source: draft-ietf-httpbis/latest/p6-cache.xml @ 161

Last change on this file since 161 was 157, checked in by fielding@…, 15 years ago

editorial: Move caching intro in overview to introduction along
with the prerequisite definitions. Move definition of delta-seconds
to where it is first used (Age). Add a couple glue sentences.
Replace redundant "the HTTP/1.1 protocol" with "HTTP/1.1".

  • Property svn:eol-style set to native
File size: 127.5 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 "January">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY messaging                   "<xref target='Part1' xmlns:x=''/>">
18  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x=''/>">
19  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x=''/>">
20  <!ENTITY entity-header-fields        "<xref target='Part4' x:rel='#header.fields' xmlns:x=''/>">
21  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#entity.tags' xmlns:x=''/>">
22  <!ENTITY full-date                   "<xref target='Part1' x:rel='' xmlns:x=''/>">
23  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x=''/>">
24  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x=''/>">
25  <!ENTITY header-date                 "<xref target='Part1' x:rel='' xmlns:x=''/>">
26  <!ENTITY weak-and-strong-validators  "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x=''/>">
27  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x=''/>">
28  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x=''/>">
29  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
30  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x=''/>">
32<?rfc toc="yes" ?>
33<?rfc symrefs="yes" ?>
34<?rfc sortrefs="yes" ?>
35<?rfc compact="yes"?>
36<?rfc subcompact="no" ?>
37<?rfc linkmailto="no" ?>
38<?rfc editing="no" ?>
39<?rfc-ext allow-markup-in-artwork="yes" ?>
40<?rfc-ext include-references-in-index="yes" ?>
41<rfc obsoletes="2616" category="std"
42     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
43     xmlns:x=''>
46  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
48  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
49    <organization abbrev="Day Software">Day Software</organization>
50    <address>
51      <postal>
52        <street>23 Corporate Plaza DR, Suite 280</street>
53        <city>Newport Beach</city>
54        <region>CA</region>
55        <code>92660</code>
56        <country>USA</country>
57      </postal>
58      <phone>+1-949-706-5300</phone>
59      <facsimile>+1-949-706-5305</facsimile>
60      <email></email>
61      <uri></uri>
62    </address>
63  </author>
65  <author initials="J." surname="Gettys" fullname="Jim Gettys">
66    <organization>One Laptop per Child</organization>
67    <address>
68      <postal>
69        <street>21 Oak Knoll Road</street>
70        <city>Carlisle</city>
71        <region>MA</region>
72        <code>01741</code>
73        <country>USA</country>
74      </postal>
75      <email></email>
76      <uri></uri>
77    </address>
78  </author>
80  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
81    <organization abbrev="HP">Hewlett-Packard Company</organization>
82    <address>
83      <postal>
84        <street>HP Labs, Large Scale Systems Group</street>
85        <street>1501 Page Mill Road, MS 1177</street>
86        <city>Palo Alto</city>
87        <region>CA</region>
88        <code>94304</code>
89        <country>USA</country>
90      </postal>
91      <email></email>
92    </address>
93  </author>
95  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
96    <organization abbrev="Microsoft">Microsoft Corporation</organization>
97    <address>
98      <postal>
99        <street>1 Microsoft Way</street>
100        <city>Redmond</city>
101        <region>WA</region>
102        <code>98052</code>
103        <country>USA</country>
104      </postal>
105      <email></email>
106    </address>
107  </author>
109  <author initials="L." surname="Masinter" fullname="Larry Masinter">
110    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
111    <address>
112      <postal>
113        <street>345 Park Ave</street>
114        <city>San Jose</city>
115        <region>CA</region>
116        <code>95110</code>
117        <country>USA</country>
118      </postal>
119      <email></email>
120      <uri></uri>
121    </address>
122  </author>
124  <author initials="P." surname="Leach" fullname="Paul J. Leach">
125    <organization abbrev="Microsoft">Microsoft Corporation</organization>
126    <address>
127      <postal>
128        <street>1 Microsoft Way</street>
129        <city>Redmond</city>
130        <region>WA</region>
131        <code>98052</code>
132      </postal>
133      <email></email>
134    </address>
135  </author>
137  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
138    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
139    <address>
140      <postal>
141        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
142        <street>The Stata Center, Building 32</street>
143        <street>32 Vassar Street</street>
144        <city>Cambridge</city>
145        <region>MA</region>
146        <code>02139</code>
147        <country>USA</country>
148      </postal>
149      <email></email>
150      <uri></uri>
151    </address>
152  </author>
154  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
155    <organization abbrev="W3C">World Wide Web Consortium</organization>
156    <address>
157      <postal>
158        <street>W3C / ERCIM</street>
159        <street>2004, rte des Lucioles</street>
160        <city>Sophia-Antipolis</city>
161        <region>AM</region>
162        <code>06902</code>
163        <country>France</country>
164      </postal>
165      <email></email>
166      <uri></uri>
167    </address>
168  </author>
170  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
171    <organization abbrev="greenbytes">greenbytes GmbH</organization>
172    <address>
173      <postal>
174        <street>Hafenweg 16</street>
175        <city>Muenster</city><region>NW</region><code>48155</code>
176        <country>Germany</country>
177      </postal>
178      <phone>+49 251 2807760</phone>   
179      <facsimile>+49 251 2807761</facsimile>   
180      <email></email>       
181      <uri></uri>     
182    </address>
183  </author>
185  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
189   The Hypertext Transfer Protocol (HTTP) is an application-level
190   protocol for distributed, collaborative, hypermedia information
191   systems. HTTP has been in use by the World Wide Web global information
192   initiative since 1990. This document is Part 6 of the seven-part specification
193   that defines the protocol referred to as "HTTP/1.1" and, taken together,
194   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
195   and the associated header fields that control cache behavior or indicate
196   cacheable response messages.
200<note title="Editorial Note (To be removed by RFC Editor)">
201  <t>
202    Discussion of this draft should take place on the HTTPBIS working group
203    mailing list ( The current issues list is
204    at <eref target=""/>
205    and related documents (including fancy diffs) can be found at
206    <eref target=""/>.
207  </t>
208  <t>
209    This draft incorporates those issue resolutions that were either
210    collected in the original RFC2616 errata list (<eref target=""/>),
211    or which were agreed upon on the mailing list between October 2006 and
212    November 2007 (as published in "draft-lafon-rfc2616bis-03").
213  </t>
217<section title="Introduction" anchor="caching">
219   HTTP is typically used for distributed information systems, where
220   performance can be improved by the use of response caches. The
221   HTTP/1.1 protocol includes a number of elements intended to make
222   caching work as well as possible. Because these elements are
223   inextricable from other aspects of the protocol, and because they
224   interact with each other, it is useful to describe the basic caching
225   design of HTTP separately from the detailed descriptions of methods,
226   headers, response codes, etc.  This document defines aspects of HTTP
227   related to caching response messages.
230<section title="Purpose" anchor="intro.purpose">
231<iref item="cache"/>
233   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
234   and the subsystem that controls its message storage, retrieval, and
235   deletion. A cache stores cacheable responses in order to reduce the
236   response time and network bandwidth consumption on future, equivalent
237   requests. Any client or server may include a cache, though a cache
238   cannot be used by a server that is acting as a tunnel.
241   Caching would be useless if it did not significantly improve
242   performance. The goal of caching in HTTP/1.1 is to eliminate the need
243   to send requests in many cases, and to eliminate the need to send
244   full responses in many other cases. The former reduces the number of
245   network round-trips required for many operations; we use an
246   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
247   latter reduces network bandwidth requirements; we use a "validation"
248   mechanism for this purpose (see <xref target="validation.model"/>).
250<iref item="semantically transparent"/>
252   A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
253   respect to a particular response, when its use affects neither the
254   requesting client nor the origin server, except to improve
255   performance. When a cache is semantically transparent, the client
256   receives exactly the same response (except for hop-by-hop headers)
257   that it would have received had its request been handled directly
258   by the origin server.
261   In an ideal world, all interactions with an HTTP cache would be
262   semantically transparent.  However, for some resources, semantic
263   transparency is not always necessary and can be effectively traded
264   for the sake of bandwidth scaling, disconnected operation, and
265   high availability.  HTTP/1.1 allows origin servers, caches,
266   and clients to explicitly reduce transparency when necessary.
267   However, because non-transparent operation may confuse non-expert
268   users and might be incompatible with certain server applications
269   (such as those for ordering merchandise), the protocol requires that
270   transparency be relaxed
271  <list style="symbols">
272     <t>only by an explicit protocol-level request when relaxed by
273        client or origin server</t>
275     <t>only with an explicit warning to the end user when relaxed by
276        cache or client</t>
277  </list>
280   Therefore, HTTP/1.1 provides these important elements:
281  <list style="numbers">
282      <t>Protocol features that provide full semantic transparency when
283         this is required by all parties.</t>
285      <t>Protocol features that allow an origin server or user agent to
286         explicitly request and control non-transparent operation.</t>
288      <t>Protocol features that allow a cache to attach warnings to
289         responses that do not preserve the requested approximation of
290         semantic transparency.</t>
291  </list>
294   A basic principle is that it must be possible for the clients to
295   detect any potential relaxation of semantic transparency.
296  <list><t>
297      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
298      design decisions not explicitly discussed in this specification.
299      If a decision might affect semantic transparency, the implementor
300      ought to err on the side of maintaining transparency unless a
301      careful and complete analysis shows significant benefits in
302      breaking transparency.
303    </t></list>
307<section title="Terminology" anchor="intro.terminology">
309   This specification uses a number of terms to refer to the roles
310   played by participants in, and objects of, HTTP caching.
313  <iref item="cacheable"/>
314  <x:dfn>cacheable</x:dfn>
315  <list>
316    <t>
317      A response is cacheable if a cache is allowed to store a copy of
318      the response message for use in answering subsequent requests.
319      Even if a resource is cacheable, there may
320      be additional constraints on whether a cache can use the cached
321      copy for a particular request.
322    </t>
323  </list>
326  <iref item="first-hand"/>
327  <x:dfn>first-hand</x:dfn>
328  <list>
329    <t>
330      A response is first-hand if it comes directly and without
331      unnecessary delay from the origin server, perhaps via one or more
332      proxies. A response is also first-hand if its validity has just
333      been checked directly with the origin server.
334    </t>
335  </list>
338  <iref item="explicit expiration time"/>
339  <x:dfn>explicit expiration time</x:dfn>
340  <list>
341    <t>
342      The time at which the origin server intends that an entity should
343      no longer be returned by a cache without further validation.
344    </t>
345  </list>
348  <iref item="heuristic expiration time"/>
349  <x:dfn>heuristic expiration time</x:dfn>
350  <list>
351    <t>
352      An expiration time assigned by a cache when no explicit expiration
353      time is available.
354    </t>
355  </list>
358  <iref item="age"/>
359  <x:dfn>age</x:dfn>
360  <list>
361    <t>
362      The age of a response is the time since it was sent by, or
363      successfully validated with, the origin server.
364    </t>
365  </list>
368  <iref item="freshness lifetime"/>
369  <x:dfn>freshness lifetime</x:dfn>
370  <list>
371    <t>
372      The length of time between the generation of a response and its
373      expiration time.
374    </t>
375  </list>
378  <iref item="fresh"/>
379  <x:dfn>fresh</x:dfn>
380  <list>
381    <t>
382      A response is fresh if its age has not yet exceeded its freshness
383      lifetime.
384    </t>
385  </list>
388  <iref item="stale"/>
389  <x:dfn>stale</x:dfn>
390  <list>
391    <t>
392      A response is stale if its age has passed its freshness lifetime.
393    </t>
394  </list>
397  <iref item="validator"/>
398  <x:dfn>validator</x:dfn>
399  <list>
400    <t>
401      A protocol element (e.g., an entity tag or a Last-Modified time)
402      that is used to find out whether a cache entry is an equivalent
403      copy of an entity.
404    </t>
405  </list>
409<section title="Requirements" anchor="intro.requirements">
411   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
412   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
413   document are to be interpreted as described in <xref target="RFC2119"/>.
416   An implementation is not compliant if it fails to satisfy one or more
417   of the &MUST; or &REQUIRED; level requirements for the protocols it
418   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
419   level and all the &SHOULD; level requirements for its protocols is said
420   to be "unconditionally compliant"; one that satisfies all the &MUST;
421   level requirements but not all the &SHOULD; level requirements for its
422   protocols is said to be "conditionally compliant."
427<section title="Overview" anchor="caching.overview">
428<section title="Cache Correctness" anchor="cache.correctness">
430   A correct cache &MUST; respond to a request with the most up-to-date
431   response held by the cache that is appropriate to the request (see
432   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
433   <xref target="disambiguating.multiple.responses" format="counter"/>,
434   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
435   conditions:
436  <list style="numbers">
437      <t>It has been checked for equivalence with what the origin server
438         would have returned by revalidating the response with the
439         origin server (<xref target="validation.model"/>);</t>
441      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
442         this means it meets the least restrictive freshness requirement
443         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
444         the origin server so specifies, it is the freshness requirement
445         of the origin server alone.
447         If a stored response is not "fresh enough" by the most
448         restrictive freshness requirement of both the client and the
449         origin server, in carefully considered circumstances the cache
450         &MAY; still return the response with the appropriate Warning
451         header (see Sections <xref target="" format="counter"/>
452         and <xref target="header.warning" format="counter"/>), unless such a response
453         is prohibited (e.g., by a "no-store" cache-directive, or by a
454         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
456      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
457         or error (4xx or 5xx) response message.</t>
458  </list>
461   If the cache can not communicate with the origin server, then a
462   correct cache &SHOULD; respond as above if the response can be
463   correctly served from the cache; if not it &MUST; return an error or
464   warning indicating that there was a communication failure.
467   If a cache receives a response (either an entire response, or a 304
468   (Not Modified) response) that it would normally forward to the
469   requesting client, and the received response is no longer fresh, the
470   cache &SHOULD; forward it to the requesting client without adding a new
471   Warning (but without removing any existing Warning headers). A cache
472   &SHOULD-NOT;  attempt to revalidate a response simply because that
473   response became stale in transit; this might lead to an infinite
474   loop. A user agent that receives a stale response without a Warning
475   &MAY; display a warning indication to the user.
479<section title="Warnings" anchor="warnings">
481   Whenever a cache returns a response that is neither first-hand nor
482   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
483   &MUST; attach a warning to that effect, using a Warning general-header.
484   The Warning header and the currently defined warnings are described
485   in <xref target="header.warning"/>. The warning allows clients to take appropriate
486   action.
489   Warnings &MAY; be used for other purposes, both cache-related and
490   otherwise. The use of a warning, rather than an error status code,
491   distinguish these responses from true failures.
494   Warnings are assigned three digit warn-codes. The first digit
495   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
496   stored cache entry after a successful revalidation:
499  <list style="hanging">
500    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
501     the response, and so &MUST; be deleted after a successful
502     revalidation. 1xx warn-codes &MAY; be generated by a cache only when
503     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
505    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
506     headers that is not rectified by a revalidation (for example, a
507     lossy compression of the entity bodies) and which &MUST-NOT; be
508     deleted after a successful revalidation.</t>
509    </list>
512   See <xref target="header.warning"/> for the definitions of the codes themselves.
515   HTTP/1.0 caches will cache all Warnings in responses, without
516   deleting the ones in the first category. Warnings in responses that
517   are passed to HTTP/1.0 caches carry an extra warning-date field,
518   which prevents a future HTTP/1.1 recipient from believing an
519   erroneously cached Warning.
522   Warnings also carry a warning text. The text &MAY; be in any
523   appropriate natural language (perhaps based on the client's Accept
524   headers), and include an &OPTIONAL; indication of what character set is
525   used.
528   Multiple warnings &MAY; be attached to a response (either by the origin
529   server or by a cache), including multiple warnings with the same code
530   number. For example, a server might provide the same warning with
531   texts in both English and Basque.
534   When multiple warnings are attached to a response, it might not be
535   practical or reasonable to display all of them to the user. This
536   version of HTTP does not specify strict priority rules for deciding
537   which warnings to display and in what order, but does suggest some
538   heuristics.
542<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
544   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
545   times and validators) are implicit directives to caches. In some
546   cases, a server or client might need to provide explicit directives
547   to the HTTP caches. We use the Cache-Control header for this purpose.
550   The Cache-Control header allows a client or server to transmit a
551   variety of directives in either requests or responses. These
552   directives typically override the default caching algorithms. As a
553   general rule, if there is any apparent conflict between header
554   values, the most restrictive interpretation is applied (that is, the
555   one that is most likely to preserve semantic transparency). However,
556   in some cases, cache-control directives are explicitly specified as
557   weakening the approximation of semantic transparency (for example,
558   "max-stale" or "public").
561   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
565<section title="Explicit User Agent Warnings" anchor="">
567   Many user agents make it possible for users to override the basic
568   caching mechanisms. For example, the user agent might allow the user
569   to specify that cached entities (even explicitly stale ones) are
570   never validated. Or the user agent might habitually add "Cache-Control:
571   max-stale=3600" to every request. The user agent &SHOULD-NOT;
572   default to either non-transparent behavior, or behavior that results
573   in abnormally ineffective caching, but &MAY; be explicitly configured
574   to do so by an explicit action of the user.
577   If the user has overridden the basic caching mechanisms, the user
578   agent &SHOULD; explicitly indicate to the user whenever this results in
579   the display of information that might not meet the server's
580   transparency requirements (in particular, if the displayed entity is
581   known to be stale). Since the protocol normally allows the user agent
582   to determine if responses are stale or not, this indication need only
583   be displayed when this actually happens. The indication need not be a
584   dialog box; it could be an icon (for example, a picture of a rotting
585   fish) or some other indicator.
588   If the user has overridden the caching mechanisms in a way that would
589   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
590   continually indicate this state to the user (for example, by a
591   display of a picture of currency in flames) so that the user does not
592   inadvertently consume excess resources or suffer from excessive
593   latency.
597<section title="Exceptions to the Rules and Warnings" anchor="">
599   In some cases, the operator of a cache &MAY; choose to configure it to
600   return stale responses even when not requested by clients. This
601   decision ought not be made lightly, but may be necessary for reasons
602   of availability or performance, especially when the cache is poorly
603   connected to the origin server. Whenever a cache returns a stale
604   response, it &MUST; mark it as such (using a Warning header) enabling
605   the client software to alert the user that there might be a potential
606   problem.
609   It also allows the user agent to take steps to obtain a first-hand or
610   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
611   response if the client explicitly requests a first-hand or fresh one,
612   unless it is impossible to comply for technical or policy reasons.
616<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
618   While the origin server (and to a lesser extent, intermediate caches,
619   by their contribution to the age of a response) are the primary
620   source of expiration information, in some cases the client might need
621   to control a cache's decision about whether to return a cached
622   response without validating it. Clients do this using several
623   directives of the Cache-Control header.
626   A client's request &MAY; specify the maximum age it is willing to
627   accept of an unvalidated response; specifying a value of zero forces
628   the cache(s) to revalidate all responses. A client &MAY; also specify
629   the minimum time remaining before a response expires. Both of these
630   options increase constraints on the behavior of caches, and so cannot
631   further relax the cache's approximation of semantic transparency.
634   A client &MAY; also specify that it will accept stale responses, up to
635   some maximum amount of staleness. This loosens the constraints on the
636   caches, and so might violate the origin server's specified
637   constraints on semantic transparency, but might be necessary to
638   support disconnected operation, or high availability in the face of
639   poor connectivity.
644<section title="Expiration Model" anchor="expiration.model">
646<section title="Server-Specified Expiration" anchor="server-specified.expiration">
648   HTTP caching works best when caches can entirely avoid making
649   requests to the origin server. The primary mechanism for avoiding
650   requests is for an origin server to provide an explicit expiration
651   time in the future, indicating that a response &MAY; be used to satisfy
652   subsequent requests. In other words, a cache can return a fresh
653   response without first contacting the server.
656   Our expectation is that servers will assign future explicit
657   expiration times to responses in the belief that the entity is not
658   likely to change, in a semantically significant way, before the
659   expiration time is reached. This normally preserves semantic
660   transparency, as long as the server's expiration times are carefully
661   chosen.
664   The expiration mechanism applies only to responses taken from a cache
665   and not to first-hand responses forwarded immediately to the
666   requesting client.
669   If an origin server wishes to force a semantically transparent cache
670   to validate every request, it &MAY; assign an explicit expiration time
671   in the past. This means that the response is always stale, and so the
672   cache &SHOULD; validate it before using it for subsequent requests. See
673   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
676   If an origin server wishes to force any HTTP/1.1 cache, no matter how
677   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
678   cache-control directive (see <xref target="header.cache-control"/>).
681   Servers specify explicit expiration times using either the Expires
682   header, or the max-age directive of the Cache-Control header.
685   An expiration time cannot be used to force a user agent to refresh
686   its display or reload a resource; its semantics apply only to caching
687   mechanisms, and such mechanisms need only check a resource's
688   expiration status when a new request for that resource is initiated.
689   See <xref target="history.lists"/> for an explanation of the difference between caches
690   and history mechanisms.
694<section title="Heuristic Expiration" anchor="heuristic.expiration">
696   Since origin servers do not always provide explicit expiration times,
697   HTTP caches typically assign heuristic expiration times, employing
698   algorithms that use other header values (such as the Last-Modified
699   time) to estimate a plausible expiration time. The HTTP/1.1
700   specification does not provide specific algorithms, but does impose
701   worst-case constraints on their results. Since heuristic expiration
702   times might compromise semantic transparency, they ought to be used
703   cautiously, and we encourage origin servers to provide explicit
704   expiration times as much as possible.
708<section title="Age Calculations" anchor="age.calculations">
710   In order to know if a cached entry is fresh, a cache needs to know if
711   its age exceeds its freshness lifetime. We discuss how to calculate
712   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
713   the age of a response or cache entry.
716   In this discussion, we use the term "now" to mean "the current value
717   of the clock at the host performing the calculation." Hosts that use
718   HTTP, but especially hosts running origin servers and caches, &SHOULD;
719   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
720   a globally accurate time standard.
723   HTTP/1.1 requires origin servers to send a Date header, if possible,
724   with every response, giving the time at which the response was
725   generated (see &header-date;). We use the term "date_value" to denote
726   the value of the Date header, in a form appropriate for arithmetic
727   operations.
730   HTTP/1.1 uses the Age response-header to convey the estimated age of
731   the response message when obtained from a cache. The Age field value
732   is the cache's estimate of the amount of time since the response was
733   generated or revalidated by the origin server.
736   In essence, the Age value is the sum of the time that the response
737   has been resident in each of the caches along the path from the
738   origin server, plus the amount of time it has been in transit along
739   network paths.
742   We use the term "age_value" to denote the value of the Age header, in
743   a form appropriate for arithmetic operations.
746   A response's age can be calculated in two entirely independent ways:
747  <list style="numbers">
748      <t>now minus date_value, if the local clock is reasonably well
749         synchronized to the origin server's clock. If the result is
750         negative, the result is replaced by zero.</t>
752      <t>age_value, if all of the caches along the response path
753         implement HTTP/1.1.</t>
754  </list>
757   Given that we have two independent ways to compute the age of a
758   response when it is received, we can combine these as
760<figure><artwork type="code">
761    corrected_received_age = max(now - date_value, age_value)
764   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
765   paths, one gets a reliable (conservative) result.
768   Because of network-imposed delays, some significant interval might
769   pass between the time that a server generates a response and the time
770   it is received at the next outbound cache or client. If uncorrected,
771   this delay could result in improperly low ages.
774   Because the request that resulted in the returned Age value must have
775   been initiated prior to that Age value's generation, we can correct
776   for delays imposed by the network by recording the time at which the
777   request was initiated. Then, when an Age value is received, it &MUST;
778   be interpreted relative to the time the request was initiated, not
779   the time that the response was received. This algorithm results in
780   conservative behavior no matter how much delay is experienced. So, we
781   compute:
783<figure><artwork type="code">
784   corrected_initial_age = corrected_received_age
785                         + (now - request_time)
788   where "request_time" is the time (according to the local clock) when
789   the request that elicited this response was sent.
792   Summary of age calculation algorithm, when a cache receives a
793   response:
795<figure><artwork type="code">
796   /*
797    * age_value
798    *      is the value of Age: header received by the cache with
799    *              this response.
800    * date_value
801    *      is the value of the origin server's Date: header
802    * request_time
803    *      is the (local) time when the cache made the request
804    *              that resulted in this cached response
805    * response_time
806    *      is the (local) time when the cache received the
807    *              response
808    * now
809    *      is the current (local) time
810    */
812   apparent_age = max(0, response_time - date_value);
813   corrected_received_age = max(apparent_age, age_value);
814   response_delay = response_time - request_time;
815   corrected_initial_age = corrected_received_age + response_delay;
816   resident_time = now - response_time;
817   current_age   = corrected_initial_age + resident_time;
820   The current_age of a cache entry is calculated by adding the amount
821   of time (in seconds) since the cache entry was last validated by the
822   origin server to the corrected_initial_age. When a response is
823   generated from a cache entry, the cache &MUST; include a single Age
824   header field in the response with a value equal to the cache entry's
825   current_age.
828   The presence of an Age header field in a response implies that a
829   response is not first-hand. However, the converse is not true, since
830   the lack of an Age header field in a response does not imply that the
831   response is first-hand unless all caches along the request path are
832   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
833   the Age header field).
837<section title="Expiration Calculations" anchor="expiration.calculations">
839   In order to decide whether a response is fresh or stale, we need to
840   compare its freshness lifetime to its age. The age is calculated as
841   described in <xref target="age.calculations"/>; this section describes how to calculate
842   the freshness lifetime, and to determine if a response has expired.
843   In the discussion below, the values can be represented in any form
844   appropriate for arithmetic operations.
847   We use the term "expires_value" to denote the value of the Expires
848   header. We use the term "max_age_value" to denote an appropriate
849   value of the number of seconds carried by the "max-age" directive of
850   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
853   The max-age directive takes priority over Expires, so if max-age is
854   present in a response, the calculation is simply:
856<figure><artwork type="code">
857   freshness_lifetime = max_age_value
860   Otherwise, if Expires is present in the response, the calculation is:
862<figure><artwork type="code">
863   freshness_lifetime = expires_value - date_value
866   Note that neither of these calculations is vulnerable to clock skew,
867   since all of the information comes from the origin server.
870   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
871   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
872   does not include other restrictions on caching, the cache &MAY; compute
873   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
874   113 to any response whose age is more than 24 hours if such warning
875   has not already been added.
878   Also, if the response does have a Last-Modified time, the heuristic
879   expiration value &SHOULD; be no more than some fraction of the interval
880   since that time. A typical setting of this fraction might be 10%.
883   The calculation to determine if a response has expired is quite
884   simple:
886<figure><artwork type="code">
887   response_is_fresh = (freshness_lifetime &gt; current_age)
891<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
893   Because expiration values are assigned optimistically, it is possible
894   for two caches to contain fresh values for the same resource that are
895   different.
898   If a client performing a retrieval receives a non-first-hand response
899   for a request that was already fresh in its own cache, and the Date
900   header in its existing cache entry is newer than the Date on the new
901   response, then the client &MAY; ignore the response. If so, it &MAY;
902   retry the request with a "Cache-Control: max-age=0" directive (see
903   <xref target="header.cache-control"/>), to force a check with the origin server.
906   If a cache has two fresh responses for the same representation with
907   different validators, it &MUST; use the one with the more recent Date
908   header. This situation might arise because the cache is pooling
909   responses from other caches, or because a client has asked for a
910   reload or a revalidation of an apparently fresh cache entry.
914<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
916   Because a client might be receiving responses via multiple paths, so
917   that some responses flow through one set of caches and other
918   responses flow through a different set of caches, a client might
919   receive responses in an order different from that in which the origin
920   server sent them. We would like the client to use the most recently
921   generated response, even if older responses are still apparently
922   fresh.
925   Neither the entity tag nor the expiration value can impose an
926   ordering on responses, since it is possible that a later response
927   intentionally carries an earlier expiration time. The Date values are
928   ordered to a granularity of one second.
931   When a client tries to revalidate a cache entry, and the response it
932   receives contains a Date header that appears to be older than the one
933   for the existing entry, then the client &SHOULD; repeat the request
934   unconditionally, and include
936<figure><artwork type="example">
937    Cache-Control: max-age=0
940   to force any intermediate caches to validate their copies directly
941   with the origin server, or
943<figure><artwork type="example">
944    Cache-Control: no-cache
947   to force any intermediate caches to obtain a new copy from the origin
948   server.
951   If the Date values are equal, then the client &MAY; use either response
952   (or &MAY;, if it is being extremely prudent, request a new response).
953   Servers &MUST-NOT; depend on clients being able to choose
954   deterministically between responses generated during the same second,
955   if their expiration times overlap.
960<section title="Validation Model" anchor="validation.model">
962   When a cache has a stale entry that it would like to use as a
963   response to a client's request, it first has to check with the origin
964   server (or possibly an intermediate cache with a fresh response) to
965   see if its cached entry is still usable. We call this "validating"
966   the cache entry. Since we do not want to have to pay the overhead of
967   retransmitting the full response if the cached entry is good, and we
968   do not want to pay the overhead of an extra round trip if the cached
969   entry is invalid, the HTTP/1.1 protocol supports the use of
970   conditional methods.
973   The key protocol features for supporting conditional methods are
974   those concerned with "cache validators." When an origin server
975   generates a full response, it attaches some sort of validator to it,
976   which is kept with the cache entry. When a client (user agent or
977   proxy cache) makes a conditional request for a resource for which it
978   has a cache entry, it includes the associated validator in the
979   request.
982   The server then checks that validator against the current validator
983   for the entity, and, if they match (see &weak-and-strong-validators;), it responds
984   with a special status code (usually, 304 (Not Modified)) and no
985   entity-body. Otherwise, it returns a full response (including
986   entity-body). Thus, we avoid transmitting the full response if the
987   validator matches, and we avoid an extra round trip if it does not
988   match.
991   In HTTP/1.1, a conditional request looks exactly the same as a normal
992   request for the same resource, except that it carries a special
993   header (which includes the validator) that implicitly turns the
994   method (usually, GET) into a conditional.
997   The protocol includes both positive and negative senses of cache-validating
998   conditions. That is, it is possible to request either that
999   a method be performed if and only if a validator matches or if and
1000   only if no validators match.
1001  <list><t>
1002      <x:h>Note:</x:h> a response that lacks a validator may still be cached, and
1003      served from cache until it expires, unless this is explicitly
1004      prohibited by a cache-control directive. However, a cache cannot
1005      do a conditional retrieval if it does not have a validator for the
1006      entity, which means it will not be refreshable after it expires.
1007  </t></list>
1010<section title="Last-Modified Dates" anchor="last-modified.dates">
1012   The Last-Modified entity-header field value is often used as a cache
1013   validator. In simple terms, a cache entry is considered to be valid
1014   if the entity has not been modified since the Last-Modified value.
1018<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
1020   The ETag response-header field value, an entity tag, provides for an
1021   "opaque" cache validator. This might allow more reliable validation
1022   in situations where it is inconvenient to store modification dates,
1023   where the one-second resolution of HTTP date values is not
1024   sufficient, or where the origin server wishes to avoid certain
1025   paradoxes that might arise from the use of modification dates.
1028   Entity Tags are described in &entity-tags;. The headers used with entity
1029   tags are described in &entity-header-fields;.
1033<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
1035   The principle behind entity tags is that only the service author
1036   knows the semantics of a resource well enough to select an
1037   appropriate cache validation mechanism, and the specification of any
1038   validator comparison function more complex than byte-equality would
1039   open up a can of worms. Thus, comparisons of any other headers
1040   (except Last-Modified, for compatibility with HTTP/1.0) are never
1041   used for purposes of validating a cache entry.
1046<section title="Response Cacheability" anchor="response.cacheability">
1048   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1049   directive, a caching system &MAY; always store a successful response
1050   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1051   if it is fresh, and &MAY; return it after successful validation. If
1052   there is neither a cache validator nor an explicit expiration time
1053   associated with a response, we do not expect it to be cached, but
1054   certain caches &MAY; violate this expectation (for example, when little
1055   or no network connectivity is available). A client can usually detect
1056   that such a response was taken from a cache by comparing the Date
1057   header to the current time.
1058  <list><t>
1059      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1060      without providing any Warning.
1061  </t></list>
1064   However, in some cases it might be inappropriate for a cache to
1065   retain an entity, or to return it in response to a subsequent
1066   request. This might be because absolute semantic transparency is
1067   deemed necessary by the service author, or because of security or
1068   privacy considerations. Certain cache-control directives are
1069   therefore provided so that the server can indicate that certain
1070   resource entities, or portions thereof, are not to be cached
1071   regardless of other considerations.
1074   Note that &header-authorization; normally prevents a shared cache from saving
1075   and returning a response to a previous request if that request
1076   included an Authorization header.
1079   A response received with a status code of 200, 203, 206, 300, 301 or
1080   410 &MAY; be stored by a cache and used in reply to a subsequent
1081   request, subject to the expiration mechanism, unless a cache-control
1082   directive prohibits caching. However, a cache that does not support
1083   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1084   Content) responses.
1087   A response received with any other status code (e.g. status codes 302
1088   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1089   unless there are cache-control directives or another header(s) that
1090   explicitly allow it. For example, these include the following: an
1091   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1092   "proxy-revalidate", "public" or "private" cache-control
1093   directive (<xref target="header.cache-control"/>).
1097<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1099   The purpose of an HTTP cache is to store information received in
1100   response to requests for use in responding to future requests. In
1101   many cases, a cache simply returns the appropriate parts of a
1102   response to the requester. However, if the cache holds a cache entry
1103   based on a previous response, it might have to combine parts of a new
1104   response with what is held in the cache entry.
1107<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1109   For the purpose of defining the behavior of caches and non-caching
1110   proxies, we divide HTTP headers into two categories:
1111  <list style="symbols">
1112      <t>End-to-end headers, which are  transmitted to the ultimate
1113        recipient of a request or response. End-to-end headers in
1114        responses &MUST; be stored as part of a cache entry and &MUST; be
1115        transmitted in any response formed from a cache entry.</t>
1117      <t>Hop-by-hop headers, which are meaningful only for a single
1118        transport-level connection, and are not stored by caches or
1119        forwarded by proxies.</t>
1120  </list>
1123   The following HTTP/1.1 headers are hop-by-hop headers:
1124  <list style="symbols">
1125      <t>Connection</t>
1126      <t>Keep-Alive</t>
1127      <t>Proxy-Authenticate</t>
1128      <t>Proxy-Authorization</t>
1129      <t>TE</t>
1130      <t>Trailer</t>
1131      <t>Transfer-Encoding</t>
1132      <t>Upgrade</t>
1133  </list>
1136   All other headers defined by HTTP/1.1 are end-to-end headers.
1139   Other hop-by-hop headers &MUST; be listed in a Connection header
1140   (&header-connection;).
1144<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1146   Some features of the HTTP/1.1 protocol, such as Digest
1147   Authentication, depend on the value of certain end-to-end headers. A
1148   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1149   definition of that header requires or specifically allows that.
1152   A transparent proxy &MUST-NOT; modify any of the following fields in a
1153   request or response, and it &MUST-NOT; add any of these fields if not
1154   already present:
1155  <list style="symbols">
1156      <t>Content-Location</t>
1157      <t>Content-MD5</t>
1158      <t>ETag</t>
1159      <t>Last-Modified</t>
1160  </list>
1163   A transparent proxy &MUST-NOT; modify any of the following fields in a
1164   response:
1165  <list style="symbols">
1166    <t>Expires</t>
1167  </list>
1170   but it &MAY; add any of these fields if not already present. If an
1171   Expires header is added, it &MUST; be given a field-value identical to
1172   that of the Date header in that response.
1175   A  proxy &MUST-NOT; modify or add any of the following fields in a
1176   message that contains the no-transform cache-control directive, or in
1177   any request:
1178  <list style="symbols">
1179    <t>Content-Encoding</t>
1180    <t>Content-Range</t>
1181    <t>Content-Type</t>
1182  </list>
1185   A non-transparent proxy &MAY; modify or add these fields to a message
1186   that does not include no-transform, but if it does so, it &MUST; add a
1187   Warning 214 (Transformation applied) if one does not already appear
1188   in the message (see <xref target="header.warning"/>).
1189  <list><t>
1190      Warning: unnecessary modification of end-to-end headers might
1191      cause authentication failures if stronger authentication
1192      mechanisms are introduced in later versions of HTTP. Such
1193      authentication mechanisms &MAY; rely on the values of header fields
1194      not listed here.
1195    </t></list>
1198   The Content-Length field of a request or response is added or deleted
1199   according to the rules in &message-length;. A transparent proxy &MUST;
1200   preserve the entity-length (&entity-length;) of the entity-body,
1201   although it &MAY; change the transfer-length (&message-length;).
1205<section title="Combining Headers" anchor="combining.headers">
1207   When a cache makes a validating request to a server, and the server
1208   provides a 304 (Not Modified) response or a 206 (Partial Content)
1209   response, the cache then constructs a response to send to the
1210   requesting client.
1213   If the status code is 304 (Not Modified), the cache uses the entity-body
1214   stored in the cache entry as the entity-body of this outgoing
1215   response. If the status code is 206 (Partial Content) and the ETag or
1216   Last-Modified headers match exactly, the cache &MAY; combine the
1217   contents stored in the cache entry with the new contents received in
1218   the response and use the result as the entity-body of this outgoing
1219   response, (see &combining-byte-ranges;).
1222   The end-to-end headers stored in the cache entry are used for the
1223   constructed response, except that
1224  <list style="symbols">
1225    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1226      &MUST; be deleted from the cache entry and the forwarded response.</t>
1227    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1228        in the cache entry and the forwarded response.</t>
1229    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1230        replace the corresponding headers from the cache entry.</t>
1231  </list>
1234   Unless the cache decides to remove the cache entry, it &MUST; also
1235   replace the end-to-end headers stored with the cache entry with
1236   corresponding headers received in the incoming response, except for
1237   Warning headers as described immediately above. If a header field-name
1238   in the incoming response matches more than one header in the
1239   cache entry, all such old headers &MUST; be replaced.
1242   In other words, the set of end-to-end headers received in the
1243   incoming response overrides all corresponding end-to-end headers
1244   stored with the cache entry (except for stored Warning headers with
1245   warn-code 1xx, which are deleted even if not overridden).
1246  <list><t>
1247      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1248      Modified) or a 206 (Partial Content) response to update any header
1249      associated with a previous response for the same entity or sub-ranges
1250      thereof, although it might not always be meaningful or
1251      correct to do so. This rule does not allow an origin server to use
1252      a 304 (Not Modified) or a 206 (Partial Content) response to
1253      entirely delete a header that it had provided with a previous
1254      response.
1255  </t></list>
1261<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1263   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1264   by the presence of a Vary header field in a response, alters the
1265   conditions and procedure by which a cache can use the response for
1266   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1267   field by servers.
1270   A server &SHOULD; use the Vary header field to inform a cache of what
1271   request-header fields were used to select among multiple
1272   representations of a cacheable response subject to server-driven
1273   negotiation. The set of header fields named by the Vary field value
1274   is known as the "selecting" request-headers.
1277   When the cache receives a subsequent request whose Request-URI
1278   specifies one or more cache entries including a Vary header field,
1279   the cache &MUST-NOT; use such a cache entry to construct a response to
1280   the new request unless all of the selecting request-headers present
1281   in the new request match the corresponding stored request-headers in
1282   the original request.
1285   The selecting request-headers from two requests are defined to match
1286   if and only if the selecting request-headers in the first request can
1287   be transformed to the selecting request-headers in the second request
1288   by adding or removing linear white space (LWS) at places where this
1289   is allowed by the corresponding BNF, and/or combining multiple
1290   message-header fields with the same field name following the rules
1291   about message headers in &message-headers;.
1294   A Vary header field-value of "*" always fails to match and subsequent
1295   requests on that resource can only be properly interpreted by the
1296   origin server.
1299   If the selecting request header fields for the cached entry do not
1300   match the selecting request header fields of the new request, then
1301   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1302   it first relays the new request to the origin server in a conditional
1303   request and the server responds with 304 (Not Modified), including an
1304   entity tag or Content-Location that indicates the entity to be used.
1307   If an entity tag was assigned to a cached representation, the
1308   forwarded request &SHOULD; be conditional and include the entity tags
1309   in an If-None-Match header field from all its cache entries for the
1310   resource. This conveys to the server the set of entities currently
1311   held by the cache, so that if any one of these entities matches the
1312   requested entity, the server can use the ETag header field in its 304
1313   (Not Modified) response to tell the cache which entry is appropriate.
1314   If the entity-tag of the new response matches that of an existing
1315   entry, the new response &SHOULD; be used to update the header fields of
1316   the existing entry, and the result &MUST; be returned to the client.
1319   If any of the existing cache entries contains only partial content
1320   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1321   the If-None-Match header field unless the request is for a range that
1322   would be fully satisfied by that entry.
1325   If a cache receives a successful response whose Content-Location
1326   field matches that of an existing cache entry for the same Request-URI,
1327   whose entity-tag differs from that of the existing entry, and
1328   whose Date is more recent than that of the existing entry, the
1329   existing entry &SHOULD-NOT;  be returned in response to future requests
1330   and &SHOULD; be deleted from the cache.
1334<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1336   For reasons of security and privacy, it is necessary to make a
1337   distinction between "shared" and "non-shared" caches. A non-shared
1338   cache is one that is accessible only to a single user. Accessibility
1339   in this case &SHOULD; be enforced by appropriate security mechanisms.
1340   All other caches are considered to be "shared." Other sections of
1341   this specification place certain constraints on the operation of
1342   shared caches in order to prevent loss of privacy or failure of
1343   access controls.
1347<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1349   A cache that receives an incomplete response (for example, with fewer
1350   bytes of data than specified in a Content-Length header) &MAY; store
1351   the response. However, the cache &MUST; treat this as a partial
1352   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1353   the result might be a full response or might still be
1354   partial. A cache &MUST-NOT; return a partial response to a client
1355   without explicitly marking it as such, using the 206 (Partial
1356   Content) status code. A cache &MUST-NOT; return a partial response
1357   using a status code of 200 (OK).
1360   If a cache receives a 5xx response while attempting to revalidate an
1361   entry, it &MAY; either forward this response to the requesting client,
1362   or act as if the server failed to respond. In the latter case, it &MAY;
1363   return a previously received response unless the cached entry
1364   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1368<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1370   Unless the origin server explicitly prohibits the caching of their
1371   responses, the application of GET and HEAD methods to any resources
1372   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1373   these responses are taken from a cache. They &MAY; still have side
1374   effects, but a cache is not required to consider such side effects in
1375   its caching decisions. Caches are always expected to observe an
1376   origin server's explicit restrictions on caching.
1379   We note one exception to this rule: since some applications have
1380   traditionally used GETs and HEADs with query URLs (those containing a
1381   "?" in the rel_path part) to perform operations with significant side
1382   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1383   the server provides an explicit expiration time. This specifically
1384   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT;
1385   be taken from a cache. See &safe-methods; for related information.
1389<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1391   The effect of certain methods performed on a resource at the origin
1392   server might cause one or more existing cache entries to become non-transparently
1393   invalid. That is, although they might continue to be
1394   "fresh," they do not accurately reflect what the origin server would
1395   return for a new request on that resource.
1398   There is no way for the HTTP protocol to guarantee that all such
1399   cache entries are marked invalid. For example, the request that
1400   caused the change at the origin server might not have gone through
1401   the proxy where a cache entry is stored. However, several rules help
1402   reduce the likelihood of erroneous behavior.
1405   In this section, the phrase "invalidate an entity" means that the
1406   cache will either remove all instances of that entity from its
1407   storage, or will mark these as "invalid" and in need of a mandatory
1408   revalidation before they can be returned in response to a subsequent
1409   request.
1412   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1413   either the entity referred to by the Request-URI, or by the Location
1414   or Content-Location headers (if present). These methods are:
1415  <list style="symbols">
1416      <t>PUT</t>
1417      <t>DELETE</t>
1418      <t>POST</t>
1419  </list>
1422   An invalidation based
1423   on the URI in a Location or Content-Location header &MUST-NOT; be
1424   performed if the host part of that URI differs from the host part
1425   in the Request-URI. This helps prevent denial of service attacks.
1428   A cache that passes through requests for methods it does not
1429   understand &SHOULD; invalidate any entities referred to by the
1430   Request-URI.
1434<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1436   All methods that might be expected to cause modifications to the
1437   origin server's resources &MUST; be written through to the origin
1438   server. This currently includes all methods except for GET and HEAD.
1439   A cache &MUST-NOT; reply to such a request from a client before having
1440   transmitted the request to the inbound server, and having received a
1441   corresponding response from the inbound server. This does not prevent
1442   a proxy cache from sending a 100 (Continue) response before the
1443   inbound server has sent its final reply.
1446   The alternative (known as "write-back" or "copy-back" caching) is not
1447   allowed in HTTP/1.1, due to the difficulty of providing consistent
1448   updates and the problems arising from server, cache, or network
1449   failure prior to write-back.
1453<section title="Cache Replacement" anchor="cache.replacement">
1455   If a new cacheable (see Sections <xref target="" format="counter"/>,
1456   <xref target="disambiguating.expiration.values" format="counter"/>,
1457   <xref target="disambiguating.multiple.responses" format="counter"/>
1458   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1459   response is received from a resource while any existing responses for
1460   the same resource are cached, the cache &SHOULD; use the new response
1461   to reply to the current request. It &MAY; insert it into cache storage
1462   and &MAY;, if it meets all other requirements, use it to respond to any
1463   future requests that would previously have caused the old response to
1464   be returned. If it inserts the new response into cache storage  the
1465   rules in <xref target="combining.headers"/> apply.
1466  <list><t>
1467      <x:h>Note:</x:h> a new response that has an older Date header value than
1468      existing cached responses is not cacheable.
1469  </t></list>
1473<section title="History Lists" anchor="history.lists">
1475   User agents often have history mechanisms, such as "Back" buttons and
1476   history lists, which can be used to redisplay an entity retrieved
1477   earlier in a session.
1480   History mechanisms and caches are different. In particular history
1481   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1482   the current state of a resource. Rather, a history mechanism is meant
1483   to show exactly what the user saw at the time when the resource was
1484   retrieved.
1487   By default, an expiration time does not apply to history mechanisms.
1488   If the entity is still in storage, a history mechanism &SHOULD; display
1489   it even if the entity has expired, unless the user has specifically
1490   configured the agent to refresh expired history documents.
1493   This is not to be construed to prohibit the history mechanism from
1494   telling the user that a view might be stale.
1495  <list><t>
1496      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1497      viewing stale resources, this will tend to force service authors
1498      to avoid using HTTP expiration controls and cache controls when
1499      they would otherwise like to. Service authors may consider it
1500      important that users not be presented with error messages or
1501      warning messages when they use navigation controls (such as BACK)
1502      to view previously fetched resources. Even though sometimes such
1503      resources ought not be cached, or ought to expire quickly, user
1504      interface considerations may force service authors to resort to
1505      other means of preventing caching (e.g. "once-only" URLs) in order
1506      not to suffer the effects of improperly functioning history
1507      mechanisms.
1508  </t></list>
1512<section title="Header Field Definitions" anchor="header.fields">
1514   This section defines the syntax and semantics of HTTP/1.1 header fields
1515   related to caching.
1518   For entity-header fields, both sender and recipient refer to either the
1519   client or the server, depending on who sends and who receives the entity.
1522<section title="Age" anchor="header.age">
1523  <iref primary="true" item="Age header" x:for-anchor=""/>
1524  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1526      The Age response-header field conveys the sender's estimate of the
1527      amount of time since the response (or its revalidation) was
1528      generated at the origin server. A cached response is "fresh" if
1529      its age does not exceed its freshness lifetime. Age values are
1530      calculated as specified in <xref target="age.calculations"/>.
1532<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1533  Age = "Age" ":" age-value
1534  age-value = delta-seconds
1537      Age values are non-negative decimal integers, representing time in
1538      seconds.
1540<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
1541  delta-seconds  = 1*DIGIT
1544      If a cache receives a value larger than the largest positive
1545      integer it can represent, or if any of its age calculations
1546      overflows, it &MUST; transmit an Age header with a value of
1547      2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
1548      include an Age header field in every response generated from its
1549      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1550      bits of range.
1554<section title="Cache-Control" anchor="header.cache-control">
1555  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1556  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1558   The Cache-Control general-header field is used to specify directives
1559   that &MUST; be obeyed by all caching mechanisms along the
1560   request/response chain. The directives specify behavior intended to
1561   prevent caches from adversely interfering with the request or
1562   response. These directives typically override the default caching
1563   algorithms. Cache directives are unidirectional in that the presence
1564   of a directive in a request does not imply that the same directive is
1565   to be given in the response.
1566  <list><t>
1567      Note that HTTP/1.0 caches might not implement Cache-Control and
1568      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1569  </t></list>
1572   Cache directives &MUST; be passed through by a proxy or gateway
1573   application, regardless of their significance to that application,
1574   since the directives might be applicable to all recipients along the
1575   request/response chain. It is not possible to specify a cache-directive
1576   for a specific cache.
1578<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><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"/>
1579  Cache-Control   = "Cache-Control" ":" 1#cache-directive
1581  cache-directive = cache-request-directive
1582       | cache-response-directive
1584  cache-request-directive =
1585         "no-cache"                          ; <xref target=""/>
1586       | "no-store"                          ; <xref target=""/>
1587       | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1588       | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1589       | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1590       | "no-transform"                      ; <xref target="no-transform.directive"/>
1591       | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1592       | cache-extension                     ; <xref target="cache.control.extensions"/>
1594  cache-response-directive =
1595         "public"                               ; <xref target=""/>
1596       | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; <xref target=""/>
1597       | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]; <xref target=""/>
1598       | "no-store"                             ; <xref target=""/>
1599       | "no-transform"                         ; <xref target="no-transform.directive"/>
1600       | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1601       | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1602       | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1603       | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1604       | cache-extension                        ; <xref target="cache.control.extensions"/>
1606  cache-extension = token [ "=" ( token | quoted-string ) ]
1609   When a directive appears without any 1#field-name parameter, the
1610   directive applies to the entire request or response. When such a
1611   directive appears with a 1#field-name parameter, it applies only to
1612   the named field or fields, and not to the rest of the request or
1613   response. This mechanism supports extensibility; implementations of
1614   future versions of the HTTP protocol might apply these directives to
1615   header fields not defined in HTTP/1.1.
1618   The cache-control directives can be broken down into these general
1619   categories:
1620  <list style="symbols">
1621     <t>Restrictions on what are cacheable; these may only be imposed by
1622        the origin server.</t>
1624     <t>Restrictions on what may be stored by a cache; these may be
1625        imposed by either the origin server or the user agent.</t>
1627     <t>Modifications of the basic expiration mechanism; these may be
1628        imposed by either the origin server or the user agent.</t>
1630     <t>Controls over cache revalidation and reload; these may only be
1631        imposed by a user agent.</t>
1633     <t>Control over transformation of entities.</t>
1635     <t>Extensions to the caching system.</t>
1636  </list>
1639<section title="What is Cacheable" anchor="">
1641   By default, a response is cacheable if the requirements of the
1642   request method, request header fields, and the response status
1643   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1644   for cacheability. The following Cache-Control response directives
1645   allow an origin server to override the default cacheability of a
1646   response:
1649  <iref item="Cache Directives" subitem="public" primary="true"/>
1650  <iref item="public" subitem="Cache Directive" primary="true"/>
1651   public
1652  <list><t>
1653      Indicates that the response &MAY; be cached by any cache, even if it
1654      would normally be non-cacheable or cacheable only within a non-shared
1655      cache. (See also Authorization, &header-authorization;, for
1656      additional details.)
1657  </t></list>
1660  <iref item="Cache Directives" subitem="private" primary="true"/>
1661  <iref item="private" subitem="Cache Directive" primary="true"/>
1662   private
1663  <list><t>
1664      Indicates that all or part of the response message is intended for
1665      a single user and &MUST-NOT; be cached by a shared cache. This
1666      allows an origin server to state that the specified parts of the
1667      response are intended for only one user and are not a valid
1668      response for requests by other users. A private (non-shared) cache
1669      &MAY; cache the response.
1670    </t><t>
1671       <x:h>Note:</x:h> This usage of the word private only controls where the
1672       response may be cached, and cannot ensure the privacy of the
1673       message content.
1674  </t></list>
1677  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1678  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1679   no-cache
1680  <list><t>
1681       If the no-cache directive does not specify a field-name, then a
1682      cache &MUST-NOT; use the response to satisfy a subsequent request
1683      without successful revalidation with the origin server. This
1684      allows an origin server to prevent caching even by caches that
1685      have been configured to return stale responses to client requests.
1686    </t><t>
1687      If the no-cache directive does specify one or more field-names,
1688      then a cache &MAY; use the response to satisfy a subsequent request,
1689      subject to any other restrictions on caching. However, the
1690      specified field-name(s) &MUST-NOT; be sent in the response to a
1691      subsequent request without successful revalidation with the origin
1692      server. This allows an origin server to prevent the re-use of
1693      certain header fields in a response, while still allowing caching
1694      of the rest of the response.
1695    <list><t>
1696       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1697       directive.
1698    </t></list>
1699  </t></list>
1703<section title="What May be Stored by Caches" anchor="">
1705  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1706  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1707   no-store
1708  <list><t>   
1709      The purpose of the no-store directive is to prevent the
1710      inadvertent release or retention of sensitive information (for
1711      example, on backup tapes). The no-store directive applies to the
1712      entire message, and &MAY; be sent either in a response or in a
1713      request. If sent in a request, a cache &MUST-NOT; store any part of
1714      either this request or any response to it. If sent in a response,
1715      a cache &MUST-NOT; store any part of either this response or the
1716      request that elicited it. This directive applies to both non-shared
1717      and shared caches. "&MUST-NOT; store" in this context means
1718      that the cache &MUST-NOT; intentionally store the information in
1719      non-volatile storage, and &MUST; make a best-effort attempt to
1720      remove the information from volatile storage as promptly as
1721      possible after forwarding it.
1722  </t><t>
1723      Even when this directive is associated with a response, users
1724      might explicitly store such a response outside of the caching
1725      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1726      such responses as part of their normal operation.
1727  </t><t>
1728      The purpose of this directive is to meet the stated requirements
1729      of certain users and service authors who are concerned about
1730      accidental releases of information via unanticipated accesses to
1731      cache data structures. While the use of this directive might
1732      improve privacy in some cases, we caution that it is NOT in any
1733      way a reliable or sufficient mechanism for ensuring privacy. In
1734      particular, malicious or compromised caches might not recognize or
1735      obey this directive, and communications networks might be
1736      vulnerable to eavesdropping.
1737  </t></list>
1741<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1743   The expiration time of an entity &MAY; be specified by the origin
1744   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1745   it &MAY; be specified using the max-age directive in a response. When
1746   the max-age cache-control directive is present in a cached response,
1747   the response is stale if its current age is greater than the age
1748   value given (in seconds) at the time of a new request for that
1749   resource. The max-age directive on a response implies that the
1750   response is cacheable (i.e., "public") unless some other, more
1751   restrictive cache directive is also present.
1754   If a response includes both an Expires header and a max-age
1755   directive, the max-age directive overrides the Expires header, even
1756   if the Expires header is more restrictive. This rule allows an origin
1757   server to provide, for a given response, a longer expiration time to
1758   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1759   useful if certain HTTP/1.0 caches improperly calculate ages or
1760   expiration times, perhaps due to desynchronized clocks.
1763   Many HTTP/1.0 cache implementations will treat an Expires value that
1764   is less than or equal to the response Date value as being equivalent
1765   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1766   cache receives such a response, and the response does not include a
1767   Cache-Control header field, it &SHOULD; consider the response to be
1768   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1769  <list><t>
1770       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1771       cache control feature, such as the "private" directive, on a
1772       network including older caches that do not understand that
1773       feature. The origin server will need to combine the new feature
1774       with an Expires field whose value is less than or equal to the
1775       Date value. This will prevent older caches from improperly
1776       caching the response.
1777  </t></list>
1780  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1781  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1782   s-maxage
1783  <list><t>
1784       If a response includes an s-maxage directive, then for a shared
1785       cache (but not for a private cache), the maximum age specified by
1786       this directive overrides the maximum age specified by either the
1787       max-age directive or the Expires header. The s-maxage directive
1788       also implies the semantics of the proxy-revalidate directive (see
1789       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1790       entry after it becomes stale to respond to a subsequent request
1791       without first revalidating it with the origin server. The s-maxage
1792       directive is always ignored by a private cache.
1793  </t></list>
1796   Note that most older caches, not compliant with this specification,
1797   do not implement any cache-control directives. An origin server
1798   wishing to use a cache-control directive that restricts, but does not
1799   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1800   requirement that the max-age directive overrides the Expires header,
1801   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1802   max-age directive.
1805   Other directives allow a user agent to modify the basic expiration
1806   mechanism. These directives &MAY; be specified on a request:
1809  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1810  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1811   max-age
1812  <list><t>
1813      Indicates that the client is willing to accept a response whose
1814      age is no greater than the specified time in seconds. Unless max-stale
1815      directive is also included, the client is not willing to
1816      accept a stale response.
1817  </t></list>
1820  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1821  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1822   min-fresh
1823  <list><t>
1824      Indicates that the client is willing to accept a response whose
1825      freshness lifetime is no less than its current age plus the
1826      specified time in seconds. That is, the client wants a response
1827      that will still be fresh for at least the specified number of
1828      seconds.
1829  </t></list>
1832  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1833  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1834   max-stale
1835  <list><t>
1836      Indicates that the client is willing to accept a response that has
1837      exceeded its expiration time. If max-stale is assigned a value,
1838      then the client is willing to accept a response that has exceeded
1839      its expiration time by no more than the specified number of
1840      seconds. If no value is assigned to max-stale, then the client is
1841      willing to accept a stale response of any age.
1842  </t></list>
1845   If a cache returns a stale response, either because of a max-stale
1846   directive on a request, or because the cache is configured to
1847   override the expiration time of a response, the cache &MUST; attach a
1848   Warning header to the stale response, using Warning 110 (Response is
1849   stale).
1852   A cache &MAY; be configured to return stale responses without
1853   validation, but only if this does not conflict with any "MUST"-level
1854   requirements concerning cache validation (e.g., a "must-revalidate"
1855   cache-control directive).
1858   If both the new request and the cached entry include "max-age"
1859   directives, then the lesser of the two values is used for determining
1860   the freshness of the cached entry for that request.
1864<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1866   Sometimes a user agent might want or need to insist that a cache
1867   revalidate its cache entry with the origin server (and not just with
1868   the next cache along the path to the origin server), or to reload its
1869   cache entry from the origin server. End-to-end revalidation might be
1870   necessary if either the cache or the origin server has overestimated
1871   the expiration time of the cached response. End-to-end reload may be
1872   necessary if the cache entry has become corrupted for some reason.
1875   End-to-end revalidation may be requested either when the client does
1876   not have its own local cached copy, in which case we call it
1877   "unspecified end-to-end revalidation", or when the client does have a
1878   local cached copy, in which case we call it "specific end-to-end
1879   revalidation."
1882   The client can specify these three kinds of action using Cache-Control
1883   request directives:
1886   End-to-end reload
1887  <list><t>
1888      The request includes a "no-cache" cache-control directive or, for
1889      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1890      names &MUST-NOT; be included with the no-cache directive in a
1891      request. The server &MUST-NOT; use a cached copy when responding to
1892      such a request.
1893  </t></list>
1896   Specific end-to-end revalidation
1897  <list><t>
1898      The request includes a "max-age=0" cache-control directive, which
1899      forces each cache along the path to the origin server to
1900      revalidate its own entry, if any, with the next cache or server.
1901      The initial request includes a cache-validating conditional with
1902      the client's current validator.
1903  </t></list>
1906   Unspecified end-to-end revalidation
1907  <list><t>
1908      The request includes "max-age=0" cache-control directive, which
1909      forces each cache along the path to the origin server to
1910      revalidate its own entry, if any, with the next cache or server.
1911      The initial request does not include a cache-validating
1912      conditional; the first cache along the path (if any) that holds a
1913      cache entry for this resource includes a cache-validating
1914      conditional with its current validator.
1915  </t></list>
1918  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1919  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1920   max-age
1921  <list><t>
1922      When an intermediate cache is forced, by means of a max-age=0
1923      directive, to revalidate its own cache entry, and the client has
1924      supplied its own validator in the request, the supplied validator
1925      might differ from the validator currently stored with the cache
1926      entry. In this case, the cache &MAY; use either validator in making
1927      its own request without affecting semantic transparency.
1928  </t><t>
1929      However, the choice of validator might affect performance. The
1930      best approach is for the intermediate cache to use its own
1931      validator when making its request. If the server replies with 304
1932      (Not Modified), then the cache can return its now validated copy
1933      to the client with a 200 (OK) response. If the server replies with
1934      a new entity and cache validator, however, the intermediate cache
1935      can compare the returned validator with the one provided in the
1936      client's request, using the strong comparison function. If the
1937      client's validator is equal to the origin server's, then the
1938      intermediate cache simply returns 304 (Not Modified). Otherwise,
1939      it returns the new entity with a 200 (OK) response.
1940  </t><t>
1941      If a request includes the no-cache directive, it &SHOULD-NOT;
1942      include min-fresh, max-stale, or max-age.
1943  </t></list>
1946  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1947  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1948   only-if-cached
1949  <list><t>
1950      In some cases, such as times of extremely poor network
1951      connectivity, a client may want a cache to return only those
1952      responses that it currently has stored, and not to reload or
1953      revalidate with the origin server. To do this, the client may
1954      include the only-if-cached directive in a request. If it receives
1955      this directive, a cache &SHOULD; either respond using a cached entry
1956      that is consistent with the other constraints of the request, or
1957      respond with a 504 (Gateway Timeout) status. However, if a group
1958      of caches is being operated as a unified system with good internal
1959      connectivity, such a request &MAY; be forwarded within that group of
1960      caches.
1961  </t></list>
1964  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1965  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1966   must-revalidate
1967  <list><t>
1968      Because a cache &MAY; be configured to ignore a server's specified
1969      expiration time, and because a client request &MAY; include a max-stale
1970      directive (which has a similar effect), the protocol also
1971      includes a mechanism for the origin server to require revalidation
1972      of a cache entry on any subsequent use. When the must-revalidate
1973      directive is present in a response received by a cache, that cache
1974      &MUST-NOT; use the entry after it becomes stale to respond to a
1975      subsequent request without first revalidating it with the origin
1976      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1977      time, if, based solely on the origin server's Expires or max-age
1978      value, the cached response is stale.)
1979  </t><t>
1980      The must-revalidate directive is necessary to support reliable
1981      operation for certain protocol features. In all circumstances an
1982      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1983      particular, if the cache cannot reach the origin server for any
1984      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1985  </t><t>
1986      Servers &SHOULD; send the must-revalidate directive if and only if
1987      failure to revalidate a request on the entity could result in
1988      incorrect operation, such as a silently unexecuted financial
1989      transaction. Recipients &MUST-NOT; take any automated action that
1990      violates this directive, and &MUST-NOT; automatically provide an
1991      unvalidated copy of the entity if revalidation fails.
1992  </t><t>
1993      Although this is not recommended, user agents operating under
1994      severe connectivity constraints &MAY; violate this directive but, if
1995      so, &MUST; explicitly warn the user that an unvalidated response has
1996      been provided. The warning &MUST; be provided on each unvalidated
1997      access, and &SHOULD; require explicit user confirmation.
1998  </t></list>
2001  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
2002  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
2003   proxy-revalidate
2004  <list><t>
2005      The proxy-revalidate directive has the same meaning as the must-revalidate
2006      directive, except that it does not apply to non-shared
2007      user agent caches. It can be used on a response to an
2008      authenticated request to permit the user's cache to store and
2009      later return the response without needing to revalidate it (since
2010      it has already been authenticated once by that user), while still
2011      requiring proxies that service many users to revalidate each time
2012      (in order to make sure that each user has been authenticated).
2013      Note that such authenticated responses also need the public cache
2014      control directive in order to allow them to be cached at all.
2015  </t></list>
2019<section title="No-Transform Directive" anchor="no-transform.directive">
2021  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2022  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2023   no-transform
2024  <list><t>
2025      Implementors of intermediate caches (proxies) have found it useful
2026      to convert the media type of certain entity bodies. A non-transparent
2027      proxy might, for example, convert between image
2028      formats in order to save cache space or to reduce the amount of
2029      traffic on a slow link.
2030  </t><t>
2031      Serious operational problems occur, however, when these
2032      transformations are applied to entity bodies intended for certain
2033      kinds of applications. For example, applications for medical
2034      imaging, scientific data analysis and those using end-to-end
2035      authentication, all depend on receiving an entity body that is bit
2036      for bit identical to the original entity-body.
2037  </t><t>
2038      Therefore, if a message includes the no-transform directive, an
2039      intermediate cache or proxy &MUST-NOT; change those headers that are
2040      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2041      directive. This implies that the cache or proxy &MUST-NOT; change
2042      any aspect of the entity-body that is specified by these headers,
2043      including the value of the entity-body itself.
2044  </t></list>
2048<section title="Cache Control Extensions" anchor="cache.control.extensions">
2050   The Cache-Control header field can be extended through the use of one
2051   or more cache-extension tokens, each with an optional assigned value.
2052   Informational extensions (those which do not require a change in
2053   cache behavior) &MAY; be added without changing the semantics of other
2054   directives. Behavioral extensions are designed to work by acting as
2055   modifiers to the existing base of cache directives. Both the new
2056   directive and the standard directive are supplied, such that
2057   applications which do not understand the new directive will default
2058   to the behavior specified by the standard directive, and those that
2059   understand the new directive will recognize it as modifying the
2060   requirements associated with the standard directive. In this way,
2061   extensions to the cache-control directives can be made without
2062   requiring changes to the base protocol.
2065   This extension mechanism depends on an HTTP cache obeying all of the
2066   cache-control directives defined for its native HTTP-version, obeying
2067   certain extensions, and ignoring all directives that it does not
2068   understand.
2071   For example, consider a hypothetical new response directive called
2072   community which acts as a modifier to the private directive. We
2073   define this new directive to mean that, in addition to any non-shared
2074   cache, any cache which is shared only by members of the community
2075   named within its value may cache the response. An origin server
2076   wishing to allow the UCI community to use an otherwise private
2077   response in their shared cache(s) could do so by including
2079<figure><artwork type="example">
2080    Cache-Control: private, community="UCI"
2083   A cache seeing this header field will act correctly even if the cache
2084   does not understand the community cache-extension, since it will also
2085   see and understand the private directive and thus default to the safe
2086   behavior.
2089   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2090   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2091   be combined with standard directives (or the response's default
2092   cacheability) such that the cache behavior will remain minimally
2093   correct even if the cache does not understand the extension(s).
2098<section title="Expires" anchor="header.expires">
2099  <iref primary="true" item="Expires header" x:for-anchor=""/>
2100  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2102   The Expires entity-header field gives the date/time after which the
2103   response is considered stale. A stale cache entry may not normally be
2104   returned by a cache (either a proxy cache or a user agent cache)
2105   unless it is first validated with the origin server (or with an
2106   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2107   for further discussion of the expiration model.
2110   The presence of an Expires field does not imply that the original
2111   resource will change or cease to exist at, before, or after that
2112   time.
2115   The format is an absolute date and time as defined by HTTP-date in
2116   &full-date;; it &MUST; be sent in rfc1123-date format.
2118<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2119  Expires = "Expires" ":" HTTP-date
2122   An example of its use is
2124<figure><artwork type="example">
2125   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2128  <list><t>
2129      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2130      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2131      Expires field.
2132  </t></list>
2135   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2136   especially including the value "0", as in the past (i.e., "already
2137   expired").
2140   To mark a response as "already expired," an origin server sends an
2141   Expires date that is equal to the Date header value. (See the rules
2142   for expiration calculations in <xref target="expiration.calculations"/>.)
2145   To mark a response as "never expires," an origin server sends an
2146   Expires date approximately one year from the time the response is
2147   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2148   year in the future.
2151   The presence of an Expires header field with a date value of some
2152   time in the future on a response that otherwise would by default be
2153   non-cacheable indicates that the response is cacheable, unless
2154   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2158<section title="Pragma" anchor="header.pragma">
2159  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2160  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2162   The Pragma general-header field is used to include implementation-specific
2163   directives that might apply to any recipient along the
2164   request/response chain. All pragma directives specify optional
2165   behavior from the viewpoint of the protocol; however, some systems
2166   &MAY; require that behavior be consistent with the directives.
2168<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
2169  Pragma            = "Pragma" ":" 1#pragma-directive
2170  pragma-directive  = "no-cache" | extension-pragma
2171  extension-pragma  = token [ "=" ( token | quoted-string ) ]
2174   When the no-cache directive is present in a request message, an
2175   application &SHOULD; forward the request toward the origin server even
2176   if it has a cached copy of what is being requested. This pragma
2177   directive has the same semantics as the no-cache cache-directive (see
2178   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2179   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2180   request is sent to a server not known to be HTTP/1.1 compliant.
2183   Pragma directives &MUST; be passed through by a proxy or gateway
2184   application, regardless of their significance to that application,
2185   since the directives might be applicable to all recipients along the
2186   request/response chain. It is not possible to specify a pragma for a
2187   specific recipient; however, any pragma directive not relevant to a
2188   recipient &SHOULD; be ignored by that recipient.
2191   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2192   sent "Cache-Control: no-cache". No new Pragma directives will be
2193   defined in HTTP.
2194  <list><t>
2195      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
2196      response-header field is not actually specified, it does not provide a
2197      reliable replacement for "Cache-Control: no-cache" in a response.
2198  </t></list>
2202<section title="Vary" anchor="header.vary">
2203  <iref primary="true" item="Vary header" x:for-anchor=""/>
2204  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2206   The Vary field value indicates the set of request-header fields that
2207   fully determines, while the response is fresh, whether a cache is
2208   permitted to use the response to reply to a subsequent request
2209   without revalidation. For uncacheable or stale responses, the Vary
2210   field value advises the user agent about the criteria that were used
2211   to select the representation. A Vary field value of "*" implies that
2212   a cache cannot determine from the request headers of a subsequent
2213   request whether this response is the appropriate representation. See
2214   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2216<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2217  Vary  = "Vary" ":" ( "*" | 1#field-name )
2220   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2221   cacheable response that is subject to server-driven negotiation.
2222   Doing so allows a cache to properly interpret future requests on that
2223   resource and informs the user agent about the presence of negotiation
2224   on that resource. A server &MAY; include a Vary header field with a
2225   non-cacheable response that is subject to server-driven negotiation,
2226   since this might provide the user agent with useful information about
2227   the dimensions over which the response varies at the time of the
2228   response.
2231   A Vary field value consisting of a list of field-names signals that
2232   the representation selected for the response is based on a selection
2233   algorithm which considers ONLY the listed request-header field values
2234   in selecting the most appropriate representation. A cache &MAY; assume
2235   that the same selection will be made for future requests with the
2236   same values for the listed field names, for the duration of time for
2237   which the response is fresh.
2240   The field-names given are not limited to the set of standard
2241   request-header fields defined by this specification. Field names are
2242   case-insensitive.
2245   A Vary field value of "*" signals that unspecified parameters not
2246   limited to the request-headers (e.g., the network address of the
2247   client), play a role in the selection of the response representation.
2248   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2249   generated by an origin server.
2253<section title="Warning" anchor="header.warning">
2254  <iref primary="true" item="Warning header" x:for-anchor=""/>
2255  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2257   The Warning general-header field is used to carry additional
2258   information about the status or transformation of a message which
2259   might not be reflected in the message. This information is typically
2260   used to warn about a possible lack of semantic transparency from
2261   caching operations or transformations applied to the entity body of
2262   the message.
2265   Warning headers are sent with responses using:
2267<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><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"/>
2268  Warning    = "Warning" ":" 1#warning-value
2270  warning-value = warn-code SP warn-agent SP warn-text
2271                                        [SP warn-date]
2273  warn-code  = 3DIGIT
2274  warn-agent = ( host [ ":" port ] ) | pseudonym
2275                  ; the name or pseudonym of the server adding
2276                  ; the Warning header, for use in debugging
2277  warn-text  = quoted-string
2278  warn-date  = DQUOTE HTTP-date DQUOTE
2281   A response &MAY; carry more than one Warning header.
2284   The warn-text &SHOULD; be in a natural language and character set that
2285   is most likely to be intelligible to the human user receiving the
2286   response. This decision &MAY; be based on any available knowledge, such
2287   as the location of the cache or user, the Accept-Language field in a
2288   request, the Content-Language field in a response, etc. The default
2289   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2292   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2293   in the warn-text using the method described in <xref target="RFC2047"/>.
2296   Warning headers can in general be applied to any message, however
2297   some specific warn-codes are specific to caches and can only be
2298   applied to response messages. New Warning headers &SHOULD; be added
2299   after any existing Warning headers. A cache &MUST-NOT; delete any
2300   Warning header that it received with a message. However, if a cache
2301   successfully validates a cache entry, it &SHOULD; remove any Warning
2302   headers previously attached to that entry except as specified for
2303   specific Warning codes. It &MUST; then add any Warning headers received
2304   in the validating response. In other words, Warning headers are those
2305   that would be attached to the most recent relevant response.
2308   When multiple Warning headers are attached to a response, the user
2309   agent ought to inform the user of as many of them as possible, in the
2310   order that they appear in the response. If it is not possible to
2311   inform the user of all of the warnings, the user agent &SHOULD; follow
2312   these heuristics:
2313  <list style="symbols">
2314    <t>Warnings that appear early in the response take priority over
2315        those appearing later in the response.</t>
2317    <t>Warnings in the user's preferred character set take priority
2318        over warnings in other character sets but with identical warn-codes
2319        and warn-agents.</t>
2320  </list>
2323   Systems that generate multiple Warning headers &SHOULD; order them with
2324   this user agent behavior in mind.
2327   Requirements for the behavior of caches with respect to Warnings are
2328   stated in <xref target="warnings"/>.
2331   This is a list of the currently-defined warn-codes, each with a
2332   recommended warn-text in English, and a description of its meaning.
2335   110 Response is stale
2336  <list><t>
2337     &MUST; be included whenever the returned response is stale.
2338  </t></list>
2341   111 Revalidation failed
2342  <list><t>
2343     &MUST; be included if a cache returns a stale response because an
2344     attempt to revalidate the response failed, due to an inability to
2345     reach the server.
2346  </t></list>
2349   112 Disconnected operation
2350  <list><t>
2351     &SHOULD; be included if the cache is intentionally disconnected from
2352     the rest of the network for a period of time.
2353  </t></list>
2356   113 Heuristic expiration
2357  <list><t>
2358     &MUST; be included if the cache heuristically chose a freshness
2359     lifetime greater than 24 hours and the response's age is greater
2360     than 24 hours.
2361  </t></list>
2364   199 Miscellaneous warning
2365  <list><t>
2366     The warning text &MAY; include arbitrary information to be presented
2367     to a human user, or logged. A system receiving this warning &MUST-NOT;
2368     take any automated action, besides presenting the warning to
2369     the user.
2370  </t></list>
2373   214 Transformation applied
2374  <list><t>
2375     &MUST; be added by an intermediate cache or proxy if it applies any
2376     transformation changing the content-coding (as specified in the
2377     Content-Encoding header) or media-type (as specified in the
2378     Content-Type header) of the response, or the entity-body of the
2379     response, unless this Warning code already appears in the response.
2380  </t></list>
2383   299 Miscellaneous persistent warning
2384  <list><t>
2385     The warning text &MAY; include arbitrary information to be presented
2386     to a human user, or logged. A system receiving this warning &MUST-NOT;
2387     take any automated action.
2388  </t></list>
2391   If an implementation sends a message with one or more Warning headers
2392   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2393   each warning-value a warn-date that matches the date in the response.
2396   If an implementation receives a message with a warning-value that
2397   includes a warn-date, and that warn-date is different from the Date
2398   value in the response, then that warning-value &MUST; be deleted from
2399   the message before storing, forwarding, or using it. (This prevents
2400   bad consequences of naive caching of Warning header fields.) If all
2401   of the warning-values are deleted for this reason, the Warning header
2402   &MUST; be deleted as well.
2408<section title="IANA Considerations" anchor="IANA.considerations">
2410   TBD.
2414<section title="Security Considerations" anchor="security.considerations">
2416   Caching proxies provide additional potential vulnerabilities, since
2417   the contents of the cache represent an attractive target for
2418   malicious exploitation. Because cache contents persist after an HTTP
2419   request is complete, an attack on the cache can reveal information
2420   long after a user believes that the information has been removed from
2421   the network. Therefore, cache contents should be protected as
2422   sensitive information.
2426<section title="Acknowledgments" anchor="ack">
2428   Much of the content and presentation of the caching design is due to
2429   suggestions and comments from individuals including: Shel Kaphan,
2430   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2436<references title="Normative References">
2438<reference anchor="ISO-8859-1">
2439  <front>
2440    <title>
2441     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2442    </title>
2443    <author>
2444      <organization>International Organization for Standardization</organization>
2445    </author>
2446    <date year="1998"/>
2447  </front>
2448  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2451<reference anchor="Part1">
2452  <front>
2453    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2454    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2455      <organization abbrev="Day Software">Day Software</organization>
2456      <address><email></email></address>
2457    </author>
2458    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2459      <organization>One Laptop per Child</organization>
2460      <address><email></email></address>
2461    </author>
2462    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2463      <organization abbrev="HP">Hewlett-Packard Company</organization>
2464      <address><email></email></address>
2465    </author>
2466    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2467      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2468      <address><email></email></address>
2469    </author>
2470    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2471      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2472      <address><email></email></address>
2473    </author>
2474    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2475      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2476      <address><email></email></address>
2477    </author>
2478    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2479      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2480      <address><email></email></address>
2481    </author>
2482    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2483      <organization abbrev="W3C">World Wide Web Consortium</organization>
2484      <address><email></email></address>
2485    </author>
2486    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2487      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2488      <address><email></email></address>
2489    </author>
2490    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2491  </front>
2492  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2493  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2496<reference anchor="Part2">
2497  <front>
2498    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2499    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2500      <organization abbrev="Day Software">Day Software</organization>
2501      <address><email></email></address>
2502    </author>
2503    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2504      <organization>One Laptop per Child</organization>
2505      <address><email></email></address>
2506    </author>
2507    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2508      <organization abbrev="HP">Hewlett-Packard Company</organization>
2509      <address><email></email></address>
2510    </author>
2511    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2512      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2513      <address><email></email></address>
2514    </author>
2515    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2516      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2517      <address><email></email></address>
2518    </author>
2519    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2520      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2521      <address><email></email></address>
2522    </author>
2523    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2524      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2525      <address><email></email></address>
2526    </author>
2527    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2528      <organization abbrev="W3C">World Wide Web Consortium</organization>
2529      <address><email></email></address>
2530    </author>
2531    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2532      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2533      <address><email></email></address>
2534    </author>
2535    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2536  </front>
2537  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2538  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2541<reference anchor="Part3">
2542  <front>
2543    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2544    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2545      <organization abbrev="Day Software">Day Software</organization>
2546      <address><email></email></address>
2547    </author>
2548    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2549      <organization>One Laptop per Child</organization>
2550      <address><email></email></address>
2551    </author>
2552    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2553      <organization abbrev="HP">Hewlett-Packard Company</organization>
2554      <address><email></email></address>
2555    </author>
2556    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2557      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2558      <address><email></email></address>
2559    </author>
2560    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2561      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2562      <address><email></email></address>
2563    </author>
2564    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2565      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2566      <address><email></email></address>
2567    </author>
2568    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2569      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2570      <address><email></email></address>
2571    </author>
2572    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2573      <organization abbrev="W3C">World Wide Web Consortium</organization>
2574      <address><email></email></address>
2575    </author>
2576    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2577      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2578      <address><email></email></address>
2579    </author>
2580    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2581  </front>
2582  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2583  <x:source href="p3-payload.xml" basename="p3-payload"/>
2586<reference anchor="Part4">
2587  <front>
2588    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2589    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2590      <organization abbrev="Day Software">Day Software</organization>
2591      <address><email></email></address>
2592    </author>
2593    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2594      <organization>One Laptop per Child</organization>
2595      <address><email></email></address>
2596    </author>
2597    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2598      <organization abbrev="HP">Hewlett-Packard Company</organization>
2599      <address><email></email></address>
2600    </author>
2601    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2602      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2603      <address><email></email></address>
2604    </author>
2605    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2606      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2607      <address><email></email></address>
2608    </author>
2609    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2610      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2611      <address><email></email></address>
2612    </author>
2613    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2614      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2615      <address><email></email></address>
2616    </author>
2617    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2618      <organization abbrev="W3C">World Wide Web Consortium</organization>
2619      <address><email></email></address>
2620    </author>
2621    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2622      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2623      <address><email></email></address>
2624    </author>
2625    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2626  </front>
2627  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2628  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2631<reference anchor="Part5">
2632  <front>
2633    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2634    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2635      <organization abbrev="Day Software">Day Software</organization>
2636      <address><email></email></address>
2637    </author>
2638    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2639      <organization>One Laptop per Child</organization>
2640      <address><email></email></address>
2641    </author>
2642    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2643      <organization abbrev="HP">Hewlett-Packard Company</organization>
2644      <address><email></email></address>
2645    </author>
2646    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2647      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2648      <address><email></email></address>
2649    </author>
2650    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2651      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2652      <address><email></email></address>
2653    </author>
2654    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2655      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2656      <address><email></email></address>
2657    </author>
2658    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2659      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2660      <address><email></email></address>
2661    </author>
2662    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2663      <organization abbrev="W3C">World Wide Web Consortium</organization>
2664      <address><email></email></address>
2665    </author>
2666    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2667      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2668      <address><email></email></address>
2669    </author>
2670    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2671  </front>
2672  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2673  <x:source href="p5-range.xml" basename="p5-range"/>
2676<reference anchor="Part7">
2677  <front>
2678    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2679    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2680      <organization abbrev="Day Software">Day Software</organization>
2681      <address><email></email></address>
2682    </author>
2683    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2684      <organization>One Laptop per Child</organization>
2685      <address><email></email></address>
2686    </author>
2687    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2688      <organization abbrev="HP">Hewlett-Packard Company</organization>
2689      <address><email></email></address>
2690    </author>
2691    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2692      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2693      <address><email></email></address>
2694    </author>
2695    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2696      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2697      <address><email></email></address>
2698    </author>
2699    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2700      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2701      <address><email></email></address>
2702    </author>
2703    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2704      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2705      <address><email></email></address>
2706    </author>
2707    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2708      <organization abbrev="W3C">World Wide Web Consortium</organization>
2709      <address><email></email></address>
2710    </author>
2711    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2712      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2713      <address><email></email></address>
2714    </author>
2715    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2716  </front>
2717  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2718  <x:source href="p7-auth.xml" basename="p7-auth"/>
2721<reference anchor="RFC2047">
2722  <front>
2723    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2724    <author initials="K." surname="Moore" fullname="Keith Moore">
2725      <organization>University of Tennessee</organization>
2726      <address><email></email></address>
2727    </author>
2728    <date month="November" year="1996"/>
2729  </front>
2730  <seriesInfo name="RFC" value="2047"/>
2733<reference anchor="RFC2119">
2734  <front>
2735    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2736    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2737      <organization>Harvard University</organization>
2738      <address><email></email></address>
2739    </author>
2740    <date month="March" year="1997"/>
2741  </front>
2742  <seriesInfo name="BCP" value="14"/>
2743  <seriesInfo name="RFC" value="2119"/>
2748<references title="Informative References">
2750<reference anchor="RFC1305">
2751  <front>
2752    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2753    <author initials="D." surname="Mills" fullname="David L. Mills">
2754      <organization>University of Delaware, Electrical Engineering Department</organization>
2755      <address><email></email></address>
2756    </author>
2757    <date month="March" year="1992"/>
2758  </front>
2759  <seriesInfo name="RFC" value="1305"/>
2762<reference anchor="RFC2616">
2763  <front>
2764    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2765    <author initials="R." surname="Fielding" fullname="R. Fielding">
2766      <organization>University of California, Irvine</organization>
2767      <address><email></email></address>
2768    </author>
2769    <author initials="J." surname="Gettys" fullname="J. Gettys">
2770      <organization>W3C</organization>
2771      <address><email></email></address>
2772    </author>
2773    <author initials="J." surname="Mogul" fullname="J. Mogul">
2774      <organization>Compaq Computer Corporation</organization>
2775      <address><email></email></address>
2776    </author>
2777    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2778      <organization>MIT Laboratory for Computer Science</organization>
2779      <address><email></email></address>
2780    </author>
2781    <author initials="L." surname="Masinter" fullname="L. Masinter">
2782      <organization>Xerox Corporation</organization>
2783      <address><email></email></address>
2784    </author>
2785    <author initials="P." surname="Leach" fullname="P. Leach">
2786      <organization>Microsoft Corporation</organization>
2787      <address><email></email></address>
2788    </author>
2789    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2790      <organization>W3C</organization>
2791      <address><email></email></address>
2792    </author>
2793    <date month="June" year="1999"/>
2794  </front>
2795  <seriesInfo name="RFC" value="2616"/>
2800<section title="Compatibility with Previous Versions" anchor="compatibility">
2802<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2804   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2805   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2806   <xref target="header.cache-control" format="counter"/>,
2807   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2810   Transfer-coding and message lengths all interact in ways that
2811   required fixing exactly when chunked encoding is used (to allow for
2812   transfer encoding that may not be self delimiting); it was important
2813   to straighten out exactly how message lengths are computed.
2814   (<xref target="non-modifiable.headers"/>,
2815   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2818   Proxies should be able to add Content-Length when appropriate.
2819   (<xref target="non-modifiable.headers"/>)
2822   Range request responses would become very verbose if all meta-data
2823   were always returned; by allowing the server to only send needed
2824   headers in a 206 response, this problem can be avoided.
2825   (<xref target="combining.headers"/>)
2828   The Cache-Control: max-age directive was not properly defined for
2829   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2832   Warnings could be cached incorrectly, or not updated appropriately.
2833   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2834   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2835   and <xref target="header.warning" format="counter"/>) Warning
2836   also needed to be a general header, as PUT or other methods may have
2837   need for it in requests.
2841<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2843  Clarify denial of service attack avoidance requirement.
2844  (<xref target="invalidation.after.updates.or.deletions"/>)
2850<section title="Change Log (to be removed by RFC Editor before publication)">
2852<section title="Since RFC2616">
2854  Extracted relevant partitions from <xref target="RFC2616"/>.
2858<section title="Since draft-ietf-httpbis-p6-cache-00">
2860  Closed issues:
2861  <list style="symbols">
2862    <t>
2863      <eref target=""/>:
2864      "Trailer"
2865      (<eref target=""/>)
2866    </t>
2867    <t>
2868      <eref target=""/>:
2869      "Invalidation after Update or Delete"
2870      (<eref target=""/>)
2871    </t>
2872    <t>
2873      <eref target=""/>:
2874      "Normative and Informative references"
2875    </t>
2876    <t>
2877      <eref target=""/>:
2878      "Date reference typo"
2879    </t>
2880    <t>
2881      <eref target=""/>:
2882      "Connection header text"
2883    </t>
2884    <t>
2885      <eref target=""/>:
2886      "Informative references"
2887    </t>
2888    <t>
2889      <eref target=""/>:
2890      "ISO-8859-1 Reference"
2891    </t>
2892    <t>
2893      <eref target=""/>:
2894      "Normative up-to-date references"
2895    </t>
2896    <t>
2897      <eref target=""/>:
2898      "typo in 13.2.2"
2899    </t>
2900  </list>
2903  Other changes:
2904  <list style="symbols">
2905    <t>
2906      Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target=""/>)
2907    </t>
2908  </list>
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