source: draft-ietf-httpbis/latest/p6-cache.xml @ 746

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Update to latest version of xml2rfc and rfc2629.xslt, bump document dates

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='rfc2629.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 "January">
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 messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' 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 message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36]>
37<?rfc toc="yes" ?>
38<?rfc symrefs="yes" ?>
39<?rfc sortrefs="yes" ?>
40<?rfc compact="yes"?>
41<?rfc subcompact="no" ?>
42<?rfc linkmailto="no" ?>
43<?rfc editing="no" ?>
44<?rfc comments="yes"?>
45<?rfc inline="yes"?>
46<?rfc-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 fullname="Jim Gettys" initials="J." surname="Gettys">
72    <organization>One Laptop per Child</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@laptop.org</email>
82      <uri>http://www.laptop.org/</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 mailing list
214  (ietf-http-wg@w3.org). The current issues list is at <eref
215  target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
216  (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.
217</t>
218<t>
219  The changes in this draft are summarized in <xref target="changes.since.08" />.
220</t>
221</note>
222
223  </front>
224  <middle>
225
226<section anchor="caching" title="Introduction">
227<t>
228  HTTP is typically used for distributed information systems, where performance can be
229  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
230  caching and reusing response messages.
231</t>
232
233<section anchor="intro.purpose" title="Purpose">
234<iref item="cache" />
235<t>
236  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
237  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
238  in order to reduce the response time and network bandwidth consumption on future,
239  equivalent requests. Any client or server may include a cache, though a cache cannot be
240  used by a server that is acting as a tunnel.
241</t>
242<t>
243  Caching would be useless if it did not significantly improve performance. The goal of
244  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
245  some cases, a stored response can be reused without the need for a network request,
246  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
247  (see <xref target="expiration.model" />). Even when a new request is required, it is often
248  possible to reuse all or parts of the payload of a prior response to satisfy the request,
249  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
250  purpose (see <xref target="validation.model" />).
251</t>
252</section>
253
254<section anchor="intro.terminology" title="Terminology">
255<t>
256  This specification uses a number of terms to refer to the roles played by participants
257  in, and objects of, HTTP caching.
258</t>
259<t>
260  <iref item="cacheable" />
261  <x:dfn>cacheable</x:dfn>
262  <list>
263    <t>A response is cacheable if a cache is allowed to store a copy of the response message
264      for use in answering subsequent requests. Even when a response is cacheable, there may
265      be additional constraints on whether a cache can use the cached copy to satisfy a
266      particular request.</t>
267  </list>
268</t>
269<t>
270  <iref item="explicit expiration time" />
271  <x:dfn>explicit expiration time</x:dfn>
272  <list>
273    <t>The time at which the origin server intends that an entity should no longer be
274      returned by a cache without further validation.</t>
275  </list>
276</t>
277<t>
278  <iref item="heuristic expiration time" />
279  <x:dfn>heuristic expiration time</x:dfn>
280  <list>
281    <t>An expiration time assigned by a cache when no explicit expiration time is
282    available.</t>
283  </list>
284</t>
285<t>
286  <iref item="age" />
287  <x:dfn>age</x:dfn>
288  <list>
289    <t>The age of a response is the time since it was sent by, or successfully validated
290      with, the origin server.</t>
291  </list>
292</t>
293<t>
294  <iref item="first-hand" />
295  <x:dfn>first-hand</x:dfn>
296  <list>
297    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
298    0.</t>
299  </list>
300</t>
301<t>
302  <iref item="freshness lifetime" />
303  <x:dfn>freshness lifetime</x:dfn>
304  <list>
305    <t>The length of time between the generation of a response and its expiration time. </t>
306  </list>
307</t>
308<t>
309  <iref item="fresh" />
310  <x:dfn>fresh</x:dfn>
311  <list>
312    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
313  </list>
314</t>
315<t>
316  <iref item="stale" />
317  <x:dfn>stale</x:dfn>
318  <list>
319    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
320  </list>
321</t>
322<t>
323  <iref item="validator" />
324  <x:dfn>validator</x:dfn>
325  <list>
326    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
327      out whether a stored response is an equivalent copy of an entity.</t>
328  </list>
329</t>
330<t anchor="shared.and.non-shared.caches">
331  <iref item="validator" />
332  <x:dfn>shared cache</x:dfn>
333  <list>
334    <t>A cache that is accessible to more than one user. A non-shared cache is
335      dedicated to a single user.</t>
336  </list>
337</t>
338</section>
339
340<section anchor="intro.requirements" title="Requirements">
341<t>
342  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
343  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
344  described in <xref target="RFC2119" />.
345</t>
346<t>
347  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
348  or &REQUIRED; level requirements for the protocols it implements. An implementation
349  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
350  requirements for its protocols is said to be "unconditionally compliant"; one that
351  satisfies all the &MUST; level requirements but not all the &SHOULD; level
352  requirements for its protocols is said to be "conditionally compliant."
353</t>
354</section>
355
356<section title="Syntax Notation" anchor="notation">
357  <x:anchor-alias value="ALPHA"/>
358  <x:anchor-alias value="CR"/>
359  <x:anchor-alias value="DIGIT"/>
360  <x:anchor-alias value="DQUOTE"/>
361  <x:anchor-alias value="LF"/>
362  <x:anchor-alias value="OCTET"/>
363  <x:anchor-alias value="SP"/>
364  <x:anchor-alias value="VCHAR"/>
365  <x:anchor-alias value="WSP"/>
366<t>
367  This specification uses the ABNF syntax defined in &notation; (which
368  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
369  <xref target="collected.abnf"/> shows the collected ABNF, with the list
370  rule expanded.
371</t>
372<t>
373  The following core rules are included by
374  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
375  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
376  DIGIT (decimal 0-9), DQUOTE (double quote),
377  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
378  OCTET (any 8-bit sequence of data), SP (space),
379  VCHAR (any visible USASCII character),
380  and WSP (whitespace).
381</t>
382
383<section title="Core Rules" anchor="core.rules">
384  <x:anchor-alias value="quoted-string"/>
385  <x:anchor-alias value="token"/>
386  <x:anchor-alias value="OWS"/>
387<t>
388  The core rules below are defined in &basic-rules;:
389</t>
390<figure><artwork type="abnf2616">
391  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
392  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
393  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
394</artwork></figure>
395</section>
396
397<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
398  <x:anchor-alias value="field-name"/>
399  <x:anchor-alias value="HTTP-date"/>
400  <x:anchor-alias value="port"/>
401  <x:anchor-alias value="pseudonym"/>
402  <x:anchor-alias value="uri-host"/>
403<t>
404  The ABNF rules below are defined in other parts:
405</t>
406<figure><!--Part1--><artwork type="abnf2616">
407  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
408  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
409  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
410  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
411  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
412</artwork></figure>
413</section>
414
415</section>
416</section>
417
418<section anchor="caching.overview" title="Cache Operation">
419
420<section anchor="response.cacheability" title="Response Cacheability">
421<t>
422  A cache &MUST-NOT; store a response to any request, unless:
423  <list style="symbols">
424    <t>The request method is understood by the cache and defined as being cacheable, and</t>
425    <t>the response status code is understood by the cache, and</t>
426    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
427       appear in request or response headers, and</t>
428    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
429       does not appear in the response, if the cache is shared, and</t>
430    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
431       the cache is shared (unless the "public" directive is present; see <xref
432       target="header.cache-control" />), and</t>
433    <t>the response either:
434      <list style="symbols">
435        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
436        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
437        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
438        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
439        <t>has a status code that can be served with heuristic freshness (see <xref
440           target="heuristic.freshness" />).</t>
441      </list>
442    </t>
443  </list>
444</t>
445<t>
446  In this context, a cache has "understood" a request method or a response status
447  code if it recognises it and implements any cache-specific behaviour. In
448  particular, 206 Partial Content responses cannot be cached by an
449  implementation that does not handle partial content
450  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
451</t>
452<t>
453  Note that in normal operation, most caches will not store a response that has neither a
454  cache validator nor an explicit expiration time, as such responses are not usually
455  useful to store. However, caches are not prohibited from storing such responses.
456</t>
457
458<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
459<t>
460  A cache that receives an incomplete response (for example, with fewer bytes of data
461  than specified in a Content-Length header) can store the response, but &MUST; 
462  treat it as a partial response &partial;. Partial responses
463  can be combined as described in &combining-byte-ranges;; the result might be a
464  full response or might still be partial. A cache &MUST-NOT; return a partial
465  response to a client without explicitly marking it as such using the 206 (Partial
466  Content) status code.
467</t>
468<t>
469  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
470  incomplete or partial responses.
471</t>
472</section>
473
474</section>
475
476
477<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
478<t>
479  For a presented request, a cache &MUST-NOT; return a stored response, unless:
480  <list style="symbols">
481    <t>The presented Request-URI and that of the stored response match
482      (<cref anchor="TODO-Request-URI">Need to find a new term for this, as Part
483      1 doesn't define Request-URI anymore; the new term request-target does not
484      work for this. (see <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196" />)</cref>), and</t>
485    <t>the request method associated with the stored response allows it to be
486      used for the presented request, and</t>
487    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
488      target="caching.negotiated.responses" />), and</t>
489    <t>the presented request and stored response are free from directives that would prevent
490      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
491      and</t>
492    <t>the stored response is either:
493      <list style="symbols">
494        <t>fresh (see <xref target="expiration.model" />), or</t>
495        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
496        <t>successfully validated (see <xref target="validation.model" />).</t>
497      </list>
498    </t>
499  </list>
500</t>
501<t>
502  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
503</t>
504<t>
505  When a stored response is used to satisfy a request, caches &MUST; include a
506  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
507  current_age; see <xref target="age.calculations" />.
508  <cref>DISCUSS: this currently includes successfully validated responses.</cref>
509</t>
510<t>
511  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
512  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
513  corresponding response.
514</t>
515<t>
516  Also, note that unsafe requests might invalidate already stored responses; see
517  <xref target="invalidation.after.updates.or.deletions" />.
518</t>
519<t>
520  Caches &MUST; use the most recent response (as determined by the Date header) when
521  more than one suitable response is stored. They can also forward a request with
522  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
523  use.
524</t>
525<t>
526  <cref anchor="TODO-header-properties">end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1</cref>
527</t>
528</section>
529
530<section anchor="expiration.model" title="Freshness Model">
531<t>
532  When a response is "fresh" in the cache, it can be used to satisfy subsequent
533  requests without contacting the origin server, thereby improving efficiency.
534</t>
535<t>
536  The primary mechanism for determining freshness is for an origin server to provide an
537  explicit expiration time in the future, using either the Expires header (<xref
538  target="header.expires" />) or the max-age response cache directive (<xref
539  target="cache-response-directive" />). Generally, origin servers will assign future
540  explicit expiration times to responses in the belief that the entity is not likely to
541  change in a semantically significant way before the expiration time is reached.
542</t>
543<t>
544  If an origin server wishes to force a cache to validate every request, it can
545  assign an explicit expiration time in the past. This means that the response is always
546  stale, so that caches should validate it before using it for subsequent requests.
547  <cref>This wording may cause confusion, because the response may still be served stale.</cref>
548</t>
549<t>
550  Since origin servers do not always provide explicit expiration times, HTTP caches may
551  also assign heuristic expiration times when they are not specified, employing algorithms that
552  use other header values (such as the Last-Modified time) to estimate a plausible
553  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
554  impose worst-case constraints on their results.
555</t>
556<figure>
557<preamble>
558  The calculation to determine if a response is fresh is:
559</preamble>
560<artwork type="code">
561   response_is_fresh = (freshness_lifetime &gt; current_age)
562</artwork>
563</figure>
564
565<t>
566  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
567  the current_age is defined in <xref target="age.calculations" />.
568</t>
569<t>
570  Additionally, clients may need to influence freshness calculation. They can do this using
571  several request cache directives, with the effect of either increasing or loosening
572  constraints on freshness. See <xref target="cache-request-directive" />.
573</t>
574<t>
575  <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
576  freshness.</cref>
577</t>
578<t>
579  Note that freshness applies only to cache operation; it cannot be used to force a user agent
580  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
581  the difference between caches and history mechanisms.
582</t>
583
584<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
585<t>
586  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
587  response by using the first match of:
588  <list style="symbols">
589    <t>If the cache is shared and the s-maxage response cache directive (<xref
590      target="cache-response-directive" />) is present, use its value, or</t>
591    <t>If the max-age response cache directive (<xref target="cache-response-directive"
592      />) is present, use its value, or</t>
593    <t>If the Expires response header (<xref target="header.expires" />) is present, use
594      its value minus the value of the Date response header, or</t>
595    <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
596      may be used; see <xref target="heuristic.freshness" />.</t>
597  </list>
598</t>
599<t>
600  Note that this calculation is not vulnerable to clock skew, since all of the
601  information comes from the origin server.
602</t>
603
604<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
605<t>
606  If no explicit expiration time is present in a stored response that has a status code
607  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
608  calculated. Heuristics &MUST-NOT; be used for other response status codes.
609</t>
610<t>
611  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
612  attach a Warning header with a 113 warn-code to the response if its current_age is
613  more than 24 hours and such a warning is not already present.
614</t>
615<t>
616  Also, if the response has a Last-Modified header (&header-last-modified;), the
617  heuristic expiration value &SHOULD; be no more than some fraction of the interval
618  since that time. A typical setting of this fraction might be 10%.
619</t>
620<t>
621  <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
622</t>
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 term "age_value" denotes the value of the Age header, in a form appropriate for
637  arithmetic operations.
638</t>
639<t>
640  HTTP/1.1 requires origin servers to send a Date header, if possible, with every
641  response, giving the time at which the response was generated (see &header-date;).
642  The term "date_value" denotes the value of the Date header, in a form appropriate for
643  arithmetic operations.
644</t>
645<t>
646  The term "now" means "the current value of the clock at the host performing the
647  calculation." Hosts that use HTTP, but especially hosts running origin servers and
648  caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
649  synchronize their clocks to a globally accurate time standard.
650</t>
651<t>
652  A response's age can be calculated in two entirely independent ways:
653  <list style="numbers">
654    <t>now minus date_value, if the local clock is reasonably well synchronized to the
655      origin server's clock. If the result is negative, the result is replaced by zero.</t>
656    <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
657  </list>
658</t>
659<figure>
660<preamble>These are combined as</preamble>
661<artwork type="code">
662    corrected_received_age = max(now - date_value, age_value)
663</artwork></figure>
664<t>
665  When an Age value is received, it &MUST; be interpreted relative to the time the
666  request was initiated, not the time that the response was received.
667</t>
668<figure><artwork type="code">
669   corrected_initial_age = corrected_received_age
670                         + (now - request_time)
671</artwork></figure>
672<t>
673  where "request_time" is the time (according to the local clock) when the request that
674  elicited this response was sent.
675</t>
676<t>
677  The current_age of a stored response can then be calculated by adding the amount of
678  time (in seconds) since the stored response was last validated by the origin server to
679  the corrected_initial_age.
680</t>
681<t>
682  In summary:
683</t>
684<figure><artwork type="code">
685  age_value     - Age header field-value received with the response
686  date_value    - Date header field-value received with the response
687  request_time  - local time when the cache made the request
688                 resulting in the stored response
689  response_time - local time when the cache received the response
690  now           - current local time
691 
692  apparent_age = max(0, response_time - date_value);
693  corrected_received_age = max(apparent_age, age_value);
694  response_delay = response_time - request_time;
695  corrected_initial_age = corrected_received_age + response_delay;
696  resident_time = now - response_time;
697  current_age   = corrected_initial_age + resident_time;
698</artwork></figure>
699</section>
700
701<section anchor="serving.stale.responses" title="Serving Stale Responses">
702<t>
703  A "stale" response is one that either has explicit expiry information, or is allowed to
704  have heuristic expiry calculated, but is not fresh according to the calculations in
705  <xref target="expiration.model" />.
706</t>
707<t>
708  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
709  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
710  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
711  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
712</t>
713<t>
714  Caches &SHOULD-NOT; return stale responses unless they are
715  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
716  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
717</t>
718<t>
719  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
720  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
721  the cache is disconnected.
722</t>
723<t>
724  If a cache receives a first-hand response (either an entire response, or a 304 (Not
725  Modified) response) that it would normally forward to the requesting client, and the
726  received response is no longer fresh, the cache &SHOULD; forward it to the
727  requesting client without adding a new Warning (but without removing any existing
728  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
729  that response became stale in transit.
730</t>
731</section>
732</section>
733
734<section anchor="validation.model" title="Validation Model">
735<t>
736  When a cache has one or more stored responses for a requested URI, but cannot 
737  serve any of them (e.g., because they are not fresh, or one cannot be selected;
738  see <xref target="caching.negotiated.responses"/>),
739  it can use the conditional request mechanism &conditional; in the forwarded
740  request to give the origin server an opportunity to both select a valid stored
741  response to be used, and to update it. This process is known as "validating"
742  or "revalidating" the stored response.
743</t>
744<t>
745  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
746  header whose value is that of the Last-Modified header from the selected
747  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
748</t>
749<t>
750  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
751  is that of the ETag header(s) from all responses stored for the requested URI,
752  if present. However, if any of the stored responses contains only partial
753  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
754  field unless the request is for a range that would be fully satisfied by
755  that stored response.
756</t>
757<t>
758  A 304 (Not Modified) response status code indicates that the stored
759  response can be updated and reused; see <xref target="combining.headers"/>.
760</t>
761<t>
762  A full response (i.e., one with a response body) indicates that none 
763  of the stored responses nominated in the conditional request is
764  suitable. Instead, the full response is used both to satisfy the
765  request and replace the stored response. <cref>Should there be a requirement here?</cref>
766</t>
767<t>
768  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
769  either forward this response to the requesting client, or act as if the server failed to
770  respond. In the latter case, it &MAY; return a previously stored response (see <xref
771  target="serving.stale.responses" />).
772</t>
773</section>
774
775<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
776<t>
777  Because unsafe methods (&safe-methods;) have the potential for changing state on the
778  origin server, intervening caches can use them to keep their contents
779  up-to-date.
780</t>
781<t>
782  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
783  as the URI(s) in the Location and Content-Location headers (if present):
784  <list style="symbols">
785    <t>PUT</t>
786    <t>DELETE</t>
787    <t>POST</t>
788  </list>
789</t>
790<t>
791  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
792  be performed if the host part of that URI differs from the host part in the Request-URI.
793  This helps prevent denial of service attacks.
794</t>
795<t>
796  <cref>TODO: "host part" needs to be specified better.</cref>
797</t>
798<t>
799  A cache that passes through requests for methods it does not understand &SHOULD;
800  invalidate the Request-URI.
801</t>
802<t>
803  Here, "invalidate" means that the cache will either remove all stored responses related
804  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
805  before they can be returned in response to a subsequent request.
806</t>
807<t>
808  Note that this does not guarantee that all appropriate responses are invalidated. For
809  example, the request that caused the change at the origin server might not have gone
810  through the cache where a response is stored.
811</t>
812<t>
813  <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
814</t>
815</section>
816
817<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
818<t>
819  When a cache receives a request that can be satisfied by a stored response
820  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
821  response unless all of the selecting request-headers nominated by the Vary header match
822  in both the original request (i.e., that associated with the stored response),
823  and the presented request.
824</t>
825<t>
826  The selecting request-headers from two requests are defined to match if and only if the
827  selecting request-headers in the first request can be transformed to the selecting
828  request-headers in the second request by adding or removing linear white space
829  <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
830  combining multiple message-header fields with the same field name following the rules
831  about header fields in &header-fields;.
832</t>
833<t>
834  If a header field is absent from a request, it can only match another request
835  if it is also absent there.
836</t>
837<t>
838  A Vary header field-value of "*" always fails to match, and subsequent requests to that
839  resource can only be properly interpreted by the origin server.
840</t>
841<t>
842  The stored response with matching selecting request-headers is known as the
843  selected response.
844</t>
845<t>
846  If no selected response is available, the cache &MAY; forward the presented
847  request to the origin server in a conditional request; see <xref target="validation.model"/>.
848</t>
849</section>
850
851<section anchor="combining.headers" title="Combining Responses">
852<t>
853  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
854  (in this section, the "new" response"), it needs to created an updated response by combining
855  the stored response with the new one, so that the updated response can be used to satisfy the request.
856</t>
857<t>
858  If the new response contains an ETag, it identifies the stored 
859  response to use. <cref>may need language about Content-Location 
860  here</cref><cref>cover case where INM with multiple etags was sent</cref>
861</t>
862<t>
863  If the status code is 206 (partial content), both the stored and new 
864  responses &MUST; have validators, and those validators &MUST; match using the strong 
865  comparison function (see &weak-and-strong;). Otherwise, the 
866  responses &MUST-NOT; be combined.
867</t>
868<t>
869  The stored response headers are used as those of the updated response, except that
870  <list style="symbols">
871    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
872      &MUST; be deleted from the stored response and the updated response.</t>
873    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
874      response and the updated response.</t>
875    <t>any headers provided in the new response &MUST; replace the corresponding
876      headers from the stored response.</t>
877  </list>
878</t>
879<t>
880  If a header field-name in the new response matches more than one 
881  header in the stored response, all such stored headers &MUST; be replaced.
882</t>
883<t>
884  The updated response can [[<cref>requirement?</cref>]] be used to replace the 
885  stored response in cache. In the case of a 206 response, the combined 
886  entity-body &MAY; be stored.
887</t>
888<t>
889  <cref>ISSUE: discuss how to handle HEAD updates</cref>
890</t>
891</section>
892
893</section>
894
895<section anchor="header.fields" title="Header Field Definitions">
896<t>
897  This section defines the syntax and semantics of HTTP/1.1 header fields
898  related to caching.
899</t>
900<t>
901  For entity-header fields, both sender and recipient refer to either the client or the
902  server, depending on who sends and who receives the entity.
903</t>
904
905<section anchor="header.age" title="Age">
906  <iref item="Age header" primary="true" x:for-anchor="" />
907  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
908  <x:anchor-alias value="Age"/>
909  <x:anchor-alias value="Age-v"/>
910  <x:anchor-alias value="age-value"/>
911<t>
912  The "Age" response-header field conveys the sender's estimate of the amount
913  of time since the response was generated or successfully validated at the
914  origin server. Age values are calculated as specified in
915  <xref target="age.calculations" />.
916</t>
917<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
918  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
919  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
920</artwork></figure>
921<t anchor="rule.delta-seconds">
922  <x:anchor-alias value="delta-seconds" />
923  Age field-values are non-negative integers, representing time in seconds.
924</t>
925<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
926  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
927</artwork></figure>
928<t>
929  If a cache receives a value larger than the largest positive integer it can represent, or
930  if any of its age calculations overflows, it &MUST; transmit an Age header with a
931  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
932  of at least 31 bits of range.
933</t>
934<t>
935  The presence of an Age header field in a response implies that a response is not
936  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
937  Age header field.
938</t>
939</section>
940
941<section anchor="header.cache-control" title="Cache-Control">
942  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
943  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
944  <x:anchor-alias value="Cache-Control"/>
945  <x:anchor-alias value="Cache-Control-v"/>
946  <x:anchor-alias value="cache-directive"/>
947  <x:anchor-alias value="cache-extension"/>
948  <x:anchor-alias value="cache-request-directive"/>
949  <x:anchor-alias value="cache-response-directive"/>
950<t>
951  The "Cache-Control" general-header field is used to specify directives that
952  &MUST; be obeyed by all caches along the request/response chain. Such cache
953  directives are unidirectional in that the presence of a directive in a
954  request does not imply that the same directive is to be given in the
955  response.
956</t>
957<x:note>
958  <t>
959    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
960    Pragma: no-cache (see <xref target="header.pragma" />).
961  </t>
962</x:note>
963<t>
964  Cache directives &MUST; be passed through by a proxy or gateway application,
965  regardless of their significance to that application, since the directives might be
966  applicable to all recipients along the request/response chain. It is not possible to
967  target a directive to a specific cache.
968</t>
969<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"/>
970  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
971  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
972
973  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
974     / <x:ref>cache-response-directive</x:ref>
975
976  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
977</artwork></figure>
978
979<section anchor="cache-request-directive" title="Request Cache-Control Directives">
980  <x:anchor-alias value="cache-request-directive" />
981
982<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
983  <x:ref>cache-request-directive</x:ref> =
984       "no-cache"
985     / "no-store"
986     / "max-age" "=" <x:ref>delta-seconds</x:ref>
987     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
988     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
989     / "no-transform"
990     / "only-if-cached"
991     / <x:ref>cache-extension</x:ref>
992</artwork></figure>
993
994<t>
995  <iref item="Cache Directives" primary="true" subitem="no-cache" />
996  <iref item="no-cache" primary="true" subitem="Cache Directive" />
997  no-cache
998  <list>
999    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1000      used to satisfy the request without successful validation on the origin server.</t>
1001  </list>
1002</t>
1003<t>
1004  <iref item="Cache Directives" primary="true" subitem="no-store" />
1005  <iref item="no-store" primary="true" subitem="Cache Directive" />
1006  no-store
1007  <list>
1008    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1009      of either this request or any response to it. This directive applies to both
1010      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1011      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1012      and &MUST; make a best-effort attempt to remove the information from volatile
1013      storage as promptly as possible after forwarding it.</t>
1014    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1015      particular, malicious or compromised caches might not recognize or obey this
1016      directive, and communications networks may be vulnerable to eavesdropping.</t>
1017  </list>
1018</t>
1019<t>
1020  <iref item="Cache Directives" primary="true" subitem="max-age" />
1021  <iref item="max-age" primary="true" subitem="Cache Directive" />
1022  max-age
1023  <list>
1024    <t>The max-age request directive indicates that the client is willing to accept a
1025      response whose age is no greater than the specified time in seconds. Unless
1026      max-stale directive is also included, the client is not willing to accept a stale
1027      response.</t>
1028  </list>
1029</t>
1030<t>
1031  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1032  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1033  max-stale
1034  <list>
1035    <t>The max-stale request directive indicates that the client is willing to accept a
1036      response that has exceeded its expiration time. If max-stale is assigned a value,
1037      then the client is willing to accept a response that has exceeded its expiration
1038      time by no more than the specified number of seconds. If no value is assigned to
1039      max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
1040  </list>
1041</t>
1042<t>
1043  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1044  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1045  min-fresh
1046  <list>
1047    <t>The min-fresh request directive indicates that the client is willing to accept a
1048      response whose freshness lifetime is no less than its current age plus the specified
1049      time in seconds. That is, the client wants a response that will still be fresh for
1050      at least the specified number of seconds.</t>
1051  </list>
1052</t>
1053<t>
1054  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1055  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1056  no-transform
1057  <list>
1058    <t>The no-transform request directive indicates that an intermediate cache or proxy
1059      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1060      headers, nor the request entity-body.</t>
1061  </list>
1062</t>
1063<t>
1064  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1065  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1066  only-if-cached
1067  <list>
1068    <t>The only-if-cached request directive indicates that the client only wishes to
1069      return a stored response. If it receives this directive, a cache &SHOULD; either
1070      respond using a stored response that is consistent with the other constraints of the
1071      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1072      being operated as a unified system with good internal connectivity, such a request
1073      &MAY; be forwarded within that group of caches.</t>
1074  </list>
1075</t>
1076</section>
1077
1078<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1079  <x:anchor-alias value="cache-response-directive" />
1080
1081<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1082  <x:ref>cache-response-directive</x:ref> =
1083       "public"
1084     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1085     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1086     / "no-store"
1087     / "no-transform"
1088     / "must-revalidate"
1089     / "proxy-revalidate"
1090     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1091     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1092     / <x:ref>cache-extension</x:ref>
1093</artwork></figure>
1094
1095<t>
1096  <iref item="Cache Directives" primary="true" subitem="public" />
1097  <iref item="public" primary="true" subitem="Cache Directive" />
1098  public
1099  <list>
1100    <t>The public response directive indicates that the response &MAY; be cached, even
1101      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1102      (See also Authorization, &header-authorization;, for additional details.) </t>
1103  </list>
1104</t>
1105<t>
1106  <iref item="Cache Directives" primary="true" subitem="private" />
1107  <iref item="private" primary="true" subitem="Cache Directive" />
1108  private
1109  <list>
1110    <t>The private response directive indicates that the response message is intended for
1111      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1112      cache &MAY; store the response.</t>
1113    <t>If the private response directive specifies one or more field-names, this
1114      requirement is limited to the field-values associated with the listed response
1115      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1116      cache, whereas the remainder of the response message &MAY; be.</t>
1117    <t>
1118      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1119      be stored, and cannot ensure the privacy of the message content.
1120      Also, private response directives with field-names are often handled by
1121      implementations as if an unqualified private directive was received; i.e.,
1122      the special handling for the qualified form is not widely implemented.
1123</t>
1124  </list>
1125</t>
1126<t>
1127  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1128  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1129  no-cache
1130  <list>
1131    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1132      satisfy a subsequent request without successful validation on the origin server.
1133      This allows an origin server to prevent caching even by caches that have been
1134      configured to return stale responses.</t>
1135    <t>If the no-cache response directive specifies one or more field-names, this
1136      requirement is limited to the field-values associated with the listed response
1137      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1138      to a subsequent request without successful validation on the origin server. This
1139      allows an origin server to prevent the re-use of certain header fields in a
1140      response, while still allowing caching of the rest of the response.</t>
1141    <t>
1142      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1143      Also, no-cache response directives with field-names are often handled by
1144      implementations as if an unqualified no-cache directive was received; i.e.,
1145      the special handling for the qualified form is not widely implemented.
1146    </t>
1147  </list>
1148</t>
1149
1150<t>
1151  <iref item="Cache Directives" primary="true" subitem="no-store" />
1152  <iref item="no-store" primary="true" subitem="Cache Directive" />
1153  no-store
1154  <list>
1155    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1156      part of either the immediate request or response. This directive applies to both
1157      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1158      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1159      and &MUST; make a best-effort attempt to remove the information from volatile
1160      storage as promptly as possible after forwarding it.</t>
1161    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1162      particular, malicious or compromised caches might not recognize or obey this
1163      directive, and communications networks may be vulnerable to eavesdropping.</t>
1164  </list>
1165</t>
1166<t>
1167  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1168  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1169  must-revalidate
1170  <list>
1171    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1172     used to satisfy subsequent requests without successful validation on the origin server.</t>
1173    <t>The must-revalidate directive is necessary to support reliable operation for
1174      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1175      the must-revalidate directive; in particular, if the cache cannot reach the origin
1176      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1177    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1178      validate a request on the entity could result in incorrect operation, such as a
1179      silently unexecuted financial transaction.</t>
1180  </list>
1181</t>
1182<t>
1183  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1184  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1185  proxy-revalidate
1186  <list>
1187    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1188      response directive, except that it does not apply to non-shared caches.</t>
1189  </list>
1190</t>
1191<t>
1192  <iref item="Cache Directives" primary="true" subitem="max-age" />
1193  <iref item="max-age" primary="true" subitem="Cache Directive" />
1194  max-age
1195  <list>
1196    <t>The max-age response directive indicates that response is to be considered stale
1197      after its age is greater than the specified number of seconds.</t>
1198  </list>
1199</t>
1200<t>
1201  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1202  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1203  s-maxage
1204  <list>
1205    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1206      specified by this directive overrides the maximum age specified by either the
1207      max-age directive or the Expires header. The s-maxage directive also implies the
1208      semantics of the proxy-revalidate response directive.</t>
1209  </list>
1210</t>
1211<t>
1212  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1213  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1214  no-transform
1215  <list>
1216    <t>The no-transform response directive indicates that an intermediate cache or proxy
1217      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1218      headers, nor the response entity-body.</t>
1219  </list>
1220</t>
1221
1222</section>
1223
1224<section anchor="cache.control.extensions" title="Cache Control Extensions">
1225<t>
1226  The Cache-Control header field can be extended through the use of one or more
1227  cache-extension tokens, each with an optional value. Informational extensions (those
1228  that do not require a change in cache behavior) can be added without changing the
1229  semantics of other directives. Behavioral extensions are designed to work by acting as
1230  modifiers to the existing base of cache directives. Both the new directive and the
1231  standard directive are supplied, such that applications that do not understand the new
1232  directive will default to the behavior specified by the standard directive, and those
1233  that understand the new directive will recognize it as modifying the requirements
1234  associated with the standard directive. In this way, extensions to the cache-control
1235  directives can be made without requiring changes to the base protocol.
1236</t>
1237<t>
1238  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1239  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1240  all directives that it does not understand.
1241</t>
1242<t>
1243  For example, consider a hypothetical new response directive called "community" that
1244  acts as a modifier to the private directive. We define this new directive to mean that,
1245  in addition to any non-shared cache, any cache that is shared only by members of the
1246  community named within its value may cache the response. An origin server wishing to
1247  allow the UCI community to use an otherwise private response in their shared cache(s)
1248  could do so by including
1249</t>
1250<figure><artwork type="example">
1251  Cache-Control: private, community="UCI"
1252</artwork></figure>
1253<t>
1254  A cache seeing this header field will act correctly even if the cache does not
1255  understand the community cache-extension, since it will also see and understand the
1256  private directive and thus default to the safe behavior.
1257</t>
1258<t>
1259  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1260  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1261  directives (or the response's default cacheability) such that the cache behavior will
1262  remain minimally correct even if the cache does not understand the extension(s).
1263</t>
1264</section>
1265
1266</section>
1267
1268<section anchor="header.expires" title="Expires">
1269  <iref item="Expires header" primary="true" x:for-anchor="" />
1270  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1271  <x:anchor-alias value="Expires"/>
1272  <x:anchor-alias value="Expires-v"/>
1273<t>
1274  The "Expires" entity-header field gives the date/time after which the response is
1275  considered stale. See <xref target="expiration.model" /> for further discussion of the
1276  freshness model.
1277</t>
1278<t>
1279  The presence of an Expires field does not imply that the original resource will change or
1280  cease to exist at, before, or after that time.
1281</t>
1282<t>
1283  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1284  it &MUST; be sent in rfc1123-date format.
1285</t>
1286<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1287  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1288  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1289</artwork></figure>
1290<figure>
1291  <preamble>For example</preamble>
1292<artwork type="example">
1293  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1294</artwork></figure>
1295<x:note>
1296  <t>
1297    <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
1298    directive (see <xref target="cache-response-directive" />), that directive overrides
1299    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1300  </t>
1301</x:note>
1302<t>
1303  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1304</t>
1305<t>
1306  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1307  including the value "0", as in the past (i.e., "already expired").
1308</t>
1309</section>
1310
1311<section anchor="header.pragma" title="Pragma">
1312  <iref item="Pragma header" primary="true" x:for-anchor="" />
1313  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1314  <x:anchor-alias value="extension-pragma"/>
1315  <x:anchor-alias value="Pragma"/>
1316  <x:anchor-alias value="Pragma-v"/>
1317  <x:anchor-alias value="pragma-directive"/>
1318<t>
1319  The "Pragma" general-header field is used to include implementation-specific directives
1320  that might apply to any recipient along the request/response chain. All pragma directives
1321  specify optional behavior from the viewpoint of the protocol; however, some systems
1322  &MAY; require that behavior be consistent with the directives.
1323</t>
1324<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"/>
1325  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1326  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1327  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1328  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1329</artwork></figure>
1330<t>
1331  When the no-cache directive is present in a request message, an application &SHOULD;
1332  forward the request toward the origin server even if it has a cached copy of what is being
1333  requested. This pragma directive has the same semantics as the no-cache response directive
1334  (see <xref target="cache-response-directive" />) and is defined here for backward
1335  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1336  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1337  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1338</t>
1339<x:note>
1340  <t>
1341    <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
1342    is not actually specified, it does not provide a reliable replacement for
1343    "Cache-Control: no-cache" in a response.
1344  </t>
1345</x:note>
1346<t>
1347  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1348</t>
1349</section>
1350
1351<section anchor="header.vary" title="Vary">
1352  <iref item="Vary header" primary="true" x:for-anchor="" />
1353  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1354  <x:anchor-alias value="Vary"/>
1355  <x:anchor-alias value="Vary-v"/>
1356<t>
1357  The "Vary" response-header field conveys the set of request-header fields
1358  that were used to select the representation.
1359</t>
1360<t>
1361  Caches use this information, in part, to determine whether a stored response
1362  can be used to satisfy a given request; see
1363  <xref target="caching.negotiated.responses" />.
1364  determines, while the response is fresh, whether a cache is permitted to use the
1365  response to reply to a subsequent request without validation; see <xref 
1366  target="caching.negotiated.responses" />.
1367</t>
1368<t>
1369  In uncacheable or stale responses, the Vary field value advises the user agent about
1370  the criteria that were used to select the representation.
1371</t>
1372<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1373  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1374  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1375</artwork></figure>
1376<t>
1377  The set of header fields named by the Vary field value is known as the selecting
1378  request-headers.
1379</t>
1380<t>
1381  Servers &SHOULD; include a Vary header field with any cacheable response that is
1382  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1383  requests on that resource and informs the user agent about the presence of negotiation on
1384  that resource. A server &MAY; include a Vary header field with a non-cacheable
1385  response that is subject to server-driven negotiation, since this might provide the user
1386  agent with useful information about the dimensions over which the response varies at the
1387  time of the response.
1388</t>
1389<t>
1390  A Vary field value of "*" signals that unspecified parameters not limited to the
1391  request-headers (e.g., the network address of the client), play a role in the selection of
1392  the response representation; therefore, a cache cannot determine whether this response is
1393  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1394  it may only be generated by an origin server.
1395</t>
1396<t>
1397  The field-names given are not limited to the set of standard request-header fields
1398  defined by this specification. Field names are case-insensitive.
1399</t>
1400</section>
1401
1402<section anchor="header.warning" title="Warning">
1403  <iref item="Warning header" primary="true" x:for-anchor="" />
1404  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1405  <x:anchor-alias value="Warning"/>
1406  <x:anchor-alias value="Warning-v"/>
1407  <x:anchor-alias value="warning-value"/>
1408  <x:anchor-alias value="warn-agent"/>
1409  <x:anchor-alias value="warn-code"/>
1410  <x:anchor-alias value="warn-date"/>
1411  <x:anchor-alias value="warn-text"/>
1412<t>
1413  The "Warning" general-header field is used to carry additional information about the status
1414  or transformation of a message that might not be reflected in the message. This
1415  information is typically used to warn about possible incorrectness introduced by caching
1416  operations or transformations applied to the entity body of the message.
1417</t>
1418<t>
1419  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1420  warning, rather than an error status code, distinguishes these responses from true failures.
1421</t>
1422<t>
1423  Warning headers can in general be applied to any message, however some warn-codes are
1424  specific to caches and can only be applied to response messages.
1425</t>
1426<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"/>
1427  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1428  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1429 
1430  <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>
1431                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1432 
1433  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1434  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1435                  ; the name or pseudonym of the server adding
1436                  ; the Warning header, for use in debugging
1437  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1438  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1439</artwork></figure>
1440<t>
1441  Multiple warnings can be attached to a response (either by the origin server or by
1442  a cache), including multiple warnings with the same code number, only differing
1443  in warn-text.
1444</t>
1445<t>
1446  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1447  possible, in the order that they appear in the response.
1448</t>
1449<t>
1450  Systems that generate multiple Warning headers &SHOULD; order them with this user
1451  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1452  Warning headers.
1453</t>
1454<t>
1455  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1456  Warning is required to be deleted from a stored response after validation:
1457  <list style="symbols">
1458    <t>1xx Warnings describe the freshness or validation status of the response, and so
1459      &MUST; be deleted by caches after validation. They can only be generated by a cache
1460      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1461    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
1462      not rectified by a validation (for example, a lossy compression of the entity bodies)
1463      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1464      returned, in which case they &MUST; be.</t>
1465  </list>
1466</t>
1467<t>
1468  If an implementation sends a message with one or more Warning headers to a receiver whose
1469  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1470  warn-date that matches the Date header in the message.
1471</t>
1472<t>
1473  If an implementation receives a message with a warning-value that includes a warn-date,
1474  and that warn-date is different from the Date value in the response, then that
1475  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1476  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1477  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1478  well.
1479</t>
1480<t>
1481  The following warn-codes are defined by this specification, each with a recommended
1482  warn-text in English, and a description of its meaning.
1483</t>
1484<t><?rfc needLines="4"?>
1485  110 Response is stale
1486  <list>
1487    <t>&SHOULD; be included whenever the returned response is stale.</t>
1488  </list>
1489</t>
1490<t><?rfc needLines="4"?>
1491  111 Revalidation failed
1492  <list>
1493    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1494      validate the response failed, due to an inability to reach the server.</t>
1495  </list>
1496</t>
1497<t><?rfc needLines="4"?>
1498  112 Disconnected operation
1499  <list>
1500    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1501      the network for a period of time.</t>
1502  </list>
1503</t>
1504<t><?rfc needLines="4"?>
1505  113 Heuristic expiration
1506  <list>
1507    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1508      greater than 24 hours and the response's age is greater than 24 hours.</t>
1509  </list>
1510</t>
1511<t><?rfc needLines="4"?>
1512  199 Miscellaneous warning
1513  <list>
1514    <t>The warning text can include arbitrary information to be presented to a human
1515      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1516      action, besides presenting the warning to the user.</t>
1517  </list>
1518</t>
1519<t><?rfc needLines="4"?>
1520  214 Transformation applied
1521  <list>
1522    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1523      transformation changing the content-coding (as specified in the Content-Encoding
1524      header) or media-type (as specified in the Content-Type header) of the response, or
1525      the entity-body of the response, unless this Warning code already appears in the
1526      response.</t>
1527  </list>
1528</t>
1529<t><?rfc needLines="4"?>
1530  299 Miscellaneous persistent warning
1531  <list>
1532    <t>The warning text can include arbitrary information to be presented to a human
1533      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1534      action.</t>
1535  </list>
1536</t>
1537</section>
1538
1539</section>
1540
1541<section anchor="history.lists" title="History Lists">
1542<t>
1543  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1544  can be used to redisplay an entity retrieved earlier in a session.
1545</t>
1546<t>
1547  History mechanisms and caches are different. In particular history mechanisms
1548  &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1549  history mechanism is meant to show exactly what the user saw at the time when the resource
1550  was retrieved.
1551</t>
1552<t>
1553  By default, an expiration time does not apply to history mechanisms. If the entity is still
1554  in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1555  unless the user has specifically configured the agent to refresh expired history documents.
1556</t>
1557<t>
1558  This is not to be construed to prohibit the history mechanism from telling the user that a
1559  view might be stale.
1560</t>
1561<x:note>
1562  <t>
1563    <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1564    stale resources, this will tend to force service authors to avoid using HTTP expiration
1565    controls and cache controls when they would otherwise like to. Service authors may
1566    consider it important that users not be presented with error messages or warning
1567    messages when they use navigation controls (such as BACK) to view previously fetched
1568    resources. Even though sometimes such resources ought not be cached, or ought to expire
1569    quickly, user interface considerations may force service authors to resort to other
1570    means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1571    of improperly functioning history mechanisms.
1572  </t>
1573</x:note>
1574</section>
1575
1576
1577<section anchor="IANA.considerations" title="IANA Considerations">
1578
1579<section anchor="message.header.registration" title="Message Header Registration">
1580<t>
1581  The Message Header Registry located at <eref
1582  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1583  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1584</t>
1585<?BEGININC p6-cache.iana-headers ?>
1586<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1587<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1588   <ttcol>Header Field Name</ttcol>
1589   <ttcol>Protocol</ttcol>
1590   <ttcol>Status</ttcol>
1591   <ttcol>Reference</ttcol>
1592
1593   <c>Age</c>
1594   <c>http</c>
1595   <c>standard</c>
1596   <c>
1597      <xref target="header.age"/>
1598   </c>
1599   <c>Cache-Control</c>
1600   <c>http</c>
1601   <c>standard</c>
1602   <c>
1603      <xref target="header.cache-control"/>
1604   </c>
1605   <c>Expires</c>
1606   <c>http</c>
1607   <c>standard</c>
1608   <c>
1609      <xref target="header.expires"/>
1610   </c>
1611   <c>Pragma</c>
1612   <c>http</c>
1613   <c>standard</c>
1614   <c>
1615      <xref target="header.pragma"/>
1616   </c>
1617   <c>Vary</c>
1618   <c>http</c>
1619   <c>standard</c>
1620   <c>
1621      <xref target="header.vary"/>
1622   </c>
1623   <c>Warning</c>
1624   <c>http</c>
1625   <c>standard</c>
1626   <c>
1627      <xref target="header.warning"/>
1628   </c>
1629</texttable>
1630<!--(END)-->
1631<?ENDINC p6-cache.iana-headers ?>
1632<t>
1633  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1634</t>
1635</section>
1636
1637</section>
1638
1639<section anchor="security.considerations" title="Security Considerations">
1640<t>
1641  Caches expose additional potential vulnerabilities, since the contents of the cache
1642  represent an attractive target for malicious exploitation. Because cache contents persist
1643  after an HTTP request is complete, an attack on the cache can reveal information long after
1644  a user believes that the information has been removed from the network. Therefore, cache
1645  contents should be protected as sensitive information.
1646</t>
1647</section>
1648
1649<section anchor="ack" title="Acknowledgments">
1650<t>
1651  Much of the content and presentation of the caching design is due to suggestions and
1652  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1653  and Larry Masinter.
1654</t>
1655</section>
1656
1657</middle>
1658
1659<back>
1660<references title="Normative References">
1661
1662  <reference anchor="Part1">
1663    <front>
1664      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1665      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1666        <organization abbrev="Day Software">Day Software</organization>
1667        <address><email>fielding@gbiv.com</email></address>
1668      </author>
1669      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1670        <organization>One Laptop per Child</organization>
1671        <address><email>jg@laptop.org</email></address>
1672      </author>
1673      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1674        <organization abbrev="HP">Hewlett-Packard Company</organization>
1675        <address><email>JeffMogul@acm.org</email></address>
1676      </author>
1677      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1678        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1679        <address><email>henrikn@microsoft.com</email></address>
1680      </author>
1681      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1682        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1683        <address><email>LMM@acm.org</email></address>
1684      </author>
1685      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1686        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1687        <address><email>paulle@microsoft.com</email></address>
1688      </author>
1689      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1690        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1691        <address><email>timbl@w3.org</email></address>
1692      </author>
1693      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1694        <organization abbrev="W3C">World Wide Web Consortium</organization>
1695        <address><email>ylafon@w3.org</email></address>
1696      </author>
1697      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1698        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1699        <address><email>julian.reschke@greenbytes.de</email></address>
1700      </author>
1701      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1702    </front>
1703    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1704    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1705  </reference>
1706
1707  <reference anchor="Part2">
1708    <front>
1709      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1710      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1711        <organization abbrev="Day Software">Day Software</organization>
1712        <address><email>fielding@gbiv.com</email></address>
1713      </author>
1714      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1715        <organization>One Laptop per Child</organization>
1716        <address><email>jg@laptop.org</email></address>
1717      </author>
1718      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1719        <organization abbrev="HP">Hewlett-Packard Company</organization>
1720        <address><email>JeffMogul@acm.org</email></address>
1721      </author>
1722      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1723        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1724        <address><email>henrikn@microsoft.com</email></address>
1725      </author>
1726      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1727        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1728        <address><email>LMM@acm.org</email></address>
1729      </author>
1730      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1731        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1732        <address><email>paulle@microsoft.com</email></address>
1733      </author>
1734      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1735        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1736        <address><email>timbl@w3.org</email></address>
1737      </author>
1738      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1739        <organization abbrev="W3C">World Wide Web Consortium</organization>
1740        <address><email>ylafon@w3.org</email></address>
1741      </author>
1742      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1743        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1744        <address><email>julian.reschke@greenbytes.de</email></address>
1745      </author>
1746      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1747    </front>
1748    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1749    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1750  </reference>
1751
1752  <reference anchor="Part3">
1753    <front>
1754      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1755      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1756        <organization abbrev="Day Software">Day Software</organization>
1757        <address><email>fielding@gbiv.com</email></address>
1758      </author>
1759      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1760        <organization>One Laptop per Child</organization>
1761        <address><email>jg@laptop.org</email></address>
1762      </author>
1763      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1764        <organization abbrev="HP">Hewlett-Packard Company</organization>
1765        <address><email>JeffMogul@acm.org</email></address>
1766      </author>
1767      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1768        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1769        <address><email>henrikn@microsoft.com</email></address>
1770      </author>
1771      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1772        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1773        <address><email>LMM@acm.org</email></address>
1774      </author>
1775      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1776        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1777        <address><email>paulle@microsoft.com</email></address>
1778      </author>
1779      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1780        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1781        <address><email>timbl@w3.org</email></address>
1782      </author>
1783      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1784        <organization abbrev="W3C">World Wide Web Consortium</organization>
1785        <address><email>ylafon@w3.org</email></address>
1786      </author>
1787      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1788        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1789        <address><email>julian.reschke@greenbytes.de</email></address>
1790      </author>
1791      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1792    </front>
1793    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1794    <x:source basename="p3-payload" href="p3-payload.xml" />
1795  </reference>
1796
1797  <reference anchor="Part4">
1798    <front>
1799      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1800      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1801        <organization abbrev="Day Software">Day Software</organization>
1802        <address><email>fielding@gbiv.com</email></address>
1803      </author>
1804      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1805        <organization>One Laptop per Child</organization>
1806        <address><email>jg@laptop.org</email></address>
1807      </author>
1808      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1809        <organization abbrev="HP">Hewlett-Packard Company</organization>
1810        <address><email>JeffMogul@acm.org</email></address>
1811      </author>
1812      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1813        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1814        <address><email>henrikn@microsoft.com</email></address>
1815      </author>
1816      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1817        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1818        <address><email>LMM@acm.org</email></address>
1819      </author>
1820      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1821        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1822        <address><email>paulle@microsoft.com</email></address>
1823      </author>
1824      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1825        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1826        <address><email>timbl@w3.org</email></address>
1827      </author>
1828      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1829        <organization abbrev="W3C">World Wide Web Consortium</organization>
1830        <address><email>ylafon@w3.org</email></address>
1831      </author>
1832      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1833        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1834        <address><email>julian.reschke@greenbytes.de</email></address>
1835      </author>
1836      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1837    </front>
1838    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1839    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1840  </reference>
1841
1842  <reference anchor="Part5">
1843    <front>
1844      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1845      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1846        <organization abbrev="Day Software">Day Software</organization>
1847        <address><email>fielding@gbiv.com</email></address>
1848      </author>
1849      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1850        <organization>One Laptop per Child</organization>
1851        <address><email>jg@laptop.org</email></address>
1852      </author>
1853      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1854        <organization abbrev="HP">Hewlett-Packard Company</organization>
1855        <address><email>JeffMogul@acm.org</email></address>
1856      </author>
1857      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1858        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1859        <address><email>henrikn@microsoft.com</email></address>
1860      </author>
1861      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1862        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1863        <address><email>LMM@acm.org</email></address>
1864      </author>
1865      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1866        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1867        <address><email>paulle@microsoft.com</email></address>
1868      </author>
1869      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1870        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1871        <address><email>timbl@w3.org</email></address>
1872      </author>
1873      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1874        <organization abbrev="W3C">World Wide Web Consortium</organization>
1875        <address><email>ylafon@w3.org</email></address>
1876      </author>
1877      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1878        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1879        <address><email>julian.reschke@greenbytes.de</email></address>
1880      </author>
1881      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1882    </front>
1883    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1884    <x:source basename="p5-range" href="p5-range.xml" />
1885  </reference>
1886
1887  <reference anchor="Part7">
1888    <front>
1889      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1890      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1891        <organization abbrev="Day Software">Day Software</organization>
1892        <address><email>fielding@gbiv.com</email></address>
1893      </author>
1894      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1895        <organization>One Laptop per Child</organization>
1896        <address><email>jg@laptop.org</email></address>
1897      </author>
1898      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1899        <organization abbrev="HP">Hewlett-Packard Company</organization>
1900        <address><email>JeffMogul@acm.org</email></address>
1901      </author>
1902      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1903        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1904        <address><email>henrikn@microsoft.com</email></address>
1905      </author>
1906      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1907        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1908        <address><email>LMM@acm.org</email></address>
1909      </author>
1910      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1911        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1912        <address><email>paulle@microsoft.com</email></address>
1913      </author>
1914      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1915        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1916        <address><email>timbl@w3.org</email></address>
1917      </author>
1918      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1919        <organization abbrev="W3C">World Wide Web Consortium</organization>
1920        <address><email>ylafon@w3.org</email></address>
1921      </author>
1922      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1923        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1924        <address><email>julian.reschke@greenbytes.de</email></address>
1925      </author>
1926      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1927    </front>
1928    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1929    <x:source basename="p7-auth" href="p7-auth.xml" />
1930  </reference>
1931
1932  <reference anchor="RFC2119">
1933    <front>
1934      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1935      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1936        <organization>Harvard University</organization>
1937        <address><email>sob@harvard.edu</email></address>
1938      </author>
1939      <date month="March" year="1997" />
1940    </front>
1941    <seriesInfo name="BCP" value="14" />
1942    <seriesInfo name="RFC" value="2119" />
1943  </reference>
1944
1945  <reference anchor="RFC5234">
1946    <front>
1947      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1948      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1949        <organization>Brandenburg InternetWorking</organization>
1950        <address>
1951          <email>dcrocker@bbiw.net</email>
1952        </address> 
1953      </author>
1954      <author initials="P." surname="Overell" fullname="Paul Overell">
1955        <organization>THUS plc.</organization>
1956        <address>
1957          <email>paul.overell@thus.net</email>
1958        </address>
1959      </author>
1960      <date month="January" year="2008"/>
1961    </front>
1962    <seriesInfo name="STD" value="68"/>
1963    <seriesInfo name="RFC" value="5234"/>
1964  </reference>
1965 
1966</references>
1967
1968<references title="Informative References">
1969
1970  <reference anchor="RFC1305">
1971    <front>
1972      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1973      <author fullname="David L. Mills" initials="D." surname="Mills">
1974        <organization>University of Delaware, Electrical Engineering Department</organization>
1975        <address><email>mills@udel.edu</email></address>
1976      </author>
1977      <date month="March" year="1992" />
1978    </front>
1979    <seriesInfo name="RFC" value="1305" />
1980  </reference>
1981
1982  <reference anchor="RFC2616">
1983    <front>
1984      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1985      <author fullname="R. Fielding" initials="R." surname="Fielding">
1986        <organization>University of California, Irvine</organization>
1987        <address><email>fielding@ics.uci.edu</email></address>
1988      </author>
1989      <author fullname="J. Gettys" initials="J." surname="Gettys">
1990        <organization>W3C</organization>
1991        <address><email>jg@w3.org</email></address>
1992      </author>
1993      <author fullname="J. Mogul" initials="J." surname="Mogul">
1994        <organization>Compaq Computer Corporation</organization>
1995        <address><email>mogul@wrl.dec.com</email></address>
1996      </author>
1997      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1998        <organization>MIT Laboratory for Computer Science</organization>
1999        <address><email>frystyk@w3.org</email></address>
2000      </author>
2001      <author fullname="L. Masinter" initials="L." surname="Masinter">
2002        <organization>Xerox Corporation</organization>
2003        <address><email>masinter@parc.xerox.com</email></address>
2004      </author>
2005      <author fullname="P. Leach" initials="P." surname="Leach">
2006        <organization>Microsoft Corporation</organization>
2007        <address><email>paulle@microsoft.com</email></address>
2008      </author>
2009      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2010        <organization>W3C</organization>
2011        <address><email>timbl@w3.org</email></address>
2012      </author>
2013      <date month="June" year="1999" />
2014    </front>
2015    <seriesInfo name="RFC" value="2616" />
2016  </reference>
2017
2018  <reference anchor="RFC3864">
2019    <front>
2020      <title>Registration Procedures for Message Header Fields</title>
2021      <author fullname="G. Klyne" initials="G." surname="Klyne">
2022        <organization>Nine by Nine</organization>
2023        <address><email>GK-IETF@ninebynine.org</email></address>
2024      </author>
2025      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2026        <organization>BEA Systems</organization>
2027        <address><email>mnot@pobox.com</email></address>
2028      </author>
2029      <author fullname="J. Mogul" initials="J." surname="Mogul">
2030        <organization>HP Labs</organization>
2031        <address><email>JeffMogul@acm.org</email></address>
2032      </author>
2033      <date month="September" year="2004" />
2034    </front>
2035    <seriesInfo name="BCP" value="90" />
2036    <seriesInfo name="RFC" value="3864" />
2037  </reference>
2038
2039</references>
2040
2041<section anchor="compatibility" title="Compatibility with Previous Versions">
2042
2043<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2044<t>
2045  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2046  this missing case.
2047  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2048</t>
2049<t>
2050  Transfer-coding and message lengths all interact in ways that required fixing exactly
2051  when chunked encoding is used (to allow for transfer encoding that may not be self
2052  delimiting); it was important to straighten out exactly how message lengths are computed.
2053  (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)
2054  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2055</t>
2056<t>
2057  Proxies should be able to add Content-Length when appropriate.
2058  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2059</t>
2060<t
2061  >Range request responses would become very verbose if all meta-data were always returned;
2062  by allowing the server to only send needed headers in a 206 response, this problem can be
2063  avoided.
2064  (<xref target="combining.headers" />)
2065</t>
2066<t>
2067  The Cache-Control: max-age directive was not properly defined for responses.
2068  (<xref target="cache-response-directive"/>)
2069</t>
2070<t>
2071  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2072    format="counter" target="expiration.model" />, <xref format="counter"
2073    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2074  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2075  header, as PUT or other methods may have need for it in requests.
2076</t>
2077</section>
2078
2079<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2080<t>
2081  Remove requirement to consider Content-Location in successful responses
2082  in order to determine the appropriate response to use.
2083  (<xref target="validation.model" />)
2084</t>
2085<t>
2086  Clarify denial of service attack avoidance requirement.
2087  (<xref target="invalidation.after.updates.or.deletions" />)
2088</t>
2089<t>
2090  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2091  anymore, as these aspects never were implemented.
2092  (<xref target="header.warning" />)
2093</t>
2094</section>
2095
2096    </section>
2097
2098<?BEGININC p6-cache.abnf-appendix ?>
2099<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2100<figure>
2101<artwork type="abnf" name="p6-cache.parsed-abnf">
2102<x:ref>Age</x:ref> = "Age:" OWS Age-v
2103<x:ref>Age-v</x:ref> = delta-seconds
2104
2105<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2106<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2107 cache-directive ] )
2108
2109<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2110<x:ref>Expires-v</x:ref> = HTTP-date
2111
2112<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2113
2114<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2115
2116<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2117<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2118 pragma-directive ] )
2119
2120<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2121<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2122 ] ) )
2123
2124<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2125<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2126 ] )
2127
2128<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2129<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2130<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2131 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2132 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2133 cache-extension
2134<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2135 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2136 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2137 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2138 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2139 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2140
2141<x:ref>delta-seconds</x:ref> = 1*DIGIT
2142
2143<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2144
2145<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2146
2147<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2148<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2149<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2150
2151<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2152
2153<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2154
2155<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2156
2157<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2158<x:ref>warn-code</x:ref> = 3DIGIT
2159<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2160<x:ref>warn-text</x:ref> = quoted-string
2161<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2162 ]
2163</artwork>
2164</figure>
2165<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2166; Age defined but not used
2167; Cache-Control defined but not used
2168; Expires defined but not used
2169; Pragma defined but not used
2170; Vary defined but not used
2171; Warning defined but not used
2172</artwork></figure></section>
2173<?ENDINC p6-cache.abnf-appendix ?>
2174
2175<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2176
2177<section title="Since RFC2616">
2178  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2179</section>
2180
2181<section title="Since draft-ietf-httpbis-p6-cache-00">
2182<t>
2183  Closed issues:
2184  <list style="symbols">
2185    <t>
2186      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2187    <t>
2188      <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>
2189    <t>
2190      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2191    <t>
2192      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2193    <t>
2194      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2195    <t>
2196      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2197    <t>
2198      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2199    <t>
2200      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2201    <t>
2202      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2203  </list>
2204</t>
2205<t>
2206  Other changes:
2207  <list style="symbols">
2208    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2209        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2210  </list>
2211</t>
2212</section>
2213
2214<section title="Since draft-ietf-httpbis-p6-cache-01">
2215<t>
2216  Closed issues:
2217  <list style="symbols">
2218    <t>
2219      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2220  </list>
2221</t>
2222<t>
2223  Other changes:
2224  <list style="symbols">
2225    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2226      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2227    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2228      specification.</t>
2229  </list>
2230</t>
2231</section>
2232
2233<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2234<t>
2235  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2236  <list style="symbols">
2237    <t>Reference RFC 3984, and update header registrations for headers defined in this
2238      document.</t>
2239  </list>
2240</t>
2241</section>
2242
2243<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2244<t>
2245  Closed issues:
2246  <list style="symbols">
2247    <t>
2248      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2249  </list>
2250</t>
2251</section>
2252
2253<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2254<t>
2255  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2256  <list style="symbols"> 
2257    <t>
2258      Use "/" instead of "|" for alternatives.
2259    </t>
2260    <t>
2261      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2262      whitespace ("OWS") and required whitespace ("RWS").
2263    </t>
2264    <t>
2265      Rewrite ABNFs to spell out whitespace rules, factor out
2266      header value format definitions.
2267    </t>
2268  </list>
2269</t>
2270</section>
2271
2272<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2273<t>
2274  This is a total rewrite of this part of the specification.
2275</t>
2276<t>
2277  Affected issues:
2278  <list style="symbols">
2279    <t>
2280      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2281    <t>
2282      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2283    <t>
2284      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2285    <t>
2286      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2287  </list>
2288</t>
2289<t>
2290  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2291  <list style="symbols"> 
2292    <t>
2293      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2294    </t>
2295  </list>
2296</t>
2297</section>
2298
2299<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2300<t>
2301  Closed issues:
2302  <list style="symbols"> 
2303    <t>
2304      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2305      "base for numeric protocol elements"
2306    </t>
2307  </list>
2308</t>
2309<t>
2310  Affected issues:
2311  <list style="symbols">
2312    <t>
2313      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2314      Vary and non-existant headers
2315    </t>
2316  </list>
2317</t>
2318</section>
2319
2320<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2321<t>
2322  Closed issues:
2323  <list style="symbols"> 
2324    <t>
2325      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2326      "Definition of 1xx Warn-Codes"
2327    </t>
2328    <t>
2329      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2330      "Content-Location on 304 responses"
2331    </t>
2332    <t>
2333      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2334      "private and no-cache CC directives with headers"
2335    </t>
2336    <t>
2337      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2338      "RFC2047 and warn-text"
2339    </t>
2340  </list>
2341</t>
2342</section>
2343
2344<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2345<t>
2346  Affected issues:
2347  <list style="symbols">
2348    <t>
2349      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2350      Status codes and caching
2351    </t>
2352  </list>
2353</t>
2354</section>
2355
2356</section>
2357  </back>
2358</rfc>
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