source: draft-ietf-httpbis/latest/p6-cache.xml @ 973

Last change on this file since 973 was 973, checked in by julian.reschke@…, 9 years ago

prepare publication of -11 drafts on 2010-08-04.

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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 "August">
16  <!ENTITY ID-YEAR "2010">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY effective-request-uri      "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36]>
37<?rfc toc="yes" ?>
38<?rfc symrefs="yes" ?>
39<?rfc sortrefs="yes" ?>
40<?rfc compact="yes"?>
41<?rfc subcompact="no" ?>
42<?rfc linkmailto="no" ?>
43<?rfc editing="no" ?>
44<?rfc comments="yes"?>
45<?rfc inline="yes"?>
46<?rfc rfcedstyle="yes"?>
47<?rfc-ext allow-markup-in-artwork="yes" ?>
48<?rfc-ext include-references-in-index="yes" ?>
49<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
50  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
51<front>
52
53  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
54
55  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
56    <organization abbrev="Day Software">Day Software</organization>
57    <address>
58      <postal>
59        <street>23 Corporate Plaza DR, Suite 280</street>
60        <city>Newport Beach</city>
61        <region>CA</region>
62        <code>92660</code>
63        <country>USA</country>
64      </postal>
65      <phone>+1-949-706-5300</phone>
66      <facsimile>+1-949-706-5305</facsimile>
67      <email>fielding@gbiv.com</email>
68      <uri>http://roy.gbiv.com/</uri>
69    </address>
70  </author>
71
72  <author initials="J." surname="Gettys" fullname="Jim Gettys">
73    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
74    <address>
75      <postal>
76        <street>21 Oak Knoll Road</street>
77        <city>Carlisle</city>
78        <region>MA</region>
79        <code>01741</code>
80        <country>USA</country>
81      </postal>
82      <email>jg@freedesktop.org</email>
83      <uri>http://gettys.wordpress.com/</uri>
84    </address>
85  </author>
86
87  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
88    <organization abbrev="HP">Hewlett-Packard Company</organization>
89    <address>
90      <postal>
91        <street>HP Labs, Large Scale Systems Group</street>
92        <street>1501 Page Mill Road, MS 1177</street>
93        <city>Palo Alto</city>
94        <region>CA</region>
95        <code>94304</code>
96        <country>USA</country>
97      </postal>
98      <email>JeffMogul@acm.org</email>
99    </address>
100  </author>
101
102  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
103    <organization abbrev="Microsoft">Microsoft Corporation</organization>
104    <address>
105      <postal>
106        <street>1 Microsoft Way</street>
107        <city>Redmond</city>
108        <region>WA</region>
109        <code>98052</code>
110        <country>USA</country>
111      </postal>
112      <email>henrikn@microsoft.com</email>
113    </address>
114  </author>
115
116  <author fullname="Larry Masinter" initials="L." surname="Masinter">
117    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
118    <address>
119      <postal>
120        <street>345 Park Ave</street>
121        <city>San Jose</city>
122        <region>CA</region>
123        <code>95110</code>
124        <country>USA</country>
125      </postal>
126      <email>LMM@acm.org</email>
127      <uri>http://larry.masinter.net/</uri>
128    </address>
129  </author>
130
131  <author fullname="Paul J. Leach" initials="P." surname="Leach">
132    <organization abbrev="Microsoft">Microsoft Corporation</organization>
133    <address>
134      <postal>
135        <street>1 Microsoft Way</street>
136        <city>Redmond</city>
137        <region>WA</region>
138        <code>98052</code>
139      </postal>
140      <email>paulle@microsoft.com</email>
141    </address>
142  </author>
143
144  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
145    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
146    <address>
147      <postal>
148        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
149        <street>The Stata Center, Building 32</street>
150        <street>32 Vassar Street</street>
151        <city>Cambridge</city>
152        <region>MA</region>
153        <code>02139</code>
154        <country>USA</country>
155      </postal>
156      <email>timbl@w3.org</email>
157      <uri>http://www.w3.org/People/Berners-Lee/</uri>
158    </address>
159  </author>
160
161  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
162    <organization abbrev="W3C">World Wide Web Consortium</organization>
163    <address>
164      <postal>
165        <street>W3C / ERCIM</street>
166        <street>2004, rte des Lucioles</street>
167        <city>Sophia-Antipolis</city>
168        <region>AM</region>
169        <code>06902</code>
170        <country>France</country>
171      </postal>
172      <email>ylafon@w3.org</email>
173      <uri>http://www.raubacapeu.net/people/yves/</uri>
174    </address>
175  </author>
176
177  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
178    <address>
179      <email>mnot@mnot.net</email>
180      <uri>http://www.mnot.net/</uri>
181    </address>
182  </author>
183
184  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
185    <organization abbrev="greenbytes">greenbytes GmbH</organization>
186    <address>
187      <postal>
188        <street>Hafenweg 16</street>
189        <city>Muenster</city><region>NW</region><code>48155</code>
190        <country>Germany</country>
191      </postal>
192      <phone>+49 251 2807760</phone>
193      <facsimile>+49 251 2807761</facsimile>
194      <email>julian.reschke@greenbytes.de</email>
195      <uri>http://greenbytes.de/tech/webdav/</uri>
196    </address>
197  </author>
198
199  <date month="&ID-MONTH;" year="&ID-YEAR;" />
200  <workgroup>HTTPbis Working Group</workgroup>
201
202<abstract>
203<t>
204   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
205   distributed, collaborative, hypermedia information systems. This document
206   is Part 6 of the seven-part specification that defines the protocol
207   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6
208   defines requirements on HTTP caches and the associated header fields that
209   control cache behavior or indicate cacheable response messages.
210</t>
211</abstract>
212
213<note title="Editorial Note (To be removed by RFC Editor)">
214   <t>
215      Discussion of this draft should take place on the HTTPBIS working group
216      mailing list (ietf-http-wg@w3.org). The current issues list is at <eref
217      target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
218      documents (including fancy diffs) can be found at <eref
219      target="http://tools.ietf.org/wg/httpbis/"/>.
220   </t>
221   <t>
222      The changes in this draft are summarized in <xref
223      target="changes.since.11"/>.
224   </t>
225</note>
226
227   </front>
228   <middle>
229
230<section anchor="caching" title="Introduction">
231<t>
232   HTTP is typically used for distributed information systems, where
233   performance can be improved by the use of response caches. This document
234   defines aspects of HTTP/1.1 related to caching and reusing response
235   messages.
236</t>
237
238<section anchor="intro.purpose" title="Purpose">
239<iref item="cache" />
240<t>
241   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
242   subsystem that controls its message storage, retrieval, and deletion. A
243   cache stores cacheable responses in order to reduce the response time and
244   network bandwidth consumption on future, equivalent requests. Any client or
245   server &MAY; employ a cache, though a cache cannot be used by a server that
246   is acting as a tunnel.
247</t>
248<t>
249   Caching would be useless if it did not significantly improve performance.
250   The goal of caching in HTTP/1.1 is to reuse a prior response message to
251   satisfy a current request. In some cases, a stored response can be reused
252   without the need for a network request, reducing latency and network
253   round-trips; a "freshness" mechanism is used for this purpose (see <xref
254   target="expiration.model" />). Even when a new request is required, it is
255   often possible to reuse all or parts of the payload of a prior response to
256   satisfy the request, thereby reducing network bandwidth usage; a
257   "validation" mechanism is used for this purpose (see <xref
258   target="validation.model" />).
259</t>
260</section>
261
262<section anchor="intro.terminology" title="Terminology">
263<t>
264   This specification uses a number of terms to refer to the roles played by
265   participants in, and objects of, HTTP caching.
266</t>
267<t>
268   <iref item="cacheable" />
269   <x:dfn>cacheable</x:dfn>
270   <list>
271      <t>A response is cacheable if a cache is allowed to store a copy of the
272      response message for use in answering subsequent requests. Even when a
273      response is cacheable, there might be additional constraints on whether
274      a cache can use the cached copy to satisfy a particular request.</t>
275   </list>
276</t>
277<t>
278   <iref item="explicit expiration time" />
279   <x:dfn>explicit expiration time</x:dfn>
280   <list>
281      <t>The time at which the origin server intends that a representation
282      no longer be returned by a cache without further validation.</t>
283   </list>
284</t>
285<t>
286   <iref item="heuristic expiration time" />
287   <x:dfn>heuristic expiration time</x:dfn>
288   <list>
289      <t>An expiration time assigned by a cache when no explicit expiration
290      time is available.</t>
291   </list>
292</t>
293<t>
294   <iref item="age" />
295   <x:dfn>age</x:dfn>
296   <list>
297      <t>The age of a response is the time since it was sent by, or
298      successfully validated with, the origin server.</t>
299   </list>
300</t>
301<t>
302   <iref item="first-hand" />
303   <x:dfn>first-hand</x:dfn>
304   <list>
305      <t>A response is first-hand if the freshness model is not in use; i.e.,
306      its age is 0.</t>
307   </list>
308</t>
309<t>
310   <iref item="freshness lifetime" />
311   <x:dfn>freshness lifetime</x:dfn>
312   <list>
313      <t>The length of time between the generation of a response and its
314      expiration time.</t>
315   </list>
316</t>
317<t>
318   <iref item="fresh" />
319   <x:dfn>fresh</x:dfn>
320   <list>
321      <t>A response is fresh if its age has not yet exceeded its freshness
322      lifetime.</t>
323   </list>
324</t>
325<t>
326   <iref item="stale" />
327   <x:dfn>stale</x:dfn>
328   <list>
329      <t>A response is stale if its age has passed its freshness lifetime
330      (either explicit or heuristic).</t>
331   </list>
332</t>
333<t>
334   <iref item="validator" />
335   <x:dfn>validator</x:dfn>
336   <list>
337      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
338      is used to find out whether a stored response has an equivalent copy of
339      a representation.</t>
340   </list>
341</t>
342<t anchor="shared.and.non-shared.caches">
343   <iref item="validator" />
344   <x:dfn>shared cache</x:dfn>
345   <list>
346      <t>A cache that is accessible to more than one user. A non-shared cache
347      is dedicated to a single user.</t>
348   </list>
349</t>
350</section>
351
352<section anchor="intro.requirements" title="Requirements">
353<t>
354   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
355   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
356   document are to be interpreted as described in <xref target="RFC2119"/>.
357</t>
358<t>
359   An implementation is not compliant if it fails to satisfy one or more of
360   the "MUST" or "REQUIRED" level requirements for the protocols it
361   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
362   level and all the "SHOULD" level requirements for its protocols is said to
363   be "unconditionally compliant"; one that satisfies all the "MUST" level
364   requirements but not all the "SHOULD" level requirements for its protocols
365   is said to be "conditionally compliant".
366</t>
367</section>
368
369<section title="Syntax Notation" anchor="notation">
370   <x:anchor-alias value="ALPHA"/>
371   <x:anchor-alias value="CR"/>
372   <x:anchor-alias value="DIGIT"/>
373   <x:anchor-alias value="DQUOTE"/>
374   <x:anchor-alias value="LF"/>
375   <x:anchor-alias value="OCTET"/>
376   <x:anchor-alias value="SP"/>
377   <x:anchor-alias value="VCHAR"/>
378   <x:anchor-alias value="WSP"/>
379<t>
380   This specification uses the ABNF syntax defined in &notation; (which
381   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
382   <xref target="collected.abnf"/> shows the collected ABNF, with the list
383   rule expanded.
384</t>
385<t>
386   The following core rules are included by reference, as defined in <xref
387   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
388   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
389   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
390   sequence of data), SP (space), VCHAR (any visible USASCII character), and
391   WSP (whitespace).
392</t>
393
394<section title="Core Rules" anchor="core.rules">
395   <x:anchor-alias value="quoted-string"/>
396   <x:anchor-alias value="token"/>
397   <x:anchor-alias value="OWS"/>
398<t>
399   The core rules below are defined in &basic-rules;:
400</t>
401<figure><artwork type="abnf2616">
402  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
403  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
404  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
405</artwork></figure>
406</section>
407
408<section title="ABNF Rules defined in other Parts of the Specification"
409    anchor="abnf.dependencies">
410   <x:anchor-alias value="field-name"/>
411   <x:anchor-alias value="HTTP-date"/>
412   <x:anchor-alias value="port"/>
413   <x:anchor-alias value="pseudonym"/>
414   <x:anchor-alias value="uri-host"/>
415<t>
416   The ABNF rules below are defined in other parts:
417</t>
418<figure><!--Part1--><artwork type="abnf2616">
419  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
420  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
421  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
422  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
423  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
424</artwork></figure>
425</section>
426
427</section>
428</section>
429
430<section anchor="caching.overview" title="Cache Operation">
431
432<section anchor="response.cacheability" title="Response Cacheability">
433<t>
434   A cache &MUST-NOT; store a response to any request, unless:
435   <list style="symbols">
436      <t>The request method is understood by the cache and defined as being
437      cacheable, and</t>
438      <t>the response status code is understood by the cache, and</t>
439      <t>the "no-store" cache directive (see <xref
440      target="header.cache-control" />) does not appear in request or response
441      headers, and</t>
442      <t>the "private" cache response directive (see <xref
443      target="cache-response-directive" /> does not appear in the response, if
444      the cache is shared, and</t>
445      <t>the "Authorization" header (see &header-authorization;) does not
446      appear in the request, if the cache is shared, unless the response
447      explicitly allows it (see <xref target="caching.authenticated.responses"
448      />), and</t>
449      <t>the response either:
450         <list style="symbols">
451            <t>contains an Expires header (see <xref target="header.expires"
452            />), or</t>
453            <t>contains a max-age response cache directive (see <xref
454            target="cache-response-directive" />), or</t>
455            <t>contains a s-maxage response cache directive and the cache is
456            shared, or</t>
457            <t>contains a Cache Control Extension (see <xref
458            target="cache.control.extensions" />) that allows it to be cached,
459            or</t>
460            <t>has a status code that can be served with heuristic freshness
461            (see <xref target="heuristic.freshness" />).</t>
462         </list>
463      </t>
464   </list>
465</t>
466<t>
467   In this context, a cache has "understood" a request method or a response
468   status code if it recognises it and implements any cache-specific
469   behaviour. In particular, 206 Partial Content responses cannot be cached by
470   an implementation that does not handle partial content (see <xref
471   target="errors.or.incomplete.response.cache.behavior" />).
472</t>
473<t>
474   Note that in normal operation, most caches will not store a response that
475   has neither a cache validator nor an explicit expiration time, as such
476   responses are not usually useful to store. However, caches are not
477   prohibited from storing such responses.
478</t>
479
480<section anchor="errors.or.incomplete.response.cache.behavior" 
481   title="Storing Partial and Incomplete Responses">
482<t>
483   A cache that receives an incomplete response (for example, with fewer bytes
484   of data than specified in a Content-Length header) can store the response,
485   but &MUST; treat it as a partial response &partial;. Partial responses can
486   be combined as described in &combining-byte-ranges;; the result might be a
487   full response or might still be partial. A cache &MUST-NOT; return a
488   partial response to a client without explicitly marking it as such using
489   the 206 (Partial Content) status code.
490</t>
491<t>
492   A cache that does not support the Range and Content-Range headers
493   &MUST-NOT; store incomplete or partial responses.
494</t>
495</section>
496
497</section>
498
499
500<section anchor="constructing.responses.from.caches" 
501   title="Constructing Responses from Caches">
502<t>
503   For a presented request, a cache &MUST-NOT; return a stored response,
504   unless:
505   <list style="symbols">
506      <t>The presented effective request URI (&effective-request-uri;) and
507      that of the stored response match, and</t>
508      <t>the request method associated with the stored response allows it to
509      be used for the presented request, and</t>
510      <t>selecting request-headers nominated by the stored response (if any)
511      match those presented (see <xref target="caching.negotiated.responses"
512      />), and</t>
513      <t>the presented request and stored response are free from directives
514      that would prevent its use (see <xref target="header.cache-control" />
515      and <xref target="header.pragma"/>), and</t>
516      <t>the stored response is either:
517         <list style="symbols">
518            <t>fresh (see <xref target="expiration.model" />), or</t>
519            <t>allowed to be served stale (see <xref
520            target="serving.stale.responses" />), or</t>
521            <t>successfully validated (see <xref target="validation.model"
522            />).</t>
523         </list>
524      </t>
525  </list>
526</t>
527<t>
528   When a stored response is used to satisfy a request without validation,
529   caches &MUST; include a single Age header field (<xref target="header.age"
530   />) in the response with a value equal to the stored response's
531   current_age; see <xref target="age.calculations" />.
532</t>
533<t>
534   Requests with methods that are unsafe (&safe-methods;) &MUST; be written
535   through the cache to the origin server; i.e., a cache must not reply to
536   such a request before having forwarded the request and having received a
537   corresponding response.
538</t>
539<t>
540   Also, note that unsafe requests might invalidate already stored responses;
541   see <xref target="invalidation.after.updates.or.deletions" />.
542</t>
543<t>
544   Caches &MUST; use the most recent response (as determined by the Date
545   header) when more than one suitable response is stored. They can also
546   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
547   no-cache" to disambiguate which response to use.
548</t>
549</section>
550
551<section anchor="expiration.model" title="Freshness Model">
552<t>
553   When a response is "fresh" in the cache, it can be used to satisfy
554   subsequent requests without contacting the origin server, thereby improving
555   efficiency.
556</t>
557<t>
558   The primary mechanism for determining freshness is for an origin server to
559   provide an explicit expiration time in the future, using either the Expires
560   header (<xref target="header.expires" />) or the max-age response cache
561   directive (<xref target="cache-response-directive" />). Generally, origin
562   servers will assign future explicit expiration times to responses in the
563   belief that the representation is not likely to change in a semantically
564   significant way before the expiration time is reached.
565</t>
566<t>
567   If an origin server wishes to force a cache to validate every request, it
568   can assign an explicit expiration time in the past to indicate that the
569   response is already stale.  Compliant caches will validate the cached response
570   before reusing it for subsequent requests.
571</t>
572<t>
573   Since origin servers do not always provide explicit expiration times, HTTP
574   caches &MAY; assign heuristic expiration times when explicit times are not
575   specified, employing algorithms that use other header values (such as the
576   Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1
577   specification does not provide specific algorithms, but does impose
578   worst-case constraints on their results.
579</t>
580<figure>
581<preamble>
582  The calculation to determine if a response is fresh is:
583</preamble>
584<artwork type="code">
585   response_is_fresh = (freshness_lifetime &gt; current_age)
586</artwork>
587</figure>
588<t>
589   The freshness_lifetime is defined in <xref
590   target="calculating.freshness.lifetime" />; the current_age is defined in
591   <xref target="age.calculations" />.
592</t>
593<t>
594   Additionally, clients might need to influence freshness calculation. They
595   can do this using several request cache directives, with the effect of
596   either increasing or loosening constraints on freshness. See <xref
597   target="cache-request-directive" />.
598</t>
599<t>
600   <cref anchor="ISSUE-no-req-for-directives">there are not requirements
601   directly applying to cache-request-directives and freshness.</cref>
602</t>
603<t>
604   Note that freshness applies only to cache operation; it cannot be used to
605   force a user agent to refresh its display or reload a resource. See <xref
606   target="history.lists" /> for an explanation of the difference between
607   caches and history mechanisms.
608</t>
609
610<section anchor="calculating.freshness.lifetime" 
611   title="Calculating Freshness Lifetime">
612<t>
613   A cache can calculate the freshness lifetime (denoted as
614   freshness_lifetime) of a response by using the first match of:
615   <list style="symbols">
616      <t>If the cache is shared and the s-maxage response cache directive
617      (<xref target="cache-response-directive" />) is present, use its value,
618      or</t>
619      <t>If the max-age response cache directive (<xref
620      target="cache-response-directive" />) is present, use its value, or</t>
621      <t>If the Expires response header (<xref target="header.expires" />) is
622      present, use its value minus the value of the Date response header,
623      or</t>
624      <t>Otherwise, no explicit expiration time is present in the response. A
625      heuristic freshness lifetime might be applicable; see <xref
626      target="heuristic.freshness" />.</t>
627   </list>
628</t>
629<t>
630   Note that this calculation is not vulnerable to clock skew, since all of
631   the information comes from the origin server.
632</t>
633
634<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
635<t>
636   If no explicit expiration time is present in a stored response that has a
637   status code whose definition allows heuristic freshness to be used
638   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
639   410), a heuristic expiration time &MAY; be calculated. Heuristics
640   &MUST-NOT; be used for response status codes that do not explicitly allow
641   it.
642</t>
643<t>
644   When a heuristic is used to calculate freshness lifetime, the cache
645   &SHOULD; attach a Warning header with a 113 warn-code to the response if
646   its current_age is more than 24 hours and such a warning is not already
647   present.
648</t>
649<t>
650   Also, if the response has a Last-Modified header (&header-last-modified;),
651   the heuristic expiration value &SHOULD; be no more than some fraction of
652   the interval since that time. A typical setting of this fraction might be
653   10%.
654</t>
655<x:note>
656   <t>
657      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
658      x:sec="13.9"/>) required that caches do not calculate heuristic
659      freshness for URLs with query components (i.e., those containing '?').
660      In practice, this has not been widely implemented. Therefore, servers
661      are encouraged to send explicit directives (e.g., Cache-Control:
662      no-cache) if they wish to preclude caching.
663   </t>
664</x:note>
665</section>
666</section>
667
668<section anchor="age.calculations" title="Calculating Age">
669<t>
670   HTTP/1.1 uses the Age response-header to convey the estimated age of the
671   response message when obtained from a cache. The Age field value is the
672   cache's estimate of the amount of time since the response was generated or
673   validated by the origin server. In essence, the Age value is the sum of the
674   time that the response has been resident in each of the caches along the
675   path from the origin server, plus the amount of time it has been in transit
676   along network paths.
677</t>
678<t>
679   The following data is used for the age calculation:
680</t>
681<t>
682   <x:dfn>age_value</x:dfn>
683   <list>
684      <t>
685         The term "age_value" denotes the value of the Age header (<xref
686         target="header.age"/>), in a form appropriate for arithmetic
687         operation; or 0, if not available.
688      </t>
689   </list>
690</t>
691<t>
692   <x:dfn>date_value</x:dfn>
693   <list>
694      <t>
695         HTTP/1.1 requires origin servers to send a Date header, if possible,
696         with every response, giving the time at which the response was
697         generated. The term "date_value" denotes the value of the Date
698         header, in a form appropriate for arithmetic operations. See
699         &header-date; for the definition of the Date header, and for
700         requirements regarding responses without a Date response header.
701      </t>
702   </list>
703</t>
704<t>
705   <x:dfn>now</x:dfn>
706   <list>
707      <t>
708         The term "now" means "the current value of the clock at the host
709         performing the calculation". Hosts that use HTTP, but especially
710         hosts running origin servers and caches, &SHOULD; use NTP (<xref
711         target="RFC1305"/>) or some similar protocol to synchronize their
712         clocks to a globally accurate time standard.
713      </t>
714   </list>
715</t>
716<t>
717   <x:dfn>request_time</x:dfn>
718   <list>
719      <t>
720         The current value of the clock at the host at the time the request
721         resulting in the stored response was made.
722      </t>
723   </list>
724</t>
725<t>
726   <x:dfn>response_time</x:dfn>
727   <list>
728      <t>
729         The current value of the clock at the host at the time the response
730         was received.
731      </t>
732   </list>
733</t>
734<t>
735   A response's age can be calculated in two entirely independent ways:
736   <list style="numbers">
737      <t>the "apparent_age": response_time minus date_value, if the local
738      clock is reasonably well synchronized to the origin server's clock. If
739      the result is negative, the result is replaced by zero.</t>
740      <t>the "corrected_age_value", if all of the caches along the response
741      path implement HTTP/1.1; note this value &MUST; be interpreted relative
742      to the time the request was initiated, not the time that the response
743      was received.</t>
744   </list>
745</t>
746<figure>
747<artwork type="code">
748  apparent_age = max(0, response_time - date_value);
749
750  response_delay = response_time - request_time;
751  corrected_age_value = age_value + response_delay; 
752</artwork>
753</figure>
754<figure>
755<preamble>These are combined as</preamble>
756<artwork type="code">
757  corrected_initial_age = max(apparent_age, corrected_age_value);
758</artwork></figure>
759<t>
760   The current_age of a stored response can then be calculated by adding the
761   amount of time (in seconds) since the stored response was last validated by
762   the origin server to the corrected_initial_age.
763</t>
764<figure><artwork type="code">
765  resident_time = now - response_time;
766  current_age = corrected_initial_age + resident_time;
767</artwork></figure>
768</section>
769
770<section anchor="serving.stale.responses" title="Serving Stale Responses">
771<t>
772   A "stale" response is one that either has explicit expiry information or is
773   allowed to have heuristic expiry calculated, but is not fresh according to
774   the calculations in <xref target="expiration.model" />.
775</t>
776<t>
777   Caches &MUST-NOT; return a stale response if it is prohibited by an
778   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
779   directive, a "must-revalidate" cache-response-directive, or an applicable
780   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
781   target="cache-response-directive"/>).
782</t>
783<t>
784   Caches &SHOULD-NOT; return stale responses unless they are disconnected
785   (i.e., it cannot contact the origin server or otherwise find a forward
786   path) or otherwise explicitly allowed (e.g., the max-stale request
787   directive; see <xref target="cache-request-directive" />).
788</t>
789<t>
790   Stale responses &SHOULD; have a Warning header with the 110 warn-code (see
791   <xref target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be
792   sent on stale responses if the cache is disconnected.
793</t>
794<t>
795   If a cache receives a first-hand response (either an entire response, or a
796   304 (Not Modified) response) that it would normally forward to the
797   requesting client, and the received response is no longer fresh, the cache
798   &SHOULD; forward it to the requesting client without adding a new Warning
799   (but without removing any existing Warning headers). A cache &SHOULD-NOT;
800   attempt to validate a response simply because that response became stale in
801   transit.
802</t>
803</section>
804</section>
805
806<section anchor="validation.model" title="Validation Model">
807<t>
808   When a cache has one or more stored responses for a requested URI, but
809   cannot serve any of them (e.g., because they are not fresh, or one cannot
810   be selected; see <xref target="caching.negotiated.responses"/>), it can use
811   the conditional request mechanism &conditional; in the forwarded request to
812   give the origin server an opportunity to both select a valid stored
813   response to be used, and to update it. This process is known as
814   "validating" or "revalidating" the stored response.
815</t>
816<t>
817   When sending such a conditional request, the cache &SHOULD; add an
818   If-Modified-Since header whose value is that of the Last-Modified header
819   from the selected (see <xref target="caching.negotiated.responses"/>)
820   stored response, if available.
821</t>
822<t>
823   Additionally, the cache &SHOULD; add an If-None-Match header whose value is
824   that of the ETag header(s) from all responses stored for the requested URI,
825   if present. However, if any of the stored responses contains only partial
826   content, its entity-tag &SHOULD-NOT; be included in the If-None-Match
827   header field unless the request is for a range that would be fully
828   satisfied by that stored response.
829</t>
830<t>
831   A 304 (Not Modified) response status code indicates that the stored
832   response can be updated and reused; see <xref target="combining.headers"/>.
833</t>
834<t>
835   A full response (i.e., one with a response body) indicates that none of the
836   stored responses nominated in the conditional request is suitable. Instead,
837   the full response &SHOULD; be used to satisfy the request and &MAY; replace
838   the stored response.
839</t>
840<t>
841   If a cache receives a 5xx response while attempting to validate a response,
842   it &MAY; either forward this response to the requesting client, or act as
843   if the server failed to respond. In the latter case, it &MAY; return a
844   previously stored response (see <xref target="serving.stale.responses" />).
845</t>
846</section>
847
848<section anchor="invalidation.after.updates.or.deletions" 
849   title="Request Methods that Invalidate">
850<t>
851   Because unsafe methods (&safe-methods;) have the potential for changing
852   state on the origin server, intervening caches can use them to keep their
853   contents up-to-date.
854</t>
855<t>
856   The following HTTP methods &MUST; cause a cache to invalidate the effective
857   Request URI (&effective-request-uri;) as well as the URI(s) in the Location
858   and Content-Location headers (if present):
859   <list style="symbols">
860      <t>PUT</t>
861      <t>DELETE</t>
862      <t>POST</t>
863   </list>
864</t>
865<t>
866   An invalidation based on a URI from a Location or Content-Location header
867   &MUST-NOT; be performed if the host part of that URI differs from the host
868   part in the effective request URI (&effective-request-uri;). This helps
869   prevent denial of service attacks.
870</t>
871<t>
872   A cache that passes through requests for methods it does not understand
873   &SHOULD; invalidate the effective request URI (&effective-request-uri;).
874</t>
875<t>
876   Here, "invalidate" means that the cache will either remove all stored
877   responses related to the effective request URI, or will mark these as
878   "invalid" and in need of a mandatory validation before they can be returned
879   in response to a subsequent request.
880</t>
881<t>
882   Note that this does not guarantee that all appropriate responses are
883   invalidated. For example, the request that caused the change at the origin
884   server might not have gone through the cache where a response is stored.
885</t>
886</section>
887
888<section anchor="caching.authenticated.responses" 
889   title="Shared Caching of Authenticated Responses">
890
891<t>
892   Shared caches &MUST-NOT; use a cached response to a request with an
893   Authorization header (&header-authorization;) to satisfy any subsequent
894   request unless a cache directive that allows such responses to be stored is
895   present in the response.
896</t>
897
898<t>
899   In this specification, the following Cache-Control response directives
900   (<xref target="cache-response-directive"/>) have such an effect:
901   must-revalidate, public, s-maxage.
902</t>
903
904<t>
905   Note that cached responses that contain the "must-revalidate" and/or
906   "s-maxage" response directives are not allowed to be served stale (<xref
907   target="serving.stale.responses"/>) by shared caches. In particular, a
908   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
909   used to satisfy a subsequent request without revalidating it on the origin
910   server.
911</t>
912</section>
913
914<section anchor="caching.negotiated.responses" 
915   title="Caching Negotiated Responses">
916<t>
917   When a cache receives a request that can be satisfied by a stored response
918   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
919   use that response unless all of the selecting request-headers nominated by
920   the Vary header match in both the original request (i.e., that associated
921   with the stored response), and the presented request.
922</t>
923<t>
924   The selecting request-headers from two requests are defined to match if and
925   only if those in the first request can be transformed to those in the
926   second request by applying any of the following:
927   <list style="symbols">
928      <t>
929         adding or removing whitespace, where allowed in the header's syntax
930      </t>
931      <t>
932         combining multiple message-header fields with the same field name
933         (see &header-fields;)
934      </t>
935      <t>
936         normalizing both header values in a way that is known to have
937         identical semantics, according to the header's specification (e.g.,
938         re-ordering field values when order is not significant;
939         case-normalization, where values are defined to be case-insensitive)
940      </t>
941  </list>
942</t>
943<t>
944   If (after any normalization that might take place) a header field is absent
945   from a request, it can only match another request if it is also absent
946   there.
947</t>
948<t>
949   A Vary header field-value of "*" always fails to match, and subsequent
950   requests to that resource can only be properly interpreted by the origin
951   server.
952</t>
953<t>
954   The stored response with matching selecting request-headers is known as the
955   selected response.
956</t>
957<t>
958   If no selected response is available, the cache &MAY; forward the presented
959   request to the origin server in a conditional request; see <xref
960   target="validation.model"/>.
961</t>
962</section>
963
964<section anchor="combining.headers" title="Combining Responses">
965<t>
966   When a cache receives a 304 (Not Modified) response or a 206 (Partial
967   Content) response (in this section, the "new" response"), it needs to
968   created an updated response by combining the stored response with the new
969   one, so that the updated response can be used to satisfy the request, and
970   potentially update the cached response.
971</t>
972<t>
973   If the new response contains an ETag, it identifies the stored response to
974   use. <cref anchor="TODO-mention-CL">might need language about
975   Content-Location here</cref><cref
976   anchor="TODO-select-for-combine">Shouldn't this be the selected
977   response?</cref>
978</t>
979<t>
980   If the new response's status code is 206 (partial content), both the stored
981   and new responses &MUST; have validators, and those validators &MUST; match
982   using the strong comparison function (see &weak-and-strong;). Otherwise,
983   the responses &MUST-NOT; be combined.
984</t>
985<t>
986   The stored response headers are used as those of the updated response,
987   except that
988   <list style="symbols">
989      <t>any stored Warning headers with warn-code 1xx (see <xref
990      target="header.warning" />) &MUST; be deleted.</t>
991      <t>any stored Warning headers with warn-code 2xx &MUST; be retained.</t>
992      <t>any other headers provided in the new response &MUST; replace all
993      instances of the corresponding headers from the stored response.</t>
994   </list>
995</t>
996<t>
997   The updated response headers &MUST; be used to replace those of the stored
998   response in cache (unless the stored response is removed from cache). In
999   the case of a 206 response, the combined representation &MAY; be stored.
1000</t>
1001</section>
1002
1003</section>
1004
1005<section anchor="header.fields" title="Header Field Definitions">
1006<t>
1007   This section defines the syntax and semantics of HTTP/1.1 header fields
1008   related to caching.
1009</t>
1010
1011<section anchor="header.age" title="Age">
1012   <iref item="Age header" primary="true" x:for-anchor="" />
1013   <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
1014   <x:anchor-alias value="Age"/>
1015   <x:anchor-alias value="Age-v"/>
1016   <x:anchor-alias value="age-value"/>
1017<t>
1018   The "Age" response-header field conveys the sender's estimate of the amount
1019   of time since the response was generated or successfully validated at the
1020   origin server. Age values are calculated as specified in <xref
1021   target="age.calculations" />.
1022</t>
1023<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
1024  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
1025  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
1026</artwork></figure>
1027<t anchor="rule.delta-seconds">
1028  <x:anchor-alias value="delta-seconds" />
1029  Age field-values are non-negative integers, representing time in seconds.
1030</t>
1031<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
1032  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
1033</artwork></figure>
1034<t>
1035   If a cache receives a value larger than the largest positive integer it can
1036   represent, or if any of its age calculations overflows, it &MUST; transmit
1037   an Age header with a field-value of 2147483648 (2<x:sup>31</x:sup>). Caches
1038   &SHOULD; use an arithmetic type of at least 31 bits of range.
1039</t>
1040<t>
1041   The presence of an Age header field in a response implies that a response
1042   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1043   might not implement the Age header field.
1044</t>
1045</section>
1046
1047<section anchor="header.cache-control" title="Cache-Control">
1048   <iref item="Cache-Control header" primary="true" x:for-anchor="" />
1049   <iref item="Headers" primary="true" subitem="Cache-Control" 
1050      x:for-anchor="" />
1051   <x:anchor-alias value="Cache-Control"/>
1052   <x:anchor-alias value="Cache-Control-v"/>
1053   <x:anchor-alias value="cache-directive"/>
1054   <x:anchor-alias value="cache-extension"/>
1055   <x:anchor-alias value="cache-request-directive"/>
1056   <x:anchor-alias value="cache-response-directive"/>
1057<t>
1058   The "Cache-Control" general-header field is used to specify directives for
1059   caches along the request/response chain. Such cache directives are
1060   unidirectional in that the presence of a directive in a request does not
1061   imply that the same directive is to be given in the response.
1062</t>
1063<t>
1064   HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control
1065   directives defined in this section. See <xref
1066   target="cache.control.extensions"/> for information about how Cache-Control
1067   directives defined elsewhere are handled.
1068</t>
1069<x:note>
1070   <t>
1071       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1072       might only implement Pragma: no-cache (see <xref target="header.pragma"
1073       />).
1074   </t>
1075</x:note>
1076<t>
1077   Cache directives &MUST; be passed through by a proxy or gateway
1078   application, regardless of their significance to that application, since
1079   the directives might be applicable to all recipients along the
1080   request/response chain. It is not possible to target a directive to a
1081   specific cache.
1082</t>
1083<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1084  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1085  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1086
1087  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1088     / <x:ref>cache-response-directive</x:ref>
1089
1090  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1091</artwork></figure>
1092
1093<section anchor="cache-request-directive" 
1094   title="Request Cache-Control Directives">
1095   <x:anchor-alias value="cache-request-directive" />
1096
1097<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1098   subitem="cache-request-directive" />
1099  <x:ref>cache-request-directive</x:ref> =
1100       "no-cache"
1101     / "no-store"
1102     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1103     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1104     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1105     / "no-transform"
1106     / "only-if-cached"
1107     / <x:ref>cache-extension</x:ref>
1108</artwork></figure>
1109
1110<t>
1111   <x:dfn>no-cache</x:dfn>
1112   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1113   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1114   <list>
1115      <t>The no-cache request directive indicates that a stored response
1116      &MUST-NOT; be used to satisfy the request without successful validation
1117      on the origin server.</t> 
1118   </list>
1119</t>
1120<t>
1121   <x:dfn>no-store</x:dfn>
1122   <iref item="Cache Directives" primary="true" subitem="no-store" />
1123   <iref item="no-store" primary="true" subitem="Cache Directive" />
1124   <list>
1125      <t>The no-store request directive indicates that a cache &MUST-NOT;
1126      store any part of either this request or any response to it. This
1127      directive applies to both non-shared and shared caches. "&MUST-NOT;
1128      store" in this context means that the cache &MUST-NOT; intentionally
1129      store the information in non-volatile storage, and &MUST; make a
1130      best-effort attempt to remove the information from volatile storage as
1131      promptly as possible after forwarding it.</t>
1132      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1133      privacy. In particular, malicious or compromised caches might not
1134      recognize or obey this directive, and communications networks might be
1135      vulnerable to eavesdropping.</t>
1136   </list>
1137</t>
1138<t>
1139   <x:dfn>max-age</x:dfn>
1140   <iref item="Cache Directives" primary="true" subitem="max-age" />
1141   <iref item="max-age" primary="true" subitem="Cache Directive" />
1142   <list>
1143      <t>The max-age request directive indicates that the client is willing to
1144      accept a response whose age is no greater than the specified time in
1145      seconds. Unless the max-stale request directive is also present, the
1146      client is not willing to accept a stale response.</t>
1147   </list>
1148</t>
1149<t>
1150   <x:dfn>max-stale</x:dfn>
1151   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1152   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1153   <list>
1154      <t>The max-stale request directive indicates that the client is willing
1155      to accept a response that has exceeded its expiration time. If max-stale
1156      is assigned a value, then the client is willing to accept a response
1157      that has exceeded its expiration time by no more than the specified
1158      number of seconds. If no value is assigned to max-stale, then the client
1159      is willing to accept a stale response of any age.</t>
1160   </list>
1161</t>
1162<t>
1163   <x:dfn>min-fresh</x:dfn>
1164   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1165   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1166   <list>
1167      <t>The min-fresh request directive indicates that the client is willing
1168      to accept a response whose freshness lifetime is no less than its
1169      current age plus the specified time in seconds. That is, the client
1170      wants a response that will still be fresh for at least the specified
1171      number of seconds.</t>
1172   </list>
1173</t>
1174<t>
1175   <x:dfn>no-transform</x:dfn>
1176   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1177   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1178   <list>
1179      <t>The no-transform request directive indicates that an intermediate
1180      cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
1181      Content-Type request headers, nor the request representation.</t>
1182   </list>
1183</t>
1184<t>
1185   <x:dfn>only-if-cached</x:dfn>
1186   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1187   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1188   <list>
1189      <t>The only-if-cached request directive indicates that the client only
1190      wishes to return a stored response. If it receives this directive, a
1191      cache &SHOULD; either respond using a stored response that is consistent
1192      with the other constraints of the request, or respond with a 504
1193      (Gateway Timeout) status code. If a group of caches is being operated as
1194      a unified system with good internal connectivity, such a request &MAY;
1195      be forwarded within that group of caches.</t>
1196   </list>
1197</t>
1198</section>
1199
1200<section anchor="cache-response-directive" 
1201   title="Response Cache-Control Directives">
1202   <x:anchor-alias value="cache-response-directive" />
1203
1204<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1205   subitem="cache-response-directive" />
1206  <x:ref>cache-response-directive</x:ref> =
1207       "public"
1208     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1209     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1210     / "no-store"
1211     / "no-transform"
1212     / "must-revalidate"
1213     / "proxy-revalidate"
1214     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1215     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1216     / <x:ref>cache-extension</x:ref>
1217</artwork></figure>
1218
1219<t>
1220   <x:dfn>public</x:dfn>
1221   <iref item="Cache Directives" primary="true" subitem="public" />
1222   <iref item="public" primary="true" subitem="Cache Directive" />
1223   <list>
1224      <t>The public response directive indicates that the response &MAY; be
1225      cached, even if it would normally be non-cacheable or cacheable only
1226      within a non-shared cache. (See also Authorization,
1227      &header-authorization;, for additional details.) </t>
1228  </list>
1229</t>
1230<t>
1231   <x:dfn>private</x:dfn>
1232   <iref item="Cache Directives" primary="true" subitem="private" />
1233   <iref item="private" primary="true" subitem="Cache Directive" />
1234   <list>
1235      <t>The private response directive indicates that the response message is
1236      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1237      private (non-shared) cache &MAY; store the response.</t>
1238      <t>If the private response directive specifies one or more field-names,
1239      this requirement is limited to the field-values associated with the
1240      listed response headers. That is, the specified field-names(s)
1241      &MUST-NOT; be stored by a shared cache, whereas the remainder of the
1242      response message &MAY; be.</t>
1243      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1244      the response can be stored; it cannot ensure the privacy of the message
1245      content. Also, private response directives with field-names are often
1246      handled by implementations as if an unqualified private directive was
1247      received; i.e., the special handling for the qualified form is not
1248      widely implemented.</t>
1249   </list>
1250</t>
1251<t>
1252   <x:dfn>no-cache</x:dfn>
1253   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1254   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1255   <list>
1256      <t>The no-cache response directive indicates that the response MUST NOT
1257      be used to satisfy a subsequent request without successful validation on
1258      the origin server. This allows an origin server to prevent a cache from
1259      using it to satisfy a request without contacting it, even by caches that
1260      have been configured to return stale responses.</t>
1261      <t>If the no-cache response directive specifies one or more field-names,
1262      this requirement is limited to the field-values associated with the
1263      listed response headers. That is, the specified field-name(s) &MUST-NOT;
1264      be sent in the response to a subsequent request without successful
1265      validation on the origin server. This allows an origin server to prevent
1266      the re-use of certain header fields in a response, while still allowing
1267      caching of the rest of the response.</t>
1268      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1269      this directive. Also, no-cache response directives with field-names are
1270      often handled by implementations as if an unqualified no-cache directive
1271      was received; i.e., the special handling for the qualified form is not
1272      widely implemented. </t>
1273   </list>
1274</t>
1275<t>
1276   <x:dfn>no-store</x:dfn>
1277   <iref item="Cache Directives" primary="true" subitem="no-store" />
1278   <iref item="no-store" primary="true" subitem="Cache Directive" />
1279   <list>
1280      <t>The no-store response directive indicates that a cache &MUST-NOT;
1281      store any part of either the immediate request or response. This
1282      directive applies to both non-shared and shared caches. "&MUST-NOT;
1283      store" in this context means that the cache &MUST-NOT; intentionally
1284      store the information in non-volatile storage, and &MUST; make a
1285      best-effort attempt to remove the information from volatile storage as
1286      promptly as possible after forwarding it.</t>
1287      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1288      privacy. In particular, malicious or compromised caches might not
1289      recognize or obey this directive, and communications networks might be
1290      vulnerable to eavesdropping.</t>
1291   </list>
1292</t>
1293<t>
1294   <x:dfn>must-revalidate</x:dfn>
1295   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1296   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1297   <list>
1298      <t>The must-revalidate response directive indicates that once it has
1299      become stale, the response &MUST-NOT; be used to satisfy subsequent
1300      requests without successful validation on the origin server.</t>
1301      <t>The must-revalidate directive is necessary to support reliable
1302      operation for certain protocol features. In all circumstances an
1303      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in particular,
1304      if the cache cannot reach the origin server for any reason, it &MUST;
1305      generate a 504 (Gateway Timeout) response.</t>
1306      <t>Servers &SHOULD; send the must-revalidate directive if and only if
1307      failure to validate a request on the representation could result in
1308      incorrect operation, such as a silently unexecuted financial
1309      transaction.</t>
1310   </list>
1311</t>
1312<t>
1313   <x:dfn>proxy-revalidate</x:dfn>
1314   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1315   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1316   <list>
1317      <t>The proxy-revalidate response directive has the same meaning as the
1318      must-revalidate response directive, except that it does not apply to
1319      non-shared caches.</t>
1320   </list>
1321</t>
1322<t>
1323   <x:dfn>max-age</x:dfn>
1324   <iref item="Cache Directives" primary="true" subitem="max-age" />
1325   <iref item="max-age" primary="true" subitem="Cache Directive" />
1326   <list>
1327      <t>The max-age response directive indicates that response is to be
1328      considered stale after its age is greater than the specified number of
1329      seconds.</t>
1330   </list>
1331</t>
1332<t>
1333   <x:dfn>s-maxage</x:dfn>
1334   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1335   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1336   <list>
1337      <t>The s-maxage response directive indicates that, in shared caches, the
1338      maximum age specified by this directive overrides the maximum age
1339      specified by either the max-age directive or the Expires header. The
1340      s-maxage directive also implies the semantics of the proxy-revalidate
1341      response directive.</t>
1342   </list>
1343</t>
1344<t>
1345   <x:dfn>no-transform</x:dfn>
1346   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1347   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1348   <list>
1349      <t>The no-transform response directive indicates that an intermediate
1350      cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
1351      Content-Type response headers, nor the response representation.</t>
1352   </list>
1353</t>
1354
1355</section>
1356
1357<section anchor="cache.control.extensions" title="Cache Control Extensions">
1358<t>
1359   The Cache-Control header field can be extended through the use of one or
1360   more cache-extension tokens, each with an optional value. Informational
1361   extensions (those that do not require a change in cache behavior) can be
1362   added without changing the semantics of other directives. Behavioral
1363   extensions are designed to work by acting as modifiers to the existing base
1364   of cache directives. Both the new directive and the standard directive are
1365   supplied, such that applications that do not understand the new directive
1366   will default to the behavior specified by the standard directive, and those
1367   that understand the new directive will recognize it as modifying the
1368   requirements associated with the standard directive. In this way,
1369   extensions to the cache-control directives can be made without requiring
1370   changes to the base protocol.
1371</t>
1372<t>
1373   This extension mechanism depends on an HTTP cache obeying all of the
1374   cache-control directives defined for its native HTTP-version, obeying
1375   certain extensions, and ignoring all directives that it does not
1376   understand.
1377</t>
1378<t>
1379   For example, consider a hypothetical new response directive called
1380   "community" that acts as a modifier to the private directive. We define
1381   this new directive to mean that, in addition to any non-shared cache, any
1382   cache that is shared only by members of the community named within its
1383   value may cache the response. An origin server wishing to allow the UCI
1384   community to use an otherwise private response in their shared cache(s)
1385   could do so by including
1386</t>
1387<figure><artwork type="example">
1388  Cache-Control: private, community="UCI"
1389</artwork></figure>
1390<t>
1391   A cache seeing this header field will act correctly even if the cache does
1392   not understand the community cache-extension, since it will also see and
1393   understand the private directive and thus default to the safe behavior.
1394</t>
1395<t>
1396   Unrecognized cache directives &MUST; be ignored; it is assumed that any
1397   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1398   combined with standard directives (or the response's default cacheability)
1399   such that the cache behavior will remain minimally correct even if the
1400   cache does not understand the extension(s).
1401</t>
1402<t>
1403   The HTTP Cache Directive Registry defines the name space for the cache
1404   directives.
1405</t>
1406<t>
1407   Registrations &MUST; include the following fields:
1408   <list style="symbols">
1409      <t>Cache Directive Name</t>
1410      <t>Pointer to specification text</t>
1411   </list>
1412</t>
1413<t>
1414   Values to be added to this name space are subject to IETF review (<xref
1415   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1416</t>
1417<t>
1418   The registry itself is maintained at <eref
1419   target="http://www.iana.org/assignments/http-cache-directives"/>.
1420</t>
1421</section>
1422
1423</section>
1424
1425<section anchor="header.expires" title="Expires">
1426   <iref item="Expires header" primary="true" x:for-anchor="" />
1427   <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1428   <x:anchor-alias value="Expires"/>
1429   <x:anchor-alias value="Expires-v"/>
1430<t>
1431   The "Expires" header field gives the date/time after which the
1432   response is considered stale. See <xref target="expiration.model" /> for
1433   further discussion of the freshness model.
1434</t>
1435<t>
1436   The presence of an Expires field does not imply that the original resource
1437   will change or cease to exist at, before, or after that time.
1438</t>
1439<t>
1440   The field-value is an absolute date and time as defined by HTTP-date in
1441   &full-date;; it &MUST; be sent in rfc1123-date format.
1442</t>
1443<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1444  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1445  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1446</artwork></figure>
1447<figure>
1448  <preamble>For example</preamble>
1449<artwork type="example">
1450  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1451</artwork></figure>
1452<x:note>
1453   <t>
1454       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1455       max-age directive (see <xref target="cache-response-directive" />),
1456       that directive overrides the Expires field. Likewise, the s-maxage
1457       directive overrides Expires in shared caches.
1458   </t>
1459</x:note>
1460<t>
1461   HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the
1462   future.
1463</t>
1464<t>
1465   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
1466   especially including the value "0", as in the past (i.e., "already
1467   expired").
1468</t>
1469</section>
1470
1471<section anchor="header.pragma" title="Pragma">
1472   <iref item="Pragma header" primary="true" x:for-anchor="" />
1473   <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1474   <x:anchor-alias value="extension-pragma"/>
1475   <x:anchor-alias value="Pragma"/>
1476   <x:anchor-alias value="Pragma-v"/>
1477   <x:anchor-alias value="pragma-directive"/>
1478<t>
1479   The "Pragma" general-header field is used to include
1480   implementation-specific directives that might apply to any recipient along
1481   the request/response chain. All pragma directives specify optional behavior
1482   from the viewpoint of the protocol; however, some systems &MAY; require
1483   that behavior be consistent with the directives.
1484</t>
1485<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1486  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1487  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1488  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1489  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1490</artwork></figure>
1491<t>
1492   When the no-cache directive is present in a request message, an application
1493   &SHOULD; forward the request toward the origin server even if it has a
1494   cached copy of what is being requested. This pragma directive has the same
1495   semantics as the no-cache response directive (see <xref
1496   target="cache-response-directive" />) and is defined here for backward
1497   compatibility with HTTP/1.0. Clients &SHOULD; include both header fields
1498   when a no-cache request is sent to a server not known to be HTTP/1.1
1499   compliant. HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the
1500   client had sent "Cache-Control: no-cache".
1501</t>
1502<x:note>
1503   <t>
1504      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a
1505      response-header field is not actually specified, it does not provide a
1506      reliable replacement for "Cache-Control: no-cache" in a response.
1507   </t>
1508</x:note>
1509<t>
1510   This mechanism is deprecated; no new Pragma directives will be defined in
1511   HTTP.
1512</t>
1513</section>
1514
1515<section anchor="header.vary" title="Vary">
1516   <iref item="Vary header" primary="true" x:for-anchor="" />
1517   <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1518   <x:anchor-alias value="Vary"/>
1519   <x:anchor-alias value="Vary-v"/>
1520<t>
1521   The "Vary" response-header field conveys the set of request-header fields
1522   that were used to select the representation.
1523</t>
1524<t>
1525   Caches use this information, in part, to determine whether a stored
1526   response can be used to satisfy a given request; see <xref
1527   target="caching.negotiated.responses" />. determines, while the response is
1528   fresh, whether a cache is permitted to use the response to reply to a
1529   subsequent request without validation; see <xref
1530   target="caching.negotiated.responses" />.
1531</t>
1532<t>
1533   In uncacheable or stale responses, the Vary field value advises the user
1534   agent about the criteria that were used to select the representation.
1535</t>
1536<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1537  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1538  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1539</artwork></figure>
1540<t>
1541   The set of header fields named by the Vary field value is known as the
1542   selecting request-headers.
1543</t>
1544<t>
1545   Servers &SHOULD; include a Vary header field with any cacheable response
1546   that is subject to server-driven negotiation. Doing so allows a cache to
1547   properly interpret future requests on that resource and informs the user
1548   agent about the presence of negotiation on that resource. A server &MAY;
1549   include a Vary header field with a non-cacheable response that is subject
1550   to server-driven negotiation, since this might provide the user agent with
1551   useful information about the dimensions over which the response varies at
1552   the time of the response.
1553</t>
1554<t>
1555   A Vary field value of "*" signals that unspecified parameters not limited
1556   to the request-headers (e.g., the network address of the client), play a
1557   role in the selection of the response representation; therefore, a cache
1558   cannot determine whether this response is appropriate. The "*" value
1559   &MUST-NOT; be generated by a proxy server.
1560</t>
1561<t>
1562   The field-names given are not limited to the set of standard request-header
1563   fields defined by this specification. Field names are case-insensitive.
1564</t>
1565</section>
1566
1567<section anchor="header.warning" title="Warning">
1568   <iref item="Warning header" primary="true" x:for-anchor="" />
1569   <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1570   <x:anchor-alias value="Warning"/>
1571   <x:anchor-alias value="Warning-v"/>
1572   <x:anchor-alias value="warning-value"/>
1573   <x:anchor-alias value="warn-agent"/>
1574   <x:anchor-alias value="warn-code"/>
1575   <x:anchor-alias value="warn-date"/>
1576   <x:anchor-alias value="warn-text"/>
1577<t>
1578   The "Warning" general-header field is used to carry additional information
1579   about the status or transformation of a message that might not be reflected
1580   in the message. This information is typically used to warn about possible
1581   incorrectness introduced by caching operations or transformations applied
1582   to the payload of the message.
1583</t>
1584<t>
1585   Warnings can be used for other purposes, both cache-related and otherwise.
1586   The use of a warning, rather than an error status code, distinguishes these
1587   responses from true failures.
1588</t>
1589<t>
1590   Warning headers can in general be applied to any message, however some
1591   warn-codes are specific to caches and can only be applied to response
1592   messages.
1593</t>
1594<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
1595  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1596  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1597 
1598  <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>
1599                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1600 
1601  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1602  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1603                  ; the name or pseudonym of the server adding
1604                  ; the Warning header, for use in debugging
1605  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1606  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1607</artwork></figure>
1608<t>
1609   Multiple warnings can be attached to a response (either by the origin
1610   server or by a cache), including multiple warnings with the same code
1611   number, only differing in warn-text.
1612</t>
1613<t>
1614   When this occurs, the user agent &SHOULD; inform the user of as many of
1615   them as possible, in the order that they appear in the response.
1616</t>
1617<t>
1618   Systems that generate multiple Warning headers &SHOULD; order them with
1619   this user agent behavior in mind. New Warning headers &SHOULD; be added
1620   after any existing Warning headers.
1621</t>
1622<t>
1623   Warnings are assigned three digit warn-codes. The first digit indicates
1624   whether the Warning is required to be deleted from a stored response after
1625   validation:
1626   <list style="symbols">
1627      <t>1xx Warnings describe the freshness or validation status of the
1628      response, and so &MUST; be deleted by caches after validation. They can
1629      only be generated by a cache when validating a cached entry, and
1630      &MUST-NOT; be generated in any other situation.</t>
1631      <t>2xx Warnings describe some aspect of the representation that is not
1632      rectified by a validation (for example, a lossy compression of the
1633      representation) and &MUST-NOT; be deleted by caches after validation,
1634      unless a full response is returned, in which case they &MUST; be.</t>
1635   </list>
1636</t>
1637<t>
1638   If an implementation sends a message with one or more Warning headers to a
1639   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1640   in each warning-value a warn-date that matches the Date header in the
1641   message.
1642</t>
1643<t>
1644   If an implementation receives a message with a warning-value that includes
1645   a warn-date, and that warn-date is different from the Date value in the
1646   response, then that warning-value &MUST; be deleted from the message before
1647   storing, forwarding, or using it. (preventing the consequences of naive
1648   caching of Warning header fields.) If all of the warning-values are deleted
1649   for this reason, the Warning header &MUST; be deleted as well.
1650</t>
1651<t>
1652   The following warn-codes are defined by this specification, each with a
1653   recommended warn-text in English, and a description of its meaning.
1654</t>
1655<t><?rfc needLines="4"?>
1656   110 Response is stale
1657   <list>
1658      <t>&SHOULD; be included whenever the returned response is stale.</t>
1659   </list>
1660</t>
1661<t><?rfc needLines="4"?>
1662   111 Revalidation failed
1663   <list>
1664      <t>&SHOULD; be included if a cache returns a stale response because an
1665      attempt to validate the response failed, due to an inability to reach
1666      the server.</t>
1667   </list>
1668</t>
1669<t><?rfc needLines="4"?>
1670   112 Disconnected operation
1671   <list>
1672      <t>&SHOULD; be included if the cache is intentionally disconnected from
1673      the rest of the network for a period of time.</t>
1674   </list>
1675</t>
1676<t><?rfc needLines="4"?>
1677   113 Heuristic expiration
1678   <list>
1679      <t>&SHOULD; be included if the cache heuristically chose a freshness
1680      lifetime greater than 24 hours and the response's age is greater than 24
1681      hours.</t>
1682   </list>
1683</t>
1684<t><?rfc needLines="4"?>
1685   199 Miscellaneous warning
1686   <list>
1687      <t>The warning text can include arbitrary information to be presented to
1688      a human user, or logged. A system receiving this warning &MUST-NOT; take
1689      any automated action, besides presenting the warning to the user.</t>
1690   </list>
1691</t>
1692<t><?rfc needLines="4"?>
1693   214 Transformation applied
1694   <list>
1695      <t>&MUST; be added by an intermediate proxy if it applies any
1696      transformation to the representation, such as changing the
1697      content-coding, media-type, or modifying the representation data, unless
1698      this Warning code already appears in the response.</t>
1699   </list>
1700</t>
1701<t><?rfc needLines="4"?>
1702   299 Miscellaneous persistent warning
1703   <list>
1704      <t>The warning text can include arbitrary information to be presented to
1705      a human user, or logged. A system receiving this warning &MUST-NOT; take
1706      any automated action.</t>
1707   </list>
1708</t>
1709</section>
1710
1711</section>
1712
1713<section anchor="history.lists" title="History Lists">
1714<t>
1715   User agents often have history mechanisms, such as "Back" buttons and
1716   history lists, that can be used to redisplay a representation retrieved
1717   earlier in a session.
1718</t>
1719<t>
1720   The freshness model (<xref target="expiration.model"/>) does not
1721   necessarily apply to history mechanisms. I.e., a history mechanism can
1722   display a previous representation even if it has expired.
1723</t>
1724<t>
1725   This does not prohibit the history mechanism from telling the user that a
1726   view might be stale, or from honoring cache directives (e.g.,
1727   Cache-Control: no-store).
1728</t>
1729</section>
1730
1731
1732<section anchor="IANA.considerations" title="IANA Considerations">
1733
1734<section title="Cache Directive Registry" 
1735   anchor="cache.directive.registration">
1736<t>
1737   The registration procedure for HTTP Cache Directives is defined by <xref
1738   target="cache.control.extensions"/> of this document.
1739</t>
1740<t>
1741   The HTTP Cache Directive Registry shall be created at <eref
1742   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1743   populated with the registrations below:
1744</t>
1745<?BEGININC p6-cache.cache-directives ?>
1746<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1747<texttable xmlns:my="#my" align="left" suppress-title="true"
1748           anchor="iana.cache.directive.registration.table">
1749   <ttcol>Cache Directive</ttcol>
1750   <ttcol>Reference</ttcol>
1751
1752   <c>max-age</c>
1753   <c>
1754      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1755   </c>
1756   <c>max-stale</c>
1757   <c>
1758      <xref target="cache-request-directive"/>
1759   </c>
1760   <c>min-fresh</c>
1761   <c>
1762      <xref target="cache-request-directive"/>
1763   </c>
1764   <c>must-revalidate</c>
1765   <c>
1766      <xref target="cache-response-directive"/>
1767   </c>
1768   <c>no-cache</c>
1769   <c>
1770      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1771   </c>
1772   <c>no-store</c>
1773   <c>
1774      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1775   </c>
1776   <c>no-transform</c>
1777   <c>
1778      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1779   </c>
1780   <c>only-if-cached</c>
1781   <c>
1782      <xref target="cache-request-directive"/>
1783   </c>
1784   <c>private</c>
1785   <c>
1786      <xref target="cache-response-directive"/>
1787   </c>
1788   <c>proxy-revalidate</c>
1789   <c>
1790      <xref target="cache-response-directive"/>
1791   </c>
1792   <c>public</c>
1793   <c>
1794      <xref target="cache-response-directive"/>
1795   </c>
1796   <c>s-maxage</c>
1797   <c>
1798      <xref target="cache-response-directive"/>
1799   </c>
1800   <c>stale-if-error</c>
1801   <c>
1802      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1803   </c>
1804   <c>stale-while-revalidate</c>
1805   <c>
1806      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1807   </c>
1808</texttable>
1809<!--(END)-->
1810<?ENDINC p6-cache.cache-directives ?>
1811</section>
1812
1813<section title="Header Field Registration" anchor="header.field.registration">
1814<t>
1815  The Message Header Field Registry located at <eref 
1816  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1817  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
1818</t>
1819<?BEGININC p6-cache.iana-headers ?>
1820<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1821<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1822   <ttcol>Header Field Name</ttcol>
1823   <ttcol>Protocol</ttcol>
1824   <ttcol>Status</ttcol>
1825   <ttcol>Reference</ttcol>
1826
1827   <c>Age</c>
1828   <c>http</c>
1829   <c>standard</c>
1830   <c>
1831      <xref target="header.age"/>
1832   </c>
1833   <c>Cache-Control</c>
1834   <c>http</c>
1835   <c>standard</c>
1836   <c>
1837      <xref target="header.cache-control"/>
1838   </c>
1839   <c>Expires</c>
1840   <c>http</c>
1841   <c>standard</c>
1842   <c>
1843      <xref target="header.expires"/>
1844   </c>
1845   <c>Pragma</c>
1846   <c>http</c>
1847   <c>standard</c>
1848   <c>
1849      <xref target="header.pragma"/>
1850   </c>
1851   <c>Vary</c>
1852   <c>http</c>
1853   <c>standard</c>
1854   <c>
1855      <xref target="header.vary"/>
1856   </c>
1857   <c>Warning</c>
1858   <c>http</c>
1859   <c>standard</c>
1860   <c>
1861      <xref target="header.warning"/>
1862   </c>
1863</texttable>
1864<!--(END)-->
1865<?ENDINC p6-cache.iana-headers ?>
1866<t>
1867   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1868   Force".
1869</t>
1870</section>
1871
1872</section>
1873
1874<section anchor="security.considerations" title="Security Considerations">
1875<t>
1876   Caches expose additional potential vulnerabilities, since the contents of
1877   the cache represent an attractive target for malicious exploitation.
1878   Because cache contents persist after an HTTP request is complete, an attack
1879   on the cache can reveal information long after a user believes that the
1880   information has been removed from the network. Therefore, cache contents
1881   need to be protected as sensitive information.
1882</t>
1883</section>
1884
1885<section anchor="ack" title="Acknowledgments">
1886<t>
1887   Much of the content and presentation of the caching design is due to
1888   suggestions and comments from individuals including: Shel Kaphan, Paul
1889   Leach, Koen Holtman, David Morris, and Larry Masinter.
1890</t>
1891</section>
1892
1893</middle>
1894
1895<back>
1896<references title="Normative References">
1897
1898  <reference anchor="Part1">
1899    <front>
1900      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1901      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1902        <organization abbrev="Day Software">Day Software</organization>
1903        <address><email>fielding@gbiv.com</email></address>
1904      </author>
1905      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1906        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1907        <address><email>jg@freedesktop.org</email></address>
1908      </author>
1909      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1910        <organization abbrev="HP">Hewlett-Packard Company</organization>
1911        <address><email>JeffMogul@acm.org</email></address>
1912      </author>
1913      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1914        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1915        <address><email>henrikn@microsoft.com</email></address>
1916      </author>
1917      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1918        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1919        <address><email>LMM@acm.org</email></address>
1920      </author>
1921      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1922        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1923        <address><email>paulle@microsoft.com</email></address>
1924      </author>
1925      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1926        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1927        <address><email>timbl@w3.org</email></address>
1928      </author>
1929      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1930        <organization abbrev="W3C">World Wide Web Consortium</organization>
1931        <address><email>ylafon@w3.org</email></address>
1932      </author>
1933      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1934        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1935        <address><email>julian.reschke@greenbytes.de</email></address>
1936      </author>
1937      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1938    </front>
1939    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1940    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1941  </reference>
1942
1943  <reference anchor="Part2">
1944    <front>
1945      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1946      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1947        <organization abbrev="Day Software">Day Software</organization>
1948        <address><email>fielding@gbiv.com</email></address>
1949      </author>
1950      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1951        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1952        <address><email>jg@freedesktop.org</email></address>
1953      </author>
1954      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1955        <organization abbrev="HP">Hewlett-Packard Company</organization>
1956        <address><email>JeffMogul@acm.org</email></address>
1957      </author>
1958      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1959        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1960        <address><email>henrikn@microsoft.com</email></address>
1961      </author>
1962      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1963        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1964        <address><email>LMM@acm.org</email></address>
1965      </author>
1966      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1967        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1968        <address><email>paulle@microsoft.com</email></address>
1969      </author>
1970      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1971        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1972        <address><email>timbl@w3.org</email></address>
1973      </author>
1974      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1975        <organization abbrev="W3C">World Wide Web Consortium</organization>
1976        <address><email>ylafon@w3.org</email></address>
1977      </author>
1978      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1979        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1980        <address><email>julian.reschke@greenbytes.de</email></address>
1981      </author>
1982      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1983    </front>
1984    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1985    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1986  </reference>
1987
1988  <reference anchor="Part4">
1989    <front>
1990      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1991      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1992        <organization abbrev="Day Software">Day Software</organization>
1993        <address><email>fielding@gbiv.com</email></address>
1994      </author>
1995      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1996        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1997        <address><email>jg@freedesktop.org</email></address>
1998      </author>
1999      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2000        <organization abbrev="HP">Hewlett-Packard Company</organization>
2001        <address><email>JeffMogul@acm.org</email></address>
2002      </author>
2003      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2004        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2005        <address><email>henrikn@microsoft.com</email></address>
2006      </author>
2007      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2008        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2009        <address><email>LMM@acm.org</email></address>
2010      </author>
2011      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2012        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2013        <address><email>paulle@microsoft.com</email></address>
2014      </author>
2015      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2016        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2017        <address><email>timbl@w3.org</email></address>
2018      </author>
2019      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2020        <organization abbrev="W3C">World Wide Web Consortium</organization>
2021        <address><email>ylafon@w3.org</email></address>
2022      </author>
2023      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2024        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2025        <address><email>julian.reschke@greenbytes.de</email></address>
2026      </author>
2027      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2028    </front>
2029    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2030    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2031  </reference>
2032
2033  <reference anchor="Part5">
2034    <front>
2035      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2036      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2037        <organization abbrev="Day Software">Day Software</organization>
2038        <address><email>fielding@gbiv.com</email></address>
2039      </author>
2040      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2041        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2042        <address><email>jg@freedesktop.org</email></address>
2043      </author>
2044      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2045        <organization abbrev="HP">Hewlett-Packard Company</organization>
2046        <address><email>JeffMogul@acm.org</email></address>
2047      </author>
2048      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2049        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2050        <address><email>henrikn@microsoft.com</email></address>
2051      </author>
2052      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2053        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2054        <address><email>LMM@acm.org</email></address>
2055      </author>
2056      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2057        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2058        <address><email>paulle@microsoft.com</email></address>
2059      </author>
2060      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2061        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2062        <address><email>timbl@w3.org</email></address>
2063      </author>
2064      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2065        <organization abbrev="W3C">World Wide Web Consortium</organization>
2066        <address><email>ylafon@w3.org</email></address>
2067      </author>
2068      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2069        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2070        <address><email>julian.reschke@greenbytes.de</email></address>
2071      </author>
2072      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2073    </front>
2074    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2075    <x:source basename="p5-range" href="p5-range.xml" />
2076  </reference>
2077
2078  <reference anchor="Part7">
2079    <front>
2080      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2081      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2082        <organization abbrev="Day Software">Day Software</organization>
2083        <address><email>fielding@gbiv.com</email></address>
2084      </author>
2085      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2086        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2087        <address><email>jg@freedesktop.org</email></address>
2088      </author>
2089      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2090        <organization abbrev="HP">Hewlett-Packard Company</organization>
2091        <address><email>JeffMogul@acm.org</email></address>
2092      </author>
2093      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2094        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2095        <address><email>henrikn@microsoft.com</email></address>
2096      </author>
2097      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2098        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2099        <address><email>LMM@acm.org</email></address>
2100      </author>
2101      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2102        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2103        <address><email>paulle@microsoft.com</email></address>
2104      </author>
2105      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2106        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2107        <address><email>timbl@w3.org</email></address>
2108      </author>
2109      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2110        <organization abbrev="W3C">World Wide Web Consortium</organization>
2111        <address><email>ylafon@w3.org</email></address>
2112      </author>
2113      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2114        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2115        <address><email>julian.reschke@greenbytes.de</email></address>
2116      </author>
2117      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2118    </front>
2119    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2120    <x:source basename="p7-auth" href="p7-auth.xml" />
2121  </reference>
2122
2123  <reference anchor="RFC2119">
2124    <front>
2125      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2126      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2127        <organization>Harvard University</organization>
2128        <address><email>sob@harvard.edu</email></address>
2129      </author>
2130      <date month="March" year="1997" />
2131    </front>
2132    <seriesInfo name="BCP" value="14" />
2133    <seriesInfo name="RFC" value="2119" />
2134  </reference>
2135
2136  <reference anchor="RFC5234">
2137    <front>
2138      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2139      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2140        <organization>Brandenburg InternetWorking</organization>
2141        <address>
2142          <email>dcrocker@bbiw.net</email>
2143        </address> 
2144      </author>
2145      <author initials="P." surname="Overell" fullname="Paul Overell">
2146        <organization>THUS plc.</organization>
2147        <address>
2148          <email>paul.overell@thus.net</email>
2149        </address>
2150      </author>
2151      <date month="January" year="2008"/>
2152    </front>
2153    <seriesInfo name="STD" value="68"/>
2154    <seriesInfo name="RFC" value="5234"/>
2155  </reference>
2156 
2157</references>
2158
2159<references title="Informative References">
2160
2161  <reference anchor="RFC1305">
2162    <front>
2163      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2164      <author fullname="David L. Mills" initials="D." surname="Mills">
2165        <organization>University of Delaware, Electrical Engineering Department</organization>
2166        <address><email>mills@udel.edu</email></address>
2167      </author>
2168      <date month="March" year="1992" />
2169    </front>
2170    <seriesInfo name="RFC" value="1305" />
2171  </reference>
2172
2173  <reference anchor="RFC2616">
2174    <front>
2175      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2176      <author fullname="R. Fielding" initials="R." surname="Fielding">
2177        <organization>University of California, Irvine</organization>
2178        <address><email>fielding@ics.uci.edu</email></address>
2179      </author>
2180      <author fullname="J. Gettys" initials="J." surname="Gettys">
2181        <organization>W3C</organization>
2182        <address><email>jg@w3.org</email></address>
2183      </author>
2184      <author fullname="J. Mogul" initials="J." surname="Mogul">
2185        <organization>Compaq Computer Corporation</organization>
2186        <address><email>mogul@wrl.dec.com</email></address>
2187      </author>
2188      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2189        <organization>MIT Laboratory for Computer Science</organization>
2190        <address><email>frystyk@w3.org</email></address>
2191      </author>
2192      <author fullname="L. Masinter" initials="L." surname="Masinter">
2193        <organization>Xerox Corporation</organization>
2194        <address><email>masinter@parc.xerox.com</email></address>
2195      </author>
2196      <author fullname="P. Leach" initials="P." surname="Leach">
2197        <organization>Microsoft Corporation</organization>
2198        <address><email>paulle@microsoft.com</email></address>
2199      </author>
2200      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2201        <organization>W3C</organization>
2202        <address><email>timbl@w3.org</email></address>
2203      </author>
2204      <date month="June" year="1999" />
2205    </front>
2206    <seriesInfo name="RFC" value="2616" />
2207  </reference>
2208
2209  <reference anchor="RFC3864">
2210    <front>
2211      <title>Registration Procedures for Message Header Fields</title>
2212      <author fullname="G. Klyne" initials="G." surname="Klyne">
2213        <organization>Nine by Nine</organization>
2214        <address><email>GK-IETF@ninebynine.org</email></address>
2215      </author>
2216      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2217        <organization>BEA Systems</organization>
2218        <address><email>mnot@pobox.com</email></address>
2219      </author>
2220      <author fullname="J. Mogul" initials="J." surname="Mogul">
2221        <organization>HP Labs</organization>
2222        <address><email>JeffMogul@acm.org</email></address>
2223      </author>
2224      <date month="September" year="2004" />
2225    </front>
2226    <seriesInfo name="BCP" value="90" />
2227    <seriesInfo name="RFC" value="3864" />
2228  </reference>
2229
2230  <reference anchor='RFC5226'>
2231    <front>
2232      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2233      <author initials='T.' surname='Narten' fullname='T. Narten'>
2234        <organization>IBM</organization>
2235        <address><email>narten@us.ibm.com</email></address>
2236      </author>
2237      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2238        <organization>Google</organization>
2239        <address><email>Harald@Alvestrand.no</email></address>
2240      </author>
2241      <date year='2008' month='May' />
2242    </front>
2243    <seriesInfo name='BCP' value='26' />
2244    <seriesInfo name='RFC' value='5226' />
2245  </reference>
2246
2247  <reference anchor='RFC5861'>
2248    <front>
2249      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2250      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2251        <organization>Yahoo! Inc.</organization>
2252        <address><email>mnot@yahoo-inc.com</email></address>
2253      </author>
2254      <date month="April" year="2010"/>
2255    </front>
2256    <seriesInfo name='RFC' value='5861' />
2257  </reference>
2258
2259</references>
2260
2261<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2262<t>
2263  Make the specified age calculation algorithm less conservative.
2264  (<xref target="age.calculations"/>)
2265</t>
2266<t>
2267  Remove requirement to consider Content-Location in successful responses
2268  in order to determine the appropriate response to use.
2269  (<xref target="validation.model" />)
2270</t>
2271<t>
2272  Clarify denial of service attack avoidance requirement.
2273  (<xref target="invalidation.after.updates.or.deletions" />)
2274</t>
2275<t>
2276  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2277  anymore, as these aspects never were implemented.
2278  (<xref target="header.warning" />)
2279</t>
2280</section>
2281
2282<?BEGININC p6-cache.abnf-appendix ?>
2283<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2284<figure>
2285<artwork type="abnf" name="p6-cache.parsed-abnf">
2286<x:ref>Age</x:ref> = "Age:" OWS Age-v
2287<x:ref>Age-v</x:ref> = delta-seconds
2288
2289<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2290<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2291 cache-directive ] )
2292
2293<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2294<x:ref>Expires-v</x:ref> = HTTP-date
2295
2296<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2297
2298<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2299
2300<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2301<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2302 pragma-directive ] )
2303
2304<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2305<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2306 ] ) )
2307
2308<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2309<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2310 ] )
2311
2312<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2313<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2314<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2315 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2316 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2317 cache-extension
2318<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2319 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2320 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2321 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2322 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2323 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2324
2325<x:ref>delta-seconds</x:ref> = 1*DIGIT
2326
2327<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2328
2329<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2330
2331<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2332<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2333<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2334
2335<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2336
2337<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2338
2339<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2340
2341<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2342<x:ref>warn-code</x:ref> = 3DIGIT
2343<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2344<x:ref>warn-text</x:ref> = quoted-string
2345<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2346 ]
2347</artwork>
2348</figure>
2349<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2350; Age defined but not used
2351; Cache-Control defined but not used
2352; Expires defined but not used
2353; Pragma defined but not used
2354; Vary defined but not used
2355; Warning defined but not used
2356</artwork></figure></section>
2357<?ENDINC p6-cache.abnf-appendix ?>
2358
2359<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2360
2361<section title="Since RFC2616">
2362  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2363</section>
2364
2365<section title="Since draft-ietf-httpbis-p6-cache-00">
2366<t>
2367  Closed issues:
2368  <list style="symbols">
2369    <t>
2370      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2371    <t>
2372      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
2373    <t>
2374      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2375    <t>
2376      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2377    <t>
2378      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2379    <t>
2380      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2381    <t>
2382      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2383    <t>
2384      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2385    <t>
2386      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2387  </list>
2388</t>
2389<t>
2390  Other changes:
2391  <list style="symbols">
2392    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2393        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2394  </list>
2395</t>
2396</section>
2397
2398<section title="Since draft-ietf-httpbis-p6-cache-01">
2399<t>
2400  Closed issues:
2401  <list style="symbols">
2402    <t>
2403      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2404  </list>
2405</t>
2406<t>
2407  Other changes:
2408  <list style="symbols">
2409    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2410      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2411    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2412      specification.</t>
2413  </list>
2414</t>
2415</section>
2416
2417<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2418<t>
2419  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2420  <list style="symbols">
2421    <t>Reference RFC 3984, and update header registrations for headers defined in this
2422      document.</t>
2423  </list>
2424</t>
2425</section>
2426
2427<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2428<t>
2429  Closed issues:
2430  <list style="symbols">
2431    <t>
2432      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2433  </list>
2434</t>
2435</section>
2436
2437<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2438<t>
2439  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2440  <list style="symbols"> 
2441    <t>
2442      Use "/" instead of "|" for alternatives.
2443    </t>
2444    <t>
2445      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2446      whitespace ("OWS") and required whitespace ("RWS").
2447    </t>
2448    <t>
2449      Rewrite ABNFs to spell out whitespace rules, factor out
2450      header value format definitions.
2451    </t>
2452  </list>
2453</t>
2454</section>
2455
2456<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2457<t>
2458  This is a total rewrite of this part of the specification.
2459</t>
2460<t>
2461  Affected issues:
2462  <list style="symbols">
2463    <t>
2464      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2465    <t>
2466      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2467    <t>
2468      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2469    <t>
2470      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2471  </list>
2472</t>
2473<t>
2474  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2475  <list style="symbols"> 
2476    <t>
2477      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2478    </t>
2479  </list>
2480</t>
2481</section>
2482
2483<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2484<t>
2485  Closed issues:
2486  <list style="symbols"> 
2487    <t>
2488      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2489      "base for numeric protocol elements"
2490    </t>
2491  </list>
2492</t>
2493<t>
2494  Affected issues:
2495  <list style="symbols">
2496    <t>
2497      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2498      Vary and non-existant headers
2499    </t>
2500  </list>
2501</t>
2502</section>
2503
2504<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2505<t>
2506  Closed issues:
2507  <list style="symbols"> 
2508    <t>
2509      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2510      "Definition of 1xx Warn-Codes"
2511    </t>
2512    <t>
2513      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2514      "Content-Location on 304 responses"
2515    </t>
2516    <t>
2517      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2518      "private and no-cache CC directives with headers"
2519    </t>
2520    <t>
2521      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2522      "RFC2047 and warn-text"
2523    </t>
2524  </list>
2525</t>
2526</section>
2527
2528<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2529<t>
2530  Closed issues:
2531  <list style="symbols"> 
2532    <t>
2533      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2534      "serving negotiated responses from cache: header-specific canonicalization"
2535    </t>
2536    <t>
2537      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2538      "Effect of CC directives on history lists"
2539    </t>
2540  </list>
2541</t>
2542<t>
2543  Affected issues:
2544  <list style="symbols">
2545    <t>
2546      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2547      Status codes and caching
2548    </t>
2549  </list>
2550</t>
2551<t>
2552  Partly resolved issues:
2553  <list style="symbols"> 
2554    <t>
2555      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2556      "Placement of 13.5.1 and 13.5.2"
2557    </t>
2558  </list>
2559</t>
2560</section>
2561
2562<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2563<t>
2564  Closed issues:
2565  <list style="symbols"> 
2566    <t>
2567      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2568      "Age calculation"
2569    </t>
2570    <t>
2571      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2572      "Clarify differences between / requirements for request and response CC directives"
2573    </t>
2574        <t>
2575                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2576                "Caching authenticated responses"
2577        </t>
2578    <t>
2579      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2580      "IANA registry for cache-control directives"
2581    </t>
2582    <t>
2583      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2584      "Heuristic caching of URLs with query components"
2585    </t>
2586  </list>
2587</t>
2588<t>
2589  Partly resolved issues:
2590  <list style="symbols"> 
2591    <t>
2592      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2593      "Term for the requested resource's URI"
2594    </t>
2595  </list>
2596</t>
2597</section>
2598
2599<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2600<t>
2601  Closed issues:
2602  <list style="symbols"> 
2603    <t>
2604      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2605      "Clarify entity / representation / variant terminology"
2606    </t>
2607    <t>
2608      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2609      "consider removing the 'changes from 2068' sections"
2610    </t>
2611    <t>
2612      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2613      "Allowing heuristic caching for new status codes"
2614    </t>
2615    <t>
2616      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2617      "Allowing heuristic caching for new status codes"
2618    </t>
2619    <t>
2620      Clean up TODOs and prose in "Combining Responses."
2621    </t>
2622  </list>
2623</t>
2624</section>
2625
2626<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2627<t>
2628  None yet.
2629</t>
2630</section>
2631
2632</section>
2633  </back>
2634</rfc>
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