source: draft-ietf-httpbis/latest/p6-cache.xml @ 965

Last change on this file since 965 was 965, checked in by fielding@…, 9 years ago

Addresses #109: Clarify entity / representation / variant terminology

Removed entity-header adjective and ABNF and clarified distinction
between payload and representation.

Uncapitalize the phrase effective request URI so that it doesn't
dominate the prose, and define the term "target resource" to be
used instead of the "resource identified by the effective request URI".

  • Property svn:eol-style set to native
File size: 106.0 KB
Line 
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 "July">
16  <!ENTITY ID-YEAR "2010">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY effective-request-uri      "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36]>
37<?rfc toc="yes" ?>
38<?rfc symrefs="yes" ?>
39<?rfc sortrefs="yes" ?>
40<?rfc compact="yes"?>
41<?rfc subcompact="no" ?>
42<?rfc linkmailto="no" ?>
43<?rfc editing="no" ?>
44<?rfc comments="yes"?>
45<?rfc inline="yes"?>
46<?rfc rfcedstyle="yes"?>
47<?rfc-ext allow-markup-in-artwork="yes" ?>
48<?rfc-ext include-references-in-index="yes" ?>
49<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
50  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
51<front>
52
53  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
54
55  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
56    <organization abbrev="Day Software">Day Software</organization>
57    <address>
58      <postal>
59        <street>23 Corporate Plaza DR, Suite 280</street>
60        <city>Newport Beach</city>
61        <region>CA</region>
62        <code>92660</code>
63        <country>USA</country>
64      </postal>
65      <phone>+1-949-706-5300</phone>
66      <facsimile>+1-949-706-5305</facsimile>
67      <email>fielding@gbiv.com</email>
68      <uri>http://roy.gbiv.com/</uri>
69    </address>
70  </author>
71
72  <author initials="J." surname="Gettys" fullname="Jim Gettys">
73    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
74    <address>
75      <postal>
76        <street>21 Oak Knoll Road</street>
77        <city>Carlisle</city>
78        <region>MA</region>
79        <code>01741</code>
80        <country>USA</country>
81      </postal>
82      <email>jg@freedesktop.org</email>
83      <uri>http://gettys.wordpress.com/</uri>
84    </address>
85  </author>
86
87  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
88    <organization abbrev="HP">Hewlett-Packard Company</organization>
89    <address>
90      <postal>
91        <street>HP Labs, Large Scale Systems Group</street>
92        <street>1501 Page Mill Road, MS 1177</street>
93        <city>Palo Alto</city>
94        <region>CA</region>
95        <code>94304</code>
96        <country>USA</country>
97      </postal>
98      <email>JeffMogul@acm.org</email>
99    </address>
100  </author>
101
102  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
103    <organization abbrev="Microsoft">Microsoft Corporation</organization>
104    <address>
105      <postal>
106        <street>1 Microsoft Way</street>
107        <city>Redmond</city>
108        <region>WA</region>
109        <code>98052</code>
110        <country>USA</country>
111      </postal>
112      <email>henrikn@microsoft.com</email>
113    </address>
114  </author>
115
116  <author fullname="Larry Masinter" initials="L." surname="Masinter">
117    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
118    <address>
119      <postal>
120        <street>345 Park Ave</street>
121        <city>San Jose</city>
122        <region>CA</region>
123        <code>95110</code>
124        <country>USA</country>
125      </postal>
126      <email>LMM@acm.org</email>
127      <uri>http://larry.masinter.net/</uri>
128    </address>
129  </author>
130
131  <author fullname="Paul J. Leach" initials="P." surname="Leach">
132    <organization abbrev="Microsoft">Microsoft Corporation</organization>
133    <address>
134      <postal>
135        <street>1 Microsoft Way</street>
136        <city>Redmond</city>
137        <region>WA</region>
138        <code>98052</code>
139      </postal>
140      <email>paulle@microsoft.com</email>
141    </address>
142  </author>
143
144  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
145    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
146    <address>
147      <postal>
148        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
149        <street>The Stata Center, Building 32</street>
150        <street>32 Vassar Street</street>
151        <city>Cambridge</city>
152        <region>MA</region>
153        <code>02139</code>
154        <country>USA</country>
155      </postal>
156      <email>timbl@w3.org</email>
157      <uri>http://www.w3.org/People/Berners-Lee/</uri>
158    </address>
159  </author>
160
161  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
162    <organization abbrev="W3C">World Wide Web Consortium</organization>
163    <address>
164      <postal>
165        <street>W3C / ERCIM</street>
166        <street>2004, rte des Lucioles</street>
167        <city>Sophia-Antipolis</city>
168        <region>AM</region>
169        <code>06902</code>
170        <country>France</country>
171      </postal>
172      <email>ylafon@w3.org</email>
173      <uri>http://www.raubacapeu.net/people/yves/</uri>
174    </address>
175  </author>
176
177  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
178    <address>
179      <email>mnot@mnot.net</email>
180      <uri>http://www.mnot.net/</uri>
181    </address>
182  </author>
183
184  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
185    <organization abbrev="greenbytes">greenbytes GmbH</organization>
186    <address>
187      <postal>
188        <street>Hafenweg 16</street>
189        <city>Muenster</city><region>NW</region><code>48155</code>
190        <country>Germany</country>
191      </postal>
192      <phone>+49 251 2807760</phone>
193      <facsimile>+49 251 2807761</facsimile>
194      <email>julian.reschke@greenbytes.de</email>
195      <uri>http://greenbytes.de/tech/webdav/</uri>
196    </address>
197  </author>
198
199  <date month="&ID-MONTH;" year="&ID-YEAR;" />
200  <workgroup>HTTPbis Working Group</workgroup>
201
202<abstract>
203<t>
204   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
205   distributed, collaborative, hypermedia information systems. This document
206   is Part 6 of the seven-part specification that defines the protocol
207   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6
208   defines requirements on HTTP caches and the associated header fields that
209   control cache behavior or indicate cacheable response messages.
210</t>
211</abstract>
212
213<note title="Editorial Note (To be removed by RFC Editor)">
214   <t>
215      Discussion of this draft should take place on the HTTPBIS working group
216      mailing list (ietf-http-wg@w3.org). The current issues list is at <eref
217      target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
218      documents (including fancy diffs) can be found at <eref
219      target="http://tools.ietf.org/wg/httpbis/"/>.
220   </t>
221   <t>
222      The changes in this draft are summarized in <xref
223      target="changes.since.10"/>.
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      should no longer be returned by a cache without further validation.</t>
283   </list>
284</t>
285<t>
286   <iref item="heuristic expiration time" />
287   <x:dfn>heuristic expiration time</x:dfn>
288   <list>
289      <t>An expiration time assigned by a cache when no explicit expiration
290      time is available.</t>
291   </list>
292</t>
293<t>
294   <iref item="age" />
295   <x:dfn>age</x:dfn>
296   <list>
297      <t>The age of a response is the time since it was sent by, or
298      successfully validated with, the origin server.</t>
299   </list>
300</t>
301<t>
302   <iref item="first-hand" />
303   <x:dfn>first-hand</x:dfn>
304   <list>
305      <t>A response is first-hand if the freshness model is not in use; i.e.,
306      its age is 0.</t>
307   </list>
308</t>
309<t>
310   <iref item="freshness lifetime" />
311   <x:dfn>freshness lifetime</x:dfn>
312   <list>
313      <t>The length of time between the generation of a response and its
314      expiration time.</t>
315   </list>
316</t>
317<t>
318   <iref item="fresh" />
319   <x:dfn>fresh</x:dfn>
320   <list>
321      <t>A response is fresh if its age has not yet exceeded its freshness
322      lifetime.</t>
323   </list>
324</t>
325<t>
326   <iref item="stale" />
327   <x:dfn>stale</x:dfn>
328   <list>
329      <t>A response is stale if its age has passed its freshness lifetime
330      (either explicit or heuristic).</t>
331   </list>
332</t>
333<t>
334   <iref item="validator" />
335   <x:dfn>validator</x:dfn>
336   <list>
337      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
338      is used to find out whether a stored response has an equivalent copy of
339      a representation.</t>
340   </list>
341</t>
342<t anchor="shared.and.non-shared.caches">
343   <iref item="validator" />
344   <x:dfn>shared cache</x:dfn>
345   <list>
346      <t>A cache that is accessible to more than one user. A non-shared cache
347      is dedicated to a single user.</t>
348   </list>
349</t>
350</section>
351
352<section anchor="intro.requirements" title="Requirements">
353<t>
354   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
355   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
356   document are to be interpreted as described in <xref target="RFC2119"/>.
357</t>
358<t>
359   An implementation is not compliant if it fails to satisfy one or more of
360   the "MUST" or "REQUIRED" level requirements for the protocols it
361   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
362   level and all the "SHOULD" level requirements for its protocols is said to
363   be "unconditionally compliant"; one that satisfies all the "MUST" level
364   requirements but not all the "SHOULD" level requirements for its protocols
365   is said to be "conditionally compliant".
366</t>
367</section>
368
369<section title="Syntax Notation" anchor="notation">
370   <x:anchor-alias value="ALPHA"/>
371   <x:anchor-alias value="CR"/>
372   <x:anchor-alias value="DIGIT"/>
373   <x:anchor-alias value="DQUOTE"/>
374   <x:anchor-alias value="LF"/>
375   <x:anchor-alias value="OCTET"/>
376   <x:anchor-alias value="SP"/>
377   <x:anchor-alias value="VCHAR"/>
378   <x:anchor-alias value="WSP"/>
379<t>
380   This specification uses the ABNF syntax defined in &notation; (which
381   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
382   <xref target="collected.abnf"/> shows the collected ABNF, with the list
383   rule expanded.
384</t>
385<t>
386   The following core rules are included by reference, as defined in <xref
387   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
388   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
389   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
390   sequence of data), SP (space), VCHAR (any visible USASCII character), and
391   WSP (whitespace).
392</t>
393
394<section title="Core Rules" anchor="core.rules">
395   <x:anchor-alias value="quoted-string"/>
396   <x:anchor-alias value="token"/>
397   <x:anchor-alias value="OWS"/>
398<t>
399   The core rules below are defined in &basic-rules;:
400</t>
401<figure><artwork type="abnf2616">
402  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
403  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
404  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
405</artwork></figure>
406</section>
407
408<section title="ABNF Rules defined in other Parts of the Specification"
409    anchor="abnf.dependencies">
410   <x:anchor-alias value="field-name"/>
411   <x:anchor-alias value="HTTP-date"/>
412   <x:anchor-alias value="port"/>
413   <x:anchor-alias value="pseudonym"/>
414   <x:anchor-alias value="uri-host"/>
415<t>
416   The ABNF rules below are defined in other parts:
417</t>
418<figure><!--Part1--><artwork type="abnf2616">
419  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
420  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
421  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
422  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
423  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
424</artwork></figure>
425</section>
426
427</section>
428</section>
429
430<section anchor="caching.overview" title="Cache Operation">
431
432<section anchor="response.cacheability" title="Response Cacheability">
433<t>
434   A cache &MUST-NOT; store a response to any request, unless:
435   <list style="symbols">
436      <t>The request method is understood by the cache and defined as being
437      cacheable, and</t>
438      <t>the response status code is understood by the cache, and</t>
439      <t>the "no-store" cache directive (see <xref
440      target="header.cache-control" />) does not appear in request or response
441      headers, and</t>
442      <t>the "private" cache response directive (see <xref
443      target="cache-response-directive" /> does not appear in the response, if
444      the cache is shared, and</t>
445      <t>the "Authorization" header (see &header-authorization;) does not
446      appear in the request, if the cache is shared, unless the response
447      explicitly allows it (see <xref target="caching.authenticated.responses"
448      />), and</t>
449      <t>the response either:
450         <list style="symbols">
451            <t>contains an Expires header (see <xref target="header.expires"
452            />), or</t>
453            <t>contains a max-age response cache directive (see <xref
454            target="cache-response-directive" />), or</t>
455            <t>contains a s-maxage response cache directive and the cache is
456            shared, or</t>
457            <t>contains a Cache Control Extension (see <xref
458            target="cache.control.extensions" />) that allows it to be cached,
459            or</t>
460            <t>has a status code that can be served with heuristic freshness
461            (see <xref target="heuristic.freshness" />).</t>
462         </list>
463      </t>
464   </list>
465</t>
466<t>
467   In this context, a cache has "understood" a request method or a response
468   status code if it recognises it and implements any cache-specific
469   behaviour. In particular, 206 Partial Content responses cannot be cached by
470   an implementation that does not handle partial content (see <xref
471   target="errors.or.incomplete.response.cache.behavior" />).
472</t>
473<t>
474   Note that in normal operation, most caches will not store a response that
475   has neither a cache validator nor an explicit expiration time, as such
476   responses are not usually useful to store. However, caches are not
477   prohibited from storing such responses.
478</t>
479
480<section anchor="errors.or.incomplete.response.cache.behavior" 
481   title="Storing Partial and Incomplete Responses">
482<t>
483   A cache that receives an incomplete response (for example, with fewer bytes
484   of data than specified in a Content-Length header) can store the response,
485   but &MUST; treat it as a partial response &partial;. Partial responses can
486   be combined as described in &combining-byte-ranges;; the result might be a
487   full response or might still be partial. A cache &MUST-NOT; return a
488   partial response to a client without explicitly marking it as such using
489   the 206 (Partial Content) status code.
490</t>
491<t>
492   A cache that does not support the Range and Content-Range headers
493   &MUST-NOT; store incomplete or partial responses.
494</t>
495</section>
496
497</section>
498
499
500<section anchor="constructing.responses.from.caches" 
501   title="Constructing Responses from Caches">
502<t>
503   For a presented request, a cache &MUST-NOT; return a stored response,
504   unless:
505   <list style="symbols">
506      <t>The presented effective request URI (&effective-request-uri;) and
507      that of the stored response match, and</t>
508      <t>the request method associated with the stored response allows it to
509      be used for the presented request, and</t>
510      <t>selecting request-headers nominated by the stored response (if any)
511      match those presented (see <xref target="caching.negotiated.responses"
512      />), and</t>
513      <t>the presented request and stored response are free from directives
514      that would prevent its use (see <xref target="header.cache-control" />
515      and <xref target="header.pragma"/>), and</t>
516      <t>the stored response is either:
517         <list style="symbols">
518            <t>fresh (see <xref target="expiration.model" />), or</t>
519            <t>allowed to be served stale (see <xref
520            target="serving.stale.responses" />), or</t>
521            <t>successfully validated (see <xref target="validation.model"
522            />).</t>
523         </list>
524      </t>
525  </list>
526</t>
527<t>
528   When a stored response is used to satisfy a request without validation,
529   caches &MUST; include a single Age header field (<xref target="header.age"
530   />) in the response with a value equal to the stored response's
531   current_age; see <xref target="age.calculations" />.
532</t>
533<t>
534   Requests with methods that are unsafe (&safe-methods;) &MUST; be written
535   through the cache to the origin server; i.e., a cache must not reply to
536   such a request before having forwarded the request and having received a
537   corresponding response.
538</t>
539<t>
540   Also, note that unsafe requests might invalidate already stored responses;
541   see <xref target="invalidation.after.updates.or.deletions" />.
542</t>
543<t>
544   Caches &MUST; use the most recent response (as determined by the Date
545   header) when more than one suitable response is stored. They can also
546   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
547   no-cache" to disambiguate which response to use.
548</t>
549</section>
550
551<section anchor="expiration.model" title="Freshness Model">
552<t>
553   When a response is "fresh" in the cache, it can be used to satisfy
554   subsequent requests without contacting the origin server, thereby improving
555   efficiency.
556</t>
557<t>
558   The primary mechanism for determining freshness is for an origin server to
559   provide an explicit expiration time in the future, using either the Expires
560   header (<xref target="header.expires" />) or the max-age response cache
561   directive (<xref target="cache-response-directive" />). Generally, origin
562   servers will assign future explicit expiration times to responses in the
563   belief that the representation is not likely to change in a semantically
564   significant way before the expiration time is reached.
565</t>
566<t>
567   If an origin server wishes to force a cache to validate every request, it
568   can assign an explicit expiration time in the past. This means that the
569   response is always stale, so that caches should validate it before using it
570   for subsequent requests. <cref anchor="TODO-response-stale">This wording
571   might cause confusion, because the response might still be served
572   stale.</cref>
573</t>
574<t>
575   Since origin servers do not always provide explicit expiration times, HTTP
576   caches &MAY; assign heuristic expiration times when explicit times are not
577   specified, employing algorithms that use other header values (such as the
578   Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1
579   specification does not provide specific algorithms, but does impose
580   worst-case constraints on their results.
581</t>
582<figure>
583<preamble>
584  The calculation to determine if a response is fresh is:
585</preamble>
586<artwork type="code">
587   response_is_fresh = (freshness_lifetime &gt; current_age)
588</artwork>
589</figure>
590<t>
591   The freshness_lifetime is defined in <xref
592   target="calculating.freshness.lifetime" />; the current_age is defined in
593   <xref target="age.calculations" />.
594</t>
595<t>
596   Additionally, clients might need to influence freshness calculation. They
597   can do this using several request cache directives, with the effect of
598   either increasing or loosening constraints on freshness. See <xref
599   target="cache-request-directive" />.
600</t>
601<t>
602   <cref anchor="ISSUE-no-req-for-directives">there are not requirements
603   directly applying to cache-request-directives and freshness.</cref>
604</t>
605<t>
606   Note that freshness applies only to cache operation; it cannot be used to
607   force a user agent to refresh its display or reload a resource. See <xref
608   target="history.lists" /> for an explanation of the difference between
609   caches and history mechanisms.
610</t>
611
612<section anchor="calculating.freshness.lifetime" 
613   title="Calculating Freshness Lifetime">
614<t>
615   A cache can calculate the freshness lifetime (denoted as
616   freshness_lifetime) of a response by using the first match of:
617   <list style="symbols">
618      <t>If the cache is shared and the s-maxage response cache directive
619      (<xref target="cache-response-directive" />) is present, use its value,
620      or</t>
621      <t>If the max-age response cache directive (<xref
622      target="cache-response-directive" />) is present, use its value, or</t>
623      <t>If the Expires response header (<xref target="header.expires" />) is
624      present, use its value minus the value of the Date response header,
625      or</t>
626      <t>Otherwise, no explicit expiration time is present in the response. A
627      heuristic freshness lifetime might be applicable; see <xref
628      target="heuristic.freshness" />.</t>
629   </list>
630</t>
631<t>
632   Note that this calculation is not vulnerable to clock skew, since all of
633   the information comes from the origin server.
634</t>
635
636<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
637<t>
638   If no explicit expiration time is present in a stored response that has a
639   status code whose definition allows heuristic freshness to be used
640   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
641   410), a heuristic expiration time &MAY; be calculated. Heuristics
642   &MUST-NOT; be used for response status codes that do not explicitly allow
643   it.
644</t>
645<t>
646   When a heuristic is used to calculate freshness lifetime, the cache
647   &SHOULD; attach a Warning header with a 113 warn-code to the response if
648   its current_age is more than 24 hours and such a warning is not already
649   present.
650</t>
651<t>
652   Also, if the response has a Last-Modified header (&header-last-modified;),
653   the heuristic expiration value &SHOULD; be no more than some fraction of
654   the interval since that time. A typical setting of this fraction might be
655   10%.
656</t>
657<x:note>
658   <t>
659      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
660      x:sec="13.9"/>) required that caches do not calculate heuristic
661      freshness for URLs with query components (i.e., those containing '?').
662      In practice, this has not been widely implemented. Therefore, servers
663      are encouraged to send explicit directives (e.g., Cache-Control:
664      no-cache) if they wish to preclude caching.
665   </t>
666</x:note>
667</section>
668</section>
669
670<section anchor="age.calculations" title="Calculating Age">
671<t>
672   HTTP/1.1 uses the Age response-header to convey the estimated age of the
673   response message when obtained from a cache. The Age field value is the
674   cache's estimate of the amount of time since the response was generated or
675   validated by the origin server. In essence, the Age value is the sum of the
676   time that the response has been resident in each of the caches along the
677   path from the origin server, plus the amount of time it has been in transit
678   along network paths.
679</t>
680<t>
681   The following data is used for the age calculation:
682</t>
683<t>
684   <x:dfn>age_value</x:dfn>
685   <list>
686      <t>
687         The term "age_value" denotes the value of the Age header (<xref
688         target="header.age"/>), in a form appropriate for arithmetic
689         operation; or 0, if not available.
690      </t>
691   </list>
692</t>
693<t>
694   <x:dfn>date_value</x:dfn>
695   <list>
696      <t>
697         HTTP/1.1 requires origin servers to send a Date header, if possible,
698         with every response, giving the time at which the response was
699         generated. The term "date_value" denotes the value of the Date
700         header, in a form appropriate for arithmetic operations. See
701         &header-date; for the definition of the Date header, and for
702         requirements regarding responses without a Date response header.
703      </t>
704   </list>
705</t>
706<t>
707   <x:dfn>now</x:dfn>
708   <list>
709      <t>
710         The term "now" means "the current value of the clock at the host
711         performing the calculation". Hosts that use HTTP, but especially
712         hosts running origin servers and caches, &SHOULD; use NTP (<xref
713         target="RFC1305"/>) or some similar protocol to synchronize their
714         clocks to a globally accurate time standard.
715      </t>
716   </list>
717</t>
718<t>
719   <x:dfn>request_time</x:dfn>
720   <list>
721      <t>
722         The current value of the clock at the host at the time the request
723         resulting in the stored response was made.
724      </t>
725   </list>
726</t>
727<t>
728   <x:dfn>response_time</x:dfn>
729   <list>
730      <t>
731         The current value of the clock at the host at the time the response
732         was received.
733      </t>
734   </list>
735</t>
736<t>
737   A response's age can be calculated in two entirely independent ways:
738   <list style="numbers">
739      <t>the "apparent_age": response_time minus date_value, if the local
740      clock is reasonably well synchronized to the origin server's clock. If
741      the result is negative, the result is replaced by zero.</t>
742      <t>the "corrected_age_value", if all of the caches along the response
743      path implement HTTP/1.1; note this value &MUST; be interpreted relative
744      to the time the request was initiated, not the time that the response
745      was received.</t>
746   </list>
747</t>
748<figure>
749<artwork type="code">
750  apparent_age = max(0, response_time - date_value);
751
752  response_delay = response_time - request_time;
753  corrected_age_value = age_value + response_delay; 
754</artwork>
755</figure>
756<figure>
757<preamble>These are combined as</preamble>
758<artwork type="code">
759  corrected_initial_age = max(apparent_age, corrected_age_value);
760</artwork></figure>
761<t>
762   The current_age of a stored response can then be calculated by adding the
763   amount of time (in seconds) since the stored response was last validated by
764   the origin server to the corrected_initial_age.
765</t>
766<figure><artwork type="code">
767  resident_time = now - response_time;
768  current_age = corrected_initial_age + resident_time;
769</artwork></figure>
770</section>
771
772<section anchor="serving.stale.responses" title="Serving Stale Responses">
773<t>
774   A "stale" response is one that either has explicit expiry information or is
775   allowed to have heuristic expiry calculated, but is not fresh according to
776   the calculations in <xref target="expiration.model" />.
777</t>
778<t>
779   Caches &MUST-NOT; return a stale response if it is prohibited by an
780   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
781   directive, a "must-revalidate" cache-response-directive, or an applicable
782   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
783   target="cache-response-directive"/>).
784</t>
785<t>
786   Caches &SHOULD-NOT; return stale responses unless they are disconnected
787   (i.e., it cannot contact the origin server or otherwise find a forward
788   path) or otherwise explicitly allowed (e.g., the max-stale request
789   directive; see <xref target="cache-request-directive" />).
790</t>
791<t>
792   Stale responses &SHOULD; have a Warning header with the 110 warn-code (see
793   <xref target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be
794   sent on stale responses if the cache is disconnected.
795</t>
796<t>
797   If a cache receives a first-hand response (either an entire response, or a
798   304 (Not Modified) response) that it would normally forward to the
799   requesting client, and the received response is no longer fresh, the cache
800   &SHOULD; forward it to the requesting client without adding a new Warning
801   (but without removing any existing Warning headers). A cache &SHOULD-NOT;
802   attempt to validate a response simply because that response became stale in
803   transit.
804</t>
805</section>
806</section>
807
808<section anchor="validation.model" title="Validation Model">
809<t>
810   When a cache has one or more stored responses for a requested URI, but
811   cannot serve any of them (e.g., because they are not fresh, or one cannot
812   be selected; see <xref target="caching.negotiated.responses"/>), it can use
813   the conditional request mechanism &conditional; in the forwarded request to
814   give the origin server an opportunity to both select a valid stored
815   response to be used, and to update it. This process is known as
816   "validating" or "revalidating" the stored response.
817</t>
818<t>
819   When sending such a conditional request, the cache &SHOULD; add an
820   If-Modified-Since header whose value is that of the Last-Modified header
821   from the selected (see <xref target="caching.negotiated.responses"/>)
822   stored response, if available.
823</t>
824<t>
825   Additionally, the cache &SHOULD; add an If-None-Match header whose value is
826   that of the ETag header(s) from all responses stored for the requested URI,
827   if present. However, if any of the stored responses contains only partial
828   content, its entity-tag &SHOULD-NOT; be included in the If-None-Match
829   header field unless the request is for a range that would be fully
830   satisfied by that stored response.
831</t>
832<t>
833   A 304 (Not Modified) response status code indicates that the stored
834   response can be updated and reused; see <xref target="combining.headers"/>.
835</t>
836<t>
837   A full response (i.e., one with a response body) indicates that none of the
838   stored responses nominated in the conditional request is suitable. Instead,
839   the full response &SHOULD; be used to satisfy the request and &MAY; replace
840   the stored response.
841</t>
842<t>
843   If a cache receives a 5xx response while attempting to validate a response,
844   it &MAY; either forward this response to the requesting client, or act as
845   if the server failed to respond. In the latter case, it &MAY; return a
846   previously stored response (see <xref target="serving.stale.responses" />).
847</t>
848</section>
849
850<section anchor="invalidation.after.updates.or.deletions" 
851   title="Request Methods that Invalidate">
852<t>
853   Because unsafe methods (&safe-methods;) have the potential for changing
854   state on the origin server, intervening caches can use them to keep their
855   contents up-to-date.
856</t>
857<t>
858   The following HTTP methods &MUST; cause a cache to invalidate the Effective
859   Request URI (&effective-request-uri;) as well as the URI(s) in the Location
860   and Content-Location headers (if present):
861   <list style="symbols">
862      <t>PUT</t>
863      <t>DELETE</t>
864      <t>POST</t>
865   </list>
866</t>
867<t>
868   An invalidation based on a URI from a Location or Content-Location header
869   &MUST-NOT; be performed if the host part of that URI differs from the host
870   part in the effective request URI (&effective-request-uri;). This helps
871   prevent denial of service attacks.
872</t>
873<t>
874   A cache that passes through requests for methods it does not understand
875   &SHOULD; invalidate the effective request URI (&effective-request-uri;).
876</t>
877<t>
878   Here, "invalidate" means that the cache will either remove all stored
879   responses related to the effective request URI, or will mark these as
880   "invalid" and in need of a mandatory validation before they can be returned
881   in response to a subsequent request.
882</t>
883<t>
884   Note that this does not guarantee that all appropriate responses are
885   invalidated. For example, the request that caused the change at the origin
886   server might not have gone through the cache where a response is stored.
887</t>
888</section>
889
890<section anchor="caching.authenticated.responses" 
891   title="Shared Caching of Authenticated Responses">
892
893<t>
894   Shared caches &MUST-NOT; use a cached response to a request with an
895   Authorization header (&header-authorization;) to satisfy any subsequent
896   request unless a cache directive that allows such responses to be stored is
897   present in the response.
898</t>
899
900<t>
901   In this specification, the following Cache-Control response directives
902   (<xref target="cache-response-directive"/>) have such an effect:
903   must-revalidate, public, s-maxage.
904</t>
905
906<t>
907   Note that cached responses that contain the "must-revalidate" and/or
908   "s-maxage" response directives are not allowed to be served stale (<xref
909   target="serving.stale.responses"/>) by shared caches. In particular, a
910   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
911   used to satisfy a subsequent request without revalidating it on the origin
912   server.
913</t>
914</section>
915
916<section anchor="caching.negotiated.responses" 
917   title="Caching Negotiated Responses">
918<t>
919   When a cache receives a request that can be satisfied by a stored response
920   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
921   use that response unless all of the selecting request-headers nominated by
922   the Vary header match in both the original request (i.e., that associated
923   with the stored response), and the presented request.
924</t>
925<t>
926   The selecting request-headers from two requests are defined to match if and
927   only if those in the first request can be transformed to those in the
928   second request by applying any of the following:
929   <list style="symbols">
930      <t>
931         adding or removing whitespace, where allowed in the header's syntax
932      </t>
933      <t>
934         combining multiple message-header fields with the same field name
935         (see &header-fields;)
936      </t>
937      <t>
938         normalizing both header values in a way that is known to have
939         identical semantics, according to the header's specification (e.g.,
940         re-ordering field values when order is not significant;
941         case-normalization, where values are defined to be case-insensitive)
942      </t>
943  </list>
944</t>
945<t>
946   If (after any normalization that might take place) a header field is absent
947   from a request, it can only match another request if it is also absent
948   there.
949</t>
950<t>
951   A Vary header field-value of "*" always fails to match, and subsequent
952   requests to that resource can only be properly interpreted by the origin
953   server.
954</t>
955<t>
956   The stored response with matching selecting request-headers is known as the
957   selected response.
958</t>
959<t>
960   If no selected response is available, the cache &MAY; forward the presented
961   request to the origin server in a conditional request; see <xref
962   target="validation.model"/>.
963</t>
964</section>
965
966<section anchor="combining.headers" title="Combining Responses">
967<t>
968   When a cache receives a 304 (Not Modified) response or a 206 (Partial
969   Content) response (in this section, the "new" response"), it needs to
970   created an updated response by combining the stored response with the new
971   one, so that the updated response can be used to satisfy the request, and
972   potentially update the cached response.
973</t>
974<t>
975   If the new response contains an ETag, it identifies the stored response to
976   use. <cref anchor="TODO-mention-CL">might need language about
977   Content-Location here</cref><cref
978   anchor="TODO-select-for-combine">Shouldn't this be the selected
979   response?</cref>
980</t>
981<t>
982   If the new response's status code is 206 (partial content), both the stored
983   and new responses &MUST; have validators, and those validators &MUST; match
984   using the strong comparison function (see &weak-and-strong;). Otherwise,
985   the responses &MUST-NOT; be combined.
986</t>
987<t>
988   The stored response headers are used as those of the updated response,
989   except that
990   <list style="symbols">
991      <t>any stored Warning headers with warn-code 1xx (see <xref
992      target="header.warning" />) &MUST; be deleted.</t>
993      <t>any stored Warning headers with warn-code 2xx &MUST; be retained.</t>
994      <t>any other headers provided in the new response &MUST; replace all
995      instances of the corresponding headers from the stored response.</t>
996   </list>
997</t>
998<t>
999   The updated response headers &MUST; be used to replace those of the stored
1000   response in cache (unless the stored response is removed from cache). In
1001   the case of a 206 response, the combined representation &MAY; be stored.
1002</t>
1003</section>
1004
1005</section>
1006
1007<section anchor="header.fields" title="Header Field Definitions">
1008<t>
1009   This section defines the syntax and semantics of HTTP/1.1 header fields
1010   related to caching.
1011</t>
1012
1013<section anchor="header.age" title="Age">
1014   <iref item="Age header" primary="true" x:for-anchor="" />
1015   <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
1016   <x:anchor-alias value="Age"/>
1017   <x:anchor-alias value="Age-v"/>
1018   <x:anchor-alias value="age-value"/>
1019<t>
1020   The "Age" response-header field conveys the sender's estimate of the amount
1021   of time since the response was generated or successfully validated at the
1022   origin server. Age values are calculated as specified in <xref
1023   target="age.calculations" />.
1024</t>
1025<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
1026  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
1027  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
1028</artwork></figure>
1029<t anchor="rule.delta-seconds">
1030  <x:anchor-alias value="delta-seconds" />
1031  Age field-values are non-negative integers, representing time in seconds.
1032</t>
1033<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
1034  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
1035</artwork></figure>
1036<t>
1037   If a cache receives a value larger than the largest positive integer it can
1038   represent, or if any of its age calculations overflows, it &MUST; transmit
1039   an Age header with a field-value of 2147483648 (2<x:sup>31</x:sup>). Caches
1040   &SHOULD; use an arithmetic type of at least 31 bits of range.
1041</t>
1042<t>
1043   The presence of an Age header field in a response implies that a response
1044   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1045   might not implement the Age header field.
1046</t>
1047</section>
1048
1049<section anchor="header.cache-control" title="Cache-Control">
1050   <iref item="Cache-Control header" primary="true" x:for-anchor="" />
1051   <iref item="Headers" primary="true" subitem="Cache-Control" 
1052      x:for-anchor="" />
1053   <x:anchor-alias value="Cache-Control"/>
1054   <x:anchor-alias value="Cache-Control-v"/>
1055   <x:anchor-alias value="cache-directive"/>
1056   <x:anchor-alias value="cache-extension"/>
1057   <x:anchor-alias value="cache-request-directive"/>
1058   <x:anchor-alias value="cache-response-directive"/>
1059<t>
1060   The "Cache-Control" general-header field is used to specify directives for
1061   caches along the request/response chain. Such cache directives are
1062   unidirectional in that the presence of a directive in a request does not
1063   imply that the same directive is to be given in the response.
1064</t>
1065<t>
1066   HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control
1067   directives defined in this section. See <xref
1068   target="cache.control.extensions"/> for information about how Cache-Control
1069   directives defined elsewhere are handled.
1070</t>
1071<x:note>
1072   <t>
1073       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1074       might only implement Pragma: no-cache (see <xref target="header.pragma"
1075       />).
1076   </t>
1077</x:note>
1078<t>
1079   Cache directives &MUST; be passed through by a proxy or gateway
1080   application, regardless of their significance to that application, since
1081   the directives might be applicable to all recipients along the
1082   request/response chain. It is not possible to target a directive to a
1083   specific cache.
1084</t>
1085<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"/>
1086  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1087  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1088
1089  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1090     / <x:ref>cache-response-directive</x:ref>
1091
1092  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1093</artwork></figure>
1094
1095<section anchor="cache-request-directive" 
1096   title="Request Cache-Control Directives">
1097   <x:anchor-alias value="cache-request-directive" />
1098
1099<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1100   subitem="cache-request-directive" />
1101  <x:ref>cache-request-directive</x:ref> =
1102       "no-cache"
1103     / "no-store"
1104     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1105     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1106     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1107     / "no-transform"
1108     / "only-if-cached"
1109     / <x:ref>cache-extension</x:ref>
1110</artwork></figure>
1111
1112<t>
1113   <x:dfn>no-cache</x:dfn>
1114   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1115   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1116   <list>
1117      <t>The no-cache request directive indicates that a stored response
1118      &MUST-NOT; be used to satisfy the request without successful validation
1119      on the origin server.</t> 
1120   </list>
1121</t>
1122<t>
1123   <x:dfn>no-store</x:dfn>
1124   <iref item="Cache Directives" primary="true" subitem="no-store" />
1125   <iref item="no-store" primary="true" subitem="Cache Directive" />
1126   <list>
1127      <t>The no-store request directive indicates that a cache &MUST-NOT;
1128      store any part of either this request or any response to it. This
1129      directive applies to both non-shared and shared caches. "&MUST-NOT;
1130      store" in this context means that the cache &MUST-NOT; intentionally
1131      store the information in non-volatile storage, and &MUST; make a
1132      best-effort attempt to remove the information from volatile storage as
1133      promptly as possible after forwarding it.</t>
1134      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1135      privacy. In particular, malicious or compromised caches might not
1136      recognize or obey this directive, and communications networks might be
1137      vulnerable to eavesdropping.</t>
1138   </list>
1139</t>
1140<t>
1141   <x:dfn>max-age</x:dfn>
1142   <iref item="Cache Directives" primary="true" subitem="max-age" />
1143   <iref item="max-age" primary="true" subitem="Cache Directive" />
1144   <list>
1145      <t>The max-age request directive indicates that the client is willing to
1146      accept a response whose age is no greater than the specified time in
1147      seconds. Unless the max-stale request directive is also present, the
1148      client is not willing to accept a stale response.</t>
1149   </list>
1150</t>
1151<t>
1152   <x:dfn>max-stale</x:dfn>
1153   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1154   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1155   <list>
1156      <t>The max-stale request directive indicates that the client is willing
1157      to accept a response that has exceeded its expiration time. If max-stale
1158      is assigned a value, then the client is willing to accept a response
1159      that has exceeded its expiration time by no more than the specified
1160      number of seconds. If no value is assigned to max-stale, then the client
1161      is willing to accept a stale response of any age.</t>
1162   </list>
1163</t>
1164<t>
1165   <x:dfn>min-fresh</x:dfn>
1166   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1167   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1168   <list>
1169      <t>The min-fresh request directive indicates that the client is willing
1170      to accept a response whose freshness lifetime is no less than its
1171      current age plus the specified time in seconds. That is, the client
1172      wants a response that will still be fresh for at least the specified
1173      number of seconds.</t>
1174   </list>
1175</t>
1176<t>
1177   <x:dfn>no-transform</x:dfn>
1178   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1179   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1180   <list>
1181      <t>The no-transform request directive indicates that an intermediate
1182      cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
1183      Content-Type request headers, nor the request representation.</t>
1184   </list>
1185</t>
1186<t>
1187   <x:dfn>only-if-cached</x:dfn>
1188   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1189   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1190   <list>
1191      <t>The only-if-cached request directive indicates that the client only
1192      wishes to return a stored response. If it receives this directive, a
1193      cache &SHOULD; either respond using a stored response that is consistent
1194      with the other constraints of the request, or respond with a 504
1195      (Gateway Timeout) status code. If a group of caches is being operated as
1196      a unified system with good internal connectivity, such a request &MAY;
1197      be forwarded within that group of caches.</t>
1198   </list>
1199</t>
1200</section>
1201
1202<section anchor="cache-response-directive" 
1203   title="Response Cache-Control Directives">
1204   <x:anchor-alias value="cache-response-directive" />
1205
1206<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1207   subitem="cache-response-directive" />
1208  <x:ref>cache-response-directive</x:ref> =
1209       "public"
1210     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1211     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1212     / "no-store"
1213     / "no-transform"
1214     / "must-revalidate"
1215     / "proxy-revalidate"
1216     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1217     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1218     / <x:ref>cache-extension</x:ref>
1219</artwork></figure>
1220
1221<t>
1222   <x:dfn>public</x:dfn>
1223   <iref item="Cache Directives" primary="true" subitem="public" />
1224   <iref item="public" primary="true" subitem="Cache Directive" />
1225   <list>
1226      <t>The public response directive indicates that the response &MAY; be
1227      cached, even if it would normally be non-cacheable or cacheable only
1228      within a non-shared cache. (See also Authorization,
1229      &header-authorization;, for additional details.) </t>
1230  </list>
1231</t>
1232<t>
1233   <x:dfn>private</x:dfn>
1234   <iref item="Cache Directives" primary="true" subitem="private" />
1235   <iref item="private" primary="true" subitem="Cache Directive" />
1236   <list>
1237      <t>The private response directive indicates that the response message is
1238      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1239      private (non-shared) cache &MAY; store the response.</t>
1240      <t>If the private response directive specifies one or more field-names,
1241      this requirement is limited to the field-values associated with the
1242      listed response headers. That is, the specified field-names(s)
1243      &MUST-NOT; be stored by a shared cache, whereas the remainder of the
1244      response message &MAY; be.</t>
1245      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1246      the response can be stored; it cannot ensure the privacy of the message
1247      content. Also, private response directives with field-names are often
1248      handled by implementations as if an unqualified private directive was
1249      received; i.e., the special handling for the qualified form is not
1250      widely implemented.</t>
1251   </list>
1252</t>
1253<t>
1254   <x:dfn>no-cache</x:dfn>
1255   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1256   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1257   <list>
1258      <t>The no-cache response directive indicates that the response MUST NOT
1259      be used to satisfy a subsequent request without successful validation on
1260      the origin server. This allows an origin server to prevent a cache from
1261      using it to satisfy a request without contacting it, even by caches that
1262      have been configured to return stale responses.</t>
1263      <t>If the no-cache response directive specifies one or more field-names,
1264      this requirement is limited to the field-values associated with the
1265      listed response headers. That is, the specified field-name(s) &MUST-NOT;
1266      be sent in the response to a subsequent request without successful
1267      validation on the origin server. This allows an origin server to prevent
1268      the re-use of certain header fields in a response, while still allowing
1269      caching of the rest of the response.</t>
1270      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1271      this directive. Also, no-cache response directives with field-names are
1272      often handled by implementations as if an unqualified no-cache directive
1273      was received; i.e., the special handling for the qualified form is not
1274      widely implemented. </t>
1275   </list>
1276</t>
1277<t>
1278   <x:dfn>no-store</x:dfn>
1279   <iref item="Cache Directives" primary="true" subitem="no-store" />
1280   <iref item="no-store" primary="true" subitem="Cache Directive" />
1281   <list>
1282      <t>The no-store response directive indicates that a cache &MUST-NOT;
1283      store any part of either the immediate request or response. This
1284      directive applies to both non-shared and shared caches. "&MUST-NOT;
1285      store" in this context means that the cache &MUST-NOT; intentionally
1286      store the information in non-volatile storage, and &MUST; make a
1287      best-effort attempt to remove the information from volatile storage as
1288      promptly as possible after forwarding it.</t>
1289      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1290      privacy. In particular, malicious or compromised caches might not
1291      recognize or obey this directive, and communications networks might be
1292      vulnerable to eavesdropping.</t>
1293   </list>
1294</t>
1295<t>
1296   <x:dfn>must-revalidate</x:dfn>
1297   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1298   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1299   <list>
1300      <t>The must-revalidate response directive indicates that once it has
1301      become stale, the response &MUST-NOT; be used to satisfy subsequent
1302      requests without successful validation on the origin server.</t>
1303      <t>The must-revalidate directive is necessary to support reliable
1304      operation for certain protocol features. In all circumstances an
1305      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in particular,
1306      if the cache cannot reach the origin server for any reason, it &MUST;
1307      generate a 504 (Gateway Timeout) response.</t>
1308      <t>Servers &SHOULD; send the must-revalidate directive if and only if
1309      failure to validate a request on the representation could result in
1310      incorrect operation, such as a silently unexecuted financial
1311      transaction.</t>
1312   </list>
1313</t>
1314<t>
1315   <x:dfn>proxy-revalidate</x:dfn>
1316   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1317   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1318   <list>
1319      <t>The proxy-revalidate response directive has the same meaning as the
1320      must-revalidate response directive, except that it does not apply to
1321      non-shared caches.</t>
1322   </list>
1323</t>
1324<t>
1325   <x:dfn>max-age</x:dfn>
1326   <iref item="Cache Directives" primary="true" subitem="max-age" />
1327   <iref item="max-age" primary="true" subitem="Cache Directive" />
1328   <list>
1329      <t>The max-age response directive indicates that response is to be
1330      considered stale after its age is greater than the specified number of
1331      seconds.</t>
1332   </list>
1333</t>
1334<t>
1335   <x:dfn>s-maxage</x:dfn>
1336   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1337   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1338   <list>
1339      <t>The s-maxage response directive indicates that, in shared caches, the
1340      maximum age specified by this directive overrides the maximum age
1341      specified by either the max-age directive or the Expires header. The
1342      s-maxage directive also implies the semantics of the proxy-revalidate
1343      response directive.</t>
1344   </list>
1345</t>
1346<t>
1347   <x:dfn>no-transform</x:dfn>
1348   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1349   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1350   <list>
1351      <t>The no-transform response directive indicates that an intermediate
1352      cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
1353      Content-Type response headers, nor the response representation.</t>
1354   </list>
1355</t>
1356
1357</section>
1358
1359<section anchor="cache.control.extensions" title="Cache Control Extensions">
1360<t>
1361   The Cache-Control header field can be extended through the use of one or
1362   more cache-extension tokens, each with an optional value. Informational
1363   extensions (those that do not require a change in cache behavior) can be
1364   added without changing the semantics of other directives. Behavioral
1365   extensions are designed to work by acting as modifiers to the existing base
1366   of cache directives. Both the new directive and the standard directive are
1367   supplied, such that applications that do not understand the new directive
1368   will default to the behavior specified by the standard directive, and those
1369   that understand the new directive will recognize it as modifying the
1370   requirements associated with the standard directive. In this way,
1371   extensions to the cache-control directives can be made without requiring
1372   changes to the base protocol.
1373</t>
1374<t>
1375   This extension mechanism depends on an HTTP cache obeying all of the
1376   cache-control directives defined for its native HTTP-version, obeying
1377   certain extensions, and ignoring all directives that it does not
1378   understand.
1379</t>
1380<t>
1381   For example, consider a hypothetical new response directive called
1382   "community" that acts as a modifier to the private directive. We define
1383   this new directive to mean that, in addition to any non-shared cache, any
1384   cache that is shared only by members of the community named within its
1385   value may cache the response. An origin server wishing to allow the UCI
1386   community to use an otherwise private response in their shared cache(s)
1387   could do so by including
1388</t>
1389<figure><artwork type="example">
1390  Cache-Control: private, community="UCI"
1391</artwork></figure>
1392<t>
1393   A cache seeing this header field will act correctly even if the cache does
1394   not understand the community cache-extension, since it will also see and
1395   understand the private directive and thus default to the safe behavior.
1396</t>
1397<t>
1398   Unrecognized cache directives &MUST; be ignored; it is assumed that any
1399   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1400   combined with standard directives (or the response's default cacheability)
1401   such that the cache behavior will remain minimally correct even if the
1402   cache does not understand the extension(s).
1403</t>
1404<t>
1405   The HTTP Cache Directive Registry defines the name space for the cache
1406   directives.
1407</t>
1408<t>
1409   Registrations &MUST; include the following fields:
1410   <list style="symbols">
1411      <t>Cache Directive Name</t>
1412      <t>Pointer to specification text</t>
1413   </list>
1414</t>
1415<t>
1416   Values to be added to this name space are subject to IETF review (<xref
1417   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1418</t>
1419<t>
1420   The registry itself is maintained at <eref
1421   target="http://www.iana.org/assignments/http-cache-directives"/>.
1422</t>
1423</section>
1424
1425</section>
1426
1427<section anchor="header.expires" title="Expires">
1428   <iref item="Expires header" primary="true" x:for-anchor="" />
1429   <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1430   <x:anchor-alias value="Expires"/>
1431   <x:anchor-alias value="Expires-v"/>
1432<t>
1433   The "Expires" header field gives the date/time after which the
1434   response is considered stale. See <xref target="expiration.model" /> for
1435   further discussion of the freshness model.
1436</t>
1437<t>
1438   The presence of an Expires field does not imply that the original resource
1439   will change or cease to exist at, before, or after that time.
1440</t>
1441<t>
1442   The field-value is an absolute date and time as defined by HTTP-date in
1443   &full-date;; it &MUST; be sent in rfc1123-date format.
1444</t>
1445<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1446  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1447  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1448</artwork></figure>
1449<figure>
1450  <preamble>For example</preamble>
1451<artwork type="example">
1452  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1453</artwork></figure>
1454<x:note>
1455   <t>
1456       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1457       max-age directive (see <xref target="cache-response-directive" />),
1458       that directive overrides the Expires field. Likewise, the s-maxage
1459       directive overrides Expires in shared caches.
1460   </t>
1461</x:note>
1462<t>
1463   HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the
1464   future.
1465</t>
1466<t>
1467   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
1468   especially including the value "0", as in the past (i.e., "already
1469   expired").
1470</t>
1471</section>
1472
1473<section anchor="header.pragma" title="Pragma">
1474   <iref item="Pragma header" primary="true" x:for-anchor="" />
1475   <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1476   <x:anchor-alias value="extension-pragma"/>
1477   <x:anchor-alias value="Pragma"/>
1478   <x:anchor-alias value="Pragma-v"/>
1479   <x:anchor-alias value="pragma-directive"/>
1480<t>
1481   The "Pragma" general-header field is used to include
1482   implementation-specific directives that might apply to any recipient along
1483   the request/response chain. All pragma directives specify optional behavior
1484   from the viewpoint of the protocol; however, some systems &MAY; require
1485   that behavior be consistent with the directives.
1486</t>
1487<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"/>
1488  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1489  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1490  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1491  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1492</artwork></figure>
1493<t>
1494   When the no-cache directive is present in a request message, an application
1495   &SHOULD; forward the request toward the origin server even if it has a
1496   cached copy of what is being requested. This pragma directive has the same
1497   semantics as the no-cache response directive (see <xref
1498   target="cache-response-directive" />) and is defined here for backward
1499   compatibility with HTTP/1.0. Clients &SHOULD; include both header fields
1500   when a no-cache request is sent to a server not known to be HTTP/1.1
1501   compliant. HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the
1502   client had sent "Cache-Control: no-cache".
1503</t>
1504<x:note>
1505   <t>
1506      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a
1507      response-header field is not actually specified, it does not provide a
1508      reliable replacement for "Cache-Control: no-cache" in a response.
1509   </t>
1510</x:note>
1511<t>
1512   This mechanism is deprecated; no new Pragma directives will be defined in
1513   HTTP.
1514</t>
1515</section>
1516
1517<section anchor="header.vary" title="Vary">
1518   <iref item="Vary header" primary="true" x:for-anchor="" />
1519   <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1520   <x:anchor-alias value="Vary"/>
1521   <x:anchor-alias value="Vary-v"/>
1522<t>
1523   The "Vary" response-header field conveys the set of request-header fields
1524   that were used to select the representation.
1525</t>
1526<t>
1527   Caches use this information, in part, to determine whether a stored
1528   response can be used to satisfy a given request; see <xref
1529   target="caching.negotiated.responses" />. determines, while the response is
1530   fresh, whether a cache is permitted to use the response to reply to a
1531   subsequent request without validation; see <xref
1532   target="caching.negotiated.responses" />.
1533</t>
1534<t>
1535   In uncacheable or stale responses, the Vary field value advises the user
1536   agent about the criteria that were used to select the representation.
1537</t>
1538<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1539  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1540  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1541</artwork></figure>
1542<t>
1543   The set of header fields named by the Vary field value is known as the
1544   selecting request-headers.
1545</t>
1546<t>
1547   Servers &SHOULD; include a Vary header field with any cacheable response
1548   that is subject to server-driven negotiation. Doing so allows a cache to
1549   properly interpret future requests on that resource and informs the user
1550   agent about the presence of negotiation on that resource. A server &MAY;
1551   include a Vary header field with a non-cacheable response that is subject
1552   to server-driven negotiation, since this might provide the user agent with
1553   useful information about the dimensions over which the response varies at
1554   the time of the response.
1555</t>
1556<t>
1557   A Vary field value of "*" signals that unspecified parameters not limited
1558   to the request-headers (e.g., the network address of the client), play a
1559   role in the selection of the response representation; therefore, a cache
1560   cannot determine whether this response is appropriate. The "*" value
1561   &MUST-NOT; be generated by a proxy server.
1562</t>
1563<t>
1564   The field-names given are not limited to the set of standard request-header
1565   fields defined by this specification. Field names are case-insensitive.
1566</t>
1567</section>
1568
1569<section anchor="header.warning" title="Warning">
1570   <iref item="Warning header" primary="true" x:for-anchor="" />
1571   <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1572   <x:anchor-alias value="Warning"/>
1573   <x:anchor-alias value="Warning-v"/>
1574   <x:anchor-alias value="warning-value"/>
1575   <x:anchor-alias value="warn-agent"/>
1576   <x:anchor-alias value="warn-code"/>
1577   <x:anchor-alias value="warn-date"/>
1578   <x:anchor-alias value="warn-text"/>
1579<t>
1580   The "Warning" general-header field is used to carry additional information
1581   about the status or transformation of a message that might not be reflected
1582   in the message. This information is typically used to warn about possible
1583   incorrectness introduced by caching operations or transformations applied
1584   to the payload of the message.
1585</t>
1586<t>
1587   Warnings can be used for other purposes, both cache-related and otherwise.
1588   The use of a warning, rather than an error status code, distinguishes these
1589   responses from true failures.
1590</t>
1591<t>
1592   Warning headers can in general be applied to any message, however some
1593   warn-codes are specific to caches and can only be applied to response
1594   messages.
1595</t>
1596<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"/>
1597  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1598  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1599 
1600  <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>
1601                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1602 
1603  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1604  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1605                  ; the name or pseudonym of the server adding
1606                  ; the Warning header, for use in debugging
1607  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1608  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1609</artwork></figure>
1610<t>
1611   Multiple warnings can be attached to a response (either by the origin
1612   server or by a cache), including multiple warnings with the same code
1613   number, only differing in warn-text.
1614</t>
1615<t>
1616   When this occurs, the user agent &SHOULD; inform the user of as many of
1617   them as possible, in the order that they appear in the response.
1618</t>
1619<t>
1620   Systems that generate multiple Warning headers &SHOULD; order them with
1621   this user agent behavior in mind. New Warning headers &SHOULD; be added
1622   after any existing Warning headers.
1623</t>
1624<t>
1625   Warnings are assigned three digit warn-codes. The first digit indicates
1626   whether the Warning is required to be deleted from a stored response after
1627   validation:
1628   <list style="symbols">
1629      <t>1xx Warnings describe the freshness or validation status of the
1630      response, and so &MUST; be deleted by caches after validation. They can
1631      only be generated by a cache when validating a cached entry, and
1632      &MUST-NOT; be generated in any other situation.</t>
1633      <t>2xx Warnings describe some aspect of the representation that is not
1634      rectified by a validation (for example, a lossy compression of the
1635      representation) and &MUST-NOT; be deleted by caches after validation,
1636      unless a full response is returned, in which case they &MUST; be.</t>
1637   </list>
1638</t>
1639<t>
1640   If an implementation sends a message with one or more Warning headers to a
1641   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1642   in each warning-value a warn-date that matches the Date header in the
1643   message.
1644</t>
1645<t>
1646   If an implementation receives a message with a warning-value that includes
1647   a warn-date, and that warn-date is different from the Date value in the
1648   response, then that warning-value &MUST; be deleted from the message before
1649   storing, forwarding, or using it. (preventing the consequences of naive
1650   caching of Warning header fields.) If all of the warning-values are deleted
1651   for this reason, the Warning header &MUST; be deleted as well.
1652</t>
1653<t>
1654   The following warn-codes are defined by this specification, each with a
1655   recommended warn-text in English, and a description of its meaning.
1656</t>
1657<t><?rfc needLines="4"?>
1658   110 Response is stale
1659   <list>
1660      <t>&SHOULD; be included whenever the returned response is stale.</t>
1661   </list>
1662</t>
1663<t><?rfc needLines="4"?>
1664   111 Revalidation failed
1665   <list>
1666      <t>&SHOULD; be included if a cache returns a stale response because an
1667      attempt to validate the response failed, due to an inability to reach
1668      the server.</t>
1669   </list>
1670</t>
1671<t><?rfc needLines="4"?>
1672   112 Disconnected operation
1673   <list>
1674      <t>&SHOULD; be included if the cache is intentionally disconnected from
1675      the rest of the network for a period of time.</t>
1676   </list>
1677</t>
1678<t><?rfc needLines="4"?>
1679   113 Heuristic expiration
1680   <list>
1681      <t>&SHOULD; be included if the cache heuristically chose a freshness
1682      lifetime greater than 24 hours and the response's age is greater than 24
1683      hours.</t>
1684   </list>
1685</t>
1686<t><?rfc needLines="4"?>
1687   199 Miscellaneous warning
1688   <list>
1689      <t>The warning text can include arbitrary information to be presented to
1690      a human user, or logged. A system receiving this warning &MUST-NOT; take
1691      any automated action, besides presenting the warning to the user.</t>
1692   </list>
1693</t>
1694<t><?rfc needLines="4"?>
1695   214 Transformation applied
1696   <list>
1697      <t>&MUST; be added by an intermediate proxy if it applies any
1698      transformation to the representation, such as changing the
1699      content-coding, media-type, or modifying the representation data, unless
1700      this Warning code already appears in the response.</t>
1701   </list>
1702</t>
1703<t><?rfc needLines="4"?>
1704   299 Miscellaneous persistent warning
1705   <list>
1706      <t>The warning text can include arbitrary information to be presented to
1707      a human user, or logged. A system receiving this warning &MUST-NOT; take
1708      any automated action.</t>
1709   </list>
1710</t>
1711</section>
1712
1713</section>
1714
1715<section anchor="history.lists" title="History Lists">
1716<t>
1717   User agents often have history mechanisms, such as "Back" buttons and
1718   history lists, that can be used to redisplay a representation retrieved
1719   earlier in a session.
1720</t>
1721<t>
1722   The freshness model (<xref target="expiration.model"/>) does not
1723   necessarily apply to history mechanisms. I.e., a history mechanism can
1724   display a previous representation even if it has expired.
1725</t>
1726<t>
1727   This does not prohibit the history mechanism from telling the user that a
1728   view might be stale, or from honoring cache directives (e.g.,
1729   Cache-Control: no-store).
1730</t>
1731</section>
1732
1733
1734<section anchor="IANA.considerations" title="IANA Considerations">
1735
1736<section title="Cache Directive Registry" 
1737   anchor="cache.directive.registration">
1738<t>
1739   The registration procedure for HTTP Cache Directives is defined by <xref
1740   target="cache.control.extensions"/> of this document.
1741</t>
1742<t>
1743   The HTTP Cache Directive Registry should be created at <eref
1744   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1745   populated with the registrations below:
1746</t>
1747<?BEGININC p6-cache.cache-directives ?>
1748<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1749<texttable xmlns:my="#my" align="left" suppress-title="true"
1750           anchor="iana.cache.directive.registration.table">
1751   <ttcol>Cache Directive</ttcol>
1752   <ttcol>Reference</ttcol>
1753
1754   <c>max-age</c>
1755   <c>
1756      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1757   </c>
1758   <c>max-stale</c>
1759   <c>
1760      <xref target="cache-request-directive"/>
1761   </c>
1762   <c>min-fresh</c>
1763   <c>
1764      <xref target="cache-request-directive"/>
1765   </c>
1766   <c>must-revalidate</c>
1767   <c>
1768      <xref target="cache-response-directive"/>
1769   </c>
1770   <c>no-cache</c>
1771   <c>
1772      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1773   </c>
1774   <c>no-store</c>
1775   <c>
1776      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1777   </c>
1778   <c>no-transform</c>
1779   <c>
1780      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1781   </c>
1782   <c>only-if-cached</c>
1783   <c>
1784      <xref target="cache-request-directive"/>
1785   </c>
1786   <c>private</c>
1787   <c>
1788      <xref target="cache-response-directive"/>
1789   </c>
1790   <c>proxy-revalidate</c>
1791   <c>
1792      <xref target="cache-response-directive"/>
1793   </c>
1794   <c>public</c>
1795   <c>
1796      <xref target="cache-response-directive"/>
1797   </c>
1798   <c>s-maxage</c>
1799   <c>
1800      <xref target="cache-response-directive"/>
1801   </c>
1802   <c>stale-if-error</c>
1803   <c>
1804      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1805   </c>
1806   <c>stale-while-revalidate</c>
1807   <c>
1808      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1809   </c>
1810</texttable>
1811<!--(END)-->
1812<?ENDINC p6-cache.cache-directives ?>
1813</section>
1814
1815<section title="Header Field Registration" anchor="header.field.registration">
1816<t>
1817  The Message Header Field Registry located at <eref 
1818  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1819  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1820</t>
1821<?BEGININC p6-cache.iana-headers ?>
1822<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1823<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1824   <ttcol>Header Field Name</ttcol>
1825   <ttcol>Protocol</ttcol>
1826   <ttcol>Status</ttcol>
1827   <ttcol>Reference</ttcol>
1828
1829   <c>Age</c>
1830   <c>http</c>
1831   <c>standard</c>
1832   <c>
1833      <xref target="header.age"/>
1834   </c>
1835   <c>Cache-Control</c>
1836   <c>http</c>
1837   <c>standard</c>
1838   <c>
1839      <xref target="header.cache-control"/>
1840   </c>
1841   <c>Expires</c>
1842   <c>http</c>
1843   <c>standard</c>
1844   <c>
1845      <xref target="header.expires"/>
1846   </c>
1847   <c>Pragma</c>
1848   <c>http</c>
1849   <c>standard</c>
1850   <c>
1851      <xref target="header.pragma"/>
1852   </c>
1853   <c>Vary</c>
1854   <c>http</c>
1855   <c>standard</c>
1856   <c>
1857      <xref target="header.vary"/>
1858   </c>
1859   <c>Warning</c>
1860   <c>http</c>
1861   <c>standard</c>
1862   <c>
1863      <xref target="header.warning"/>
1864   </c>
1865</texttable>
1866<!--(END)-->
1867<?ENDINC p6-cache.iana-headers ?>
1868<t>
1869   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1870   Force".
1871</t>
1872</section>
1873
1874</section>
1875
1876<section anchor="security.considerations" title="Security Considerations">
1877<t>
1878   Caches expose additional potential vulnerabilities, since the contents of
1879   the cache represent an attractive target for malicious exploitation.
1880   Because cache contents persist after an HTTP request is complete, an attack
1881   on the cache can reveal information long after a user believes that the
1882   information has been removed from the network. Therefore, cache contents
1883   should be protected as sensitive information.
1884</t>
1885</section>
1886
1887<section anchor="ack" title="Acknowledgments">
1888<t>
1889   Much of the content and presentation of the caching design is due to
1890   suggestions and comments from individuals including: Shel Kaphan, Paul
1891   Leach, Koen Holtman, David Morris, and Larry Masinter.
1892</t>
1893</section>
1894
1895</middle>
1896
1897<back>
1898<references title="Normative References">
1899
1900  <reference anchor="Part1">
1901    <front>
1902      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1903      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1904        <organization abbrev="Day Software">Day Software</organization>
1905        <address><email>fielding@gbiv.com</email></address>
1906      </author>
1907      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1908        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1909        <address><email>jg@freedesktop.org</email></address>
1910      </author>
1911      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1912        <organization abbrev="HP">Hewlett-Packard Company</organization>
1913        <address><email>JeffMogul@acm.org</email></address>
1914      </author>
1915      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1916        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1917        <address><email>henrikn@microsoft.com</email></address>
1918      </author>
1919      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1920        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1921        <address><email>LMM@acm.org</email></address>
1922      </author>
1923      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1924        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1925        <address><email>paulle@microsoft.com</email></address>
1926      </author>
1927      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1928        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1929        <address><email>timbl@w3.org</email></address>
1930      </author>
1931      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1932        <organization abbrev="W3C">World Wide Web Consortium</organization>
1933        <address><email>ylafon@w3.org</email></address>
1934      </author>
1935      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1936        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1937        <address><email>julian.reschke@greenbytes.de</email></address>
1938      </author>
1939      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1940    </front>
1941    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1942    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1943  </reference>
1944
1945  <reference anchor="Part2">
1946    <front>
1947      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1948      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1949        <organization abbrev="Day Software">Day Software</organization>
1950        <address><email>fielding@gbiv.com</email></address>
1951      </author>
1952      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1953        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1954        <address><email>jg@freedesktop.org</email></address>
1955      </author>
1956      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1957        <organization abbrev="HP">Hewlett-Packard Company</organization>
1958        <address><email>JeffMogul@acm.org</email></address>
1959      </author>
1960      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1961        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1962        <address><email>henrikn@microsoft.com</email></address>
1963      </author>
1964      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1965        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1966        <address><email>LMM@acm.org</email></address>
1967      </author>
1968      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1969        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1970        <address><email>paulle@microsoft.com</email></address>
1971      </author>
1972      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1973        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1974        <address><email>timbl@w3.org</email></address>
1975      </author>
1976      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1977        <organization abbrev="W3C">World Wide Web Consortium</organization>
1978        <address><email>ylafon@w3.org</email></address>
1979      </author>
1980      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1981        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1982        <address><email>julian.reschke@greenbytes.de</email></address>
1983      </author>
1984      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1985    </front>
1986    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1987    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1988  </reference>
1989
1990  <reference anchor="Part4">
1991    <front>
1992      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1993      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1994        <organization abbrev="Day Software">Day Software</organization>
1995        <address><email>fielding@gbiv.com</email></address>
1996      </author>
1997      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1998        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1999        <address><email>jg@freedesktop.org</email></address>
2000      </author>
2001      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2002        <organization abbrev="HP">Hewlett-Packard Company</organization>
2003        <address><email>JeffMogul@acm.org</email></address>
2004      </author>
2005      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2006        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2007        <address><email>henrikn@microsoft.com</email></address>
2008      </author>
2009      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2010        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2011        <address><email>LMM@acm.org</email></address>
2012      </author>
2013      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2014        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2015        <address><email>paulle@microsoft.com</email></address>
2016      </author>
2017      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2018        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2019        <address><email>timbl@w3.org</email></address>
2020      </author>
2021      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2022        <organization abbrev="W3C">World Wide Web Consortium</organization>
2023        <address><email>ylafon@w3.org</email></address>
2024      </author>
2025      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2026        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2027        <address><email>julian.reschke@greenbytes.de</email></address>
2028      </author>
2029      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2030    </front>
2031    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2032    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2033  </reference>
2034
2035  <reference anchor="Part5">
2036    <front>
2037      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2038      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2039        <organization abbrev="Day Software">Day Software</organization>
2040        <address><email>fielding@gbiv.com</email></address>
2041      </author>
2042      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2043        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2044        <address><email>jg@freedesktop.org</email></address>
2045      </author>
2046      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2047        <organization abbrev="HP">Hewlett-Packard Company</organization>
2048        <address><email>JeffMogul@acm.org</email></address>
2049      </author>
2050      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2051        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2052        <address><email>henrikn@microsoft.com</email></address>
2053      </author>
2054      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2055        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2056        <address><email>LMM@acm.org</email></address>
2057      </author>
2058      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2059        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2060        <address><email>paulle@microsoft.com</email></address>
2061      </author>
2062      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2063        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2064        <address><email>timbl@w3.org</email></address>
2065      </author>
2066      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2067        <organization abbrev="W3C">World Wide Web Consortium</organization>
2068        <address><email>ylafon@w3.org</email></address>
2069      </author>
2070      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2071        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2072        <address><email>julian.reschke@greenbytes.de</email></address>
2073      </author>
2074      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2075    </front>
2076    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2077    <x:source basename="p5-range" href="p5-range.xml" />
2078  </reference>
2079
2080  <reference anchor="Part7">
2081    <front>
2082      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2083      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2084        <organization abbrev="Day Software">Day Software</organization>
2085        <address><email>fielding@gbiv.com</email></address>
2086      </author>
2087      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2088        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2089        <address><email>jg@freedesktop.org</email></address>
2090      </author>
2091      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2092        <organization abbrev="HP">Hewlett-Packard Company</organization>
2093        <address><email>JeffMogul@acm.org</email></address>
2094      </author>
2095      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2096        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2097        <address><email>henrikn@microsoft.com</email></address>
2098      </author>
2099      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2100        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2101        <address><email>LMM@acm.org</email></address>
2102      </author>
2103      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2104        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2105        <address><email>paulle@microsoft.com</email></address>
2106      </author>
2107      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2108        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2109        <address><email>timbl@w3.org</email></address>
2110      </author>
2111      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2112        <organization abbrev="W3C">World Wide Web Consortium</organization>
2113        <address><email>ylafon@w3.org</email></address>
2114      </author>
2115      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2116        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2117        <address><email>julian.reschke@greenbytes.de</email></address>
2118      </author>
2119      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2120    </front>
2121    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2122    <x:source basename="p7-auth" href="p7-auth.xml" />
2123  </reference>
2124
2125  <reference anchor="RFC2119">
2126    <front>
2127      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2128      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2129        <organization>Harvard University</organization>
2130        <address><email>sob@harvard.edu</email></address>
2131      </author>
2132      <date month="March" year="1997" />
2133    </front>
2134    <seriesInfo name="BCP" value="14" />
2135    <seriesInfo name="RFC" value="2119" />
2136  </reference>
2137
2138  <reference anchor="RFC5234">
2139    <front>
2140      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2141      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2142        <organization>Brandenburg InternetWorking</organization>
2143        <address>
2144          <email>dcrocker@bbiw.net</email>
2145        </address> 
2146      </author>
2147      <author initials="P." surname="Overell" fullname="Paul Overell">
2148        <organization>THUS plc.</organization>
2149        <address>
2150          <email>paul.overell@thus.net</email>
2151        </address>
2152      </author>
2153      <date month="January" year="2008"/>
2154    </front>
2155    <seriesInfo name="STD" value="68"/>
2156    <seriesInfo name="RFC" value="5234"/>
2157  </reference>
2158 
2159</references>
2160
2161<references title="Informative References">
2162
2163  <reference anchor="RFC1305">
2164    <front>
2165      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2166      <author fullname="David L. Mills" initials="D." surname="Mills">
2167        <organization>University of Delaware, Electrical Engineering Department</organization>
2168        <address><email>mills@udel.edu</email></address>
2169      </author>
2170      <date month="March" year="1992" />
2171    </front>
2172    <seriesInfo name="RFC" value="1305" />
2173  </reference>
2174
2175  <reference anchor="RFC2616">
2176    <front>
2177      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2178      <author fullname="R. Fielding" initials="R." surname="Fielding">
2179        <organization>University of California, Irvine</organization>
2180        <address><email>fielding@ics.uci.edu</email></address>
2181      </author>
2182      <author fullname="J. Gettys" initials="J." surname="Gettys">
2183        <organization>W3C</organization>
2184        <address><email>jg@w3.org</email></address>
2185      </author>
2186      <author fullname="J. Mogul" initials="J." surname="Mogul">
2187        <organization>Compaq Computer Corporation</organization>
2188        <address><email>mogul@wrl.dec.com</email></address>
2189      </author>
2190      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2191        <organization>MIT Laboratory for Computer Science</organization>
2192        <address><email>frystyk@w3.org</email></address>
2193      </author>
2194      <author fullname="L. Masinter" initials="L." surname="Masinter">
2195        <organization>Xerox Corporation</organization>
2196        <address><email>masinter@parc.xerox.com</email></address>
2197      </author>
2198      <author fullname="P. Leach" initials="P." surname="Leach">
2199        <organization>Microsoft Corporation</organization>
2200        <address><email>paulle@microsoft.com</email></address>
2201      </author>
2202      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2203        <organization>W3C</organization>
2204        <address><email>timbl@w3.org</email></address>
2205      </author>
2206      <date month="June" year="1999" />
2207    </front>
2208    <seriesInfo name="RFC" value="2616" />
2209  </reference>
2210
2211  <reference anchor="RFC3864">
2212    <front>
2213      <title>Registration Procedures for Message Header Fields</title>
2214      <author fullname="G. Klyne" initials="G." surname="Klyne">
2215        <organization>Nine by Nine</organization>
2216        <address><email>GK-IETF@ninebynine.org</email></address>
2217      </author>
2218      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2219        <organization>BEA Systems</organization>
2220        <address><email>mnot@pobox.com</email></address>
2221      </author>
2222      <author fullname="J. Mogul" initials="J." surname="Mogul">
2223        <organization>HP Labs</organization>
2224        <address><email>JeffMogul@acm.org</email></address>
2225      </author>
2226      <date month="September" year="2004" />
2227    </front>
2228    <seriesInfo name="BCP" value="90" />
2229    <seriesInfo name="RFC" value="3864" />
2230  </reference>
2231
2232  <reference anchor='RFC5226'>
2233    <front>
2234      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2235      <author initials='T.' surname='Narten' fullname='T. Narten'>
2236        <organization>IBM</organization>
2237        <address><email>narten@us.ibm.com</email></address>
2238      </author>
2239      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2240        <organization>Google</organization>
2241        <address><email>Harald@Alvestrand.no</email></address>
2242      </author>
2243      <date year='2008' month='May' />
2244    </front>
2245    <seriesInfo name='BCP' value='26' />
2246    <seriesInfo name='RFC' value='5226' />
2247  </reference>
2248
2249  <reference anchor='RFC5861'>
2250    <front>
2251      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2252      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2253        <organization>Yahoo! Inc.</organization>
2254        <address><email>mnot@yahoo-inc.com</email></address>
2255      </author>
2256      <date month="April" year="2010"/>
2257    </front>
2258    <seriesInfo name='RFC' value='5861' />
2259  </reference>
2260
2261</references>
2262
2263<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2264<t>
2265  Make the specified age calculation algorithm less conservative.
2266  (<xref target="age.calculations"/>)
2267</t>
2268<t>
2269  Remove requirement to consider Content-Location in successful responses
2270  in order to determine the appropriate response to use.
2271  (<xref target="validation.model" />)
2272</t>
2273<t>
2274  Clarify denial of service attack avoidance requirement.
2275  (<xref target="invalidation.after.updates.or.deletions" />)
2276</t>
2277<t>
2278  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2279  anymore, as these aspects never were implemented.
2280  (<xref target="header.warning" />)
2281</t>
2282</section>
2283
2284<?BEGININC p6-cache.abnf-appendix ?>
2285<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2286<figure>
2287<artwork type="abnf" name="p6-cache.parsed-abnf">
2288<x:ref>Age</x:ref> = "Age:" OWS Age-v
2289<x:ref>Age-v</x:ref> = delta-seconds
2290
2291<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2292<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2293 cache-directive ] )
2294
2295<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2296<x:ref>Expires-v</x:ref> = HTTP-date
2297
2298<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2299
2300<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2301
2302<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2303<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2304 pragma-directive ] )
2305
2306<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2307<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2308 ] ) )
2309
2310<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2311<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2312 ] )
2313
2314<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2315<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2316<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2317 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2318 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2319 cache-extension
2320<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2321 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2322 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2323 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2324 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2325 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2326
2327<x:ref>delta-seconds</x:ref> = 1*DIGIT
2328
2329<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2330
2331<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2332
2333<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2334<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2335<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2336
2337<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2338
2339<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2340
2341<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2342
2343<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2344<x:ref>warn-code</x:ref> = 3DIGIT
2345<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2346<x:ref>warn-text</x:ref> = quoted-string
2347<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2348 ]
2349</artwork>
2350</figure>
2351<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2352; Age defined but not used
2353; Cache-Control defined but not used
2354; Expires defined but not used
2355; Pragma defined but not used
2356; Vary defined but not used
2357; Warning defined but not used
2358</artwork></figure></section>
2359<?ENDINC p6-cache.abnf-appendix ?>
2360
2361<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2362
2363<section title="Since RFC2616">
2364  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2365</section>
2366
2367<section title="Since draft-ietf-httpbis-p6-cache-00">
2368<t>
2369  Closed issues:
2370  <list style="symbols">
2371    <t>
2372      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2373    <t>
2374      <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>
2375    <t>
2376      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2377    <t>
2378      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2379    <t>
2380      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2381    <t>
2382      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2383    <t>
2384      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2385    <t>
2386      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2387    <t>
2388      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2389  </list>
2390</t>
2391<t>
2392  Other changes:
2393  <list style="symbols">
2394    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2395        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2396  </list>
2397</t>
2398</section>
2399
2400<section title="Since draft-ietf-httpbis-p6-cache-01">
2401<t>
2402  Closed issues:
2403  <list style="symbols">
2404    <t>
2405      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2406  </list>
2407</t>
2408<t>
2409  Other changes:
2410  <list style="symbols">
2411    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2412      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2413    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2414      specification.</t>
2415  </list>
2416</t>
2417</section>
2418
2419<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2420<t>
2421  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2422  <list style="symbols">
2423    <t>Reference RFC 3984, and update header registrations for headers defined in this
2424      document.</t>
2425  </list>
2426</t>
2427</section>
2428
2429<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2430<t>
2431  Closed issues:
2432  <list style="symbols">
2433    <t>
2434      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2435  </list>
2436</t>
2437</section>
2438
2439<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2440<t>
2441  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2442  <list style="symbols"> 
2443    <t>
2444      Use "/" instead of "|" for alternatives.
2445    </t>
2446    <t>
2447      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2448      whitespace ("OWS") and required whitespace ("RWS").
2449    </t>
2450    <t>
2451      Rewrite ABNFs to spell out whitespace rules, factor out
2452      header value format definitions.
2453    </t>
2454  </list>
2455</t>
2456</section>
2457
2458<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2459<t>
2460  This is a total rewrite of this part of the specification.
2461</t>
2462<t>
2463  Affected issues:
2464  <list style="symbols">
2465    <t>
2466      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2467    <t>
2468      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2469    <t>
2470      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2471    <t>
2472      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2473  </list>
2474</t>
2475<t>
2476  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2477  <list style="symbols"> 
2478    <t>
2479      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2480    </t>
2481  </list>
2482</t>
2483</section>
2484
2485<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2486<t>
2487  Closed issues:
2488  <list style="symbols"> 
2489    <t>
2490      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2491      "base for numeric protocol elements"
2492    </t>
2493  </list>
2494</t>
2495<t>
2496  Affected issues:
2497  <list style="symbols">
2498    <t>
2499      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2500      Vary and non-existant headers
2501    </t>
2502  </list>
2503</t>
2504</section>
2505
2506<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2507<t>
2508  Closed issues:
2509  <list style="symbols"> 
2510    <t>
2511      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2512      "Definition of 1xx Warn-Codes"
2513    </t>
2514    <t>
2515      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2516      "Content-Location on 304 responses"
2517    </t>
2518    <t>
2519      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2520      "private and no-cache CC directives with headers"
2521    </t>
2522    <t>
2523      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2524      "RFC2047 and warn-text"
2525    </t>
2526  </list>
2527</t>
2528</section>
2529
2530<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2531<t>
2532  Closed issues:
2533  <list style="symbols"> 
2534    <t>
2535      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2536      "serving negotiated responses from cache: header-specific canonicalization"
2537    </t>
2538    <t>
2539      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2540      "Effect of CC directives on history lists"
2541    </t>
2542  </list>
2543</t>
2544<t>
2545  Affected issues:
2546  <list style="symbols">
2547    <t>
2548      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2549      Status codes and caching
2550    </t>
2551  </list>
2552</t>
2553<t>
2554  Partly resolved issues:
2555  <list style="symbols"> 
2556    <t>
2557      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2558      "Placement of 13.5.1 and 13.5.2"
2559    </t>
2560  </list>
2561</t>
2562</section>
2563
2564<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2565<t>
2566  Closed issues:
2567  <list style="symbols"> 
2568    <t>
2569      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2570      "Age calculation"
2571    </t>
2572    <t>
2573      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2574      "Clarify differences between / requirements for request and response CC directives"
2575    </t>
2576        <t>
2577                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2578                "Caching authenticated responses"
2579        </t>
2580    <t>
2581      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2582      "IANA registry for cache-control directives"
2583    </t>
2584    <t>
2585      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2586      "Heuristic caching of URLs with query components"
2587    </t>
2588  </list>
2589</t>
2590<t>
2591  Partly resolved issues:
2592  <list style="symbols"> 
2593    <t>
2594      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2595      "Term for the requested resource's URI"
2596    </t>
2597  </list>
2598</t>
2599</section>
2600
2601<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2602<t>
2603  Closed issues:
2604  <list style="symbols"> 
2605    <t>
2606      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2607      "Clarify entity / representation / variant terminology"
2608    </t>
2609    <t>
2610      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2611      "consider removing the 'changes from 2068' sections"
2612    </t>
2613    <t>
2614      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2615      "Allowing heuristic caching for new status codes"
2616    </t>
2617    <t>
2618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2619      "Allowing heuristic caching for new status codes"
2620    </t>
2621    <t>
2622      Clean up TODOs and prose in "Combining Responses."
2623    </t>
2624  </list>
2625</t>
2626</section>
2627
2628</section>
2629  </back>
2630</rfc>
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