source: draft-ietf-httpbis/latest/p6-cache.xml @ 1924

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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 "October">
16  <!ENTITY ID-YEAR "2012">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY architecture                "<xref target='Part1' x:rel='#architecture' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY conformance                 "<xref target='Part1' x:rel='#conformance' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY whitespace                  "<xref target='Part1' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY field-components            "<xref target='Part1' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY http-date                   "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-date                 "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-vary                 "<xref target='Part2' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46]>
47<?rfc toc="yes" ?>
48<?rfc symrefs="yes" ?>
49<?rfc sortrefs="yes" ?>
50<?rfc compact="yes"?>
51<?rfc subcompact="no" ?>
52<?rfc linkmailto="no" ?>
53<?rfc editing="no" ?>
54<?rfc comments="yes"?>
55<?rfc inline="yes"?>
56<?rfc rfcedstyle="yes"?>
57<?rfc-ext allow-markup-in-artwork="yes" ?>
58<?rfc-ext include-references-in-index="yes" ?>
59<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
60  obsoletes="2616" x:maturity-level="proposed" xmlns:x="http://purl.org/net/xml2rfc/ext">
61<x:link rel="prev" basename="p5-range"/>
62<x:link rel="next" basename="p7-auth"/>
63<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
64<front>
65
66  <title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
67
68  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
69    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
70    <address>
71      <postal>
72        <street>345 Park Ave</street>
73        <city>San Jose</city>
74        <region>CA</region>
75        <code>95110</code>
76        <country>USA</country>
77      </postal>
78      <email>fielding@gbiv.com</email>
79      <uri>http://roy.gbiv.com/</uri>
80    </address>
81  </author>
82
83  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
84    <organization>Akamai</organization>
85    <address>
86      <email>mnot@mnot.net</email>
87      <uri>http://www.mnot.net/</uri>
88    </address>
89  </author>
90
91  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
92    <organization abbrev="greenbytes">greenbytes GmbH</organization>
93    <address>
94      <postal>
95        <street>Hafenweg 16</street>
96        <city>Muenster</city><region>NW</region><code>48155</code>
97        <country>Germany</country>
98      </postal>
99      <email>julian.reschke@greenbytes.de</email>
100      <uri>http://greenbytes.de/tech/webdav/</uri>
101    </address>
102  </author>
103
104  <date month="&ID-MONTH;" year="&ID-YEAR;" />
105  <workgroup>HTTPbis Working Group</workgroup>
106
107<abstract>
108<t>
109   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
110   distributed, collaborative, hypertext information systems. This document
111   defines requirements on HTTP caches and the associated header fields
112   that control cache behavior or indicate cacheable response messages.
113</t>
114</abstract>
115
116<note title="Editorial Note (To be removed by RFC Editor)">
117  <t>
118    Discussion of this draft takes place on the HTTPBIS working group
119    mailing list (ietf-http-wg@w3.org), which is archived at
120    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
121  </t>
122  <t>
123    The current issues list is at
124    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
125    documents (including fancy diffs) can be found at
126    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
127  </t>
128  <t>
129    The changes in this draft are summarized in <xref target="changes.since.20"/>.
130  </t>
131</note>
132
133   </front>
134   <middle>
135
136<section anchor="caching" title="Introduction">
137<t>
138   HTTP is typically used for distributed information systems, where
139   performance can be improved by the use of response caches. This document
140   defines aspects of HTTP/1.1 related to caching and reusing response
141   messages.
142</t>
143
144<section anchor="intro.purpose" title="Purpose">
145<iref item="cache" />
146<t>
147   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
148   subsystem that controls its message storage, retrieval, and deletion. A
149   cache stores cacheable responses in order to reduce the response time and
150   network bandwidth consumption on future, equivalent requests. Any client or
151   server &MAY; employ a cache, though a cache cannot be used by a server that
152   is acting as a tunnel.
153</t>
154<t>
155   The goal of caching in HTTP/1.1 is to significantly improve performance
156   by reusing a prior response message to satisfy a current request.
157   A stored response is considered "fresh", as defined in
158   <xref target="expiration.model" />, if the response can be reused without
159   "validation" (checking with the origin server to see if the cached response
160   remains valid for this request).  A fresh cache response can therefore
161   reduce both latency and network transfers each time it is reused.
162   When a cached response is not fresh, it might still be reusable if it can
163   be freshened by validation (<xref target="validation.model" />) or if the
164   origin is unavailable.
165</t>
166</section>
167
168<section anchor="intro.terminology" title="Terminology">
169<t>
170   This specification uses a number of terms to refer to the roles played by
171   participants in, and objects of, HTTP caching.
172</t>
173<t>
174   <iref item="cache" />
175   <x:dfn>cache</x:dfn>
176   <list>
177      <t>A conformant implementation of a HTTP cache. Note that this implies
178        an HTTP/1.1 cache; this specification does not define conformance
179        for HTTP/1.0 caches.</t>
180   </list>
181</t>
182<t anchor="shared.and.non-shared.caches">
183   <iref item="shared cache" />
184   <x:dfn>shared cache</x:dfn>
185   <list>
186      <t>A cache that stores responses to be reused by more than one user;
187         usually (but not always) deployed as part of an intermediary.</t>
188   </list>
189</t>
190<t>
191   <iref item="private cache" />
192   <x:dfn>private cache</x:dfn>
193   <list>
194      <t>A cache that is dedicated to a single user.</t>
195   </list>
196</t>
197<t>
198   <iref item="cacheable" />
199   <x:dfn>cacheable</x:dfn>
200   <list>
201      <t>A response is cacheable if a cache is allowed to store a copy of the
202      response message for use in answering subsequent requests. Even when a
203      response is cacheable, there might be additional constraints on whether
204      a cache can use the stored copy to satisfy a particular request.</t>
205   </list>
206</t>
207<t>
208   <iref item="explicit expiration time" />
209   <x:dfn>explicit expiration time</x:dfn>
210   <list>
211      <t>The time at which the origin server intends that a representation
212      no longer be returned by a cache without further validation.</t>
213   </list>
214</t>
215<t>
216   <iref item="heuristic expiration time" />
217   <x:dfn>heuristic expiration time</x:dfn>
218   <list>
219      <t>An expiration time assigned by a cache when no explicit expiration
220      time is available.</t>
221   </list>
222</t>
223<t>
224   <iref item="age" />
225   <x:dfn>age</x:dfn>
226   <list>
227      <t>The age of a response is the time since it was sent by, or
228      successfully validated with, the origin server.</t>
229   </list>
230</t>
231<t>
232   <iref item="first-hand" />
233   <x:dfn>first-hand</x:dfn>
234   <list>
235      <t>A response is first-hand if the freshness model is not in use; i.e.,
236      its age is 0.</t>
237   </list>
238</t>
239<t>
240   <iref item="freshness lifetime" />
241   <x:dfn>freshness lifetime</x:dfn>
242   <list>
243      <t>The length of time between the generation of a response and its
244      expiration time.</t>
245   </list>
246</t>
247<t>
248   <iref item="fresh" />
249   <x:dfn>fresh</x:dfn>
250   <list>
251      <t>A response is fresh if its age has not yet exceeded its freshness
252      lifetime.</t>
253   </list>
254</t>
255<t>
256   <iref item="stale" />
257   <x:dfn>stale</x:dfn>
258   <list>
259      <t>A response is stale if its age has passed its freshness lifetime
260      (either explicit or heuristic).</t>
261   </list>
262</t>
263<t>
264   <iref item="validator" />
265   <x:dfn>validator</x:dfn>
266   <list>
267      <t>A protocol element (e.g., an entity-tag or a <x:ref>Last-Modified</x:ref>
268      time) that is used to find out whether a stored response is an equivalent
269      copy of a representation. See &weak-and-strong;.</t>
270   </list>
271</t>
272<t>
273   <iref item="strong validator" />
274   <iref item="validator" subitem="strong" />
275   <x:dfn>strong validator</x:dfn>
276   <list>
277      <t>A validator that is defined by the origin server such that its
278         current value will change if the representation data changes; i.e.,
279         an entity-tag that is not marked as weak (&entity-tags;) or,
280         if no entity-tag is provided, a <x:ref>Last-Modified</x:ref> value
281         that is strong in the sense defined by &lastmod-comparison;.</t>
282   </list>
283</t>
284</section>
285
286<section title="Conformance and Error Handling" anchor="conformance">
287<t>
288   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
289   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
290   document are to be interpreted as described in <xref target="RFC2119"/>.
291</t>
292<t>
293   Conformance criteria and considerations regarding error handling
294   are defined in &conformance;.
295</t>
296</section>
297
298<section title="Syntax Notation" anchor="notation">
299<t>
300   This specification uses the Augmented Backus-Naur Form (ABNF) notation
301   of <xref target="RFC5234"/> with the list rule extension defined in
302   &notation;. <xref target="imported.abnf"/> describes rules imported from
303   other documents. <xref target="collected.abnf"/> shows the collected ABNF
304   with the list rule expanded.
305</t>
306
307<section title="Delta Seconds" anchor="delta-seconds">
308<t>
309   The delta-seconds rule specifies a non-negative integer, representing time
310   in seconds.
311</t>
312<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
313  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
314</artwork></figure>
315<t>
316   If an implementation receives a delta-seconds value larger than the largest
317   positive integer it can represent, or if any of its subsequent calculations
318   overflows, it &MUST; consider the value to be 2147483648
319   (2<x:sup>31</x:sup>). Recipients parsing a delta-seconds value &MUST; use
320   an arithmetic type of at least 31 bits of range, and senders &MUST-NOT;
321   send delta-seconds with a value greater than 2147483648.
322</t>
323</section>
324
325</section>
326</section>
327
328<section anchor="caching.overview" title="Overview of Cache Operation">
329<iref item="cache entry" />
330<iref item="cache key" />
331<t>
332   Proper cache operation preserves the semantics of HTTP transfers
333   (&semantics;) while eliminating the transfer of information already held
334   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
335   we assume that reusing the cached response is desirable and that such
336   reuse is the default behavior when no requirement or locally-desired
337   configuration prevents it.  Therefore, HTTP cache requirements are focused
338   on preventing a cache from either storing a non-reusable response or
339   reusing a stored response inappropriately.
340</t>
341<t>
342   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
343   HTTP responses corresponding to prior requests that used the same key. The
344   most common form of cache entry is a successful result of a retrieval
345   request: i.e., a <x:ref>200 (OK)</x:ref> response containing a
346   representation of the resource identified by the request target. However,
347   it is also possible to cache negative results (e.g., <x:ref>404 (Not
348   Found)</x:ref>, incomplete results (e.g., <x:ref>206 (Partial
349   Content)</x:ref>), and responses to methods other than GET if the method's
350   definition allows such caching and defines something suitable for use as a
351   cache key.
352</t>
353<t>
354   The default <x:dfn>cache key</x:dfn> consists of the request method and
355   target URI.  However, since HTTP caches in common use today are typically
356   limited to caching responses to GET, many implementations simply decline
357   other methods and use only the URI as the key.
358</t>
359<t>
360   If a request target is subject to content negotiation, its cache entry
361   might consist of multiple stored responses, each differentiated by a
362   secondary key for the values of the original request's selecting header
363   fields (<xref target="caching.negotiated.responses"/>).
364</t>
365</section>
366
367<section anchor="response.cacheability" title="Storing Responses in Caches">
368<t>
369   A cache &MUST-NOT; store a response to any request, unless:
370   <list style="symbols">
371      <t>The request method is understood by the cache and defined as being
372      cacheable, and</t>
373      <t>the response status code is understood by the cache, and</t>
374      <t>the "no-store" cache directive (see <xref
375      target="header.cache-control" />) does not appear in request or response
376      header fields, and</t>
377      <t>the "private" cache response directive (see <xref
378      target="cache-response-directive.private" />) does not appear in the
379      response, if the cache is shared, and</t>
380      <t>the <x:ref>Authorization</x:ref> header field (see
381      &header-authorization;) does not appear in the request, if the cache is
382      shared, unless the response explicitly allows it (see <xref
383      target="caching.authenticated.responses" />), and</t>
384      <t>the response either:
385         <list style="symbols">
386            <t>contains an <x:ref>Expires</x:ref> header field (see
387            <xref target="header.expires"/>), or</t>
388            <t>contains a max-age response cache directive (see <xref
389            target="cache-response-directive.max-age" />), or</t>
390            <t>contains a s-maxage response cache directive and the cache is
391            shared, or</t>
392            <t>contains a Cache Control Extension (see <xref
393            target="cache.control.extensions" />) that allows it to be cached,
394            or</t>
395            <t>has a status code that can be served with heuristic freshness
396            (see <xref target="heuristic.freshness" />).</t>
397         </list>
398      </t>
399   </list>
400</t>
401<t>
402   Note that any of the requirements listed above can be overridden by a
403   cache-control extension; see <xref target="cache.control.extensions" />.
404</t>
405<t>
406   In this context, a cache has "understood" a request method or a response
407   status code if it recognizes it and implements any cache-specific
408   behavior.
409</t>
410<t>
411   Note that, in normal operation, many caches will not store a response that
412   has neither a cache validator nor an explicit expiration time, as such
413   responses are not usually useful to store. However, caches are not
414   prohibited from storing such responses.
415</t>
416
417<section anchor="incomplete.responses" title="Storing Incomplete Responses">
418<t>
419   A response message is considered complete when all of the octets indicated
420   by the message framing (&messaging;) are received prior to the connection
421   being closed. If the request is GET, the response status is <x:ref>200
422   (OK)</x:ref>, and the entire response header block has been received, a
423   cache &MAY; store an incomplete response message body if the cache entry is
424   recorded as incomplete. Likewise, a <x:ref>206 (Partial Content)</x:ref>
425   response &MAY; be stored as if it were an incomplete <x:ref>200
426   (OK)</x:ref> cache entry. However, a cache &MUST-NOT; store incomplete or
427   partial content responses if it does not support the <x:ref>Range</x:ref>
428   and <x:ref>Content-Range</x:ref> header fields or if it does not understand
429   the range units used in those fields.
430</t>
431<t>
432   A cache &MAY; complete a stored incomplete response by making a subsequent
433   range request (&partial;) and combining the successful response with the
434   stored entry, as defined in <xref target="combining.responses"/>. A cache
435   &MUST-NOT; use an incomplete response to answer requests unless the
436   response has been made complete or the request is partial and specifies a
437   range that is wholly within the incomplete response. A cache &MUST-NOT;
438   send a partial response to a client without explicitly marking it as such
439   using the <x:ref>206 (Partial Content)</x:ref> status code.
440</t>
441</section>
442
443
444<section anchor="caching.authenticated.responses" 
445   title="Storing Responses to Authenticated Requests">
446<t>
447   A shared cache &MUST-NOT; use a cached response to a request with an
448   <x:ref>Authorization</x:ref> header field (&header-authorization;) to
449   satisfy any subsequent request unless a cache directive that allows such
450   responses to be stored is present in the response.
451</t>
452<t>
453   In this specification, the following <x:ref>Cache-Control</x:ref> response
454   directives (<xref target="cache-response-directive"/>) have such an effect:
455   must-revalidate, public, s-maxage.
456</t>
457<t>
458   Note that cached responses that contain the "must-revalidate" and/or
459   "s-maxage" response directives are not allowed to be served stale (<xref
460   target="serving.stale.responses"/>) by shared caches. In particular, a
461   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
462   used to satisfy a subsequent request without revalidating it on the origin
463   server.
464</t>
465</section>
466</section>
467
468
469<section anchor="constructing.responses.from.caches" 
470   title="Constructing Responses from Caches">
471<t>
472   For a presented request, a cache &MUST-NOT; return a stored response,
473   unless:
474   <list style="symbols">
475      <t>The presented effective request URI (&effective-request-uri;) and
476      that of the stored response match, and</t>
477      <t>the request method associated with the stored response allows it to
478      be used for the presented request, and</t>
479      <t>selecting header fields nominated by the stored response (if any)
480      match those presented (see <xref target="caching.negotiated.responses"
481      />), and</t>
482      <t>the presented request does not contain the no-cache pragma (<xref
483      target="header.pragma"/>), nor the no-cache cache directive (<xref
484      target="cache-request-directive"/>), unless the stored response is
485      successfully validated (<xref target="validation.model"/>), and</t>
486      <t>the stored response does not contain the no-cache cache directive
487      (<xref target="cache-response-directive.no-cache"/>), unless it is
488      successfully validated (<xref target="validation.model"/>), and</t>
489      <t>the stored response is either:
490         <list style="symbols">
491            <t>fresh (see <xref target="expiration.model" />), or</t>
492            <t>allowed to be served stale (see <xref
493            target="serving.stale.responses" />), or</t>
494            <t>successfully validated (see <xref target="validation.model"
495            />).</t>
496         </list>
497      </t>
498  </list>
499</t>
500<t>
501   Note that any of the requirements listed above can be overridden by a
502   cache-control extension; see <xref target="cache.control.extensions" />.
503</t>
504<t>
505   When a stored response is used to satisfy a request without validation,
506   a cache &MUST; include a single <x:ref>Age</x:ref> header field
507   (<xref target="header.age"/>) in the response with a value equal to the
508   stored response's current_age; see <xref target="age.calculations" />.
509</t>
510<t>
511   A cache &MUST; write through requests with methods that are unsafe
512   (&safe-methods;) to the origin server; i.e., a cache is not allowed to
513   generate a reply to such a request before having forwarded the request and
514   having received a corresponding response.
515</t>
516<t>
517   Also, note that unsafe requests might invalidate already stored responses;
518   see <xref target="invalidation.after.updates.or.deletions" />.
519</t>
520<t>
521   When more than one suitable response is stored, a cache &MUST; use the
522   most recent response (as determined by the <x:ref>Date</x:ref> header
523   field). It can also forward a request with "Cache-Control: max-age=0" or
524   "Cache-Control: no-cache" to disambiguate which response to use.
525</t>
526<t>
527   A cache that does not have a clock available &MUST-NOT; use stored
528   responses without revalidating them on every use. A cache, especially a
529   shared cache, &SHOULD; use a mechanism, such as NTP <xref
530   target="RFC1305"/>, to synchronize its clock with a reliable external
531   standard.
532</t>
533
534
535<section anchor="expiration.model" title="Freshness Model">
536<t>
537   When a response is "fresh" in the cache, it can be used to satisfy
538   subsequent requests without contacting the origin server, thereby improving
539   efficiency.
540</t>
541<t>
542   The primary mechanism for determining freshness is for an origin server to
543   provide an explicit expiration time in the future, using either the
544   <x:ref>Expires</x:ref> header field (<xref target="header.expires" />) or
545   the max-age response cache directive (<xref
546   target="cache-response-directive.max-age" />). Generally, origin servers will
547   assign future explicit expiration times to responses in the belief that the
548   representation is not likely to change in a semantically significant way
549   before the expiration time is reached.
550</t>
551<t>
552   If an origin server wishes to force a cache to validate every request, it
553   can assign an explicit expiration time in the past to indicate that the
554   response is already stale. Compliant caches will normally validate the
555   cached response before reusing it for subsequent requests (see <xref
556   target="serving.stale.responses" />).
557</t>
558<t>
559   Since origin servers do not always provide explicit expiration times, a
560   cache &MAY; assign a heuristic expiration time when an explicit time is not
561   specified, employing algorithms that use other header field values (such as
562   the <x:ref>Last-Modified</x:ref> time) to estimate a plausible expiration
563   time. This specification does not provide specific algorithms, but does
564   impose worst-case constraints on their results.
565</t>
566<figure>
567<preamble>
568  The calculation to determine if a response is fresh is:
569</preamble>
570<artwork type="code">
571   response_is_fresh = (freshness_lifetime &gt; current_age)
572</artwork>
573</figure>
574<t>
575   The freshness_lifetime is defined in <xref
576   target="calculating.freshness.lifetime" />; the current_age is defined in
577   <xref target="age.calculations" />.
578</t>
579<t>
580   Additionally, clients can influence freshness calculation &mdash; either
581   constraining it relaxing it &mdash; by using the max-age and min-fresh
582   request cache directives. See <xref target="cache-request-directive" /> 
583   for details.
584</t>
585<t>
586   Note that freshness applies only to cache operation; it cannot be used to
587   force a user agent to refresh its display or reload a resource. See <xref
588   target="history.lists" /> for an explanation of the difference between
589   caches and history mechanisms.
590</t>
591
592<section anchor="calculating.freshness.lifetime" 
593   title="Calculating Freshness Lifetime">
594<t>
595   A cache can calculate the freshness lifetime (denoted as
596   freshness_lifetime) of a response by using the first match of:
597   <list style="symbols">
598      <t>If the cache is shared and the s-maxage response cache directive
599      (<xref target="cache-response-directive.s-maxage" />) is present, use its value,
600      or</t>
601      <t>If the max-age response cache directive (<xref
602      target="cache-response-directive.max-age" />) is present, use its value, or</t>
603      <t>If the <x:ref>Expires</x:ref> response header field
604      (<xref target="header.expires" />) is present, use its value minus the
605      value of the <x:ref>Date</x:ref> response header field, or</t>
606      <t>Otherwise, no explicit expiration time is present in the response. A
607      heuristic freshness lifetime might be applicable; see <xref
608      target="heuristic.freshness" />.</t>
609   </list>
610</t>
611<t>
612   Note that this calculation is not vulnerable to clock skew, since all of
613   the information comes from the origin server.
614</t>
615<t>
616   When there is more than one value present for a given directive (e.g., two
617   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
618   directives), it is considered invalid. Caches are encouraged to consider
619   responses that have invalid freshness information to be stale.
620</t>
621</section>
622
623<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
624<t>
625   If no explicit expiration time is present in a stored response that has a
626   status code whose definition allows heuristic freshness to be used
627   (including the following in &status-codes;: <x:ref>200 (OK)</x:ref>,
628   <x:ref>203 (Non-Authoritative Information)</x:ref>, <x:ref>206 (Partial
629   Content)</x:ref>, <x:ref>300 (Multiple Choices)</x:ref>, <x:ref>301 (Moved
630   Permanently)</x:ref> and <x:ref>410 (Gone)</x:ref>), a cache &MAY;
631   calculate a heuristic expiration time. A cache &MUST-NOT; use heuristics to
632   determine freshness for responses with status codes that do not explicitly
633   allow it.
634</t>
635<t>
636   When a heuristic is used to calculate freshness lifetime, a cache &SHOULD;
637   attach a <x:ref>Warning</x:ref> header field with a 113 warn-code to the
638   response if its current_age is more than 24 hours and such a warning is not
639   already present.
640</t>
641<t>
642   Also, if the response has a <x:ref>Last-Modified</x:ref> header field
643   (&header-last-modified;), caches are encouraged to use a heuristic
644   expiration value that is no more than some fraction of the interval since
645   that time. A typical setting of this fraction might be 10%.
646</t>
647<x:note>
648   <t>
649      &Note; <xref target="RFC2616" x:fmt="of" x:sec="13.9"/> prohibited caches
650      from calculating heuristic freshness for URIs with query components
651      (i.e., those containing '?'). In practice, this has not been widely
652      implemented. Therefore, servers are encouraged to send explicit
653      directives (e.g., Cache-Control: no-cache) if they wish to preclude
654      caching.
655   </t>
656</x:note>
657</section>
658
659<section anchor="age.calculations" title="Calculating Age">
660<t>
661   HTTP/1.1 uses the <x:ref>Age</x:ref> header field to convey the estimated
662   age of the response message when obtained from a cache. The Age field value
663   is the cache's estimate of the amount of time since the response was
664   generated or validated by the origin server. In essence, the Age value is
665   the sum of the time that the response has been resident in each of the
666   caches along the path from the origin server, plus the amount of time it
667   has been in transit along network paths.
668</t>
669<t>
670   The following data is used for the age calculation:
671</t>
672<t>
673   <x:dfn>age_value</x:dfn>
674   <list>
675      <t>
676         The term "age_value" denotes the value of the <x:ref>Age</x:ref>
677         header field (<xref target="header.age"/>), in a form appropriate for
678         arithmetic operation; or 0, if not available.
679      </t>
680   </list>
681</t>
682<t>
683   <x:dfn>date_value</x:dfn>
684   <list>
685      <t>
686         HTTP/1.1 requires origin servers to send a <x:ref>Date</x:ref> header
687         field, if possible, with every response, giving the time at which the
688         response was generated. The term "date_value" denotes the value of
689         the Date header field, in a form appropriate for arithmetic
690         operations. See &header-date; for the definition of the Date header
691         field, and for requirements regarding responses without it.
692      </t>
693   </list>
694</t>
695<t>
696   <x:dfn>now</x:dfn>
697   <list>
698      <t>
699         The term "now" means "the current value of the clock at the host
700         performing the calculation". A cache &SHOULD; use NTP (<xref
701         target="RFC1305"/>) or some similar protocol to synchronize its
702         clocks to a globally accurate time standard.
703      </t>
704   </list>
705</t>
706<t>
707   <x:dfn>request_time</x:dfn>
708   <list>
709      <t>
710         The current value of the clock at the host at the time the request
711         resulting in the stored response was made.
712      </t>
713   </list>
714</t>
715<t>
716   <x:dfn>response_time</x:dfn>
717   <list>
718      <t>
719         The current value of the clock at the host at the time the response
720         was received.
721      </t>
722   </list>
723</t>
724<t>
725   A response's age can be calculated in two entirely independent ways:
726   <list style="numbers">
727      <t>the "apparent_age": response_time minus date_value, if the local
728      clock is reasonably well synchronized to the origin server's clock. If
729      the result is negative, the result is replaced by zero.</t>
730      <t>the "corrected_age_value", if all of the caches along the response
731      path implement HTTP/1.1. A cache &MUST; interpret this value relative
732      to the time the request was initiated, not the time that the response
733      was received.</t>
734   </list>
735</t>
736<figure>
737<artwork type="code">
738  apparent_age = max(0, response_time - date_value);
739
740  response_delay = response_time - request_time;
741  corrected_age_value = age_value + response_delay; 
742</artwork>
743</figure>
744<figure>
745<preamble>These &SHOULD; be combined as</preamble>
746<artwork type="code">
747  corrected_initial_age = max(apparent_age, corrected_age_value);
748</artwork></figure>
749<t>
750   unless the cache is confident in the value of the <x:ref>Age</x:ref> header
751   field (e.g., because there are no HTTP/1.0 hops in the <x:ref>Via</x:ref>
752   header field), in which case the corrected_age_value &MAY; be used as the
753   corrected_initial_age.</t>
754<t>
755   The current_age of a stored response can then be calculated by adding the
756   amount of time (in seconds) since the stored response was last validated by
757   the origin server to the corrected_initial_age.
758</t>
759<figure><artwork type="code">
760  resident_time = now - response_time;
761  current_age = corrected_initial_age + resident_time;
762</artwork></figure>
763<t>
764   Additionally, to avoid common problems in date parsing:
765</t>
766<t>
767  <list style="symbols">
768     <t>Recipients &SHOULD; assume that an RFC-850 date
769        which appears to be more than 50 years in the future is in fact
770        in the past (this helps solve the "year 2000" problem).</t>
771
772     <t>Although all date formats are specified to be case-sensitive,
773        recipients &SHOULD; match day, week and timezone names
774        case-insensitively.</t>
775             
776     <t>An implementation &MAY; internally represent a parsed
777        <x:ref>Expires</x:ref> date as earlier than the proper value, but
778        &MUST-NOT; internally represent a parsed Expires date as later than the
779        proper value.</t>
780
781     <t>Recipients &MUST; perform all expiration-related calculations in GMT.
782        The local time zone &MUST-NOT; influence the calculation or comparison
783        of an age or expiration time.</t>
784
785     <t>Caches &SHOULD; consider dates with time zones other than "GMT"
786        invalid.</t>
787  </list>
788</t>
789</section>
790
791<section anchor="serving.stale.responses" title="Serving Stale Responses">
792<t>
793   A "stale" response is one that either has explicit expiry information or is
794   allowed to have heuristic expiry calculated, but is not fresh according to
795   the calculations in <xref target="expiration.model" />.
796</t>
797<t>
798   A cache &MUST-NOT; return a stale response if it is prohibited by an
799   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
800   directive, a "must-revalidate" cache-response-directive, or an applicable
801   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
802   target="cache-response-directive"/>).
803</t>
804<t>
805   A cache &MUST-NOT; return stale responses unless it is disconnected
806   (i.e., it cannot contact the origin server or otherwise find a forward
807   path) or doing so is explicitly allowed (e.g., by the max-stale request
808   directive; see <xref target="cache-request-directive" />).
809</t>
810<t>
811   A cache &SHOULD; append a <x:ref>Warning</x:ref> header field with the 110
812   warn-code (see <xref target="header.warning"/>) to stale responses.
813   Likewise, a cache &SHOULD; add the 112 warn-code to stale responses if the
814   cache is disconnected.
815</t>
816<t>
817   If a cache receives a first-hand response (either an entire response, or a
818   <x:ref>304 (Not Modified)</x:ref> response) that it would normally forward
819   to the requesting client, and the received response is no longer fresh, the
820   cache can forward it to the requesting client without adding a new
821   <x:ref>Warning</x:ref> (but without removing any existing Warning header
822   fields). A cache shouldn't attempt to validate a response simply because
823   that response became stale in transit.
824</t>
825</section>
826</section>
827
828<section anchor="validation.model" title="Validation Model">
829<t>
830   When a cache has one or more stored responses for a requested URI, but
831   cannot serve any of them (e.g., because they are not fresh, or one cannot
832   be selected; see <xref target="caching.negotiated.responses"/>), it can use
833   the conditional request mechanism &conditional; in the forwarded request to
834   give the origin server an opportunity to both select a valid stored
835   response to be used, and to update it. This process is known as
836   "validating" or "revalidating" the stored response.
837</t>
838<t>
839   When sending such a conditional request, a cache adds an
840   <x:ref>If-Modified-Since</x:ref> header field whose value is that of the
841   <x:ref>Last-Modified</x:ref> header field from the selected
842   (see <xref target="caching.negotiated.responses"/>) stored response, if
843   available.
844</t>
845<t>
846   Additionally, a cache can add an <x:ref>If-None-Match</x:ref> header field
847   whose value is that of the <x:ref>ETag</x:ref> header field(s) from all
848   responses stored for the requested URI, if present. However, if any of the
849   stored responses contains only partial content, the cache shouldn't
850   include its entity-tag in the If-None-Match header field unless the request
851   is for a range that would be fully satisfied by that stored response.
852</t>
853
854<t>Cache handling of a response to a conditional request is dependent upon its
855status code:</t>
856
857<t>
858   <list style="symbols">
859      <t>
860         A <x:ref>304 (Not Modified)</x:ref> response status code indicates
861         that the stored response can be updated and reused; see <xref
862         target="freshening.responses"/>.
863      </t>
864      <t>
865         A full response (i.e., one with a payload body) indicates that none
866         of the stored responses nominated in the conditional request is
867         suitable. Instead, the cache can use the full response to
868         satisfy the request and &MAY; replace the stored response(s).
869      </t>
870      <t>
871         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
872         response while attempting to validate a response, it can either
873         forward this response to the requesting client, or act as if the
874         server failed to respond. In the latter case, it can return a
875         previously stored response (see <xref
876         target="serving.stale.responses" />).
877      </t>
878   </list>
879</t>
880
881<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
882<t>
883   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and
884   already has one or more stored <x:ref>200 (OK)</x:ref> responses for the
885   same cache key, the cache needs to identify which of the stored responses
886   are updated by this new response and then update the stored response(s)
887   with the new information provided in the <x:ref>304</x:ref> response.
888   <list style="symbols">
889    <t>
890     If the new response contains a strong validator, then that strong
891     validator identifies the selected representation.  All of the stored
892     responses with the same strong validator are selected.
893     If none of the stored responses contain the same strong validator, then
894     this new response corresponds to a new selected representation and
895     &MUST-NOT; update the existing stored responses.
896    </t>
897    <t>
898     If the new response contains a weak validator and that validator
899     corresponds to one of the cache's stored responses, then the most
900     recent of those matching stored responses is selected.
901    </t>
902    <t>
903     If the new response does not include any form of validator, there is
904     only one stored response, and that stored response also lacks a
905     validator, then that stored response is selected.
906    </t>
907   </list>
908</t>
909<t>
910   If a stored response is selected for update, the cache &MUST;:
911   <list style="symbols">
912      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
913         with warn-code 1xx (see <xref target="header.warning" />);</t>
914      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
915         with warn-code 2xx; and,</t>
916      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
917         response to replace all instances of the corresponding header
918         fields in the stored response.</t>
919   </list>
920</t>
921</section>
922
923</section>
924
925<section anchor="caching.negotiated.responses" 
926   title="Using Negotiated Responses">
927<t>
928   When a cache receives a request that can be satisfied by a stored response
929   that has a <x:ref>Vary</x:ref> header field (&header-vary;),
930   it &MUST-NOT; use that response unless all of the selecting header fields
931   nominated by the Vary header field match in both the original request
932   (i.e., that associated with the stored response), and the presented
933   request.
934</t>
935<t>
936   The selecting header fields from two requests are defined to match if and
937   only if those in the first request can be transformed to those in the
938   second request by applying any of the following:
939   <list style="symbols">
940      <t>
941         adding or removing whitespace, where allowed in the header field's
942         syntax
943      </t>
944      <t>
945         combining multiple header fields with the same field name
946         (see &header-fields;)
947      </t>
948      <t>
949         normalizing both header field values in a way that is known to have
950         identical semantics, according to the header field's specification
951         (e.g., re-ordering field values when order is not significant;
952         case-normalization, where values are defined to be case-insensitive)
953      </t>
954  </list>
955</t>
956<t>
957   If (after any normalization that might take place) a header field is absent
958   from a request, it can only match another request if it is also absent
959   there.
960</t>
961<t>
962   A <x:ref>Vary</x:ref> header field-value of "*" always fails to match, and
963   subsequent requests to that resource can only be properly interpreted by the
964   origin server.
965</t>
966<t>
967   The stored response with matching selecting header fields is known as the
968   selected response.
969</t>
970<t>
971   If multiple selected responses are available, the most recent response
972   (as determined by the <x:ref>Date</x:ref> header field) is used; see <xref 
973   target="constructing.responses.from.caches"/>.
974</t>
975<t>
976   If no selected response is available, the cache can forward the presented
977   request to the origin server in a conditional request; see <xref
978   target="validation.model"/>.
979</t>
980</section>
981
982
983<section anchor="combining.responses" title="Combining Partial Content">
984<t>
985   A response might transfer only a partial representation if the
986   connection closed prematurely or if the request used one or more Range
987   specifiers (&partial;).  After several such transfers, a cache might have
988   received several ranges of the same representation.  A cache &MAY; combine
989   these ranges into a single stored response, and reuse that response to
990   satisfy later requests, if they all share the same strong validator and
991   the cache complies with the client requirements in &combining-byte-ranges;.
992</t>
993<t>
994   When combining the new response with one or more stored responses, a
995   cache &MUST;:
996   <list style="symbols">
997      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
998         with warn-code 1xx (see <xref target="header.warning" />);</t>
999      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1000         with warn-code 2xx; and,</t>
1001      <t>use other header fields provided in the new response, aside
1002         from <x:ref>Content-Range</x:ref>, to replace all instances of the
1003         corresponding header fields in the stored response.</t>
1004   </list>
1005</t>
1006</section>
1007</section>
1008
1009
1010<section anchor="head.effects" title="Updating Caches with HEAD Responses">
1011<t>
1012   A response to the HEAD method is identical to what an equivalent request
1013   made with a GET would have been, except it lacks a body. This property
1014   of HEAD responses is used to both invalidate and update cached GET
1015   responses.
1016</t>
1017<t>
1018   If one or more stored GET responses can be selected (as per <xref
1019   target="caching.negotiated.responses"/>) for a HEAD request, and the
1020   <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> or
1021   <x:ref>Last-Modified</x:ref> value of a HEAD response differs from that in a
1022   selected GET response, the cache &MUST; consider that selected response to
1023   be stale.
1024</t>
1025<t>
1026   If the <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> and
1027   <x:ref>Last-Modified</x:ref> values of a HEAD response (when present) are
1028   the same as that in a selected GET response (as per
1029   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update
1030   the remaining header fields in the stored response using the following
1031   rules:
1032   <list style="symbols">
1033      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1034         with warn-code 1xx (see <xref target="header.warning" />);</t>
1035      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1036         with warn-code 2xx; and,</t>
1037      <t>use other header fields provided in the response to replace
1038         all instances of the corresponding header fields in the stored
1039         response.</t>
1040   </list>
1041</t>
1042
1043</section>
1044
1045
1046<section anchor="invalidation.after.updates.or.deletions" 
1047   title="Request Methods that Invalidate">
1048<t>
1049   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1050   have the potential for changing state on the origin server, intervening
1051   caches can use them to keep their contents up-to-date.
1052</t>
1053<t>
1054   A cache &MUST; invalidate the effective Request URI
1055   (&effective-request-uri;) as well as the URI(s) in the
1056   <x:ref>Location</x:ref> and <x:ref>Content-Location</x:ref> response header
1057   fields (if present) when a non-error response to a request with an unsafe
1058   method is received.
1059</t>
1060<t>
1061   However, a cache &MUST-NOT; invalidate a URI from a <x:ref>Location</x:ref>
1062   or <x:ref>Content-Location</x:ref> response header field if the host part of
1063   that URI differs from the host part in the effective request URI
1064   (&effective-request-uri;). This helps prevent denial of service attacks.
1065</t>
1066<t>
1067   A cache &MUST; invalidate the effective request URI
1068   (&effective-request-uri;) when it receives a non-error response
1069   to a request with a method whose safety is unknown.
1070</t>
1071<t>
1072   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref>
1073   or <x:ref>3xx (Redirection)</x:ref> status code. "Invalidate" means that
1074   the cache will either remove all stored responses related to the effective
1075   request URI, or will mark these as "invalid" and in need of a mandatory
1076   validation before they can be returned in response to a subsequent request.
1077</t>
1078<t>
1079   Note that this does not guarantee that all appropriate responses are
1080   invalidated. For example, the request that caused the change at the origin
1081   server might not have gone through the cache where a response is stored.
1082</t>
1083</section>
1084
1085
1086
1087
1088<section anchor="header.field.definitions" title="Header Field Definitions">
1089<t>
1090   This section defines the syntax and semantics of HTTP/1.1 header fields
1091   related to caching.
1092</t>
1093
1094<section anchor="header.age" title="Age">
1095   <iref item="Age header field" primary="true" x:for-anchor="" />
1096   <x:anchor-alias value="Age"/>
1097   <x:anchor-alias value="age-value"/>
1098<t>
1099   The "Age" header field conveys the sender's estimate of the amount
1100   of time since the response was generated or successfully validated at the
1101   origin server. Age values are calculated as specified in <xref
1102   target="age.calculations" />.
1103</t>
1104<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1105  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1106</artwork></figure>
1107<t>
1108  Age field-values are non-negative integers, representing time in seconds
1109  (see <xref target="delta-seconds"/>).
1110</t>
1111<t>
1112   The presence of an Age header field in a response implies that a response
1113   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1114   might not implement the Age header field.
1115</t>
1116</section>
1117
1118<section anchor="header.cache-control" title="Cache-Control">
1119   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1120   <x:anchor-alias value="Cache-Control"/>
1121   <x:anchor-alias value="cache-directive"/>
1122<t>
1123   The "Cache-Control" header field is used to specify directives for
1124   caches along the request/response chain. Such cache directives are
1125   unidirectional in that the presence of a directive in a request does not
1126   imply that the same directive is to be given in the response.
1127</t>
1128<t>
1129   A cache &MUST; obey the requirements of the Cache-Control
1130   directives defined in this section. See <xref
1131   target="cache.control.extensions"/> for information about how Cache-Control
1132   directives defined elsewhere are handled.
1133</t>
1134<x:note>
1135   <t>
1136       &Note; HTTP/1.0 caches might not implement Cache-Control and
1137       might only implement Pragma: no-cache (see <xref target="header.pragma"
1138       />).
1139   </t>
1140</x:note>
1141<t>
1142   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1143   through in forwarded messages, regardless of their
1144   significance to that application, since the directives might be applicable
1145   to all recipients along the request/response chain. It is not possible to
1146   target a directive to a specific cache.
1147</t>
1148<t>
1149   Cache directives are identified by a token, to be compared case-insensitively,
1150   and have an optional argument, that can use both token and quoted-string
1151   syntax. For the directives defined below that define arguments, recipients
1152   ought to accept both forms, even if one is documented to be preferred. For
1153   any directive not defined by this specification, recipients &MUST; accept
1154   both forms.
1155</t>
1156<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1157  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1158
1159  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1160</artwork></figure>
1161<t>
1162   For the cache directives defined below, no argument is defined (nor allowed)
1163   otherwise stated otherwise.
1164</t>
1165
1166<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1167
1168<section title="no-cache" anchor="cache-request-directive.no-cache">
1169   <iref item="no-cache (cache directive)" primary="true" />
1170<t>
1171   The "no-cache" request directive indicates that a cache &MUST-NOT; 
1172   use a stored response to satisfy the request without successful
1173   validation on the origin server.
1174</t>
1175</section>
1176 
1177<section title="no-store" anchor="cache-request-directive.no-store">
1178   <iref item="no-store (cache directive)" primary="true" />
1179<t>
1180   The "no-store" request directive indicates that a cache &MUST-NOT;
1181   store any part of either this request or any response to it. This
1182   directive applies to both private and shared caches. "&MUST-NOT;
1183   store" in this context means that the cache &MUST-NOT; intentionally
1184   store the information in non-volatile storage, and &MUST; make a
1185   best-effort attempt to remove the information from volatile storage as
1186   promptly as possible after forwarding it.
1187</t>
1188<t>
1189   This directive is NOT a reliable or sufficient mechanism for ensuring
1190   privacy. In particular, malicious or compromised caches might not
1191   recognize or obey this directive, and communications networks might be
1192   vulnerable to eavesdropping.
1193</t>
1194<t>
1195   Note that if a request containing this directive is satisfied from a
1196   cache, the no-store request directive does not apply to the already
1197   stored response.
1198</t>
1199</section>
1200
1201<section title="max-age" anchor="cache-request-directive.max-age">
1202   <iref item="max-age (cache directive)" primary="true" />
1203<t>
1204   Argument syntax:
1205   <list>
1206      <t>
1207        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1208      </t>
1209   </list>
1210</t>
1211<t>
1212   The "max-age" request directive indicates that the client is unwilling to
1213   accept a response whose age is greater than the specified number of
1214   seconds. Unless the max-stale request directive is also present, the
1215   client is not willing to accept a stale response.
1216</t>
1217<t>
1218   &Note; This directive uses the token form of the argument syntax;
1219   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1220   quoted-string form.
1221</t>
1222</section>
1223
1224<section title="max-stale" anchor="cache-request-directive.max-stale">
1225   <iref item="max-stale (cache directive)" primary="true" />
1226<t>
1227   Argument syntax:
1228   <list>
1229      <t>
1230        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1231      </t>
1232   </list>
1233</t>
1234<t>
1235   The "max-stale" request directive indicates that the client is willing
1236   to accept a response that has exceeded its expiration time. If max-stale
1237   is assigned a value, then the client is willing to accept a response
1238   that has exceeded its expiration time by no more than the specified
1239   number of seconds. If no value is assigned to max-stale, then the client
1240   is willing to accept a stale response of any age.
1241</t>
1242<t>
1243   &Note; This directive uses the token form of the argument syntax;
1244   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1245   quoted-string form.
1246</t>
1247</section>
1248
1249<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1250   <iref item="min-fresh (cache directive)" primary="true" />
1251<t>
1252   Argument syntax:
1253   <list>
1254      <t>
1255        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1256      </t>
1257   </list>
1258</t>
1259<t>
1260   The "min-fresh" request directive indicates that the client is willing
1261   to accept a response whose freshness lifetime is no less than its
1262   current age plus the specified time in seconds. That is, the client
1263   wants a response that will still be fresh for at least the specified
1264   number of seconds.
1265</t>
1266<t>
1267   &Note; This directive uses the token form of the argument syntax;
1268   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1269   quoted-string form.
1270</t>
1271</section>
1272
1273<section title="no-transform" anchor="cache-request-directive.no-transform">
1274   <iref item="no-transform (cache directive)" primary="true" />
1275<t>
1276   The "no-transform" request directive indicates that an intermediary
1277   (whether or not it implements a cache) &MUST-NOT; change the
1278   <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Range</x:ref> or
1279   <x:ref>Content-Type</x:ref> request header fields, nor the request
1280   representation.
1281</t>
1282</section>
1283
1284<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1285   <iref item="only-if-cached (cache directive)" primary="true" />
1286<t>
1287   The "only-if-cached" request directive indicates that the client only wishes
1288   to obtain a stored response. If it receives this directive, a cache &SHOULD;
1289   either respond using a stored response that is consistent with the other
1290   constraints of the request, or respond with a <x:ref>504 (Gateway
1291   Timeout)</x:ref> status code. If a group of caches is being operated as a
1292   unified system with good internal connectivity, a member cache &MAY;
1293   forward such a request within that group of caches.
1294</t>
1295</section>
1296</section>
1297
1298<section anchor="cache-response-directive" 
1299   title="Response Cache-Control Directives">
1300   <x:anchor-alias value="cache-response-directive" />
1301
1302<section title="public" anchor="cache-response-directive.only-if-cached">
1303   <iref item="public (cache directive)" primary="true" />
1304<t>
1305   The "public" response directive indicates that a response whose
1306   associated request contains an 'Authentication' header &MAY; be
1307   stored (see <xref target="caching.authenticated.responses" />).
1308</t>
1309</section>
1310
1311<section title="private" anchor="cache-response-directive.private">
1312   <iref item="private (cache directive)" primary="true" />
1313<t>
1314   Argument syntax:
1315   <list>
1316      <t>
1317        #<x:ref>field-name</x:ref>
1318      </t>
1319   </list>
1320</t>
1321<t>
1322   The "private" response directive indicates that the response message is
1323   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1324   private cache &MAY; store the response.
1325</t>
1326<t>
1327   If the private response directive specifies one or more field-names,
1328   this requirement is limited to the field-values associated with the
1329   listed response header fields. That is, a shared cache &MUST-NOT; store
1330   the specified field-names(s), whereas it &MAY; store the remainder of the
1331   response message.
1332</t>
1333<t>
1334   The field-names given are not limited to the set of standard header
1335   fields defined by this specification. Field names are case-insensitive.
1336</t>
1337<t>
1338   &Note; This usage of the word "private" only controls
1339   where the response can be stored; it cannot ensure the privacy of the
1340   message content. Also, private response directives with field-names are
1341   often handled by implementations as if an unqualified private directive
1342   was received; i.e., the special handling for the qualified form is not
1343   widely implemented.
1344</t>
1345<t>
1346   &Note; This directive uses the quoted-string form of the argument syntax.
1347   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1348   needed for single-entry lists).
1349</t>
1350</section>
1351
1352<section title="no-cache" anchor="cache-response-directive.no-cache">
1353   <iref item="no-cache (cache directive)" primary="true" />
1354<t>
1355   Argument syntax:
1356   <list>
1357      <t>
1358        #<x:ref>field-name</x:ref>
1359      </t>
1360   </list>
1361</t>
1362<t>
1363   The "no-cache" response directive indicates that the response &MUST-NOT;
1364   be used to satisfy a subsequent request without successful validation on
1365   the origin server. This allows an origin server to prevent a cache from
1366   using it to satisfy a request without contacting it, even by caches that
1367   have been configured to return stale responses.
1368</t>
1369<t>
1370   If the no-cache response directive specifies one or more field-names,
1371   then a cache &MAY; use the response to satisfy a subsequent request,
1372   subject to any other restrictions on caching. However, any header fields
1373   in the response that have the field-name(s) listed &MUST-NOT; be sent
1374   in the response to a subsequent request without successful revalidation
1375   with the origin server. This allows an origin server to prevent the
1376   re-use of certain header fields in a response, while still allowing
1377   caching of the rest of the response.
1378</t> 
1379<t>
1380   The field-names given are not limited to the set of standard header
1381   fields defined by this specification. Field names are case-insensitive.
1382</t>
1383<t>
1384   &Note; Many HTTP/1.0 caches will not recognize or obey
1385   this directive. Also, no-cache response directives with field-names are
1386   often handled by implementations as if an unqualified no-cache directive
1387   was received; i.e., the special handling for the qualified form is not
1388   widely implemented.
1389</t>
1390<t>
1391   &Note; This directive uses the quoted-string form of the argument syntax.
1392   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1393   needed for single-entry lists).
1394</t>
1395</section>
1396
1397<section title="no-store" anchor="cache-response-directive.no-store">
1398   <iref item="no-store (cache directive)" primary="true" />
1399<t>
1400   The "no-store" response directive indicates that a cache &MUST-NOT;
1401   store any part of either the immediate request or response. This
1402   directive applies to both private and shared caches. "&MUST-NOT;
1403   store" in this context means that the cache &MUST-NOT; intentionally
1404   store the information in non-volatile storage, and &MUST; make a
1405   best-effort attempt to remove the information from volatile storage as
1406   promptly as possible after forwarding it.
1407</t>
1408<t>
1409   This directive is NOT a reliable or sufficient mechanism for ensuring
1410   privacy. In particular, malicious or compromised caches might not
1411   recognize or obey this directive, and communications networks might be
1412   vulnerable to eavesdropping.
1413</t>
1414</section>
1415
1416<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1417   <iref item="must-revalidate (cache directive)" primary="true" />
1418<t>
1419   The "must-revalidate" response directive indicates that once it has
1420   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1421   requests without successful validation on the origin server.
1422</t>
1423<t>
1424   The must-revalidate directive is necessary to support reliable
1425   operation for certain protocol features. In all circumstances a
1426   cache &MUST; obey the must-revalidate directive; in particular,
1427   if a cache cannot reach the origin server for any reason, it &MUST;
1428   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1429</t>
1430<t>
1431   The must-revalidate directive ought to be used by servers if and only
1432   if failure to validate a request on the representation could result in
1433   incorrect operation, such as a silently unexecuted financial
1434   transaction.
1435</t>
1436</section>
1437
1438<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1439   <iref item="proxy-revalidate (cache directive)" primary="true" />
1440<t>
1441   The "proxy-revalidate" response directive has the same meaning as the
1442   must-revalidate response directive, except that it does not apply to
1443   private caches.
1444</t>
1445</section>
1446
1447<section title="max-age" anchor="cache-response-directive.max-age">
1448   <iref item="max-age (cache directive)" primary="true" />
1449<t>
1450   Argument syntax:
1451   <list>
1452      <t>
1453        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1454      </t>
1455   </list>
1456</t>
1457<t>
1458   The "max-age" response directive indicates that the response is to be
1459   considered stale after its age is greater than the specified number of
1460   seconds.
1461</t>
1462<t>
1463   &Note; This directive uses the token form of the argument syntax;
1464   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1465   quoted-string form.
1466</t>
1467</section>     
1468
1469<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1470   <iref item="s-maxage (cache directive)" primary="true" />
1471<t>
1472   Argument syntax:
1473   <list>
1474      <t>
1475        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1476      </t>
1477   </list>
1478</t>
1479<t>
1480   The "s-maxage" response directive indicates that, in shared caches, the
1481   maximum age specified by this directive overrides the maximum age
1482   specified by either the max-age directive or the <x:ref>Expires</x:ref>
1483   header field. The s-maxage directive also implies the semantics of the
1484   proxy-revalidate response directive.
1485</t>
1486<t>
1487   &Note; This directive uses the token form of the argument syntax;
1488   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1489   quoted-string form.
1490</t>
1491</section>
1492
1493<section title="no-transform" anchor="cache-response-directive.no-transform">
1494   <iref item="no-transform (cache directive)" primary="true" />
1495<t>
1496   The "no-transform" response directive indicates that an intermediary
1497   (regardless of whether it implements a cache) &MUST-NOT; change the
1498   <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Range</x:ref> or
1499   <x:ref>Content-Type</x:ref> response header fields, nor the response
1500   representation.
1501</t>
1502</section>
1503
1504</section>
1505
1506<section anchor="cache.control.extensions" title="Cache Control Extensions">
1507<t>
1508   The Cache-Control header field can be extended through the use of one or
1509   more cache-extension tokens, each with an optional value. Informational
1510   extensions (those that do not require a change in cache behavior) can be
1511   added without changing the semantics of other directives. Behavioral
1512   extensions are designed to work by acting as modifiers to the existing base
1513   of cache directives. Both the new directive and the standard directive are
1514   supplied, such that applications that do not understand the new directive
1515   will default to the behavior specified by the standard directive, and those
1516   that understand the new directive will recognize it as modifying the
1517   requirements associated with the standard directive. In this way,
1518   extensions to the cache-control directives can be made without requiring
1519   changes to the base protocol.
1520</t>
1521<t>
1522   This extension mechanism depends on an HTTP cache obeying all of the
1523   cache-control directives defined for its native HTTP-version, obeying
1524   certain extensions, and ignoring all directives that it does not
1525   understand.
1526</t>
1527<t>
1528   For example, consider a hypothetical new response directive called
1529   "community" that acts as a modifier to the private directive. We define
1530   this new directive to mean that, in addition to any private cache, any
1531   cache that is shared only by members of the community named within its
1532   value is allowed to cache the response. An origin server wishing to allow
1533   the UCI community to use an otherwise private response in their shared
1534   cache(s) could do so by including
1535</t>
1536<figure><artwork type="example">
1537  Cache-Control: private, community="UCI"
1538</artwork></figure>
1539<t>
1540   A cache seeing this header field will act correctly even if the cache does
1541   not understand the community cache-extension, since it will also see and
1542   understand the private directive and thus default to the safe behavior.
1543</t>
1544<t>
1545   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1546   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1547   combined with standard directives (or the response's default cacheability)
1548   such that the cache behavior will remain minimally correct even if the
1549   cache does not understand the extension(s).
1550</t>
1551<t>
1552   New extension directives ought to consider defining:
1553</t>
1554<t>
1555   <list style="symbols">
1556      <t>What it means for a directive to be specified multiple times,</t>
1557      <t>When the directive does not take an argument, what it means when an
1558      argument is present,</t>
1559      <t>When the directive requires an argument, what it means when it is
1560      missing.</t>
1561   </list>
1562</t>
1563<t>
1564   The HTTP Cache Directive Registry defines the name space for the cache
1565   directives.
1566</t>
1567<t>
1568   A registration &MUST; include the following fields:
1569   <list style="symbols">
1570      <t>Cache Directive Name</t>
1571      <t>Pointer to specification text</t>
1572   </list>
1573</t>
1574<t>
1575   Values to be added to this name space require IETF Review (see <xref
1576   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1577</t>
1578<t>
1579   The registry itself is maintained at <eref
1580   target="http://www.iana.org/assignments/http-cache-directives"/>.
1581</t>
1582</section>
1583
1584</section>
1585
1586<section anchor="header.expires" title="Expires">
1587   <iref item="Expires header field" primary="true" x:for-anchor="" />
1588   <x:anchor-alias value="Expires"/>
1589<t>
1590   The "Expires" header field gives the date/time after which the
1591   response is considered stale. See <xref target="expiration.model" /> for
1592   further discussion of the freshness model.
1593</t>
1594<t>
1595   The presence of an Expires field does not imply that the original resource
1596   will change or cease to exist at, before, or after that time.
1597</t>
1598<t>
1599   The field-value is an absolute date and time as defined by HTTP-date in
1600   &http-date;; a sender &MUST; use the rfc1123-date format.
1601</t>
1602<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1603  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1604</artwork></figure>
1605<figure>
1606  <preamble>For example</preamble>
1607<artwork type="example">
1608  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1609</artwork></figure>
1610<t>
1611   A cache &MUST; treat other invalid date formats,
1612   especially including the value "0", as in the past (i.e., "already
1613   expired").
1614</t>
1615<x:note>
1616   <t>
1617       &Note; If a response includes a <x:ref>Cache-Control</x:ref> field with
1618       the max-age directive (see <xref target="cache-response-directive.max-age" />),
1619       that directive overrides the Expires field. Likewise, the s-maxage
1620       directive (<xref target="cache-response-directive.s-maxage" />) overrides
1621       the <x:ref>Expires</x:ref> header fieldin shared caches.
1622   </t>
1623</x:note>
1624<t>
1625   Historically, HTTP required the Expires field-value to be no more than a
1626   year in the future. While longer freshness lifetimes are no longer
1627   prohibited, extremely large values have been demonstrated to cause
1628   problems (e.g., clock overflows due to use of 32-bit integers for
1629   time values), and many caches will evict a response far sooner than
1630   that. Therefore, senders ought not produce them.
1631</t>
1632<t>
1633   An origin server without a clock &MUST-NOT; assign Expires
1634   values to a response unless these values were associated
1635   with the resource by a system or user with a reliable clock. It &MAY;
1636   assign an Expires value that is known, at or before server
1637   configuration time, to be in the past (this allows "pre-expiration"
1638   of responses without storing separate Expires values for each
1639   resource).
1640</t>
1641</section>
1642
1643<section anchor="header.pragma" title="Pragma">
1644   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1645   <x:anchor-alias value="extension-pragma"/>
1646   <x:anchor-alias value="Pragma"/>
1647   <x:anchor-alias value="pragma-directive"/>
1648<t>
1649   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1650   caches, so that clients can specify a "no-cache" request that they will
1651   understand (as <x:ref>Cache-Control</x:ref> was not defined until HTTP/1.1).
1652   When the Cache-Control header field is also present and understood in a
1653   request, Pragma is ignored.
1654</t>
1655<t>
1656   In HTTP/1.0, Pragma was defined as an extensible field for
1657   implementation-specified directives for recipients. This specification
1658   deprecates such extensions to improve interoperability.
1659</t>
1660<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1661  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1662  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1663  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1664</artwork></figure>
1665<t>
1666   When the <x:ref>Cache-Control</x:ref> header field is not present in a
1667   request, the no-cache request pragma-directive &MUST; have the same effect
1668   on caches as if "Cache-Control: no-cache" were present (see <xref
1669   target="cache-request-directive" />).
1670</t>
1671<t>
1672   When sending a no-cache request, a client ought to include both the pragma
1673   and cache-control directives, unless Cache-Control: no-cache is
1674   purposefully omitted to target other <x:ref>Cache-Control</x:ref> response
1675   directives at HTTP/1.1 caches. For example:
1676</t>
1677<figure>
1678<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1679GET / HTTP/1.1
1680Host: www.example.com
1681Cache-Control: max-age=30
1682Pragma: no-cache
1683
1684</artwork>
1685</figure>
1686<t>
1687   will constrain HTTP/1.1 caches to serve a response no older than 30
1688   seconds, while precluding implementations that do not understand
1689   <x:ref>Cache-Control</x:ref> from serving a cached response.
1690</t>
1691<x:note>
1692   <t>
1693      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1694      specified, it does not provide a reliable replacement for
1695      "Cache-Control: no-cache" in them.
1696   </t>
1697</x:note>
1698</section>
1699
1700<section anchor="header.warning" title="Warning">
1701   <iref item="Warning header field" primary="true" x:for-anchor="" />
1702   <x:anchor-alias value="Warning"/>
1703   <x:anchor-alias value="warning-value"/>
1704   <x:anchor-alias value="warn-agent"/>
1705   <x:anchor-alias value="warn-code"/>
1706   <x:anchor-alias value="warn-date"/>
1707   <x:anchor-alias value="warn-text"/>
1708<t>
1709   The "Warning" header field is used to carry additional information
1710   about the status or transformation of a message that might not be reflected
1711   in the message. This information is typically used to warn about possible
1712   incorrectness introduced by caching operations or transformations applied
1713   to the payload of the message.
1714</t>
1715<t>
1716   Warnings can be used for other purposes, both cache-related and otherwise.
1717   The use of a warning, rather than an error status code, distinguishes these
1718   responses from true failures.
1719</t>
1720<t>
1721   Warning header fields can in general be applied to any message, however some
1722   warn-codes are specific to caches and can only be applied to response
1723   messages.
1724</t>
1725<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><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"/>
1726  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1727 
1728  <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>
1729                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1730 
1731  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1732  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1733                  ; the name or pseudonym of the server adding
1734                  ; the Warning header field, for use in debugging
1735  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1736  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1737</artwork></figure>
1738<t>
1739   Multiple warnings can be attached to a response (either by the origin
1740   server or by a cache), including multiple warnings with the same code
1741   number, only differing in warn-text.
1742</t>
1743<t>
1744   When this occurs, the user agent &SHOULD; inform the user of as many of
1745   them as possible, in the order that they appear in the response.
1746</t>
1747<t>
1748   Systems that generate multiple Warning header fields are encouraged to
1749   order them with this user agent behavior in mind. New Warning header fields
1750   are added after any existing Warning header fields.
1751</t>
1752<t>
1753   Warnings are assigned three digit warn-codes. The first digit indicates
1754   whether the Warning is required to be deleted from a stored response after
1755   validation:
1756   <list style="symbols">
1757      <t>1xx Warnings describe the freshness or validation status of the
1758      response, and so &MUST; be deleted by a cache after validation. They can
1759      only be generated by a cache when validating a cached entry, and
1760      &MUST-NOT; be generated in any other situation.</t>
1761      <t>2xx Warnings describe some aspect of the representation that is not
1762      rectified by a validation (for example, a lossy compression of the
1763      representation) and &MUST-NOT; be deleted by a cache after validation,
1764      unless a full response is returned, in which case they &MUST; be.</t>
1765   </list>
1766</t>
1767<t>
1768   If an implementation sends a message with one or more Warning header fields
1769   to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST;
1770   include in each warning-value a warn-date that matches the
1771   <x:ref>Date</x:ref> header field in the message.
1772</t>
1773<t>
1774   If a system receives a message with a warning-value that includes a
1775   warn-date, and that warn-date is different from the <x:ref>Date</x:ref>
1776   value in the response, then that warning-value &MUST; be deleted from the
1777   message before storing, forwarding, or using it. (preventing the
1778   consequences of naive caching of Warning header fields.) If all of the
1779   warning-values are deleted for this reason, the Warning header field &MUST;
1780   be deleted as well.
1781</t>
1782<t>
1783   The following warn-codes are defined by this specification, each with a
1784   recommended warn-text in English, and a description of its meaning.
1785</t>
1786
1787<section title="110 Response is Stale" anchor="warn.110">
1788  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1789<t>
1790   A cache &SHOULD; include this whenever the returned response is stale.
1791</t>
1792</section>
1793
1794<section title="111 Revalidation Failed" anchor="warn.111">
1795  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1796<t>
1797   A cache &SHOULD; include this when returning a stale response because an
1798   attempt to validate the response failed, due to an inability to reach
1799   the server.
1800</t>
1801</section>
1802
1803<section title="112 Disconnected Operation" anchor="warn.112">
1804  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1805<t>
1806   A cache &SHOULD; include this if it is intentionally disconnected from
1807   the rest of the network for a period of time.
1808</t>
1809</section>
1810
1811<section title="113 Heuristic Expiration" anchor="warn.113">
1812  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1813<t>
1814   A cache &SHOULD; include this if it heuristically chose a freshness
1815   lifetime greater than 24 hours and the response's age is greater than 24
1816   hours.
1817</t>
1818</section>
1819
1820<section title="199 Miscellaneous Warning" anchor="warn.199">
1821  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
1822<t>
1823   The warning text can include arbitrary information to be presented to
1824   a human user, or logged. A system receiving this warning &MUST-NOT; take
1825   any automated action, besides presenting the warning to the user.
1826</t>
1827</section>
1828
1829<section title="214 Transformation Applied" anchor="warn.214">
1830  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
1831<t>
1832   &MUST; be added by a proxy if it applies any transformation to the
1833   representation, such as changing the content-coding, media-type, or
1834   modifying the representation data, unless this Warning code already appears
1835   in the response.
1836</t>
1837</section>
1838
1839<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1840  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
1841<t>
1842   The warning text can include arbitrary information to be presented to
1843   a human user, or logged. A system receiving this warning &MUST-NOT; take
1844   any automated action.
1845</t>
1846</section>
1847
1848<section title="Warn Code Extensions" anchor="warn.code.extensions">
1849<t>
1850   The HTTP Warn Code Registry defines the name space for warn codes.
1851</t>
1852<t>
1853   A registration &MUST; include the following fields:
1854   <list style="symbols">
1855      <t>Warn Code (3 digits)</t>
1856      <t>Short Description</t>
1857      <t>Pointer to specification text</t>
1858   </list>
1859</t>
1860<t>
1861   Values to be added to this name space require IETF Review (see <xref
1862   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1863</t>
1864<t>
1865   The registry itself is maintained at <eref
1866   target="http://www.iana.org/assignments/http-warn-codes"/>.
1867</t>
1868</section>
1869</section>
1870</section>
1871
1872<section anchor="history.lists" title="History Lists">
1873<t>
1874   User agents often have history mechanisms, such as "Back" buttons and
1875   history lists, that can be used to redisplay a representation retrieved
1876   earlier in a session.
1877</t>
1878<t>
1879   The freshness model (<xref target="expiration.model"/>) does not
1880   necessarily apply to history mechanisms. I.e., a history mechanism can
1881   display a previous representation even if it has expired.
1882</t>
1883<t>
1884   This does not prohibit the history mechanism from telling the user that a
1885   view might be stale, or from honoring cache directives (e.g.,
1886   Cache-Control: no-store).
1887</t>
1888</section>
1889
1890
1891<section anchor="IANA.considerations" title="IANA Considerations">
1892
1893<section title="Cache Directive Registry" 
1894   anchor="cache.directive.registration">
1895<t>
1896   The registration procedure for HTTP Cache Directives is defined by <xref
1897   target="cache.control.extensions"/> of this document.
1898</t>
1899<t>
1900   The HTTP Cache Directive Registry shall be created at <eref
1901   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1902   populated with the registrations below:
1903</t>
1904<?BEGININC p6-cache.cache-directives ?>
1905<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1906<texttable xmlns:my="#my" align="left" suppress-title="true"
1907           anchor="iana.cache.directive.registration.table">
1908   <ttcol>Cache Directive</ttcol>
1909   <ttcol>Reference</ttcol>
1910
1911   <c>max-age</c>
1912   <c>
1913      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1914   </c>
1915   <c>max-stale</c>
1916   <c>
1917      <xref target="cache-request-directive.max-stale"/>
1918   </c>
1919   <c>min-fresh</c>
1920   <c>
1921      <xref target="cache-request-directive.min-fresh"/>
1922   </c>
1923   <c>must-revalidate</c>
1924   <c>
1925      <xref target="cache-response-directive.must-revalidate"/>
1926   </c>
1927   <c>no-cache</c>
1928   <c>
1929      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1930   </c>
1931   <c>no-store</c>
1932   <c>
1933      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1934   </c>
1935   <c>no-transform</c>
1936   <c>
1937      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1938   </c>
1939   <c>only-if-cached</c>
1940   <c>
1941      <xref target="cache-request-directive.only-if-cached"/>
1942   </c>
1943   <c>private</c>
1944   <c>
1945      <xref target="cache-response-directive.private"/>
1946   </c>
1947   <c>proxy-revalidate</c>
1948   <c>
1949      <xref target="cache-response-directive.proxy-revalidate"/>
1950   </c>
1951   <c>public</c>
1952   <c>
1953      <xref target="cache-response-directive.only-if-cached"/>
1954   </c>
1955   <c>s-maxage</c>
1956   <c>
1957      <xref target="cache-response-directive.s-maxage"/>
1958   </c>
1959   <c>stale-if-error</c>
1960   <c>
1961      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1962   </c>
1963   <c>stale-while-revalidate</c>
1964   <c>
1965      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1966   </c>
1967</texttable>
1968<!--(END)-->
1969<?ENDINC p6-cache.cache-directives ?>
1970</section>
1971
1972<section title="Warn Code Registry" 
1973   anchor="warn.code.registration">
1974<t>
1975   The registration procedure for HTTP Warn Codes is defined by <xref
1976   target="warn.code.extensions"/> of this document.
1977</t>
1978<t>
1979   The HTTP Warn Code Registry shall be created at <eref
1980   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1981   populated with the registrations below:
1982</t>
1983<?BEGININC p6-cache.iana-warn-codes ?>
1984<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1985<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1986   <ttcol>Warn Code</ttcol>
1987   <ttcol>Short Description</ttcol>
1988   <ttcol>Reference</ttcol>
1989   <c>110</c>
1990   <c>Response is Stale</c>
1991   <c>
1992      <xref target="warn.110"/>
1993   </c>
1994   <c>111</c>
1995   <c>Revalidation Failed</c>
1996   <c>
1997      <xref target="warn.111"/>
1998   </c>
1999   <c>112</c>
2000   <c>Disconnected Operation</c>
2001   <c>
2002      <xref target="warn.112"/>
2003   </c>
2004   <c>113</c>
2005   <c>Heuristic Expiration</c>
2006   <c>
2007      <xref target="warn.113"/>
2008   </c>
2009   <c>199</c>
2010   <c>Miscellaneous Warning</c>
2011   <c>
2012      <xref target="warn.199"/>
2013   </c>
2014   <c>214</c>
2015   <c>Transformation Applied</c>
2016   <c>
2017      <xref target="warn.214"/>
2018   </c>
2019   <c>299</c>
2020   <c>Miscellaneous Persistent Warning</c>
2021   <c>
2022      <xref target="warn.299"/>
2023   </c>
2024</texttable>
2025<!--(END)-->
2026<?ENDINC p6-cache.iana-warn-codes ?>
2027</section>
2028
2029<section title="Header Field Registration" anchor="header.field.registration">
2030<t>
2031  The Message Header Field Registry located at <eref 
2032  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2033  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
2034</t>
2035<?BEGININC p6-cache.iana-headers ?>
2036<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2037<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2038   <ttcol>Header Field Name</ttcol>
2039   <ttcol>Protocol</ttcol>
2040   <ttcol>Status</ttcol>
2041   <ttcol>Reference</ttcol>
2042
2043   <c>Age</c>
2044   <c>http</c>
2045   <c>standard</c>
2046   <c>
2047      <xref target="header.age"/>
2048   </c>
2049   <c>Cache-Control</c>
2050   <c>http</c>
2051   <c>standard</c>
2052   <c>
2053      <xref target="header.cache-control"/>
2054   </c>
2055   <c>Expires</c>
2056   <c>http</c>
2057   <c>standard</c>
2058   <c>
2059      <xref target="header.expires"/>
2060   </c>
2061   <c>Pragma</c>
2062   <c>http</c>
2063   <c>standard</c>
2064   <c>
2065      <xref target="header.pragma"/>
2066   </c>
2067   <c>Warning</c>
2068   <c>http</c>
2069   <c>standard</c>
2070   <c>
2071      <xref target="header.warning"/>
2072   </c>
2073</texttable>
2074<!--(END)-->
2075<?ENDINC p6-cache.iana-headers ?>
2076<t>
2077   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2078   Force".
2079</t>
2080</section>
2081</section>
2082
2083<section anchor="security.considerations" title="Security Considerations">
2084<t>
2085   Caches expose additional potential vulnerabilities, since the contents of
2086   the cache represent an attractive target for malicious exploitation.
2087   Because cache contents persist after an HTTP request is complete, an attack
2088   on the cache can reveal information long after a user believes that the
2089   information has been removed from the network. Therefore, cache contents
2090   need to be protected as sensitive information.
2091</t>
2092<t>
2093   Implementation flaws might allow attackers to insert content into a cache
2094   ("cache poisoning"), leading to compromise of clients that trust that
2095   content. Because of their nature, these attacks are difficult to mitigate.
2096</t>
2097<t>
2098   Likewise, implementation flaws (as well as misunderstanding of cache
2099   operation) might lead to caching of sensitive information (e.g.,
2100   authentication credentials) that is thought to be private, exposing it to
2101   unauthorised parties.
2102</t>
2103<t>
2104   Note that the Set-Cookie response header <xref target="RFC6265"/> does not
2105   inhibit caching; a cacheable response with a Set-Cookie header can be (and
2106   often is) used to satisfy subsequent requests to caches. Servers who wish
2107   to control caching of these responses are encouraged to emit appropriate
2108   Cache-Control response headers.
2109</t>
2110
2111</section>
2112
2113<section title="Acknowledgments" anchor="acks">
2114<t>
2115  See &acks;.
2116</t>
2117</section>
2118
2119</middle>
2120
2121<back>
2122<references title="Normative References">
2123
2124  <reference anchor="Part1">
2125    <front>
2126      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2127      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2128        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2129        <address><email>fielding@gbiv.com</email></address>
2130      </author>
2131      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2132        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2133        <address><email>julian.reschke@greenbytes.de</email></address>
2134      </author>
2135      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2136    </front>
2137    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2138    <x:source basename="p1-messaging" href="p1-messaging.xml">
2139      <x:defines>Content-Length</x:defines>
2140      <x:defines>Via</x:defines>
2141    </x:source>
2142  </reference>
2143
2144  <reference anchor="Part2">
2145    <front>
2146      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2147      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2148        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2149        <address><email>fielding@gbiv.com</email></address>
2150      </author>
2151      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2152        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2153        <address><email>julian.reschke@greenbytes.de</email></address>
2154      </author>
2155      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2156    </front>
2157    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2158    <x:source basename="p2-semantics" href="p2-semantics.xml">
2159      <x:defines>2xx (Successful)</x:defines>
2160      <x:defines>200 (OK)</x:defines>
2161      <x:defines>203 (Non-Authoritative Information)</x:defines>
2162      <x:defines>3xx (Redirection)</x:defines>
2163      <x:defines>300 (Multiple Choices)</x:defines>
2164      <x:defines>301 (Moved Permanently)</x:defines>
2165      <x:defines>404 (Not Found)</x:defines>
2166      <x:defines>410 (Gone)</x:defines>
2167      <x:defines>5xx (Server Error)</x:defines>
2168      <x:defines>504 (Gateway Timeout)</x:defines>
2169      <x:defines>Content-Encoding</x:defines>
2170      <x:defines>Content-Location</x:defines>
2171      <x:defines>Content-Type</x:defines>
2172      <x:defines>Date</x:defines>
2173      <x:defines>Location</x:defines>
2174      <x:defines>Vary</x:defines>
2175    </x:source>
2176  </reference>
2177
2178  <reference anchor="Part4">
2179    <front>
2180      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2181      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2182        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2183        <address><email>fielding@gbiv.com</email></address>
2184      </author>
2185      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2186        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2187        <address><email>julian.reschke@greenbytes.de</email></address>
2188      </author>
2189      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2190    </front>
2191    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2192    <x:source basename="p4-conditional" href="p4-conditional.xml">
2193      <x:defines>304</x:defines>
2194      <x:defines>304 (Not Modified)</x:defines>
2195      <x:defines>ETag</x:defines>
2196      <x:defines>If-Modified-Since</x:defines>
2197      <x:defines>If-None-Match</x:defines>
2198      <x:defines>Last-Modified</x:defines>
2199    </x:source>
2200  </reference>
2201
2202  <reference anchor="Part5">
2203    <front>
2204      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2205      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2206        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2207        <address><email>fielding@gbiv.com</email></address>
2208      </author>
2209      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2210        <organization abbrev="W3C">World Wide Web Consortium</organization>
2211        <address><email>ylafon@w3.org</email></address>
2212      </author>
2213      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2214        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2215        <address><email>julian.reschke@greenbytes.de</email></address>
2216      </author>
2217      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2218    </front>
2219    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2220    <x:source basename="p5-range" href="p5-range.xml">
2221      <x:defines>206 (Partial Content)</x:defines>
2222      <x:defines>Content-Range</x:defines>
2223      <x:defines>Range</x:defines>
2224    </x:source>
2225  </reference>
2226
2227  <reference anchor="Part7">
2228    <front>
2229      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2230      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2231        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2232        <address><email>fielding@gbiv.com</email></address>
2233      </author>
2234      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2235        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2236        <address><email>julian.reschke@greenbytes.de</email></address>
2237      </author>
2238      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2239    </front>
2240    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2241    <x:source basename="p7-auth" href="p7-auth.xml">
2242      <x:defines>Authorization</x:defines>
2243    </x:source>
2244  </reference>
2245
2246  <reference anchor="RFC2119">
2247    <front>
2248      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2249      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2250        <organization>Harvard University</organization>
2251        <address><email>sob@harvard.edu</email></address>
2252      </author>
2253      <date month="March" year="1997" />
2254    </front>
2255    <seriesInfo name="BCP" value="14" />
2256    <seriesInfo name="RFC" value="2119" />
2257  </reference>
2258
2259  <reference anchor="RFC5234">
2260    <front>
2261      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2262      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2263        <organization>Brandenburg InternetWorking</organization>
2264        <address>
2265          <email>dcrocker@bbiw.net</email>
2266        </address> 
2267      </author>
2268      <author initials="P." surname="Overell" fullname="Paul Overell">
2269        <organization>THUS plc.</organization>
2270        <address>
2271          <email>paul.overell@thus.net</email>
2272        </address>
2273      </author>
2274      <date month="January" year="2008"/>
2275    </front>
2276    <seriesInfo name="STD" value="68"/>
2277    <seriesInfo name="RFC" value="5234"/>
2278  </reference>
2279 
2280</references>
2281
2282<references title="Informative References">
2283
2284  <reference anchor="RFC1305">
2285    <front>
2286      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2287      <author fullname="David L. Mills" initials="D." surname="Mills">
2288        <organization>University of Delaware, Electrical Engineering Department</organization>
2289        <address><email>mills@udel.edu</email></address>
2290      </author>
2291      <date month="March" year="1992" />
2292    </front>
2293    <seriesInfo name="RFC" value="1305" />
2294  </reference>
2295
2296  <reference anchor="RFC2616">
2297    <front>
2298      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2299      <author fullname="R. Fielding" initials="R." surname="Fielding">
2300        <organization>University of California, Irvine</organization>
2301        <address><email>fielding@ics.uci.edu</email></address>
2302      </author>
2303      <author fullname="J. Gettys" initials="J." surname="Gettys">
2304        <organization>W3C</organization>
2305        <address><email>jg@w3.org</email></address>
2306      </author>
2307      <author fullname="J. Mogul" initials="J." surname="Mogul">
2308        <organization>Compaq Computer Corporation</organization>
2309        <address><email>mogul@wrl.dec.com</email></address>
2310      </author>
2311      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2312        <organization>MIT Laboratory for Computer Science</organization>
2313        <address><email>frystyk@w3.org</email></address>
2314      </author>
2315      <author fullname="L. Masinter" initials="L." surname="Masinter">
2316        <organization>Xerox Corporation</organization>
2317        <address><email>masinter@parc.xerox.com</email></address>
2318      </author>
2319      <author fullname="P. Leach" initials="P." surname="Leach">
2320        <organization>Microsoft Corporation</organization>
2321        <address><email>paulle@microsoft.com</email></address>
2322      </author>
2323      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2324        <organization>W3C</organization>
2325        <address><email>timbl@w3.org</email></address>
2326      </author>
2327      <date month="June" year="1999" />
2328    </front>
2329    <seriesInfo name="RFC" value="2616" />
2330  </reference>
2331
2332  <reference anchor="RFC3864">
2333    <front>
2334      <title>Registration Procedures for Message Header Fields</title>
2335      <author fullname="G. Klyne" initials="G." surname="Klyne">
2336        <organization>Nine by Nine</organization>
2337        <address><email>GK-IETF@ninebynine.org</email></address>
2338      </author>
2339      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2340        <organization>BEA Systems</organization>
2341        <address><email>mnot@pobox.com</email></address>
2342      </author>
2343      <author fullname="J. Mogul" initials="J." surname="Mogul">
2344        <organization>HP Labs</organization>
2345        <address><email>JeffMogul@acm.org</email></address>
2346      </author>
2347      <date month="September" year="2004" />
2348    </front>
2349    <seriesInfo name="BCP" value="90" />
2350    <seriesInfo name="RFC" value="3864" />
2351  </reference>
2352
2353  <reference anchor='RFC5226'>
2354    <front>
2355      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2356      <author initials='T.' surname='Narten' fullname='T. Narten'>
2357        <organization>IBM</organization>
2358        <address><email>narten@us.ibm.com</email></address>
2359      </author>
2360      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2361        <organization>Google</organization>
2362        <address><email>Harald@Alvestrand.no</email></address>
2363      </author>
2364      <date year='2008' month='May' />
2365    </front>
2366    <seriesInfo name='BCP' value='26' />
2367    <seriesInfo name='RFC' value='5226' />
2368  </reference>
2369
2370  <reference anchor='RFC5861'>
2371    <front>
2372      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2373      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2374        <organization>Yahoo! Inc.</organization>
2375        <address><email>mnot@yahoo-inc.com</email></address>
2376      </author>
2377      <date month="April" year="2010"/>
2378    </front>
2379    <seriesInfo name='RFC' value='5861' />
2380  </reference>
2381
2382  <reference anchor="RFC6265">
2383    <front>
2384      <title>HTTP State Management Mechanism</title>
2385      <author initials="A." surname="Barth" fullname="Adam Barth">
2386        <organization abbrev="U.C. Berkeley">
2387          University of California, Berkeley
2388        </organization>
2389        <address><email>abarth@eecs.berkeley.edu</email></address>
2390      </author>
2391      <date year="2011" month="April" />
2392    </front>
2393    <seriesInfo name="RFC" value="6265"/>
2394  </reference>
2395
2396</references>
2397
2398<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2399<t>
2400  Make the specified age calculation algorithm less conservative.
2401  (<xref target="age.calculations"/>)
2402</t>
2403<t>
2404  Remove requirement to consider "<x:ref>Content-Location</x:ref>" in successful
2405  responses in order to determine the appropriate response to use.
2406  (<xref target="validation.model" />)
2407</t>
2408<t>
2409  Clarify denial of service attack avoidance requirement.
2410  (<xref target="invalidation.after.updates.or.deletions" />)
2411</t>
2412<t>
2413  Do not mention RFC 2047 encoding and multiple languages in "<x:ref>Warning</x:ref>"
2414  header fields anymore, as these aspects never were implemented.
2415  (<xref target="header.warning" />)
2416</t>
2417<t>
2418  Introduce Cache Directive and Warn Code Registries.
2419  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
2420</t>
2421</section>
2422
2423<section title="Imported ABNF" anchor="imported.abnf">
2424   <x:anchor-alias value="ALPHA"/>
2425   <x:anchor-alias value="CR"/>
2426   <x:anchor-alias value="DIGIT"/>
2427   <x:anchor-alias value="DQUOTE"/>
2428   <x:anchor-alias value="LF"/>
2429   <x:anchor-alias value="OCTET"/>
2430   <x:anchor-alias value="SP"/>
2431   <x:anchor-alias value="VCHAR"/>
2432   <x:anchor-alias value="quoted-string"/>
2433   <x:anchor-alias value="token"/>
2434   <x:anchor-alias value="OWS"/>
2435   <x:anchor-alias value="field-name"/>
2436   <x:anchor-alias value="HTTP-date"/>
2437   <x:anchor-alias value="port"/>
2438   <x:anchor-alias value="pseudonym"/>
2439   <x:anchor-alias value="uri-host"/>
2440<t>
2441   The following core rules are included by reference, as defined in <xref
2442   target="RFC5234" x:fmt="of" x:sec="B.1"/>: ALPHA (letters), CR (carriage
2443   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2444   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2445   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2446</t>
2447<t>
2448   The rules below are defined in <xref target="Part1"/>:
2449</t>
2450<figure><artwork type="abnf2616">
2451  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
2452  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
2453  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
2454  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
2455
2456  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
2457  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
2458  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
2459</artwork></figure>
2460<t>
2461   The rules below are defined in other parts:
2462</t>
2463<figure><artwork type="abnf2616">
2464  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
2465</artwork></figure>
2466</section>
2467
2468<?BEGININC p6-cache.abnf-appendix ?>
2469<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2470<figure>
2471<artwork type="abnf" name="p6-cache.parsed-abnf">
2472<x:ref>Age</x:ref> = delta-seconds
2473
2474<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2475 cache-directive ] )
2476
2477<x:ref>Expires</x:ref> = HTTP-date
2478
2479<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8.1.1.1&gt;
2480
2481<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
2482
2483<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2484 pragma-directive ] )
2485
2486<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2487 )
2488
2489<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2490
2491<x:ref>delta-seconds</x:ref> = 1*DIGIT
2492
2493<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2494
2495<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2496
2497<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2498<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2499<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 5.7&gt;
2500
2501<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.4&gt;
2502
2503<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.4&gt;
2504
2505<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2506
2507<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2508<x:ref>warn-code</x:ref> = 3DIGIT
2509<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2510<x:ref>warn-text</x:ref> = quoted-string
2511<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2512 ]
2513</artwork>
2514</figure>
2515</section>
2516<?ENDINC p6-cache.abnf-appendix ?>
2517
2518<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2519<t>
2520  Changes up to the first Working Group Last Call draft are summarized
2521  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2522</t>
2523
2524<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2525<t>
2526  Closed issues:
2527  <list style="symbols">
2528    <t>
2529      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2530      "untangle Cache-Control ABNF"
2531    </t>
2532    <t>
2533      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2534      "Multiple values in Cache-Control header fields"
2535    </t>
2536    <t>
2537      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2538      "Case sensitivity of header fields in CC values"
2539    </t>
2540    <t>
2541      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2542      "Spurious 'MAYs'"
2543    </t>
2544    <t>
2545      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2546      "enhance considerations for new cache control directives"
2547    </t>
2548    <t>
2549      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2550      "ABNF requirements for recipients"
2551    </t>
2552    <t>
2553      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2554      "note introduction of new IANA registries as normative changes"
2555    </t>
2556    <t>
2557      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
2558      "broken prose in description of 'Vary'"
2559    </t>
2560  </list>
2561</t>
2562</section>
2563
2564<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
2565<t>
2566  Closed issues:
2567  <list style="symbols">
2568    <t>
2569      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
2570      "'Most Conservative'"
2571    </t>
2572  </list>
2573</t>
2574<t>
2575  Other changes:
2576  <list style="symbols">
2577    <t>
2578      Conformance criteria and considerations regarding error handling are
2579      now defined in Part 1.
2580    </t>
2581    <t>
2582      Move definition of "Vary" header field into Part 2.
2583    </t>
2584    <t>
2585      Add security considerations with respect to cache poisoning and
2586      the "Set-Cookie" header field.
2587    </t>
2588  </list>
2589</t>
2590</section>
2591
2592</section>
2593  </back>
2594</rfc>
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