source: draft-ietf-httpbis/latest/p6-cache.xml @ 1517

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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">
[1497]15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2012">
[1452]17  <!ENTITY architecture               "<xref target='Part1' x:rel='#architecture' xmlns:x=''/>">
[424]18  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x=''/>">
[1399]19  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x=''/>">
[205]20  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x=''/>">
[1396]21  <!ENTITY field-rules                 "<xref target='Part1' x:rel='#field.rules' xmlns:x=''/>">
[538]22  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x=''/>">
[1399]23  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x=''/>">
[31]24  <!ENTITY messaging                   "<xref target='Part1' xmlns:x=''/>">
[1374]25  <!ENTITY semantics                   "<xref target='Part2' xmlns:x=''/>">
[183]26  <!ENTITY conditional                 "<xref target='Part4' xmlns:x=''/>">
[538]27  <!ENTITY partial                     "<xref target='Part5' xmlns:x=''/>">
[31]28  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x=''/>">
[1436]29  <!ENTITY http-date                   "<xref target='Part2' x:rel='' xmlns:x=''/>">
[31]30  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x=''/>">
31  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x=''/>">
[1436]32  <!ENTITY header-date                 "<xref target='Part2' x:rel='' xmlns:x=''/>">
[205]33  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x=''/>">
[538]34  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
[697]35  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x=''/>">
[31]36  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
[1374]37  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x=''/>">
[607]38  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x=''/>">
[1374]39  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x=''/>">
[914]40  <!ENTITY status-codes                "<xref target='Part2' x:rel='' xmlns:x=''/>">
[943]41  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x=''/>">
43<?rfc toc="yes" ?>
[29]44<?rfc symrefs="yes" ?>
45<?rfc sortrefs="yes" ?>
[8]46<?rfc compact="yes"?>
47<?rfc subcompact="no" ?>
48<?rfc linkmailto="no" ?>
49<?rfc editing="no" ?>
[203]50<?rfc comments="yes"?>
51<?rfc inline="yes"?>
[799]52<?rfc rfcedstyle="yes"?>
[8]53<?rfc-ext allow-markup-in-artwork="yes" ?>
54<?rfc-ext include-references-in-index="yes" ?>
[538]55<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
[1477]56  obsoletes="2616" x:maturity-level="proposed" xmlns:x="">
[1472]57<x:link rel="prev" basename="p5-range"/>
58<x:link rel="next" basename="p7-auth"/>
[547]61  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
[547]63  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]64    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[547]65    <address>
[8]66      <postal>
[1106]67        <street>345 Park Ave</street>
68        <city>San Jose</city>
[8]69        <region>CA</region>
[1106]70        <code>95110</code>
[29]71        <country>USA</country>
[8]72      </postal>
[29]73      <email></email>
74      <uri></uri>
[8]75    </address>
[547]76  </author>
[844]78  <author initials="J." surname="Gettys" fullname="Jim Gettys">
79    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
[547]80    <address>
[8]81      <postal>
[29]82        <street>21 Oak Knoll Road</street>
83        <city>Carlisle</city>
[8]84        <region>MA</region>
[29]85        <code>01741</code>
86        <country>USA</country>
[8]87      </postal>
[844]88      <email></email>
89      <uri></uri>
[8]90    </address>
[547]91  </author>
[547]93  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
94    <organization abbrev="HP">Hewlett-Packard Company</organization>
95    <address>
[8]96      <postal>
[29]97        <street>HP Labs, Large Scale Systems Group</street>
98        <street>1501 Page Mill Road, MS 1177</street>
[8]99        <city>Palo Alto</city>
100        <region>CA</region>
[29]101        <code>94304</code>
102        <country>USA</country>
[8]103      </postal>
[29]104      <email></email>
[8]105    </address>
[547]106  </author>
[547]108  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
109    <organization abbrev="Microsoft">Microsoft Corporation</organization>
110    <address>
[8]111      <postal>
[29]112        <street>1 Microsoft Way</street>
113        <city>Redmond</city>
114        <region>WA</region>
115        <code>98052</code>
116        <country>USA</country>
[8]117      </postal>
[29]118      <email></email>
[8]119    </address>
[547]120  </author>
[547]122  <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]123    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[547]124    <address>
[8]125      <postal>
[29]126        <street>345 Park Ave</street>
127        <city>San Jose</city>
[8]128        <region>CA</region>
[29]129        <code>95110</code>
130        <country>USA</country>
[8]131      </postal>
[29]132      <email></email>
133      <uri></uri>
[8]134    </address>
[547]135  </author>
[547]137  <author fullname="Paul J. Leach" initials="P." surname="Leach">
138    <organization abbrev="Microsoft">Microsoft Corporation</organization>
139    <address>
[8]140      <postal>
141        <street>1 Microsoft Way</street>
142        <city>Redmond</city>
143        <region>WA</region>
144        <code>98052</code>
145      </postal>
146      <email></email>
147    </address>
[547]148  </author>
[547]150  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
151    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
152    <address>
[8]153      <postal>
[34]154        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
155        <street>The Stata Center, Building 32</street>
156        <street>32 Vassar Street</street>
[8]157        <city>Cambridge</city>
158        <region>MA</region>
159        <code>02139</code>
[29]160        <country>USA</country>
[8]161      </postal>
162      <email></email>
[34]163      <uri></uri>
[8]164    </address>
[547]165  </author>
[547]167  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
168    <organization abbrev="W3C">World Wide Web Consortium</organization>
169    <address>
[93]170      <postal>
171        <street>W3C / ERCIM</street>
172        <street>2004, rte des Lucioles</street>
173        <city>Sophia-Antipolis</city>
174        <region>AM</region>
175        <code>06902</code>
176        <country>France</country>
177      </postal>
178      <email></email>
179      <uri></uri>
180    </address>
[547]181  </author>
[601]183  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
[1438]184    <organization>Rackspace</organization>
[607]185    <address>
186      <email></email>
187      <uri></uri>
[601]188    </address>
189  </author>
[547]191  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
192    <organization abbrev="greenbytes">greenbytes GmbH</organization>
193    <address>
[95]194      <postal>
195        <street>Hafenweg 16</street>
196        <city>Muenster</city><region>NW</region><code>48155</code>
197        <country>Germany</country>
198      </postal>
[607]199      <phone>+49 251 2807760</phone>
200      <facsimile>+49 251 2807761</facsimile>
201      <email></email>
202      <uri></uri>
[95]203    </address>
[547]204  </author>
[547]206  <date month="&ID-MONTH;" year="&ID-YEAR;" />
207  <workgroup>HTTPbis Working Group</workgroup>
[949]211   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
[1373]212   distributed, collaborative, hypertext information systems. HTTP has been in
213   use by the World Wide Web global information initiative since 1990. This
214   document is Part 6 of the seven-part specification that defines the protocol
215   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
218   Part 6 defines requirements on HTTP caches and the associated header fields
219   that control cache behavior or indicate cacheable response messages.
[540]223<note title="Editorial Note (To be removed by RFC Editor)">
[1268]224  <t>
225    Discussion of this draft should take place on the HTTPBIS working group
226    mailing list (, which is archived at
227    <eref target=""/>.
228  </t>
229  <t>
230    The current issues list is at
231    <eref target=""/> and related
232    documents (including fancy diffs) can be found at
233    <eref target=""/>.
234  </t>
235  <t>
[1499]236    The changes in this draft are summarized in <xref target="changes.since.18"/>.
[1268]237  </t>
[949]240   </front>
241   <middle>
[540]243<section anchor="caching" title="Introduction">
[949]245   HTTP is typically used for distributed information systems, where
246   performance can be improved by the use of response caches. This document
247   defines aspects of HTTP/1.1 related to caching and reusing response
248   messages.
[540]251<section anchor="intro.purpose" title="Purpose">
252<iref item="cache" />
[949]254   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
255   subsystem that controls its message storage, retrieval, and deletion. A
256   cache stores cacheable responses in order to reduce the response time and
257   network bandwidth consumption on future, equivalent requests. Any client or
258   server &MAY; employ a cache, though a cache cannot be used by a server that
259   is acting as a tunnel.
[1374]262   The goal of caching in HTTP/1.1 is to significantly improve performance
263   by reusing a prior response message to satisfy a current request.
264   A stored response is considered "fresh", as defined in
265   <xref target="expiration.model" />, if the response can be reused without
266   "validation" (checking with the origin server to see if the cached response
267   remains valid for this request).  A fresh cache response can therefore
268   reduce both latency and network transfers each time it is reused.
269   When a cached response is not fresh, it might still be reusable if it can
270   be freshened by validation (<xref target="validation.model" />) or if the
271   origin is unavailable.
[540]275<section anchor="intro.terminology" title="Terminology">
[949]277   This specification uses a number of terms to refer to the roles played by
278   participants in, and objects of, HTTP caching.
[1112]281   <iref item="cache" />
282   <x:dfn>cache</x:dfn>
283   <list>
284      <t>A conformant implementation of a HTTP cache. Note that this implies
285        an HTTP/1.1 cache; this specification does not define conformance
286        for HTTP/1.0 caches.</t>
287   </list>
289<t anchor="shared.and.non-shared.caches">
290   <iref item="shared cache" />
291   <x:dfn>shared cache</x:dfn>
292   <list>
[1374]293      <t>A cache that stores responses to be reused by more than one user;
294         usually (but not always) deployed as part of an intermediary.</t>
[1112]295   </list>
298   <iref item="private cache" />
299   <x:dfn>private cache</x:dfn>
300   <list>
301      <t>A cache that is dedicated to a single user.</t>
302   </list>
[949]305   <iref item="cacheable" />
306   <x:dfn>cacheable</x:dfn>
307   <list>
308      <t>A response is cacheable if a cache is allowed to store a copy of the
309      response message for use in answering subsequent requests. Even when a
310      response is cacheable, there might be additional constraints on whether
[1112]311      a cache can use the stored copy to satisfy a particular request.</t>
[949]312   </list>
[949]315   <iref item="explicit expiration time" />
316   <x:dfn>explicit expiration time</x:dfn>
317   <list>
318      <t>The time at which the origin server intends that a representation
[969]319      no longer be returned by a cache without further validation.</t>
[949]320   </list>
[949]323   <iref item="heuristic expiration time" />
324   <x:dfn>heuristic expiration time</x:dfn>
325   <list>
326      <t>An expiration time assigned by a cache when no explicit expiration
327      time is available.</t>
328   </list>
[949]331   <iref item="age" />
332   <x:dfn>age</x:dfn>
333   <list>
334      <t>The age of a response is the time since it was sent by, or
335      successfully validated with, the origin server.</t>
336   </list>
[949]339   <iref item="first-hand" />
340   <x:dfn>first-hand</x:dfn>
341   <list>
342      <t>A response is first-hand if the freshness model is not in use; i.e.,
343      its age is 0.</t>
344   </list>
[949]347   <iref item="freshness lifetime" />
348   <x:dfn>freshness lifetime</x:dfn>
349   <list>
350      <t>The length of time between the generation of a response and its
351      expiration time.</t>
352   </list>
[949]355   <iref item="fresh" />
356   <x:dfn>fresh</x:dfn>
357   <list>
358      <t>A response is fresh if its age has not yet exceeded its freshness
359      lifetime.</t>
360   </list>
[949]363   <iref item="stale" />
364   <x:dfn>stale</x:dfn>
365   <list>
366      <t>A response is stale if its age has passed its freshness lifetime
367      (either explicit or heuristic).</t>
368   </list>
[949]371   <iref item="validator" />
372   <x:dfn>validator</x:dfn>
373   <list>
374      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
[1112]375      is used to find out whether a stored response is an equivalent copy of
[1374]376      a representation. See &weak-and-strong;.</t>
[949]377   </list>
380   <iref item="strong validator" />
[1376]381   <iref item="validator" subitem="strong" />
[1374]382   <x:dfn>strong validator</x:dfn>
383   <list>
384      <t>A validator that is defined by the origin server such that its
385         current value will change if the representation body changes; i.e.,
386         an entity-tag that is not marked as weak (&entity-tags;) or,
387         if no entity-tag is provided, a Last-Modified value that is strong
388         in the sense defined by &lastmod-comparison;.</t>
389   </list>
[1452]393<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
[847]395   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
396   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
397   document are to be interpreted as described in <xref target="RFC2119"/>.
[1452]400   This document defines conformance criteria for several roles in HTTP
401   communication, including Senders, Recipients, Clients, Servers, User-Agents,
402   Origin Servers, Intermediaries, Proxies and Gateways. See &architecture;
403   for definitions of these terms.
406   An implementation is considered conformant if it complies with all of the
407   requirements associated with its role(s). Note that SHOULD-level requirements
408   are relevant here, unless one of the documented exceptions is applicable.
411   This document also uses ABNF to define valid protocol elements
412   (<xref target="notation"/>). In addition to the prose requirements placed
413   upon them, Senders &MUST-NOT; generate protocol elements that are invalid.
416   Unless noted otherwise, Recipients &MAY; take steps to recover a usable
417   protocol element from an invalid construct. However, HTTP does not define
418   specific error handling mechanisms, except in cases where it has direct
419   impact on security. This is because different uses of the protocol require
420   different error handling strategies; for example, a Web browser may wish to
421   transparently recover from a response where the Location header field
422   doesn't parse according to the ABNF, whereby in a systems control protocol
423   using HTTP, this type of error recovery could lead to dangerous consequences.
[424]427<section title="Syntax Notation" anchor="notation">
[949]428   <x:anchor-alias value="ALPHA"/>
429   <x:anchor-alias value="CR"/>
430   <x:anchor-alias value="DIGIT"/>
431   <x:anchor-alias value="DQUOTE"/>
432   <x:anchor-alias value="LF"/>
433   <x:anchor-alias value="OCTET"/>
434   <x:anchor-alias value="SP"/>
435   <x:anchor-alias value="VCHAR"/>
[949]437   This specification uses the ABNF syntax defined in &notation; (which
438   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
439   <xref target="collected.abnf"/> shows the collected ABNF, with the list
440   rule expanded.
[949]443   The following core rules are included by reference, as defined in <xref
444   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
445   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
446   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
[1425]447   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
[424]450<section title="Core Rules" anchor="core.rules">
[949]451   <x:anchor-alias value="quoted-string"/>
452   <x:anchor-alias value="token"/>
453   <x:anchor-alias value="OWS"/>
[1396]455   The core rules below are defined in <xref target="Part1"/>:
457<figure><artwork type="abnf2616">
[364]458  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
[1396]459  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-rules;&gt;
460  <x:ref>token</x:ref>         = &lt;token, defined in &field-rules;&gt;
[949]464<section title="ABNF Rules defined in other Parts of the Specification"
465    anchor="abnf.dependencies">
466   <x:anchor-alias value="field-name"/>
467   <x:anchor-alias value="HTTP-date"/>
468   <x:anchor-alias value="port"/>
469   <x:anchor-alias value="pseudonym"/>
470   <x:anchor-alias value="uri-host"/>
[949]472   The ABNF rules below are defined in other parts:
[207]474<figure><!--Part1--><artwork type="abnf2616">
[647]475  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
[1436]476  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
[374]477  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
[229]478  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
[374]479  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
[1323]484<section title="Delta Seconds" anchor="delta-seconds">
486   The delta-seconds rule specifies a non-negative integer, representing time
487   in seconds.
489<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
490  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
493   If an implementation receives a delta-seconds value larger than the largest
494   positive integer it can represent, or if any of its subsequent calculations
495   overflows, it &MUST; consider the value to be 2147483648 (2<x:sup>31</x:sup>).
[1433]496   Recipients parsing a delta-seconds value &MUST; use an arithmetic type of
[1323]497   at least 31 bits of range, and senders &MUST-NOT; send delta-seconds with a
498   value greater than 2147483648.
[540]504<section anchor="caching.overview" title="Cache Operation">
[1374]505<iref item="cache entry" />
506<iref item="cache key" />
508   Proper cache operation preserves the semantics of HTTP transfers
509   (&semantics;) while eliminating the transfer of information already held
510   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
511   we assume that reusing the cached response is desirable and that such
512   reuse is the default behavior when no requirement or locally-desired
513   configuration prevents it.  Therefore, HTTP cache requirements are focused
514   on preventing a cache from either storing a non-reusable response or
515   reusing a stored response inappropriately.
518   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
519   HTTP responses corresponding to prior requests that used the same key.
520   The most common form of cache entry is a successful result of a retrieval
521   request: i.e., a 200 (OK) response containing a representation of the
522   resource identified by the request target.  However, it is also possible
523   to cache negative results (e.g., 404 not found), incomplete results
524   (e.g., 206 partial content), and responses to safe methods other than
525   GET if the method's definition allows such caching and defines something
526   suitable for use as a cache key.
529   The default <x:dfn>cache key</x:dfn> consists of the request method and
530   target URI.  However, since HTTP caches in common use today are typically
531   limited to caching responses to GET, most implementations simply decline
532   other methods and use only the URI as the key.
535   If a request target is subject to content negotiation, its cache entry
536   might consist of multiple stored responses, each differentiated by a
537   secondary key for the values of the original request's selecting header
538   fields (<xref target="caching.negotiated.responses"/>).
[540]541<section anchor="response.cacheability" title="Response Cacheability">
[949]543   A cache &MUST-NOT; store a response to any request, unless:
544   <list style="symbols">
545      <t>The request method is understood by the cache and defined as being
546      cacheable, and</t>
547      <t>the response status code is understood by the cache, and</t>
548      <t>the "no-store" cache directive (see <xref
549      target="header.cache-control" />) does not appear in request or response
[994]550      header fields, and</t>
[949]551      <t>the "private" cache response directive (see <xref
[1514]552      target="cache-response-directive" />) does not appear in the response, if
[949]553      the cache is shared, and</t>
[994]554      <t>the "Authorization" header field (see &header-authorization;) does not
[949]555      appear in the request, if the cache is shared, unless the response
556      explicitly allows it (see <xref target="caching.authenticated.responses"
557      />), and</t>
558      <t>the response either:
559         <list style="symbols">
[994]560            <t>contains an Expires header field (see <xref target="header.expires"
[949]561            />), or</t>
562            <t>contains a max-age response cache directive (see <xref
563            target="cache-response-directive" />), or</t>
564            <t>contains a s-maxage response cache directive and the cache is
565            shared, or</t>
566            <t>contains a Cache Control Extension (see <xref
567            target="cache.control.extensions" />) that allows it to be cached,
568            or</t>
569            <t>has a status code that can be served with heuristic freshness
570            (see <xref target="heuristic.freshness" />).</t>
571         </list>
572      </t>
573   </list>
[1292]576   Note that any of the requirements listed above can be overridden by a
[1291]577   cache-control extension; see <xref target="cache.control.extensions" />.
[949]580   In this context, a cache has "understood" a request method or a response
[1374]581   status code if it recognizes it and implements any cache-specific
582   behavior.
[1374]585   Note that, in normal operation, most caches will not store a response that
[949]586   has neither a cache validator nor an explicit expiration time, as such
587   responses are not usually useful to store. However, caches are not
588   prohibited from storing such responses.
[1374]591   A response message is considered complete when all of the octets
592   indicated by the message framing (&messaging;) are received
593   prior to the connection being closed.
594   If the request is GET, the response status is 200 (OK), and the entire
595   response header block has been received, a cache &MAY; store an incomplete
596   response message-body if the cache entry is recorded as incomplete.
597   Likewise, a 206 (Partial Content) response &MAY; be stored as if it were
598   an incomplete 200 (OK) cache entry.  However, a cache &MUST-NOT; store
599   incomplete or partial content responses if it does not support the Range
600   and Content-Range header fields or if it does not understand the
601   range units used in those fields.
[1374]604   A cache &MAY; complete a stored incomplete response by making a subsequent
605   range request (&partial;) and combining the successful response with the
606   stored entry, as defined in <xref target="combining.responses"/>.
607   A cache &MUST-NOT; use an incomplete response to answer requests
608   unless the response has been made complete or the request is partial and
609   specifies a range that is wholly within the incomplete response.
610   A cache &MUST-NOT; send a partial response to a client without explicitly
611   marking it as such using the 206 (Partial Content) status code.
[949]616<section anchor="constructing.responses.from.caches"
617   title="Constructing Responses from Caches">
[949]619   For a presented request, a cache &MUST-NOT; return a stored response,
620   unless:
621   <list style="symbols">
[965]622      <t>The presented effective request URI (&effective-request-uri;) and
[949]623      that of the stored response match, and</t>
624      <t>the request method associated with the stored response allows it to
625      be used for the presented request, and</t>
[1163]626      <t>selecting header fields nominated by the stored response (if any)
[949]627      match those presented (see <xref target="caching.negotiated.responses"
628      />), and</t>
[1483]629      <t>the presented request does not contain the no-cache pragma (<xref
630      target="header.pragma"/>), nor the no-cache cache directive (<xref
631      target="cache-request-directive"/>), unless the stored response is
632      successfully validated (<xref target="validation.model"/>), and</t>
633      <t>the stored response does not contain the no-cache cache directive
634      (<xref target="cache-response-directive"/>), unless it is successfully
635      validated (<xref target="validation.model"/>), and</t>
[949]636      <t>the stored response is either:
637         <list style="symbols">
638            <t>fresh (see <xref target="expiration.model" />), or</t>
639            <t>allowed to be served stale (see <xref
640            target="serving.stale.responses" />), or</t>
641            <t>successfully validated (see <xref target="validation.model"
642            />).</t>
643         </list>
644      </t>
[540]645  </list>
[1292]648   Note that any of the requirements listed above can be overridden by a
[1291]649   cache-control extension; see <xref target="cache.control.extensions" />.
[949]652   When a stored response is used to satisfy a request without validation,
[1112]653   a cache &MUST; include a single Age header field (<xref target="header.age"
[949]654   />) in the response with a value equal to the stored response's
655   current_age; see <xref target="age.calculations" />.
[1112]658   A cache &MUST; write through requests with methods that are unsafe
659   (&safe-methods;) to the origin server; i.e., a cache must not generate
660   a reply to such a request before having forwarded the request and having
661   received a corresponding response.
[949]664   Also, note that unsafe requests might invalidate already stored responses;
665   see <xref target="invalidation.after.updates.or.deletions" />.
[1277]668   When more than one suitable response is stored, a cache &MUST; use the
669   most recent response (as determined by the Date header field). It can also
[949]670   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
671   no-cache" to disambiguate which response to use.
[1184]674   A cache that does not have a clock available &MUST-NOT; use stored responses
[1112]675   without revalidating them on every use. A cache, especially a shared
[1044]676   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
677   synchronize its clock with a reliable external standard.
[540]682<section anchor="expiration.model" title="Freshness Model">
[949]684   When a response is "fresh" in the cache, it can be used to satisfy
685   subsequent requests without contacting the origin server, thereby improving
686   efficiency.
[949]689   The primary mechanism for determining freshness is for an origin server to
690   provide an explicit expiration time in the future, using either the Expires
[994]691   header field (<xref target="header.expires" />) or the max-age response cache
[949]692   directive (<xref target="cache-response-directive" />). Generally, origin
693   servers will assign future explicit expiration times to responses in the
694   belief that the representation is not likely to change in a semantically
695   significant way before the expiration time is reached.
[949]698   If an origin server wishes to force a cache to validate every request, it
[969]699   can assign an explicit expiration time in the past to indicate that the
[1113]700   response is already stale. Compliant caches will normally validate the
701   cached response before reusing it for subsequent requests (see <xref
702   target="serving.stale.responses" />).
[1112]705   Since origin servers do not always provide explicit expiration times,
706   a cache &MAY; assign a heuristic expiration time when an explicit time is not
707   specified, employing algorithms that use other header field values (such as the
708   Last-Modified time) to estimate a plausible expiration time. This
[949]709   specification does not provide specific algorithms, but does impose
710   worst-case constraints on their results.
[575]714  The calculation to determine if a response is fresh is:
716<artwork type="code">
[538]717   response_is_fresh = (freshness_lifetime &gt; current_age)
[949]721   The freshness_lifetime is defined in <xref
722   target="calculating.freshness.lifetime" />; the current_age is defined in
723   <xref target="age.calculations" />.
[1483]726   Additionally, clients can influence freshness calculation -- either
727   constraining it relaxing it -- by using the max-age and min-fresh
728   request cache directives. See <xref target="cache-request-directive" />
729   for details.
[949]732   Note that freshness applies only to cache operation; it cannot be used to
733   force a user agent to refresh its display or reload a resource. See <xref
734   target="history.lists" /> for an explanation of the difference between
735   caches and history mechanisms.
[949]738<section anchor="calculating.freshness.lifetime"
739   title="Calculating Freshness Lifetime">
[949]741   A cache can calculate the freshness lifetime (denoted as
742   freshness_lifetime) of a response by using the first match of:
743   <list style="symbols">
744      <t>If the cache is shared and the s-maxage response cache directive
745      (<xref target="cache-response-directive" />) is present, use its value,
746      or</t>
747      <t>If the max-age response cache directive (<xref
[540]748      target="cache-response-directive" />) is present, use its value, or</t>
[994]749      <t>If the Expires response header field (<xref target="header.expires" />) is
750      present, use its value minus the value of the Date response header field,
[949]751      or</t>
752      <t>Otherwise, no explicit expiration time is present in the response. A
753      heuristic freshness lifetime might be applicable; see <xref
754      target="heuristic.freshness" />.</t>
755   </list>
[949]758   Note that this calculation is not vulnerable to clock skew, since all of
759   the information comes from the origin server.
[540]762<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
[949]764   If no explicit expiration time is present in a stored response that has a
765   status code whose definition allows heuristic freshness to be used
766   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
[1112]767   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT;
768   use heuristics to determine freshness for responses with status codes that do
769   not explicitly allow it.
[1112]772   When a heuristic is used to calculate freshness lifetime, a cache
[994]773   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
[949]774   its current_age is more than 24 hours and such a warning is not already
775   present.
[1433]778   Also, if the response has a Last-Modified header field
779   (&header-last-modified;), caches are encouraged to use a heuristic
780   expiration value that is no more than some fraction of the interval since
781   that time. A typical setting of this fraction might be 10%.
[949]784   <t>
785      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
786      x:sec="13.9"/>) required that caches do not calculate heuristic
[1173]787      freshness for URIs with query components (i.e., those containing '?').
[949]788      In practice, this has not been widely implemented. Therefore, servers
789      are encouraged to send explicit directives (e.g., Cache-Control:
790      no-cache) if they wish to preclude caching.
791   </t>
[540]796<section anchor="age.calculations" title="Calculating Age">
[1163]798   HTTP/1.1 uses the Age header field to convey the estimated age of the
[949]799   response message when obtained from a cache. The Age field value is the
800   cache's estimate of the amount of time since the response was generated or
801   validated by the origin server. In essence, the Age value is the sum of the
802   time that the response has been resident in each of the caches along the
803   path from the origin server, plus the amount of time it has been in transit
804   along network paths.
[949]807   The following data is used for the age calculation:
[949]810   <x:dfn>age_value</x:dfn>
811   <list>
812      <t>
[994]813         The term "age_value" denotes the value of the Age header field (<xref
[949]814         target="header.age"/>), in a form appropriate for arithmetic
815         operation; or 0, if not available.
816      </t>
817   </list>
[949]820   <x:dfn>date_value</x:dfn>
821   <list>
822      <t>
[994]823         HTTP/1.1 requires origin servers to send a Date header field, if possible,
[949]824         with every response, giving the time at which the response was
825         generated. The term "date_value" denotes the value of the Date
[994]826         header field, in a form appropriate for arithmetic operations. See
827         &header-date; for the definition of the Date header field, and for
828         requirements regarding responses without it.
[949]829      </t>
830   </list>
[949]833   <x:dfn>now</x:dfn>
834   <list>
835      <t>
836         The term "now" means "the current value of the clock at the host
[1112]837         performing the calculation". A cache &SHOULD; use NTP (<xref
838         target="RFC1305"/>) or some similar protocol to synchronize its
[949]839         clocks to a globally accurate time standard.
840      </t>
841   </list>
[949]844   <x:dfn>request_time</x:dfn>
845   <list>
846      <t>
847         The current value of the clock at the host at the time the request
848         resulting in the stored response was made.
849      </t>
850   </list>
[949]853   <x:dfn>response_time</x:dfn>
854   <list>
855      <t>
856         The current value of the clock at the host at the time the response
857         was received.
858      </t>
859   </list>
[949]862   A response's age can be calculated in two entirely independent ways:
863   <list style="numbers">
864      <t>the "apparent_age": response_time minus date_value, if the local
865      clock is reasonably well synchronized to the origin server's clock. If
866      the result is negative, the result is replaced by zero.</t>
867      <t>the "corrected_age_value", if all of the caches along the response
[1112]868      path implement HTTP/1.1. A cache &MUST; interpret this value relative
[949]869      to the time the request was initiated, not the time that the response
870      was received.</t>
871   </list>
874<artwork type="code">
[786]875  apparent_age = max(0, response_time - date_value);
[786]877  response_delay = response_time - request_time;
[792]878  corrected_age_value = age_value + response_delay; 
[1484]882<preamble>These &SHOULD; be combined as</preamble>
[792]883<artwork type="code">
884  corrected_initial_age = max(apparent_age, corrected_age_value);
[1484]887   unless the cache is confident in the value of the Age header (e.g., because
888   there are no HTTP/1.0 hops in the Via header), in which case the
889   corrected_age_value &MAY; be used as the corrected_initial_age.</t>
[949]891   The current_age of a stored response can then be calculated by adding the
892   amount of time (in seconds) since the stored response was last validated by
893   the origin server to the corrected_initial_age.
[786]895<figure><artwork type="code">
[540]896  resident_time = now - response_time;
[786]897  current_age = corrected_initial_age + resident_time;
[1418]900   Additionally, to avoid common problems in date parsing:
903  <list style="symbols">
904     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
905        which appears to be more than 50 years in the future is in fact
906        in the past (this helps solve the "year 2000" problem).</t>
908     <t>Although all date formats are specified to be case-sensitive,
909        recipients &SHOULD; match day, week and timezone names
910        case-insensitively.</t>
912     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
913        Expires date as earlier than the proper value, but &MUST-NOT;
914        internally represent a parsed Expires date as later than the
915        proper value.</t>
917     <t>All expiration-related calculations &MUST; be done in GMT. The
918        local time zone &MUST-NOT; influence the calculation or comparison
919        of an age or expiration time.</t>
921     <t>If an HTTP header field incorrectly carries a date value with a time
922        zone other than GMT, it &MUST; be converted into GMT using the
923        most conservative possible conversion.</t>
924  </list>
[540]928<section anchor="serving.stale.responses" title="Serving Stale Responses">
[949]930   A "stale" response is one that either has explicit expiry information or is
931   allowed to have heuristic expiry calculated, but is not fresh according to
932   the calculations in <xref target="expiration.model" />.
[1112]935   A cache &MUST-NOT; return a stale response if it is prohibited by an
[949]936   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
937   directive, a "must-revalidate" cache-response-directive, or an applicable
938   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
939   target="cache-response-directive"/>).
[1433]942   A cache &MUST-NOT; return stale responses unless it is disconnected
[949]943   (i.e., it cannot contact the origin server or otherwise find a forward
[1264]944   path) or doing so is explicitly allowed (e.g., by the max-stale request
[949]945   directive; see <xref target="cache-request-directive" />).
[1112]948   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
949   <xref target="header.warning" />) to stale responses. Likewise, a cache
950   &SHOULD; add the 112 warn-code to stale responses if the cache is
951   disconnected.
[949]954   If a cache receives a first-hand response (either an entire response, or a
955   304 (Not Modified) response) that it would normally forward to the
956   requesting client, and the received response is no longer fresh, the cache
[1433]957   can forward it to the requesting client without adding a new Warning (but
958   without removing any existing Warning header fields). A cache shouldn't
[949]959   attempt to validate a response simply because that response became stale in
960   transit.
[540]965<section anchor="validation.model" title="Validation Model">
[949]967   When a cache has one or more stored responses for a requested URI, but
968   cannot serve any of them (e.g., because they are not fresh, or one cannot
969   be selected; see <xref target="caching.negotiated.responses"/>), it can use
970   the conditional request mechanism &conditional; in the forwarded request to
971   give the origin server an opportunity to both select a valid stored
972   response to be used, and to update it. This process is known as
973   "validating" or "revalidating" the stored response.
[1433]976   When sending such a conditional request, a cache adds an If-Modified-Since
977   header field whose value is that of the Last-Modified header field from the
978   selected (see <xref target="caching.negotiated.responses"/>) stored
979   response, if available.
[1433]982   Additionally, a cache can add an If-None-Match header field whose value is
983   that of the ETag header field(s) from all responses stored for the
984   requested URI, if present. However, if any of the stored responses contains
985   only partial content, the cache shouldn't include its entity-tag in the
986   If-None-Match header field unless the request is for a range that would be
987   fully satisfied by that stored response.
990<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
[1419]993   <list style="symbols">
994      <t>
995         A 304 (Not Modified) response status code indicates that the stored
996         response can be updated and reused; see <xref
997         target="freshening.responses"/>.
998      </t>
999      <t>
1000         A full response (i.e., one with a response body) indicates that none
1001         of the stored responses nominated in the conditional request is
[1433]1002         suitable. Instead, the cache can use the full response to
1003         satisfy the request and &MAY; replace the stored response(s).
[1419]1004      </t>
1005      <t>
1006         However, if a cache receives a 5xx response while attempting to
[1433]1007         validate a response, it can either forward this response to the
[1419]1008         requesting client, or act as if the server failed to respond. In the
[1433]1009         latter case, it can return a previously stored response (see <xref
[1419]1010         target="serving.stale.responses" />).
1011      </t>
1012   </list>
1015<section anchor="freshening.responses" title="Freshening Responses">
[1419]1017   When a cache receives a 304 (Not Modified) response and already has one
1018   or more stored 200 (OK) responses for the same cache key, the cache needs
1019   to identify which of the stored responses are updated by this new response
1020   and then update the stored response(s) with the new information provided in
1021   the 304 response.
1022   <list style="symbols">
1023    <t>
1024     If the new response contains a strong validator, then that strong
1025     validator identifies the selected representation.  All of the stored
1026     responses with the same strong validator are selected.
1027     If none of the stored responses contain the same strong validator, then
1028     this new response corresponds to a new selected representation and
1029     &MUST-NOT; update the existing stored responses.
1030    </t>
1031    <t>
1032     If the new response contains a weak validator and that validator
1033     corresponds to one of the cache's stored responses, then the most
1034     recent of those matching stored responses is selected.
1035    </t>
1036    <t>
1037     If the new response does not include any form of validator, there is
1038     only one stored response, and that stored response also lacks a
1039     validator, then that stored response is selected.
1040    </t>
1041   </list>
[1419]1044   If a stored response is selected for update, the cache &MUST;:
1045   <list style="symbols">
1046      <t>delete any Warning header fields in the stored response with
1047         warn-code 1xx (see <xref target="header.warning" />);</t>
1048      <t>retain any Warning header fields in the stored response with
1049         warn-code 2xx; and,</t>
1050      <t>use other header fields provided in the 304 response to replace
1051         all instances of the corresponding header fields in the stored
1052         response.</t>
1053   </list>
[949]1059<section anchor="invalidation.after.updates.or.deletions"
1060   title="Request Methods that Invalidate">
[1290]1062   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1063   have the potential for changing state on the origin server, intervening
1064   caches can use them to keep their contents up-to-date.
[1112]1067   A cache &MUST; invalidate the effective Request URI
1068   (&effective-request-uri;) as well as the URI(s) in the Location
[1314]1069   and Content-Location header fields (if present) when a non-error
1070   response to a request with an unsafe method is received.
[1112]1073   However, a cache &MUST-NOT; invalidate a URI from a
1074   Location or Content-Location header field if the host part of that URI
1075   differs from the host part in the effective request URI
1076   (&effective-request-uri;). This helps prevent denial of service attacks.
[1433]1079   A cache &MUST; invalidate the effective request URI
[1314]1080   (&effective-request-uri;) when it receives a non-error response
1081   to a request with a method whose safety is unknown.
[1314]1084   Here, a "non-error response" is one with a 2xx or 3xx status code.
1085   "Invalidate" means that the cache will either remove all stored
[965]1086   responses related to the effective request URI, or will mark these as
[949]1087   "invalid" and in need of a mandatory validation before they can be returned
1088   in response to a subsequent request.
[949]1091   Note that this does not guarantee that all appropriate responses are
1092   invalidated. For example, the request that caused the change at the origin
1093   server might not have gone through the cache where a response is stored.
[949]1097<section anchor="caching.authenticated.responses"
1098   title="Shared Caching of Authenticated Responses">
[1112]1101   A shared cache &MUST-NOT; use a cached response to a request with an
[994]1102   Authorization header field (&header-authorization;) to satisfy any subsequent
[949]1103   request unless a cache directive that allows such responses to be stored is
1104   present in the response.
1108   In this specification, the following Cache-Control response directives
1109   (<xref target="cache-response-directive"/>) have such an effect:
1110   must-revalidate, public, s-maxage.
1114   Note that cached responses that contain the "must-revalidate" and/or
1115   "s-maxage" response directives are not allowed to be served stale (<xref
1116   target="serving.stale.responses"/>) by shared caches. In particular, a
1117   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1118   used to satisfy a subsequent request without revalidating it on the origin
1119   server.
[949]1123<section anchor="caching.negotiated.responses"
1124   title="Caching Negotiated Responses">
[949]1126   When a cache receives a request that can be satisfied by a stored response
1127   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
[1163]1128   use that response unless all of the selecting header fields nominated by
[994]1129   the Vary header field match in both the original request (i.e., that associated
[949]1130   with the stored response), and the presented request.
[1163]1133   The selecting header fields from two requests are defined to match if and
[949]1134   only if those in the first request can be transformed to those in the
1135   second request by applying any of the following:
1136   <list style="symbols">
1137      <t>
[994]1138         adding or removing whitespace, where allowed in the header field's syntax
[949]1139      </t>
1140      <t>
[1163]1141         combining multiple header fields with the same field name
[949]1142         (see &header-fields;)
1143      </t>
1144      <t>
[994]1145         normalizing both header field values in a way that is known to have
1146         identical semantics, according to the header field's specification (e.g.,
[949]1147         re-ordering field values when order is not significant;
1148         case-normalization, where values are defined to be case-insensitive)
1149      </t>
[771]1150  </list>
[949]1153   If (after any normalization that might take place) a header field is absent
1154   from a request, it can only match another request if it is also absent
1155   there.
[949]1158   A Vary header field-value of "*" always fails to match, and subsequent
1159   requests to that resource can only be properly interpreted by the origin
1160   server.
[1163]1163   The stored response with matching selecting header fields is known as the
[949]1164   selected response.
[1292]1167   If multiple selected responses are available, the most recent response
1168   (as determined by the Date header field) is used; see <xref
1169   target="constructing.responses.from.caches"/>.
[1433]1172   If no selected response is available, the cache can forward the presented
[949]1173   request to the origin server in a conditional request; see <xref
1174   target="validation.model"/>.
[1374]1178<section anchor="combining.responses" title="Combining Partial Content">
[1374]1180   A response might transfer only a partial representation if the
1181   connection closed prematurely or if the request used one or more Range
1182   specifiers (&partial;).  After several such transfers, a cache might have
1183   received several ranges of the same representation.  A cache &MAY; combine
1184   these ranges into a single stored response, and reuse that response to
1185   satisfy later requests, if they all share the same strong validator and
1186   the cache complies with the client requirements in &combining-byte-ranges;.
[1374]1189   When combining the new response with one or more stored responses, a
1190   cache &MUST;:
1191   <list style="symbols">
1192      <t>delete any Warning header fields in the stored response with
1193         warn-code 1xx (see <xref target="header.warning" />);</t>
1194      <t>retain any Warning header fields in the stored response with
1195         warn-code 2xx; and,</t>
1196      <t>use other header fields provided in the new response, aside
1197         from Content-Range, to replace all instances of the corresponding
1198         header fields in the stored response.</t>
1199   </list>
[1415]1206<section anchor="header.field.definitions" title="Header Field Definitions">
[949]1208   This section defines the syntax and semantics of HTTP/1.1 header fields
1209   related to caching.
[540]1212<section anchor="header.age" title="Age">
[1120]1213   <iref item="Age header field" primary="true" x:for-anchor="" />
1214   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
[949]1215   <x:anchor-alias value="Age"/>
1216   <x:anchor-alias value="age-value"/>
[1163]1218   The "Age" header field conveys the sender's estimate of the amount
[949]1219   of time since the response was generated or successfully validated at the
1220   origin server. Age values are calculated as specified in <xref
1221   target="age.calculations" />.
[1231]1223<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1224  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
[1323]1227  Age field-values are non-negative integers, representing time in seconds
1228  (see <xref target="delta-seconds"/>).
[949]1231   The presence of an Age header field in a response implies that a response
1232   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1233   might not implement the Age header field.
[540]1237<section anchor="header.cache-control" title="Cache-Control">
[1120]1238   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1239   <iref item="Header Fields" primary="true" subitem="Cache-Control"
[949]1240      x:for-anchor="" />
1241   <x:anchor-alias value="Cache-Control"/>
1242   <x:anchor-alias value="cache-directive"/>
1243   <x:anchor-alias value="cache-extension"/>
1244   <x:anchor-alias value="cache-request-directive"/>
1245   <x:anchor-alias value="cache-response-directive"/>
[1163]1247   The "Cache-Control" header field is used to specify directives for
[949]1248   caches along the request/response chain. Such cache directives are
1249   unidirectional in that the presence of a directive in a request does not
1250   imply that the same directive is to be given in the response.
[1112]1253   A cache &MUST; obey the requirements of the Cache-Control
[949]1254   directives defined in this section. See <xref
1255   target="cache.control.extensions"/> for information about how Cache-Control
1256   directives defined elsewhere are handled.
[949]1259   <t>
1260       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1261       might only implement Pragma: no-cache (see <xref target="header.pragma"
1262       />).
1263   </t>
[1173]1266   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1267   through in forwarded messages, regardless of their
[1112]1268   significance to that application, since the directives might be applicable
1269   to all recipients along the request/response chain. It is not possible to
1270   target a directive to a specific cache.
[1449]1272<t>Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument.</t>
[1231]1273<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1274  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
[229]1276  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
[334]1277     / <x:ref>cache-response-directive</x:ref>
[334]1279  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
[949]1282<section anchor="cache-request-directive"
1283   title="Request Cache-Control Directives">
1284   <x:anchor-alias value="cache-request-directive" />
[949]1286<figure><artwork type="abnf2616"><iref item="Grammar" primary="true"
1287   subitem="cache-request-directive" />
[538]1288  <x:ref>cache-request-directive</x:ref> =
1289       "no-cache"
1290     / "no-store"
1291     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1292     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1293     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1294     / "no-transform"
1295     / "only-if-cached"
1296     / <x:ref>cache-extension</x:ref>
[949]1300   <x:dfn>no-cache</x:dfn>
1301   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1302   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1303   <list>
[1112]1304      <t>The no-cache request directive indicates that a cache &MUST-NOT;
1305      use a stored response to satisfy the request without successful
1306      validation on the origin server.</t>
[949]1307   </list>
[949]1310   <x:dfn>no-store</x:dfn>
1311   <iref item="Cache Directives" primary="true" subitem="no-store" />
1312   <iref item="no-store" primary="true" subitem="Cache Directive" />
1313   <list>
1314      <t>The no-store request directive indicates that a cache &MUST-NOT;
1315      store any part of either this request or any response to it. This
[1112]1316      directive applies to both private and shared caches. "&MUST-NOT;
[949]1317      store" in this context means that the cache &MUST-NOT; intentionally
1318      store the information in non-volatile storage, and &MUST; make a
1319      best-effort attempt to remove the information from volatile storage as
1320      promptly as possible after forwarding it.</t>
1321      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1322      privacy. In particular, malicious or compromised caches might not
1323      recognize or obey this directive, and communications networks might be
1324      vulnerable to eavesdropping.</t>
[1110]1325      <t>Note that if a request containing this directive is satisfied from a
1326      cache, the no-store request directive does not apply to the already
1327      stored response.</t>
[949]1328   </list>
[949]1331   <x:dfn>max-age</x:dfn>
1332   <iref item="Cache Directives" primary="true" subitem="max-age" />
1333   <iref item="max-age" primary="true" subitem="Cache Directive" />
1334   <list>
[1273]1335      <t>The max-age request directive indicates that the client is unwilling to
1336      accept a response whose age is greater than the specified number of
[949]1337      seconds. Unless the max-stale request directive is also present, the
1338      client is not willing to accept a stale response.</t>
1339   </list>
[949]1342   <x:dfn>max-stale</x:dfn>
1343   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1344   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1345   <list>
1346      <t>The max-stale request directive indicates that the client is willing
1347      to accept a response that has exceeded its expiration time. If max-stale
1348      is assigned a value, then the client is willing to accept a response
1349      that has exceeded its expiration time by no more than the specified
1350      number of seconds. If no value is assigned to max-stale, then the client
1351      is willing to accept a stale response of any age.</t>
1352   </list>
[949]1355   <x:dfn>min-fresh</x:dfn>
1356   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1357   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1358   <list>
1359      <t>The min-fresh request directive indicates that the client is willing
1360      to accept a response whose freshness lifetime is no less than its
1361      current age plus the specified time in seconds. That is, the client
1362      wants a response that will still be fresh for at least the specified
1363      number of seconds.</t>
1364   </list>
[949]1367   <x:dfn>no-transform</x:dfn>
1368   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1369   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1370   <list>
[1112]1371      <t>The no-transform request directive indicates that an intermediary
1372        (whether or not it implements a cache) &MUST-NOT; change the
1373        Content-Encoding, Content-Range or Content-Type request header fields,
1374        nor the request representation.</t>
[949]1375   </list>
[949]1378   <x:dfn>only-if-cached</x:dfn>
1379   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1380   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1381   <list>
1382      <t>The only-if-cached request directive indicates that the client only
[1278]1383      wishes to obtain a stored response. If it receives this directive, a
[949]1384      cache &SHOULD; either respond using a stored response that is consistent
1385      with the other constraints of the request, or respond with a 504
1386      (Gateway Timeout) status code. If a group of caches is being operated as
[1112]1387      a unified system with good internal connectivity, a member cache &MAY;
1388      forward such a request within that group of caches.</t>
[949]1389   </list>
[949]1393<section anchor="cache-response-directive"
1394   title="Response Cache-Control Directives">
1395   <x:anchor-alias value="cache-response-directive" />
[949]1397<figure><artwork type="abnf2616"><iref item="Grammar" primary="true"
1398   subitem="cache-response-directive" />
[538]1399  <x:ref>cache-response-directive</x:ref> =
1400       "public"
1401     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1402     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1403     / "no-store"
1404     / "no-transform"
1405     / "must-revalidate"
1406     / "proxy-revalidate"
1407     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1408     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1409     / <x:ref>cache-extension</x:ref>
[949]1413   <x:dfn>public</x:dfn>
1414   <iref item="Cache Directives" primary="true" subitem="public" />
1415   <iref item="public" primary="true" subitem="Cache Directive" />
1416   <list>
[1129]1417      <t>The public response directive indicates that a response whose
1418        associated request contains an 'Authentication' header &MAY; be
1419        stored (see <xref target="caching.authenticated.responses" />).</t>
[540]1420  </list>
[949]1423   <x:dfn>private</x:dfn>
1424   <iref item="Cache Directives" primary="true" subitem="private" />
1425   <iref item="private" primary="true" subitem="Cache Directive" />
1426   <list>
1427      <t>The private response directive indicates that the response message is
1428      intended for a single user and &MUST-NOT; be stored by a shared cache. A
[1112]1429      private cache &MAY; store the response.</t>
[949]1430      <t>If the private response directive specifies one or more field-names,
1431      this requirement is limited to the field-values associated with the
[1112]1432      listed response header fields. That is, a shared cache &MUST-NOT; store
1433      the specified field-names(s), whereas it &MAY; store the remainder of the
1434      response message.</t>
[949]1435      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1436      the response can be stored; it cannot ensure the privacy of the message
1437      content. Also, private response directives with field-names are often
1438      handled by implementations as if an unqualified private directive was
1439      received; i.e., the special handling for the qualified form is not
1440      widely implemented.</t>
1441   </list>
[949]1444   <x:dfn>no-cache</x:dfn>
1445   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1446   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1447   <list>
1448      <t>The no-cache response directive indicates that the response MUST NOT
1449      be used to satisfy a subsequent request without successful validation on
1450      the origin server. This allows an origin server to prevent a cache from
1451      using it to satisfy a request without contacting it, even by caches that
1452      have been configured to return stale responses.</t>
1453      <t>If the no-cache response directive specifies one or more field-names,
1454      this requirement is limited to the field-values associated with the
[1112]1455      listed response header fields. That is, a cache &MUST-NOT; send the
1456      specified field-name(s) in the response to a subsequent request without successful
[949]1457      validation on the origin server. This allows an origin server to prevent
1458      the re-use of certain header fields in a response, while still allowing
1459      caching of the rest of the response.</t>
1460      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1461      this directive. Also, no-cache response directives with field-names are
1462      often handled by implementations as if an unqualified no-cache directive
1463      was received; i.e., the special handling for the qualified form is not
1464      widely implemented. </t>
1465   </list>
[949]1468   <x:dfn>no-store</x:dfn>
1469   <iref item="Cache Directives" primary="true" subitem="no-store" />
1470   <iref item="no-store" primary="true" subitem="Cache Directive" />
1471   <list>
1472      <t>The no-store response directive indicates that a cache &MUST-NOT;
1473      store any part of either the immediate request or response. This
[1112]1474      directive applies to both private and shared caches. "&MUST-NOT;
[949]1475      store" in this context means that the cache &MUST-NOT; intentionally
1476      store the information in non-volatile storage, and &MUST; make a
1477      best-effort attempt to remove the information from volatile storage as
1478      promptly as possible after forwarding it.</t>
1479      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1480      privacy. In particular, malicious or compromised caches might not
1481      recognize or obey this directive, and communications networks might be
1482      vulnerable to eavesdropping.</t>
1483   </list>
[949]1486   <x:dfn>must-revalidate</x:dfn>
1487   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1488   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1489   <list>
1490      <t>The must-revalidate response directive indicates that once it has
[1112]1491      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
[949]1492      requests without successful validation on the origin server.</t>
1493      <t>The must-revalidate directive is necessary to support reliable
[1112]1494      operation for certain protocol features. In all circumstances a
1495      cache &MUST; obey the must-revalidate directive; in particular,
1496      if a cache cannot reach the origin server for any reason, it &MUST;
[949]1497      generate a 504 (Gateway Timeout) response.</t>
[1433]1498      <t>The must-revalidate directive ought to be used by servers if and only
1499      if failure to validate a request on the representation could result in
[949]1500      incorrect operation, such as a silently unexecuted financial
1501      transaction.</t>
1502   </list>
[949]1505   <x:dfn>proxy-revalidate</x:dfn>
1506   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1507   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1508   <list>
1509      <t>The proxy-revalidate response directive has the same meaning as the
1510      must-revalidate response directive, except that it does not apply to
[1112]1511      private caches.</t>
[949]1512   </list>
[949]1515   <x:dfn>max-age</x:dfn>
1516   <iref item="Cache Directives" primary="true" subitem="max-age" />
1517   <iref item="max-age" primary="true" subitem="Cache Directive" />
1518   <list>
[1270]1519      <t>The max-age response directive indicates that the response is to be
[949]1520      considered stale after its age is greater than the specified number of
1521      seconds.</t>
1522   </list>
[949]1525   <x:dfn>s-maxage</x:dfn>
1526   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1527   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1528   <list>
1529      <t>The s-maxage response directive indicates that, in shared caches, the
1530      maximum age specified by this directive overrides the maximum age
[994]1531      specified by either the max-age directive or the Expires header field. The
[949]1532      s-maxage directive also implies the semantics of the proxy-revalidate
1533      response directive.</t>
1534   </list>
[949]1537   <x:dfn>no-transform</x:dfn>
1538   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1539   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1540   <list>
[1112]1541      <t>The no-transform response directive indicates that an intermediary
1542      (regardless of whether it implements a cache) &MUST-NOT; change the
1543      Content-Encoding, Content-Range or Content-Type response header fields,
1544      nor the response representation.</t>
[949]1545   </list>
[540]1550<section anchor="cache.control.extensions" title="Cache Control Extensions">
[949]1552   The Cache-Control header field can be extended through the use of one or
1553   more cache-extension tokens, each with an optional value. Informational
1554   extensions (those that do not require a change in cache behavior) can be
1555   added without changing the semantics of other directives. Behavioral
1556   extensions are designed to work by acting as modifiers to the existing base
1557   of cache directives. Both the new directive and the standard directive are
1558   supplied, such that applications that do not understand the new directive
1559   will default to the behavior specified by the standard directive, and those
1560   that understand the new directive will recognize it as modifying the
1561   requirements associated with the standard directive. In this way,
1562   extensions to the cache-control directives can be made without requiring
1563   changes to the base protocol.
[949]1566   This extension mechanism depends on an HTTP cache obeying all of the
1567   cache-control directives defined for its native HTTP-version, obeying
1568   certain extensions, and ignoring all directives that it does not
1569   understand.
[949]1572   For example, consider a hypothetical new response directive called
1573   "community" that acts as a modifier to the private directive. We define
[1112]1574   this new directive to mean that, in addition to any private cache, any
[949]1575   cache that is shared only by members of the community named within its
1576   value may cache the response. An origin server wishing to allow the UCI
1577   community to use an otherwise private response in their shared cache(s)
1578   could do so by including
1580<figure><artwork type="example">
[458]1581  Cache-Control: private, community="UCI"
[949]1584   A cache seeing this header field will act correctly even if the cache does
1585   not understand the community cache-extension, since it will also see and
1586   understand the private directive and thus default to the safe behavior.
[1285]1589   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
[949]1590   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1591   combined with standard directives (or the response's default cacheability)
1592   such that the cache behavior will remain minimally correct even if the
1593   cache does not understand the extension(s).
[949]1596   The HTTP Cache Directive Registry defines the name space for the cache
1597   directives.
[1112]1600   A registration &MUST; include the following fields:
[949]1601   <list style="symbols">
1602      <t>Cache Directive Name</t>
1603      <t>Pointer to specification text</t>
1604   </list>
[949]1607   Values to be added to this name space are subject to IETF review (<xref
1608   target="RFC5226" x:fmt="," x:sec="4.1"/>).
[949]1611   The registry itself is maintained at <eref
1612   target=""/>.
[540]1618<section anchor="header.expires" title="Expires">
[1120]1619   <iref item="Expires header field" primary="true" x:for-anchor="" />
1620   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
[949]1621   <x:anchor-alias value="Expires"/>
[965]1623   The "Expires" header field gives the date/time after which the
[949]1624   response is considered stale. See <xref target="expiration.model" /> for
1625   further discussion of the freshness model.
[949]1628   The presence of an Expires field does not imply that the original resource
1629   will change or cease to exist at, before, or after that time.
[949]1632   The field-value is an absolute date and time as defined by HTTP-date in
[1436]1633   &http-date;; a sender &MUST; use the rfc1123-date format.
[1231]1635<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1636  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1639  <preamble>For example</preamble>
1640<artwork type="example">
[364]1641  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1644   A cache &MUST; treat other invalid date formats,
1645   especially including the value "0", as in the past (i.e., "already
1646   expired").
[949]1649   <t>
1650       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1651       max-age directive (see <xref target="cache-response-directive" />),
1652       that directive overrides the Expires field. Likewise, the s-maxage
1653       directive overrides Expires in shared caches.
1654   </t>
[1400]1657   Historically, HTTP required the Expires field-value to be no more than a
1658   year in the future. While longer freshness lifetimes are no longer
1659   prohibited, extremely large values have been demonstrated to cause
1660   problems (e.g., clock overflows due to use of 32-bit integers for
1661   time values), and most caches will evict a response far sooner than
1662   that. Therefore, senders ought not produce them.
1665   An origin server without a clock &MUST-NOT; assign Expires
1666   values to a response unless these values were associated
1667   with the resource by a system or user with a reliable clock. It &MAY;
1668   assign an Expires value that is known, at or before server
1669   configuration time, to be in the past (this allows "pre-expiration"
1670   of responses without storing separate Expires values for each
1671   resource).
[540]1675<section anchor="header.pragma" title="Pragma">
[1120]1676   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1677   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
[949]1678   <x:anchor-alias value="extension-pragma"/>
1679   <x:anchor-alias value="Pragma"/>
1680   <x:anchor-alias value="pragma-directive"/>
[1287]1682   The "Pragma" header field allows backwards compatibility with HTTP/1.0
[1284]1683   caches, so that clients can specify a "no-cache" request that they will
1684   understand (as Cache-Control was not defined until HTTP/1.1). When the
1685   Cache-Control header is also present and understood in a request, Pragma is
1686   ignored.
[1287]1689   In HTTP/1.0, Pragma was defined as an extensible field for
[1284]1690   implementation-specified directives for recipients. This specification
1691   deprecates such extensions to improve interoperability.
[1231]1693<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"/>
1694  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1695  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1696  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
[1287]1699   When the Cache-Control header is not present in a request, the no-cache
[1284]1700   request pragma-directive &MUST; have the same effect on caches as if
1701   "Cache-Control: no-cache" were present (see <xref
1702   target="cache-request-directive" />).
[1433]1705   When sending a no-cache request, a client ought to include both the pragma
1706   and cache-control directives, unless Cache-Control: no-cache is
1707   purposefully omitted to target other Cache-Control response directives at
1708   HTTP/1.1 caches. For example:
[1287]1711<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
[1284]1712GET / HTTP/1.1
1714Cache-Control: max-age=30
1715Pragma: no-cache
1720   will constrain HTTP/1.1 caches to serve a response no older than 30
1721   seconds, while precluding implementations that do not understand
1722   Cache-Control from serving a cached response.
[949]1725   <t>
[1287]1726      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
[1284]1727      specified, it does not provide a reliable replacement for
1728      "Cache-Control: no-cache" in them.
[1287]1729   </t>
[540]1733<section anchor="header.vary" title="Vary">
[1120]1734   <iref item="Vary header field" primary="true" x:for-anchor="" />
1735   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
[949]1736   <x:anchor-alias value="Vary"/>
[1163]1738   The "Vary" header field conveys the set of header fields
[949]1739   that were used to select the representation.
[949]1742   Caches use this information, in part, to determine whether a stored
1743   response can be used to satisfy a given request; see <xref
1744   target="caching.negotiated.responses" />. determines, while the response is
1745   fresh, whether a cache is permitted to use the response to reply to a
1746   subsequent request without validation; see <xref
1747   target="caching.negotiated.responses" />.
[949]1750   In uncacheable or stale responses, the Vary field value advises the user
1751   agent about the criteria that were used to select the representation.
[1231]1753<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1754  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
[949]1757   The set of header fields named by the Vary field value is known as the
[1163]1758   selecting header fields.
[1112]1761   A server &SHOULD; include a Vary header field with any cacheable response
[949]1762   that is subject to server-driven negotiation. Doing so allows a cache to
1763   properly interpret future requests on that resource and informs the user
1764   agent about the presence of negotiation on that resource. A server &MAY;
1765   include a Vary header field with a non-cacheable response that is subject
1766   to server-driven negotiation, since this might provide the user agent with
1767   useful information about the dimensions over which the response varies at
1768   the time of the response.
[949]1771   A Vary field value of "*" signals that unspecified parameters not limited
[1163]1772   to the header fields (e.g., the network address of the client), play a
[949]1773   role in the selection of the response representation; therefore, a cache
[1112]1774   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1775   generate the "*" value.
[1163]1778   The field-names given are not limited to the set of standard header
[949]1779   fields defined by this specification. Field names are case-insensitive.
[540]1783<section anchor="header.warning" title="Warning">
[1120]1784   <iref item="Warning header field" primary="true" x:for-anchor="" />
1785   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
[949]1786   <x:anchor-alias value="Warning"/>
1787   <x:anchor-alias value="warning-value"/>
1788   <x:anchor-alias value="warn-agent"/>
1789   <x:anchor-alias value="warn-code"/>
1790   <x:anchor-alias value="warn-date"/>
1791   <x:anchor-alias value="warn-text"/>
[1163]1793   The "Warning" header field is used to carry additional information
[949]1794   about the status or transformation of a message that might not be reflected
1795   in the message. This information is typically used to warn about possible
1796   incorrectness introduced by caching operations or transformations applied
1797   to the payload of the message.
[949]1800   Warnings can be used for other purposes, both cache-related and otherwise.
1801   The use of a warning, rather than an error status code, distinguishes these
1802   responses from true failures.
[994]1805   Warning header fields can in general be applied to any message, however some
[949]1806   warn-codes are specific to caches and can only be applied to response
1807   messages.
[1231]1809<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"/>
1810  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
[229]1812  <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>
1813                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
[229]1815  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
[334]1816  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
[135]1817                  ; the name or pseudonym of the server adding
[994]1818                  ; the Warning header field, for use in debugging
[229]1819  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1820  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
[949]1823   Multiple warnings can be attached to a response (either by the origin
1824   server or by a cache), including multiple warnings with the same code
1825   number, only differing in warn-text.
[949]1828   When this occurs, the user agent &SHOULD; inform the user of as many of
1829   them as possible, in the order that they appear in the response.
[1433]1832   Systems that generate multiple Warning header fields are encouraged to
1833   order them with this user agent behavior in mind. New Warning header fields
1834   are added after any existing Warning headers fields.
[949]1837   Warnings are assigned three digit warn-codes. The first digit indicates
1838   whether the Warning is required to be deleted from a stored response after
1839   validation:
1840   <list style="symbols">
1841      <t>1xx Warnings describe the freshness or validation status of the
[1112]1842      response, and so &MUST; be deleted by a cache after validation. They can
[949]1843      only be generated by a cache when validating a cached entry, and
1844      &MUST-NOT; be generated in any other situation.</t>
1845      <t>2xx Warnings describe some aspect of the representation that is not
1846      rectified by a validation (for example, a lossy compression of the
[1112]1847      representation) and &MUST-NOT; be deleted by a cache after validation,
[949]1848      unless a full response is returned, in which case they &MUST; be.</t>
1849   </list>
[994]1852   If an implementation sends a message with one or more Warning header fields to a
[949]1853   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
[994]1854   in each warning-value a warn-date that matches the Date header field in the
[949]1855   message.
[1112]1858   If a system receives a message with a warning-value that includes
[949]1859   a warn-date, and that warn-date is different from the Date value in the
1860   response, then that warning-value &MUST; be deleted from the message before
1861   storing, forwarding, or using it. (preventing the consequences of naive
1862   caching of Warning header fields.) If all of the warning-values are deleted
[994]1863   for this reason, the Warning header field &MUST; be deleted as well.
[949]1866   The following warn-codes are defined by this specification, each with a
1867   recommended warn-text in English, and a description of its meaning.
1870<section title="110 Response is Stale" anchor="warn.110">
1871  <iref primary="true" item="110 Response is Stale (warn code)" x:for-anchor=""/>
1872  <iref primary="true" item="Warn Codes" subitem="110 Response is Stale" x:for-anchor=""/>
1874   A cache &SHOULD; include this whenever the returned response is stale.
1878<section title="111 Revalidation Failed" anchor="warn.111">
1879  <iref primary="true" item="111 Revalidation Failed (warn code)" x:for-anchor=""/>
1880  <iref primary="true" item="Warn Codes" subitem="111 Revalidation Failed" x:for-anchor=""/>
1882   A cache &SHOULD; include this when returning a stale response because an
1883   attempt to validate the response failed, due to an inability to reach
1884   the server.
1888<section title="112 Disconnected Operation" anchor="warn.112">
1889  <iref primary="true" item="112 Disconnected Operation (warn code)" x:for-anchor=""/>
1890  <iref primary="true" item="Warn Codes" subitem="112 Disconnected Operation" x:for-anchor=""/>
1892   A cache &SHOULD; include this if it is intentionally disconnected from
1893   the rest of the network for a period of time.
1897<section title="113 Heuristic Expiration" anchor="warn.113">
1898  <iref primary="true" item="113 Heuristic Expiration (warn code)" x:for-anchor=""/>
1899  <iref primary="true" item="Warn Codes" subitem="113 Heuristic Expiration" x:for-anchor=""/>
1901   A cache &SHOULD; include this if it heuristically chose a freshness
1902   lifetime greater than 24 hours and the response's age is greater than 24
1903   hours.
1907<section title="199 Miscellaneous Warning" anchor="warn.199">
1908  <iref primary="true" item="199 Miscellaneous Warning (warn code)" x:for-anchor=""/>
1909  <iref primary="true" item="Warn Codes" subitem="199 Miscellaneous Warning" x:for-anchor=""/>
1911   The warning text can include arbitrary information to be presented to
1912   a human user, or logged. A system receiving this warning &MUST-NOT; take
1913   any automated action, besides presenting the warning to the user.
1917<section title="214 Transformation Applied" anchor="warn.214">
1918  <iref primary="true" item="214 Transformation Applied (warn code)" x:for-anchor=""/>
1919  <iref primary="true" item="Warn Codes" subitem="214 Transformation Applied" x:for-anchor=""/>
1921   &MUST; be added by a proxy if it applies any transformation to the
1922   representation, such as changing the content-coding, media-type, or
1923   modifying the representation data, unless this Warning code already appears
1924   in the response.
1928<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
1929  <iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)" x:for-anchor=""/>
1930  <iref primary="true" item="Warn Codes" subitem="299 Miscellaneous Persistent Warning" x:for-anchor=""/>
1932   The warning text can include arbitrary information to be presented to
1933   a human user, or logged. A system receiving this warning &MUST-NOT; take
1934   any automated action.
[1485]1938<section title="Warn Code Extensions" anchor="warn.code.extensions">
1940   The HTTP Warn Code Registry defines the name space for warn codes.
1943   A registration &MUST; include the following fields:
1944   <list style="symbols">
1945      <t>Warn Code (3 digits)</t>
1946      <t>Short Description</t>
1947      <t>Pointer to specification text</t>
1948   </list>
1951   Values to be added to this name space are subject to IETF review (<xref
1952   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1955   The registry itself is maintained at <eref
1956   target=""/>.
[540]1962<section anchor="history.lists" title="History Lists">
[949]1964   User agents often have history mechanisms, such as "Back" buttons and
1965   history lists, that can be used to redisplay a representation retrieved
1966   earlier in a session.
[949]1969   The freshness model (<xref target="expiration.model"/>) does not
1970   necessarily apply to history mechanisms. I.e., a history mechanism can
1971   display a previous representation even if it has expired.
1974   This does not prohibit the history mechanism from telling the user that a
1975   view might be stale, or from honoring cache directives (e.g.,
1976   Cache-Control: no-store).
[540]1981<section anchor="IANA.considerations" title="IANA Considerations">
[949]1983<section title="Cache Directive Registry"
1984   anchor="cache.directive.registration">
[949]1986   The registration procedure for HTTP Cache Directives is defined by <xref
1987   target="cache.control.extensions"/> of this document.
[969]1990   The HTTP Cache Directive Registry shall be created at <eref
[949]1991   target=""/> and be
1992   populated with the registrations below:
1994<?BEGININC p6-cache.cache-directives ?>
1995<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
[825]1996<texttable xmlns:my="#my" align="left" suppress-title="true"
[817]1997           anchor="iana.cache.directive.registration.table">
1998   <ttcol>Cache Directive</ttcol>
1999   <ttcol>Reference</ttcol>
2001   <c>max-age</c>
2002   <c>
2003      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
2004   </c>
2005   <c>max-stale</c>
2006   <c>
2007      <xref target="cache-request-directive"/>
2008   </c>
2009   <c>min-fresh</c>
2010   <c>
2011      <xref target="cache-request-directive"/>
2012   </c>
2013   <c>must-revalidate</c>
2014   <c>
2015      <xref target="cache-response-directive"/>
2016   </c>
2017   <c>no-cache</c>
2018   <c>
2019      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
2020   </c>
2021   <c>no-store</c>
2022   <c>
2023      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
2024   </c>
2025   <c>no-transform</c>
2026   <c>
2027      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
2028   </c>
2029   <c>only-if-cached</c>
2030   <c>
2031      <xref target="cache-request-directive"/>
2032   </c>
2033   <c>private</c>
2034   <c>
2035      <xref target="cache-response-directive"/>
2036   </c>
2037   <c>proxy-revalidate</c>
2038   <c>
2039      <xref target="cache-response-directive"/>
2040   </c>
2041   <c>public</c>
2042   <c>
2043      <xref target="cache-response-directive"/>
2044   </c>
2045   <c>s-maxage</c>
2046   <c>
2047      <xref target="cache-response-directive"/>
2048   </c>
[825]2049   <c>stale-if-error</c>
2050   <c>
2051      <xref xmlns:x="" target="RFC5861" x:fmt="," x:sec="4"/>
2052   </c>
2053   <c>stale-while-revalidate</c>
2054   <c>
2055      <xref xmlns:x="" target="RFC5861" x:fmt="," x:sec="3"/>
2056   </c>
2059<?ENDINC p6-cache.cache-directives ?>
[1485]2062<section title="Warn Code Registry"
2063   anchor="warn.code.registration">
2065   The registration procedure for HTTP Warn Codes is defined by <xref
2066   target="warn.code.extensions"/> of this document.
2069   The HTTP Warn Code Registry shall be created at <eref
2070   target=""/> and be
2071   populated with the registrations below:
[1487]2073<?BEGININC p6-cache.iana-warn-codes ?>
2074<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
2075<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
[1485]2076   <ttcol>Warn Code</ttcol>
2077   <ttcol>Short Description</ttcol>
2078   <ttcol>Reference</ttcol>
2079   <c>110</c>
[1487]2080   <c>Response is Stale</c>
2081   <c>
2082      <xref target="warn.110"/>
2083   </c>
[1485]2084   <c>111</c>
[1487]2085   <c>Revalidation Failed</c>
2086   <c>
2087      <xref target="warn.111"/>
2088   </c>
[1485]2089   <c>112</c>
[1487]2090   <c>Disconnected Operation</c>
2091   <c>
2092      <xref target="warn.112"/>
2093   </c>
[1485]2094   <c>113</c>
[1487]2095   <c>Heuristic Expiration</c>
2096   <c>
2097      <xref target="warn.113"/>
2098   </c>
[1485]2099   <c>199</c>
[1487]2100   <c>Miscellaneous Warning</c>
2101   <c>
2102      <xref target="warn.199"/>
2103   </c>
[1485]2104   <c>214</c>
[1487]2105   <c>Transformation Applied</c>
2106   <c>
2107      <xref target="warn.214"/>
2108   </c>
[1485]2109   <c>299</c>
[1487]2110   <c>Miscellaneous Persistent Warning</c>
2111   <c>
2112      <xref target="warn.299"/>
2113   </c>
2116<?ENDINC p6-cache.iana-warn-codes ?>
[921]2119<section title="Header Field Registration" anchor="header.field.registration">
[949]2121  The Message Header Field Registry located at <eref 
[540]2122  target="" />
[969]2123  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
[680]2125<?BEGININC p6-cache.iana-headers ?>
[540]2126<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
[680]2127<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2128   <ttcol>Header Field Name</ttcol>
2129   <ttcol>Protocol</ttcol>
2130   <ttcol>Status</ttcol>
2131   <ttcol>Reference</ttcol>
[680]2133   <c>Age</c>
2134   <c>http</c>
2135   <c>standard</c>
2136   <c>
2137      <xref target="header.age"/>
2138   </c>
2139   <c>Cache-Control</c>
2140   <c>http</c>
2141   <c>standard</c>
2142   <c>
2143      <xref target="header.cache-control"/>
2144   </c>
2145   <c>Expires</c>
2146   <c>http</c>
2147   <c>standard</c>
2148   <c>
2149      <xref target="header.expires"/>
2150   </c>
2151   <c>Pragma</c>
2152   <c>http</c>
2153   <c>standard</c>
2154   <c>
2155      <xref target="header.pragma"/>
2156   </c>
2157   <c>Vary</c>
2158   <c>http</c>
2159   <c>standard</c>
2160   <c>
2161      <xref target="header.vary"/>
2162   </c>
2163   <c>Warning</c>
2164   <c>http</c>
2165   <c>standard</c>
2166   <c>
2167      <xref target="header.warning"/>
2168   </c>
[680]2171<?ENDINC p6-cache.iana-headers ?>
[949]2173   The change controller is: "IETF ( - Internet Engineering Task
2174   Force".
[540]2179<section anchor="security.considerations" title="Security Considerations">
[949]2181   Caches expose additional potential vulnerabilities, since the contents of
2182   the cache represent an attractive target for malicious exploitation.
2183   Because cache contents persist after an HTTP request is complete, an attack
2184   on the cache can reveal information long after a user believes that the
2185   information has been removed from the network. Therefore, cache contents
[969]2186   need to be protected as sensitive information.
[1364]2190<section title="Acknowledgments" anchor="acks">
[1364]2192  See &acks;.
2199<references title="Normative References">
[540]2201  <reference anchor="Part1">
2202    <front>
2203      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2204      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2205        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2206        <address><email></email></address>
2207      </author>
2208      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2209        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2210        <address><email></email></address>
[540]2211      </author>
2212      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2213        <organization abbrev="HP">Hewlett-Packard Company</organization>
2214        <address><email></email></address>
2215      </author>
2216      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2217        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2218        <address><email></email></address>
2219      </author>
2220      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2221        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2222        <address><email></email></address>
2223      </author>
2224      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2225        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2226        <address><email></email></address>
2227      </author>
2228      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2229        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2230        <address><email></email></address>
2231      </author>
2232      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2233        <organization abbrev="W3C">World Wide Web Consortium</organization>
2234        <address><email></email></address>
2235      </author>
2236      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2237        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2238        <address><email></email></address>
2239      </author>
2240      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2241    </front>
2242    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2243    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2244  </reference>
[540]2246  <reference anchor="Part2">
2247    <front>
2248      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2249      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2250        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2251        <address><email></email></address>
2252      </author>
2253      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2254        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2255        <address><email></email></address>
[540]2256      </author>
2257      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2258        <organization abbrev="HP">Hewlett-Packard Company</organization>
2259        <address><email></email></address>
2260      </author>
2261      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2262        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2263        <address><email></email></address>
2264      </author>
2265      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2266        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2267        <address><email></email></address>
2268      </author>
2269      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2270        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2271        <address><email></email></address>
2272      </author>
2273      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2274        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2275        <address><email></email></address>
2276      </author>
2277      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2278        <organization abbrev="W3C">World Wide Web Consortium</organization>
2279        <address><email></email></address>
2280      </author>
2281      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2282        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2283        <address><email></email></address>
2284      </author>
2285      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2286    </front>
2287    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2288    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2289  </reference>
[540]2291  <reference anchor="Part4">
2292    <front>
2293      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2294      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2295        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2296        <address><email></email></address>
2297      </author>
2298      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2299        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2300        <address><email></email></address>
[540]2301      </author>
2302      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2303        <organization abbrev="HP">Hewlett-Packard Company</organization>
2304        <address><email></email></address>
2305      </author>
2306      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2307        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2308        <address><email></email></address>
2309      </author>
2310      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2311        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2312        <address><email></email></address>
2313      </author>
2314      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2315        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2316        <address><email></email></address>
2317      </author>
2318      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2319        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2320        <address><email></email></address>
2321      </author>
2322      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2323        <organization abbrev="W3C">World Wide Web Consortium</organization>
2324        <address><email></email></address>
2325      </author>
2326      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2327        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2328        <address><email></email></address>
2329      </author>
2330      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2331    </front>
2332    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2333    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2334  </reference>
[540]2336  <reference anchor="Part5">
2337    <front>
2338      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2339      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2340        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2341        <address><email></email></address>
2342      </author>
2343      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2344        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2345        <address><email></email></address>
[540]2346      </author>
2347      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2348        <organization abbrev="HP">Hewlett-Packard Company</organization>
2349        <address><email></email></address>
2350      </author>
2351      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2352        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2353        <address><email></email></address>
2354      </author>
2355      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2356        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2357        <address><email></email></address>
2358      </author>
2359      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2360        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2361        <address><email></email></address>
2362      </author>
2363      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2364        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2365        <address><email></email></address>
2366      </author>
2367      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2368        <organization abbrev="W3C">World Wide Web Consortium</organization>
2369        <address><email></email></address>
2370      </author>
2371      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2372        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2373        <address><email></email></address>
2374      </author>
2375      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2376    </front>
2377    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2378    <x:source basename="p5-range" href="p5-range.xml" />
2379  </reference>
[540]2381  <reference anchor="Part7">
2382    <front>
2383      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2384      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2385        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2386        <address><email></email></address>
2387      </author>
2388      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2389        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2390        <address><email></email></address>
[540]2391      </author>
2392      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2393        <organization abbrev="HP">Hewlett-Packard Company</organization>
2394        <address><email></email></address>
2395      </author>
2396      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2397        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2398        <address><email></email></address>
2399      </author>
2400      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2401        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2402        <address><email></email></address>
2403      </author>
2404      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2405        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2406        <address><email></email></address>
2407      </author>
2408      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2409        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2410        <address><email></email></address>
2411      </author>
2412      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2413        <organization abbrev="W3C">World Wide Web Consortium</organization>
2414        <address><email></email></address>
2415      </author>
2416      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2417        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2418        <address><email></email></address>
2419      </author>
2420      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2421    </front>
2422    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2423    <x:source basename="p7-auth" href="p7-auth.xml" />
2424  </reference>
[540]2426  <reference anchor="RFC2119">
2427    <front>
2428      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2429      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2430        <organization>Harvard University</organization>
2431        <address><email></email></address>
2432      </author>
2433      <date month="March" year="1997" />
2434    </front>
2435    <seriesInfo name="BCP" value="14" />
2436    <seriesInfo name="RFC" value="2119" />
2437  </reference>
[540]2439  <reference anchor="RFC5234">
2440    <front>
2441      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2442      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2443        <organization>Brandenburg InternetWorking</organization>
2444        <address>
[728]2445          <email></email>
2446        </address> 
[540]2447      </author>
2448      <author initials="P." surname="Overell" fullname="Paul Overell">
2449        <organization>THUS plc.</organization>
2450        <address>
[728]2451          <email></email>
2452        </address>
[540]2453      </author>
2454      <date month="January" year="2008"/>
2455    </front>
2456    <seriesInfo name="STD" value="68"/>
2457    <seriesInfo name="RFC" value="5234"/>
2458  </reference>
[540]2462<references title="Informative References">
[540]2464  <reference anchor="RFC1305">
2465    <front>
2466      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2467      <author fullname="David L. Mills" initials="D." surname="Mills">
2468        <organization>University of Delaware, Electrical Engineering Department</organization>
2469        <address><email></email></address>
2470      </author>
2471      <date month="March" year="1992" />
2472    </front>
2473    <seriesInfo name="RFC" value="1305" />
2474  </reference>
[540]2476  <reference anchor="RFC2616">
2477    <front>
2478      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2479      <author fullname="R. Fielding" initials="R." surname="Fielding">
2480        <organization>University of California, Irvine</organization>
2481        <address><email></email></address>
2482      </author>
2483      <author fullname="J. Gettys" initials="J." surname="Gettys">
2484        <organization>W3C</organization>
2485        <address><email></email></address>
2486      </author>
2487      <author fullname="J. Mogul" initials="J." surname="Mogul">
2488        <organization>Compaq Computer Corporation</organization>
2489        <address><email></email></address>
2490      </author>
2491      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2492        <organization>MIT Laboratory for Computer Science</organization>
2493        <address><email></email></address>
2494      </author>
2495      <author fullname="L. Masinter" initials="L." surname="Masinter">
2496        <organization>Xerox Corporation</organization>
2497        <address><email></email></address>
2498      </author>
2499      <author fullname="P. Leach" initials="P." surname="Leach">
2500        <organization>Microsoft Corporation</organization>
2501        <address><email></email></address>
2502      </author>
2503      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2504        <organization>W3C</organization>
2505        <address><email></email></address>
2506      </author>
2507      <date month="June" year="1999" />
2508    </front>
2509    <seriesInfo name="RFC" value="2616" />
2510  </reference>
2512  <reference anchor="RFC3864">
2513    <front>
2514      <title>Registration Procedures for Message Header Fields</title>
2515      <author fullname="G. Klyne" initials="G." surname="Klyne">
2516        <organization>Nine by Nine</organization>
2517        <address><email></email></address>
2518      </author>
2519      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2520        <organization>BEA Systems</organization>
2521        <address><email></email></address>
2522      </author>
2523      <author fullname="J. Mogul" initials="J." surname="Mogul">
2524        <organization>HP Labs</organization>
2525        <address><email></email></address>
2526      </author>
2527      <date month="September" year="2004" />
2528    </front>
2529    <seriesInfo name="BCP" value="90" />
2530    <seriesInfo name="RFC" value="3864" />
2531  </reference>
[817]2533  <reference anchor='RFC5226'>
2534    <front>
2535      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2536      <author initials='T.' surname='Narten' fullname='T. Narten'>
2537        <organization>IBM</organization>
2538        <address><email></email></address>
2539      </author>
2540      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2541        <organization>Google</organization>
2542        <address><email></email></address>
2543      </author>
2544      <date year='2008' month='May' />
2545    </front>
2546    <seriesInfo name='BCP' value='26' />
2547    <seriesInfo name='RFC' value='5226' />
2548  </reference>
[825]2550  <reference anchor='RFC5861'>
2551    <front>
2552      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2553      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2554        <organization>Yahoo! Inc.</organization>
2555        <address><email></email></address>
2556      </author>
2557      <date month="April" year="2010"/>
2558    </front>
2559    <seriesInfo name='RFC' value='5861' />
2560  </reference>
2564<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
[795]2566  Make the specified age calculation algorithm less conservative.
2567  (<xref target="age.calculations"/>)
[691]2570  Remove requirement to consider Content-Location in successful responses
2571  in order to determine the appropriate response to use.
2572  (<xref target="validation.model" />)
[540]2575  Clarify denial of service attack avoidance requirement.
2576  (<xref target="invalidation.after.updates.or.deletions" />)
[1231]2579  Change ABNF productions for header fields to only define the field value.
[1415]2580  (<xref target="header.field.definitions"/>)
[994]2583  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
[669]2584  anymore, as these aspects never were implemented.
2585  (<xref target="header.warning" />)
[680]2589<?BEGININC p6-cache.abnf-appendix ?>
[427]2590<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
2592<artwork type="abnf" name="p6-cache.parsed-abnf">
[1231]2593<x:ref>Age</x:ref> = delta-seconds
[1231]2595<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
[425]2596 cache-directive ] )
[1231]2598<x:ref>Expires</x:ref> = HTTP-date
[1436]2600<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
2602<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
[1231]2604<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
[425]2605 pragma-directive ] )
[1231]2607<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2608 ) )
[1231]2610<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2611 )
[427]2613<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2614<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2615<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
[425]2616 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2617 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2618 cache-extension
[427]2619<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
[425]2620 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2621 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2622 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2623 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2624 ) / ( "s-maxage=" delta-seconds ) / cache-extension
[427]2626<x:ref>delta-seconds</x:ref> = 1*DIGIT
2628<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
[678]2630<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
[1318]2632<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
[427]2633<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
[1436]2634<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.8&gt;
[1396]2636<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
[1396]2638<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.3&gt;
[1318]2640<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2642<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2643<x:ref>warn-code</x:ref> = 3DIGIT
2644<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2645<x:ref>warn-text</x:ref> = quoted-string
2646<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
[425]2647 ]
[532]2650<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2651; Age defined but not used
[425]2652; Cache-Control defined but not used
2653; Expires defined but not used
2654; Pragma defined but not used
2655; Vary defined but not used
2656; Warning defined but not used
[680]2658<?ENDINC p6-cache.abnf-appendix ?>
[540]2660<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
[1002]2662<section title="Since RFC 2616">
[540]2663  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
[540]2666<section title="Since draft-ietf-httpbis-p6-cache-00">
2668  Closed issues:
2669  <list style="symbols">
2670    <t>
[542]2671      <eref target="" />: "Trailer" (<eref target="" />)</t>
[540]2672    <t>
2673      <eref target="" />: "Invalidation after Update or Delete" (<eref target="" />)</t>
2674    <t>
2675      <eref target="" />: "Normative and Informative references"</t>
2676    <t>
2677      <eref target="" />: "Date reference typo"</t>
2678    <t>
2679      <eref target="" />: "Connection header text"</t>
2680    <t>
2681      <eref target="" />: "Informative references"</t>
2682    <t>
2683      <eref target="" />: "ISO-8859-1 Reference"</t>
2684    <t>
2685      <eref target="" />: "Normative up-to-date references"</t>
2686    <t>
2687      <eref target="" />: "typo in 13.2.2"</t>
2688  </list>
2691  Other changes:
2692  <list style="symbols">
2693    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2694        target="" />)</t>
2695  </list>
[540]2699<section title="Since draft-ietf-httpbis-p6-cache-01">
2701  Closed issues:
2702  <list style="symbols">
2703    <t>
2704      <eref target="" />: "rel_path not used"</t>
2705  </list>
2708  Other changes:
2709  <list style="symbols">
2710    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2711      on <eref target="" />)</t>
2712    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2713      specification.</t>
2714  </list>
[540]2718<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
[994]2720  Ongoing work on IANA Message Header Field Registration (<eref target="" />):
[540]2721  <list style="symbols">
[994]2722    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
[540]2723      document.</t>
2724  </list>
[540]2728<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2730  Closed issues:
2731  <list style="symbols">
2732    <t>
2733      <eref target="" />: "Vary header classification"</t>
2734  </list>
[540]2738<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2740  Ongoing work on ABNF conversion (<eref target=""/>):
2741  <list style="symbols">
2742    <t>
2743      Use "/" instead of "|" for alternatives.
2744    </t>
2745    <t>
2746      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2747      whitespace ("OWS") and required whitespace ("RWS").
2748    </t>
2749    <t>
2750      Rewrite ABNFs to spell out whitespace rules, factor out
[994]2751      header field value format definitions.
[540]2752    </t>
2753  </list>
[540]2757<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2759  This is a total rewrite of this part of the specification.
2762  Affected issues:
2763  <list style="symbols">
2764    <t>
2765      <eref target="" />: "Definition of 1xx Warn-Codes"</t>
2766    <t>
[839]2767      <eref target="" />: "Placement of 13.5.1 and 13.5.2"</t>
[542]2768    <t>
[839]2769      <eref target="" />: "The role of Warning and Semantic Transparency in Caching"</t>
[542]2770    <t>
[839]2771      <eref target="" />: "Methods and Caching"</t>
[542]2772  </list>
[543]2775  In addition: Final work on ABNF conversion (<eref target=""/>):
[542]2776  <list style="symbols">
2777    <t>
2778      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2779    </t>
2780  </list>
[547]2784<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
[576]2786  Closed issues:
2787  <list style="symbols">
2788    <t>
2789      <eref target=""/>:
2790      "base for numeric protocol elements"
2791    </t>
2792  </list>
2795  Affected issues:
2796  <list style="symbols">
2797    <t>
2798      <eref target=""/>:
[1166]2799      "Vary and non-existant headers"
[607]2800    </t>
2801  </list>
[604]2805<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
[669]2807  Closed issues:
2808  <list style="symbols">
2809    <t>
[711]2810      <eref target="" />:
2811      "Definition of 1xx Warn-Codes"
2812    </t>
2813    <t>
[691]2814      <eref target=""/>:
2815      "Content-Location on 304 responses"
2816    </t>
2817    <t>
[711]2818      <eref target="" />:
2819      "private and no-cache CC directives with headers"
2820    </t>
2821    <t>
[669]2822      <eref target=""/>:
2823      "RFC2047 and warn-text"
2824    </t>
2825  </list>
[720]2829<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
[766]2831  Closed issues:
2832  <list style="symbols">
2833    <t>
[771]2834      <eref target="" />:
2835      "serving negotiated responses from cache: header-specific canonicalization"
2836    </t>
2837    <t>
[766]2838      <eref target="" />:
2839      "Effect of CC directives on history lists"
2840    </t>
[1291]2841    <t>
[1292]2842      <eref target="" />:
2843      "Cache Extensions can override no-store, etc."
2844    </t>
[766]2845  </list>
[722]2848  Affected issues:
2849  <list style="symbols">
2850    <t>
2851      <eref target=""/>:
2852      Status codes and caching
2853    </t>
2854  </list>
2857  Partly resolved issues:
2858  <list style="symbols">
2859    <t>
2860      <eref target=""/>:
2861      "Placement of 13.5.1 and 13.5.2"
2862    </t>
2863  </list>
[773]2867<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
[792]2869  Closed issues:
2870  <list style="symbols">
2871    <t>
2872      <eref target="" />:
2873      "Age calculation"
2874    </t>
[817]2875    <t>
[827]2876      <eref target="" />:
2877      "Clarify differences between / requirements for request and response CC directives"
2878    </t>
2879    <t>
[1287]2880      <eref target="" />:
2881      "Caching authenticated responses"
2882    </t>
2883    <t>
[817]2884      <eref target="" />:
2885      "IANA registry for cache-control directives"
2886    </t>
[828]2887    <t>
2888      <eref target="" />:
2889      "Heuristic caching of URLs with query components"
2890    </t>
[792]2891  </list>
2894  Partly resolved issues:
2895  <list style="symbols">
2896    <t>
2897      <eref target=""/>:
2898      "Term for the requested resource's URI"
2899    </t>
2900  </list>
[841]2904<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
[911]2906  Closed issues:
2907  <list style="symbols">
2908    <t>
[965]2909      <eref target=""/>:
2910      "Clarify entity / representation / variant terminology"
2911    </t>
2912    <t>
[911]2913      <eref target=""/>:
2914      "consider removing the 'changes from 2068' sections"
2915    </t>
[916]2916    <t>
2917      <eref target=""/>:
2918      "Allowing heuristic caching for new status codes"
2919    </t>
[944]2920    <t>
2921      Clean up TODOs and prose in "Combining Responses."
2922    </t>
[911]2923  </list>
[973]2927<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
[1045]2929  Closed issues:
2930  <list style="symbols">
2931    <t>
2932      <eref target=""/>:
2933      "Text about clock requirement for caches belongs in p6"
2934    </t>
2935  </list>
[1052]2939<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
[1130]2941  Closed issues:
2942  <list style="symbols">
2943    <t>
[1165]2944      <eref target=""/>:
2945      "Header Classification"
2946    </t>
2947    <t>
[1130]2948      <eref target=""/>:
2949      "Clarify 'public'"
2950    </t>
2951  </list>
[1180]2955<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
[1231]2957  Closed issues:
2958  <list style="symbols">
2959    <t>
2960      <eref target=""/>:
2961      "untangle ABNFs for header fields"
2962    </t>
2963  </list>
[1271]2967<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
[1284]2969  Closed issues:
[1287]2970  <list style="symbols">
2971    <t>
[1292]2972      <eref target=""/>:
2973      "Mismatch Vary"
2974    </t>
2975    <t>
[1319]2976      <eref target=""/>:
2977      "Cache Invalidation only happens upon successful responses"
2978    </t>
2979    <t>
[1323]2980      <eref target=""/>:
2981      "Recommend minimum sizes for protocol elements"
2982    </t>
2983    <t>
[1292]2984      <eref target=""/>:
2985      "Proxies don't 'understand' methods"
2986    </t>
[1314]2987    <t>
[1333]2988      <eref target=""/>:
2989      "Cache Extensions can override no-store, etc."
2990    </t>
2991    <t>
[1319]2992      <eref target=""/>:
2993      "Pragma"
[1315]2994    </t>
[1287]2995  </list>
[1326]2999<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
[1400]3001  Closed issues:
3002  <list style="symbols">
3003    <t>
3004      <eref target=""/>:
3005      "Motivate one-year limit for Expires"
3006    </t>
3007  </list>
[1401]3011<section title="Since draft-ietf-httpbis-p6-cache-16" anchor="changes.since.16">
[1449]3013  Closed issues:
3014  <list style="symbols">
3015    <t>
[1452]3016      <eref target=""/>:
3017      "Document HTTP's error-handling philosophy"
3018    </t>
3019    <t>
[1449]3020      <eref target=""/>:
3021      "Cache-Control directive case sensitivity"
3022    </t>
3023  </list>
[1467]3027<section title="Since draft-ietf-httpbis-p6-cache-17" anchor="changes.since.17">
[1483]3029  Closed issues:
3030  <list style="symbols">
3031    <t>
3032      <eref target=""/>:
3033      "Interaction of request and response Cache-Control"
3034    </t>
[1484]3035    <t>
3036      <eref target=""/>:
3037      "Refining age for 1.1 proxy chains"
3038    </t>
[1485]3039    <t>
3040      <eref target=""/>:
3041      "warn-code registry"
3042    </t>
[1483]3043  </list>
[1499]3047<section title="Since draft-ietf-httpbis-p6-cache-18" anchor="changes.since.18">
3049  None yet.
[538]3054  </back>
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