source: draft-ietf-httpbis/latest/p6-cache.xml @ 203

Last change on this file since 203 was 203, checked in by julian.reschke@…, 12 years ago

Add processing instructions for inline comments throughout, and use them for the IANA TBDs.

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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 "latest">
15  <!ENTITY ID-MONTH "February">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29]>
30<?rfc toc="yes" ?>
31<?rfc symrefs="yes" ?>
32<?rfc sortrefs="yes" ?>
33<?rfc compact="yes"?>
34<?rfc subcompact="no" ?>
35<?rfc linkmailto="no" ?>
36<?rfc editing="no" ?>
37<?rfc comments="yes"?>
38<?rfc inline="yes"?>
39<?rfc comments="yes"?>
40<?rfc inline="yes"?>
41<?rfc-ext allow-markup-in-artwork="yes" ?>
42<?rfc-ext include-references-in-index="yes" ?>
43<rfc obsoletes="2616" category="std"
44     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
45     xmlns:x='http://purl.org/net/xml2rfc/ext'>
46<front>
47
48  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
49
50  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
51    <organization abbrev="Day Software">Day Software</organization>
52    <address>
53      <postal>
54        <street>23 Corporate Plaza DR, Suite 280</street>
55        <city>Newport Beach</city>
56        <region>CA</region>
57        <code>92660</code>
58        <country>USA</country>
59      </postal>
60      <phone>+1-949-706-5300</phone>
61      <facsimile>+1-949-706-5305</facsimile>
62      <email>fielding@gbiv.com</email>
63      <uri>http://roy.gbiv.com/</uri>
64    </address>
65  </author>
66
67  <author initials="J." surname="Gettys" fullname="Jim Gettys">
68    <organization>One Laptop per Child</organization>
69    <address>
70      <postal>
71        <street>21 Oak Knoll Road</street>
72        <city>Carlisle</city>
73        <region>MA</region>
74        <code>01741</code>
75        <country>USA</country>
76      </postal>
77      <email>jg@laptop.org</email>
78      <uri>http://www.laptop.org/</uri>
79    </address>
80  </author>
81 
82  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
83    <organization abbrev="HP">Hewlett-Packard Company</organization>
84    <address>
85      <postal>
86        <street>HP Labs, Large Scale Systems Group</street>
87        <street>1501 Page Mill Road, MS 1177</street>
88        <city>Palo Alto</city>
89        <region>CA</region>
90        <code>94304</code>
91        <country>USA</country>
92      </postal>
93      <email>JeffMogul@acm.org</email>
94    </address>
95  </author>
96
97  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
98    <organization abbrev="Microsoft">Microsoft Corporation</organization>
99    <address>
100      <postal>
101        <street>1 Microsoft Way</street>
102        <city>Redmond</city>
103        <region>WA</region>
104        <code>98052</code>
105        <country>USA</country>
106      </postal>
107      <email>henrikn@microsoft.com</email>
108    </address>
109  </author>
110
111  <author initials="L." surname="Masinter" fullname="Larry Masinter">
112    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
113    <address>
114      <postal>
115        <street>345 Park Ave</street>
116        <city>San Jose</city>
117        <region>CA</region>
118        <code>95110</code>
119        <country>USA</country>
120      </postal>
121      <email>LMM@acm.org</email>
122      <uri>http://larry.masinter.net/</uri>
123    </address>
124  </author>
125 
126  <author initials="P." surname="Leach" fullname="Paul J. Leach">
127    <organization abbrev="Microsoft">Microsoft Corporation</organization>
128    <address>
129      <postal>
130        <street>1 Microsoft Way</street>
131        <city>Redmond</city>
132        <region>WA</region>
133        <code>98052</code>
134      </postal>
135      <email>paulle@microsoft.com</email>
136    </address>
137  </author>
138   
139  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
140    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
141    <address>
142      <postal>
143        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
144        <street>The Stata Center, Building 32</street>
145        <street>32 Vassar Street</street>
146        <city>Cambridge</city>
147        <region>MA</region>
148        <code>02139</code>
149        <country>USA</country>
150      </postal>
151      <email>timbl@w3.org</email>
152      <uri>http://www.w3.org/People/Berners-Lee/</uri>
153    </address>
154  </author>
155
156  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
157    <organization abbrev="W3C">World Wide Web Consortium</organization>
158    <address>
159      <postal>
160        <street>W3C / ERCIM</street>
161        <street>2004, rte des Lucioles</street>
162        <city>Sophia-Antipolis</city>
163        <region>AM</region>
164        <code>06902</code>
165        <country>France</country>
166      </postal>
167      <email>ylafon@w3.org</email>
168      <uri>http://www.raubacapeu.net/people/yves/</uri>
169    </address>
170  </author>
171
172  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
173    <organization abbrev="greenbytes">greenbytes GmbH</organization>
174    <address>
175      <postal>
176        <street>Hafenweg 16</street>
177        <city>Muenster</city><region>NW</region><code>48155</code>
178        <country>Germany</country>
179      </postal>
180      <phone>+49 251 2807760</phone>   
181      <facsimile>+49 251 2807761</facsimile>   
182      <email>julian.reschke@greenbytes.de</email>       
183      <uri>http://greenbytes.de/tech/webdav/</uri>     
184    </address>
185  </author>
186
187  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
188
189<abstract>
190<t>
191   The Hypertext Transfer Protocol (HTTP) is an application-level
192   protocol for distributed, collaborative, hypermedia information
193   systems. HTTP has been in use by the World Wide Web global information
194   initiative since 1990. This document is Part 6 of the seven-part specification
195   that defines the protocol referred to as "HTTP/1.1" and, taken together,
196   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
197   and the associated header fields that control cache behavior or indicate
198   cacheable response messages.
199</t>
200</abstract>
201
202<note title="Editorial Note (To be removed by RFC Editor)">
203  <t>
204    Discussion of this draft should take place on the HTTPBIS working group
205    mailing list (ietf-http-wg@w3.org). The current issues list is
206    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
207    and related documents (including fancy diffs) can be found at
208    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
209  </t>
210  <t>
211    This draft incorporates those issue resolutions that were either
212    collected in the original RFC2616 errata list (<eref target="http://purl.org/NET/http-errata"/>),
213    or which were agreed upon on the mailing list between October 2006 and
214    November 2007 (as published in "draft-lafon-rfc2616bis-03").
215  </t>
216</note>
217</front>
218<middle>
219<section title="Introduction" anchor="caching">
220<t>
221   HTTP is typically used for distributed information systems, where
222   performance can be improved by the use of response caches, and includes
223   a number of elements intended to make caching work as well as possible.
224   Because these elements interact with each other, it is useful to describe
225   the caching design of HTTP separately.  This document defines aspects of
226   HTTP/1.1 related to caching and reusing response messages.
227</t>
228
229<section title="Purpose" anchor="intro.purpose">
230<iref item="cache"/>
231<t>
232   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
233   and the subsystem that controls its message storage, retrieval, and
234   deletion. A cache stores cacheable responses in order to reduce the
235   response time and network bandwidth consumption on future, equivalent
236   requests. Any client or server may include a cache, though a cache
237   cannot be used by a server that is acting as a tunnel.
238</t>
239<t>
240   Caching would be useless if it did not significantly improve
241   performance. The goal of caching in HTTP/1.1 is to reuse a prior response
242   message to satisfy a current request.  In some cases, the existing response
243   can be reused without the need for a network request, reducing latency and
244   network round-trips; we use an "expiration" mechanism for this purpose
245   (see <xref target="expiration.model"/>).  Even when a new request is required,
246   it is often possible to reuse all or parts of the payload of a prior response
247   to satisfy the request, thereby reducing network bandwidth usage; we use a
248   "validation" mechanism for this purpose (see <xref target="validation.model"/>).
249</t>
250<iref item="semantically transparent"/>
251<t>
252   A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
253   respect to a particular response, when its use affects neither the
254   requesting client nor the origin server, except to improve
255   performance. When a cache is semantically transparent, the client
256   receives exactly the same response status and payload
257   that it would have received had its request been handled directly
258   by the origin server.
259</t>
260<t>
261   In an ideal world, all interactions with an HTTP cache would be
262   semantically transparent.  However, for some resources, semantic
263   transparency is not always necessary and can be effectively traded
264   for the sake of bandwidth scaling, disconnected operation, and
265   high availability.  HTTP/1.1 allows origin servers, caches,
266   and clients to explicitly reduce transparency when necessary.
267   However, because non-transparent operation may confuse non-expert
268   users and might be incompatible with certain server applications
269   (such as those for ordering merchandise), the protocol requires that
270   transparency be relaxed
271  <list style="symbols">
272     <t>only by an explicit protocol-level request when relaxed by
273        client or origin server</t>
274
275     <t>only with an explicit warning to the end user when relaxed by
276        cache or client</t>
277  </list>
278</t>
279<t>
280   Therefore, HTTP/1.1 provides these important elements:
281  <list style="numbers">
282      <t>Protocol features that provide full semantic transparency when
283         this is required by all parties.</t>
284
285      <t>Protocol features that allow an origin server or user agent to
286         explicitly request and control non-transparent operation.</t>
287
288      <t>Protocol features that allow a cache to attach warnings to
289         responses that do not preserve the requested approximation of
290         semantic transparency.</t>
291  </list>
292</t>
293<t>
294   A basic principle is that it must be possible for the clients to
295   detect any potential relaxation of semantic transparency.
296  <list><t>
297      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
298      design decisions not explicitly discussed in this specification.
299      If a decision might affect semantic transparency, the implementor
300      ought to err on the side of maintaining transparency unless a
301      careful and complete analysis shows significant benefits in
302      breaking transparency.
303    </t></list>
304</t>
305</section>
306
307<section title="Terminology" anchor="intro.terminology">
308<t>
309   This specification uses a number of terms to refer to the roles
310   played by participants in, and objects of, HTTP caching.
311</t>
312<t>
313  <iref item="cacheable"/>
314  <x:dfn>cacheable</x:dfn>
315  <list>
316    <t>
317      A response is cacheable if a cache is allowed to store a copy of
318      the response message for use in answering subsequent requests.
319      Even when a response is cacheable, there may
320      be additional constraints on whether a cache can use the cached
321      copy for a particular request.
322    </t>
323  </list>
324</t>
325<t>
326  <iref item="first-hand"/>
327  <x:dfn>first-hand</x:dfn>
328  <list>
329    <t>
330      A response is first-hand if it comes directly and without
331      unnecessary delay from the origin server, perhaps via one or more
332      proxies. A response is also first-hand if its validity has just
333      been checked directly with the origin server.
334    </t>
335  </list>
336</t>
337<t>
338  <iref item="explicit expiration time"/>
339  <x:dfn>explicit expiration time</x:dfn>
340  <list>
341    <t>
342      The time at which the origin server intends that an entity should
343      no longer be returned by a cache without further validation.
344    </t>
345  </list>
346</t>
347<t>
348  <iref item="heuristic expiration time"/>
349  <x:dfn>heuristic expiration time</x:dfn>
350  <list>
351    <t>
352      An expiration time assigned by a cache when no explicit expiration
353      time is available.
354    </t>
355  </list>
356</t>
357<t>
358  <iref item="age"/>
359  <x:dfn>age</x:dfn>
360  <list>
361    <t>
362      The age of a response is the time since it was sent by, or
363      successfully validated with, the origin server.
364    </t>
365  </list>
366</t>
367<t>
368  <iref item="freshness lifetime"/>
369  <x:dfn>freshness lifetime</x:dfn>
370  <list>
371    <t>
372      The length of time between the generation of a response and its
373      expiration time.
374    </t>
375  </list>
376</t>
377<t>
378  <iref item="fresh"/>
379  <x:dfn>fresh</x:dfn>
380  <list>
381    <t>
382      A response is fresh if its age has not yet exceeded its freshness
383      lifetime.
384    </t>
385  </list>
386</t>
387<t>
388  <iref item="stale"/>
389  <x:dfn>stale</x:dfn>
390  <list>
391    <t>
392      A response is stale if its age has passed its freshness lifetime.
393    </t>
394  </list>
395</t>
396<t>
397  <iref item="validator"/>
398  <x:dfn>validator</x:dfn>
399  <list>
400    <t>
401      A protocol element (e.g., an entity tag or a Last-Modified time)
402      that is used to find out whether a cache entry is an equivalent
403      copy of an entity.
404    </t>
405  </list>
406</t>
407</section>
408
409<section title="Requirements" anchor="intro.requirements">
410<t>
411   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
412   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
413   document are to be interpreted as described in <xref target="RFC2119"/>.
414</t>
415<t>
416   An implementation is not compliant if it fails to satisfy one or more
417   of the &MUST; or &REQUIRED; level requirements for the protocols it
418   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
419   level and all the &SHOULD; level requirements for its protocols is said
420   to be "unconditionally compliant"; one that satisfies all the &MUST;
421   level requirements but not all the &SHOULD; level requirements for its
422   protocols is said to be "conditionally compliant."
423</t>
424</section>
425</section>
426
427<section title="Overview" anchor="caching.overview">
428<section title="Cache Correctness" anchor="cache.correctness">
429<t>
430   A correct cache &MUST; respond to a request with the most up-to-date
431   response held by the cache that is appropriate to the request (see
432   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
433   <xref target="disambiguating.multiple.responses" format="counter"/>,
434   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
435   conditions:
436  <list style="numbers">
437      <t>It has been checked for equivalence with what the origin server
438         would have returned by revalidating the response with the
439         origin server (<xref target="validation.model"/>);</t>
440
441      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
442         this means it meets the least restrictive freshness requirement
443         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
444         the origin server so specifies, it is the freshness requirement
445         of the origin server alone.
446
447         If a stored response is not "fresh enough" by the most
448         restrictive freshness requirement of both the client and the
449         origin server, in carefully considered circumstances the cache
450         &MAY; still return the response with the appropriate Warning
451         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
452         and <xref target="header.warning" format="counter"/>), unless such a response
453         is prohibited (e.g., by a "no-store" cache-directive, or by a
454         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
455
456      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
457         or error (4xx or 5xx) response message.</t>
458  </list>
459</t>
460<t>
461   If the cache can not communicate with the origin server, then a
462   correct cache &SHOULD; respond as above if the response can be
463   correctly served from the cache; if not it &MUST; return an error or
464   warning indicating that there was a communication failure.
465</t>
466<t>
467   If a cache receives a response (either an entire response, or a 304
468   (Not Modified) response) that it would normally forward to the
469   requesting client, and the received response is no longer fresh, the
470   cache &SHOULD; forward it to the requesting client without adding a new
471   Warning (but without removing any existing Warning headers). A cache
472   &SHOULD-NOT;  attempt to revalidate a response simply because that
473   response became stale in transit; this might lead to an infinite
474   loop. A user agent that receives a stale response without a Warning
475   &MAY; display a warning indication to the user.
476</t>
477</section>
478
479<section title="Warnings" anchor="warnings">
480<t>
481   Whenever a cache returns a response that is neither first-hand nor
482   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
483   &MUST; attach a warning to that effect, using a Warning general-header.
484   The Warning header and the currently defined warnings are described
485   in <xref target="header.warning"/>. The warning allows clients to take appropriate
486   action.
487</t>
488<t>
489   Warnings &MAY; be used for other purposes, both cache-related and
490   otherwise. The use of a warning, rather than an error status code,
491   distinguish these responses from true failures.
492</t>
493<t>
494   Warnings are assigned three digit warn-codes. The first digit
495   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
496   stored cache entry after a successful revalidation:
497</t>
498<t>
499  <list style="hanging">
500    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
501     the response, and so &MUST; be deleted after a successful
502     revalidation. 1xx warn-codes &MAY; be generated by a cache only when
503     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
504
505    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
506     headers that is not rectified by a revalidation (for example, a
507     lossy compression of the entity bodies) and which &MUST-NOT; be
508     deleted after a successful revalidation.</t>
509    </list>
510</t>
511<t>
512   See <xref target="header.warning"/> for the definitions of the codes themselves.
513</t>
514<t>
515   HTTP/1.0 caches will cache all Warnings in responses, without
516   deleting the ones in the first category. Warnings in responses that
517   are passed to HTTP/1.0 caches carry an extra warning-date field,
518   which prevents a future HTTP/1.1 recipient from believing an
519   erroneously cached Warning.
520</t>
521<t>
522   Warnings also carry a warning text. The text &MAY; be in any
523   appropriate natural language (perhaps based on the client's Accept
524   headers), and include an &OPTIONAL; indication of what character set is
525   used.
526</t>
527<t>
528   Multiple warnings &MAY; be attached to a response (either by the origin
529   server or by a cache), including multiple warnings with the same code
530   number. For example, a server might provide the same warning with
531   texts in both English and Basque.
532</t>
533<t>
534   When multiple warnings are attached to a response, it might not be
535   practical or reasonable to display all of them to the user. This
536   version of HTTP does not specify strict priority rules for deciding
537   which warnings to display and in what order, but does suggest some
538   heuristics.
539</t>
540</section>
541
542<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
543<t>
544   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
545   times and validators) are implicit directives to caches. In some
546   cases, a server or client might need to provide explicit directives
547   to the HTTP caches. We use the Cache-Control header for this purpose.
548</t>
549<t>
550   The Cache-Control header allows a client or server to transmit a
551   variety of directives in either requests or responses. These
552   directives typically override the default caching algorithms. As a
553   general rule, if there is any apparent conflict between header
554   values, the most restrictive interpretation is applied (that is, the
555   one that is most likely to preserve semantic transparency). However,
556   in some cases, cache-control directives are explicitly specified as
557   weakening the approximation of semantic transparency (for example,
558   "max-stale" or "public").
559</t>
560<t>
561   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
562</t>
563</section>
564
565<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
566<t>
567   Many user agents make it possible for users to override the basic
568   caching mechanisms. For example, the user agent might allow the user
569   to specify that cached entities (even explicitly stale ones) are
570   never validated. Or the user agent might habitually add "Cache-Control:
571   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
572   default to either non-transparent behavior, or behavior that results
573   in abnormally ineffective caching, but &MAY; be explicitly configured
574   to do so by an explicit action of the user.
575</t>
576<t>
577   If the user has overridden the basic caching mechanisms, the user
578   agent &SHOULD; explicitly indicate to the user whenever this results in
579   the display of information that might not meet the server's
580   transparency requirements (in particular, if the displayed entity is
581   known to be stale). Since the protocol normally allows the user agent
582   to determine if responses are stale or not, this indication need only
583   be displayed when this actually happens. The indication need not be a
584   dialog box; it could be an icon (for example, a picture of a rotting
585   fish) or some other indicator.
586</t>
587<t>
588   If the user has overridden the caching mechanisms in a way that would
589   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
590   continually indicate this state to the user (for example, by a
591   display of a picture of currency in flames) so that the user does not
592   inadvertently consume excess resources or suffer from excessive
593   latency.
594</t>
595</section>
596
597<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
598<t>
599   In some cases, the operator of a cache &MAY; choose to configure it to
600   return stale responses even when not requested by clients. This
601   decision ought not be made lightly, but may be necessary for reasons
602   of availability or performance, especially when the cache is poorly
603   connected to the origin server. Whenever a cache returns a stale
604   response, it &MUST; mark it as such (using a Warning header) enabling
605   the client software to alert the user that there might be a potential
606   problem.
607</t>
608<t>
609   It also allows the user agent to take steps to obtain a first-hand or
610   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
611   response if the client explicitly requests a first-hand or fresh one,
612   unless it is impossible to comply for technical or policy reasons.
613</t>
614</section>
615
616<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
617<t>
618   While the origin server (and to a lesser extent, intermediate caches,
619   by their contribution to the age of a response) are the primary
620   source of expiration information, in some cases the client might need
621   to control a cache's decision about whether to return a cached
622   response without validating it. Clients do this using several
623   directives of the Cache-Control header.
624</t>
625<t>
626   A client's request &MAY; specify the maximum age it is willing to
627   accept of an unvalidated response; specifying a value of zero forces
628   the cache(s) to revalidate all responses. A client &MAY; also specify
629   the minimum time remaining before a response expires. Both of these
630   options increase constraints on the behavior of caches, and so cannot
631   further relax the cache's approximation of semantic transparency.
632</t>
633<t>
634   A client &MAY; also specify that it will accept stale responses, up to
635   some maximum amount of staleness. This loosens the constraints on the
636   caches, and so might violate the origin server's specified
637   constraints on semantic transparency, but might be necessary to
638   support disconnected operation, or high availability in the face of
639   poor connectivity.
640</t>
641</section>
642</section>
643
644<section title="Expiration Model" anchor="expiration.model">
645
646<section title="Server-Specified Expiration" anchor="server-specified.expiration">
647<t>
648   HTTP caching works best when caches can entirely avoid making
649   requests to the origin server. The primary mechanism for avoiding
650   requests is for an origin server to provide an explicit expiration
651   time in the future, indicating that a response &MAY; be used to satisfy
652   subsequent requests. In other words, a cache can return a fresh
653   response without first contacting the server.
654</t>
655<t>
656   Our expectation is that servers will assign future explicit
657   expiration times to responses in the belief that the entity is not
658   likely to change, in a semantically significant way, before the
659   expiration time is reached. This normally preserves semantic
660   transparency, as long as the server's expiration times are carefully
661   chosen.
662</t>
663<t>
664   The expiration mechanism applies only to responses taken from a cache
665   and not to first-hand responses forwarded immediately to the
666   requesting client.
667</t>
668<t>
669   If an origin server wishes to force a semantically transparent cache
670   to validate every request, it &MAY; assign an explicit expiration time
671   in the past. This means that the response is always stale, and so the
672   cache &SHOULD; validate it before using it for subsequent requests. See
673   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
674</t>
675<t>
676   If an origin server wishes to force any HTTP/1.1 cache, no matter how
677   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
678   cache-control directive (see <xref target="header.cache-control"/>).
679</t>
680<t>
681   Servers specify explicit expiration times using either the Expires
682   header, or the max-age directive of the Cache-Control header.
683</t>
684<t>
685   An expiration time cannot be used to force a user agent to refresh
686   its display or reload a resource; its semantics apply only to caching
687   mechanisms, and such mechanisms need only check a resource's
688   expiration status when a new request for that resource is initiated.
689   See <xref target="history.lists"/> for an explanation of the difference between caches
690   and history mechanisms.
691</t>
692</section>
693
694<section title="Heuristic Expiration" anchor="heuristic.expiration">
695<t>
696   Since origin servers do not always provide explicit expiration times,
697   HTTP caches typically assign heuristic expiration times, employing
698   algorithms that use other header values (such as the Last-Modified
699   time) to estimate a plausible expiration time. The HTTP/1.1
700   specification does not provide specific algorithms, but does impose
701   worst-case constraints on their results. Since heuristic expiration
702   times might compromise semantic transparency, they ought to be used
703   cautiously, and we encourage origin servers to provide explicit
704   expiration times as much as possible.
705</t>
706</section>
707
708<section title="Age Calculations" anchor="age.calculations">
709<t>
710   In order to know if a cached entry is fresh, a cache needs to know if
711   its age exceeds its freshness lifetime. We discuss how to calculate
712   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
713   the age of a response or cache entry.
714</t>
715<t>
716   In this discussion, we use the term "now" to mean "the current value
717   of the clock at the host performing the calculation." Hosts that use
718   HTTP, but especially hosts running origin servers and caches, &SHOULD;
719   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
720   a globally accurate time standard.
721</t>
722<t>
723   HTTP/1.1 requires origin servers to send a Date header, if possible,
724   with every response, giving the time at which the response was
725   generated (see &header-date;). We use the term "date_value" to denote
726   the value of the Date header, in a form appropriate for arithmetic
727   operations.
728</t>
729<t>
730   HTTP/1.1 uses the Age response-header to convey the estimated age of
731   the response message when obtained from a cache. The Age field value
732   is the cache's estimate of the amount of time since the response was
733   generated or revalidated by the origin server.
734</t>
735<t>
736   In essence, the Age value is the sum of the time that the response
737   has been resident in each of the caches along the path from the
738   origin server, plus the amount of time it has been in transit along
739   network paths.
740</t>
741<t>
742   We use the term "age_value" to denote the value of the Age header, in
743   a form appropriate for arithmetic operations.
744</t>
745<t>
746   A response's age can be calculated in two entirely independent ways:
747  <list style="numbers">
748      <t>now minus date_value, if the local clock is reasonably well
749         synchronized to the origin server's clock. If the result is
750         negative, the result is replaced by zero.</t>
751
752      <t>age_value, if all of the caches along the response path
753         implement HTTP/1.1.</t>
754  </list>
755</t>
756<t>
757   Given that we have two independent ways to compute the age of a
758   response when it is received, we can combine these as
759</t>
760<figure><artwork type="code">
761    corrected_received_age = max(now - date_value, age_value)
762</artwork></figure>
763<t>
764   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
765   paths, one gets a reliable (conservative) result.
766</t>
767<t>
768   Because of network-imposed delays, some significant interval might
769   pass between the time that a server generates a response and the time
770   it is received at the next outbound cache or client. If uncorrected,
771   this delay could result in improperly low ages.
772</t>
773<t>
774   Because the request that resulted in the returned Age value must have
775   been initiated prior to that Age value's generation, we can correct
776   for delays imposed by the network by recording the time at which the
777   request was initiated. Then, when an Age value is received, it &MUST;
778   be interpreted relative to the time the request was initiated, not
779   the time that the response was received. This algorithm results in
780   conservative behavior no matter how much delay is experienced. So, we
781   compute:
782</t>
783<figure><artwork type="code">
784   corrected_initial_age = corrected_received_age
785                         + (now - request_time)
786</artwork></figure>
787<t>
788   where "request_time" is the time (according to the local clock) when
789   the request that elicited this response was sent.
790</t>
791<t>
792   Summary of age calculation algorithm, when a cache receives a
793   response:
794</t>
795<figure><artwork type="code">
796   /*
797    * age_value
798    *      is the value of Age: header received by the cache with
799    *              this response.
800    * date_value
801    *      is the value of the origin server's Date: header
802    * request_time
803    *      is the (local) time when the cache made the request
804    *              that resulted in this cached response
805    * response_time
806    *      is the (local) time when the cache received the
807    *              response
808    * now
809    *      is the current (local) time
810    */
811
812   apparent_age = max(0, response_time - date_value);
813   corrected_received_age = max(apparent_age, age_value);
814   response_delay = response_time - request_time;
815   corrected_initial_age = corrected_received_age + response_delay;
816   resident_time = now - response_time;
817   current_age   = corrected_initial_age + resident_time;
818</artwork></figure>
819<t>
820   The current_age of a cache entry is calculated by adding the amount
821   of time (in seconds) since the cache entry was last validated by the
822   origin server to the corrected_initial_age. When a response is
823   generated from a cache entry, the cache &MUST; include a single Age
824   header field in the response with a value equal to the cache entry's
825   current_age.
826</t>
827<t>
828   The presence of an Age header field in a response implies that a
829   response is not first-hand. However, the converse is not true, since
830   the lack of an Age header field in a response does not imply that the
831   response is first-hand unless all caches along the request path are
832   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
833   the Age header field).
834</t>
835</section>
836
837<section title="Expiration Calculations" anchor="expiration.calculations">
838<t>
839   In order to decide whether a response is fresh or stale, we need to
840   compare its freshness lifetime to its age. The age is calculated as
841   described in <xref target="age.calculations"/>; this section describes how to calculate
842   the freshness lifetime, and to determine if a response has expired.
843   In the discussion below, the values can be represented in any form
844   appropriate for arithmetic operations.
845</t>
846<t>
847   We use the term "expires_value" to denote the value of the Expires
848   header. We use the term "max_age_value" to denote an appropriate
849   value of the number of seconds carried by the "max-age" directive of
850   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
851</t>
852<t>
853   The max-age directive takes priority over Expires, so if max-age is
854   present in a response, the calculation is simply:
855</t>
856<figure><artwork type="code">
857   freshness_lifetime = max_age_value
858</artwork></figure>
859<t>
860   Otherwise, if Expires is present in the response, the calculation is:
861</t>
862<figure><artwork type="code">
863   freshness_lifetime = expires_value - date_value
864</artwork></figure>
865<t>
866   Note that neither of these calculations is vulnerable to clock skew,
867   since all of the information comes from the origin server.
868</t>
869<t>
870   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
871   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
872   does not include other restrictions on caching, the cache &MAY; compute
873   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
874   113 to any response whose age is more than 24 hours if such warning
875   has not already been added.
876</t>
877<t>
878   Also, if the response does have a Last-Modified time, the heuristic
879   expiration value &SHOULD; be no more than some fraction of the interval
880   since that time. A typical setting of this fraction might be 10%.
881</t>
882<t>
883   The calculation to determine if a response has expired is quite
884   simple:
885</t>
886<figure><artwork type="code">
887   response_is_fresh = (freshness_lifetime &gt; current_age)
888</artwork></figure>
889</section>
890
891<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
892<t>
893   Because expiration values are assigned optimistically, it is possible
894   for two caches to contain fresh values for the same resource that are
895   different.
896</t>
897<t>
898   If a client performing a retrieval receives a non-first-hand response
899   for a request that was already fresh in its own cache, and the Date
900   header in its existing cache entry is newer than the Date on the new
901   response, then the client &MAY; ignore the response. If so, it &MAY;
902   retry the request with a "Cache-Control: max-age=0" directive (see
903   <xref target="header.cache-control"/>), to force a check with the origin server.
904</t>
905<t>
906   If a cache has two fresh responses for the same representation with
907   different validators, it &MUST; use the one with the more recent Date
908   header. This situation might arise because the cache is pooling
909   responses from other caches, or because a client has asked for a
910   reload or a revalidation of an apparently fresh cache entry.
911</t>
912</section>
913
914<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
915<t>
916   Because a client might be receiving responses via multiple paths, so
917   that some responses flow through one set of caches and other
918   responses flow through a different set of caches, a client might
919   receive responses in an order different from that in which the origin
920   server sent them. We would like the client to use the most recently
921   generated response, even if older responses are still apparently
922   fresh.
923</t>
924<t>
925   Neither the entity tag nor the expiration value can impose an
926   ordering on responses, since it is possible that a later response
927   intentionally carries an earlier expiration time. The Date values are
928   ordered to a granularity of one second.
929</t>
930<t>
931   When a client tries to revalidate a cache entry, and the response it
932   receives contains a Date header that appears to be older than the one
933   for the existing entry, then the client &SHOULD; repeat the request
934   unconditionally, and include
935</t>
936<figure><artwork type="example">
937    Cache-Control: max-age=0
938</artwork></figure>
939<t>
940   to force any intermediate caches to validate their copies directly
941   with the origin server, or
942</t>
943<figure><artwork type="example">
944    Cache-Control: no-cache
945</artwork></figure>
946<t>
947   to force any intermediate caches to obtain a new copy from the origin
948   server.
949</t>
950<t>
951   If the Date values are equal, then the client &MAY; use either response
952   (or &MAY;, if it is being extremely prudent, request a new response).
953   Servers &MUST-NOT; depend on clients being able to choose
954   deterministically between responses generated during the same second,
955   if their expiration times overlap.
956</t>
957</section>
958</section>
959
960<section title="Validation Model" anchor="validation.model">
961<t>
962   When a cache has a stale entry that it would like to use as a
963   response to a client's request, it first has to check with the origin
964   server (or possibly an intermediate cache with a fresh response) to
965   see if its cached entry is still usable. We call this "validating"
966   the cache entry.
967</t>
968<t>
969   HTTP's conditional request mechanism, defined in &conditional;, is
970   used to avoid retransmitting the response payload when the cached entry
971   is valid.  When a cached response includes one or more "cache validators,"
972   such as the field values of an ETag or Last-Modified header field, then
973   a validating GET request &SHOULD; be made conditional to those field values.
974   The server checks the conditional request's validator against the current
975   state of the requested resource and, if they match, the server responds
976   with a 304 (Not Modified) status code to indicate that the cached response
977   can be refreshed and reused without retransmitting the response payload.
978   If the validator does not match the current state of the requested
979   resource, then the server returns a full response, including payload,
980   so that the request can be satisfied and the cache entry supplanted
981   without the need for an additional network round-trip.
982</t>
983</section>
984
985<section title="Response Cacheability" anchor="response.cacheability">
986<t>
987   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
988   directive, a caching system &MAY; always store a successful response
989   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
990   if it is fresh, and &MAY; return it after successful validation. If
991   there is neither a cache validator nor an explicit expiration time
992   associated with a response, we do not expect it to be cached, but
993   certain caches &MAY; violate this expectation (for example, when little
994   or no network connectivity is available). A client can usually detect
995   that such a response was taken from a cache by comparing the Date
996   header to the current time.
997  <list><t>
998      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
999      without providing any Warning.
1000  </t></list>
1001</t>
1002<t>
1003   However, in some cases it might be inappropriate for a cache to
1004   retain an entity, or to return it in response to a subsequent
1005   request. This might be because absolute semantic transparency is
1006   deemed necessary by the service author, or because of security or
1007   privacy considerations. Certain cache-control directives are
1008   therefore provided so that the server can indicate that certain
1009   resource entities, or portions thereof, are not to be cached
1010   regardless of other considerations.
1011</t>
1012<t>
1013   Note that &header-authorization; normally prevents a shared cache from saving
1014   and returning a response to a previous request if that request
1015   included an Authorization header.
1016</t>
1017<t>
1018   A response received with a status code of 200, 203, 206, 300, 301 or
1019   410 &MAY; be stored by a cache and used in reply to a subsequent
1020   request, subject to the expiration mechanism, unless a cache-control
1021   directive prohibits caching. However, a cache that does not support
1022   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1023   Content) responses.
1024</t>
1025<t>
1026   A response received with any other status code (e.g. status codes 302
1027   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1028   unless there are cache-control directives or another header(s) that
1029   explicitly allow it. For example, these include the following: an
1030   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1031   "proxy-revalidate", "public" or "private" cache-control
1032   directive (<xref target="header.cache-control"/>).
1033</t>
1034</section>
1035
1036<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1037<t>
1038   The purpose of an HTTP cache is to store information received in
1039   response to requests for use in responding to future requests. In
1040   many cases, a cache simply returns the appropriate parts of a
1041   response to the requester. However, if the cache holds a cache entry
1042   based on a previous response, it might have to combine parts of a new
1043   response with what is held in the cache entry.
1044</t>
1045
1046<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1047<t>
1048   For the purpose of defining the behavior of caches and non-caching
1049   proxies, we divide HTTP headers into two categories:
1050  <list style="symbols">
1051      <t>End-to-end headers, which are  transmitted to the ultimate
1052        recipient of a request or response. End-to-end headers in
1053        responses &MUST; be stored as part of a cache entry and &MUST; be
1054        transmitted in any response formed from a cache entry.</t>
1055
1056      <t>Hop-by-hop headers, which are meaningful only for a single
1057        transport-level connection, and are not stored by caches or
1058        forwarded by proxies.</t>
1059  </list>
1060</t>
1061<t>
1062   The following HTTP/1.1 headers are hop-by-hop headers:
1063  <list style="symbols">
1064      <t>Connection</t>
1065      <t>Keep-Alive</t>
1066      <t>Proxy-Authenticate</t>
1067      <t>Proxy-Authorization</t>
1068      <t>TE</t>
1069      <t>Trailer</t>
1070      <t>Transfer-Encoding</t>
1071      <t>Upgrade</t>
1072  </list>
1073</t>
1074<t>
1075   All other headers defined by HTTP/1.1 are end-to-end headers.
1076</t>
1077<t>
1078   Other hop-by-hop headers &MUST; be listed in a Connection header
1079   (&header-connection;).
1080</t>
1081</section>
1082
1083<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1084<t>
1085   Some features of HTTP/1.1, such as Digest
1086   Authentication, depend on the value of certain end-to-end headers. A
1087   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1088   definition of that header requires or specifically allows that.
1089</t>
1090<t>
1091   A transparent proxy &MUST-NOT; modify any of the following fields in a
1092   request or response, and it &MUST-NOT; add any of these fields if not
1093   already present:
1094  <list style="symbols">
1095      <t>Content-Location</t>
1096      <t>Content-MD5</t>
1097      <t>ETag</t>
1098      <t>Last-Modified</t>
1099  </list>
1100</t>
1101<t>
1102   A transparent proxy &MUST-NOT; modify any of the following fields in a
1103   response:
1104  <list style="symbols">
1105    <t>Expires</t>
1106  </list>
1107</t>
1108<t>
1109   but it &MAY; add any of these fields if not already present. If an
1110   Expires header is added, it &MUST; be given a field-value identical to
1111   that of the Date header in that response.
1112</t>
1113<t>
1114   A  proxy &MUST-NOT; modify or add any of the following fields in a
1115   message that contains the no-transform cache-control directive, or in
1116   any request:
1117  <list style="symbols">
1118    <t>Content-Encoding</t>
1119    <t>Content-Range</t>
1120    <t>Content-Type</t>
1121  </list>
1122</t>
1123<t>
1124   A non-transparent proxy &MAY; modify or add these fields to a message
1125   that does not include no-transform, but if it does so, it &MUST; add a
1126   Warning 214 (Transformation applied) if one does not already appear
1127   in the message (see <xref target="header.warning"/>).
1128  <list><t>
1129      Warning: unnecessary modification of end-to-end headers might
1130      cause authentication failures if stronger authentication
1131      mechanisms are introduced in later versions of HTTP. Such
1132      authentication mechanisms &MAY; rely on the values of header fields
1133      not listed here.
1134    </t></list>
1135</t>
1136<t>
1137   The Content-Length field of a request or response is added or deleted
1138   according to the rules in &message-length;. A transparent proxy &MUST;
1139   preserve the entity-length (&entity-length;) of the entity-body,
1140   although it &MAY; change the transfer-length (&message-length;).
1141</t>
1142</section>
1143
1144<section title="Combining Headers" anchor="combining.headers">
1145<t>
1146   When a cache makes a validating request to a server, and the server
1147   provides a 304 (Not Modified) response or a 206 (Partial Content)
1148   response, the cache then constructs a response to send to the
1149   requesting client.
1150</t>
1151<t>
1152   If the status code is 304 (Not Modified), the cache uses the entity-body
1153   stored in the cache entry as the entity-body of this outgoing
1154   response. If the status code is 206 (Partial Content) and the ETag or
1155   Last-Modified headers match exactly, the cache &MAY; combine the
1156   contents stored in the cache entry with the new contents received in
1157   the response and use the result as the entity-body of this outgoing
1158   response, (see &combining-byte-ranges;).
1159</t>
1160<t>
1161   The end-to-end headers stored in the cache entry are used for the
1162   constructed response, except that
1163  <list style="symbols">
1164    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1165      &MUST; be deleted from the cache entry and the forwarded response.</t>
1166    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1167        in the cache entry and the forwarded response.</t>
1168    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1169        replace the corresponding headers from the cache entry.</t>
1170  </list>
1171</t>
1172<t>
1173   Unless the cache decides to remove the cache entry, it &MUST; also
1174   replace the end-to-end headers stored with the cache entry with
1175   corresponding headers received in the incoming response, except for
1176   Warning headers as described immediately above. If a header field-name
1177   in the incoming response matches more than one header in the
1178   cache entry, all such old headers &MUST; be replaced.
1179</t>
1180<t>
1181   In other words, the set of end-to-end headers received in the
1182   incoming response overrides all corresponding end-to-end headers
1183   stored with the cache entry (except for stored Warning headers with
1184   warn-code 1xx, which are deleted even if not overridden).
1185  <list><t>
1186      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1187      Modified) or a 206 (Partial Content) response to update any header
1188      associated with a previous response for the same entity or sub-ranges
1189      thereof, although it might not always be meaningful or
1190      correct to do so. This rule does not allow an origin server to use
1191      a 304 (Not Modified) or a 206 (Partial Content) response to
1192      entirely delete a header that it had provided with a previous
1193      response.
1194  </t></list>
1195</t>
1196</section>
1197
1198</section>
1199
1200<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1201<t>
1202   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1203   by the presence of a Vary header field in a response, alters the
1204   conditions and procedure by which a cache can use the response for
1205   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1206   field by servers.
1207</t>
1208<t>
1209   A server &SHOULD; use the Vary header field to inform a cache of what
1210   request-header fields were used to select among multiple
1211   representations of a cacheable response subject to server-driven
1212   negotiation. The set of header fields named by the Vary field value
1213   is known as the "selecting" request-headers.
1214</t>
1215<t>
1216   When the cache receives a subsequent request whose Request-URI
1217   specifies one or more cache entries including a Vary header field,
1218   the cache &MUST-NOT; use such a cache entry to construct a response to
1219   the new request unless all of the selecting request-headers present
1220   in the new request match the corresponding stored request-headers in
1221   the original request.
1222</t>
1223<t>
1224   The selecting request-headers from two requests are defined to match
1225   if and only if the selecting request-headers in the first request can
1226   be transformed to the selecting request-headers in the second request
1227   by adding or removing linear white space (LWS) at places where this
1228   is allowed by the corresponding BNF, and/or combining multiple
1229   message-header fields with the same field name following the rules
1230   about message headers in &message-headers;.
1231</t>
1232<t>
1233   A Vary header field-value of "*" always fails to match and subsequent
1234   requests on that resource can only be properly interpreted by the
1235   origin server.
1236</t>
1237<t>
1238   If the selecting request header fields for the cached entry do not
1239   match the selecting request header fields of the new request, then
1240   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1241   it first relays the new request to the origin server in a conditional
1242   request and the server responds with 304 (Not Modified), including an
1243   entity tag or Content-Location that indicates the entity to be used.
1244</t>
1245<t>
1246   If an entity tag was assigned to a cached representation, the
1247   forwarded request &SHOULD; be conditional and include the entity tags
1248   in an If-None-Match header field from all its cache entries for the
1249   resource. This conveys to the server the set of entities currently
1250   held by the cache, so that if any one of these entities matches the
1251   requested entity, the server can use the ETag header field in its 304
1252   (Not Modified) response to tell the cache which entry is appropriate.
1253   If the entity-tag of the new response matches that of an existing
1254   entry, the new response &SHOULD; be used to update the header fields of
1255   the existing entry, and the result &MUST; be returned to the client.
1256</t>
1257<t>
1258   If any of the existing cache entries contains only partial content
1259   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1260   the If-None-Match header field unless the request is for a range that
1261   would be fully satisfied by that entry.
1262</t>
1263<t>
1264   If a cache receives a successful response whose Content-Location
1265   field matches that of an existing cache entry for the same Request-URI,
1266   whose entity-tag differs from that of the existing entry, and
1267   whose Date is more recent than that of the existing entry, the
1268   existing entry &SHOULD-NOT;  be returned in response to future requests
1269   and &SHOULD; be deleted from the cache.
1270</t>
1271</section>
1272
1273<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1274<t>
1275   For reasons of security and privacy, it is necessary to make a
1276   distinction between "shared" and "non-shared" caches. A non-shared
1277   cache is one that is accessible only to a single user. Accessibility
1278   in this case &SHOULD; be enforced by appropriate security mechanisms.
1279   All other caches are considered to be "shared." Other sections of
1280   this specification place certain constraints on the operation of
1281   shared caches in order to prevent loss of privacy or failure of
1282   access controls.
1283</t>
1284</section>
1285
1286<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1287<t>
1288   A cache that receives an incomplete response (for example, with fewer
1289   bytes of data than specified in a Content-Length header) &MAY; store
1290   the response. However, the cache &MUST; treat this as a partial
1291   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1292   the result might be a full response or might still be
1293   partial. A cache &MUST-NOT; return a partial response to a client
1294   without explicitly marking it as such, using the 206 (Partial
1295   Content) status code. A cache &MUST-NOT; return a partial response
1296   using a status code of 200 (OK).
1297</t>
1298<t>
1299   If a cache receives a 5xx response while attempting to revalidate an
1300   entry, it &MAY; either forward this response to the requesting client,
1301   or act as if the server failed to respond. In the latter case, it &MAY;
1302   return a previously received response unless the cached entry
1303   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1304</t>
1305</section>
1306
1307<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1308<t>
1309   Unless the origin server explicitly prohibits the caching of their
1310   responses, the application of GET and HEAD methods to any resources
1311   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1312   these responses are taken from a cache. They &MAY; still have side
1313   effects, but a cache is not required to consider such side effects in
1314   its caching decisions. Caches are always expected to observe an
1315   origin server's explicit restrictions on caching.
1316</t>
1317<t>
1318   We note one exception to this rule: since some applications have
1319   traditionally used GET and HEAD requests with URLs containing a query part
1320   to perform operations with significant side
1321   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1322   the server provides an explicit expiration time. This specifically
1323   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1324   be taken from a cache. See &safe-methods; for related information.
1325</t>
1326</section>
1327
1328<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1329<t>
1330   The effect of certain methods performed on a resource at the origin
1331   server might cause one or more existing cache entries to become non-transparently
1332   invalid. That is, although they might continue to be
1333   "fresh," they do not accurately reflect what the origin server would
1334   return for a new request on that resource.
1335</t>
1336<t>
1337   There is no way for HTTP to guarantee that all such
1338   cache entries are marked invalid. For example, the request that
1339   caused the change at the origin server might not have gone through
1340   the proxy where a cache entry is stored. However, several rules help
1341   reduce the likelihood of erroneous behavior.
1342</t>
1343<t>
1344   In this section, the phrase "invalidate an entity" means that the
1345   cache will either remove all instances of that entity from its
1346   storage, or will mark these as "invalid" and in need of a mandatory
1347   revalidation before they can be returned in response to a subsequent
1348   request.
1349</t>
1350<t>
1351   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1352   either the entity referred to by the Request-URI, or by the Location
1353   or Content-Location headers (if present). These methods are:
1354  <list style="symbols">
1355      <t>PUT</t>
1356      <t>DELETE</t>
1357      <t>POST</t>
1358  </list>
1359</t> 
1360<t>
1361   An invalidation based
1362   on the URI in a Location or Content-Location header &MUST-NOT; be
1363   performed if the host part of that URI differs from the host part
1364   in the Request-URI. This helps prevent denial of service attacks.
1365</t>
1366<t>
1367   A cache that passes through requests for methods it does not
1368   understand &SHOULD; invalidate any entities referred to by the
1369   Request-URI.
1370</t>
1371</section>
1372
1373<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1374<t>
1375   All methods that might be expected to cause modifications to the
1376   origin server's resources &MUST; be written through to the origin
1377   server. This currently includes all methods except for GET and HEAD.
1378   A cache &MUST-NOT; reply to such a request from a client before having
1379   transmitted the request to the inbound server, and having received a
1380   corresponding response from the inbound server. This does not prevent
1381   a proxy cache from sending a 100 (Continue) response before the
1382   inbound server has sent its final reply.
1383</t>
1384<t>
1385   The alternative (known as "write-back" or "copy-back" caching) is not
1386   allowed in HTTP/1.1, due to the difficulty of providing consistent
1387   updates and the problems arising from server, cache, or network
1388   failure prior to write-back.
1389</t>
1390</section>
1391
1392<section title="Cache Replacement" anchor="cache.replacement">
1393<t>
1394   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1395   <xref target="disambiguating.expiration.values" format="counter"/>,
1396   <xref target="disambiguating.multiple.responses" format="counter"/>
1397   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1398   response is received from a resource while any existing responses for
1399   the same resource are cached, the cache &SHOULD; use the new response
1400   to reply to the current request. It &MAY; insert it into cache storage
1401   and &MAY;, if it meets all other requirements, use it to respond to any
1402   future requests that would previously have caused the old response to
1403   be returned. If it inserts the new response into cache storage  the
1404   rules in <xref target="combining.headers"/> apply.
1405  <list><t>
1406      <x:h>Note:</x:h> a new response that has an older Date header value than
1407      existing cached responses is not cacheable.
1408  </t></list>
1409</t>
1410</section>
1411
1412<section title="History Lists" anchor="history.lists">
1413<t>
1414   User agents often have history mechanisms, such as "Back" buttons and
1415   history lists, which can be used to redisplay an entity retrieved
1416   earlier in a session.
1417</t>
1418<t>
1419   History mechanisms and caches are different. In particular history
1420   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1421   the current state of a resource. Rather, a history mechanism is meant
1422   to show exactly what the user saw at the time when the resource was
1423   retrieved.
1424</t>
1425<t>
1426   By default, an expiration time does not apply to history mechanisms.
1427   If the entity is still in storage, a history mechanism &SHOULD; display
1428   it even if the entity has expired, unless the user has specifically
1429   configured the agent to refresh expired history documents.
1430</t>
1431<t>
1432   This is not to be construed to prohibit the history mechanism from
1433   telling the user that a view might be stale.
1434  <list><t>
1435      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1436      viewing stale resources, this will tend to force service authors
1437      to avoid using HTTP expiration controls and cache controls when
1438      they would otherwise like to. Service authors may consider it
1439      important that users not be presented with error messages or
1440      warning messages when they use navigation controls (such as BACK)
1441      to view previously fetched resources. Even though sometimes such
1442      resources ought not be cached, or ought to expire quickly, user
1443      interface considerations may force service authors to resort to
1444      other means of preventing caching (e.g. "once-only" URLs) in order
1445      not to suffer the effects of improperly functioning history
1446      mechanisms.
1447  </t></list>
1448</t>
1449</section>
1450
1451<section title="Header Field Definitions" anchor="header.fields">
1452<t>
1453   This section defines the syntax and semantics of HTTP/1.1 header fields
1454   related to caching.
1455</t>
1456<t>
1457   For entity-header fields, both sender and recipient refer to either the
1458   client or the server, depending on who sends and who receives the entity.
1459</t>
1460
1461<section title="Age" anchor="header.age">
1462  <iref primary="true" item="Age header" x:for-anchor=""/>
1463  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1464<t>
1465      The Age response-header field conveys the sender's estimate of the
1466      amount of time since the response (or its revalidation) was
1467      generated at the origin server. A cached response is "fresh" if
1468      its age does not exceed its freshness lifetime. Age values are
1469      calculated as specified in <xref target="age.calculations"/>.
1470</t>
1471<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1472  Age = "Age" ":" age-value
1473  age-value = delta-seconds
1474</artwork></figure>
1475<t>
1476      Age values are non-negative decimal integers, representing time in
1477      seconds.
1478</t>
1479<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
1480  delta-seconds  = 1*DIGIT
1481</artwork></figure>
1482<t>
1483      If a cache receives a value larger than the largest positive
1484      integer it can represent, or if any of its age calculations
1485      overflows, it &MUST; transmit an Age header with a value of
1486      2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
1487      include an Age header field in every response generated from its
1488      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1489      bits of range.
1490</t>
1491</section>
1492
1493<section title="Cache-Control" anchor="header.cache-control">
1494  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1495  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1496<t>
1497   The Cache-Control general-header field is used to specify directives
1498   that &MUST; be obeyed by all caching mechanisms along the
1499   request/response chain. The directives specify behavior intended to
1500   prevent caches from adversely interfering with the request or
1501   response. These directives typically override the default caching
1502   algorithms. Cache directives are unidirectional in that the presence
1503   of a directive in a request does not imply that the same directive is
1504   to be given in the response.
1505  <list><t>
1506      Note that HTTP/1.0 caches might not implement Cache-Control and
1507      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1508  </t></list>
1509</t>
1510<t>
1511   Cache directives &MUST; be passed through by a proxy or gateway
1512   application, regardless of their significance to that application,
1513   since the directives might be applicable to all recipients along the
1514   request/response chain. It is not possible to specify a cache-directive
1515   for a specific cache.
1516</t>
1517<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"/>
1518  Cache-Control   = "Cache-Control" ":" 1#cache-directive
1519
1520  cache-directive = cache-request-directive
1521       | cache-response-directive
1522
1523  cache-request-directive =
1524         "no-cache"                          ; <xref target="what.is.cacheable"/>
1525       | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1526       | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1527       | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1528       | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1529       | "no-transform"                      ; <xref target="no-transform.directive"/>
1530       | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1531       | cache-extension                     ; <xref target="cache.control.extensions"/>
1532
1533  cache-response-directive =
1534         "public"                               ; <xref target="what.is.cacheable"/>
1535       | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; <xref target="what.is.cacheable"/>
1536       | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]; <xref target="what.is.cacheable"/>
1537       | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1538       | "no-transform"                         ; <xref target="no-transform.directive"/>
1539       | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1540       | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1541       | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1542       | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1543       | cache-extension                        ; <xref target="cache.control.extensions"/>
1544
1545  cache-extension = token [ "=" ( token | quoted-string ) ]
1546</artwork></figure>
1547<t>
1548   When a directive appears without any 1#field-name parameter, the
1549   directive applies to the entire request or response. When such a
1550   directive appears with a 1#field-name parameter, it applies only to
1551   the named field or fields, and not to the rest of the request or
1552   response. This mechanism supports extensibility; implementations of
1553   future versions of HTTP might apply these directives to
1554   header fields not defined in HTTP/1.1.
1555</t>
1556<t>
1557   The cache-control directives can be broken down into these general
1558   categories:
1559  <list style="symbols">
1560     <t>Restrictions on what are cacheable; these may only be imposed by
1561        the origin server.</t>
1562
1563     <t>Restrictions on what may be stored by a cache; these may be
1564        imposed by either the origin server or the user agent.</t>
1565
1566     <t>Modifications of the basic expiration mechanism; these may be
1567        imposed by either the origin server or the user agent.</t>
1568
1569     <t>Controls over cache revalidation and reload; these may only be
1570        imposed by a user agent.</t>
1571
1572     <t>Control over transformation of entities.</t>
1573
1574     <t>Extensions to the caching system.</t>
1575  </list>
1576</t>
1577
1578<section title="What is Cacheable" anchor="what.is.cacheable">
1579<t>
1580   By default, a response is cacheable if the requirements of the
1581   request method, request header fields, and the response status
1582   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1583   for cacheability. The following Cache-Control response directives
1584   allow an origin server to override the default cacheability of a
1585   response:
1586</t>
1587<t>
1588  <iref item="Cache Directives" subitem="public" primary="true"/>
1589  <iref item="public" subitem="Cache Directive" primary="true"/>
1590   public
1591  <list><t>
1592      Indicates that the response &MAY; be cached by any cache, even if it
1593      would normally be non-cacheable or cacheable only within a non-shared
1594      cache. (See also Authorization, &header-authorization;, for
1595      additional details.)
1596  </t></list>
1597</t>
1598<t>
1599  <iref item="Cache Directives" subitem="private" primary="true"/>
1600  <iref item="private" subitem="Cache Directive" primary="true"/>
1601   private
1602  <list><t>
1603      Indicates that all or part of the response message is intended for
1604      a single user and &MUST-NOT; be cached by a shared cache. This
1605      allows an origin server to state that the specified parts of the
1606      response are intended for only one user and are not a valid
1607      response for requests by other users. A private (non-shared) cache
1608      &MAY; cache the response.
1609    </t><t>
1610       <x:h>Note:</x:h> This usage of the word private only controls where the
1611       response may be cached, and cannot ensure the privacy of the
1612       message content.
1613  </t></list>
1614</t>
1615<t>
1616  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1617  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1618   no-cache
1619  <list><t>
1620       If the no-cache directive does not specify a field-name, then a
1621      cache &MUST-NOT; use the response to satisfy a subsequent request
1622      without successful revalidation with the origin server. This
1623      allows an origin server to prevent caching even by caches that
1624      have been configured to return stale responses to client requests.
1625    </t><t>
1626      If the no-cache directive does specify one or more field-names,
1627      then a cache &MAY; use the response to satisfy a subsequent request,
1628      subject to any other restrictions on caching. However, the
1629      specified field-name(s) &MUST-NOT; be sent in the response to a
1630      subsequent request without successful revalidation with the origin
1631      server. This allows an origin server to prevent the re-use of
1632      certain header fields in a response, while still allowing caching
1633      of the rest of the response.
1634    <list><t>
1635       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1636       directive.
1637    </t></list>
1638  </t></list>
1639</t>
1640</section>
1641
1642<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1643<t>
1644  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1645  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1646   no-store
1647  <list><t>   
1648      The purpose of the no-store directive is to prevent the
1649      inadvertent release or retention of sensitive information (for
1650      example, on backup tapes). The no-store directive applies to the
1651      entire message, and &MAY; be sent either in a response or in a
1652      request. If sent in a request, a cache &MUST-NOT; store any part of
1653      either this request or any response to it. If sent in a response,
1654      a cache &MUST-NOT; store any part of either this response or the
1655      request that elicited it. This directive applies to both non-shared
1656      and shared caches. "&MUST-NOT; store" in this context means
1657      that the cache &MUST-NOT; intentionally store the information in
1658      non-volatile storage, and &MUST; make a best-effort attempt to
1659      remove the information from volatile storage as promptly as
1660      possible after forwarding it.
1661  </t><t>
1662      Even when this directive is associated with a response, users
1663      might explicitly store such a response outside of the caching
1664      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1665      such responses as part of their normal operation.
1666  </t><t>
1667      The purpose of this directive is to meet the stated requirements
1668      of certain users and service authors who are concerned about
1669      accidental releases of information via unanticipated accesses to
1670      cache data structures. While the use of this directive might
1671      improve privacy in some cases, we caution that it is NOT in any
1672      way a reliable or sufficient mechanism for ensuring privacy. In
1673      particular, malicious or compromised caches might not recognize or
1674      obey this directive, and communications networks might be
1675      vulnerable to eavesdropping.
1676  </t></list>
1677</t>
1678</section>
1679
1680<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1681<t>
1682   The expiration time of an entity &MAY; be specified by the origin
1683   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1684   it &MAY; be specified using the max-age directive in a response. When
1685   the max-age cache-control directive is present in a cached response,
1686   the response is stale if its current age is greater than the age
1687   value given (in seconds) at the time of a new request for that
1688   resource. The max-age directive on a response implies that the
1689   response is cacheable (i.e., "public") unless some other, more
1690   restrictive cache directive is also present.
1691</t>
1692<t>
1693   If a response includes both an Expires header and a max-age
1694   directive, the max-age directive overrides the Expires header, even
1695   if the Expires header is more restrictive. This rule allows an origin
1696   server to provide, for a given response, a longer expiration time to
1697   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1698   useful if certain HTTP/1.0 caches improperly calculate ages or
1699   expiration times, perhaps due to desynchronized clocks.
1700</t>
1701<t>
1702   Many HTTP/1.0 cache implementations will treat an Expires value that
1703   is less than or equal to the response Date value as being equivalent
1704   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1705   cache receives such a response, and the response does not include a
1706   Cache-Control header field, it &SHOULD; consider the response to be
1707   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1708  <list><t>
1709       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1710       cache control feature, such as the "private" directive, on a
1711       network including older caches that do not understand that
1712       feature. The origin server will need to combine the new feature
1713       with an Expires field whose value is less than or equal to the
1714       Date value. This will prevent older caches from improperly
1715       caching the response.
1716  </t></list>
1717</t>
1718<t>
1719  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1720  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1721   s-maxage
1722  <list><t>
1723       If a response includes an s-maxage directive, then for a shared
1724       cache (but not for a private cache), the maximum age specified by
1725       this directive overrides the maximum age specified by either the
1726       max-age directive or the Expires header. The s-maxage directive
1727       also implies the semantics of the proxy-revalidate directive (see
1728       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1729       entry after it becomes stale to respond to a subsequent request
1730       without first revalidating it with the origin server. The s-maxage
1731       directive is always ignored by a private cache.
1732  </t></list>
1733</t>
1734<t>
1735   Note that most older caches, not compliant with this specification,
1736   do not implement any cache-control directives. An origin server
1737   wishing to use a cache-control directive that restricts, but does not
1738   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1739   requirement that the max-age directive overrides the Expires header,
1740   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1741   max-age directive.
1742</t>
1743<t>
1744   Other directives allow a user agent to modify the basic expiration
1745   mechanism. These directives &MAY; be specified on a request:
1746</t>
1747<t>
1748  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1749  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1750   max-age
1751  <list><t>
1752      Indicates that the client is willing to accept a response whose
1753      age is no greater than the specified time in seconds. Unless max-stale
1754      directive is also included, the client is not willing to
1755      accept a stale response.
1756  </t></list>
1757</t>
1758<t>
1759  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1760  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1761   min-fresh
1762  <list><t>
1763      Indicates that the client is willing to accept a response whose
1764      freshness lifetime is no less than its current age plus the
1765      specified time in seconds. That is, the client wants a response
1766      that will still be fresh for at least the specified number of
1767      seconds.
1768  </t></list>
1769</t>
1770<t>
1771  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1772  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1773   max-stale
1774  <list><t>
1775      Indicates that the client is willing to accept a response that has
1776      exceeded its expiration time. If max-stale is assigned a value,
1777      then the client is willing to accept a response that has exceeded
1778      its expiration time by no more than the specified number of
1779      seconds. If no value is assigned to max-stale, then the client is
1780      willing to accept a stale response of any age.
1781  </t></list>
1782</t>
1783<t>
1784   If a cache returns a stale response, either because of a max-stale
1785   directive on a request, or because the cache is configured to
1786   override the expiration time of a response, the cache &MUST; attach a
1787   Warning header to the stale response, using Warning 110 (Response is
1788   stale).
1789</t>
1790<t>
1791   A cache &MAY; be configured to return stale responses without
1792   validation, but only if this does not conflict with any "MUST"-level
1793   requirements concerning cache validation (e.g., a "must-revalidate"
1794   cache-control directive).
1795</t>
1796<t>
1797   If both the new request and the cached entry include "max-age"
1798   directives, then the lesser of the two values is used for determining
1799   the freshness of the cached entry for that request.
1800</t>
1801</section>
1802
1803<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1804<t>
1805   Sometimes a user agent might want or need to insist that a cache
1806   revalidate its cache entry with the origin server (and not just with
1807   the next cache along the path to the origin server), or to reload its
1808   cache entry from the origin server. End-to-end revalidation might be
1809   necessary if either the cache or the origin server has overestimated
1810   the expiration time of the cached response. End-to-end reload may be
1811   necessary if the cache entry has become corrupted for some reason.
1812</t>
1813<t>
1814   End-to-end revalidation may be requested either when the client does
1815   not have its own local cached copy, in which case we call it
1816   "unspecified end-to-end revalidation", or when the client does have a
1817   local cached copy, in which case we call it "specific end-to-end
1818   revalidation."
1819</t>
1820<t>
1821   The client can specify these three kinds of action using Cache-Control
1822   request directives:
1823</t>
1824<t>
1825   End-to-end reload
1826  <list><t>
1827      The request includes a "no-cache" cache-control directive or, for
1828      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1829      names &MUST-NOT; be included with the no-cache directive in a
1830      request. The server &MUST-NOT; use a cached copy when responding to
1831      such a request.
1832  </t></list>
1833</t>
1834<t>
1835   Specific end-to-end revalidation
1836  <list><t>
1837      The request includes a "max-age=0" cache-control directive, which
1838      forces each cache along the path to the origin server to
1839      revalidate its own entry, if any, with the next cache or server.
1840      The initial request includes a cache-validating conditional with
1841      the client's current validator.
1842  </t></list>
1843</t>
1844<t>
1845   Unspecified end-to-end revalidation
1846  <list><t>
1847      The request includes "max-age=0" cache-control directive, which
1848      forces each cache along the path to the origin server to
1849      revalidate its own entry, if any, with the next cache or server.
1850      The initial request does not include a cache-validating
1851      conditional; the first cache along the path (if any) that holds a
1852      cache entry for this resource includes a cache-validating
1853      conditional with its current validator.
1854  </t></list>
1855</t>
1856<t>
1857  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1858  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1859   max-age
1860  <list><t>
1861      When an intermediate cache is forced, by means of a max-age=0
1862      directive, to revalidate its own cache entry, and the client has
1863      supplied its own validator in the request, the supplied validator
1864      might differ from the validator currently stored with the cache
1865      entry. In this case, the cache &MAY; use either validator in making
1866      its own request without affecting semantic transparency.
1867  </t><t>
1868      However, the choice of validator might affect performance. The
1869      best approach is for the intermediate cache to use its own
1870      validator when making its request. If the server replies with 304
1871      (Not Modified), then the cache can return its now validated copy
1872      to the client with a 200 (OK) response. If the server replies with
1873      a new entity and cache validator, however, the intermediate cache
1874      can compare the returned validator with the one provided in the
1875      client's request, using the strong comparison function. If the
1876      client's validator is equal to the origin server's, then the
1877      intermediate cache simply returns 304 (Not Modified). Otherwise,
1878      it returns the new entity with a 200 (OK) response.
1879  </t><t>
1880      If a request includes the no-cache directive, it &SHOULD-NOT; 
1881      include min-fresh, max-stale, or max-age.
1882  </t></list>
1883</t>
1884<t>
1885  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1886  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1887   only-if-cached
1888  <list><t>
1889      In some cases, such as times of extremely poor network
1890      connectivity, a client may want a cache to return only those
1891      responses that it currently has stored, and not to reload or
1892      revalidate with the origin server. To do this, the client may
1893      include the only-if-cached directive in a request. If it receives
1894      this directive, a cache &SHOULD; either respond using a cached entry
1895      that is consistent with the other constraints of the request, or
1896      respond with a 504 (Gateway Timeout) status. However, if a group
1897      of caches is being operated as a unified system with good internal
1898      connectivity, such a request &MAY; be forwarded within that group of
1899      caches.
1900  </t></list>
1901</t>
1902<t>
1903  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1904  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1905   must-revalidate
1906  <list><t>
1907      Because a cache &MAY; be configured to ignore a server's specified
1908      expiration time, and because a client request &MAY; include a max-stale
1909      directive (which has a similar effect), the protocol also
1910      includes a mechanism for the origin server to require revalidation
1911      of a cache entry on any subsequent use. When the must-revalidate
1912      directive is present in a response received by a cache, that cache
1913      &MUST-NOT; use the entry after it becomes stale to respond to a
1914      subsequent request without first revalidating it with the origin
1915      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1916      time, if, based solely on the origin server's Expires or max-age
1917      value, the cached response is stale.)
1918  </t><t>
1919      The must-revalidate directive is necessary to support reliable
1920      operation for certain protocol features. In all circumstances an
1921      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1922      particular, if the cache cannot reach the origin server for any
1923      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1924  </t><t>
1925      Servers &SHOULD; send the must-revalidate directive if and only if
1926      failure to revalidate a request on the entity could result in
1927      incorrect operation, such as a silently unexecuted financial
1928      transaction. Recipients &MUST-NOT; take any automated action that
1929      violates this directive, and &MUST-NOT; automatically provide an
1930      unvalidated copy of the entity if revalidation fails.
1931  </t><t>
1932      Although this is not recommended, user agents operating under
1933      severe connectivity constraints &MAY; violate this directive but, if
1934      so, &MUST; explicitly warn the user that an unvalidated response has
1935      been provided. The warning &MUST; be provided on each unvalidated
1936      access, and &SHOULD; require explicit user confirmation.
1937  </t></list>
1938</t>
1939<t>
1940  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1941  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
1942   proxy-revalidate
1943  <list><t>
1944      The proxy-revalidate directive has the same meaning as the must-revalidate
1945      directive, except that it does not apply to non-shared
1946      user agent caches. It can be used on a response to an
1947      authenticated request to permit the user's cache to store and
1948      later return the response without needing to revalidate it (since
1949      it has already been authenticated once by that user), while still
1950      requiring proxies that service many users to revalidate each time
1951      (in order to make sure that each user has been authenticated).
1952      Note that such authenticated responses also need the public cache
1953      control directive in order to allow them to be cached at all.
1954  </t></list>
1955</t>
1956</section>
1957
1958<section title="No-Transform Directive" anchor="no-transform.directive">
1959<t>
1960  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
1961  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
1962   no-transform
1963  <list><t>
1964      Implementors of intermediate caches (proxies) have found it useful
1965      to convert the media type of certain entity bodies. A non-transparent
1966      proxy might, for example, convert between image
1967      formats in order to save cache space or to reduce the amount of
1968      traffic on a slow link.
1969  </t><t>
1970      Serious operational problems occur, however, when these
1971      transformations are applied to entity bodies intended for certain
1972      kinds of applications. For example, applications for medical
1973      imaging, scientific data analysis and those using end-to-end
1974      authentication, all depend on receiving an entity body that is bit
1975      for bit identical to the original entity-body.
1976  </t><t>
1977      Therefore, if a message includes the no-transform directive, an
1978      intermediate cache or proxy &MUST-NOT; change those headers that are
1979      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
1980      directive. This implies that the cache or proxy &MUST-NOT; change
1981      any aspect of the entity-body that is specified by these headers,
1982      including the value of the entity-body itself.
1983  </t></list>
1984</t>
1985</section>
1986
1987<section title="Cache Control Extensions" anchor="cache.control.extensions">
1988<t>
1989   The Cache-Control header field can be extended through the use of one
1990   or more cache-extension tokens, each with an optional assigned value.
1991   Informational extensions (those which do not require a change in
1992   cache behavior) &MAY; be added without changing the semantics of other
1993   directives. Behavioral extensions are designed to work by acting as
1994   modifiers to the existing base of cache directives. Both the new
1995   directive and the standard directive are supplied, such that
1996   applications which do not understand the new directive will default
1997   to the behavior specified by the standard directive, and those that
1998   understand the new directive will recognize it as modifying the
1999   requirements associated with the standard directive. In this way,
2000   extensions to the cache-control directives can be made without
2001   requiring changes to the base protocol.
2002</t>
2003<t>
2004   This extension mechanism depends on an HTTP cache obeying all of the
2005   cache-control directives defined for its native HTTP-version, obeying
2006   certain extensions, and ignoring all directives that it does not
2007   understand.
2008</t>
2009<t>
2010   For example, consider a hypothetical new response directive called
2011   community which acts as a modifier to the private directive. We
2012   define this new directive to mean that, in addition to any non-shared
2013   cache, any cache which is shared only by members of the community
2014   named within its value may cache the response. An origin server
2015   wishing to allow the UCI community to use an otherwise private
2016   response in their shared cache(s) could do so by including
2017</t>
2018<figure><artwork type="example">
2019    Cache-Control: private, community="UCI"
2020</artwork></figure>
2021<t>
2022   A cache seeing this header field will act correctly even if the cache
2023   does not understand the community cache-extension, since it will also
2024   see and understand the private directive and thus default to the safe
2025   behavior.
2026</t>
2027<t>
2028   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2029   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2030   be combined with standard directives (or the response's default
2031   cacheability) such that the cache behavior will remain minimally
2032   correct even if the cache does not understand the extension(s).
2033</t>
2034</section>
2035</section>
2036
2037<section title="Expires" anchor="header.expires">
2038  <iref primary="true" item="Expires header" x:for-anchor=""/>
2039  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2040<t>
2041   The Expires entity-header field gives the date/time after which the
2042   response is considered stale. A stale cache entry may not normally be
2043   returned by a cache (either a proxy cache or a user agent cache)
2044   unless it is first validated with the origin server (or with an
2045   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2046   for further discussion of the expiration model.
2047</t>
2048<t>
2049   The presence of an Expires field does not imply that the original
2050   resource will change or cease to exist at, before, or after that
2051   time.
2052</t>
2053<t>
2054   The format is an absolute date and time as defined by HTTP-date in
2055   &full-date;; it &MUST; be sent in rfc1123-date format.
2056</t>
2057<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2058  Expires = "Expires" ":" HTTP-date
2059</artwork></figure>
2060<t>
2061   An example of its use is
2062</t>
2063<figure><artwork type="example">
2064   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2065</artwork></figure>
2066<t>
2067  <list><t>
2068      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2069      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2070      Expires field.
2071  </t></list>
2072</t>
2073<t>
2074   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2075   especially including the value "0", as in the past (i.e., "already
2076   expired").
2077</t>
2078<t>
2079   To mark a response as "already expired," an origin server sends an
2080   Expires date that is equal to the Date header value. (See the rules
2081   for expiration calculations in <xref target="expiration.calculations"/>.)
2082</t>
2083<t>
2084   To mark a response as "never expires," an origin server sends an
2085   Expires date approximately one year from the time the response is
2086   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2087   year in the future.
2088</t>
2089<t>
2090   The presence of an Expires header field with a date value of some
2091   time in the future on a response that otherwise would by default be
2092   non-cacheable indicates that the response is cacheable, unless
2093   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2094</t>
2095</section>
2096
2097<section title="Pragma" anchor="header.pragma">
2098  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2099  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2100<t>
2101   The Pragma general-header field is used to include implementation-specific
2102   directives that might apply to any recipient along the
2103   request/response chain. All pragma directives specify optional
2104   behavior from the viewpoint of the protocol; however, some systems
2105   &MAY; require that behavior be consistent with the directives.
2106</t>
2107<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"/>
2108  Pragma            = "Pragma" ":" 1#pragma-directive
2109  pragma-directive  = "no-cache" | extension-pragma
2110  extension-pragma  = token [ "=" ( token | quoted-string ) ]
2111</artwork></figure>
2112<t>
2113   When the no-cache directive is present in a request message, an
2114   application &SHOULD; forward the request toward the origin server even
2115   if it has a cached copy of what is being requested. This pragma
2116   directive has the same semantics as the no-cache cache-directive (see
2117   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2118   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2119   request is sent to a server not known to be HTTP/1.1 compliant.
2120</t>
2121<t>
2122   Pragma directives &MUST; be passed through by a proxy or gateway
2123   application, regardless of their significance to that application,
2124   since the directives might be applicable to all recipients along the
2125   request/response chain. It is not possible to specify a pragma for a
2126   specific recipient; however, any pragma directive not relevant to a
2127   recipient &SHOULD; be ignored by that recipient.
2128</t>
2129<t>
2130   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2131   sent "Cache-Control: no-cache". No new Pragma directives will be
2132   defined in HTTP.
2133  <list><t>
2134      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
2135      response-header field is not actually specified, it does not provide a
2136      reliable replacement for "Cache-Control: no-cache" in a response.
2137  </t></list>
2138</t>
2139</section>
2140
2141<section title="Vary" anchor="header.vary">
2142  <iref primary="true" item="Vary header" x:for-anchor=""/>
2143  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2144<t>
2145   The Vary field value indicates the set of request-header fields that
2146   fully determines, while the response is fresh, whether a cache is
2147   permitted to use the response to reply to a subsequent request
2148   without revalidation. For uncacheable or stale responses, the Vary
2149   field value advises the user agent about the criteria that were used
2150   to select the representation. A Vary field value of "*" implies that
2151   a cache cannot determine from the request headers of a subsequent
2152   request whether this response is the appropriate representation. See
2153   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2154</t>
2155<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2156  Vary  = "Vary" ":" ( "*" | 1#field-name )
2157</artwork></figure>
2158<t>
2159   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2160   cacheable response that is subject to server-driven negotiation.
2161   Doing so allows a cache to properly interpret future requests on that
2162   resource and informs the user agent about the presence of negotiation
2163   on that resource. A server &MAY; include a Vary header field with a
2164   non-cacheable response that is subject to server-driven negotiation,
2165   since this might provide the user agent with useful information about
2166   the dimensions over which the response varies at the time of the
2167   response.
2168</t>
2169<t>
2170   A Vary field value consisting of a list of field-names signals that
2171   the representation selected for the response is based on a selection
2172   algorithm which considers ONLY the listed request-header field values
2173   in selecting the most appropriate representation. A cache &MAY; assume
2174   that the same selection will be made for future requests with the
2175   same values for the listed field names, for the duration of time for
2176   which the response is fresh.
2177</t>
2178<t>
2179   The field-names given are not limited to the set of standard
2180   request-header fields defined by this specification. Field names are
2181   case-insensitive.
2182</t>
2183<t>
2184   A Vary field value of "*" signals that unspecified parameters not
2185   limited to the request-headers (e.g., the network address of the
2186   client), play a role in the selection of the response representation.
2187   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2188   generated by an origin server.
2189</t>
2190</section>
2191
2192<section title="Warning" anchor="header.warning">
2193  <iref primary="true" item="Warning header" x:for-anchor=""/>
2194  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2195<t>
2196   The Warning general-header field is used to carry additional
2197   information about the status or transformation of a message which
2198   might not be reflected in the message. This information is typically
2199   used to warn about a possible lack of semantic transparency from
2200   caching operations or transformations applied to the entity body of
2201   the message.
2202</t>
2203<t>
2204   Warning headers are sent with responses using:
2205</t>
2206<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"/>
2207  Warning    = "Warning" ":" 1#warning-value
2208 
2209  warning-value = warn-code SP warn-agent SP warn-text
2210                                        [SP warn-date]
2211 
2212  warn-code  = 3DIGIT
2213  warn-agent = ( uri-host [ ":" port ] ) | pseudonym
2214                  ; the name or pseudonym of the server adding
2215                  ; the Warning header, for use in debugging
2216  warn-text  = quoted-string
2217  warn-date  = DQUOTE HTTP-date DQUOTE
2218</artwork></figure>
2219<t>
2220   A response &MAY; carry more than one Warning header.
2221</t>
2222<t>
2223   The warn-text &SHOULD; be in a natural language and character set that
2224   is most likely to be intelligible to the human user receiving the
2225   response. This decision &MAY; be based on any available knowledge, such
2226   as the location of the cache or user, the Accept-Language field in a
2227   request, the Content-Language field in a response, etc. The default
2228   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2229</t>
2230<t>
2231   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2232   in the warn-text using the method described in <xref target="RFC2047"/>.
2233</t>
2234<t>
2235   Warning headers can in general be applied to any message, however
2236   some specific warn-codes are specific to caches and can only be
2237   applied to response messages. New Warning headers &SHOULD; be added
2238   after any existing Warning headers. A cache &MUST-NOT; delete any
2239   Warning header that it received with a message. However, if a cache
2240   successfully validates a cache entry, it &SHOULD; remove any Warning
2241   headers previously attached to that entry except as specified for
2242   specific Warning codes. It &MUST; then add any Warning headers received
2243   in the validating response. In other words, Warning headers are those
2244   that would be attached to the most recent relevant response.
2245</t>
2246<t>
2247   When multiple Warning headers are attached to a response, the user
2248   agent ought to inform the user of as many of them as possible, in the
2249   order that they appear in the response. If it is not possible to
2250   inform the user of all of the warnings, the user agent &SHOULD; follow
2251   these heuristics:
2252  <list style="symbols">
2253    <t>Warnings that appear early in the response take priority over
2254        those appearing later in the response.</t>
2255
2256    <t>Warnings in the user's preferred character set take priority
2257        over warnings in other character sets but with identical warn-codes
2258        and warn-agents.</t>
2259  </list>
2260</t>
2261<t>
2262   Systems that generate multiple Warning headers &SHOULD; order them with
2263   this user agent behavior in mind.
2264</t>
2265<t>
2266   Requirements for the behavior of caches with respect to Warnings are
2267   stated in <xref target="warnings"/>.
2268</t>
2269<t>
2270   This is a list of the currently-defined warn-codes, each with a
2271   recommended warn-text in English, and a description of its meaning.
2272</t>
2273<t>
2274   110 Response is stale
2275  <list><t>
2276     &MUST; be included whenever the returned response is stale.
2277  </t></list>
2278</t>
2279<t>
2280   111 Revalidation failed
2281  <list><t>
2282     &MUST; be included if a cache returns a stale response because an
2283     attempt to revalidate the response failed, due to an inability to
2284     reach the server.
2285  </t></list>
2286</t>
2287<t>
2288   112 Disconnected operation
2289  <list><t>
2290     &SHOULD; be included if the cache is intentionally disconnected from
2291     the rest of the network for a period of time.
2292  </t></list>
2293</t>
2294<t>
2295   113 Heuristic expiration
2296  <list><t>
2297     &MUST; be included if the cache heuristically chose a freshness
2298     lifetime greater than 24 hours and the response's age is greater
2299     than 24 hours.
2300  </t></list>
2301</t>
2302<t>
2303   199 Miscellaneous warning
2304  <list><t>
2305     The warning text &MAY; include arbitrary information to be presented
2306     to a human user, or logged. A system receiving this warning &MUST-NOT;
2307     take any automated action, besides presenting the warning to
2308     the user.
2309  </t></list>
2310</t>
2311<t>
2312   214 Transformation applied
2313  <list><t>
2314     &MUST; be added by an intermediate cache or proxy if it applies any
2315     transformation changing the content-coding (as specified in the
2316     Content-Encoding header) or media-type (as specified in the
2317     Content-Type header) of the response, or the entity-body of the
2318     response, unless this Warning code already appears in the response.
2319  </t></list>
2320</t>
2321<t>
2322   299 Miscellaneous persistent warning
2323  <list><t>
2324     The warning text &MAY; include arbitrary information to be presented
2325     to a human user, or logged. A system receiving this warning &MUST-NOT;
2326     take any automated action.
2327  </t></list>
2328</t>
2329<t>
2330   If an implementation sends a message with one or more Warning headers
2331   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2332   each warning-value a warn-date that matches the date in the response.
2333</t>
2334<t>
2335   If an implementation receives a message with a warning-value that
2336   includes a warn-date, and that warn-date is different from the Date
2337   value in the response, then that warning-value &MUST; be deleted from
2338   the message before storing, forwarding, or using it. (This prevents
2339   bad consequences of naive caching of Warning header fields.) If all
2340   of the warning-values are deleted for this reason, the Warning header
2341   &MUST; be deleted as well.
2342</t>
2343</section>
2344
2345</section>
2346
2347<section title="IANA Considerations" anchor="IANA.considerations">
2348<t>
2349   <cref>TBD.</cref>
2350</t>
2351</section>
2352
2353<section title="Security Considerations" anchor="security.considerations">
2354<t>
2355   Caching proxies provide additional potential vulnerabilities, since
2356   the contents of the cache represent an attractive target for
2357   malicious exploitation. Because cache contents persist after an HTTP
2358   request is complete, an attack on the cache can reveal information
2359   long after a user believes that the information has been removed from
2360   the network. Therefore, cache contents should be protected as
2361   sensitive information.
2362</t>
2363</section>
2364
2365<section title="Acknowledgments" anchor="ack">
2366<t>
2367   Much of the content and presentation of the caching design is due to
2368   suggestions and comments from individuals including: Shel Kaphan,
2369   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2370</t>
2371</section>
2372</middle>
2373<back>
2374
2375<references title="Normative References">
2376
2377<reference anchor="ISO-8859-1">
2378  <front>
2379    <title>
2380     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2381    </title>
2382    <author>
2383      <organization>International Organization for Standardization</organization>
2384    </author>
2385    <date year="1998"/>
2386  </front>
2387  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2388</reference>
2389
2390<reference anchor="Part1">
2391  <front>
2392    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2393    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2394      <organization abbrev="Day Software">Day Software</organization>
2395      <address><email>fielding@gbiv.com</email></address>
2396    </author>
2397    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2398      <organization>One Laptop per Child</organization>
2399      <address><email>jg@laptop.org</email></address>
2400    </author>
2401    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2402      <organization abbrev="HP">Hewlett-Packard Company</organization>
2403      <address><email>JeffMogul@acm.org</email></address>
2404    </author>
2405    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2406      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2407      <address><email>henrikn@microsoft.com</email></address>
2408    </author>
2409    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2410      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2411      <address><email>LMM@acm.org</email></address>
2412    </author>
2413    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2414      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2415      <address><email>paulle@microsoft.com</email></address>
2416    </author>
2417    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2418      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2419      <address><email>timbl@w3.org</email></address>
2420    </author>
2421    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2422      <organization abbrev="W3C">World Wide Web Consortium</organization>
2423      <address><email>ylafon@w3.org</email></address>
2424    </author>
2425    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2426      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2427      <address><email>julian.reschke@greenbytes.de</email></address>
2428    </author>
2429    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2430  </front>
2431  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2432  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2433</reference>
2434
2435<reference anchor="Part2">
2436  <front>
2437    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2438    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2439      <organization abbrev="Day Software">Day Software</organization>
2440      <address><email>fielding@gbiv.com</email></address>
2441    </author>
2442    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2443      <organization>One Laptop per Child</organization>
2444      <address><email>jg@laptop.org</email></address>
2445    </author>
2446    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2447      <organization abbrev="HP">Hewlett-Packard Company</organization>
2448      <address><email>JeffMogul@acm.org</email></address>
2449    </author>
2450    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2451      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2452      <address><email>henrikn@microsoft.com</email></address>
2453    </author>
2454    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2455      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2456      <address><email>LMM@acm.org</email></address>
2457    </author>
2458    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2459      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2460      <address><email>paulle@microsoft.com</email></address>
2461    </author>
2462    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2463      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2464      <address><email>timbl@w3.org</email></address>
2465    </author>
2466    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2467      <organization abbrev="W3C">World Wide Web Consortium</organization>
2468      <address><email>ylafon@w3.org</email></address>
2469    </author>
2470    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2471      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2472      <address><email>julian.reschke@greenbytes.de</email></address>
2473    </author>
2474    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2475  </front>
2476  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2477  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2478</reference>
2479
2480<reference anchor="Part3">
2481  <front>
2482    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2483    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2484      <organization abbrev="Day Software">Day Software</organization>
2485      <address><email>fielding@gbiv.com</email></address>
2486    </author>
2487    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2488      <organization>One Laptop per Child</organization>
2489      <address><email>jg@laptop.org</email></address>
2490    </author>
2491    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2492      <organization abbrev="HP">Hewlett-Packard Company</organization>
2493      <address><email>JeffMogul@acm.org</email></address>
2494    </author>
2495    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2496      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2497      <address><email>henrikn@microsoft.com</email></address>
2498    </author>
2499    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2500      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2501      <address><email>LMM@acm.org</email></address>
2502    </author>
2503    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2504      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2505      <address><email>paulle@microsoft.com</email></address>
2506    </author>
2507    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2508      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2509      <address><email>timbl@w3.org</email></address>
2510    </author>
2511    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2512      <organization abbrev="W3C">World Wide Web Consortium</organization>
2513      <address><email>ylafon@w3.org</email></address>
2514    </author>
2515    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2516      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2517      <address><email>julian.reschke@greenbytes.de</email></address>
2518    </author>
2519    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2520  </front>
2521  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2522  <x:source href="p3-payload.xml" basename="p3-payload"/>
2523</reference>
2524
2525<reference anchor="Part4">
2526  <front>
2527    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2528    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2529      <organization abbrev="Day Software">Day Software</organization>
2530      <address><email>fielding@gbiv.com</email></address>
2531    </author>
2532    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2533      <organization>One Laptop per Child</organization>
2534      <address><email>jg@laptop.org</email></address>
2535    </author>
2536    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2537      <organization abbrev="HP">Hewlett-Packard Company</organization>
2538      <address><email>JeffMogul@acm.org</email></address>
2539    </author>
2540    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2541      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2542      <address><email>henrikn@microsoft.com</email></address>
2543    </author>
2544    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2545      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2546      <address><email>LMM@acm.org</email></address>
2547    </author>
2548    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2549      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2550      <address><email>paulle@microsoft.com</email></address>
2551    </author>
2552    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2553      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2554      <address><email>timbl@w3.org</email></address>
2555    </author>
2556    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2557      <organization abbrev="W3C">World Wide Web Consortium</organization>
2558      <address><email>ylafon@w3.org</email></address>
2559    </author>
2560    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2561      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2562      <address><email>julian.reschke@greenbytes.de</email></address>
2563    </author>
2564    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2565  </front>
2566  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2567  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2568</reference>
2569
2570<reference anchor="Part5">
2571  <front>
2572    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2573    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2574      <organization abbrev="Day Software">Day Software</organization>
2575      <address><email>fielding@gbiv.com</email></address>
2576    </author>
2577    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2578      <organization>One Laptop per Child</organization>
2579      <address><email>jg@laptop.org</email></address>
2580    </author>
2581    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2582      <organization abbrev="HP">Hewlett-Packard Company</organization>
2583      <address><email>JeffMogul@acm.org</email></address>
2584    </author>
2585    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2586      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2587      <address><email>henrikn@microsoft.com</email></address>
2588    </author>
2589    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2590      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2591      <address><email>LMM@acm.org</email></address>
2592    </author>
2593    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2594      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2595      <address><email>paulle@microsoft.com</email></address>
2596    </author>
2597    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2598      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2599      <address><email>timbl@w3.org</email></address>
2600    </author>
2601    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2602      <organization abbrev="W3C">World Wide Web Consortium</organization>
2603      <address><email>ylafon@w3.org</email></address>
2604    </author>
2605    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2606      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2607      <address><email>julian.reschke@greenbytes.de</email></address>
2608    </author>
2609    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2610  </front>
2611  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2612  <x:source href="p5-range.xml" basename="p5-range"/>
2613</reference>
2614
2615<reference anchor="Part7">
2616  <front>
2617    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2618    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2619      <organization abbrev="Day Software">Day Software</organization>
2620      <address><email>fielding@gbiv.com</email></address>
2621    </author>
2622    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2623      <organization>One Laptop per Child</organization>
2624      <address><email>jg@laptop.org</email></address>
2625    </author>
2626    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2627      <organization abbrev="HP">Hewlett-Packard Company</organization>
2628      <address><email>JeffMogul@acm.org</email></address>
2629    </author>
2630    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2631      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2632      <address><email>henrikn@microsoft.com</email></address>
2633    </author>
2634    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2635      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2636      <address><email>LMM@acm.org</email></address>
2637    </author>
2638    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2639      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2640      <address><email>paulle@microsoft.com</email></address>
2641    </author>
2642    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2643      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2644      <address><email>timbl@w3.org</email></address>
2645    </author>
2646    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2647      <organization abbrev="W3C">World Wide Web Consortium</organization>
2648      <address><email>ylafon@w3.org</email></address>
2649    </author>
2650    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2651      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2652      <address><email>julian.reschke@greenbytes.de</email></address>
2653    </author>
2654    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2655  </front>
2656  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2657  <x:source href="p7-auth.xml" basename="p7-auth"/>
2658</reference>
2659
2660<reference anchor="RFC2047">
2661  <front>
2662    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2663    <author initials="K." surname="Moore" fullname="Keith Moore">
2664      <organization>University of Tennessee</organization>
2665      <address><email>moore@cs.utk.edu</email></address>
2666    </author>
2667    <date month="November" year="1996"/>
2668  </front>
2669  <seriesInfo name="RFC" value="2047"/>
2670</reference>
2671
2672<reference anchor="RFC2119">
2673  <front>
2674    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2675    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2676      <organization>Harvard University</organization>
2677      <address><email>sob@harvard.edu</email></address>
2678    </author>
2679    <date month="March" year="1997"/>
2680  </front>
2681  <seriesInfo name="BCP" value="14"/>
2682  <seriesInfo name="RFC" value="2119"/>
2683</reference>
2684
2685</references>
2686
2687<references title="Informative References">
2688
2689<reference anchor="RFC1305">
2690  <front>
2691    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2692    <author initials="D." surname="Mills" fullname="David L. Mills">
2693      <organization>University of Delaware, Electrical Engineering Department</organization>
2694      <address><email>mills@udel.edu</email></address>
2695    </author>
2696    <date month="March" year="1992"/>
2697  </front>
2698  <seriesInfo name="RFC" value="1305"/>
2699</reference>
2700
2701<reference anchor="RFC2616">
2702  <front>
2703    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2704    <author initials="R." surname="Fielding" fullname="R. Fielding">
2705      <organization>University of California, Irvine</organization>
2706      <address><email>fielding@ics.uci.edu</email></address>
2707    </author>
2708    <author initials="J." surname="Gettys" fullname="J. Gettys">
2709      <organization>W3C</organization>
2710      <address><email>jg@w3.org</email></address>
2711    </author>
2712    <author initials="J." surname="Mogul" fullname="J. Mogul">
2713      <organization>Compaq Computer Corporation</organization>
2714      <address><email>mogul@wrl.dec.com</email></address>
2715    </author>
2716    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2717      <organization>MIT Laboratory for Computer Science</organization>
2718      <address><email>frystyk@w3.org</email></address>
2719    </author>
2720    <author initials="L." surname="Masinter" fullname="L. Masinter">
2721      <organization>Xerox Corporation</organization>
2722      <address><email>masinter@parc.xerox.com</email></address>
2723    </author>
2724    <author initials="P." surname="Leach" fullname="P. Leach">
2725      <organization>Microsoft Corporation</organization>
2726      <address><email>paulle@microsoft.com</email></address>
2727    </author>
2728    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2729      <organization>W3C</organization>
2730      <address><email>timbl@w3.org</email></address>
2731    </author>
2732    <date month="June" year="1999"/>
2733  </front>
2734  <seriesInfo name="RFC" value="2616"/>
2735</reference>
2736
2737</references>
2738
2739<section title="Compatibility with Previous Versions" anchor="compatibility">
2740
2741<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2742<t>
2743   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2744   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2745   <xref target="header.cache-control" format="counter"/>,
2746   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2747</t>
2748<t>
2749   Transfer-coding and message lengths all interact in ways that
2750   required fixing exactly when chunked encoding is used (to allow for
2751   transfer encoding that may not be self delimiting); it was important
2752   to straighten out exactly how message lengths are computed.
2753   (<xref target="non-modifiable.headers"/>,
2754   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2755</t>
2756<t>
2757   Proxies should be able to add Content-Length when appropriate.
2758   (<xref target="non-modifiable.headers"/>)
2759</t>
2760<t>
2761   Range request responses would become very verbose if all meta-data
2762   were always returned; by allowing the server to only send needed
2763   headers in a 206 response, this problem can be avoided.
2764   (<xref target="combining.headers"/>)
2765</t>
2766<t>
2767   The Cache-Control: max-age directive was not properly defined for
2768   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2769</t>
2770<t>
2771   Warnings could be cached incorrectly, or not updated appropriately.
2772   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2773   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2774   and <xref target="header.warning" format="counter"/>) Warning
2775   also needed to be a general header, as PUT or other methods may have
2776   need for it in requests.
2777</t>
2778</section>
2779
2780<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2781<t>
2782  Clarify denial of service attack avoidance requirement.
2783  (<xref target="invalidation.after.updates.or.deletions"/>)
2784</t>
2785</section>
2786
2787</section>
2788
2789<section title="Change Log (to be removed by RFC Editor before publication)">
2790
2791<section title="Since RFC2616">
2792<t>
2793  Extracted relevant partitions from <xref target="RFC2616"/>.
2794</t>
2795</section>
2796
2797<section title="Since draft-ietf-httpbis-p6-cache-00">
2798<t>
2799  Closed issues:
2800  <list style="symbols"> 
2801    <t>
2802      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
2803      "Trailer"
2804      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
2805    </t>
2806    <t>
2807      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
2808      "Invalidation after Update or Delete"
2809      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
2810    </t>
2811    <t>
2812      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2813      "Normative and Informative references"
2814    </t>
2815    <t>
2816      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
2817      "Date reference typo"
2818    </t>
2819    <t>
2820      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
2821      "Connection header text"
2822    </t>
2823    <t>
2824      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2825      "Informative references"
2826    </t>
2827    <t>
2828      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2829      "ISO-8859-1 Reference"
2830    </t>
2831    <t>
2832      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2833      "Normative up-to-date references"
2834    </t>
2835    <t>
2836      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
2837      "typo in 13.2.2"
2838    </t>
2839  </list>
2840</t>
2841<t>
2842  Other changes:
2843  <list style="symbols"> 
2844    <t>
2845      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"/>)
2846    </t>
2847  </list>
2848</t>
2849</section>
2850
2851<section title="Since draft-ietf-httpbis-p6-cache-01">
2852<t>
2853  Closed issues:
2854  <list style="symbols"> 
2855    <t>
2856      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
2857      "rel_path not used"
2858    </t>
2859  </list>
2860</t>
2861<t>
2862  Other changes:
2863  <list style="symbols"> 
2864    <t>
2865       Get rid of duplicate BNF rule names ("host" -> "uri-host")
2866       (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
2867    </t>
2868  </list>
2869</t>
2870</section>
2871
2872</section>
2873
2874</back>
2875</rfc>
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