source: draft-ietf-httpbis/latest/p6-cache.xml @ 139

Last change on this file since 139 was 139, checked in by julian.reschke@…, 12 years ago

Move change note from p1 to p6 (where the changed text lives)

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File size: 127.3 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY entity-header-fields        "<xref target='Part4' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#entity.tags' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY weak-and-strong-validators  "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31]>
32<?rfc toc="yes" ?>
33<?rfc symrefs="yes" ?>
34<?rfc sortrefs="yes" ?>
35<?rfc compact="yes"?>
36<?rfc subcompact="no" ?>
37<?rfc linkmailto="no" ?>
38<?rfc editing="no" ?>
39<?rfc-ext allow-markup-in-artwork="yes" ?>
40<?rfc-ext include-references-in-index="yes" ?>
41<rfc obsoletes="2068, 2616" category="std"
42     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
43     xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit">
44<front>
45
46  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
47
48  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
49    <organization abbrev="Day Software">Day Software</organization>
50    <address>
51      <postal>
52        <street>23 Corporate Plaza DR, Suite 280</street>
53        <city>Newport Beach</city>
54        <region>CA</region>
55        <code>92660</code>
56        <country>USA</country>
57      </postal>
58      <phone>+1-949-706-5300</phone>
59      <facsimile>+1-949-706-5305</facsimile>
60      <email>fielding@gbiv.com</email>
61      <uri>http://roy.gbiv.com/</uri>
62    </address>
63  </author>
64
65  <author initials="J." surname="Gettys" fullname="Jim Gettys">
66    <organization>One Laptop per Child</organization>
67    <address>
68      <postal>
69        <street>21 Oak Knoll Road</street>
70        <city>Carlisle</city>
71        <region>MA</region>
72        <code>01741</code>
73        <country>USA</country>
74      </postal>
75      <email>jg@laptop.org</email>
76      <uri>http://www.laptop.org/</uri>
77    </address>
78  </author>
79 
80  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
81    <organization abbrev="HP">Hewlett-Packard Company</organization>
82    <address>
83      <postal>
84        <street>HP Labs, Large Scale Systems Group</street>
85        <street>1501 Page Mill Road, MS 1177</street>
86        <city>Palo Alto</city>
87        <region>CA</region>
88        <code>94304</code>
89        <country>USA</country>
90      </postal>
91      <email>JeffMogul@acm.org</email>
92    </address>
93  </author>
94
95  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
96    <organization abbrev="Microsoft">Microsoft Corporation</organization>
97    <address>
98      <postal>
99        <street>1 Microsoft Way</street>
100        <city>Redmond</city>
101        <region>WA</region>
102        <code>98052</code>
103        <country>USA</country>
104      </postal>
105      <email>henrikn@microsoft.com</email>
106    </address>
107  </author>
108
109  <author initials="L." surname="Masinter" fullname="Larry Masinter">
110    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
111    <address>
112      <postal>
113        <street>345 Park Ave</street>
114        <city>San Jose</city>
115        <region>CA</region>
116        <code>95110</code>
117        <country>USA</country>
118      </postal>
119      <email>LMM@acm.org</email>
120      <uri>http://larry.masinter.net/</uri>
121    </address>
122  </author>
123 
124  <author initials="P." surname="Leach" fullname="Paul J. Leach">
125    <organization abbrev="Microsoft">Microsoft Corporation</organization>
126    <address>
127      <postal>
128        <street>1 Microsoft Way</street>
129        <city>Redmond</city>
130        <region>WA</region>
131        <code>98052</code>
132      </postal>
133      <email>paulle@microsoft.com</email>
134    </address>
135  </author>
136   
137  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
138    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
139    <address>
140      <postal>
141        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
142        <street>The Stata Center, Building 32</street>
143        <street>32 Vassar Street</street>
144        <city>Cambridge</city>
145        <region>MA</region>
146        <code>02139</code>
147        <country>USA</country>
148      </postal>
149      <email>timbl@w3.org</email>
150      <uri>http://www.w3.org/People/Berners-Lee/</uri>
151    </address>
152  </author>
153
154  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
155    <organization abbrev="W3C">World Wide Web Consortium</organization>
156    <address>
157      <postal>
158        <street>W3C / ERCIM</street>
159        <street>2004, rte des Lucioles</street>
160        <city>Sophia-Antipolis</city>
161        <region>AM</region>
162        <code>06902</code>
163        <country>France</country>
164      </postal>
165      <email>ylafon@w3.org</email>
166      <uri>http://www.raubacapeu.net/people/yves/</uri>
167    </address>
168  </author>
169
170  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
171    <organization abbrev="greenbytes">greenbytes GmbH</organization>
172    <address>
173      <postal>
174        <street>Hafenweg 16</street>
175        <city>Muenster</city><region>NW</region><code>48155</code>
176        <country>Germany</country>
177      </postal>
178      <phone>+49 251 2807760</phone>   
179      <facsimile>+49 251 2807761</facsimile>   
180      <email>julian.reschke@greenbytes.de</email>       
181      <uri>http://greenbytes.de/tech/webdav/</uri>     
182    </address>
183  </author>
184
185  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
186
187<abstract>
188<t>
189   The Hypertext Transfer Protocol (HTTP) is an application-level
190   protocol for distributed, collaborative, hypermedia information
191   systems. HTTP has been in use by the World Wide Web global information
192   initiative since 1990. This document is Part 6 of the seven-part specification
193   that defines the protocol referred to as "HTTP/1.1" and, taken together,
194   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
195   and the associated header fields that control cache behavior or indicate
196   cacheable response messages.
197</t>
198</abstract>
199
200<note title="Editorial Note (To be removed by RFC Editor)">
201  <t>
202    Discussion of this draft should take place on the HTTPBIS working group
203    mailing list (ietf-http-wg@w3.org). The current issues list is
204    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
205    and related documents (including fancy diffs) can be found at
206    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
207  </t>
208</note>
209</front>
210<middle>
211<section title="Introduction" anchor="introduction">
212<t>
213   This document will define aspects of HTTP related to caching response
214   messages.  Right now it only includes the extracted relevant sections
215   of <xref target="RFC2616" x:fmt="none">RFC 2616</xref> without edit.
216</t>
217
218<section title="Requirements" anchor="intro.requirements">
219<t>
220   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
221   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
222   document are to be interpreted as described in <xref target="RFC2119"/>.
223</t>
224<t>
225   An implementation is not compliant if it fails to satisfy one or more
226   of the &MUST; or &REQUIRED; level requirements for the protocols it
227   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
228   level and all the &SHOULD; level requirements for its protocols is said
229   to be "unconditionally compliant"; one that satisfies all the &MUST;
230   level requirements but not all the &SHOULD; level requirements for its
231   protocols is said to be "conditionally compliant."
232</t>
233</section>
234
235<section title="Terminology" anchor="intro.terminology">
236<t>
237   This specification uses a number of terms to refer to the roles
238   played by participants in, and objects of, the HTTP communication.
239</t>
240<t>
241  <iref item="cache"/>
242  <x:dfn>cache</x:dfn>
243  <list>
244    <t>
245      A program's local store of response messages and the subsystem
246      that controls its message storage, retrieval, and deletion. A
247      cache stores cacheable responses in order to reduce the response
248      time and network bandwidth consumption on future, equivalent
249      requests. Any client or server may include a cache, though a cache
250      cannot be used by a server that is acting as a tunnel.
251    </t>
252  </list>
253</t>
254<t>
255  <iref item="cacheable"/>
256  <x:dfn>cacheable</x:dfn>
257  <list>
258    <t>
259      A response is cacheable if a cache is allowed to store a copy of
260      the response message for use in answering subsequent requests. The
261      rules for determining the cacheability of HTTP responses are
262      defined in <xref target="caching"/>. Even if a resource is cacheable, there may
263      be additional constraints on whether a cache can use the cached
264      copy for a particular request.
265    </t>
266  </list>
267</t>
268<t>
269  <iref item="first-hand"/>
270  <x:dfn>first-hand</x:dfn>
271  <list>
272    <t>
273      A response is first-hand if it comes directly and without
274      unnecessary delay from the origin server, perhaps via one or more
275      proxies. A response is also first-hand if its validity has just
276      been checked directly with the origin server.
277    </t>
278  </list>
279</t>
280<t>
281  <iref item="explicit expiration time"/>
282  <x:dfn>explicit expiration time</x:dfn>
283  <list>
284    <t>
285      The time at which the origin server intends that an entity should
286      no longer be returned by a cache without further validation.
287    </t>
288  </list>
289</t>
290<t>
291  <iref item="heuristic expiration time"/>
292  <x:dfn>heuristic expiration time</x:dfn>
293  <list>
294    <t>
295      An expiration time assigned by a cache when no explicit expiration
296      time is available.
297    </t>
298  </list>
299</t>
300<t>
301  <iref item="age"/>
302  <x:dfn>age</x:dfn>
303  <list>
304    <t>
305      The age of a response is the time since it was sent by, or
306      successfully validated with, the origin server.
307    </t>
308  </list>
309</t>
310<t>
311  <iref item="freshness lifetime"/>
312  <x:dfn>freshness lifetime</x:dfn>
313  <list>
314    <t>
315      The length of time between the generation of a response and its
316      expiration time.
317    </t>
318  </list>
319</t>
320<t>
321  <iref item="fresh"/>
322  <x:dfn>fresh</x:dfn>
323  <list>
324    <t>
325      A response is fresh if its age has not yet exceeded its freshness
326      lifetime.
327    </t>
328  </list>
329</t>
330<t>
331  <iref item="stale"/>
332  <x:dfn>stale</x:dfn>
333  <list>
334    <t>
335      A response is stale if its age has passed its freshness lifetime.
336    </t>
337  </list>
338</t>
339<t>
340  <iref item="semantically transparent"/>
341  <x:dfn>semantically transparent</x:dfn>
342  <list>
343    <t>
344      A cache behaves in a "semantically transparent" manner, with
345      respect to a particular response, when its use affects neither the
346      requesting client nor the origin server, except to improve
347      performance. When a cache is semantically transparent, the client
348      receives exactly the same response (except for hop-by-hop headers)
349      that it would have received had its request been handled directly
350      by the origin server.
351    </t>
352  </list>
353</t>
354<t>
355  <iref item="validator"/>
356  <x:dfn>validator</x:dfn>
357  <list>
358    <t>
359      A protocol element (e.g., an entity tag or a Last-Modified time)
360      that is used to find out whether a cache entry is an equivalent
361      copy of an entity.
362    </t>
363  </list>
364</t>
365</section>
366
367<section title="Delta Seconds" anchor="delta.seconds">
368<t>
369   Some HTTP header fields allow a time value to be specified as an
370   integer number of seconds, represented in decimal, after the time
371   that the message was received.
372</t>
373<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
374  delta-seconds  = 1*DIGIT
375</artwork></figure>
376</section>
377</section>
378
379<section title="Caching in HTTP" anchor="caching">
380<section title="Overview" anchor="caching.overview">
381<t>
382   HTTP is typically used for distributed information systems, where
383   performance can be improved by the use of response caches. The
384   HTTP/1.1 protocol includes a number of elements intended to make
385   caching work as well as possible. Because these elements are
386   inextricable from other aspects of the protocol, and because they
387   interact with each other, it is useful to describe the basic caching
388   design of HTTP separately from the detailed descriptions of methods,
389   headers, response codes, etc.
390</t>
391<t>
392   Caching would be useless if it did not significantly improve
393   performance. The goal of caching in HTTP/1.1 is to eliminate the need
394   to send requests in many cases, and to eliminate the need to send
395   full responses in many other cases. The former reduces the number of
396   network round-trips required for many operations; we use an
397   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
398   latter reduces network bandwidth requirements; we use a "validation"
399   mechanism for this purpose (see <xref target="validation.model"/>).
400</t>
401<t>
402   Requirements for performance, availability, and disconnected
403   operation require us to be able to relax the goal of semantic
404   transparency. The HTTP/1.1 protocol allows origin servers, caches,
405   and clients to explicitly reduce transparency when necessary.
406   However, because non-transparent operation may confuse non-expert
407   users, and might be incompatible with certain server applications
408   (such as those for ordering merchandise), the protocol requires that
409   transparency be relaxed
410  <list style="symbols">
411     <t>only by an explicit protocol-level request when relaxed by
412        client or origin server</t>
413
414     <t>only with an explicit warning to the end user when relaxed by
415        cache or client</t>
416  </list>
417</t>
418<t>
419   Therefore, the HTTP/1.1 protocol provides these important elements:
420  <list style="numbers">
421      <t>Protocol features that provide full semantic transparency when
422         this is required by all parties.</t>
423
424      <t>Protocol features that allow an origin server or user agent to
425         explicitly request and control non-transparent operation.</t>
426
427      <t>Protocol features that allow a cache to attach warnings to
428         responses that do not preserve the requested approximation of
429         semantic transparency.</t>
430  </list>
431</t>
432<t>
433   A basic principle is that it must be possible for the clients to
434   detect any potential relaxation of semantic transparency.
435  <list><t>
436      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
437      design decisions not explicitly discussed in this specification.
438      If a decision might affect semantic transparency, the implementor
439      ought to err on the side of maintaining transparency unless a
440      careful and complete analysis shows significant benefits in
441      breaking transparency.
442    </t></list>
443</t>
444
445<section title="Cache Correctness" anchor="cache.correctness">
446<t>
447   A correct cache &MUST; respond to a request with the most up-to-date
448   response held by the cache that is appropriate to the request (see
449   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
450   <xref target="disambiguating.multiple.responses" format="counter"/>,
451   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
452   conditions:
453  <list style="numbers">
454      <t>It has been checked for equivalence with what the origin server
455         would have returned by revalidating the response with the
456         origin server (<xref target="validation.model"/>);</t>
457
458      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
459         this means it meets the least restrictive freshness requirement
460         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
461         the origin server so specifies, it is the freshness requirement
462         of the origin server alone.
463
464         If a stored response is not "fresh enough" by the most
465         restrictive freshness requirement of both the client and the
466         origin server, in carefully considered circumstances the cache
467         &MAY; still return the response with the appropriate Warning
468         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
469         and <xref target="header.warning" format="counter"/>), unless such a response
470         is prohibited (e.g., by a "no-store" cache-directive, or by a
471         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
472
473      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
474         or error (4xx or 5xx) response message.</t>
475  </list>
476</t>
477<t>
478   If the cache can not communicate with the origin server, then a
479   correct cache &SHOULD; respond as above if the response can be
480   correctly served from the cache; if not it &MUST; return an error or
481   warning indicating that there was a communication failure.
482</t>
483<t>
484   If a cache receives a response (either an entire response, or a 304
485   (Not Modified) response) that it would normally forward to the
486   requesting client, and the received response is no longer fresh, the
487   cache &SHOULD; forward it to the requesting client without adding a new
488   Warning (but without removing any existing Warning headers). A cache
489   &SHOULD-NOT;  attempt to revalidate a response simply because that
490   response became stale in transit; this might lead to an infinite
491   loop. A user agent that receives a stale response without a Warning
492   &MAY; display a warning indication to the user.
493</t>
494</section>
495
496<section title="Warnings" anchor="warnings">
497<t>
498   Whenever a cache returns a response that is neither first-hand nor
499   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
500   &MUST; attach a warning to that effect, using a Warning general-header.
501   The Warning header and the currently defined warnings are described
502   in <xref target="header.warning"/>. The warning allows clients to take appropriate
503   action.
504</t>
505<t>
506   Warnings &MAY; be used for other purposes, both cache-related and
507   otherwise. The use of a warning, rather than an error status code,
508   distinguish these responses from true failures.
509</t>
510<t>
511   Warnings are assigned three digit warn-codes. The first digit
512   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
513   stored cache entry after a successful revalidation:
514</t>
515<t>
516  <list style="hanging">
517    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
518     the response, and so &MUST; be deleted after a successful
519     revalidation. 1XX warn-codes &MAY; be generated by a cache only when
520     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
521
522    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
523     headers that is not rectified by a revalidation (for example, a
524     lossy compression of the entity bodies) and which &MUST-NOT; be
525     deleted after a successful revalidation.</t>
526    </list>
527</t>
528<t>
529   See <xref target="header.warning"/> for the definitions of the codes themselves.
530</t>
531<t>
532   HTTP/1.0 caches will cache all Warnings in responses, without
533   deleting the ones in the first category. Warnings in responses that
534   are passed to HTTP/1.0 caches carry an extra warning-date field,
535   which prevents a future HTTP/1.1 recipient from believing an
536   erroneously cached Warning.
537</t>
538<t>
539   Warnings also carry a warning text. The text &MAY; be in any
540   appropriate natural language (perhaps based on the client's Accept
541   headers), and include an &OPTIONAL; indication of what character set is
542   used.
543</t>
544<t>
545   Multiple warnings &MAY; be attached to a response (either by the origin
546   server or by a cache), including multiple warnings with the same code
547   number. For example, a server might provide the same warning with
548   texts in both English and Basque.
549</t>
550<t>
551   When multiple warnings are attached to a response, it might not be
552   practical or reasonable to display all of them to the user. This
553   version of HTTP does not specify strict priority rules for deciding
554   which warnings to display and in what order, but does suggest some
555   heuristics.
556</t>
557</section>
558
559<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
560<t>
561   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
562   times and validators) are implicit directives to caches. In some
563   cases, a server or client might need to provide explicit directives
564   to the HTTP caches. We use the Cache-Control header for this purpose.
565</t>
566<t>
567   The Cache-Control header allows a client or server to transmit a
568   variety of directives in either requests or responses. These
569   directives typically override the default caching algorithms. As a
570   general rule, if there is any apparent conflict between header
571   values, the most restrictive interpretation is applied (that is, the
572   one that is most likely to preserve semantic transparency). However,
573   in some cases, cache-control directives are explicitly specified as
574   weakening the approximation of semantic transparency (for example,
575   "max-stale" or "public").
576</t>
577<t>
578   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
579</t>
580</section>
581
582<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
583<t>
584   Many user agents make it possible for users to override the basic
585   caching mechanisms. For example, the user agent might allow the user
586   to specify that cached entities (even explicitly stale ones) are
587   never validated. Or the user agent might habitually add "Cache-Control:
588   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
589   default to either non-transparent behavior, or behavior that results
590   in abnormally ineffective caching, but &MAY; be explicitly configured
591   to do so by an explicit action of the user.
592</t>
593<t>
594   If the user has overridden the basic caching mechanisms, the user
595   agent &SHOULD; explicitly indicate to the user whenever this results in
596   the display of information that might not meet the server's
597   transparency requirements (in particular, if the displayed entity is
598   known to be stale). Since the protocol normally allows the user agent
599   to determine if responses are stale or not, this indication need only
600   be displayed when this actually happens. The indication need not be a
601   dialog box; it could be an icon (for example, a picture of a rotting
602   fish) or some other indicator.
603</t>
604<t>
605   If the user has overridden the caching mechanisms in a way that would
606   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
607   continually indicate this state to the user (for example, by a
608   display of a picture of currency in flames) so that the user does not
609   inadvertently consume excess resources or suffer from excessive
610   latency.
611</t>
612</section>
613
614<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
615<t>
616   In some cases, the operator of a cache &MAY; choose to configure it to
617   return stale responses even when not requested by clients. This
618   decision ought not be made lightly, but may be necessary for reasons
619   of availability or performance, especially when the cache is poorly
620   connected to the origin server. Whenever a cache returns a stale
621   response, it &MUST; mark it as such (using a Warning header) enabling
622   the client software to alert the user that there might be a potential
623   problem.
624</t>
625<t>
626   It also allows the user agent to take steps to obtain a first-hand or
627   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
628   response if the client explicitly requests a first-hand or fresh one,
629   unless it is impossible to comply for technical or policy reasons.
630</t>
631</section>
632
633<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
634<t>
635   While the origin server (and to a lesser extent, intermediate caches,
636   by their contribution to the age of a response) are the primary
637   source of expiration information, in some cases the client might need
638   to control a cache's decision about whether to return a cached
639   response without validating it. Clients do this using several
640   directives of the Cache-Control header.
641</t>
642<t>
643   A client's request &MAY; specify the maximum age it is willing to
644   accept of an unvalidated response; specifying a value of zero forces
645   the cache(s) to revalidate all responses. A client &MAY; also specify
646   the minimum time remaining before a response expires. Both of these
647   options increase constraints on the behavior of caches, and so cannot
648   further relax the cache's approximation of semantic transparency.
649</t>
650<t>
651   A client &MAY; also specify that it will accept stale responses, up to
652   some maximum amount of staleness. This loosens the constraints on the
653   caches, and so might violate the origin server's specified
654   constraints on semantic transparency, but might be necessary to
655   support disconnected operation, or high availability in the face of
656   poor connectivity.
657</t>
658</section>
659</section>
660
661<section title="Expiration Model" anchor="expiration.model">
662
663<section title="Server-Specified Expiration" anchor="server-specified.expiration">
664<t>
665   HTTP caching works best when caches can entirely avoid making
666   requests to the origin server. The primary mechanism for avoiding
667   requests is for an origin server to provide an explicit expiration
668   time in the future, indicating that a response &MAY; be used to satisfy
669   subsequent requests. In other words, a cache can return a fresh
670   response without first contacting the server.
671</t>
672<t>
673   Our expectation is that servers will assign future explicit
674   expiration times to responses in the belief that the entity is not
675   likely to change, in a semantically significant way, before the
676   expiration time is reached. This normally preserves semantic
677   transparency, as long as the server's expiration times are carefully
678   chosen.
679</t>
680<t>
681   The expiration mechanism applies only to responses taken from a cache
682   and not to first-hand responses forwarded immediately to the
683   requesting client.
684</t>
685<t>
686   If an origin server wishes to force a semantically transparent cache
687   to validate every request, it &MAY; assign an explicit expiration time
688   in the past. This means that the response is always stale, and so the
689   cache &SHOULD; validate it before using it for subsequent requests. See
690   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
691</t>
692<t>
693   If an origin server wishes to force any HTTP/1.1 cache, no matter how
694   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
695   cache-control directive (see <xref target="header.cache-control"/>).
696</t>
697<t>
698   Servers specify explicit expiration times using either the Expires
699   header, or the max-age directive of the Cache-Control header.
700</t>
701<t>
702   An expiration time cannot be used to force a user agent to refresh
703   its display or reload a resource; its semantics apply only to caching
704   mechanisms, and such mechanisms need only check a resource's
705   expiration status when a new request for that resource is initiated.
706   See <xref target="history.lists"/> for an explanation of the difference between caches
707   and history mechanisms.
708</t>
709</section>
710
711<section title="Heuristic Expiration" anchor="heuristic.expiration">
712<t>
713   Since origin servers do not always provide explicit expiration times,
714   HTTP caches typically assign heuristic expiration times, employing
715   algorithms that use other header values (such as the Last-Modified
716   time) to estimate a plausible expiration time. The HTTP/1.1
717   specification does not provide specific algorithms, but does impose
718   worst-case constraints on their results. Since heuristic expiration
719   times might compromise semantic transparency, they ought to be used
720   cautiously, and we encourage origin servers to provide explicit
721   expiration times as much as possible.
722</t>
723</section>
724
725<section title="Age Calculations" anchor="age.calculations">
726<t>
727   In order to know if a cached entry is fresh, a cache needs to know if
728   its age exceeds its freshness lifetime. We discuss how to calculate
729   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
730   the age of a response or cache entry.
731</t>
732<t>
733   In this discussion, we use the term "now" to mean "the current value
734   of the clock at the host performing the calculation." Hosts that use
735   HTTP, but especially hosts running origin servers and caches, &SHOULD;
736   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
737   a globally accurate time standard.
738</t>
739<t>
740   HTTP/1.1 requires origin servers to send a Date header, if possible,
741   with every response, giving the time at which the response was
742   generated (see &header-date;). We use the term "date_value" to denote
743   the value of the Date header, in a form appropriate for arithmetic
744   operations.
745</t>
746<t>
747   HTTP/1.1 uses the Age response-header to convey the estimated age of
748   the response message when obtained from a cache. The Age field value
749   is the cache's estimate of the amount of time since the response was
750   generated or revalidated by the origin server.
751</t>
752<t>
753   In essence, the Age value is the sum of the time that the response
754   has been resident in each of the caches along the path from the
755   origin server, plus the amount of time it has been in transit along
756   network paths.
757</t>
758<t>
759   We use the term "age_value" to denote the value of the Age header, in
760   a form appropriate for arithmetic operations.
761</t>
762<t>
763   A response's age can be calculated in two entirely independent ways:
764  <list style="numbers">
765      <t>now minus date_value, if the local clock is reasonably well
766         synchronized to the origin server's clock. If the result is
767         negative, the result is replaced by zero.</t>
768
769      <t>age_value, if all of the caches along the response path
770         implement HTTP/1.1.</t>
771  </list>
772</t>
773<t>
774   Given that we have two independent ways to compute the age of a
775   response when it is received, we can combine these as
776</t>
777<figure><artwork type="code">
778    corrected_received_age = max(now - date_value, age_value)
779</artwork></figure>
780<t>
781   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
782   paths, one gets a reliable (conservative) result.
783</t>
784<t>
785   Because of network-imposed delays, some significant interval might
786   pass between the time that a server generates a response and the time
787   it is received at the next outbound cache or client. If uncorrected,
788   this delay could result in improperly low ages.
789</t>
790<t>
791   Because the request that resulted in the returned Age value must have
792   been initiated prior to that Age value's generation, we can correct
793   for delays imposed by the network by recording the time at which the
794   request was initiated. Then, when an Age value is received, it &MUST;
795   be interpreted relative to the time the request was initiated, not
796   the time that the response was received. This algorithm results in
797   conservative behavior no matter how much delay is experienced. So, we
798   compute:
799</t>
800<figure><artwork type="code">
801   corrected_initial_age = corrected_received_age
802                         + (now - request_time)
803</artwork></figure>
804<t>
805   where "request_time" is the time (according to the local clock) when
806   the request that elicited this response was sent.
807</t>
808<t>
809   Summary of age calculation algorithm, when a cache receives a
810   response:
811</t>
812<figure><artwork type="code">
813   /*
814    * age_value
815    *      is the value of Age: header received by the cache with
816    *              this response.
817    * date_value
818    *      is the value of the origin server's Date: header
819    * request_time
820    *      is the (local) time when the cache made the request
821    *              that resulted in this cached response
822    * response_time
823    *      is the (local) time when the cache received the
824    *              response
825    * now
826    *      is the current (local) time
827    */
828
829   apparent_age = max(0, response_time - date_value);
830   corrected_received_age = max(apparent_age, age_value);
831   response_delay = response_time - request_time;
832   corrected_initial_age = corrected_received_age + response_delay;
833   resident_time = now - response_time;
834   current_age   = corrected_initial_age + resident_time;
835</artwork></figure>
836<t>
837   The current_age of a cache entry is calculated by adding the amount
838   of time (in seconds) since the cache entry was last validated by the
839   origin server to the corrected_initial_age. When a response is
840   generated from a cache entry, the cache &MUST; include a single Age
841   header field in the response with a value equal to the cache entry's
842   current_age.
843</t>
844<t>
845   The presence of an Age header field in a response implies that a
846   response is not first-hand. However, the converse is not true, since
847   the lack of an Age header field in a response does not imply that the
848   response is first-hand unless all caches along the request path are
849   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
850   the Age header field).
851</t>
852</section>
853
854<section title="Expiration Calculations" anchor="expiration.calculations">
855<t>
856   In order to decide whether a response is fresh or stale, we need to
857   compare its freshness lifetime to its age. The age is calculated as
858   described in <xref target="age.calculations"/>; this section describes how to calculate
859   the freshness lifetime, and to determine if a response has expired.
860   In the discussion below, the values can be represented in any form
861   appropriate for arithmetic operations.
862</t>
863<t>
864   We use the term "expires_value" to denote the value of the Expires
865   header. We use the term "max_age_value" to denote an appropriate
866   value of the number of seconds carried by the "max-age" directive of
867   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
868</t>
869<t>
870   The max-age directive takes priority over Expires, so if max-age is
871   present in a response, the calculation is simply:
872</t>
873<figure><artwork type="code">
874   freshness_lifetime = max_age_value
875</artwork></figure>
876<t>
877   Otherwise, if Expires is present in the response, the calculation is:
878</t>
879<figure><artwork type="code">
880   freshness_lifetime = expires_value - date_value
881</artwork></figure>
882<t>
883   Note that neither of these calculations is vulnerable to clock skew,
884   since all of the information comes from the origin server.
885</t>
886<t>
887   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
888   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
889   does not include other restrictions on caching, the cache &MAY; compute
890   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
891   113 to any response whose age is more than 24 hours if such warning
892   has not already been added.
893</t>
894<t>
895   Also, if the response does have a Last-Modified time, the heuristic
896   expiration value &SHOULD; be no more than some fraction of the interval
897   since that time. A typical setting of this fraction might be 10%.
898</t>
899<t>
900   The calculation to determine if a response has expired is quite
901   simple:
902</t>
903<figure><artwork type="code">
904   response_is_fresh = (freshness_lifetime &gt; current_age)
905</artwork></figure>
906</section>
907
908<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
909<t>
910   Because expiration values are assigned optimistically, it is possible
911   for two caches to contain fresh values for the same resource that are
912   different.
913</t>
914<t>
915   If a client performing a retrieval receives a non-first-hand response
916   for a request that was already fresh in its own cache, and the Date
917   header in its existing cache entry is newer than the Date on the new
918   response, then the client &MAY; ignore the response. If so, it &MAY;
919   retry the request with a "Cache-Control: max-age=0" directive (see
920   <xref target="header.cache-control"/>), to force a check with the origin server.
921</t>
922<t>
923   If a cache has two fresh responses for the same representation with
924   different validators, it &MUST; use the one with the more recent Date
925   header. This situation might arise because the cache is pooling
926   responses from other caches, or because a client has asked for a
927   reload or a revalidation of an apparently fresh cache entry.
928</t>
929</section>
930
931<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
932<t>
933   Because a client might be receiving responses via multiple paths, so
934   that some responses flow through one set of caches and other
935   responses flow through a different set of caches, a client might
936   receive responses in an order different from that in which the origin
937   server sent them. We would like the client to use the most recently
938   generated response, even if older responses are still apparently
939   fresh.
940</t>
941<t>
942   Neither the entity tag nor the expiration value can impose an
943   ordering on responses, since it is possible that a later response
944   intentionally carries an earlier expiration time. The Date values are
945   ordered to a granularity of one second.
946</t>
947<t>
948   When a client tries to revalidate a cache entry, and the response it
949   receives contains a Date header that appears to be older than the one
950   for the existing entry, then the client &SHOULD; repeat the request
951   unconditionally, and include
952</t>
953<figure><artwork type="example">
954    Cache-Control: max-age=0
955</artwork></figure>
956<t>
957   to force any intermediate caches to validate their copies directly
958   with the origin server, or
959</t>
960<figure><artwork type="example">
961    Cache-Control: no-cache
962</artwork></figure>
963<t>
964   to force any intermediate caches to obtain a new copy from the origin
965   server.
966</t>
967<t>
968   If the Date values are equal, then the client &MAY; use either response
969   (or &MAY;, if it is being extremely prudent, request a new response).
970   Servers &MUST-NOT; depend on clients being able to choose
971   deterministically between responses generated during the same second,
972   if their expiration times overlap.
973</t>
974</section>
975</section>
976
977<section title="Validation Model" anchor="validation.model">
978<t>
979   When a cache has a stale entry that it would like to use as a
980   response to a client's request, it first has to check with the origin
981   server (or possibly an intermediate cache with a fresh response) to
982   see if its cached entry is still usable. We call this "validating"
983   the cache entry. Since we do not want to have to pay the overhead of
984   retransmitting the full response if the cached entry is good, and we
985   do not want to pay the overhead of an extra round trip if the cached
986   entry is invalid, the HTTP/1.1 protocol supports the use of
987   conditional methods.
988</t>
989<t>
990   The key protocol features for supporting conditional methods are
991   those concerned with "cache validators." When an origin server
992   generates a full response, it attaches some sort of validator to it,
993   which is kept with the cache entry. When a client (user agent or
994   proxy cache) makes a conditional request for a resource for which it
995   has a cache entry, it includes the associated validator in the
996   request.
997</t>
998<t>
999   The server then checks that validator against the current validator
1000   for the entity, and, if they match (see &weak-and-strong-validators;), it responds
1001   with a special status code (usually, 304 (Not Modified)) and no
1002   entity-body. Otherwise, it returns a full response (including
1003   entity-body). Thus, we avoid transmitting the full response if the
1004   validator matches, and we avoid an extra round trip if it does not
1005   match.
1006</t>
1007<t>
1008   In HTTP/1.1, a conditional request looks exactly the same as a normal
1009   request for the same resource, except that it carries a special
1010   header (which includes the validator) that implicitly turns the
1011   method (usually, GET) into a conditional.
1012</t>
1013<t>
1014   The protocol includes both positive and negative senses of cache-validating
1015   conditions. That is, it is possible to request either that
1016   a method be performed if and only if a validator matches or if and
1017   only if no validators match.
1018  <list><t>
1019      <x:h>Note:</x:h> a response that lacks a validator may still be cached, and
1020      served from cache until it expires, unless this is explicitly
1021      prohibited by a cache-control directive. However, a cache cannot
1022      do a conditional retrieval if it does not have a validator for the
1023      entity, which means it will not be refreshable after it expires.
1024  </t></list>
1025</t>
1026
1027<section title="Last-Modified Dates" anchor="last-modified.dates">
1028<t>
1029   The Last-Modified entity-header field value is often used as a cache
1030   validator. In simple terms, a cache entry is considered to be valid
1031   if the entity has not been modified since the Last-Modified value.
1032</t>
1033</section>
1034
1035<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
1036<t>
1037   The ETag response-header field value, an entity tag, provides for an
1038   "opaque" cache validator. This might allow more reliable validation
1039   in situations where it is inconvenient to store modification dates,
1040   where the one-second resolution of HTTP date values is not
1041   sufficient, or where the origin server wishes to avoid certain
1042   paradoxes that might arise from the use of modification dates.
1043</t>
1044<t>
1045   Entity Tags are described in &entity-tags;. The headers used with entity
1046   tags are described in &entity-header-fields;.
1047</t>
1048</section>
1049
1050<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
1051<t>
1052   The principle behind entity tags is that only the service author
1053   knows the semantics of a resource well enough to select an
1054   appropriate cache validation mechanism, and the specification of any
1055   validator comparison function more complex than byte-equality would
1056   open up a can of worms. Thus, comparisons of any other headers
1057   (except Last-Modified, for compatibility with HTTP/1.0) are never
1058   used for purposes of validating a cache entry.
1059</t>
1060</section>
1061</section>
1062
1063<section title="Response Cacheability" anchor="response.cacheability">
1064<t>
1065   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1066   directive, a caching system &MAY; always store a successful response
1067   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1068   if it is fresh, and &MAY; return it after successful validation. If
1069   there is neither a cache validator nor an explicit expiration time
1070   associated with a response, we do not expect it to be cached, but
1071   certain caches &MAY; violate this expectation (for example, when little
1072   or no network connectivity is available). A client can usually detect
1073   that such a response was taken from a cache by comparing the Date
1074   header to the current time.
1075  <list><t>
1076      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1077      without providing any Warning.
1078  </t></list>
1079</t>
1080<t>
1081   However, in some cases it might be inappropriate for a cache to
1082   retain an entity, or to return it in response to a subsequent
1083   request. This might be because absolute semantic transparency is
1084   deemed necessary by the service author, or because of security or
1085   privacy considerations. Certain cache-control directives are
1086   therefore provided so that the server can indicate that certain
1087   resource entities, or portions thereof, are not to be cached
1088   regardless of other considerations.
1089</t>
1090<t>
1091   Note that &header-authorization; normally prevents a shared cache from saving
1092   and returning a response to a previous request if that request
1093   included an Authorization header.
1094</t>
1095<t>
1096   A response received with a status code of 200, 203, 206, 300, 301 or
1097   410 &MAY; be stored by a cache and used in reply to a subsequent
1098   request, subject to the expiration mechanism, unless a cache-control
1099   directive prohibits caching. However, a cache that does not support
1100   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1101   Content) responses.
1102</t>
1103<t>
1104   A response received with any other status code (e.g. status codes 302
1105   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1106   unless there are cache-control directives or another header(s) that
1107   explicitly allow it. For example, these include the following: an
1108   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1109   "proxy-revalidate", "public" or "private" cache-control
1110   directive (<xref target="header.cache-control"/>).
1111</t>
1112</section>
1113
1114<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1115<t>
1116   The purpose of an HTTP cache is to store information received in
1117   response to requests for use in responding to future requests. In
1118   many cases, a cache simply returns the appropriate parts of a
1119   response to the requester. However, if the cache holds a cache entry
1120   based on a previous response, it might have to combine parts of a new
1121   response with what is held in the cache entry.
1122</t>
1123
1124<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1125<t>
1126   For the purpose of defining the behavior of caches and non-caching
1127   proxies, we divide HTTP headers into two categories:
1128  <list style="symbols">
1129      <t>End-to-end headers, which are  transmitted to the ultimate
1130        recipient of a request or response. End-to-end headers in
1131        responses &MUST; be stored as part of a cache entry and &MUST; be
1132        transmitted in any response formed from a cache entry.</t>
1133
1134      <t>Hop-by-hop headers, which are meaningful only for a single
1135        transport-level connection, and are not stored by caches or
1136        forwarded by proxies.</t>
1137  </list>
1138</t>
1139<t>
1140   The following HTTP/1.1 headers are hop-by-hop headers:
1141  <list style="symbols">
1142      <t>Connection</t>
1143      <t>Keep-Alive</t>
1144      <t>Proxy-Authenticate</t>
1145      <t>Proxy-Authorization</t>
1146      <t>TE</t>
1147      <t>Trailer</t>
1148      <t>Transfer-Encoding</t>
1149      <t>Upgrade</t>
1150  </list>
1151</t>
1152<t>
1153   All other headers defined by HTTP/1.1 are end-to-end headers.
1154</t>
1155<t>
1156   Other hop-by-hop headers &MUST; be listed in a Connection header
1157   (&header-connection;).
1158</t>
1159</section>
1160
1161<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1162<t>
1163   Some features of the HTTP/1.1 protocol, such as Digest
1164   Authentication, depend on the value of certain end-to-end headers. A
1165   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1166   definition of that header requires or specifically allows that.
1167</t>
1168<t>
1169   A transparent proxy &MUST-NOT; modify any of the following fields in a
1170   request or response, and it &MUST-NOT; add any of these fields if not
1171   already present:
1172  <list style="symbols">
1173      <t>Content-Location</t>
1174      <t>Content-MD5</t>
1175      <t>ETag</t>
1176      <t>Last-Modified</t>
1177  </list>
1178</t>
1179<t>
1180   A transparent proxy &MUST-NOT; modify any of the following fields in a
1181   response:
1182  <list style="symbols">
1183    <t>Expires</t>
1184  </list>
1185</t>
1186<t>
1187   but it &MAY; add any of these fields if not already present. If an
1188   Expires header is added, it &MUST; be given a field-value identical to
1189   that of the Date header in that response.
1190</t>
1191<t>
1192   A  proxy &MUST-NOT; modify or add any of the following fields in a
1193   message that contains the no-transform cache-control directive, or in
1194   any request:
1195  <list style="symbols">
1196    <t>Content-Encoding</t>
1197    <t>Content-Range</t>
1198    <t>Content-Type</t>
1199  </list>
1200</t>
1201<t>
1202   A non-transparent proxy &MAY; modify or add these fields to a message
1203   that does not include no-transform, but if it does so, it &MUST; add a
1204   Warning 214 (Transformation applied) if one does not already appear
1205   in the message (see <xref target="header.warning"/>).
1206  <list><t>
1207      Warning: unnecessary modification of end-to-end headers might
1208      cause authentication failures if stronger authentication
1209      mechanisms are introduced in later versions of HTTP. Such
1210      authentication mechanisms &MAY; rely on the values of header fields
1211      not listed here.
1212    </t></list>
1213</t>
1214<t>
1215   The Content-Length field of a request or response is added or deleted
1216   according to the rules in &message-length;. A transparent proxy &MUST;
1217   preserve the entity-length (&entity-length;) of the entity-body,
1218   although it &MAY; change the transfer-length (&message-length;).
1219</t>
1220</section>
1221
1222<section title="Combining Headers" anchor="combining.headers">
1223<t>
1224   When a cache makes a validating request to a server, and the server
1225   provides a 304 (Not Modified) response or a 206 (Partial Content)
1226   response, the cache then constructs a response to send to the
1227   requesting client.
1228</t>
1229<t>
1230   If the status code is 304 (Not Modified), the cache uses the entity-body
1231   stored in the cache entry as the entity-body of this outgoing
1232   response. If the status code is 206 (Partial Content) and the ETag or
1233   Last-Modified headers match exactly, the cache &MAY; combine the
1234   contents stored in the cache entry with the new contents received in
1235   the response and use the result as the entity-body of this outgoing
1236   response, (see &combining-byte-ranges;).
1237</t>
1238<t>
1239   The end-to-end headers stored in the cache entry are used for the
1240   constructed response, except that
1241  <list style="symbols">
1242    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1243      &MUST; be deleted from the cache entry and the forwarded response.</t>
1244    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1245        in the cache entry and the forwarded response.</t>
1246    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1247        replace the corresponding headers from the cache entry.</t>
1248  </list>
1249</t>
1250<t>
1251   Unless the cache decides to remove the cache entry, it &MUST; also
1252   replace the end-to-end headers stored with the cache entry with
1253   corresponding headers received in the incoming response, except for
1254   Warning headers as described immediately above. If a header field-name
1255   in the incoming response matches more than one header in the
1256   cache entry, all such old headers &MUST; be replaced.
1257</t>
1258<t>
1259   In other words, the set of end-to-end headers received in the
1260   incoming response overrides all corresponding end-to-end headers
1261   stored with the cache entry (except for stored Warning headers with
1262   warn-code 1xx, which are deleted even if not overridden).
1263  <list><t>
1264      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1265      Modified) or a 206 (Partial Content) response to update any header
1266      associated with a previous response for the same entity or sub-ranges
1267      thereof, although it might not always be meaningful or
1268      correct to do so. This rule does not allow an origin server to use
1269      a 304 (Not Modified) or a 206 (Partial Content) response to
1270      entirely delete a header that it had provided with a previous
1271      response.
1272  </t></list>
1273</t>
1274</section>
1275
1276</section>
1277
1278<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1279<t>
1280   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1281   by the presence of a Vary header field in a response, alters the
1282   conditions and procedure by which a cache can use the response for
1283   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1284   field by servers.
1285</t>
1286<t>
1287   A server &SHOULD; use the Vary header field to inform a cache of what
1288   request-header fields were used to select among multiple
1289   representations of a cacheable response subject to server-driven
1290   negotiation. The set of header fields named by the Vary field value
1291   is known as the "selecting" request-headers.
1292</t>
1293<t>
1294   When the cache receives a subsequent request whose Request-URI
1295   specifies one or more cache entries including a Vary header field,
1296   the cache &MUST-NOT; use such a cache entry to construct a response to
1297   the new request unless all of the selecting request-headers present
1298   in the new request match the corresponding stored request-headers in
1299   the original request.
1300</t>
1301<t>
1302   The selecting request-headers from two requests are defined to match
1303   if and only if the selecting request-headers in the first request can
1304   be transformed to the selecting request-headers in the second request
1305   by adding or removing linear white space (LWS) at places where this
1306   is allowed by the corresponding BNF, and/or combining multiple
1307   message-header fields with the same field name following the rules
1308   about message headers in &message-headers;.
1309</t>
1310<t>
1311   A Vary header field-value of "*" always fails to match and subsequent
1312   requests on that resource can only be properly interpreted by the
1313   origin server.
1314</t>
1315<t>
1316   If the selecting request header fields for the cached entry do not
1317   match the selecting request header fields of the new request, then
1318   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1319   it first relays the new request to the origin server in a conditional
1320   request and the server responds with 304 (Not Modified), including an
1321   entity tag or Content-Location that indicates the entity to be used.
1322</t>
1323<t>
1324   If an entity tag was assigned to a cached representation, the
1325   forwarded request &SHOULD; be conditional and include the entity tags
1326   in an If-None-Match header field from all its cache entries for the
1327   resource. This conveys to the server the set of entities currently
1328   held by the cache, so that if any one of these entities matches the
1329   requested entity, the server can use the ETag header field in its 304
1330   (Not Modified) response to tell the cache which entry is appropriate.
1331   If the entity-tag of the new response matches that of an existing
1332   entry, the new response &SHOULD; be used to update the header fields of
1333   the existing entry, and the result &MUST; be returned to the client.
1334</t>
1335<t>
1336   If any of the existing cache entries contains only partial content
1337   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1338   the If-None-Match header field unless the request is for a range that
1339   would be fully satisfied by that entry.
1340</t>
1341<t>
1342   If a cache receives a successful response whose Content-Location
1343   field matches that of an existing cache entry for the same Request-URI,
1344   whose entity-tag differs from that of the existing entry, and
1345   whose Date is more recent than that of the existing entry, the
1346   existing entry &SHOULD-NOT;  be returned in response to future requests
1347   and &SHOULD; be deleted from the cache.
1348</t>
1349</section>
1350
1351<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1352<t>
1353   For reasons of security and privacy, it is necessary to make a
1354   distinction between "shared" and "non-shared" caches. A non-shared
1355   cache is one that is accessible only to a single user. Accessibility
1356   in this case &SHOULD; be enforced by appropriate security mechanisms.
1357   All other caches are considered to be "shared." Other sections of
1358   this specification place certain constraints on the operation of
1359   shared caches in order to prevent loss of privacy or failure of
1360   access controls.
1361</t>
1362</section>
1363
1364<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1365<t>
1366   A cache that receives an incomplete response (for example, with fewer
1367   bytes of data than specified in a Content-Length header) &MAY; store
1368   the response. However, the cache &MUST; treat this as a partial
1369   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1370   the result might be a full response or might still be
1371   partial. A cache &MUST-NOT; return a partial response to a client
1372   without explicitly marking it as such, using the 206 (Partial
1373   Content) status code. A cache &MUST-NOT; return a partial response
1374   using a status code of 200 (OK).
1375</t>
1376<t>
1377   If a cache receives a 5xx response while attempting to revalidate an
1378   entry, it &MAY; either forward this response to the requesting client,
1379   or act as if the server failed to respond. In the latter case, it &MAY;
1380   return a previously received response unless the cached entry
1381   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1382</t>
1383</section>
1384
1385<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1386<t>
1387   Unless the origin server explicitly prohibits the caching of their
1388   responses, the application of GET and HEAD methods to any resources
1389   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1390   these responses are taken from a cache. They &MAY; still have side
1391   effects, but a cache is not required to consider such side effects in
1392   its caching decisions. Caches are always expected to observe an
1393   origin server's explicit restrictions on caching.
1394</t>
1395<t>
1396   We note one exception to this rule: since some applications have
1397   traditionally used GETs and HEADs with query URLs (those containing a
1398   "?" in the rel_path part) to perform operations with significant side
1399   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1400   the server provides an explicit expiration time. This specifically
1401   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1402   be taken from a cache. See &safe-methods; for related information.
1403</t>
1404</section>
1405
1406<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1407<t>
1408   The effect of certain methods performed on a resource at the origin
1409   server might cause one or more existing cache entries to become non-transparently
1410   invalid. That is, although they might continue to be
1411   "fresh," they do not accurately reflect what the origin server would
1412   return for a new request on that resource.
1413</t>
1414<t>
1415   There is no way for the HTTP protocol to guarantee that all such
1416   cache entries are marked invalid. For example, the request that
1417   caused the change at the origin server might not have gone through
1418   the proxy where a cache entry is stored. However, several rules help
1419   reduce the likelihood of erroneous behavior.
1420</t>
1421<t>
1422   In this section, the phrase "invalidate an entity" means that the
1423   cache will either remove all instances of that entity from its
1424   storage, or will mark these as "invalid" and in need of a mandatory
1425   revalidation before they can be returned in response to a subsequent
1426   request.
1427</t>
1428<t>
1429   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1430   either the entity referred to by the Request-URI, or by the Location
1431   or Content-Location headers (if present). These methods are:
1432  <list style="symbols">
1433      <t>PUT</t>
1434      <t>DELETE</t>
1435      <t>POST</t>
1436  </list>
1437</t> 
1438<t>
1439   An invalidation based
1440   on the URI in a Location or Content-Location header &MUST-NOT; be
1441   performed if the host part of that URI differs from the host part
1442   in the Request-URI. This helps prevent denial of service attacks.
1443</t>
1444<t>
1445   A cache that passes through requests for methods it does not
1446   understand &SHOULD; invalidate any entities referred to by the
1447   Request-URI.
1448</t>
1449</section>
1450
1451<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1452<t>
1453   All methods that might be expected to cause modifications to the
1454   origin server's resources &MUST; be written through to the origin
1455   server. This currently includes all methods except for GET and HEAD.
1456   A cache &MUST-NOT; reply to such a request from a client before having
1457   transmitted the request to the inbound server, and having received a
1458   corresponding response from the inbound server. This does not prevent
1459   a proxy cache from sending a 100 (Continue) response before the
1460   inbound server has sent its final reply.
1461</t>
1462<t>
1463   The alternative (known as "write-back" or "copy-back" caching) is not
1464   allowed in HTTP/1.1, due to the difficulty of providing consistent
1465   updates and the problems arising from server, cache, or network
1466   failure prior to write-back.
1467</t>
1468</section>
1469
1470<section title="Cache Replacement" anchor="cache.replacement">
1471<t>
1472   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1473   <xref target="disambiguating.expiration.values" format="counter"/>,
1474   <xref target="disambiguating.multiple.responses" format="counter"/>
1475   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1476   response is received from a resource while any existing responses for
1477   the same resource are cached, the cache &SHOULD; use the new response
1478   to reply to the current request. It &MAY; insert it into cache storage
1479   and &MAY;, if it meets all other requirements, use it to respond to any
1480   future requests that would previously have caused the old response to
1481   be returned. If it inserts the new response into cache storage  the
1482   rules in <xref target="combining.headers"/> apply.
1483  <list><t>
1484      <x:h>Note:</x:h> a new response that has an older Date header value than
1485      existing cached responses is not cacheable.
1486  </t></list>
1487</t>
1488</section>
1489
1490<section title="History Lists" anchor="history.lists">
1491<t>
1492   User agents often have history mechanisms, such as "Back" buttons and
1493   history lists, which can be used to redisplay an entity retrieved
1494   earlier in a session.
1495</t>
1496<t>
1497   History mechanisms and caches are different. In particular history
1498   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1499   the current state of a resource. Rather, a history mechanism is meant
1500   to show exactly what the user saw at the time when the resource was
1501   retrieved.
1502</t>
1503<t>
1504   By default, an expiration time does not apply to history mechanisms.
1505   If the entity is still in storage, a history mechanism &SHOULD; display
1506   it even if the entity has expired, unless the user has specifically
1507   configured the agent to refresh expired history documents.
1508</t>
1509<t>
1510   This is not to be construed to prohibit the history mechanism from
1511   telling the user that a view might be stale.
1512  <list><t>
1513      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1514      viewing stale resources, this will tend to force service authors
1515      to avoid using HTTP expiration controls and cache controls when
1516      they would otherwise like to. Service authors may consider it
1517      important that users not be presented with error messages or
1518      warning messages when they use navigation controls (such as BACK)
1519      to view previously fetched resources. Even though sometimes such
1520      resources ought not to cached, or ought to expire quickly, user
1521      interface considerations may force service authors to resort to
1522      other means of preventing caching (e.g. "once-only" URLs) in order
1523      not to suffer the effects of improperly functioning history
1524      mechanisms.
1525  </t></list>
1526</t>
1527</section>
1528</section>
1529
1530<section title="Header Field Definitions" anchor="header.fields">
1531<t>
1532   This section defines the syntax and semantics of HTTP/1.1 header fields
1533   related to caching.
1534</t>
1535<t>
1536   For entity-header fields, both sender and recipient refer to either the
1537   client or the server, depending on who sends and who receives the entity.
1538</t>
1539
1540<section title="Age" anchor="header.age">
1541  <iref primary="true" item="Age header" x:for-anchor=""/>
1542  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1543<t>
1544      The Age response-header field conveys the sender's estimate of the
1545      amount of time since the response (or its revalidation) was
1546      generated at the origin server. A cached response is "fresh" if
1547      its age does not exceed its freshness lifetime. Age values are
1548      calculated as specified in <xref target="age.calculations"/>.
1549</t>
1550<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1551  Age = "Age" ":" age-value
1552  age-value = delta-seconds
1553</artwork></figure>
1554<t>
1555      Age values are non-negative decimal integers, representing time in
1556      seconds.
1557</t>
1558<t>
1559      If a cache receives a value larger than the largest positive
1560      integer it can represent, or if any of its age calculations
1561      overflows, it &MUST; transmit an Age header with a value of
1562      2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
1563      include an Age header field in every response generated from its
1564      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1565      bits of range.
1566</t>
1567</section>
1568
1569<section title="Cache-Control" anchor="header.cache-control">
1570  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1571  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1572<t>
1573   The Cache-Control general-header field is used to specify directives
1574   that &MUST; be obeyed by all caching mechanisms along the
1575   request/response chain. The directives specify behavior intended to
1576   prevent caches from adversely interfering with the request or
1577   response. These directives typically override the default caching
1578   algorithms. Cache directives are unidirectional in that the presence
1579   of a directive in a request does not imply that the same directive is
1580   to be given in the response.
1581  <list><t>
1582      Note that HTTP/1.0 caches might not implement Cache-Control and
1583      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1584  </t></list>
1585</t>
1586<t>
1587   Cache directives &MUST; be passed through by a proxy or gateway
1588   application, regardless of their significance to that application,
1589   since the directives might be applicable to all recipients along the
1590   request/response chain. It is not possible to specify a cache-directive
1591   for a specific cache.
1592</t>
1593<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"/>
1594  Cache-Control   = "Cache-Control" ":" 1#cache-directive
1595
1596  cache-directive = cache-request-directive
1597       | cache-response-directive
1598
1599  cache-request-directive =
1600         "no-cache"                          ; <xref target="what.is.cacheable"/>
1601       | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1602       | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1603       | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1604       | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1605       | "no-transform"                      ; <xref target="no-transform.directive"/>
1606       | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1607       | cache-extension                     ; <xref target="cache.control.extensions"/>
1608
1609  cache-response-directive =
1610         "public"                               ; <xref target="what.is.cacheable"/>
1611       | "private" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ] ; <xref target="what.is.cacheable"/>
1612       | "no-cache" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ]; <xref target="what.is.cacheable"/>
1613       | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1614       | "no-transform"                         ; <xref target="no-transform.directive"/>
1615       | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1616       | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1617       | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1618       | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1619       | cache-extension                        ; <xref target="cache.control.extensions"/>
1620
1621  cache-extension = token [ "=" ( token | quoted-string ) ]
1622</artwork></figure>
1623<t>
1624   When a directive appears without any 1#field-name parameter, the
1625   directive applies to the entire request or response. When such a
1626   directive appears with a 1#field-name parameter, it applies only to
1627   the named field or fields, and not to the rest of the request or
1628   response. This mechanism supports extensibility; implementations of
1629   future versions of the HTTP protocol might apply these directives to
1630   header fields not defined in HTTP/1.1.
1631</t>
1632<t>
1633   The cache-control directives can be broken down into these general
1634   categories:
1635  <list style="symbols">
1636     <t>Restrictions on what are cacheable; these may only be imposed by
1637        the origin server.</t>
1638
1639     <t>Restrictions on what may be stored by a cache; these may be
1640        imposed by either the origin server or the user agent.</t>
1641
1642     <t>Modifications of the basic expiration mechanism; these may be
1643        imposed by either the origin server or the user agent.</t>
1644
1645     <t>Controls over cache revalidation and reload; these may only be
1646        imposed by a user agent.</t>
1647
1648     <t>Control over transformation of entities.</t>
1649
1650     <t>Extensions to the caching system.</t>
1651  </list>
1652</t>
1653
1654<section title="What is Cacheable" anchor="what.is.cacheable">
1655<t>
1656   By default, a response is cacheable if the requirements of the
1657   request method, request header fields, and the response status
1658   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1659   for cacheability. The following Cache-Control response directives
1660   allow an origin server to override the default cacheability of a
1661   response:
1662</t>
1663<t>
1664  <iref item="Cache Directives" subitem="public" primary="true"/>
1665  <iref item="public" subitem="Cache Directive" primary="true"/>
1666   public
1667  <list><t>
1668      Indicates that the response &MAY; be cached by any cache, even if it
1669      would normally be non-cacheable or cacheable only within a non-shared
1670      cache. (See also Authorization, &header-authorization;, for
1671      additional details.)
1672  </t></list>
1673</t>
1674<t>
1675  <iref item="Cache Directives" subitem="private" primary="true"/>
1676  <iref item="private" subitem="Cache Directive" primary="true"/>
1677   private
1678  <list><t>
1679      Indicates that all or part of the response message is intended for
1680      a single user and &MUST-NOT; be cached by a shared cache. This
1681      allows an origin server to state that the specified parts of the
1682      response are intended for only one user and are not a valid
1683      response for requests by other users. A private (non-shared) cache
1684      &MAY; cache the response.
1685    </t><t>
1686       <x:h>Note:</x:h> This usage of the word private only controls where the
1687       response may be cached, and cannot ensure the privacy of the
1688       message content.
1689  </t></list>
1690</t>
1691<t>
1692  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1693  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1694   no-cache
1695  <list><t>
1696       If the no-cache directive does not specify a field-name, then a
1697      cache &MUST-NOT; use the response to satisfy a subsequent request
1698      without successful revalidation with the origin server. This
1699      allows an origin server to prevent caching even by caches that
1700      have been configured to return stale responses to client requests.
1701    </t><t>
1702      If the no-cache directive does specify one or more field-names,
1703      then a cache &MAY; use the response to satisfy a subsequent request,
1704      subject to any other restrictions on caching. However, the
1705      specified field-name(s) &MUST-NOT; be sent in the response to a
1706      subsequent request without successful revalidation with the origin
1707      server. This allows an origin server to prevent the re-use of
1708      certain header fields in a response, while still allowing caching
1709      of the rest of the response.
1710    <list><t>
1711       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1712       directive.
1713    </t></list>
1714  </t></list>
1715</t>
1716</section>
1717
1718<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1719<t>
1720  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1721  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1722   no-store
1723  <list><t>   
1724      The purpose of the no-store directive is to prevent the
1725      inadvertent release or retention of sensitive information (for
1726      example, on backup tapes). The no-store directive applies to the
1727      entire message, and &MAY; be sent either in a response or in a
1728      request. If sent in a request, a cache &MUST-NOT; store any part of
1729      either this request or any response to it. If sent in a response,
1730      a cache &MUST-NOT; store any part of either this response or the
1731      request that elicited it. This directive applies to both non-shared
1732      and shared caches. "&MUST-NOT; store" in this context means
1733      that the cache &MUST-NOT; intentionally store the information in
1734      non-volatile storage, and &MUST; make a best-effort attempt to
1735      remove the information from volatile storage as promptly as
1736      possible after forwarding it.
1737  </t><t>
1738      Even when this directive is associated with a response, users
1739      might explicitly store such a response outside of the caching
1740      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1741      such responses as part of their normal operation.
1742  </t><t>
1743      The purpose of this directive is to meet the stated requirements
1744      of certain users and service authors who are concerned about
1745      accidental releases of information via unanticipated accesses to
1746      cache data structures. While the use of this directive might
1747      improve privacy in some cases, we caution that it is NOT in any
1748      way a reliable or sufficient mechanism for ensuring privacy. In
1749      particular, malicious or compromised caches might not recognize or
1750      obey this directive, and communications networks might be
1751      vulnerable to eavesdropping.
1752  </t></list>
1753</t>
1754</section>
1755
1756<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1757<t>
1758   The expiration time of an entity &MAY; be specified by the origin
1759   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1760   it &MAY; be specified using the max-age directive in a response. When
1761   the max-age cache-control directive is present in a cached response,
1762   the response is stale if its current age is greater than the age
1763   value given (in seconds) at the time of a new request for that
1764   resource. The max-age directive on a response implies that the
1765   response is cacheable (i.e., "public") unless some other, more
1766   restrictive cache directive is also present.
1767</t>
1768<t>
1769   If a response includes both an Expires header and a max-age
1770   directive, the max-age directive overrides the Expires header, even
1771   if the Expires header is more restrictive. This rule allows an origin
1772   server to provide, for a given response, a longer expiration time to
1773   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1774   useful if certain HTTP/1.0 caches improperly calculate ages or
1775   expiration times, perhaps due to desynchronized clocks.
1776</t>
1777<t>
1778   Many HTTP/1.0 cache implementations will treat an Expires value that
1779   is less than or equal to the response Date value as being equivalent
1780   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1781   cache receives such a response, and the response does not include a
1782   Cache-Control header field, it &SHOULD; consider the response to be
1783   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1784  <list><t>
1785       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1786       cache control feature, such as the "private" directive, on a
1787       network including older caches that do not understand that
1788       feature. The origin server will need to combine the new feature
1789       with an Expires field whose value is less than or equal to the
1790       Date value. This will prevent older caches from improperly
1791       caching the response.
1792  </t></list>
1793</t>
1794<t>
1795  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1796  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1797   s-maxage
1798  <list><t>
1799       If a response includes an s-maxage directive, then for a shared
1800       cache (but not for a private cache), the maximum age specified by
1801       this directive overrides the maximum age specified by either the
1802       max-age directive or the Expires header. The s-maxage directive
1803       also implies the semantics of the proxy-revalidate directive (see
1804       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1805       entry after it becomes stale to respond to a subsequent request
1806       without first revalidating it with the origin server. The s-maxage
1807       directive is always ignored by a private cache.
1808  </t></list>
1809</t>
1810<t>
1811   Note that most older caches, not compliant with this specification,
1812   do not implement any cache-control directives. An origin server
1813   wishing to use a cache-control directive that restricts, but does not
1814   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1815   requirement that the max-age directive overrides the Expires header,
1816   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1817   max-age directive.
1818</t>
1819<t>
1820   Other directives allow a user agent to modify the basic expiration
1821   mechanism. These directives &MAY; be specified on a request:
1822</t>
1823<t>
1824  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1825  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1826   max-age
1827  <list><t>
1828      Indicates that the client is willing to accept a response whose
1829      age is no greater than the specified time in seconds. Unless max-stale
1830      directive is also included, the client is not willing to
1831      accept a stale response.
1832  </t></list>
1833</t>
1834<t>
1835  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1836  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1837   min-fresh
1838  <list><t>
1839      Indicates that the client is willing to accept a response whose
1840      freshness lifetime is no less than its current age plus the
1841      specified time in seconds. That is, the client wants a response
1842      that will still be fresh for at least the specified number of
1843      seconds.
1844  </t></list>
1845</t>
1846<t>
1847  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1848  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1849   max-stale
1850  <list><t>
1851      Indicates that the client is willing to accept a response that has
1852      exceeded its expiration time. If max-stale is assigned a value,
1853      then the client is willing to accept a response that has exceeded
1854      its expiration time by no more than the specified number of
1855      seconds. If no value is assigned to max-stale, then the client is
1856      willing to accept a stale response of any age.
1857  </t></list>
1858</t>
1859<t>
1860   If a cache returns a stale response, either because of a max-stale
1861   directive on a request, or because the cache is configured to
1862   override the expiration time of a response, the cache &MUST; attach a
1863   Warning header to the stale response, using Warning 110 (Response is
1864   stale).
1865</t>
1866<t>
1867   A cache &MAY; be configured to return stale responses without
1868   validation, but only if this does not conflict with any "MUST"-level
1869   requirements concerning cache validation (e.g., a "must-revalidate"
1870   cache-control directive).
1871</t>
1872<t>
1873   If both the new request and the cached entry include "max-age"
1874   directives, then the lesser of the two values is used for determining
1875   the freshness of the cached entry for that request.
1876</t>
1877</section>
1878
1879<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1880<t>
1881   Sometimes a user agent might want or need to insist that a cache
1882   revalidate its cache entry with the origin server (and not just with
1883   the next cache along the path to the origin server), or to reload its
1884   cache entry from the origin server. End-to-end revalidation might be
1885   necessary if either the cache or the origin server has overestimated
1886   the expiration time of the cached response. End-to-end reload may be
1887   necessary if the cache entry has become corrupted for some reason.
1888</t>
1889<t>
1890   End-to-end revalidation may be requested either when the client does
1891   not have its own local cached copy, in which case we call it
1892   "unspecified end-to-end revalidation", or when the client does have a
1893   local cached copy, in which case we call it "specific end-to-end
1894   revalidation."
1895</t>
1896<t>
1897   The client can specify these three kinds of action using Cache-Control
1898   request directives:
1899</t>
1900<t>
1901   End-to-end reload
1902  <list><t>
1903      The request includes a "no-cache" cache-control directive or, for
1904      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1905      names &MUST-NOT; be included with the no-cache directive in a
1906      request. The server &MUST-NOT; use a cached copy when responding to
1907      such a request.
1908  </t></list>
1909</t>
1910<t>
1911   Specific end-to-end revalidation
1912  <list><t>
1913      The request includes a "max-age=0" cache-control directive, which
1914      forces each cache along the path to the origin server to
1915      revalidate its own entry, if any, with the next cache or server.
1916      The initial request includes a cache-validating conditional with
1917      the client's current validator.
1918  </t></list>
1919</t>
1920<t>
1921   Unspecified end-to-end revalidation
1922  <list><t>
1923      The request includes "max-age=0" cache-control directive, which
1924      forces each cache along the path to the origin server to
1925      revalidate its own entry, if any, with the next cache or server.
1926      The initial request does not include a cache-validating
1927      conditional; the first cache along the path (if any) that holds a
1928      cache entry for this resource includes a cache-validating
1929      conditional with its current validator.
1930  </t></list>
1931</t>
1932<t>
1933  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1934  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1935   max-age
1936  <list><t>
1937      When an intermediate cache is forced, by means of a max-age=0
1938      directive, to revalidate its own cache entry, and the client has
1939      supplied its own validator in the request, the supplied validator
1940      might differ from the validator currently stored with the cache
1941      entry. In this case, the cache &MAY; use either validator in making
1942      its own request without affecting semantic transparency.
1943  </t><t>
1944      However, the choice of validator might affect performance. The
1945      best approach is for the intermediate cache to use its own
1946      validator when making its request. If the server replies with 304
1947      (Not Modified), then the cache can return its now validated copy
1948      to the client with a 200 (OK) response. If the server replies with
1949      a new entity and cache validator, however, the intermediate cache
1950      can compare the returned validator with the one provided in the
1951      client's request, using the strong comparison function. If the
1952      client's validator is equal to the origin server's, then the
1953      intermediate cache simply returns 304 (Not Modified). Otherwise,
1954      it returns the new entity with a 200 (OK) response.
1955  </t><t>
1956      If a request includes the no-cache directive, it &SHOULD-NOT; 
1957      include min-fresh, max-stale, or max-age.
1958  </t></list>
1959</t>
1960<t>
1961  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1962  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1963   only-if-cached
1964  <list><t>
1965      In some cases, such as times of extremely poor network
1966      connectivity, a client may want a cache to return only those
1967      responses that it currently has stored, and not to reload or
1968      revalidate with the origin server. To do this, the client may
1969      include the only-if-cached directive in a request. If it receives
1970      this directive, a cache &SHOULD; either respond using a cached entry
1971      that is consistent with the other constraints of the request, or
1972      respond with a 504 (Gateway Timeout) status. However, if a group
1973      of caches is being operated as a unified system with good internal
1974      connectivity, such a request &MAY; be forwarded within that group of
1975      caches.
1976  </t></list>
1977</t>
1978<t>
1979  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1980  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1981   must-revalidate
1982  <list><t>
1983      Because a cache &MAY; be configured to ignore a server's specified
1984      expiration time, and because a client request &MAY; include a max-stale
1985      directive (which has a similar effect), the protocol also
1986      includes a mechanism for the origin server to require revalidation
1987      of a cache entry on any subsequent use. When the must-revalidate
1988      directive is present in a response received by a cache, that cache
1989      &MUST-NOT; use the entry after it becomes stale to respond to a
1990      subsequent request without first revalidating it with the origin
1991      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1992      time, if, based solely on the origin server's Expires or max-age
1993      value, the cached response is stale.)
1994  </t><t>
1995      The must-revalidate directive is necessary to support reliable
1996      operation for certain protocol features. In all circumstances an
1997      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1998      particular, if the cache cannot reach the origin server for any
1999      reason, it &MUST; generate a 504 (Gateway Timeout) response.
2000  </t><t>
2001      Servers &SHOULD; send the must-revalidate directive if and only if
2002      failure to revalidate a request on the entity could result in
2003      incorrect operation, such as a silently unexecuted financial
2004      transaction. Recipients &MUST-NOT; take any automated action that
2005      violates this directive, and &MUST-NOT; automatically provide an
2006      unvalidated copy of the entity if revalidation fails.
2007  </t><t>
2008      Although this is not recommended, user agents operating under
2009      severe connectivity constraints &MAY; violate this directive but, if
2010      so, &MUST; explicitly warn the user that an unvalidated response has
2011      been provided. The warning &MUST; be provided on each unvalidated
2012      access, and &SHOULD; require explicit user confirmation.
2013  </t></list>
2014</t>
2015<t>
2016  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
2017  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
2018   proxy-revalidate
2019  <list><t>
2020      The proxy-revalidate directive has the same meaning as the must-revalidate
2021      directive, except that it does not apply to non-shared
2022      user agent caches. It can be used on a response to an
2023      authenticated request to permit the user's cache to store and
2024      later return the response without needing to revalidate it (since
2025      it has already been authenticated once by that user), while still
2026      requiring proxies that service many users to revalidate each time
2027      (in order to make sure that each user has been authenticated).
2028      Note that such authenticated responses also need the public cache
2029      control directive in order to allow them to be cached at all.
2030  </t></list>
2031</t>
2032</section>
2033
2034<section title="No-Transform Directive" anchor="no-transform.directive">
2035<t>
2036  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2037  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2038   no-transform
2039  <list><t>
2040      Implementors of intermediate caches (proxies) have found it useful
2041      to convert the media type of certain entity bodies. A non-transparent
2042      proxy might, for example, convert between image
2043      formats in order to save cache space or to reduce the amount of
2044      traffic on a slow link.
2045  </t><t>
2046      Serious operational problems occur, however, when these
2047      transformations are applied to entity bodies intended for certain
2048      kinds of applications. For example, applications for medical
2049      imaging, scientific data analysis and those using end-to-end
2050      authentication, all depend on receiving an entity body that is bit
2051      for bit identical to the original entity-body.
2052  </t><t>
2053      Therefore, if a message includes the no-transform directive, an
2054      intermediate cache or proxy &MUST-NOT; change those headers that are
2055      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2056      directive. This implies that the cache or proxy &MUST-NOT; change
2057      any aspect of the entity-body that is specified by these headers,
2058      including the value of the entity-body itself.
2059  </t></list>
2060</t>
2061</section>
2062
2063<section title="Cache Control Extensions" anchor="cache.control.extensions">
2064<t>
2065   The Cache-Control header field can be extended through the use of one
2066   or more cache-extension tokens, each with an optional assigned value.
2067   Informational extensions (those which do not require a change in
2068   cache behavior) &MAY; be added without changing the semantics of other
2069   directives. Behavioral extensions are designed to work by acting as
2070   modifiers to the existing base of cache directives. Both the new
2071   directive and the standard directive are supplied, such that
2072   applications which do not understand the new directive will default
2073   to the behavior specified by the standard directive, and those that
2074   understand the new directive will recognize it as modifying the
2075   requirements associated with the standard directive. In this way,
2076   extensions to the cache-control directives can be made without
2077   requiring changes to the base protocol.
2078</t>
2079<t>
2080   This extension mechanism depends on an HTTP cache obeying all of the
2081   cache-control directives defined for its native HTTP-version, obeying
2082   certain extensions, and ignoring all directives that it does not
2083   understand.
2084</t>
2085<t>
2086   For example, consider a hypothetical new response directive called
2087   community which acts as a modifier to the private directive. We
2088   define this new directive to mean that, in addition to any non-shared
2089   cache, any cache which is shared only by members of the community
2090   named within its value may cache the response. An origin server
2091   wishing to allow the UCI community to use an otherwise private
2092   response in their shared cache(s) could do so by including
2093</t>
2094<figure><artwork type="example">
2095    Cache-Control: private, community="UCI"
2096</artwork></figure>
2097<t>
2098   A cache seeing this header field will act correctly even if the cache
2099   does not understand the community cache-extension, since it will also
2100   see and understand the private directive and thus default to the safe
2101   behavior.
2102</t>
2103<t>
2104   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2105   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2106   be combined with standard directives (or the response's default
2107   cacheability) such that the cache behavior will remain minimally
2108   correct even if the cache does not understand the extension(s).
2109</t>
2110</section>
2111</section>
2112
2113<section title="Expires" anchor="header.expires">
2114  <iref primary="true" item="Expires header" x:for-anchor=""/>
2115  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2116<t>
2117   The Expires entity-header field gives the date/time after which the
2118   response is considered stale. A stale cache entry may not normally be
2119   returned by a cache (either a proxy cache or a user agent cache)
2120   unless it is first validated with the origin server (or with an
2121   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2122   for further discussion of the expiration model.
2123</t>
2124<t>
2125   The presence of an Expires field does not imply that the original
2126   resource will change or cease to exist at, before, or after that
2127   time.
2128</t>
2129<t>
2130   The format is an absolute date and time as defined by HTTP-date in
2131   &full-date;; it &MUST; be sent in rfc1123-date format.
2132</t>
2133<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2134  Expires = "Expires" ":" HTTP-date
2135</artwork></figure>
2136<t>
2137   An example of its use is
2138</t>
2139<figure><artwork type="example">
2140   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2141</artwork></figure>
2142<t>
2143  <list><t>
2144      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2145      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2146      Expires field.
2147  </t></list>
2148</t>
2149<t>
2150   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2151   especially including the value "0", as in the past (i.e., "already
2152   expired").
2153</t>
2154<t>
2155   To mark a response as "already expired," an origin server sends an
2156   Expires date that is equal to the Date header value. (See the rules
2157   for expiration calculations in <xref target="expiration.calculations"/>.)
2158</t>
2159<t>
2160   To mark a response as "never expires," an origin server sends an
2161   Expires date approximately one year from the time the response is
2162   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2163   year in the future.
2164</t>
2165<t>
2166   The presence of an Expires header field with a date value of some
2167   time in the future on a response that otherwise would by default be
2168   non-cacheable indicates that the response is cacheable, unless
2169   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2170</t>
2171</section>
2172
2173<section title="Pragma" anchor="header.pragma">
2174  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2175  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2176<t>
2177   The Pragma general-header field is used to include implementation-specific
2178   directives that might apply to any recipient along the
2179   request/response chain. All pragma directives specify optional
2180   behavior from the viewpoint of the protocol; however, some systems
2181   &MAY; require that behavior be consistent with the directives.
2182</t>
2183<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"/>
2184  Pragma            = "Pragma" ":" 1#pragma-directive
2185  pragma-directive  = "no-cache" | extension-pragma
2186  extension-pragma  = token [ "=" ( token | quoted-string ) ]
2187</artwork></figure>
2188<t>
2189   When the no-cache directive is present in a request message, an
2190   application &SHOULD; forward the request toward the origin server even
2191   if it has a cached copy of what is being requested. This pragma
2192   directive has the same semantics as the no-cache cache-directive (see
2193   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2194   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2195   request is sent to a server not known to be HTTP/1.1 compliant.
2196</t>
2197<t>
2198   Pragma directives &MUST; be passed through by a proxy or gateway
2199   application, regardless of their significance to that application,
2200   since the directives might be applicable to all recipients along the
2201   request/response chain. It is not possible to specify a pragma for a
2202   specific recipient; however, any pragma directive not relevant to a
2203   recipient &SHOULD; be ignored by that recipient.
2204</t>
2205<t>
2206   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2207   sent "Cache-Control: no-cache". No new Pragma directives will be
2208   defined in HTTP.
2209  <list><t>
2210      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache as a response
2211      header field is not actually specified, it does not provide a
2212      reliable replacement for "Cache-Control: no-cache" in a response
2213  </t></list>
2214</t>
2215</section>
2216
2217<section title="Vary" anchor="header.vary">
2218  <iref primary="true" item="Vary header" x:for-anchor=""/>
2219  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2220<t>
2221   The Vary field value indicates the set of request-header fields that
2222   fully determines, while the response is fresh, whether a cache is
2223   permitted to use the response to reply to a subsequent request
2224   without revalidation. For uncacheable or stale responses, the Vary
2225   field value advises the user agent about the criteria that were used
2226   to select the representation. A Vary field value of "*" implies that
2227   a cache cannot determine from the request headers of a subsequent
2228   request whether this response is the appropriate representation. See
2229   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2230</t>
2231<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2232  Vary  = "Vary" ":" ( "*" | 1#field-name )
2233</artwork></figure>
2234<t>
2235   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2236   cacheable response that is subject to server-driven negotiation.
2237   Doing so allows a cache to properly interpret future requests on that
2238   resource and informs the user agent about the presence of negotiation
2239   on that resource. A server &MAY; include a Vary header field with a
2240   non-cacheable response that is subject to server-driven negotiation,
2241   since this might provide the user agent with useful information about
2242   the dimensions over which the response varies at the time of the
2243   response.
2244</t>
2245<t>
2246   A Vary field value consisting of a list of field-names signals that
2247   the representation selected for the response is based on a selection
2248   algorithm which considers ONLY the listed request-header field values
2249   in selecting the most appropriate representation. A cache &MAY; assume
2250   that the same selection will be made for future requests with the
2251   same values for the listed field names, for the duration of time for
2252   which the response is fresh.
2253</t>
2254<t>
2255   The field-names given are not limited to the set of standard
2256   request-header fields defined by this specification. Field names are
2257   case-insensitive.
2258</t>
2259<t>
2260   A Vary field value of "*" signals that unspecified parameters not
2261   limited to the request-headers (e.g., the network address of the
2262   client), play a role in the selection of the response representation.
2263   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2264   generated by an origin server.
2265</t>
2266</section>
2267
2268<section title="Warning" anchor="header.warning">
2269  <iref primary="true" item="Warning header" x:for-anchor=""/>
2270  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2271<t>
2272   The Warning general-header field is used to carry additional
2273   information about the status or transformation of a message which
2274   might not be reflected in the message. This information is typically
2275   used to warn about a possible lack of semantic transparency from
2276   caching operations or transformations applied to the entity body of
2277   the message.
2278</t>
2279<t>
2280   Warning headers are sent with responses using:
2281</t>
2282<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"/>
2283  Warning    = "Warning" ":" 1#warning-value
2284 
2285  warning-value = warn-code SP warn-agent SP warn-text
2286                                        [SP warn-date]
2287 
2288  warn-code  = 3DIGIT
2289  warn-agent = ( host [ ":" port ] ) | pseudonym
2290                  ; the name or pseudonym of the server adding
2291                  ; the Warning header, for use in debugging
2292  warn-text  = quoted-string
2293  warn-date  = &lt;"&gt; HTTP-date &lt;"&gt;
2294</artwork></figure>
2295<t>
2296   A response &MAY; carry more than one Warning header.
2297</t>
2298<t>
2299   The warn-text &SHOULD; be in a natural language and character set that
2300   is most likely to be intelligible to the human user receiving the
2301   response. This decision &MAY; be based on any available knowledge, such
2302   as the location of the cache or user, the Accept-Language field in a
2303   request, the Content-Language field in a response, etc. The default
2304   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2305</t>
2306<t>
2307   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2308   in the warn-text using the method described in <xref target="RFC2047"/>.
2309</t>
2310<t>
2311   Warning headers can in general be applied to any message, however
2312   some specific warn-codes are specific to caches and can only be
2313   applied to response messages. New Warning headers &SHOULD; be added
2314   after any existing Warning headers. A cache &MUST-NOT; delete any
2315   Warning header that it received with a message. However, if a cache
2316   successfully validates a cache entry, it &SHOULD; remove any Warning
2317   headers previously attached to that entry except as specified for
2318   specific Warning codes. It &MUST; then add any Warning headers received
2319   in the validating response. In other words, Warning headers are those
2320   that would be attached to the most recent relevant response.
2321</t>
2322<t>
2323   When multiple Warning headers are attached to a response, the user
2324   agent ought to inform the user of as many of them as possible, in the
2325   order that they appear in the response. If it is not possible to
2326   inform the user of all of the warnings, the user agent &SHOULD; follow
2327   these heuristics:
2328  <list style="symbols">
2329    <t>Warnings that appear early in the response take priority over
2330        those appearing later in the response.</t>
2331
2332    <t>Warnings in the user's preferred character set take priority
2333        over warnings in other character sets but with identical warn-codes
2334        and warn-agents.</t>
2335  </list>
2336</t>
2337<t>
2338   Systems that generate multiple Warning headers &SHOULD; order them with
2339   this user agent behavior in mind.
2340</t>
2341<t>
2342   Requirements for the behavior of caches with respect to Warnings are
2343   stated in <xref target="warnings"/>.
2344</t>
2345<t>
2346   This is a list of the currently-defined warn-codes, each with a
2347   recommended warn-text in English, and a description of its meaning.
2348</t>
2349<t>
2350   110 Response is stale
2351  <list><t>
2352     &MUST; be included whenever the returned response is stale.
2353  </t></list>
2354</t>
2355<t>
2356   111 Revalidation failed
2357  <list><t>
2358     &MUST; be included if a cache returns a stale response because an
2359     attempt to revalidate the response failed, due to an inability to
2360     reach the server.
2361  </t></list>
2362</t>
2363<t>
2364   112 Disconnected operation
2365  <list><t>
2366     &SHOULD; be included if the cache is intentionally disconnected from
2367     the rest of the network for a period of time.
2368  </t></list>
2369</t>
2370<t>
2371   113 Heuristic expiration
2372  <list><t>
2373     &MUST; be included if the cache heuristically chose a freshness
2374     lifetime greater than 24 hours and the response's age is greater
2375     than 24 hours.
2376  </t></list>
2377</t>
2378<t>
2379   199 Miscellaneous warning
2380  <list><t>
2381     The warning text &MAY; include arbitrary information to be presented
2382     to a human user, or logged. A system receiving this warning &MUST-NOT;
2383     take any automated action, besides presenting the warning to
2384     the user.
2385  </t></list>
2386</t>
2387<t>
2388   214 Transformation applied
2389  <list><t>
2390     &MUST; be added by an intermediate cache or proxy if it applies any
2391     transformation changing the content-coding (as specified in the
2392     Content-Encoding header) or media-type (as specified in the
2393     Content-Type header) of the response, or the entity-body of the
2394     response, unless this Warning code already appears in the response.
2395  </t></list>
2396</t>
2397<t>
2398   299 Miscellaneous persistent warning
2399  <list><t>
2400     The warning text &MAY; include arbitrary information to be presented
2401     to a human user, or logged. A system receiving this warning &MUST-NOT;
2402     take any automated action.
2403  </t></list>
2404</t>
2405<t>
2406   If an implementation sends a message with one or more Warning headers
2407   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2408   each warning-value a warn-date that matches the date in the response.
2409</t>
2410<t>
2411   If an implementation receives a message with a warning-value that
2412   includes a warn-date, and that warn-date is different from the Date
2413   value in the response, then that warning-value &MUST; be deleted from
2414   the message before storing, forwarding, or using it. (This prevents
2415   bad consequences of naive caching of Warning header fields.) If all
2416   of the warning-values are deleted for this reason, the Warning header
2417   &MUST; be deleted as well.
2418</t>
2419</section>
2420
2421</section>
2422
2423<section title="IANA Considerations" anchor="IANA.considerations">
2424<t>
2425   TBD.
2426</t>
2427</section>
2428
2429<section title="Security Considerations" anchor="security.considerations">
2430<t>
2431   Caching proxies provide additional potential vulnerabilities, since
2432   the contents of the cache represent an attractive target for
2433   malicious exploitation. Because cache contents persist after an HTTP
2434   request is complete, an attack on the cache can reveal information
2435   long after a user believes that the information has been removed from
2436   the network. Therefore, cache contents should be protected as
2437   sensitive information.
2438</t>
2439</section>
2440
2441<section title="Acknowledgments" anchor="ack">
2442<t>
2443   Much of the content and presentation of the caching design is due to
2444   suggestions and comments from individuals including: Shel Kaphan,
2445   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2446</t>
2447</section>
2448</middle>
2449<back>
2450
2451<references title="Normative References">
2452
2453<reference anchor="ISO-8859-1">
2454  <front>
2455    <title>
2456     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2457    </title>
2458    <author>
2459      <organization>International Organization for Standardization</organization>
2460    </author>
2461    <date year="1998"/>
2462  </front>
2463  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2464</reference>
2465
2466<reference anchor="Part1">
2467  <front>
2468    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2469    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2470      <organization abbrev="Day Software">Day Software</organization>
2471      <address><email>fielding@gbiv.com</email></address>
2472    </author>
2473    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2474      <organization>One Laptop per Child</organization>
2475      <address><email>jg@laptop.org</email></address>
2476    </author>
2477    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2478      <organization abbrev="HP">Hewlett-Packard Company</organization>
2479      <address><email>JeffMogul@acm.org</email></address>
2480    </author>
2481    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2482      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2483      <address><email>henrikn@microsoft.com</email></address>
2484    </author>
2485    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2486      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2487      <address><email>LMM@acm.org</email></address>
2488    </author>
2489    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2490      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2491      <address><email>paulle@microsoft.com</email></address>
2492    </author>
2493    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2494      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2495      <address><email>timbl@w3.org</email></address>
2496    </author>
2497    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2498      <organization abbrev="W3C">World Wide Web Consortium</organization>
2499      <address><email>ylafon@w3.org</email></address>
2500    </author>
2501    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2502      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2503      <address><email>julian.reschke@greenbytes.de</email></address>
2504    </author>
2505    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2506  </front>
2507  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2508  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2509</reference>
2510
2511<reference anchor="Part2">
2512  <front>
2513    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2514    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2515      <organization abbrev="Day Software">Day Software</organization>
2516      <address><email>fielding@gbiv.com</email></address>
2517    </author>
2518    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2519      <organization>One Laptop per Child</organization>
2520      <address><email>jg@laptop.org</email></address>
2521    </author>
2522    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2523      <organization abbrev="HP">Hewlett-Packard Company</organization>
2524      <address><email>JeffMogul@acm.org</email></address>
2525    </author>
2526    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2527      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2528      <address><email>henrikn@microsoft.com</email></address>
2529    </author>
2530    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2531      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2532      <address><email>LMM@acm.org</email></address>
2533    </author>
2534    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2535      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2536      <address><email>paulle@microsoft.com</email></address>
2537    </author>
2538    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2539      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2540      <address><email>timbl@w3.org</email></address>
2541    </author>
2542    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2543      <organization abbrev="W3C">World Wide Web Consortium</organization>
2544      <address><email>ylafon@w3.org</email></address>
2545    </author>
2546    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2547      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2548      <address><email>julian.reschke@greenbytes.de</email></address>
2549    </author>
2550    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2551  </front>
2552  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2553  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2554</reference>
2555
2556<reference anchor="Part3">
2557  <front>
2558    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2559    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2560      <organization abbrev="Day Software">Day Software</organization>
2561      <address><email>fielding@gbiv.com</email></address>
2562    </author>
2563    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2564      <organization>One Laptop per Child</organization>
2565      <address><email>jg@laptop.org</email></address>
2566    </author>
2567    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2568      <organization abbrev="HP">Hewlett-Packard Company</organization>
2569      <address><email>JeffMogul@acm.org</email></address>
2570    </author>
2571    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2572      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2573      <address><email>henrikn@microsoft.com</email></address>
2574    </author>
2575    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2576      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2577      <address><email>LMM@acm.org</email></address>
2578    </author>
2579    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2580      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2581      <address><email>paulle@microsoft.com</email></address>
2582    </author>
2583    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2584      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2585      <address><email>timbl@w3.org</email></address>
2586    </author>
2587    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2588      <organization abbrev="W3C">World Wide Web Consortium</organization>
2589      <address><email>ylafon@w3.org</email></address>
2590    </author>
2591    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2592      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2593      <address><email>julian.reschke@greenbytes.de</email></address>
2594    </author>
2595    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2596  </front>
2597  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2598  <x:source href="p3-payload.xml" basename="p3-payload"/>
2599</reference>
2600
2601<reference anchor="Part4">
2602  <front>
2603    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2604    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2605      <organization abbrev="Day Software">Day Software</organization>
2606      <address><email>fielding@gbiv.com</email></address>
2607    </author>
2608    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2609      <organization>One Laptop per Child</organization>
2610      <address><email>jg@laptop.org</email></address>
2611    </author>
2612    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2613      <organization abbrev="HP">Hewlett-Packard Company</organization>
2614      <address><email>JeffMogul@acm.org</email></address>
2615    </author>
2616    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2617      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2618      <address><email>henrikn@microsoft.com</email></address>
2619    </author>
2620    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2621      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2622      <address><email>LMM@acm.org</email></address>
2623    </author>
2624    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2625      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2626      <address><email>paulle@microsoft.com</email></address>
2627    </author>
2628    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2629      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2630      <address><email>timbl@w3.org</email></address>
2631    </author>
2632    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2633      <organization abbrev="W3C">World Wide Web Consortium</organization>
2634      <address><email>ylafon@w3.org</email></address>
2635    </author>
2636    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2637      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2638      <address><email>julian.reschke@greenbytes.de</email></address>
2639    </author>
2640    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2641  </front>
2642  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2643  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2644</reference>
2645
2646<reference anchor="Part5">
2647  <front>
2648    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2649    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2650      <organization abbrev="Day Software">Day Software</organization>
2651      <address><email>fielding@gbiv.com</email></address>
2652    </author>
2653    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2654      <organization>One Laptop per Child</organization>
2655      <address><email>jg@laptop.org</email></address>
2656    </author>
2657    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2658      <organization abbrev="HP">Hewlett-Packard Company</organization>
2659      <address><email>JeffMogul@acm.org</email></address>
2660    </author>
2661    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2662      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2663      <address><email>henrikn@microsoft.com</email></address>
2664    </author>
2665    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2666      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2667      <address><email>LMM@acm.org</email></address>
2668    </author>
2669    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2670      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2671      <address><email>paulle@microsoft.com</email></address>
2672    </author>
2673    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2674      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2675      <address><email>timbl@w3.org</email></address>
2676    </author>
2677    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2678      <organization abbrev="W3C">World Wide Web Consortium</organization>
2679      <address><email>ylafon@w3.org</email></address>
2680    </author>
2681    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2682      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2683      <address><email>julian.reschke@greenbytes.de</email></address>
2684    </author>
2685    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2686  </front>
2687  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2688  <x:source href="p5-range.xml" basename="p5-range"/>
2689</reference>
2690
2691<reference anchor="Part7">
2692  <front>
2693    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2694    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2695      <organization abbrev="Day Software">Day Software</organization>
2696      <address><email>fielding@gbiv.com</email></address>
2697    </author>
2698    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2699      <organization>One Laptop per Child</organization>
2700      <address><email>jg@laptop.org</email></address>
2701    </author>
2702    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2703      <organization abbrev="HP">Hewlett-Packard Company</organization>
2704      <address><email>JeffMogul@acm.org</email></address>
2705    </author>
2706    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2707      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2708      <address><email>henrikn@microsoft.com</email></address>
2709    </author>
2710    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2711      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2712      <address><email>LMM@acm.org</email></address>
2713    </author>
2714    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2715      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2716      <address><email>paulle@microsoft.com</email></address>
2717    </author>
2718    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2719      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2720      <address><email>timbl@w3.org</email></address>
2721    </author>
2722    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2723      <organization abbrev="W3C">World Wide Web Consortium</organization>
2724      <address><email>ylafon@w3.org</email></address>
2725    </author>
2726    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2727      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2728      <address><email>julian.reschke@greenbytes.de</email></address>
2729    </author>
2730    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2731  </front>
2732  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2733  <x:source href="p7-auth.xml" basename="p7-auth"/>
2734</reference>
2735
2736<reference anchor="RFC2047">
2737  <front>
2738    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2739    <author initials="K." surname="Moore" fullname="Keith Moore">
2740      <organization>University of Tennessee</organization>
2741      <address><email>moore@cs.utk.edu</email></address>
2742    </author>
2743    <date month="November" year="1996"/>
2744  </front>
2745  <seriesInfo name="RFC" value="2047"/>
2746</reference>
2747
2748<reference anchor="RFC2119">
2749  <front>
2750    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2751    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2752      <organization>Harvard University</organization>
2753      <address><email>sob@harvard.edu</email></address>
2754    </author>
2755    <date month="March" year="1997"/>
2756  </front>
2757  <seriesInfo name="BCP" value="14"/>
2758  <seriesInfo name="RFC" value="2119"/>
2759</reference>
2760
2761</references>
2762
2763<references title="Informative References">
2764
2765<reference anchor="RFC1305">
2766  <front>
2767    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2768    <author initials="D." surname="Mills" fullname="David L. Mills">
2769      <organization>University of Delaware, Electrical Engineering Department</organization>
2770      <address><email>mills@udel.edu</email></address>
2771    </author>
2772    <date month="March" year="1992"/>
2773  </front>
2774  <seriesInfo name="RFC" value="1305"/>
2775</reference>
2776
2777<reference anchor="RFC2616">
2778  <front>
2779    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2780    <author initials="R." surname="Fielding" fullname="R. Fielding">
2781      <organization>University of California, Irvine</organization>
2782      <address><email>fielding@ics.uci.edu</email></address>
2783    </author>
2784    <author initials="J." surname="Gettys" fullname="J. Gettys">
2785      <organization>W3C</organization>
2786      <address><email>jg@w3.org</email></address>
2787    </author>
2788    <author initials="J." surname="Mogul" fullname="J. Mogul">
2789      <organization>Compaq Computer Corporation</organization>
2790      <address><email>mogul@wrl.dec.com</email></address>
2791    </author>
2792    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2793      <organization>MIT Laboratory for Computer Science</organization>
2794      <address><email>frystyk@w3.org</email></address>
2795    </author>
2796    <author initials="L." surname="Masinter" fullname="L. Masinter">
2797      <organization>Xerox Corporation</organization>
2798      <address><email>masinter@parc.xerox.com</email></address>
2799    </author>
2800    <author initials="P." surname="Leach" fullname="P. Leach">
2801      <organization>Microsoft Corporation</organization>
2802      <address><email>paulle@microsoft.com</email></address>
2803    </author>
2804    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2805      <organization>W3C</organization>
2806      <address><email>timbl@w3.org</email></address>
2807    </author>
2808    <date month="June" year="1999"/>
2809  </front>
2810  <seriesInfo name="RFC" value="2616"/>
2811</reference>
2812
2813</references>
2814
2815<section title="Compatibility with Previous Versions" anchor="compatibility">
2816
2817<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2818<t>
2819   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2820   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2821   <xref target="header.cache-control" format="counter"/>,
2822   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2823</t>
2824<t>
2825   Transfer-coding and message lengths all interact in ways that
2826   required fixing exactly when chunked encoding is used (to allow for
2827   transfer encoding that may not be self delimiting); it was important
2828   to straighten out exactly how message lengths are computed.
2829   (<xref target="non-modifiable.headers"/>,
2830   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2831</t>
2832<t>
2833   Proxies should be able to add Content-Length when appropriate.
2834   (<xref target="non-modifiable.headers"/>)
2835</t>
2836<t>
2837   The Cache-Control: max-age directive was not properly defined for
2838   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2839</t>
2840<t>
2841   Warnings could be cached incorrectly, or not updated appropriately.
2842   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2843   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2844   and <xref target="header.warning" format="counter"/>) Warning
2845   also needed to be a general header, as PUT or other methods may have
2846   need for it in requests.
2847</t>
2848</section>
2849
2850<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2851<t>
2852  Clarify denial of service attack avoidance requirement.
2853  (<xref target="invalidation.after.updates.or.deletions"/>)
2854</t>
2855</section>
2856
2857</section>
2858
2859<section title="Change Log (to be removed by RFC Editor before publication)">
2860
2861<section title="Since RFC2616">
2862<t>
2863  Extracted relevant partitions from <xref target="RFC2616"/>.
2864</t>
2865</section>
2866
2867<section title="Since draft-ietf-httpbis-p6-cache-00">
2868<t>
2869  Closed issues:
2870  <list style="symbols"> 
2871    <t>
2872      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
2873      "Trailer"
2874      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
2875    </t>
2876    <t>
2877      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
2878      "Invalidation after Update or Delete"
2879      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
2880    </t>
2881    <t>
2882      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
2883      "Date reference typo"
2884    </t>
2885    <t>
2886      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
2887      "Connection header text"
2888    </t>
2889    <t>
2890      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2891      "Informative references"
2892    </t>
2893    <t>
2894      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2895      "ISO-8859-1 Reference"
2896    </t>
2897    <t>
2898      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2899      "Normative up-to-date references"
2900    </t>
2901    <t>
2902      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
2903      "typo in 13.2.2"
2904    </t>
2905  </list>
2906</t>
2907<t>
2908  Other changes:
2909  <list style="symbols">
2910    <t>
2911      Categorize references as "Normative" or "Informative".
2912    </t>
2913  </list>
2914</t>
2915</section>
2916
2917</section>
2918
2919</back>
2920</rfc>
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