< draft-ietf-httpbis-p6-cache-21.txt   draft-ietf-httpbis-p6-cache-22.txt >
HTTPbis Working Group R. Fielding, Ed. HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 2616 (if approved) M. Nottingham, Ed. Obsoletes: 2616 (if approved) M. Nottingham, Ed.
Intended status: Standards Track Akamai Intended status: Standards Track Akamai
Expires: April 7, 2013 J. Reschke, Ed. Expires: August 27, 2013 J. Reschke, Ed.
greenbytes greenbytes
October 4, 2012 February 23, 2013
Hypertext Transfer Protocol (HTTP/1.1): Caching Hypertext Transfer Protocol (HTTP/1.1): Caching
draft-ietf-httpbis-p6-cache-21 draft-ietf-httpbis-p6-cache-22
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information protocol for distributed, collaborative, hypertext information
systems. This document defines requirements on HTTP caches and the systems. This document defines requirements on HTTP caches and the
associated header fields that control cache behavior or indicate associated header fields that control cache behavior or indicate
cacheable response messages. cacheable response messages.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>. <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at The current issues list is at
<http://tools.ietf.org/wg/httpbis/trac/report/3> and related <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>. <http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix D.2. The changes in this draft are summarized in Appendix D.3.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 7, 2013. This Internet-Draft will expire on August 27, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Conformance and Error Handling . . . . . . . . . . . . . . 6 1.3. Conformance and Error Handling . . . . . . . . . . . . . . 6
1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6 1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6
1.4.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 6 1.4.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 6
2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6
3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7
3.1. Storing Incomplete Responses . . . . . . . . . . . . . . . 8 3.1. Storing Incomplete Responses . . . . . . . . . . . . . . . 8
3.2. Storing Responses to Authenticated Requests . . . . . . . 8 3.2. Storing Responses to Authenticated Requests . . . . . . . 9
4. Constructing Responses from Caches . . . . . . . . . . . . . . 9 4. Constructing Responses from Caches . . . . . . . . . . . . . . 9
4.1. Freshness Model . . . . . . . . . . . . . . . . . . . . . 10 4.1. Freshness Model . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11 4.1.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11
4.1.2. Calculating Heuristic Freshness . . . . . . . . . . . 12 4.1.2. Calculating Heuristic Freshness . . . . . . . . . . . 12
4.1.3. Calculating Age . . . . . . . . . . . . . . . . . . . 12 4.1.3. Calculating Age . . . . . . . . . . . . . . . . . . . 12
4.1.4. Serving Stale Responses . . . . . . . . . . . . . . . 14 4.1.4. Serving Stale Responses . . . . . . . . . . . . . . . 14
4.2. Validation Model . . . . . . . . . . . . . . . . . . . . . 15 4.2. Validation Model . . . . . . . . . . . . . . . . . . . . . 15
4.2.1. Freshening Responses with 304 Not Modified . . . . . . 16 4.2.1. Freshening Responses with 304 Not Modified . . . . . . 16
4.3. Using Negotiated Responses . . . . . . . . . . . . . . . . 16 4.3. Using Negotiated Responses . . . . . . . . . . . . . . . . 17
4.4. Combining Partial Content . . . . . . . . . . . . . . . . 17 4.4. Combining Partial Content . . . . . . . . . . . . . . . . 17
5. Updating Caches with HEAD Responses . . . . . . . . . . . . . 18 5. Updating Caches with HEAD Responses . . . . . . . . . . . . . 18
6. Request Methods that Invalidate . . . . . . . . . . . . . . . 18 6. Request Methods that Invalidate . . . . . . . . . . . . . . . 19
7. Header Field Definitions . . . . . . . . . . . . . . . . . . . 19 7. Header Field Definitions . . . . . . . . . . . . . . . . . . . 19
7.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 7.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 20 7.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 20
7.2.1. Request Cache-Control Directives . . . . . . . . . . . 20 7.2.1. Request Cache-Control Directives . . . . . . . . . . . 20
7.2.2. Response Cache-Control Directives . . . . . . . . . . 22 7.2.2. Response Cache-Control Directives . . . . . . . . . . 22
7.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 25 7.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 26
7.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27 7.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.5.1. 110 Response is Stale . . . . . . . . . . . . . . . . 30 7.5.1. 110 Response is Stale . . . . . . . . . . . . . . . . 30
7.5.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 30 7.5.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 30
7.5.3. 112 Disconnected Operation . . . . . . . . . . . . . . 30 7.5.3. 112 Disconnected Operation . . . . . . . . . . . . . . 30
7.5.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 30 7.5.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 30
7.5.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 30 7.5.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 31
7.5.6. 214 Transformation Applied . . . . . . . . . . . . . . 30 7.5.6. 214 Transformation Applied . . . . . . . . . . . . . . 31
7.5.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 31 7.5.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 31
7.5.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 31 7.5.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 31
8. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 31 8. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 31 9.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 32
9.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 32 9.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 32
9.3. Header Field Registration . . . . . . . . . . . . . . . . 33 9.3. Header Field Registration . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33 10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
12.1. Normative References . . . . . . . . . . . . . . . . . . . 34 12.1. Normative References . . . . . . . . . . . . . . . . . . . 34
12.2. Informative References . . . . . . . . . . . . . . . . . . 34 12.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 35 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 35
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 35 Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 37
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 37 Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 38
Appendix D. Change Log (to be removed by RFC Editor before Appendix D. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 38 publication) . . . . . . . . . . . . . . . . . . . . 39
D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 38 D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 39
D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 38 D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 39
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 D.3. Since draft-ietf-httpbis-p6-cache-21 . . . . . . . . . . . 40
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction 1. Introduction
HTTP is typically used for distributed information systems, where HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing document defines aspects of HTTP/1.1 related to caching and reusing
response messages. response messages.
1.1. Purpose 1.1. Purpose
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reusable if it can be freshened by validation (Section 4.2) or if the reusable if it can be freshened by validation (Section 4.2) or if the
origin is unavailable. origin is unavailable.
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.
cache cache
A conformant implementation of a HTTP cache. Note that this A conformant implementation of an HTTP cache. Note that this
implies an HTTP/1.1 cache; this specification does not define implies an HTTP/1.1 cache; this specification does not define
conformance for HTTP/1.0 caches. conformance for HTTP/1.0 caches.
shared cache shared cache
A cache that stores responses to be reused by more than one user; A cache that stores responses to be reused by more than one user;
usually (but not always) deployed as part of an intermediary. usually (but not always) deployed as part of an intermediary.
private cache private cache
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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 might be additional Even when a response is cacheable, there might be additional
constraints on whether a cache can use the stored copy to satisfy constraints on whether a cache can use the stored copy to satisfy
a particular request. a particular request.
explicit expiration time explicit expiration time
The time at which the origin server intends that a representation The time at which the origin server intends that a stored response
no longer be returned by a cache without further validation. no longer be used 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.
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in the cache. Although caching is an entirely OPTIONAL feature of in the cache. Although caching is an entirely OPTIONAL feature of
HTTP, we assume that reusing the cached response is desirable and HTTP, we assume that reusing the cached response is desirable and
that such reuse is the default behavior when no requirement or that such reuse is the default behavior when no requirement or
locally-desired configuration prevents it. Therefore, HTTP cache locally-desired configuration prevents it. Therefore, HTTP cache
requirements are focused on preventing a cache from either storing a requirements are focused on preventing a cache from either storing a
non-reusable response or reusing a stored response inappropriately. non-reusable response or reusing a stored response inappropriately.
Each cache entry consists of a cache key and one or more HTTP Each cache entry consists of a cache key and one or more HTTP
responses corresponding to prior requests that used the same key. responses corresponding to prior requests that used the same key.
The most common form of cache entry is a successful result of a The most common form of cache entry is a successful result of a
retrieval request: i.e., a 200 (OK) response containing a retrieval request: i.e., a 200 (OK) response to a GET request, which
representation of the resource identified by the request target. contains a representation of the resource identified by the request
However, it is also possible to cache negative results (e.g., 404 target (Section 4.3.1 of [Part2]). However, it is also possible to
(Not Found), incomplete results (e.g., 206 (Partial Content)), and cache permanent redirects, negative results (e.g., 404 (Not Found)),
responses to methods other than GET if the method's definition allows incomplete results (e.g., 206 (Partial Content)), and responses to
such caching and defines something suitable for use as a cache key. methods other than GET if the method's definition allows such caching
and defines something suitable for use as a cache key.
The default cache key consists of the request method and target URI. The default cache key consists of the request method and target URI.
However, since HTTP caches in common use today are typically limited However, since HTTP caches in common use today are typically limited
to caching responses to GET, many implementations simply decline to caching responses to GET, many implementations simply decline
other methods and use only the URI as the key. other methods and use only the URI as the key.
If a request target is subject to content negotiation, its cache If a request target is subject to content negotiation, its cache
entry might consist of multiple stored responses, each differentiated entry might consist of multiple stored responses, each differentiated
by a secondary key for the values of the original request's selecting by a secondary key for the values of the original request's selecting
header fields (Section 4.3). header fields (Section 4.3).
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* contains a max-age response cache directive (see * contains a max-age response cache directive (see
Section 7.2.2.7), or Section 7.2.2.7), or
* contains a s-maxage response cache directive and the cache is * contains a s-maxage response cache directive and the cache is
shared, or shared, or
* contains a Cache Control Extension (see Section 7.2.3) that * contains a Cache Control Extension (see Section 7.2.3) that
allows it to be cached, or allows it to be cached, or
* has a status code that can be served with heuristic freshness * has a status code that is defined as cacheable (see
(see Section 4.1.2). Section 4.1.2), or
* contains a public response cache directive (see
Section 7.2.2.1).
Note that any of the requirements listed above can be overridden by a Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 7.2.3. cache-control extension; see Section 7.2.3.
In this context, a cache has "understood" a request method or a In this context, a cache has "understood" a request method or a
response status code if it recognizes it and implements any cache- response status code if it recognizes it and implements any cache-
specific behavior. specific behavior.
Note that, in normal operation, many caches will not store a response Note that, in normal operation, many caches will not store a response
that has neither a cache validator nor an explicit expiration time, that has neither a cache validator nor an explicit expiration time,
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Note that cached responses that contain the "must-revalidate" and/or Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale "s-maxage" response directives are not allowed to be served stale
(Section 4.1.4) by shared caches. In particular, a response with (Section 4.1.4) by shared caches. In particular, a response with
either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
satisfy a subsequent request without revalidating it on the origin satisfy a subsequent request without revalidating it on the origin
server. server.
4. Constructing Responses from Caches 4. Constructing Responses from Caches
For a presented request, a cache MUST NOT return a stored response, For a presented request, a cache MUST NOT send a stored response,
unless: unless:
o The presented effective request URI (Section 5.5 of [Part1]) and o The presented effective request URI (Section 5.5 of [Part1]) and
that of the stored response match, and that of the stored response match, and
o the request method associated with the stored response allows it o the request method associated with the stored response allows it
to be used for the presented request, and to be used for the presented request, and
o selecting header fields nominated by the stored response (if any) o selecting header fields nominated by the stored response (if any)
match those presented (see Section 4.3), and match those presented (see Section 4.3), and
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o the stored response does not contain the no-cache cache directive o the stored response does not contain the no-cache cache directive
(Section 7.2.2.3), unless it is successfully validated (Section 7.2.2.3), unless it is successfully validated
(Section 4.2), and (Section 4.2), and
o the stored response is either: o the stored response is either:
* fresh (see Section 4.1), or * fresh (see Section 4.1), or
* allowed to be served stale (see Section 4.1.4), or * allowed to be served stale (see Section 4.1.4), or
* successfully validated (see Section 4.2). * successfully validated (see Section 4.2).
Note that any of the requirements listed above can be overridden by a Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 7.2.3. cache-control extension; see Section 7.2.3.
When a stored response is used to satisfy a request without When a stored response is used to satisfy a request without
validation, a cache MUST include a single Age header field validation, a cache MUST send a single Age header field (Section 7.1)
(Section 7.1) in the response with a value equal to the stored in the response with a value equal to the stored response's
response's current_age; see Section 4.1.3. current_age; see Section 4.1.3.
A cache MUST write through requests with methods that are unsafe A cache MUST write through requests with methods that are unsafe
(Section 5.2.1 of [Part2]) to the origin server; i.e., a cache is not (Section 4.2.1 of [Part2]) to the origin server; i.e., a cache is not
allowed to generate a reply to such a request before having forwarded allowed to generate a reply to such a request before having forwarded
the request and having received a corresponding response. the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already stored Also, note that unsafe requests might invalidate already stored
responses; see Section 6. responses; see Section 6.
When more than one suitable response is stored, a cache MUST use the When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It most recent response (as determined by the Date header field). It
can also forward a request with "Cache-Control: max-age=0" or "Cache- can also forward a request with "Cache-Control: max-age=0" or "Cache-
Control: no-cache" to disambiguate which response to use. Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT use stored A cache that does not have a clock available MUST NOT use stored
responses without revalidating them on every use. A cache, responses without revalidating them on every use.
especially a shared cache, SHOULD use a mechanism, such as NTP
[RFC1305], to synchronize its clock with a reliable external
standard.
4.1. Freshness Model 4.1. Freshness Model
When a response is "fresh" in the cache, it can be used to satisfy When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby subsequent requests without contacting the origin server, thereby
improving efficiency. improving efficiency.
The primary mechanism for determining freshness is for an origin The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using server to provide an explicit expiration time in the future, using
either the Expires header field (Section 7.3) or the max-age response either the Expires header field (Section 7.3) or the max-age response
cache directive (Section 7.2.2.7). Generally, origin servers will cache directive (Section 7.2.2.7). Generally, origin servers will
assign future explicit expiration times to responses in the belief assign future explicit expiration times to responses in the belief
that the representation is not likely to change in a semantically that the representation is not likely to change in a semantically
significant way before the expiration time is reached. significant way before the expiration time is reached.
If an origin server wishes to force a cache to validate every If an origin server wishes to force a cache to validate every
request, it can assign an explicit expiration time in the past to request, it can assign an explicit expiration time in the past to
indicate that the response is already stale. Compliant caches will indicate that the response is already stale. Compliant caches will
normally validate the cached response before reusing it for normally validate a stale cached response before reusing it for
subsequent requests (see Section 4.1.4). subsequent requests (see Section 4.1.4).
Since origin servers do not always provide explicit expiration times, Since origin servers do not always provide explicit expiration times,
a cache MAY assign a heuristic expiration time when an explicit time caches are also allowed to use a heuristic to determine an expiration
is not specified, employing algorithms that use other header field time under certain circumstances (see Section 4.1.2).
values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results.
The calculation to determine if a response is fresh is: The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age) response_is_fresh = (freshness_lifetime > current_age)
The freshness_lifetime is defined in Section 4.1.1; the current_age The freshness_lifetime is defined in Section 4.1.1; the current_age
is defined in Section 4.1.3. is defined in Section 4.1.3.
Additionally, clients can influence freshness calculation -- either Clients can send the max-age or min-fresh cache directives in a
constraining it relaxing it -- by using the max-age and min-fresh request to constrain or relax freshness calculations for the
request cache directives. See Section 7.2.1 for details. corresponding response (Section 7.2.1).
Note that freshness applies only to cache operation; it cannot be Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a used to force a user agent to refresh its display or reload a
resource. See Section 8 for an explanation of the difference between resource. See Section 8 for an explanation of the difference between
caches and history mechanisms. caches and history mechanisms.
4.1.1. Calculating Freshness Lifetime 4.1.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of: freshness_lifetime) of a response by using the first match of:
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of the information comes from the origin server. of the information comes from the origin server.
When there is more than one value present for a given directive When there is more than one value present for a given directive
(e.g., two Expires header fields, multiple Cache-Control: max-age (e.g., two Expires header fields, multiple Cache-Control: max-age
directives), it is considered invalid. Caches are encouraged to directives), it is considered invalid. Caches are encouraged to
consider responses that have invalid freshness information to be consider responses that have invalid freshness information to be
stale. stale.
4.1.2. Calculating Heuristic Freshness 4.1.2. Calculating Heuristic Freshness
If no explicit expiration time is present in a stored response that Since origin servers do not always provide explicit expiration times,
has a status code whose definition allows heuristic freshness to be a cache MAY assign a heuristic expiration time when an explicit time
used (including the following in Section 7 of [Part2]: 200 (OK), 203 is not specified, employing algorithms that use other header field
(Non-Authoritative Information), 206 (Partial Content), 300 (Multiple values (such as the Last-Modified time) to estimate a plausible
Choices), 301 (Moved Permanently) and 410 (Gone)), a cache MAY expiration time. This specification does not provide specific
calculate a heuristic expiration time. A cache MUST NOT use algorithms, but does impose worst-case constraints on their results.
heuristics to determine freshness for responses with status codes
that do not explicitly allow it. A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because
of the requirements in Section 3, this means that, effectively,
heuristics can only be used on responses without explicit freshness
whose status codes are defined as cacheable, and responses without
explicit freshness that have been marked as explicitly cacheable
(e.g., with a "public" response cache directive).
If the response has a Last-Modified header field (Section 2.2 of
[Part4]), caches are encouraged to use a heuristic expiration value
that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache When a heuristic is used to calculate freshness lifetime, a cache
SHOULD attach a Warning header field with a 113 warn-code to the SHOULD attach a Warning header field with a 113 warn-code to the
response if its current_age is more than 24 hours and such a warning response if its current_age is more than 24 hours and such a warning
is not already present. is not already present.
Also, if the response has a Last-Modified header field (Section 2.2
of [Part4]), caches are encouraged to use a heuristic expiration
value that is no more than some fraction of the interval since that
time. A typical setting of this fraction might be 10%.
Note: Section 13.9 of [RFC2616] prohibited caches from calculating Note: Section 13.9 of [RFC2616] prohibited caches from calculating
heuristic freshness for URIs with query components (i.e., those heuristic freshness for URIs with query components (i.e., those
containing '?'). In practice, this has not been widely containing '?'). In practice, this has not been widely
implemented. Therefore, servers are encouraged to send explicit implemented. Therefore, servers are encouraged to send explicit
directives (e.g., Cache-Control: no-cache) if they wish to directives (e.g., Cache-Control: no-cache) if they wish to
preclude caching. preclude caching.
4.1.3. Calculating Age 4.1.3. Calculating Age
HTTP/1.1 uses the Age header field to convey the estimated age of the The Age header field is used to convey an estimated age of the
response message when obtained from a cache. The Age field value is 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 the cache's estimate of the number of seconds since the response was
generated or validated by the origin server. In essence, the Age 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 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 each of the caches along the path from the origin server, plus the
amount of time it has been in transit along network paths. amount of time it has been in transit along network paths.
The following data is used for the age calculation: The following data is used for the age calculation:
age_value age_value
The term "age_value" denotes the value of the Age header field The term "age_value" denotes the value of the Age header field
(Section 7.1), in a form appropriate for arithmetic operation; or (Section 7.1), in a form appropriate for arithmetic operation; or
0, if not available. 0, if not available.
date_value date_value
HTTP/1.1 requires origin servers to send a Date header field, if The term "date_value" denotes the value of the Date header field,
possible, with every response, giving the time at which the in a form appropriate for arithmetic operations. See Section
response was generated. The term "date_value" denotes the value 7.1.1.2 of [Part2] for the definition of the Date header field,
of the Date header field, in a form appropriate for arithmetic and for requirements regarding responses without it.
operations. See Section 8.1.1.2 of [Part2] for the definition of
the Date header field, and for requirements regarding responses
without it.
now now
The term "now" means "the current value of the clock at the host The term "now" means "the current value of the clock at the host
performing the calculation". A cache SHOULD use NTP ([RFC1305]) performing the calculation". A host ought to use NTP ([RFC1305])
or some similar protocol to synchronize its clocks to a globally or some similar protocol to synchronize its clocks to Coordinated
accurate time standard. Universal Time.
request_time request_time
The current value of the clock at the host at the time the request The current value of the clock at the host at the time the request
resulting in the stored response was made. resulting in the stored response was made.
response_time response_time
The current value of the clock at the host at the time the The current value of the clock at the host at the time the
response was received. response was received.
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The current_age of a stored response can then be calculated by adding The current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last the amount of time (in seconds) since the stored response was last
validated by the origin server to the corrected_initial_age. validated by the origin server to the corrected_initial_age.
resident_time = now - response_time; resident_time = now - response_time;
current_age = corrected_initial_age + resident_time; current_age = corrected_initial_age + resident_time;
Additionally, to avoid common problems in date parsing: Additionally, to avoid common problems in date parsing:
o Recipients SHOULD assume that an RFC-850 date which appears to be
more than 50 years in the future is in fact in the past (this
helps solve the "year 2000" problem).
o Although all date formats are specified to be case-sensitive, o Although all date formats are specified to be case-sensitive,
recipients SHOULD match day, week and timezone names case- cache recipients SHOULD match day, week and timezone names case-
insensitively. insensitively.
o An implementation MAY internally represent a parsed Expires date o If a cache recipient's internal implementation of time has less
as earlier than the proper value, but MUST NOT internally resolution than the value of an HTTP-date, the recipient MUST
represent a parsed Expires date as later than the proper value. internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value.
o Recipients MUST perform all expiration-related calculations in o Cache recipients MUST NOT allow local time zones to influence the
GMT. The local time zone MUST NOT influence the calculation or calculation or comparison of an age or expiration time.
comparison of an age or expiration time.
o Caches SHOULD consider dates with time zones other than "GMT" o Cache recipients SHOULD consider a date with a zone abbreviation
invalid. other than "GMT" to be invalid for calculating expiration.
4.1.4. Serving Stale Responses 4.1.4. Serving Stale Responses
A "stale" response is one that either has explicit expiry information A "stale" response is one that either has explicit expiry information
or is allowed to have heuristic expiry calculated, but is not fresh or is allowed to have heuristic expiry calculated, but is not fresh
according to the calculations in Section 4.1. according to the calculations in Section 4.1.
A cache MUST NOT return a stale response if it is prohibited by an A cache MUST NOT send a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache" explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
cache directive, a "must-revalidate" cache-response-directive, or an cache directive, a "must-revalidate" cache-response-directive, or an
applicable "s-maxage" or "proxy-revalidate" cache-response-directive; applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 7.2.2). see Section 7.2.2).
A cache MUST NOT return stale responses unless it is disconnected A cache MUST NOT send stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a (i.e., it cannot contact the origin server or otherwise find a
forward path) or doing so is explicitly allowed (e.g., by the max- forward path) or doing so is explicitly allowed (e.g., by the max-
stale request directive; see Section 7.2.1). stale request directive; see Section 7.2.1).
A cache SHOULD append a Warning header field with the 110 warn-code A cache SHOULD append a Warning header field with the 110 warn-code
(see Section 7.5) to stale responses. Likewise, a cache SHOULD add (see Section 7.5) to stale responses. Likewise, a cache SHOULD add
the 112 warn-code to stale responses if the cache is disconnected. the 112 warn-code to stale responses if the cache is disconnected.
If a cache receives a first-hand response (either an entire response, If a cache receives a first-hand response (either an entire response,
or a 304 (Not Modified) response) that it would normally forward to or a 304 (Not Modified) response) that it would normally forward to
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stored response can be updated and reused; see Section 4.2.1. stored response can be updated and reused; see Section 4.2.1.
o A full response (i.e., one with a payload body) indicates that o A full response (i.e., one with a payload body) indicates that
none of the stored responses nominated in the conditional request none of the stored responses nominated in the conditional request
is suitable. Instead, the cache can use the full response to is suitable. Instead, the cache can use the full response to
satisfy the request and MAY replace the stored response(s). satisfy the request and MAY replace the stored response(s).
o However, if a cache receives a 5xx (Server Error) response while o However, if a cache receives a 5xx (Server Error) response while
attempting to validate a response, it can either forward this attempting to validate a response, it can either forward this
response to the requesting client, or act as if the server failed response to the requesting client, or act as if the server failed
to respond. In the latter case, it can return a previously stored to respond. In the latter case, it can send a previously stored
response (see Section 4.1.4). response (see Section 4.1.4).
4.2.1. Freshening Responses with 304 Not Modified 4.2.1. Freshening Responses with 304 Not Modified
When a cache receives a 304 (Not Modified) response and already has When a cache receives a 304 (Not Modified) response and already has
one or more stored 200 (OK) responses for the same cache key, the one or more stored 200 (OK) responses for the same cache key, the
cache needs to identify which of the stored responses are updated by cache needs to identify which of the stored responses are updated by
this new response and then update the stored response(s) with the new this new response and then update the stored response(s) with the new
information provided in the 304 response. information provided in the 304 response.
The stored response to update is identified by using the first match
(if any) of:
o If the new response contains a strong validator, then that strong o If the new response contains a strong validator, then that strong
validator identifies the selected representation. All of the validator identifies the selected representation. All of the
stored responses with the same strong validator are selected. If stored responses with the same strong validator are selected. If
none of the stored responses contain the same strong validator, none of the stored responses contain the same strong validator,
then this new response corresponds to a new selected then the new response MUST NOT be used to update any stored
representation and MUST NOT update the existing stored responses. responses.
o If the new response contains a weak validator and that validator o If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected. recent of those matching stored responses is selected.
o If the new response does not include any form of validator, there o If the new response does not include any form of validator (such
is only one stored response, and that stored response also lacks a as in the case where a client generates an If-Modified-Since
validator, then that stored response is selected. request from a source other than the Last-Modified response header
field), and there is only one stored response, and that stored
response also lacks a validator, then that stored response is
selected.
If a stored response is selected for update, the cache MUST: If a stored response is selected for update, the cache MUST:
o delete any Warning header fields in the stored response with warn- o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5); code 1xx (see Section 7.5);
o retain any Warning header fields in the stored response with warn- o retain any Warning header fields in the stored response with warn-
code 2xx; and, code 2xx; and,
o use other header fields provided in the 304 (Not Modified) o use other header fields provided in the 304 (Not Modified)
response to replace all instances of the corresponding header response to replace all instances of the corresponding header
fields in the stored response. fields in the stored response.
4.3. Using Negotiated Responses 4.3. Using Negotiated Responses
When a cache receives a request that can be satisfied by a stored When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 8.2.1 of [Part2]), it response that has a Vary header field (Section 7.1.4 of [Part2]), it
MUST NOT use that response unless all of the selecting header fields MUST NOT use that response unless all of the selecting header fields
nominated by the Vary header field match in both the original request nominated by the Vary header field match in both the original request
(i.e., that associated with the stored response), and the presented (i.e., that associated with the stored response), and the presented
request. request.
The selecting header fields from two requests are defined to match if The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in and only if those in the first request can be transformed to those in
the second request by applying any of the following: the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's o adding or removing whitespace, where allowed in the header field's
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subsequent requests to that resource can only be properly interpreted subsequent requests to that resource can only be properly interpreted
by the origin server. by the origin server.
The stored response with matching selecting header fields is known as The stored response with matching selecting header fields is known as
the selected response. the selected response.
If multiple selected responses are available, the most recent If multiple selected responses are available, the most recent
response (as determined by the Date header field) is used; see response (as determined by the Date header field) is used; see
Section 4. Section 4.
If no selected response is available, the cache can forward the If no selected response is available, the cache cannot satisfy the
presented request to the origin server in a conditional request; see presented request. Typically, it is forwarded to the origin server
Section 4.2. in a (possibly conditional; see Section 4.2) request.
4.4. Combining Partial Content 4.4. Combining Partial Content
A response might transfer only a partial representation if the A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more connection closed prematurely or if the request used one or more
Range specifiers ([Part5]). After several such transfers, a cache Range specifiers ([Part5]). After several such transfers, a cache
might have received several ranges of the same representation. A might have received several ranges of the same representation. A
cache MAY combine these ranges into a single stored response, and cache MAY combine these ranges into a single stored response, and
reuse that response to satisfy later requests, if they all share the reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client same strong validator and the cache complies with the client
requirements in Section 4.2 of [Part5]. requirements in Section 4.3 of [Part5].
When combining the new response with one or more stored responses, a When combining the new response with one or more stored responses, a
cache MUST: cache MUST:
o delete any Warning header fields in the stored response with warn- o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5); code 1xx (see Section 7.5);
o retain any Warning header fields in the stored response with warn- o retain any Warning header fields in the stored response with warn-
code 2xx; and, code 2xx; and,
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o retain any Warning header fields in the stored response with warn- o retain any Warning header fields in the stored response with warn-
code 2xx; and, code 2xx; and,
o use other header fields provided in the response to replace all o use other header fields provided in the response to replace all
instances of the corresponding header fields in the stored instances of the corresponding header fields in the stored
response. response.
6. Request Methods that Invalidate 6. Request Methods that Invalidate
Because unsafe request methods (Section 5.2.1 of [Part2]) such as Because unsafe request methods (Section 4.2.1 of [Part2]) such as
PUT, POST or DELETE have the potential for changing state on the PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches can use them to keep their contents origin server, intervening caches can use them to keep their contents
up-to-date. up-to-date.
A cache MUST invalidate the effective Request URI (Section 5.5 of A cache MUST invalidate the effective Request URI (Section 5.5 of
[Part1]) as well as the URI(s) in the Location and Content-Location [Part1]) as well as the URI(s) in the Location and Content-Location
response header fields (if present) when a non-error response to a response header fields (if present) when a non-error response to a
request with an unsafe method is received. request with an unsafe method is received.
However, a cache MUST NOT invalidate a URI from a Location or However, a cache MUST NOT invalidate a URI from a Location or
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of [Part1]). This helps prevent denial of service attacks. of [Part1]). This helps prevent denial of service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of A cache MUST invalidate the effective request URI (Section 5.5 of
[Part1]) when it receives a non-error response to a request with a [Part1]) when it receives a non-error response to a request with a
method whose safety is unknown. method whose safety is unknown.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx Here, a "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. "Invalidate" means that the cache will (Redirection) status code. "Invalidate" means that the cache will
either remove all stored responses related to the effective request either remove all stored responses related to the effective request
URI, or will mark these as "invalid" and in need of a mandatory URI, or will mark these as "invalid" and in need of a mandatory
validation before they can be returned in response to a subsequent validation before they can be sent in response to a subsequent
request. request.
Note that this does not guarantee that all appropriate responses are Note that this does not guarantee that all appropriate responses are
invalidated. For example, the request that caused the change at the invalidated. For example, the request that caused the change at the
origin server might not have gone through the cache where a response origin server might not have gone through the cache where a response
is stored. is stored.
7. Header Field Definitions 7. 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
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client wants a response that will still be fresh for at least the client wants a response that will still be fresh for at least the
specified number of seconds. specified number of seconds.
Note: This directive uses the token form of the argument syntax; Note: This directive uses the token form of the argument syntax;
e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use
the quoted-string form. the quoted-string form.
7.2.1.6. no-transform 7.2.1.6. no-transform
The "no-transform" request directive indicates that an intermediary The "no-transform" request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT change the Content- (whether or not it implements a cache) MUST NOT transform the
Encoding, Content-Range or Content-Type request header fields, nor payload, as defined in Section 5.7.2 of [Part1].
the request representation.
7.2.1.7. only-if-cached 7.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a wishes to obtain a stored response. If it receives this directive, a
cache SHOULD either respond using a stored response that is cache SHOULD either respond using a stored response that is
consistent with the other constraints of the request, or respond with consistent with the other constraints of the request, or respond with
a 504 (Gateway Timeout) status code. If a group of caches is being a 504 (Gateway Timeout) status code. If a group of caches is being
operated as a unified system with good internal connectivity, a operated as a unified system with good internal connectivity, a
member cache MAY forward such a request within that group of caches. member cache MAY forward such a request within that group of caches.
7.2.2. Response Cache-Control Directives 7.2.2. Response Cache-Control Directives
7.2.2.1. public 7.2.2.1. public
The "public" response directive indicates that a response whose The "public" response directive indicates that any cache MAY store
associated request contains an 'Authentication' header MAY be stored the response, even if the response would normally be non-cacheable or
(see Section 3.2). cacheable only within a non-shared cache. (See Section 3.2 for
additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable.)
7.2.2.2. private 7.2.2.2. private
Argument syntax: Argument syntax:
#field-name #field-name
The "private" response directive indicates that the response message The "private" response directive indicates that the response message
is intended for a single user and MUST NOT be stored by a shared is intended for a single user and MUST NOT be stored by a shared
cache. A private cache MAY store the response. cache. A private cache MAY store the response and reuse it for later
requests, even if the response would normally be non-cacheable.
If the private response directive specifies one or more field-names, If the private response directive specifies one or more field-names,
this requirement is limited to the field-values associated with the this requirement is limited to the field-values associated with the
listed response header fields. That is, a shared cache MUST NOT listed response header fields. That is, a shared cache MUST NOT
store the specified field-names(s), whereas it MAY store the store the specified field-names(s), whereas it MAY store the
remainder of the response message. remainder of the response message.
The field-names given are not limited to the set of standard header The field-names given are not limited to the set of standard header
fields defined by this specification. Field names are case- fields defined by this specification. Field names are case-
insensitive. insensitive.
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7.2.2.3. no-cache 7.2.2.3. no-cache
Argument syntax: Argument syntax:
#field-name #field-name
The "no-cache" response directive indicates that the response MUST The "no-cache" response directive indicates that the response MUST
NOT be used to satisfy a subsequent request without successful NOT be used to satisfy a subsequent request without successful
validation on the origin server. This allows an origin server to validation on the origin server. This allows an origin server to
prevent a cache from using it to satisfy a request without contacting prevent a cache from using it to satisfy a request without contacting
it, even by caches that have been configured to return stale it, even by caches that have been configured to send stale responses.
responses.
If the no-cache response directive specifies one or more field-names, If the no-cache response directive specifies one or more field-names,
then a cache MAY use the response to satisfy a subsequent request, then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, any header subject to any other restrictions on caching. However, any header
fields in the response that have the field-name(s) listed MUST NOT be fields in the response that have the field-name(s) listed MUST NOT be
sent in the response to a subsequent request without successful sent in the response to a subsequent request without successful
revalidation with the origin server. This allows an origin server to revalidation with the origin server. This allows an origin server to
prevent the re-use of certain header fields in a response, while prevent the re-use of certain header fields in a response, while
still allowing caching of the rest of the response. still allowing caching of the rest of the response.
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field. The s-maxage directive also implies the semantics of the field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive. proxy-revalidate response directive.
Note: This directive uses the token form of the argument syntax; Note: This directive uses the token form of the argument syntax;
e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the
quoted-string form. quoted-string form.
7.2.2.9. no-transform 7.2.2.9. no-transform
The "no-transform" response directive indicates that an intermediary The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT change the (regardless of whether it implements a cache) MUST NOT transform the
Content-Encoding, Content-Range or Content-Type response header payload, as defined in Section 5.7.2 of [Part1].
fields, nor the response representation.
7.2.3. Cache Control Extensions 7.2.3. Cache Control Extensions
The Cache-Control header field can be extended through the use of one The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional value. or more cache-extension tokens, each with an optional value.
Informational extensions (those that do not require a change in cache Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other behavior) can be added without changing the semantics of other
directives. Behavioral extensions are designed to work by acting as directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the new modifiers to the existing base of cache directives. Both the new
directive and the standard directive are supplied, such that directive and the standard directive are supplied, such that
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7.3. Expires 7.3. Expires
The "Expires" header field gives the date/time after which the The "Expires" header field gives the date/time after which the
response is considered stale. See Section 4.1 for further discussion response is considered stale. See Section 4.1 for further discussion
of the freshness model. of the freshness model.
The presence of an Expires field does not imply that the original The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that resource will change or cease to exist at, before, or after that
time. time.
The field-value is an absolute date and time as defined by HTTP-date The Expires value is an HTTP-date timestamp, as defined in Section
in Section 8.1.1.1 of [Part2]; a sender MUST use the rfc1123-date 7.1.1.1 of [Part2].
format.
Expires = HTTP-date Expires = HTTP-date
For example For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache MUST treat other invalid date formats, especially including A cache recipient MUST interpret invalid date formats, especially the
the value "0", as in the past (i.e., "already expired"). value "0", as representing a time in the past (i.e., "already
expired").
Note: If a response includes a Cache-Control field with the max- If a response includes a Cache-Control field with the max-age
age directive (see Section 7.2.2.7), that directive overrides the directive (Section 7.2.2.7), a recipient MUST ignore the Expires
Expires field. Likewise, the s-maxage directive (Section 7.2.2.8) field. Likewise, if a response includes the s-maxage directive
overrides the Expires header fieldin shared caches. (Section 7.2.2.8), a shared cache recipient MUST ignore the Expires
field. In both these cases, the value in Expires is only intended
for recipients that have not yet implemented the Cache-Control field.
An origin server without a clock MUST NOT generate an Expires field
unless its value represents a fixed time in the past (always expired)
or its value has been associated with the resource by a system or
user with a reliable clock.
Historically, HTTP required the Expires field-value to be no more Historically, HTTP required the Expires field-value to be no more
than a year in the future. While longer freshness lifetimes are no than a year in the future. While longer freshness lifetimes are no
longer prohibited, extremely large values have been demonstrated to longer prohibited, extremely large values have been demonstrated to
cause problems (e.g., clock overflows due to use of 32-bit integers cause problems (e.g., clock overflows due to use of 32-bit integers
for time values), and many caches will evict a response far sooner for time values), and many caches will evict a response far sooner
than that. Therefore, senders ought not produce them. than that.
An origin server without a clock MUST NOT assign Expires values to a
response unless these values were associated with the resource by a
system or user with a reliable clock. It MAY assign an Expires value
that is known, at or before server configuration time, to be in the
past (this allows "pre-expiration" of responses without storing
separate Expires values for each resource).
7.4. Pragma 7.4. Pragma
The "Pragma" header field allows backwards compatibility with The "Pragma" header field allows backwards compatibility with
HTTP/1.0 caches, so that clients can specify a "no-cache" request HTTP/1.0 caches, so that clients can specify a "no-cache" request
that they will understand (as Cache-Control was not defined until that they will understand (as Cache-Control was not defined until
HTTP/1.1). When the Cache-Control header field is also present and HTTP/1.1). When the Cache-Control header field is also present and
understood in a request, Pragma is ignored. understood in a request, Pragma is ignored.
In HTTP/1.0, Pragma was defined as an extensible field for In HTTP/1.0, Pragma was defined as an extensible field for
skipping to change at page 29, line 50 skipping to change at page 30, line 13
response after validation: response after validation:
o 1xx Warnings describe the freshness or validation status of the o 1xx Warnings describe the freshness or validation status of the
response, and so MUST be deleted by a cache after validation. response, and so MUST be deleted by a cache after validation.
They can only be generated by a cache when validating a cached They can only be generated by a cache when validating a cached
entry, and MUST NOT be generated in any other situation. entry, and MUST NOT be generated in any other situation.
o 2xx Warnings describe some aspect of the representation that is o 2xx Warnings describe some aspect of the representation that is
not rectified by a validation (for example, a lossy compression of not rectified by a validation (for example, a lossy compression of
the representation) and MUST NOT be deleted by a cache after the representation) and MUST NOT be deleted by a cache after
validation, unless a full response is returned, in which case they validation, unless a full response is sent, in which case they
MUST be. MUST be.
If an implementation sends a message with one or more Warning header If an implementation sends a message with one or more Warning header
fields to a receiver whose version is HTTP/1.0 or lower, then the fields to a receiver whose version is HTTP/1.0 or lower, then the
sender MUST include in each warning-value a warn-date that matches sender MUST include in each warning-value a warn-date that matches
the Date header field in the message. the Date header field in the message.
If a system receives a message with a warning-value that includes a If a system receives a message with a warning-value that includes a
warn-date, and that warn-date is different from the Date value in the 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 response, then that warning-value MUST be deleted from the message
before storing, forwarding, or using it. (preventing the consequences before storing, forwarding, or using it. (preventing the consequences
of naive caching of Warning header fields.) If all of the warning- of naive caching of Warning header fields.) If all of the warning-
values are deleted for this reason, the Warning header field MUST be values are deleted for this reason, the Warning header field MUST be
deleted as well. deleted as well.
The following warn-codes are defined by this specification, each with The following warn-codes are defined by this specification, each with
a recommended warn-text in English, and a description of its meaning. a recommended warn-text in English, and a description of its meaning.
7.5.1. 110 Response is Stale 7.5.1. 110 Response is Stale
A cache SHOULD include this whenever the returned response is stale. A cache SHOULD generate this whenever the sent response is stale.
7.5.2. 111 Revalidation Failed 7.5.2. 111 Revalidation Failed
A cache SHOULD include this when returning a stale response because A cache SHOULD generate this when sending a stale response because an
an attempt to validate the response failed, due to an inability to attempt to validate the response failed, due to an inability to reach
reach the server. the server.
7.5.3. 112 Disconnected Operation 7.5.3. 112 Disconnected Operation
A cache SHOULD include this if it is intentionally disconnected from A cache SHOULD generate this if it is intentionally disconnected from
the rest of the network for a period of time. the rest of the network for a period of time.
7.5.4. 113 Heuristic Expiration 7.5.4. 113 Heuristic Expiration
A cache SHOULD include this if it heuristically chose a freshness A cache SHOULD generate this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than lifetime greater than 24 hours and the response's age is greater than
24 hours. 24 hours.
7.5.5. 199 Miscellaneous Warning 7.5.5. 199 Miscellaneous Warning
The warning text can include arbitrary information to be presented to The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT a human user, or logged. A system receiving this warning MUST NOT
take any automated action, besides presenting the warning to the take any automated action, besides presenting the warning to the
user. user.
skipping to change at page 33, line 9 skipping to change at page 33, line 21
| 113 | Heuristic Expiration | Section 7.5.4 | | 113 | Heuristic Expiration | Section 7.5.4 |
| 199 | Miscellaneous Warning | Section 7.5.5 | | 199 | Miscellaneous Warning | Section 7.5.5 |
| 214 | Transformation Applied | Section 7.5.6 | | 214 | Transformation Applied | Section 7.5.6 |
| 299 | Miscellaneous Persistent Warning | Section 7.5.7 | | 299 | Miscellaneous Persistent Warning | Section 7.5.7 |
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
9.3. Header Field Registration 9.3. Header Field Registration
The Message Header Field Registry located at <http://www.iana.org/ The Message Header Field Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> shall be assignments/message-headers/message-header-index.html> shall be
updated with the permanent registrations below (see [RFC3864]): updated with the permanent registrations below (see [BCP90]):
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference | | Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
| Age | http | standard | Section 7.1 | | Age | http | standard | Section 7.1 |
| Cache-Control | http | standard | Section 7.2 | | Cache-Control | http | standard | Section 7.2 |
| Expires | http | standard | Section 7.3 | | Expires | http | standard | Section 7.3 |
| Pragma | http | standard | Section 7.4 | | Pragma | http | standard | Section 7.4 |
| Warning | http | standard | Section 7.5 | | Warning | http | standard | Section 7.5 |
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force". Engineering Task Force".
10. Security Considerations 10. Security Considerations
This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP/1.1 caching.
More general security considerations are addressed in HTTP messaging
[Part1] and semantics [Part2].
Caches expose additional potential vulnerabilities, since the Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive network. Therefore, cache contents need to be protected as sensitive
information. information.
Furthermore, the very use of a cache can bring about privacy
concerns. For example, if two users share a cache, and the first one
browses to a site, the second may be able to detect that the other
has been to that site, because the resources from it load more
quickly, thanks to the cache.
Implementation flaws might allow attackers to insert content into a Implementation flaws might allow attackers to insert content into a
cache ("cache poisoning"), leading to compromise of clients that cache ("cache poisoning"), leading to compromise of clients that
trust that content. Because of their nature, these attacks are trust that content. Because of their nature, these attacks are
difficult to mitigate. difficult to mitigate.
Likewise, implementation flaws (as well as misunderstanding of cache Likewise, implementation flaws (as well as misunderstanding of cache
operation) might lead to caching of sensitive information (e.g., operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing authentication credentials) that is thought to be private, exposing
it to unauthorised parties. it to unauthorized parties.
Note that the Set-Cookie response header [RFC6265] does not inhibit Note that the Set-Cookie response header [RFC6265] does not inhibit
caching; a cacheable response with a Set-Cookie header can be (and caching; a cacheable response with a Set-Cookie header can be (and
often is) used to satisfy subsequent requests to caches. Servers who often is) used to satisfy subsequent requests to caches. Servers who
wish to control caching of these responses are encouraged to emit wish to control caching of these responses are encouraged to emit
appropriate Cache-Control response headers. appropriate Cache-Control response headers.
11. Acknowledgments 11. Acknowledgments
See Section 9 of [Part1]. See Section 9 of [Part1].
12. References 12. References
12.1. Normative References 12.1. Normative References
[Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-21 (work in progress), draft-ietf-httpbis-p1-messaging-22 (work in progress),
October 2012. February 2013.
[Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", Protocol (HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-21 (work in progress), draft-ietf-httpbis-p2-semantics-22 (work in progress),
October 2012. February 2013.
[Part4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [Part4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", Protocol (HTTP/1.1): Conditional Requests",
draft-ietf-httpbis-p4-conditional-21 (work in progress), draft-ietf-httpbis-p4-conditional-22 (work in progress),
October 2012. February 2013.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., [Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests", "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
draft-ietf-httpbis-p5-range-21 (work in progress), draft-ietf-httpbis-p5-range-22 (work in progress),
October 2012. February 2013.
[Part7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [Part7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", Protocol (HTTP/1.1): Authentication",
draft-ietf-httpbis-p7-auth-21 (work in progress), draft-ietf-httpbis-p7-auth-22 (work in progress),
October 2012. February 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
12.2. Informative References 12.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[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
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, April 2010. Content", RFC 5861, April 2010.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011. April 2011.
Appendix A. Changes from RFC 2616 Appendix A. Changes from RFC 2616
Make the specified age calculation algorithm less conservative. Caching-related text has been substantially rewritten for clarity.
The algorithm for calculating age is now less conservative.
(Section 4.1.3) (Section 4.1.3)
Remove requirement to consider "Content-Location" in successful Caches are now required to handle dates with timezones as if they're
responses in order to determine the appropriate response to use. invalid, because it's not possible to accurately guess.
(Section 4.1.3)
The Content-Location response header field is no longer used to
determine the appropriate response to use when validating.
(Section 4.2) (Section 4.2)
The algorithm for selecting a cached negotiated response to use has
been clarified in several ways. In particular, it now explicitly
allows header-specific canonicalization when processing selecting
header fields. (Section 4.3)
Clarify denial of service attack avoidance requirement. (Section 6) Requirements regarding denial of service attack avoidance when
performing invalidation have been clarified. (Section 6)
Do not mention RFC 2047 encoding and multiple languages in "Warning" Cache invalidation only occurs when a successful response is
header fields anymore, as these aspects never were implemented. received. (Section 6)
The conditions under which an authenticated response can be cached
have been clarified. (Section 3.2)
The one-year limit on Expires header field values has been removed;
instead, the reasoning for using a sensible value is given.
(Section 7.3)
The Pragma header field is now only defined for backwards
compatibility; future pragmas are deprecated. (Section 7.4)
Cache directives are explicitly defined to be case-insensitive.
(Section 7.2)
Handling of multiple instances of cache directives when only one is
expected is now defined. (Section 7.2)
The qualified forms of the private and no-cache cache directives are
noted to not be widely implemented; e.g., "private=foo" is
interpreted by many caches as simply "private". Additionally, the
meaning of the qualified form of no-cache has been clarified.
(Section 7.2.2)
The "no-store" cache request directive doesn't apply to responses;
i.e., a cache can satisfy a request with no-store on it, and does not
invalidate it. (Section 7.2.1.2)
The "no-cache" response cache directive's meaning has been clarified.
(Section 7.2.2.3)
New status codes can now define that caches are allowed to use
heuristic freshness with them. (Section 4.1.2)
Caches are now allow to calculate heuristic freshness for URLs with
query components. (Section 4.1.2)
Some requirements regarding production of the Warning header have
been relaxed, as it is not widely implemented. (Section 7.5)
The Warning header field no longer uses RFC 2047 encoding, nor allows
multiple languages, as these aspects were not implemented.
(Section 7.5) (Section 7.5)
Introduce Cache Directive and Warn Code Registries. (Section 7.2.3 This specification introduces the Cache Directive and Warn Code
and Section 7.5.8) Registries, and defines considerations for new cache directives.
(Section 7.2.3 and Section 7.5.8)
Appendix B. Imported ABNF Appendix B. Imported ABNF
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character). character).
The rules below are defined in [Part1]: The rules below are defined in [Part1]:
OWS = <OWS, defined in [Part1], Section 3.2.1> OWS = <OWS, defined in [Part1], Section 3.2.3>
field-name = <field-name, defined in [Part1], Section 3.2> field-name = <field-name, defined in [Part1], Section 3.2>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.4> quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
token = <token, defined in [Part1], Section 3.2.4> token = <token, defined in [Part1], Section 3.2.6>
port = <port, defined in [Part1], Section 2.7> port = <port, defined in [Part1], Section 2.7>
pseudonym = <pseudonym, defined in [Part1], Section 5.7> pseudonym = <pseudonym, defined in [Part1], Section 5.7.1>
uri-host = <uri-host, defined in [Part1], Section 2.7> uri-host = <uri-host, defined in [Part1], Section 2.7>
The rules below are defined in other parts: The rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in [Part2], Section 8.1.1.1> HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
Appendix C. Collected ABNF Appendix C. Collected ABNF
Age = delta-seconds Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] ) cache-directive ] )
Expires = HTTP-date Expires = HTTP-date
HTTP-date = <HTTP-date, defined in [Part2], Section 8.1.1.1> HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
OWS = <OWS, defined in [Part1], Section 3.2.1> OWS = <OWS, defined in [Part1], Section 3.2.3>
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] ) pragma-directive ] )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
) )
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ] extension-pragma = token [ "=" ( token / quoted-string ) ]
field-name = <field-name, defined in [Part1], Section 3.2> field-name = <field-name, defined in [Part1], Section 3.2>
port = <port, defined in [Part1], Section 2.7> port = <port, defined in [Part1], Section 2.7>
pragma-directive = "no-cache" / extension-pragma pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, defined in [Part1], Section 5.7> pseudonym = <pseudonym, defined in [Part1], Section 5.7.1>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.4> quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
token = <token, defined in [Part1], Section 3.2.4> token = <token, defined in [Part1], Section 3.2.6>
uri-host = <uri-host, defined in [Part1], Section 2.7> uri-host = <uri-host, defined in [Part1], Section 2.7>
warn-agent = ( uri-host [ ":" port ] ) / pseudonym warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
] ]
skipping to change at page 39, line 8 skipping to change at page 40, line 8
Other changes: Other changes:
o Conformance criteria and considerations regarding error handling o Conformance criteria and considerations regarding error handling
are now defined in Part 1. are now defined in Part 1.
o Move definition of "Vary" header field into Part 2. o Move definition of "Vary" header field into Part 2.
o Add security considerations with respect to cache poisoning and o Add security considerations with respect to cache poisoning and
the "Set-Cookie" header field. the "Set-Cookie" header field.
D.3. Since draft-ietf-httpbis-p6-cache-21
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
heuristic caching for new status codes"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/406>: "304 without
validator"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/418>: "No-Transform"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/430>: "Revert prior
change to the meaning of the public cache response directive.
Index Index
1 1
110 Response is Stale (warn code) 30 110 Response is Stale (warn code) 30
111 Revalidation Failed (warn code) 30 111 Revalidation Failed (warn code) 30
112 Disconnected Operation (warn code) 30 112 Disconnected Operation (warn code) 30
113 Heuristic Expiration (warn code) 30 113 Heuristic Expiration (warn code) 30
199 Miscellaneous Warning (warn code) 30 199 Miscellaneous Warning (warn code) 31
2 2
214 Transformation Applied (warn code) 30 214 Transformation Applied (warn code) 31
299 Miscellaneous Persistent Warning (warn code) 31 299 Miscellaneous Persistent Warning (warn code) 31
A A
age 5 age 5
Age header field 19 Age header field 19
C C
cache 4 cache 4
cache entry 6 cache entry 6
cache key 6 cache key 6
skipping to change at page 40, line 20 skipping to change at page 41, line 35
heuristic expiration time 5 heuristic expiration time 5
M M
max-age (cache directive) 21, 25 max-age (cache directive) 21, 25
max-stale (cache directive) 21 max-stale (cache directive) 21
min-fresh (cache directive) 22 min-fresh (cache directive) 22
must-revalidate (cache directive) 24 must-revalidate (cache directive) 24
N N
no-cache (cache directive) 20, 23 no-cache (cache directive) 20, 23
no-store (cache directive) 20, 24 no-store (cache directive) 21, 24
no-transform (cache directive) 22, 25 no-transform (cache directive) 22, 25
O O
only-if-cached (cache directive) 22 only-if-cached (cache directive) 22
P P
Pragma header field 28 Pragma header field 28
private (cache directive) 22 private (cache directive) 23
private cache 4 private cache 4
proxy-revalidate (cache directive) 24 proxy-revalidate (cache directive) 25
public (cache directive) 22 public (cache directive) 22
S S
s-maxage (cache directive) 25 s-maxage (cache directive) 25
shared cache 4 shared cache 4
stale 5 stale 5
strong validator 6 strong validator 6
V V
validator 5 validator 5
strong 6 strong 6
W W
Warning header field 28 Warning header field 29
Authors' Addresses Authors' Addresses
Roy T. Fielding (editor) Roy T. Fielding (editor)
Adobe Systems Incorporated Adobe Systems Incorporated
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
USA USA
EMail: fielding@gbiv.com EMail: fielding@gbiv.com
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