source: draft-ietf-httpbis/latest-roy/p6-cache.xml @ 489

Last change on this file since 489 was 489, checked in by mnot@…, 11 years ago

add todo so we don't forget about method cacheability.

  • Property svn:eol-style set to native
File size: 109.2 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "March">
16  <!ENTITY ID-YEAR "2009">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35]>
36<?rfc toc="yes" ?>
37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
45<?rfc-ext allow-markup-in-artwork="yes" ?>
46<?rfc-ext include-references-in-index="yes" ?>
47<?oxygen RNGSchema="../../rfc2629xslt/rfc2629-ext.rnc" type="compact"?>
48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
49  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
50  <front>
51
52    <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
53
54    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
55      <organization abbrev="Day Software">Day Software</organization>
56      <address>
57      <postal>
58        <street>23 Corporate Plaza DR, Suite 280</street>
59        <city>Newport Beach</city>
60        <region>CA</region>
61        <code>92660</code>
62        <country>USA</country>
63      </postal>
64      <phone>+1-949-706-5300</phone>
65      <facsimile>+1-949-706-5305</facsimile>
66      <email>fielding@gbiv.com</email>
67      <uri>http://roy.gbiv.com/</uri>
68    </address>
69    </author>
70
71    <author fullname="Jim Gettys" initials="J." surname="Gettys">
72      <organization>One Laptop per Child</organization>
73      <address>
74      <postal>
75        <street>21 Oak Knoll Road</street>
76        <city>Carlisle</city>
77        <region>MA</region>
78        <code>01741</code>
79        <country>USA</country>
80      </postal>
81      <email>jg@laptop.org</email>
82      <uri>http://www.laptop.org/</uri>
83    </address>
84    </author>
85
86    <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
87      <organization abbrev="HP">Hewlett-Packard Company</organization>
88      <address>
89      <postal>
90        <street>HP Labs, Large Scale Systems Group</street>
91        <street>1501 Page Mill Road, MS 1177</street>
92        <city>Palo Alto</city>
93        <region>CA</region>
94        <code>94304</code>
95        <country>USA</country>
96      </postal>
97      <email>JeffMogul@acm.org</email>
98    </address>
99    </author>
100
101    <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
102      <organization abbrev="Microsoft">Microsoft Corporation</organization>
103      <address>
104      <postal>
105        <street>1 Microsoft Way</street>
106        <city>Redmond</city>
107        <region>WA</region>
108        <code>98052</code>
109        <country>USA</country>
110      </postal>
111      <email>henrikn@microsoft.com</email>
112    </address>
113    </author>
114
115    <author fullname="Larry Masinter" initials="L." surname="Masinter">
116      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
117      <address>
118      <postal>
119        <street>345 Park Ave</street>
120        <city>San Jose</city>
121        <region>CA</region>
122        <code>95110</code>
123        <country>USA</country>
124      </postal>
125      <email>LMM@acm.org</email>
126      <uri>http://larry.masinter.net/</uri>
127    </address>
128    </author>
129
130    <author fullname="Paul J. Leach" initials="P." surname="Leach">
131      <organization abbrev="Microsoft">Microsoft Corporation</organization>
132      <address>
133      <postal>
134        <street>1 Microsoft Way</street>
135        <city>Redmond</city>
136        <region>WA</region>
137        <code>98052</code>
138      </postal>
139      <email>paulle@microsoft.com</email>
140    </address>
141    </author>
142
143    <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
144      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
145      <address>
146      <postal>
147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
148        <street>The Stata Center, Building 32</street>
149        <street>32 Vassar Street</street>
150        <city>Cambridge</city>
151        <region>MA</region>
152        <code>02139</code>
153        <country>USA</country>
154      </postal>
155      <email>timbl@w3.org</email>
156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
157    </address>
158    </author>
159
160    <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
161      <organization abbrev="W3C">World Wide Web Consortium</organization>
162      <address>
163      <postal>
164        <street>W3C / ERCIM</street>
165        <street>2004, rte des Lucioles</street>
166        <city>Sophia-Antipolis</city>
167        <region>AM</region>
168        <code>06902</code>
169        <country>France</country>
170      </postal>
171      <email>ylafon@w3.org</email>
172      <uri>http://www.raubacapeu.net/people/yves/</uri>
173    </address>
174    </author>
175
176    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
177      <organization abbrev="greenbytes">greenbytes GmbH</organization>
178      <address>
179      <postal>
180        <street>Hafenweg 16</street>
181        <city>Muenster</city><region>NW</region><code>48155</code>
182        <country>Germany</country>
183      </postal>
184      <phone>+49 251 2807760</phone>   
185      <facsimile>+49 251 2807761</facsimile>   
186      <email>julian.reschke@greenbytes.de</email>       
187      <uri>http://greenbytes.de/tech/webdav/</uri>     
188    </address>
189    </author>
190
191    <date month="&ID-MONTH;" year="&ID-YEAR;" />
192    <workgroup>HTTPbis Working Group</workgroup>
193
194    <abstract>
195      <t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
196        collaborative, hypermedia information systems. This document is Part 6 of the seven-part
197        specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
198        obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
199        fields that control cache behavior or indicate cacheable response messages.</t>
200    </abstract>
201
202    <note title="Editorial Note (To be removed by RFC Editor)">
203      <t>Discussion of this draft should take place on the HTTPBIS working group mailing list
204        (ietf-http-wg@w3.org). The current issues list is at <eref
205          target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
206        (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.</t>
207      <t>The changes in this draft are summarized in <xref target="changes.since.04" />.</t>
208    </note>
209  </front>
210  <middle>
211    <section anchor="caching" title="Introduction">
212      <t>HTTP is typically used for distributed information systems, where performance can be
213        improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
214        caching and reusing response messages.</t>
215
216      <section anchor="intro.purpose" title="Purpose">
217        <iref item="cache" />
218        <t>An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
219          controls its message storage, retrieval, and deletion. A cache stores cacheable responses
220          in order to reduce the response time and network bandwidth consumption on future,
221          equivalent requests. Any client or server may include a cache, though a cache cannot be
222          used by a server that is acting as a tunnel.</t>
223        <t>Caching would be useless if it did not significantly improve performance. The goal of
224          caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
225          some cases, a stored response can be reused without the need for a network request,
226          reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
227          (see <xref target="expiration.model" />). Even when a new request is required, it is often
228          possible to reuse all or parts of the payload of a prior response to satisfy the request,
229          thereby reducing network bandwidth usage; a "validation" mechanism is used for this
230          purpose (see <xref target="validation.model" />).</t>
231      </section>
232
233      <section anchor="intro.terminology" title="Terminology">
234        <t>This specification uses a number of terms to refer to the roles played by participants
235          in, and objects of, HTTP caching.</t>
236        <t>
237          <iref item="cacheable" />
238          <x:dfn>cacheable</x:dfn>
239          <list>
240            <t>A response is cacheable if a cache is allowed to store a copy of the response message
241              for use in answering subsequent requests. Even when a response is cacheable, there may
242              be additional constraints on whether a cache can use the cached copy to satisfy a
243              particular request.</t>
244          </list>
245        </t>
246        <t>
247          <iref item="explicit expiration time" />
248          <x:dfn>explicit expiration time</x:dfn>
249          <list>
250            <t>The time at which the origin server intends that an entity should no longer be
251              returned by a cache without further validation.</t>
252          </list>
253        </t>
254        <t>
255          <iref item="heuristic expiration time" />
256          <x:dfn>heuristic expiration time</x:dfn>
257          <list>
258            <t>An expiration time assigned by a cache when no explicit expiration time is
259            available.</t>
260          </list>
261        </t>
262        <t>
263          <iref item="age" />
264          <x:dfn>age</x:dfn>
265          <list>
266            <t>The age of a response is the time since it was sent by, or successfully validated
267              with, the origin server.</t>
268          </list>
269        </t>
270        <t>
271          <iref item="first-hand" />
272          <x:dfn>first-hand</x:dfn>
273          <list>
274            <t>A response is first-hand if the freshness model is not in use; i.e., its age is
275            0.</t>
276          </list>
277        </t>
278        <t>
279          <iref item="freshness lifetime" />
280          <x:dfn>freshness lifetime</x:dfn>
281          <list>
282            <t>The length of time between the generation of a response and its expiration time. </t>
283          </list>
284        </t>
285        <t>
286          <iref item="fresh" />
287          <x:dfn>fresh</x:dfn>
288          <list>
289            <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
290          </list>
291        </t>
292        <t>
293          <iref item="stale" />
294          <x:dfn>stale</x:dfn>
295          <list>
296            <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
297          </list>
298        </t>
299        <t>
300          <iref item="validator" />
301          <x:dfn>validator</x:dfn>
302          <list>
303            <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
304              out whether a stored response is an equivalent copy of an entity.</t>
305          </list>
306        </t>
307        <t>
308          <iref item="validator" />
309          <x:dfn>shared cache</x:dfn>
310          <list>
311            <t>A cache that is accessible to more than one user. A non-shared cache is
312              dedicated to a single user.</t>
313          </list>
314        </t>
315      </section>
316
317
318      <section anchor="intro.requirements" title="Requirements">
319        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
320          NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
321          described in <xref target="RFC2119" />.</t>
322        <t>An implementation is not compliant if it fails to satisfy one or more of the &MUST;
323          or &REQUIRED; level requirements for the protocols it implements. An implementation
324          that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
325          requirements for its protocols is said to be "unconditionally compliant"; one that
326          satisfies all the &MUST; level requirements but not all the &SHOULD; level
327          requirements for its protocols is said to be "conditionally compliant."</t>
328      </section>
329
330
331<section title="Syntax Notation" anchor="notation">
332  <x:anchor-alias value="ALPHA"/>
333  <x:anchor-alias value="CR"/>
334  <x:anchor-alias value="DIGIT"/>
335  <x:anchor-alias value="DQUOTE"/>
336  <x:anchor-alias value="LF"/>
337  <x:anchor-alias value="OCTET"/>
338  <x:anchor-alias value="SP"/>
339  <x:anchor-alias value="VCHAR"/>
340  <x:anchor-alias value="WSP"/>
341<t>
342  This specification uses the ABNF syntax defined in &notation;.
343  The following core rules are included by
344  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
345  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
346  DIGIT (decimal 0-9), DQUOTE (double quote),
347  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
348  OCTET (any 8-bit sequence of data), SP (space),
349  VCHAR (any visible USASCII character),
350  and WSP (whitespace).
351</t>
352
353<section title="Core Rules" anchor="core.rules">
354  <x:anchor-alias value="quoted-string"/>
355  <x:anchor-alias value="token"/>
356  <x:anchor-alias value="OWS"/>
357<t>
358  The core rules below are defined in &basic-rules;:
359</t>
360<figure><artwork type="abnf2616">
361  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
362  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
363  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
364</artwork></figure>
365</section>
366
367<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
368  <x:anchor-alias value="field-name"/>
369  <x:anchor-alias value="HTTP-date"/>
370  <x:anchor-alias value="port"/>
371  <x:anchor-alias value="pseudonym"/>
372  <x:anchor-alias value="uri-host"/>
373<t>
374  The ABNF rules below are defined in other parts:
375</t>
376<figure><!--Part1--><artwork type="abnf2616">
377  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
378  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
379  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
380  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
381  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
382</artwork></figure>
383</section>
384
385</section>
386</section>
387
388    <section anchor="caching.overview" title="Cache Operation">
389
390      <section anchor="response.cacheability" title="Response Cacheability">
391        <t>A cache &MAY; store a response to any request, provided that: <list style="symbols">
392                <t>The request method is defined as being cacheable, and</t>
393            <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
394              appear in request or response headers, and</t>
395            <t>the "private" cache response directive (see <xref target="header.cache-control" />
396            does not appear in the response, if the cache is a shared cache, and</t>
397            <t>the cache understands partial responses, if the response is partial or incomplete
398              (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
399          </list>
400        </t>
401        <t>Note that in normal operation, most caches will not store a response that has neither a
402          cache validator nor an explicit expiration time, as such responses are not usually
403          useful to store. However, caches are not prohibited from storing such responses.</t>
404
405        <section anchor="errors.or.incomplete.response.cache.behavior"
406          title="Storing Partial and Incomplete Responses">
407          <t>A cache that receives an incomplete response (for example, with fewer bytes of data
408            than specified in a Content-Length header) &MAY; store the response. However, the
409            cache &MUST; treat this as a partial response &partial;. Partial responses
410            &MAY; be combined as described in &combining-byte-ranges;; the result might be a
411            full response or might still be partial. A cache &MUST-NOT; return a partial
412            response to a client without explicitly marking it as such using the 206 (Partial
413            Content) status code.</t>
414          <t>A cache that does not support the Range and Content-Range headers &MUST-NOT; store
415            incomplete or partial responses.</t>
416        </section>
417
418      </section>
419
420
421      <section anchor="constructing.responses.from.caches"
422        title="Constructing Responses from Caches">
423        <t>For a presented request, a non-shared cache &MAY; return a stored response, provided
424          that: <list style="symbols">
425            <t>The presented Request-URI and that of the stored response match (see
426              <cref>TBD</cref>), and</t>
427            <t>the request method associated with the stored response allows it to be
428              used for the presented request, and</t>
429            <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
430                target="caching.negotiated.responses" />), and</t>
431            <t>the stored response is either:
432            <list style="symbols">
433                <t>fresh (see <xref target="expiration.model" />), or</t>
434                <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
435                <t>successfully validated (see <xref target="validation.model" />).</t>
436            </list>and</t>
437            <t>the presented request and stored response are free from directives that would prevent
438              its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"
439             />).</t>
440          </list>
441        </t>
442        <t>A shared cache &MAY; return a stored response, provided that: <list style="symbols">
443            <t>The criteria for non-shared caches above are met (taking into account directives specific to shared
444              caches; see <xref target="header.cache-control" />), and</t>
445            <t>the stored response was not associated with an authenticated request (see
446              &header-authorization;), unless explicitly allowed (see <xref
447                target="header.cache-control" />).</t>
448          </list>
449        </t>
450        <t><cref>TODO: define method cacheability for GET, HEAD and POST in p2-semantics.</cref></t>
451        <t>All responses satisfied from cache include an appropriate Age header field; see
452            <xref target="header.age" />. <cref>DISCUSS: this currently includes successfully validated responses.</cref></t>
453        <t>All request methods other than GET and HEAD &MUST; be written through the cache to
454          the origin server. Note that such requests might invalidate already stored responses; see
455            <xref target="invalidation.after.updates.or.deletions" />.</t>
456        <t>Caches &SHOULD; use the most recent response (as determined by the Date header) when
457          more than one applicable response is stored. They &MAY; also send a request with
458          "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
459          use.</t>
460        <t>
461          <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in
462            p1</cref>
463        </t>
464      </section>
465
466
467
468
469      <section anchor="expiration.model" title="Freshness Model">
470
471        <t>When a response is "fresh" in the cache, it can be used to satisfy subsequent
472          requests without contacting the origin server, thereby improving efficiency.</t>
473        <t>This mechanism applies only to responses taken from a cache and not to first-hand responses.
474          It cannot be used to force a user agent to refresh its display or reload a resource; its
475          semantics apply only to caches. See <xref target="history.lists" /> for an explanation of
476          the difference between caches and history mechanisms.</t>
477        <t>The primary mechanism for avoiding requests is for an origin server to provide an
478          explicit expiration time in the future, using either the Expires header (<xref
479            target="header.expires" />) or the max-age response cache directive (<xref
480            target="cache-response-directive" />). Generally, origin servers will assign future
481          explicit expiration times to responses in the belief that the entity is not likely to
482          change in a semantically significant way before the expiration time is reached.</t>
483        <t>If an origin server wishes to force a cache to validate every request, it &MAY;
484          assign an explicit expiration time in the past. This means that the response is always
485          stale, so that caches should validate it before using it for subsequent requests. <cref>This wording may cause confusion, because the response may still be served stale.</cref></t>
486        <t>Since origin servers do not always provide explicit expiration times, HTTP caches may
487          assign heuristic expiration times when they are not specified, employing algorithms that
488          use other header values (such as the Last-Modified time) to estimate a plausible
489          expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
490          impose worst-case constraints on their results.</t>
491        <t>The calculation to determine if a response has expired is:</t>
492        <figure>
493          <artwork type="code">
494   response_is_fresh = (freshness_lifetime &gt; current_age)
495</artwork>
496        </figure>
497
498        <t>The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
499          the current_age is defined in <xref target="age.calculations" />.</t>
500        <t>Additionally, clients may need to influence freshness calculation. They can do this using
501          several request cache directives, with the effect of either increasing or loosening
502          constraints on freshness. See <xref target="cache-request-directive" />.</t>
503        <t>
504          <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
505            freshness.</cref>
506        </t>
507
508        <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
509          <t>A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
510            response by using the first match of: <list style="symbols">
511              <t>If the cache is shared and the s-maxage response cache directive (<xref
512                  target="cache-response-directive" />) is present, use its value, or</t>
513              <t>If the max-age response cache directive (<xref target="cache-response-directive"
514                 />) is present, use its value, or</t>
515              <t>If the Expires response header (<xref target="header.expires" />) is present, use
516                its value minus the value of the Date response header, or</t>
517              <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
518                may be used; see <xref target="heuristic.freshness" />.</t>
519            </list>
520          </t>
521          <t>Note that this calculation is not vulnerable to clock skew, since all of the
522            information comes from the origin server.</t>
523
524          <section anchor="heuristic.freshness" title="Using Heuristic Freshness">
525            <t>If no explicit expiration time is present in a stored response that has a status code
526              of 200, 203, 206, 300, 301 or 410, a heuristic expiration time &MAY; be
527              calculated. Heuristics &MUST-NOT; be used for other response status codes. </t>
528            <t> When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
529              attach a Warning header with a 113 warn-code to the response if its current_age is
530              more than 24 hours and such a warning is not already present.</t>
531            <t>Also, if the response has a Last-Modified header (&header-last-modified;), the
532              heuristic expiration value &SHOULD; be no more than some fraction of the interval
533              since that time. A typical setting of this fraction might be 10%.</t>
534            <t>
535              <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
536            </t>
537          </section>
538        </section>
539
540        <section anchor="age.calculations" title="Calculating Age">
541          <t>HTTP/1.1 uses the Age response-header to convey the estimated age of the response
542            message when obtained from a cache. The Age field value is the cache's estimate of the
543            amount of time since the response was generated or validated by the origin server. In
544            essence, the Age value is the sum of the time that the response has been resident in
545            each of the caches along the path from the origin server, plus the amount of time it has
546            been in transit along network paths.</t>
547          <t>When a stored response is used to satisfy a request, the cache &MUST; include a
548            single Age header field in the response with a value equal to the stored response's
549            current_age, calculated using the algorithm described in this section.</t>
550          <t>The term "age_value" denotes the value of the Age header, in a form appropriate for
551            arithmetic operations.</t>
552          <t>HTTP/1.1 requires origin servers to send a Date header, if possible, with every
553            response, giving the time at which the response was generated (see &header-date;).
554            The term "date_value" denotes the value of the Date header, in a form appropriate for
555            arithmetic operations.</t>
556          <t>The term "now" means "the current value of the clock at the host performing the
557            calculation." Hosts that use HTTP, but especially hosts running origin servers and
558            caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
559            synchronize their clocks to a globally accurate time standard.</t>
560          <t>A response's age can be calculated in two entirely independent ways: <list
561              style="numbers">
562              <t>now minus date_value, if the local clock is reasonably well synchronized to the
563                origin server's clock. If the result is negative, the result is replaced by zero.</t>
564
565              <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
566            </list>
567          </t>
568          <t>These are combined as</t>
569          <figure>
570            <artwork type="code">
571    corrected_received_age = max(now - date_value, age_value)
572</artwork>
573          </figure>
574          <t>When an Age value is received, it &MUST; be interpreted relative to the time the
575            request was initiated, not the time that the response was received.</t>
576          <figure>
577            <artwork type="code">
578   corrected_initial_age = corrected_received_age
579                         + (now - request_time)
580</artwork>
581          </figure>
582          <t>where "request_time" is the time (according to the local clock) when the request that
583            elicited this response was sent.</t>
584          <t>The current_age of a stored response can then be calculated by adding the amount of
585            time (in seconds) since the stored response was last validated by the origin server to
586            the corrected_initial_age.</t>
587          <t>In summary:</t>
588          <figure>
589            <artwork type="code">
590   /*
591    * age_value
592    *      is the value of Age: header received by the cache with
593    *              this response.
594    * date_value
595    *      is the value of the origin server's Date: header
596    * request_time
597    *      is the (local) time when the cache made the request
598    *              that resulted in this stored response
599    * response_time
600    *      is the (local) time when the cache received the
601    *              response
602    * now
603    *      is the current (local) time
604    */
605
606   apparent_age = max(0, response_time - date_value);
607   corrected_received_age = max(apparent_age, age_value);
608   response_delay = response_time - request_time;
609   corrected_initial_age = corrected_received_age + response_delay;
610   resident_time = now - response_time;
611   current_age   = corrected_initial_age + resident_time;
612</artwork>
613          </figure>
614        </section>
615
616        <section anchor="serving.stale.responses" title="Serving Stale Responses">
617          <t>A "stale" response is one that either has explicit expiry information, or is allowed to
618            have heuristic expiry calculated, but is not fresh according to the calculations in
619              <xref target="expiration.model" />.</t>
620          <t>Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
621            in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
622            "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
623            "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"
624             />). </t>
625          <t>Caches &MAY; return a stale response if disconnected or explicitly allowed (e.g.,
626            the max-stale request directive; see <xref target="cache-request-directive" />).</t>
627          <t>Otherwise, caches &SHOULD-NOT; return stale responses.</t>
628          <t>Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
629              target="header.warning" />).</t>
630          <t>If a cache receives a first-hand response (either an entire response, or a 304 (Not
631            Modified) response) that it would normally forward to the requesting client, and the
632            received response is no longer fresh, the cache &SHOULD; forward it to the
633            requesting client without adding a new Warning (but without removing any existing
634            Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
635            that response became stale in transit.</t>
636        </section>
637      </section>
638
639
640      <section anchor="validation.model" title="Validation Model">
641        <t>When a cache has a stale response that it would like to use, it &SHOULD; first check
642          with the origin server (or possibly an intermediate cache) to see if
643          it is still usable. This is called "validating" or "revalidating" the stored response.</t>
644        <t>HTTP's conditional request mechanism, defined in &conditional;, is used to avoid
645          retransmitting the response payload when the stored response is valid. When a stored
646          response includes one or more "cache validators", such as the field values of an ETag or
647          Last-Modified header field, then a validating GET request &SHOULD; be made conditional
648          to those field values; a 304 (Not Modified) response status code indicates that the stored
649          response can be updated and reused; see <xref target="combining.headers"/>.</t>
650        <t>If the cache receives a full response (i.e., one with payload), it is used to satisfy the
651        request and replace the stored response.</t>
652        <t>If a cache receives a 5xx response while attempting to validate a response, it &MAY;
653          either forward this response to the requesting client, or act as if the server failed to
654          respond. In the latter case, it &MAY; return a previously stored response unless the
655          stored response includes the "must-revalidate" cache directive (see <xref
656            target="cache-response-directive" />).</t>
657      </section>
658
659      <section anchor="invalidation.after.updates.or.deletions"
660        title="Request Methods that Invalidate">
661        <t>Because unsafe methods &safe-methods; have the potential for changing state on the
662          origin server, intervening caches can use them to keep their contents
663          up-to-date.</t>
664        <t>The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
665          as the Location and Content-Location headers (if present): <list style="symbols">
666            <t>PUT</t>
667            <t>DELETE</t>
668            <t>POST</t>
669          </list>
670        </t>
671        <t>An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
672          be performed if the host part of that URI differs from the host part in the Request-URI.
673          This helps prevent denial of service attacks.</t>
674        <t>
675          <cref>TODO: "host part" needs to be specified better.</cref>
676        </t>
677        <t>A cache that passes through requests for methods it does not understand &SHOULD;
678          invalidate the Request-URI.</t>
679        <t>Here, "invalidate" means that the cache will either remove all stored responses related
680          to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
681          before they can be returned in response to a subsequent request.</t>
682        <t>Note that this does not guarantee that all appropriate responses are invalidated. For
683          example, the request that caused the change at the origin server might not have gone
684          through the cache where a response is stored.</t>
685        <t>
686          <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
687        </t>
688      </section>
689
690
691
692
693
694      <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
695        <t>Use of server-driven content negotiation (&server-driven-negotiation;) alters
696          the conditions and procedure by which a cache can use the response for subsequent
697          requests.</t>
698        <t>When the cache receives a request which may be satisfied by a stored response
699          that includes a Vary header field, it &MUST-NOT; use the stored response to satisfy the request unless
700          all of the selecting request-headers present in the new request match the corresponding
701          stored request-headers from the original request.</t>
702        <t>The selecting request-headers from two requests are defined to match if and only if the
703          selecting request-headers in the first request can be transformed to the selecting
704          request-headers in the second request by adding or removing linear white space
705          <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
706          combining multiple message-header fields with the same field name following the rules
707          about message headers in &message-headers;. <cref>DISCUSS: header-specific canonicalisation</cref></t>
708        <t>A Vary header field-value of "*" always fails to match, and subsequent requests to that
709          resource can only be properly interpreted by the origin server.</t>
710        <t>If they fail to match, the cache &MAY; forward the presented request to the origin
711          server in a conditional request, which &SHOULD; include all ETags stored with
712          potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
713          includes an entity tag or Content-Location that indicates the entity to be used, that
714          cached response &MUST; be used to satisfy the presented request, and &SHOULD;
715          be used to update the corresponding stored response; see <xref target="combining.headers"/>.</t>
716        <t>If any of the stored responses contains only partial content, its entity-tag &SHOULD-NOT; 
717          be included in the If-None-Match header field unless the request is for a range that would
718          be fully satisfied by that stored response.</t>
719        <t>If a cache receives a successful response whose Content-Location field matches that of an
720          existing stored response for the same Request-URI, whose entity-tag differs from that of
721          the existing stored response, and whose Date is more recent than that of the existing
722          response, the existing response &SHOULD-NOT; be returned in response to future
723          requests and &SHOULD; be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref></t>
724      </section>
725
726
727      <section anchor="combining.headers" title="Combining Responses">
728        <t>When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
729          it needs to update the stored response with the new one, so that the updated response can
730          be sent to the client.</t>
731        <t>If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
732          the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
733          Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
734          the stored response with the updated entity-body received in the response and use the
735          result as the updated entity-body (see &combining-byte-ranges;).</t>
736        <t>The stored response headers are used for the updated response, except that <list
737            style="symbols">
738            <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
739              &MUST; be deleted from the stored response and the forwarded response.</t>
740            <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
741              response and the forwarded response.</t>
742            <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
743              headers from the stored response.</t>
744          </list>
745        </t>
746        <t>A cache &MUST; also replace stored headers with corresponding headers received in the
747          incoming response, except for Warning headers as described immediately above. If a header
748          field-name in the incoming response matches more than one header in the stored response,
749          all such old headers &MUST; be replaced. it &MAY; store the combined
750        entity-body.</t>
751        <t><cref>ISSUE: discuss how to handle HEAD updates</cref></t>
752      </section>
753
754    </section>
755
756
757
758
759    <section anchor="header.fields" title="Header Field Definitions">
760      <t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
761      <t>For entity-header fields, both sender and recipient refer to either the client or the
762        server, depending on who sends and who receives the entity.</t>
763
764      <section anchor="header.age" title="Age">
765        <iref item="Age header" primary="true" x:for-anchor="" />
766        <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
767        <x:anchor-alias value="Age"/>
768        <x:anchor-alias value="Age-v"/>
769        <x:anchor-alias value="age-value"/>
770        <t>      The response-header field "Age" conveys the sender's estimate of the amount of time since
771          the response (or its validation) was generated at the origin server. Age values are
772          calculated as specified in <xref target="age.calculations" />.</t>
773<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
774  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
775  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
776</artwork></figure>
777        <t anchor="rule.delta-seconds">
778          <x:anchor-alias value="delta-seconds" /> Age field-values are non-negative decimal
779          integers, representing time in seconds.</t>
780        <figure>
781          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
782  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
783</artwork>
784        </figure>
785        <t>If a cache receives a value larger than the largest positive integer it can represent, or
786          if any of its age calculations overflows, it &MUST; transmit an Age header with a
787          field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
788          of at least 31 bits of range.</t>
789        <t>The presence of an Age header field in a response implies that a response is not
790          first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
791          Age header field.</t>
792      </section>
793
794      <section anchor="header.cache-control" title="Cache-Control">
795        <iref item="Cache-Control header" primary="true" x:for-anchor="" />
796        <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
797        <x:anchor-alias value="Cache-Control"/>
798        <x:anchor-alias value="Cache-Control-v"/>
799        <x:anchor-alias value="cache-directive"/>
800        <x:anchor-alias value="cache-extension"/>
801        <x:anchor-alias value="cache-request-directive"/>
802        <x:anchor-alias value="cache-response-directive"/>
803        <t>The general-header field "Cache-Control" is used to specify directives that &MUST; be
804          obeyed by all caches along the request/response chain. The directives specify behavior
805          intended to prevent caches from adversely interfering with the request or response. Cache
806          directives are unidirectional in that the presence of a directive in a request does not
807          imply that the same directive is to be given in the response. <list>
808            <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
809              Pragma: no-cache (see <xref target="header.pragma" />).</t>
810          </list>
811        </t>
812        <t>Cache directives &MUST; be passed through by a proxy or gateway application,
813          regardless of their significance to that application, since the directives might be
814          applicable to all recipients along the request/response chain. It is not possible to
815          target a directive to a specific cache.</t>
816<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"/>
817  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
818  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
819
820  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
821     / <x:ref>cache-response-directive</x:ref>
822
823  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
824</artwork></figure>
825
826        <section anchor="cache-request-directive" title="Request Cache-Control Directives">
827          <x:anchor-alias value="cache-request-directive" />
828
829          <figure>
830            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
831  <x:ref>cache-request-directive</x:ref> =
832       "no-cache"
833     / "no-store"
834     / "max-age" "=" <x:ref>delta-seconds</x:ref>
835     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
836     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
837     / "no-transform"
838     / "only-if-cached"
839     / <x:ref>cache-extension</x:ref>
840</artwork>
841          </figure>
842
843
844          <t>
845            <iref item="Cache Directives" primary="true" subitem="no-cache" />
846            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
847              <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
848                used to satisfy the request without successful validation on the origin server. </t>
849            </list>
850          </t>
851          <t>
852            <iref item="Cache Directives" primary="true" subitem="no-store" />
853            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
854              <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
855                of either this request or any response to it. This directive applies to both
856                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
857                cache &MUST-NOT; intentionally store the information in non-volatile storage,
858                and &MUST; make a best-effort attempt to remove the information from volatile
859                storage as promptly as possible after forwarding it.</t>
860              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
861                particular, malicious or compromised caches might not recognize or obey this
862                directive, and communications networks may be vulnerable to eavesdropping.</t>
863            </list>
864          </t>
865          <t>
866            <iref item="Cache Directives" primary="true" subitem="max-age" />
867            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
868              <t>The max-age request directive indicates that the client is willing to accept a
869                response whose age is no greater than the specified time in seconds. Unless
870                max-stale directive is also included, the client is not willing to accept a stale
871                response.</t>
872            </list>
873          </t>
874          <t>
875            <iref item="Cache Directives" primary="true" subitem="max-stale" />
876            <iref item="max-stale" primary="true" subitem="Cache Directive" /> max-stale <list>
877              <t>The max-stale request directive indicates that the client is willing to accept a
878                response that has exceeded its expiration time. If max-stale is assigned a value,
879                then the client is willing to accept a response that has exceeded its expiration
880                time by no more than the specified number of seconds. If no value is assigned to
881                max-stale, then the client is willing to accept a stale response of any age.</t>
882            </list>
883          </t>
884          <t>
885            <iref item="Cache Directives" primary="true" subitem="min-fresh" />
886            <iref item="min-fresh" primary="true" subitem="Cache Directive" /> min-fresh <list>
887              <t>The min-fresh request directive indicates that the client is willing to accept a
888                response whose freshness lifetime is no less than its current age plus the specified
889                time in seconds. That is, the client wants a response that will still be fresh for
890                at least the specified number of seconds.</t>
891            </list>
892          </t>
893          <t>
894            <iref item="Cache Directives" primary="true" subitem="no-transform" />
895            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
896              <t>The no-transform request directive indicates that an intermediate cache or proxy
897                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
898                headers, nor the request entity-body.</t>
899            </list>
900          </t>
901
902          <t>
903            <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
904            <iref item="only-if-cached" primary="true" subitem="Cache Directive" /> only-if-cached <list>
905              <t>The only-if-cached request directive indicates that the client only wishes to
906                return a stored response. If it receives this directive, a cache &SHOULD; either
907                respond using a stored response that is consistent with the other constraints of the
908                request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
909                being operated as a unified system with good internal connectivity, such a request
910                &MAY; be forwarded within that group of caches.</t>
911            </list>
912          </t>
913        </section>
914
915        <section anchor="cache-response-directive" title="Response Cache-Control Directives">
916          <x:anchor-alias value="cache-response-directive" />
917
918          <figure>
919            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
920  <x:ref>cache-response-directive</x:ref> =
921       "public"
922     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
923     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
924     / "no-store"
925     / "no-transform"
926     / "must-revalidate"
927     / "proxy-revalidate"
928     / "max-age" "=" <x:ref>delta-seconds</x:ref>
929     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
930     / <x:ref>cache-extension</x:ref>
931</artwork>
932          </figure>
933
934          <t>
935            <iref item="Cache Directives" primary="true" subitem="public" />
936            <iref item="public" primary="true" subitem="Cache Directive" /> public <list>
937              <t>The public response directive indicates that the response &MAY; be cached, even
938                if it would normally be non-cacheable or cacheable only within a non-shared cache.
939                (See also Authorization, &header-authorization;, for additional details.) </t>
940            </list>
941          </t>
942
943          <t>
944            <iref item="Cache Directives" primary="true" subitem="private" />
945            <iref item="private" primary="true" subitem="Cache Directive" /> private <list>
946              <t>The private response directive indicates that the response message is intended for
947                a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
948                cache &MAY; store the response.</t>
949              <t>If the private response directive specifies one or more field-names, this
950                requirement is limited to the field-values associated with the listed response
951                headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
952                cache, whereas the remainder of the response message &MAY; be.</t>
953              <t>
954                <x:h>Note:</x:h> This usage of the word private only controls where the response may
955                be stored, and cannot ensure the privacy of the message content.</t>
956            </list>
957          </t>
958          <t>
959            <iref item="Cache Directives" primary="true" subitem="no-cache" />
960            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
961              <t>The no-cache response directive indicates that a response &MUST-NOT; be used to
962                satisfy a subsequent request without successful validation on the origin server.
963                This allows an origin server to prevent caching even by caches that have been
964                configured to return stale responses.</t>
965              <t>If the no-cache response directive specifies one or more field-names, this
966                requirement is limited to the field-values assosicated with the listed response
967                headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
968                to a subsequent request without successful validation on the origin server. This
969                allows an origin server to prevent the re-use of certain header fields in a
970                response, while still allowing caching of the rest of the response.</t>
971              <t>
972                <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
973              </t>
974            </list>
975          </t>
976
977          <t>
978            <iref item="Cache Directives" primary="true" subitem="no-store" />
979            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
980              <t>The no-store response directive indicates that a cache &MUST-NOT; store any
981                part of either the immediate request or response. This directive applies to both
982                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
983                cache &MUST-NOT; intentionally store the information in non-volatile storage,
984                and &MUST; make a best-effort attempt to remove the information from volatile
985                storage as promptly as possible after forwarding it.</t>
986              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
987                particular, malicious or compromised caches might not recognize or obey this
988                directive, and communications networks may be vulnerable to eavesdropping.</t>
989            </list>
990          </t>
991          <t>
992            <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
993            <iref item="must-revalidate" primary="true" subitem="Cache Directive" /> must-revalidate <list>
994              <t>The must-revalidate response directive indicates that validation is required before
995                the response is used by a cache to satisfy any request.</t>
996              <t>When the present, caches &MUST-NOT; use a stored after it becomes stale to
997                respond to a subsequent request without first validating it with the origin server.</t>
998              <t>The must-revalidate directive is necessary to support reliable operation for
999                certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1000                the must-revalidate directive; in particular, if the cache cannot reach the origin
1001                server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1002              <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1003                validate a request on the entity could result in incorrect operation, such as a
1004                silently unexecuted financial transaction.</t>
1005            </list>
1006          </t>
1007          <t>
1008            <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1009            <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1010            proxy-revalidate <list>
1011              <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1012                response directive, except that it does not apply to non-shared caches.</t>
1013            </list>
1014          </t>
1015          <t>
1016            <iref item="Cache Directives" primary="true" subitem="max-age" />
1017            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
1018              <t>The max-age response directive indicates that response is to be considered stale
1019                after its age is greater than the specified number of seconds.</t>
1020            </list>
1021          </t>
1022          <t>
1023            <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1024            <iref item="s-maxage" primary="true" subitem="Cache Directive" /> s-maxage <list>
1025              <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1026                specified by this directive overrides the maximum age specified by either the
1027                max-age directive or the Expires header. The s-maxage directive also implies the
1028                semantics of the proxy-revalidate response directive.</t>
1029            </list>
1030          </t>
1031          <t>
1032            <iref item="Cache Directives" primary="true" subitem="no-transform" />
1033            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
1034              <t>The no-transform response directive indicates that an intermediate cache or proxy
1035                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1036                headers, nor the response entity-body.</t>
1037            </list>
1038          </t>
1039
1040        </section>
1041
1042        <section anchor="cache.control.extensions" title="Cache Control Extensions">
1043          <t>The Cache-Control header field can be extended through the use of one or more
1044            cache-extension tokens, each with an optional value. Informational extensions (those
1045            which do not require a change in cache behavior) &MAY; be added without changing the
1046            semantics of other directives. Behavioral extensions are designed to work by acting as
1047            modifiers to the existing base of cache directives. Both the new directive and the
1048            standard directive are supplied, such that applications which do not understand the new
1049            directive will default to the behavior specified by the standard directive, and those
1050            that understand the new directive will recognize it as modifying the requirements
1051            associated with the standard directive. In this way, extensions to the cache-control
1052            directives can be made without requiring changes to the base protocol.</t>
1053          <t>This extension mechanism depends on an HTTP cache obeying all of the cache-control
1054            directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1055            all directives that it does not understand.</t>
1056          <t>For example, consider a hypothetical new response directive called "community" which
1057            acts as a modifier to the private directive. We define this new directive to mean that,
1058            in addition to any non-shared cache, any cache which is shared only by members of the
1059            community named within its value may cache the response. An origin server wishing to
1060            allow the UCI community to use an otherwise private response in their shared cache(s)
1061            could do so by including</t>
1062          <figure>
1063            <artwork type="example">
1064  Cache-Control: private, community="UCI"
1065</artwork>
1066          </figure>
1067          <t>A cache seeing this header field will act correctly even if the cache does not
1068            understand the community cache-extension, since it will also see and understand the
1069            private directive and thus default to the safe behavior.</t>
1070          <t>Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1071            directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1072            directives (or the response's default cacheability) such that the cache behavior will
1073            remain minimally correct even if the cache does not understand the extension(s).</t>
1074        </section>
1075
1076      </section>
1077
1078      <section anchor="header.expires" title="Expires">
1079        <iref item="Expires header" primary="true" x:for-anchor="" />
1080        <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1081        <x:anchor-alias value="Expires"/>
1082        <x:anchor-alias value="Expires-v"/>
1083        <t>The entity-header field "Expires" gives the date/time after which the response is
1084          considered stale. See <xref target="expiration.model" /> for further discussion of the
1085          expiration model.</t>
1086        <t>The presence of an Expires field does not imply that the original resource will change or
1087          cease to exist at, before, or after that time.</t>
1088        <t>The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1089          it &MUST; be sent in rfc1123-date format.</t>
1090<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1091  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1092  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1093</artwork></figure>
1094        <t>For example</t>
1095        <figure>
1096          <artwork type="example">
1097  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1098</artwork>
1099        </figure>
1100        <t>
1101          <list>
1102            <t>
1103              <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
1104              directive (see <xref target="cache-response-directive" />), that directive overrides
1105              the Expires field.</t>
1106          </list>
1107        </t>
1108        <t>HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.</t>
1109        <t>HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1110          including the value "0", as in the past (i.e., "already expired").</t>
1111      </section>
1112
1113      <section anchor="header.pragma" title="Pragma">
1114        <iref item="Pragma header" primary="true" x:for-anchor="" />
1115        <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1116        <x:anchor-alias value="extension-pragma"/>
1117        <x:anchor-alias value="Pragma"/>
1118        <x:anchor-alias value="Pragma-v"/>
1119        <x:anchor-alias value="pragma-directive"/>
1120        <t>The general-header field "Pragma" is used to include implementation-specific directives
1121          that might apply to any recipient along the request/response chain. All pragma directives
1122          specify optional behavior from the viewpoint of the protocol; however, some systems
1123          &MAY; require that behavior be consistent with the directives.</t>
1124<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"/>
1125  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1126  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1127  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1128  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1129</artwork></figure>
1130        <t>When the no-cache directive is present in a request message, an application &SHOULD;
1131          forward the request toward the origin server even if it has a cached copy of what is being
1132          requested. This pragma directive has the same semantics as the no-cache response directive
1133          (see <xref target="cache-response-directive" />) and is defined here for backward
1134          compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1135          no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1136          &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".</t>
1137        <t>
1138          <list>
1139            <t>
1140              <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
1141              is not actually specified, it does not provide a reliable replacement for
1142              "Cache-Control: no-cache" in a response.</t>
1143          </list>
1144        </t>
1145        <t>This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</t>
1146      </section>
1147
1148      <section anchor="header.vary" title="Vary">
1149        <iref item="Vary header" primary="true" x:for-anchor="" />
1150        <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1151        <x:anchor-alias value="Vary"/>
1152        <x:anchor-alias value="Vary-v"/>
1153        <t>The "Vary" response-header field's value indicates the set of request-header fields that
1154          fully determines, while the response is fresh, whether a cache is permitted to use the
1155          response to reply to a subsequent request without validation. For uncacheable or stale
1156          responses, the Vary field value advises the user agent about the criteria that were used
1157          to select the representation. A Vary field value of "*" implies that a cache cannot
1158          determine from the request headers of a subsequent request whether this response is the
1159          appropriate representation. See <xref target="caching.negotiated.responses" /> for use of
1160          the Vary header field by caches.</t>
1161<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1162  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1163  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1164</artwork></figure>
1165        <t>The set of header fields named by the Vary field value is known as the "selecting"
1166          request-headers.</t>
1167        <t>Servers &SHOULD; include a Vary header field with any cacheable response that is
1168          subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1169          requests on that resource and informs the user agent about the presence of negotiation on
1170          that resource. A server &MAY; include a Vary header field with a non-cacheable
1171          response that is subject to server-driven negotiation, since this might provide the user
1172          agent with useful information about the dimensions over which the response varies at the
1173          time of the response.</t>
1174        <t>A Vary field value of "*" signals that unspecified parameters not limited to the
1175          request-headers (e.g., the network address of the client), play a role in the selection of
1176          the response representation. The "*" value &MUST-NOT; be generated by a proxy server;
1177          it may only be generated by an origin server.</t>
1178        <t>The field-names given are not limited to the set of standard request-header fields
1179          defined by this specification. Field names are case-insensitive.</t>
1180      </section>
1181
1182      <section anchor="header.warning" title="Warning">
1183        <iref item="Warning header" primary="true" x:for-anchor="" />
1184        <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1185        <x:anchor-alias value="Warning"/>
1186        <x:anchor-alias value="Warning-v"/>
1187        <x:anchor-alias value="warning-value"/>
1188        <x:anchor-alias value="warn-agent"/>
1189        <x:anchor-alias value="warn-code"/>
1190        <x:anchor-alias value="warn-date"/>
1191        <x:anchor-alias value="warn-text"/>
1192        <t>The general-header field "Warning" is used to carry additional information about the status
1193          or transformation of a message which might not be reflected in the message. This
1194          information is typically used to warn about possible incorrectness introduced by caching
1195          operations or transformations applied to the entity body of the message.</t>
1196        <t>Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1197          warning, rather than an error status code, distinguish these responses from true failures.</t>
1198
1199        <t>Warning headers can in general be applied to any message, however some warn-codes are
1200          specific to caches and can only be applied to response messages.</t>
1201
1202<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"/>
1203  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1204  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1205 
1206  <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>
1207                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1208 
1209  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1210  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1211                  ; the name or pseudonym of the server adding
1212                  ; the Warning header, for use in debugging
1213  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1214  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1215</artwork></figure>
1216
1217        <t>Multiple warnings &MAY; be attached to a response (either by the origin server or by
1218          a cache), including multiple warnings with the same code number. For example, a server
1219          might provide the same warning with texts in both English and Basque.</t>
1220        <t>When this occurs, the user agent &SHOULD; inform the user of as many of them as
1221          possible, in the order that they appear in the response. If it is not possible to inform
1222          the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
1223            <list style="symbols">
1224            <t>Warnings that appear early in the response take priority over those appearing later
1225              in the response.</t>
1226
1227            <t>Warnings in the user's preferred character set take priority over warnings in other
1228              character sets but with identical warn-codes and warn-agents.</t>
1229          </list>
1230        </t>
1231        <t>Systems that generate multiple Warning headers &SHOULD; order them with this user
1232          agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1233          Warning headers.</t>
1234        <t>Warnings are assigned three digit warn-codes. The first digit indicates whether the
1235          Warning is required to be deleted from a stored response after validation: <list
1236            style="symbols">
1237            <t>1xx Warnings that describe the freshness or validation status of the response, and so
1238              &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
1239              except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
1240            <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1241              not rectified by a validation (for example, a lossy compression of the entity bodies)
1242              and which &MUST-NOT; be deleted by caches after validation, unless a full response is
1243              returned, in which case they &MUST; be.</t>
1244          </list>
1245        </t>
1246        <t>The warn-text &SHOULD; be in a natural language and character set that is most likely
1247          to be intelligible to the human user receiving the response. This decision can be based on
1248          any available knowledge, such as the location of the cache or user, the Accept-Language
1249          field in a request, the Content-Language field in a response, etc. The default language is
1250          English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).</t>
1251        <t>If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
1252          warn-text using the method described in <xref target="RFC2047" />.</t>
1253        <t>If an implementation sends a message with one or more Warning headers to a receiver whose
1254          version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1255          warn-date that matches the Date header in the message.</t>
1256        <t>If an implementation receives a message with a warning-value that includes a warn-date,
1257          and that warn-date is different from the Date value in the response, then that
1258          warning-value &MUST; be deleted from the message before storing, forwarding, or using
1259          it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1260          warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1261          well.</t>
1262        <t>The following warn-codes are defined by this specification, each with a recommended
1263          warn-text in English, and a description of its meaning.</t>
1264        <t>110 Response is stale <list>
1265            <t>&SHOULD; be included whenever the returned response is stale.</t>
1266          </list>
1267        </t>
1268        <t>111 Revalidation failed <list>
1269            <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1270              validate the response failed, due to an inability to reach the server.</t>
1271          </list>
1272        </t>
1273        <t>112 Disconnected operation <list>
1274            <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1275              the network for a period of time.</t>
1276          </list>
1277        </t>
1278        <t>113 Heuristic expiration <list>
1279            <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1280              greater than 24 hours and the response's age is greater than 24 hours.</t>
1281          </list>
1282        </t>
1283        <t>199 Miscellaneous warning <list>
1284            <t>The warning text &MAY; include arbitrary information to be presented to a human
1285              user, or logged. A system receiving this warning &MUST-NOT; take any automated
1286              action, besides presenting the warning to the user.</t>
1287          </list>
1288        </t>
1289        <t>214 Transformation applied <list>
1290            <t>&MUST; be added by an intermediate cache or proxy if it applies any
1291              transformation changing the content-coding (as specified in the Content-Encoding
1292              header) or media-type (as specified in the Content-Type header) of the response, or
1293              the entity-body of the response, unless this Warning code already appears in the
1294              response.</t>
1295          </list>
1296        </t>
1297        <t>299 Miscellaneous persistent warning <list>
1298            <t>The warning text &MAY; include arbitrary information to be presented to a human
1299              user, or logged. A system receiving this warning &MUST-NOT; take any automated
1300              action.</t>
1301          </list>
1302        </t>
1303      </section>
1304
1305    </section>
1306
1307
1308    <section anchor="history.lists" title="History Lists">
1309      <t>User agents often have history mechanisms, such as "Back" buttons and history lists, which
1310        can be used to redisplay an entity retrieved earlier in a session.</t>
1311      <t>History mechanisms and caches are different. In particular history mechanisms
1312        &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1313        history mechanism is meant to show exactly what the user saw at the time when the resource
1314        was retrieved.</t>
1315      <t>By default, an expiration time does not apply to history mechanisms. If the entity is still
1316        in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1317        unless the user has specifically configured the agent to refresh expired history documents.</t>
1318      <t>This is not to be construed to prohibit the history mechanism from telling the user that a
1319        view might be stale. <list>
1320          <t>
1321            <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1322            stale resources, this will tend to force service authors to avoid using HTTP expiration
1323            controls and cache controls when they would otherwise like to. Service authors may
1324            consider it important that users not be presented with error messages or warning
1325            messages when they use navigation controls (such as BACK) to view previously fetched
1326            resources. Even though sometimes such resources ought not be cached, or ought to expire
1327            quickly, user interface considerations may force service authors to resort to other
1328            means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1329            of improperly functioning history mechanisms.</t>
1330        </list>
1331      </t>
1332    </section>
1333
1334
1335    <section anchor="IANA.considerations" title="IANA Considerations">
1336      <section anchor="message.header.registration" title="Message Header Registration">
1337        <t>The Message Header Registry located at <eref
1338            target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1339          should be updated with the permanent registrations below (see <xref target="RFC3864" />):</t>
1340        <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1341        <texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1342          <ttcol>Header Field Name</ttcol>
1343          <ttcol>Protocol</ttcol>
1344          <ttcol>Status</ttcol>
1345          <ttcol>Reference</ttcol>
1346
1347          <c>Age</c>
1348          <c>http</c>
1349          <c>standard</c>
1350          <c>
1351            <xref target="header.age" />
1352          </c>
1353          <c>Cache-Control</c>
1354          <c>http</c>
1355          <c>standard</c>
1356          <c>
1357            <xref target="header.cache-control" />
1358          </c>
1359          <c>Expires</c>
1360          <c>http</c>
1361          <c>standard</c>
1362          <c>
1363            <xref target="header.expires" />
1364          </c>
1365          <c>Pragma</c>
1366          <c>http</c>
1367          <c>standard</c>
1368          <c>
1369            <xref target="header.pragma" />
1370          </c>
1371          <c>Vary</c>
1372          <c>http</c>
1373          <c>standard</c>
1374          <c>
1375            <xref target="header.vary" />
1376          </c>
1377          <c>Warning</c>
1378          <c>http</c>
1379          <c>standard</c>
1380          <c>
1381            <xref target="header.warning" />
1382          </c>
1383        </texttable>
1384        <!--(END)-->
1385        <t>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</t>
1386      </section>
1387    </section>
1388
1389    <section anchor="security.considerations" title="Security Considerations">
1390      <t>Caches expose additional potential vulnerabilities, since the contents of the cache
1391        represent an attractive target for malicious exploitation. Because cache contents persist
1392        after an HTTP request is complete, an attack on the cache can reveal information long after
1393        a user believes that the information has been removed from the network. Therefore, cache
1394        contents should be protected as sensitive information.</t>
1395    </section>
1396
1397    <section anchor="ack" title="Acknowledgments">
1398      <t>Much of the content and presentation of the caching design is due to suggestions and
1399        comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1400        and Larry Masinter.</t>
1401    </section>
1402  </middle>
1403
1404  <back>
1405    <references title="Normative References">
1406
1407      <reference anchor="ISO-8859-1">
1408        <front>
1409          <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1410            1: Latin alphabet No. 1 </title>
1411          <author>
1412            <organization>International Organization for Standardization</organization>
1413          </author>
1414          <date year="1998" />
1415        </front>
1416        <seriesInfo name="ISO/IEC" value="8859-1:1998" />
1417      </reference>
1418
1419      <reference anchor="Part1">
1420        <front>
1421          <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1422          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1423            <organization abbrev="Day Software">Day Software</organization>
1424            <address><email>fielding@gbiv.com</email></address>
1425          </author>
1426          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1427            <organization>One Laptop per Child</organization>
1428            <address><email>jg@laptop.org</email></address>
1429          </author>
1430          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1431            <organization abbrev="HP">Hewlett-Packard Company</organization>
1432            <address><email>JeffMogul@acm.org</email></address>
1433          </author>
1434          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1435            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1436            <address><email>henrikn@microsoft.com</email></address>
1437          </author>
1438          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1439            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1440            <address><email>LMM@acm.org</email></address>
1441          </author>
1442          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1443            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1444            <address><email>paulle@microsoft.com</email></address>
1445          </author>
1446          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1447            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1448            <address><email>timbl@w3.org</email></address>
1449          </author>
1450          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1451            <organization abbrev="W3C">World Wide Web Consortium</organization>
1452            <address><email>ylafon@w3.org</email></address>
1453          </author>
1454          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1455            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1456            <address><email>julian.reschke@greenbytes.de</email></address>
1457          </author>
1458          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1459        </front>
1460        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1461        <x:source basename="p1-messaging" href="p1-messaging.xml" />
1462      </reference>
1463
1464      <reference anchor="Part2">
1465        <front>
1466          <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1467          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1468            <organization abbrev="Day Software">Day Software</organization>
1469            <address><email>fielding@gbiv.com</email></address>
1470          </author>
1471          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1472            <organization>One Laptop per Child</organization>
1473            <address><email>jg@laptop.org</email></address>
1474          </author>
1475          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1476            <organization abbrev="HP">Hewlett-Packard Company</organization>
1477            <address><email>JeffMogul@acm.org</email></address>
1478          </author>
1479          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1480            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1481            <address><email>henrikn@microsoft.com</email></address>
1482          </author>
1483          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1484            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1485            <address><email>LMM@acm.org</email></address>
1486          </author>
1487          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1488            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1489            <address><email>paulle@microsoft.com</email></address>
1490          </author>
1491          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1492            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1493            <address><email>timbl@w3.org</email></address>
1494          </author>
1495          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1496            <organization abbrev="W3C">World Wide Web Consortium</organization>
1497            <address><email>ylafon@w3.org</email></address>
1498          </author>
1499          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1500            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1501            <address><email>julian.reschke@greenbytes.de</email></address>
1502          </author>
1503          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1504        </front>
1505        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1506        <x:source basename="p2-semantics" href="p2-semantics.xml" />
1507      </reference>
1508
1509      <reference anchor="Part3">
1510        <front>
1511          <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1512          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1513            <organization abbrev="Day Software">Day Software</organization>
1514            <address><email>fielding@gbiv.com</email></address>
1515          </author>
1516          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1517            <organization>One Laptop per Child</organization>
1518            <address><email>jg@laptop.org</email></address>
1519          </author>
1520          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1521            <organization abbrev="HP">Hewlett-Packard Company</organization>
1522            <address><email>JeffMogul@acm.org</email></address>
1523          </author>
1524          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1525            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1526            <address><email>henrikn@microsoft.com</email></address>
1527          </author>
1528          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1529            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1530            <address><email>LMM@acm.org</email></address>
1531          </author>
1532          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1533            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1534            <address><email>paulle@microsoft.com</email></address>
1535          </author>
1536          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1537            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1538            <address><email>timbl@w3.org</email></address>
1539          </author>
1540          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1541            <organization abbrev="W3C">World Wide Web Consortium</organization>
1542            <address><email>ylafon@w3.org</email></address>
1543          </author>
1544          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1545            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1546            <address><email>julian.reschke@greenbytes.de</email></address>
1547          </author>
1548          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1549        </front>
1550        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1551        <x:source basename="p3-payload" href="p3-payload.xml" />
1552      </reference>
1553
1554      <reference anchor="Part4">
1555        <front>
1556          <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1557          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1558            <organization abbrev="Day Software">Day Software</organization>
1559            <address><email>fielding@gbiv.com</email></address>
1560          </author>
1561          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1562            <organization>One Laptop per Child</organization>
1563            <address><email>jg@laptop.org</email></address>
1564          </author>
1565          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1566            <organization abbrev="HP">Hewlett-Packard Company</organization>
1567            <address><email>JeffMogul@acm.org</email></address>
1568          </author>
1569          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1570            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1571            <address><email>henrikn@microsoft.com</email></address>
1572          </author>
1573          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1574            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1575            <address><email>LMM@acm.org</email></address>
1576          </author>
1577          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1578            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1579            <address><email>paulle@microsoft.com</email></address>
1580          </author>
1581          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1582            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1583            <address><email>timbl@w3.org</email></address>
1584          </author>
1585          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1586            <organization abbrev="W3C">World Wide Web Consortium</organization>
1587            <address><email>ylafon@w3.org</email></address>
1588          </author>
1589          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1590            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1591            <address><email>julian.reschke@greenbytes.de</email></address>
1592          </author>
1593          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1594        </front>
1595        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1596        <x:source basename="p4-conditional" href="p4-conditional.xml" />
1597      </reference>
1598
1599      <reference anchor="Part5">
1600        <front>
1601          <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1602          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1603            <organization abbrev="Day Software">Day Software</organization>
1604            <address><email>fielding@gbiv.com</email></address>
1605          </author>
1606          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1607            <organization>One Laptop per Child</organization>
1608            <address><email>jg@laptop.org</email></address>
1609          </author>
1610          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1611            <organization abbrev="HP">Hewlett-Packard Company</organization>
1612            <address><email>JeffMogul@acm.org</email></address>
1613          </author>
1614          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1615            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1616            <address><email>henrikn@microsoft.com</email></address>
1617          </author>
1618          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1619            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1620            <address><email>LMM@acm.org</email></address>
1621          </author>
1622          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1623            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1624            <address><email>paulle@microsoft.com</email></address>
1625          </author>
1626          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1627            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1628            <address><email>timbl@w3.org</email></address>
1629          </author>
1630          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1631            <organization abbrev="W3C">World Wide Web Consortium</organization>
1632            <address><email>ylafon@w3.org</email></address>
1633          </author>
1634          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1635            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1636            <address><email>julian.reschke@greenbytes.de</email></address>
1637          </author>
1638          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1639        </front>
1640        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1641        <x:source basename="p5-range" href="p5-range.xml" />
1642      </reference>
1643
1644      <reference anchor="Part7">
1645        <front>
1646          <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1647          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1648            <organization abbrev="Day Software">Day Software</organization>
1649            <address><email>fielding@gbiv.com</email></address>
1650          </author>
1651          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1652            <organization>One Laptop per Child</organization>
1653            <address><email>jg@laptop.org</email></address>
1654          </author>
1655          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1656            <organization abbrev="HP">Hewlett-Packard Company</organization>
1657            <address><email>JeffMogul@acm.org</email></address>
1658          </author>
1659          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1660            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1661            <address><email>henrikn@microsoft.com</email></address>
1662          </author>
1663          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1664            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1665            <address><email>LMM@acm.org</email></address>
1666          </author>
1667          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1668            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1669            <address><email>paulle@microsoft.com</email></address>
1670          </author>
1671          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1672            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1673            <address><email>timbl@w3.org</email></address>
1674          </author>
1675          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1676            <organization abbrev="W3C">World Wide Web Consortium</organization>
1677            <address><email>ylafon@w3.org</email></address>
1678          </author>
1679          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1680            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1681            <address><email>julian.reschke@greenbytes.de</email></address>
1682          </author>
1683          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1684        </front>
1685        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1686        <x:source basename="p7-auth" href="p7-auth.xml" />
1687      </reference>
1688
1689      <reference anchor="RFC2047">
1690        <front>
1691          <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1692            Part Three: Message Header Extensions for Non-ASCII Text</title>
1693          <author fullname="Keith Moore" initials="K." surname="Moore">
1694            <organization>University of Tennessee</organization>
1695            <address><email>moore@cs.utk.edu</email></address>
1696          </author>
1697          <date month="November" year="1996" />
1698        </front>
1699        <seriesInfo name="RFC" value="2047" />
1700      </reference>
1701
1702      <reference anchor="RFC2119">
1703        <front>
1704          <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1705          <author fullname="Scott Bradner" initials="S." surname="Bradner">
1706            <organization>Harvard University</organization>
1707            <address><email>sob@harvard.edu</email></address>
1708          </author>
1709          <date month="March" year="1997" />
1710        </front>
1711        <seriesInfo name="BCP" value="14" />
1712        <seriesInfo name="RFC" value="2119" />
1713      </reference>
1714
1715      <reference anchor="RFC5234">
1716        <front>
1717          <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1718          <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1719            <organization>Brandenburg InternetWorking</organization>
1720            <address>
1721            <postal>
1722            <street>675 Spruce Dr.</street>
1723            <city>Sunnyvale</city>
1724            <region>CA</region>
1725            <code>94086</code>
1726            <country>US</country></postal>
1727            <phone>+1.408.246.8253</phone>
1728            <email>dcrocker@bbiw.net</email></address> 
1729          </author>
1730          <author initials="P." surname="Overell" fullname="Paul Overell">
1731            <organization>THUS plc.</organization>
1732            <address>
1733            <postal>
1734            <street>1/2 Berkeley Square</street>
1735            <street>99 Berkely Street</street>
1736            <city>Glasgow</city>
1737            <code>G3 7HR</code>
1738            <country>UK</country></postal>
1739            <email>paul.overell@thus.net</email></address>
1740          </author>
1741          <date month="January" year="2008"/>
1742        </front>
1743        <seriesInfo name="STD" value="68"/>
1744        <seriesInfo name="RFC" value="5234"/>
1745      </reference>
1746     
1747    </references>
1748
1749    <references title="Informative References">
1750
1751      <reference anchor="RFC1305">
1752        <front>
1753          <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1754          <author fullname="David L. Mills" initials="D." surname="Mills">
1755            <organization>University of Delaware, Electrical Engineering Department</organization>
1756            <address><email>mills@udel.edu</email></address>
1757          </author>
1758          <date month="March" year="1992" />
1759        </front>
1760        <seriesInfo name="RFC" value="1305" />
1761      </reference>
1762
1763      <reference anchor="RFC2616">
1764        <front>
1765          <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1766          <author fullname="R. Fielding" initials="R." surname="Fielding">
1767            <organization>University of California, Irvine</organization>
1768            <address><email>fielding@ics.uci.edu</email></address>
1769          </author>
1770          <author fullname="J. Gettys" initials="J." surname="Gettys">
1771            <organization>W3C</organization>
1772            <address><email>jg@w3.org</email></address>
1773          </author>
1774          <author fullname="J. Mogul" initials="J." surname="Mogul">
1775            <organization>Compaq Computer Corporation</organization>
1776            <address><email>mogul@wrl.dec.com</email></address>
1777          </author>
1778          <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1779            <organization>MIT Laboratory for Computer Science</organization>
1780            <address><email>frystyk@w3.org</email></address>
1781          </author>
1782          <author fullname="L. Masinter" initials="L." surname="Masinter">
1783            <organization>Xerox Corporation</organization>
1784            <address><email>masinter@parc.xerox.com</email></address>
1785          </author>
1786          <author fullname="P. Leach" initials="P." surname="Leach">
1787            <organization>Microsoft Corporation</organization>
1788            <address><email>paulle@microsoft.com</email></address>
1789          </author>
1790          <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1791            <organization>W3C</organization>
1792            <address><email>timbl@w3.org</email></address>
1793          </author>
1794          <date month="June" year="1999" />
1795        </front>
1796        <seriesInfo name="RFC" value="2616" />
1797      </reference>
1798
1799      <reference anchor="RFC3864">
1800        <front>
1801          <title>Registration Procedures for Message Header Fields</title>
1802          <author fullname="G. Klyne" initials="G." surname="Klyne">
1803            <organization>Nine by Nine</organization>
1804            <address><email>GK-IETF@ninebynine.org</email></address>
1805          </author>
1806          <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1807            <organization>BEA Systems</organization>
1808            <address><email>mnot@pobox.com</email></address>
1809          </author>
1810          <author fullname="J. Mogul" initials="J." surname="Mogul">
1811            <organization>HP Labs</organization>
1812            <address><email>JeffMogul@acm.org</email></address>
1813          </author>
1814          <date month="September" year="2004" />
1815        </front>
1816        <seriesInfo name="BCP" value="90" />
1817        <seriesInfo name="RFC" value="3864" />
1818      </reference>
1819
1820    </references>
1821
1822    <section anchor="compatibility" title="Compatibility with Previous Versions">
1823
1824      <section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
1825        <t>A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
1826          this missing case. (Sections <xref format="counter" target="response.cacheability" />,
1827            <xref format="counter" target="header.cache-control" />).</t>
1828        <t>Transfer-coding and message lengths all interact in ways that required fixing exactly
1829          when chunked encoding is used (to allow for transfer encoding that may not be self
1830          delimiting); it was important to straighten out exactly how message lengths are computed.
1831          (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)</t>
1832        <t>Proxies should be able to add Content-Length when appropriate.</t>
1833        <t>Range request responses would become very verbose if all meta-data were always returned;
1834          by allowing the server to only send needed headers in a 206 response, this problem can be
1835          avoided. (<xref target="combining.headers" />)</t>
1836        <t>The Cache-Control: max-age directive was not properly defined for responses.</t>
1837        <t>Warnings could be cached incorrectly, or not updated appropriately. <xref
1838            format="counter" target="expiration.model" />, <xref format="counter"
1839            target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
1840          and <xref format="counter" target="header.warning" />) Warning also needed to be a general
1841          header, as PUT or other methods may have need for it in requests.</t>
1842      </section>
1843
1844      <section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
1845        <t>Clarify denial of service attack avoidance requirement. (<xref
1846            target="invalidation.after.updates.or.deletions" />)</t>
1847      </section>
1848
1849    </section>
1850
1851<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1852<figure>
1853<artwork type="abnf" name="p6-cache.parsed-abnf">
1854<x:ref>Age</x:ref> = "Age:" OWS Age-v
1855<x:ref>Age-v</x:ref> = delta-seconds
1856
1857<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
1858<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
1859 cache-directive ] )
1860
1861<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
1862<x:ref>Expires-v</x:ref> = HTTP-date
1863
1864<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
1865
1866<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1867
1868<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
1869<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1870 pragma-directive ] )
1871
1872<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
1873<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1874 ] ) )
1875
1876<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
1877<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1878 ] )
1879
1880<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
1881<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
1882<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
1883 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1884 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1885 cache-extension
1886<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
1887 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1888 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1889 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1890 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1891 ) / ( "s-maxage=" delta-seconds ) / cache-extension
1892
1893<x:ref>delta-seconds</x:ref> = 1*DIGIT
1894
1895<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
1896
1897<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
1898
1899<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
1900<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
1901<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
1902
1903<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1904
1905<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
1906
1907<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
1908
1909<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
1910<x:ref>warn-code</x:ref> = 3DIGIT
1911<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
1912<x:ref>warn-text</x:ref> = quoted-string
1913<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
1914 ]
1915
1916
1917</artwork>
1918</figure>
1919<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">; Age defined but not used
1920; Cache-Control defined but not used
1921; Expires defined but not used
1922; Pragma defined but not used
1923; Vary defined but not used
1924; Warning defined but not used
1925</artwork></figure></section>
1926
1927    <section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
1928
1929      <section title="Since RFC2616">
1930        <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
1931      </section>
1932
1933      <section title="Since draft-ietf-httpbis-p6-cache-00">
1934        <t>Closed issues: <list style="symbols">
1935            <t>
1936              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer"
1937                (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
1938            <t>
1939              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation
1940              after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
1941            <t>
1942              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and
1943              Informative references"</t>
1944            <t>
1945              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date
1946              reference typo"</t>
1947            <t>
1948              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection
1949              header text"</t>
1950            <t>
1951              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative
1952              references"</t>
1953            <t>
1954              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1
1955              Reference"</t>
1956            <t>
1957              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative
1958              up-to-date references"</t>
1959            <t>
1960              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in
1961              13.2.2"</t>
1962          </list>
1963        </t>
1964        <t>Other changes: <list style="symbols">
1965            <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
1966                target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
1967          </list>
1968        </t>
1969      </section>
1970
1971      <section title="Since draft-ietf-httpbis-p6-cache-01">
1972        <t>Closed issues: <list style="symbols">
1973            <t>
1974              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not
1975              used"</t>
1976          </list>
1977        </t>
1978        <t>Other changes: <list style="symbols">
1979            <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
1980              on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
1981            <t>Add explicit references to BNF syntax and rules imported from other parts of the
1982              specification.</t>
1983          </list>
1984        </t>
1985      </section>
1986
1987      <section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
1988        <t>Ongoing work on IANA Message Header Registration (<eref
1989            target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40" />): <list style="symbols">
1990            <t>Reference RFC 3984, and update header registrations for headers defined in this
1991              document.</t>
1992          </list>
1993        </t>
1994      </section>
1995
1996      <section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
1997        <t>Closed issues: <list style="symbols">
1998            <t>
1999              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header
2000              classification"</t>
2001          </list>
2002        </t>
2003      </section>
2004
2005      <section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2006      <t>
2007      </t>
2008      </section>
2009
2010      <section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2011      <t>
2012        This is a total rewrite.
2013      </t>
2014      </section>
2015
2016    </section>
2017  </back>
2018</rfc>
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