source: draft-ietf-httpbis/latest-roy/p6-cache.xml @ 509

Last change on this file since 509 was 509, checked in by mnot@…, 11 years ago

Note requirement to generate 112 disconnected operation warn-code in serving.stale.responses.

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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 anchor="shared.and.non-shared.caches">
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>Request methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
454          the origin server; i.e., A cache must not reply to such a request before having forwarded the request and having received a
455   corresponding response.</t>
456                <t>Also, note that unsafe requests might invalidate already stored responses; see
457            <xref target="invalidation.after.updates.or.deletions" />.</t>
458        <t>Caches &SHOULD; use the most recent response (as determined by the Date header) when
459          more than one suitable response is stored. They &MAY; also forward a request with
460          "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
461          use.</t>
462        <t>
463          <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in
464            p1</cref>
465        </t>
466      </section>
467
468
469
470
471      <section anchor="expiration.model" title="Freshness Model">
472
473        <t>When a response is "fresh" in the cache, it can be used to satisfy subsequent
474          requests without contacting the origin server, thereby improving efficiency.</t>
475        <t>The primary mechanism for determining freshness is for an origin server to provide an
476          explicit expiration time in the future, using either the Expires header (<xref
477            target="header.expires" />) or the max-age response cache directive (<xref
478            target="cache-response-directive" />). Generally, origin servers will assign future
479          explicit expiration times to responses in the belief that the entity is not likely to
480          change in a semantically significant way before the expiration time is reached.</t>
481        <t>If an origin server wishes to force a cache to validate every request, it &MAY;
482          assign an explicit expiration time in the past. This means that the response is always
483          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>
484        <t>Since origin servers do not always provide explicit expiration times, HTTP caches may
485          also assign heuristic expiration times when they are not specified, employing algorithms that
486          use other header values (such as the Last-Modified time) to estimate a plausible
487          expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
488          impose worst-case constraints on their results.</t>
489        <t>The calculation to determine if a response is fresh is:</t>
490        <figure>
491          <artwork type="code">
492   response_is_fresh = (freshness_lifetime &gt; current_age)
493</artwork>
494        </figure>
495
496        <t>The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
497          the current_age is defined in <xref target="age.calculations" />.</t>
498        <t>Additionally, clients may need to influence freshness calculation. They can do this using
499          several request cache directives, with the effect of either increasing or loosening
500          constraints on freshness. See <xref target="cache-request-directive" />.</t>
501        <t>Note that reshness applies only to cache operation; it cannot be used to force a user agent
502        to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
503          the difference between caches and history mechanisms.</t>
504        <t>
505          <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
506            freshness.</cref>
507        </t>
508
509        <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
510          <t>A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
511            response by using the first match of: <list style="symbols">
512              <t>If the cache is shared and the s-maxage response cache directive (<xref
513                  target="cache-response-directive" />) is present, use its value, or</t>
514              <t>If the max-age response cache directive (<xref target="cache-response-directive"
515                 />) is present, use its value, or</t>
516              <t>If the Expires response header (<xref target="header.expires" />) is present, use
517                its value minus the value of the Date response header, or</t>
518              <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
519                may be used; see <xref target="heuristic.freshness" />.</t>
520            </list>
521          </t>
522          <t>Note that this calculation is not vulnerable to clock skew, since all of the
523            information comes from the origin server.</t>
524
525          <section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
526            <t>If no explicit expiration time is present in a stored response that has a status code
527              of 200, 203, 206, 300, 301 or 410, a heuristic expiration time &MAY; be
528              calculated. Heuristics &MUST-NOT; be used for other response status codes. </t>
529            <t> When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
530              attach a Warning header with a 113 warn-code to the response if its current_age is
531              more than 24 hours and such a warning is not already present.</t>
532            <t>Also, if the response has a Last-Modified header (&header-last-modified;), the
533              heuristic expiration value &SHOULD; be no more than some fraction of the interval
534              since that time. A typical setting of this fraction might be 10%.</t>
535            <t>
536              <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
537            </t>
538          </section>
539        </section>
540
541        <section anchor="age.calculations" title="Calculating Age">
542          <t>HTTP/1.1 uses the Age response-header to convey the estimated age of the response
543            message when obtained from a cache. The Age field value is the cache's estimate of the
544            amount of time since the response was generated or validated by the origin server. In
545            essence, the Age value is the sum of the time that the response has been resident in
546            each of the caches along the path from the origin server, plus the amount of time it has
547            been in transit along network paths.</t>
548          <t>When a stored response is used to satisfy a request, the cache &MUST; include a
549            single Age header field in the response with a value equal to the stored response's
550            current_age, calculated using the algorithm described in this section.</t>
551          <t>The term "age_value" denotes the value of the Age header, in a form appropriate for
552            arithmetic operations.</t>
553          <t>HTTP/1.1 requires origin servers to send a Date header, if possible, with every
554            response, giving the time at which the response was generated (see &header-date;).
555            The term "date_value" denotes the value of the Date header, in a form appropriate for
556            arithmetic operations.</t>
557          <t>The term "now" means "the current value of the clock at the host performing the
558            calculation." Hosts that use HTTP, but especially hosts running origin servers and
559            caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
560            synchronize their clocks to a globally accurate time standard.</t>
561          <t>A response's age can be calculated in two entirely independent ways: <list
562              style="numbers">
563              <t>now minus date_value, if the local clock is reasonably well synchronized to the
564                origin server's clock. If the result is negative, the result is replaced by zero.</t>
565
566              <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
567            </list>
568          </t>
569          <t>These are combined as</t>
570          <figure>
571            <artwork type="code">
572    corrected_received_age = max(now - date_value, age_value)
573</artwork>
574          </figure>
575          <t>When an Age value is received, it &MUST; be interpreted relative to the time the
576            request was initiated, not the time that the response was received.</t>
577          <figure>
578            <artwork type="code">
579   corrected_initial_age = corrected_received_age
580                         + (now - request_time)
581</artwork>
582          </figure>
583          <t>where "request_time" is the time (according to the local clock) when the request that
584            elicited this response was sent.</t>
585          <t>The current_age of a stored response can then be calculated by adding the amount of
586            time (in seconds) since the stored response was last validated by the origin server to
587            the corrected_initial_age.</t>
588          <t>In summary:</t>
589          <figure>
590            <artwork type="code">
591   /*
592    * age_value
593    *      is the value of Age: header received by the cache with
594    *              this response.
595    * date_value
596    *      is the value of the origin server's Date: header
597    * request_time
598    *      is the (local) time when the cache made the request
599    *              that resulted in this stored response
600    * response_time
601    *      is the (local) time when the cache received the
602    *              response
603    * now
604    *      is the current (local) time
605    */
606
607   apparent_age = max(0, response_time - date_value);
608   corrected_received_age = max(apparent_age, age_value);
609   response_delay = response_time - request_time;
610   corrected_initial_age = corrected_received_age + response_delay;
611   resident_time = now - response_time;
612   current_age   = corrected_initial_age + resident_time;
613</artwork>
614          </figure>
615        </section>
616
617        <section anchor="serving.stale.responses" title="Serving Stale Responses">
618          <t>A "stale" response is one that either has explicit expiry information, or is allowed to
619            have heuristic expiry calculated, but is not fresh according to the calculations in
620              <xref target="expiration.model" />.</t>
621          <t>Caches &MAY; return a stale response if disconnected or explicitly allowed (e.g.,
622            the max-stale request directive; see <xref target="cache-request-directive" />).</t>
623          <t>Otherwise, caches &SHOULD-NOT; return stale responses.</t>
624          <t>Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
625            in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
626            "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
627            "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"
628             />). </t>
629          <t>Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
630              target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent if
631              the cache is disconnected.</t>
632          <t>If a cache receives a first-hand response (either an entire response, or a 304 (Not
633            Modified) response) that it would normally forward to the requesting client, and the
634            received response is no longer fresh, the cache &SHOULD; forward it to the
635            requesting client without adding a new Warning (but without removing any existing
636            Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
637            that response became stale in transit.</t>
638        </section>
639      </section>
640
641
642      <section anchor="validation.model" title="Validation Model">
643        <t>Checking with the origin server to see if a stale or otherwise unusable cached response
644        can be reused is called "validating" or "revalidating." Doing so potentially avoids
645        the overhead of retransmitting the response body when the stored response is valid.</t>
646        <t>HTTP's conditional request mechanism &conditional; is used for this purpose. When a stored
647          response includes one or more validators, such as the field values of an ETag or
648          Last-Modified header field, then a validating request &SHOULD; be made conditional
649          to those field values.</t>
650         <t>A 304 (Not Modified) response status code indicates that the stored
651          response can be updated and reused; see <xref target="combining.headers"/>.</t>
652        <t>If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the
653        request and replace the stored response. <cref>Should there be a requirement here?</cref></t>
654        <t>If a cache receives a 5xx response while attempting to validate a response, it &MAY;
655          either forward this response to the requesting client, or act as if the server failed to
656          respond. In the latter case, it &MAY; return a previously stored response unless the
657          stored response includes the "must-revalidate" cache directive (see <xref
658            target="serving.stale.responses" />).</t>
659      </section>
660
661      <section anchor="invalidation.after.updates.or.deletions"
662        title="Request Methods that Invalidate">
663        <t>Because unsafe methods (&safe-methods;) have the potential for changing state on the
664          origin server, intervening caches can use them to keep their contents
665          up-to-date.</t>
666        <t>The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
667          as the Location and Content-Location headers (if present): <list style="symbols">
668            <t>PUT</t>
669            <t>DELETE</t>
670            <t>POST</t>
671          </list>
672        </t>
673        <t>An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
674          be performed if the host part of that URI differs from the host part in the Request-URI.
675          This helps prevent denial of service attacks.</t>
676        <t>
677          <cref>TODO: "host part" needs to be specified better.</cref>
678        </t>
679        <t>A cache that passes through requests for methods it does not understand &SHOULD;
680          invalidate the Request-URI.</t>
681        <t>Here, "invalidate" means that the cache will either remove all stored responses related
682          to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
683          before they can be returned in response to a subsequent request.</t>
684        <t>Note that this does not guarantee that all appropriate responses are invalidated. For
685          example, the request that caused the change at the origin server might not have gone
686          through the cache where a response is stored.</t>
687        <t>
688          <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
689        </t>
690      </section>
691
692
693
694
695
696      <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
697        <t>Use of server-driven content negotiation (&server-driven-negotiation;) alters
698          the conditions and procedure by which a cache can use the response for subsequent
699          requests.</t>
700        <t>When the cache receives a request which may be satisfied by a stored response
701          that includes a Vary header field <xref target="header.vary"/>, it &MUST-NOT; use the stored response to satisfy the request unless
702          all of the selecting request-headers present in the new request match the corresponding
703          stored request-headers from the original request.</t>
704        <t>The selecting request-headers from two requests are defined to match if and only if the
705          selecting request-headers in the first request can be transformed to the selecting
706          request-headers in the second request by adding or removing linear white space
707          <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
708          combining multiple message-header fields with the same field name following the rules
709          about message headers in &message-headers;. <cref>DISCUSS: header-specific canonicalisation</cref></t>
710        <t>A Vary header field-value of "*" always fails to match, and subsequent requests to that
711          resource can only be properly interpreted by the origin server.</t>
712        <t>If no stored response matches, the cache &MAY; forward the presented request to the origin
713          server in a conditional request, which &SHOULD; include all ETags stored with
714          potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
715          includes an entity tag or Content-Location that indicates the entity to be used, that
716          cached response &MUST; be used to satisfy the presented request, and &SHOULD;
717          be used to update the corresponding stored response; see <xref target="combining.headers"/>.</t>
718        <t>If any of the stored responses contains only partial content, its entity-tag &SHOULD-NOT; 
719          be included in the If-None-Match header field unless the request is for a range that would
720          be fully satisfied by that stored response.</t>
721        <t>If a cache receives a successful response whose Content-Location field matches that of an
722          existing stored response for the same Request-URI, whose entity-tag differs from that of
723          the existing stored response, and whose Date is more recent than that of the existing
724          response, the existing response &SHOULD-NOT; be returned in response to future
725          requests and &SHOULD; be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref></t>
726      </section>
727
728
729      <section anchor="combining.headers" title="Combining Responses">
730        <t>When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
731          it needs to update the stored response with the new one, so that the updated response can
732          be sent to the client.</t>
733        <t>If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
734          the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
735          Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
736          the stored response with the updated entity-body received in the response and use the
737          result as the updated entity-body (see &combining-byte-ranges;).</t>
738        <t>The stored response headers are used for the updated response, except that <list
739            style="symbols">
740            <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
741              &MUST; be deleted from the stored response and the forwarded response.</t>
742            <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
743              response and the forwarded response.</t>
744            <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
745              headers from the stored response.</t>
746          </list>
747        </t>
748        <t>A cache &MUST; also replace stored headers with corresponding headers received in the
749          incoming response, except for Warning headers as described immediately above. If a header
750          field-name in the incoming response matches more than one header in the stored response,
751          all such old headers &MUST; be replaced. it &MAY; store the combined
752        entity-body.</t>
753        <t><cref>ISSUE: discuss how to handle HEAD updates</cref></t>
754      </section>
755
756    </section>
757
758
759
760
761    <section anchor="header.fields" title="Header Field Definitions">
762      <t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
763      <t>For entity-header fields, both sender and recipient refer to either the client or the
764        server, depending on who sends and who receives the entity.</t>
765
766      <section anchor="header.age" title="Age">
767        <iref item="Age header" primary="true" x:for-anchor="" />
768        <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
769        <x:anchor-alias value="Age"/>
770        <x:anchor-alias value="Age-v"/>
771        <x:anchor-alias value="age-value"/>
772        <t>      The response-header field "Age" conveys the sender's estimate of the amount of time since
773          the response (or its validation) was generated at the origin server. Age values are
774          calculated as specified in <xref target="age.calculations" />.</t>
775<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
776  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
777  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
778</artwork></figure>
779        <t anchor="rule.delta-seconds">
780          <x:anchor-alias value="delta-seconds" /> Age field-values are non-negative decimal
781          integers, representing time in seconds.</t>
782        <figure>
783          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
784  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
785</artwork>
786        </figure>
787        <t>If a cache receives a value larger than the largest positive integer it can represent, or
788          if any of its age calculations overflows, it &MUST; transmit an Age header with a
789          field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
790          of at least 31 bits of range.</t>
791        <t>The presence of an Age header field in a response implies that a response is not
792          first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
793          Age header field.</t>
794      </section>
795
796      <section anchor="header.cache-control" title="Cache-Control">
797        <iref item="Cache-Control header" primary="true" x:for-anchor="" />
798        <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
799        <x:anchor-alias value="Cache-Control"/>
800        <x:anchor-alias value="Cache-Control-v"/>
801        <x:anchor-alias value="cache-directive"/>
802        <x:anchor-alias value="cache-extension"/>
803        <x:anchor-alias value="cache-request-directive"/>
804        <x:anchor-alias value="cache-response-directive"/>
805        <t>The general-header field "Cache-Control" is used to specify directives that &MUST; be
806          obeyed by all caches along the request/response chain. The directives specify behavior
807          intended to prevent caches from adversely interfering with the request or response. Cache
808          directives are unidirectional in that the presence of a directive in a request does not
809          imply that the same directive is to be given in the response. <list>
810            <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
811              Pragma: no-cache (see <xref target="header.pragma" />).</t>
812          </list>
813        </t>
814        <t>Cache directives &MUST; be passed through by a proxy or gateway application,
815          regardless of their significance to that application, since the directives might be
816          applicable to all recipients along the request/response chain. It is not possible to
817          target a directive to a specific cache.</t>
818<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"/>
819  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
820  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
821
822  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
823     / <x:ref>cache-response-directive</x:ref>
824
825  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
826</artwork></figure>
827
828        <section anchor="cache-request-directive" title="Request Cache-Control Directives">
829          <x:anchor-alias value="cache-request-directive" />
830
831          <figure>
832            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
833  <x:ref>cache-request-directive</x:ref> =
834       "no-cache"
835     / "no-store"
836     / "max-age" "=" <x:ref>delta-seconds</x:ref>
837     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
838     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
839     / "no-transform"
840     / "only-if-cached"
841     / <x:ref>cache-extension</x:ref>
842</artwork>
843          </figure>
844
845
846          <t>
847            <iref item="Cache Directives" primary="true" subitem="no-cache" />
848            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
849              <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
850                used to satisfy the request without successful validation on the origin server. </t>
851            </list>
852          </t>
853          <t>
854            <iref item="Cache Directives" primary="true" subitem="no-store" />
855            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
856              <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
857                of either this request or any response to it. This directive applies to both
858                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
859                cache &MUST-NOT; intentionally store the information in non-volatile storage,
860                and &MUST; make a best-effort attempt to remove the information from volatile
861                storage as promptly as possible after forwarding it.</t>
862              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
863                particular, malicious or compromised caches might not recognize or obey this
864                directive, and communications networks may be vulnerable to eavesdropping.</t>
865            </list>
866          </t>
867          <t>
868            <iref item="Cache Directives" primary="true" subitem="max-age" />
869            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
870              <t>The max-age request directive indicates that the client is willing to accept a
871                response whose age is no greater than the specified time in seconds. Unless
872                max-stale directive is also included, the client is not willing to accept a stale
873                response.</t>
874            </list>
875          </t>
876          <t>
877            <iref item="Cache Directives" primary="true" subitem="max-stale" />
878            <iref item="max-stale" primary="true" subitem="Cache Directive" /> max-stale <list>
879              <t>The max-stale request directive indicates that the client is willing to accept a
880                response that has exceeded its expiration time. If max-stale is assigned a value,
881                then the client is willing to accept a response that has exceeded its expiration
882                time by no more than the specified number of seconds. If no value is assigned to
883                max-stale, then the client is willing to accept a stale response of any age. <cref>of any staleness? -- mnot </cref></t>
884            </list>
885          </t>
886          <t>
887            <iref item="Cache Directives" primary="true" subitem="min-fresh" />
888            <iref item="min-fresh" primary="true" subitem="Cache Directive" /> min-fresh <list>
889              <t>The min-fresh request directive indicates that the client is willing to accept a
890                response whose freshness lifetime is no less than its current age plus the specified
891                time in seconds. That is, the client wants a response that will still be fresh for
892                at least the specified number of seconds.</t>
893            </list>
894          </t>
895          <t>
896            <iref item="Cache Directives" primary="true" subitem="no-transform" />
897            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
898              <t>The no-transform request directive indicates that an intermediate cache or proxy
899                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
900                headers, nor the request entity-body.</t>
901            </list>
902          </t>
903
904          <t>
905            <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
906            <iref item="only-if-cached" primary="true" subitem="Cache Directive" /> only-if-cached <list>
907              <t>The only-if-cached request directive indicates that the client only wishes to
908                return a stored response. If it receives this directive, a cache &SHOULD; either
909                respond using a stored response that is consistent with the other constraints of the
910                request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
911                being operated as a unified system with good internal connectivity, such a request
912                &MAY; be forwarded within that group of caches.</t>
913            </list>
914          </t>
915        </section>
916
917        <section anchor="cache-response-directive" title="Response Cache-Control Directives">
918          <x:anchor-alias value="cache-response-directive" />
919
920          <figure>
921            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
922  <x:ref>cache-response-directive</x:ref> =
923       "public"
924     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
925     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
926     / "no-store"
927     / "no-transform"
928     / "must-revalidate"
929     / "proxy-revalidate"
930     / "max-age" "=" <x:ref>delta-seconds</x:ref>
931     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
932     / <x:ref>cache-extension</x:ref>
933</artwork>
934          </figure>
935
936          <t>
937            <iref item="Cache Directives" primary="true" subitem="public" />
938            <iref item="public" primary="true" subitem="Cache Directive" /> public <list>
939              <t>The public response directive indicates that the response &MAY; be cached, even
940                if it would normally be non-cacheable or cacheable only within a non-shared cache.
941                (See also Authorization, &header-authorization;, for additional details.) </t>
942            </list>
943          </t>
944
945          <t>
946            <iref item="Cache Directives" primary="true" subitem="private" />
947            <iref item="private" primary="true" subitem="Cache Directive" /> private <list>
948              <t>The private response directive indicates that the response message is intended for
949                a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
950                cache &MAY; store the response.</t>
951              <t>If the private response directive specifies one or more field-names, this
952                requirement is limited to the field-values associated with the listed response
953                headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
954                cache, whereas the remainder of the response message &MAY; be.</t>
955              <t>
956                <x:h>Note:</x:h> This usage of the word private only controls where the response may
957                be stored, and cannot ensure the privacy of the message content.</t>
958            </list>
959          </t>
960          <t>
961            <iref item="Cache Directives" primary="true" subitem="no-cache" />
962            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
963              <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
964                satisfy a subsequent request without successful validation on the origin server.
965                This allows an origin server to prevent caching even by caches that have been
966                configured to return stale responses.</t>
967              <t>If the no-cache response directive specifies one or more field-names, this
968                requirement is limited to the field-values assosicated with the listed response
969                headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
970                to a subsequent request without successful validation on the origin server. This
971                allows an origin server to prevent the re-use of certain header fields in a
972                response, while still allowing caching of the rest of the response.</t>
973              <t>
974                <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
975              </t>
976            </list>
977          </t>
978
979          <t>
980            <iref item="Cache Directives" primary="true" subitem="no-store" />
981            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
982              <t>The no-store response directive indicates that a cache &MUST-NOT; store any
983                part of either the immediate request or response. This directive applies to both
984                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
985                cache &MUST-NOT; intentionally store the information in non-volatile storage,
986                and &MUST; make a best-effort attempt to remove the information from volatile
987                storage as promptly as possible after forwarding it.</t>
988              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
989                particular, malicious or compromised caches might not recognize or obey this
990                directive, and communications networks may be vulnerable to eavesdropping.</t>
991            </list>
992          </t>
993          <t>
994            <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
995            <iref item="must-revalidate" primary="true" subitem="Cache Directive" /> must-revalidate <list>
996              <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
997               used to satisfy subsequent requests without successful validation on 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          freshness 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. Likewise, the s-maxage directive overrides Expires in shared caches.</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          determines, while the response is fresh, whether a cache is permitted to use the
1155          response to reply to a subsequent request without validation; see <xref 
1156          target="caching.negotiated.responses" />.</t>
1157        <t>In uncacheable or stale responses, the Vary field value advises the user agent about
1158          the criteria that were used to select the representation.</t>
1159<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1160  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1161  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1162</artwork></figure>
1163        <t>The set of header fields named by the Vary field value is known as the selecting
1164          request-headers.</t>
1165        <t>Servers &SHOULD; include a Vary header field with any cacheable response that is
1166          subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1167          requests on that resource and informs the user agent about the presence of negotiation on
1168          that resource. A server &MAY; include a Vary header field with a non-cacheable
1169          response that is subject to server-driven negotiation, since this might provide the user
1170          agent with useful information about the dimensions over which the response varies at the
1171          time of the response.</t>
1172        <t>A Vary field value of "*" signals that unspecified parameters not limited to the
1173          request-headers (e.g., the network address of the client), play a role in the selection of
1174          the response representation; therefore, a cache cannot determine whether this response is
1175          appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1176          it may only be generated by an origin server.</t>
1177        <t>The field-names given are not limited to the set of standard request-header fields
1178          defined by this specification. Field names are case-insensitive.</t>
1179      </section>
1180
1181      <section anchor="header.warning" title="Warning">
1182        <iref item="Warning header" primary="true" x:for-anchor="" />
1183        <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1184        <x:anchor-alias value="Warning"/>
1185        <x:anchor-alias value="Warning-v"/>
1186        <x:anchor-alias value="warning-value"/>
1187        <x:anchor-alias value="warn-agent"/>
1188        <x:anchor-alias value="warn-code"/>
1189        <x:anchor-alias value="warn-date"/>
1190        <x:anchor-alias value="warn-text"/>
1191        <t>The general-header field "Warning" is used to carry additional information about the status
1192          or transformation of a message which might not be reflected in the message. This
1193          information is typically used to warn about possible incorrectness introduced by caching
1194          operations or transformations applied to the entity body of the message.</t>
1195        <t>Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1196          warning, rather than an error status code, distinguish these responses from true failures.</t>
1197
1198        <t>Warning headers can in general be applied to any message, however some warn-codes are
1199          specific to caches and can only be applied to response messages.</t>
1200
1201<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"/>
1202  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1203  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1204 
1205  <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>
1206                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1207 
1208  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1209  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1210                  ; the name or pseudonym of the server adding
1211                  ; the Warning header, for use in debugging
1212  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1213  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1214</artwork></figure>
1215
1216        <t>Multiple warnings &MAY; be attached to a response (either by the origin server or by
1217          a cache), including multiple warnings with the same code number. For example, a server
1218          might provide the same warning with texts in both English and Basque.</t>
1219        <t>When this occurs, the user agent &SHOULD; inform the user of as many of them as
1220          possible, in the order that they appear in the response. If it is not possible to inform
1221          the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
1222            <list style="symbols">
1223            <t>Warnings that appear early in the response take priority over those appearing later
1224              in the response.</t>
1225
1226            <t>Warnings in the user's preferred character set take priority over warnings in other
1227              character sets but with identical warn-codes and warn-agents.</t>
1228          </list>
1229        </t>
1230        <t>Systems that generate multiple Warning headers &SHOULD; order them with this user
1231          agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1232          Warning headers.</t>
1233        <t>Warnings are assigned three digit warn-codes. The first digit indicates whether the
1234          Warning is required to be deleted from a stored response after validation: <list
1235            style="symbols">
1236            <t>1xx Warnings that describe the freshness or validation status of the response, and so
1237              &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
1238              except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
1239            <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1240              not rectified by a validation (for example, a lossy compression of the entity bodies)
1241              and which &MUST-NOT; be deleted by caches after validation, unless a full response is
1242              returned, in which case they &MUST; be.</t>
1243          </list>
1244        </t>
1245        <t>The warn-text &SHOULD; be in a natural language and character set that is most likely
1246          to be intelligible to the human user receiving the response. This decision can be based on
1247          any available knowledge, such as the location of the cache or user, the Accept-Language
1248          field in a request, the Content-Language field in a response, etc. The default language is
1249          English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).</t>
1250        <t>If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
1251          warn-text using the method described in <xref target="RFC2047" />.</t>
1252        <t>If an implementation sends a message with one or more Warning headers to a receiver whose
1253          version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1254          warn-date that matches the Date header in the message.</t>
1255        <t>If an implementation receives a message with a warning-value that includes a warn-date,
1256          and that warn-date is different from the Date value in the response, then that
1257          warning-value &MUST; be deleted from the message before storing, forwarding, or using
1258          it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1259          warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1260          well.</t>
1261        <t>The following warn-codes are defined by this specification, each with a recommended
1262          warn-text in English, and a description of its meaning.</t>
1263        <t>110 Response is stale <list>
1264            <t>&SHOULD; be included whenever the returned response is stale.</t>
1265          </list>
1266        </t>
1267        <t>111 Revalidation failed <list>
1268            <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1269              validate the response failed, due to an inability to reach the server.</t>
1270          </list>
1271        </t>
1272        <t>112 Disconnected operation <list>
1273            <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1274              the network for a period of time.</t>
1275          </list>
1276        </t>
1277        <t>113 Heuristic expiration <list>
1278            <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1279              greater than 24 hours and the response's age is greater than 24 hours.</t>
1280          </list>
1281        </t>
1282        <t>199 Miscellaneous warning <list>
1283            <t>The warning text &MAY; include arbitrary information to be presented to a human
1284              user, or logged. A system receiving this warning &MUST-NOT; take any automated
1285              action, besides presenting the warning to the user.</t>
1286          </list>
1287        </t>
1288        <t>214 Transformation applied <list>
1289            <t>&MUST; be added by an intermediate cache or proxy if it applies any
1290              transformation changing the content-coding (as specified in the Content-Encoding
1291              header) or media-type (as specified in the Content-Type header) of the response, or
1292              the entity-body of the response, unless this Warning code already appears in the
1293              response.</t>
1294          </list>
1295        </t>
1296        <t>299 Miscellaneous persistent warning <list>
1297            <t>The warning text &MAY; include arbitrary information to be presented to a human
1298              user, or logged. A system receiving this warning &MUST-NOT; take any automated
1299              action.</t>
1300          </list>
1301        </t>
1302      </section>
1303
1304    </section>
1305
1306
1307    <section anchor="history.lists" title="History Lists">
1308      <t>User agents often have history mechanisms, such as "Back" buttons and history lists, which
1309        can be used to redisplay an entity retrieved earlier in a session.</t>
1310      <t>History mechanisms and caches are different. In particular history mechanisms
1311        &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1312        history mechanism is meant to show exactly what the user saw at the time when the resource
1313        was retrieved.</t>
1314      <t>By default, an expiration time does not apply to history mechanisms. If the entity is still
1315        in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1316        unless the user has specifically configured the agent to refresh expired history documents.</t>
1317      <t>This is not to be construed to prohibit the history mechanism from telling the user that a
1318        view might be stale. <list>
1319          <t>
1320            <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1321            stale resources, this will tend to force service authors to avoid using HTTP expiration
1322            controls and cache controls when they would otherwise like to. Service authors may
1323            consider it important that users not be presented with error messages or warning
1324            messages when they use navigation controls (such as BACK) to view previously fetched
1325            resources. Even though sometimes such resources ought not be cached, or ought to expire
1326            quickly, user interface considerations may force service authors to resort to other
1327            means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1328            of improperly functioning history mechanisms.</t>
1329        </list>
1330      </t>
1331    </section>
1332
1333
1334    <section anchor="IANA.considerations" title="IANA Considerations">
1335      <section anchor="message.header.registration" title="Message Header Registration">
1336        <t>The Message Header Registry located at <eref
1337            target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1338          should be updated with the permanent registrations below (see <xref target="RFC3864" />):</t>
1339        <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1340        <texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1341          <ttcol>Header Field Name</ttcol>
1342          <ttcol>Protocol</ttcol>
1343          <ttcol>Status</ttcol>
1344          <ttcol>Reference</ttcol>
1345
1346          <c>Age</c>
1347          <c>http</c>
1348          <c>standard</c>
1349          <c>
1350            <xref target="header.age" />
1351          </c>
1352          <c>Cache-Control</c>
1353          <c>http</c>
1354          <c>standard</c>
1355          <c>
1356            <xref target="header.cache-control" />
1357          </c>
1358          <c>Expires</c>
1359          <c>http</c>
1360          <c>standard</c>
1361          <c>
1362            <xref target="header.expires" />
1363          </c>
1364          <c>Pragma</c>
1365          <c>http</c>
1366          <c>standard</c>
1367          <c>
1368            <xref target="header.pragma" />
1369          </c>
1370          <c>Vary</c>
1371          <c>http</c>
1372          <c>standard</c>
1373          <c>
1374            <xref target="header.vary" />
1375          </c>
1376          <c>Warning</c>
1377          <c>http</c>
1378          <c>standard</c>
1379          <c>
1380            <xref target="header.warning" />
1381          </c>
1382        </texttable>
1383        <!--(END)-->
1384        <t>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</t>
1385      </section>
1386    </section>
1387
1388    <section anchor="security.considerations" title="Security Considerations">
1389      <t>Caches expose additional potential vulnerabilities, since the contents of the cache
1390        represent an attractive target for malicious exploitation. Because cache contents persist
1391        after an HTTP request is complete, an attack on the cache can reveal information long after
1392        a user believes that the information has been removed from the network. Therefore, cache
1393        contents should be protected as sensitive information.</t>
1394    </section>
1395
1396    <section anchor="ack" title="Acknowledgments">
1397      <t>Much of the content and presentation of the caching design is due to suggestions and
1398        comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1399        and Larry Masinter.</t>
1400    </section>
1401  </middle>
1402
1403  <back>
1404    <references title="Normative References">
1405
1406      <reference anchor="ISO-8859-1">
1407        <front>
1408          <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1409            1: Latin alphabet No. 1 </title>
1410          <author>
1411            <organization>International Organization for Standardization</organization>
1412          </author>
1413          <date year="1998" />
1414        </front>
1415        <seriesInfo name="ISO/IEC" value="8859-1:1998" />
1416      </reference>
1417
1418      <reference anchor="Part1">
1419        <front>
1420          <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1421          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1422            <organization abbrev="Day Software">Day Software</organization>
1423            <address><email>fielding@gbiv.com</email></address>
1424          </author>
1425          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1426            <organization>One Laptop per Child</organization>
1427            <address><email>jg@laptop.org</email></address>
1428          </author>
1429          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1430            <organization abbrev="HP">Hewlett-Packard Company</organization>
1431            <address><email>JeffMogul@acm.org</email></address>
1432          </author>
1433          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1434            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1435            <address><email>henrikn@microsoft.com</email></address>
1436          </author>
1437          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1438            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1439            <address><email>LMM@acm.org</email></address>
1440          </author>
1441          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1442            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1443            <address><email>paulle@microsoft.com</email></address>
1444          </author>
1445          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1446            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1447            <address><email>timbl@w3.org</email></address>
1448          </author>
1449          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1450            <organization abbrev="W3C">World Wide Web Consortium</organization>
1451            <address><email>ylafon@w3.org</email></address>
1452          </author>
1453          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1454            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1455            <address><email>julian.reschke@greenbytes.de</email></address>
1456          </author>
1457          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1458        </front>
1459        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1460        <x:source basename="p1-messaging" href="p1-messaging.xml" />
1461      </reference>
1462
1463      <reference anchor="Part2">
1464        <front>
1465          <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1466          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1467            <organization abbrev="Day Software">Day Software</organization>
1468            <address><email>fielding@gbiv.com</email></address>
1469          </author>
1470          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1471            <organization>One Laptop per Child</organization>
1472            <address><email>jg@laptop.org</email></address>
1473          </author>
1474          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1475            <organization abbrev="HP">Hewlett-Packard Company</organization>
1476            <address><email>JeffMogul@acm.org</email></address>
1477          </author>
1478          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1479            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1480            <address><email>henrikn@microsoft.com</email></address>
1481          </author>
1482          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1483            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1484            <address><email>LMM@acm.org</email></address>
1485          </author>
1486          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1487            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1488            <address><email>paulle@microsoft.com</email></address>
1489          </author>
1490          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1491            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1492            <address><email>timbl@w3.org</email></address>
1493          </author>
1494          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1495            <organization abbrev="W3C">World Wide Web Consortium</organization>
1496            <address><email>ylafon@w3.org</email></address>
1497          </author>
1498          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1499            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1500            <address><email>julian.reschke@greenbytes.de</email></address>
1501          </author>
1502          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1503        </front>
1504        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1505        <x:source basename="p2-semantics" href="p2-semantics.xml" />
1506      </reference>
1507
1508      <reference anchor="Part3">
1509        <front>
1510          <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1511          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1512            <organization abbrev="Day Software">Day Software</organization>
1513            <address><email>fielding@gbiv.com</email></address>
1514          </author>
1515          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1516            <organization>One Laptop per Child</organization>
1517            <address><email>jg@laptop.org</email></address>
1518          </author>
1519          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1520            <organization abbrev="HP">Hewlett-Packard Company</organization>
1521            <address><email>JeffMogul@acm.org</email></address>
1522          </author>
1523          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1524            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1525            <address><email>henrikn@microsoft.com</email></address>
1526          </author>
1527          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1528            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1529            <address><email>LMM@acm.org</email></address>
1530          </author>
1531          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1532            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1533            <address><email>paulle@microsoft.com</email></address>
1534          </author>
1535          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1536            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1537            <address><email>timbl@w3.org</email></address>
1538          </author>
1539          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1540            <organization abbrev="W3C">World Wide Web Consortium</organization>
1541            <address><email>ylafon@w3.org</email></address>
1542          </author>
1543          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1544            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1545            <address><email>julian.reschke@greenbytes.de</email></address>
1546          </author>
1547          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1548        </front>
1549        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1550        <x:source basename="p3-payload" href="p3-payload.xml" />
1551      </reference>
1552
1553      <reference anchor="Part4">
1554        <front>
1555          <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1556          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1557            <organization abbrev="Day Software">Day Software</organization>
1558            <address><email>fielding@gbiv.com</email></address>
1559          </author>
1560          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1561            <organization>One Laptop per Child</organization>
1562            <address><email>jg@laptop.org</email></address>
1563          </author>
1564          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1565            <organization abbrev="HP">Hewlett-Packard Company</organization>
1566            <address><email>JeffMogul@acm.org</email></address>
1567          </author>
1568          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1569            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1570            <address><email>henrikn@microsoft.com</email></address>
1571          </author>
1572          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1573            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1574            <address><email>LMM@acm.org</email></address>
1575          </author>
1576          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1577            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1578            <address><email>paulle@microsoft.com</email></address>
1579          </author>
1580          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1581            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1582            <address><email>timbl@w3.org</email></address>
1583          </author>
1584          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1585            <organization abbrev="W3C">World Wide Web Consortium</organization>
1586            <address><email>ylafon@w3.org</email></address>
1587          </author>
1588          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1589            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1590            <address><email>julian.reschke@greenbytes.de</email></address>
1591          </author>
1592          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1593        </front>
1594        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1595        <x:source basename="p4-conditional" href="p4-conditional.xml" />
1596      </reference>
1597
1598      <reference anchor="Part5">
1599        <front>
1600          <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1601          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1602            <organization abbrev="Day Software">Day Software</organization>
1603            <address><email>fielding@gbiv.com</email></address>
1604          </author>
1605          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1606            <organization>One Laptop per Child</organization>
1607            <address><email>jg@laptop.org</email></address>
1608          </author>
1609          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1610            <organization abbrev="HP">Hewlett-Packard Company</organization>
1611            <address><email>JeffMogul@acm.org</email></address>
1612          </author>
1613          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1614            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1615            <address><email>henrikn@microsoft.com</email></address>
1616          </author>
1617          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1618            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1619            <address><email>LMM@acm.org</email></address>
1620          </author>
1621          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1622            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1623            <address><email>paulle@microsoft.com</email></address>
1624          </author>
1625          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1626            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1627            <address><email>timbl@w3.org</email></address>
1628          </author>
1629          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1630            <organization abbrev="W3C">World Wide Web Consortium</organization>
1631            <address><email>ylafon@w3.org</email></address>
1632          </author>
1633          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1634            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1635            <address><email>julian.reschke@greenbytes.de</email></address>
1636          </author>
1637          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1638        </front>
1639        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1640        <x:source basename="p5-range" href="p5-range.xml" />
1641      </reference>
1642
1643      <reference anchor="Part7">
1644        <front>
1645          <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1646          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1647            <organization abbrev="Day Software">Day Software</organization>
1648            <address><email>fielding@gbiv.com</email></address>
1649          </author>
1650          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1651            <organization>One Laptop per Child</organization>
1652            <address><email>jg@laptop.org</email></address>
1653          </author>
1654          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1655            <organization abbrev="HP">Hewlett-Packard Company</organization>
1656            <address><email>JeffMogul@acm.org</email></address>
1657          </author>
1658          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1659            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1660            <address><email>henrikn@microsoft.com</email></address>
1661          </author>
1662          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1663            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1664            <address><email>LMM@acm.org</email></address>
1665          </author>
1666          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1667            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1668            <address><email>paulle@microsoft.com</email></address>
1669          </author>
1670          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1671            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1672            <address><email>timbl@w3.org</email></address>
1673          </author>
1674          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1675            <organization abbrev="W3C">World Wide Web Consortium</organization>
1676            <address><email>ylafon@w3.org</email></address>
1677          </author>
1678          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1679            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1680            <address><email>julian.reschke@greenbytes.de</email></address>
1681          </author>
1682          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1683        </front>
1684        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1685        <x:source basename="p7-auth" href="p7-auth.xml" />
1686      </reference>
1687
1688      <reference anchor="RFC2047">
1689        <front>
1690          <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1691            Part Three: Message Header Extensions for Non-ASCII Text</title>
1692          <author fullname="Keith Moore" initials="K." surname="Moore">
1693            <organization>University of Tennessee</organization>
1694            <address><email>moore@cs.utk.edu</email></address>
1695          </author>
1696          <date month="November" year="1996" />
1697        </front>
1698        <seriesInfo name="RFC" value="2047" />
1699      </reference>
1700
1701      <reference anchor="RFC2119">
1702        <front>
1703          <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1704          <author fullname="Scott Bradner" initials="S." surname="Bradner">
1705            <organization>Harvard University</organization>
1706            <address><email>sob@harvard.edu</email></address>
1707          </author>
1708          <date month="March" year="1997" />
1709        </front>
1710        <seriesInfo name="BCP" value="14" />
1711        <seriesInfo name="RFC" value="2119" />
1712      </reference>
1713
1714      <reference anchor="RFC5234">
1715        <front>
1716          <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1717          <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1718            <organization>Brandenburg InternetWorking</organization>
1719            <address>
1720            <postal>
1721            <street>675 Spruce Dr.</street>
1722            <city>Sunnyvale</city>
1723            <region>CA</region>
1724            <code>94086</code>
1725            <country>US</country></postal>
1726            <phone>+1.408.246.8253</phone>
1727            <email>dcrocker@bbiw.net</email></address> 
1728          </author>
1729          <author initials="P." surname="Overell" fullname="Paul Overell">
1730            <organization>THUS plc.</organization>
1731            <address>
1732            <postal>
1733            <street>1/2 Berkeley Square</street>
1734            <street>99 Berkely Street</street>
1735            <city>Glasgow</city>
1736            <code>G3 7HR</code>
1737            <country>UK</country></postal>
1738            <email>paul.overell@thus.net</email></address>
1739          </author>
1740          <date month="January" year="2008"/>
1741        </front>
1742        <seriesInfo name="STD" value="68"/>
1743        <seriesInfo name="RFC" value="5234"/>
1744      </reference>
1745     
1746    </references>
1747
1748    <references title="Informative References">
1749
1750      <reference anchor="RFC1305">
1751        <front>
1752          <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1753          <author fullname="David L. Mills" initials="D." surname="Mills">
1754            <organization>University of Delaware, Electrical Engineering Department</organization>
1755            <address><email>mills@udel.edu</email></address>
1756          </author>
1757          <date month="March" year="1992" />
1758        </front>
1759        <seriesInfo name="RFC" value="1305" />
1760      </reference>
1761
1762      <reference anchor="RFC2616">
1763        <front>
1764          <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1765          <author fullname="R. Fielding" initials="R." surname="Fielding">
1766            <organization>University of California, Irvine</organization>
1767            <address><email>fielding@ics.uci.edu</email></address>
1768          </author>
1769          <author fullname="J. Gettys" initials="J." surname="Gettys">
1770            <organization>W3C</organization>
1771            <address><email>jg@w3.org</email></address>
1772          </author>
1773          <author fullname="J. Mogul" initials="J." surname="Mogul">
1774            <organization>Compaq Computer Corporation</organization>
1775            <address><email>mogul@wrl.dec.com</email></address>
1776          </author>
1777          <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1778            <organization>MIT Laboratory for Computer Science</organization>
1779            <address><email>frystyk@w3.org</email></address>
1780          </author>
1781          <author fullname="L. Masinter" initials="L." surname="Masinter">
1782            <organization>Xerox Corporation</organization>
1783            <address><email>masinter@parc.xerox.com</email></address>
1784          </author>
1785          <author fullname="P. Leach" initials="P." surname="Leach">
1786            <organization>Microsoft Corporation</organization>
1787            <address><email>paulle@microsoft.com</email></address>
1788          </author>
1789          <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1790            <organization>W3C</organization>
1791            <address><email>timbl@w3.org</email></address>
1792          </author>
1793          <date month="June" year="1999" />
1794        </front>
1795        <seriesInfo name="RFC" value="2616" />
1796      </reference>
1797
1798      <reference anchor="RFC3864">
1799        <front>
1800          <title>Registration Procedures for Message Header Fields</title>
1801          <author fullname="G. Klyne" initials="G." surname="Klyne">
1802            <organization>Nine by Nine</organization>
1803            <address><email>GK-IETF@ninebynine.org</email></address>
1804          </author>
1805          <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1806            <organization>BEA Systems</organization>
1807            <address><email>mnot@pobox.com</email></address>
1808          </author>
1809          <author fullname="J. Mogul" initials="J." surname="Mogul">
1810            <organization>HP Labs</organization>
1811            <address><email>JeffMogul@acm.org</email></address>
1812          </author>
1813          <date month="September" year="2004" />
1814        </front>
1815        <seriesInfo name="BCP" value="90" />
1816        <seriesInfo name="RFC" value="3864" />
1817      </reference>
1818
1819    </references>
1820
1821    <section anchor="compatibility" title="Compatibility with Previous Versions">
1822
1823      <section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
1824        <t>A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
1825          this missing case. (Sections <xref format="counter" target="response.cacheability" />,
1826            <xref format="counter" target="header.cache-control" />).</t>
1827        <t>Transfer-coding and message lengths all interact in ways that required fixing exactly
1828          when chunked encoding is used (to allow for transfer encoding that may not be self
1829          delimiting); it was important to straighten out exactly how message lengths are computed.
1830          (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)</t>
1831        <t>Proxies should be able to add Content-Length when appropriate.</t>
1832        <t>Range request responses would become very verbose if all meta-data were always returned;
1833          by allowing the server to only send needed headers in a 206 response, this problem can be
1834          avoided. (<xref target="combining.headers" />)</t>
1835        <t>The Cache-Control: max-age directive was not properly defined for responses.</t>
1836        <t>Warnings could be cached incorrectly, or not updated appropriately. <xref
1837            format="counter" target="expiration.model" />, <xref format="counter"
1838            target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
1839          and <xref format="counter" target="header.warning" />) Warning also needed to be a general
1840          header, as PUT or other methods may have need for it in requests.</t>
1841      </section>
1842
1843      <section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
1844        <t>Clarify denial of service attack avoidance requirement. (<xref
1845            target="invalidation.after.updates.or.deletions" />)</t>
1846      </section>
1847
1848    </section>
1849
1850<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1851<figure>
1852<artwork type="abnf" name="p6-cache.parsed-abnf">
1853<x:ref>Age</x:ref> = "Age:" OWS Age-v
1854<x:ref>Age-v</x:ref> = delta-seconds
1855
1856<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
1857<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
1858 cache-directive ] )
1859
1860<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
1861<x:ref>Expires-v</x:ref> = HTTP-date
1862
1863<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
1864
1865<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1866
1867<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
1868<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1869 pragma-directive ] )
1870
1871<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
1872<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1873 ] ) )
1874
1875<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
1876<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1877 ] )
1878
1879<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
1880<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
1881<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
1882 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1883 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1884 cache-extension
1885<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
1886 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1887 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1888 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1889 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1890 ) / ( "s-maxage=" delta-seconds ) / cache-extension
1891
1892<x:ref>delta-seconds</x:ref> = 1*DIGIT
1893
1894<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
1895
1896<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
1897
1898<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
1899<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
1900<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
1901
1902<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1903
1904<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
1905
1906<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
1907
1908<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
1909<x:ref>warn-code</x:ref> = 3DIGIT
1910<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
1911<x:ref>warn-text</x:ref> = quoted-string
1912<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
1913 ]
1914
1915
1916</artwork>
1917</figure>
1918<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">; Age defined but not used
1919; Cache-Control defined but not used
1920; Expires defined but not used
1921; Pragma defined but not used
1922; Vary defined but not used
1923; Warning defined but not used
1924</artwork></figure></section>
1925
1926    <section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
1927
1928      <section title="Since RFC2616">
1929        <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
1930      </section>
1931
1932      <section title="Since draft-ietf-httpbis-p6-cache-00">
1933        <t>Closed issues: <list style="symbols">
1934            <t>
1935              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer"
1936                (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
1937            <t>
1938              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation
1939              after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
1940            <t>
1941              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and
1942              Informative references"</t>
1943            <t>
1944              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date
1945              reference typo"</t>
1946            <t>
1947              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection
1948              header text"</t>
1949            <t>
1950              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative
1951              references"</t>
1952            <t>
1953              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1
1954              Reference"</t>
1955            <t>
1956              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative
1957              up-to-date references"</t>
1958            <t>
1959              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in
1960              13.2.2"</t>
1961          </list>
1962        </t>
1963        <t>Other changes: <list style="symbols">
1964            <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
1965                target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
1966          </list>
1967        </t>
1968      </section>
1969
1970      <section title="Since draft-ietf-httpbis-p6-cache-01">
1971        <t>Closed issues: <list style="symbols">
1972            <t>
1973              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not
1974              used"</t>
1975          </list>
1976        </t>
1977        <t>Other changes: <list style="symbols">
1978            <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
1979              on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
1980            <t>Add explicit references to BNF syntax and rules imported from other parts of the
1981              specification.</t>
1982          </list>
1983        </t>
1984      </section>
1985
1986      <section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
1987        <t>Ongoing work on IANA Message Header Registration (<eref
1988            target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40" />): <list style="symbols">
1989            <t>Reference RFC 3984, and update header registrations for headers defined in this
1990              document.</t>
1991          </list>
1992        </t>
1993      </section>
1994
1995      <section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
1996        <t>Closed issues: <list style="symbols">
1997            <t>
1998              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header
1999              classification"</t>
2000          </list>
2001        </t>
2002      </section>
2003
2004      <section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2005      <t>
2006      </t>
2007      </section>
2008
2009      <section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2010      <t>
2011        This is a total rewrite.
2012      </t>
2013      </section>
2014
2015    </section>
2016  </back>
2017</rfc>
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