source: draft-ietf-httpbis/latest/p6-cache.xml @ 153

Last change on this file since 153 was 153, checked in by julian.reschke@…, 12 years ago

Update "Editorial Note" stating that we now have incorporated everything that was in draft-lafon-rfc2616bis-03.

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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 "January">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY entity-header-fields        "<xref target='Part4' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#entity.tags' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY weak-and-strong-validators  "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31]>
32<?rfc toc="yes" ?>
33<?rfc symrefs="yes" ?>
34<?rfc sortrefs="yes" ?>
35<?rfc compact="yes"?>
36<?rfc subcompact="no" ?>
37<?rfc linkmailto="no" ?>
38<?rfc editing="no" ?>
39<?rfc-ext allow-markup-in-artwork="yes" ?>
40<?rfc-ext include-references-in-index="yes" ?>
41<rfc obsoletes="2616" category="std"
42     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
43     xmlns:x='http://purl.org/net/xml2rfc/ext'>
44<front>
45
46  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
47
48  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
49    <organization abbrev="Day Software">Day Software</organization>
50    <address>
51      <postal>
52        <street>23 Corporate Plaza DR, Suite 280</street>
53        <city>Newport Beach</city>
54        <region>CA</region>
55        <code>92660</code>
56        <country>USA</country>
57      </postal>
58      <phone>+1-949-706-5300</phone>
59      <facsimile>+1-949-706-5305</facsimile>
60      <email>fielding@gbiv.com</email>
61      <uri>http://roy.gbiv.com/</uri>
62    </address>
63  </author>
64
65  <author initials="J." surname="Gettys" fullname="Jim Gettys">
66    <organization>One Laptop per Child</organization>
67    <address>
68      <postal>
69        <street>21 Oak Knoll Road</street>
70        <city>Carlisle</city>
71        <region>MA</region>
72        <code>01741</code>
73        <country>USA</country>
74      </postal>
75      <email>jg@laptop.org</email>
76      <uri>http://www.laptop.org/</uri>
77    </address>
78  </author>
79 
80  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
81    <organization abbrev="HP">Hewlett-Packard Company</organization>
82    <address>
83      <postal>
84        <street>HP Labs, Large Scale Systems Group</street>
85        <street>1501 Page Mill Road, MS 1177</street>
86        <city>Palo Alto</city>
87        <region>CA</region>
88        <code>94304</code>
89        <country>USA</country>
90      </postal>
91      <email>JeffMogul@acm.org</email>
92    </address>
93  </author>
94
95  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
96    <organization abbrev="Microsoft">Microsoft Corporation</organization>
97    <address>
98      <postal>
99        <street>1 Microsoft Way</street>
100        <city>Redmond</city>
101        <region>WA</region>
102        <code>98052</code>
103        <country>USA</country>
104      </postal>
105      <email>henrikn@microsoft.com</email>
106    </address>
107  </author>
108
109  <author initials="L." surname="Masinter" fullname="Larry Masinter">
110    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
111    <address>
112      <postal>
113        <street>345 Park Ave</street>
114        <city>San Jose</city>
115        <region>CA</region>
116        <code>95110</code>
117        <country>USA</country>
118      </postal>
119      <email>LMM@acm.org</email>
120      <uri>http://larry.masinter.net/</uri>
121    </address>
122  </author>
123 
124  <author initials="P." surname="Leach" fullname="Paul J. Leach">
125    <organization abbrev="Microsoft">Microsoft Corporation</organization>
126    <address>
127      <postal>
128        <street>1 Microsoft Way</street>
129        <city>Redmond</city>
130        <region>WA</region>
131        <code>98052</code>
132      </postal>
133      <email>paulle@microsoft.com</email>
134    </address>
135  </author>
136   
137  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
138    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
139    <address>
140      <postal>
141        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
142        <street>The Stata Center, Building 32</street>
143        <street>32 Vassar Street</street>
144        <city>Cambridge</city>
145        <region>MA</region>
146        <code>02139</code>
147        <country>USA</country>
148      </postal>
149      <email>timbl@w3.org</email>
150      <uri>http://www.w3.org/People/Berners-Lee/</uri>
151    </address>
152  </author>
153
154  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
155    <organization abbrev="W3C">World Wide Web Consortium</organization>
156    <address>
157      <postal>
158        <street>W3C / ERCIM</street>
159        <street>2004, rte des Lucioles</street>
160        <city>Sophia-Antipolis</city>
161        <region>AM</region>
162        <code>06902</code>
163        <country>France</country>
164      </postal>
165      <email>ylafon@w3.org</email>
166      <uri>http://www.raubacapeu.net/people/yves/</uri>
167    </address>
168  </author>
169
170  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
171    <organization abbrev="greenbytes">greenbytes GmbH</organization>
172    <address>
173      <postal>
174        <street>Hafenweg 16</street>
175        <city>Muenster</city><region>NW</region><code>48155</code>
176        <country>Germany</country>
177      </postal>
178      <phone>+49 251 2807760</phone>   
179      <facsimile>+49 251 2807761</facsimile>   
180      <email>julian.reschke@greenbytes.de</email>       
181      <uri>http://greenbytes.de/tech/webdav/</uri>     
182    </address>
183  </author>
184
185  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
186
187<abstract>
188<t>
189   The Hypertext Transfer Protocol (HTTP) is an application-level
190   protocol for distributed, collaborative, hypermedia information
191   systems. HTTP has been in use by the World Wide Web global information
192   initiative since 1990. This document is Part 6 of the seven-part specification
193   that defines the protocol referred to as "HTTP/1.1" and, taken together,
194   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
195   and the associated header fields that control cache behavior or indicate
196   cacheable response messages.
197</t>
198</abstract>
199
200<note title="Editorial Note (To be removed by RFC Editor)">
201  <t>
202    Discussion of this draft should take place on the HTTPBIS working group
203    mailing list (ietf-http-wg@w3.org). The current issues list is
204    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
205    and related documents (including fancy diffs) can be found at
206    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
207  </t>
208  <t>
209    This draft incorporates those issue resolutions that were either
210    collected in the original RFC2616 errata list (<eref target="http://purl.org/NET/http-errata"/>),
211    or which were agreed upon on the mailing list between October 2006 and
212    November 2007 (as published in "draft-lafon-rfc2616bis-03").
213  </t>
214</note>
215</front>
216<middle>
217<section title="Introduction" anchor="introduction">
218<t>
219   This document will define aspects of HTTP related to caching response
220   messages.  Right now it only includes the extracted relevant sections
221   of <xref target="RFC2616" x:fmt="none">RFC 2616</xref> without edit.
222</t>
223
224<section title="Requirements" anchor="intro.requirements">
225<t>
226   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
227   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
228   document are to be interpreted as described in <xref target="RFC2119"/>.
229</t>
230<t>
231   An implementation is not compliant if it fails to satisfy one or more
232   of the &MUST; or &REQUIRED; level requirements for the protocols it
233   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
234   level and all the &SHOULD; level requirements for its protocols is said
235   to be "unconditionally compliant"; one that satisfies all the &MUST;
236   level requirements but not all the &SHOULD; level requirements for its
237   protocols is said to be "conditionally compliant."
238</t>
239</section>
240
241<section title="Terminology" anchor="intro.terminology">
242<t>
243   This specification uses a number of terms to refer to the roles
244   played by participants in, and objects of, the HTTP communication.
245</t>
246<t>
247  <iref item="cache"/>
248  <x:dfn>cache</x:dfn>
249  <list>
250    <t>
251      A program's local store of response messages and the subsystem
252      that controls its message storage, retrieval, and deletion. A
253      cache stores cacheable responses in order to reduce the response
254      time and network bandwidth consumption on future, equivalent
255      requests. Any client or server may include a cache, though a cache
256      cannot be used by a server that is acting as a tunnel.
257    </t>
258  </list>
259</t>
260<t>
261  <iref item="cacheable"/>
262  <x:dfn>cacheable</x:dfn>
263  <list>
264    <t>
265      A response is cacheable if a cache is allowed to store a copy of
266      the response message for use in answering subsequent requests. The
267      rules for determining the cacheability of HTTP responses are
268      defined in <xref target="caching"/>. Even if a resource is cacheable, there may
269      be additional constraints on whether a cache can use the cached
270      copy for a particular request.
271    </t>
272  </list>
273</t>
274<t>
275  <iref item="first-hand"/>
276  <x:dfn>first-hand</x:dfn>
277  <list>
278    <t>
279      A response is first-hand if it comes directly and without
280      unnecessary delay from the origin server, perhaps via one or more
281      proxies. A response is also first-hand if its validity has just
282      been checked directly with the origin server.
283    </t>
284  </list>
285</t>
286<t>
287  <iref item="explicit expiration time"/>
288  <x:dfn>explicit expiration time</x:dfn>
289  <list>
290    <t>
291      The time at which the origin server intends that an entity should
292      no longer be returned by a cache without further validation.
293    </t>
294  </list>
295</t>
296<t>
297  <iref item="heuristic expiration time"/>
298  <x:dfn>heuristic expiration time</x:dfn>
299  <list>
300    <t>
301      An expiration time assigned by a cache when no explicit expiration
302      time is available.
303    </t>
304  </list>
305</t>
306<t>
307  <iref item="age"/>
308  <x:dfn>age</x:dfn>
309  <list>
310    <t>
311      The age of a response is the time since it was sent by, or
312      successfully validated with, the origin server.
313    </t>
314  </list>
315</t>
316<t>
317  <iref item="freshness lifetime"/>
318  <x:dfn>freshness lifetime</x:dfn>
319  <list>
320    <t>
321      The length of time between the generation of a response and its
322      expiration time.
323    </t>
324  </list>
325</t>
326<t>
327  <iref item="fresh"/>
328  <x:dfn>fresh</x:dfn>
329  <list>
330    <t>
331      A response is fresh if its age has not yet exceeded its freshness
332      lifetime.
333    </t>
334  </list>
335</t>
336<t>
337  <iref item="stale"/>
338  <x:dfn>stale</x:dfn>
339  <list>
340    <t>
341      A response is stale if its age has passed its freshness lifetime.
342    </t>
343  </list>
344</t>
345<t>
346  <iref item="semantically transparent"/>
347  <x:dfn>semantically transparent</x:dfn>
348  <list>
349    <t>
350      A cache behaves in a "semantically transparent" manner, with
351      respect to a particular response, when its use affects neither the
352      requesting client nor the origin server, except to improve
353      performance. When a cache is semantically transparent, the client
354      receives exactly the same response (except for hop-by-hop headers)
355      that it would have received had its request been handled directly
356      by the origin server.
357    </t>
358  </list>
359</t>
360<t>
361  <iref item="validator"/>
362  <x:dfn>validator</x:dfn>
363  <list>
364    <t>
365      A protocol element (e.g., an entity tag or a Last-Modified time)
366      that is used to find out whether a cache entry is an equivalent
367      copy of an entity.
368    </t>
369  </list>
370</t>
371</section>
372
373<section title="Delta Seconds" anchor="delta.seconds">
374<t>
375   Some HTTP header fields allow a time value to be specified as an
376   integer number of seconds, represented in decimal, after the time
377   that the message was received.
378</t>
379<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
380  delta-seconds  = 1*DIGIT
381</artwork></figure>
382</section>
383</section>
384
385<section title="Caching in HTTP" anchor="caching">
386<section title="Overview" anchor="caching.overview">
387<t>
388   HTTP is typically used for distributed information systems, where
389   performance can be improved by the use of response caches. The
390   HTTP/1.1 protocol includes a number of elements intended to make
391   caching work as well as possible. Because these elements are
392   inextricable from other aspects of the protocol, and because they
393   interact with each other, it is useful to describe the basic caching
394   design of HTTP separately from the detailed descriptions of methods,
395   headers, response codes, etc.
396</t>
397<t>
398   Caching would be useless if it did not significantly improve
399   performance. The goal of caching in HTTP/1.1 is to eliminate the need
400   to send requests in many cases, and to eliminate the need to send
401   full responses in many other cases. The former reduces the number of
402   network round-trips required for many operations; we use an
403   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
404   latter reduces network bandwidth requirements; we use a "validation"
405   mechanism for this purpose (see <xref target="validation.model"/>).
406</t>
407<t>
408   Requirements for performance, availability, and disconnected
409   operation require us to be able to relax the goal of semantic
410   transparency. The HTTP/1.1 protocol allows origin servers, caches,
411   and clients to explicitly reduce transparency when necessary.
412   However, because non-transparent operation may confuse non-expert
413   users, and might be incompatible with certain server applications
414   (such as those for ordering merchandise), the protocol requires that
415   transparency be relaxed
416  <list style="symbols">
417     <t>only by an explicit protocol-level request when relaxed by
418        client or origin server</t>
419
420     <t>only with an explicit warning to the end user when relaxed by
421        cache or client</t>
422  </list>
423</t>
424<t>
425   Therefore, the HTTP/1.1 protocol provides these important elements:
426  <list style="numbers">
427      <t>Protocol features that provide full semantic transparency when
428         this is required by all parties.</t>
429
430      <t>Protocol features that allow an origin server or user agent to
431         explicitly request and control non-transparent operation.</t>
432
433      <t>Protocol features that allow a cache to attach warnings to
434         responses that do not preserve the requested approximation of
435         semantic transparency.</t>
436  </list>
437</t>
438<t>
439   A basic principle is that it must be possible for the clients to
440   detect any potential relaxation of semantic transparency.
441  <list><t>
442      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
443      design decisions not explicitly discussed in this specification.
444      If a decision might affect semantic transparency, the implementor
445      ought to err on the side of maintaining transparency unless a
446      careful and complete analysis shows significant benefits in
447      breaking transparency.
448    </t></list>
449</t>
450
451<section title="Cache Correctness" anchor="cache.correctness">
452<t>
453   A correct cache &MUST; respond to a request with the most up-to-date
454   response held by the cache that is appropriate to the request (see
455   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
456   <xref target="disambiguating.multiple.responses" format="counter"/>,
457   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
458   conditions:
459  <list style="numbers">
460      <t>It has been checked for equivalence with what the origin server
461         would have returned by revalidating the response with the
462         origin server (<xref target="validation.model"/>);</t>
463
464      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
465         this means it meets the least restrictive freshness requirement
466         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
467         the origin server so specifies, it is the freshness requirement
468         of the origin server alone.
469
470         If a stored response is not "fresh enough" by the most
471         restrictive freshness requirement of both the client and the
472         origin server, in carefully considered circumstances the cache
473         &MAY; still return the response with the appropriate Warning
474         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
475         and <xref target="header.warning" format="counter"/>), unless such a response
476         is prohibited (e.g., by a "no-store" cache-directive, or by a
477         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
478
479      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
480         or error (4xx or 5xx) response message.</t>
481  </list>
482</t>
483<t>
484   If the cache can not communicate with the origin server, then a
485   correct cache &SHOULD; respond as above if the response can be
486   correctly served from the cache; if not it &MUST; return an error or
487   warning indicating that there was a communication failure.
488</t>
489<t>
490   If a cache receives a response (either an entire response, or a 304
491   (Not Modified) response) that it would normally forward to the
492   requesting client, and the received response is no longer fresh, the
493   cache &SHOULD; forward it to the requesting client without adding a new
494   Warning (but without removing any existing Warning headers). A cache
495   &SHOULD-NOT;  attempt to revalidate a response simply because that
496   response became stale in transit; this might lead to an infinite
497   loop. A user agent that receives a stale response without a Warning
498   &MAY; display a warning indication to the user.
499</t>
500</section>
501
502<section title="Warnings" anchor="warnings">
503<t>
504   Whenever a cache returns a response that is neither first-hand nor
505   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
506   &MUST; attach a warning to that effect, using a Warning general-header.
507   The Warning header and the currently defined warnings are described
508   in <xref target="header.warning"/>. The warning allows clients to take appropriate
509   action.
510</t>
511<t>
512   Warnings &MAY; be used for other purposes, both cache-related and
513   otherwise. The use of a warning, rather than an error status code,
514   distinguish these responses from true failures.
515</t>
516<t>
517   Warnings are assigned three digit warn-codes. The first digit
518   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
519   stored cache entry after a successful revalidation:
520</t>
521<t>
522  <list style="hanging">
523    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
524     the response, and so &MUST; be deleted after a successful
525     revalidation. 1xx warn-codes &MAY; be generated by a cache only when
526     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
527
528    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
529     headers that is not rectified by a revalidation (for example, a
530     lossy compression of the entity bodies) and which &MUST-NOT; be
531     deleted after a successful revalidation.</t>
532    </list>
533</t>
534<t>
535   See <xref target="header.warning"/> for the definitions of the codes themselves.
536</t>
537<t>
538   HTTP/1.0 caches will cache all Warnings in responses, without
539   deleting the ones in the first category. Warnings in responses that
540   are passed to HTTP/1.0 caches carry an extra warning-date field,
541   which prevents a future HTTP/1.1 recipient from believing an
542   erroneously cached Warning.
543</t>
544<t>
545   Warnings also carry a warning text. The text &MAY; be in any
546   appropriate natural language (perhaps based on the client's Accept
547   headers), and include an &OPTIONAL; indication of what character set is
548   used.
549</t>
550<t>
551   Multiple warnings &MAY; be attached to a response (either by the origin
552   server or by a cache), including multiple warnings with the same code
553   number. For example, a server might provide the same warning with
554   texts in both English and Basque.
555</t>
556<t>
557   When multiple warnings are attached to a response, it might not be
558   practical or reasonable to display all of them to the user. This
559   version of HTTP does not specify strict priority rules for deciding
560   which warnings to display and in what order, but does suggest some
561   heuristics.
562</t>
563</section>
564
565<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
566<t>
567   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
568   times and validators) are implicit directives to caches. In some
569   cases, a server or client might need to provide explicit directives
570   to the HTTP caches. We use the Cache-Control header for this purpose.
571</t>
572<t>
573   The Cache-Control header allows a client or server to transmit a
574   variety of directives in either requests or responses. These
575   directives typically override the default caching algorithms. As a
576   general rule, if there is any apparent conflict between header
577   values, the most restrictive interpretation is applied (that is, the
578   one that is most likely to preserve semantic transparency). However,
579   in some cases, cache-control directives are explicitly specified as
580   weakening the approximation of semantic transparency (for example,
581   "max-stale" or "public").
582</t>
583<t>
584   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
585</t>
586</section>
587
588<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
589<t>
590   Many user agents make it possible for users to override the basic
591   caching mechanisms. For example, the user agent might allow the user
592   to specify that cached entities (even explicitly stale ones) are
593   never validated. Or the user agent might habitually add "Cache-Control:
594   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
595   default to either non-transparent behavior, or behavior that results
596   in abnormally ineffective caching, but &MAY; be explicitly configured
597   to do so by an explicit action of the user.
598</t>
599<t>
600   If the user has overridden the basic caching mechanisms, the user
601   agent &SHOULD; explicitly indicate to the user whenever this results in
602   the display of information that might not meet the server's
603   transparency requirements (in particular, if the displayed entity is
604   known to be stale). Since the protocol normally allows the user agent
605   to determine if responses are stale or not, this indication need only
606   be displayed when this actually happens. The indication need not be a
607   dialog box; it could be an icon (for example, a picture of a rotting
608   fish) or some other indicator.
609</t>
610<t>
611   If the user has overridden the caching mechanisms in a way that would
612   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
613   continually indicate this state to the user (for example, by a
614   display of a picture of currency in flames) so that the user does not
615   inadvertently consume excess resources or suffer from excessive
616   latency.
617</t>
618</section>
619
620<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
621<t>
622   In some cases, the operator of a cache &MAY; choose to configure it to
623   return stale responses even when not requested by clients. This
624   decision ought not be made lightly, but may be necessary for reasons
625   of availability or performance, especially when the cache is poorly
626   connected to the origin server. Whenever a cache returns a stale
627   response, it &MUST; mark it as such (using a Warning header) enabling
628   the client software to alert the user that there might be a potential
629   problem.
630</t>
631<t>
632   It also allows the user agent to take steps to obtain a first-hand or
633   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
634   response if the client explicitly requests a first-hand or fresh one,
635   unless it is impossible to comply for technical or policy reasons.
636</t>
637</section>
638
639<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
640<t>
641   While the origin server (and to a lesser extent, intermediate caches,
642   by their contribution to the age of a response) are the primary
643   source of expiration information, in some cases the client might need
644   to control a cache's decision about whether to return a cached
645   response without validating it. Clients do this using several
646   directives of the Cache-Control header.
647</t>
648<t>
649   A client's request &MAY; specify the maximum age it is willing to
650   accept of an unvalidated response; specifying a value of zero forces
651   the cache(s) to revalidate all responses. A client &MAY; also specify
652   the minimum time remaining before a response expires. Both of these
653   options increase constraints on the behavior of caches, and so cannot
654   further relax the cache's approximation of semantic transparency.
655</t>
656<t>
657   A client &MAY; also specify that it will accept stale responses, up to
658   some maximum amount of staleness. This loosens the constraints on the
659   caches, and so might violate the origin server's specified
660   constraints on semantic transparency, but might be necessary to
661   support disconnected operation, or high availability in the face of
662   poor connectivity.
663</t>
664</section>
665</section>
666
667<section title="Expiration Model" anchor="expiration.model">
668
669<section title="Server-Specified Expiration" anchor="server-specified.expiration">
670<t>
671   HTTP caching works best when caches can entirely avoid making
672   requests to the origin server. The primary mechanism for avoiding
673   requests is for an origin server to provide an explicit expiration
674   time in the future, indicating that a response &MAY; be used to satisfy
675   subsequent requests. In other words, a cache can return a fresh
676   response without first contacting the server.
677</t>
678<t>
679   Our expectation is that servers will assign future explicit
680   expiration times to responses in the belief that the entity is not
681   likely to change, in a semantically significant way, before the
682   expiration time is reached. This normally preserves semantic
683   transparency, as long as the server's expiration times are carefully
684   chosen.
685</t>
686<t>
687   The expiration mechanism applies only to responses taken from a cache
688   and not to first-hand responses forwarded immediately to the
689   requesting client.
690</t>
691<t>
692   If an origin server wishes to force a semantically transparent cache
693   to validate every request, it &MAY; assign an explicit expiration time
694   in the past. This means that the response is always stale, and so the
695   cache &SHOULD; validate it before using it for subsequent requests. See
696   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
697</t>
698<t>
699   If an origin server wishes to force any HTTP/1.1 cache, no matter how
700   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
701   cache-control directive (see <xref target="header.cache-control"/>).
702</t>
703<t>
704   Servers specify explicit expiration times using either the Expires
705   header, or the max-age directive of the Cache-Control header.
706</t>
707<t>
708   An expiration time cannot be used to force a user agent to refresh
709   its display or reload a resource; its semantics apply only to caching
710   mechanisms, and such mechanisms need only check a resource's
711   expiration status when a new request for that resource is initiated.
712   See <xref target="history.lists"/> for an explanation of the difference between caches
713   and history mechanisms.
714</t>
715</section>
716
717<section title="Heuristic Expiration" anchor="heuristic.expiration">
718<t>
719   Since origin servers do not always provide explicit expiration times,
720   HTTP caches typically assign heuristic expiration times, employing
721   algorithms that use other header values (such as the Last-Modified
722   time) to estimate a plausible expiration time. The HTTP/1.1
723   specification does not provide specific algorithms, but does impose
724   worst-case constraints on their results. Since heuristic expiration
725   times might compromise semantic transparency, they ought to be used
726   cautiously, and we encourage origin servers to provide explicit
727   expiration times as much as possible.
728</t>
729</section>
730
731<section title="Age Calculations" anchor="age.calculations">
732<t>
733   In order to know if a cached entry is fresh, a cache needs to know if
734   its age exceeds its freshness lifetime. We discuss how to calculate
735   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
736   the age of a response or cache entry.
737</t>
738<t>
739   In this discussion, we use the term "now" to mean "the current value
740   of the clock at the host performing the calculation." Hosts that use
741   HTTP, but especially hosts running origin servers and caches, &SHOULD;
742   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
743   a globally accurate time standard.
744</t>
745<t>
746   HTTP/1.1 requires origin servers to send a Date header, if possible,
747   with every response, giving the time at which the response was
748   generated (see &header-date;). We use the term "date_value" to denote
749   the value of the Date header, in a form appropriate for arithmetic
750   operations.
751</t>
752<t>
753   HTTP/1.1 uses the Age response-header to convey the estimated age of
754   the response message when obtained from a cache. The Age field value
755   is the cache's estimate of the amount of time since the response was
756   generated or revalidated by the origin server.
757</t>
758<t>
759   In essence, the Age value is the sum of the time that the response
760   has been resident in each of the caches along the path from the
761   origin server, plus the amount of time it has been in transit along
762   network paths.
763</t>
764<t>
765   We use the term "age_value" to denote the value of the Age header, in
766   a form appropriate for arithmetic operations.
767</t>
768<t>
769   A response's age can be calculated in two entirely independent ways:
770  <list style="numbers">
771      <t>now minus date_value, if the local clock is reasonably well
772         synchronized to the origin server's clock. If the result is
773         negative, the result is replaced by zero.</t>
774
775      <t>age_value, if all of the caches along the response path
776         implement HTTP/1.1.</t>
777  </list>
778</t>
779<t>
780   Given that we have two independent ways to compute the age of a
781   response when it is received, we can combine these as
782</t>
783<figure><artwork type="code">
784    corrected_received_age = max(now - date_value, age_value)
785</artwork></figure>
786<t>
787   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
788   paths, one gets a reliable (conservative) result.
789</t>
790<t>
791   Because of network-imposed delays, some significant interval might
792   pass between the time that a server generates a response and the time
793   it is received at the next outbound cache or client. If uncorrected,
794   this delay could result in improperly low ages.
795</t>
796<t>
797   Because the request that resulted in the returned Age value must have
798   been initiated prior to that Age value's generation, we can correct
799   for delays imposed by the network by recording the time at which the
800   request was initiated. Then, when an Age value is received, it &MUST;
801   be interpreted relative to the time the request was initiated, not
802   the time that the response was received. This algorithm results in
803   conservative behavior no matter how much delay is experienced. So, we
804   compute:
805</t>
806<figure><artwork type="code">
807   corrected_initial_age = corrected_received_age
808                         + (now - request_time)
809</artwork></figure>
810<t>
811   where "request_time" is the time (according to the local clock) when
812   the request that elicited this response was sent.
813</t>
814<t>
815   Summary of age calculation algorithm, when a cache receives a
816   response:
817</t>
818<figure><artwork type="code">
819   /*
820    * age_value
821    *      is the value of Age: header received by the cache with
822    *              this response.
823    * date_value
824    *      is the value of the origin server's Date: header
825    * request_time
826    *      is the (local) time when the cache made the request
827    *              that resulted in this cached response
828    * response_time
829    *      is the (local) time when the cache received the
830    *              response
831    * now
832    *      is the current (local) time
833    */
834
835   apparent_age = max(0, response_time - date_value);
836   corrected_received_age = max(apparent_age, age_value);
837   response_delay = response_time - request_time;
838   corrected_initial_age = corrected_received_age + response_delay;
839   resident_time = now - response_time;
840   current_age   = corrected_initial_age + resident_time;
841</artwork></figure>
842<t>
843   The current_age of a cache entry is calculated by adding the amount
844   of time (in seconds) since the cache entry was last validated by the
845   origin server to the corrected_initial_age. When a response is
846   generated from a cache entry, the cache &MUST; include a single Age
847   header field in the response with a value equal to the cache entry's
848   current_age.
849</t>
850<t>
851   The presence of an Age header field in a response implies that a
852   response is not first-hand. However, the converse is not true, since
853   the lack of an Age header field in a response does not imply that the
854   response is first-hand unless all caches along the request path are
855   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
856   the Age header field).
857</t>
858</section>
859
860<section title="Expiration Calculations" anchor="expiration.calculations">
861<t>
862   In order to decide whether a response is fresh or stale, we need to
863   compare its freshness lifetime to its age. The age is calculated as
864   described in <xref target="age.calculations"/>; this section describes how to calculate
865   the freshness lifetime, and to determine if a response has expired.
866   In the discussion below, the values can be represented in any form
867   appropriate for arithmetic operations.
868</t>
869<t>
870   We use the term "expires_value" to denote the value of the Expires
871   header. We use the term "max_age_value" to denote an appropriate
872   value of the number of seconds carried by the "max-age" directive of
873   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
874</t>
875<t>
876   The max-age directive takes priority over Expires, so if max-age is
877   present in a response, the calculation is simply:
878</t>
879<figure><artwork type="code">
880   freshness_lifetime = max_age_value
881</artwork></figure>
882<t>
883   Otherwise, if Expires is present in the response, the calculation is:
884</t>
885<figure><artwork type="code">
886   freshness_lifetime = expires_value - date_value
887</artwork></figure>
888<t>
889   Note that neither of these calculations is vulnerable to clock skew,
890   since all of the information comes from the origin server.
891</t>
892<t>
893   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
894   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
895   does not include other restrictions on caching, the cache &MAY; compute
896   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
897   113 to any response whose age is more than 24 hours if such warning
898   has not already been added.
899</t>
900<t>
901   Also, if the response does have a Last-Modified time, the heuristic
902   expiration value &SHOULD; be no more than some fraction of the interval
903   since that time. A typical setting of this fraction might be 10%.
904</t>
905<t>
906   The calculation to determine if a response has expired is quite
907   simple:
908</t>
909<figure><artwork type="code">
910   response_is_fresh = (freshness_lifetime &gt; current_age)
911</artwork></figure>
912</section>
913
914<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
915<t>
916   Because expiration values are assigned optimistically, it is possible
917   for two caches to contain fresh values for the same resource that are
918   different.
919</t>
920<t>
921   If a client performing a retrieval receives a non-first-hand response
922   for a request that was already fresh in its own cache, and the Date
923   header in its existing cache entry is newer than the Date on the new
924   response, then the client &MAY; ignore the response. If so, it &MAY;
925   retry the request with a "Cache-Control: max-age=0" directive (see
926   <xref target="header.cache-control"/>), to force a check with the origin server.
927</t>
928<t>
929   If a cache has two fresh responses for the same representation with
930   different validators, it &MUST; use the one with the more recent Date
931   header. This situation might arise because the cache is pooling
932   responses from other caches, or because a client has asked for a
933   reload or a revalidation of an apparently fresh cache entry.
934</t>
935</section>
936
937<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
938<t>
939   Because a client might be receiving responses via multiple paths, so
940   that some responses flow through one set of caches and other
941   responses flow through a different set of caches, a client might
942   receive responses in an order different from that in which the origin
943   server sent them. We would like the client to use the most recently
944   generated response, even if older responses are still apparently
945   fresh.
946</t>
947<t>
948   Neither the entity tag nor the expiration value can impose an
949   ordering on responses, since it is possible that a later response
950   intentionally carries an earlier expiration time. The Date values are
951   ordered to a granularity of one second.
952</t>
953<t>
954   When a client tries to revalidate a cache entry, and the response it
955   receives contains a Date header that appears to be older than the one
956   for the existing entry, then the client &SHOULD; repeat the request
957   unconditionally, and include
958</t>
959<figure><artwork type="example">
960    Cache-Control: max-age=0
961</artwork></figure>
962<t>
963   to force any intermediate caches to validate their copies directly
964   with the origin server, or
965</t>
966<figure><artwork type="example">
967    Cache-Control: no-cache
968</artwork></figure>
969<t>
970   to force any intermediate caches to obtain a new copy from the origin
971   server.
972</t>
973<t>
974   If the Date values are equal, then the client &MAY; use either response
975   (or &MAY;, if it is being extremely prudent, request a new response).
976   Servers &MUST-NOT; depend on clients being able to choose
977   deterministically between responses generated during the same second,
978   if their expiration times overlap.
979</t>
980</section>
981</section>
982
983<section title="Validation Model" anchor="validation.model">
984<t>
985   When a cache has a stale entry that it would like to use as a
986   response to a client's request, it first has to check with the origin
987   server (or possibly an intermediate cache with a fresh response) to
988   see if its cached entry is still usable. We call this "validating"
989   the cache entry. Since we do not want to have to pay the overhead of
990   retransmitting the full response if the cached entry is good, and we
991   do not want to pay the overhead of an extra round trip if the cached
992   entry is invalid, the HTTP/1.1 protocol supports the use of
993   conditional methods.
994</t>
995<t>
996   The key protocol features for supporting conditional methods are
997   those concerned with "cache validators." When an origin server
998   generates a full response, it attaches some sort of validator to it,
999   which is kept with the cache entry. When a client (user agent or
1000   proxy cache) makes a conditional request for a resource for which it
1001   has a cache entry, it includes the associated validator in the
1002   request.
1003</t>
1004<t>
1005   The server then checks that validator against the current validator
1006   for the entity, and, if they match (see &weak-and-strong-validators;), it responds
1007   with a special status code (usually, 304 (Not Modified)) and no
1008   entity-body. Otherwise, it returns a full response (including
1009   entity-body). Thus, we avoid transmitting the full response if the
1010   validator matches, and we avoid an extra round trip if it does not
1011   match.
1012</t>
1013<t>
1014   In HTTP/1.1, a conditional request looks exactly the same as a normal
1015   request for the same resource, except that it carries a special
1016   header (which includes the validator) that implicitly turns the
1017   method (usually, GET) into a conditional.
1018</t>
1019<t>
1020   The protocol includes both positive and negative senses of cache-validating
1021   conditions. That is, it is possible to request either that
1022   a method be performed if and only if a validator matches or if and
1023   only if no validators match.
1024  <list><t>
1025      <x:h>Note:</x:h> a response that lacks a validator may still be cached, and
1026      served from cache until it expires, unless this is explicitly
1027      prohibited by a cache-control directive. However, a cache cannot
1028      do a conditional retrieval if it does not have a validator for the
1029      entity, which means it will not be refreshable after it expires.
1030  </t></list>
1031</t>
1032
1033<section title="Last-Modified Dates" anchor="last-modified.dates">
1034<t>
1035   The Last-Modified entity-header field value is often used as a cache
1036   validator. In simple terms, a cache entry is considered to be valid
1037   if the entity has not been modified since the Last-Modified value.
1038</t>
1039</section>
1040
1041<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
1042<t>
1043   The ETag response-header field value, an entity tag, provides for an
1044   "opaque" cache validator. This might allow more reliable validation
1045   in situations where it is inconvenient to store modification dates,
1046   where the one-second resolution of HTTP date values is not
1047   sufficient, or where the origin server wishes to avoid certain
1048   paradoxes that might arise from the use of modification dates.
1049</t>
1050<t>
1051   Entity Tags are described in &entity-tags;. The headers used with entity
1052   tags are described in &entity-header-fields;.
1053</t>
1054</section>
1055
1056<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
1057<t>
1058   The principle behind entity tags is that only the service author
1059   knows the semantics of a resource well enough to select an
1060   appropriate cache validation mechanism, and the specification of any
1061   validator comparison function more complex than byte-equality would
1062   open up a can of worms. Thus, comparisons of any other headers
1063   (except Last-Modified, for compatibility with HTTP/1.0) are never
1064   used for purposes of validating a cache entry.
1065</t>
1066</section>
1067</section>
1068
1069<section title="Response Cacheability" anchor="response.cacheability">
1070<t>
1071   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1072   directive, a caching system &MAY; always store a successful response
1073   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1074   if it is fresh, and &MAY; return it after successful validation. If
1075   there is neither a cache validator nor an explicit expiration time
1076   associated with a response, we do not expect it to be cached, but
1077   certain caches &MAY; violate this expectation (for example, when little
1078   or no network connectivity is available). A client can usually detect
1079   that such a response was taken from a cache by comparing the Date
1080   header to the current time.
1081  <list><t>
1082      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1083      without providing any Warning.
1084  </t></list>
1085</t>
1086<t>
1087   However, in some cases it might be inappropriate for a cache to
1088   retain an entity, or to return it in response to a subsequent
1089   request. This might be because absolute semantic transparency is
1090   deemed necessary by the service author, or because of security or
1091   privacy considerations. Certain cache-control directives are
1092   therefore provided so that the server can indicate that certain
1093   resource entities, or portions thereof, are not to be cached
1094   regardless of other considerations.
1095</t>
1096<t>
1097   Note that &header-authorization; normally prevents a shared cache from saving
1098   and returning a response to a previous request if that request
1099   included an Authorization header.
1100</t>
1101<t>
1102   A response received with a status code of 200, 203, 206, 300, 301 or
1103   410 &MAY; be stored by a cache and used in reply to a subsequent
1104   request, subject to the expiration mechanism, unless a cache-control
1105   directive prohibits caching. However, a cache that does not support
1106   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1107   Content) responses.
1108</t>
1109<t>
1110   A response received with any other status code (e.g. status codes 302
1111   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1112   unless there are cache-control directives or another header(s) that
1113   explicitly allow it. For example, these include the following: an
1114   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1115   "proxy-revalidate", "public" or "private" cache-control
1116   directive (<xref target="header.cache-control"/>).
1117</t>
1118</section>
1119
1120<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1121<t>
1122   The purpose of an HTTP cache is to store information received in
1123   response to requests for use in responding to future requests. In
1124   many cases, a cache simply returns the appropriate parts of a
1125   response to the requester. However, if the cache holds a cache entry
1126   based on a previous response, it might have to combine parts of a new
1127   response with what is held in the cache entry.
1128</t>
1129
1130<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1131<t>
1132   For the purpose of defining the behavior of caches and non-caching
1133   proxies, we divide HTTP headers into two categories:
1134  <list style="symbols">
1135      <t>End-to-end headers, which are  transmitted to the ultimate
1136        recipient of a request or response. End-to-end headers in
1137        responses &MUST; be stored as part of a cache entry and &MUST; be
1138        transmitted in any response formed from a cache entry.</t>
1139
1140      <t>Hop-by-hop headers, which are meaningful only for a single
1141        transport-level connection, and are not stored by caches or
1142        forwarded by proxies.</t>
1143  </list>
1144</t>
1145<t>
1146   The following HTTP/1.1 headers are hop-by-hop headers:
1147  <list style="symbols">
1148      <t>Connection</t>
1149      <t>Keep-Alive</t>
1150      <t>Proxy-Authenticate</t>
1151      <t>Proxy-Authorization</t>
1152      <t>TE</t>
1153      <t>Trailer</t>
1154      <t>Transfer-Encoding</t>
1155      <t>Upgrade</t>
1156  </list>
1157</t>
1158<t>
1159   All other headers defined by HTTP/1.1 are end-to-end headers.
1160</t>
1161<t>
1162   Other hop-by-hop headers &MUST; be listed in a Connection header
1163   (&header-connection;).
1164</t>
1165</section>
1166
1167<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1168<t>
1169   Some features of the HTTP/1.1 protocol, such as Digest
1170   Authentication, depend on the value of certain end-to-end headers. A
1171   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1172   definition of that header requires or specifically allows that.
1173</t>
1174<t>
1175   A transparent proxy &MUST-NOT; modify any of the following fields in a
1176   request or response, and it &MUST-NOT; add any of these fields if not
1177   already present:
1178  <list style="symbols">
1179      <t>Content-Location</t>
1180      <t>Content-MD5</t>
1181      <t>ETag</t>
1182      <t>Last-Modified</t>
1183  </list>
1184</t>
1185<t>
1186   A transparent proxy &MUST-NOT; modify any of the following fields in a
1187   response:
1188  <list style="symbols">
1189    <t>Expires</t>
1190  </list>
1191</t>
1192<t>
1193   but it &MAY; add any of these fields if not already present. If an
1194   Expires header is added, it &MUST; be given a field-value identical to
1195   that of the Date header in that response.
1196</t>
1197<t>
1198   A  proxy &MUST-NOT; modify or add any of the following fields in a
1199   message that contains the no-transform cache-control directive, or in
1200   any request:
1201  <list style="symbols">
1202    <t>Content-Encoding</t>
1203    <t>Content-Range</t>
1204    <t>Content-Type</t>
1205  </list>
1206</t>
1207<t>
1208   A non-transparent proxy &MAY; modify or add these fields to a message
1209   that does not include no-transform, but if it does so, it &MUST; add a
1210   Warning 214 (Transformation applied) if one does not already appear
1211   in the message (see <xref target="header.warning"/>).
1212  <list><t>
1213      Warning: unnecessary modification of end-to-end headers might
1214      cause authentication failures if stronger authentication
1215      mechanisms are introduced in later versions of HTTP. Such
1216      authentication mechanisms &MAY; rely on the values of header fields
1217      not listed here.
1218    </t></list>
1219</t>
1220<t>
1221   The Content-Length field of a request or response is added or deleted
1222   according to the rules in &message-length;. A transparent proxy &MUST;
1223   preserve the entity-length (&entity-length;) of the entity-body,
1224   although it &MAY; change the transfer-length (&message-length;).
1225</t>
1226</section>
1227
1228<section title="Combining Headers" anchor="combining.headers">
1229<t>
1230   When a cache makes a validating request to a server, and the server
1231   provides a 304 (Not Modified) response or a 206 (Partial Content)
1232   response, the cache then constructs a response to send to the
1233   requesting client.
1234</t>
1235<t>
1236   If the status code is 304 (Not Modified), the cache uses the entity-body
1237   stored in the cache entry as the entity-body of this outgoing
1238   response. If the status code is 206 (Partial Content) and the ETag or
1239   Last-Modified headers match exactly, the cache &MAY; combine the
1240   contents stored in the cache entry with the new contents received in
1241   the response and use the result as the entity-body of this outgoing
1242   response, (see &combining-byte-ranges;).
1243</t>
1244<t>
1245   The end-to-end headers stored in the cache entry are used for the
1246   constructed response, except that
1247  <list style="symbols">
1248    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1249      &MUST; be deleted from the cache entry and the forwarded response.</t>
1250    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1251        in the cache entry and the forwarded response.</t>
1252    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1253        replace the corresponding headers from the cache entry.</t>
1254  </list>
1255</t>
1256<t>
1257   Unless the cache decides to remove the cache entry, it &MUST; also
1258   replace the end-to-end headers stored with the cache entry with
1259   corresponding headers received in the incoming response, except for
1260   Warning headers as described immediately above. If a header field-name
1261   in the incoming response matches more than one header in the
1262   cache entry, all such old headers &MUST; be replaced.
1263</t>
1264<t>
1265   In other words, the set of end-to-end headers received in the
1266   incoming response overrides all corresponding end-to-end headers
1267   stored with the cache entry (except for stored Warning headers with
1268   warn-code 1xx, which are deleted even if not overridden).
1269  <list><t>
1270      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1271      Modified) or a 206 (Partial Content) response to update any header
1272      associated with a previous response for the same entity or sub-ranges
1273      thereof, although it might not always be meaningful or
1274      correct to do so. This rule does not allow an origin server to use
1275      a 304 (Not Modified) or a 206 (Partial Content) response to
1276      entirely delete a header that it had provided with a previous
1277      response.
1278  </t></list>
1279</t>
1280</section>
1281
1282</section>
1283
1284<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1285<t>
1286   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1287   by the presence of a Vary header field in a response, alters the
1288   conditions and procedure by which a cache can use the response for
1289   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1290   field by servers.
1291</t>
1292<t>
1293   A server &SHOULD; use the Vary header field to inform a cache of what
1294   request-header fields were used to select among multiple
1295   representations of a cacheable response subject to server-driven
1296   negotiation. The set of header fields named by the Vary field value
1297   is known as the "selecting" request-headers.
1298</t>
1299<t>
1300   When the cache receives a subsequent request whose Request-URI
1301   specifies one or more cache entries including a Vary header field,
1302   the cache &MUST-NOT; use such a cache entry to construct a response to
1303   the new request unless all of the selecting request-headers present
1304   in the new request match the corresponding stored request-headers in
1305   the original request.
1306</t>
1307<t>
1308   The selecting request-headers from two requests are defined to match
1309   if and only if the selecting request-headers in the first request can
1310   be transformed to the selecting request-headers in the second request
1311   by adding or removing linear white space (LWS) at places where this
1312   is allowed by the corresponding BNF, and/or combining multiple
1313   message-header fields with the same field name following the rules
1314   about message headers in &message-headers;.
1315</t>
1316<t>
1317   A Vary header field-value of "*" always fails to match and subsequent
1318   requests on that resource can only be properly interpreted by the
1319   origin server.
1320</t>
1321<t>
1322   If the selecting request header fields for the cached entry do not
1323   match the selecting request header fields of the new request, then
1324   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1325   it first relays the new request to the origin server in a conditional
1326   request and the server responds with 304 (Not Modified), including an
1327   entity tag or Content-Location that indicates the entity to be used.
1328</t>
1329<t>
1330   If an entity tag was assigned to a cached representation, the
1331   forwarded request &SHOULD; be conditional and include the entity tags
1332   in an If-None-Match header field from all its cache entries for the
1333   resource. This conveys to the server the set of entities currently
1334   held by the cache, so that if any one of these entities matches the
1335   requested entity, the server can use the ETag header field in its 304
1336   (Not Modified) response to tell the cache which entry is appropriate.
1337   If the entity-tag of the new response matches that of an existing
1338   entry, the new response &SHOULD; be used to update the header fields of
1339   the existing entry, and the result &MUST; be returned to the client.
1340</t>
1341<t>
1342   If any of the existing cache entries contains only partial content
1343   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1344   the If-None-Match header field unless the request is for a range that
1345   would be fully satisfied by that entry.
1346</t>
1347<t>
1348   If a cache receives a successful response whose Content-Location
1349   field matches that of an existing cache entry for the same Request-URI,
1350   whose entity-tag differs from that of the existing entry, and
1351   whose Date is more recent than that of the existing entry, the
1352   existing entry &SHOULD-NOT;  be returned in response to future requests
1353   and &SHOULD; be deleted from the cache.
1354</t>
1355</section>
1356
1357<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1358<t>
1359   For reasons of security and privacy, it is necessary to make a
1360   distinction between "shared" and "non-shared" caches. A non-shared
1361   cache is one that is accessible only to a single user. Accessibility
1362   in this case &SHOULD; be enforced by appropriate security mechanisms.
1363   All other caches are considered to be "shared." Other sections of
1364   this specification place certain constraints on the operation of
1365   shared caches in order to prevent loss of privacy or failure of
1366   access controls.
1367</t>
1368</section>
1369
1370<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1371<t>
1372   A cache that receives an incomplete response (for example, with fewer
1373   bytes of data than specified in a Content-Length header) &MAY; store
1374   the response. However, the cache &MUST; treat this as a partial
1375   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1376   the result might be a full response or might still be
1377   partial. A cache &MUST-NOT; return a partial response to a client
1378   without explicitly marking it as such, using the 206 (Partial
1379   Content) status code. A cache &MUST-NOT; return a partial response
1380   using a status code of 200 (OK).
1381</t>
1382<t>
1383   If a cache receives a 5xx response while attempting to revalidate an
1384   entry, it &MAY; either forward this response to the requesting client,
1385   or act as if the server failed to respond. In the latter case, it &MAY;
1386   return a previously received response unless the cached entry
1387   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1388</t>
1389</section>
1390
1391<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1392<t>
1393   Unless the origin server explicitly prohibits the caching of their
1394   responses, the application of GET and HEAD methods to any resources
1395   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1396   these responses are taken from a cache. They &MAY; still have side
1397   effects, but a cache is not required to consider such side effects in
1398   its caching decisions. Caches are always expected to observe an
1399   origin server's explicit restrictions on caching.
1400</t>
1401<t>
1402   We note one exception to this rule: since some applications have
1403   traditionally used GETs and HEADs with query URLs (those containing a
1404   "?" in the rel_path part) to perform operations with significant side
1405   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1406   the server provides an explicit expiration time. This specifically
1407   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1408   be taken from a cache. See &safe-methods; for related information.
1409</t>
1410</section>
1411
1412<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1413<t>
1414   The effect of certain methods performed on a resource at the origin
1415   server might cause one or more existing cache entries to become non-transparently
1416   invalid. That is, although they might continue to be
1417   "fresh," they do not accurately reflect what the origin server would
1418   return for a new request on that resource.
1419</t>
1420<t>
1421   There is no way for the HTTP protocol to guarantee that all such
1422   cache entries are marked invalid. For example, the request that
1423   caused the change at the origin server might not have gone through
1424   the proxy where a cache entry is stored. However, several rules help
1425   reduce the likelihood of erroneous behavior.
1426</t>
1427<t>
1428   In this section, the phrase "invalidate an entity" means that the
1429   cache will either remove all instances of that entity from its
1430   storage, or will mark these as "invalid" and in need of a mandatory
1431   revalidation before they can be returned in response to a subsequent
1432   request.
1433</t>
1434<t>
1435   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1436   either the entity referred to by the Request-URI, or by the Location
1437   or Content-Location headers (if present). These methods are:
1438  <list style="symbols">
1439      <t>PUT</t>
1440      <t>DELETE</t>
1441      <t>POST</t>
1442  </list>
1443</t> 
1444<t>
1445   An invalidation based
1446   on the URI in a Location or Content-Location header &MUST-NOT; be
1447   performed if the host part of that URI differs from the host part
1448   in the Request-URI. This helps prevent denial of service attacks.
1449</t>
1450<t>
1451   A cache that passes through requests for methods it does not
1452   understand &SHOULD; invalidate any entities referred to by the
1453   Request-URI.
1454</t>
1455</section>
1456
1457<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1458<t>
1459   All methods that might be expected to cause modifications to the
1460   origin server's resources &MUST; be written through to the origin
1461   server. This currently includes all methods except for GET and HEAD.
1462   A cache &MUST-NOT; reply to such a request from a client before having
1463   transmitted the request to the inbound server, and having received a
1464   corresponding response from the inbound server. This does not prevent
1465   a proxy cache from sending a 100 (Continue) response before the
1466   inbound server has sent its final reply.
1467</t>
1468<t>
1469   The alternative (known as "write-back" or "copy-back" caching) is not
1470   allowed in HTTP/1.1, due to the difficulty of providing consistent
1471   updates and the problems arising from server, cache, or network
1472   failure prior to write-back.
1473</t>
1474</section>
1475
1476<section title="Cache Replacement" anchor="cache.replacement">
1477<t>
1478   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1479   <xref target="disambiguating.expiration.values" format="counter"/>,
1480   <xref target="disambiguating.multiple.responses" format="counter"/>
1481   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1482   response is received from a resource while any existing responses for
1483   the same resource are cached, the cache &SHOULD; use the new response
1484   to reply to the current request. It &MAY; insert it into cache storage
1485   and &MAY;, if it meets all other requirements, use it to respond to any
1486   future requests that would previously have caused the old response to
1487   be returned. If it inserts the new response into cache storage  the
1488   rules in <xref target="combining.headers"/> apply.
1489  <list><t>
1490      <x:h>Note:</x:h> a new response that has an older Date header value than
1491      existing cached responses is not cacheable.
1492  </t></list>
1493</t>
1494</section>
1495
1496<section title="History Lists" anchor="history.lists">
1497<t>
1498   User agents often have history mechanisms, such as "Back" buttons and
1499   history lists, which can be used to redisplay an entity retrieved
1500   earlier in a session.
1501</t>
1502<t>
1503   History mechanisms and caches are different. In particular history
1504   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1505   the current state of a resource. Rather, a history mechanism is meant
1506   to show exactly what the user saw at the time when the resource was
1507   retrieved.
1508</t>
1509<t>
1510   By default, an expiration time does not apply to history mechanisms.
1511   If the entity is still in storage, a history mechanism &SHOULD; display
1512   it even if the entity has expired, unless the user has specifically
1513   configured the agent to refresh expired history documents.
1514</t>
1515<t>
1516   This is not to be construed to prohibit the history mechanism from
1517   telling the user that a view might be stale.
1518  <list><t>
1519      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1520      viewing stale resources, this will tend to force service authors
1521      to avoid using HTTP expiration controls and cache controls when
1522      they would otherwise like to. Service authors may consider it
1523      important that users not be presented with error messages or
1524      warning messages when they use navigation controls (such as BACK)
1525      to view previously fetched resources. Even though sometimes such
1526      resources ought not be cached, or ought to expire quickly, user
1527      interface considerations may force service authors to resort to
1528      other means of preventing caching (e.g. "once-only" URLs) in order
1529      not to suffer the effects of improperly functioning history
1530      mechanisms.
1531  </t></list>
1532</t>
1533</section>
1534</section>
1535
1536<section title="Header Field Definitions" anchor="header.fields">
1537<t>
1538   This section defines the syntax and semantics of HTTP/1.1 header fields
1539   related to caching.
1540</t>
1541<t>
1542   For entity-header fields, both sender and recipient refer to either the
1543   client or the server, depending on who sends and who receives the entity.
1544</t>
1545
1546<section title="Age" anchor="header.age">
1547  <iref primary="true" item="Age header" x:for-anchor=""/>
1548  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1549<t>
1550      The Age response-header field conveys the sender's estimate of the
1551      amount of time since the response (or its revalidation) was
1552      generated at the origin server. A cached response is "fresh" if
1553      its age does not exceed its freshness lifetime. Age values are
1554      calculated as specified in <xref target="age.calculations"/>.
1555</t>
1556<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1557  Age = "Age" ":" age-value
1558  age-value = delta-seconds
1559</artwork></figure>
1560<t>
1561      Age values are non-negative decimal integers, representing time in
1562      seconds.
1563</t>
1564<t>
1565      If a cache receives a value larger than the largest positive
1566      integer it can represent, or if any of its age calculations
1567      overflows, it &MUST; transmit an Age header with a value of
1568      2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
1569      include an Age header field in every response generated from its
1570      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1571      bits of range.
1572</t>
1573</section>
1574
1575<section title="Cache-Control" anchor="header.cache-control">
1576  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1577  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1578<t>
1579   The Cache-Control general-header field is used to specify directives
1580   that &MUST; be obeyed by all caching mechanisms along the
1581   request/response chain. The directives specify behavior intended to
1582   prevent caches from adversely interfering with the request or
1583   response. These directives typically override the default caching
1584   algorithms. Cache directives are unidirectional in that the presence
1585   of a directive in a request does not imply that the same directive is
1586   to be given in the response.
1587  <list><t>
1588      Note that HTTP/1.0 caches might not implement Cache-Control and
1589      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1590  </t></list>
1591</t>
1592<t>
1593   Cache directives &MUST; be passed through by a proxy or gateway
1594   application, regardless of their significance to that application,
1595   since the directives might be applicable to all recipients along the
1596   request/response chain. It is not possible to specify a cache-directive
1597   for a specific cache.
1598</t>
1599<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"/>
1600  Cache-Control   = "Cache-Control" ":" 1#cache-directive
1601
1602  cache-directive = cache-request-directive
1603       | cache-response-directive
1604
1605  cache-request-directive =
1606         "no-cache"                          ; <xref target="what.is.cacheable"/>
1607       | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1608       | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1609       | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1610       | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1611       | "no-transform"                      ; <xref target="no-transform.directive"/>
1612       | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1613       | cache-extension                     ; <xref target="cache.control.extensions"/>
1614
1615  cache-response-directive =
1616         "public"                               ; <xref target="what.is.cacheable"/>
1617       | "private" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ] ; <xref target="what.is.cacheable"/>
1618       | "no-cache" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ]; <xref target="what.is.cacheable"/>
1619       | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1620       | "no-transform"                         ; <xref target="no-transform.directive"/>
1621       | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1622       | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1623       | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1624       | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1625       | cache-extension                        ; <xref target="cache.control.extensions"/>
1626
1627  cache-extension = token [ "=" ( token | quoted-string ) ]
1628</artwork></figure>
1629<t>
1630   When a directive appears without any 1#field-name parameter, the
1631   directive applies to the entire request or response. When such a
1632   directive appears with a 1#field-name parameter, it applies only to
1633   the named field or fields, and not to the rest of the request or
1634   response. This mechanism supports extensibility; implementations of
1635   future versions of the HTTP protocol might apply these directives to
1636   header fields not defined in HTTP/1.1.
1637</t>
1638<t>
1639   The cache-control directives can be broken down into these general
1640   categories:
1641  <list style="symbols">
1642     <t>Restrictions on what are cacheable; these may only be imposed by
1643        the origin server.</t>
1644
1645     <t>Restrictions on what may be stored by a cache; these may be
1646        imposed by either the origin server or the user agent.</t>
1647
1648     <t>Modifications of the basic expiration mechanism; these may be
1649        imposed by either the origin server or the user agent.</t>
1650
1651     <t>Controls over cache revalidation and reload; these may only be
1652        imposed by a user agent.</t>
1653
1654     <t>Control over transformation of entities.</t>
1655
1656     <t>Extensions to the caching system.</t>
1657  </list>
1658</t>
1659
1660<section title="What is Cacheable" anchor="what.is.cacheable">
1661<t>
1662   By default, a response is cacheable if the requirements of the
1663   request method, request header fields, and the response status
1664   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1665   for cacheability. The following Cache-Control response directives
1666   allow an origin server to override the default cacheability of a
1667   response:
1668</t>
1669<t>
1670  <iref item="Cache Directives" subitem="public" primary="true"/>
1671  <iref item="public" subitem="Cache Directive" primary="true"/>
1672   public
1673  <list><t>
1674      Indicates that the response &MAY; be cached by any cache, even if it
1675      would normally be non-cacheable or cacheable only within a non-shared
1676      cache. (See also Authorization, &header-authorization;, for
1677      additional details.)
1678  </t></list>
1679</t>
1680<t>
1681  <iref item="Cache Directives" subitem="private" primary="true"/>
1682  <iref item="private" subitem="Cache Directive" primary="true"/>
1683   private
1684  <list><t>
1685      Indicates that all or part of the response message is intended for
1686      a single user and &MUST-NOT; be cached by a shared cache. This
1687      allows an origin server to state that the specified parts of the
1688      response are intended for only one user and are not a valid
1689      response for requests by other users. A private (non-shared) cache
1690      &MAY; cache the response.
1691    </t><t>
1692       <x:h>Note:</x:h> This usage of the word private only controls where the
1693       response may be cached, and cannot ensure the privacy of the
1694       message content.
1695  </t></list>
1696</t>
1697<t>
1698  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1699  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1700   no-cache
1701  <list><t>
1702       If the no-cache directive does not specify a field-name, then a
1703      cache &MUST-NOT; use the response to satisfy a subsequent request
1704      without successful revalidation with the origin server. This
1705      allows an origin server to prevent caching even by caches that
1706      have been configured to return stale responses to client requests.
1707    </t><t>
1708      If the no-cache directive does specify one or more field-names,
1709      then a cache &MAY; use the response to satisfy a subsequent request,
1710      subject to any other restrictions on caching. However, the
1711      specified field-name(s) &MUST-NOT; be sent in the response to a
1712      subsequent request without successful revalidation with the origin
1713      server. This allows an origin server to prevent the re-use of
1714      certain header fields in a response, while still allowing caching
1715      of the rest of the response.
1716    <list><t>
1717       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1718       directive.
1719    </t></list>
1720  </t></list>
1721</t>
1722</section>
1723
1724<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1725<t>
1726  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1727  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1728   no-store
1729  <list><t>   
1730      The purpose of the no-store directive is to prevent the
1731      inadvertent release or retention of sensitive information (for
1732      example, on backup tapes). The no-store directive applies to the
1733      entire message, and &MAY; be sent either in a response or in a
1734      request. If sent in a request, a cache &MUST-NOT; store any part of
1735      either this request or any response to it. If sent in a response,
1736      a cache &MUST-NOT; store any part of either this response or the
1737      request that elicited it. This directive applies to both non-shared
1738      and shared caches. "&MUST-NOT; store" in this context means
1739      that the cache &MUST-NOT; intentionally store the information in
1740      non-volatile storage, and &MUST; make a best-effort attempt to
1741      remove the information from volatile storage as promptly as
1742      possible after forwarding it.
1743  </t><t>
1744      Even when this directive is associated with a response, users
1745      might explicitly store such a response outside of the caching
1746      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1747      such responses as part of their normal operation.
1748  </t><t>
1749      The purpose of this directive is to meet the stated requirements
1750      of certain users and service authors who are concerned about
1751      accidental releases of information via unanticipated accesses to
1752      cache data structures. While the use of this directive might
1753      improve privacy in some cases, we caution that it is NOT in any
1754      way a reliable or sufficient mechanism for ensuring privacy. In
1755      particular, malicious or compromised caches might not recognize or
1756      obey this directive, and communications networks might be
1757      vulnerable to eavesdropping.
1758  </t></list>
1759</t>
1760</section>
1761
1762<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1763<t>
1764   The expiration time of an entity &MAY; be specified by the origin
1765   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1766   it &MAY; be specified using the max-age directive in a response. When
1767   the max-age cache-control directive is present in a cached response,
1768   the response is stale if its current age is greater than the age
1769   value given (in seconds) at the time of a new request for that
1770   resource. The max-age directive on a response implies that the
1771   response is cacheable (i.e., "public") unless some other, more
1772   restrictive cache directive is also present.
1773</t>
1774<t>
1775   If a response includes both an Expires header and a max-age
1776   directive, the max-age directive overrides the Expires header, even
1777   if the Expires header is more restrictive. This rule allows an origin
1778   server to provide, for a given response, a longer expiration time to
1779   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1780   useful if certain HTTP/1.0 caches improperly calculate ages or
1781   expiration times, perhaps due to desynchronized clocks.
1782</t>
1783<t>
1784   Many HTTP/1.0 cache implementations will treat an Expires value that
1785   is less than or equal to the response Date value as being equivalent
1786   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1787   cache receives such a response, and the response does not include a
1788   Cache-Control header field, it &SHOULD; consider the response to be
1789   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1790  <list><t>
1791       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1792       cache control feature, such as the "private" directive, on a
1793       network including older caches that do not understand that
1794       feature. The origin server will need to combine the new feature
1795       with an Expires field whose value is less than or equal to the
1796       Date value. This will prevent older caches from improperly
1797       caching the response.
1798  </t></list>
1799</t>
1800<t>
1801  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1802  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1803   s-maxage
1804  <list><t>
1805       If a response includes an s-maxage directive, then for a shared
1806       cache (but not for a private cache), the maximum age specified by
1807       this directive overrides the maximum age specified by either the
1808       max-age directive or the Expires header. The s-maxage directive
1809       also implies the semantics of the proxy-revalidate directive (see
1810       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1811       entry after it becomes stale to respond to a subsequent request
1812       without first revalidating it with the origin server. The s-maxage
1813       directive is always ignored by a private cache.
1814  </t></list>
1815</t>
1816<t>
1817   Note that most older caches, not compliant with this specification,
1818   do not implement any cache-control directives. An origin server
1819   wishing to use a cache-control directive that restricts, but does not
1820   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1821   requirement that the max-age directive overrides the Expires header,
1822   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1823   max-age directive.
1824</t>
1825<t>
1826   Other directives allow a user agent to modify the basic expiration
1827   mechanism. These directives &MAY; be specified on a request:
1828</t>
1829<t>
1830  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1831  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1832   max-age
1833  <list><t>
1834      Indicates that the client is willing to accept a response whose
1835      age is no greater than the specified time in seconds. Unless max-stale
1836      directive is also included, the client is not willing to
1837      accept a stale response.
1838  </t></list>
1839</t>
1840<t>
1841  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1842  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1843   min-fresh
1844  <list><t>
1845      Indicates that the client is willing to accept a response whose
1846      freshness lifetime is no less than its current age plus the
1847      specified time in seconds. That is, the client wants a response
1848      that will still be fresh for at least the specified number of
1849      seconds.
1850  </t></list>
1851</t>
1852<t>
1853  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1854  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1855   max-stale
1856  <list><t>
1857      Indicates that the client is willing to accept a response that has
1858      exceeded its expiration time. If max-stale is assigned a value,
1859      then the client is willing to accept a response that has exceeded
1860      its expiration time by no more than the specified number of
1861      seconds. If no value is assigned to max-stale, then the client is
1862      willing to accept a stale response of any age.
1863  </t></list>
1864</t>
1865<t>
1866   If a cache returns a stale response, either because of a max-stale
1867   directive on a request, or because the cache is configured to
1868   override the expiration time of a response, the cache &MUST; attach a
1869   Warning header to the stale response, using Warning 110 (Response is
1870   stale).
1871</t>
1872<t>
1873   A cache &MAY; be configured to return stale responses without
1874   validation, but only if this does not conflict with any "MUST"-level
1875   requirements concerning cache validation (e.g., a "must-revalidate"
1876   cache-control directive).
1877</t>
1878<t>
1879   If both the new request and the cached entry include "max-age"
1880   directives, then the lesser of the two values is used for determining
1881   the freshness of the cached entry for that request.
1882</t>
1883</section>
1884
1885<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1886<t>
1887   Sometimes a user agent might want or need to insist that a cache
1888   revalidate its cache entry with the origin server (and not just with
1889   the next cache along the path to the origin server), or to reload its
1890   cache entry from the origin server. End-to-end revalidation might be
1891   necessary if either the cache or the origin server has overestimated
1892   the expiration time of the cached response. End-to-end reload may be
1893   necessary if the cache entry has become corrupted for some reason.
1894</t>
1895<t>
1896   End-to-end revalidation may be requested either when the client does
1897   not have its own local cached copy, in which case we call it
1898   "unspecified end-to-end revalidation", or when the client does have a
1899   local cached copy, in which case we call it "specific end-to-end
1900   revalidation."
1901</t>
1902<t>
1903   The client can specify these three kinds of action using Cache-Control
1904   request directives:
1905</t>
1906<t>
1907   End-to-end reload
1908  <list><t>
1909      The request includes a "no-cache" cache-control directive or, for
1910      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1911      names &MUST-NOT; be included with the no-cache directive in a
1912      request. The server &MUST-NOT; use a cached copy when responding to
1913      such a request.
1914  </t></list>
1915</t>
1916<t>
1917   Specific end-to-end revalidation
1918  <list><t>
1919      The request includes a "max-age=0" cache-control directive, which
1920      forces each cache along the path to the origin server to
1921      revalidate its own entry, if any, with the next cache or server.
1922      The initial request includes a cache-validating conditional with
1923      the client's current validator.
1924  </t></list>
1925</t>
1926<t>
1927   Unspecified end-to-end revalidation
1928  <list><t>
1929      The request includes "max-age=0" cache-control directive, which
1930      forces each cache along the path to the origin server to
1931      revalidate its own entry, if any, with the next cache or server.
1932      The initial request does not include a cache-validating
1933      conditional; the first cache along the path (if any) that holds a
1934      cache entry for this resource includes a cache-validating
1935      conditional with its current validator.
1936  </t></list>
1937</t>
1938<t>
1939  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1940  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1941   max-age
1942  <list><t>
1943      When an intermediate cache is forced, by means of a max-age=0
1944      directive, to revalidate its own cache entry, and the client has
1945      supplied its own validator in the request, the supplied validator
1946      might differ from the validator currently stored with the cache
1947      entry. In this case, the cache &MAY; use either validator in making
1948      its own request without affecting semantic transparency.
1949  </t><t>
1950      However, the choice of validator might affect performance. The
1951      best approach is for the intermediate cache to use its own
1952      validator when making its request. If the server replies with 304
1953      (Not Modified), then the cache can return its now validated copy
1954      to the client with a 200 (OK) response. If the server replies with
1955      a new entity and cache validator, however, the intermediate cache
1956      can compare the returned validator with the one provided in the
1957      client's request, using the strong comparison function. If the
1958      client's validator is equal to the origin server's, then the
1959      intermediate cache simply returns 304 (Not Modified). Otherwise,
1960      it returns the new entity with a 200 (OK) response.
1961  </t><t>
1962      If a request includes the no-cache directive, it &SHOULD-NOT; 
1963      include min-fresh, max-stale, or max-age.
1964  </t></list>
1965</t>
1966<t>
1967  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1968  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1969   only-if-cached
1970  <list><t>
1971      In some cases, such as times of extremely poor network
1972      connectivity, a client may want a cache to return only those
1973      responses that it currently has stored, and not to reload or
1974      revalidate with the origin server. To do this, the client may
1975      include the only-if-cached directive in a request. If it receives
1976      this directive, a cache &SHOULD; either respond using a cached entry
1977      that is consistent with the other constraints of the request, or
1978      respond with a 504 (Gateway Timeout) status. However, if a group
1979      of caches is being operated as a unified system with good internal
1980      connectivity, such a request &MAY; be forwarded within that group of
1981      caches.
1982  </t></list>
1983</t>
1984<t>
1985  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1986  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1987   must-revalidate
1988  <list><t>
1989      Because a cache &MAY; be configured to ignore a server's specified
1990      expiration time, and because a client request &MAY; include a max-stale
1991      directive (which has a similar effect), the protocol also
1992      includes a mechanism for the origin server to require revalidation
1993      of a cache entry on any subsequent use. When the must-revalidate
1994      directive is present in a response received by a cache, that cache
1995      &MUST-NOT; use the entry after it becomes stale to respond to a
1996      subsequent request without first revalidating it with the origin
1997      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1998      time, if, based solely on the origin server's Expires or max-age
1999      value, the cached response is stale.)
2000  </t><t>
2001      The must-revalidate directive is necessary to support reliable
2002      operation for certain protocol features. In all circumstances an
2003      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
2004      particular, if the cache cannot reach the origin server for any
2005      reason, it &MUST; generate a 504 (Gateway Timeout) response.
2006  </t><t>
2007      Servers &SHOULD; send the must-revalidate directive if and only if
2008      failure to revalidate a request on the entity could result in
2009      incorrect operation, such as a silently unexecuted financial
2010      transaction. Recipients &MUST-NOT; take any automated action that
2011      violates this directive, and &MUST-NOT; automatically provide an
2012      unvalidated copy of the entity if revalidation fails.
2013  </t><t>
2014      Although this is not recommended, user agents operating under
2015      severe connectivity constraints &MAY; violate this directive but, if
2016      so, &MUST; explicitly warn the user that an unvalidated response has
2017      been provided. The warning &MUST; be provided on each unvalidated
2018      access, and &SHOULD; require explicit user confirmation.
2019  </t></list>
2020</t>
2021<t>
2022  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
2023  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
2024   proxy-revalidate
2025  <list><t>
2026      The proxy-revalidate directive has the same meaning as the must-revalidate
2027      directive, except that it does not apply to non-shared
2028      user agent caches. It can be used on a response to an
2029      authenticated request to permit the user's cache to store and
2030      later return the response without needing to revalidate it (since
2031      it has already been authenticated once by that user), while still
2032      requiring proxies that service many users to revalidate each time
2033      (in order to make sure that each user has been authenticated).
2034      Note that such authenticated responses also need the public cache
2035      control directive in order to allow them to be cached at all.
2036  </t></list>
2037</t>
2038</section>
2039
2040<section title="No-Transform Directive" anchor="no-transform.directive">
2041<t>
2042  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2043  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2044   no-transform
2045  <list><t>
2046      Implementors of intermediate caches (proxies) have found it useful
2047      to convert the media type of certain entity bodies. A non-transparent
2048      proxy might, for example, convert between image
2049      formats in order to save cache space or to reduce the amount of
2050      traffic on a slow link.
2051  </t><t>
2052      Serious operational problems occur, however, when these
2053      transformations are applied to entity bodies intended for certain
2054      kinds of applications. For example, applications for medical
2055      imaging, scientific data analysis and those using end-to-end
2056      authentication, all depend on receiving an entity body that is bit
2057      for bit identical to the original entity-body.
2058  </t><t>
2059      Therefore, if a message includes the no-transform directive, an
2060      intermediate cache or proxy &MUST-NOT; change those headers that are
2061      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2062      directive. This implies that the cache or proxy &MUST-NOT; change
2063      any aspect of the entity-body that is specified by these headers,
2064      including the value of the entity-body itself.
2065  </t></list>
2066</t>
2067</section>
2068
2069<section title="Cache Control Extensions" anchor="cache.control.extensions">
2070<t>
2071   The Cache-Control header field can be extended through the use of one
2072   or more cache-extension tokens, each with an optional assigned value.
2073   Informational extensions (those which do not require a change in
2074   cache behavior) &MAY; be added without changing the semantics of other
2075   directives. Behavioral extensions are designed to work by acting as
2076   modifiers to the existing base of cache directives. Both the new
2077   directive and the standard directive are supplied, such that
2078   applications which do not understand the new directive will default
2079   to the behavior specified by the standard directive, and those that
2080   understand the new directive will recognize it as modifying the
2081   requirements associated with the standard directive. In this way,
2082   extensions to the cache-control directives can be made without
2083   requiring changes to the base protocol.
2084</t>
2085<t>
2086   This extension mechanism depends on an HTTP cache obeying all of the
2087   cache-control directives defined for its native HTTP-version, obeying
2088   certain extensions, and ignoring all directives that it does not
2089   understand.
2090</t>
2091<t>
2092   For example, consider a hypothetical new response directive called
2093   community which acts as a modifier to the private directive. We
2094   define this new directive to mean that, in addition to any non-shared
2095   cache, any cache which is shared only by members of the community
2096   named within its value may cache the response. An origin server
2097   wishing to allow the UCI community to use an otherwise private
2098   response in their shared cache(s) could do so by including
2099</t>
2100<figure><artwork type="example">
2101    Cache-Control: private, community="UCI"
2102</artwork></figure>
2103<t>
2104   A cache seeing this header field will act correctly even if the cache
2105   does not understand the community cache-extension, since it will also
2106   see and understand the private directive and thus default to the safe
2107   behavior.
2108</t>
2109<t>
2110   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2111   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2112   be combined with standard directives (or the response's default
2113   cacheability) such that the cache behavior will remain minimally
2114   correct even if the cache does not understand the extension(s).
2115</t>
2116</section>
2117</section>
2118
2119<section title="Expires" anchor="header.expires">
2120  <iref primary="true" item="Expires header" x:for-anchor=""/>
2121  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2122<t>
2123   The Expires entity-header field gives the date/time after which the
2124   response is considered stale. A stale cache entry may not normally be
2125   returned by a cache (either a proxy cache or a user agent cache)
2126   unless it is first validated with the origin server (or with an
2127   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2128   for further discussion of the expiration model.
2129</t>
2130<t>
2131   The presence of an Expires field does not imply that the original
2132   resource will change or cease to exist at, before, or after that
2133   time.
2134</t>
2135<t>
2136   The format is an absolute date and time as defined by HTTP-date in
2137   &full-date;; it &MUST; be sent in rfc1123-date format.
2138</t>
2139<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2140  Expires = "Expires" ":" HTTP-date
2141</artwork></figure>
2142<t>
2143   An example of its use is
2144</t>
2145<figure><artwork type="example">
2146   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2147</artwork></figure>
2148<t>
2149  <list><t>
2150      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2151      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2152      Expires field.
2153  </t></list>
2154</t>
2155<t>
2156   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2157   especially including the value "0", as in the past (i.e., "already
2158   expired").
2159</t>
2160<t>
2161   To mark a response as "already expired," an origin server sends an
2162   Expires date that is equal to the Date header value. (See the rules
2163   for expiration calculations in <xref target="expiration.calculations"/>.)
2164</t>
2165<t>
2166   To mark a response as "never expires," an origin server sends an
2167   Expires date approximately one year from the time the response is
2168   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2169   year in the future.
2170</t>
2171<t>
2172   The presence of an Expires header field with a date value of some
2173   time in the future on a response that otherwise would by default be
2174   non-cacheable indicates that the response is cacheable, unless
2175   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2176</t>
2177</section>
2178
2179<section title="Pragma" anchor="header.pragma">
2180  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2181  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2182<t>
2183   The Pragma general-header field is used to include implementation-specific
2184   directives that might apply to any recipient along the
2185   request/response chain. All pragma directives specify optional
2186   behavior from the viewpoint of the protocol; however, some systems
2187   &MAY; require that behavior be consistent with the directives.
2188</t>
2189<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"/>
2190  Pragma            = "Pragma" ":" 1#pragma-directive
2191  pragma-directive  = "no-cache" | extension-pragma
2192  extension-pragma  = token [ "=" ( token | quoted-string ) ]
2193</artwork></figure>
2194<t>
2195   When the no-cache directive is present in a request message, an
2196   application &SHOULD; forward the request toward the origin server even
2197   if it has a cached copy of what is being requested. This pragma
2198   directive has the same semantics as the no-cache cache-directive (see
2199   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2200   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2201   request is sent to a server not known to be HTTP/1.1 compliant.
2202</t>
2203<t>
2204   Pragma directives &MUST; be passed through by a proxy or gateway
2205   application, regardless of their significance to that application,
2206   since the directives might be applicable to all recipients along the
2207   request/response chain. It is not possible to specify a pragma for a
2208   specific recipient; however, any pragma directive not relevant to a
2209   recipient &SHOULD; be ignored by that recipient.
2210</t>
2211<t>
2212   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2213   sent "Cache-Control: no-cache". No new Pragma directives will be
2214   defined in HTTP.
2215  <list><t>
2216      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
2217      response-header field is not actually specified, it does not provide a
2218      reliable replacement for "Cache-Control: no-cache" in a response.
2219  </t></list>
2220</t>
2221</section>
2222
2223<section title="Vary" anchor="header.vary">
2224  <iref primary="true" item="Vary header" x:for-anchor=""/>
2225  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2226<t>
2227   The Vary field value indicates the set of request-header fields that
2228   fully determines, while the response is fresh, whether a cache is
2229   permitted to use the response to reply to a subsequent request
2230   without revalidation. For uncacheable or stale responses, the Vary
2231   field value advises the user agent about the criteria that were used
2232   to select the representation. A Vary field value of "*" implies that
2233   a cache cannot determine from the request headers of a subsequent
2234   request whether this response is the appropriate representation. See
2235   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2236</t>
2237<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2238  Vary  = "Vary" ":" ( "*" | 1#field-name )
2239</artwork></figure>
2240<t>
2241   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2242   cacheable response that is subject to server-driven negotiation.
2243   Doing so allows a cache to properly interpret future requests on that
2244   resource and informs the user agent about the presence of negotiation
2245   on that resource. A server &MAY; include a Vary header field with a
2246   non-cacheable response that is subject to server-driven negotiation,
2247   since this might provide the user agent with useful information about
2248   the dimensions over which the response varies at the time of the
2249   response.
2250</t>
2251<t>
2252   A Vary field value consisting of a list of field-names signals that
2253   the representation selected for the response is based on a selection
2254   algorithm which considers ONLY the listed request-header field values
2255   in selecting the most appropriate representation. A cache &MAY; assume
2256   that the same selection will be made for future requests with the
2257   same values for the listed field names, for the duration of time for
2258   which the response is fresh.
2259</t>
2260<t>
2261   The field-names given are not limited to the set of standard
2262   request-header fields defined by this specification. Field names are
2263   case-insensitive.
2264</t>
2265<t>
2266   A Vary field value of "*" signals that unspecified parameters not
2267   limited to the request-headers (e.g., the network address of the
2268   client), play a role in the selection of the response representation.
2269   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2270   generated by an origin server.
2271</t>
2272</section>
2273
2274<section title="Warning" anchor="header.warning">
2275  <iref primary="true" item="Warning header" x:for-anchor=""/>
2276  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2277<t>
2278   The Warning general-header field is used to carry additional
2279   information about the status or transformation of a message which
2280   might not be reflected in the message. This information is typically
2281   used to warn about a possible lack of semantic transparency from
2282   caching operations or transformations applied to the entity body of
2283   the message.
2284</t>
2285<t>
2286   Warning headers are sent with responses using:
2287</t>
2288<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"/>
2289  Warning    = "Warning" ":" 1#warning-value
2290 
2291  warning-value = warn-code SP warn-agent SP warn-text
2292                                        [SP warn-date]
2293 
2294  warn-code  = 3DIGIT
2295  warn-agent = ( host [ ":" port ] ) | pseudonym
2296                  ; the name or pseudonym of the server adding
2297                  ; the Warning header, for use in debugging
2298  warn-text  = quoted-string
2299  warn-date  = &lt;"&gt; HTTP-date &lt;"&gt;
2300</artwork></figure>
2301<t>
2302   A response &MAY; carry more than one Warning header.
2303</t>
2304<t>
2305   The warn-text &SHOULD; be in a natural language and character set that
2306   is most likely to be intelligible to the human user receiving the
2307   response. This decision &MAY; be based on any available knowledge, such
2308   as the location of the cache or user, the Accept-Language field in a
2309   request, the Content-Language field in a response, etc. The default
2310   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2311</t>
2312<t>
2313   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2314   in the warn-text using the method described in <xref target="RFC2047"/>.
2315</t>
2316<t>
2317   Warning headers can in general be applied to any message, however
2318   some specific warn-codes are specific to caches and can only be
2319   applied to response messages. New Warning headers &SHOULD; be added
2320   after any existing Warning headers. A cache &MUST-NOT; delete any
2321   Warning header that it received with a message. However, if a cache
2322   successfully validates a cache entry, it &SHOULD; remove any Warning
2323   headers previously attached to that entry except as specified for
2324   specific Warning codes. It &MUST; then add any Warning headers received
2325   in the validating response. In other words, Warning headers are those
2326   that would be attached to the most recent relevant response.
2327</t>
2328<t>
2329   When multiple Warning headers are attached to a response, the user
2330   agent ought to inform the user of as many of them as possible, in the
2331   order that they appear in the response. If it is not possible to
2332   inform the user of all of the warnings, the user agent &SHOULD; follow
2333   these heuristics:
2334  <list style="symbols">
2335    <t>Warnings that appear early in the response take priority over
2336        those appearing later in the response.</t>
2337
2338    <t>Warnings in the user's preferred character set take priority
2339        over warnings in other character sets but with identical warn-codes
2340        and warn-agents.</t>
2341  </list>
2342</t>
2343<t>
2344   Systems that generate multiple Warning headers &SHOULD; order them with
2345   this user agent behavior in mind.
2346</t>
2347<t>
2348   Requirements for the behavior of caches with respect to Warnings are
2349   stated in <xref target="warnings"/>.
2350</t>
2351<t>
2352   This is a list of the currently-defined warn-codes, each with a
2353   recommended warn-text in English, and a description of its meaning.
2354</t>
2355<t>
2356   110 Response is stale
2357  <list><t>
2358     &MUST; be included whenever the returned response is stale.
2359  </t></list>
2360</t>
2361<t>
2362   111 Revalidation failed
2363  <list><t>
2364     &MUST; be included if a cache returns a stale response because an
2365     attempt to revalidate the response failed, due to an inability to
2366     reach the server.
2367  </t></list>
2368</t>
2369<t>
2370   112 Disconnected operation
2371  <list><t>
2372     &SHOULD; be included if the cache is intentionally disconnected from
2373     the rest of the network for a period of time.
2374  </t></list>
2375</t>
2376<t>
2377   113 Heuristic expiration
2378  <list><t>
2379     &MUST; be included if the cache heuristically chose a freshness
2380     lifetime greater than 24 hours and the response's age is greater
2381     than 24 hours.
2382  </t></list>
2383</t>
2384<t>
2385   199 Miscellaneous warning
2386  <list><t>
2387     The warning text &MAY; include arbitrary information to be presented
2388     to a human user, or logged. A system receiving this warning &MUST-NOT;
2389     take any automated action, besides presenting the warning to
2390     the user.
2391  </t></list>
2392</t>
2393<t>
2394   214 Transformation applied
2395  <list><t>
2396     &MUST; be added by an intermediate cache or proxy if it applies any
2397     transformation changing the content-coding (as specified in the
2398     Content-Encoding header) or media-type (as specified in the
2399     Content-Type header) of the response, or the entity-body of the
2400     response, unless this Warning code already appears in the response.
2401  </t></list>
2402</t>
2403<t>
2404   299 Miscellaneous persistent warning
2405  <list><t>
2406     The warning text &MAY; include arbitrary information to be presented
2407     to a human user, or logged. A system receiving this warning &MUST-NOT;
2408     take any automated action.
2409  </t></list>
2410</t>
2411<t>
2412   If an implementation sends a message with one or more Warning headers
2413   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2414   each warning-value a warn-date that matches the date in the response.
2415</t>
2416<t>
2417   If an implementation receives a message with a warning-value that
2418   includes a warn-date, and that warn-date is different from the Date
2419   value in the response, then that warning-value &MUST; be deleted from
2420   the message before storing, forwarding, or using it. (This prevents
2421   bad consequences of naive caching of Warning header fields.) If all
2422   of the warning-values are deleted for this reason, the Warning header
2423   &MUST; be deleted as well.
2424</t>
2425</section>
2426
2427</section>
2428
2429<section title="IANA Considerations" anchor="IANA.considerations">
2430<t>
2431   TBD.
2432</t>
2433</section>
2434
2435<section title="Security Considerations" anchor="security.considerations">
2436<t>
2437   Caching proxies provide additional potential vulnerabilities, since
2438   the contents of the cache represent an attractive target for
2439   malicious exploitation. Because cache contents persist after an HTTP
2440   request is complete, an attack on the cache can reveal information
2441   long after a user believes that the information has been removed from
2442   the network. Therefore, cache contents should be protected as
2443   sensitive information.
2444</t>
2445</section>
2446
2447<section title="Acknowledgments" anchor="ack">
2448<t>
2449   Much of the content and presentation of the caching design is due to
2450   suggestions and comments from individuals including: Shel Kaphan,
2451   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2452</t>
2453</section>
2454</middle>
2455<back>
2456
2457<references title="Normative References">
2458
2459<reference anchor="ISO-8859-1">
2460  <front>
2461    <title>
2462     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2463    </title>
2464    <author>
2465      <organization>International Organization for Standardization</organization>
2466    </author>
2467    <date year="1998"/>
2468  </front>
2469  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2470</reference>
2471
2472<reference anchor="Part1">
2473  <front>
2474    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2475    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2476      <organization abbrev="Day Software">Day Software</organization>
2477      <address><email>fielding@gbiv.com</email></address>
2478    </author>
2479    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2480      <organization>One Laptop per Child</organization>
2481      <address><email>jg@laptop.org</email></address>
2482    </author>
2483    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2484      <organization abbrev="HP">Hewlett-Packard Company</organization>
2485      <address><email>JeffMogul@acm.org</email></address>
2486    </author>
2487    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2488      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2489      <address><email>henrikn@microsoft.com</email></address>
2490    </author>
2491    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2492      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2493      <address><email>LMM@acm.org</email></address>
2494    </author>
2495    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2496      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2497      <address><email>paulle@microsoft.com</email></address>
2498    </author>
2499    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2500      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2501      <address><email>timbl@w3.org</email></address>
2502    </author>
2503    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2504      <organization abbrev="W3C">World Wide Web Consortium</organization>
2505      <address><email>ylafon@w3.org</email></address>
2506    </author>
2507    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2508      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2509      <address><email>julian.reschke@greenbytes.de</email></address>
2510    </author>
2511    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2512  </front>
2513  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2514  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2515</reference>
2516
2517<reference anchor="Part2">
2518  <front>
2519    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2520    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2521      <organization abbrev="Day Software">Day Software</organization>
2522      <address><email>fielding@gbiv.com</email></address>
2523    </author>
2524    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2525      <organization>One Laptop per Child</organization>
2526      <address><email>jg@laptop.org</email></address>
2527    </author>
2528    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2529      <organization abbrev="HP">Hewlett-Packard Company</organization>
2530      <address><email>JeffMogul@acm.org</email></address>
2531    </author>
2532    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2533      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2534      <address><email>henrikn@microsoft.com</email></address>
2535    </author>
2536    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2537      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2538      <address><email>LMM@acm.org</email></address>
2539    </author>
2540    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2541      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2542      <address><email>paulle@microsoft.com</email></address>
2543    </author>
2544    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2545      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2546      <address><email>timbl@w3.org</email></address>
2547    </author>
2548    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2549      <organization abbrev="W3C">World Wide Web Consortium</organization>
2550      <address><email>ylafon@w3.org</email></address>
2551    </author>
2552    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2553      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2554      <address><email>julian.reschke@greenbytes.de</email></address>
2555    </author>
2556    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2557  </front>
2558  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2559  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2560</reference>
2561
2562<reference anchor="Part3">
2563  <front>
2564    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2565    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2566      <organization abbrev="Day Software">Day Software</organization>
2567      <address><email>fielding@gbiv.com</email></address>
2568    </author>
2569    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2570      <organization>One Laptop per Child</organization>
2571      <address><email>jg@laptop.org</email></address>
2572    </author>
2573    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2574      <organization abbrev="HP">Hewlett-Packard Company</organization>
2575      <address><email>JeffMogul@acm.org</email></address>
2576    </author>
2577    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2578      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2579      <address><email>henrikn@microsoft.com</email></address>
2580    </author>
2581    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2582      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2583      <address><email>LMM@acm.org</email></address>
2584    </author>
2585    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2586      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2587      <address><email>paulle@microsoft.com</email></address>
2588    </author>
2589    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2590      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2591      <address><email>timbl@w3.org</email></address>
2592    </author>
2593    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2594      <organization abbrev="W3C">World Wide Web Consortium</organization>
2595      <address><email>ylafon@w3.org</email></address>
2596    </author>
2597    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2598      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2599      <address><email>julian.reschke@greenbytes.de</email></address>
2600    </author>
2601    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2602  </front>
2603  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2604  <x:source href="p3-payload.xml" basename="p3-payload"/>
2605</reference>
2606
2607<reference anchor="Part4">
2608  <front>
2609    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2610    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2611      <organization abbrev="Day Software">Day Software</organization>
2612      <address><email>fielding@gbiv.com</email></address>
2613    </author>
2614    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2615      <organization>One Laptop per Child</organization>
2616      <address><email>jg@laptop.org</email></address>
2617    </author>
2618    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2619      <organization abbrev="HP">Hewlett-Packard Company</organization>
2620      <address><email>JeffMogul@acm.org</email></address>
2621    </author>
2622    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2623      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2624      <address><email>henrikn@microsoft.com</email></address>
2625    </author>
2626    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2627      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2628      <address><email>LMM@acm.org</email></address>
2629    </author>
2630    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2631      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2632      <address><email>paulle@microsoft.com</email></address>
2633    </author>
2634    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2635      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2636      <address><email>timbl@w3.org</email></address>
2637    </author>
2638    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2639      <organization abbrev="W3C">World Wide Web Consortium</organization>
2640      <address><email>ylafon@w3.org</email></address>
2641    </author>
2642    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2643      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2644      <address><email>julian.reschke@greenbytes.de</email></address>
2645    </author>
2646    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2647  </front>
2648  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2649  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2650</reference>
2651
2652<reference anchor="Part5">
2653  <front>
2654    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2655    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2656      <organization abbrev="Day Software">Day Software</organization>
2657      <address><email>fielding@gbiv.com</email></address>
2658    </author>
2659    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2660      <organization>One Laptop per Child</organization>
2661      <address><email>jg@laptop.org</email></address>
2662    </author>
2663    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2664      <organization abbrev="HP">Hewlett-Packard Company</organization>
2665      <address><email>JeffMogul@acm.org</email></address>
2666    </author>
2667    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2668      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2669      <address><email>henrikn@microsoft.com</email></address>
2670    </author>
2671    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2672      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2673      <address><email>LMM@acm.org</email></address>
2674    </author>
2675    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2676      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2677      <address><email>paulle@microsoft.com</email></address>
2678    </author>
2679    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2680      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2681      <address><email>timbl@w3.org</email></address>
2682    </author>
2683    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2684      <organization abbrev="W3C">World Wide Web Consortium</organization>
2685      <address><email>ylafon@w3.org</email></address>
2686    </author>
2687    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2688      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2689      <address><email>julian.reschke@greenbytes.de</email></address>
2690    </author>
2691    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2692  </front>
2693  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2694  <x:source href="p5-range.xml" basename="p5-range"/>
2695</reference>
2696
2697<reference anchor="Part7">
2698  <front>
2699    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2700    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2701      <organization abbrev="Day Software">Day Software</organization>
2702      <address><email>fielding@gbiv.com</email></address>
2703    </author>
2704    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2705      <organization>One Laptop per Child</organization>
2706      <address><email>jg@laptop.org</email></address>
2707    </author>
2708    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2709      <organization abbrev="HP">Hewlett-Packard Company</organization>
2710      <address><email>JeffMogul@acm.org</email></address>
2711    </author>
2712    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2713      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2714      <address><email>henrikn@microsoft.com</email></address>
2715    </author>
2716    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2717      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2718      <address><email>LMM@acm.org</email></address>
2719    </author>
2720    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2721      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2722      <address><email>paulle@microsoft.com</email></address>
2723    </author>
2724    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2725      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2726      <address><email>timbl@w3.org</email></address>
2727    </author>
2728    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2729      <organization abbrev="W3C">World Wide Web Consortium</organization>
2730      <address><email>ylafon@w3.org</email></address>
2731    </author>
2732    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2733      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2734      <address><email>julian.reschke@greenbytes.de</email></address>
2735    </author>
2736    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2737  </front>
2738  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2739  <x:source href="p7-auth.xml" basename="p7-auth"/>
2740</reference>
2741
2742<reference anchor="RFC2047">
2743  <front>
2744    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2745    <author initials="K." surname="Moore" fullname="Keith Moore">
2746      <organization>University of Tennessee</organization>
2747      <address><email>moore@cs.utk.edu</email></address>
2748    </author>
2749    <date month="November" year="1996"/>
2750  </front>
2751  <seriesInfo name="RFC" value="2047"/>
2752</reference>
2753
2754<reference anchor="RFC2119">
2755  <front>
2756    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2757    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2758      <organization>Harvard University</organization>
2759      <address><email>sob@harvard.edu</email></address>
2760    </author>
2761    <date month="March" year="1997"/>
2762  </front>
2763  <seriesInfo name="BCP" value="14"/>
2764  <seriesInfo name="RFC" value="2119"/>
2765</reference>
2766
2767</references>
2768
2769<references title="Informative References">
2770
2771<reference anchor="RFC1305">
2772  <front>
2773    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2774    <author initials="D." surname="Mills" fullname="David L. Mills">
2775      <organization>University of Delaware, Electrical Engineering Department</organization>
2776      <address><email>mills@udel.edu</email></address>
2777    </author>
2778    <date month="March" year="1992"/>
2779  </front>
2780  <seriesInfo name="RFC" value="1305"/>
2781</reference>
2782
2783<reference anchor="RFC2616">
2784  <front>
2785    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2786    <author initials="R." surname="Fielding" fullname="R. Fielding">
2787      <organization>University of California, Irvine</organization>
2788      <address><email>fielding@ics.uci.edu</email></address>
2789    </author>
2790    <author initials="J." surname="Gettys" fullname="J. Gettys">
2791      <organization>W3C</organization>
2792      <address><email>jg@w3.org</email></address>
2793    </author>
2794    <author initials="J." surname="Mogul" fullname="J. Mogul">
2795      <organization>Compaq Computer Corporation</organization>
2796      <address><email>mogul@wrl.dec.com</email></address>
2797    </author>
2798    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2799      <organization>MIT Laboratory for Computer Science</organization>
2800      <address><email>frystyk@w3.org</email></address>
2801    </author>
2802    <author initials="L." surname="Masinter" fullname="L. Masinter">
2803      <organization>Xerox Corporation</organization>
2804      <address><email>masinter@parc.xerox.com</email></address>
2805    </author>
2806    <author initials="P." surname="Leach" fullname="P. Leach">
2807      <organization>Microsoft Corporation</organization>
2808      <address><email>paulle@microsoft.com</email></address>
2809    </author>
2810    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2811      <organization>W3C</organization>
2812      <address><email>timbl@w3.org</email></address>
2813    </author>
2814    <date month="June" year="1999"/>
2815  </front>
2816  <seriesInfo name="RFC" value="2616"/>
2817</reference>
2818
2819</references>
2820
2821<section title="Compatibility with Previous Versions" anchor="compatibility">
2822
2823<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2824<t>
2825   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2826   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2827   <xref target="header.cache-control" format="counter"/>,
2828   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2829</t>
2830<t>
2831   Transfer-coding and message lengths all interact in ways that
2832   required fixing exactly when chunked encoding is used (to allow for
2833   transfer encoding that may not be self delimiting); it was important
2834   to straighten out exactly how message lengths are computed.
2835   (<xref target="non-modifiable.headers"/>,
2836   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2837</t>
2838<t>
2839   Proxies should be able to add Content-Length when appropriate.
2840   (<xref target="non-modifiable.headers"/>)
2841</t>
2842<t>
2843   Range request responses would become very verbose if all meta-data
2844   were always returned; by allowing the server to only send needed
2845   headers in a 206 response, this problem can be avoided.
2846   (<xref target="combining.headers"/>)
2847</t>
2848<t>
2849   The Cache-Control: max-age directive was not properly defined for
2850   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2851</t>
2852<t>
2853   Warnings could be cached incorrectly, or not updated appropriately.
2854   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2855   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2856   and <xref target="header.warning" format="counter"/>) Warning
2857   also needed to be a general header, as PUT or other methods may have
2858   need for it in requests.
2859</t>
2860</section>
2861
2862<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2863<t>
2864  Clarify denial of service attack avoidance requirement.
2865  (<xref target="invalidation.after.updates.or.deletions"/>)
2866</t>
2867</section>
2868
2869</section>
2870
2871<section title="Change Log (to be removed by RFC Editor before publication)">
2872
2873<section title="Since RFC2616">
2874<t>
2875  Extracted relevant partitions from <xref target="RFC2616"/>.
2876</t>
2877</section>
2878
2879<section title="Since draft-ietf-httpbis-p6-cache-00">
2880<t>
2881  Closed issues:
2882  <list style="symbols"> 
2883    <t>
2884      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
2885      "Trailer"
2886      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
2887    </t>
2888    <t>
2889      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
2890      "Invalidation after Update or Delete"
2891      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
2892    </t>
2893    <t>
2894      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2895      "Normative and Informative references"
2896    </t>
2897    <t>
2898      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
2899      "Date reference typo"
2900    </t>
2901    <t>
2902      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
2903      "Connection header text"
2904    </t>
2905    <t>
2906      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2907      "Informative references"
2908    </t>
2909    <t>
2910      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2911      "ISO-8859-1 Reference"
2912    </t>
2913    <t>
2914      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2915      "Normative up-to-date references"
2916    </t>
2917    <t>
2918      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
2919      "typo in 13.2.2"
2920    </t>
2921  </list>
2922</t>
2923</section>
2924
2925</section>
2926
2927</back>
2928</rfc>
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