source: draft-ietf-httpbis/latest/p6-cache.xml @ 1793

Last change on this file since 1793 was 1793, checked in by julian.reschke@…, 7 years ago

make links to cache directive definitions more specific

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[29]1<?xml version="1.0" encoding="utf-8"?>
[768]2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
[8]3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
[29]14  <!ENTITY ID-VERSION "latest">
[1697]15  <!ENTITY ID-MONTH "July">
[1497]16  <!ENTITY ID-YEAR "2012">
[1692]17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x=''>Note:</x:h>">
[1518]19  <!ENTITY architecture                "<xref target='Part1' x:rel='#architecture' xmlns:x=''/>">
[424]20  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x=''/>">
[1399]21  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x=''/>">
[1518]22  <!ENTITY whitespace                  "<xref target='Part1' x:rel='#whitespace' xmlns:x=''/>">
23  <!ENTITY field-components            "<xref target='Part1' x:rel='#field.components' xmlns:x=''/>">
[538]24  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x=''/>">
[1399]25  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x=''/>">
[31]26  <!ENTITY messaging                   "<xref target='Part1' xmlns:x=''/>">
[1374]27  <!ENTITY semantics                   "<xref target='Part2' xmlns:x=''/>">
[183]28  <!ENTITY conditional                 "<xref target='Part4' xmlns:x=''/>">
[538]29  <!ENTITY partial                     "<xref target='Part5' xmlns:x=''/>">
[31]30  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x=''/>">
[1436]31  <!ENTITY http-date                   "<xref target='Part2' x:rel='' xmlns:x=''/>">
[31]32  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x=''/>">
33  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x=''/>">
[1436]34  <!ENTITY header-date                 "<xref target='Part2' x:rel='' xmlns:x=''/>">
[205]35  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x=''/>">
[538]36  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
[697]37  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x=''/>">
[31]38  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
[1374]39  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x=''/>">
[607]40  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x=''/>">
[1374]41  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x=''/>">
[1635]42  <!ENTITY status-codes                "<xref target='Part2' x:rel='' xmlns:x=''/>">
[943]43  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x=''/>">
45<?rfc toc="yes" ?>
[29]46<?rfc symrefs="yes" ?>
47<?rfc sortrefs="yes" ?>
[8]48<?rfc compact="yes"?>
49<?rfc subcompact="no" ?>
50<?rfc linkmailto="no" ?>
51<?rfc editing="no" ?>
[203]52<?rfc comments="yes"?>
53<?rfc inline="yes"?>
[799]54<?rfc rfcedstyle="yes"?>
[8]55<?rfc-ext allow-markup-in-artwork="yes" ?>
56<?rfc-ext include-references-in-index="yes" ?>
[538]57<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
[1477]58  obsoletes="2616" x:maturity-level="proposed" xmlns:x="">
[1472]59<x:link rel="prev" basename="p5-range"/>
60<x:link rel="next" basename="p7-auth"/>
[1522]61<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
[547]64  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
[547]66  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]67    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[547]68    <address>
[8]69      <postal>
[1106]70        <street>345 Park Ave</street>
71        <city>San Jose</city>
[8]72        <region>CA</region>
[1106]73        <code>95110</code>
[29]74        <country>USA</country>
[8]75      </postal>
[29]76      <email></email>
77      <uri></uri>
[8]78    </address>
[547]79  </author>
[547]81  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
82    <organization abbrev="W3C">World Wide Web Consortium</organization>
83    <address>
[93]84      <postal>
85        <street>W3C / ERCIM</street>
86        <street>2004, rte des Lucioles</street>
87        <city>Sophia-Antipolis</city>
88        <region>AM</region>
89        <code>06902</code>
90        <country>France</country>
91      </postal>
92      <email></email>
93      <uri></uri>
94    </address>
[547]95  </author>
[601]97  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
[1438]98    <organization>Rackspace</organization>
[607]99    <address>
100      <email></email>
101      <uri></uri>
[601]102    </address>
103  </author>
[547]105  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
106    <organization abbrev="greenbytes">greenbytes GmbH</organization>
107    <address>
[95]108      <postal>
109        <street>Hafenweg 16</street>
110        <city>Muenster</city><region>NW</region><code>48155</code>
111        <country>Germany</country>
112      </postal>
[607]113      <email></email>
114      <uri></uri>
[95]115    </address>
[547]116  </author>
[547]118  <date month="&ID-MONTH;" year="&ID-YEAR;" />
119  <workgroup>HTTPbis Working Group</workgroup>
[949]123   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
[1373]124   distributed, collaborative, hypertext information systems. HTTP has been in
125   use by the World Wide Web global information initiative since 1990. This
126   document is Part 6 of the seven-part specification that defines the protocol
127   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
130   Part 6 defines requirements on HTTP caches and the associated header fields
131   that control cache behavior or indicate cacheable response messages.
[540]135<note title="Editorial Note (To be removed by RFC Editor)">
[1268]136  <t>
[1764]137    Discussion of this draft takes place on the HTTPBIS working group
[1268]138    mailing list (, which is archived at
139    <eref target=""/>.
140  </t>
141  <t>
142    The current issues list is at
143    <eref target=""/> and related
144    documents (including fancy diffs) can be found at
145    <eref target=""/>.
146  </t>
147  <t>
[1592]148    The changes in this draft are summarized in <xref target="changes.since.19"/>.
[1268]149  </t>
[949]152   </front>
153   <middle>
[540]155<section anchor="caching" title="Introduction">
[949]157   HTTP is typically used for distributed information systems, where
158   performance can be improved by the use of response caches. This document
159   defines aspects of HTTP/1.1 related to caching and reusing response
160   messages.
[540]163<section anchor="intro.purpose" title="Purpose">
164<iref item="cache" />
[949]166   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
167   subsystem that controls its message storage, retrieval, and deletion. A
168   cache stores cacheable responses in order to reduce the response time and
169   network bandwidth consumption on future, equivalent requests. Any client or
170   server &MAY; employ a cache, though a cache cannot be used by a server that
171   is acting as a tunnel.
[1374]174   The goal of caching in HTTP/1.1 is to significantly improve performance
175   by reusing a prior response message to satisfy a current request.
176   A stored response is considered "fresh", as defined in
177   <xref target="expiration.model" />, if the response can be reused without
178   "validation" (checking with the origin server to see if the cached response
179   remains valid for this request).  A fresh cache response can therefore
180   reduce both latency and network transfers each time it is reused.
181   When a cached response is not fresh, it might still be reusable if it can
182   be freshened by validation (<xref target="validation.model" />) or if the
183   origin is unavailable.
[540]187<section anchor="intro.terminology" title="Terminology">
[949]189   This specification uses a number of terms to refer to the roles played by
190   participants in, and objects of, HTTP caching.
[1112]193   <iref item="cache" />
194   <x:dfn>cache</x:dfn>
195   <list>
196      <t>A conformant implementation of a HTTP cache. Note that this implies
197        an HTTP/1.1 cache; this specification does not define conformance
198        for HTTP/1.0 caches.</t>
199   </list>
201<t anchor="shared.and.non-shared.caches">
202   <iref item="shared cache" />
203   <x:dfn>shared cache</x:dfn>
204   <list>
[1374]205      <t>A cache that stores responses to be reused by more than one user;
206         usually (but not always) deployed as part of an intermediary.</t>
[1112]207   </list>
210   <iref item="private cache" />
211   <x:dfn>private cache</x:dfn>
212   <list>
213      <t>A cache that is dedicated to a single user.</t>
214   </list>
[949]217   <iref item="cacheable" />
218   <x:dfn>cacheable</x:dfn>
219   <list>
220      <t>A response is cacheable if a cache is allowed to store a copy of the
221      response message for use in answering subsequent requests. Even when a
222      response is cacheable, there might be additional constraints on whether
[1112]223      a cache can use the stored copy to satisfy a particular request.</t>
[949]224   </list>
[949]227   <iref item="explicit expiration time" />
228   <x:dfn>explicit expiration time</x:dfn>
229   <list>
230      <t>The time at which the origin server intends that a representation
[969]231      no longer be returned by a cache without further validation.</t>
[949]232   </list>
[949]235   <iref item="heuristic expiration time" />
236   <x:dfn>heuristic expiration time</x:dfn>
237   <list>
238      <t>An expiration time assigned by a cache when no explicit expiration
239      time is available.</t>
240   </list>
[949]243   <iref item="age" />
244   <x:dfn>age</x:dfn>
245   <list>
246      <t>The age of a response is the time since it was sent by, or
247      successfully validated with, the origin server.</t>
248   </list>
[949]251   <iref item="first-hand" />
252   <x:dfn>first-hand</x:dfn>
253   <list>
254      <t>A response is first-hand if the freshness model is not in use; i.e.,
255      its age is 0.</t>
256   </list>
[949]259   <iref item="freshness lifetime" />
260   <x:dfn>freshness lifetime</x:dfn>
261   <list>
262      <t>The length of time between the generation of a response and its
263      expiration time.</t>
264   </list>
[949]267   <iref item="fresh" />
268   <x:dfn>fresh</x:dfn>
269   <list>
270      <t>A response is fresh if its age has not yet exceeded its freshness
271      lifetime.</t>
272   </list>
[949]275   <iref item="stale" />
276   <x:dfn>stale</x:dfn>
277   <list>
278      <t>A response is stale if its age has passed its freshness lifetime
279      (either explicit or heuristic).</t>
280   </list>
[949]283   <iref item="validator" />
284   <x:dfn>validator</x:dfn>
285   <list>
[1739]286      <t>A protocol element (e.g., an entity-tag or a <x:ref>Last-Modified</x:ref>
287      time) that is used to find out whether a stored response is an equivalent
288      copy of a representation. See &weak-and-strong;.</t>
[949]289   </list>
292   <iref item="strong validator" />
[1376]293   <iref item="validator" subitem="strong" />
[1374]294   <x:dfn>strong validator</x:dfn>
295   <list>
296      <t>A validator that is defined by the origin server such that its
297         current value will change if the representation body changes; i.e.,
298         an entity-tag that is not marked as weak (&entity-tags;) or,
[1739]299         if no entity-tag is provided, a <x:ref>Last-Modified</x:ref> value
300         that is strong in the sense defined by &lastmod-comparison;.</t>
[1374]301   </list>
[1452]305<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
[847]307   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
308   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
309   document are to be interpreted as described in <xref target="RFC2119"/>.
[1770]312   This specification targets conformance criteria according to the role of
313   a participant in HTTP communication.  Hence, HTTP requirements are placed
314   on senders, recipients, clients, servers, user agents, intermediaries,
315   origin servers, proxies, gateways, or caches, depending on what behavior
316   is being constrained by the requirement. See &architecture; for definitions
317   of these terms.
[1770]320   The verb "generate" is used instead of "send" where a requirement
321   differentiates between creating a protocol element and merely forwarding a
322   received element downstream.
[1452]325   An implementation is considered conformant if it complies with all of the
[1770]326   requirements associated with the roles it partakes in HTTP. Note that
327   SHOULD-level requirements are relevant here, unless one of the documented
328   exceptions is applicable.
331   This document also uses ABNF to define valid protocol elements
[1770]332   (<xref target="notation"/>).
333   In addition to the prose requirements placed upon them, senders &MUST-NOT;
334   generate protocol elements that do not match the grammar defined by the
335   ABNF rules for those protocol elements that are applicable to the sender's
336   role. If a received protocol element is processed, the recipient &MUST; be
337   able to parse any value that would match the ABNF rules for that protocol
338   element, excluding only those rules not applicable to the recipient's role.
[1770]341   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
342   protocol element from an invalid construct.  HTTP does not define
343   specific error handling mechanisms except when they have a direct impact
344   on security, since different applications of the protocol require
345   different error handling strategies.  For example, a Web browser might
346   wish to transparently recover from a response where the
347   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
348   whereas a systems control client might consider any form of error recovery
349   to be dangerous.
[424]353<section title="Syntax Notation" anchor="notation">
[949]354   <x:anchor-alias value="ALPHA"/>
355   <x:anchor-alias value="CR"/>
356   <x:anchor-alias value="DIGIT"/>
357   <x:anchor-alias value="DQUOTE"/>
358   <x:anchor-alias value="LF"/>
359   <x:anchor-alias value="OCTET"/>
360   <x:anchor-alias value="SP"/>
361   <x:anchor-alias value="VCHAR"/>
[1518]363   This specification uses the Augmented Backus-Naur Form (ABNF) notation
364   of <xref target="RFC5234"/> with the list rule extension defined in
365   &notation;<xref target="collected.abnf"/> shows the collected ABNF
366   with the list rule expanded.
[949]369   The following core rules are included by reference, as defined in <xref
370   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
371   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
372   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
[1425]373   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
[424]376<section title="Core Rules" anchor="core.rules">
[949]377   <x:anchor-alias value="quoted-string"/>
378   <x:anchor-alias value="token"/>
379   <x:anchor-alias value="OWS"/>
[1396]381   The core rules below are defined in <xref target="Part1"/>:
383<figure><artwork type="abnf2616">
[1518]384  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
385  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-components;&gt;
386  <x:ref>token</x:ref>         = &lt;token, defined in &field-components;&gt;
[949]390<section title="ABNF Rules defined in other Parts of the Specification"
391    anchor="abnf.dependencies">
392   <x:anchor-alias value="field-name"/>
393   <x:anchor-alias value="HTTP-date"/>
394   <x:anchor-alias value="port"/>
395   <x:anchor-alias value="pseudonym"/>
396   <x:anchor-alias value="uri-host"/>
[949]398   The ABNF rules below are defined in other parts:
[207]400<figure><!--Part1--><artwork type="abnf2616">
[647]401  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
[1436]402  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
[374]403  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
[229]404  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
[374]405  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
[1323]409<section title="Delta Seconds" anchor="delta-seconds">
411   The delta-seconds rule specifies a non-negative integer, representing time
412   in seconds.
414<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
415  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
418   If an implementation receives a delta-seconds value larger than the largest
419   positive integer it can represent, or if any of its subsequent calculations
[1746]420   overflows, it &MUST; consider the value to be 2147483648
421   (2<x:sup>31</x:sup>). Recipients parsing a delta-seconds value &MUST; use
422   an arithmetic type of at least 31 bits of range, and senders &MUST-NOT;
423   send delta-seconds with a value greater than 2147483648.
[1751]430<section anchor="caching.overview" title="Overview of Cache Operation">
[1374]431<iref item="cache entry" />
432<iref item="cache key" />
434   Proper cache operation preserves the semantics of HTTP transfers
435   (&semantics;) while eliminating the transfer of information already held
436   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
437   we assume that reusing the cached response is desirable and that such
438   reuse is the default behavior when no requirement or locally-desired
439   configuration prevents it.  Therefore, HTTP cache requirements are focused
440   on preventing a cache from either storing a non-reusable response or
441   reusing a stored response inappropriately.
444   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
[1746]445   HTTP responses corresponding to prior requests that used the same key. The
446   most common form of cache entry is a successful result of a retrieval
447   request: i.e., a <x:ref>200 (OK)</x:ref> response containing a
448   representation of the resource identified by the request target. However,
449   it is also possible to cache negative results (e.g., <x:ref>404 (Not
450   Found)</x:ref>, incomplete results (e.g., <x:ref>206 (Partial
[1754]451   Content)</x:ref>), and responses to methods other than GET if the method's
452   definition allows such caching and defines something suitable for use as a
453   cache key.
456   The default <x:dfn>cache key</x:dfn> consists of the request method and
457   target URI.  However, since HTTP caches in common use today are typically
[1752]458   limited to caching responses to GET, many implementations simply decline
[1374]459   other methods and use only the URI as the key.
462   If a request target is subject to content negotiation, its cache entry
463   might consist of multiple stored responses, each differentiated by a
464   secondary key for the values of the original request's selecting header
465   fields (<xref target="caching.negotiated.responses"/>).
[1751]469<section anchor="response.cacheability" title="Storing Responses in Caches">
[949]471   A cache &MUST-NOT; store a response to any request, unless:
472   <list style="symbols">
473      <t>The request method is understood by the cache and defined as being
474      cacheable, and</t>
475      <t>the response status code is understood by the cache, and</t>
476      <t>the "no-store" cache directive (see <xref
477      target="header.cache-control" />) does not appear in request or response
[994]478      header fields, and</t>
[949]479      <t>the "private" cache response directive (see <xref
[1793]480      target="cache-response-directive.private" />) does not appear in the
481      response, if the cache is shared, and</t>
[1746]482      <t>the <x:ref>Authorization</x:ref> header field (see
483      &header-authorization;) does not appear in the request, if the cache is
484      shared, unless the response explicitly allows it (see <xref
485      target="caching.authenticated.responses" />), and</t>
[949]486      <t>the response either:
487         <list style="symbols">
[1737]488            <t>contains an <x:ref>Expires</x:ref> header field (see
489            <xref target="header.expires"/>), or</t>
[949]490            <t>contains a max-age response cache directive (see <xref
[1793]491            target="cache-response-directive.max-age" />), or</t>
[949]492            <t>contains a s-maxage response cache directive and the cache is
493            shared, or</t>
494            <t>contains a Cache Control Extension (see <xref
495            target="cache.control.extensions" />) that allows it to be cached,
496            or</t>
497            <t>has a status code that can be served with heuristic freshness
498            (see <xref target="heuristic.freshness" />).</t>
499         </list>
500      </t>
501   </list>
[1292]504   Note that any of the requirements listed above can be overridden by a
[1291]505   cache-control extension; see <xref target="cache.control.extensions" />.
[949]508   In this context, a cache has "understood" a request method or a response
[1374]509   status code if it recognizes it and implements any cache-specific
510   behavior.
[1752]513   Note that, in normal operation, many caches will not store a response that
[949]514   has neither a cache validator nor an explicit expiration time, as such
515   responses are not usually useful to store. However, caches are not
516   prohibited from storing such responses.
519<section anchor="incomplete.responses" title="Storing Incomplete Responses">
[1746]521   A response message is considered complete when all of the octets indicated
522   by the message framing (&messaging;) are received prior to the connection
523   being closed. If the request is GET, the response status is <x:ref>200
524   (OK)</x:ref>, and the entire response header block has been received, a
525   cache &MAY; store an incomplete response message body if the cache entry is
526   recorded as incomplete. Likewise, a <x:ref>206 (Partial Content)</x:ref>
527   response &MAY; be stored as if it were an incomplete <x:ref>200
528   (OK)</x:ref> cache entry. However, a cache &MUST-NOT; store incomplete or
529   partial content responses if it does not support the <x:ref>Range</x:ref>
530   and <x:ref>Content-Range</x:ref> header fields or if it does not understand
531   the range units used in those fields.
[1374]534   A cache &MAY; complete a stored incomplete response by making a subsequent
535   range request (&partial;) and combining the successful response with the
[1746]536   stored entry, as defined in <xref target="combining.responses"/>. A cache
537   &MUST-NOT; use an incomplete response to answer requests unless the
538   response has been made complete or the request is partial and specifies a
539   range that is wholly within the incomplete response. A cache &MUST-NOT;
540   send a partial response to a client without explicitly marking it as such
541   using the <x:ref>206 (Partial Content)</x:ref> status code.
[1751]546<section anchor="caching.authenticated.responses" 
547   title="Storing Responses to Authenticated Requests">
549   A shared cache &MUST-NOT; use a cached response to a request with an
550   <x:ref>Authorization</x:ref> header field (&header-authorization;) to
551   satisfy any subsequent request unless a cache directive that allows such
552   responses to be stored is present in the response.
555   In this specification, the following <x:ref>Cache-Control</x:ref> response
556   directives (<xref target="cache-response-directive"/>) have such an effect:
557   must-revalidate, public, s-maxage.
560   Note that cached responses that contain the "must-revalidate" and/or
561   "s-maxage" response directives are not allowed to be served stale (<xref
562   target="serving.stale.responses"/>) by shared caches. In particular, a
563   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
564   used to satisfy a subsequent request without revalidating it on the origin
565   server.
[949]571<section anchor="constructing.responses.from.caches" 
572   title="Constructing Responses from Caches">
[949]574   For a presented request, a cache &MUST-NOT; return a stored response,
575   unless:
576   <list style="symbols">
[965]577      <t>The presented effective request URI (&effective-request-uri;) and
[949]578      that of the stored response match, and</t>
579      <t>the request method associated with the stored response allows it to
580      be used for the presented request, and</t>
[1163]581      <t>selecting header fields nominated by the stored response (if any)
[949]582      match those presented (see <xref target="caching.negotiated.responses"
583      />), and</t>
[1483]584      <t>the presented request does not contain the no-cache pragma (<xref
585      target="header.pragma"/>), nor the no-cache cache directive (<xref
586      target="cache-request-directive"/>), unless the stored response is
587      successfully validated (<xref target="validation.model"/>), and</t>
588      <t>the stored response does not contain the no-cache cache directive
[1793]589      (<xref target=""/>), unless it is
590      successfully validated (<xref target="validation.model"/>), and</t>
[949]591      <t>the stored response is either:
592         <list style="symbols">
593            <t>fresh (see <xref target="expiration.model" />), or</t>
594            <t>allowed to be served stale (see <xref
595            target="serving.stale.responses" />), or</t>
596            <t>successfully validated (see <xref target="validation.model"
597            />).</t>
598         </list>
599      </t>
[540]600  </list>
[1292]603   Note that any of the requirements listed above can be overridden by a
[1291]604   cache-control extension; see <xref target="cache.control.extensions" />.
[949]607   When a stored response is used to satisfy a request without validation,
[1737]608   a cache &MUST; include a single <x:ref>Age</x:ref> header field
609   (<xref target="header.age"/>) in the response with a value equal to the
610   stored response's current_age; see <xref target="age.calculations" />.
[1746]613   A cache &MUST; write through requests with methods that are unsafe
614   (&safe-methods;) to the origin server; i.e., a cache is not allowed to
615   generate a reply to such a request before having forwarded the request and
616   having received a corresponding response.
[949]619   Also, note that unsafe requests might invalidate already stored responses;
620   see <xref target="invalidation.after.updates.or.deletions" />.
[1277]623   When more than one suitable response is stored, a cache &MUST; use the
[1740]624   most recent response (as determined by the <x:ref>Date</x:ref> header
625   field). It can also forward a request with "Cache-Control: max-age=0" or
626   "Cache-Control: no-cache" to disambiguate which response to use.
[1746]629   A cache that does not have a clock available &MUST-NOT; use stored
630   responses without revalidating them on every use. A cache, especially a
631   shared cache, &SHOULD; use a mechanism, such as NTP <xref
632   target="RFC1305"/>, to synchronize its clock with a reliable external
633   standard.
[540]637<section anchor="expiration.model" title="Freshness Model">
[949]639   When a response is "fresh" in the cache, it can be used to satisfy
640   subsequent requests without contacting the origin server, thereby improving
641   efficiency.
[949]644   The primary mechanism for determining freshness is for an origin server to
[1746]645   provide an explicit expiration time in the future, using either the
646   <x:ref>Expires</x:ref> header field (<xref target="header.expires" />) or
647   the max-age response cache directive (<xref
[1793]648   target="cache-response-directive.max-age" />). Generally, origin servers will
[1746]649   assign future explicit expiration times to responses in the belief that the
650   representation is not likely to change in a semantically significant way
651   before the expiration time is reached.
[949]654   If an origin server wishes to force a cache to validate every request, it
[969]655   can assign an explicit expiration time in the past to indicate that the
[1113]656   response is already stale. Compliant caches will normally validate the
657   cached response before reusing it for subsequent requests (see <xref
658   target="serving.stale.responses" />).
[1746]661   Since origin servers do not always provide explicit expiration times, a
662   cache &MAY; assign a heuristic expiration time when an explicit time is not
663   specified, employing algorithms that use other header field values (such as
664   the <x:ref>Last-Modified</x:ref> time) to estimate a plausible expiration
665   time. This specification does not provide specific algorithms, but does
666   impose worst-case constraints on their results.
[575]670  The calculation to determine if a response is fresh is:
672<artwork type="code">
[538]673   response_is_fresh = (freshness_lifetime &gt; current_age)
[949]677   The freshness_lifetime is defined in <xref
678   target="calculating.freshness.lifetime" />; the current_age is defined in
679   <xref target="age.calculations" />.
[1692]682   Additionally, clients can influence freshness calculation &mdash; either
683   constraining it relaxing it &mdash; by using the max-age and min-fresh
[1483]684   request cache directives. See <xref target="cache-request-directive" /> 
685   for details.
[949]688   Note that freshness applies only to cache operation; it cannot be used to
689   force a user agent to refresh its display or reload a resource. See <xref
690   target="history.lists" /> for an explanation of the difference between
691   caches and history mechanisms.
[949]694<section anchor="calculating.freshness.lifetime" 
695   title="Calculating Freshness Lifetime">
[949]697   A cache can calculate the freshness lifetime (denoted as
698   freshness_lifetime) of a response by using the first match of:
699   <list style="symbols">
700      <t>If the cache is shared and the s-maxage response cache directive
[1793]701      (<xref target="cache-response-directive.s-maxage" />) is present, use its value,
[949]702      or</t>
703      <t>If the max-age response cache directive (<xref
[1793]704      target="cache-response-directive.max-age" />) is present, use its value, or</t>
[1737]705      <t>If the <x:ref>Expires</x:ref> response header field
706      (<xref target="header.expires" />) is present, use its value minus the
[1740]707      value of the <x:ref>Date</x:ref> response header field, or</t>
[949]708      <t>Otherwise, no explicit expiration time is present in the response. A
709      heuristic freshness lifetime might be applicable; see <xref
710      target="heuristic.freshness" />.</t>
711   </list>
[949]714   Note that this calculation is not vulnerable to clock skew, since all of
715   the information comes from the origin server.
718   When there is more than one value present for a given directive (e.g., two
[1737]719   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
720   directives), it is considered invalid. Caches are encouraged to consider
721   responses that have invalid freshness information to be stale.
[540]725<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
[949]727   If no explicit expiration time is present in a stored response that has a
728   status code whose definition allows heuristic freshness to be used
[1746]729   (including the following in &status-codes;: <x:ref>200 (OK)</x:ref>,
730   <x:ref>203 (Non-Authoritative Information)</x:ref>, <x:ref>206 (Partial
731   Content)</x:ref>, <x:ref>300 (Multiple Choices)</x:ref>, <x:ref>301 (Moved
732   Permanently)</x:ref> and <x:ref>410 (Gone)</x:ref>), a cache &MAY;
733   calculate a heuristic expiration time. A cache &MUST-NOT; use heuristics to
734   determine freshness for responses with status codes that do not explicitly
735   allow it.
[1737]738   When a heuristic is used to calculate freshness lifetime, a cache &SHOULD;
739   attach a <x:ref>Warning</x:ref> header field with a 113 warn-code to the
740   response if its current_age is more than 24 hours and such a warning is not
741   already present.
[1739]744   Also, if the response has a <x:ref>Last-Modified</x:ref> header field
[1433]745   (&header-last-modified;), caches are encouraged to use a heuristic
746   expiration value that is no more than some fraction of the interval since
747   that time. A typical setting of this fraction might be 10%.
[949]750   <t>
[1755]751      &Note; <xref target="RFC2616" x:fmt="of" x:sec="13.9"/> prohibited caches
[1753]752      from calculating heuristic freshness for URIs with query components
753      (i.e., those containing '?'). In practice, this has not been widely
754      implemented. Therefore, servers are encouraged to send explicit
755      directives (e.g., Cache-Control: no-cache) if they wish to preclude
756      caching.
[949]757   </t>
[540]761<section anchor="age.calculations" title="Calculating Age">
[1737]763   HTTP/1.1 uses the <x:ref>Age</x:ref> header field to convey the estimated
764   age of the response message when obtained from a cache. The Age field value
[1746]765   is the cache's estimate of the amount of time since the response was
766   generated or validated by the origin server. In essence, the Age value is
767   the sum of the time that the response has been resident in each of the
768   caches along the path from the origin server, plus the amount of time it
769   has been in transit along network paths.
[949]772   The following data is used for the age calculation:
[949]775   <x:dfn>age_value</x:dfn>
776   <list>
777      <t>
[1737]778         The term "age_value" denotes the value of the <x:ref>Age</x:ref>
779         header field (<xref target="header.age"/>), in a form appropriate for
780         arithmetic operation; or 0, if not available.
[949]781      </t>
782   </list>
[949]785   <x:dfn>date_value</x:dfn>
786   <list>
787      <t>
[1740]788         HTTP/1.1 requires origin servers to send a <x:ref>Date</x:ref> header
789         field, if possible, with every response, giving the time at which the
[1746]790         response was generated. The term "date_value" denotes the value of
791         the Date header field, in a form appropriate for arithmetic
792         operations. See &header-date; for the definition of the Date header
793         field, and for requirements regarding responses without it.
[949]794      </t>
795   </list>
[949]798   <x:dfn>now</x:dfn>
799   <list>
800      <t>
801         The term "now" means "the current value of the clock at the host
[1112]802         performing the calculation". A cache &SHOULD; use NTP (<xref
803         target="RFC1305"/>) or some similar protocol to synchronize its
[949]804         clocks to a globally accurate time standard.
805      </t>
806   </list>
[949]809   <x:dfn>request_time</x:dfn>
810   <list>
811      <t>
812         The current value of the clock at the host at the time the request
813         resulting in the stored response was made.
814      </t>
815   </list>
[949]818   <x:dfn>response_time</x:dfn>
819   <list>
820      <t>
821         The current value of the clock at the host at the time the response
822         was received.
823      </t>
824   </list>
[949]827   A response's age can be calculated in two entirely independent ways:
828   <list style="numbers">
829      <t>the "apparent_age": response_time minus date_value, if the local
830      clock is reasonably well synchronized to the origin server's clock. If
831      the result is negative, the result is replaced by zero.</t>
832      <t>the "corrected_age_value", if all of the caches along the response
[1112]833      path implement HTTP/1.1. A cache &MUST; interpret this value relative
[949]834      to the time the request was initiated, not the time that the response
835      was received.</t>
836   </list>
839<artwork type="code">
[786]840  apparent_age = max(0, response_time - date_value);
[786]842  response_delay = response_time - request_time;
[792]843  corrected_age_value = age_value + response_delay; 
[1484]847<preamble>These &SHOULD; be combined as</preamble>
[792]848<artwork type="code">
849  corrected_initial_age = max(apparent_age, corrected_age_value);
[1737]852   unless the cache is confident in the value of the <x:ref>Age</x:ref> header
[1741]853   field (e.g., because there are no HTTP/1.0 hops in the <x:ref>Via</x:ref>
854   header field), in which case the corrected_age_value &MAY; be used as the
[1737]855   corrected_initial_age.</t>
[949]857   The current_age of a stored response can then be calculated by adding the
858   amount of time (in seconds) since the stored response was last validated by
859   the origin server to the corrected_initial_age.
[786]861<figure><artwork type="code">
[540]862  resident_time = now - response_time;
[786]863  current_age = corrected_initial_age + resident_time;
[1418]866   Additionally, to avoid common problems in date parsing:
869  <list style="symbols">
870     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
871        which appears to be more than 50 years in the future is in fact
872        in the past (this helps solve the "year 2000" problem).</t>
874     <t>Although all date formats are specified to be case-sensitive,
875        recipients &SHOULD; match day, week and timezone names
876        case-insensitively.</t>
878     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
[1737]879        <x:ref>Expires</x:ref> date as earlier than the proper value, but
880        &MUST-NOT; internally represent a parsed Expires date as later than the
[1390]881        proper value.</t>
883     <t>All expiration-related calculations &MUST; be done in GMT. The
884        local time zone &MUST-NOT; influence the calculation or comparison
885        of an age or expiration time.</t>
887     <t>If an HTTP header field incorrectly carries a date value with a time
888        zone other than GMT, it &MUST; be converted into GMT using the
889        most conservative possible conversion.</t>
890  </list>
[540]894<section anchor="serving.stale.responses" title="Serving Stale Responses">
[949]896   A "stale" response is one that either has explicit expiry information or is
897   allowed to have heuristic expiry calculated, but is not fresh according to
898   the calculations in <xref target="expiration.model" />.
[1112]901   A cache &MUST-NOT; return a stale response if it is prohibited by an
[949]902   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
903   directive, a "must-revalidate" cache-response-directive, or an applicable
904   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
905   target="cache-response-directive"/>).
[1433]908   A cache &MUST-NOT; return stale responses unless it is disconnected
[949]909   (i.e., it cannot contact the origin server or otherwise find a forward
[1264]910   path) or doing so is explicitly allowed (e.g., by the max-stale request
[949]911   directive; see <xref target="cache-request-directive" />).
[1737]914   A cache &SHOULD; append a <x:ref>Warning</x:ref> header field with the 110
915   warn-code (see <xref target="header.warning"/>) to stale responses.
916   Likewise, a cache &SHOULD; add the 112 warn-code to stale responses if the
917   cache is disconnected.
[949]920   If a cache receives a first-hand response (either an entire response, or a
[1746]921   <x:ref>304 (Not Modified)</x:ref> response) that it would normally forward
922   to the requesting client, and the received response is no longer fresh, the
923   cache can forward it to the requesting client without adding a new
924   <x:ref>Warning</x:ref> (but without removing any existing Warning header
925   fields). A cache shouldn't attempt to validate a response simply because
926   that response became stale in transit.
[540]931<section anchor="validation.model" title="Validation Model">
[949]933   When a cache has one or more stored responses for a requested URI, but
934   cannot serve any of them (e.g., because they are not fresh, or one cannot
935   be selected; see <xref target="caching.negotiated.responses"/>), it can use
936   the conditional request mechanism &conditional; in the forwarded request to
937   give the origin server an opportunity to both select a valid stored
938   response to be used, and to update it. This process is known as
939   "validating" or "revalidating" the stored response.
[1739]942   When sending such a conditional request, a cache adds an
943   <x:ref>If-Modified-Since</x:ref> header field whose value is that of the
944   <x:ref>Last-Modified</x:ref> header field from the selected
945   (see <xref target="caching.negotiated.responses"/>) stored response, if
946   available.
[1739]949   Additionally, a cache can add an <x:ref>If-None-Match</x:ref> header field
950   whose value is that of the <x:ref>ETag</x:ref> header field(s) from all
951   responses stored for the requested URI, if present. However, if any of the
952   stored responses contains only partial content, the cache shouldn't
953   include its entity-tag in the If-None-Match header field unless the request
954   is for a range that would be fully satisfied by that stored response.
[1746]957<t>Cache handling of a response to a conditional request is dependent upon its
958status code:</t>
[1419]961   <list style="symbols">
962      <t>
[1746]963         A <x:ref>304 (Not Modified)</x:ref> response status code indicates
964         that the stored response can be updated and reused; see <xref
[1419]965         target="freshening.responses"/>.
966      </t>
967      <t>
968         A full response (i.e., one with a response body) indicates that none
969         of the stored responses nominated in the conditional request is
[1433]970         suitable. Instead, the cache can use the full response to
971         satisfy the request and &MAY; replace the stored response(s).
[1419]972      </t>
973      <t>
[1735]974         However, if a cache receives a <x:ref>5xx (Server Error)</x:ref>
975         response while attempting to validate a response, it can either
976         forward this response to the requesting client, or act as if the
977         server failed to respond. In the latter case, it can return a
[1746]978         previously stored response (see <xref
979         target="serving.stale.responses" />).
[1419]980      </t>
981   </list>
[1554]984<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
[1746]986   When a cache receives a <x:ref>304 (Not Modified)</x:ref> response and
987   already has one or more stored <x:ref>200 (OK)</x:ref> responses for the
988   same cache key, the cache needs to identify which of the stored responses
989   are updated by this new response and then update the stored response(s)
990   with the new information provided in the <x:ref>304</x:ref> response.
[1419]991   <list style="symbols">
992    <t>
993     If the new response contains a strong validator, then that strong
994     validator identifies the selected representation.  All of the stored
995     responses with the same strong validator are selected.
996     If none of the stored responses contain the same strong validator, then
997     this new response corresponds to a new selected representation and
998     &MUST-NOT; update the existing stored responses.
999    </t>
1000    <t>
1001     If the new response contains a weak validator and that validator
1002     corresponds to one of the cache's stored responses, then the most
1003     recent of those matching stored responses is selected.
1004    </t>
1005    <t>
1006     If the new response does not include any form of validator, there is
1007     only one stored response, and that stored response also lacks a
1008     validator, then that stored response is selected.
1009    </t>
1010   </list>
[1419]1013   If a stored response is selected for update, the cache &MUST;:
1014   <list style="symbols">
[1737]1015      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1016         with warn-code 1xx (see <xref target="header.warning" />);</t>
1017      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1018         with warn-code 2xx; and,</t>
[1734]1019      <t>use other header fields provided in the <x:ref>304 (Not Modified)</x:ref>
1020         response to replace all instances of the corresponding header
1021         fields in the stored response.</t>
[1419]1022   </list>
[949]1028<section anchor="caching.negotiated.responses" 
[1751]1029   title="Using Negotiated Responses">
[949]1031   When a cache receives a request that can be satisfied by a stored response
[1737]1032   that has a <x:ref>Vary</x:ref> header field (<xref target="header.vary"/>),
1033   it &MUST-NOT; use that response unless all of the selecting header fields
[1746]1034   nominated by the Vary header field match in both the original request
1035   (i.e., that associated with the stored response), and the presented
1036   request.
[1163]1039   The selecting header fields from two requests are defined to match if and
[949]1040   only if those in the first request can be transformed to those in the
1041   second request by applying any of the following:
1042   <list style="symbols">
1043      <t>
[1746]1044         adding or removing whitespace, where allowed in the header field's
1045         syntax
[949]1046      </t>
1047      <t>
[1163]1048         combining multiple header fields with the same field name
[949]1049         (see &header-fields;)
1050      </t>
1051      <t>
[994]1052         normalizing both header field values in a way that is known to have
[1746]1053         identical semantics, according to the header field's specification
1054         (e.g., re-ordering field values when order is not significant;
[949]1055         case-normalization, where values are defined to be case-insensitive)
1056      </t>
[771]1057  </list>
[949]1060   If (after any normalization that might take place) a header field is absent
1061   from a request, it can only match another request if it is also absent
1062   there.
[1737]1065   A <x:ref>Vary</x:ref> header field-value of "*" always fails to match, and
1066   subsequent requests to that resource can only be properly interpreted by the
1067   origin server.
[1163]1070   The stored response with matching selecting header fields is known as the
[949]1071   selected response.
[1292]1074   If multiple selected responses are available, the most recent response
[1740]1075   (as determined by the <x:ref>Date</x:ref> header field) is used; see <xref 
[1292]1076   target="constructing.responses.from.caches"/>.
[1433]1079   If no selected response is available, the cache can forward the presented
[949]1080   request to the origin server in a conditional request; see <xref
1081   target="validation.model"/>.
[1374]1086<section anchor="combining.responses" title="Combining Partial Content">
[1374]1088   A response might transfer only a partial representation if the
1089   connection closed prematurely or if the request used one or more Range
1090   specifiers (&partial;).  After several such transfers, a cache might have
1091   received several ranges of the same representation.  A cache &MAY; combine
1092   these ranges into a single stored response, and reuse that response to
1093   satisfy later requests, if they all share the same strong validator and
1094   the cache complies with the client requirements in &combining-byte-ranges;.
[1374]1097   When combining the new response with one or more stored responses, a
1098   cache &MUST;:
1099   <list style="symbols">
[1737]1100      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1101         with warn-code 1xx (see <xref target="header.warning" />);</t>
1102      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1103         with warn-code 2xx; and,</t>
[1374]1104      <t>use other header fields provided in the new response, aside
[1738]1105         from <x:ref>Content-Range</x:ref>, to replace all instances of the
1106         corresponding header fields in the stored response.</t>
[1374]1107   </list>
[1751]1113<section anchor="head.effects" title="Updating Caches with HEAD Responses">
1115   A response to the HEAD method is identical to what an equivalent request
1116   made with a GET would have been, except it lacks a body. This property
1117   of HEAD responses is used to both invalidate and update cached GET
1118   responses.
1121   If one or more stored GET responses can be selected (as per <xref
1122   target="caching.negotiated.responses"/>) for a HEAD request, and the
1123   <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> or
1124   <x:ref>Last-Modified</x:ref> value of a HEAD response differs from that in a
1125   selected GET response, the cache &MUST; consider that selected response to
1126   be stale.
1129   If the <x:ref>Content-Length</x:ref>, <x:ref>ETag</x:ref> and
1130   <x:ref>Last-Modified</x:ref> values of a HEAD response (when present) are
1131   the same as that in a selected GET response (as per
1132   <xref target="caching.negotiated.responses"/>), the cache &SHOULD; update
[1772]1133   the remaining header fields in the stored response using the following
1134   rules:
[1751]1135   <list style="symbols">
1136      <t>delete any <x:ref>Warning</x:ref> header fields in the stored response
1137         with warn-code 1xx (see <xref target="header.warning" />);</t>
1138      <t>retain any <x:ref>Warning</x:ref> header fields in the stored response
1139         with warn-code 2xx; and,</t>
1140      <t>use other header fields provided in the response to replace
1141         all instances of the corresponding header fields in the stored
1142         response.</t>
1143   </list>
1149<section anchor="invalidation.after.updates.or.deletions" 
1150   title="Request Methods that Invalidate">
1152   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1153   have the potential for changing state on the origin server, intervening
1154   caches can use them to keep their contents up-to-date.
1157   A cache &MUST; invalidate the effective Request URI
1158   (&effective-request-uri;) as well as the URI(s) in the
1159   <x:ref>Location</x:ref> and <x:ref>Content-Location</x:ref> response header
1160   fields (if present) when a non-error response to a request with an unsafe
1161   method is received.
1164   However, a cache &MUST-NOT; invalidate a URI from a <x:ref>Location</x:ref>
1165   or <x:ref>Content-Location</x:ref> response header field if the host part of
1166   that URI differs from the host part in the effective request URI
1167   (&effective-request-uri;). This helps prevent denial of service attacks.
1170   A cache &MUST; invalidate the effective request URI
1171   (&effective-request-uri;) when it receives a non-error response
1172   to a request with a method whose safety is unknown.
1175   Here, a "non-error response" is one with a <x:ref>2xx (Successful)</x:ref>
1176   or <x:ref>3xx (Redirection)</x:ref> status code. "Invalidate" means that
1177   the cache will either remove all stored responses related to the effective
1178   request URI, or will mark these as "invalid" and in need of a mandatory
1179   validation before they can be returned in response to a subsequent request.
1182   Note that this does not guarantee that all appropriate responses are
1183   invalidated. For example, the request that caused the change at the origin
1184   server might not have gone through the cache where a response is stored.
[1415]1191<section anchor="header.field.definitions" title="Header Field Definitions">
[949]1193   This section defines the syntax and semantics of HTTP/1.1 header fields
1194   related to caching.
[540]1197<section anchor="header.age" title="Age">
[1120]1198   <iref item="Age header field" primary="true" x:for-anchor="" />
1199   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
[949]1200   <x:anchor-alias value="Age"/>
1201   <x:anchor-alias value="age-value"/>
[1163]1203   The "Age" header field conveys the sender's estimate of the amount
[949]1204   of time since the response was generated or successfully validated at the
1205   origin server. Age values are calculated as specified in <xref
1206   target="age.calculations" />.
[1231]1208<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1209  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
[1323]1212  Age field-values are non-negative integers, representing time in seconds
1213  (see <xref target="delta-seconds"/>).
[949]1216   The presence of an Age header field in a response implies that a response
1217   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1218   might not implement the Age header field.
[540]1222<section anchor="header.cache-control" title="Cache-Control">
[1120]1223   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1224   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
[949]1225      x:for-anchor="" />
1226   <x:anchor-alias value="Cache-Control"/>
1227   <x:anchor-alias value="cache-directive"/>
[1163]1229   The "Cache-Control" header field is used to specify directives for
[949]1230   caches along the request/response chain. Such cache directives are
1231   unidirectional in that the presence of a directive in a request does not
1232   imply that the same directive is to be given in the response.
[1112]1235   A cache &MUST; obey the requirements of the Cache-Control
[949]1236   directives defined in this section. See <xref
1237   target="cache.control.extensions"/> for information about how Cache-Control
1238   directives defined elsewhere are handled.
[949]1241   <t>
[1692]1242       &Note; HTTP/1.0 caches might not implement Cache-Control and
[949]1243       might only implement Pragma: no-cache (see <xref target="header.pragma"
1244       />).
1245   </t>
[1173]1248   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1249   through in forwarded messages, regardless of their
[1112]1250   significance to that application, since the directives might be applicable
1251   to all recipients along the request/response chain. It is not possible to
1252   target a directive to a specific cache.
1255   Cache directives are identified by a token, to be compared case-insensitively,
1256   and have an optional argument, that can use both token and quoted-string
1257   syntax. For the directives defined below that define arguments, recipients
1258   ought to accept both forms, even if one is documented to be preferred. For
1259   any directive not defined by this specification, recipients &MUST; accept
1260   both forms.
1262<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/>
[1231]1263  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
[1695]1265  <x:ref>cache-directive</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1268   For the cache directives defined below, no argument is defined (nor allowed)
1269   otherwise stated otherwise.
[1779]1272<section title="Request Cache-Control Directives" anchor="cache-request-directive">
[1695]1274<section title="no-cache" anchor="">
[949]1275   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1276   <iref item="no-cache" primary="true" subitem="Cache Directive" />
[1779]1278   The "no-cache" request directive indicates that a cache &MUST-NOT; 
[1695]1279   use a stored response to satisfy the request without successful
1280   validation on the origin server.
1284<section title="no-store" anchor="">
[949]1285   <iref item="Cache Directives" primary="true" subitem="no-store" />
1286   <iref item="no-store" primary="true" subitem="Cache Directive" />
[1779]1288   The "no-store" request directive indicates that a cache &MUST-NOT;
[1695]1289   store any part of either this request or any response to it. This
1290   directive applies to both private and shared caches. "&MUST-NOT;
1291   store" in this context means that the cache &MUST-NOT; intentionally
1292   store the information in non-volatile storage, and &MUST; make a
1293   best-effort attempt to remove the information from volatile storage as
1294   promptly as possible after forwarding it.
[1695]1297   This directive is NOT a reliable or sufficient mechanism for ensuring
1298   privacy. In particular, malicious or compromised caches might not
1299   recognize or obey this directive, and communications networks might be
1300   vulnerable to eavesdropping.
1303   Note that if a request containing this directive is satisfied from a
1304   cache, the no-store request directive does not apply to the already
1305   stored response.
1309<section title="max-age" anchor="cache-request-directive.max-age">
[949]1310   <iref item="Cache Directives" primary="true" subitem="max-age" />
1311   <iref item="max-age" primary="true" subitem="Cache Directive" />
1313   Argument syntax:
[949]1314   <list>
[1695]1315      <t>
1316        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1317      </t>
[949]1318   </list>
[1779]1321   The "max-age" request directive indicates that the client is unwilling to
[1695]1322   accept a response whose age is greater than the specified number of
1323   seconds. Unless the max-stale request directive is also present, the
1324   client is not willing to accept a stale response.
1327   &Note; This directive uses the token form of the argument syntax;
1328   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1329   quoted-string form.
1333<section title="max-stale" anchor="cache-request-directive.max-stale">
[949]1334   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1335   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1337   Argument syntax:
[949]1338   <list>
[1695]1339      <t>
1340        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1341      </t>
[949]1342   </list>
[1779]1345   The "max-stale" request directive indicates that the client is willing
[1695]1346   to accept a response that has exceeded its expiration time. If max-stale
1347   is assigned a value, then the client is willing to accept a response
1348   that has exceeded its expiration time by no more than the specified
1349   number of seconds. If no value is assigned to max-stale, then the client
1350   is willing to accept a stale response of any age.
1353   &Note; This directive uses the token form of the argument syntax;
1354   e.g., 'max-stale=10', not 'max-stale="10"'. Senders &SHOULD-NOT; use the
1355   quoted-string form.
1359<section title="min-fresh" anchor="cache-request-directive.min-fresh">
[949]1360   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1361   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1363   Argument syntax:
[949]1364   <list>
[1695]1365      <t>
1366        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1367      </t>
[949]1368   </list>
[1779]1371   The "min-fresh" request directive indicates that the client is willing
[1695]1372   to accept a response whose freshness lifetime is no less than its
1373   current age plus the specified time in seconds. That is, the client
1374   wants a response that will still be fresh for at least the specified
1375   number of seconds.
1378   &Note; This directive uses the token form of the argument syntax;
1379   e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders &SHOULD-NOT; use the
1380   quoted-string form.
1384<section title="no-transform" anchor="">
[949]1385   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1386   <iref item="no-transform" primary="true" subitem="Cache Directive" />
[1779]1388   The "no-transform" request directive indicates that an intermediary
[1695]1389   (whether or not it implements a cache) &MUST-NOT; change the
[1740]1390   <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Range</x:ref> or
1391   <x:ref>Content-Type</x:ref> request header fields, nor the request
1392   representation.
1396<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
[949]1397   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1398   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
[1779]1400   The "only-if-cached" request directive indicates that the client only wishes
[1775]1401   to obtain a stored response. If it receives this directive, a cache &SHOULD;
1402   either respond using a stored response that is consistent with the other
[1746]1403   constraints of the request, or respond with a <x:ref>504 (Gateway
1404   Timeout)</x:ref> status code. If a group of caches is being operated as a
1405   unified system with good internal connectivity, a member cache &MAY;
[1695]1406   forward such a request within that group of caches.
[949]1411<section anchor="cache-response-directive" 
1412   title="Response Cache-Control Directives">
1413   <x:anchor-alias value="cache-response-directive" />
[1695]1415<section title="public" anchor="cache-response-directive.only-if-cached">
[949]1416   <iref item="Cache Directives" primary="true" subitem="public" />
1417   <iref item="public" primary="true" subitem="Cache Directive" />
[1779]1419   The "public" response directive indicates that a response whose
[1695]1420   associated request contains an 'Authentication' header &MAY; be
1421   stored (see <xref target="caching.authenticated.responses" />).
1425<section title="private" anchor="cache-response-directive.private">
[949]1426   <iref item="Cache Directives" primary="true" subitem="private" />
1427   <iref item="private" primary="true" subitem="Cache Directive" />
1429   Argument syntax:
[949]1430   <list>
[1695]1431      <t>
1432        #<x:ref>field-name</x:ref>
[1664]1433      </t>
[949]1434   </list>
[1779]1437   The "private" response directive indicates that the response message is
[1695]1438   intended for a single user and &MUST-NOT; be stored by a shared cache. A
1439   private cache &MAY; store the response.
1442   If the private response directive specifies one or more field-names,
1443   this requirement is limited to the field-values associated with the
1444   listed response header fields. That is, a shared cache &MUST-NOT; store
1445   the specified field-names(s), whereas it &MAY; store the remainder of the
1446   response message.
1449   The field-names given are not limited to the set of standard header
1450   fields defined by this specification. Field names are case-insensitive.
1453   &Note; This usage of the word "private" only controls
1454   where the response can be stored; it cannot ensure the privacy of the
1455   message content. Also, private response directives with field-names are
1456   often handled by implementations as if an unqualified private directive
1457   was received; i.e., the special handling for the qualified form is not
1458   widely implemented.
[1746]1461   &Note; This directive uses the quoted-string form of the argument syntax.
1462   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1463   needed for single-entry lists).
1467<section title="no-cache" anchor="">
[949]1468   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1469   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1471   Argument syntax:
[949]1472   <list>
[1695]1473      <t>
1474        #<x:ref>field-name</x:ref>
[1664]1475      </t>
[949]1476   </list>
[1779]1479   The "no-cache" response directive indicates that the response &MUST-NOT;
[1695]1480   be used to satisfy a subsequent request without successful validation on
1481   the origin server. This allows an origin server to prevent a cache from
1482   using it to satisfy a request without contacting it, even by caches that
1483   have been configured to return stale responses.
1486   If the no-cache response directive specifies one or more field-names,
1487   then a cache &MAY; use the response to satisfy a subsequent request,
1488   subject to any other restrictions on caching. However, any header fields
1489   in the response that have the field-name(s) listed &MUST-NOT; be sent
1490   in the response to a subsequent request without successful revalidation
1491   with the origin server. This allows an origin server to prevent the
1492   re-use of certain header fields in a response, while still allowing
1493   caching of the rest of the response.
1496   The field-names given are not limited to the set of standard header
1497   fields defined by this specification. Field names are case-insensitive.
[1752]1500   &Note; Many HTTP/1.0 caches will not recognize or obey
[1695]1501   this directive. Also, no-cache response directives with field-names are
1502   often handled by implementations as if an unqualified no-cache directive
1503   was received; i.e., the special handling for the qualified form is not
1504   widely implemented.
[1746]1507   &Note; This directive uses the quoted-string form of the argument syntax.
1508   Senders &SHOULD-NOT; use the token form (even if quoting appears not to be
1509   needed for single-entry lists).
1513<section title="no-store" anchor="">
[949]1514   <iref item="Cache Directives" primary="true" subitem="no-store" />
1515   <iref item="no-store" primary="true" subitem="Cache Directive" />
[1779]1517   The "no-store" response directive indicates that a cache &MUST-NOT;
[1695]1518   store any part of either the immediate request or response. This
1519   directive applies to both private and shared caches. "&MUST-NOT;
1520   store" in this context means that the cache &MUST-NOT; intentionally
1521   store the information in non-volatile storage, and &MUST; make a
1522   best-effort attempt to remove the information from volatile storage as
1523   promptly as possible after forwarding it.
[1695]1526   This directive is NOT a reliable or sufficient mechanism for ensuring
1527   privacy. In particular, malicious or compromised caches might not
1528   recognize or obey this directive, and communications networks might be
1529   vulnerable to eavesdropping.
1533<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
[949]1534   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1535   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
[1779]1537   The "must-revalidate" response directive indicates that once it has
[1695]1538   become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1539   requests without successful validation on the origin server.
[1695]1542   The must-revalidate directive is necessary to support reliable
1543   operation for certain protocol features. In all circumstances a
1544   cache &MUST; obey the must-revalidate directive; in particular,
1545   if a cache cannot reach the origin server for any reason, it &MUST;
[1710]1546   generate a <x:ref>504 (Gateway Timeout)</x:ref> response.
1549   The must-revalidate directive ought to be used by servers if and only
1550   if failure to validate a request on the representation could result in
1551   incorrect operation, such as a silently unexecuted financial
1552   transaction.
1556<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
[949]1557   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1558   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
[1779]1560   The "proxy-revalidate" response directive has the same meaning as the
[1695]1561   must-revalidate response directive, except that it does not apply to
1562   private caches.
1566<section title="max-age" anchor="cache-response-directive.max-age">
[949]1567   <iref item="Cache Directives" primary="true" subitem="max-age" />
1568   <iref item="max-age" primary="true" subitem="Cache Directive" />
1570   Argument syntax:
[949]1571   <list>
[1695]1572      <t>
1573        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1574      </t>
[949]1575   </list>
[1779]1578   The "max-age" response directive indicates that the response is to be
[1695]1579   considered stale after its age is greater than the specified number of
1580   seconds.
1583   &Note; This directive uses the token form of the argument syntax;
1584   e.g., 'max-age=5', not 'max-age="5"'. Senders &SHOULD-NOT; use the
1585   quoted-string form.
1589<section title="s-maxage" anchor="cache-response-directive.s-maxage">
[949]1590   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1591   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1593   Argument syntax:
[949]1594   <list>
[1695]1595      <t>
1596        <x:ref>delta-seconds</x:ref> (see <xref target="delta-seconds"/>)
1597      </t>
[949]1598   </list>
[1779]1601   The "s-maxage" response directive indicates that, in shared caches, the
[1695]1602   maximum age specified by this directive overrides the maximum age
[1737]1603   specified by either the max-age directive or the <x:ref>Expires</x:ref>
1604   header field. The s-maxage directive also implies the semantics of the
1605   proxy-revalidate response directive.
1608   &Note; This directive uses the token form of the argument syntax;
1609   e.g., 's-maxage=10', not 's-maxage="10"'. Senders &SHOULD-NOT; use the
1610   quoted-string form.
1614<section title="no-transform" anchor="">
[949]1615   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1616   <iref item="no-transform" primary="true" subitem="Cache Directive" />
[1779]1618   The "no-transform" response directive indicates that an intermediary
[1695]1619   (regardless of whether it implements a cache) &MUST-NOT; change the
[1740]1620   <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Range</x:ref> or
1621   <x:ref>Content-Type</x:ref> response header fields, nor the response
1622   representation.
[540]1628<section anchor="cache.control.extensions" title="Cache Control Extensions">
[949]1630   The Cache-Control header field can be extended through the use of one or
1631   more cache-extension tokens, each with an optional value. Informational
1632   extensions (those that do not require a change in cache behavior) can be
1633   added without changing the semantics of other directives. Behavioral
1634   extensions are designed to work by acting as modifiers to the existing base
1635   of cache directives. Both the new directive and the standard directive are
1636   supplied, such that applications that do not understand the new directive
1637   will default to the behavior specified by the standard directive, and those
1638   that understand the new directive will recognize it as modifying the
1639   requirements associated with the standard directive. In this way,
1640   extensions to the cache-control directives can be made without requiring
1641   changes to the base protocol.
[949]1644   This extension mechanism depends on an HTTP cache obeying all of the
1645   cache-control directives defined for its native HTTP-version, obeying
1646   certain extensions, and ignoring all directives that it does not
1647   understand.
[949]1650   For example, consider a hypothetical new response directive called
1651   "community" that acts as a modifier to the private directive. We define
[1112]1652   this new directive to mean that, in addition to any private cache, any
[949]1653   cache that is shared only by members of the community named within its
[1707]1654   value is allowed to cache the response. An origin server wishing to allow
1655   the UCI community to use an otherwise private response in their shared
1656   cache(s) could do so by including
1658<figure><artwork type="example">
[458]1659  Cache-Control: private, community="UCI"
[949]1662   A cache seeing this header field will act correctly even if the cache does
1663   not understand the community cache-extension, since it will also see and
1664   understand the private directive and thus default to the safe behavior.
[1285]1667   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
[949]1668   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1669   combined with standard directives (or the response's default cacheability)
1670   such that the cache behavior will remain minimally correct even if the
1671   cache does not understand the extension(s).
[1701]1674   New extension directives ought to consider defining:
1677   <list style="symbols">
1678      <t>What it means for a directive to be specified multiple times,</t>
1679      <t>When the directive does not take an argument, what it means when an
1680      argument is present,</t>
1681      <t>When the directive requires an argument, what it means when it is
1682      missing.</t>
1683   </list>
[949]1686   The HTTP Cache Directive Registry defines the name space for the cache
1687   directives.
[1112]1690   A registration &MUST; include the following fields:
[949]1691   <list style="symbols">
1692      <t>Cache Directive Name</t>
1693      <t>Pointer to specification text</t>
1694   </list>
[1567]1697   Values to be added to this name space require IETF Review (see <xref
[949]1698   target="RFC5226" x:fmt="," x:sec="4.1"/>).
[949]1701   The registry itself is maintained at <eref
1702   target=""/>.
[540]1708<section anchor="header.expires" title="Expires">
[1120]1709   <iref item="Expires header field" primary="true" x:for-anchor="" />
1710   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
[949]1711   <x:anchor-alias value="Expires"/>
[965]1713   The "Expires" header field gives the date/time after which the
[949]1714   response is considered stale. See <xref target="expiration.model" /> for
1715   further discussion of the freshness model.
[949]1718   The presence of an Expires field does not imply that the original resource
1719   will change or cease to exist at, before, or after that time.
[949]1722   The field-value is an absolute date and time as defined by HTTP-date in
[1436]1723   &http-date;; a sender &MUST; use the rfc1123-date format.
[1231]1725<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1726  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1729  <preamble>For example</preamble>
1730<artwork type="example">
[364]1731  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1734   A cache &MUST; treat other invalid date formats,
1735   especially including the value "0", as in the past (i.e., "already
1736   expired").
[949]1739   <t>
[1737]1740       &Note; If a response includes a <x:ref>Cache-Control</x:ref> field with
[1793]1741       the max-age directive (see <xref target="cache-response-directive.max-age" />),
[949]1742       that directive overrides the Expires field. Likewise, the s-maxage
[1793]1743       directive (<xref target="cache-response-directive.s-maxage" />) overrides
1744       the <x:ref>Expires</x:ref> header fieldin shared caches.
[949]1745   </t>
[1400]1748   Historically, HTTP required the Expires field-value to be no more than a
1749   year in the future. While longer freshness lifetimes are no longer
1750   prohibited, extremely large values have been demonstrated to cause
1751   problems (e.g., clock overflows due to use of 32-bit integers for
[1752]1752   time values), and many caches will evict a response far sooner than
[1400]1753   that. Therefore, senders ought not produce them.
1756   An origin server without a clock &MUST-NOT; assign Expires
1757   values to a response unless these values were associated
1758   with the resource by a system or user with a reliable clock. It &MAY;
1759   assign an Expires value that is known, at or before server
1760   configuration time, to be in the past (this allows "pre-expiration"
1761   of responses without storing separate Expires values for each
1762   resource).
[540]1766<section anchor="header.pragma" title="Pragma">
[1120]1767   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1768   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
[949]1769   <x:anchor-alias value="extension-pragma"/>
1770   <x:anchor-alias value="Pragma"/>
1771   <x:anchor-alias value="pragma-directive"/>
[1287]1773   The "Pragma" header field allows backwards compatibility with HTTP/1.0
[1284]1774   caches, so that clients can specify a "no-cache" request that they will
[1737]1775   understand (as <x:ref>Cache-Control</x:ref> was not defined until HTTP/1.1).
1776   When the Cache-Control header field is also present and understood in a
1777   request, Pragma is ignored.
[1287]1780   In HTTP/1.0, Pragma was defined as an extensible field for
[1284]1781   implementation-specified directives for recipients. This specification
1782   deprecates such extensions to improve interoperability.
[1231]1784<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"/>
1785  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1786  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1787  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
[1737]1790   When the <x:ref>Cache-Control</x:ref> header field is not present in a
1791   request, the no-cache request pragma-directive &MUST; have the same effect
1792   on caches as if "Cache-Control: no-cache" were present (see <xref
[1284]1793   target="cache-request-directive" />).
[1433]1796   When sending a no-cache request, a client ought to include both the pragma
1797   and cache-control directives, unless Cache-Control: no-cache is
[1737]1798   purposefully omitted to target other <x:ref>Cache-Control</x:ref> response
1799   directives at HTTP/1.1 caches. For example:
[1287]1802<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
[1284]1803GET / HTTP/1.1
1805Cache-Control: max-age=30
1806Pragma: no-cache
1811   will constrain HTTP/1.1 caches to serve a response no older than 30
1812   seconds, while precluding implementations that do not understand
[1737]1813   <x:ref>Cache-Control</x:ref> from serving a cached response.
[949]1816   <t>
[1692]1817      &Note; Because the meaning of "Pragma: no-cache" in responses is not
[1284]1818      specified, it does not provide a reliable replacement for
1819      "Cache-Control: no-cache" in them.
[1287]1820   </t>
[540]1824<section anchor="header.vary" title="Vary">
[1120]1825   <iref item="Vary header field" primary="true" x:for-anchor="" />
1826   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
[949]1827   <x:anchor-alias value="Vary"/>
[1163]1829   The "Vary" header field conveys the set of header fields
[949]1830   that were used to select the representation.
[949]1833   Caches use this information, in part, to determine whether a stored
1834   response can be used to satisfy a given request; see <xref
1835   target="caching.negotiated.responses" />. determines, while the response is
1836   fresh, whether a cache is permitted to use the response to reply to a
1837   subsequent request without validation; see <xref
1838   target="caching.negotiated.responses" />.
[949]1841   In uncacheable or stale responses, the Vary field value advises the user
1842   agent about the criteria that were used to select the representation.
[1231]1844<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1845  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
[949]1848   The set of header fields named by the Vary field value is known as the
[1163]1849   selecting header fields.
[1112]1852   A server &SHOULD; include a Vary header field with any cacheable response
[949]1853   that is subject to server-driven negotiation. Doing so allows a cache to
1854   properly interpret future requests on that resource and informs the user
1855   agent about the presence of negotiation on that resource. A server &MAY;
1856   include a Vary header field with a non-cacheable response that is subject
1857   to server-driven negotiation, since this might provide the user agent with
1858   useful information about the dimensions over which the response varies at
1859   the time of the response.
[949]1862   A Vary field value of "*" signals that unspecified parameters not limited
[1163]1863   to the header fields (e.g., the network address of the client), play a
[949]1864   role in the selection of the response representation; therefore, a cache
[1112]1865   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1866   generate the "*" value.
[1163]1869   The field-names given are not limited to the set of standard header
[949]1870   fields defined by this specification. Field names are case-insensitive.
[540]1874<section anchor="header.warning" title="Warning">
[1120]1875   <iref item="Warning header field" primary="true" x:for-anchor="" />
1876   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
[949]1877   <x:anchor-alias value="Warning"/>
1878   <x:anchor-alias value="warning-value"/>
1879   <x:anchor-alias value="warn-agent"/>
1880   <x:anchor-alias value="warn-code"/>
1881   <x:anchor-alias value="warn-date"/>
1882   <x:anchor-alias value="warn-text"/>
[1163]1884   The "Warning" header field is used to carry additional information
[949]1885   about the status or transformation of a message that might not be reflected
1886   in the message. This information is typically used to warn about possible
1887   incorrectness introduced by caching operations or transformations applied
1888   to the payload of the message.
[949]1891   Warnings can be used for other purposes, both cache-related and otherwise.
1892   The use of a warning, rather than an error status code, distinguishes these
1893   responses from true failures.
[994]1896   Warning header fields can in general be applied to any message, however some
[949]1897   warn-codes are specific to caches and can only be applied to response
1898   messages.
[1231]1900<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"/>
1901  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
[229]1903  <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>
1904                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
[229]1906  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
[334]1907  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
[135]1908                  ; the name or pseudonym of the server adding
[994]1909                  ; the Warning header field, for use in debugging
[229]1910  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1911  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
[949]1914   Multiple warnings can be attached to a response (either by the origin
1915   server or by a cache), including multiple warnings with the same code
1916   number, only differing in warn-text.
[949]1919   When this occurs, the user agent &SHOULD; inform the user of as many of
1920   them as possible, in the order that they appear in the response.
[1433]1923   Systems that generate multiple Warning header fields are encouraged to
1924   order them with this user agent behavior in mind. New Warning header fields
[1772]1925   are added after any existing Warning header fields.
[949]1928   Warnings are assigned three digit warn-codes. The first digit indicates
1929   whether the Warning is required to be deleted from a stored response after
1930   validation:
1931   <list style="symbols">
1932      <t>1xx Warnings describe the freshness or validation status of the
[1112]1933      response, and so &MUST; be deleted by a cache after validation. They can
[949]1934      only be generated by a cache when validating a cached entry, and
1935      &MUST-NOT; be generated in any other situation.</t>
1936      <t>2xx Warnings describe some aspect of the representation that is not
1937      rectified by a validation (for example, a lossy compression of the
[1112]1938      representation) and &MUST-NOT; be deleted by a cache after validation,
[949]1939      unless a full response is returned, in which case they &MUST; be.</t>
1940   </list>
[1746]1943   If an implementation sends a message with one or more Warning header fields
1944   to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST;
1945   include in each warning-value a warn-date that matches the
1946   <x:ref>Date</x:ref> header field in the message.
[1746]1949   If a system receives a message with a warning-value that includes a
1950   warn-date, and that warn-date is different from the <x:ref>Date</x:ref>
[1740]1951   value in the response, then that warning-value &MUST; be deleted from the
[1746]1952   message before storing, forwarding, or using it. (preventing the
1953   consequences of naive caching of Warning header fields.) If all of the
1954   warning-values are deleted for this reason, the Warning header field &MUST;
1955   be deleted as well.
[949]1958   The following warn-codes are defined by this specification, each with a
1959   recommended warn-text in English, and a description of its meaning.
1962<section title="110 Response is Stale" anchor="warn.110">
1963  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1964  <iref primary="true" item="Warn Codes" subitem="110 Response is Stale" x:for-anchor=""/>
1966   A cache &SHOULD; include this whenever the returned response is stale.
1970<section title="111 Revalidation Failed" anchor="warn.111">
1971  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1972  <iref primary="true" item="Warn Codes" subitem="111 Revalidation Failed" x:for-anchor=""/>
1974   A cache &SHOULD; include this when returning a stale response because an
1975   attempt to validate the response failed, due to an inability to reach
1976   the server.
1980<section title="112 Disconnected Operation" anchor="warn.112">
1981  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1982  <iref primary="true" item="Warn Codes" subitem="112 Disconnected Operation" x:for-anchor=""/>
1984   A cache &SHOULD; include this if it is intentionally disconnected from
1985   the rest of the network for a period of time.
1989<section title="113 Heuristic Expiration" anchor="warn.113">
1990  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1991  <iref primary="true" item="Warn Codes" subitem="113 Heuristic Expiration" x:for-anchor=""/>
1993   A cache &SHOULD; include this if it heuristically chose a freshness
1994   lifetime greater than 24 hours and the response's age is greater than 24
1995   hours.
1999<section title="199 Miscellaneous Warning" anchor="warn.199">
2000  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
2001  <iref primary="true" item="Warn Codes" subitem="199 Miscellaneous Warning" x:for-anchor=""/>
2003   The warning text can include arbitrary information to be presented to
2004   a human user, or logged. A system receiving this warning &MUST-NOT; take
2005   any automated action, besides presenting the warning to the user.
2009<section title="214 Transformation Applied" anchor="warn.214">
2010  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
2011  <iref primary="true" item="Warn Codes" subitem="214 Transformation Applied" x:for-anchor=""/>
2013   &MUST; be added by a proxy if it applies any transformation to the
2014   representation, such as changing the content-coding, media-type, or
2015   modifying the representation data, unless this Warning code already appears
2016   in the response.
2020<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
2021  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
2022  <iref primary="true" item="Warn Codes" subitem="299 Miscellaneous Persistent Warning" x:for-anchor=""/>
2024   The warning text can include arbitrary information to be presented to
2025   a human user, or logged. A system receiving this warning &MUST-NOT; take
2026   any automated action.
[1485]2030<section title="Warn Code Extensions" anchor="warn.code.extensions">
2032   The HTTP Warn Code Registry defines the name space for warn codes.
2035   A registration &MUST; include the following fields:
2036   <list style="symbols">
2037      <t>Warn Code (3 digits)</t>
2038      <t>Short Description</t>
2039      <t>Pointer to specification text</t>
2040   </list>
[1567]2043   Values to be added to this name space require IETF Review (see <xref
[1485]2044   target="RFC5226" x:fmt="," x:sec="4.1"/>).
2047   The registry itself is maintained at <eref
2048   target=""/>.
[540]2054<section anchor="history.lists" title="History Lists">
[949]2056   User agents often have history mechanisms, such as "Back" buttons and
2057   history lists, that can be used to redisplay a representation retrieved
2058   earlier in a session.
[949]2061   The freshness model (<xref target="expiration.model"/>) does not
2062   necessarily apply to history mechanisms. I.e., a history mechanism can
2063   display a previous representation even if it has expired.
2066   This does not prohibit the history mechanism from telling the user that a
2067   view might be stale, or from honoring cache directives (e.g.,
2068   Cache-Control: no-store).
[540]2073<section anchor="IANA.considerations" title="IANA Considerations">
[949]2075<section title="Cache Directive Registry" 
2076   anchor="cache.directive.registration">
[949]2078   The registration procedure for HTTP Cache Directives is defined by <xref
2079   target="cache.control.extensions"/> of this document.
[969]2082   The HTTP Cache Directive Registry shall be created at <eref
[949]2083   target=""/> and be
2084   populated with the registrations below:
2086<?BEGININC p6-cache.cache-directives ?>
2087<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
[825]2088<texttable xmlns:my="#my" align="left" suppress-title="true"
[817]2089           anchor="iana.cache.directive.registration.table">
2090   <ttcol>Cache Directive</ttcol>
2091   <ttcol>Reference</ttcol>
2093   <c>max-age</c>
2094   <c>
[1695]2095      <xref target="cache-request-directive.max-age"/>, <xref target="cache-response-directive.max-age"/>
[817]2096   </c>
2097   <c>max-stale</c>
2098   <c>
[1695]2099      <xref target="cache-request-directive.max-stale"/>
[817]2100   </c>
2101   <c>min-fresh</c>
2102   <c>
[1695]2103      <xref target="cache-request-directive.min-fresh"/>
[817]2104   </c>
2105   <c>must-revalidate</c>
2106   <c>
[1695]2107      <xref target="cache-response-directive.must-revalidate"/>
[817]2108   </c>
2109   <c>no-cache</c>
2110   <c>
[1695]2111      <xref target=""/>, <xref target=""/>
[817]2112   </c>
2113   <c>no-store</c>
2114   <c>
[1695]2115      <xref target=""/>, <xref target=""/>
[817]2116   </c>
2117   <c>no-transform</c>
2118   <c>
[1695]2119      <xref target=""/>, <xref target=""/>
[817]2120   </c>
2121   <c>only-if-cached</c>
2122   <c>
[1695]2123      <xref target="cache-request-directive.only-if-cached"/>
[817]2124   </c>
2125   <c>private</c>
2126   <c>
[1695]2127      <xref target="cache-response-directive.private"/>
[817]2128   </c>
2129   <c>proxy-revalidate</c>
2130   <c>
[1695]2131      <xref target="cache-response-directive.proxy-revalidate"/>
[817]2132   </c>
2133   <c>public</c>
2134   <c>
[1695]2135      <xref target="cache-response-directive.only-if-cached"/>
[817]2136   </c>
2137   <c>s-maxage</c>
2138   <c>
[1695]2139      <xref target="cache-response-directive.s-maxage"/>
[817]2140   </c>
[825]2141   <c>stale-if-error</c>
2142   <c>
2143      <xref xmlns:x="" target="RFC5861" x:fmt="," x:sec="4"/>
2144   </c>
2145   <c>stale-while-revalidate</c>
2146   <c>
2147      <xref xmlns:x="" target="RFC5861" x:fmt="," x:sec="3"/>
2148   </c>
2151<?ENDINC p6-cache.cache-directives ?>
[1485]2154<section title="Warn Code Registry" 
2155   anchor="warn.code.registration">
2157   The registration procedure for HTTP Warn Codes is defined by <xref
2158   target="warn.code.extensions"/> of this document.
2161   The HTTP Warn Code Registry shall be created at <eref
2162   target=""/> and be
2163   populated with the registrations below:
[1487]2165<?BEGININC p6-cache.iana-warn-codes ?>
2166<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
2167<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
[1485]2168   <ttcol>Warn Code</ttcol>
2169   <ttcol>Short Description</ttcol>
2170   <ttcol>Reference</ttcol>
2171   <c>110</c>
[1487]2172   <c>Response is Stale</c>
2173   <c>
2174      <xref target="warn.110"/>
2175   </c>
[1485]2176   <c>111</c>
[1487]2177   <c>Revalidation Failed</c>
2178   <c>
2179      <xref target="warn.111"/>
2180   </c>
[1485]2181   <c>112</c>
[1487]2182   <c>Disconnected Operation</c>
2183   <c>
2184      <xref target="warn.112"/>
2185   </c>
[1485]2186   <c>113</c>
[1487]2187   <c>Heuristic Expiration</c>
2188   <c>
2189      <xref target="warn.113"/>
2190   </c>
[1485]2191   <c>199</c>
[1487]2192   <c>Miscellaneous Warning</c>
2193   <c>
2194      <xref target="warn.199"/>
2195   </c>
[1485]2196   <c>214</c>
[1487]2197   <c>Transformation Applied</c>
2198   <c>
2199      <xref target="warn.214"/>
2200   </c>
[1485]2201   <c>299</c>
[1487]2202   <c>Miscellaneous Persistent Warning</c>
2203   <c>
2204      <xref target="warn.299"/>
2205   </c>
2208<?ENDINC p6-cache.iana-warn-codes ?>
[921]2211<section title="Header Field Registration" anchor="header.field.registration">
[949]2213  The Message Header Field Registry located at <eref 
[540]2214  target="" />
[969]2215  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
[680]2217<?BEGININC p6-cache.iana-headers ?>
[540]2218<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
[680]2219<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2220   <ttcol>Header Field Name</ttcol>
2221   <ttcol>Protocol</ttcol>
2222   <ttcol>Status</ttcol>
2223   <ttcol>Reference</ttcol>
[680]2225   <c>Age</c>
2226   <c>http</c>
2227   <c>standard</c>
2228   <c>
2229      <xref target="header.age"/>
2230   </c>
2231   <c>Cache-Control</c>
2232   <c>http</c>
2233   <c>standard</c>
2234   <c>
2235      <xref target="header.cache-control"/>
2236   </c>
2237   <c>Expires</c>
2238   <c>http</c>
2239   <c>standard</c>
2240   <c>
2241      <xref target="header.expires"/>
2242   </c>
2243   <c>Pragma</c>
2244   <c>http</c>
2245   <c>standard</c>
2246   <c>
2247      <xref target="header.pragma"/>
2248   </c>
2249   <c>Vary</c>
2250   <c>http</c>
2251   <c>standard</c>
2252   <c>
2253      <xref target="header.vary"/>
2254   </c>
2255   <c>Warning</c>
2256   <c>http</c>
2257   <c>standard</c>
2258   <c>
2259      <xref target="header.warning"/>
2260   </c>
[680]2263<?ENDINC p6-cache.iana-headers ?>
[949]2265   The change controller is: "IETF ( - Internet Engineering Task
2266   Force".
[540]2271<section anchor="security.considerations" title="Security Considerations">
[949]2273   Caches expose additional potential vulnerabilities, since the contents of
2274   the cache represent an attractive target for malicious exploitation.
2275   Because cache contents persist after an HTTP request is complete, an attack
2276   on the cache can reveal information long after a user believes that the
2277   information has been removed from the network. Therefore, cache contents
[969]2278   need to be protected as sensitive information.
[1364]2282<section title="Acknowledgments" anchor="acks">
[1364]2284  See &acks;.
2291<references title="Normative References">
[540]2293  <reference anchor="Part1">
2294    <front>
[1726]2295      <title>HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
[540]2296      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2297        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2298        <address><email></email></address>
2299      </author>
2300      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2301        <organization abbrev="W3C">World Wide Web Consortium</organization>
2302        <address><email></email></address>
2303      </author>
2304      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2305        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2306        <address><email></email></address>
2307      </author>
2308      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2309    </front>
2310    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
[1741]2311    <x:source basename="p1-messaging" href="p1-messaging.xml">
2312      <x:defines>Content-Length</x:defines>
2313      <x:defines>Via</x:defines>
2314    </x:source>
[540]2315  </reference>
[540]2317  <reference anchor="Part2">
2318    <front>
[1726]2319      <title>HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation</title>
[540]2320      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2321        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2322        <address><email></email></address>
2323      </author>
2324      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2325        <organization abbrev="W3C">World Wide Web Consortium</organization>
2326        <address><email></email></address>
2327      </author>
2328      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2329        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2330        <address><email></email></address>
2331      </author>
2332      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2333    </front>
2334    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
[1710]2335    <x:source basename="p2-semantics" href="p2-semantics.xml">
[1735]2336      <x:defines>2xx (Successful)</x:defines>
[1715]2337      <x:defines>200 (OK)</x:defines>
[1735]2338      <x:defines>203 (Non-Authoritative Information)</x:defines>
2339      <x:defines>3xx (Redirection)</x:defines>
2340      <x:defines>300 (Multiple Choices)</x:defines>
2341      <x:defines>301 (Moved Permanently)</x:defines>
[1713]2342      <x:defines>404 (Not Found)</x:defines>
[1735]2343      <x:defines>410 (Gone)</x:defines>
2344      <x:defines>5xx (Server Error)</x:defines>
[1710]2345      <x:defines>504 (Gateway Timeout)</x:defines>
[1740]2346      <x:defines>Content-Encoding</x:defines>
2347      <x:defines>Content-Location</x:defines>
2348      <x:defines>Content-Type</x:defines>
2349      <x:defines>Date</x:defines>
2350      <x:defines>Location</x:defines>
[1710]2351    </x:source>
[540]2352  </reference>
[540]2354  <reference anchor="Part4">
2355    <front>
[1726]2356      <title>HTTP/1.1, part 4: Conditional Requests</title>
[540]2357      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2358        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2359        <address><email></email></address>
2360      </author>
2361      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2362        <organization abbrev="W3C">World Wide Web Consortium</organization>
2363        <address><email></email></address>
2364      </author>
2365      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2366        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2367        <address><email></email></address>
2368      </author>
2369      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2370    </front>
2371    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
[1708]2372    <x:source basename="p4-conditional" href="p4-conditional.xml">
[1734]2373      <x:defines>304</x:defines>
[1708]2374      <x:defines>304 (Not Modified)</x:defines>
[1739]2375      <x:defines>ETag</x:defines>
2376      <x:defines>If-Modified-Since</x:defines>
2377      <x:defines>If-None-Match</x:defines>
2378      <x:defines>Last-Modified</x:defines>
[1708]2379    </x:source>
[540]2380  </reference>
[540]2382  <reference anchor="Part5">
2383    <front>
[1726]2384      <title>HTTP/1.1, part 5: Range Requests and Partial Responses</title>
[540]2385      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2386        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2387        <address><email></email></address>
2388      </author>
2389      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2390        <organization abbrev="W3C">World Wide Web Consortium</organization>
2391        <address><email></email></address>
2392      </author>
2393      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2394        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2395        <address><email></email></address>
2396      </author>
2397      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2398    </front>
2399    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
[1708]2400    <x:source basename="p5-range" href="p5-range.xml">
2401      <x:defines>206 (Partial Content)</x:defines>
[1738]2402      <x:defines>Content-Range</x:defines>
2403      <x:defines>Range</x:defines>
[1708]2404    </x:source>
[540]2405  </reference>
[540]2407  <reference anchor="Part7">
2408    <front>
[1726]2409      <title>HTTP/1.1, part 7: Authentication</title>
[540]2410      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2411        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2412        <address><email></email></address>
2413      </author>
2414      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2415        <organization abbrev="W3C">World Wide Web Consortium</organization>
2416        <address><email></email></address>
2417      </author>
2418      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2419        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2420        <address><email></email></address>
2421      </author>
2422      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2423    </front>
2424    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
[1736]2425    <x:source basename="p7-auth" href="p7-auth.xml">
2426      <x:defines>Authorization</x:defines>
2427    </x:source>
[540]2428  </reference>
[540]2430  <reference anchor="RFC2119">
2431    <front>
2432      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2433      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2434        <organization>Harvard University</organization>
2435        <address><email></email></address>
2436      </author>
2437      <date month="March" year="1997" />
2438    </front>
2439    <seriesInfo name="BCP" value="14" />
2440    <seriesInfo name="RFC" value="2119" />
2441  </reference>
[540]2443  <reference anchor="RFC5234">
2444    <front>
2445      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2446      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2447        <organization>Brandenburg InternetWorking</organization>
2448        <address>
[728]2449          <email></email>
2450        </address> 
[540]2451      </author>
2452      <author initials="P." surname="Overell" fullname="Paul Overell">
2453        <organization>THUS plc.</organization>
2454        <address>
[728]2455          <email></email>
2456        </address>
[540]2457      </author>
2458      <date month="January" year="2008"/>
2459    </front>
2460    <seriesInfo name="STD" value="68"/>
2461    <seriesInfo name="RFC" value="5234"/>
2462  </reference>
[540]2466<references title="Informative References">
[540]2468  <reference anchor="RFC1305">
2469    <front>
2470      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2471      <author fullname="David L. Mills" initials="D." surname="Mills">
2472        <organization>University of Delaware, Electrical Engineering Department</organization>
2473        <address><email></email></address>
2474      </author>
2475      <date month="March" year="1992" />
2476    </front>
2477    <seriesInfo name="RFC" value="1305" />
2478  </reference>
[540]2480  <reference anchor="RFC2616">
2481    <front>
2482      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2483      <author fullname="R. Fielding" initials="R." surname="Fielding">
2484        <organization>University of California, Irvine</organization>
2485        <address><email></email></address>
2486      </author>
2487      <author fullname="J. Gettys" initials="J." surname="Gettys">
2488        <organization>W3C</organization>
2489        <address><email></email></address>
2490      </author>
2491      <author fullname="J. Mogul" initials="J." surname="Mogul">
2492        <organization>Compaq Computer Corporation</organization>
2493        <address><email></email></address>
2494      </author>
2495      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2496        <organization>MIT Laboratory for Computer Science</organization>
2497        <address><email></email></address>
2498      </author>
2499      <author fullname="L. Masinter" initials="L." surname="Masinter">
2500        <organization>Xerox Corporation</organization>
2501        <address><email></email></address>
2502      </author>
2503      <author fullname="P. Leach" initials="P." surname="Leach">
2504        <organization>Microsoft Corporation</organization>
2505        <address><email></email></address>
2506      </author>
2507      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2508        <organization>W3C</organization>
2509        <address><email></email></address>
2510      </author>
2511      <date month="June" year="1999" />
2512    </front>
2513    <seriesInfo name="RFC" value="2616" />
2514  </reference>
2516  <reference anchor="RFC3864">
2517    <front>
2518      <title>Registration Procedures for Message Header Fields</title>
2519      <author fullname="G. Klyne" initials="G." surname="Klyne">
2520        <organization>Nine by Nine</organization>
2521        <address><email></email></address>
2522      </author>
2523      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2524        <organization>BEA Systems</organization>
2525        <address><email></email></address>
2526      </author>
2527      <author fullname="J. Mogul" initials="J." surname="Mogul">
2528        <organization>HP Labs</organization>
2529        <address><email></email></address>
2530      </author>
2531      <date month="September" year="2004" />
2532    </front>
2533    <seriesInfo name="BCP" value="90" />
2534    <seriesInfo name="RFC" value="3864" />
2535  </reference>
[817]2537  <reference anchor='RFC5226'>
2538    <front>
2539      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2540      <author initials='T.' surname='Narten' fullname='T. Narten'>
2541        <organization>IBM</organization>
2542        <address><email></email></address>
2543      </author>
2544      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2545        <organization>Google</organization>
2546        <address><email></email></address>
2547      </author>
2548      <date year='2008' month='May' />
2549    </front>
2550    <seriesInfo name='BCP' value='26' />
2551    <seriesInfo name='RFC' value='5226' />
2552  </reference>
[825]2554  <reference anchor='RFC5861'>
2555    <front>
2556      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2557      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2558        <organization>Yahoo! Inc.</organization>
2559        <address><email></email></address>
2560      </author>
2561      <date month="April" year="2010"/>
2562    </front>
2563    <seriesInfo name='RFC' value='5861' />
2564  </reference>
2568<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
[795]2570  Make the specified age calculation algorithm less conservative.
2571  (<xref target="age.calculations"/>)
[1740]2574  Remove requirement to consider <x:ref>Content-Location</x:ref> in successful
2575  responses in order to determine the appropriate response to use.
[691]2576  (<xref target="validation.model" />)
[540]2579  Clarify denial of service attack avoidance requirement.
2580  (<xref target="invalidation.after.updates.or.deletions" />)
[1231]2583  Change ABNF productions for header fields to only define the field value.
[1415]2584  (<xref target="header.field.definitions"/>)
[1737]2587  Do not mention RFC 2047 encoding and multiple languages in <x:ref>Warning</x:ref>
2588  header fields anymore, as these aspects never were implemented.
[669]2589  (<xref target="header.warning" />)
2592  Introduce Cache Directive and Warn Code Registries.
2593  (<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
[680]2597<?BEGININC p6-cache.abnf-appendix ?>
[427]2598<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
2600<artwork type="abnf" name="p6-cache.parsed-abnf">
[1231]2601<x:ref>Age</x:ref> = delta-seconds
[1231]2603<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
[425]2604 cache-directive ] )
[1231]2606<x:ref>Expires</x:ref> = HTTP-date
[1663]2608<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 5.1&gt;
[1523]2610<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
[1231]2612<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
[425]2613 pragma-directive ] )
[1231]2615<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2616 ) )
[1231]2618<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2619 )
[1695]2621<x:ref>cache-directive</x:ref> = token [ "=" ( token / quoted-string ) ]
[427]2623<x:ref>delta-seconds</x:ref> = 1*DIGIT
2625<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
[678]2627<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
[1769]2629<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.8&gt;
[427]2630<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
[1587]2631<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 6.2&gt;
[1523]2633<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.4&gt;
[1523]2635<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.4&gt;
[1769]2637<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.8&gt;
2639<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2640<x:ref>warn-code</x:ref> = 3DIGIT
2641<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2642<x:ref>warn-text</x:ref> = quoted-string
2643<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
[425]2644 ]
[680]2648<?ENDINC p6-cache.abnf-appendix ?>
[540]2650<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
[1624]2652  Changes up to the first Working Group Last Call draft are summarized
2653  in <eref target=""/>.
[1592]2656<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
[1666]2658  Closed issues:
2659  <list style="symbols">
2660    <t>
[1695]2661      <eref target=""/>:
2662      "untangle Cache-Control ABNF"
2663    </t>
2664    <t>
[1731]2665      <eref target=""/>:
[1772]2666      "Multiple values in Cache-Control header fields"
[1731]2667    </t>
2668    <t>
[1666]2669      <eref target=""/>:
2670      "Case sensitivity of header fields in CC values"
2671    </t>
2672    <t>
2673      <eref target=""/>:
[1776]2674      "Spurious 'MAYs'"
[1666]2675    </t>
[1682]2676    <t>
[1702]2677      <eref target=""/>:
2678      "enhance considerations for new cache control directives"
2679    </t>
2680    <t>
[1682]2681      <eref target=""/>:
2682      "ABNF requirements for recipients"
2683    </t>
[1760]2684    <t>
2685      <eref target=""/>:
2686      "note introduction of new IANA registries as normative changes"
2687    </t>
[1666]2688  </list>
[538]2693  </back>
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