source: draft-ietf-httpbis/latest/p6-cache.xml @ 938

Last change on this file since 938 was 938, checked in by mnot@…, 10 years ago

Remove "host part" TODO; now #236.

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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 "July">
16  <!ENTITY ID-YEAR "2010">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY effective-request-uri      "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35]>
36<?rfc toc="yes" ?>
37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
45<?rfc rfcedstyle="yes"?>
46<?rfc-ext allow-markup-in-artwork="yes" ?>
47<?rfc-ext include-references-in-index="yes" ?>
48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
49  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
50<front>
51
52  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
53
54  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
55    <organization abbrev="Day Software">Day Software</organization>
56    <address>
57      <postal>
58        <street>23 Corporate Plaza DR, Suite 280</street>
59        <city>Newport Beach</city>
60        <region>CA</region>
61        <code>92660</code>
62        <country>USA</country>
63      </postal>
64      <phone>+1-949-706-5300</phone>
65      <facsimile>+1-949-706-5305</facsimile>
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 Systems">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 distributed,
204  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
205  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
206  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
207  fields that control cache behavior or indicate cacheable response messages.
208</t>
209</abstract>
210
211<note title="Editorial Note (To be removed by RFC Editor)">
212  <t>
213    Discussion of this draft should take place on the HTTPBIS working group
214    mailing list (ietf-http-wg@w3.org). The current issues list is
215    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
216    and related documents (including fancy diffs) can be found at
217    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
218  </t>
219  <t>
220    The changes in this draft are summarized in <xref target="changes.since.10"/>.
221  </t>
222</note>
223
224  </front>
225  <middle>
226
227<section anchor="caching" title="Introduction">
228<t>
229  HTTP is typically used for distributed information systems, where performance can be
230  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
231  caching and reusing response messages.
232</t>
233
234<section anchor="intro.purpose" title="Purpose">
235<iref item="cache" />
236<t>
237  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
238  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
239  in order to reduce the response time and network bandwidth consumption on future,
240  equivalent requests. Any client or server &MAY; employ a cache, though a cache cannot be
241  used by a server that is acting as a tunnel.
242</t>
243<t>
244  Caching would be useless if it did not significantly improve performance. The goal of
245  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
246  some cases, a stored response can be reused without the need for a network request,
247  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
248  (see <xref target="expiration.model" />). Even when a new request is required, it is often
249  possible to reuse all or parts of the payload of a prior response to satisfy the request,
250  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
251  purpose (see <xref target="validation.model" />).
252</t>
253</section>
254
255<section anchor="intro.terminology" title="Terminology">
256<t>
257  This specification uses a number of terms to refer to the roles played by participants
258  in, and objects of, HTTP caching.
259</t>
260<t>
261  <iref item="cacheable" />
262  <x:dfn>cacheable</x:dfn>
263  <list>
264    <t>A response is cacheable if a cache is allowed to store a copy of the response message
265      for use in answering subsequent requests. Even when a response is cacheable, there might
266      be additional constraints on whether a cache can use the cached copy to satisfy a
267      particular request.</t>
268  </list>
269</t>
270<t>
271  <iref item="explicit expiration time" />
272  <x:dfn>explicit expiration time</x:dfn>
273  <list>
274    <t>The time at which the origin server intends that a representation should no longer be
275      returned by a cache without further validation.</t>
276  </list>
277</t>
278<t>
279  <iref item="heuristic expiration time" />
280  <x:dfn>heuristic expiration time</x:dfn>
281  <list>
282    <t>An expiration time assigned by a cache when no explicit expiration time is
283    available.</t>
284  </list>
285</t>
286<t>
287  <iref item="age" />
288  <x:dfn>age</x:dfn>
289  <list>
290    <t>The age of a response is the time since it was sent by, or successfully validated
291      with, the origin server.</t>
292  </list>
293</t>
294<t>
295  <iref item="first-hand" />
296  <x:dfn>first-hand</x:dfn>
297  <list>
298    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
299    0.</t>
300  </list>
301</t>
302<t>
303  <iref item="freshness lifetime" />
304  <x:dfn>freshness lifetime</x:dfn>
305  <list>
306    <t>The length of time between the generation of a response and its expiration time. </t>
307  </list>
308</t>
309<t>
310  <iref item="fresh" />
311  <x:dfn>fresh</x:dfn>
312  <list>
313    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
314  </list>
315</t>
316<t>
317  <iref item="stale" />
318  <x:dfn>stale</x:dfn>
319  <list>
320    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
321  </list>
322</t>
323<t>
324  <iref item="validator" />
325  <x:dfn>validator</x:dfn>
326  <list>
327    <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that is used to find
328      out whether a stored response has an equivalent copy of a representation.</t>
329  </list>
330</t>
331<t anchor="shared.and.non-shared.caches">
332  <iref item="validator" />
333  <x:dfn>shared cache</x:dfn>
334  <list>
335    <t>A cache that is accessible to more than one user. A non-shared cache is
336      dedicated to a single user.</t>
337  </list>
338</t>
339</section>
340
341<section anchor="intro.requirements" title="Requirements">
342<t>
343   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
344   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
345   document are to be interpreted as described in <xref target="RFC2119"/>.
346</t>
347<t>
348   An implementation is not compliant if it fails to satisfy one or more
349   of the "MUST" or "REQUIRED" level requirements for the protocols it
350   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
351   level and all the "SHOULD" level requirements for its protocols is said
352   to be "unconditionally compliant"; one that satisfies all the "MUST"
353   level requirements but not all the "SHOULD" level requirements for its
354   protocols is said to be "conditionally compliant".
355</t>
356</section>
357
358<section title="Syntax Notation" anchor="notation">
359  <x:anchor-alias value="ALPHA"/>
360  <x:anchor-alias value="CR"/>
361  <x:anchor-alias value="DIGIT"/>
362  <x:anchor-alias value="DQUOTE"/>
363  <x:anchor-alias value="LF"/>
364  <x:anchor-alias value="OCTET"/>
365  <x:anchor-alias value="SP"/>
366  <x:anchor-alias value="VCHAR"/>
367  <x:anchor-alias value="WSP"/>
368<t>
369  This specification uses the ABNF syntax defined in &notation; (which
370  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
371  <xref target="collected.abnf"/> shows the collected ABNF, with the list
372  rule expanded.
373</t>
374<t>
375  The following core rules are included by
376  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
377  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
378  DIGIT (decimal 0-9), DQUOTE (double quote),
379  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
380  OCTET (any 8-bit sequence of data), SP (space),
381  VCHAR (any visible USASCII character),
382  and WSP (whitespace).
383</t>
384
385<section title="Core Rules" anchor="core.rules">
386  <x:anchor-alias value="quoted-string"/>
387  <x:anchor-alias value="token"/>
388  <x:anchor-alias value="OWS"/>
389<t>
390  The core rules below are defined in &basic-rules;:
391</t>
392<figure><artwork type="abnf2616">
393  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
394  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
395  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
396</artwork></figure>
397</section>
398
399<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
400  <x:anchor-alias value="field-name"/>
401  <x:anchor-alias value="HTTP-date"/>
402  <x:anchor-alias value="port"/>
403  <x:anchor-alias value="pseudonym"/>
404  <x:anchor-alias value="uri-host"/>
405<t>
406  The ABNF rules below are defined in other parts:
407</t>
408<figure><!--Part1--><artwork type="abnf2616">
409  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
410  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
411  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
412  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
413  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
414</artwork></figure>
415</section>
416
417</section>
418</section>
419
420<section anchor="caching.overview" title="Cache Operation">
421
422<section anchor="response.cacheability" title="Response Cacheability">
423<t>
424  A cache &MUST-NOT; store a response to any request, unless:
425  <list style="symbols">
426    <t>The request method is understood by the cache and defined as being cacheable, and</t>
427    <t>the response status code is understood by the cache, and</t>
428    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
429       appear in request or response headers, and</t>
430    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
431       does not appear in the response, if the cache is shared, and</t>
432    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
433       the cache is shared, unless the response explicitly allows it (see <xref
434       target="caching.authenticated.responses" />), and</t>
435    <t>the response either:
436      <list style="symbols">
437        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
438        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
439        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
440        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
441        <t>has a status code that can be served with heuristic freshness (see <xref
442           target="heuristic.freshness" />).</t>
443      </list>
444    </t>
445  </list>
446</t>
447<t>
448  In this context, a cache has "understood" a request method or a response status
449  code if it recognises it and implements any cache-specific behaviour. In
450  particular, 206 Partial Content responses cannot be cached by an
451  implementation that does not handle partial content
452  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
453</t>
454<t>
455  Note that in normal operation, most caches will not store a response that has neither a
456  cache validator nor an explicit expiration time, as such responses are not usually
457  useful to store. However, caches are not prohibited from storing such responses.
458</t>
459
460<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
461<t>
462  A cache that receives an incomplete response (for example, with fewer bytes of data
463  than specified in a Content-Length header) can store the response, but &MUST; 
464  treat it as a partial response &partial;. Partial responses
465  can be combined as described in &combining-byte-ranges;; the result might be a
466  full response or might still be partial. A cache &MUST-NOT; return a partial
467  response to a client without explicitly marking it as such using the 206 (Partial
468  Content) status code.
469</t>
470<t>
471  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
472  incomplete or partial responses.
473</t>
474</section>
475
476</section>
477
478
479<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
480<t>
481  For a presented request, a cache &MUST-NOT; return a stored response, unless:
482  <list style="symbols">
483    <t>The presented Effective Request URI (&effective-request-uri;) and that of the stored response match, and</t>
484    <t>the request method associated with the stored response allows it to be
485      used for the presented request, and</t>
486    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
487      target="caching.negotiated.responses" />), and</t>
488    <t>the presented request and stored response are free from directives that would prevent
489      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
490      and</t>
491    <t>the stored response is either:
492      <list style="symbols">
493        <t>fresh (see <xref target="expiration.model" />), or</t>
494        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
495        <t>successfully validated (see <xref target="validation.model" />).</t>
496      </list>
497    </t>
498  </list>
499</t>
500<t>
501  When a stored response is used to satisfy a request without validation, caches &MUST; include a
502  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
503  current_age; see <xref target="age.calculations" />.
504</t>
505<t>
506  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
507  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
508  corresponding response.
509</t>
510<t>
511  Also, note that unsafe requests might invalidate already stored responses; see
512  <xref target="invalidation.after.updates.or.deletions" />.
513</t>
514<t>
515  Caches &MUST; use the most recent response (as determined by the Date header) when
516  more than one suitable response is stored. They can also forward a request with
517  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
518  use.
519</t>
520</section>
521
522<section anchor="expiration.model" title="Freshness Model">
523<t>
524  When a response is "fresh" in the cache, it can be used to satisfy subsequent
525  requests without contacting the origin server, thereby improving efficiency.
526</t>
527<t>
528  The primary mechanism for determining freshness is for an origin server to provide an
529  explicit expiration time in the future, using either the Expires header (<xref
530  target="header.expires" />) or the max-age response cache directive (<xref
531  target="cache-response-directive" />). Generally, origin servers will assign future
532  explicit expiration times to responses in the belief that the representation is not likely to
533  change in a semantically significant way before the expiration time is reached.
534</t>
535<t>
536  If an origin server wishes to force a cache to validate every request, it can
537  assign an explicit expiration time in the past. This means that the response is always
538  stale, so that caches should validate it before using it for subsequent requests.
539  <cref anchor="TODO-response-stale">This wording might cause confusion, because the response might still be served stale.</cref>
540</t>
541<t>
542  Since origin servers do not always provide explicit expiration times, HTTP caches &MAY;
543  assign heuristic expiration times when explicit times are not specified, employing algorithms that
544  use other header values (such as the Last-Modified time) to estimate a plausible
545  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
546  impose worst-case constraints on their results.
547</t>
548<figure>
549<preamble>
550  The calculation to determine if a response is fresh is:
551</preamble>
552<artwork type="code">
553   response_is_fresh = (freshness_lifetime &gt; current_age)
554</artwork>
555</figure>
556<t>
557  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
558  the current_age is defined in <xref target="age.calculations" />.
559</t>
560<t>
561  Additionally, clients might need to influence freshness calculation. They can do this using
562  several request cache directives, with the effect of either increasing or loosening
563  constraints on freshness. See <xref target="cache-request-directive" />.
564</t>
565<t>
566  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
567  freshness.</cref>
568</t>
569<t>
570  Note that freshness applies only to cache operation; it cannot be used to force a user agent
571  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
572  the difference between caches and history mechanisms.
573</t>
574
575<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
576<t>
577  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
578  response by using the first match of:
579  <list style="symbols">
580    <t>If the cache is shared and the s-maxage response cache directive (<xref
581      target="cache-response-directive" />) is present, use its value, or</t>
582    <t>If the max-age response cache directive (<xref target="cache-response-directive"
583      />) is present, use its value, or</t>
584    <t>If the Expires response header (<xref target="header.expires" />) is present, use
585      its value minus the value of the Date response header, or</t>
586    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
587      freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
588  </list>
589</t>
590<t>
591  Note that this calculation is not vulnerable to clock skew, since all of the
592  information comes from the origin server.
593</t>
594
595<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
596<t>
597  If no explicit expiration time is present in a stored response that has a status code
598  whose definition allows heuristic freshness to be used (including the following
599  in &status-codes;: 200, 203, 206, 300, 301 and 410), a heuristic expiration time &MAY; be
600  calculated. Heuristics &MUST-NOT; be used for response status codes that do not explicitly
601  allow it.
602</t>
603<t>
604  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
605  attach a Warning header with a 113 warn-code to the response if its current_age is
606  more than 24 hours and such a warning is not already present.
607</t>
608<t>
609  Also, if the response has a Last-Modified header (&header-last-modified;), the
610  heuristic expiration value &SHOULD; be no more than some fraction of the interval
611  since that time. A typical setting of this fraction might be 10%.
612</t>
613<x:note>
614  <t>
615    <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt="," x:sec="13.9"/>)
616    required that caches do not calculate heuristic freshness for URLs with
617    query components (i.e., those containing '?'). In practice, this has not
618    been widely implemented. Therefore, servers are encouraged to send explicit
619    directives (e.g., Cache-Control: no-cache) if they wish to preclude
620    caching.
621  </t>
622</x:note>
623</section>
624</section>
625
626<section anchor="age.calculations" title="Calculating Age">
627<t>
628  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
629  message when obtained from a cache. The Age field value is the cache's estimate of the
630  amount of time since the response was generated or validated by the origin server. In
631  essence, the Age value is the sum of the time that the response has been resident in
632  each of the caches along the path from the origin server, plus the amount of time it has
633  been in transit along network paths.
634</t>
635<t>
636  The following data is used for the age calculation:
637</t>
638<t>
639  <x:dfn>age_value</x:dfn>
640  <list>
641    <t>
642      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
643      in a form appropriate for arithmetic operation; or 0, if not available.
644    </t>
645  </list>
646</t>
647<t>
648  <x:dfn>date_value</x:dfn>
649  <list>
650    <t>
651      HTTP/1.1 requires origin servers to send a Date header, if possible,
652      with every response, giving the time at which the response was generated.
653      The term "date_value" denotes the value of the Date header, in a form
654      appropriate for arithmetic operations. See &header-date; for the definition
655      of the Date header, and for requirements regarding responses without a
656      Date response header.
657    </t>
658  </list>
659</t>
660<t>
661  <x:dfn>now</x:dfn>
662  <list>
663    <t>
664      The term "now" means "the current value of the clock at the host
665      performing the calculation". Hosts that use HTTP, but especially hosts
666      running origin servers and caches, &SHOULD; use NTP
667      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
668      clocks to a globally accurate time standard.
669    </t>
670  </list>
671</t>
672<t>
673  <x:dfn>request_time</x:dfn>
674  <list>
675    <t>
676      The current value of the clock at the host at the time the request
677      resulting in the stored response was made.
678    </t>
679  </list>
680</t>
681<t>
682  <x:dfn>response_time</x:dfn>
683  <list>
684    <t>
685      The current value of the clock at the host at the time the response was
686      received.
687    </t>
688  </list>
689</t>
690<t>
691  A response's age can be calculated in two entirely independent ways:
692  <list style="numbers">
693    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
694      origin server's clock. If the result is negative, the result is replaced by zero.</t>
695    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
696      note this value &MUST; be interpreted relative to the time the
697      request was initiated, not the time that the response was received.</t>
698  </list>
699</t>
700<figure>
701<artwork type="code">
702  apparent_age = max(0, response_time - date_value);
703
704  response_delay = response_time - request_time;
705  corrected_age_value = age_value + response_delay; 
706</artwork>
707</figure>
708<figure>
709<preamble>These are combined as</preamble>
710<artwork type="code">
711  corrected_initial_age = max(apparent_age, corrected_age_value);
712</artwork></figure>
713<t>
714  The current_age of a stored response can then be calculated by adding the amount of
715  time (in seconds) since the stored response was last validated by the origin server to
716  the corrected_initial_age.
717</t>
718<figure><artwork type="code">
719  resident_time = now - response_time;
720  current_age = corrected_initial_age + resident_time;
721</artwork></figure>
722</section>
723
724<section anchor="serving.stale.responses" title="Serving Stale Responses">
725<t>
726  A "stale" response is one that either has explicit expiry information or is allowed to
727  have heuristic expiry calculated, but is not fresh according to the calculations in
728  <xref target="expiration.model" />.
729</t>
730<t>
731  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
732  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
733  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
734  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
735</t>
736<t>
737  Caches &SHOULD-NOT; return stale responses unless they are
738  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
739  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
740</t>
741<t>
742  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
743  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
744  the cache is disconnected.
745</t>
746<t>
747  If a cache receives a first-hand response (either an entire response, or a 304 (Not
748  Modified) response) that it would normally forward to the requesting client, and the
749  received response is no longer fresh, the cache &SHOULD; forward it to the
750  requesting client without adding a new Warning (but without removing any existing
751  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
752  that response became stale in transit.
753</t>
754</section>
755</section>
756
757<section anchor="validation.model" title="Validation Model">
758<t>
759  When a cache has one or more stored responses for a requested URI, but cannot 
760  serve any of them (e.g., because they are not fresh, or one cannot be selected;
761  see <xref target="caching.negotiated.responses"/>),
762  it can use the conditional request mechanism &conditional; in the forwarded
763  request to give the origin server an opportunity to both select a valid stored
764  response to be used, and to update it. This process is known as "validating"
765  or "revalidating" the stored response.
766</t>
767<t>
768  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
769  header whose value is that of the Last-Modified header from the selected
770  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
771</t>
772<t>
773  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
774  is that of the ETag header(s) from all responses stored for the requested URI,
775  if present. However, if any of the stored responses contains only partial
776  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
777  field unless the request is for a range that would be fully satisfied by
778  that stored response.
779</t>
780<t>
781  A 304 (Not Modified) response status code indicates that the stored
782  response can be updated and reused; see <xref target="combining.headers"/>.
783</t>
784<t>
785  A full response (i.e., one with a response body) indicates that none 
786  of the stored responses nominated in the conditional request is
787  suitable. Instead, the full response &SHOULD; be used to satisfy the
788  request and &MAY; replace the stored response.
789</t>
790<t>
791  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
792  either forward this response to the requesting client, or act as if the server failed to
793  respond. In the latter case, it &MAY; return a previously stored response (see <xref
794  target="serving.stale.responses" />).
795</t>
796</section>
797
798<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
799<t>
800  Because unsafe methods (&safe-methods;) have the potential for changing state on the
801  origin server, intervening caches can use them to keep their contents
802  up-to-date.
803</t>
804<t>
805  The following HTTP methods &MUST; cause a cache to invalidate the Effective Request URI (&effective-request-uri;) as well
806  as the URI(s) in the Location and Content-Location headers (if present):
807  <list style="symbols">
808    <t>PUT</t>
809    <t>DELETE</t>
810    <t>POST</t>
811  </list>
812</t>
813<t>
814  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
815  be performed if the host part of that URI differs from the host part in the Effective Request URI (&effective-request-uri;).
816  This helps prevent denial of service attacks.
817</t>
818<t>
819  A cache that passes through requests for methods it does not understand &SHOULD;
820  invalidate the Effective Request URI (&effective-request-uri;).
821</t>
822<t>
823  Here, "invalidate" means that the cache will either remove all stored responses related
824  to the Effective Request URI, or will mark these as "invalid" and in need of a mandatory validation
825  before they can be returned in response to a subsequent request.
826</t>
827<t>
828  Note that this does not guarantee that all appropriate responses are invalidated. For
829  example, the request that caused the change at the origin server might not have gone
830  through the cache where a response is stored.
831</t>
832</section>
833
834<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
835
836<t>Shared caches &MUST-NOT; use a cached response to a request with an Authorization header (&header-authorization;) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.</t>
837
838<t>In this specification, the following Cache-Control response directives (<xref target="cache-response-directive"/>) have such an effect: must-revalidate, public, s-maxage.</t>
839
840<t>Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating it on the origin server.</t>
841</section>
842
843<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
844<t>
845  When a cache receives a request that can be satisfied by a stored response
846  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
847  response unless all of the selecting request-headers nominated by the Vary header match
848  in both the original request (i.e., that associated with the stored response),
849  and the presented request.
850</t>
851<t>
852  The selecting request-headers from two requests are defined to match
853  if and only if those in the first request can be transformed to those in the
854  second request by applying any of the following:
855  <list style="symbols">
856    <t>
857      adding or removing whitespace, where allowed in the header's syntax
858    </t>
859    <t>
860      combining multiple message-header fields with the same field name (see
861      &header-fields;)
862    </t>
863    <t>
864      normalizing both header values in a way that is known to have identical
865      semantics, according to the header's specification (e.g., re-ordering field values
866      when order is not significant; case-normalization, where values are defined to be
867      case-insensitive)   
868    </t>
869  </list>
870</t>
871<t>
872  If (after any normalization that might take place) a header field is absent
873  from a request, it can only match another request if it is also absent there.
874</t>
875<t>
876  A Vary header field-value of "*" always fails to match, and subsequent requests to that
877  resource can only be properly interpreted by the origin server.
878</t>
879<t>
880  The stored response with matching selecting request-headers is known as the
881  selected response.
882</t>
883<t>
884  If no selected response is available, the cache &MAY; forward the presented
885  request to the origin server in a conditional request; see <xref target="validation.model"/>.
886</t>
887</section>
888
889<section anchor="combining.headers" title="Combining Responses">
890<t>
891  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
892  (in this section, the "new" response"), it needs to created an updated response by combining
893  the stored response with the new one, so that the updated response can be used to satisfy
894  the request, and potentially update the cached response.
895</t>
896<t>
897  If the new response contains an ETag, it identifies the stored 
898  response to use. <cref anchor="TODO-mention-CL">might need language about Content-Location 
899  here</cref><cref anchor="TODO-select-for-combine">Shouldn't this be the selected response?</cref>
900</t>
901<t>
902  If the new response's status code is 206 (partial content), both the stored and new 
903  responses &MUST; have validators, and those validators &MUST; match using the strong 
904  comparison function (see &weak-and-strong;). Otherwise, the 
905  responses &MUST-NOT; be combined.
906</t>
907<t>
908  The stored response headers are used as those of the updated response, except that
909  <list style="symbols">
910    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
911      &MUST; be deleted.</t>
912    <t>any stored Warning headers with warn-code 2xx &MUST; be retained.</t>
913    <t>any other headers provided in the new response &MUST; replace all instances of
914          the corresponding headers from the stored response.</t>
915  </list>
916</t>
917<t>
918  The updated response headers &MUST; be used to replace those of the 
919  stored response in cache (unless the stored response is removed from cache). In the
920  case of a 206 response, the combined representation &MAY; be stored.
921</t>
922</section>
923
924</section>
925
926<section anchor="header.fields" title="Header Field Definitions">
927<t>
928  This section defines the syntax and semantics of HTTP/1.1 header fields
929  related to caching.
930</t>
931<t>
932  For entity-header fields, both sender and recipient refer to either the client or the
933  server, depending on who sends and who receives the message.
934</t>
935
936<section anchor="header.age" title="Age">
937  <iref item="Age header" primary="true" x:for-anchor="" />
938  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
939  <x:anchor-alias value="Age"/>
940  <x:anchor-alias value="Age-v"/>
941  <x:anchor-alias value="age-value"/>
942<t>
943  The "Age" response-header field conveys the sender's estimate of the amount
944  of time since the response was generated or successfully validated at the
945  origin server. Age values are calculated as specified in
946  <xref target="age.calculations" />.
947</t>
948<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
949  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
950  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
951</artwork></figure>
952<t anchor="rule.delta-seconds">
953  <x:anchor-alias value="delta-seconds" />
954  Age field-values are non-negative integers, representing time in seconds.
955</t>
956<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
957  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
958</artwork></figure>
959<t>
960  If a cache receives a value larger than the largest positive integer it can represent, or
961  if any of its age calculations overflows, it &MUST; transmit an Age header with a
962  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
963  of at least 31 bits of range.
964</t>
965<t>
966  The presence of an Age header field in a response implies that a response is not
967  first-hand. However, the converse is not true, since HTTP/1.0 caches might not implement the
968  Age header field.
969</t>
970</section>
971
972<section anchor="header.cache-control" title="Cache-Control">
973  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
974  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
975  <x:anchor-alias value="Cache-Control"/>
976  <x:anchor-alias value="Cache-Control-v"/>
977  <x:anchor-alias value="cache-directive"/>
978  <x:anchor-alias value="cache-extension"/>
979  <x:anchor-alias value="cache-request-directive"/>
980  <x:anchor-alias value="cache-response-directive"/>
981<t>
982  The "Cache-Control" general-header field is used to specify directives for
983  caches along the request/response chain. Such cache directives are
984  unidirectional in that the presence of a directive in a request does not
985  imply that the same directive is to be given in the response.
986</t>
987<t>
988  HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control directives
989  defined in this section. See <xref target="cache.control.extensions"/> for
990  information about how Cache-Control directives defined elsewhere are handled.
991</t>
992<x:note>
993  <t>
994    <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
995    might only implement Pragma: no-cache (see <xref target="header.pragma" />).
996  </t>
997</x:note>
998<t>
999  Cache directives &MUST; be passed through by a proxy or gateway application,
1000  regardless of their significance to that application, since the directives might be
1001  applicable to all recipients along the request/response chain. It is not possible to
1002  target a directive to a specific cache.
1003</t>
1004<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1005  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1006  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1007
1008  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1009     / <x:ref>cache-response-directive</x:ref>
1010
1011  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1012</artwork></figure>
1013
1014<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1015  <x:anchor-alias value="cache-request-directive" />
1016
1017<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
1018  <x:ref>cache-request-directive</x:ref> =
1019       "no-cache"
1020     / "no-store"
1021     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1022     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1023     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1024     / "no-transform"
1025     / "only-if-cached"
1026     / <x:ref>cache-extension</x:ref>
1027</artwork></figure>
1028
1029<t>
1030  <x:dfn>no-cache</x:dfn>
1031  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1032  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1033  <list>
1034    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1035      used to satisfy the request without successful validation on the origin server.</t>
1036  </list>
1037</t>
1038<t>
1039  <x:dfn>no-store</x:dfn>
1040  <iref item="Cache Directives" primary="true" subitem="no-store" />
1041  <iref item="no-store" primary="true" subitem="Cache Directive" />
1042  <list>
1043    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1044      of either this request or any response to it. This directive applies to both
1045      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1046      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1047      and &MUST; make a best-effort attempt to remove the information from volatile
1048      storage as promptly as possible after forwarding it.</t>
1049    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1050      particular, malicious or compromised caches might not recognize or obey this
1051      directive, and communications networks might be vulnerable to eavesdropping.</t>
1052  </list>
1053</t>
1054<t>
1055  <x:dfn>max-age</x:dfn>
1056  <iref item="Cache Directives" primary="true" subitem="max-age" />
1057  <iref item="max-age" primary="true" subitem="Cache Directive" />
1058  <list>
1059    <t>The max-age request directive indicates that the client is willing to accept a
1060      response whose age is no greater than the specified time in seconds. Unless
1061      the max-stale request directive is also present, the client is not willing to accept a stale
1062      response.</t>
1063  </list>
1064</t>
1065<t>
1066  <x:dfn>max-stale</x:dfn>
1067  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1068  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1069  <list>
1070    <t>The max-stale request directive indicates that the client is willing to accept a
1071      response that has exceeded its expiration time. If max-stale is assigned a value,
1072      then the client is willing to accept a response that has exceeded its expiration
1073      time by no more than the specified number of seconds. If no value is assigned to
1074      max-stale, then the client is willing to accept a stale response of any age.</t>
1075  </list>
1076</t>
1077<t>
1078  <x:dfn>min-fresh</x:dfn>
1079  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1080  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1081  <list>
1082    <t>The min-fresh request directive indicates that the client is willing to accept a
1083      response whose freshness lifetime is no less than its current age plus the specified
1084      time in seconds. That is, the client wants a response that will still be fresh for
1085      at least the specified number of seconds.</t>
1086  </list>
1087</t>
1088<t>
1089  <x:dfn>no-transform</x:dfn>
1090  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1091  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1092  <list>
1093    <t>The no-transform request directive indicates that an intermediate cache or proxy
1094      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1095      headers, nor the request representation.</t>
1096  </list>
1097</t>
1098<t>
1099  <x:dfn>only-if-cached</x:dfn>
1100  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1101  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1102  <list>
1103    <t>The only-if-cached request directive indicates that the client only wishes to
1104      return a stored response. If it receives this directive, a cache &SHOULD; either
1105      respond using a stored response that is consistent with the other constraints of the
1106      request, or respond with a 504 (Gateway Timeout) status code. If a group of caches is
1107      being operated as a unified system with good internal connectivity, such a request
1108      &MAY; be forwarded within that group of caches.</t>
1109  </list>
1110</t>
1111</section>
1112
1113<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1114  <x:anchor-alias value="cache-response-directive" />
1115
1116<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1117  <x:ref>cache-response-directive</x:ref> =
1118       "public"
1119     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1120     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1121     / "no-store"
1122     / "no-transform"
1123     / "must-revalidate"
1124     / "proxy-revalidate"
1125     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1126     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1127     / <x:ref>cache-extension</x:ref>
1128</artwork></figure>
1129
1130<t>
1131  <x:dfn>public</x:dfn>
1132  <iref item="Cache Directives" primary="true" subitem="public" />
1133  <iref item="public" primary="true" subitem="Cache Directive" />
1134  <list>
1135    <t>The public response directive indicates that the response &MAY; be cached, even
1136      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1137      (See also Authorization, &header-authorization;, for additional details.) </t>
1138  </list>
1139</t>
1140<t>
1141  <x:dfn>private</x:dfn>
1142  <iref item="Cache Directives" primary="true" subitem="private" />
1143  <iref item="private" primary="true" subitem="Cache Directive" />
1144  <list>
1145    <t>The private response directive indicates that the response message is intended for
1146      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1147      cache &MAY; store the response.</t>
1148    <t>If the private response directive specifies one or more field-names, this
1149      requirement is limited to the field-values associated with the listed response
1150      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1151      cache, whereas the remainder of the response message &MAY; be.</t>
1152    <t>
1153      <x:h>Note:</x:h> This usage of the word private only controls where the response can
1154      be stored; it cannot ensure the privacy of the message content.
1155      Also, private response directives with field-names are often handled by
1156      implementations as if an unqualified private directive was received; i.e.,
1157      the special handling for the qualified form is not widely implemented.</t>
1158  </list>
1159</t>
1160<t>
1161  <x:dfn>no-cache</x:dfn>
1162  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1163  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1164  <list>
1165    <t>The no-cache response directive indicates that the response MUST NOT be used
1166          to satisfy a subsequent request without successful validation on the origin
1167          server. This allows an origin server to prevent a cache from using it to satisfy
1168          a request without contacting it, even by caches that have been configured to
1169          return stale responses.</t>
1170    <t>If the no-cache response directive specifies one or more field-names, this
1171      requirement is limited to the field-values associated with the listed response
1172      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1173      to a subsequent request without successful validation on the origin server. This
1174      allows an origin server to prevent the re-use of certain header fields in a
1175      response, while still allowing caching of the rest of the response.</t>
1176    <t>
1177      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1178      Also, no-cache response directives with field-names are often handled by
1179      implementations as if an unqualified no-cache directive was received; i.e.,
1180      the special handling for the qualified form is not widely implemented.
1181    </t>
1182  </list>
1183</t>
1184<t>
1185  <x:dfn>no-store</x:dfn>
1186  <iref item="Cache Directives" primary="true" subitem="no-store" />
1187  <iref item="no-store" primary="true" subitem="Cache Directive" />
1188  <list>
1189    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1190      part of either the immediate request or response. This directive applies to both
1191      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1192      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1193      and &MUST; make a best-effort attempt to remove the information from volatile
1194      storage as promptly as possible after forwarding it.</t>
1195    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1196      particular, malicious or compromised caches might not recognize or obey this
1197      directive, and communications networks might be vulnerable to eavesdropping.</t>
1198  </list>
1199</t>
1200<t>
1201  <x:dfn>must-revalidate</x:dfn>
1202  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1203  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1204  <list>
1205    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1206     used to satisfy subsequent requests without successful validation on the origin server.</t>
1207    <t>The must-revalidate directive is necessary to support reliable operation for
1208      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1209      the must-revalidate directive; in particular, if the cache cannot reach the origin
1210      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1211    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1212      validate a request on the representation could result in incorrect operation, such as a
1213      silently unexecuted financial transaction.</t>
1214  </list>
1215</t>
1216<t>
1217  <x:dfn>proxy-revalidate</x:dfn>
1218  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1219  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1220  <list>
1221    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1222      response directive, except that it does not apply to non-shared caches.</t>
1223  </list>
1224</t>
1225<t>
1226  <x:dfn>max-age</x:dfn>
1227  <iref item="Cache Directives" primary="true" subitem="max-age" />
1228  <iref item="max-age" primary="true" subitem="Cache Directive" />
1229  <list>
1230    <t>The max-age response directive indicates that response is to be considered stale
1231      after its age is greater than the specified number of seconds.</t>
1232  </list>
1233</t>
1234<t>
1235  <x:dfn>s-maxage</x:dfn>
1236  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1237  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1238  <list>
1239    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1240      specified by this directive overrides the maximum age specified by either the
1241      max-age directive or the Expires header. The s-maxage directive also implies the
1242      semantics of the proxy-revalidate response directive.</t>
1243  </list>
1244</t>
1245<t>
1246  <x:dfn>no-transform</x:dfn>
1247  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1248  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1249  <list>
1250    <t>The no-transform response directive indicates that an intermediate cache or proxy
1251      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1252      headers, nor the response representation.</t>
1253  </list>
1254</t>
1255
1256</section>
1257
1258<section anchor="cache.control.extensions" title="Cache Control Extensions">
1259<t>
1260  The Cache-Control header field can be extended through the use of one or more
1261  cache-extension tokens, each with an optional value. Informational extensions (those
1262  that do not require a change in cache behavior) can be added without changing the
1263  semantics of other directives. Behavioral extensions are designed to work by acting as
1264  modifiers to the existing base of cache directives. Both the new directive and the
1265  standard directive are supplied, such that applications that do not understand the new
1266  directive will default to the behavior specified by the standard directive, and those
1267  that understand the new directive will recognize it as modifying the requirements
1268  associated with the standard directive. In this way, extensions to the cache-control
1269  directives can be made without requiring changes to the base protocol.
1270</t>
1271<t>
1272  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1273  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1274  all directives that it does not understand.
1275</t>
1276<t>
1277  For example, consider a hypothetical new response directive called "community" that
1278  acts as a modifier to the private directive. We define this new directive to mean that,
1279  in addition to any non-shared cache, any cache that is shared only by members of the
1280  community named within its value may cache the response. An origin server wishing to
1281  allow the UCI community to use an otherwise private response in their shared cache(s)
1282  could do so by including
1283</t>
1284<figure><artwork type="example">
1285  Cache-Control: private, community="UCI"
1286</artwork></figure>
1287<t>
1288  A cache seeing this header field will act correctly even if the cache does not
1289  understand the community cache-extension, since it will also see and understand the
1290  private directive and thus default to the safe behavior.
1291</t>
1292<t>
1293  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1294  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1295  directives (or the response's default cacheability) such that the cache behavior will
1296  remain minimally correct even if the cache does not understand the extension(s).
1297</t>
1298<t>
1299  The HTTP Cache Directive Registry defines the name space for the cache
1300  directives.
1301</t>
1302<t>
1303  Registrations &MUST; include the following fields:
1304  <list style="symbols">
1305    <t>Cache Directive Name</t>
1306    <t>Pointer to specification text</t>
1307  </list>
1308</t>
1309<t>
1310  Values to be added to this name space are subject to IETF review
1311  (<xref target="RFC5226" x:fmt="," x:sec="4.1"/>).
1312</t>
1313<t>
1314  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1315</t>
1316</section>
1317
1318</section>
1319
1320<section anchor="header.expires" title="Expires">
1321  <iref item="Expires header" primary="true" x:for-anchor="" />
1322  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1323  <x:anchor-alias value="Expires"/>
1324  <x:anchor-alias value="Expires-v"/>
1325<t>
1326  The "Expires" entity-header field gives the date/time after which the response is
1327  considered stale. See <xref target="expiration.model" /> for further discussion of the
1328  freshness model.
1329</t>
1330<t>
1331  The presence of an Expires field does not imply that the original resource will change or
1332  cease to exist at, before, or after that time.
1333</t>
1334<t>
1335  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1336  it &MUST; be sent in rfc1123-date format.
1337</t>
1338<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1339  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1340  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1341</artwork></figure>
1342<figure>
1343  <preamble>For example</preamble>
1344<artwork type="example">
1345  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1346</artwork></figure>
1347<x:note>
1348  <t>
1349    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1350    directive (see <xref target="cache-response-directive" />), that directive overrides
1351    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1352  </t>
1353</x:note>
1354<t>
1355  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1356</t>
1357<t>
1358  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1359  including the value "0", as in the past (i.e., "already expired").
1360</t>
1361</section>
1362
1363<section anchor="header.pragma" title="Pragma">
1364  <iref item="Pragma header" primary="true" x:for-anchor="" />
1365  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1366  <x:anchor-alias value="extension-pragma"/>
1367  <x:anchor-alias value="Pragma"/>
1368  <x:anchor-alias value="Pragma-v"/>
1369  <x:anchor-alias value="pragma-directive"/>
1370<t>
1371  The "Pragma" general-header field is used to include implementation-specific directives
1372  that might apply to any recipient along the request/response chain. All pragma directives
1373  specify optional behavior from the viewpoint of the protocol; however, some systems
1374  &MAY; require that behavior be consistent with the directives.
1375</t>
1376<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1377  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1378  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1379  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1380  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1381</artwork></figure>
1382<t>
1383  When the no-cache directive is present in a request message, an application &SHOULD;
1384  forward the request toward the origin server even if it has a cached copy of what is being
1385  requested. This pragma directive has the same semantics as the no-cache response directive
1386  (see <xref target="cache-response-directive" />) and is defined here for backward
1387  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1388  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1389  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1390</t>
1391<x:note>
1392  <t>
1393    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1394    is not actually specified, it does not provide a reliable replacement for
1395    "Cache-Control: no-cache" in a response.
1396  </t>
1397</x:note>
1398<t>
1399  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1400</t>
1401</section>
1402
1403<section anchor="header.vary" title="Vary">
1404  <iref item="Vary header" primary="true" x:for-anchor="" />
1405  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1406  <x:anchor-alias value="Vary"/>
1407  <x:anchor-alias value="Vary-v"/>
1408<t>
1409  The "Vary" response-header field conveys the set of request-header fields
1410  that were used to select the representation.
1411</t>
1412<t>
1413  Caches use this information, in part, to determine whether a stored response
1414  can be used to satisfy a given request; see
1415  <xref target="caching.negotiated.responses" />.
1416  determines, while the response is fresh, whether a cache is permitted to use the
1417  response to reply to a subsequent request without validation; see <xref 
1418  target="caching.negotiated.responses" />.
1419</t>
1420<t>
1421  In uncacheable or stale responses, the Vary field value advises the user agent about
1422  the criteria that were used to select the representation.
1423</t>
1424<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1425  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1426  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1427</artwork></figure>
1428<t>
1429  The set of header fields named by the Vary field value is known as the selecting
1430  request-headers.
1431</t>
1432<t>
1433  Servers &SHOULD; include a Vary header field with any cacheable response that is
1434  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1435  requests on that resource and informs the user agent about the presence of negotiation on
1436  that resource. A server &MAY; include a Vary header field with a non-cacheable
1437  response that is subject to server-driven negotiation, since this might provide the user
1438  agent with useful information about the dimensions over which the response varies at the
1439  time of the response.
1440</t>
1441<t>
1442  A Vary field value of "*" signals that unspecified parameters not limited to the
1443  request-headers (e.g., the network address of the client), play a role in the selection of
1444  the response representation; therefore, a cache cannot determine whether this response is
1445  appropriate. The "*" value &MUST-NOT; be generated by a proxy server.
1446</t>
1447<t>
1448  The field-names given are not limited to the set of standard request-header fields
1449  defined by this specification. Field names are case-insensitive.
1450</t>
1451</section>
1452
1453<section anchor="header.warning" title="Warning">
1454  <iref item="Warning header" primary="true" x:for-anchor="" />
1455  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1456  <x:anchor-alias value="Warning"/>
1457  <x:anchor-alias value="Warning-v"/>
1458  <x:anchor-alias value="warning-value"/>
1459  <x:anchor-alias value="warn-agent"/>
1460  <x:anchor-alias value="warn-code"/>
1461  <x:anchor-alias value="warn-date"/>
1462  <x:anchor-alias value="warn-text"/>
1463<t>
1464  The "Warning" general-header field is used to carry additional information about the status
1465  or transformation of a message that might not be reflected in the message. This
1466  information is typically used to warn about possible incorrectness introduced by caching
1467  operations or transformations applied to the payload of the message.
1468</t>
1469<t>
1470  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1471  warning, rather than an error status code, distinguishes these responses from true failures.
1472</t>
1473<t>
1474  Warning headers can in general be applied to any message, however some warn-codes are
1475  specific to caches and can only be applied to response messages.
1476</t>
1477<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
1478  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1479  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1480 
1481  <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>
1482                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1483 
1484  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1485  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1486                  ; the name or pseudonym of the server adding
1487                  ; the Warning header, for use in debugging
1488  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1489  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1490</artwork></figure>
1491<t>
1492  Multiple warnings can be attached to a response (either by the origin server or by
1493  a cache), including multiple warnings with the same code number, only differing
1494  in warn-text.
1495</t>
1496<t>
1497  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1498  possible, in the order that they appear in the response.
1499</t>
1500<t>
1501  Systems that generate multiple Warning headers &SHOULD; order them with this user
1502  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1503  Warning headers.
1504</t>
1505<t>
1506  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1507  Warning is required to be deleted from a stored response after validation:
1508  <list style="symbols">
1509    <t>1xx Warnings describe the freshness or validation status of the response, and so
1510      &MUST; be deleted by caches after validation. They can only be generated by a cache
1511      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1512    <t>2xx Warnings describe some aspect of the representation that is
1513      not rectified by a validation (for example, a lossy compression of the representation)
1514      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1515      returned, in which case they &MUST; be.</t>
1516  </list>
1517</t>
1518<t>
1519  If an implementation sends a message with one or more Warning headers to a receiver whose
1520  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1521  warn-date that matches the Date header in the message.
1522</t>
1523<t>
1524  If an implementation receives a message with a warning-value that includes a warn-date,
1525  and that warn-date is different from the Date value in the response, then that
1526  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1527  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1528  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1529  well.
1530</t>
1531<t>
1532  The following warn-codes are defined by this specification, each with a recommended
1533  warn-text in English, and a description of its meaning.
1534</t>
1535<t><?rfc needLines="4"?>
1536  110 Response is stale
1537  <list>
1538    <t>&SHOULD; be included whenever the returned response is stale.</t>
1539  </list>
1540</t>
1541<t><?rfc needLines="4"?>
1542  111 Revalidation failed
1543  <list>
1544    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1545      validate the response failed, due to an inability to reach the server.</t>
1546  </list>
1547</t>
1548<t><?rfc needLines="4"?>
1549  112 Disconnected operation
1550  <list>
1551    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1552      the network for a period of time.</t>
1553  </list>
1554</t>
1555<t><?rfc needLines="4"?>
1556  113 Heuristic expiration
1557  <list>
1558    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1559      greater than 24 hours and the response's age is greater than 24 hours.</t>
1560  </list>
1561</t>
1562<t><?rfc needLines="4"?>
1563  199 Miscellaneous warning
1564  <list>
1565    <t>The warning text can include arbitrary information to be presented to a human
1566      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1567      action, besides presenting the warning to the user.</t>
1568  </list>
1569</t>
1570<t><?rfc needLines="4"?>
1571  214 Transformation applied
1572  <list>
1573    <t>&MUST; be added by an intermediate proxy if it applies any
1574      transformation to the representation, such as changing the content-coding,
1575      media-type, or modifying the representation data, unless this Warning code
1576      already appears in the response.</t>
1577  </list>
1578</t>
1579<t><?rfc needLines="4"?>
1580  299 Miscellaneous persistent warning
1581  <list>
1582    <t>The warning text can include arbitrary information to be presented to a human
1583      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1584      action.</t>
1585  </list>
1586</t>
1587</section>
1588
1589</section>
1590
1591<section anchor="history.lists" title="History Lists">
1592<t>
1593  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1594  can be used to redisplay a representation retrieved earlier in a session.
1595</t>
1596<t>
1597  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1598  a history mechanism can display a previous representation even if it has expired.
1599</t>
1600  <t>
1601  This does not prohibit the history mechanism from telling the user that a
1602  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1603  </t>
1604</section>
1605
1606
1607<section anchor="IANA.considerations" title="IANA Considerations">
1608
1609<section title="Cache Directive Registry" anchor="cache.directive.registration">
1610<t>
1611  The registration procedure for HTTP Cache Directives is defined by
1612  <xref target="cache.control.extensions"/> of this document.
1613</t>
1614<t>
1615   The HTTP Cache Directive Registry should be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/>
1616   and be populated with the registrations below:
1617</t>
1618<?BEGININC p6-cache.cache-directives ?>
1619<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1620<texttable xmlns:my="#my" align="left" suppress-title="true"
1621           anchor="iana.cache.directive.registration.table">
1622   <ttcol>Cache Directive</ttcol>
1623   <ttcol>Reference</ttcol>
1624
1625   <c>max-age</c>
1626   <c>
1627      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1628   </c>
1629   <c>max-stale</c>
1630   <c>
1631      <xref target="cache-request-directive"/>
1632   </c>
1633   <c>min-fresh</c>
1634   <c>
1635      <xref target="cache-request-directive"/>
1636   </c>
1637   <c>must-revalidate</c>
1638   <c>
1639      <xref target="cache-response-directive"/>
1640   </c>
1641   <c>no-cache</c>
1642   <c>
1643      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1644   </c>
1645   <c>no-store</c>
1646   <c>
1647      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1648   </c>
1649   <c>no-transform</c>
1650   <c>
1651      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1652   </c>
1653   <c>only-if-cached</c>
1654   <c>
1655      <xref target="cache-request-directive"/>
1656   </c>
1657   <c>private</c>
1658   <c>
1659      <xref target="cache-response-directive"/>
1660   </c>
1661   <c>proxy-revalidate</c>
1662   <c>
1663      <xref target="cache-response-directive"/>
1664   </c>
1665   <c>public</c>
1666   <c>
1667      <xref target="cache-response-directive"/>
1668   </c>
1669   <c>s-maxage</c>
1670   <c>
1671      <xref target="cache-response-directive"/>
1672   </c>
1673   <c>stale-if-error</c>
1674   <c>
1675      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1676   </c>
1677   <c>stale-while-revalidate</c>
1678   <c>
1679      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1680   </c>
1681</texttable>
1682<!--(END)-->
1683<?ENDINC p6-cache.cache-directives ?>
1684</section>
1685
1686<section title="Header Field Registration" anchor="header.field.registration">
1687<t>
1688  The Message Header Field Registry located at <eref
1689  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1690  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1691</t>
1692<?BEGININC p6-cache.iana-headers ?>
1693<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1694<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1695   <ttcol>Header Field Name</ttcol>
1696   <ttcol>Protocol</ttcol>
1697   <ttcol>Status</ttcol>
1698   <ttcol>Reference</ttcol>
1699
1700   <c>Age</c>
1701   <c>http</c>
1702   <c>standard</c>
1703   <c>
1704      <xref target="header.age"/>
1705   </c>
1706   <c>Cache-Control</c>
1707   <c>http</c>
1708   <c>standard</c>
1709   <c>
1710      <xref target="header.cache-control"/>
1711   </c>
1712   <c>Expires</c>
1713   <c>http</c>
1714   <c>standard</c>
1715   <c>
1716      <xref target="header.expires"/>
1717   </c>
1718   <c>Pragma</c>
1719   <c>http</c>
1720   <c>standard</c>
1721   <c>
1722      <xref target="header.pragma"/>
1723   </c>
1724   <c>Vary</c>
1725   <c>http</c>
1726   <c>standard</c>
1727   <c>
1728      <xref target="header.vary"/>
1729   </c>
1730   <c>Warning</c>
1731   <c>http</c>
1732   <c>standard</c>
1733   <c>
1734      <xref target="header.warning"/>
1735   </c>
1736</texttable>
1737<!--(END)-->
1738<?ENDINC p6-cache.iana-headers ?>
1739<t>
1740  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1741</t>
1742</section>
1743
1744</section>
1745
1746<section anchor="security.considerations" title="Security Considerations">
1747<t>
1748  Caches expose additional potential vulnerabilities, since the contents of the cache
1749  represent an attractive target for malicious exploitation. Because cache contents persist
1750  after an HTTP request is complete, an attack on the cache can reveal information long after
1751  a user believes that the information has been removed from the network. Therefore, cache
1752  contents should be protected as sensitive information.
1753</t>
1754</section>
1755
1756<section anchor="ack" title="Acknowledgments">
1757<t>
1758  Much of the content and presentation of the caching design is due to suggestions and
1759  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1760  and Larry Masinter.
1761</t>
1762</section>
1763
1764</middle>
1765
1766<back>
1767<references title="Normative References">
1768
1769  <reference anchor="Part1">
1770    <front>
1771      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1772      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1773        <organization abbrev="Day Software">Day Software</organization>
1774        <address><email>fielding@gbiv.com</email></address>
1775      </author>
1776      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1777        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1778        <address><email>jg@freedesktop.org</email></address>
1779      </author>
1780      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1781        <organization abbrev="HP">Hewlett-Packard Company</organization>
1782        <address><email>JeffMogul@acm.org</email></address>
1783      </author>
1784      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1785        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1786        <address><email>henrikn@microsoft.com</email></address>
1787      </author>
1788      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1789        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1790        <address><email>LMM@acm.org</email></address>
1791      </author>
1792      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1793        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1794        <address><email>paulle@microsoft.com</email></address>
1795      </author>
1796      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1797        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1798        <address><email>timbl@w3.org</email></address>
1799      </author>
1800      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1801        <organization abbrev="W3C">World Wide Web Consortium</organization>
1802        <address><email>ylafon@w3.org</email></address>
1803      </author>
1804      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1805        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1806        <address><email>julian.reschke@greenbytes.de</email></address>
1807      </author>
1808      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1809    </front>
1810    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1811    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1812  </reference>
1813
1814  <reference anchor="Part2">
1815    <front>
1816      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1817      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1818        <organization abbrev="Day Software">Day Software</organization>
1819        <address><email>fielding@gbiv.com</email></address>
1820      </author>
1821      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1822        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1823        <address><email>jg@freedesktop.org</email></address>
1824      </author>
1825      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1826        <organization abbrev="HP">Hewlett-Packard Company</organization>
1827        <address><email>JeffMogul@acm.org</email></address>
1828      </author>
1829      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1830        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1831        <address><email>henrikn@microsoft.com</email></address>
1832      </author>
1833      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1834        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1835        <address><email>LMM@acm.org</email></address>
1836      </author>
1837      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1838        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1839        <address><email>paulle@microsoft.com</email></address>
1840      </author>
1841      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1842        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1843        <address><email>timbl@w3.org</email></address>
1844      </author>
1845      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1846        <organization abbrev="W3C">World Wide Web Consortium</organization>
1847        <address><email>ylafon@w3.org</email></address>
1848      </author>
1849      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1850        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1851        <address><email>julian.reschke@greenbytes.de</email></address>
1852      </author>
1853      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1854    </front>
1855    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1856    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1857  </reference>
1858
1859  <reference anchor="Part4">
1860    <front>
1861      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1862      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1863        <organization abbrev="Day Software">Day Software</organization>
1864        <address><email>fielding@gbiv.com</email></address>
1865      </author>
1866      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1867        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1868        <address><email>jg@freedesktop.org</email></address>
1869      </author>
1870      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1871        <organization abbrev="HP">Hewlett-Packard Company</organization>
1872        <address><email>JeffMogul@acm.org</email></address>
1873      </author>
1874      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1875        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1876        <address><email>henrikn@microsoft.com</email></address>
1877      </author>
1878      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1879        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1880        <address><email>LMM@acm.org</email></address>
1881      </author>
1882      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1883        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1884        <address><email>paulle@microsoft.com</email></address>
1885      </author>
1886      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1887        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1888        <address><email>timbl@w3.org</email></address>
1889      </author>
1890      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1891        <organization abbrev="W3C">World Wide Web Consortium</organization>
1892        <address><email>ylafon@w3.org</email></address>
1893      </author>
1894      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1895        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1896        <address><email>julian.reschke@greenbytes.de</email></address>
1897      </author>
1898      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1899    </front>
1900    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1901    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1902  </reference>
1903
1904  <reference anchor="Part5">
1905    <front>
1906      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1907      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1908        <organization abbrev="Day Software">Day Software</organization>
1909        <address><email>fielding@gbiv.com</email></address>
1910      </author>
1911      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1912        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1913        <address><email>jg@freedesktop.org</email></address>
1914      </author>
1915      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1916        <organization abbrev="HP">Hewlett-Packard Company</organization>
1917        <address><email>JeffMogul@acm.org</email></address>
1918      </author>
1919      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1920        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1921        <address><email>henrikn@microsoft.com</email></address>
1922      </author>
1923      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1924        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1925        <address><email>LMM@acm.org</email></address>
1926      </author>
1927      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1928        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1929        <address><email>paulle@microsoft.com</email></address>
1930      </author>
1931      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1932        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1933        <address><email>timbl@w3.org</email></address>
1934      </author>
1935      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1936        <organization abbrev="W3C">World Wide Web Consortium</organization>
1937        <address><email>ylafon@w3.org</email></address>
1938      </author>
1939      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1940        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1941        <address><email>julian.reschke@greenbytes.de</email></address>
1942      </author>
1943      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1944    </front>
1945    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1946    <x:source basename="p5-range" href="p5-range.xml" />
1947  </reference>
1948
1949  <reference anchor="Part7">
1950    <front>
1951      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1952      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1953        <organization abbrev="Day Software">Day Software</organization>
1954        <address><email>fielding@gbiv.com</email></address>
1955      </author>
1956      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1957        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1958        <address><email>jg@freedesktop.org</email></address>
1959      </author>
1960      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1961        <organization abbrev="HP">Hewlett-Packard Company</organization>
1962        <address><email>JeffMogul@acm.org</email></address>
1963      </author>
1964      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1965        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1966        <address><email>henrikn@microsoft.com</email></address>
1967      </author>
1968      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1969        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1970        <address><email>LMM@acm.org</email></address>
1971      </author>
1972      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1973        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1974        <address><email>paulle@microsoft.com</email></address>
1975      </author>
1976      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1977        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1978        <address><email>timbl@w3.org</email></address>
1979      </author>
1980      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1981        <organization abbrev="W3C">World Wide Web Consortium</organization>
1982        <address><email>ylafon@w3.org</email></address>
1983      </author>
1984      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1985        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1986        <address><email>julian.reschke@greenbytes.de</email></address>
1987      </author>
1988      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1989    </front>
1990    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1991    <x:source basename="p7-auth" href="p7-auth.xml" />
1992  </reference>
1993
1994  <reference anchor="RFC2119">
1995    <front>
1996      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1997      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1998        <organization>Harvard University</organization>
1999        <address><email>sob@harvard.edu</email></address>
2000      </author>
2001      <date month="March" year="1997" />
2002    </front>
2003    <seriesInfo name="BCP" value="14" />
2004    <seriesInfo name="RFC" value="2119" />
2005  </reference>
2006
2007  <reference anchor="RFC5234">
2008    <front>
2009      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2010      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2011        <organization>Brandenburg InternetWorking</organization>
2012        <address>
2013          <email>dcrocker@bbiw.net</email>
2014        </address> 
2015      </author>
2016      <author initials="P." surname="Overell" fullname="Paul Overell">
2017        <organization>THUS plc.</organization>
2018        <address>
2019          <email>paul.overell@thus.net</email>
2020        </address>
2021      </author>
2022      <date month="January" year="2008"/>
2023    </front>
2024    <seriesInfo name="STD" value="68"/>
2025    <seriesInfo name="RFC" value="5234"/>
2026  </reference>
2027 
2028</references>
2029
2030<references title="Informative References">
2031
2032  <reference anchor="RFC1305">
2033    <front>
2034      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2035      <author fullname="David L. Mills" initials="D." surname="Mills">
2036        <organization>University of Delaware, Electrical Engineering Department</organization>
2037        <address><email>mills@udel.edu</email></address>
2038      </author>
2039      <date month="March" year="1992" />
2040    </front>
2041    <seriesInfo name="RFC" value="1305" />
2042  </reference>
2043
2044  <reference anchor="RFC2616">
2045    <front>
2046      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2047      <author fullname="R. Fielding" initials="R." surname="Fielding">
2048        <organization>University of California, Irvine</organization>
2049        <address><email>fielding@ics.uci.edu</email></address>
2050      </author>
2051      <author fullname="J. Gettys" initials="J." surname="Gettys">
2052        <organization>W3C</organization>
2053        <address><email>jg@w3.org</email></address>
2054      </author>
2055      <author fullname="J. Mogul" initials="J." surname="Mogul">
2056        <organization>Compaq Computer Corporation</organization>
2057        <address><email>mogul@wrl.dec.com</email></address>
2058      </author>
2059      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2060        <organization>MIT Laboratory for Computer Science</organization>
2061        <address><email>frystyk@w3.org</email></address>
2062      </author>
2063      <author fullname="L. Masinter" initials="L." surname="Masinter">
2064        <organization>Xerox Corporation</organization>
2065        <address><email>masinter@parc.xerox.com</email></address>
2066      </author>
2067      <author fullname="P. Leach" initials="P." surname="Leach">
2068        <organization>Microsoft Corporation</organization>
2069        <address><email>paulle@microsoft.com</email></address>
2070      </author>
2071      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2072        <organization>W3C</organization>
2073        <address><email>timbl@w3.org</email></address>
2074      </author>
2075      <date month="June" year="1999" />
2076    </front>
2077    <seriesInfo name="RFC" value="2616" />
2078  </reference>
2079
2080  <reference anchor="RFC3864">
2081    <front>
2082      <title>Registration Procedures for Message Header Fields</title>
2083      <author fullname="G. Klyne" initials="G." surname="Klyne">
2084        <organization>Nine by Nine</organization>
2085        <address><email>GK-IETF@ninebynine.org</email></address>
2086      </author>
2087      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2088        <organization>BEA Systems</organization>
2089        <address><email>mnot@pobox.com</email></address>
2090      </author>
2091      <author fullname="J. Mogul" initials="J." surname="Mogul">
2092        <organization>HP Labs</organization>
2093        <address><email>JeffMogul@acm.org</email></address>
2094      </author>
2095      <date month="September" year="2004" />
2096    </front>
2097    <seriesInfo name="BCP" value="90" />
2098    <seriesInfo name="RFC" value="3864" />
2099  </reference>
2100
2101  <reference anchor='RFC5226'>
2102    <front>
2103      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2104      <author initials='T.' surname='Narten' fullname='T. Narten'>
2105        <organization>IBM</organization>
2106        <address><email>narten@us.ibm.com</email></address>
2107      </author>
2108      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2109        <organization>Google</organization>
2110        <address><email>Harald@Alvestrand.no</email></address>
2111      </author>
2112      <date year='2008' month='May' />
2113    </front>
2114    <seriesInfo name='BCP' value='26' />
2115    <seriesInfo name='RFC' value='5226' />
2116  </reference>
2117
2118  <reference anchor='RFC5861'>
2119    <front>
2120      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2121      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2122        <organization>Yahoo! Inc.</organization>
2123        <address><email>mnot@yahoo-inc.com</email></address>
2124      </author>
2125      <date month="April" year="2010"/>
2126    </front>
2127    <seriesInfo name='RFC' value='5861' />
2128  </reference>
2129
2130</references>
2131
2132<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2133<t>
2134  Make the specified age calculation algorithm less conservative.
2135  (<xref target="age.calculations"/>)
2136</t>
2137<t>
2138  Remove requirement to consider Content-Location in successful responses
2139  in order to determine the appropriate response to use.
2140  (<xref target="validation.model" />)
2141</t>
2142<t>
2143  Clarify denial of service attack avoidance requirement.
2144  (<xref target="invalidation.after.updates.or.deletions" />)
2145</t>
2146<t>
2147  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2148  anymore, as these aspects never were implemented.
2149  (<xref target="header.warning" />)
2150</t>
2151</section>
2152
2153<?BEGININC p6-cache.abnf-appendix ?>
2154<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2155<figure>
2156<artwork type="abnf" name="p6-cache.parsed-abnf">
2157<x:ref>Age</x:ref> = "Age:" OWS Age-v
2158<x:ref>Age-v</x:ref> = delta-seconds
2159
2160<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2161<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2162 cache-directive ] )
2163
2164<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2165<x:ref>Expires-v</x:ref> = HTTP-date
2166
2167<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2168
2169<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2170
2171<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2172<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2173 pragma-directive ] )
2174
2175<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2176<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2177 ] ) )
2178
2179<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2180<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2181 ] )
2182
2183<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2184<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2185<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2186 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2187 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2188 cache-extension
2189<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2190 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2191 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2192 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2193 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2194 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2195
2196<x:ref>delta-seconds</x:ref> = 1*DIGIT
2197
2198<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2199
2200<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2201
2202<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2203<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2204<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2205
2206<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2207
2208<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2209
2210<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2211
2212<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2213<x:ref>warn-code</x:ref> = 3DIGIT
2214<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2215<x:ref>warn-text</x:ref> = quoted-string
2216<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2217 ]
2218</artwork>
2219</figure>
2220<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2221; Age defined but not used
2222; Cache-Control defined but not used
2223; Expires defined but not used
2224; Pragma defined but not used
2225; Vary defined but not used
2226; Warning defined but not used
2227</artwork></figure></section>
2228<?ENDINC p6-cache.abnf-appendix ?>
2229
2230<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2231
2232<section title="Since RFC2616">
2233  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2234</section>
2235
2236<section title="Since draft-ietf-httpbis-p6-cache-00">
2237<t>
2238  Closed issues:
2239  <list style="symbols">
2240    <t>
2241      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2242    <t>
2243      <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>
2244    <t>
2245      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2246    <t>
2247      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2248    <t>
2249      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2250    <t>
2251      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2252    <t>
2253      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2254    <t>
2255      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2256    <t>
2257      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2258  </list>
2259</t>
2260<t>
2261  Other changes:
2262  <list style="symbols">
2263    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2264        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2265  </list>
2266</t>
2267</section>
2268
2269<section title="Since draft-ietf-httpbis-p6-cache-01">
2270<t>
2271  Closed issues:
2272  <list style="symbols">
2273    <t>
2274      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2275  </list>
2276</t>
2277<t>
2278  Other changes:
2279  <list style="symbols">
2280    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2281      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2282    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2283      specification.</t>
2284  </list>
2285</t>
2286</section>
2287
2288<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2289<t>
2290  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2291  <list style="symbols">
2292    <t>Reference RFC 3984, and update header registrations for headers defined in this
2293      document.</t>
2294  </list>
2295</t>
2296</section>
2297
2298<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2299<t>
2300  Closed issues:
2301  <list style="symbols">
2302    <t>
2303      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2304  </list>
2305</t>
2306</section>
2307
2308<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2309<t>
2310  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2311  <list style="symbols"> 
2312    <t>
2313      Use "/" instead of "|" for alternatives.
2314    </t>
2315    <t>
2316      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2317      whitespace ("OWS") and required whitespace ("RWS").
2318    </t>
2319    <t>
2320      Rewrite ABNFs to spell out whitespace rules, factor out
2321      header value format definitions.
2322    </t>
2323  </list>
2324</t>
2325</section>
2326
2327<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2328<t>
2329  This is a total rewrite of this part of the specification.
2330</t>
2331<t>
2332  Affected issues:
2333  <list style="symbols">
2334    <t>
2335      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2336    <t>
2337      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2338    <t>
2339      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2340    <t>
2341      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2342  </list>
2343</t>
2344<t>
2345  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2346  <list style="symbols"> 
2347    <t>
2348      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2349    </t>
2350  </list>
2351</t>
2352</section>
2353
2354<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2355<t>
2356  Closed issues:
2357  <list style="symbols"> 
2358    <t>
2359      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2360      "base for numeric protocol elements"
2361    </t>
2362  </list>
2363</t>
2364<t>
2365  Affected issues:
2366  <list style="symbols">
2367    <t>
2368      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2369      Vary and non-existant headers
2370    </t>
2371  </list>
2372</t>
2373</section>
2374
2375<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2376<t>
2377  Closed issues:
2378  <list style="symbols"> 
2379    <t>
2380      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2381      "Definition of 1xx Warn-Codes"
2382    </t>
2383    <t>
2384      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2385      "Content-Location on 304 responses"
2386    </t>
2387    <t>
2388      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2389      "private and no-cache CC directives with headers"
2390    </t>
2391    <t>
2392      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2393      "RFC2047 and warn-text"
2394    </t>
2395  </list>
2396</t>
2397</section>
2398
2399<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2400<t>
2401  Closed issues:
2402  <list style="symbols"> 
2403    <t>
2404      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2405      "serving negotiated responses from cache: header-specific canonicalization"
2406    </t>
2407    <t>
2408      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2409      "Effect of CC directives on history lists"
2410    </t>
2411  </list>
2412</t>
2413<t>
2414  Affected issues:
2415  <list style="symbols">
2416    <t>
2417      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2418      Status codes and caching
2419    </t>
2420  </list>
2421</t>
2422<t>
2423  Partly resolved issues:
2424  <list style="symbols"> 
2425    <t>
2426      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2427      "Placement of 13.5.1 and 13.5.2"
2428    </t>
2429  </list>
2430</t>
2431</section>
2432
2433<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2434<t>
2435  Closed issues:
2436  <list style="symbols"> 
2437    <t>
2438      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2439      "Age calculation"
2440    </t>
2441    <t>
2442      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2443      "Clarify differences between / requirements for request and response CC directives"
2444    </t>
2445        <t>
2446                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2447                "Caching authenticated responses"
2448        </t>
2449    <t>
2450      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2451      "IANA registry for cache-control directives"
2452    </t>
2453    <t>
2454      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2455      "Heuristic caching of URLs with query components"
2456    </t>
2457  </list>
2458</t>
2459<t>
2460  Partly resolved issues:
2461  <list style="symbols"> 
2462    <t>
2463      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2464      "Term for the requested resource's URI"
2465    </t>
2466  </list>
2467</t>
2468</section>
2469
2470<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2471<t>
2472  Closed issues:
2473  <list style="symbols"> 
2474    <t>
2475      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2476      "consider removing the 'changes from 2068' sections"
2477    </t>
2478    <t>
2479      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2480      "Allowing heuristic caching for new status codes"
2481    </t>
2482        <t>
2483          Clean up TODOs and prose in "Combining Responses."
2484        </t>
2485  </list>
2486</t>
2487</section>
2488
2489</section>
2490  </back>
2491</rfc>
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