source: draft-ietf-httpbis/latest/p6-cache.xml @ 1287

Last change on this file since 1287 was 1287, checked in by julian.reschke@…, 9 years ago

editorial: remove tabs, tune markup, regen HTML

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