source: draft-ietf-httpbis/latest/p6-cache.xml @ 1368

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bump up document dates; update to latest version of rfc2629.xslt.

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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 "August">
16  <!ENTITY ID-YEAR "2011">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY acks                       "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY effective-request-uri      "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37]>
38<?rfc toc="yes" ?>
39<?rfc symrefs="yes" ?>
40<?rfc sortrefs="yes" ?>
41<?rfc compact="yes"?>
42<?rfc subcompact="no" ?>
43<?rfc linkmailto="no" ?>
44<?rfc editing="no" ?>
45<?rfc comments="yes"?>
46<?rfc inline="yes"?>
47<?rfc rfcedstyle="yes"?>
48<?rfc-ext allow-markup-in-artwork="yes" ?>
49<?rfc-ext include-references-in-index="yes" ?>
50<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
51  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
52<front>
53
54  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
55
56  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
57    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
58    <address>
59      <postal>
60        <street>345 Park Ave</street>
61        <city>San Jose</city>
62        <region>CA</region>
63        <code>95110</code>
64        <country>USA</country>
65      </postal>
66      <email>fielding@gbiv.com</email>
67      <uri>http://roy.gbiv.com/</uri>
68    </address>
69  </author>
70
71  <author initials="J." surname="Gettys" fullname="Jim Gettys">
72    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
73    <address>
74      <postal>
75        <street>21 Oak Knoll Road</street>
76        <city>Carlisle</city>
77        <region>MA</region>
78        <code>01741</code>
79        <country>USA</country>
80      </postal>
81      <email>jg@freedesktop.org</email>
82      <uri>http://gettys.wordpress.com/</uri>
83    </address>
84  </author>
85
86  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
87    <organization abbrev="HP">Hewlett-Packard Company</organization>
88    <address>
89      <postal>
90        <street>HP Labs, Large Scale Systems Group</street>
91        <street>1501 Page Mill Road, MS 1177</street>
92        <city>Palo Alto</city>
93        <region>CA</region>
94        <code>94304</code>
95        <country>USA</country>
96      </postal>
97      <email>JeffMogul@acm.org</email>
98    </address>
99  </author>
100
101  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
102    <organization abbrev="Microsoft">Microsoft Corporation</organization>
103    <address>
104      <postal>
105        <street>1 Microsoft Way</street>
106        <city>Redmond</city>
107        <region>WA</region>
108        <code>98052</code>
109        <country>USA</country>
110      </postal>
111      <email>henrikn@microsoft.com</email>
112    </address>
113  </author>
114
115  <author fullname="Larry Masinter" initials="L." surname="Masinter">
116    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
117    <address>
118      <postal>
119        <street>345 Park Ave</street>
120        <city>San Jose</city>
121        <region>CA</region>
122        <code>95110</code>
123        <country>USA</country>
124      </postal>
125      <email>LMM@acm.org</email>
126      <uri>http://larry.masinter.net/</uri>
127    </address>
128  </author>
129
130  <author fullname="Paul J. Leach" initials="P." surname="Leach">
131    <organization abbrev="Microsoft">Microsoft Corporation</organization>
132    <address>
133      <postal>
134        <street>1 Microsoft Way</street>
135        <city>Redmond</city>
136        <region>WA</region>
137        <code>98052</code>
138      </postal>
139      <email>paulle@microsoft.com</email>
140    </address>
141  </author>
142
143  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
144    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
145    <address>
146      <postal>
147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
148        <street>The Stata Center, Building 32</street>
149        <street>32 Vassar Street</street>
150        <city>Cambridge</city>
151        <region>MA</region>
152        <code>02139</code>
153        <country>USA</country>
154      </postal>
155      <email>timbl@w3.org</email>
156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
157    </address>
158  </author>
159
160  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
161    <organization abbrev="W3C">World Wide Web Consortium</organization>
162    <address>
163      <postal>
164        <street>W3C / ERCIM</street>
165        <street>2004, rte des Lucioles</street>
166        <city>Sophia-Antipolis</city>
167        <region>AM</region>
168        <code>06902</code>
169        <country>France</country>
170      </postal>
171      <email>ylafon@w3.org</email>
172      <uri>http://www.raubacapeu.net/people/yves/</uri>
173    </address>
174  </author>
175
176  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
177    <address>
178      <email>mnot@mnot.net</email>
179      <uri>http://www.mnot.net/</uri>
180    </address>
181  </author>
182
183  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
184    <organization abbrev="greenbytes">greenbytes GmbH</organization>
185    <address>
186      <postal>
187        <street>Hafenweg 16</street>
188        <city>Muenster</city><region>NW</region><code>48155</code>
189        <country>Germany</country>
190      </postal>
191      <phone>+49 251 2807760</phone>
192      <facsimile>+49 251 2807761</facsimile>
193      <email>julian.reschke@greenbytes.de</email>
194      <uri>http://greenbytes.de/tech/webdav/</uri>
195    </address>
196  </author>
197
198  <date month="&ID-MONTH;" year="&ID-YEAR;" />
199  <workgroup>HTTPbis Working Group</workgroup>
200
201<abstract>
202<t>
203   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
204   distributed, collaborative, hypermedia information systems. This document
205   is Part 6 of the seven-part specification that defines the protocol
206   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6
207   defines requirements on HTTP caches and the associated header fields that
208   control cache behavior or indicate cacheable response messages.
209</t>
210</abstract>
211
212<note title="Editorial Note (To be removed by RFC Editor)">
213  <t>
214    Discussion of this draft should take place on the HTTPBIS working group
215    mailing list (ietf-http-wg@w3.org), which is archived at
216    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
217  </t>
218  <t>
219    The current issues list is at
220    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
221    documents (including fancy diffs) can be found at
222    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
223  </t>
224  <t>
225    The changes in this draft are summarized in <xref target="changes.since.15"/>.
226  </t>
227</note>
228
229   </front>
230   <middle>
231
232<section anchor="caching" title="Introduction">
233<t>
234   HTTP is typically used for distributed information systems, where
235   performance can be improved by the use of response caches. This document
236   defines aspects of HTTP/1.1 related to caching and reusing response
237   messages.
238</t>
239
240<section anchor="intro.purpose" title="Purpose">
241<iref item="cache" />
242<t>
243   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
244   subsystem that controls its message storage, retrieval, and deletion. A
245   cache stores cacheable responses in order to reduce the response time and
246   network bandwidth consumption on future, equivalent requests. Any client or
247   server &MAY; employ a cache, though a cache cannot be used by a server that
248   is acting as a tunnel.
249</t>
250<t>
251   Caching would be useless if it did not significantly improve performance.
252   The goal of caching in HTTP/1.1 is to reuse a prior response message to
253   satisfy a current request. In some cases, a stored response can be reused
254   without the need for a network request, reducing latency and network
255   round-trips; a "freshness" mechanism is used for this purpose (see <xref
256   target="expiration.model" />). Even when a new request is required, it is
257   often possible to reuse all or parts of the payload of a prior response to
258   satisfy the request, thereby reducing network bandwidth usage; a
259   "validation" mechanism is used for this purpose (see <xref
260   target="validation.model" />).
261</t>
262</section>
263
264<section anchor="intro.terminology" title="Terminology">
265<t>
266   This specification uses a number of terms to refer to the roles played by
267   participants in, and objects of, HTTP caching.
268</t>
269<t>
270   <iref item="cache" />
271   <x:dfn>cache</x:dfn>
272   <list>
273      <t>A conformant implementation of a HTTP cache. Note that this implies
274        an HTTP/1.1 cache; this specification does not define conformance
275        for HTTP/1.0 caches.</t>
276   </list>
277</t>
278<t anchor="shared.and.non-shared.caches">
279   <iref item="shared cache" />
280   <x:dfn>shared cache</x:dfn>
281   <list>
282      <t>A cache that is accessible to more than one user; usually (but
283        not always) deployed as part of an intermediary.</t>
284   </list>
285</t>
286<t>
287   <iref item="private cache" />
288   <x:dfn>private cache</x:dfn>
289   <list>
290      <t>A cache that is dedicated to a single user.</t>
291   </list>
292</t>
293<t>
294   <iref item="cacheable" />
295   <x:dfn>cacheable</x:dfn>
296   <list>
297      <t>A response is cacheable if a cache is allowed to store a copy of the
298      response message for use in answering subsequent requests. Even when a
299      response is cacheable, there might be additional constraints on whether
300      a cache can use the stored copy to satisfy a particular request.</t>
301   </list>
302</t>
303<t>
304   <iref item="explicit expiration time" />
305   <x:dfn>explicit expiration time</x:dfn>
306   <list>
307      <t>The time at which the origin server intends that a representation
308      no longer be returned by a cache without further validation.</t>
309   </list>
310</t>
311<t>
312   <iref item="heuristic expiration time" />
313   <x:dfn>heuristic expiration time</x:dfn>
314   <list>
315      <t>An expiration time assigned by a cache when no explicit expiration
316      time is available.</t>
317   </list>
318</t>
319<t>
320   <iref item="age" />
321   <x:dfn>age</x:dfn>
322   <list>
323      <t>The age of a response is the time since it was sent by, or
324      successfully validated with, the origin server.</t>
325   </list>
326</t>
327<t>
328   <iref item="first-hand" />
329   <x:dfn>first-hand</x:dfn>
330   <list>
331      <t>A response is first-hand if the freshness model is not in use; i.e.,
332      its age is 0.</t>
333   </list>
334</t>
335<t>
336   <iref item="freshness lifetime" />
337   <x:dfn>freshness lifetime</x:dfn>
338   <list>
339      <t>The length of time between the generation of a response and its
340      expiration time.</t>
341   </list>
342</t>
343<t>
344   <iref item="fresh" />
345   <x:dfn>fresh</x:dfn>
346   <list>
347      <t>A response is fresh if its age has not yet exceeded its freshness
348      lifetime.</t>
349   </list>
350</t>
351<t>
352   <iref item="stale" />
353   <x:dfn>stale</x:dfn>
354   <list>
355      <t>A response is stale if its age has passed its freshness lifetime
356      (either explicit or heuristic).</t>
357   </list>
358</t>
359<t>
360   <iref item="validator" />
361   <x:dfn>validator</x:dfn>
362   <list>
363      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
364      is used to find out whether a stored response is an equivalent copy of
365      a representation.</t>
366   </list>
367</t>
368</section>
369
370<section anchor="intro.requirements" title="Requirements">
371<t>
372   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
373   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
374   document are to be interpreted as described in <xref target="RFC2119"/>.
375</t>
376<t>
377   An implementation is not compliant if it fails to satisfy one or more of
378   the "MUST" or "REQUIRED" level requirements for the protocols it
379   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
380   level and all the "SHOULD" level requirements for its protocols is said to
381   be "unconditionally compliant"; one that satisfies all the "MUST" level
382   requirements but not all the "SHOULD" level requirements for its protocols
383   is said to be "conditionally compliant".
384</t>
385</section>
386
387<section title="Syntax Notation" anchor="notation">
388   <x:anchor-alias value="ALPHA"/>
389   <x:anchor-alias value="CR"/>
390   <x:anchor-alias value="DIGIT"/>
391   <x:anchor-alias value="DQUOTE"/>
392   <x:anchor-alias value="LF"/>
393   <x:anchor-alias value="OCTET"/>
394   <x:anchor-alias value="SP"/>
395   <x:anchor-alias value="VCHAR"/>
396   <x:anchor-alias value="WSP"/>
397<t>
398   This specification uses the ABNF syntax defined in &notation; (which
399   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
400   <xref target="collected.abnf"/> shows the collected ABNF, with the list
401   rule expanded.
402</t>
403<t>
404   The following core rules are included by reference, as defined in <xref
405   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
406   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
407   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
408   sequence of data), SP (space), VCHAR (any visible USASCII character), and
409   WSP (whitespace).
410</t>
411
412<section title="Core Rules" anchor="core.rules">
413   <x:anchor-alias value="quoted-string"/>
414   <x:anchor-alias value="token"/>
415   <x:anchor-alias value="OWS"/>
416<t>
417   The core rules below are defined in &basic-rules;:
418</t>
419<figure><artwork type="abnf2616">
420  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
421  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
422  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
423</artwork></figure>
424</section>
425
426<section title="ABNF Rules defined in other Parts of the Specification"
427    anchor="abnf.dependencies">
428   <x:anchor-alias value="field-name"/>
429   <x:anchor-alias value="HTTP-date"/>
430   <x:anchor-alias value="port"/>
431   <x:anchor-alias value="pseudonym"/>
432   <x:anchor-alias value="uri-host"/>
433<t>
434   The ABNF rules below are defined in other parts:
435</t>
436<figure><!--Part1--><artwork type="abnf2616">
437  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
438  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
439  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
440  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
441  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
442</artwork></figure>
443</section>
444</section>
445
446<section title="Delta Seconds" anchor="delta-seconds">
447<t>
448   The delta-seconds rule specifies a non-negative integer, representing time
449   in seconds.
450</t>
451<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
452  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
453</artwork></figure>
454<t>
455   If an implementation receives a delta-seconds value larger than the largest
456   positive integer it can represent, or if any of its subsequent calculations
457   overflows, it &MUST; consider the value to be 2147483648 (2<x:sup>31</x:sup>).
458   Recipients parsing a delta-seconds value &SHOULD; use an arithmetic type of
459   at least 31 bits of range, and senders &MUST-NOT; send delta-seconds with a
460   value greater than 2147483648.
461</t>
462</section>
463
464</section>
465
466<section anchor="caching.overview" title="Cache Operation">
467
468<section anchor="response.cacheability" title="Response Cacheability">
469<t>
470   A cache &MUST-NOT; store a response to any request, unless:
471   <list style="symbols">
472      <t>The request method is understood by the cache and defined as being
473      cacheable, and</t>
474      <t>the response status code is understood by the cache, and</t>
475      <t>the "no-store" cache directive (see <xref
476      target="header.cache-control" />) does not appear in request or response
477      header fields, and</t>
478      <t>the "private" cache response directive (see <xref
479      target="cache-response-directive" /> does not appear in the response, if
480      the cache is shared, and</t>
481      <t>the "Authorization" header field (see &header-authorization;) does not
482      appear in the request, if the cache is shared, unless the response
483      explicitly allows it (see <xref target="caching.authenticated.responses"
484      />), and</t>
485      <t>the response either:
486         <list style="symbols">
487            <t>contains an Expires header field (see <xref target="header.expires"
488            />), or</t>
489            <t>contains a max-age response cache directive (see <xref
490            target="cache-response-directive" />), or</t>
491            <t>contains a s-maxage response cache directive and the cache is
492            shared, or</t>
493            <t>contains a Cache Control Extension (see <xref
494            target="cache.control.extensions" />) that allows it to be cached,
495            or</t>
496            <t>has a status code that can be served with heuristic freshness
497            (see <xref target="heuristic.freshness" />).</t>
498         </list>
499      </t>
500   </list>
501</t>
502<t>
503   Note that any of the requirements listed above can be overridden by a
504   cache-control extension; see <xref target="cache.control.extensions" />.
505</t>
506<t>
507   In this context, a cache has "understood" a request method or a response
508   status code if it recognises it and implements any cache-specific
509   behavior. In particular, 206 Partial Content responses cannot be cached by
510   an implementation that does not handle partial content (see <xref
511   target="errors.or.incomplete.response.cache.behavior" />).
512</t>
513<t>
514   Note that in normal operation, most caches will not store a response that
515   has neither a cache validator nor an explicit expiration time, as such
516   responses are not usually useful to store. However, caches are not
517   prohibited from storing such responses.
518</t>
519
520<section anchor="errors.or.incomplete.response.cache.behavior" 
521   title="Storing Partial and Incomplete Responses">
522<t>
523   A cache that receives an incomplete response (for example, with fewer bytes
524   of data than specified in a Content-Length header field) can store the response,
525   but &MUST; treat it as a partial response &partial;. Partial responses can
526   be combined as described in &combining-byte-ranges;; the result might be a
527   full response or might still be partial. A cache &MUST-NOT; return a
528   partial response to a client without explicitly marking it as such using
529   the 206 (Partial Content) status code.
530</t>
531<t>
532   A cache that does not support the Range and Content-Range header fields
533   &MUST-NOT; store incomplete or partial responses.
534</t>
535</section>
536
537</section>
538
539
540<section anchor="constructing.responses.from.caches" 
541   title="Constructing Responses from Caches">
542<t>
543   For a presented request, a cache &MUST-NOT; return a stored response,
544   unless:
545   <list style="symbols">
546      <t>The presented effective request URI (&effective-request-uri;) and
547      that of the stored response match, and</t>
548      <t>the request method associated with the stored response allows it to
549      be used for the presented request, and</t>
550      <t>selecting header fields nominated by the stored response (if any)
551      match those presented (see <xref target="caching.negotiated.responses"
552      />), and</t>
553      <t>the presented request and stored response are free from directives
554      that would prevent its use (see <xref target="header.cache-control" />
555      and <xref target="header.pragma"/>), and</t>
556      <t>the stored response is either:
557         <list style="symbols">
558            <t>fresh (see <xref target="expiration.model" />), or</t>
559            <t>allowed to be served stale (see <xref
560            target="serving.stale.responses" />), or</t>
561            <t>successfully validated (see <xref target="validation.model"
562            />).</t>
563         </list>
564      </t>
565  </list>
566</t>
567<t>
568   Note that any of the requirements listed above can be overridden by a
569   cache-control extension; see <xref target="cache.control.extensions" />.
570</t>
571<t>
572   When a stored response is used to satisfy a request without validation,
573   a cache &MUST; include a single Age header field (<xref target="header.age"
574   />) in the response with a value equal to the stored response's
575   current_age; see <xref target="age.calculations" />.
576</t>
577<t>
578   A cache &MUST; write through requests with methods that are unsafe
579   (&safe-methods;) to the origin server; i.e., a cache must not generate
580   a reply to such a request before having forwarded the request and having
581   received a corresponding response.
582</t>
583<t>
584   Also, note that unsafe requests might invalidate already stored responses;
585   see <xref target="invalidation.after.updates.or.deletions" />.
586</t>
587<t>
588   When more than one suitable response is stored, a cache &MUST; use the
589   most recent response (as determined by the Date header field). It can also
590   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
591   no-cache" to disambiguate which response to use.
592</t>
593<t>
594   A cache that does not have a clock available &MUST-NOT; use stored responses
595   without revalidating them on every use. A cache, especially a shared
596   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
597   synchronize its clock with a reliable external standard.
598</t>
599
600</section>
601
602<section anchor="expiration.model" title="Freshness Model">
603<t>
604   When a response is "fresh" in the cache, it can be used to satisfy
605   subsequent requests without contacting the origin server, thereby improving
606   efficiency.
607</t>
608<t>
609   The primary mechanism for determining freshness is for an origin server to
610   provide an explicit expiration time in the future, using either the Expires
611   header field (<xref target="header.expires" />) or the max-age response cache
612   directive (<xref target="cache-response-directive" />). Generally, origin
613   servers will assign future explicit expiration times to responses in the
614   belief that the representation is not likely to change in a semantically
615   significant way before the expiration time is reached.
616</t>
617<t>
618   If an origin server wishes to force a cache to validate every request, it
619   can assign an explicit expiration time in the past to indicate that the
620   response is already stale. Compliant caches will normally validate the
621   cached response before reusing it for subsequent requests (see <xref
622   target="serving.stale.responses" />).
623</t>
624<t>
625   Since origin servers do not always provide explicit expiration times,
626   a cache &MAY; assign a heuristic expiration time when an explicit time is not
627   specified, employing algorithms that use other header field values (such as the
628   Last-Modified time) to estimate a plausible expiration time. This
629   specification does not provide specific algorithms, but does impose
630   worst-case constraints on their results.
631</t>
632<figure>
633<preamble>
634  The calculation to determine if a response is fresh is:
635</preamble>
636<artwork type="code">
637   response_is_fresh = (freshness_lifetime &gt; current_age)
638</artwork>
639</figure>
640<t>
641   The freshness_lifetime is defined in <xref
642   target="calculating.freshness.lifetime" />; the current_age is defined in
643   <xref target="age.calculations" />.
644</t>
645<t>
646   Additionally, clients might need to influence freshness calculation. They
647   can do this using several request cache directives, with the effect of
648   either increasing or loosening constraints on freshness. See <xref
649   target="cache-request-directive" />.
650</t>
651<t>
652   Note that freshness applies only to cache operation; it cannot be used to
653   force a user agent to refresh its display or reload a resource. See <xref
654   target="history.lists" /> for an explanation of the difference between
655   caches and history mechanisms.
656</t>
657
658<section anchor="calculating.freshness.lifetime" 
659   title="Calculating Freshness Lifetime">
660<t>
661   A cache can calculate the freshness lifetime (denoted as
662   freshness_lifetime) of a response by using the first match of:
663   <list style="symbols">
664      <t>If the cache is shared and the s-maxage response cache directive
665      (<xref target="cache-response-directive" />) is present, use its value,
666      or</t>
667      <t>If the max-age response cache directive (<xref
668      target="cache-response-directive" />) is present, use its value, or</t>
669      <t>If the Expires response header field (<xref target="header.expires" />) is
670      present, use its value minus the value of the Date response header field,
671      or</t>
672      <t>Otherwise, no explicit expiration time is present in the response. A
673      heuristic freshness lifetime might be applicable; see <xref
674      target="heuristic.freshness" />.</t>
675   </list>
676</t>
677<t>
678   Note that this calculation is not vulnerable to clock skew, since all of
679   the information comes from the origin server.
680</t>
681
682<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
683<t>
684   If no explicit expiration time is present in a stored response that has a
685   status code whose definition allows heuristic freshness to be used
686   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
687   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
688   use heuristics to determine freshness for responses with status codes that do
689   not explicitly allow it.
690</t>
691<t>
692   When a heuristic is used to calculate freshness lifetime, a cache
693   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
694   its current_age is more than 24 hours and such a warning is not already
695   present.
696</t>
697<t>
698   Also, if the response has a Last-Modified header field (&header-last-modified;),
699   a cache &SHOULD-NOT; use a heuristic expiration value that is more than some
700   fraction of the interval since that time. A typical setting of this fraction
701   might be 10%.
702</t>
703<x:note>
704   <t>
705      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
706      x:sec="13.9"/>) required that caches do not calculate heuristic
707      freshness for URIs with query components (i.e., those containing '?').
708      In practice, this has not been widely implemented. Therefore, servers
709      are encouraged to send explicit directives (e.g., Cache-Control:
710      no-cache) if they wish to preclude caching.
711   </t>
712</x:note>
713</section>
714</section>
715
716<section anchor="age.calculations" title="Calculating Age">
717<t>
718   HTTP/1.1 uses the Age header field to convey the estimated age of the
719   response message when obtained from a cache. The Age field value is the
720   cache's estimate of the amount of time since the response was generated or
721   validated by the origin server. In essence, the Age value is the sum of the
722   time that the response has been resident in each of the caches along the
723   path from the origin server, plus the amount of time it has been in transit
724   along network paths.
725</t>
726<t>
727   The following data is used for the age calculation:
728</t>
729<t>
730   <x:dfn>age_value</x:dfn>
731   <list>
732      <t>
733         The term "age_value" denotes the value of the Age header field (<xref
734         target="header.age"/>), in a form appropriate for arithmetic
735         operation; or 0, if not available.
736      </t>
737   </list>
738</t>
739<t>
740   <x:dfn>date_value</x:dfn>
741   <list>
742      <t>
743         HTTP/1.1 requires origin servers to send a Date header field, if possible,
744         with every response, giving the time at which the response was
745         generated. The term "date_value" denotes the value of the Date
746         header field, in a form appropriate for arithmetic operations. See
747         &header-date; for the definition of the Date header field, and for
748         requirements regarding responses without it.
749      </t>
750   </list>
751</t>
752<t>
753   <x:dfn>now</x:dfn>
754   <list>
755      <t>
756         The term "now" means "the current value of the clock at the host
757         performing the calculation". A cache &SHOULD; use NTP (<xref
758         target="RFC1305"/>) or some similar protocol to synchronize its
759         clocks to a globally accurate time standard.
760      </t>
761   </list>
762</t>
763<t>
764   <x:dfn>request_time</x:dfn>
765   <list>
766      <t>
767         The current value of the clock at the host at the time the request
768         resulting in the stored response was made.
769      </t>
770   </list>
771</t>
772<t>
773   <x:dfn>response_time</x:dfn>
774   <list>
775      <t>
776         The current value of the clock at the host at the time the response
777         was received.
778      </t>
779   </list>
780</t>
781<t>
782   A response's age can be calculated in two entirely independent ways:
783   <list style="numbers">
784      <t>the "apparent_age": response_time minus date_value, if the local
785      clock is reasonably well synchronized to the origin server's clock. If
786      the result is negative, the result is replaced by zero.</t>
787      <t>the "corrected_age_value", if all of the caches along the response
788      path implement HTTP/1.1. A cache &MUST; interpret this value relative
789      to the time the request was initiated, not the time that the response
790      was received.</t>
791   </list>
792</t>
793<figure>
794<artwork type="code">
795  apparent_age = max(0, response_time - date_value);
796
797  response_delay = response_time - request_time;
798  corrected_age_value = age_value + response_delay; 
799</artwork>
800</figure>
801<figure>
802<preamble>These are combined as</preamble>
803<artwork type="code">
804  corrected_initial_age = max(apparent_age, corrected_age_value);
805</artwork></figure>
806<t>
807   The current_age of a stored response can then be calculated by adding the
808   amount of time (in seconds) since the stored response was last validated by
809   the origin server to the corrected_initial_age.
810</t>
811<figure><artwork type="code">
812  resident_time = now - response_time;
813  current_age = corrected_initial_age + resident_time;
814</artwork></figure>
815</section>
816
817<section anchor="serving.stale.responses" title="Serving Stale Responses">
818<t>
819   A "stale" response is one that either has explicit expiry information or is
820   allowed to have heuristic expiry calculated, but is not fresh according to
821   the calculations in <xref target="expiration.model" />.
822</t>
823<t>
824   A cache &MUST-NOT; return a stale response if it is prohibited by an
825   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
826   directive, a "must-revalidate" cache-response-directive, or an applicable
827   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
828   target="cache-response-directive"/>).
829</t>
830<t>
831   A cache &SHOULD-NOT; return stale responses unless it is disconnected
832   (i.e., it cannot contact the origin server or otherwise find a forward
833   path) or doing so is explicitly allowed (e.g., by the max-stale request
834   directive; see <xref target="cache-request-directive" />).
835</t>
836<t>
837   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
838   <xref target="header.warning" />) to stale responses. Likewise, a cache
839   &SHOULD; add the 112 warn-code to stale responses if the cache is
840   disconnected.
841</t>
842<t>
843   If a cache receives a first-hand response (either an entire response, or a
844   304 (Not Modified) response) that it would normally forward to the
845   requesting client, and the received response is no longer fresh, the cache
846   &SHOULD; forward it to the requesting client without adding a new Warning
847   (but without removing any existing Warning header fields). A cache &SHOULD-NOT;
848   attempt to validate a response simply because that response became stale in
849   transit.
850</t>
851</section>
852</section>
853
854<section anchor="validation.model" title="Validation Model">
855<t>
856   When a cache has one or more stored responses for a requested URI, but
857   cannot serve any of them (e.g., because they are not fresh, or one cannot
858   be selected; see <xref target="caching.negotiated.responses"/>), it can use
859   the conditional request mechanism &conditional; in the forwarded request to
860   give the origin server an opportunity to both select a valid stored
861   response to be used, and to update it. This process is known as
862   "validating" or "revalidating" the stored response.
863</t>
864<t>
865   When sending such a conditional request, a cache &SHOULD; add an
866   If-Modified-Since header field whose value is that of the Last-Modified header
867   field from the selected (see <xref target="caching.negotiated.responses"/>)
868   stored response, if available.
869</t>
870<t>
871   Additionally, a cache &SHOULD; add an If-None-Match header field whose value is
872   that of the ETag header field(s) from all responses stored for the requested URI,
873   if present. However, if any of the stored responses contains only partial
874   content, the cache &SHOULD-NOT; include its entity-tag in the If-None-Match
875   header field unless the request is for a range that would be fully
876   satisfied by that stored response.
877</t>
878<t>
879   A 304 (Not Modified) response status code indicates that the stored
880   response can be updated and reused; see <xref target="combining.responses"/>.
881</t>
882<t>
883   A full response (i.e., one with a response body) indicates that none of the
884   stored responses nominated in the conditional request is suitable. Instead,
885   a cache &SHOULD; use the full response to satisfy the request and &MAY; 
886   replace the stored response.
887</t>
888<t>
889   If a cache receives a 5xx response while attempting to validate a response,
890   it &MAY; either forward this response to the requesting client, or act as
891   if the server failed to respond. In the latter case, it &MAY; return a
892   previously stored response (see <xref target="serving.stale.responses" />).
893</t>
894</section>
895
896<section anchor="invalidation.after.updates.or.deletions" 
897   title="Request Methods that Invalidate">
898<t>
899   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
900   have the potential for changing state on the origin server, intervening
901   caches can use them to keep their contents up-to-date.
902</t>
903<t>
904   A cache &MUST; invalidate the effective Request URI
905   (&effective-request-uri;) as well as the URI(s) in the Location
906   and Content-Location header fields (if present) when a non-error
907   response to a request with an unsafe method is received.
908</t>
909<t>
910   However, a cache &MUST-NOT; invalidate a URI from a
911   Location or Content-Location header field if the host part of that URI
912   differs from the host part in the effective request URI
913   (&effective-request-uri;). This helps prevent denial of service attacks.
914</t>
915<t>
916   A cache &SHOULD; invalidate the effective request URI
917   (&effective-request-uri;) when it receives a non-error response
918   to a request with a method whose safety is unknown.
919</t>
920<t>
921   Here, a "non-error response" is one with a 2xx or 3xx status code.
922   "Invalidate" means that the cache will either remove all stored
923   responses related to the effective request URI, or will mark these as
924   "invalid" and in need of a mandatory validation before they can be returned
925   in response to a subsequent request.
926</t>
927<t>
928   Note that this does not guarantee that all appropriate responses are
929   invalidated. For example, the request that caused the change at the origin
930   server might not have gone through the cache where a response is stored.
931</t>
932</section>
933
934<section anchor="caching.authenticated.responses" 
935   title="Shared Caching of Authenticated Responses">
936
937<t>
938   A shared cache &MUST-NOT; use a cached response to a request with an
939   Authorization header field (&header-authorization;) to satisfy any subsequent
940   request unless a cache directive that allows such responses to be stored is
941   present in the response.
942</t>
943
944<t>
945   In this specification, the following Cache-Control response directives
946   (<xref target="cache-response-directive"/>) have such an effect:
947   must-revalidate, public, s-maxage.
948</t>
949
950<t>
951   Note that cached responses that contain the "must-revalidate" and/or
952   "s-maxage" response directives are not allowed to be served stale (<xref
953   target="serving.stale.responses"/>) by shared caches. In particular, a
954   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
955   used to satisfy a subsequent request without revalidating it on the origin
956   server.
957</t>
958</section>
959
960<section anchor="caching.negotiated.responses" 
961   title="Caching Negotiated Responses">
962<t>
963   When a cache receives a request that can be satisfied by a stored response
964   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
965   use that response unless all of the selecting header fields nominated by
966   the Vary header field match in both the original request (i.e., that associated
967   with the stored response), and the presented request.
968</t>
969<t>
970   The selecting header fields from two requests are defined to match if and
971   only if those in the first request can be transformed to those in the
972   second request by applying any of the following:
973   <list style="symbols">
974      <t>
975         adding or removing whitespace, where allowed in the header field's syntax
976      </t>
977      <t>
978         combining multiple header fields with the same field name
979         (see &header-fields;)
980      </t>
981      <t>
982         normalizing both header field values in a way that is known to have
983         identical semantics, according to the header field's specification (e.g.,
984         re-ordering field values when order is not significant;
985         case-normalization, where values are defined to be case-insensitive)
986      </t>
987  </list>
988</t>
989<t>
990   If (after any normalization that might take place) a header field is absent
991   from a request, it can only match another request if it is also absent
992   there.
993</t>
994<t>
995   A Vary header field-value of "*" always fails to match, and subsequent
996   requests to that resource can only be properly interpreted by the origin
997   server.
998</t>
999<t>
1000   The stored response with matching selecting header fields is known as the
1001   selected response.
1002</t>
1003<t>
1004   If multiple selected responses are available, the most recent response
1005   (as determined by the Date header field) is used; see <xref 
1006   target="constructing.responses.from.caches"/>.
1007</t>
1008<t>
1009   If no selected response is available, the cache &MAY; forward the presented
1010   request to the origin server in a conditional request; see <xref
1011   target="validation.model"/>.
1012</t>
1013</section>
1014
1015<section anchor="combining.responses" title="Combining Responses">
1016<t>
1017   When a cache receives a 304 (Not Modified) response or a 206 (Partial
1018   Content) response (in this section, the "new" response"), it needs to
1019   create an updated response by combining the stored response with the new
1020   one, so that the updated response can be used to satisfy the request, and
1021   potentially update the cached response.
1022</t>
1023<t>
1024   If the new response contains an ETag, it identifies the stored response to
1025   use. <cref anchor="TODO-mention-CL">might need language about
1026   Content-Location here</cref><cref
1027   anchor="TODO-select-for-combine">Shouldn't this be the selected
1028   response?</cref>
1029</t>
1030<t>
1031   When the new response's status code is 206 (partial content), a cache
1032   &MUST-NOT; combine it with the old response if either response does not
1033   have a validator, and &MUST-NOT; combine it with the old response when
1034   those validators do not match with the strong comparison function
1035   (see &weak-and-strong;).
1036</t>
1037<t>
1038   The stored response header fields are used as those of the updated response,
1039   except that
1040   <list style="symbols">
1041      <t>a cache &MUST; delete any stored Warning header fields with warn-code 1xx (see <xref
1042      target="header.warning" />).</t>
1043      <t>a cache &MUST; retain any stored Warning header fields with warn-code 2xx.</t>
1044      <t>a cache &MUST; use other header fields provided in the new response to replace all
1045      instances of the corresponding header fields from the stored response.</t>
1046   </list>
1047</t>
1048<t>
1049   A cache &MUST; use the updated response header fields to replace those of the stored
1050   response (unless the stored response is removed). In
1051   the case of a 206 response, a cache &MAY; store the combined representation.
1052</t>
1053</section>
1054
1055</section>
1056
1057<section anchor="header.fields" title="Header Field Definitions">
1058<t>
1059   This section defines the syntax and semantics of HTTP/1.1 header fields
1060   related to caching.
1061</t>
1062
1063<section anchor="header.age" title="Age">
1064   <iref item="Age header field" primary="true" x:for-anchor="" />
1065   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1066   <x:anchor-alias value="Age"/>
1067   <x:anchor-alias value="age-value"/>
1068<t>
1069   The "Age" header field conveys the sender's estimate of the amount
1070   of time since the response was generated or successfully validated at the
1071   origin server. Age values are calculated as specified in <xref
1072   target="age.calculations" />.
1073</t>
1074<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1075  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1076</artwork></figure>
1077<t>
1078  Age field-values are non-negative integers, representing time in seconds
1079  (see <xref target="delta-seconds"/>).
1080</t>
1081<t>
1082   The presence of an Age header field in a response implies that a response
1083   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1084   might not implement the Age header field.
1085</t>
1086</section>
1087
1088<section anchor="header.cache-control" title="Cache-Control">
1089   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1090   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1091      x:for-anchor="" />
1092   <x:anchor-alias value="Cache-Control"/>
1093   <x:anchor-alias value="cache-directive"/>
1094   <x:anchor-alias value="cache-extension"/>
1095   <x:anchor-alias value="cache-request-directive"/>
1096   <x:anchor-alias value="cache-response-directive"/>
1097<t>
1098   The "Cache-Control" header field is used to specify directives for
1099   caches along the request/response chain. Such cache directives are
1100   unidirectional in that the presence of a directive in a request does not
1101   imply that the same directive is to be given in the response.
1102</t>
1103<t>
1104   A cache &MUST; obey the requirements of the Cache-Control
1105   directives defined in this section. See <xref
1106   target="cache.control.extensions"/> for information about how Cache-Control
1107   directives defined elsewhere are handled.
1108</t>
1109<x:note>
1110   <t>
1111       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1112       might only implement Pragma: no-cache (see <xref target="header.pragma"
1113       />).
1114   </t>
1115</x:note>
1116<t>
1117   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1118   through in forwarded messages, regardless of their
1119   significance to that application, since the directives might be applicable
1120   to all recipients along the request/response chain. It is not possible to
1121   target a directive to a specific cache.
1122</t>
1123<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1124  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1125
1126  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1127     / <x:ref>cache-response-directive</x:ref>
1128
1129  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1130</artwork></figure>
1131
1132<section anchor="cache-request-directive" 
1133   title="Request Cache-Control Directives">
1134   <x:anchor-alias value="cache-request-directive" />
1135
1136<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1137   subitem="cache-request-directive" />
1138  <x:ref>cache-request-directive</x:ref> =
1139       "no-cache"
1140     / "no-store"
1141     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1142     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1143     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1144     / "no-transform"
1145     / "only-if-cached"
1146     / <x:ref>cache-extension</x:ref>
1147</artwork></figure>
1148
1149<t>
1150   <x:dfn>no-cache</x:dfn>
1151   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1152   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1153   <list>
1154      <t>The no-cache request directive indicates that a cache &MUST-NOT; 
1155      use a stored response to satisfy the request without successful
1156      validation on the origin server.</t> 
1157   </list>
1158</t>
1159<t>
1160   <x:dfn>no-store</x:dfn>
1161   <iref item="Cache Directives" primary="true" subitem="no-store" />
1162   <iref item="no-store" primary="true" subitem="Cache Directive" />
1163   <list>
1164      <t>The no-store request directive indicates that a cache &MUST-NOT;
1165      store any part of either this request or any response to it. This
1166      directive applies to both private and shared caches. "&MUST-NOT;
1167      store" in this context means that the cache &MUST-NOT; intentionally
1168      store the information in non-volatile storage, and &MUST; make a
1169      best-effort attempt to remove the information from volatile storage as
1170      promptly as possible after forwarding it.</t>
1171      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1172      privacy. In particular, malicious or compromised caches might not
1173      recognize or obey this directive, and communications networks might be
1174      vulnerable to eavesdropping.</t>
1175      <t>Note that if a request containing this directive is satisfied from a
1176      cache, the no-store request directive does not apply to the already
1177      stored response.</t>
1178   </list>
1179</t>
1180<t>
1181   <x:dfn>max-age</x:dfn>
1182   <iref item="Cache Directives" primary="true" subitem="max-age" />
1183   <iref item="max-age" primary="true" subitem="Cache Directive" />
1184   <list>
1185      <t>The max-age request directive indicates that the client is unwilling to
1186      accept a response whose age is greater than the specified number of
1187      seconds. Unless the max-stale request directive is also present, the
1188      client is not willing to accept a stale response.</t>
1189   </list>
1190</t>
1191<t>
1192   <x:dfn>max-stale</x:dfn>
1193   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1194   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1195   <list>
1196      <t>The max-stale request directive indicates that the client is willing
1197      to accept a response that has exceeded its expiration time. If max-stale
1198      is assigned a value, then the client is willing to accept a response
1199      that has exceeded its expiration time by no more than the specified
1200      number of seconds. If no value is assigned to max-stale, then the client
1201      is willing to accept a stale response of any age.</t>
1202   </list>
1203</t>
1204<t>
1205   <x:dfn>min-fresh</x:dfn>
1206   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1207   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1208   <list>
1209      <t>The min-fresh request directive indicates that the client is willing
1210      to accept a response whose freshness lifetime is no less than its
1211      current age plus the specified time in seconds. That is, the client
1212      wants a response that will still be fresh for at least the specified
1213      number of seconds.</t>
1214   </list>
1215</t>
1216<t>
1217   <x:dfn>no-transform</x:dfn>
1218   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1219   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1220   <list>
1221      <t>The no-transform request directive indicates that an intermediary
1222        (whether or not it implements a cache) &MUST-NOT; change the
1223        Content-Encoding, Content-Range or Content-Type request header fields,
1224        nor the request representation.</t>
1225   </list>
1226</t>
1227<t>
1228   <x:dfn>only-if-cached</x:dfn>
1229   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1230   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1231   <list>
1232      <t>The only-if-cached request directive indicates that the client only
1233      wishes to obtain a stored response. If it receives this directive, a
1234      cache &SHOULD; either respond using a stored response that is consistent
1235      with the other constraints of the request, or respond with a 504
1236      (Gateway Timeout) status code. If a group of caches is being operated as
1237      a unified system with good internal connectivity, a member cache &MAY;
1238      forward such a request within that group of caches.</t>
1239   </list>
1240</t>
1241</section>
1242
1243<section anchor="cache-response-directive" 
1244   title="Response Cache-Control Directives">
1245   <x:anchor-alias value="cache-response-directive" />
1246
1247<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1248   subitem="cache-response-directive" />
1249  <x:ref>cache-response-directive</x:ref> =
1250       "public"
1251     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1252     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1253     / "no-store"
1254     / "no-transform"
1255     / "must-revalidate"
1256     / "proxy-revalidate"
1257     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1258     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1259     / <x:ref>cache-extension</x:ref>
1260</artwork></figure>
1261
1262<t>
1263   <x:dfn>public</x:dfn>
1264   <iref item="Cache Directives" primary="true" subitem="public" />
1265   <iref item="public" primary="true" subitem="Cache Directive" />
1266   <list>
1267      <t>The public response directive indicates that a response whose
1268        associated request contains an 'Authentication' header &MAY; be
1269        stored (see <xref target="caching.authenticated.responses" />).</t>
1270  </list>
1271</t>
1272<t>
1273   <x:dfn>private</x:dfn>
1274   <iref item="Cache Directives" primary="true" subitem="private" />
1275   <iref item="private" primary="true" subitem="Cache Directive" />
1276   <list>
1277      <t>The private response directive indicates that the response message is
1278      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1279      private cache &MAY; store the response.</t>
1280      <t>If the private response directive specifies one or more field-names,
1281      this requirement is limited to the field-values associated with the
1282      listed response header fields. That is, a shared cache &MUST-NOT; store
1283      the specified field-names(s), whereas it &MAY; store the remainder of the
1284      response message.</t>
1285      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1286      the response can be stored; it cannot ensure the privacy of the message
1287      content. Also, private response directives with field-names are often
1288      handled by implementations as if an unqualified private directive was
1289      received; i.e., the special handling for the qualified form is not
1290      widely implemented.</t>
1291   </list>
1292</t>
1293<t>
1294   <x:dfn>no-cache</x:dfn>
1295   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1296   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1297   <list>
1298      <t>The no-cache response directive indicates that the response MUST NOT
1299      be used to satisfy a subsequent request without successful validation on
1300      the origin server. This allows an origin server to prevent a cache from
1301      using it to satisfy a request without contacting it, even by caches that
1302      have been configured to return stale responses.</t>
1303      <t>If the no-cache response directive specifies one or more field-names,
1304      this requirement is limited to the field-values associated with the
1305      listed response header fields. That is, a cache &MUST-NOT; send the
1306      specified field-name(s) in the response to a subsequent request without successful
1307      validation on the origin server. This allows an origin server to prevent
1308      the re-use of certain header fields in a response, while still allowing
1309      caching of the rest of the response.</t>
1310      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1311      this directive. Also, no-cache response directives with field-names are
1312      often handled by implementations as if an unqualified no-cache directive
1313      was received; i.e., the special handling for the qualified form is not
1314      widely implemented. </t>
1315   </list>
1316</t>
1317<t>
1318   <x:dfn>no-store</x:dfn>
1319   <iref item="Cache Directives" primary="true" subitem="no-store" />
1320   <iref item="no-store" primary="true" subitem="Cache Directive" />
1321   <list>
1322      <t>The no-store response directive indicates that a cache &MUST-NOT;
1323      store any part of either the immediate request or response. This
1324      directive applies to both private and shared caches. "&MUST-NOT;
1325      store" in this context means that the cache &MUST-NOT; intentionally
1326      store the information in non-volatile storage, and &MUST; make a
1327      best-effort attempt to remove the information from volatile storage as
1328      promptly as possible after forwarding it.</t>
1329      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1330      privacy. In particular, malicious or compromised caches might not
1331      recognize or obey this directive, and communications networks might be
1332      vulnerable to eavesdropping.</t>
1333   </list>
1334</t>
1335<t>
1336   <x:dfn>must-revalidate</x:dfn>
1337   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1338   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1339   <list>
1340      <t>The must-revalidate response directive indicates that once it has
1341      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1342      requests without successful validation on the origin server.</t>
1343      <t>The must-revalidate directive is necessary to support reliable
1344      operation for certain protocol features. In all circumstances a
1345      cache &MUST; obey the must-revalidate directive; in particular,
1346      if a cache cannot reach the origin server for any reason, it &MUST;
1347      generate a 504 (Gateway Timeout) response.</t>
1348      <t>A server &SHOULD; send the must-revalidate directive if and only if
1349      failure to validate a request on the representation could result in
1350      incorrect operation, such as a silently unexecuted financial
1351      transaction.</t>
1352   </list>
1353</t>
1354<t>
1355   <x:dfn>proxy-revalidate</x:dfn>
1356   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1357   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1358   <list>
1359      <t>The proxy-revalidate response directive has the same meaning as the
1360      must-revalidate response directive, except that it does not apply to
1361      private caches.</t>
1362   </list>
1363</t>
1364<t>
1365   <x:dfn>max-age</x:dfn>
1366   <iref item="Cache Directives" primary="true" subitem="max-age" />
1367   <iref item="max-age" primary="true" subitem="Cache Directive" />
1368   <list>
1369      <t>The max-age response directive indicates that the response is to be
1370      considered stale after its age is greater than the specified number of
1371      seconds.</t>
1372   </list>
1373</t>
1374<t>
1375   <x:dfn>s-maxage</x:dfn>
1376   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1377   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1378   <list>
1379      <t>The s-maxage response directive indicates that, in shared caches, the
1380      maximum age specified by this directive overrides the maximum age
1381      specified by either the max-age directive or the Expires header field. The
1382      s-maxage directive also implies the semantics of the proxy-revalidate
1383      response directive.</t>
1384   </list>
1385</t>
1386<t>
1387   <x:dfn>no-transform</x:dfn>
1388   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1389   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1390   <list>
1391      <t>The no-transform response directive indicates that an intermediary
1392      (regardless of whether it implements a cache) &MUST-NOT; change the
1393      Content-Encoding, Content-Range or Content-Type response header fields,
1394      nor the response representation.</t>
1395   </list>
1396</t>
1397
1398</section>
1399
1400<section anchor="cache.control.extensions" title="Cache Control Extensions">
1401<t>
1402   The Cache-Control header field can be extended through the use of one or
1403   more cache-extension tokens, each with an optional value. Informational
1404   extensions (those that do not require a change in cache behavior) can be
1405   added without changing the semantics of other directives. Behavioral
1406   extensions are designed to work by acting as modifiers to the existing base
1407   of cache directives. Both the new directive and the standard directive are
1408   supplied, such that applications that do not understand the new directive
1409   will default to the behavior specified by the standard directive, and those
1410   that understand the new directive will recognize it as modifying the
1411   requirements associated with the standard directive. In this way,
1412   extensions to the cache-control directives can be made without requiring
1413   changes to the base protocol.
1414</t>
1415<t>
1416   This extension mechanism depends on an HTTP cache obeying all of the
1417   cache-control directives defined for its native HTTP-version, obeying
1418   certain extensions, and ignoring all directives that it does not
1419   understand.
1420</t>
1421<t>
1422   For example, consider a hypothetical new response directive called
1423   "community" that acts as a modifier to the private directive. We define
1424   this new directive to mean that, in addition to any private cache, any
1425   cache that is shared only by members of the community named within its
1426   value may cache the response. An origin server wishing to allow the UCI
1427   community to use an otherwise private response in their shared cache(s)
1428   could do so by including
1429</t>
1430<figure><artwork type="example">
1431  Cache-Control: private, community="UCI"
1432</artwork></figure>
1433<t>
1434   A cache seeing this header field will act correctly even if the cache does
1435   not understand the community cache-extension, since it will also see and
1436   understand the private directive and thus default to the safe behavior.
1437</t>
1438<t>
1439   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1440   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1441   combined with standard directives (or the response's default cacheability)
1442   such that the cache behavior will remain minimally correct even if the
1443   cache does not understand the extension(s).
1444</t>
1445<t>
1446   The HTTP Cache Directive Registry defines the name space for the cache
1447   directives.
1448</t>
1449<t>
1450   A registration &MUST; include the following fields:
1451   <list style="symbols">
1452      <t>Cache Directive Name</t>
1453      <t>Pointer to specification text</t>
1454   </list>
1455</t>
1456<t>
1457   Values to be added to this name space are subject to IETF review (<xref
1458   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1459</t>
1460<t>
1461   The registry itself is maintained at <eref
1462   target="http://www.iana.org/assignments/http-cache-directives"/>.
1463</t>
1464</section>
1465
1466</section>
1467
1468<section anchor="header.expires" title="Expires">
1469   <iref item="Expires header field" primary="true" x:for-anchor="" />
1470   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1471   <x:anchor-alias value="Expires"/>
1472<t>
1473   The "Expires" header field gives the date/time after which the
1474   response is considered stale. See <xref target="expiration.model" /> for
1475   further discussion of the freshness model.
1476</t>
1477<t>
1478   The presence of an Expires field does not imply that the original resource
1479   will change or cease to exist at, before, or after that time.
1480</t>
1481<t>
1482   The field-value is an absolute date and time as defined by HTTP-date in
1483   &full-date;; a sender &MUST; use the rfc1123-date format.
1484</t>
1485<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1486  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1487</artwork></figure>
1488<figure>
1489  <preamble>For example</preamble>
1490<artwork type="example">
1491  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1492</artwork></figure>
1493<x:note>
1494   <t>
1495       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1496       max-age directive (see <xref target="cache-response-directive" />),
1497       that directive overrides the Expires field. Likewise, the s-maxage
1498       directive overrides Expires in shared caches.
1499   </t>
1500</x:note>
1501<t>
1502   A server &SHOULD-NOT; send Expires dates more than one year in the
1503   future.
1504</t>
1505<t>
1506   A cache &MUST; treat other invalid date formats,
1507   especially including the value "0", as in the past (i.e., "already
1508   expired").
1509</t>
1510</section>
1511
1512<section anchor="header.pragma" title="Pragma">
1513   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1514   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1515   <x:anchor-alias value="extension-pragma"/>
1516   <x:anchor-alias value="Pragma"/>
1517   <x:anchor-alias value="pragma-directive"/>
1518<t>
1519   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1520   caches, so that clients can specify a "no-cache" request that they will
1521   understand (as Cache-Control was not defined until HTTP/1.1). When the
1522   Cache-Control header is also present and understood in a request, Pragma is
1523   ignored.
1524</t>
1525<t>
1526   In HTTP/1.0, Pragma was defined as an extensible field for
1527   implementation-specified directives for recipients. This specification
1528   deprecates such extensions to improve interoperability.
1529</t>
1530<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"/>
1531  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1532  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1533  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1534</artwork></figure>
1535<t>
1536   When the Cache-Control header is not present in a request, the no-cache
1537   request pragma-directive &MUST; have the same effect on caches as if
1538   "Cache-Control: no-cache" were present (see <xref
1539   target="cache-request-directive" />).
1540</t>
1541<t>
1542   When sending a no-cache request, a client &SHOULD; include both pragma and
1543   cache-control directives unless Cache-Control: no-cache is purposefully
1544   omitted to target other Cache-Control response directives at HTTP/1.1
1545   caches. For example:
1546</t>
1547<figure>
1548<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1549GET / HTTP/1.1
1550Host: www.example.com
1551Cache-Control: max-age=30
1552Pragma: no-cache
1553
1554</artwork>
1555</figure>
1556<t>
1557   will constrain HTTP/1.1 caches to serve a response no older than 30
1558   seconds, while precluding implementations that do not understand
1559   Cache-Control from serving a cached response.
1560</t>
1561<x:note>
1562   <t>
1563      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
1564      specified, it does not provide a reliable replacement for
1565      "Cache-Control: no-cache" in them.
1566   </t>
1567</x:note>
1568</section>
1569
1570<section anchor="header.vary" title="Vary">
1571   <iref item="Vary header field" primary="true" x:for-anchor="" />
1572   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1573   <x:anchor-alias value="Vary"/>
1574<t>
1575   The "Vary" header field conveys the set of header fields
1576   that were used to select the representation.
1577</t>
1578<t>
1579   Caches use this information, in part, to determine whether a stored
1580   response can be used to satisfy a given request; see <xref
1581   target="caching.negotiated.responses" />. determines, while the response is
1582   fresh, whether a cache is permitted to use the response to reply to a
1583   subsequent request without validation; see <xref
1584   target="caching.negotiated.responses" />.
1585</t>
1586<t>
1587   In uncacheable or stale responses, the Vary field value advises the user
1588   agent about the criteria that were used to select the representation.
1589</t>
1590<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1591  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1592</artwork></figure>
1593<t>
1594   The set of header fields named by the Vary field value is known as the
1595   selecting header fields.
1596</t>
1597<t>
1598   A server &SHOULD; include a Vary header field with any cacheable response
1599   that is subject to server-driven negotiation. Doing so allows a cache to
1600   properly interpret future requests on that resource and informs the user
1601   agent about the presence of negotiation on that resource. A server &MAY;
1602   include a Vary header field with a non-cacheable response that is subject
1603   to server-driven negotiation, since this might provide the user agent with
1604   useful information about the dimensions over which the response varies at
1605   the time of the response.
1606</t>
1607<t>
1608   A Vary field value of "*" signals that unspecified parameters not limited
1609   to the header fields (e.g., the network address of the client), play a
1610   role in the selection of the response representation; therefore, a cache
1611   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1612   generate the "*" value.
1613</t>
1614<t>
1615   The field-names given are not limited to the set of standard header
1616   fields defined by this specification. Field names are case-insensitive.
1617</t>
1618</section>
1619
1620<section anchor="header.warning" title="Warning">
1621   <iref item="Warning header field" primary="true" x:for-anchor="" />
1622   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1623   <x:anchor-alias value="Warning"/>
1624   <x:anchor-alias value="warning-value"/>
1625   <x:anchor-alias value="warn-agent"/>
1626   <x:anchor-alias value="warn-code"/>
1627   <x:anchor-alias value="warn-date"/>
1628   <x:anchor-alias value="warn-text"/>
1629<t>
1630   The "Warning" header field is used to carry additional information
1631   about the status or transformation of a message that might not be reflected
1632   in the message. This information is typically used to warn about possible
1633   incorrectness introduced by caching operations or transformations applied
1634   to the payload of the message.
1635</t>
1636<t>
1637   Warnings can be used for other purposes, both cache-related and otherwise.
1638   The use of a warning, rather than an error status code, distinguishes these
1639   responses from true failures.
1640</t>
1641<t>
1642   Warning header fields can in general be applied to any message, however some
1643   warn-codes are specific to caches and can only be applied to response
1644   messages.
1645</t>
1646<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"/>
1647  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1648 
1649  <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>
1650                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1651 
1652  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1653  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1654                  ; the name or pseudonym of the server adding
1655                  ; the Warning header field, for use in debugging
1656  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1657  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1658</artwork></figure>
1659<t>
1660   Multiple warnings can be attached to a response (either by the origin
1661   server or by a cache), including multiple warnings with the same code
1662   number, only differing in warn-text.
1663</t>
1664<t>
1665   When this occurs, the user agent &SHOULD; inform the user of as many of
1666   them as possible, in the order that they appear in the response.
1667</t>
1668<t>
1669   Systems that generate multiple Warning header fields &SHOULD; order them with
1670   this user agent behavior in mind. New Warning header fields &SHOULD; be added
1671   after any existing Warning headers fields.
1672</t>
1673<t>
1674   Warnings are assigned three digit warn-codes. The first digit indicates
1675   whether the Warning is required to be deleted from a stored response after
1676   validation:
1677   <list style="symbols">
1678      <t>1xx Warnings describe the freshness or validation status of the
1679      response, and so &MUST; be deleted by a cache after validation. They can
1680      only be generated by a cache when validating a cached entry, and
1681      &MUST-NOT; be generated in any other situation.</t>
1682      <t>2xx Warnings describe some aspect of the representation that is not
1683      rectified by a validation (for example, a lossy compression of the
1684      representation) and &MUST-NOT; be deleted by a cache after validation,
1685      unless a full response is returned, in which case they &MUST; be.</t>
1686   </list>
1687</t>
1688<t>
1689   If an implementation sends a message with one or more Warning header fields to a
1690   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1691   in each warning-value a warn-date that matches the Date header field in the
1692   message.
1693</t>
1694<t>
1695   If a system receives a message with a warning-value that includes
1696   a warn-date, and that warn-date is different from the Date value in the
1697   response, then that warning-value &MUST; be deleted from the message before
1698   storing, forwarding, or using it. (preventing the consequences of naive
1699   caching of Warning header fields.) If all of the warning-values are deleted
1700   for this reason, the Warning header field &MUST; be deleted as well.
1701</t>
1702<t>
1703   The following warn-codes are defined by this specification, each with a
1704   recommended warn-text in English, and a description of its meaning.
1705</t>
1706<t><?rfc needLines="4"?>
1707   110 Response is stale
1708   <list>
1709      <t>A cache &SHOULD; include this whenever the returned response is stale.</t>
1710   </list>
1711</t>
1712<t><?rfc needLines="4"?>
1713   111 Revalidation failed
1714   <list>
1715      <t>A cache &SHOULD; include this when returning a stale response because an
1716      attempt to validate the response failed, due to an inability to reach
1717      the server.</t>
1718   </list>
1719</t>
1720<t><?rfc needLines="4"?>
1721   112 Disconnected operation
1722   <list>
1723      <t>A cache &SHOULD; b include this if it is intentionally disconnected from
1724      the rest of the network for a period of time.</t>
1725   </list>
1726</t>
1727<t><?rfc needLines="4"?>
1728   113 Heuristic expiration
1729   <list>
1730      <t>A cache &SHOULD; include this if it heuristically chose a freshness
1731      lifetime greater than 24 hours and the response's age is greater than 24
1732      hours.</t>
1733   </list>
1734</t>
1735<t><?rfc needLines="4"?>
1736   199 Miscellaneous warning
1737   <list>
1738      <t>The warning text can include arbitrary information to be presented to
1739      a human user, or logged. A system receiving this warning &MUST-NOT; take
1740      any automated action, besides presenting the warning to the user.</t>
1741   </list>
1742</t>
1743<t><?rfc needLines="4"?>
1744   214 Transformation applied
1745   <list>
1746      <t>&MUST; be added by a proxy if it applies any
1747      transformation to the representation, such as changing the
1748      content-coding, media-type, or modifying the representation data, unless
1749      this Warning code already appears in the response.</t>
1750   </list>
1751</t>
1752<t><?rfc needLines="4"?>
1753   299 Miscellaneous persistent warning
1754   <list>
1755      <t>The warning text can include arbitrary information to be presented to
1756      a human user, or logged. A system receiving this warning &MUST-NOT; take
1757      any automated action.</t>
1758   </list>
1759</t>
1760</section>
1761
1762</section>
1763
1764<section anchor="history.lists" title="History Lists">
1765<t>
1766   User agents often have history mechanisms, such as "Back" buttons and
1767   history lists, that can be used to redisplay a representation retrieved
1768   earlier in a session.
1769</t>
1770<t>
1771   The freshness model (<xref target="expiration.model"/>) does not
1772   necessarily apply to history mechanisms. I.e., a history mechanism can
1773   display a previous representation even if it has expired.
1774</t>
1775<t>
1776   This does not prohibit the history mechanism from telling the user that a
1777   view might be stale, or from honoring cache directives (e.g.,
1778   Cache-Control: no-store).
1779</t>
1780</section>
1781
1782
1783<section anchor="IANA.considerations" title="IANA Considerations">
1784
1785<section title="Cache Directive Registry" 
1786   anchor="cache.directive.registration">
1787<t>
1788   The registration procedure for HTTP Cache Directives is defined by <xref
1789   target="cache.control.extensions"/> of this document.
1790</t>
1791<t>
1792   The HTTP Cache Directive Registry shall be created at <eref
1793   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1794   populated with the registrations below:
1795</t>
1796<?BEGININC p6-cache.cache-directives ?>
1797<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1798<texttable xmlns:my="#my" align="left" suppress-title="true"
1799           anchor="iana.cache.directive.registration.table">
1800   <ttcol>Cache Directive</ttcol>
1801   <ttcol>Reference</ttcol>
1802
1803   <c>max-age</c>
1804   <c>
1805      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1806   </c>
1807   <c>max-stale</c>
1808   <c>
1809      <xref target="cache-request-directive"/>
1810   </c>
1811   <c>min-fresh</c>
1812   <c>
1813      <xref target="cache-request-directive"/>
1814   </c>
1815   <c>must-revalidate</c>
1816   <c>
1817      <xref target="cache-response-directive"/>
1818   </c>
1819   <c>no-cache</c>
1820   <c>
1821      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1822   </c>
1823   <c>no-store</c>
1824   <c>
1825      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1826   </c>
1827   <c>no-transform</c>
1828   <c>
1829      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1830   </c>
1831   <c>only-if-cached</c>
1832   <c>
1833      <xref target="cache-request-directive"/>
1834   </c>
1835   <c>private</c>
1836   <c>
1837      <xref target="cache-response-directive"/>
1838   </c>
1839   <c>proxy-revalidate</c>
1840   <c>
1841      <xref target="cache-response-directive"/>
1842   </c>
1843   <c>public</c>
1844   <c>
1845      <xref target="cache-response-directive"/>
1846   </c>
1847   <c>s-maxage</c>
1848   <c>
1849      <xref target="cache-response-directive"/>
1850   </c>
1851   <c>stale-if-error</c>
1852   <c>
1853      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1854   </c>
1855   <c>stale-while-revalidate</c>
1856   <c>
1857      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1858   </c>
1859</texttable>
1860<!--(END)-->
1861<?ENDINC p6-cache.cache-directives ?>
1862</section>
1863
1864<section title="Header Field Registration" anchor="header.field.registration">
1865<t>
1866  The Message Header Field Registry located at <eref 
1867  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1868  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
1869</t>
1870<?BEGININC p6-cache.iana-headers ?>
1871<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1872<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1873   <ttcol>Header Field Name</ttcol>
1874   <ttcol>Protocol</ttcol>
1875   <ttcol>Status</ttcol>
1876   <ttcol>Reference</ttcol>
1877
1878   <c>Age</c>
1879   <c>http</c>
1880   <c>standard</c>
1881   <c>
1882      <xref target="header.age"/>
1883   </c>
1884   <c>Cache-Control</c>
1885   <c>http</c>
1886   <c>standard</c>
1887   <c>
1888      <xref target="header.cache-control"/>
1889   </c>
1890   <c>Expires</c>
1891   <c>http</c>
1892   <c>standard</c>
1893   <c>
1894      <xref target="header.expires"/>
1895   </c>
1896   <c>Pragma</c>
1897   <c>http</c>
1898   <c>standard</c>
1899   <c>
1900      <xref target="header.pragma"/>
1901   </c>
1902   <c>Vary</c>
1903   <c>http</c>
1904   <c>standard</c>
1905   <c>
1906      <xref target="header.vary"/>
1907   </c>
1908   <c>Warning</c>
1909   <c>http</c>
1910   <c>standard</c>
1911   <c>
1912      <xref target="header.warning"/>
1913   </c>
1914</texttable>
1915<!--(END)-->
1916<?ENDINC p6-cache.iana-headers ?>
1917<t>
1918   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1919   Force".
1920</t>
1921</section>
1922
1923</section>
1924
1925<section anchor="security.considerations" title="Security Considerations">
1926<t>
1927   Caches expose additional potential vulnerabilities, since the contents of
1928   the cache represent an attractive target for malicious exploitation.
1929   Because cache contents persist after an HTTP request is complete, an attack
1930   on the cache can reveal information long after a user believes that the
1931   information has been removed from the network. Therefore, cache contents
1932   need to be protected as sensitive information.
1933</t>
1934</section>
1935
1936<section title="Acknowledgments" anchor="acks">
1937<t>
1938  See &acks;.
1939</t>
1940</section>
1941
1942</middle>
1943
1944<back>
1945<references title="Normative References">
1946
1947  <reference anchor="Part1">
1948    <front>
1949      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1950      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1951        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1952        <address><email>fielding@gbiv.com</email></address>
1953      </author>
1954      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1955        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1956        <address><email>jg@freedesktop.org</email></address>
1957      </author>
1958      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1959        <organization abbrev="HP">Hewlett-Packard Company</organization>
1960        <address><email>JeffMogul@acm.org</email></address>
1961      </author>
1962      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1963        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1964        <address><email>henrikn@microsoft.com</email></address>
1965      </author>
1966      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1967        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1968        <address><email>LMM@acm.org</email></address>
1969      </author>
1970      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1971        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1972        <address><email>paulle@microsoft.com</email></address>
1973      </author>
1974      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1975        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1976        <address><email>timbl@w3.org</email></address>
1977      </author>
1978      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1979        <organization abbrev="W3C">World Wide Web Consortium</organization>
1980        <address><email>ylafon@w3.org</email></address>
1981      </author>
1982      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1983        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1984        <address><email>julian.reschke@greenbytes.de</email></address>
1985      </author>
1986      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1987    </front>
1988    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1989    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1990  </reference>
1991
1992  <reference anchor="Part2">
1993    <front>
1994      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1995      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1996        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1997        <address><email>fielding@gbiv.com</email></address>
1998      </author>
1999      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2000        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2001        <address><email>jg@freedesktop.org</email></address>
2002      </author>
2003      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2004        <organization abbrev="HP">Hewlett-Packard Company</organization>
2005        <address><email>JeffMogul@acm.org</email></address>
2006      </author>
2007      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2008        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2009        <address><email>henrikn@microsoft.com</email></address>
2010      </author>
2011      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2012        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2013        <address><email>LMM@acm.org</email></address>
2014      </author>
2015      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2016        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2017        <address><email>paulle@microsoft.com</email></address>
2018      </author>
2019      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2020        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2021        <address><email>timbl@w3.org</email></address>
2022      </author>
2023      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2024        <organization abbrev="W3C">World Wide Web Consortium</organization>
2025        <address><email>ylafon@w3.org</email></address>
2026      </author>
2027      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2028        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2029        <address><email>julian.reschke@greenbytes.de</email></address>
2030      </author>
2031      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2032    </front>
2033    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2034    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2035  </reference>
2036
2037  <reference anchor="Part4">
2038    <front>
2039      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2040      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2041        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2042        <address><email>fielding@gbiv.com</email></address>
2043      </author>
2044      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2045        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2046        <address><email>jg@freedesktop.org</email></address>
2047      </author>
2048      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2049        <organization abbrev="HP">Hewlett-Packard Company</organization>
2050        <address><email>JeffMogul@acm.org</email></address>
2051      </author>
2052      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2053        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2054        <address><email>henrikn@microsoft.com</email></address>
2055      </author>
2056      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2057        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2058        <address><email>LMM@acm.org</email></address>
2059      </author>
2060      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2061        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2062        <address><email>paulle@microsoft.com</email></address>
2063      </author>
2064      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2065        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2066        <address><email>timbl@w3.org</email></address>
2067      </author>
2068      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2069        <organization abbrev="W3C">World Wide Web Consortium</organization>
2070        <address><email>ylafon@w3.org</email></address>
2071      </author>
2072      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2073        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2074        <address><email>julian.reschke@greenbytes.de</email></address>
2075      </author>
2076      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2077    </front>
2078    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2079    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2080  </reference>
2081
2082  <reference anchor="Part5">
2083    <front>
2084      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2085      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2086        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2087        <address><email>fielding@gbiv.com</email></address>
2088      </author>
2089      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2090        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2091        <address><email>jg@freedesktop.org</email></address>
2092      </author>
2093      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2094        <organization abbrev="HP">Hewlett-Packard Company</organization>
2095        <address><email>JeffMogul@acm.org</email></address>
2096      </author>
2097      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2098        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2099        <address><email>henrikn@microsoft.com</email></address>
2100      </author>
2101      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2102        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2103        <address><email>LMM@acm.org</email></address>
2104      </author>
2105      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2106        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2107        <address><email>paulle@microsoft.com</email></address>
2108      </author>
2109      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2110        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2111        <address><email>timbl@w3.org</email></address>
2112      </author>
2113      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2114        <organization abbrev="W3C">World Wide Web Consortium</organization>
2115        <address><email>ylafon@w3.org</email></address>
2116      </author>
2117      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2118        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2119        <address><email>julian.reschke@greenbytes.de</email></address>
2120      </author>
2121      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2122    </front>
2123    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2124    <x:source basename="p5-range" href="p5-range.xml" />
2125  </reference>
2126
2127  <reference anchor="Part7">
2128    <front>
2129      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2130      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2131        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2132        <address><email>fielding@gbiv.com</email></address>
2133      </author>
2134      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2135        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2136        <address><email>jg@freedesktop.org</email></address>
2137      </author>
2138      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2139        <organization abbrev="HP">Hewlett-Packard Company</organization>
2140        <address><email>JeffMogul@acm.org</email></address>
2141      </author>
2142      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2143        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2144        <address><email>henrikn@microsoft.com</email></address>
2145      </author>
2146      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2147        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2148        <address><email>LMM@acm.org</email></address>
2149      </author>
2150      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2151        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2152        <address><email>paulle@microsoft.com</email></address>
2153      </author>
2154      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2155        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2156        <address><email>timbl@w3.org</email></address>
2157      </author>
2158      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2159        <organization abbrev="W3C">World Wide Web Consortium</organization>
2160        <address><email>ylafon@w3.org</email></address>
2161      </author>
2162      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2163        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2164        <address><email>julian.reschke@greenbytes.de</email></address>
2165      </author>
2166      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2167    </front>
2168    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2169    <x:source basename="p7-auth" href="p7-auth.xml" />
2170  </reference>
2171
2172  <reference anchor="RFC2119">
2173    <front>
2174      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2175      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2176        <organization>Harvard University</organization>
2177        <address><email>sob@harvard.edu</email></address>
2178      </author>
2179      <date month="March" year="1997" />
2180    </front>
2181    <seriesInfo name="BCP" value="14" />
2182    <seriesInfo name="RFC" value="2119" />
2183  </reference>
2184
2185  <reference anchor="RFC5234">
2186    <front>
2187      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2188      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2189        <organization>Brandenburg InternetWorking</organization>
2190        <address>
2191          <email>dcrocker@bbiw.net</email>
2192        </address> 
2193      </author>
2194      <author initials="P." surname="Overell" fullname="Paul Overell">
2195        <organization>THUS plc.</organization>
2196        <address>
2197          <email>paul.overell@thus.net</email>
2198        </address>
2199      </author>
2200      <date month="January" year="2008"/>
2201    </front>
2202    <seriesInfo name="STD" value="68"/>
2203    <seriesInfo name="RFC" value="5234"/>
2204  </reference>
2205 
2206</references>
2207
2208<references title="Informative References">
2209
2210  <reference anchor="RFC1305">
2211    <front>
2212      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2213      <author fullname="David L. Mills" initials="D." surname="Mills">
2214        <organization>University of Delaware, Electrical Engineering Department</organization>
2215        <address><email>mills@udel.edu</email></address>
2216      </author>
2217      <date month="March" year="1992" />
2218    </front>
2219    <seriesInfo name="RFC" value="1305" />
2220  </reference>
2221
2222  <reference anchor="RFC2616">
2223    <front>
2224      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2225      <author fullname="R. Fielding" initials="R." surname="Fielding">
2226        <organization>University of California, Irvine</organization>
2227        <address><email>fielding@ics.uci.edu</email></address>
2228      </author>
2229      <author fullname="J. Gettys" initials="J." surname="Gettys">
2230        <organization>W3C</organization>
2231        <address><email>jg@w3.org</email></address>
2232      </author>
2233      <author fullname="J. Mogul" initials="J." surname="Mogul">
2234        <organization>Compaq Computer Corporation</organization>
2235        <address><email>mogul@wrl.dec.com</email></address>
2236      </author>
2237      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2238        <organization>MIT Laboratory for Computer Science</organization>
2239        <address><email>frystyk@w3.org</email></address>
2240      </author>
2241      <author fullname="L. Masinter" initials="L." surname="Masinter">
2242        <organization>Xerox Corporation</organization>
2243        <address><email>masinter@parc.xerox.com</email></address>
2244      </author>
2245      <author fullname="P. Leach" initials="P." surname="Leach">
2246        <organization>Microsoft Corporation</organization>
2247        <address><email>paulle@microsoft.com</email></address>
2248      </author>
2249      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2250        <organization>W3C</organization>
2251        <address><email>timbl@w3.org</email></address>
2252      </author>
2253      <date month="June" year="1999" />
2254    </front>
2255    <seriesInfo name="RFC" value="2616" />
2256  </reference>
2257
2258  <reference anchor="RFC3864">
2259    <front>
2260      <title>Registration Procedures for Message Header Fields</title>
2261      <author fullname="G. Klyne" initials="G." surname="Klyne">
2262        <organization>Nine by Nine</organization>
2263        <address><email>GK-IETF@ninebynine.org</email></address>
2264      </author>
2265      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2266        <organization>BEA Systems</organization>
2267        <address><email>mnot@pobox.com</email></address>
2268      </author>
2269      <author fullname="J. Mogul" initials="J." surname="Mogul">
2270        <organization>HP Labs</organization>
2271        <address><email>JeffMogul@acm.org</email></address>
2272      </author>
2273      <date month="September" year="2004" />
2274    </front>
2275    <seriesInfo name="BCP" value="90" />
2276    <seriesInfo name="RFC" value="3864" />
2277  </reference>
2278
2279  <reference anchor='RFC5226'>
2280    <front>
2281      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2282      <author initials='T.' surname='Narten' fullname='T. Narten'>
2283        <organization>IBM</organization>
2284        <address><email>narten@us.ibm.com</email></address>
2285      </author>
2286      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2287        <organization>Google</organization>
2288        <address><email>Harald@Alvestrand.no</email></address>
2289      </author>
2290      <date year='2008' month='May' />
2291    </front>
2292    <seriesInfo name='BCP' value='26' />
2293    <seriesInfo name='RFC' value='5226' />
2294  </reference>
2295
2296  <reference anchor='RFC5861'>
2297    <front>
2298      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2299      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2300        <organization>Yahoo! Inc.</organization>
2301        <address><email>mnot@yahoo-inc.com</email></address>
2302      </author>
2303      <date month="April" year="2010"/>
2304    </front>
2305    <seriesInfo name='RFC' value='5861' />
2306  </reference>
2307
2308</references>
2309
2310<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2311<t>
2312  Make the specified age calculation algorithm less conservative.
2313  (<xref target="age.calculations"/>)
2314</t>
2315<t>
2316  Remove requirement to consider Content-Location in successful responses
2317  in order to determine the appropriate response to use.
2318  (<xref target="validation.model" />)
2319</t>
2320<t>
2321  Clarify denial of service attack avoidance requirement.
2322  (<xref target="invalidation.after.updates.or.deletions" />)
2323</t>
2324<t>
2325  Change ABNF productions for header fields to only define the field value.
2326  (<xref target="header.fields"/>)
2327</t>
2328<t>
2329  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2330  anymore, as these aspects never were implemented.
2331  (<xref target="header.warning" />)
2332</t>
2333</section>
2334
2335<?BEGININC p6-cache.abnf-appendix ?>
2336<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2337<figure>
2338<artwork type="abnf" name="p6-cache.parsed-abnf">
2339<x:ref>Age</x:ref> = delta-seconds
2340
2341<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2342 cache-directive ] )
2343
2344<x:ref>Expires</x:ref> = HTTP-date
2345
2346<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2347
2348<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2349
2350<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2351 pragma-directive ] )
2352
2353<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2354 ) )
2355
2356<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2357 )
2358
2359<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2360<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2361<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2362 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2363 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2364 cache-extension
2365<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2366 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2367 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2368 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2369 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2370 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2371
2372<x:ref>delta-seconds</x:ref> = 1*DIGIT
2373
2374<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2375
2376<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2377
2378<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2379<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2380<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2381
2382<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2383
2384<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2385
2386<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2387
2388<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2389<x:ref>warn-code</x:ref> = 3DIGIT
2390<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2391<x:ref>warn-text</x:ref> = quoted-string
2392<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2393 ]
2394</artwork>
2395</figure>
2396<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2397; Age defined but not used
2398; Cache-Control defined but not used
2399; Expires defined but not used
2400; Pragma defined but not used
2401; Vary defined but not used
2402; Warning defined but not used
2403</artwork></figure></section>
2404<?ENDINC p6-cache.abnf-appendix ?>
2405
2406<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2407
2408<section title="Since RFC 2616">
2409  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2410</section>
2411
2412<section title="Since draft-ietf-httpbis-p6-cache-00">
2413<t>
2414  Closed issues:
2415  <list style="symbols">
2416    <t>
2417      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2418    <t>
2419      <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>
2420    <t>
2421      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2422    <t>
2423      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2424    <t>
2425      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2426    <t>
2427      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2428    <t>
2429      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2430    <t>
2431      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2432    <t>
2433      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2434  </list>
2435</t>
2436<t>
2437  Other changes:
2438  <list style="symbols">
2439    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2440        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2441  </list>
2442</t>
2443</section>
2444
2445<section title="Since draft-ietf-httpbis-p6-cache-01">
2446<t>
2447  Closed issues:
2448  <list style="symbols">
2449    <t>
2450      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2451  </list>
2452</t>
2453<t>
2454  Other changes:
2455  <list style="symbols">
2456    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2457      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2458    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2459      specification.</t>
2460  </list>
2461</t>
2462</section>
2463
2464<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2465<t>
2466  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2467  <list style="symbols">
2468    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2469      document.</t>
2470  </list>
2471</t>
2472</section>
2473
2474<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2475<t>
2476  Closed issues:
2477  <list style="symbols">
2478    <t>
2479      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2480  </list>
2481</t>
2482</section>
2483
2484<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2485<t>
2486  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2487  <list style="symbols"> 
2488    <t>
2489      Use "/" instead of "|" for alternatives.
2490    </t>
2491    <t>
2492      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2493      whitespace ("OWS") and required whitespace ("RWS").
2494    </t>
2495    <t>
2496      Rewrite ABNFs to spell out whitespace rules, factor out
2497      header field value format definitions.
2498    </t>
2499  </list>
2500</t>
2501</section>
2502
2503<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2504<t>
2505  This is a total rewrite of this part of the specification.
2506</t>
2507<t>
2508  Affected issues:
2509  <list style="symbols">
2510    <t>
2511      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2512    <t>
2513      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2514    <t>
2515      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2516    <t>
2517      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2518  </list>
2519</t>
2520<t>
2521  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2522  <list style="symbols"> 
2523    <t>
2524      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2525    </t>
2526  </list>
2527</t>
2528</section>
2529
2530<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2531<t>
2532  Closed issues:
2533  <list style="symbols"> 
2534    <t>
2535      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2536      "base for numeric protocol elements"
2537    </t>
2538  </list>
2539</t>
2540<t>
2541  Affected issues:
2542  <list style="symbols">
2543    <t>
2544      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2545      "Vary and non-existant headers"
2546    </t>
2547  </list>
2548</t>
2549</section>
2550
2551<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2552<t>
2553  Closed issues:
2554  <list style="symbols"> 
2555    <t>
2556      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2557      "Definition of 1xx Warn-Codes"
2558    </t>
2559    <t>
2560      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2561      "Content-Location on 304 responses"
2562    </t>
2563    <t>
2564      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2565      "private and no-cache CC directives with headers"
2566    </t>
2567    <t>
2568      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2569      "RFC2047 and warn-text"
2570    </t>
2571  </list>
2572</t>
2573</section>
2574
2575<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2576<t>
2577  Closed issues:
2578  <list style="symbols"> 
2579    <t>
2580      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2581      "serving negotiated responses from cache: header-specific canonicalization"
2582    </t>
2583    <t>
2584      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2585      "Effect of CC directives on history lists"
2586    </t>
2587    <t>
2588      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291" />:
2589      "Cache Extensions can override no-store, etc."
2590    </t>
2591  </list>
2592</t>
2593<t>
2594  Affected issues:
2595  <list style="symbols">
2596    <t>
2597      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2598      Status codes and caching
2599    </t>
2600  </list>
2601</t>
2602<t>
2603  Partly resolved issues:
2604  <list style="symbols"> 
2605    <t>
2606      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2607      "Placement of 13.5.1 and 13.5.2"
2608    </t>
2609  </list>
2610</t>
2611</section>
2612
2613<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2614<t>
2615  Closed issues:
2616  <list style="symbols"> 
2617    <t>
2618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2619      "Age calculation"
2620    </t>
2621    <t>
2622      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2623      "Clarify differences between / requirements for request and response CC directives"
2624    </t>
2625    <t>
2626      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2627      "Caching authenticated responses"
2628    </t>
2629    <t>
2630      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2631      "IANA registry for cache-control directives"
2632    </t>
2633    <t>
2634      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2635      "Heuristic caching of URLs with query components"
2636    </t>
2637  </list>
2638</t>
2639<t>
2640  Partly resolved issues:
2641  <list style="symbols"> 
2642    <t>
2643      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2644      "Term for the requested resource's URI"
2645    </t>
2646  </list>
2647</t>
2648</section>
2649
2650<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2651<t>
2652  Closed issues:
2653  <list style="symbols"> 
2654    <t>
2655      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2656      "Clarify entity / representation / variant terminology"
2657    </t>
2658    <t>
2659      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2660      "consider removing the 'changes from 2068' sections"
2661    </t>
2662    <t>
2663      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2664      "Allowing heuristic caching for new status codes"
2665    </t>
2666    <t>
2667      Clean up TODOs and prose in "Combining Responses."
2668    </t>
2669  </list>
2670</t>
2671</section>
2672
2673<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2674<t>
2675  Closed issues:
2676  <list style="symbols"> 
2677    <t>
2678      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2679      "Text about clock requirement for caches belongs in p6"
2680    </t>
2681  </list>
2682</t>
2683</section>
2684
2685<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2686<t>
2687  Closed issues:
2688  <list style="symbols"> 
2689    <t>
2690      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2691      "Header Classification"
2692    </t>
2693    <t>
2694      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2695      "Clarify 'public'"
2696    </t>
2697  </list>
2698</t>
2699</section>
2700
2701<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2702<t>
2703  Closed issues:
2704  <list style="symbols">
2705    <t>
2706      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2707      "untangle ABNFs for header fields"
2708    </t>
2709  </list>
2710</t>
2711</section>
2712
2713<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2714<t>
2715  Closed issues:
2716  <list style="symbols">
2717    <t>
2718      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2719      "Mismatch Vary"
2720    </t>
2721    <t>
2722      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2723      "Cache Invalidation only happens upon successful responses"
2724    </t>
2725    <t>
2726      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2727      "Recommend minimum sizes for protocol elements"
2728    </t>
2729    <t>
2730      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2731      "Proxies don't 'understand' methods"
2732    </t>
2733    <t>
2734      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2735      "Cache Extensions can override no-store, etc."
2736    </t>
2737    <t>
2738      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2739      "Pragma"
2740    </t>
2741  </list>
2742</t>
2743</section>
2744
2745<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
2746<t>
2747  None yet.
2748</t>
2749</section>
2750
2751</section>
2752  </back>
2753</rfc>
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