source: draft-ietf-httpbis/latest/p6-cache.xml @ 2082

Last change on this file since 2082 was 2082, checked in by fielding@…, 7 years ago

Yet another attempt to explain HTTP-date, remove redundant requirements in sections that use HTTP-date, and correct some inconsistent requirements regarding time zones; related to #375 and [2077]

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  • Property svn:mime-type set to text/xml
File size: 105.4 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2013">
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  <!ENTITY transformations             "<xref target='Part1' x:rel='#message.transformations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47]>
48<?rfc toc="yes" ?>
49<?rfc symrefs="yes" ?>
50<?rfc sortrefs="yes" ?>
51<?rfc compact="yes"?>
52<?rfc subcompact="no" ?>
53<?rfc linkmailto="no" ?>
54<?rfc editing="no" ?>
55<?rfc comments="yes"?>
56<?rfc inline="yes"?>
57<?rfc rfcedstyle="yes"?>
58<?rfc-ext allow-markup-in-artwork="yes" ?>
59<?rfc-ext include-references-in-index="yes" ?>
60<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
61  obsoletes="2616" x:maturity-level="proposed" xmlns:x="http://purl.org/net/xml2rfc/ext">
62<x:link rel="prev" basename="p5-range"/>
63<x:link rel="next" basename="p7-auth"/>
64<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
65<front>
66
67  <title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
68
69  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
70    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
71    <address>
72      <postal>
73        <street>345 Park Ave</street>
74        <city>San Jose</city>
75        <region>CA</region>
76        <code>95110</code>
77        <country>USA</country>
78      </postal>
79      <email>fielding@gbiv.com</email>
80      <uri>http://roy.gbiv.com/</uri>
81    </address>
82  </author>
83
84  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
85    <organization>Akamai</organization>
86    <address>
87      <email>mnot@mnot.net</email>
88      <uri>http://www.mnot.net/</uri>
89    </address>
90  </author>
91
92  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
93    <organization abbrev="greenbytes">greenbytes GmbH</organization>
94    <address>
95      <postal>
96        <street>Hafenweg 16</street>
97        <city>Muenster</city><region>NW</region><code>48155</code>
98        <country>Germany</country>
99      </postal>
100      <email>julian.reschke@greenbytes.de</email>
101      <uri>http://greenbytes.de/tech/webdav/</uri>
102    </address>
103  </author>
104
105  <date month="&ID-MONTH;" year="&ID-YEAR;" />
106  <workgroup>HTTPbis Working Group</workgroup>
107
108<abstract>
109<t>
110   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
111   distributed, collaborative, hypertext information systems. This document
112   defines requirements on HTTP caches and the associated header fields
113   that control cache behavior or indicate cacheable response messages.
114</t>
115</abstract>
116
117<note title="Editorial Note (To be removed by RFC Editor)">
118  <t>
119    Discussion of this draft takes place on the HTTPBIS working group
120    mailing list (ietf-http-wg@w3.org), which is archived at
121    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
122  </t>
123  <t>
124    The current issues list is at
125    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
126    documents (including fancy diffs) can be found at
127    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
128  </t>
129  <t>
130    The changes in this draft are summarized in <xref target="changes.since.21"/>.
131  </t>
132</note>
133
134   </front>
135   <middle>
136
137<section anchor="caching" title="Introduction">
138<t>
139   HTTP is typically used for distributed information systems, where
140   performance can be improved by the use of response caches. This document
141   defines aspects of HTTP/1.1 related to caching and reusing response
142   messages.
143</t>
144
145<section anchor="intro.purpose" title="Purpose">
146<iref item="cache" />
147<t>
148   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
149   subsystem that controls its message storage, retrieval, and deletion. A
150   cache stores cacheable responses in order to reduce the response time and
151   network bandwidth consumption on future, equivalent requests. Any client or
152   server &MAY; employ a cache, though a cache cannot be used by a server that
153   is acting as a tunnel.
154</t>
155<t>
156   The goal of caching in HTTP/1.1 is to significantly improve performance
157   by reusing a prior response message to satisfy a current request.
158   A stored response is considered "fresh", as defined in
159   <xref target="expiration.model" />, if the response can be reused without
160   "validation" (checking with the origin server to see if the cached response
161   remains valid for this request).  A fresh cache response can therefore
162   reduce both latency and network transfers each time it is reused.
163   When a cached response is not fresh, it might still be reusable if it can
164   be freshened by validation (<xref target="validation.model" />) or if the
165   origin is unavailable.
166</t>
167</section>
168
169<section anchor="intro.terminology" title="Terminology">
170<t>
171   This specification uses a number of terms to refer to the roles played by
172   participants in, and objects of, HTTP caching.
173</t>
174<t>
175   <iref item="cache" />
176   <x:dfn>cache</x:dfn>
177   <list>
178      <t>A conformant implementation of an HTTP cache. Note that this implies
179        an HTTP/1.1 cache; this specification does not define conformance
180        for HTTP/1.0 caches.</t>
181   </list>
182</t>
183<t anchor="shared.and.non-shared.caches">
184   <iref item="shared cache" />
185   <x:dfn>shared cache</x:dfn>
186   <list>
187      <t>A cache that stores responses to be reused by more than one user;
188         usually (but not always) deployed as part of an intermediary.</t>
189   </list>
190</t>
191<t>
192   <iref item="private cache" />
193   <x:dfn>private cache</x:dfn>
194   <list>
195      <t>A cache that is dedicated to a single user.</t>
196   </list>
197</t>
198<t>
199   <iref item="cacheable" />
200   <x:dfn>cacheable</x:dfn>
201   <list>
202      <t>A response is cacheable if a cache is allowed to store a copy of the
203      response message for use in answering subsequent requests. Even when a
204      response is cacheable, there might be additional constraints on whether
205      a cache can use the stored copy to satisfy a particular request.</t>
206   </list>
207</t>
208<t>
209   <iref item="explicit expiration time" />
210   <x:dfn>explicit expiration time</x:dfn>
211   <list>
212      <t>The time at which the origin server intends that a representation
213      no longer be used by a cache without further validation.</t>
214   </list>
215</t>
216<t>
217   <iref item="heuristic expiration time" />
218   <x:dfn>heuristic expiration time</x:dfn>
219   <list>
220      <t>An expiration time assigned by a cache when no explicit expiration
221      time is available.</t>
222   </list>
223</t>
224<t>
225   <iref item="age" />
226   <x:dfn>age</x:dfn>
227   <list>
228      <t>The age of a response is the time since it was sent by, or
229      successfully validated with, the origin server.</t>
230   </list>
231</t>
232<t>
233   <iref item="first-hand" />
234   <x:dfn>first-hand</x:dfn>
235   <list>
236      <t>A response is first-hand if the freshness model is not in use; i.e.,
237      its age is 0.</t>
238   </list>
239</t>
240<t>
241   <iref item="freshness lifetime" />
242   <x:dfn>freshness lifetime</x:dfn>
243   <list>
244      <t>The length of time between the generation of a response and its
245      expiration time.</t>
246   </list>
247</t>
248<t>
249   <iref item="fresh" />
250   <x:dfn>fresh</x:dfn>
251   <list>
252      <t>A response is fresh if its age has not yet exceeded its freshness
253      lifetime.</t>
254   </list>
255</t>
256<t>
257   <iref item="stale" />
258   <x:dfn>stale</x:dfn>
259   <list>
260      <t>A response is stale if its age has passed its freshness lifetime
261      (either explicit or heuristic).</t>
262   </list>
263</t>
264<t>
265   <iref item="validator" />
266   <x:dfn>validator</x:dfn>
267   <list>
268      <t>A protocol element (e.g., an entity-tag or a <x:ref>Last-Modified</x:ref>
269      time) that is used to find out whether a stored response is an equivalent
270      copy of a representation. See &weak-and-strong;.</t>
271   </list>
272</t>
273<t>
274   <iref item="strong validator" />
275   <iref item="validator" subitem="strong" />
276   <x:dfn>strong validator</x:dfn>
277   <list>
278      <t>A validator that is defined by the origin server such that its
279         current value will change if the representation data changes; i.e.,
280         an entity-tag that is not marked as weak (&entity-tags;) or,
281         if no entity-tag is provided, a <x:ref>Last-Modified</x:ref> value
282         that is strong in the sense defined by &lastmod-comparison;.</t>
283   </list>
284</t>
285</section>
286
287<section title="Conformance and Error Handling" anchor="conformance">
288<t>
289   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
290   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
291   document are to be interpreted as described in <xref target="RFC2119"/>.
292</t>
293<t>
294   Conformance criteria and considerations regarding error handling
295   are defined in &conformance;.
296</t>
297</section>
298
299<section title="Syntax Notation" anchor="notation">
300<t>
301   This specification uses the Augmented Backus-Naur Form (ABNF) notation
302   of <xref target="RFC5234"/> with the list rule extension defined in
303   &notation;. <xref target="imported.abnf"/> describes rules imported from
304   other documents. <xref target="collected.abnf"/> shows the collected ABNF
305   with the list rule expanded.
306</t>
307
308<section title="Delta Seconds" anchor="delta-seconds">
309<t>
310   The delta-seconds rule specifies a non-negative integer, representing time
311   in seconds.
312</t>
313<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
314  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
315</artwork></figure>
316<t>
317   If an implementation receives a delta-seconds value larger than the largest
318   positive integer it can represent, or if any of its subsequent calculations
319   overflows, it &MUST; consider the value to be 2147483648
320   (2<x:sup>31</x:sup>). Recipients parsing a delta-seconds value &MUST; use
321   an arithmetic type of at least 31 bits of range, and senders &MUST-NOT;
322   send delta-seconds with a value greater than 2147483648.
323</t>
324</section>
325
326</section>
327</section>
328
329<section anchor="caching.overview" title="Overview of Cache Operation">
330<iref item="cache entry" />
331<iref item="cache key" />
332<t>
333   Proper cache operation preserves the semantics of HTTP transfers
334   (&semantics;) while eliminating the transfer of information already held
335   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
336   we assume that reusing the cached response is desirable and that such
337   reuse is the default behavior when no requirement or locally-desired
338   configuration prevents it.  Therefore, HTTP cache requirements are focused
339   on preventing a cache from either storing a non-reusable response or
340   reusing a stored response inappropriately.
341</t>
342<t>
343   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
344   HTTP responses corresponding to prior requests that used the same key. The
345   most common form of cache entry is a successful result of a retrieval
346   request: i.e., a <x:ref>200 (OK)</x:ref> response containing a
347   representation of the resource identified by the request target. However,
348   it is also possible to cache negative results (e.g., <x:ref>404 (Not
349   Found)</x:ref>, incomplete results (e.g., <x:ref>206 (Partial
350   Content)</x:ref>), and responses to methods other than GET if the method's
351   definition allows such caching and defines something suitable for use as a
352   cache key.
353</t>
354<t>
355   The default <x:dfn>cache key</x:dfn> consists of the request method and
356   target URI.  However, since HTTP caches in common use today are typically
357   limited to caching responses to GET, many implementations simply decline
358   other methods and use only the URI as the key.
359</t>
360<t>
361   If a request target is subject to content negotiation, its cache entry
362   might consist of multiple stored responses, each differentiated by a
363   secondary key for the values of the original request's selecting header
364   fields (<xref target="caching.negotiated.responses"/>).
365</t>
366</section>
367
368<section anchor="response.cacheability" title="Storing Responses in Caches">
369<t>
370   A cache &MUST-NOT; store a response to any request, unless:
371   <list style="symbols">
372      <t>The request method is understood by the cache and defined as being
373      cacheable, and</t>
374      <t>the response status code is understood by the cache, and</t>
375      <t>the "no-store" cache directive (see <xref
376      target="header.cache-control" />) does not appear in request or response
377      header fields, and</t>
378      <t>the "private" cache response directive (see <xref
379      target="cache-response-directive.private" />) does not appear in the
380      response, if the cache is shared, and</t>
381      <t>the <x:ref>Authorization</x:ref> header field (see
382      &header-authorization;) does not appear in the request, if the cache is
383      shared, unless the response explicitly allows it (see <xref
384      target="caching.authenticated.responses" />), and</t>
385      <t>the response either:
386         <list style="symbols">
387            <t>contains an <x:ref>Expires</x:ref> header field (see
388            <xref target="header.expires"/>), or</t>
389            <t>contains a max-age response cache directive (see <xref
390            target="cache-response-directive.max-age" />), or</t>
391            <t>contains a s-maxage response cache directive and the cache is
392            shared, or</t>
393            <t>contains a Cache Control Extension (see <xref
394            target="cache.control.extensions" />) that allows it to be cached,
395            or</t>
396            <t>has a status code that can be served with heuristic freshness
397            (see <xref target="heuristic.freshness" />).</t>
398         </list>
399      </t>
400   </list>
401</t>
402<t>
403   Note that any of the requirements listed above can be overridden by a
404   cache-control extension; see <xref target="cache.control.extensions" />.
405</t>
406<t>
407   In this context, a cache has "understood" a request method or a response
408   status code if it recognizes it and implements any cache-specific
409   behavior.
410</t>
411<t>
412   Note that, in normal operation, many caches will not store a response that
413   has neither a cache validator nor an explicit expiration time, as such
414   responses are not usually useful to store. However, caches are not
415   prohibited from storing such responses.
416</t>
417
418<section anchor="incomplete.responses" title="Storing Incomplete Responses">
419<t>
420   A response message is considered complete when all of the octets indicated
421   by the message framing (&messaging;) are received prior to the connection
422   being closed. If the request is GET, the response status is <x:ref>200
423   (OK)</x:ref>, and the entire response header block has been received, a
424   cache &MAY; store an incomplete response message body if the cache entry is
425   recorded as incomplete. Likewise, a <x:ref>206 (Partial Content)</x:ref>
426   response &MAY; be stored as if it were an incomplete <x:ref>200
427   (OK)</x:ref> cache entry. However, a cache &MUST-NOT; store incomplete or
428   partial content responses if it does not support the <x:ref>Range</x:ref>
429   and <x:ref>Content-Range</x:ref> header fields or if it does not understand
430   the range units used in those fields.
431</t>
432<t>
433   A cache &MAY; complete a stored incomplete response by making a subsequent
434   range request (&partial;) and combining the successful response with the
435   stored entry, as defined in <xref target="combining.responses"/>. A cache
436   &MUST-NOT; use an incomplete response to answer requests unless the
437   response has been made complete or the request is partial and specifies a
438   range that is wholly within the incomplete response. A cache &MUST-NOT;
439   send a partial response to a client without explicitly marking it as such
440   using the <x:ref>206 (Partial Content)</x:ref> status code.
441</t>
442</section>
443
444
445<section anchor="caching.authenticated.responses" 
446   title="Storing Responses to Authenticated Requests">
447<t>
448   A shared cache &MUST-NOT; use a cached response to a request with an
449   <x:ref>Authorization</x:ref> header field (&header-authorization;) to
450   satisfy any subsequent request unless a cache directive that allows such
451   responses to be stored is present in the response.
452</t>
453<t>
454   In this specification, the following <x:ref>Cache-Control</x:ref> response
455   directives (<xref target="cache-response-directive"/>) have such an effect:
456   must-revalidate, public, s-maxage.
457</t>
458<t>
459   Note that cached responses that contain the "must-revalidate" and/or
460   "s-maxage" response directives are not allowed to be served stale (<xref
461   target="serving.stale.responses"/>) by shared caches. In particular, a
462   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
463   used to satisfy a subsequent request without revalidating it on the origin
464   server.
465</t>
466</section>
467</section>
468
469
470<section anchor="constructing.responses.from.caches" 
471   title="Constructing Responses from Caches">
472<t>
473   For a presented request, a cache &MUST-NOT; send a stored response,
474   unless:
475   <list style="symbols">
476      <t>The presented effective request URI (&effective-request-uri;) and
477      that of the stored response match, and</t>
478      <t>the request method associated with the stored response allows it to
479      be used for the presented request, and</t>
480      <t>selecting header fields nominated by the stored response (if any)
481      match those presented (see <xref target="caching.negotiated.responses"
482      />), and</t>
483      <t>the presented request does not contain the no-cache pragma (<xref
484      target="header.pragma"/>), nor the no-cache cache directive (<xref
485      target="cache-request-directive"/>), unless the stored response is
486      successfully validated (<xref target="validation.model"/>), and</t>
487      <t>the stored response does not contain the no-cache cache directive
488      (<xref target="cache-response-directive.no-cache"/>), unless it is
489      successfully validated (<xref target="validation.model"/>), and</t>
490      <t>the stored response is either:
491         <list style="symbols">
492            <t>fresh (see <xref target="expiration.model" />), or</t>
493            <t>allowed to be served stale (see <xref
494            target="serving.stale.responses" />), or</t>
495            <t>successfully validated (see <xref target="validation.model"
496            />).</t>
497         </list>
498      </t>
499  </list>
500</t>
501<t>
502   Note that any of the requirements listed above can be overridden by a
503   cache-control extension; see <xref target="cache.control.extensions" />.
504</t>
505<t>
506   When a stored response is used to satisfy a request without validation,
507   a cache &MUST; send a single <x:ref>Age</x:ref> header field
508   (<xref target="header.age"/>) in the response with a value equal to the
509   stored response's current_age; see <xref target="age.calculations" />.
510</t>
511<t>
512   A cache &MUST; write through requests with methods that are unsafe
513   (&safe-methods;) to the origin server; i.e., a cache is not allowed to
514   generate a reply to such a request before having forwarded the request and
515   having received a corresponding response.
516</t>
517<t>
518   Also, note that unsafe requests might invalidate already stored responses;
519   see <xref target="invalidation.after.updates.or.deletions" />.
520</t>
521<t>
522   When more than one suitable response is stored, a cache &MUST; use the
523   most recent response (as determined by the <x:ref>Date</x:ref> header
524   field). It can also forward a request with "Cache-Control: max-age=0" or
525   "Cache-Control: no-cache" to disambiguate which response to use.
526</t>
527<t>
528   A cache that does not have a clock available &MUST-NOT; use stored
529   responses without revalidating them on every use.
530</t>
531
532
533<section anchor="expiration.model" title="Freshness Model">
534<t>
535   When a response is "fresh" in the cache, it can be used to satisfy
536   subsequent requests without contacting the origin server, thereby improving
537   efficiency.
538</t>
539<t>
540   The primary mechanism for determining freshness is for an origin server to
541   provide an explicit expiration time in the future, using either the
542   <x:ref>Expires</x:ref> header field (<xref target="header.expires" />) or
543   the max-age response cache directive (<xref
544   target="cache-response-directive.max-age" />). Generally, origin servers will
545   assign future explicit expiration times to responses in the belief that the
546   representation is not likely to change in a semantically significant way
547   before the expiration time is reached.
548</t>
549<t>
550   If an origin server wishes to force a cache to validate every request, it
551   can assign an explicit expiration time in the past to indicate that the
552   response is already stale. Compliant caches will normally validate a stale
553   cached response before reusing it for subsequent requests (see <xref
554   target="serving.stale.responses" />).
555</t>
556<t>
557   Since origin servers do not always provide explicit expiration times, a
558   cache &MAY; assign a heuristic expiration time when an explicit time is not
559   specified, employing algorithms that use other header field values (such as
560   the <x:ref>Last-Modified</x:ref> time) to estimate a plausible expiration
561   time. This specification does not provide specific algorithms, but does
562   impose worst-case constraints on their results.
563</t>
564<figure>
565<preamble>
566  The calculation to determine if a response is fresh is:
567</preamble>
568<artwork type="code">
569   response_is_fresh = (freshness_lifetime &gt; current_age)
570</artwork>
571</figure>
572<t>
573   The freshness_lifetime is defined in <xref
574   target="calculating.freshness.lifetime" />; the current_age is defined in
575   <xref target="age.calculations" />.
576</t>
577<t>
578   Clients can send the max-age or min-fresh cache directives in a request to
579   constrain or relax freshness calculations for the corresponding response
580   (<xref target="cache-request-directive" />).
581</t>
582<t>
583   Note that freshness applies only to cache operation; it cannot be used to
584   force a user agent to refresh its display or reload a resource. See <xref
585   target="history.lists" /> for an explanation of the difference between
586   caches and history mechanisms.
587</t>
588
589<section anchor="calculating.freshness.lifetime" 
590   title="Calculating Freshness Lifetime">
591<t>
592   A cache can calculate the freshness lifetime (denoted as
593   freshness_lifetime) of a response by using the first match of:
594   <list style="symbols">
595      <t>If the cache is shared and the s-maxage response cache directive
596      (<xref target="cache-response-directive.s-maxage" />) is present, use its value,
597      or</t>
598      <t>If the max-age response cache directive (<xref
599      target="cache-response-directive.max-age" />) is present, use its value, or</t>
600      <t>If the <x:ref>Expires</x:ref> response header field
601      (<xref target="header.expires" />) is present, use its value minus the
602      value of the <x:ref>Date</x:ref> response header field, or</t>
603      <t>Otherwise, no explicit expiration time is present in the response. A
604      heuristic freshness lifetime might be applicable; see <xref
605      target="heuristic.freshness" />.</t>
606   </list>
607</t>
608<t>
609   Note that this calculation is not vulnerable to clock skew, since all of
610   the information comes from the origin server.
611</t>
612<t>
613   When there is more than one value present for a given directive (e.g., two
614   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
615   directives), it is considered invalid. Caches are encouraged to consider
616   responses that have invalid freshness information to be stale.
617</t>
618</section>
619
620<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
621<t>
622   If no explicit expiration time is present in a stored response that has a
623   status code whose definition allows heuristic freshness to be used
624   (including the following in &status-codes;: <x:ref>200 (OK)</x:ref>,
625   <x:ref>203 (Non-Authoritative Information)</x:ref>, <x:ref>206 (Partial
626   Content)</x:ref>, <x:ref>300 (Multiple Choices)</x:ref>, <x:ref>301 (Moved
627   Permanently)</x:ref> and <x:ref>410 (Gone)</x:ref>), a cache &MAY;
628   calculate a heuristic expiration time. A cache &MUST-NOT; use heuristics to
629   determine freshness for responses with status codes that do not explicitly
630   allow it.
631</t>
632<t>
633   When a heuristic is used to calculate freshness lifetime, a cache &SHOULD;
634   attach a <x:ref>Warning</x:ref> header field with a 113 warn-code to the
635   response if its current_age is more than 24 hours and such a warning is not
636   already present.
637</t>
638<t>
639   Also, if the response has a <x:ref>Last-Modified</x:ref> header field
640   (&header-last-modified;), caches are encouraged to use a heuristic
641   expiration value that is no more than some fraction of the interval since
642   that time. A typical setting of this fraction might be 10%.
643</t>
644<x:note>
645   <t>
646      &Note; <xref target="RFC2616" x:fmt="of" x:sec="13.9"/> prohibited caches
647      from calculating heuristic freshness for URIs with query components
648      (i.e., those containing '?'). In practice, this has not been widely
649      implemented. Therefore, servers are encouraged to send explicit
650      directives (e.g., Cache-Control: no-cache) if they wish to preclude
651      caching.
652   </t>
653</x:note>
654</section>
655
656<section anchor="age.calculations" title="Calculating Age">
657<t>
658   The <x:ref>Age</x:ref> header field is used to convey an estimated
659   age of the response message when obtained from a cache. The Age field value
660   is the cache's estimate of the number of seconds since the response was
661   generated or validated by the origin server. In essence, the Age value is
662   the sum of the time that the response has been resident in each of the
663   caches along the path from the origin server, plus the amount of time it
664   has been in transit along network paths.
665</t>
666<t>
667   The following data is used for the age calculation:
668</t>
669<t>
670   <x:dfn>age_value</x:dfn>
671   <list>
672      <t>
673         The term "age_value" denotes the value of the <x:ref>Age</x:ref>
674         header field (<xref target="header.age"/>), in a form appropriate for
675         arithmetic operation; or 0, if not available.
676      </t>
677   </list>
678</t>
679<t>
680   <x:dfn>date_value</x:dfn>
681   <list>
682      <t>
683         The term "date_value" denotes the value of
684         the Date header field, in a form appropriate for arithmetic
685         operations. See &header-date; for the definition of the Date header
686         field, and for requirements regarding responses without it.
687      </t>
688   </list>
689</t>
690<t>
691   <x:dfn>now</x:dfn>
692   <list>
693      <t>
694         The term "now" means "the current value of the clock at the host
695         performing the calculation". A host ought to use NTP (<xref
696         target="RFC1305"/>) or some similar protocol to synchronize its
697         clocks to Coordinated Universal Time.
698      </t>
699   </list>
700</t>
701<t>
702   <x:dfn>request_time</x:dfn>
703   <list>
704      <t>
705         The current value of the clock at the host at the time the request
706         resulting in the stored response was made.
707      </t>
708   </list>
709</t>
710<t>
711   <x:dfn>response_time</x:dfn>
712   <list>
713      <t>
714         The current value of the clock at the host at the time the response
715         was received.
716      </t>
717   </list>
718</t>
719<t>
720   A response's age can be calculated in two entirely independent ways:
721   <list style="numbers">
722      <t>the "apparent_age": response_time minus date_value, if the local
723      clock is reasonably well synchronized to the origin server's clock. If
724      the result is negative, the result is replaced by zero.</t>
725      <t>the "corrected_age_value", if all of the caches along the response
726      path implement HTTP/1.1. A cache &MUST; interpret this value relative
727      to the time the request was initiated, not the time that the response
728      was received.</t>
729   </list>
730</t>
731<figure>
732<artwork type="code">
733  apparent_age = max(0, response_time - date_value);
734
735  response_delay = response_time - request_time;
736  corrected_age_value = age_value + response_delay; 
737</artwork>
738</figure>
739<figure>
740<preamble>These &SHOULD; be combined as</preamble>
741<artwork type="code">
742  corrected_initial_age = max(apparent_age, corrected_age_value);
743</artwork></figure>
744<t>
745   unless the cache is confident in the value of the <x:ref>Age</x:ref> header
746   field (e.g., because there are no HTTP/1.0 hops in the <x:ref>Via</x:ref>
747   header field), in which case the corrected_age_value &MAY; be used as the
748   corrected_initial_age.</t>
749<t>
750   The current_age of a stored response can then be calculated by adding the
751   amount of time (in seconds) since the stored response was last validated by
752   the origin server to the corrected_initial_age.
753</t>
754<figure><artwork type="code">
755  resident_time = now - response_time;
756  current_age = corrected_initial_age + resident_time;
757</artwork></figure>
758<t>
759   Additionally, to avoid common problems in date parsing:
760</t>
761<t>
762  <list style="symbols">
763     <t>Although all date formats are specified to be case-sensitive,
764        cache recipients &SHOULD; match day, week and timezone names
765        case-insensitively.</t>
766             
767     <t>If a cache recipient's internal implementation of time has less
768        resolution than the value of an HTTP-date, the recipient &MUST;
769        internally represent a parsed <x:ref>Expires</x:ref> date as the
770        nearest time equal to or earlier than the received value.</t>
771
772     <t>Cache recipients &MUST-NOT; allow local time zones to influence the
773        calculation or comparison of an age or expiration time.</t>
774
775     <t>Cache recipients &SHOULD; consider a date with a zone abbreviation
776        other than "GMT" to be invalid for calculating expiration.</t>
777  </list>
778</t>
779</section>
780
781<section anchor="serving.stale.responses" title="Serving Stale Responses">
782<t>
783   A "stale" response is one that either has explicit expiry information or is
784   allowed to have heuristic expiry calculated, but is not fresh according to
785   the calculations in <xref target="expiration.model" />.
786</t>
787<t>
788   A cache &MUST-NOT; send a stale response if it is prohibited by an
789   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
790   directive, a "must-revalidate" cache-response-directive, or an applicable
791   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
792   target="cache-response-directive"/>).
793</t>
794<t>
795   A cache &MUST-NOT; send stale responses unless it is disconnected
796   (i.e., it cannot contact the origin server or otherwise find a forward
797   path) or doing so is explicitly allowed (e.g., by the max-stale request
798   directive; see <xref target="cache-request-directive" />).
799</t>
800<t>
801   A cache &SHOULD; append a <x:ref>Warning</x:ref> header field with the 110
802   warn-code (see <xref target="header.warning"/>) to stale responses.
803   Likewise, a cache &SHOULD; add the 112 warn-code to stale responses if the
804   cache is disconnected.
805</t>
806<t>
807   If a cache receives a first-hand response (either an entire response, or a
808   <x:ref>304 (Not Modified)</x:ref> response) that it would normally forward
809   to the requesting client, and the received response is no longer fresh, the
810   cache can forward it to the requesting client without adding a new
811   <x:ref>Warning</x:ref> (but without removing any existing Warning header
812   fields). A cache shouldn't attempt to validate a response simply because
813   that response became stale in transit.
814</t>
815</section>
816</section>
817
818<section anchor="validation.model" title="Validation Model">
819<t>
820   When a cache has one or more stored responses for a requested URI, but
821   cannot serve any of them (e.g., because they are not fresh, or one cannot
822   be selected; see <xref target="caching.negotiated.responses"/>), it can use
823   the conditional request mechanism &conditional; in the forwarded request to
824   give the origin server an opportunity to both select a valid stored
825   response to be used, and to update it. This process is known as
826   "validating" or "revalidating" the stored response.
827</t>
828<t>
829   When sending such a conditional request, a cache adds an
830   <x:ref>If-Modified-Since</x:ref> header field whose value is that of the
831   <x:ref>Last-Modified</x:ref> header field from the selected
832   (see <xref target="caching.negotiated.responses"/>) stored response, if
833   available.
834</t>
835<t>
836   Additionally, a cache can add an <x:ref>If-None-Match</x:ref> header field
837   whose value is that of the <x:ref>ETag</x:ref> header field(s) from all
838   responses stored for the requested URI, if present. However, if any of the
839   stored responses contains only partial content, the cache shouldn't
840   include its entity-tag in the If-None-Match header field unless the request
841   is for a range that would be fully satisfied by that stored response.
842</t>
843
844<t>Cache handling of a response to a conditional request is dependent upon its
845status code:</t>
846
847<t>
848   <list style="symbols">
849      <t>
850         A <x:ref>304 (Not Modified)</x:ref> response status code indicates
851         that the stored response can be updated and reused; see <xref
852         target="freshening.responses"/>.
853      </t>
854      <t>
855         A full response (i.e., one with a payload body) indicates that none
856         of the stored responses nominated in the conditional request is
857         suitable. Instead, the cache can use the full response to
858         satisfy the request and &MAY; replace the stored response(s).
859      </t>
860      <t>
861         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
862         response while attempting to validate a response, it can either
863         forward this response to the requesting client, or act as if the
864         server failed to respond. In the latter case, it can send a
865         previously stored response (see <xref
866         target="serving.stale.responses" />).
867      </t>
868   </list>
869</t>
870
871<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
872<t>
873   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and
874   already has one or more stored <x:ref>200 (OK)</x:ref> responses for the
875   same cache key, the cache needs to identify which of the stored responses
876   are updated by this new response and then update the stored response(s)
877   with the new information provided in the <x:ref>304</x:ref> response.
878   <list style="symbols">
879    <t>
880     If the new response contains a strong validator, then that strong
881     validator identifies the selected representation.  All of the stored
882     responses with the same strong validator are selected.
883     If none of the stored responses contain the same strong validator,
884     then the new response &MUST-NOT; be used to update any stored responses.
885    </t>
886    <t>
887     If the new response contains a weak validator and that validator
888     corresponds to one of the cache's stored responses, then the most
889     recent of those matching stored responses is selected.
890    </t>
891    <t>
892     If the new response does not include any form of validator, there is
893     only one stored response, and that stored response also lacks a
894     validator, then that stored response is selected.
895    </t>
896   </list>
897</t>
898<t>
899   If a stored response is selected for update, the cache &MUST;:
900   <list style="symbols">
901      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
902         with warn-code 1xx (see <xref target="header.warning" />);</t>
903      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
904         with warn-code 2xx; and,</t>
905      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
906         response to replace all instances of the corresponding header
907         fields in the stored response.</t>
908   </list>
909</t>
910</section>
911
912</section>
913
914<section anchor="caching.negotiated.responses" 
915   title="Using Negotiated Responses">
916<t>
917   When a cache receives a request that can be satisfied by a stored response
918   that has a <x:ref>Vary</x:ref> header field (&header-vary;),
919   it &MUST-NOT; use that response unless all of the selecting header fields
920   nominated by the Vary header field match in both the original request
921   (i.e., that associated with the stored response), and the presented
922   request.
923</t>
924<t>
925   The selecting header fields from two requests are defined to match if and
926   only if those in the first request can be transformed to those in the
927   second request by applying any of the following:
928   <list style="symbols">
929      <t>
930         adding or removing whitespace, where allowed in the header field's
931         syntax
932      </t>
933      <t>
934         combining multiple header fields with the same field name
935         (see &header-fields;)
936      </t>
937      <t>
938         normalizing both header field values in a way that is known to have
939         identical semantics, according to the header field's specification
940         (e.g., re-ordering field values when order is not significant;
941         case-normalization, where values are defined to be case-insensitive)
942      </t>
943  </list>
944</t>
945<t>
946   If (after any normalization that might take place) a header field is absent
947   from a request, it can only match another request if it is also absent
948   there.
949</t>
950<t>
951   A <x:ref>Vary</x:ref> header field-value of "*" always fails to match, and
952   subsequent requests to that resource can only be properly interpreted by the
953   origin server.
954</t>
955<t>
956   The stored response with matching selecting header fields is known as the
957   selected response.
958</t>
959<t>
960   If multiple selected responses are available, the most recent response
961   (as determined by the <x:ref>Date</x:ref> header field) is used; see <xref 
962   target="constructing.responses.from.caches"/>.
963</t>
964<t>
965   If no selected response is available, the cache cannot satisfy the
966   presented request. Typically, it is forwarded to the origin server
967   in a (possibly conditional; see <xref target="validation.model"/>) request.
968</t>
969</section>
970
971
972<section anchor="combining.responses" title="Combining Partial Content">
973<t>
974   A response might transfer only a partial representation if the
975   connection closed prematurely or if the request used one or more Range
976   specifiers (&partial;).  After several such transfers, a cache might have
977   received several ranges of the same representation.  A cache &MAY; combine
978   these ranges into a single stored response, and reuse that response to
979   satisfy later requests, if they all share the same strong validator and
980   the cache complies with the client requirements in &combining-byte-ranges;.
981</t>
982<t>
983   When combining the new response with one or more stored responses, a
984   cache &MUST;:
985   <list style="symbols">
986      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
987         with warn-code 1xx (see <xref target="header.warning" />);</t>
988      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
989         with warn-code 2xx; and,</t>
990      <t>use other header fields provided in the new response, aside
991         from <x:ref>Content-Range</x:ref>, to replace all instances of the
992         corresponding header fields in the stored response.</t>
993   </list>
994</t>
995</section>
996</section>
997
998
999<section anchor="head.effects" title="Updating Caches with HEAD Responses">
1000<t>
1001   A response to the HEAD method is identical to what an equivalent request
1002   made with a GET would have been, except it lacks a body. This property
1003   of HEAD responses is used to both invalidate and update cached GET
1004   responses.
1005</t>
1006<t>
1007   If one or more stored GET responses can be selected (as per <xref
1008   target="caching.negotiated.responses"/>) for a HEAD request, and the
1009   <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> or
1010   <x:ref>Last-Modified</x:ref> value of a HEAD response differs from that in a
1011   selected GET response, the cache &MUST; consider that selected response to
1012   be stale.
1013</t>
1014<t>
1015   If the <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> and
1016   <x:ref>Last-Modified</x:ref> values of a HEAD response (when present) are
1017   the same as that in a selected GET response (as per
1018   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update
1019   the remaining header fields in the stored response using the following
1020   rules:
1021   <list style="symbols">
1022      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1023         with warn-code 1xx (see <xref target="header.warning" />);</t>
1024      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1025         with warn-code 2xx; and,</t>
1026      <t>use other header fields provided in the response to replace
1027         all instances of the corresponding header fields in the stored
1028         response.</t>
1029   </list>
1030</t>
1031
1032</section>
1033
1034
1035<section anchor="invalidation.after.updates.or.deletions" 
1036   title="Request Methods that Invalidate">
1037<t>
1038   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1039   have the potential for changing state on the origin server, intervening
1040   caches can use them to keep their contents up-to-date.
1041</t>
1042<t>
1043   A cache &MUST; invalidate the effective Request URI
1044   (&effective-request-uri;) as well as the URI(s) in the
1045   <x:ref>Location</x:ref> and <x:ref>Content-Location</x:ref> response header
1046   fields (if present) when a non-error response to a request with an unsafe
1047   method is received.
1048</t>
1049<t>
1050   However, a cache &MUST-NOT; invalidate a URI from a <x:ref>Location</x:ref>
1051   or <x:ref>Content-Location</x:ref> response header field if the host part of
1052   that URI differs from the host part in the effective request URI
1053   (&effective-request-uri;). This helps prevent denial of service attacks.
1054</t>
1055<t>
1056   A cache &MUST; invalidate the effective request URI
1057   (&effective-request-uri;) when it receives a non-error response
1058   to a request with a method whose safety is unknown.
1059</t>
1060<t>
1061   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref>
1062   or <x:ref>3xx (Redirection)</x:ref> status code. "Invalidate" means that
1063   the cache will either remove all stored responses related to the effective
1064   request URI, or will mark these as "invalid" and in need of a mandatory
1065   validation before they can be sent in response to a subsequent request.
1066</t>
1067<t>
1068   Note that this does not guarantee that all appropriate responses are
1069   invalidated. For example, the request that caused the change at the origin
1070   server might not have gone through the cache where a response is stored.
1071</t>
1072</section>
1073
1074
1075
1076
1077<section anchor="header.field.definitions" title="Header Field Definitions">
1078<t>
1079   This section defines the syntax and semantics of HTTP/1.1 header fields
1080   related to caching.
1081</t>
1082
1083<section anchor="header.age" title="Age">
1084   <iref item="Age header field" primary="true" x:for-anchor="" />
1085   <x:anchor-alias value="Age"/>
1086   <x:anchor-alias value="age-value"/>
1087<t>
1088   The "Age" header field conveys the sender's estimate of the amount
1089   of time since the response was generated or successfully validated at the
1090   origin server. Age values are calculated as specified in <xref
1091   target="age.calculations" />.
1092</t>
1093<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1094  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1095</artwork></figure>
1096<t>
1097  Age field-values are non-negative integers, representing time in seconds
1098  (see <xref target="delta-seconds"/>).
1099</t>
1100<t>
1101   The presence of an Age header field in a response implies that a response
1102   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1103   might not implement the Age header field.
1104</t>
1105</section>
1106
1107<section anchor="header.cache-control" title="Cache-Control">
1108   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1109   <x:anchor-alias value="Cache-Control"/>
1110   <x:anchor-alias value="cache-directive"/>
1111<t>
1112   The "Cache-Control" header field is used to specify directives for
1113   caches along the request/response chain. Such cache directives are
1114   unidirectional in that the presence of a directive in a request does not
1115   imply that the same directive is to be given in the response.
1116</t>
1117<t>
1118   A cache &MUST; obey the requirements of the Cache-Control
1119   directives defined in this section. See <xref
1120   target="cache.control.extensions"/> for information about how Cache-Control
1121   directives defined elsewhere are handled.
1122</t>
1123<x:note>
1124   <t>
1125       &Note; HTTP/1.0 caches might not implement Cache-Control and
1126       might only implement Pragma: no-cache (see <xref target="header.pragma"
1127       />).
1128   </t>
1129</x:note>
1130<t>
1131   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1132   through in forwarded messages, regardless of their
1133   significance to that application, since the directives might be applicable
1134   to all recipients along the request/response chain. It is not possible to
1135   target a directive to a specific cache.
1136</t>
1137<t>
1138   Cache directives are identified by a token, to be compared case-insensitively,
1139   and have an optional argument, that can use both token and quoted-string
1140   syntax. For the directives defined below that define arguments, recipients
1141   ought to accept both forms, even if one is documented to be preferred. For
1142   any directive not defined by this specification, recipients &MUST; accept
1143   both forms.
1144</t>
1145<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1146  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1147
1148  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1149</artwork></figure>
1150<t>
1151   For the cache directives defined below, no argument is defined (nor allowed)
1152   otherwise stated otherwise.
1153</t>
1154
1155<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1156
1157<section title="no-cache" anchor="cache-request-directive.no-cache">
1158   <iref item="no-cache (cache directive)" primary="true" />
1159<t>
1160   The "no-cache" request directive indicates that a cache &MUST-NOT; 
1161   use a stored response to satisfy the request without successful
1162   validation on the origin server.
1163</t>
1164</section>
1165 
1166<section title="no-store" anchor="cache-request-directive.no-store">
1167   <iref item="no-store (cache directive)" primary="true" />
1168<t>
1169   The "no-store" request directive indicates that a cache &MUST-NOT;
1170   store any part of either this request or any response to it. This
1171   directive applies to both private and shared caches. "&MUST-NOT;
1172   store" in this context means that the cache &MUST-NOT; intentionally
1173   store the information in non-volatile storage, and &MUST; make a
1174   best-effort attempt to remove the information from volatile storage as
1175   promptly as possible after forwarding it.
1176</t>
1177<t>
1178   This directive is NOT a reliable or sufficient mechanism for ensuring
1179   privacy. In particular, malicious or compromised caches might not
1180   recognize or obey this directive, and communications networks might be
1181   vulnerable to eavesdropping.
1182</t>
1183<t>
1184   Note that if a request containing this directive is satisfied from a
1185   cache, the no-store request directive does not apply to the already
1186   stored response.
1187</t>
1188</section>
1189
1190<section title="max-age" anchor="cache-request-directive.max-age">
1191   <iref item="max-age (cache directive)" primary="true" />
1192<t>
1193   Argument syntax:
1194   <list>
1195      <t>
1196        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1197      </t>
1198   </list>
1199</t>
1200<t>
1201   The "max-age" request directive indicates that the client is unwilling to
1202   accept a response whose age is greater than the specified number of
1203   seconds. Unless the max-stale request directive is also present, the
1204   client is not willing to accept a stale response.
1205</t>
1206<t>
1207   &Note; This directive uses the token form of the argument syntax;
1208   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1209   quoted-string form.
1210</t>
1211</section>
1212
1213<section title="max-stale" anchor="cache-request-directive.max-stale">
1214   <iref item="max-stale (cache directive)" primary="true" />
1215<t>
1216   Argument syntax:
1217   <list>
1218      <t>
1219        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1220      </t>
1221   </list>
1222</t>
1223<t>
1224   The "max-stale" request directive indicates that the client is willing
1225   to accept a response that has exceeded its expiration time. If max-stale
1226   is assigned a value, then the client is willing to accept a response
1227   that has exceeded its expiration time by no more than the specified
1228   number of seconds. If no value is assigned to max-stale, then the client
1229   is willing to accept a stale response of any age.
1230</t>
1231<t>
1232   &Note; This directive uses the token form of the argument syntax;
1233   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1234   quoted-string form.
1235</t>
1236</section>
1237
1238<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1239   <iref item="min-fresh (cache directive)" primary="true" />
1240<t>
1241   Argument syntax:
1242   <list>
1243      <t>
1244        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1245      </t>
1246   </list>
1247</t>
1248<t>
1249   The "min-fresh" request directive indicates that the client is willing
1250   to accept a response whose freshness lifetime is no less than its
1251   current age plus the specified time in seconds. That is, the client
1252   wants a response that will still be fresh for at least the specified
1253   number of seconds.
1254</t>
1255<t>
1256   &Note; This directive uses the token form of the argument syntax;
1257   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1258   quoted-string form.
1259</t>
1260</section>
1261
1262<section title="no-transform" anchor="cache-request-directive.no-transform">
1263   <iref item="no-transform (cache directive)" primary="true" />
1264<t>
1265   The "no-transform" request directive indicates that an intermediary
1266   (whether or not it implements a cache) &MUST-NOT; transform the payload,
1267   as defined in &transformations;.
1268</t>
1269</section>
1270
1271<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1272   <iref item="only-if-cached (cache directive)" primary="true" />
1273<t>
1274   The "only-if-cached" request directive indicates that the client only wishes
1275   to obtain a stored response. If it receives this directive, a cache &SHOULD;
1276   either respond using a stored response that is consistent with the other
1277   constraints of the request, or respond with a <x:ref>504 (Gateway
1278   Timeout)</x:ref> status code. If a group of caches is being operated as a
1279   unified system with good internal connectivity, a member cache &MAY;
1280   forward such a request within that group of caches.
1281</t>
1282</section>
1283</section>
1284
1285<section anchor="cache-response-directive" 
1286   title="Response Cache-Control Directives">
1287   <x:anchor-alias value="cache-response-directive" />
1288
1289<section title="public" anchor="cache-response-directive.public">
1290   <iref item="public (cache directive)" primary="true" />
1291<t>
1292   The "public" response directive indicates that a response whose
1293   associated request contains an 'Authentication' header &MAY; be
1294   stored (see <xref target="caching.authenticated.responses" />).
1295</t>
1296</section>
1297
1298<section title="private" anchor="cache-response-directive.private">
1299   <iref item="private (cache directive)" primary="true" />
1300<t>
1301   Argument syntax:
1302   <list>
1303      <t>
1304        #<x:ref>field-name</x:ref>
1305      </t>
1306   </list>
1307</t>
1308<t>
1309   The "private" response directive indicates that the response message is
1310   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1311   private cache &MAY; store the response.
1312</t>
1313<t>
1314   If the private response directive specifies one or more field-names,
1315   this requirement is limited to the field-values associated with the
1316   listed response header fields. That is, a shared cache &MUST-NOT; store
1317   the specified field-names(s), whereas it &MAY; store the remainder of the
1318   response message.
1319</t>
1320<t>
1321   The field-names given are not limited to the set of standard header
1322   fields defined by this specification. Field names are case-insensitive.
1323</t>
1324<t>
1325   &Note; This usage of the word "private" only controls
1326   where the response can be stored; it cannot ensure the privacy of the
1327   message content. Also, private response directives with field-names are
1328   often handled by implementations as if an unqualified private directive
1329   was received; i.e., the special handling for the qualified form is not
1330   widely implemented.
1331</t>
1332<t>
1333   &Note; This directive uses the quoted-string form of the argument syntax.
1334   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1335   needed for single-entry lists).
1336</t>
1337</section>
1338
1339<section title="no-cache" anchor="cache-response-directive.no-cache">
1340   <iref item="no-cache (cache directive)" primary="true" />
1341<t>
1342   Argument syntax:
1343   <list>
1344      <t>
1345        #<x:ref>field-name</x:ref>
1346      </t>
1347   </list>
1348</t>
1349<t>
1350   The "no-cache" response directive indicates that the response &MUST-NOT;
1351   be used to satisfy a subsequent request without successful validation on
1352   the origin server. This allows an origin server to prevent a cache from
1353   using it to satisfy a request without contacting it, even by caches that
1354   have been configured to send stale responses.
1355</t>
1356<t>
1357   If the no-cache response directive specifies one or more field-names,
1358   then a cache &MAY; use the response to satisfy a subsequent request,
1359   subject to any other restrictions on caching. However, any header fields
1360   in the response that have the field-name(s) listed &MUST-NOT; be sent
1361   in the response to a subsequent request without successful revalidation
1362   with the origin server. This allows an origin server to prevent the
1363   re-use of certain header fields in a response, while still allowing
1364   caching of the rest of the response.
1365</t> 
1366<t>
1367   The field-names given are not limited to the set of standard header
1368   fields defined by this specification. Field names are case-insensitive.
1369</t>
1370<t>
1371   &Note; Many HTTP/1.0 caches will not recognize or obey
1372   this directive. Also, no-cache response directives with field-names are
1373   often handled by implementations as if an unqualified no-cache directive
1374   was received; i.e., the special handling for the qualified form is not
1375   widely implemented.
1376</t>
1377<t>
1378   &Note; This directive uses the quoted-string form of the argument syntax.
1379   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1380   needed for single-entry lists).
1381</t>
1382</section>
1383
1384<section title="no-store" anchor="cache-response-directive.no-store">
1385   <iref item="no-store (cache directive)" primary="true" />
1386<t>
1387   The "no-store" response directive indicates that a cache &MUST-NOT;
1388   store any part of either the immediate request or response. This
1389   directive applies to both private and shared caches. "&MUST-NOT;
1390   store" in this context means that the cache &MUST-NOT; intentionally
1391   store the information in non-volatile storage, and &MUST; make a
1392   best-effort attempt to remove the information from volatile storage as
1393   promptly as possible after forwarding it.
1394</t>
1395<t>
1396   This directive is NOT a reliable or sufficient mechanism for ensuring
1397   privacy. In particular, malicious or compromised caches might not
1398   recognize or obey this directive, and communications networks might be
1399   vulnerable to eavesdropping.
1400</t>
1401</section>
1402
1403<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1404   <iref item="must-revalidate (cache directive)" primary="true" />
1405<t>
1406   The "must-revalidate" response directive indicates that once it has
1407   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1408   requests without successful validation on the origin server.
1409</t>
1410<t>
1411   The must-revalidate directive is necessary to support reliable
1412   operation for certain protocol features. In all circumstances a
1413   cache &MUST; obey the must-revalidate directive; in particular,
1414   if a cache cannot reach the origin server for any reason, it &MUST;
1415   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1416</t>
1417<t>
1418   The must-revalidate directive ought to be used by servers if and only
1419   if failure to validate a request on the representation could result in
1420   incorrect operation, such as a silently unexecuted financial
1421   transaction.
1422</t>
1423</section>
1424
1425<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1426   <iref item="proxy-revalidate (cache directive)" primary="true" />
1427<t>
1428   The "proxy-revalidate" response directive has the same meaning as the
1429   must-revalidate response directive, except that it does not apply to
1430   private caches.
1431</t>
1432</section>
1433
1434<section title="max-age" anchor="cache-response-directive.max-age">
1435   <iref item="max-age (cache directive)" primary="true" />
1436<t>
1437   Argument syntax:
1438   <list>
1439      <t>
1440        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1441      </t>
1442   </list>
1443</t>
1444<t>
1445   The "max-age" response directive indicates that the response is to be
1446   considered stale after its age is greater than the specified number of
1447   seconds.
1448</t>
1449<t>
1450   &Note; This directive uses the token form of the argument syntax;
1451   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1452   quoted-string form.
1453</t>
1454</section>     
1455
1456<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1457   <iref item="s-maxage (cache directive)" primary="true" />
1458<t>
1459   Argument syntax:
1460   <list>
1461      <t>
1462        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1463      </t>
1464   </list>
1465</t>
1466<t>
1467   The "s-maxage" response directive indicates that, in shared caches, the
1468   maximum age specified by this directive overrides the maximum age
1469   specified by either the max-age directive or the <x:ref>Expires</x:ref>
1470   header field. The s-maxage directive also implies the semantics of the
1471   proxy-revalidate response directive.
1472</t>
1473<t>
1474   &Note; This directive uses the token form of the argument syntax;
1475   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1476   quoted-string form.
1477</t>
1478</section>
1479
1480<section title="no-transform" anchor="cache-response-directive.no-transform">
1481   <iref item="no-transform (cache directive)" primary="true" />
1482<t>
1483   The "no-transform" response directive indicates that an intermediary
1484   (regardless of whether it implements a cache) &MUST-NOT; transform the
1485   payload, as defined in &transformations;.
1486</t>
1487</section>
1488
1489</section>
1490
1491<section anchor="cache.control.extensions" title="Cache Control Extensions">
1492<t>
1493   The Cache-Control header field can be extended through the use of one or
1494   more cache-extension tokens, each with an optional value. Informational
1495   extensions (those that do not require a change in cache behavior) can be
1496   added without changing the semantics of other directives. Behavioral
1497   extensions are designed to work by acting as modifiers to the existing base
1498   of cache directives. Both the new directive and the standard directive are
1499   supplied, such that applications that do not understand the new directive
1500   will default to the behavior specified by the standard directive, and those
1501   that understand the new directive will recognize it as modifying the
1502   requirements associated with the standard directive. In this way,
1503   extensions to the cache-control directives can be made without requiring
1504   changes to the base protocol.
1505</t>
1506<t>
1507   This extension mechanism depends on an HTTP cache obeying all of the
1508   cache-control directives defined for its native HTTP-version, obeying
1509   certain extensions, and ignoring all directives that it does not
1510   understand.
1511</t>
1512<t>
1513   For example, consider a hypothetical new response directive called
1514   "community" that acts as a modifier to the private directive. We define
1515   this new directive to mean that, in addition to any private cache, any
1516   cache that is shared only by members of the community named within its
1517   value is allowed to cache the response. An origin server wishing to allow
1518   the UCI community to use an otherwise private response in their shared
1519   cache(s) could do so by including
1520</t>
1521<figure><artwork type="example">
1522  Cache-Control: private, community="UCI"
1523</artwork></figure>
1524<t>
1525   A cache seeing this header field will act correctly even if the cache does
1526   not understand the community cache-extension, since it will also see and
1527   understand the private directive and thus default to the safe behavior.
1528</t>
1529<t>
1530   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1531   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1532   combined with standard directives (or the response's default cacheability)
1533   such that the cache behavior will remain minimally correct even if the
1534   cache does not understand the extension(s).
1535</t>
1536<t>
1537   New extension directives ought to consider defining:
1538</t>
1539<t>
1540   <list style="symbols">
1541      <t>What it means for a directive to be specified multiple times,</t>
1542      <t>When the directive does not take an argument, what it means when an
1543      argument is present,</t>
1544      <t>When the directive requires an argument, what it means when it is
1545      missing.</t>
1546   </list>
1547</t>
1548<t>
1549   The HTTP Cache Directive Registry defines the name space for the cache
1550   directives.
1551</t>
1552<t>
1553   A registration &MUST; include the following fields:
1554   <list style="symbols">
1555      <t>Cache Directive Name</t>
1556      <t>Pointer to specification text</t>
1557   </list>
1558</t>
1559<t>
1560   Values to be added to this name space require IETF Review (see <xref
1561   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1562</t>
1563<t>
1564   The registry itself is maintained at <eref
1565   target="http://www.iana.org/assignments/http-cache-directives"/>.
1566</t>
1567</section>
1568
1569</section>
1570
1571<section anchor="header.expires" title="Expires">
1572   <iref item="Expires header field" primary="true" x:for-anchor="" />
1573   <x:anchor-alias value="Expires"/>
1574<t>
1575   The "Expires" header field gives the date/time after which the
1576   response is considered stale. See <xref target="expiration.model" /> for
1577   further discussion of the freshness model.
1578</t>
1579<t>
1580   The presence of an Expires field does not imply that the original resource
1581   will change or cease to exist at, before, or after that time.
1582</t>
1583<t>
1584   The Expires value is an HTTP-date timestamp, as defined in &http-date;.
1585</t>
1586<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1587  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1588</artwork></figure>
1589<figure>
1590  <preamble>For example</preamble>
1591<artwork type="example">
1592  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1593</artwork></figure>
1594<t>
1595   A cache recipient &MUST; interpret invalid date formats, especially the
1596   value "0", as representing a time in the past (i.e., "already expired").
1597</t>
1598<t>
1599   If a response includes a <x:ref>Cache-Control</x:ref> field with
1600   the max-age directive (<xref target="cache-response-directive.max-age"/>),
1601   a recipient &MUST; ignore the Expires field.
1602   Likewise, if a response includes the s-maxage directive
1603   (<xref target="cache-response-directive.s-maxage" />), a shared cache
1604   recipient &MUST; ignore the Expires field. In both these cases, the value
1605   in Expires is only intended for recipients that have not yet implemented
1606   the Cache-Control field.
1607</t>
1608<t>
1609   An origin server without a clock &MUST-NOT; generate an Expires field
1610   unless its value represents a fixed time in the past (always expired)
1611   or its value has been associated with the resource by a system or user
1612   with a reliable clock.
1613</t>
1614<t>
1615   Historically, HTTP required the Expires field-value to be no more than a
1616   year in the future. While longer freshness lifetimes are no longer
1617   prohibited, extremely large values have been demonstrated to cause
1618   problems (e.g., clock overflows due to use of 32-bit integers for
1619   time values), and many caches will evict a response far sooner than
1620   that.
1621</t>
1622</section>
1623
1624<section anchor="header.pragma" title="Pragma">
1625   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1626   <x:anchor-alias value="extension-pragma"/>
1627   <x:anchor-alias value="Pragma"/>
1628   <x:anchor-alias value="pragma-directive"/>
1629<t>
1630   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1631   caches, so that clients can specify a "no-cache" request that they will
1632   understand (as <x:ref>Cache-Control</x:ref> was not defined until HTTP/1.1).
1633   When the Cache-Control header field is also present and understood in a
1634   request, Pragma is ignored.
1635</t>
1636<t>
1637   In HTTP/1.0, Pragma was defined as an extensible field for
1638   implementation-specified directives for recipients. This specification
1639   deprecates such extensions to improve interoperability.
1640</t>
1641<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"/>
1642  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1643  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1644  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1645</artwork></figure>
1646<t>
1647   When the <x:ref>Cache-Control</x:ref> header field is not present in a
1648   request, the no-cache request pragma-directive &MUST; have the same effect
1649   on caches as if "Cache-Control: no-cache" were present (see <xref
1650   target="cache-request-directive" />).
1651</t>
1652<t>
1653   When sending a no-cache request, a client ought to include both the pragma
1654   and cache-control directives, unless Cache-Control: no-cache is
1655   purposefully omitted to target other <x:ref>Cache-Control</x:ref> response
1656   directives at HTTP/1.1 caches. For example:
1657</t>
1658<figure>
1659<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1660GET / HTTP/1.1
1661Host: www.example.com
1662Cache-Control: max-age=30
1663Pragma: no-cache
1664
1665</artwork>
1666</figure>
1667<t>
1668   will constrain HTTP/1.1 caches to serve a response no older than 30
1669   seconds, while precluding implementations that do not understand
1670   <x:ref>Cache-Control</x:ref> from serving a cached response.
1671</t>
1672<x:note>
1673   <t>
1674      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1675      specified, it does not provide a reliable replacement for
1676      "Cache-Control: no-cache" in them.
1677   </t>
1678</x:note>
1679</section>
1680
1681<section anchor="header.warning" title="Warning">
1682   <iref item="Warning header field" primary="true" x:for-anchor="" />
1683   <x:anchor-alias value="Warning"/>
1684   <x:anchor-alias value="warning-value"/>
1685   <x:anchor-alias value="warn-agent"/>
1686   <x:anchor-alias value="warn-code"/>
1687   <x:anchor-alias value="warn-date"/>
1688   <x:anchor-alias value="warn-text"/>
1689<t>
1690   The "Warning" header field is used to carry additional information
1691   about the status or transformation of a message that might not be reflected
1692   in the message. This information is typically used to warn about possible
1693   incorrectness introduced by caching operations or transformations applied
1694   to the payload of the message.
1695</t>
1696<t>
1697   Warnings can be used for other purposes, both cache-related and otherwise.
1698   The use of a warning, rather than an error status code, distinguishes these
1699   responses from true failures.
1700</t>
1701<t>
1702   Warning header fields can in general be applied to any message, however some
1703   warn-codes are specific to caches and can only be applied to response
1704   messages.
1705</t>
1706<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"/>
1707  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1708 
1709  <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>
1710                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1711 
1712  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1713  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1714                  ; the name or pseudonym of the server adding
1715                  ; the Warning header field, for use in debugging
1716  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1717  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1718</artwork></figure>
1719<t>
1720   Multiple warnings can be attached to a response (either by the origin
1721   server or by a cache), including multiple warnings with the same code
1722   number, only differing in warn-text.
1723</t>
1724<t>
1725   When this occurs, the user agent &SHOULD; inform the user of as many of
1726   them as possible, in the order that they appear in the response.
1727</t>
1728<t>
1729   Systems that generate multiple Warning header fields are encouraged to
1730   order them with this user agent behavior in mind. New Warning header fields
1731   are added after any existing Warning header fields.
1732</t>
1733<t>
1734   Warnings are assigned three digit warn-codes. The first digit indicates
1735   whether the Warning is required to be deleted from a stored response after
1736   validation:
1737   <list style="symbols">
1738      <t>1xx Warnings describe the freshness or validation status of the
1739      response, and so &MUST; be deleted by a cache after validation. They can
1740      only be generated by a cache when validating a cached entry, and
1741      &MUST-NOT; be generated in any other situation.</t>
1742      <t>2xx Warnings describe some aspect of the representation that is not
1743      rectified by a validation (for example, a lossy compression of the
1744      representation) and &MUST-NOT; be deleted by a cache after validation,
1745      unless a full response is sent, in which case they &MUST; be.</t>
1746   </list>
1747</t>
1748<t>
1749   If an implementation sends a message with one or more Warning header fields
1750   to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST;
1751   include in each warning-value a warn-date that matches the
1752   <x:ref>Date</x:ref> header field in the message.
1753</t>
1754<t>
1755   If a system receives a message with a warning-value that includes a
1756   warn-date, and that warn-date is different from the <x:ref>Date</x:ref>
1757   value in the response, then that warning-value &MUST; be deleted from the
1758   message before storing, forwarding, or using it. (preventing the
1759   consequences of naive caching of Warning header fields.) If all of the
1760   warning-values are deleted for this reason, the Warning header field &MUST;
1761   be deleted as well.
1762</t>
1763<t>
1764   The following warn-codes are defined by this specification, each with a
1765   recommended warn-text in English, and a description of its meaning.
1766</t>
1767
1768<section title="110 Response is Stale" anchor="warn.110">
1769  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1770<t>
1771   A cache &SHOULD; generate this whenever the sent response is stale.
1772</t>
1773</section>
1774
1775<section title="111 Revalidation Failed" anchor="warn.111">
1776  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1777<t>
1778   A cache &SHOULD; generate this when sending a stale response because an
1779   attempt to validate the response failed, due to an inability to reach
1780   the server.
1781</t>
1782</section>
1783
1784<section title="112 Disconnected Operation" anchor="warn.112">
1785  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1786<t>
1787   A cache &SHOULD; generate this if it is intentionally disconnected from
1788   the rest of the network for a period of time.
1789</t>
1790</section>
1791
1792<section title="113 Heuristic Expiration" anchor="warn.113">
1793  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1794<t>
1795   A cache &SHOULD; generate this if it heuristically chose a freshness
1796   lifetime greater than 24 hours and the response's age is greater than 24
1797   hours.
1798</t>
1799</section>
1800
1801<section title="199 Miscellaneous Warning" anchor="warn.199">
1802  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
1803<t>
1804   The warning text can include arbitrary information to be presented to
1805   a human user, or logged. A system receiving this warning &MUST-NOT; take
1806   any automated action, besides presenting the warning to the user.
1807</t>
1808</section>
1809
1810<section title="214 Transformation Applied" anchor="warn.214">
1811  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
1812<t>
1813   &MUST; be added by a proxy if it applies any transformation to the
1814   representation, such as changing the content-coding, media-type, or
1815   modifying the representation data, unless this Warning code already appears
1816   in the response.
1817</t>
1818</section>
1819
1820<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1821  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
1822<t>
1823   The warning text can include arbitrary information to be presented to
1824   a human user, or logged. A system receiving this warning &MUST-NOT; take
1825   any automated action.
1826</t>
1827</section>
1828
1829<section title="Warn Code Extensions" anchor="warn.code.extensions">
1830<t>
1831   The HTTP Warn Code Registry defines the name space for warn codes.
1832</t>
1833<t>
1834   A registration &MUST; include the following fields:
1835   <list style="symbols">
1836      <t>Warn Code (3 digits)</t>
1837      <t>Short Description</t>
1838      <t>Pointer to specification text</t>
1839   </list>
1840</t>
1841<t>
1842   Values to be added to this name space require IETF Review (see <xref
1843   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1844</t>
1845<t>
1846   The registry itself is maintained at <eref
1847   target="http://www.iana.org/assignments/http-warn-codes"/>.
1848</t>
1849</section>
1850</section>
1851</section>
1852
1853<section anchor="history.lists" title="History Lists">
1854<t>
1855   User agents often have history mechanisms, such as "Back" buttons and
1856   history lists, that can be used to redisplay a representation retrieved
1857   earlier in a session.
1858</t>
1859<t>
1860   The freshness model (<xref target="expiration.model"/>) does not
1861   necessarily apply to history mechanisms. I.e., a history mechanism can
1862   display a previous representation even if it has expired.
1863</t>
1864<t>
1865   This does not prohibit the history mechanism from telling the user that a
1866   view might be stale, or from honoring cache directives (e.g.,
1867   Cache-Control: no-store).
1868</t>
1869</section>
1870
1871
1872<section anchor="IANA.considerations" title="IANA Considerations">
1873
1874<section title="Cache Directive Registry" 
1875   anchor="cache.directive.registration">
1876<t>
1877   The registration procedure for HTTP Cache Directives is defined by <xref
1878   target="cache.control.extensions"/> of this document.
1879</t>
1880<t>
1881   The HTTP Cache Directive Registry shall be created at <eref
1882   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1883   populated with the registrations below:
1884</t>
1885<?BEGININC p6-cache.cache-directives ?>
1886<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1887<texttable xmlns:my="#my" align="left" suppress-title="true"
1888           anchor="iana.cache.directive.registration.table">
1889   <ttcol>Cache Directive</ttcol>
1890   <ttcol>Reference</ttcol>
1891
1892   <c>max-age</c>
1893   <c>
1894      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1895   </c>
1896   <c>max-stale</c>
1897   <c>
1898      <xref target="cache-request-directive.max-stale"/>
1899   </c>
1900   <c>min-fresh</c>
1901   <c>
1902      <xref target="cache-request-directive.min-fresh"/>
1903   </c>
1904   <c>must-revalidate</c>
1905   <c>
1906      <xref target="cache-response-directive.must-revalidate"/>
1907   </c>
1908   <c>no-cache</c>
1909   <c>
1910      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1911   </c>
1912   <c>no-store</c>
1913   <c>
1914      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1915   </c>
1916   <c>no-transform</c>
1917   <c>
1918      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1919   </c>
1920   <c>only-if-cached</c>
1921   <c>
1922      <xref target="cache-request-directive.only-if-cached"/>
1923   </c>
1924   <c>private</c>
1925   <c>
1926      <xref target="cache-response-directive.private"/>
1927   </c>
1928   <c>proxy-revalidate</c>
1929   <c>
1930      <xref target="cache-response-directive.proxy-revalidate"/>
1931   </c>
1932   <c>public</c>
1933   <c>
1934      <xref target="cache-response-directive.public"/>
1935   </c>
1936   <c>s-maxage</c>
1937   <c>
1938      <xref target="cache-response-directive.s-maxage"/>
1939   </c>
1940   <c>stale-if-error</c>
1941   <c>
1942      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1943   </c>
1944   <c>stale-while-revalidate</c>
1945   <c>
1946      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1947   </c>
1948</texttable>
1949<!--(END)-->
1950<?ENDINC p6-cache.cache-directives ?>
1951</section>
1952
1953<section title="Warn Code Registry" 
1954   anchor="warn.code.registration">
1955<t>
1956   The registration procedure for HTTP Warn Codes is defined by <xref
1957   target="warn.code.extensions"/> of this document.
1958</t>
1959<t>
1960   The HTTP Warn Code Registry shall be created at <eref
1961   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1962   populated with the registrations below:
1963</t>
1964<?BEGININC p6-cache.iana-warn-codes ?>
1965<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1966<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1967   <ttcol>Warn Code</ttcol>
1968   <ttcol>Short Description</ttcol>
1969   <ttcol>Reference</ttcol>
1970   <c>110</c>
1971   <c>Response is Stale</c>
1972   <c>
1973      <xref target="warn.110"/>
1974   </c>
1975   <c>111</c>
1976   <c>Revalidation Failed</c>
1977   <c>
1978      <xref target="warn.111"/>
1979   </c>
1980   <c>112</c>
1981   <c>Disconnected Operation</c>
1982   <c>
1983      <xref target="warn.112"/>
1984   </c>
1985   <c>113</c>
1986   <c>Heuristic Expiration</c>
1987   <c>
1988      <xref target="warn.113"/>
1989   </c>
1990   <c>199</c>
1991   <c>Miscellaneous Warning</c>
1992   <c>
1993      <xref target="warn.199"/>
1994   </c>
1995   <c>214</c>
1996   <c>Transformation Applied</c>
1997   <c>
1998      <xref target="warn.214"/>
1999   </c>
2000   <c>299</c>
2001   <c>Miscellaneous Persistent Warning</c>
2002   <c>
2003      <xref target="warn.299"/>
2004   </c>
2005</texttable>
2006<!--(END)-->
2007<?ENDINC p6-cache.iana-warn-codes ?>
2008</section>
2009
2010<section title="Header Field Registration" anchor="header.field.registration">
2011<t>
2012  The Message Header Field Registry located at <eref 
2013  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2014  shall be updated with the permanent registrations below (see <xref target="BCP90" />):
2015</t>
2016<?BEGININC p6-cache.iana-headers ?>
2017<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2018<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2019   <ttcol>Header Field Name</ttcol>
2020   <ttcol>Protocol</ttcol>
2021   <ttcol>Status</ttcol>
2022   <ttcol>Reference</ttcol>
2023
2024   <c>Age</c>
2025   <c>http</c>
2026   <c>standard</c>
2027   <c>
2028      <xref target="header.age"/>
2029   </c>
2030   <c>Cache-Control</c>
2031   <c>http</c>
2032   <c>standard</c>
2033   <c>
2034      <xref target="header.cache-control"/>
2035   </c>
2036   <c>Expires</c>
2037   <c>http</c>
2038   <c>standard</c>
2039   <c>
2040      <xref target="header.expires"/>
2041   </c>
2042   <c>Pragma</c>
2043   <c>http</c>
2044   <c>standard</c>
2045   <c>
2046      <xref target="header.pragma"/>
2047   </c>
2048   <c>Warning</c>
2049   <c>http</c>
2050   <c>standard</c>
2051   <c>
2052      <xref target="header.warning"/>
2053   </c>
2054</texttable>
2055<!--(END)-->
2056<?ENDINC p6-cache.iana-headers ?>
2057<t>
2058   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2059   Force".
2060</t>
2061</section>
2062</section>
2063
2064<section anchor="security.considerations" title="Security Considerations">
2065<t>
2066   This section is meant to inform developers, information providers, and
2067   users of known security concerns specific to HTTP/1.1 caching.
2068   More general security considerations are addressed in HTTP messaging
2069   &messaging; and semantics &semantics;.
2070</t>
2071<t>
2072   Caches expose additional potential vulnerabilities, since the contents of
2073   the cache represent an attractive target for malicious exploitation.
2074   Because cache contents persist after an HTTP request is complete, an attack
2075   on the cache can reveal information long after a user believes that the
2076   information has been removed from the network. Therefore, cache contents
2077   need to be protected as sensitive information.
2078</t>
2079<t>
2080   Furthermore, the very use of a cache can bring about privacy concerns. For
2081   example, if two users share a cache, and the first one browses to a site,
2082   the second may be able to detect that the other has been to that site,
2083   because the resources from it load more quickly, thanks to the cache.
2084</t>
2085<t>
2086   Implementation flaws might allow attackers to insert content into a cache
2087   ("cache poisoning"), leading to compromise of clients that trust that
2088   content. Because of their nature, these attacks are difficult to mitigate.
2089</t>
2090<t>
2091   Likewise, implementation flaws (as well as misunderstanding of cache
2092   operation) might lead to caching of sensitive information (e.g.,
2093   authentication credentials) that is thought to be private, exposing it to
2094   unauthorized parties.
2095</t>
2096<t>
2097   Note that the Set-Cookie response header <xref target="RFC6265"/> does not
2098   inhibit caching; a cacheable response with a Set-Cookie header can be (and
2099   often is) used to satisfy subsequent requests to caches. Servers who wish
2100   to control caching of these responses are encouraged to emit appropriate
2101   Cache-Control response headers.
2102</t>
2103
2104</section>
2105
2106<section title="Acknowledgments" anchor="acks">
2107<t>
2108  See &acks;.
2109</t>
2110</section>
2111
2112</middle>
2113
2114<back>
2115<references title="Normative References">
2116
2117  <reference anchor="Part1">
2118    <front>
2119      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2120      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2121        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2122        <address><email>fielding@gbiv.com</email></address>
2123      </author>
2124      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2125        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2126        <address><email>julian.reschke@greenbytes.de</email></address>
2127      </author>
2128      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2129    </front>
2130    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2131    <x:source basename="p1-messaging" href="p1-messaging.xml">
2132      <x:defines>Content-Length</x:defines>
2133      <x:defines>Via</x:defines>
2134    </x:source>
2135  </reference>
2136
2137  <reference anchor="Part2">
2138    <front>
2139      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2140      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2141        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2142        <address><email>fielding@gbiv.com</email></address>
2143      </author>
2144      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2145        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2146        <address><email>julian.reschke@greenbytes.de</email></address>
2147      </author>
2148      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2149    </front>
2150    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2151    <x:source basename="p2-semantics" href="p2-semantics.xml">
2152      <x:defines>2xx (Successful)</x:defines>
2153      <x:defines>200 (OK)</x:defines>
2154      <x:defines>203 (Non-Authoritative Information)</x:defines>
2155      <x:defines>3xx (Redirection)</x:defines>
2156      <x:defines>300 (Multiple Choices)</x:defines>
2157      <x:defines>301 (Moved Permanently)</x:defines>
2158      <x:defines>404 (Not Found)</x:defines>
2159      <x:defines>410 (Gone)</x:defines>
2160      <x:defines>5xx (Server Error)</x:defines>
2161      <x:defines>504 (Gateway Timeout)</x:defines>
2162      <x:defines>Content-Encoding</x:defines>
2163      <x:defines>Content-Location</x:defines>
2164      <x:defines>Content-Type</x:defines>
2165      <x:defines>Date</x:defines>
2166      <x:defines>Location</x:defines>
2167      <x:defines>Vary</x:defines>
2168    </x:source>
2169  </reference>
2170
2171  <reference anchor="Part4">
2172    <front>
2173      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2174      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2175        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2176        <address><email>fielding@gbiv.com</email></address>
2177      </author>
2178      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2179        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2180        <address><email>julian.reschke@greenbytes.de</email></address>
2181      </author>
2182      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2183    </front>
2184    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2185    <x:source basename="p4-conditional" href="p4-conditional.xml">
2186      <x:defines>304</x:defines>
2187      <x:defines>304 (Not Modified)</x:defines>
2188      <x:defines>ETag</x:defines>
2189      <x:defines>If-Modified-Since</x:defines>
2190      <x:defines>If-None-Match</x:defines>
2191      <x:defines>Last-Modified</x:defines>
2192    </x:source>
2193  </reference>
2194
2195  <reference anchor="Part5">
2196    <front>
2197      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2198      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2199        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2200        <address><email>fielding@gbiv.com</email></address>
2201      </author>
2202      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2203        <organization abbrev="W3C">World Wide Web Consortium</organization>
2204        <address><email>ylafon@w3.org</email></address>
2205      </author>
2206      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2207        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2208        <address><email>julian.reschke@greenbytes.de</email></address>
2209      </author>
2210      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2211    </front>
2212    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2213    <x:source basename="p5-range" href="p5-range.xml">
2214      <x:defines>206 (Partial Content)</x:defines>
2215      <x:defines>Content-Range</x:defines>
2216      <x:defines>Range</x:defines>
2217    </x:source>
2218  </reference>
2219
2220  <reference anchor="Part7">
2221    <front>
2222      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2223      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2224        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2225        <address><email>fielding@gbiv.com</email></address>
2226      </author>
2227      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2228        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2229        <address><email>julian.reschke@greenbytes.de</email></address>
2230      </author>
2231      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2232    </front>
2233    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2234    <x:source basename="p7-auth" href="p7-auth.xml">
2235      <x:defines>Authorization</x:defines>
2236    </x:source>
2237  </reference>
2238
2239  <reference anchor="RFC2119">
2240    <front>
2241      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2242      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2243        <organization>Harvard University</organization>
2244        <address><email>sob@harvard.edu</email></address>
2245      </author>
2246      <date month="March" year="1997" />
2247    </front>
2248    <seriesInfo name="BCP" value="14" />
2249    <seriesInfo name="RFC" value="2119" />
2250  </reference>
2251
2252  <reference anchor="RFC5234">
2253    <front>
2254      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2255      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2256        <organization>Brandenburg InternetWorking</organization>
2257        <address>
2258          <email>dcrocker@bbiw.net</email>
2259        </address> 
2260      </author>
2261      <author initials="P." surname="Overell" fullname="Paul Overell">
2262        <organization>THUS plc.</organization>
2263        <address>
2264          <email>paul.overell@thus.net</email>
2265        </address>
2266      </author>
2267      <date month="January" year="2008"/>
2268    </front>
2269    <seriesInfo name="STD" value="68"/>
2270    <seriesInfo name="RFC" value="5234"/>
2271  </reference>
2272 
2273</references>
2274
2275<references title="Informative References">
2276
2277  <reference anchor="RFC1305">
2278    <front>
2279      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2280      <author fullname="David L. Mills" initials="D." surname="Mills">
2281        <organization>University of Delaware, Electrical Engineering Department</organization>
2282        <address><email>mills@udel.edu</email></address>
2283      </author>
2284      <date month="March" year="1992" />
2285    </front>
2286    <seriesInfo name="RFC" value="1305" />
2287  </reference>
2288
2289  <reference anchor="RFC2616">
2290    <front>
2291      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2292      <author fullname="R. Fielding" initials="R." surname="Fielding">
2293        <organization>University of California, Irvine</organization>
2294        <address><email>fielding@ics.uci.edu</email></address>
2295      </author>
2296      <author fullname="J. Gettys" initials="J." surname="Gettys">
2297        <organization>W3C</organization>
2298        <address><email>jg@w3.org</email></address>
2299      </author>
2300      <author fullname="J. Mogul" initials="J." surname="Mogul">
2301        <organization>Compaq Computer Corporation</organization>
2302        <address><email>mogul@wrl.dec.com</email></address>
2303      </author>
2304      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2305        <organization>MIT Laboratory for Computer Science</organization>
2306        <address><email>frystyk@w3.org</email></address>
2307      </author>
2308      <author fullname="L. Masinter" initials="L." surname="Masinter">
2309        <organization>Xerox Corporation</organization>
2310        <address><email>masinter@parc.xerox.com</email></address>
2311      </author>
2312      <author fullname="P. Leach" initials="P." surname="Leach">
2313        <organization>Microsoft Corporation</organization>
2314        <address><email>paulle@microsoft.com</email></address>
2315      </author>
2316      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2317        <organization>W3C</organization>
2318        <address><email>timbl@w3.org</email></address>
2319      </author>
2320      <date month="June" year="1999" />
2321    </front>
2322    <seriesInfo name="RFC" value="2616" />
2323  </reference>
2324
2325  <reference anchor="BCP90">
2326    <front>
2327      <title>Registration Procedures for Message Header Fields</title>
2328      <author fullname="G. Klyne" initials="G." surname="Klyne">
2329        <organization>Nine by Nine</organization>
2330        <address><email>GK-IETF@ninebynine.org</email></address>
2331      </author>
2332      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2333        <organization>BEA Systems</organization>
2334        <address><email>mnot@pobox.com</email></address>
2335      </author>
2336      <author fullname="J. Mogul" initials="J." surname="Mogul">
2337        <organization>HP Labs</organization>
2338        <address><email>JeffMogul@acm.org</email></address>
2339      </author>
2340      <date month="September" year="2004" />
2341    </front>
2342    <seriesInfo name="BCP" value="90" />
2343    <seriesInfo name="RFC" value="3864" />
2344  </reference>
2345
2346  <reference anchor='RFC5226'>
2347    <front>
2348      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2349      <author initials='T.' surname='Narten' fullname='T. Narten'>
2350        <organization>IBM</organization>
2351        <address><email>narten@us.ibm.com</email></address>
2352      </author>
2353      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2354        <organization>Google</organization>
2355        <address><email>Harald@Alvestrand.no</email></address>
2356      </author>
2357      <date year='2008' month='May' />
2358    </front>
2359    <seriesInfo name='BCP' value='26' />
2360    <seriesInfo name='RFC' value='5226' />
2361  </reference>
2362
2363  <reference anchor='RFC5861'>
2364    <front>
2365      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2366      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2367        <organization>Yahoo! Inc.</organization>
2368        <address><email>mnot@yahoo-inc.com</email></address>
2369      </author>
2370      <date month="April" year="2010"/>
2371    </front>
2372    <seriesInfo name='RFC' value='5861' />
2373  </reference>
2374
2375  <reference anchor="RFC6265">
2376    <front>
2377      <title>HTTP State Management Mechanism</title>
2378      <author initials="A." surname="Barth" fullname="Adam Barth">
2379        <organization abbrev="U.C. Berkeley">
2380          University of California, Berkeley
2381        </organization>
2382        <address><email>abarth@eecs.berkeley.edu</email></address>
2383      </author>
2384      <date year="2011" month="April" />
2385    </front>
2386    <seriesInfo name="RFC" value="6265"/>
2387  </reference>
2388
2389</references>
2390
2391<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2392<t>
2393  Caching-related text has been substantially rewritten for clarity.
2394</t>
2395<t>
2396  The algorithm for calculating age is now less conservative.
2397  (<xref target="age.calculations"/>)
2398</t>
2399<t>
2400  Caches are now required to handle dates with timezones as if they're
2401  invalid, because it's not possible to accurately guess.
2402  (<xref target="age.calculations"/>)
2403</t>
2404<t>
2405  The <x:ref>Content-Location</x:ref> response header field is no longer used
2406  to determine the appropriate response to use when validating.
2407  (<xref target="validation.model" />)
2408</t>
2409<t>
2410  The algorithm for selecting a cached negotiated response to use has been
2411  clarified in several ways. In particular, it now explicitly allows
2412  header-specific canonicalization when processing selecting header fields.
2413  (<xref target="caching.negotiated.responses" />)
2414</t>
2415<t>
2416  Requirements regarding denial of service attack avoidance when performing
2417  invalidation have been clarified.
2418  (<xref target="invalidation.after.updates.or.deletions" />)
2419</t>
2420<t>
2421  Cache invalidation only occurs when a successful response is received.
2422  (<xref target="invalidation.after.updates.or.deletions" />)
2423</t>
2424<t>
2425  The conditions under which an authenticated response can be cached have been
2426  clarified.
2427  (<xref target="caching.authenticated.responses" />)
2428</t>
2429<t>
2430  The one-year limit on Expires header field values has been removed; instead,
2431  the reasoning for using a sensible value is given.
2432  (<xref target="header.expires" />)
2433</t>
2434<t>
2435  The Pragma header field is now only defined for backwards compatibility;
2436  future pragmas are deprecated.
2437  (<xref target="header.pragma" />)
2438</t>
2439<t>
2440  Cache directives are explicitly defined to be case-insensitive.
2441  (<xref target="header.cache-control" />)
2442</t>
2443<t>
2444  Handling of multiple instances of cache directives when only one is
2445  expected is now defined.
2446  (<xref target="header.cache-control" />)
2447</t>
2448<t>
2449  The qualified forms of the private and no-cache cache directives are noted
2450  to not be widely implemented; e.g., "private=foo" is interpreted by many
2451  caches as simply "private". Additionally, the meaning of the qualified
2452  form of no-cache has been clarified.
2453  (<xref target="cache-response-directive" />)
2454</t>
2455<t>
2456  The "no-store" cache request directive doesn't apply to responses; i.e.,
2457  a cache can satisfy a request with no-store on it, and does not invalidate
2458  it.
2459  (<xref target="cache-request-directive.no-store" />)
2460</t>
2461<t>
2462  The meaning of the "public" cache response directive has been refined to
2463  be less vague.
2464  (<xref target="cache-response-directive.public" />)
2465</t>
2466<t>
2467  The "no-cache" response cache directive's meaning has been clarified.
2468  (<xref target="cache-response-directive.no-cache" />)
2469</t>
2470<t>
2471  New status codes can now define that caches are allowed to use heuristic
2472  freshness with them.
2473  (<xref target="heuristic.freshness" />)
2474</t>
2475<t>
2476  Caches are now allow to calculate heuristic freshness for URLs with query
2477  components.
2478  (<xref target="heuristic.freshness" />)
2479</t>
2480<t>
2481  Some requirements regarding production of the <x:ref>Warning</x:ref> 
2482  header have been relaxed, as it is not widely implemented.
2483  (<xref target="header.warning" />)
2484</t>
2485<t>
2486  The <x:ref>Warning</x:ref> header field no longer uses RFC 2047 encoding,
2487  nor allows multiple languages, as these aspects were not implemented.
2488  (<xref target="header.warning" />)
2489</t>
2490<t>
2491  This specification introduces the Cache Directive and Warn Code Registries,
2492  and defines considerations for new cache directives.
2493  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
2494</t>
2495</section>
2496
2497<section title="Imported ABNF" anchor="imported.abnf">
2498   <x:anchor-alias value="ALPHA"/>
2499   <x:anchor-alias value="CR"/>
2500   <x:anchor-alias value="DIGIT"/>
2501   <x:anchor-alias value="DQUOTE"/>
2502   <x:anchor-alias value="LF"/>
2503   <x:anchor-alias value="OCTET"/>
2504   <x:anchor-alias value="SP"/>
2505   <x:anchor-alias value="VCHAR"/>
2506   <x:anchor-alias value="quoted-string"/>
2507   <x:anchor-alias value="token"/>
2508   <x:anchor-alias value="OWS"/>
2509   <x:anchor-alias value="field-name"/>
2510   <x:anchor-alias value="HTTP-date"/>
2511   <x:anchor-alias value="port"/>
2512   <x:anchor-alias value="pseudonym"/>
2513   <x:anchor-alias value="uri-host"/>
2514<t>
2515   The following core rules are included by reference, as defined in <xref
2516   target="RFC5234" x:fmt="of" x:sec="B.1"/>: ALPHA (letters), CR (carriage
2517   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2518   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2519   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2520</t>
2521<t>
2522   The rules below are defined in <xref target="Part1"/>:
2523</t>
2524<figure><artwork type="abnf2616">
2525  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
2526  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
2527  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
2528  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
2529
2530  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
2531  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
2532  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
2533</artwork></figure>
2534<t>
2535   The rules below are defined in other parts:
2536</t>
2537<figure><artwork type="abnf2616">
2538  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
2539</artwork></figure>
2540</section>
2541
2542<?BEGININC p6-cache.abnf-appendix ?>
2543<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2544<figure>
2545<artwork type="abnf" name="p6-cache.parsed-abnf">
2546<x:ref>Age</x:ref> = delta-seconds
2547
2548<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2549 cache-directive ] )
2550
2551<x:ref>Expires</x:ref> = HTTP-date
2552
2553<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
2554
2555<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
2556
2557<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2558 pragma-directive ] )
2559
2560<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2561 )
2562
2563<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2564
2565<x:ref>delta-seconds</x:ref> = 1*DIGIT
2566
2567<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2568
2569<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2570
2571<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2572<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2573<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 5.7.1&gt;
2574
2575<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.6&gt;
2576
2577<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.6&gt;
2578
2579<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2580
2581<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2582<x:ref>warn-code</x:ref> = 3DIGIT
2583<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2584<x:ref>warn-text</x:ref> = quoted-string
2585<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2586 ]
2587</artwork>
2588</figure>
2589</section>
2590<?ENDINC p6-cache.abnf-appendix ?>
2591
2592<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2593<t>
2594  Changes up to the first Working Group Last Call draft are summarized
2595  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2596</t>
2597
2598<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2599<t>
2600  Closed issues:
2601  <list style="symbols">
2602    <t>
2603      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2604      "untangle Cache-Control ABNF"
2605    </t>
2606    <t>
2607      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2608      "Multiple values in Cache-Control header fields"
2609    </t>
2610    <t>
2611      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2612      "Case sensitivity of header fields in CC values"
2613    </t>
2614    <t>
2615      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2616      "Spurious 'MAYs'"
2617    </t>
2618    <t>
2619      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2620      "enhance considerations for new cache control directives"
2621    </t>
2622    <t>
2623      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2624      "ABNF requirements for recipients"
2625    </t>
2626    <t>
2627      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2628      "note introduction of new IANA registries as normative changes"
2629    </t>
2630    <t>
2631      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
2632      "broken prose in description of 'Vary'"
2633    </t>
2634  </list>
2635</t>
2636</section>
2637
2638<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
2639<t>
2640  Closed issues:
2641  <list style="symbols">
2642    <t>
2643      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
2644      "'Most Conservative'"
2645    </t>
2646  </list>
2647</t>
2648<t>
2649  Other changes:
2650  <list style="symbols">
2651    <t>
2652      Conformance criteria and considerations regarding error handling are
2653      now defined in Part 1.
2654    </t>
2655    <t>
2656      Move definition of "Vary" header field into Part 2.
2657    </t>
2658    <t>
2659      Add security considerations with respect to cache poisoning and
2660      the "Set-Cookie" header field.
2661    </t>
2662  </list>
2663</t>
2664</section>
2665
2666<section title="Since draft-ietf-httpbis-p6-cache-21" anchor="changes.since.21">
2667<t>
2668  Closed issues:
2669  <list style="symbols">
2670    <t>
2671      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
2672      "No-Transform"
2673    </t>
2674  </list>
2675</t>
2676</section>
2677
2678</section>
2679  </back>
2680</rfc>
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