source: draft-ietf-httpbis/latest/p6-cache.xml @ 1099

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2011">
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.12"/>.
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      header fields, 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 field (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 field (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 field) 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 header fields
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-header fields 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 field) 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<t>
550   An HTTP implementation without a clock &MUST-NOT; used stored responses
551   without revalidating them on every use. An HTTP cache, especially a shared
552   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
553   synchronize its clock with a reliable external standard.
554</t>
555
556</section>
557
558<section anchor="expiration.model" title="Freshness Model">
559<t>
560   When a response is "fresh" in the cache, it can be used to satisfy
561   subsequent requests without contacting the origin server, thereby improving
562   efficiency.
563</t>
564<t>
565   The primary mechanism for determining freshness is for an origin server to
566   provide an explicit expiration time in the future, using either the Expires
567   header field (<xref target="header.expires" />) or the max-age response cache
568   directive (<xref target="cache-response-directive" />). Generally, origin
569   servers will assign future explicit expiration times to responses in the
570   belief that the representation is not likely to change in a semantically
571   significant way before the expiration time is reached.
572</t>
573<t>
574   If an origin server wishes to force a cache to validate every request, it
575   can assign an explicit expiration time in the past to indicate that the
576   response is already stale.  Compliant caches will validate the cached response
577   before reusing it for subsequent requests.
578</t>
579<t>
580   Since origin servers do not always provide explicit expiration times, HTTP
581   caches &MAY; assign heuristic expiration times when explicit times are not
582   specified, employing algorithms that use other heade field values (such as the
583   Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1
584   specification does not provide specific algorithms, but does impose
585   worst-case constraints on their results.
586</t>
587<figure>
588<preamble>
589  The calculation to determine if a response is fresh is:
590</preamble>
591<artwork type="code">
592   response_is_fresh = (freshness_lifetime &gt; current_age)
593</artwork>
594</figure>
595<t>
596   The freshness_lifetime is defined in <xref
597   target="calculating.freshness.lifetime" />; the current_age is defined in
598   <xref target="age.calculations" />.
599</t>
600<t>
601   Additionally, clients might need to influence freshness calculation. They
602   can do this using several request cache directives, with the effect of
603   either increasing or loosening constraints on freshness. See <xref
604   target="cache-request-directive" />.
605</t>
606<t>
607   <cref anchor="ISSUE-no-req-for-directives">there are not requirements
608   directly applying to cache-request-directives and freshness.</cref>
609</t>
610<t>
611   Note that freshness applies only to cache operation; it cannot be used to
612   force a user agent to refresh its display or reload a resource. See <xref
613   target="history.lists" /> for an explanation of the difference between
614   caches and history mechanisms.
615</t>
616
617<section anchor="calculating.freshness.lifetime" 
618   title="Calculating Freshness Lifetime">
619<t>
620   A cache can calculate the freshness lifetime (denoted as
621   freshness_lifetime) of a response by using the first match of:
622   <list style="symbols">
623      <t>If the cache is shared and the s-maxage response cache directive
624      (<xref target="cache-response-directive" />) is present, use its value,
625      or</t>
626      <t>If the max-age response cache directive (<xref
627      target="cache-response-directive" />) is present, use its value, or</t>
628      <t>If the Expires response header field (<xref target="header.expires" />) is
629      present, use its value minus the value of the Date response header field,
630      or</t>
631      <t>Otherwise, no explicit expiration time is present in the response. A
632      heuristic freshness lifetime might be applicable; see <xref
633      target="heuristic.freshness" />.</t>
634   </list>
635</t>
636<t>
637   Note that this calculation is not vulnerable to clock skew, since all of
638   the information comes from the origin server.
639</t>
640
641<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
642<t>
643   If no explicit expiration time is present in a stored response that has a
644   status code whose definition allows heuristic freshness to be used
645   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
646   410), a heuristic expiration time &MAY; be calculated. Heuristics
647   &MUST-NOT; be used for response status codes that do not explicitly allow
648   it.
649</t>
650<t>
651   When a heuristic is used to calculate freshness lifetime, the cache
652   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
653   its current_age is more than 24 hours and such a warning is not already
654   present.
655</t>
656<t>
657   Also, if the response has a Last-Modified header field (&header-last-modified;),
658   the heuristic expiration value &SHOULD; be no more than some fraction of
659   the interval since that time. A typical setting of this fraction might be
660   10%.
661</t>
662<x:note>
663   <t>
664      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
665      x:sec="13.9"/>) required that caches do not calculate heuristic
666      freshness for URLs with query components (i.e., those containing '?').
667      In practice, this has not been widely implemented. Therefore, servers
668      are encouraged to send explicit directives (e.g., Cache-Control:
669      no-cache) if they wish to preclude caching.
670   </t>
671</x:note>
672</section>
673</section>
674
675<section anchor="age.calculations" title="Calculating Age">
676<t>
677   HTTP/1.1 uses the Age response-header field to convey the estimated age of the
678   response message when obtained from a cache. The Age field value is the
679   cache's estimate of the amount of time since the response was generated or
680   validated by the origin server. In essence, the Age value is the sum of the
681   time that the response has been resident in each of the caches along the
682   path from the origin server, plus the amount of time it has been in transit
683   along network paths.
684</t>
685<t>
686   The following data is used for the age calculation:
687</t>
688<t>
689   <x:dfn>age_value</x:dfn>
690   <list>
691      <t>
692         The term "age_value" denotes the value of the Age header field (<xref
693         target="header.age"/>), in a form appropriate for arithmetic
694         operation; or 0, if not available.
695      </t>
696   </list>
697</t>
698<t>
699   <x:dfn>date_value</x:dfn>
700   <list>
701      <t>
702         HTTP/1.1 requires origin servers to send a Date header field, if possible,
703         with every response, giving the time at which the response was
704         generated. The term "date_value" denotes the value of the Date
705         header field, in a form appropriate for arithmetic operations. See
706         &header-date; for the definition of the Date header field, and for
707         requirements regarding responses without it.
708      </t>
709   </list>
710</t>
711<t>
712   <x:dfn>now</x:dfn>
713   <list>
714      <t>
715         The term "now" means "the current value of the clock at the host
716         performing the calculation". Hosts that use HTTP, but especially
717         hosts running origin servers and caches, &SHOULD; use NTP (<xref
718         target="RFC1305"/>) or some similar protocol to synchronize their
719         clocks to a globally accurate time standard.
720      </t>
721   </list>
722</t>
723<t>
724   <x:dfn>request_time</x:dfn>
725   <list>
726      <t>
727         The current value of the clock at the host at the time the request
728         resulting in the stored response was made.
729      </t>
730   </list>
731</t>
732<t>
733   <x:dfn>response_time</x:dfn>
734   <list>
735      <t>
736         The current value of the clock at the host at the time the response
737         was received.
738      </t>
739   </list>
740</t>
741<t>
742   A response's age can be calculated in two entirely independent ways:
743   <list style="numbers">
744      <t>the "apparent_age": response_time minus date_value, if the local
745      clock is reasonably well synchronized to the origin server's clock. If
746      the result is negative, the result is replaced by zero.</t>
747      <t>the "corrected_age_value", if all of the caches along the response
748      path implement HTTP/1.1; note this value &MUST; be interpreted relative
749      to the time the request was initiated, not the time that the response
750      was received.</t>
751   </list>
752</t>
753<figure>
754<artwork type="code">
755  apparent_age = max(0, response_time - date_value);
756
757  response_delay = response_time - request_time;
758  corrected_age_value = age_value + response_delay; 
759</artwork>
760</figure>
761<figure>
762<preamble>These are combined as</preamble>
763<artwork type="code">
764  corrected_initial_age = max(apparent_age, corrected_age_value);
765</artwork></figure>
766<t>
767   The current_age of a stored response can then be calculated by adding the
768   amount of time (in seconds) since the stored response was last validated by
769   the origin server to the corrected_initial_age.
770</t>
771<figure><artwork type="code">
772  resident_time = now - response_time;
773  current_age = corrected_initial_age + resident_time;
774</artwork></figure>
775</section>
776
777<section anchor="serving.stale.responses" title="Serving Stale Responses">
778<t>
779   A "stale" response is one that either has explicit expiry information or is
780   allowed to have heuristic expiry calculated, but is not fresh according to
781   the calculations in <xref target="expiration.model" />.
782</t>
783<t>
784   Caches &MUST-NOT; return a stale response if it is prohibited by an
785   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
786   directive, a "must-revalidate" cache-response-directive, or an applicable
787   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
788   target="cache-response-directive"/>).
789</t>
790<t>
791   Caches &SHOULD-NOT; return stale responses unless they are disconnected
792   (i.e., it cannot contact the origin server or otherwise find a forward
793   path) or otherwise explicitly allowed (e.g., the max-stale request
794   directive; see <xref target="cache-request-directive" />).
795</t>
796<t>
797   Stale responses &SHOULD; have a Warning header field with the 110 warn-code (see
798   <xref target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be
799   sent on stale responses if the cache is disconnected.
800</t>
801<t>
802   If a cache receives a first-hand response (either an entire response, or a
803   304 (Not Modified) response) that it would normally forward to the
804   requesting client, and the received response is no longer fresh, the cache
805   &SHOULD; forward it to the requesting client without adding a new Warning
806   (but without removing any existing Warning header fields). A cache &SHOULD-NOT;
807   attempt to validate a response simply because that response became stale in
808   transit.
809</t>
810</section>
811</section>
812
813<section anchor="validation.model" title="Validation Model">
814<t>
815   When a cache has one or more stored responses for a requested URI, but
816   cannot serve any of them (e.g., because they are not fresh, or one cannot
817   be selected; see <xref target="caching.negotiated.responses"/>), it can use
818   the conditional request mechanism &conditional; in the forwarded request to
819   give the origin server an opportunity to both select a valid stored
820   response to be used, and to update it. This process is known as
821   "validating" or "revalidating" the stored response.
822</t>
823<t>
824   When sending such a conditional request, the cache &SHOULD; add an
825   If-Modified-Since header field whose value is that of the Last-Modified header
826   field from the selected (see <xref target="caching.negotiated.responses"/>)
827   stored response, if available.
828</t>
829<t>
830   Additionally, the cache &SHOULD; add an If-None-Match header field whose value is
831   that of the ETag header field(s) from all responses stored for the requested URI,
832   if present. However, if any of the stored responses contains only partial
833   content, its entity-tag &SHOULD-NOT; be included in the If-None-Match
834   header field unless the request is for a range that would be fully
835   satisfied by that stored response.
836</t>
837<t>
838   A 304 (Not Modified) response status code indicates that the stored
839   response can be updated and reused; see <xref target="combining.responses"/>.
840</t>
841<t>
842   A full response (i.e., one with a response body) indicates that none of the
843   stored responses nominated in the conditional request is suitable. Instead,
844   the full response &SHOULD; be used to satisfy the request and &MAY; replace
845   the stored response.
846</t>
847<t>
848   If a cache receives a 5xx response while attempting to validate a response,
849   it &MAY; either forward this response to the requesting client, or act as
850   if the server failed to respond. In the latter case, it &MAY; return a
851   previously stored response (see <xref target="serving.stale.responses" />).
852</t>
853</section>
854
855<section anchor="invalidation.after.updates.or.deletions" 
856   title="Request Methods that Invalidate">
857<t>
858   Because unsafe methods (&safe-methods;) have the potential for changing
859   state on the origin server, intervening caches can use them to keep their
860   contents up-to-date.
861</t>
862<t>
863   The following HTTP methods &MUST; cause a cache to invalidate the effective
864   Request URI (&effective-request-uri;) as well as the URI(s) in the Location
865   and Content-Location header fields (if present):
866   <list style="symbols">
867      <t>PUT</t>
868      <t>DELETE</t>
869      <t>POST</t>
870   </list>
871</t>
872<t>
873   An invalidation based on a URI from a Location or Content-Location header field
874   &MUST-NOT; be performed if the host part of that URI differs from the host
875   part in the effective request URI (&effective-request-uri;). This helps
876   prevent denial of service attacks.
877</t>
878<t>
879   A cache that passes through requests for methods it does not understand
880   &SHOULD; invalidate the effective request URI (&effective-request-uri;).
881</t>
882<t>
883   Here, "invalidate" means that the cache will either remove all stored
884   responses related to the effective request URI, or will mark these as
885   "invalid" and in need of a mandatory validation before they can be returned
886   in response to a subsequent request.
887</t>
888<t>
889   Note that this does not guarantee that all appropriate responses are
890   invalidated. For example, the request that caused the change at the origin
891   server might not have gone through the cache where a response is stored.
892</t>
893</section>
894
895<section anchor="caching.authenticated.responses" 
896   title="Shared Caching of Authenticated Responses">
897
898<t>
899   Shared caches &MUST-NOT; use a cached response to a request with an
900   Authorization header field (&header-authorization;) to satisfy any subsequent
901   request unless a cache directive that allows such responses to be stored is
902   present in the response.
903</t>
904
905<t>
906   In this specification, the following Cache-Control response directives
907   (<xref target="cache-response-directive"/>) have such an effect:
908   must-revalidate, public, s-maxage.
909</t>
910
911<t>
912   Note that cached responses that contain the "must-revalidate" and/or
913   "s-maxage" response directives are not allowed to be served stale (<xref
914   target="serving.stale.responses"/>) by shared caches. In particular, a
915   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
916   used to satisfy a subsequent request without revalidating it on the origin
917   server.
918</t>
919</section>
920
921<section anchor="caching.negotiated.responses" 
922   title="Caching Negotiated Responses">
923<t>
924   When a cache receives a request that can be satisfied by a stored response
925   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
926   use that response unless all of the selecting request-header fields nominated by
927   the Vary header field match in both the original request (i.e., that associated
928   with the stored response), and the presented request.
929</t>
930<t>
931   The selecting request-header fields from two requests are defined to match if and
932   only if those in the first request can be transformed to those in the
933   second request by applying any of the following:
934   <list style="symbols">
935      <t>
936         adding or removing whitespace, where allowed in the header field's syntax
937      </t>
938      <t>
939         combining multiple message-header fields with the same field name
940         (see &header-fields;)
941      </t>
942      <t>
943         normalizing both header field values in a way that is known to have
944         identical semantics, according to the header field's specification (e.g.,
945         re-ordering field values when order is not significant;
946         case-normalization, where values are defined to be case-insensitive)
947      </t>
948  </list>
949</t>
950<t>
951   If (after any normalization that might take place) a header field is absent
952   from a request, it can only match another request if it is also absent
953   there.
954</t>
955<t>
956   A Vary header field-value of "*" always fails to match, and subsequent
957   requests to that resource can only be properly interpreted by the origin
958   server.
959</t>
960<t>
961   The stored response with matching selecting request-header fields is known as the
962   selected response.
963</t>
964<t>
965   If no selected response is available, the cache &MAY; forward the presented
966   request to the origin server in a conditional request; see <xref
967   target="validation.model"/>.
968</t>
969</section>
970
971<section anchor="combining.responses" title="Combining Responses">
972<t>
973   When a cache receives a 304 (Not Modified) response or a 206 (Partial
974   Content) response (in this section, the "new" response"), it needs to
975   create an updated response by combining the stored response with the new
976   one, so that the updated response can be used to satisfy the request, and
977   potentially update the cached response.
978</t>
979<t>
980   If the new response contains an ETag, it identifies the stored response to
981   use. <cref anchor="TODO-mention-CL">might need language about
982   Content-Location here</cref><cref
983   anchor="TODO-select-for-combine">Shouldn't this be the selected
984   response?</cref>
985</t>
986<t>
987   If the new response's status code is 206 (partial content), both the stored
988   and new responses &MUST; have validators, and those validators &MUST; match
989   using the strong comparison function (see &weak-and-strong;). Otherwise,
990   the responses &MUST-NOT; be combined.
991</t>
992<t>
993   The stored response header fields are used as those of the updated response,
994   except that
995   <list style="symbols">
996      <t>any stored Warning header fields with warn-code 1xx (see <xref
997      target="header.warning" />) &MUST; be deleted.</t>
998      <t>any stored Warning header fields with warn-code 2xx &MUST; be retained.</t>
999      <t>any other header fields provided in the new response &MUST; replace all
1000      instances of the corresponding header fields from the stored response.</t>
1001   </list>
1002</t>
1003<t>
1004   The updated response header fields &MUST; be used to replace those of the stored
1005   response in cache (unless the stored response is removed from cache). In
1006   the case of a 206 response, the combined representation &MAY; be stored.
1007</t>
1008</section>
1009
1010</section>
1011
1012<section anchor="header.fields" title="Header Field Definitions">
1013<t>
1014   This section defines the syntax and semantics of HTTP/1.1 header fields
1015   related to caching.
1016</t>
1017
1018<section anchor="header.age" title="Age">
1019   <iref item="Age header" primary="true" x:for-anchor="" />
1020   <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
1021   <x:anchor-alias value="Age"/>
1022   <x:anchor-alias value="Age-v"/>
1023   <x:anchor-alias value="age-value"/>
1024<t>
1025   The "Age" response-header field conveys the sender's estimate of the amount
1026   of time since the response was generated or successfully validated at the
1027   origin server. Age values are calculated as specified in <xref
1028   target="age.calculations" />.
1029</t>
1030<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
1031  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
1032  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
1033</artwork></figure>
1034<t anchor="rule.delta-seconds">
1035  <x:anchor-alias value="delta-seconds" />
1036  Age field-values are non-negative integers, representing time in seconds.
1037</t>
1038<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
1039  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
1040</artwork></figure>
1041<t>
1042   If a cache receives a value larger than the largest positive integer it can
1043   represent, or if any of its age calculations overflows, it &MUST; transmit
1044   an Age header field with a field-value of 2147483648 (2<x:sup>31</x:sup>). Caches
1045   &SHOULD; use an arithmetic type of at least 31 bits of range.
1046</t>
1047<t>
1048   The presence of an Age header field in a response implies that a response
1049   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1050   might not implement the Age header field.
1051</t>
1052</section>
1053
1054<section anchor="header.cache-control" title="Cache-Control">
1055   <iref item="Cache-Control header" primary="true" x:for-anchor="" />
1056   <iref item="Headers" primary="true" subitem="Cache-Control" 
1057      x:for-anchor="" />
1058   <x:anchor-alias value="Cache-Control"/>
1059   <x:anchor-alias value="Cache-Control-v"/>
1060   <x:anchor-alias value="cache-directive"/>
1061   <x:anchor-alias value="cache-extension"/>
1062   <x:anchor-alias value="cache-request-directive"/>
1063   <x:anchor-alias value="cache-response-directive"/>
1064<t>
1065   The "Cache-Control" general-header field is used to specify directives for
1066   caches along the request/response chain. Such cache directives are
1067   unidirectional in that the presence of a directive in a request does not
1068   imply that the same directive is to be given in the response.
1069</t>
1070<t>
1071   HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control
1072   directives defined in this section. See <xref
1073   target="cache.control.extensions"/> for information about how Cache-Control
1074   directives defined elsewhere are handled.
1075</t>
1076<x:note>
1077   <t>
1078       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1079       might only implement Pragma: no-cache (see <xref target="header.pragma"
1080       />).
1081   </t>
1082</x:note>
1083<t>
1084   Cache directives &MUST; be passed through by a proxy or gateway
1085   application, regardless of their significance to that application, since
1086   the directives might be applicable to all recipients along the
1087   request/response chain. It is not possible to target a directive to a
1088   specific cache.
1089</t>
1090<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"/>
1091  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1092  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1093
1094  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1095     / <x:ref>cache-response-directive</x:ref>
1096
1097  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1098</artwork></figure>
1099
1100<section anchor="cache-request-directive" 
1101   title="Request Cache-Control Directives">
1102   <x:anchor-alias value="cache-request-directive" />
1103
1104<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1105   subitem="cache-request-directive" />
1106  <x:ref>cache-request-directive</x:ref> =
1107       "no-cache"
1108     / "no-store"
1109     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1110     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1111     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1112     / "no-transform"
1113     / "only-if-cached"
1114     / <x:ref>cache-extension</x:ref>
1115</artwork></figure>
1116
1117<t>
1118   <x:dfn>no-cache</x:dfn>
1119   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1120   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1121   <list>
1122      <t>The no-cache request directive indicates that a stored response
1123      &MUST-NOT; be used to satisfy the request without successful validation
1124      on the origin server.</t> 
1125   </list>
1126</t>
1127<t>
1128   <x:dfn>no-store</x:dfn>
1129   <iref item="Cache Directives" primary="true" subitem="no-store" />
1130   <iref item="no-store" primary="true" subitem="Cache Directive" />
1131   <list>
1132      <t>The no-store request directive indicates that a cache &MUST-NOT;
1133      store any part of either this request or any response to it. This
1134      directive applies to both non-shared and shared caches. "&MUST-NOT;
1135      store" in this context means that the cache &MUST-NOT; intentionally
1136      store the information in non-volatile storage, and &MUST; make a
1137      best-effort attempt to remove the information from volatile storage as
1138      promptly as possible after forwarding it.</t>
1139      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1140      privacy. In particular, malicious or compromised caches might not
1141      recognize or obey this directive, and communications networks might be
1142      vulnerable to eavesdropping.</t>
1143   </list>
1144</t>
1145<t>
1146   <x:dfn>max-age</x:dfn>
1147   <iref item="Cache Directives" primary="true" subitem="max-age" />
1148   <iref item="max-age" primary="true" subitem="Cache Directive" />
1149   <list>
1150      <t>The max-age request directive indicates that the client is willing to
1151      accept a response whose age is no greater than the specified time in
1152      seconds. Unless the max-stale request directive is also present, the
1153      client is not willing to accept a stale response.</t>
1154   </list>
1155</t>
1156<t>
1157   <x:dfn>max-stale</x:dfn>
1158   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1159   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1160   <list>
1161      <t>The max-stale request directive indicates that the client is willing
1162      to accept a response that has exceeded its expiration time. If max-stale
1163      is assigned a value, then the client is willing to accept a response
1164      that has exceeded its expiration time by no more than the specified
1165      number of seconds. If no value is assigned to max-stale, then the client
1166      is willing to accept a stale response of any age.</t>
1167   </list>
1168</t>
1169<t>
1170   <x:dfn>min-fresh</x:dfn>
1171   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1172   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1173   <list>
1174      <t>The min-fresh request directive indicates that the client is willing
1175      to accept a response whose freshness lifetime is no less than its
1176      current age plus the specified time in seconds. That is, the client
1177      wants a response that will still be fresh for at least the specified
1178      number of seconds.</t>
1179   </list>
1180</t>
1181<t>
1182   <x:dfn>no-transform</x:dfn>
1183   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1184   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1185   <list>
1186      <t>The no-transform request directive indicates that an intermediate
1187      cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
1188      Content-Type request header fields, nor the request representation.</t>
1189   </list>
1190</t>
1191<t>
1192   <x:dfn>only-if-cached</x:dfn>
1193   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1194   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1195   <list>
1196      <t>The only-if-cached request directive indicates that the client only
1197      wishes to return a stored response. If it receives this directive, a
1198      cache &SHOULD; either respond using a stored response that is consistent
1199      with the other constraints of the request, or respond with a 504
1200      (Gateway Timeout) status code. If a group of caches is being operated as
1201      a unified system with good internal connectivity, such a request &MAY;
1202      be forwarded within that group of caches.</t>
1203   </list>
1204</t>
1205</section>
1206
1207<section anchor="cache-response-directive" 
1208   title="Response Cache-Control Directives">
1209   <x:anchor-alias value="cache-response-directive" />
1210
1211<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1212   subitem="cache-response-directive" />
1213  <x:ref>cache-response-directive</x:ref> =
1214       "public"
1215     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1216     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1217     / "no-store"
1218     / "no-transform"
1219     / "must-revalidate"
1220     / "proxy-revalidate"
1221     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1222     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1223     / <x:ref>cache-extension</x:ref>
1224</artwork></figure>
1225
1226<t>
1227   <x:dfn>public</x:dfn>
1228   <iref item="Cache Directives" primary="true" subitem="public" />
1229   <iref item="public" primary="true" subitem="Cache Directive" />
1230   <list>
1231      <t>The public response directive indicates that the response &MAY; be
1232      cached, even if it would normally be non-cacheable or cacheable only
1233      within a non-shared cache. (See also Authorization,
1234      &header-authorization;, for additional details.) </t>
1235  </list>
1236</t>
1237<t>
1238   <x:dfn>private</x:dfn>
1239   <iref item="Cache Directives" primary="true" subitem="private" />
1240   <iref item="private" primary="true" subitem="Cache Directive" />
1241   <list>
1242      <t>The private response directive indicates that the response message is
1243      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1244      private (non-shared) cache &MAY; store the response.</t>
1245      <t>If the private response directive specifies one or more field-names,
1246      this requirement is limited to the field-values associated with the
1247      listed response header fields. That is, the specified field-names(s)
1248      &MUST-NOT; be stored by a shared cache, whereas the remainder of the
1249      response message &MAY; be.</t>
1250      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1251      the response can be stored; it cannot ensure the privacy of the message
1252      content. Also, private response directives with field-names are often
1253      handled by implementations as if an unqualified private directive was
1254      received; i.e., the special handling for the qualified form is not
1255      widely implemented.</t>
1256   </list>
1257</t>
1258<t>
1259   <x:dfn>no-cache</x:dfn>
1260   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1261   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1262   <list>
1263      <t>The no-cache response directive indicates that the response MUST NOT
1264      be used to satisfy a subsequent request without successful validation on
1265      the origin server. This allows an origin server to prevent a cache from
1266      using it to satisfy a request without contacting it, even by caches that
1267      have been configured to return stale responses.</t>
1268      <t>If the no-cache response directive specifies one or more field-names,
1269      this requirement is limited to the field-values associated with the
1270      listed response header fields. That is, the specified field-name(s) &MUST-NOT;
1271      be sent in the response to a subsequent request without successful
1272      validation on the origin server. This allows an origin server to prevent
1273      the re-use of certain header fields in a response, while still allowing
1274      caching of the rest of the response.</t>
1275      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1276      this directive. Also, no-cache response directives with field-names are
1277      often handled by implementations as if an unqualified no-cache directive
1278      was received; i.e., the special handling for the qualified form is not
1279      widely implemented. </t>
1280   </list>
1281</t>
1282<t>
1283   <x:dfn>no-store</x:dfn>
1284   <iref item="Cache Directives" primary="true" subitem="no-store" />
1285   <iref item="no-store" primary="true" subitem="Cache Directive" />
1286   <list>
1287      <t>The no-store response directive indicates that a cache &MUST-NOT;
1288      store any part of either the immediate request or response. This
1289      directive applies to both non-shared and shared caches. "&MUST-NOT;
1290      store" in this context means that the cache &MUST-NOT; intentionally
1291      store the information in non-volatile storage, and &MUST; make a
1292      best-effort attempt to remove the information from volatile storage as
1293      promptly as possible after forwarding it.</t>
1294      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1295      privacy. In particular, malicious or compromised caches might not
1296      recognize or obey this directive, and communications networks might be
1297      vulnerable to eavesdropping.</t>
1298   </list>
1299</t>
1300<t>
1301   <x:dfn>must-revalidate</x:dfn>
1302   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1303   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1304   <list>
1305      <t>The must-revalidate response directive indicates that once it has
1306      become stale, the response &MUST-NOT; be used to satisfy subsequent
1307      requests without successful validation on the origin server.</t>
1308      <t>The must-revalidate directive is necessary to support reliable
1309      operation for certain protocol features. In all circumstances an
1310      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in particular,
1311      if the cache cannot reach the origin server for any reason, it &MUST;
1312      generate a 504 (Gateway Timeout) response.</t>
1313      <t>Servers &SHOULD; send the must-revalidate directive if and only if
1314      failure to validate a request on the representation could result in
1315      incorrect operation, such as a silently unexecuted financial
1316      transaction.</t>
1317   </list>
1318</t>
1319<t>
1320   <x:dfn>proxy-revalidate</x:dfn>
1321   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1322   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1323   <list>
1324      <t>The proxy-revalidate response directive has the same meaning as the
1325      must-revalidate response directive, except that it does not apply to
1326      non-shared caches.</t>
1327   </list>
1328</t>
1329<t>
1330   <x:dfn>max-age</x:dfn>
1331   <iref item="Cache Directives" primary="true" subitem="max-age" />
1332   <iref item="max-age" primary="true" subitem="Cache Directive" />
1333   <list>
1334      <t>The max-age response directive indicates that response is to be
1335      considered stale after its age is greater than the specified number of
1336      seconds.</t>
1337   </list>
1338</t>
1339<t>
1340   <x:dfn>s-maxage</x:dfn>
1341   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1342   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1343   <list>
1344      <t>The s-maxage response directive indicates that, in shared caches, the
1345      maximum age specified by this directive overrides the maximum age
1346      specified by either the max-age directive or the Expires header field. The
1347      s-maxage directive also implies the semantics of the proxy-revalidate
1348      response directive.</t>
1349   </list>
1350</t>
1351<t>
1352   <x:dfn>no-transform</x:dfn>
1353   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1354   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1355   <list>
1356      <t>The no-transform response directive indicates that an intermediate
1357      cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
1358      Content-Type response header fields, nor the response representation.</t>
1359   </list>
1360</t>
1361
1362</section>
1363
1364<section anchor="cache.control.extensions" title="Cache Control Extensions">
1365<t>
1366   The Cache-Control header field can be extended through the use of one or
1367   more cache-extension tokens, each with an optional value. Informational
1368   extensions (those that do not require a change in cache behavior) can be
1369   added without changing the semantics of other directives. Behavioral
1370   extensions are designed to work by acting as modifiers to the existing base
1371   of cache directives. Both the new directive and the standard directive are
1372   supplied, such that applications that do not understand the new directive
1373   will default to the behavior specified by the standard directive, and those
1374   that understand the new directive will recognize it as modifying the
1375   requirements associated with the standard directive. In this way,
1376   extensions to the cache-control directives can be made without requiring
1377   changes to the base protocol.
1378</t>
1379<t>
1380   This extension mechanism depends on an HTTP cache obeying all of the
1381   cache-control directives defined for its native HTTP-version, obeying
1382   certain extensions, and ignoring all directives that it does not
1383   understand.
1384</t>
1385<t>
1386   For example, consider a hypothetical new response directive called
1387   "community" that acts as a modifier to the private directive. We define
1388   this new directive to mean that, in addition to any non-shared cache, any
1389   cache that is shared only by members of the community named within its
1390   value may cache the response. An origin server wishing to allow the UCI
1391   community to use an otherwise private response in their shared cache(s)
1392   could do so by including
1393</t>
1394<figure><artwork type="example">
1395  Cache-Control: private, community="UCI"
1396</artwork></figure>
1397<t>
1398   A cache seeing this header field will act correctly even if the cache does
1399   not understand the community cache-extension, since it will also see and
1400   understand the private directive and thus default to the safe behavior.
1401</t>
1402<t>
1403   Unrecognized cache directives &MUST; be ignored; it is assumed that any
1404   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1405   combined with standard directives (or the response's default cacheability)
1406   such that the cache behavior will remain minimally correct even if the
1407   cache does not understand the extension(s).
1408</t>
1409<t>
1410   The HTTP Cache Directive Registry defines the name space for the cache
1411   directives.
1412</t>
1413<t>
1414   Registrations &MUST; include the following fields:
1415   <list style="symbols">
1416      <t>Cache Directive Name</t>
1417      <t>Pointer to specification text</t>
1418   </list>
1419</t>
1420<t>
1421   Values to be added to this name space are subject to IETF review (<xref
1422   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1423</t>
1424<t>
1425   The registry itself is maintained at <eref
1426   target="http://www.iana.org/assignments/http-cache-directives"/>.
1427</t>
1428</section>
1429
1430</section>
1431
1432<section anchor="header.expires" title="Expires">
1433   <iref item="Expires header" primary="true" x:for-anchor="" />
1434   <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1435   <x:anchor-alias value="Expires"/>
1436   <x:anchor-alias value="Expires-v"/>
1437<t>
1438   The "Expires" header field gives the date/time after which the
1439   response is considered stale. See <xref target="expiration.model" /> for
1440   further discussion of the freshness model.
1441</t>
1442<t>
1443   The presence of an Expires field does not imply that the original resource
1444   will change or cease to exist at, before, or after that time.
1445</t>
1446<t>
1447   The field-value is an absolute date and time as defined by HTTP-date in
1448   &full-date;; it &MUST; be sent in rfc1123-date format.
1449</t>
1450<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1451  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1452  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1453</artwork></figure>
1454<figure>
1455  <preamble>For example</preamble>
1456<artwork type="example">
1457  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1458</artwork></figure>
1459<x:note>
1460   <t>
1461       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1462       max-age directive (see <xref target="cache-response-directive" />),
1463       that directive overrides the Expires field. Likewise, the s-maxage
1464       directive overrides Expires in shared caches.
1465   </t>
1466</x:note>
1467<t>
1468   HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the
1469   future.
1470</t>
1471<t>
1472   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
1473   especially including the value "0", as in the past (i.e., "already
1474   expired").
1475</t>
1476</section>
1477
1478<section anchor="header.pragma" title="Pragma">
1479   <iref item="Pragma header" primary="true" x:for-anchor="" />
1480   <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1481   <x:anchor-alias value="extension-pragma"/>
1482   <x:anchor-alias value="Pragma"/>
1483   <x:anchor-alias value="Pragma-v"/>
1484   <x:anchor-alias value="pragma-directive"/>
1485<t>
1486   The "Pragma" general-header field is used to include
1487   implementation-specific directives that might apply to any recipient along
1488   the request/response chain. All pragma directives specify optional behavior
1489   from the viewpoint of the protocol; however, some systems &MAY; require
1490   that behavior be consistent with the directives.
1491</t>
1492<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"/>
1493  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1494  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1495  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1496  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1497</artwork></figure>
1498<t>
1499   When the no-cache directive is present in a request message, an application
1500   &SHOULD; forward the request toward the origin server even if it has a
1501   cached copy of what is being requested. This pragma directive has the same
1502   semantics as the no-cache response directive (see <xref
1503   target="cache-response-directive" />) and is defined here for backward
1504   compatibility with HTTP/1.0. Clients &SHOULD; include both header fields
1505   when a no-cache request is sent to a server not known to be HTTP/1.1
1506   compliant. HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the
1507   client had sent "Cache-Control: no-cache".
1508</t>
1509<x:note>
1510   <t>
1511      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a
1512      response-header field is not actually specified, it does not provide a
1513      reliable replacement for "Cache-Control: no-cache" in a response.
1514   </t>
1515</x:note>
1516<t>
1517   This mechanism is deprecated; no new Pragma directives will be defined in
1518   HTTP.
1519</t>
1520</section>
1521
1522<section anchor="header.vary" title="Vary">
1523   <iref item="Vary header" primary="true" x:for-anchor="" />
1524   <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1525   <x:anchor-alias value="Vary"/>
1526   <x:anchor-alias value="Vary-v"/>
1527<t>
1528   The "Vary" response-header field conveys the set of request-header fields
1529   that were used to select the representation.
1530</t>
1531<t>
1532   Caches use this information, in part, to determine whether a stored
1533   response can be used to satisfy a given request; see <xref
1534   target="caching.negotiated.responses" />. determines, while the response is
1535   fresh, whether a cache is permitted to use the response to reply to a
1536   subsequent request without validation; see <xref
1537   target="caching.negotiated.responses" />.
1538</t>
1539<t>
1540   In uncacheable or stale responses, the Vary field value advises the user
1541   agent about the criteria that were used to select the representation.
1542</t>
1543<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1544  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1545  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1546</artwork></figure>
1547<t>
1548   The set of header fields named by the Vary field value is known as the
1549   selecting request-header fields.
1550</t>
1551<t>
1552   Servers &SHOULD; include a Vary header field with any cacheable response
1553   that is subject to server-driven negotiation. Doing so allows a cache to
1554   properly interpret future requests on that resource and informs the user
1555   agent about the presence of negotiation on that resource. A server &MAY;
1556   include a Vary header field with a non-cacheable response that is subject
1557   to server-driven negotiation, since this might provide the user agent with
1558   useful information about the dimensions over which the response varies at
1559   the time of the response.
1560</t>
1561<t>
1562   A Vary field value of "*" signals that unspecified parameters not limited
1563   to the request-header fields (e.g., the network address of the client), play a
1564   role in the selection of the response representation; therefore, a cache
1565   cannot determine whether this response is appropriate. The "*" value
1566   &MUST-NOT; be generated by a proxy server.
1567</t>
1568<t>
1569   The field-names given are not limited to the set of standard request-header
1570   fields defined by this specification. Field names are case-insensitive.
1571</t>
1572</section>
1573
1574<section anchor="header.warning" title="Warning">
1575   <iref item="Warning header" primary="true" x:for-anchor="" />
1576   <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1577   <x:anchor-alias value="Warning"/>
1578   <x:anchor-alias value="Warning-v"/>
1579   <x:anchor-alias value="warning-value"/>
1580   <x:anchor-alias value="warn-agent"/>
1581   <x:anchor-alias value="warn-code"/>
1582   <x:anchor-alias value="warn-date"/>
1583   <x:anchor-alias value="warn-text"/>
1584<t>
1585   The "Warning" general-header field is used to carry additional information
1586   about the status or transformation of a message that might not be reflected
1587   in the message. This information is typically used to warn about possible
1588   incorrectness introduced by caching operations or transformations applied
1589   to the payload of the message.
1590</t>
1591<t>
1592   Warnings can be used for other purposes, both cache-related and otherwise.
1593   The use of a warning, rather than an error status code, distinguishes these
1594   responses from true failures.
1595</t>
1596<t>
1597   Warning header fields can in general be applied to any message, however some
1598   warn-codes are specific to caches and can only be applied to response
1599   messages.
1600</t>
1601<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"/>
1602  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1603  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1604 
1605  <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>
1606                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1607 
1608  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1609  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1610                  ; the name or pseudonym of the server adding
1611                  ; the Warning header field, for use in debugging
1612  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1613  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1614</artwork></figure>
1615<t>
1616   Multiple warnings can be attached to a response (either by the origin
1617   server or by a cache), including multiple warnings with the same code
1618   number, only differing in warn-text.
1619</t>
1620<t>
1621   When this occurs, the user agent &SHOULD; inform the user of as many of
1622   them as possible, in the order that they appear in the response.
1623</t>
1624<t>
1625   Systems that generate multiple Warning header fields &SHOULD; order them with
1626   this user agent behavior in mind. New Warning header fields &SHOULD; be added
1627   after any existing Warning headers fields.
1628</t>
1629<t>
1630   Warnings are assigned three digit warn-codes. The first digit indicates
1631   whether the Warning is required to be deleted from a stored response after
1632   validation:
1633   <list style="symbols">
1634      <t>1xx Warnings describe the freshness or validation status of the
1635      response, and so &MUST; be deleted by caches after validation. They can
1636      only be generated by a cache when validating a cached entry, and
1637      &MUST-NOT; be generated in any other situation.</t>
1638      <t>2xx Warnings describe some aspect of the representation that is not
1639      rectified by a validation (for example, a lossy compression of the
1640      representation) and &MUST-NOT; be deleted by caches after validation,
1641      unless a full response is returned, in which case they &MUST; be.</t>
1642   </list>
1643</t>
1644<t>
1645   If an implementation sends a message with one or more Warning header fields to a
1646   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1647   in each warning-value a warn-date that matches the Date header field in the
1648   message.
1649</t>
1650<t>
1651   If an implementation receives a message with a warning-value that includes
1652   a warn-date, and that warn-date is different from the Date value in the
1653   response, then that warning-value &MUST; be deleted from the message before
1654   storing, forwarding, or using it. (preventing the consequences of naive
1655   caching of Warning header fields.) If all of the warning-values are deleted
1656   for this reason, the Warning header field &MUST; be deleted as well.
1657</t>
1658<t>
1659   The following warn-codes are defined by this specification, each with a
1660   recommended warn-text in English, and a description of its meaning.
1661</t>
1662<t><?rfc needLines="4"?>
1663   110 Response is stale
1664   <list>
1665      <t>&SHOULD; be included whenever the returned response is stale.</t>
1666   </list>
1667</t>
1668<t><?rfc needLines="4"?>
1669   111 Revalidation failed
1670   <list>
1671      <t>&SHOULD; be included if a cache returns a stale response because an
1672      attempt to validate the response failed, due to an inability to reach
1673      the server.</t>
1674   </list>
1675</t>
1676<t><?rfc needLines="4"?>
1677   112 Disconnected operation
1678   <list>
1679      <t>&SHOULD; be included if the cache is intentionally disconnected from
1680      the rest of the network for a period of time.</t>
1681   </list>
1682</t>
1683<t><?rfc needLines="4"?>
1684   113 Heuristic expiration
1685   <list>
1686      <t>&SHOULD; be included if the cache heuristically chose a freshness
1687      lifetime greater than 24 hours and the response's age is greater than 24
1688      hours.</t>
1689   </list>
1690</t>
1691<t><?rfc needLines="4"?>
1692   199 Miscellaneous warning
1693   <list>
1694      <t>The warning text can include arbitrary information to be presented to
1695      a human user, or logged. A system receiving this warning &MUST-NOT; take
1696      any automated action, besides presenting the warning to the user.</t>
1697   </list>
1698</t>
1699<t><?rfc needLines="4"?>
1700   214 Transformation applied
1701   <list>
1702      <t>&MUST; be added by an intermediate proxy if it applies any
1703      transformation to the representation, such as changing the
1704      content-coding, media-type, or modifying the representation data, unless
1705      this Warning code already appears in the response.</t>
1706   </list>
1707</t>
1708<t><?rfc needLines="4"?>
1709   299 Miscellaneous persistent warning
1710   <list>
1711      <t>The warning text can include arbitrary information to be presented to
1712      a human user, or logged. A system receiving this warning &MUST-NOT; take
1713      any automated action.</t>
1714   </list>
1715</t>
1716</section>
1717
1718</section>
1719
1720<section anchor="history.lists" title="History Lists">
1721<t>
1722   User agents often have history mechanisms, such as "Back" buttons and
1723   history lists, that can be used to redisplay a representation retrieved
1724   earlier in a session.
1725</t>
1726<t>
1727   The freshness model (<xref target="expiration.model"/>) does not
1728   necessarily apply to history mechanisms. I.e., a history mechanism can
1729   display a previous representation even if it has expired.
1730</t>
1731<t>
1732   This does not prohibit the history mechanism from telling the user that a
1733   view might be stale, or from honoring cache directives (e.g.,
1734   Cache-Control: no-store).
1735</t>
1736</section>
1737
1738
1739<section anchor="IANA.considerations" title="IANA Considerations">
1740
1741<section title="Cache Directive Registry" 
1742   anchor="cache.directive.registration">
1743<t>
1744   The registration procedure for HTTP Cache Directives is defined by <xref
1745   target="cache.control.extensions"/> of this document.
1746</t>
1747<t>
1748   The HTTP Cache Directive Registry shall be created at <eref
1749   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1750   populated with the registrations below:
1751</t>
1752<?BEGININC p6-cache.cache-directives ?>
1753<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1754<texttable xmlns:my="#my" align="left" suppress-title="true"
1755           anchor="iana.cache.directive.registration.table">
1756   <ttcol>Cache Directive</ttcol>
1757   <ttcol>Reference</ttcol>
1758
1759   <c>max-age</c>
1760   <c>
1761      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1762   </c>
1763   <c>max-stale</c>
1764   <c>
1765      <xref target="cache-request-directive"/>
1766   </c>
1767   <c>min-fresh</c>
1768   <c>
1769      <xref target="cache-request-directive"/>
1770   </c>
1771   <c>must-revalidate</c>
1772   <c>
1773      <xref target="cache-response-directive"/>
1774   </c>
1775   <c>no-cache</c>
1776   <c>
1777      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1778   </c>
1779   <c>no-store</c>
1780   <c>
1781      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1782   </c>
1783   <c>no-transform</c>
1784   <c>
1785      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1786   </c>
1787   <c>only-if-cached</c>
1788   <c>
1789      <xref target="cache-request-directive"/>
1790   </c>
1791   <c>private</c>
1792   <c>
1793      <xref target="cache-response-directive"/>
1794   </c>
1795   <c>proxy-revalidate</c>
1796   <c>
1797      <xref target="cache-response-directive"/>
1798   </c>
1799   <c>public</c>
1800   <c>
1801      <xref target="cache-response-directive"/>
1802   </c>
1803   <c>s-maxage</c>
1804   <c>
1805      <xref target="cache-response-directive"/>
1806   </c>
1807   <c>stale-if-error</c>
1808   <c>
1809      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1810   </c>
1811   <c>stale-while-revalidate</c>
1812   <c>
1813      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1814   </c>
1815</texttable>
1816<!--(END)-->
1817<?ENDINC p6-cache.cache-directives ?>
1818</section>
1819
1820<section title="Header Field Registration" anchor="header.field.registration">
1821<t>
1822  The Message Header Field Registry located at <eref 
1823  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1824  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
1825</t>
1826<?BEGININC p6-cache.iana-headers ?>
1827<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1828<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1829   <ttcol>Header Field Name</ttcol>
1830   <ttcol>Protocol</ttcol>
1831   <ttcol>Status</ttcol>
1832   <ttcol>Reference</ttcol>
1833
1834   <c>Age</c>
1835   <c>http</c>
1836   <c>standard</c>
1837   <c>
1838      <xref target="header.age"/>
1839   </c>
1840   <c>Cache-Control</c>
1841   <c>http</c>
1842   <c>standard</c>
1843   <c>
1844      <xref target="header.cache-control"/>
1845   </c>
1846   <c>Expires</c>
1847   <c>http</c>
1848   <c>standard</c>
1849   <c>
1850      <xref target="header.expires"/>
1851   </c>
1852   <c>Pragma</c>
1853   <c>http</c>
1854   <c>standard</c>
1855   <c>
1856      <xref target="header.pragma"/>
1857   </c>
1858   <c>Vary</c>
1859   <c>http</c>
1860   <c>standard</c>
1861   <c>
1862      <xref target="header.vary"/>
1863   </c>
1864   <c>Warning</c>
1865   <c>http</c>
1866   <c>standard</c>
1867   <c>
1868      <xref target="header.warning"/>
1869   </c>
1870</texttable>
1871<!--(END)-->
1872<?ENDINC p6-cache.iana-headers ?>
1873<t>
1874   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1875   Force".
1876</t>
1877</section>
1878
1879</section>
1880
1881<section anchor="security.considerations" title="Security Considerations">
1882<t>
1883   Caches expose additional potential vulnerabilities, since the contents of
1884   the cache represent an attractive target for malicious exploitation.
1885   Because cache contents persist after an HTTP request is complete, an attack
1886   on the cache can reveal information long after a user believes that the
1887   information has been removed from the network. Therefore, cache contents
1888   need to be protected as sensitive information.
1889</t>
1890</section>
1891
1892<section anchor="ack" title="Acknowledgments">
1893<t>
1894   Much of the content and presentation of the caching design is due to
1895   suggestions and comments from individuals including: Shel Kaphan, Paul
1896   Leach, Koen Holtman, David Morris, and Larry Masinter.
1897</t>
1898</section>
1899
1900</middle>
1901
1902<back>
1903<references title="Normative References">
1904
1905  <reference anchor="Part1">
1906    <front>
1907      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1908      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1909        <organization abbrev="Day Software">Day Software</organization>
1910        <address><email>fielding@gbiv.com</email></address>
1911      </author>
1912      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1913        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1914        <address><email>jg@freedesktop.org</email></address>
1915      </author>
1916      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1917        <organization abbrev="HP">Hewlett-Packard Company</organization>
1918        <address><email>JeffMogul@acm.org</email></address>
1919      </author>
1920      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1921        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1922        <address><email>henrikn@microsoft.com</email></address>
1923      </author>
1924      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1925        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1926        <address><email>LMM@acm.org</email></address>
1927      </author>
1928      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1929        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1930        <address><email>paulle@microsoft.com</email></address>
1931      </author>
1932      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1933        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1934        <address><email>timbl@w3.org</email></address>
1935      </author>
1936      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1937        <organization abbrev="W3C">World Wide Web Consortium</organization>
1938        <address><email>ylafon@w3.org</email></address>
1939      </author>
1940      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1941        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1942        <address><email>julian.reschke@greenbytes.de</email></address>
1943      </author>
1944      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1945    </front>
1946    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1947    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1948  </reference>
1949
1950  <reference anchor="Part2">
1951    <front>
1952      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1953      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1954        <organization abbrev="Day Software">Day Software</organization>
1955        <address><email>fielding@gbiv.com</email></address>
1956      </author>
1957      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1958        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1959        <address><email>jg@freedesktop.org</email></address>
1960      </author>
1961      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1962        <organization abbrev="HP">Hewlett-Packard Company</organization>
1963        <address><email>JeffMogul@acm.org</email></address>
1964      </author>
1965      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1966        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1967        <address><email>henrikn@microsoft.com</email></address>
1968      </author>
1969      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1970        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1971        <address><email>LMM@acm.org</email></address>
1972      </author>
1973      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1974        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1975        <address><email>paulle@microsoft.com</email></address>
1976      </author>
1977      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1978        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1979        <address><email>timbl@w3.org</email></address>
1980      </author>
1981      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1982        <organization abbrev="W3C">World Wide Web Consortium</organization>
1983        <address><email>ylafon@w3.org</email></address>
1984      </author>
1985      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1986        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1987        <address><email>julian.reschke@greenbytes.de</email></address>
1988      </author>
1989      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1990    </front>
1991    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1992    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1993  </reference>
1994
1995  <reference anchor="Part4">
1996    <front>
1997      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1998      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1999        <organization abbrev="Day Software">Day Software</organization>
2000        <address><email>fielding@gbiv.com</email></address>
2001      </author>
2002      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2003        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2004        <address><email>jg@freedesktop.org</email></address>
2005      </author>
2006      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2007        <organization abbrev="HP">Hewlett-Packard Company</organization>
2008        <address><email>JeffMogul@acm.org</email></address>
2009      </author>
2010      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2011        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2012        <address><email>henrikn@microsoft.com</email></address>
2013      </author>
2014      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2015        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2016        <address><email>LMM@acm.org</email></address>
2017      </author>
2018      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2019        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2020        <address><email>paulle@microsoft.com</email></address>
2021      </author>
2022      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2023        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2024        <address><email>timbl@w3.org</email></address>
2025      </author>
2026      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2027        <organization abbrev="W3C">World Wide Web Consortium</organization>
2028        <address><email>ylafon@w3.org</email></address>
2029      </author>
2030      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2031        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2032        <address><email>julian.reschke@greenbytes.de</email></address>
2033      </author>
2034      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2035    </front>
2036    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2037    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2038  </reference>
2039
2040  <reference anchor="Part5">
2041    <front>
2042      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2043      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2044        <organization abbrev="Day Software">Day Software</organization>
2045        <address><email>fielding@gbiv.com</email></address>
2046      </author>
2047      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2048        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2049        <address><email>jg@freedesktop.org</email></address>
2050      </author>
2051      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2052        <organization abbrev="HP">Hewlett-Packard Company</organization>
2053        <address><email>JeffMogul@acm.org</email></address>
2054      </author>
2055      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2056        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2057        <address><email>henrikn@microsoft.com</email></address>
2058      </author>
2059      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2060        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2061        <address><email>LMM@acm.org</email></address>
2062      </author>
2063      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2064        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2065        <address><email>paulle@microsoft.com</email></address>
2066      </author>
2067      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2068        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2069        <address><email>timbl@w3.org</email></address>
2070      </author>
2071      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2072        <organization abbrev="W3C">World Wide Web Consortium</organization>
2073        <address><email>ylafon@w3.org</email></address>
2074      </author>
2075      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2076        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2077        <address><email>julian.reschke@greenbytes.de</email></address>
2078      </author>
2079      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2080    </front>
2081    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2082    <x:source basename="p5-range" href="p5-range.xml" />
2083  </reference>
2084
2085  <reference anchor="Part7">
2086    <front>
2087      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2088      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2089        <organization abbrev="Day Software">Day Software</organization>
2090        <address><email>fielding@gbiv.com</email></address>
2091      </author>
2092      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2093        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2094        <address><email>jg@freedesktop.org</email></address>
2095      </author>
2096      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2097        <organization abbrev="HP">Hewlett-Packard Company</organization>
2098        <address><email>JeffMogul@acm.org</email></address>
2099      </author>
2100      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2101        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2102        <address><email>henrikn@microsoft.com</email></address>
2103      </author>
2104      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2105        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2106        <address><email>LMM@acm.org</email></address>
2107      </author>
2108      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2109        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2110        <address><email>paulle@microsoft.com</email></address>
2111      </author>
2112      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2113        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2114        <address><email>timbl@w3.org</email></address>
2115      </author>
2116      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2117        <organization abbrev="W3C">World Wide Web Consortium</organization>
2118        <address><email>ylafon@w3.org</email></address>
2119      </author>
2120      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2121        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2122        <address><email>julian.reschke@greenbytes.de</email></address>
2123      </author>
2124      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2125    </front>
2126    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2127    <x:source basename="p7-auth" href="p7-auth.xml" />
2128  </reference>
2129
2130  <reference anchor="RFC2119">
2131    <front>
2132      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2133      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2134        <organization>Harvard University</organization>
2135        <address><email>sob@harvard.edu</email></address>
2136      </author>
2137      <date month="March" year="1997" />
2138    </front>
2139    <seriesInfo name="BCP" value="14" />
2140    <seriesInfo name="RFC" value="2119" />
2141  </reference>
2142
2143  <reference anchor="RFC5234">
2144    <front>
2145      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2146      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2147        <organization>Brandenburg InternetWorking</organization>
2148        <address>
2149          <email>dcrocker@bbiw.net</email>
2150        </address> 
2151      </author>
2152      <author initials="P." surname="Overell" fullname="Paul Overell">
2153        <organization>THUS plc.</organization>
2154        <address>
2155          <email>paul.overell@thus.net</email>
2156        </address>
2157      </author>
2158      <date month="January" year="2008"/>
2159    </front>
2160    <seriesInfo name="STD" value="68"/>
2161    <seriesInfo name="RFC" value="5234"/>
2162  </reference>
2163 
2164</references>
2165
2166<references title="Informative References">
2167
2168  <reference anchor="RFC1305">
2169    <front>
2170      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2171      <author fullname="David L. Mills" initials="D." surname="Mills">
2172        <organization>University of Delaware, Electrical Engineering Department</organization>
2173        <address><email>mills@udel.edu</email></address>
2174      </author>
2175      <date month="March" year="1992" />
2176    </front>
2177    <seriesInfo name="RFC" value="1305" />
2178  </reference>
2179
2180  <reference anchor="RFC2616">
2181    <front>
2182      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2183      <author fullname="R. Fielding" initials="R." surname="Fielding">
2184        <organization>University of California, Irvine</organization>
2185        <address><email>fielding@ics.uci.edu</email></address>
2186      </author>
2187      <author fullname="J. Gettys" initials="J." surname="Gettys">
2188        <organization>W3C</organization>
2189        <address><email>jg@w3.org</email></address>
2190      </author>
2191      <author fullname="J. Mogul" initials="J." surname="Mogul">
2192        <organization>Compaq Computer Corporation</organization>
2193        <address><email>mogul@wrl.dec.com</email></address>
2194      </author>
2195      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2196        <organization>MIT Laboratory for Computer Science</organization>
2197        <address><email>frystyk@w3.org</email></address>
2198      </author>
2199      <author fullname="L. Masinter" initials="L." surname="Masinter">
2200        <organization>Xerox Corporation</organization>
2201        <address><email>masinter@parc.xerox.com</email></address>
2202      </author>
2203      <author fullname="P. Leach" initials="P." surname="Leach">
2204        <organization>Microsoft Corporation</organization>
2205        <address><email>paulle@microsoft.com</email></address>
2206      </author>
2207      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2208        <organization>W3C</organization>
2209        <address><email>timbl@w3.org</email></address>
2210      </author>
2211      <date month="June" year="1999" />
2212    </front>
2213    <seriesInfo name="RFC" value="2616" />
2214  </reference>
2215
2216  <reference anchor="RFC3864">
2217    <front>
2218      <title>Registration Procedures for Message Header Fields</title>
2219      <author fullname="G. Klyne" initials="G." surname="Klyne">
2220        <organization>Nine by Nine</organization>
2221        <address><email>GK-IETF@ninebynine.org</email></address>
2222      </author>
2223      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2224        <organization>BEA Systems</organization>
2225        <address><email>mnot@pobox.com</email></address>
2226      </author>
2227      <author fullname="J. Mogul" initials="J." surname="Mogul">
2228        <organization>HP Labs</organization>
2229        <address><email>JeffMogul@acm.org</email></address>
2230      </author>
2231      <date month="September" year="2004" />
2232    </front>
2233    <seriesInfo name="BCP" value="90" />
2234    <seriesInfo name="RFC" value="3864" />
2235  </reference>
2236
2237  <reference anchor='RFC5226'>
2238    <front>
2239      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2240      <author initials='T.' surname='Narten' fullname='T. Narten'>
2241        <organization>IBM</organization>
2242        <address><email>narten@us.ibm.com</email></address>
2243      </author>
2244      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2245        <organization>Google</organization>
2246        <address><email>Harald@Alvestrand.no</email></address>
2247      </author>
2248      <date year='2008' month='May' />
2249    </front>
2250    <seriesInfo name='BCP' value='26' />
2251    <seriesInfo name='RFC' value='5226' />
2252  </reference>
2253
2254  <reference anchor='RFC5861'>
2255    <front>
2256      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2257      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2258        <organization>Yahoo! Inc.</organization>
2259        <address><email>mnot@yahoo-inc.com</email></address>
2260      </author>
2261      <date month="April" year="2010"/>
2262    </front>
2263    <seriesInfo name='RFC' value='5861' />
2264  </reference>
2265
2266</references>
2267
2268<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2269<t>
2270  Make the specified age calculation algorithm less conservative.
2271  (<xref target="age.calculations"/>)
2272</t>
2273<t>
2274  Remove requirement to consider Content-Location in successful responses
2275  in order to determine the appropriate response to use.
2276  (<xref target="validation.model" />)
2277</t>
2278<t>
2279  Clarify denial of service attack avoidance requirement.
2280  (<xref target="invalidation.after.updates.or.deletions" />)
2281</t>
2282<t>
2283  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2284  anymore, as these aspects never were implemented.
2285  (<xref target="header.warning" />)
2286</t>
2287</section>
2288
2289<?BEGININC p6-cache.abnf-appendix ?>
2290<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2291<figure>
2292<artwork type="abnf" name="p6-cache.parsed-abnf">
2293<x:ref>Age</x:ref> = "Age:" OWS Age-v
2294<x:ref>Age-v</x:ref> = delta-seconds
2295
2296<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2297<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2298 cache-directive ] )
2299
2300<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2301<x:ref>Expires-v</x:ref> = HTTP-date
2302
2303<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2304
2305<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2306
2307<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2308<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2309 pragma-directive ] )
2310
2311<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2312<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2313 ] ) )
2314
2315<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2316<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2317 ] )
2318
2319<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2320<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2321<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2322 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2323 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2324 cache-extension
2325<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2326 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2327 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2328 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2329 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2330 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2331
2332<x:ref>delta-seconds</x:ref> = 1*DIGIT
2333
2334<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2335
2336<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2337
2338<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2339<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2340<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2341
2342<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2343
2344<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2345
2346<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2347
2348<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2349<x:ref>warn-code</x:ref> = 3DIGIT
2350<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2351<x:ref>warn-text</x:ref> = quoted-string
2352<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2353 ]
2354</artwork>
2355</figure>
2356<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2357; Age defined but not used
2358; Cache-Control defined but not used
2359; Expires defined but not used
2360; Pragma defined but not used
2361; Vary defined but not used
2362; Warning defined but not used
2363</artwork></figure></section>
2364<?ENDINC p6-cache.abnf-appendix ?>
2365
2366<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2367
2368<section title="Since RFC 2616">
2369  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2370</section>
2371
2372<section title="Since draft-ietf-httpbis-p6-cache-00">
2373<t>
2374  Closed issues:
2375  <list style="symbols">
2376    <t>
2377      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2378    <t>
2379      <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>
2380    <t>
2381      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2382    <t>
2383      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2384    <t>
2385      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2386    <t>
2387      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2388    <t>
2389      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2390    <t>
2391      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2392    <t>
2393      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2394  </list>
2395</t>
2396<t>
2397  Other changes:
2398  <list style="symbols">
2399    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2400        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2401  </list>
2402</t>
2403</section>
2404
2405<section title="Since draft-ietf-httpbis-p6-cache-01">
2406<t>
2407  Closed issues:
2408  <list style="symbols">
2409    <t>
2410      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2411  </list>
2412</t>
2413<t>
2414  Other changes:
2415  <list style="symbols">
2416    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2417      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2418    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2419      specification.</t>
2420  </list>
2421</t>
2422</section>
2423
2424<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2425<t>
2426  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2427  <list style="symbols">
2428    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2429      document.</t>
2430  </list>
2431</t>
2432</section>
2433
2434<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2435<t>
2436  Closed issues:
2437  <list style="symbols">
2438    <t>
2439      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2440  </list>
2441</t>
2442</section>
2443
2444<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2445<t>
2446  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2447  <list style="symbols"> 
2448    <t>
2449      Use "/" instead of "|" for alternatives.
2450    </t>
2451    <t>
2452      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2453      whitespace ("OWS") and required whitespace ("RWS").
2454    </t>
2455    <t>
2456      Rewrite ABNFs to spell out whitespace rules, factor out
2457      header field value format definitions.
2458    </t>
2459  </list>
2460</t>
2461</section>
2462
2463<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2464<t>
2465  This is a total rewrite of this part of the specification.
2466</t>
2467<t>
2468  Affected issues:
2469  <list style="symbols">
2470    <t>
2471      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2472    <t>
2473      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2474    <t>
2475      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2476    <t>
2477      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2478  </list>
2479</t>
2480<t>
2481  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2482  <list style="symbols"> 
2483    <t>
2484      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2485    </t>
2486  </list>
2487</t>
2488</section>
2489
2490<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2491<t>
2492  Closed issues:
2493  <list style="symbols"> 
2494    <t>
2495      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2496      "base for numeric protocol elements"
2497    </t>
2498  </list>
2499</t>
2500<t>
2501  Affected issues:
2502  <list style="symbols">
2503    <t>
2504      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2505      WVary and non-existant headers"
2506    </t>
2507  </list>
2508</t>
2509</section>
2510
2511<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2512<t>
2513  Closed issues:
2514  <list style="symbols"> 
2515    <t>
2516      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2517      "Definition of 1xx Warn-Codes"
2518    </t>
2519    <t>
2520      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2521      "Content-Location on 304 responses"
2522    </t>
2523    <t>
2524      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2525      "private and no-cache CC directives with headers"
2526    </t>
2527    <t>
2528      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2529      "RFC2047 and warn-text"
2530    </t>
2531  </list>
2532</t>
2533</section>
2534
2535<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2536<t>
2537  Closed issues:
2538  <list style="symbols"> 
2539    <t>
2540      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2541      "serving negotiated responses from cache: header-specific canonicalization"
2542    </t>
2543    <t>
2544      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2545      "Effect of CC directives on history lists"
2546    </t>
2547  </list>
2548</t>
2549<t>
2550  Affected issues:
2551  <list style="symbols">
2552    <t>
2553      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2554      Status codes and caching
2555    </t>
2556  </list>
2557</t>
2558<t>
2559  Partly resolved issues:
2560  <list style="symbols"> 
2561    <t>
2562      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2563      "Placement of 13.5.1 and 13.5.2"
2564    </t>
2565  </list>
2566</t>
2567</section>
2568
2569<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2570<t>
2571  Closed issues:
2572  <list style="symbols"> 
2573    <t>
2574      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2575      "Age calculation"
2576    </t>
2577    <t>
2578      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2579      "Clarify differences between / requirements for request and response CC directives"
2580    </t>
2581        <t>
2582                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2583                "Caching authenticated responses"
2584        </t>
2585    <t>
2586      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2587      "IANA registry for cache-control directives"
2588    </t>
2589    <t>
2590      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2591      "Heuristic caching of URLs with query components"
2592    </t>
2593  </list>
2594</t>
2595<t>
2596  Partly resolved issues:
2597  <list style="symbols"> 
2598    <t>
2599      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2600      "Term for the requested resource's URI"
2601    </t>
2602  </list>
2603</t>
2604</section>
2605
2606<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2607<t>
2608  Closed issues:
2609  <list style="symbols"> 
2610    <t>
2611      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2612      "Clarify entity / representation / variant terminology"
2613    </t>
2614    <t>
2615      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2616      "consider removing the 'changes from 2068' sections"
2617    </t>
2618    <t>
2619      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2620      "Allowing heuristic caching for new status codes"
2621    </t>
2622    <t>
2623      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2624      "Allowing heuristic caching for new status codes"
2625    </t>
2626    <t>
2627      Clean up TODOs and prose in "Combining Responses."
2628    </t>
2629  </list>
2630</t>
2631</section>
2632
2633<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2634<t>
2635  Closed issues:
2636  <list style="symbols"> 
2637    <t>
2638      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2639      "Text about clock requirement for caches belongs in p6"
2640    </t>
2641  </list>
2642</t>
2643</section>
2644
2645<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2646<t>
2647  None yet.
2648</t>
2649</section>
2650
2651</section>
2652  </back>
2653</rfc>
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