source: draft-ietf-httpbis/latest/p6-cache.xml @ 267

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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 "June">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY notation-abnf               "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY general-syntax              "<xref target='Part1' x:rel='#general.syntax' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33]>
34<?rfc toc="yes" ?>
35<?rfc symrefs="yes" ?>
36<?rfc sortrefs="yes" ?>
37<?rfc compact="yes"?>
38<?rfc subcompact="no" ?>
39<?rfc linkmailto="no" ?>
40<?rfc editing="no" ?>
41<?rfc comments="yes"?>
42<?rfc inline="yes"?>
43<?rfc-ext allow-markup-in-artwork="yes" ?>
44<?rfc-ext include-references-in-index="yes" ?>
45<rfc obsoletes="2616" category="std"
46     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
47     xmlns:x='http://purl.org/net/xml2rfc/ext'>
48<front>
49
50  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
51
52  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
53    <organization abbrev="Day Software">Day Software</organization>
54    <address>
55      <postal>
56        <street>23 Corporate Plaza DR, Suite 280</street>
57        <city>Newport Beach</city>
58        <region>CA</region>
59        <code>92660</code>
60        <country>USA</country>
61      </postal>
62      <phone>+1-949-706-5300</phone>
63      <facsimile>+1-949-706-5305</facsimile>
64      <email>fielding@gbiv.com</email>
65      <uri>http://roy.gbiv.com/</uri>
66    </address>
67  </author>
68
69  <author initials="J." surname="Gettys" fullname="Jim Gettys">
70    <organization>One Laptop per Child</organization>
71    <address>
72      <postal>
73        <street>21 Oak Knoll Road</street>
74        <city>Carlisle</city>
75        <region>MA</region>
76        <code>01741</code>
77        <country>USA</country>
78      </postal>
79      <email>jg@laptop.org</email>
80      <uri>http://www.laptop.org/</uri>
81    </address>
82  </author>
83 
84  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
85    <organization abbrev="HP">Hewlett-Packard Company</organization>
86    <address>
87      <postal>
88        <street>HP Labs, Large Scale Systems Group</street>
89        <street>1501 Page Mill Road, MS 1177</street>
90        <city>Palo Alto</city>
91        <region>CA</region>
92        <code>94304</code>
93        <country>USA</country>
94      </postal>
95      <email>JeffMogul@acm.org</email>
96    </address>
97  </author>
98
99  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107        <country>USA</country>
108      </postal>
109      <email>henrikn@microsoft.com</email>
110    </address>
111  </author>
112
113  <author initials="L." surname="Masinter" fullname="Larry Masinter">
114    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
115    <address>
116      <postal>
117        <street>345 Park Ave</street>
118        <city>San Jose</city>
119        <region>CA</region>
120        <code>95110</code>
121        <country>USA</country>
122      </postal>
123      <email>LMM@acm.org</email>
124      <uri>http://larry.masinter.net/</uri>
125    </address>
126  </author>
127 
128  <author initials="P." surname="Leach" fullname="Paul J. Leach">
129    <organization abbrev="Microsoft">Microsoft Corporation</organization>
130    <address>
131      <postal>
132        <street>1 Microsoft Way</street>
133        <city>Redmond</city>
134        <region>WA</region>
135        <code>98052</code>
136      </postal>
137      <email>paulle@microsoft.com</email>
138    </address>
139  </author>
140   
141  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
142    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
143    <address>
144      <postal>
145        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
146        <street>The Stata Center, Building 32</street>
147        <street>32 Vassar Street</street>
148        <city>Cambridge</city>
149        <region>MA</region>
150        <code>02139</code>
151        <country>USA</country>
152      </postal>
153      <email>timbl@w3.org</email>
154      <uri>http://www.w3.org/People/Berners-Lee/</uri>
155    </address>
156  </author>
157
158  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
159    <organization abbrev="W3C">World Wide Web Consortium</organization>
160    <address>
161      <postal>
162        <street>W3C / ERCIM</street>
163        <street>2004, rte des Lucioles</street>
164        <city>Sophia-Antipolis</city>
165        <region>AM</region>
166        <code>06902</code>
167        <country>France</country>
168      </postal>
169      <email>ylafon@w3.org</email>
170      <uri>http://www.raubacapeu.net/people/yves/</uri>
171    </address>
172  </author>
173
174  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
175    <organization abbrev="greenbytes">greenbytes GmbH</organization>
176    <address>
177      <postal>
178        <street>Hafenweg 16</street>
179        <city>Muenster</city><region>NW</region><code>48155</code>
180        <country>Germany</country>
181      </postal>
182      <phone>+49 251 2807760</phone>   
183      <facsimile>+49 251 2807761</facsimile>   
184      <email>julian.reschke@greenbytes.de</email>       
185      <uri>http://greenbytes.de/tech/webdav/</uri>     
186    </address>
187  </author>
188
189  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
190
191<abstract>
192<t>
193   The Hypertext Transfer Protocol (HTTP) is an application-level
194   protocol for distributed, collaborative, hypermedia information
195   systems. HTTP has been in use by the World Wide Web global information
196   initiative since 1990. This document is Part 6 of the seven-part specification
197   that defines the protocol referred to as "HTTP/1.1" and, taken together,
198   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
199   and the associated header fields that control cache behavior or indicate
200   cacheable response messages.
201</t>
202</abstract>
203
204<note title="Editorial Note (To be removed by RFC Editor)">
205  <t>
206    Discussion of this draft should take place on the HTTPBIS working group
207    mailing list (ietf-http-wg@w3.org). The current issues list is
208    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
209    and related documents (including fancy diffs) can be found at
210    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
211  </t>
212  <t>
213    The changes in this draft are summarized in <xref target="changes.since.02"/>.
214  </t>
215</note>
216</front>
217<middle>
218<section title="Introduction" anchor="caching">
219<t>
220   HTTP is typically used for distributed information systems, where
221   performance can be improved by the use of response caches, and includes
222   a number of elements intended to make caching work as well as possible.
223   Because these elements interact with each other, it is useful to describe
224   the caching design of HTTP separately.  This document defines aspects of
225   HTTP/1.1 related to caching and reusing response messages.
226</t>
227
228<section title="Purpose" anchor="intro.purpose">
229<iref item="cache"/>
230<t>
231   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
232   and the subsystem that controls its message storage, retrieval, and
233   deletion. A cache stores cacheable responses in order to reduce the
234   response time and network bandwidth consumption on future, equivalent
235   requests. Any client or server may include a cache, though a cache
236   cannot be used by a server that is acting as a tunnel.
237</t>
238<t>
239   Caching would be useless if it did not significantly improve
240   performance. The goal of caching in HTTP/1.1 is to reuse a prior response
241   message to satisfy a current request.  In some cases, the existing response
242   can be reused without the need for a network request, reducing latency and
243   network round-trips; we use an "expiration" mechanism for this purpose
244   (see <xref target="expiration.model"/>).  Even when a new request is required,
245   it is often possible to reuse all or parts of the payload of a prior response
246   to satisfy the request, thereby reducing network bandwidth usage; we use a
247   "validation" mechanism for this purpose (see <xref target="validation.model"/>).
248</t>
249<iref item="semantically transparent"/>
250<t>
251   A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
252   respect to a particular response, when its use affects neither the
253   requesting client nor the origin server, except to improve
254   performance. When a cache is semantically transparent, the client
255   receives exactly the same response status and payload
256   that it would have received had its request been handled directly
257   by the origin server.
258</t>
259<t>
260   In an ideal world, all interactions with an HTTP cache would be
261   semantically transparent.  However, for some resources, semantic
262   transparency is not always necessary and can be effectively traded
263   for the sake of bandwidth scaling, disconnected operation, and
264   high availability.  HTTP/1.1 allows origin servers, caches,
265   and clients to explicitly reduce transparency when necessary.
266   However, because non-transparent operation may confuse non-expert
267   users and might be incompatible with certain server applications
268   (such as those for ordering merchandise), the protocol requires that
269   transparency be relaxed
270  <list style="symbols">
271     <t>only by an explicit protocol-level request when relaxed by
272        client or origin server</t>
273
274     <t>only with an explicit warning to the end user when relaxed by
275        cache or client</t>
276  </list>
277</t>
278<t>
279   Therefore, HTTP/1.1 provides these important elements:
280  <list style="numbers">
281      <t>Protocol features that provide full semantic transparency when
282         this is required by all parties.</t>
283
284      <t>Protocol features that allow an origin server or user agent to
285         explicitly request and control non-transparent operation.</t>
286
287      <t>Protocol features that allow a cache to attach warnings to
288         responses that do not preserve the requested approximation of
289         semantic transparency.</t>
290  </list>
291</t>
292<t>
293   A basic principle is that it must be possible for the clients to
294   detect any potential relaxation of semantic transparency.
295  <list><t>
296      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
297      design decisions not explicitly discussed in this specification.
298      If a decision might affect semantic transparency, the implementor
299      ought to err on the side of maintaining transparency unless a
300      careful and complete analysis shows significant benefits in
301      breaking transparency.
302    </t></list>
303</t>
304</section>
305
306<section title="Terminology" anchor="intro.terminology">
307<t>
308   This specification uses a number of terms to refer to the roles
309   played by participants in, and objects of, HTTP caching.
310</t>
311<t>
312  <iref item="cacheable"/>
313  <x:dfn>cacheable</x:dfn>
314  <list>
315    <t>
316      A response is cacheable if a cache is allowed to store a copy of
317      the response message for use in answering subsequent requests.
318      Even when a response is cacheable, there may
319      be additional constraints on whether a cache can use the cached
320      copy for a particular request.
321    </t>
322  </list>
323</t>
324<t>
325  <iref item="first-hand"/>
326  <x:dfn>first-hand</x:dfn>
327  <list>
328    <t>
329      A response is first-hand if it comes directly and without
330      unnecessary delay from the origin server, perhaps via one or more
331      proxies. A response is also first-hand if its validity has just
332      been checked directly with the origin server.
333    </t>
334  </list>
335</t>
336<t>
337  <iref item="explicit expiration time"/>
338  <x:dfn>explicit expiration time</x:dfn>
339  <list>
340    <t>
341      The time at which the origin server intends that an entity should
342      no longer be returned by a cache without further validation.
343    </t>
344  </list>
345</t>
346<t>
347  <iref item="heuristic expiration time"/>
348  <x:dfn>heuristic expiration time</x:dfn>
349  <list>
350    <t>
351      An expiration time assigned by a cache when no explicit expiration
352      time is available.
353    </t>
354  </list>
355</t>
356<t>
357  <iref item="age"/>
358  <x:dfn>age</x:dfn>
359  <list>
360    <t>
361      The age of a response is the time since it was sent by, or
362      successfully validated with, the origin server.
363    </t>
364  </list>
365</t>
366<t>
367  <iref item="freshness lifetime"/>
368  <x:dfn>freshness lifetime</x:dfn>
369  <list>
370    <t>
371      The length of time between the generation of a response and its
372      expiration time.
373    </t>
374  </list>
375</t>
376<t>
377  <iref item="fresh"/>
378  <x:dfn>fresh</x:dfn>
379  <list>
380    <t>
381      A response is fresh if its age has not yet exceeded its freshness
382      lifetime.
383    </t>
384  </list>
385</t>
386<t>
387  <iref item="stale"/>
388  <x:dfn>stale</x:dfn>
389  <list>
390    <t>
391      A response is stale if its age has passed its freshness lifetime.
392    </t>
393  </list>
394</t>
395<t>
396  <iref item="validator"/>
397  <x:dfn>validator</x:dfn>
398  <list>
399    <t>
400      A protocol element (e.g., an entity tag or a Last-Modified time)
401      that is used to find out whether a cache entry is an equivalent
402      copy of an entity.
403    </t>
404  </list>
405</t>
406</section>
407
408<section title="Requirements" anchor="intro.requirements">
409<t>
410   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
411   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
412   document are to be interpreted as described in <xref target="RFC2119"/>.
413</t>
414<t>
415   An implementation is not compliant if it fails to satisfy one or more
416   of the &MUST; or &REQUIRED; level requirements for the protocols it
417   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
418   level and all the &SHOULD; level requirements for its protocols is said
419   to be "unconditionally compliant"; one that satisfies all the &MUST;
420   level requirements but not all the &SHOULD; level requirements for its
421   protocols is said to be "conditionally compliant."
422</t>
423</section>
424</section>
425
426
427<section title="Notational Conventions and Generic Grammar" anchor="notation">
428  <x:anchor-alias value="DIGIT"/>
429  <x:anchor-alias value="DQUOTE"/>
430  <x:anchor-alias value="quoted-string"/>
431  <x:anchor-alias value="SP"/>
432  <x:anchor-alias value="token"/>
433<t>
434  This specification uses the ABNF syntax defined in &notation-abnf; and
435  the core rules defined in &basic-rules;:
436  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
437  <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>.</cref>
438</t>
439<figure><artwork type="abnf2616">
440  <x:ref>DIGIT</x:ref>         = &lt;DIGIT, defined in &basic-rules;&gt;
441  <x:ref>DQUOTE</x:ref>        = &lt;DQUOTE, defined in &basic-rules;&gt;
442  <x:ref>SP</x:ref>            = &lt;SP, defined in &basic-rules;&gt;
443</artwork></figure>
444<figure><artwork type="abnf2616">
445  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
446  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
447</artwork></figure>
448<t anchor="abnf.dependencies">
449  <x:anchor-alias value="field-name"/>
450  <x:anchor-alias value="HTTP-date"/>
451  <x:anchor-alias value="port"/>
452  <x:anchor-alias value="pseudonym"/>
453  <x:anchor-alias value="uri-host"/>
454  The ABNF rules below are defined in other parts:
455</t>
456<figure><!--Part1--><artwork type="abnf2616">
457  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
458  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
459  <x:ref>port</x:ref>          = &lt;port, defined in &general-syntax;&gt;
460  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
461  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &general-syntax;&gt;
462</artwork></figure>
463</section>
464
465<section title="Overview" anchor="caching.overview">
466<section title="Cache Correctness" anchor="cache.correctness">
467<t>
468   A correct cache &MUST; respond to a request with the most up-to-date
469   response held by the cache that is appropriate to the request (see
470   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
471   <xref target="disambiguating.multiple.responses" format="counter"/>,
472   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
473   conditions:
474  <list style="numbers">
475      <t>It has been checked for equivalence with what the origin server
476         would have returned by revalidating the response with the
477         origin server (<xref target="validation.model"/>);</t>
478
479      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
480         this means it meets the least restrictive freshness requirement
481         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
482         the origin server so specifies, it is the freshness requirement
483         of the origin server alone.
484
485         If a stored response is not "fresh enough" by the most
486         restrictive freshness requirement of both the client and the
487         origin server, in carefully considered circumstances the cache
488         &MAY; still return the response with the appropriate Warning
489         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
490         and <xref target="header.warning" format="counter"/>), unless such a response
491         is prohibited (e.g., by a "no-store" cache-directive, or by a
492         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
493
494      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
495         or error (4xx or 5xx) response message.</t>
496  </list>
497</t>
498<t>
499   If the cache can not communicate with the origin server, then a
500   correct cache &SHOULD; respond as above if the response can be
501   correctly served from the cache; if not it &MUST; return an error or
502   warning indicating that there was a communication failure.
503</t>
504<t>
505   If a cache receives a response (either an entire response, or a 304
506   (Not Modified) response) that it would normally forward to the
507   requesting client, and the received response is no longer fresh, the
508   cache &SHOULD; forward it to the requesting client without adding a new
509   Warning (but without removing any existing Warning headers). A cache
510   &SHOULD-NOT;  attempt to revalidate a response simply because that
511   response became stale in transit; this might lead to an infinite
512   loop. A user agent that receives a stale response without a Warning
513   &MAY; display a warning indication to the user.
514</t>
515</section>
516
517<section title="Warnings" anchor="warnings">
518<t>
519   Whenever a cache returns a response that is neither first-hand nor
520   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
521   &MUST; attach a warning to that effect, using a Warning general-header.
522   The Warning header and the currently defined warnings are described
523   in <xref target="header.warning"/>. The warning allows clients to take appropriate
524   action.
525</t>
526<t>
527   Warnings &MAY; be used for other purposes, both cache-related and
528   otherwise. The use of a warning, rather than an error status code,
529   distinguish these responses from true failures.
530</t>
531<t>
532   Warnings are assigned three digit warn-codes. The first digit
533   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
534   stored cache entry after a successful revalidation:
535</t>
536<t>
537  <list style="hanging">
538    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
539     the response, and so &MUST; be deleted after a successful
540     revalidation. 1xx warn-codes &MAY; be generated by a cache only when
541     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
542
543    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
544     headers that is not rectified by a revalidation (for example, a
545     lossy compression of the entity bodies) and which &MUST-NOT; be
546     deleted after a successful revalidation.</t>
547    </list>
548</t>
549<t>
550   See <xref target="header.warning"/> for the definitions of the codes themselves.
551</t>
552<t>
553   HTTP/1.0 caches will cache all Warnings in responses, without
554   deleting the ones in the first category. Warnings in responses that
555   are passed to HTTP/1.0 caches carry an extra warning-date field,
556   which prevents a future HTTP/1.1 recipient from believing an
557   erroneously cached Warning.
558</t>
559<t>
560   Warnings also carry a warning text. The text &MAY; be in any
561   appropriate natural language (perhaps based on the client's Accept
562   headers), and include an &OPTIONAL; indication of what character set is
563   used.
564</t>
565<t>
566   Multiple warnings &MAY; be attached to a response (either by the origin
567   server or by a cache), including multiple warnings with the same code
568   number. For example, a server might provide the same warning with
569   texts in both English and Basque.
570</t>
571<t>
572   When multiple warnings are attached to a response, it might not be
573   practical or reasonable to display all of them to the user. This
574   version of HTTP does not specify strict priority rules for deciding
575   which warnings to display and in what order, but does suggest some
576   heuristics.
577</t>
578</section>
579
580<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
581<t>
582   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
583   times and validators) are implicit directives to caches. In some
584   cases, a server or client might need to provide explicit directives
585   to the HTTP caches. We use the Cache-Control header for this purpose.
586</t>
587<t>
588   The Cache-Control header allows a client or server to transmit a
589   variety of directives in either requests or responses. These
590   directives typically override the default caching algorithms. As a
591   general rule, if there is any apparent conflict between header
592   values, the most restrictive interpretation is applied (that is, the
593   one that is most likely to preserve semantic transparency). However,
594   in some cases, cache-control directives are explicitly specified as
595   weakening the approximation of semantic transparency (for example,
596   "max-stale" or "public").
597</t>
598<t>
599   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
600</t>
601</section>
602
603<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
604<t>
605   Many user agents make it possible for users to override the basic
606   caching mechanisms. For example, the user agent might allow the user
607   to specify that cached entities (even explicitly stale ones) are
608   never validated. Or the user agent might habitually add "Cache-Control:
609   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
610   default to either non-transparent behavior, or behavior that results
611   in abnormally ineffective caching, but &MAY; be explicitly configured
612   to do so by an explicit action of the user.
613</t>
614<t>
615   If the user has overridden the basic caching mechanisms, the user
616   agent &SHOULD; explicitly indicate to the user whenever this results in
617   the display of information that might not meet the server's
618   transparency requirements (in particular, if the displayed entity is
619   known to be stale). Since the protocol normally allows the user agent
620   to determine if responses are stale or not, this indication need only
621   be displayed when this actually happens. The indication need not be a
622   dialog box; it could be an icon (for example, a picture of a rotting
623   fish) or some other indicator.
624</t>
625<t>
626   If the user has overridden the caching mechanisms in a way that would
627   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
628   continually indicate this state to the user (for example, by a
629   display of a picture of currency in flames) so that the user does not
630   inadvertently consume excess resources or suffer from excessive
631   latency.
632</t>
633</section>
634
635<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
636<t>
637   In some cases, the operator of a cache &MAY; choose to configure it to
638   return stale responses even when not requested by clients. This
639   decision ought not be made lightly, but may be necessary for reasons
640   of availability or performance, especially when the cache is poorly
641   connected to the origin server. Whenever a cache returns a stale
642   response, it &MUST; mark it as such (using a Warning header) enabling
643   the client software to alert the user that there might be a potential
644   problem.
645</t>
646<t>
647   It also allows the user agent to take steps to obtain a first-hand or
648   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
649   response if the client explicitly requests a first-hand or fresh one,
650   unless it is impossible to comply for technical or policy reasons.
651</t>
652</section>
653
654<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
655<t>
656   While the origin server (and to a lesser extent, intermediate caches,
657   by their contribution to the age of a response) are the primary
658   source of expiration information, in some cases the client might need
659   to control a cache's decision about whether to return a cached
660   response without validating it. Clients do this using several
661   directives of the Cache-Control header.
662</t>
663<t>
664   A client's request &MAY; specify the maximum age it is willing to
665   accept of an unvalidated response; specifying a value of zero forces
666   the cache(s) to revalidate all responses. A client &MAY; also specify
667   the minimum time remaining before a response expires. Both of these
668   options increase constraints on the behavior of caches, and so cannot
669   further relax the cache's approximation of semantic transparency.
670</t>
671<t>
672   A client &MAY; also specify that it will accept stale responses, up to
673   some maximum amount of staleness. This loosens the constraints on the
674   caches, and so might violate the origin server's specified
675   constraints on semantic transparency, but might be necessary to
676   support disconnected operation, or high availability in the face of
677   poor connectivity.
678</t>
679</section>
680</section>
681
682<section title="Expiration Model" anchor="expiration.model">
683
684<section title="Server-Specified Expiration" anchor="server-specified.expiration">
685<t>
686   HTTP caching works best when caches can entirely avoid making
687   requests to the origin server. The primary mechanism for avoiding
688   requests is for an origin server to provide an explicit expiration
689   time in the future, indicating that a response &MAY; be used to satisfy
690   subsequent requests. In other words, a cache can return a fresh
691   response without first contacting the server.
692</t>
693<t>
694   Our expectation is that servers will assign future explicit
695   expiration times to responses in the belief that the entity is not
696   likely to change, in a semantically significant way, before the
697   expiration time is reached. This normally preserves semantic
698   transparency, as long as the server's expiration times are carefully
699   chosen.
700</t>
701<t>
702   The expiration mechanism applies only to responses taken from a cache
703   and not to first-hand responses forwarded immediately to the
704   requesting client.
705</t>
706<t>
707   If an origin server wishes to force a semantically transparent cache
708   to validate every request, it &MAY; assign an explicit expiration time
709   in the past. This means that the response is always stale, and so the
710   cache &SHOULD; validate it before using it for subsequent requests. See
711   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
712</t>
713<t>
714   If an origin server wishes to force any HTTP/1.1 cache, no matter how
715   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
716   cache-control directive (see <xref target="header.cache-control"/>).
717</t>
718<t>
719   Servers specify explicit expiration times using either the Expires
720   header, or the max-age directive of the Cache-Control header.
721</t>
722<t>
723   An expiration time cannot be used to force a user agent to refresh
724   its display or reload a resource; its semantics apply only to caching
725   mechanisms, and such mechanisms need only check a resource's
726   expiration status when a new request for that resource is initiated.
727   See <xref target="history.lists"/> for an explanation of the difference between caches
728   and history mechanisms.
729</t>
730</section>
731
732<section title="Heuristic Expiration" anchor="heuristic.expiration">
733<t>
734   Since origin servers do not always provide explicit expiration times,
735   HTTP caches typically assign heuristic expiration times, employing
736   algorithms that use other header values (such as the Last-Modified
737   time) to estimate a plausible expiration time. The HTTP/1.1
738   specification does not provide specific algorithms, but does impose
739   worst-case constraints on their results. Since heuristic expiration
740   times might compromise semantic transparency, they ought to be used
741   cautiously, and we encourage origin servers to provide explicit
742   expiration times as much as possible.
743</t>
744</section>
745
746<section title="Age Calculations" anchor="age.calculations">
747<t>
748   In order to know if a cached entry is fresh, a cache needs to know if
749   its age exceeds its freshness lifetime. We discuss how to calculate
750   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
751   the age of a response or cache entry.
752</t>
753<t>
754   In this discussion, we use the term "now" to mean "the current value
755   of the clock at the host performing the calculation." Hosts that use
756   HTTP, but especially hosts running origin servers and caches, &SHOULD;
757   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
758   a globally accurate time standard.
759</t>
760<t>
761   HTTP/1.1 requires origin servers to send a Date header, if possible,
762   with every response, giving the time at which the response was
763   generated (see &header-date;). We use the term "date_value" to denote
764   the value of the Date header, in a form appropriate for arithmetic
765   operations.
766</t>
767<t>
768   HTTP/1.1 uses the Age response-header to convey the estimated age of
769   the response message when obtained from a cache. The Age field value
770   is the cache's estimate of the amount of time since the response was
771   generated or revalidated by the origin server.
772</t>
773<t>
774   In essence, the Age value is the sum of the time that the response
775   has been resident in each of the caches along the path from the
776   origin server, plus the amount of time it has been in transit along
777   network paths.
778</t>
779<t>
780   We use the term "age_value" to denote the value of the Age header, in
781   a form appropriate for arithmetic operations.
782</t>
783<t>
784   A response's age can be calculated in two entirely independent ways:
785  <list style="numbers">
786      <t>now minus date_value, if the local clock is reasonably well
787         synchronized to the origin server's clock. If the result is
788         negative, the result is replaced by zero.</t>
789
790      <t>age_value, if all of the caches along the response path
791         implement HTTP/1.1.</t>
792  </list>
793</t>
794<t>
795   Given that we have two independent ways to compute the age of a
796   response when it is received, we can combine these as
797</t>
798<figure><artwork type="code">
799    corrected_received_age = max(now - date_value, age_value)
800</artwork></figure>
801<t>
802   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
803   paths, one gets a reliable (conservative) result.
804</t>
805<t>
806   Because of network-imposed delays, some significant interval might
807   pass between the time that a server generates a response and the time
808   it is received at the next outbound cache or client. If uncorrected,
809   this delay could result in improperly low ages.
810</t>
811<t>
812   Because the request that resulted in the returned Age value must have
813   been initiated prior to that Age value's generation, we can correct
814   for delays imposed by the network by recording the time at which the
815   request was initiated. Then, when an Age value is received, it &MUST;
816   be interpreted relative to the time the request was initiated, not
817   the time that the response was received. This algorithm results in
818   conservative behavior no matter how much delay is experienced. So, we
819   compute:
820</t>
821<figure><artwork type="code">
822   corrected_initial_age = corrected_received_age
823                         + (now - request_time)
824</artwork></figure>
825<t>
826   where "request_time" is the time (according to the local clock) when
827   the request that elicited this response was sent.
828</t>
829<t>
830   Summary of age calculation algorithm, when a cache receives a
831   response:
832</t>
833<figure><artwork type="code">
834   /*
835    * age_value
836    *      is the value of Age: header received by the cache with
837    *              this response.
838    * date_value
839    *      is the value of the origin server's Date: header
840    * request_time
841    *      is the (local) time when the cache made the request
842    *              that resulted in this cached response
843    * response_time
844    *      is the (local) time when the cache received the
845    *              response
846    * now
847    *      is the current (local) time
848    */
849
850   apparent_age = max(0, response_time - date_value);
851   corrected_received_age = max(apparent_age, age_value);
852   response_delay = response_time - request_time;
853   corrected_initial_age = corrected_received_age + response_delay;
854   resident_time = now - response_time;
855   current_age   = corrected_initial_age + resident_time;
856</artwork></figure>
857<t>
858   The current_age of a cache entry is calculated by adding the amount
859   of time (in seconds) since the cache entry was last validated by the
860   origin server to the corrected_initial_age. When a response is
861   generated from a cache entry, the cache &MUST; include a single Age
862   header field in the response with a value equal to the cache entry's
863   current_age.
864</t>
865<t>
866   The presence of an Age header field in a response implies that a
867   response is not first-hand. However, the converse is not true, since
868   the lack of an Age header field in a response does not imply that the
869   response is first-hand unless all caches along the request path are
870   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
871   the Age header field).
872</t>
873</section>
874
875<section title="Expiration Calculations" anchor="expiration.calculations">
876<t>
877   In order to decide whether a response is fresh or stale, we need to
878   compare its freshness lifetime to its age. The age is calculated as
879   described in <xref target="age.calculations"/>; this section describes how to calculate
880   the freshness lifetime, and to determine if a response has expired.
881   In the discussion below, the values can be represented in any form
882   appropriate for arithmetic operations.
883</t>
884<t>
885   We use the term "expires_value" to denote the value of the Expires
886   header. We use the term "max_age_value" to denote an appropriate
887   value of the number of seconds carried by the "max-age" directive of
888   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
889</t>
890<t>
891   The max-age directive takes priority over Expires, so if max-age is
892   present in a response, the calculation is simply:
893</t>
894<figure><artwork type="code">
895   freshness_lifetime = max_age_value
896</artwork></figure>
897<t>
898   Otherwise, if Expires is present in the response, the calculation is:
899</t>
900<figure><artwork type="code">
901   freshness_lifetime = expires_value - date_value
902</artwork></figure>
903<t>
904   Note that neither of these calculations is vulnerable to clock skew,
905   since all of the information comes from the origin server.
906</t>
907<t>
908   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
909   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
910   does not include other restrictions on caching, the cache &MAY; compute
911   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
912   113 to any response whose age is more than 24 hours if such warning
913   has not already been added.
914</t>
915<t>
916   Also, if the response does have a Last-Modified time, the heuristic
917   expiration value &SHOULD; be no more than some fraction of the interval
918   since that time. A typical setting of this fraction might be 10%.
919</t>
920<t>
921   The calculation to determine if a response has expired is quite
922   simple:
923</t>
924<figure><artwork type="code">
925   response_is_fresh = (freshness_lifetime &gt; current_age)
926</artwork></figure>
927</section>
928
929<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
930<t>
931   Because expiration values are assigned optimistically, it is possible
932   for two caches to contain fresh values for the same resource that are
933   different.
934</t>
935<t>
936   If a client performing a retrieval receives a non-first-hand response
937   for a request that was already fresh in its own cache, and the Date
938   header in its existing cache entry is newer than the Date on the new
939   response, then the client &MAY; ignore the response. If so, it &MAY;
940   retry the request with a "Cache-Control: max-age=0" directive (see
941   <xref target="header.cache-control"/>), to force a check with the origin server.
942</t>
943<t>
944   If a cache has two fresh responses for the same representation with
945   different validators, it &MUST; use the one with the more recent Date
946   header. This situation might arise because the cache is pooling
947   responses from other caches, or because a client has asked for a
948   reload or a revalidation of an apparently fresh cache entry.
949</t>
950</section>
951
952<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
953<t>
954   Because a client might be receiving responses via multiple paths, so
955   that some responses flow through one set of caches and other
956   responses flow through a different set of caches, a client might
957   receive responses in an order different from that in which the origin
958   server sent them. We would like the client to use the most recently
959   generated response, even if older responses are still apparently
960   fresh.
961</t>
962<t>
963   Neither the entity tag nor the expiration value can impose an
964   ordering on responses, since it is possible that a later response
965   intentionally carries an earlier expiration time. The Date values are
966   ordered to a granularity of one second.
967</t>
968<t>
969   When a client tries to revalidate a cache entry, and the response it
970   receives contains a Date header that appears to be older than the one
971   for the existing entry, then the client &SHOULD; repeat the request
972   unconditionally, and include
973</t>
974<figure><artwork type="example">
975    Cache-Control: max-age=0
976</artwork></figure>
977<t>
978   to force any intermediate caches to validate their copies directly
979   with the origin server, or
980</t>
981<figure><artwork type="example">
982    Cache-Control: no-cache
983</artwork></figure>
984<t>
985   to force any intermediate caches to obtain a new copy from the origin
986   server.
987</t>
988<t>
989   If the Date values are equal, then the client &MAY; use either response
990   (or &MAY;, if it is being extremely prudent, request a new response).
991   Servers &MUST-NOT; depend on clients being able to choose
992   deterministically between responses generated during the same second,
993   if their expiration times overlap.
994</t>
995</section>
996</section>
997
998<section title="Validation Model" anchor="validation.model">
999<t>
1000   When a cache has a stale entry that it would like to use as a
1001   response to a client's request, it first has to check with the origin
1002   server (or possibly an intermediate cache with a fresh response) to
1003   see if its cached entry is still usable. We call this "validating"
1004   the cache entry.
1005</t>
1006<t>
1007   HTTP's conditional request mechanism, defined in &conditional;, is
1008   used to avoid retransmitting the response payload when the cached entry
1009   is valid.  When a cached response includes one or more "cache validators,"
1010   such as the field values of an ETag or Last-Modified header field, then
1011   a validating GET request &SHOULD; be made conditional to those field values.
1012   The server checks the conditional request's validator against the current
1013   state of the requested resource and, if they match, the server responds
1014   with a 304 (Not Modified) status code to indicate that the cached response
1015   can be refreshed and reused without retransmitting the response payload.
1016   If the validator does not match the current state of the requested
1017   resource, then the server returns a full response, including payload,
1018   so that the request can be satisfied and the cache entry supplanted
1019   without the need for an additional network round-trip.
1020</t>
1021</section>
1022
1023<section title="Response Cacheability" anchor="response.cacheability">
1024<t>
1025   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1026   directive, a caching system &MAY; always store a successful response
1027   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1028   if it is fresh, and &MAY; return it after successful validation. If
1029   there is neither a cache validator nor an explicit expiration time
1030   associated with a response, we do not expect it to be cached, but
1031   certain caches &MAY; violate this expectation (for example, when little
1032   or no network connectivity is available). A client can usually detect
1033   that such a response was taken from a cache by comparing the Date
1034   header to the current time.
1035  <list><t>
1036      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1037      without providing any Warning.
1038  </t></list>
1039</t>
1040<t>
1041   However, in some cases it might be inappropriate for a cache to
1042   retain an entity, or to return it in response to a subsequent
1043   request. This might be because absolute semantic transparency is
1044   deemed necessary by the service author, or because of security or
1045   privacy considerations. Certain cache-control directives are
1046   therefore provided so that the server can indicate that certain
1047   resource entities, or portions thereof, are not to be cached
1048   regardless of other considerations.
1049</t>
1050<t>
1051   Note that &header-authorization; normally prevents a shared cache from saving
1052   and returning a response to a previous request if that request
1053   included an Authorization header.
1054</t>
1055<t>
1056   A response received with a status code of 200, 203, 206, 300, 301 or
1057   410 &MAY; be stored by a cache and used in reply to a subsequent
1058   request, subject to the expiration mechanism, unless a cache-control
1059   directive prohibits caching. However, a cache that does not support
1060   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1061   Content) responses.
1062</t>
1063<t>
1064   A response received with any other status code (e.g. status codes 302
1065   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1066   unless there are cache-control directives or another header(s) that
1067   explicitly allow it. For example, these include the following: an
1068   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1069   "proxy-revalidate", "public" or "private" cache-control
1070   directive (<xref target="header.cache-control"/>).
1071</t>
1072</section>
1073
1074<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1075<t>
1076   The purpose of an HTTP cache is to store information received in
1077   response to requests for use in responding to future requests. In
1078   many cases, a cache simply returns the appropriate parts of a
1079   response to the requester. However, if the cache holds a cache entry
1080   based on a previous response, it might have to combine parts of a new
1081   response with what is held in the cache entry.
1082</t>
1083
1084<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1085<t>
1086   For the purpose of defining the behavior of caches and non-caching
1087   proxies, we divide HTTP headers into two categories:
1088  <list style="symbols">
1089      <t>End-to-end headers, which are  transmitted to the ultimate
1090        recipient of a request or response. End-to-end headers in
1091        responses &MUST; be stored as part of a cache entry and &MUST; be
1092        transmitted in any response formed from a cache entry.</t>
1093
1094      <t>Hop-by-hop headers, which are meaningful only for a single
1095        transport-level connection, and are not stored by caches or
1096        forwarded by proxies.</t>
1097  </list>
1098</t>
1099<t>
1100   The following HTTP/1.1 headers are hop-by-hop headers:
1101  <list style="symbols">
1102      <t>Connection</t>
1103      <t>Keep-Alive</t>
1104      <t>Proxy-Authenticate</t>
1105      <t>Proxy-Authorization</t>
1106      <t>TE</t>
1107      <t>Trailer</t>
1108      <t>Transfer-Encoding</t>
1109      <t>Upgrade</t>
1110  </list>
1111</t>
1112<t>
1113   All other headers defined by HTTP/1.1 are end-to-end headers.
1114</t>
1115<t>
1116   Other hop-by-hop headers &MUST; be listed in a Connection header
1117   (&header-connection;).
1118</t>
1119</section>
1120
1121<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1122<t>
1123   Some features of HTTP/1.1, such as Digest
1124   Authentication, depend on the value of certain end-to-end headers. A
1125   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1126   definition of that header requires or specifically allows that.
1127</t>
1128<t>
1129   A transparent proxy &MUST-NOT; modify any of the following fields in a
1130   request or response, and it &MUST-NOT; add any of these fields if not
1131   already present:
1132  <list style="symbols">
1133      <t>Content-Location</t>
1134      <t>Content-MD5</t>
1135      <t>ETag</t>
1136      <t>Last-Modified</t>
1137  </list>
1138</t>
1139<t>
1140   A transparent proxy &MUST-NOT; modify any of the following fields in a
1141   response:
1142  <list style="symbols">
1143    <t>Expires</t>
1144  </list>
1145</t>
1146<t>
1147   but it &MAY; add any of these fields if not already present. If an
1148   Expires header is added, it &MUST; be given a field-value identical to
1149   that of the Date header in that response.
1150</t>
1151<t>
1152   A  proxy &MUST-NOT; modify or add any of the following fields in a
1153   message that contains the no-transform cache-control directive, or in
1154   any request:
1155  <list style="symbols">
1156    <t>Content-Encoding</t>
1157    <t>Content-Range</t>
1158    <t>Content-Type</t>
1159  </list>
1160</t>
1161<t>
1162   A non-transparent proxy &MAY; modify or add these fields to a message
1163   that does not include no-transform, but if it does so, it &MUST; add a
1164   Warning 214 (Transformation applied) if one does not already appear
1165   in the message (see <xref target="header.warning"/>).
1166  <list><t>
1167      Warning: unnecessary modification of end-to-end headers might
1168      cause authentication failures if stronger authentication
1169      mechanisms are introduced in later versions of HTTP. Such
1170      authentication mechanisms &MAY; rely on the values of header fields
1171      not listed here.
1172    </t></list>
1173</t>
1174<t>
1175   The Content-Length field of a request or response is added or deleted
1176   according to the rules in &message-length;. A transparent proxy &MUST;
1177   preserve the entity-length (&entity-length;) of the entity-body,
1178   although it &MAY; change the transfer-length (&message-length;).
1179</t>
1180</section>
1181
1182<section title="Combining Headers" anchor="combining.headers">
1183<t>
1184   When a cache makes a validating request to a server, and the server
1185   provides a 304 (Not Modified) response or a 206 (Partial Content)
1186   response, the cache then constructs a response to send to the
1187   requesting client.
1188</t>
1189<t>
1190   If the status code is 304 (Not Modified), the cache uses the entity-body
1191   stored in the cache entry as the entity-body of this outgoing
1192   response. If the status code is 206 (Partial Content) and the ETag or
1193   Last-Modified headers match exactly, the cache &MAY; combine the
1194   contents stored in the cache entry with the new contents received in
1195   the response and use the result as the entity-body of this outgoing
1196   response, (see &combining-byte-ranges;).
1197</t>
1198<t>
1199   The end-to-end headers stored in the cache entry are used for the
1200   constructed response, except that
1201  <list style="symbols">
1202    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1203      &MUST; be deleted from the cache entry and the forwarded response.</t>
1204    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1205        in the cache entry and the forwarded response.</t>
1206    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1207        replace the corresponding headers from the cache entry.</t>
1208  </list>
1209</t>
1210<t>
1211   Unless the cache decides to remove the cache entry, it &MUST; also
1212   replace the end-to-end headers stored with the cache entry with
1213   corresponding headers received in the incoming response, except for
1214   Warning headers as described immediately above. If a header field-name
1215   in the incoming response matches more than one header in the
1216   cache entry, all such old headers &MUST; be replaced.
1217</t>
1218<t>
1219   In other words, the set of end-to-end headers received in the
1220   incoming response overrides all corresponding end-to-end headers
1221   stored with the cache entry (except for stored Warning headers with
1222   warn-code 1xx, which are deleted even if not overridden).
1223  <list><t>
1224      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1225      Modified) or a 206 (Partial Content) response to update any header
1226      associated with a previous response for the same entity or sub-ranges
1227      thereof, although it might not always be meaningful or
1228      correct to do so. This rule does not allow an origin server to use
1229      a 304 (Not Modified) or a 206 (Partial Content) response to
1230      entirely delete a header that it had provided with a previous
1231      response.
1232  </t></list>
1233</t>
1234</section>
1235
1236</section>
1237
1238<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1239<t>
1240   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1241   by the presence of a Vary header field in a response, alters the
1242   conditions and procedure by which a cache can use the response for
1243   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1244   field by servers.
1245</t>
1246<t>
1247   A server &SHOULD; use the Vary header field to inform a cache of what
1248   request-header fields were used to select among multiple
1249   representations of a cacheable response subject to server-driven
1250   negotiation. The set of header fields named by the Vary field value
1251   is known as the "selecting" request-headers.
1252</t>
1253<t>
1254   When the cache receives a subsequent request whose Request-URI
1255   specifies one or more cache entries including a Vary header field,
1256   the cache &MUST-NOT; use such a cache entry to construct a response to
1257   the new request unless all of the selecting request-headers present
1258   in the new request match the corresponding stored request-headers in
1259   the original request.
1260</t>
1261<t>
1262   The selecting request-headers from two requests are defined to match
1263   if and only if the selecting request-headers in the first request can
1264   be transformed to the selecting request-headers in the second request
1265   by adding or removing linear white space (LWS) at places where this
1266   is allowed by the corresponding BNF, and/or combining multiple
1267   message-header fields with the same field name following the rules
1268   about message headers in &message-headers;.
1269</t>
1270<t>
1271   A Vary header field-value of "*" always fails to match and subsequent
1272   requests on that resource can only be properly interpreted by the
1273   origin server.
1274</t>
1275<t>
1276   If the selecting request header fields for the cached entry do not
1277   match the selecting request header fields of the new request, then
1278   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1279   it first relays the new request to the origin server in a conditional
1280   request and the server responds with 304 (Not Modified), including an
1281   entity tag or Content-Location that indicates the entity to be used.
1282</t>
1283<t>
1284   If an entity tag was assigned to a cached representation, the
1285   forwarded request &SHOULD; be conditional and include the entity tags
1286   in an If-None-Match header field from all its cache entries for the
1287   resource. This conveys to the server the set of entities currently
1288   held by the cache, so that if any one of these entities matches the
1289   requested entity, the server can use the ETag header field in its 304
1290   (Not Modified) response to tell the cache which entry is appropriate.
1291   If the entity-tag of the new response matches that of an existing
1292   entry, the new response &SHOULD; be used to update the header fields of
1293   the existing entry, and the result &MUST; be returned to the client.
1294</t>
1295<t>
1296   If any of the existing cache entries contains only partial content
1297   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1298   the If-None-Match header field unless the request is for a range that
1299   would be fully satisfied by that entry.
1300</t>
1301<t>
1302   If a cache receives a successful response whose Content-Location
1303   field matches that of an existing cache entry for the same Request-URI,
1304   whose entity-tag differs from that of the existing entry, and
1305   whose Date is more recent than that of the existing entry, the
1306   existing entry &SHOULD-NOT;  be returned in response to future requests
1307   and &SHOULD; be deleted from the cache.
1308</t>
1309</section>
1310
1311<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1312<t>
1313   For reasons of security and privacy, it is necessary to make a
1314   distinction between "shared" and "non-shared" caches. A non-shared
1315   cache is one that is accessible only to a single user. Accessibility
1316   in this case &SHOULD; be enforced by appropriate security mechanisms.
1317   All other caches are considered to be "shared." Other sections of
1318   this specification place certain constraints on the operation of
1319   shared caches in order to prevent loss of privacy or failure of
1320   access controls.
1321</t>
1322</section>
1323
1324<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1325<t>
1326   A cache that receives an incomplete response (for example, with fewer
1327   bytes of data than specified in a Content-Length header) &MAY; store
1328   the response. However, the cache &MUST; treat this as a partial
1329   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1330   the result might be a full response or might still be
1331   partial. A cache &MUST-NOT; return a partial response to a client
1332   without explicitly marking it as such, using the 206 (Partial
1333   Content) status code. A cache &MUST-NOT; return a partial response
1334   using a status code of 200 (OK).
1335</t>
1336<t>
1337   If a cache receives a 5xx response while attempting to revalidate an
1338   entry, it &MAY; either forward this response to the requesting client,
1339   or act as if the server failed to respond. In the latter case, it &MAY;
1340   return a previously received response unless the cached entry
1341   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1342</t>
1343</section>
1344
1345<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1346<t>
1347   Unless the origin server explicitly prohibits the caching of their
1348   responses, the application of GET and HEAD methods to any resources
1349   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1350   these responses are taken from a cache. They &MAY; still have side
1351   effects, but a cache is not required to consider such side effects in
1352   its caching decisions. Caches are always expected to observe an
1353   origin server's explicit restrictions on caching.
1354</t>
1355<t>
1356   We note one exception to this rule: since some applications have
1357   traditionally used GET and HEAD requests with URLs containing a query part
1358   to perform operations with significant side
1359   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1360   the server provides an explicit expiration time. This specifically
1361   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1362   be taken from a cache. See &safe-methods; for related information.
1363</t>
1364</section>
1365
1366<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1367<t>
1368   The effect of certain methods performed on a resource at the origin
1369   server might cause one or more existing cache entries to become non-transparently
1370   invalid. That is, although they might continue to be
1371   "fresh," they do not accurately reflect what the origin server would
1372   return for a new request on that resource.
1373</t>
1374<t>
1375   There is no way for HTTP to guarantee that all such
1376   cache entries are marked invalid. For example, the request that
1377   caused the change at the origin server might not have gone through
1378   the proxy where a cache entry is stored. However, several rules help
1379   reduce the likelihood of erroneous behavior.
1380</t>
1381<t>
1382   In this section, the phrase "invalidate an entity" means that the
1383   cache will either remove all instances of that entity from its
1384   storage, or will mark these as "invalid" and in need of a mandatory
1385   revalidation before they can be returned in response to a subsequent
1386   request.
1387</t>
1388<t>
1389   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1390   either the entity referred to by the Request-URI, or by the Location
1391   or Content-Location headers (if present). These methods are:
1392  <list style="symbols">
1393      <t>PUT</t>
1394      <t>DELETE</t>
1395      <t>POST</t>
1396  </list>
1397</t> 
1398<t>
1399   An invalidation based
1400   on the URI in a Location or Content-Location header &MUST-NOT; be
1401   performed if the host part of that URI differs from the host part
1402   in the Request-URI. This helps prevent denial of service attacks.
1403</t>
1404<t>
1405   A cache that passes through requests for methods it does not
1406   understand &SHOULD; invalidate any entities referred to by the
1407   Request-URI.
1408</t>
1409</section>
1410
1411<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1412<t>
1413   All methods that might be expected to cause modifications to the
1414   origin server's resources &MUST; be written through to the origin
1415   server. This currently includes all methods except for GET and HEAD.
1416   A cache &MUST-NOT; reply to such a request from a client before having
1417   transmitted the request to the inbound server, and having received a
1418   corresponding response from the inbound server. This does not prevent
1419   a proxy cache from sending a 100 (Continue) response before the
1420   inbound server has sent its final reply.
1421</t>
1422<t>
1423   The alternative (known as "write-back" or "copy-back" caching) is not
1424   allowed in HTTP/1.1, due to the difficulty of providing consistent
1425   updates and the problems arising from server, cache, or network
1426   failure prior to write-back.
1427</t>
1428</section>
1429
1430<section title="Cache Replacement" anchor="cache.replacement">
1431<t>
1432   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1433   <xref target="disambiguating.expiration.values" format="counter"/>,
1434   <xref target="disambiguating.multiple.responses" format="counter"/>
1435   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1436   response is received from a resource while any existing responses for
1437   the same resource are cached, the cache &SHOULD; use the new response
1438   to reply to the current request. It &MAY; insert it into cache storage
1439   and &MAY;, if it meets all other requirements, use it to respond to any
1440   future requests that would previously have caused the old response to
1441   be returned. If it inserts the new response into cache storage  the
1442   rules in <xref target="combining.headers"/> apply.
1443  <list><t>
1444      <x:h>Note:</x:h> a new response that has an older Date header value than
1445      existing cached responses is not cacheable.
1446  </t></list>
1447</t>
1448</section>
1449
1450<section title="History Lists" anchor="history.lists">
1451<t>
1452   User agents often have history mechanisms, such as "Back" buttons and
1453   history lists, which can be used to redisplay an entity retrieved
1454   earlier in a session.
1455</t>
1456<t>
1457   History mechanisms and caches are different. In particular history
1458   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1459   the current state of a resource. Rather, a history mechanism is meant
1460   to show exactly what the user saw at the time when the resource was
1461   retrieved.
1462</t>
1463<t>
1464   By default, an expiration time does not apply to history mechanisms.
1465   If the entity is still in storage, a history mechanism &SHOULD; display
1466   it even if the entity has expired, unless the user has specifically
1467   configured the agent to refresh expired history documents.
1468</t>
1469<t>
1470   This is not to be construed to prohibit the history mechanism from
1471   telling the user that a view might be stale.
1472  <list><t>
1473      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1474      viewing stale resources, this will tend to force service authors
1475      to avoid using HTTP expiration controls and cache controls when
1476      they would otherwise like to. Service authors may consider it
1477      important that users not be presented with error messages or
1478      warning messages when they use navigation controls (such as BACK)
1479      to view previously fetched resources. Even though sometimes such
1480      resources ought not be cached, or ought to expire quickly, user
1481      interface considerations may force service authors to resort to
1482      other means of preventing caching (e.g. "once-only" URLs) in order
1483      not to suffer the effects of improperly functioning history
1484      mechanisms.
1485  </t></list>
1486</t>
1487</section>
1488
1489<section title="Header Field Definitions" anchor="header.fields">
1490<t>
1491   This section defines the syntax and semantics of HTTP/1.1 header fields
1492   related to caching.
1493</t>
1494<t>
1495   For entity-header fields, both sender and recipient refer to either the
1496   client or the server, depending on who sends and who receives the entity.
1497</t>
1498
1499<section title="Age" anchor="header.age">
1500  <iref primary="true" item="Age header" x:for-anchor=""/>
1501  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1502  <x:anchor-alias value="Age"/>
1503  <x:anchor-alias value="age-value"/>
1504<t>
1505      The Age response-header field conveys the sender's estimate of the
1506      amount of time since the response (or its revalidation) was
1507      generated at the origin server. A cached response is "fresh" if
1508      its age does not exceed its freshness lifetime. Age values are
1509      calculated as specified in <xref target="age.calculations"/>.
1510</t>
1511<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1512  <x:ref>Age</x:ref> = "Age" ":" <x:ref>age-value</x:ref>
1513  <x:ref>age-value</x:ref> = <x:ref>delta-seconds</x:ref>
1514</artwork></figure>
1515<t anchor="rule.delta-seconds">
1516  <x:anchor-alias value="delta-seconds"/>
1517      Age values are non-negative decimal integers, representing time in
1518      seconds.
1519</t>
1520<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
1521  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
1522</artwork></figure>
1523<t>
1524      If a cache receives a value larger than the largest positive
1525      integer it can represent, or if any of its age calculations
1526      overflows, it &MUST; transmit an Age header with a value of
1527      2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache &MUST;
1528      include an Age header field in every response generated from its
1529      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1530      bits of range.
1531</t>
1532</section>
1533
1534<section title="Cache-Control" anchor="header.cache-control">
1535  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1536  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1537  <x:anchor-alias value="Cache-Control"/>
1538  <x:anchor-alias value="cache-directive"/>
1539  <x:anchor-alias value="cache-extension"/>
1540  <x:anchor-alias value="cache-request-directive"/>
1541  <x:anchor-alias value="cache-response-directive"/>
1542<t>
1543   The Cache-Control general-header field is used to specify directives
1544   that &MUST; be obeyed by all caching mechanisms along the
1545   request/response chain. The directives specify behavior intended to
1546   prevent caches from adversely interfering with the request or
1547   response. These directives typically override the default caching
1548   algorithms. Cache directives are unidirectional in that the presence
1549   of a directive in a request does not imply that the same directive is
1550   to be given in the response.
1551  <list><t>
1552      Note that HTTP/1.0 caches might not implement Cache-Control and
1553      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1554  </t></list>
1555</t>
1556<t>
1557   Cache directives &MUST; be passed through by a proxy or gateway
1558   application, regardless of their significance to that application,
1559   since the directives might be applicable to all recipients along the
1560   request/response chain. It is not possible to specify a cache-directive
1561   for a specific cache.
1562</t>
1563<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><iref primary="true" item="Grammar" subitem="cache-request-directive"/><iref primary="true" item="Grammar" subitem="cache-response-directive"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1564  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" 1#<x:ref>cache-directive</x:ref>
1565
1566  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1567     | <x:ref>cache-response-directive</x:ref>
1568
1569  <x:ref>cache-request-directive</x:ref> =
1570       "no-cache"                          ; <xref target="what.is.cacheable"/>
1571     | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1572     | "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"/>
1573     | "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1574     | "min-fresh" "=" <x:ref>delta-seconds</x:ref>       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1575     | "no-transform"                      ; <xref target="no-transform.directive"/>
1576     | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1577     | <x:ref>cache-extension</x:ref>                     ; <xref target="cache.control.extensions"/>
1578
1579  <x:ref>cache-response-directive</x:ref> =
1580       "public"                               ; <xref target="what.is.cacheable"/>
1581     | "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
1582     | "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
1583     | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1584     | "no-transform"                         ; <xref target="no-transform.directive"/>
1585     | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1586     | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1587     | "max-age" "=" <x:ref>delta-seconds</x:ref>            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1588     | "s-maxage" "=" <x:ref>delta-seconds</x:ref>           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1589     | <x:ref>cache-extension</x:ref>                        ; <xref target="cache.control.extensions"/>
1590
1591  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> | <x:ref>quoted-string</x:ref> ) ]
1592</artwork></figure>
1593<t>
1594   When a directive appears without any 1#field-name parameter, the
1595   directive applies to the entire request or response. When such a
1596   directive appears with a 1#field-name parameter, it applies only to
1597   the named field or fields, and not to the rest of the request or
1598   response. This mechanism supports extensibility; implementations of
1599   future versions of HTTP might apply these directives to
1600   header fields not defined in HTTP/1.1.
1601</t>
1602<t>
1603   The cache-control directives can be broken down into these general
1604   categories:
1605  <list style="symbols">
1606     <t>Restrictions on what are cacheable; these may only be imposed by
1607        the origin server.</t>
1608
1609     <t>Restrictions on what may be stored by a cache; these may be
1610        imposed by either the origin server or the user agent.</t>
1611
1612     <t>Modifications of the basic expiration mechanism; these may be
1613        imposed by either the origin server or the user agent.</t>
1614
1615     <t>Controls over cache revalidation and reload; these may only be
1616        imposed by a user agent.</t>
1617
1618     <t>Control over transformation of entities.</t>
1619
1620     <t>Extensions to the caching system.</t>
1621  </list>
1622</t>
1623
1624<section title="What is Cacheable" anchor="what.is.cacheable">
1625<t>
1626   By default, a response is cacheable if the requirements of the
1627   request method, request header fields, and the response status
1628   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1629   for cacheability. The following Cache-Control response directives
1630   allow an origin server to override the default cacheability of a
1631   response:
1632</t>
1633<t>
1634  <iref item="Cache Directives" subitem="public" primary="true"/>
1635  <iref item="public" subitem="Cache Directive" primary="true"/>
1636   public
1637  <list><t>
1638      Indicates that the response &MAY; be cached by any cache, even if it
1639      would normally be non-cacheable or cacheable only within a non-shared
1640      cache. (See also Authorization, &header-authorization;, for
1641      additional details.)
1642  </t></list>
1643</t>
1644<t>
1645  <iref item="Cache Directives" subitem="private" primary="true"/>
1646  <iref item="private" subitem="Cache Directive" primary="true"/>
1647   private
1648  <list><t>
1649      Indicates that all or part of the response message is intended for
1650      a single user and &MUST-NOT; be cached by a shared cache. This
1651      allows an origin server to state that the specified parts of the
1652      response are intended for only one user and are not a valid
1653      response for requests by other users. A private (non-shared) cache
1654      &MAY; cache the response.
1655    </t><t>
1656       <x:h>Note:</x:h> This usage of the word private only controls where the
1657       response may be cached, and cannot ensure the privacy of the
1658       message content.
1659  </t></list>
1660</t>
1661<t>
1662  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1663  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1664   no-cache
1665  <list><t>
1666       If the no-cache directive does not specify a field-name, then a
1667      cache &MUST-NOT; use the response to satisfy a subsequent request
1668      without successful revalidation with the origin server. This
1669      allows an origin server to prevent caching even by caches that
1670      have been configured to return stale responses to client requests.
1671    </t><t>
1672      If the no-cache directive does specify one or more field-names,
1673      then a cache &MAY; use the response to satisfy a subsequent request,
1674      subject to any other restrictions on caching. However, the
1675      specified field-name(s) &MUST-NOT; be sent in the response to a
1676      subsequent request without successful revalidation with the origin
1677      server. This allows an origin server to prevent the re-use of
1678      certain header fields in a response, while still allowing caching
1679      of the rest of the response.
1680    <list><t>
1681       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1682       directive.
1683    </t></list>
1684  </t></list>
1685</t>
1686</section>
1687
1688<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1689<t>
1690  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1691  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1692   no-store
1693  <list><t>   
1694      The purpose of the no-store directive is to prevent the
1695      inadvertent release or retention of sensitive information (for
1696      example, on backup tapes). The no-store directive applies to the
1697      entire message, and &MAY; be sent either in a response or in a
1698      request. If sent in a request, a cache &MUST-NOT; store any part of
1699      either this request or any response to it. If sent in a response,
1700      a cache &MUST-NOT; store any part of either this response or the
1701      request that elicited it. This directive applies to both non-shared
1702      and shared caches. "&MUST-NOT; store" in this context means
1703      that the cache &MUST-NOT; intentionally store the information in
1704      non-volatile storage, and &MUST; make a best-effort attempt to
1705      remove the information from volatile storage as promptly as
1706      possible after forwarding it.
1707  </t><t>
1708      Even when this directive is associated with a response, users
1709      might explicitly store such a response outside of the caching
1710      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1711      such responses as part of their normal operation.
1712  </t><t>
1713      The purpose of this directive is to meet the stated requirements
1714      of certain users and service authors who are concerned about
1715      accidental releases of information via unanticipated accesses to
1716      cache data structures. While the use of this directive might
1717      improve privacy in some cases, we caution that it is NOT in any
1718      way a reliable or sufficient mechanism for ensuring privacy. In
1719      particular, malicious or compromised caches might not recognize or
1720      obey this directive, and communications networks might be
1721      vulnerable to eavesdropping.
1722  </t></list>
1723</t>
1724</section>
1725
1726<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1727<t>
1728   The expiration time of an entity &MAY; be specified by the origin
1729   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1730   it &MAY; be specified using the max-age directive in a response. When
1731   the max-age cache-control directive is present in a cached response,
1732   the response is stale if its current age is greater than the age
1733   value given (in seconds) at the time of a new request for that
1734   resource. The max-age directive on a response implies that the
1735   response is cacheable (i.e., "public") unless some other, more
1736   restrictive cache directive is also present.
1737</t>
1738<t>
1739   If a response includes both an Expires header and a max-age
1740   directive, the max-age directive overrides the Expires header, even
1741   if the Expires header is more restrictive. This rule allows an origin
1742   server to provide, for a given response, a longer expiration time to
1743   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1744   useful if certain HTTP/1.0 caches improperly calculate ages or
1745   expiration times, perhaps due to desynchronized clocks.
1746</t>
1747<t>
1748   Many HTTP/1.0 cache implementations will treat an Expires value that
1749   is less than or equal to the response Date value as being equivalent
1750   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1751   cache receives such a response, and the response does not include a
1752   Cache-Control header field, it &SHOULD; consider the response to be
1753   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1754  <list><t>
1755       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1756       cache control feature, such as the "private" directive, on a
1757       network including older caches that do not understand that
1758       feature. The origin server will need to combine the new feature
1759       with an Expires field whose value is less than or equal to the
1760       Date value. This will prevent older caches from improperly
1761       caching the response.
1762  </t></list>
1763</t>
1764<t>
1765  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1766  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1767   s-maxage
1768  <list><t>
1769       If a response includes an s-maxage directive, then for a shared
1770       cache (but not for a private cache), the maximum age specified by
1771       this directive overrides the maximum age specified by either the
1772       max-age directive or the Expires header. The s-maxage directive
1773       also implies the semantics of the proxy-revalidate directive (see
1774       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1775       entry after it becomes stale to respond to a subsequent request
1776       without first revalidating it with the origin server. The s-maxage
1777       directive is always ignored by a private cache.
1778  </t></list>
1779</t>
1780<t>
1781   Note that most older caches, not compliant with this specification,
1782   do not implement any cache-control directives. An origin server
1783   wishing to use a cache-control directive that restricts, but does not
1784   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1785   requirement that the max-age directive overrides the Expires header,
1786   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1787   max-age directive.
1788</t>
1789<t>
1790   Other directives allow a user agent to modify the basic expiration
1791   mechanism. These directives &MAY; be specified on a request:
1792</t>
1793<t>
1794  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1795  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1796   max-age
1797  <list><t>
1798      Indicates that the client is willing to accept a response whose
1799      age is no greater than the specified time in seconds. Unless max-stale
1800      directive is also included, the client is not willing to
1801      accept a stale response.
1802  </t></list>
1803</t>
1804<t>
1805  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1806  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1807   min-fresh
1808  <list><t>
1809      Indicates that the client is willing to accept a response whose
1810      freshness lifetime is no less than its current age plus the
1811      specified time in seconds. That is, the client wants a response
1812      that will still be fresh for at least the specified number of
1813      seconds.
1814  </t></list>
1815</t>
1816<t>
1817  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1818  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1819   max-stale
1820  <list><t>
1821      Indicates that the client is willing to accept a response that has
1822      exceeded its expiration time. If max-stale is assigned a value,
1823      then the client is willing to accept a response that has exceeded
1824      its expiration time by no more than the specified number of
1825      seconds. If no value is assigned to max-stale, then the client is
1826      willing to accept a stale response of any age.
1827  </t></list>
1828</t>
1829<t>
1830   If a cache returns a stale response, either because of a max-stale
1831   directive on a request, or because the cache is configured to
1832   override the expiration time of a response, the cache &MUST; attach a
1833   Warning header to the stale response, using Warning 110 (Response is
1834   stale).
1835</t>
1836<t>
1837   A cache &MAY; be configured to return stale responses without
1838   validation, but only if this does not conflict with any "MUST"-level
1839   requirements concerning cache validation (e.g., a "must-revalidate"
1840   cache-control directive).
1841</t>
1842<t>
1843   If both the new request and the cached entry include "max-age"
1844   directives, then the lesser of the two values is used for determining
1845   the freshness of the cached entry for that request.
1846</t>
1847</section>
1848
1849<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1850<t>
1851   Sometimes a user agent might want or need to insist that a cache
1852   revalidate its cache entry with the origin server (and not just with
1853   the next cache along the path to the origin server), or to reload its
1854   cache entry from the origin server. End-to-end revalidation might be
1855   necessary if either the cache or the origin server has overestimated
1856   the expiration time of the cached response. End-to-end reload may be
1857   necessary if the cache entry has become corrupted for some reason.
1858</t>
1859<t>
1860   End-to-end revalidation may be requested either when the client does
1861   not have its own local cached copy, in which case we call it
1862   "unspecified end-to-end revalidation", or when the client does have a
1863   local cached copy, in which case we call it "specific end-to-end
1864   revalidation."
1865</t>
1866<t>
1867   The client can specify these three kinds of action using Cache-Control
1868   request directives:
1869</t>
1870<t>
1871   End-to-end reload
1872  <list><t>
1873      The request includes a "no-cache" cache-control directive or, for
1874      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1875      names &MUST-NOT; be included with the no-cache directive in a
1876      request. The server &MUST-NOT; use a cached copy when responding to
1877      such a request.
1878  </t></list>
1879</t>
1880<t>
1881   Specific end-to-end revalidation
1882  <list><t>
1883      The request includes a "max-age=0" cache-control directive, which
1884      forces each cache along the path to the origin server to
1885      revalidate its own entry, if any, with the next cache or server.
1886      The initial request includes a cache-validating conditional with
1887      the client's current validator.
1888  </t></list>
1889</t>
1890<t>
1891   Unspecified end-to-end revalidation
1892  <list><t>
1893      The request includes "max-age=0" cache-control directive, which
1894      forces each cache along the path to the origin server to
1895      revalidate its own entry, if any, with the next cache or server.
1896      The initial request does not include a cache-validating
1897      conditional; the first cache along the path (if any) that holds a
1898      cache entry for this resource includes a cache-validating
1899      conditional with its current validator.
1900  </t></list>
1901</t>
1902<t>
1903  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1904  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1905   max-age
1906  <list><t>
1907      When an intermediate cache is forced, by means of a max-age=0
1908      directive, to revalidate its own cache entry, and the client has
1909      supplied its own validator in the request, the supplied validator
1910      might differ from the validator currently stored with the cache
1911      entry. In this case, the cache &MAY; use either validator in making
1912      its own request without affecting semantic transparency.
1913  </t><t>
1914      However, the choice of validator might affect performance. The
1915      best approach is for the intermediate cache to use its own
1916      validator when making its request. If the server replies with 304
1917      (Not Modified), then the cache can return its now validated copy
1918      to the client with a 200 (OK) response. If the server replies with
1919      a new entity and cache validator, however, the intermediate cache
1920      can compare the returned validator with the one provided in the
1921      client's request, using the strong comparison function. If the
1922      client's validator is equal to the origin server's, then the
1923      intermediate cache simply returns 304 (Not Modified). Otherwise,
1924      it returns the new entity with a 200 (OK) response.
1925  </t><t>
1926      If a request includes the no-cache directive, it &SHOULD-NOT; 
1927      include min-fresh, max-stale, or max-age.
1928  </t></list>
1929</t>
1930<t>
1931  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1932  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1933   only-if-cached
1934  <list><t>
1935      In some cases, such as times of extremely poor network
1936      connectivity, a client may want a cache to return only those
1937      responses that it currently has stored, and not to reload or
1938      revalidate with the origin server. To do this, the client may
1939      include the only-if-cached directive in a request. If it receives
1940      this directive, a cache &SHOULD; either respond using a cached entry
1941      that is consistent with the other constraints of the request, or
1942      respond with a 504 (Gateway Timeout) status. However, if a group
1943      of caches is being operated as a unified system with good internal
1944      connectivity, such a request &MAY; be forwarded within that group of
1945      caches.
1946  </t></list>
1947</t>
1948<t>
1949  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1950  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1951   must-revalidate
1952  <list><t>
1953      Because a cache &MAY; be configured to ignore a server's specified
1954      expiration time, and because a client request &MAY; include a max-stale
1955      directive (which has a similar effect), the protocol also
1956      includes a mechanism for the origin server to require revalidation
1957      of a cache entry on any subsequent use. When the must-revalidate
1958      directive is present in a response received by a cache, that cache
1959      &MUST-NOT; use the entry after it becomes stale to respond to a
1960      subsequent request without first revalidating it with the origin
1961      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1962      time, if, based solely on the origin server's Expires or max-age
1963      value, the cached response is stale.)
1964  </t><t>
1965      The must-revalidate directive is necessary to support reliable
1966      operation for certain protocol features. In all circumstances an
1967      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1968      particular, if the cache cannot reach the origin server for any
1969      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1970  </t><t>
1971      Servers &SHOULD; send the must-revalidate directive if and only if
1972      failure to revalidate a request on the entity could result in
1973      incorrect operation, such as a silently unexecuted financial
1974      transaction. Recipients &MUST-NOT; take any automated action that
1975      violates this directive, and &MUST-NOT; automatically provide an
1976      unvalidated copy of the entity if revalidation fails.
1977  </t><t>
1978      Although this is not recommended, user agents operating under
1979      severe connectivity constraints &MAY; violate this directive but, if
1980      so, &MUST; explicitly warn the user that an unvalidated response has
1981      been provided. The warning &MUST; be provided on each unvalidated
1982      access, and &SHOULD; require explicit user confirmation.
1983  </t></list>
1984</t>
1985<t>
1986  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1987  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
1988   proxy-revalidate
1989  <list><t>
1990      The proxy-revalidate directive has the same meaning as the must-revalidate
1991      directive, except that it does not apply to non-shared
1992      user agent caches. It can be used on a response to an
1993      authenticated request to permit the user's cache to store and
1994      later return the response without needing to revalidate it (since
1995      it has already been authenticated once by that user), while still
1996      requiring proxies that service many users to revalidate each time
1997      (in order to make sure that each user has been authenticated).
1998      Note that such authenticated responses also need the public cache
1999      control directive in order to allow them to be cached at all.
2000  </t></list>
2001</t>
2002</section>
2003
2004<section title="No-Transform Directive" anchor="no-transform.directive">
2005<t>
2006  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2007  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2008   no-transform
2009  <list><t>
2010      Implementors of intermediate caches (proxies) have found it useful
2011      to convert the media type of certain entity bodies. A non-transparent
2012      proxy might, for example, convert between image
2013      formats in order to save cache space or to reduce the amount of
2014      traffic on a slow link.
2015  </t><t>
2016      Serious operational problems occur, however, when these
2017      transformations are applied to entity bodies intended for certain
2018      kinds of applications. For example, applications for medical
2019      imaging, scientific data analysis and those using end-to-end
2020      authentication, all depend on receiving an entity body that is bit
2021      for bit identical to the original entity-body.
2022  </t><t>
2023      Therefore, if a message includes the no-transform directive, an
2024      intermediate cache or proxy &MUST-NOT; change those headers that are
2025      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2026      directive. This implies that the cache or proxy &MUST-NOT; change
2027      any aspect of the entity-body that is specified by these headers,
2028      including the value of the entity-body itself.
2029  </t></list>
2030</t>
2031</section>
2032
2033<section title="Cache Control Extensions" anchor="cache.control.extensions">
2034<t>
2035   The Cache-Control header field can be extended through the use of one
2036   or more cache-extension tokens, each with an optional assigned value.
2037   Informational extensions (those which do not require a change in
2038   cache behavior) &MAY; be added without changing the semantics of other
2039   directives. Behavioral extensions are designed to work by acting as
2040   modifiers to the existing base of cache directives. Both the new
2041   directive and the standard directive are supplied, such that
2042   applications which do not understand the new directive will default
2043   to the behavior specified by the standard directive, and those that
2044   understand the new directive will recognize it as modifying the
2045   requirements associated with the standard directive. In this way,
2046   extensions to the cache-control directives can be made without
2047   requiring changes to the base protocol.
2048</t>
2049<t>
2050   This extension mechanism depends on an HTTP cache obeying all of the
2051   cache-control directives defined for its native HTTP-version, obeying
2052   certain extensions, and ignoring all directives that it does not
2053   understand.
2054</t>
2055<t>
2056   For example, consider a hypothetical new response directive called
2057   community which acts as a modifier to the private directive. We
2058   define this new directive to mean that, in addition to any non-shared
2059   cache, any cache which is shared only by members of the community
2060   named within its value may cache the response. An origin server
2061   wishing to allow the UCI community to use an otherwise private
2062   response in their shared cache(s) could do so by including
2063</t>
2064<figure><artwork type="example">
2065    Cache-Control: private, community="UCI"
2066</artwork></figure>
2067<t>
2068   A cache seeing this header field will act correctly even if the cache
2069   does not understand the community cache-extension, since it will also
2070   see and understand the private directive and thus default to the safe
2071   behavior.
2072</t>
2073<t>
2074   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2075   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2076   be combined with standard directives (or the response's default
2077   cacheability) such that the cache behavior will remain minimally
2078   correct even if the cache does not understand the extension(s).
2079</t>
2080</section>
2081</section>
2082
2083<section title="Expires" anchor="header.expires">
2084  <iref primary="true" item="Expires header" x:for-anchor=""/>
2085  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2086  <x:anchor-alias value="Expires"/>
2087<t>
2088   The Expires entity-header field gives the date/time after which the
2089   response is considered stale. A stale cache entry may not normally be
2090   returned by a cache (either a proxy cache or a user agent cache)
2091   unless it is first validated with the origin server (or with an
2092   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2093   for further discussion of the expiration model.
2094</t>
2095<t>
2096   The presence of an Expires field does not imply that the original
2097   resource will change or cease to exist at, before, or after that
2098   time.
2099</t>
2100<t>
2101   The format is an absolute date and time as defined by HTTP-date in
2102   &full-date;; it &MUST; be sent in rfc1123-date format.
2103</t>
2104<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2105  <x:ref>Expires</x:ref> = "Expires" ":" <x:ref>HTTP-date</x:ref>
2106</artwork></figure>
2107<t>
2108   An example of its use is
2109</t>
2110<figure><artwork type="example">
2111   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2112</artwork></figure>
2113<t>
2114  <list><t>
2115      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2116      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2117      Expires field.
2118  </t></list>
2119</t>
2120<t>
2121   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2122   especially including the value "0", as in the past (i.e., "already
2123   expired").
2124</t>
2125<t>
2126   To mark a response as "already expired," an origin server sends an
2127   Expires date that is equal to the Date header value. (See the rules
2128   for expiration calculations in <xref target="expiration.calculations"/>.)
2129</t>
2130<t>
2131   To mark a response as "never expires," an origin server sends an
2132   Expires date approximately one year from the time the response is
2133   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2134   year in the future.
2135</t>
2136<t>
2137   The presence of an Expires header field with a date value of some
2138   time in the future on a response that otherwise would by default be
2139   non-cacheable indicates that the response is cacheable, unless
2140   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2141</t>
2142</section>
2143
2144<section title="Pragma" anchor="header.pragma">
2145  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2146  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2147  <x:anchor-alias value="extension-pragma"/>
2148  <x:anchor-alias value="Pragma"/>
2149  <x:anchor-alias value="pragma-directive"/>
2150<t>
2151   The Pragma general-header field is used to include implementation-specific
2152   directives that might apply to any recipient along the
2153   request/response chain. All pragma directives specify optional
2154   behavior from the viewpoint of the protocol; however, some systems
2155   &MAY; require that behavior be consistent with the directives.
2156</t>
2157<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
2158  <x:ref>Pragma</x:ref>            = "Pragma" ":" 1#<x:ref>pragma-directive</x:ref>
2159  <x:ref>pragma-directive</x:ref>  = "no-cache" | <x:ref>extension-pragma</x:ref>
2160  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> | <x:ref>quoted-string</x:ref> ) ]
2161</artwork></figure>
2162<t>
2163   When the no-cache directive is present in a request message, an
2164   application &SHOULD; forward the request toward the origin server even
2165   if it has a cached copy of what is being requested. This pragma
2166   directive has the same semantics as the no-cache cache-directive (see
2167   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2168   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2169   request is sent to a server not known to be HTTP/1.1 compliant.
2170</t>
2171<t>
2172   Pragma directives &MUST; be passed through by a proxy or gateway
2173   application, regardless of their significance to that application,
2174   since the directives might be applicable to all recipients along the
2175   request/response chain. It is not possible to specify a pragma for a
2176   specific recipient; however, any pragma directive not relevant to a
2177   recipient &SHOULD; be ignored by that recipient.
2178</t>
2179<t>
2180   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2181   sent "Cache-Control: no-cache". No new Pragma directives will be
2182   defined in HTTP.
2183  <list><t>
2184      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
2185      response-header field is not actually specified, it does not provide a
2186      reliable replacement for "Cache-Control: no-cache" in a response.
2187  </t></list>
2188</t>
2189</section>
2190
2191<section title="Vary" anchor="header.vary">
2192  <iref primary="true" item="Vary header" x:for-anchor=""/>
2193  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2194  <x:anchor-alias value="Vary"/>
2195<t>
2196   The Vary field value indicates the set of request-header fields that
2197   fully determines, while the response is fresh, whether a cache is
2198   permitted to use the response to reply to a subsequent request
2199   without revalidation. For uncacheable or stale responses, the Vary
2200   field value advises the user agent about the criteria that were used
2201   to select the representation. A Vary field value of "*" implies that
2202   a cache cannot determine from the request headers of a subsequent
2203   request whether this response is the appropriate representation. See
2204   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2205</t>
2206<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2207  <x:ref>Vary</x:ref>  = "Vary" ":" ( "*" | 1#<x:ref>field-name</x:ref> )
2208</artwork></figure>
2209<t>
2210   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2211   cacheable response that is subject to server-driven negotiation.
2212   Doing so allows a cache to properly interpret future requests on that
2213   resource and informs the user agent about the presence of negotiation
2214   on that resource. A server &MAY; include a Vary header field with a
2215   non-cacheable response that is subject to server-driven negotiation,
2216   since this might provide the user agent with useful information about
2217   the dimensions over which the response varies at the time of the
2218   response.
2219</t>
2220<t>
2221   A Vary field value consisting of a list of field-names signals that
2222   the representation selected for the response is based on a selection
2223   algorithm which considers ONLY the listed request-header field values
2224   in selecting the most appropriate representation. A cache &MAY; assume
2225   that the same selection will be made for future requests with the
2226   same values for the listed field names, for the duration of time for
2227   which the response is fresh.
2228</t>
2229<t>
2230   The field-names given are not limited to the set of standard
2231   request-header fields defined by this specification. Field names are
2232   case-insensitive.
2233</t>
2234<t>
2235   A Vary field value of "*" signals that unspecified parameters not
2236   limited to the request-headers (e.g., the network address of the
2237   client), play a role in the selection of the response representation.
2238   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2239   generated by an origin server.
2240</t>
2241</section>
2242
2243<section title="Warning" anchor="header.warning">
2244  <iref primary="true" item="Warning header" x:for-anchor=""/>
2245  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2246  <x:anchor-alias value="Warning"/>
2247  <x:anchor-alias value="warning-value"/>
2248  <x:anchor-alias value="warn-agent"/>
2249  <x:anchor-alias value="warn-code"/>
2250  <x:anchor-alias value="warn-date"/>
2251  <x:anchor-alias value="warn-text"/>
2252<t>
2253   The Warning general-header field is used to carry additional
2254   information about the status or transformation of a message which
2255   might not be reflected in the message. This information is typically
2256   used to warn about a possible lack of semantic transparency from
2257   caching operations or transformations applied to the entity body of
2258   the message.
2259</t>
2260<t>
2261   Warning headers are sent with responses using:
2262</t>
2263<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
2264  <x:ref>Warning</x:ref>    = "Warning" ":" 1#<x:ref>warning-value</x:ref>
2265 
2266  <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>
2267                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
2268 
2269  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
2270  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) | <x:ref>pseudonym</x:ref>
2271                  ; the name or pseudonym of the server adding
2272                  ; the Warning header, for use in debugging
2273  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
2274  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
2275</artwork></figure>
2276<t>
2277   A response &MAY; carry more than one Warning header.
2278</t>
2279<t>
2280   The warn-text &SHOULD; be in a natural language and character set that
2281   is most likely to be intelligible to the human user receiving the
2282   response. This decision &MAY; be based on any available knowledge, such
2283   as the location of the cache or user, the Accept-Language field in a
2284   request, the Content-Language field in a response, etc. The default
2285   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2286</t>
2287<t>
2288   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2289   in the warn-text using the method described in <xref target="RFC2047"/>.
2290</t>
2291<t>
2292   Warning headers can in general be applied to any message, however
2293   some specific warn-codes are specific to caches and can only be
2294   applied to response messages. New Warning headers &SHOULD; be added
2295   after any existing Warning headers. A cache &MUST-NOT; delete any
2296   Warning header that it received with a message. However, if a cache
2297   successfully validates a cache entry, it &SHOULD; remove any Warning
2298   headers previously attached to that entry except as specified for
2299   specific Warning codes. It &MUST; then add any Warning headers received
2300   in the validating response. In other words, Warning headers are those
2301   that would be attached to the most recent relevant response.
2302</t>
2303<t>
2304   When multiple Warning headers are attached to a response, the user
2305   agent ought to inform the user of as many of them as possible, in the
2306   order that they appear in the response. If it is not possible to
2307   inform the user of all of the warnings, the user agent &SHOULD; follow
2308   these heuristics:
2309  <list style="symbols">
2310    <t>Warnings that appear early in the response take priority over
2311        those appearing later in the response.</t>
2312
2313    <t>Warnings in the user's preferred character set take priority
2314        over warnings in other character sets but with identical warn-codes
2315        and warn-agents.</t>
2316  </list>
2317</t>
2318<t>
2319   Systems that generate multiple Warning headers &SHOULD; order them with
2320   this user agent behavior in mind.
2321</t>
2322<t>
2323   Requirements for the behavior of caches with respect to Warnings are
2324   stated in <xref target="warnings"/>.
2325</t>
2326<t>
2327   This is a list of the currently-defined warn-codes, each with a
2328   recommended warn-text in English, and a description of its meaning.
2329</t>
2330<t>
2331   110 Response is stale
2332  <list><t>
2333     &MUST; be included whenever the returned response is stale.
2334  </t></list>
2335</t>
2336<t>
2337   111 Revalidation failed
2338  <list><t>
2339     &MUST; be included if a cache returns a stale response because an
2340     attempt to revalidate the response failed, due to an inability to
2341     reach the server.
2342  </t></list>
2343</t>
2344<t>
2345   112 Disconnected operation
2346  <list><t>
2347     &SHOULD; be included if the cache is intentionally disconnected from
2348     the rest of the network for a period of time.
2349  </t></list>
2350</t>
2351<t>
2352   113 Heuristic expiration
2353  <list><t>
2354     &MUST; be included if the cache heuristically chose a freshness
2355     lifetime greater than 24 hours and the response's age is greater
2356     than 24 hours.
2357  </t></list>
2358</t>
2359<t>
2360   199 Miscellaneous warning
2361  <list><t>
2362     The warning text &MAY; include arbitrary information to be presented
2363     to a human user, or logged. A system receiving this warning &MUST-NOT;
2364     take any automated action, besides presenting the warning to
2365     the user.
2366  </t></list>
2367</t>
2368<t>
2369   214 Transformation applied
2370  <list><t>
2371     &MUST; be added by an intermediate cache or proxy if it applies any
2372     transformation changing the content-coding (as specified in the
2373     Content-Encoding header) or media-type (as specified in the
2374     Content-Type header) of the response, or the entity-body of the
2375     response, unless this Warning code already appears in the response.
2376  </t></list>
2377</t>
2378<t>
2379   299 Miscellaneous persistent warning
2380  <list><t>
2381     The warning text &MAY; include arbitrary information to be presented
2382     to a human user, or logged. A system receiving this warning &MUST-NOT;
2383     take any automated action.
2384  </t></list>
2385</t>
2386<t>
2387   If an implementation sends a message with one or more Warning headers
2388   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2389   each warning-value a warn-date that matches the date in the response.
2390</t>
2391<t>
2392   If an implementation receives a message with a warning-value that
2393   includes a warn-date, and that warn-date is different from the Date
2394   value in the response, then that warning-value &MUST; be deleted from
2395   the message before storing, forwarding, or using it. (This prevents
2396   bad consequences of naive caching of Warning header fields.) If all
2397   of the warning-values are deleted for this reason, the Warning header
2398   &MUST; be deleted as well.
2399</t>
2400</section>
2401
2402</section>
2403
2404<section title="IANA Considerations" anchor="IANA.considerations">
2405<section title="Message Header Registration" anchor="message.header.registration">
2406<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2407<!--(START)-->
2408<t>
2409    The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2410    with the permanent registrations below (see <xref target="RFC3864"/>):
2411  </t>
2412<texttable>
2413   <ttcol>Header Field Name</ttcol>
2414   <ttcol>Protocol</ttcol>
2415   <ttcol>Status</ttcol>
2416   <ttcol>Reference</ttcol>
2417
2418   <c>Age</c>
2419   <c>http</c>
2420   <c>standard</c>
2421   <c>
2422      <xref target="header.age"/>
2423   </c>
2424
2425   <c>Cache-Control</c>
2426   <c>http</c>
2427   <c>standard</c>
2428   <c>
2429      <xref target="header.cache-control"/>
2430   </c>
2431
2432   <c>Expires</c>
2433   <c>http</c>
2434   <c>standard</c>
2435   <c>
2436      <xref target="header.expires"/>
2437   </c>
2438
2439   <c>Pragma</c>
2440   <c>http</c>
2441   <c>standard</c>
2442   <c>
2443      <xref target="header.pragma"/>
2444   </c>
2445
2446   <c>Vary</c>
2447   <c>http</c>
2448   <c>standard</c>
2449   <c>
2450      <xref target="header.vary"/>
2451   </c>
2452
2453   <c>Warning</c>
2454   <c>http</c>
2455   <c>standard</c>
2456   <c>
2457      <xref target="header.warning"/>
2458   </c>
2459</texttable>
2460<t>
2461    The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2462  </t>
2463<!--(END)-->
2464</section>
2465</section>
2466
2467<section title="Security Considerations" anchor="security.considerations">
2468<t>
2469   Caching proxies provide additional potential vulnerabilities, since
2470   the contents of the cache represent an attractive target for
2471   malicious exploitation. Because cache contents persist after an HTTP
2472   request is complete, an attack on the cache can reveal information
2473   long after a user believes that the information has been removed from
2474   the network. Therefore, cache contents should be protected as
2475   sensitive information.
2476</t>
2477</section>
2478
2479<section title="Acknowledgments" anchor="ack">
2480<t>
2481   Much of the content and presentation of the caching design is due to
2482   suggestions and comments from individuals including: Shel Kaphan,
2483   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2484</t>
2485</section>
2486</middle>
2487<back>
2488
2489<references title="Normative References">
2490
2491<reference anchor="ISO-8859-1">
2492  <front>
2493    <title>
2494     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2495    </title>
2496    <author>
2497      <organization>International Organization for Standardization</organization>
2498    </author>
2499    <date year="1998"/>
2500  </front>
2501  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2502</reference>
2503
2504<reference anchor="Part1">
2505  <front>
2506    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2507    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2508      <organization abbrev="Day Software">Day Software</organization>
2509      <address><email>fielding@gbiv.com</email></address>
2510    </author>
2511    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2512      <organization>One Laptop per Child</organization>
2513      <address><email>jg@laptop.org</email></address>
2514    </author>
2515    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2516      <organization abbrev="HP">Hewlett-Packard Company</organization>
2517      <address><email>JeffMogul@acm.org</email></address>
2518    </author>
2519    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2520      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2521      <address><email>henrikn@microsoft.com</email></address>
2522    </author>
2523    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2524      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2525      <address><email>LMM@acm.org</email></address>
2526    </author>
2527    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2528      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2529      <address><email>paulle@microsoft.com</email></address>
2530    </author>
2531    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2532      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2533      <address><email>timbl@w3.org</email></address>
2534    </author>
2535    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2536      <organization abbrev="W3C">World Wide Web Consortium</organization>
2537      <address><email>ylafon@w3.org</email></address>
2538    </author>
2539    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2540      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2541      <address><email>julian.reschke@greenbytes.de</email></address>
2542    </author>
2543    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2544  </front>
2545  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2546  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2547</reference>
2548
2549<reference anchor="Part2">
2550  <front>
2551    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2552    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2553      <organization abbrev="Day Software">Day Software</organization>
2554      <address><email>fielding@gbiv.com</email></address>
2555    </author>
2556    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2557      <organization>One Laptop per Child</organization>
2558      <address><email>jg@laptop.org</email></address>
2559    </author>
2560    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2561      <organization abbrev="HP">Hewlett-Packard Company</organization>
2562      <address><email>JeffMogul@acm.org</email></address>
2563    </author>
2564    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2565      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2566      <address><email>henrikn@microsoft.com</email></address>
2567    </author>
2568    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2569      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2570      <address><email>LMM@acm.org</email></address>
2571    </author>
2572    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2573      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2574      <address><email>paulle@microsoft.com</email></address>
2575    </author>
2576    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2577      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2578      <address><email>timbl@w3.org</email></address>
2579    </author>
2580    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2581      <organization abbrev="W3C">World Wide Web Consortium</organization>
2582      <address><email>ylafon@w3.org</email></address>
2583    </author>
2584    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2585      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2586      <address><email>julian.reschke@greenbytes.de</email></address>
2587    </author>
2588    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2589  </front>
2590  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2591  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2592</reference>
2593
2594<reference anchor="Part3">
2595  <front>
2596    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2597    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2598      <organization abbrev="Day Software">Day Software</organization>
2599      <address><email>fielding@gbiv.com</email></address>
2600    </author>
2601    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2602      <organization>One Laptop per Child</organization>
2603      <address><email>jg@laptop.org</email></address>
2604    </author>
2605    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2606      <organization abbrev="HP">Hewlett-Packard Company</organization>
2607      <address><email>JeffMogul@acm.org</email></address>
2608    </author>
2609    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2610      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2611      <address><email>henrikn@microsoft.com</email></address>
2612    </author>
2613    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2614      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2615      <address><email>LMM@acm.org</email></address>
2616    </author>
2617    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2618      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2619      <address><email>paulle@microsoft.com</email></address>
2620    </author>
2621    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2622      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2623      <address><email>timbl@w3.org</email></address>
2624    </author>
2625    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2626      <organization abbrev="W3C">World Wide Web Consortium</organization>
2627      <address><email>ylafon@w3.org</email></address>
2628    </author>
2629    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2630      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2631      <address><email>julian.reschke@greenbytes.de</email></address>
2632    </author>
2633    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2634  </front>
2635  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2636  <x:source href="p3-payload.xml" basename="p3-payload"/>
2637</reference>
2638
2639<reference anchor="Part4">
2640  <front>
2641    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2642    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2643      <organization abbrev="Day Software">Day Software</organization>
2644      <address><email>fielding@gbiv.com</email></address>
2645    </author>
2646    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2647      <organization>One Laptop per Child</organization>
2648      <address><email>jg@laptop.org</email></address>
2649    </author>
2650    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2651      <organization abbrev="HP">Hewlett-Packard Company</organization>
2652      <address><email>JeffMogul@acm.org</email></address>
2653    </author>
2654    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2655      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2656      <address><email>henrikn@microsoft.com</email></address>
2657    </author>
2658    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2659      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2660      <address><email>LMM@acm.org</email></address>
2661    </author>
2662    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2663      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2664      <address><email>paulle@microsoft.com</email></address>
2665    </author>
2666    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2667      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2668      <address><email>timbl@w3.org</email></address>
2669    </author>
2670    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2671      <organization abbrev="W3C">World Wide Web Consortium</organization>
2672      <address><email>ylafon@w3.org</email></address>
2673    </author>
2674    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2675      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2676      <address><email>julian.reschke@greenbytes.de</email></address>
2677    </author>
2678    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2679  </front>
2680  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2681  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2682</reference>
2683
2684<reference anchor="Part5">
2685  <front>
2686    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2687    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2688      <organization abbrev="Day Software">Day Software</organization>
2689      <address><email>fielding@gbiv.com</email></address>
2690    </author>
2691    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2692      <organization>One Laptop per Child</organization>
2693      <address><email>jg@laptop.org</email></address>
2694    </author>
2695    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2696      <organization abbrev="HP">Hewlett-Packard Company</organization>
2697      <address><email>JeffMogul@acm.org</email></address>
2698    </author>
2699    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2700      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2701      <address><email>henrikn@microsoft.com</email></address>
2702    </author>
2703    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2704      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2705      <address><email>LMM@acm.org</email></address>
2706    </author>
2707    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2708      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2709      <address><email>paulle@microsoft.com</email></address>
2710    </author>
2711    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2712      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2713      <address><email>timbl@w3.org</email></address>
2714    </author>
2715    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2716      <organization abbrev="W3C">World Wide Web Consortium</organization>
2717      <address><email>ylafon@w3.org</email></address>
2718    </author>
2719    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2720      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2721      <address><email>julian.reschke@greenbytes.de</email></address>
2722    </author>
2723    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2724  </front>
2725  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2726  <x:source href="p5-range.xml" basename="p5-range"/>
2727</reference>
2728
2729<reference anchor="Part7">
2730  <front>
2731    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2732    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2733      <organization abbrev="Day Software">Day Software</organization>
2734      <address><email>fielding@gbiv.com</email></address>
2735    </author>
2736    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2737      <organization>One Laptop per Child</organization>
2738      <address><email>jg@laptop.org</email></address>
2739    </author>
2740    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2741      <organization abbrev="HP">Hewlett-Packard Company</organization>
2742      <address><email>JeffMogul@acm.org</email></address>
2743    </author>
2744    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2745      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2746      <address><email>henrikn@microsoft.com</email></address>
2747    </author>
2748    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2749      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2750      <address><email>LMM@acm.org</email></address>
2751    </author>
2752    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2753      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2754      <address><email>paulle@microsoft.com</email></address>
2755    </author>
2756    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2757      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2758      <address><email>timbl@w3.org</email></address>
2759    </author>
2760    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2761      <organization abbrev="W3C">World Wide Web Consortium</organization>
2762      <address><email>ylafon@w3.org</email></address>
2763    </author>
2764    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2765      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2766      <address><email>julian.reschke@greenbytes.de</email></address>
2767    </author>
2768    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2769  </front>
2770  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2771  <x:source href="p7-auth.xml" basename="p7-auth"/>
2772</reference>
2773
2774<reference anchor="RFC2047">
2775  <front>
2776    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2777    <author initials="K." surname="Moore" fullname="Keith Moore">
2778      <organization>University of Tennessee</organization>
2779      <address><email>moore@cs.utk.edu</email></address>
2780    </author>
2781    <date month="November" year="1996"/>
2782  </front>
2783  <seriesInfo name="RFC" value="2047"/>
2784</reference>
2785
2786<reference anchor="RFC2119">
2787  <front>
2788    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2789    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2790      <organization>Harvard University</organization>
2791      <address><email>sob@harvard.edu</email></address>
2792    </author>
2793    <date month="March" year="1997"/>
2794  </front>
2795  <seriesInfo name="BCP" value="14"/>
2796  <seriesInfo name="RFC" value="2119"/>
2797</reference>
2798
2799</references>
2800
2801<references title="Informative References">
2802
2803<reference anchor="RFC1305">
2804  <front>
2805    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2806    <author initials="D." surname="Mills" fullname="David L. Mills">
2807      <organization>University of Delaware, Electrical Engineering Department</organization>
2808      <address><email>mills@udel.edu</email></address>
2809    </author>
2810    <date month="March" year="1992"/>
2811  </front>
2812  <seriesInfo name="RFC" value="1305"/>
2813</reference>
2814
2815<reference anchor="RFC2616">
2816  <front>
2817    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2818    <author initials="R." surname="Fielding" fullname="R. Fielding">
2819      <organization>University of California, Irvine</organization>
2820      <address><email>fielding@ics.uci.edu</email></address>
2821    </author>
2822    <author initials="J." surname="Gettys" fullname="J. Gettys">
2823      <organization>W3C</organization>
2824      <address><email>jg@w3.org</email></address>
2825    </author>
2826    <author initials="J." surname="Mogul" fullname="J. Mogul">
2827      <organization>Compaq Computer Corporation</organization>
2828      <address><email>mogul@wrl.dec.com</email></address>
2829    </author>
2830    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2831      <organization>MIT Laboratory for Computer Science</organization>
2832      <address><email>frystyk@w3.org</email></address>
2833    </author>
2834    <author initials="L." surname="Masinter" fullname="L. Masinter">
2835      <organization>Xerox Corporation</organization>
2836      <address><email>masinter@parc.xerox.com</email></address>
2837    </author>
2838    <author initials="P." surname="Leach" fullname="P. Leach">
2839      <organization>Microsoft Corporation</organization>
2840      <address><email>paulle@microsoft.com</email></address>
2841    </author>
2842    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2843      <organization>W3C</organization>
2844      <address><email>timbl@w3.org</email></address>
2845    </author>
2846    <date month="June" year="1999"/>
2847  </front>
2848  <seriesInfo name="RFC" value="2616"/>
2849</reference>
2850
2851<reference anchor='RFC3864'>
2852  <front>
2853    <title>Registration Procedures for Message Header Fields</title>
2854    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
2855      <organization>Nine by Nine</organization>
2856      <address><email>GK-IETF@ninebynine.org</email></address>
2857    </author>
2858    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
2859      <organization>BEA Systems</organization>
2860      <address><email>mnot@pobox.com</email></address>
2861    </author>
2862    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
2863      <organization>HP Labs</organization>
2864      <address><email>JeffMogul@acm.org</email></address>
2865    </author>
2866    <date year='2004' month='September' />
2867  </front>
2868  <seriesInfo name='BCP' value='90' />
2869  <seriesInfo name='RFC' value='3864' />
2870</reference>
2871
2872</references>
2873
2874<section title="Compatibility with Previous Versions" anchor="compatibility">
2875
2876<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2877<t>
2878   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2879   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2880   <xref target="header.cache-control" format="counter"/>,
2881   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2882</t>
2883<t>
2884   Transfer-coding and message lengths all interact in ways that
2885   required fixing exactly when chunked encoding is used (to allow for
2886   transfer encoding that may not be self delimiting); it was important
2887   to straighten out exactly how message lengths are computed.
2888   (<xref target="non-modifiable.headers"/>,
2889   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2890</t>
2891<t>
2892   Proxies should be able to add Content-Length when appropriate.
2893   (<xref target="non-modifiable.headers"/>)
2894</t>
2895<t>
2896   Range request responses would become very verbose if all meta-data
2897   were always returned; by allowing the server to only send needed
2898   headers in a 206 response, this problem can be avoided.
2899   (<xref target="combining.headers"/>)
2900</t>
2901<t>
2902   The Cache-Control: max-age directive was not properly defined for
2903   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2904</t>
2905<t>
2906   Warnings could be cached incorrectly, or not updated appropriately.
2907   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2908   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2909   and <xref target="header.warning" format="counter"/>) Warning
2910   also needed to be a general header, as PUT or other methods may have
2911   need for it in requests.
2912</t>
2913</section>
2914
2915<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2916<t>
2917  Clarify denial of service attack avoidance requirement.
2918  (<xref target="invalidation.after.updates.or.deletions"/>)
2919</t>
2920</section>
2921
2922</section>
2923
2924<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
2925
2926<section title="Since RFC2616">
2927<t>
2928  Extracted relevant partitions from <xref target="RFC2616"/>.
2929</t>
2930</section>
2931
2932<section title="Since draft-ietf-httpbis-p6-cache-00">
2933<t>
2934  Closed issues:
2935  <list style="symbols"> 
2936    <t>
2937      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
2938      "Trailer"
2939      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
2940    </t>
2941    <t>
2942      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
2943      "Invalidation after Update or Delete"
2944      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
2945    </t>
2946    <t>
2947      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2948      "Normative and Informative references"
2949    </t>
2950    <t>
2951      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
2952      "Date reference typo"
2953    </t>
2954    <t>
2955      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
2956      "Connection header text"
2957    </t>
2958    <t>
2959      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2960      "Informative references"
2961    </t>
2962    <t>
2963      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2964      "ISO-8859-1 Reference"
2965    </t>
2966    <t>
2967      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2968      "Normative up-to-date references"
2969    </t>
2970    <t>
2971      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
2972      "typo in 13.2.2"
2973    </t>
2974  </list>
2975</t>
2976<t>
2977  Other changes:
2978  <list style="symbols"> 
2979    <t>
2980      Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
2981    </t>
2982  </list>
2983</t>
2984</section>
2985
2986<section title="Since draft-ietf-httpbis-p6-cache-01">
2987<t>
2988  Closed issues:
2989  <list style="symbols"> 
2990    <t>
2991      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
2992      "rel_path not used"
2993    </t>
2994  </list>
2995</t>
2996<t>
2997  Other changes:
2998  <list style="symbols"> 
2999    <t>
3000       Get rid of duplicate BNF rule names ("host" -> "uri-host")
3001       (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
3002    </t>
3003    <t>
3004      Add explicit references to BNF syntax and rules imported from other parts of the specification.
3005    </t>
3006  </list>
3007</t>
3008</section>
3009
3010<section title="Since draft-ietf-httpbis-p6-cache-02" anchor="changes.since.02">
3011<t>
3012  Ongoing work on IANA Message Header Registration (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
3013  <list style="symbols"> 
3014    <t>
3015      Reference RFC 3984, and update header registrations for headers defined
3016      in this document.
3017    </t>
3018  </list>
3019</t>
3020</section>
3021
3022<section title="Since draft-ietf-httpbis-p6-cache-03" anchor="changes.since.03">
3023<t>
3024</t>
3025</section>
3026
3027</section>
3028
3029</back>
3030</rfc>
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