source: draft-ietf-httpbis/01/p6-cache.xml @ 165

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set date and version for draft 01

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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='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "01">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY entity-header-fields        "<xref target='Part4' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#entity.tags' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY weak-and-strong-validators  "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31]>
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='http://purl.org/net/xml2rfc/ext'>
44<front>
45
46  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
47
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>fielding@gbiv.com</email>
61      <uri>http://roy.gbiv.com/</uri>
62    </address>
63  </author>
64
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>jg@laptop.org</email>
76      <uri>http://www.laptop.org/</uri>
77    </address>
78  </author>
79 
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>JeffMogul@acm.org</email>
92    </address>
93  </author>
94
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>henrikn@microsoft.com</email>
106    </address>
107  </author>
108
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>LMM@acm.org</email>
120      <uri>http://larry.masinter.net/</uri>
121    </address>
122  </author>
123 
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>paulle@microsoft.com</email>
134    </address>
135  </author>
136   
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>timbl@w3.org</email>
150      <uri>http://www.w3.org/People/Berners-Lee/</uri>
151    </address>
152  </author>
153
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>ylafon@w3.org</email>
166      <uri>http://www.raubacapeu.net/people/yves/</uri>
167    </address>
168  </author>
169
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>julian.reschke@greenbytes.de</email>       
181      <uri>http://greenbytes.de/tech/webdav/</uri>     
182    </address>
183  </author>
184
185  <date month="&ID-MONTH;" year="&ID-YEAR;" day="12"/>
186
187<abstract>
188<t>
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.
197</t>
198</abstract>
199
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 (ietf-http-wg@w3.org). The current issues list is
204    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
205    and related documents (including fancy diffs) can be found at
206    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
207  </t>
208  <t>
209    This draft incorporates those issue resolutions that were either
210    collected in the original RFC2616 errata list (<eref target="http://purl.org/NET/http-errata"/>),
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>
214</note>
215</front>
216<middle>
217<section title="Introduction" anchor="caching">
218<t>
219   HTTP is typically used for distributed information systems, where
220   performance can be improved by the use of response caches, and includes
221   a number of elements intended to make caching work as well as possible.
222   Because these elements interact with each other, it is useful to describe
223   the caching design of HTTP separately.  This document defines aspects of
224   HTTP/1.1 related to caching and reusing response messages.
225</t>
226
227<section title="Purpose" anchor="intro.purpose">
228<iref item="cache"/>
229<t>
230   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
231   and the subsystem that controls its message storage, retrieval, and
232   deletion. A cache stores cacheable responses in order to reduce the
233   response time and network bandwidth consumption on future, equivalent
234   requests. Any client or server may include a cache, though a cache
235   cannot be used by a server that is acting as a tunnel.
236</t>
237<t>
238   Caching would be useless if it did not significantly improve
239   performance. The goal of caching in HTTP/1.1 is to eliminate the need
240   to send requests in many cases, and to eliminate the need to send
241   full responses in many other cases. The former reduces the number of
242   network round-trips required for many operations; we use an
243   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
244   latter reduces network bandwidth requirements; we use a "validation"
245   mechanism for this purpose (see <xref target="validation.model"/>).
246</t>
247<iref item="semantically transparent"/>
248<t>
249   A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
250   respect to a particular response, when its use affects neither the
251   requesting client nor the origin server, except to improve
252   performance. When a cache is semantically transparent, the client
253   receives exactly the same response (except for hop-by-hop headers)
254   that it would have received had its request been handled directly
255   by the origin server.
256</t>
257<t>
258   In an ideal world, all interactions with an HTTP cache would be
259   semantically transparent.  However, for some resources, semantic
260   transparency is not always necessary and can be effectively traded
261   for the sake of bandwidth scaling, disconnected operation, and
262   high availability.  HTTP/1.1 allows origin servers, caches,
263   and clients to explicitly reduce transparency when necessary.
264   However, because non-transparent operation may confuse non-expert
265   users and might be incompatible with certain server applications
266   (such as those for ordering merchandise), the protocol requires that
267   transparency be relaxed
268  <list style="symbols">
269     <t>only by an explicit protocol-level request when relaxed by
270        client or origin server</t>
271
272     <t>only with an explicit warning to the end user when relaxed by
273        cache or client</t>
274  </list>
275</t>
276<t>
277   Therefore, HTTP/1.1 provides these important elements:
278  <list style="numbers">
279      <t>Protocol features that provide full semantic transparency when
280         this is required by all parties.</t>
281
282      <t>Protocol features that allow an origin server or user agent to
283         explicitly request and control non-transparent operation.</t>
284
285      <t>Protocol features that allow a cache to attach warnings to
286         responses that do not preserve the requested approximation of
287         semantic transparency.</t>
288  </list>
289</t>
290<t>
291   A basic principle is that it must be possible for the clients to
292   detect any potential relaxation of semantic transparency.
293  <list><t>
294      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
295      design decisions not explicitly discussed in this specification.
296      If a decision might affect semantic transparency, the implementor
297      ought to err on the side of maintaining transparency unless a
298      careful and complete analysis shows significant benefits in
299      breaking transparency.
300    </t></list>
301</t>
302</section>
303
304<section title="Terminology" anchor="intro.terminology">
305<t>
306   This specification uses a number of terms to refer to the roles
307   played by participants in, and objects of, HTTP caching.
308</t>
309<t>
310  <iref item="cacheable"/>
311  <x:dfn>cacheable</x:dfn>
312  <list>
313    <t>
314      A response is cacheable if a cache is allowed to store a copy of
315      the response message for use in answering subsequent requests.
316      Even if a resource is cacheable, there may
317      be additional constraints on whether a cache can use the cached
318      copy for a particular request.
319    </t>
320  </list>
321</t>
322<t>
323  <iref item="first-hand"/>
324  <x:dfn>first-hand</x:dfn>
325  <list>
326    <t>
327      A response is first-hand if it comes directly and without
328      unnecessary delay from the origin server, perhaps via one or more
329      proxies. A response is also first-hand if its validity has just
330      been checked directly with the origin server.
331    </t>
332  </list>
333</t>
334<t>
335  <iref item="explicit expiration time"/>
336  <x:dfn>explicit expiration time</x:dfn>
337  <list>
338    <t>
339      The time at which the origin server intends that an entity should
340      no longer be returned by a cache without further validation.
341    </t>
342  </list>
343</t>
344<t>
345  <iref item="heuristic expiration time"/>
346  <x:dfn>heuristic expiration time</x:dfn>
347  <list>
348    <t>
349      An expiration time assigned by a cache when no explicit expiration
350      time is available.
351    </t>
352  </list>
353</t>
354<t>
355  <iref item="age"/>
356  <x:dfn>age</x:dfn>
357  <list>
358    <t>
359      The age of a response is the time since it was sent by, or
360      successfully validated with, the origin server.
361    </t>
362  </list>
363</t>
364<t>
365  <iref item="freshness lifetime"/>
366  <x:dfn>freshness lifetime</x:dfn>
367  <list>
368    <t>
369      The length of time between the generation of a response and its
370      expiration time.
371    </t>
372  </list>
373</t>
374<t>
375  <iref item="fresh"/>
376  <x:dfn>fresh</x:dfn>
377  <list>
378    <t>
379      A response is fresh if its age has not yet exceeded its freshness
380      lifetime.
381    </t>
382  </list>
383</t>
384<t>
385  <iref item="stale"/>
386  <x:dfn>stale</x:dfn>
387  <list>
388    <t>
389      A response is stale if its age has passed its freshness lifetime.
390    </t>
391  </list>
392</t>
393<t>
394  <iref item="validator"/>
395  <x:dfn>validator</x:dfn>
396  <list>
397    <t>
398      A protocol element (e.g., an entity tag or a Last-Modified time)
399      that is used to find out whether a cache entry is an equivalent
400      copy of an entity.
401    </t>
402  </list>
403</t>
404</section>
405
406<section title="Requirements" anchor="intro.requirements">
407<t>
408   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
409   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
410   document are to be interpreted as described in <xref target="RFC2119"/>.
411</t>
412<t>
413   An implementation is not compliant if it fails to satisfy one or more
414   of the &MUST; or &REQUIRED; level requirements for the protocols it
415   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
416   level and all the &SHOULD; level requirements for its protocols is said
417   to be "unconditionally compliant"; one that satisfies all the &MUST;
418   level requirements but not all the &SHOULD; level requirements for its
419   protocols is said to be "conditionally compliant."
420</t>
421</section>
422</section>
423
424<section title="Overview" anchor="caching.overview">
425<section title="Cache Correctness" anchor="cache.correctness">
426<t>
427   A correct cache &MUST; respond to a request with the most up-to-date
428   response held by the cache that is appropriate to the request (see
429   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
430   <xref target="disambiguating.multiple.responses" format="counter"/>,
431   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
432   conditions:
433  <list style="numbers">
434      <t>It has been checked for equivalence with what the origin server
435         would have returned by revalidating the response with the
436         origin server (<xref target="validation.model"/>);</t>
437
438      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
439         this means it meets the least restrictive freshness requirement
440         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
441         the origin server so specifies, it is the freshness requirement
442         of the origin server alone.
443
444         If a stored response is not "fresh enough" by the most
445         restrictive freshness requirement of both the client and the
446         origin server, in carefully considered circumstances the cache
447         &MAY; still return the response with the appropriate Warning
448         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
449         and <xref target="header.warning" format="counter"/>), unless such a response
450         is prohibited (e.g., by a "no-store" cache-directive, or by a
451         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
452
453      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
454         or error (4xx or 5xx) response message.</t>
455  </list>
456</t>
457<t>
458   If the cache can not communicate with the origin server, then a
459   correct cache &SHOULD; respond as above if the response can be
460   correctly served from the cache; if not it &MUST; return an error or
461   warning indicating that there was a communication failure.
462</t>
463<t>
464   If a cache receives a response (either an entire response, or a 304
465   (Not Modified) response) that it would normally forward to the
466   requesting client, and the received response is no longer fresh, the
467   cache &SHOULD; forward it to the requesting client without adding a new
468   Warning (but without removing any existing Warning headers). A cache
469   &SHOULD-NOT;  attempt to revalidate a response simply because that
470   response became stale in transit; this might lead to an infinite
471   loop. A user agent that receives a stale response without a Warning
472   &MAY; display a warning indication to the user.
473</t>
474</section>
475
476<section title="Warnings" anchor="warnings">
477<t>
478   Whenever a cache returns a response that is neither first-hand nor
479   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
480   &MUST; attach a warning to that effect, using a Warning general-header.
481   The Warning header and the currently defined warnings are described
482   in <xref target="header.warning"/>. The warning allows clients to take appropriate
483   action.
484</t>
485<t>
486   Warnings &MAY; be used for other purposes, both cache-related and
487   otherwise. The use of a warning, rather than an error status code,
488   distinguish these responses from true failures.
489</t>
490<t>
491   Warnings are assigned three digit warn-codes. The first digit
492   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
493   stored cache entry after a successful revalidation:
494</t>
495<t>
496  <list style="hanging">
497    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
498     the response, and so &MUST; be deleted after a successful
499     revalidation. 1xx warn-codes &MAY; be generated by a cache only when
500     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
501
502    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
503     headers that is not rectified by a revalidation (for example, a
504     lossy compression of the entity bodies) and which &MUST-NOT; be
505     deleted after a successful revalidation.</t>
506    </list>
507</t>
508<t>
509   See <xref target="header.warning"/> for the definitions of the codes themselves.
510</t>
511<t>
512   HTTP/1.0 caches will cache all Warnings in responses, without
513   deleting the ones in the first category. Warnings in responses that
514   are passed to HTTP/1.0 caches carry an extra warning-date field,
515   which prevents a future HTTP/1.1 recipient from believing an
516   erroneously cached Warning.
517</t>
518<t>
519   Warnings also carry a warning text. The text &MAY; be in any
520   appropriate natural language (perhaps based on the client's Accept
521   headers), and include an &OPTIONAL; indication of what character set is
522   used.
523</t>
524<t>
525   Multiple warnings &MAY; be attached to a response (either by the origin
526   server or by a cache), including multiple warnings with the same code
527   number. For example, a server might provide the same warning with
528   texts in both English and Basque.
529</t>
530<t>
531   When multiple warnings are attached to a response, it might not be
532   practical or reasonable to display all of them to the user. This
533   version of HTTP does not specify strict priority rules for deciding
534   which warnings to display and in what order, but does suggest some
535   heuristics.
536</t>
537</section>
538
539<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
540<t>
541   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
542   times and validators) are implicit directives to caches. In some
543   cases, a server or client might need to provide explicit directives
544   to the HTTP caches. We use the Cache-Control header for this purpose.
545</t>
546<t>
547   The Cache-Control header allows a client or server to transmit a
548   variety of directives in either requests or responses. These
549   directives typically override the default caching algorithms. As a
550   general rule, if there is any apparent conflict between header
551   values, the most restrictive interpretation is applied (that is, the
552   one that is most likely to preserve semantic transparency). However,
553   in some cases, cache-control directives are explicitly specified as
554   weakening the approximation of semantic transparency (for example,
555   "max-stale" or "public").
556</t>
557<t>
558   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
559</t>
560</section>
561
562<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
563<t>
564   Many user agents make it possible for users to override the basic
565   caching mechanisms. For example, the user agent might allow the user
566   to specify that cached entities (even explicitly stale ones) are
567   never validated. Or the user agent might habitually add "Cache-Control:
568   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
569   default to either non-transparent behavior, or behavior that results
570   in abnormally ineffective caching, but &MAY; be explicitly configured
571   to do so by an explicit action of the user.
572</t>
573<t>
574   If the user has overridden the basic caching mechanisms, the user
575   agent &SHOULD; explicitly indicate to the user whenever this results in
576   the display of information that might not meet the server's
577   transparency requirements (in particular, if the displayed entity is
578   known to be stale). Since the protocol normally allows the user agent
579   to determine if responses are stale or not, this indication need only
580   be displayed when this actually happens. The indication need not be a
581   dialog box; it could be an icon (for example, a picture of a rotting
582   fish) or some other indicator.
583</t>
584<t>
585   If the user has overridden the caching mechanisms in a way that would
586   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
587   continually indicate this state to the user (for example, by a
588   display of a picture of currency in flames) so that the user does not
589   inadvertently consume excess resources or suffer from excessive
590   latency.
591</t>
592</section>
593
594<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
595<t>
596   In some cases, the operator of a cache &MAY; choose to configure it to
597   return stale responses even when not requested by clients. This
598   decision ought not be made lightly, but may be necessary for reasons
599   of availability or performance, especially when the cache is poorly
600   connected to the origin server. Whenever a cache returns a stale
601   response, it &MUST; mark it as such (using a Warning header) enabling
602   the client software to alert the user that there might be a potential
603   problem.
604</t>
605<t>
606   It also allows the user agent to take steps to obtain a first-hand or
607   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
608   response if the client explicitly requests a first-hand or fresh one,
609   unless it is impossible to comply for technical or policy reasons.
610</t>
611</section>
612
613<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
614<t>
615   While the origin server (and to a lesser extent, intermediate caches,
616   by their contribution to the age of a response) are the primary
617   source of expiration information, in some cases the client might need
618   to control a cache's decision about whether to return a cached
619   response without validating it. Clients do this using several
620   directives of the Cache-Control header.
621</t>
622<t>
623   A client's request &MAY; specify the maximum age it is willing to
624   accept of an unvalidated response; specifying a value of zero forces
625   the cache(s) to revalidate all responses. A client &MAY; also specify
626   the minimum time remaining before a response expires. Both of these
627   options increase constraints on the behavior of caches, and so cannot
628   further relax the cache's approximation of semantic transparency.
629</t>
630<t>
631   A client &MAY; also specify that it will accept stale responses, up to
632   some maximum amount of staleness. This loosens the constraints on the
633   caches, and so might violate the origin server's specified
634   constraints on semantic transparency, but might be necessary to
635   support disconnected operation, or high availability in the face of
636   poor connectivity.
637</t>
638</section>
639</section>
640
641<section title="Expiration Model" anchor="expiration.model">
642
643<section title="Server-Specified Expiration" anchor="server-specified.expiration">
644<t>
645   HTTP caching works best when caches can entirely avoid making
646   requests to the origin server. The primary mechanism for avoiding
647   requests is for an origin server to provide an explicit expiration
648   time in the future, indicating that a response &MAY; be used to satisfy
649   subsequent requests. In other words, a cache can return a fresh
650   response without first contacting the server.
651</t>
652<t>
653   Our expectation is that servers will assign future explicit
654   expiration times to responses in the belief that the entity is not
655   likely to change, in a semantically significant way, before the
656   expiration time is reached. This normally preserves semantic
657   transparency, as long as the server's expiration times are carefully
658   chosen.
659</t>
660<t>
661   The expiration mechanism applies only to responses taken from a cache
662   and not to first-hand responses forwarded immediately to the
663   requesting client.
664</t>
665<t>
666   If an origin server wishes to force a semantically transparent cache
667   to validate every request, it &MAY; assign an explicit expiration time
668   in the past. This means that the response is always stale, and so the
669   cache &SHOULD; validate it before using it for subsequent requests. See
670   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
671</t>
672<t>
673   If an origin server wishes to force any HTTP/1.1 cache, no matter how
674   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
675   cache-control directive (see <xref target="header.cache-control"/>).
676</t>
677<t>
678   Servers specify explicit expiration times using either the Expires
679   header, or the max-age directive of the Cache-Control header.
680</t>
681<t>
682   An expiration time cannot be used to force a user agent to refresh
683   its display or reload a resource; its semantics apply only to caching
684   mechanisms, and such mechanisms need only check a resource's
685   expiration status when a new request for that resource is initiated.
686   See <xref target="history.lists"/> for an explanation of the difference between caches
687   and history mechanisms.
688</t>
689</section>
690
691<section title="Heuristic Expiration" anchor="heuristic.expiration">
692<t>
693   Since origin servers do not always provide explicit expiration times,
694   HTTP caches typically assign heuristic expiration times, employing
695   algorithms that use other header values (such as the Last-Modified
696   time) to estimate a plausible expiration time. The HTTP/1.1
697   specification does not provide specific algorithms, but does impose
698   worst-case constraints on their results. Since heuristic expiration
699   times might compromise semantic transparency, they ought to be used
700   cautiously, and we encourage origin servers to provide explicit
701   expiration times as much as possible.
702</t>
703</section>
704
705<section title="Age Calculations" anchor="age.calculations">
706<t>
707   In order to know if a cached entry is fresh, a cache needs to know if
708   its age exceeds its freshness lifetime. We discuss how to calculate
709   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
710   the age of a response or cache entry.
711</t>
712<t>
713   In this discussion, we use the term "now" to mean "the current value
714   of the clock at the host performing the calculation." Hosts that use
715   HTTP, but especially hosts running origin servers and caches, &SHOULD;
716   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
717   a globally accurate time standard.
718</t>
719<t>
720   HTTP/1.1 requires origin servers to send a Date header, if possible,
721   with every response, giving the time at which the response was
722   generated (see &header-date;). We use the term "date_value" to denote
723   the value of the Date header, in a form appropriate for arithmetic
724   operations.
725</t>
726<t>
727   HTTP/1.1 uses the Age response-header to convey the estimated age of
728   the response message when obtained from a cache. The Age field value
729   is the cache's estimate of the amount of time since the response was
730   generated or revalidated by the origin server.
731</t>
732<t>
733   In essence, the Age value is the sum of the time that the response
734   has been resident in each of the caches along the path from the
735   origin server, plus the amount of time it has been in transit along
736   network paths.
737</t>
738<t>
739   We use the term "age_value" to denote the value of the Age header, in
740   a form appropriate for arithmetic operations.
741</t>
742<t>
743   A response's age can be calculated in two entirely independent ways:
744  <list style="numbers">
745      <t>now minus date_value, if the local clock is reasonably well
746         synchronized to the origin server's clock. If the result is
747         negative, the result is replaced by zero.</t>
748
749      <t>age_value, if all of the caches along the response path
750         implement HTTP/1.1.</t>
751  </list>
752</t>
753<t>
754   Given that we have two independent ways to compute the age of a
755   response when it is received, we can combine these as
756</t>
757<figure><artwork type="code">
758    corrected_received_age = max(now - date_value, age_value)
759</artwork></figure>
760<t>
761   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
762   paths, one gets a reliable (conservative) result.
763</t>
764<t>
765   Because of network-imposed delays, some significant interval might
766   pass between the time that a server generates a response and the time
767   it is received at the next outbound cache or client. If uncorrected,
768   this delay could result in improperly low ages.
769</t>
770<t>
771   Because the request that resulted in the returned Age value must have
772   been initiated prior to that Age value's generation, we can correct
773   for delays imposed by the network by recording the time at which the
774   request was initiated. Then, when an Age value is received, it &MUST;
775   be interpreted relative to the time the request was initiated, not
776   the time that the response was received. This algorithm results in
777   conservative behavior no matter how much delay is experienced. So, we
778   compute:
779</t>
780<figure><artwork type="code">
781   corrected_initial_age = corrected_received_age
782                         + (now - request_time)
783</artwork></figure>
784<t>
785   where "request_time" is the time (according to the local clock) when
786   the request that elicited this response was sent.
787</t>
788<t>
789   Summary of age calculation algorithm, when a cache receives a
790   response:
791</t>
792<figure><artwork type="code">
793   /*
794    * age_value
795    *      is the value of Age: header received by the cache with
796    *              this response.
797    * date_value
798    *      is the value of the origin server's Date: header
799    * request_time
800    *      is the (local) time when the cache made the request
801    *              that resulted in this cached response
802    * response_time
803    *      is the (local) time when the cache received the
804    *              response
805    * now
806    *      is the current (local) time
807    */
808
809   apparent_age = max(0, response_time - date_value);
810   corrected_received_age = max(apparent_age, age_value);
811   response_delay = response_time - request_time;
812   corrected_initial_age = corrected_received_age + response_delay;
813   resident_time = now - response_time;
814   current_age   = corrected_initial_age + resident_time;
815</artwork></figure>
816<t>
817   The current_age of a cache entry is calculated by adding the amount
818   of time (in seconds) since the cache entry was last validated by the
819   origin server to the corrected_initial_age. When a response is
820   generated from a cache entry, the cache &MUST; include a single Age
821   header field in the response with a value equal to the cache entry's
822   current_age.
823</t>
824<t>
825   The presence of an Age header field in a response implies that a
826   response is not first-hand. However, the converse is not true, since
827   the lack of an Age header field in a response does not imply that the
828   response is first-hand unless all caches along the request path are
829   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
830   the Age header field).
831</t>
832</section>
833
834<section title="Expiration Calculations" anchor="expiration.calculations">
835<t>
836   In order to decide whether a response is fresh or stale, we need to
837   compare its freshness lifetime to its age. The age is calculated as
838   described in <xref target="age.calculations"/>; this section describes how to calculate
839   the freshness lifetime, and to determine if a response has expired.
840   In the discussion below, the values can be represented in any form
841   appropriate for arithmetic operations.
842</t>
843<t>
844   We use the term "expires_value" to denote the value of the Expires
845   header. We use the term "max_age_value" to denote an appropriate
846   value of the number of seconds carried by the "max-age" directive of
847   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
848</t>
849<t>
850   The max-age directive takes priority over Expires, so if max-age is
851   present in a response, the calculation is simply:
852</t>
853<figure><artwork type="code">
854   freshness_lifetime = max_age_value
855</artwork></figure>
856<t>
857   Otherwise, if Expires is present in the response, the calculation is:
858</t>
859<figure><artwork type="code">
860   freshness_lifetime = expires_value - date_value
861</artwork></figure>
862<t>
863   Note that neither of these calculations is vulnerable to clock skew,
864   since all of the information comes from the origin server.
865</t>
866<t>
867   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
868   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
869   does not include other restrictions on caching, the cache &MAY; compute
870   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
871   113 to any response whose age is more than 24 hours if such warning
872   has not already been added.
873</t>
874<t>
875   Also, if the response does have a Last-Modified time, the heuristic
876   expiration value &SHOULD; be no more than some fraction of the interval
877   since that time. A typical setting of this fraction might be 10%.
878</t>
879<t>
880   The calculation to determine if a response has expired is quite
881   simple:
882</t>
883<figure><artwork type="code">
884   response_is_fresh = (freshness_lifetime &gt; current_age)
885</artwork></figure>
886</section>
887
888<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
889<t>
890   Because expiration values are assigned optimistically, it is possible
891   for two caches to contain fresh values for the same resource that are
892   different.
893</t>
894<t>
895   If a client performing a retrieval receives a non-first-hand response
896   for a request that was already fresh in its own cache, and the Date
897   header in its existing cache entry is newer than the Date on the new
898   response, then the client &MAY; ignore the response. If so, it &MAY;
899   retry the request with a "Cache-Control: max-age=0" directive (see
900   <xref target="header.cache-control"/>), to force a check with the origin server.
901</t>
902<t>
903   If a cache has two fresh responses for the same representation with
904   different validators, it &MUST; use the one with the more recent Date
905   header. This situation might arise because the cache is pooling
906   responses from other caches, or because a client has asked for a
907   reload or a revalidation of an apparently fresh cache entry.
908</t>
909</section>
910
911<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
912<t>
913   Because a client might be receiving responses via multiple paths, so
914   that some responses flow through one set of caches and other
915   responses flow through a different set of caches, a client might
916   receive responses in an order different from that in which the origin
917   server sent them. We would like the client to use the most recently
918   generated response, even if older responses are still apparently
919   fresh.
920</t>
921<t>
922   Neither the entity tag nor the expiration value can impose an
923   ordering on responses, since it is possible that a later response
924   intentionally carries an earlier expiration time. The Date values are
925   ordered to a granularity of one second.
926</t>
927<t>
928   When a client tries to revalidate a cache entry, and the response it
929   receives contains a Date header that appears to be older than the one
930   for the existing entry, then the client &SHOULD; repeat the request
931   unconditionally, and include
932</t>
933<figure><artwork type="example">
934    Cache-Control: max-age=0
935</artwork></figure>
936<t>
937   to force any intermediate caches to validate their copies directly
938   with the origin server, or
939</t>
940<figure><artwork type="example">
941    Cache-Control: no-cache
942</artwork></figure>
943<t>
944   to force any intermediate caches to obtain a new copy from the origin
945   server.
946</t>
947<t>
948   If the Date values are equal, then the client &MAY; use either response
949   (or &MAY;, if it is being extremely prudent, request a new response).
950   Servers &MUST-NOT; depend on clients being able to choose
951   deterministically between responses generated during the same second,
952   if their expiration times overlap.
953</t>
954</section>
955</section>
956
957<section title="Validation Model" anchor="validation.model">
958<t>
959   When a cache has a stale entry that it would like to use as a
960   response to a client's request, it first has to check with the origin
961   server (or possibly an intermediate cache with a fresh response) to
962   see if its cached entry is still usable. We call this "validating"
963   the cache entry. Since we do not want to have to pay the overhead of
964   retransmitting the full response if the cached entry is good, and we
965   do not want to pay the overhead of an extra round trip if the cached
966   entry is invalid, the HTTP/1.1 protocol supports the use of
967   conditional methods.
968</t>
969<t>
970   The key protocol features for supporting conditional methods are
971   those concerned with "cache validators." When an origin server
972   generates a full response, it attaches some sort of validator to it,
973   which is kept with the cache entry. When a client (user agent or
974   proxy cache) makes a conditional request for a resource for which it
975   has a cache entry, it includes the associated validator in the
976   request.
977</t>
978<t>
979   The server then checks that validator against the current validator
980   for the entity, and, if they match (see &weak-and-strong-validators;), it responds
981   with a special status code (usually, 304 (Not Modified)) and no
982   entity-body. Otherwise, it returns a full response (including
983   entity-body). Thus, we avoid transmitting the full response if the
984   validator matches, and we avoid an extra round trip if it does not
985   match.
986</t>
987<t>
988   In HTTP/1.1, a conditional request looks exactly the same as a normal
989   request for the same resource, except that it carries a special
990   header (which includes the validator) that implicitly turns the
991   method (usually, GET) into a conditional.
992</t>
993<t>
994   The protocol includes both positive and negative senses of cache-validating
995   conditions. That is, it is possible to request either that
996   a method be performed if and only if a validator matches or if and
997   only if no validators match.
998  <list><t>
999      <x:h>Note:</x:h> a response that lacks a validator may still be cached, and
1000      served from cache until it expires, unless this is explicitly
1001      prohibited by a cache-control directive. However, a cache cannot
1002      do a conditional retrieval if it does not have a validator for the
1003      entity, which means it will not be refreshable after it expires.
1004  </t></list>
1005</t>
1006
1007<section title="Last-Modified Dates" anchor="last-modified.dates">
1008<t>
1009   The Last-Modified entity-header field value is often used as a cache
1010   validator. In simple terms, a cache entry is considered to be valid
1011   if the entity has not been modified since the Last-Modified value.
1012</t>
1013</section>
1014
1015<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
1016<t>
1017   The ETag response-header field value, an entity tag, provides for an
1018   "opaque" cache validator. This might allow more reliable validation
1019   in situations where it is inconvenient to store modification dates,
1020   where the one-second resolution of HTTP date values is not
1021   sufficient, or where the origin server wishes to avoid certain
1022   paradoxes that might arise from the use of modification dates.
1023</t>
1024<t>
1025   Entity Tags are described in &entity-tags;. The headers used with entity
1026   tags are described in &entity-header-fields;.
1027</t>
1028</section>
1029
1030<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
1031<t>
1032   The principle behind entity tags is that only the service author
1033   knows the semantics of a resource well enough to select an
1034   appropriate cache validation mechanism, and the specification of any
1035   validator comparison function more complex than byte-equality would
1036   open up a can of worms. Thus, comparisons of any other headers
1037   (except Last-Modified, for compatibility with HTTP/1.0) are never
1038   used for purposes of validating a cache entry.
1039</t>
1040</section>
1041</section>
1042
1043<section title="Response Cacheability" anchor="response.cacheability">
1044<t>
1045   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1046   directive, a caching system &MAY; always store a successful response
1047   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1048   if it is fresh, and &MAY; return it after successful validation. If
1049   there is neither a cache validator nor an explicit expiration time
1050   associated with a response, we do not expect it to be cached, but
1051   certain caches &MAY; violate this expectation (for example, when little
1052   or no network connectivity is available). A client can usually detect
1053   that such a response was taken from a cache by comparing the Date
1054   header to the current time.
1055  <list><t>
1056      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1057      without providing any Warning.
1058  </t></list>
1059</t>
1060<t>
1061   However, in some cases it might be inappropriate for a cache to
1062   retain an entity, or to return it in response to a subsequent
1063   request. This might be because absolute semantic transparency is
1064   deemed necessary by the service author, or because of security or
1065   privacy considerations. Certain cache-control directives are
1066   therefore provided so that the server can indicate that certain
1067   resource entities, or portions thereof, are not to be cached
1068   regardless of other considerations.
1069</t>
1070<t>
1071   Note that &header-authorization; normally prevents a shared cache from saving
1072   and returning a response to a previous request if that request
1073   included an Authorization header.
1074</t>
1075<t>
1076   A response received with a status code of 200, 203, 206, 300, 301 or
1077   410 &MAY; be stored by a cache and used in reply to a subsequent
1078   request, subject to the expiration mechanism, unless a cache-control
1079   directive prohibits caching. However, a cache that does not support
1080   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1081   Content) responses.
1082</t>
1083<t>
1084   A response received with any other status code (e.g. status codes 302
1085   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1086   unless there are cache-control directives or another header(s) that
1087   explicitly allow it. For example, these include the following: an
1088   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1089   "proxy-revalidate", "public" or "private" cache-control
1090   directive (<xref target="header.cache-control"/>).
1091</t>
1092</section>
1093
1094<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1095<t>
1096   The purpose of an HTTP cache is to store information received in
1097   response to requests for use in responding to future requests. In
1098   many cases, a cache simply returns the appropriate parts of a
1099   response to the requester. However, if the cache holds a cache entry
1100   based on a previous response, it might have to combine parts of a new
1101   response with what is held in the cache entry.
1102</t>
1103
1104<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1105<t>
1106   For the purpose of defining the behavior of caches and non-caching
1107   proxies, we divide HTTP headers into two categories:
1108  <list style="symbols">
1109      <t>End-to-end headers, which are  transmitted to the ultimate
1110        recipient of a request or response. End-to-end headers in
1111        responses &MUST; be stored as part of a cache entry and &MUST; be
1112        transmitted in any response formed from a cache entry.</t>
1113
1114      <t>Hop-by-hop headers, which are meaningful only for a single
1115        transport-level connection, and are not stored by caches or
1116        forwarded by proxies.</t>
1117  </list>
1118</t>
1119<t>
1120   The following HTTP/1.1 headers are hop-by-hop headers:
1121  <list style="symbols">
1122      <t>Connection</t>
1123      <t>Keep-Alive</t>
1124      <t>Proxy-Authenticate</t>
1125      <t>Proxy-Authorization</t>
1126      <t>TE</t>
1127      <t>Trailer</t>
1128      <t>Transfer-Encoding</t>
1129      <t>Upgrade</t>
1130  </list>
1131</t>
1132<t>
1133   All other headers defined by HTTP/1.1 are end-to-end headers.
1134</t>
1135<t>
1136   Other hop-by-hop headers &MUST; be listed in a Connection header
1137   (&header-connection;).
1138</t>
1139</section>
1140
1141<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1142<t>
1143   Some features of the HTTP/1.1 protocol, such as Digest
1144   Authentication, depend on the value of certain end-to-end headers. A
1145   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1146   definition of that header requires or specifically allows that.
1147</t>
1148<t>
1149   A transparent proxy &MUST-NOT; modify any of the following fields in a
1150   request or response, and it &MUST-NOT; add any of these fields if not
1151   already present:
1152  <list style="symbols">
1153      <t>Content-Location</t>
1154      <t>Content-MD5</t>
1155      <t>ETag</t>
1156      <t>Last-Modified</t>
1157  </list>
1158</t>
1159<t>
1160   A transparent proxy &MUST-NOT; modify any of the following fields in a
1161   response:
1162  <list style="symbols">
1163    <t>Expires</t>
1164  </list>
1165</t>
1166<t>
1167   but it &MAY; add any of these fields if not already present. If an
1168   Expires header is added, it &MUST; be given a field-value identical to
1169   that of the Date header in that response.
1170</t>
1171<t>
1172   A  proxy &MUST-NOT; modify or add any of the following fields in a
1173   message that contains the no-transform cache-control directive, or in
1174   any request:
1175  <list style="symbols">
1176    <t>Content-Encoding</t>
1177    <t>Content-Range</t>
1178    <t>Content-Type</t>
1179  </list>
1180</t>
1181<t>
1182   A non-transparent proxy &MAY; modify or add these fields to a message
1183   that does not include no-transform, but if it does so, it &MUST; add a
1184   Warning 214 (Transformation applied) if one does not already appear
1185   in the message (see <xref target="header.warning"/>).
1186  <list><t>
1187      Warning: unnecessary modification of end-to-end headers might
1188      cause authentication failures if stronger authentication
1189      mechanisms are introduced in later versions of HTTP. Such
1190      authentication mechanisms &MAY; rely on the values of header fields
1191      not listed here.
1192    </t></list>
1193</t>
1194<t>
1195   The Content-Length field of a request or response is added or deleted
1196   according to the rules in &message-length;. A transparent proxy &MUST;
1197   preserve the entity-length (&entity-length;) of the entity-body,
1198   although it &MAY; change the transfer-length (&message-length;).
1199</t>
1200</section>
1201
1202<section title="Combining Headers" anchor="combining.headers">
1203<t>
1204   When a cache makes a validating request to a server, and the server
1205   provides a 304 (Not Modified) response or a 206 (Partial Content)
1206   response, the cache then constructs a response to send to the
1207   requesting client.
1208</t>
1209<t>
1210   If the status code is 304 (Not Modified), the cache uses the entity-body
1211   stored in the cache entry as the entity-body of this outgoing
1212   response. If the status code is 206 (Partial Content) and the ETag or
1213   Last-Modified headers match exactly, the cache &MAY; combine the
1214   contents stored in the cache entry with the new contents received in
1215   the response and use the result as the entity-body of this outgoing
1216   response, (see &combining-byte-ranges;).
1217</t>
1218<t>
1219   The end-to-end headers stored in the cache entry are used for the
1220   constructed response, except that
1221  <list style="symbols">
1222    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1223      &MUST; be deleted from the cache entry and the forwarded response.</t>
1224    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1225        in the cache entry and the forwarded response.</t>
1226    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1227        replace the corresponding headers from the cache entry.</t>
1228  </list>
1229</t>
1230<t>
1231   Unless the cache decides to remove the cache entry, it &MUST; also
1232   replace the end-to-end headers stored with the cache entry with
1233   corresponding headers received in the incoming response, except for
1234   Warning headers as described immediately above. If a header field-name
1235   in the incoming response matches more than one header in the
1236   cache entry, all such old headers &MUST; be replaced.
1237</t>
1238<t>
1239   In other words, the set of end-to-end headers received in the
1240   incoming response overrides all corresponding end-to-end headers
1241   stored with the cache entry (except for stored Warning headers with
1242   warn-code 1xx, which are deleted even if not overridden).
1243  <list><t>
1244      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1245      Modified) or a 206 (Partial Content) response to update any header
1246      associated with a previous response for the same entity or sub-ranges
1247      thereof, although it might not always be meaningful or
1248      correct to do so. This rule does not allow an origin server to use
1249      a 304 (Not Modified) or a 206 (Partial Content) response to
1250      entirely delete a header that it had provided with a previous
1251      response.
1252  </t></list>
1253</t>
1254</section>
1255
1256</section>
1257
1258<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1259<t>
1260   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1261   by the presence of a Vary header field in a response, alters the
1262   conditions and procedure by which a cache can use the response for
1263   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1264   field by servers.
1265</t>
1266<t>
1267   A server &SHOULD; use the Vary header field to inform a cache of what
1268   request-header fields were used to select among multiple
1269   representations of a cacheable response subject to server-driven
1270   negotiation. The set of header fields named by the Vary field value
1271   is known as the "selecting" request-headers.
1272</t>
1273<t>
1274   When the cache receives a subsequent request whose Request-URI
1275   specifies one or more cache entries including a Vary header field,
1276   the cache &MUST-NOT; use such a cache entry to construct a response to
1277   the new request unless all of the selecting request-headers present
1278   in the new request match the corresponding stored request-headers in
1279   the original request.
1280</t>
1281<t>
1282   The selecting request-headers from two requests are defined to match
1283   if and only if the selecting request-headers in the first request can
1284   be transformed to the selecting request-headers in the second request
1285   by adding or removing linear white space (LWS) at places where this
1286   is allowed by the corresponding BNF, and/or combining multiple
1287   message-header fields with the same field name following the rules
1288   about message headers in &message-headers;.
1289</t>
1290<t>
1291   A Vary header field-value of "*" always fails to match and subsequent
1292   requests on that resource can only be properly interpreted by the
1293   origin server.
1294</t>
1295<t>
1296   If the selecting request header fields for the cached entry do not
1297   match the selecting request header fields of the new request, then
1298   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1299   it first relays the new request to the origin server in a conditional
1300   request and the server responds with 304 (Not Modified), including an
1301   entity tag or Content-Location that indicates the entity to be used.
1302</t>
1303<t>
1304   If an entity tag was assigned to a cached representation, the
1305   forwarded request &SHOULD; be conditional and include the entity tags
1306   in an If-None-Match header field from all its cache entries for the
1307   resource. This conveys to the server the set of entities currently
1308   held by the cache, so that if any one of these entities matches the
1309   requested entity, the server can use the ETag header field in its 304
1310   (Not Modified) response to tell the cache which entry is appropriate.
1311   If the entity-tag of the new response matches that of an existing
1312   entry, the new response &SHOULD; be used to update the header fields of
1313   the existing entry, and the result &MUST; be returned to the client.
1314</t>
1315<t>
1316   If any of the existing cache entries contains only partial content
1317   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1318   the If-None-Match header field unless the request is for a range that
1319   would be fully satisfied by that entry.
1320</t>
1321<t>
1322   If a cache receives a successful response whose Content-Location
1323   field matches that of an existing cache entry for the same Request-URI,
1324   whose entity-tag differs from that of the existing entry, and
1325   whose Date is more recent than that of the existing entry, the
1326   existing entry &SHOULD-NOT;  be returned in response to future requests
1327   and &SHOULD; be deleted from the cache.
1328</t>
1329</section>
1330
1331<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1332<t>
1333   For reasons of security and privacy, it is necessary to make a
1334   distinction between "shared" and "non-shared" caches. A non-shared
1335   cache is one that is accessible only to a single user. Accessibility
1336   in this case &SHOULD; be enforced by appropriate security mechanisms.
1337   All other caches are considered to be "shared." Other sections of
1338   this specification place certain constraints on the operation of
1339   shared caches in order to prevent loss of privacy or failure of
1340   access controls.
1341</t>
1342</section>
1343
1344<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1345<t>
1346   A cache that receives an incomplete response (for example, with fewer
1347   bytes of data than specified in a Content-Length header) &MAY; store
1348   the response. However, the cache &MUST; treat this as a partial
1349   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1350   the result might be a full response or might still be
1351   partial. A cache &MUST-NOT; return a partial response to a client
1352   without explicitly marking it as such, using the 206 (Partial
1353   Content) status code. A cache &MUST-NOT; return a partial response
1354   using a status code of 200 (OK).
1355</t>
1356<t>
1357   If a cache receives a 5xx response while attempting to revalidate an
1358   entry, it &MAY; either forward this response to the requesting client,
1359   or act as if the server failed to respond. In the latter case, it &MAY;
1360   return a previously received response unless the cached entry
1361   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1362</t>
1363</section>
1364
1365<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1366<t>
1367   Unless the origin server explicitly prohibits the caching of their
1368   responses, the application of GET and HEAD methods to any resources
1369   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1370   these responses are taken from a cache. They &MAY; still have side
1371   effects, but a cache is not required to consider such side effects in
1372   its caching decisions. Caches are always expected to observe an
1373   origin server's explicit restrictions on caching.
1374</t>
1375<t>
1376   We note one exception to this rule: since some applications have
1377   traditionally used GETs and HEADs with query URLs (those containing a
1378   "?" in the rel_path part) to perform operations with significant side
1379   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1380   the server provides an explicit expiration time. This specifically
1381   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1382   be taken from a cache. See &safe-methods; for related information.
1383</t>
1384</section>
1385
1386<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1387<t>
1388   The effect of certain methods performed on a resource at the origin
1389   server might cause one or more existing cache entries to become non-transparently
1390   invalid. That is, although they might continue to be
1391   "fresh," they do not accurately reflect what the origin server would
1392   return for a new request on that resource.
1393</t>
1394<t>
1395   There is no way for the HTTP protocol to guarantee that all such
1396   cache entries are marked invalid. For example, the request that
1397   caused the change at the origin server might not have gone through
1398   the proxy where a cache entry is stored. However, several rules help
1399   reduce the likelihood of erroneous behavior.
1400</t>
1401<t>
1402   In this section, the phrase "invalidate an entity" means that the
1403   cache will either remove all instances of that entity from its
1404   storage, or will mark these as "invalid" and in need of a mandatory
1405   revalidation before they can be returned in response to a subsequent
1406   request.
1407</t>
1408<t>
1409   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1410   either the entity referred to by the Request-URI, or by the Location
1411   or Content-Location headers (if present). These methods are:
1412  <list style="symbols">
1413      <t>PUT</t>
1414      <t>DELETE</t>
1415      <t>POST</t>
1416  </list>
1417</t> 
1418<t>
1419   An invalidation based
1420   on the URI in a Location or Content-Location header &MUST-NOT; be
1421   performed if the host part of that URI differs from the host part
1422   in the Request-URI. This helps prevent denial of service attacks.
1423</t>
1424<t>
1425   A cache that passes through requests for methods it does not
1426   understand &SHOULD; invalidate any entities referred to by the
1427   Request-URI.
1428</t>
1429</section>
1430
1431<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1432<t>
1433   All methods that might be expected to cause modifications to the
1434   origin server's resources &MUST; be written through to the origin
1435   server. This currently includes all methods except for GET and HEAD.
1436   A cache &MUST-NOT; reply to such a request from a client before having
1437   transmitted the request to the inbound server, and having received a
1438   corresponding response from the inbound server. This does not prevent
1439   a proxy cache from sending a 100 (Continue) response before the
1440   inbound server has sent its final reply.
1441</t>
1442<t>
1443   The alternative (known as "write-back" or "copy-back" caching) is not
1444   allowed in HTTP/1.1, due to the difficulty of providing consistent
1445   updates and the problems arising from server, cache, or network
1446   failure prior to write-back.
1447</t>
1448</section>
1449
1450<section title="Cache Replacement" anchor="cache.replacement">
1451<t>
1452   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1453   <xref target="disambiguating.expiration.values" format="counter"/>,
1454   <xref target="disambiguating.multiple.responses" format="counter"/>
1455   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1456   response is received from a resource while any existing responses for
1457   the same resource are cached, the cache &SHOULD; use the new response
1458   to reply to the current request. It &MAY; insert it into cache storage
1459   and &MAY;, if it meets all other requirements, use it to respond to any
1460   future requests that would previously have caused the old response to
1461   be returned. If it inserts the new response into cache storage  the
1462   rules in <xref target="combining.headers"/> apply.
1463  <list><t>
1464      <x:h>Note:</x:h> a new response that has an older Date header value than
1465      existing cached responses is not cacheable.
1466  </t></list>
1467</t>
1468</section>
1469
1470<section title="History Lists" anchor="history.lists">
1471<t>
1472   User agents often have history mechanisms, such as "Back" buttons and
1473   history lists, which can be used to redisplay an entity retrieved
1474   earlier in a session.
1475</t>
1476<t>
1477   History mechanisms and caches are different. In particular history
1478   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1479   the current state of a resource. Rather, a history mechanism is meant
1480   to show exactly what the user saw at the time when the resource was
1481   retrieved.
1482</t>
1483<t>
1484   By default, an expiration time does not apply to history mechanisms.
1485   If the entity is still in storage, a history mechanism &SHOULD; display
1486   it even if the entity has expired, unless the user has specifically
1487   configured the agent to refresh expired history documents.
1488</t>
1489<t>
1490   This is not to be construed to prohibit the history mechanism from
1491   telling the user that a view might be stale.
1492  <list><t>
1493      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1494      viewing stale resources, this will tend to force service authors
1495      to avoid using HTTP expiration controls and cache controls when
1496      they would otherwise like to. Service authors may consider it
1497      important that users not be presented with error messages or
1498      warning messages when they use navigation controls (such as BACK)
1499      to view previously fetched resources. Even though sometimes such
1500      resources ought not be cached, or ought to expire quickly, user
1501      interface considerations may force service authors to resort to
1502      other means of preventing caching (e.g. "once-only" URLs) in order
1503      not to suffer the effects of improperly functioning history
1504      mechanisms.
1505  </t></list>
1506</t>
1507</section>
1508
1509<section title="Header Field Definitions" anchor="header.fields">
1510<t>
1511   This section defines the syntax and semantics of HTTP/1.1 header fields
1512   related to caching.
1513</t>
1514<t>
1515   For entity-header fields, both sender and recipient refer to either the
1516   client or the server, depending on who sends and who receives the entity.
1517</t>
1518
1519<section title="Age" anchor="header.age">
1520  <iref primary="true" item="Age header" x:for-anchor=""/>
1521  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1522<t>
1523      The Age response-header field conveys the sender's estimate of the
1524      amount of time since the response (or its revalidation) was
1525      generated at the origin server. A cached response is "fresh" if
1526      its age does not exceed its freshness lifetime. Age values are
1527      calculated as specified in <xref target="age.calculations"/>.
1528</t>
1529<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1530  Age = "Age" ":" age-value
1531  age-value = delta-seconds
1532</artwork></figure>
1533<t>
1534      Age values are non-negative decimal integers, representing time in
1535      seconds.
1536</t>
1537<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
1538  delta-seconds  = 1*DIGIT
1539</artwork></figure>
1540<t>
1541      If a cache receives a value larger than the largest positive
1542      integer it can represent, or if any of its age calculations
1543      overflows, it &MUST; transmit an Age header with a value of
1544      2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
1545      include an Age header field in every response generated from its
1546      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1547      bits of range.
1548</t>
1549</section>
1550
1551<section title="Cache-Control" anchor="header.cache-control">
1552  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1553  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1554<t>
1555   The Cache-Control general-header field is used to specify directives
1556   that &MUST; be obeyed by all caching mechanisms along the
1557   request/response chain. The directives specify behavior intended to
1558   prevent caches from adversely interfering with the request or
1559   response. These directives typically override the default caching
1560   algorithms. Cache directives are unidirectional in that the presence
1561   of a directive in a request does not imply that the same directive is
1562   to be given in the response.
1563  <list><t>
1564      Note that HTTP/1.0 caches might not implement Cache-Control and
1565      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1566  </t></list>
1567</t>
1568<t>
1569   Cache directives &MUST; be passed through by a proxy or gateway
1570   application, regardless of their significance to that application,
1571   since the directives might be applicable to all recipients along the
1572   request/response chain. It is not possible to specify a cache-directive
1573   for a specific cache.
1574</t>
1575<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"/>
1576  Cache-Control   = "Cache-Control" ":" 1#cache-directive
1577
1578  cache-directive = cache-request-directive
1579       | cache-response-directive
1580
1581  cache-request-directive =
1582         "no-cache"                          ; <xref target="what.is.cacheable"/>
1583       | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1584       | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1585       | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1586       | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1587       | "no-transform"                      ; <xref target="no-transform.directive"/>
1588       | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1589       | cache-extension                     ; <xref target="cache.control.extensions"/>
1590
1591  cache-response-directive =
1592         "public"                               ; <xref target="what.is.cacheable"/>
1593       | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; <xref target="what.is.cacheable"/>
1594       | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]; <xref target="what.is.cacheable"/>
1595       | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1596       | "no-transform"                         ; <xref target="no-transform.directive"/>
1597       | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1598       | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1599       | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1600       | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1601       | cache-extension                        ; <xref target="cache.control.extensions"/>
1602
1603  cache-extension = token [ "=" ( token | quoted-string ) ]
1604</artwork></figure>
1605<t>
1606   When a directive appears without any 1#field-name parameter, the
1607   directive applies to the entire request or response. When such a
1608   directive appears with a 1#field-name parameter, it applies only to
1609   the named field or fields, and not to the rest of the request or
1610   response. This mechanism supports extensibility; implementations of
1611   future versions of the HTTP protocol might apply these directives to
1612   header fields not defined in HTTP/1.1.
1613</t>
1614<t>
1615   The cache-control directives can be broken down into these general
1616   categories:
1617  <list style="symbols">
1618     <t>Restrictions on what are cacheable; these may only be imposed by
1619        the origin server.</t>
1620
1621     <t>Restrictions on what may be stored by a cache; these may be
1622        imposed by either the origin server or the user agent.</t>
1623
1624     <t>Modifications of the basic expiration mechanism; these may be
1625        imposed by either the origin server or the user agent.</t>
1626
1627     <t>Controls over cache revalidation and reload; these may only be
1628        imposed by a user agent.</t>
1629
1630     <t>Control over transformation of entities.</t>
1631
1632     <t>Extensions to the caching system.</t>
1633  </list>
1634</t>
1635
1636<section title="What is Cacheable" anchor="what.is.cacheable">
1637<t>
1638   By default, a response is cacheable if the requirements of the
1639   request method, request header fields, and the response status
1640   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1641   for cacheability. The following Cache-Control response directives
1642   allow an origin server to override the default cacheability of a
1643   response:
1644</t>
1645<t>
1646  <iref item="Cache Directives" subitem="public" primary="true"/>
1647  <iref item="public" subitem="Cache Directive" primary="true"/>
1648   public
1649  <list><t>
1650      Indicates that the response &MAY; be cached by any cache, even if it
1651      would normally be non-cacheable or cacheable only within a non-shared
1652      cache. (See also Authorization, &header-authorization;, for
1653      additional details.)
1654  </t></list>
1655</t>
1656<t>
1657  <iref item="Cache Directives" subitem="private" primary="true"/>
1658  <iref item="private" subitem="Cache Directive" primary="true"/>
1659   private
1660  <list><t>
1661      Indicates that all or part of the response message is intended for
1662      a single user and &MUST-NOT; be cached by a shared cache. This
1663      allows an origin server to state that the specified parts of the
1664      response are intended for only one user and are not a valid
1665      response for requests by other users. A private (non-shared) cache
1666      &MAY; cache the response.
1667    </t><t>
1668       <x:h>Note:</x:h> This usage of the word private only controls where the
1669       response may be cached, and cannot ensure the privacy of the
1670       message content.
1671  </t></list>
1672</t>
1673<t>
1674  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1675  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1676   no-cache
1677  <list><t>
1678       If the no-cache directive does not specify a field-name, then a
1679      cache &MUST-NOT; use the response to satisfy a subsequent request
1680      without successful revalidation with the origin server. This
1681      allows an origin server to prevent caching even by caches that
1682      have been configured to return stale responses to client requests.
1683    </t><t>
1684      If the no-cache directive does specify one or more field-names,
1685      then a cache &MAY; use the response to satisfy a subsequent request,
1686      subject to any other restrictions on caching. However, the
1687      specified field-name(s) &MUST-NOT; be sent in the response to a
1688      subsequent request without successful revalidation with the origin
1689      server. This allows an origin server to prevent the re-use of
1690      certain header fields in a response, while still allowing caching
1691      of the rest of the response.
1692    <list><t>
1693       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1694       directive.
1695    </t></list>
1696  </t></list>
1697</t>
1698</section>
1699
1700<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1701<t>
1702  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1703  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1704   no-store
1705  <list><t>   
1706      The purpose of the no-store directive is to prevent the
1707      inadvertent release or retention of sensitive information (for
1708      example, on backup tapes). The no-store directive applies to the
1709      entire message, and &MAY; be sent either in a response or in a
1710      request. If sent in a request, a cache &MUST-NOT; store any part of
1711      either this request or any response to it. If sent in a response,
1712      a cache &MUST-NOT; store any part of either this response or the
1713      request that elicited it. This directive applies to both non-shared
1714      and shared caches. "&MUST-NOT; store" in this context means
1715      that the cache &MUST-NOT; intentionally store the information in
1716      non-volatile storage, and &MUST; make a best-effort attempt to
1717      remove the information from volatile storage as promptly as
1718      possible after forwarding it.
1719  </t><t>
1720      Even when this directive is associated with a response, users
1721      might explicitly store such a response outside of the caching
1722      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1723      such responses as part of their normal operation.
1724  </t><t>
1725      The purpose of this directive is to meet the stated requirements
1726      of certain users and service authors who are concerned about
1727      accidental releases of information via unanticipated accesses to
1728      cache data structures. While the use of this directive might
1729      improve privacy in some cases, we caution that it is NOT in any
1730      way a reliable or sufficient mechanism for ensuring privacy. In
1731      particular, malicious or compromised caches might not recognize or
1732      obey this directive, and communications networks might be
1733      vulnerable to eavesdropping.
1734  </t></list>
1735</t>
1736</section>
1737
1738<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1739<t>
1740   The expiration time of an entity &MAY; be specified by the origin
1741   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1742   it &MAY; be specified using the max-age directive in a response. When
1743   the max-age cache-control directive is present in a cached response,
1744   the response is stale if its current age is greater than the age
1745   value given (in seconds) at the time of a new request for that
1746   resource. The max-age directive on a response implies that the
1747   response is cacheable (i.e., "public") unless some other, more
1748   restrictive cache directive is also present.
1749</t>
1750<t>
1751   If a response includes both an Expires header and a max-age
1752   directive, the max-age directive overrides the Expires header, even
1753   if the Expires header is more restrictive. This rule allows an origin
1754   server to provide, for a given response, a longer expiration time to
1755   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1756   useful if certain HTTP/1.0 caches improperly calculate ages or
1757   expiration times, perhaps due to desynchronized clocks.
1758</t>
1759<t>
1760   Many HTTP/1.0 cache implementations will treat an Expires value that
1761   is less than or equal to the response Date value as being equivalent
1762   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1763   cache receives such a response, and the response does not include a
1764   Cache-Control header field, it &SHOULD; consider the response to be
1765   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1766  <list><t>
1767       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1768       cache control feature, such as the "private" directive, on a
1769       network including older caches that do not understand that
1770       feature. The origin server will need to combine the new feature
1771       with an Expires field whose value is less than or equal to the
1772       Date value. This will prevent older caches from improperly
1773       caching the response.
1774  </t></list>
1775</t>
1776<t>
1777  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1778  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1779   s-maxage
1780  <list><t>
1781       If a response includes an s-maxage directive, then for a shared
1782       cache (but not for a private cache), the maximum age specified by
1783       this directive overrides the maximum age specified by either the
1784       max-age directive or the Expires header. The s-maxage directive
1785       also implies the semantics of the proxy-revalidate directive (see
1786       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1787       entry after it becomes stale to respond to a subsequent request
1788       without first revalidating it with the origin server. The s-maxage
1789       directive is always ignored by a private cache.
1790  </t></list>
1791</t>
1792<t>
1793   Note that most older caches, not compliant with this specification,
1794   do not implement any cache-control directives. An origin server
1795   wishing to use a cache-control directive that restricts, but does not
1796   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1797   requirement that the max-age directive overrides the Expires header,
1798   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1799   max-age directive.
1800</t>
1801<t>
1802   Other directives allow a user agent to modify the basic expiration
1803   mechanism. These directives &MAY; be specified on a request:
1804</t>
1805<t>
1806  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1807  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1808   max-age
1809  <list><t>
1810      Indicates that the client is willing to accept a response whose
1811      age is no greater than the specified time in seconds. Unless max-stale
1812      directive is also included, the client is not willing to
1813      accept a stale response.
1814  </t></list>
1815</t>
1816<t>
1817  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1818  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1819   min-fresh
1820  <list><t>
1821      Indicates that the client is willing to accept a response whose
1822      freshness lifetime is no less than its current age plus the
1823      specified time in seconds. That is, the client wants a response
1824      that will still be fresh for at least the specified number of
1825      seconds.
1826  </t></list>
1827</t>
1828<t>
1829  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1830  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1831   max-stale
1832  <list><t>
1833      Indicates that the client is willing to accept a response that has
1834      exceeded its expiration time. If max-stale is assigned a value,
1835      then the client is willing to accept a response that has exceeded
1836      its expiration time by no more than the specified number of
1837      seconds. If no value is assigned to max-stale, then the client is
1838      willing to accept a stale response of any age.
1839  </t></list>
1840</t>
1841<t>
1842   If a cache returns a stale response, either because of a max-stale
1843   directive on a request, or because the cache is configured to
1844   override the expiration time of a response, the cache &MUST; attach a
1845   Warning header to the stale response, using Warning 110 (Response is
1846   stale).
1847</t>
1848<t>
1849   A cache &MAY; be configured to return stale responses without
1850   validation, but only if this does not conflict with any "MUST"-level
1851   requirements concerning cache validation (e.g., a "must-revalidate"
1852   cache-control directive).
1853</t>
1854<t>
1855   If both the new request and the cached entry include "max-age"
1856   directives, then the lesser of the two values is used for determining
1857   the freshness of the cached entry for that request.
1858</t>
1859</section>
1860
1861<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1862<t>
1863   Sometimes a user agent might want or need to insist that a cache
1864   revalidate its cache entry with the origin server (and not just with
1865   the next cache along the path to the origin server), or to reload its
1866   cache entry from the origin server. End-to-end revalidation might be
1867   necessary if either the cache or the origin server has overestimated
1868   the expiration time of the cached response. End-to-end reload may be
1869   necessary if the cache entry has become corrupted for some reason.
1870</t>
1871<t>
1872   End-to-end revalidation may be requested either when the client does
1873   not have its own local cached copy, in which case we call it
1874   "unspecified end-to-end revalidation", or when the client does have a
1875   local cached copy, in which case we call it "specific end-to-end
1876   revalidation."
1877</t>
1878<t>
1879   The client can specify these three kinds of action using Cache-Control
1880   request directives:
1881</t>
1882<t>
1883   End-to-end reload
1884  <list><t>
1885      The request includes a "no-cache" cache-control directive or, for
1886      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1887      names &MUST-NOT; be included with the no-cache directive in a
1888      request. The server &MUST-NOT; use a cached copy when responding to
1889      such a request.
1890  </t></list>
1891</t>
1892<t>
1893   Specific end-to-end revalidation
1894  <list><t>
1895      The request includes a "max-age=0" cache-control directive, which
1896      forces each cache along the path to the origin server to
1897      revalidate its own entry, if any, with the next cache or server.
1898      The initial request includes a cache-validating conditional with
1899      the client's current validator.
1900  </t></list>
1901</t>
1902<t>
1903   Unspecified end-to-end revalidation
1904  <list><t>
1905      The request includes "max-age=0" cache-control directive, which
1906      forces each cache along the path to the origin server to
1907      revalidate its own entry, if any, with the next cache or server.
1908      The initial request does not include a cache-validating
1909      conditional; the first cache along the path (if any) that holds a
1910      cache entry for this resource includes a cache-validating
1911      conditional with its current validator.
1912  </t></list>
1913</t>
1914<t>
1915  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1916  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1917   max-age
1918  <list><t>
1919      When an intermediate cache is forced, by means of a max-age=0
1920      directive, to revalidate its own cache entry, and the client has
1921      supplied its own validator in the request, the supplied validator
1922      might differ from the validator currently stored with the cache
1923      entry. In this case, the cache &MAY; use either validator in making
1924      its own request without affecting semantic transparency.
1925  </t><t>
1926      However, the choice of validator might affect performance. The
1927      best approach is for the intermediate cache to use its own
1928      validator when making its request. If the server replies with 304
1929      (Not Modified), then the cache can return its now validated copy
1930      to the client with a 200 (OK) response. If the server replies with
1931      a new entity and cache validator, however, the intermediate cache
1932      can compare the returned validator with the one provided in the
1933      client's request, using the strong comparison function. If the
1934      client's validator is equal to the origin server's, then the
1935      intermediate cache simply returns 304 (Not Modified). Otherwise,
1936      it returns the new entity with a 200 (OK) response.
1937  </t><t>
1938      If a request includes the no-cache directive, it &SHOULD-NOT; 
1939      include min-fresh, max-stale, or max-age.
1940  </t></list>
1941</t>
1942<t>
1943  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1944  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1945   only-if-cached
1946  <list><t>
1947      In some cases, such as times of extremely poor network
1948      connectivity, a client may want a cache to return only those
1949      responses that it currently has stored, and not to reload or
1950      revalidate with the origin server. To do this, the client may
1951      include the only-if-cached directive in a request. If it receives
1952      this directive, a cache &SHOULD; either respond using a cached entry
1953      that is consistent with the other constraints of the request, or
1954      respond with a 504 (Gateway Timeout) status. However, if a group
1955      of caches is being operated as a unified system with good internal
1956      connectivity, such a request &MAY; be forwarded within that group of
1957      caches.
1958  </t></list>
1959</t>
1960<t>
1961  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1962  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1963   must-revalidate
1964  <list><t>
1965      Because a cache &MAY; be configured to ignore a server's specified
1966      expiration time, and because a client request &MAY; include a max-stale
1967      directive (which has a similar effect), the protocol also
1968      includes a mechanism for the origin server to require revalidation
1969      of a cache entry on any subsequent use. When the must-revalidate
1970      directive is present in a response received by a cache, that cache
1971      &MUST-NOT; use the entry after it becomes stale to respond to a
1972      subsequent request without first revalidating it with the origin
1973      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1974      time, if, based solely on the origin server's Expires or max-age
1975      value, the cached response is stale.)
1976  </t><t>
1977      The must-revalidate directive is necessary to support reliable
1978      operation for certain protocol features. In all circumstances an
1979      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1980      particular, if the cache cannot reach the origin server for any
1981      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1982  </t><t>
1983      Servers &SHOULD; send the must-revalidate directive if and only if
1984      failure to revalidate a request on the entity could result in
1985      incorrect operation, such as a silently unexecuted financial
1986      transaction. Recipients &MUST-NOT; take any automated action that
1987      violates this directive, and &MUST-NOT; automatically provide an
1988      unvalidated copy of the entity if revalidation fails.
1989  </t><t>
1990      Although this is not recommended, user agents operating under
1991      severe connectivity constraints &MAY; violate this directive but, if
1992      so, &MUST; explicitly warn the user that an unvalidated response has
1993      been provided. The warning &MUST; be provided on each unvalidated
1994      access, and &SHOULD; require explicit user confirmation.
1995  </t></list>
1996</t>
1997<t>
1998  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1999  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
2000   proxy-revalidate
2001  <list><t>
2002      The proxy-revalidate directive has the same meaning as the must-revalidate
2003      directive, except that it does not apply to non-shared
2004      user agent caches. It can be used on a response to an
2005      authenticated request to permit the user's cache to store and
2006      later return the response without needing to revalidate it (since
2007      it has already been authenticated once by that user), while still
2008      requiring proxies that service many users to revalidate each time
2009      (in order to make sure that each user has been authenticated).
2010      Note that such authenticated responses also need the public cache
2011      control directive in order to allow them to be cached at all.
2012  </t></list>
2013</t>
2014</section>
2015
2016<section title="No-Transform Directive" anchor="no-transform.directive">
2017<t>
2018  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2019  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2020   no-transform
2021  <list><t>
2022      Implementors of intermediate caches (proxies) have found it useful
2023      to convert the media type of certain entity bodies. A non-transparent
2024      proxy might, for example, convert between image
2025      formats in order to save cache space or to reduce the amount of
2026      traffic on a slow link.
2027  </t><t>
2028      Serious operational problems occur, however, when these
2029      transformations are applied to entity bodies intended for certain
2030      kinds of applications. For example, applications for medical
2031      imaging, scientific data analysis and those using end-to-end
2032      authentication, all depend on receiving an entity body that is bit
2033      for bit identical to the original entity-body.
2034  </t><t>
2035      Therefore, if a message includes the no-transform directive, an
2036      intermediate cache or proxy &MUST-NOT; change those headers that are
2037      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2038      directive. This implies that the cache or proxy &MUST-NOT; change
2039      any aspect of the entity-body that is specified by these headers,
2040      including the value of the entity-body itself.
2041  </t></list>
2042</t>
2043</section>
2044
2045<section title="Cache Control Extensions" anchor="cache.control.extensions">
2046<t>
2047   The Cache-Control header field can be extended through the use of one
2048   or more cache-extension tokens, each with an optional assigned value.
2049   Informational extensions (those which do not require a change in
2050   cache behavior) &MAY; be added without changing the semantics of other
2051   directives. Behavioral extensions are designed to work by acting as
2052   modifiers to the existing base of cache directives. Both the new
2053   directive and the standard directive are supplied, such that
2054   applications which do not understand the new directive will default
2055   to the behavior specified by the standard directive, and those that
2056   understand the new directive will recognize it as modifying the
2057   requirements associated with the standard directive. In this way,
2058   extensions to the cache-control directives can be made without
2059   requiring changes to the base protocol.
2060</t>
2061<t>
2062   This extension mechanism depends on an HTTP cache obeying all of the
2063   cache-control directives defined for its native HTTP-version, obeying
2064   certain extensions, and ignoring all directives that it does not
2065   understand.
2066</t>
2067<t>
2068   For example, consider a hypothetical new response directive called
2069   community which acts as a modifier to the private directive. We
2070   define this new directive to mean that, in addition to any non-shared
2071   cache, any cache which is shared only by members of the community
2072   named within its value may cache the response. An origin server
2073   wishing to allow the UCI community to use an otherwise private
2074   response in their shared cache(s) could do so by including
2075</t>
2076<figure><artwork type="example">
2077    Cache-Control: private, community="UCI"
2078</artwork></figure>
2079<t>
2080   A cache seeing this header field will act correctly even if the cache
2081   does not understand the community cache-extension, since it will also
2082   see and understand the private directive and thus default to the safe
2083   behavior.
2084</t>
2085<t>
2086   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2087   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2088   be combined with standard directives (or the response's default
2089   cacheability) such that the cache behavior will remain minimally
2090   correct even if the cache does not understand the extension(s).
2091</t>
2092</section>
2093</section>
2094
2095<section title="Expires" anchor="header.expires">
2096  <iref primary="true" item="Expires header" x:for-anchor=""/>
2097  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2098<t>
2099   The Expires entity-header field gives the date/time after which the
2100   response is considered stale. A stale cache entry may not normally be
2101   returned by a cache (either a proxy cache or a user agent cache)
2102   unless it is first validated with the origin server (or with an
2103   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2104   for further discussion of the expiration model.
2105</t>
2106<t>
2107   The presence of an Expires field does not imply that the original
2108   resource will change or cease to exist at, before, or after that
2109   time.
2110</t>
2111<t>
2112   The format is an absolute date and time as defined by HTTP-date in
2113   &full-date;; it &MUST; be sent in rfc1123-date format.
2114</t>
2115<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2116  Expires = "Expires" ":" HTTP-date
2117</artwork></figure>
2118<t>
2119   An example of its use is
2120</t>
2121<figure><artwork type="example">
2122   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2123</artwork></figure>
2124<t>
2125  <list><t>
2126      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2127      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2128      Expires field.
2129  </t></list>
2130</t>
2131<t>
2132   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2133   especially including the value "0", as in the past (i.e., "already
2134   expired").
2135</t>
2136<t>
2137   To mark a response as "already expired," an origin server sends an
2138   Expires date that is equal to the Date header value. (See the rules
2139   for expiration calculations in <xref target="expiration.calculations"/>.)
2140</t>
2141<t>
2142   To mark a response as "never expires," an origin server sends an
2143   Expires date approximately one year from the time the response is
2144   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2145   year in the future.
2146</t>
2147<t>
2148   The presence of an Expires header field with a date value of some
2149   time in the future on a response that otherwise would by default be
2150   non-cacheable indicates that the response is cacheable, unless
2151   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2152</t>
2153</section>
2154
2155<section title="Pragma" anchor="header.pragma">
2156  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2157  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2158<t>
2159   The Pragma general-header field is used to include implementation-specific
2160   directives that might apply to any recipient along the
2161   request/response chain. All pragma directives specify optional
2162   behavior from the viewpoint of the protocol; however, some systems
2163   &MAY; require that behavior be consistent with the directives.
2164</t>
2165<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"/>
2166  Pragma            = "Pragma" ":" 1#pragma-directive
2167  pragma-directive  = "no-cache" | extension-pragma
2168  extension-pragma  = token [ "=" ( token | quoted-string ) ]
2169</artwork></figure>
2170<t>
2171   When the no-cache directive is present in a request message, an
2172   application &SHOULD; forward the request toward the origin server even
2173   if it has a cached copy of what is being requested. This pragma
2174   directive has the same semantics as the no-cache cache-directive (see
2175   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2176   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2177   request is sent to a server not known to be HTTP/1.1 compliant.
2178</t>
2179<t>
2180   Pragma directives &MUST; be passed through by a proxy or gateway
2181   application, regardless of their significance to that application,
2182   since the directives might be applicable to all recipients along the
2183   request/response chain. It is not possible to specify a pragma for a
2184   specific recipient; however, any pragma directive not relevant to a
2185   recipient &SHOULD; be ignored by that recipient.
2186</t>
2187<t>
2188   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2189   sent "Cache-Control: no-cache". No new Pragma directives will be
2190   defined in HTTP.
2191  <list><t>
2192      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
2193      response-header field is not actually specified, it does not provide a
2194      reliable replacement for "Cache-Control: no-cache" in a response.
2195  </t></list>
2196</t>
2197</section>
2198
2199<section title="Vary" anchor="header.vary">
2200  <iref primary="true" item="Vary header" x:for-anchor=""/>
2201  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2202<t>
2203   The Vary field value indicates the set of request-header fields that
2204   fully determines, while the response is fresh, whether a cache is
2205   permitted to use the response to reply to a subsequent request
2206   without revalidation. For uncacheable or stale responses, the Vary
2207   field value advises the user agent about the criteria that were used
2208   to select the representation. A Vary field value of "*" implies that
2209   a cache cannot determine from the request headers of a subsequent
2210   request whether this response is the appropriate representation. See
2211   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2212</t>
2213<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2214  Vary  = "Vary" ":" ( "*" | 1#field-name )
2215</artwork></figure>
2216<t>
2217   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2218   cacheable response that is subject to server-driven negotiation.
2219   Doing so allows a cache to properly interpret future requests on that
2220   resource and informs the user agent about the presence of negotiation
2221   on that resource. A server &MAY; include a Vary header field with a
2222   non-cacheable response that is subject to server-driven negotiation,
2223   since this might provide the user agent with useful information about
2224   the dimensions over which the response varies at the time of the
2225   response.
2226</t>
2227<t>
2228   A Vary field value consisting of a list of field-names signals that
2229   the representation selected for the response is based on a selection
2230   algorithm which considers ONLY the listed request-header field values
2231   in selecting the most appropriate representation. A cache &MAY; assume
2232   that the same selection will be made for future requests with the
2233   same values for the listed field names, for the duration of time for
2234   which the response is fresh.
2235</t>
2236<t>
2237   The field-names given are not limited to the set of standard
2238   request-header fields defined by this specification. Field names are
2239   case-insensitive.
2240</t>
2241<t>
2242   A Vary field value of "*" signals that unspecified parameters not
2243   limited to the request-headers (e.g., the network address of the
2244   client), play a role in the selection of the response representation.
2245   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2246   generated by an origin server.
2247</t>
2248</section>
2249
2250<section title="Warning" anchor="header.warning">
2251  <iref primary="true" item="Warning header" x:for-anchor=""/>
2252  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2253<t>
2254   The Warning general-header field is used to carry additional
2255   information about the status or transformation of a message which
2256   might not be reflected in the message. This information is typically
2257   used to warn about a possible lack of semantic transparency from
2258   caching operations or transformations applied to the entity body of
2259   the message.
2260</t>
2261<t>
2262   Warning headers are sent with responses using:
2263</t>
2264<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"/>
2265  Warning    = "Warning" ":" 1#warning-value
2266 
2267  warning-value = warn-code SP warn-agent SP warn-text
2268                                        [SP warn-date]
2269 
2270  warn-code  = 3DIGIT
2271  warn-agent = ( host [ ":" port ] ) | pseudonym
2272                  ; the name or pseudonym of the server adding
2273                  ; the Warning header, for use in debugging
2274  warn-text  = quoted-string
2275  warn-date  = DQUOTE HTTP-date DQUOTE
2276</artwork></figure>
2277<t>
2278   A response &MAY; carry more than one Warning header.
2279</t>
2280<t>
2281   The warn-text &SHOULD; be in a natural language and character set that
2282   is most likely to be intelligible to the human user receiving the
2283   response. This decision &MAY; be based on any available knowledge, such
2284   as the location of the cache or user, the Accept-Language field in a
2285   request, the Content-Language field in a response, etc. The default
2286   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2287</t>
2288<t>
2289   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2290   in the warn-text using the method described in <xref target="RFC2047"/>.
2291</t>
2292<t>
2293   Warning headers can in general be applied to any message, however
2294   some specific warn-codes are specific to caches and can only be
2295   applied to response messages. New Warning headers &SHOULD; be added
2296   after any existing Warning headers. A cache &MUST-NOT; delete any
2297   Warning header that it received with a message. However, if a cache
2298   successfully validates a cache entry, it &SHOULD; remove any Warning
2299   headers previously attached to that entry except as specified for
2300   specific Warning codes. It &MUST; then add any Warning headers received
2301   in the validating response. In other words, Warning headers are those
2302   that would be attached to the most recent relevant response.
2303</t>
2304<t>
2305   When multiple Warning headers are attached to a response, the user
2306   agent ought to inform the user of as many of them as possible, in the
2307   order that they appear in the response. If it is not possible to
2308   inform the user of all of the warnings, the user agent &SHOULD; follow
2309   these heuristics:
2310  <list style="symbols">
2311    <t>Warnings that appear early in the response take priority over
2312        those appearing later in the response.</t>
2313
2314    <t>Warnings in the user's preferred character set take priority
2315        over warnings in other character sets but with identical warn-codes
2316        and warn-agents.</t>
2317  </list>
2318</t>
2319<t>
2320   Systems that generate multiple Warning headers &SHOULD; order them with
2321   this user agent behavior in mind.
2322</t>
2323<t>
2324   Requirements for the behavior of caches with respect to Warnings are
2325   stated in <xref target="warnings"/>.
2326</t>
2327<t>
2328   This is a list of the currently-defined warn-codes, each with a
2329   recommended warn-text in English, and a description of its meaning.
2330</t>
2331<t>
2332   110 Response is stale
2333  <list><t>
2334     &MUST; be included whenever the returned response is stale.
2335  </t></list>
2336</t>
2337<t>
2338   111 Revalidation failed
2339  <list><t>
2340     &MUST; be included if a cache returns a stale response because an
2341     attempt to revalidate the response failed, due to an inability to
2342     reach the server.
2343  </t></list>
2344</t>
2345<t>
2346   112 Disconnected operation
2347  <list><t>
2348     &SHOULD; be included if the cache is intentionally disconnected from
2349     the rest of the network for a period of time.
2350  </t></list>
2351</t>
2352<t>
2353   113 Heuristic expiration
2354  <list><t>
2355     &MUST; be included if the cache heuristically chose a freshness
2356     lifetime greater than 24 hours and the response's age is greater
2357     than 24 hours.
2358  </t></list>
2359</t>
2360<t>
2361   199 Miscellaneous warning
2362  <list><t>
2363     The warning text &MAY; include arbitrary information to be presented
2364     to a human user, or logged. A system receiving this warning &MUST-NOT;
2365     take any automated action, besides presenting the warning to
2366     the user.
2367  </t></list>
2368</t>
2369<t>
2370   214 Transformation applied
2371  <list><t>
2372     &MUST; be added by an intermediate cache or proxy if it applies any
2373     transformation changing the content-coding (as specified in the
2374     Content-Encoding header) or media-type (as specified in the
2375     Content-Type header) of the response, or the entity-body of the
2376     response, unless this Warning code already appears in the response.
2377  </t></list>
2378</t>
2379<t>
2380   299 Miscellaneous persistent warning
2381  <list><t>
2382     The warning text &MAY; include arbitrary information to be presented
2383     to a human user, or logged. A system receiving this warning &MUST-NOT;
2384     take any automated action.
2385  </t></list>
2386</t>
2387<t>
2388   If an implementation sends a message with one or more Warning headers
2389   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2390   each warning-value a warn-date that matches the date in the response.
2391</t>
2392<t>
2393   If an implementation receives a message with a warning-value that
2394   includes a warn-date, and that warn-date is different from the Date
2395   value in the response, then that warning-value &MUST; be deleted from
2396   the message before storing, forwarding, or using it. (This prevents
2397   bad consequences of naive caching of Warning header fields.) If all
2398   of the warning-values are deleted for this reason, the Warning header
2399   &MUST; be deleted as well.
2400</t>
2401</section>
2402
2403</section>
2404
2405<section title="IANA Considerations" anchor="IANA.considerations">
2406<t>
2407   TBD.
2408</t>
2409</section>
2410
2411<section title="Security Considerations" anchor="security.considerations">
2412<t>
2413   Caching proxies provide additional potential vulnerabilities, since
2414   the contents of the cache represent an attractive target for
2415   malicious exploitation. Because cache contents persist after an HTTP
2416   request is complete, an attack on the cache can reveal information
2417   long after a user believes that the information has been removed from
2418   the network. Therefore, cache contents should be protected as
2419   sensitive information.
2420</t>
2421</section>
2422
2423<section title="Acknowledgments" anchor="ack">
2424<t>
2425   Much of the content and presentation of the caching design is due to
2426   suggestions and comments from individuals including: Shel Kaphan,
2427   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2428</t>
2429</section>
2430</middle>
2431<back>
2432
2433<references title="Normative References">
2434
2435<reference anchor="ISO-8859-1">
2436  <front>
2437    <title>
2438     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2439    </title>
2440    <author>
2441      <organization>International Organization for Standardization</organization>
2442    </author>
2443    <date year="1998"/>
2444  </front>
2445  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2446</reference>
2447
2448<reference anchor="Part1">
2449  <front>
2450    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2451    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2452      <organization abbrev="Day Software">Day Software</organization>
2453      <address><email>fielding@gbiv.com</email></address>
2454    </author>
2455    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2456      <organization>One Laptop per Child</organization>
2457      <address><email>jg@laptop.org</email></address>
2458    </author>
2459    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2460      <organization abbrev="HP">Hewlett-Packard Company</organization>
2461      <address><email>JeffMogul@acm.org</email></address>
2462    </author>
2463    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2464      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2465      <address><email>henrikn@microsoft.com</email></address>
2466    </author>
2467    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2468      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2469      <address><email>LMM@acm.org</email></address>
2470    </author>
2471    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2472      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2473      <address><email>paulle@microsoft.com</email></address>
2474    </author>
2475    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2476      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2477      <address><email>timbl@w3.org</email></address>
2478    </author>
2479    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2480      <organization abbrev="W3C">World Wide Web Consortium</organization>
2481      <address><email>ylafon@w3.org</email></address>
2482    </author>
2483    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2484      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2485      <address><email>julian.reschke@greenbytes.de</email></address>
2486    </author>
2487    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2488  </front>
2489  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2490  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2491</reference>
2492
2493<reference anchor="Part2">
2494  <front>
2495    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2496    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2497      <organization abbrev="Day Software">Day Software</organization>
2498      <address><email>fielding@gbiv.com</email></address>
2499    </author>
2500    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2501      <organization>One Laptop per Child</organization>
2502      <address><email>jg@laptop.org</email></address>
2503    </author>
2504    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2505      <organization abbrev="HP">Hewlett-Packard Company</organization>
2506      <address><email>JeffMogul@acm.org</email></address>
2507    </author>
2508    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2509      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2510      <address><email>henrikn@microsoft.com</email></address>
2511    </author>
2512    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2513      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2514      <address><email>LMM@acm.org</email></address>
2515    </author>
2516    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2517      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2518      <address><email>paulle@microsoft.com</email></address>
2519    </author>
2520    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2521      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2522      <address><email>timbl@w3.org</email></address>
2523    </author>
2524    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2525      <organization abbrev="W3C">World Wide Web Consortium</organization>
2526      <address><email>ylafon@w3.org</email></address>
2527    </author>
2528    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2529      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2530      <address><email>julian.reschke@greenbytes.de</email></address>
2531    </author>
2532    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2533  </front>
2534  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2535  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2536</reference>
2537
2538<reference anchor="Part3">
2539  <front>
2540    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2541    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2542      <organization abbrev="Day Software">Day Software</organization>
2543      <address><email>fielding@gbiv.com</email></address>
2544    </author>
2545    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2546      <organization>One Laptop per Child</organization>
2547      <address><email>jg@laptop.org</email></address>
2548    </author>
2549    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2550      <organization abbrev="HP">Hewlett-Packard Company</organization>
2551      <address><email>JeffMogul@acm.org</email></address>
2552    </author>
2553    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2554      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2555      <address><email>henrikn@microsoft.com</email></address>
2556    </author>
2557    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2558      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2559      <address><email>LMM@acm.org</email></address>
2560    </author>
2561    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2562      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2563      <address><email>paulle@microsoft.com</email></address>
2564    </author>
2565    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2566      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2567      <address><email>timbl@w3.org</email></address>
2568    </author>
2569    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2570      <organization abbrev="W3C">World Wide Web Consortium</organization>
2571      <address><email>ylafon@w3.org</email></address>
2572    </author>
2573    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2574      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2575      <address><email>julian.reschke@greenbytes.de</email></address>
2576    </author>
2577    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2578  </front>
2579  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2580  <x:source href="p3-payload.xml" basename="p3-payload"/>
2581</reference>
2582
2583<reference anchor="Part4">
2584  <front>
2585    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2586    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2587      <organization abbrev="Day Software">Day Software</organization>
2588      <address><email>fielding@gbiv.com</email></address>
2589    </author>
2590    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2591      <organization>One Laptop per Child</organization>
2592      <address><email>jg@laptop.org</email></address>
2593    </author>
2594    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2595      <organization abbrev="HP">Hewlett-Packard Company</organization>
2596      <address><email>JeffMogul@acm.org</email></address>
2597    </author>
2598    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2599      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2600      <address><email>henrikn@microsoft.com</email></address>
2601    </author>
2602    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2603      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2604      <address><email>LMM@acm.org</email></address>
2605    </author>
2606    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2607      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2608      <address><email>paulle@microsoft.com</email></address>
2609    </author>
2610    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2611      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2612      <address><email>timbl@w3.org</email></address>
2613    </author>
2614    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2615      <organization abbrev="W3C">World Wide Web Consortium</organization>
2616      <address><email>ylafon@w3.org</email></address>
2617    </author>
2618    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2619      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2620      <address><email>julian.reschke@greenbytes.de</email></address>
2621    </author>
2622    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2623  </front>
2624  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2625  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2626</reference>
2627
2628<reference anchor="Part5">
2629  <front>
2630    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2631    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2632      <organization abbrev="Day Software">Day Software</organization>
2633      <address><email>fielding@gbiv.com</email></address>
2634    </author>
2635    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2636      <organization>One Laptop per Child</organization>
2637      <address><email>jg@laptop.org</email></address>
2638    </author>
2639    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2640      <organization abbrev="HP">Hewlett-Packard Company</organization>
2641      <address><email>JeffMogul@acm.org</email></address>
2642    </author>
2643    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2644      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2645      <address><email>henrikn@microsoft.com</email></address>
2646    </author>
2647    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2648      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2649      <address><email>LMM@acm.org</email></address>
2650    </author>
2651    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2652      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2653      <address><email>paulle@microsoft.com</email></address>
2654    </author>
2655    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2656      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2657      <address><email>timbl@w3.org</email></address>
2658    </author>
2659    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2660      <organization abbrev="W3C">World Wide Web Consortium</organization>
2661      <address><email>ylafon@w3.org</email></address>
2662    </author>
2663    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2664      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2665      <address><email>julian.reschke@greenbytes.de</email></address>
2666    </author>
2667    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2668  </front>
2669  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2670  <x:source href="p5-range.xml" basename="p5-range"/>
2671</reference>
2672
2673<reference anchor="Part7">
2674  <front>
2675    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2676    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2677      <organization abbrev="Day Software">Day Software</organization>
2678      <address><email>fielding@gbiv.com</email></address>
2679    </author>
2680    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2681      <organization>One Laptop per Child</organization>
2682      <address><email>jg@laptop.org</email></address>
2683    </author>
2684    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2685      <organization abbrev="HP">Hewlett-Packard Company</organization>
2686      <address><email>JeffMogul@acm.org</email></address>
2687    </author>
2688    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2689      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2690      <address><email>henrikn@microsoft.com</email></address>
2691    </author>
2692    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2693      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2694      <address><email>LMM@acm.org</email></address>
2695    </author>
2696    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2697      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2698      <address><email>paulle@microsoft.com</email></address>
2699    </author>
2700    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2701      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2702      <address><email>timbl@w3.org</email></address>
2703    </author>
2704    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2705      <organization abbrev="W3C">World Wide Web Consortium</organization>
2706      <address><email>ylafon@w3.org</email></address>
2707    </author>
2708    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2709      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2710      <address><email>julian.reschke@greenbytes.de</email></address>
2711    </author>
2712    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2713  </front>
2714  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2715  <x:source href="p7-auth.xml" basename="p7-auth"/>
2716</reference>
2717
2718<reference anchor="RFC2047">
2719  <front>
2720    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2721    <author initials="K." surname="Moore" fullname="Keith Moore">
2722      <organization>University of Tennessee</organization>
2723      <address><email>moore@cs.utk.edu</email></address>
2724    </author>
2725    <date month="November" year="1996"/>
2726  </front>
2727  <seriesInfo name="RFC" value="2047"/>
2728</reference>
2729
2730<reference anchor="RFC2119">
2731  <front>
2732    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2733    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2734      <organization>Harvard University</organization>
2735      <address><email>sob@harvard.edu</email></address>
2736    </author>
2737    <date month="March" year="1997"/>
2738  </front>
2739  <seriesInfo name="BCP" value="14"/>
2740  <seriesInfo name="RFC" value="2119"/>
2741</reference>
2742
2743</references>
2744
2745<references title="Informative References">
2746
2747<reference anchor="RFC1305">
2748  <front>
2749    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2750    <author initials="D." surname="Mills" fullname="David L. Mills">
2751      <organization>University of Delaware, Electrical Engineering Department</organization>
2752      <address><email>mills@udel.edu</email></address>
2753    </author>
2754    <date month="March" year="1992"/>
2755  </front>
2756  <seriesInfo name="RFC" value="1305"/>
2757</reference>
2758
2759<reference anchor="RFC2616">
2760  <front>
2761    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2762    <author initials="R." surname="Fielding" fullname="R. Fielding">
2763      <organization>University of California, Irvine</organization>
2764      <address><email>fielding@ics.uci.edu</email></address>
2765    </author>
2766    <author initials="J." surname="Gettys" fullname="J. Gettys">
2767      <organization>W3C</organization>
2768      <address><email>jg@w3.org</email></address>
2769    </author>
2770    <author initials="J." surname="Mogul" fullname="J. Mogul">
2771      <organization>Compaq Computer Corporation</organization>
2772      <address><email>mogul@wrl.dec.com</email></address>
2773    </author>
2774    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2775      <organization>MIT Laboratory for Computer Science</organization>
2776      <address><email>frystyk@w3.org</email></address>
2777    </author>
2778    <author initials="L." surname="Masinter" fullname="L. Masinter">
2779      <organization>Xerox Corporation</organization>
2780      <address><email>masinter@parc.xerox.com</email></address>
2781    </author>
2782    <author initials="P." surname="Leach" fullname="P. Leach">
2783      <organization>Microsoft Corporation</organization>
2784      <address><email>paulle@microsoft.com</email></address>
2785    </author>
2786    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2787      <organization>W3C</organization>
2788      <address><email>timbl@w3.org</email></address>
2789    </author>
2790    <date month="June" year="1999"/>
2791  </front>
2792  <seriesInfo name="RFC" value="2616"/>
2793</reference>
2794
2795</references>
2796
2797<section title="Compatibility with Previous Versions" anchor="compatibility">
2798
2799<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2800<t>
2801   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2802   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2803   <xref target="header.cache-control" format="counter"/>,
2804   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2805</t>
2806<t>
2807   Transfer-coding and message lengths all interact in ways that
2808   required fixing exactly when chunked encoding is used (to allow for
2809   transfer encoding that may not be self delimiting); it was important
2810   to straighten out exactly how message lengths are computed.
2811   (<xref target="non-modifiable.headers"/>,
2812   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2813</t>
2814<t>
2815   Proxies should be able to add Content-Length when appropriate.
2816   (<xref target="non-modifiable.headers"/>)
2817</t>
2818<t>
2819   Range request responses would become very verbose if all meta-data
2820   were always returned; by allowing the server to only send needed
2821   headers in a 206 response, this problem can be avoided.
2822   (<xref target="combining.headers"/>)
2823</t>
2824<t>
2825   The Cache-Control: max-age directive was not properly defined for
2826   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2827</t>
2828<t>
2829   Warnings could be cached incorrectly, or not updated appropriately.
2830   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2831   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2832   and <xref target="header.warning" format="counter"/>) Warning
2833   also needed to be a general header, as PUT or other methods may have
2834   need for it in requests.
2835</t>
2836</section>
2837
2838<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2839<t>
2840  Clarify denial of service attack avoidance requirement.
2841  (<xref target="invalidation.after.updates.or.deletions"/>)
2842</t>
2843</section>
2844
2845</section>
2846
2847<section title="Change Log (to be removed by RFC Editor before publication)">
2848
2849<section title="Since RFC2616">
2850<t>
2851  Extracted relevant partitions from <xref target="RFC2616"/>.
2852</t>
2853</section>
2854
2855<section title="Since draft-ietf-httpbis-p6-cache-00">
2856<t>
2857  Closed issues:
2858  <list style="symbols"> 
2859    <t>
2860      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
2861      "Trailer"
2862      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
2863    </t>
2864    <t>
2865      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
2866      "Invalidation after Update or Delete"
2867      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
2868    </t>
2869    <t>
2870      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2871      "Normative and Informative references"
2872    </t>
2873    <t>
2874      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
2875      "Date reference typo"
2876    </t>
2877    <t>
2878      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
2879      "Connection header text"
2880    </t>
2881    <t>
2882      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2883      "Informative references"
2884    </t>
2885    <t>
2886      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2887      "ISO-8859-1 Reference"
2888    </t>
2889    <t>
2890      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2891      "Normative up-to-date references"
2892    </t>
2893    <t>
2894      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
2895      "typo in 13.2.2"
2896    </t>
2897  </list>
2898</t>
2899<t>
2900  Other changes:
2901  <list style="symbols"> 
2902    <t>
2903      Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
2904    </t>
2905  </list>
2906</t>
2907</section>
2908
2909</section>
2910
2911</back>
2912</rfc>
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