source: draft-ietf-httpbis/latest/p6-cache.xml @ 792

Last change on this file since 792 was 792, checked in by julian.reschke@…, 10 years ago

make age calculation a bit less conservative (related to #29)

  • Property svn:eol-style set to native
File size: 99.0 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "March">
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 full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34]>
35<?rfc toc="yes" ?>
36<?rfc symrefs="yes" ?>
37<?rfc sortrefs="yes" ?>
38<?rfc compact="yes"?>
39<?rfc subcompact="no" ?>
40<?rfc linkmailto="no" ?>
41<?rfc editing="no" ?>
42<?rfc comments="yes"?>
43<?rfc inline="yes"?>
44<?rfc-ext allow-markup-in-artwork="yes" ?>
45<?rfc-ext include-references-in-index="yes" ?>
46<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
47  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
48<front>
49
50  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
51
52  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
53    <organization abbrev="Day Software">Day Software</organization>
54    <address>
55      <postal>
56        <street>23 Corporate Plaza DR, Suite 280</street>
57        <city>Newport Beach</city>
58        <region>CA</region>
59        <code>92660</code>
60        <country>USA</country>
61      </postal>
62      <phone>+1-949-706-5300</phone>
63      <facsimile>+1-949-706-5305</facsimile>
64      <email>fielding@gbiv.com</email>
65      <uri>http://roy.gbiv.com/</uri>
66    </address>
67  </author>
68
69  <author fullname="Jim Gettys" initials="J." surname="Gettys">
70    <organization>One Laptop per Child</organization>
71    <address>
72      <postal>
73        <street>21 Oak Knoll Road</street>
74        <city>Carlisle</city>
75        <region>MA</region>
76        <code>01741</code>
77        <country>USA</country>
78      </postal>
79      <email>jg@laptop.org</email>
80      <uri>http://www.laptop.org/</uri>
81    </address>
82  </author>
83
84  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
85    <organization abbrev="HP">Hewlett-Packard Company</organization>
86    <address>
87      <postal>
88        <street>HP Labs, Large Scale Systems Group</street>
89        <street>1501 Page Mill Road, MS 1177</street>
90        <city>Palo Alto</city>
91        <region>CA</region>
92        <code>94304</code>
93        <country>USA</country>
94      </postal>
95      <email>JeffMogul@acm.org</email>
96    </address>
97  </author>
98
99  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107        <country>USA</country>
108      </postal>
109      <email>henrikn@microsoft.com</email>
110    </address>
111  </author>
112
113  <author fullname="Larry Masinter" initials="L." surname="Masinter">
114    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
115    <address>
116      <postal>
117        <street>345 Park Ave</street>
118        <city>San Jose</city>
119        <region>CA</region>
120        <code>95110</code>
121        <country>USA</country>
122      </postal>
123      <email>LMM@acm.org</email>
124      <uri>http://larry.masinter.net/</uri>
125    </address>
126  </author>
127
128  <author fullname="Paul J. Leach" initials="P." surname="Leach">
129    <organization abbrev="Microsoft">Microsoft Corporation</organization>
130    <address>
131      <postal>
132        <street>1 Microsoft Way</street>
133        <city>Redmond</city>
134        <region>WA</region>
135        <code>98052</code>
136      </postal>
137      <email>paulle@microsoft.com</email>
138    </address>
139  </author>
140
141  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
142    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
143    <address>
144      <postal>
145        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
146        <street>The Stata Center, Building 32</street>
147        <street>32 Vassar Street</street>
148        <city>Cambridge</city>
149        <region>MA</region>
150        <code>02139</code>
151        <country>USA</country>
152      </postal>
153      <email>timbl@w3.org</email>
154      <uri>http://www.w3.org/People/Berners-Lee/</uri>
155    </address>
156  </author>
157
158  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
159    <organization abbrev="W3C">World Wide Web Consortium</organization>
160    <address>
161      <postal>
162        <street>W3C / ERCIM</street>
163        <street>2004, rte des Lucioles</street>
164        <city>Sophia-Antipolis</city>
165        <region>AM</region>
166        <code>06902</code>
167        <country>France</country>
168      </postal>
169      <email>ylafon@w3.org</email>
170      <uri>http://www.raubacapeu.net/people/yves/</uri>
171    </address>
172  </author>
173
174  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
175    <address>
176      <email>mnot@mnot.net</email>
177      <uri>http://www.mnot.net/</uri>
178    </address>
179  </author>
180
181  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
182    <organization abbrev="greenbytes">greenbytes GmbH</organization>
183    <address>
184      <postal>
185        <street>Hafenweg 16</street>
186        <city>Muenster</city><region>NW</region><code>48155</code>
187        <country>Germany</country>
188      </postal>
189      <phone>+49 251 2807760</phone>
190      <facsimile>+49 251 2807761</facsimile>
191      <email>julian.reschke@greenbytes.de</email>
192      <uri>http://greenbytes.de/tech/webdav/</uri>
193    </address>
194  </author>
195
196  <date month="&ID-MONTH;" year="&ID-YEAR;" />
197  <workgroup>HTTPbis Working Group</workgroup>
198
199<abstract>
200<t>
201  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
202  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
203  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
204  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
205  fields that control cache behavior or indicate cacheable response messages.
206</t>
207</abstract>
208
209<note title="Editorial Note (To be removed by RFC Editor)">
210<t>
211  Discussion of this draft should take place on the HTTPBIS working group mailing list
212  (ietf-http-wg@w3.org). The current issues list is at <eref
213  target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
214  (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.
215</t>
216<t>
217  The changes in this draft are summarized in <xref target="changes.since.09"/>.
218</t>
219</note>
220
221  </front>
222  <middle>
223
224<section anchor="caching" title="Introduction">
225<t>
226  HTTP is typically used for distributed information systems, where performance can be
227  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
228  caching and reusing response messages.
229</t>
230
231<section anchor="intro.purpose" title="Purpose">
232<iref item="cache" />
233<t>
234  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
235  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
236  in order to reduce the response time and network bandwidth consumption on future,
237  equivalent requests. Any client or server may include a cache, though a cache cannot be
238  used by a server that is acting as a tunnel.
239</t>
240<t>
241  Caching would be useless if it did not significantly improve performance. The goal of
242  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
243  some cases, a stored response can be reused without the need for a network request,
244  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
245  (see <xref target="expiration.model" />). Even when a new request is required, it is often
246  possible to reuse all or parts of the payload of a prior response to satisfy the request,
247  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
248  purpose (see <xref target="validation.model" />).
249</t>
250</section>
251
252<section anchor="intro.terminology" title="Terminology">
253<t>
254  This specification uses a number of terms to refer to the roles played by participants
255  in, and objects of, HTTP caching.
256</t>
257<t>
258  <iref item="cacheable" />
259  <x:dfn>cacheable</x:dfn>
260  <list>
261    <t>A response is cacheable if a cache is allowed to store a copy of the response message
262      for use in answering subsequent requests. Even when a response is cacheable, there may
263      be additional constraints on whether a cache can use the cached copy to satisfy a
264      particular request.</t>
265  </list>
266</t>
267<t>
268  <iref item="explicit expiration time" />
269  <x:dfn>explicit expiration time</x:dfn>
270  <list>
271    <t>The time at which the origin server intends that an entity should no longer be
272      returned by a cache without further validation.</t>
273  </list>
274</t>
275<t>
276  <iref item="heuristic expiration time" />
277  <x:dfn>heuristic expiration time</x:dfn>
278  <list>
279    <t>An expiration time assigned by a cache when no explicit expiration time is
280    available.</t>
281  </list>
282</t>
283<t>
284  <iref item="age" />
285  <x:dfn>age</x:dfn>
286  <list>
287    <t>The age of a response is the time since it was sent by, or successfully validated
288      with, the origin server.</t>
289  </list>
290</t>
291<t>
292  <iref item="first-hand" />
293  <x:dfn>first-hand</x:dfn>
294  <list>
295    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
296    0.</t>
297  </list>
298</t>
299<t>
300  <iref item="freshness lifetime" />
301  <x:dfn>freshness lifetime</x:dfn>
302  <list>
303    <t>The length of time between the generation of a response and its expiration time. </t>
304  </list>
305</t>
306<t>
307  <iref item="fresh" />
308  <x:dfn>fresh</x:dfn>
309  <list>
310    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
311  </list>
312</t>
313<t>
314  <iref item="stale" />
315  <x:dfn>stale</x:dfn>
316  <list>
317    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
318  </list>
319</t>
320<t>
321  <iref item="validator" />
322  <x:dfn>validator</x:dfn>
323  <list>
324    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
325      out whether a stored response is an equivalent copy of an entity.</t>
326  </list>
327</t>
328<t anchor="shared.and.non-shared.caches">
329  <iref item="validator" />
330  <x:dfn>shared cache</x:dfn>
331  <list>
332    <t>A cache that is accessible to more than one user. A non-shared cache is
333      dedicated to a single user.</t>
334  </list>
335</t>
336</section>
337
338<section anchor="intro.requirements" title="Requirements">
339<t>
340  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
341  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
342  described in <xref target="RFC2119" />.
343</t>
344<t>
345  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
346  or &REQUIRED; level requirements for the protocols it implements. An implementation
347  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
348  requirements for its protocols is said to be "unconditionally compliant"; one that
349  satisfies all the &MUST; level requirements but not all the &SHOULD; level
350  requirements for its protocols is said to be "conditionally compliant."
351</t>
352</section>
353
354<section title="Syntax Notation" anchor="notation">
355  <x:anchor-alias value="ALPHA"/>
356  <x:anchor-alias value="CR"/>
357  <x:anchor-alias value="DIGIT"/>
358  <x:anchor-alias value="DQUOTE"/>
359  <x:anchor-alias value="LF"/>
360  <x:anchor-alias value="OCTET"/>
361  <x:anchor-alias value="SP"/>
362  <x:anchor-alias value="VCHAR"/>
363  <x:anchor-alias value="WSP"/>
364<t>
365  This specification uses the ABNF syntax defined in &notation; (which
366  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
367  <xref target="collected.abnf"/> shows the collected ABNF, with the list
368  rule expanded.
369</t>
370<t>
371  The following core rules are included by
372  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
373  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
374  DIGIT (decimal 0-9), DQUOTE (double quote),
375  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
376  OCTET (any 8-bit sequence of data), SP (space),
377  VCHAR (any visible USASCII character),
378  and WSP (whitespace).
379</t>
380
381<section title="Core Rules" anchor="core.rules">
382  <x:anchor-alias value="quoted-string"/>
383  <x:anchor-alias value="token"/>
384  <x:anchor-alias value="OWS"/>
385<t>
386  The core rules below are defined in &basic-rules;:
387</t>
388<figure><artwork type="abnf2616">
389  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
390  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
391  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
392</artwork></figure>
393</section>
394
395<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
396  <x:anchor-alias value="field-name"/>
397  <x:anchor-alias value="HTTP-date"/>
398  <x:anchor-alias value="port"/>
399  <x:anchor-alias value="pseudonym"/>
400  <x:anchor-alias value="uri-host"/>
401<t>
402  The ABNF rules below are defined in other parts:
403</t>
404<figure><!--Part1--><artwork type="abnf2616">
405  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
406  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
407  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
408  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
409  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
410</artwork></figure>
411</section>
412
413</section>
414</section>
415
416<section anchor="caching.overview" title="Cache Operation">
417
418<section anchor="response.cacheability" title="Response Cacheability">
419<t>
420  A cache &MUST-NOT; store a response to any request, unless:
421  <list style="symbols">
422    <t>The request method is understood by the cache and defined as being cacheable, and</t>
423    <t>the response status code is understood by the cache, and</t>
424    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
425       appear in request or response headers, and</t>
426    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
427       does not appear in the response, if the cache is shared, and</t>
428    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
429       the cache is shared (unless the "public" directive is present; see <xref
430       target="header.cache-control" />), and</t>
431    <t>the response either:
432      <list style="symbols">
433        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
434        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
435        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
436        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
437        <t>has a status code that can be served with heuristic freshness (see <xref
438           target="heuristic.freshness" />).</t>
439      </list>
440    </t>
441  </list>
442</t>
443<t>
444  In this context, a cache has "understood" a request method or a response status
445  code if it recognises it and implements any cache-specific behaviour. In
446  particular, 206 Partial Content responses cannot be cached by an
447  implementation that does not handle partial content
448  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
449</t>
450<t>
451  Note that in normal operation, most caches will not store a response that has neither a
452  cache validator nor an explicit expiration time, as such responses are not usually
453  useful to store. However, caches are not prohibited from storing such responses.
454</t>
455
456<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
457<t>
458  A cache that receives an incomplete response (for example, with fewer bytes of data
459  than specified in a Content-Length header) can store the response, but &MUST; 
460  treat it as a partial response &partial;. Partial responses
461  can be combined as described in &combining-byte-ranges;; the result might be a
462  full response or might still be partial. A cache &MUST-NOT; return a partial
463  response to a client without explicitly marking it as such using the 206 (Partial
464  Content) status code.
465</t>
466<t>
467  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
468  incomplete or partial responses.
469</t>
470</section>
471
472</section>
473
474
475<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
476<t>
477  For a presented request, a cache &MUST-NOT; return a stored response, unless:
478  <list style="symbols">
479    <t>The presented Request-URI and that of the stored response match
480      (<cref anchor="TODO-Request-URI">Need to find a new term for this, as Part
481      1 doesn't define Request-URI anymore; the new term request-target does not
482      work for this. (see <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196" />)</cref>), and</t>
483    <t>the request method associated with the stored response allows it to be
484      used for the presented request, and</t>
485    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
486      target="caching.negotiated.responses" />), and</t>
487    <t>the presented request and stored response are free from directives that would prevent
488      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
489      and</t>
490    <t>the stored response is either:
491      <list style="symbols">
492        <t>fresh (see <xref target="expiration.model" />), or</t>
493        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
494        <t>successfully validated (see <xref target="validation.model" />).</t>
495      </list>
496    </t>
497  </list>
498</t>
499<t>
500  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
501</t>
502<t>
503  When a stored response is used to satisfy a request, caches &MUST; include a
504  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
505  current_age; see <xref target="age.calculations" />.
506  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
507</t>
508<t>
509  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
510  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
511  corresponding response.
512</t>
513<t>
514  Also, note that unsafe requests might invalidate already stored responses; see
515  <xref target="invalidation.after.updates.or.deletions" />.
516</t>
517<t>
518  Caches &MUST; use the most recent response (as determined by the Date header) when
519  more than one suitable response is stored. They can also forward a request with
520  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
521  use.
522</t>
523</section>
524
525<section anchor="expiration.model" title="Freshness Model">
526<t>
527  When a response is "fresh" in the cache, it can be used to satisfy subsequent
528  requests without contacting the origin server, thereby improving efficiency.
529</t>
530<t>
531  The primary mechanism for determining freshness is for an origin server to provide an
532  explicit expiration time in the future, using either the Expires header (<xref
533  target="header.expires" />) or the max-age response cache directive (<xref
534  target="cache-response-directive" />). Generally, origin servers will assign future
535  explicit expiration times to responses in the belief that the entity is not likely to
536  change in a semantically significant way before the expiration time is reached.
537</t>
538<t>
539  If an origin server wishes to force a cache to validate every request, it can
540  assign an explicit expiration time in the past. This means that the response is always
541  stale, so that caches should validate it before using it for subsequent requests.
542  <cref anchor="TODO-response-stale">This wording may cause confusion, because the response may still be served stale.</cref>
543</t>
544<t>
545  Since origin servers do not always provide explicit expiration times, HTTP caches may
546  also assign heuristic expiration times when they are not specified, employing algorithms that
547  use other header values (such as the Last-Modified time) to estimate a plausible
548  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
549  impose worst-case constraints on their results.
550</t>
551<figure>
552<preamble>
553  The calculation to determine if a response is fresh is:
554</preamble>
555<artwork type="code">
556   response_is_fresh = (freshness_lifetime &gt; current_age)
557</artwork>
558</figure>
559<t>
560  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
561  the current_age is defined in <xref target="age.calculations" />.
562</t>
563<t>
564  Additionally, clients may need to influence freshness calculation. They can do this using
565  several request cache directives, with the effect of either increasing or loosening
566  constraints on freshness. See <xref target="cache-request-directive" />.
567</t>
568<t>
569  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
570  freshness.</cref>
571</t>
572<t>
573  Note that freshness applies only to cache operation; it cannot be used to force a user agent
574  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
575  the difference between caches and history mechanisms.
576</t>
577
578<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
579<t>
580  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
581  response by using the first match of:
582  <list style="symbols">
583    <t>If the cache is shared and the s-maxage response cache directive (<xref
584      target="cache-response-directive" />) is present, use its value, or</t>
585    <t>If the max-age response cache directive (<xref target="cache-response-directive"
586      />) is present, use its value, or</t>
587    <t>If the Expires response header (<xref target="header.expires" />) is present, use
588      its value minus the value of the Date response header, or</t>
589    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
590      freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
591  </list>
592</t>
593<t>
594  Note that this calculation is not vulnerable to clock skew, since all of the
595  information comes from the origin server.
596</t>
597
598<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
599<t>
600  If no explicit expiration time is present in a stored response that has a status code
601  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
602  calculated. Heuristics &MUST-NOT; be used for other response status codes.
603</t>
604<t>
605  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
606  attach a Warning header with a 113 warn-code to the response if its current_age is
607  more than 24 hours and such a warning is not already present.
608</t>
609<t>
610  Also, if the response has a Last-Modified header (&header-last-modified;), the
611  heuristic expiration value &SHOULD; be no more than some fraction of the interval
612  since that time. A typical setting of this fraction might be 10%.
613</t>
614<t>
615  <cref anchor="REVIEW-query-string-heuristics">took away HTTP/1.0 query string heuristic uncacheability.</cref>
616</t>
617</section>
618</section>
619
620<section anchor="age.calculations" title="Calculating Age">
621<t>
622  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
623  message when obtained from a cache. The Age field value is the cache's estimate of the
624  amount of time since the response was generated or validated by the origin server. In
625  essence, the Age value is the sum of the time that the response has been resident in
626  each of the caches along the path from the origin server, plus the amount of time it has
627  been in transit along network paths.
628</t>
629<t>
630  The following data is used for the age calculation:
631</t>
632<t>
633  <x:dfn>age_value</x:dfn>
634  <list>
635    <t>
636      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
637      in a form appropriate for arithmetic operation; or 0, if not available.
638    </t>
639  </list>
640</t>
641<t>
642  <x:dfn>date_value</x:dfn>
643  <list>
644    <t>
645      HTTP/1.1 requires origin servers to send a Date header, if possible,
646      with every response, giving the time at which the response was generated.
647      The term "date_value" denotes the value of the Date header, in a form
648      appropriate for arithmetic operations. See &header-date; for the definition
649      of the Date header, and for requirements regarding responses without a
650      Date response header.
651    </t>
652  </list>
653</t>
654<t>
655  <x:dfn>now</x:dfn>
656  <list>
657    <t>
658      The term "now" means "the current value of the clock at the host
659      performing the calculation". Hosts that use HTTP, but especially hosts
660      running origin servers and caches, &SHOULD; use NTP
661      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
662      clocks to a globally accurate time standard.
663    </t>
664  </list>
665</t>
666<t>
667  <x:dfn>request_time</x:dfn>
668  <list>
669    <t>
670      The current value of the clock at the host at the time the request
671      resulting in the stored response was made.
672    </t>
673  </list>
674</t>
675<t>
676  <x:dfn>response_time</x:dfn>
677  <list>
678    <t>
679      The current value of the clock at the host at the time the response was
680      received.
681    </t>
682  </list>
683</t>
684<t>
685  A response's age can be calculated in two entirely independent ways:
686  <list style="numbers">
687    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
688      origin server's clock. If the result is negative, the result is replaced by zero.</t>
689    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
690      note this value &MUST; be interpreted relative to the time the
691      request was initiated, not the time that the response was received.</t>
692  </list>
693</t>
694<figure>
695<artwork type="code">
696  apparent_age = max(0, response_time - date_value);
697
698  response_delay = response_time - request_time;
699  corrected_age_value = age_value + response_delay; 
700</artwork>
701</figure>
702<figure>
703<preamble>These are combined as</preamble>
704<artwork type="code">
705  corrected_initial_age = max(apparent_age, corrected_age_value);
706</artwork></figure>
707<t>
708  The current_age of a stored response can then be calculated by adding the amount of
709  time (in seconds) since the stored response was last validated by the origin server to
710  the corrected_initial_age.
711</t>
712<figure><artwork type="code">
713  resident_time = now - response_time;
714  current_age = corrected_initial_age + resident_time;
715</artwork></figure>
716</section>
717
718<section anchor="serving.stale.responses" title="Serving Stale Responses">
719<t>
720  A "stale" response is one that either has explicit expiry information or is allowed to
721  have heuristic expiry calculated, but is not fresh according to the calculations in
722  <xref target="expiration.model" />.
723</t>
724<t>
725  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
726  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
727  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
728  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
729</t>
730<t>
731  Caches &SHOULD-NOT; return stale responses unless they are
732  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
733  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
734</t>
735<t>
736  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
737  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
738  the cache is disconnected.
739</t>
740<t>
741  If a cache receives a first-hand response (either an entire response, or a 304 (Not
742  Modified) response) that it would normally forward to the requesting client, and the
743  received response is no longer fresh, the cache &SHOULD; forward it to the
744  requesting client without adding a new Warning (but without removing any existing
745  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
746  that response became stale in transit.
747</t>
748</section>
749</section>
750
751<section anchor="validation.model" title="Validation Model">
752<t>
753  When a cache has one or more stored responses for a requested URI, but cannot 
754  serve any of them (e.g., because they are not fresh, or one cannot be selected;
755  see <xref target="caching.negotiated.responses"/>),
756  it can use the conditional request mechanism &conditional; in the forwarded
757  request to give the origin server an opportunity to both select a valid stored
758  response to be used, and to update it. This process is known as "validating"
759  or "revalidating" the stored response.
760</t>
761<t>
762  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
763  header whose value is that of the Last-Modified header from the selected
764  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
765</t>
766<t>
767  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
768  is that of the ETag header(s) from all responses stored for the requested URI,
769  if present. However, if any of the stored responses contains only partial
770  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
771  field unless the request is for a range that would be fully satisfied by
772  that stored response.
773</t>
774<t>
775  A 304 (Not Modified) response status code indicates that the stored
776  response can be updated and reused; see <xref target="combining.headers"/>.
777</t>
778<t>
779  A full response (i.e., one with a response body) indicates that none 
780  of the stored responses nominated in the conditional request is
781  suitable. Instead, the full response is used both to satisfy the
782  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
783</t>
784<t>
785  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
786  either forward this response to the requesting client, or act as if the server failed to
787  respond. In the latter case, it &MAY; return a previously stored response (see <xref
788  target="serving.stale.responses" />).
789</t>
790</section>
791
792<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
793<t>
794  Because unsafe methods (&safe-methods;) have the potential for changing state on the
795  origin server, intervening caches can use them to keep their contents
796  up-to-date.
797</t>
798<t>
799  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
800  as the URI(s) in the Location and Content-Location headers (if present):
801  <list style="symbols">
802    <t>PUT</t>
803    <t>DELETE</t>
804    <t>POST</t>
805  </list>
806</t>
807<t>
808  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
809  be performed if the host part of that URI differs from the host part in the Request-URI.
810  This helps prevent denial of service attacks.
811</t>
812<t>
813  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
814</t>
815<t>
816  A cache that passes through requests for methods it does not understand &SHOULD;
817  invalidate the Request-URI.
818</t>
819<t>
820  Here, "invalidate" means that the cache will either remove all stored responses related
821  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
822  before they can be returned in response to a subsequent request.
823</t>
824<t>
825  Note that this does not guarantee that all appropriate responses are invalidated. For
826  example, the request that caused the change at the origin server might not have gone
827  through the cache where a response is stored.
828</t>
829<t>
830  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
831</t>
832</section>
833
834<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
835<t>
836  When a cache receives a request that can be satisfied by a stored response
837  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
838  response unless all of the selecting request-headers nominated by the Vary header match
839  in both the original request (i.e., that associated with the stored response),
840  and the presented request.
841</t>
842<t>
843  The selecting request-headers from two requests are defined to match
844  if and only if those in the first request can be transformed to those in the
845  second request by applying any of the following:
846  <list style="symbols">
847    <t>
848      adding or removing whitespace, where allowed in the header's syntax
849    </t>
850    <t>
851      combining multiple message-header fields with the same field name (see
852      &header-fields;)
853    </t>
854    <t>
855      normalizing both header values in a way that is known to have identical
856      semantics, according to the header's specification (e.g., re-ordering field values
857      when order is not significant; case-normalization, where values are defined to be
858      case-insensitive)   
859    </t>
860  </list>
861</t>
862<t>
863  If (after any normalisation that may take place) a header field is absent
864  from a request, it can only match another request if it is also absent there.
865</t>
866<t>
867  A Vary header field-value of "*" always fails to match, and subsequent requests to that
868  resource can only be properly interpreted by the origin server.
869</t>
870<t>
871  The stored response with matching selecting request-headers is known as the
872  selected response.
873</t>
874<t>
875  If no selected response is available, the cache &MAY; forward the presented
876  request to the origin server in a conditional request; see <xref target="validation.model"/>.
877</t>
878</section>
879
880<section anchor="combining.headers" title="Combining Responses">
881<t>
882  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
883  (in this section, the "new" response"), it needs to created an updated response by combining
884  the stored response with the new one, so that the updated response can be used to satisfy the request.
885</t>
886<t>
887  If the new response contains an ETag, it identifies the stored 
888  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
889  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
890</t>
891<t>
892  If the status code is 206 (partial content), both the stored and new 
893  responses &MUST; have validators, and those validators &MUST; match using the strong 
894  comparison function (see &weak-and-strong;). Otherwise, the 
895  responses &MUST-NOT; be combined.
896</t>
897<t>
898  The stored response headers are used as those of the updated response, except that
899  <list style="symbols">
900    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
901      &MUST; be deleted from the stored response and the updated response.</t>
902    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
903      response and the updated response.</t>
904    <t>any headers provided in the new response &MUST; replace the corresponding
905      headers from the stored response.</t>
906  </list>
907</t>
908<t>
909  If a header field-name in the new response matches more than one 
910  header in the stored response, all such stored headers &MUST; be replaced.
911</t>
912<t>
913  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
914  stored response in cache. In the case of a 206 response, the combined 
915  entity-body &MAY; be stored.
916</t>
917<t>
918  <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
919</t>
920</section>
921
922</section>
923
924<section anchor="header.fields" title="Header Field Definitions">
925<t>
926  This section defines the syntax and semantics of HTTP/1.1 header fields
927  related to caching.
928</t>
929<t>
930  For entity-header fields, both sender and recipient refer to either the client or the
931  server, depending on who sends and who receives the entity.
932</t>
933
934<section anchor="header.age" title="Age">
935  <iref item="Age header" primary="true" x:for-anchor="" />
936  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
937  <x:anchor-alias value="Age"/>
938  <x:anchor-alias value="Age-v"/>
939  <x:anchor-alias value="age-value"/>
940<t>
941  The "Age" response-header field conveys the sender's estimate of the amount
942  of time since the response was generated or successfully validated at the
943  origin server. Age values are calculated as specified in
944  <xref target="age.calculations" />.
945</t>
946<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
947  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
948  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
949</artwork></figure>
950<t anchor="rule.delta-seconds">
951  <x:anchor-alias value="delta-seconds" />
952  Age field-values are non-negative integers, representing time in seconds.
953</t>
954<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
955  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
956</artwork></figure>
957<t>
958  If a cache receives a value larger than the largest positive integer it can represent, or
959  if any of its age calculations overflows, it &MUST; transmit an Age header with a
960  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
961  of at least 31 bits of range.
962</t>
963<t>
964  The presence of an Age header field in a response implies that a response is not
965  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
966  Age header field.
967</t>
968</section>
969
970<section anchor="header.cache-control" title="Cache-Control">
971  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
972  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
973  <x:anchor-alias value="Cache-Control"/>
974  <x:anchor-alias value="Cache-Control-v"/>
975  <x:anchor-alias value="cache-directive"/>
976  <x:anchor-alias value="cache-extension"/>
977  <x:anchor-alias value="cache-request-directive"/>
978  <x:anchor-alias value="cache-response-directive"/>
979<t>
980  The "Cache-Control" general-header field is used to specify directives that
981  &MUST; be obeyed by all caches along the request/response chain. Such cache
982  directives are unidirectional in that the presence of a directive in a
983  request does not imply that the same directive is to be given in the
984  response.
985</t>
986<x:note>
987  <t>
988    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
989    Pragma: no-cache (see <xref target="header.pragma" />).
990  </t>
991</x:note>
992<t>
993  Cache directives &MUST; be passed through by a proxy or gateway application,
994  regardless of their significance to that application, since the directives might be
995  applicable to all recipients along the request/response chain. It is not possible to
996  target a directive to a specific cache.
997</t>
998<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"/>
999  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1000  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1001
1002  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1003     / <x:ref>cache-response-directive</x:ref>
1004
1005  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1006</artwork></figure>
1007
1008<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1009  <x:anchor-alias value="cache-request-directive" />
1010
1011<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
1012  <x:ref>cache-request-directive</x:ref> =
1013       "no-cache"
1014     / "no-store"
1015     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1016     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1017     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1018     / "no-transform"
1019     / "only-if-cached"
1020     / <x:ref>cache-extension</x:ref>
1021</artwork></figure>
1022
1023<t>
1024  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1025  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1026  no-cache
1027  <list>
1028    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1029      used to satisfy the request without successful validation on the origin server.</t>
1030  </list>
1031</t>
1032<t>
1033  <iref item="Cache Directives" primary="true" subitem="no-store" />
1034  <iref item="no-store" primary="true" subitem="Cache Directive" />
1035  no-store
1036  <list>
1037    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1038      of either this request or any response to it. This directive applies to both
1039      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1040      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1041      and &MUST; make a best-effort attempt to remove the information from volatile
1042      storage as promptly as possible after forwarding it.</t>
1043    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1044      particular, malicious or compromised caches might not recognize or obey this
1045      directive, and communications networks may be vulnerable to eavesdropping.</t>
1046  </list>
1047</t>
1048<t>
1049  <iref item="Cache Directives" primary="true" subitem="max-age" />
1050  <iref item="max-age" primary="true" subitem="Cache Directive" />
1051  max-age
1052  <list>
1053    <t>The max-age request directive indicates that the client is willing to accept a
1054      response whose age is no greater than the specified time in seconds. Unless
1055      the max-stale request directive is also present, the client is not willing to accept a stale
1056      response.</t>
1057  </list>
1058</t>
1059<t>
1060  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1061  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1062  max-stale
1063  <list>
1064    <t>The max-stale request directive indicates that the client is willing to accept a
1065      response that has exceeded its expiration time. If max-stale is assigned a value,
1066      then the client is willing to accept a response that has exceeded its expiration
1067      time by no more than the specified number of seconds. If no value is assigned to
1068      max-stale, then the client is willing to accept a stale response of any age. <cref anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
1069  </list>
1070</t>
1071<t>
1072  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1073  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1074  min-fresh
1075  <list>
1076    <t>The min-fresh request directive indicates that the client is willing to accept a
1077      response whose freshness lifetime is no less than its current age plus the specified
1078      time in seconds. That is, the client wants a response that will still be fresh for
1079      at least the specified number of seconds.</t>
1080  </list>
1081</t>
1082<t>
1083  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1084  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1085  no-transform
1086  <list>
1087    <t>The no-transform request directive indicates that an intermediate cache or proxy
1088      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1089      headers, nor the request entity-body.</t>
1090  </list>
1091</t>
1092<t>
1093  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1094  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1095  only-if-cached
1096  <list>
1097    <t>The only-if-cached request directive indicates that the client only wishes to
1098      return a stored response. If it receives this directive, a cache &SHOULD; either
1099      respond using a stored response that is consistent with the other constraints of the
1100      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1101      being operated as a unified system with good internal connectivity, such a request
1102      &MAY; be forwarded within that group of caches.</t>
1103  </list>
1104</t>
1105</section>
1106
1107<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1108  <x:anchor-alias value="cache-response-directive" />
1109
1110<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1111  <x:ref>cache-response-directive</x:ref> =
1112       "public"
1113     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1114     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1115     / "no-store"
1116     / "no-transform"
1117     / "must-revalidate"
1118     / "proxy-revalidate"
1119     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1120     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1121     / <x:ref>cache-extension</x:ref>
1122</artwork></figure>
1123
1124<t>
1125  <iref item="Cache Directives" primary="true" subitem="public" />
1126  <iref item="public" primary="true" subitem="Cache Directive" />
1127  public
1128  <list>
1129    <t>The public response directive indicates that the response &MAY; be cached, even
1130      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1131      (See also Authorization, &header-authorization;, for additional details.) </t>
1132  </list>
1133</t>
1134<t>
1135  <iref item="Cache Directives" primary="true" subitem="private" />
1136  <iref item="private" primary="true" subitem="Cache Directive" />
1137  private
1138  <list>
1139    <t>The private response directive indicates that the response message is intended for
1140      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1141      cache &MAY; store the response.</t>
1142    <t>If the private response directive specifies one or more field-names, this
1143      requirement is limited to the field-values associated with the listed response
1144      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1145      cache, whereas the remainder of the response message &MAY; be.</t>
1146    <t>
1147      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1148      be stored, and cannot ensure the privacy of the message content.
1149      Also, private response directives with field-names are often handled by
1150      implementations as if an unqualified private directive was received; i.e.,
1151      the special handling for the qualified form is not widely implemented.
1152</t>
1153  </list>
1154</t>
1155<t>
1156  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1157  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1158  no-cache
1159  <list>
1160    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1161      satisfy a subsequent request without successful validation on the origin server.
1162      This allows an origin server to prevent caching even by caches that have been
1163      configured to return stale responses.</t>
1164    <t>If the no-cache response directive specifies one or more field-names, this
1165      requirement is limited to the field-values associated with the listed response
1166      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1167      to a subsequent request without successful validation on the origin server. This
1168      allows an origin server to prevent the re-use of certain header fields in a
1169      response, while still allowing caching of the rest of the response.</t>
1170    <t>
1171      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1172      Also, no-cache response directives with field-names are often handled by
1173      implementations as if an unqualified no-cache directive was received; i.e.,
1174      the special handling for the qualified form is not widely implemented.
1175    </t>
1176  </list>
1177</t>
1178
1179<t>
1180  <iref item="Cache Directives" primary="true" subitem="no-store" />
1181  <iref item="no-store" primary="true" subitem="Cache Directive" />
1182  no-store
1183  <list>
1184    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1185      part of either the immediate request or response. This directive applies to both
1186      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1187      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1188      and &MUST; make a best-effort attempt to remove the information from volatile
1189      storage as promptly as possible after forwarding it.</t>
1190    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1191      particular, malicious or compromised caches might not recognize or obey this
1192      directive, and communications networks may be vulnerable to eavesdropping.</t>
1193  </list>
1194</t>
1195<t>
1196  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1197  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1198  must-revalidate
1199  <list>
1200    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1201     used to satisfy subsequent requests without successful validation on the origin server.</t>
1202    <t>The must-revalidate directive is necessary to support reliable operation for
1203      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1204      the must-revalidate directive; in particular, if the cache cannot reach the origin
1205      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1206    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1207      validate a request on the entity could result in incorrect operation, such as a
1208      silently unexecuted financial transaction.</t>
1209  </list>
1210</t>
1211<t>
1212  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1213  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1214  proxy-revalidate
1215  <list>
1216    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1217      response directive, except that it does not apply to non-shared caches.</t>
1218  </list>
1219</t>
1220<t>
1221  <iref item="Cache Directives" primary="true" subitem="max-age" />
1222  <iref item="max-age" primary="true" subitem="Cache Directive" />
1223  max-age
1224  <list>
1225    <t>The max-age response directive indicates that response is to be considered stale
1226      after its age is greater than the specified number of seconds.</t>
1227  </list>
1228</t>
1229<t>
1230  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1231  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1232  s-maxage
1233  <list>
1234    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1235      specified by this directive overrides the maximum age specified by either the
1236      max-age directive or the Expires header. The s-maxage directive also implies the
1237      semantics of the proxy-revalidate response directive.</t>
1238  </list>
1239</t>
1240<t>
1241  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1242  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1243  no-transform
1244  <list>
1245    <t>The no-transform response directive indicates that an intermediate cache or proxy
1246      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1247      headers, nor the response entity-body.</t>
1248  </list>
1249</t>
1250
1251</section>
1252
1253<section anchor="cache.control.extensions" title="Cache Control Extensions">
1254<t>
1255  The Cache-Control header field can be extended through the use of one or more
1256  cache-extension tokens, each with an optional value. Informational extensions (those
1257  that do not require a change in cache behavior) can be added without changing the
1258  semantics of other directives. Behavioral extensions are designed to work by acting as
1259  modifiers to the existing base of cache directives. Both the new directive and the
1260  standard directive are supplied, such that applications that do not understand the new
1261  directive will default to the behavior specified by the standard directive, and those
1262  that understand the new directive will recognize it as modifying the requirements
1263  associated with the standard directive. In this way, extensions to the cache-control
1264  directives can be made without requiring changes to the base protocol.
1265</t>
1266<t>
1267  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1268  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1269  all directives that it does not understand.
1270</t>
1271<t>
1272  For example, consider a hypothetical new response directive called "community" that
1273  acts as a modifier to the private directive. We define this new directive to mean that,
1274  in addition to any non-shared cache, any cache that is shared only by members of the
1275  community named within its value may cache the response. An origin server wishing to
1276  allow the UCI community to use an otherwise private response in their shared cache(s)
1277  could do so by including
1278</t>
1279<figure><artwork type="example">
1280  Cache-Control: private, community="UCI"
1281</artwork></figure>
1282<t>
1283  A cache seeing this header field will act correctly even if the cache does not
1284  understand the community cache-extension, since it will also see and understand the
1285  private directive and thus default to the safe behavior.
1286</t>
1287<t>
1288  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1289  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1290  directives (or the response's default cacheability) such that the cache behavior will
1291  remain minimally correct even if the cache does not understand the extension(s).
1292</t>
1293</section>
1294
1295</section>
1296
1297<section anchor="header.expires" title="Expires">
1298  <iref item="Expires header" primary="true" x:for-anchor="" />
1299  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1300  <x:anchor-alias value="Expires"/>
1301  <x:anchor-alias value="Expires-v"/>
1302<t>
1303  The "Expires" entity-header field gives the date/time after which the response is
1304  considered stale. See <xref target="expiration.model" /> for further discussion of the
1305  freshness model.
1306</t>
1307<t>
1308  The presence of an Expires field does not imply that the original resource will change or
1309  cease to exist at, before, or after that time.
1310</t>
1311<t>
1312  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1313  it &MUST; be sent in rfc1123-date format.
1314</t>
1315<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1316  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1317  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1318</artwork></figure>
1319<figure>
1320  <preamble>For example</preamble>
1321<artwork type="example">
1322  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1323</artwork></figure>
1324<x:note>
1325  <t>
1326    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1327    directive (see <xref target="cache-response-directive" />), that directive overrides
1328    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1329  </t>
1330</x:note>
1331<t>
1332  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1333</t>
1334<t>
1335  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1336  including the value "0", as in the past (i.e., "already expired").
1337</t>
1338</section>
1339
1340<section anchor="header.pragma" title="Pragma">
1341  <iref item="Pragma header" primary="true" x:for-anchor="" />
1342  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1343  <x:anchor-alias value="extension-pragma"/>
1344  <x:anchor-alias value="Pragma"/>
1345  <x:anchor-alias value="Pragma-v"/>
1346  <x:anchor-alias value="pragma-directive"/>
1347<t>
1348  The "Pragma" general-header field is used to include implementation-specific directives
1349  that might apply to any recipient along the request/response chain. All pragma directives
1350  specify optional behavior from the viewpoint of the protocol; however, some systems
1351  &MAY; require that behavior be consistent with the directives.
1352</t>
1353<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"/>
1354  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1355  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1356  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1357  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1358</artwork></figure>
1359<t>
1360  When the no-cache directive is present in a request message, an application &SHOULD;
1361  forward the request toward the origin server even if it has a cached copy of what is being
1362  requested. This pragma directive has the same semantics as the no-cache response directive
1363  (see <xref target="cache-response-directive" />) and is defined here for backward
1364  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1365  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1366  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1367</t>
1368<x:note>
1369  <t>
1370    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1371    is not actually specified, it does not provide a reliable replacement for
1372    "Cache-Control: no-cache" in a response.
1373  </t>
1374</x:note>
1375<t>
1376  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1377</t>
1378</section>
1379
1380<section anchor="header.vary" title="Vary">
1381  <iref item="Vary header" primary="true" x:for-anchor="" />
1382  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1383  <x:anchor-alias value="Vary"/>
1384  <x:anchor-alias value="Vary-v"/>
1385<t>
1386  The "Vary" response-header field conveys the set of request-header fields
1387  that were used to select the representation.
1388</t>
1389<t>
1390  Caches use this information, in part, to determine whether a stored response
1391  can be used to satisfy a given request; see
1392  <xref target="caching.negotiated.responses" />.
1393  determines, while the response is fresh, whether a cache is permitted to use the
1394  response to reply to a subsequent request without validation; see <xref 
1395  target="caching.negotiated.responses" />.
1396</t>
1397<t>
1398  In uncacheable or stale responses, the Vary field value advises the user agent about
1399  the criteria that were used to select the representation.
1400</t>
1401<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1402  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1403  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1404</artwork></figure>
1405<t>
1406  The set of header fields named by the Vary field value is known as the selecting
1407  request-headers.
1408</t>
1409<t>
1410  Servers &SHOULD; include a Vary header field with any cacheable response that is
1411  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1412  requests on that resource and informs the user agent about the presence of negotiation on
1413  that resource. A server &MAY; include a Vary header field with a non-cacheable
1414  response that is subject to server-driven negotiation, since this might provide the user
1415  agent with useful information about the dimensions over which the response varies at the
1416  time of the response.
1417</t>
1418<t>
1419  A Vary field value of "*" signals that unspecified parameters not limited to the
1420  request-headers (e.g., the network address of the client), play a role in the selection of
1421  the response representation; therefore, a cache cannot determine whether this response is
1422  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1423  it may only be generated by an origin server.
1424</t>
1425<t>
1426  The field-names given are not limited to the set of standard request-header fields
1427  defined by this specification. Field names are case-insensitive.
1428</t>
1429</section>
1430
1431<section anchor="header.warning" title="Warning">
1432  <iref item="Warning header" primary="true" x:for-anchor="" />
1433  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1434  <x:anchor-alias value="Warning"/>
1435  <x:anchor-alias value="Warning-v"/>
1436  <x:anchor-alias value="warning-value"/>
1437  <x:anchor-alias value="warn-agent"/>
1438  <x:anchor-alias value="warn-code"/>
1439  <x:anchor-alias value="warn-date"/>
1440  <x:anchor-alias value="warn-text"/>
1441<t>
1442  The "Warning" general-header field is used to carry additional information about the status
1443  or transformation of a message that might not be reflected in the message. This
1444  information is typically used to warn about possible incorrectness introduced by caching
1445  operations or transformations applied to the entity body of the message.
1446</t>
1447<t>
1448  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1449  warning, rather than an error status code, distinguishes these responses from true failures.
1450</t>
1451<t>
1452  Warning headers can in general be applied to any message, however some warn-codes are
1453  specific to caches and can only be applied to response messages.
1454</t>
1455<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"/>
1456  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1457  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1458 
1459  <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>
1460                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1461 
1462  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1463  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1464                  ; the name or pseudonym of the server adding
1465                  ; the Warning header, for use in debugging
1466  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1467  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1468</artwork></figure>
1469<t>
1470  Multiple warnings can be attached to a response (either by the origin server or by
1471  a cache), including multiple warnings with the same code number, only differing
1472  in warn-text.
1473</t>
1474<t>
1475  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1476  possible, in the order that they appear in the response.
1477</t>
1478<t>
1479  Systems that generate multiple Warning headers &SHOULD; order them with this user
1480  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1481  Warning headers.
1482</t>
1483<t>
1484  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1485  Warning is required to be deleted from a stored response after validation:
1486  <list style="symbols">
1487    <t>1xx Warnings describe the freshness or validation status of the response, and so
1488      &MUST; be deleted by caches after validation. They can only be generated by a cache
1489      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1490    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
1491      not rectified by a validation (for example, a lossy compression of the entity bodies)
1492      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1493      returned, in which case they &MUST; be.</t>
1494  </list>
1495</t>
1496<t>
1497  If an implementation sends a message with one or more Warning headers to a receiver whose
1498  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1499  warn-date that matches the Date header in the message.
1500</t>
1501<t>
1502  If an implementation receives a message with a warning-value that includes a warn-date,
1503  and that warn-date is different from the Date value in the response, then that
1504  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1505  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1506  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1507  well.
1508</t>
1509<t>
1510  The following warn-codes are defined by this specification, each with a recommended
1511  warn-text in English, and a description of its meaning.
1512</t>
1513<t><?rfc needLines="4"?>
1514  110 Response is stale
1515  <list>
1516    <t>&SHOULD; be included whenever the returned response is stale.</t>
1517  </list>
1518</t>
1519<t><?rfc needLines="4"?>
1520  111 Revalidation failed
1521  <list>
1522    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1523      validate the response failed, due to an inability to reach the server.</t>
1524  </list>
1525</t>
1526<t><?rfc needLines="4"?>
1527  112 Disconnected operation
1528  <list>
1529    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1530      the network for a period of time.</t>
1531  </list>
1532</t>
1533<t><?rfc needLines="4"?>
1534  113 Heuristic expiration
1535  <list>
1536    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1537      greater than 24 hours and the response's age is greater than 24 hours.</t>
1538  </list>
1539</t>
1540<t><?rfc needLines="4"?>
1541  199 Miscellaneous warning
1542  <list>
1543    <t>The warning text can include arbitrary information to be presented to a human
1544      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1545      action, besides presenting the warning to the user.</t>
1546  </list>
1547</t>
1548<t><?rfc needLines="4"?>
1549  214 Transformation applied
1550  <list>
1551    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1552      transformation changing the content-coding (as specified in the Content-Encoding
1553      header) or media-type (as specified in the Content-Type header) of the response, or
1554      the entity-body of the response, unless this Warning code already appears in the
1555      response.</t>
1556  </list>
1557</t>
1558<t><?rfc needLines="4"?>
1559  299 Miscellaneous persistent warning
1560  <list>
1561    <t>The warning text can include arbitrary information to be presented to a human
1562      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1563      action.</t>
1564  </list>
1565</t>
1566</section>
1567
1568</section>
1569
1570<section anchor="history.lists" title="History Lists">
1571<t>
1572  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1573  can be used to redisplay an entity retrieved earlier in a session.
1574</t>
1575<t>
1576  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1577  a history mechanism can display a previous representation even if it has expired.
1578</t>
1579  <t>
1580  This does not prohibit the history mechanism from telling the user that a
1581  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1582  </t>
1583</section>
1584
1585
1586<section anchor="IANA.considerations" title="IANA Considerations">
1587
1588<section anchor="message.header.registration" title="Message Header Registration">
1589<t>
1590  The Message Header Registry located at <eref
1591  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1592  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1593</t>
1594<?BEGININC p6-cache.iana-headers ?>
1595<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1596<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1597   <ttcol>Header Field Name</ttcol>
1598   <ttcol>Protocol</ttcol>
1599   <ttcol>Status</ttcol>
1600   <ttcol>Reference</ttcol>
1601
1602   <c>Age</c>
1603   <c>http</c>
1604   <c>standard</c>
1605   <c>
1606      <xref target="header.age"/>
1607   </c>
1608   <c>Cache-Control</c>
1609   <c>http</c>
1610   <c>standard</c>
1611   <c>
1612      <xref target="header.cache-control"/>
1613   </c>
1614   <c>Expires</c>
1615   <c>http</c>
1616   <c>standard</c>
1617   <c>
1618      <xref target="header.expires"/>
1619   </c>
1620   <c>Pragma</c>
1621   <c>http</c>
1622   <c>standard</c>
1623   <c>
1624      <xref target="header.pragma"/>
1625   </c>
1626   <c>Vary</c>
1627   <c>http</c>
1628   <c>standard</c>
1629   <c>
1630      <xref target="header.vary"/>
1631   </c>
1632   <c>Warning</c>
1633   <c>http</c>
1634   <c>standard</c>
1635   <c>
1636      <xref target="header.warning"/>
1637   </c>
1638</texttable>
1639<!--(END)-->
1640<?ENDINC p6-cache.iana-headers ?>
1641<t>
1642  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1643</t>
1644</section>
1645
1646</section>
1647
1648<section anchor="security.considerations" title="Security Considerations">
1649<t>
1650  Caches expose additional potential vulnerabilities, since the contents of the cache
1651  represent an attractive target for malicious exploitation. Because cache contents persist
1652  after an HTTP request is complete, an attack on the cache can reveal information long after
1653  a user believes that the information has been removed from the network. Therefore, cache
1654  contents should be protected as sensitive information.
1655</t>
1656</section>
1657
1658<section anchor="ack" title="Acknowledgments">
1659<t>
1660  Much of the content and presentation of the caching design is due to suggestions and
1661  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1662  and Larry Masinter.
1663</t>
1664</section>
1665
1666</middle>
1667
1668<back>
1669<references title="Normative References">
1670
1671  <reference anchor="Part1">
1672    <front>
1673      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1674      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1675        <organization abbrev="Day Software">Day Software</organization>
1676        <address><email>fielding@gbiv.com</email></address>
1677      </author>
1678      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1679        <organization>One Laptop per Child</organization>
1680        <address><email>jg@laptop.org</email></address>
1681      </author>
1682      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1683        <organization abbrev="HP">Hewlett-Packard Company</organization>
1684        <address><email>JeffMogul@acm.org</email></address>
1685      </author>
1686      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1687        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1688        <address><email>henrikn@microsoft.com</email></address>
1689      </author>
1690      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1691        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1692        <address><email>LMM@acm.org</email></address>
1693      </author>
1694      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1695        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1696        <address><email>paulle@microsoft.com</email></address>
1697      </author>
1698      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1699        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1700        <address><email>timbl@w3.org</email></address>
1701      </author>
1702      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1703        <organization abbrev="W3C">World Wide Web Consortium</organization>
1704        <address><email>ylafon@w3.org</email></address>
1705      </author>
1706      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1707        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1708        <address><email>julian.reschke@greenbytes.de</email></address>
1709      </author>
1710      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1711    </front>
1712    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1713    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1714  </reference>
1715
1716  <reference anchor="Part2">
1717    <front>
1718      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1719      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1720        <organization abbrev="Day Software">Day Software</organization>
1721        <address><email>fielding@gbiv.com</email></address>
1722      </author>
1723      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1724        <organization>One Laptop per Child</organization>
1725        <address><email>jg@laptop.org</email></address>
1726      </author>
1727      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1728        <organization abbrev="HP">Hewlett-Packard Company</organization>
1729        <address><email>JeffMogul@acm.org</email></address>
1730      </author>
1731      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1732        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1733        <address><email>henrikn@microsoft.com</email></address>
1734      </author>
1735      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1736        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1737        <address><email>LMM@acm.org</email></address>
1738      </author>
1739      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1740        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1741        <address><email>paulle@microsoft.com</email></address>
1742      </author>
1743      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1744        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1745        <address><email>timbl@w3.org</email></address>
1746      </author>
1747      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1748        <organization abbrev="W3C">World Wide Web Consortium</organization>
1749        <address><email>ylafon@w3.org</email></address>
1750      </author>
1751      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1752        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1753        <address><email>julian.reschke@greenbytes.de</email></address>
1754      </author>
1755      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1756    </front>
1757    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1758    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1759  </reference>
1760
1761  <reference anchor="Part4">
1762    <front>
1763      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1764      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1765        <organization abbrev="Day Software">Day Software</organization>
1766        <address><email>fielding@gbiv.com</email></address>
1767      </author>
1768      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1769        <organization>One Laptop per Child</organization>
1770        <address><email>jg@laptop.org</email></address>
1771      </author>
1772      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1773        <organization abbrev="HP">Hewlett-Packard Company</organization>
1774        <address><email>JeffMogul@acm.org</email></address>
1775      </author>
1776      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1777        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1778        <address><email>henrikn@microsoft.com</email></address>
1779      </author>
1780      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1781        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1782        <address><email>LMM@acm.org</email></address>
1783      </author>
1784      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1785        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1786        <address><email>paulle@microsoft.com</email></address>
1787      </author>
1788      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1789        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1790        <address><email>timbl@w3.org</email></address>
1791      </author>
1792      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1793        <organization abbrev="W3C">World Wide Web Consortium</organization>
1794        <address><email>ylafon@w3.org</email></address>
1795      </author>
1796      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1797        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1798        <address><email>julian.reschke@greenbytes.de</email></address>
1799      </author>
1800      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1801    </front>
1802    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1803    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1804  </reference>
1805
1806  <reference anchor="Part5">
1807    <front>
1808      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1809      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1810        <organization abbrev="Day Software">Day Software</organization>
1811        <address><email>fielding@gbiv.com</email></address>
1812      </author>
1813      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1814        <organization>One Laptop per Child</organization>
1815        <address><email>jg@laptop.org</email></address>
1816      </author>
1817      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1818        <organization abbrev="HP">Hewlett-Packard Company</organization>
1819        <address><email>JeffMogul@acm.org</email></address>
1820      </author>
1821      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1822        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1823        <address><email>henrikn@microsoft.com</email></address>
1824      </author>
1825      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1826        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1827        <address><email>LMM@acm.org</email></address>
1828      </author>
1829      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1830        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1831        <address><email>paulle@microsoft.com</email></address>
1832      </author>
1833      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1834        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1835        <address><email>timbl@w3.org</email></address>
1836      </author>
1837      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1838        <organization abbrev="W3C">World Wide Web Consortium</organization>
1839        <address><email>ylafon@w3.org</email></address>
1840      </author>
1841      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1842        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1843        <address><email>julian.reschke@greenbytes.de</email></address>
1844      </author>
1845      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1846    </front>
1847    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1848    <x:source basename="p5-range" href="p5-range.xml" />
1849  </reference>
1850
1851  <reference anchor="Part7">
1852    <front>
1853      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1854      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1855        <organization abbrev="Day Software">Day Software</organization>
1856        <address><email>fielding@gbiv.com</email></address>
1857      </author>
1858      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1859        <organization>One Laptop per Child</organization>
1860        <address><email>jg@laptop.org</email></address>
1861      </author>
1862      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1863        <organization abbrev="HP">Hewlett-Packard Company</organization>
1864        <address><email>JeffMogul@acm.org</email></address>
1865      </author>
1866      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1867        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1868        <address><email>henrikn@microsoft.com</email></address>
1869      </author>
1870      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1871        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1872        <address><email>LMM@acm.org</email></address>
1873      </author>
1874      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1875        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1876        <address><email>paulle@microsoft.com</email></address>
1877      </author>
1878      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1879        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1880        <address><email>timbl@w3.org</email></address>
1881      </author>
1882      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1883        <organization abbrev="W3C">World Wide Web Consortium</organization>
1884        <address><email>ylafon@w3.org</email></address>
1885      </author>
1886      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1887        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1888        <address><email>julian.reschke@greenbytes.de</email></address>
1889      </author>
1890      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1891    </front>
1892    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1893    <x:source basename="p7-auth" href="p7-auth.xml" />
1894  </reference>
1895
1896  <reference anchor="RFC2119">
1897    <front>
1898      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1899      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1900        <organization>Harvard University</organization>
1901        <address><email>sob@harvard.edu</email></address>
1902      </author>
1903      <date month="March" year="1997" />
1904    </front>
1905    <seriesInfo name="BCP" value="14" />
1906    <seriesInfo name="RFC" value="2119" />
1907  </reference>
1908
1909  <reference anchor="RFC5234">
1910    <front>
1911      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1912      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1913        <organization>Brandenburg InternetWorking</organization>
1914        <address>
1915          <email>dcrocker@bbiw.net</email>
1916        </address> 
1917      </author>
1918      <author initials="P." surname="Overell" fullname="Paul Overell">
1919        <organization>THUS plc.</organization>
1920        <address>
1921          <email>paul.overell@thus.net</email>
1922        </address>
1923      </author>
1924      <date month="January" year="2008"/>
1925    </front>
1926    <seriesInfo name="STD" value="68"/>
1927    <seriesInfo name="RFC" value="5234"/>
1928  </reference>
1929 
1930</references>
1931
1932<references title="Informative References">
1933
1934  <reference anchor="RFC1305">
1935    <front>
1936      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1937      <author fullname="David L. Mills" initials="D." surname="Mills">
1938        <organization>University of Delaware, Electrical Engineering Department</organization>
1939        <address><email>mills@udel.edu</email></address>
1940      </author>
1941      <date month="March" year="1992" />
1942    </front>
1943    <seriesInfo name="RFC" value="1305" />
1944  </reference>
1945
1946  <reference anchor="RFC2616">
1947    <front>
1948      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1949      <author fullname="R. Fielding" initials="R." surname="Fielding">
1950        <organization>University of California, Irvine</organization>
1951        <address><email>fielding@ics.uci.edu</email></address>
1952      </author>
1953      <author fullname="J. Gettys" initials="J." surname="Gettys">
1954        <organization>W3C</organization>
1955        <address><email>jg@w3.org</email></address>
1956      </author>
1957      <author fullname="J. Mogul" initials="J." surname="Mogul">
1958        <organization>Compaq Computer Corporation</organization>
1959        <address><email>mogul@wrl.dec.com</email></address>
1960      </author>
1961      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1962        <organization>MIT Laboratory for Computer Science</organization>
1963        <address><email>frystyk@w3.org</email></address>
1964      </author>
1965      <author fullname="L. Masinter" initials="L." surname="Masinter">
1966        <organization>Xerox Corporation</organization>
1967        <address><email>masinter@parc.xerox.com</email></address>
1968      </author>
1969      <author fullname="P. Leach" initials="P." surname="Leach">
1970        <organization>Microsoft Corporation</organization>
1971        <address><email>paulle@microsoft.com</email></address>
1972      </author>
1973      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1974        <organization>W3C</organization>
1975        <address><email>timbl@w3.org</email></address>
1976      </author>
1977      <date month="June" year="1999" />
1978    </front>
1979    <seriesInfo name="RFC" value="2616" />
1980  </reference>
1981
1982  <reference anchor="RFC3864">
1983    <front>
1984      <title>Registration Procedures for Message Header Fields</title>
1985      <author fullname="G. Klyne" initials="G." surname="Klyne">
1986        <organization>Nine by Nine</organization>
1987        <address><email>GK-IETF@ninebynine.org</email></address>
1988      </author>
1989      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1990        <organization>BEA Systems</organization>
1991        <address><email>mnot@pobox.com</email></address>
1992      </author>
1993      <author fullname="J. Mogul" initials="J." surname="Mogul">
1994        <organization>HP Labs</organization>
1995        <address><email>JeffMogul@acm.org</email></address>
1996      </author>
1997      <date month="September" year="2004" />
1998    </front>
1999    <seriesInfo name="BCP" value="90" />
2000    <seriesInfo name="RFC" value="3864" />
2001  </reference>
2002
2003</references>
2004
2005<section anchor="compatibility" title="Compatibility with Previous Versions">
2006
2007<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2008<t>
2009  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2010  this missing case.
2011  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2012</t>
2013<t>
2014  Range request responses would become very verbose if all meta-data were always returned;
2015  by allowing the server to only send needed headers in a 206 response, this problem can be
2016  avoided.
2017  (<xref target="combining.headers" />)
2018</t>
2019<t>
2020  The Cache-Control: max-age directive was not properly defined for responses.
2021  (<xref target="cache-response-directive"/>)
2022</t>
2023<t>
2024  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2025    format="counter" target="expiration.model" />, <xref format="counter"
2026    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2027  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2028  header, as PUT or other methods may have need for it in requests.
2029</t>
2030</section>
2031
2032<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2033<t>
2034  Remove requirement to consider Content-Location in successful responses
2035  in order to determine the appropriate response to use.
2036  (<xref target="validation.model" />)
2037</t>
2038<t>
2039  Clarify denial of service attack avoidance requirement.
2040  (<xref target="invalidation.after.updates.or.deletions" />)
2041</t>
2042<t>
2043  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2044  anymore, as these aspects never were implemented.
2045  (<xref target="header.warning" />)
2046</t>
2047</section>
2048
2049    </section>
2050
2051<?BEGININC p6-cache.abnf-appendix ?>
2052<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2053<figure>
2054<artwork type="abnf" name="p6-cache.parsed-abnf">
2055<x:ref>Age</x:ref> = "Age:" OWS Age-v
2056<x:ref>Age-v</x:ref> = delta-seconds
2057
2058<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2059<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2060 cache-directive ] )
2061
2062<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2063<x:ref>Expires-v</x:ref> = HTTP-date
2064
2065<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2066
2067<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2068
2069<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2070<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2071 pragma-directive ] )
2072
2073<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2074<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2075 ] ) )
2076
2077<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2078<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2079 ] )
2080
2081<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2082<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2083<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2084 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2085 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2086 cache-extension
2087<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2088 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2089 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2090 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2091 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2092 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2093
2094<x:ref>delta-seconds</x:ref> = 1*DIGIT
2095
2096<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2097
2098<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2099
2100<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2101<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2102<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2103
2104<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2105
2106<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2107
2108<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2109
2110<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2111<x:ref>warn-code</x:ref> = 3DIGIT
2112<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2113<x:ref>warn-text</x:ref> = quoted-string
2114<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2115 ]
2116</artwork>
2117</figure>
2118<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2119; Age defined but not used
2120; Cache-Control defined but not used
2121; Expires defined but not used
2122; Pragma defined but not used
2123; Vary defined but not used
2124; Warning defined but not used
2125</artwork></figure></section>
2126<?ENDINC p6-cache.abnf-appendix ?>
2127
2128<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2129
2130<section title="Since RFC2616">
2131  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2132</section>
2133
2134<section title="Since draft-ietf-httpbis-p6-cache-00">
2135<t>
2136  Closed issues:
2137  <list style="symbols">
2138    <t>
2139      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2140    <t>
2141      <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>
2142    <t>
2143      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2144    <t>
2145      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2146    <t>
2147      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2148    <t>
2149      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2150    <t>
2151      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2152    <t>
2153      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2154    <t>
2155      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2156  </list>
2157</t>
2158<t>
2159  Other changes:
2160  <list style="symbols">
2161    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2162        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2163  </list>
2164</t>
2165</section>
2166
2167<section title="Since draft-ietf-httpbis-p6-cache-01">
2168<t>
2169  Closed issues:
2170  <list style="symbols">
2171    <t>
2172      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2173  </list>
2174</t>
2175<t>
2176  Other changes:
2177  <list style="symbols">
2178    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2179      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2180    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2181      specification.</t>
2182  </list>
2183</t>
2184</section>
2185
2186<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2187<t>
2188  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2189  <list style="symbols">
2190    <t>Reference RFC 3984, and update header registrations for headers defined in this
2191      document.</t>
2192  </list>
2193</t>
2194</section>
2195
2196<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2197<t>
2198  Closed issues:
2199  <list style="symbols">
2200    <t>
2201      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2202  </list>
2203</t>
2204</section>
2205
2206<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2207<t>
2208  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2209  <list style="symbols"> 
2210    <t>
2211      Use "/" instead of "|" for alternatives.
2212    </t>
2213    <t>
2214      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2215      whitespace ("OWS") and required whitespace ("RWS").
2216    </t>
2217    <t>
2218      Rewrite ABNFs to spell out whitespace rules, factor out
2219      header value format definitions.
2220    </t>
2221  </list>
2222</t>
2223</section>
2224
2225<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2226<t>
2227  This is a total rewrite of this part of the specification.
2228</t>
2229<t>
2230  Affected issues:
2231  <list style="symbols">
2232    <t>
2233      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2234    <t>
2235      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2236    <t>
2237      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2238    <t>
2239      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2240  </list>
2241</t>
2242<t>
2243  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2244  <list style="symbols"> 
2245    <t>
2246      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2247    </t>
2248  </list>
2249</t>
2250</section>
2251
2252<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2253<t>
2254  Closed issues:
2255  <list style="symbols"> 
2256    <t>
2257      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2258      "base for numeric protocol elements"
2259    </t>
2260  </list>
2261</t>
2262<t>
2263  Affected issues:
2264  <list style="symbols">
2265    <t>
2266      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2267      Vary and non-existant headers
2268    </t>
2269  </list>
2270</t>
2271</section>
2272
2273<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2274<t>
2275  Closed issues:
2276  <list style="symbols"> 
2277    <t>
2278      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2279      "Definition of 1xx Warn-Codes"
2280    </t>
2281    <t>
2282      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2283      "Content-Location on 304 responses"
2284    </t>
2285    <t>
2286      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2287      "private and no-cache CC directives with headers"
2288    </t>
2289    <t>
2290      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2291      "RFC2047 and warn-text"
2292    </t>
2293  </list>
2294</t>
2295</section>
2296
2297<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2298<t>
2299  Closed issues:
2300  <list style="symbols"> 
2301    <t>
2302      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2303      "serving negotiated responses from cache: header-specific canonicalization"
2304    </t>
2305    <t>
2306      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2307      "Effect of CC directives on history lists"
2308    </t>
2309  </list>
2310</t>
2311<t>
2312  Affected issues:
2313  <list style="symbols">
2314    <t>
2315      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2316      Status codes and caching
2317    </t>
2318  </list>
2319</t>
2320<t>
2321  Partly resolved issues:
2322  <list style="symbols"> 
2323    <t>
2324      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2325      "Placement of 13.5.1 and 13.5.2"
2326    </t>
2327  </list>
2328</t>
2329</section>
2330
2331<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2332<t>
2333  Closed issues:
2334  <list style="symbols"> 
2335    <t>
2336      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2337      "Age calculation"
2338    </t>
2339  </list>
2340</t>
2341</section>
2342
2343</section>
2344  </back>
2345</rfc>
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