source: draft-ietf-httpbis/latest/p6-cache.xml @ 2027

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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 "December">
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.21"/>.
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 a stale
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   Clients can send the max-age or min-fresh cache directives in a request to
581   constrain or relax freshness calculations for the corresponding response
582   (<xref target="cache-request-directive" />).
583</t>
584<t>
585   Note that freshness applies only to cache operation; it cannot be used to
586   force a user agent to refresh its display or reload a resource. See <xref
587   target="history.lists" /> for an explanation of the difference between
588   caches and history mechanisms.
589</t>
590
591<section anchor="calculating.freshness.lifetime" 
592   title="Calculating Freshness Lifetime">
593<t>
594   A cache can calculate the freshness lifetime (denoted as
595   freshness_lifetime) of a response by using the first match of:
596   <list style="symbols">
597      <t>If the cache is shared and the s-maxage response cache directive
598      (<xref target="cache-response-directive.s-maxage" />) is present, use its value,
599      or</t>
600      <t>If the max-age response cache directive (<xref
601      target="cache-response-directive.max-age" />) is present, use its value, or</t>
602      <t>If the <x:ref>Expires</x:ref> response header field
603      (<xref target="header.expires" />) is present, use its value minus the
604      value of the <x:ref>Date</x:ref> response header field, or</t>
605      <t>Otherwise, no explicit expiration time is present in the response. A
606      heuristic freshness lifetime might be applicable; see <xref
607      target="heuristic.freshness" />.</t>
608   </list>
609</t>
610<t>
611   Note that this calculation is not vulnerable to clock skew, since all of
612   the information comes from the origin server.
613</t>
614<t>
615   When there is more than one value present for a given directive (e.g., two
616   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
617   directives), it is considered invalid. Caches are encouraged to consider
618   responses that have invalid freshness information to be stale.
619</t>
620</section>
621
622<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
623<t>
624   If no explicit expiration time is present in a stored response that has a
625   status code whose definition allows heuristic freshness to be used
626   (including the following in &status-codes;: <x:ref>200 (OK)</x:ref>,
627   <x:ref>203 (Non-Authoritative Information)</x:ref>, <x:ref>206 (Partial
628   Content)</x:ref>, <x:ref>300 (Multiple Choices)</x:ref>, <x:ref>301 (Moved
629   Permanently)</x:ref> and <x:ref>410 (Gone)</x:ref>), a cache &MAY;
630   calculate a heuristic expiration time. A cache &MUST-NOT; use heuristics to
631   determine freshness for responses with status codes that do not explicitly
632   allow it.
633</t>
634<t>
635   When a heuristic is used to calculate freshness lifetime, a cache &SHOULD;
636   attach a <x:ref>Warning</x:ref> header field with a 113 warn-code to the
637   response if its current_age is more than 24 hours and such a warning is not
638   already present.
639</t>
640<t>
641   Also, if the response has a <x:ref>Last-Modified</x:ref> header field
642   (&header-last-modified;), caches are encouraged to use a heuristic
643   expiration value that is no more than some fraction of the interval since
644   that time. A typical setting of this fraction might be 10%.
645</t>
646<x:note>
647   <t>
648      &Note; <xref target="RFC2616" x:fmt="of" x:sec="13.9"/> prohibited caches
649      from calculating heuristic freshness for URIs with query components
650      (i.e., those containing '?'). In practice, this has not been widely
651      implemented. Therefore, servers are encouraged to send explicit
652      directives (e.g., Cache-Control: no-cache) if they wish to preclude
653      caching.
654   </t>
655</x:note>
656</section>
657
658<section anchor="age.calculations" title="Calculating Age">
659<t>
660   HTTP/1.1 uses the <x:ref>Age</x:ref> header field to convey the estimated
661   age of the response message when obtained from a cache. The Age field value
662   is the cache's estimate of the amount of time since the response was
663   generated or validated by the origin server. In essence, the Age value is
664   the sum of the time that the response has been resident in each of the
665   caches along the path from the origin server, plus the amount of time it
666   has been in transit along network paths.
667</t>
668<t>
669   The following data is used for the age calculation:
670</t>
671<t>
672   <x:dfn>age_value</x:dfn>
673   <list>
674      <t>
675         The term "age_value" denotes the value of the <x:ref>Age</x:ref>
676         header field (<xref target="header.age"/>), in a form appropriate for
677         arithmetic operation; or 0, if not available.
678      </t>
679   </list>
680</t>
681<t>
682   <x:dfn>date_value</x:dfn>
683   <list>
684      <t>
685         HTTP/1.1 requires origin servers to send a <x:ref>Date</x:ref> header
686         field, if possible, with every response, giving the time at which the
687         response was generated. The term "date_value" denotes the value of
688         the Date header field, in a form appropriate for arithmetic
689         operations. See &header-date; for the definition of the Date header
690         field, and for requirements regarding responses without it.
691      </t>
692   </list>
693</t>
694<t>
695   <x:dfn>now</x:dfn>
696   <list>
697      <t>
698         The term "now" means "the current value of the clock at the host
699         performing the calculation". A cache &SHOULD; use NTP (<xref
700         target="RFC1305"/>) or some similar protocol to synchronize its
701         clocks to a globally accurate time standard.
702      </t>
703   </list>
704</t>
705<t>
706   <x:dfn>request_time</x:dfn>
707   <list>
708      <t>
709         The current value of the clock at the host at the time the request
710         resulting in the stored response was made.
711      </t>
712   </list>
713</t>
714<t>
715   <x:dfn>response_time</x:dfn>
716   <list>
717      <t>
718         The current value of the clock at the host at the time the response
719         was received.
720      </t>
721   </list>
722</t>
723<t>
724   A response's age can be calculated in two entirely independent ways:
725   <list style="numbers">
726      <t>the "apparent_age": response_time minus date_value, if the local
727      clock is reasonably well synchronized to the origin server's clock. If
728      the result is negative, the result is replaced by zero.</t>
729      <t>the "corrected_age_value", if all of the caches along the response
730      path implement HTTP/1.1. A cache &MUST; interpret this value relative
731      to the time the request was initiated, not the time that the response
732      was received.</t>
733   </list>
734</t>
735<figure>
736<artwork type="code">
737  apparent_age = max(0, response_time - date_value);
738
739  response_delay = response_time - request_time;
740  corrected_age_value = age_value + response_delay; 
741</artwork>
742</figure>
743<figure>
744<preamble>These &SHOULD; be combined as</preamble>
745<artwork type="code">
746  corrected_initial_age = max(apparent_age, corrected_age_value);
747</artwork></figure>
748<t>
749   unless the cache is confident in the value of the <x:ref>Age</x:ref> header
750   field (e.g., because there are no HTTP/1.0 hops in the <x:ref>Via</x:ref>
751   header field), in which case the corrected_age_value &MAY; be used as the
752   corrected_initial_age.</t>
753<t>
754   The current_age of a stored response can then be calculated by adding the
755   amount of time (in seconds) since the stored response was last validated by
756   the origin server to the corrected_initial_age.
757</t>
758<figure><artwork type="code">
759  resident_time = now - response_time;
760  current_age = corrected_initial_age + resident_time;
761</artwork></figure>
762<t>
763   Additionally, to avoid common problems in date parsing:
764</t>
765<t>
766  <list style="symbols">
767     <t>Recipients &SHOULD; assume that an RFC-850 date
768        which appears to be more than 50 years in the future is in fact
769        in the past (this helps solve the "year 2000" problem).</t>
770
771     <t>Although all date formats are specified to be case-sensitive,
772        recipients &SHOULD; match day, week and timezone names
773        case-insensitively.</t>
774             
775     <t>An implementation &MAY; internally represent a parsed
776        <x:ref>Expires</x:ref> date as earlier than the proper value, but
777        &MUST-NOT; internally represent a parsed Expires date as later than the
778        proper value.</t>
779
780     <t>Recipients &MUST; perform all expiration-related calculations in GMT.
781        The local time zone &MUST-NOT; influence the calculation or comparison
782        of an age or expiration time.</t>
783
784     <t>Caches &SHOULD; consider dates with time zones other than "GMT"
785        invalid.</t>
786  </list>
787</t>
788</section>
789
790<section anchor="serving.stale.responses" title="Serving Stale Responses">
791<t>
792   A "stale" response is one that either has explicit expiry information or is
793   allowed to have heuristic expiry calculated, but is not fresh according to
794   the calculations in <xref target="expiration.model" />.
795</t>
796<t>
797   A cache &MUST-NOT; return a stale response if it is prohibited by an
798   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
799   directive, a "must-revalidate" cache-response-directive, or an applicable
800   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
801   target="cache-response-directive"/>).
802</t>
803<t>
804   A cache &MUST-NOT; return stale responses unless it is disconnected
805   (i.e., it cannot contact the origin server or otherwise find a forward
806   path) or doing so is explicitly allowed (e.g., by the max-stale request
807   directive; see <xref target="cache-request-directive" />).
808</t>
809<t>
810   A cache &SHOULD; append a <x:ref>Warning</x:ref> header field with the 110
811   warn-code (see <xref target="header.warning"/>) to stale responses.
812   Likewise, a cache &SHOULD; add the 112 warn-code to stale responses if the
813   cache is disconnected.
814</t>
815<t>
816   If a cache receives a first-hand response (either an entire response, or a
817   <x:ref>304 (Not Modified)</x:ref> response) that it would normally forward
818   to the requesting client, and the received response is no longer fresh, the
819   cache can forward it to the requesting client without adding a new
820   <x:ref>Warning</x:ref> (but without removing any existing Warning header
821   fields). A cache shouldn't attempt to validate a response simply because
822   that response became stale in transit.
823</t>
824</section>
825</section>
826
827<section anchor="validation.model" title="Validation Model">
828<t>
829   When a cache has one or more stored responses for a requested URI, but
830   cannot serve any of them (e.g., because they are not fresh, or one cannot
831   be selected; see <xref target="caching.negotiated.responses"/>), it can use
832   the conditional request mechanism &conditional; in the forwarded request to
833   give the origin server an opportunity to both select a valid stored
834   response to be used, and to update it. This process is known as
835   "validating" or "revalidating" the stored response.
836</t>
837<t>
838   When sending such a conditional request, a cache adds an
839   <x:ref>If-Modified-Since</x:ref> header field whose value is that of the
840   <x:ref>Last-Modified</x:ref> header field from the selected
841   (see <xref target="caching.negotiated.responses"/>) stored response, if
842   available.
843</t>
844<t>
845   Additionally, a cache can add an <x:ref>If-None-Match</x:ref> header field
846   whose value is that of the <x:ref>ETag</x:ref> header field(s) from all
847   responses stored for the requested URI, if present. However, if any of the
848   stored responses contains only partial content, the cache shouldn't
849   include its entity-tag in the If-None-Match header field unless the request
850   is for a range that would be fully satisfied by that stored response.
851</t>
852
853<t>Cache handling of a response to a conditional request is dependent upon its
854status code:</t>
855
856<t>
857   <list style="symbols">
858      <t>
859         A <x:ref>304 (Not Modified)</x:ref> response status code indicates
860         that the stored response can be updated and reused; see <xref
861         target="freshening.responses"/>.
862      </t>
863      <t>
864         A full response (i.e., one with a payload body) indicates that none
865         of the stored responses nominated in the conditional request is
866         suitable. Instead, the cache can use the full response to
867         satisfy the request and &MAY; replace the stored response(s).
868      </t>
869      <t>
870         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
871         response while attempting to validate a response, it can either
872         forward this response to the requesting client, or act as if the
873         server failed to respond. In the latter case, it can return a
874         previously stored response (see <xref
875         target="serving.stale.responses" />).
876      </t>
877   </list>
878</t>
879
880<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
881<t>
882   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and
883   already has one or more stored <x:ref>200 (OK)</x:ref> responses for the
884   same cache key, the cache needs to identify which of the stored responses
885   are updated by this new response and then update the stored response(s)
886   with the new information provided in the <x:ref>304</x:ref> response.
887   <list style="symbols">
888    <t>
889     If the new response contains a strong validator, then that strong
890     validator identifies the selected representation.  All of the stored
891     responses with the same strong validator are selected.
892     If none of the stored responses contain the same strong validator,
893     then the new response &MUST-NOT; be used to update any stored responses.
894    </t>
895    <t>
896     If the new response contains a weak validator and that validator
897     corresponds to one of the cache's stored responses, then the most
898     recent of those matching stored responses is selected.
899    </t>
900    <t>
901     If the new response does not include any form of validator, there is
902     only one stored response, and that stored response also lacks a
903     validator, then that stored response is selected.
904    </t>
905   </list>
906</t>
907<t>
908   If a stored response is selected for update, the cache &MUST;:
909   <list style="symbols">
910      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
911         with warn-code 1xx (see <xref target="header.warning" />);</t>
912      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
913         with warn-code 2xx; and,</t>
914      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
915         response to replace all instances of the corresponding header
916         fields in the stored response.</t>
917   </list>
918</t>
919</section>
920
921</section>
922
923<section anchor="caching.negotiated.responses" 
924   title="Using Negotiated Responses">
925<t>
926   When a cache receives a request that can be satisfied by a stored response
927   that has a <x:ref>Vary</x:ref> header field (&header-vary;),
928   it &MUST-NOT; use that response unless all of the selecting header fields
929   nominated by the Vary header field match in both the original request
930   (i.e., that associated with the stored response), and the presented
931   request.
932</t>
933<t>
934   The selecting header fields from two requests are defined to match if and
935   only if those in the first request can be transformed to those in the
936   second request by applying any of the following:
937   <list style="symbols">
938      <t>
939         adding or removing whitespace, where allowed in the header field's
940         syntax
941      </t>
942      <t>
943         combining multiple header fields with the same field name
944         (see &header-fields;)
945      </t>
946      <t>
947         normalizing both header field values in a way that is known to have
948         identical semantics, according to the header field's specification
949         (e.g., re-ordering field values when order is not significant;
950         case-normalization, where values are defined to be case-insensitive)
951      </t>
952  </list>
953</t>
954<t>
955   If (after any normalization that might take place) a header field is absent
956   from a request, it can only match another request if it is also absent
957   there.
958</t>
959<t>
960   A <x:ref>Vary</x:ref> header field-value of "*" always fails to match, and
961   subsequent requests to that resource can only be properly interpreted by the
962   origin server.
963</t>
964<t>
965   The stored response with matching selecting header fields is known as the
966   selected response.
967</t>
968<t>
969   If multiple selected responses are available, the most recent response
970   (as determined by the <x:ref>Date</x:ref> header field) is used; see <xref 
971   target="constructing.responses.from.caches"/>.
972</t>
973<t>
974   If no selected response is available, the cache cannot satisfy the
975   presented request. Typically, it is forwarded to the origin server
976   in a (possibly conditional; see <xref target="validation.model"/>) request.
977</t>
978</section>
979
980
981<section anchor="combining.responses" title="Combining Partial Content">
982<t>
983   A response might transfer only a partial representation if the
984   connection closed prematurely or if the request used one or more Range
985   specifiers (&partial;).  After several such transfers, a cache might have
986   received several ranges of the same representation.  A cache &MAY; combine
987   these ranges into a single stored response, and reuse that response to
988   satisfy later requests, if they all share the same strong validator and
989   the cache complies with the client requirements in &combining-byte-ranges;.
990</t>
991<t>
992   When combining the new response with one or more stored responses, a
993   cache &MUST;:
994   <list style="symbols">
995      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
996         with warn-code 1xx (see <xref target="header.warning" />);</t>
997      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
998         with warn-code 2xx; and,</t>
999      <t>use other header fields provided in the new response, aside
1000         from <x:ref>Content-Range</x:ref>, to replace all instances of the
1001         corresponding header fields in the stored response.</t>
1002   </list>
1003</t>
1004</section>
1005</section>
1006
1007
1008<section anchor="head.effects" title="Updating Caches with HEAD Responses">
1009<t>
1010   A response to the HEAD method is identical to what an equivalent request
1011   made with a GET would have been, except it lacks a body. This property
1012   of HEAD responses is used to both invalidate and update cached GET
1013   responses.
1014</t>
1015<t>
1016   If one or more stored GET responses can be selected (as per <xref
1017   target="caching.negotiated.responses"/>) for a HEAD request, and the
1018   <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> or
1019   <x:ref>Last-Modified</x:ref> value of a HEAD response differs from that in a
1020   selected GET response, the cache &MUST; consider that selected response to
1021   be stale.
1022</t>
1023<t>
1024   If the <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> and
1025   <x:ref>Last-Modified</x:ref> values of a HEAD response (when present) are
1026   the same as that in a selected GET response (as per
1027   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update
1028   the remaining header fields in the stored response using the following
1029   rules:
1030   <list style="symbols">
1031      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1032         with warn-code 1xx (see <xref target="header.warning" />);</t>
1033      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1034         with warn-code 2xx; and,</t>
1035      <t>use other header fields provided in the response to replace
1036         all instances of the corresponding header fields in the stored
1037         response.</t>
1038   </list>
1039</t>
1040
1041</section>
1042
1043
1044<section anchor="invalidation.after.updates.or.deletions" 
1045   title="Request Methods that Invalidate">
1046<t>
1047   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1048   have the potential for changing state on the origin server, intervening
1049   caches can use them to keep their contents up-to-date.
1050</t>
1051<t>
1052   A cache &MUST; invalidate the effective Request URI
1053   (&effective-request-uri;) as well as the URI(s) in the
1054   <x:ref>Location</x:ref> and <x:ref>Content-Location</x:ref> response header
1055   fields (if present) when a non-error response to a request with an unsafe
1056   method is received.
1057</t>
1058<t>
1059   However, a cache &MUST-NOT; invalidate a URI from a <x:ref>Location</x:ref>
1060   or <x:ref>Content-Location</x:ref> response header field if the host part of
1061   that URI differs from the host part in the effective request URI
1062   (&effective-request-uri;). This helps prevent denial of service attacks.
1063</t>
1064<t>
1065   A cache &MUST; invalidate the effective request URI
1066   (&effective-request-uri;) when it receives a non-error response
1067   to a request with a method whose safety is unknown.
1068</t>
1069<t>
1070   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref>
1071   or <x:ref>3xx (Redirection)</x:ref> status code. "Invalidate" means that
1072   the cache will either remove all stored responses related to the effective
1073   request URI, or will mark these as "invalid" and in need of a mandatory
1074   validation before they can be returned in response to a subsequent request.
1075</t>
1076<t>
1077   Note that this does not guarantee that all appropriate responses are
1078   invalidated. For example, the request that caused the change at the origin
1079   server might not have gone through the cache where a response is stored.
1080</t>
1081</section>
1082
1083
1084
1085
1086<section anchor="header.field.definitions" title="Header Field Definitions">
1087<t>
1088   This section defines the syntax and semantics of HTTP/1.1 header fields
1089   related to caching.
1090</t>
1091
1092<section anchor="header.age" title="Age">
1093   <iref item="Age header field" primary="true" x:for-anchor="" />
1094   <x:anchor-alias value="Age"/>
1095   <x:anchor-alias value="age-value"/>
1096<t>
1097   The "Age" header field conveys the sender's estimate of the amount
1098   of time since the response was generated or successfully validated at the
1099   origin server. Age values are calculated as specified in <xref
1100   target="age.calculations" />.
1101</t>
1102<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1103  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1104</artwork></figure>
1105<t>
1106  Age field-values are non-negative integers, representing time in seconds
1107  (see <xref target="delta-seconds"/>).
1108</t>
1109<t>
1110   The presence of an Age header field in a response implies that a response
1111   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1112   might not implement the Age header field.
1113</t>
1114</section>
1115
1116<section anchor="header.cache-control" title="Cache-Control">
1117   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1118   <x:anchor-alias value="Cache-Control"/>
1119   <x:anchor-alias value="cache-directive"/>
1120<t>
1121   The "Cache-Control" header field is used to specify directives for
1122   caches along the request/response chain. Such cache directives are
1123   unidirectional in that the presence of a directive in a request does not
1124   imply that the same directive is to be given in the response.
1125</t>
1126<t>
1127   A cache &MUST; obey the requirements of the Cache-Control
1128   directives defined in this section. See <xref
1129   target="cache.control.extensions"/> for information about how Cache-Control
1130   directives defined elsewhere are handled.
1131</t>
1132<x:note>
1133   <t>
1134       &Note; HTTP/1.0 caches might not implement Cache-Control and
1135       might only implement Pragma: no-cache (see <xref target="header.pragma"
1136       />).
1137   </t>
1138</x:note>
1139<t>
1140   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1141   through in forwarded messages, regardless of their
1142   significance to that application, since the directives might be applicable
1143   to all recipients along the request/response chain. It is not possible to
1144   target a directive to a specific cache.
1145</t>
1146<t>
1147   Cache directives are identified by a token, to be compared case-insensitively,
1148   and have an optional argument, that can use both token and quoted-string
1149   syntax. For the directives defined below that define arguments, recipients
1150   ought to accept both forms, even if one is documented to be preferred. For
1151   any directive not defined by this specification, recipients &MUST; accept
1152   both forms.
1153</t>
1154<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1155  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1156
1157  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1158</artwork></figure>
1159<t>
1160   For the cache directives defined below, no argument is defined (nor allowed)
1161   otherwise stated otherwise.
1162</t>
1163
1164<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1165
1166<section title="no-cache" anchor="cache-request-directive.no-cache">
1167   <iref item="no-cache (cache directive)" primary="true" />
1168<t>
1169   The "no-cache" request directive indicates that a cache &MUST-NOT; 
1170   use a stored response to satisfy the request without successful
1171   validation on the origin server.
1172</t>
1173</section>
1174 
1175<section title="no-store" anchor="cache-request-directive.no-store">
1176   <iref item="no-store (cache directive)" primary="true" />
1177<t>
1178   The "no-store" request directive indicates that a cache &MUST-NOT;
1179   store any part of either this request or any response to it. This
1180   directive applies to both private and shared caches. "&MUST-NOT;
1181   store" in this context means that the cache &MUST-NOT; intentionally
1182   store the information in non-volatile storage, and &MUST; make a
1183   best-effort attempt to remove the information from volatile storage as
1184   promptly as possible after forwarding it.
1185</t>
1186<t>
1187   This directive is NOT a reliable or sufficient mechanism for ensuring
1188   privacy. In particular, malicious or compromised caches might not
1189   recognize or obey this directive, and communications networks might be
1190   vulnerable to eavesdropping.
1191</t>
1192<t>
1193   Note that if a request containing this directive is satisfied from a
1194   cache, the no-store request directive does not apply to the already
1195   stored response.
1196</t>
1197</section>
1198
1199<section title="max-age" anchor="cache-request-directive.max-age">
1200   <iref item="max-age (cache directive)" primary="true" />
1201<t>
1202   Argument syntax:
1203   <list>
1204      <t>
1205        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1206      </t>
1207   </list>
1208</t>
1209<t>
1210   The "max-age" request directive indicates that the client is unwilling to
1211   accept a response whose age is greater than the specified number of
1212   seconds. Unless the max-stale request directive is also present, the
1213   client is not willing to accept a stale response.
1214</t>
1215<t>
1216   &Note; This directive uses the token form of the argument syntax;
1217   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1218   quoted-string form.
1219</t>
1220</section>
1221
1222<section title="max-stale" anchor="cache-request-directive.max-stale">
1223   <iref item="max-stale (cache directive)" primary="true" />
1224<t>
1225   Argument syntax:
1226   <list>
1227      <t>
1228        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1229      </t>
1230   </list>
1231</t>
1232<t>
1233   The "max-stale" request directive indicates that the client is willing
1234   to accept a response that has exceeded its expiration time. If max-stale
1235   is assigned a value, then the client is willing to accept a response
1236   that has exceeded its expiration time by no more than the specified
1237   number of seconds. If no value is assigned to max-stale, then the client
1238   is willing to accept a stale response of any age.
1239</t>
1240<t>
1241   &Note; This directive uses the token form of the argument syntax;
1242   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1243   quoted-string form.
1244</t>
1245</section>
1246
1247<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1248   <iref item="min-fresh (cache directive)" primary="true" />
1249<t>
1250   Argument syntax:
1251   <list>
1252      <t>
1253        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1254      </t>
1255   </list>
1256</t>
1257<t>
1258   The "min-fresh" request directive indicates that the client is willing
1259   to accept a response whose freshness lifetime is no less than its
1260   current age plus the specified time in seconds. That is, the client
1261   wants a response that will still be fresh for at least the specified
1262   number of seconds.
1263</t>
1264<t>
1265   &Note; This directive uses the token form of the argument syntax;
1266   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1267   quoted-string form.
1268</t>
1269</section>
1270
1271<section title="no-transform" anchor="cache-request-directive.no-transform">
1272   <iref item="no-transform (cache directive)" primary="true" />
1273<t>
1274   The "no-transform" request directive indicates that an intermediary
1275   (whether or not it implements a cache) &MUST-NOT; change the
1276   <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Range</x:ref> or
1277   <x:ref>Content-Type</x:ref> request header fields, nor the request
1278   representation.
1279</t>
1280</section>
1281
1282<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1283   <iref item="only-if-cached (cache directive)" primary="true" />
1284<t>
1285   The "only-if-cached" request directive indicates that the client only wishes
1286   to obtain a stored response. If it receives this directive, a cache &SHOULD;
1287   either respond using a stored response that is consistent with the other
1288   constraints of the request, or respond with a <x:ref>504 (Gateway
1289   Timeout)</x:ref> status code. If a group of caches is being operated as a
1290   unified system with good internal connectivity, a member cache &MAY;
1291   forward such a request within that group of caches.
1292</t>
1293</section>
1294</section>
1295
1296<section anchor="cache-response-directive" 
1297   title="Response Cache-Control Directives">
1298   <x:anchor-alias value="cache-response-directive" />
1299
1300<section title="public" anchor="cache-response-directive.public">
1301   <iref item="public (cache directive)" primary="true" />
1302<t>
1303   The "public" response directive indicates that a response whose
1304   associated request contains an 'Authentication' header &MAY; be
1305   stored (see <xref target="caching.authenticated.responses" />).
1306</t>
1307</section>
1308
1309<section title="private" anchor="cache-response-directive.private">
1310   <iref item="private (cache directive)" primary="true" />
1311<t>
1312   Argument syntax:
1313   <list>
1314      <t>
1315        #<x:ref>field-name</x:ref>
1316      </t>
1317   </list>
1318</t>
1319<t>
1320   The "private" response directive indicates that the response message is
1321   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1322   private cache &MAY; store the response.
1323</t>
1324<t>
1325   If the private response directive specifies one or more field-names,
1326   this requirement is limited to the field-values associated with the
1327   listed response header fields. That is, a shared cache &MUST-NOT; store
1328   the specified field-names(s), whereas it &MAY; store the remainder of the
1329   response message.
1330</t>
1331<t>
1332   The field-names given are not limited to the set of standard header
1333   fields defined by this specification. Field names are case-insensitive.
1334</t>
1335<t>
1336   &Note; This usage of the word "private" only controls
1337   where the response can be stored; it cannot ensure the privacy of the
1338   message content. Also, private response directives with field-names are
1339   often handled by implementations as if an unqualified private directive
1340   was received; i.e., the special handling for the qualified form is not
1341   widely implemented.
1342</t>
1343<t>
1344   &Note; This directive uses the quoted-string form of the argument syntax.
1345   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1346   needed for single-entry lists).
1347</t>
1348</section>
1349
1350<section title="no-cache" anchor="cache-response-directive.no-cache">
1351   <iref item="no-cache (cache directive)" primary="true" />
1352<t>
1353   Argument syntax:
1354   <list>
1355      <t>
1356        #<x:ref>field-name</x:ref>
1357      </t>
1358   </list>
1359</t>
1360<t>
1361   The "no-cache" response directive indicates that the response &MUST-NOT;
1362   be used to satisfy a subsequent request without successful validation on
1363   the origin server. This allows an origin server to prevent a cache from
1364   using it to satisfy a request without contacting it, even by caches that
1365   have been configured to return stale responses.
1366</t>
1367<t>
1368   If the no-cache response directive specifies one or more field-names,
1369   then a cache &MAY; use the response to satisfy a subsequent request,
1370   subject to any other restrictions on caching. However, any header fields
1371   in the response that have the field-name(s) listed &MUST-NOT; be sent
1372   in the response to a subsequent request without successful revalidation
1373   with the origin server. This allows an origin server to prevent the
1374   re-use of certain header fields in a response, while still allowing
1375   caching of the rest of the response.
1376</t> 
1377<t>
1378   The field-names given are not limited to the set of standard header
1379   fields defined by this specification. Field names are case-insensitive.
1380</t>
1381<t>
1382   &Note; Many HTTP/1.0 caches will not recognize or obey
1383   this directive. Also, no-cache response directives with field-names are
1384   often handled by implementations as if an unqualified no-cache directive
1385   was received; i.e., the special handling for the qualified form is not
1386   widely implemented.
1387</t>
1388<t>
1389   &Note; This directive uses the quoted-string form of the argument syntax.
1390   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1391   needed for single-entry lists).
1392</t>
1393</section>
1394
1395<section title="no-store" anchor="cache-response-directive.no-store">
1396   <iref item="no-store (cache directive)" primary="true" />
1397<t>
1398   The "no-store" response directive indicates that a cache &MUST-NOT;
1399   store any part of either the immediate request or response. This
1400   directive applies to both private and shared caches. "&MUST-NOT;
1401   store" in this context means that the cache &MUST-NOT; intentionally
1402   store the information in non-volatile storage, and &MUST; make a
1403   best-effort attempt to remove the information from volatile storage as
1404   promptly as possible after forwarding it.
1405</t>
1406<t>
1407   This directive is NOT a reliable or sufficient mechanism for ensuring
1408   privacy. In particular, malicious or compromised caches might not
1409   recognize or obey this directive, and communications networks might be
1410   vulnerable to eavesdropping.
1411</t>
1412</section>
1413
1414<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1415   <iref item="must-revalidate (cache directive)" primary="true" />
1416<t>
1417   The "must-revalidate" response directive indicates that once it has
1418   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1419   requests without successful validation on the origin server.
1420</t>
1421<t>
1422   The must-revalidate directive is necessary to support reliable
1423   operation for certain protocol features. In all circumstances a
1424   cache &MUST; obey the must-revalidate directive; in particular,
1425   if a cache cannot reach the origin server for any reason, it &MUST;
1426   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1427</t>
1428<t>
1429   The must-revalidate directive ought to be used by servers if and only
1430   if failure to validate a request on the representation could result in
1431   incorrect operation, such as a silently unexecuted financial
1432   transaction.
1433</t>
1434</section>
1435
1436<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1437   <iref item="proxy-revalidate (cache directive)" primary="true" />
1438<t>
1439   The "proxy-revalidate" response directive has the same meaning as the
1440   must-revalidate response directive, except that it does not apply to
1441   private caches.
1442</t>
1443</section>
1444
1445<section title="max-age" anchor="cache-response-directive.max-age">
1446   <iref item="max-age (cache directive)" primary="true" />
1447<t>
1448   Argument syntax:
1449   <list>
1450      <t>
1451        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1452      </t>
1453   </list>
1454</t>
1455<t>
1456   The "max-age" response directive indicates that the response is to be
1457   considered stale after its age is greater than the specified number of
1458   seconds.
1459</t>
1460<t>
1461   &Note; This directive uses the token form of the argument syntax;
1462   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1463   quoted-string form.
1464</t>
1465</section>     
1466
1467<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1468   <iref item="s-maxage (cache directive)" primary="true" />
1469<t>
1470   Argument syntax:
1471   <list>
1472      <t>
1473        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1474      </t>
1475   </list>
1476</t>
1477<t>
1478   The "s-maxage" response directive indicates that, in shared caches, the
1479   maximum age specified by this directive overrides the maximum age
1480   specified by either the max-age directive or the <x:ref>Expires</x:ref>
1481   header field. The s-maxage directive also implies the semantics of the
1482   proxy-revalidate response directive.
1483</t>
1484<t>
1485   &Note; This directive uses the token form of the argument syntax;
1486   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1487   quoted-string form.
1488</t>
1489</section>
1490
1491<section title="no-transform" anchor="cache-response-directive.no-transform">
1492   <iref item="no-transform (cache directive)" primary="true" />
1493<t>
1494   The "no-transform" response directive indicates that an intermediary
1495   (regardless of whether it implements a cache) &MUST-NOT; change the
1496   <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Range</x:ref> or
1497   <x:ref>Content-Type</x:ref> response header fields, nor the response
1498   representation.
1499</t>
1500</section>
1501
1502</section>
1503
1504<section anchor="cache.control.extensions" title="Cache Control Extensions">
1505<t>
1506   The Cache-Control header field can be extended through the use of one or
1507   more cache-extension tokens, each with an optional value. Informational
1508   extensions (those that do not require a change in cache behavior) can be
1509   added without changing the semantics of other directives. Behavioral
1510   extensions are designed to work by acting as modifiers to the existing base
1511   of cache directives. Both the new directive and the standard directive are
1512   supplied, such that applications that do not understand the new directive
1513   will default to the behavior specified by the standard directive, and those
1514   that understand the new directive will recognize it as modifying the
1515   requirements associated with the standard directive. In this way,
1516   extensions to the cache-control directives can be made without requiring
1517   changes to the base protocol.
1518</t>
1519<t>
1520   This extension mechanism depends on an HTTP cache obeying all of the
1521   cache-control directives defined for its native HTTP-version, obeying
1522   certain extensions, and ignoring all directives that it does not
1523   understand.
1524</t>
1525<t>
1526   For example, consider a hypothetical new response directive called
1527   "community" that acts as a modifier to the private directive. We define
1528   this new directive to mean that, in addition to any private cache, any
1529   cache that is shared only by members of the community named within its
1530   value is allowed to cache the response. An origin server wishing to allow
1531   the UCI community to use an otherwise private response in their shared
1532   cache(s) could do so by including
1533</t>
1534<figure><artwork type="example">
1535  Cache-Control: private, community="UCI"
1536</artwork></figure>
1537<t>
1538   A cache seeing this header field will act correctly even if the cache does
1539   not understand the community cache-extension, since it will also see and
1540   understand the private directive and thus default to the safe behavior.
1541</t>
1542<t>
1543   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1544   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1545   combined with standard directives (or the response's default cacheability)
1546   such that the cache behavior will remain minimally correct even if the
1547   cache does not understand the extension(s).
1548</t>
1549<t>
1550   New extension directives ought to consider defining:
1551</t>
1552<t>
1553   <list style="symbols">
1554      <t>What it means for a directive to be specified multiple times,</t>
1555      <t>When the directive does not take an argument, what it means when an
1556      argument is present,</t>
1557      <t>When the directive requires an argument, what it means when it is
1558      missing.</t>
1559   </list>
1560</t>
1561<t>
1562   The HTTP Cache Directive Registry defines the name space for the cache
1563   directives.
1564</t>
1565<t>
1566   A registration &MUST; include the following fields:
1567   <list style="symbols">
1568      <t>Cache Directive Name</t>
1569      <t>Pointer to specification text</t>
1570   </list>
1571</t>
1572<t>
1573   Values to be added to this name space require IETF Review (see <xref
1574   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1575</t>
1576<t>
1577   The registry itself is maintained at <eref
1578   target="http://www.iana.org/assignments/http-cache-directives"/>.
1579</t>
1580</section>
1581
1582</section>
1583
1584<section anchor="header.expires" title="Expires">
1585   <iref item="Expires header field" primary="true" x:for-anchor="" />
1586   <x:anchor-alias value="Expires"/>
1587<t>
1588   The "Expires" header field gives the date/time after which the
1589   response is considered stale. See <xref target="expiration.model" /> for
1590   further discussion of the freshness model.
1591</t>
1592<t>
1593   The presence of an Expires field does not imply that the original resource
1594   will change or cease to exist at, before, or after that time.
1595</t>
1596<t>
1597   The field-value is an absolute date and time as defined by HTTP-date in
1598   &http-date;; a sender &MUST; use the rfc1123-date format.
1599</t>
1600<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1601  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1602</artwork></figure>
1603<figure>
1604  <preamble>For example</preamble>
1605<artwork type="example">
1606  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1607</artwork></figure>
1608<t>
1609   A cache &MUST; treat other invalid date formats,
1610   especially including the value "0", as in the past (i.e., "already
1611   expired").
1612</t>
1613<x:note>
1614   <t>
1615       &Note; If a response includes a <x:ref>Cache-Control</x:ref> field with
1616       the max-age directive (see <xref target="cache-response-directive.max-age" />),
1617       that directive overrides the Expires field. Likewise, the s-maxage
1618       directive (<xref target="cache-response-directive.s-maxage" />) overrides
1619       the <x:ref>Expires</x:ref> header field in shared caches.
1620   </t>
1621</x:note>
1622<t>
1623   Historically, HTTP required the Expires field-value to be no more than a
1624   year in the future. While longer freshness lifetimes are no longer
1625   prohibited, extremely large values have been demonstrated to cause
1626   problems (e.g., clock overflows due to use of 32-bit integers for
1627   time values), and many caches will evict a response far sooner than
1628   that. Therefore, senders ought not produce them.
1629</t>
1630<t>
1631   An origin server without a clock &MUST-NOT; assign Expires
1632   values to a response unless these values were associated
1633   with the resource by a system or user with a reliable clock. It &MAY;
1634   assign an Expires value that is known, at or before server
1635   configuration time, to be in the past (this allows "pre-expiration"
1636   of responses without storing separate Expires values for each
1637   resource).
1638</t>
1639</section>
1640
1641<section anchor="header.pragma" title="Pragma">
1642   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1643   <x:anchor-alias value="extension-pragma"/>
1644   <x:anchor-alias value="Pragma"/>
1645   <x:anchor-alias value="pragma-directive"/>
1646<t>
1647   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1648   caches, so that clients can specify a "no-cache" request that they will
1649   understand (as <x:ref>Cache-Control</x:ref> was not defined until HTTP/1.1).
1650   When the Cache-Control header field is also present and understood in a
1651   request, Pragma is ignored.
1652</t>
1653<t>
1654   In HTTP/1.0, Pragma was defined as an extensible field for
1655   implementation-specified directives for recipients. This specification
1656   deprecates such extensions to improve interoperability.
1657</t>
1658<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"/>
1659  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1660  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1661  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1662</artwork></figure>
1663<t>
1664   When the <x:ref>Cache-Control</x:ref> header field is not present in a
1665   request, the no-cache request pragma-directive &MUST; have the same effect
1666   on caches as if "Cache-Control: no-cache" were present (see <xref
1667   target="cache-request-directive" />).
1668</t>
1669<t>
1670   When sending a no-cache request, a client ought to include both the pragma
1671   and cache-control directives, unless Cache-Control: no-cache is
1672   purposefully omitted to target other <x:ref>Cache-Control</x:ref> response
1673   directives at HTTP/1.1 caches. For example:
1674</t>
1675<figure>
1676<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1677GET / HTTP/1.1
1678Host: www.example.com
1679Cache-Control: max-age=30
1680Pragma: no-cache
1681
1682</artwork>
1683</figure>
1684<t>
1685   will constrain HTTP/1.1 caches to serve a response no older than 30
1686   seconds, while precluding implementations that do not understand
1687   <x:ref>Cache-Control</x:ref> from serving a cached response.
1688</t>
1689<x:note>
1690   <t>
1691      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1692      specified, it does not provide a reliable replacement for
1693      "Cache-Control: no-cache" in them.
1694   </t>
1695</x:note>
1696</section>
1697
1698<section anchor="header.warning" title="Warning">
1699   <iref item="Warning header field" primary="true" x:for-anchor="" />
1700   <x:anchor-alias value="Warning"/>
1701   <x:anchor-alias value="warning-value"/>
1702   <x:anchor-alias value="warn-agent"/>
1703   <x:anchor-alias value="warn-code"/>
1704   <x:anchor-alias value="warn-date"/>
1705   <x:anchor-alias value="warn-text"/>
1706<t>
1707   The "Warning" header field is used to carry additional information
1708   about the status or transformation of a message that might not be reflected
1709   in the message. This information is typically used to warn about possible
1710   incorrectness introduced by caching operations or transformations applied
1711   to the payload of the message.
1712</t>
1713<t>
1714   Warnings can be used for other purposes, both cache-related and otherwise.
1715   The use of a warning, rather than an error status code, distinguishes these
1716   responses from true failures.
1717</t>
1718<t>
1719   Warning header fields can in general be applied to any message, however some
1720   warn-codes are specific to caches and can only be applied to response
1721   messages.
1722</t>
1723<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"/>
1724  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1725 
1726  <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>
1727                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1728 
1729  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1730  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1731                  ; the name or pseudonym of the server adding
1732                  ; the Warning header field, for use in debugging
1733  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1734  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1735</artwork></figure>
1736<t>
1737   Multiple warnings can be attached to a response (either by the origin
1738   server or by a cache), including multiple warnings with the same code
1739   number, only differing in warn-text.
1740</t>
1741<t>
1742   When this occurs, the user agent &SHOULD; inform the user of as many of
1743   them as possible, in the order that they appear in the response.
1744</t>
1745<t>
1746   Systems that generate multiple Warning header fields are encouraged to
1747   order them with this user agent behavior in mind. New Warning header fields
1748   are added after any existing Warning header fields.
1749</t>
1750<t>
1751   Warnings are assigned three digit warn-codes. The first digit indicates
1752   whether the Warning is required to be deleted from a stored response after
1753   validation:
1754   <list style="symbols">
1755      <t>1xx Warnings describe the freshness or validation status of the
1756      response, and so &MUST; be deleted by a cache after validation. They can
1757      only be generated by a cache when validating a cached entry, and
1758      &MUST-NOT; be generated in any other situation.</t>
1759      <t>2xx Warnings describe some aspect of the representation that is not
1760      rectified by a validation (for example, a lossy compression of the
1761      representation) and &MUST-NOT; be deleted by a cache after validation,
1762      unless a full response is returned, in which case they &MUST; be.</t>
1763   </list>
1764</t>
1765<t>
1766   If an implementation sends a message with one or more Warning header fields
1767   to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST;
1768   include in each warning-value a warn-date that matches the
1769   <x:ref>Date</x:ref> header field in the message.
1770</t>
1771<t>
1772   If a system receives a message with a warning-value that includes a
1773   warn-date, and that warn-date is different from the <x:ref>Date</x:ref>
1774   value in the response, then that warning-value &MUST; be deleted from the
1775   message before storing, forwarding, or using it. (preventing the
1776   consequences of naive caching of Warning header fields.) If all of the
1777   warning-values are deleted for this reason, the Warning header field &MUST;
1778   be deleted as well.
1779</t>
1780<t>
1781   The following warn-codes are defined by this specification, each with a
1782   recommended warn-text in English, and a description of its meaning.
1783</t>
1784
1785<section title="110 Response is Stale" anchor="warn.110">
1786  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1787<t>
1788   A cache &SHOULD; include this whenever the returned response is stale.
1789</t>
1790</section>
1791
1792<section title="111 Revalidation Failed" anchor="warn.111">
1793  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1794<t>
1795   A cache &SHOULD; include this when returning a stale response because an
1796   attempt to validate the response failed, due to an inability to reach
1797   the server.
1798</t>
1799</section>
1800
1801<section title="112 Disconnected Operation" anchor="warn.112">
1802  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1803<t>
1804   A cache &SHOULD; include this if it is intentionally disconnected from
1805   the rest of the network for a period of time.
1806</t>
1807</section>
1808
1809<section title="113 Heuristic Expiration" anchor="warn.113">
1810  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1811<t>
1812   A cache &SHOULD; include this if it heuristically chose a freshness
1813   lifetime greater than 24 hours and the response's age is greater than 24
1814   hours.
1815</t>
1816</section>
1817
1818<section title="199 Miscellaneous Warning" anchor="warn.199">
1819  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
1820<t>
1821   The warning text can include arbitrary information to be presented to
1822   a human user, or logged. A system receiving this warning &MUST-NOT; take
1823   any automated action, besides presenting the warning to the user.
1824</t>
1825</section>
1826
1827<section title="214 Transformation Applied" anchor="warn.214">
1828  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
1829<t>
1830   &MUST; be added by a proxy if it applies any transformation to the
1831   representation, such as changing the content-coding, media-type, or
1832   modifying the representation data, unless this Warning code already appears
1833   in the response.
1834</t>
1835</section>
1836
1837<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1838  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
1839<t>
1840   The warning text can include arbitrary information to be presented to
1841   a human user, or logged. A system receiving this warning &MUST-NOT; take
1842   any automated action.
1843</t>
1844</section>
1845
1846<section title="Warn Code Extensions" anchor="warn.code.extensions">
1847<t>
1848   The HTTP Warn Code Registry defines the name space for warn codes.
1849</t>
1850<t>
1851   A registration &MUST; include the following fields:
1852   <list style="symbols">
1853      <t>Warn Code (3 digits)</t>
1854      <t>Short Description</t>
1855      <t>Pointer to specification text</t>
1856   </list>
1857</t>
1858<t>
1859   Values to be added to this name space require IETF Review (see <xref
1860   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1861</t>
1862<t>
1863   The registry itself is maintained at <eref
1864   target="http://www.iana.org/assignments/http-warn-codes"/>.
1865</t>
1866</section>
1867</section>
1868</section>
1869
1870<section anchor="history.lists" title="History Lists">
1871<t>
1872   User agents often have history mechanisms, such as "Back" buttons and
1873   history lists, that can be used to redisplay a representation retrieved
1874   earlier in a session.
1875</t>
1876<t>
1877   The freshness model (<xref target="expiration.model"/>) does not
1878   necessarily apply to history mechanisms. I.e., a history mechanism can
1879   display a previous representation even if it has expired.
1880</t>
1881<t>
1882   This does not prohibit the history mechanism from telling the user that a
1883   view might be stale, or from honoring cache directives (e.g.,
1884   Cache-Control: no-store).
1885</t>
1886</section>
1887
1888
1889<section anchor="IANA.considerations" title="IANA Considerations">
1890
1891<section title="Cache Directive Registry" 
1892   anchor="cache.directive.registration">
1893<t>
1894   The registration procedure for HTTP Cache Directives is defined by <xref
1895   target="cache.control.extensions"/> of this document.
1896</t>
1897<t>
1898   The HTTP Cache Directive Registry shall be created at <eref
1899   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1900   populated with the registrations below:
1901</t>
1902<?BEGININC p6-cache.cache-directives ?>
1903<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1904<texttable xmlns:my="#my" align="left" suppress-title="true"
1905           anchor="iana.cache.directive.registration.table">
1906   <ttcol>Cache Directive</ttcol>
1907   <ttcol>Reference</ttcol>
1908
1909   <c>max-age</c>
1910   <c>
1911      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1912   </c>
1913   <c>max-stale</c>
1914   <c>
1915      <xref target="cache-request-directive.max-stale"/>
1916   </c>
1917   <c>min-fresh</c>
1918   <c>
1919      <xref target="cache-request-directive.min-fresh"/>
1920   </c>
1921   <c>must-revalidate</c>
1922   <c>
1923      <xref target="cache-response-directive.must-revalidate"/>
1924   </c>
1925   <c>no-cache</c>
1926   <c>
1927      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1928   </c>
1929   <c>no-store</c>
1930   <c>
1931      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1932   </c>
1933   <c>no-transform</c>
1934   <c>
1935      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1936   </c>
1937   <c>only-if-cached</c>
1938   <c>
1939      <xref target="cache-request-directive.only-if-cached"/>
1940   </c>
1941   <c>private</c>
1942   <c>
1943      <xref target="cache-response-directive.private"/>
1944   </c>
1945   <c>proxy-revalidate</c>
1946   <c>
1947      <xref target="cache-response-directive.proxy-revalidate"/>
1948   </c>
1949   <c>public</c>
1950   <c>
1951      <xref target="cache-response-directive.public"/>
1952   </c>
1953   <c>s-maxage</c>
1954   <c>
1955      <xref target="cache-response-directive.s-maxage"/>
1956   </c>
1957   <c>stale-if-error</c>
1958   <c>
1959      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1960   </c>
1961   <c>stale-while-revalidate</c>
1962   <c>
1963      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1964   </c>
1965</texttable>
1966<!--(END)-->
1967<?ENDINC p6-cache.cache-directives ?>
1968</section>
1969
1970<section title="Warn Code Registry" 
1971   anchor="warn.code.registration">
1972<t>
1973   The registration procedure for HTTP Warn Codes is defined by <xref
1974   target="warn.code.extensions"/> of this document.
1975</t>
1976<t>
1977   The HTTP Warn Code Registry shall be created at <eref
1978   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1979   populated with the registrations below:
1980</t>
1981<?BEGININC p6-cache.iana-warn-codes ?>
1982<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1983<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1984   <ttcol>Warn Code</ttcol>
1985   <ttcol>Short Description</ttcol>
1986   <ttcol>Reference</ttcol>
1987   <c>110</c>
1988   <c>Response is Stale</c>
1989   <c>
1990      <xref target="warn.110"/>
1991   </c>
1992   <c>111</c>
1993   <c>Revalidation Failed</c>
1994   <c>
1995      <xref target="warn.111"/>
1996   </c>
1997   <c>112</c>
1998   <c>Disconnected Operation</c>
1999   <c>
2000      <xref target="warn.112"/>
2001   </c>
2002   <c>113</c>
2003   <c>Heuristic Expiration</c>
2004   <c>
2005      <xref target="warn.113"/>
2006   </c>
2007   <c>199</c>
2008   <c>Miscellaneous Warning</c>
2009   <c>
2010      <xref target="warn.199"/>
2011   </c>
2012   <c>214</c>
2013   <c>Transformation Applied</c>
2014   <c>
2015      <xref target="warn.214"/>
2016   </c>
2017   <c>299</c>
2018   <c>Miscellaneous Persistent Warning</c>
2019   <c>
2020      <xref target="warn.299"/>
2021   </c>
2022</texttable>
2023<!--(END)-->
2024<?ENDINC p6-cache.iana-warn-codes ?>
2025</section>
2026
2027<section title="Header Field Registration" anchor="header.field.registration">
2028<t>
2029  The Message Header Field Registry located at <eref 
2030  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2031  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
2032</t>
2033<?BEGININC p6-cache.iana-headers ?>
2034<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2035<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2036   <ttcol>Header Field Name</ttcol>
2037   <ttcol>Protocol</ttcol>
2038   <ttcol>Status</ttcol>
2039   <ttcol>Reference</ttcol>
2040
2041   <c>Age</c>
2042   <c>http</c>
2043   <c>standard</c>
2044   <c>
2045      <xref target="header.age"/>
2046   </c>
2047   <c>Cache-Control</c>
2048   <c>http</c>
2049   <c>standard</c>
2050   <c>
2051      <xref target="header.cache-control"/>
2052   </c>
2053   <c>Expires</c>
2054   <c>http</c>
2055   <c>standard</c>
2056   <c>
2057      <xref target="header.expires"/>
2058   </c>
2059   <c>Pragma</c>
2060   <c>http</c>
2061   <c>standard</c>
2062   <c>
2063      <xref target="header.pragma"/>
2064   </c>
2065   <c>Warning</c>
2066   <c>http</c>
2067   <c>standard</c>
2068   <c>
2069      <xref target="header.warning"/>
2070   </c>
2071</texttable>
2072<!--(END)-->
2073<?ENDINC p6-cache.iana-headers ?>
2074<t>
2075   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2076   Force".
2077</t>
2078</section>
2079</section>
2080
2081<section anchor="security.considerations" title="Security Considerations">
2082<t>
2083   Caches expose additional potential vulnerabilities, since the contents of
2084   the cache represent an attractive target for malicious exploitation.
2085   Because cache contents persist after an HTTP request is complete, an attack
2086   on the cache can reveal information long after a user believes that the
2087   information has been removed from the network. Therefore, cache contents
2088   need to be protected as sensitive information.
2089</t>
2090<t>
2091   Furthermore, the very use of a cache can bring about privacy concerns. For
2092   example, if two users share a cache, and the first one browses to a site,
2093   the second may be able to detect that the other has been to that site,
2094   because the resources from it load more quickly, thanks to the cache.
2095</t>
2096<t>
2097   Implementation flaws might allow attackers to insert content into a cache
2098   ("cache poisoning"), leading to compromise of clients that trust that
2099   content. Because of their nature, these attacks are difficult to mitigate.
2100</t>
2101<t>
2102   Likewise, implementation flaws (as well as misunderstanding of cache
2103   operation) might lead to caching of sensitive information (e.g.,
2104   authentication credentials) that is thought to be private, exposing it to
2105   unauthorized parties.
2106</t>
2107<t>
2108   Note that the Set-Cookie response header <xref target="RFC6265"/> does not
2109   inhibit caching; a cacheable response with a Set-Cookie header can be (and
2110   often is) used to satisfy subsequent requests to caches. Servers who wish
2111   to control caching of these responses are encouraged to emit appropriate
2112   Cache-Control response headers.
2113</t>
2114
2115</section>
2116
2117<section title="Acknowledgments" anchor="acks">
2118<t>
2119  See &acks;.
2120</t>
2121</section>
2122
2123</middle>
2124
2125<back>
2126<references title="Normative References">
2127
2128  <reference anchor="Part1">
2129    <front>
2130      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2131      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2132        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2133        <address><email>fielding@gbiv.com</email></address>
2134      </author>
2135      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2136        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2137        <address><email>julian.reschke@greenbytes.de</email></address>
2138      </author>
2139      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2140    </front>
2141    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2142    <x:source basename="p1-messaging" href="p1-messaging.xml">
2143      <x:defines>Content-Length</x:defines>
2144      <x:defines>Via</x:defines>
2145    </x:source>
2146  </reference>
2147
2148  <reference anchor="Part2">
2149    <front>
2150      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2151      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2152        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2153        <address><email>fielding@gbiv.com</email></address>
2154      </author>
2155      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2156        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2157        <address><email>julian.reschke@greenbytes.de</email></address>
2158      </author>
2159      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2160    </front>
2161    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2162    <x:source basename="p2-semantics" href="p2-semantics.xml">
2163      <x:defines>2xx (Successful)</x:defines>
2164      <x:defines>200 (OK)</x:defines>
2165      <x:defines>203 (Non-Authoritative Information)</x:defines>
2166      <x:defines>3xx (Redirection)</x:defines>
2167      <x:defines>300 (Multiple Choices)</x:defines>
2168      <x:defines>301 (Moved Permanently)</x:defines>
2169      <x:defines>404 (Not Found)</x:defines>
2170      <x:defines>410 (Gone)</x:defines>
2171      <x:defines>5xx (Server Error)</x:defines>
2172      <x:defines>504 (Gateway Timeout)</x:defines>
2173      <x:defines>Content-Encoding</x:defines>
2174      <x:defines>Content-Location</x:defines>
2175      <x:defines>Content-Type</x:defines>
2176      <x:defines>Date</x:defines>
2177      <x:defines>Location</x:defines>
2178      <x:defines>Vary</x:defines>
2179    </x:source>
2180  </reference>
2181
2182  <reference anchor="Part4">
2183    <front>
2184      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2185      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2186        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2187        <address><email>fielding@gbiv.com</email></address>
2188      </author>
2189      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2190        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2191        <address><email>julian.reschke@greenbytes.de</email></address>
2192      </author>
2193      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2194    </front>
2195    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2196    <x:source basename="p4-conditional" href="p4-conditional.xml">
2197      <x:defines>304</x:defines>
2198      <x:defines>304 (Not Modified)</x:defines>
2199      <x:defines>ETag</x:defines>
2200      <x:defines>If-Modified-Since</x:defines>
2201      <x:defines>If-None-Match</x:defines>
2202      <x:defines>Last-Modified</x:defines>
2203    </x:source>
2204  </reference>
2205
2206  <reference anchor="Part5">
2207    <front>
2208      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2209      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2210        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2211        <address><email>fielding@gbiv.com</email></address>
2212      </author>
2213      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2214        <organization abbrev="W3C">World Wide Web Consortium</organization>
2215        <address><email>ylafon@w3.org</email></address>
2216      </author>
2217      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2218        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2219        <address><email>julian.reschke@greenbytes.de</email></address>
2220      </author>
2221      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2222    </front>
2223    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2224    <x:source basename="p5-range" href="p5-range.xml">
2225      <x:defines>206 (Partial Content)</x:defines>
2226      <x:defines>Content-Range</x:defines>
2227      <x:defines>Range</x:defines>
2228    </x:source>
2229  </reference>
2230
2231  <reference anchor="Part7">
2232    <front>
2233      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2234      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2235        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2236        <address><email>fielding@gbiv.com</email></address>
2237      </author>
2238      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2239        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2240        <address><email>julian.reschke@greenbytes.de</email></address>
2241      </author>
2242      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2243    </front>
2244    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2245    <x:source basename="p7-auth" href="p7-auth.xml">
2246      <x:defines>Authorization</x:defines>
2247    </x:source>
2248  </reference>
2249
2250  <reference anchor="RFC2119">
2251    <front>
2252      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2253      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2254        <organization>Harvard University</organization>
2255        <address><email>sob@harvard.edu</email></address>
2256      </author>
2257      <date month="March" year="1997" />
2258    </front>
2259    <seriesInfo name="BCP" value="14" />
2260    <seriesInfo name="RFC" value="2119" />
2261  </reference>
2262
2263  <reference anchor="RFC5234">
2264    <front>
2265      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2266      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2267        <organization>Brandenburg InternetWorking</organization>
2268        <address>
2269          <email>dcrocker@bbiw.net</email>
2270        </address> 
2271      </author>
2272      <author initials="P." surname="Overell" fullname="Paul Overell">
2273        <organization>THUS plc.</organization>
2274        <address>
2275          <email>paul.overell@thus.net</email>
2276        </address>
2277      </author>
2278      <date month="January" year="2008"/>
2279    </front>
2280    <seriesInfo name="STD" value="68"/>
2281    <seriesInfo name="RFC" value="5234"/>
2282  </reference>
2283 
2284</references>
2285
2286<references title="Informative References">
2287
2288  <reference anchor="RFC1305">
2289    <front>
2290      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2291      <author fullname="David L. Mills" initials="D." surname="Mills">
2292        <organization>University of Delaware, Electrical Engineering Department</organization>
2293        <address><email>mills@udel.edu</email></address>
2294      </author>
2295      <date month="March" year="1992" />
2296    </front>
2297    <seriesInfo name="RFC" value="1305" />
2298  </reference>
2299
2300  <reference anchor="RFC2616">
2301    <front>
2302      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2303      <author fullname="R. Fielding" initials="R." surname="Fielding">
2304        <organization>University of California, Irvine</organization>
2305        <address><email>fielding@ics.uci.edu</email></address>
2306      </author>
2307      <author fullname="J. Gettys" initials="J." surname="Gettys">
2308        <organization>W3C</organization>
2309        <address><email>jg@w3.org</email></address>
2310      </author>
2311      <author fullname="J. Mogul" initials="J." surname="Mogul">
2312        <organization>Compaq Computer Corporation</organization>
2313        <address><email>mogul@wrl.dec.com</email></address>
2314      </author>
2315      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2316        <organization>MIT Laboratory for Computer Science</organization>
2317        <address><email>frystyk@w3.org</email></address>
2318      </author>
2319      <author fullname="L. Masinter" initials="L." surname="Masinter">
2320        <organization>Xerox Corporation</organization>
2321        <address><email>masinter@parc.xerox.com</email></address>
2322      </author>
2323      <author fullname="P. Leach" initials="P." surname="Leach">
2324        <organization>Microsoft Corporation</organization>
2325        <address><email>paulle@microsoft.com</email></address>
2326      </author>
2327      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2328        <organization>W3C</organization>
2329        <address><email>timbl@w3.org</email></address>
2330      </author>
2331      <date month="June" year="1999" />
2332    </front>
2333    <seriesInfo name="RFC" value="2616" />
2334  </reference>
2335
2336  <reference anchor="RFC3864">
2337    <front>
2338      <title>Registration Procedures for Message Header Fields</title>
2339      <author fullname="G. Klyne" initials="G." surname="Klyne">
2340        <organization>Nine by Nine</organization>
2341        <address><email>GK-IETF@ninebynine.org</email></address>
2342      </author>
2343      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2344        <organization>BEA Systems</organization>
2345        <address><email>mnot@pobox.com</email></address>
2346      </author>
2347      <author fullname="J. Mogul" initials="J." surname="Mogul">
2348        <organization>HP Labs</organization>
2349        <address><email>JeffMogul@acm.org</email></address>
2350      </author>
2351      <date month="September" year="2004" />
2352    </front>
2353    <seriesInfo name="BCP" value="90" />
2354    <seriesInfo name="RFC" value="3864" />
2355  </reference>
2356
2357  <reference anchor='RFC5226'>
2358    <front>
2359      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2360      <author initials='T.' surname='Narten' fullname='T. Narten'>
2361        <organization>IBM</organization>
2362        <address><email>narten@us.ibm.com</email></address>
2363      </author>
2364      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2365        <organization>Google</organization>
2366        <address><email>Harald@Alvestrand.no</email></address>
2367      </author>
2368      <date year='2008' month='May' />
2369    </front>
2370    <seriesInfo name='BCP' value='26' />
2371    <seriesInfo name='RFC' value='5226' />
2372  </reference>
2373
2374  <reference anchor='RFC5861'>
2375    <front>
2376      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2377      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2378        <organization>Yahoo! Inc.</organization>
2379        <address><email>mnot@yahoo-inc.com</email></address>
2380      </author>
2381      <date month="April" year="2010"/>
2382    </front>
2383    <seriesInfo name='RFC' value='5861' />
2384  </reference>
2385
2386  <reference anchor="RFC6265">
2387    <front>
2388      <title>HTTP State Management Mechanism</title>
2389      <author initials="A." surname="Barth" fullname="Adam Barth">
2390        <organization abbrev="U.C. Berkeley">
2391          University of California, Berkeley
2392        </organization>
2393        <address><email>abarth@eecs.berkeley.edu</email></address>
2394      </author>
2395      <date year="2011" month="April" />
2396    </front>
2397    <seriesInfo name="RFC" value="6265"/>
2398  </reference>
2399
2400</references>
2401
2402<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2403<t>
2404  Caching-related text has been substantially rewritten for clarity.
2405</t>
2406<t>
2407  The algorithm for calculating age is now less conservative.
2408  (<xref target="age.calculations"/>)
2409</t>
2410<t>
2411  Caches are now required to handle dates with timezones as if they're
2412  invalid, because it's not possible to accurately guess.
2413  (<xref target="age.calculations"/>)
2414</t>
2415<t>
2416  The <x:ref>Content-Location</x:ref> response header field is no longer used
2417  to determine the appropriate response to use when validating.
2418  (<xref target="validation.model" />)
2419</t>
2420<t>
2421  The algorithm for selecting a cached negotiated response to use has been
2422  clarified in several ways. In particular, it now explicitly allows
2423  header-specific canonicalization when processing selecting header fields.
2424  (<xref target="caching.negotiated.responses" />)
2425</t>
2426<t>
2427  Requirements regarding denial of service attack avoidance when performing
2428  invalidation have been clarified.
2429  (<xref target="invalidation.after.updates.or.deletions" />)
2430</t>
2431<t>
2432  Cache invalidation only occurs when a successful response is received.
2433  (<xref target="invalidation.after.updates.or.deletions" />)
2434</t>
2435<t>
2436  The conditions under which an authenticated response can be cached have been
2437  clarified.
2438  (<xref target="caching.authenticated.responses" />)
2439</t>
2440<t>
2441  The one-year limit on Expires header field values has been removed; instead,
2442  the reasoning for using a sensible value is given.
2443  (<xref target="header.expires" />)
2444</t>
2445<t>
2446  The Pragma header field is now only defined for backwards compatibility;
2447  future pragmas are deprecated.
2448  (<xref target="header.pragma" />)
2449</t>
2450<t>
2451  Cache directives are explicitly defined to be case-insensitive.
2452  (<xref target="header.cache-control" />)
2453</t>
2454<t>
2455  Handling of multiple instances of cache directives when only one is
2456  expected is now defined.
2457  (<xref target="header.cache-control" />)
2458</t>
2459<t>
2460  The qualified forms of the private and no-cache cache directives are noted
2461  to not be widely implemented; e.g., "private=foo" is interpreted by many
2462  caches as simply "private". Additionally, the meaning of the qualified
2463  form of no-cache has been clarified.
2464  (<xref target="cache-response-directive" />)
2465</t>
2466<t>
2467  The "no-store" cache request directive doesn't apply to responses; i.e.,
2468  a cache can satisfy a request with no-store on it, and does not invalidate
2469  it.
2470  (<xref target="cache-request-directive.no-store" />)
2471</t>
2472<t>
2473  The meaning of the "public" cache response directive has been refined to
2474  be less vague.
2475  (<xref target="cache-response-directive.public" />)
2476</t>
2477<t>
2478  The "no-cache" response cache directive's meaning has been clarified.
2479  (<xref target="cache-response-directive.no-cache" />)
2480</t>
2481<t>
2482  New status codes can now define that caches are allowed to use heuristic
2483  freshness with them.
2484  (<xref target="heuristic.freshness" />)
2485</t>
2486<t>
2487  Caches are now allow to calculate heuristic freshness for URLs with query
2488  components.
2489  (<xref target="heuristic.freshness" />)
2490</t>
2491<t>
2492  Some requirements regarding production of the <x:ref>Warning</x:ref> 
2493  header have been relaxed, as it is not widely implemented.
2494  (<xref target="header.warning" />)
2495</t>
2496<t>
2497  The <x:ref>Warning</x:ref> header field no longer uses RFC 2047 encoding,
2498  nor allows multiple languages, as these aspects were not implemented.
2499  (<xref target="header.warning" />)
2500</t>
2501<t>
2502  This specification introduces the Cache Directive and Warn Code Registries,
2503  and defines considerations for new cache directives.
2504  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
2505</t>
2506</section>
2507
2508<section title="Imported ABNF" anchor="imported.abnf">
2509   <x:anchor-alias value="ALPHA"/>
2510   <x:anchor-alias value="CR"/>
2511   <x:anchor-alias value="DIGIT"/>
2512   <x:anchor-alias value="DQUOTE"/>
2513   <x:anchor-alias value="LF"/>
2514   <x:anchor-alias value="OCTET"/>
2515   <x:anchor-alias value="SP"/>
2516   <x:anchor-alias value="VCHAR"/>
2517   <x:anchor-alias value="quoted-string"/>
2518   <x:anchor-alias value="token"/>
2519   <x:anchor-alias value="OWS"/>
2520   <x:anchor-alias value="field-name"/>
2521   <x:anchor-alias value="HTTP-date"/>
2522   <x:anchor-alias value="port"/>
2523   <x:anchor-alias value="pseudonym"/>
2524   <x:anchor-alias value="uri-host"/>
2525<t>
2526   The following core rules are included by reference, as defined in <xref
2527   target="RFC5234" x:fmt="of" x:sec="B.1"/>: ALPHA (letters), CR (carriage
2528   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2529   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2530   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2531</t>
2532<t>
2533   The rules below are defined in <xref target="Part1"/>:
2534</t>
2535<figure><artwork type="abnf2616">
2536  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
2537  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
2538  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
2539  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
2540
2541  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
2542  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
2543  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
2544</artwork></figure>
2545<t>
2546   The rules below are defined in other parts:
2547</t>
2548<figure><artwork type="abnf2616">
2549  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
2550</artwork></figure>
2551</section>
2552
2553<?BEGININC p6-cache.abnf-appendix ?>
2554<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2555<figure>
2556<artwork type="abnf" name="p6-cache.parsed-abnf">
2557<x:ref>Age</x:ref> = delta-seconds
2558
2559<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2560 cache-directive ] )
2561
2562<x:ref>Expires</x:ref> = HTTP-date
2563
2564<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8.1.1.1&gt;
2565
2566<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
2567
2568<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2569 pragma-directive ] )
2570
2571<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2572 )
2573
2574<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2575
2576<x:ref>delta-seconds</x:ref> = 1*DIGIT
2577
2578<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2579
2580<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2581
2582<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2583<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2584<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 5.7&gt;
2585
2586<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.4&gt;
2587
2588<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.4&gt;
2589
2590<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2591
2592<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2593<x:ref>warn-code</x:ref> = 3DIGIT
2594<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2595<x:ref>warn-text</x:ref> = quoted-string
2596<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2597 ]
2598</artwork>
2599</figure>
2600</section>
2601<?ENDINC p6-cache.abnf-appendix ?>
2602
2603<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2604<t>
2605  Changes up to the first Working Group Last Call draft are summarized
2606  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2607</t>
2608
2609<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2610<t>
2611  Closed issues:
2612  <list style="symbols">
2613    <t>
2614      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2615      "untangle Cache-Control ABNF"
2616    </t>
2617    <t>
2618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2619      "Multiple values in Cache-Control header fields"
2620    </t>
2621    <t>
2622      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2623      "Case sensitivity of header fields in CC values"
2624    </t>
2625    <t>
2626      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2627      "Spurious 'MAYs'"
2628    </t>
2629    <t>
2630      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2631      "enhance considerations for new cache control directives"
2632    </t>
2633    <t>
2634      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2635      "ABNF requirements for recipients"
2636    </t>
2637    <t>
2638      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2639      "note introduction of new IANA registries as normative changes"
2640    </t>
2641    <t>
2642      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
2643      "broken prose in description of 'Vary'"
2644    </t>
2645  </list>
2646</t>
2647</section>
2648
2649<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
2650<t>
2651  Closed issues:
2652  <list style="symbols">
2653    <t>
2654      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
2655      "'Most Conservative'"
2656    </t>
2657  </list>
2658</t>
2659<t>
2660  Other changes:
2661  <list style="symbols">
2662    <t>
2663      Conformance criteria and considerations regarding error handling are
2664      now defined in Part 1.
2665    </t>
2666    <t>
2667      Move definition of "Vary" header field into Part 2.
2668    </t>
2669    <t>
2670      Add security considerations with respect to cache poisoning and
2671      the "Set-Cookie" header field.
2672    </t>
2673  </list>
2674</t>
2675</section>
2676
2677<section title="Since draft-ietf-httpbis-p6-cache-21" anchor="changes.since.21">
2678<t>
2679  None yet.
2680</t>
2681</section>
2682
2683</section>
2684  </back>
2685</rfc>
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