source: draft-ietf-httpbis/latest-roy/p6-cache.xml @ 476

Last change on this file since 476 was 476, checked in by mnot@…, 11 years ago

move validation to part of 'constructing response' list.

  • Property svn:eol-style set to native
File size: 110.4 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "March">
16  <!ENTITY ID-YEAR "2009">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35]>
36<?rfc toc="yes" ?>
37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
45<?rfc-ext allow-markup-in-artwork="yes" ?>
46<?rfc-ext include-references-in-index="yes" ?>
47<?oxygen RNGSchema="../../rfc2629xslt/rfc2629-ext.rnc" type="compact"?>
48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
49  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
50  <front>
51
52    <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
53
54    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
55      <organization abbrev="Day Software">Day Software</organization>
56      <address>
57      <postal>
58        <street>23 Corporate Plaza DR, Suite 280</street>
59        <city>Newport Beach</city>
60        <region>CA</region>
61        <code>92660</code>
62        <country>USA</country>
63      </postal>
64      <phone>+1-949-706-5300</phone>
65      <facsimile>+1-949-706-5305</facsimile>
66      <email>fielding@gbiv.com</email>
67      <uri>http://roy.gbiv.com/</uri>
68    </address>
69    </author>
70
71    <author fullname="Jim Gettys" initials="J." surname="Gettys">
72      <organization>One Laptop per Child</organization>
73      <address>
74      <postal>
75        <street>21 Oak Knoll Road</street>
76        <city>Carlisle</city>
77        <region>MA</region>
78        <code>01741</code>
79        <country>USA</country>
80      </postal>
81      <email>jg@laptop.org</email>
82      <uri>http://www.laptop.org/</uri>
83    </address>
84    </author>
85
86    <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
87      <organization abbrev="HP">Hewlett-Packard Company</organization>
88      <address>
89      <postal>
90        <street>HP Labs, Large Scale Systems Group</street>
91        <street>1501 Page Mill Road, MS 1177</street>
92        <city>Palo Alto</city>
93        <region>CA</region>
94        <code>94304</code>
95        <country>USA</country>
96      </postal>
97      <email>JeffMogul@acm.org</email>
98    </address>
99    </author>
100
101    <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
102      <organization abbrev="Microsoft">Microsoft Corporation</organization>
103      <address>
104      <postal>
105        <street>1 Microsoft Way</street>
106        <city>Redmond</city>
107        <region>WA</region>
108        <code>98052</code>
109        <country>USA</country>
110      </postal>
111      <email>henrikn@microsoft.com</email>
112    </address>
113    </author>
114
115    <author fullname="Larry Masinter" initials="L." surname="Masinter">
116      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
117      <address>
118      <postal>
119        <street>345 Park Ave</street>
120        <city>San Jose</city>
121        <region>CA</region>
122        <code>95110</code>
123        <country>USA</country>
124      </postal>
125      <email>LMM@acm.org</email>
126      <uri>http://larry.masinter.net/</uri>
127    </address>
128    </author>
129
130    <author fullname="Paul J. Leach" initials="P." surname="Leach">
131      <organization abbrev="Microsoft">Microsoft Corporation</organization>
132      <address>
133      <postal>
134        <street>1 Microsoft Way</street>
135        <city>Redmond</city>
136        <region>WA</region>
137        <code>98052</code>
138      </postal>
139      <email>paulle@microsoft.com</email>
140    </address>
141    </author>
142
143    <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
144      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
145      <address>
146      <postal>
147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
148        <street>The Stata Center, Building 32</street>
149        <street>32 Vassar Street</street>
150        <city>Cambridge</city>
151        <region>MA</region>
152        <code>02139</code>
153        <country>USA</country>
154      </postal>
155      <email>timbl@w3.org</email>
156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
157    </address>
158    </author>
159
160    <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
161      <organization abbrev="W3C">World Wide Web Consortium</organization>
162      <address>
163      <postal>
164        <street>W3C / ERCIM</street>
165        <street>2004, rte des Lucioles</street>
166        <city>Sophia-Antipolis</city>
167        <region>AM</region>
168        <code>06902</code>
169        <country>France</country>
170      </postal>
171      <email>ylafon@w3.org</email>
172      <uri>http://www.raubacapeu.net/people/yves/</uri>
173    </address>
174    </author>
175
176    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
177      <organization abbrev="greenbytes">greenbytes GmbH</organization>
178      <address>
179      <postal>
180        <street>Hafenweg 16</street>
181        <city>Muenster</city><region>NW</region><code>48155</code>
182        <country>Germany</country>
183      </postal>
184      <phone>+49 251 2807760</phone>   
185      <facsimile>+49 251 2807761</facsimile>   
186      <email>julian.reschke@greenbytes.de</email>       
187      <uri>http://greenbytes.de/tech/webdav/</uri>     
188    </address>
189    </author>
190
191    <date month="&ID-MONTH;" year="&ID-YEAR;" />
192    <workgroup>HTTPbis Working Group</workgroup>
193
194    <abstract>
195      <t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
196        collaborative, hypermedia information systems. This document is Part 6 of the seven-part
197        specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
198        obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
199        fields that control cache behavior or indicate cacheable response messages.</t>
200    </abstract>
201
202    <note title="Editorial Note (To be removed by RFC Editor)">
203      <t>Discussion of this draft should take place on the HTTPBIS working group mailing list
204        (ietf-http-wg@w3.org). The current issues list is at <eref
205          target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
206        (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.</t>
207      <t>The changes in this draft are summarized in <xref target="changes.since.04" />.</t>
208    </note>
209  </front>
210  <middle>
211    <section anchor="caching" title="Introduction">
212      <t>HTTP is typically used for distributed information systems, where performance can be
213        improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
214        caching and reusing response messages.</t>
215
216      <section anchor="intro.purpose" title="Purpose">
217        <iref item="cache" />
218        <t>An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
219          controls its message storage, retrieval, and deletion. A cache stores cacheable responses
220          in order to reduce the response time and network bandwidth consumption on future,
221          equivalent requests. Any client or server may include a cache, though a cache cannot be
222          used by a server that is acting as a tunnel.</t>
223        <t>Caching would be useless if it did not significantly improve performance. The goal of
224          caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
225          some cases, a stored response can be reused without the need for a network request,
226          reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
227          (see <xref target="expiration.model" />). Even when a new request is required, it is often
228          possible to reuse all or parts of the payload of a prior response to satisfy the request,
229          thereby reducing network bandwidth usage; a "validation" mechanism is used for this
230          purpose (see <xref target="validation.model" />).</t>
231      </section>
232
233      <section anchor="intro.terminology" title="Terminology">
234        <t>This specification uses a number of terms to refer to the roles played by participants
235          in, and objects of, HTTP caching.</t>
236        <t>
237          <iref item="cacheable" />
238          <x:dfn>cacheable</x:dfn>
239          <list>
240            <t>A response is cacheable if a cache is allowed to store a copy of the response message
241              for use in answering subsequent requests. Even when a response is cacheable, there may
242              be additional constraints on whether a cache can use the cached copy to satisfy a
243              particular request.</t>
244          </list>
245        </t>
246        <t>
247          <iref item="explicit expiration time" />
248          <x:dfn>explicit expiration time</x:dfn>
249          <list>
250            <t>The time at which the origin server intends that an entity should no longer be
251              returned by a cache without further validation.</t>
252          </list>
253        </t>
254        <t>
255          <iref item="heuristic expiration time" />
256          <x:dfn>heuristic expiration time</x:dfn>
257          <list>
258            <t>An expiration time assigned by a cache when no explicit expiration time is
259            available.</t>
260          </list>
261        </t>
262        <t>
263          <iref item="age" />
264          <x:dfn>age</x:dfn>
265          <list>
266            <t>The age of a response is the time since it was sent by, or successfully validated
267              with, the origin server.</t>
268          </list>
269        </t>
270        <t>
271          <iref item="first-hand" />
272          <x:dfn>first-hand</x:dfn>
273          <list>
274            <t>A response is first-hand if the freshness model is not in use; i.e., its age is
275            0.</t>
276          </list>
277        </t>
278        <t>
279          <iref item="freshness lifetime" />
280          <x:dfn>freshness lifetime</x:dfn>
281          <list>
282            <t>The length of time between the generation of a response and its expiration time. </t>
283          </list>
284        </t>
285        <t>
286          <iref item="fresh" />
287          <x:dfn>fresh</x:dfn>
288          <list>
289            <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
290          </list>
291        </t>
292        <t>
293          <iref item="stale" />
294          <x:dfn>stale</x:dfn>
295          <list>
296            <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
297          </list>
298        </t>
299        <t>
300          <iref item="validator" />
301          <x:dfn>validator</x:dfn>
302          <list>
303            <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
304              out whether a stored response is an equivalent copy of an entity.</t>
305          </list>
306        </t>
307        <t>
308          <iref item="validator" />
309          <x:dfn>shared cache</x:dfn>
310          <list>
311            <t>A cache that is accessible to more than one user. A non-shared cache is
312              dedicated to a single user.</t>
313          </list>
314        </t>
315      </section>
316
317
318      <section anchor="intro.requirements" title="Requirements">
319        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
320          NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
321          described in <xref target="RFC2119" />.</t>
322        <t>An implementation is not compliant if it fails to satisfy one or more of the &MUST;
323          or &REQUIRED; level requirements for the protocols it implements. An implementation
324          that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
325          requirements for its protocols is said to be "unconditionally compliant"; one that
326          satisfies all the &MUST; level requirements but not all the &SHOULD; level
327          requirements for its protocols is said to be "conditionally compliant."</t>
328      </section>
329
330
331<section title="Syntax Notation" anchor="notation">
332  <x:anchor-alias value="ALPHA"/>
333  <x:anchor-alias value="CR"/>
334  <x:anchor-alias value="DIGIT"/>
335  <x:anchor-alias value="DQUOTE"/>
336  <x:anchor-alias value="LF"/>
337  <x:anchor-alias value="OCTET"/>
338  <x:anchor-alias value="SP"/>
339  <x:anchor-alias value="VCHAR"/>
340  <x:anchor-alias value="WSP"/>
341<t>
342  This specification uses the ABNF syntax defined in &notation;.
343  The following core rules are included by
344  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
345  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
346  DIGIT (decimal 0-9), DQUOTE (double quote),
347  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
348  OCTET (any 8-bit sequence of data), SP (space),
349  VCHAR (any visible USASCII character),
350  and WSP (whitespace).
351</t>
352
353<section title="Core Rules" anchor="core.rules">
354  <x:anchor-alias value="quoted-string"/>
355  <x:anchor-alias value="token"/>
356  <x:anchor-alias value="OWS"/>
357<t>
358  The core rules below are defined in &basic-rules;:
359</t>
360<figure><artwork type="abnf2616">
361  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
362  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
363  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
364</artwork></figure>
365</section>
366
367<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
368  <x:anchor-alias value="field-name"/>
369  <x:anchor-alias value="HTTP-date"/>
370  <x:anchor-alias value="port"/>
371  <x:anchor-alias value="pseudonym"/>
372  <x:anchor-alias value="uri-host"/>
373<t>
374  The ABNF rules below are defined in other parts:
375</t>
376<figure><!--Part1--><artwork type="abnf2616">
377  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
378  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
379  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
380  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
381  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
382</artwork></figure>
383</section>
384
385</section>
386</section>
387
388    <section anchor="caching.overview" title="Cache Operation">
389
390      <section anchor="response.cacheability" title="Response Cacheability">
391        <t>A cache &MAY; store a response to any request, provided that: <list style="symbols">
392                <t>The request method is defined as being cacheable, and</t>
393            <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
394              appear in request or response headers, and</t>
395            <t>the cache understands partial responses, if the response is partial or incomplete
396              (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
397          </list>
398        </t>
399        <t>Note that in normal operation, most caches will not store a response that has neither a
400          cache validator nor an explicit expiration time, as such responses are not usually
401          useful to store. However, caches are not prohibited from storing such responses.</t>
402
403                <t>
404                        <cref>DISCUSS: A strict reading of 2616 is that a shared cache can store a private response,
405                        but never use it. I suspect that this may just be sloppiness in use of terminology, not intent;
406                        should this be disallowed? --mnot</cref>
407                </t>
408
409        <section anchor="errors.or.incomplete.response.cache.behavior"
410          title="Storing Incomplete Responses">
411          <t>A cache that receives an incomplete response (for example, with fewer bytes of data
412            than specified in a Content-Length header) &MAY; store the response. However, the
413            cache &MUST; treat this as a partial response &partial;. Partial responses
414            &MAY; be combined as described in &combining-byte-ranges;; the result might be a
415            full response or might still be partial. A cache &MUST-NOT; return a partial
416            response to a client without explicitly marking it as such using the 206 (Partial
417            Content) status code.</t>
418          <t>A cache that does not support the Range and Content-Range headers &MUST-NOT; store
419            incomplete or partial responses.</t>
420        </section>
421
422      </section>
423
424      <section anchor="constructing.responses.from.caches"
425        title="Constructing Responses from Caches">
426        <t>For a given request, a non-shared cache &MAY; return a stored response, provided
427          that: <list style="symbols">
428            <t>the presented target-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 stored response is either:
435            <list style="symbols">
436                <t>fresh (see <xref target="expiration.model" />), or</t>
437                <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
438                <t>successfully validated (see <xref target="validation.model" />).</t>
439            </list>, and</t>
440            <t>the presented request and stored response are free from directives that would prevent
441              its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"
442             />).</t>
443          </list>
444        </t>
445        <t>A shared cache &MAY; return a stored response, provided that: <list style="symbols">
446            <t>the criteria for non-shared caches above are met (taking into account directives specific to shared
447              caches; see <xref target="header.cache-control" />), and</t>
448            <t>the stored response was not associated with an authenticated request (see
449              &header-authorization;), unless explicitly allowed (see <xref
450                target="header.cache-control" />).</t>
451          </list>
452        </t>
453        <t>All responses satisfied from cache &MUST; include an appropriate Age header field
454            (<xref target="header.age" />).</t>
455        <t>All request methods other than GET and HEAD &MUST; be written through the cache to
456          the origin server. Note that such requests might invalidate already stored responses; see
457            <xref target="invalidation.after.updates.or.deletions" />.</t>
458        <t>Caches &SHOULD; use the most recent response (as determined by the Date header) when
459          more than one applicable response is stored. They &MAY; also send a request with
460          "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
461          use.</t>
462        <t>
463          <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in
464            p1</cref>
465        </t>
466      </section>
467
468
469
470
471      <section anchor="expiration.model" title="Freshness Model">
472
473        <t>HTTP caching works best when caches can entirely avoid making requests to the origin
474          server. When a response is "fresh" in the cache, it can be used to satisfy subsequent
475          requests without contacting the origin server. This is also referred to as "expiration."<cref source="JRE">What exactly is called 'expiration'?</cref>.</t>
476        <t>Expiration applies only to responses taken from a cache and not to first-hand responses.
477          It cannot be used to force a user agent to refresh its display or reload a resource; its
478          semantics apply only to caches. See <xref target="history.lists" /> for an explanation of
479          the difference between caches and history mechanisms.</t>
480        <t>The primary mechanism for avoiding requests is for an origin server to provide an
481          explicit expiration time in the future, using either the Expires header (<xref
482            target="header.expires" />) or the max-age response cache directive (<xref
483            target="cache-response-directive" />). Generally, origin servers will assign future
484          explicit expiration times to responses in the belief that the entity is not likely to
485          change in a semantically significant way before the expiration time is reached. This
486          normally preserves cache correctness, as long as the server's expiration times are
487          carefully chosen.</t>
488        <t>If an origin server wishes to force a cache to validate every request, it &MAY;
489          assign an explicit expiration time in the past. This means that the response is always
490          stale, so that caches should validate it before using it for subsequent requests.</t>
491        <t>Since origin servers do not always provide explicit expiration times, HTTP caches may
492          assign heuristic expiration times when they are not specified, employing algorithms that
493          use other header values (such as the Last-Modified time) to estimate a plausible
494          expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
495          impose worst-case constraints on their results.</t>
496        <t>The calculation to determine if a response has expired is:</t>
497        <figure>
498          <artwork type="code">
499   response_is_fresh = (freshness_lifetime &gt; current_age)
500</artwork>
501        </figure>
502
503        <t>The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
504          the current_age is defined in <xref target="age.calculations" />.</t>
505        <t>Additionally, clients may need to influence freshness calculation. They can do this using
506          several request cache directives, with the effect of either increasing or loosening
507          constraints on freshness. See <xref target="cache-request-directive" />.</t>
508        <t>
509          <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
510            freshness.</cref>
511        </t>
512
513        <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
514          <t>A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
515            response by using the first match of: <list style="symbols">
516              <t>If the cache is shared and the s-maxage response cache directive (<xref
517                  target="cache-response-directive" />) is present, use its value, or</t>
518              <t>If the max-age response cache directive (<xref target="cache-response-directive"
519                 />) is present, use its value, or</t>
520              <t>If the Expires response header (<xref target="header.expires" />) is present, use
521                its value minus the value of the Date response header, or</t>
522              <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
523                may be used; see <xref target="heuristic.freshness" />.</t>
524            </list>
525          </t>
526          <t>Note that this calculation is not vulnerable to clock skew, since all of the
527            information comes from the origin server.</t>
528
529          <section anchor="heuristic.freshness" title="Using Heuristic Freshness">
530            <t>If no explicit expiration time is present in a stored response that has a status code
531              of 200, 203, 206, 300, 301 or 410, a heuristic expiration time &MAY; be
532              calculated. Heuristics &MUST-NOT; be used for other response status codes. </t>
533            <t> When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
534              attach a Warning header with a 113 warn-code to the response if its current_age is
535              more than 24 hours and such a warning is not already present.</t>
536            <t>Also, if the response has a Last-Modified header (&header-last-modified;), the
537              heuristic expiration value &SHOULD; be no more than some fraction of the interval
538              since that time. A typical setting of this fraction might be 10%.</t>
539            <t>
540              <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
541            </t>
542          </section>
543        </section>
544
545        <section anchor="age.calculations" title="Calculating Age">
546          <t>HTTP/1.1 uses the Age response-header to convey the estimated age of the response
547            message when obtained from a cache. The Age field value is the cache's estimate of the
548            amount of time since the response was generated or validated by the origin server. In
549            essence, the Age value is the sum of the time that the response has been resident in
550            each of the caches along the path from the origin server, plus the amount of time it has
551            been in transit along network paths.</t>
552          <t>When a response is generated from a stored response, the cache &MUST; include a
553            single Age header field in the response with a value equal to the stored response's
554            current_age, calculated using the algorithm described in this section.</t>
555          <t>The term "age_value" denotes the value of the Age header, in a form appropriate for
556            arithmetic operations.</t>
557          <t>HTTP/1.1 requires origin servers to send a Date header, if possible, with every
558            response, giving the time at which the response was generated (see &header-date;).
559            The term "date_value" denotes the value of the Date header, in a form appropriate for
560            arithmetic operations.</t>
561          <t>The term "now" means "the current value of the clock at the host performing the
562            calculation." Hosts that use HTTP, but especially hosts running origin servers and
563            caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
564            synchronize their clocks to a globally accurate time standard.</t>
565          <t>A response's age can be calculated in two entirely independent ways: <list
566              style="numbers">
567              <t>now minus date_value, if the local clock is reasonably well synchronized to the
568                origin server's clock. If the result is negative, the result is replaced by zero.</t>
569
570              <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
571            </list>
572          </t>
573          <t>These are combined as</t>
574          <figure>
575            <artwork type="code">
576    corrected_received_age = max(now - date_value, age_value)
577</artwork>
578          </figure>
579          <t>When an Age value is received, it &MUST; be interpreted relative to the time the
580            request was initiated, not the time that the response was received.</t>
581          <figure>
582            <artwork type="code">
583   corrected_initial_age = corrected_received_age
584                         + (now - request_time)
585</artwork>
586          </figure>
587          <t>where "request_time" is the time (according to the local clock) when the request that
588            elicited this response was sent.</t>
589          <t>The current_age of a stored response can then be calculated by adding the amount of
590            time (in seconds) since the stored response was last validated by the origin server to
591            the corrected_initial_age.</t>
592          <t>In summary:</t>
593          <figure>
594            <artwork type="code">
595   /*
596    * age_value
597    *      is the value of Age: header received by the cache with
598    *              this response.
599    * date_value
600    *      is the value of the origin server's Date: header
601    * request_time
602    *      is the (local) time when the cache made the request
603    *              that resulted in this stored response
604    * response_time
605    *      is the (local) time when the cache received the
606    *              response
607    * now
608    *      is the current (local) time
609    */
610
611   apparent_age = max(0, response_time - date_value);
612   corrected_received_age = max(apparent_age, age_value);
613   response_delay = response_time - request_time;
614   corrected_initial_age = corrected_received_age + response_delay;
615   resident_time = now - response_time;
616   current_age   = corrected_initial_age + resident_time;
617</artwork>
618          </figure>
619        </section>
620
621        <section anchor="serving.stale.responses" title="Serving Stale Responses">
622          <t>A "stale" response is one that either has explicit expiry information, or is allowed to
623            have heuristic expiry calculated, but is not fresh according to the calculations in
624              <xref target="expiration.model" />. </t>
625          <t>Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
626            in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
627            "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
628            "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"
629             />). </t>
630          <t>Caches &MAY; return a stale response if disconnected or explicitly allowed (e.g.,
631            the max-stale request directive; see <xref target="cache-request-directive" />).</t>
632          <t>Otherwise, caches &SHOULD-NOT; return stale responses.</t>
633          <t>Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
634              target="header.warning" />).</t>
635          <t>If a cache receives a first-hand response (either an entire response, or a 304 (Not
636            Modified) response) that it would normally forward to the requesting client, and the
637            received response is no longer fresh, the cache &SHOULD; forward it to the
638            requesting client without adding a new Warning (but without removing any existing
639            Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
640            that response became stale in transit.</t>
641        </section>
642      </section>
643
644
645      <section anchor="validation.model" title="Validation Model">
646        <t>When a cache has a stale response that it would like to use, it &SHOULD; first check
647          with the origin server (or possibly an intermediate cache with a fresh response) to see if
648          it is still usable. This is called "validating" or "revalidating" the stored response.</t>
649        <t>HTTP's conditional request mechanism, defined in &conditional;, is used to avoid
650          retransmitting the response payload when the stored response is valid. When a stored
651          response includes one or more "cache validators", such as the field values of an ETag or
652          Last-Modified header field, then a validating GET request &SHOULD; be made conditional
653          to those field values. The server checks the conditional request's validator against the
654          current state of the requested resource and, if they match, the server responds with a 304
655          (Not Modified) status code to indicate that the stored response can be refreshed and
656          reused without retransmitting the response payload. If the validator does not match the
657          current state of the requested resource, then the server returns a full response,
658          including payload, so that the request can be satisfied and the stored response supplanted
659          without the need for an additional network round-trip.</t>
660        <t>See <xref target="combining.headers" /> regarding combining cached headers with those in
661          a 304 response.</t>
662        <t>If a cache receives a 5xx response while attempting to validate a response, it &MAY;
663          either forward this response to the requesting client, or act as if the server failed to
664          respond. In the latter case, it &MAY; return a previously received response unless the
665          stored response includes the "must-revalidate" cache directive (see <xref
666            target="cache-response-directive" />).</t>
667      </section>
668
669      <section anchor="invalidation.after.updates.or.deletions"
670        title="Request Methods that Invalidate">
671        <t>Because unsafe methods &safe-methods; have the potential for changing state on the
672          origin server, intervening caches have the opportunity to use them to keep their contents
673          up-to-date.</t>
674        <t>The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
675          as the Location and Content-Location headers (if present): <list style="symbols">
676            <t>PUT</t>
677            <t>DELETE</t>
678            <t>POST</t>
679          </list>
680        </t>
681        <t>An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
682          be performed if the host part of that URI differs from the host part in the Request-URI.
683          This helps prevent denial of service attacks.</t>
684        <t>
685          <cref>TODO: "host part" needs to be specified better.</cref>
686        </t>
687        <t>A cache that passes through requests for methods it does not understand &SHOULD;
688          invalidate the Request-URI.</t>
689        <t>Here, "invalidate" means that the cache will either remove all stored responses related
690          to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
691          before they can be returned in response to a subsequent request.</t>
692        <t>Note that this does not guarantee that all appropriate responses are invalidated. For
693          example, the request that caused the change at the origin server might not have gone
694          through the cache where a response is stored.</t>
695        <t>
696          <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
697        </t>
698      </section>
699
700
701
702
703
704      <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
705        <t>Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
706          by the presence of a Vary header field (<xref target="header.vary" />) in a response, alters
707          the conditions and procedure by which a cache can use the response for subsequent
708          requests.</t>
709        <t>When the cache receives a subsequent request which may be satisfied by a stored responses
710          that include a Vary header field, it &MUST-NOT; use it to satisfy the request unless
711          all of the selecting request-headers present in the new request match the corresponding
712          stored request-headers from the original request.</t>
713        <t>The selecting request-headers from two requests are defined to match if and only if the
714          selecting request-headers in the first request can be transformed to the selecting
715          request-headers in the second request by adding or removing linear white space
716          <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
717          combining multiple message-header fields with the same field name following the rules
718          about message headers in &message-headers;.</t>
719        <t>A Vary header field-value of "*" always fails to match, and subsequent requests on that
720          resource can only be properly interpreted by the origin server.</t>
721        <t>If the selecting request-headers for the stored response do not match the selecting
722          request header fields of the new request, then the cache &MUST-NOT; use the stored
723          response to satisfy the request unless it first relays the new request to the origin
724          server in a conditional request and the server responds with 304 (Not Modified), including
725          an entity tag or Content-Location that indicates the entity to be used.</t>
726        <t>If one or more applicable stored response has an entity tag, the forwarded request
727          &SHOULD; be conditional and include all of these entity tags in an If-None-Match
728          header field. This conveys to the server the set of entities currently stored by the
729          cache, so that if any one of these entities matches the requested entity, the server can
730          use the ETag header field in its 304 (Not Modified) response to tell the cache which
731          stored response is appropriate. If the entity-tag of the new response matches that of an
732          existing stored resopnse, the new response &SHOULD; be used to update its header
733          fields, and the result &MUST; be returned to the client.</t>
734        <t>If any of the existing stored responses contains only partial content for the associated
735          entity, its entity-tag &SHOULD-NOT; be included in the If-None-Match header field
736          unless the request is for a range that would be fully satisfied by that stored response.</t>
737        <t>If a cache receives a successful response whose Content-Location field matches that of an
738          existing stored response for the same Request-URI, whose entity-tag differs from that of
739          the existing stored response, and whose Date is more recent than that of the existing
740          response, the existing response &SHOULD-NOT; be returned in response to future
741          requests and &SHOULD; be deleted from the cache.</t>
742        <t>
743          <cref>TODO: this still needs work.</cref>
744        </t>
745      </section>
746
747
748      <section anchor="combining.headers" title="Combining Responses">
749        <t>When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
750          it needs to update the stored response with the new one, so that the updated response can
751          be sent to the client.</t>
752        <t>If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
753          the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
754          Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
755          the stored response with the updated entity-body received in the response and use the
756          result as the updated entity-body (see &combining-byte-ranges;).</t>
757        <t>The stored response headers are used for the updated response, except that <list
758            style="symbols">
759            <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
760              &MUST; be deleted from the stored response and the forwarded response.</t>
761            <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
762              response and the forwarded response.</t>
763            <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
764              headers from the stored response.</t>
765          </list>
766        </t>
767        <t>A cache &MUST; also replace stored headers with corresponding headers received in the
768          incoming response, except for Warning headers as described immediately above. If a header
769          field-name in the incoming response matches more than one header in the stored response,
770          all such old headers &MUST; be replaced. it &MAY; store the combined
771        entity-body.</t>
772        <t><cref>ISSUE: discuss how to handle HEAD updates</cref></t>
773      </section>
774
775    </section>
776
777
778
779
780    <section anchor="header.fields" title="Header Field Definitions">
781      <t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
782      <t>For entity-header fields, both sender and recipient refer to either the client or the
783        server, depending on who sends and who receives the entity.</t>
784
785      <section anchor="header.age" title="Age">
786        <iref item="Age header" primary="true" x:for-anchor="" />
787        <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
788        <x:anchor-alias value="Age"/>
789        <x:anchor-alias value="Age-v"/>
790        <x:anchor-alias value="age-value"/>
791        <t>      The response-header field "Age" conveys the sender's estimate of the amount of time since
792          the response (or its validation) was generated at the origin server. Age values are
793          calculated as specified in <xref target="age.calculations" />.</t>
794<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
795  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
796  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
797</artwork></figure>
798        <t anchor="rule.delta-seconds">
799          <x:anchor-alias value="delta-seconds" /> Age field-values are non-negative decimal
800          integers, representing time in seconds.</t>
801        <figure>
802          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
803  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
804</artwork>
805        </figure>
806        <t>If a cache receives a value larger than the largest positive integer it can represent, or
807          if any of its age calculations overflows, it &MUST; transmit an Age header with a
808          field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
809          of at least 31 bits of range.</t>
810        <t>The presence of an Age header field in a response implies that a response is not
811          first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
812          Age header field.</t>
813      </section>
814
815      <section anchor="header.cache-control" title="Cache-Control">
816        <iref item="Cache-Control header" primary="true" x:for-anchor="" />
817        <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
818        <x:anchor-alias value="Cache-Control"/>
819        <x:anchor-alias value="Cache-Control-v"/>
820        <x:anchor-alias value="cache-directive"/>
821        <x:anchor-alias value="cache-extension"/>
822        <x:anchor-alias value="cache-request-directive"/>
823        <x:anchor-alias value="cache-response-directive"/>
824        <t>The general-header field "Cache-Control" is used to specify directives that &MUST; be
825          obeyed by all caches along the request/response chain. The directives specify behavior
826          intended to prevent caches from adversely interfering with the request or response. Cache
827          directives are unidirectional in that the presence of a directive in a request does not
828          imply that the same directive is to be given in the response. <list>
829            <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
830              Pragma: no-cache (see <xref target="header.pragma" />).</t>
831          </list>
832        </t>
833        <t>Cache directives &MUST; be passed through by a proxy or gateway application,
834          regardless of their significance to that application, since the directives might be
835          applicable to all recipients along the request/response chain. It is not possible to
836          target a directive to a specific cache.</t>
837<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"/>
838  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
839  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
840
841  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
842     / <x:ref>cache-response-directive</x:ref>
843
844  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
845</artwork></figure>
846
847        <section anchor="cache-request-directive" title="Request Cache-Control Directives">
848          <x:anchor-alias value="cache-request-directive" />
849
850          <figure>
851            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
852  <x:ref>cache-request-directive</x:ref> =
853       "no-cache"
854     / "no-store"
855     / "max-age" "=" <x:ref>delta-seconds</x:ref>
856     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
857     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
858     / "no-transform"
859     / "only-if-cached"
860     / <x:ref>cache-extension</x:ref>
861</artwork>
862          </figure>
863
864
865          <t>
866            <iref item="Cache Directives" primary="true" subitem="no-cache" />
867            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
868              <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
869                used to satisfy the request without successful validation on the origin server. </t>
870            </list>
871          </t>
872          <t>
873            <iref item="Cache Directives" primary="true" subitem="no-store" />
874            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
875              <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
876                of either this request or any response to it. This directive applies to both
877                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
878                cache &MUST-NOT; intentionally store the information in non-volatile storage,
879                and &MUST; make a best-effort attempt to remove the information from volatile
880                storage as promptly as possible after forwarding it.</t>
881              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
882                particular, malicious or compromised caches might not recognize or obey this
883                directive, and communications networks may be vulnerable to eavesdropping.</t>
884            </list>
885          </t>
886          <t>
887            <iref item="Cache Directives" primary="true" subitem="max-age" />
888            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
889              <t>The max-age request directive indicates that the client is willing to accept a
890                response whose age is no greater than the specified time in seconds. Unless
891                max-stale directive is also included, the client is not willing to accept a stale
892                response.</t>
893            </list>
894          </t>
895          <t>
896            <iref item="Cache Directives" primary="true" subitem="max-stale" />
897            <iref item="max-stale" primary="true" subitem="Cache Directive" /> max-stale <list>
898              <t>The max-stale request directive indicates that the client is willing to accept a
899                response that has exceeded its expiration time. If max-stale is assigned a value,
900                then the client is willing to accept a response that has exceeded its expiration
901                time by no more than the specified number of seconds. If no value is assigned to
902                max-stale, then the client is willing to accept a stale response of any age.</t>
903            </list>
904          </t>
905          <t>
906            <iref item="Cache Directives" primary="true" subitem="min-fresh" />
907            <iref item="min-fresh" primary="true" subitem="Cache Directive" /> min-fresh <list>
908              <t>The min-fresh request directive indicates that the client is willing to accept a
909                response whose freshness lifetime is no less than its current age plus the specified
910                time in seconds. That is, the client wants a response that will still be fresh for
911                at least the specified number of seconds.</t>
912            </list>
913          </t>
914          <t>
915            <iref item="Cache Directives" primary="true" subitem="no-transform" />
916            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
917              <t>The no-transform request directive indicates that an intermediate cache or proxy
918                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
919                headers, nor the request entity-body.</t>
920            </list>
921          </t>
922
923          <t>
924            <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
925            <iref item="only-if-cached" primary="true" subitem="Cache Directive" /> only-if-cached <list>
926              <t>The only-if-cached request directive indicates that the client only wishes to
927                return a stored response. If it receives this directive, a cache &SHOULD; either
928                respond using a stored response that is consistent with the other constraints of the
929                request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
930                being operated as a unified system with good internal connectivity, such a request
931                &MAY; be forwarded within that group of caches.</t>
932            </list>
933          </t>
934        </section>
935
936        <section anchor="cache-response-directive" title="Response Cache-Control Directives">
937          <x:anchor-alias value="cache-response-directive" />
938
939          <figure>
940            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
941  <x:ref>cache-response-directive</x:ref> =
942       "public"
943     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
944     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
945     / "no-store"
946     / "no-transform"
947     / "must-revalidate"
948     / "proxy-revalidate"
949     / "max-age" "=" <x:ref>delta-seconds</x:ref>
950     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
951     / <x:ref>cache-extension</x:ref>
952</artwork>
953          </figure>
954
955          <t>
956            <iref item="Cache Directives" primary="true" subitem="public" />
957            <iref item="public" primary="true" subitem="Cache Directive" /> public <list>
958              <t>The public response directive indicates that the response &MAY; be cached, even
959                if it would normally be non-cacheable or cacheable only within a non-shared cache.
960                (See also Authorization, &header-authorization;, for additional details.) </t>
961            </list>
962          </t>
963
964          <t>
965            <iref item="Cache Directives" primary="true" subitem="private" />
966            <iref item="private" primary="true" subitem="Cache Directive" /> private <list>
967              <t>The private response directive indicates that the response message is intended for
968                a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
969                cache &MAY; store the response.</t>
970              <t>If the private response directive specifies one or more field-names, this
971                requirement is limited to the field-values associated with the listed response
972                headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
973                cache, whereas the remainder of the response message &MAY; be.</t>
974              <t>
975                <x:h>Note:</x:h> This usage of the word private only controls where the response may
976                be stored, and cannot ensure the privacy of the message content.</t>
977            </list>
978          </t>
979          <t>
980            <iref item="Cache Directives" primary="true" subitem="no-cache" />
981            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
982              <t>The no-cache response directive indicates that a response &MUST-NOT; be used to
983                satisfy a subsequent request without successful validation on the origin server.
984                This allows an origin server to prevent caching even by caches that have been
985                configured to return stale responses.</t>
986              <t>If the no-cache response directive specifies one or more field-names, this
987                requirement is limited to the field-values assosicated with the listed response
988                headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
989                to a subsequent request without successful validation on the origin server. This
990                allows an origin server to prevent the re-use of certain header fields in a
991                response, while still allowing caching of the rest of the response.</t>
992              <t>
993                <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
994              </t>
995            </list>
996          </t>
997
998          <t>
999            <iref item="Cache Directives" primary="true" subitem="no-store" />
1000            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
1001              <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1002                part of either the immediate request or response. This directive applies to both
1003                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1004                cache &MUST-NOT; intentionally store the information in non-volatile storage,
1005                and &MUST; make a best-effort attempt to remove the information from volatile
1006                storage as promptly as possible after forwarding it.</t>
1007              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1008                particular, malicious or compromised caches might not recognize or obey this
1009                directive, and communications networks may be vulnerable to eavesdropping.</t>
1010            </list>
1011          </t>
1012          <t>
1013            <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1014            <iref item="must-revalidate" primary="true" subitem="Cache Directive" /> must-revalidate <list>
1015              <t>The must-revalidate response directive indicates that validation is required before
1016                the response is used by a cache to satisfy any request.</t>
1017              <t>When the present, caches &MUST-NOT; use a stored after it becomes stale to
1018                respond to a subsequent request without first validating it with the origin server.</t>
1019              <t>The must-revalidate directive is necessary to support reliable operation for
1020                certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1021                the must-revalidate directive; in particular, if the cache cannot reach the origin
1022                server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1023              <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1024                validate a request on the entity could result in incorrect operation, such as a
1025                silently unexecuted financial transaction.</t>
1026            </list>
1027          </t>
1028          <t>
1029            <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1030            <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1031            proxy-revalidate <list>
1032              <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1033                response directive, except that it does not apply to non-shared caches.</t>
1034            </list>
1035          </t>
1036          <t>
1037            <iref item="Cache Directives" primary="true" subitem="max-age" />
1038            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
1039              <t>The max-age response directive indicates that response is to be considered stale
1040                after its age is greater than the specified number of seconds.</t>
1041            </list>
1042          </t>
1043          <t>
1044            <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1045            <iref item="s-maxage" primary="true" subitem="Cache Directive" /> s-maxage <list>
1046              <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1047                specified by this directive overrides the maximum age specified by either the
1048                max-age directive or the Expires header. The s-maxage directive also implies the
1049                semantics of the proxy-revalidate response directive.</t>
1050            </list>
1051          </t>
1052          <t>
1053            <iref item="Cache Directives" primary="true" subitem="no-transform" />
1054            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
1055              <t>The no-transform response directive indicates that an intermediate cache or proxy
1056                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1057                headers, nor the response entity-body.</t>
1058            </list>
1059          </t>
1060
1061        </section>
1062
1063        <section anchor="cache.control.extensions" title="Cache Control Extensions">
1064          <t>The Cache-Control header field can be extended through the use of one or more
1065            cache-extension tokens, each with an optional value. Informational extensions (those
1066            which do not require a change in cache behavior) &MAY; be added without changing the
1067            semantics of other directives. Behavioral extensions are designed to work by acting as
1068            modifiers to the existing base of cache directives. Both the new directive and the
1069            standard directive are supplied, such that applications which do not understand the new
1070            directive will default to the behavior specified by the standard directive, and those
1071            that understand the new directive will recognize it as modifying the requirements
1072            associated with the standard directive. In this way, extensions to the cache-control
1073            directives can be made without requiring changes to the base protocol.</t>
1074          <t>This extension mechanism depends on an HTTP cache obeying all of the cache-control
1075            directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1076            all directives that it does not understand.</t>
1077          <t>For example, consider a hypothetical new response directive called "community" which
1078            acts as a modifier to the private directive. We define this new directive to mean that,
1079            in addition to any non-shared cache, any cache which is shared only by members of the
1080            community named within its value may cache the response. An origin server wishing to
1081            allow the UCI community to use an otherwise private response in their shared cache(s)
1082            could do so by including</t>
1083          <figure>
1084            <artwork type="example">
1085  Cache-Control: private, community="UCI"
1086</artwork>
1087          </figure>
1088          <t>A cache seeing this header field will act correctly even if the cache does not
1089            understand the community cache-extension, since it will also see and understand the
1090            private directive and thus default to the safe behavior.</t>
1091          <t>Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1092            directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1093            directives (or the response's default cacheability) such that the cache behavior will
1094            remain minimally correct even if the cache does not understand the extension(s).</t>
1095        </section>
1096
1097      </section>
1098
1099      <section anchor="header.expires" title="Expires">
1100        <iref item="Expires header" primary="true" x:for-anchor="" />
1101        <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1102        <x:anchor-alias value="Expires"/>
1103        <x:anchor-alias value="Expires-v"/>
1104        <t>The entity-header field "Expires" gives the date/time after which the response is
1105          considered stale. See <xref target="expiration.model" /> for further discussion of the
1106          expiration model.</t>
1107        <t>The presence of an Expires field does not imply that the original resource will change or
1108          cease to exist at, before, or after that time.</t>
1109        <t>The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1110          it &MUST; be sent in rfc1123-date format.</t>
1111<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1112  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1113  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1114</artwork></figure>
1115        <t>For example</t>
1116        <figure>
1117          <artwork type="example">
1118  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1119</artwork>
1120        </figure>
1121        <t>
1122          <list>
1123            <t>
1124              <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
1125              directive (see <xref target="cache-response-directive" />), that directive overrides
1126              the Expires field.</t>
1127          </list>
1128        </t>
1129        <t>HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.</t>
1130        <t>HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1131          including the value "0", as in the past (i.e., "already expired").</t>
1132      </section>
1133
1134      <section anchor="header.pragma" title="Pragma">
1135        <iref item="Pragma header" primary="true" x:for-anchor="" />
1136        <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1137        <x:anchor-alias value="extension-pragma"/>
1138        <x:anchor-alias value="Pragma"/>
1139        <x:anchor-alias value="Pragma-v"/>
1140        <x:anchor-alias value="pragma-directive"/>
1141        <t>The general-header field "Pragma" is used to include implementation-specific directives
1142          that might apply to any recipient along the request/response chain. All pragma directives
1143          specify optional behavior from the viewpoint of the protocol; however, some systems
1144          &MAY; require that behavior be consistent with the directives.</t>
1145<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"/>
1146  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1147  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1148  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1149  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1150</artwork></figure>
1151        <t>When the no-cache directive is present in a request message, an application &SHOULD;
1152          forward the request toward the origin server even if it has a cached copy of what is being
1153          requested. This pragma directive has the same semantics as the no-cache response directive
1154          (see <xref target="cache-response-directive" />) and is defined here for backward
1155          compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1156          no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1157          &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".</t>
1158        <t>
1159          <list>
1160            <t>
1161              <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
1162              is not actually specified, it does not provide a reliable replacement for
1163              "Cache-Control: no-cache" in a response.</t>
1164          </list>
1165        </t>
1166        <t>This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</t>
1167      </section>
1168
1169      <section anchor="header.vary" title="Vary">
1170        <iref item="Vary header" primary="true" x:for-anchor="" />
1171        <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1172        <x:anchor-alias value="Vary"/>
1173        <x:anchor-alias value="Vary-v"/>
1174        <t>The "Vary" response-header field's value indicates the set of request-header fields that
1175          fully determines, while the response is fresh, whether a cache is permitted to use the
1176          response to reply to a subsequent request without validation. For uncacheable or stale
1177          responses, the Vary field value advises the user agent about the criteria that were used
1178          to select the representation. A Vary field value of "*" implies that a cache cannot
1179          determine from the request headers of a subsequent request whether this response is the
1180          appropriate representation. See <xref target="caching.negotiated.responses" /> for use of
1181          the Vary header field by caches.</t>
1182<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1183  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1184  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1185</artwork></figure>
1186        <t>The set of header fields named by the Vary field value is known as the "selecting"
1187          request-headers.</t>
1188        <t>Servers &SHOULD; include a Vary header field with any cacheable response that is
1189          subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1190          requests on that resource and informs the user agent about the presence of negotiation on
1191          that resource. A server &MAY; include a Vary header field with a non-cacheable
1192          response that is subject to server-driven negotiation, since this might provide the user
1193          agent with useful information about the dimensions over which the response varies at the
1194          time of the response.</t>
1195        <t>A Vary field value of "*" signals that unspecified parameters not limited to the
1196          request-headers (e.g., the network address of the client), play a role in the selection of
1197          the response representation. The "*" value &MUST-NOT; be generated by a proxy server;
1198          it may only be generated by an origin server.</t>
1199        <t>The field-names given are not limited to the set of standard request-header fields
1200          defined by this specification. Field names are case-insensitive.</t>
1201      </section>
1202
1203      <section anchor="header.warning" title="Warning">
1204        <iref item="Warning header" primary="true" x:for-anchor="" />
1205        <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1206        <x:anchor-alias value="Warning"/>
1207        <x:anchor-alias value="Warning-v"/>
1208        <x:anchor-alias value="warning-value"/>
1209        <x:anchor-alias value="warn-agent"/>
1210        <x:anchor-alias value="warn-code"/>
1211        <x:anchor-alias value="warn-date"/>
1212        <x:anchor-alias value="warn-text"/>
1213        <t>The general-header field "Warning" is used to carry additional information about the status
1214          or transformation of a message which might not be reflected in the message. This
1215          information is typically used to warn about possible incorrectness introduced by caching
1216          operations or transformations applied to the entity body of the message.</t>
1217        <t>Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1218          warning, rather than an error status code, distinguish these responses from true failures.</t>
1219
1220        <t>Warning headers can in general be applied to any message, however some warn-codes are
1221          specific to caches and can only be applied to response messages.</t>
1222
1223<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"/>
1224  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1225  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1226 
1227  <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>
1228                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1229 
1230  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1231  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1232                  ; the name or pseudonym of the server adding
1233                  ; the Warning header, for use in debugging
1234  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1235  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1236</artwork></figure>
1237
1238        <t>Multiple warnings &MAY; be attached to a response (either by the origin server or by
1239          a cache), including multiple warnings with the same code number. For example, a server
1240          might provide the same warning with texts in both English and Basque.</t>
1241        <t>When this occurs, the user agent &SHOULD; inform the user of as many of them as
1242          possible, in the order that they appear in the response. If it is not possible to inform
1243          the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
1244            <list style="symbols">
1245            <t>Warnings that appear early in the response take priority over those appearing later
1246              in the response.</t>
1247
1248            <t>Warnings in the user's preferred character set take priority over warnings in other
1249              character sets but with identical warn-codes and warn-agents.</t>
1250          </list>
1251        </t>
1252        <t>Systems that generate multiple Warning headers &SHOULD; order them with this user
1253          agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1254          Warning headers.</t>
1255        <t>Warnings are assigned three digit warn-codes. The first digit indicates whether the
1256          Warning is required to be deleted from a stored response after validation: <list
1257            style="symbols">
1258            <t>1xx Warnings that describe the freshness or validation status of the response, and so
1259              &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
1260              except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
1261            <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1262              not rectified by a validation (for example, a lossy compression of the entity bodies)
1263              and which &MUST-NOT; be deleted by caches after validation, unless a full response is
1264              returned, in which case they &MUST; be.</t>
1265          </list>
1266        </t>
1267        <t>The warn-text &SHOULD; be in a natural language and character set that is most likely
1268          to be intelligible to the human user receiving the response. This decision can be based on
1269          any available knowledge, such as the location of the cache or user, the Accept-Language
1270          field in a request, the Content-Language field in a response, etc. The default language is
1271          English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).</t>
1272        <t>If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
1273          warn-text using the method described in <xref target="RFC2047" />.</t>
1274        <t>If an implementation sends a message with one or more Warning headers to a receiver whose
1275          version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1276          warn-date that matches the Date header in the message.</t>
1277        <t>If an implementation receives a message with a warning-value that includes a warn-date,
1278          and that warn-date is different from the Date value in the response, then that
1279          warning-value &MUST; be deleted from the message before storing, forwarding, or using
1280          it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1281          warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1282          well.</t>
1283        <t>The following warn-codes are defined by this specification, each with a recommended
1284          warn-text in English, and a description of its meaning.</t>
1285        <t>110 Response is stale <list>
1286            <t>&SHOULD; be included whenever the returned response is stale.</t>
1287          </list>
1288        </t>
1289        <t>111 Revalidation failed <list>
1290            <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1291              validate the response failed, due to an inability to reach the server.</t>
1292          </list>
1293        </t>
1294        <t>112 Disconnected operation <list>
1295            <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1296              the network for a period of time.</t>
1297          </list>
1298        </t>
1299        <t>113 Heuristic expiration <list>
1300            <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1301              greater than 24 hours and the response's age is greater than 24 hours.</t>
1302          </list>
1303        </t>
1304        <t>199 Miscellaneous warning <list>
1305            <t>The warning text &MAY; include arbitrary information to be presented to a human
1306              user, or logged. A system receiving this warning &MUST-NOT; take any automated
1307              action, besides presenting the warning to the user.</t>
1308          </list>
1309        </t>
1310        <t>214 Transformation applied <list>
1311            <t>&MUST; be added by an intermediate cache or proxy if it applies any
1312              transformation changing the content-coding (as specified in the Content-Encoding
1313              header) or media-type (as specified in the Content-Type header) of the response, or
1314              the entity-body of the response, unless this Warning code already appears in the
1315              response.</t>
1316          </list>
1317        </t>
1318        <t>299 Miscellaneous persistent warning <list>
1319            <t>The warning text &MAY; include arbitrary information to be presented to a human
1320              user, or logged. A system receiving this warning &MUST-NOT; take any automated
1321              action.</t>
1322          </list>
1323        </t>
1324      </section>
1325
1326    </section>
1327
1328
1329    <section anchor="history.lists" title="History Lists">
1330      <t>User agents often have history mechanisms, such as "Back" buttons and history lists, which
1331        can be used to redisplay an entity retrieved earlier in a session.</t>
1332      <t>History mechanisms and caches are different. In particular history mechanisms
1333        &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1334        history mechanism is meant to show exactly what the user saw at the time when the resource
1335        was retrieved.</t>
1336      <t>By default, an expiration time does not apply to history mechanisms. If the entity is still
1337        in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1338        unless the user has specifically configured the agent to refresh expired history documents.</t>
1339      <t>This is not to be construed to prohibit the history mechanism from telling the user that a
1340        view might be stale. <list>
1341          <t>
1342            <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1343            stale resources, this will tend to force service authors to avoid using HTTP expiration
1344            controls and cache controls when they would otherwise like to. Service authors may
1345            consider it important that users not be presented with error messages or warning
1346            messages when they use navigation controls (such as BACK) to view previously fetched
1347            resources. Even though sometimes such resources ought not be cached, or ought to expire
1348            quickly, user interface considerations may force service authors to resort to other
1349            means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1350            of improperly functioning history mechanisms.</t>
1351        </list>
1352      </t>
1353    </section>
1354
1355
1356    <section anchor="IANA.considerations" title="IANA Considerations">
1357      <section anchor="message.header.registration" title="Message Header Registration">
1358        <t>The Message Header Registry located at <eref
1359            target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1360          should be updated with the permanent registrations below (see <xref target="RFC3864" />):</t>
1361        <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1362        <texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1363          <ttcol>Header Field Name</ttcol>
1364          <ttcol>Protocol</ttcol>
1365          <ttcol>Status</ttcol>
1366          <ttcol>Reference</ttcol>
1367
1368          <c>Age</c>
1369          <c>http</c>
1370          <c>standard</c>
1371          <c>
1372            <xref target="header.age" />
1373          </c>
1374          <c>Cache-Control</c>
1375          <c>http</c>
1376          <c>standard</c>
1377          <c>
1378            <xref target="header.cache-control" />
1379          </c>
1380          <c>Expires</c>
1381          <c>http</c>
1382          <c>standard</c>
1383          <c>
1384            <xref target="header.expires" />
1385          </c>
1386          <c>Pragma</c>
1387          <c>http</c>
1388          <c>standard</c>
1389          <c>
1390            <xref target="header.pragma" />
1391          </c>
1392          <c>Vary</c>
1393          <c>http</c>
1394          <c>standard</c>
1395          <c>
1396            <xref target="header.vary" />
1397          </c>
1398          <c>Warning</c>
1399          <c>http</c>
1400          <c>standard</c>
1401          <c>
1402            <xref target="header.warning" />
1403          </c>
1404        </texttable>
1405        <!--(END)-->
1406        <t>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</t>
1407      </section>
1408    </section>
1409
1410    <section anchor="security.considerations" title="Security Considerations">
1411      <t>Caches expose additional potential vulnerabilities, since the contents of the cache
1412        represent an attractive target for malicious exploitation. Because cache contents persist
1413        after an HTTP request is complete, an attack on the cache can reveal information long after
1414        a user believes that the information has been removed from the network. Therefore, cache
1415        contents should be protected as sensitive information.</t>
1416    </section>
1417
1418    <section anchor="ack" title="Acknowledgments">
1419      <t>Much of the content and presentation of the caching design is due to suggestions and
1420        comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1421        and Larry Masinter.</t>
1422    </section>
1423  </middle>
1424
1425  <back>
1426    <references title="Normative References">
1427
1428      <reference anchor="ISO-8859-1">
1429        <front>
1430          <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1431            1: Latin alphabet No. 1 </title>
1432          <author>
1433            <organization>International Organization for Standardization</organization>
1434          </author>
1435          <date year="1998" />
1436        </front>
1437        <seriesInfo name="ISO/IEC" value="8859-1:1998" />
1438      </reference>
1439
1440      <reference anchor="Part1">
1441        <front>
1442          <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1443          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1444            <organization abbrev="Day Software">Day Software</organization>
1445            <address><email>fielding@gbiv.com</email></address>
1446          </author>
1447          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1448            <organization>One Laptop per Child</organization>
1449            <address><email>jg@laptop.org</email></address>
1450          </author>
1451          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1452            <organization abbrev="HP">Hewlett-Packard Company</organization>
1453            <address><email>JeffMogul@acm.org</email></address>
1454          </author>
1455          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1456            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1457            <address><email>henrikn@microsoft.com</email></address>
1458          </author>
1459          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1460            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1461            <address><email>LMM@acm.org</email></address>
1462          </author>
1463          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1464            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1465            <address><email>paulle@microsoft.com</email></address>
1466          </author>
1467          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1468            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1469            <address><email>timbl@w3.org</email></address>
1470          </author>
1471          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1472            <organization abbrev="W3C">World Wide Web Consortium</organization>
1473            <address><email>ylafon@w3.org</email></address>
1474          </author>
1475          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1476            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1477            <address><email>julian.reschke@greenbytes.de</email></address>
1478          </author>
1479          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1480        </front>
1481        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1482        <x:source basename="p1-messaging" href="p1-messaging.xml" />
1483      </reference>
1484
1485      <reference anchor="Part2">
1486        <front>
1487          <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1488          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1489            <organization abbrev="Day Software">Day Software</organization>
1490            <address><email>fielding@gbiv.com</email></address>
1491          </author>
1492          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1493            <organization>One Laptop per Child</organization>
1494            <address><email>jg@laptop.org</email></address>
1495          </author>
1496          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1497            <organization abbrev="HP">Hewlett-Packard Company</organization>
1498            <address><email>JeffMogul@acm.org</email></address>
1499          </author>
1500          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1501            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1502            <address><email>henrikn@microsoft.com</email></address>
1503          </author>
1504          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1505            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1506            <address><email>LMM@acm.org</email></address>
1507          </author>
1508          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1509            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1510            <address><email>paulle@microsoft.com</email></address>
1511          </author>
1512          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1513            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1514            <address><email>timbl@w3.org</email></address>
1515          </author>
1516          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1517            <organization abbrev="W3C">World Wide Web Consortium</organization>
1518            <address><email>ylafon@w3.org</email></address>
1519          </author>
1520          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1521            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1522            <address><email>julian.reschke@greenbytes.de</email></address>
1523          </author>
1524          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1525        </front>
1526        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1527        <x:source basename="p2-semantics" href="p2-semantics.xml" />
1528      </reference>
1529
1530      <reference anchor="Part3">
1531        <front>
1532          <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1533          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1534            <organization abbrev="Day Software">Day Software</organization>
1535            <address><email>fielding@gbiv.com</email></address>
1536          </author>
1537          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1538            <organization>One Laptop per Child</organization>
1539            <address><email>jg@laptop.org</email></address>
1540          </author>
1541          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1542            <organization abbrev="HP">Hewlett-Packard Company</organization>
1543            <address><email>JeffMogul@acm.org</email></address>
1544          </author>
1545          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1546            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1547            <address><email>henrikn@microsoft.com</email></address>
1548          </author>
1549          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1550            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1551            <address><email>LMM@acm.org</email></address>
1552          </author>
1553          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1554            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1555            <address><email>paulle@microsoft.com</email></address>
1556          </author>
1557          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1558            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1559            <address><email>timbl@w3.org</email></address>
1560          </author>
1561          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1562            <organization abbrev="W3C">World Wide Web Consortium</organization>
1563            <address><email>ylafon@w3.org</email></address>
1564          </author>
1565          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1566            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1567            <address><email>julian.reschke@greenbytes.de</email></address>
1568          </author>
1569          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1570        </front>
1571        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1572        <x:source basename="p3-payload" href="p3-payload.xml" />
1573      </reference>
1574
1575      <reference anchor="Part4">
1576        <front>
1577          <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1578          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1579            <organization abbrev="Day Software">Day Software</organization>
1580            <address><email>fielding@gbiv.com</email></address>
1581          </author>
1582          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1583            <organization>One Laptop per Child</organization>
1584            <address><email>jg@laptop.org</email></address>
1585          </author>
1586          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1587            <organization abbrev="HP">Hewlett-Packard Company</organization>
1588            <address><email>JeffMogul@acm.org</email></address>
1589          </author>
1590          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1591            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1592            <address><email>henrikn@microsoft.com</email></address>
1593          </author>
1594          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1595            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1596            <address><email>LMM@acm.org</email></address>
1597          </author>
1598          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1599            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1600            <address><email>paulle@microsoft.com</email></address>
1601          </author>
1602          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1603            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1604            <address><email>timbl@w3.org</email></address>
1605          </author>
1606          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1607            <organization abbrev="W3C">World Wide Web Consortium</organization>
1608            <address><email>ylafon@w3.org</email></address>
1609          </author>
1610          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1611            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1612            <address><email>julian.reschke@greenbytes.de</email></address>
1613          </author>
1614          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1615        </front>
1616        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1617        <x:source basename="p4-conditional" href="p4-conditional.xml" />
1618      </reference>
1619
1620      <reference anchor="Part5">
1621        <front>
1622          <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1623          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1624            <organization abbrev="Day Software">Day Software</organization>
1625            <address><email>fielding@gbiv.com</email></address>
1626          </author>
1627          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1628            <organization>One Laptop per Child</organization>
1629            <address><email>jg@laptop.org</email></address>
1630          </author>
1631          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1632            <organization abbrev="HP">Hewlett-Packard Company</organization>
1633            <address><email>JeffMogul@acm.org</email></address>
1634          </author>
1635          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1636            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1637            <address><email>henrikn@microsoft.com</email></address>
1638          </author>
1639          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1640            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1641            <address><email>LMM@acm.org</email></address>
1642          </author>
1643          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1644            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1645            <address><email>paulle@microsoft.com</email></address>
1646          </author>
1647          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1648            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1649            <address><email>timbl@w3.org</email></address>
1650          </author>
1651          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1652            <organization abbrev="W3C">World Wide Web Consortium</organization>
1653            <address><email>ylafon@w3.org</email></address>
1654          </author>
1655          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1656            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1657            <address><email>julian.reschke@greenbytes.de</email></address>
1658          </author>
1659          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1660        </front>
1661        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1662        <x:source basename="p5-range" href="p5-range.xml" />
1663      </reference>
1664
1665      <reference anchor="Part7">
1666        <front>
1667          <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1668          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1669            <organization abbrev="Day Software">Day Software</organization>
1670            <address><email>fielding@gbiv.com</email></address>
1671          </author>
1672          <author fullname="Jim Gettys" initials="J." surname="Gettys">
1673            <organization>One Laptop per Child</organization>
1674            <address><email>jg@laptop.org</email></address>
1675          </author>
1676          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1677            <organization abbrev="HP">Hewlett-Packard Company</organization>
1678            <address><email>JeffMogul@acm.org</email></address>
1679          </author>
1680          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1681            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1682            <address><email>henrikn@microsoft.com</email></address>
1683          </author>
1684          <author fullname="Larry Masinter" initials="L." surname="Masinter">
1685            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1686            <address><email>LMM@acm.org</email></address>
1687          </author>
1688          <author fullname="Paul J. Leach" initials="P." surname="Leach">
1689            <organization abbrev="Microsoft">Microsoft Corporation</organization>
1690            <address><email>paulle@microsoft.com</email></address>
1691          </author>
1692          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1693            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1694            <address><email>timbl@w3.org</email></address>
1695          </author>
1696          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1697            <organization abbrev="W3C">World Wide Web Consortium</organization>
1698            <address><email>ylafon@w3.org</email></address>
1699          </author>
1700          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1701            <organization abbrev="greenbytes">greenbytes GmbH</organization>
1702            <address><email>julian.reschke@greenbytes.de</email></address>
1703          </author>
1704          <date month="&ID-MONTH;" year="&ID-YEAR;" />
1705        </front>
1706        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1707        <x:source basename="p7-auth" href="p7-auth.xml" />
1708      </reference>
1709
1710      <reference anchor="RFC2047">
1711        <front>
1712          <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1713            Part Three: Message Header Extensions for Non-ASCII Text</title>
1714          <author fullname="Keith Moore" initials="K." surname="Moore">
1715            <organization>University of Tennessee</organization>
1716            <address><email>moore@cs.utk.edu</email></address>
1717          </author>
1718          <date month="November" year="1996" />
1719        </front>
1720        <seriesInfo name="RFC" value="2047" />
1721      </reference>
1722
1723      <reference anchor="RFC2119">
1724        <front>
1725          <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1726          <author fullname="Scott Bradner" initials="S." surname="Bradner">
1727            <organization>Harvard University</organization>
1728            <address><email>sob@harvard.edu</email></address>
1729          </author>
1730          <date month="March" year="1997" />
1731        </front>
1732        <seriesInfo name="BCP" value="14" />
1733        <seriesInfo name="RFC" value="2119" />
1734      </reference>
1735
1736      <reference anchor="RFC5234">
1737        <front>
1738          <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1739          <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1740            <organization>Brandenburg InternetWorking</organization>
1741            <address>
1742            <postal>
1743            <street>675 Spruce Dr.</street>
1744            <city>Sunnyvale</city>
1745            <region>CA</region>
1746            <code>94086</code>
1747            <country>US</country></postal>
1748            <phone>+1.408.246.8253</phone>
1749            <email>dcrocker@bbiw.net</email></address> 
1750          </author>
1751          <author initials="P." surname="Overell" fullname="Paul Overell">
1752            <organization>THUS plc.</organization>
1753            <address>
1754            <postal>
1755            <street>1/2 Berkeley Square</street>
1756            <street>99 Berkely Street</street>
1757            <city>Glasgow</city>
1758            <code>G3 7HR</code>
1759            <country>UK</country></postal>
1760            <email>paul.overell@thus.net</email></address>
1761          </author>
1762          <date month="January" year="2008"/>
1763        </front>
1764        <seriesInfo name="STD" value="68"/>
1765        <seriesInfo name="RFC" value="5234"/>
1766      </reference>
1767     
1768    </references>
1769
1770    <references title="Informative References">
1771
1772      <reference anchor="RFC1305">
1773        <front>
1774          <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1775          <author fullname="David L. Mills" initials="D." surname="Mills">
1776            <organization>University of Delaware, Electrical Engineering Department</organization>
1777            <address><email>mills@udel.edu</email></address>
1778          </author>
1779          <date month="March" year="1992" />
1780        </front>
1781        <seriesInfo name="RFC" value="1305" />
1782      </reference>
1783
1784      <reference anchor="RFC2616">
1785        <front>
1786          <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1787          <author fullname="R. Fielding" initials="R." surname="Fielding">
1788            <organization>University of California, Irvine</organization>
1789            <address><email>fielding@ics.uci.edu</email></address>
1790          </author>
1791          <author fullname="J. Gettys" initials="J." surname="Gettys">
1792            <organization>W3C</organization>
1793            <address><email>jg@w3.org</email></address>
1794          </author>
1795          <author fullname="J. Mogul" initials="J." surname="Mogul">
1796            <organization>Compaq Computer Corporation</organization>
1797            <address><email>mogul@wrl.dec.com</email></address>
1798          </author>
1799          <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1800            <organization>MIT Laboratory for Computer Science</organization>
1801            <address><email>frystyk@w3.org</email></address>
1802          </author>
1803          <author fullname="L. Masinter" initials="L." surname="Masinter">
1804            <organization>Xerox Corporation</organization>
1805            <address><email>masinter@parc.xerox.com</email></address>
1806          </author>
1807          <author fullname="P. Leach" initials="P." surname="Leach">
1808            <organization>Microsoft Corporation</organization>
1809            <address><email>paulle@microsoft.com</email></address>
1810          </author>
1811          <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1812            <organization>W3C</organization>
1813            <address><email>timbl@w3.org</email></address>
1814          </author>
1815          <date month="June" year="1999" />
1816        </front>
1817        <seriesInfo name="RFC" value="2616" />
1818      </reference>
1819
1820      <reference anchor="RFC3864">
1821        <front>
1822          <title>Registration Procedures for Message Header Fields</title>
1823          <author fullname="G. Klyne" initials="G." surname="Klyne">
1824            <organization>Nine by Nine</organization>
1825            <address><email>GK-IETF@ninebynine.org</email></address>
1826          </author>
1827          <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1828            <organization>BEA Systems</organization>
1829            <address><email>mnot@pobox.com</email></address>
1830          </author>
1831          <author fullname="J. Mogul" initials="J." surname="Mogul">
1832            <organization>HP Labs</organization>
1833            <address><email>JeffMogul@acm.org</email></address>
1834          </author>
1835          <date month="September" year="2004" />
1836        </front>
1837        <seriesInfo name="BCP" value="90" />
1838        <seriesInfo name="RFC" value="3864" />
1839      </reference>
1840
1841    </references>
1842
1843    <section anchor="compatibility" title="Compatibility with Previous Versions">
1844
1845      <section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
1846        <t>A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
1847          this missing case. (Sections <xref format="counter" target="response.cacheability" />,
1848            <xref format="counter" target="header.cache-control" />).</t>
1849        <t>Transfer-coding and message lengths all interact in ways that required fixing exactly
1850          when chunked encoding is used (to allow for transfer encoding that may not be self
1851          delimiting); it was important to straighten out exactly how message lengths are computed.
1852          (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)</t>
1853        <t>Proxies should be able to add Content-Length when appropriate.</t>
1854        <t>Range request responses would become very verbose if all meta-data were always returned;
1855          by allowing the server to only send needed headers in a 206 response, this problem can be
1856          avoided. (<xref target="combining.headers" />)</t>
1857        <t>The Cache-Control: max-age directive was not properly defined for responses.</t>
1858        <t>Warnings could be cached incorrectly, or not updated appropriately. <xref
1859            format="counter" target="expiration.model" />, <xref format="counter"
1860            target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
1861          and <xref format="counter" target="header.warning" />) Warning also needed to be a general
1862          header, as PUT or other methods may have need for it in requests.</t>
1863      </section>
1864
1865      <section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
1866        <t>Clarify denial of service attack avoidance requirement. (<xref
1867            target="invalidation.after.updates.or.deletions" />)</t>
1868      </section>
1869
1870    </section>
1871
1872<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1873<figure>
1874<artwork type="abnf" name="p6-cache.parsed-abnf">
1875<x:ref>Age</x:ref> = "Age:" OWS Age-v
1876<x:ref>Age-v</x:ref> = delta-seconds
1877
1878<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
1879<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
1880 cache-directive ] )
1881
1882<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
1883<x:ref>Expires-v</x:ref> = HTTP-date
1884
1885<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
1886
1887<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1888
1889<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
1890<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1891 pragma-directive ] )
1892
1893<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
1894<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1895 ] ) )
1896
1897<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
1898<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1899 ] )
1900
1901<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
1902<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
1903<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
1904 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1905 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1906 cache-extension
1907<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
1908 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1909 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1910 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1911 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1912 ) / ( "s-maxage=" delta-seconds ) / cache-extension
1913
1914<x:ref>delta-seconds</x:ref> = 1*DIGIT
1915
1916<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
1917
1918<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
1919
1920<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
1921<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
1922<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
1923
1924<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1925
1926<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
1927
1928<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
1929
1930<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
1931<x:ref>warn-code</x:ref> = 3DIGIT
1932<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
1933<x:ref>warn-text</x:ref> = quoted-string
1934<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
1935 ]
1936
1937
1938</artwork>
1939</figure>
1940<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">; Age defined but not used
1941; Cache-Control defined but not used
1942; Expires defined but not used
1943; Pragma defined but not used
1944; Vary defined but not used
1945; Warning defined but not used
1946</artwork></figure></section>
1947
1948    <section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
1949
1950      <section title="Since RFC2616">
1951        <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
1952      </section>
1953
1954      <section title="Since draft-ietf-httpbis-p6-cache-00">
1955        <t>Closed issues: <list style="symbols">
1956            <t>
1957              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer"
1958                (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
1959            <t>
1960              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation
1961              after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
1962            <t>
1963              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and
1964              Informative references"</t>
1965            <t>
1966              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date
1967              reference typo"</t>
1968            <t>
1969              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection
1970              header text"</t>
1971            <t>
1972              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative
1973              references"</t>
1974            <t>
1975              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1
1976              Reference"</t>
1977            <t>
1978              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative
1979              up-to-date references"</t>
1980            <t>
1981              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in
1982              13.2.2"</t>
1983          </list>
1984        </t>
1985        <t>Other changes: <list style="symbols">
1986            <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
1987                target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
1988          </list>
1989        </t>
1990      </section>
1991
1992      <section title="Since draft-ietf-httpbis-p6-cache-01">
1993        <t>Closed issues: <list style="symbols">
1994            <t>
1995              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not
1996              used"</t>
1997          </list>
1998        </t>
1999        <t>Other changes: <list style="symbols">
2000            <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2001              on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2002            <t>Add explicit references to BNF syntax and rules imported from other parts of the
2003              specification.</t>
2004          </list>
2005        </t>
2006      </section>
2007
2008      <section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2009        <t>Ongoing work on IANA Message Header Registration (<eref
2010            target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40" />): <list style="symbols">
2011            <t>Reference RFC 3984, and update header registrations for headers defined in this
2012              document.</t>
2013          </list>
2014        </t>
2015      </section>
2016
2017      <section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2018        <t>Closed issues: <list style="symbols">
2019            <t>
2020              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header
2021              classification"</t>
2022          </list>
2023        </t>
2024      </section>
2025
2026      <section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2027      <t>
2028      </t>
2029      </section>
2030
2031      <section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2032      <t>
2033        This is a total rewrite.
2034      </t>
2035      </section>
2036
2037    </section>
2038  </back>
2039</rfc>
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