source: draft-ietf-httpbis/latest/p6-cache.xml @ 1303

Last change on this file since 1303 was 1303, checked in by julian.reschke@…, 9 years ago

update rfc2629.xslt, bump up document dates

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "June">
16  <!ENTITY ID-YEAR "2011">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY effective-request-uri      "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36]>
37<?rfc toc="yes" ?>
38<?rfc symrefs="yes" ?>
39<?rfc sortrefs="yes" ?>
40<?rfc compact="yes"?>
41<?rfc subcompact="no" ?>
42<?rfc linkmailto="no" ?>
43<?rfc editing="no" ?>
44<?rfc comments="yes"?>
45<?rfc inline="yes"?>
46<?rfc rfcedstyle="yes"?>
47<?rfc-ext allow-markup-in-artwork="yes" ?>
48<?rfc-ext include-references-in-index="yes" ?>
49<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
50  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
51<front>
52
53  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
54
55  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
56    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
57    <address>
58      <postal>
59        <street>345 Park Ave</street>
60        <city>San Jose</city>
61        <region>CA</region>
62        <code>95110</code>
63        <country>USA</country>
64      </postal>
65      <email>fielding@gbiv.com</email>
66      <uri>http://roy.gbiv.com/</uri>
67    </address>
68  </author>
69
70  <author initials="J." surname="Gettys" fullname="Jim Gettys">
71    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
72    <address>
73      <postal>
74        <street>21 Oak Knoll Road</street>
75        <city>Carlisle</city>
76        <region>MA</region>
77        <code>01741</code>
78        <country>USA</country>
79      </postal>
80      <email>jg@freedesktop.org</email>
81      <uri>http://gettys.wordpress.com/</uri>
82    </address>
83  </author>
84
85  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
86    <organization abbrev="HP">Hewlett-Packard Company</organization>
87    <address>
88      <postal>
89        <street>HP Labs, Large Scale Systems Group</street>
90        <street>1501 Page Mill Road, MS 1177</street>
91        <city>Palo Alto</city>
92        <region>CA</region>
93        <code>94304</code>
94        <country>USA</country>
95      </postal>
96      <email>JeffMogul@acm.org</email>
97    </address>
98  </author>
99
100  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108        <country>USA</country>
109      </postal>
110      <email>henrikn@microsoft.com</email>
111    </address>
112  </author>
113
114  <author fullname="Larry Masinter" initials="L." surname="Masinter">
115    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
116    <address>
117      <postal>
118        <street>345 Park Ave</street>
119        <city>San Jose</city>
120        <region>CA</region>
121        <code>95110</code>
122        <country>USA</country>
123      </postal>
124      <email>LMM@acm.org</email>
125      <uri>http://larry.masinter.net/</uri>
126    </address>
127  </author>
128
129  <author fullname="Paul J. Leach" initials="P." surname="Leach">
130    <organization abbrev="Microsoft">Microsoft Corporation</organization>
131    <address>
132      <postal>
133        <street>1 Microsoft Way</street>
134        <city>Redmond</city>
135        <region>WA</region>
136        <code>98052</code>
137      </postal>
138      <email>paulle@microsoft.com</email>
139    </address>
140  </author>
141
142  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
143    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
144    <address>
145      <postal>
146        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
147        <street>The Stata Center, Building 32</street>
148        <street>32 Vassar Street</street>
149        <city>Cambridge</city>
150        <region>MA</region>
151        <code>02139</code>
152        <country>USA</country>
153      </postal>
154      <email>timbl@w3.org</email>
155      <uri>http://www.w3.org/People/Berners-Lee/</uri>
156    </address>
157  </author>
158
159  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
160    <organization abbrev="W3C">World Wide Web Consortium</organization>
161    <address>
162      <postal>
163        <street>W3C / ERCIM</street>
164        <street>2004, rte des Lucioles</street>
165        <city>Sophia-Antipolis</city>
166        <region>AM</region>
167        <code>06902</code>
168        <country>France</country>
169      </postal>
170      <email>ylafon@w3.org</email>
171      <uri>http://www.raubacapeu.net/people/yves/</uri>
172    </address>
173  </author>
174
175  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
176    <address>
177      <email>mnot@mnot.net</email>
178      <uri>http://www.mnot.net/</uri>
179    </address>
180  </author>
181
182  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>
191      <facsimile>+49 251 2807761</facsimile>
192      <email>julian.reschke@greenbytes.de</email>
193      <uri>http://greenbytes.de/tech/webdav/</uri>
194    </address>
195  </author>
196
197  <date month="&ID-MONTH;" year="&ID-YEAR;" />
198  <workgroup>HTTPbis Working Group</workgroup>
199
200<abstract>
201<t>
202   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
203   distributed, collaborative, hypermedia information systems. This document
204   is Part 6 of the seven-part specification that defines the protocol
205   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6
206   defines requirements on HTTP caches and the associated header fields that
207   control cache behavior or indicate cacheable response messages.
208</t>
209</abstract>
210
211<note title="Editorial Note (To be removed by RFC Editor)">
212  <t>
213    Discussion of this draft should take place on the HTTPBIS working group
214    mailing list (ietf-http-wg@w3.org), which is archived at
215    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
216  </t>
217  <t>
218    The current issues list is at
219    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
220    documents (including fancy diffs) can be found at
221    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
222  </t>
223  <t>
224    The changes in this draft are summarized in <xref target="changes.since.14"/>.
225  </t>
226</note>
227
228   </front>
229   <middle>
230
231<section anchor="caching" title="Introduction">
232<t>
233   HTTP is typically used for distributed information systems, where
234   performance can be improved by the use of response caches. This document
235   defines aspects of HTTP/1.1 related to caching and reusing response
236   messages.
237</t>
238
239<section anchor="intro.purpose" title="Purpose">
240<iref item="cache" />
241<t>
242   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
243   subsystem that controls its message storage, retrieval, and deletion. A
244   cache stores cacheable responses in order to reduce the response time and
245   network bandwidth consumption on future, equivalent requests. Any client or
246   server &MAY; employ a cache, though a cache cannot be used by a server that
247   is acting as a tunnel.
248</t>
249<t>
250   Caching would be useless if it did not significantly improve performance.
251   The goal of caching in HTTP/1.1 is to reuse a prior response message to
252   satisfy a current request. In some cases, a stored response can be reused
253   without the need for a network request, reducing latency and network
254   round-trips; a "freshness" mechanism is used for this purpose (see <xref
255   target="expiration.model" />). Even when a new request is required, it is
256   often possible to reuse all or parts of the payload of a prior response to
257   satisfy the request, thereby reducing network bandwidth usage; a
258   "validation" mechanism is used for this purpose (see <xref
259   target="validation.model" />).
260</t>
261</section>
262
263<section anchor="intro.terminology" title="Terminology">
264<t>
265   This specification uses a number of terms to refer to the roles played by
266   participants in, and objects of, HTTP caching.
267</t>
268<t>
269   <iref item="cache" />
270   <x:dfn>cache</x:dfn>
271   <list>
272      <t>A conformant implementation of a HTTP cache. Note that this implies
273        an HTTP/1.1 cache; this specification does not define conformance
274        for HTTP/1.0 caches.</t>
275   </list>
276</t>
277<t anchor="shared.and.non-shared.caches">
278   <iref item="shared cache" />
279   <x:dfn>shared cache</x:dfn>
280   <list>
281      <t>A cache that is accessible to more than one user; usually (but
282        not always) deployed as part of an intermediary.</t>
283   </list>
284</t>
285<t>
286   <iref item="private cache" />
287   <x:dfn>private cache</x:dfn>
288   <list>
289      <t>A cache that is dedicated to a single user.</t>
290   </list>
291</t>
292<t>
293   <iref item="cacheable" />
294   <x:dfn>cacheable</x:dfn>
295   <list>
296      <t>A response is cacheable if a cache is allowed to store a copy of the
297      response message for use in answering subsequent requests. Even when a
298      response is cacheable, there might be additional constraints on whether
299      a cache can use the stored copy to satisfy a particular request.</t>
300   </list>
301</t>
302<t>
303   <iref item="explicit expiration time" />
304   <x:dfn>explicit expiration time</x:dfn>
305   <list>
306      <t>The time at which the origin server intends that a representation
307      no longer be returned by a cache without further validation.</t>
308   </list>
309</t>
310<t>
311   <iref item="heuristic expiration time" />
312   <x:dfn>heuristic expiration time</x:dfn>
313   <list>
314      <t>An expiration time assigned by a cache when no explicit expiration
315      time is available.</t>
316   </list>
317</t>
318<t>
319   <iref item="age" />
320   <x:dfn>age</x:dfn>
321   <list>
322      <t>The age of a response is the time since it was sent by, or
323      successfully validated with, the origin server.</t>
324   </list>
325</t>
326<t>
327   <iref item="first-hand" />
328   <x:dfn>first-hand</x:dfn>
329   <list>
330      <t>A response is first-hand if the freshness model is not in use; i.e.,
331      its age is 0.</t>
332   </list>
333</t>
334<t>
335   <iref item="freshness lifetime" />
336   <x:dfn>freshness lifetime</x:dfn>
337   <list>
338      <t>The length of time between the generation of a response and its
339      expiration time.</t>
340   </list>
341</t>
342<t>
343   <iref item="fresh" />
344   <x:dfn>fresh</x:dfn>
345   <list>
346      <t>A response is fresh if its age has not yet exceeded its freshness
347      lifetime.</t>
348   </list>
349</t>
350<t>
351   <iref item="stale" />
352   <x:dfn>stale</x:dfn>
353   <list>
354      <t>A response is stale if its age has passed its freshness lifetime
355      (either explicit or heuristic).</t>
356   </list>
357</t>
358<t>
359   <iref item="validator" />
360   <x:dfn>validator</x:dfn>
361   <list>
362      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
363      is used to find out whether a stored response is an equivalent copy of
364      a representation.</t>
365   </list>
366</t>
367</section>
368
369<section anchor="intro.requirements" title="Requirements">
370<t>
371   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
372   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
373   document are to be interpreted as described in <xref target="RFC2119"/>.
374</t>
375<t>
376   An implementation is not compliant if it fails to satisfy one or more of
377   the "MUST" or "REQUIRED" level requirements for the protocols it
378   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
379   level and all the "SHOULD" level requirements for its protocols is said to
380   be "unconditionally compliant"; one that satisfies all the "MUST" level
381   requirements but not all the "SHOULD" level requirements for its protocols
382   is said to be "conditionally compliant".
383</t>
384</section>
385
386<section title="Syntax Notation" anchor="notation">
387   <x:anchor-alias value="ALPHA"/>
388   <x:anchor-alias value="CR"/>
389   <x:anchor-alias value="DIGIT"/>
390   <x:anchor-alias value="DQUOTE"/>
391   <x:anchor-alias value="LF"/>
392   <x:anchor-alias value="OCTET"/>
393   <x:anchor-alias value="SP"/>
394   <x:anchor-alias value="VCHAR"/>
395   <x:anchor-alias value="WSP"/>
396<t>
397   This specification uses the ABNF syntax defined in &notation; (which
398   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
399   <xref target="collected.abnf"/> shows the collected ABNF, with the list
400   rule expanded.
401</t>
402<t>
403   The following core rules are included by reference, as defined in <xref
404   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
405   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
406   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
407   sequence of data), SP (space), VCHAR (any visible USASCII character), and
408   WSP (whitespace).
409</t>
410
411<section title="Core Rules" anchor="core.rules">
412   <x:anchor-alias value="quoted-string"/>
413   <x:anchor-alias value="token"/>
414   <x:anchor-alias value="OWS"/>
415<t>
416   The core rules below are defined in &basic-rules;:
417</t>
418<figure><artwork type="abnf2616">
419  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
420  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
421  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
422</artwork></figure>
423</section>
424
425<section title="ABNF Rules defined in other Parts of the Specification"
426    anchor="abnf.dependencies">
427   <x:anchor-alias value="field-name"/>
428   <x:anchor-alias value="HTTP-date"/>
429   <x:anchor-alias value="port"/>
430   <x:anchor-alias value="pseudonym"/>
431   <x:anchor-alias value="uri-host"/>
432<t>
433   The ABNF rules below are defined in other parts:
434</t>
435<figure><!--Part1--><artwork type="abnf2616">
436  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
437  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
438  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
439  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
440  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
441</artwork></figure>
442</section>
443
444</section>
445</section>
446
447<section anchor="caching.overview" title="Cache Operation">
448
449<section anchor="response.cacheability" title="Response Cacheability">
450<t>
451   A cache &MUST-NOT; store a response to any request, unless:
452   <list style="symbols">
453      <t>The request method is understood by the cache and defined as being
454      cacheable, and</t>
455      <t>the response status code is understood by the cache, and</t>
456      <t>the "no-store" cache directive (see <xref
457      target="header.cache-control" />) does not appear in request or response
458      header fields, and</t>
459      <t>the "private" cache response directive (see <xref
460      target="cache-response-directive" /> does not appear in the response, if
461      the cache is shared, and</t>
462      <t>the "Authorization" header field (see &header-authorization;) does not
463      appear in the request, if the cache is shared, unless the response
464      explicitly allows it (see <xref target="caching.authenticated.responses"
465      />), and</t>
466      <t>the response either:
467         <list style="symbols">
468            <t>contains an Expires header field (see <xref target="header.expires"
469            />), or</t>
470            <t>contains a max-age response cache directive (see <xref
471            target="cache-response-directive" />), or</t>
472            <t>contains a s-maxage response cache directive and the cache is
473            shared, or</t>
474            <t>contains a Cache Control Extension (see <xref
475            target="cache.control.extensions" />) that allows it to be cached,
476            or</t>
477            <t>has a status code that can be served with heuristic freshness
478            (see <xref target="heuristic.freshness" />).</t>
479         </list>
480      </t>
481   </list>
482</t>
483<t>
484   Note that any of the requirements listed above can be overridden by a
485   cache-control extension; see <xref target="cache.control.extensions" />.
486</t>
487<t>
488   In this context, a cache has "understood" a request method or a response
489   status code if it recognises it and implements any cache-specific
490   behavior. In particular, 206 Partial Content responses cannot be cached by
491   an implementation that does not handle partial content (see <xref
492   target="errors.or.incomplete.response.cache.behavior" />).
493</t>
494<t>
495   Note that in normal operation, most caches will not store a response that
496   has neither a cache validator nor an explicit expiration time, as such
497   responses are not usually useful to store. However, caches are not
498   prohibited from storing such responses.
499</t>
500
501<section anchor="errors.or.incomplete.response.cache.behavior" 
502   title="Storing Partial and Incomplete Responses">
503<t>
504   A cache that receives an incomplete response (for example, with fewer bytes
505   of data than specified in a Content-Length header field) can store the response,
506   but &MUST; treat it as a partial response &partial;. Partial responses can
507   be combined as described in &combining-byte-ranges;; the result might be a
508   full response or might still be partial. A cache &MUST-NOT; return a
509   partial response to a client without explicitly marking it as such using
510   the 206 (Partial Content) status code.
511</t>
512<t>
513   A cache that does not support the Range and Content-Range header fields
514   &MUST-NOT; store incomplete or partial responses.
515</t>
516</section>
517
518</section>
519
520
521<section anchor="constructing.responses.from.caches" 
522   title="Constructing Responses from Caches">
523<t>
524   For a presented request, a cache &MUST-NOT; return a stored response,
525   unless:
526   <list style="symbols">
527      <t>The presented effective request URI (&effective-request-uri;) and
528      that of the stored response match, and</t>
529      <t>the request method associated with the stored response allows it to
530      be used for the presented request, and</t>
531      <t>selecting header fields nominated by the stored response (if any)
532      match those presented (see <xref target="caching.negotiated.responses"
533      />), and</t>
534      <t>the presented request and stored response are free from directives
535      that would prevent its use (see <xref target="header.cache-control" />
536      and <xref target="header.pragma"/>), and</t>
537      <t>the stored response is either:
538         <list style="symbols">
539            <t>fresh (see <xref target="expiration.model" />), or</t>
540            <t>allowed to be served stale (see <xref
541            target="serving.stale.responses" />), or</t>
542            <t>successfully validated (see <xref target="validation.model"
543            />).</t>
544         </list>
545      </t>
546  </list>
547</t>
548<t>
549   Note that any of the requirements listed above can be overridden by a
550   cache-control extension; see <xref target="cache.control.extensions" />.
551</t>
552<t>
553   When a stored response is used to satisfy a request without validation,
554   a cache &MUST; include a single Age header field (<xref target="header.age"
555   />) in the response with a value equal to the stored response's
556   current_age; see <xref target="age.calculations" />.
557</t>
558<t>
559   A cache &MUST; write through requests with methods that are unsafe
560   (&safe-methods;) to the origin server; i.e., a cache must not generate
561   a reply to such a request before having forwarded the request and having
562   received a corresponding response.
563</t>
564<t>
565   Also, note that unsafe requests might invalidate already stored responses;
566   see <xref target="invalidation.after.updates.or.deletions" />.
567</t>
568<t>
569   When more than one suitable response is stored, a cache &MUST; use the
570   most recent response (as determined by the Date header field). It can also
571   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
572   no-cache" to disambiguate which response to use.
573</t>
574<t>
575   A cache that does not have a clock available &MUST-NOT; use stored responses
576   without revalidating them on every use. A cache, especially a shared
577   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
578   synchronize its clock with a reliable external standard.
579</t>
580
581</section>
582
583<section anchor="expiration.model" title="Freshness Model">
584<t>
585   When a response is "fresh" in the cache, it can be used to satisfy
586   subsequent requests without contacting the origin server, thereby improving
587   efficiency.
588</t>
589<t>
590   The primary mechanism for determining freshness is for an origin server to
591   provide an explicit expiration time in the future, using either the Expires
592   header field (<xref target="header.expires" />) or the max-age response cache
593   directive (<xref target="cache-response-directive" />). Generally, origin
594   servers will assign future explicit expiration times to responses in the
595   belief that the representation is not likely to change in a semantically
596   significant way before the expiration time is reached.
597</t>
598<t>
599   If an origin server wishes to force a cache to validate every request, it
600   can assign an explicit expiration time in the past to indicate that the
601   response is already stale. Compliant caches will normally validate the
602   cached response before reusing it for subsequent requests (see <xref
603   target="serving.stale.responses" />).
604</t>
605<t>
606   Since origin servers do not always provide explicit expiration times,
607   a cache &MAY; assign a heuristic expiration time when an explicit time is not
608   specified, employing algorithms that use other header field values (such as the
609   Last-Modified time) to estimate a plausible expiration time. This
610   specification does not provide specific algorithms, but does impose
611   worst-case constraints on their results.
612</t>
613<figure>
614<preamble>
615  The calculation to determine if a response is fresh is:
616</preamble>
617<artwork type="code">
618   response_is_fresh = (freshness_lifetime &gt; current_age)
619</artwork>
620</figure>
621<t>
622   The freshness_lifetime is defined in <xref
623   target="calculating.freshness.lifetime" />; the current_age is defined in
624   <xref target="age.calculations" />.
625</t>
626<t>
627   Additionally, clients might need to influence freshness calculation. They
628   can do this using several request cache directives, with the effect of
629   either increasing or loosening constraints on freshness. See <xref
630   target="cache-request-directive" />.
631</t>
632<t>
633   Note that freshness applies only to cache operation; it cannot be used to
634   force a user agent to refresh its display or reload a resource. See <xref
635   target="history.lists" /> for an explanation of the difference between
636   caches and history mechanisms.
637</t>
638
639<section anchor="calculating.freshness.lifetime" 
640   title="Calculating Freshness Lifetime">
641<t>
642   A cache can calculate the freshness lifetime (denoted as
643   freshness_lifetime) of a response by using the first match of:
644   <list style="symbols">
645      <t>If the cache is shared and the s-maxage response cache directive
646      (<xref target="cache-response-directive" />) is present, use its value,
647      or</t>
648      <t>If the max-age response cache directive (<xref
649      target="cache-response-directive" />) is present, use its value, or</t>
650      <t>If the Expires response header field (<xref target="header.expires" />) is
651      present, use its value minus the value of the Date response header field,
652      or</t>
653      <t>Otherwise, no explicit expiration time is present in the response. A
654      heuristic freshness lifetime might be applicable; see <xref
655      target="heuristic.freshness" />.</t>
656   </list>
657</t>
658<t>
659   Note that this calculation is not vulnerable to clock skew, since all of
660   the information comes from the origin server.
661</t>
662
663<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
664<t>
665   If no explicit expiration time is present in a stored response that has a
666   status code whose definition allows heuristic freshness to be used
667   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
668   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
669   use heuristics to determine freshness for responses with status codes that do
670   not explicitly allow it.
671</t>
672<t>
673   When a heuristic is used to calculate freshness lifetime, a cache
674   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
675   its current_age is more than 24 hours and such a warning is not already
676   present.
677</t>
678<t>
679   Also, if the response has a Last-Modified header field (&header-last-modified;),
680   a cache &SHOULD-NOT; use a heuristic expiration value that is more than some
681   fraction of the interval since that time. A typical setting of this fraction
682   might be 10%.
683</t>
684<x:note>
685   <t>
686      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
687      x:sec="13.9"/>) required that caches do not calculate heuristic
688      freshness for URIs with query components (i.e., those containing '?').
689      In practice, this has not been widely implemented. Therefore, servers
690      are encouraged to send explicit directives (e.g., Cache-Control:
691      no-cache) if they wish to preclude caching.
692   </t>
693</x:note>
694</section>
695</section>
696
697<section anchor="age.calculations" title="Calculating Age">
698<t>
699   HTTP/1.1 uses the Age header field to convey the estimated age of the
700   response message when obtained from a cache. The Age field value is the
701   cache's estimate of the amount of time since the response was generated or
702   validated by the origin server. In essence, the Age value is the sum of the
703   time that the response has been resident in each of the caches along the
704   path from the origin server, plus the amount of time it has been in transit
705   along network paths.
706</t>
707<t>
708   The following data is used for the age calculation:
709</t>
710<t>
711   <x:dfn>age_value</x:dfn>
712   <list>
713      <t>
714         The term "age_value" denotes the value of the Age header field (<xref
715         target="header.age"/>), in a form appropriate for arithmetic
716         operation; or 0, if not available.
717      </t>
718   </list>
719</t>
720<t>
721   <x:dfn>date_value</x:dfn>
722   <list>
723      <t>
724         HTTP/1.1 requires origin servers to send a Date header field, if possible,
725         with every response, giving the time at which the response was
726         generated. The term "date_value" denotes the value of the Date
727         header field, in a form appropriate for arithmetic operations. See
728         &header-date; for the definition of the Date header field, and for
729         requirements regarding responses without it.
730      </t>
731   </list>
732</t>
733<t>
734   <x:dfn>now</x:dfn>
735   <list>
736      <t>
737         The term "now" means "the current value of the clock at the host
738         performing the calculation". A cache &SHOULD; use NTP (<xref
739         target="RFC1305"/>) or some similar protocol to synchronize its
740         clocks to a globally accurate time standard.
741      </t>
742   </list>
743</t>
744<t>
745   <x:dfn>request_time</x:dfn>
746   <list>
747      <t>
748         The current value of the clock at the host at the time the request
749         resulting in the stored response was made.
750      </t>
751   </list>
752</t>
753<t>
754   <x:dfn>response_time</x:dfn>
755   <list>
756      <t>
757         The current value of the clock at the host at the time the response
758         was received.
759      </t>
760   </list>
761</t>
762<t>
763   A response's age can be calculated in two entirely independent ways:
764   <list style="numbers">
765      <t>the "apparent_age": response_time minus date_value, if the local
766      clock is reasonably well synchronized to the origin server's clock. If
767      the result is negative, the result is replaced by zero.</t>
768      <t>the "corrected_age_value", if all of the caches along the response
769      path implement HTTP/1.1. A cache &MUST; interpret this value relative
770      to the time the request was initiated, not the time that the response
771      was received.</t>
772   </list>
773</t>
774<figure>
775<artwork type="code">
776  apparent_age = max(0, response_time - date_value);
777
778  response_delay = response_time - request_time;
779  corrected_age_value = age_value + response_delay; 
780</artwork>
781</figure>
782<figure>
783<preamble>These are combined as</preamble>
784<artwork type="code">
785  corrected_initial_age = max(apparent_age, corrected_age_value);
786</artwork></figure>
787<t>
788   The current_age of a stored response can then be calculated by adding the
789   amount of time (in seconds) since the stored response was last validated by
790   the origin server to the corrected_initial_age.
791</t>
792<figure><artwork type="code">
793  resident_time = now - response_time;
794  current_age = corrected_initial_age + resident_time;
795</artwork></figure>
796</section>
797
798<section anchor="serving.stale.responses" title="Serving Stale Responses">
799<t>
800   A "stale" response is one that either has explicit expiry information or is
801   allowed to have heuristic expiry calculated, but is not fresh according to
802   the calculations in <xref target="expiration.model" />.
803</t>
804<t>
805   A cache &MUST-NOT; return a stale response if it is prohibited by an
806   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
807   directive, a "must-revalidate" cache-response-directive, or an applicable
808   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
809   target="cache-response-directive"/>).
810</t>
811<t>
812   A cache &SHOULD-NOT; return stale responses unless it is disconnected
813   (i.e., it cannot contact the origin server or otherwise find a forward
814   path) or doing so is explicitly allowed (e.g., by the max-stale request
815   directive; see <xref target="cache-request-directive" />).
816</t>
817<t>
818   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
819   <xref target="header.warning" />) to stale responses. Likewise, a cache
820   &SHOULD; add the 112 warn-code to stale responses if the cache is
821   disconnected.
822</t>
823<t>
824   If a cache receives a first-hand response (either an entire response, or a
825   304 (Not Modified) response) that it would normally forward to the
826   requesting client, and the received response is no longer fresh, the cache
827   &SHOULD; forward it to the requesting client without adding a new Warning
828   (but without removing any existing Warning header fields). A cache &SHOULD-NOT;
829   attempt to validate a response simply because that response became stale in
830   transit.
831</t>
832</section>
833</section>
834
835<section anchor="validation.model" title="Validation Model">
836<t>
837   When a cache has one or more stored responses for a requested URI, but
838   cannot serve any of them (e.g., because they are not fresh, or one cannot
839   be selected; see <xref target="caching.negotiated.responses"/>), it can use
840   the conditional request mechanism &conditional; in the forwarded request to
841   give the origin server an opportunity to both select a valid stored
842   response to be used, and to update it. This process is known as
843   "validating" or "revalidating" the stored response.
844</t>
845<t>
846   When sending such a conditional request, a cache &SHOULD; add an
847   If-Modified-Since header field whose value is that of the Last-Modified header
848   field from the selected (see <xref target="caching.negotiated.responses"/>)
849   stored response, if available.
850</t>
851<t>
852   Additionally, a cache &SHOULD; add an If-None-Match header field whose value is
853   that of the ETag header field(s) from all responses stored for the requested URI,
854   if present. However, if any of the stored responses contains only partial
855   content, the cache &SHOULD-NOT; include its entity-tag in the If-None-Match
856   header field unless the request is for a range that would be fully
857   satisfied by that stored response.
858</t>
859<t>
860   A 304 (Not Modified) response status code indicates that the stored
861   response can be updated and reused; see <xref target="combining.responses"/>.
862</t>
863<t>
864   A full response (i.e., one with a response body) indicates that none of the
865   stored responses nominated in the conditional request is suitable. Instead,
866   a cache &SHOULD; use the full response to satisfy the request and &MAY; 
867   replace the stored response.
868</t>
869<t>
870   If a cache receives a 5xx response while attempting to validate a response,
871   it &MAY; either forward this response to the requesting client, or act as
872   if the server failed to respond. In the latter case, it &MAY; return a
873   previously stored response (see <xref target="serving.stale.responses" />).
874</t>
875</section>
876
877<section anchor="invalidation.after.updates.or.deletions" 
878   title="Request Methods that Invalidate">
879<t>
880   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
881   have the potential for changing state on the origin server, intervening
882   caches can use them to keep their contents up-to-date.
883</t>
884<t>
885   A cache &MUST; invalidate the effective Request URI
886   (&effective-request-uri;) as well as the URI(s) in the Location
887   and Content-Location header fields (if present) when requests with
888   unsafe methods are received.
889</t>
890<t>
891   However, a cache &MUST-NOT; invalidate a URI from a
892   Location or Content-Location header field if the host part of that URI
893   differs from the host part in the effective request URI
894   (&effective-request-uri;). This helps prevent denial of service attacks.
895</t>
896<t>
897   A cache &SHOULD; invalidate the effective request URI
898   (&effective-request-uri;) when passing through requests with methods
899   whose safety is unknown.
900</t>
901<t>
902   Here, "invalidate" means that the cache will either remove all stored
903   responses related to the effective request URI, or will mark these as
904   "invalid" and in need of a mandatory validation before they can be returned
905   in response to a subsequent request.
906</t>
907<t>
908   Note that this does not guarantee that all appropriate responses are
909   invalidated. For example, the request that caused the change at the origin
910   server might not have gone through the cache where a response is stored.
911</t>
912</section>
913
914<section anchor="caching.authenticated.responses" 
915   title="Shared Caching of Authenticated Responses">
916
917<t>
918   A shared cache &MUST-NOT; use a cached response to a request with an
919   Authorization header field (&header-authorization;) to satisfy any subsequent
920   request unless a cache directive that allows such responses to be stored is
921   present in the response.
922</t>
923
924<t>
925   In this specification, the following Cache-Control response directives
926   (<xref target="cache-response-directive"/>) have such an effect:
927   must-revalidate, public, s-maxage.
928</t>
929
930<t>
931   Note that cached responses that contain the "must-revalidate" and/or
932   "s-maxage" response directives are not allowed to be served stale (<xref
933   target="serving.stale.responses"/>) by shared caches. In particular, a
934   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
935   used to satisfy a subsequent request without revalidating it on the origin
936   server.
937</t>
938</section>
939
940<section anchor="caching.negotiated.responses" 
941   title="Caching Negotiated Responses">
942<t>
943   When a cache receives a request that can be satisfied by a stored response
944   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
945   use that response unless all of the selecting header fields nominated by
946   the Vary header field match in both the original request (i.e., that associated
947   with the stored response), and the presented request.
948</t>
949<t>
950   The selecting header fields from two requests are defined to match if and
951   only if those in the first request can be transformed to those in the
952   second request by applying any of the following:
953   <list style="symbols">
954      <t>
955         adding or removing whitespace, where allowed in the header field's syntax
956      </t>
957      <t>
958         combining multiple header fields with the same field name
959         (see &header-fields;)
960      </t>
961      <t>
962         normalizing both header field values in a way that is known to have
963         identical semantics, according to the header field's specification (e.g.,
964         re-ordering field values when order is not significant;
965         case-normalization, where values are defined to be case-insensitive)
966      </t>
967  </list>
968</t>
969<t>
970   If (after any normalization that might take place) a header field is absent
971   from a request, it can only match another request if it is also absent
972   there.
973</t>
974<t>
975   A Vary header field-value of "*" always fails to match, and subsequent
976   requests to that resource can only be properly interpreted by the origin
977   server.
978</t>
979<t>
980   The stored response with matching selecting header fields is known as the
981   selected response.
982</t>
983<t>
984   If multiple selected responses are available, the most recent response
985   (as determined by the Date header field) is used; see <xref 
986   target="constructing.responses.from.caches"/>.
987</t>
988<t>
989   If no selected response is available, the cache &MAY; forward the presented
990   request to the origin server in a conditional request; see <xref
991   target="validation.model"/>.
992</t>
993</section>
994
995<section anchor="combining.responses" title="Combining Responses">
996<t>
997   When a cache receives a 304 (Not Modified) response or a 206 (Partial
998   Content) response (in this section, the "new" response"), it needs to
999   create an updated response by combining the stored response with the new
1000   one, so that the updated response can be used to satisfy the request, and
1001   potentially update the cached response.
1002</t>
1003<t>
1004   If the new response contains an ETag, it identifies the stored response to
1005   use. <cref anchor="TODO-mention-CL">might need language about
1006   Content-Location here</cref><cref
1007   anchor="TODO-select-for-combine">Shouldn't this be the selected
1008   response?</cref>
1009</t>
1010<t>
1011   When the new response's status code is 206 (partial content), a cache
1012   &MUST-NOT; combine it with the old response if either response does not
1013   have a validator, and &MUST-NOT; combine it with the old response when
1014   those validators do not match with the strong comparison function
1015   (see &weak-and-strong;).
1016</t>
1017<t>
1018   The stored response header fields are used as those of the updated response,
1019   except that
1020   <list style="symbols">
1021      <t>a cache &MUST; delete any stored Warning header fields with warn-code 1xx (see <xref
1022      target="header.warning" />).</t>
1023      <t>a cache &MUST; retain any stored Warning header fields with warn-code 2xx.</t>
1024      <t>a cache &MUST; use other header fields provided in the new response to replace all
1025      instances of the corresponding header fields from the stored response.</t>
1026   </list>
1027</t>
1028<t>
1029   A cache &MUST; use the updated response header fields to replace those of the stored
1030   response (unless the stored response is removed). In
1031   the case of a 206 response, a cache &MAY; store the combined representation.
1032</t>
1033</section>
1034
1035</section>
1036
1037<section anchor="header.fields" title="Header Field Definitions">
1038<t>
1039   This section defines the syntax and semantics of HTTP/1.1 header fields
1040   related to caching.
1041</t>
1042
1043<section anchor="header.age" title="Age">
1044   <iref item="Age header field" primary="true" x:for-anchor="" />
1045   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1046   <x:anchor-alias value="Age"/>
1047   <x:anchor-alias value="age-value"/>
1048<t>
1049   The "Age" header field conveys the sender's estimate of the amount
1050   of time since the response was generated or successfully validated at the
1051   origin server. Age values are calculated as specified in <xref
1052   target="age.calculations" />.
1053</t>
1054<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1055  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1056</artwork></figure>
1057<t anchor="rule.delta-seconds">
1058  <x:anchor-alias value="delta-seconds" />
1059  Age field-values are non-negative integers, representing time in seconds.
1060</t>
1061<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
1062  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
1063</artwork></figure>
1064<t>
1065   If a cache receives a value larger than the largest positive integer it can
1066   represent, or if any of its age calculations overflows, it &MUST; transmit
1067   an Age header field with a field-value of 2147483648 (2<x:sup>31</x:sup>).
1068   Recipients parsing the Age header field-value &SHOULD; use an arithmetic type of
1069   at least 31 bits of range.
1070</t>
1071<t>
1072   The presence of an Age header field in a response implies that a response
1073   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1074   might not implement the Age header field.
1075</t>
1076</section>
1077
1078<section anchor="header.cache-control" title="Cache-Control">
1079   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1080   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1081      x:for-anchor="" />
1082   <x:anchor-alias value="Cache-Control"/>
1083   <x:anchor-alias value="cache-directive"/>
1084   <x:anchor-alias value="cache-extension"/>
1085   <x:anchor-alias value="cache-request-directive"/>
1086   <x:anchor-alias value="cache-response-directive"/>
1087<t>
1088   The "Cache-Control" header field is used to specify directives for
1089   caches along the request/response chain. Such cache directives are
1090   unidirectional in that the presence of a directive in a request does not
1091   imply that the same directive is to be given in the response.
1092</t>
1093<t>
1094   A cache &MUST; obey the requirements of the Cache-Control
1095   directives defined in this section. See <xref
1096   target="cache.control.extensions"/> for information about how Cache-Control
1097   directives defined elsewhere are handled.
1098</t>
1099<x:note>
1100   <t>
1101       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1102       might only implement Pragma: no-cache (see <xref target="header.pragma"
1103       />).
1104   </t>
1105</x:note>
1106<t>
1107   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1108   through in forwarded messages, regardless of their
1109   significance to that application, since the directives might be applicable
1110   to all recipients along the request/response chain. It is not possible to
1111   target a directive to a specific cache.
1112</t>
1113<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1114  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1115
1116  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1117     / <x:ref>cache-response-directive</x:ref>
1118
1119  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1120</artwork></figure>
1121
1122<section anchor="cache-request-directive" 
1123   title="Request Cache-Control Directives">
1124   <x:anchor-alias value="cache-request-directive" />
1125
1126<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1127   subitem="cache-request-directive" />
1128  <x:ref>cache-request-directive</x:ref> =
1129       "no-cache"
1130     / "no-store"
1131     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1132     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1133     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1134     / "no-transform"
1135     / "only-if-cached"
1136     / <x:ref>cache-extension</x:ref>
1137</artwork></figure>
1138
1139<t>
1140   <x:dfn>no-cache</x:dfn>
1141   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1142   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1143   <list>
1144      <t>The no-cache request directive indicates that a cache &MUST-NOT; 
1145      use a stored response to satisfy the request without successful
1146      validation on the origin server.</t> 
1147   </list>
1148</t>
1149<t>
1150   <x:dfn>no-store</x:dfn>
1151   <iref item="Cache Directives" primary="true" subitem="no-store" />
1152   <iref item="no-store" primary="true" subitem="Cache Directive" />
1153   <list>
1154      <t>The no-store request directive indicates that a cache &MUST-NOT;
1155      store any part of either this request or any response to it. This
1156      directive applies to both private and shared caches. "&MUST-NOT;
1157      store" in this context means that the cache &MUST-NOT; intentionally
1158      store the information in non-volatile storage, and &MUST; make a
1159      best-effort attempt to remove the information from volatile storage as
1160      promptly as possible after forwarding it.</t>
1161      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1162      privacy. In particular, malicious or compromised caches might not
1163      recognize or obey this directive, and communications networks might be
1164      vulnerable to eavesdropping.</t>
1165      <t>Note that if a request containing this directive is satisfied from a
1166      cache, the no-store request directive does not apply to the already
1167      stored response.</t>
1168   </list>
1169</t>
1170<t>
1171   <x:dfn>max-age</x:dfn>
1172   <iref item="Cache Directives" primary="true" subitem="max-age" />
1173   <iref item="max-age" primary="true" subitem="Cache Directive" />
1174   <list>
1175      <t>The max-age request directive indicates that the client is unwilling to
1176      accept a response whose age is greater than the specified number of
1177      seconds. Unless the max-stale request directive is also present, the
1178      client is not willing to accept a stale response.</t>
1179   </list>
1180</t>
1181<t>
1182   <x:dfn>max-stale</x:dfn>
1183   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1184   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1185   <list>
1186      <t>The max-stale request directive indicates that the client is willing
1187      to accept a response that has exceeded its expiration time. If max-stale
1188      is assigned a value, then the client is willing to accept a response
1189      that has exceeded its expiration time by no more than the specified
1190      number of seconds. If no value is assigned to max-stale, then the client
1191      is willing to accept a stale response of any age.</t>
1192   </list>
1193</t>
1194<t>
1195   <x:dfn>min-fresh</x:dfn>
1196   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1197   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1198   <list>
1199      <t>The min-fresh request directive indicates that the client is willing
1200      to accept a response whose freshness lifetime is no less than its
1201      current age plus the specified time in seconds. That is, the client
1202      wants a response that will still be fresh for at least the specified
1203      number of seconds.</t>
1204   </list>
1205</t>
1206<t>
1207   <x:dfn>no-transform</x:dfn>
1208   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1209   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1210   <list>
1211      <t>The no-transform request directive indicates that an intermediary
1212        (whether or not it implements a cache) &MUST-NOT; change the
1213        Content-Encoding, Content-Range or Content-Type request header fields,
1214        nor the request representation.</t>
1215   </list>
1216</t>
1217<t>
1218   <x:dfn>only-if-cached</x:dfn>
1219   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1220   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1221   <list>
1222      <t>The only-if-cached request directive indicates that the client only
1223      wishes to obtain a stored response. If it receives this directive, a
1224      cache &SHOULD; either respond using a stored response that is consistent
1225      with the other constraints of the request, or respond with a 504
1226      (Gateway Timeout) status code. If a group of caches is being operated as
1227      a unified system with good internal connectivity, a member cache &MAY;
1228      forward such a request within that group of caches.</t>
1229   </list>
1230</t>
1231</section>
1232
1233<section anchor="cache-response-directive" 
1234   title="Response Cache-Control Directives">
1235   <x:anchor-alias value="cache-response-directive" />
1236
1237<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1238   subitem="cache-response-directive" />
1239  <x:ref>cache-response-directive</x:ref> =
1240       "public"
1241     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1242     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1243     / "no-store"
1244     / "no-transform"
1245     / "must-revalidate"
1246     / "proxy-revalidate"
1247     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1248     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1249     / <x:ref>cache-extension</x:ref>
1250</artwork></figure>
1251
1252<t>
1253   <x:dfn>public</x:dfn>
1254   <iref item="Cache Directives" primary="true" subitem="public" />
1255   <iref item="public" primary="true" subitem="Cache Directive" />
1256   <list>
1257      <t>The public response directive indicates that a response whose
1258        associated request contains an 'Authentication' header &MAY; be
1259        stored (see <xref target="caching.authenticated.responses" />).</t>
1260  </list>
1261</t>
1262<t>
1263   <x:dfn>private</x:dfn>
1264   <iref item="Cache Directives" primary="true" subitem="private" />
1265   <iref item="private" primary="true" subitem="Cache Directive" />
1266   <list>
1267      <t>The private response directive indicates that the response message is
1268      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1269      private cache &MAY; store the response.</t>
1270      <t>If the private response directive specifies one or more field-names,
1271      this requirement is limited to the field-values associated with the
1272      listed response header fields. That is, a shared cache &MUST-NOT; store
1273      the specified field-names(s), whereas it &MAY; store the remainder of the
1274      response message.</t>
1275      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1276      the response can be stored; it cannot ensure the privacy of the message
1277      content. Also, private response directives with field-names are often
1278      handled by implementations as if an unqualified private directive was
1279      received; i.e., the special handling for the qualified form is not
1280      widely implemented.</t>
1281   </list>
1282</t>
1283<t>
1284   <x:dfn>no-cache</x:dfn>
1285   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1286   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1287   <list>
1288      <t>The no-cache response directive indicates that the response MUST NOT
1289      be used to satisfy a subsequent request without successful validation on
1290      the origin server. This allows an origin server to prevent a cache from
1291      using it to satisfy a request without contacting it, even by caches that
1292      have been configured to return stale responses.</t>
1293      <t>If the no-cache response directive specifies one or more field-names,
1294      this requirement is limited to the field-values associated with the
1295      listed response header fields. That is, a cache &MUST-NOT; send the
1296      specified field-name(s) in the response to a subsequent request without successful
1297      validation on the origin server. This allows an origin server to prevent
1298      the re-use of certain header fields in a response, while still allowing
1299      caching of the rest of the response.</t>
1300      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1301      this directive. Also, no-cache response directives with field-names are
1302      often handled by implementations as if an unqualified no-cache directive
1303      was received; i.e., the special handling for the qualified form is not
1304      widely implemented. </t>
1305   </list>
1306</t>
1307<t>
1308   <x:dfn>no-store</x:dfn>
1309   <iref item="Cache Directives" primary="true" subitem="no-store" />
1310   <iref item="no-store" primary="true" subitem="Cache Directive" />
1311   <list>
1312      <t>The no-store response directive indicates that a cache &MUST-NOT;
1313      store any part of either the immediate request or response. This
1314      directive applies to both private and shared caches. "&MUST-NOT;
1315      store" in this context means that the cache &MUST-NOT; intentionally
1316      store the information in non-volatile storage, and &MUST; make a
1317      best-effort attempt to remove the information from volatile storage as
1318      promptly as possible after forwarding it.</t>
1319      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1320      privacy. In particular, malicious or compromised caches might not
1321      recognize or obey this directive, and communications networks might be
1322      vulnerable to eavesdropping.</t>
1323   </list>
1324</t>
1325<t>
1326   <x:dfn>must-revalidate</x:dfn>
1327   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1328   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1329   <list>
1330      <t>The must-revalidate response directive indicates that once it has
1331      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1332      requests without successful validation on the origin server.</t>
1333      <t>The must-revalidate directive is necessary to support reliable
1334      operation for certain protocol features. In all circumstances a
1335      cache &MUST; obey the must-revalidate directive; in particular,
1336      if a cache cannot reach the origin server for any reason, it &MUST;
1337      generate a 504 (Gateway Timeout) response.</t>
1338      <t>A server &SHOULD; send the must-revalidate directive if and only if
1339      failure to validate a request on the representation could result in
1340      incorrect operation, such as a silently unexecuted financial
1341      transaction.</t>
1342   </list>
1343</t>
1344<t>
1345   <x:dfn>proxy-revalidate</x:dfn>
1346   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1347   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1348   <list>
1349      <t>The proxy-revalidate response directive has the same meaning as the
1350      must-revalidate response directive, except that it does not apply to
1351      private caches.</t>
1352   </list>
1353</t>
1354<t>
1355   <x:dfn>max-age</x:dfn>
1356   <iref item="Cache Directives" primary="true" subitem="max-age" />
1357   <iref item="max-age" primary="true" subitem="Cache Directive" />
1358   <list>
1359      <t>The max-age response directive indicates that the response is to be
1360      considered stale after its age is greater than the specified number of
1361      seconds.</t>
1362   </list>
1363</t>
1364<t>
1365   <x:dfn>s-maxage</x:dfn>
1366   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1367   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1368   <list>
1369      <t>The s-maxage response directive indicates that, in shared caches, the
1370      maximum age specified by this directive overrides the maximum age
1371      specified by either the max-age directive or the Expires header field. The
1372      s-maxage directive also implies the semantics of the proxy-revalidate
1373      response directive.</t>
1374   </list>
1375</t>
1376<t>
1377   <x:dfn>no-transform</x:dfn>
1378   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1379   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1380   <list>
1381      <t>The no-transform response directive indicates that an intermediary
1382      (regardless of whether it implements a cache) &MUST-NOT; change the
1383      Content-Encoding, Content-Range or Content-Type response header fields,
1384      nor the response representation.</t>
1385   </list>
1386</t>
1387
1388</section>
1389
1390<section anchor="cache.control.extensions" title="Cache Control Extensions">
1391<t>
1392   The Cache-Control header field can be extended through the use of one or
1393   more cache-extension tokens, each with an optional value. Informational
1394   extensions (those that do not require a change in cache behavior) can be
1395   added without changing the semantics of other directives. Behavioral
1396   extensions are designed to work by acting as modifiers to the existing base
1397   of cache directives. Both the new directive and the standard directive are
1398   supplied, such that applications that do not understand the new directive
1399   will default to the behavior specified by the standard directive, and those
1400   that understand the new directive will recognize it as modifying the
1401   requirements associated with the standard directive. In this way,
1402   extensions to the cache-control directives can be made without requiring
1403   changes to the base protocol.
1404</t>
1405<t>
1406   This extension mechanism depends on an HTTP cache obeying all of the
1407   cache-control directives defined for its native HTTP-version, obeying
1408   certain extensions, and ignoring all directives that it does not
1409   understand.
1410</t>
1411<t>
1412   For example, consider a hypothetical new response directive called
1413   "community" that acts as a modifier to the private directive. We define
1414   this new directive to mean that, in addition to any private cache, any
1415   cache that is shared only by members of the community named within its
1416   value may cache the response. An origin server wishing to allow the UCI
1417   community to use an otherwise private response in their shared cache(s)
1418   could do so by including
1419</t>
1420<figure><artwork type="example">
1421  Cache-Control: private, community="UCI"
1422</artwork></figure>
1423<t>
1424   A cache seeing this header field will act correctly even if the cache does
1425   not understand the community cache-extension, since it will also see and
1426   understand the private directive and thus default to the safe behavior.
1427</t>
1428<t>
1429   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1430   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1431   combined with standard directives (or the response's default cacheability)
1432   such that the cache behavior will remain minimally correct even if the
1433   cache does not understand the extension(s).
1434</t>
1435<t>
1436   The HTTP Cache Directive Registry defines the name space for the cache
1437   directives.
1438</t>
1439<t>
1440   A registration &MUST; include the following fields:
1441   <list style="symbols">
1442      <t>Cache Directive Name</t>
1443      <t>Pointer to specification text</t>
1444   </list>
1445</t>
1446<t>
1447   Values to be added to this name space are subject to IETF review (<xref
1448   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1449</t>
1450<t>
1451   The registry itself is maintained at <eref
1452   target="http://www.iana.org/assignments/http-cache-directives"/>.
1453</t>
1454</section>
1455
1456</section>
1457
1458<section anchor="header.expires" title="Expires">
1459   <iref item="Expires header field" primary="true" x:for-anchor="" />
1460   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1461   <x:anchor-alias value="Expires"/>
1462<t>
1463   The "Expires" header field gives the date/time after which the
1464   response is considered stale. See <xref target="expiration.model" /> for
1465   further discussion of the freshness model.
1466</t>
1467<t>
1468   The presence of an Expires field does not imply that the original resource
1469   will change or cease to exist at, before, or after that time.
1470</t>
1471<t>
1472   The field-value is an absolute date and time as defined by HTTP-date in
1473   &full-date;; a sender &MUST; use the rfc1123-date format.
1474</t>
1475<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1476  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1477</artwork></figure>
1478<figure>
1479  <preamble>For example</preamble>
1480<artwork type="example">
1481  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1482</artwork></figure>
1483<x:note>
1484   <t>
1485       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1486       max-age directive (see <xref target="cache-response-directive" />),
1487       that directive overrides the Expires field. Likewise, the s-maxage
1488       directive overrides Expires in shared caches.
1489   </t>
1490</x:note>
1491<t>
1492   A server &SHOULD-NOT; send Expires dates more than one year in the
1493   future.
1494</t>
1495<t>
1496   A cache &MUST; treat other invalid date formats,
1497   especially including the value "0", as in the past (i.e., "already
1498   expired").
1499</t>
1500</section>
1501
1502<section anchor="header.pragma" title="Pragma">
1503   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1504   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1505   <x:anchor-alias value="extension-pragma"/>
1506   <x:anchor-alias value="Pragma"/>
1507   <x:anchor-alias value="pragma-directive"/>
1508<t>
1509   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1510   caches, so that clients can specify a "no-cache" request that they will
1511   understand (as Cache-Control was not defined until HTTP/1.1). When the
1512   Cache-Control header is also present and understood in a request, Pragma is
1513   ignored.
1514</t>
1515<t>
1516   In HTTP/1.0, Pragma was defined as an extensible field for
1517   implementation-specified directives for recipients. This specification
1518   deprecates such extensions to improve interoperability.
1519</t>
1520<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1521  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1522  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1523  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1524</artwork></figure>
1525<t>
1526   When the Cache-Control header is not present in a request, the no-cache
1527   request pragma-directive &MUST; have the same effect on caches as if
1528   "Cache-Control: no-cache" were present (see <xref
1529   target="cache-request-directive" />).
1530</t>
1531<t>
1532   When sending a no-cache request, a client &SHOULD; include both pragma and
1533   cache-control directives unless Cache-Control: no-cache is purposefully
1534   omitted to target other Cache-Control response directives at HTTP/1.1
1535   caches. For example:
1536</t>
1537<figure>
1538<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1539GET / HTTP/1.1
1540Host: www.example.com
1541Cache-Control: max-age=30
1542Pragma: no-cache
1543
1544</artwork>
1545</figure>
1546<t>
1547   will constrain HTTP/1.1 caches to serve a response no older than 30
1548   seconds, while precluding implementations that do not understand
1549   Cache-Control from serving a cached response.
1550</t>
1551<x:note>
1552   <t>
1553      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
1554      specified, it does not provide a reliable replacement for
1555      "Cache-Control: no-cache" in them.
1556   </t>
1557</x:note>
1558</section>
1559
1560<section anchor="header.vary" title="Vary">
1561   <iref item="Vary header field" primary="true" x:for-anchor="" />
1562   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1563   <x:anchor-alias value="Vary"/>
1564<t>
1565   The "Vary" header field conveys the set of header fields
1566   that were used to select the representation.
1567</t>
1568<t>
1569   Caches use this information, in part, to determine whether a stored
1570   response can be used to satisfy a given request; see <xref
1571   target="caching.negotiated.responses" />. determines, while the response is
1572   fresh, whether a cache is permitted to use the response to reply to a
1573   subsequent request without validation; see <xref
1574   target="caching.negotiated.responses" />.
1575</t>
1576<t>
1577   In uncacheable or stale responses, the Vary field value advises the user
1578   agent about the criteria that were used to select the representation.
1579</t>
1580<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1581  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1582</artwork></figure>
1583<t>
1584   The set of header fields named by the Vary field value is known as the
1585   selecting header fields.
1586</t>
1587<t>
1588   A server &SHOULD; include a Vary header field with any cacheable response
1589   that is subject to server-driven negotiation. Doing so allows a cache to
1590   properly interpret future requests on that resource and informs the user
1591   agent about the presence of negotiation on that resource. A server &MAY;
1592   include a Vary header field with a non-cacheable response that is subject
1593   to server-driven negotiation, since this might provide the user agent with
1594   useful information about the dimensions over which the response varies at
1595   the time of the response.
1596</t>
1597<t>
1598   A Vary field value of "*" signals that unspecified parameters not limited
1599   to the header fields (e.g., the network address of the client), play a
1600   role in the selection of the response representation; therefore, a cache
1601   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1602   generate the "*" value.
1603</t>
1604<t>
1605   The field-names given are not limited to the set of standard header
1606   fields defined by this specification. Field names are case-insensitive.
1607</t>
1608</section>
1609
1610<section anchor="header.warning" title="Warning">
1611   <iref item="Warning header field" primary="true" x:for-anchor="" />
1612   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1613   <x:anchor-alias value="Warning"/>
1614   <x:anchor-alias value="warning-value"/>
1615   <x:anchor-alias value="warn-agent"/>
1616   <x:anchor-alias value="warn-code"/>
1617   <x:anchor-alias value="warn-date"/>
1618   <x:anchor-alias value="warn-text"/>
1619<t>
1620   The "Warning" header field is used to carry additional information
1621   about the status or transformation of a message that might not be reflected
1622   in the message. This information is typically used to warn about possible
1623   incorrectness introduced by caching operations or transformations applied
1624   to the payload of the message.
1625</t>
1626<t>
1627   Warnings can be used for other purposes, both cache-related and otherwise.
1628   The use of a warning, rather than an error status code, distinguishes these
1629   responses from true failures.
1630</t>
1631<t>
1632   Warning header fields can in general be applied to any message, however some
1633   warn-codes are specific to caches and can only be applied to response
1634   messages.
1635</t>
1636<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
1637  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1638 
1639  <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>
1640                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1641 
1642  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1643  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1644                  ; the name or pseudonym of the server adding
1645                  ; the Warning header field, for use in debugging
1646  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1647  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1648</artwork></figure>
1649<t>
1650   Multiple warnings can be attached to a response (either by the origin
1651   server or by a cache), including multiple warnings with the same code
1652   number, only differing in warn-text.
1653</t>
1654<t>
1655   When this occurs, the user agent &SHOULD; inform the user of as many of
1656   them as possible, in the order that they appear in the response.
1657</t>
1658<t>
1659   Systems that generate multiple Warning header fields &SHOULD; order them with
1660   this user agent behavior in mind. New Warning header fields &SHOULD; be added
1661   after any existing Warning headers fields.
1662</t>
1663<t>
1664   Warnings are assigned three digit warn-codes. The first digit indicates
1665   whether the Warning is required to be deleted from a stored response after
1666   validation:
1667   <list style="symbols">
1668      <t>1xx Warnings describe the freshness or validation status of the
1669      response, and so &MUST; be deleted by a cache after validation. They can
1670      only be generated by a cache when validating a cached entry, and
1671      &MUST-NOT; be generated in any other situation.</t>
1672      <t>2xx Warnings describe some aspect of the representation that is not
1673      rectified by a validation (for example, a lossy compression of the
1674      representation) and &MUST-NOT; be deleted by a cache after validation,
1675      unless a full response is returned, in which case they &MUST; be.</t>
1676   </list>
1677</t>
1678<t>
1679   If an implementation sends a message with one or more Warning header fields to a
1680   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1681   in each warning-value a warn-date that matches the Date header field in the
1682   message.
1683</t>
1684<t>
1685   If a system receives a message with a warning-value that includes
1686   a warn-date, and that warn-date is different from the Date value in the
1687   response, then that warning-value &MUST; be deleted from the message before
1688   storing, forwarding, or using it. (preventing the consequences of naive
1689   caching of Warning header fields.) If all of the warning-values are deleted
1690   for this reason, the Warning header field &MUST; be deleted as well.
1691</t>
1692<t>
1693   The following warn-codes are defined by this specification, each with a
1694   recommended warn-text in English, and a description of its meaning.
1695</t>
1696<t><?rfc needLines="4"?>
1697   110 Response is stale
1698   <list>
1699      <t>A cache &SHOULD; include this whenever the returned response is stale.</t>
1700   </list>
1701</t>
1702<t><?rfc needLines="4"?>
1703   111 Revalidation failed
1704   <list>
1705      <t>A cache &SHOULD; include this when returning a stale response because an
1706      attempt to validate the response failed, due to an inability to reach
1707      the server.</t>
1708   </list>
1709</t>
1710<t><?rfc needLines="4"?>
1711   112 Disconnected operation
1712   <list>
1713      <t>A cache &SHOULD; b include this if it is intentionally disconnected from
1714      the rest of the network for a period of time.</t>
1715   </list>
1716</t>
1717<t><?rfc needLines="4"?>
1718   113 Heuristic expiration
1719   <list>
1720      <t>A cache &SHOULD; include this if it heuristically chose a freshness
1721      lifetime greater than 24 hours and the response's age is greater than 24
1722      hours.</t>
1723   </list>
1724</t>
1725<t><?rfc needLines="4"?>
1726   199 Miscellaneous warning
1727   <list>
1728      <t>The warning text can include arbitrary information to be presented to
1729      a human user, or logged. A system receiving this warning &MUST-NOT; take
1730      any automated action, besides presenting the warning to the user.</t>
1731   </list>
1732</t>
1733<t><?rfc needLines="4"?>
1734   214 Transformation applied
1735   <list>
1736      <t>&MUST; be added by a proxy if it applies any
1737      transformation to the representation, such as changing the
1738      content-coding, media-type, or modifying the representation data, unless
1739      this Warning code already appears in the response.</t>
1740   </list>
1741</t>
1742<t><?rfc needLines="4"?>
1743   299 Miscellaneous persistent warning
1744   <list>
1745      <t>The warning text can include arbitrary information to be presented to
1746      a human user, or logged. A system receiving this warning &MUST-NOT; take
1747      any automated action.</t>
1748   </list>
1749</t>
1750</section>
1751
1752</section>
1753
1754<section anchor="history.lists" title="History Lists">
1755<t>
1756   User agents often have history mechanisms, such as "Back" buttons and
1757   history lists, that can be used to redisplay a representation retrieved
1758   earlier in a session.
1759</t>
1760<t>
1761   The freshness model (<xref target="expiration.model"/>) does not
1762   necessarily apply to history mechanisms. I.e., a history mechanism can
1763   display a previous representation even if it has expired.
1764</t>
1765<t>
1766   This does not prohibit the history mechanism from telling the user that a
1767   view might be stale, or from honoring cache directives (e.g.,
1768   Cache-Control: no-store).
1769</t>
1770</section>
1771
1772
1773<section anchor="IANA.considerations" title="IANA Considerations">
1774
1775<section title="Cache Directive Registry" 
1776   anchor="cache.directive.registration">
1777<t>
1778   The registration procedure for HTTP Cache Directives is defined by <xref
1779   target="cache.control.extensions"/> of this document.
1780</t>
1781<t>
1782   The HTTP Cache Directive Registry shall be created at <eref
1783   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1784   populated with the registrations below:
1785</t>
1786<?BEGININC p6-cache.cache-directives ?>
1787<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1788<texttable xmlns:my="#my" align="left" suppress-title="true"
1789           anchor="iana.cache.directive.registration.table">
1790   <ttcol>Cache Directive</ttcol>
1791   <ttcol>Reference</ttcol>
1792
1793   <c>max-age</c>
1794   <c>
1795      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1796   </c>
1797   <c>max-stale</c>
1798   <c>
1799      <xref target="cache-request-directive"/>
1800   </c>
1801   <c>min-fresh</c>
1802   <c>
1803      <xref target="cache-request-directive"/>
1804   </c>
1805   <c>must-revalidate</c>
1806   <c>
1807      <xref target="cache-response-directive"/>
1808   </c>
1809   <c>no-cache</c>
1810   <c>
1811      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1812   </c>
1813   <c>no-store</c>
1814   <c>
1815      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1816   </c>
1817   <c>no-transform</c>
1818   <c>
1819      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1820   </c>
1821   <c>only-if-cached</c>
1822   <c>
1823      <xref target="cache-request-directive"/>
1824   </c>
1825   <c>private</c>
1826   <c>
1827      <xref target="cache-response-directive"/>
1828   </c>
1829   <c>proxy-revalidate</c>
1830   <c>
1831      <xref target="cache-response-directive"/>
1832   </c>
1833   <c>public</c>
1834   <c>
1835      <xref target="cache-response-directive"/>
1836   </c>
1837   <c>s-maxage</c>
1838   <c>
1839      <xref target="cache-response-directive"/>
1840   </c>
1841   <c>stale-if-error</c>
1842   <c>
1843      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1844   </c>
1845   <c>stale-while-revalidate</c>
1846   <c>
1847      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1848   </c>
1849</texttable>
1850<!--(END)-->
1851<?ENDINC p6-cache.cache-directives ?>
1852</section>
1853
1854<section title="Header Field Registration" anchor="header.field.registration">
1855<t>
1856  The Message Header Field Registry located at <eref 
1857  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1858  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
1859</t>
1860<?BEGININC p6-cache.iana-headers ?>
1861<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1862<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1863   <ttcol>Header Field Name</ttcol>
1864   <ttcol>Protocol</ttcol>
1865   <ttcol>Status</ttcol>
1866   <ttcol>Reference</ttcol>
1867
1868   <c>Age</c>
1869   <c>http</c>
1870   <c>standard</c>
1871   <c>
1872      <xref target="header.age"/>
1873   </c>
1874   <c>Cache-Control</c>
1875   <c>http</c>
1876   <c>standard</c>
1877   <c>
1878      <xref target="header.cache-control"/>
1879   </c>
1880   <c>Expires</c>
1881   <c>http</c>
1882   <c>standard</c>
1883   <c>
1884      <xref target="header.expires"/>
1885   </c>
1886   <c>Pragma</c>
1887   <c>http</c>
1888   <c>standard</c>
1889   <c>
1890      <xref target="header.pragma"/>
1891   </c>
1892   <c>Vary</c>
1893   <c>http</c>
1894   <c>standard</c>
1895   <c>
1896      <xref target="header.vary"/>
1897   </c>
1898   <c>Warning</c>
1899   <c>http</c>
1900   <c>standard</c>
1901   <c>
1902      <xref target="header.warning"/>
1903   </c>
1904</texttable>
1905<!--(END)-->
1906<?ENDINC p6-cache.iana-headers ?>
1907<t>
1908   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1909   Force".
1910</t>
1911</section>
1912
1913</section>
1914
1915<section anchor="security.considerations" title="Security Considerations">
1916<t>
1917   Caches expose additional potential vulnerabilities, since the contents of
1918   the cache represent an attractive target for malicious exploitation.
1919   Because cache contents persist after an HTTP request is complete, an attack
1920   on the cache can reveal information long after a user believes that the
1921   information has been removed from the network. Therefore, cache contents
1922   need to be protected as sensitive information.
1923</t>
1924</section>
1925
1926<section anchor="ack" title="Acknowledgments">
1927<t>
1928   Much of the content and presentation of the caching design is due to
1929   suggestions and comments from individuals including: Shel Kaphan, Paul
1930   Leach, Koen Holtman, David Morris, and Larry Masinter.
1931</t>
1932</section>
1933
1934</middle>
1935
1936<back>
1937<references title="Normative References">
1938
1939  <reference anchor="Part1">
1940    <front>
1941      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1942      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1943        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1944        <address><email>fielding@gbiv.com</email></address>
1945      </author>
1946      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1947        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1948        <address><email>jg@freedesktop.org</email></address>
1949      </author>
1950      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1951        <organization abbrev="HP">Hewlett-Packard Company</organization>
1952        <address><email>JeffMogul@acm.org</email></address>
1953      </author>
1954      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1955        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1956        <address><email>henrikn@microsoft.com</email></address>
1957      </author>
1958      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1959        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1960        <address><email>LMM@acm.org</email></address>
1961      </author>
1962      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1963        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1964        <address><email>paulle@microsoft.com</email></address>
1965      </author>
1966      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1967        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1968        <address><email>timbl@w3.org</email></address>
1969      </author>
1970      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1971        <organization abbrev="W3C">World Wide Web Consortium</organization>
1972        <address><email>ylafon@w3.org</email></address>
1973      </author>
1974      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1975        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1976        <address><email>julian.reschke@greenbytes.de</email></address>
1977      </author>
1978      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1979    </front>
1980    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1981    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1982  </reference>
1983
1984  <reference anchor="Part2">
1985    <front>
1986      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1987      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1988        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1989        <address><email>fielding@gbiv.com</email></address>
1990      </author>
1991      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1992        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1993        <address><email>jg@freedesktop.org</email></address>
1994      </author>
1995      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1996        <organization abbrev="HP">Hewlett-Packard Company</organization>
1997        <address><email>JeffMogul@acm.org</email></address>
1998      </author>
1999      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2000        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2001        <address><email>henrikn@microsoft.com</email></address>
2002      </author>
2003      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2004        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2005        <address><email>LMM@acm.org</email></address>
2006      </author>
2007      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2008        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2009        <address><email>paulle@microsoft.com</email></address>
2010      </author>
2011      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2012        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2013        <address><email>timbl@w3.org</email></address>
2014      </author>
2015      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2016        <organization abbrev="W3C">World Wide Web Consortium</organization>
2017        <address><email>ylafon@w3.org</email></address>
2018      </author>
2019      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2020        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2021        <address><email>julian.reschke@greenbytes.de</email></address>
2022      </author>
2023      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2024    </front>
2025    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2026    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2027  </reference>
2028
2029  <reference anchor="Part4">
2030    <front>
2031      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2032      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2033        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2034        <address><email>fielding@gbiv.com</email></address>
2035      </author>
2036      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2037        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2038        <address><email>jg@freedesktop.org</email></address>
2039      </author>
2040      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2041        <organization abbrev="HP">Hewlett-Packard Company</organization>
2042        <address><email>JeffMogul@acm.org</email></address>
2043      </author>
2044      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2045        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2046        <address><email>henrikn@microsoft.com</email></address>
2047      </author>
2048      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2049        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2050        <address><email>LMM@acm.org</email></address>
2051      </author>
2052      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2053        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2054        <address><email>paulle@microsoft.com</email></address>
2055      </author>
2056      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2057        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2058        <address><email>timbl@w3.org</email></address>
2059      </author>
2060      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2061        <organization abbrev="W3C">World Wide Web Consortium</organization>
2062        <address><email>ylafon@w3.org</email></address>
2063      </author>
2064      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2065        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2066        <address><email>julian.reschke@greenbytes.de</email></address>
2067      </author>
2068      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2069    </front>
2070    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2071    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2072  </reference>
2073
2074  <reference anchor="Part5">
2075    <front>
2076      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2077      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2078        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2079        <address><email>fielding@gbiv.com</email></address>
2080      </author>
2081      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2082        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2083        <address><email>jg@freedesktop.org</email></address>
2084      </author>
2085      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2086        <organization abbrev="HP">Hewlett-Packard Company</organization>
2087        <address><email>JeffMogul@acm.org</email></address>
2088      </author>
2089      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2090        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2091        <address><email>henrikn@microsoft.com</email></address>
2092      </author>
2093      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2094        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2095        <address><email>LMM@acm.org</email></address>
2096      </author>
2097      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2098        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2099        <address><email>paulle@microsoft.com</email></address>
2100      </author>
2101      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2102        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2103        <address><email>timbl@w3.org</email></address>
2104      </author>
2105      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2106        <organization abbrev="W3C">World Wide Web Consortium</organization>
2107        <address><email>ylafon@w3.org</email></address>
2108      </author>
2109      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2110        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2111        <address><email>julian.reschke@greenbytes.de</email></address>
2112      </author>
2113      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2114    </front>
2115    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2116    <x:source basename="p5-range" href="p5-range.xml" />
2117  </reference>
2118
2119  <reference anchor="Part7">
2120    <front>
2121      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2122      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2123        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2124        <address><email>fielding@gbiv.com</email></address>
2125      </author>
2126      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2127        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2128        <address><email>jg@freedesktop.org</email></address>
2129      </author>
2130      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2131        <organization abbrev="HP">Hewlett-Packard Company</organization>
2132        <address><email>JeffMogul@acm.org</email></address>
2133      </author>
2134      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2135        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2136        <address><email>henrikn@microsoft.com</email></address>
2137      </author>
2138      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2139        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2140        <address><email>LMM@acm.org</email></address>
2141      </author>
2142      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2143        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2144        <address><email>paulle@microsoft.com</email></address>
2145      </author>
2146      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2147        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2148        <address><email>timbl@w3.org</email></address>
2149      </author>
2150      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2151        <organization abbrev="W3C">World Wide Web Consortium</organization>
2152        <address><email>ylafon@w3.org</email></address>
2153      </author>
2154      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2155        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2156        <address><email>julian.reschke@greenbytes.de</email></address>
2157      </author>
2158      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2159    </front>
2160    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2161    <x:source basename="p7-auth" href="p7-auth.xml" />
2162  </reference>
2163
2164  <reference anchor="RFC2119">
2165    <front>
2166      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2167      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2168        <organization>Harvard University</organization>
2169        <address><email>sob@harvard.edu</email></address>
2170      </author>
2171      <date month="March" year="1997" />
2172    </front>
2173    <seriesInfo name="BCP" value="14" />
2174    <seriesInfo name="RFC" value="2119" />
2175  </reference>
2176
2177  <reference anchor="RFC5234">
2178    <front>
2179      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2180      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2181        <organization>Brandenburg InternetWorking</organization>
2182        <address>
2183          <email>dcrocker@bbiw.net</email>
2184        </address> 
2185      </author>
2186      <author initials="P." surname="Overell" fullname="Paul Overell">
2187        <organization>THUS plc.</organization>
2188        <address>
2189          <email>paul.overell@thus.net</email>
2190        </address>
2191      </author>
2192      <date month="January" year="2008"/>
2193    </front>
2194    <seriesInfo name="STD" value="68"/>
2195    <seriesInfo name="RFC" value="5234"/>
2196  </reference>
2197 
2198</references>
2199
2200<references title="Informative References">
2201
2202  <reference anchor="RFC1305">
2203    <front>
2204      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2205      <author fullname="David L. Mills" initials="D." surname="Mills">
2206        <organization>University of Delaware, Electrical Engineering Department</organization>
2207        <address><email>mills@udel.edu</email></address>
2208      </author>
2209      <date month="March" year="1992" />
2210    </front>
2211    <seriesInfo name="RFC" value="1305" />
2212  </reference>
2213
2214  <reference anchor="RFC2616">
2215    <front>
2216      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2217      <author fullname="R. Fielding" initials="R." surname="Fielding">
2218        <organization>University of California, Irvine</organization>
2219        <address><email>fielding@ics.uci.edu</email></address>
2220      </author>
2221      <author fullname="J. Gettys" initials="J." surname="Gettys">
2222        <organization>W3C</organization>
2223        <address><email>jg@w3.org</email></address>
2224      </author>
2225      <author fullname="J. Mogul" initials="J." surname="Mogul">
2226        <organization>Compaq Computer Corporation</organization>
2227        <address><email>mogul@wrl.dec.com</email></address>
2228      </author>
2229      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2230        <organization>MIT Laboratory for Computer Science</organization>
2231        <address><email>frystyk@w3.org</email></address>
2232      </author>
2233      <author fullname="L. Masinter" initials="L." surname="Masinter">
2234        <organization>Xerox Corporation</organization>
2235        <address><email>masinter@parc.xerox.com</email></address>
2236      </author>
2237      <author fullname="P. Leach" initials="P." surname="Leach">
2238        <organization>Microsoft Corporation</organization>
2239        <address><email>paulle@microsoft.com</email></address>
2240      </author>
2241      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2242        <organization>W3C</organization>
2243        <address><email>timbl@w3.org</email></address>
2244      </author>
2245      <date month="June" year="1999" />
2246    </front>
2247    <seriesInfo name="RFC" value="2616" />
2248  </reference>
2249
2250  <reference anchor="RFC3864">
2251    <front>
2252      <title>Registration Procedures for Message Header Fields</title>
2253      <author fullname="G. Klyne" initials="G." surname="Klyne">
2254        <organization>Nine by Nine</organization>
2255        <address><email>GK-IETF@ninebynine.org</email></address>
2256      </author>
2257      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2258        <organization>BEA Systems</organization>
2259        <address><email>mnot@pobox.com</email></address>
2260      </author>
2261      <author fullname="J. Mogul" initials="J." surname="Mogul">
2262        <organization>HP Labs</organization>
2263        <address><email>JeffMogul@acm.org</email></address>
2264      </author>
2265      <date month="September" year="2004" />
2266    </front>
2267    <seriesInfo name="BCP" value="90" />
2268    <seriesInfo name="RFC" value="3864" />
2269  </reference>
2270
2271  <reference anchor='RFC5226'>
2272    <front>
2273      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2274      <author initials='T.' surname='Narten' fullname='T. Narten'>
2275        <organization>IBM</organization>
2276        <address><email>narten@us.ibm.com</email></address>
2277      </author>
2278      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2279        <organization>Google</organization>
2280        <address><email>Harald@Alvestrand.no</email></address>
2281      </author>
2282      <date year='2008' month='May' />
2283    </front>
2284    <seriesInfo name='BCP' value='26' />
2285    <seriesInfo name='RFC' value='5226' />
2286  </reference>
2287
2288  <reference anchor='RFC5861'>
2289    <front>
2290      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2291      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2292        <organization>Yahoo! Inc.</organization>
2293        <address><email>mnot@yahoo-inc.com</email></address>
2294      </author>
2295      <date month="April" year="2010"/>
2296    </front>
2297    <seriesInfo name='RFC' value='5861' />
2298  </reference>
2299
2300</references>
2301
2302<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2303<t>
2304  Make the specified age calculation algorithm less conservative.
2305  (<xref target="age.calculations"/>)
2306</t>
2307<t>
2308  Remove requirement to consider Content-Location in successful responses
2309  in order to determine the appropriate response to use.
2310  (<xref target="validation.model" />)
2311</t>
2312<t>
2313  Clarify denial of service attack avoidance requirement.
2314  (<xref target="invalidation.after.updates.or.deletions" />)
2315</t>
2316<t>
2317  Change ABNF productions for header fields to only define the field value.
2318  (<xref target="header.fields"/>)
2319</t>
2320<t>
2321  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2322  anymore, as these aspects never were implemented.
2323  (<xref target="header.warning" />)
2324</t>
2325</section>
2326
2327<?BEGININC p6-cache.abnf-appendix ?>
2328<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2329<figure>
2330<artwork type="abnf" name="p6-cache.parsed-abnf">
2331<x:ref>Age</x:ref> = delta-seconds
2332
2333<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2334 cache-directive ] )
2335
2336<x:ref>Expires</x:ref> = HTTP-date
2337
2338<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2339
2340<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2341
2342<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2343 pragma-directive ] )
2344
2345<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2346 ) )
2347
2348<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2349 )
2350
2351<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2352<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2353<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2354 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2355 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2356 cache-extension
2357<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2358 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2359 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2360 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2361 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2362 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2363
2364<x:ref>delta-seconds</x:ref> = 1*DIGIT
2365
2366<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2367
2368<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2369
2370<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2371<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2372<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2373
2374<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2375
2376<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2377
2378<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2379
2380<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2381<x:ref>warn-code</x:ref> = 3DIGIT
2382<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2383<x:ref>warn-text</x:ref> = quoted-string
2384<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2385 ]
2386</artwork>
2387</figure>
2388<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2389; Age defined but not used
2390; Cache-Control defined but not used
2391; Expires defined but not used
2392; Pragma defined but not used
2393; Vary defined but not used
2394; Warning defined but not used
2395</artwork></figure></section>
2396<?ENDINC p6-cache.abnf-appendix ?>
2397
2398<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2399
2400<section title="Since RFC 2616">
2401  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2402</section>
2403
2404<section title="Since draft-ietf-httpbis-p6-cache-00">
2405<t>
2406  Closed issues:
2407  <list style="symbols">
2408    <t>
2409      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2410    <t>
2411      <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>
2412    <t>
2413      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2414    <t>
2415      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2416    <t>
2417      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2418    <t>
2419      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2420    <t>
2421      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2422    <t>
2423      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2424    <t>
2425      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2426  </list>
2427</t>
2428<t>
2429  Other changes:
2430  <list style="symbols">
2431    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2432        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2433  </list>
2434</t>
2435</section>
2436
2437<section title="Since draft-ietf-httpbis-p6-cache-01">
2438<t>
2439  Closed issues:
2440  <list style="symbols">
2441    <t>
2442      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2443  </list>
2444</t>
2445<t>
2446  Other changes:
2447  <list style="symbols">
2448    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2449      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2450    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2451      specification.</t>
2452  </list>
2453</t>
2454</section>
2455
2456<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2457<t>
2458  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2459  <list style="symbols">
2460    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2461      document.</t>
2462  </list>
2463</t>
2464</section>
2465
2466<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2467<t>
2468  Closed issues:
2469  <list style="symbols">
2470    <t>
2471      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2472  </list>
2473</t>
2474</section>
2475
2476<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2477<t>
2478  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2479  <list style="symbols"> 
2480    <t>
2481      Use "/" instead of "|" for alternatives.
2482    </t>
2483    <t>
2484      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2485      whitespace ("OWS") and required whitespace ("RWS").
2486    </t>
2487    <t>
2488      Rewrite ABNFs to spell out whitespace rules, factor out
2489      header field value format definitions.
2490    </t>
2491  </list>
2492</t>
2493</section>
2494
2495<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2496<t>
2497  This is a total rewrite of this part of the specification.
2498</t>
2499<t>
2500  Affected issues:
2501  <list style="symbols">
2502    <t>
2503      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2504    <t>
2505      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2506    <t>
2507      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2508    <t>
2509      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2510  </list>
2511</t>
2512<t>
2513  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2514  <list style="symbols"> 
2515    <t>
2516      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2517    </t>
2518  </list>
2519</t>
2520</section>
2521
2522<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2523<t>
2524  Closed issues:
2525  <list style="symbols"> 
2526    <t>
2527      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2528      "base for numeric protocol elements"
2529    </t>
2530  </list>
2531</t>
2532<t>
2533  Affected issues:
2534  <list style="symbols">
2535    <t>
2536      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2537      "Vary and non-existant headers"
2538    </t>
2539  </list>
2540</t>
2541</section>
2542
2543<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2544<t>
2545  Closed issues:
2546  <list style="symbols"> 
2547    <t>
2548      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2549      "Definition of 1xx Warn-Codes"
2550    </t>
2551    <t>
2552      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2553      "Content-Location on 304 responses"
2554    </t>
2555    <t>
2556      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2557      "private and no-cache CC directives with headers"
2558    </t>
2559    <t>
2560      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2561      "RFC2047 and warn-text"
2562    </t>
2563  </list>
2564</t>
2565</section>
2566
2567<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2568<t>
2569  Closed issues:
2570  <list style="symbols"> 
2571    <t>
2572      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2573      "serving negotiated responses from cache: header-specific canonicalization"
2574    </t>
2575    <t>
2576      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2577      "Effect of CC directives on history lists"
2578    </t>
2579    <t>
2580      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291" />:
2581      "Cache Extensions can override no-store, etc."
2582    </t>
2583  </list>
2584</t>
2585<t>
2586  Affected issues:
2587  <list style="symbols">
2588    <t>
2589      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2590      Status codes and caching
2591    </t>
2592  </list>
2593</t>
2594<t>
2595  Partly resolved issues:
2596  <list style="symbols"> 
2597    <t>
2598      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2599      "Placement of 13.5.1 and 13.5.2"
2600    </t>
2601  </list>
2602</t>
2603</section>
2604
2605<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2606<t>
2607  Closed issues:
2608  <list style="symbols"> 
2609    <t>
2610      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2611      "Age calculation"
2612    </t>
2613    <t>
2614      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2615      "Clarify differences between / requirements for request and response CC directives"
2616    </t>
2617    <t>
2618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2619      "Caching authenticated responses"
2620    </t>
2621    <t>
2622      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2623      "IANA registry for cache-control directives"
2624    </t>
2625    <t>
2626      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2627      "Heuristic caching of URLs with query components"
2628    </t>
2629  </list>
2630</t>
2631<t>
2632  Partly resolved issues:
2633  <list style="symbols"> 
2634    <t>
2635      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2636      "Term for the requested resource's URI"
2637    </t>
2638  </list>
2639</t>
2640</section>
2641
2642<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2643<t>
2644  Closed issues:
2645  <list style="symbols"> 
2646    <t>
2647      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2648      "Clarify entity / representation / variant terminology"
2649    </t>
2650    <t>
2651      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2652      "consider removing the 'changes from 2068' sections"
2653    </t>
2654    <t>
2655      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2656      "Allowing heuristic caching for new status codes"
2657    </t>
2658    <t>
2659      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2660      "Allowing heuristic caching for new status codes"
2661    </t>
2662    <t>
2663      Clean up TODOs and prose in "Combining Responses."
2664    </t>
2665  </list>
2666</t>
2667</section>
2668
2669<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2670<t>
2671  Closed issues:
2672  <list style="symbols"> 
2673    <t>
2674      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2675      "Text about clock requirement for caches belongs in p6"
2676    </t>
2677  </list>
2678</t>
2679</section>
2680
2681<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2682<t>
2683  Closed issues:
2684  <list style="symbols"> 
2685    <t>
2686      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2687      "Header Classification"
2688    </t>
2689    <t>
2690      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2691      "Clarify 'public'"
2692    </t>
2693  </list>
2694</t>
2695</section>
2696
2697<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2698<t>
2699  Closed issues:
2700  <list style="symbols">
2701    <t>
2702      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2703      "untangle ABNFs for header fields"
2704    </t>
2705  </list>
2706</t>
2707</section>
2708
2709<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2710<t>
2711  Closed issues:
2712  <list style="symbols">
2713    <t>
2714      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2715      "Pragma"
2716    </t>
2717    <t>
2718      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2719      "Mismatch Vary"
2720    </t>
2721    <t>
2722      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2723      "Proxies don't 'understand' methods"
2724    </t>
2725  </list>
2726</t>
2727</section>
2728
2729</section>
2730  </back>
2731</rfc>
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