source: draft-ietf-httpbis/latest/p6-cache.xml @ 1112

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Work on terminology and requirements language. #234

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