source: draft-ietf-httpbis/latest/p6-cache.xml @ 778

Last change on this file since 778 was 778, checked in by julian.reschke@…, 10 years ago

aesthetic changes, preparing for work on #29.

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