source: draft-ietf-httpbis/latest/p6-cache.xml @ 538

Last change on this file since 538 was 538, checked in by julian.reschke@…, 11 years ago

Move P6 from ./latest-roy to ./latest

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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.05" />.</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 &MUST-NOT; store a response to any request, unless: <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 shared, and</t>
397            <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
398             the cache is shared (unless the "public" directive is present; see <xref
399                target="header.cache-control" />), and</t>           
400            <t>the cache understands partial responses, if the response is partial or incomplete
401              (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
402          </list>
403        </t>
404        <t>Note that in normal operation, most caches will not store a response that has neither a
405          cache validator nor an explicit expiration time, as such responses are not usually
406          useful to store. However, caches are not prohibited from storing such responses.</t>
407
408        <section anchor="errors.or.incomplete.response.cache.behavior"
409          title="Storing Partial and Incomplete Responses">
410          <t>A cache that receives an incomplete response (for example, with fewer bytes of data
411            than specified in a Content-Length header) can store the response, but &MUST; 
412            treat it as a partial response &partial;. Partial responses
413            can be combined as described in &combining-byte-ranges;; the result might be a
414            full response or might still be partial. A cache &MUST-NOT; return a partial
415            response to a client without explicitly marking it as such using the 206 (Partial
416            Content) status code.</t>
417          <t>A cache that does not support the Range and Content-Range headers &MUST-NOT; store
418            incomplete or partial responses.</t>
419        </section>
420
421      </section>
422
423
424      <section anchor="constructing.responses.from.caches"
425        title="Constructing Responses from Caches">
426        <t>For a presented request, a cache &MUST-NOT; return a stored response, unless:
427          <list style="symbols">
428            <t>The presented Request-URI and that of the stored response match (see
429              <cref>TBD</cref>), and</t>
430            <t>the request method associated with the stored response allows it to be
431              used for the presented request, and</t>
432            <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
433                target="caching.negotiated.responses" />), and</t>
434            <t>the presented request and stored response are free from directives that would prevent
435              its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"
436             />), and</t>
437            <t>the stored response is either:
438            <list style="symbols">
439                <t>fresh (see <xref target="expiration.model" />), or</t>
440                <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
441                <t>successfully validated (see <xref target="validation.model" />).</t>
442            </list></t>
443          </list>
444        </t>
445        <t><cref>TODO: define method cacheability for GET, HEAD and POST in p2-semantics.</cref></t>
446        <t>When a stored response is used to satisfy a request, caches &MUST; include a
447          single Age header field <xref target="header.age" /> in the response with a value equal to the stored response's
448          current_age; see <xref target="age.calculations" />.
449                <cref>DISCUSS: this currently includes successfully validated responses.</cref></t>
450        <t>Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
451          the origin server; i.e., A cache must not reply to such a request before having forwarded the request and having received a
452   corresponding response.</t>
453                <t>Also, note that unsafe requests might invalidate already stored responses; see
454            <xref target="invalidation.after.updates.or.deletions" />.</t>
455        <t>Caches &MUST; use the most recent response (as determined by the Date header) when
456          more than one suitable response is stored. They can also forward a request with
457          "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
458          use.</t>
459        <t>
460          <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in
461            p1</cref>
462        </t>
463      </section>
464
465
466
467
468      <section anchor="expiration.model" title="Freshness Model">
469
470        <t>When a response is "fresh" in the cache, it can be used to satisfy subsequent
471          requests without contacting the origin server, thereby improving efficiency.</t>
472        <t>The primary mechanism for determining freshness is for an origin server to provide an
473          explicit expiration time in the future, using either the Expires header (<xref
474            target="header.expires" />) or the max-age response cache directive (<xref
475            target="cache-response-directive" />). Generally, origin servers will assign future
476          explicit expiration times to responses in the belief that the entity is not likely to
477          change in a semantically significant way before the expiration time is reached.</t>
478        <t>If an origin server wishes to force a cache to validate every request, it can
479          assign an explicit expiration time in the past. This means that the response is always
480          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>
481        <t>Since origin servers do not always provide explicit expiration times, HTTP caches may
482          also assign heuristic expiration times when they are not specified, employing algorithms that
483          use other header values (such as the Last-Modified time) to estimate a plausible
484          expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
485          impose worst-case constraints on their results.</t>
486        <t>The calculation to determine if a response is fresh is:</t>
487        <figure>
488          <artwork type="code">
489   response_is_fresh = (freshness_lifetime &gt; current_age)
490</artwork>
491        </figure>
492
493        <t>The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
494          the current_age is defined in <xref target="age.calculations" />.</t>
495        <t>Additionally, clients may need to influence freshness calculation. They can do this using
496          several request cache directives, with the effect of either increasing or loosening
497          constraints on freshness. See <xref target="cache-request-directive" />.</t>
498        <t>
499          <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
500            freshness.</cref>
501        </t>
502        <t>Note that freshness applies only to cache operation; it cannot be used to force a user agent
503        to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
504          the difference between caches and history mechanisms.</t>
505
506        <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
507          <t>A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
508            response by using the first match of: <list style="symbols">
509              <t>If the cache is shared and the s-maxage response cache directive (<xref
510                  target="cache-response-directive" />) is present, use its value, or</t>
511              <t>If the max-age response cache directive (<xref target="cache-response-directive"
512                 />) is present, use its value, or</t>
513              <t>If the Expires response header (<xref target="header.expires" />) is present, use
514                its value minus the value of the Date response header, or</t>
515              <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
516                may be used; see <xref target="heuristic.freshness" />.</t>
517            </list>
518          </t>
519          <t>Note that this calculation is not vulnerable to clock skew, since all of the
520            information comes from the origin server.</t>
521
522          <section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
523            <t>If no explicit expiration time is present in a stored response that has a status code
524              of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
525              calculated. Heuristics &MUST-NOT; be used for other response status codes. </t>
526            <t> When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
527              attach a Warning header with a 113 warn-code to the response if its current_age is
528              more than 24 hours and such a warning is not already present.</t>
529            <t>Also, if the response has a Last-Modified header (&header-last-modified;), the
530              heuristic expiration value &SHOULD; be no more than some fraction of the interval
531              since that time. A typical setting of this fraction might be 10%.</t>
532            <t>
533              <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
534            </t>
535          </section>
536        </section>
537
538        <section anchor="age.calculations" title="Calculating Age">
539          <t>HTTP/1.1 uses the Age response-header to convey the estimated age of the response
540            message when obtained from a cache. The Age field value is the cache's estimate of the
541            amount of time since the response was generated or validated by the origin server. In
542            essence, the Age value is the sum of the time that the response has been resident in
543            each of the caches along the path from the origin server, plus the amount of time it has
544            been in transit along network paths.</t>
545          <t>The term "age_value" denotes the value of the Age header, in a form appropriate for
546            arithmetic operations.</t>
547          <t>HTTP/1.1 requires origin servers to send a Date header, if possible, with every
548            response, giving the time at which the response was generated (see &header-date;).
549            The term "date_value" denotes the value of the Date header, in a form appropriate for
550            arithmetic operations.</t>
551          <t>The term "now" means "the current value of the clock at the host performing the
552            calculation." Hosts that use HTTP, but especially hosts running origin servers and
553            caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
554            synchronize their clocks to a globally accurate time standard.</t>
555          <t>A response's age can be calculated in two entirely independent ways: <list
556              style="numbers">
557              <t>now minus date_value, if the local clock is reasonably well synchronized to the
558                origin server's clock. If the result is negative, the result is replaced by zero.</t>
559
560              <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
561            </list>
562          </t>
563          <t>These are combined as</t>
564          <figure>
565            <artwork type="code">
566    corrected_received_age = max(now - date_value, age_value)
567</artwork>
568          </figure>
569          <t>When an Age value is received, it &MUST; be interpreted relative to the time the
570            request was initiated, not the time that the response was received.</t>
571          <figure>
572            <artwork type="code">
573   corrected_initial_age = corrected_received_age
574                         + (now - request_time)
575</artwork>
576          </figure>
577          <t>where "request_time" is the time (according to the local clock) when the request that
578            elicited this response was sent.</t>
579          <t>The current_age of a stored response can then be calculated by adding the amount of
580            time (in seconds) since the stored response was last validated by the origin server to
581            the corrected_initial_age.</t>
582          <t>In summary:</t>
583          <figure>
584            <artwork type="code">
585   age_value     - Age header field-value received with the response
586   date_value    - Date header field-value received with the response
587   request_time  - local time when the cache made the request
588                   resulting in the stored response
589   response_time - local time when the cache received the response
590   now           - current local time
591
592   apparent_age = max(0, response_time - date_value);
593   corrected_received_age = max(apparent_age, age_value);
594   response_delay = response_time - request_time;
595   corrected_initial_age = corrected_received_age + response_delay;
596   resident_time = now - response_time;
597   current_age   = corrected_initial_age + resident_time;
598</artwork>
599          </figure>
600        </section>
601
602        <section anchor="serving.stale.responses" title="Serving Stale Responses">
603          <t>A "stale" response is one that either has explicit expiry information, or is allowed to
604            have heuristic expiry calculated, but is not fresh according to the calculations in
605              <xref target="expiration.model" />.</t>
606          <t>Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
607            in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
608            "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
609            "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"
610             />). </t>
611          <t>Caches &SHOULD-NOT; return stale responses unless they are
612          disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
613          or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />)..</t>
614          <t>Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
615              target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
616              the cache is disconnected.</t>
617          <t>If a cache receives a first-hand response (either an entire response, or a 304 (Not
618            Modified) response) that it would normally forward to the requesting client, and the
619            received response is no longer fresh, the cache &SHOULD; forward it to the
620            requesting client without adding a new Warning (but without removing any existing
621            Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
622            that response became stale in transit.</t>
623        </section>
624      </section>
625
626
627      <section anchor="validation.model" title="Validation Model">
628        <t>Checking with the origin server to see if a stale or otherwise unusable cached response
629        can be reused is called "validating" or "revalidating." Doing so potentially avoids
630        the overhead of retransmitting the response body when the stored response is valid.</t>
631        <t>HTTP's conditional request mechanism &conditional; is used for this purpose. When a stored
632          response includes one or more validators, such as the field values of an ETag or
633          Last-Modified header field, then a validating request &SHOULD; be made conditional
634          to those field values.</t>
635         <t>A 304 (Not Modified) response status code indicates that the stored
636          response can be updated and reused; see <xref target="combining.headers"/>.</t>
637        <t>If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the
638        request and replace the stored response. <cref>Should there be a requirement here?</cref></t>
639        <t>If a cache receives a 5xx response while attempting to validate a response, it &MAY;
640          either forward this response to the requesting client, or act as if the server failed to
641          respond. In the latter case, it &MAY; return a previously stored response (which &SHOULD; include the
642          111 warn-code; see <xref target="header.warning"/>) unless the
643          stored response includes the "must-revalidate" cache directive (see <xref
644            target="serving.stale.responses" />).</t>
645      </section>
646
647      <section anchor="invalidation.after.updates.or.deletions"
648        title="Request Methods that Invalidate">
649        <t>Because unsafe methods (&safe-methods;) have the potential for changing state on the
650          origin server, intervening caches can use them to keep their contents
651          up-to-date.</t>
652        <t>The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
653          as the Location and Content-Location headers (if present): <list style="symbols">
654            <t>PUT</t>
655            <t>DELETE</t>
656            <t>POST</t>
657          </list>
658        </t>
659        <t>An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
660          be performed if the host part of that URI differs from the host part in the Request-URI.
661          This helps prevent denial of service attacks.</t>
662        <t>
663          <cref>TODO: "host part" needs to be specified better.</cref>
664        </t>
665        <t>A cache that passes through requests for methods it does not understand &SHOULD;
666          invalidate the Request-URI.</t>
667        <t>Here, "invalidate" means that the cache will either remove all stored responses related
668          to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
669          before they can be returned in response to a subsequent request.</t>
670        <t>Note that this does not guarantee that all appropriate responses are invalidated. For
671          example, the request that caused the change at the origin server might not have gone
672          through the cache where a response is stored.</t>
673        <t>
674          <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
675        </t>
676      </section>
677
678
679
680
681
682      <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
683        <t>Use of server-driven content negotiation (&server-driven-negotiation;) alters
684          the conditions under which a cache can use the response for subsequent
685          requests.</t>
686        <t>When a cache receives a request that can be satisfied by a stored response
687          that includes a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that response unless
688          all of the selecting request-headers in the presented request match the corresponding
689          stored request-headers from the original request.</t>
690        <t>The selecting request-headers from two requests are defined to match if and only if the
691          selecting request-headers in the first request can be transformed to the selecting
692          request-headers in the second request by adding or removing linear white space
693          <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
694          combining multiple message-header fields with the same field name following the rules
695          about message headers in &message-headers;. <cref>DISCUSS: header-specific canonicalisation</cref></t>
696        <t>A Vary header field-value of "*" always fails to match, and subsequent requests to that
697          resource can only be properly interpreted by the origin server.</t>
698        <t>If no stored response matches, the cache &MAY; forward the presented request to the origin
699          server in a conditional request, and &SHOULD; include all ETags stored with
700          potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
701          includes an entity tag or Content-Location that indicates the entity to be used, that
702          cached response &MUST; be used to satisfy the presented request, and &SHOULD;
703          be used to update the corresponding stored response; see <xref target="combining.headers"/>.</t>
704        <t>If any of the stored responses contains only partial content, its entity-tag &SHOULD-NOT; 
705          be included in the If-None-Match header field unless the request is for a range that would
706          be fully satisfied by that stored response.</t>
707        <t>If a cache receives a successful response whose Content-Location field matches that of an
708          existing stored response for the same Request-URI, whose entity-tag differs from that of
709          the existing stored response, and whose Date is more recent than that of the existing
710          response, the existing response &SHOULD-NOT; be returned in response to future
711          requests and &SHOULD; be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref></t>
712      </section>
713
714
715      <section anchor="combining.headers" title="Combining Responses">
716        <t>When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
717          it needs to update the stored response with the new one, so that the updated response can
718          be sent to the client.</t>
719        <t>If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
720          the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
721          Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
722          the stored response with the updated entity-body received in the response and use the
723          result as the updated entity-body (see &combining-byte-ranges;).</t>
724        <t>The stored response headers are used for the updated response, except that <list
725            style="symbols">
726            <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
727              &MUST; be deleted from the stored response and the forwarded response.</t>
728            <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
729              response and the forwarded response.</t>
730            <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
731              headers from the stored response.</t>
732          </list>
733        </t>
734        <t>A cache &MUST; also replace any stored headers with corresponding headers received in the
735          incoming response, except for Warning headers as described immediately above. If a header
736          field-name in the incoming response matches more than one header in the stored response,
737          all such old headers &MUST; be replaced. It &MAY; store the combined
738        entity-body.</t>
739        <t><cref>ISSUE: discuss how to handle HEAD updates</cref></t>
740      </section>
741
742    </section>
743
744
745
746
747    <section anchor="header.fields" title="Header Field Definitions">
748      <t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
749      <t>For entity-header fields, both sender and recipient refer to either the client or the
750        server, depending on who sends and who receives the entity.</t>
751
752      <section anchor="header.age" title="Age">
753        <iref item="Age header" primary="true" x:for-anchor="" />
754        <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
755        <x:anchor-alias value="Age"/>
756        <x:anchor-alias value="Age-v"/>
757        <x:anchor-alias value="age-value"/>
758        <t>      The response-header field "Age" conveys the sender's estimate of the amount of time since
759          the response (or its validation) was generated at the origin server. Age values are
760          calculated as specified in <xref target="age.calculations" />.</t>
761<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
762  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
763  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
764</artwork></figure>
765        <t anchor="rule.delta-seconds">
766          <x:anchor-alias value="delta-seconds" /> Age field-values are non-negative decimal
767          integers, representing time in seconds.</t>
768        <figure>
769          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
770  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
771</artwork>
772        </figure>
773        <t>If a cache receives a value larger than the largest positive integer it can represent, or
774          if any of its age calculations overflows, it &MUST; transmit an Age header with a
775          field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
776          of at least 31 bits of range.</t>
777        <t>The presence of an Age header field in a response implies that a response is not
778          first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
779          Age header field.</t>
780      </section>
781
782      <section anchor="header.cache-control" title="Cache-Control">
783        <iref item="Cache-Control header" primary="true" x:for-anchor="" />
784        <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
785        <x:anchor-alias value="Cache-Control"/>
786        <x:anchor-alias value="Cache-Control-v"/>
787        <x:anchor-alias value="cache-directive"/>
788        <x:anchor-alias value="cache-extension"/>
789        <x:anchor-alias value="cache-request-directive"/>
790        <x:anchor-alias value="cache-response-directive"/>
791        <t>The general-header field "Cache-Control" is used to specify directives that &MUST; be
792          obeyed by all caches along the request/response chain. The directives specify behavior
793          intended to prevent caches from adversely interfering with the request or response. Cache
794          directives are unidirectional in that the presence of a directive in a request does not
795          imply that the same directive is to be given in the response. <list>
796            <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
797              Pragma: no-cache (see <xref target="header.pragma" />).</t>
798          </list>
799        </t>
800        <t>Cache directives &MUST; be passed through by a proxy or gateway application,
801          regardless of their significance to that application, since the directives might be
802          applicable to all recipients along the request/response chain. It is not possible to
803          target a directive to a specific cache.</t>
804<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"/>
805  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
806  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
807
808  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
809     / <x:ref>cache-response-directive</x:ref>
810
811  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
812</artwork></figure>
813
814        <section anchor="cache-request-directive" title="Request Cache-Control Directives">
815          <x:anchor-alias value="cache-request-directive" />
816
817          <figure>
818            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
819  <x:ref>cache-request-directive</x:ref> =
820       "no-cache"
821     / "no-store"
822     / "max-age" "=" <x:ref>delta-seconds</x:ref>
823     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
824     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
825     / "no-transform"
826     / "only-if-cached"
827     / <x:ref>cache-extension</x:ref>
828</artwork>
829          </figure>
830
831
832          <t>
833            <iref item="Cache Directives" primary="true" subitem="no-cache" />
834            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
835              <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
836                used to satisfy the request without successful validation on the origin server. </t>
837            </list>
838          </t>
839          <t>
840            <iref item="Cache Directives" primary="true" subitem="no-store" />
841            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
842              <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
843                of either this request or any response to it. This directive applies to both
844                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
845                cache &MUST-NOT; intentionally store the information in non-volatile storage,
846                and &MUST; make a best-effort attempt to remove the information from volatile
847                storage as promptly as possible after forwarding it.</t>
848              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
849                particular, malicious or compromised caches might not recognize or obey this
850                directive, and communications networks may be vulnerable to eavesdropping.</t>
851            </list>
852          </t>
853          <t>
854            <iref item="Cache Directives" primary="true" subitem="max-age" />
855            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
856              <t>The max-age request directive indicates that the client is willing to accept a
857                response whose age is no greater than the specified time in seconds. Unless
858                max-stale directive is also included, the client is not willing to accept a stale
859                response.</t>
860            </list>
861          </t>
862          <t>
863            <iref item="Cache Directives" primary="true" subitem="max-stale" />
864            <iref item="max-stale" primary="true" subitem="Cache Directive" /> max-stale <list>
865              <t>The max-stale request directive indicates that the client is willing to accept a
866                response that has exceeded its expiration time. If max-stale is assigned a value,
867                then the client is willing to accept a response that has exceeded its expiration
868                time by no more than the specified number of seconds. If no value is assigned to
869                max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
870            </list>
871          </t>
872          <t>
873            <iref item="Cache Directives" primary="true" subitem="min-fresh" />
874            <iref item="min-fresh" primary="true" subitem="Cache Directive" /> min-fresh <list>
875              <t>The min-fresh request directive indicates that the client is willing to accept a
876                response whose freshness lifetime is no less than its current age plus the specified
877                time in seconds. That is, the client wants a response that will still be fresh for
878                at least the specified number of seconds.</t>
879            </list>
880          </t>
881          <t>
882            <iref item="Cache Directives" primary="true" subitem="no-transform" />
883            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
884              <t>The no-transform request directive indicates that an intermediate cache or proxy
885                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
886                headers, nor the request entity-body.</t>
887            </list>
888          </t>
889
890          <t>
891            <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
892            <iref item="only-if-cached" primary="true" subitem="Cache Directive" /> only-if-cached <list>
893              <t>The only-if-cached request directive indicates that the client only wishes to
894                return a stored response. If it receives this directive, a cache &SHOULD; either
895                respond using a stored response that is consistent with the other constraints of the
896                request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
897                being operated as a unified system with good internal connectivity, such a request
898                &MAY; be forwarded within that group of caches.</t>
899            </list>
900          </t>
901        </section>
902
903        <section anchor="cache-response-directive" title="Response Cache-Control Directives">
904          <x:anchor-alias value="cache-response-directive" />
905
906          <figure>
907            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
908  <x:ref>cache-response-directive</x:ref> =
909       "public"
910     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
911     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
912     / "no-store"
913     / "no-transform"
914     / "must-revalidate"
915     / "proxy-revalidate"
916     / "max-age" "=" <x:ref>delta-seconds</x:ref>
917     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
918     / <x:ref>cache-extension</x:ref>
919</artwork>
920          </figure>
921
922          <t>
923            <iref item="Cache Directives" primary="true" subitem="public" />
924            <iref item="public" primary="true" subitem="Cache Directive" /> public <list>
925              <t>The public response directive indicates that the response &MAY; be cached, even
926                if it would normally be non-cacheable or cacheable only within a non-shared cache.
927                (See also Authorization, &header-authorization;, for additional details.) </t>
928            </list>
929          </t>
930
931          <t>
932            <iref item="Cache Directives" primary="true" subitem="private" />
933            <iref item="private" primary="true" subitem="Cache Directive" /> private <list>
934              <t>The private response directive indicates that the response message is intended for
935                a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
936                cache &MAY; store the response.</t>
937              <t>If the private response directive specifies one or more field-names, this
938                requirement is limited to the field-values associated with the listed response
939                headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
940                cache, whereas the remainder of the response message &MAY; be.</t>
941              <t>
942                <x:h>Note:</x:h> This usage of the word private only controls where the response may
943                be stored, and cannot ensure the privacy of the message content.</t>
944            </list>
945          </t>
946          <t>
947            <iref item="Cache Directives" primary="true" subitem="no-cache" />
948            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
949              <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
950                satisfy a subsequent request without successful validation on the origin server.
951                This allows an origin server to prevent caching even by caches that have been
952                configured to return stale responses.</t>
953              <t>If the no-cache response directive specifies one or more field-names, this
954                requirement is limited to the field-values assosicated with the listed response
955                headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
956                to a subsequent request without successful validation on the origin server. This
957                allows an origin server to prevent the re-use of certain header fields in a
958                response, while still allowing caching of the rest of the response.</t>
959              <t>
960                <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
961              </t>
962            </list>
963          </t>
964
965          <t>
966            <iref item="Cache Directives" primary="true" subitem="no-store" />
967            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
968              <t>The no-store response directive indicates that a cache &MUST-NOT; store any
969                part of either the immediate request or response. This directive applies to both
970                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
971                cache &MUST-NOT; intentionally store the information in non-volatile storage,
972                and &MUST; make a best-effort attempt to remove the information from volatile
973                storage as promptly as possible after forwarding it.</t>
974              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
975                particular, malicious or compromised caches might not recognize or obey this
976                directive, and communications networks may be vulnerable to eavesdropping.</t>
977            </list>
978          </t>
979          <t>
980            <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
981            <iref item="must-revalidate" primary="true" subitem="Cache Directive" /> must-revalidate <list>
982              <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
983               used to satisfy subsequent requests without successful validation on the origin server.</t>
984              <t>The must-revalidate directive is necessary to support reliable operation for
985                certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
986                the must-revalidate directive; in particular, if the cache cannot reach the origin
987                server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
988              <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
989                validate a request on the entity could result in incorrect operation, such as a
990                silently unexecuted financial transaction.</t>
991            </list>
992          </t>
993          <t>
994            <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
995            <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
996            proxy-revalidate <list>
997              <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
998                response directive, except that it does not apply to non-shared caches.</t>
999            </list>
1000          </t>
1001          <t>
1002            <iref item="Cache Directives" primary="true" subitem="max-age" />
1003            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
1004              <t>The max-age response directive indicates that response is to be considered stale
1005                after its age is greater than the specified number of seconds.</t>
1006            </list>
1007          </t>
1008          <t>
1009            <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1010            <iref item="s-maxage" primary="true" subitem="Cache Directive" /> s-maxage <list>
1011              <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1012                specified by this directive overrides the maximum age specified by either the
1013                max-age directive or the Expires header. The s-maxage directive also implies the
1014                semantics of the proxy-revalidate response directive.</t>
1015            </list>
1016          </t>
1017          <t>
1018            <iref item="Cache Directives" primary="true" subitem="no-transform" />
1019            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
1020              <t>The no-transform response directive indicates that an intermediate cache or proxy
1021                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1022                headers, nor the response entity-body.</t>
1023            </list>
1024          </t>
1025
1026        </section>
1027
1028        <section anchor="cache.control.extensions" title="Cache Control Extensions">
1029          <t>The Cache-Control header field can be extended through the use of one or more
1030            cache-extension tokens, each with an optional value. Informational extensions (those
1031            that do not require a change in cache behavior) can be added without changing the
1032            semantics of other directives. Behavioral extensions are designed to work by acting as
1033            modifiers to the existing base of cache directives. Both the new directive and the
1034            standard directive are supplied, such that applications that do not understand the new
1035            directive will default to the behavior specified by the standard directive, and those
1036            that understand the new directive will recognize it as modifying the requirements
1037            associated with the standard directive. In this way, extensions to the cache-control
1038            directives can be made without requiring changes to the base protocol.</t>
1039          <t>This extension mechanism depends on an HTTP cache obeying all of the cache-control
1040            directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1041            all directives that it does not understand.</t>
1042          <t>For example, consider a hypothetical new response directive called "community" that
1043            acts as a modifier to the private directive. We define this new directive to mean that,
1044            in addition to any non-shared cache, any cache that is shared only by members of the
1045            community named within its value may cache the response. An origin server wishing to
1046            allow the UCI community to use an otherwise private response in their shared cache(s)
1047            could do so by including</t>
1048          <figure>
1049            <artwork type="example">
1050  Cache-Control: private, community="UCI"
1051</artwork>
1052          </figure>
1053          <t>A cache seeing this header field will act correctly even if the cache does not
1054            understand the community cache-extension, since it will also see and understand the
1055            private directive and thus default to the safe behavior.</t>
1056          <t>Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1057            directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1058            directives (or the response's default cacheability) such that the cache behavior will
1059            remain minimally correct even if the cache does not understand the extension(s).</t>
1060        </section>
1061
1062      </section>
1063
1064      <section anchor="header.expires" title="Expires">
1065        <iref item="Expires header" primary="true" x:for-anchor="" />
1066        <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1067        <x:anchor-alias value="Expires"/>
1068        <x:anchor-alias value="Expires-v"/>
1069        <t>The entity-header field "Expires" gives the date/time after which the response is
1070          considered stale. See <xref target="expiration.model" /> for further discussion of the
1071          freshness model.</t>
1072        <t>The presence of an Expires field does not imply that the original resource will change or
1073          cease to exist at, before, or after that time.</t>
1074        <t>The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1075          it &MUST; be sent in rfc1123-date format.</t>
1076<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1077  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1078  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1079</artwork></figure>
1080        <t>For example</t>
1081        <figure>
1082          <artwork type="example">
1083  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1084</artwork>
1085        </figure>
1086        <t>
1087          <list>
1088            <t>
1089              <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
1090              directive (see <xref target="cache-response-directive" />), that directive overrides
1091              the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.</t>
1092          </list>
1093        </t>
1094        <t>HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.</t>
1095        <t>HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1096          including the value "0", as in the past (i.e., "already expired").</t>
1097      </section>
1098
1099      <section anchor="header.pragma" title="Pragma">
1100        <iref item="Pragma header" primary="true" x:for-anchor="" />
1101        <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1102        <x:anchor-alias value="extension-pragma"/>
1103        <x:anchor-alias value="Pragma"/>
1104        <x:anchor-alias value="Pragma-v"/>
1105        <x:anchor-alias value="pragma-directive"/>
1106        <t>The general-header field "Pragma" is used to include implementation-specific directives
1107          that might apply to any recipient along the request/response chain. All pragma directives
1108          specify optional behavior from the viewpoint of the protocol; however, some systems
1109          &MAY; require that behavior be consistent with the directives.</t>
1110<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"/>
1111  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1112  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1113  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1114  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1115</artwork></figure>
1116        <t>When the no-cache directive is present in a request message, an application &SHOULD;
1117          forward the request toward the origin server even if it has a cached copy of what is being
1118          requested. This pragma directive has the same semantics as the no-cache response directive
1119          (see <xref target="cache-response-directive" />) and is defined here for backward
1120          compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1121          no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1122          &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".</t>
1123        <t>
1124          <list>
1125            <t>
1126              <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
1127              is not actually specified, it does not provide a reliable replacement for
1128              "Cache-Control: no-cache" in a response.</t>
1129          </list>
1130        </t>
1131        <t>This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</t>
1132      </section>
1133
1134      <section anchor="header.vary" title="Vary">
1135        <iref item="Vary header" primary="true" x:for-anchor="" />
1136        <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1137        <x:anchor-alias value="Vary"/>
1138        <x:anchor-alias value="Vary-v"/>
1139        <t>The "Vary" response-header field's value indicates the set of request-header fields that
1140          determines, while the response is fresh, whether a cache is permitted to use the
1141          response to reply to a subsequent request without validation; see <xref 
1142          target="caching.negotiated.responses" />.</t>
1143        <t>In uncacheable or stale responses, the Vary field value advises the user agent about
1144          the criteria that were used to select the representation.</t>
1145<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1146  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1147  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1148</artwork></figure>
1149        <t>The set of header fields named by the Vary field value is known as the selecting
1150          request-headers.</t>
1151        <t>Servers &SHOULD; include a Vary header field with any cacheable response that is
1152          subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1153          requests on that resource and informs the user agent about the presence of negotiation on
1154          that resource. A server &MAY; include a Vary header field with a non-cacheable
1155          response that is subject to server-driven negotiation, since this might provide the user
1156          agent with useful information about the dimensions over which the response varies at the
1157          time of the response.</t>
1158        <t>A Vary field value of "*" signals that unspecified parameters not limited to the
1159          request-headers (e.g., the network address of the client), play a role in the selection of
1160          the response representation; therefore, a cache cannot determine whether this response is
1161          appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1162          it may only be generated by an origin server.</t>
1163        <t>The field-names given are not limited to the set of standard request-header fields
1164          defined by this specification. Field names are case-insensitive.</t>
1165      </section>
1166
1167      <section anchor="header.warning" title="Warning">
1168        <iref item="Warning header" primary="true" x:for-anchor="" />
1169        <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1170        <x:anchor-alias value="Warning"/>
1171        <x:anchor-alias value="Warning-v"/>
1172        <x:anchor-alias value="warning-value"/>
1173        <x:anchor-alias value="warn-agent"/>
1174        <x:anchor-alias value="warn-code"/>
1175        <x:anchor-alias value="warn-date"/>
1176        <x:anchor-alias value="warn-text"/>
1177        <t>The general-header field "Warning" is used to carry additional information about the status
1178          or transformation of a message that might not be reflected in the message. This
1179          information is typically used to warn about possible incorrectness introduced by caching
1180          operations or transformations applied to the entity body of the message.</t>
1181        <t>Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1182          warning, rather than an error status code, distinguish these responses from true failures.</t>
1183
1184        <t>Warning headers can in general be applied to any message, however some warn-codes are
1185          specific to caches and can only be applied to response messages.</t>
1186
1187<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"/>
1188  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1189  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1190 
1191  <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>
1192                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1193 
1194  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1195  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1196                  ; the name or pseudonym of the server adding
1197                  ; the Warning header, for use in debugging
1198  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1199  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1200</artwork></figure>
1201
1202        <t>Multiple warnings can be attached to a response (either by the origin server or by
1203          a cache), including multiple warnings with the same code number. For example, a server
1204          might provide the same warning with texts in both English and Basque.</t>
1205        <t>When this occurs, the user agent &SHOULD; inform the user of as many of them as
1206          possible, in the order that they appear in the response. If it is not possible to inform
1207          the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
1208            <list style="symbols">
1209            <t>Warnings that appear early in the response take priority over those appearing later
1210              in the response.</t>
1211
1212            <t>Warnings in the user's preferred character set take priority over warnings in other
1213              character sets but with identical warn-codes and warn-agents.</t>
1214          </list>
1215        </t>
1216        <t>Systems that generate multiple Warning headers &SHOULD; order them with this user
1217          agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1218          Warning headers.</t>
1219        <t>Warnings are assigned three digit warn-codes. The first digit indicates whether the
1220          Warning is required to be deleted from a stored response after validation: <list
1221            style="symbols">
1222            <t>1xx Warnings that describe the freshness or validation status of the response, and so
1223              &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
1224              except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
1225            <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1226              not rectified by a validation (for example, a lossy compression of the entity bodies)
1227              and &MUST-NOT; be deleted by caches after validation, unless a full response is
1228              returned, in which case they &MUST; be.</t>
1229          </list>
1230        </t>
1231        <t>The warn-text &SHOULD; be in a natural language and character set that is most likely
1232          to be intelligible to the human user receiving the response. This decision can be based on
1233          any available knowledge, such as the location of the cache or user, the Accept-Language
1234          field in a request, the Content-Language field in a response, etc. The default language is
1235          English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).</t>
1236        <t>If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
1237          warn-text using the method described in <xref target="RFC2047" />.</t>
1238        <t>If an implementation sends a message with one or more Warning headers to a receiver whose
1239          version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1240          warn-date that matches the Date header in the message.</t>
1241        <t>If an implementation receives a message with a warning-value that includes a warn-date,
1242          and that warn-date is different from the Date value in the response, then that
1243          warning-value &MUST; be deleted from the message before storing, forwarding, or using
1244          it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1245          warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1246          well.</t>
1247        <t>The following warn-codes are defined by this specification, each with a recommended
1248          warn-text in English, and a description of its meaning.</t>
1249        <t>110 Response is stale <list>
1250            <t>&SHOULD; be included whenever the returned response is stale.</t>
1251          </list>
1252        </t>
1253        <t>111 Revalidation failed <list>
1254            <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1255              validate the response failed, due to an inability to reach the server.</t>
1256          </list>
1257        </t>
1258        <t>112 Disconnected operation <list>
1259            <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1260              the network for a period of time.</t>
1261          </list>
1262        </t>
1263        <t>113 Heuristic expiration <list>
1264            <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1265              greater than 24 hours and the response's age is greater than 24 hours.</t>
1266          </list>
1267        </t>
1268        <t>199 Miscellaneous warning <list>
1269            <t>The warning text can include arbitrary information to be presented to a human
1270              user, or logged. A system receiving this warning &MUST-NOT; take any automated
1271              action, besides presenting the warning to the user.</t>
1272          </list>
1273        </t>
1274        <t>214 Transformation applied <list>
1275            <t>&MUST; be added by an intermediate cache or proxy if it applies any
1276              transformation changing the content-coding (as specified in the Content-Encoding
1277              header) or media-type (as specified in the Content-Type header) of the response, or
1278              the entity-body of the response, unless this Warning code already appears in the
1279              response.</t>
1280          </list>
1281        </t>
1282        <t>299 Miscellaneous persistent warning <list>
1283            <t>The warning text can 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.</t>
1286          </list>
1287        </t>
1288      </section>
1289
1290    </section>
1291
1292
1293    <section anchor="history.lists" title="History Lists">
1294      <t>User agents often have history mechanisms, such as "Back" buttons and history lists, that
1295        can be used to redisplay an entity retrieved earlier in a session.</t>
1296      <t>History mechanisms and caches are different. In particular history mechanisms
1297        &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1298        history mechanism is meant to show exactly what the user saw at the time when the resource
1299        was retrieved.</t>
1300      <t>By default, an expiration time does not apply to history mechanisms. If the entity is still
1301        in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1302        unless the user has specifically configured the agent to refresh expired history documents.</t>
1303      <t>This is not to be construed to prohibit the history mechanism from telling the user that a
1304        view might be stale. <list>
1305          <t>
1306            <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1307            stale resources, this will tend to force service authors to avoid using HTTP expiration
1308            controls and cache controls when they would otherwise like to. Service authors may
1309            consider it important that users not be presented with error messages or warning
1310            messages when they use navigation controls (such as BACK) to view previously fetched
1311            resources. Even though sometimes such resources ought not be cached, or ought to expire
1312            quickly, user interface considerations may force service authors to resort to other
1313            means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1314            of improperly functioning history mechanisms.</t>
1315        </list>
1316      </t>
1317    </section>
1318
1319
1320    <section anchor="IANA.considerations" title="IANA Considerations">
1321      <section anchor="message.header.registration" title="Message Header Registration">
1322        <t>The Message Header Registry located at <eref
1323            target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1324          should be updated with the permanent registrations below (see <xref target="RFC3864" />):</t>
1325        <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1326        <texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1327          <ttcol>Header Field Name</ttcol>
1328          <ttcol>Protocol</ttcol>
1329          <ttcol>Status</ttcol>
1330          <ttcol>Reference</ttcol>
1331
1332          <c>Age</c>
1333          <c>http</c>
1334          <c>standard</c>
1335          <c>
1336            <xref target="header.age" />
1337          </c>
1338          <c>Cache-Control</c>
1339          <c>http</c>
1340          <c>standard</c>
1341          <c>
1342            <xref target="header.cache-control" />
1343          </c>
1344          <c>Expires</c>
1345          <c>http</c>
1346          <c>standard</c>
1347          <c>
1348            <xref target="header.expires" />
1349          </c>
1350          <c>Pragma</c>
1351          <c>http</c>
1352          <c>standard</c>
1353          <c>
1354            <xref target="header.pragma" />
1355          </c>
1356          <c>Vary</c>
1357          <c>http</c>
1358          <c>standard</c>
1359          <c>
1360            <xref target="header.vary" />
1361          </c>
1362          <c>Warning</c>
1363          <c>http</c>
1364          <c>standard</c>
1365          <c>
1366            <xref target="header.warning" />
1367          </c>
1368        </texttable>
1369        <!--(END)-->
1370        <t>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</t>
1371      </section>
1372    </section>
1373
1374    <section anchor="security.considerations" title="Security Considerations">
1375      <t>Caches expose additional potential vulnerabilities, since the contents of the cache
1376        represent an attractive target for malicious exploitation. Because cache contents persist
1377        after an HTTP request is complete, an attack on the cache can reveal information long after
1378        a user believes that the information has been removed from the network. Therefore, cache
1379        contents should be protected as sensitive information.</t>
1380    </section>
1381
1382    <section anchor="ack" title="Acknowledgments">
1383      <t>Much of the content and presentation of the caching design is due to suggestions and
1384        comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1385        and Larry Masinter.</t>
1386    </section>
1387  </middle>
1388
1389  <back>
1390    <references title="Normative References">
1391
1392      <reference anchor="ISO-8859-1">
1393        <front>
1394          <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1395            1: Latin alphabet No. 1 </title>
1396          <author>
1397            <organization>International Organization for Standardization</organization>
1398          </author>
1399          <date year="1998" />
1400        </front>
1401        <seriesInfo name="ISO/IEC" value="8859-1:1998" />
1402      </reference>
1403
1404      <reference anchor="Part1">
1405        <front>
1406          <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1407          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1408            <organization abbrev="Day Software">Day Software</organization>
1409            <address><email>fielding@gbiv.com</email></address>
1410          </author>
1411          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1412            <organization>One Laptop per Child</organization>
1413            <address><email>jg@laptop.org</email></address>
1414          </author>
1415          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1416            <organization abbrev="HP">Hewlett-Packard Company</organization>
1417            <address><email>JeffMogul@acm.org</email></address>
1418          </author>
1419          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1420            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1421            <address><email>henrikn@microsoft.com</email></address>
1422          </author>
1423          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1424            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1425            <address><email>LMM@acm.org</email></address>
1426          </author>
1427          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1428            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1429            <address><email>paulle@microsoft.com</email></address>
1430          </author>
1431          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1432            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1433            <address><email>timbl@w3.org</email></address>
1434          </author>
1435          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1436            <organization abbrev="W3C">World Wide Web Consortium</organization>
1437            <address><email>ylafon@w3.org</email></address>
1438          </author>
1439          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1440            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1441            <address><email>julian.reschke@greenbytes.de</email></address>
1442          </author>
1443          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1444        </front>
1445        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1446        <x:source basename="p1-messaging" href="p1-messaging.xml" />
1447      </reference>
1448
1449      <reference anchor="Part2">
1450        <front>
1451          <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1452          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1453            <organization abbrev="Day Software">Day Software</organization>
1454            <address><email>fielding@gbiv.com</email></address>
1455          </author>
1456          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1457            <organization>One Laptop per Child</organization>
1458            <address><email>jg@laptop.org</email></address>
1459          </author>
1460          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1461            <organization abbrev="HP">Hewlett-Packard Company</organization>
1462            <address><email>JeffMogul@acm.org</email></address>
1463          </author>
1464          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1465            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1466            <address><email>henrikn@microsoft.com</email></address>
1467          </author>
1468          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1469            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1470            <address><email>LMM@acm.org</email></address>
1471          </author>
1472          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1473            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1474            <address><email>paulle@microsoft.com</email></address>
1475          </author>
1476          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1477            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1478            <address><email>timbl@w3.org</email></address>
1479          </author>
1480          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1481            <organization abbrev="W3C">World Wide Web Consortium</organization>
1482            <address><email>ylafon@w3.org</email></address>
1483          </author>
1484          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1485            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1486            <address><email>julian.reschke@greenbytes.de</email></address>
1487          </author>
1488          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1489        </front>
1490        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1491        <x:source basename="p2-semantics" href="p2-semantics.xml" />
1492      </reference>
1493
1494      <reference anchor="Part3">
1495        <front>
1496          <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1497          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1498            <organization abbrev="Day Software">Day Software</organization>
1499            <address><email>fielding@gbiv.com</email></address>
1500          </author>
1501          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1502            <organization>One Laptop per Child</organization>
1503            <address><email>jg@laptop.org</email></address>
1504          </author>
1505          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1506            <organization abbrev="HP">Hewlett-Packard Company</organization>
1507            <address><email>JeffMogul@acm.org</email></address>
1508          </author>
1509          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1510            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1511            <address><email>henrikn@microsoft.com</email></address>
1512          </author>
1513          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1514            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1515            <address><email>LMM@acm.org</email></address>
1516          </author>
1517          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1518            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1519            <address><email>paulle@microsoft.com</email></address>
1520          </author>
1521          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1522            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1523            <address><email>timbl@w3.org</email></address>
1524          </author>
1525          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1526            <organization abbrev="W3C">World Wide Web Consortium</organization>
1527            <address><email>ylafon@w3.org</email></address>
1528          </author>
1529          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1530            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1531            <address><email>julian.reschke@greenbytes.de</email></address>
1532          </author>
1533          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1534        </front>
1535        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1536        <x:source basename="p3-payload" href="p3-payload.xml" />
1537      </reference>
1538
1539      <reference anchor="Part4">
1540        <front>
1541          <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1542          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1543            <organization abbrev="Day Software">Day Software</organization>
1544            <address><email>fielding@gbiv.com</email></address>
1545          </author>
1546          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1547            <organization>One Laptop per Child</organization>
1548            <address><email>jg@laptop.org</email></address>
1549          </author>
1550          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1551            <organization abbrev="HP">Hewlett-Packard Company</organization>
1552            <address><email>JeffMogul@acm.org</email></address>
1553          </author>
1554          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1555            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1556            <address><email>henrikn@microsoft.com</email></address>
1557          </author>
1558          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1559            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1560            <address><email>LMM@acm.org</email></address>
1561          </author>
1562          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1563            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1564            <address><email>paulle@microsoft.com</email></address>
1565          </author>
1566          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1567            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1568            <address><email>timbl@w3.org</email></address>
1569          </author>
1570          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1571            <organization abbrev="W3C">World Wide Web Consortium</organization>
1572            <address><email>ylafon@w3.org</email></address>
1573          </author>
1574          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1575            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1576            <address><email>julian.reschke@greenbytes.de</email></address>
1577          </author>
1578          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1579        </front>
1580        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1581        <x:source basename="p4-conditional" href="p4-conditional.xml" />
1582      </reference>
1583
1584      <reference anchor="Part5">
1585        <front>
1586          <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1587          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1588            <organization abbrev="Day Software">Day Software</organization>
1589            <address><email>fielding@gbiv.com</email></address>
1590          </author>
1591          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1592            <organization>One Laptop per Child</organization>
1593            <address><email>jg@laptop.org</email></address>
1594          </author>
1595          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1596            <organization abbrev="HP">Hewlett-Packard Company</organization>
1597            <address><email>JeffMogul@acm.org</email></address>
1598          </author>
1599          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1600            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1601            <address><email>henrikn@microsoft.com</email></address>
1602          </author>
1603          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1604            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1605            <address><email>LMM@acm.org</email></address>
1606          </author>
1607          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1608            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1609            <address><email>paulle@microsoft.com</email></address>
1610          </author>
1611          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1612            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1613            <address><email>timbl@w3.org</email></address>
1614          </author>
1615          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1616            <organization abbrev="W3C">World Wide Web Consortium</organization>
1617            <address><email>ylafon@w3.org</email></address>
1618          </author>
1619          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1620            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1621            <address><email>julian.reschke@greenbytes.de</email></address>
1622          </author>
1623          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1624        </front>
1625        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1626        <x:source basename="p5-range" href="p5-range.xml" />
1627      </reference>
1628
1629      <reference anchor="Part7">
1630        <front>
1631          <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1632          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1633            <organization abbrev="Day Software">Day Software</organization>
1634            <address><email>fielding@gbiv.com</email></address>
1635          </author>
1636          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1637            <organization>One Laptop per Child</organization>
1638            <address><email>jg@laptop.org</email></address>
1639          </author>
1640          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1641            <organization abbrev="HP">Hewlett-Packard Company</organization>
1642            <address><email>JeffMogul@acm.org</email></address>
1643          </author>
1644          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1645            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1646            <address><email>henrikn@microsoft.com</email></address>
1647          </author>
1648          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1649            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1650            <address><email>LMM@acm.org</email></address>
1651          </author>
1652          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1653            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1654            <address><email>paulle@microsoft.com</email></address>
1655          </author>
1656          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1657            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1658            <address><email>timbl@w3.org</email></address>
1659          </author>
1660          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1661            <organization abbrev="W3C">World Wide Web Consortium</organization>
1662            <address><email>ylafon@w3.org</email></address>
1663          </author>
1664          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1665            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1666            <address><email>julian.reschke@greenbytes.de</email></address>
1667          </author>
1668          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1669        </front>
1670        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1671        <x:source basename="p7-auth" href="p7-auth.xml" />
1672      </reference>
1673
1674      <reference anchor="RFC2047">
1675        <front>
1676          <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1677            Part Three: Message Header Extensions for Non-ASCII Text</title>
1678          <author fullname="Keith Moore" initials="K." surname="Moore">
1679            <organization>University of Tennessee</organization>
1680            <address><email>moore@cs.utk.edu</email></address>
1681          </author>
1682          <date month="November" year="1996" />
1683        </front>
1684        <seriesInfo name="RFC" value="2047" />
1685      </reference>
1686
1687      <reference anchor="RFC2119">
1688        <front>
1689          <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1690          <author fullname="Scott Bradner" initials="S." surname="Bradner">
1691            <organization>Harvard University</organization>
1692            <address><email>sob@harvard.edu</email></address>
1693          </author>
1694          <date month="March" year="1997" />
1695        </front>
1696        <seriesInfo name="BCP" value="14" />
1697        <seriesInfo name="RFC" value="2119" />
1698      </reference>
1699
1700      <reference anchor="RFC5234">
1701        <front>
1702          <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1703          <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1704            <organization>Brandenburg InternetWorking</organization>
1705            <address>
1706            <postal>
1707            <street>675 Spruce Dr.</street>
1708            <city>Sunnyvale</city>
1709            <region>CA</region>
1710            <code>94086</code>
1711            <country>US</country></postal>
1712            <phone>+1.408.246.8253</phone>
1713            <email>dcrocker@bbiw.net</email></address> 
1714          </author>
1715          <author initials="P." surname="Overell" fullname="Paul Overell">
1716            <organization>THUS plc.</organization>
1717            <address>
1718            <postal>
1719            <street>1/2 Berkeley Square</street>
1720            <street>99 Berkely Street</street>
1721            <city>Glasgow</city>
1722            <code>G3 7HR</code>
1723            <country>UK</country></postal>
1724            <email>paul.overell@thus.net</email></address>
1725          </author>
1726          <date month="January" year="2008"/>
1727        </front>
1728        <seriesInfo name="STD" value="68"/>
1729        <seriesInfo name="RFC" value="5234"/>
1730      </reference>
1731     
1732    </references>
1733
1734    <references title="Informative References">
1735
1736      <reference anchor="RFC1305">
1737        <front>
1738          <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1739          <author fullname="David L. Mills" initials="D." surname="Mills">
1740            <organization>University of Delaware, Electrical Engineering Department</organization>
1741            <address><email>mills@udel.edu</email></address>
1742          </author>
1743          <date month="March" year="1992" />
1744        </front>
1745        <seriesInfo name="RFC" value="1305" />
1746      </reference>
1747
1748      <reference anchor="RFC2616">
1749        <front>
1750          <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1751          <author fullname="R. Fielding" initials="R." surname="Fielding">
1752            <organization>University of California, Irvine</organization>
1753            <address><email>fielding@ics.uci.edu</email></address>
1754          </author>
1755          <author fullname="J. Gettys" initials="J." surname="Gettys">
1756            <organization>W3C</organization>
1757            <address><email>jg@w3.org</email></address>
1758          </author>
1759          <author fullname="J. Mogul" initials="J." surname="Mogul">
1760            <organization>Compaq Computer Corporation</organization>
1761            <address><email>mogul@wrl.dec.com</email></address>
1762          </author>
1763          <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1764            <organization>MIT Laboratory for Computer Science</organization>
1765            <address><email>frystyk@w3.org</email></address>
1766          </author>
1767          <author fullname="L. Masinter" initials="L." surname="Masinter">
1768            <organization>Xerox Corporation</organization>
1769            <address><email>masinter@parc.xerox.com</email></address>
1770          </author>
1771          <author fullname="P. Leach" initials="P." surname="Leach">
1772            <organization>Microsoft Corporation</organization>
1773            <address><email>paulle@microsoft.com</email></address>
1774          </author>
1775          <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1776            <organization>W3C</organization>
1777            <address><email>timbl@w3.org</email></address>
1778          </author>
1779          <date month="June" year="1999" />
1780        </front>
1781        <seriesInfo name="RFC" value="2616" />
1782      </reference>
1783
1784      <reference anchor="RFC3864">
1785        <front>
1786          <title>Registration Procedures for Message Header Fields</title>
1787          <author fullname="G. Klyne" initials="G." surname="Klyne">
1788            <organization>Nine by Nine</organization>
1789            <address><email>GK-IETF@ninebynine.org</email></address>
1790          </author>
1791          <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1792            <organization>BEA Systems</organization>
1793            <address><email>mnot@pobox.com</email></address>
1794          </author>
1795          <author fullname="J. Mogul" initials="J." surname="Mogul">
1796            <organization>HP Labs</organization>
1797            <address><email>JeffMogul@acm.org</email></address>
1798          </author>
1799          <date month="September" year="2004" />
1800        </front>
1801        <seriesInfo name="BCP" value="90" />
1802        <seriesInfo name="RFC" value="3864" />
1803      </reference>
1804
1805    </references>
1806
1807    <section anchor="compatibility" title="Compatibility with Previous Versions">
1808
1809      <section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
1810        <t>A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
1811          this missing case. (Sections <xref format="counter" target="response.cacheability" />,
1812            <xref format="counter" target="header.cache-control" />).</t>
1813        <t>Transfer-coding and message lengths all interact in ways that required fixing exactly
1814          when chunked encoding is used (to allow for transfer encoding that may not be self
1815          delimiting); it was important to straighten out exactly how message lengths are computed.
1816          (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)
1817          <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
1818          </t>
1819        <t>Proxies should be able to add Content-Length when appropriate.
1820          <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
1821        </t>
1822        <t>Range request responses would become very verbose if all meta-data were always returned;
1823          by allowing the server to only send needed headers in a 206 response, this problem can be
1824          avoided. (<xref target="combining.headers" />)</t>
1825        <t>The Cache-Control: max-age directive was not properly defined for responses. (<xref target="cache-response-directive"/>)</t>
1826        <t>Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
1827            format="counter" target="expiration.model" />, <xref format="counter"
1828            target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
1829          and <xref format="counter" target="header.warning" />) Warning also needed to be a general
1830          header, as PUT or other methods may have need for it in requests.</t>
1831      </section>
1832
1833      <section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
1834        <t>Clarify denial of service attack avoidance requirement. (<xref
1835            target="invalidation.after.updates.or.deletions" />)</t>
1836      </section>
1837
1838    </section>
1839
1840<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1841<figure>
1842<artwork type="abnf" name="p6-cache.parsed-abnf">
1843<x:ref>Age</x:ref> = "Age:" OWS Age-v
1844<x:ref>Age-v</x:ref> = delta-seconds
1845
1846<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
1847<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
1848 cache-directive ] )
1849
1850<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
1851<x:ref>Expires-v</x:ref> = HTTP-date
1852
1853<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
1854
1855<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1856
1857<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
1858<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1859 pragma-directive ] )
1860
1861<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
1862<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1863 ] ) )
1864
1865<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
1866<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1867 ] )
1868
1869<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
1870<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
1871<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
1872 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1873 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1874 cache-extension
1875<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
1876 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1877 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1878 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1879 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1880 ) / ( "s-maxage=" delta-seconds ) / cache-extension
1881
1882<x:ref>delta-seconds</x:ref> = 1*DIGIT
1883
1884<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
1885
1886<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
1887
1888<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
1889<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
1890<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
1891
1892<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1893
1894<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
1895
1896<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
1897
1898<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
1899<x:ref>warn-code</x:ref> = 3DIGIT
1900<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
1901<x:ref>warn-text</x:ref> = quoted-string
1902<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
1903 ]
1904
1905
1906</artwork>
1907</figure>
1908<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1909; Age defined but not used
1910; Cache-Control defined but not used
1911; Expires defined but not used
1912; Pragma defined but not used
1913; Vary defined but not used
1914; Warning defined but not used
1915</artwork></figure></section>
1916
1917    <section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
1918
1919      <section title="Since RFC2616">
1920        <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
1921      </section>
1922
1923      <section title="Since draft-ietf-httpbis-p6-cache-00">
1924        <t>Closed issues: <list style="symbols">
1925            <t>
1926              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer"
1927                (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
1928            <t>
1929              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation
1930              after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
1931            <t>
1932              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and
1933              Informative references"</t>
1934            <t>
1935              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date
1936              reference typo"</t>
1937            <t>
1938              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection
1939              header text"</t>
1940            <t>
1941              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative
1942              references"</t>
1943            <t>
1944              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1
1945              Reference"</t>
1946            <t>
1947              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative
1948              up-to-date references"</t>
1949            <t>
1950              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in
1951              13.2.2"</t>
1952          </list>
1953        </t>
1954        <t>Other changes: <list style="symbols">
1955            <t>Use names of RFC4234 core rule DQUOTE (work in progress on <eref
1956                target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
1957          </list>
1958        </t>
1959      </section>
1960
1961      <section title="Since draft-ietf-httpbis-p6-cache-01">
1962        <t>Closed issues: <list style="symbols">
1963            <t>
1964              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not
1965              used"</t>
1966          </list>
1967        </t>
1968        <t>Other changes: <list style="symbols">
1969            <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
1970              on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
1971            <t>Add explicit references to BNF syntax and rules imported from other parts of the
1972              specification.</t>
1973          </list>
1974        </t>
1975      </section>
1976
1977      <section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
1978        <t>Ongoing work on IANA Message Header Registration (<eref
1979            target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />): <list style="symbols">
1980            <t>Reference RFC 3984, and update header registrations for headers defined in this
1981              document.</t>
1982          </list>
1983        </t>
1984      </section>
1985
1986      <section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
1987        <t>Closed issues: <list style="symbols">
1988            <t>
1989              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header
1990              classification"</t>
1991          </list>
1992        </t>
1993      </section>
1994
1995      <section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
1996        <t>
1997          Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1998          <list style="symbols"> 
1999            <t>
2000              Use "/" instead of "|" for alternatives.
2001            </t>
2002            <t>
2003              Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2004              whitespace ("OWS") and required whitespace ("RWS").
2005            </t>
2006            <t>
2007              Rewrite ABNFs to spell out whitespace rules, factor out
2008              header value format definitions.
2009            </t>
2010          </list>
2011        </t>
2012      </section>
2013
2014      <section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2015        <t>
2016          This is a total rewrite of this part of the specification.
2017        </t>
2018        <t>
2019          In addition: Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2020          <list style="symbols"> 
2021            <t>
2022              Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2023            </t>
2024          </list>
2025        </t>
2026      </section>
2027
2028    </section>
2029  </back>
2030</rfc>
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