Warning.
Warnings also carry a warning text. The text MAY be in any o selecting request-headers nominated by the stored response (if
appropriate natural language (perhaps based on the client's Accept any) match those presented (see Section 2.6), and
headers), and include an OPTIONAL indication of what character set is o the presented request and stored response are free from directives
used. that would prevent its use (see Section 3.2 and Section 3.4), and
Multiple warnings MAY be attached to a response (either by the origin o the stored response is either:
server or by a cache), including multiple warnings with the same code
number. For example, a server might provide the same warning with
texts in both English and Basque.
When multiple warnings are attached to a response, it might not be * fresh (see Section 2.3), or
practical or reasonable to display all of them to the user. This
version of HTTP does not specify strict priority rules for deciding
which warnings to display and in what order, but does suggest some
heuristics.
3.3. Cache-control Mechanisms * allowed to be served stale (see Section 2.3.3), or
The basic cache mechanisms in HTTP/1.1 (server-specified expiration * successfully validated (see Section 2.4).
times and validators) are implicit directives to caches. In some
cases, a server or client might need to provide explicit directives
to the HTTP caches. We use the Cache-Control header for this
purpose.
The Cache-Control header allows a client or server to transmit a [[anchor2: TODO: define method cacheability for GET, HEAD and POST in
variety of directives in either requests or responses. These p2-semantics.]]
directives typically override the default caching algorithms. As a
general rule, if there is any apparent conflict between header
values, the most restrictive interpretation is applied (that is, the
one that is most likely to preserve semantic transparency). However,
in some cases, cache-control directives are explicitly specified as
weakening the approximation of semantic transparency (for example,
"max-stale" or "public").
The cache-control directives are described in detail in Section 16.2. When a stored response is used to satisfy a request, caches MUST
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.
[[anchor3: DISCUSS: this currently includes successfully validated
responses.]]
3.4. Explicit User Agent Warnings 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
not reply to such a request before having forwarded the request and
having received a corresponding response.
Many user agents make it possible for users to override the basic Also, note that unsafe requests might invalidate already stored
caching mechanisms. For example, the user agent might allow the user responses; see Section 2.5.
to specify that cached entities (even explicitly stale ones) are
never validated. Or the user agent might habitually add "Cache-
Control: max-stale=3600" to every request. The user agent SHOULD NOT
default to either non-transparent behavior, or behavior that results
in abnormally ineffective caching, but MAY be explicitly configured
to do so by an explicit action of the user.
If the user has overridden the basic caching mechanisms, the user Caches MUST use the most recent response (as determined by the Date
agent SHOULD explicitly indicate to the user whenever this results in header) when more than one suitable response is stored. They can
the display of information that might not meet the server's also forward a request with "Cache-Control: max-age=0" or "Cache-
transparency requirements (in particular, if the displayed entity is Control: no-cache" to disambiguate which response to use.
known to be stale). Since the protocol normally allows the user
agent to determine if responses are stale or not, this indication
need only be displayed when this actually happens. The indication
need not be a dialog box; it could be an icon (for example, a picture
of a rotting fish) or some other indicator.
If the user has overridden the caching mechanisms in a way that would [[anchor4: TODO: end-to-end and hop-by-hop headers, non-modifiable
abnormally reduce the effectiveness of caches, the user agent SHOULD headers removed; re-spec in p1]]
continually indicate this state to the user (for example, by a
display of a picture of currency in flames) so that the user does not
inadvertently consume excess resources or suffer from excessive
latency.
3.5. Exceptions to the Rules and Warnings 2.3. Freshness Model
In some cases, the operator of a cache MAY choose to configure it to When a response is "fresh" in the cache, it can be used to satisfy
return stale responses even when not requested by clients. This subsequent requests without contacting the origin server, thereby
decision ought not be made lightly, but may be necessary for reasons improving efficiency.
of availability or performance, especially when the cache is poorly
connected to the origin server. Whenever a cache returns a stale
response, it MUST mark it as such (using a Warning header) enabling
the client software to alert the user that there might be a potential
problem.
It also allows the user agent to take steps to obtain a first-hand or The primary mechanism for determining freshness is for an origin
fresh response. For this reason, a cache SHOULD NOT return a stale server to provide an explicit expiration time in the future, using
response if the client explicitly requests a first-hand or fresh one, either the Expires header (Section 3.3) or the max-age response cache
unless it is impossible to comply for technical or policy reasons. directive (Section 3.2.2). Generally, origin servers will assign
future explicit expiration times to responses in the belief that the
entity is not likely to change in a semantically significant way
before the expiration time is reached.
3.6. Client-controlled Behavior If an origin server wishes to force a cache to validate every
request, it can assign an explicit expiration time in the past. This
means that the response is always stale, so that caches should
validate it before using it for subsequent requests. [[anchor5: This
wording may cause confusion, because the response may still be served
stale.]]
While the origin server (and to a lesser extent, intermediate caches, Since origin servers do not always provide explicit expiration times,
by their contribution to the age of a response) are the primary HTTP caches may also assign heuristic expiration times when they are
source of expiration information, in some cases the client might need not specified, employing algorithms that use other header values
to control a cache's decision about whether to return a cached (such as the Last-Modified time) to estimate a plausible expiration
response without validating it. Clients do this using several time. The HTTP/1.1 specification does not provide specific
directives of the Cache-Control header. algorithms, but does impose worst-case constraints on their results.
A client's request MAY specify the maximum age it is willing to The calculation to determine if a response is fresh is:
accept of an unvalidated response; specifying a value of zero forces
the cache(s) to revalidate all responses. A client MAY also specify
the minimum time remaining before a response expires. Both of these
options increase constraints on the behavior of caches, and so cannot
further relax the cache's approximation of semantic transparency.
A client MAY also specify that it will accept stale responses, up to response_is_fresh = (freshness_lifetime > current_age)
some maximum amount of staleness. This loosens the constraints on
the caches, and so might violate the origin server's specified
constraints on semantic transparency, but might be necessary to
support disconnected operation, or high availability in the face of
poor connectivity.
4. Expiration Model The freshness_lifetime is defined in Section 2.3.1; the current_age
is defined in Section 2.3.2.
4.1. Server-Specified Expiration Additionally, clients may need to influence freshness calculation.
They can do this using several request cache directives, with the
effect of either increasing or loosening constraints on freshness.
See Section 3.2.1.
HTTP caching works best when caches can entirely avoid making [[anchor6: ISSUE: there are not requirements directly applying to
requests to the origin server. The primary mechanism for avoiding cache-request-directives and freshness.]]
requests is for an origin server to provide an explicit expiration
time in the future, indicating that a response MAY be used to satisfy
subsequent requests. In other words, a cache can return a fresh
response without first contacting the server.
Our expectation is that servers will assign future explicit Note that freshness applies only to cache operation; it cannot be
expiration times to responses in the belief that the entity is not used to force a user agent to refresh its display or reload a
likely to change, in a semantically significant way, before the resource. See Section 4 for an explanation of the difference between
expiration time is reached. This normally preserves semantic caches and history mechanisms.
transparency, as long as the server's expiration times are carefully
chosen.
The expiration mechanism applies only to responses taken from a cache 2.3.1. Calculating Freshness Lifetime
and not to first-hand responses forwarded immediately to the
requesting client.
If an origin server wishes to force a semantically transparent cache A cache can calculate the freshness lifetime (denoted as
to validate every request, it MAY assign an explicit expiration time freshness_lifetime) of a response by using the first match of:
in the past. This means that the response is always stale, and so
the cache SHOULD validate it before using it for subsequent requests.
See Section 16.2.4 for a more restrictive way to force revalidation.
If an origin server wishes to force any HTTP/1.1 cache, no matter how o If the cache is shared and the s-maxage response cache directive
it is configured, to validate every request, it SHOULD use the "must- (Section 3.2.2) is present, use its value, or
revalidate" cache-control directive (see Section 16.2).
Servers specify explicit expiration times using either the Expires o If the max-age response cache directive (Section 3.2.2) is
header, or the max-age directive of the Cache-Control header. present, use its value, or
o If the Expires response header (Section 3.3) is present, use its
value minus the value of the Date response header, or
An expiration time cannot be used to force a user agent to refresh o Otherwise, no explicit expiration time is present in the response,
its display or reload a resource; its semantics apply only to caching but a heuristic may be used; see Section 2.3.1.1.
mechanisms, and such mechanisms need only check a resource's
expiration status when a new request for that resource is initiated.
See Section 15 for an explanation of the difference between caches
and history mechanisms.
4.2. Heuristic Expiration Note that this calculation is not vulnerable to clock skew, since all
of the information comes from the origin server.
Since origin servers do not always provide explicit expiration times, 2.3.1.1. Calculating Heuristic Freshness
HTTP caches typically assign heuristic expiration times, employing
algorithms that use other header values (such as the Last-Modified
time) to estimate a plausible expiration time. The HTTP/1.1
specification does not provide specific algorithms, but does impose
worst-case constraints on their results. Since heuristic expiration
times might compromise semantic transparency, they ought to be used
cautiously, and we encourage origin servers to provide explicit
expiration times as much as possible.
4.3. Age Calculations 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
expiration time can be calculated. Heuristics MUST NOT be used for
other response status codes.
In order to know if a cached entry is fresh, a cache needs to know if When a heuristic is used to calculate freshness lifetime, the cache
its age exceeds its freshness lifetime. We discuss how to calculate SHOULD attach a Warning header with a 113 warn-code to the response
the latter in Section 4.4; this section describes how to calculate if its current_age is more than 24 hours and such a warning is not
the age of a response or cache entry. already present.
In this discussion, we use the term "now" to mean "the current value Also, if the response has a Last-Modified header (Section 6.6 of
of the clock at the host performing the calculation." Hosts that use [Part4]), the heuristic expiration value SHOULD be no more than some
HTTP, but especially hosts running origin servers and caches, SHOULD fraction of the interval since that time. A typical setting of this
use NTP [RFC1305] or some similar protocol to synchronize their fraction might be 10%.
clocks to a globally accurate time standard.
HTTP/1.1 requires origin servers to send a Date header, if possible, [[anchor7: REVIEW: took away HTTP/1.0 query string heuristic
with every response, giving the time at which the response was uncacheability.]]
generated (see Section 8.3 of [Part1]). We use the term "date_value"
to denote the value of the Date header, in a form appropriate for 2.3.2. Calculating Age
arithmetic operations.
HTTP/1.1 uses the Age response-header to convey the estimated age of HTTP/1.1 uses the Age response-header to convey the estimated age of
the response message when obtained from a cache. The Age field value the response message when obtained from a cache. The Age field value
is the cache's estimate of the amount of time since the response was is the cache's estimate of the amount of time since the response was
generated or revalidated by the origin server. generated or validated by the origin server. In essence, the Age
value is the sum of the time that the response has been resident in
each of the caches along the path from the origin server, plus the
amount of time it has been in transit along network paths.
In essence, the Age value is the sum of the time that the response The term "age_value" denotes the value of the Age header, in a form
has been resident in each of the caches along the path from the appropriate for arithmetic operations.
origin server, plus the amount of time it has been in transit along
network paths.
We use the term "age_value" to denote the value of the Age header, in HTTP/1.1 requires origin servers to send a Date header, if possible,
a form appropriate for arithmetic operations. with every response, giving the time at which the response was
generated (see Section 8.3 of [Part1]). The term "date_value"
denotes the value of the Date header, in a form appropriate for
arithmetic operations.
The term "now" means "the current value of the clock at the host
performing the calculation." Hosts that use HTTP, but especially
hosts running origin servers and caches, SHOULD use NTP [RFC1305] or
some similar protocol to synchronize their clocks to a globally
accurate time standard.
A response's age can be calculated in two entirely independent ways: A response's age can be calculated in two entirely independent ways:
1. now minus date_value, if the local clock is reasonably well 1. now minus date_value, if the local clock is reasonably well
synchronized to the origin server's clock. If the result is synchronized to the origin server's clock. If the result is
negative, the result is replaced by zero. negative, the result is replaced by zero.
2. age_value, if all of the caches along the response path implement 2. age_value, if all of the caches along the response path implement
HTTP/1.1. HTTP/1.1.
Given that we have two independent ways to compute the age of a These are combined as
response when it is received, we can combine these as
corrected_received_age = max(now - date_value, age_value) corrected_received_age = max(now - date_value, age_value)
and as long as we have either nearly synchronized clocks or all- When an Age value is received, it MUST be interpreted relative to the
HTTP/1.1 paths, one gets a reliable (conservative) result. time the request was initiated, not the time that the response was
received.
Because of network-imposed delays, some significant interval might
pass between the time that a server generates a response and the time
it is received at the next outbound cache or client. If uncorrected,
this delay could result in improperly low ages.
Because the request that resulted in the returned Age value must have
been initiated prior to that Age value's generation, we can correct
for delays imposed by the network by recording the time at which the
request was initiated. Then, when an Age value is received, it MUST
be interpreted relative to the time the request was initiated, not
the time that the response was received. This algorithm results in
conservative behavior no matter how much delay is experienced. So,
we compute:
corrected_initial_age = corrected_received_age corrected_initial_age = corrected_received_age
+ (now - request_time) + (now - request_time)
where "request_time" is the time (according to the local clock) when where "request_time" is the time (according to the local clock) when
the request that elicited this response was sent. the request that elicited this response was sent.
Summary of age calculation algorithm, when a cache receives a The current_age of a stored response can then be calculated by adding
response: the amount of time (in seconds) since the stored response was last
validated by the origin server to the corrected_initial_age.
/* In summary:
* age_value
* is the value of Age: header received by the cache with age_value - Age header field-value received with the response
* this response. date_value - Date header field-value received with the response
* date_value request_time - local time when the cache made the request
* is the value of the origin server's Date: header resulting in the stored response
* request_time response_time - local time when the cache received the response
* is the (local) time when the cache made the request now - current local time
* that resulted in this cached response
* response_time
* is the (local) time when the cache received the
* response
* now
* is the current (local) time
*/
apparent_age = max(0, response_time - date_value); apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value); corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time; response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay; corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time; resident_time = now - response_time;
current_age = corrected_initial_age + resident_time; current_age = corrected_initial_age + resident_time;
The current_age of a cache entry is calculated by adding the amount 2.3.3. Serving Stale Responses
of time (in seconds) since the cache entry was last validated by the
origin server to the corrected_initial_age. When a response is
generated from a cache entry, the cache MUST include a single Age
header field in the response with a value equal to the cache entry's
current_age.
The presence of an Age header field in a response implies that a
response is not first-hand. However, the converse is not true, since
the lack of an Age header field in a response does not imply that the
response is first-hand unless all caches along the request path are
compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
the Age header field).
4.4. Expiration Calculations
In order to decide whether a response is fresh or stale, we need to
compare its freshness lifetime to its age. The age is calculated as
described in Section 4.3; this section describes how to calculate the
freshness lifetime, and to determine if a response has expired. In
the discussion below, the values can be represented in any form
appropriate for arithmetic operations.
We use the term "expires_value" to denote the value of the Expires
header. We use the term "max_age_value" to denote an appropriate
value of the number of seconds carried by the "max-age" directive of
the Cache-Control header in a response (see Section 16.2.3).
The max-age directive takes priority over Expires, so if max-age is
present in a response, the calculation is simply:
freshness_lifetime = max_age_value
Otherwise, if Expires is present in the response, the calculation is:
freshness_lifetime = expires_value - date_value
Note that neither of these calculations is vulnerable to clock skew,
since all of the information comes from the origin server.
If none of Expires, Cache-Control: max-age, or Cache-Control:
s-maxage (see Section 16.2.3) appears in the response, and the
response does not include other restrictions on caching, the cache
MAY compute a freshness lifetime using a heuristic. The cache MUST
attach Warning 113 to any response whose age is more than 24 hours if
such warning has not already been added.
Also, if the response does have a Last-Modified time, the heuristic
expiration value SHOULD be no more than some fraction of the interval
since that time. A typical setting of this fraction might be 10%.
The calculation to determine if a response has expired is quite
simple:
response_is_fresh = (freshness_lifetime > current_age)
4.5. Disambiguating Expiration Values
Because expiration values are assigned optimistically, it is possible
for two caches to contain fresh values for the same resource that are
different.
If a client performing a retrieval receives a non-first-hand response
for a request that was already fresh in its own cache, and the Date
header in its existing cache entry is newer than the Date on the new
response, then the client MAY ignore the response. If so, it MAY
retry the request with a "Cache-Control: max-age=0" directive (see
Section 16.2), to force a check with the origin server.
If a cache has two fresh responses for the same representation with
different validators, it MUST use the one with the more recent Date
header. This situation might arise because the cache is pooling
responses from other caches, or because a client has asked for a
reload or a revalidation of an apparently fresh cache entry.
4.6. Disambiguating Multiple Responses
Because a client might be receiving responses via multiple paths, so
that some responses flow through one set of caches and other
responses flow through a different set of caches, a client might
receive responses in an order different from that in which the origin
server sent them. We would like the client to use the most recently
generated response, even if older responses are still apparently
fresh.
Neither the entity tag nor the expiration value can impose an
ordering on responses, since it is possible that a later response
intentionally carries an earlier expiration time. The Date values
are ordered to a granularity of one second.
When a client tries to revalidate a cache entry, and the response it
receives contains a Date header that appears to be older than the one
for the existing entry, then the client SHOULD repeat the request
unconditionally, and include
Cache-Control: max-age=0
to force any intermediate caches to validate their copies directly
with the origin server, or
Cache-Control: no-cache
to force any intermediate caches to obtain a new copy from the origin
server.
If the Date values are equal, then the client MAY use either response
(or MAY, if it is being extremely prudent, request a new response).
Servers MUST NOT depend on clients being able to choose
deterministically between responses generated during the same second,
if their expiration times overlap.
5. Validation Model
When a cache has a stale entry that it would like to use as a
response to a client's request, it first has to check with the origin
server (or possibly an intermediate cache with a fresh response) to
see if its cached entry is still usable. We call this "validating"
the cache entry.
HTTP's conditional request mechanism, defined in [Part4], is used to
avoid retransmitting the response payload when the cached entry is
valid. When a cached response includes one or more "cache
validators," such as the field values of an ETag or Last-Modified
header field, then a validating GET request SHOULD be made
conditional to those field values. The server checks the conditional
request's validator against the current state of the requested
resource and, if they match, the server responds with a 304 (Not
Modified) status code to indicate that the cached response can be
refreshed and reused without retransmitting the response payload. If
the validator does not match the current state of the requested
resource, then the server returns a full response, including payload,
so that the request can be satisfied and the cache entry supplanted
without the need for an additional network round-trip.
6. Response Cacheability
Unless specifically constrained by a cache-control (Section 16.2)
directive, a caching system MAY always store a successful response
(see Section 10) as a cache entry, MAY return it without validation
if it is fresh, and MAY return it after successful validation. If
there is neither a cache validator nor an explicit expiration time
associated with a response, we do not expect it to be cached, but
certain caches MAY violate this expectation (for example, when little
or no network connectivity is available). A client can usually
detect that such a response was taken from a cache by comparing the
Date header to the current time.
Note: some HTTP/1.0 caches are known to violate this expectation
without providing any Warning.
However, in some cases it might be inappropriate for a cache to
retain an entity, or to return it in response to a subsequent
request. This might be because absolute semantic transparency is
deemed necessary by the service author, or because of security or
privacy considerations. Certain cache-control directives are
therefore provided so that the server can indicate that certain
resource entities, or portions thereof, are not to be cached
regardless of other considerations.
Note that Section 4.1 of [Part7] normally prevents a shared cache
from saving and returning a response to a previous request if that
request included an Authorization header.
A response received with a status code of 200, 203, 206, 300, 301 or
410 MAY be stored by a cache and used in reply to a subsequent
request, subject to the expiration mechanism, unless a cache-control
directive prohibits caching. However, a cache that does not support
the Range and Content-Range headers MUST NOT cache 206 (Partial
Content) responses.
A response received with any other status code (e.g. status codes 302
and 307) MUST NOT be returned in a reply to a subsequent request
unless there are cache-control directives or another header(s) that
explicitly allow it. For example, these include the following: an
Expires header (Section 16.3); a "max-age", "s-maxage", "must-
revalidate", "proxy-revalidate", "public" or "private" cache-control
directive (Section 16.2).
7. Constructing Responses From Caches
The purpose of an HTTP cache is to store information received in
response to requests for use in responding to future requests. In
many cases, a cache simply returns the appropriate parts of a
response to the requester. However, if the cache holds a cache entry
based on a previous response, it might have to combine parts of a new
response with what is held in the cache entry.
7.1. End-to-end and Hop-by-hop Headers
For the purpose of defining the behavior of caches and non-caching
proxies, we divide HTTP headers into two categories:
o End-to-end headers, which are transmitted to the ultimate
recipient of a request or response. End-to-end headers in
responses MUST be stored as part of a cache entry and MUST be
transmitted in any response formed from a cache entry.
o Hop-by-hop headers, which are meaningful only for a single
transport-level connection, and are not stored by caches or
forwarded by proxies.
The following HTTP/1.1 headers are hop-by-hop headers:
o Connection
o Keep-Alive
o Proxy-Authenticate
o Proxy-Authorization
o TE
o Trailer
o Transfer-Encoding
o Upgrade
All other headers defined by HTTP/1.1 are end-to-end headers.
Other hop-by-hop headers MUST be listed in a Connection header
(Section 8.1 of [Part1]).
7.2. Non-modifiable Headers
Some features of HTTP/1.1, such as Digest Authentication, depend on
the value of certain end-to-end headers. A transparent proxy SHOULD
NOT modify an end-to-end header unless the definition of that header
requires or specifically allows that.
A transparent proxy MUST NOT modify any of the following fields in a
request or response, and it MUST NOT add any of these fields if not
already present:
o Content-Location
o Content-MD5
o ETag A "stale" response is one that either has explicit expiry
information, or is allowed to have heuristic expiry calculated, but
is not fresh according to the calculations in Section 2.3.
o Last-Modified Caches MUST NOT return a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
cache directive, a "must-revalidate" cache-response-directive, or an
applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 3.2.2).
A transparent proxy MUST NOT modify any of the following fields in a Caches SHOULD NOT return stale responses unless they are disconnected
response: (i.e., it cannot contact the origin server or otherwise find a
forward path) or otherwise explicitly allowed (e.g., the max-stale
request directive; see Section 3.2.1).
o Expires Stale responses SHOULD have a Warning header with the 110 warn-code
(see Section 3.6). Likewise, the 112 warn-code SHOULD be sent on
stale responses if the cache is disconnected.
but it MAY add any of these fields if not already present. If an If a cache receives a first-hand response (either an entire response,
Expires header is added, it MUST be given a field-value identical to or a 304 (Not Modified) response) that it would normally forward to
that of the Date header in that response. the requesting client, and the received response is no longer fresh,
the cache SHOULD forward it to the requesting client without adding a
new Warning (but without removing any existing Warning headers). A
cache SHOULD NOT attempt to validate a response simply because that
response became stale in transit.
A proxy MUST NOT modify or add any of the following fields in a 2.4. Validation Model
message that contains the no-transform cache-control directive, or in
any request:
o Content-Encoding Checking with the origin server to see if a stale or otherwise
unusable cached response can be reused is called "validating" or
"revalidating." Doing so potentially avoids the overhead of
retransmitting the response body when the stored response is valid.
o Content-Range HTTP's conditional request mechanism [Part4] is used for this
o Content-Type purpose. When a stored response includes one or more validators,
such as the field values of an ETag or Last-Modified header field,
then a validating request SHOULD be made conditional to those field
values.
A non-transparent proxy MAY modify or add these fields to a message A 304 (Not Modified) response status code indicates that the stored
that does not include no-transform, but if it does so, it MUST add a response can be updated and reused; see Section 2.7.
Warning 214 (Transformation applied) if one does not already appear
in the message (see Section 16.6).
Warning: unnecessary modification of end-to-end headers might If instead the cache receives a full response (i.e., one with a
cause authentication failures if stronger authentication response body), it is used to satisfy the request and replace the
mechanisms are introduced in later versions of HTTP. Such stored response. [[anchor8: Should there be a requirement here?]]
authentication mechanisms MAY rely on the values of header fields If a cache receives a 5xx response while attempting to validate a
not listed here. response, it MAY either forward this response to the requesting
client, or act as if the server failed to respond. In the latter
case, it MAY return a previously stored response (which SHOULD
include the 111 warn-code; see Section 3.6) unless the stored
response includes the "must-revalidate" cache directive (see
Section 2.3.3).
The Content-Length field of a request or response is added or deleted 2.5. Request Methods that Invalidate
according to the rules in Section 4.4 of [Part1]. A transparent
proxy MUST preserve the entity-length (Section 4.2.2 of [Part3]) of
the entity-body, although it MAY change the transfer-length (Section
4.4 of [Part1]).
7.3. Combining Headers Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
for changing state on the origin server, intervening caches can use
them to keep their contents up-to-date.
When a cache makes a validating request to a server, and the server The following HTTP methods MUST cause a cache to invalidate the
provides a 304 (Not Modified) response or a 206 (Partial Content) Request-URI as well as the Location and Content-Location headers (if
response, the cache then constructs a response to send to the present):
requesting client.
If the status code is 304 (Not Modified), the cache uses the entity- o PUT
body stored in the cache entry as the entity-body of this outgoing
response. If the status code is 206 (Partial Content) and the ETag
or Last-Modified headers match exactly, the cache MAY combine the
contents stored in the cache entry with the new contents received in
the response and use the result as the entity-body of this outgoing
response, (see Section 5 of [Part5]).
The end-to-end headers stored in the cache entry are used for the o DELETE
constructed response, except that
o any stored Warning headers with warn-code 1xx (see Section 16.6) o POST
MUST be deleted from the cache entry and the forwarded response.
o any stored Warning headers with warn-code 2xx MUST be retained in An invalidation based on the URI in a Location or Content-Location
the cache entry and the forwarded response. header MUST NOT be performed if the host part of that URI differs
from the host part in the Request-URI. This helps prevent denial of
service attacks.
o any end-to-end headers provided in the 304 or 206 response MUST [[anchor9: TODO: "host part" needs to be specified better.]]
replace the corresponding headers from the cache entry.
Unless the cache decides to remove the cache entry, it MUST also A cache that passes through requests for methods it does not
replace the end-to-end headers stored with the cache entry with understand SHOULD invalidate the Request-URI.
corresponding headers received in the incoming response, except for
Warning headers as described immediately above. If a header field-
name in the incoming response matches more than one header in the
cache entry, all such old headers MUST be replaced.
In other words, the set of end-to-end headers received in the Here, "invalidate" means that the cache will either remove all stored
incoming response overrides all corresponding end-to-end headers responses related to the Request-URI, or will mark these as "invalid"
stored with the cache entry (except for stored Warning headers with and in need of a mandatory validation before they can be returned in
warn-code 1xx, which are deleted even if not overridden). response to a subsequent request.
Note: this rule allows an origin server to use a 304 (Not Note that this does not guarantee that all appropriate responses are
Modified) or a 206 (Partial Content) response to update any header invalidated. For example, the request that caused the change at the
associated with a previous response for the same entity or sub- origin server might not have gone through the cache where a response
ranges thereof, although it might not always be meaningful or is stored.
correct to do so. This rule does not allow an origin server to
use a 304 (Not Modified) or a 206 (Partial Content) response to
entirely delete a header that it had provided with a previous
response.
8. Caching Negotiated Responses [[anchor10: TODO: specify that only successful (2xx, 3xx?) responses
invalidate.]]
Use of server-driven content negotiation (Section 5.1 of [Part3]), as 2.6. Caching Negotiated Responses
indicated by the presence of a Vary header field in a response,
alters the conditions and procedure by which a cache can use the
response for subsequent requests. See Section 16.5 for use of the
Vary header field by servers.
A server SHOULD use the Vary header field to inform a cache of what Use of server-driven content negotiation (Section 4.1 of [Part3])
request-header fields were used to select among multiple alters the conditions under which a cache can use the response for
representations of a cacheable response subject to server-driven subsequent requests.
negotiation. The set of header fields named by the Vary field value
is known as the "selecting" request-headers.
When the cache receives a subsequent request whose Request-URI When a cache receives a request that can be satisfied by a stored
specifies one or more cache entries including a Vary header field, response that includes a Vary header field (Section 3.5), it MUST NOT
the cache MUST NOT use such a cache entry to construct a response to use that response unless all of the selecting request-headers in the
the new request unless all of the selecting request-headers present presented request match the corresponding stored request-headers from
in the new request match the corresponding stored request-headers in
the original request. the original request.
The selecting request-headers from two requests are defined to match The selecting request-headers from two requests are defined to match
if and only if the selecting request-headers in the first request can if and only if the selecting request-headers in the first request can
be transformed to the selecting request-headers in the second request be transformed to the selecting request-headers in the second request
by adding or removing linear white space (LWS) at places where this by adding or removing linear white space [[anchor11: [ref]]] at
is allowed by the corresponding BNF, and/or combining multiple places where this is allowed by the corresponding ABNF, and/or
message-header fields with the same field name following the rules combining multiple message-header fields with the same field name
about message headers in Section 4.2 of [Part1]. following the rules about message headers in Section 4.2 of [Part1].
[[anchor12: DISCUSS: header-specific canonicalisation]]
A Vary header field-value of "*" always fails to match and subsequent
requests on that resource can only be properly interpreted by the
origin server.
If the selecting request header fields for the cached entry do not A Vary header field-value of "*" always fails to match, and
match the selecting request header fields of the new request, then subsequent requests to that resource can only be properly interpreted
the cache MUST NOT use a cached entry to satisfy the request unless by the origin server.
it first relays the new request to the origin server in a conditional
request and the server responds with 304 (Not Modified), including an
entity tag or Content-Location that indicates the entity to be used.
If an entity tag was assigned to a cached representation, the If no stored response matches, the cache MAY forward the presented
forwarded request SHOULD be conditional and include the entity tags request to the origin server in a conditional request, and SHOULD
in an If-None-Match header field from all its cache entries for the include all ETags stored with potentially suitable responses in an
resource. This conveys to the server the set of entities currently If-None-Match request header. If the server responds with 304 (Not
held by the cache, so that if any one of these entities matches the Modified) and includes an entity tag or Content-Location that
requested entity, the server can use the ETag header field in its 304 indicates the entity to be used, that cached response MUST be used to
(Not Modified) response to tell the cache which entry is appropriate. satisfy the presented request, and SHOULD be used to update the
If the entity-tag of the new response matches that of an existing corresponding stored response; see Section 2.7.
entry, the new response SHOULD be used to update the header fields of
the existing entry, and the result MUST be returned to the client.
If any of the existing cache entries contains only partial content If any of the stored responses contains only partial content, its
for the associated entity, its entity-tag SHOULD NOT be included in entity-tag SHOULD NOT be included in the If-None-Match header field
the If-None-Match header field unless the request is for a range that unless the request is for a range that would be fully satisfied by
would be fully satisfied by that entry. that stored response.
If a cache receives a successful response whose Content-Location If a cache receives a successful response whose Content-Location
field matches that of an existing cache entry for the same Request- field matches that of an existing stored response for the same
URI, whose entity-tag differs from that of the existing entry, and Request-URI, whose entity-tag differs from that of the existing
whose Date is more recent than that of the existing entry, the stored response, and whose Date is more recent than that of the
existing entry SHOULD NOT be returned in response to future requests existing response, the existing response SHOULD NOT be returned in
and SHOULD be deleted from the cache. response to future requests and SHOULD be deleted from the
cache.[[anchor13: DISCUSS: Not sure if this is necessary.]]
9. Shared and Non-Shared Caches
For reasons of security and privacy, it is necessary to make a
distinction between "shared" and "non-shared" caches. A non-shared
cache is one that is accessible only to a single user. Accessibility
in this case SHOULD be enforced by appropriate security mechanisms.
All other caches are considered to be "shared." Other sections of
this specification place certain constraints on the operation of
shared caches in order to prevent loss of privacy or failure of
access controls.
10. Errors or Incomplete Response Cache Behavior
A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-Length header) MAY store
the response. However, the cache MUST treat this as a partial
response. Partial responses MAY be combined as described in Section
5 of [Part5]; the result might be a full response or might still be
partial. A cache MUST NOT return a partial response to a client
without explicitly marking it as such, using the 206 (Partial
Content) status code. A cache MUST NOT return a partial response
using a status code of 200 (OK).
If a cache receives a 5xx response while attempting to revalidate an
entry, it MAY either forward this response to the requesting client,
or act as if the server failed to respond. In the latter case, it
MAY return a previously received response unless the cached entry
includes the "must-revalidate" cache-control directive (see
Section 16.2).
11. Side Effects of GET and HEAD
Unless the origin server explicitly prohibits the caching of their
responses, the application of GET and HEAD methods to any resources
SHOULD NOT have side effects that would lead to erroneous behavior if
these responses are taken from a cache. They MAY still have side
effects, but a cache is not required to consider such side effects in
its caching decisions. Caches are always expected to observe an
origin server's explicit restrictions on caching.
We note one exception to this rule: since some applications have
traditionally used GET and HEAD requests with URLs containing a query
part to perform operations with significant side effects, caches MUST
NOT treat responses to such URIs as fresh unless the server provides
an explicit expiration time. This specifically means that responses
from HTTP/1.0 servers for such URIs SHOULD NOT be taken from a cache.
See Section 8.1.1 of [Part2] for related information.
12. Invalidation After Updates or Deletions
The effect of certain methods performed on a resource at the origin
server might cause one or more existing cache entries to become non-
transparently invalid. That is, although they might continue to be
"fresh," they do not accurately reflect what the origin server would
return for a new request on that resource.
There is no way for HTTP to guarantee that all such cache entries are
marked invalid. For example, the request that caused the change at
the origin server might not have gone through the proxy where a cache
entry is stored. However, several rules help reduce the likelihood
of erroneous behavior.
In this section, the phrase "invalidate an entity" means that the
cache will either remove all instances of that entity from its
storage, or will mark these as "invalid" and in need of a mandatory
revalidation before they can be returned in response to a subsequent
request.
Some HTTP methods MUST cause a cache to invalidate an entity. This
is either the entity referred to by the Request-URI, or by the
Location or Content-Location headers (if present). These methods
are:
o PUT
o DELETE
o POST
An invalidation based on the URI in a Location or Content-Location
header MUST NOT be performed if the host part of that URI differs
from the host part in the Request-URI. This helps prevent denial of
service attacks.
A cache that passes through requests for methods it does not
understand SHOULD invalidate any entities referred to by the Request-
URI.
13. Write-Through Mandatory
All methods that might be expected to cause modifications to the
origin server's resources MUST be written through to the origin
server. This currently includes all methods except for GET and HEAD.
A cache MUST NOT reply to such a request from a client before having
transmitted the request to the inbound server, and having received a
corresponding response from the inbound server. This does not
prevent a proxy cache from sending a 100 (Continue) response before
the inbound server has sent its final reply.
The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network
failure prior to write-back.
14. Cache Replacement 2.7. Combining Responses
If a new cacheable (see Sections 16.2.2, 4.5, 4.6 and 10) response is When a cache receives a 304 (Not Modified) response or a 206 (Partial
received from a resource while any existing responses for the same Content) response, it needs to update the stored response with the
resource are cached, the cache SHOULD use the new response to reply new one, so that the updated response can be sent to the client.
to the current request. It MAY insert it into cache storage and MAY,
if it meets all other requirements, use it to respond to any future
requests that would previously have caused the old response to be
returned. If it inserts the new response into cache storage the
rules in Section 7.3 apply.
Note: a new response that has an older Date header value than If the status code is 304 (Not Modified), the cache SHOULD use the
existing cached responses is not cacheable. stored entity-body as the updated entity-body. If the status code is
206 (Partial Content) and the ETag or Last-Modified headers match
exactly, the cache MAY combine the stored entity-body in the stored
response with the updated entity-body received in the response and
use the result as the updated entity-body (see Section 4 of [Part5]).
15. History Lists The stored response headers are used for the updated response, except
that
User agents often have history mechanisms, such as "Back" buttons and o any stored Warning headers with warn-code 1xx (see Section 3.6)
history lists, which can be used to redisplay an entity retrieved MUST be deleted from the stored response and the forwarded
earlier in a session. response.
History mechanisms and caches are different. In particular history o any stored Warning headers with warn-code 2xx MUST be retained in
mechanisms SHOULD NOT try to show a semantically transparent view of the stored response and the forwarded response.
the current state of a resource. Rather, a history mechanism is
meant to show exactly what the user saw at the time when the resource
was retrieved.
By default, an expiration time does not apply to history mechanisms. o any headers provided in the 304 or 206 response MUST replace the
If the entity is still in storage, a history mechanism SHOULD display corresponding headers from the stored response.
it even if the entity has expired, unless the user has specifically
configured the agent to refresh expired history documents.
This is not to be construed to prohibit the history mechanism from A cache MUST also replace any stored headers with corresponding
telling the user that a view might be stale. headers received in the incoming response, except for Warning headers
as described immediately above. If a header field-name in the
incoming response matches more than one header in the stored
response, all such old headers MUST be replaced. It MAY store the
combined entity-body.
Note: if history list mechanisms unnecessarily prevent users from [[anchor14: ISSUE: discuss how to handle HEAD updates]]
viewing stale resources, this will tend to force service authors
to avoid using HTTP expiration controls and cache controls when
they would otherwise like to. Service authors may consider it
important that users not be presented with error messages or
warning messages when they use navigation controls (such as BACK)
to view previously fetched resources. Even though sometimes such
resources ought not be cached, or ought to expire quickly, user
interface considerations may force service authors to resort to
other means of preventing caching (e.g. "once-only" URLs) in order
not to suffer the effects of improperly functioning history
mechanisms.
16. 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 For entity-header fields, both sender and recipient refer to either
the client or the server, depending on who sends and who receives the the client or the server, depending on who sends and who receives the
entity. entity.
16.1. Age 3.1. Age
The response-header field "Age" conveys the sender's estimate of the The response-header field "Age" conveys the sender's estimate of the
amount of time since the response (or its revalidation) was generated amount of time since the response (or its validation) was generated
at the origin server. A cached response is "fresh" if its age does at the origin server. Age values are calculated as specified in
not exceed its freshness lifetime. Age values are calculated as Section 2.3.2.
specified in Section 4.3.
Age = "Age" ":" OWS Age-v Age = "Age" ":" OWS Age-v
Age-v = delta-seconds Age-v = delta-seconds
Age values are non-negative decimal integers, representing time in Age field-values are non-negative decimal integers, representing time
seconds. in 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 value of 2147483648 (2^31). An MUST transmit an Age header with a field-value of 2147483648 (2^31).
HTTP/1.1 server that includes a cache MUST include an Age header Caches SHOULD use an arithmetic type of at least 31 bits of range.
field in every response generated from its own cache. Caches SHOULD
use an arithmetic type of at least 31 bits of range.
16.2. Cache-Control 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
HTTP/1.0 caches may not implement the Age header field.
3.2. Cache-Control
The general-header field "Cache-Control" is used to specify The general-header field "Cache-Control" is used to specify
directives that MUST be obeyed by all caching mechanisms along the directives that MUST be obeyed by all caches along the request/
request/response chain. The directives specify behavior intended to response chain. The directives specify behavior intended to prevent
prevent caches from adversely interfering with the request or caches from adversely interfering with the request or response.
response. These directives typically override the default caching Cache directives are unidirectional in that the presence of a
algorithms. Cache directives are unidirectional in that the presence directive in a request does not imply that the same directive is to
of a directive in a request does not imply that the same directive is be given in the response.
to be given in the response.
Note that HTTP/1.0 caches might not implement Cache-Control and Note that HTTP/1.0 caches might not implement Cache-Control and
might only implement Pragma: no-cache (see Section 16.4). might only implement Pragma: no-cache (see Section 3.4).
Cache directives MUST be passed through by a proxy or gateway Cache directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application, application, regardless of their significance to that application,
since the directives might be applicable to all recipients along the since the directives might be applicable to all recipients along the
request/response chain. It is not possible to specify a cache- request/response chain. It is not possible to target a directive to
directive for a specific cache. a specific cache.
Cache-Control = "Cache-Control" ":" OWS Cache-Control-v Cache-Control = "Cache-Control" ":" OWS Cache-Control-v
Cache-Control-v = 1#cache-directive Cache-Control-v = 1#cache-directive
cache-directive = cache-request-directive cache-directive = cache-request-directive
/ cache-response-directive / cache-response-directive
cache-request-directive =
"no-cache" ; Section 16.2.1
/ "no-store" ; Section 16.2.2
/ "max-age" "=" delta-seconds ; Section 16.2.3, 16.2.4
/ "max-stale" [ "=" delta-seconds ] ; Section 16.2.3
/ "min-fresh" "=" delta-seconds ; Section 16.2.3
/ "no-transform" ; Section 16.2.5
/ "only-if-cached" ; Section 16.2.4
/ cache-extension ; Section 16.2.6
cache-response-directive =
"public" ; Section 16.2.1
/ "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
/ "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
/ "no-store" ; Section 16.2.2
/ "no-transform" ; Section 16.2.5
/ "must-revalidate" ; Section 16.2.4
/ "proxy-revalidate" ; Section 16.2.4
/ "max-age" "=" delta-seconds ; Section 16.2.3
/ "s-maxage" "=" delta-seconds ; Section 16.2.3
/ cache-extension ; Section 16.2.6
cache-extension = token [ "=" ( token / quoted-string ) ] cache-extension = token [ "=" ( token / quoted-string ) ]
When a directive appears without any 1#field-name parameter, the 3.2.1. Request Cache-Control Directives
directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to
the named field or fields, and not to the rest of the request or
response. This mechanism supports extensibility; implementations of
future versions of HTTP might apply these directives to header fields
not defined in HTTP/1.1.
The cache-control directives can be broken down into these general
categories:
o Restrictions on what are cacheable; these may only be imposed by
the origin server.
o Restrictions on what may be stored by a cache; these may be
imposed by either the origin server or the user agent.
o Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent.
o Controls over cache revalidation and reload; these may only be
imposed by a user agent.
o Control over transformation of entities.
o Extensions to the caching system.
16.2.1. What is Cacheable
By default, a response is cacheable if the requirements of the
request method, request header fields, and the response status
indicate that it is cacheable. Section 6 summarizes these defaults
for cacheability. The following Cache-Control response directives
allow an origin server to override the default cacheability of a
response:
public
Indicates that the response MAY be cached by any cache, even if it
would normally be non-cacheable or cacheable only within a non-
shared cache. (See also Authorization, Section 4.1 of [Part7],
for additional details.)
private
Indicates that all or part of the response message is intended for
a single user and MUST NOT be cached by a shared cache. This
allows an origin server to state that the specified parts of the
response are intended for only one user and are not a valid
response for requests by other users. A private (non-shared)
cache MAY cache the response.
Note: This usage of the word private only controls where the cache-request-directive =
response may be cached, and cannot ensure the privacy of the "no-cache"
message content. / "no-store"
/ "max-age" "=" delta-seconds
/ "max-stale" [ "=" delta-seconds ]
/ "min-fresh" "=" delta-seconds
/ "no-transform"
/ "only-if-cached"
/ cache-extension
no-cache no-cache
If the no-cache directive does not specify a field-name, then a The no-cache request directive indicates that a stored response
cache MUST NOT use the response to satisfy a subsequent request MUST NOT be used to satisfy the request without successful
without successful revalidation with the origin server. This validation on the origin server.
allows an origin server to prevent caching even by caches that
have been configured to return stale responses to client requests.
If the no-cache directive does specify one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, the
specified field-name(s) MUST NOT be sent in the response to a
subsequent request without successful revalidation with the origin
server. This allows an origin server to prevent the re-use of
certain header fields in a response, while still allowing caching
of the rest of the response.
Note: Most HTTP/1.0 caches will not recognize or obey this
directive.
16.2.2. What May be Stored by Caches
no-store no-store
The purpose of the no-store directive is to prevent the The no-store request directive indicates that a cache MUST NOT
inadvertent release or retention of sensitive information (for store any part of either this request or any response to it. This
example, on backup tapes). The no-store directive applies to the directive applies to both non-shared and shared caches. "MUST NOT
entire message, and MAY be sent either in a response or in a store" in this context means that the cache MUST NOT intentionally
request. If sent in a request, a cache MUST NOT store any part of store the information in non-volatile storage, and MUST make a
either this request or any response to it. If sent in a response, best-effort attempt to remove the information from volatile
a cache MUST NOT store any part of either this response or the storage as promptly as possible after forwarding it.
request that elicited it. This directive applies to both non-
shared and shared caches. "MUST NOT store" in this context means
that the cache MUST NOT intentionally store the information in
non-volatile storage, and MUST make a best-effort attempt to
remove the information from volatile storage as promptly as
possible after forwarding it.
Even when this directive is associated with a response, users
might explicitly store such a response outside of the caching
system (e.g., with a "Save As" dialog). History buffers MAY store
such responses as part of their normal operation.
The purpose of this directive is to meet the stated requirements
of certain users and service authors who are concerned about
accidental releases of information via unanticipated accesses to
cache data structures. While the use of this directive might
improve privacy in some cases, we caution that it is NOT in any
way a reliable or sufficient mechanism for ensuring privacy. In
particular, malicious or compromised caches might not recognize or
obey this directive, and communications networks might be
vulnerable to eavesdropping.
16.2.3. Modifications of the Basic Expiration Mechanism
The expiration time of an entity MAY be specified by the origin
server using the Expires header (see Section 16.3). Alternatively,
it MAY be specified using the max-age directive in a response. When
the max-age cache-control directive is present in a cached response,
the response is stale if its current age is greater than the age
value given (in seconds) at the time of a new request for that
resource. The max-age directive on a response implies that the
response is cacheable (i.e., "public") unless some other, more
restrictive cache directive is also present.
If a response includes both an Expires header and a max-age
directive, the max-age directive overrides the Expires header, even
if the Expires header is more restrictive. This rule allows an
origin server to provide, for a given response, a longer expiration
time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This
might be useful if certain HTTP/1.0 caches improperly calculate ages
or expiration times, perhaps due to desynchronized clocks.
Many HTTP/1.0 cache implementations will treat an Expires value that
is less than or equal to the response Date value as being equivalent
to the Cache-Control response directive "no-cache". If an HTTP/1.1
cache receives such a response, and the response does not include a
Cache-Control header field, it SHOULD consider the response to be
non-cacheable in order to retain compatibility with HTTP/1.0 servers.
Note: An origin server might wish to use a relatively new HTTP
cache control feature, such as the "private" directive, on a
network including older caches that do not understand that
feature. The origin server will need to combine the new feature
with an Expires field whose value is less than or equal to the
Date value. This will prevent older caches from improperly
caching the response.
s-maxage
If a response includes an s-maxage directive, then for a shared This directive is NOT a reliable or sufficient mechanism for
cache (but not for a private cache), the maximum age specified by ensuring privacy. In particular, malicious or compromised caches
this directive overrides the maximum age specified by either the might not recognize or obey this directive, and communications
max-age directive or the Expires header. The s-maxage directive networks may be vulnerable to eavesdropping.
also implies the semantics of the proxy-revalidate directive (see
Section 16.2.4), i.e., that the shared cache must not use the
entry after it becomes stale to respond to a subsequent request
without first revalidating it with the origin server. The
s-maxage directive is always ignored by a private cache.
Note that most older caches, not compliant with this specification, max-age
do not implement any cache-control directives. An origin server
wishing to use a cache-control directive that restricts, but does not
prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
requirement that the max-age directive overrides the Expires header,
and the fact that pre-HTTP/1.1-compliant caches do not observe the
max-age directive.
Other directives allow a user agent to modify the basic expiration The max-age request directive indicates that the client is willing
mechanism. These directives MAY be specified on a request: to accept a response whose age is no greater than the specified
time in seconds. Unless max-stale directive is also included, the
client is not willing to accept a stale response.
max-age max-stale
Indicates that the client is willing to accept a response whose The max-stale request directive indicates that the client is
age is no greater than the specified time in seconds. Unless max- willing to accept a response that has exceeded its expiration
stale directive is also included, the client is not willing to time. If max-stale is assigned a value, then the client is
accept a stale response. willing to accept a response that has exceeded its expiration time
by no more than the specified number of seconds. If no value is
assigned to max-stale, then the client is willing to accept a
stale response of any age. [[anchor15: of any staleness? --mnot]]
min-fresh min-fresh
Indicates that the client is willing to accept a response whose The min-fresh request directive indicates that the client is
freshness lifetime is no less than its current age plus the willing to accept a response whose freshness lifetime is no less
specified time in seconds. That is, the client wants a response than its current age plus the specified time in seconds. That is,
that will still be fresh for at least the specified number of the client wants a response that will still be fresh for at least
seconds. the specified number of seconds.
max-stale
Indicates that the client is willing to accept a response that has no-transform
exceeded its expiration time. If max-stale is assigned a value,
then the client is willing to accept a response that has exceeded
its expiration time by no more than the specified number of
seconds. If no value is assigned to max-stale, then the client is
willing to accept a stale response of any age.
If a cache returns a stale response, either because of a max-stale The no-transform request directive indicates that an intermediate
directive on a request, or because the cache is configured to cache or proxy MUST NOT change the Content-Encoding, Content-Range
override the expiration time of a response, the cache MUST attach a or Content-Type request headers, nor the request entity-body.
Warning header to the stale response, using Warning 110 (Response is
stale).
A cache MAY be configured to return stale responses without only-if-cached
validation, but only if this does not conflict with any "MUST"-level
requirements concerning cache validation (e.g., a "must-revalidate"
cache-control directive).
If both the new request and the cached entry include "max-age" The only-if-cached request directive indicates that the client
directives, then the lesser of the two values is used for determining only wishes to return a stored response. If it receives this
the freshness of the cached entry for that request. directive, a cache SHOULD either respond using a stored response
that is consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status. If a group of caches
is being operated as a unified system with good internal
connectivity, such a request MAY be forwarded within that group of
caches.
16.2.4. Cache Revalidation and Reload Controls 3.2.2. Response Cache-Control Directives
Sometimes a user agent might want or need to insist that a cache cache-response-directive =
revalidate its cache entry with the origin server (and not just with "public"
the next cache along the path to the origin server), or to reload its / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
cache entry from the origin server. End-to-end revalidation might be / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
necessary if either the cache or the origin server has overestimated / "no-store"
the expiration time of the cached response. End-to-end reload may be / "no-transform"
necessary if the cache entry has become corrupted for some reason. / "must-revalidate"
/ "proxy-revalidate"
/ "max-age" "=" delta-seconds
/ "s-maxage" "=" delta-seconds
/ cache-extension
End-to-end revalidation may be requested either when the client does public
not have its own local cached copy, in which case we call it
"unspecified end-to-end revalidation", or when the client does have a
local cached copy, in which case we call it "specific end-to-end
revalidation."
The client can specify these three kinds of action using Cache- The public response directive indicates that the response MAY be
Control request directives: cached, even if it would normally be non-cacheable or cacheable
only within a non-shared cache. (See also Authorization, Section
3.1 of [Part7], for additional details.)
End-to-end reload private
The request includes a "no-cache" cache-control directive or, for The private response directive indicates that the response message
compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field is intended for a single user and MUST NOT be stored by a shared
names MUST NOT be included with the no-cache directive in a cache. A private (non-shared) cache MAY store the response.
request. The server MUST NOT use a cached copy when responding to
such a request.
Specific end-to-end revalidation If the private response directive specifies one or more field-
names, this requirement is limited to the field-values associated
with the listed response headers. That is, the specified field-
names(s) MUST NOT be stored by a shared cache, whereas the
remainder of the response message MAY be.
The request includes a "max-age=0" cache-control directive, which Note: This usage of the word private only controls where the
forces each cache along the path to the origin server to response may be stored, and cannot ensure the privacy of the
revalidate its own entry, if any, with the next cache or server. message content.
The initial request includes a cache-validating conditional with
the client's current validator.
Unspecified end-to-end revalidation no-cache
The request includes "max-age=0" cache-control directive, which The no-cache response directive indicates that the response MUST
forces each cache along the path to the origin server to NOT be used to satisfy a subsequent request without successful
revalidate its own entry, if any, with the next cache or server. validation on the origin server. This allows an origin server to
The initial request does not include a cache-validating prevent caching even by caches that have been configured to return
conditional; the first cache along the path (if any) that holds a stale responses.
cache entry for this resource includes a cache-validating
conditional with its current validator.
max-age If the no-cache response directive specifies one or more field-
When an intermediate cache is forced, by means of a max-age=0 names, this requirement is limited to the field-values assosicated
directive, to revalidate its own cache entry, and the client has with the listed response headers. That is, the specified field-
supplied its own validator in the request, the supplied validator name(s) MUST NOT be sent in the response to a subsequent request
might differ from the validator currently stored with the cache without successful validation on the origin server. This allows
entry. In this case, the cache MAY use either validator in making an origin server to prevent the re-use of certain header fields in
its own request without affecting semantic transparency. a response, while still allowing caching of the rest of the
response.
However, the choice of validator might affect performance. The Note: Most HTTP/1.0 caches will not recognize or obey this
best approach is for the intermediate cache to use its own directive.
validator when making its request. If the server replies with 304
(Not Modified), then the cache can return its now validated copy
to the client with a 200 (OK) response. If the server replies
with a new entity and cache validator, however, the intermediate
cache can compare the returned validator with the one provided in
the client's request, using the strong comparison function. If
the client's validator is equal to the origin server's, then the
intermediate cache simply returns 304 (Not Modified). Otherwise,
it returns the new entity with a 200 (OK) response.
If a request includes the no-cache directive, it SHOULD NOT no-store
include min-fresh, max-stale, or max-age.
only-if-cached The no-store response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This
directive applies to both non-shared and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile
storage as promptly as possible after forwarding it.
In some cases, such as times of extremely poor network This directive is NOT a reliable or sufficient mechanism for
connectivity, a client may want a cache to return only those ensuring privacy. In particular, malicious or compromised caches
responses that it currently has stored, and not to reload or might not recognize or obey this directive, and communications
revalidate with the origin server. To do this, the client may networks may be vulnerable to eavesdropping.
include the only-if-cached directive in a request. If it receives
this directive, a cache SHOULD either respond using a cached entry
that is consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status. However, if a group
of caches is being operated as a unified system with good internal
connectivity, such a request MAY be forwarded within that group of
caches.
must-revalidate must-revalidate
Because a cache MAY be configured to ignore a server's specified The must-revalidate response directive indicates that once it has
expiration time, and because a client request MAY include a max- become stale, the response MUST NOT be used to satisfy subsequent
stale directive (which has a similar effect), the protocol also requests without successful validation on the origin server.
includes a mechanism for the origin server to require revalidation
of a cache entry on any subsequent use. When the must-revalidate
directive is present in a response received by a cache, that cache
MUST NOT use the entry after it becomes stale to respond to a
subsequent request without first revalidating it with the origin
server. (I.e., the cache MUST do an end-to-end revalidation every
time, if, based solely on the origin server's Expires or max-age
value, the cached response is stale.)
The must-revalidate directive is necessary to support reliable 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 revalidate a request on the entity could result in failure to validate a request on the entity could result in
incorrect operation, such as a silently unexecuted financial incorrect operation, such as a silently unexecuted financial
transaction. Recipients MUST NOT take any automated action that transaction.
violates this directive, and MUST NOT automatically provide an
unvalidated copy of the entity if revalidation fails.
Although this is not recommended, user agents operating under
severe connectivity constraints MAY violate this directive but, if
so, MUST explicitly warn the user that an unvalidated response has
been provided. The warning MUST be provided on each unvalidated
access, and SHOULD require explicit user confirmation.
proxy-revalidate proxy-revalidate
The proxy-revalidate directive has the same meaning as the must- The proxy-revalidate response directive has the same meaning as
revalidate directive, except that it does not apply to non-shared the must-revalidate response directive, except that it does not
user agent caches. It can be used on a response to an apply to non-shared caches.
authenticated request to permit the user's cache to store and
later return the response without needing to revalidate it (since
it has already been authenticated once by that user), while still
requiring proxies that service many users to revalidate each time
(in order to make sure that each user has been authenticated).
Note that such authenticated responses also need the public cache
control directive in order to allow them to be cached at all.
16.2.5. No-Transform Directive max-age
The max-age response directive indicates that response is to be
considered stale after its age is greater than the specified
number of seconds.
no-transform s-maxage
Implementors of intermediate caches (proxies) have found it useful The s-maxage response directive indicates that, in shared caches,
to convert the media type of certain entity bodies. A non- the maximum age specified by this directive overrides the maximum
transparent proxy might, for example, convert between image age specified by either the max-age directive or the Expires
formats in order to save cache space or to reduce the amount of header. The s-maxage directive also implies the semantics of the
traffic on a slow link. proxy-revalidate response directive.
Serious operational problems occur, however, when these no-transform
transformations are applied to entity bodies intended for certain
kinds of applications. For example, applications for medical
imaging, scientific data analysis and those using end-to-end
authentication, all depend on receiving an entity body that is bit
for bit identical to the original entity-body.
Therefore, if a message includes the no-transform directive, an The no-transform response directive indicates that an intermediate
intermediate cache or proxy MUST NOT change those headers that are cache or proxy MUST NOT change the Content-Encoding, Content-Range
listed in Section 7.2 as being subject to the no-transform or Content-Type response headers, nor the response entity-body.
directive. This implies that the cache or proxy MUST NOT change
any aspect of the entity-body that is specified by these headers,
including the value of the entity-body itself.
16.2.6. 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 assigned value. or more cache-extension tokens, each with an optional value.
Informational extensions (those which do not require a change in Informational extensions (those that do not require a change in cache
cache behavior) MAY 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
applications which do not understand the new directive will default applications that do not understand the new directive will default to
to the behavior specified by the standard directive, and those that the behavior specified by the standard directive, and those that
understand the new directive will recognize it as modifying the understand the new directive will recognize it as modifying the
requirements associated with the standard directive. In this way, requirements associated with the standard directive. In this way,
extensions to the cache-control directives can be made without extensions to the cache-control directives can be made without
requiring changes to the base protocol. requiring changes to the base protocol.
This extension mechanism depends on an HTTP cache obeying all of the This extension mechanism depends on an HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not certain extensions, and ignoring all directives that it does not
understand. understand.
For example, consider a hypothetical new response directive called For example, consider a hypothetical new response directive called
community which acts as a modifier to the private directive. We "community" that acts as a modifier to the private directive. We
define this new directive to mean that, in addition to any non-shared define this new directive to mean that, in addition to any non-shared
cache, any cache which is shared only by members of the community cache, any cache that is shared only by members of the community
named within its value may cache the response. An origin server named within its value may cache the response. An origin server
wishing to allow the UCI community to use an otherwise private wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including response in their shared cache(s) could do so by including
Cache-Control: private, community="UCI" Cache-Control: private, community="UCI"
A cache seeing this header field will act correctly even if the cache A cache seeing this header field will act correctly even if the cache
does not understand the community cache-extension, since it will also does not understand the community cache-extension, since it will also
see and understand the private directive and thus default to the safe see and understand the private directive and thus default to the safe
behavior. behavior.
Unrecognized cache-directives MUST be ignored; it is assumed that any Unrecognized cache directives MUST be ignored; it is assumed that any
cache-directive likely to be unrecognized by an HTTP/1.1 cache will cache directive likely to be unrecognized by an HTTP/1.1 cache will
be combined with standard directives (or the response's default be combined with standard directives (or the response's default
cacheability) such that the cache behavior will remain minimally cacheability) such that the cache behavior will remain minimally
correct even if the cache does not understand the extension(s). correct even if the cache does not understand the extension(s).
16.3. Expires 3.3. Expires
The Expires entity-header field gives the date/time after which the The entity-header field "Expires" gives the date/time after which the
response is considered stale. A stale cache entry may not normally response is considered stale. See Section 2.3 for further discussion
be returned by a cache (either a proxy cache or a user agent cache) of the freshness model.
unless it is first validated with the origin server (or with an
intermediate cache that has a fresh copy of the entity). See
Section 4 for further discussion of the expiration 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 format is an absolute date and time as defined by HTTP-date in The field-value is an absolute date and time as defined by HTTP-date
Section 3.3.1 of [Part1]; it MUST be sent in rfc1123-date format. in Section 3.2.1 of [Part1]; it MUST be sent in rfc1123-date format.
Expires = "Expires" ":" OWS Expires-v Expires = "Expires" ":" OWS Expires-v
Expires-v = HTTP-date Expires-v = HTTP-date
An example of its use is For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
Note: if a response includes a Cache-Control field with the max- Note: if a response includes a Cache-Control field with the max-
age directive (see Section 16.2.3), that directive overrides the age directive (see Section 3.2.2), that directive overrides the
Expires field. Expires field. Likewise, the s-maxage directive overrides Expires
in shared caches.
HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
the future.
HTTP/1.1 clients and caches MUST treat other invalid date formats, HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already especially including the value "0", as in the past (i.e., "already
expired"). expired").
To mark a response as "already expired," an origin server sends an 3.4. Pragma
Expires date that is equal to the Date header value. (See the rules
for expiration calculations in Section 4.4.)
To mark a response as "never expires," an origin server sends an
Expires date approximately one year from the time the response is
sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one
year in the future.
The presence of an Expires header field with a date value of some
time in the future on a response that otherwise would by default be
non-cacheable indicates that the response is cacheable, unless
indicated otherwise by a Cache-Control header field (Section 16.2).
16.4. Pragma
The general-header field "Pragma" is used to include implementation- The general-header field "Pragma" is used to include implementation-
specific directives that might apply to any recipient along the specific directives that might apply to any recipient along the
request/response chain. All pragma directives specify optional request/response chain. All pragma directives specify optional
behavior from the viewpoint of the protocol; however, some systems behavior from the viewpoint of the protocol; however, some systems
MAY require that behavior be consistent with the directives. MAY require that behavior be consistent with the directives.
Pragma = "Pragma" ":" OWS Pragma-v Pragma = "Pragma" ":" OWS Pragma-v
Pragma-v = 1#pragma-directive Pragma-v = 1#pragma-directive
pragma-directive = "no-cache" / extension-pragma pragma-directive = "no-cache" / extension-pragma
extension-pragma = token [ "=" ( token / quoted-string ) ] extension-pragma = token [ "=" ( token / quoted-string ) ]
When the no-cache directive is present in a request message, an When the no-cache directive is present in a request message, an
application SHOULD forward the request toward the origin server even application SHOULD forward the request toward the origin server even
if it has a cached copy of what is being requested. This pragma if it has a cached copy of what is being requested. This pragma
directive has the same semantics as the no-cache cache-directive (see directive has the same semantics as the no-cache response directive
Section 16.2) and is defined here for backward compatibility with (see Section 3.2.2) and is defined here for backward compatibility
HTTP/1.0. Clients SHOULD include both header fields when a no-cache with HTTP/1.0. Clients SHOULD include both header fields when a no-
request is sent to a server not known to be HTTP/1.1 compliant. cache request is sent to a server not known to be HTTP/1.1 compliant.
Pragma directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives might be applicable to all recipients along the
request/response chain. It is not possible to specify a pragma for a
specific recipient; however, any pragma directive not relevant to a
recipient SHOULD be ignored by that recipient.
HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
sent "Cache-Control: no-cache". No new Pragma directives will be sent "Cache-Control: no-cache".
defined in HTTP.
Note: because the meaning of "Pragma: no-cache" as a response- Note: because the meaning of "Pragma: no-cache" as a response-
header field is not actually specified, it does not provide a header field is not actually specified, it does not provide a
reliable replacement for "Cache-Control: no-cache" in a response. reliable replacement for "Cache-Control: no-cache" in a response.
16.5. Vary This mechanism is deprecated; no new Pragma directives will be
defined in HTTP.
3.5. Vary
The "Vary" response-header field's value indicates the set of The "Vary" response-header field's value indicates the set of
request-header fields that fully determines, while the response is request-header fields that determines, while the response is fresh,
fresh, whether a cache is permitted to use the response to reply to a whether a cache is permitted to use the response to reply to a
subsequent request without revalidation. For uncacheable or stale subsequent request without validation; see Section 2.6.
responses, the Vary field value advises the user agent about the
criteria that were used to select the representation. A Vary field In uncacheable or stale responses, the Vary field value advises the
value of "*" implies that a cache cannot determine from the request user agent about the criteria that were used to select the
headers of a subsequent request whether this response is the representation.
appropriate representation. See Section 8 for use of the Vary header
field by caches.
Vary = "Vary" ":" OWS Vary-v Vary = "Vary" ":" OWS Vary-v
Vary-v = "*" / 1#field-name Vary-v = "*" / 1#field-name
An HTTP/1.1 server SHOULD include a Vary header field with any The set of header fields named by the Vary field value is known as
cacheable response that is subject to server-driven negotiation. the selecting request-headers.
Doing so allows a cache to properly interpret future requests on that
resource and informs the user agent about the presence of negotiation Servers SHOULD include a Vary header field with any cacheable
on that resource. A server MAY include a Vary header field with a response that is subject to server-driven negotiation. Doing so
non-cacheable response that is subject to server-driven negotiation, allows a cache to properly interpret future requests on that resource
and informs the user agent about the presence of negotiation on that
resource. A server MAY include a Vary header field with a non-
cacheable response that is subject to server-driven negotiation,
since this might provide the user agent with useful information about 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 consisting of a list of field-names signals that A Vary field value of "*" signals that unspecified parameters not
the representation selected for the response is based on a selection limited to the request-headers (e.g., the network address of the
algorithm which considers ONLY the listed request-header field values client), play a role in the selection of the response representation;
in selecting the most appropriate representation. A cache MAY assume therefore, a cache cannot determine whether this response is
that the same selection will be made for future requests with the appropriate. The "*" value MUST NOT be generated by a proxy server;
same values for the listed field names, for the duration of time for it may only be generated by an origin server.
which the response is fresh.
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.
A Vary field value of "*" signals that unspecified parameters not 3.6. Warning
limited to the request-headers (e.g., the network address of the
client), play a role in the selection of the response representation.
The "*" value MUST NOT be generated by a proxy server; it may only be
generated by an origin server.
16.6. Warning
The general-header field "Warning" is used to carry additional The general-header field "Warning" is used to carry additional
information about the status or transformation of a message which 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 a possible lack of semantic transparency from used to warn about possible incorrectness introduced by caching
caching operations or transformations applied to the entity body of operations or transformations applied to the entity body of the
the message. message.
Warning headers are sent with responses using: Warnings can be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguish these responses from true failures.
Warning headers can in general be applied to any message, however
some warn-codes are specific to caches and can only be applied to
response messages.
Warning = "Warning" ":" OWS Warning-v Warning = "Warning" ":" OWS Warning-v
Warning-v = 1#warning-value Warning-v = 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text warning-value = warn-code SP warn-agent SP warn-text
[SP warn-date] [SP warn-date]
warn-code = 3DIGIT warn-code = 3DIGIT
warn-agent = ( uri-host [ ":" port ] ) / pseudonym warn-agent = ( uri-host [ ":" port ] ) / pseudonym
; the name or pseudonym of the server adding ; the name or pseudonym of the server adding
; the Warning header, for use in debugging ; the Warning header, for use in debugging
warn-text = quoted-string warn-text = quoted-string
warn-date = DQUOTE HTTP-date DQUOTE warn-date = DQUOTE HTTP-date DQUOTE
A response MAY carry more than one Warning header. Multiple warnings can be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number. For example, a server might provide the same warning with
texts in both English and Basque.
When this occurs, the user agent SHOULD inform the user of as many of
them as possible, in the order that they appear in the response. If
it is not possible to inform the user of all of the warnings, the
user agent SHOULD follow these heuristics:
o Warnings that appear early in the response take priority over
those appearing later in the response.
o Warnings in the user's preferred character set take priority over
warnings in other character sets but with identical warn-codes and
warn-agents.
Systems that generate multiple Warning headers SHOULD order them with
this user agent behavior in mind. New Warning headers SHOULD be
added after any existing Warning headers.
Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning is required to be deleted from a stored
response after validation:
o 1xx Warnings that describe the freshness or validation status of
the response, and so MUST be deleted by caches after validation.
They MUST NOT be generated by a cache except when validating a
cached entry, and MUST NOT be generated by clients.
o 2xx Warnings that describe some aspect of the entity body or
entity headers that is not rectified by a validation (for example,
a lossy compression of the entity bodies) and MUST NOT be deleted
by caches after validation, unless a full response is returned, in
which case they MUST be.
The warn-text SHOULD be in a natural language and character set that The warn-text SHOULD be in a natural language and character set that
is most likely to be intelligible to the human user receiving the is most likely to be intelligible to the human user receiving the
response. This decision MAY be based on any available knowledge, response. This decision can be based on any available knowledge,
such as the location of the cache or user, the Accept-Language field such as the location of the cache or user, the Accept-Language field
in a request, the Content-Language field in a response, etc. The in a request, the Content-Language field in a response, etc. The
default language is English and the default character set is ISO- default language is English and the default character set is ISO-
8859-1 ([ISO-8859-1]). 8859-1 ([ISO-8859-1]).
If a character set other than ISO-8859-1 is used, it MUST be encoded If a character set other than ISO-8859-1 is used, it MUST be encoded
in the warn-text using the method described in [RFC2047]. in the warn-text using the method described in [RFC2047].
Warning headers can in general be applied to any message, however If an implementation sends a message with one or more Warning headers
some specific warn-codes are specific to caches and can only be to a receiver whose version is HTTP/1.0 or lower, then the sender
applied to response messages. New Warning headers SHOULD be added MUST include in each warning-value a warn-date that matches the Date
after any existing Warning headers. A cache MUST NOT delete any header in the message.
Warning header that it received with a message. However, if a cache
successfully validates a cache entry, it SHOULD remove any Warning
headers previously attached to that entry except as specified for
specific Warning codes. It MUST then add any Warning headers
received in the validating response. In other words, Warning headers
are those that would be attached to the most recent relevant
response.
When multiple Warning headers are attached to a response, the user
agent ought to inform the user of as many of them as possible, in the
order that they appear in the response. If it is not possible to
inform the user of all of the warnings, the user agent SHOULD follow
these heuristics:
o Warnings that appear early in the response take priority over
those appearing later in the response.
o Warnings in the user's preferred character set take priority over
warnings in other character sets but with identical warn-codes and
warn-agents.
Systems that generate multiple Warning headers SHOULD order them with
this user agent behavior in mind.
Requirements for the behavior of caches with respect to Warnings are If an implementation receives a message with a warning-value that
stated in Section 3.2. includes a warn-date, and that warn-date is different from the Date
value in the response, then that warning-value MUST be deleted from
the message before storing, forwarding, or using it. (preventing the
consequences of naive caching of Warning header fields.) If all of
the warning-values are deleted for this reason, the Warning header
MUST be deleted as well.
This is a list of the currently-defined warn-codes, each with a The following warn-codes are defined by this specification, each with
recommended warn-text in English, and a description of its meaning. a recommended warn-text in English, and a description of its meaning.
110 Response is stale 110 Response is stale
MUST be included whenever the returned response is stale. SHOULD be included whenever the returned response is stale.
111 Revalidation failed 111 Revalidation failed
MUST be included if a cache returns a stale response because an SHOULD be included if a cache returns a stale response because an
attempt to revalidate the response failed, due to an inability to attempt to validate the response failed, due to an inability to
reach the server. reach the server.
112 Disconnected operation 112 Disconnected operation
SHOULD be included if the cache is intentionally disconnected from SHOULD be included if the cache is intentionally disconnected from
the rest of the network for a period of time. the rest of the network for a period of time.
113 Heuristic expiration 113 Heuristic expiration
MUST be included if the cache heuristically chose a freshness SHOULD be included if the cache heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater lifetime greater than 24 hours and the response's age is greater
than 24 hours. than 24 hours.
199 Miscellaneous warning 199 Miscellaneous warning
The warning text MAY 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 cache or proxy if it applies any
transformation changing the content-coding (as specified in the transformation changing the content-coding (as specified in the
Content-Encoding header) or media-type (as specified in the Content-Encoding header) or media-type (as specified in the
Content-Type header) of the response, or the entity-body of the Content-Type header) of the response, or the entity-body of the
response, unless this Warning code already appears in the response, unless this Warning code already appears in the
response. response.
299 Miscellaneous persistent warning 299 Miscellaneous persistent warning
The warning text MAY 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.
If an implementation sends a message with one or more Warning headers 4. History Lists
whose version is HTTP/1.0 or lower, then the sender MUST include in
each warning-value a warn-date that matches the date in the response.
If an implementation receives a message with a warning-value that User agents often have history mechanisms, such as "Back" buttons and
includes a warn-date, and that warn-date is different from the Date history lists, that can be used to redisplay an entity retrieved
value in the response, then that warning-value MUST be deleted from earlier in a session.
the message before storing, forwarding, or using it. (This prevents
bad consequences of naive caching of Warning header fields.) If all
of the warning-values are deleted for this reason, the Warning header
MUST be deleted as well.
17. IANA Considerations History mechanisms and caches are different. In particular history
mechanisms SHOULD NOT try to show a correct view of the current state
of a resource. Rather, a history mechanism is meant to show exactly
what the user saw at the time when the resource was retrieved.
17.1. Message Header Registration By default, an expiration time does not apply to history mechanisms.
If the entity is still in storage, a history mechanism SHOULD display
it even if the entity has expired, unless the user has specifically
configured the agent to refresh expired history documents.
This is not to be construed to prohibit the history mechanism from
telling the user that a view might be stale.
Note: if history list mechanisms unnecessarily prevent users from
viewing stale resources, this will tend to force service authors
to avoid using HTTP expiration controls and cache controls when
they would otherwise like to. Service authors may consider it
important that users not be presented with error messages or
warning messages when they use navigation controls (such as BACK)
to view previously fetched resources. Even though sometimes such
resources ought not be cached, or ought to expire quickly, user
interface considerations may force service authors to resort to
other means of preventing caching (e.g. "once-only" URLs) in order
not to suffer the effects of improperly functioning history
mechanisms.
5. IANA Considerations
5.1. Message Header Registration
The Message Header Registry located at <http://www.iana.org/ The Message Header Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> should be assignments/message-headers/message-header-index.html> should 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 16.1 | | Age | http | standard | Section 3.1 |
| Cache-Control | http | standard | Section 16.2 | | Cache-Control | http | standard | Section 3.2 |
| Expires | http | standard | Section 16.3 | | Expires | http | standard | Section 3.3 |
| Pragma | http | standard | Section 16.4 | | Pragma | http | standard | Section 3.4 |
| Vary | http | standard | Section 16.5 | | Vary | http | standard | Section 3.5 |
| Warning | http | standard | Section 16.6 | | Warning | http | standard | Section 3.6 |
+-------------------+----------+----------+--------------+ +-------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force". Engineering Task Force".
18. Security Considerations 6. Security Considerations
Caching proxies provide additional potential vulnerabilities, since Caches expose additional potential vulnerabilities, since the
the contents of the cache represent an attractive target for contents of the cache represent an attractive target for malicious
malicious exploitation. Because cache contents persist after an HTTP exploitation. Because cache contents persist after an HTTP request
request is complete, an attack on the cache can reveal information is complete, an attack on the cache can reveal information long after
long after a user believes that the information has been removed from a user believes that the information has been removed from the
the network. Therefore, cache contents should be protected as network. Therefore, cache contents should be protected as sensitive
sensitive information. information.
19. 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.
20. References 8. References
20.1. Normative References 8.1. Normative References
[ISO-8859-1] [ISO-8859-1]
International Organization for Standardization, International Organization for Standardization,
"Information technology -- 8-bit single-byte coded graphic "Information technology -- 8-bit single-byte coded graphic
character sets -- Part 1: Latin alphabet No. 1", ISO/ character sets -- Part 1: Latin alphabet No. 1", ISO/
IEC 8859-1:1998, 1998. IEC 8859-1:1998, 1998.
[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-05 and Message Parsing", draft-ietf-httpbis-p1-messaging-06
(work in progress), November 2008. (work in progress), March 2009.
[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-05 (work in Semantics", draft-ietf-httpbis-p2-semantics-06 (work in
progress), November 2008. progress), March 2009.
[Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part3] 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 3: Message Payload and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
and Content Negotiation", draft-ietf-httpbis-p3-payload-05 and Content Negotiation", draft-ietf-httpbis-p3-payload-06
(work in progress), November 2008. (work in progress), March 2009.
[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-05 (work in Requests", draft-ietf-httpbis-p4-conditional-06 (work in
progress), November 2008. progress), March 2009.
[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-05 (work Partial Responses", draft-ietf-httpbis-p5-range-06 (work
in progress), November 2008. in progress), March 2009.
[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-05 (work in progress), draft-ietf-httpbis-p7-auth-06 (work in progress),
November 2008. March 2009.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text", Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996. RFC 2047, November 1996.
[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.
20.2. Informative References [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
8.2. Informative References
[RFC1305] Mills, D., "Network Time Protocol (Version 3) [RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992. Specification, Implementation", RFC 1305, March 1992.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004. September 2004.
Appendix A. Compatibility with Previous Versions Appendix A. Compatibility with Previous Versions
A.1. Changes from RFC 2068 A.1. Changes from RFC 2068
A case was missed in the Cache-Control model of HTTP/1.1; s-maxage A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
was introduced to add this missing case. (Sections 6, 16.2, 16.2.3) was introduced to add this missing case. (Sections 2.1, 3.2).
Transfer-coding and message lengths all interact in ways that Transfer-coding and message lengths all interact in ways that
required fixing exactly when chunked encoding is used (to allow for required fixing exactly when chunked encoding is used (to allow for
transfer encoding that may not be self delimiting); it was important transfer encoding that may not be self delimiting); it was important
to straighten out exactly how message lengths are computed. to straighten out exactly how message lengths are computed. (see also
(Section 7.2, see also [Part1], [Part3] and [Part5]) [Part1], [Part3] and [Part5]) [[anchor18: This used to refer to the
text about non-modifiable headers, and will have to be updated later
on. --jre]]
Proxies should be able to add Content-Length when appropriate. Proxies should be able to add Content-Length when appropriate.
(Section 7.2) [[anchor19: This used to refer to the text about non-modifiable
headers, and will have to be updated later on. --jre]]
Range request responses would become very verbose if all meta-data Range request responses would become very verbose if all meta-data
were always returned; by allowing the server to only send needed were always returned; by allowing the server to only send needed
headers in a 206 response, this problem can be avoided. headers in a 206 response, this problem can be avoided.
(Section 7.3) (Section 2.7)
The Cache-Control: max-age directive was not properly defined for The Cache-Control: max-age directive was not properly defined for
responses. (Section 16.2.3) responses. (Section 3.2.2)
Warnings could be cached incorrectly, or not updated appropriately. Warnings could be cached incorrectly, or not updated appropriately.
(Section 3.2, 4.4, 7.2, 7.3, 16.2.3, and 16.6) Warning also needed to (Section 2.3, 2.7, 3.2, and 3.6) Warning also needed to be a general
be a general header, as PUT or other methods may have need for it in header, as PUT or other methods may have need for it in requests.
requests.
A.2. Changes from RFC 2616 A.2. Changes from RFC 2616
Clarify denial of service attack avoidance requirement. (Section 12) Clarify denial of service attack avoidance requirement.
(Section 2.5)
Appendix B. Change Log (to be removed by RFC Editor before publication) Appendix B. Collected ABNF
B.1. Since RFC2616 Age = "Age:" OWS Age-v
Age-v = delta-seconds
Cache-Control = "Cache-Control:" OWS Cache-Control-v
Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] )
Expires = "Expires:" OWS Expires-v
Expires-v = HTTP-date
HTTP-date = <HTTP-date, defined in [Part1], Section 3.2.1>
OWS = <OWS, defined in [Part1], Section 1.2.2>
Pragma = "Pragma:" OWS Pragma-v
Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] )
Vary = "Vary:" OWS Vary-v
Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
] ) )
Warning = "Warning:" OWS Warning-v
Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
] )
cache-directive = cache-request-directive / cache-response-directive
cache-extension = token [ "=" ( token / quoted-string ) ]
cache-request-directive = "no-cache" / "no-store" / ( "max-age="
delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
"min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
cache-extension
cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
"no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
"must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
) / ( "s-maxage=" delta-seconds ) / cache-extension
delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ]
field-name = <field-name, defined in [Part1], Section 4.2>
port = <port, defined in [Part1], Section 2.1>
pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, defined in [Part1], Section 8.9>
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
token = <token, defined in [Part1], Section 1.2.2>
uri-host = <uri-host, defined in [Part1], Section 2.1>
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
]
ABNF diagnostics:
; Age defined but not used
; Cache-Control defined but not used
; Expires defined but not used
; Pragma defined but not used
; Vary defined but not used
; Warning defined but not used
Appendix C. Change Log (to be removed by RFC Editor before publication)
C.1. Since RFC2616
Extracted relevant partitions from [RFC2616]. Extracted relevant partitions from [RFC2616].
B.2. Since draft-ietf-httpbis-p6-cache-00 C.2. Since draft-ietf-httpbis-p6-cache-00
Closed issues: Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer" o <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
(<http://purl.org/NET/http-errata#trailer-hop>) (<http://purl.org/NET/http-errata#trailer-hop>)
o <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation o <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
after Update or Delete" after Update or Delete"
(<http://purl.org/NET/http-errata#invalidupd>) (<http://purl.org/NET/http-errata#invalidupd>)
skipping to change at page 46, line 18 skipping to change at page 34, line 21
to-date references" to-date references"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in o <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
13.2.2" 13.2.2"
Other changes: Other changes:
o Use names of RFC4234 core rules DQUOTE and HTAB (work in progress o Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>) on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
B.3. Since draft-ietf-httpbis-p6-cache-01 C.3. Since draft-ietf-httpbis-p6-cache-01
Closed issues: Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not o <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
used" used"
Other changes: Other changes:
o Get rid of duplicate BNF rule names ("host" -> "uri-host") (work o Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>) in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
o Add explicit references to BNF syntax and rules imported from o Add explicit references to BNF syntax and rules imported from
other parts of the specification. other parts of the specification.
B.4. Since draft-ietf-httpbis-p6-cache-02 C.4. Since draft-ietf-httpbis-p6-cache-02
Ongoing work on IANA Message Header Registration Ongoing work on IANA Message Header Registration
(<http://tools.ietf.org/wg/httpbis/trac/ticket/40>): (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
o Reference RFC 3984, and update header registrations for headers o Reference RFC 3984, and update header registrations for headers
defined in this document. defined in this document.
B.5. Since draft-ietf-httpbis-p6-cache-03 C.5. Since draft-ietf-httpbis-p6-cache-03
Closed issues: Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header o <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
classification" classification"
B.6. Since draft-ietf-httpbis-p6-cache-04 C.6. Since draft-ietf-httpbis-p6-cache-04
Ongoing work on ABNF conversion Ongoing work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Use "/" instead of "|" for alternatives. o Use "/" instead of "|" for alternatives.
o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS"). whitespace ("OWS") and required whitespace ("RWS").
o Rewrite ABNFs to spell out whitespace rules, factor out header o Rewrite ABNFs to spell out whitespace rules, factor out header
value format definitions. value format definitions.
C.7. Since draft-ietf-httpbis-p6-cache-05
This is a total rewrite of this part of the specification.
Affected issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1xx Warn-Codes"
o <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement
of 13.5.1 and 13.5.2"
o <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role
of Warning and Semantic Transparency in Caching"
o <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods
and Caching"
In addition: Final work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Add appendix containing collected and expanded ABNF, reorganize
ABNF introduction.
Index Index
A A
age 7 age 6
Age header 27 Age header 17
C C
cache 5 cache 5
Cache Directives Cache Directives
max-age 32-33 max-age 18, 21
max-stale 32 max-stale 19
min-fresh 32 min-fresh 19
must-revalidate 34 must-revalidate 21
no-cache 29 no-cache 18, 20
no-store 30 no-store 18, 21
no-transform 35 no-transform 19, 22
only-if-cached 34 only-if-cached 19
private 29 private 20
proxy-revalidate 35 proxy-revalidate 21
public 29 public 20
s-maxage 31 s-maxage 22
Cache-Control header 27 Cache-Control header 17
cacheable 6 cacheable 5
E E
Expires header 37 Expires header 23
explicit expiration time 7 explicit expiration time 5
F F
first-hand 6 first-hand 6
fresh 7 fresh 6
freshness lifetime 7 freshness lifetime 6
G G
Grammar Grammar
Age 27 Age 17
Age-v 27 Age-v 17
Cache-Control 28 Cache-Control 18
Cache-Control-v 28 Cache-Control-v 18
cache-directive 28 cache-extension 18
cache-extension 28 cache-request-directive 18
cache-request-directive 28 cache-response-directive 20
cache-response-directive 28 delta-seconds 17
delta-seconds 27 Expires 23
Expires 37 Expires-v 23
Expires-v 37 extension-pragma 24
extension-pragma 38 Pragma 24
Pragma 38 pragma-directive 24
pragma-directive 38 Pragma-v 24
Pragma-v 38 Vary 24
Vary 39 Vary-v 24
Vary-v 39 warn-agent 25
warn-agent 40 warn-code 25
warn-code 40 warn-date 25
warn-date 40 warn-text 25
warn-text 40 Warning 25
Warning 40 Warning-v 25
Warning-v 40 warning-value 25
warning-value 40
H H
Headers Headers
Age 27 Age 17
Cache-Control 27 Cache-Control 17
Expires 37 Expires 23
Pragma 38 Pragma 23
Vary 38 Vary 24
Warning 39 Warning 25
heuristic expiration time 7 heuristic expiration time 5
M M
max-age max-age
Cache Directive 32-33 Cache Directive 18, 21
max-stale max-stale
Cache Directive 32 Cache Directive 19
min-fresh min-fresh
Cache Directive 32 Cache Directive 19
must-revalidate must-revalidate
Cache Directive 34 Cache Directive 21
N N
no-cache no-cache
Cache Directive 29 Cache Directive 18, 20
no-store no-store
Cache Directive 30 Cache Directive 18, 21
no-transform no-transform
Cache Directive 35 Cache Directive 19, 22
O O
only-if-cached only-if-cached
Cache Directive 34 Cache Directive 19
P P
Pragma header 38 Pragma header 23
private private
Cache Directive 29 Cache Directive 20
proxy-revalidate proxy-revalidate
Cache Directive 35 Cache Directive 21
public public
Cache Directive 29 Cache Directive 20
S S
s-maxage s-maxage
Cache Directive 31 Cache Directive 22
semantically transparent 5 stale 6
stale 7
V V
validator 7 validator 6
Vary header 38 Vary header 24
W W
Warning header 39 Warning header 25
Authors' Addresses Authors' Addresses
Roy T. Fielding (editor) Roy T. Fielding (editor)
Day Software Day Software
23 Corporate Plaza DR, Suite 280 23 Corporate Plaza DR, Suite 280
Newport Beach, CA 92660 Newport Beach, CA 92660
USA USA
Phone: +1-949-706-5300 Phone: +1-949-706-5300
skipping to change at page 52, line 4 skipping to change at line 1775
Julian F. Reschke (editor) Julian F. Reschke (editor)
greenbytes GmbH greenbytes GmbH
Hafenweg 16 Hafenweg 16
Muenster, NW 48155 Muenster, NW 48155
Germany Germany
Phone: +49 251 2807760 Phone: +49 251 2807760
Fax: +49 251 2807761 Fax: +49 251 2807761
Email: julian.reschke@greenbytes.de Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/ URI: http://greenbytes.de/tech/webdav/
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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