source: draft-ietf-httpbis/latest/p6-cache.xml @ 1273

Last change on this file since 1273 was 1273, checked in by mnot@…, 9 years ago

Align max-age description language.

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