source: draft-ietf-httpbis/latest/p6-cache.xml @ 816

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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 "May">
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 rfcedstyle="yes"?>
45<?rfc-ext allow-markup-in-artwork="yes" ?>
46<?rfc-ext include-references-in-index="yes" ?>
47<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
48  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
49<front>
50
51  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
52
53  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
54    <organization abbrev="Day Software">Day Software</organization>
55    <address>
56      <postal>
57        <street>23 Corporate Plaza DR, Suite 280</street>
58        <city>Newport Beach</city>
59        <region>CA</region>
60        <code>92660</code>
61        <country>USA</country>
62      </postal>
63      <phone>+1-949-706-5300</phone>
64      <facsimile>+1-949-706-5305</facsimile>
65      <email>fielding@gbiv.com</email>
66      <uri>http://roy.gbiv.com/</uri>
67    </address>
68  </author>
69
70  <author fullname="Jim Gettys" initials="J." surname="Gettys">
71    <organization>One Laptop per Child</organization>
72    <address>
73      <postal>
74        <street>21 Oak Knoll Road</street>
75        <city>Carlisle</city>
76        <region>MA</region>
77        <code>01741</code>
78        <country>USA</country>
79      </postal>
80      <email>jg@laptop.org</email>
81      <uri>http://www.laptop.org/</uri>
82    </address>
83  </author>
84
85  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
86    <organization abbrev="HP">Hewlett-Packard Company</organization>
87    <address>
88      <postal>
89        <street>HP Labs, Large Scale Systems Group</street>
90        <street>1501 Page Mill Road, MS 1177</street>
91        <city>Palo Alto</city>
92        <region>CA</region>
93        <code>94304</code>
94        <country>USA</country>
95      </postal>
96      <email>JeffMogul@acm.org</email>
97    </address>
98  </author>
99
100  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108        <country>USA</country>
109      </postal>
110      <email>henrikn@microsoft.com</email>
111    </address>
112  </author>
113
114  <author fullname="Larry Masinter" initials="L." surname="Masinter">
115    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
116    <address>
117      <postal>
118        <street>345 Park Ave</street>
119        <city>San Jose</city>
120        <region>CA</region>
121        <code>95110</code>
122        <country>USA</country>
123      </postal>
124      <email>LMM@acm.org</email>
125      <uri>http://larry.masinter.net/</uri>
126    </address>
127  </author>
128
129  <author fullname="Paul J. Leach" initials="P." surname="Leach">
130    <organization abbrev="Microsoft">Microsoft Corporation</organization>
131    <address>
132      <postal>
133        <street>1 Microsoft Way</street>
134        <city>Redmond</city>
135        <region>WA</region>
136        <code>98052</code>
137      </postal>
138      <email>paulle@microsoft.com</email>
139    </address>
140  </author>
141
142  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
143    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
144    <address>
145      <postal>
146        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
147        <street>The Stata Center, Building 32</street>
148        <street>32 Vassar Street</street>
149        <city>Cambridge</city>
150        <region>MA</region>
151        <code>02139</code>
152        <country>USA</country>
153      </postal>
154      <email>timbl@w3.org</email>
155      <uri>http://www.w3.org/People/Berners-Lee/</uri>
156    </address>
157  </author>
158
159  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
160    <organization abbrev="W3C">World Wide Web Consortium</organization>
161    <address>
162      <postal>
163        <street>W3C / ERCIM</street>
164        <street>2004, rte des Lucioles</street>
165        <city>Sophia-Antipolis</city>
166        <region>AM</region>
167        <code>06902</code>
168        <country>France</country>
169      </postal>
170      <email>ylafon@w3.org</email>
171      <uri>http://www.raubacapeu.net/people/yves/</uri>
172    </address>
173  </author>
174
175  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
176    <address>
177      <email>mnot@mnot.net</email>
178      <uri>http://www.mnot.net/</uri>
179    </address>
180  </author>
181
182  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>
191      <facsimile>+49 251 2807761</facsimile>
192      <email>julian.reschke@greenbytes.de</email>
193      <uri>http://greenbytes.de/tech/webdav/</uri>
194    </address>
195  </author>
196
197  <date month="&ID-MONTH;" year="&ID-YEAR;" />
198  <workgroup>HTTPbis Working Group</workgroup>
199
200<abstract>
201<t>
202  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
203  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
204  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
205  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
206  fields that control cache behavior or indicate cacheable response messages.
207</t>
208</abstract>
209
210<note title="Editorial Note (To be removed by RFC Editor)">
211<t>
212  Discussion of this draft should take place on the HTTPBIS working group mailing list
213  (ietf-http-wg@w3.org). The current issues list is at <eref
214  target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
215  (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.
216</t>
217<t>
218  The changes in this draft are summarized in <xref target="changes.since.09"/>.
219</t>
220</note>
221
222  </front>
223  <middle>
224
225<section anchor="caching" title="Introduction">
226<t>
227  HTTP is typically used for distributed information systems, where performance can be
228  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
229  caching and reusing response messages.
230</t>
231
232<section anchor="intro.purpose" title="Purpose">
233<iref item="cache" />
234<t>
235  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
236  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
237  in order to reduce the response time and network bandwidth consumption on future,
238  equivalent requests. Any client or server may include a cache, though a cache cannot be
239  used by a server that is acting as a tunnel.
240</t>
241<t>
242  Caching would be useless if it did not significantly improve performance. The goal of
243  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
244  some cases, a stored response can be reused without the need for a network request,
245  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
246  (see <xref target="expiration.model" />). Even when a new request is required, it is often
247  possible to reuse all or parts of the payload of a prior response to satisfy the request,
248  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
249  purpose (see <xref target="validation.model" />).
250</t>
251</section>
252
253<section anchor="intro.terminology" title="Terminology">
254<t>
255  This specification uses a number of terms to refer to the roles played by participants
256  in, and objects of, HTTP caching.
257</t>
258<t>
259  <iref item="cacheable" />
260  <x:dfn>cacheable</x:dfn>
261  <list>
262    <t>A response is cacheable if a cache is allowed to store a copy of the response message
263      for use in answering subsequent requests. Even when a response is cacheable, there may
264      be additional constraints on whether a cache can use the cached copy to satisfy a
265      particular request.</t>
266  </list>
267</t>
268<t>
269  <iref item="explicit expiration time" />
270  <x:dfn>explicit expiration time</x:dfn>
271  <list>
272    <t>The time at which the origin server intends that an entity should no longer be
273      returned by a cache without further validation.</t>
274  </list>
275</t>
276<t>
277  <iref item="heuristic expiration time" />
278  <x:dfn>heuristic expiration time</x:dfn>
279  <list>
280    <t>An expiration time assigned by a cache when no explicit expiration time is
281    available.</t>
282  </list>
283</t>
284<t>
285  <iref item="age" />
286  <x:dfn>age</x:dfn>
287  <list>
288    <t>The age of a response is the time since it was sent by, or successfully validated
289      with, the origin server.</t>
290  </list>
291</t>
292<t>
293  <iref item="first-hand" />
294  <x:dfn>first-hand</x:dfn>
295  <list>
296    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
297    0.</t>
298  </list>
299</t>
300<t>
301  <iref item="freshness lifetime" />
302  <x:dfn>freshness lifetime</x:dfn>
303  <list>
304    <t>The length of time between the generation of a response and its expiration time. </t>
305  </list>
306</t>
307<t>
308  <iref item="fresh" />
309  <x:dfn>fresh</x:dfn>
310  <list>
311    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
312  </list>
313</t>
314<t>
315  <iref item="stale" />
316  <x:dfn>stale</x:dfn>
317  <list>
318    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
319  </list>
320</t>
321<t>
322  <iref item="validator" />
323  <x:dfn>validator</x:dfn>
324  <list>
325    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
326      out whether a stored response is an equivalent copy of an entity.</t>
327  </list>
328</t>
329<t anchor="shared.and.non-shared.caches">
330  <iref item="validator" />
331  <x:dfn>shared cache</x:dfn>
332  <list>
333    <t>A cache that is accessible to more than one user. A non-shared cache is
334      dedicated to a single user.</t>
335  </list>
336</t>
337</section>
338
339<section anchor="intro.requirements" title="Requirements">
340<t>
341  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
342  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
343  described in <xref target="RFC2119" />.
344</t>
345<t>
346  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
347  or &REQUIRED; level requirements for the protocols it implements. An implementation
348  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
349  requirements for its protocols is said to be "unconditionally compliant"; one that
350  satisfies all the &MUST; level requirements but not all the &SHOULD; level
351  requirements for its protocols is said to be "conditionally compliant."
352</t>
353</section>
354
355<section title="Syntax Notation" anchor="notation">
356  <x:anchor-alias value="ALPHA"/>
357  <x:anchor-alias value="CR"/>
358  <x:anchor-alias value="DIGIT"/>
359  <x:anchor-alias value="DQUOTE"/>
360  <x:anchor-alias value="LF"/>
361  <x:anchor-alias value="OCTET"/>
362  <x:anchor-alias value="SP"/>
363  <x:anchor-alias value="VCHAR"/>
364  <x:anchor-alias value="WSP"/>
365<t>
366  This specification uses the ABNF syntax defined in &notation; (which
367  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
368  <xref target="collected.abnf"/> shows the collected ABNF, with the list
369  rule expanded.
370</t>
371<t>
372  The following core rules are included by
373  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
374  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
375  DIGIT (decimal 0-9), DQUOTE (double quote),
376  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
377  OCTET (any 8-bit sequence of data), SP (space),
378  VCHAR (any visible USASCII character),
379  and WSP (whitespace).
380</t>
381
382<section title="Core Rules" anchor="core.rules">
383  <x:anchor-alias value="quoted-string"/>
384  <x:anchor-alias value="token"/>
385  <x:anchor-alias value="OWS"/>
386<t>
387  The core rules below are defined in &basic-rules;:
388</t>
389<figure><artwork type="abnf2616">
390  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
391  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
392  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
393</artwork></figure>
394</section>
395
396<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
397  <x:anchor-alias value="field-name"/>
398  <x:anchor-alias value="HTTP-date"/>
399  <x:anchor-alias value="port"/>
400  <x:anchor-alias value="pseudonym"/>
401  <x:anchor-alias value="uri-host"/>
402<t>
403  The ABNF rules below are defined in other parts:
404</t>
405<figure><!--Part1--><artwork type="abnf2616">
406  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
407  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
408  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
409  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
410  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
411</artwork></figure>
412</section>
413
414</section>
415</section>
416
417<section anchor="caching.overview" title="Cache Operation">
418
419<section anchor="response.cacheability" title="Response Cacheability">
420<t>
421  A cache &MUST-NOT; store a response to any request, unless:
422  <list style="symbols">
423    <t>The request method is understood by the cache and defined as being cacheable, and</t>
424    <t>the response status code is understood by the cache, and</t>
425    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
426       appear in request or response headers, and</t>
427    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
428       does not appear in the response, if the cache is shared, and</t>
429    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
430       the cache is shared (unless the "public" directive is present; see <xref
431       target="header.cache-control" />), and</t>
432    <t>the response either:
433      <list style="symbols">
434        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
435        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
436        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
437        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
438        <t>has a status code that can be served with heuristic freshness (see <xref
439           target="heuristic.freshness" />).</t>
440      </list>
441    </t>
442  </list>
443</t>
444<t>
445  In this context, a cache has "understood" a request method or a response status
446  code if it recognises it and implements any cache-specific behaviour. In
447  particular, 206 Partial Content responses cannot be cached by an
448  implementation that does not handle partial content
449  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
450</t>
451<t>
452  Note that in normal operation, most caches will not store a response that has neither a
453  cache validator nor an explicit expiration time, as such responses are not usually
454  useful to store. However, caches are not prohibited from storing such responses.
455</t>
456
457<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
458<t>
459  A cache that receives an incomplete response (for example, with fewer bytes of data
460  than specified in a Content-Length header) can store the response, but &MUST; 
461  treat it as a partial response &partial;. Partial responses
462  can be combined as described in &combining-byte-ranges;; the result might be a
463  full response or might still be partial. A cache &MUST-NOT; return a partial
464  response to a client without explicitly marking it as such using the 206 (Partial
465  Content) status code.
466</t>
467<t>
468  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
469  incomplete or partial responses.
470</t>
471</section>
472
473</section>
474
475
476<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
477<t>
478  For a presented request, a cache &MUST-NOT; return a stored response, unless:
479  <list style="symbols">
480    <t>The presented Request-URI and that of the stored response match
481      (<cref anchor="TODO-Request-URI">Need to find a new term for this, as Part
482      1 doesn't define Request-URI anymore; the new term request-target does not
483      work for this. (see <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196" />)</cref>), and</t>
484    <t>the request method associated with the stored response allows it to be
485      used for the presented request, and</t>
486    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
487      target="caching.negotiated.responses" />), and</t>
488    <t>the presented request and stored response are free from directives that would prevent
489      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
490      and</t>
491    <t>the stored response is either:
492      <list style="symbols">
493        <t>fresh (see <xref target="expiration.model" />), or</t>
494        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
495        <t>successfully validated (see <xref target="validation.model" />).</t>
496      </list>
497    </t>
498  </list>
499</t>
500<t>
501  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
502</t>
503<t>
504  When a stored response is used to satisfy a request, caches &MUST; include a
505  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
506  current_age; see <xref target="age.calculations" />.
507  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
508</t>
509<t>
510  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
511  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
512  corresponding response.
513</t>
514<t>
515  Also, note that unsafe requests might invalidate already stored responses; see
516  <xref target="invalidation.after.updates.or.deletions" />.
517</t>
518<t>
519  Caches &MUST; use the most recent response (as determined by the Date header) when
520  more than one suitable response is stored. They can also forward a request with
521  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
522  use.
523</t>
524</section>
525
526<section anchor="expiration.model" title="Freshness Model">
527<t>
528  When a response is "fresh" in the cache, it can be used to satisfy subsequent
529  requests without contacting the origin server, thereby improving efficiency.
530</t>
531<t>
532  The primary mechanism for determining freshness is for an origin server to provide an
533  explicit expiration time in the future, using either the Expires header (<xref
534  target="header.expires" />) or the max-age response cache directive (<xref
535  target="cache-response-directive" />). Generally, origin servers will assign future
536  explicit expiration times to responses in the belief that the entity is not likely to
537  change in a semantically significant way before the expiration time is reached.
538</t>
539<t>
540  If an origin server wishes to force a cache to validate every request, it can
541  assign an explicit expiration time in the past. This means that the response is always
542  stale, so that caches should validate it before using it for subsequent requests.
543  <cref anchor="TODO-response-stale">This wording may cause confusion, because the response may still be served stale.</cref>
544</t>
545<t>
546  Since origin servers do not always provide explicit expiration times, HTTP caches may
547  also assign heuristic expiration times when they are not specified, employing algorithms that
548  use other header values (such as the Last-Modified time) to estimate a plausible
549  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
550  impose worst-case constraints on their results.
551</t>
552<figure>
553<preamble>
554  The calculation to determine if a response is fresh is:
555</preamble>
556<artwork type="code">
557   response_is_fresh = (freshness_lifetime &gt; current_age)
558</artwork>
559</figure>
560<t>
561  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
562  the current_age is defined in <xref target="age.calculations" />.
563</t>
564<t>
565  Additionally, clients may need to influence freshness calculation. They can do this using
566  several request cache directives, with the effect of either increasing or loosening
567  constraints on freshness. See <xref target="cache-request-directive" />.
568</t>
569<t>
570  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
571  freshness.</cref>
572</t>
573<t>
574  Note that freshness applies only to cache operation; it cannot be used to force a user agent
575  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
576  the difference between caches and history mechanisms.
577</t>
578
579<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
580<t>
581  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
582  response by using the first match of:
583  <list style="symbols">
584    <t>If the cache is shared and the s-maxage response cache directive (<xref
585      target="cache-response-directive" />) is present, use its value, or</t>
586    <t>If the max-age response cache directive (<xref target="cache-response-directive"
587      />) is present, use its value, or</t>
588    <t>If the Expires response header (<xref target="header.expires" />) is present, use
589      its value minus the value of the Date response header, or</t>
590    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
591      freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
592  </list>
593</t>
594<t>
595  Note that this calculation is not vulnerable to clock skew, since all of the
596  information comes from the origin server.
597</t>
598
599<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
600<t>
601  If no explicit expiration time is present in a stored response that has a status code
602  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
603  calculated. Heuristics &MUST-NOT; be used for other response status codes.
604</t>
605<t>
606  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
607  attach a Warning header with a 113 warn-code to the response if its current_age is
608  more than 24 hours and such a warning is not already present.
609</t>
610<t>
611  Also, if the response has a Last-Modified header (&header-last-modified;), the
612  heuristic expiration value &SHOULD; be no more than some fraction of the interval
613  since that time. A typical setting of this fraction might be 10%.
614</t>
615<t>
616  <cref anchor="REVIEW-query-string-heuristics">took away HTTP/1.0 query string heuristic uncacheability.</cref>
617</t>
618</section>
619</section>
620
621<section anchor="age.calculations" title="Calculating Age">
622<t>
623  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
624  message when obtained from a cache. The Age field value is the cache's estimate of the
625  amount of time since the response was generated or validated by the origin server. In
626  essence, the Age value is the sum of the time that the response has been resident in
627  each of the caches along the path from the origin server, plus the amount of time it has
628  been in transit along network paths.
629</t>
630<t>
631  The following data is used for the age calculation:
632</t>
633<t>
634  <x:dfn>age_value</x:dfn>
635  <list>
636    <t>
637      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
638      in a form appropriate for arithmetic operation; or 0, if not available.
639    </t>
640  </list>
641</t>
642<t>
643  <x:dfn>date_value</x:dfn>
644  <list>
645    <t>
646      HTTP/1.1 requires origin servers to send a Date header, if possible,
647      with every response, giving the time at which the response was generated.
648      The term "date_value" denotes the value of the Date header, in a form
649      appropriate for arithmetic operations. See &header-date; for the definition
650      of the Date header, and for requirements regarding responses without a
651      Date response header.
652    </t>
653  </list>
654</t>
655<t>
656  <x:dfn>now</x:dfn>
657  <list>
658    <t>
659      The term "now" means "the current value of the clock at the host
660      performing the calculation". Hosts that use HTTP, but especially hosts
661      running origin servers and caches, &SHOULD; use NTP
662      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
663      clocks to a globally accurate time standard.
664    </t>
665  </list>
666</t>
667<t>
668  <x:dfn>request_time</x:dfn>
669  <list>
670    <t>
671      The current value of the clock at the host at the time the request
672      resulting in the stored response was made.
673    </t>
674  </list>
675</t>
676<t>
677  <x:dfn>response_time</x:dfn>
678  <list>
679    <t>
680      The current value of the clock at the host at the time the response was
681      received.
682    </t>
683  </list>
684</t>
685<t>
686  A response's age can be calculated in two entirely independent ways:
687  <list style="numbers">
688    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
689      origin server's clock. If the result is negative, the result is replaced by zero.</t>
690    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
691      note this value &MUST; be interpreted relative to the time the
692      request was initiated, not the time that the response was received.</t>
693  </list>
694</t>
695<figure>
696<artwork type="code">
697  apparent_age = max(0, response_time - date_value);
698
699  response_delay = response_time - request_time;
700  corrected_age_value = age_value + response_delay; 
701</artwork>
702</figure>
703<figure>
704<preamble>These are combined as</preamble>
705<artwork type="code">
706  corrected_initial_age = max(apparent_age, corrected_age_value);
707</artwork></figure>
708<t>
709  The current_age of a stored response can then be calculated by adding the amount of
710  time (in seconds) since the stored response was last validated by the origin server to
711  the corrected_initial_age.
712</t>
713<figure><artwork type="code">
714  resident_time = now - response_time;
715  current_age = corrected_initial_age + resident_time;
716</artwork></figure>
717</section>
718
719<section anchor="serving.stale.responses" title="Serving Stale Responses">
720<t>
721  A "stale" response is one that either has explicit expiry information or is allowed to
722  have heuristic expiry calculated, but is not fresh according to the calculations in
723  <xref target="expiration.model" />.
724</t>
725<t>
726  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
727  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
728  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
729  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
730</t>
731<t>
732  Caches &SHOULD-NOT; return stale responses unless they are
733  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
734  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
735</t>
736<t>
737  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
738  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
739  the cache is disconnected.
740</t>
741<t>
742  If a cache receives a first-hand response (either an entire response, or a 304 (Not
743  Modified) response) that it would normally forward to the requesting client, and the
744  received response is no longer fresh, the cache &SHOULD; forward it to the
745  requesting client without adding a new Warning (but without removing any existing
746  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
747  that response became stale in transit.
748</t>
749</section>
750</section>
751
752<section anchor="validation.model" title="Validation Model">
753<t>
754  When a cache has one or more stored responses for a requested URI, but cannot 
755  serve any of them (e.g., because they are not fresh, or one cannot be selected;
756  see <xref target="caching.negotiated.responses"/>),
757  it can use the conditional request mechanism &conditional; in the forwarded
758  request to give the origin server an opportunity to both select a valid stored
759  response to be used, and to update it. This process is known as "validating"
760  or "revalidating" the stored response.
761</t>
762<t>
763  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
764  header whose value is that of the Last-Modified header from the selected
765  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
766</t>
767<t>
768  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
769  is that of the ETag header(s) from all responses stored for the requested URI,
770  if present. However, if any of the stored responses contains only partial
771  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
772  field unless the request is for a range that would be fully satisfied by
773  that stored response.
774</t>
775<t>
776  A 304 (Not Modified) response status code indicates that the stored
777  response can be updated and reused; see <xref target="combining.headers"/>.
778</t>
779<t>
780  A full response (i.e., one with a response body) indicates that none 
781  of the stored responses nominated in the conditional request is
782  suitable. Instead, the full response is used both to satisfy the
783  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
784</t>
785<t>
786  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
787  either forward this response to the requesting client, or act as if the server failed to
788  respond. In the latter case, it &MAY; return a previously stored response (see <xref
789  target="serving.stale.responses" />).
790</t>
791</section>
792
793<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
794<t>
795  Because unsafe methods (&safe-methods;) have the potential for changing state on the
796  origin server, intervening caches can use them to keep their contents
797  up-to-date.
798</t>
799<t>
800  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
801  as the URI(s) in the Location and Content-Location headers (if present):
802  <list style="symbols">
803    <t>PUT</t>
804    <t>DELETE</t>
805    <t>POST</t>
806  </list>
807</t>
808<t>
809  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
810  be performed if the host part of that URI differs from the host part in the Request-URI.
811  This helps prevent denial of service attacks.
812</t>
813<t>
814  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
815</t>
816<t>
817  A cache that passes through requests for methods it does not understand &SHOULD;
818  invalidate the Request-URI.
819</t>
820<t>
821  Here, "invalidate" means that the cache will either remove all stored responses related
822  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
823  before they can be returned in response to a subsequent request.
824</t>
825<t>
826  Note that this does not guarantee that all appropriate responses are invalidated. For
827  example, the request that caused the change at the origin server might not have gone
828  through the cache where a response is stored.
829</t>
830<t>
831  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
832</t>
833</section>
834
835<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
836<t>
837  When a cache receives a request that can be satisfied by a stored response
838  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
839  response unless all of the selecting request-headers nominated by the Vary header match
840  in both the original request (i.e., that associated with the stored response),
841  and the presented request.
842</t>
843<t>
844  The selecting request-headers from two requests are defined to match
845  if and only if those in the first request can be transformed to those in the
846  second request by applying any of the following:
847  <list style="symbols">
848    <t>
849      adding or removing whitespace, where allowed in the header's syntax
850    </t>
851    <t>
852      combining multiple message-header fields with the same field name (see
853      &header-fields;)
854    </t>
855    <t>
856      normalizing both header values in a way that is known to have identical
857      semantics, according to the header's specification (e.g., re-ordering field values
858      when order is not significant; case-normalization, where values are defined to be
859      case-insensitive)   
860    </t>
861  </list>
862</t>
863<t>
864  If (after any normalisation that may take place) a header field is absent
865  from a request, it can only match another request if it is also absent there.
866</t>
867<t>
868  A Vary header field-value of "*" always fails to match, and subsequent requests to that
869  resource can only be properly interpreted by the origin server.
870</t>
871<t>
872  The stored response with matching selecting request-headers is known as the
873  selected response.
874</t>
875<t>
876  If no selected response is available, the cache &MAY; forward the presented
877  request to the origin server in a conditional request; see <xref target="validation.model"/>.
878</t>
879</section>
880
881<section anchor="combining.headers" title="Combining Responses">
882<t>
883  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
884  (in this section, the "new" response"), it needs to created an updated response by combining
885  the stored response with the new one, so that the updated response can be used to satisfy the request.
886</t>
887<t>
888  If the new response contains an ETag, it identifies the stored 
889  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
890  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
891</t>
892<t>
893  If the status code is 206 (partial content), both the stored and new 
894  responses &MUST; have validators, and those validators &MUST; match using the strong 
895  comparison function (see &weak-and-strong;). Otherwise, the 
896  responses &MUST-NOT; be combined.
897</t>
898<t>
899  The stored response headers are used as those of the updated response, except that
900  <list style="symbols">
901    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
902      &MUST; be deleted from the stored response and the updated response.</t>
903    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
904      response and the updated response.</t>
905    <t>any headers provided in the new response &MUST; replace the corresponding
906      headers from the stored response.</t>
907  </list>
908</t>
909<t>
910  If a header field-name in the new response matches more than one 
911  header in the stored response, all such stored headers &MUST; be replaced.
912</t>
913<t>
914  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
915  stored response in cache. In the case of a 206 response, the combined 
916  entity-body &MAY; be stored.
917</t>
918<t>
919  <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
920</t>
921</section>
922
923</section>
924
925<section anchor="header.fields" title="Header Field Definitions">
926<t>
927  This section defines the syntax and semantics of HTTP/1.1 header fields
928  related to caching.
929</t>
930<t>
931  For entity-header fields, both sender and recipient refer to either the client or the
932  server, depending on who sends and who receives the entity.
933</t>
934
935<section anchor="header.age" title="Age">
936  <iref item="Age header" primary="true" x:for-anchor="" />
937  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
938  <x:anchor-alias value="Age"/>
939  <x:anchor-alias value="Age-v"/>
940  <x:anchor-alias value="age-value"/>
941<t>
942  The "Age" response-header field conveys the sender's estimate of the amount
943  of time since the response was generated or successfully validated at the
944  origin server. Age values are calculated as specified in
945  <xref target="age.calculations" />.
946</t>
947<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
948  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
949  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
950</artwork></figure>
951<t anchor="rule.delta-seconds">
952  <x:anchor-alias value="delta-seconds" />
953  Age field-values are non-negative integers, representing time in seconds.
954</t>
955<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
956  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
957</artwork></figure>
958<t>
959  If a cache receives a value larger than the largest positive integer it can represent, or
960  if any of its age calculations overflows, it &MUST; transmit an Age header with a
961  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
962  of at least 31 bits of range.
963</t>
964<t>
965  The presence of an Age header field in a response implies that a response is not
966  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
967  Age header field.
968</t>
969</section>
970
971<section anchor="header.cache-control" title="Cache-Control">
972  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
973  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
974  <x:anchor-alias value="Cache-Control"/>
975  <x:anchor-alias value="Cache-Control-v"/>
976  <x:anchor-alias value="cache-directive"/>
977  <x:anchor-alias value="cache-extension"/>
978  <x:anchor-alias value="cache-request-directive"/>
979  <x:anchor-alias value="cache-response-directive"/>
980<t>
981  The "Cache-Control" general-header field is used to specify directives that
982  &MUST; be obeyed by all caches along the request/response chain. Such cache
983  directives are unidirectional in that the presence of a directive in a
984  request does not imply that the same directive is to be given in the
985  response.
986</t>
987<x:note>
988  <t>
989    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
990    Pragma: no-cache (see <xref target="header.pragma" />).
991  </t>
992</x:note>
993<t>
994  Cache directives &MUST; be passed through by a proxy or gateway application,
995  regardless of their significance to that application, since the directives might be
996  applicable to all recipients along the request/response chain. It is not possible to
997  target a directive to a specific cache.
998</t>
999<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"/>
1000  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1001  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1002
1003  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1004     / <x:ref>cache-response-directive</x:ref>
1005
1006  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1007</artwork></figure>
1008
1009<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1010  <x:anchor-alias value="cache-request-directive" />
1011
1012<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
1013  <x:ref>cache-request-directive</x:ref> =
1014       "no-cache"
1015     / "no-store"
1016     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1017     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1018     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1019     / "no-transform"
1020     / "only-if-cached"
1021     / <x:ref>cache-extension</x:ref>
1022</artwork></figure>
1023
1024<t>
1025  <x:dfn>no-cache</x:dfn>
1026  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1027  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1028  <list>
1029    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1030      used to satisfy the request without successful validation on the origin server.</t>
1031  </list>
1032</t>
1033<t>
1034  <x:dfn>no-store</x:dfn>
1035  <iref item="Cache Directives" primary="true" subitem="no-store" />
1036  <iref item="no-store" primary="true" subitem="Cache Directive" />
1037  <list>
1038    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1039      of either this request or any response to it. This directive applies to both
1040      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1041      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1042      and &MUST; make a best-effort attempt to remove the information from volatile
1043      storage as promptly as possible after forwarding it.</t>
1044    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1045      particular, malicious or compromised caches might not recognize or obey this
1046      directive, and communications networks may be vulnerable to eavesdropping.</t>
1047  </list>
1048</t>
1049<t>
1050  <x:dfn>max-age</x:dfn>
1051  <iref item="Cache Directives" primary="true" subitem="max-age" />
1052  <iref item="max-age" primary="true" subitem="Cache Directive" />
1053  <list>
1054    <t>The max-age request directive indicates that the client is willing to accept a
1055      response whose age is no greater than the specified time in seconds. Unless
1056      the max-stale request directive is also present, the client is not willing to accept a stale
1057      response.</t>
1058  </list>
1059</t>
1060<t>
1061  <x:dfn>max-stale</x:dfn>
1062  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1063  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1064  <list>
1065    <t>The max-stale request directive indicates that the client is willing to accept a
1066      response that has exceeded its expiration time. If max-stale is assigned a value,
1067      then the client is willing to accept a response that has exceeded its expiration
1068      time by no more than the specified number of seconds. If no value is assigned to
1069      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>
1070  </list>
1071</t>
1072<t>
1073  <x:dfn>min-fresh</x:dfn>
1074  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1075  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1076  <list>
1077    <t>The min-fresh request directive indicates that the client is willing to accept a
1078      response whose freshness lifetime is no less than its current age plus the specified
1079      time in seconds. That is, the client wants a response that will still be fresh for
1080      at least the specified number of seconds.</t>
1081  </list>
1082</t>
1083<t>
1084  <x:dfn>no-transform</x:dfn>
1085  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1086  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1087  <list>
1088    <t>The no-transform request directive indicates that an intermediate cache or proxy
1089      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1090      headers, nor the request entity-body.</t>
1091  </list>
1092</t>
1093<t>
1094  <x:dfn>only-if-cached</x:dfn>
1095  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1096  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1097  <list>
1098    <t>The only-if-cached request directive indicates that the client only wishes to
1099      return a stored response. If it receives this directive, a cache &SHOULD; either
1100      respond using a stored response that is consistent with the other constraints of the
1101      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1102      being operated as a unified system with good internal connectivity, such a request
1103      &MAY; be forwarded within that group of caches.</t>
1104  </list>
1105</t>
1106</section>
1107
1108<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1109  <x:anchor-alias value="cache-response-directive" />
1110
1111<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1112  <x:ref>cache-response-directive</x:ref> =
1113       "public"
1114     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1115     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1116     / "no-store"
1117     / "no-transform"
1118     / "must-revalidate"
1119     / "proxy-revalidate"
1120     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1121     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1122     / <x:ref>cache-extension</x:ref>
1123</artwork></figure>
1124
1125<t>
1126  <x:dfn>public</x:dfn>
1127  <iref item="Cache Directives" primary="true" subitem="public" />
1128  <iref item="public" primary="true" subitem="Cache Directive" />
1129  <list>
1130    <t>The public response directive indicates that the response &MAY; be cached, even
1131      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1132      (See also Authorization, &header-authorization;, for additional details.) </t>
1133  </list>
1134</t>
1135<t>
1136  <x:dfn>private</x:dfn>
1137  <iref item="Cache Directives" primary="true" subitem="private" />
1138  <iref item="private" primary="true" subitem="Cache Directive" />
1139  <list>
1140    <t>The private response directive indicates that the response message is intended for
1141      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1142      cache &MAY; store the response.</t>
1143    <t>If the private response directive specifies one or more field-names, this
1144      requirement is limited to the field-values associated with the listed response
1145      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1146      cache, whereas the remainder of the response message &MAY; be.</t>
1147    <t>
1148      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1149      be stored, and cannot ensure the privacy of the message content.
1150      Also, private response directives with field-names are often handled by
1151      implementations as if an unqualified private directive was received; i.e.,
1152      the special handling for the qualified form is not widely implemented.</t>
1153  </list>
1154</t>
1155<t>
1156  <x:dfn>no-cache</x:dfn>
1157  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1158  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1159  <list>
1160    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1161      satisfy a subsequent request without successful validation on the origin server.
1162      This allows an origin server to prevent caching even by caches that have been
1163      configured to return stale responses.</t>
1164    <t>If the no-cache response directive specifies one or more field-names, this
1165      requirement is limited to the field-values associated with the listed response
1166      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1167      to a subsequent request without successful validation on the origin server. This
1168      allows an origin server to prevent the re-use of certain header fields in a
1169      response, while still allowing caching of the rest of the response.</t>
1170    <t>
1171      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1172      Also, no-cache response directives with field-names are often handled by
1173      implementations as if an unqualified no-cache directive was received; i.e.,
1174      the special handling for the qualified form is not widely implemented.
1175    </t>
1176  </list>
1177</t>
1178<t>
1179  <x:dfn>no-store</x:dfn>
1180  <iref item="Cache Directives" primary="true" subitem="no-store" />
1181  <iref item="no-store" primary="true" subitem="Cache Directive" />
1182  <list>
1183    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1184      part of either the immediate request or response. This directive applies to both
1185      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1186      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1187      and &MUST; make a best-effort attempt to remove the information from volatile
1188      storage as promptly as possible after forwarding it.</t>
1189    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1190      particular, malicious or compromised caches might not recognize or obey this
1191      directive, and communications networks may be vulnerable to eavesdropping.</t>
1192  </list>
1193</t>
1194<t>
1195  <x:dfn>must-revalidate</x:dfn>
1196  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1197  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1198  <list>
1199    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1200     used to satisfy subsequent requests without successful validation on the origin server.</t>
1201    <t>The must-revalidate directive is necessary to support reliable operation for
1202      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1203      the must-revalidate directive; in particular, if the cache cannot reach the origin
1204      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1205    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1206      validate a request on the entity could result in incorrect operation, such as a
1207      silently unexecuted financial transaction.</t>
1208  </list>
1209</t>
1210<t>
1211  <x:dfn>proxy-revalidate</x:dfn>
1212  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1213  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1214  <list>
1215    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1216      response directive, except that it does not apply to non-shared caches.</t>
1217  </list>
1218</t>
1219<t>
1220  <x:dfn>max-age</x:dfn>
1221  <iref item="Cache Directives" primary="true" subitem="max-age" />
1222  <iref item="max-age" primary="true" subitem="Cache Directive" />
1223  <list>
1224    <t>The max-age response directive indicates that response is to be considered stale
1225      after its age is greater than the specified number of seconds.</t>
1226  </list>
1227</t>
1228<t>
1229  <x:dfn>s-maxage</x:dfn>
1230  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1231  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1232  <list>
1233    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1234      specified by this directive overrides the maximum age specified by either the
1235      max-age directive or the Expires header. The s-maxage directive also implies the
1236      semantics of the proxy-revalidate response directive.</t>
1237  </list>
1238</t>
1239<t>
1240  <x:dfn>no-transform</x:dfn>
1241  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1242  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1243  <list>
1244    <t>The no-transform response directive indicates that an intermediate cache or proxy
1245      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1246      headers, nor the response entity-body.</t>
1247  </list>
1248</t>
1249
1250</section>
1251
1252<section anchor="cache.control.extensions" title="Cache Control Extensions">
1253<t>
1254  The Cache-Control header field can be extended through the use of one or more
1255  cache-extension tokens, each with an optional value. Informational extensions (those
1256  that do not require a change in cache behavior) can be added without changing the
1257  semantics of other directives. Behavioral extensions are designed to work by acting as
1258  modifiers to the existing base of cache directives. Both the new directive and the
1259  standard directive are supplied, such that applications that do not understand the new
1260  directive will default to the behavior specified by the standard directive, and those
1261  that understand the new directive will recognize it as modifying the requirements
1262  associated with the standard directive. In this way, extensions to the cache-control
1263  directives can be made without requiring changes to the base protocol.
1264</t>
1265<t>
1266  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1267  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1268  all directives that it does not understand.
1269</t>
1270<t>
1271  For example, consider a hypothetical new response directive called "community" that
1272  acts as a modifier to the private directive. We define this new directive to mean that,
1273  in addition to any non-shared cache, any cache that is shared only by members of the
1274  community named within its value may cache the response. An origin server wishing to
1275  allow the UCI community to use an otherwise private response in their shared cache(s)
1276  could do so by including
1277</t>
1278<figure><artwork type="example">
1279  Cache-Control: private, community="UCI"
1280</artwork></figure>
1281<t>
1282  A cache seeing this header field will act correctly even if the cache does not
1283  understand the community cache-extension, since it will also see and understand the
1284  private directive and thus default to the safe behavior.
1285</t>
1286<t>
1287  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1288  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1289  directives (or the response's default cacheability) such that the cache behavior will
1290  remain minimally correct even if the cache does not understand the extension(s).
1291</t>
1292</section>
1293
1294</section>
1295
1296<section anchor="header.expires" title="Expires">
1297  <iref item="Expires header" primary="true" x:for-anchor="" />
1298  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1299  <x:anchor-alias value="Expires"/>
1300  <x:anchor-alias value="Expires-v"/>
1301<t>
1302  The "Expires" entity-header field gives the date/time after which the response is
1303  considered stale. See <xref target="expiration.model" /> for further discussion of the
1304  freshness model.
1305</t>
1306<t>
1307  The presence of an Expires field does not imply that the original resource will change or
1308  cease to exist at, before, or after that time.
1309</t>
1310<t>
1311  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1312  it &MUST; be sent in rfc1123-date format.
1313</t>
1314<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1315  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1316  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1317</artwork></figure>
1318<figure>
1319  <preamble>For example</preamble>
1320<artwork type="example">
1321  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1322</artwork></figure>
1323<x:note>
1324  <t>
1325    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1326    directive (see <xref target="cache-response-directive" />), that directive overrides
1327    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1328  </t>
1329</x:note>
1330<t>
1331  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1332</t>
1333<t>
1334  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1335  including the value "0", as in the past (i.e., "already expired").
1336</t>
1337</section>
1338
1339<section anchor="header.pragma" title="Pragma">
1340  <iref item="Pragma header" primary="true" x:for-anchor="" />
1341  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1342  <x:anchor-alias value="extension-pragma"/>
1343  <x:anchor-alias value="Pragma"/>
1344  <x:anchor-alias value="Pragma-v"/>
1345  <x:anchor-alias value="pragma-directive"/>
1346<t>
1347  The "Pragma" general-header field is used to include implementation-specific directives
1348  that might apply to any recipient along the request/response chain. All pragma directives
1349  specify optional behavior from the viewpoint of the protocol; however, some systems
1350  &MAY; require that behavior be consistent with the directives.
1351</t>
1352<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"/>
1353  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1354  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1355  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1356  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1357</artwork></figure>
1358<t>
1359  When the no-cache directive is present in a request message, an application &SHOULD;
1360  forward the request toward the origin server even if it has a cached copy of what is being
1361  requested. This pragma directive has the same semantics as the no-cache response directive
1362  (see <xref target="cache-response-directive" />) and is defined here for backward
1363  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1364  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1365  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1366</t>
1367<x:note>
1368  <t>
1369    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1370    is not actually specified, it does not provide a reliable replacement for
1371    "Cache-Control: no-cache" in a response.
1372  </t>
1373</x:note>
1374<t>
1375  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1376</t>
1377</section>
1378
1379<section anchor="header.vary" title="Vary">
1380  <iref item="Vary header" primary="true" x:for-anchor="" />
1381  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1382  <x:anchor-alias value="Vary"/>
1383  <x:anchor-alias value="Vary-v"/>
1384<t>
1385  The "Vary" response-header field conveys the set of request-header fields
1386  that were used to select the representation.
1387</t>
1388<t>
1389  Caches use this information, in part, to determine whether a stored response
1390  can be used to satisfy a given request; see
1391  <xref target="caching.negotiated.responses" />.
1392  determines, while the response is fresh, whether a cache is permitted to use the
1393  response to reply to a subsequent request without validation; see <xref 
1394  target="caching.negotiated.responses" />.
1395</t>
1396<t>
1397  In uncacheable or stale responses, the Vary field value advises the user agent about
1398  the criteria that were used to select the representation.
1399</t>
1400<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1401  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1402  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1403</artwork></figure>
1404<t>
1405  The set of header fields named by the Vary field value is known as the selecting
1406  request-headers.
1407</t>
1408<t>
1409  Servers &SHOULD; include a Vary header field with any cacheable response that is
1410  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1411  requests on that resource and informs the user agent about the presence of negotiation on
1412  that resource. A server &MAY; include a Vary header field with a non-cacheable
1413  response that is subject to server-driven negotiation, since this might provide the user
1414  agent with useful information about the dimensions over which the response varies at the
1415  time of the response.
1416</t>
1417<t>
1418  A Vary field value of "*" signals that unspecified parameters not limited to the
1419  request-headers (e.g., the network address of the client), play a role in the selection of
1420  the response representation; therefore, a cache cannot determine whether this response is
1421  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1422  it may only be generated by an origin server.
1423</t>
1424<t>
1425  The field-names given are not limited to the set of standard request-header fields
1426  defined by this specification. Field names are case-insensitive.
1427</t>
1428</section>
1429
1430<section anchor="header.warning" title="Warning">
1431  <iref item="Warning header" primary="true" x:for-anchor="" />
1432  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1433  <x:anchor-alias value="Warning"/>
1434  <x:anchor-alias value="Warning-v"/>
1435  <x:anchor-alias value="warning-value"/>
1436  <x:anchor-alias value="warn-agent"/>
1437  <x:anchor-alias value="warn-code"/>
1438  <x:anchor-alias value="warn-date"/>
1439  <x:anchor-alias value="warn-text"/>
1440<t>
1441  The "Warning" general-header field is used to carry additional information about the status
1442  or transformation of a message that might not be reflected in the message. This
1443  information is typically used to warn about possible incorrectness introduced by caching
1444  operations or transformations applied to the entity body of the message.
1445</t>
1446<t>
1447  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1448  warning, rather than an error status code, distinguishes these responses from true failures.
1449</t>
1450<t>
1451  Warning headers can in general be applied to any message, however some warn-codes are
1452  specific to caches and can only be applied to response messages.
1453</t>
1454<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"/>
1455  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1456  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1457 
1458  <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>
1459                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1460 
1461  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1462  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1463                  ; the name or pseudonym of the server adding
1464                  ; the Warning header, for use in debugging
1465  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1466  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1467</artwork></figure>
1468<t>
1469  Multiple warnings can be attached to a response (either by the origin server or by
1470  a cache), including multiple warnings with the same code number, only differing
1471  in warn-text.
1472</t>
1473<t>
1474  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1475  possible, in the order that they appear in the response.
1476</t>
1477<t>
1478  Systems that generate multiple Warning headers &SHOULD; order them with this user
1479  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1480  Warning headers.
1481</t>
1482<t>
1483  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1484  Warning is required to be deleted from a stored response after validation:
1485  <list style="symbols">
1486    <t>1xx Warnings describe the freshness or validation status of the response, and so
1487      &MUST; be deleted by caches after validation. They can only be generated by a cache
1488      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1489    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
1490      not rectified by a validation (for example, a lossy compression of the entity bodies)
1491      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1492      returned, in which case they &MUST; be.</t>
1493  </list>
1494</t>
1495<t>
1496  If an implementation sends a message with one or more Warning headers to a receiver whose
1497  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1498  warn-date that matches the Date header in the message.
1499</t>
1500<t>
1501  If an implementation receives a message with a warning-value that includes a warn-date,
1502  and that warn-date is different from the Date value in the response, then that
1503  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1504  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1505  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1506  well.
1507</t>
1508<t>
1509  The following warn-codes are defined by this specification, each with a recommended
1510  warn-text in English, and a description of its meaning.
1511</t>
1512<t><?rfc needLines="4"?>
1513  110 Response is stale
1514  <list>
1515    <t>&SHOULD; be included whenever the returned response is stale.</t>
1516  </list>
1517</t>
1518<t><?rfc needLines="4"?>
1519  111 Revalidation failed
1520  <list>
1521    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1522      validate the response failed, due to an inability to reach the server.</t>
1523  </list>
1524</t>
1525<t><?rfc needLines="4"?>
1526  112 Disconnected operation
1527  <list>
1528    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1529      the network for a period of time.</t>
1530  </list>
1531</t>
1532<t><?rfc needLines="4"?>
1533  113 Heuristic expiration
1534  <list>
1535    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1536      greater than 24 hours and the response's age is greater than 24 hours.</t>
1537  </list>
1538</t>
1539<t><?rfc needLines="4"?>
1540  199 Miscellaneous warning
1541  <list>
1542    <t>The warning text can include arbitrary information to be presented to a human
1543      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1544      action, besides presenting the warning to the user.</t>
1545  </list>
1546</t>
1547<t><?rfc needLines="4"?>
1548  214 Transformation applied
1549  <list>
1550    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1551      transformation changing the content-coding (as specified in the Content-Encoding
1552      header) or media-type (as specified in the Content-Type header) of the response, or
1553      the entity-body of the response, unless this Warning code already appears in the
1554      response.</t>
1555  </list>
1556</t>
1557<t><?rfc needLines="4"?>
1558  299 Miscellaneous persistent warning
1559  <list>
1560    <t>The warning text can include arbitrary information to be presented to a human
1561      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1562      action.</t>
1563  </list>
1564</t>
1565</section>
1566
1567</section>
1568
1569<section anchor="history.lists" title="History Lists">
1570<t>
1571  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1572  can be used to redisplay an entity retrieved earlier in a session.
1573</t>
1574<t>
1575  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1576  a history mechanism can display a previous representation even if it has expired.
1577</t>
1578  <t>
1579  This does not prohibit the history mechanism from telling the user that a
1580  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1581  </t>
1582</section>
1583
1584
1585<section anchor="IANA.considerations" title="IANA Considerations">
1586
1587<section anchor="message.header.registration" title="Message Header Registration">
1588<t>
1589  The Message Header Registry located at <eref
1590  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1591  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1592</t>
1593<?BEGININC p6-cache.iana-headers ?>
1594<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1595<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1596   <ttcol>Header Field Name</ttcol>
1597   <ttcol>Protocol</ttcol>
1598   <ttcol>Status</ttcol>
1599   <ttcol>Reference</ttcol>
1600
1601   <c>Age</c>
1602   <c>http</c>
1603   <c>standard</c>
1604   <c>
1605      <xref target="header.age"/>
1606   </c>
1607   <c>Cache-Control</c>
1608   <c>http</c>
1609   <c>standard</c>
1610   <c>
1611      <xref target="header.cache-control"/>
1612   </c>
1613   <c>Expires</c>
1614   <c>http</c>
1615   <c>standard</c>
1616   <c>
1617      <xref target="header.expires"/>
1618   </c>
1619   <c>Pragma</c>
1620   <c>http</c>
1621   <c>standard</c>
1622   <c>
1623      <xref target="header.pragma"/>
1624   </c>
1625   <c>Vary</c>
1626   <c>http</c>
1627   <c>standard</c>
1628   <c>
1629      <xref target="header.vary"/>
1630   </c>
1631   <c>Warning</c>
1632   <c>http</c>
1633   <c>standard</c>
1634   <c>
1635      <xref target="header.warning"/>
1636   </c>
1637</texttable>
1638<!--(END)-->
1639<?ENDINC p6-cache.iana-headers ?>
1640<t>
1641  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1642</t>
1643</section>
1644
1645</section>
1646
1647<section anchor="security.considerations" title="Security Considerations">
1648<t>
1649  Caches expose additional potential vulnerabilities, since the contents of the cache
1650  represent an attractive target for malicious exploitation. Because cache contents persist
1651  after an HTTP request is complete, an attack on the cache can reveal information long after
1652  a user believes that the information has been removed from the network. Therefore, cache
1653  contents should be protected as sensitive information.
1654</t>
1655</section>
1656
1657<section anchor="ack" title="Acknowledgments">
1658<t>
1659  Much of the content and presentation of the caching design is due to suggestions and
1660  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1661  and Larry Masinter.
1662</t>
1663</section>
1664
1665</middle>
1666
1667<back>
1668<references title="Normative References">
1669
1670  <reference anchor="Part1">
1671    <front>
1672      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1673      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1674        <organization abbrev="Day Software">Day Software</organization>
1675        <address><email>fielding@gbiv.com</email></address>
1676      </author>
1677      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1678        <organization>One Laptop per Child</organization>
1679        <address><email>jg@laptop.org</email></address>
1680      </author>
1681      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1682        <organization abbrev="HP">Hewlett-Packard Company</organization>
1683        <address><email>JeffMogul@acm.org</email></address>
1684      </author>
1685      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1686        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1687        <address><email>henrikn@microsoft.com</email></address>
1688      </author>
1689      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1690        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1691        <address><email>LMM@acm.org</email></address>
1692      </author>
1693      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1694        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1695        <address><email>paulle@microsoft.com</email></address>
1696      </author>
1697      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1698        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1699        <address><email>timbl@w3.org</email></address>
1700      </author>
1701      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1702        <organization abbrev="W3C">World Wide Web Consortium</organization>
1703        <address><email>ylafon@w3.org</email></address>
1704      </author>
1705      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1706        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1707        <address><email>julian.reschke@greenbytes.de</email></address>
1708      </author>
1709      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1710    </front>
1711    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1712    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1713  </reference>
1714
1715  <reference anchor="Part2">
1716    <front>
1717      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1718      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1719        <organization abbrev="Day Software">Day Software</organization>
1720        <address><email>fielding@gbiv.com</email></address>
1721      </author>
1722      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1723        <organization>One Laptop per Child</organization>
1724        <address><email>jg@laptop.org</email></address>
1725      </author>
1726      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1727        <organization abbrev="HP">Hewlett-Packard Company</organization>
1728        <address><email>JeffMogul@acm.org</email></address>
1729      </author>
1730      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1731        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1732        <address><email>henrikn@microsoft.com</email></address>
1733      </author>
1734      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1735        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1736        <address><email>LMM@acm.org</email></address>
1737      </author>
1738      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1739        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1740        <address><email>paulle@microsoft.com</email></address>
1741      </author>
1742      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1743        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1744        <address><email>timbl@w3.org</email></address>
1745      </author>
1746      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1747        <organization abbrev="W3C">World Wide Web Consortium</organization>
1748        <address><email>ylafon@w3.org</email></address>
1749      </author>
1750      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1751        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1752        <address><email>julian.reschke@greenbytes.de</email></address>
1753      </author>
1754      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1755    </front>
1756    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1757    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1758  </reference>
1759
1760  <reference anchor="Part4">
1761    <front>
1762      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1763      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1764        <organization abbrev="Day Software">Day Software</organization>
1765        <address><email>fielding@gbiv.com</email></address>
1766      </author>
1767      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1768        <organization>One Laptop per Child</organization>
1769        <address><email>jg@laptop.org</email></address>
1770      </author>
1771      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1772        <organization abbrev="HP">Hewlett-Packard Company</organization>
1773        <address><email>JeffMogul@acm.org</email></address>
1774      </author>
1775      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1776        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1777        <address><email>henrikn@microsoft.com</email></address>
1778      </author>
1779      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1780        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1781        <address><email>LMM@acm.org</email></address>
1782      </author>
1783      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1784        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1785        <address><email>paulle@microsoft.com</email></address>
1786      </author>
1787      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1788        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1789        <address><email>timbl@w3.org</email></address>
1790      </author>
1791      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1792        <organization abbrev="W3C">World Wide Web Consortium</organization>
1793        <address><email>ylafon@w3.org</email></address>
1794      </author>
1795      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1796        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1797        <address><email>julian.reschke@greenbytes.de</email></address>
1798      </author>
1799      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1800    </front>
1801    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1802    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1803  </reference>
1804
1805  <reference anchor="Part5">
1806    <front>
1807      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1808      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1809        <organization abbrev="Day Software">Day Software</organization>
1810        <address><email>fielding@gbiv.com</email></address>
1811      </author>
1812      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1813        <organization>One Laptop per Child</organization>
1814        <address><email>jg@laptop.org</email></address>
1815      </author>
1816      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1817        <organization abbrev="HP">Hewlett-Packard Company</organization>
1818        <address><email>JeffMogul@acm.org</email></address>
1819      </author>
1820      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1821        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1822        <address><email>henrikn@microsoft.com</email></address>
1823      </author>
1824      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1825        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1826        <address><email>LMM@acm.org</email></address>
1827      </author>
1828      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1829        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1830        <address><email>paulle@microsoft.com</email></address>
1831      </author>
1832      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1833        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1834        <address><email>timbl@w3.org</email></address>
1835      </author>
1836      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1837        <organization abbrev="W3C">World Wide Web Consortium</organization>
1838        <address><email>ylafon@w3.org</email></address>
1839      </author>
1840      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1841        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1842        <address><email>julian.reschke@greenbytes.de</email></address>
1843      </author>
1844      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1845    </front>
1846    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1847    <x:source basename="p5-range" href="p5-range.xml" />
1848  </reference>
1849
1850  <reference anchor="Part7">
1851    <front>
1852      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1853      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1854        <organization abbrev="Day Software">Day Software</organization>
1855        <address><email>fielding@gbiv.com</email></address>
1856      </author>
1857      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1858        <organization>One Laptop per Child</organization>
1859        <address><email>jg@laptop.org</email></address>
1860      </author>
1861      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1862        <organization abbrev="HP">Hewlett-Packard Company</organization>
1863        <address><email>JeffMogul@acm.org</email></address>
1864      </author>
1865      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1866        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1867        <address><email>henrikn@microsoft.com</email></address>
1868      </author>
1869      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1870        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1871        <address><email>LMM@acm.org</email></address>
1872      </author>
1873      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1874        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1875        <address><email>paulle@microsoft.com</email></address>
1876      </author>
1877      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1878        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1879        <address><email>timbl@w3.org</email></address>
1880      </author>
1881      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1882        <organization abbrev="W3C">World Wide Web Consortium</organization>
1883        <address><email>ylafon@w3.org</email></address>
1884      </author>
1885      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1886        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1887        <address><email>julian.reschke@greenbytes.de</email></address>
1888      </author>
1889      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1890    </front>
1891    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1892    <x:source basename="p7-auth" href="p7-auth.xml" />
1893  </reference>
1894
1895  <reference anchor="RFC2119">
1896    <front>
1897      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1898      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1899        <organization>Harvard University</organization>
1900        <address><email>sob@harvard.edu</email></address>
1901      </author>
1902      <date month="March" year="1997" />
1903    </front>
1904    <seriesInfo name="BCP" value="14" />
1905    <seriesInfo name="RFC" value="2119" />
1906  </reference>
1907
1908  <reference anchor="RFC5234">
1909    <front>
1910      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1911      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1912        <organization>Brandenburg InternetWorking</organization>
1913        <address>
1914          <email>dcrocker@bbiw.net</email>
1915        </address> 
1916      </author>
1917      <author initials="P." surname="Overell" fullname="Paul Overell">
1918        <organization>THUS plc.</organization>
1919        <address>
1920          <email>paul.overell@thus.net</email>
1921        </address>
1922      </author>
1923      <date month="January" year="2008"/>
1924    </front>
1925    <seriesInfo name="STD" value="68"/>
1926    <seriesInfo name="RFC" value="5234"/>
1927  </reference>
1928 
1929</references>
1930
1931<references title="Informative References">
1932
1933  <reference anchor="RFC1305">
1934    <front>
1935      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1936      <author fullname="David L. Mills" initials="D." surname="Mills">
1937        <organization>University of Delaware, Electrical Engineering Department</organization>
1938        <address><email>mills@udel.edu</email></address>
1939      </author>
1940      <date month="March" year="1992" />
1941    </front>
1942    <seriesInfo name="RFC" value="1305" />
1943  </reference>
1944
1945  <reference anchor="RFC2616">
1946    <front>
1947      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1948      <author fullname="R. Fielding" initials="R." surname="Fielding">
1949        <organization>University of California, Irvine</organization>
1950        <address><email>fielding@ics.uci.edu</email></address>
1951      </author>
1952      <author fullname="J. Gettys" initials="J." surname="Gettys">
1953        <organization>W3C</organization>
1954        <address><email>jg@w3.org</email></address>
1955      </author>
1956      <author fullname="J. Mogul" initials="J." surname="Mogul">
1957        <organization>Compaq Computer Corporation</organization>
1958        <address><email>mogul@wrl.dec.com</email></address>
1959      </author>
1960      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1961        <organization>MIT Laboratory for Computer Science</organization>
1962        <address><email>frystyk@w3.org</email></address>
1963      </author>
1964      <author fullname="L. Masinter" initials="L." surname="Masinter">
1965        <organization>Xerox Corporation</organization>
1966        <address><email>masinter@parc.xerox.com</email></address>
1967      </author>
1968      <author fullname="P. Leach" initials="P." surname="Leach">
1969        <organization>Microsoft Corporation</organization>
1970        <address><email>paulle@microsoft.com</email></address>
1971      </author>
1972      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1973        <organization>W3C</organization>
1974        <address><email>timbl@w3.org</email></address>
1975      </author>
1976      <date month="June" year="1999" />
1977    </front>
1978    <seriesInfo name="RFC" value="2616" />
1979  </reference>
1980
1981  <reference anchor="RFC3864">
1982    <front>
1983      <title>Registration Procedures for Message Header Fields</title>
1984      <author fullname="G. Klyne" initials="G." surname="Klyne">
1985        <organization>Nine by Nine</organization>
1986        <address><email>GK-IETF@ninebynine.org</email></address>
1987      </author>
1988      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1989        <organization>BEA Systems</organization>
1990        <address><email>mnot@pobox.com</email></address>
1991      </author>
1992      <author fullname="J. Mogul" initials="J." surname="Mogul">
1993        <organization>HP Labs</organization>
1994        <address><email>JeffMogul@acm.org</email></address>
1995      </author>
1996      <date month="September" year="2004" />
1997    </front>
1998    <seriesInfo name="BCP" value="90" />
1999    <seriesInfo name="RFC" value="3864" />
2000  </reference>
2001
2002</references>
2003
2004<section anchor="compatibility" title="Compatibility with Previous Versions">
2005
2006<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2007<t>
2008  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2009  this missing case.
2010  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2011</t>
2012<t>
2013  Range request responses would become very verbose if all meta-data were always returned;
2014  by allowing the server to only send needed headers in a 206 response, this problem can be
2015  avoided.
2016  (<xref target="combining.headers" />)
2017</t>
2018<t>
2019  The Cache-Control: max-age directive was not properly defined for responses.
2020  (<xref target="cache-response-directive"/>)
2021</t>
2022<t>
2023  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2024    format="counter" target="expiration.model" />, <xref format="counter"
2025    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2026  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2027  header, as PUT or other methods may have need for it in requests.
2028</t>
2029</section>
2030
2031<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2032<t>
2033  Make the specified age calculation algorithm less conservative.
2034  (<xref target="age.calculations"/>)
2035</t>
2036<t>
2037  Remove requirement to consider Content-Location in successful responses
2038  in order to determine the appropriate response to use.
2039  (<xref target="validation.model" />)
2040</t>
2041<t>
2042  Clarify denial of service attack avoidance requirement.
2043  (<xref target="invalidation.after.updates.or.deletions" />)
2044</t>
2045<t>
2046  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2047  anymore, as these aspects never were implemented.
2048  (<xref target="header.warning" />)
2049</t>
2050</section>
2051
2052    </section>
2053
2054<?BEGININC p6-cache.abnf-appendix ?>
2055<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2056<figure>
2057<artwork type="abnf" name="p6-cache.parsed-abnf">
2058<x:ref>Age</x:ref> = "Age:" OWS Age-v
2059<x:ref>Age-v</x:ref> = delta-seconds
2060
2061<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2062<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2063 cache-directive ] )
2064
2065<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2066<x:ref>Expires-v</x:ref> = HTTP-date
2067
2068<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2069
2070<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2071
2072<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2073<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2074 pragma-directive ] )
2075
2076<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2077<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2078 ] ) )
2079
2080<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2081<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2082 ] )
2083
2084<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2085<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2086<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2087 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2088 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2089 cache-extension
2090<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2091 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2092 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2093 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2094 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2095 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2096
2097<x:ref>delta-seconds</x:ref> = 1*DIGIT
2098
2099<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2100
2101<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2102
2103<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2104<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2105<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2106
2107<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2108
2109<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2110
2111<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2112
2113<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2114<x:ref>warn-code</x:ref> = 3DIGIT
2115<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2116<x:ref>warn-text</x:ref> = quoted-string
2117<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2118 ]
2119</artwork>
2120</figure>
2121<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2122; Age defined but not used
2123; Cache-Control defined but not used
2124; Expires defined but not used
2125; Pragma defined but not used
2126; Vary defined but not used
2127; Warning defined but not used
2128</artwork></figure></section>
2129<?ENDINC p6-cache.abnf-appendix ?>
2130
2131<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2132
2133<section title="Since RFC2616">
2134  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2135</section>
2136
2137<section title="Since draft-ietf-httpbis-p6-cache-00">
2138<t>
2139  Closed issues:
2140  <list style="symbols">
2141    <t>
2142      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2143    <t>
2144      <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>
2145    <t>
2146      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2147    <t>
2148      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2149    <t>
2150      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2151    <t>
2152      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2153    <t>
2154      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2155    <t>
2156      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2157    <t>
2158      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2159  </list>
2160</t>
2161<t>
2162  Other changes:
2163  <list style="symbols">
2164    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2165        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2166  </list>
2167</t>
2168</section>
2169
2170<section title="Since draft-ietf-httpbis-p6-cache-01">
2171<t>
2172  Closed issues:
2173  <list style="symbols">
2174    <t>
2175      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2176  </list>
2177</t>
2178<t>
2179  Other changes:
2180  <list style="symbols">
2181    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2182      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2183    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2184      specification.</t>
2185  </list>
2186</t>
2187</section>
2188
2189<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2190<t>
2191  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2192  <list style="symbols">
2193    <t>Reference RFC 3984, and update header registrations for headers defined in this
2194      document.</t>
2195  </list>
2196</t>
2197</section>
2198
2199<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2200<t>
2201  Closed issues:
2202  <list style="symbols">
2203    <t>
2204      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2205  </list>
2206</t>
2207</section>
2208
2209<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2210<t>
2211  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2212  <list style="symbols"> 
2213    <t>
2214      Use "/" instead of "|" for alternatives.
2215    </t>
2216    <t>
2217      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2218      whitespace ("OWS") and required whitespace ("RWS").
2219    </t>
2220    <t>
2221      Rewrite ABNFs to spell out whitespace rules, factor out
2222      header value format definitions.
2223    </t>
2224  </list>
2225</t>
2226</section>
2227
2228<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2229<t>
2230  This is a total rewrite of this part of the specification.
2231</t>
2232<t>
2233  Affected issues:
2234  <list style="symbols">
2235    <t>
2236      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2237    <t>
2238      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2239    <t>
2240      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2241    <t>
2242      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2243  </list>
2244</t>
2245<t>
2246  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2247  <list style="symbols"> 
2248    <t>
2249      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2250    </t>
2251  </list>
2252</t>
2253</section>
2254
2255<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2256<t>
2257  Closed issues:
2258  <list style="symbols"> 
2259    <t>
2260      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2261      "base for numeric protocol elements"
2262    </t>
2263  </list>
2264</t>
2265<t>
2266  Affected issues:
2267  <list style="symbols">
2268    <t>
2269      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2270      Vary and non-existant headers
2271    </t>
2272  </list>
2273</t>
2274</section>
2275
2276<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2277<t>
2278  Closed issues:
2279  <list style="symbols"> 
2280    <t>
2281      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2282      "Definition of 1xx Warn-Codes"
2283    </t>
2284    <t>
2285      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2286      "Content-Location on 304 responses"
2287    </t>
2288    <t>
2289      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2290      "private and no-cache CC directives with headers"
2291    </t>
2292    <t>
2293      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2294      "RFC2047 and warn-text"
2295    </t>
2296  </list>
2297</t>
2298</section>
2299
2300<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2301<t>
2302  Closed issues:
2303  <list style="symbols"> 
2304    <t>
2305      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2306      "serving negotiated responses from cache: header-specific canonicalization"
2307    </t>
2308    <t>
2309      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2310      "Effect of CC directives on history lists"
2311    </t>
2312  </list>
2313</t>
2314<t>
2315  Affected issues:
2316  <list style="symbols">
2317    <t>
2318      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2319      Status codes and caching
2320    </t>
2321  </list>
2322</t>
2323<t>
2324  Partly resolved issues:
2325  <list style="symbols"> 
2326    <t>
2327      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2328      "Placement of 13.5.1 and 13.5.2"
2329    </t>
2330  </list>
2331</t>
2332</section>
2333
2334<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2335<t>
2336  Closed issues:
2337  <list style="symbols"> 
2338    <t>
2339      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2340      "Age calculation"
2341    </t>
2342  </list>
2343</t>
2344</section>
2345
2346</section>
2347  </back>
2348</rfc>
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