source: draft-ietf-httpbis/latest/p6-cache.xml @ 1290

Last change on this file since 1290 was 1290, checked in by mnot@…, 9 years ago

Don't talk about "understanding" methods in invalidation; #289

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