source: draft-ietf-httpbis/latest/p6-cache.xml @ 2385

Last change on this file since 2385 was 2385, checked in by julian.reschke@…, 7 years ago

Note changes for [2384] (see #486)

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 115.4 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "September">
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 abnf-extension              "<xref target='Part1' x:rel='#abnf.extension' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY whitespace                  "<xref target='Part1' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY field-components            "<xref target='Part1' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY conditional-precedence      "<xref target='Part4' x:rel='#precedence' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY http-date                   "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-date                 "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-etag                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-if-match             "<xref target='Part4' x:rel='#header.if-match' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-if-none-match        "<xref target='Part4' x:rel='#header.if-none-match' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-if-modified-since    "<xref target='Part4' x:rel='#header.if-modified-since' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-if-unmodified-since  "<xref target='Part4' x:rel='#header.if-unmodified-since' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-if-range             "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY header-vary                 "<xref target='Part2' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY GET                         "<xref target='Part2' x:rel='#GET' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY transformations             "<xref target='Part1' x:rel='#message.transformations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56]>
57<?rfc toc="yes" ?>
58<?rfc symrefs="yes" ?>
59<?rfc sortrefs="yes" ?>
60<?rfc compact="yes"?>
61<?rfc subcompact="no" ?>
62<?rfc linkmailto="no" ?>
63<?rfc editing="no" ?>
64<?rfc comments="yes"?>
65<?rfc inline="yes"?>
66<?rfc rfcedstyle="yes"?>
67<?rfc-ext allow-markup-in-artwork="yes" ?>
68<?rfc-ext include-references-in-index="yes" ?>
69<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
70  obsoletes="2616" x:maturity-level="proposed" xmlns:x="http://purl.org/net/xml2rfc/ext">
71<x:link rel="prev" basename="p5-range"/>
72<x:link rel="next" basename="p7-auth"/>
73<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
74<front>
75
76  <title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
77
78  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
79    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
80    <address>
81      <postal>
82        <street>345 Park Ave</street>
83        <city>San Jose</city>
84        <region>CA</region>
85        <code>95110</code>
86        <country>USA</country>
87      </postal>
88      <email>fielding@gbiv.com</email>
89      <uri>http://roy.gbiv.com/</uri>
90    </address>
91  </author>
92
93  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
94    <organization>Akamai</organization>
95    <address>
96      <email>mnot@mnot.net</email>
97      <uri>http://www.mnot.net/</uri>
98    </address>
99  </author>
100
101  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
102    <organization abbrev="greenbytes">greenbytes GmbH</organization>
103    <address>
104      <postal>
105        <street>Hafenweg 16</street>
106        <city>Muenster</city><region>NW</region><code>48155</code>
107        <country>Germany</country>
108      </postal>
109      <email>julian.reschke@greenbytes.de</email>
110      <uri>http://greenbytes.de/tech/webdav/</uri>
111    </address>
112  </author>
113
114  <date month="&ID-MONTH;" year="&ID-YEAR;" />
115  <workgroup>HTTPbis Working Group</workgroup>
116
117<abstract>
118<t>
119   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
120   distributed, collaborative, hypertext information systems. This document
121   defines requirements on HTTP caches and the associated header fields
122   that control cache behavior or indicate cacheable response messages.
123</t>
124</abstract>
125
126<note title="Editorial Note (To be removed by RFC Editor)">
127  <t>
128    Discussion of this draft takes place on the HTTPBIS working group
129    mailing list (ietf-http-wg@w3.org), which is archived at
130    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
131  </t>
132  <t>
133    The current issues list is at
134    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
135    documents (including fancy diffs) can be found at
136    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
137  </t>
138  <t>
139    The changes in this draft are summarized in <xref target="changes.since.23"/>.
140  </t>
141</note>
142
143   </front>
144   <middle>
145
146<section anchor="caching" title="Introduction">
147<t>
148   HTTP is typically used for distributed information systems, where
149   performance can be improved by the use of response caches. This document
150   defines aspects of HTTP/1.1 related to caching and reusing response
151   messages.
152</t>
153
154<iref item="cache" />
155<t>
156   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
157   subsystem that controls storage, retrieval, and deletion of messages in it.
158   A cache stores cacheable responses in order to reduce the response time and
159   network bandwidth consumption on future, equivalent requests. Any client or
160   server &MAY; employ a cache, though a cache cannot be used by a server that
161   is acting as a tunnel.
162</t>
163<iref item="shared cache" />
164<iref item="private cache" />
165<t anchor="shared.and.non-shared.caches">
166   A <x:dfn>shared cache</x:dfn> is a cache that stores responses to be reused
167   by more than one user; shared caches are usually (but not always) deployed
168   as a part of an intermediary. A <x:dfn>private cache</x:dfn>, in contrast,
169   is dedicated to a single user.
170</t>
171<t>
172   The goal of caching in HTTP/1.1 is to significantly improve performance
173   by reusing a prior response message to satisfy a current request.
174   A stored response is considered "fresh", as defined in
175   <xref target="expiration.model" />, if the response can be reused without
176   "validation" (checking with the origin server to see if the cached response
177   remains valid for this request).  A fresh response can therefore
178   reduce both latency and network overhead each time it is reused.
179   When a cached response is not fresh, it might still be reusable if it can
180   be freshened by validation (<xref target="validation.model" />) or if the
181   origin is unavailable (<xref target="serving.stale.responses" />).
182</t>
183
184<section title="Conformance and Error Handling" anchor="conformance">
185<t>
186   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
187   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
188   document are to be interpreted as described in <xref target="RFC2119"/>.
189</t>
190<t>
191   Conformance criteria and considerations regarding error handling
192   are defined in &conformance;.
193</t>
194</section>
195
196<section title="Syntax Notation" anchor="notation">
197<t>
198   This specification uses the Augmented Backus-Naur Form (ABNF) notation
199   of <xref target="RFC5234"/> with the list rule extension defined in
200   &abnf-extension;. <xref target="imported.abnf"/> describes rules imported from
201   other documents. <xref target="collected.abnf"/> shows the collected ABNF
202   with the list rule expanded.
203</t>
204
205<section title="Delta Seconds" anchor="delta-seconds">
206<t>
207   The delta-seconds rule specifies a non-negative integer, representing time
208   in seconds.
209</t>
210<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
211  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
212</artwork></figure>
213<t>
214   If a cache receives a delta-seconds value larger than the largest
215   positive integer it can represent, or if any of its subsequent calculations
216   overflows, it &MUST; consider the value to be 2147483648
217   (2<x:sup>31</x:sup>). Recipients parsing a delta-seconds value &MUST; use
218   an arithmetic type of at least 31 bits of range, and senders &MUST-NOT;
219   generate delta-seconds with a value greater than 2147483648.
220</t>
221</section>
222
223</section>
224</section>
225
226<section anchor="caching.overview" title="Overview of Cache Operation">
227<iref item="cache entry" />
228<iref item="cache key" />
229<t>
230   Proper cache operation preserves the semantics of HTTP transfers
231   (&semantics;) while eliminating the transfer of information already held
232   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
233   we assume that reusing the cached response is desirable and that such
234   reuse is the default behavior when no requirement or local
235   configuration prevents it.  Therefore, HTTP cache requirements are focused
236   on preventing a cache from either storing a non-reusable response or
237   reusing a stored response inappropriately, rather than mandating that
238   caches always store and reuse particular responses.
239</t>
240<t>
241   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
242   HTTP responses corresponding to prior requests that used the same key. The
243   most common form of cache entry is a successful result of a retrieval
244   request: i.e., a <x:ref>200 (OK)</x:ref> response to a GET request, which
245   contains a representation of the resource identified by the request target
246   (&GET;). However, it is also possible to cache permanent redirects,
247   negative results (e.g., <x:ref>404 (Not Found)</x:ref>),
248   incomplete results (e.g., <x:ref>206 (Partial Content)</x:ref>), and
249   responses to methods other than GET if the method's definition allows such
250   caching and defines something suitable for use as a cache key.
251</t>
252<iref item="cache key" />
253<t>
254   The primary <x:dfn>cache key</x:dfn> consists of the request method and
255   target URI.  However, since HTTP caches in common use today are typically
256   limited to caching responses to GET, many caches simply decline
257   other methods and use only the URI as the primary cache key.
258</t>
259<t>
260   If a request target is subject to content negotiation, its cache entry
261   might consist of multiple stored responses, each differentiated by a
262   secondary key for the values of the original request's selecting header
263   fields (<xref target="caching.negotiated.responses"/>).
264</t>
265</section>
266
267<section anchor="response.cacheability" title="Storing Responses in Caches">
268<t>
269   A cache &MUST-NOT; store a response to any request, unless:
270   <list style="symbols">
271      <t>The request method is understood by the cache and defined as being
272      cacheable, and</t>
273      <t>the response status code is understood by the cache, and</t>
274      <t>the "no-store" cache directive (see <xref
275      target="header.cache-control" />) does not appear in request or response
276      header fields, and</t>
277      <t>the "private" cache response directive (see <xref
278      target="cache-response-directive.private" />) does not appear in the
279      response, if the cache is shared, and</t>
280      <t>the <x:ref>Authorization</x:ref> header field (see
281      &header-authorization;) does not appear in the request, if the cache is
282      shared, unless the response explicitly allows it (see <xref
283      target="caching.authenticated.responses" />), and</t>
284      <t>the response either:
285         <list style="symbols">
286            <t>contains an <x:ref>Expires</x:ref> header field (see
287            <xref target="header.expires"/>), or</t>
288            <t>contains a max-age response cache directive (see <xref
289            target="cache-response-directive.max-age" />), or</t>
290            <t>contains a s-maxage response cache directive (see <xref
291            target="cache-response-directive.s-maxage" />) and the cache is
292            shared, or</t>
293            <t>contains a Cache Control Extension (see <xref
294            target="cache.control.extensions" />) that allows it to be cached,
295            or</t>
296            <t>has a status code that is defined as cacheable
297            (see <xref target="heuristic.freshness" />), or</t>
298            <t>contains a public response cache directive (see <xref
299            target="cache-response-directive.public"/>).</t>
300         </list>
301      </t>
302   </list>
303</t>
304<t>
305   Note that any of the requirements listed above can be overridden by a
306   cache-control extension; see <xref target="cache.control.extensions" />.
307</t>
308<t>
309   In this context, a cache has "understood" a request method or a response
310   status code if it recognizes it and implements all specified
311   caching-related behavior.
312</t>
313<t>
314   Note that, in normal operation, some caches will not store a response that
315   has neither a cache validator nor an explicit expiration time, as such
316   responses are not usually useful to store. However, caches are not
317   prohibited from storing such responses.
318</t>
319
320<section anchor="incomplete.responses" title="Storing Incomplete Responses">
321<t>
322   A response message is considered complete when all of the octets indicated
323   by the message framing (&messaging;) are received prior to the connection
324   being closed. If the request is GET, the response status is <x:ref>200
325   (OK)</x:ref>, and the entire response header block has been received, a
326   cache &MAY; store an incomplete response message body if the cache entry is
327   recorded as incomplete. Likewise, a <x:ref>206 (Partial Content)</x:ref>
328   response &MAY; be stored as if it were an incomplete <x:ref>200
329   (OK)</x:ref> cache entry. However, a cache &MUST-NOT; store incomplete or
330   partial content responses if it does not support the <x:ref>Range</x:ref>
331   and <x:ref>Content-Range</x:ref> header fields or if it does not understand
332   the range units used in those fields.
333</t>
334<t>
335   A cache &MAY; complete a stored incomplete response by making a subsequent
336   range request (&partial;) and combining the successful response with the
337   stored entry, as defined in <xref target="combining.responses"/>. A cache
338   &MUST-NOT; use an incomplete response to answer requests unless the
339   response has been made complete or the request is partial and specifies a
340   range that is wholly within the incomplete response. A cache &MUST-NOT;
341   send a partial response to a client without explicitly marking it as such
342   using the <x:ref>206 (Partial Content)</x:ref> status code.
343</t>
344</section>
345
346
347<section anchor="caching.authenticated.responses"
348   title="Storing Responses to Authenticated Requests">
349<t>
350   A shared cache &MUST-NOT; use a cached response to a request with an
351   <x:ref>Authorization</x:ref> header field (&header-authorization;) to
352   satisfy any subsequent request unless a cache directive that allows such
353   responses to be stored is present in the response.
354</t>
355<t>
356   In this specification, the following <x:ref>Cache-Control</x:ref> response
357   directives (<xref target="cache-response-directive"/>) have such an effect:
358   must-revalidate, public, s-maxage.
359</t>
360<t>
361   Note that cached responses that contain the "must-revalidate" and/or
362   "s-maxage" response directives are not allowed to be served stale (<xref
363   target="serving.stale.responses"/>) by shared caches. In particular, a
364   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
365   used to satisfy a subsequent request without revalidating it on the origin
366   server.
367</t>
368</section>
369
370<section anchor="combining.responses" title="Combining Partial Content">
371<t>
372   A response might transfer only a partial representation if the
373   connection closed prematurely or if the request used one or more Range
374   specifiers (&partial;).  After several such transfers, a cache might have
375   received several ranges of the same representation.  A cache &MAY; combine
376   these ranges into a single stored response, and reuse that response to
377   satisfy later requests, if they all share the same strong validator and
378   the cache complies with the client requirements in &combining-byte-ranges;.
379</t>
380<t>
381   When combining the new response with one or more stored responses, a
382   cache &MUST;:
383   <list style="symbols">
384      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
385         with warn-code 1xx (see <xref target="header.warning" />);</t>
386      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
387         with warn-code 2xx; and,</t>
388      <t>use other header fields provided in the new response, aside
389         from <x:ref>Content-Range</x:ref>, to replace all instances of the
390         corresponding header fields in the stored response.</t>
391   </list>
392</t>
393</section>
394
395</section>
396
397
398<section anchor="constructing.responses.from.caches"
399   title="Constructing Responses from Caches">
400<t>
401   When presented with a request, a cache &MUST-NOT; reuse a stored response,
402   unless:
403   <list style="symbols">
404      <t>The presented effective request URI (&effective-request-uri;) and
405      that of the stored response match, and</t>
406      <t>the request method associated with the stored response allows it to
407      be used for the presented request, and</t>
408      <t>selecting header fields nominated by the stored response (if any)
409      match those presented (see <xref target="caching.negotiated.responses"
410      />), and</t>
411      <t>the presented request does not contain the no-cache pragma (<xref
412      target="header.pragma"/>), nor the no-cache cache directive (<xref
413      target="cache-request-directive"/>), unless the stored response is
414      successfully validated (<xref target="validation.model"/>), and</t>
415      <t>the stored response does not contain the no-cache cache directive
416      (<xref target="cache-response-directive.no-cache"/>), unless it is
417      successfully validated (<xref target="validation.model"/>), and</t>
418      <t>the stored response is either:
419         <list style="symbols">
420            <t>fresh (see <xref target="expiration.model" />), or</t>
421            <t>allowed to be served stale (see <xref
422            target="serving.stale.responses" />), or</t>
423            <t>successfully validated (see <xref target="validation.model"
424            />).</t>
425         </list>
426      </t>
427  </list>
428</t>
429<t>
430   Note that any of the requirements listed above can be overridden by a
431   cache-control extension; see <xref target="cache.control.extensions" />.
432</t>
433<t>
434   When a stored response is used to satisfy a request without validation, a
435   cache &MUST; generate an <x:ref>Age</x:ref> header field (<xref
436   target="header.age"/>), replacing any present in the response with a value
437   equal to the stored response's current_age; see <xref
438   target="age.calculations" />.
439</t>
440<t>
441   A cache &MUST; write through requests with methods that are unsafe
442   (&safe-methods;) to the origin server; i.e., a cache is not allowed to
443   generate a reply to such a request before having forwarded the request and
444   having received a corresponding response.
445</t>
446<t>
447   Also, note that unsafe requests might invalidate already stored responses;
448   see <xref target="invalidation" />.
449</t>
450<t>
451   When more than one suitable response is stored, a cache &MUST; use the
452   most recent response (as determined by the <x:ref>Date</x:ref> header
453   field). It can also forward the request with "Cache-Control: max-age=0" or
454   "Cache-Control: no-cache" to disambiguate which response to use.
455</t>
456<t>
457   A cache that does not have a clock available &MUST-NOT; use stored
458   responses without revalidating them upon every use.
459</t>
460
461<section anchor="caching.negotiated.responses"
462   title="Calculating Secondary Keys with Vary">
463<t>
464   When a cache receives a request that can be satisfied by a stored response
465   that has a <x:ref>Vary</x:ref> header field (&header-vary;),
466   it &MUST-NOT; use that response unless all of the selecting header fields
467   nominated by the Vary header field match in both the original request
468   (i.e., that associated with the stored response), and the presented
469   request.
470</t>
471<t>
472   The selecting header fields from two requests are defined to match if and
473   only if those in the first request can be transformed to those in the
474   second request by applying any of the following:
475   <list style="symbols">
476      <t>
477         adding or removing whitespace, where allowed in the header field's
478         syntax
479      </t>
480      <t>
481         combining multiple header fields with the same field name
482         (see &header-fields;)
483      </t>
484      <t>
485         normalizing both header field values in a way that is known to have
486         identical semantics, according to the header field's specification
487         (e.g., re-ordering field values when order is not significant;
488         case-normalization, where values are defined to be case-insensitive)
489      </t>
490  </list>
491</t>
492<t>
493   If (after any normalization that might take place) a header field is absent
494   from a request, it can only match another request if it is also absent
495   there.
496</t>
497<t>
498   A <x:ref>Vary</x:ref> header field-value of "*" always fails to match.
499</t>
500<t>
501   The stored response with matching selecting header fields is known as the
502   selected response.
503</t>
504<t>
505   If multiple selected responses are available (potentially including
506   responses without a Vary header field), the cache will need to choose one to use.
507   When a selecting header field has a known mechanism for doing so (e.g., qvalues on
508   <x:ref>Accept</x:ref> and similar request header fields), that mechanism &MAY; be
509   used to select preferred responses; of the remainder, the most recent
510   response (as determined by the <x:ref>Date</x:ref> header field) is used, as
511   per <xref target="constructing.responses.from.caches"/>.
512</t>
513<t>
514   If no selected response is available, the cache cannot satisfy the
515   presented request. Typically, it is forwarded to the origin server
516   in a (possibly conditional; see <xref target="validation.model"/>) request.
517</t>
518</section>
519
520<section anchor="expiration.model" title="Freshness">
521<iref item="fresh" />
522<iref item="stale" />
523<t>
524   A <x:dfn>fresh</x:dfn> response is one whose age has not yet exceeded its
525   freshness lifetime. Conversely, a <x:dfn>stale</x:dfn>
526   response is one where it has.
527</t>
528<iref item="freshness lifetime" />
529<iref item="explicit expiration time" />
530<iref item="heuristic expiration time" />
531<t>
532   A response's <x:dfn>freshness lifetime</x:dfn> is the length of time
533   between its generation by the origin server and its expiration time. An
534   <x:dfn>explicit expiration time</x:dfn> is the time at which the origin
535   server intends that a stored response can no longer be used by a cache
536   without further validation, whereas a <x:dfn>heuristic expiration
537   time</x:dfn> is assigned by a cache when no explicit expiration time is
538   available.
539</t>
540<iref item="age" />
541<t>
542   A response's <x:dfn>age</x:dfn> is the time that has passed since it was
543   generated by, or successfully validated with, the origin server. 
544</t>
545<t>
546   When a response is "fresh" in the cache, it can be used to satisfy
547   subsequent requests without contacting the origin server, thereby improving
548   efficiency.
549</t>
550<t>
551   The primary mechanism for determining freshness is for an origin server to
552   provide an explicit expiration time in the future, using either the
553   <x:ref>Expires</x:ref> header field (<xref target="header.expires" />) or
554   the max-age response cache directive (<xref
555   target="cache-response-directive.max-age" />). Generally, origin servers
556   will assign future explicit expiration times to responses in the belief
557   that the representation is not likely to change in a semantically
558   significant way before the expiration time is reached.
559</t>
560<t>
561   If an origin server wishes to force a cache to validate every request, it
562   can assign an explicit expiration time in the past to indicate that the
563   response is already stale. Compliant caches will normally validate a stale
564   cached response before reusing it for subsequent requests (see <xref
565   target="serving.stale.responses" />).
566</t>
567<t>
568   Since origin servers do not always provide explicit expiration times,
569   caches are also allowed to use a heuristic to determine an expiration time
570   under certain circumstances (see <xref target="heuristic.freshness"/>).
571</t>
572<figure>
573<preamble>
574  The calculation to determine if a response is fresh is:
575</preamble>
576<artwork type="code">
577   response_is_fresh = (freshness_lifetime &gt; current_age)
578</artwork>
579</figure>
580<t>
581   freshness_lifetime is defined in <xref
582   target="calculating.freshness.lifetime" />; current_age is defined in
583   <xref target="age.calculations" />.
584</t>
585<t>
586   Clients can send the max-age or min-fresh cache directives in a request to
587   constrain or relax freshness calculations for the corresponding response
588   (<xref target="cache-request-directive" />).
589</t>
590<t>
591   When calculating freshness, to avoid common problems in date parsing:
592</t>
593<t>
594  <list style="symbols">
595     <t>Although all date formats are specified to be case-sensitive,
596        cache recipients &SHOULD; match day, week and timezone names
597        case-insensitively.</t>
598             
599     <t>If a cache recipient's internal implementation of time has less
600        resolution than the value of an HTTP-date, the recipient &MUST;
601        internally represent a parsed <x:ref>Expires</x:ref> date as the
602        nearest time equal to or earlier than the received value.</t>
603
604     <t>Cache recipients &MUST-NOT; allow local time zones to influence the
605        calculation or comparison of an age or expiration time.</t>
606
607     <t>Cache recipients &SHOULD; consider a date with a zone abbreviation
608        other than "GMT" to be invalid for calculating expiration.</t>
609  </list>
610</t>
611<t>
612   Note that freshness applies only to cache operation; it cannot be used to
613   force a user agent to refresh its display or reload a resource. See <xref
614   target="history.lists" /> for an explanation of the difference between
615   caches and history mechanisms.
616</t>
617
618<section anchor="calculating.freshness.lifetime"
619   title="Calculating Freshness Lifetime">
620<t>
621   A cache can calculate the freshness lifetime (denoted as
622   freshness_lifetime) of a response by using the first match of:
623   <list style="symbols">
624      <t>If the cache is shared and the s-maxage response cache directive
625      (<xref target="cache-response-directive.s-maxage" />) is present, use its value,
626      or</t>
627      <t>If the max-age response cache directive (<xref
628      target="cache-response-directive.max-age" />) is present, use its value, or</t>
629      <t>If the <x:ref>Expires</x:ref> response header field
630      (<xref target="header.expires" />) is present, use its value minus the
631      value of the <x:ref>Date</x:ref> response header field, or</t>
632      <t>Otherwise, no explicit expiration time is present in the response. A
633      heuristic freshness lifetime might be applicable; see <xref
634      target="heuristic.freshness" />.</t>
635   </list>
636</t>
637<t>
638   Note that this calculation is not vulnerable to clock skew, since all of
639   the information comes from the origin server.
640</t>
641<t>
642   When there is more than one value present for a given directive (e.g., two
643   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
644   directives), the directive's value is considered invalid. Caches are
645   encouraged to consider responses that have invalid freshness information to
646   be stale.
647</t>
648</section>
649
650<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
651<t>
652   Since origin servers do not always provide explicit expiration times, a
653   cache &MAY; assign a heuristic expiration time when an explicit time is not
654   specified, employing algorithms that use other header field values (such as
655   the <x:ref>Last-Modified</x:ref> time) to estimate a plausible expiration
656   time. This specification does not provide specific algorithms, but does
657   impose worst-case constraints on their results.
658</t>
659<t>
660   A cache &MUST-NOT; use heuristics to determine freshness when an explicit
661   expiration time is present in the stored response. Because of the
662   requirements in <xref target="response.cacheability"/>, this means that,
663   effectively, heuristics can only be used on responses without explicit
664   freshness whose status codes are defined as cacheable, and responses
665   without explicit freshness that have been marked as explicitly cacheable
666   (e.g., with a "public" response cache directive).
667</t>
668<t>
669   If the response has a <x:ref>Last-Modified</x:ref> header field
670   (&header-last-modified;), caches are encouraged to use a heuristic
671   expiration value that is no more than some fraction of the interval since
672   that time. A typical setting of this fraction might be 10%.
673</t>
674<t>
675   When a heuristic is used to calculate freshness lifetime, a cache &SHOULD;
676   generate a <x:ref>Warning</x:ref> header field with a 113 warn-code in the
677   response if its current_age is more than 24 hours and such a warning is not
678   already present.
679</t>
680<x:note>
681   <t>
682      &Note; <xref target="RFC2616" x:fmt="of" x:sec="13.9"/> prohibited caches
683      from calculating heuristic freshness for URIs with query components
684      (i.e., those containing '?'). In practice, this has not been widely
685      implemented. Therefore, origin servers are encouraged to send explicit
686      directives (e.g., Cache-Control: no-cache) if they wish to preclude
687      caching.
688   </t>
689</x:note>
690</section>
691
692<section anchor="age.calculations" title="Calculating Age">
693<t>
694   The <x:ref>Age</x:ref> header field is used to convey an estimated
695   age of the response message when obtained from a cache. The Age field value
696   is the cache's estimate of the number of seconds since the response was
697   generated or validated by the origin server. In essence, the Age value is
698   the sum of the time that the response has been resident in each of the
699   caches along the path from the origin server, plus the amount of time it
700   has been in transit along network paths.
701</t>
702<t>
703   The following data is used for the age calculation:
704</t>
705<t>
706   <x:dfn>age_value</x:dfn>
707   <list>
708      <t>
709         The term "age_value" denotes the value of the <x:ref>Age</x:ref>
710         header field (<xref target="header.age"/>), in a form appropriate for
711         arithmetic operation; or 0, if not available.
712      </t>
713   </list>
714</t>
715<t>
716   <x:dfn>date_value</x:dfn>
717   <list>
718      <t>
719         The term "date_value" denotes the value of
720         the Date header field, in a form appropriate for arithmetic
721         operations. See &header-date; for the definition of the Date header
722         field, and for requirements regarding responses without it.
723      </t>
724   </list>
725</t>
726<t>
727   <x:dfn>now</x:dfn>
728   <list>
729      <t>
730         The term "now" means "the current value of the clock at the host
731         performing the calculation". A host ought to use NTP (<xref
732         target="RFC1305"/>) or some similar protocol to synchronize its
733         clocks to Coordinated Universal Time.
734      </t>
735   </list>
736</t>
737<t>
738   <x:dfn>request_time</x:dfn>
739   <list>
740      <t>
741         The current value of the clock at the host at the time the request
742         resulting in the stored response was made.
743      </t>
744   </list>
745</t>
746<t>
747   <x:dfn>response_time</x:dfn>
748   <list>
749      <t>
750         The current value of the clock at the host at the time the response
751         was received.
752      </t>
753   </list>
754</t>
755<t>
756   A response's age can be calculated in two entirely independent ways:
757   <list style="numbers">
758      <t>the "apparent_age": response_time minus date_value, if the local
759      clock is reasonably well synchronized to the origin server's clock. If
760      the result is negative, the result is replaced by zero.</t>
761      <t>the "corrected_age_value", if all of the caches along the response
762      path implement HTTP/1.1. A cache &MUST; interpret this value relative
763      to the time the request was initiated, not the time that the response
764      was received.</t>
765   </list>
766</t>
767<figure>
768<artwork type="code">
769  apparent_age = max(0, response_time - date_value);
770
771  response_delay = response_time - request_time;
772  corrected_age_value = age_value + response_delay; 
773</artwork>
774</figure>
775<figure>
776<preamble>These are combined as</preamble>
777<artwork type="code">
778  corrected_initial_age = max(apparent_age, corrected_age_value);
779</artwork></figure>
780<t>
781   unless the cache is confident in the value of the <x:ref>Age</x:ref> header
782   field (e.g., because there are no HTTP/1.0 hops in the <x:ref>Via</x:ref>
783   header field), in which case the corrected_age_value &MAY; be used as the
784   corrected_initial_age.
785</t>
786<t>
787   The current_age of a stored response can then be calculated by adding the
788   amount of time (in seconds) since the stored response was last validated by
789   the origin server to the corrected_initial_age.
790</t>
791<figure><artwork type="code">
792  resident_time = now - response_time;
793  current_age = corrected_initial_age + resident_time;
794</artwork></figure>
795</section>
796
797<section anchor="serving.stale.responses" title="Serving Stale Responses">
798<t>
799   A "stale" response is one that either has explicit expiry information or is
800   allowed to have heuristic expiry calculated, but is not fresh according to
801   the calculations in <xref target="expiration.model" />.
802</t>
803<t>
804   A cache &MUST-NOT; generate a stale response if it is prohibited by an
805   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
806   directive, a "must-revalidate" cache-response-directive, or an applicable
807   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
808   target="cache-response-directive"/>).
809</t>
810<t>
811   A cache &MUST-NOT; send stale responses unless it is disconnected
812   (i.e., it cannot contact the origin server or otherwise find a forward
813   path) or doing so is explicitly allowed (e.g., by the max-stale request
814   directive; see <xref target="cache-request-directive" />).
815</t>
816<t>
817   A cache &SHOULD; generate a <x:ref>Warning</x:ref> header field with the
818   110 warn-code (see <xref target="header.warning"/>) in stale responses.
819   Likewise, a cache &SHOULD; generate a 112 warn-code in stale responses
820   if the cache is disconnected.
821</t>
822<t>
823   A cache &SHOULD-NOT; generate a new <x:ref>Warning</x:ref> header field
824   when forwarding a response that does not have an <x:ref>Age</x:ref> header
825   field, even if the response is already stale.  A cache need not validate
826   a response that merely became stale in transit.
827</t>
828</section>
829</section>
830
831<section anchor="validation.model" title="Validation">
832<t>
833   When a cache has one or more stored responses for a requested URI, but
834   cannot serve any of them (e.g., because they are not fresh, or one cannot
835   be selected; see <xref target="caching.negotiated.responses"/>), it can use
836   the conditional request mechanism &conditional; in the forwarded request to
837   give the next inbound server an opportunity to select a valid stored
838   response to use, updating the stored metadata in the process, or to replace
839   the stored response(s) with a new response. This process is known as
840   "validating" or "revalidating" the stored response.
841</t>
842
843<section anchor="validation.sent" title="Sending a Validation Request"><iref item="validator" />
844<t>
845   When sending a conditional request for cache validation, a cache sends one
846   or more precondition header fields containing <x:dfn>validator</x:dfn>
847   metadata from its stored response(s), which is then compared by recipients
848   to determine whether a stored response is equivalent to a current
849   representation of the resource.
850</t>
851<t>
852   One such validator is the timestamp given in a <x:ref>Last-Modified</x:ref>
853   header field (&header-last-modified;), which can be used in an
854   <x:ref>If-Modified-Since</x:ref> header field for response validation, or
855   in an <x:ref>If-Unmodified-Since</x:ref> or <x:ref>If-Range</x:ref> header
856   field for representation selection (i.e., the client is referring
857   specifically to a previously obtained representation with that timestamp).
858</t>
859<t>
860   Another validator is the entity-tag given in an <x:ref>ETag</x:ref> header
861   field (&header-etag;). One or more entity-tags, indicating one or more
862   stored responses, can be used in an <x:ref>If-None-Match</x:ref> header
863   field for response validation, or in an <x:ref>If-Match</x:ref> or
864   <x:ref>If-Range</x:ref> header field for representation selection (i.e.,
865   the client is referring specifically to one or more previously obtained
866   representations with the listed entity-tags).
867</t>
868</section>
869
870<section anchor="validation.received" title="Handling a Received Validation Request">
871<t>
872   Each client in the request chain may have its own cache, so it is common
873   for a cache at an intermediary or origin server to receive conditional
874   requests from other (outbound) caches. Likewise, some user agents make use
875   of conditional requests to limit data transfers to recently modified
876   representations or to complete the transfer of partially retrieved
877   representations.
878</t>
879<t>
880   If the request semantics can be satisfied with a cached response and a
881   recipient cache has at least one response stored for this primary cache
882   key, the cache &MUST; evaluate received precondition header fields as part
883   of its selection process for determining a suitable response.
884   A cache &MUST-NOT; evaluate received precondition header fields in a
885   request with semantics that cannot be satisfied with a cached response, or
886   for which the cache has no prior stored responses, since such preconditions
887   are intended for some other (inbound) server.
888</t>
889<t>
890   The proper evaluation of conditional requests by a cache depends on the
891   received precondition header fields and their precedence, as defined in
892   &conditional-precedence;.
893</t>
894<t>
895   A request containing an <x:ref>If-Match</x:ref> header field
896   (&header-if-match;) indicates that the client wants to receive an error
897   response if the cache has a stored response that cannot be used to answer
898   this request.
899   The cache &MUST; generate a <x:ref>412 (Precondition Failed)</x:ref>
900   response if the field-value is "*" and no suitable response is stored, or
901   if the field-value is a list of entity-tags and none of them match the
902   entity-tag of a suitable stored response.
903   Otherwise, the cache &MUST; generate a <x:ref>2xx (Successful)</x:ref>
904   response that reuses the most recent of its matching stored responses to
905   satisfy the request.
906</t>
907<t>
908   A request containing an <x:ref>If-Unmodified-Since</x:ref> header field
909   (&header-if-unmodified-since;) indicates that the client wants to receive
910   an error response if the cache has a stored response that cannot be used
911   to answer this request.
912   The cache &MUST; generate a <x:ref>412 (Precondition Failed)</x:ref>
913   response if one of the following is true:
914   1) a stored response has a <x:ref>Last-Modified</x:ref> field-value that
915   is more recent than the conditional timestamp;
916   2) no <x:ref>Last-Modified</x:ref> field is present, but a stored
917   response has a <x:ref>Date</x:ref> field-value that is more recent than the
918   conditional timestamp; or,
919   3) neither <x:ref>Last-Modified</x:ref> nor <x:ref>Date</x:ref> is present,
920   but the cache recorded a stored response as having been received at a
921   time more recent than the conditional timestamp.
922   Otherwise, the cache &MUST; generate a <x:ref>2xx (Successful)</x:ref>
923   response that reuses the most recent of its matching stored responses to
924   satisfy the request.
925</t>
926<t>
927   A request containing an <x:ref>If-None-Match</x:ref> header field
928   (&header-if-none-match;) indicates that the client wants to validate one
929   or more of its stored responses.
930   If the field-value is "*" and any suitable response is
931   stored, or the field-value is a list of entity-tags and at least one of
932   them match the entity-tag of a suitable stored response, the cache &MUST;
933   generate a <x:ref>304 (Not Modified)</x:ref> response using the most
934   suitable of those matching responses as the selected representation.
935   Otherwise, if the cache has one or more suitable stored responses that
936   do not match the condition, the cache &MUST; generate a
937   <x:ref>2xx (Successful)</x:ref> response that reuses the most suitable of
938   those stored responses to satisfy the request.
939   Finally, if the cache has no suitable stored responses, the cache &SHOULD;
940   forward the conditional request toward the origin server; if the received
941   condition contains a list of entity-tags and the cache has its own set of
942   stored responses for that primary cache key, the cache &SHOULD; take the
943   union of the received set with the set of entity-tags for its own stored
944   set of responses (fresh or stale) and generate an
945   <x:ref>If-None-Match</x:ref> header field containing that union when it
946   forwards the request toward the origin server.
947   However, if a stored responses contains only partial content, the cache
948   &MUST-NOT; include its entity-tag in the union unless the request is for
949   a range that would be fully satisfied by that stored response.
950</t>
951<t>
952   A request containing an <x:ref>If-Modified-Since</x:ref> header field
953   (&header-if-modified-since;) indicates that the client wants to validate
954   one or more of its stored responses by modification date if the stored
955   responses have no entity-tag or the recipient does not implement
956   <x:ref>If-None-Match</x:ref>.
957   The cache &MUST; generate a <x:ref>2xx (Successful)</x:ref> response,
958   reusing the most recent of its stored responses to satisfy the request,
959   if one of the following is true:
960   1) a stored response has a <x:ref>Last-Modified</x:ref> field-value that
961   is more recent than the conditional timestamp;
962   2) no <x:ref>Last-Modified</x:ref> field is present, but a stored
963   response has a <x:ref>Date</x:ref> field-value that is more recent than the
964   conditional timestamp; or,
965   3) neither <x:ref>Last-Modified</x:ref> nor <x:ref>Date</x:ref> is present,
966   but the cache recorded a stored response as having been received at a
967   time more recent than the conditional timestamp.
968   Otherwise, if the cache has one or more suitable stored responses, the
969   cache &MUST; generate a <x:ref>304 (Not Modified)</x:ref> response using
970   the most recent of its suitable stored responses as the selected
971   representation.
972   Finally, if the cache has no suitable stored responses, the cache &SHOULD;
973   forward the conditional request toward the origin server.
974</t>
975<t>
976   A cache that implements partial responses to range requests, as defined in
977   &partial;, also needs to evaluate a received <x:ref>If-Range</x:ref> header
978   field (&header-if-range;) in respect to its selected stored response.
979</t>
980</section>
981
982<section anchor="validation.response" title="Handling a Validation Response">
983<t>
984   Cache handling of a response to a conditional request is dependent upon its
985   status code:
986</t>
987<t>
988   <list style="symbols">
989      <t>
990         A <x:ref>304 (Not Modified)</x:ref> response status code indicates
991         that the stored response can be updated and reused; see <xref
992         target="freshening.responses"/>.
993      </t>
994      <t>
995         A full response (i.e., one with a payload body) indicates that none
996         of the stored responses nominated in the conditional request is
997         suitable. Instead, the cache &MUST; use the full response to
998         satisfy the request and &MAY; replace the stored response(s).
999      </t>
1000      <t>
1001         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
1002         response while attempting to validate a response, it can either
1003         forward this response to the requesting client, or act as if the
1004         server failed to respond. In the latter case, the cache &MAY; send a
1005         previously stored response (see <xref
1006         target="serving.stale.responses" />).
1007      </t>
1008   </list>
1009</t>
1010</section>
1011
1012<section anchor="freshening.responses" title="Freshening Stored Responses upon Validation">
1013<t>
1014   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and
1015   already has one or more stored <x:ref>200 (OK)</x:ref> responses for the
1016   same cache key, the cache needs to identify which of the stored responses
1017   are updated by this new response and then update the stored response(s)
1018   with the new information provided in the <x:ref>304</x:ref> response.
1019</t>
1020<iref item="strong validator" />
1021<t>
1022   The stored response to update is identified by using the first match (if
1023   any) of:
1024   <list style="symbols">
1025    <t>
1026     If the new response contains a <x:dfn>strong validator</x:dfn> (see
1027     &weak-and-strong;), then that strong validator identifies the selected
1028     representation for update. All of the stored responses with the same
1029     strong validator are selected. If none of the stored responses contain the
1030     same strong validator, then the cache &MUST-NOT; use the new response to
1031     update any stored responses.
1032    </t>
1033    <t>
1034     If the new response contains a weak validator and that validator
1035     corresponds to one of the cache's stored responses, then the most
1036     recent of those matching stored responses is selected for update.
1037    </t>
1038    <t>
1039     If the new response does not include any form of validator (such as in
1040     the case where a client generates an If-Modified-Since request from a
1041     source other than the Last-Modified response header field), and there is
1042     only one stored response, and that stored response also lacks a
1043     validator, then that stored response is selected for update.
1044    </t>
1045   </list>
1046</t>
1047<t>
1048   If a stored response is selected for update, the cache &MUST;:
1049   <list style="symbols">
1050      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1051         with warn-code 1xx (see <xref target="header.warning" />);</t>
1052      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1053         with warn-code 2xx; and,</t>
1054      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
1055         response to replace all instances of the corresponding header
1056         fields in the stored response.</t>
1057   </list>
1058</t>
1059</section>
1060
1061<section anchor="head.effects" title="Freshening Responses via HEAD">
1062<t>
1063   A response to the HEAD method is identical to what an equivalent request
1064   made with a GET would have been, except it lacks a body. This property
1065   of HEAD responses can be used to invalidate or update a cached GET
1066   response if the more efficient conditional GET request mechanism is not
1067   available (due to no validators being present in the stored response) or
1068   if transmission of the representation body is not desired even if it has
1069   changed.
1070</t>
1071<t>
1072   If one or more stored GET responses can be selected (as per <xref
1073   target="caching.negotiated.responses"/>) for a HEAD request, and the
1074   <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> or
1075   <x:ref>Last-Modified</x:ref> value of a HEAD response differs from that in a
1076   selected GET response, the cache &MUST; consider that selected response to
1077   be stale.
1078</t>
1079<t>
1080   If the <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> and
1081   <x:ref>Last-Modified</x:ref> values of a HEAD response (when present) are
1082   the same as that in a selected GET response (as per
1083   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update
1084   the remaining header fields in the stored response using the following
1085   rules:
1086   <list style="symbols">
1087      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1088         with warn-code 1xx (see <xref target="header.warning" />);</t>
1089      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1090         with warn-code 2xx; and,</t>
1091      <t>use other header fields provided in the HEAD response to replace
1092         all instances of the corresponding header fields in the stored
1093         response.</t>
1094   </list>
1095</t>
1096</section>
1097</section>
1098
1099
1100<section anchor="invalidation" title="Invalidation">
1101<t>
1102   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1103   have the potential for changing state on the origin server, intervening
1104   caches can use them to keep their contents up-to-date.
1105</t>
1106<t>
1107   A cache &MUST; invalidate the effective Request URI
1108   (&effective-request-uri;) as well as the URI(s) in the
1109   <x:ref>Location</x:ref> and <x:ref>Content-Location</x:ref> response header
1110   fields (if present) when a non-error response to a request with an unsafe
1111   method is received.
1112</t>
1113<t>
1114   However, a cache &MUST-NOT; invalidate a URI from a <x:ref>Location</x:ref>
1115   or <x:ref>Content-Location</x:ref> response header field if the host part of
1116   that URI differs from the host part in the effective request URI
1117   (&effective-request-uri;). This helps prevent denial of service attacks.
1118</t>
1119<t>
1120   A cache &MUST; invalidate the effective request URI
1121   (&effective-request-uri;) when it receives a non-error response
1122   to a request with a method whose safety is unknown.
1123</t>
1124<t>
1125   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref>
1126   or <x:ref>3xx (Redirection)</x:ref> status code. "Invalidate" means that
1127   the cache will either remove all stored responses related to the effective
1128   request URI, or will mark these as "invalid" and in need of a mandatory
1129   validation before they can be sent in response to a subsequent request.
1130</t>
1131<t>
1132   Note that this does not guarantee that all appropriate responses are
1133   invalidated. For example, a state-changing request might invalidate
1134   responses in the caches it travels through, but relevant responses still
1135   might be stored in other caches that it has not.</t>
1136</section>
1137</section>
1138
1139
1140
1141<section anchor="header.field.definitions" title="Header Field Definitions">
1142<t>
1143   This section defines the syntax and semantics of HTTP/1.1 header fields
1144   related to caching.
1145</t>
1146
1147<section anchor="header.age" title="Age">
1148   <iref item="Age header field" primary="true" x:for-anchor="" />
1149   <x:anchor-alias value="Age"/>
1150   <x:anchor-alias value="age-value"/>
1151<t>
1152   The "Age" header field conveys the sender's estimate of the amount
1153   of time since the response was generated or successfully validated at the
1154   origin server. Age values are calculated as specified in <xref
1155   target="age.calculations" />.
1156</t>
1157<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1158  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1159</artwork></figure>
1160<t>
1161   The Age field-value is a non-negative integer, representing time in seconds
1162   (see <xref target="delta-seconds"/>).
1163</t>
1164<t>
1165   The presence of an Age header field implies that the response was not
1166   generated or validated by the origin server for this request. However,
1167   lack of an Age header field does not imply the origin was contacted, since
1168   the response might have been received from an HTTP/1.0 cache that does not
1169   implement Age.
1170</t>
1171</section>
1172
1173<section anchor="header.cache-control" title="Cache-Control">
1174   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1175   <x:anchor-alias value="Cache-Control"/>
1176   <x:anchor-alias value="cache-directive"/>
1177<t>
1178   The "Cache-Control" header field is used to specify directives for
1179   caches along the request/response chain. Such cache directives are
1180   unidirectional in that the presence of a directive in a request does not
1181   imply that the same directive is to be given in the response.
1182</t>
1183<t>
1184   A cache &MUST; obey the requirements of the Cache-Control
1185   directives defined in this section. See <xref
1186   target="cache.control.extensions"/> for information about how Cache-Control
1187   directives defined elsewhere are handled.
1188</t>
1189<x:note>
1190   <t>
1191       &Note; Some HTTP/1.0 caches might not implement Cache-Control.
1192   </t>
1193</x:note>
1194<t>
1195   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1196   through in forwarded messages, regardless of their
1197   significance to that application, since the directives might be applicable
1198   to all recipients along the request/response chain. It is not possible to
1199   target a directive to a specific cache.
1200</t>
1201<t>
1202   Cache directives are identified by a token, to be compared case-insensitively,
1203   and have an optional argument, that can use both token and quoted-string
1204   syntax. For the directives defined below that define arguments, recipients
1205   ought to accept both forms, even if one is documented to be preferred. For
1206   any directive not defined by this specification, recipients &MUST; accept
1207   both forms.
1208</t>
1209<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
1210  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1211
1212  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1213</artwork></figure>
1214<t>
1215   For the cache directives defined below, no argument is defined (nor allowed)
1216   unless stated otherwise.
1217</t>
1218
1219<section title="Request Cache-Control Directives" anchor="cache-request-directive">
1220
1221<section title="max-age" anchor="cache-request-directive.max-age">
1222   <iref item="max-age (cache directive)" primary="true" />
1223<t>
1224   Argument syntax:
1225   <list>
1226      <t>
1227        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1228      </t>
1229   </list>
1230</t>
1231<t>
1232   The "max-age" request directive indicates that the client is unwilling to
1233   accept a response whose age is greater than the specified number of
1234   seconds. Unless the max-stale request directive is also present, the
1235   client is not willing to accept a stale response.
1236</t>
1237<t>
1238   &Note; This directive uses the token form of the argument syntax;
1239   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1240   quoted-string form.
1241</t>
1242</section>
1243
1244<section title="max-stale" anchor="cache-request-directive.max-stale">
1245   <iref item="max-stale (cache directive)" primary="true" />
1246<t>
1247   Argument syntax:
1248   <list>
1249      <t>
1250        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1251      </t>
1252   </list>
1253</t>
1254<t>
1255   The "max-stale" request directive indicates that the client is willing
1256   to accept a response that has exceeded its freshness lifetime. If max-stale
1257   is assigned a value, then the client is willing to accept a response
1258   that has exceeded its freshness lifetime by no more than the specified
1259   number of seconds. If no value is assigned to max-stale, then the client
1260   is willing to accept a stale response of any age.
1261</t>
1262<t>
1263   &Note; This directive uses the token form of the argument syntax;
1264   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1265   quoted-string form.
1266</t>
1267</section>
1268
1269<section title="min-fresh" anchor="cache-request-directive.min-fresh">
1270   <iref item="min-fresh (cache directive)" primary="true" />
1271<t>
1272   Argument syntax:
1273   <list>
1274      <t>
1275        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1276      </t>
1277   </list>
1278</t>
1279<t>
1280   The "min-fresh" request directive indicates that the client is willing
1281   to accept a response whose freshness lifetime is no less than its
1282   current age plus the specified time in seconds. That is, the client
1283   wants a response that will still be fresh for at least the specified
1284   number of seconds.
1285</t>
1286<t>
1287   &Note; This directive uses the token form of the argument syntax;
1288   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1289   quoted-string form.
1290</t>
1291</section>
1292
1293<section title="no-cache" anchor="cache-request-directive.no-cache">
1294   <iref item="no-cache (cache directive)" primary="true" />
1295<t>
1296   The "no-cache" request directive indicates that a cache &MUST-NOT;
1297   use a stored response to satisfy the request without successful
1298   validation on the origin server.
1299</t>
1300</section>
1301 
1302<section title="no-store" anchor="cache-request-directive.no-store">
1303   <iref item="no-store (cache directive)" primary="true" />
1304<t>
1305   The "no-store" request directive indicates that a cache &MUST-NOT;
1306   store any part of either this request or any response to it. This
1307   directive applies to both private and shared caches. "&MUST-NOT;
1308   store" in this context means that the cache &MUST-NOT; intentionally
1309   store the information in non-volatile storage, and &MUST; make a
1310   best-effort attempt to remove the information from volatile storage as
1311   promptly as possible after forwarding it.
1312</t>
1313<t>
1314   This directive is NOT a reliable or sufficient mechanism for ensuring
1315   privacy. In particular, malicious or compromised caches might not
1316   recognize or obey this directive, and communications networks might be
1317   vulnerable to eavesdropping.
1318</t>
1319<t>
1320   Note that if a request containing this directive is satisfied from a
1321   cache, the no-store request directive does not apply to the already
1322   stored response.
1323</t>
1324</section>
1325
1326<section title="no-transform" anchor="cache-request-directive.no-transform">
1327   <iref item="no-transform (cache directive)" primary="true" />
1328<t>
1329   The "no-transform" request directive indicates that an intermediary
1330   (whether or not it implements a cache) &MUST-NOT; transform the payload,
1331   as defined in &transformations;.
1332</t>
1333</section>
1334
1335<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
1336   <iref item="only-if-cached (cache directive)" primary="true" />
1337<t>
1338   The "only-if-cached" request directive indicates that the client only wishes
1339   to obtain a stored response. If it receives this directive, a cache &SHOULD;
1340   either respond using a stored response that is consistent with the other
1341   constraints of the request, or respond with a <x:ref>504 (Gateway
1342   Timeout)</x:ref> status code. If a group of caches is being operated as a
1343   unified system with good internal connectivity, a member cache &MAY;
1344   forward such a request within that group of caches.
1345</t>
1346</section>
1347</section>
1348
1349<section anchor="cache-response-directive"
1350   title="Response Cache-Control Directives">
1351   <x:anchor-alias value="cache-response-directive" />
1352
1353<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
1354   <iref item="must-revalidate (cache directive)" primary="true" />
1355<t>
1356   The "must-revalidate" response directive indicates that once it has
1357   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1358   requests without successful validation on the origin server.
1359</t>
1360<t>
1361   The must-revalidate directive is necessary to support reliable
1362   operation for certain protocol features. In all circumstances a
1363   cache &MUST; obey the must-revalidate directive; in particular,
1364   if a cache cannot reach the origin server for any reason, it &MUST;
1365   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1366</t>
1367<t>
1368   The must-revalidate directive ought to be used by servers if and only
1369   if failure to validate a request on the representation could result in
1370   incorrect operation, such as a silently unexecuted financial
1371   transaction.
1372</t>
1373</section>
1374
1375<section title="no-cache" anchor="cache-response-directive.no-cache">
1376   <iref item="no-cache (cache directive)" primary="true" />
1377<t>
1378   Argument syntax:
1379   <list>
1380      <t>
1381        #<x:ref>field-name</x:ref>
1382      </t>
1383   </list>
1384</t>
1385<t>
1386   The "no-cache" response directive indicates that the response &MUST-NOT;
1387   be used to satisfy a subsequent request without successful validation on
1388   the origin server. This allows an origin server to prevent a cache from
1389   using it to satisfy a request without contacting it, even by caches that
1390   have been configured to send stale responses.
1391</t>
1392<t>
1393   If the no-cache response directive specifies one or more field-names,
1394   then a cache &MAY; use the response to satisfy a subsequent request,
1395   subject to any other restrictions on caching. However, any header fields
1396   in the response that have the field-name(s) listed &MUST-NOT; be sent
1397   in the response to a subsequent request without successful revalidation
1398   with the origin server. This allows an origin server to prevent the
1399   re-use of certain header fields in a response, while still allowing
1400   caching of the rest of the response.
1401</t>
1402<t>
1403   The field-names given are not limited to the set of header
1404   fields defined by this specification. Field names are case-insensitive.
1405</t>
1406<t>
1407   &Note; Although it has been back-ported to many implementations, some
1408   HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache
1409   response directives with field-names are often handled by caches
1410   as if an unqualified no-cache directive was received; i.e., the special
1411   handling for the qualified form is not widely implemented.
1412</t>
1413<t>
1414   &Note; This directive uses the quoted-string form of the argument syntax.
1415   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1416   needed for single-entry lists).
1417</t>
1418</section>
1419
1420<section title="no-store" anchor="cache-response-directive.no-store">
1421   <iref item="no-store (cache directive)" primary="true" />
1422<t>
1423   The "no-store" response directive indicates that a cache &MUST-NOT;
1424   store any part of either the immediate request or response. This
1425   directive applies to both private and shared caches. "&MUST-NOT;
1426   store" in this context means that the cache &MUST-NOT; intentionally
1427   store the information in non-volatile storage, and &MUST; make a
1428   best-effort attempt to remove the information from volatile storage as
1429   promptly as possible after forwarding it.
1430</t>
1431<t>
1432   This directive is NOT a reliable or sufficient mechanism for ensuring
1433   privacy. In particular, malicious or compromised caches might not
1434   recognize or obey this directive, and communications networks might be
1435   vulnerable to eavesdropping.
1436</t>
1437</section>
1438
1439<section title="no-transform" anchor="cache-response-directive.no-transform">
1440   <iref item="no-transform (cache directive)" primary="true" />
1441<t>
1442   The "no-transform" response directive indicates that an intermediary
1443   (regardless of whether it implements a cache) &MUST-NOT; transform the
1444   payload, as defined in &transformations;.
1445</t>
1446</section>
1447
1448<section title="public" anchor="cache-response-directive.public">
1449   <iref item="public (cache directive)" primary="true" />
1450<t>
1451   The "public" response directive indicates that any cache &MAY; store the
1452   response, even if the response would normally be non-cacheable or cacheable
1453   only within a non-shared cache. (See <xref
1454   target="caching.authenticated.responses"/> for additional details related
1455   to the use of public in response to a request containing
1456   <x:ref>Authorization</x:ref>, and <xref target="response.cacheability"/>
1457   for details of how public affects responses that would normally not be
1458   stored, due to their status codes not being defined as cacheable.)
1459</t>
1460</section>
1461
1462<section title="private" anchor="cache-response-directive.private">
1463   <iref item="private (cache directive)" primary="true" />
1464<t>
1465   Argument syntax:
1466   <list>
1467      <t>
1468        #<x:ref>field-name</x:ref>
1469      </t>
1470   </list>
1471</t>
1472<t>
1473   The "private" response directive indicates that the response message is
1474   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1475   private cache &MAY; store the response and reuse it for later requests,
1476   even if the response would normally be non-cacheable.
1477</t>
1478<t>
1479   If the private response directive specifies one or more field-names,
1480   this requirement is limited to the field-values associated with the
1481   listed response header fields. That is, a shared cache &MUST-NOT; store
1482   the specified field-names(s), whereas it &MAY; store the remainder of the
1483   response message.
1484</t>
1485<t>
1486   The field-names given are not limited to the set of header
1487   fields defined by this specification. Field names are case-insensitive.
1488</t>
1489<t>
1490   &Note; This usage of the word "private" only controls
1491   where the response can be stored; it cannot ensure the privacy of the
1492   message content. Also, private response directives with field-names are
1493   often handled by caches as if an unqualified private directive
1494   was received; i.e., the special handling for the qualified form is not
1495   widely implemented.
1496</t>
1497<t>
1498   &Note; This directive uses the quoted-string form of the argument syntax.
1499   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1500   needed for single-entry lists).
1501</t>
1502</section>
1503
1504<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
1505   <iref item="proxy-revalidate (cache directive)" primary="true" />
1506<t>
1507   The "proxy-revalidate" response directive has the same meaning as the
1508   must-revalidate response directive, except that it does not apply to
1509   private caches.
1510</t>
1511</section>
1512
1513<section title="max-age" anchor="cache-response-directive.max-age">
1514   <iref item="max-age (cache directive)" primary="true" />
1515<t>
1516   Argument syntax:
1517   <list>
1518      <t>
1519        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1520      </t>
1521   </list>
1522</t>
1523<t>
1524   The "max-age" response directive indicates that the response is to be
1525   considered stale after its age is greater than the specified number of
1526   seconds.
1527</t>
1528<t>
1529   &Note; This directive uses the token form of the argument syntax;
1530   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1531   quoted-string form.
1532</t>
1533</section>     
1534
1535<section title="s-maxage" anchor="cache-response-directive.s-maxage">
1536   <iref item="s-maxage (cache directive)" primary="true" />
1537<t>
1538   Argument syntax:
1539   <list>
1540      <t>
1541        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1542      </t>
1543   </list>
1544</t>
1545<t>
1546   The "s-maxage" response directive indicates that, in shared caches, the
1547   maximum age specified by this directive overrides the maximum age
1548   specified by either the max-age directive or the <x:ref>Expires</x:ref>
1549   header field. The s-maxage directive also implies the semantics of the
1550   proxy-revalidate response directive.
1551</t>
1552<t>
1553   &Note; This directive uses the token form of the argument syntax;
1554   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1555   quoted-string form.
1556</t>
1557</section>
1558
1559</section>
1560
1561<section anchor="cache.control.extensions" title="Cache Control Extensions">
1562<t>
1563   The Cache-Control header field can be extended through the use of one or
1564   more cache-extension tokens, each with an optional value.
1565</t>
1566<t>
1567   Informational extensions (those that do not require a change in cache
1568   behavior) can be added without changing the semantics of other directives.
1569   Behavioral extensions are designed to work by acting as modifiers to the
1570   existing base of cache directives.
1571</t>
1572<t>  
1573   Both the new directive and the standard directive are supplied, such that
1574   applications that do not understand the new directive will default to the
1575   behavior specified by the standard directive, and those that understand the
1576   new directive will recognize it as modifying the requirements associated
1577   with the standard directive. In this way, extensions to the cache-control
1578   directives can be made without requiring changes to the base protocol.
1579</t>
1580<t>
1581   This extension mechanism depends on an HTTP cache obeying all of the
1582   cache-control directives defined for its native HTTP-version, obeying
1583   certain extensions, and ignoring all directives that it does not
1584   understand.
1585</t>
1586<t>
1587   For example, consider a hypothetical new response directive called
1588   "community" that acts as a modifier to the private directive. We define
1589   this new directive to mean that, in addition to any private cache, any
1590   cache that is shared only by members of the community named within its
1591   value is allowed to cache the response. An origin server wishing to allow
1592   the UCI community to use an otherwise private response in their shared
1593   cache(s) could do so by including
1594</t>
1595<figure><artwork type="example">
1596  Cache-Control: private, community="UCI"
1597</artwork></figure>
1598<t>
1599   A cache seeing this header field will act correctly even if the cache does
1600   not understand the community cache-extension, since it will also see and
1601   understand the private directive and thus default to the safe behavior.
1602</t>
1603<t>
1604   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1605   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1606   combined with standard directives (or the response's default cacheability)
1607   such that the cache behavior will remain minimally correct even if the
1608   cache does not understand the extension(s).
1609</t>
1610</section>
1611
1612</section>
1613
1614<section anchor="header.expires" title="Expires">
1615   <iref item="Expires header field" primary="true" x:for-anchor="" />
1616   <x:anchor-alias value="Expires"/>
1617<t>
1618   The "Expires" header field gives the date/time after which the
1619   response is considered stale. See <xref target="expiration.model" /> for
1620   further discussion of the freshness model.
1621</t>
1622<t>
1623   The presence of an Expires field does not imply that the original resource
1624   will change or cease to exist at, before, or after that time.
1625</t>
1626<t>
1627   The Expires value is an HTTP-date timestamp, as defined in &http-date;.
1628</t>
1629<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1630  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1631</artwork></figure>
1632<figure>
1633  <preamble>For example</preamble>
1634<artwork type="example">
1635  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1636</artwork></figure>
1637<t>
1638   A cache recipient &MUST; interpret invalid date formats, especially the
1639   value "0", as representing a time in the past (i.e., "already expired").
1640</t>
1641<t>
1642   If a response includes a <x:ref>Cache-Control</x:ref> field with
1643   the max-age directive (<xref target="cache-response-directive.max-age"/>),
1644   a recipient &MUST; ignore the Expires field.
1645   Likewise, if a response includes the s-maxage directive
1646   (<xref target="cache-response-directive.s-maxage" />), a shared cache
1647   recipient &MUST; ignore the Expires field. In both these cases, the value
1648   in Expires is only intended for recipients that have not yet implemented
1649   the Cache-Control field.
1650</t>
1651<t>
1652   An origin server without a clock &MUST-NOT; generate an Expires field
1653   unless its value represents a fixed time in the past (always expired)
1654   or its value has been associated with the resource by a system or user
1655   with a reliable clock.
1656</t>
1657<t>
1658   Historically, HTTP required the Expires field-value to be no more than a
1659   year in the future. While longer freshness lifetimes are no longer
1660   prohibited, extremely large values have been demonstrated to cause
1661   problems (e.g., clock overflows due to use of 32-bit integers for
1662   time values), and many caches will evict a response far sooner than
1663   that.
1664</t>
1665</section>
1666
1667<section anchor="header.pragma" title="Pragma">
1668   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1669   <x:anchor-alias value="extension-pragma"/>
1670   <x:anchor-alias value="Pragma"/>
1671   <x:anchor-alias value="pragma-directive"/>
1672<t>
1673   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1674   caches, so that clients can specify a "no-cache" request that they will
1675   understand (as <x:ref>Cache-Control</x:ref> was not defined until HTTP/1.1).
1676   When the Cache-Control header field is also present and understood in a
1677   request, Pragma is ignored.
1678</t>
1679<t>
1680   In HTTP/1.0, Pragma was defined as an extensible field for
1681   implementation-specified directives for recipients. This specification
1682   deprecates such extensions to improve interoperability.
1683</t>
1684<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"/>
1685  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1686  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1687  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1688</artwork></figure>
1689<t>
1690   When the <x:ref>Cache-Control</x:ref> header field is not present in a
1691   request, caches &MUST; consider the no-cache request pragma-directive as
1692   having the same effect as if "Cache-Control: no-cache" were present (see
1693   <xref target="cache-request-directive" />).
1694</t>
1695<t>
1696   When sending a no-cache request, a client ought to include both the pragma
1697   and cache-control directives, unless Cache-Control: no-cache is
1698   purposefully omitted to target other <x:ref>Cache-Control</x:ref> response
1699   directives at HTTP/1.1 caches. For example:
1700</t>
1701<figure>
1702<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1703GET / HTTP/1.1
1704Host: www.example.com
1705Cache-Control: max-age=30
1706Pragma: no-cache
1707
1708</artwork>
1709</figure>
1710<t>
1711   will constrain HTTP/1.1 caches to serve a response no older than 30
1712   seconds, while precluding implementations that do not understand
1713   <x:ref>Cache-Control</x:ref> from serving a cached response.
1714</t>
1715<x:note>
1716   <t>
1717      &Note; Because the meaning of "Pragma: no-cache" in responses is not
1718      specified, it does not provide a reliable replacement for
1719      "Cache-Control: no-cache" in them.
1720   </t>
1721</x:note>
1722</section>
1723
1724<section anchor="header.warning" title="Warning">
1725   <iref item="Warning header field" primary="true" x:for-anchor="" />
1726   <x:anchor-alias value="Warning"/>
1727   <x:anchor-alias value="warning-value"/>
1728   <x:anchor-alias value="warn-agent"/>
1729   <x:anchor-alias value="warn-code"/>
1730   <x:anchor-alias value="warn-date"/>
1731   <x:anchor-alias value="warn-text"/>
1732<t>
1733   The "Warning" header field is used to carry additional information
1734   about the status or transformation of a message that might not be reflected
1735   in the status code. This information is typically used to warn about possible
1736   incorrectness introduced by caching operations or transformations applied
1737   to the payload of the message.
1738</t>
1739<t>
1740   Warnings can be used for other purposes, both cache-related and otherwise.
1741   The use of a warning, rather than an error status code, distinguishes these
1742   responses from true failures.
1743</t>
1744<t>
1745   Warning header fields can in general be applied to any message, however some
1746   warn-codes are specific to caches and can only be applied to response
1747   messages.
1748</t>
1749<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"/>
1750  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1751 
1752  <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>
1753                                        [ <x:ref>SP</x:ref> <x:ref>warn-date</x:ref> ]
1754 
1755  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1756  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1757                  ; the name or pseudonym of the server adding
1758                  ; the Warning header field, for use in debugging
1759  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1760  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1761</artwork></figure>
1762<t>
1763   Multiple warnings can be generated in a response (either by the origin
1764   server or by a cache), including multiple warnings with the same warn-code
1765   number that only differ in warn-text.
1766</t>
1767<t>
1768   A user agent that receives one or more Warning header fields &SHOULD;
1769   inform the user of as many of them as possible, in the order that they
1770   appear in the response. Senders that generate multiple Warning header
1771   fields are encouraged to order them with this user agent behavior in mind.
1772   A sender that generates new Warning header fields &MUST; append them after
1773   any existing Warning header fields.
1774</t>
1775<t>
1776   Warnings are assigned three digit warn-codes. The first digit indicates
1777   whether the Warning is required to be deleted from a stored response after
1778   validation:
1779   <list style="symbols">
1780      <t>1xx warn-codes describe the freshness or validation status of the
1781      response, and so &MUST; be deleted by a cache after validation. They can
1782      only be generated by a cache when validating a cached entry, and
1783      &MUST-NOT; be generated in any other situation.</t>
1784      <t>2xx warn-codes describe some aspect of the representation that is not
1785      rectified by a validation (for example, a lossy compression of the
1786      representation) and &MUST-NOT; be deleted by a cache after validation,
1787      unless a full response is sent, in which case they &MUST; be.</t>
1788   </list>
1789</t>
1790<t>
1791   If a sender generates one or more Warning header fields in a message to be
1792   sent to a recipient known to implement only HTTP/1.0, the sender &MUST;
1793   include in each generated warning-value a warn-date that matches the
1794   <x:ref>Date</x:ref> header field in the message.
1795</t>
1796<t>
1797   If a recipient that uses, evaluates, or displays Warning header fields
1798   receives a warn-date that is different from the <x:ref>Date</x:ref> value
1799   in the same message, the recipient &MUST; exclude the warning-value
1800   containing that warn-date before storing, forwarding, or using the message.
1801   This allows recipients to exclude warning-values that were improperly
1802   retained after a cache validation.
1803   If all of the warning-values are excluded, the recipient &MUST; exclude
1804   the Warning header field as well.
1805</t>
1806<t>
1807   The following warn-codes are defined by this specification, each with a
1808   recommended warn-text in English, and a description of its meaning.
1809   The procedure for defining additional warn codes is described in
1810   <xref target="warn.code.registry.procedure"/>.
1811</t>
1812
1813<section title='Warning: 110 means "Response is Stale"' anchor="warn.110">
1814  <iref primary="true" item="110 (warn-code)" x:for-anchor=""/>
1815  <iref primary="true" item="Response is Stale (warn-text)" x:for-anchor=""/>
1816<t>
1817   A cache &SHOULD; generate this whenever the sent response is stale.
1818</t>
1819</section>
1820
1821<section title='Warning: 111 means "Revalidation Failed"' anchor="warn.111">
1822  <iref primary="true" item="111 (warn-code)" x:for-anchor=""/>
1823  <iref primary="true" item="Revalidation Failed (warn-text)" x:for-anchor=""/>
1824<t>
1825   A cache &SHOULD; generate this when sending a stale response because an
1826   attempt to validate the response failed, due to an inability to reach
1827   the server.
1828</t>
1829</section>
1830
1831<section title='Warning: 112 means "Disconnected Operation"' anchor="warn.112">
1832  <iref primary="true" item="112 (warn-code)" x:for-anchor=""/>
1833  <iref primary="true" item="Disconnected Operation (warn-text)" x:for-anchor=""/>
1834<t>
1835   A cache &SHOULD; generate this if it is intentionally disconnected from
1836   the rest of the network for a period of time.
1837</t>
1838</section>
1839
1840<section title='Warning: 113 means "Heuristic Expiration"' anchor="warn.113">
1841  <iref primary="true" item="113 (warn-code)" x:for-anchor=""/>
1842  <iref primary="true" item="Heuristic Expiration (warn-text)" x:for-anchor=""/>
1843<t>
1844   A cache &SHOULD; generate this if it heuristically chose a freshness
1845   lifetime greater than 24 hours and the response's age is greater than 24
1846   hours.
1847</t>
1848</section>
1849
1850<section title='Warning: 199 means "Miscellaneous Warning"' anchor="warn.199">
1851  <iref primary="true" item="199 (warn-code)" x:for-anchor=""/>
1852  <iref primary="true" item="Miscellaneous Warning (warn-text)" x:for-anchor=""/>
1853<t>
1854   The warning text can include arbitrary information to be presented to
1855   a human user, or logged. A system receiving this warning &MUST-NOT; take
1856   any automated action, besides presenting the warning to the user.
1857</t>
1858</section>
1859
1860<section title='Warning: 214 means "Transformation Applied"' anchor="warn.214">
1861  <iref primary="true" item="214 (warn-code)" x:for-anchor=""/>
1862  <iref primary="true" item="Transformation Applied (warn-text)" x:for-anchor=""/>
1863<t>
1864   &MUST; be added by a proxy if it applies any transformation to the
1865   representation, such as changing the content-coding, media-type, or
1866   modifying the representation data, unless this Warning code already appears
1867   in the response.
1868</t>
1869</section>
1870
1871<section title='Warning: 299 means "Miscellaneous Persistent Warning"' anchor="warn.299">
1872  <iref primary="true" item="299 (warn-code)" x:for-anchor=""/>
1873  <iref primary="true" item="Miscellaneous Persistent Warning (warn-text)" x:for-anchor=""/>
1874<t>
1875   The warning text can include arbitrary information to be presented to
1876   a human user, or logged. A system receiving this warning &MUST-NOT; take
1877   any automated action.
1878</t>
1879</section>
1880</section>
1881</section>
1882
1883<section anchor="history.lists" title="History Lists">
1884<t>
1885   User agents often have history mechanisms, such as "Back" buttons and
1886   history lists, that can be used to redisplay a representation retrieved
1887   earlier in a session.
1888</t>
1889<t>
1890   The freshness model (<xref target="expiration.model"/>) does not
1891   necessarily apply to history mechanisms. I.e., a history mechanism can
1892   display a previous representation even if it has expired.
1893</t>
1894<t>
1895   This does not prohibit the history mechanism from telling the user that a
1896   view might be stale, or from honoring cache directives (e.g.,
1897   Cache-Control: no-store).
1898</t>
1899</section>
1900
1901
1902<section title="IANA Considerations" anchor="iana.considerations">
1903
1904<section title="Cache Directive Registry" anchor="cache.directive.registry">
1905<t>
1906   The HTTP Cache Directive Registry defines the name space for the
1907   cache directives. It will be created and maintained at
1908   <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1909</t>
1910
1911<section title="Procedure" anchor="cache.directive.registry.procedure">
1912<t>
1913   A registration &MUST; include the following fields:
1914   <list style="symbols">
1915      <t>Cache Directive Name</t>
1916      <t>Pointer to specification text</t>
1917   </list>
1918</t>
1919<t>
1920   Values to be added to this name space require IETF Review (see <xref
1921   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1922</t>
1923</section>
1924
1925<section title="Considerations for New Cache Control Directives" anchor="cache.directive.considerations">
1926<t>
1927   New extension directives ought to consider defining:
1928</t>
1929<t>
1930   <list style="symbols">
1931      <t>What it means for a directive to be specified multiple times,</t>
1932      <t>When the directive does not take an argument, what it means when an
1933      argument is present,</t>
1934      <t>When the directive requires an argument, what it means when it is
1935      missing,</t>
1936      <t>Whether the directive is specific to requests, responses, or able
1937        to be used in either.</t>
1938   </list>
1939</t>
1940<t>
1941  See also <xref target="cache.control.extensions"/>.
1942</t>
1943</section>
1944
1945<section title="Registrations" anchor="cache.directive.registration">
1946<t>
1947  The HTTP Cache Directive Registry shall be populated with the registrations below:
1948</t>
1949<?BEGININC p6-cache.cache-directives ?>
1950<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1951<texttable xmlns:my="#my" align="left" suppress-title="true"
1952           anchor="iana.cache.directive.registration.table">
1953   <ttcol>Cache Directive</ttcol>
1954   <ttcol>Reference</ttcol>
1955
1956   <c>max-age</c>
1957   <c>
1958      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
1959   </c>
1960   <c>max-stale</c>
1961   <c>
1962      <xref target="cache-request-directive.max-stale"/>
1963   </c>
1964   <c>min-fresh</c>
1965   <c>
1966      <xref target="cache-request-directive.min-fresh"/>
1967   </c>
1968   <c>must-revalidate</c>
1969   <c>
1970      <xref target="cache-response-directive.must-revalidate"/>
1971   </c>
1972   <c>no-cache</c>
1973   <c>
1974      <xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
1975   </c>
1976   <c>no-store</c>
1977   <c>
1978      <xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
1979   </c>
1980   <c>no-transform</c>
1981   <c>
1982      <xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
1983   </c>
1984   <c>only-if-cached</c>
1985   <c>
1986      <xref target="cache-request-directive.only-if-cached"/>
1987   </c>
1988   <c>private</c>
1989   <c>
1990      <xref target="cache-response-directive.private"/>
1991   </c>
1992   <c>proxy-revalidate</c>
1993   <c>
1994      <xref target="cache-response-directive.proxy-revalidate"/>
1995   </c>
1996   <c>public</c>
1997   <c>
1998      <xref target="cache-response-directive.public"/>
1999   </c>
2000   <c>s-maxage</c>
2001   <c>
2002      <xref target="cache-response-directive.s-maxage"/>
2003   </c>
2004   <c>stale-if-error</c>
2005   <c>
2006      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
2007   </c>
2008   <c>stale-while-revalidate</c>
2009   <c>
2010      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
2011   </c>
2012</texttable>
2013<!--(END)-->
2014<?ENDINC p6-cache.cache-directives ?>
2015</section>
2016</section>
2017
2018<section title="Warn Code Registry" anchor="warn.code.registry">
2019<t>
2020   The HTTP Warn Code Registry defines the name space for warn codes.
2021   It will be created and maintained at
2022   <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
2023</t>
2024
2025<section title="Procedure" anchor="warn.code.registry.procedure">
2026<t>
2027   A registration &MUST; include the following fields:
2028   <list style="symbols">
2029      <t>Warn Code (3 digits)</t>
2030      <t>Short Description</t>
2031      <t>Pointer to specification text</t>
2032   </list>
2033</t>
2034<t>
2035   Values to be added to this name space require IETF Review (see <xref
2036   target="RFC5226" x:fmt="," x:sec="4.1"/>).
2037</t>
2038</section>
2039
2040<section title="Registrations" anchor="warn.code.registration">
2041<t>
2042  The HTTP Warn Code Registry shall be populated with the registrations below:
2043</t>
2044<?BEGININC p6-cache.iana-warn-codes ?>
2045<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
2046<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
2047   <ttcol>Warn Code</ttcol>
2048   <ttcol>Short Description</ttcol>
2049   <ttcol>Reference</ttcol>
2050   <c>110</c>
2051   <c>Response is Stale</c>
2052   <c>
2053      <xref target="warn.110"/>
2054   </c>
2055   <c>111</c>
2056   <c>Revalidation Failed</c>
2057   <c>
2058      <xref target="warn.111"/>
2059   </c>
2060   <c>112</c>
2061   <c>Disconnected Operation</c>
2062   <c>
2063      <xref target="warn.112"/>
2064   </c>
2065   <c>113</c>
2066   <c>Heuristic Expiration</c>
2067   <c>
2068      <xref target="warn.113"/>
2069   </c>
2070   <c>199</c>
2071   <c>Miscellaneous Warning</c>
2072   <c>
2073      <xref target="warn.199"/>
2074   </c>
2075   <c>214</c>
2076   <c>Transformation Applied</c>
2077   <c>
2078      <xref target="warn.214"/>
2079   </c>
2080   <c>299</c>
2081   <c>Miscellaneous Persistent Warning</c>
2082   <c>
2083      <xref target="warn.299"/>
2084   </c>
2085</texttable>
2086<!--(END)-->
2087<?ENDINC p6-cache.iana-warn-codes ?>
2088</section>
2089</section>
2090
2091<section title="Header Field Registration" anchor="header.field.registration">
2092<t>
2093   HTTP header fields are registered within the Message Header Field Registry
2094   maintained at
2095   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
2096</t>
2097<t>
2098   This document defines the following HTTP header fields, so their
2099   associated registry entries shall be updated according to the permanent
2100   registrations below (see <xref target="BCP90"/>):
2101</t>
2102<?BEGININC p6-cache.iana-headers ?>
2103<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2104<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2105   <ttcol>Header Field Name</ttcol>
2106   <ttcol>Protocol</ttcol>
2107   <ttcol>Status</ttcol>
2108   <ttcol>Reference</ttcol>
2109
2110   <c>Age</c>
2111   <c>http</c>
2112   <c>standard</c>
2113   <c>
2114      <xref target="header.age"/>
2115   </c>
2116   <c>Cache-Control</c>
2117   <c>http</c>
2118   <c>standard</c>
2119   <c>
2120      <xref target="header.cache-control"/>
2121   </c>
2122   <c>Expires</c>
2123   <c>http</c>
2124   <c>standard</c>
2125   <c>
2126      <xref target="header.expires"/>
2127   </c>
2128   <c>Pragma</c>
2129   <c>http</c>
2130   <c>standard</c>
2131   <c>
2132      <xref target="header.pragma"/>
2133   </c>
2134   <c>Warning</c>
2135   <c>http</c>
2136   <c>standard</c>
2137   <c>
2138      <xref target="header.warning"/>
2139   </c>
2140</texttable>
2141<!--(END)-->
2142<?ENDINC p6-cache.iana-headers ?>
2143<t>
2144   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2145   Force".
2146</t>
2147</section>
2148</section>
2149
2150<section anchor="security.considerations" title="Security Considerations">
2151<t>
2152   This section is meant to inform developers, information providers, and
2153   users of known security concerns specific to HTTP/1.1 caching.
2154   More general security considerations are addressed in HTTP messaging
2155   &messaging; and semantics &semantics;.
2156</t>
2157<t>
2158   Caches expose additional potential vulnerabilities, since the contents of
2159   the cache represent an attractive target for malicious exploitation.
2160   Because cache contents persist after an HTTP request is complete, an attack
2161   on the cache can reveal information long after a user believes that the
2162   information has been removed from the network. Therefore, cache contents
2163   need to be protected as sensitive information.
2164</t>
2165<t>
2166   Furthermore, the very use of a cache can bring about privacy concerns. For
2167   example, if two users share a cache, and the first one browses to a site,
2168   the second may be able to detect that the other has been to that site,
2169   because the resources from it load more quickly, thanks to the cache.
2170</t>
2171<t>
2172   Implementation flaws might allow attackers to insert content into a cache
2173   ("cache poisoning"), leading to compromise of clients that trust that
2174   content. Because of their nature, these attacks are difficult to mitigate.
2175</t>
2176<t>
2177   Likewise, implementation flaws (as well as misunderstanding of cache
2178   operation) might lead to caching of sensitive information (e.g.,
2179   authentication credentials) that is thought to be private, exposing it to
2180   unauthorized parties.
2181</t>
2182<t>
2183   Note that the Set-Cookie response header field <xref target="RFC6265"/> does not
2184   inhibit caching; a cacheable response with a Set-Cookie header field can be (and
2185   often is) used to satisfy subsequent requests to caches. Servers who wish
2186   to control caching of these responses are encouraged to emit appropriate
2187   Cache-Control response header fields.
2188</t>
2189
2190</section>
2191
2192<section title="Acknowledgments" anchor="acks">
2193<t>
2194  See &acks;.
2195</t>
2196</section>
2197
2198</middle>
2199
2200<back>
2201<references title="Normative References">
2202
2203  <reference anchor="Part1">
2204    <front>
2205      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
2206      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2207        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2208        <address><email>fielding@gbiv.com</email></address>
2209      </author>
2210      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2211        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2212        <address><email>julian.reschke@greenbytes.de</email></address>
2213      </author>
2214      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2215    </front>
2216    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2217    <x:source basename="p1-messaging" href="p1-messaging.xml">
2218      <x:defines>Content-Length</x:defines>
2219      <x:defines>Via</x:defines>
2220    </x:source>
2221  </reference>
2222
2223  <reference anchor="Part2">
2224    <front>
2225      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
2226      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2227        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2228        <address><email>fielding@gbiv.com</email></address>
2229      </author>
2230      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2231        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2232        <address><email>julian.reschke@greenbytes.de</email></address>
2233      </author>
2234      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2235    </front>
2236    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2237    <x:source basename="p2-semantics" href="p2-semantics.xml">
2238      <x:defines>2xx (Successful)</x:defines>
2239      <x:defines>200 (OK)</x:defines>
2240      <x:defines>203 (Non-Authoritative Information)</x:defines>
2241      <x:defines>3xx (Redirection)</x:defines>
2242      <x:defines>300 (Multiple Choices)</x:defines>
2243      <x:defines>301 (Moved Permanently)</x:defines>
2244      <x:defines>404 (Not Found)</x:defines>
2245      <x:defines>410 (Gone)</x:defines>
2246      <x:defines>5xx (Server Error)</x:defines>
2247      <x:defines>504 (Gateway Timeout)</x:defines>
2248      <x:defines>Accept</x:defines>
2249      <x:defines>Content-Encoding</x:defines>
2250      <x:defines>Content-Location</x:defines>
2251      <x:defines>Content-Type</x:defines>
2252      <x:defines>Date</x:defines>
2253      <x:defines>Location</x:defines>
2254      <x:defines>Vary</x:defines>
2255    </x:source>
2256  </reference>
2257
2258  <reference anchor="Part4">
2259    <front>
2260      <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
2261      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2262        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2263        <address><email>fielding@gbiv.com</email></address>
2264      </author>
2265      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2266        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2267        <address><email>julian.reschke@greenbytes.de</email></address>
2268      </author>
2269      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2270    </front>
2271    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2272    <x:source basename="p4-conditional" href="p4-conditional.xml">
2273      <x:defines>304</x:defines>
2274      <x:defines>304 (Not Modified)</x:defines>
2275      <x:defines>412 (Precondition Failed)</x:defines>
2276      <x:defines>ETag</x:defines>
2277      <x:defines>If-Modified-Since</x:defines>
2278      <x:defines>If-Unmodified-Since</x:defines>
2279      <x:defines>If-Match</x:defines>
2280      <x:defines>If-None-Match</x:defines>
2281      <x:defines>Last-Modified</x:defines>
2282    </x:source>
2283  </reference>
2284
2285  <reference anchor="Part5">
2286    <front>
2287      <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
2288      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2289        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2290        <address><email>fielding@gbiv.com</email></address>
2291      </author>
2292      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2293        <organization abbrev="W3C">World Wide Web Consortium</organization>
2294        <address><email>ylafon@w3.org</email></address>
2295      </author>
2296      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2297        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2298        <address><email>julian.reschke@greenbytes.de</email></address>
2299      </author>
2300      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2301    </front>
2302    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2303    <x:source basename="p5-range" href="p5-range.xml">
2304      <x:defines>206 (Partial Content)</x:defines>
2305      <x:defines>Content-Range</x:defines>
2306      <x:defines>If-Range</x:defines>
2307      <x:defines>Range</x:defines>
2308    </x:source>
2309  </reference>
2310
2311  <reference anchor="Part7">
2312    <front>
2313      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
2314      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2315        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2316        <address><email>fielding@gbiv.com</email></address>
2317      </author>
2318      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2319        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2320        <address><email>julian.reschke@greenbytes.de</email></address>
2321      </author>
2322      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2323    </front>
2324    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2325    <x:source basename="p7-auth" href="p7-auth.xml">
2326      <x:defines>Authorization</x:defines>
2327    </x:source>
2328  </reference>
2329
2330  <reference anchor="RFC2119">
2331    <front>
2332      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2333      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2334        <organization>Harvard University</organization>
2335        <address><email>sob@harvard.edu</email></address>
2336      </author>
2337      <date month="March" year="1997" />
2338    </front>
2339    <seriesInfo name="BCP" value="14" />
2340    <seriesInfo name="RFC" value="2119" />
2341  </reference>
2342
2343  <reference anchor="RFC5234">
2344    <front>
2345      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2346      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2347        <organization>Brandenburg InternetWorking</organization>
2348        <address>
2349          <email>dcrocker@bbiw.net</email>
2350        </address> 
2351      </author>
2352      <author initials="P." surname="Overell" fullname="Paul Overell">
2353        <organization>THUS plc.</organization>
2354        <address>
2355          <email>paul.overell@thus.net</email>
2356        </address>
2357      </author>
2358      <date month="January" year="2008"/>
2359    </front>
2360    <seriesInfo name="STD" value="68"/>
2361    <seriesInfo name="RFC" value="5234"/>
2362  </reference>
2363 
2364</references>
2365
2366<references title="Informative References">
2367
2368  <reference anchor="RFC1305">
2369    <front>
2370      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2371      <author fullname="David L. Mills" initials="D." surname="Mills">
2372        <organization>University of Delaware, Electrical Engineering Department</organization>
2373        <address><email>mills@udel.edu</email></address>
2374      </author>
2375      <date month="March" year="1992" />
2376    </front>
2377    <seriesInfo name="RFC" value="1305" />
2378  </reference>
2379
2380  <reference anchor="RFC2616">
2381    <front>
2382      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2383      <author fullname="R. Fielding" initials="R." surname="Fielding">
2384        <organization>University of California, Irvine</organization>
2385        <address><email>fielding@ics.uci.edu</email></address>
2386      </author>
2387      <author fullname="J. Gettys" initials="J." surname="Gettys">
2388        <organization>W3C</organization>
2389        <address><email>jg@w3.org</email></address>
2390      </author>
2391      <author fullname="J. Mogul" initials="J." surname="Mogul">
2392        <organization>Compaq Computer Corporation</organization>
2393        <address><email>mogul@wrl.dec.com</email></address>
2394      </author>
2395      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2396        <organization>MIT Laboratory for Computer Science</organization>
2397        <address><email>frystyk@w3.org</email></address>
2398      </author>
2399      <author fullname="L. Masinter" initials="L." surname="Masinter">
2400        <organization>Xerox Corporation</organization>
2401        <address><email>masinter@parc.xerox.com</email></address>
2402      </author>
2403      <author fullname="P. Leach" initials="P." surname="Leach">
2404        <organization>Microsoft Corporation</organization>
2405        <address><email>paulle@microsoft.com</email></address>
2406      </author>
2407      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2408        <organization>W3C</organization>
2409        <address><email>timbl@w3.org</email></address>
2410      </author>
2411      <date month="June" year="1999" />
2412    </front>
2413    <seriesInfo name="RFC" value="2616" />
2414  </reference>
2415
2416  <reference anchor="BCP90">
2417    <front>
2418      <title>Registration Procedures for Message Header Fields</title>
2419      <author fullname="G. Klyne" initials="G." surname="Klyne">
2420        <organization>Nine by Nine</organization>
2421        <address><email>GK-IETF@ninebynine.org</email></address>
2422      </author>
2423      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2424        <organization>BEA Systems</organization>
2425        <address><email>mnot@pobox.com</email></address>
2426      </author>
2427      <author fullname="J. Mogul" initials="J." surname="Mogul">
2428        <organization>HP Labs</organization>
2429        <address><email>JeffMogul@acm.org</email></address>
2430      </author>
2431      <date month="September" year="2004" />
2432    </front>
2433    <seriesInfo name="BCP" value="90" />
2434    <seriesInfo name="RFC" value="3864" />
2435  </reference>
2436
2437  <reference anchor='RFC5226'>
2438    <front>
2439      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2440      <author initials='T.' surname='Narten' fullname='T. Narten'>
2441        <organization>IBM</organization>
2442        <address><email>narten@us.ibm.com</email></address>
2443      </author>
2444      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2445        <organization>Google</organization>
2446        <address><email>Harald@Alvestrand.no</email></address>
2447      </author>
2448      <date year='2008' month='May' />
2449    </front>
2450    <seriesInfo name='BCP' value='26' />
2451    <seriesInfo name='RFC' value='5226' />
2452  </reference>
2453
2454  <reference anchor='RFC5861'>
2455    <front>
2456      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2457      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2458        <organization>Yahoo! Inc.</organization>
2459        <address><email>mnot@yahoo-inc.com</email></address>
2460      </author>
2461      <date month="April" year="2010"/>
2462    </front>
2463    <seriesInfo name='RFC' value='5861' />
2464  </reference>
2465
2466  <reference anchor="RFC6265">
2467    <front>
2468      <title>HTTP State Management Mechanism</title>
2469      <author initials="A." surname="Barth" fullname="Adam Barth">
2470        <organization abbrev="U.C. Berkeley">
2471          University of California, Berkeley
2472        </organization>
2473        <address><email>abarth@eecs.berkeley.edu</email></address>
2474      </author>
2475      <date year="2011" month="April" />
2476    </front>
2477    <seriesInfo name="RFC" value="6265"/>
2478  </reference>
2479
2480</references>
2481
2482<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2483<t>
2484  The specification has been substantially rewritten for clarity.
2485</t>
2486<t>
2487  The conditions under which an authenticated response can be cached have been
2488  clarified.
2489  (<xref target="caching.authenticated.responses" />)
2490</t>
2491<t>
2492  New status codes can now define that caches are allowed to use heuristic
2493  freshness with them.
2494  Caches are now allowed to calculate heuristic freshness for URIs with query
2495  components.
2496  (<xref target="heuristic.freshness" />)
2497</t>
2498<t>
2499  The algorithm for calculating age is now less conservative.
2500  Caches are now required to handle dates with timezones as if they're
2501  invalid, because it's not possible to accurately guess.
2502  (<xref target="age.calculations"/>)
2503</t>
2504<t>
2505  The <x:ref>Content-Location</x:ref> response header field is no longer used
2506  to determine the appropriate response to use when validating.
2507  (<xref target="validation.model" />)
2508</t>
2509<t>
2510  The algorithm for selecting a cached negotiated response to use has been
2511  clarified in several ways. In particular, it now explicitly allows
2512  header-specific canonicalization when processing selecting header fields.
2513  (<xref target="caching.negotiated.responses" />)
2514</t>
2515<t>
2516  Requirements regarding denial of service attack avoidance when performing
2517  invalidation have been clarified.
2518  (<xref target="invalidation" />)
2519</t>
2520<t>
2521  Cache invalidation only occurs when a successful response is received.
2522  (<xref target="invalidation" />)
2523</t>
2524<t>
2525  Cache directives are explicitly defined to be case-insensitive.
2526  Handling of multiple instances of cache directives when only one is
2527  expected is now defined.
2528  (<xref target="header.cache-control" />)
2529</t>
2530<t>
2531  The "no-store" cache request directive doesn't apply to responses; i.e.,
2532  a cache can satisfy a request with no-store on it, and does not invalidate
2533  it.
2534  (<xref target="cache-request-directive.no-store" />)
2535</t>
2536<t>
2537  The qualified forms of the private and no-cache cache directives are noted
2538  to not be widely implemented; e.g., "private=foo" is interpreted by many
2539  caches as simply "private". Additionally, the meaning of the qualified
2540  form of no-cache has been clarified.
2541  (<xref target="cache-response-directive" />)
2542</t>
2543<t>
2544  The "no-cache" response cache directive's meaning has been clarified.
2545  (<xref target="cache-response-directive.no-cache" />)
2546</t>
2547<t>
2548  The one-year limit on <x:ref>Expires</x:ref> header field values has been removed; instead,
2549  the reasoning for using a sensible value is given.
2550  (<xref target="header.expires" />)
2551</t>
2552<t>
2553  The <x:ref>Pragma</x:ref> header field is now only defined for backwards compatibility;
2554  future pragmas are deprecated.
2555  (<xref target="header.pragma" />)
2556</t>
2557<t>
2558  Some requirements regarding production and processing of the <x:ref>Warning</x:ref>
2559  header fields have been relaxed, as it is not widely implemented.
2560  Furthermore, the <x:ref>Warning</x:ref> header field no longer uses RFC 2047 encoding,
2561  nor allows multiple languages, as these aspects were not implemented.
2562  (<xref target="header.warning" />)
2563</t>
2564<t>
2565  This specification introduces the Cache Directive and Warn Code Registries,
2566  and defines considerations for new cache directives.
2567  (<xref target="cache.directive.registry"/> and <xref target="warn.code.registry"/>)
2568</t>
2569</section>
2570
2571<section title="Imported ABNF" anchor="imported.abnf">
2572   <x:anchor-alias value="ALPHA"/>
2573   <x:anchor-alias value="CR"/>
2574   <x:anchor-alias value="DIGIT"/>
2575   <x:anchor-alias value="DQUOTE"/>
2576   <x:anchor-alias value="LF"/>
2577   <x:anchor-alias value="OCTET"/>
2578   <x:anchor-alias value="SP"/>
2579   <x:anchor-alias value="VCHAR"/>
2580   <x:anchor-alias value="quoted-string"/>
2581   <x:anchor-alias value="token"/>
2582   <x:anchor-alias value="OWS"/>
2583   <x:anchor-alias value="field-name"/>
2584   <x:anchor-alias value="HTTP-date"/>
2585   <x:anchor-alias value="port"/>
2586   <x:anchor-alias value="pseudonym"/>
2587   <x:anchor-alias value="uri-host"/>
2588<t>
2589   The following core rules are included by reference, as defined in <xref
2590   target="RFC5234" x:fmt="of" x:sec="B.1"/>: ALPHA (letters), CR (carriage
2591   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
2592   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
2593   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2594</t>
2595<t>
2596   The rules below are defined in <xref target="Part1"/>:
2597</t>
2598<figure><artwork type="abnf2616">
2599  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
2600  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
2601  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
2602  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
2603
2604  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
2605  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
2606  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
2607</artwork></figure>
2608<t>
2609   The rules below are defined in other parts:
2610</t>
2611<figure><artwork type="abnf2616">
2612  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
2613</artwork></figure>
2614</section>
2615
2616<?BEGININC p6-cache.abnf-appendix ?>
2617<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2618<t>
2619  In the collected ABNF below, list rules are expanded as per <xref target="Part1" x:rel="#notation"/>.
2620</t><figure>
2621<artwork type="abnf" name="p6-cache.parsed-abnf">
2622<x:ref>Age</x:ref> = delta-seconds
2623
2624<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2625 cache-directive ] )
2626
2627<x:ref>Expires</x:ref> = HTTP-date
2628
2629<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
2630
2631<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
2632
2633<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2634 pragma-directive ] )
2635
2636<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2637 )
2638
2639<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
2640
2641<x:ref>delta-seconds</x:ref> = 1*DIGIT
2642
2643<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2644
2645<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2646
2647<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2648<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2649<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 5.7.1&gt;
2650
2651<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.6&gt;
2652
2653<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.6&gt;
2654
2655<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2656
2657<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2658<x:ref>warn-code</x:ref> = 3DIGIT
2659<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2660<x:ref>warn-text</x:ref> = quoted-string
2661<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2662 ]
2663</artwork>
2664</figure>
2665</section>
2666<?ENDINC p6-cache.abnf-appendix ?>
2667
2668<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2669<t>
2670  Changes up to the first Working Group Last Call draft are summarized
2671  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
2672</t>
2673
2674<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
2675<t>
2676  Closed issues:
2677  <list style="symbols">
2678    <t>
2679      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
2680      "untangle Cache-Control ABNF"
2681    </t>
2682    <t>
2683      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
2684      "Multiple values in Cache-Control header fields"
2685    </t>
2686    <t>
2687      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
2688      "Case sensitivity of header fields in CC values"
2689    </t>
2690    <t>
2691      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
2692      "Spurious 'MAYs'"
2693    </t>
2694    <t>
2695      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
2696      "enhance considerations for new cache control directives"
2697    </t>
2698    <t>
2699      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
2700      "ABNF requirements for recipients"
2701    </t>
2702    <t>
2703      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
2704      "note introduction of new IANA registries as normative changes"
2705    </t>
2706    <t>
2707      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
2708      "broken prose in description of 'Vary'"
2709    </t>
2710  </list>
2711</t>
2712</section>
2713
2714<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
2715<t>
2716  Closed issues:
2717  <list style="symbols">
2718    <t>
2719      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
2720      "'Most Conservative'"
2721    </t>
2722  </list>
2723</t>
2724<t>
2725  Other changes:
2726  <list style="symbols">
2727    <t>
2728      Conformance criteria and considerations regarding error handling are
2729      now defined in Part 1.
2730    </t>
2731    <t>
2732      Move definition of "Vary" header field into Part 2.
2733    </t>
2734    <t>
2735      Add security considerations with respect to cache poisoning and
2736      the "Set-Cookie" header field.
2737    </t>
2738  </list>
2739</t>
2740</section>
2741
2742<section title="Since draft-ietf-httpbis-p6-cache-21" anchor="changes.since.21">
2743<t>
2744  Closed issues:
2745  <list style="symbols">
2746    <t>
2747      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2748      "Allowing heuristic caching for new status codes"
2749    </t>
2750    <t>
2751      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
2752      "304 without validator"
2753    </t>
2754    <t>
2755      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
2756      "No-Transform"
2757    </t>
2758    <t>
2759      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/430"/>:
2760      "Revert prior change to the meaning of the public cache response
2761       directive.
2762    </t>
2763  </list>
2764</t>
2765</section>
2766
2767<section title="Since draft-ietf-httpbis-p6-cache-22" anchor="changes.since.22">
2768<t>
2769  Closed issues:
2770  <list style="symbols">
2771    <t>
2772      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/436"/>:
2773      "explain list expansion in ABNF appendices"
2774    </t>
2775    <t>
2776      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/453"/>:
2777      "Returning the freshest response"
2778    </t>
2779    <t>
2780      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/464"/>:
2781      "placement of extension point considerations"
2782    </t>
2783    <t>
2784      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/469"/>:
2785      "Editorial notes for p6"
2786    </t>
2787    <t>
2788      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/471"/>:
2789      "Vary and future requests"
2790    </t>
2791  </list>
2792</t>
2793</section>
2794
2795<section title="Since draft-ietf-httpbis-p6-cache-23" anchor="changes.since.23">
2796<t>
2797  Closed issues:
2798  <list style="symbols">
2799    <t>
2800      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/486"/>:
2801      "Requiring proxies to process warn-date"
2802    </t>
2803  </list>
2804</t>
2805</section>
2806</section>
2807  </back>
2808</rfc>
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