source: draft-ietf-httpbis/latest/p6-cache.xml @ 100

Last change on this file since 100 was 100, checked in by fielding@…, 12 years ago

Second part of resolution for #48: Date reference typo

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1<?xml version="1.0" encoding="utf-8"?>
2<!DOCTYPE rfc [
3  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
4  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
5  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
6  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
7  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
8  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
9  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
10  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
11  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
12  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
13  <!ENTITY ID-VERSION "latest">
14  <!ENTITY ID-MONTH "December">
15  <!ENTITY ID-YEAR "2007">
16  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
17  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#entity.tags' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY weak-and-strong-validators  "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29]>
30<?rfc toc="yes" ?>
31<?rfc symrefs="yes" ?>
32<?rfc sortrefs="yes" ?>
33<?rfc compact="yes"?>
34<?rfc subcompact="no" ?>
35<?rfc linkmailto="no" ?>
36<?rfc editing="no" ?>
37<?rfc-ext allow-markup-in-artwork="yes" ?>
38<?rfc-ext include-references-in-index="yes" ?>
39<rfc obsoletes="2068, 2616" category="std"
40     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
41     xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit">
42<front>
43
44  <title abbrev="HTTP/1.1, part 6">HTTP/1.1, part 6: Caching</title>
45
46  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
47    <organization abbrev="Day Software">Day Software</organization>
48    <address>
49      <postal>
50        <street>23 Corporate Plaza DR, Suite 280</street>
51        <city>Newport Beach</city>
52        <region>CA</region>
53        <code>92660</code>
54        <country>USA</country>
55      </postal>
56      <phone>+1-949-706-5300</phone>
57      <facsimile>+1-949-706-5305</facsimile>
58      <email>fielding@gbiv.com</email>
59      <uri>http://roy.gbiv.com/</uri>
60    </address>
61  </author>
62
63  <author initials="J." surname="Gettys" fullname="Jim Gettys">
64    <organization>One Laptop per Child</organization>
65    <address>
66      <postal>
67        <street>21 Oak Knoll Road</street>
68        <city>Carlisle</city>
69        <region>MA</region>
70        <code>01741</code>
71        <country>USA</country>
72      </postal>
73      <email>jg@laptop.org</email>
74      <uri>http://www.laptop.org/</uri>
75    </address>
76  </author>
77 
78  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
79    <organization abbrev="HP">Hewlett-Packard Company</organization>
80    <address>
81      <postal>
82        <street>HP Labs, Large Scale Systems Group</street>
83        <street>1501 Page Mill Road, MS 1177</street>
84        <city>Palo Alto</city>
85        <region>CA</region>
86        <code>94304</code>
87        <country>USA</country>
88      </postal>
89      <email>JeffMogul@acm.org</email>
90    </address>
91  </author>
92
93  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
94    <organization abbrev="Microsoft">Microsoft Corporation</organization>
95    <address>
96      <postal>
97        <street>1 Microsoft Way</street>
98        <city>Redmond</city>
99        <region>WA</region>
100        <code>98052</code>
101        <country>USA</country>
102      </postal>
103      <email>henrikn@microsoft.com</email>
104    </address>
105  </author>
106
107  <author initials="L." surname="Masinter" fullname="Larry Masinter">
108    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
109    <address>
110      <postal>
111        <street>345 Park Ave</street>
112        <city>San Jose</city>
113        <region>CA</region>
114        <code>95110</code>
115        <country>USA</country>
116      </postal>
117      <email>LMM@acm.org</email>
118      <uri>http://larry.masinter.net/</uri>
119    </address>
120  </author>
121 
122  <author initials="P." surname="Leach" fullname="Paul J. Leach">
123    <organization abbrev="Microsoft">Microsoft Corporation</organization>
124    <address>
125      <postal>
126        <street>1 Microsoft Way</street>
127        <city>Redmond</city>
128        <region>WA</region>
129        <code>98052</code>
130      </postal>
131      <email>paulle@microsoft.com</email>
132    </address>
133  </author>
134   
135  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
136    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
137    <address>
138      <postal>
139        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
140        <street>The Stata Center, Building 32</street>
141        <street>32 Vassar Street</street>
142        <city>Cambridge</city>
143        <region>MA</region>
144        <code>02139</code>
145        <country>USA</country>
146      </postal>
147      <email>timbl@w3.org</email>
148      <uri>http://www.w3.org/People/Berners-Lee/</uri>
149    </address>
150  </author>
151
152  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
153    <organization abbrev="W3C">World Wide Web Consortium</organization>
154    <address>
155      <postal>
156        <street>W3C / ERCIM</street>
157        <street>2004, rte des Lucioles</street>
158        <city>Sophia-Antipolis</city>
159        <region>AM</region>
160        <code>06902</code>
161        <country>France</country>
162      </postal>
163      <email>ylafon@w3.org</email>
164      <uri>http://www.raubacapeu.net/people/yves/</uri>
165    </address>
166  </author>
167
168  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
169    <organization abbrev="greenbytes">greenbytes GmbH</organization>
170    <address>
171      <postal>
172        <street>Hafenweg 16</street>
173        <city>Muenster</city><region>NW</region><code>48155</code>
174        <country>Germany</country>
175      </postal>
176      <phone>+49 251 2807760</phone>   
177      <facsimile>+49 251 2807761</facsimile>   
178      <email>julian.reschke@greenbytes.de</email>       
179      <uri>http://greenbytes.de/tech/webdav/</uri>     
180    </address>
181  </author>
182
183  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
184
185<abstract>
186<t>
187   The Hypertext Transfer Protocol (HTTP) is an application-level
188   protocol for distributed, collaborative, hypermedia information
189   systems. HTTP has been in use by the World Wide Web global information
190   initiative since 1990. This document is Part 6 of the seven-part specification
191   that defines the protocol referred to as "HTTP/1.1" and, taken together,
192   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
193   and the associated header fields that control cache behavior or indicate
194   cacheable response messages.
195</t>
196</abstract>
197
198<note title="Editorial Note (To be removed by RFC Editor)">
199  <t>
200    This version of the HTTP specification contains only minimal editorial
201    changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
202    and authors' addresses).  All other changes are due to partitioning the
203    original into seven mostly independent parts.  The intent is for readers
204    of future drafts to able to use draft 00 as the basis for comparison
205    when the WG makes later changes to the specification text.  This draft
206    will shortly be followed by draft 01 (containing the first round of changes
207    that have already been agreed to on the mailing list). There is no point in
208    reviewing this draft other than to verify that the partitioning has been
209    done correctly.  Roy T. Fielding, Yves Lafon, and Julian Reschke
210    will be the editors after draft 00 is submitted.
211  </t>
212  <t>
213    Discussion of this draft should take place on the HTTPBIS working group
214    mailing list (ietf-http-wg@w3.org). The current issues list is
215    at <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
216    and related documents (including fancy diffs) can be found at
217    <eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
218  </t>
219</note>
220</front>
221<middle>
222<section title="Introduction" anchor="introduction">
223<t>
224   This document will define aspects of HTTP related to caching response
225   messages.  Right now it only includes the extracted relevant sections
226   of <xref target="RFC2616" x:fmt="none">RFC 2616</xref> without edit.
227</t>
228
229<section title="Requirements" anchor="intro.requirements">
230<t>
231   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
232   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
233   document are to be interpreted as described in <xref target="RFC2119"/>.
234</t>
235<t>
236   An implementation is not compliant if it fails to satisfy one or more
237   of the &MUST; or &REQUIRED; level requirements for the protocols it
238   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
239   level and all the &SHOULD; level requirements for its protocols is said
240   to be "unconditionally compliant"; one that satisfies all the &MUST;
241   level requirements but not all the &SHOULD; level requirements for its
242   protocols is said to be "conditionally compliant."
243</t>
244</section>
245
246<section title="Terminology" anchor="intro.terminology">
247<t>
248   This specification uses a number of terms to refer to the roles
249   played by participants in, and objects of, the HTTP communication.
250</t>
251<t>
252  <iref item="cache"/>
253  <x:dfn>cache</x:dfn>
254  <list>
255    <t>
256      A program's local store of response messages and the subsystem
257      that controls its message storage, retrieval, and deletion. A
258      cache stores cacheable responses in order to reduce the response
259      time and network bandwidth consumption on future, equivalent
260      requests. Any client or server may include a cache, though a cache
261      cannot be used by a server that is acting as a tunnel.
262    </t>
263  </list>
264</t>
265<t>
266  <iref item="cacheable"/>
267  <x:dfn>cacheable</x:dfn>
268  <list>
269    <t>
270      A response is cacheable if a cache is allowed to store a copy of
271      the response message for use in answering subsequent requests. The
272      rules for determining the cacheability of HTTP responses are
273      defined in <xref target="caching"/>. Even if a resource is cacheable, there may
274      be additional constraints on whether a cache can use the cached
275      copy for a particular request.
276    </t>
277  </list>
278</t>
279<t>
280  <iref item="first-hand"/>
281  <x:dfn>first-hand</x:dfn>
282  <list>
283    <t>
284      A response is first-hand if it comes directly and without
285      unnecessary delay from the origin server, perhaps via one or more
286      proxies. A response is also first-hand if its validity has just
287      been checked directly with the origin server.
288    </t>
289  </list>
290</t>
291<t>
292  <iref item="explicit expiration time"/>
293  <x:dfn>explicit expiration time</x:dfn>
294  <list>
295    <t>
296      The time at which the origin server intends that an entity should
297      no longer be returned by a cache without further validation.
298    </t>
299  </list>
300</t>
301<t>
302  <iref item="heuristic expiration time"/>
303  <x:dfn>heuristic expiration time</x:dfn>
304  <list>
305    <t>
306      An expiration time assigned by a cache when no explicit expiration
307      time is available.
308    </t>
309  </list>
310</t>
311<t>
312  <iref item="age"/>
313  <x:dfn>age</x:dfn>
314  <list>
315    <t>
316      The age of a response is the time since it was sent by, or
317      successfully validated with, the origin server.
318    </t>
319  </list>
320</t>
321<t>
322  <iref item="freshness lifetime"/>
323  <x:dfn>freshness lifetime</x:dfn>
324  <list>
325    <t>
326      The length of time between the generation of a response and its
327      expiration time.
328    </t>
329  </list>
330</t>
331<t>
332  <iref item="fresh"/>
333  <x:dfn>fresh</x:dfn>
334  <list>
335    <t>
336      A response is fresh if its age has not yet exceeded its freshness
337      lifetime.
338    </t>
339  </list>
340</t>
341<t>
342  <iref item="stale"/>
343  <x:dfn>stale</x:dfn>
344  <list>
345    <t>
346      A response is stale if its age has passed its freshness lifetime.
347    </t>
348  </list>
349</t>
350<t>
351  <iref item="semantically transparent"/>
352  <x:dfn>semantically transparent</x:dfn>
353  <list>
354    <t>
355      A cache behaves in a "semantically transparent" manner, with
356      respect to a particular response, when its use affects neither the
357      requesting client nor the origin server, except to improve
358      performance. When a cache is semantically transparent, the client
359      receives exactly the same response (except for hop-by-hop headers)
360      that it would have received had its request been handled directly
361      by the origin server.
362    </t>
363  </list>
364</t>
365<t>
366  <iref item="validator"/>
367  <x:dfn>validator</x:dfn>
368  <list>
369    <t>
370      A protocol element (e.g., an entity tag or a Last-Modified time)
371      that is used to find out whether a cache entry is an equivalent
372      copy of an entity.
373    </t>
374  </list>
375</t>
376</section>
377
378<section title="Delta Seconds" anchor="delta.seconds">
379<t>
380   Some HTTP header fields allow a time value to be specified as an
381   integer number of seconds, represented in decimal, after the time
382   that the message was received.
383</t>
384<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
385    delta-seconds  = 1*DIGIT
386</artwork></figure>
387</section>
388</section>
389
390<section title="Caching in HTTP" anchor="caching">
391<section title="Overview" anchor="caching.overview">
392<t>
393   HTTP is typically used for distributed information systems, where
394   performance can be improved by the use of response caches. The
395   HTTP/1.1 protocol includes a number of elements intended to make
396   caching work as well as possible. Because these elements are
397   inextricable from other aspects of the protocol, and because they
398   interact with each other, it is useful to describe the basic caching
399   design of HTTP separately from the detailed descriptions of methods,
400   headers, response codes, etc.
401</t>
402<t>
403   Caching would be useless if it did not significantly improve
404   performance. The goal of caching in HTTP/1.1 is to eliminate the need
405   to send requests in many cases, and to eliminate the need to send
406   full responses in many other cases. The former reduces the number of
407   network round-trips required for many operations; we use an
408   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
409   latter reduces network bandwidth requirements; we use a "validation"
410   mechanism for this purpose (see <xref target="validation.model"/>).
411</t>
412<t>
413   Requirements for performance, availability, and disconnected
414   operation require us to be able to relax the goal of semantic
415   transparency. The HTTP/1.1 protocol allows origin servers, caches,
416   and clients to explicitly reduce transparency when necessary.
417   However, because non-transparent operation may confuse non-expert
418   users, and might be incompatible with certain server applications
419   (such as those for ordering merchandise), the protocol requires that
420   transparency be relaxed
421  <list style="symbols">
422     <t>only by an explicit protocol-level request when relaxed by
423        client or origin server</t>
424
425     <t>only with an explicit warning to the end user when relaxed by
426        cache or client</t>
427  </list>
428</t>
429<t>
430   Therefore, the HTTP/1.1 protocol provides these important elements:
431  <list style="numbers">
432      <t>Protocol features that provide full semantic transparency when
433         this is required by all parties.</t>
434
435      <t>Protocol features that allow an origin server or user agent to
436         explicitly request and control non-transparent operation.</t>
437
438      <t>Protocol features that allow a cache to attach warnings to
439         responses that do not preserve the requested approximation of
440         semantic transparency.</t>
441  </list>
442</t>
443<t>
444   A basic principle is that it must be possible for the clients to
445   detect any potential relaxation of semantic transparency.
446  <list><t>
447      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
448      design decisions not explicitly discussed in this specification.
449      If a decision might affect semantic transparency, the implementor
450      ought to err on the side of maintaining transparency unless a
451      careful and complete analysis shows significant benefits in
452      breaking transparency.
453    </t></list>
454</t>
455
456<section title="Cache Correctness" anchor="cache.correctness">
457<t>
458   A correct cache &MUST; respond to a request with the most up-to-date
459   response held by the cache that is appropriate to the request (see
460   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
461   <xref target="disambiguating.multiple.responses" format="counter"/>,
462   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
463   conditions:
464  <list style="numbers">
465      <t>It has been checked for equivalence with what the origin server
466         would have returned by revalidating the response with the
467         origin server (<xref target="validation.model"/>);</t>
468
469      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
470         this means it meets the least restrictive freshness requirement
471         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
472         the origin server so specifies, it is the freshness requirement
473         of the origin server alone.
474
475         If a stored response is not "fresh enough" by the most
476         restrictive freshness requirement of both the client and the
477         origin server, in carefully considered circumstances the cache
478         &MAY; still return the response with the appropriate Warning
479         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
480         and <xref target="header.warning" format="counter"/>), unless such a response
481         is prohibited (e.g., by a "no-store" cache-directive, or by a
482         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
483
484      <t>It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
485         or error (4xx or 5xx) response message.</t>
486  </list>
487</t>
488<t>
489   If the cache can not communicate with the origin server, then a
490   correct cache &SHOULD; respond as above if the response can be
491   correctly served from the cache; if not it &MUST; return an error or
492   warning indicating that there was a communication failure.
493</t>
494<t>
495   If a cache receives a response (either an entire response, or a 304
496   (Not Modified) response) that it would normally forward to the
497   requesting client, and the received response is no longer fresh, the
498   cache &SHOULD; forward it to the requesting client without adding a new
499   Warning (but without removing any existing Warning headers). A cache
500   &SHOULD-NOT;  attempt to revalidate a response simply because that
501   response became stale in transit; this might lead to an infinite
502   loop. A user agent that receives a stale response without a Warning
503   &MAY; display a warning indication to the user.
504</t>
505</section>
506
507<section title="Warnings" anchor="warnings">
508<t>
509   Whenever a cache returns a response that is neither first-hand nor
510   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
511   &MUST; attach a warning to that effect, using a Warning general-header.
512   The Warning header and the currently defined warnings are described
513   in <xref target="header.warning"/>. The warning allows clients to take appropriate
514   action.
515</t>
516<t>
517   Warnings &MAY; be used for other purposes, both cache-related and
518   otherwise. The use of a warning, rather than an error status code,
519   distinguish these responses from true failures.
520</t>
521<t>
522   Warnings are assigned three digit warn-codes. The first digit
523   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
524   stored cache entry after a successful revalidation:
525</t>
526<t>
527  <list style="hanging">
528    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
529     the response, and so &MUST; be deleted after a successful
530     revalidation. 1XX warn-codes &MAY; be generated by a cache only when
531     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
532
533    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
534     headers that is not rectified by a revalidation (for example, a
535     lossy compression of the entity bodies) and which &MUST-NOT; be
536     deleted after a successful revalidation.</t>
537    </list>
538</t>
539<t>
540   See <xref target="header.warning"/> for the definitions of the codes themselves.
541</t>
542<t>
543   HTTP/1.0 caches will cache all Warnings in responses, without
544   deleting the ones in the first category. Warnings in responses that
545   are passed to HTTP/1.0 caches carry an extra warning-date field,
546   which prevents a future HTTP/1.1 recipient from believing an
547   erroneously cached Warning.
548</t>
549<t>
550   Warnings also carry a warning text. The text &MAY; be in any
551   appropriate natural language (perhaps based on the client's Accept
552   headers), and include an &OPTIONAL; indication of what character set is
553   used.
554</t>
555<t>
556   Multiple warnings &MAY; be attached to a response (either by the origin
557   server or by a cache), including multiple warnings with the same code
558   number. For example, a server might provide the same warning with
559   texts in both English and Basque.
560</t>
561<t>
562   When multiple warnings are attached to a response, it might not be
563   practical or reasonable to display all of them to the user. This
564   version of HTTP does not specify strict priority rules for deciding
565   which warnings to display and in what order, but does suggest some
566   heuristics.
567</t>
568</section>
569
570<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
571<t>
572   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
573   times and validators) are implicit directives to caches. In some
574   cases, a server or client might need to provide explicit directives
575   to the HTTP caches. We use the Cache-Control header for this purpose.
576</t>
577<t>
578   The Cache-Control header allows a client or server to transmit a
579   variety of directives in either requests or responses. These
580   directives typically override the default caching algorithms. As a
581   general rule, if there is any apparent conflict between header
582   values, the most restrictive interpretation is applied (that is, the
583   one that is most likely to preserve semantic transparency). However,
584   in some cases, cache-control directives are explicitly specified as
585   weakening the approximation of semantic transparency (for example,
586   "max-stale" or "public").
587</t>
588<t>
589   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
590</t>
591</section>
592
593<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
594<t>
595   Many user agents make it possible for users to override the basic
596   caching mechanisms. For example, the user agent might allow the user
597   to specify that cached entities (even explicitly stale ones) are
598   never validated. Or the user agent might habitually add "Cache-Control:
599   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
600   default to either non-transparent behavior, or behavior that results
601   in abnormally ineffective caching, but &MAY; be explicitly configured
602   to do so by an explicit action of the user.
603</t>
604<t>
605   If the user has overridden the basic caching mechanisms, the user
606   agent &SHOULD; explicitly indicate to the user whenever this results in
607   the display of information that might not meet the server's
608   transparency requirements (in particular, if the displayed entity is
609   known to be stale). Since the protocol normally allows the user agent
610   to determine if responses are stale or not, this indication need only
611   be displayed when this actually happens. The indication need not be a
612   dialog box; it could be an icon (for example, a picture of a rotting
613   fish) or some other indicator.
614</t>
615<t>
616   If the user has overridden the caching mechanisms in a way that would
617   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
618   continually indicate this state to the user (for example, by a
619   display of a picture of currency in flames) so that the user does not
620   inadvertently consume excess resources or suffer from excessive
621   latency.
622</t>
623</section>
624
625<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
626<t>
627   In some cases, the operator of a cache &MAY; choose to configure it to
628   return stale responses even when not requested by clients. This
629   decision ought not be made lightly, but may be necessary for reasons
630   of availability or performance, especially when the cache is poorly
631   connected to the origin server. Whenever a cache returns a stale
632   response, it &MUST; mark it as such (using a Warning header) enabling
633   the client software to alert the user that there might be a potential
634   problem.
635</t>
636<t>
637   It also allows the user agent to take steps to obtain a first-hand or
638   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
639   response if the client explicitly requests a first-hand or fresh one,
640   unless it is impossible to comply for technical or policy reasons.
641</t>
642</section>
643
644<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
645<t>
646   While the origin server (and to a lesser extent, intermediate caches,
647   by their contribution to the age of a response) are the primary
648   source of expiration information, in some cases the client might need
649   to control a cache's decision about whether to return a cached
650   response without validating it. Clients do this using several
651   directives of the Cache-Control header.
652</t>
653<t>
654   A client's request &MAY; specify the maximum age it is willing to
655   accept of an unvalidated response; specifying a value of zero forces
656   the cache(s) to revalidate all responses. A client &MAY; also specify
657   the minimum time remaining before a response expires. Both of these
658   options increase constraints on the behavior of caches, and so cannot
659   further relax the cache's approximation of semantic transparency.
660</t>
661<t>
662   A client &MAY; also specify that it will accept stale responses, up to
663   some maximum amount of staleness. This loosens the constraints on the
664   caches, and so might violate the origin server's specified
665   constraints on semantic transparency, but might be necessary to
666   support disconnected operation, or high availability in the face of
667   poor connectivity.
668</t>
669</section>
670</section>
671
672<section title="Expiration Model" anchor="expiration.model">
673
674<section title="Server-Specified Expiration" anchor="server-specified.expiration">
675<t>
676   HTTP caching works best when caches can entirely avoid making
677   requests to the origin server. The primary mechanism for avoiding
678   requests is for an origin server to provide an explicit expiration
679   time in the future, indicating that a response &MAY; be used to satisfy
680   subsequent requests. In other words, a cache can return a fresh
681   response without first contacting the server.
682</t>
683<t>
684   Our expectation is that servers will assign future explicit
685   expiration times to responses in the belief that the entity is not
686   likely to change, in a semantically significant way, before the
687   expiration time is reached. This normally preserves semantic
688   transparency, as long as the server's expiration times are carefully
689   chosen.
690</t>
691<t>
692   The expiration mechanism applies only to responses taken from a cache
693   and not to first-hand responses forwarded immediately to the
694   requesting client.
695</t>
696<t>
697   If an origin server wishes to force a semantically transparent cache
698   to validate every request, it &MAY; assign an explicit expiration time
699   in the past. This means that the response is always stale, and so the
700   cache &SHOULD; validate it before using it for subsequent requests. See
701   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
702</t>
703<t>
704   If an origin server wishes to force any HTTP/1.1 cache, no matter how
705   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
706   cache-control directive (see <xref target="header.cache-control"/>).
707</t>
708<t>
709   Servers specify explicit expiration times using either the Expires
710   header, or the max-age directive of the Cache-Control header.
711</t>
712<t>
713   An expiration time cannot be used to force a user agent to refresh
714   its display or reload a resource; its semantics apply only to caching
715   mechanisms, and such mechanisms need only check a resource's
716   expiration status when a new request for that resource is initiated.
717   See <xref target="history.lists"/> for an explanation of the difference between caches
718   and history mechanisms.
719</t>
720</section>
721
722<section title="Heuristic Expiration" anchor="heuristic.expiration">
723<t>
724   Since origin servers do not always provide explicit expiration times,
725   HTTP caches typically assign heuristic expiration times, employing
726   algorithms that use other header values (such as the Last-Modified
727   time) to estimate a plausible expiration time. The HTTP/1.1
728   specification does not provide specific algorithms, but does impose
729   worst-case constraints on their results. Since heuristic expiration
730   times might compromise semantic transparency, they ought to be used
731   cautiously, and we encourage origin servers to provide explicit
732   expiration times as much as possible.
733</t>
734</section>
735
736<section title="Age Calculations" anchor="age.calculations">
737<t>
738   In order to know if a cached entry is fresh, a cache needs to know if
739   its age exceeds its freshness lifetime. We discuss how to calculate
740   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
741   the age of a response or cache entry.
742</t>
743<t>
744   In this discussion, we use the term "now" to mean "the current value
745   of the clock at the host performing the calculation." Hosts that use
746   HTTP, but especially hosts running origin servers and caches, &SHOULD;
747   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
748   a globally accurate time standard.
749</t>
750<t>
751   HTTP/1.1 requires origin servers to send a Date header, if possible,
752   with every response, giving the time at which the response was
753   generated (see &header-date;). We use the term "date_value" to denote
754   the value of the Date header, in a form appropriate for arithmetic
755   operations.
756</t>
757<t>
758   HTTP/1.1 uses the Age response-header to convey the estimated age of
759   the response message when obtained from a cache. The Age field value
760   is the cache's estimate of the amount of time since the response was
761   generated or revalidated by the origin server.
762</t>
763<t>
764   In essence, the Age value is the sum of the time that the response
765   has been resident in each of the caches along the path from the
766   origin server, plus the amount of time it has been in transit along
767   network paths.
768</t>
769<t>
770   We use the term "age_value" to denote the value of the Age header, in
771   a form appropriate for arithmetic operations.
772</t>
773<t>
774   A response's age can be calculated in two entirely independent ways:
775  <list style="numbers">
776      <t>now minus date_value, if the local clock is reasonably well
777         synchronized to the origin server's clock. If the result is
778         negative, the result is replaced by zero.</t>
779
780      <t>age_value, if all of the caches along the response path
781         implement HTTP/1.1.</t>
782  </list>
783</t>
784<t>
785   Given that we have two independent ways to compute the age of a
786   response when it is received, we can combine these as
787</t>
788<figure><artwork type="code">
789    corrected_received_age = max(now - date_value, age_value)
790</artwork></figure>
791<t>
792   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
793   paths, one gets a reliable (conservative) result.
794</t>
795<t>
796   Because of network-imposed delays, some significant interval might
797   pass between the time that a server generates a response and the time
798   it is received at the next outbound cache or client. If uncorrected,
799   this delay could result in improperly low ages.
800</t>
801<t>
802   Because the request that resulted in the returned Age value must have
803   been initiated prior to that Age value's generation, we can correct
804   for delays imposed by the network by recording the time at which the
805   request was initiated. Then, when an Age value is received, it &MUST;
806   be interpreted relative to the time the request was initiated, not
807   the time that the response was received. This algorithm results in
808   conservative behavior no matter how much delay is experienced. So, we
809   compute:
810</t>
811<figure><artwork type="code">
812   corrected_initial_age = corrected_received_age
813                         + (now - request_time)
814</artwork></figure>
815<t>
816   where "request_time" is the time (according to the local clock) when
817   the request that elicited this response was sent.
818</t>
819<t>
820   Summary of age calculation algorithm, when a cache receives a
821   response:
822</t>
823<figure><artwork type="code">
824   /*
825    * age_value
826    *      is the value of Age: header received by the cache with
827    *              this response.
828    * date_value
829    *      is the value of the origin server's Date: header
830    * request_time
831    *      is the (local) time when the cache made the request
832    *              that resulted in this cached response
833    * response_time
834    *      is the (local) time when the cache received the
835    *              response
836    * now
837    *      is the current (local) time
838    */
839
840   apparent_age = max(0, response_time - date_value);
841   corrected_received_age = max(apparent_age, age_value);
842   response_delay = response_time - request_time;
843   corrected_initial_age = corrected_received_age + response_delay;
844   resident_time = now - response_time;
845   current_age   = corrected_initial_age + resident_time;
846</artwork></figure>
847<t>
848   The current_age of a cache entry is calculated by adding the amount
849   of time (in seconds) since the cache entry was last validated by the
850   origin server to the corrected_initial_age. When a response is
851   generated from a cache entry, the cache &MUST; include a single Age
852   header field in the response with a value equal to the cache entry's
853   current_age.
854</t>
855<t>
856   The presence of an Age header field in a response implies that a
857   response is not first-hand. However, the converse is not true, since
858   the lack of an Age header field in a response does not imply that the
859   response is first-hand unless all caches along the request path are
860   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
861   the Age header field).
862</t>
863</section>
864
865<section title="Expiration Calculations" anchor="expiration.calculations">
866<t>
867   In order to decide whether a response is fresh or stale, we need to
868   compare its freshness lifetime to its age. The age is calculated as
869   described in <xref target="age.calculations"/>; this section describes how to calculate
870   the freshness lifetime, and to determine if a response has expired.
871   In the discussion below, the values can be represented in any form
872   appropriate for arithmetic operations.
873</t>
874<t>
875   We use the term "expires_value" to denote the value of the Expires
876   header. We use the term "max_age_value" to denote an appropriate
877   value of the number of seconds carried by the "max-age" directive of
878   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
879</t>
880<t>
881   The max-age directive takes priority over Expires, so if max-age is
882   present in a response, the calculation is simply:
883</t>
884<figure><artwork type="code">
885   freshness_lifetime = max_age_value
886</artwork></figure>
887<t>
888   Otherwise, if Expires is present in the response, the calculation is:
889</t>
890<figure><artwork type="code">
891   freshness_lifetime = expires_value - date_value
892</artwork></figure>
893<t>
894   Note that neither of these calculations is vulnerable to clock skew,
895   since all of the information comes from the origin server.
896</t>
897<t>
898   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
899   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
900   does not include other restrictions on caching, the cache &MAY; compute
901   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
902   113 to any response whose age is more than 24 hours if such warning
903   has not already been added.
904</t>
905<t>
906   Also, if the response does have a Last-Modified time, the heuristic
907   expiration value &SHOULD; be no more than some fraction of the interval
908   since that time. A typical setting of this fraction might be 10%.
909</t>
910<t>
911   The calculation to determine if a response has expired is quite
912   simple:
913</t>
914<figure><artwork type="code">
915   response_is_fresh = (freshness_lifetime &gt; current_age)
916</artwork></figure>
917</section>
918
919<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
920<t>
921   Because expiration values are assigned optimistically, it is possible
922   for two caches to contain fresh values for the same resource that are
923   different.
924</t>
925<t>
926   If a client performing a retrieval receives a non-first-hand response
927   for a request that was already fresh in its own cache, and the Date
928   header in its existing cache entry is newer than the Date on the new
929   response, then the client &MAY; ignore the response. If so, it &MAY;
930   retry the request with a "Cache-Control: max-age=0" directive (see
931   <xref target="header.cache-control"/>), to force a check with the origin server.
932</t>
933<t>
934   If a cache has two fresh responses for the same representation with
935   different validators, it &MUST; use the one with the more recent Date
936   header. This situation might arise because the cache is pooling
937   responses from other caches, or because a client has asked for a
938   reload or a revalidation of an apparently fresh cache entry.
939</t>
940</section>
941
942<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
943<t>
944   Because a client might be receiving responses via multiple paths, so
945   that some responses flow through one set of caches and other
946   responses flow through a different set of caches, a client might
947   receive responses in an order different from that in which the origin
948   server sent them. We would like the client to use the most recently
949   generated response, even if older responses are still apparently
950   fresh.
951</t>
952<t>
953   Neither the entity tag nor the expiration value can impose an
954   ordering on responses, since it is possible that a later response
955   intentionally carries an earlier expiration time. The Date values are
956   ordered to a granularity of one second.
957</t>
958<t>
959   When a client tries to revalidate a cache entry, and the response it
960   receives contains a Date header that appears to be older than the one
961   for the existing entry, then the client &SHOULD; repeat the request
962   unconditionally, and include
963</t>
964<figure><artwork type="example">
965    Cache-Control: max-age=0
966</artwork></figure>
967<t>
968   to force any intermediate caches to validate their copies directly
969   with the origin server, or
970</t>
971<figure><artwork type="example">
972    Cache-Control: no-cache
973</artwork></figure>
974<t>
975   to force any intermediate caches to obtain a new copy from the origin
976   server.
977</t>
978<t>
979   If the Date values are equal, then the client &MAY; use either response
980   (or &MAY;, if it is being extremely prudent, request a new response).
981   Servers &MUST-NOT; depend on clients being able to choose
982   deterministically between responses generated during the same second,
983   if their expiration times overlap.
984</t>
985</section>
986</section>
987
988<section title="Validation Model" anchor="validation.model">
989<t>
990   When a cache has a stale entry that it would like to use as a
991   response to a client's request, it first has to check with the origin
992   server (or possibly an intermediate cache with a fresh response) to
993   see if its cached entry is still usable. We call this "validating"
994   the cache entry. Since we do not want to have to pay the overhead of
995   retransmitting the full response if the cached entry is good, and we
996   do not want to pay the overhead of an extra round trip if the cached
997   entry is invalid, the HTTP/1.1 protocol supports the use of
998   conditional methods.
999</t>
1000<t>
1001   The key protocol features for supporting conditional methods are
1002   those concerned with "cache validators." When an origin server
1003   generates a full response, it attaches some sort of validator to it,
1004   which is kept with the cache entry. When a client (user agent or
1005   proxy cache) makes a conditional request for a resource for which it
1006   has a cache entry, it includes the associated validator in the
1007   request.
1008</t>
1009<t>
1010   The server then checks that validator against the current validator
1011   for the entity, and, if they match (see &weak-and-strong-validators;), it responds
1012   with a special status code (usually, 304 (Not Modified)) and no
1013   entity-body. Otherwise, it returns a full response (including
1014   entity-body). Thus, we avoid transmitting the full response if the
1015   validator matches, and we avoid an extra round trip if it does not
1016   match.
1017</t>
1018<t>
1019   In HTTP/1.1, a conditional request looks exactly the same as a normal
1020   request for the same resource, except that it carries a special
1021   header (which includes the validator) that implicitly turns the
1022   method (usually, GET) into a conditional.
1023</t>
1024<t>
1025   The protocol includes both positive and negative senses of cache-validating
1026   conditions. That is, it is possible to request either that
1027   a method be performed if and only if a validator matches or if and
1028   only if no validators match.
1029  <list><t>
1030      <x:h>Note:</x:h> a response that lacks a validator may still be cached, and
1031      served from cache until it expires, unless this is explicitly
1032      prohibited by a cache-control directive. However, a cache cannot
1033      do a conditional retrieval if it does not have a validator for the
1034      entity, which means it will not be refreshable after it expires.
1035  </t></list>
1036</t>
1037
1038<section title="Last-Modified Dates" anchor="last-modified.dates">
1039<t>
1040   The Last-Modified entity-header field value is often used as a cache
1041   validator. In simple terms, a cache entry is considered to be valid
1042   if the entity has not been modified since the Last-Modified value.
1043</t>
1044</section>
1045
1046<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
1047<t>
1048   The ETag response-header field value, an entity tag, provides for an
1049   "opaque" cache validator. This might allow more reliable validation
1050   in situations where it is inconvenient to store modification dates,
1051   where the one-second resolution of HTTP date values is not
1052   sufficient, or where the origin server wishes to avoid certain
1053   paradoxes that might arise from the use of modification dates.
1054</t>
1055<t>
1056   Entity Tags are described in &entity-tags;.
1057</t>
1058</section>
1059
1060<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
1061<t>
1062   The principle behind entity tags is that only the service author
1063   knows the semantics of a resource well enough to select an
1064   appropriate cache validation mechanism, and the specification of any
1065   validator comparison function more complex than byte-equality would
1066   open up a can of worms. Thus, comparisons of any other headers
1067   (except Last-Modified, for compatibility with HTTP/1.0) are never
1068   used for purposes of validating a cache entry.
1069</t>
1070</section>
1071</section>
1072
1073<section title="Response Cacheability" anchor="response.cacheability">
1074<t>
1075   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1076   directive, a caching system &MAY; always store a successful response
1077   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1078   if it is fresh, and &MAY; return it after successful validation. If
1079   there is neither a cache validator nor an explicit expiration time
1080   associated with a response, we do not expect it to be cached, but
1081   certain caches &MAY; violate this expectation (for example, when little
1082   or no network connectivity is available). A client can usually detect
1083   that such a response was taken from a cache by comparing the Date
1084   header to the current time.
1085  <list><t>
1086      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1087      without providing any Warning.
1088  </t></list>
1089</t>
1090<t>
1091   However, in some cases it might be inappropriate for a cache to
1092   retain an entity, or to return it in response to a subsequent
1093   request. This might be because absolute semantic transparency is
1094   deemed necessary by the service author, or because of security or
1095   privacy considerations. Certain cache-control directives are
1096   therefore provided so that the server can indicate that certain
1097   resource entities, or portions thereof, are not to be cached
1098   regardless of other considerations.
1099</t>
1100<t>
1101   Note that &header-authorization; normally prevents a shared cache from saving
1102   and returning a response to a previous request if that request
1103   included an Authorization header.
1104</t>
1105<t>
1106   A response received with a status code of 200, 203, 206, 300, 301 or
1107   410 &MAY; be stored by a cache and used in reply to a subsequent
1108   request, subject to the expiration mechanism, unless a cache-control
1109   directive prohibits caching. However, a cache that does not support
1110   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1111   Content) responses.
1112</t>
1113<t>
1114   A response received with any other status code (e.g. status codes 302
1115   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1116   unless there are cache-control directives or another header(s) that
1117   explicitly allow it. For example, these include the following: an
1118   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1119   "proxy-revalidate", "public" or "private" cache-control
1120   directive (<xref target="header.cache-control"/>).
1121</t>
1122</section>
1123
1124<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1125<t>
1126   The purpose of an HTTP cache is to store information received in
1127   response to requests for use in responding to future requests. In
1128   many cases, a cache simply returns the appropriate parts of a
1129   response to the requester. However, if the cache holds a cache entry
1130   based on a previous response, it might have to combine parts of a new
1131   response with what is held in the cache entry.
1132</t>
1133
1134<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1135<t>
1136   For the purpose of defining the behavior of caches and non-caching
1137   proxies, we divide HTTP headers into two categories:
1138  <list style="symbols">
1139      <t>End-to-end headers, which are  transmitted to the ultimate
1140        recipient of a request or response. End-to-end headers in
1141        responses &MUST; be stored as part of a cache entry and &MUST; be
1142        transmitted in any response formed from a cache entry.</t>
1143
1144      <t>Hop-by-hop headers, which are meaningful only for a single
1145        transport-level connection, and are not stored by caches or
1146        forwarded by proxies.</t>
1147  </list>
1148</t>
1149<t>
1150   The following HTTP/1.1 headers are hop-by-hop headers:
1151  <list style="symbols">
1152      <t>Connection</t>
1153      <t>Keep-Alive</t>
1154      <t>Proxy-Authenticate</t>
1155      <t>Proxy-Authorization</t>
1156      <t>TE</t>
1157      <t>Trailer</t>
1158      <t>Transfer-Encoding</t>
1159      <t>Upgrade</t>
1160  </list>
1161</t>
1162<t>
1163   All other headers defined by HTTP/1.1 are end-to-end headers.
1164</t>
1165<t>
1166   Other hop-by-hop headers &MUST; be listed in a Connection header
1167   (&header-connection;).
1168</t>
1169</section>
1170
1171<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1172<t>
1173   Some features of the HTTP/1.1 protocol, such as Digest
1174   Authentication, depend on the value of certain end-to-end headers. A
1175   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1176   definition of that header requires or specifically allows that.
1177</t>
1178<t>
1179   A transparent proxy &MUST-NOT; modify any of the following fields in a
1180   request or response, and it &MUST-NOT; add any of these fields if not
1181   already present:
1182  <list style="symbols">
1183      <t>Content-Location</t>
1184      <t>Content-MD5</t>
1185      <t>ETag</t>
1186      <t>Last-Modified</t>
1187  </list>
1188</t>
1189<t>
1190   A transparent proxy &MUST-NOT; modify any of the following fields in a
1191   response:
1192  <list style="symbols">
1193    <t>Expires</t>
1194  </list>
1195</t>
1196<t>
1197   but it &MAY; add any of these fields if not already present. If an
1198   Expires header is added, it &MUST; be given a field-value identical to
1199   that of the Date header in that response.
1200</t>
1201<t>
1202   A  proxy &MUST-NOT; modify or add any of the following fields in a
1203   message that contains the no-transform cache-control directive, or in
1204   any request:
1205  <list style="symbols">
1206    <t>Content-Encoding</t>
1207    <t>Content-Range</t>
1208    <t>Content-Type</t>
1209  </list>
1210</t>
1211<t>
1212   A non-transparent proxy &MAY; modify or add these fields to a message
1213   that does not include no-transform, but if it does so, it &MUST; add a
1214   Warning 214 (Transformation applied) if one does not already appear
1215   in the message (see <xref target="header.warning"/>).
1216  <list><t>
1217      Warning: unnecessary modification of end-to-end headers might
1218      cause authentication failures if stronger authentication
1219      mechanisms are introduced in later versions of HTTP. Such
1220      authentication mechanisms &MAY; rely on the values of header fields
1221      not listed here.
1222    </t></list>
1223</t>
1224<t>
1225   The Content-Length field of a request or response is added or deleted
1226   according to the rules in &message-length;. A transparent proxy &MUST;
1227   preserve the entity-length (&entity-length;) of the entity-body,
1228   although it &MAY; change the transfer-length (&message-length;).
1229</t>
1230</section>
1231
1232<section title="Combining Headers" anchor="combining.headers">
1233<t>
1234   When a cache makes a validating request to a server, and the server
1235   provides a 304 (Not Modified) response or a 206 (Partial Content)
1236   response, the cache then constructs a response to send to the
1237   requesting client.
1238</t>
1239<t>
1240   If the status code is 304 (Not Modified), the cache uses the entity-body
1241   stored in the cache entry as the entity-body of this outgoing
1242   response. If the status code is 206 (Partial Content) and the ETag or
1243   Last-Modified headers match exactly, the cache &MAY; combine the
1244   contents stored in the cache entry with the new contents received in
1245   the response and use the result as the entity-body of this outgoing
1246   response, (see &combining-byte-ranges;).
1247</t>
1248<t>
1249   The end-to-end headers stored in the cache entry are used for the
1250   constructed response, except that
1251  <list style="symbols">
1252    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1253      &MUST; be deleted from the cache entry and the forwarded response.</t>
1254    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1255        in the cache entry and the forwarded response.</t>
1256    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1257        replace the corresponding headers from the cache entry.</t>
1258  </list>
1259</t>
1260<t>
1261   Unless the cache decides to remove the cache entry, it &MUST; also
1262   replace the end-to-end headers stored with the cache entry with
1263   corresponding headers received in the incoming response, except for
1264   Warning headers as described immediately above. If a header field-name
1265   in the incoming response matches more than one header in the
1266   cache entry, all such old headers &MUST; be replaced.
1267</t>
1268<t>
1269   In other words, the set of end-to-end headers received in the
1270   incoming response overrides all corresponding end-to-end headers
1271   stored with the cache entry (except for stored Warning headers with
1272   warn-code 1xx, which are deleted even if not overridden).
1273  <list><t>
1274      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1275      Modified) or a 206 (Partial Content) response to update any header
1276      associated with a previous response for the same entity or sub-ranges
1277      thereof, although it might not always be meaningful or
1278      correct to do so. This rule does not allow an origin server to use
1279      a 304 (Not Modified) or a 206 (Partial Content) response to
1280      entirely delete a header that it had provided with a previous
1281      response.
1282  </t></list>
1283</t>
1284</section>
1285
1286</section>
1287
1288<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1289<t>
1290   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1291   by the presence of a Vary header field in a response, alters the
1292   conditions and procedure by which a cache can use the response for
1293   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1294   field by servers.
1295</t>
1296<t>
1297   A server &SHOULD; use the Vary header field to inform a cache of what
1298   request-header fields were used to select among multiple
1299   representations of a cacheable response subject to server-driven
1300   negotiation. The set of header fields named by the Vary field value
1301   is known as the "selecting" request-headers.
1302</t>
1303<t>
1304   When the cache receives a subsequent request whose Request-URI
1305   specifies one or more cache entries including a Vary header field,
1306   the cache &MUST-NOT; use such a cache entry to construct a response to
1307   the new request unless all of the selecting request-headers present
1308   in the new request match the corresponding stored request-headers in
1309   the original request.
1310</t>
1311<t>
1312   The selecting request-headers from two requests are defined to match
1313   if and only if the selecting request-headers in the first request can
1314   be transformed to the selecting request-headers in the second request
1315   by adding or removing linear white space (LWS) at places where this
1316   is allowed by the corresponding BNF, and/or combining multiple
1317   message-header fields with the same field name following the rules
1318   about message headers in &message-headers;.
1319</t>
1320<t>
1321   A Vary header field-value of "*" always fails to match and subsequent
1322   requests on that resource can only be properly interpreted by the
1323   origin server.
1324</t>
1325<t>
1326   If the selecting request header fields for the cached entry do not
1327   match the selecting request header fields of the new request, then
1328   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1329   it first relays the new request to the origin server in a conditional
1330   request and the server responds with 304 (Not Modified), including an
1331   entity tag or Content-Location that indicates the entity to be used.
1332</t>
1333<t>
1334   If an entity tag was assigned to a cached representation, the
1335   forwarded request &SHOULD; be conditional and include the entity tags
1336   in an If-None-Match header field from all its cache entries for the
1337   resource. This conveys to the server the set of entities currently
1338   held by the cache, so that if any one of these entities matches the
1339   requested entity, the server can use the ETag header field in its 304
1340   (Not Modified) response to tell the cache which entry is appropriate.
1341   If the entity-tag of the new response matches that of an existing
1342   entry, the new response &SHOULD; be used to update the header fields of
1343   the existing entry, and the result &MUST; be returned to the client.
1344</t>
1345<t>
1346   If any of the existing cache entries contains only partial content
1347   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1348   the If-None-Match header field unless the request is for a range that
1349   would be fully satisfied by that entry.
1350</t>
1351<t>
1352   If a cache receives a successful response whose Content-Location
1353   field matches that of an existing cache entry for the same Request-URI,
1354   whose entity-tag differs from that of the existing entry, and
1355   whose Date is more recent than that of the existing entry, the
1356   existing entry &SHOULD-NOT;  be returned in response to future requests
1357   and &SHOULD; be deleted from the cache.
1358</t>
1359</section>
1360
1361<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1362<t>
1363   For reasons of security and privacy, it is necessary to make a
1364   distinction between "shared" and "non-shared" caches. A non-shared
1365   cache is one that is accessible only to a single user. Accessibility
1366   in this case &SHOULD; be enforced by appropriate security mechanisms.
1367   All other caches are considered to be "shared." Other sections of
1368   this specification place certain constraints on the operation of
1369   shared caches in order to prevent loss of privacy or failure of
1370   access controls.
1371</t>
1372</section>
1373
1374<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1375<t>
1376   A cache that receives an incomplete response (for example, with fewer
1377   bytes of data than specified in a Content-Length header) &MAY; store
1378   the response. However, the cache &MUST; treat this as a partial
1379   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1380   the result might be a full response or might still be
1381   partial. A cache &MUST-NOT; return a partial response to a client
1382   without explicitly marking it as such, using the 206 (Partial
1383   Content) status code. A cache &MUST-NOT; return a partial response
1384   using a status code of 200 (OK).
1385</t>
1386<t>
1387   If a cache receives a 5xx response while attempting to revalidate an
1388   entry, it &MAY; either forward this response to the requesting client,
1389   or act as if the server failed to respond. In the latter case, it &MAY;
1390   return a previously received response unless the cached entry
1391   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1392</t>
1393</section>
1394
1395<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1396<t>
1397   Unless the origin server explicitly prohibits the caching of their
1398   responses, the application of GET and HEAD methods to any resources
1399   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1400   these responses are taken from a cache. They &MAY; still have side
1401   effects, but a cache is not required to consider such side effects in
1402   its caching decisions. Caches are always expected to observe an
1403   origin server's explicit restrictions on caching.
1404</t>
1405<t>
1406   We note one exception to this rule: since some applications have
1407   traditionally used GETs and HEADs with query URLs (those containing a
1408   "?" in the rel_path part) to perform operations with significant side
1409   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1410   the server provides an explicit expiration time. This specifically
1411   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1412   be taken from a cache. See &safe-methods; for related information.
1413</t>
1414</section>
1415
1416<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1417<t>
1418   The effect of certain methods performed on a resource at the origin
1419   server might cause one or more existing cache entries to become non-transparently
1420   invalid. That is, although they might continue to be
1421   "fresh," they do not accurately reflect what the origin server would
1422   return for a new request on that resource.
1423</t>
1424<t>
1425   There is no way for the HTTP protocol to guarantee that all such
1426   cache entries are marked invalid. For example, the request that
1427   caused the change at the origin server might not have gone through
1428   the proxy where a cache entry is stored. However, several rules help
1429   reduce the likelihood of erroneous behavior.
1430</t>
1431<t>
1432   In this section, the phrase "invalidate an entity" means that the
1433   cache will either remove all instances of that entity from its
1434   storage, or will mark these as "invalid" and in need of a mandatory
1435   revalidation before they can be returned in response to a subsequent
1436   request.
1437</t>
1438<t>
1439   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1440   either the entity referred to by the Request-URI, or by the Location
1441   or Content-Location headers (if present). These methods are:
1442  <list style="symbols">
1443      <t>PUT</t>
1444      <t>DELETE</t>
1445      <t>POST</t>
1446  </list>
1447</t> 
1448<t>
1449   An invalidation based
1450   on the URI in a Location or Content-Location header &MUST-NOT; be
1451   performed if the host part of that URI differs from the host part
1452   in the Request-URI. This helps prevent denial of service attacks.
1453</t>
1454<t>
1455   A cache that passes through requests for methods it does not
1456   understand &SHOULD; invalidate any entities referred to by the
1457   Request-URI.
1458</t>
1459</section>
1460
1461<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1462<t>
1463   All methods that might be expected to cause modifications to the
1464   origin server's resources &MUST; be written through to the origin
1465   server. This currently includes all methods except for GET and HEAD.
1466   A cache &MUST-NOT; reply to such a request from a client before having
1467   transmitted the request to the inbound server, and having received a
1468   corresponding response from the inbound server. This does not prevent
1469   a proxy cache from sending a 100 (Continue) response before the
1470   inbound server has sent its final reply.
1471</t>
1472<t>
1473   The alternative (known as "write-back" or "copy-back" caching) is not
1474   allowed in HTTP/1.1, due to the difficulty of providing consistent
1475   updates and the problems arising from server, cache, or network
1476   failure prior to write-back.
1477</t>
1478</section>
1479
1480<section title="Cache Replacement" anchor="cache.replacement">
1481<t>
1482   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1483   <xref target="disambiguating.expiration.values" format="counter"/>,
1484   <xref target="disambiguating.multiple.responses" format="counter"/>
1485   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1486   response is received from a resource while any existing responses for
1487   the same resource are cached, the cache &SHOULD; use the new response
1488   to reply to the current request. It &MAY; insert it into cache storage
1489   and &MAY;, if it meets all other requirements, use it to respond to any
1490   future requests that would previously have caused the old response to
1491   be returned. If it inserts the new response into cache storage  the
1492   rules in <xref target="combining.headers"/> apply.
1493  <list><t>
1494      <x:h>Note:</x:h> a new response that has an older Date header value than
1495      existing cached responses is not cacheable.
1496  </t></list>
1497</t>
1498</section>
1499
1500<section title="History Lists" anchor="history.lists">
1501<t>
1502   User agents often have history mechanisms, such as "Back" buttons and
1503   history lists, which can be used to redisplay an entity retrieved
1504   earlier in a session.
1505</t>
1506<t>
1507   History mechanisms and caches are different. In particular history
1508   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1509   the current state of a resource. Rather, a history mechanism is meant
1510   to show exactly what the user saw at the time when the resource was
1511   retrieved.
1512</t>
1513<t>
1514   By default, an expiration time does not apply to history mechanisms.
1515   If the entity is still in storage, a history mechanism &SHOULD; display
1516   it even if the entity has expired, unless the user has specifically
1517   configured the agent to refresh expired history documents.
1518</t>
1519<t>
1520   This is not to be construed to prohibit the history mechanism from
1521   telling the user that a view might be stale.
1522  <list><t>
1523      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1524      viewing stale resources, this will tend to force service authors
1525      to avoid using HTTP expiration controls and cache controls when
1526      they would otherwise like to. Service authors may consider it
1527      important that users not be presented with error messages or
1528      warning messages when they use navigation controls (such as BACK)
1529      to view previously fetched resources. Even though sometimes such
1530      resources ought not to cached, or ought to expire quickly, user
1531      interface considerations may force service authors to resort to
1532      other means of preventing caching (e.g. "once-only" URLs) in order
1533      not to suffer the effects of improperly functioning history
1534      mechanisms.
1535  </t></list>
1536</t>
1537</section>
1538</section>
1539
1540<section title="Header Field Definitions" anchor="header.fields">
1541<t>
1542   This section defines the syntax and semantics of all standard
1543   HTTP/1.1 header fields. For entity-header fields, both sender and
1544   recipient refer to either the client or the server, depending on who
1545   sends and who receives the entity.
1546</t>
1547
1548<section title="Age" anchor="header.age">
1549  <iref primary="true" item="Age header" x:for-anchor=""/>
1550  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1551<t>
1552      The Age response-header field conveys the sender's estimate of the
1553      amount of time since the response (or its revalidation) was
1554      generated at the origin server. A cached response is "fresh" if
1555      its age does not exceed its freshness lifetime. Age values are
1556      calculated as specified in <xref target="age.calculations"/>.
1557</t>
1558<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1559        Age = "Age" ":" age-value
1560        age-value = delta-seconds
1561</artwork></figure>
1562<t>
1563      Age values are non-negative decimal integers, representing time in
1564      seconds.
1565</t>
1566<t>
1567      If a cache receives a value larger than the largest positive
1568      integer it can represent, or if any of its age calculations
1569      overflows, it &MUST; transmit an Age header with a value of
1570      2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
1571      include an Age header field in every response generated from its
1572      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1573      bits of range.
1574</t>
1575</section>
1576
1577<section title="Cache-Control" anchor="header.cache-control">
1578  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1579  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1580<t>
1581   The Cache-Control general-header field is used to specify directives
1582   that &MUST; be obeyed by all caching mechanisms along the
1583   request/response chain. The directives specify behavior intended to
1584   prevent caches from adversely interfering with the request or
1585   response. These directives typically override the default caching
1586   algorithms. Cache directives are unidirectional in that the presence
1587   of a directive in a request does not imply that the same directive is
1588   to be given in the response.
1589  <list><t>
1590      Note that HTTP/1.0 caches might not implement Cache-Control and
1591      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1592  </t></list>
1593</t>
1594<t>
1595   Cache directives &MUST; be passed through by a proxy or gateway
1596   application, regardless of their significance to that application,
1597   since the directives might be applicable to all recipients along the
1598   request/response chain. It is not possible to specify a cache-directive
1599   for a specific cache.
1600</t>
1601<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"/>
1602   Cache-Control   = "Cache-Control" ":" 1#cache-directive
1603
1604   cache-directive = cache-request-directive
1605        | cache-response-directive
1606
1607   cache-request-directive =
1608          "no-cache"                          ; <xref target="what.is.cacheable"/>
1609        | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1610        | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1611        | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1612        | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1613        | "no-transform"                      ; <xref target="no-transform.directive"/>
1614        | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1615        | cache-extension                     ; <xref target="cache.control.extensions"/>
1616
1617    cache-response-directive =
1618          "public"                               ; <xref target="what.is.cacheable"/>
1619        | "private" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ] ; <xref target="what.is.cacheable"/>
1620        | "no-cache" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ]; <xref target="what.is.cacheable"/>
1621        | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1622        | "no-transform"                         ; <xref target="no-transform.directive"/>
1623        | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1624        | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1625        | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1626        | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1627        | cache-extension                        ; <xref target="cache.control.extensions"/>
1628
1629   cache-extension = token [ "=" ( token | quoted-string ) ]
1630</artwork></figure>
1631<t>
1632   When a directive appears without any 1#field-name parameter, the
1633   directive applies to the entire request or response. When such a
1634   directive appears with a 1#field-name parameter, it applies only to
1635   the named field or fields, and not to the rest of the request or
1636   response. This mechanism supports extensibility; implementations of
1637   future versions of the HTTP protocol might apply these directives to
1638   header fields not defined in HTTP/1.1.
1639</t>
1640<t>
1641   The cache-control directives can be broken down into these general
1642   categories:
1643  <list style="symbols">
1644     <t>Restrictions on what are cacheable; these may only be imposed by
1645        the origin server.</t>
1646
1647     <t>Restrictions on what may be stored by a cache; these may be
1648        imposed by either the origin server or the user agent.</t>
1649
1650     <t>Modifications of the basic expiration mechanism; these may be
1651        imposed by either the origin server or the user agent.</t>
1652
1653     <t>Controls over cache revalidation and reload; these may only be
1654        imposed by a user agent.</t>
1655
1656     <t>Control over transformation of entities.</t>
1657
1658     <t>Extensions to the caching system.</t>
1659  </list>
1660</t>
1661
1662<section title="What is Cacheable" anchor="what.is.cacheable">
1663<t>
1664   By default, a response is cacheable if the requirements of the
1665   request method, request header fields, and the response status
1666   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1667   for cacheability. The following Cache-Control response directives
1668   allow an origin server to override the default cacheability of a
1669   response:
1670</t>
1671<t>
1672  <iref item="Cache Directives" subitem="public" primary="true"/>
1673  <iref item="public" subitem="Cache Directive" primary="true"/>
1674   public
1675  <list><t>
1676      Indicates that the response &MAY; be cached by any cache, even if it
1677      would normally be non-cacheable or cacheable only within a non-shared
1678      cache. (See also Authorization, &header-authorization;, for
1679      additional details.)
1680  </t></list>
1681</t>
1682<t>
1683  <iref item="Cache Directives" subitem="private" primary="true"/>
1684  <iref item="private" subitem="Cache Directive" primary="true"/>
1685   private
1686  <list><t>
1687      Indicates that all or part of the response message is intended for
1688      a single user and &MUST-NOT; be cached by a shared cache. This
1689      allows an origin server to state that the specified parts of the
1690      response are intended for only one user and are not a valid
1691      response for requests by other users. A private (non-shared) cache
1692      &MAY; cache the response.
1693    </t><t>
1694       <x:h>Note:</x:h> This usage of the word private only controls where the
1695       response may be cached, and cannot ensure the privacy of the
1696       message content.
1697  </t></list>
1698</t>
1699<t>
1700  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1701  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1702   no-cache
1703  <list><t>
1704       If the no-cache directive does not specify a field-name, then a
1705      cache &MUST-NOT; use the response to satisfy a subsequent request
1706      without successful revalidation with the origin server. This
1707      allows an origin server to prevent caching even by caches that
1708      have been configured to return stale responses to client requests.
1709    </t><t>
1710      If the no-cache directive does specify one or more field-names,
1711      then a cache &MAY; use the response to satisfy a subsequent request,
1712      subject to any other restrictions on caching. However, the
1713      specified field-name(s) &MUST-NOT; be sent in the response to a
1714      subsequent request without successful revalidation with the origin
1715      server. This allows an origin server to prevent the re-use of
1716      certain header fields in a response, while still allowing caching
1717      of the rest of the response.
1718    <list><t>
1719       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1720       directive.
1721    </t></list>
1722  </t></list>
1723</t>
1724</section>
1725
1726<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1727<t>
1728  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1729  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1730   no-store
1731  <list><t>   
1732      The purpose of the no-store directive is to prevent the
1733      inadvertent release or retention of sensitive information (for
1734      example, on backup tapes). The no-store directive applies to the
1735      entire message, and &MAY; be sent either in a response or in a
1736      request. If sent in a request, a cache &MUST-NOT; store any part of
1737      either this request or any response to it. If sent in a response,
1738      a cache &MUST-NOT; store any part of either this response or the
1739      request that elicited it. This directive applies to both non-shared
1740      and shared caches. "&MUST-NOT; store" in this context means
1741      that the cache &MUST-NOT; intentionally store the information in
1742      non-volatile storage, and &MUST; make a best-effort attempt to
1743      remove the information from volatile storage as promptly as
1744      possible after forwarding it.
1745  </t><t>
1746      Even when this directive is associated with a response, users
1747      might explicitly store such a response outside of the caching
1748      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1749      such responses as part of their normal operation.
1750  </t><t>
1751      The purpose of this directive is to meet the stated requirements
1752      of certain users and service authors who are concerned about
1753      accidental releases of information via unanticipated accesses to
1754      cache data structures. While the use of this directive might
1755      improve privacy in some cases, we caution that it is NOT in any
1756      way a reliable or sufficient mechanism for ensuring privacy. In
1757      particular, malicious or compromised caches might not recognize or
1758      obey this directive, and communications networks might be
1759      vulnerable to eavesdropping.
1760  </t></list>
1761</t>
1762</section>
1763
1764<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1765<t>
1766   The expiration time of an entity &MAY; be specified by the origin
1767   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1768   it &MAY; be specified using the max-age directive in a response. When
1769   the max-age cache-control directive is present in a cached response,
1770   the response is stale if its current age is greater than the age
1771   value given (in seconds) at the time of a new request for that
1772   resource. The max-age directive on a response implies that the
1773   response is cacheable (i.e., "public") unless some other, more
1774   restrictive cache directive is also present.
1775</t>
1776<t>
1777   If a response includes both an Expires header and a max-age
1778   directive, the max-age directive overrides the Expires header, even
1779   if the Expires header is more restrictive. This rule allows an origin
1780   server to provide, for a given response, a longer expiration time to
1781   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1782   useful if certain HTTP/1.0 caches improperly calculate ages or
1783   expiration times, perhaps due to desynchronized clocks.
1784</t>
1785<t>
1786   Many HTTP/1.0 cache implementations will treat an Expires value that
1787   is less than or equal to the response Date value as being equivalent
1788   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1789   cache receives such a response, and the response does not include a
1790   Cache-Control header field, it &SHOULD; consider the response to be
1791   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1792  <list><t>
1793       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1794       cache control feature, such as the "private" directive, on a
1795       network including older caches that do not understand that
1796       feature. The origin server will need to combine the new feature
1797       with an Expires field whose value is less than or equal to the
1798       Date value. This will prevent older caches from improperly
1799       caching the response.
1800  </t></list>
1801</t>
1802<t>
1803  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1804  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1805   s-maxage
1806  <list><t>
1807       If a response includes an s-maxage directive, then for a shared
1808       cache (but not for a private cache), the maximum age specified by
1809       this directive overrides the maximum age specified by either the
1810       max-age directive or the Expires header. The s-maxage directive
1811       also implies the semantics of the proxy-revalidate directive (see
1812       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1813       entry after it becomes stale to respond to a subsequent request
1814       without first revalidating it with the origin server. The s-maxage
1815       directive is always ignored by a private cache.
1816  </t></list>
1817</t>
1818<t>
1819   Note that most older caches, not compliant with this specification,
1820   do not implement any cache-control directives. An origin server
1821   wishing to use a cache-control directive that restricts, but does not
1822   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1823   requirement that the max-age directive overrides the Expires header,
1824   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1825   max-age directive.
1826</t>
1827<t>
1828   Other directives allow a user agent to modify the basic expiration
1829   mechanism. These directives &MAY; be specified on a request:
1830</t>
1831<t>
1832  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1833  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1834   max-age
1835  <list><t>
1836      Indicates that the client is willing to accept a response whose
1837      age is no greater than the specified time in seconds. Unless max-stale
1838      directive is also included, the client is not willing to
1839      accept a stale response.
1840  </t></list>
1841</t>
1842<t>
1843  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1844  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1845   min-fresh
1846  <list><t>
1847      Indicates that the client is willing to accept a response whose
1848      freshness lifetime is no less than its current age plus the
1849      specified time in seconds. That is, the client wants a response
1850      that will still be fresh for at least the specified number of
1851      seconds.
1852  </t></list>
1853</t>
1854<t>
1855  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1856  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1857   max-stale
1858  <list><t>
1859      Indicates that the client is willing to accept a response that has
1860      exceeded its expiration time. If max-stale is assigned a value,
1861      then the client is willing to accept a response that has exceeded
1862      its expiration time by no more than the specified number of
1863      seconds. If no value is assigned to max-stale, then the client is
1864      willing to accept a stale response of any age.
1865  </t></list>
1866</t>
1867<t>
1868   If a cache returns a stale response, either because of a max-stale
1869   directive on a request, or because the cache is configured to
1870   override the expiration time of a response, the cache &MUST; attach a
1871   Warning header to the stale response, using Warning 110 (Response is
1872   stale).
1873</t>
1874<t>
1875   A cache &MAY; be configured to return stale responses without
1876   validation, but only if this does not conflict with any "MUST"-level
1877   requirements concerning cache validation (e.g., a "must-revalidate"
1878   cache-control directive).
1879</t>
1880<t>
1881   If both the new request and the cached entry include "max-age"
1882   directives, then the lesser of the two values is used for determining
1883   the freshness of the cached entry for that request.
1884</t>
1885</section>
1886
1887<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1888<t>
1889   Sometimes a user agent might want or need to insist that a cache
1890   revalidate its cache entry with the origin server (and not just with
1891   the next cache along the path to the origin server), or to reload its
1892   cache entry from the origin server. End-to-end revalidation might be
1893   necessary if either the cache or the origin server has overestimated
1894   the expiration time of the cached response. End-to-end reload may be
1895   necessary if the cache entry has become corrupted for some reason.
1896</t>
1897<t>
1898   End-to-end revalidation may be requested either when the client does
1899   not have its own local cached copy, in which case we call it
1900   "unspecified end-to-end revalidation", or when the client does have a
1901   local cached copy, in which case we call it "specific end-to-end
1902   revalidation."
1903</t>
1904<t>
1905   The client can specify these three kinds of action using Cache-Control
1906   request directives:
1907</t>
1908<t>
1909   End-to-end reload
1910  <list><t>
1911      The request includes a "no-cache" cache-control directive or, for
1912      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1913      names &MUST-NOT; be included with the no-cache directive in a
1914      request. The server &MUST-NOT; use a cached copy when responding to
1915      such a request.
1916  </t></list>
1917</t>
1918<t>
1919   Specific end-to-end revalidation
1920  <list><t>
1921      The request includes a "max-age=0" cache-control directive, which
1922      forces each cache along the path to the origin server to
1923      revalidate its own entry, if any, with the next cache or server.
1924      The initial request includes a cache-validating conditional with
1925      the client's current validator.
1926  </t></list>
1927</t>
1928<t>
1929   Unspecified end-to-end revalidation
1930  <list><t>
1931      The request includes "max-age=0" cache-control directive, which
1932      forces each cache along the path to the origin server to
1933      revalidate its own entry, if any, with the next cache or server.
1934      The initial request does not include a cache-validating
1935      conditional; the first cache along the path (if any) that holds a
1936      cache entry for this resource includes a cache-validating
1937      conditional with its current validator.
1938  </t></list>
1939</t>
1940<t>
1941  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1942  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1943   max-age
1944  <list><t>
1945      When an intermediate cache is forced, by means of a max-age=0
1946      directive, to revalidate its own cache entry, and the client has
1947      supplied its own validator in the request, the supplied validator
1948      might differ from the validator currently stored with the cache
1949      entry. In this case, the cache &MAY; use either validator in making
1950      its own request without affecting semantic transparency.
1951  </t><t>
1952      However, the choice of validator might affect performance. The
1953      best approach is for the intermediate cache to use its own
1954      validator when making its request. If the server replies with 304
1955      (Not Modified), then the cache can return its now validated copy
1956      to the client with a 200 (OK) response. If the server replies with
1957      a new entity and cache validator, however, the intermediate cache
1958      can compare the returned validator with the one provided in the
1959      client's request, using the strong comparison function. If the
1960      client's validator is equal to the origin server's, then the
1961      intermediate cache simply returns 304 (Not Modified). Otherwise,
1962      it returns the new entity with a 200 (OK) response.
1963  </t><t>
1964      If a request includes the no-cache directive, it &SHOULD-NOT; 
1965      include min-fresh, max-stale, or max-age.
1966  </t></list>
1967</t>
1968<t>
1969  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1970  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1971   only-if-cached
1972  <list><t>
1973      In some cases, such as times of extremely poor network
1974      connectivity, a client may want a cache to return only those
1975      responses that it currently has stored, and not to reload or
1976      revalidate with the origin server. To do this, the client may
1977      include the only-if-cached directive in a request. If it receives
1978      this directive, a cache &SHOULD; either respond using a cached entry
1979      that is consistent with the other constraints of the request, or
1980      respond with a 504 (Gateway Timeout) status. However, if a group
1981      of caches is being operated as a unified system with good internal
1982      connectivity, such a request &MAY; be forwarded within that group of
1983      caches.
1984  </t></list>
1985</t>
1986<t>
1987  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1988  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1989   must-revalidate
1990  <list><t>
1991      Because a cache &MAY; be configured to ignore a server's specified
1992      expiration time, and because a client request &MAY; include a max-stale
1993      directive (which has a similar effect), the protocol also
1994      includes a mechanism for the origin server to require revalidation
1995      of a cache entry on any subsequent use. When the must-revalidate
1996      directive is present in a response received by a cache, that cache
1997      &MUST-NOT; use the entry after it becomes stale to respond to a
1998      subsequent request without first revalidating it with the origin
1999      server. (I.e., the cache &MUST; do an end-to-end revalidation every
2000      time, if, based solely on the origin server's Expires or max-age
2001      value, the cached response is stale.)
2002  </t><t>
2003      The must-revalidate directive is necessary to support reliable
2004      operation for certain protocol features. In all circumstances an
2005      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
2006      particular, if the cache cannot reach the origin server for any
2007      reason, it &MUST; generate a 504 (Gateway Timeout) response.
2008  </t><t>
2009      Servers &SHOULD; send the must-revalidate directive if and only if
2010      failure to revalidate a request on the entity could result in
2011      incorrect operation, such as a silently unexecuted financial
2012      transaction. Recipients &MUST-NOT; take any automated action that
2013      violates this directive, and &MUST-NOT; automatically provide an
2014      unvalidated copy of the entity if revalidation fails.
2015  </t><t>
2016      Although this is not recommended, user agents operating under
2017      severe connectivity constraints &MAY; violate this directive but, if
2018      so, &MUST; explicitly warn the user that an unvalidated response has
2019      been provided. The warning &MUST; be provided on each unvalidated
2020      access, and &SHOULD; require explicit user confirmation.
2021  </t></list>
2022</t>
2023<t>
2024  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
2025  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
2026   proxy-revalidate
2027  <list><t>
2028      The proxy-revalidate directive has the same meaning as the must-revalidate
2029      directive, except that it does not apply to non-shared
2030      user agent caches. It can be used on a response to an
2031      authenticated request to permit the user's cache to store and
2032      later return the response without needing to revalidate it (since
2033      it has already been authenticated once by that user), while still
2034      requiring proxies that service many users to revalidate each time
2035      (in order to make sure that each user has been authenticated).
2036      Note that such authenticated responses also need the public cache
2037      control directive in order to allow them to be cached at all.
2038  </t></list>
2039</t>
2040</section>
2041
2042<section title="No-Transform Directive" anchor="no-transform.directive">
2043<t>
2044  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2045  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2046   no-transform
2047  <list><t>
2048      Implementors of intermediate caches (proxies) have found it useful
2049      to convert the media type of certain entity bodies. A non-transparent
2050      proxy might, for example, convert between image
2051      formats in order to save cache space or to reduce the amount of
2052      traffic on a slow link.
2053  </t><t>
2054      Serious operational problems occur, however, when these
2055      transformations are applied to entity bodies intended for certain
2056      kinds of applications. For example, applications for medical
2057      imaging, scientific data analysis and those using end-to-end
2058      authentication, all depend on receiving an entity body that is bit
2059      for bit identical to the original entity-body.
2060  </t><t>
2061      Therefore, if a message includes the no-transform directive, an
2062      intermediate cache or proxy &MUST-NOT; change those headers that are
2063      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2064      directive. This implies that the cache or proxy &MUST-NOT; change
2065      any aspect of the entity-body that is specified by these headers,
2066      including the value of the entity-body itself.
2067  </t></list>
2068</t>
2069</section>
2070
2071<section title="Cache Control Extensions" anchor="cache.control.extensions">
2072<t>
2073   The Cache-Control header field can be extended through the use of one
2074   or more cache-extension tokens, each with an optional assigned value.
2075   Informational extensions (those which do not require a change in
2076   cache behavior) &MAY; be added without changing the semantics of other
2077   directives. Behavioral extensions are designed to work by acting as
2078   modifiers to the existing base of cache directives. Both the new
2079   directive and the standard directive are supplied, such that
2080   applications which do not understand the new directive will default
2081   to the behavior specified by the standard directive, and those that
2082   understand the new directive will recognize it as modifying the
2083   requirements associated with the standard directive. In this way,
2084   extensions to the cache-control directives can be made without
2085   requiring changes to the base protocol.
2086</t>
2087<t>
2088   This extension mechanism depends on an HTTP cache obeying all of the
2089   cache-control directives defined for its native HTTP-version, obeying
2090   certain extensions, and ignoring all directives that it does not
2091   understand.
2092</t>
2093<t>
2094   For example, consider a hypothetical new response directive called
2095   community which acts as a modifier to the private directive. We
2096   define this new directive to mean that, in addition to any non-shared
2097   cache, any cache which is shared only by members of the community
2098   named within its value may cache the response. An origin server
2099   wishing to allow the UCI community to use an otherwise private
2100   response in their shared cache(s) could do so by including
2101</t>
2102<figure><artwork type="example">
2103    Cache-Control: private, community="UCI"
2104</artwork></figure>
2105<t>
2106   A cache seeing this header field will act correctly even if the cache
2107   does not understand the community cache-extension, since it will also
2108   see and understand the private directive and thus default to the safe
2109   behavior.
2110</t>
2111<t>
2112   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2113   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2114   be combined with standard directives (or the response's default
2115   cacheability) such that the cache behavior will remain minimally
2116   correct even if the cache does not understand the extension(s).
2117</t>
2118</section>
2119</section>
2120
2121<section title="Expires" anchor="header.expires">
2122  <iref primary="true" item="Expires header" x:for-anchor=""/>
2123  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2124<t>
2125   The Expires entity-header field gives the date/time after which the
2126   response is considered stale. A stale cache entry may not normally be
2127   returned by a cache (either a proxy cache or a user agent cache)
2128   unless it is first validated with the origin server (or with an
2129   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2130   for further discussion of the expiration model.
2131</t>
2132<t>
2133   The presence of an Expires field does not imply that the original
2134   resource will change or cease to exist at, before, or after that
2135   time.
2136</t>
2137<t>
2138   The format is an absolute date and time as defined by HTTP-date in
2139   &full-date;; it &MUST; be sent in rfc1123-date format.
2140</t>
2141<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2142   Expires = "Expires" ":" HTTP-date
2143</artwork></figure>
2144<t>
2145   An example of its use is
2146</t>
2147<figure><artwork type="example">
2148   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2149</artwork></figure>
2150<t>
2151  <list><t>
2152      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2153      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2154      Expires field.
2155  </t></list>
2156</t>
2157<t>
2158   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2159   especially including the value "0", as in the past (i.e., "already
2160   expired").
2161</t>
2162<t>
2163   To mark a response as "already expired," an origin server sends an
2164   Expires date that is equal to the Date header value. (See the rules
2165   for expiration calculations in <xref target="expiration.calculations"/>.)
2166</t>
2167<t>
2168   To mark a response as "never expires," an origin server sends an
2169   Expires date approximately one year from the time the response is
2170   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2171   year in the future.
2172</t>
2173<t>
2174   The presence of an Expires header field with a date value of some
2175   time in the future on a response that otherwise would by default be
2176   non-cacheable indicates that the response is cacheable, unless
2177   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2178</t>
2179</section>
2180
2181<section title="Pragma" anchor="header.pragma">
2182  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2183  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2184<t>
2185   The Pragma general-header field is used to include implementation-specific
2186   directives that might apply to any recipient along the
2187   request/response chain. All pragma directives specify optional
2188   behavior from the viewpoint of the protocol; however, some systems
2189   &MAY; require that behavior be consistent with the directives.
2190</t>
2191<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"/>
2192    Pragma            = "Pragma" ":" 1#pragma-directive
2193    pragma-directive  = "no-cache" | extension-pragma
2194    extension-pragma  = token [ "=" ( token | quoted-string ) ]
2195</artwork></figure>
2196<t>
2197   When the no-cache directive is present in a request message, an
2198   application &SHOULD; forward the request toward the origin server even
2199   if it has a cached copy of what is being requested. This pragma
2200   directive has the same semantics as the no-cache cache-directive (see
2201   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2202   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2203   request is sent to a server not known to be HTTP/1.1 compliant.
2204</t>
2205<t>
2206   Pragma directives &MUST; be passed through by a proxy or gateway
2207   application, regardless of their significance to that application,
2208   since the directives might be applicable to all recipients along the
2209   request/response chain. It is not possible to specify a pragma for a
2210   specific recipient; however, any pragma directive not relevant to a
2211   recipient &SHOULD; be ignored by that recipient.
2212</t>
2213<t>
2214   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2215   sent "Cache-Control: no-cache". No new Pragma directives will be
2216   defined in HTTP.
2217  <list><t>
2218      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache as a response
2219      header field is not actually specified, it does not provide a
2220      reliable replacement for "Cache-Control: no-cache" in a response
2221  </t></list>
2222</t>
2223</section>
2224
2225<section title="Vary" anchor="header.vary">
2226  <iref primary="true" item="Vary header" x:for-anchor=""/>
2227  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2228<t>
2229   The Vary field value indicates the set of request-header fields that
2230   fully determines, while the response is fresh, whether a cache is
2231   permitted to use the response to reply to a subsequent request
2232   without revalidation. For uncacheable or stale responses, the Vary
2233   field value advises the user agent about the criteria that were used
2234   to select the representation. A Vary field value of "*" implies that
2235   a cache cannot determine from the request headers of a subsequent
2236   request whether this response is the appropriate representation. See
2237   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2238</t>
2239<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2240    Vary  = "Vary" ":" ( "*" | 1#field-name )
2241</artwork></figure>
2242<t>
2243   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2244   cacheable response that is subject to server-driven negotiation.
2245   Doing so allows a cache to properly interpret future requests on that
2246   resource and informs the user agent about the presence of negotiation
2247   on that resource. A server &MAY; include a Vary header field with a
2248   non-cacheable response that is subject to server-driven negotiation,
2249   since this might provide the user agent with useful information about
2250   the dimensions over which the response varies at the time of the
2251   response.
2252</t>
2253<t>
2254   A Vary field value consisting of a list of field-names signals that
2255   the representation selected for the response is based on a selection
2256   algorithm which considers ONLY the listed request-header field values
2257   in selecting the most appropriate representation. A cache &MAY; assume
2258   that the same selection will be made for future requests with the
2259   same values for the listed field names, for the duration of time for
2260   which the response is fresh.
2261</t>
2262<t>
2263   The field-names given are not limited to the set of standard
2264   request-header fields defined by this specification. Field names are
2265   case-insensitive.
2266</t>
2267<t>
2268   A Vary field value of "*" signals that unspecified parameters not
2269   limited to the request-headers (e.g., the network address of the
2270   client), play a role in the selection of the response representation.
2271   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2272   generated by an origin server.
2273</t>
2274</section>
2275
2276<section title="Warning" anchor="header.warning">
2277  <iref primary="true" item="Warning header" x:for-anchor=""/>
2278  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2279<t>
2280   The Warning general-header field is used to carry additional
2281   information about the status or transformation of a message which
2282   might not be reflected in the message. This information is typically
2283   used to warn about a possible lack of semantic transparency from
2284   caching operations or transformations applied to the entity body of
2285   the message.
2286</t>
2287<t>
2288   Warning headers are sent with responses using:
2289</t>
2290<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"/>
2291    Warning    = "Warning" ":" 1#warning-value
2292
2293    warning-value = warn-code SP warn-agent SP warn-text
2294                                          [SP warn-date]
2295
2296    warn-code  = 3DIGIT
2297    warn-agent = ( host [ ":" port ] ) | pseudonym
2298                    ; the name or pseudonym of the server adding
2299                    ; the Warning header, for use in debugging
2300    warn-text  = quoted-string
2301    warn-date  = &lt;"&gt; HTTP-date &lt;"&gt;
2302</artwork></figure>
2303<t>
2304   A response &MAY; carry more than one Warning header.
2305</t>
2306<t>
2307   The warn-text &SHOULD; be in a natural language and character set that
2308   is most likely to be intelligible to the human user receiving the
2309   response. This decision &MAY; be based on any available knowledge, such
2310   as the location of the cache or user, the Accept-Language field in a
2311   request, the Content-Language field in a response, etc. The default
2312   language is English and the default character set is ISO-8859-1.
2313</t>
2314<t>
2315   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2316   in the warn-text using the method described in <xref target="RFC2047"/>.
2317</t>
2318<t>
2319   Warning headers can in general be applied to any message, however
2320   some specific warn-codes are specific to caches and can only be
2321   applied to response messages. New Warning headers &SHOULD; be added
2322   after any existing Warning headers. A cache &MUST-NOT; delete any
2323   Warning header that it received with a message. However, if a cache
2324   successfully validates a cache entry, it &SHOULD; remove any Warning
2325   headers previously attached to that entry except as specified for
2326   specific Warning codes. It &MUST; then add any Warning headers received
2327   in the validating response. In other words, Warning headers are those
2328   that would be attached to the most recent relevant response.
2329</t>
2330<t>
2331   When multiple Warning headers are attached to a response, the user
2332   agent ought to inform the user of as many of them as possible, in the
2333   order that they appear in the response. If it is not possible to
2334   inform the user of all of the warnings, the user agent &SHOULD; follow
2335   these heuristics:
2336  <list style="symbols">
2337    <t>Warnings that appear early in the response take priority over
2338        those appearing later in the response.</t>
2339
2340    <t>Warnings in the user's preferred character set take priority
2341        over warnings in other character sets but with identical warn-codes
2342        and warn-agents.</t>
2343  </list>
2344</t>
2345<t>
2346   Systems that generate multiple Warning headers &SHOULD; order them with
2347   this user agent behavior in mind.
2348</t>
2349<t>
2350   Requirements for the behavior of caches with respect to Warnings are
2351   stated in <xref target="warnings"/>.
2352</t>
2353<t>
2354   This is a list of the currently-defined warn-codes, each with a
2355   recommended warn-text in English, and a description of its meaning.
2356</t>
2357<t>
2358   110 Response is stale
2359  <list><t>
2360     &MUST; be included whenever the returned response is stale.
2361  </t></list>
2362</t>
2363<t>
2364   111 Revalidation failed
2365  <list><t>
2366     &MUST; be included if a cache returns a stale response because an
2367     attempt to revalidate the response failed, due to an inability to
2368     reach the server.
2369  </t></list>
2370</t>
2371<t>
2372   112 Disconnected operation
2373  <list><t>
2374     &SHOULD; be included if the cache is intentionally disconnected from
2375     the rest of the network for a period of time.
2376  </t></list>
2377</t>
2378<t>
2379   113 Heuristic expiration
2380  <list><t>
2381     &MUST; be included if the cache heuristically chose a freshness
2382     lifetime greater than 24 hours and the response's age is greater
2383     than 24 hours.
2384  </t></list>
2385</t>
2386<t>
2387   199 Miscellaneous warning
2388  <list><t>
2389     The warning text &MAY; include arbitrary information to be presented
2390     to a human user, or logged. A system receiving this warning &MUST-NOT;
2391     take any automated action, besides presenting the warning to
2392     the user.
2393  </t></list>
2394</t>
2395<t>
2396   214 Transformation applied
2397  <list><t>
2398     &MUST; be added by an intermediate cache or proxy if it applies any
2399     transformation changing the content-coding (as specified in the
2400     Content-Encoding header) or media-type (as specified in the
2401     Content-Type header) of the response, or the entity-body of the
2402     response, unless this Warning code already appears in the response.
2403  </t></list>
2404</t>
2405<t>
2406   299 Miscellaneous persistent warning
2407  <list><t>
2408     The warning text &MAY; include arbitrary information to be presented
2409     to a human user, or logged. A system receiving this warning &MUST-NOT;
2410     take any automated action.
2411  </t></list>
2412</t>
2413<t>
2414   If an implementation sends a message with one or more Warning headers
2415   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2416   each warning-value a warn-date that matches the date in the response.
2417</t>
2418<t>
2419   If an implementation receives a message with a warning-value that
2420   includes a warn-date, and that warn-date is different from the Date
2421   value in the response, then that warning-value &MUST; be deleted from
2422   the message before storing, forwarding, or using it. (This prevents
2423   bad consequences of naive caching of Warning header fields.) If all
2424   of the warning-values are deleted for this reason, the Warning header
2425   &MUST; be deleted as well.
2426</t>
2427</section>
2428
2429</section>
2430
2431<section title="IANA Considerations" anchor="IANA.considerations">
2432<t>
2433   TBD.
2434</t>
2435</section>
2436
2437<section title="Security Considerations" anchor="security.considerations">
2438<t>
2439   Caching proxies provide additional potential vulnerabilities, since
2440   the contents of the cache represent an attractive target for
2441   malicious exploitation. Because cache contents persist after an HTTP
2442   request is complete, an attack on the cache can reveal information
2443   long after a user believes that the information has been removed from
2444   the network. Therefore, cache contents should be protected as
2445   sensitive information.
2446</t>
2447</section>
2448
2449<section title="Acknowledgments" anchor="ack">
2450<t>
2451   Much of the content and presentation of the caching design is due to
2452   suggestions and comments from individuals including: Shel Kaphan,
2453   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2454</t>
2455</section>
2456</middle>
2457<back>
2458<references>
2459
2460<reference anchor="Part1">
2461   <front>
2462      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2463      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2464         <organization abbrev="Day Software">Day Software</organization>
2465         <address><email>fielding@gbiv.com</email></address>
2466      </author>
2467      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2468         <organization>One Laptop per Child</organization>
2469         <address><email>jg@laptop.org</email></address>
2470      </author>
2471      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2472         <organization abbrev="HP">Hewlett-Packard Company</organization>
2473         <address><email>JeffMogul@acm.org</email></address>
2474      </author>
2475      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2476         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2477         <address><email>henrikn@microsoft.com</email></address>
2478      </author>
2479      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2480         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2481         <address><email>LMM@acm.org</email></address>
2482      </author>
2483      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2484         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2485         <address><email>paulle@microsoft.com</email></address>
2486      </author>
2487      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2488         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2489         <address><email>timbl@w3.org</email></address>
2490      </author>
2491      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2492         <organization abbrev="W3C">World Wide Web Consortium</organization>
2493         <address><email>ylafon@w3.org</email></address>
2494      </author>
2495      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2496         <organization abbrev="greenbytes">greenbytes GmbH</organization>
2497         <address><email>julian.reschke@greenbytes.de</email></address>
2498      </author>
2499      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2500   </front>
2501   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2502   <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2503</reference>
2504
2505<reference anchor="Part2">
2506   <front>
2507      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2508      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2509         <organization abbrev="Day Software">Day Software</organization>
2510         <address><email>fielding@gbiv.com</email></address>
2511      </author>
2512      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2513         <organization>One Laptop per Child</organization>
2514         <address><email>jg@laptop.org</email></address>
2515      </author>
2516      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2517         <organization abbrev="HP">Hewlett-Packard Company</organization>
2518         <address><email>JeffMogul@acm.org</email></address>
2519      </author>
2520      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2521         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2522         <address><email>henrikn@microsoft.com</email></address>
2523      </author>
2524      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2525         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2526         <address><email>LMM@acm.org</email></address>
2527      </author>
2528      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2529         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2530         <address><email>paulle@microsoft.com</email></address>
2531      </author>
2532      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2533         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2534         <address><email>timbl@w3.org</email></address>
2535      </author>
2536      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2537         <organization abbrev="W3C">World Wide Web Consortium</organization>
2538         <address><email>ylafon@w3.org</email></address>
2539      </author>
2540      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2541         <organization abbrev="greenbytes">greenbytes GmbH</organization>
2542         <address><email>julian.reschke@greenbytes.de</email></address>
2543      </author>
2544      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2545   </front>
2546   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2547   <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2548</reference>
2549
2550<reference anchor="Part3">
2551   <front>
2552      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2553      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2554         <organization abbrev="Day Software">Day Software</organization>
2555         <address><email>fielding@gbiv.com</email></address>
2556      </author>
2557      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2558         <organization>One Laptop per Child</organization>
2559         <address><email>jg@laptop.org</email></address>
2560      </author>
2561      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2562         <organization abbrev="HP">Hewlett-Packard Company</organization>
2563         <address><email>JeffMogul@acm.org</email></address>
2564      </author>
2565      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2566         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2567         <address><email>henrikn@microsoft.com</email></address>
2568      </author>
2569      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2570         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2571         <address><email>LMM@acm.org</email></address>
2572      </author>
2573      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2574         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2575         <address><email>paulle@microsoft.com</email></address>
2576      </author>
2577      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2578         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2579         <address><email>timbl@w3.org</email></address>
2580      </author>
2581      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2582         <organization abbrev="W3C">World Wide Web Consortium</organization>
2583         <address><email>ylafon@w3.org</email></address>
2584      </author>
2585      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2586         <organization abbrev="greenbytes">greenbytes GmbH</organization>
2587         <address><email>julian.reschke@greenbytes.de</email></address>
2588      </author>
2589      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2590   </front>
2591   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2592   <x:source href="p3-payload.xml" basename="p3-payload"/>
2593</reference>
2594
2595<reference anchor="Part4">
2596   <front>
2597      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2598      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2599         <organization abbrev="Day Software">Day Software</organization>
2600         <address><email>fielding@gbiv.com</email></address>
2601      </author>
2602      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2603         <organization>One Laptop per Child</organization>
2604         <address><email>jg@laptop.org</email></address>
2605      </author>
2606      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2607         <organization abbrev="HP">Hewlett-Packard Company</organization>
2608         <address><email>JeffMogul@acm.org</email></address>
2609      </author>
2610      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2611         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2612         <address><email>henrikn@microsoft.com</email></address>
2613      </author>
2614      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2615         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2616         <address><email>LMM@acm.org</email></address>
2617      </author>
2618      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2619         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2620         <address><email>paulle@microsoft.com</email></address>
2621      </author>
2622      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2623         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2624         <address><email>timbl@w3.org</email></address>
2625      </author>
2626      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2627         <organization abbrev="W3C">World Wide Web Consortium</organization>
2628         <address><email>ylafon@w3.org</email></address>
2629      </author>
2630      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2631         <organization abbrev="greenbytes">greenbytes GmbH</organization>
2632         <address><email>julian.reschke@greenbytes.de</email></address>
2633      </author>
2634      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2635   </front>
2636   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2637   <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2638</reference>
2639
2640<reference anchor="Part5">
2641   <front>
2642      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2643      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2644         <organization abbrev="Day Software">Day Software</organization>
2645         <address><email>fielding@gbiv.com</email></address>
2646      </author>
2647      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2648         <organization>One Laptop per Child</organization>
2649         <address><email>jg@laptop.org</email></address>
2650      </author>
2651      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2652         <organization abbrev="HP">Hewlett-Packard Company</organization>
2653         <address><email>JeffMogul@acm.org</email></address>
2654      </author>
2655      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2656         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2657         <address><email>henrikn@microsoft.com</email></address>
2658      </author>
2659      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2660         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2661         <address><email>LMM@acm.org</email></address>
2662      </author>
2663      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2664         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2665         <address><email>paulle@microsoft.com</email></address>
2666      </author>
2667      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2668         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2669         <address><email>timbl@w3.org</email></address>
2670      </author>
2671      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2672         <organization abbrev="W3C">World Wide Web Consortium</organization>
2673         <address><email>ylafon@w3.org</email></address>
2674      </author>
2675      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2676         <organization abbrev="greenbytes">greenbytes GmbH</organization>
2677         <address><email>julian.reschke@greenbytes.de</email></address>
2678      </author>
2679      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2680   </front>
2681   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2682   <x:source href="p5-range.xml" basename="p5-range"/>
2683</reference>
2684
2685<reference anchor="Part7">
2686   <front>
2687      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2688      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2689         <organization abbrev="Day Software">Day Software</organization>
2690         <address><email>fielding@gbiv.com</email></address>
2691      </author>
2692      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2693         <organization>One Laptop per Child</organization>
2694         <address><email>jg@laptop.org</email></address>
2695      </author>
2696      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2697         <organization abbrev="HP">Hewlett-Packard Company</organization>
2698         <address><email>JeffMogul@acm.org</email></address>
2699      </author>
2700      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2701         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2702         <address><email>henrikn@microsoft.com</email></address>
2703      </author>
2704      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2705         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2706         <address><email>LMM@acm.org</email></address>
2707      </author>
2708      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2709         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2710         <address><email>paulle@microsoft.com</email></address>
2711      </author>
2712      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2713         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2714         <address><email>timbl@w3.org</email></address>
2715      </author>
2716      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2717         <organization abbrev="W3C">World Wide Web Consortium</organization>
2718         <address><email>ylafon@w3.org</email></address>
2719      </author>
2720      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2721         <organization abbrev="greenbytes">greenbytes GmbH</organization>
2722         <address><email>julian.reschke@greenbytes.de</email></address>
2723      </author>
2724      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2725   </front>
2726   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2727   <x:source href="p7-auth.xml" basename="p7-auth"/>
2728</reference>
2729
2730<reference anchor="RFC2119">
2731  <front>
2732    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2733    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2734      <organization>Harvard University</organization>
2735      <address><email>sob@harvard.edu</email></address>
2736    </author>
2737    <date month="March" year="1997"/>
2738  </front>
2739  <seriesInfo name="BCP" value="14"/>
2740  <seriesInfo name="RFC" value="2119"/>
2741</reference>
2742
2743<reference anchor="RFC2616">
2744   <front>
2745      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2746      <author initials="R." surname="Fielding" fullname="R. Fielding">
2747         <organization>University of California, Irvine</organization>
2748         <address><email>fielding@ics.uci.edu</email></address>
2749      </author>
2750      <author initials="J." surname="Gettys" fullname="J. Gettys">
2751         <organization>W3C</organization>
2752         <address><email>jg@w3.org</email></address>
2753      </author>
2754      <author initials="J." surname="Mogul" fullname="J. Mogul">
2755         <organization>Compaq Computer Corporation</organization>
2756         <address><email>mogul@wrl.dec.com</email></address>
2757      </author>
2758      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2759         <organization>MIT Laboratory for Computer Science</organization>
2760         <address><email>frystyk@w3.org</email></address>
2761      </author>
2762      <author initials="L." surname="Masinter" fullname="L. Masinter">
2763         <organization>Xerox Corporation</organization>
2764         <address><email>masinter@parc.xerox.com</email></address>
2765      </author>
2766      <author initials="P." surname="Leach" fullname="P. Leach">
2767         <organization>Microsoft Corporation</organization>
2768         <address><email>paulle@microsoft.com</email></address>
2769      </author>
2770      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2771         <organization>W3C</organization>
2772         <address><email>timbl@w3.org</email></address>
2773      </author>
2774      <date month="June" year="1999"/>
2775   </front>
2776   <seriesInfo name="RFC" value="2616"/>
2777</reference>
2778
2779<reference anchor="RFC2047">
2780  <front>
2781    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2782    <author initials="K." surname="Moore" fullname="Keith Moore">
2783      <organization>University of Tennessee</organization>
2784      <address><email>moore@cs.utk.edu</email></address>
2785    </author>
2786    <date month="November" year="1996"/>
2787  </front>
2788  <seriesInfo name="RFC" value="2047"/>
2789</reference>
2790
2791<reference anchor="RFC1305">
2792<front>
2793<title>Network Time Protocol (Version 3) Specification, Implementation</title>
2794<author initials="D." surname="Mills" fullname="David L. Mills">
2795<organization>University of Delaware, Electrical Engineering Department</organization>
2796<address>
2797<postal>
2798<street/>
2799<city>Newark</city>
2800<region>DE</region>
2801<code>19716</code>
2802<country>US</country></postal>
2803<phone>+1 302 451 8247</phone>
2804<email>mills@udel.edu</email></address></author>
2805<date month="March" year="1992"/>
2806<abstract>
2807<t>This document describes the Network Time Protocol (NTP), specifies its normal structure and summarizes information useful for its implementation. NTP provides the mechanisms to synchronize time and coordinate time distribution in a large, diverse internet operating at rates from mundane to lightwave. It uses a returnable-time design in which a distributed subnet of time servers operating in a self-organizing, hierarchical-master-slave configuration synchronizes local clocks within the subnet and to national time standards via wire or radio. The servers can also redistribute reference time via local routing algorithms and time daemons.</t></abstract></front>
2808<seriesInfo name="RFC" value="1305"/>
2809</reference>
2810
2811</references>
2812
2813<section title="Compatibility with Previous Versions" anchor="compatibility">
2814
2815<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2816<t>
2817   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2818   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2819   <xref target="header.cache-control" format="counter"/>,
2820   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2821</t>
2822<t>
2823   The Cache-Control: max-age directive was not properly defined for
2824   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2825</t>
2826<t>
2827   Warnings could be cached incorrectly, or not updated appropriately.
2828   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2829   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2830   and <xref target="header.warning" format="counter"/>) Warning
2831   also needed to be a general header, as PUT or other methods may have
2832   need for it in requests.
2833</t>
2834</section>
2835
2836<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2837</section>
2838
2839</section>
2840
2841</back>
2842</rfc>
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