source: draft-ietf-httpbis/latest-roy/p6-cache.xml @ 348

Last change on this file since 348 was 348, checked in by fielding@…, 11 years ago

replace xref to general-syntax with xref to uri

  • Property svn:eol-style set to native
File size: 129.8 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 "November">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY notation-abnf               "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33]>
34<?rfc toc="yes" ?>
35<?rfc symrefs="yes" ?>
36<?rfc sortrefs="yes" ?>
37<?rfc compact="yes"?>
38<?rfc subcompact="no" ?>
39<?rfc linkmailto="no" ?>
40<?rfc editing="no" ?>
41<?rfc comments="yes"?>
42<?rfc inline="yes"?>
43<?rfc-ext allow-markup-in-artwork="yes" ?>
44<?rfc-ext include-references-in-index="yes" ?>
45<rfc obsoletes="2616" category="std" x:maturity-level="draft"
46     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
47     xmlns:x='http://purl.org/net/xml2rfc/ext'>
48<front>
49
50  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
51
52  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
53    <organization abbrev="Day Software">Day Software</organization>
54    <address>
55      <postal>
56        <street>23 Corporate Plaza DR, Suite 280</street>
57        <city>Newport Beach</city>
58        <region>CA</region>
59        <code>92660</code>
60        <country>USA</country>
61      </postal>
62      <phone>+1-949-706-5300</phone>
63      <facsimile>+1-949-706-5305</facsimile>
64      <email>fielding@gbiv.com</email>
65      <uri>http://roy.gbiv.com/</uri>
66    </address>
67  </author>
68
69  <author initials="J." surname="Gettys" fullname="Jim Gettys">
70    <organization>One Laptop per Child</organization>
71    <address>
72      <postal>
73        <street>21 Oak Knoll Road</street>
74        <city>Carlisle</city>
75        <region>MA</region>
76        <code>01741</code>
77        <country>USA</country>
78      </postal>
79      <email>jg@laptop.org</email>
80      <uri>http://www.laptop.org/</uri>
81    </address>
82  </author>
83 
84  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
85    <organization abbrev="HP">Hewlett-Packard Company</organization>
86    <address>
87      <postal>
88        <street>HP Labs, Large Scale Systems Group</street>
89        <street>1501 Page Mill Road, MS 1177</street>
90        <city>Palo Alto</city>
91        <region>CA</region>
92        <code>94304</code>
93        <country>USA</country>
94      </postal>
95      <email>JeffMogul@acm.org</email>
96    </address>
97  </author>
98
99  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107        <country>USA</country>
108      </postal>
109      <email>henrikn@microsoft.com</email>
110    </address>
111  </author>
112
113  <author initials="L." surname="Masinter" fullname="Larry Masinter">
114    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
115    <address>
116      <postal>
117        <street>345 Park Ave</street>
118        <city>San Jose</city>
119        <region>CA</region>
120        <code>95110</code>
121        <country>USA</country>
122      </postal>
123      <email>LMM@acm.org</email>
124      <uri>http://larry.masinter.net/</uri>
125    </address>
126  </author>
127 
128  <author initials="P." surname="Leach" fullname="Paul J. Leach">
129    <organization abbrev="Microsoft">Microsoft Corporation</organization>
130    <address>
131      <postal>
132        <street>1 Microsoft Way</street>
133        <city>Redmond</city>
134        <region>WA</region>
135        <code>98052</code>
136      </postal>
137      <email>paulle@microsoft.com</email>
138    </address>
139  </author>
140   
141  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
142    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
143    <address>
144      <postal>
145        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
146        <street>The Stata Center, Building 32</street>
147        <street>32 Vassar Street</street>
148        <city>Cambridge</city>
149        <region>MA</region>
150        <code>02139</code>
151        <country>USA</country>
152      </postal>
153      <email>timbl@w3.org</email>
154      <uri>http://www.w3.org/People/Berners-Lee/</uri>
155    </address>
156  </author>
157
158  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
159    <organization abbrev="W3C">World Wide Web Consortium</organization>
160    <address>
161      <postal>
162        <street>W3C / ERCIM</street>
163        <street>2004, rte des Lucioles</street>
164        <city>Sophia-Antipolis</city>
165        <region>AM</region>
166        <code>06902</code>
167        <country>France</country>
168      </postal>
169      <email>ylafon@w3.org</email>
170      <uri>http://www.raubacapeu.net/people/yves/</uri>
171    </address>
172  </author>
173
174  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
175    <organization abbrev="greenbytes">greenbytes GmbH</organization>
176    <address>
177      <postal>
178        <street>Hafenweg 16</street>
179        <city>Muenster</city><region>NW</region><code>48155</code>
180        <country>Germany</country>
181      </postal>
182      <phone>+49 251 2807760</phone>   
183      <facsimile>+49 251 2807761</facsimile>   
184      <email>julian.reschke@greenbytes.de</email>       
185      <uri>http://greenbytes.de/tech/webdav/</uri>     
186    </address>
187  </author>
188
189  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
190
191<abstract>
192<t>
193   The Hypertext Transfer Protocol (HTTP) is an application-level
194   protocol for distributed, collaborative, hypermedia information
195   systems.
196   This document is Part 6 of the seven-part specification
197   that defines the protocol referred to as "HTTP/1.1" and, taken together,
198   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
199   and the associated header fields that control cache behavior or indicate
200   cacheable response messages.
201</t>
202</abstract>
203
204<note title="Editorial Note (To be removed by RFC Editor)">
205  <t>
206    Discussion of this draft should take place on the HTTPBIS working group
207    mailing list (ietf-http-wg@w3.org). The current issues list is
208    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
209    and related documents (including fancy diffs) can be found at
210    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
211  </t>
212  <t>
213    The changes in this draft are summarized in <xref target="changes.since.04"/>.
214  </t>
215</note>
216</front>
217<middle>
218<section title="Introduction" anchor="caching">
219<t>
220   HTTP is typically used for distributed information systems, where
221   performance can be improved by the use of response caches.  This document
222   defines aspects of HTTP/1.1 related to caching and reusing response messages.
223</t>
224
225<section title="Purpose" anchor="intro.purpose">
226<iref item="cache"/>
227<t>
228   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
229   and the subsystem that controls its message storage, retrieval, and
230   deletion. A cache stores cacheable responses in order to reduce the
231   response time and network bandwidth consumption on future, equivalent
232   requests. Any client or server may include a cache, though a cache
233   cannot be used by a server that is acting as a tunnel.
234</t>
235<t>
236   Caching would be useless if it did not significantly improve
237   performance. The goal of caching in HTTP/1.1 is to reuse a prior response
238   message to satisfy a current request.  In some cases, the existing response
239   can be reused without the need for a network request, reducing latency and
240   network round-trips; we use an "expiration" mechanism for this purpose
241   (see <xref target="expiration.model"/>).  Even when a new request is required,
242   it is often possible to reuse all or parts of the payload of a prior response
243   to satisfy the request, thereby reducing network bandwidth usage; we use a
244   "validation" mechanism for this purpose (see <xref target="validation.model"/>).
245</t>
246<iref item="semantically transparent"/>
247<t>
248   A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
249   respect to a particular response, when its use affects neither the
250   requesting client nor the origin server, except to improve
251   performance. When a cache is semantically transparent, the client
252   receives exactly the same response status and payload
253   that it would have received had its request been handled directly
254   by the origin server.
255</t>
256<t>
257   In an ideal world, all interactions with an HTTP cache would be
258   semantically transparent.  However, for some resources, semantic
259   transparency is not always necessary and can be effectively traded
260   for the sake of bandwidth scaling, disconnected operation, and
261   high availability.  HTTP/1.1 allows origin servers, caches,
262   and clients to explicitly reduce transparency when necessary.
263   However, because non-transparent operation may confuse non-expert
264   users and might be incompatible with certain server applications
265   (such as those for ordering merchandise), the protocol requires that
266   transparency be relaxed
267  <list style="symbols">
268     <t>only by an explicit protocol-level request when relaxed by
269        client or origin server</t>
270
271     <t>only with an explicit warning to the end user when relaxed by
272        cache or client</t>
273  </list>
274</t>
275<t>
276   Therefore, HTTP/1.1 provides these important elements:
277  <list style="numbers">
278      <t>Protocol features that provide full semantic transparency when
279         this is required by all parties.</t>
280
281      <t>Protocol features that allow an origin server or user agent to
282         explicitly request and control non-transparent operation.</t>
283
284      <t>Protocol features that allow a cache to attach warnings to
285         responses that do not preserve the requested approximation of
286         semantic transparency.</t>
287  </list>
288</t>
289<t>
290   A basic principle is that it must be possible for the clients to
291   detect any potential relaxation of semantic transparency.
292  <list><t>
293      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
294      design decisions not explicitly discussed in this specification.
295      If a decision might affect semantic transparency, the implementor
296      ought to err on the side of maintaining transparency unless a
297      careful and complete analysis shows significant benefits in
298      breaking transparency.
299    </t></list>
300</t>
301</section>
302
303<section title="Terminology" anchor="intro.terminology">
304<t>
305   This specification uses a number of terms to refer to the roles
306   played by participants in, and objects of, HTTP caching.
307</t>
308<t>
309  <iref item="cacheable"/>
310  <x:dfn>cacheable</x:dfn>
311  <list>
312    <t>
313      A response is cacheable if a cache is allowed to store a copy of
314      the response message for use in answering subsequent requests.
315      Even when a response is cacheable, there may
316      be additional constraints on whether a cache can use the cached
317      copy for a particular request.
318    </t>
319  </list>
320</t>
321<t>
322  <iref item="first-hand"/>
323  <x:dfn>first-hand</x:dfn>
324  <list>
325    <t>
326      A response is first-hand if it comes directly and without
327      unnecessary delay from the origin server, perhaps via one or more
328      proxies. A response is also first-hand if its validity has just
329      been checked directly with the origin server.
330    </t>
331  </list>
332</t>
333<t>
334  <iref item="explicit expiration time"/>
335  <x:dfn>explicit expiration time</x:dfn>
336  <list>
337    <t>
338      The time at which the origin server intends that an entity should
339      no longer be returned by a cache without further validation.
340    </t>
341  </list>
342</t>
343<t>
344  <iref item="heuristic expiration time"/>
345  <x:dfn>heuristic expiration time</x:dfn>
346  <list>
347    <t>
348      An expiration time assigned by a cache when no explicit expiration
349      time is available.
350    </t>
351  </list>
352</t>
353<t>
354  <iref item="age"/>
355  <x:dfn>age</x:dfn>
356  <list>
357    <t>
358      The age of a response is the time since it was sent by, or
359      successfully validated with, the origin server.
360    </t>
361  </list>
362</t>
363<t>
364  <iref item="freshness lifetime"/>
365  <x:dfn>freshness lifetime</x:dfn>
366  <list>
367    <t>
368      The length of time between the generation of a response and its
369      expiration time.
370    </t>
371  </list>
372</t>
373<t>
374  <iref item="fresh"/>
375  <x:dfn>fresh</x:dfn>
376  <list>
377    <t>
378      A response is fresh if its age has not yet exceeded its freshness
379      lifetime.
380    </t>
381  </list>
382</t>
383<t>
384  <iref item="stale"/>
385  <x:dfn>stale</x:dfn>
386  <list>
387    <t>
388      A response is stale if its age has passed its freshness lifetime.
389    </t>
390  </list>
391</t>
392<t>
393  <iref item="validator"/>
394  <x:dfn>validator</x:dfn>
395  <list>
396    <t>
397      A protocol element (e.g., an entity tag or a Last-Modified time)
398      that is used to find out whether a cache entry is an equivalent
399      copy of an entity.
400    </t>
401  </list>
402</t>
403</section>
404
405<section title="Requirements" anchor="intro.requirements">
406<t>
407   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
408   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
409   document are to be interpreted as described in <xref target="RFC2119"/>.
410</t>
411<t>
412   An implementation is not compliant if it fails to satisfy one or more
413   of the &MUST; or &REQUIRED; level requirements for the protocols it
414   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
415   level and all the &SHOULD; level requirements for its protocols is said
416   to be "unconditionally compliant"; one that satisfies all the &MUST;
417   level requirements but not all the &SHOULD; level requirements for its
418   protocols is said to be "conditionally compliant."
419</t>
420</section>
421</section>
422
423
424<section title="Notational Conventions and Generic Grammar" anchor="notation">
425  <x:anchor-alias value="DIGIT"/>
426  <x:anchor-alias value="DQUOTE"/>
427  <x:anchor-alias value="quoted-string"/>
428  <x:anchor-alias value="SP"/>
429  <x:anchor-alias value="token"/>
430<t>
431  This specification uses the ABNF syntax defined in &notation-abnf; and
432  the core rules defined in &basic-rules;:
433  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
434  <eref target="http://ietf.org/wg/httpbis/trac/ticket/36"/>.</cref>
435</t>
436<figure><artwork type="abnf2616">
437  <x:ref>DIGIT</x:ref>         = &lt;DIGIT, defined in &basic-rules;&gt;
438  <x:ref>DQUOTE</x:ref>        = &lt;DQUOTE, defined in &basic-rules;&gt;
439  <x:ref>SP</x:ref>            = &lt;SP, defined in &basic-rules;&gt;
440</artwork></figure>
441<figure><artwork type="abnf2616">
442  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
443  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
444</artwork></figure>
445<t anchor="abnf.dependencies">
446  <x:anchor-alias value="field-name"/>
447  <x:anchor-alias value="HTTP-date"/>
448  <x:anchor-alias value="port"/>
449  <x:anchor-alias value="pseudonym"/>
450  <x:anchor-alias value="uri-host"/>
451  The ABNF rules below are defined in other parts:
452</t>
453<figure><!--Part1--><artwork type="abnf2616">
454  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
455  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
456  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
457  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
458  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
459</artwork></figure>
460</section>
461
462<section title="Overview" anchor="caching.overview">
463<section title="Cache Correctness" anchor="cache.correctness">
464<t>
465   A correct cache &MUST; respond to a request with the most up-to-date
466   response held by the cache that is appropriate to the request (see
467   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
468   <xref target="disambiguating.multiple.responses" format="counter"/>,
469   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
470   conditions:
471  <list style="numbers">
472      <t>It has been checked for equivalence with what the origin server
473         would have returned by revalidating the response with the
474         origin server (<xref target="validation.model"/>);</t>
475
476      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
477         this means it meets the least restrictive freshness requirement
478         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
479         the origin server so specifies, it is the freshness requirement
480         of the origin server alone.
481
482         If a stored response is not "fresh enough" by the most
483         restrictive freshness requirement of both the client and the
484         origin server, in carefully considered circumstances the cache
485         &MAY; still return the response with the appropriate Warning
486         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
487         and <xref target="header.warning" format="counter"/>), unless such a response
488         is prohibited (e.g., by a "no-store" cache-directive, or by a
489         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
490
491      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
492         or error (4xx or 5xx) response message.</t>
493  </list>
494</t>
495<t>
496   If the cache can not communicate with the origin server, then a
497   correct cache &SHOULD; respond as above if the response can be
498   correctly served from the cache; if not it &MUST; return an error or
499   warning indicating that there was a communication failure.
500</t>
501<t>
502   If a cache receives a response (either an entire response, or a 304
503   (Not Modified) response) that it would normally forward to the
504   requesting client, and the received response is no longer fresh, the
505   cache &SHOULD; forward it to the requesting client without adding a new
506   Warning (but without removing any existing Warning headers). A cache
507   &SHOULD-NOT;  attempt to revalidate a response simply because that
508   response became stale in transit; this might lead to an infinite
509   loop. A user agent that receives a stale response without a Warning
510   &MAY; display a warning indication to the user.
511</t>
512</section>
513
514<section title="Warnings" anchor="warnings">
515<t>
516   Whenever a cache returns a response that is neither first-hand nor
517   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
518   &MUST; attach a warning to that effect, using a Warning general-header.
519   The Warning header and the currently defined warnings are described
520   in <xref target="header.warning"/>.
521</t>
522</section>
523
524<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
525<t>
526   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
527   times and validators) are implicit directives to caches. In some
528   cases, a server or client might need to provide explicit directives
529   to the HTTP caches. We use the Cache-Control header for this purpose.
530</t>
531<t>
532   The Cache-Control header allows a client or server to transmit a
533   variety of directives in either requests or responses. These
534   directives typically override the default caching algorithms. As a
535   general rule, if there is any apparent conflict between header
536   values, the most restrictive interpretation is applied (that is, the
537   one that is most likely to preserve semantic transparency). However,
538   in some cases, cache-control directives are explicitly specified as
539   weakening the approximation of semantic transparency (for example,
540   "max-stale" or "public").
541</t>
542<t>
543   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
544</t>
545</section>
546
547<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
548<t>
549   Many user agents make it possible for users to override the basic
550   caching mechanisms. For example, the user agent might allow the user
551   to specify that cached entities (even explicitly stale ones) are
552   never validated. Or the user agent might habitually add "Cache-Control:
553   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
554   default to either non-transparent behavior, or behavior that results
555   in abnormally ineffective caching, but &MAY; be explicitly configured
556   to do so by an explicit action of the user.
557</t>
558<t>
559   If the user has overridden the basic caching mechanisms, the user
560   agent &SHOULD; explicitly indicate to the user whenever this results in
561   the display of information that might not meet the server's
562   transparency requirements (in particular, if the displayed entity is
563   known to be stale). Since the protocol normally allows the user agent
564   to determine if responses are stale or not, this indication need only
565   be displayed when this actually happens. The indication need not be a
566   dialog box; it could be an icon (for example, a picture of a rotting
567   fish) or some other indicator.
568</t>
569<t>
570   If the user has overridden the caching mechanisms in a way that would
571   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
572   continually indicate this state to the user (for example, by a
573   display of a picture of currency in flames) so that the user does not
574   inadvertently consume excess resources or suffer from excessive
575   latency.
576</t>
577</section>
578
579<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
580<t>
581   In some cases, the operator of a cache &MAY; choose to configure it to
582   return stale responses even when not requested by clients. This
583   decision ought not be made lightly, but may be necessary for reasons
584   of availability or performance, especially when the cache is poorly
585   connected to the origin server. Whenever a cache returns a stale
586   response, it &MUST; mark it as such (using a Warning header) enabling
587   the client software to alert the user that there might be a potential
588   problem.
589</t>
590<t>
591   It also allows the user agent to take steps to obtain a first-hand or
592   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
593   response if the client explicitly requests a first-hand or fresh one,
594   unless it is impossible to comply for technical or policy reasons.
595</t>
596</section>
597
598<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
599<t>
600   While the origin server (and to a lesser extent, intermediate caches,
601   by their contribution to the age of a response) are the primary
602   source of expiration information, in some cases the client might need
603   to control a cache's decision about whether to return a cached
604   response without validating it. Clients do this using several
605   directives of the Cache-Control header.
606</t>
607<t>
608   A client's request &MAY; specify the maximum age it is willing to
609   accept of an unvalidated response; specifying a value of zero forces
610   the cache(s) to revalidate all responses. A client &MAY; also specify
611   the minimum time remaining before a response expires. Both of these
612   options increase constraints on the behavior of caches, and so cannot
613   further relax the cache's approximation of semantic transparency.
614</t>
615<t>
616   A client &MAY; also specify that it will accept stale responses, up to
617   some maximum amount of staleness. This loosens the constraints on the
618   caches, and so might violate the origin server's specified
619   constraints on semantic transparency, but might be necessary to
620   support disconnected operation, or high availability in the face of
621   poor connectivity.
622</t>
623</section>
624</section>
625
626<section title="Expiration Model" anchor="expiration.model">
627
628<section title="Server-Specified Expiration" anchor="server-specified.expiration">
629<t>
630   HTTP caching works best when caches can entirely avoid making
631   requests to the origin server. The primary mechanism for avoiding
632   requests is for an origin server to provide an explicit expiration
633   time in the future, indicating that a response &MAY; be used to satisfy
634   subsequent requests. In other words, a cache can return a fresh
635   response without first contacting the server.
636</t>
637<t>
638   Our expectation is that servers will assign future explicit
639   expiration times to responses in the belief that the entity is not
640   likely to change, in a semantically significant way, before the
641   expiration time is reached. This normally preserves semantic
642   transparency, as long as the server's expiration times are carefully
643   chosen.
644</t>
645<t>
646   The expiration mechanism applies only to responses taken from a cache
647   and not to first-hand responses forwarded immediately to the
648   requesting client.
649</t>
650<t>
651   If an origin server wishes to force a semantically transparent cache
652   to validate every request, it &MAY; assign an explicit expiration time
653   in the past. This means that the response is always stale, and so the
654   cache &SHOULD; validate it before using it for subsequent requests. See
655   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
656</t>
657<t>
658   If an origin server wishes to force any HTTP/1.1 cache, no matter how
659   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
660   cache-control directive (see <xref target="header.cache-control"/>).
661</t>
662<t>
663   Servers specify explicit expiration times using either the Expires
664   header, or the max-age directive of the Cache-Control header.
665</t>
666<t>
667   An expiration time cannot be used to force a user agent to refresh
668   its display or reload a resource; its semantics apply only to caching
669   mechanisms, and such mechanisms need only check a resource's
670   expiration status when a new request for that resource is initiated.
671   See <xref target="history.lists"/> for an explanation of the difference between caches
672   and history mechanisms.
673</t>
674</section>
675
676<section title="Heuristic Expiration" anchor="heuristic.expiration">
677<t>
678   Since origin servers do not always provide explicit expiration times,
679   HTTP caches typically assign heuristic expiration times, employing
680   algorithms that use other header values (such as the Last-Modified
681   time) to estimate a plausible expiration time. The HTTP/1.1
682   specification does not provide specific algorithms, but does impose
683   worst-case constraints on their results. Since heuristic expiration
684   times might compromise semantic transparency, they ought to be used
685   cautiously, and we encourage origin servers to provide explicit
686   expiration times as much as possible.
687</t>
688</section>
689
690<section title="Age Calculations" anchor="age.calculations">
691<t>
692   In order to know if a cached entry is fresh, a cache needs to know if
693   its age exceeds its freshness lifetime. We discuss how to calculate
694   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
695   the age of a response or cache entry.
696</t>
697<t>
698   In this discussion, we use the term "now" to mean "the current value
699   of the clock at the host performing the calculation." Hosts that use
700   HTTP, but especially hosts running origin servers and caches, &SHOULD;
701   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
702   a globally accurate time standard.
703</t>
704<t>
705   HTTP/1.1 requires origin servers to send a Date header, if possible,
706   with every response, giving the time at which the response was
707   generated (see &header-date;). We use the term "date_value" to denote
708   the value of the Date header, in a form appropriate for arithmetic
709   operations.
710</t>
711<t>
712   HTTP/1.1 uses the Age response-header to convey the estimated age of
713   the response message when obtained from a cache. The Age field value
714   is the cache's estimate of the amount of time since the response was
715   generated or revalidated by the origin server.
716</t>
717<t>
718   In essence, the Age value is the sum of the time that the response
719   has been resident in each of the caches along the path from the
720   origin server, plus the amount of time it has been in transit along
721   network paths.
722</t>
723<t>
724   We use the term "age_value" to denote the value of the Age header, in
725   a form appropriate for arithmetic operations.
726</t>
727<t>
728   A response's age can be calculated in two entirely independent ways:
729  <list style="numbers">
730      <t>now minus date_value, if the local clock is reasonably well
731         synchronized to the origin server's clock. If the result is
732         negative, the result is replaced by zero.</t>
733
734      <t>age_value, if all of the caches along the response path
735         implement HTTP/1.1.</t>
736  </list>
737</t>
738<t>
739   Given that we have two independent ways to compute the age of a
740   response when it is received, we can combine these as
741</t>
742<figure><artwork type="code">
743    corrected_received_age = max(now - date_value, age_value)
744</artwork></figure>
745<t>
746   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
747   paths, one gets a reliable (conservative) result.
748</t>
749<t>
750   Because of network-imposed delays, some significant interval might
751   pass between the time that a server generates a response and the time
752   it is received at the next outbound cache or client. If uncorrected,
753   this delay could result in improperly low ages.
754</t>
755<t>
756   Because the request that resulted in the returned Age value must have
757   been initiated prior to that Age value's generation, we can correct
758   for delays imposed by the network by recording the time at which the
759   request was initiated. Then, when an Age value is received, it &MUST;
760   be interpreted relative to the time the request was initiated, not
761   the time that the response was received. This algorithm results in
762   conservative behavior no matter how much delay is experienced. So, we
763   compute:
764</t>
765<figure><artwork type="code">
766   corrected_initial_age = corrected_received_age
767                         + (now - request_time)
768</artwork></figure>
769<t>
770   where "request_time" is the time (according to the local clock) when
771   the request that elicited this response was sent.
772</t>
773<t>
774   Summary of age calculation algorithm, when a cache receives a
775   response:
776</t>
777<figure><artwork type="code">
778   /*
779    * age_value
780    *      is the value of Age: header received by the cache with
781    *              this response.
782    * date_value
783    *      is the value of the origin server's Date: header
784    * request_time
785    *      is the (local) time when the cache made the request
786    *              that resulted in this cached response
787    * response_time
788    *      is the (local) time when the cache received the
789    *              response
790    * now
791    *      is the current (local) time
792    */
793
794   apparent_age = max(0, response_time - date_value);
795   corrected_received_age = max(apparent_age, age_value);
796   response_delay = response_time - request_time;
797   corrected_initial_age = corrected_received_age + response_delay;
798   resident_time = now - response_time;
799   current_age   = corrected_initial_age + resident_time;
800</artwork></figure>
801<t>
802   The current_age of a cache entry is calculated by adding the amount
803   of time (in seconds) since the cache entry was last validated by the
804   origin server to the corrected_initial_age. When a response is
805   generated from a cache entry, the cache &MUST; include a single Age
806   header field in the response with a value equal to the cache entry's
807   current_age.
808</t>
809<t>
810   The presence of an Age header field in a response implies that a
811   response is not first-hand. However, the converse is not true, since
812   the lack of an Age header field in a response does not imply that the
813   response is first-hand unless all caches along the request path are
814   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
815   the Age header field).
816</t>
817</section>
818
819<section title="Expiration Calculations" anchor="expiration.calculations">
820<t>
821   In order to decide whether a response is fresh or stale, we need to
822   compare its freshness lifetime to its age. The age is calculated as
823   described in <xref target="age.calculations"/>; this section describes how to calculate
824   the freshness lifetime, and to determine if a response has expired.
825   In the discussion below, the values can be represented in any form
826   appropriate for arithmetic operations.
827</t>
828<t>
829   We use the term "expires_value" to denote the value of the Expires
830   header. We use the term "max_age_value" to denote an appropriate
831   value of the number of seconds carried by the "max-age" directive of
832   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
833</t>
834<t>
835   The max-age directive takes priority over Expires, so if max-age is
836   present in a response, the calculation is simply:
837</t>
838<figure><artwork type="code">
839   freshness_lifetime = max_age_value
840</artwork></figure>
841<t>
842   Otherwise, if Expires is present in the response, the calculation is:
843</t>
844<figure><artwork type="code">
845   freshness_lifetime = expires_value - date_value
846</artwork></figure>
847<t>
848   Note that neither of these calculations is vulnerable to clock skew,
849   since all of the information comes from the origin server.
850</t>
851<t>
852   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
853   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
854   does not include other restrictions on caching, the cache &MAY; compute
855   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
856   113 to any response whose age is more than 24 hours if such warning
857   has not already been added.
858</t>
859<t>
860   Also, if the response does have a Last-Modified time, the heuristic
861   expiration value &SHOULD; be no more than some fraction of the interval
862   since that time. A typical setting of this fraction might be 10%.
863</t>
864<t>
865   The calculation to determine if a response has expired is quite
866   simple:
867</t>
868<figure><artwork type="code">
869   response_is_fresh = (freshness_lifetime &gt; current_age)
870</artwork></figure>
871</section>
872
873<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
874<t>
875   Because expiration values are assigned optimistically, it is possible
876   for two caches to contain fresh values for the same resource that are
877   different.
878</t>
879<t>
880   If a client performing a retrieval receives a non-first-hand response
881   for a request that was already fresh in its own cache, and the Date
882   header in its existing cache entry is newer than the Date on the new
883   response, then the client &MAY; ignore the response. If so, it &MAY;
884   retry the request with a "Cache-Control: max-age=0" directive (see
885   <xref target="header.cache-control"/>), to force a check with the origin server.
886</t>
887<t>
888   If a cache has two fresh responses for the same representation with
889   different validators, it &MUST; use the one with the more recent Date
890   header. This situation might arise because the cache is pooling
891   responses from other caches, or because a client has asked for a
892   reload or a revalidation of an apparently fresh cache entry.
893</t>
894</section>
895
896<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
897<t>
898   Because a client might be receiving responses via multiple paths, so
899   that some responses flow through one set of caches and other
900   responses flow through a different set of caches, a client might
901   receive responses in an order different from that in which the origin
902   server sent them. We would like the client to use the most recently
903   generated response, even if older responses are still apparently
904   fresh.
905</t>
906<t>
907   Neither the entity tag nor the expiration value can impose an
908   ordering on responses, since it is possible that a later response
909   intentionally carries an earlier expiration time. The Date values are
910   ordered to a granularity of one second.
911</t>
912<t>
913   When a client tries to revalidate a cache entry, and the response it
914   receives contains a Date header that appears to be older than the one
915   for the existing entry, then the client &SHOULD; repeat the request
916   unconditionally, and include
917</t>
918<figure><artwork type="example">
919    Cache-Control: max-age=0
920</artwork></figure>
921<t>
922   to force any intermediate caches to validate their copies directly
923   with the origin server, or
924</t>
925<figure><artwork type="example">
926    Cache-Control: no-cache
927</artwork></figure>
928<t>
929   to force any intermediate caches to obtain a new copy from the origin
930   server.
931</t>
932<t>
933   If the Date values are equal, then the client &MAY; use either response
934   (or &MAY;, if it is being extremely prudent, request a new response).
935   Servers &MUST-NOT; depend on clients being able to choose
936   deterministically between responses generated during the same second,
937   if their expiration times overlap.
938</t>
939</section>
940</section>
941
942<section title="Validation Model" anchor="validation.model">
943<t>
944   When a cache has a stale entry that it would like to use as a
945   response to a client's request, it first has to check with the origin
946   server (or possibly an intermediate cache with a fresh response) to
947   see if its cached entry is still usable. We call this "validating"
948   the cache entry.
949</t>
950<t>
951   HTTP's conditional request mechanism, defined in &conditional;, is
952   used to avoid retransmitting the response payload when the cached entry
953   is valid.  When a cached response includes one or more "cache validators,"
954   such as the field values of an ETag or Last-Modified header field, then
955   a validating GET request &SHOULD; be made conditional to those field values.
956   The server checks the conditional request's validator against the current
957   state of the requested resource and, if they match, the server responds
958   with a 304 (Not Modified) status code to indicate that the cached response
959   can be refreshed and reused without retransmitting the response payload.
960   If the validator does not match the current state of the requested
961   resource, then the server returns a full response, including payload,
962   so that the request can be satisfied and the cache entry supplanted
963   without the need for an additional network round-trip.
964</t>
965</section>
966
967<section title="Response Cacheability" anchor="response.cacheability">
968<t>
969   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
970   directive, a caching system &MAY; always store a successful response
971   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
972   if it is fresh, and &MAY; return it after successful validation. If
973   there is neither a cache validator nor an explicit expiration time
974   associated with a response, we do not expect it to be cached, but
975   certain caches &MAY; violate this expectation (for example, when little
976   or no network connectivity is available). A client can usually detect
977   that such a response was taken from a cache by comparing the Date
978   header to the current time.
979  <list><t>
980      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
981      without providing any Warning.
982  </t></list>
983</t>
984<t>
985   However, in some cases it might be inappropriate for a cache to
986   retain an entity, or to return it in response to a subsequent
987   request. This might be because absolute semantic transparency is
988   deemed necessary by the service author, or because of security or
989   privacy considerations. Certain cache-control directives are
990   therefore provided so that the server can indicate that certain
991   resource entities, or portions thereof, are not to be cached
992   regardless of other considerations.
993</t>
994<t>
995   Note that &header-authorization; normally prevents a shared cache from saving
996   and returning a response to a previous request if that request
997   included an Authorization header.
998</t>
999<t>
1000   A response received with a status code of 200, 203, 206, 300, 301 or
1001   410 &MAY; be stored by a cache and used in reply to a subsequent
1002   request, subject to the expiration mechanism, unless a cache-control
1003   directive prohibits caching. However, a cache that does not support
1004   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1005   Content) responses.
1006</t>
1007<t>
1008   A response received with any other status code (e.g. status codes 302
1009   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1010   unless there are cache-control directives or another header(s) that
1011   explicitly allow it. For example, these include the following: an
1012   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1013   "proxy-revalidate", "public" or "private" cache-control
1014   directive (<xref target="header.cache-control"/>).
1015</t>
1016</section>
1017
1018<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1019<t>
1020   The purpose of an HTTP cache is to store information received in
1021   response to requests for use in responding to future requests. In
1022   many cases, a cache simply returns the appropriate parts of a
1023   response to the requester. However, if the cache holds a cache entry
1024   based on a previous response, it might have to combine parts of a new
1025   response with what is held in the cache entry.
1026</t>
1027
1028<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1029<t>
1030   For the purpose of defining the behavior of caches and non-caching
1031   proxies, we divide HTTP headers into two categories:
1032  <list style="symbols">
1033      <t>End-to-end headers, which are  transmitted to the ultimate
1034        recipient of a request or response. End-to-end headers in
1035        responses &MUST; be stored as part of a cache entry and &MUST; be
1036        transmitted in any response formed from a cache entry.</t>
1037
1038      <t>Hop-by-hop headers, which are meaningful only for a single
1039        transport-level connection, and are not stored by caches or
1040        forwarded by proxies.</t>
1041  </list>
1042</t>
1043<t>
1044   The following HTTP/1.1 headers are hop-by-hop headers:
1045  <list style="symbols">
1046      <t>Connection</t>
1047      <t>Keep-Alive</t>
1048      <t>Proxy-Authenticate</t>
1049      <t>Proxy-Authorization</t>
1050      <t>TE</t>
1051      <t>Trailer</t>
1052      <t>Transfer-Encoding</t>
1053      <t>Upgrade</t>
1054  </list>
1055</t>
1056<t>
1057   All other headers defined by HTTP/1.1 are end-to-end headers.
1058</t>
1059<t>
1060   Other hop-by-hop headers &MUST; be listed in a Connection header
1061   (&header-connection;).
1062</t>
1063</section>
1064
1065<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1066<t>
1067   Some features of HTTP/1.1, such as Digest
1068   Authentication, depend on the value of certain end-to-end headers. A
1069   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1070   definition of that header requires or specifically allows that.
1071</t>
1072<t>
1073   A transparent proxy &MUST-NOT; modify any of the following fields in a
1074   request or response, and it &MUST-NOT; add any of these fields if not
1075   already present:
1076  <list style="symbols">
1077      <t>Content-Location</t>
1078      <t>Content-MD5</t>
1079      <t>ETag</t>
1080      <t>Last-Modified</t>
1081  </list>
1082</t>
1083<t>
1084   A transparent proxy &MUST-NOT; modify any of the following fields in a
1085   response:
1086  <list style="symbols">
1087    <t>Expires</t>
1088  </list>
1089</t>
1090<t>
1091   but it &MAY; add any of these fields if not already present. If an
1092   Expires header is added, it &MUST; be given a field-value identical to
1093   that of the Date header in that response.
1094</t>
1095<t>
1096   A  proxy &MUST-NOT; modify or add any of the following fields in a
1097   message that contains the no-transform cache-control directive, or in
1098   any request:
1099  <list style="symbols">
1100    <t>Content-Encoding</t>
1101    <t>Content-Range</t>
1102    <t>Content-Type</t>
1103  </list>
1104</t>
1105<t>
1106   A non-transparent proxy &MAY; modify or add these fields to a message
1107   that does not include no-transform, but if it does so, it &MUST; add a
1108   Warning 214 (Transformation applied) if one does not already appear
1109   in the message (see <xref target="header.warning"/>).
1110  <list><t>
1111      Warning: unnecessary modification of end-to-end headers might
1112      cause authentication failures if stronger authentication
1113      mechanisms are introduced in later versions of HTTP. Such
1114      authentication mechanisms &MAY; rely on the values of header fields
1115      not listed here.
1116    </t></list>
1117</t>
1118<t>
1119   The Content-Length field of a request or response is added or deleted
1120   according to the rules in &message-length;. A transparent proxy &MUST;
1121   preserve the entity-length (&entity-length;) of the entity-body,
1122   although it &MAY; change the transfer-length (&message-length;).
1123</t>
1124</section>
1125
1126<section title="Combining Headers" anchor="combining.headers">
1127<t>
1128   When a cache makes a validating request to a server, and the server
1129   provides a 304 (Not Modified) response or a 206 (Partial Content)
1130   response, the cache then constructs a response to send to the
1131   requesting client.
1132</t>
1133<t>
1134   If the status code is 304 (Not Modified), the cache uses the entity-body
1135   stored in the cache entry as the entity-body of this outgoing
1136   response. If the status code is 206 (Partial Content) and the ETag or
1137   Last-Modified headers match exactly, the cache &MAY; combine the
1138   contents stored in the cache entry with the new contents received in
1139   the response and use the result as the entity-body of this outgoing
1140   response, (see &combining-byte-ranges;).
1141</t>
1142<t>
1143   The end-to-end headers stored in the cache entry are used for the
1144   constructed response, except that
1145  <list style="symbols">
1146    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1147      &MUST; be deleted from the cache entry and the forwarded response.</t>
1148    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1149        in the cache entry and the forwarded response.</t>
1150    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1151        replace the corresponding headers from the cache entry.</t>
1152  </list>
1153</t>
1154<t>
1155   Unless the cache decides to remove the cache entry, it &MUST; also
1156   replace the end-to-end headers stored with the cache entry with
1157   corresponding headers received in the incoming response, except for
1158   Warning headers as described immediately above. If a header field-name
1159   in the incoming response matches more than one header in the
1160   cache entry, all such old headers &MUST; be replaced.
1161</t>
1162<t>
1163   In other words, the set of end-to-end headers received in the
1164   incoming response overrides all corresponding end-to-end headers
1165   stored with the cache entry (except for stored Warning headers with
1166   warn-code 1xx, which are deleted even if not overridden).
1167  <list><t>
1168      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1169      Modified) or a 206 (Partial Content) response to update any header
1170      associated with a previous response for the same entity or sub-ranges
1171      thereof, although it might not always be meaningful or
1172      correct to do so. This rule does not allow an origin server to use
1173      a 304 (Not Modified) or a 206 (Partial Content) response to
1174      entirely delete a header that it had provided with a previous
1175      response.
1176  </t></list>
1177</t>
1178</section>
1179
1180</section>
1181
1182<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1183<t>
1184   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1185   by the presence of a Vary header field in a response, alters the
1186   conditions and procedure by which a cache can use the response for
1187   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1188   field by servers.
1189</t>
1190<t>
1191   A server &SHOULD; use the Vary header field to inform a cache of what
1192   request-header fields were used to select among multiple
1193   representations of a cacheable response subject to server-driven
1194   negotiation. The set of header fields named by the Vary field value
1195   is known as the "selecting" request-headers.
1196</t>
1197<t>
1198   When the cache receives a subsequent request whose Request-URI
1199   specifies one or more cache entries including a Vary header field,
1200   the cache &MUST-NOT; use such a cache entry to construct a response to
1201   the new request unless all of the selecting request-headers present
1202   in the new request match the corresponding stored request-headers in
1203   the original request.
1204</t>
1205<t>
1206   The selecting request-headers from two requests are defined to match
1207   if and only if the selecting request-headers in the first request can
1208   be transformed to the selecting request-headers in the second request
1209   by adding or removing linear white space (LWS) at places where this
1210   is allowed by the corresponding BNF, and/or combining multiple
1211   message-header fields with the same field name following the rules
1212   about message headers in &message-headers;.
1213</t>
1214<t>
1215   A Vary header field-value of "*" always fails to match and subsequent
1216   requests on that resource can only be properly interpreted by the
1217   origin server.
1218</t>
1219<t>
1220   If the selecting request header fields for the cached entry do not
1221   match the selecting request header fields of the new request, then
1222   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1223   it first relays the new request to the origin server in a conditional
1224   request and the server responds with 304 (Not Modified), including an
1225   entity tag or Content-Location that indicates the entity to be used.
1226</t>
1227<t>
1228   If an entity tag was assigned to a cached representation, the
1229   forwarded request &SHOULD; be conditional and include the entity tags
1230   in an If-None-Match header field from all its cache entries for the
1231   resource. This conveys to the server the set of entities currently
1232   held by the cache, so that if any one of these entities matches the
1233   requested entity, the server can use the ETag header field in its 304
1234   (Not Modified) response to tell the cache which entry is appropriate.
1235   If the entity-tag of the new response matches that of an existing
1236   entry, the new response &SHOULD; be used to update the header fields of
1237   the existing entry, and the result &MUST; be returned to the client.
1238</t>
1239<t>
1240   If any of the existing cache entries contains only partial content
1241   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1242   the If-None-Match header field unless the request is for a range that
1243   would be fully satisfied by that entry.
1244</t>
1245<t>
1246   If a cache receives a successful response whose Content-Location
1247   field matches that of an existing cache entry for the same Request-URI,
1248   whose entity-tag differs from that of the existing entry, and
1249   whose Date is more recent than that of the existing entry, the
1250   existing entry &SHOULD-NOT;  be returned in response to future requests
1251   and &SHOULD; be deleted from the cache.
1252</t>
1253</section>
1254
1255<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1256<t>
1257   For reasons of security and privacy, it is necessary to make a
1258   distinction between "shared" and "non-shared" caches. A non-shared
1259   cache is one that is accessible only to a single user. Accessibility
1260   in this case &SHOULD; be enforced by appropriate security mechanisms.
1261   All other caches are considered to be "shared." Other sections of
1262   this specification place certain constraints on the operation of
1263   shared caches in order to prevent loss of privacy or failure of
1264   access controls.
1265</t>
1266</section>
1267
1268<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1269<t>
1270   A cache that receives an incomplete response (for example, with fewer
1271   bytes of data than specified in a Content-Length header) &MAY; store
1272   the response. However, the cache &MUST; treat this as a partial
1273   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1274   the result might be a full response or might still be
1275   partial. A cache &MUST-NOT; return a partial response to a client
1276   without explicitly marking it as such, using the 206 (Partial
1277   Content) status code. A cache &MUST-NOT; return a partial response
1278   using a status code of 200 (OK).
1279</t>
1280<t>
1281   If a cache receives a 5xx response while attempting to revalidate an
1282   entry, it &MAY; either forward this response to the requesting client,
1283   or act as if the server failed to respond. In the latter case, it &MAY;
1284   return a previously received response unless the cached entry
1285   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1286</t>
1287</section>
1288
1289<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1290<t>
1291   Unless the origin server explicitly prohibits the caching of their
1292   responses, the application of GET and HEAD methods to any resources
1293   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1294   these responses are taken from a cache. They &MAY; still have side
1295   effects, but a cache is not required to consider such side effects in
1296   its caching decisions. Caches are always expected to observe an
1297   origin server's explicit restrictions on caching.
1298</t>
1299<t>
1300   We note one exception to this rule: since some applications have
1301   traditionally used GET and HEAD requests with URLs containing a query part
1302   to perform operations with significant side
1303   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1304   the server provides an explicit expiration time. This specifically
1305   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1306   be taken from a cache. See &safe-methods; for related information.
1307</t>
1308</section>
1309
1310<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1311<t>
1312   The effect of certain methods performed on a resource at the origin
1313   server might cause one or more existing cache entries to become non-transparently
1314   invalid. That is, although they might continue to be
1315   "fresh," they do not accurately reflect what the origin server would
1316   return for a new request on that resource.
1317</t>
1318<t>
1319   There is no way for HTTP to guarantee that all such
1320   cache entries are marked invalid. For example, the request that
1321   caused the change at the origin server might not have gone through
1322   the proxy where a cache entry is stored. However, several rules help
1323   reduce the likelihood of erroneous behavior.
1324</t>
1325<t>
1326   In this section, the phrase "invalidate an entity" means that the
1327   cache will either remove all instances of that entity from its
1328   storage, or will mark these as "invalid" and in need of a mandatory
1329   revalidation before they can be returned in response to a subsequent
1330   request.
1331</t>
1332<t>
1333   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1334   either the entity referred to by the Request-URI, or by the Location
1335   or Content-Location headers (if present). These methods are:
1336  <list style="symbols">
1337      <t>PUT</t>
1338      <t>DELETE</t>
1339      <t>POST</t>
1340  </list>
1341</t> 
1342<t>
1343   An invalidation based
1344   on the URI in a Location or Content-Location header &MUST-NOT; be
1345   performed if the host part of that URI differs from the host part
1346   in the Request-URI. This helps prevent denial of service attacks.
1347</t>
1348<t>
1349   A cache that passes through requests for methods it does not
1350   understand &SHOULD; invalidate any entities referred to by the
1351   Request-URI.
1352</t>
1353</section>
1354
1355<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1356<t>
1357   All methods that might be expected to cause modifications to the
1358   origin server's resources &MUST; be written through to the origin
1359   server. This currently includes all methods except for GET and HEAD.
1360   A cache &MUST-NOT; reply to such a request from a client before having
1361   transmitted the request to the inbound server, and having received a
1362   corresponding response from the inbound server. This does not prevent
1363   a proxy cache from sending a 100 (Continue) response before the
1364   inbound server has sent its final reply.
1365</t>
1366<t>
1367   The alternative (known as "write-back" or "copy-back" caching) is not
1368   allowed in HTTP/1.1, due to the difficulty of providing consistent
1369   updates and the problems arising from server, cache, or network
1370   failure prior to write-back.
1371</t>
1372</section>
1373
1374<section title="Cache Replacement" anchor="cache.replacement">
1375<t>
1376   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1377   <xref target="disambiguating.expiration.values" format="counter"/>,
1378   <xref target="disambiguating.multiple.responses" format="counter"/>
1379   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1380   response is received from a resource while any existing responses for
1381   the same resource are cached, the cache &SHOULD; use the new response
1382   to reply to the current request. It &MAY; insert it into cache storage
1383   and &MAY;, if it meets all other requirements, use it to respond to any
1384   future requests that would previously have caused the old response to
1385   be returned. If it inserts the new response into cache storage  the
1386   rules in <xref target="combining.headers"/> apply.
1387  <list><t>
1388      <x:h>Note:</x:h> a new response that has an older Date header value than
1389      existing cached responses is not cacheable.
1390  </t></list>
1391</t>
1392</section>
1393
1394<section title="History Lists" anchor="history.lists">
1395<t>
1396   User agents often have history mechanisms, such as "Back" buttons and
1397   history lists, which can be used to redisplay an entity retrieved
1398   earlier in a session.
1399</t>
1400<t>
1401   History mechanisms and caches are different. In particular history
1402   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1403   the current state of a resource. Rather, a history mechanism is meant
1404   to show exactly what the user saw at the time when the resource was
1405   retrieved.
1406</t>
1407<t>
1408   By default, an expiration time does not apply to history mechanisms.
1409   If the entity is still in storage, a history mechanism &SHOULD; display
1410   it even if the entity has expired, unless the user has specifically
1411   configured the agent to refresh expired history documents.
1412</t>
1413<t>
1414   This is not to be construed to prohibit the history mechanism from
1415   telling the user that a view might be stale.
1416  <list><t>
1417      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1418      viewing stale resources, this will tend to force service authors
1419      to avoid using HTTP expiration controls and cache controls when
1420      they would otherwise like to. Service authors may consider it
1421      important that users not be presented with error messages or
1422      warning messages when they use navigation controls (such as BACK)
1423      to view previously fetched resources. Even though sometimes such
1424      resources ought not be cached, or ought to expire quickly, user
1425      interface considerations may force service authors to resort to
1426      other means of preventing caching (e.g. "once-only" URLs) in order
1427      not to suffer the effects of improperly functioning history
1428      mechanisms.
1429  </t></list>
1430</t>
1431</section>
1432
1433<section title="Header Field Definitions" anchor="header.fields">
1434<t>
1435   This section defines the syntax and semantics of HTTP/1.1 header fields
1436   related to caching.
1437</t>
1438<t>
1439   For entity-header fields, both sender and recipient refer to either the
1440   client or the server, depending on who sends and who receives the entity.
1441</t>
1442
1443<section title="Age" anchor="header.age">
1444  <iref primary="true" item="Age header" x:for-anchor=""/>
1445  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1446  <x:anchor-alias value="Age"/>
1447  <x:anchor-alias value="age-value"/>
1448<t>
1449      The Age response-header field conveys the sender's estimate of the
1450      amount of time since the response (or its revalidation) was
1451      generated at the origin server. A cached response is "fresh" if
1452      its age does not exceed its freshness lifetime. Age values are
1453      calculated as specified in <xref target="age.calculations"/>.
1454</t>
1455<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1456  <x:ref>Age</x:ref> = "Age" ":" <x:ref>age-value</x:ref>
1457  <x:ref>age-value</x:ref> = <x:ref>delta-seconds</x:ref>
1458</artwork></figure>
1459<t anchor="rule.delta-seconds">
1460  <x:anchor-alias value="delta-seconds"/>
1461      Age values are non-negative decimal integers, representing time in
1462      seconds.
1463</t>
1464<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
1465  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
1466</artwork></figure>
1467<t>
1468      If a cache receives a value larger than the largest positive
1469      integer it can represent, or if any of its age calculations
1470      overflows, it &MUST; transmit an Age header with a value of
1471      2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache &MUST;
1472      include an Age header field in every response generated from its
1473      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1474      bits of range.
1475</t>
1476</section>
1477
1478<section title="Cache-Control" anchor="header.cache-control">
1479  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1480  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1481  <x:anchor-alias value="Cache-Control"/>
1482  <x:anchor-alias value="cache-directive"/>
1483  <x:anchor-alias value="cache-extension"/>
1484  <x:anchor-alias value="cache-request-directive"/>
1485  <x:anchor-alias value="cache-response-directive"/>
1486<t>
1487   The Cache-Control general-header field is used to specify directives
1488   that &MUST; be obeyed by all caching mechanisms along the
1489   request/response chain. The directives specify behavior intended to
1490   prevent caches from adversely interfering with the request or
1491   response. These directives typically override the default caching
1492   algorithms. Cache directives are unidirectional in that the presence
1493   of a directive in a request does not imply that the same directive is
1494   to be given in the response.
1495  <list><t>
1496      Note that HTTP/1.0 caches might not implement Cache-Control and
1497      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1498  </t></list>
1499</t>
1500<t>
1501   Cache directives &MUST; be passed through by a proxy or gateway
1502   application, regardless of their significance to that application,
1503   since the directives might be applicable to all recipients along the
1504   request/response chain. It is not possible to specify a cache-directive
1505   for a specific cache.
1506</t>
1507<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"/>
1508  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" 1#<x:ref>cache-directive</x:ref>
1509
1510  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1511     / <x:ref>cache-response-directive</x:ref>
1512
1513  <x:ref>cache-request-directive</x:ref> =
1514       "no-cache"                          ; <xref target="what.is.cacheable"/>
1515     / "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1516     / "max-age" "=" <x:ref>delta-seconds</x:ref>         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1517     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1518     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1519     / "no-transform"                      ; <xref target="no-transform.directive"/>
1520     / "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1521     / <x:ref>cache-extension</x:ref>                     ; <xref target="cache.control.extensions"/>
1522
1523  <x:ref>cache-response-directive</x:ref> =
1524       "public"                               ; <xref target="what.is.cacheable"/>
1525     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
1526     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
1527     / "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1528     / "no-transform"                         ; <xref target="no-transform.directive"/>
1529     / "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1530     / "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1531     / "max-age" "=" <x:ref>delta-seconds</x:ref>            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1532     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1533     / <x:ref>cache-extension</x:ref>                        ; <xref target="cache.control.extensions"/>
1534
1535  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1536</artwork></figure>
1537<t>
1538   When a directive appears without any 1#field-name parameter, the
1539   directive applies to the entire request or response. When such a
1540   directive appears with a 1#field-name parameter, it applies only to
1541   the named field or fields, and not to the rest of the request or
1542   response. This mechanism supports extensibility; implementations of
1543   future versions of HTTP might apply these directives to
1544   header fields not defined in HTTP/1.1.
1545</t>
1546<t>
1547   The cache-control directives can be broken down into these general
1548   categories:
1549  <list style="symbols">
1550     <t>Restrictions on what are cacheable; these may only be imposed by
1551        the origin server.</t>
1552
1553     <t>Restrictions on what may be stored by a cache; these may be
1554        imposed by either the origin server or the user agent.</t>
1555
1556     <t>Modifications of the basic expiration mechanism; these may be
1557        imposed by either the origin server or the user agent.</t>
1558
1559     <t>Controls over cache revalidation and reload; these may only be
1560        imposed by a user agent.</t>
1561
1562     <t>Control over transformation of entities.</t>
1563
1564     <t>Extensions to the caching system.</t>
1565  </list>
1566</t>
1567
1568<section title="What is Cacheable" anchor="what.is.cacheable">
1569<t>
1570   By default, a response is cacheable if the requirements of the
1571   request method, request header fields, and the response status
1572   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1573   for cacheability. The following Cache-Control response directives
1574   allow an origin server to override the default cacheability of a
1575   response:
1576</t>
1577<t>
1578  <iref item="Cache Directives" subitem="public" primary="true"/>
1579  <iref item="public" subitem="Cache Directive" primary="true"/>
1580   public
1581  <list><t>
1582      Indicates that the response &MAY; be cached by any cache, even if it
1583      would normally be non-cacheable or cacheable only within a non-shared
1584      cache. (See also Authorization, &header-authorization;, for
1585      additional details.)
1586  </t></list>
1587</t>
1588<t>
1589  <iref item="Cache Directives" subitem="private" primary="true"/>
1590  <iref item="private" subitem="Cache Directive" primary="true"/>
1591   private
1592  <list><t>
1593      Indicates that all or part of the response message is intended for
1594      a single user and &MUST-NOT; be cached by a shared cache. This
1595      allows an origin server to state that the specified parts of the
1596      response are intended for only one user and are not a valid
1597      response for requests by other users. A private (non-shared) cache
1598      &MAY; cache the response.
1599    </t><t>
1600       <x:h>Note:</x:h> This usage of the word private only controls where the
1601       response may be cached, and cannot ensure the privacy of the
1602       message content.
1603  </t></list>
1604</t>
1605<t>
1606  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1607  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1608   no-cache
1609  <list><t>
1610       If the no-cache directive does not specify a field-name, then a
1611      cache &MUST-NOT; use the response to satisfy a subsequent request
1612      without successful revalidation with the origin server. This
1613      allows an origin server to prevent caching even by caches that
1614      have been configured to return stale responses to client requests.
1615    </t><t>
1616      If the no-cache directive does specify one or more field-names,
1617      then a cache &MAY; use the response to satisfy a subsequent request,
1618      subject to any other restrictions on caching. However, the
1619      specified field-name(s) &MUST-NOT; be sent in the response to a
1620      subsequent request without successful revalidation with the origin
1621      server. This allows an origin server to prevent the re-use of
1622      certain header fields in a response, while still allowing caching
1623      of the rest of the response.
1624    <list><t>
1625       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1626       directive.
1627    </t></list>
1628  </t></list>
1629</t>
1630</section>
1631
1632<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1633<t>
1634  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1635  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1636   no-store
1637  <list><t>   
1638      The purpose of the no-store directive is to prevent the
1639      inadvertent release or retention of sensitive information (for
1640      example, on backup tapes). The no-store directive applies to the
1641      entire message, and &MAY; be sent either in a response or in a
1642      request. If sent in a request, a cache &MUST-NOT; store any part of
1643      either this request or any response to it. If sent in a response,
1644      a cache &MUST-NOT; store any part of either this response or the
1645      request that elicited it. This directive applies to both non-shared
1646      and shared caches. "&MUST-NOT; store" in this context means
1647      that the cache &MUST-NOT; intentionally store the information in
1648      non-volatile storage, and &MUST; make a best-effort attempt to
1649      remove the information from volatile storage as promptly as
1650      possible after forwarding it.
1651  </t><t>
1652      Even when this directive is associated with a response, users
1653      might explicitly store such a response outside of the caching
1654      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1655      such responses as part of their normal operation.
1656  </t><t>
1657      The purpose of this directive is to meet the stated requirements
1658      of certain users and service authors who are concerned about
1659      accidental releases of information via unanticipated accesses to
1660      cache data structures. While the use of this directive might
1661      improve privacy in some cases, we caution that it is NOT in any
1662      way a reliable or sufficient mechanism for ensuring privacy. In
1663      particular, malicious or compromised caches might not recognize or
1664      obey this directive, and communications networks might be
1665      vulnerable to eavesdropping.
1666  </t></list>
1667</t>
1668</section>
1669
1670<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1671<t>
1672   The expiration time of an entity &MAY; be specified by the origin
1673   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1674   it &MAY; be specified using the max-age directive in a response. When
1675   the max-age cache-control directive is present in a cached response,
1676   the response is stale if its current age is greater than the age
1677   value given (in seconds) at the time of a new request for that
1678   resource. The max-age directive on a response implies that the
1679   response is cacheable (i.e., "public") unless some other, more
1680   restrictive cache directive is also present.
1681</t>
1682<t>
1683   If a response includes both an Expires header and a max-age
1684   directive, the max-age directive overrides the Expires header, even
1685   if the Expires header is more restrictive. This rule allows an origin
1686   server to provide, for a given response, a longer expiration time to
1687   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1688   useful if certain HTTP/1.0 caches improperly calculate ages or
1689   expiration times, perhaps due to desynchronized clocks.
1690</t>
1691<t>
1692   Many HTTP/1.0 cache implementations will treat an Expires value that
1693   is less than or equal to the response Date value as being equivalent
1694   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1695   cache receives such a response, and the response does not include a
1696   Cache-Control header field, it &SHOULD; consider the response to be
1697   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1698  <list><t>
1699       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1700       cache control feature, such as the "private" directive, on a
1701       network including older caches that do not understand that
1702       feature. The origin server will need to combine the new feature
1703       with an Expires field whose value is less than or equal to the
1704       Date value. This will prevent older caches from improperly
1705       caching the response.
1706  </t></list>
1707</t>
1708<t>
1709  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1710  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1711   s-maxage
1712  <list><t>
1713       If a response includes an s-maxage directive, then for a shared
1714       cache (but not for a private cache), the maximum age specified by
1715       this directive overrides the maximum age specified by either the
1716       max-age directive or the Expires header. The s-maxage directive
1717       also implies the semantics of the proxy-revalidate directive (see
1718       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1719       entry after it becomes stale to respond to a subsequent request
1720       without first revalidating it with the origin server. The s-maxage
1721       directive is always ignored by a private cache.
1722  </t></list>
1723</t>
1724<t>
1725   Note that most older caches, not compliant with this specification,
1726   do not implement any cache-control directives. An origin server
1727   wishing to use a cache-control directive that restricts, but does not
1728   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1729   requirement that the max-age directive overrides the Expires header,
1730   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1731   max-age directive.
1732</t>
1733<t>
1734   Other directives allow a user agent to modify the basic expiration
1735   mechanism. These directives &MAY; be specified on a request:
1736</t>
1737<t>
1738  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1739  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1740   max-age
1741  <list><t>
1742      Indicates that the client is willing to accept a response whose
1743      age is no greater than the specified time in seconds. Unless max-stale
1744      directive is also included, the client is not willing to
1745      accept a stale response.
1746  </t></list>
1747</t>
1748<t>
1749  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1750  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1751   min-fresh
1752  <list><t>
1753      Indicates that the client is willing to accept a response whose
1754      freshness lifetime is no less than its current age plus the
1755      specified time in seconds. That is, the client wants a response
1756      that will still be fresh for at least the specified number of
1757      seconds.
1758  </t></list>
1759</t>
1760<t>
1761  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1762  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1763   max-stale
1764  <list><t>
1765      Indicates that the client is willing to accept a response that has
1766      exceeded its expiration time. If max-stale is assigned a value,
1767      then the client is willing to accept a response that has exceeded
1768      its expiration time by no more than the specified number of
1769      seconds. If no value is assigned to max-stale, then the client is
1770      willing to accept a stale response of any age.
1771  </t></list>
1772</t>
1773<t>
1774   If a cache returns a stale response, either because of a max-stale
1775   directive on a request, or because the cache is configured to
1776   override the expiration time of a response, the cache &MUST; attach a
1777   Warning header to the stale response, using Warning 110 (Response is
1778   stale).
1779</t>
1780<t>
1781   A cache &MAY; be configured to return stale responses without
1782   validation, but only if this does not conflict with any "MUST"-level
1783   requirements concerning cache validation (e.g., a "must-revalidate"
1784   cache-control directive).
1785</t>
1786<t>
1787   If both the new request and the cached entry include "max-age"
1788   directives, then the lesser of the two values is used for determining
1789   the freshness of the cached entry for that request.
1790</t>
1791</section>
1792
1793<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1794<t>
1795   Sometimes a user agent might want or need to insist that a cache
1796   revalidate its cache entry with the origin server (and not just with
1797   the next cache along the path to the origin server), or to reload its
1798   cache entry from the origin server. End-to-end revalidation might be
1799   necessary if either the cache or the origin server has overestimated
1800   the expiration time of the cached response. End-to-end reload may be
1801   necessary if the cache entry has become corrupted for some reason.
1802</t>
1803<t>
1804   End-to-end revalidation may be requested either when the client does
1805   not have its own local cached copy, in which case we call it
1806   "unspecified end-to-end revalidation", or when the client does have a
1807   local cached copy, in which case we call it "specific end-to-end
1808   revalidation."
1809</t>
1810<t>
1811   The client can specify these three kinds of action using Cache-Control
1812   request directives:
1813</t>
1814<t>
1815   End-to-end reload
1816  <list><t>
1817      The request includes a "no-cache" cache-control directive or, for
1818      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1819      names &MUST-NOT; be included with the no-cache directive in a
1820      request. The server &MUST-NOT; use a cached copy when responding to
1821      such a request.
1822  </t></list>
1823</t>
1824<t>
1825   Specific end-to-end revalidation
1826  <list><t>
1827      The request includes a "max-age=0" cache-control directive, which
1828      forces each cache along the path to the origin server to
1829      revalidate its own entry, if any, with the next cache or server.
1830      The initial request includes a cache-validating conditional with
1831      the client's current validator.
1832  </t></list>
1833</t>
1834<t>
1835   Unspecified end-to-end revalidation
1836  <list><t>
1837      The request includes "max-age=0" cache-control directive, which
1838      forces each cache along the path to the origin server to
1839      revalidate its own entry, if any, with the next cache or server.
1840      The initial request does not include a cache-validating
1841      conditional; the first cache along the path (if any) that holds a
1842      cache entry for this resource includes a cache-validating
1843      conditional with its current validator.
1844  </t></list>
1845</t>
1846<t>
1847  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1848  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1849   max-age
1850  <list><t>
1851      When an intermediate cache is forced, by means of a max-age=0
1852      directive, to revalidate its own cache entry, and the client has
1853      supplied its own validator in the request, the supplied validator
1854      might differ from the validator currently stored with the cache
1855      entry. In this case, the cache &MAY; use either validator in making
1856      its own request without affecting semantic transparency.
1857  </t><t>
1858      However, the choice of validator might affect performance. The
1859      best approach is for the intermediate cache to use its own
1860      validator when making its request. If the server replies with 304
1861      (Not Modified), then the cache can return its now validated copy
1862      to the client with a 200 (OK) response. If the server replies with
1863      a new entity and cache validator, however, the intermediate cache
1864      can compare the returned validator with the one provided in the
1865      client's request, using the strong comparison function. If the
1866      client's validator is equal to the origin server's, then the
1867      intermediate cache simply returns 304 (Not Modified). Otherwise,
1868      it returns the new entity with a 200 (OK) response.
1869  </t><t>
1870      If a request includes the no-cache directive, it &SHOULD-NOT; 
1871      include min-fresh, max-stale, or max-age.
1872  </t></list>
1873</t>
1874<t>
1875  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1876  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1877   only-if-cached
1878  <list><t>
1879      In some cases, such as times of extremely poor network
1880      connectivity, a client may want a cache to return only those
1881      responses that it currently has stored, and not to reload or
1882      revalidate with the origin server. To do this, the client may
1883      include the only-if-cached directive in a request. If it receives
1884      this directive, a cache &SHOULD; either respond using a cached entry
1885      that is consistent with the other constraints of the request, or
1886      respond with a 504 (Gateway Timeout) status. However, if a group
1887      of caches is being operated as a unified system with good internal
1888      connectivity, such a request &MAY; be forwarded within that group of
1889      caches.
1890  </t></list>
1891</t>
1892<t>
1893  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1894  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1895   must-revalidate
1896  <list><t>
1897      Because a cache &MAY; be configured to ignore a server's specified
1898      expiration time, and because a client request &MAY; include a max-stale
1899      directive (which has a similar effect), the protocol also
1900      includes a mechanism for the origin server to require revalidation
1901      of a cache entry on any subsequent use. When the must-revalidate
1902      directive is present in a response received by a cache, that cache
1903      &MUST-NOT; use the entry after it becomes stale to respond to a
1904      subsequent request without first revalidating it with the origin
1905      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1906      time, if, based solely on the origin server's Expires or max-age
1907      value, the cached response is stale.)
1908  </t><t>
1909      The must-revalidate directive is necessary to support reliable
1910      operation for certain protocol features. In all circumstances an
1911      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1912      particular, if the cache cannot reach the origin server for any
1913      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1914  </t><t>
1915      Servers &SHOULD; send the must-revalidate directive if and only if
1916      failure to revalidate a request on the entity could result in
1917      incorrect operation, such as a silently unexecuted financial
1918      transaction. Recipients &MUST-NOT; take any automated action that
1919      violates this directive, and &MUST-NOT; automatically provide an
1920      unvalidated copy of the entity if revalidation fails.
1921  </t><t>
1922      Although this is not recommended, user agents operating under
1923      severe connectivity constraints &MAY; violate this directive but, if
1924      so, &MUST; explicitly warn the user that an unvalidated response has
1925      been provided. The warning &MUST; be provided on each unvalidated
1926      access, and &SHOULD; require explicit user confirmation.
1927  </t></list>
1928</t>
1929<t>
1930  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1931  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
1932   proxy-revalidate
1933  <list><t>
1934      The proxy-revalidate directive has the same meaning as the must-revalidate
1935      directive, except that it does not apply to non-shared
1936      user agent caches. It can be used on a response to an
1937      authenticated request to permit the user's cache to store and
1938      later return the response without needing to revalidate it (since
1939      it has already been authenticated once by that user), while still
1940      requiring proxies that service many users to revalidate each time
1941      (in order to make sure that each user has been authenticated).
1942      Note that such authenticated responses also need the public cache
1943      control directive in order to allow them to be cached at all.
1944  </t></list>
1945</t>
1946</section>
1947
1948<section title="No-Transform Directive" anchor="no-transform.directive">
1949<t>
1950  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
1951  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
1952   no-transform
1953  <list><t>
1954      Implementors of intermediate caches (proxies) have found it useful
1955      to convert the media type of certain entity bodies. A non-transparent
1956      proxy might, for example, convert between image
1957      formats in order to save cache space or to reduce the amount of
1958      traffic on a slow link.
1959  </t><t>
1960      Serious operational problems occur, however, when these
1961      transformations are applied to entity bodies intended for certain
1962      kinds of applications. For example, applications for medical
1963      imaging, scientific data analysis and those using end-to-end
1964      authentication, all depend on receiving an entity body that is bit
1965      for bit identical to the original entity-body.
1966  </t><t>
1967      Therefore, if a message includes the no-transform directive, an
1968      intermediate cache or proxy &MUST-NOT; change those headers that are
1969      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
1970      directive. This implies that the cache or proxy &MUST-NOT; change
1971      any aspect of the entity-body that is specified by these headers,
1972      including the value of the entity-body itself.
1973  </t></list>
1974</t>
1975</section>
1976
1977<section title="Cache Control Extensions" anchor="cache.control.extensions">
1978<t>
1979   The Cache-Control header field can be extended through the use of one
1980   or more cache-extension tokens, each with an optional assigned value.
1981   Informational extensions (those which do not require a change in
1982   cache behavior) &MAY; be added without changing the semantics of other
1983   directives. Behavioral extensions are designed to work by acting as
1984   modifiers to the existing base of cache directives. Both the new
1985   directive and the standard directive are supplied, such that
1986   applications which do not understand the new directive will default
1987   to the behavior specified by the standard directive, and those that
1988   understand the new directive will recognize it as modifying the
1989   requirements associated with the standard directive. In this way,
1990   extensions to the cache-control directives can be made without
1991   requiring changes to the base protocol.
1992</t>
1993<t>
1994   This extension mechanism depends on an HTTP cache obeying all of the
1995   cache-control directives defined for its native HTTP-version, obeying
1996   certain extensions, and ignoring all directives that it does not
1997   understand.
1998</t>
1999<t>
2000   For example, consider a hypothetical new response directive called
2001   community which acts as a modifier to the private directive. We
2002   define this new directive to mean that, in addition to any non-shared
2003   cache, any cache which is shared only by members of the community
2004   named within its value may cache the response. An origin server
2005   wishing to allow the UCI community to use an otherwise private
2006   response in their shared cache(s) could do so by including
2007</t>
2008<figure><artwork type="example">
2009    Cache-Control: private, community="UCI"
2010</artwork></figure>
2011<t>
2012   A cache seeing this header field will act correctly even if the cache
2013   does not understand the community cache-extension, since it will also
2014   see and understand the private directive and thus default to the safe
2015   behavior.
2016</t>
2017<t>
2018   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2019   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2020   be combined with standard directives (or the response's default
2021   cacheability) such that the cache behavior will remain minimally
2022   correct even if the cache does not understand the extension(s).
2023</t>
2024</section>
2025</section>
2026
2027<section title="Expires" anchor="header.expires">
2028  <iref primary="true" item="Expires header" x:for-anchor=""/>
2029  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2030  <x:anchor-alias value="Expires"/>
2031<t>
2032   The Expires entity-header field gives the date/time after which the
2033   response is considered stale. A stale cache entry may not normally be
2034   returned by a cache (either a proxy cache or a user agent cache)
2035   unless it is first validated with the origin server (or with an
2036   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2037   for further discussion of the expiration model.
2038</t>
2039<t>
2040   The presence of an Expires field does not imply that the original
2041   resource will change or cease to exist at, before, or after that
2042   time.
2043</t>
2044<t>
2045   The format is an absolute date and time as defined by HTTP-date in
2046   &full-date;; it &MUST; be sent in rfc1123-date format.
2047</t>
2048<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2049  <x:ref>Expires</x:ref> = "Expires" ":" <x:ref>HTTP-date</x:ref>
2050</artwork></figure>
2051<t>
2052   An example of its use is
2053</t>
2054<figure><artwork type="example">
2055   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2056</artwork></figure>
2057<t>
2058  <list><t>
2059      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2060      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2061      Expires field.
2062  </t></list>
2063</t>
2064<t>
2065   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2066   especially including the value "0", as in the past (i.e., "already
2067   expired").
2068</t>
2069<t>
2070   To mark a response as "already expired," an origin server sends an
2071   Expires date that is equal to the Date header value. (See the rules
2072   for expiration calculations in <xref target="expiration.calculations"/>.)
2073</t>
2074<t>
2075   To mark a response as "never expires," an origin server sends an
2076   Expires date approximately one year from the time the response is
2077   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2078   year in the future.
2079</t>
2080<t>
2081   The presence of an Expires header field with a date value of some
2082   time in the future on a response that otherwise would by default be
2083   non-cacheable indicates that the response is cacheable, unless
2084   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2085</t>
2086</section>
2087
2088<section title="Pragma" anchor="header.pragma">
2089  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2090  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2091  <x:anchor-alias value="extension-pragma"/>
2092  <x:anchor-alias value="Pragma"/>
2093  <x:anchor-alias value="pragma-directive"/>
2094<t>
2095   The Pragma general-header field is used to include implementation-specific
2096   directives that might apply to any recipient along the
2097   request/response chain. All pragma directives specify optional
2098   behavior from the viewpoint of the protocol; however, some systems
2099   &MAY; require that behavior be consistent with the directives.
2100</t>
2101<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"/>
2102  <x:ref>Pragma</x:ref>            = "Pragma" ":" 1#<x:ref>pragma-directive</x:ref>
2103  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
2104  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
2105</artwork></figure>
2106<t>
2107   When the no-cache directive is present in a request message, an
2108   application &SHOULD; forward the request toward the origin server even
2109   if it has a cached copy of what is being requested. This pragma
2110   directive has the same semantics as the no-cache cache-directive (see
2111   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2112   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2113   request is sent to a server not known to be HTTP/1.1 compliant.
2114</t>
2115<t>
2116   Pragma directives &MUST; be passed through by a proxy or gateway
2117   application, regardless of their significance to that application,
2118   since the directives might be applicable to all recipients along the
2119   request/response chain. It is not possible to specify a pragma for a
2120   specific recipient; however, any pragma directive not relevant to a
2121   recipient &SHOULD; be ignored by that recipient.
2122</t>
2123<t>
2124   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2125   sent "Cache-Control: no-cache". No new Pragma directives will be
2126   defined in HTTP.
2127  <list><t>
2128      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
2129      response-header field is not actually specified, it does not provide a
2130      reliable replacement for "Cache-Control: no-cache" in a response.
2131  </t></list>
2132</t>
2133</section>
2134
2135<section title="Vary" anchor="header.vary">
2136  <iref primary="true" item="Vary header" x:for-anchor=""/>
2137  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2138  <x:anchor-alias value="Vary"/>
2139<t>
2140   The Vary response-header field's value indicates the set of request-header
2141   fields that fully determines, while the response is fresh, whether a cache
2142   is permitted to use the response to reply to a subsequent request
2143   without revalidation. For uncacheable or stale responses, the Vary
2144   field value advises the user agent about the criteria that were used
2145   to select the representation. A Vary field value of "*" implies that
2146   a cache cannot determine from the request headers of a subsequent
2147   request whether this response is the appropriate representation. See
2148   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2149</t>
2150<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2151  <x:ref>Vary</x:ref>  = "Vary" ":" ( "*" / 1#<x:ref>field-name</x:ref> )
2152</artwork></figure>
2153<t>
2154   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2155   cacheable response that is subject to server-driven negotiation.
2156   Doing so allows a cache to properly interpret future requests on that
2157   resource and informs the user agent about the presence of negotiation
2158   on that resource. A server &MAY; include a Vary header field with a
2159   non-cacheable response that is subject to server-driven negotiation,
2160   since this might provide the user agent with useful information about
2161   the dimensions over which the response varies at the time of the
2162   response.
2163</t>
2164<t>
2165   A Vary field value consisting of a list of field-names signals that
2166   the representation selected for the response is based on a selection
2167   algorithm which considers ONLY the listed request-header field values
2168   in selecting the most appropriate representation. A cache &MAY; assume
2169   that the same selection will be made for future requests with the
2170   same values for the listed field names, for the duration of time for
2171   which the response is fresh.
2172</t>
2173<t>
2174   The field-names given are not limited to the set of standard
2175   request-header fields defined by this specification. Field names are
2176   case-insensitive.
2177</t>
2178<t>
2179   A Vary field value of "*" signals that unspecified parameters not
2180   limited to the request-headers (e.g., the network address of the
2181   client), play a role in the selection of the response representation.
2182   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2183   generated by an origin server.
2184</t>
2185</section>
2186
2187<section title="Warning" anchor="header.warning">
2188  <iref primary="true" item="Warning header" x:for-anchor=""/>
2189  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2190  <x:anchor-alias value="Warning"/>
2191  <x:anchor-alias value="warning-value"/>
2192  <x:anchor-alias value="warn-agent"/>
2193  <x:anchor-alias value="warn-code"/>
2194  <x:anchor-alias value="warn-date"/>
2195  <x:anchor-alias value="warn-text"/>
2196<t>
2197   The Warning general-header field is used to carry additional
2198   information about the status or transformation of a message which
2199   might not be reflected in the message. This information is typically
2200   used to warn about a possible lack of semantic transparency from
2201   caching operations or transformations applied to the entity body of
2202   the message.
2203</t>
2204<t>
2205   Warnings MAY be used for other purposes, both cache-related and
2206   otherwise.  The use of a warning, rather than an error status code,
2207   distinguish these responses from true failures.
2208</t>
2209
2210<t>
2211   Warning headers can in general be applied to any message, however
2212   some warn-codes are specific to caches and can only be
2213   applied to response messages.
2214</t>
2215
2216<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"/>
2217  <x:ref>Warning</x:ref>    = "Warning" ":" 1#<x:ref>warning-value</x:ref>
2218 
2219  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
2220                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
2221 
2222  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
2223  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2224                  ; the name or pseudonym of the server adding
2225                  ; the Warning header, for use in debugging
2226  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
2227  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
2228</artwork></figure>
2229
2230<t>
2231   Multiple warnings &MAY; be attached to a response (either by the origin
2232   server or by a cache), including multiple warnings with the same code
2233   number.  For example, a server might provide the same warning with
2234   texts in both English and Basque.
2235</t>
2236<t>
2237   When this occurs, the user agent ought to inform the user of as many
2238   of them as possible, in the order that they appear in the response. 
2239   If it is not possible to inform the user of all of the warnings,
2240   the user agent SHOULD follow these heuristics:
2241  <list style="symbols">
2242    <t>Warnings that appear early in the response take priority over
2243        those appearing later in the response.</t>
2244
2245    <t>Warnings in the user's preferred character set take priority
2246        over warnings in other character sets but with identical warn-codes
2247        and warn-agents.</t>
2248  </list>
2249</t>
2250<t>
2251   Systems that generate multiple Warning headers &SHOULD; order them with
2252   this user agent behavior in mind. New Warning headers &SHOULD; be added
2253  after any existing Warning headers.
2254</t>
2255<t>
2256   Warnings are assigned three digit warn-codes.  The first digit
2257   indicates whether the Warning is required to be deleted from a
2258   stored cache entry after validation:
2259   <list style="symbols">
2260         <t>1xx  Warnings that describe the freshness or revalidation status of
2261             the response, and so MUST be deleted by caches after validation.</t>
2262         <t> 2xx  Warnings that describe some aspect of the entity body or entity
2263             headers that is not rectified by a revalidation (for example, a
2264             lossy compression of the entity bodies) and which MUST NOT be
2265             deleted by caches after validation, unless a full response is
2266             returned, in which case they MUST be.</t>
2267   </list>
2268</t>
2269<t>
2270   The warn-text &SHOULD; be in a natural language and character set that
2271   is most likely to be intelligible to the human user receiving the
2272   response. This decision &MAY; be based on any available knowledge, such
2273   as the location of the cache or user, the Accept-Language field in a
2274   request, the Content-Language field in a response, etc. The default
2275   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2276</t>
2277<t>
2278   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2279   in the warn-text using the method described in <xref target="RFC2047"/>.
2280</t>
2281<t>
2282   If an implementation sends a message with one or more Warning headers
2283   to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST;
2284   include in each warning-value a warn-date that matches the Date header
2285   in the message.
2286</t>
2287<t>
2288   If an implementation receives a message with a warning-value that
2289   includes a warn-date, and that warn-date is different from the Date
2290   value in the response, then that warning-value &MUST; be deleted from
2291   the message before storing, forwarding, or using it.  (This prevents
2292   bad consequences of naive caching of Warning header fields.)  If all
2293   of the warning-values are deleted for this reason, the Warning header
2294   &MUST; be deleted as well.
2295</t>
2296<t>
2297   The following warn-codes are defined by this specification, each with a
2298   recommended warn-text in English, and a description of its meaning.
2299</t>
2300<t>
2301   110 Response is stale
2302  <list><t>
2303     &MUST; be included whenever the returned response is stale.
2304  </t></list>
2305</t>
2306<t>
2307   111 Revalidation failed
2308  <list><t>
2309     &MUST; be included if a cache returns a stale response because an
2310     attempt to revalidate the response failed, due to an inability to
2311     reach the server.
2312  </t></list>
2313</t>
2314<t>
2315   112 Disconnected operation
2316  <list><t>
2317     &SHOULD; be included if the cache is intentionally disconnected from
2318     the rest of the network for a period of time.
2319  </t></list>
2320</t>
2321<t>
2322   113 Heuristic expiration
2323  <list><t>
2324     &MUST; be included if the cache heuristically chose a freshness
2325     lifetime greater than 24 hours and the response's age is greater
2326     than 24 hours.
2327  </t></list>
2328</t>
2329<t>
2330   199 Miscellaneous warning
2331  <list><t>
2332     The warning text &MAY; include arbitrary information to be presented
2333     to a human user, or logged. A system receiving this warning &MUST-NOT;
2334     take any automated action, besides presenting the warning to
2335     the user.
2336  </t></list>
2337</t>
2338<t>
2339   214 Transformation applied
2340  <list><t>
2341     &MUST; be added by an intermediate cache or proxy if it applies any
2342     transformation changing the content-coding (as specified in the
2343     Content-Encoding header) or media-type (as specified in the
2344     Content-Type header) of the response, or the entity-body of the
2345     response, unless this Warning code already appears in the response.
2346  </t></list>
2347</t>
2348<t>
2349   299 Miscellaneous persistent warning
2350  <list><t>
2351     The warning text &MAY; include arbitrary information to be presented
2352     to a human user, or logged. A system receiving this warning &MUST-NOT;
2353     take any automated action.
2354  </t></list>
2355</t>
2356</section>
2357
2358</section>
2359
2360<section title="IANA Considerations" anchor="IANA.considerations">
2361<section title="Message Header Registration" anchor="message.header.registration">
2362<t>
2363   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2364   with the permanent registrations below (see <xref target="RFC3864"/>):
2365</t>
2366<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2367<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2368   <ttcol>Header Field Name</ttcol>
2369   <ttcol>Protocol</ttcol>
2370   <ttcol>Status</ttcol>
2371   <ttcol>Reference</ttcol>
2372
2373   <c>Age</c>
2374   <c>http</c>
2375   <c>standard</c>
2376   <c>
2377      <xref target="header.age"/>
2378   </c>
2379   <c>Cache-Control</c>
2380   <c>http</c>
2381   <c>standard</c>
2382   <c>
2383      <xref target="header.cache-control"/>
2384   </c>
2385   <c>Expires</c>
2386   <c>http</c>
2387   <c>standard</c>
2388   <c>
2389      <xref target="header.expires"/>
2390   </c>
2391   <c>Pragma</c>
2392   <c>http</c>
2393   <c>standard</c>
2394   <c>
2395      <xref target="header.pragma"/>
2396   </c>
2397   <c>Vary</c>
2398   <c>http</c>
2399   <c>standard</c>
2400   <c>
2401      <xref target="header.vary"/>
2402   </c>
2403   <c>Warning</c>
2404   <c>http</c>
2405   <c>standard</c>
2406   <c>
2407      <xref target="header.warning"/>
2408   </c>
2409</texttable>
2410<!--(END)-->
2411<t>
2412   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2413</t>
2414</section>
2415</section>
2416
2417<section title="Security Considerations" anchor="security.considerations">
2418<t>
2419   Caching proxies provide additional potential vulnerabilities, since
2420   the contents of the cache represent an attractive target for
2421   malicious exploitation. Because cache contents persist after an HTTP
2422   request is complete, an attack on the cache can reveal information
2423   long after a user believes that the information has been removed from
2424   the network. Therefore, cache contents should be protected as
2425   sensitive information.
2426</t>
2427</section>
2428
2429<section title="Acknowledgments" anchor="ack">
2430<t>
2431   Much of the content and presentation of the caching design is due to
2432   suggestions and comments from individuals including: Shel Kaphan,
2433   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2434</t>
2435</section>
2436</middle>
2437<back>
2438
2439<references title="Normative References">
2440
2441<reference anchor="ISO-8859-1">
2442  <front>
2443    <title>
2444     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2445    </title>
2446    <author>
2447      <organization>International Organization for Standardization</organization>
2448    </author>
2449    <date year="1998"/>
2450  </front>
2451  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2452</reference>
2453
2454<reference anchor="Part1">
2455  <front>
2456    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2457    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2458      <organization abbrev="Day Software">Day Software</organization>
2459      <address><email>fielding@gbiv.com</email></address>
2460    </author>
2461    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2462      <organization>One Laptop per Child</organization>
2463      <address><email>jg@laptop.org</email></address>
2464    </author>
2465    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2466      <organization abbrev="HP">Hewlett-Packard Company</organization>
2467      <address><email>JeffMogul@acm.org</email></address>
2468    </author>
2469    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2470      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2471      <address><email>henrikn@microsoft.com</email></address>
2472    </author>
2473    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2474      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2475      <address><email>LMM@acm.org</email></address>
2476    </author>
2477    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2478      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2479      <address><email>paulle@microsoft.com</email></address>
2480    </author>
2481    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2482      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2483      <address><email>timbl@w3.org</email></address>
2484    </author>
2485    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2486      <organization abbrev="W3C">World Wide Web Consortium</organization>
2487      <address><email>ylafon@w3.org</email></address>
2488    </author>
2489    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2490      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2491      <address><email>julian.reschke@greenbytes.de</email></address>
2492    </author>
2493    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2494  </front>
2495  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2496  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2497</reference>
2498
2499<reference anchor="Part2">
2500  <front>
2501    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2502    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2503      <organization abbrev="Day Software">Day Software</organization>
2504      <address><email>fielding@gbiv.com</email></address>
2505    </author>
2506    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2507      <organization>One Laptop per Child</organization>
2508      <address><email>jg@laptop.org</email></address>
2509    </author>
2510    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2511      <organization abbrev="HP">Hewlett-Packard Company</organization>
2512      <address><email>JeffMogul@acm.org</email></address>
2513    </author>
2514    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2515      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2516      <address><email>henrikn@microsoft.com</email></address>
2517    </author>
2518    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2519      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2520      <address><email>LMM@acm.org</email></address>
2521    </author>
2522    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2523      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2524      <address><email>paulle@microsoft.com</email></address>
2525    </author>
2526    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2527      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2528      <address><email>timbl@w3.org</email></address>
2529    </author>
2530    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2531      <organization abbrev="W3C">World Wide Web Consortium</organization>
2532      <address><email>ylafon@w3.org</email></address>
2533    </author>
2534    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2535      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2536      <address><email>julian.reschke@greenbytes.de</email></address>
2537    </author>
2538    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2539  </front>
2540  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2541  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2542</reference>
2543
2544<reference anchor="Part3">
2545  <front>
2546    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2547    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2548      <organization abbrev="Day Software">Day Software</organization>
2549      <address><email>fielding@gbiv.com</email></address>
2550    </author>
2551    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2552      <organization>One Laptop per Child</organization>
2553      <address><email>jg@laptop.org</email></address>
2554    </author>
2555    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2556      <organization abbrev="HP">Hewlett-Packard Company</organization>
2557      <address><email>JeffMogul@acm.org</email></address>
2558    </author>
2559    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2560      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2561      <address><email>henrikn@microsoft.com</email></address>
2562    </author>
2563    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2564      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2565      <address><email>LMM@acm.org</email></address>
2566    </author>
2567    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2568      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2569      <address><email>paulle@microsoft.com</email></address>
2570    </author>
2571    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2572      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2573      <address><email>timbl@w3.org</email></address>
2574    </author>
2575    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2576      <organization abbrev="W3C">World Wide Web Consortium</organization>
2577      <address><email>ylafon@w3.org</email></address>
2578    </author>
2579    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2580      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2581      <address><email>julian.reschke@greenbytes.de</email></address>
2582    </author>
2583    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2584  </front>
2585  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2586  <x:source href="p3-payload.xml" basename="p3-payload"/>
2587</reference>
2588
2589<reference anchor="Part4">
2590  <front>
2591    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2592    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2593      <organization abbrev="Day Software">Day Software</organization>
2594      <address><email>fielding@gbiv.com</email></address>
2595    </author>
2596    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2597      <organization>One Laptop per Child</organization>
2598      <address><email>jg@laptop.org</email></address>
2599    </author>
2600    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2601      <organization abbrev="HP">Hewlett-Packard Company</organization>
2602      <address><email>JeffMogul@acm.org</email></address>
2603    </author>
2604    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2605      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2606      <address><email>henrikn@microsoft.com</email></address>
2607    </author>
2608    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2609      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2610      <address><email>LMM@acm.org</email></address>
2611    </author>
2612    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2613      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2614      <address><email>paulle@microsoft.com</email></address>
2615    </author>
2616    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2617      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2618      <address><email>timbl@w3.org</email></address>
2619    </author>
2620    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2621      <organization abbrev="W3C">World Wide Web Consortium</organization>
2622      <address><email>ylafon@w3.org</email></address>
2623    </author>
2624    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2625      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2626      <address><email>julian.reschke@greenbytes.de</email></address>
2627    </author>
2628    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2629  </front>
2630  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2631  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2632</reference>
2633
2634<reference anchor="Part5">
2635  <front>
2636    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2637    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2638      <organization abbrev="Day Software">Day Software</organization>
2639      <address><email>fielding@gbiv.com</email></address>
2640    </author>
2641    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2642      <organization>One Laptop per Child</organization>
2643      <address><email>jg@laptop.org</email></address>
2644    </author>
2645    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2646      <organization abbrev="HP">Hewlett-Packard Company</organization>
2647      <address><email>JeffMogul@acm.org</email></address>
2648    </author>
2649    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2650      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2651      <address><email>henrikn@microsoft.com</email></address>
2652    </author>
2653    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2654      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2655      <address><email>LMM@acm.org</email></address>
2656    </author>
2657    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2658      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2659      <address><email>paulle@microsoft.com</email></address>
2660    </author>
2661    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2662      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2663      <address><email>timbl@w3.org</email></address>
2664    </author>
2665    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2666      <organization abbrev="W3C">World Wide Web Consortium</organization>
2667      <address><email>ylafon@w3.org</email></address>
2668    </author>
2669    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2670      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2671      <address><email>julian.reschke@greenbytes.de</email></address>
2672    </author>
2673    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2674  </front>
2675  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2676  <x:source href="p5-range.xml" basename="p5-range"/>
2677</reference>
2678
2679<reference anchor="Part7">
2680  <front>
2681    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2682    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2683      <organization abbrev="Day Software">Day Software</organization>
2684      <address><email>fielding@gbiv.com</email></address>
2685    </author>
2686    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2687      <organization>One Laptop per Child</organization>
2688      <address><email>jg@laptop.org</email></address>
2689    </author>
2690    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2691      <organization abbrev="HP">Hewlett-Packard Company</organization>
2692      <address><email>JeffMogul@acm.org</email></address>
2693    </author>
2694    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2695      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2696      <address><email>henrikn@microsoft.com</email></address>
2697    </author>
2698    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2699      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2700      <address><email>LMM@acm.org</email></address>
2701    </author>
2702    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2703      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2704      <address><email>paulle@microsoft.com</email></address>
2705    </author>
2706    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2707      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2708      <address><email>timbl@w3.org</email></address>
2709    </author>
2710    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2711      <organization abbrev="W3C">World Wide Web Consortium</organization>
2712      <address><email>ylafon@w3.org</email></address>
2713    </author>
2714    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2715      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2716      <address><email>julian.reschke@greenbytes.de</email></address>
2717    </author>
2718    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2719  </front>
2720  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2721  <x:source href="p7-auth.xml" basename="p7-auth"/>
2722</reference>
2723
2724<reference anchor="RFC2047">
2725  <front>
2726    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2727    <author initials="K." surname="Moore" fullname="Keith Moore">
2728      <organization>University of Tennessee</organization>
2729      <address><email>moore@cs.utk.edu</email></address>
2730    </author>
2731    <date month="November" year="1996"/>
2732  </front>
2733  <seriesInfo name="RFC" value="2047"/>
2734</reference>
2735
2736<reference anchor="RFC2119">
2737  <front>
2738    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2739    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2740      <organization>Harvard University</organization>
2741      <address><email>sob@harvard.edu</email></address>
2742    </author>
2743    <date month="March" year="1997"/>
2744  </front>
2745  <seriesInfo name="BCP" value="14"/>
2746  <seriesInfo name="RFC" value="2119"/>
2747</reference>
2748
2749</references>
2750
2751<references title="Informative References">
2752
2753<reference anchor="RFC1305">
2754  <front>
2755    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2756    <author initials="D." surname="Mills" fullname="David L. Mills">
2757      <organization>University of Delaware, Electrical Engineering Department</organization>
2758      <address><email>mills@udel.edu</email></address>
2759    </author>
2760    <date month="March" year="1992"/>
2761  </front>
2762  <seriesInfo name="RFC" value="1305"/>
2763</reference>
2764
2765<reference anchor="RFC2616">
2766  <front>
2767    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2768    <author initials="R." surname="Fielding" fullname="R. Fielding">
2769      <organization>University of California, Irvine</organization>
2770      <address><email>fielding@ics.uci.edu</email></address>
2771    </author>
2772    <author initials="J." surname="Gettys" fullname="J. Gettys">
2773      <organization>W3C</organization>
2774      <address><email>jg@w3.org</email></address>
2775    </author>
2776    <author initials="J." surname="Mogul" fullname="J. Mogul">
2777      <organization>Compaq Computer Corporation</organization>
2778      <address><email>mogul@wrl.dec.com</email></address>
2779    </author>
2780    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2781      <organization>MIT Laboratory for Computer Science</organization>
2782      <address><email>frystyk@w3.org</email></address>
2783    </author>
2784    <author initials="L." surname="Masinter" fullname="L. Masinter">
2785      <organization>Xerox Corporation</organization>
2786      <address><email>masinter@parc.xerox.com</email></address>
2787    </author>
2788    <author initials="P." surname="Leach" fullname="P. Leach">
2789      <organization>Microsoft Corporation</organization>
2790      <address><email>paulle@microsoft.com</email></address>
2791    </author>
2792    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2793      <organization>W3C</organization>
2794      <address><email>timbl@w3.org</email></address>
2795    </author>
2796    <date month="June" year="1999"/>
2797  </front>
2798  <seriesInfo name="RFC" value="2616"/>
2799</reference>
2800
2801<reference anchor='RFC3864'>
2802  <front>
2803    <title>Registration Procedures for Message Header Fields</title>
2804    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
2805      <organization>Nine by Nine</organization>
2806      <address><email>GK-IETF@ninebynine.org</email></address>
2807    </author>
2808    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
2809      <organization>BEA Systems</organization>
2810      <address><email>mnot@pobox.com</email></address>
2811    </author>
2812    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
2813      <organization>HP Labs</organization>
2814      <address><email>JeffMogul@acm.org</email></address>
2815    </author>
2816    <date year='2004' month='September' />
2817  </front>
2818  <seriesInfo name='BCP' value='90' />
2819  <seriesInfo name='RFC' value='3864' />
2820</reference>
2821
2822</references>
2823
2824<section title="Compatibility with Previous Versions" anchor="compatibility">
2825
2826<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2827<t>
2828   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2829   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2830   <xref target="header.cache-control" format="counter"/>,
2831   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2832</t>
2833<t>
2834   Transfer-coding and message lengths all interact in ways that
2835   required fixing exactly when chunked encoding is used (to allow for
2836   transfer encoding that may not be self delimiting); it was important
2837   to straighten out exactly how message lengths are computed.
2838   (<xref target="non-modifiable.headers"/>,
2839   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2840</t>
2841<t>
2842   Proxies should be able to add Content-Length when appropriate.
2843   (<xref target="non-modifiable.headers"/>)
2844</t>
2845<t>
2846   Range request responses would become very verbose if all meta-data
2847   were always returned; by allowing the server to only send needed
2848   headers in a 206 response, this problem can be avoided.
2849   (<xref target="combining.headers"/>)
2850</t>
2851<t>
2852   The Cache-Control: max-age directive was not properly defined for
2853   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2854</t>
2855<t>
2856   Warnings could be cached incorrectly, or not updated appropriately.
2857   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2858   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2859   and <xref target="header.warning" format="counter"/>) Warning
2860   also needed to be a general header, as PUT or other methods may have
2861   need for it in requests.
2862</t>
2863</section>
2864
2865<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2866<t>
2867  Clarify denial of service attack avoidance requirement.
2868  (<xref target="invalidation.after.updates.or.deletions"/>)
2869</t>
2870</section>
2871
2872</section>
2873
2874<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
2875
2876<section title="Since RFC2616">
2877<t>
2878  Extracted relevant partitions from <xref target="RFC2616"/>.
2879</t>
2880</section>
2881
2882<section title="Since draft-ietf-httpbis-p6-cache-00">
2883<t>
2884  Closed issues:
2885  <list style="symbols"> 
2886    <t>
2887      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
2888      "Trailer"
2889      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
2890    </t>
2891    <t>
2892      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
2893      "Invalidation after Update or Delete"
2894      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
2895    </t>
2896    <t>
2897      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2898      "Normative and Informative references"
2899    </t>
2900    <t>
2901      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
2902      "Date reference typo"
2903    </t>
2904    <t>
2905      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
2906      "Connection header text"
2907    </t>
2908    <t>
2909      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2910      "Informative references"
2911    </t>
2912    <t>
2913      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2914      "ISO-8859-1 Reference"
2915    </t>
2916    <t>
2917      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2918      "Normative up-to-date references"
2919    </t>
2920    <t>
2921      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
2922      "typo in 13.2.2"
2923    </t>
2924  </list>
2925</t>
2926<t>
2927  Other changes:
2928  <list style="symbols"> 
2929    <t>
2930      Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
2931    </t>
2932  </list>
2933</t>
2934</section>
2935
2936<section title="Since draft-ietf-httpbis-p6-cache-01">
2937<t>
2938  Closed issues:
2939  <list style="symbols"> 
2940    <t>
2941      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
2942      "rel_path not used"
2943    </t>
2944  </list>
2945</t>
2946<t>
2947  Other changes:
2948  <list style="symbols"> 
2949    <t>
2950       Get rid of duplicate BNF rule names ("host" -> "uri-host")
2951       (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
2952    </t>
2953    <t>
2954      Add explicit references to BNF syntax and rules imported from other parts of the specification.
2955    </t>
2956  </list>
2957</t>
2958</section>
2959
2960<section title="Since draft-ietf-httpbis-p6-cache-02" anchor="changes.since.02">
2961<t>
2962  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2963  <list style="symbols"> 
2964    <t>
2965      Reference RFC 3984, and update header registrations for headers defined
2966      in this document.
2967    </t>
2968  </list>
2969</t>
2970</section>
2971
2972<section title="Since draft-ietf-httpbis-p6-cache-03" anchor="changes.since.03">
2973<t>
2974  Closed issues:
2975  <list style="symbols"> 
2976    <t>
2977      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>:
2978      "Vary header classification"
2979    </t>
2980  </list>
2981</t>
2982</section>
2983
2984<section title="Since draft-ietf-httpbis-p6-cache-04" anchor="changes.since.04">
2985<t>
2986  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2987  <list style="symbols"> 
2988    <t>
2989      Use "/" instead of "|" for alternatives.
2990    </t>
2991  </list>
2992</t>
2993</section>
2994
2995</section>
2996
2997</back>
2998</rfc>
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