source: draft-ietf-httpbis/latest/p6-cache.xml @ 366

Last change on this file since 366 was 366, checked in by julian.reschke@…, 14 years ago

Rewrite header ABNFs to spell out whitespace rules, factor out value format definitions. (related to #36)

  • Property svn:eol-style set to native
File size: 133.4 KB
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "November">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY notation-abnf               "<xref target='Part1' x:rel='#notation.abnf' xmlns:x=''/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x=''/>">
19  <!ENTITY general-syntax              "<xref target='Part1' x:rel='#general.syntax' xmlns:x=''/>">
20  <!ENTITY messaging                   "<xref target='Part1' xmlns:x=''/>">
21  <!ENTITY conditional                 "<xref target='Part4' xmlns:x=''/>">
22  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x=''/>">
23  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x=''/>">
24  <!ENTITY full-date                   "<xref target='Part1' x:rel='' xmlns:x=''/>">
25  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x=''/>">
26  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x=''/>">
27  <!ENTITY header-date                 "<xref target='Part1' x:rel='' xmlns:x=''/>">
28  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x=''/>">
29  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x=''/>">
30  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x=''/>">
31  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
32  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x=''/>">
34<?rfc toc="yes" ?>
35<?rfc symrefs="yes" ?>
36<?rfc sortrefs="yes" ?>
37<?rfc compact="yes"?>
38<?rfc subcompact="no" ?>
39<?rfc linkmailto="no" ?>
40<?rfc editing="no" ?>
41<?rfc comments="yes"?>
42<?rfc inline="yes"?>
43<?rfc-ext allow-markup-in-artwork="yes" ?>
44<?rfc-ext include-references-in-index="yes" ?>
45<rfc obsoletes="2616" category="std" x:maturity-level="draft"
46     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
47     xmlns:x=''>
50  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
52  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
53    <organization abbrev="Day Software">Day Software</organization>
54    <address>
55      <postal>
56        <street>23 Corporate Plaza DR, Suite 280</street>
57        <city>Newport Beach</city>
58        <region>CA</region>
59        <code>92660</code>
60        <country>USA</country>
61      </postal>
62      <phone>+1-949-706-5300</phone>
63      <facsimile>+1-949-706-5305</facsimile>
64      <email></email>
65      <uri></uri>
66    </address>
67  </author>
69  <author initials="J." surname="Gettys" fullname="Jim Gettys">
70    <organization>One Laptop per Child</organization>
71    <address>
72      <postal>
73        <street>21 Oak Knoll Road</street>
74        <city>Carlisle</city>
75        <region>MA</region>
76        <code>01741</code>
77        <country>USA</country>
78      </postal>
79      <email></email>
80      <uri></uri>
81    </address>
82  </author>
84  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
85    <organization abbrev="HP">Hewlett-Packard Company</organization>
86    <address>
87      <postal>
88        <street>HP Labs, Large Scale Systems Group</street>
89        <street>1501 Page Mill Road, MS 1177</street>
90        <city>Palo Alto</city>
91        <region>CA</region>
92        <code>94304</code>
93        <country>USA</country>
94      </postal>
95      <email></email>
96    </address>
97  </author>
99  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107        <country>USA</country>
108      </postal>
109      <email></email>
110    </address>
111  </author>
113  <author initials="L." surname="Masinter" fullname="Larry Masinter">
114    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
115    <address>
116      <postal>
117        <street>345 Park Ave</street>
118        <city>San Jose</city>
119        <region>CA</region>
120        <code>95110</code>
121        <country>USA</country>
122      </postal>
123      <email></email>
124      <uri></uri>
125    </address>
126  </author>
128  <author initials="P." surname="Leach" fullname="Paul J. Leach">
129    <organization abbrev="Microsoft">Microsoft Corporation</organization>
130    <address>
131      <postal>
132        <street>1 Microsoft Way</street>
133        <city>Redmond</city>
134        <region>WA</region>
135        <code>98052</code>
136      </postal>
137      <email></email>
138    </address>
139  </author>
141  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
142    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
143    <address>
144      <postal>
145        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
146        <street>The Stata Center, Building 32</street>
147        <street>32 Vassar Street</street>
148        <city>Cambridge</city>
149        <region>MA</region>
150        <code>02139</code>
151        <country>USA</country>
152      </postal>
153      <email></email>
154      <uri></uri>
155    </address>
156  </author>
158  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
159    <organization abbrev="W3C">World Wide Web Consortium</organization>
160    <address>
161      <postal>
162        <street>W3C / ERCIM</street>
163        <street>2004, rte des Lucioles</street>
164        <city>Sophia-Antipolis</city>
165        <region>AM</region>
166        <code>06902</code>
167        <country>France</country>
168      </postal>
169      <email></email>
170      <uri></uri>
171    </address>
172  </author>
174  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
175    <organization abbrev="greenbytes">greenbytes GmbH</organization>
176    <address>
177      <postal>
178        <street>Hafenweg 16</street>
179        <city>Muenster</city><region>NW</region><code>48155</code>
180        <country>Germany</country>
181      </postal>
182      <phone>+49 251 2807760</phone>   
183      <facsimile>+49 251 2807761</facsimile>   
184      <email></email>       
185      <uri></uri>     
186    </address>
187  </author>
189  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
193   The Hypertext Transfer Protocol (HTTP) is an application-level
194   protocol for distributed, collaborative, hypermedia information
195   systems. HTTP has been in use by the World Wide Web global information
196   initiative since 1990. This document is Part 6 of the seven-part specification
197   that defines the protocol referred to as "HTTP/1.1" and, taken together,
198   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
199   and the associated header fields that control cache behavior or indicate
200   cacheable response messages.
204<note title="Editorial Note (To be removed by RFC Editor)">
205  <t>
206    Discussion of this draft should take place on the HTTPBIS working group
207    mailing list ( The current issues list is
208    at <eref target=""/>
209    and related documents (including fancy diffs) can be found at
210    <eref target=""/>.
211  </t>
212  <t>
213    The changes in this draft are summarized in <xref target="changes.since.04"/>.
214  </t>
218<section title="Introduction" anchor="caching">
220   HTTP is typically used for distributed information systems, where
221   performance can be improved by the use of response caches, and includes
222   a number of elements intended to make caching work as well as possible.
223   Because these elements interact with each other, it is useful to describe
224   the caching design of HTTP separately.  This document defines aspects of
225   HTTP/1.1 related to caching and reusing response messages.
228<section title="Purpose" anchor="intro.purpose">
229<iref item="cache"/>
231   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
232   and the subsystem that controls its message storage, retrieval, and
233   deletion. A cache stores cacheable responses in order to reduce the
234   response time and network bandwidth consumption on future, equivalent
235   requests. Any client or server may include a cache, though a cache
236   cannot be used by a server that is acting as a tunnel.
239   Caching would be useless if it did not significantly improve
240   performance. The goal of caching in HTTP/1.1 is to reuse a prior response
241   message to satisfy a current request.  In some cases, the existing response
242   can be reused without the need for a network request, reducing latency and
243   network round-trips; we use an "expiration" mechanism for this purpose
244   (see <xref target="expiration.model"/>).  Even when a new request is required,
245   it is often possible to reuse all or parts of the payload of a prior response
246   to satisfy the request, thereby reducing network bandwidth usage; we use a
247   "validation" mechanism for this purpose (see <xref target="validation.model"/>).
249<iref item="semantically transparent"/>
251   A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
252   respect to a particular response, when its use affects neither the
253   requesting client nor the origin server, except to improve
254   performance. When a cache is semantically transparent, the client
255   receives exactly the same response status and payload
256   that it would have received had its request been handled directly
257   by the origin server.
260   In an ideal world, all interactions with an HTTP cache would be
261   semantically transparent.  However, for some resources, semantic
262   transparency is not always necessary and can be effectively traded
263   for the sake of bandwidth scaling, disconnected operation, and
264   high availability.  HTTP/1.1 allows origin servers, caches,
265   and clients to explicitly reduce transparency when necessary.
266   However, because non-transparent operation may confuse non-expert
267   users and might be incompatible with certain server applications
268   (such as those for ordering merchandise), the protocol requires that
269   transparency be relaxed
270  <list style="symbols">
271     <t>only by an explicit protocol-level request when relaxed by
272        client or origin server</t>
274     <t>only with an explicit warning to the end user when relaxed by
275        cache or client</t>
276  </list>
279   Therefore, HTTP/1.1 provides these important elements:
280  <list style="numbers">
281      <t>Protocol features that provide full semantic transparency when
282         this is required by all parties.</t>
284      <t>Protocol features that allow an origin server or user agent to
285         explicitly request and control non-transparent operation.</t>
287      <t>Protocol features that allow a cache to attach warnings to
288         responses that do not preserve the requested approximation of
289         semantic transparency.</t>
290  </list>
293   A basic principle is that it must be possible for the clients to
294   detect any potential relaxation of semantic transparency.
295  <list><t>
296      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
297      design decisions not explicitly discussed in this specification.
298      If a decision might affect semantic transparency, the implementor
299      ought to err on the side of maintaining transparency unless a
300      careful and complete analysis shows significant benefits in
301      breaking transparency.
302    </t></list>
306<section title="Terminology" anchor="intro.terminology">
308   This specification uses a number of terms to refer to the roles
309   played by participants in, and objects of, HTTP caching.
312  <iref item="cacheable"/>
313  <x:dfn>cacheable</x:dfn>
314  <list>
315    <t>
316      A response is cacheable if a cache is allowed to store a copy of
317      the response message for use in answering subsequent requests.
318      Even when a response is cacheable, there may
319      be additional constraints on whether a cache can use the cached
320      copy for a particular request.
321    </t>
322  </list>
325  <iref item="first-hand"/>
326  <x:dfn>first-hand</x:dfn>
327  <list>
328    <t>
329      A response is first-hand if it comes directly and without
330      unnecessary delay from the origin server, perhaps via one or more
331      proxies. A response is also first-hand if its validity has just
332      been checked directly with the origin server.
333    </t>
334  </list>
337  <iref item="explicit expiration time"/>
338  <x:dfn>explicit expiration time</x:dfn>
339  <list>
340    <t>
341      The time at which the origin server intends that an entity should
342      no longer be returned by a cache without further validation.
343    </t>
344  </list>
347  <iref item="heuristic expiration time"/>
348  <x:dfn>heuristic expiration time</x:dfn>
349  <list>
350    <t>
351      An expiration time assigned by a cache when no explicit expiration
352      time is available.
353    </t>
354  </list>
357  <iref item="age"/>
358  <x:dfn>age</x:dfn>
359  <list>
360    <t>
361      The age of a response is the time since it was sent by, or
362      successfully validated with, the origin server.
363    </t>
364  </list>
367  <iref item="freshness lifetime"/>
368  <x:dfn>freshness lifetime</x:dfn>
369  <list>
370    <t>
371      The length of time between the generation of a response and its
372      expiration time.
373    </t>
374  </list>
377  <iref item="fresh"/>
378  <x:dfn>fresh</x:dfn>
379  <list>
380    <t>
381      A response is fresh if its age has not yet exceeded its freshness
382      lifetime.
383    </t>
384  </list>
387  <iref item="stale"/>
388  <x:dfn>stale</x:dfn>
389  <list>
390    <t>
391      A response is stale if its age has passed its freshness lifetime.
392    </t>
393  </list>
396  <iref item="validator"/>
397  <x:dfn>validator</x:dfn>
398  <list>
399    <t>
400      A protocol element (e.g., an entity tag or a Last-Modified time)
401      that is used to find out whether a cache entry is an equivalent
402      copy of an entity.
403    </t>
404  </list>
408<section title="Requirements" anchor="intro.requirements">
410   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
411   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
412   document are to be interpreted as described in <xref target="RFC2119"/>.
415   An implementation is not compliant if it fails to satisfy one or more
416   of the &MUST; or &REQUIRED; level requirements for the protocols it
417   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
418   level and all the &SHOULD; level requirements for its protocols is said
419   to be "unconditionally compliant"; one that satisfies all the &MUST;
420   level requirements but not all the &SHOULD; level requirements for its
421   protocols is said to be "conditionally compliant."
427<section title="Notational Conventions and Generic Grammar" anchor="notation">
428  <x:anchor-alias value="DIGIT"/>
429  <x:anchor-alias value="DQUOTE"/>
430  <x:anchor-alias value="quoted-string"/>
431  <x:anchor-alias value="SP"/>
432  <x:anchor-alias value="token"/>
433  <x:anchor-alias value="BWS"/>
434  <x:anchor-alias value="OWS"/>
435  <x:anchor-alias value="RWS"/>
437  This specification uses the ABNF syntax defined in &notation-abnf; and
438  the core rules defined in &basic-rules;:
439  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
440  <eref target=""/>.</cref>
442<figure><artwork type="abnf2616">
443  <x:ref>DIGIT</x:ref>         = &lt;DIGIT, defined in &basic-rules;&gt;
444  <x:ref>DQUOTE</x:ref>        = &lt;DQUOTE, defined in &basic-rules;&gt;
445  <x:ref>SP</x:ref>            = &lt;SP, defined in &basic-rules;&gt;
447<figure><artwork type="abnf2616">
448  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
449  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
450  <x:ref>BWS</x:ref>           = &lt;BWS, defined in &basic-rules;&gt;
451  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
452  <x:ref>RWS</x:ref>           = &lt;RWS, defined in &basic-rules;&gt;
454<t anchor="abnf.dependencies">
455  <x:anchor-alias value="field-name"/>
456  <x:anchor-alias value="HTTP-date"/>
457  <x:anchor-alias value="port"/>
458  <x:anchor-alias value="pseudonym"/>
459  <x:anchor-alias value="uri-host"/>
460  The ABNF rules below are defined in other parts:
462<figure><!--Part1--><artwork type="abnf2616">
463  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
464  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
465  <x:ref>port</x:ref>          = &lt;port, defined in &general-syntax;&gt;
466  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
467  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &general-syntax;&gt;
471<section title="Overview" anchor="caching.overview">
472<section title="Cache Correctness" anchor="cache.correctness">
474   A correct cache &MUST; respond to a request with the most up-to-date
475   response held by the cache that is appropriate to the request (see
476   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
477   <xref target="disambiguating.multiple.responses" format="counter"/>,
478   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
479   conditions:
480  <list style="numbers">
481      <t>It has been checked for equivalence with what the origin server
482         would have returned by revalidating the response with the
483         origin server (<xref target="validation.model"/>);</t>
485      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
486         this means it meets the least restrictive freshness requirement
487         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
488         the origin server so specifies, it is the freshness requirement
489         of the origin server alone.
491         If a stored response is not "fresh enough" by the most
492         restrictive freshness requirement of both the client and the
493         origin server, in carefully considered circumstances the cache
494         &MAY; still return the response with the appropriate Warning
495         header (see Sections <xref target="" format="counter"/>
496         and <xref target="header.warning" format="counter"/>), unless such a response
497         is prohibited (e.g., by a "no-store" cache-directive, or by a
498         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
500      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
501         or error (4xx or 5xx) response message.</t>
502  </list>
505   If the cache can not communicate with the origin server, then a
506   correct cache &SHOULD; respond as above if the response can be
507   correctly served from the cache; if not it &MUST; return an error or
508   warning indicating that there was a communication failure.
511   If a cache receives a response (either an entire response, or a 304
512   (Not Modified) response) that it would normally forward to the
513   requesting client, and the received response is no longer fresh, the
514   cache &SHOULD; forward it to the requesting client without adding a new
515   Warning (but without removing any existing Warning headers). A cache
516   &SHOULD-NOT;  attempt to revalidate a response simply because that
517   response became stale in transit; this might lead to an infinite
518   loop. A user agent that receives a stale response without a Warning
519   &MAY; display a warning indication to the user.
523<section title="Warnings" anchor="warnings">
525   Whenever a cache returns a response that is neither first-hand nor
526   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
527   &MUST; attach a warning to that effect, using a Warning general-header.
528   The Warning header and the currently defined warnings are described
529   in <xref target="header.warning"/>. The warning allows clients to take appropriate
530   action.
533   Warnings &MAY; be used for other purposes, both cache-related and
534   otherwise. The use of a warning, rather than an error status code,
535   distinguish these responses from true failures.
538   Warnings are assigned three digit warn-codes. The first digit
539   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
540   stored cache entry after a successful revalidation:
543  <list style="hanging">
544    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
545     the response, and so &MUST; be deleted after a successful
546     revalidation. 1xx warn-codes &MAY; be generated by a cache only when
547     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
549    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
550     headers that is not rectified by a revalidation (for example, a
551     lossy compression of the entity bodies) and which &MUST-NOT; be
552     deleted after a successful revalidation.</t>
553    </list>
556   See <xref target="header.warning"/> for the definitions of the codes themselves.
559   HTTP/1.0 caches will cache all Warnings in responses, without
560   deleting the ones in the first category. Warnings in responses that
561   are passed to HTTP/1.0 caches carry an extra warning-date field,
562   which prevents a future HTTP/1.1 recipient from believing an
563   erroneously cached Warning.
566   Warnings also carry a warning text. The text &MAY; be in any
567   appropriate natural language (perhaps based on the client's Accept
568   headers), and include an &OPTIONAL; indication of what character set is
569   used.
572   Multiple warnings &MAY; be attached to a response (either by the origin
573   server or by a cache), including multiple warnings with the same code
574   number. For example, a server might provide the same warning with
575   texts in both English and Basque.
578   When multiple warnings are attached to a response, it might not be
579   practical or reasonable to display all of them to the user. This
580   version of HTTP does not specify strict priority rules for deciding
581   which warnings to display and in what order, but does suggest some
582   heuristics.
586<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
588   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
589   times and validators) are implicit directives to caches. In some
590   cases, a server or client might need to provide explicit directives
591   to the HTTP caches. We use the Cache-Control header for this purpose.
594   The Cache-Control header allows a client or server to transmit a
595   variety of directives in either requests or responses. These
596   directives typically override the default caching algorithms. As a
597   general rule, if there is any apparent conflict between header
598   values, the most restrictive interpretation is applied (that is, the
599   one that is most likely to preserve semantic transparency). However,
600   in some cases, cache-control directives are explicitly specified as
601   weakening the approximation of semantic transparency (for example,
602   "max-stale" or "public").
605   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
609<section title="Explicit User Agent Warnings" anchor="">
611   Many user agents make it possible for users to override the basic
612   caching mechanisms. For example, the user agent might allow the user
613   to specify that cached entities (even explicitly stale ones) are
614   never validated. Or the user agent might habitually add "Cache-Control:
615   max-stale=3600" to every request. The user agent &SHOULD-NOT;
616   default to either non-transparent behavior, or behavior that results
617   in abnormally ineffective caching, but &MAY; be explicitly configured
618   to do so by an explicit action of the user.
621   If the user has overridden the basic caching mechanisms, the user
622   agent &SHOULD; explicitly indicate to the user whenever this results in
623   the display of information that might not meet the server's
624   transparency requirements (in particular, if the displayed entity is
625   known to be stale). Since the protocol normally allows the user agent
626   to determine if responses are stale or not, this indication need only
627   be displayed when this actually happens. The indication need not be a
628   dialog box; it could be an icon (for example, a picture of a rotting
629   fish) or some other indicator.
632   If the user has overridden the caching mechanisms in a way that would
633   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
634   continually indicate this state to the user (for example, by a
635   display of a picture of currency in flames) so that the user does not
636   inadvertently consume excess resources or suffer from excessive
637   latency.
641<section title="Exceptions to the Rules and Warnings" anchor="">
643   In some cases, the operator of a cache &MAY; choose to configure it to
644   return stale responses even when not requested by clients. This
645   decision ought not be made lightly, but may be necessary for reasons
646   of availability or performance, especially when the cache is poorly
647   connected to the origin server. Whenever a cache returns a stale
648   response, it &MUST; mark it as such (using a Warning header) enabling
649   the client software to alert the user that there might be a potential
650   problem.
653   It also allows the user agent to take steps to obtain a first-hand or
654   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
655   response if the client explicitly requests a first-hand or fresh one,
656   unless it is impossible to comply for technical or policy reasons.
660<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
662   While the origin server (and to a lesser extent, intermediate caches,
663   by their contribution to the age of a response) are the primary
664   source of expiration information, in some cases the client might need
665   to control a cache's decision about whether to return a cached
666   response without validating it. Clients do this using several
667   directives of the Cache-Control header.
670   A client's request &MAY; specify the maximum age it is willing to
671   accept of an unvalidated response; specifying a value of zero forces
672   the cache(s) to revalidate all responses. A client &MAY; also specify
673   the minimum time remaining before a response expires. Both of these
674   options increase constraints on the behavior of caches, and so cannot
675   further relax the cache's approximation of semantic transparency.
678   A client &MAY; also specify that it will accept stale responses, up to
679   some maximum amount of staleness. This loosens the constraints on the
680   caches, and so might violate the origin server's specified
681   constraints on semantic transparency, but might be necessary to
682   support disconnected operation, or high availability in the face of
683   poor connectivity.
688<section title="Expiration Model" anchor="expiration.model">
690<section title="Server-Specified Expiration" anchor="server-specified.expiration">
692   HTTP caching works best when caches can entirely avoid making
693   requests to the origin server. The primary mechanism for avoiding
694   requests is for an origin server to provide an explicit expiration
695   time in the future, indicating that a response &MAY; be used to satisfy
696   subsequent requests. In other words, a cache can return a fresh
697   response without first contacting the server.
700   Our expectation is that servers will assign future explicit
701   expiration times to responses in the belief that the entity is not
702   likely to change, in a semantically significant way, before the
703   expiration time is reached. This normally preserves semantic
704   transparency, as long as the server's expiration times are carefully
705   chosen.
708   The expiration mechanism applies only to responses taken from a cache
709   and not to first-hand responses forwarded immediately to the
710   requesting client.
713   If an origin server wishes to force a semantically transparent cache
714   to validate every request, it &MAY; assign an explicit expiration time
715   in the past. This means that the response is always stale, and so the
716   cache &SHOULD; validate it before using it for subsequent requests. See
717   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
720   If an origin server wishes to force any HTTP/1.1 cache, no matter how
721   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
722   cache-control directive (see <xref target="header.cache-control"/>).
725   Servers specify explicit expiration times using either the Expires
726   header, or the max-age directive of the Cache-Control header.
729   An expiration time cannot be used to force a user agent to refresh
730   its display or reload a resource; its semantics apply only to caching
731   mechanisms, and such mechanisms need only check a resource's
732   expiration status when a new request for that resource is initiated.
733   See <xref target="history.lists"/> for an explanation of the difference between caches
734   and history mechanisms.
738<section title="Heuristic Expiration" anchor="heuristic.expiration">
740   Since origin servers do not always provide explicit expiration times,
741   HTTP caches typically assign heuristic expiration times, employing
742   algorithms that use other header values (such as the Last-Modified
743   time) to estimate a plausible expiration time. The HTTP/1.1
744   specification does not provide specific algorithms, but does impose
745   worst-case constraints on their results. Since heuristic expiration
746   times might compromise semantic transparency, they ought to be used
747   cautiously, and we encourage origin servers to provide explicit
748   expiration times as much as possible.
752<section title="Age Calculations" anchor="age.calculations">
754   In order to know if a cached entry is fresh, a cache needs to know if
755   its age exceeds its freshness lifetime. We discuss how to calculate
756   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
757   the age of a response or cache entry.
760   In this discussion, we use the term "now" to mean "the current value
761   of the clock at the host performing the calculation." Hosts that use
762   HTTP, but especially hosts running origin servers and caches, &SHOULD;
763   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
764   a globally accurate time standard.
767   HTTP/1.1 requires origin servers to send a Date header, if possible,
768   with every response, giving the time at which the response was
769   generated (see &header-date;). We use the term "date_value" to denote
770   the value of the Date header, in a form appropriate for arithmetic
771   operations.
774   HTTP/1.1 uses the Age response-header to convey the estimated age of
775   the response message when obtained from a cache. The Age field value
776   is the cache's estimate of the amount of time since the response was
777   generated or revalidated by the origin server.
780   In essence, the Age value is the sum of the time that the response
781   has been resident in each of the caches along the path from the
782   origin server, plus the amount of time it has been in transit along
783   network paths.
786   We use the term "age_value" to denote the value of the Age header, in
787   a form appropriate for arithmetic operations.
790   A response's age can be calculated in two entirely independent ways:
791  <list style="numbers">
792      <t>now minus date_value, if the local clock is reasonably well
793         synchronized to the origin server's clock. If the result is
794         negative, the result is replaced by zero.</t>
796      <t>age_value, if all of the caches along the response path
797         implement HTTP/1.1.</t>
798  </list>
801   Given that we have two independent ways to compute the age of a
802   response when it is received, we can combine these as
804<figure><artwork type="code">
805    corrected_received_age = max(now - date_value, age_value)
808   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
809   paths, one gets a reliable (conservative) result.
812   Because of network-imposed delays, some significant interval might
813   pass between the time that a server generates a response and the time
814   it is received at the next outbound cache or client. If uncorrected,
815   this delay could result in improperly low ages.
818   Because the request that resulted in the returned Age value must have
819   been initiated prior to that Age value's generation, we can correct
820   for delays imposed by the network by recording the time at which the
821   request was initiated. Then, when an Age value is received, it &MUST;
822   be interpreted relative to the time the request was initiated, not
823   the time that the response was received. This algorithm results in
824   conservative behavior no matter how much delay is experienced. So, we
825   compute:
827<figure><artwork type="code">
828   corrected_initial_age = corrected_received_age
829                         + (now - request_time)
832   where "request_time" is the time (according to the local clock) when
833   the request that elicited this response was sent.
836   Summary of age calculation algorithm, when a cache receives a
837   response:
839<figure><artwork type="code">
840   /*
841    * age_value
842    *      is the value of Age: header received by the cache with
843    *              this response.
844    * date_value
845    *      is the value of the origin server's Date: header
846    * request_time
847    *      is the (local) time when the cache made the request
848    *              that resulted in this cached response
849    * response_time
850    *      is the (local) time when the cache received the
851    *              response
852    * now
853    *      is the current (local) time
854    */
856   apparent_age = max(0, response_time - date_value);
857   corrected_received_age = max(apparent_age, age_value);
858   response_delay = response_time - request_time;
859   corrected_initial_age = corrected_received_age + response_delay;
860   resident_time = now - response_time;
861   current_age   = corrected_initial_age + resident_time;
864   The current_age of a cache entry is calculated by adding the amount
865   of time (in seconds) since the cache entry was last validated by the
866   origin server to the corrected_initial_age. When a response is
867   generated from a cache entry, the cache &MUST; include a single Age
868   header field in the response with a value equal to the cache entry's
869   current_age.
872   The presence of an Age header field in a response implies that a
873   response is not first-hand. However, the converse is not true, since
874   the lack of an Age header field in a response does not imply that the
875   response is first-hand unless all caches along the request path are
876   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
877   the Age header field).
881<section title="Expiration Calculations" anchor="expiration.calculations">
883   In order to decide whether a response is fresh or stale, we need to
884   compare its freshness lifetime to its age. The age is calculated as
885   described in <xref target="age.calculations"/>; this section describes how to calculate
886   the freshness lifetime, and to determine if a response has expired.
887   In the discussion below, the values can be represented in any form
888   appropriate for arithmetic operations.
891   We use the term "expires_value" to denote the value of the Expires
892   header. We use the term "max_age_value" to denote an appropriate
893   value of the number of seconds carried by the "max-age" directive of
894   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
897   The max-age directive takes priority over Expires, so if max-age is
898   present in a response, the calculation is simply:
900<figure><artwork type="code">
901   freshness_lifetime = max_age_value
904   Otherwise, if Expires is present in the response, the calculation is:
906<figure><artwork type="code">
907   freshness_lifetime = expires_value - date_value
910   Note that neither of these calculations is vulnerable to clock skew,
911   since all of the information comes from the origin server.
914   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
915   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
916   does not include other restrictions on caching, the cache &MAY; compute
917   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
918   113 to any response whose age is more than 24 hours if such warning
919   has not already been added.
922   Also, if the response does have a Last-Modified time, the heuristic
923   expiration value &SHOULD; be no more than some fraction of the interval
924   since that time. A typical setting of this fraction might be 10%.
927   The calculation to determine if a response has expired is quite
928   simple:
930<figure><artwork type="code">
931   response_is_fresh = (freshness_lifetime &gt; current_age)
935<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
937   Because expiration values are assigned optimistically, it is possible
938   for two caches to contain fresh values for the same resource that are
939   different.
942   If a client performing a retrieval receives a non-first-hand response
943   for a request that was already fresh in its own cache, and the Date
944   header in its existing cache entry is newer than the Date on the new
945   response, then the client &MAY; ignore the response. If so, it &MAY;
946   retry the request with a "Cache-Control: max-age=0" directive (see
947   <xref target="header.cache-control"/>), to force a check with the origin server.
950   If a cache has two fresh responses for the same representation with
951   different validators, it &MUST; use the one with the more recent Date
952   header. This situation might arise because the cache is pooling
953   responses from other caches, or because a client has asked for a
954   reload or a revalidation of an apparently fresh cache entry.
958<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
960   Because a client might be receiving responses via multiple paths, so
961   that some responses flow through one set of caches and other
962   responses flow through a different set of caches, a client might
963   receive responses in an order different from that in which the origin
964   server sent them. We would like the client to use the most recently
965   generated response, even if older responses are still apparently
966   fresh.
969   Neither the entity tag nor the expiration value can impose an
970   ordering on responses, since it is possible that a later response
971   intentionally carries an earlier expiration time. The Date values are
972   ordered to a granularity of one second.
975   When a client tries to revalidate a cache entry, and the response it
976   receives contains a Date header that appears to be older than the one
977   for the existing entry, then the client &SHOULD; repeat the request
978   unconditionally, and include
980<figure><artwork type="example">
981    Cache-Control: max-age=0
984   to force any intermediate caches to validate their copies directly
985   with the origin server, or
987<figure><artwork type="example">
988    Cache-Control: no-cache
991   to force any intermediate caches to obtain a new copy from the origin
992   server.
995   If the Date values are equal, then the client &MAY; use either response
996   (or &MAY;, if it is being extremely prudent, request a new response).
997   Servers &MUST-NOT; depend on clients being able to choose
998   deterministically between responses generated during the same second,
999   if their expiration times overlap.
1004<section title="Validation Model" anchor="validation.model">
1006   When a cache has a stale entry that it would like to use as a
1007   response to a client's request, it first has to check with the origin
1008   server (or possibly an intermediate cache with a fresh response) to
1009   see if its cached entry is still usable. We call this "validating"
1010   the cache entry.
1013   HTTP's conditional request mechanism, defined in &conditional;, is
1014   used to avoid retransmitting the response payload when the cached entry
1015   is valid.  When a cached response includes one or more "cache validators,"
1016   such as the field values of an ETag or Last-Modified header field, then
1017   a validating GET request &SHOULD; be made conditional to those field values.
1018   The server checks the conditional request's validator against the current
1019   state of the requested resource and, if they match, the server responds
1020   with a 304 (Not Modified) status code to indicate that the cached response
1021   can be refreshed and reused without retransmitting the response payload.
1022   If the validator does not match the current state of the requested
1023   resource, then the server returns a full response, including payload,
1024   so that the request can be satisfied and the cache entry supplanted
1025   without the need for an additional network round-trip.
1029<section title="Response Cacheability" anchor="response.cacheability">
1031   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1032   directive, a caching system &MAY; always store a successful response
1033   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1034   if it is fresh, and &MAY; return it after successful validation. If
1035   there is neither a cache validator nor an explicit expiration time
1036   associated with a response, we do not expect it to be cached, but
1037   certain caches &MAY; violate this expectation (for example, when little
1038   or no network connectivity is available). A client can usually detect
1039   that such a response was taken from a cache by comparing the Date
1040   header to the current time.
1041  <list><t>
1042      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1043      without providing any Warning.
1044  </t></list>
1047   However, in some cases it might be inappropriate for a cache to
1048   retain an entity, or to return it in response to a subsequent
1049   request. This might be because absolute semantic transparency is
1050   deemed necessary by the service author, or because of security or
1051   privacy considerations. Certain cache-control directives are
1052   therefore provided so that the server can indicate that certain
1053   resource entities, or portions thereof, are not to be cached
1054   regardless of other considerations.
1057   Note that &header-authorization; normally prevents a shared cache from saving
1058   and returning a response to a previous request if that request
1059   included an Authorization header.
1062   A response received with a status code of 200, 203, 206, 300, 301 or
1063   410 &MAY; be stored by a cache and used in reply to a subsequent
1064   request, subject to the expiration mechanism, unless a cache-control
1065   directive prohibits caching. However, a cache that does not support
1066   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1067   Content) responses.
1070   A response received with any other status code (e.g. status codes 302
1071   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1072   unless there are cache-control directives or another header(s) that
1073   explicitly allow it. For example, these include the following: an
1074   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1075   "proxy-revalidate", "public" or "private" cache-control
1076   directive (<xref target="header.cache-control"/>).
1080<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1082   The purpose of an HTTP cache is to store information received in
1083   response to requests for use in responding to future requests. In
1084   many cases, a cache simply returns the appropriate parts of a
1085   response to the requester. However, if the cache holds a cache entry
1086   based on a previous response, it might have to combine parts of a new
1087   response with what is held in the cache entry.
1090<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1092   For the purpose of defining the behavior of caches and non-caching
1093   proxies, we divide HTTP headers into two categories:
1094  <list style="symbols">
1095      <t>End-to-end headers, which are  transmitted to the ultimate
1096        recipient of a request or response. End-to-end headers in
1097        responses &MUST; be stored as part of a cache entry and &MUST; be
1098        transmitted in any response formed from a cache entry.</t>
1100      <t>Hop-by-hop headers, which are meaningful only for a single
1101        transport-level connection, and are not stored by caches or
1102        forwarded by proxies.</t>
1103  </list>
1106   The following HTTP/1.1 headers are hop-by-hop headers:
1107  <list style="symbols">
1108      <t>Connection</t>
1109      <t>Keep-Alive</t>
1110      <t>Proxy-Authenticate</t>
1111      <t>Proxy-Authorization</t>
1112      <t>TE</t>
1113      <t>Trailer</t>
1114      <t>Transfer-Encoding</t>
1115      <t>Upgrade</t>
1116  </list>
1119   All other headers defined by HTTP/1.1 are end-to-end headers.
1122   Other hop-by-hop headers &MUST; be listed in a Connection header
1123   (&header-connection;).
1127<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1129   Some features of HTTP/1.1, such as Digest
1130   Authentication, depend on the value of certain end-to-end headers. A
1131   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1132   definition of that header requires or specifically allows that.
1135   A transparent proxy &MUST-NOT; modify any of the following fields in a
1136   request or response, and it &MUST-NOT; add any of these fields if not
1137   already present:
1138  <list style="symbols">
1139      <t>Content-Location</t>
1140      <t>Content-MD5</t>
1141      <t>ETag</t>
1142      <t>Last-Modified</t>
1143  </list>
1146   A transparent proxy &MUST-NOT; modify any of the following fields in a
1147   response:
1148  <list style="symbols">
1149    <t>Expires</t>
1150  </list>
1153   but it &MAY; add any of these fields if not already present. If an
1154   Expires header is added, it &MUST; be given a field-value identical to
1155   that of the Date header in that response.
1158   A  proxy &MUST-NOT; modify or add any of the following fields in a
1159   message that contains the no-transform cache-control directive, or in
1160   any request:
1161  <list style="symbols">
1162    <t>Content-Encoding</t>
1163    <t>Content-Range</t>
1164    <t>Content-Type</t>
1165  </list>
1168   A non-transparent proxy &MAY; modify or add these fields to a message
1169   that does not include no-transform, but if it does so, it &MUST; add a
1170   Warning 214 (Transformation applied) if one does not already appear
1171   in the message (see <xref target="header.warning"/>).
1172  <list><t>
1173      Warning: unnecessary modification of end-to-end headers might
1174      cause authentication failures if stronger authentication
1175      mechanisms are introduced in later versions of HTTP. Such
1176      authentication mechanisms &MAY; rely on the values of header fields
1177      not listed here.
1178    </t></list>
1181   The Content-Length field of a request or response is added or deleted
1182   according to the rules in &message-length;. A transparent proxy &MUST;
1183   preserve the entity-length (&entity-length;) of the entity-body,
1184   although it &MAY; change the transfer-length (&message-length;).
1188<section title="Combining Headers" anchor="combining.headers">
1190   When a cache makes a validating request to a server, and the server
1191   provides a 304 (Not Modified) response or a 206 (Partial Content)
1192   response, the cache then constructs a response to send to the
1193   requesting client.
1196   If the status code is 304 (Not Modified), the cache uses the entity-body
1197   stored in the cache entry as the entity-body of this outgoing
1198   response. If the status code is 206 (Partial Content) and the ETag or
1199   Last-Modified headers match exactly, the cache &MAY; combine the
1200   contents stored in the cache entry with the new contents received in
1201   the response and use the result as the entity-body of this outgoing
1202   response, (see &combining-byte-ranges;).
1205   The end-to-end headers stored in the cache entry are used for the
1206   constructed response, except that
1207  <list style="symbols">
1208    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1209      &MUST; be deleted from the cache entry and the forwarded response.</t>
1210    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1211        in the cache entry and the forwarded response.</t>
1212    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1213        replace the corresponding headers from the cache entry.</t>
1214  </list>
1217   Unless the cache decides to remove the cache entry, it &MUST; also
1218   replace the end-to-end headers stored with the cache entry with
1219   corresponding headers received in the incoming response, except for
1220   Warning headers as described immediately above. If a header field-name
1221   in the incoming response matches more than one header in the
1222   cache entry, all such old headers &MUST; be replaced.
1225   In other words, the set of end-to-end headers received in the
1226   incoming response overrides all corresponding end-to-end headers
1227   stored with the cache entry (except for stored Warning headers with
1228   warn-code 1xx, which are deleted even if not overridden).
1229  <list><t>
1230      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1231      Modified) or a 206 (Partial Content) response to update any header
1232      associated with a previous response for the same entity or sub-ranges
1233      thereof, although it might not always be meaningful or
1234      correct to do so. This rule does not allow an origin server to use
1235      a 304 (Not Modified) or a 206 (Partial Content) response to
1236      entirely delete a header that it had provided with a previous
1237      response.
1238  </t></list>
1244<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1246   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1247   by the presence of a Vary header field in a response, alters the
1248   conditions and procedure by which a cache can use the response for
1249   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1250   field by servers.
1253   A server &SHOULD; use the Vary header field to inform a cache of what
1254   request-header fields were used to select among multiple
1255   representations of a cacheable response subject to server-driven
1256   negotiation. The set of header fields named by the Vary field value
1257   is known as the "selecting" request-headers.
1260   When the cache receives a subsequent request whose Request-URI
1261   specifies one or more cache entries including a Vary header field,
1262   the cache &MUST-NOT; use such a cache entry to construct a response to
1263   the new request unless all of the selecting request-headers present
1264   in the new request match the corresponding stored request-headers in
1265   the original request.
1268   The selecting request-headers from two requests are defined to match
1269   if and only if the selecting request-headers in the first request can
1270   be transformed to the selecting request-headers in the second request
1271   by adding or removing linear white space (LWS) at places where this
1272   is allowed by the corresponding BNF, and/or combining multiple
1273   message-header fields with the same field name following the rules
1274   about message headers in &message-headers;.
1277   A Vary header field-value of "*" always fails to match and subsequent
1278   requests on that resource can only be properly interpreted by the
1279   origin server.
1282   If the selecting request header fields for the cached entry do not
1283   match the selecting request header fields of the new request, then
1284   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1285   it first relays the new request to the origin server in a conditional
1286   request and the server responds with 304 (Not Modified), including an
1287   entity tag or Content-Location that indicates the entity to be used.
1290   If an entity tag was assigned to a cached representation, the
1291   forwarded request &SHOULD; be conditional and include the entity tags
1292   in an If-None-Match header field from all its cache entries for the
1293   resource. This conveys to the server the set of entities currently
1294   held by the cache, so that if any one of these entities matches the
1295   requested entity, the server can use the ETag header field in its 304
1296   (Not Modified) response to tell the cache which entry is appropriate.
1297   If the entity-tag of the new response matches that of an existing
1298   entry, the new response &SHOULD; be used to update the header fields of
1299   the existing entry, and the result &MUST; be returned to the client.
1302   If any of the existing cache entries contains only partial content
1303   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1304   the If-None-Match header field unless the request is for a range that
1305   would be fully satisfied by that entry.
1308   If a cache receives a successful response whose Content-Location
1309   field matches that of an existing cache entry for the same Request-URI,
1310   whose entity-tag differs from that of the existing entry, and
1311   whose Date is more recent than that of the existing entry, the
1312   existing entry &SHOULD-NOT;  be returned in response to future requests
1313   and &SHOULD; be deleted from the cache.
1317<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1319   For reasons of security and privacy, it is necessary to make a
1320   distinction between "shared" and "non-shared" caches. A non-shared
1321   cache is one that is accessible only to a single user. Accessibility
1322   in this case &SHOULD; be enforced by appropriate security mechanisms.
1323   All other caches are considered to be "shared." Other sections of
1324   this specification place certain constraints on the operation of
1325   shared caches in order to prevent loss of privacy or failure of
1326   access controls.
1330<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1332   A cache that receives an incomplete response (for example, with fewer
1333   bytes of data than specified in a Content-Length header) &MAY; store
1334   the response. However, the cache &MUST; treat this as a partial
1335   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1336   the result might be a full response or might still be
1337   partial. A cache &MUST-NOT; return a partial response to a client
1338   without explicitly marking it as such, using the 206 (Partial
1339   Content) status code. A cache &MUST-NOT; return a partial response
1340   using a status code of 200 (OK).
1343   If a cache receives a 5xx response while attempting to revalidate an
1344   entry, it &MAY; either forward this response to the requesting client,
1345   or act as if the server failed to respond. In the latter case, it &MAY;
1346   return a previously received response unless the cached entry
1347   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1351<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1353   Unless the origin server explicitly prohibits the caching of their
1354   responses, the application of GET and HEAD methods to any resources
1355   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1356   these responses are taken from a cache. They &MAY; still have side
1357   effects, but a cache is not required to consider such side effects in
1358   its caching decisions. Caches are always expected to observe an
1359   origin server's explicit restrictions on caching.
1362   We note one exception to this rule: since some applications have
1363   traditionally used GET and HEAD requests with URLs containing a query part
1364   to perform operations with significant side
1365   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1366   the server provides an explicit expiration time. This specifically
1367   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT;
1368   be taken from a cache. See &safe-methods; for related information.
1372<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1374   The effect of certain methods performed on a resource at the origin
1375   server might cause one or more existing cache entries to become non-transparently
1376   invalid. That is, although they might continue to be
1377   "fresh," they do not accurately reflect what the origin server would
1378   return for a new request on that resource.
1381   There is no way for HTTP to guarantee that all such
1382   cache entries are marked invalid. For example, the request that
1383   caused the change at the origin server might not have gone through
1384   the proxy where a cache entry is stored. However, several rules help
1385   reduce the likelihood of erroneous behavior.
1388   In this section, the phrase "invalidate an entity" means that the
1389   cache will either remove all instances of that entity from its
1390   storage, or will mark these as "invalid" and in need of a mandatory
1391   revalidation before they can be returned in response to a subsequent
1392   request.
1395   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1396   either the entity referred to by the Request-URI, or by the Location
1397   or Content-Location headers (if present). These methods are:
1398  <list style="symbols">
1399      <t>PUT</t>
1400      <t>DELETE</t>
1401      <t>POST</t>
1402  </list>
1405   An invalidation based
1406   on the URI in a Location or Content-Location header &MUST-NOT; be
1407   performed if the host part of that URI differs from the host part
1408   in the Request-URI. This helps prevent denial of service attacks.
1411   A cache that passes through requests for methods it does not
1412   understand &SHOULD; invalidate any entities referred to by the
1413   Request-URI.
1417<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1419   All methods that might be expected to cause modifications to the
1420   origin server's resources &MUST; be written through to the origin
1421   server. This currently includes all methods except for GET and HEAD.
1422   A cache &MUST-NOT; reply to such a request from a client before having
1423   transmitted the request to the inbound server, and having received a
1424   corresponding response from the inbound server. This does not prevent
1425   a proxy cache from sending a 100 (Continue) response before the
1426   inbound server has sent its final reply.
1429   The alternative (known as "write-back" or "copy-back" caching) is not
1430   allowed in HTTP/1.1, due to the difficulty of providing consistent
1431   updates and the problems arising from server, cache, or network
1432   failure prior to write-back.
1436<section title="Cache Replacement" anchor="cache.replacement">
1438   If a new cacheable (see Sections <xref target="" format="counter"/>,
1439   <xref target="disambiguating.expiration.values" format="counter"/>,
1440   <xref target="disambiguating.multiple.responses" format="counter"/>
1441   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1442   response is received from a resource while any existing responses for
1443   the same resource are cached, the cache &SHOULD; use the new response
1444   to reply to the current request. It &MAY; insert it into cache storage
1445   and &MAY;, if it meets all other requirements, use it to respond to any
1446   future requests that would previously have caused the old response to
1447   be returned. If it inserts the new response into cache storage  the
1448   rules in <xref target="combining.headers"/> apply.
1449  <list><t>
1450      <x:h>Note:</x:h> a new response that has an older Date header value than
1451      existing cached responses is not cacheable.
1452  </t></list>
1456<section title="History Lists" anchor="history.lists">
1458   User agents often have history mechanisms, such as "Back" buttons and
1459   history lists, which can be used to redisplay an entity retrieved
1460   earlier in a session.
1463   History mechanisms and caches are different. In particular history
1464   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1465   the current state of a resource. Rather, a history mechanism is meant
1466   to show exactly what the user saw at the time when the resource was
1467   retrieved.
1470   By default, an expiration time does not apply to history mechanisms.
1471   If the entity is still in storage, a history mechanism &SHOULD; display
1472   it even if the entity has expired, unless the user has specifically
1473   configured the agent to refresh expired history documents.
1476   This is not to be construed to prohibit the history mechanism from
1477   telling the user that a view might be stale.
1478  <list><t>
1479      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1480      viewing stale resources, this will tend to force service authors
1481      to avoid using HTTP expiration controls and cache controls when
1482      they would otherwise like to. Service authors may consider it
1483      important that users not be presented with error messages or
1484      warning messages when they use navigation controls (such as BACK)
1485      to view previously fetched resources. Even though sometimes such
1486      resources ought not be cached, or ought to expire quickly, user
1487      interface considerations may force service authors to resort to
1488      other means of preventing caching (e.g. "once-only" URLs) in order
1489      not to suffer the effects of improperly functioning history
1490      mechanisms.
1491  </t></list>
1495<section title="Header Field Definitions" anchor="header.fields">
1497   This section defines the syntax and semantics of HTTP/1.1 header fields
1498   related to caching.
1501   For entity-header fields, both sender and recipient refer to either the
1502   client or the server, depending on who sends and who receives the entity.
1505<section title="Age" anchor="header.age">
1506  <iref primary="true" item="Age header" x:for-anchor=""/>
1507  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1508  <x:anchor-alias value="Age"/>
1509  <x:anchor-alias value="Age-v"/>
1510  <x:anchor-alias value="age-value"/>
1512      The response-header field "Age" conveys the sender's estimate of the
1513      amount of time since the response (or its revalidation) was
1514      generated at the origin server. A cached response is "fresh" if
1515      its age does not exceed its freshness lifetime. Age values are
1516      calculated as specified in <xref target="age.calculations"/>.
1518<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
1519  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
1520  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
1522<t anchor="">
1523  <x:anchor-alias value="delta-seconds"/>
1524      Age values are non-negative decimal integers, representing time in
1525      seconds.
1527<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
1528  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
1531      If a cache receives a value larger than the largest positive
1532      integer it can represent, or if any of its age calculations
1533      overflows, it &MUST; transmit an Age header with a value of
1534      2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache &MUST;
1535      include an Age header field in every response generated from its
1536      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1537      bits of range.
1541<section title="Cache-Control" anchor="header.cache-control">
1542  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1543  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1544  <x:anchor-alias value="Cache-Control"/>
1545  <x:anchor-alias value="Cache-Control-v"/>
1546  <x:anchor-alias value="cache-directive"/>
1547  <x:anchor-alias value="cache-extension"/>
1548  <x:anchor-alias value="cache-request-directive"/>
1549  <x:anchor-alias value="cache-response-directive"/>
1551   The general-header field "Cache-Control" is used to specify directives
1552   that &MUST; be obeyed by all caching mechanisms along the
1553   request/response chain. The directives specify behavior intended to
1554   prevent caches from adversely interfering with the request or
1555   response. These directives typically override the default caching
1556   algorithms. Cache directives are unidirectional in that the presence
1557   of a directive in a request does not imply that the same directive is
1558   to be given in the response.
1559  <list><t>
1560      Note that HTTP/1.0 caches might not implement Cache-Control and
1561      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1562  </t></list>
1565   Cache directives &MUST; be passed through by a proxy or gateway
1566   application, regardless of their significance to that application,
1567   since the directives might be applicable to all recipients along the
1568   request/response chain. It is not possible to specify a cache-directive
1569   for a specific cache.
1571<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-directive"/><iref primary="true" item="Grammar" subitem="cache-request-directive"/><iref primary="true" item="Grammar" subitem="cache-response-directive"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1572  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1573  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1575  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1576     / <x:ref>cache-response-directive</x:ref>
1578  <x:ref>cache-request-directive</x:ref> =
1579       "no-cache"                          ; <xref target=""/>
1580     / "no-store"                          ; <xref target=""/>
1581     / "max-age" "=" <x:ref>delta-seconds</x:ref>         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1582     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1583     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1584     / "no-transform"                      ; <xref target="no-transform.directive"/>
1585     / "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1586     / <x:ref>cache-extension</x:ref>                     ; <xref target="cache.control.extensions"/>
1588  <x:ref>cache-response-directive</x:ref> =
1589       "public"                               ; <xref target=""/>
1590     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target=""/>
1591     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target=""/>
1592     / "no-store"                             ; <xref target=""/>
1593     / "no-transform"                         ; <xref target="no-transform.directive"/>
1594     / "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1595     / "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1596     / "max-age" "=" <x:ref>delta-seconds</x:ref>            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1597     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1598     / <x:ref>cache-extension</x:ref>                        ; <xref target="cache.control.extensions"/>
1600  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1603   When a directive appears without any 1#field-name parameter, the
1604   directive applies to the entire request or response. When such a
1605   directive appears with a 1#field-name parameter, it applies only to
1606   the named field or fields, and not to the rest of the request or
1607   response. This mechanism supports extensibility; implementations of
1608   future versions of HTTP might apply these directives to
1609   header fields not defined in HTTP/1.1.
1612   The cache-control directives can be broken down into these general
1613   categories:
1614  <list style="symbols">
1615     <t>Restrictions on what are cacheable; these may only be imposed by
1616        the origin server.</t>
1618     <t>Restrictions on what may be stored by a cache; these may be
1619        imposed by either the origin server or the user agent.</t>
1621     <t>Modifications of the basic expiration mechanism; these may be
1622        imposed by either the origin server or the user agent.</t>
1624     <t>Controls over cache revalidation and reload; these may only be
1625        imposed by a user agent.</t>
1627     <t>Control over transformation of entities.</t>
1629     <t>Extensions to the caching system.</t>
1630  </list>
1633<section title="What is Cacheable" anchor="">
1635   By default, a response is cacheable if the requirements of the
1636   request method, request header fields, and the response status
1637   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1638   for cacheability. The following Cache-Control response directives
1639   allow an origin server to override the default cacheability of a
1640   response:
1643  <iref item="Cache Directives" subitem="public" primary="true"/>
1644  <iref item="public" subitem="Cache Directive" primary="true"/>
1645   public
1646  <list><t>
1647      Indicates that the response &MAY; be cached by any cache, even if it
1648      would normally be non-cacheable or cacheable only within a non-shared
1649      cache. (See also Authorization, &header-authorization;, for
1650      additional details.)
1651  </t></list>
1654  <iref item="Cache Directives" subitem="private" primary="true"/>
1655  <iref item="private" subitem="Cache Directive" primary="true"/>
1656   private
1657  <list><t>
1658      Indicates that all or part of the response message is intended for
1659      a single user and &MUST-NOT; be cached by a shared cache. This
1660      allows an origin server to state that the specified parts of the
1661      response are intended for only one user and are not a valid
1662      response for requests by other users. A private (non-shared) cache
1663      &MAY; cache the response.
1664    </t><t>
1665       <x:h>Note:</x:h> This usage of the word private only controls where the
1666       response may be cached, and cannot ensure the privacy of the
1667       message content.
1668  </t></list>
1671  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1672  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1673   no-cache
1674  <list><t>
1675       If the no-cache directive does not specify a field-name, then a
1676      cache &MUST-NOT; use the response to satisfy a subsequent request
1677      without successful revalidation with the origin server. This
1678      allows an origin server to prevent caching even by caches that
1679      have been configured to return stale responses to client requests.
1680    </t><t>
1681      If the no-cache directive does specify one or more field-names,
1682      then a cache &MAY; use the response to satisfy a subsequent request,
1683      subject to any other restrictions on caching. However, the
1684      specified field-name(s) &MUST-NOT; be sent in the response to a
1685      subsequent request without successful revalidation with the origin
1686      server. This allows an origin server to prevent the re-use of
1687      certain header fields in a response, while still allowing caching
1688      of the rest of the response.
1689    <list><t>
1690       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1691       directive.
1692    </t></list>
1693  </t></list>
1697<section title="What May be Stored by Caches" anchor="">
1699  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1700  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1701   no-store
1702  <list><t>   
1703      The purpose of the no-store directive is to prevent the
1704      inadvertent release or retention of sensitive information (for
1705      example, on backup tapes). The no-store directive applies to the
1706      entire message, and &MAY; be sent either in a response or in a
1707      request. If sent in a request, a cache &MUST-NOT; store any part of
1708      either this request or any response to it. If sent in a response,
1709      a cache &MUST-NOT; store any part of either this response or the
1710      request that elicited it. This directive applies to both non-shared
1711      and shared caches. "&MUST-NOT; store" in this context means
1712      that the cache &MUST-NOT; intentionally store the information in
1713      non-volatile storage, and &MUST; make a best-effort attempt to
1714      remove the information from volatile storage as promptly as
1715      possible after forwarding it.
1716  </t><t>
1717      Even when this directive is associated with a response, users
1718      might explicitly store such a response outside of the caching
1719      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1720      such responses as part of their normal operation.
1721  </t><t>
1722      The purpose of this directive is to meet the stated requirements
1723      of certain users and service authors who are concerned about
1724      accidental releases of information via unanticipated accesses to
1725      cache data structures. While the use of this directive might
1726      improve privacy in some cases, we caution that it is NOT in any
1727      way a reliable or sufficient mechanism for ensuring privacy. In
1728      particular, malicious or compromised caches might not recognize or
1729      obey this directive, and communications networks might be
1730      vulnerable to eavesdropping.
1731  </t></list>
1735<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1737   The expiration time of an entity &MAY; be specified by the origin
1738   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1739   it &MAY; be specified using the max-age directive in a response. When
1740   the max-age cache-control directive is present in a cached response,
1741   the response is stale if its current age is greater than the age
1742   value given (in seconds) at the time of a new request for that
1743   resource. The max-age directive on a response implies that the
1744   response is cacheable (i.e., "public") unless some other, more
1745   restrictive cache directive is also present.
1748   If a response includes both an Expires header and a max-age
1749   directive, the max-age directive overrides the Expires header, even
1750   if the Expires header is more restrictive. This rule allows an origin
1751   server to provide, for a given response, a longer expiration time to
1752   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1753   useful if certain HTTP/1.0 caches improperly calculate ages or
1754   expiration times, perhaps due to desynchronized clocks.
1757   Many HTTP/1.0 cache implementations will treat an Expires value that
1758   is less than or equal to the response Date value as being equivalent
1759   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1760   cache receives such a response, and the response does not include a
1761   Cache-Control header field, it &SHOULD; consider the response to be
1762   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1763  <list><t>
1764       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1765       cache control feature, such as the "private" directive, on a
1766       network including older caches that do not understand that
1767       feature. The origin server will need to combine the new feature
1768       with an Expires field whose value is less than or equal to the
1769       Date value. This will prevent older caches from improperly
1770       caching the response.
1771  </t></list>
1774  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1775  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1776   s-maxage
1777  <list><t>
1778       If a response includes an s-maxage directive, then for a shared
1779       cache (but not for a private cache), the maximum age specified by
1780       this directive overrides the maximum age specified by either the
1781       max-age directive or the Expires header. The s-maxage directive
1782       also implies the semantics of the proxy-revalidate directive (see
1783       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1784       entry after it becomes stale to respond to a subsequent request
1785       without first revalidating it with the origin server. The s-maxage
1786       directive is always ignored by a private cache.
1787  </t></list>
1790   Note that most older caches, not compliant with this specification,
1791   do not implement any cache-control directives. An origin server
1792   wishing to use a cache-control directive that restricts, but does not
1793   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1794   requirement that the max-age directive overrides the Expires header,
1795   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1796   max-age directive.
1799   Other directives allow a user agent to modify the basic expiration
1800   mechanism. These directives &MAY; be specified on a request:
1803  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1804  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1805   max-age
1806  <list><t>
1807      Indicates that the client is willing to accept a response whose
1808      age is no greater than the specified time in seconds. Unless max-stale
1809      directive is also included, the client is not willing to
1810      accept a stale response.
1811  </t></list>
1814  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1815  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1816   min-fresh
1817  <list><t>
1818      Indicates that the client is willing to accept a response whose
1819      freshness lifetime is no less than its current age plus the
1820      specified time in seconds. That is, the client wants a response
1821      that will still be fresh for at least the specified number of
1822      seconds.
1823  </t></list>
1826  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1827  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1828   max-stale
1829  <list><t>
1830      Indicates that the client is willing to accept a response that has
1831      exceeded its expiration time. If max-stale is assigned a value,
1832      then the client is willing to accept a response that has exceeded
1833      its expiration time by no more than the specified number of
1834      seconds. If no value is assigned to max-stale, then the client is
1835      willing to accept a stale response of any age.
1836  </t></list>
1839   If a cache returns a stale response, either because of a max-stale
1840   directive on a request, or because the cache is configured to
1841   override the expiration time of a response, the cache &MUST; attach a
1842   Warning header to the stale response, using Warning 110 (Response is
1843   stale).
1846   A cache &MAY; be configured to return stale responses without
1847   validation, but only if this does not conflict with any "MUST"-level
1848   requirements concerning cache validation (e.g., a "must-revalidate"
1849   cache-control directive).
1852   If both the new request and the cached entry include "max-age"
1853   directives, then the lesser of the two values is used for determining
1854   the freshness of the cached entry for that request.
1858<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1860   Sometimes a user agent might want or need to insist that a cache
1861   revalidate its cache entry with the origin server (and not just with
1862   the next cache along the path to the origin server), or to reload its
1863   cache entry from the origin server. End-to-end revalidation might be
1864   necessary if either the cache or the origin server has overestimated
1865   the expiration time of the cached response. End-to-end reload may be
1866   necessary if the cache entry has become corrupted for some reason.
1869   End-to-end revalidation may be requested either when the client does
1870   not have its own local cached copy, in which case we call it
1871   "unspecified end-to-end revalidation", or when the client does have a
1872   local cached copy, in which case we call it "specific end-to-end
1873   revalidation."
1876   The client can specify these three kinds of action using Cache-Control
1877   request directives:
1880   End-to-end reload
1881  <list><t>
1882      The request includes a "no-cache" cache-control directive or, for
1883      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1884      names &MUST-NOT; be included with the no-cache directive in a
1885      request. The server &MUST-NOT; use a cached copy when responding to
1886      such a request.
1887  </t></list>
1890   Specific end-to-end revalidation
1891  <list><t>
1892      The request includes a "max-age=0" cache-control directive, which
1893      forces each cache along the path to the origin server to
1894      revalidate its own entry, if any, with the next cache or server.
1895      The initial request includes a cache-validating conditional with
1896      the client's current validator.
1897  </t></list>
1900   Unspecified end-to-end revalidation
1901  <list><t>
1902      The request includes "max-age=0" cache-control directive, which
1903      forces each cache along the path to the origin server to
1904      revalidate its own entry, if any, with the next cache or server.
1905      The initial request does not include a cache-validating
1906      conditional; the first cache along the path (if any) that holds a
1907      cache entry for this resource includes a cache-validating
1908      conditional with its current validator.
1909  </t></list>
1912  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1913  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1914   max-age
1915  <list><t>
1916      When an intermediate cache is forced, by means of a max-age=0
1917      directive, to revalidate its own cache entry, and the client has
1918      supplied its own validator in the request, the supplied validator
1919      might differ from the validator currently stored with the cache
1920      entry. In this case, the cache &MAY; use either validator in making
1921      its own request without affecting semantic transparency.
1922  </t><t>
1923      However, the choice of validator might affect performance. The
1924      best approach is for the intermediate cache to use its own
1925      validator when making its request. If the server replies with 304
1926      (Not Modified), then the cache can return its now validated copy
1927      to the client with a 200 (OK) response. If the server replies with
1928      a new entity and cache validator, however, the intermediate cache
1929      can compare the returned validator with the one provided in the
1930      client's request, using the strong comparison function. If the
1931      client's validator is equal to the origin server's, then the
1932      intermediate cache simply returns 304 (Not Modified). Otherwise,
1933      it returns the new entity with a 200 (OK) response.
1934  </t><t>
1935      If a request includes the no-cache directive, it &SHOULD-NOT;
1936      include min-fresh, max-stale, or max-age.
1937  </t></list>
1940  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1941  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1942   only-if-cached
1943  <list><t>
1944      In some cases, such as times of extremely poor network
1945      connectivity, a client may want a cache to return only those
1946      responses that it currently has stored, and not to reload or
1947      revalidate with the origin server. To do this, the client may
1948      include the only-if-cached directive in a request. If it receives
1949      this directive, a cache &SHOULD; either respond using a cached entry
1950      that is consistent with the other constraints of the request, or
1951      respond with a 504 (Gateway Timeout) status. However, if a group
1952      of caches is being operated as a unified system with good internal
1953      connectivity, such a request &MAY; be forwarded within that group of
1954      caches.
1955  </t></list>
1958  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1959  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1960   must-revalidate
1961  <list><t>
1962      Because a cache &MAY; be configured to ignore a server's specified
1963      expiration time, and because a client request &MAY; include a max-stale
1964      directive (which has a similar effect), the protocol also
1965      includes a mechanism for the origin server to require revalidation
1966      of a cache entry on any subsequent use. When the must-revalidate
1967      directive is present in a response received by a cache, that cache
1968      &MUST-NOT; use the entry after it becomes stale to respond to a
1969      subsequent request without first revalidating it with the origin
1970      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1971      time, if, based solely on the origin server's Expires or max-age
1972      value, the cached response is stale.)
1973  </t><t>
1974      The must-revalidate directive is necessary to support reliable
1975      operation for certain protocol features. In all circumstances an
1976      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1977      particular, if the cache cannot reach the origin server for any
1978      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1979  </t><t>
1980      Servers &SHOULD; send the must-revalidate directive if and only if
1981      failure to revalidate a request on the entity could result in
1982      incorrect operation, such as a silently unexecuted financial
1983      transaction. Recipients &MUST-NOT; take any automated action that
1984      violates this directive, and &MUST-NOT; automatically provide an
1985      unvalidated copy of the entity if revalidation fails.
1986  </t><t>
1987      Although this is not recommended, user agents operating under
1988      severe connectivity constraints &MAY; violate this directive but, if
1989      so, &MUST; explicitly warn the user that an unvalidated response has
1990      been provided. The warning &MUST; be provided on each unvalidated
1991      access, and &SHOULD; require explicit user confirmation.
1992  </t></list>
1995  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1996  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
1997   proxy-revalidate
1998  <list><t>
1999      The proxy-revalidate directive has the same meaning as the must-revalidate
2000      directive, except that it does not apply to non-shared
2001      user agent caches. It can be used on a response to an
2002      authenticated request to permit the user's cache to store and
2003      later return the response without needing to revalidate it (since
2004      it has already been authenticated once by that user), while still
2005      requiring proxies that service many users to revalidate each time
2006      (in order to make sure that each user has been authenticated).
2007      Note that such authenticated responses also need the public cache
2008      control directive in order to allow them to be cached at all.
2009  </t></list>
2013<section title="No-Transform Directive" anchor="no-transform.directive">
2015  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2016  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2017   no-transform
2018  <list><t>
2019      Implementors of intermediate caches (proxies) have found it useful
2020      to convert the media type of certain entity bodies. A non-transparent
2021      proxy might, for example, convert between image
2022      formats in order to save cache space or to reduce the amount of
2023      traffic on a slow link.
2024  </t><t>
2025      Serious operational problems occur, however, when these
2026      transformations are applied to entity bodies intended for certain
2027      kinds of applications. For example, applications for medical
2028      imaging, scientific data analysis and those using end-to-end
2029      authentication, all depend on receiving an entity body that is bit
2030      for bit identical to the original entity-body.
2031  </t><t>
2032      Therefore, if a message includes the no-transform directive, an
2033      intermediate cache or proxy &MUST-NOT; change those headers that are
2034      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2035      directive. This implies that the cache or proxy &MUST-NOT; change
2036      any aspect of the entity-body that is specified by these headers,
2037      including the value of the entity-body itself.
2038  </t></list>
2042<section title="Cache Control Extensions" anchor="cache.control.extensions">
2044   The Cache-Control header field can be extended through the use of one
2045   or more cache-extension tokens, each with an optional assigned value.
2046   Informational extensions (those which do not require a change in
2047   cache behavior) &MAY; be added without changing the semantics of other
2048   directives. Behavioral extensions are designed to work by acting as
2049   modifiers to the existing base of cache directives. Both the new
2050   directive and the standard directive are supplied, such that
2051   applications which do not understand the new directive will default
2052   to the behavior specified by the standard directive, and those that
2053   understand the new directive will recognize it as modifying the
2054   requirements associated with the standard directive. In this way,
2055   extensions to the cache-control directives can be made without
2056   requiring changes to the base protocol.
2059   This extension mechanism depends on an HTTP cache obeying all of the
2060   cache-control directives defined for its native HTTP-version, obeying
2061   certain extensions, and ignoring all directives that it does not
2062   understand.
2065   For example, consider a hypothetical new response directive called
2066   community which acts as a modifier to the private directive. We
2067   define this new directive to mean that, in addition to any non-shared
2068   cache, any cache which is shared only by members of the community
2069   named within its value may cache the response. An origin server
2070   wishing to allow the UCI community to use an otherwise private
2071   response in their shared cache(s) could do so by including
2073<figure><artwork type="example">
2074    Cache-Control: private, community="UCI"
2077   A cache seeing this header field will act correctly even if the cache
2078   does not understand the community cache-extension, since it will also
2079   see and understand the private directive and thus default to the safe
2080   behavior.
2083   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2084   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2085   be combined with standard directives (or the response's default
2086   cacheability) such that the cache behavior will remain minimally
2087   correct even if the cache does not understand the extension(s).
2092<section title="Expires" anchor="header.expires">
2093  <iref primary="true" item="Expires header" x:for-anchor=""/>
2094  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2095  <x:anchor-alias value="Expires"/>
2096  <x:anchor-alias value="Expires-v"/>
2098   The Expires entity-header field gives the date/time after which the
2099   response is considered stale. A stale cache entry may not normally be
2100   returned by a cache (either a proxy cache or a user agent cache)
2101   unless it is first validated with the origin server (or with an
2102   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2103   for further discussion of the expiration model.
2106   The presence of an Expires field does not imply that the original
2107   resource will change or cease to exist at, before, or after that
2108   time.
2111   The format is an absolute date and time as defined by HTTP-date in
2112   &full-date;; it &MUST; be sent in rfc1123-date format.
2114<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
2115  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
2116  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
2119   An example of its use is
2121<figure><artwork type="example">
2122  Expires: Thu, 01 Dec 1994 16:00:00 GMT
2125  <list><t>
2126      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2127      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2128      Expires field.
2129  </t></list>
2132   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2133   especially including the value "0", as in the past (i.e., "already
2134   expired").
2137   To mark a response as "already expired," an origin server sends an
2138   Expires date that is equal to the Date header value. (See the rules
2139   for expiration calculations in <xref target="expiration.calculations"/>.)
2142   To mark a response as "never expires," an origin server sends an
2143   Expires date approximately one year from the time the response is
2144   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2145   year in the future.
2148   The presence of an Expires header field with a date value of some
2149   time in the future on a response that otherwise would by default be
2150   non-cacheable indicates that the response is cacheable, unless
2151   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2155<section title="Pragma" anchor="header.pragma">
2156  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2157  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2158  <x:anchor-alias value="extension-pragma"/>
2159  <x:anchor-alias value="Pragma"/>
2160  <x:anchor-alias value="Pragma-v"/>
2161  <x:anchor-alias value="pragma-directive"/>
2163   The general-header field "Pragma" is used to include implementation-specific
2164   directives that might apply to any recipient along the
2165   request/response chain. All pragma directives specify optional
2166   behavior from the viewpoint of the protocol; however, some systems
2167   &MAY; require that behavior be consistent with the directives.
2169<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
2170  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
2171  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
2172  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
2173  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
2176   When the no-cache directive is present in a request message, an
2177   application &SHOULD; forward the request toward the origin server even
2178   if it has a cached copy of what is being requested. This pragma
2179   directive has the same semantics as the no-cache cache-directive (see
2180   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2181   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2182   request is sent to a server not known to be HTTP/1.1 compliant.
2185   Pragma directives &MUST; be passed through by a proxy or gateway
2186   application, regardless of their significance to that application,
2187   since the directives might be applicable to all recipients along the
2188   request/response chain. It is not possible to specify a pragma for a
2189   specific recipient; however, any pragma directive not relevant to a
2190   recipient &SHOULD; be ignored by that recipient.
2193   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2194   sent "Cache-Control: no-cache". No new Pragma directives will be
2195   defined in HTTP.
2196  <list><t>
2197      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
2198      response-header field is not actually specified, it does not provide a
2199      reliable replacement for "Cache-Control: no-cache" in a response.
2200  </t></list>
2204<section title="Vary" anchor="header.vary">
2205  <iref primary="true" item="Vary header" x:for-anchor=""/>
2206  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2207  <x:anchor-alias value="Vary"/>
2208  <x:anchor-alias value="Vary-v"/>
2210   The "Vary" response-header field's value indicates the set of request-header
2211   fields that fully determines, while the response is fresh, whether a cache
2212   is permitted to use the response to reply to a subsequent request
2213   without revalidation. For uncacheable or stale responses, the Vary
2214   field value advises the user agent about the criteria that were used
2215   to select the representation. A Vary field value of "*" implies that
2216   a cache cannot determine from the request headers of a subsequent
2217   request whether this response is the appropriate representation. See
2218   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2220<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
2221  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
2222  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
2225   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2226   cacheable response that is subject to server-driven negotiation.
2227   Doing so allows a cache to properly interpret future requests on that
2228   resource and informs the user agent about the presence of negotiation
2229   on that resource. A server &MAY; include a Vary header field with a
2230   non-cacheable response that is subject to server-driven negotiation,
2231   since this might provide the user agent with useful information about
2232   the dimensions over which the response varies at the time of the
2233   response.
2236   A Vary field value consisting of a list of field-names signals that
2237   the representation selected for the response is based on a selection
2238   algorithm which considers ONLY the listed request-header field values
2239   in selecting the most appropriate representation. A cache &MAY; assume
2240   that the same selection will be made for future requests with the
2241   same values for the listed field names, for the duration of time for
2242   which the response is fresh.
2245   The field-names given are not limited to the set of standard
2246   request-header fields defined by this specification. Field names are
2247   case-insensitive.
2250   A Vary field value of "*" signals that unspecified parameters not
2251   limited to the request-headers (e.g., the network address of the
2252   client), play a role in the selection of the response representation.
2253   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2254   generated by an origin server.
2258<section title="Warning" anchor="header.warning">
2259  <iref primary="true" item="Warning header" x:for-anchor=""/>
2260  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2261  <x:anchor-alias value="Warning"/>
2262  <x:anchor-alias value="Warning-v"/>
2263  <x:anchor-alias value="warning-value"/>
2264  <x:anchor-alias value="warn-agent"/>
2265  <x:anchor-alias value="warn-code"/>
2266  <x:anchor-alias value="warn-date"/>
2267  <x:anchor-alias value="warn-text"/>
2269   The general-header field "Warning" is used to carry additional
2270   information about the status or transformation of a message which
2271   might not be reflected in the message. This information is typically
2272   used to warn about a possible lack of semantic transparency from
2273   caching operations or transformations applied to the entity body of
2274   the message.
2277   Warning headers are sent with responses using:
2279<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
2280  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
2281  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
2283  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
2284                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
2286  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
2287  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2288                  ; the name or pseudonym of the server adding
2289                  ; the Warning header, for use in debugging
2290  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
2291  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
2294   A response &MAY; carry more than one Warning header.
2297   The warn-text &SHOULD; be in a natural language and character set that
2298   is most likely to be intelligible to the human user receiving the
2299   response. This decision &MAY; be based on any available knowledge, such
2300   as the location of the cache or user, the Accept-Language field in a
2301   request, the Content-Language field in a response, etc. The default
2302   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2305   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2306   in the warn-text using the method described in <xref target="RFC2047"/>.
2309   Warning headers can in general be applied to any message, however
2310   some specific warn-codes are specific to caches and can only be
2311   applied to response messages. New Warning headers &SHOULD; be added
2312   after any existing Warning headers. A cache &MUST-NOT; delete any
2313   Warning header that it received with a message. However, if a cache
2314   successfully validates a cache entry, it &SHOULD; remove any Warning
2315   headers previously attached to that entry except as specified for
2316   specific Warning codes. It &MUST; then add any Warning headers received
2317   in the validating response. In other words, Warning headers are those
2318   that would be attached to the most recent relevant response.
2321   When multiple Warning headers are attached to a response, the user
2322   agent ought to inform the user of as many of them as possible, in the
2323   order that they appear in the response. If it is not possible to
2324   inform the user of all of the warnings, the user agent &SHOULD; follow
2325   these heuristics:
2326  <list style="symbols">
2327    <t>Warnings that appear early in the response take priority over
2328        those appearing later in the response.</t>
2330    <t>Warnings in the user's preferred character set take priority
2331        over warnings in other character sets but with identical warn-codes
2332        and warn-agents.</t>
2333  </list>
2336   Systems that generate multiple Warning headers &SHOULD; order them with
2337   this user agent behavior in mind.
2340   Requirements for the behavior of caches with respect to Warnings are
2341   stated in <xref target="warnings"/>.
2344   This is a list of the currently-defined warn-codes, each with a
2345   recommended warn-text in English, and a description of its meaning.
2348   110 Response is stale
2349  <list><t>
2350     &MUST; be included whenever the returned response is stale.
2351  </t></list>
2354   111 Revalidation failed
2355  <list><t>
2356     &MUST; be included if a cache returns a stale response because an
2357     attempt to revalidate the response failed, due to an inability to
2358     reach the server.
2359  </t></list>
2362   112 Disconnected operation
2363  <list><t>
2364     &SHOULD; be included if the cache is intentionally disconnected from
2365     the rest of the network for a period of time.
2366  </t></list>
2369   113 Heuristic expiration
2370  <list><t>
2371     &MUST; be included if the cache heuristically chose a freshness
2372     lifetime greater than 24 hours and the response's age is greater
2373     than 24 hours.
2374  </t></list>
2377   199 Miscellaneous warning
2378  <list><t>
2379     The warning text &MAY; include arbitrary information to be presented
2380     to a human user, or logged. A system receiving this warning &MUST-NOT;
2381     take any automated action, besides presenting the warning to
2382     the user.
2383  </t></list>
2386   214 Transformation applied
2387  <list><t>
2388     &MUST; be added by an intermediate cache or proxy if it applies any
2389     transformation changing the content-coding (as specified in the
2390     Content-Encoding header) or media-type (as specified in the
2391     Content-Type header) of the response, or the entity-body of the
2392     response, unless this Warning code already appears in the response.
2393  </t></list>
2396   299 Miscellaneous persistent warning
2397  <list><t>
2398     The warning text &MAY; include arbitrary information to be presented
2399     to a human user, or logged. A system receiving this warning &MUST-NOT;
2400     take any automated action.
2401  </t></list>
2404   If an implementation sends a message with one or more Warning headers
2405   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2406   each warning-value a warn-date that matches the date in the response.
2409   If an implementation receives a message with a warning-value that
2410   includes a warn-date, and that warn-date is different from the Date
2411   value in the response, then that warning-value &MUST; be deleted from
2412   the message before storing, forwarding, or using it. (This prevents
2413   bad consequences of naive caching of Warning header fields.) If all
2414   of the warning-values are deleted for this reason, the Warning header
2415   &MUST; be deleted as well.
2421<section title="IANA Considerations" anchor="IANA.considerations">
2422<section title="Message Header Registration" anchor="message.header.registration">
2424   The Message Header Registry located at <eref target=""/> should be updated
2425   with the permanent registrations below (see <xref target="RFC3864"/>):
2427<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2428<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2429   <ttcol>Header Field Name</ttcol>
2430   <ttcol>Protocol</ttcol>
2431   <ttcol>Status</ttcol>
2432   <ttcol>Reference</ttcol>
2434   <c>Age</c>
2435   <c>http</c>
2436   <c>standard</c>
2437   <c>
2438      <xref target="header.age"/>
2439   </c>
2440   <c>Cache-Control</c>
2441   <c>http</c>
2442   <c>standard</c>
2443   <c>
2444      <xref target="header.cache-control"/>
2445   </c>
2446   <c>Expires</c>
2447   <c>http</c>
2448   <c>standard</c>
2449   <c>
2450      <xref target="header.expires"/>
2451   </c>
2452   <c>Pragma</c>
2453   <c>http</c>
2454   <c>standard</c>
2455   <c>
2456      <xref target="header.pragma"/>
2457   </c>
2458   <c>Vary</c>
2459   <c>http</c>
2460   <c>standard</c>
2461   <c>
2462      <xref target="header.vary"/>
2463   </c>
2464   <c>Warning</c>
2465   <c>http</c>
2466   <c>standard</c>
2467   <c>
2468      <xref target="header.warning"/>
2469   </c>
2473   The change controller is: "IETF ( - Internet Engineering Task Force".
2478<section title="Security Considerations" anchor="security.considerations">
2480   Caching proxies provide additional potential vulnerabilities, since
2481   the contents of the cache represent an attractive target for
2482   malicious exploitation. Because cache contents persist after an HTTP
2483   request is complete, an attack on the cache can reveal information
2484   long after a user believes that the information has been removed from
2485   the network. Therefore, cache contents should be protected as
2486   sensitive information.
2490<section title="Acknowledgments" anchor="ack">
2492   Much of the content and presentation of the caching design is due to
2493   suggestions and comments from individuals including: Shel Kaphan,
2494   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2500<references title="Normative References">
2502<reference anchor="ISO-8859-1">
2503  <front>
2504    <title>
2505     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2506    </title>
2507    <author>
2508      <organization>International Organization for Standardization</organization>
2509    </author>
2510    <date year="1998"/>
2511  </front>
2512  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2515<reference anchor="Part1">
2516  <front>
2517    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2518    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2519      <organization abbrev="Day Software">Day Software</organization>
2520      <address><email></email></address>
2521    </author>
2522    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2523      <organization>One Laptop per Child</organization>
2524      <address><email></email></address>
2525    </author>
2526    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2527      <organization abbrev="HP">Hewlett-Packard Company</organization>
2528      <address><email></email></address>
2529    </author>
2530    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2531      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2532      <address><email></email></address>
2533    </author>
2534    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2535      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2536      <address><email></email></address>
2537    </author>
2538    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2539      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2540      <address><email></email></address>
2541    </author>
2542    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2543      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2544      <address><email></email></address>
2545    </author>
2546    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2547      <organization abbrev="W3C">World Wide Web Consortium</organization>
2548      <address><email></email></address>
2549    </author>
2550    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2551      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2552      <address><email></email></address>
2553    </author>
2554    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2555  </front>
2556  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2557  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2560<reference anchor="Part2">
2561  <front>
2562    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2563    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2564      <organization abbrev="Day Software">Day Software</organization>
2565      <address><email></email></address>
2566    </author>
2567    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2568      <organization>One Laptop per Child</organization>
2569      <address><email></email></address>
2570    </author>
2571    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2572      <organization abbrev="HP">Hewlett-Packard Company</organization>
2573      <address><email></email></address>
2574    </author>
2575    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2576      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2577      <address><email></email></address>
2578    </author>
2579    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2580      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2581      <address><email></email></address>
2582    </author>
2583    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2584      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2585      <address><email></email></address>
2586    </author>
2587    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2588      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2589      <address><email></email></address>
2590    </author>
2591    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2592      <organization abbrev="W3C">World Wide Web Consortium</organization>
2593      <address><email></email></address>
2594    </author>
2595    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2596      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2597      <address><email></email></address>
2598    </author>
2599    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2600  </front>
2601  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2602  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2605<reference anchor="Part3">
2606  <front>
2607    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2608    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2609      <organization abbrev="Day Software">Day Software</organization>
2610      <address><email></email></address>
2611    </author>
2612    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2613      <organization>One Laptop per Child</organization>
2614      <address><email></email></address>
2615    </author>
2616    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2617      <organization abbrev="HP">Hewlett-Packard Company</organization>
2618      <address><email></email></address>
2619    </author>
2620    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2621      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2622      <address><email></email></address>
2623    </author>
2624    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2625      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2626      <address><email></email></address>
2627    </author>
2628    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2629      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2630      <address><email></email></address>
2631    </author>
2632    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2633      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2634      <address><email></email></address>
2635    </author>
2636    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2637      <organization abbrev="W3C">World Wide Web Consortium</organization>
2638      <address><email></email></address>
2639    </author>
2640    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2641      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2642      <address><email></email></address>
2643    </author>
2644    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2645  </front>
2646  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2647  <x:source href="p3-payload.xml" basename="p3-payload"/>
2650<reference anchor="Part4">
2651  <front>
2652    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2653    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2654      <organization abbrev="Day Software">Day Software</organization>
2655      <address><email></email></address>
2656    </author>
2657    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2658      <organization>One Laptop per Child</organization>
2659      <address><email></email></address>
2660    </author>
2661    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2662      <organization abbrev="HP">Hewlett-Packard Company</organization>
2663      <address><email></email></address>
2664    </author>
2665    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2666      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2667      <address><email></email></address>
2668    </author>
2669    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2670      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2671      <address><email></email></address>
2672    </author>
2673    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2674      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2675      <address><email></email></address>
2676    </author>
2677    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2678      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2679      <address><email></email></address>
2680    </author>
2681    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2682      <organization abbrev="W3C">World Wide Web Consortium</organization>
2683      <address><email></email></address>
2684    </author>
2685    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2686      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2687      <address><email></email></address>
2688    </author>
2689    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2690  </front>
2691  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2692  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2695<reference anchor="Part5">
2696  <front>
2697    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2698    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2699      <organization abbrev="Day Software">Day Software</organization>
2700      <address><email></email></address>
2701    </author>
2702    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2703      <organization>One Laptop per Child</organization>
2704      <address><email></email></address>
2705    </author>
2706    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2707      <organization abbrev="HP">Hewlett-Packard Company</organization>
2708      <address><email></email></address>
2709    </author>
2710    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2711      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2712      <address><email></email></address>
2713    </author>
2714    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2715      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2716      <address><email></email></address>
2717    </author>
2718    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2719      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2720      <address><email></email></address>
2721    </author>
2722    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2723      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2724      <address><email></email></address>
2725    </author>
2726    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2727      <organization abbrev="W3C">World Wide Web Consortium</organization>
2728      <address><email></email></address>
2729    </author>
2730    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2731      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2732      <address><email></email></address>
2733    </author>
2734    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2735  </front>
2736  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2737  <x:source href="p5-range.xml" basename="p5-range"/>
2740<reference anchor="Part7">
2741  <front>
2742    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2743    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2744      <organization abbrev="Day Software">Day Software</organization>
2745      <address><email></email></address>
2746    </author>
2747    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2748      <organization>One Laptop per Child</organization>
2749      <address><email></email></address>
2750    </author>
2751    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2752      <organization abbrev="HP">Hewlett-Packard Company</organization>
2753      <address><email></email></address>
2754    </author>
2755    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2756      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2757      <address><email></email></address>
2758    </author>
2759    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2760      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2761      <address><email></email></address>
2762    </author>
2763    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2764      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2765      <address><email></email></address>
2766    </author>
2767    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2768      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2769      <address><email></email></address>
2770    </author>
2771    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2772      <organization abbrev="W3C">World Wide Web Consortium</organization>
2773      <address><email></email></address>
2774    </author>
2775    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2776      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2777      <address><email></email></address>
2778    </author>
2779    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2780  </front>
2781  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2782  <x:source href="p7-auth.xml" basename="p7-auth"/>
2785<reference anchor="RFC2047">
2786  <front>
2787    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2788    <author initials="K." surname="Moore" fullname="Keith Moore">
2789      <organization>University of Tennessee</organization>
2790      <address><email></email></address>
2791    </author>
2792    <date month="November" year="1996"/>
2793  </front>
2794  <seriesInfo name="RFC" value="2047"/>
2797<reference anchor="RFC2119">
2798  <front>
2799    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2800    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2801      <organization>Harvard University</organization>
2802      <address><email></email></address>
2803    </author>
2804    <date month="March" year="1997"/>
2805  </front>
2806  <seriesInfo name="BCP" value="14"/>
2807  <seriesInfo name="RFC" value="2119"/>
2812<references title="Informative References">
2814<reference anchor="RFC1305">
2815  <front>
2816    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2817    <author initials="D." surname="Mills" fullname="David L. Mills">
2818      <organization>University of Delaware, Electrical Engineering Department</organization>
2819      <address><email></email></address>
2820    </author>
2821    <date month="March" year="1992"/>
2822  </front>
2823  <seriesInfo name="RFC" value="1305"/>
2826<reference anchor="RFC2616">
2827  <front>
2828    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2829    <author initials="R." surname="Fielding" fullname="R. Fielding">
2830      <organization>University of California, Irvine</organization>
2831      <address><email></email></address>
2832    </author>
2833    <author initials="J." surname="Gettys" fullname="J. Gettys">
2834      <organization>W3C</organization>
2835      <address><email></email></address>
2836    </author>
2837    <author initials="J." surname="Mogul" fullname="J. Mogul">
2838      <organization>Compaq Computer Corporation</organization>
2839      <address><email></email></address>
2840    </author>
2841    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2842      <organization>MIT Laboratory for Computer Science</organization>
2843      <address><email></email></address>
2844    </author>
2845    <author initials="L." surname="Masinter" fullname="L. Masinter">
2846      <organization>Xerox Corporation</organization>
2847      <address><email></email></address>
2848    </author>
2849    <author initials="P." surname="Leach" fullname="P. Leach">
2850      <organization>Microsoft Corporation</organization>
2851      <address><email></email></address>
2852    </author>
2853    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2854      <organization>W3C</organization>
2855      <address><email></email></address>
2856    </author>
2857    <date month="June" year="1999"/>
2858  </front>
2859  <seriesInfo name="RFC" value="2616"/>
2862<reference anchor='RFC3864'>
2863  <front>
2864    <title>Registration Procedures for Message Header Fields</title>
2865    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
2866      <organization>Nine by Nine</organization>
2867      <address><email></email></address>
2868    </author>
2869    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
2870      <organization>BEA Systems</organization>
2871      <address><email></email></address>
2872    </author>
2873    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
2874      <organization>HP Labs</organization>
2875      <address><email></email></address>
2876    </author>
2877    <date year='2004' month='September' />
2878  </front>
2879  <seriesInfo name='BCP' value='90' />
2880  <seriesInfo name='RFC' value='3864' />
2885<section title="Compatibility with Previous Versions" anchor="compatibility">
2887<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2889   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2890   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2891   <xref target="header.cache-control" format="counter"/>,
2892   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2895   Transfer-coding and message lengths all interact in ways that
2896   required fixing exactly when chunked encoding is used (to allow for
2897   transfer encoding that may not be self delimiting); it was important
2898   to straighten out exactly how message lengths are computed.
2899   (<xref target="non-modifiable.headers"/>,
2900   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2903   Proxies should be able to add Content-Length when appropriate.
2904   (<xref target="non-modifiable.headers"/>)
2907   Range request responses would become very verbose if all meta-data
2908   were always returned; by allowing the server to only send needed
2909   headers in a 206 response, this problem can be avoided.
2910   (<xref target="combining.headers"/>)
2913   The Cache-Control: max-age directive was not properly defined for
2914   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2917   Warnings could be cached incorrectly, or not updated appropriately.
2918   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2919   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2920   and <xref target="header.warning" format="counter"/>) Warning
2921   also needed to be a general header, as PUT or other methods may have
2922   need for it in requests.
2926<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2928  Clarify denial of service attack avoidance requirement.
2929  (<xref target="invalidation.after.updates.or.deletions"/>)
2935<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
2937<section title="Since RFC2616">
2939  Extracted relevant partitions from <xref target="RFC2616"/>.
2943<section title="Since draft-ietf-httpbis-p6-cache-00">
2945  Closed issues:
2946  <list style="symbols">
2947    <t>
2948      <eref target=""/>:
2949      "Trailer"
2950      (<eref target=""/>)
2951    </t>
2952    <t>
2953      <eref target=""/>:
2954      "Invalidation after Update or Delete"
2955      (<eref target=""/>)
2956    </t>
2957    <t>
2958      <eref target=""/>:
2959      "Normative and Informative references"
2960    </t>
2961    <t>
2962      <eref target=""/>:
2963      "Date reference typo"
2964    </t>
2965    <t>
2966      <eref target=""/>:
2967      "Connection header text"
2968    </t>
2969    <t>
2970      <eref target=""/>:
2971      "Informative references"
2972    </t>
2973    <t>
2974      <eref target=""/>:
2975      "ISO-8859-1 Reference"
2976    </t>
2977    <t>
2978      <eref target=""/>:
2979      "Normative up-to-date references"
2980    </t>
2981    <t>
2982      <eref target=""/>:
2983      "typo in 13.2.2"
2984    </t>
2985  </list>
2988  Other changes:
2989  <list style="symbols">
2990    <t>
2991      Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target=""/>)
2992    </t>
2993  </list>
2997<section title="Since draft-ietf-httpbis-p6-cache-01">
2999  Closed issues:
3000  <list style="symbols">
3001    <t>
3002      <eref target=""/>:
3003      "rel_path not used"
3004    </t>
3005  </list>
3008  Other changes:
3009  <list style="symbols">
3010    <t>
3011       Get rid of duplicate BNF rule names ("host" -> "uri-host")
3012       (work in progress on <eref target=""/>)
3013    </t>
3014    <t>
3015      Add explicit references to BNF syntax and rules imported from other parts of the specification.
3016    </t>
3017  </list>
3021<section title="Since draft-ietf-httpbis-p6-cache-02" anchor="changes.since.02">
3023  Ongoing work on IANA Message Header Registration (<eref target=""/>):
3024  <list style="symbols">
3025    <t>
3026      Reference RFC 3984, and update header registrations for headers defined
3027      in this document.
3028    </t>
3029  </list>
3033<section title="Since draft-ietf-httpbis-p6-cache-03" anchor="changes.since.03">
3035  Closed issues:
3036  <list style="symbols">
3037    <t>
3038      <eref target=""/>:
3039      "Vary header classification"
3040    </t>
3041  </list>
3045<section title="Since draft-ietf-httpbis-p6-cache-04" anchor="changes.since.04">
3047  Ongoing work on ABNF conversion (<eref target=""/>):
3048  <list style="symbols">
3049    <t>
3050      Use "/" instead of "|" for alternatives.
3051    </t>
3052    <t>
3053      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
3054      whitespace ("OWS") and required whitespace ("RWS").
3055    </t>
3056    <t>
3057      Rewrite ABNFs to spell out whitespace rules, factor out
3058      header value format definitions.
3059    </t>
3060  </list>
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