source: draft-ietf-httpbis/latest/p6-cache.xml @ 784

Last change on this file since 784 was 784, checked in by julian.reschke@…, 10 years ago

re-organize field descriptions for age calculation, preparing for work on #29.

  • Property svn:eol-style set to native
File size: 99.1 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>now 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>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
690  </list>
691</t>
692<figure>
693<preamble>These are combined as</preamble>
694<artwork type="code">
695  corrected_received_age = max(now - date_value, age_value)
696</artwork></figure>
697<t>
698  When an Age value is received, it &MUST; be interpreted relative to the time the
699  request was initiated, not the time that the response was received.
700</t>
701<figure><artwork type="code">
702  corrected_initial_age = corrected_received_age
703                          + (now - request_time)
704</artwork></figure>
705<t>
706  The current_age of a stored response can then be calculated by adding the amount of
707  time (in seconds) since the stored response was last validated by the origin server to
708  the corrected_initial_age.
709</t>
710<figure>
711<preamble>With these definitions, the current_age can be calculated as per:</preamble>
712<artwork type="code">
713  apparent_age = max(0, response_time - date_value);
714  corrected_received_age = max(apparent_age, age_value);
715  response_delay = response_time - request_time;
716  corrected_initial_age = corrected_received_age + response_delay;
717  resident_time = now - response_time;
718  current_age   = corrected_initial_age + resident_time;
719</artwork></figure>
720</section>
721
722<section anchor="serving.stale.responses" title="Serving Stale Responses">
723<t>
724  A "stale" response is one that either has explicit expiry information or is allowed to
725  have heuristic expiry calculated, but is not fresh according to the calculations in
726  <xref target="expiration.model" />.
727</t>
728<t>
729  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
730  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
731  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
732  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
733</t>
734<t>
735  Caches &SHOULD-NOT; return stale responses unless they are
736  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
737  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
738</t>
739<t>
740  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
741  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
742  the cache is disconnected.
743</t>
744<t>
745  If a cache receives a first-hand response (either an entire response, or a 304 (Not
746  Modified) response) that it would normally forward to the requesting client, and the
747  received response is no longer fresh, the cache &SHOULD; forward it to the
748  requesting client without adding a new Warning (but without removing any existing
749  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
750  that response became stale in transit.
751</t>
752</section>
753</section>
754
755<section anchor="validation.model" title="Validation Model">
756<t>
757  When a cache has one or more stored responses for a requested URI, but cannot 
758  serve any of them (e.g., because they are not fresh, or one cannot be selected;
759  see <xref target="caching.negotiated.responses"/>),
760  it can use the conditional request mechanism &conditional; in the forwarded
761  request to give the origin server an opportunity to both select a valid stored
762  response to be used, and to update it. This process is known as "validating"
763  or "revalidating" the stored response.
764</t>
765<t>
766  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
767  header whose value is that of the Last-Modified header from the selected
768  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
769</t>
770<t>
771  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
772  is that of the ETag header(s) from all responses stored for the requested URI,
773  if present. However, if any of the stored responses contains only partial
774  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
775  field unless the request is for a range that would be fully satisfied by
776  that stored response.
777</t>
778<t>
779  A 304 (Not Modified) response status code indicates that the stored
780  response can be updated and reused; see <xref target="combining.headers"/>.
781</t>
782<t>
783  A full response (i.e., one with a response body) indicates that none 
784  of the stored responses nominated in the conditional request is
785  suitable. Instead, the full response is used both to satisfy the
786  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
787</t>
788<t>
789  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
790  either forward this response to the requesting client, or act as if the server failed to
791  respond. In the latter case, it &MAY; return a previously stored response (see <xref
792  target="serving.stale.responses" />).
793</t>
794</section>
795
796<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
797<t>
798  Because unsafe methods (&safe-methods;) have the potential for changing state on the
799  origin server, intervening caches can use them to keep their contents
800  up-to-date.
801</t>
802<t>
803  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
804  as the URI(s) in the Location and Content-Location headers (if present):
805  <list style="symbols">
806    <t>PUT</t>
807    <t>DELETE</t>
808    <t>POST</t>
809  </list>
810</t>
811<t>
812  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
813  be performed if the host part of that URI differs from the host part in the Request-URI.
814  This helps prevent denial of service attacks.
815</t>
816<t>
817  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
818</t>
819<t>
820  A cache that passes through requests for methods it does not understand &SHOULD;
821  invalidate the Request-URI.
822</t>
823<t>
824  Here, "invalidate" means that the cache will either remove all stored responses related
825  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
826  before they can be returned in response to a subsequent request.
827</t>
828<t>
829  Note that this does not guarantee that all appropriate responses are invalidated. For
830  example, the request that caused the change at the origin server might not have gone
831  through the cache where a response is stored.
832</t>
833<t>
834  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
835</t>
836</section>
837
838<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
839<t>
840  When a cache receives a request that can be satisfied by a stored response
841  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
842  response unless all of the selecting request-headers nominated by the Vary header match
843  in both the original request (i.e., that associated with the stored response),
844  and the presented request.
845</t>
846<t>
847  The selecting request-headers from two requests are defined to match
848  if and only if those in the first request can be transformed to those in the
849  second request by applying any of the following:
850  <list style="symbols">
851    <t>
852      adding or removing whitespace, where allowed in the header's syntax
853    </t>
854    <t>
855      combining multiple message-header fields with the same field name (see
856      &header-fields;)
857    </t>
858    <t>
859      normalizing both header values in a way that is known to have identical
860      semantics, according to the header's specification (e.g., re-ordering field values
861      when order is not significant; case-normalization, where values are defined to be
862      case-insensitive)   
863    </t>
864  </list>
865</t>
866<t>
867  If (after any normalisation that may take place) a header field is absent
868  from a request, it can only match another request if it is also absent there.
869</t>
870<t>
871  A Vary header field-value of "*" always fails to match, and subsequent requests to that
872  resource can only be properly interpreted by the origin server.
873</t>
874<t>
875  The stored response with matching selecting request-headers is known as the
876  selected response.
877</t>
878<t>
879  If no selected response is available, the cache &MAY; forward the presented
880  request to the origin server in a conditional request; see <xref target="validation.model"/>.
881</t>
882</section>
883
884<section anchor="combining.headers" title="Combining Responses">
885<t>
886  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
887  (in this section, the "new" response"), it needs to created an updated response by combining
888  the stored response with the new one, so that the updated response can be used to satisfy the request.
889</t>
890<t>
891  If the new response contains an ETag, it identifies the stored 
892  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
893  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
894</t>
895<t>
896  If the status code is 206 (partial content), both the stored and new 
897  responses &MUST; have validators, and those validators &MUST; match using the strong 
898  comparison function (see &weak-and-strong;). Otherwise, the 
899  responses &MUST-NOT; be combined.
900</t>
901<t>
902  The stored response headers are used as those of the updated response, except that
903  <list style="symbols">
904    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
905      &MUST; be deleted from the stored response and the updated response.</t>
906    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
907      response and the updated response.</t>
908    <t>any headers provided in the new response &MUST; replace the corresponding
909      headers from the stored response.</t>
910  </list>
911</t>
912<t>
913  If a header field-name in the new response matches more than one 
914  header in the stored response, all such stored headers &MUST; be replaced.
915</t>
916<t>
917  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
918  stored response in cache. In the case of a 206 response, the combined 
919  entity-body &MAY; be stored.
920</t>
921<t>
922  <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
923</t>
924</section>
925
926</section>
927
928<section anchor="header.fields" title="Header Field Definitions">
929<t>
930  This section defines the syntax and semantics of HTTP/1.1 header fields
931  related to caching.
932</t>
933<t>
934  For entity-header fields, both sender and recipient refer to either the client or the
935  server, depending on who sends and who receives the entity.
936</t>
937
938<section anchor="header.age" title="Age">
939  <iref item="Age header" primary="true" x:for-anchor="" />
940  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
941  <x:anchor-alias value="Age"/>
942  <x:anchor-alias value="Age-v"/>
943  <x:anchor-alias value="age-value"/>
944<t>
945  The "Age" response-header field conveys the sender's estimate of the amount
946  of time since the response was generated or successfully validated at the
947  origin server. Age values are calculated as specified in
948  <xref target="age.calculations" />.
949</t>
950<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
951  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
952  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
953</artwork></figure>
954<t anchor="rule.delta-seconds">
955  <x:anchor-alias value="delta-seconds" />
956  Age field-values are non-negative integers, representing time in seconds.
957</t>
958<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
959  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
960</artwork></figure>
961<t>
962  If a cache receives a value larger than the largest positive integer it can represent, or
963  if any of its age calculations overflows, it &MUST; transmit an Age header with a
964  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
965  of at least 31 bits of range.
966</t>
967<t>
968  The presence of an Age header field in a response implies that a response is not
969  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
970  Age header field.
971</t>
972</section>
973
974<section anchor="header.cache-control" title="Cache-Control">
975  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
976  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
977  <x:anchor-alias value="Cache-Control"/>
978  <x:anchor-alias value="Cache-Control-v"/>
979  <x:anchor-alias value="cache-directive"/>
980  <x:anchor-alias value="cache-extension"/>
981  <x:anchor-alias value="cache-request-directive"/>
982  <x:anchor-alias value="cache-response-directive"/>
983<t>
984  The "Cache-Control" general-header field is used to specify directives that
985  &MUST; be obeyed by all caches along the request/response chain. Such cache
986  directives are unidirectional in that the presence of a directive in a
987  request does not imply that the same directive is to be given in the
988  response.
989</t>
990<x:note>
991  <t>
992    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
993    Pragma: no-cache (see <xref target="header.pragma" />).
994  </t>
995</x:note>
996<t>
997  Cache directives &MUST; be passed through by a proxy or gateway application,
998  regardless of their significance to that application, since the directives might be
999  applicable to all recipients along the request/response chain. It is not possible to
1000  target a directive to a specific cache.
1001</t>
1002<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"/>
1003  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1004  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1005
1006  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1007     / <x:ref>cache-response-directive</x:ref>
1008
1009  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1010</artwork></figure>
1011
1012<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1013  <x:anchor-alias value="cache-request-directive" />
1014
1015<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
1016  <x:ref>cache-request-directive</x:ref> =
1017       "no-cache"
1018     / "no-store"
1019     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1020     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1021     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1022     / "no-transform"
1023     / "only-if-cached"
1024     / <x:ref>cache-extension</x:ref>
1025</artwork></figure>
1026
1027<t>
1028  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1029  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1030  no-cache
1031  <list>
1032    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1033      used to satisfy the request without successful validation on the origin server.</t>
1034  </list>
1035</t>
1036<t>
1037  <iref item="Cache Directives" primary="true" subitem="no-store" />
1038  <iref item="no-store" primary="true" subitem="Cache Directive" />
1039  no-store
1040  <list>
1041    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1042      of either this request or any response to it. This directive applies to both
1043      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1044      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1045      and &MUST; make a best-effort attempt to remove the information from volatile
1046      storage as promptly as possible after forwarding it.</t>
1047    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1048      particular, malicious or compromised caches might not recognize or obey this
1049      directive, and communications networks may be vulnerable to eavesdropping.</t>
1050  </list>
1051</t>
1052<t>
1053  <iref item="Cache Directives" primary="true" subitem="max-age" />
1054  <iref item="max-age" primary="true" subitem="Cache Directive" />
1055  max-age
1056  <list>
1057    <t>The max-age request directive indicates that the client is willing to accept a
1058      response whose age is no greater than the specified time in seconds. Unless
1059      the max-stale request directive is also present, the client is not willing to accept a stale
1060      response.</t>
1061  </list>
1062</t>
1063<t>
1064  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1065  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1066  max-stale
1067  <list>
1068    <t>The max-stale request directive indicates that the client is willing to accept a
1069      response that has exceeded its expiration time. If max-stale is assigned a value,
1070      then the client is willing to accept a response that has exceeded its expiration
1071      time by no more than the specified number of seconds. If no value is assigned to
1072      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>
1073  </list>
1074</t>
1075<t>
1076  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1077  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1078  min-fresh
1079  <list>
1080    <t>The min-fresh request directive indicates that the client is willing to accept a
1081      response whose freshness lifetime is no less than its current age plus the specified
1082      time in seconds. That is, the client wants a response that will still be fresh for
1083      at least the specified number of seconds.</t>
1084  </list>
1085</t>
1086<t>
1087  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1088  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1089  no-transform
1090  <list>
1091    <t>The no-transform request directive indicates that an intermediate cache or proxy
1092      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1093      headers, nor the request entity-body.</t>
1094  </list>
1095</t>
1096<t>
1097  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1098  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1099  only-if-cached
1100  <list>
1101    <t>The only-if-cached request directive indicates that the client only wishes to
1102      return a stored response. If it receives this directive, a cache &SHOULD; either
1103      respond using a stored response that is consistent with the other constraints of the
1104      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1105      being operated as a unified system with good internal connectivity, such a request
1106      &MAY; be forwarded within that group of caches.</t>
1107  </list>
1108</t>
1109</section>
1110
1111<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1112  <x:anchor-alias value="cache-response-directive" />
1113
1114<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1115  <x:ref>cache-response-directive</x:ref> =
1116       "public"
1117     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1118     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1119     / "no-store"
1120     / "no-transform"
1121     / "must-revalidate"
1122     / "proxy-revalidate"
1123     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1124     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1125     / <x:ref>cache-extension</x:ref>
1126</artwork></figure>
1127
1128<t>
1129  <iref item="Cache Directives" primary="true" subitem="public" />
1130  <iref item="public" primary="true" subitem="Cache Directive" />
1131  public
1132  <list>
1133    <t>The public response directive indicates that the response &MAY; be cached, even
1134      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1135      (See also Authorization, &header-authorization;, for additional details.) </t>
1136  </list>
1137</t>
1138<t>
1139  <iref item="Cache Directives" primary="true" subitem="private" />
1140  <iref item="private" primary="true" subitem="Cache Directive" />
1141  private
1142  <list>
1143    <t>The private response directive indicates that the response message is intended for
1144      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1145      cache &MAY; store the response.</t>
1146    <t>If the private response directive specifies one or more field-names, this
1147      requirement is limited to the field-values associated with the listed response
1148      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1149      cache, whereas the remainder of the response message &MAY; be.</t>
1150    <t>
1151      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1152      be stored, and cannot ensure the privacy of the message content.
1153      Also, private response directives with field-names are often handled by
1154      implementations as if an unqualified private directive was received; i.e.,
1155      the special handling for the qualified form is not widely implemented.
1156</t>
1157  </list>
1158</t>
1159<t>
1160  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1161  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1162  no-cache
1163  <list>
1164    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1165      satisfy a subsequent request without successful validation on the origin server.
1166      This allows an origin server to prevent caching even by caches that have been
1167      configured to return stale responses.</t>
1168    <t>If the no-cache response directive specifies one or more field-names, this
1169      requirement is limited to the field-values associated with the listed response
1170      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1171      to a subsequent request without successful validation on the origin server. This
1172      allows an origin server to prevent the re-use of certain header fields in a
1173      response, while still allowing caching of the rest of the response.</t>
1174    <t>
1175      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1176      Also, no-cache response directives with field-names are often handled by
1177      implementations as if an unqualified no-cache directive was received; i.e.,
1178      the special handling for the qualified form is not widely implemented.
1179    </t>
1180  </list>
1181</t>
1182
1183<t>
1184  <iref item="Cache Directives" primary="true" subitem="no-store" />
1185  <iref item="no-store" primary="true" subitem="Cache Directive" />
1186  no-store
1187  <list>
1188    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1189      part of either the immediate request or response. This directive applies to both
1190      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1191      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1192      and &MUST; make a best-effort attempt to remove the information from volatile
1193      storage as promptly as possible after forwarding it.</t>
1194    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1195      particular, malicious or compromised caches might not recognize or obey this
1196      directive, and communications networks may be vulnerable to eavesdropping.</t>
1197  </list>
1198</t>
1199<t>
1200  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1201  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1202  must-revalidate
1203  <list>
1204    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1205     used to satisfy subsequent requests without successful validation on the origin server.</t>
1206    <t>The must-revalidate directive is necessary to support reliable operation for
1207      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1208      the must-revalidate directive; in particular, if the cache cannot reach the origin
1209      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1210    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1211      validate a request on the entity could result in incorrect operation, such as a
1212      silently unexecuted financial transaction.</t>
1213  </list>
1214</t>
1215<t>
1216  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1217  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1218  proxy-revalidate
1219  <list>
1220    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1221      response directive, except that it does not apply to non-shared caches.</t>
1222  </list>
1223</t>
1224<t>
1225  <iref item="Cache Directives" primary="true" subitem="max-age" />
1226  <iref item="max-age" primary="true" subitem="Cache Directive" />
1227  max-age
1228  <list>
1229    <t>The max-age response directive indicates that response is to be considered stale
1230      after its age is greater than the specified number of seconds.</t>
1231  </list>
1232</t>
1233<t>
1234  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1235  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1236  s-maxage
1237  <list>
1238    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1239      specified by this directive overrides the maximum age specified by either the
1240      max-age directive or the Expires header. The s-maxage directive also implies the
1241      semantics of the proxy-revalidate response directive.</t>
1242  </list>
1243</t>
1244<t>
1245  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1246  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1247  no-transform
1248  <list>
1249    <t>The no-transform response directive indicates that an intermediate cache or proxy
1250      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1251      headers, nor the response entity-body.</t>
1252  </list>
1253</t>
1254
1255</section>
1256
1257<section anchor="cache.control.extensions" title="Cache Control Extensions">
1258<t>
1259  The Cache-Control header field can be extended through the use of one or more
1260  cache-extension tokens, each with an optional value. Informational extensions (those
1261  that do not require a change in cache behavior) can be added without changing the
1262  semantics of other directives. Behavioral extensions are designed to work by acting as
1263  modifiers to the existing base of cache directives. Both the new directive and the
1264  standard directive are supplied, such that applications that do not understand the new
1265  directive will default to the behavior specified by the standard directive, and those
1266  that understand the new directive will recognize it as modifying the requirements
1267  associated with the standard directive. In this way, extensions to the cache-control
1268  directives can be made without requiring changes to the base protocol.
1269</t>
1270<t>
1271  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1272  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1273  all directives that it does not understand.
1274</t>
1275<t>
1276  For example, consider a hypothetical new response directive called "community" that
1277  acts as a modifier to the private directive. We define this new directive to mean that,
1278  in addition to any non-shared cache, any cache that is shared only by members of the
1279  community named within its value may cache the response. An origin server wishing to
1280  allow the UCI community to use an otherwise private response in their shared cache(s)
1281  could do so by including
1282</t>
1283<figure><artwork type="example">
1284  Cache-Control: private, community="UCI"
1285</artwork></figure>
1286<t>
1287  A cache seeing this header field will act correctly even if the cache does not
1288  understand the community cache-extension, since it will also see and understand the
1289  private directive and thus default to the safe behavior.
1290</t>
1291<t>
1292  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1293  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1294  directives (or the response's default cacheability) such that the cache behavior will
1295  remain minimally correct even if the cache does not understand the extension(s).
1296</t>
1297</section>
1298
1299</section>
1300
1301<section anchor="header.expires" title="Expires">
1302  <iref item="Expires header" primary="true" x:for-anchor="" />
1303  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1304  <x:anchor-alias value="Expires"/>
1305  <x:anchor-alias value="Expires-v"/>
1306<t>
1307  The "Expires" entity-header field gives the date/time after which the response is
1308  considered stale. See <xref target="expiration.model" /> for further discussion of the
1309  freshness model.
1310</t>
1311<t>
1312  The presence of an Expires field does not imply that the original resource will change or
1313  cease to exist at, before, or after that time.
1314</t>
1315<t>
1316  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1317  it &MUST; be sent in rfc1123-date format.
1318</t>
1319<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1320  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1321  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1322</artwork></figure>
1323<figure>
1324  <preamble>For example</preamble>
1325<artwork type="example">
1326  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1327</artwork></figure>
1328<x:note>
1329  <t>
1330    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1331    directive (see <xref target="cache-response-directive" />), that directive overrides
1332    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1333  </t>
1334</x:note>
1335<t>
1336  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1337</t>
1338<t>
1339  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1340  including the value "0", as in the past (i.e., "already expired").
1341</t>
1342</section>
1343
1344<section anchor="header.pragma" title="Pragma">
1345  <iref item="Pragma header" primary="true" x:for-anchor="" />
1346  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1347  <x:anchor-alias value="extension-pragma"/>
1348  <x:anchor-alias value="Pragma"/>
1349  <x:anchor-alias value="Pragma-v"/>
1350  <x:anchor-alias value="pragma-directive"/>
1351<t>
1352  The "Pragma" general-header field is used to include implementation-specific directives
1353  that might apply to any recipient along the request/response chain. All pragma directives
1354  specify optional behavior from the viewpoint of the protocol; however, some systems
1355  &MAY; require that behavior be consistent with the directives.
1356</t>
1357<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"/>
1358  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1359  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1360  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1361  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1362</artwork></figure>
1363<t>
1364  When the no-cache directive is present in a request message, an application &SHOULD;
1365  forward the request toward the origin server even if it has a cached copy of what is being
1366  requested. This pragma directive has the same semantics as the no-cache response directive
1367  (see <xref target="cache-response-directive" />) and is defined here for backward
1368  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1369  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1370  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1371</t>
1372<x:note>
1373  <t>
1374    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1375    is not actually specified, it does not provide a reliable replacement for
1376    "Cache-Control: no-cache" in a response.
1377  </t>
1378</x:note>
1379<t>
1380  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1381</t>
1382</section>
1383
1384<section anchor="header.vary" title="Vary">
1385  <iref item="Vary header" primary="true" x:for-anchor="" />
1386  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1387  <x:anchor-alias value="Vary"/>
1388  <x:anchor-alias value="Vary-v"/>
1389<t>
1390  The "Vary" response-header field conveys the set of request-header fields
1391  that were used to select the representation.
1392</t>
1393<t>
1394  Caches use this information, in part, to determine whether a stored response
1395  can be used to satisfy a given request; see
1396  <xref target="caching.negotiated.responses" />.
1397  determines, while the response is fresh, whether a cache is permitted to use the
1398  response to reply to a subsequent request without validation; see <xref 
1399  target="caching.negotiated.responses" />.
1400</t>
1401<t>
1402  In uncacheable or stale responses, the Vary field value advises the user agent about
1403  the criteria that were used to select the representation.
1404</t>
1405<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1406  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1407  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1408</artwork></figure>
1409<t>
1410  The set of header fields named by the Vary field value is known as the selecting
1411  request-headers.
1412</t>
1413<t>
1414  Servers &SHOULD; include a Vary header field with any cacheable response that is
1415  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1416  requests on that resource and informs the user agent about the presence of negotiation on
1417  that resource. A server &MAY; include a Vary header field with a non-cacheable
1418  response that is subject to server-driven negotiation, since this might provide the user
1419  agent with useful information about the dimensions over which the response varies at the
1420  time of the response.
1421</t>
1422<t>
1423  A Vary field value of "*" signals that unspecified parameters not limited to the
1424  request-headers (e.g., the network address of the client), play a role in the selection of
1425  the response representation; therefore, a cache cannot determine whether this response is
1426  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1427  it may only be generated by an origin server.
1428</t>
1429<t>
1430  The field-names given are not limited to the set of standard request-header fields
1431  defined by this specification. Field names are case-insensitive.
1432</t>
1433</section>
1434
1435<section anchor="header.warning" title="Warning">
1436  <iref item="Warning header" primary="true" x:for-anchor="" />
1437  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1438  <x:anchor-alias value="Warning"/>
1439  <x:anchor-alias value="Warning-v"/>
1440  <x:anchor-alias value="warning-value"/>
1441  <x:anchor-alias value="warn-agent"/>
1442  <x:anchor-alias value="warn-code"/>
1443  <x:anchor-alias value="warn-date"/>
1444  <x:anchor-alias value="warn-text"/>
1445<t>
1446  The "Warning" general-header field is used to carry additional information about the status
1447  or transformation of a message that might not be reflected in the message. This
1448  information is typically used to warn about possible incorrectness introduced by caching
1449  operations or transformations applied to the entity body of the message.
1450</t>
1451<t>
1452  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1453  warning, rather than an error status code, distinguishes these responses from true failures.
1454</t>
1455<t>
1456  Warning headers can in general be applied to any message, however some warn-codes are
1457  specific to caches and can only be applied to response messages.
1458</t>
1459<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"/>
1460  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1461  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1462 
1463  <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>
1464                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1465 
1466  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1467  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1468                  ; the name or pseudonym of the server adding
1469                  ; the Warning header, for use in debugging
1470  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1471  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1472</artwork></figure>
1473<t>
1474  Multiple warnings can be attached to a response (either by the origin server or by
1475  a cache), including multiple warnings with the same code number, only differing
1476  in warn-text.
1477</t>
1478<t>
1479  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1480  possible, in the order that they appear in the response.
1481</t>
1482<t>
1483  Systems that generate multiple Warning headers &SHOULD; order them with this user
1484  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1485  Warning headers.
1486</t>
1487<t>
1488  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1489  Warning is required to be deleted from a stored response after validation:
1490  <list style="symbols">
1491    <t>1xx Warnings describe the freshness or validation status of the response, and so
1492      &MUST; be deleted by caches after validation. They can only be generated by a cache
1493      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1494    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
1495      not rectified by a validation (for example, a lossy compression of the entity bodies)
1496      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1497      returned, in which case they &MUST; be.</t>
1498  </list>
1499</t>
1500<t>
1501  If an implementation sends a message with one or more Warning headers to a receiver whose
1502  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1503  warn-date that matches the Date header in the message.
1504</t>
1505<t>
1506  If an implementation receives a message with a warning-value that includes a warn-date,
1507  and that warn-date is different from the Date value in the response, then that
1508  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1509  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1510  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1511  well.
1512</t>
1513<t>
1514  The following warn-codes are defined by this specification, each with a recommended
1515  warn-text in English, and a description of its meaning.
1516</t>
1517<t><?rfc needLines="4"?>
1518  110 Response is stale
1519  <list>
1520    <t>&SHOULD; be included whenever the returned response is stale.</t>
1521  </list>
1522</t>
1523<t><?rfc needLines="4"?>
1524  111 Revalidation failed
1525  <list>
1526    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1527      validate the response failed, due to an inability to reach the server.</t>
1528  </list>
1529</t>
1530<t><?rfc needLines="4"?>
1531  112 Disconnected operation
1532  <list>
1533    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1534      the network for a period of time.</t>
1535  </list>
1536</t>
1537<t><?rfc needLines="4"?>
1538  113 Heuristic expiration
1539  <list>
1540    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1541      greater than 24 hours and the response's age is greater than 24 hours.</t>
1542  </list>
1543</t>
1544<t><?rfc needLines="4"?>
1545  199 Miscellaneous warning
1546  <list>
1547    <t>The warning text can include arbitrary information to be presented to a human
1548      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1549      action, besides presenting the warning to the user.</t>
1550  </list>
1551</t>
1552<t><?rfc needLines="4"?>
1553  214 Transformation applied
1554  <list>
1555    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1556      transformation changing the content-coding (as specified in the Content-Encoding
1557      header) or media-type (as specified in the Content-Type header) of the response, or
1558      the entity-body of the response, unless this Warning code already appears in the
1559      response.</t>
1560  </list>
1561</t>
1562<t><?rfc needLines="4"?>
1563  299 Miscellaneous persistent warning
1564  <list>
1565    <t>The warning text can include arbitrary information to be presented to a human
1566      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1567      action.</t>
1568  </list>
1569</t>
1570</section>
1571
1572</section>
1573
1574<section anchor="history.lists" title="History Lists">
1575<t>
1576  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1577  can be used to redisplay an entity retrieved earlier in a session.
1578</t>
1579<t>
1580  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1581  a history mechanism can display a previous representation even if it has expired.
1582</t>
1583  <t>
1584  This does not prohibit the history mechanism from telling the user that a
1585  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1586  </t>
1587</section>
1588
1589
1590<section anchor="IANA.considerations" title="IANA Considerations">
1591
1592<section anchor="message.header.registration" title="Message Header Registration">
1593<t>
1594  The Message Header Registry located at <eref
1595  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1596  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1597</t>
1598<?BEGININC p6-cache.iana-headers ?>
1599<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1600<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1601   <ttcol>Header Field Name</ttcol>
1602   <ttcol>Protocol</ttcol>
1603   <ttcol>Status</ttcol>
1604   <ttcol>Reference</ttcol>
1605
1606   <c>Age</c>
1607   <c>http</c>
1608   <c>standard</c>
1609   <c>
1610      <xref target="header.age"/>
1611   </c>
1612   <c>Cache-Control</c>
1613   <c>http</c>
1614   <c>standard</c>
1615   <c>
1616      <xref target="header.cache-control"/>
1617   </c>
1618   <c>Expires</c>
1619   <c>http</c>
1620   <c>standard</c>
1621   <c>
1622      <xref target="header.expires"/>
1623   </c>
1624   <c>Pragma</c>
1625   <c>http</c>
1626   <c>standard</c>
1627   <c>
1628      <xref target="header.pragma"/>
1629   </c>
1630   <c>Vary</c>
1631   <c>http</c>
1632   <c>standard</c>
1633   <c>
1634      <xref target="header.vary"/>
1635   </c>
1636   <c>Warning</c>
1637   <c>http</c>
1638   <c>standard</c>
1639   <c>
1640      <xref target="header.warning"/>
1641   </c>
1642</texttable>
1643<!--(END)-->
1644<?ENDINC p6-cache.iana-headers ?>
1645<t>
1646  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1647</t>
1648</section>
1649
1650</section>
1651
1652<section anchor="security.considerations" title="Security Considerations">
1653<t>
1654  Caches expose additional potential vulnerabilities, since the contents of the cache
1655  represent an attractive target for malicious exploitation. Because cache contents persist
1656  after an HTTP request is complete, an attack on the cache can reveal information long after
1657  a user believes that the information has been removed from the network. Therefore, cache
1658  contents should be protected as sensitive information.
1659</t>
1660</section>
1661
1662<section anchor="ack" title="Acknowledgments">
1663<t>
1664  Much of the content and presentation of the caching design is due to suggestions and
1665  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1666  and Larry Masinter.
1667</t>
1668</section>
1669
1670</middle>
1671
1672<back>
1673<references title="Normative References">
1674
1675  <reference anchor="Part1">
1676    <front>
1677      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1678      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1679        <organization abbrev="Day Software">Day Software</organization>
1680        <address><email>fielding@gbiv.com</email></address>
1681      </author>
1682      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1683        <organization>One Laptop per Child</organization>
1684        <address><email>jg@laptop.org</email></address>
1685      </author>
1686      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1687        <organization abbrev="HP">Hewlett-Packard Company</organization>
1688        <address><email>JeffMogul@acm.org</email></address>
1689      </author>
1690      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1691        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1692        <address><email>henrikn@microsoft.com</email></address>
1693      </author>
1694      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1695        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1696        <address><email>LMM@acm.org</email></address>
1697      </author>
1698      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1699        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1700        <address><email>paulle@microsoft.com</email></address>
1701      </author>
1702      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1703        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1704        <address><email>timbl@w3.org</email></address>
1705      </author>
1706      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1707        <organization abbrev="W3C">World Wide Web Consortium</organization>
1708        <address><email>ylafon@w3.org</email></address>
1709      </author>
1710      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1711        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1712        <address><email>julian.reschke@greenbytes.de</email></address>
1713      </author>
1714      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1715    </front>
1716    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1717    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1718  </reference>
1719
1720  <reference anchor="Part2">
1721    <front>
1722      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1723      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1724        <organization abbrev="Day Software">Day Software</organization>
1725        <address><email>fielding@gbiv.com</email></address>
1726      </author>
1727      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1728        <organization>One Laptop per Child</organization>
1729        <address><email>jg@laptop.org</email></address>
1730      </author>
1731      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1732        <organization abbrev="HP">Hewlett-Packard Company</organization>
1733        <address><email>JeffMogul@acm.org</email></address>
1734      </author>
1735      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1736        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1737        <address><email>henrikn@microsoft.com</email></address>
1738      </author>
1739      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1740        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1741        <address><email>LMM@acm.org</email></address>
1742      </author>
1743      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1744        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1745        <address><email>paulle@microsoft.com</email></address>
1746      </author>
1747      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1748        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1749        <address><email>timbl@w3.org</email></address>
1750      </author>
1751      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1752        <organization abbrev="W3C">World Wide Web Consortium</organization>
1753        <address><email>ylafon@w3.org</email></address>
1754      </author>
1755      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1756        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1757        <address><email>julian.reschke@greenbytes.de</email></address>
1758      </author>
1759      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1760    </front>
1761    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1762    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1763  </reference>
1764
1765  <reference anchor="Part4">
1766    <front>
1767      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1768      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1769        <organization abbrev="Day Software">Day Software</organization>
1770        <address><email>fielding@gbiv.com</email></address>
1771      </author>
1772      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1773        <organization>One Laptop per Child</organization>
1774        <address><email>jg@laptop.org</email></address>
1775      </author>
1776      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1777        <organization abbrev="HP">Hewlett-Packard Company</organization>
1778        <address><email>JeffMogul@acm.org</email></address>
1779      </author>
1780      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1781        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1782        <address><email>henrikn@microsoft.com</email></address>
1783      </author>
1784      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1785        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1786        <address><email>LMM@acm.org</email></address>
1787      </author>
1788      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1789        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1790        <address><email>paulle@microsoft.com</email></address>
1791      </author>
1792      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1793        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1794        <address><email>timbl@w3.org</email></address>
1795      </author>
1796      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1797        <organization abbrev="W3C">World Wide Web Consortium</organization>
1798        <address><email>ylafon@w3.org</email></address>
1799      </author>
1800      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1801        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1802        <address><email>julian.reschke@greenbytes.de</email></address>
1803      </author>
1804      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1805    </front>
1806    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1807    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1808  </reference>
1809
1810  <reference anchor="Part5">
1811    <front>
1812      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1813      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1814        <organization abbrev="Day Software">Day Software</organization>
1815        <address><email>fielding@gbiv.com</email></address>
1816      </author>
1817      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1818        <organization>One Laptop per Child</organization>
1819        <address><email>jg@laptop.org</email></address>
1820      </author>
1821      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1822        <organization abbrev="HP">Hewlett-Packard Company</organization>
1823        <address><email>JeffMogul@acm.org</email></address>
1824      </author>
1825      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1826        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1827        <address><email>henrikn@microsoft.com</email></address>
1828      </author>
1829      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1830        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1831        <address><email>LMM@acm.org</email></address>
1832      </author>
1833      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1834        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1835        <address><email>paulle@microsoft.com</email></address>
1836      </author>
1837      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1838        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1839        <address><email>timbl@w3.org</email></address>
1840      </author>
1841      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1842        <organization abbrev="W3C">World Wide Web Consortium</organization>
1843        <address><email>ylafon@w3.org</email></address>
1844      </author>
1845      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1846        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1847        <address><email>julian.reschke@greenbytes.de</email></address>
1848      </author>
1849      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1850    </front>
1851    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1852    <x:source basename="p5-range" href="p5-range.xml" />
1853  </reference>
1854
1855  <reference anchor="Part7">
1856    <front>
1857      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1858      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1859        <organization abbrev="Day Software">Day Software</organization>
1860        <address><email>fielding@gbiv.com</email></address>
1861      </author>
1862      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1863        <organization>One Laptop per Child</organization>
1864        <address><email>jg@laptop.org</email></address>
1865      </author>
1866      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1867        <organization abbrev="HP">Hewlett-Packard Company</organization>
1868        <address><email>JeffMogul@acm.org</email></address>
1869      </author>
1870      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1871        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1872        <address><email>henrikn@microsoft.com</email></address>
1873      </author>
1874      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1875        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1876        <address><email>LMM@acm.org</email></address>
1877      </author>
1878      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1879        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1880        <address><email>paulle@microsoft.com</email></address>
1881      </author>
1882      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1883        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1884        <address><email>timbl@w3.org</email></address>
1885      </author>
1886      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1887        <organization abbrev="W3C">World Wide Web Consortium</organization>
1888        <address><email>ylafon@w3.org</email></address>
1889      </author>
1890      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1891        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1892        <address><email>julian.reschke@greenbytes.de</email></address>
1893      </author>
1894      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1895    </front>
1896    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1897    <x:source basename="p7-auth" href="p7-auth.xml" />
1898  </reference>
1899
1900  <reference anchor="RFC2119">
1901    <front>
1902      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1903      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1904        <organization>Harvard University</organization>
1905        <address><email>sob@harvard.edu</email></address>
1906      </author>
1907      <date month="March" year="1997" />
1908    </front>
1909    <seriesInfo name="BCP" value="14" />
1910    <seriesInfo name="RFC" value="2119" />
1911  </reference>
1912
1913  <reference anchor="RFC5234">
1914    <front>
1915      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1916      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1917        <organization>Brandenburg InternetWorking</organization>
1918        <address>
1919          <email>dcrocker@bbiw.net</email>
1920        </address> 
1921      </author>
1922      <author initials="P." surname="Overell" fullname="Paul Overell">
1923        <organization>THUS plc.</organization>
1924        <address>
1925          <email>paul.overell@thus.net</email>
1926        </address>
1927      </author>
1928      <date month="January" year="2008"/>
1929    </front>
1930    <seriesInfo name="STD" value="68"/>
1931    <seriesInfo name="RFC" value="5234"/>
1932  </reference>
1933 
1934</references>
1935
1936<references title="Informative References">
1937
1938  <reference anchor="RFC1305">
1939    <front>
1940      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1941      <author fullname="David L. Mills" initials="D." surname="Mills">
1942        <organization>University of Delaware, Electrical Engineering Department</organization>
1943        <address><email>mills@udel.edu</email></address>
1944      </author>
1945      <date month="March" year="1992" />
1946    </front>
1947    <seriesInfo name="RFC" value="1305" />
1948  </reference>
1949
1950  <reference anchor="RFC2616">
1951    <front>
1952      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1953      <author fullname="R. Fielding" initials="R." surname="Fielding">
1954        <organization>University of California, Irvine</organization>
1955        <address><email>fielding@ics.uci.edu</email></address>
1956      </author>
1957      <author fullname="J. Gettys" initials="J." surname="Gettys">
1958        <organization>W3C</organization>
1959        <address><email>jg@w3.org</email></address>
1960      </author>
1961      <author fullname="J. Mogul" initials="J." surname="Mogul">
1962        <organization>Compaq Computer Corporation</organization>
1963        <address><email>mogul@wrl.dec.com</email></address>
1964      </author>
1965      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1966        <organization>MIT Laboratory for Computer Science</organization>
1967        <address><email>frystyk@w3.org</email></address>
1968      </author>
1969      <author fullname="L. Masinter" initials="L." surname="Masinter">
1970        <organization>Xerox Corporation</organization>
1971        <address><email>masinter@parc.xerox.com</email></address>
1972      </author>
1973      <author fullname="P. Leach" initials="P." surname="Leach">
1974        <organization>Microsoft Corporation</organization>
1975        <address><email>paulle@microsoft.com</email></address>
1976      </author>
1977      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1978        <organization>W3C</organization>
1979        <address><email>timbl@w3.org</email></address>
1980      </author>
1981      <date month="June" year="1999" />
1982    </front>
1983    <seriesInfo name="RFC" value="2616" />
1984  </reference>
1985
1986  <reference anchor="RFC3864">
1987    <front>
1988      <title>Registration Procedures for Message Header Fields</title>
1989      <author fullname="G. Klyne" initials="G." surname="Klyne">
1990        <organization>Nine by Nine</organization>
1991        <address><email>GK-IETF@ninebynine.org</email></address>
1992      </author>
1993      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1994        <organization>BEA Systems</organization>
1995        <address><email>mnot@pobox.com</email></address>
1996      </author>
1997      <author fullname="J. Mogul" initials="J." surname="Mogul">
1998        <organization>HP Labs</organization>
1999        <address><email>JeffMogul@acm.org</email></address>
2000      </author>
2001      <date month="September" year="2004" />
2002    </front>
2003    <seriesInfo name="BCP" value="90" />
2004    <seriesInfo name="RFC" value="3864" />
2005  </reference>
2006
2007</references>
2008
2009<section anchor="compatibility" title="Compatibility with Previous Versions">
2010
2011<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2012<t>
2013  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2014  this missing case.
2015  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2016</t>
2017<t>
2018  Range request responses would become very verbose if all meta-data were always returned;
2019  by allowing the server to only send needed headers in a 206 response, this problem can be
2020  avoided.
2021  (<xref target="combining.headers" />)
2022</t>
2023<t>
2024  The Cache-Control: max-age directive was not properly defined for responses.
2025  (<xref target="cache-response-directive"/>)
2026</t>
2027<t>
2028  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2029    format="counter" target="expiration.model" />, <xref format="counter"
2030    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2031  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2032  header, as PUT or other methods may have need for it in requests.
2033</t>
2034</section>
2035
2036<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2037<t>
2038  Remove requirement to consider Content-Location in successful responses
2039  in order to determine the appropriate response to use.
2040  (<xref target="validation.model" />)
2041</t>
2042<t>
2043  Clarify denial of service attack avoidance requirement.
2044  (<xref target="invalidation.after.updates.or.deletions" />)
2045</t>
2046<t>
2047  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2048  anymore, as these aspects never were implemented.
2049  (<xref target="header.warning" />)
2050</t>
2051</section>
2052
2053    </section>
2054
2055<?BEGININC p6-cache.abnf-appendix ?>
2056<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2057<figure>
2058<artwork type="abnf" name="p6-cache.parsed-abnf">
2059<x:ref>Age</x:ref> = "Age:" OWS Age-v
2060<x:ref>Age-v</x:ref> = delta-seconds
2061
2062<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2063<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2064 cache-directive ] )
2065
2066<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2067<x:ref>Expires-v</x:ref> = HTTP-date
2068
2069<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2070
2071<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2072
2073<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2074<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2075 pragma-directive ] )
2076
2077<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2078<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2079 ] ) )
2080
2081<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2082<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2083 ] )
2084
2085<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2086<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2087<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2088 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2089 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2090 cache-extension
2091<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2092 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2093 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2094 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2095 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2096 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2097
2098<x:ref>delta-seconds</x:ref> = 1*DIGIT
2099
2100<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2101
2102<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2103
2104<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2105<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2106<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2107
2108<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2109
2110<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2111
2112<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2113
2114<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2115<x:ref>warn-code</x:ref> = 3DIGIT
2116<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2117<x:ref>warn-text</x:ref> = quoted-string
2118<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2119 ]
2120</artwork>
2121</figure>
2122<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2123; Age defined but not used
2124; Cache-Control defined but not used
2125; Expires defined but not used
2126; Pragma defined but not used
2127; Vary defined but not used
2128; Warning defined but not used
2129</artwork></figure></section>
2130<?ENDINC p6-cache.abnf-appendix ?>
2131
2132<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2133
2134<section title="Since RFC2616">
2135  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2136</section>
2137
2138<section title="Since draft-ietf-httpbis-p6-cache-00">
2139<t>
2140  Closed issues:
2141  <list style="symbols">
2142    <t>
2143      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2144    <t>
2145      <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>
2146    <t>
2147      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2148    <t>
2149      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2150    <t>
2151      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2152    <t>
2153      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2154    <t>
2155      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2156    <t>
2157      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2158    <t>
2159      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2160  </list>
2161</t>
2162<t>
2163  Other changes:
2164  <list style="symbols">
2165    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2166        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2167  </list>
2168</t>
2169</section>
2170
2171<section title="Since draft-ietf-httpbis-p6-cache-01">
2172<t>
2173  Closed issues:
2174  <list style="symbols">
2175    <t>
2176      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2177  </list>
2178</t>
2179<t>
2180  Other changes:
2181  <list style="symbols">
2182    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2183      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2184    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2185      specification.</t>
2186  </list>
2187</t>
2188</section>
2189
2190<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2191<t>
2192  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2193  <list style="symbols">
2194    <t>Reference RFC 3984, and update header registrations for headers defined in this
2195      document.</t>
2196  </list>
2197</t>
2198</section>
2199
2200<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2201<t>
2202  Closed issues:
2203  <list style="symbols">
2204    <t>
2205      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2206  </list>
2207</t>
2208</section>
2209
2210<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2211<t>
2212  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2213  <list style="symbols"> 
2214    <t>
2215      Use "/" instead of "|" for alternatives.
2216    </t>
2217    <t>
2218      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2219      whitespace ("OWS") and required whitespace ("RWS").
2220    </t>
2221    <t>
2222      Rewrite ABNFs to spell out whitespace rules, factor out
2223      header value format definitions.
2224    </t>
2225  </list>
2226</t>
2227</section>
2228
2229<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2230<t>
2231  This is a total rewrite of this part of the specification.
2232</t>
2233<t>
2234  Affected issues:
2235  <list style="symbols">
2236    <t>
2237      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2238    <t>
2239      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2240    <t>
2241      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2242    <t>
2243      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2244  </list>
2245</t>
2246<t>
2247  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2248  <list style="symbols"> 
2249    <t>
2250      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2251    </t>
2252  </list>
2253</t>
2254</section>
2255
2256<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2257<t>
2258  Closed issues:
2259  <list style="symbols"> 
2260    <t>
2261      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2262      "base for numeric protocol elements"
2263    </t>
2264  </list>
2265</t>
2266<t>
2267  Affected issues:
2268  <list style="symbols">
2269    <t>
2270      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2271      Vary and non-existant headers
2272    </t>
2273  </list>
2274</t>
2275</section>
2276
2277<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2278<t>
2279  Closed issues:
2280  <list style="symbols"> 
2281    <t>
2282      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2283      "Definition of 1xx Warn-Codes"
2284    </t>
2285    <t>
2286      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2287      "Content-Location on 304 responses"
2288    </t>
2289    <t>
2290      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2291      "private and no-cache CC directives with headers"
2292    </t>
2293    <t>
2294      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2295      "RFC2047 and warn-text"
2296    </t>
2297  </list>
2298</t>
2299</section>
2300
2301<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2302<t>
2303  Closed issues:
2304  <list style="symbols"> 
2305    <t>
2306      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2307      "serving negotiated responses from cache: header-specific canonicalization"
2308    </t>
2309    <t>
2310      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2311      "Effect of CC directives on history lists"
2312    </t>
2313  </list>
2314</t>
2315<t>
2316  Affected issues:
2317  <list style="symbols">
2318    <t>
2319      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2320      Status codes and caching
2321    </t>
2322  </list>
2323</t>
2324<t>
2325  Partly resolved issues:
2326  <list style="symbols"> 
2327    <t>
2328      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2329      "Placement of 13.5.1 and 13.5.2"
2330    </t>
2331  </list>
2332</t>
2333</section>
2334
2335<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2336<t>
2337  No significant changes.
2338</t>
2339</section>
2340
2341</section>
2342  </back>
2343</rfc>
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