source: draft-ietf-httpbis/latest/p6-cache.xml @ 460

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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 "March">
16  <!ENTITY ID-YEAR "2009">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY notation-abnf               "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' 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  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34]>
35<?rfc toc="yes" ?>
36<?rfc symrefs="yes" ?>
37<?rfc sortrefs="yes" ?>
38<?rfc compact="yes"?>
39<?rfc subcompact="no" ?>
40<?rfc linkmailto="no" ?>
41<?rfc editing="no" ?>
42<?rfc comments="yes"?>
43<?rfc inline="yes"?>
44<?rfc-ext allow-markup-in-artwork="yes" ?>
45<?rfc-ext include-references-in-index="yes" ?>
46<rfc obsoletes="2616" category="std" x:maturity-level="draft"
47     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
48     xmlns:x='http://purl.org/net/xml2rfc/ext'>
49<front>
50
51  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
52
53  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
54    <organization abbrev="Day Software">Day Software</organization>
55    <address>
56      <postal>
57        <street>23 Corporate Plaza DR, Suite 280</street>
58        <city>Newport Beach</city>
59        <region>CA</region>
60        <code>92660</code>
61        <country>USA</country>
62      </postal>
63      <phone>+1-949-706-5300</phone>
64      <facsimile>+1-949-706-5305</facsimile>
65      <email>fielding@gbiv.com</email>
66      <uri>http://roy.gbiv.com/</uri>
67    </address>
68  </author>
69
70  <author initials="J." surname="Gettys" fullname="Jim Gettys">
71    <organization>One Laptop per Child</organization>
72    <address>
73      <postal>
74        <street>21 Oak Knoll Road</street>
75        <city>Carlisle</city>
76        <region>MA</region>
77        <code>01741</code>
78        <country>USA</country>
79      </postal>
80      <email>jg@laptop.org</email>
81      <uri>http://www.laptop.org/</uri>
82    </address>
83  </author>
84 
85  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
86    <organization abbrev="HP">Hewlett-Packard Company</organization>
87    <address>
88      <postal>
89        <street>HP Labs, Large Scale Systems Group</street>
90        <street>1501 Page Mill Road, MS 1177</street>
91        <city>Palo Alto</city>
92        <region>CA</region>
93        <code>94304</code>
94        <country>USA</country>
95      </postal>
96      <email>JeffMogul@acm.org</email>
97    </address>
98  </author>
99
100  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108        <country>USA</country>
109      </postal>
110      <email>henrikn@microsoft.com</email>
111    </address>
112  </author>
113
114  <author initials="L." surname="Masinter" fullname="Larry Masinter">
115    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
116    <address>
117      <postal>
118        <street>345 Park Ave</street>
119        <city>San Jose</city>
120        <region>CA</region>
121        <code>95110</code>
122        <country>USA</country>
123      </postal>
124      <email>LMM@acm.org</email>
125      <uri>http://larry.masinter.net/</uri>
126    </address>
127  </author>
128 
129  <author initials="P." surname="Leach" fullname="Paul J. Leach">
130    <organization abbrev="Microsoft">Microsoft Corporation</organization>
131    <address>
132      <postal>
133        <street>1 Microsoft Way</street>
134        <city>Redmond</city>
135        <region>WA</region>
136        <code>98052</code>
137      </postal>
138      <email>paulle@microsoft.com</email>
139    </address>
140  </author>
141   
142  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
143    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
144    <address>
145      <postal>
146        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
147        <street>The Stata Center, Building 32</street>
148        <street>32 Vassar Street</street>
149        <city>Cambridge</city>
150        <region>MA</region>
151        <code>02139</code>
152        <country>USA</country>
153      </postal>
154      <email>timbl@w3.org</email>
155      <uri>http://www.w3.org/People/Berners-Lee/</uri>
156    </address>
157  </author>
158
159  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
160    <organization abbrev="W3C">World Wide Web Consortium</organization>
161    <address>
162      <postal>
163        <street>W3C / ERCIM</street>
164        <street>2004, rte des Lucioles</street>
165        <city>Sophia-Antipolis</city>
166        <region>AM</region>
167        <code>06902</code>
168        <country>France</country>
169      </postal>
170      <email>ylafon@w3.org</email>
171      <uri>http://www.raubacapeu.net/people/yves/</uri>
172    </address>
173  </author>
174
175  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
176    <organization abbrev="greenbytes">greenbytes GmbH</organization>
177    <address>
178      <postal>
179        <street>Hafenweg 16</street>
180        <city>Muenster</city><region>NW</region><code>48155</code>
181        <country>Germany</country>
182      </postal>
183      <phone>+49 251 2807760</phone>   
184      <facsimile>+49 251 2807761</facsimile>   
185      <email>julian.reschke@greenbytes.de</email>       
186      <uri>http://greenbytes.de/tech/webdav/</uri>     
187    </address>
188  </author>
189
190  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
191  <workgroup>HTTPbis Working Group</workgroup>
192
193<abstract>
194<t>
195   The Hypertext Transfer Protocol (HTTP) is an application-level
196   protocol for distributed, collaborative, hypermedia information
197   systems. HTTP has been in use by the World Wide Web global information
198   initiative since 1990. This document is Part 6 of the seven-part specification
199   that defines the protocol referred to as "HTTP/1.1" and, taken together,
200   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
201   and the associated header fields that control cache behavior or indicate
202   cacheable response messages.
203</t>
204</abstract>
205
206<note title="Editorial Note (To be removed by RFC Editor)">
207  <t>
208    Discussion of this draft should take place on the HTTPBIS working group
209    mailing list (ietf-http-wg@w3.org). The current issues list is
210    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
211    and related documents (including fancy diffs) can be found at
212    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
213  </t>
214  <t>
215    The changes in this draft are summarized in <xref target="changes.since.05"/>.
216  </t>
217</note>
218</front>
219<middle>
220<section title="Introduction" anchor="caching">
221<t>
222   HTTP is typically used for distributed information systems, where
223   performance can be improved by the use of response caches, and includes
224   a number of elements intended to make caching work as well as possible.
225   Because these elements interact with each other, it is useful to describe
226   the caching design of HTTP separately.  This document defines aspects of
227   HTTP/1.1 related to caching and reusing response messages.
228</t>
229
230<section title="Purpose" anchor="intro.purpose">
231<iref item="cache"/>
232<t>
233   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
234   and the subsystem that controls its message storage, retrieval, and
235   deletion. A cache stores cacheable responses in order to reduce the
236   response time and network bandwidth consumption on future, equivalent
237   requests. Any client or server may include a cache, though a cache
238   cannot be used by a server that is acting as a tunnel.
239</t>
240<t>
241   Caching would be useless if it did not significantly improve
242   performance. The goal of caching in HTTP/1.1 is to reuse a prior response
243   message to satisfy a current request.  In some cases, the existing response
244   can be reused without the need for a network request, reducing latency and
245   network round-trips; we use an "expiration" mechanism for this purpose
246   (see <xref target="expiration.model"/>).  Even when a new request is required,
247   it is often possible to reuse all or parts of the payload of a prior response
248   to satisfy the request, thereby reducing network bandwidth usage; we use a
249   "validation" mechanism for this purpose (see <xref target="validation.model"/>).
250</t>
251<iref item="semantically transparent"/>
252<t>
253   A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
254   respect to a particular response, when its use affects neither the
255   requesting client nor the origin server, except to improve
256   performance. When a cache is semantically transparent, the client
257   receives exactly the same response status and payload
258   that it would have received had its request been handled directly
259   by the origin server.
260</t>
261<t>
262   In an ideal world, all interactions with an HTTP cache would be
263   semantically transparent.  However, for some resources, semantic
264   transparency is not always necessary and can be effectively traded
265   for the sake of bandwidth scaling, disconnected operation, and
266   high availability.  HTTP/1.1 allows origin servers, caches,
267   and clients to explicitly reduce transparency when necessary.
268   However, because non-transparent operation may confuse non-expert
269   users and might be incompatible with certain server applications
270   (such as those for ordering merchandise), the protocol requires that
271   transparency be relaxed
272  <list style="symbols">
273     <t>only by an explicit protocol-level request when relaxed by
274        client or origin server</t>
275
276     <t>only with an explicit warning to the end user when relaxed by
277        cache or client</t>
278  </list>
279</t>
280<t>
281   Therefore, HTTP/1.1 provides these important elements:
282  <list style="numbers">
283      <t>Protocol features that provide full semantic transparency when
284         this is required by all parties.</t>
285
286      <t>Protocol features that allow an origin server or user agent to
287         explicitly request and control non-transparent operation.</t>
288
289      <t>Protocol features that allow a cache to attach warnings to
290         responses that do not preserve the requested approximation of
291         semantic transparency.</t>
292  </list>
293</t>
294<t>
295   A basic principle is that it must be possible for the clients to
296   detect any potential relaxation of semantic transparency.
297  <list><t>
298      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
299      design decisions not explicitly discussed in this specification.
300      If a decision might affect semantic transparency, the implementor
301      ought to err on the side of maintaining transparency unless a
302      careful and complete analysis shows significant benefits in
303      breaking transparency.
304    </t></list>
305</t>
306</section>
307
308<section title="Terminology" anchor="intro.terminology">
309<t>
310   This specification uses a number of terms to refer to the roles
311   played by participants in, and objects of, HTTP caching.
312</t>
313<t>
314  <iref item="cacheable"/>
315  <x:dfn>cacheable</x:dfn>
316  <list>
317    <t>
318      A response is cacheable if a cache is allowed to store a copy of
319      the response message for use in answering subsequent requests.
320      Even when a response is cacheable, there may
321      be additional constraints on whether a cache can use the cached
322      copy for a particular request.
323    </t>
324  </list>
325</t>
326<t>
327  <iref item="first-hand"/>
328  <x:dfn>first-hand</x:dfn>
329  <list>
330    <t>
331      A response is first-hand if it comes directly and without
332      unnecessary delay from the origin server, perhaps via one or more
333      proxies. A response is also first-hand if its validity has just
334      been checked directly with the origin server.
335    </t>
336  </list>
337</t>
338<t>
339  <iref item="explicit expiration time"/>
340  <x:dfn>explicit expiration time</x:dfn>
341  <list>
342    <t>
343      The time at which the origin server intends that an entity should
344      no longer be returned by a cache without further validation.
345    </t>
346  </list>
347</t>
348<t>
349  <iref item="heuristic expiration time"/>
350  <x:dfn>heuristic expiration time</x:dfn>
351  <list>
352    <t>
353      An expiration time assigned by a cache when no explicit expiration
354      time is available.
355    </t>
356  </list>
357</t>
358<t>
359  <iref item="age"/>
360  <x:dfn>age</x:dfn>
361  <list>
362    <t>
363      The age of a response is the time since it was sent by, or
364      successfully validated with, the origin server.
365    </t>
366  </list>
367</t>
368<t>
369  <iref item="freshness lifetime"/>
370  <x:dfn>freshness lifetime</x:dfn>
371  <list>
372    <t>
373      The length of time between the generation of a response and its
374      expiration time.
375    </t>
376  </list>
377</t>
378<t>
379  <iref item="fresh"/>
380  <x:dfn>fresh</x:dfn>
381  <list>
382    <t>
383      A response is fresh if its age has not yet exceeded its freshness
384      lifetime.
385    </t>
386  </list>
387</t>
388<t>
389  <iref item="stale"/>
390  <x:dfn>stale</x:dfn>
391  <list>
392    <t>
393      A response is stale if its age has passed its freshness lifetime.
394    </t>
395  </list>
396</t>
397<t>
398  <iref item="validator"/>
399  <x:dfn>validator</x:dfn>
400  <list>
401    <t>
402      A protocol element (e.g., an entity tag or a Last-Modified time)
403      that is used to find out whether a cache entry is an equivalent
404      copy of an entity.
405    </t>
406  </list>
407</t>
408</section>
409
410<section title="Requirements" anchor="intro.requirements">
411<t>
412   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
413   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
414   document are to be interpreted as described in <xref target="RFC2119"/>.
415</t>
416<t>
417   An implementation is not compliant if it fails to satisfy one or more
418   of the &MUST; or &REQUIRED; level requirements for the protocols it
419   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
420   level and all the &SHOULD; level requirements for its protocols is said
421   to be "unconditionally compliant"; one that satisfies all the &MUST;
422   level requirements but not all the &SHOULD; level requirements for its
423   protocols is said to be "conditionally compliant."
424</t>
425</section>
426
427<section title="Syntax Notation" anchor="notation">
428  <x:anchor-alias value="ALPHA"/>
429  <x:anchor-alias value="CR"/>
430  <x:anchor-alias value="DIGIT"/>
431  <x:anchor-alias value="DQUOTE"/>
432  <x:anchor-alias value="LF"/>
433  <x:anchor-alias value="OCTET"/>
434  <x:anchor-alias value="SP"/>
435  <x:anchor-alias value="VCHAR"/>
436  <x:anchor-alias value="WSP"/>
437<t>
438  This specification uses the ABNF syntax defined in &notation;.
439  The following core rules are included by
440  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
441  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
442  DIGIT (decimal 0-9), DQUOTE (double quote),
443  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
444  OCTET (any 8-bit sequence of data), SP (space),
445  VCHAR (any visible USASCII character),
446  and WSP (whitespace).
447</t>
448
449<section title="Core Rules" anchor="core.rules">
450  <x:anchor-alias value="quoted-string"/>
451  <x:anchor-alias value="token"/>
452  <x:anchor-alias value="OWS"/>
453<t>
454  The core rules below are defined in &basic-rules;:
455</t>
456<figure><artwork type="abnf2616">
457  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
458  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
459  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
460</artwork></figure>
461</section>
462
463<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
464  <x:anchor-alias value="field-name"/>
465  <x:anchor-alias value="HTTP-date"/>
466  <x:anchor-alias value="port"/>
467  <x:anchor-alias value="pseudonym"/>
468  <x:anchor-alias value="uri-host"/>
469<t>
470  The ABNF rules below are defined in other parts:
471</t>
472<figure><!--Part1--><artwork type="abnf2616">
473  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
474  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
475  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
476  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
477  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
478</artwork></figure>
479</section>
480
481</section>
482
483</section>
484
485
486<section title="Overview" anchor="caching.overview">
487<section title="Cache Correctness" anchor="cache.correctness">
488<t>
489   A correct cache &MUST; respond to a request with the most up-to-date
490   response held by the cache that is appropriate to the request (see
491   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
492   <xref target="disambiguating.multiple.responses" format="counter"/>,
493   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
494   conditions:
495  <list style="numbers">
496      <t>It has been checked for equivalence with what the origin server
497         would have returned by revalidating the response with the
498         origin server (<xref target="validation.model"/>);</t>
499
500      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
501         this means it meets the least restrictive freshness requirement
502         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
503         the origin server so specifies, it is the freshness requirement
504         of the origin server alone.
505
506         If a stored response is not "fresh enough" by the most
507         restrictive freshness requirement of both the client and the
508         origin server, in carefully considered circumstances the cache
509         &MAY; still return the response with the appropriate Warning
510         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
511         and <xref target="header.warning" format="counter"/>), unless such a response
512         is prohibited (e.g., by a "no-store" cache-directive, or by a
513         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
514
515      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
516         or error (4xx or 5xx) response message.</t>
517  </list>
518</t>
519<t>
520   If the cache can not communicate with the origin server, then a
521   correct cache &SHOULD; respond as above if the response can be
522   correctly served from the cache; if not it &MUST; return an error or
523   warning indicating that there was a communication failure.
524</t>
525<t>
526   If a cache receives a response (either an entire response, or a 304
527   (Not Modified) response) that it would normally forward to the
528   requesting client, and the received response is no longer fresh, the
529   cache &SHOULD; forward it to the requesting client without adding a new
530   Warning (but without removing any existing Warning headers). A cache
531   &SHOULD-NOT;  attempt to revalidate a response simply because that
532   response became stale in transit; this might lead to an infinite
533   loop. A user agent that receives a stale response without a Warning
534   &MAY; display a warning indication to the user.
535</t>
536</section>
537
538<section title="Warnings" anchor="warnings">
539<t>
540   Whenever a cache returns a response that is neither first-hand nor
541   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
542   &MUST; attach a warning to that effect, using a Warning general-header.
543   The Warning header and the currently defined warnings are described
544   in <xref target="header.warning"/>. The warning allows clients to take appropriate
545   action.
546</t>
547<t>
548   Warnings &MAY; be used for other purposes, both cache-related and
549   otherwise. The use of a warning, rather than an error status code,
550   distinguish these responses from true failures.
551</t>
552<t>
553   Warnings are assigned three digit warn-codes. The first digit
554   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
555   stored cache entry after a successful revalidation:
556</t>
557<t>
558  <list style="hanging">
559    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
560     the response, and so &MUST; be deleted after a successful
561     revalidation. 1xx warn-codes &MAY; be generated by a cache only when
562     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
563
564    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
565     headers that is not rectified by a revalidation (for example, a
566     lossy compression of the entity bodies) and which &MUST-NOT; be
567     deleted after a successful revalidation.</t>
568    </list>
569</t>
570<t>
571   See <xref target="header.warning"/> for the definitions of the codes themselves.
572</t>
573<t>
574   HTTP/1.0 caches will cache all Warnings in responses, without
575   deleting the ones in the first category. Warnings in responses that
576   are passed to HTTP/1.0 caches carry an extra warning-date field,
577   which prevents a future HTTP/1.1 recipient from believing an
578   erroneously cached Warning.
579</t>
580<t>
581   Warnings also carry a warning text. The text &MAY; be in any
582   appropriate natural language (perhaps based on the client's Accept
583   headers), and include an &OPTIONAL; indication of what character set is
584   used.
585</t>
586<t>
587   Multiple warnings &MAY; be attached to a response (either by the origin
588   server or by a cache), including multiple warnings with the same code
589   number. For example, a server might provide the same warning with
590   texts in both English and Basque.
591</t>
592<t>
593   When multiple warnings are attached to a response, it might not be
594   practical or reasonable to display all of them to the user. This
595   version of HTTP does not specify strict priority rules for deciding
596   which warnings to display and in what order, but does suggest some
597   heuristics.
598</t>
599</section>
600
601<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
602<t>
603   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
604   times and validators) are implicit directives to caches. In some
605   cases, a server or client might need to provide explicit directives
606   to the HTTP caches. We use the Cache-Control header for this purpose.
607</t>
608<t>
609   The Cache-Control header allows a client or server to transmit a
610   variety of directives in either requests or responses. These
611   directives typically override the default caching algorithms. As a
612   general rule, if there is any apparent conflict between header
613   values, the most restrictive interpretation is applied (that is, the
614   one that is most likely to preserve semantic transparency). However,
615   in some cases, cache-control directives are explicitly specified as
616   weakening the approximation of semantic transparency (for example,
617   "max-stale" or "public").
618</t>
619<t>
620   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
621</t>
622</section>
623
624<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
625<t>
626   Many user agents make it possible for users to override the basic
627   caching mechanisms. For example, the user agent might allow the user
628   to specify that cached entities (even explicitly stale ones) are
629   never validated. Or the user agent might habitually add "Cache-Control:
630   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
631   default to either non-transparent behavior, or behavior that results
632   in abnormally ineffective caching, but &MAY; be explicitly configured
633   to do so by an explicit action of the user.
634</t>
635<t>
636   If the user has overridden the basic caching mechanisms, the user
637   agent &SHOULD; explicitly indicate to the user whenever this results in
638   the display of information that might not meet the server's
639   transparency requirements (in particular, if the displayed entity is
640   known to be stale). Since the protocol normally allows the user agent
641   to determine if responses are stale or not, this indication need only
642   be displayed when this actually happens. The indication need not be a
643   dialog box; it could be an icon (for example, a picture of a rotting
644   fish) or some other indicator.
645</t>
646<t>
647   If the user has overridden the caching mechanisms in a way that would
648   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
649   continually indicate this state to the user (for example, by a
650   display of a picture of currency in flames) so that the user does not
651   inadvertently consume excess resources or suffer from excessive
652   latency.
653</t>
654</section>
655
656<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
657<t>
658   In some cases, the operator of a cache &MAY; choose to configure it to
659   return stale responses even when not requested by clients. This
660   decision ought not be made lightly, but may be necessary for reasons
661   of availability or performance, especially when the cache is poorly
662   connected to the origin server. Whenever a cache returns a stale
663   response, it &MUST; mark it as such (using a Warning header) enabling
664   the client software to alert the user that there might be a potential
665   problem.
666</t>
667<t>
668   It also allows the user agent to take steps to obtain a first-hand or
669   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
670   response if the client explicitly requests a first-hand or fresh one,
671   unless it is impossible to comply for technical or policy reasons.
672</t>
673</section>
674
675<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
676<t>
677   While the origin server (and to a lesser extent, intermediate caches,
678   by their contribution to the age of a response) are the primary
679   source of expiration information, in some cases the client might need
680   to control a cache's decision about whether to return a cached
681   response without validating it. Clients do this using several
682   directives of the Cache-Control header.
683</t>
684<t>
685   A client's request &MAY; specify the maximum age it is willing to
686   accept of an unvalidated response; specifying a value of zero forces
687   the cache(s) to revalidate all responses. A client &MAY; also specify
688   the minimum time remaining before a response expires. Both of these
689   options increase constraints on the behavior of caches, and so cannot
690   further relax the cache's approximation of semantic transparency.
691</t>
692<t>
693   A client &MAY; also specify that it will accept stale responses, up to
694   some maximum amount of staleness. This loosens the constraints on the
695   caches, and so might violate the origin server's specified
696   constraints on semantic transparency, but might be necessary to
697   support disconnected operation, or high availability in the face of
698   poor connectivity.
699</t>
700</section>
701</section>
702
703<section title="Expiration Model" anchor="expiration.model">
704
705<section title="Server-Specified Expiration" anchor="server-specified.expiration">
706<t>
707   HTTP caching works best when caches can entirely avoid making
708   requests to the origin server. The primary mechanism for avoiding
709   requests is for an origin server to provide an explicit expiration
710   time in the future, indicating that a response &MAY; be used to satisfy
711   subsequent requests. In other words, a cache can return a fresh
712   response without first contacting the server.
713</t>
714<t>
715   Our expectation is that servers will assign future explicit
716   expiration times to responses in the belief that the entity is not
717   likely to change, in a semantically significant way, before the
718   expiration time is reached. This normally preserves semantic
719   transparency, as long as the server's expiration times are carefully
720   chosen.
721</t>
722<t>
723   The expiration mechanism applies only to responses taken from a cache
724   and not to first-hand responses forwarded immediately to the
725   requesting client.
726</t>
727<t>
728   If an origin server wishes to force a semantically transparent cache
729   to validate every request, it &MAY; assign an explicit expiration time
730   in the past. This means that the response is always stale, and so the
731   cache &SHOULD; validate it before using it for subsequent requests. See
732   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
733</t>
734<t>
735   If an origin server wishes to force any HTTP/1.1 cache, no matter how
736   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
737   cache-control directive (see <xref target="header.cache-control"/>).
738</t>
739<t>
740   Servers specify explicit expiration times using either the Expires
741   header, or the max-age directive of the Cache-Control header.
742</t>
743<t>
744   An expiration time cannot be used to force a user agent to refresh
745   its display or reload a resource; its semantics apply only to caching
746   mechanisms, and such mechanisms need only check a resource's
747   expiration status when a new request for that resource is initiated.
748   See <xref target="history.lists"/> for an explanation of the difference between caches
749   and history mechanisms.
750</t>
751</section>
752
753<section title="Heuristic Expiration" anchor="heuristic.expiration">
754<t>
755   Since origin servers do not always provide explicit expiration times,
756   HTTP caches typically assign heuristic expiration times, employing
757   algorithms that use other header values (such as the Last-Modified
758   time) to estimate a plausible expiration time. The HTTP/1.1
759   specification does not provide specific algorithms, but does impose
760   worst-case constraints on their results. Since heuristic expiration
761   times might compromise semantic transparency, they ought to be used
762   cautiously, and we encourage origin servers to provide explicit
763   expiration times as much as possible.
764</t>
765</section>
766
767<section title="Age Calculations" anchor="age.calculations">
768<t>
769   In order to know if a cached entry is fresh, a cache needs to know if
770   its age exceeds its freshness lifetime. We discuss how to calculate
771   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
772   the age of a response or cache entry.
773</t>
774<t>
775   In this discussion, we use the term "now" to mean "the current value
776   of the clock at the host performing the calculation." Hosts that use
777   HTTP, but especially hosts running origin servers and caches, &SHOULD;
778   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
779   a globally accurate time standard.
780</t>
781<t>
782   HTTP/1.1 requires origin servers to send a Date header, if possible,
783   with every response, giving the time at which the response was
784   generated (see &header-date;). We use the term "date_value" to denote
785   the value of the Date header, in a form appropriate for arithmetic
786   operations.
787</t>
788<t>
789   HTTP/1.1 uses the Age response-header to convey the estimated age of
790   the response message when obtained from a cache. The Age field value
791   is the cache's estimate of the amount of time since the response was
792   generated or revalidated by the origin server.
793</t>
794<t>
795   In essence, the Age value is the sum of the time that the response
796   has been resident in each of the caches along the path from the
797   origin server, plus the amount of time it has been in transit along
798   network paths.
799</t>
800<t>
801   We use the term "age_value" to denote the value of the Age header, in
802   a form appropriate for arithmetic operations.
803</t>
804<t>
805   A response's age can be calculated in two entirely independent ways:
806  <list style="numbers">
807      <t>now minus date_value, if the local clock is reasonably well
808         synchronized to the origin server's clock. If the result is
809         negative, the result is replaced by zero.</t>
810
811      <t>age_value, if all of the caches along the response path
812         implement HTTP/1.1.</t>
813  </list>
814</t>
815<t>
816   Given that we have two independent ways to compute the age of a
817   response when it is received, we can combine these as
818</t>
819<figure><artwork type="code">
820    corrected_received_age = max(now - date_value, age_value)
821</artwork></figure>
822<t>
823   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
824   paths, one gets a reliable (conservative) result.
825</t>
826<t>
827   Because of network-imposed delays, some significant interval might
828   pass between the time that a server generates a response and the time
829   it is received at the next outbound cache or client. If uncorrected,
830   this delay could result in improperly low ages.
831</t>
832<t>
833   Because the request that resulted in the returned Age value must have
834   been initiated prior to that Age value's generation, we can correct
835   for delays imposed by the network by recording the time at which the
836   request was initiated. Then, when an Age value is received, it &MUST;
837   be interpreted relative to the time the request was initiated, not
838   the time that the response was received. This algorithm results in
839   conservative behavior no matter how much delay is experienced. So, we
840   compute:
841</t>
842<figure><artwork type="code">
843   corrected_initial_age = corrected_received_age
844                         + (now - request_time)
845</artwork></figure>
846<t>
847   where "request_time" is the time (according to the local clock) when
848   the request that elicited this response was sent.
849</t>
850<t>
851   Summary of age calculation algorithm, when a cache receives a
852   response:
853</t>
854<figure><artwork type="code">
855   /*
856    * age_value
857    *      is the value of Age: header received by the cache with
858    *              this response.
859    * date_value
860    *      is the value of the origin server's Date: header
861    * request_time
862    *      is the (local) time when the cache made the request
863    *              that resulted in this cached response
864    * response_time
865    *      is the (local) time when the cache received the
866    *              response
867    * now
868    *      is the current (local) time
869    */
870
871   apparent_age = max(0, response_time - date_value);
872   corrected_received_age = max(apparent_age, age_value);
873   response_delay = response_time - request_time;
874   corrected_initial_age = corrected_received_age + response_delay;
875   resident_time = now - response_time;
876   current_age   = corrected_initial_age + resident_time;
877</artwork></figure>
878<t>
879   The current_age of a cache entry is calculated by adding the amount
880   of time (in seconds) since the cache entry was last validated by the
881   origin server to the corrected_initial_age. When a response is
882   generated from a cache entry, the cache &MUST; include a single Age
883   header field in the response with a value equal to the cache entry's
884   current_age.
885</t>
886<t>
887   The presence of an Age header field in a response implies that a
888   response is not first-hand. However, the converse is not true, since
889   the lack of an Age header field in a response does not imply that the
890   response is first-hand unless all caches along the request path are
891   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
892   the Age header field).
893</t>
894</section>
895
896<section title="Expiration Calculations" anchor="expiration.calculations">
897<t>
898   In order to decide whether a response is fresh or stale, we need to
899   compare its freshness lifetime to its age. The age is calculated as
900   described in <xref target="age.calculations"/>; this section describes how to calculate
901   the freshness lifetime, and to determine if a response has expired.
902   In the discussion below, the values can be represented in any form
903   appropriate for arithmetic operations.
904</t>
905<t>
906   We use the term "expires_value" to denote the value of the Expires
907   header. We use the term "max_age_value" to denote an appropriate
908   value of the number of seconds carried by the "max-age" directive of
909   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
910</t>
911<t>
912   The max-age directive takes priority over Expires, so if max-age is
913   present in a response, the calculation is simply:
914</t>
915<figure><artwork type="code">
916   freshness_lifetime = max_age_value
917</artwork></figure>
918<t>
919   Otherwise, if Expires is present in the response, the calculation is:
920</t>
921<figure><artwork type="code">
922   freshness_lifetime = expires_value - date_value
923</artwork></figure>
924<t>
925   Note that neither of these calculations is vulnerable to clock skew,
926   since all of the information comes from the origin server.
927</t>
928<t>
929   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
930   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
931   does not include other restrictions on caching, the cache &MAY; compute
932   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
933   113 to any response whose age is more than 24 hours if such warning
934   has not already been added.
935</t>
936<t>
937   Also, if the response does have a Last-Modified time, the heuristic
938   expiration value &SHOULD; be no more than some fraction of the interval
939   since that time. A typical setting of this fraction might be 10%.
940</t>
941<t>
942   The calculation to determine if a response has expired is quite
943   simple:
944</t>
945<figure><artwork type="code">
946   response_is_fresh = (freshness_lifetime &gt; current_age)
947</artwork></figure>
948</section>
949
950<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
951<t>
952   Because expiration values are assigned optimistically, it is possible
953   for two caches to contain fresh values for the same resource that are
954   different.
955</t>
956<t>
957   If a client performing a retrieval receives a non-first-hand response
958   for a request that was already fresh in its own cache, and the Date
959   header in its existing cache entry is newer than the Date on the new
960   response, then the client &MAY; ignore the response. If so, it &MAY;
961   retry the request with a "Cache-Control: max-age=0" directive (see
962   <xref target="header.cache-control"/>), to force a check with the origin server.
963</t>
964<t>
965   If a cache has two fresh responses for the same representation with
966   different validators, it &MUST; use the one with the more recent Date
967   header. This situation might arise because the cache is pooling
968   responses from other caches, or because a client has asked for a
969   reload or a revalidation of an apparently fresh cache entry.
970</t>
971</section>
972
973<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
974<t>
975   Because a client might be receiving responses via multiple paths, so
976   that some responses flow through one set of caches and other
977   responses flow through a different set of caches, a client might
978   receive responses in an order different from that in which the origin
979   server sent them. We would like the client to use the most recently
980   generated response, even if older responses are still apparently
981   fresh.
982</t>
983<t>
984   Neither the entity tag nor the expiration value can impose an
985   ordering on responses, since it is possible that a later response
986   intentionally carries an earlier expiration time. The Date values are
987   ordered to a granularity of one second.
988</t>
989<t>
990   When a client tries to revalidate a cache entry, and the response it
991   receives contains a Date header that appears to be older than the one
992   for the existing entry, then the client &SHOULD; repeat the request
993   unconditionally, and include
994</t>
995<figure><artwork type="example">
996  Cache-Control: max-age=0
997</artwork></figure>
998<t>
999   to force any intermediate caches to validate their copies directly
1000   with the origin server, or
1001</t>
1002<figure><artwork type="example">
1003  Cache-Control: no-cache
1004</artwork></figure>
1005<t>
1006   to force any intermediate caches to obtain a new copy from the origin
1007   server.
1008</t>
1009<t>
1010   If the Date values are equal, then the client &MAY; use either response
1011   (or &MAY;, if it is being extremely prudent, request a new response).
1012   Servers &MUST-NOT; depend on clients being able to choose
1013   deterministically between responses generated during the same second,
1014   if their expiration times overlap.
1015</t>
1016</section>
1017</section>
1018
1019<section title="Validation Model" anchor="validation.model">
1020<t>
1021   When a cache has a stale entry that it would like to use as a
1022   response to a client's request, it first has to check with the origin
1023   server (or possibly an intermediate cache with a fresh response) to
1024   see if its cached entry is still usable. We call this "validating"
1025   the cache entry.
1026</t>
1027<t>
1028   HTTP's conditional request mechanism, defined in &conditional;, is
1029   used to avoid retransmitting the response payload when the cached entry
1030   is valid.  When a cached response includes one or more "cache validators,"
1031   such as the field values of an ETag or Last-Modified header field, then
1032   a validating GET request &SHOULD; be made conditional to those field values.
1033   The server checks the conditional request's validator against the current
1034   state of the requested resource and, if they match, the server responds
1035   with a 304 (Not Modified) status code to indicate that the cached response
1036   can be refreshed and reused without retransmitting the response payload.
1037   If the validator does not match the current state of the requested
1038   resource, then the server returns a full response, including payload,
1039   so that the request can be satisfied and the cache entry supplanted
1040   without the need for an additional network round-trip.
1041</t>
1042</section>
1043
1044<section title="Response Cacheability" anchor="response.cacheability">
1045<t>
1046   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1047   directive, a caching system &MAY; always store a successful response
1048   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1049   if it is fresh, and &MAY; return it after successful validation. If
1050   there is neither a cache validator nor an explicit expiration time
1051   associated with a response, we do not expect it to be cached, but
1052   certain caches &MAY; violate this expectation (for example, when little
1053   or no network connectivity is available). A client can usually detect
1054   that such a response was taken from a cache by comparing the Date
1055   header to the current time.
1056  <list><t>
1057      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1058      without providing any Warning.
1059  </t></list>
1060</t>
1061<t>
1062   However, in some cases it might be inappropriate for a cache to
1063   retain an entity, or to return it in response to a subsequent
1064   request. This might be because absolute semantic transparency is
1065   deemed necessary by the service author, or because of security or
1066   privacy considerations. Certain cache-control directives are
1067   therefore provided so that the server can indicate that certain
1068   resource entities, or portions thereof, are not to be cached
1069   regardless of other considerations.
1070</t>
1071<t>
1072   Note that &header-authorization; normally prevents a shared cache from saving
1073   and returning a response to a previous request if that request
1074   included an Authorization header.
1075</t>
1076<t>
1077   A response received with a status code of 200, 203, 206, 300, 301 or
1078   410 &MAY; be stored by a cache and used in reply to a subsequent
1079   request, subject to the expiration mechanism, unless a cache-control
1080   directive prohibits caching. However, a cache that does not support
1081   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1082   Content) responses.
1083</t>
1084<t>
1085   A response received with any other status code (e.g. status codes 302
1086   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1087   unless there are cache-control directives or another header(s) that
1088   explicitly allow it. For example, these include the following: an
1089   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1090   "proxy-revalidate", "public" or "private" cache-control
1091   directive (<xref target="header.cache-control"/>).
1092</t>
1093</section>
1094
1095<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1096<t>
1097   The purpose of an HTTP cache is to store information received in
1098   response to requests for use in responding to future requests. In
1099   many cases, a cache simply returns the appropriate parts of a
1100   response to the requester. However, if the cache holds a cache entry
1101   based on a previous response, it might have to combine parts of a new
1102   response with what is held in the cache entry.
1103</t>
1104
1105<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1106<t>
1107   For the purpose of defining the behavior of caches and non-caching
1108   proxies, we divide HTTP headers into two categories:
1109  <list style="symbols">
1110      <t>End-to-end headers, which are  transmitted to the ultimate
1111        recipient of a request or response. End-to-end headers in
1112        responses &MUST; be stored as part of a cache entry and &MUST; be
1113        transmitted in any response formed from a cache entry.</t>
1114
1115      <t>Hop-by-hop headers, which are meaningful only for a single
1116        transport-level connection, and are not stored by caches or
1117        forwarded by proxies.</t>
1118  </list>
1119</t>
1120<t>
1121   The following HTTP/1.1 headers are hop-by-hop headers:
1122  <list style="symbols">
1123      <t>Connection</t>
1124      <t>Keep-Alive</t>
1125      <t>Proxy-Authenticate</t>
1126      <t>Proxy-Authorization</t>
1127      <t>TE</t>
1128      <t>Trailer</t>
1129      <t>Transfer-Encoding</t>
1130      <t>Upgrade</t>
1131  </list>
1132</t>
1133<t>
1134   All other headers defined by HTTP/1.1 are end-to-end headers.
1135</t>
1136<t>
1137   Other hop-by-hop headers &MUST; be listed in a Connection header
1138   (&header-connection;).
1139</t>
1140</section>
1141
1142<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1143<t>
1144   Some features of HTTP/1.1, such as Digest
1145   Authentication, depend on the value of certain end-to-end headers. A
1146   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1147   definition of that header requires or specifically allows that.
1148</t>
1149<t>
1150   A transparent proxy &MUST-NOT; modify any of the following fields in a
1151   request or response, and it &MUST-NOT; add any of these fields if not
1152   already present:
1153  <list style="symbols">
1154      <t>Content-Location</t>
1155      <t>Content-MD5</t>
1156      <t>ETag</t>
1157      <t>Last-Modified</t>
1158  </list>
1159</t>
1160<t>
1161   A transparent proxy &MUST-NOT; modify any of the following fields in a
1162   response:
1163  <list style="symbols">
1164    <t>Expires</t>
1165  </list>
1166</t>
1167<t>
1168   but it &MAY; add any of these fields if not already present. If an
1169   Expires header is added, it &MUST; be given a field-value identical to
1170   that of the Date header in that response.
1171</t>
1172<t>
1173   A  proxy &MUST-NOT; modify or add any of the following fields in a
1174   message that contains the no-transform cache-control directive, or in
1175   any request:
1176  <list style="symbols">
1177    <t>Content-Encoding</t>
1178    <t>Content-Range</t>
1179    <t>Content-Type</t>
1180  </list>
1181</t>
1182<t>
1183   A non-transparent proxy &MAY; modify or add these fields to a message
1184   that does not include no-transform, but if it does so, it &MUST; add a
1185   Warning 214 (Transformation applied) if one does not already appear
1186   in the message (see <xref target="header.warning"/>).
1187  <list><t>
1188      Warning: unnecessary modification of end-to-end headers might
1189      cause authentication failures if stronger authentication
1190      mechanisms are introduced in later versions of HTTP. Such
1191      authentication mechanisms &MAY; rely on the values of header fields
1192      not listed here.
1193    </t></list>
1194</t>
1195<t>
1196   The Content-Length field of a request or response is added or deleted
1197   according to the rules in &message-length;. A transparent proxy &MUST;
1198   preserve the entity-length (&entity-length;) of the entity-body,
1199   although it &MAY; change the transfer-length (&message-length;).
1200</t>
1201</section>
1202
1203<section title="Combining Headers" anchor="combining.headers">
1204<t>
1205   When a cache makes a validating request to a server, and the server
1206   provides a 304 (Not Modified) response or a 206 (Partial Content)
1207   response, the cache then constructs a response to send to the
1208   requesting client.
1209</t>
1210<t>
1211   If the status code is 304 (Not Modified), the cache uses the entity-body
1212   stored in the cache entry as the entity-body of this outgoing
1213   response. If the status code is 206 (Partial Content) and the ETag or
1214   Last-Modified headers match exactly, the cache &MAY; combine the
1215   contents stored in the cache entry with the new contents received in
1216   the response and use the result as the entity-body of this outgoing
1217   response, (see &combining-byte-ranges;).
1218</t>
1219<t>
1220   The end-to-end headers stored in the cache entry are used for the
1221   constructed response, except that
1222  <list style="symbols">
1223    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1224      &MUST; be deleted from the cache entry and the forwarded response.</t>
1225    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1226        in the cache entry and the forwarded response.</t>
1227    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1228        replace the corresponding headers from the cache entry.</t>
1229  </list>
1230</t>
1231<t>
1232   Unless the cache decides to remove the cache entry, it &MUST; also
1233   replace the end-to-end headers stored with the cache entry with
1234   corresponding headers received in the incoming response, except for
1235   Warning headers as described immediately above. If a header field-name
1236   in the incoming response matches more than one header in the
1237   cache entry, all such old headers &MUST; be replaced.
1238</t>
1239<t>
1240   In other words, the set of end-to-end headers received in the
1241   incoming response overrides all corresponding end-to-end headers
1242   stored with the cache entry (except for stored Warning headers with
1243   warn-code 1xx, which are deleted even if not overridden).
1244  <list><t>
1245      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1246      Modified) or a 206 (Partial Content) response to update any header
1247      associated with a previous response for the same entity or sub-ranges
1248      thereof, although it might not always be meaningful or
1249      correct to do so. This rule does not allow an origin server to use
1250      a 304 (Not Modified) or a 206 (Partial Content) response to
1251      entirely delete a header that it had provided with a previous
1252      response.
1253  </t></list>
1254</t>
1255</section>
1256
1257</section>
1258
1259<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1260<t>
1261   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1262   by the presence of a Vary header field in a response, alters the
1263   conditions and procedure by which a cache can use the response for
1264   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1265   field by servers.
1266</t>
1267<t>
1268   A server &SHOULD; use the Vary header field to inform a cache of what
1269   request-header fields were used to select among multiple
1270   representations of a cacheable response subject to server-driven
1271   negotiation. The set of header fields named by the Vary field value
1272   is known as the "selecting" request-headers.
1273</t>
1274<t>
1275   When the cache receives a subsequent request whose request-target
1276   specifies one or more cache entries including a Vary header field,
1277   the cache &MUST-NOT; use such a cache entry to construct a response to
1278   the new request unless all of the selecting request-headers present
1279   in the new request match the corresponding stored request-headers in
1280   the original request.
1281</t>
1282<t>
1283   The selecting request-headers from two requests are defined to match
1284   if and only if the selecting request-headers in the first request can
1285   be transformed to the selecting request-headers in the second request
1286   by adding or removing linear whitespace (LWS) at places where this
1287   is allowed by the corresponding BNF, and/or combining multiple
1288   message-header fields with the same field name following the rules
1289   about message headers in &message-headers;.
1290</t>
1291<t>
1292   A Vary header field-value of "*" always fails to match and subsequent
1293   requests on that resource can only be properly interpreted by the
1294   origin server.
1295</t>
1296<t>
1297   If the selecting request header fields for the cached entry do not
1298   match the selecting request header fields of the new request, then
1299   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1300   it first relays the new request to the origin server in a conditional
1301   request and the server responds with 304 (Not Modified), including an
1302   entity tag or Content-Location that indicates the entity to be used.
1303</t>
1304<t>
1305   If an entity tag was assigned to a cached representation, the
1306   forwarded request &SHOULD; be conditional and include the entity tags
1307   in an If-None-Match header field from all its cache entries for the
1308   resource. This conveys to the server the set of entities currently
1309   held by the cache, so that if any one of these entities matches the
1310   requested entity, the server can use the ETag header field in its 304
1311   (Not Modified) response to tell the cache which entry is appropriate.
1312   If the entity-tag of the new response matches that of an existing
1313   entry, the new response &SHOULD; be used to update the header fields of
1314   the existing entry, and the result &MUST; be returned to the client.
1315</t>
1316<t>
1317   If any of the existing cache entries contains only partial content
1318   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1319   the If-None-Match header field unless the request is for a range that
1320   would be fully satisfied by that entry.
1321</t>
1322<t>
1323   If a cache receives a successful response whose Content-Location
1324   field matches that of an existing cache entry for the same request-target,
1325   whose entity-tag differs from that of the existing entry, and
1326   whose Date is more recent than that of the existing entry, the
1327   existing entry &SHOULD-NOT;  be returned in response to future requests
1328   and &SHOULD; be deleted from the cache.
1329</t>
1330</section>
1331
1332<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1333<t>
1334   For reasons of security and privacy, it is necessary to make a
1335   distinction between "shared" and "non-shared" caches. A non-shared
1336   cache is one that is accessible only to a single user. Accessibility
1337   in this case &SHOULD; be enforced by appropriate security mechanisms.
1338   All other caches are considered to be "shared." Other sections of
1339   this specification place certain constraints on the operation of
1340   shared caches in order to prevent loss of privacy or failure of
1341   access controls.
1342</t>
1343</section>
1344
1345<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1346<t>
1347   A cache that receives an incomplete response (for example, with fewer
1348   bytes of data than specified in a Content-Length header) &MAY; store
1349   the response. However, the cache &MUST; treat this as a partial
1350   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1351   the result might be a full response or might still be
1352   partial. A cache &MUST-NOT; return a partial response to a client
1353   without explicitly marking it as such, using the 206 (Partial
1354   Content) status code. A cache &MUST-NOT; return a partial response
1355   using a status code of 200 (OK).
1356</t>
1357<t>
1358   If a cache receives a 5xx response while attempting to revalidate an
1359   entry, it &MAY; either forward this response to the requesting client,
1360   or act as if the server failed to respond. In the latter case, it &MAY;
1361   return a previously received response unless the cached entry
1362   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1363</t>
1364</section>
1365
1366<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1367<t>
1368   Unless the origin server explicitly prohibits the caching of their
1369   responses, the application of GET and HEAD methods to any resources
1370   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1371   these responses are taken from a cache. They &MAY; still have side
1372   effects, but a cache is not required to consider such side effects in
1373   its caching decisions. Caches are always expected to observe an
1374   origin server's explicit restrictions on caching.
1375</t>
1376<t>
1377   We note one exception to this rule: since some applications have
1378   traditionally used GET and HEAD requests with URLs containing a query part
1379   to perform operations with significant side
1380   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1381   the server provides an explicit expiration time. This specifically
1382   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1383   be taken from a cache. See &safe-methods; for related information.
1384</t>
1385</section>
1386
1387<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1388<t>
1389   The effect of certain methods performed on a resource at the origin
1390   server might cause one or more existing cache entries to become non-transparently
1391   invalid. That is, although they might continue to be
1392   "fresh," they do not accurately reflect what the origin server would
1393   return for a new request on that resource.
1394</t>
1395<t>
1396   There is no way for HTTP to guarantee that all such
1397   cache entries are marked invalid. For example, the request that
1398   caused the change at the origin server might not have gone through
1399   the proxy where a cache entry is stored. However, several rules help
1400   reduce the likelihood of erroneous behavior.
1401</t>
1402<t>
1403   In this section, the phrase "invalidate an entity" means that the
1404   cache will either remove all instances of that entity from its
1405   storage, or will mark these as "invalid" and in need of a mandatory
1406   revalidation before they can be returned in response to a subsequent
1407   request.
1408</t>
1409<t>
1410   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1411   either the entity referred to by the request-target, or by the Location
1412   or Content-Location headers (if present). These methods are:
1413  <list style="symbols">
1414      <t>PUT</t>
1415      <t>DELETE</t>
1416      <t>POST</t>
1417  </list>
1418</t> 
1419<t>
1420   An invalidation based
1421   on the URI in a Location or Content-Location header &MUST-NOT; be
1422   performed if the host part of that URI differs from the host part
1423   in the request-target. This helps prevent denial of service attacks.
1424</t>
1425<t>
1426   A cache that passes through requests for methods it does not
1427   understand &SHOULD; invalidate any entities referred to by the
1428   request-target.
1429</t>
1430</section>
1431
1432<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1433<t>
1434   All methods that might be expected to cause modifications to the
1435   origin server's resources &MUST; be written through to the origin
1436   server. This currently includes all methods except for GET and HEAD.
1437   A cache &MUST-NOT; reply to such a request from a client before having
1438   transmitted the request to the inbound server, and having received a
1439   corresponding response from the inbound server. This does not prevent
1440   a proxy cache from sending a 100 (Continue) response before the
1441   inbound server has sent its final reply.
1442</t>
1443<t>
1444   The alternative (known as "write-back" or "copy-back" caching) is not
1445   allowed in HTTP/1.1, due to the difficulty of providing consistent
1446   updates and the problems arising from server, cache, or network
1447   failure prior to write-back.
1448</t>
1449</section>
1450
1451<section title="Cache Replacement" anchor="cache.replacement">
1452<t>
1453   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1454   <xref target="disambiguating.expiration.values" format="counter"/>,
1455   <xref target="disambiguating.multiple.responses" format="counter"/>
1456   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1457   response is received from a resource while any existing responses for
1458   the same resource are cached, the cache &SHOULD; use the new response
1459   to reply to the current request. It &MAY; insert it into cache storage
1460   and &MAY;, if it meets all other requirements, use it to respond to any
1461   future requests that would previously have caused the old response to
1462   be returned. If it inserts the new response into cache storage  the
1463   rules in <xref target="combining.headers"/> apply.
1464  <list><t>
1465      <x:h>Note:</x:h> a new response that has an older Date header value than
1466      existing cached responses is not cacheable.
1467  </t></list>
1468</t>
1469</section>
1470
1471<section title="History Lists" anchor="history.lists">
1472<t>
1473   User agents often have history mechanisms, such as "Back" buttons and
1474   history lists, which can be used to redisplay an entity retrieved
1475   earlier in a session.
1476</t>
1477<t>
1478   History mechanisms and caches are different. In particular history
1479   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1480   the current state of a resource. Rather, a history mechanism is meant
1481   to show exactly what the user saw at the time when the resource was
1482   retrieved.
1483</t>
1484<t>
1485   By default, an expiration time does not apply to history mechanisms.
1486   If the entity is still in storage, a history mechanism &SHOULD; display
1487   it even if the entity has expired, unless the user has specifically
1488   configured the agent to refresh expired history documents.
1489</t>
1490<t>
1491   This is not to be construed to prohibit the history mechanism from
1492   telling the user that a view might be stale.
1493  <list><t>
1494      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1495      viewing stale resources, this will tend to force service authors
1496      to avoid using HTTP expiration controls and cache controls when
1497      they would otherwise like to. Service authors may consider it
1498      important that users not be presented with error messages or
1499      warning messages when they use navigation controls (such as BACK)
1500      to view previously fetched resources. Even though sometimes such
1501      resources ought not be cached, or ought to expire quickly, user
1502      interface considerations may force service authors to resort to
1503      other means of preventing caching (e.g. "once-only" URLs) in order
1504      not to suffer the effects of improperly functioning history
1505      mechanisms.
1506  </t></list>
1507</t>
1508</section>
1509
1510<section title="Header Field Definitions" anchor="header.fields">
1511<t>
1512   This section defines the syntax and semantics of HTTP/1.1 header fields
1513   related to caching.
1514</t>
1515<t>
1516   For entity-header fields, both sender and recipient refer to either the
1517   client or the server, depending on who sends and who receives the entity.
1518</t>
1519
1520<section title="Age" anchor="header.age">
1521  <iref primary="true" item="Age header" x:for-anchor=""/>
1522  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1523  <x:anchor-alias value="Age"/>
1524  <x:anchor-alias value="Age-v"/>
1525  <x:anchor-alias value="age-value"/>
1526<t>
1527      The response-header field "Age" conveys the sender's estimate of the
1528      amount of time since the response (or its revalidation) was
1529      generated at the origin server. A cached response is "fresh" if
1530      its age does not exceed its freshness lifetime. Age values are
1531      calculated as specified in <xref target="age.calculations"/>.
1532</t>
1533<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
1534  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
1535  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
1536</artwork></figure>
1537<t anchor="rule.delta-seconds">
1538  <x:anchor-alias value="delta-seconds"/>
1539      Age values are non-negative decimal integers, representing time in
1540      seconds.
1541</t>
1542<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
1543  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
1544</artwork></figure>
1545<t>
1546      If a cache receives a value larger than the largest positive
1547      integer it can represent, or if any of its age calculations
1548      overflows, it &MUST; transmit an Age header with a value of
1549      2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache &MUST;
1550      include an Age header field in every response generated from its
1551      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1552      bits of range.
1553</t>
1554</section>
1555
1556<section title="Cache-Control" anchor="header.cache-control">
1557  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1558  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1559  <x:anchor-alias value="Cache-Control"/>
1560  <x:anchor-alias value="Cache-Control-v"/>
1561  <x:anchor-alias value="cache-directive"/>
1562  <x:anchor-alias value="cache-extension"/>
1563  <x:anchor-alias value="cache-request-directive"/>
1564  <x:anchor-alias value="cache-response-directive"/>
1565<t>
1566   The general-header field "Cache-Control" is used to specify directives
1567   that &MUST; be obeyed by all caching mechanisms along the
1568   request/response chain. The directives specify behavior intended to
1569   prevent caches from adversely interfering with the request or
1570   response. These directives typically override the default caching
1571   algorithms. Cache directives are unidirectional in that the presence
1572   of a directive in a request does not imply that the same directive is
1573   to be given in the response.
1574  <list><t>
1575      Note that HTTP/1.0 caches might not implement Cache-Control and
1576      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1577  </t></list>
1578</t>
1579<t>
1580   Cache directives &MUST; be passed through by a proxy or gateway
1581   application, regardless of their significance to that application,
1582   since the directives might be applicable to all recipients along the
1583   request/response chain. It is not possible to specify a cache-directive
1584   for a specific cache.
1585</t>
1586<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><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"/>
1587  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1588  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1589
1590  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1591     / <x:ref>cache-response-directive</x:ref>
1592
1593  <x:ref>cache-request-directive</x:ref> =
1594       "no-cache"                          ; <xref target="what.is.cacheable"/>
1595     / "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1596     / "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"/>
1597     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1598     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1599     / "no-transform"                      ; <xref target="no-transform.directive"/>
1600     / "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1601     / <x:ref>cache-extension</x:ref>                     ; <xref target="cache.control.extensions"/>
1602
1603  <x:ref>cache-response-directive</x:ref> =
1604       "public"                               ; <xref target="what.is.cacheable"/>
1605     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
1606     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
1607     / "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1608     / "no-transform"                         ; <xref target="no-transform.directive"/>
1609     / "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1610     / "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1611     / "max-age" "=" <x:ref>delta-seconds</x:ref>            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1612     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1613     / <x:ref>cache-extension</x:ref>                        ; <xref target="cache.control.extensions"/>
1614
1615  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1616</artwork></figure>
1617<t>
1618   When a directive appears without any 1#field-name parameter, the
1619   directive applies to the entire request or response. When such a
1620   directive appears with a 1#field-name parameter, it applies only to
1621   the named field or fields, and not to the rest of the request or
1622   response. This mechanism supports extensibility; implementations of
1623   future versions of HTTP might apply these directives to
1624   header fields not defined in HTTP/1.1.
1625</t>
1626<t>
1627   The cache-control directives can be broken down into these general
1628   categories:
1629  <list style="symbols">
1630     <t>Restrictions on what are cacheable; these may only be imposed by
1631        the origin server.</t>
1632
1633     <t>Restrictions on what may be stored by a cache; these may be
1634        imposed by either the origin server or the user agent.</t>
1635
1636     <t>Modifications of the basic expiration mechanism; these may be
1637        imposed by either the origin server or the user agent.</t>
1638
1639     <t>Controls over cache revalidation and reload; these may only be
1640        imposed by a user agent.</t>
1641
1642     <t>Control over transformation of entities.</t>
1643
1644     <t>Extensions to the caching system.</t>
1645  </list>
1646</t>
1647
1648<section title="What is Cacheable" anchor="what.is.cacheable">
1649<t>
1650   By default, a response is cacheable if the requirements of the
1651   request method, request header fields, and the response status
1652   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1653   for cacheability. The following Cache-Control response directives
1654   allow an origin server to override the default cacheability of a
1655   response:
1656</t>
1657<t>
1658  <iref item="Cache Directives" subitem="public" primary="true"/>
1659  <iref item="public" subitem="Cache Directive" primary="true"/>
1660   public
1661  <list><t>
1662      Indicates that the response &MAY; be cached by any cache, even if it
1663      would normally be non-cacheable or cacheable only within a non-shared
1664      cache. (See also Authorization, &header-authorization;, for
1665      additional details.)
1666  </t></list>
1667</t>
1668<t>
1669  <iref item="Cache Directives" subitem="private" primary="true"/>
1670  <iref item="private" subitem="Cache Directive" primary="true"/>
1671   private
1672  <list><t>
1673      Indicates that all or part of the response message is intended for
1674      a single user and &MUST-NOT; be cached by a shared cache. This
1675      allows an origin server to state that the specified parts of the
1676      response are intended for only one user and are not a valid
1677      response for requests by other users. A private (non-shared) cache
1678      &MAY; cache the response.
1679    </t><t>
1680       <x:h>Note:</x:h> This usage of the word private only controls where the
1681       response may be cached, and cannot ensure the privacy of the
1682       message content.
1683  </t></list>
1684</t>
1685<t>
1686  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1687  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1688   no-cache
1689  <list><t>
1690       If the no-cache directive does not specify a field-name, then a
1691      cache &MUST-NOT; use the response to satisfy a subsequent request
1692      without successful revalidation with the origin server. This
1693      allows an origin server to prevent caching even by caches that
1694      have been configured to return stale responses to client requests.
1695    </t><t>
1696      If the no-cache directive does specify one or more field-names,
1697      then a cache &MAY; use the response to satisfy a subsequent request,
1698      subject to any other restrictions on caching. However, the
1699      specified field-name(s) &MUST-NOT; be sent in the response to a
1700      subsequent request without successful revalidation with the origin
1701      server. This allows an origin server to prevent the re-use of
1702      certain header fields in a response, while still allowing caching
1703      of the rest of the response.
1704    <list><t>
1705       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1706       directive.
1707    </t></list>
1708  </t></list>
1709</t>
1710</section>
1711
1712<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1713<t>
1714  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1715  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1716   no-store
1717  <list><t>   
1718      The purpose of the no-store directive is to prevent the
1719      inadvertent release or retention of sensitive information (for
1720      example, on backup tapes). The no-store directive applies to the
1721      entire message, and &MAY; be sent either in a response or in a
1722      request. If sent in a request, a cache &MUST-NOT; store any part of
1723      either this request or any response to it. If sent in a response,
1724      a cache &MUST-NOT; store any part of either this response or the
1725      request that elicited it. This directive applies to both non-shared
1726      and shared caches. "&MUST-NOT; store" in this context means
1727      that the cache &MUST-NOT; intentionally store the information in
1728      non-volatile storage, and &MUST; make a best-effort attempt to
1729      remove the information from volatile storage as promptly as
1730      possible after forwarding it.
1731  </t><t>
1732      Even when this directive is associated with a response, users
1733      might explicitly store such a response outside of the caching
1734      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1735      such responses as part of their normal operation.
1736  </t><t>
1737      The purpose of this directive is to meet the stated requirements
1738      of certain users and service authors who are concerned about
1739      accidental releases of information via unanticipated accesses to
1740      cache data structures. While the use of this directive might
1741      improve privacy in some cases, we caution that it is NOT in any
1742      way a reliable or sufficient mechanism for ensuring privacy. In
1743      particular, malicious or compromised caches might not recognize or
1744      obey this directive, and communications networks might be
1745      vulnerable to eavesdropping.
1746  </t></list>
1747</t>
1748</section>
1749
1750<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1751<t>
1752   The expiration time of an entity &MAY; be specified by the origin
1753   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1754   it &MAY; be specified using the max-age directive in a response. When
1755   the max-age cache-control directive is present in a cached response,
1756   the response is stale if its current age is greater than the age
1757   value given (in seconds) at the time of a new request for that
1758   resource. The max-age directive on a response implies that the
1759   response is cacheable (i.e., "public") unless some other, more
1760   restrictive cache directive is also present.
1761</t>
1762<t>
1763   If a response includes both an Expires header and a max-age
1764   directive, the max-age directive overrides the Expires header, even
1765   if the Expires header is more restrictive. This rule allows an origin
1766   server to provide, for a given response, a longer expiration time to
1767   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1768   useful if certain HTTP/1.0 caches improperly calculate ages or
1769   expiration times, perhaps due to desynchronized clocks.
1770</t>
1771<t>
1772   Many HTTP/1.0 cache implementations will treat an Expires value that
1773   is less than or equal to the response Date value as being equivalent
1774   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1775   cache receives such a response, and the response does not include a
1776   Cache-Control header field, it &SHOULD; consider the response to be
1777   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1778  <list><t>
1779       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1780       cache control feature, such as the "private" directive, on a
1781       network including older caches that do not understand that
1782       feature. The origin server will need to combine the new feature
1783       with an Expires field whose value is less than or equal to the
1784       Date value. This will prevent older caches from improperly
1785       caching the response.
1786  </t></list>
1787</t>
1788<t>
1789  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1790  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1791   s-maxage
1792  <list><t>
1793       If a response includes an s-maxage directive, then for a shared
1794       cache (but not for a private cache), the maximum age specified by
1795       this directive overrides the maximum age specified by either the
1796       max-age directive or the Expires header. The s-maxage directive
1797       also implies the semantics of the proxy-revalidate directive (see
1798       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1799       entry after it becomes stale to respond to a subsequent request
1800       without first revalidating it with the origin server. The s-maxage
1801       directive is always ignored by a private cache.
1802  </t></list>
1803</t>
1804<t>
1805   Note that most older caches, not compliant with this specification,
1806   do not implement any cache-control directives. An origin server
1807   wishing to use a cache-control directive that restricts, but does not
1808   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1809   requirement that the max-age directive overrides the Expires header,
1810   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1811   max-age directive.
1812</t>
1813<t>
1814   Other directives allow a user agent to modify the basic expiration
1815   mechanism. These directives &MAY; be specified on a request:
1816</t>
1817<t>
1818  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1819  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1820   max-age
1821  <list><t>
1822      Indicates that the client is willing to accept a response whose
1823      age is no greater than the specified time in seconds. Unless max-stale
1824      directive is also included, the client is not willing to
1825      accept a stale response.
1826  </t></list>
1827</t>
1828<t>
1829  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1830  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1831   min-fresh
1832  <list><t>
1833      Indicates that the client is willing to accept a response whose
1834      freshness lifetime is no less than its current age plus the
1835      specified time in seconds. That is, the client wants a response
1836      that will still be fresh for at least the specified number of
1837      seconds.
1838  </t></list>
1839</t>
1840<t>
1841  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1842  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1843   max-stale
1844  <list><t>
1845      Indicates that the client is willing to accept a response that has
1846      exceeded its expiration time. If max-stale is assigned a value,
1847      then the client is willing to accept a response that has exceeded
1848      its expiration time by no more than the specified number of
1849      seconds. If no value is assigned to max-stale, then the client is
1850      willing to accept a stale response of any age.
1851  </t></list>
1852</t>
1853<t>
1854   If a cache returns a stale response, either because of a max-stale
1855   directive on a request, or because the cache is configured to
1856   override the expiration time of a response, the cache &MUST; attach a
1857   Warning header to the stale response, using Warning 110 (Response is
1858   stale).
1859</t>
1860<t>
1861   A cache &MAY; be configured to return stale responses without
1862   validation, but only if this does not conflict with any "MUST"-level
1863   requirements concerning cache validation (e.g., a "must-revalidate"
1864   cache-control directive).
1865</t>
1866<t>
1867   If both the new request and the cached entry include "max-age"
1868   directives, then the lesser of the two values is used for determining
1869   the freshness of the cached entry for that request.
1870</t>
1871</section>
1872
1873<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1874<t>
1875   Sometimes a user agent might want or need to insist that a cache
1876   revalidate its cache entry with the origin server (and not just with
1877   the next cache along the path to the origin server), or to reload its
1878   cache entry from the origin server. End-to-end revalidation might be
1879   necessary if either the cache or the origin server has overestimated
1880   the expiration time of the cached response. End-to-end reload may be
1881   necessary if the cache entry has become corrupted for some reason.
1882</t>
1883<t>
1884   End-to-end revalidation may be requested either when the client does
1885   not have its own local cached copy, in which case we call it
1886   "unspecified end-to-end revalidation", or when the client does have a
1887   local cached copy, in which case we call it "specific end-to-end
1888   revalidation."
1889</t>
1890<t>
1891   The client can specify these three kinds of action using Cache-Control
1892   request directives:
1893</t>
1894<t>
1895   End-to-end reload
1896  <list><t>
1897      The request includes a "no-cache" cache-control directive or, for
1898      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1899      names &MUST-NOT; be included with the no-cache directive in a
1900      request. The server &MUST-NOT; use a cached copy when responding to
1901      such a request.
1902  </t></list>
1903</t>
1904<t>
1905   Specific end-to-end revalidation
1906  <list><t>
1907      The request includes a "max-age=0" cache-control directive, which
1908      forces each cache along the path to the origin server to
1909      revalidate its own entry, if any, with the next cache or server.
1910      The initial request includes a cache-validating conditional with
1911      the client's current validator.
1912  </t></list>
1913</t>
1914<t>
1915   Unspecified end-to-end revalidation
1916  <list><t>
1917      The request includes "max-age=0" cache-control directive, which
1918      forces each cache along the path to the origin server to
1919      revalidate its own entry, if any, with the next cache or server.
1920      The initial request does not include a cache-validating
1921      conditional; the first cache along the path (if any) that holds a
1922      cache entry for this resource includes a cache-validating
1923      conditional with its current validator.
1924  </t></list>
1925</t>
1926<t>
1927  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1928  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1929   max-age
1930  <list><t>
1931      When an intermediate cache is forced, by means of a max-age=0
1932      directive, to revalidate its own cache entry, and the client has
1933      supplied its own validator in the request, the supplied validator
1934      might differ from the validator currently stored with the cache
1935      entry. In this case, the cache &MAY; use either validator in making
1936      its own request without affecting semantic transparency.
1937  </t><t>
1938      However, the choice of validator might affect performance. The
1939      best approach is for the intermediate cache to use its own
1940      validator when making its request. If the server replies with 304
1941      (Not Modified), then the cache can return its now validated copy
1942      to the client with a 200 (OK) response. If the server replies with
1943      a new entity and cache validator, however, the intermediate cache
1944      can compare the returned validator with the one provided in the
1945      client's request, using the strong comparison function. If the
1946      client's validator is equal to the origin server's, then the
1947      intermediate cache simply returns 304 (Not Modified). Otherwise,
1948      it returns the new entity with a 200 (OK) response.
1949  </t><t>
1950      If a request includes the no-cache directive, it &SHOULD-NOT; 
1951      include min-fresh, max-stale, or max-age.
1952  </t></list>
1953</t>
1954<t>
1955  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1956  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1957   only-if-cached
1958  <list><t>
1959      In some cases, such as times of extremely poor network
1960      connectivity, a client may want a cache to return only those
1961      responses that it currently has stored, and not to reload or
1962      revalidate with the origin server. To do this, the client may
1963      include the only-if-cached directive in a request. If it receives
1964      this directive, a cache &SHOULD; either respond using a cached entry
1965      that is consistent with the other constraints of the request, or
1966      respond with a 504 (Gateway Timeout) status. However, if a group
1967      of caches is being operated as a unified system with good internal
1968      connectivity, such a request &MAY; be forwarded within that group of
1969      caches.
1970  </t></list>
1971</t>
1972<t>
1973  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1974  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1975   must-revalidate
1976  <list><t>
1977      Because a cache &MAY; be configured to ignore a server's specified
1978      expiration time, and because a client request &MAY; include a max-stale
1979      directive (which has a similar effect), the protocol also
1980      includes a mechanism for the origin server to require revalidation
1981      of a cache entry on any subsequent use. When the must-revalidate
1982      directive is present in a response received by a cache, that cache
1983      &MUST-NOT; use the entry after it becomes stale to respond to a
1984      subsequent request without first revalidating it with the origin
1985      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1986      time, if, based solely on the origin server's Expires or max-age
1987      value, the cached response is stale.)
1988  </t><t>
1989      The must-revalidate directive is necessary to support reliable
1990      operation for certain protocol features. In all circumstances an
1991      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1992      particular, if the cache cannot reach the origin server for any
1993      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1994  </t><t>
1995      Servers &SHOULD; send the must-revalidate directive if and only if
1996      failure to revalidate a request on the entity could result in
1997      incorrect operation, such as a silently unexecuted financial
1998      transaction. Recipients &MUST-NOT; take any automated action that
1999      violates this directive, and &MUST-NOT; automatically provide an
2000      unvalidated copy of the entity if revalidation fails.
2001  </t><t>
2002      Although this is not recommended, user agents operating under
2003      severe connectivity constraints &MAY; violate this directive but, if
2004      so, &MUST; explicitly warn the user that an unvalidated response has
2005      been provided. The warning &MUST; be provided on each unvalidated
2006      access, and &SHOULD; require explicit user confirmation.
2007  </t></list>
2008</t>
2009<t>
2010  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
2011  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
2012   proxy-revalidate
2013  <list><t>
2014      The proxy-revalidate directive has the same meaning as the must-revalidate
2015      directive, except that it does not apply to non-shared
2016      user agent caches. It can be used on a response to an
2017      authenticated request to permit the user's cache to store and
2018      later return the response without needing to revalidate it (since
2019      it has already been authenticated once by that user), while still
2020      requiring proxies that service many users to revalidate each time
2021      (in order to make sure that each user has been authenticated).
2022      Note that such authenticated responses also need the public cache
2023      control directive in order to allow them to be cached at all.
2024  </t></list>
2025</t>
2026</section>
2027
2028<section title="No-Transform Directive" anchor="no-transform.directive">
2029<t>
2030  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2031  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2032   no-transform
2033  <list><t>
2034      Implementors of intermediate caches (proxies) have found it useful
2035      to convert the media type of certain entity bodies. A non-transparent
2036      proxy might, for example, convert between image
2037      formats in order to save cache space or to reduce the amount of
2038      traffic on a slow link.
2039  </t><t>
2040      Serious operational problems occur, however, when these
2041      transformations are applied to entity bodies intended for certain
2042      kinds of applications. For example, applications for medical
2043      imaging, scientific data analysis and those using end-to-end
2044      authentication, all depend on receiving an entity body that is bit
2045      for bit identical to the original entity-body.
2046  </t><t>
2047      Therefore, if a message includes the no-transform directive, an
2048      intermediate cache or proxy &MUST-NOT; change those headers that are
2049      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2050      directive. This implies that the cache or proxy &MUST-NOT; change
2051      any aspect of the entity-body that is specified by these headers,
2052      including the value of the entity-body itself.
2053  </t></list>
2054</t>
2055</section>
2056
2057<section title="Cache Control Extensions" anchor="cache.control.extensions">
2058<t>
2059   The Cache-Control header field can be extended through the use of one
2060   or more cache-extension tokens, each with an optional assigned value.
2061   Informational extensions (those which do not require a change in
2062   cache behavior) &MAY; be added without changing the semantics of other
2063   directives. Behavioral extensions are designed to work by acting as
2064   modifiers to the existing base of cache directives. Both the new
2065   directive and the standard directive are supplied, such that
2066   applications which do not understand the new directive will default
2067   to the behavior specified by the standard directive, and those that
2068   understand the new directive will recognize it as modifying the
2069   requirements associated with the standard directive. In this way,
2070   extensions to the cache-control directives can be made without
2071   requiring changes to the base protocol.
2072</t>
2073<t>
2074   This extension mechanism depends on an HTTP cache obeying all of the
2075   cache-control directives defined for its native HTTP-version, obeying
2076   certain extensions, and ignoring all directives that it does not
2077   understand.
2078</t>
2079<t>
2080   For example, consider a hypothetical new response directive called
2081   community which acts as a modifier to the private directive. We
2082   define this new directive to mean that, in addition to any non-shared
2083   cache, any cache which is shared only by members of the community
2084   named within its value may cache the response. An origin server
2085   wishing to allow the UCI community to use an otherwise private
2086   response in their shared cache(s) could do so by including
2087</t>
2088<figure><artwork type="example">
2089  Cache-Control: private, community="UCI"
2090</artwork></figure>
2091<t>
2092   A cache seeing this header field will act correctly even if the cache
2093   does not understand the community cache-extension, since it will also
2094   see and understand the private directive and thus default to the safe
2095   behavior.
2096</t>
2097<t>
2098   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2099   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2100   be combined with standard directives (or the response's default
2101   cacheability) such that the cache behavior will remain minimally
2102   correct even if the cache does not understand the extension(s).
2103</t>
2104</section>
2105</section>
2106
2107<section title="Expires" anchor="header.expires">
2108  <iref primary="true" item="Expires header" x:for-anchor=""/>
2109  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2110  <x:anchor-alias value="Expires"/>
2111  <x:anchor-alias value="Expires-v"/>
2112<t>
2113   The entity-header field "Expires" gives the date/time after which the
2114   response is considered stale. A stale cache entry may not normally be
2115   returned by a cache (either a proxy cache or a user agent cache)
2116   unless it is first validated with the origin server (or with an
2117   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2118   for further discussion of the expiration model.
2119</t>
2120<t>
2121   The presence of an Expires field does not imply that the original
2122   resource will change or cease to exist at, before, or after that
2123   time.
2124</t>
2125<t>
2126   The format is an absolute date and time as defined by HTTP-date in
2127   &full-date;; it &MUST; be sent in rfc1123-date format.
2128</t>
2129<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
2130  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
2131  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
2132</artwork></figure>
2133<t>
2134   An example of its use is
2135</t>
2136<figure><artwork type="example">
2137  Expires: Thu, 01 Dec 1994 16:00:00 GMT
2138</artwork></figure>
2139<t>
2140  <list><t>
2141      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2142      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2143      Expires field.
2144  </t></list>
2145</t>
2146<t>
2147   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2148   especially including the value "0", as in the past (i.e., "already
2149   expired").
2150</t>
2151<t>
2152   To mark a response as "already expired," an origin server sends an
2153   Expires date that is equal to the Date header value. (See the rules
2154   for expiration calculations in <xref target="expiration.calculations"/>.)
2155</t>
2156<t>
2157   To mark a response as "never expires," an origin server sends an
2158   Expires date approximately one year from the time the response is
2159   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2160   year in the future.
2161</t>
2162<t>
2163   The presence of an Expires header field with a date value of some
2164   time in the future on a response that otherwise would by default be
2165   non-cacheable indicates that the response is cacheable, unless
2166   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2167</t>
2168</section>
2169
2170<section title="Pragma" anchor="header.pragma">
2171  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2172  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2173  <x:anchor-alias value="extension-pragma"/>
2174  <x:anchor-alias value="Pragma"/>
2175  <x:anchor-alias value="Pragma-v"/>
2176  <x:anchor-alias value="pragma-directive"/>
2177<t>
2178   The general-header field "Pragma" is used to include implementation-specific
2179   directives that might apply to any recipient along the
2180   request/response chain. All pragma directives specify optional
2181   behavior from the viewpoint of the protocol; however, some systems
2182   &MAY; require that behavior be consistent with the directives.
2183</t>
2184<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
2185  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
2186  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
2187  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
2188  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
2189</artwork></figure>
2190<t>
2191   When the no-cache directive is present in a request message, an
2192   application &SHOULD; forward the request toward the origin server even
2193   if it has a cached copy of what is being requested. This pragma
2194   directive has the same semantics as the no-cache cache-directive (see
2195   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2196   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2197   request is sent to a server not known to be HTTP/1.1 compliant.
2198</t>
2199<t>
2200   Pragma directives &MUST; be passed through by a proxy or gateway
2201   application, regardless of their significance to that application,
2202   since the directives might be applicable to all recipients along the
2203   request/response chain. It is not possible to specify a pragma for a
2204   specific recipient; however, any pragma directive not relevant to a
2205   recipient &SHOULD; be ignored by that recipient.
2206</t>
2207<t>
2208   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2209   sent "Cache-Control: no-cache". No new Pragma directives will be
2210   defined in HTTP.
2211  <list><t>
2212      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
2213      response-header field is not actually specified, it does not provide a
2214      reliable replacement for "Cache-Control: no-cache" in a response.
2215  </t></list>
2216</t>
2217</section>
2218
2219<section title="Vary" anchor="header.vary">
2220  <iref primary="true" item="Vary header" x:for-anchor=""/>
2221  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2222  <x:anchor-alias value="Vary"/>
2223  <x:anchor-alias value="Vary-v"/>
2224<t>
2225   The "Vary" response-header field's value indicates the set of request-header
2226   fields that fully determines, while the response is fresh, whether a cache
2227   is permitted to use the response to reply to a subsequent request
2228   without revalidation. For uncacheable or stale responses, the Vary
2229   field value advises the user agent about the criteria that were used
2230   to select the representation. A Vary field value of "*" implies that
2231   a cache cannot determine from the request headers of a subsequent
2232   request whether this response is the appropriate representation. See
2233   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2234</t>
2235<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
2236  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
2237  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
2238</artwork></figure>
2239<t>
2240   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2241   cacheable response that is subject to server-driven negotiation.
2242   Doing so allows a cache to properly interpret future requests on that
2243   resource and informs the user agent about the presence of negotiation
2244   on that resource. A server &MAY; include a Vary header field with a
2245   non-cacheable response that is subject to server-driven negotiation,
2246   since this might provide the user agent with useful information about
2247   the dimensions over which the response varies at the time of the
2248   response.
2249</t>
2250<t>
2251   A Vary field value consisting of a list of field-names signals that
2252   the representation selected for the response is based on a selection
2253   algorithm which considers ONLY the listed request-header field values
2254   in selecting the most appropriate representation. A cache &MAY; assume
2255   that the same selection will be made for future requests with the
2256   same values for the listed field names, for the duration of time for
2257   which the response is fresh.
2258</t>
2259<t>
2260   The field-names given are not limited to the set of standard
2261   request-header fields defined by this specification. Field names are
2262   case-insensitive.
2263</t>
2264<t>
2265   A Vary field value of "*" signals that unspecified parameters not
2266   limited to the request-headers (e.g., the network address of the
2267   client), play a role in the selection of the response representation.
2268   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2269   generated by an origin server.
2270</t>
2271</section>
2272
2273<section title="Warning" anchor="header.warning">
2274  <iref primary="true" item="Warning header" x:for-anchor=""/>
2275  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2276  <x:anchor-alias value="Warning"/>
2277  <x:anchor-alias value="Warning-v"/>
2278  <x:anchor-alias value="warning-value"/>
2279  <x:anchor-alias value="warn-agent"/>
2280  <x:anchor-alias value="warn-code"/>
2281  <x:anchor-alias value="warn-date"/>
2282  <x:anchor-alias value="warn-text"/>
2283<t>
2284   The general-header field "Warning" is used to carry additional
2285   information about the status or transformation of a message which
2286   might not be reflected in the message. This information is typically
2287   used to warn about a possible lack of semantic transparency from
2288   caching operations or transformations applied to the entity body of
2289   the message.
2290</t>
2291<t>
2292   Warning headers are sent with responses using:
2293</t>
2294<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><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"/>
2295  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
2296  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
2297 
2298  <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>
2299                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
2300 
2301  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
2302  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2303                  ; the name or pseudonym of the server adding
2304                  ; the Warning header, for use in debugging
2305  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
2306  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
2307</artwork></figure>
2308<t>
2309   A response &MAY; carry more than one Warning header.
2310</t>
2311<t>
2312   The warn-text &SHOULD; be in a natural language and character set that
2313   is most likely to be intelligible to the human user receiving the
2314   response. This decision &MAY; be based on any available knowledge, such
2315   as the location of the cache or user, the Accept-Language field in a
2316   request, the Content-Language field in a response, etc. The default
2317   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2318</t>
2319<t>
2320   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2321   in the warn-text using the method described in <xref target="RFC2047"/>.
2322</t>
2323<t>
2324   Warning headers can in general be applied to any message, however
2325   some specific warn-codes are specific to caches and can only be
2326   applied to response messages. New Warning headers &SHOULD; be added
2327   after any existing Warning headers. A cache &MUST-NOT; delete any
2328   Warning header that it received with a message. However, if a cache
2329   successfully validates a cache entry, it &SHOULD; remove any Warning
2330   headers previously attached to that entry except as specified for
2331   specific Warning codes. It &MUST; then add any Warning headers received
2332   in the validating response. In other words, Warning headers are those
2333   that would be attached to the most recent relevant response.
2334</t>
2335<t>
2336   When multiple Warning headers are attached to a response, the user
2337   agent ought to inform the user of as many of them as possible, in the
2338   order that they appear in the response. If it is not possible to
2339   inform the user of all of the warnings, the user agent &SHOULD; follow
2340   these heuristics:
2341  <list style="symbols">
2342    <t>Warnings that appear early in the response take priority over
2343        those appearing later in the response.</t>
2344
2345    <t>Warnings in the user's preferred character set take priority
2346        over warnings in other character sets but with identical warn-codes
2347        and warn-agents.</t>
2348  </list>
2349</t>
2350<t>
2351   Systems that generate multiple Warning headers &SHOULD; order them with
2352   this user agent behavior in mind.
2353</t>
2354<t>
2355   Requirements for the behavior of caches with respect to Warnings are
2356   stated in <xref target="warnings"/>.
2357</t>
2358<t>
2359   This is a list of the currently-defined warn-codes, each with a
2360   recommended warn-text in English, and a description of its meaning.
2361</t>
2362<t>
2363   110 Response is stale
2364  <list><t>
2365     &MUST; be included whenever the returned response is stale.
2366  </t></list>
2367</t>
2368<t>
2369   111 Revalidation failed
2370  <list><t>
2371     &MUST; be included if a cache returns a stale response because an
2372     attempt to revalidate the response failed, due to an inability to
2373     reach the server.
2374  </t></list>
2375</t>
2376<t>
2377   112 Disconnected operation
2378  <list><t>
2379     &SHOULD; be included if the cache is intentionally disconnected from
2380     the rest of the network for a period of time.
2381  </t></list>
2382</t>
2383<t>
2384   113 Heuristic expiration
2385  <list><t>
2386     &MUST; be included if the cache heuristically chose a freshness
2387     lifetime greater than 24 hours and the response's age is greater
2388     than 24 hours.
2389  </t></list>
2390</t>
2391<t>
2392   199 Miscellaneous warning
2393  <list><t>
2394     The warning text &MAY; include arbitrary information to be presented
2395     to a human user, or logged. A system receiving this warning &MUST-NOT;
2396     take any automated action, besides presenting the warning to
2397     the user.
2398  </t></list>
2399</t>
2400<t>
2401   214 Transformation applied
2402  <list><t>
2403     &MUST; be added by an intermediate cache or proxy if it applies any
2404     transformation changing the content-coding (as specified in the
2405     Content-Encoding header) or media-type (as specified in the
2406     Content-Type header) of the response, or the entity-body of the
2407     response, unless this Warning code already appears in the response.
2408  </t></list>
2409</t>
2410<t>
2411   299 Miscellaneous persistent warning
2412  <list><t>
2413     The warning text &MAY; include arbitrary information to be presented
2414     to a human user, or logged. A system receiving this warning &MUST-NOT;
2415     take any automated action.
2416  </t></list>
2417</t>
2418<t>
2419   If an implementation sends a message with one or more Warning headers
2420   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2421   each warning-value a warn-date that matches the date in the response.
2422</t>
2423<t>
2424   If an implementation receives a message with a warning-value that
2425   includes a warn-date, and that warn-date is different from the Date
2426   value in the response, then that warning-value &MUST; be deleted from
2427   the message before storing, forwarding, or using it. (This prevents
2428   bad consequences of naive caching of Warning header fields.) If all
2429   of the warning-values are deleted for this reason, the Warning header
2430   &MUST; be deleted as well.
2431</t>
2432</section>
2433
2434</section>
2435
2436<section title="IANA Considerations" anchor="IANA.considerations">
2437<section title="Message Header Registration" anchor="message.header.registration">
2438<t>
2439   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2440   with the permanent registrations below (see <xref target="RFC3864"/>):
2441</t>
2442<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2443<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2444   <ttcol>Header Field Name</ttcol>
2445   <ttcol>Protocol</ttcol>
2446   <ttcol>Status</ttcol>
2447   <ttcol>Reference</ttcol>
2448
2449   <c>Age</c>
2450   <c>http</c>
2451   <c>standard</c>
2452   <c>
2453      <xref target="header.age"/>
2454   </c>
2455   <c>Cache-Control</c>
2456   <c>http</c>
2457   <c>standard</c>
2458   <c>
2459      <xref target="header.cache-control"/>
2460   </c>
2461   <c>Expires</c>
2462   <c>http</c>
2463   <c>standard</c>
2464   <c>
2465      <xref target="header.expires"/>
2466   </c>
2467   <c>Pragma</c>
2468   <c>http</c>
2469   <c>standard</c>
2470   <c>
2471      <xref target="header.pragma"/>
2472   </c>
2473   <c>Vary</c>
2474   <c>http</c>
2475   <c>standard</c>
2476   <c>
2477      <xref target="header.vary"/>
2478   </c>
2479   <c>Warning</c>
2480   <c>http</c>
2481   <c>standard</c>
2482   <c>
2483      <xref target="header.warning"/>
2484   </c>
2485</texttable>
2486<!--(END)-->
2487<t>
2488   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2489</t>
2490</section>
2491</section>
2492
2493<section title="Security Considerations" anchor="security.considerations">
2494<t>
2495   Caching proxies provide additional potential vulnerabilities, since
2496   the contents of the cache represent an attractive target for
2497   malicious exploitation. Because cache contents persist after an HTTP
2498   request is complete, an attack on the cache can reveal information
2499   long after a user believes that the information has been removed from
2500   the network. Therefore, cache contents should be protected as
2501   sensitive information.
2502</t>
2503</section>
2504
2505<section title="Acknowledgments" anchor="ack">
2506<t>
2507   Much of the content and presentation of the caching design is due to
2508   suggestions and comments from individuals including: Shel Kaphan,
2509   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2510</t>
2511</section>
2512</middle>
2513<back>
2514
2515<references title="Normative References">
2516
2517<reference anchor="ISO-8859-1">
2518  <front>
2519    <title>
2520     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2521    </title>
2522    <author>
2523      <organization>International Organization for Standardization</organization>
2524    </author>
2525    <date year="1998"/>
2526  </front>
2527  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2528</reference>
2529
2530<reference anchor="Part1">
2531  <front>
2532    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2533    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2534      <organization abbrev="Day Software">Day Software</organization>
2535      <address><email>fielding@gbiv.com</email></address>
2536    </author>
2537    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2538      <organization>One Laptop per Child</organization>
2539      <address><email>jg@laptop.org</email></address>
2540    </author>
2541    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2542      <organization abbrev="HP">Hewlett-Packard Company</organization>
2543      <address><email>JeffMogul@acm.org</email></address>
2544    </author>
2545    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2546      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2547      <address><email>henrikn@microsoft.com</email></address>
2548    </author>
2549    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2550      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2551      <address><email>LMM@acm.org</email></address>
2552    </author>
2553    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2554      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2555      <address><email>paulle@microsoft.com</email></address>
2556    </author>
2557    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2558      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2559      <address><email>timbl@w3.org</email></address>
2560    </author>
2561    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2562      <organization abbrev="W3C">World Wide Web Consortium</organization>
2563      <address><email>ylafon@w3.org</email></address>
2564    </author>
2565    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2566      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2567      <address><email>julian.reschke@greenbytes.de</email></address>
2568    </author>
2569    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2570  </front>
2571  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2572  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2573</reference>
2574
2575<reference anchor="Part2">
2576  <front>
2577    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2578    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2579      <organization abbrev="Day Software">Day Software</organization>
2580      <address><email>fielding@gbiv.com</email></address>
2581    </author>
2582    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2583      <organization>One Laptop per Child</organization>
2584      <address><email>jg@laptop.org</email></address>
2585    </author>
2586    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2587      <organization abbrev="HP">Hewlett-Packard Company</organization>
2588      <address><email>JeffMogul@acm.org</email></address>
2589    </author>
2590    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2591      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2592      <address><email>henrikn@microsoft.com</email></address>
2593    </author>
2594    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2595      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2596      <address><email>LMM@acm.org</email></address>
2597    </author>
2598    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2599      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2600      <address><email>paulle@microsoft.com</email></address>
2601    </author>
2602    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2603      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2604      <address><email>timbl@w3.org</email></address>
2605    </author>
2606    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2607      <organization abbrev="W3C">World Wide Web Consortium</organization>
2608      <address><email>ylafon@w3.org</email></address>
2609    </author>
2610    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2611      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2612      <address><email>julian.reschke@greenbytes.de</email></address>
2613    </author>
2614    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2615  </front>
2616  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2617  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2618</reference>
2619
2620<reference anchor="Part3">
2621  <front>
2622    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2623    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2624      <organization abbrev="Day Software">Day Software</organization>
2625      <address><email>fielding@gbiv.com</email></address>
2626    </author>
2627    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2628      <organization>One Laptop per Child</organization>
2629      <address><email>jg@laptop.org</email></address>
2630    </author>
2631    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2632      <organization abbrev="HP">Hewlett-Packard Company</organization>
2633      <address><email>JeffMogul@acm.org</email></address>
2634    </author>
2635    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2636      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2637      <address><email>henrikn@microsoft.com</email></address>
2638    </author>
2639    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2640      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2641      <address><email>LMM@acm.org</email></address>
2642    </author>
2643    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2644      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2645      <address><email>paulle@microsoft.com</email></address>
2646    </author>
2647    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2648      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2649      <address><email>timbl@w3.org</email></address>
2650    </author>
2651    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2652      <organization abbrev="W3C">World Wide Web Consortium</organization>
2653      <address><email>ylafon@w3.org</email></address>
2654    </author>
2655    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2656      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2657      <address><email>julian.reschke@greenbytes.de</email></address>
2658    </author>
2659    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2660  </front>
2661  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2662  <x:source href="p3-payload.xml" basename="p3-payload"/>
2663</reference>
2664
2665<reference anchor="Part4">
2666  <front>
2667    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2668    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2669      <organization abbrev="Day Software">Day Software</organization>
2670      <address><email>fielding@gbiv.com</email></address>
2671    </author>
2672    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2673      <organization>One Laptop per Child</organization>
2674      <address><email>jg@laptop.org</email></address>
2675    </author>
2676    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2677      <organization abbrev="HP">Hewlett-Packard Company</organization>
2678      <address><email>JeffMogul@acm.org</email></address>
2679    </author>
2680    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2681      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2682      <address><email>henrikn@microsoft.com</email></address>
2683    </author>
2684    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2685      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2686      <address><email>LMM@acm.org</email></address>
2687    </author>
2688    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2689      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2690      <address><email>paulle@microsoft.com</email></address>
2691    </author>
2692    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2693      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2694      <address><email>timbl@w3.org</email></address>
2695    </author>
2696    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2697      <organization abbrev="W3C">World Wide Web Consortium</organization>
2698      <address><email>ylafon@w3.org</email></address>
2699    </author>
2700    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2701      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2702      <address><email>julian.reschke@greenbytes.de</email></address>
2703    </author>
2704    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2705  </front>
2706  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2707  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2708</reference>
2709
2710<reference anchor="Part5">
2711  <front>
2712    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2713    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2714      <organization abbrev="Day Software">Day Software</organization>
2715      <address><email>fielding@gbiv.com</email></address>
2716    </author>
2717    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2718      <organization>One Laptop per Child</organization>
2719      <address><email>jg@laptop.org</email></address>
2720    </author>
2721    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2722      <organization abbrev="HP">Hewlett-Packard Company</organization>
2723      <address><email>JeffMogul@acm.org</email></address>
2724    </author>
2725    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2726      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2727      <address><email>henrikn@microsoft.com</email></address>
2728    </author>
2729    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2730      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2731      <address><email>LMM@acm.org</email></address>
2732    </author>
2733    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2734      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2735      <address><email>paulle@microsoft.com</email></address>
2736    </author>
2737    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2738      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2739      <address><email>timbl@w3.org</email></address>
2740    </author>
2741    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2742      <organization abbrev="W3C">World Wide Web Consortium</organization>
2743      <address><email>ylafon@w3.org</email></address>
2744    </author>
2745    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2746      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2747      <address><email>julian.reschke@greenbytes.de</email></address>
2748    </author>
2749    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2750  </front>
2751  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2752  <x:source href="p5-range.xml" basename="p5-range"/>
2753</reference>
2754
2755<reference anchor="Part7">
2756  <front>
2757    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2758    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2759      <organization abbrev="Day Software">Day Software</organization>
2760      <address><email>fielding@gbiv.com</email></address>
2761    </author>
2762    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2763      <organization>One Laptop per Child</organization>
2764      <address><email>jg@laptop.org</email></address>
2765    </author>
2766    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2767      <organization abbrev="HP">Hewlett-Packard Company</organization>
2768      <address><email>JeffMogul@acm.org</email></address>
2769    </author>
2770    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2771      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2772      <address><email>henrikn@microsoft.com</email></address>
2773    </author>
2774    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2775      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2776      <address><email>LMM@acm.org</email></address>
2777    </author>
2778    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2779      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2780      <address><email>paulle@microsoft.com</email></address>
2781    </author>
2782    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2783      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2784      <address><email>timbl@w3.org</email></address>
2785    </author>
2786    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2787      <organization abbrev="W3C">World Wide Web Consortium</organization>
2788      <address><email>ylafon@w3.org</email></address>
2789    </author>
2790    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2791      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2792      <address><email>julian.reschke@greenbytes.de</email></address>
2793    </author>
2794    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2795  </front>
2796  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2797  <x:source href="p7-auth.xml" basename="p7-auth"/>
2798</reference>
2799
2800<reference anchor="RFC2047">
2801  <front>
2802    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2803    <author initials="K." surname="Moore" fullname="Keith Moore">
2804      <organization>University of Tennessee</organization>
2805      <address><email>moore@cs.utk.edu</email></address>
2806    </author>
2807    <date month="November" year="1996"/>
2808  </front>
2809  <seriesInfo name="RFC" value="2047"/>
2810</reference>
2811
2812<reference anchor="RFC2119">
2813  <front>
2814    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2815    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2816      <organization>Harvard University</organization>
2817      <address><email>sob@harvard.edu</email></address>
2818    </author>
2819    <date month="March" year="1997"/>
2820  </front>
2821  <seriesInfo name="BCP" value="14"/>
2822  <seriesInfo name="RFC" value="2119"/>
2823</reference>
2824
2825<reference anchor="RFC5234">
2826  <front>
2827    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2828    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2829      <organization>Brandenburg InternetWorking</organization>
2830      <address>
2831      <postal>
2832      <street>675 Spruce Dr.</street>
2833      <city>Sunnyvale</city>
2834      <region>CA</region>
2835      <code>94086</code>
2836      <country>US</country></postal>
2837      <phone>+1.408.246.8253</phone>
2838      <email>dcrocker@bbiw.net</email></address> 
2839    </author>
2840    <author initials="P." surname="Overell" fullname="Paul Overell">
2841      <organization>THUS plc.</organization>
2842      <address>
2843      <postal>
2844      <street>1/2 Berkeley Square</street>
2845      <street>99 Berkely Street</street>
2846      <city>Glasgow</city>
2847      <code>G3 7HR</code>
2848      <country>UK</country></postal>
2849      <email>paul.overell@thus.net</email></address>
2850    </author>
2851    <date month="January" year="2008"/>
2852  </front>
2853  <seriesInfo name="STD" value="68"/>
2854  <seriesInfo name="RFC" value="5234"/>
2855</reference>
2856
2857</references>
2858
2859<references title="Informative References">
2860
2861<reference anchor="RFC1305">
2862  <front>
2863    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2864    <author initials="D." surname="Mills" fullname="David L. Mills">
2865      <organization>University of Delaware, Electrical Engineering Department</organization>
2866      <address><email>mills@udel.edu</email></address>
2867    </author>
2868    <date month="March" year="1992"/>
2869  </front>
2870  <seriesInfo name="RFC" value="1305"/>
2871</reference>
2872
2873<reference anchor="RFC2616">
2874  <front>
2875    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2876    <author initials="R." surname="Fielding" fullname="R. Fielding">
2877      <organization>University of California, Irvine</organization>
2878      <address><email>fielding@ics.uci.edu</email></address>
2879    </author>
2880    <author initials="J." surname="Gettys" fullname="J. Gettys">
2881      <organization>W3C</organization>
2882      <address><email>jg@w3.org</email></address>
2883    </author>
2884    <author initials="J." surname="Mogul" fullname="J. Mogul">
2885      <organization>Compaq Computer Corporation</organization>
2886      <address><email>mogul@wrl.dec.com</email></address>
2887    </author>
2888    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2889      <organization>MIT Laboratory for Computer Science</organization>
2890      <address><email>frystyk@w3.org</email></address>
2891    </author>
2892    <author initials="L." surname="Masinter" fullname="L. Masinter">
2893      <organization>Xerox Corporation</organization>
2894      <address><email>masinter@parc.xerox.com</email></address>
2895    </author>
2896    <author initials="P." surname="Leach" fullname="P. Leach">
2897      <organization>Microsoft Corporation</organization>
2898      <address><email>paulle@microsoft.com</email></address>
2899    </author>
2900    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2901      <organization>W3C</organization>
2902      <address><email>timbl@w3.org</email></address>
2903    </author>
2904    <date month="June" year="1999"/>
2905  </front>
2906  <seriesInfo name="RFC" value="2616"/>
2907</reference>
2908
2909<reference anchor='RFC3864'>
2910  <front>
2911    <title>Registration Procedures for Message Header Fields</title>
2912    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
2913      <organization>Nine by Nine</organization>
2914      <address><email>GK-IETF@ninebynine.org</email></address>
2915    </author>
2916    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
2917      <organization>BEA Systems</organization>
2918      <address><email>mnot@pobox.com</email></address>
2919    </author>
2920    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
2921      <organization>HP Labs</organization>
2922      <address><email>JeffMogul@acm.org</email></address>
2923    </author>
2924    <date year='2004' month='September' />
2925  </front>
2926  <seriesInfo name='BCP' value='90' />
2927  <seriesInfo name='RFC' value='3864' />
2928</reference>
2929
2930</references>
2931
2932<section title="Compatibility with Previous Versions" anchor="compatibility">
2933
2934<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2935<t>
2936   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2937   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2938   <xref target="header.cache-control" format="counter"/>,
2939   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2940</t>
2941<t>
2942   Transfer-coding and message lengths all interact in ways that
2943   required fixing exactly when chunked encoding is used (to allow for
2944   transfer encoding that may not be self delimiting); it was important
2945   to straighten out exactly how message lengths are computed.
2946   (<xref target="non-modifiable.headers"/>,
2947   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2948</t>
2949<t>
2950   Proxies should be able to add Content-Length when appropriate.
2951   (<xref target="non-modifiable.headers"/>)
2952</t>
2953<t>
2954   Range request responses would become very verbose if all meta-data
2955   were always returned; by allowing the server to only send needed
2956   headers in a 206 response, this problem can be avoided.
2957   (<xref target="combining.headers"/>)
2958</t>
2959<t>
2960   The Cache-Control: max-age directive was not properly defined for
2961   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2962</t>
2963<t>
2964   Warnings could be cached incorrectly, or not updated appropriately.
2965   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2966   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2967   and <xref target="header.warning" format="counter"/>) Warning
2968   also needed to be a general header, as PUT or other methods may have
2969   need for it in requests.
2970</t>
2971</section>
2972
2973<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2974<t>
2975  Clarify denial of service attack avoidance requirement.
2976  (<xref target="invalidation.after.updates.or.deletions"/>)
2977</t>
2978</section>
2979
2980</section>
2981
2982<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2983<figure>
2984<artwork type="abnf" name="p6-cache.parsed-abnf">
2985<x:ref>Age</x:ref> = "Age:" OWS Age-v
2986<x:ref>Age-v</x:ref> = delta-seconds
2987
2988<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2989<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2990 cache-directive ] )
2991
2992<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2993<x:ref>Expires-v</x:ref> = HTTP-date
2994
2995<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
2996
2997<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2998
2999<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
3000<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
3001 pragma-directive ] )
3002
3003<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
3004<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
3005 ] ) )
3006
3007<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
3008<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
3009 ] )
3010
3011<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
3012<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
3013<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
3014 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
3015 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
3016 cache-extension
3017<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
3018 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
3019 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
3020 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
3021 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
3022 ) / ( "s-maxage=" delta-seconds ) / cache-extension
3023
3024<x:ref>delta-seconds</x:ref> = 1*DIGIT
3025
3026<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
3027
3028<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
3029
3030<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
3031<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
3032<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
3033
3034<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
3035
3036<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
3037
3038<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
3039
3040<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
3041<x:ref>warn-code</x:ref> = 3DIGIT
3042<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
3043<x:ref>warn-text</x:ref> = quoted-string
3044<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
3045 ]
3046
3047
3048</artwork>
3049</figure>
3050<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">; Age defined but not used
3051; Cache-Control defined but not used
3052; Expires defined but not used
3053; Pragma defined but not used
3054; Vary defined but not used
3055; Warning defined but not used
3056</artwork></figure></section>
3057
3058<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
3059
3060<section title="Since RFC2616">
3061<t>
3062  Extracted relevant partitions from <xref target="RFC2616"/>.
3063</t>
3064</section>
3065
3066<section title="Since draft-ietf-httpbis-p6-cache-00">
3067<t>
3068  Closed issues:
3069  <list style="symbols"> 
3070    <t>
3071      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
3072      "Trailer"
3073      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
3074    </t>
3075    <t>
3076      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
3077      "Invalidation after Update or Delete"
3078      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
3079    </t>
3080    <t>
3081      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
3082      "Normative and Informative references"
3083    </t>
3084    <t>
3085      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
3086      "Date reference typo"
3087    </t>
3088    <t>
3089      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
3090      "Connection header text"
3091    </t>
3092    <t>
3093      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
3094      "Informative references"
3095    </t>
3096    <t>
3097      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
3098      "ISO-8859-1 Reference"
3099    </t>
3100    <t>
3101      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
3102      "Normative up-to-date references"
3103    </t>
3104    <t>
3105      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
3106      "typo in 13.2.2"
3107    </t>
3108  </list>
3109</t>
3110<t>
3111  Other changes:
3112  <list style="symbols"> 
3113    <t>
3114      Use names of RFC4234 core rule DQUOTE (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
3115    </t>
3116  </list>
3117</t>
3118</section>
3119
3120<section title="Since draft-ietf-httpbis-p6-cache-01">
3121<t>
3122  Closed issues:
3123  <list style="symbols"> 
3124    <t>
3125      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
3126      "rel_path not used"
3127    </t>
3128  </list>
3129</t>
3130<t>
3131  Other changes:
3132  <list style="symbols"> 
3133    <t>
3134       Get rid of duplicate BNF rule names ("host" -> "uri-host")
3135       (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
3136    </t>
3137    <t>
3138      Add explicit references to BNF syntax and rules imported from other parts of the specification.
3139    </t>
3140  </list>
3141</t>
3142</section>
3143
3144<section title="Since draft-ietf-httpbis-p6-cache-02" anchor="changes.since.02">
3145<t>
3146  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
3147  <list style="symbols"> 
3148    <t>
3149      Reference RFC 3984, and update header registrations for headers defined
3150      in this document.
3151    </t>
3152  </list>
3153</t>
3154</section>
3155
3156<section title="Since draft-ietf-httpbis-p6-cache-03" anchor="changes.since.03">
3157<t>
3158  Closed issues:
3159  <list style="symbols"> 
3160    <t>
3161      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>:
3162      "Vary header classification"
3163    </t>
3164  </list>
3165</t>
3166</section>
3167
3168<section title="Since draft-ietf-httpbis-p6-cache-04" anchor="changes.since.04">
3169<t>
3170  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3171  <list style="symbols"> 
3172    <t>
3173      Use "/" instead of "|" for alternatives.
3174    </t>
3175    <t>
3176      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
3177      whitespace ("OWS") and required whitespace ("RWS").
3178    </t>
3179    <t>
3180      Rewrite ABNFs to spell out whitespace rules, factor out
3181      header value format definitions.
3182    </t>
3183  </list>
3184</t>
3185</section>
3186
3187<section title="Since draft-ietf-httpbis-p6-cache-05" anchor="changes.since.05">
3188<t>
3189  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3190  <list style="symbols"> 
3191    <t>
3192      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
3193    </t>
3194  </list>
3195</t>
3196</section>
3197
3198</section>
3199
3200</back>
3201</rfc>
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