source: draft-ietf-httpbis/latest/p6-cache.xml @ 2179

Last change on this file since 2179 was 2179, checked in by mnot@…, 7 years ago

Tidy up discussion of heuristic freshness, with follow-on effects in the
definition of 'public', the storage algorithm, and considerations for new
status codes. Addresses #223

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 106.8 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 "February">
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 GET                         "<xref target='Part2' x:rel='#GET' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY transformations             "<xref target='Part1' x:rel='#message.transformations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48]>
49<?rfc toc="yes" ?>
50<?rfc symrefs="yes" ?>
51<?rfc sortrefs="yes" ?>
52<?rfc compact="yes"?>
53<?rfc subcompact="no" ?>
54<?rfc linkmailto="no" ?>
55<?rfc editing="no" ?>
56<?rfc comments="yes"?>
57<?rfc inline="yes"?>
58<?rfc rfcedstyle="yes"?>
59<?rfc-ext allow-markup-in-artwork="yes" ?>
60<?rfc-ext include-references-in-index="yes" ?>
61<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
62  obsoletes="2616" x:maturity-level="proposed" xmlns:x="http://purl.org/net/xml2rfc/ext">
63<x:link rel="prev" basename="p5-range"/>
64<x:link rel="next" basename="p7-auth"/>
65<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
66<front>
67
68  <title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
69
70  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
71    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
72    <address>
73      <postal>
74        <street>345 Park Ave</street>
75        <city>San Jose</city>
76        <region>CA</region>
77        <code>95110</code>
78        <country>USA</country>
79      </postal>
80      <email>fielding@gbiv.com</email>
81      <uri>http://roy.gbiv.com/</uri>
82    </address>
83  </author>
84
85  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
86    <organization>Akamai</organization>
87    <address>
88      <email>mnot@mnot.net</email>
89      <uri>http://www.mnot.net/</uri>
90    </address>
91  </author>
92
93  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
94    <organization abbrev="greenbytes">greenbytes GmbH</organization>
95    <address>
96      <postal>
97        <street>Hafenweg 16</street>
98        <city>Muenster</city><region>NW</region><code>48155</code>
99        <country>Germany</country>
100      </postal>
101      <email>julian.reschke@greenbytes.de</email>
102      <uri>http://greenbytes.de/tech/webdav/</uri>
103    </address>
104  </author>
105
106  <date month="&ID-MONTH;" year="&ID-YEAR;" />
107  <workgroup>HTTPbis Working Group</workgroup>
108
109<abstract>
110<t>
111   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
112   distributed, collaborative, hypertext information systems. This document
113   defines requirements on HTTP caches and the associated header fields
114   that control cache behavior or indicate cacheable response messages.
115</t>
116</abstract>
117
118<note title="Editorial Note (To be removed by RFC Editor)">
119  <t>
120    Discussion of this draft takes place on the HTTPBIS working group
121    mailing list (ietf-http-wg@w3.org), which is archived at
122    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
123  </t>
124  <t>
125    The current issues list is at
126    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
127    documents (including fancy diffs) can be found at
128    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
129  </t>
130  <t>
131    The changes in this draft are summarized in <xref target="changes.since.21"/>.
132  </t>
133</note>
134
135   </front>
136   <middle>
137
138<section anchor="caching" title="Introduction">
139<t>
140   HTTP is typically used for distributed information systems, where
141   performance can be improved by the use of response caches. This document
142   defines aspects of HTTP/1.1 related to caching and reusing response
143   messages.
144</t>
145
146<section anchor="intro.purpose" title="Purpose">
147<iref item="cache" />
148<t>
149   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
150   subsystem that controls its message storage, retrieval, and deletion. A
151   cache stores cacheable responses in order to reduce the response time and
152   network bandwidth consumption on future, equivalent requests. Any client or
153   server &MAY; employ a cache, though a cache cannot be used by a server that
154   is acting as a tunnel.
155</t>
156<t>
157   The goal of caching in HTTP/1.1 is to significantly improve performance
158   by reusing a prior response message to satisfy a current request.
159   A stored response is considered "fresh", as defined in
160   <xref target="expiration.model" />, if the response can be reused without
161   "validation" (checking with the origin server to see if the cached response
162   remains valid for this request).  A fresh cache response can therefore
163   reduce both latency and network transfers each time it is reused.
164   When a cached response is not fresh, it might still be reusable if it can
165   be freshened by validation (<xref target="validation.model" />) or if the
166   origin is unavailable.
167</t>
168</section>
169
170<section anchor="intro.terminology" title="Terminology">
171<t>
172   This specification uses a number of terms to refer to the roles played by
173   participants in, and objects of, HTTP caching.
174</t>
175<t>
176   <iref item="cache" />
177   <x:dfn>cache</x:dfn>
178   <list>
179      <t>A conformant implementation of an HTTP cache. Note that this implies
180        an HTTP/1.1 cache; this specification does not define conformance
181        for HTTP/1.0 caches.</t>
182   </list>
183</t>
184<t anchor="shared.and.non-shared.caches">
185   <iref item="shared cache" />
186   <x:dfn>shared cache</x:dfn>
187   <list>
188      <t>A cache that stores responses to be reused by more than one user;
189         usually (but not always) deployed as part of an intermediary.</t>
190   </list>
191</t>
192<t>
193   <iref item="private cache" />
194   <x:dfn>private cache</x:dfn>
195   <list>
196      <t>A cache that is dedicated to a single user.</t>
197   </list>
198</t>
199<t>
200   <iref item="cacheable" />
201   <x:dfn>cacheable</x:dfn>
202   <list>
203      <t>A response is cacheable if a cache is allowed to store a copy of the
204      response message for use in answering subsequent requests. Even when a
205      response is cacheable, there might be additional constraints on whether
206      a cache can use the stored copy to satisfy a particular request.</t>
207   </list>
208</t>
209<t>
210   <iref item="explicit expiration time" />
211   <x:dfn>explicit expiration time</x:dfn>
212   <list>
213      <t>The time at which the origin server intends that a stored response
214      no longer be used by a cache without further validation.</t>
215   </list>
216</t>
217<t>
218   <iref item="heuristic expiration time" />
219   <x:dfn>heuristic expiration time</x:dfn>
220   <list>
221      <t>An expiration time assigned by a cache when no explicit expiration
222      time is available.</t>
223   </list>
224</t>
225<t>
226   <iref item="age" />
227   <x:dfn>age</x:dfn>
228   <list>
229      <t>The age of a response is the time since it was sent by, or
230      successfully validated with, the origin server.</t>
231   </list>
232</t>
233<t>
234   <iref item="first-hand" />
235   <x:dfn>first-hand</x:dfn>
236   <list>
237      <t>A response is first-hand if the freshness model is not in use; i.e.,
238      its age is 0.</t>
239   </list>
240</t>
241<t>
242   <iref item="freshness lifetime" />
243   <x:dfn>freshness lifetime</x:dfn>
244   <list>
245      <t>The length of time between the generation of a response and its
246      expiration time.</t>
247   </list>
248</t>
249<t>
250   <iref item="fresh" />
251   <x:dfn>fresh</x:dfn>
252   <list>
253      <t>A response is fresh if its age has not yet exceeded its freshness
254      lifetime.</t>
255   </list>
256</t>
257<t>
258   <iref item="stale" />
259   <x:dfn>stale</x:dfn>
260   <list>
261      <t>A response is stale if its age has passed its freshness lifetime
262      (either explicit or heuristic).</t>
263   </list>
264</t>
265<t>
266   <iref item="validator" />
267   <x:dfn>validator</x:dfn>
268   <list>
269      <t>A protocol element (e.g., an entity-tag or a <x:ref>Last-Modified</x:ref>
270      time) that is used to find out whether a stored response is an equivalent
271      copy of a representation. See &weak-and-strong;.</t>
272   </list>
273</t>
274<t>
275   <iref item="strong validator" />
276   <iref item="validator" subitem="strong" />
277   <x:dfn>strong validator</x:dfn>
278   <list>
279      <t>A validator that is defined by the origin server such that its
280         current value will change if the representation data changes; i.e.,
281         an entity-tag that is not marked as weak (&entity-tags;) or,
282         if no entity-tag is provided, a <x:ref>Last-Modified</x:ref> value
283         that is strong in the sense defined by &lastmod-comparison;.</t>
284   </list>
285</t>
286</section>
287
288<section title="Conformance and Error Handling" anchor="conformance">
289<t>
290   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
291   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
292   document are to be interpreted as described in <xref target="RFC2119"/>.
293</t>
294<t>
295   Conformance criteria and considerations regarding error handling
296   are defined in &conformance;.
297</t>
298</section>
299
300<section title="Syntax Notation" anchor="notation">
301<t>
302   This specification uses the Augmented Backus-Naur Form (ABNF) notation
303   of <xref target="RFC5234"/> with the list rule extension defined in
304   &notation;. <xref target="imported.abnf"/> describes rules imported from
305   other documents. <xref target="collected.abnf"/> shows the collected ABNF
306   with the list rule expanded.
307</t>
308
309<section title="Delta Seconds" anchor="delta-seconds">
310<t>
311   The delta-seconds rule specifies a non-negative integer, representing time
312   in seconds.
313</t>
314<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
315  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
316</artwork></figure>
317<t>
318   If an implementation receives a delta-seconds value larger than the largest
319   positive integer it can represent, or if any of its subsequent calculations
320   overflows, it &MUST; consider the value to be 2147483648
321   (2<x:sup>31</x:sup>). Recipients parsing a delta-seconds value &MUST; use
322   an arithmetic type of at least 31 bits of range, and senders &MUST-NOT;
323   send delta-seconds with a value greater than 2147483648.
324</t>
325</section>
326
327</section>
328</section>
329
330<section anchor="caching.overview" title="Overview of Cache Operation">
331<iref item="cache entry" />
332<iref item="cache key" />
333<t>
334   Proper cache operation preserves the semantics of HTTP transfers
335   (&semantics;) while eliminating the transfer of information already held
336   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
337   we assume that reusing the cached response is desirable and that such
338   reuse is the default behavior when no requirement or locally-desired
339   configuration prevents it.  Therefore, HTTP cache requirements are focused
340   on preventing a cache from either storing a non-reusable response or
341   reusing a stored response inappropriately.
342</t>
343<t>
344   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
345   HTTP responses corresponding to prior requests that used the same key. The
346   most common form of cache entry is a successful result of a retrieval
347   request: i.e., a <x:ref>200 (OK)</x:ref> response to a GET request, which
348   contains a representation of the resource identified by the request target
349   (&GET;). However, it is also possible to cache permanent redirects,
350   negative results (e.g., <x:ref>404 (Not Found)</x:ref>),
351   incomplete results (e.g., <x:ref>206 (Partial Content)</x:ref>), and
352   responses to methods other than GET if the method's definition allows such
353   caching and defines something suitable for use as a cache key.
354</t>
355<t>
356   The default <x:dfn>cache key</x:dfn> consists of the request method and
357   target URI.  However, since HTTP caches in common use today are typically
358   limited to caching responses to GET, many implementations simply decline
359   other methods and use only the URI as the key.
360</t>
361<t>
362   If a request target is subject to content negotiation, its cache entry
363   might consist of multiple stored responses, each differentiated by a
364   secondary key for the values of the original request's selecting header
365   fields (<xref target="caching.negotiated.responses"/>).
366</t>
367</section>
368
369<section anchor="response.cacheability" title="Storing Responses in Caches">
370<t>
371   A cache &MUST-NOT; store a response to any request, unless:
372   <list style="symbols">
373      <t>The request method is understood by the cache and defined as being
374      cacheable, and</t>
375      <t>the response status code is understood by the cache, and</t>
376      <t>the "no-store" cache directive (see <xref
377      target="header.cache-control" />) does not appear in request or response
378      header fields, and</t>
379      <t>the "private" cache response directive (see <xref
380      target="cache-response-directive.private" />) does not appear in the
381      response, if the cache is shared, and</t>
382      <t>the <x:ref>Authorization</x:ref> header field (see
383      &header-authorization;) does not appear in the request, if the cache is
384      shared, unless the response explicitly allows it (see <xref
385      target="caching.authenticated.responses" />), and</t>
386      <t>the response either:
387         <list style="symbols">
388            <t>contains an <x:ref>Expires</x:ref> header field (see
389            <xref target="header.expires"/>), or</t>
390            <t>contains a max-age response cache directive (see <xref
391            target="cache-response-directive.max-age" />), or</t>
392            <t>contains a s-maxage response cache directive and the cache is
393            shared, or</t>
394            <t>contains a Cache Control Extension (see <xref
395            target="cache.control.extensions" />) that allows it to be cached,
396            or</t>
397            <t>has a status code that is defined as cacheable
398            (see <xref target="heuristic.freshness" />), or</t>
399            <t>contains a public response cache directive.</t>
400         </list>
401      </t>
402   </list>
403</t>
404<t>
405   Note that any of the requirements listed above can be overridden by a
406   cache-control extension; see <xref target="cache.control.extensions" />.
407</t>
408<t>
409   In this context, a cache has "understood" a request method or a response
410   status code if it recognizes it and implements any cache-specific
411   behavior.
412</t>
413<t>
414   Note that, in normal operation, many caches will not store a response that
415   has neither a cache validator nor an explicit expiration time, as such
416   responses are not usually useful to store. However, caches are not
417   prohibited from storing such responses.
418</t>
419
420<section anchor="incomplete.responses" title="Storing Incomplete Responses">
421<t>
422   A response message is considered complete when all of the octets indicated
423   by the message framing (&messaging;) are received prior to the connection
424   being closed. If the request is GET, the response status is <x:ref>200
425   (OK)</x:ref>, and the entire response header block has been received, a
426   cache &MAY; store an incomplete response message body if the cache entry is
427   recorded as incomplete. Likewise, a <x:ref>206 (Partial Content)</x:ref>
428   response &MAY; be stored as if it were an incomplete <x:ref>200
429   (OK)</x:ref> cache entry. However, a cache &MUST-NOT; store incomplete or
430   partial content responses if it does not support the <x:ref>Range</x:ref>
431   and <x:ref>Content-Range</x:ref> header fields or if it does not understand
432   the range units used in those fields.
433</t>
434<t>
435   A cache &MAY; complete a stored incomplete response by making a subsequent
436   range request (&partial;) and combining the successful response with the
437   stored entry, as defined in <xref target="combining.responses"/>. A cache
438   &MUST-NOT; use an incomplete response to answer requests unless the
439   response has been made complete or the request is partial and specifies a
440   range that is wholly within the incomplete response. A cache &MUST-NOT;
441   send a partial response to a client without explicitly marking it as such
442   using the <x:ref>206 (Partial Content)</x:ref> status code.
443</t>
444</section>
445
446
447<section anchor="caching.authenticated.responses"
448   title="Storing Responses to Authenticated Requests">
449<t>
450   A shared cache &MUST-NOT; use a cached response to a request with an
451   <x:ref>Authorization</x:ref> header field (&header-authorization;) to
452   satisfy any subsequent request unless a cache directive that allows such
453   responses to be stored is present in the response.
454</t>
455<t>
456   In this specification, the following <x:ref>Cache-Control</x:ref> response
457   directives (<xref target="cache-response-directive"/>) have such an effect:
458   must-revalidate, public, s-maxage.
459</t>
460<t>
461   Note that cached responses that contain the "must-revalidate" and/or
462   "s-maxage" response directives are not allowed to be served stale (<xref
463   target="serving.stale.responses"/>) by shared caches. In particular, a
464   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
465   used to satisfy a subsequent request without revalidating it on the origin
466   server.
467</t>
468</section>
469</section>
470
471
472<section anchor="constructing.responses.from.caches"
473   title="Constructing Responses from Caches">
474<t>
475   For a presented request, a cache &MUST-NOT; send a stored response,
476   unless:
477   <list style="symbols">
478      <t>The presented effective request URI (&effective-request-uri;) and
479      that of the stored response match, and</t>
480      <t>the request method associated with the stored response allows it to
481      be used for the presented request, and</t>
482      <t>selecting header fields nominated by the stored response (if any)
483      match those presented (see <xref target="caching.negotiated.responses"
484      />), and</t>
485      <t>the presented request does not contain the no-cache pragma (<xref
486      target="header.pragma"/>), nor the no-cache cache directive (<xref
487      target="cache-request-directive"/>), unless the stored response is
488      successfully validated (<xref target="validation.model"/>), and</t>
489      <t>the stored response does not contain the no-cache cache directive
490      (<xref target="cache-response-directive.no-cache"/>), unless it is
491      successfully validated (<xref target="validation.model"/>), and</t>
492      <t>the stored response is either:
493         <list style="symbols">
494            <t>fresh (see <xref target="expiration.model" />), or</t>
495            <t>allowed to be served stale (see <xref
496            target="serving.stale.responses" />), or</t>
497            <t>successfully validated (see <xref target="validation.model"
498            />).</t>
499         </list>
500      </t>
501  </list>
502</t>
503<t>
504   Note that any of the requirements listed above can be overridden by a
505   cache-control extension; see <xref target="cache.control.extensions" />.
506</t>
507<t>
508   When a stored response is used to satisfy a request without validation,
509   a cache &MUST; send a single <x:ref>Age</x:ref> header field
510   (<xref target="header.age"/>) in the response with a value equal to the
511   stored response's current_age; see <xref target="age.calculations" />.
512</t>
513<t>
514   A cache &MUST; write through requests with methods that are unsafe
515   (&safe-methods;) to the origin server; i.e., a cache is not allowed to
516   generate a reply to such a request before having forwarded the request and
517   having received a corresponding response.
518</t>
519<t>
520   Also, note that unsafe requests might invalidate already stored responses;
521   see <xref target="invalidation.after.updates.or.deletions" />.
522</t>
523<t>
524   When more than one suitable response is stored, a cache &MUST; use the
525   most recent response (as determined by the <x:ref>Date</x:ref> header
526   field). It can also forward a request with "Cache-Control: max-age=0" or
527   "Cache-Control: no-cache" to disambiguate which response to use.
528</t>
529<t>
530   A cache that does not have a clock available &MUST-NOT; use stored
531   responses without revalidating them on every use.
532</t>
533
534
535<section anchor="expiration.model" title="Freshness Model">
536<t>
537   When a response is "fresh" in the cache, it can be used to satisfy
538   subsequent requests without contacting the origin server, thereby improving
539   efficiency.
540</t>
541<t>
542   The primary mechanism for determining freshness is for an origin server to
543   provide an explicit expiration time in the future, using either the
544   <x:ref>Expires</x:ref> header field (<xref target="header.expires" />) or
545   the max-age response cache directive (<xref
546   target="cache-response-directive.max-age" />). Generally, origin servers will
547   assign future explicit expiration times to responses in the belief that the
548   representation is not likely to change in a semantically significant way
549   before the expiration time is reached.
550</t>
551<t>
552   If an origin server wishes to force a cache to validate every request, it
553   can assign an explicit expiration time in the past to indicate that the
554   response is already stale. Compliant caches will normally validate a stale
555   cached response before reusing it for subsequent requests (see <xref
556   target="serving.stale.responses" />).
557</t>
558<t>
559   Since origin servers do not always provide explicit expiration times,
560   caches are also allowed to use a heuristic to determine an expiration time
561   under certain circumstances (see <xref target="heuristic.freshness"/>).
562</t>
563<figure>
564<preamble>
565  The calculation to determine if a response is fresh is:
566</preamble>
567<artwork type="code">
568   response_is_fresh = (freshness_lifetime &gt; current_age)
569</artwork>
570</figure>
571<t>
572   The freshness_lifetime is defined in <xref
573   target="calculating.freshness.lifetime" />; the current_age is defined in
574   <xref target="age.calculations" />.
575</t>
576<t>
577   Clients can send the max-age or min-fresh cache directives in a request to
578   constrain or relax freshness calculations for the corresponding response
579   (<xref target="cache-request-directive" />).
580</t>
581<t>
582   Note that freshness applies only to cache operation; it cannot be used to
583   force a user agent to refresh its display or reload a resource. See <xref
584   target="history.lists" /> for an explanation of the difference between
585   caches and history mechanisms.
586</t>
587
588<section anchor="calculating.freshness.lifetime"
589   title="Calculating Freshness Lifetime">
590<t>
591   A cache can calculate the freshness lifetime (denoted as
592   freshness_lifetime) of a response by using the first match of:
593   <list style="symbols">
594      <t>If the cache is shared and the s-maxage response cache directive
595      (<xref target="cache-response-directive.s-maxage" />) is present, use its value,
596      or</t>
597      <t>If the max-age response cache directive (<xref
598      target="cache-response-directive.max-age" />) is present, use its value, or</t>
599      <t>If the <x:ref>Expires</x:ref> response header field
600      (<xref target="header.expires" />) is present, use its value minus the
601      value of the <x:ref>Date</x:ref> response header field, or</t>
602      <t>Otherwise, no explicit expiration time is present in the response. A
603      heuristic freshness lifetime might be applicable; see <xref
604      target="heuristic.freshness" />.</t>
605   </list>
606</t>
607<t>
608   Note that this calculation is not vulnerable to clock skew, since all of
609   the information comes from the origin server.
610</t>
611<t>
612   When there is more than one value present for a given directive (e.g., two
613   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
614   directives), it is considered invalid. Caches are encouraged to consider
615   responses that have invalid freshness information to be stale.
616</t>
617</section>
618
619<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
620<t>
621   Since origin servers do not always provide explicit expiration times, a
622   cache &MAY; assign a heuristic expiration time when an explicit time is not
623   specified, employing algorithms that use other header field values (such as
624   the <x:ref>Last-Modified</x:ref> time) to estimate a plausible expiration
625   time. This specification does not provide specific algorithms, but does
626   impose worst-case constraints on their results.
627</t>
628<t>
629   A cache &MUST-NOT; use heuristics to determine freshness when an explicit
630   expiration time is present in the stored response. Because of the
631   requirements in <xref target="response.cacheability"/>, this means that,
632   effectively, heuristics can only be used on responses without explicit
633   freshness whose status codes are defined as cacheable, and responses
634   without explicit freshness that have been marked as explicitly cacheable
635   (e.g., with a "public" response cache directive).
636</t>
637<t>
638   If the response has a <x:ref>Last-Modified</x:ref> header field
639   (&header-last-modified;), caches are encouraged to use a heuristic
640   expiration value that is no more than some fraction of the interval since
641   that time. A typical setting of this fraction might be 10%.
642</t>
643<t>
644   When a heuristic is used to calculate freshness lifetime, a cache &SHOULD;
645   attach a <x:ref>Warning</x:ref> header field with a 113 warn-code to the
646   response if its current_age is more than 24 hours and such a warning is not
647   already present.
648</t>
649<x:note>
650   <t>
651      &Note; <xref target="RFC2616" x:fmt="of" x:sec="13.9"/> prohibited caches
652      from calculating heuristic freshness for URIs with query components
653      (i.e., those containing '?'). In practice, this has not been widely
654      implemented. Therefore, servers are encouraged to send explicit
655      directives (e.g., Cache-Control: no-cache) if they wish to preclude
656      caching.
657   </t>
658</x:note>
659</section>
660
661<section anchor="age.calculations" title="Calculating Age">
662<t>
663   The <x:ref>Age</x:ref> header field is used to convey an estimated
664   age of the response message when obtained from a cache. The Age field value
665   is the cache's estimate of the number of seconds since the response was
666   generated or validated by the origin server. In essence, the Age value is
667   the sum of the time that the response has been resident in each of the
668   caches along the path from the origin server, plus the amount of time it
669   has been in transit along network paths.
670</t>
671<t>
672   The following data is used for the age calculation:
673</t>
674<t>
675   <x:dfn>age_value</x:dfn>
676   <list>
677      <t>
678         The term "age_value" denotes the value of the <x:ref>Age</x:ref>
679         header field (<xref target="header.age"/>), in a form appropriate for
680         arithmetic operation; or 0, if not available.
681      </t>
682   </list>
683</t>
684<t>
685   <x:dfn>date_value</x:dfn>
686   <list>
687      <t>
688         The term "date_value" denotes the value of
689         the Date header field, in a form appropriate for arithmetic
690         operations. See &header-date; for the definition of the Date header
691         field, and for requirements regarding responses without it.
692      </t>
693   </list>
694</t>
695<t>
696   <x:dfn>now</x:dfn>
697   <list>
698      <t>
699         The term "now" means "the current value of the clock at the host
700         performing the calculation". A host ought to use NTP (<xref
701         target="RFC1305"/>) or some similar protocol to synchronize its
702         clocks to Coordinated Universal Time.
703      </t>
704   </list>
705</t>
706<t>
707   <x:dfn>request_time</x:dfn>
708   <list>
709      <t>
710         The current value of the clock at the host at the time the request
711         resulting in the stored response was made.
712      </t>
713   </list>
714</t>
715<t>
716   <x:dfn>response_time</x:dfn>
717   <list>
718      <t>
719         The current value of the clock at the host at the time the response
720         was received.
721      </t>
722   </list>
723</t>
724<t>
725   A response's age can be calculated in two entirely independent ways:
726   <list style="numbers">
727      <t>the "apparent_age": response_time minus date_value, if the local
728      clock is reasonably well synchronized to the origin server's clock. If
729      the result is negative, the result is replaced by zero.</t>
730      <t>the "corrected_age_value", if all of the caches along the response
731      path implement HTTP/1.1. A cache &MUST; interpret this value relative
732      to the time the request was initiated, not the time that the response
733      was received.</t>
734   </list>
735</t>
736<figure>
737<artwork type="code">
738  apparent_age = max(0, response_time - date_value);
739
740  response_delay = response_time - request_time;
741  corrected_age_value = age_value + response_delay; 
742</artwork>
743</figure>
744<figure>
745<preamble>These &SHOULD; be combined as</preamble>
746<artwork type="code">
747  corrected_initial_age = max(apparent_age, corrected_age_value);
748</artwork></figure>
749<t>
750   unless the cache is confident in the value of the <x:ref>Age</x:ref> header
751   field (e.g., because there are no HTTP/1.0 hops in the <x:ref>Via</x:ref>
752   header field), in which case the corrected_age_value &MAY; be used as the
753   corrected_initial_age.</t>
754<t>
755   The current_age of a stored response can then be calculated by adding the
756   amount of time (in seconds) since the stored response was last validated by
757   the origin server to the corrected_initial_age.
758</t>
759<figure><artwork type="code">
760  resident_time = now - response_time;
761  current_age = corrected_initial_age + resident_time;
762</artwork></figure>
763<t>
764   Additionally, to avoid common problems in date parsing:
765</t>
766<t>
767  <list style="symbols">
768     <t>Although all date formats are specified to be case-sensitive,
769        cache recipients &SHOULD; match day, week and timezone names
770        case-insensitively.</t>
771             
772     <t>If a cache recipient's internal implementation of time has less
773        resolution than the value of an HTTP-date, the recipient &MUST;
774        internally represent a parsed <x:ref>Expires</x:ref> date as the
775        nearest time equal to or earlier than the received value.</t>
776
777     <t>Cache recipients &MUST-NOT; allow local time zones to influence the
778        calculation or comparison of an age or expiration time.</t>
779
780     <t>Cache recipients &SHOULD; consider a date with a zone abbreviation
781        other than "GMT" to be invalid for calculating expiration.</t>
782  </list>
783</t>
784</section>
785
786<section anchor="serving.stale.responses" title="Serving Stale Responses">
787<t>
788   A "stale" response is one that either has explicit expiry information or is
789   allowed to have heuristic expiry calculated, but is not fresh according to
790   the calculations in <xref target="expiration.model" />.
791</t>
792<t>
793   A cache &MUST-NOT; send a stale response if it is prohibited by an
794   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
795   directive, a "must-revalidate" cache-response-directive, or an applicable
796   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
797   target="cache-response-directive"/>).
798</t>
799<t>
800   A cache &MUST-NOT; send stale responses unless it is disconnected
801   (i.e., it cannot contact the origin server or otherwise find a forward
802   path) or doing so is explicitly allowed (e.g., by the max-stale request
803   directive; see <xref target="cache-request-directive" />).
804</t>
805<t>
806   A cache &SHOULD; append a <x:ref>Warning</x:ref> header field with the 110
807   warn-code (see <xref target="header.warning"/>) to stale responses.
808   Likewise, a cache &SHOULD; add the 112 warn-code to stale responses if the
809   cache is disconnected.
810</t>
811<t>
812   If a cache receives a first-hand response (either an entire response, or a
813   <x:ref>304 (Not Modified)</x:ref> response) that it would normally forward
814   to the requesting client, and the received response is no longer fresh, the
815   cache can forward it to the requesting client without adding a new
816   <x:ref>Warning</x:ref> (but without removing any existing Warning header
817   fields). A cache shouldn't attempt to validate a response simply because
818   that response became stale in transit.
819</t>
820</section>
821</section>
822
823<section anchor="validation.model" title="Validation Model">
824<t>
825   When a cache has one or more stored responses for a requested URI, but
826   cannot serve any of them (e.g., because they are not fresh, or one cannot
827   be selected; see <xref target="caching.negotiated.responses"/>), it can use
828   the conditional request mechanism &conditional; in the forwarded request to
829   give the origin server an opportunity to both select a valid stored
830   response to be used, and to update it. This process is known as
831   "validating" or "revalidating" the stored response.
832</t>
833<t>
834   When sending such a conditional request, a cache adds an
835   <x:ref>If-Modified-Since</x:ref> header field whose value is that of the
836   <x:ref>Last-Modified</x:ref> header field from the selected
837   (see <xref target="caching.negotiated.responses"/>) stored response, if
838   available.
839</t>
840<t>
841   Additionally, a cache can add an <x:ref>If-None-Match</x:ref> header field
842   whose value is that of the <x:ref>ETag</x:ref> header field(s) from all
843   responses stored for the requested URI, if present. However, if any of the
844   stored responses contains only partial content, the cache shouldn't
845   include its entity-tag in the If-None-Match header field unless the request
846   is for a range that would be fully satisfied by that stored response.
847</t>
848
849<t>Cache handling of a response to a conditional request is dependent upon its
850status code:</t>
851
852<t>
853   <list style="symbols">
854      <t>
855         A <x:ref>304 (Not Modified)</x:ref> response status code indicates
856         that the stored response can be updated and reused; see <xref
857         target="freshening.responses"/>.
858      </t>
859      <t>
860         A full response (i.e., one with a payload body) indicates that none
861         of the stored responses nominated in the conditional request is
862         suitable. Instead, the cache can use the full response to
863         satisfy the request and &MAY; replace the stored response(s).
864      </t>
865      <t>
866         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
867         response while attempting to validate a response, it can either
868         forward this response to the requesting client, or act as if the
869         server failed to respond. In the latter case, it can send a
870         previously stored response (see <xref
871         target="serving.stale.responses" />).
872      </t>
873   </list>
874</t>
875
876<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
877<t>
878   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and
879   already has one or more stored <x:ref>200 (OK)</x:ref> responses for the
880   same cache key, the cache needs to identify which of the stored responses
881   are updated by this new response and then update the stored response(s)
882   with the new information provided in the <x:ref>304</x:ref> response.
883</t>
884<t>
885   The stored response to update is identified by using the first match (if
886   any) of:
887   <list style="symbols">
888    <t>
889     If the new response contains a strong validator, then that strong
890     validator identifies the selected representation.  All of the stored
891     responses with the same strong validator are selected.
892     If none of the stored responses contain the same strong validator,
893     then the new response &MUST-NOT; be used to update any stored responses.
894    </t>
895    <t>
896     If the new response contains a weak validator and that validator
897     corresponds to one of the cache's stored responses, then the most
898     recent of those matching stored responses is selected.
899    </t>
900    <t>
901     If the new response does not include any form of validator (such as in
902     the case where a client generates an If-Modified-Since request from a
903     source other than the Last-Modified response header field), and there is
904     only one stored response, and that stored response also lacks a
905     validator, then that stored response is selected.
906    </t>
907   </list>
908</t>
909<t>
910   If a stored response is selected for update, the cache &MUST;:
911   <list style="symbols">
912      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
913         with warn-code 1xx (see <xref target="header.warning" />);</t>
914      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
915         with warn-code 2xx; and,</t>
916      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
917         response to replace all instances of the corresponding header
918         fields in the stored response.</t>
919   </list>
920</t>
921</section>
922
923</section>
924
925<section anchor="caching.negotiated.responses"
926   title="Using Negotiated Responses">
927<t>
928   When a cache receives a request that can be satisfied by a stored response
929   that has a <x:ref>Vary</x:ref> header field (&header-vary;),
930   it &MUST-NOT; use that response unless all of the selecting header fields
931   nominated by the Vary header field match in both the original request
932   (i.e., that associated with the stored response), and the presented
933   request.
934</t>
935<t>
936   The selecting header fields from two requests are defined to match if and
937   only if those in the first request can be transformed to those in the
938   second request by applying any of the following:
939   <list style="symbols">
940      <t>
941         adding or removing whitespace, where allowed in the header field's
942         syntax
943      </t>
944      <t>
945         combining multiple header fields with the same field name
946         (see &header-fields;)
947      </t>
948      <t>
949         normalizing both header field values in a way that is known to have
950         identical semantics, according to the header field's specification
951         (e.g., re-ordering field values when order is not significant;
952         case-normalization, where values are defined to be case-insensitive)
953      </t>
954  </list>
955</t>
956<t>
957   If (after any normalization that might take place) a header field is absent
958   from a request, it can only match another request if it is also absent
959   there.
960</t>
961<t>
962   A <x:ref>Vary</x:ref> header field-value of "*" always fails to match, and
963   subsequent requests to that resource can only be properly interpreted by the
964   origin server.
965</t>
966<t>
967   The stored response with matching selecting header fields is known as the
968   selected response.
969</t>
970<t>
971   If multiple selected responses are available, the most recent response
972   (as determined by the <x:ref>Date</x:ref> header field) is used; see <xref
973   target="constructing.responses.from.caches"/>.
974</t>
975<t>
976   If no selected response is available, the cache cannot satisfy the
977   presented request. Typically, it is forwarded to the origin server
978   in a (possibly conditional; see <xref target="validation.model"/>) request.
979</t>
980</section>
981
982
983<section anchor="combining.responses" title="Combining Partial Content">
984<t>
985   A response might transfer only a partial representation if the
986   connection closed prematurely or if the request used one or more Range
987   specifiers (&partial;).  After several such transfers, a cache might have
988   received several ranges of the same representation.  A cache &MAY; combine
989   these ranges into a single stored response, and reuse that response to
990   satisfy later requests, if they all share the same strong validator and
991   the cache complies with the client requirements in &combining-byte-ranges;.
992</t>
993<t>
994   When combining the new response with one or more stored responses, a
995   cache &MUST;:
996   <list style="symbols">
997      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
998         with warn-code 1xx (see <xref target="header.warning" />);</t>
999      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1000         with warn-code 2xx; and,</t>
1001      <t>use other header fields provided in the new response, aside
1002         from <x:ref>Content-Range</x:ref>, to replace all instances of the
1003         corresponding header fields in the stored response.</t>
1004   </list>
1005</t>
1006</section>
1007</section>
1008
1009
1010<section anchor="head.effects" title="Updating Caches with HEAD Responses">
1011<t>
1012   A response to the HEAD method is identical to what an equivalent request
1013   made with a GET would have been, except it lacks a body. This property
1014   of HEAD responses is used to both invalidate and update cached GET
1015   responses.
1016</t>
1017<t>
1018   If one or more stored GET responses can be selected (as per <xref
1019   target="caching.negotiated.responses"/>) for a HEAD request, and the
1020   <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> or
1021   <x:ref>Last-Modified</x:ref> value of a HEAD response differs from that in a
1022   selected GET response, the cache &MUST; consider that selected response to
1023   be stale.
1024</t>
1025<t>
1026   If the <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> and
1027   <x:ref>Last-Modified</x:ref> values of a HEAD response (when present) are
1028   the same as that in a selected GET response (as per
1029   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update
1030   the remaining header fields in the stored response using the following
1031   rules:
1032   <list style="symbols">
1033      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1034         with warn-code 1xx (see <xref target="header.warning" />);</t>
1035      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1036         with warn-code 2xx; and,</t>
1037      <t>use other header fields provided in the response to replace
1038         all instances of the corresponding header fields in the stored
1039         response.</t>
1040   </list>
1041</t>
1042
1043</section>
1044
1045
1046<section anchor="invalidation.after.updates.or.deletions"
1047   title="Request Methods that Invalidate">
1048<t>
1049   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1050   have the potential for changing state on the origin server, intervening
1051   caches can use them to keep their contents up-to-date.
1052</t>
1053<t>
1054   A cache &MUST; invalidate the effective Request URI
1055   (&effective-request-uri;) as well as the URI(s) in the
1056   <x:ref>Location</x:ref> and <x:ref>Content-Location</x:ref> response header
1057   fields (if present) when a non-error response to a request with an unsafe
1058   method is received.
1059</t>
1060<t>
1061   However, a cache &MUST-NOT; invalidate a URI from a <x:ref>Location</x:ref>
1062   or <x:ref>Content-Location</x:ref> response header field if the host part of
1063   that URI differs from the host part in the effective request URI
1064   (&effective-request-uri;). This helps prevent denial of service attacks.
1065</t>
1066<t>
1067   A cache &MUST; invalidate the effective request URI
1068   (&effective-request-uri;) when it receives a non-error response
1069   to a request with a method whose safety is unknown.
1070</t>
1071<t>
1072   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref>
1073   or <x:ref>3xx (Redirection)</x:ref> status code. "Invalidate" means that
1074   the cache will either remove all stored responses related to the effective
1075   request URI, or will mark these as "invalid" and in need of a mandatory
1076   validation before they can be sent in response to a subsequent request.
1077</t>
1078<t>
1079   Note that this does not guarantee that all appropriate responses are
1080   invalidated. For example, the request that caused the change at the origin
1081   server might not have gone through the cache where a response is stored.
1082</t>
1083</section>
1084
1085
1086
1087
1088<section anchor="header.field.definitions" title="Header Field Definitions">
1089<t>
1090   This section defines the syntax and semantics of HTTP/1.1 header fields
1091   related to caching.
1092</t>
1093
1094<section anchor="header.age" title="Age">
1095   <iref item="Age header field" primary="true" x:for-anchor="" />
1096   <x:anchor-alias value="Age"/>
1097   <x:anchor-alias value="age-value"/>
1098<t>
1099   The "Age" header field conveys the sender's estimate of the amount
1100   of time since the response was generated or successfully validated at the
1101   origin server. Age values are calculated as specified in <xref
1102   target="age.calculations" />.
1103</t>
1104<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1105  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1106</artwork></figure>
1107<t>
1108  Age field-values are non-negative integers, representing time in seconds
1109  (see <xref target="delta-seconds"/>).
1110</t>
1111<t>
1112   The presence of an Age header field in a response implies that a response
1113   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1114   might not implement the Age header field.
1115</t>
1116</section>
1117
1118<section anchor="header.cache-control" title="Cache-Control">
1119   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1120   <x:anchor-alias value="Cache-Control"/>
1121   <x:anchor-alias value="cache-directive"/>
1122<t>
1123   The "Cache-Control" header field is used to specify directives for
1124   caches along the request/response chain. Such cache directives are
1125   unidirectional in that the presence of a directive in a request does not
1126   imply that the same directive is to be given in the response.
1127</t>
1128<t>
1129   A cache &MUST; obey the requirements of the Cache-Control
1130   directives defined in this section. See <xref
1131   target="cache.control.extensions"/> for information about how Cache-Control
1132   directives defined elsewhere are handled.
1133</t>
1134<x:note>
1135   <t>
1136       &Note; HTTP/1.0 caches might not implement Cache-Control and
1137       might only implement Pragma: no-cache (see <xref target="header.pragma"
1138       />).
1139   </t>
1140</x:note>
1141<t>
1142   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1143   through in forwarded messages, regardless of their
1144   significance to that application, since the directives might be applicable
1145   to all recipients along the request/response chain. It is not possible to
1146   target a directive to a specific cache.
1147</t>
1148<t>
1149   Cache directives are identified by a token, to be compared case-insensitively,
1150   and have an optional argument, that can use both token and quoted-string
1151   syntax. For the directives defined below that define arguments, recipients
1152   ought to accept both forms, even if one is documented to be preferred. For
1153   any directive not defined by this specification, recipients &MUST; accept
1154   both forms.
1155</t>
1156<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1157  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1158
1159  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1160</artwork></figure>
1161<t>
1162   For the cache directives defined below, no argument is defined (nor allowed)
1163   otherwise stated otherwise.
1164</t>
1165
1166<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1167
1168<section title="no-cache" anchor="cache-request-directive.no-cache">
1169   <iref item="no-cache (cache directive)" primary="true" />
1170<t>
1171   The "no-cache" request directive indicates that a cache &MUST-NOT;
1172   use a stored response to satisfy the request without successful
1173   validation on the origin server.
1174</t>
1175</section>
1176 
1177<section title="no-store" anchor="cache-request-directive.no-store">
1178   <iref item="no-store (cache directive)" primary="true" />
1179<t>
1180   The "no-store" request directive indicates that a cache &MUST-NOT;
1181   store any part of either this request or any response to it. This
1182   directive applies to both private and shared caches. "&MUST-NOT;
1183   store" in this context means that the cache &MUST-NOT; intentionally
1184   store the information in non-volatile storage, and &MUST; make a
1185   best-effort attempt to remove the information from volatile storage as
1186   promptly as possible after forwarding it.
1187</t>
1188<t>
1189   This directive is NOT a reliable or sufficient mechanism for ensuring
1190   privacy. In particular, malicious or compromised caches might not
1191   recognize or obey this directive, and communications networks might be
1192   vulnerable to eavesdropping.
1193</t>
1194<t>
1195   Note that if a request containing this directive is satisfied from a
1196   cache, the no-store request directive does not apply to the already
1197   stored response.
1198</t>
1199</section>
1200
1201<section title="max-age" anchor="cache-request-directive.max-age">
1202   <iref item="max-age (cache directive)" primary="true" />
1203<t>
1204   Argument syntax:
1205   <list>
1206      <t>
1207        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1208      </t>
1209   </list>
1210</t>
1211<t>
1212   The "max-age" request directive indicates that the client is unwilling to
1213   accept a response whose age is greater than the specified number of
1214   seconds. Unless the max-stale request directive is also present, the
1215   client is not willing to accept a stale response.
1216</t>
1217<t>
1218   &Note; This directive uses the token form of the argument syntax;
1219   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1220   quoted-string form.
1221</t>
1222</section>
1223
1224<section title="max-stale" anchor="cache-request-directive.max-stale">
1225   <iref item="max-stale (cache directive)" primary="true" />
1226<t>
1227   Argument syntax:
1228   <list>
1229      <t>
1230        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1231      </t>
1232   </list>
1233</t>
1234<t>
1235   The "max-stale" request directive indicates that the client is willing
1236   to accept a response that has exceeded its expiration time. If max-stale
1237   is assigned a value, then the client is willing to accept a response
1238   that has exceeded its expiration time by no more than the specified
1239   number of seconds. If no value is assigned to max-stale, then the client
1240   is willing to accept a stale response of any age.
1241</t>
1242<t>
1243   &Note; This directive uses the token form of the argument syntax;
1244   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1245   quoted-string form.
1246</t>
1247</section>
1248
1249<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1250   <iref item="min-fresh (cache directive)" primary="true" />
1251<t>
1252   Argument syntax:
1253   <list>
1254      <t>
1255        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1256      </t>
1257   </list>
1258</t>
1259<t>
1260   The "min-fresh" request directive indicates that the client is willing
1261   to accept a response whose freshness lifetime is no less than its
1262   current age plus the specified time in seconds. That is, the client
1263   wants a response that will still be fresh for at least the specified
1264   number of seconds.
1265</t>
1266<t>
1267   &Note; This directive uses the token form of the argument syntax;
1268   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1269   quoted-string form.
1270</t>
1271</section>
1272
1273<section title="no-transform" anchor="cache-request-directive.no-transform">
1274   <iref item="no-transform (cache directive)" primary="true" />
1275<t>
1276   The "no-transform" request directive indicates that an intermediary
1277   (whether or not it implements a cache) &MUST-NOT; transform the payload,
1278   as defined in &transformations;.
1279</t>
1280</section>
1281
1282<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1283   <iref item="only-if-cached (cache directive)" primary="true" />
1284<t>
1285   The "only-if-cached" request directive indicates that the client only wishes
1286   to obtain a stored response. If it receives this directive, a cache &SHOULD;
1287   either respond using a stored response that is consistent with the other
1288   constraints of the request, or respond with a <x:ref>504 (Gateway
1289   Timeout)</x:ref> status code. If a group of caches is being operated as a
1290   unified system with good internal connectivity, a member cache &MAY;
1291   forward such a request within that group of caches.
1292</t>
1293</section>
1294</section>
1295
1296<section anchor="cache-response-directive"
1297   title="Response Cache-Control Directives">
1298   <x:anchor-alias value="cache-response-directive" />
1299
1300<section title="public" anchor="cache-response-directive.public">
1301   <iref item="public (cache directive)" primary="true" />
1302<t>
1303   The "public" response directive indicates that any cache &MAY; store the
1304   response, even if the response would normally be non-cacheable or cacheable
1305   only within a non-shared cache. (See <xref
1306   target="caching.authenticated.responses"/> for additional details related
1307   to the use of public in response to a request containing
1308   <x:ref>Authorization</x:ref>, and <xref target="response.cacheability"/>
1309   for details of how public affects responses that would normally not be
1310   stored, due to their status codes not being defined as cacheable.)
1311</t>
1312</section>
1313
1314<section title="private" anchor="cache-response-directive.private">
1315   <iref item="private (cache directive)" primary="true" />
1316<t>
1317   Argument syntax:
1318   <list>
1319      <t>
1320        #<x:ref>field-name</x:ref>
1321      </t>
1322   </list>
1323</t>
1324<t>
1325   The "private" response directive indicates that the response message is
1326   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1327   private cache &MAY; store the response and reuse it for later requests,
1328   even if the response would normally be non-cacheable.
1329</t>
1330<t>
1331   If the private response directive specifies one or more field-names,
1332   this requirement is limited to the field-values associated with the
1333   listed response header fields. That is, a shared cache &MUST-NOT; store
1334   the specified field-names(s), whereas it &MAY; store the remainder of the
1335   response message.
1336</t>
1337<t>
1338   The field-names given are not limited to the set of standard header
1339   fields defined by this specification. Field names are case-insensitive.
1340</t>
1341<t>
1342   &Note; This usage of the word "private" only controls
1343   where the response can be stored; it cannot ensure the privacy of the
1344   message content. Also, private response directives with field-names are
1345   often handled by implementations as if an unqualified private directive
1346   was received; i.e., the special handling for the qualified form is not
1347   widely implemented.
1348</t>
1349<t>
1350   &Note; This directive uses the quoted-string form of the argument syntax.
1351   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1352   needed for single-entry lists).
1353</t>
1354</section>
1355
1356<section title="no-cache" anchor="cache-response-directive.no-cache">
1357   <iref item="no-cache (cache directive)" primary="true" />
1358<t>
1359   Argument syntax:
1360   <list>
1361      <t>
1362        #<x:ref>field-name</x:ref>
1363      </t>
1364   </list>
1365</t>
1366<t>
1367   The "no-cache" response directive indicates that the response &MUST-NOT;
1368   be used to satisfy a subsequent request without successful validation on
1369   the origin server. This allows an origin server to prevent a cache from
1370   using it to satisfy a request without contacting it, even by caches that
1371   have been configured to send stale responses.
1372</t>
1373<t>
1374   If the no-cache response directive specifies one or more field-names,
1375   then a cache &MAY; use the response to satisfy a subsequent request,
1376   subject to any other restrictions on caching. However, any header fields
1377   in the response that have the field-name(s) listed &MUST-NOT; be sent
1378   in the response to a subsequent request without successful revalidation
1379   with the origin server. This allows an origin server to prevent the
1380   re-use of certain header fields in a response, while still allowing
1381   caching of the rest of the response.
1382</t>
1383<t>
1384   The field-names given are not limited to the set of standard header
1385   fields defined by this specification. Field names are case-insensitive.
1386</t>
1387<t>
1388   &Note; Many HTTP/1.0 caches will not recognize or obey
1389   this directive. Also, no-cache response directives with field-names are
1390   often handled by implementations as if an unqualified no-cache directive
1391   was received; i.e., the special handling for the qualified form is not
1392   widely implemented.
1393</t>
1394<t>
1395   &Note; This directive uses the quoted-string form of the argument syntax.
1396   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1397   needed for single-entry lists).
1398</t>
1399</section>
1400
1401<section title="no-store" anchor="cache-response-directive.no-store">
1402   <iref item="no-store (cache directive)" primary="true" />
1403<t>
1404   The "no-store" response directive indicates that a cache &MUST-NOT;
1405   store any part of either the immediate request or response. This
1406   directive applies to both private and shared caches. "&MUST-NOT;
1407   store" in this context means that the cache &MUST-NOT; intentionally
1408   store the information in non-volatile storage, and &MUST; make a
1409   best-effort attempt to remove the information from volatile storage as
1410   promptly as possible after forwarding it.
1411</t>
1412<t>
1413   This directive is NOT a reliable or sufficient mechanism for ensuring
1414   privacy. In particular, malicious or compromised caches might not
1415   recognize or obey this directive, and communications networks might be
1416   vulnerable to eavesdropping.
1417</t>
1418</section>
1419
1420<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1421   <iref item="must-revalidate (cache directive)" primary="true" />
1422<t>
1423   The "must-revalidate" response directive indicates that once it has
1424   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1425   requests without successful validation on the origin server.
1426</t>
1427<t>
1428   The must-revalidate directive is necessary to support reliable
1429   operation for certain protocol features. In all circumstances a
1430   cache &MUST; obey the must-revalidate directive; in particular,
1431   if a cache cannot reach the origin server for any reason, it &MUST;
1432   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1433</t>
1434<t>
1435   The must-revalidate directive ought to be used by servers if and only
1436   if failure to validate a request on the representation could result in
1437   incorrect operation, such as a silently unexecuted financial
1438   transaction.
1439</t>
1440</section>
1441
1442<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1443   <iref item="proxy-revalidate (cache directive)" primary="true" />
1444<t>
1445   The "proxy-revalidate" response directive has the same meaning as the
1446   must-revalidate response directive, except that it does not apply to
1447   private caches.
1448</t>
1449</section>
1450
1451<section title="max-age" anchor="cache-response-directive.max-age">
1452   <iref item="max-age (cache directive)" primary="true" />
1453<t>
1454   Argument syntax:
1455   <list>
1456      <t>
1457        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1458      </t>
1459   </list>
1460</t>
1461<t>
1462   The "max-age" response directive indicates that the response is to be
1463   considered stale after its age is greater than the specified number of
1464   seconds.
1465</t>
1466<t>
1467   &Note; This directive uses the token form of the argument syntax;
1468   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1469   quoted-string form.
1470</t>
1471</section>     
1472
1473<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1474   <iref item="s-maxage (cache directive)" primary="true" />
1475<t>
1476   Argument syntax:
1477   <list>
1478      <t>
1479        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1480      </t>
1481   </list>
1482</t>
1483<t>
1484   The "s-maxage" response directive indicates that, in shared caches, the
1485   maximum age specified by this directive overrides the maximum age
1486   specified by either the max-age directive or the <x:ref>Expires</x:ref>
1487   header field. The s-maxage directive also implies the semantics of the
1488   proxy-revalidate response directive.
1489</t>
1490<t>
1491   &Note; This directive uses the token form of the argument syntax;
1492   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1493   quoted-string form.
1494</t>
1495</section>
1496
1497<section title="no-transform" anchor="cache-response-directive.no-transform">
1498   <iref item="no-transform (cache directive)" primary="true" />
1499<t>
1500   The "no-transform" response directive indicates that an intermediary
1501   (regardless of whether it implements a cache) &MUST-NOT; transform the
1502   payload, as defined in &transformations;.
1503</t>
1504</section>
1505
1506</section>
1507
1508<section anchor="cache.control.extensions" title="Cache Control Extensions">
1509<t>
1510   The Cache-Control header field can be extended through the use of one or
1511   more cache-extension tokens, each with an optional value. Informational
1512   extensions (those that do not require a change in cache behavior) can be
1513   added without changing the semantics of other directives. Behavioral
1514   extensions are designed to work by acting as modifiers to the existing base
1515   of cache directives. Both the new directive and the standard directive are
1516   supplied, such that applications that do not understand the new directive
1517   will default to the behavior specified by the standard directive, and those
1518   that understand the new directive will recognize it as modifying the
1519   requirements associated with the standard directive. In this way,
1520   extensions to the cache-control directives can be made without requiring
1521   changes to the base protocol.
1522</t>
1523<t>
1524   This extension mechanism depends on an HTTP cache obeying all of the
1525   cache-control directives defined for its native HTTP-version, obeying
1526   certain extensions, and ignoring all directives that it does not
1527   understand.
1528</t>
1529<t>
1530   For example, consider a hypothetical new response directive called
1531   "community" that acts as a modifier to the private directive. We define
1532   this new directive to mean that, in addition to any private cache, any
1533   cache that is shared only by members of the community named within its
1534   value is allowed to cache the response. An origin server wishing to allow
1535   the UCI community to use an otherwise private response in their shared
1536   cache(s) could do so by including
1537</t>
1538<figure><artwork type="example">
1539  Cache-Control: private, community="UCI"
1540</artwork></figure>
1541<t>
1542   A cache seeing this header field will act correctly even if the cache does
1543   not understand the community cache-extension, since it will also see and
1544   understand the private directive and thus default to the safe behavior.
1545</t>
1546<t>
1547   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1548   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1549   combined with standard directives (or the response's default cacheability)
1550   such that the cache behavior will remain minimally correct even if the
1551   cache does not understand the extension(s).
1552</t>
1553<t>
1554   New extension directives ought to consider defining:
1555</t>
1556<t>
1557   <list style="symbols">
1558      <t>What it means for a directive to be specified multiple times,</t>
1559      <t>When the directive does not take an argument, what it means when an
1560      argument is present,</t>
1561      <t>When the directive requires an argument, what it means when it is
1562      missing.</t>
1563   </list>
1564</t>
1565<t>
1566   The HTTP Cache Directive Registry defines the name space for the cache
1567   directives.
1568</t>
1569<t>
1570   A registration &MUST; include the following fields:
1571   <list style="symbols">
1572      <t>Cache Directive Name</t>
1573      <t>Pointer to specification text</t>
1574   </list>
1575</t>
1576<t>
1577   Values to be added to this name space require IETF Review (see <xref
1578   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1579</t>
1580<t>
1581   The registry itself is maintained at <eref
1582   target="http://www.iana.org/assignments/http-cache-directives"/>.
1583</t>
1584</section>
1585
1586</section>
1587
1588<section anchor="header.expires" title="Expires">
1589   <iref item="Expires header field" primary="true" x:for-anchor="" />
1590   <x:anchor-alias value="Expires"/>
1591<t>
1592   The "Expires" header field gives the date/time after which the
1593   response is considered stale. See <xref target="expiration.model" /> for
1594   further discussion of the freshness model.
1595</t>
1596<t>
1597   The presence of an Expires field does not imply that the original resource
1598   will change or cease to exist at, before, or after that time.
1599</t>
1600<t>
1601   The Expires value is an HTTP-date timestamp, as defined in &http-date;.
1602</t>
1603<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1604  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1605</artwork></figure>
1606<figure>
1607  <preamble>For example</preamble>
1608<artwork type="example">
1609  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1610</artwork></figure>
1611<t>
1612   A cache recipient &MUST; interpret invalid date formats, especially the
1613   value "0", as representing a time in the past (i.e., "already expired").
1614</t>
1615<t>
1616   If a response includes a <x:ref>Cache-Control</x:ref> field with
1617   the max-age directive (<xref target="cache-response-directive.max-age"/>),
1618   a recipient &MUST; ignore the Expires field.
1619   Likewise, if a response includes the s-maxage directive
1620   (<xref target="cache-response-directive.s-maxage" />), a shared cache
1621   recipient &MUST; ignore the Expires field. In both these cases, the value
1622   in Expires is only intended for recipients that have not yet implemented
1623   the Cache-Control field.
1624</t>
1625<t>
1626   An origin server without a clock &MUST-NOT; generate an Expires field
1627   unless its value represents a fixed time in the past (always expired)
1628   or its value has been associated with the resource by a system or user
1629   with a reliable clock.
1630</t>
1631<t>
1632   Historically, HTTP required the Expires field-value to be no more than a
1633   year in the future. While longer freshness lifetimes are no longer
1634   prohibited, extremely large values have been demonstrated to cause
1635   problems (e.g., clock overflows due to use of 32-bit integers for
1636   time values), and many caches will evict a response far sooner than
1637   that.
1638</t>
1639</section>
1640
1641<section anchor="header.pragma" title="Pragma">
1642   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1643   <x:anchor-alias value="extension-pragma"/>
1644   <x:anchor-alias value="Pragma"/>
1645   <x:anchor-alias value="pragma-directive"/>
1646<t>
1647   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1648   caches, so that clients can specify a "no-cache" request that they will
1649   understand (as <x:ref>Cache-Control</x:ref> was not defined until HTTP/1.1).
1650   When the Cache-Control header field is also present and understood in a
1651   request, Pragma is ignored.
1652</t>
1653<t>
1654   In HTTP/1.0, Pragma was defined as an extensible field for
1655   implementation-specified directives for recipients. This specification
1656   deprecates such extensions to improve interoperability.
1657</t>
1658<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1659  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1660  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1661  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1662</artwork></figure>
1663<t>
1664   When the <x:ref>Cache-Control</x:ref> header field is not present in a
1665   request, the no-cache request pragma-directive &MUST; have the same effect
1666   on caches as if "Cache-Control: no-cache" were present (see <xref
1667   target="cache-request-directive" />).
1668</t>
1669<t>
1670   When sending a no-cache request, a client ought to include both the pragma
1671   and cache-control directives, unless Cache-Control: no-cache is
1672   purposefully omitted to target other <x:ref>Cache-Control</x:ref> response
1673   directives at HTTP/1.1 caches. For example:
1674</t>
1675<figure>
1676<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1677GET / HTTP/1.1
1678Host: www.example.com
1679Cache-Control: max-age=30
1680Pragma: no-cache
1681
1682</artwork>
1683</figure>
1684<t>
1685   will constrain HTTP/1.1 caches to serve a response no older than 30
1686   seconds, while precluding implementations that do not understand
1687   <x:ref>Cache-Control</x:ref> from serving a cached response.
1688</t>
1689<x:note>
1690   <t>
1691      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1692      specified, it does not provide a reliable replacement for
1693      "Cache-Control: no-cache" in them.
1694   </t>
1695</x:note>
1696</section>
1697
1698<section anchor="header.warning" title="Warning">
1699   <iref item="Warning header field" primary="true" x:for-anchor="" />
1700   <x:anchor-alias value="Warning"/>
1701   <x:anchor-alias value="warning-value"/>
1702   <x:anchor-alias value="warn-agent"/>
1703   <x:anchor-alias value="warn-code"/>
1704   <x:anchor-alias value="warn-date"/>
1705   <x:anchor-alias value="warn-text"/>
1706<t>
1707   The "Warning" header field is used to carry additional information
1708   about the status or transformation of a message that might not be reflected
1709   in the message. This information is typically used to warn about possible
1710   incorrectness introduced by caching operations or transformations applied
1711   to the payload of the message.
1712</t>
1713<t>
1714   Warnings can be used for other purposes, both cache-related and otherwise.
1715   The use of a warning, rather than an error status code, distinguishes these
1716   responses from true failures.
1717</t>
1718<t>
1719   Warning header fields can in general be applied to any message, however some
1720   warn-codes are specific to caches and can only be applied to response
1721   messages.
1722</t>
1723<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
1724  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1725 
1726  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
1727                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1728 
1729  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1730  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1731                  ; the name or pseudonym of the server adding
1732                  ; the Warning header field, for use in debugging
1733  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1734  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1735</artwork></figure>
1736<t>
1737   Multiple warnings can be attached to a response (either by the origin
1738   server or by a cache), including multiple warnings with the same code
1739   number, only differing in warn-text.
1740</t>
1741<t>
1742   When this occurs, the user agent &SHOULD; inform the user of as many of
1743   them as possible, in the order that they appear in the response.
1744</t>
1745<t>
1746   Systems that generate multiple Warning header fields are encouraged to
1747   order them with this user agent behavior in mind. New Warning header fields
1748   are added after any existing Warning header fields.
1749</t>
1750<t>
1751   Warnings are assigned three digit warn-codes. The first digit indicates
1752   whether the Warning is required to be deleted from a stored response after
1753   validation:
1754   <list style="symbols">
1755      <t>1xx Warnings describe the freshness or validation status of the
1756      response, and so &MUST; be deleted by a cache after validation. They can
1757      only be generated by a cache when validating a cached entry, and
1758      &MUST-NOT; be generated in any other situation.</t>
1759      <t>2xx Warnings describe some aspect of the representation that is not
1760      rectified by a validation (for example, a lossy compression of the
1761      representation) and &MUST-NOT; be deleted by a cache after validation,
1762      unless a full response is sent, in which case they &MUST; be.</t>
1763   </list>
1764</t>
1765<t>
1766   If an implementation sends a message with one or more Warning header fields
1767   to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST;
1768   include in each warning-value a warn-date that matches the
1769   <x:ref>Date</x:ref> header field in the message.
1770</t>
1771<t>
1772   If a system receives a message with a warning-value that includes a
1773   warn-date, and that warn-date is different from the <x:ref>Date</x:ref>
1774   value in the response, then that warning-value &MUST; be deleted from the
1775   message before storing, forwarding, or using it. (preventing the
1776   consequences of naive caching of Warning header fields.) If all of the
1777   warning-values are deleted for this reason, the Warning header field &MUST;
1778   be deleted as well.
1779</t>
1780<t>
1781   The following warn-codes are defined by this specification, each with a
1782   recommended warn-text in English, and a description of its meaning.
1783</t>
1784
1785<section title="110 Response is Stale" anchor="warn.110">
1786  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1787<t>
1788   A cache &SHOULD; generate this whenever the sent response is stale.
1789</t>
1790</section>
1791
1792<section title="111 Revalidation Failed" anchor="warn.111">
1793  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1794<t>
1795   A cache &SHOULD; generate this when sending a stale response because an
1796   attempt to validate the response failed, due to an inability to reach
1797   the server.
1798</t>
1799</section>
1800
1801<section title="112 Disconnected Operation" anchor="warn.112">
1802  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1803<t>
1804   A cache &SHOULD; generate this if it is intentionally disconnected from
1805   the rest of the network for a period of time.
1806</t>
1807</section>
1808
1809<section title="113 Heuristic Expiration" anchor="warn.113">
1810  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1811<t>
1812   A cache &SHOULD; generate this if it heuristically chose a freshness
1813   lifetime greater than 24 hours and the response's age is greater than 24
1814   hours.
1815</t>
1816</section>
1817
1818<section title="199 Miscellaneous Warning" anchor="warn.199">
1819  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
1820<t>
1821   The warning text can include arbitrary information to be presented to
1822   a human user, or logged. A system receiving this warning &MUST-NOT; take
1823   any automated action, besides presenting the warning to the user.
1824</t>
1825</section>
1826
1827<section title="214 Transformation Applied" anchor="warn.214">
1828  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
1829<t>
1830   &MUST; be added by a proxy if it applies any transformation to the
1831   representation, such as changing the content-coding, media-type, or
1832   modifying the representation data, unless this Warning code already appears
1833   in the response.
1834</t>
1835</section>
1836
1837<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1838  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
1839<t>
1840   The warning text can include arbitrary information to be presented to
1841   a human user, or logged. A system receiving this warning &MUST-NOT; take
1842   any automated action.
1843</t>
1844</section>
1845
1846<section title="Warn Code Extensions" anchor="warn.code.extensions">
1847<t>
1848   The HTTP Warn Code Registry defines the name space for warn codes.
1849</t>
1850<t>
1851   A registration &MUST; include the following fields:
1852   <list style="symbols">
1853      <t>Warn Code (3 digits)</t>
1854      <t>Short Description</t>
1855      <t>Pointer to specification text</t>
1856   </list>
1857</t>
1858<t>
1859   Values to be added to this name space require IETF Review (see <xref
1860   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1861</t>
1862<t>
1863   The registry itself is maintained at <eref
1864   target="http://www.iana.org/assignments/http-warn-codes"/>.
1865</t>
1866</section>
1867</section>
1868</section>
1869
1870<section anchor="history.lists" title="History Lists">
1871<t>
1872   User agents often have history mechanisms, such as "Back" buttons and
1873   history lists, that can be used to redisplay a representation retrieved
1874   earlier in a session.
1875</t>
1876<t>
1877   The freshness model (<xref target="expiration.model"/>) does not
1878   necessarily apply to history mechanisms. I.e., a history mechanism can
1879   display a previous representation even if it has expired.
1880</t>
1881<t>
1882   This does not prohibit the history mechanism from telling the user that a
1883   view might be stale, or from honoring cache directives (e.g.,
1884   Cache-Control: no-store).
1885</t>
1886</section>
1887
1888
1889<section anchor="IANA.considerations" title="IANA Considerations">
1890
1891<section title="Cache Directive Registry"
1892   anchor="cache.directive.registration">
1893<t>
1894   The registration procedure for HTTP Cache Directives is defined by <xref
1895   target="cache.control.extensions"/> of this document.
1896</t>
1897<t>
1898   The HTTP Cache Directive Registry shall be created at <eref
1899   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1900   populated with the registrations below:
1901</t>
1902<?BEGININC p6-cache.cache-directives ?>
1903<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1904<texttable xmlns:my="#my" align="left" suppress-title="true"
1905           anchor="iana.cache.directive.registration.table">
1906   <ttcol>Cache Directive</ttcol>
1907   <ttcol>Reference</ttcol>
1908
1909   <c>max-age</c>
1910   <c>
1911      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1912   </c>
1913   <c>max-stale</c>
1914   <c>
1915      <xref target="cache-request-directive.max-stale"/>
1916   </c>
1917   <c>min-fresh</c>
1918   <c>
1919      <xref target="cache-request-directive.min-fresh"/>
1920   </c>
1921   <c>must-revalidate</c>
1922   <c>
1923      <xref target="cache-response-directive.must-revalidate"/>
1924   </c>
1925   <c>no-cache</c>
1926   <c>
1927      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1928   </c>
1929   <c>no-store</c>
1930   <c>
1931      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1932   </c>
1933   <c>no-transform</c>
1934   <c>
1935      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1936   </c>
1937   <c>only-if-cached</c>
1938   <c>
1939      <xref target="cache-request-directive.only-if-cached"/>
1940   </c>
1941   <c>private</c>
1942   <c>
1943      <xref target="cache-response-directive.private"/>
1944   </c>
1945   <c>proxy-revalidate</c>
1946   <c>
1947      <xref target="cache-response-directive.proxy-revalidate"/>
1948   </c>
1949   <c>public</c>
1950   <c>
1951      <xref target="cache-response-directive.public"/>
1952   </c>
1953   <c>s-maxage</c>
1954   <c>
1955      <xref target="cache-response-directive.s-maxage"/>
1956   </c>
1957   <c>stale-if-error</c>
1958   <c>
1959      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1960   </c>
1961   <c>stale-while-revalidate</c>
1962   <c>
1963      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1964   </c>
1965</texttable>
1966<!--(END)-->
1967<?ENDINC p6-cache.cache-directives ?>
1968</section>
1969
1970<section title="Warn Code Registry"
1971   anchor="warn.code.registration">
1972<t>
1973   The registration procedure for HTTP Warn Codes is defined by <xref
1974   target="warn.code.extensions"/> of this document.
1975</t>
1976<t>
1977   The HTTP Warn Code Registry shall be created at <eref
1978   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1979   populated with the registrations below:
1980</t>
1981<?BEGININC p6-cache.iana-warn-codes ?>
1982<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
1983<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
1984   <ttcol>Warn Code</ttcol>
1985   <ttcol>Short Description</ttcol>
1986   <ttcol>Reference</ttcol>
1987   <c>110</c>
1988   <c>Response is Stale</c>
1989   <c>
1990      <xref target="warn.110"/>
1991   </c>
1992   <c>111</c>
1993   <c>Revalidation Failed</c>
1994   <c>
1995      <xref target="warn.111"/>
1996   </c>
1997   <c>112</c>
1998   <c>Disconnected Operation</c>
1999   <c>
2000      <xref target="warn.112"/>
2001   </c>
2002   <c>113</c>
2003   <c>Heuristic Expiration</c>
2004   <c>
2005      <xref target="warn.113"/>
2006   </c>
2007   <c>199</c>
2008   <c>Miscellaneous Warning</c>
2009   <c>
2010      <xref target="warn.199"/>
2011   </c>
2012   <c>214</c>
2013   <c>Transformation Applied</c>
2014   <c>
2015      <xref target="warn.214"/>
2016   </c>
2017   <c>299</c>
2018   <c>Miscellaneous Persistent Warning</c>
2019   <c>
2020      <xref target="warn.299"/>
2021   </c>
2022</texttable>
2023<!--(END)-->
2024<?ENDINC p6-cache.iana-warn-codes ?>
2025</section>
2026
2027<section title="Header Field Registration" anchor="header.field.registration">
2028<t>
2029  The Message Header Field Registry located at <eref 
2030  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2031  shall be updated with the permanent registrations below (see <xref target="BCP90" />):
2032</t>
2033<?BEGININC p6-cache.iana-headers ?>
2034<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2035<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2036   <ttcol>Header Field Name</ttcol>
2037   <ttcol>Protocol</ttcol>
2038   <ttcol>Status</ttcol>
2039   <ttcol>Reference</ttcol>
2040
2041   <c>Age</c>
2042   <c>http</c>
2043   <c>standard</c>
2044   <c>
2045      <xref target="header.age"/>
2046   </c>
2047   <c>Cache-Control</c>
2048   <c>http</c>
2049   <c>standard</c>
2050   <c>
2051      <xref target="header.cache-control"/>
2052   </c>
2053   <c>Expires</c>
2054   <c>http</c>
2055   <c>standard</c>
2056   <c>
2057      <xref target="header.expires"/>
2058   </c>
2059   <c>Pragma</c>
2060   <c>http</c>
2061   <c>standard</c>
2062   <c>
2063      <xref target="header.pragma"/>
2064   </c>
2065   <c>Warning</c>
2066   <c>http</c>
2067   <c>standard</c>
2068   <c>
2069      <xref target="header.warning"/>
2070   </c>
2071</texttable>
2072<!--(END)-->
2073<?ENDINC p6-cache.iana-headers ?>
2074<t>
2075   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2076   Force".
2077</t>
2078</section>
2079</section>
2080
2081<section anchor="security.considerations" title="Security Considerations">
2082<t>
2083   This section is meant to inform developers, information providers, and
2084   users of known security concerns specific to HTTP/1.1 caching.
2085   More general security considerations are addressed in HTTP messaging
2086   &messaging; and semantics &semantics;.
2087</t>
2088<t>
2089   Caches expose additional potential vulnerabilities, since the contents of
2090   the cache represent an attractive target for malicious exploitation.
2091   Because cache contents persist after an HTTP request is complete, an attack
2092   on the cache can reveal information long after a user believes that the
2093   information has been removed from the network. Therefore, cache contents
2094   need to be protected as sensitive information.
2095</t>
2096<t>
2097   Furthermore, the very use of a cache can bring about privacy concerns. For
2098   example, if two users share a cache, and the first one browses to a site,
2099   the second may be able to detect that the other has been to that site,
2100   because the resources from it load more quickly, thanks to the cache.
2101</t>
2102<t>
2103   Implementation flaws might allow attackers to insert content into a cache
2104   ("cache poisoning"), leading to compromise of clients that trust that
2105   content. Because of their nature, these attacks are difficult to mitigate.
2106</t>
2107<t>
2108   Likewise, implementation flaws (as well as misunderstanding of cache
2109   operation) might lead to caching of sensitive information (e.g.,
2110   authentication credentials) that is thought to be private, exposing it to
2111   unauthorized parties.
2112</t>
2113<t>
2114   Note that the Set-Cookie response header <xref target="RFC6265"/> does not
2115   inhibit caching; a cacheable response with a Set-Cookie header can be (and
2116   often is) used to satisfy subsequent requests to caches. Servers who wish
2117   to control caching of these responses are encouraged to emit appropriate
2118   Cache-Control response headers.
2119</t>
2120
2121</section>
2122
2123<section title="Acknowledgments" anchor="acks">
2124<t>
2125  See &acks;.
2126</t>
2127</section>
2128
2129</middle>
2130
2131<back>
2132<references title="Normative References">
2133
2134  <reference anchor="Part1">
2135    <front>
2136      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2137      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2138        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2139        <address><email>fielding@gbiv.com</email></address>
2140      </author>
2141      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2142        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2143        <address><email>julian.reschke@greenbytes.de</email></address>
2144      </author>
2145      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2146    </front>
2147    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2148    <x:source basename="p1-messaging" href="p1-messaging.xml">
2149      <x:defines>Content-Length</x:defines>
2150      <x:defines>Via</x:defines>
2151    </x:source>
2152  </reference>
2153
2154  <reference anchor="Part2">
2155    <front>
2156      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2157      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2158        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2159        <address><email>fielding@gbiv.com</email></address>
2160      </author>
2161      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2162        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2163        <address><email>julian.reschke@greenbytes.de</email></address>
2164      </author>
2165      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2166    </front>
2167    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2168    <x:source basename="p2-semantics" href="p2-semantics.xml">
2169      <x:defines>2xx (Successful)</x:defines>
2170      <x:defines>200 (OK)</x:defines>
2171      <x:defines>203 (Non-Authoritative Information)</x:defines>
2172      <x:defines>3xx (Redirection)</x:defines>
2173      <x:defines>300 (Multiple Choices)</x:defines>
2174      <x:defines>301 (Moved Permanently)</x:defines>
2175      <x:defines>404 (Not Found)</x:defines>
2176      <x:defines>410 (Gone)</x:defines>
2177      <x:defines>5xx (Server Error)</x:defines>
2178      <x:defines>504 (Gateway Timeout)</x:defines>
2179      <x:defines>Content-Encoding</x:defines>
2180      <x:defines>Content-Location</x:defines>
2181      <x:defines>Content-Type</x:defines>
2182      <x:defines>Date</x:defines>
2183      <x:defines>Location</x:defines>
2184      <x:defines>Vary</x:defines>
2185    </x:source>
2186  </reference>
2187
2188  <reference anchor="Part4">
2189    <front>
2190      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2191      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2192        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2193        <address><email>fielding@gbiv.com</email></address>
2194      </author>
2195      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2196        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2197        <address><email>julian.reschke@greenbytes.de</email></address>
2198      </author>
2199      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2200    </front>
2201    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2202    <x:source basename="p4-conditional" href="p4-conditional.xml">
2203      <x:defines>304</x:defines>
2204      <x:defines>304 (Not Modified)</x:defines>
2205      <x:defines>ETag</x:defines>
2206      <x:defines>If-Modified-Since</x:defines>
2207      <x:defines>If-None-Match</x:defines>
2208      <x:defines>Last-Modified</x:defines>
2209    </x:source>
2210  </reference>
2211
2212  <reference anchor="Part5">
2213    <front>
2214      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2215      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2216        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2217        <address><email>fielding@gbiv.com</email></address>
2218      </author>
2219      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2220        <organization abbrev="W3C">World Wide Web Consortium</organization>
2221        <address><email>ylafon@w3.org</email></address>
2222      </author>
2223      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2224        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2225        <address><email>julian.reschke@greenbytes.de</email></address>
2226      </author>
2227      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2228    </front>
2229    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2230    <x:source basename="p5-range" href="p5-range.xml">
2231      <x:defines>206 (Partial Content)</x:defines>
2232      <x:defines>Content-Range</x:defines>
2233      <x:defines>Range</x:defines>
2234    </x:source>
2235  </reference>
2236
2237  <reference anchor="Part7">
2238    <front>
2239      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2240      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2241        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2242        <address><email>fielding@gbiv.com</email></address>
2243      </author>
2244      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2245        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2246        <address><email>julian.reschke@greenbytes.de</email></address>
2247      </author>
2248      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2249    </front>
2250    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2251    <x:source basename="p7-auth" href="p7-auth.xml">
2252      <x:defines>Authorization</x:defines>
2253    </x:source>
2254  </reference>
2255
2256  <reference anchor="RFC2119">
2257    <front>
2258      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2259      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2260        <organization>Harvard University</organization>
2261        <address><email>sob@harvard.edu</email></address>
2262      </author>
2263      <date month="March" year="1997" />
2264    </front>
2265    <seriesInfo name="BCP" value="14" />
2266    <seriesInfo name="RFC" value="2119" />
2267  </reference>
2268
2269  <reference anchor="RFC5234">
2270    <front>
2271      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2272      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2273        <organization>Brandenburg InternetWorking</organization>
2274        <address>
2275          <email>dcrocker@bbiw.net</email>
2276        </address> 
2277      </author>
2278      <author initials="P." surname="Overell" fullname="Paul Overell">
2279        <organization>THUS plc.</organization>
2280        <address>
2281          <email>paul.overell@thus.net</email>
2282        </address>
2283      </author>
2284      <date month="January" year="2008"/>
2285    </front>
2286    <seriesInfo name="STD" value="68"/>
2287    <seriesInfo name="RFC" value="5234"/>
2288  </reference>
2289 
2290</references>
2291
2292<references title="Informative References">
2293
2294  <reference anchor="RFC1305">
2295    <front>
2296      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2297      <author fullname="David L. Mills" initials="D." surname="Mills">
2298        <organization>University of Delaware, Electrical Engineering Department</organization>
2299        <address><email>mills@udel.edu</email></address>
2300      </author>
2301      <date month="March" year="1992" />
2302    </front>
2303    <seriesInfo name="RFC" value="1305" />
2304  </reference>
2305
2306  <reference anchor="RFC2616">
2307    <front>
2308      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2309      <author fullname="R. Fielding" initials="R." surname="Fielding">
2310        <organization>University of California, Irvine</organization>
2311        <address><email>fielding@ics.uci.edu</email></address>
2312      </author>
2313      <author fullname="J. Gettys" initials="J." surname="Gettys">
2314        <organization>W3C</organization>
2315        <address><email>jg@w3.org</email></address>
2316      </author>
2317      <author fullname="J. Mogul" initials="J." surname="Mogul">
2318        <organization>Compaq Computer Corporation</organization>
2319        <address><email>mogul@wrl.dec.com</email></address>
2320      </author>
2321      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2322        <organization>MIT Laboratory for Computer Science</organization>
2323        <address><email>frystyk@w3.org</email></address>
2324      </author>
2325      <author fullname="L. Masinter" initials="L." surname="Masinter">
2326        <organization>Xerox Corporation</organization>
2327        <address><email>masinter@parc.xerox.com</email></address>
2328      </author>
2329      <author fullname="P. Leach" initials="P." surname="Leach">
2330        <organization>Microsoft Corporation</organization>
2331        <address><email>paulle@microsoft.com</email></address>
2332      </author>
2333      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2334        <organization>W3C</organization>
2335        <address><email>timbl@w3.org</email></address>
2336      </author>
2337      <date month="June" year="1999" />
2338    </front>
2339    <seriesInfo name="RFC" value="2616" />
2340  </reference>
2341
2342  <reference anchor="BCP90">
2343    <front>
2344      <title>Registration Procedures for Message Header Fields</title>
2345      <author fullname="G. Klyne" initials="G." surname="Klyne">
2346        <organization>Nine by Nine</organization>
2347        <address><email>GK-IETF@ninebynine.org</email></address>
2348      </author>
2349      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2350        <organization>BEA Systems</organization>
2351        <address><email>mnot@pobox.com</email></address>
2352      </author>
2353      <author fullname="J. Mogul" initials="J." surname="Mogul">
2354        <organization>HP Labs</organization>
2355        <address><email>JeffMogul@acm.org</email></address>
2356      </author>
2357      <date month="September" year="2004" />
2358    </front>
2359    <seriesInfo name="BCP" value="90" />
2360    <seriesInfo name="RFC" value="3864" />
2361  </reference>
2362
2363  <reference anchor='RFC5226'>
2364    <front>
2365      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2366      <author initials='T.' surname='Narten' fullname='T. Narten'>
2367        <organization>IBM</organization>
2368        <address><email>narten@us.ibm.com</email></address>
2369      </author>
2370      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2371        <organization>Google</organization>
2372        <address><email>Harald@Alvestrand.no</email></address>
2373      </author>
2374      <date year='2008' month='May' />
2375    </front>
2376    <seriesInfo name='BCP' value='26' />
2377    <seriesInfo name='RFC' value='5226' />
2378  </reference>
2379
2380  <reference anchor='RFC5861'>
2381    <front>
2382      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2383      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2384        <organization>Yahoo! Inc.</organization>
2385        <address><email>mnot@yahoo-inc.com</email></address>
2386      </author>
2387      <date month="April" year="2010"/>
2388    </front>
2389    <seriesInfo name='RFC' value='5861' />
2390  </reference>
2391
2392  <reference anchor="RFC6265">
2393    <front>
2394      <title>HTTP State Management Mechanism</title>
2395      <author initials="A." surname="Barth" fullname="Adam Barth">
2396        <organization abbrev="U.C. Berkeley">
2397          University of California, Berkeley
2398        </organization>
2399        <address><email>abarth@eecs.berkeley.edu</email></address>
2400      </author>
2401      <date year="2011" month="April" />
2402    </front>
2403    <seriesInfo name="RFC" value="6265"/>
2404  </reference>
2405
2406</references>
2407
2408<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2409<t>
2410  Caching-related text has been substantially rewritten for clarity.
2411</t>
2412<t>
2413  The algorithm for calculating age is now less conservative.
2414  (<xref target="age.calculations"/>)
2415</t>
2416<t>
2417  Caches are now required to handle dates with timezones as if they're
2418  invalid, because it's not possible to accurately guess.
2419  (<xref target="age.calculations"/>)
2420</t>
2421<t>
2422  The <x:ref>Content-Location</x:ref> response header field is no longer used
2423  to determine the appropriate response to use when validating.
2424  (<xref target="validation.model" />)
2425</t>
2426<t>
2427  The algorithm for selecting a cached negotiated response to use has been
2428  clarified in several ways. In particular, it now explicitly allows
2429  header-specific canonicalization when processing selecting header fields.
2430  (<xref target="caching.negotiated.responses" />)
2431</t>
2432<t>
2433  Requirements regarding denial of service attack avoidance when performing
2434  invalidation have been clarified.
2435  (<xref target="invalidation.after.updates.or.deletions" />)
2436</t>
2437<t>
2438  Cache invalidation only occurs when a successful response is received.
2439  (<xref target="invalidation.after.updates.or.deletions" />)
2440</t>
2441<t>
2442  The conditions under which an authenticated response can be cached have been
2443  clarified.
2444  (<xref target="caching.authenticated.responses" />)
2445</t>
2446<t>
2447  The one-year limit on Expires header field values has been removed; instead,
2448  the reasoning for using a sensible value is given.
2449  (<xref target="header.expires" />)
2450</t>
2451<t>
2452  The Pragma header field is now only defined for backwards compatibility;
2453  future pragmas are deprecated.
2454  (<xref target="header.pragma" />)
2455</t>
2456<t>
2457  Cache directives are explicitly defined to be case-insensitive.
2458  (<xref target="header.cache-control" />)
2459</t>
2460<t>
2461  Handling of multiple instances of cache directives when only one is
2462  expected is now defined.
2463  (<xref target="header.cache-control" />)
2464</t>
2465<t>
2466  The qualified forms of the private and no-cache cache directives are noted
2467  to not be widely implemented; e.g., "private=foo" is interpreted by many
2468  caches as simply "private". Additionally, the meaning of the qualified
2469  form of no-cache has been clarified.
2470  (<xref target="cache-response-directive" />)
2471</t>
2472<t>
2473  The "no-store" cache request directive doesn't apply to responses; i.e.,
2474  a cache can satisfy a request with no-store on it, and does not invalidate
2475  it.
2476  (<xref target="cache-request-directive.no-store" />)
2477</t>
2478<t>
2479  The "no-cache" response cache directive's meaning has been clarified.
2480  (<xref target="cache-response-directive.no-cache" />)
2481</t>
2482<t>
2483  New status codes can now define that caches are allowed to use heuristic
2484  freshness with them.
2485  (<xref target="heuristic.freshness" />)
2486</t>
2487<t>
2488  Caches are now allow to calculate heuristic freshness for URLs with query
2489  components.
2490  (<xref target="heuristic.freshness" />)
2491</t>
2492<t>
2493  Some requirements regarding production of the <x:ref>Warning</x:ref>
2494  header have been relaxed, as it is not widely implemented.
2495  (<xref target="header.warning" />)
2496</t>
2497<t>
2498  The <x:ref>Warning</x:ref> header field no longer uses RFC 2047 encoding,
2499  nor allows multiple languages, as these aspects were not implemented.
2500  (<xref target="header.warning" />)
2501</t>
2502<t>
2503  This specification introduces the Cache Directive and Warn Code Registries,
2504  and defines considerations for new cache directives.
2505  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
2506</t>
2507</section>
2508
2509<section title="Imported ABNF" anchor="imported.abnf">
2510   <x:anchor-alias value="ALPHA"/>
2511   <x:anchor-alias value="CR"/>
2512   <x:anchor-alias value="DIGIT"/>
2513   <x:anchor-alias value="DQUOTE"/>
2514   <x:anchor-alias value="LF"/>
2515   <x:anchor-alias value="OCTET"/>
2516   <x:anchor-alias value="SP"/>
2517   <x:anchor-alias value="VCHAR"/>
2518   <x:anchor-alias value="quoted-string"/>
2519   <x:anchor-alias value="token"/>
2520   <x:anchor-alias value="OWS"/>
2521   <x:anchor-alias value="field-name"/>
2522   <x:anchor-alias value="HTTP-date"/>
2523   <x:anchor-alias value="port"/>
2524   <x:anchor-alias value="pseudonym"/>
2525   <x:anchor-alias value="uri-host"/>
2526<t>
2527   The following core rules are included by reference, as defined in <xref
2528   target="RFC5234" x:fmt="of" x:sec="B.1"/>: ALPHA (letters), CR (carriage
2529   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2530   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2531   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2532</t>
2533<t>
2534   The rules below are defined in <xref target="Part1"/>:
2535</t>
2536<figure><artwork type="abnf2616">
2537  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
2538  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
2539  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
2540  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
2541
2542  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
2543  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
2544  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
2545</artwork></figure>
2546<t>
2547   The rules below are defined in other parts:
2548</t>
2549<figure><artwork type="abnf2616">
2550  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
2551</artwork></figure>
2552</section>
2553
2554<?BEGININC p6-cache.abnf-appendix ?>
2555<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2556<figure>
2557<artwork type="abnf" name="p6-cache.parsed-abnf">
2558<x:ref>Age</x:ref> = delta-seconds
2559
2560<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2561 cache-directive ] )
2562
2563<x:ref>Expires</x:ref> = HTTP-date
2564
2565<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
2566
2567<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
2568
2569<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2570 pragma-directive ] )
2571
2572<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2573 )
2574
2575<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2576
2577<x:ref>delta-seconds</x:ref> = 1*DIGIT
2578
2579<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2580
2581<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2582
2583<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2584<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2585<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 5.7.1&gt;
2586
2587<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.6&gt;
2588
2589<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.6&gt;
2590
2591<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2592
2593<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2594<x:ref>warn-code</x:ref> = 3DIGIT
2595<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2596<x:ref>warn-text</x:ref> = quoted-string
2597<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2598 ]
2599</artwork>
2600</figure>
2601</section>
2602<?ENDINC p6-cache.abnf-appendix ?>
2603
2604<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2605<t>
2606  Changes up to the first Working Group Last Call draft are summarized
2607  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2608</t>
2609
2610<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2611<t>
2612  Closed issues:
2613  <list style="symbols">
2614    <t>
2615      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2616      "untangle Cache-Control ABNF"
2617    </t>
2618    <t>
2619      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2620      "Multiple values in Cache-Control header fields"
2621    </t>
2622    <t>
2623      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2624      "Case sensitivity of header fields in CC values"
2625    </t>
2626    <t>
2627      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2628      "Spurious 'MAYs'"
2629    </t>
2630    <t>
2631      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2632      "enhance considerations for new cache control directives"
2633    </t>
2634    <t>
2635      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2636      "ABNF requirements for recipients"
2637    </t>
2638    <t>
2639      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2640      "note introduction of new IANA registries as normative changes"
2641    </t>
2642    <t>
2643      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
2644      "broken prose in description of 'Vary'"
2645    </t>
2646  </list>
2647</t>
2648</section>
2649
2650<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
2651<t>
2652  Closed issues:
2653  <list style="symbols">
2654    <t>
2655      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
2656      "'Most Conservative'"
2657    </t>
2658  </list>
2659</t>
2660<t>
2661  Other changes:
2662  <list style="symbols">
2663    <t>
2664      Conformance criteria and considerations regarding error handling are
2665      now defined in Part 1.
2666    </t>
2667    <t>
2668      Move definition of "Vary" header field into Part 2.
2669    </t>
2670    <t>
2671      Add security considerations with respect to cache poisoning and
2672      the "Set-Cookie" header field.
2673    </t>
2674  </list>
2675</t>
2676</section>
2677
2678<section title="Since draft-ietf-httpbis-p6-cache-21" anchor="changes.since.21">
2679<t>
2680  Closed issues:
2681  <list style="symbols">
2682    <t>
2683      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2684      "Allowing heuristic caching for new status codes"
2685    </t>
2686    <t>
2687      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
2688      "304 without validator"
2689    </t>
2690    <t>
2691      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
2692      "No-Transform"
2693    </t>
2694    <t>
2695      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/430"/>:
2696      "Revert prior change to the meaning of the public cache response
2697       directive.
2698    </t>
2699  </list>
2700</t>
2701</section>
2702
2703<!--<section title="Since draft-ietf-httpbis-p6-cache-22" anchor="changes.since.22">
2704<t>
2705  None yet.
2706</t>
2707</section>-->
2708</section>
2709  </back>
2710</rfc>
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