source: draft-ietf-httpbis/latest/p6-cache.xml @ 1454

Last change on this file since 1454 was 1452, checked in by julian.reschke@…, 8 years ago

Rephrase description of conformance; explain how the spec handles error handling (see #186)

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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">
[1443]15  <!ENTITY ID-MONTH "October">
[1099]16  <!ENTITY ID-YEAR "2011">
[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"
56  obsoletes="2616" x:maturity-level="draft" xmlns:x="">
[547]59  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
[547]61  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]62    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[547]63    <address>
[8]64      <postal>
[1106]65        <street>345 Park Ave</street>
66        <city>San Jose</city>
[8]67        <region>CA</region>
[1106]68        <code>95110</code>
[29]69        <country>USA</country>
[8]70      </postal>
[29]71      <email></email>
72      <uri></uri>
[8]73    </address>
[547]74  </author>
[844]76  <author initials="J." surname="Gettys" fullname="Jim Gettys">
77    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
[547]78    <address>
[8]79      <postal>
[29]80        <street>21 Oak Knoll Road</street>
81        <city>Carlisle</city>
[8]82        <region>MA</region>
[29]83        <code>01741</code>
84        <country>USA</country>
[8]85      </postal>
[844]86      <email></email>
87      <uri></uri>
[8]88    </address>
[547]89  </author>
[547]91  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
92    <organization abbrev="HP">Hewlett-Packard Company</organization>
93    <address>
[8]94      <postal>
[29]95        <street>HP Labs, Large Scale Systems Group</street>
96        <street>1501 Page Mill Road, MS 1177</street>
[8]97        <city>Palo Alto</city>
98        <region>CA</region>
[29]99        <code>94304</code>
100        <country>USA</country>
[8]101      </postal>
[29]102      <email></email>
[8]103    </address>
[547]104  </author>
[547]106  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
107    <organization abbrev="Microsoft">Microsoft Corporation</organization>
108    <address>
[8]109      <postal>
[29]110        <street>1 Microsoft Way</street>
111        <city>Redmond</city>
112        <region>WA</region>
113        <code>98052</code>
114        <country>USA</country>
[8]115      </postal>
[29]116      <email></email>
[8]117    </address>
[547]118  </author>
[547]120  <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]121    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[547]122    <address>
[8]123      <postal>
[29]124        <street>345 Park Ave</street>
125        <city>San Jose</city>
[8]126        <region>CA</region>
[29]127        <code>95110</code>
128        <country>USA</country>
[8]129      </postal>
[29]130      <email></email>
131      <uri></uri>
[8]132    </address>
[547]133  </author>
[547]135  <author fullname="Paul J. Leach" initials="P." surname="Leach">
136    <organization abbrev="Microsoft">Microsoft Corporation</organization>
137    <address>
[8]138      <postal>
139        <street>1 Microsoft Way</street>
140        <city>Redmond</city>
141        <region>WA</region>
142        <code>98052</code>
143      </postal>
144      <email></email>
145    </address>
[547]146  </author>
[547]148  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
149    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
150    <address>
[8]151      <postal>
[34]152        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
153        <street>The Stata Center, Building 32</street>
154        <street>32 Vassar Street</street>
[8]155        <city>Cambridge</city>
156        <region>MA</region>
157        <code>02139</code>
[29]158        <country>USA</country>
[8]159      </postal>
160      <email></email>
[34]161      <uri></uri>
[8]162    </address>
[547]163  </author>
[547]165  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
166    <organization abbrev="W3C">World Wide Web Consortium</organization>
167    <address>
[93]168      <postal>
169        <street>W3C / ERCIM</street>
170        <street>2004, rte des Lucioles</street>
171        <city>Sophia-Antipolis</city>
172        <region>AM</region>
173        <code>06902</code>
174        <country>France</country>
175      </postal>
176      <email></email>
177      <uri></uri>
178    </address>
[547]179  </author>
[601]181  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
[1438]182    <organization>Rackspace</organization>
[607]183    <address>
184      <email></email>
185      <uri></uri>
[601]186    </address>
187  </author>
[547]189  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
190    <organization abbrev="greenbytes">greenbytes GmbH</organization>
191    <address>
[95]192      <postal>
193        <street>Hafenweg 16</street>
194        <city>Muenster</city><region>NW</region><code>48155</code>
195        <country>Germany</country>
196      </postal>
[607]197      <phone>+49 251 2807760</phone>
198      <facsimile>+49 251 2807761</facsimile>
199      <email></email>
200      <uri></uri>
[95]201    </address>
[547]202  </author>
[547]204  <date month="&ID-MONTH;" year="&ID-YEAR;" />
205  <workgroup>HTTPbis Working Group</workgroup>
[949]209   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
[1373]210   distributed, collaborative, hypertext information systems. HTTP has been in
211   use by the World Wide Web global information initiative since 1990. This
212   document is Part 6 of the seven-part specification that defines the protocol
213   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
216   Part 6 defines requirements on HTTP caches and the associated header fields
217   that control cache behavior or indicate cacheable response messages.
[540]221<note title="Editorial Note (To be removed by RFC Editor)">
[1268]222  <t>
223    Discussion of this draft should take place on the HTTPBIS working group
224    mailing list (, which is archived at
225    <eref target=""/>.
226  </t>
227  <t>
228    The current issues list is at
229    <eref target=""/> and related
230    documents (including fancy diffs) can be found at
231    <eref target=""/>.
232  </t>
233  <t>
[1401]234    The changes in this draft are summarized in <xref target="changes.since.16"/>.
[1268]235  </t>
[949]238   </front>
239   <middle>
[540]241<section anchor="caching" title="Introduction">
[949]243   HTTP is typically used for distributed information systems, where
244   performance can be improved by the use of response caches. This document
245   defines aspects of HTTP/1.1 related to caching and reusing response
246   messages.
[540]249<section anchor="intro.purpose" title="Purpose">
250<iref item="cache" />
[949]252   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
253   subsystem that controls its message storage, retrieval, and deletion. A
254   cache stores cacheable responses in order to reduce the response time and
255   network bandwidth consumption on future, equivalent requests. Any client or
256   server &MAY; employ a cache, though a cache cannot be used by a server that
257   is acting as a tunnel.
[1374]260   The goal of caching in HTTP/1.1 is to significantly improve performance
261   by reusing a prior response message to satisfy a current request.
262   A stored response is considered "fresh", as defined in
263   <xref target="expiration.model" />, if the response can be reused without
264   "validation" (checking with the origin server to see if the cached response
265   remains valid for this request).  A fresh cache response can therefore
266   reduce both latency and network transfers each time it is reused.
267   When a cached response is not fresh, it might still be reusable if it can
268   be freshened by validation (<xref target="validation.model" />) or if the
269   origin is unavailable.
[540]273<section anchor="intro.terminology" title="Terminology">
[949]275   This specification uses a number of terms to refer to the roles played by
276   participants in, and objects of, HTTP caching.
[1112]279   <iref item="cache" />
280   <x:dfn>cache</x:dfn>
281   <list>
282      <t>A conformant implementation of a HTTP cache. Note that this implies
283        an HTTP/1.1 cache; this specification does not define conformance
284        for HTTP/1.0 caches.</t>
285   </list>
287<t anchor="shared.and.non-shared.caches">
288   <iref item="shared cache" />
289   <x:dfn>shared cache</x:dfn>
290   <list>
[1374]291      <t>A cache that stores responses to be reused by more than one user;
292         usually (but not always) deployed as part of an intermediary.</t>
[1112]293   </list>
296   <iref item="private cache" />
297   <x:dfn>private cache</x:dfn>
298   <list>
299      <t>A cache that is dedicated to a single user.</t>
300   </list>
[949]303   <iref item="cacheable" />
304   <x:dfn>cacheable</x:dfn>
305   <list>
306      <t>A response is cacheable if a cache is allowed to store a copy of the
307      response message for use in answering subsequent requests. Even when a
308      response is cacheable, there might be additional constraints on whether
[1112]309      a cache can use the stored copy to satisfy a particular request.</t>
[949]310   </list>
[949]313   <iref item="explicit expiration time" />
314   <x:dfn>explicit expiration time</x:dfn>
315   <list>
316      <t>The time at which the origin server intends that a representation
[969]317      no longer be returned by a cache without further validation.</t>
[949]318   </list>
[949]321   <iref item="heuristic expiration time" />
322   <x:dfn>heuristic expiration time</x:dfn>
323   <list>
324      <t>An expiration time assigned by a cache when no explicit expiration
325      time is available.</t>
326   </list>
[949]329   <iref item="age" />
330   <x:dfn>age</x:dfn>
331   <list>
332      <t>The age of a response is the time since it was sent by, or
333      successfully validated with, the origin server.</t>
334   </list>
[949]337   <iref item="first-hand" />
338   <x:dfn>first-hand</x:dfn>
339   <list>
340      <t>A response is first-hand if the freshness model is not in use; i.e.,
341      its age is 0.</t>
342   </list>
[949]345   <iref item="freshness lifetime" />
346   <x:dfn>freshness lifetime</x:dfn>
347   <list>
348      <t>The length of time between the generation of a response and its
349      expiration time.</t>
350   </list>
[949]353   <iref item="fresh" />
354   <x:dfn>fresh</x:dfn>
355   <list>
356      <t>A response is fresh if its age has not yet exceeded its freshness
357      lifetime.</t>
358   </list>
[949]361   <iref item="stale" />
362   <x:dfn>stale</x:dfn>
363   <list>
364      <t>A response is stale if its age has passed its freshness lifetime
365      (either explicit or heuristic).</t>
366   </list>
[949]369   <iref item="validator" />
370   <x:dfn>validator</x:dfn>
371   <list>
372      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
[1112]373      is used to find out whether a stored response is an equivalent copy of
[1374]374      a representation. See &weak-and-strong;.</t>
[949]375   </list>
378   <iref item="strong validator" />
[1376]379   <iref item="validator" subitem="strong" />
[1374]380   <x:dfn>strong validator</x:dfn>
381   <list>
382      <t>A validator that is defined by the origin server such that its
383         current value will change if the representation body changes; i.e.,
384         an entity-tag that is not marked as weak (&entity-tags;) or,
385         if no entity-tag is provided, a Last-Modified value that is strong
386         in the sense defined by &lastmod-comparison;.</t>
387   </list>
[1452]391<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
[847]393   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
394   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
395   document are to be interpreted as described in <xref target="RFC2119"/>.
[1452]398   This document defines conformance criteria for several roles in HTTP
399   communication, including Senders, Recipients, Clients, Servers, User-Agents,
400   Origin Servers, Intermediaries, Proxies and Gateways. See &architecture;
401   for definitions of these terms.
404   An implementation is considered conformant if it complies with all of the
405   requirements associated with its role(s). Note that SHOULD-level requirements
406   are relevant here, unless one of the documented exceptions is applicable.
409   This document also uses ABNF to define valid protocol elements
410   (<xref target="notation"/>). In addition to the prose requirements placed
411   upon them, Senders &MUST-NOT; generate protocol elements that are invalid.
414   Unless noted otherwise, Recipients &MAY; take steps to recover a usable
415   protocol element from an invalid construct. However, HTTP does not define
416   specific error handling mechanisms, except in cases where it has direct
417   impact on security. This is because different uses of the protocol require
418   different error handling strategies; for example, a Web browser may wish to
419   transparently recover from a response where the Location header field
420   doesn't parse according to the ABNF, whereby in a systems control protocol
421   using HTTP, this type of error recovery could lead to dangerous consequences.
[424]425<section title="Syntax Notation" anchor="notation">
[949]426   <x:anchor-alias value="ALPHA"/>
427   <x:anchor-alias value="CR"/>
428   <x:anchor-alias value="DIGIT"/>
429   <x:anchor-alias value="DQUOTE"/>
430   <x:anchor-alias value="LF"/>
431   <x:anchor-alias value="OCTET"/>
432   <x:anchor-alias value="SP"/>
433   <x:anchor-alias value="VCHAR"/>
[949]435   This specification uses the ABNF syntax defined in &notation; (which
436   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
437   <xref target="collected.abnf"/> shows the collected ABNF, with the list
438   rule expanded.
[949]441   The following core rules are included by reference, as defined in <xref
442   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
443   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
444   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
[1425]445   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
[424]448<section title="Core Rules" anchor="core.rules">
[949]449   <x:anchor-alias value="quoted-string"/>
450   <x:anchor-alias value="token"/>
451   <x:anchor-alias value="OWS"/>
[1396]453   The core rules below are defined in <xref target="Part1"/>:
455<figure><artwork type="abnf2616">
[364]456  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
[1396]457  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-rules;&gt;
458  <x:ref>token</x:ref>         = &lt;token, defined in &field-rules;&gt;
[949]462<section title="ABNF Rules defined in other Parts of the Specification"
463    anchor="abnf.dependencies">
464   <x:anchor-alias value="field-name"/>
465   <x:anchor-alias value="HTTP-date"/>
466   <x:anchor-alias value="port"/>
467   <x:anchor-alias value="pseudonym"/>
468   <x:anchor-alias value="uri-host"/>
[949]470   The ABNF rules below are defined in other parts:
[207]472<figure><!--Part1--><artwork type="abnf2616">
[647]473  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
[1436]474  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
[374]475  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
[229]476  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
[374]477  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
[1323]482<section title="Delta Seconds" anchor="delta-seconds">
484   The delta-seconds rule specifies a non-negative integer, representing time
485   in seconds.
487<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
488  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
491   If an implementation receives a delta-seconds value larger than the largest
492   positive integer it can represent, or if any of its subsequent calculations
493   overflows, it &MUST; consider the value to be 2147483648 (2<x:sup>31</x:sup>).
[1433]494   Recipients parsing a delta-seconds value &MUST; use an arithmetic type of
[1323]495   at least 31 bits of range, and senders &MUST-NOT; send delta-seconds with a
496   value greater than 2147483648.
[540]502<section anchor="caching.overview" title="Cache Operation">
[1374]503<iref item="cache entry" />
504<iref item="cache key" />
506   Proper cache operation preserves the semantics of HTTP transfers
507   (&semantics;) while eliminating the transfer of information already held
508   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
509   we assume that reusing the cached response is desirable and that such
510   reuse is the default behavior when no requirement or locally-desired
511   configuration prevents it.  Therefore, HTTP cache requirements are focused
512   on preventing a cache from either storing a non-reusable response or
513   reusing a stored response inappropriately.
516   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
517   HTTP responses corresponding to prior requests that used the same key.
518   The most common form of cache entry is a successful result of a retrieval
519   request: i.e., a 200 (OK) response containing a representation of the
520   resource identified by the request target.  However, it is also possible
521   to cache negative results (e.g., 404 not found), incomplete results
522   (e.g., 206 partial content), and responses to safe methods other than
523   GET if the method's definition allows such caching and defines something
524   suitable for use as a cache key.
527   The default <x:dfn>cache key</x:dfn> consists of the request method and
528   target URI.  However, since HTTP caches in common use today are typically
529   limited to caching responses to GET, most implementations simply decline
530   other methods and use only the URI as the key.
533   If a request target is subject to content negotiation, its cache entry
534   might consist of multiple stored responses, each differentiated by a
535   secondary key for the values of the original request's selecting header
536   fields (<xref target="caching.negotiated.responses"/>).
[540]539<section anchor="response.cacheability" title="Response Cacheability">
[949]541   A cache &MUST-NOT; store a response to any request, unless:
542   <list style="symbols">
543      <t>The request method is understood by the cache and defined as being
544      cacheable, and</t>
545      <t>the response status code is understood by the cache, and</t>
546      <t>the "no-store" cache directive (see <xref
547      target="header.cache-control" />) does not appear in request or response
[994]548      header fields, and</t>
[949]549      <t>the "private" cache response directive (see <xref
550      target="cache-response-directive" /> does not appear in the response, if
551      the cache is shared, and</t>
[994]552      <t>the "Authorization" header field (see &header-authorization;) does not
[949]553      appear in the request, if the cache is shared, unless the response
554      explicitly allows it (see <xref target="caching.authenticated.responses"
555      />), and</t>
556      <t>the response either:
557         <list style="symbols">
[994]558            <t>contains an Expires header field (see <xref target="header.expires"
[949]559            />), or</t>
560            <t>contains a max-age response cache directive (see <xref
561            target="cache-response-directive" />), or</t>
562            <t>contains a s-maxage response cache directive and the cache is
563            shared, or</t>
564            <t>contains a Cache Control Extension (see <xref
565            target="cache.control.extensions" />) that allows it to be cached,
566            or</t>
567            <t>has a status code that can be served with heuristic freshness
568            (see <xref target="heuristic.freshness" />).</t>
569         </list>
570      </t>
571   </list>
[1292]574   Note that any of the requirements listed above can be overridden by a
[1291]575   cache-control extension; see <xref target="cache.control.extensions" />.
[949]578   In this context, a cache has "understood" a request method or a response
[1374]579   status code if it recognizes it and implements any cache-specific
580   behavior.
[1374]583   Note that, in normal operation, most caches will not store a response that
[949]584   has neither a cache validator nor an explicit expiration time, as such
585   responses are not usually useful to store. However, caches are not
586   prohibited from storing such responses.
[1374]589   A response message is considered complete when all of the octets
590   indicated by the message framing (&messaging;) are received
591   prior to the connection being closed.
592   If the request is GET, the response status is 200 (OK), and the entire
593   response header block has been received, a cache &MAY; store an incomplete
594   response message-body if the cache entry is recorded as incomplete.
595   Likewise, a 206 (Partial Content) response &MAY; be stored as if it were
596   an incomplete 200 (OK) cache entry.  However, a cache &MUST-NOT; store
597   incomplete or partial content responses if it does not support the Range
598   and Content-Range header fields or if it does not understand the
599   range units used in those fields.
[1374]602   A cache &MAY; complete a stored incomplete response by making a subsequent
603   range request (&partial;) and combining the successful response with the
604   stored entry, as defined in <xref target="combining.responses"/>.
605   A cache &MUST-NOT; use an incomplete response to answer requests
606   unless the response has been made complete or the request is partial and
607   specifies a range that is wholly within the incomplete response.
608   A cache &MUST-NOT; send a partial response to a client without explicitly
609   marking it as such using the 206 (Partial Content) status code.
[949]614<section anchor="constructing.responses.from.caches" 
615   title="Constructing Responses from Caches">
[949]617   For a presented request, a cache &MUST-NOT; return a stored response,
618   unless:
619   <list style="symbols">
[965]620      <t>The presented effective request URI (&effective-request-uri;) and
[949]621      that of the stored response match, and</t>
622      <t>the request method associated with the stored response allows it to
623      be used for the presented request, and</t>
[1163]624      <t>selecting header fields nominated by the stored response (if any)
[949]625      match those presented (see <xref target="caching.negotiated.responses"
626      />), and</t>
627      <t>the presented request and stored response are free from directives
628      that would prevent its use (see <xref target="header.cache-control" />
629      and <xref target="header.pragma"/>), and</t>
630      <t>the stored response is either:
631         <list style="symbols">
632            <t>fresh (see <xref target="expiration.model" />), or</t>
633            <t>allowed to be served stale (see <xref
634            target="serving.stale.responses" />), or</t>
635            <t>successfully validated (see <xref target="validation.model"
636            />).</t>
637         </list>
638      </t>
[540]639  </list>
[1292]642   Note that any of the requirements listed above can be overridden by a
[1291]643   cache-control extension; see <xref target="cache.control.extensions" />.
[949]646   When a stored response is used to satisfy a request without validation,
[1112]647   a cache &MUST; include a single Age header field (<xref target="header.age"
[949]648   />) in the response with a value equal to the stored response's
649   current_age; see <xref target="age.calculations" />.
[1112]652   A cache &MUST; write through requests with methods that are unsafe
653   (&safe-methods;) to the origin server; i.e., a cache must not generate
654   a reply to such a request before having forwarded the request and having
655   received a corresponding response.
[949]658   Also, note that unsafe requests might invalidate already stored responses;
659   see <xref target="invalidation.after.updates.or.deletions" />.
[1277]662   When more than one suitable response is stored, a cache &MUST; use the
663   most recent response (as determined by the Date header field). It can also
[949]664   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
665   no-cache" to disambiguate which response to use.
[1184]668   A cache that does not have a clock available &MUST-NOT; use stored responses
[1112]669   without revalidating them on every use. A cache, especially a shared
[1044]670   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
671   synchronize its clock with a reliable external standard.
[540]676<section anchor="expiration.model" title="Freshness Model">
[949]678   When a response is "fresh" in the cache, it can be used to satisfy
679   subsequent requests without contacting the origin server, thereby improving
680   efficiency.
[949]683   The primary mechanism for determining freshness is for an origin server to
684   provide an explicit expiration time in the future, using either the Expires
[994]685   header field (<xref target="header.expires" />) or the max-age response cache
[949]686   directive (<xref target="cache-response-directive" />). Generally, origin
687   servers will assign future explicit expiration times to responses in the
688   belief that the representation is not likely to change in a semantically
689   significant way before the expiration time is reached.
[949]692   If an origin server wishes to force a cache to validate every request, it
[969]693   can assign an explicit expiration time in the past to indicate that the
[1113]694   response is already stale. Compliant caches will normally validate the
695   cached response before reusing it for subsequent requests (see <xref
696   target="serving.stale.responses" />).
[1112]699   Since origin servers do not always provide explicit expiration times,
700   a cache &MAY; assign a heuristic expiration time when an explicit time is not
701   specified, employing algorithms that use other header field values (such as the
702   Last-Modified time) to estimate a plausible expiration time. This
[949]703   specification does not provide specific algorithms, but does impose
704   worst-case constraints on their results.
[575]708  The calculation to determine if a response is fresh is:
710<artwork type="code">
[538]711   response_is_fresh = (freshness_lifetime &gt; current_age)
[949]715   The freshness_lifetime is defined in <xref
716   target="calculating.freshness.lifetime" />; the current_age is defined in
717   <xref target="age.calculations" />.
[949]720   Additionally, clients might need to influence freshness calculation. They
721   can do this using several request cache directives, with the effect of
722   either increasing or loosening constraints on freshness. See <xref
723   target="cache-request-directive" />.
[949]726   Note that freshness applies only to cache operation; it cannot be used to
727   force a user agent to refresh its display or reload a resource. See <xref
728   target="history.lists" /> for an explanation of the difference between
729   caches and history mechanisms.
[949]732<section anchor="calculating.freshness.lifetime" 
733   title="Calculating Freshness Lifetime">
[949]735   A cache can calculate the freshness lifetime (denoted as
736   freshness_lifetime) of a response by using the first match of:
737   <list style="symbols">
738      <t>If the cache is shared and the s-maxage response cache directive
739      (<xref target="cache-response-directive" />) is present, use its value,
740      or</t>
741      <t>If the max-age response cache directive (<xref
[540]742      target="cache-response-directive" />) is present, use its value, or</t>
[994]743      <t>If the Expires response header field (<xref target="header.expires" />) is
744      present, use its value minus the value of the Date response header field,
[949]745      or</t>
746      <t>Otherwise, no explicit expiration time is present in the response. A
747      heuristic freshness lifetime might be applicable; see <xref
748      target="heuristic.freshness" />.</t>
749   </list>
[949]752   Note that this calculation is not vulnerable to clock skew, since all of
753   the information comes from the origin server.
[540]756<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
[949]758   If no explicit expiration time is present in a stored response that has a
759   status code whose definition allows heuristic freshness to be used
760   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
[1112]761   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
762   use heuristics to determine freshness for responses with status codes that do
763   not explicitly allow it.
[1112]766   When a heuristic is used to calculate freshness lifetime, a cache
[994]767   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
[949]768   its current_age is more than 24 hours and such a warning is not already
769   present.
[1433]772   Also, if the response has a Last-Modified header field
773   (&header-last-modified;), caches are encouraged to use a heuristic
774   expiration value that is no more than some fraction of the interval since
775   that time. A typical setting of this fraction might be 10%.
[949]778   <t>
779      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
780      x:sec="13.9"/>) required that caches do not calculate heuristic
[1173]781      freshness for URIs with query components (i.e., those containing '?').
[949]782      In practice, this has not been widely implemented. Therefore, servers
783      are encouraged to send explicit directives (e.g., Cache-Control:
784      no-cache) if they wish to preclude caching.
785   </t>
[540]790<section anchor="age.calculations" title="Calculating Age">
[1163]792   HTTP/1.1 uses the Age header field to convey the estimated age of the
[949]793   response message when obtained from a cache. The Age field value is the
794   cache's estimate of the amount of time since the response was generated or
795   validated by the origin server. In essence, the Age value is the sum of the
796   time that the response has been resident in each of the caches along the
797   path from the origin server, plus the amount of time it has been in transit
798   along network paths.
[949]801   The following data is used for the age calculation:
[949]804   <x:dfn>age_value</x:dfn>
805   <list>
806      <t>
[994]807         The term "age_value" denotes the value of the Age header field (<xref
[949]808         target="header.age"/>), in a form appropriate for arithmetic
809         operation; or 0, if not available.
810      </t>
811   </list>
[949]814   <x:dfn>date_value</x:dfn>
815   <list>
816      <t>
[994]817         HTTP/1.1 requires origin servers to send a Date header field, if possible,
[949]818         with every response, giving the time at which the response was
819         generated. The term "date_value" denotes the value of the Date
[994]820         header field, in a form appropriate for arithmetic operations. See
821         &header-date; for the definition of the Date header field, and for
822         requirements regarding responses without it.
[949]823      </t>
824   </list>
[949]827   <x:dfn>now</x:dfn>
828   <list>
829      <t>
830         The term "now" means "the current value of the clock at the host
[1112]831         performing the calculation". A cache &SHOULD; use NTP (<xref
832         target="RFC1305"/>) or some similar protocol to synchronize its
[949]833         clocks to a globally accurate time standard.
834      </t>
835   </list>
[949]838   <x:dfn>request_time</x:dfn>
839   <list>
840      <t>
841         The current value of the clock at the host at the time the request
842         resulting in the stored response was made.
843      </t>
844   </list>
[949]847   <x:dfn>response_time</x:dfn>
848   <list>
849      <t>
850         The current value of the clock at the host at the time the response
851         was received.
852      </t>
853   </list>
[949]856   A response's age can be calculated in two entirely independent ways:
857   <list style="numbers">
858      <t>the "apparent_age": response_time minus date_value, if the local
859      clock is reasonably well synchronized to the origin server's clock. If
860      the result is negative, the result is replaced by zero.</t>
861      <t>the "corrected_age_value", if all of the caches along the response
[1112]862      path implement HTTP/1.1. A cache &MUST; interpret this value relative
[949]863      to the time the request was initiated, not the time that the response
864      was received.</t>
865   </list>
868<artwork type="code">
[786]869  apparent_age = max(0, response_time - date_value);
[786]871  response_delay = response_time - request_time;
[792]872  corrected_age_value = age_value + response_delay; 
876<preamble>These are combined as</preamble>
877<artwork type="code">
878  corrected_initial_age = max(apparent_age, corrected_age_value);
[949]881   The current_age of a stored response can then be calculated by adding the
882   amount of time (in seconds) since the stored response was last validated by
883   the origin server to the corrected_initial_age.
[786]885<figure><artwork type="code">
[540]886  resident_time = now - response_time;
[786]887  current_age = corrected_initial_age + resident_time;
[1418]890   Additionally, to avoid common problems in date parsing:
893  <list style="symbols">
894     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
895        which appears to be more than 50 years in the future is in fact
896        in the past (this helps solve the "year 2000" problem).</t>
898     <t>Although all date formats are specified to be case-sensitive,
899        recipients &SHOULD; match day, week and timezone names
900        case-insensitively.</t>
902     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
903        Expires date as earlier than the proper value, but &MUST-NOT;
904        internally represent a parsed Expires date as later than the
905        proper value.</t>
907     <t>All expiration-related calculations &MUST; be done in GMT. The
908        local time zone &MUST-NOT; influence the calculation or comparison
909        of an age or expiration time.</t>
911     <t>If an HTTP header field incorrectly carries a date value with a time
912        zone other than GMT, it &MUST; be converted into GMT using the
913        most conservative possible conversion.</t>
914  </list>
[540]918<section anchor="serving.stale.responses" title="Serving Stale Responses">
[949]920   A "stale" response is one that either has explicit expiry information or is
921   allowed to have heuristic expiry calculated, but is not fresh according to
922   the calculations in <xref target="expiration.model" />.
[1112]925   A cache &MUST-NOT; return a stale response if it is prohibited by an
[949]926   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
927   directive, a "must-revalidate" cache-response-directive, or an applicable
928   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
929   target="cache-response-directive"/>).
[1433]932   A cache &MUST-NOT; return stale responses unless it is disconnected
[949]933   (i.e., it cannot contact the origin server or otherwise find a forward
[1264]934   path) or doing so is explicitly allowed (e.g., by the max-stale request
[949]935   directive; see <xref target="cache-request-directive" />).
[1112]938   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
939   <xref target="header.warning" />) to stale responses. Likewise, a cache
940   &SHOULD; add the 112 warn-code to stale responses if the cache is
941   disconnected.
[949]944   If a cache receives a first-hand response (either an entire response, or a
945   304 (Not Modified) response) that it would normally forward to the
946   requesting client, and the received response is no longer fresh, the cache
[1433]947   can forward it to the requesting client without adding a new Warning (but
948   without removing any existing Warning header fields). A cache shouldn't
[949]949   attempt to validate a response simply because that response became stale in
950   transit.
[540]955<section anchor="validation.model" title="Validation Model">
[949]957   When a cache has one or more stored responses for a requested URI, but
958   cannot serve any of them (e.g., because they are not fresh, or one cannot
959   be selected; see <xref target="caching.negotiated.responses"/>), it can use
960   the conditional request mechanism &conditional; in the forwarded request to
961   give the origin server an opportunity to both select a valid stored
962   response to be used, and to update it. This process is known as
963   "validating" or "revalidating" the stored response.
[1433]966   When sending such a conditional request, a cache adds an If-Modified-Since
967   header field whose value is that of the Last-Modified header field from the
968   selected (see <xref target="caching.negotiated.responses"/>) stored
969   response, if available.
[1433]972   Additionally, a cache can add an If-None-Match header field whose value is
973   that of the ETag header field(s) from all responses stored for the
974   requested URI, if present. However, if any of the stored responses contains
975   only partial content, the cache shouldn't include its entity-tag in the
976   If-None-Match header field unless the request is for a range that would be
977   fully satisfied by that stored response.
980<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
[1419]983   <list style="symbols">
984      <t>
985         A 304 (Not Modified) response status code indicates that the stored
986         response can be updated and reused; see <xref
987         target="freshening.responses"/>.
988      </t>
989      <t>
990         A full response (i.e., one with a response body) indicates that none
991         of the stored responses nominated in the conditional request is
[1433]992         suitable. Instead, the cache can use the full response to
993         satisfy the request and &MAY; replace the stored response(s).
[1419]994      </t>
995      <t>
996         However, if a cache receives a 5xx response while attempting to
[1433]997         validate a response, it can either forward this response to the
[1419]998         requesting client, or act as if the server failed to respond. In the
[1433]999         latter case, it can return a previously stored response (see <xref
[1419]1000         target="serving.stale.responses" />).
1001      </t>
1002   </list>
1005<section anchor="freshening.responses" title="Freshening Responses">
[1419]1007   When a cache receives a 304 (Not Modified) response and already has one
1008   or more stored 200 (OK) responses for the same cache key, the cache needs
1009   to identify which of the stored responses are updated by this new response
1010   and then update the stored response(s) with the new information provided in
1011   the 304 response.
1012   <list style="symbols">
1013    <t>
1014     If the new response contains a strong validator, then that strong
1015     validator identifies the selected representation.  All of the stored
1016     responses with the same strong validator are selected.
1017     If none of the stored responses contain the same strong validator, then
1018     this new response corresponds to a new selected representation and
1019     &MUST-NOT; update the existing stored responses.
1020    </t>
1021    <t>
1022     If the new response contains a weak validator and that validator
1023     corresponds to one of the cache's stored responses, then the most
1024     recent of those matching stored responses is selected.
1025    </t>
1026    <t>
1027     If the new response does not include any form of validator, there is
1028     only one stored response, and that stored response also lacks a
1029     validator, then that stored response is selected.
1030    </t>
1031   </list>
[1419]1034   If a stored response is selected for update, the cache &MUST;:
1035   <list style="symbols">
1036      <t>delete any Warning header fields in the stored response with
1037         warn-code 1xx (see <xref target="header.warning" />);</t>
1038      <t>retain any Warning header fields in the stored response with
1039         warn-code 2xx; and,</t>
1040      <t>use other header fields provided in the 304 response to replace
1041         all instances of the corresponding header fields in the stored
1042         response.</t>
1043   </list>
[949]1049<section anchor="invalidation.after.updates.or.deletions" 
1050   title="Request Methods that Invalidate">
[1290]1052   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1053   have the potential for changing state on the origin server, intervening
1054   caches can use them to keep their contents up-to-date.
[1112]1057   A cache &MUST; invalidate the effective Request URI
1058   (&effective-request-uri;) as well as the URI(s) in the Location
[1314]1059   and Content-Location header fields (if present) when a non-error
1060   response to a request with an unsafe method is received.
[1112]1063   However, a cache &MUST-NOT; invalidate a URI from a
1064   Location or Content-Location header field if the host part of that URI
1065   differs from the host part in the effective request URI
1066   (&effective-request-uri;). This helps prevent denial of service attacks.
[1433]1069   A cache &MUST; invalidate the effective request URI
[1314]1070   (&effective-request-uri;) when it receives a non-error response
1071   to a request with a method whose safety is unknown.
[1314]1074   Here, a "non-error response" is one with a 2xx or 3xx status code.
1075   "Invalidate" means that the cache will either remove all stored
[965]1076   responses related to the effective request URI, or will mark these as
[949]1077   "invalid" and in need of a mandatory validation before they can be returned
1078   in response to a subsequent request.
[949]1081   Note that this does not guarantee that all appropriate responses are
1082   invalidated. For example, the request that caused the change at the origin
1083   server might not have gone through the cache where a response is stored.
[949]1087<section anchor="caching.authenticated.responses" 
1088   title="Shared Caching of Authenticated Responses">
[1112]1091   A shared cache &MUST-NOT; use a cached response to a request with an
[994]1092   Authorization header field (&header-authorization;) to satisfy any subsequent
[949]1093   request unless a cache directive that allows such responses to be stored is
1094   present in the response.
1098   In this specification, the following Cache-Control response directives
1099   (<xref target="cache-response-directive"/>) have such an effect:
1100   must-revalidate, public, s-maxage.
1104   Note that cached responses that contain the "must-revalidate" and/or
1105   "s-maxage" response directives are not allowed to be served stale (<xref
1106   target="serving.stale.responses"/>) by shared caches. In particular, a
1107   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1108   used to satisfy a subsequent request without revalidating it on the origin
1109   server.
[949]1113<section anchor="caching.negotiated.responses" 
1114   title="Caching Negotiated Responses">
[949]1116   When a cache receives a request that can be satisfied by a stored response
1117   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
[1163]1118   use that response unless all of the selecting header fields nominated by
[994]1119   the Vary header field match in both the original request (i.e., that associated
[949]1120   with the stored response), and the presented request.
[1163]1123   The selecting header fields from two requests are defined to match if and
[949]1124   only if those in the first request can be transformed to those in the
1125   second request by applying any of the following:
1126   <list style="symbols">
1127      <t>
[994]1128         adding or removing whitespace, where allowed in the header field's syntax
[949]1129      </t>
1130      <t>
[1163]1131         combining multiple header fields with the same field name
[949]1132         (see &header-fields;)
1133      </t>
1134      <t>
[994]1135         normalizing both header field values in a way that is known to have
1136         identical semantics, according to the header field's specification (e.g.,
[949]1137         re-ordering field values when order is not significant;
1138         case-normalization, where values are defined to be case-insensitive)
1139      </t>
[771]1140  </list>
[949]1143   If (after any normalization that might take place) a header field is absent
1144   from a request, it can only match another request if it is also absent
1145   there.
[949]1148   A Vary header field-value of "*" always fails to match, and subsequent
1149   requests to that resource can only be properly interpreted by the origin
1150   server.
[1163]1153   The stored response with matching selecting header fields is known as the
[949]1154   selected response.
[1292]1157   If multiple selected responses are available, the most recent response
1158   (as determined by the Date header field) is used; see <xref 
1159   target="constructing.responses.from.caches"/>.
[1433]1162   If no selected response is available, the cache can forward the presented
[949]1163   request to the origin server in a conditional request; see <xref
1164   target="validation.model"/>.
[1374]1168<section anchor="combining.responses" title="Combining Partial Content">
[1374]1170   A response might transfer only a partial representation if the
1171   connection closed prematurely or if the request used one or more Range
1172   specifiers (&partial;).  After several such transfers, a cache might have
1173   received several ranges of the same representation.  A cache &MAY; combine
1174   these ranges into a single stored response, and reuse that response to
1175   satisfy later requests, if they all share the same strong validator and
1176   the cache complies with the client requirements in &combining-byte-ranges;.
[1374]1179   When combining the new response with one or more stored responses, a
1180   cache &MUST;:
1181   <list style="symbols">
1182      <t>delete any Warning header fields in the stored response with
1183         warn-code 1xx (see <xref target="header.warning" />);</t>
1184      <t>retain any Warning header fields in the stored response with
1185         warn-code 2xx; and,</t>
1186      <t>use other header fields provided in the new response, aside
1187         from Content-Range, to replace all instances of the corresponding
1188         header fields in the stored response.</t>
1189   </list>
[1415]1196<section anchor="header.field.definitions" title="Header Field Definitions">
[949]1198   This section defines the syntax and semantics of HTTP/1.1 header fields
1199   related to caching.
[540]1202<section anchor="header.age" title="Age">
[1120]1203   <iref item="Age header field" primary="true" x:for-anchor="" />
1204   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
[949]1205   <x:anchor-alias value="Age"/>
1206   <x:anchor-alias value="age-value"/>
[1163]1208   The "Age" header field conveys the sender's estimate of the amount
[949]1209   of time since the response was generated or successfully validated at the
1210   origin server. Age values are calculated as specified in <xref
1211   target="age.calculations" />.
[1231]1213<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1214  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
[1323]1217  Age field-values are non-negative integers, representing time in seconds
1218  (see <xref target="delta-seconds"/>).
[949]1221   The presence of an Age header field in a response implies that a response
1222   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1223   might not implement the Age header field.
[540]1227<section anchor="header.cache-control" title="Cache-Control">
[1120]1228   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1229   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
[949]1230      x:for-anchor="" />
1231   <x:anchor-alias value="Cache-Control"/>
1232   <x:anchor-alias value="cache-directive"/>
1233   <x:anchor-alias value="cache-extension"/>
1234   <x:anchor-alias value="cache-request-directive"/>
1235   <x:anchor-alias value="cache-response-directive"/>
[1163]1237   The "Cache-Control" header field is used to specify directives for
[949]1238   caches along the request/response chain. Such cache directives are
1239   unidirectional in that the presence of a directive in a request does not
1240   imply that the same directive is to be given in the response.
[1112]1243   A cache &MUST; obey the requirements of the Cache-Control
[949]1244   directives defined in this section. See <xref
1245   target="cache.control.extensions"/> for information about how Cache-Control
1246   directives defined elsewhere are handled.
[949]1249   <t>
1250       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1251       might only implement Pragma: no-cache (see <xref target="header.pragma"
1252       />).
1253   </t>
[1173]1256   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1257   through in forwarded messages, regardless of their
[1112]1258   significance to that application, since the directives might be applicable
1259   to all recipients along the request/response chain. It is not possible to
1260   target a directive to a specific cache.
[1449]1262<t>Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument.</t>
[1231]1263<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1264  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
[229]1266  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
[334]1267     / <x:ref>cache-response-directive</x:ref>
[334]1269  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
[949]1272<section anchor="cache-request-directive" 
1273   title="Request Cache-Control Directives">
1274   <x:anchor-alias value="cache-request-directive" />
[949]1276<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1277   subitem="cache-request-directive" />
[538]1278  <x:ref>cache-request-directive</x:ref> =
1279       "no-cache"
1280     / "no-store"
1281     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1282     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1283     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1284     / "no-transform"
1285     / "only-if-cached"
1286     / <x:ref>cache-extension</x:ref>
[949]1290   <x:dfn>no-cache</x:dfn>
1291   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1292   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1293   <list>
[1112]1294      <t>The no-cache request directive indicates that a cache &MUST-NOT; 
1295      use a stored response to satisfy the request without successful
1296      validation on the origin server.</t> 
[949]1297   </list>
[949]1300   <x:dfn>no-store</x:dfn>
1301   <iref item="Cache Directives" primary="true" subitem="no-store" />
1302   <iref item="no-store" primary="true" subitem="Cache Directive" />
1303   <list>
1304      <t>The no-store request directive indicates that a cache &MUST-NOT;
1305      store any part of either this request or any response to it. This
[1112]1306      directive applies to both private and shared caches. "&MUST-NOT;
[949]1307      store" in this context means that the cache &MUST-NOT; intentionally
1308      store the information in non-volatile storage, and &MUST; make a
1309      best-effort attempt to remove the information from volatile storage as
1310      promptly as possible after forwarding it.</t>
1311      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1312      privacy. In particular, malicious or compromised caches might not
1313      recognize or obey this directive, and communications networks might be
1314      vulnerable to eavesdropping.</t>
[1110]1315      <t>Note that if a request containing this directive is satisfied from a
1316      cache, the no-store request directive does not apply to the already
1317      stored response.</t>
[949]1318   </list>
[949]1321   <x:dfn>max-age</x:dfn>
1322   <iref item="Cache Directives" primary="true" subitem="max-age" />
1323   <iref item="max-age" primary="true" subitem="Cache Directive" />
1324   <list>
[1273]1325      <t>The max-age request directive indicates that the client is unwilling to
1326      accept a response whose age is greater than the specified number of
[949]1327      seconds. Unless the max-stale request directive is also present, the
1328      client is not willing to accept a stale response.</t>
1329   </list>
[949]1332   <x:dfn>max-stale</x:dfn>
1333   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1334   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1335   <list>
1336      <t>The max-stale request directive indicates that the client is willing
1337      to accept a response that has exceeded its expiration time. If max-stale
1338      is assigned a value, then the client is willing to accept a response
1339      that has exceeded its expiration time by no more than the specified
1340      number of seconds. If no value is assigned to max-stale, then the client
1341      is willing to accept a stale response of any age.</t>
1342   </list>
[949]1345   <x:dfn>min-fresh</x:dfn>
1346   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1347   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1348   <list>
1349      <t>The min-fresh request directive indicates that the client is willing
1350      to accept a response whose freshness lifetime is no less than its
1351      current age plus the specified time in seconds. That is, the client
1352      wants a response that will still be fresh for at least the specified
1353      number of seconds.</t>
1354   </list>
[949]1357   <x:dfn>no-transform</x:dfn>
1358   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1359   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1360   <list>
[1112]1361      <t>The no-transform request directive indicates that an intermediary
1362        (whether or not it implements a cache) &MUST-NOT; change the
1363        Content-Encoding, Content-Range or Content-Type request header fields,
1364        nor the request representation.</t>
[949]1365   </list>
[949]1368   <x:dfn>only-if-cached</x:dfn>
1369   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1370   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1371   <list>
1372      <t>The only-if-cached request directive indicates that the client only
[1278]1373      wishes to obtain a stored response. If it receives this directive, a
[949]1374      cache &SHOULD; either respond using a stored response that is consistent
1375      with the other constraints of the request, or respond with a 504
1376      (Gateway Timeout) status code. If a group of caches is being operated as
[1112]1377      a unified system with good internal connectivity, a member cache &MAY;
1378      forward such a request within that group of caches.</t>
[949]1379   </list>
[949]1383<section anchor="cache-response-directive" 
1384   title="Response Cache-Control Directives">
1385   <x:anchor-alias value="cache-response-directive" />
[949]1387<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1388   subitem="cache-response-directive" />
[538]1389  <x:ref>cache-response-directive</x:ref> =
1390       "public"
1391     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1392     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1393     / "no-store"
1394     / "no-transform"
1395     / "must-revalidate"
1396     / "proxy-revalidate"
1397     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1398     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1399     / <x:ref>cache-extension</x:ref>
[949]1403   <x:dfn>public</x:dfn>
1404   <iref item="Cache Directives" primary="true" subitem="public" />
1405   <iref item="public" primary="true" subitem="Cache Directive" />
1406   <list>
[1129]1407      <t>The public response directive indicates that a response whose
1408        associated request contains an 'Authentication' header &MAY; be
1409        stored (see <xref target="caching.authenticated.responses" />).</t>
[540]1410  </list>
[949]1413   <x:dfn>private</x:dfn>
1414   <iref item="Cache Directives" primary="true" subitem="private" />
1415   <iref item="private" primary="true" subitem="Cache Directive" />
1416   <list>
1417      <t>The private response directive indicates that the response message is
1418      intended for a single user and &MUST-NOT; be stored by a shared cache. A
[1112]1419      private cache &MAY; store the response.</t>
[949]1420      <t>If the private response directive specifies one or more field-names,
1421      this requirement is limited to the field-values associated with the
[1112]1422      listed response header fields. That is, a shared cache &MUST-NOT; store
1423      the specified field-names(s), whereas it &MAY; store the remainder of the
1424      response message.</t>
[949]1425      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1426      the response can be stored; it cannot ensure the privacy of the message
1427      content. Also, private response directives with field-names are often
1428      handled by implementations as if an unqualified private directive was
1429      received; i.e., the special handling for the qualified form is not
1430      widely implemented.</t>
1431   </list>
[949]1434   <x:dfn>no-cache</x:dfn>
1435   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1436   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1437   <list>
1438      <t>The no-cache response directive indicates that the response MUST NOT
1439      be used to satisfy a subsequent request without successful validation on
1440      the origin server. This allows an origin server to prevent a cache from
1441      using it to satisfy a request without contacting it, even by caches that
1442      have been configured to return stale responses.</t>
1443      <t>If the no-cache response directive specifies one or more field-names,
1444      this requirement is limited to the field-values associated with the
[1112]1445      listed response header fields. That is, a cache &MUST-NOT; send the
1446      specified field-name(s) in the response to a subsequent request without successful
[949]1447      validation on the origin server. This allows an origin server to prevent
1448      the re-use of certain header fields in a response, while still allowing
1449      caching of the rest of the response.</t>
1450      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1451      this directive. Also, no-cache response directives with field-names are
1452      often handled by implementations as if an unqualified no-cache directive
1453      was received; i.e., the special handling for the qualified form is not
1454      widely implemented. </t>
1455   </list>
[949]1458   <x:dfn>no-store</x:dfn>
1459   <iref item="Cache Directives" primary="true" subitem="no-store" />
1460   <iref item="no-store" primary="true" subitem="Cache Directive" />
1461   <list>
1462      <t>The no-store response directive indicates that a cache &MUST-NOT;
1463      store any part of either the immediate request or response. This
[1112]1464      directive applies to both private and shared caches. "&MUST-NOT;
[949]1465      store" in this context means that the cache &MUST-NOT; intentionally
1466      store the information in non-volatile storage, and &MUST; make a
1467      best-effort attempt to remove the information from volatile storage as
1468      promptly as possible after forwarding it.</t>
1469      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1470      privacy. In particular, malicious or compromised caches might not
1471      recognize or obey this directive, and communications networks might be
1472      vulnerable to eavesdropping.</t>
1473   </list>
[949]1476   <x:dfn>must-revalidate</x:dfn>
1477   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1478   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1479   <list>
1480      <t>The must-revalidate response directive indicates that once it has
[1112]1481      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
[949]1482      requests without successful validation on the origin server.</t>
1483      <t>The must-revalidate directive is necessary to support reliable
[1112]1484      operation for certain protocol features. In all circumstances a
1485      cache &MUST; obey the must-revalidate directive; in particular,
1486      if a cache cannot reach the origin server for any reason, it &MUST;
[949]1487      generate a 504 (Gateway Timeout) response.</t>
[1433]1488      <t>The must-revalidate directive ought to be used by servers if and only
1489      if failure to validate a request on the representation could result in
[949]1490      incorrect operation, such as a silently unexecuted financial
1491      transaction.</t>
1492   </list>
[949]1495   <x:dfn>proxy-revalidate</x:dfn>
1496   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1497   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1498   <list>
1499      <t>The proxy-revalidate response directive has the same meaning as the
1500      must-revalidate response directive, except that it does not apply to
[1112]1501      private caches.</t>
[949]1502   </list>
[949]1505   <x:dfn>max-age</x:dfn>
1506   <iref item="Cache Directives" primary="true" subitem="max-age" />
1507   <iref item="max-age" primary="true" subitem="Cache Directive" />
1508   <list>
[1270]1509      <t>The max-age response directive indicates that the response is to be
[949]1510      considered stale after its age is greater than the specified number of
1511      seconds.</t>
1512   </list>
[949]1515   <x:dfn>s-maxage</x:dfn>
1516   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1517   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1518   <list>
1519      <t>The s-maxage response directive indicates that, in shared caches, the
1520      maximum age specified by this directive overrides the maximum age
[994]1521      specified by either the max-age directive or the Expires header field. The
[949]1522      s-maxage directive also implies the semantics of the proxy-revalidate
1523      response directive.</t>
1524   </list>
[949]1527   <x:dfn>no-transform</x:dfn>
1528   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1529   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1530   <list>
[1112]1531      <t>The no-transform response directive indicates that an intermediary
1532      (regardless of whether it implements a cache) &MUST-NOT; change the
1533      Content-Encoding, Content-Range or Content-Type response header fields,
1534      nor the response representation.</t>
[949]1535   </list>
[540]1540<section anchor="cache.control.extensions" title="Cache Control Extensions">
[949]1542   The Cache-Control header field can be extended through the use of one or
1543   more cache-extension tokens, each with an optional value. Informational
1544   extensions (those that do not require a change in cache behavior) can be
1545   added without changing the semantics of other directives. Behavioral
1546   extensions are designed to work by acting as modifiers to the existing base
1547   of cache directives. Both the new directive and the standard directive are
1548   supplied, such that applications that do not understand the new directive
1549   will default to the behavior specified by the standard directive, and those
1550   that understand the new directive will recognize it as modifying the
1551   requirements associated with the standard directive. In this way,
1552   extensions to the cache-control directives can be made without requiring
1553   changes to the base protocol.
[949]1556   This extension mechanism depends on an HTTP cache obeying all of the
1557   cache-control directives defined for its native HTTP-version, obeying
1558   certain extensions, and ignoring all directives that it does not
1559   understand.
[949]1562   For example, consider a hypothetical new response directive called
1563   "community" that acts as a modifier to the private directive. We define
[1112]1564   this new directive to mean that, in addition to any private cache, any
[949]1565   cache that is shared only by members of the community named within its
1566   value may cache the response. An origin server wishing to allow the UCI
1567   community to use an otherwise private response in their shared cache(s)
1568   could do so by including
1570<figure><artwork type="example">
[458]1571  Cache-Control: private, community="UCI"
[949]1574   A cache seeing this header field will act correctly even if the cache does
1575   not understand the community cache-extension, since it will also see and
1576   understand the private directive and thus default to the safe behavior.
[1285]1579   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
[949]1580   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1581   combined with standard directives (or the response's default cacheability)
1582   such that the cache behavior will remain minimally correct even if the
1583   cache does not understand the extension(s).
[949]1586   The HTTP Cache Directive Registry defines the name space for the cache
1587   directives.
[1112]1590   A registration &MUST; include the following fields:
[949]1591   <list style="symbols">
1592      <t>Cache Directive Name</t>
1593      <t>Pointer to specification text</t>
1594   </list>
[949]1597   Values to be added to this name space are subject to IETF review (<xref
1598   target="RFC5226" x:fmt="," x:sec="4.1"/>).
[949]1601   The registry itself is maintained at <eref
1602   target=""/>.
[540]1608<section anchor="header.expires" title="Expires">
[1120]1609   <iref item="Expires header field" primary="true" x:for-anchor="" />
1610   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
[949]1611   <x:anchor-alias value="Expires"/>
[965]1613   The "Expires" header field gives the date/time after which the
[949]1614   response is considered stale. See <xref target="expiration.model" /> for
1615   further discussion of the freshness model.
[949]1618   The presence of an Expires field does not imply that the original resource
1619   will change or cease to exist at, before, or after that time.
[949]1622   The field-value is an absolute date and time as defined by HTTP-date in
[1436]1623   &http-date;; a sender &MUST; use the rfc1123-date format.
[1231]1625<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1626  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1629  <preamble>For example</preamble>
1630<artwork type="example">
[364]1631  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1634   A cache &MUST; treat other invalid date formats,
1635   especially including the value "0", as in the past (i.e., "already
1636   expired").
[949]1639   <t>
1640       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1641       max-age directive (see <xref target="cache-response-directive" />),
1642       that directive overrides the Expires field. Likewise, the s-maxage
1643       directive overrides Expires in shared caches.
1644   </t>
[1400]1647   Historically, HTTP required the Expires field-value to be no more than a
1648   year in the future. While longer freshness lifetimes are no longer
1649   prohibited, extremely large values have been demonstrated to cause
1650   problems (e.g., clock overflows due to use of 32-bit integers for
1651   time values), and most caches will evict a response far sooner than
1652   that. Therefore, senders ought not produce them.
1655   An origin server without a clock &MUST-NOT; assign Expires
1656   values to a response unless these values were associated
1657   with the resource by a system or user with a reliable clock. It &MAY;
1658   assign an Expires value that is known, at or before server
1659   configuration time, to be in the past (this allows "pre-expiration"
1660   of responses without storing separate Expires values for each
1661   resource).
[540]1665<section anchor="header.pragma" title="Pragma">
[1120]1666   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1667   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
[949]1668   <x:anchor-alias value="extension-pragma"/>
1669   <x:anchor-alias value="Pragma"/>
1670   <x:anchor-alias value="pragma-directive"/>
[1287]1672   The "Pragma" header field allows backwards compatibility with HTTP/1.0
[1284]1673   caches, so that clients can specify a "no-cache" request that they will
1674   understand (as Cache-Control was not defined until HTTP/1.1). When the
1675   Cache-Control header is also present and understood in a request, Pragma is
1676   ignored.
[1287]1679   In HTTP/1.0, Pragma was defined as an extensible field for
[1284]1680   implementation-specified directives for recipients. This specification
1681   deprecates such extensions to improve interoperability.
[1231]1683<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"/>
1684  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1685  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1686  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
[1287]1689   When the Cache-Control header is not present in a request, the no-cache
[1284]1690   request pragma-directive &MUST; have the same effect on caches as if
1691   "Cache-Control: no-cache" were present (see <xref
1692   target="cache-request-directive" />).
[1433]1695   When sending a no-cache request, a client ought to include both the pragma
1696   and cache-control directives, unless Cache-Control: no-cache is
1697   purposefully omitted to target other Cache-Control response directives at
1698   HTTP/1.1 caches. For example:
[1287]1701<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
[1284]1702GET / HTTP/1.1
1704Cache-Control: max-age=30
1705Pragma: no-cache
1710   will constrain HTTP/1.1 caches to serve a response no older than 30
1711   seconds, while precluding implementations that do not understand
1712   Cache-Control from serving a cached response.
[949]1715   <t>
[1287]1716      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
[1284]1717      specified, it does not provide a reliable replacement for
1718      "Cache-Control: no-cache" in them.
[1287]1719   </t>
[540]1723<section anchor="header.vary" title="Vary">
[1120]1724   <iref item="Vary header field" primary="true" x:for-anchor="" />
1725   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
[949]1726   <x:anchor-alias value="Vary"/>
[1163]1728   The "Vary" header field conveys the set of header fields
[949]1729   that were used to select the representation.
[949]1732   Caches use this information, in part, to determine whether a stored
1733   response can be used to satisfy a given request; see <xref
1734   target="caching.negotiated.responses" />. determines, while the response is
1735   fresh, whether a cache is permitted to use the response to reply to a
1736   subsequent request without validation; see <xref
1737   target="caching.negotiated.responses" />.
[949]1740   In uncacheable or stale responses, the Vary field value advises the user
1741   agent about the criteria that were used to select the representation.
[1231]1743<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1744  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
[949]1747   The set of header fields named by the Vary field value is known as the
[1163]1748   selecting header fields.
[1112]1751   A server &SHOULD; include a Vary header field with any cacheable response
[949]1752   that is subject to server-driven negotiation. Doing so allows a cache to
1753   properly interpret future requests on that resource and informs the user
1754   agent about the presence of negotiation on that resource. A server &MAY;
1755   include a Vary header field with a non-cacheable response that is subject
1756   to server-driven negotiation, since this might provide the user agent with
1757   useful information about the dimensions over which the response varies at
1758   the time of the response.
[949]1761   A Vary field value of "*" signals that unspecified parameters not limited
[1163]1762   to the header fields (e.g., the network address of the client), play a
[949]1763   role in the selection of the response representation; therefore, a cache
[1112]1764   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1765   generate the "*" value.
[1163]1768   The field-names given are not limited to the set of standard header
[949]1769   fields defined by this specification. Field names are case-insensitive.
[540]1773<section anchor="header.warning" title="Warning">
[1120]1774   <iref item="Warning header field" primary="true" x:for-anchor="" />
1775   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
[949]1776   <x:anchor-alias value="Warning"/>
1777   <x:anchor-alias value="warning-value"/>
1778   <x:anchor-alias value="warn-agent"/>
1779   <x:anchor-alias value="warn-code"/>
1780   <x:anchor-alias value="warn-date"/>
1781   <x:anchor-alias value="warn-text"/>
[1163]1783   The "Warning" header field is used to carry additional information
[949]1784   about the status or transformation of a message that might not be reflected
1785   in the message. This information is typically used to warn about possible
1786   incorrectness introduced by caching operations or transformations applied
1787   to the payload of the message.
[949]1790   Warnings can be used for other purposes, both cache-related and otherwise.
1791   The use of a warning, rather than an error status code, distinguishes these
1792   responses from true failures.
[994]1795   Warning header fields can in general be applied to any message, however some
[949]1796   warn-codes are specific to caches and can only be applied to response
1797   messages.
[1231]1799<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"/>
1800  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
[229]1802  <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>
1803                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
[229]1805  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
[334]1806  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
[135]1807                  ; the name or pseudonym of the server adding
[994]1808                  ; the Warning header field, for use in debugging
[229]1809  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1810  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
[949]1813   Multiple warnings can be attached to a response (either by the origin
1814   server or by a cache), including multiple warnings with the same code
1815   number, only differing in warn-text.
[949]1818   When this occurs, the user agent &SHOULD; inform the user of as many of
1819   them as possible, in the order that they appear in the response.
[1433]1822   Systems that generate multiple Warning header fields are encouraged to
1823   order them with this user agent behavior in mind. New Warning header fields
1824   are added after any existing Warning headers fields.
[949]1827   Warnings are assigned three digit warn-codes. The first digit indicates
1828   whether the Warning is required to be deleted from a stored response after
1829   validation:
1830   <list style="symbols">
1831      <t>1xx Warnings describe the freshness or validation status of the
[1112]1832      response, and so &MUST; be deleted by a cache after validation. They can
[949]1833      only be generated by a cache when validating a cached entry, and
1834      &MUST-NOT; be generated in any other situation.</t>
1835      <t>2xx Warnings describe some aspect of the representation that is not
1836      rectified by a validation (for example, a lossy compression of the
[1112]1837      representation) and &MUST-NOT; be deleted by a cache after validation,
[949]1838      unless a full response is returned, in which case they &MUST; be.</t>
1839   </list>
[994]1842   If an implementation sends a message with one or more Warning header fields to a
[949]1843   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
[994]1844   in each warning-value a warn-date that matches the Date header field in the
[949]1845   message.
[1112]1848   If a system receives a message with a warning-value that includes
[949]1849   a warn-date, and that warn-date is different from the Date value in the
1850   response, then that warning-value &MUST; be deleted from the message before
1851   storing, forwarding, or using it. (preventing the consequences of naive
1852   caching of Warning header fields.) If all of the warning-values are deleted
[994]1853   for this reason, the Warning header field &MUST; be deleted as well.
[949]1856   The following warn-codes are defined by this specification, each with a
1857   recommended warn-text in English, and a description of its meaning.
[541]1859<t><?rfc needLines="4"?>
[949]1860   110 Response is stale
1861   <list>
[1112]1862      <t>A cache &SHOULD; include this whenever the returned response is stale.</t>
[949]1863   </list>
[541]1865<t><?rfc needLines="4"?>
[949]1866   111 Revalidation failed
1867   <list>
[1112]1868      <t>A cache &SHOULD; include this when returning a stale response because an
[949]1869      attempt to validate the response failed, due to an inability to reach
1870      the server.</t>
1871   </list>
[541]1873<t><?rfc needLines="4"?>
[949]1874   112 Disconnected operation
1875   <list>
[1433]1876      <t>A cache &SHOULD; include this if it is intentionally disconnected from
[949]1877      the rest of the network for a period of time.</t>
1878   </list>
[541]1880<t><?rfc needLines="4"?>
[949]1881   113 Heuristic expiration
1882   <list>
[1112]1883      <t>A cache &SHOULD; include this if it heuristically chose a freshness
[949]1884      lifetime greater than 24 hours and the response's age is greater than 24
1885      hours.</t>
1886   </list>
[541]1888<t><?rfc needLines="4"?>
[949]1889   199 Miscellaneous warning
1890   <list>
1891      <t>The warning text can include arbitrary information to be presented to
1892      a human user, or logged. A system receiving this warning &MUST-NOT; take
1893      any automated action, besides presenting the warning to the user.</t>
1894   </list>
[541]1896<t><?rfc needLines="4"?>
[949]1897   214 Transformation applied
1898   <list>
[1112]1899      <t>&MUST; be added by a proxy if it applies any
[949]1900      transformation to the representation, such as changing the
1901      content-coding, media-type, or modifying the representation data, unless
1902      this Warning code already appears in the response.</t>
1903   </list>
[541]1905<t><?rfc needLines="4"?>
[949]1906   299 Miscellaneous persistent warning
1907   <list>
1908      <t>The warning text can include arbitrary information to be presented to
1909      a human user, or logged. A system receiving this warning &MUST-NOT; take
1910      any automated action.</t>
1911   </list>
[540]1917<section anchor="history.lists" title="History Lists">
[949]1919   User agents often have history mechanisms, such as "Back" buttons and
1920   history lists, that can be used to redisplay a representation retrieved
1921   earlier in a session.
[949]1924   The freshness model (<xref target="expiration.model"/>) does not
1925   necessarily apply to history mechanisms. I.e., a history mechanism can
1926   display a previous representation even if it has expired.
1929   This does not prohibit the history mechanism from telling the user that a
1930   view might be stale, or from honoring cache directives (e.g.,
1931   Cache-Control: no-store).
[540]1936<section anchor="IANA.considerations" title="IANA Considerations">
[949]1938<section title="Cache Directive Registry" 
1939   anchor="cache.directive.registration">
[949]1941   The registration procedure for HTTP Cache Directives is defined by <xref
1942   target="cache.control.extensions"/> of this document.
[969]1945   The HTTP Cache Directive Registry shall be created at <eref
[949]1946   target=""/> and be
1947   populated with the registrations below:
1949<?BEGININC p6-cache.cache-directives ?>
1950<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
[825]1951<texttable xmlns:my="#my" align="left" suppress-title="true"
[817]1952           anchor="iana.cache.directive.registration.table">
1953   <ttcol>Cache Directive</ttcol>
1954   <ttcol>Reference</ttcol>
1956   <c>max-age</c>
1957   <c>
1958      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1959   </c>
1960   <c>max-stale</c>
1961   <c>
1962      <xref target="cache-request-directive"/>
1963   </c>
1964   <c>min-fresh</c>
1965   <c>
1966      <xref target="cache-request-directive"/>
1967   </c>
1968   <c>must-revalidate</c>
1969   <c>
1970      <xref target="cache-response-directive"/>
1971   </c>
1972   <c>no-cache</c>
1973   <c>
1974      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1975   </c>
1976   <c>no-store</c>
1977   <c>
1978      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1979   </c>
1980   <c>no-transform</c>
1981   <c>
1982      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1983   </c>
1984   <c>only-if-cached</c>
1985   <c>
1986      <xref target="cache-request-directive"/>
1987   </c>
1988   <c>private</c>
1989   <c>
1990      <xref target="cache-response-directive"/>
1991   </c>
1992   <c>proxy-revalidate</c>
1993   <c>
1994      <xref target="cache-response-directive"/>
1995   </c>
1996   <c>public</c>
1997   <c>
1998      <xref target="cache-response-directive"/>
1999   </c>
2000   <c>s-maxage</c>
2001   <c>
2002      <xref target="cache-response-directive"/>
2003   </c>
[825]2004   <c>stale-if-error</c>
2005   <c>
2006      <xref xmlns:x="" target="RFC5861" x:fmt="," x:sec="4"/>
2007   </c>
2008   <c>stale-while-revalidate</c>
2009   <c>
2010      <xref xmlns:x="" target="RFC5861" x:fmt="," x:sec="3"/>
2011   </c>
2014<?ENDINC p6-cache.cache-directives ?>
[921]2017<section title="Header Field Registration" anchor="header.field.registration">
[949]2019  The Message Header Field Registry located at <eref 
[540]2020  target="" />
[969]2021  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
[680]2023<?BEGININC p6-cache.iana-headers ?>
[540]2024<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
[680]2025<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2026   <ttcol>Header Field Name</ttcol>
2027   <ttcol>Protocol</ttcol>
2028   <ttcol>Status</ttcol>
2029   <ttcol>Reference</ttcol>
[680]2031   <c>Age</c>
2032   <c>http</c>
2033   <c>standard</c>
2034   <c>
2035      <xref target="header.age"/>
2036   </c>
2037   <c>Cache-Control</c>
2038   <c>http</c>
2039   <c>standard</c>
2040   <c>
2041      <xref target="header.cache-control"/>
2042   </c>
2043   <c>Expires</c>
2044   <c>http</c>
2045   <c>standard</c>
2046   <c>
2047      <xref target="header.expires"/>
2048   </c>
2049   <c>Pragma</c>
2050   <c>http</c>
2051   <c>standard</c>
2052   <c>
2053      <xref target="header.pragma"/>
2054   </c>
2055   <c>Vary</c>
2056   <c>http</c>
2057   <c>standard</c>
2058   <c>
2059      <xref target="header.vary"/>
2060   </c>
2061   <c>Warning</c>
2062   <c>http</c>
2063   <c>standard</c>
2064   <c>
2065      <xref target="header.warning"/>
2066   </c>
[680]2069<?ENDINC p6-cache.iana-headers ?>
[949]2071   The change controller is: "IETF ( - Internet Engineering Task
2072   Force".
[540]2078<section anchor="security.considerations" title="Security Considerations">
[949]2080   Caches expose additional potential vulnerabilities, since the contents of
2081   the cache represent an attractive target for malicious exploitation.
2082   Because cache contents persist after an HTTP request is complete, an attack
2083   on the cache can reveal information long after a user believes that the
2084   information has been removed from the network. Therefore, cache contents
[969]2085   need to be protected as sensitive information.
[1364]2089<section title="Acknowledgments" anchor="acks">
[1364]2091  See &acks;.
2098<references title="Normative References">
[540]2100  <reference anchor="Part1">
2101    <front>
2102      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2103      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2104        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2105        <address><email></email></address>
2106      </author>
2107      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2108        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2109        <address><email></email></address>
[540]2110      </author>
2111      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2112        <organization abbrev="HP">Hewlett-Packard Company</organization>
2113        <address><email></email></address>
2114      </author>
2115      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2116        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2117        <address><email></email></address>
2118      </author>
2119      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2120        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2121        <address><email></email></address>
2122      </author>
2123      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2124        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2125        <address><email></email></address>
2126      </author>
2127      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2128        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2129        <address><email></email></address>
2130      </author>
2131      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2132        <organization abbrev="W3C">World Wide Web Consortium</organization>
2133        <address><email></email></address>
2134      </author>
2135      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2136        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2137        <address><email></email></address>
2138      </author>
2139      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2140    </front>
2141    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2142    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2143  </reference>
[540]2145  <reference anchor="Part2">
2146    <front>
2147      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2148      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2149        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2150        <address><email></email></address>
2151      </author>
2152      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2153        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2154        <address><email></email></address>
[540]2155      </author>
2156      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2157        <organization abbrev="HP">Hewlett-Packard Company</organization>
2158        <address><email></email></address>
2159      </author>
2160      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2161        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2162        <address><email></email></address>
2163      </author>
2164      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2165        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2166        <address><email></email></address>
2167      </author>
2168      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2169        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2170        <address><email></email></address>
2171      </author>
2172      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2173        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2174        <address><email></email></address>
2175      </author>
2176      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2177        <organization abbrev="W3C">World Wide Web Consortium</organization>
2178        <address><email></email></address>
2179      </author>
2180      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2181        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2182        <address><email></email></address>
2183      </author>
2184      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2185    </front>
2186    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2187    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2188  </reference>
[540]2190  <reference anchor="Part4">
2191    <front>
2192      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2193      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2194        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2195        <address><email></email></address>
2196      </author>
2197      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2198        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2199        <address><email></email></address>
[540]2200      </author>
2201      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2202        <organization abbrev="HP">Hewlett-Packard Company</organization>
2203        <address><email></email></address>
2204      </author>
2205      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2206        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2207        <address><email></email></address>
2208      </author>
2209      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2210        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2211        <address><email></email></address>
2212      </author>
2213      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2214        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2215        <address><email></email></address>
2216      </author>
2217      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2218        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2219        <address><email></email></address>
2220      </author>
2221      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2222        <organization abbrev="W3C">World Wide Web Consortium</organization>
2223        <address><email></email></address>
2224      </author>
2225      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2226        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2227        <address><email></email></address>
2228      </author>
2229      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2230    </front>
2231    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2232    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2233  </reference>
[540]2235  <reference anchor="Part5">
2236    <front>
2237      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2238      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2239        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2240        <address><email></email></address>
2241      </author>
2242      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2243        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2244        <address><email></email></address>
[540]2245      </author>
2246      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2247        <organization abbrev="HP">Hewlett-Packard Company</organization>
2248        <address><email></email></address>
2249      </author>
2250      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2251        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2252        <address><email></email></address>
2253      </author>
2254      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2255        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2256        <address><email></email></address>
2257      </author>
2258      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2259        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2260        <address><email></email></address>
2261      </author>
2262      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2263        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2264        <address><email></email></address>
2265      </author>
2266      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2267        <organization abbrev="W3C">World Wide Web Consortium</organization>
2268        <address><email></email></address>
2269      </author>
2270      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2271        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2272        <address><email></email></address>
2273      </author>
2274      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2275    </front>
2276    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2277    <x:source basename="p5-range" href="p5-range.xml" />
2278  </reference>
[540]2280  <reference anchor="Part7">
2281    <front>
2282      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2283      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
[1106]2284        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2285        <address><email></email></address>
2286      </author>
2287      <author fullname="Jim Gettys" initials="J." surname="Gettys">
[844]2288        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2289        <address><email></email></address>
[540]2290      </author>
2291      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2292        <organization abbrev="HP">Hewlett-Packard Company</organization>
2293        <address><email></email></address>
2294      </author>
2295      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2296        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2297        <address><email></email></address>
2298      </author>
2299      <author fullname="Larry Masinter" initials="L." surname="Masinter">
[1106]2300        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
[540]2301        <address><email></email></address>
2302      </author>
2303      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2304        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2305        <address><email></email></address>
2306      </author>
2307      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2308        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2309        <address><email></email></address>
2310      </author>
2311      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2312        <organization abbrev="W3C">World Wide Web Consortium</organization>
2313        <address><email></email></address>
2314      </author>
2315      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2316        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2317        <address><email></email></address>
2318      </author>
2319      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2320    </front>
2321    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2322    <x:source basename="p7-auth" href="p7-auth.xml" />
2323  </reference>
[540]2325  <reference anchor="RFC2119">
2326    <front>
2327      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2328      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2329        <organization>Harvard University</organization>
2330        <address><email></email></address>
2331      </author>
2332      <date month="March" year="1997" />
2333    </front>
2334    <seriesInfo name="BCP" value="14" />
2335    <seriesInfo name="RFC" value="2119" />
2336  </reference>
[540]2338  <reference anchor="RFC5234">
2339    <front>
2340      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2341      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2342        <organization>Brandenburg InternetWorking</organization>
2343        <address>
[728]2344          <email></email>
2345        </address> 
[540]2346      </author>
2347      <author initials="P." surname="Overell" fullname="Paul Overell">
2348        <organization>THUS plc.</organization>
2349        <address>
[728]2350          <email></email>
2351        </address>
[540]2352      </author>
2353      <date month="January" year="2008"/>
2354    </front>
2355    <seriesInfo name="STD" value="68"/>
2356    <seriesInfo name="RFC" value="5234"/>
2357  </reference>
[540]2361<references title="Informative References">
[540]2363  <reference anchor="RFC1305">
2364    <front>
2365      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2366      <author fullname="David L. Mills" initials="D." surname="Mills">
2367        <organization>University of Delaware, Electrical Engineering Department</organization>
2368        <address><email></email></address>
2369      </author>
2370      <date month="March" year="1992" />
2371    </front>
2372    <seriesInfo name="RFC" value="1305" />
2373  </reference>
[540]2375  <reference anchor="RFC2616">
2376    <front>
2377      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2378      <author fullname="R. Fielding" initials="R." surname="Fielding">
2379        <organization>University of California, Irvine</organization>
2380        <address><email></email></address>
2381      </author>
2382      <author fullname="J. Gettys" initials="J." surname="Gettys">
2383        <organization>W3C</organization>
2384        <address><email></email></address>
2385      </author>
2386      <author fullname="J. Mogul" initials="J." surname="Mogul">
2387        <organization>Compaq Computer Corporation</organization>
2388        <address><email></email></address>
2389      </author>
2390      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2391        <organization>MIT Laboratory for Computer Science</organization>
2392        <address><email></email></address>
2393      </author>
2394      <author fullname="L. Masinter" initials="L." surname="Masinter">
2395        <organization>Xerox Corporation</organization>
2396        <address><email></email></address>
2397      </author>
2398      <author fullname="P. Leach" initials="P." surname="Leach">
2399        <organization>Microsoft Corporation</organization>
2400        <address><email></email></address>
2401      </author>
2402      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2403        <organization>W3C</organization>
2404        <address><email></email></address>
2405      </author>
2406      <date month="June" year="1999" />
2407    </front>
2408    <seriesInfo name="RFC" value="2616" />
2409  </reference>
2411  <reference anchor="RFC3864">
2412    <front>
2413      <title>Registration Procedures for Message Header Fields</title>
2414      <author fullname="G. Klyne" initials="G." surname="Klyne">
2415        <organization>Nine by Nine</organization>
2416        <address><email></email></address>
2417      </author>
2418      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2419        <organization>BEA Systems</organization>
2420        <address><email></email></address>
2421      </author>
2422      <author fullname="J. Mogul" initials="J." surname="Mogul">
2423        <organization>HP Labs</organization>
2424        <address><email></email></address>
2425      </author>
2426      <date month="September" year="2004" />
2427    </front>
2428    <seriesInfo name="BCP" value="90" />
2429    <seriesInfo name="RFC" value="3864" />
2430  </reference>
[817]2432  <reference anchor='RFC5226'>
2433    <front>
2434      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2435      <author initials='T.' surname='Narten' fullname='T. Narten'>
2436        <organization>IBM</organization>
2437        <address><email></email></address>
2438      </author>
2439      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2440        <organization>Google</organization>
2441        <address><email></email></address>
2442      </author>
2443      <date year='2008' month='May' />
2444    </front>
2445    <seriesInfo name='BCP' value='26' />
2446    <seriesInfo name='RFC' value='5226' />
2447  </reference>
[825]2449  <reference anchor='RFC5861'>
2450    <front>
2451      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2452      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2453        <organization>Yahoo! Inc.</organization>
2454        <address><email></email></address>
2455      </author>
2456      <date month="April" year="2010"/>
2457    </front>
2458    <seriesInfo name='RFC' value='5861' />
2459  </reference>
2463<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
[795]2465  Make the specified age calculation algorithm less conservative.
2466  (<xref target="age.calculations"/>)
[691]2469  Remove requirement to consider Content-Location in successful responses
2470  in order to determine the appropriate response to use.
2471  (<xref target="validation.model" />)
[540]2474  Clarify denial of service attack avoidance requirement.
2475  (<xref target="invalidation.after.updates.or.deletions" />)
[1231]2478  Change ABNF productions for header fields to only define the field value.
[1415]2479  (<xref target="header.field.definitions"/>)
[994]2482  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
[669]2483  anymore, as these aspects never were implemented.
2484  (<xref target="header.warning" />)
[680]2488<?BEGININC p6-cache.abnf-appendix ?>
[427]2489<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
2491<artwork type="abnf" name="p6-cache.parsed-abnf">
[1231]2492<x:ref>Age</x:ref> = delta-seconds
[1231]2494<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
[425]2495 cache-directive ] )
[1231]2497<x:ref>Expires</x:ref> = HTTP-date
[1436]2499<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
2501<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
[1231]2503<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
[425]2504 pragma-directive ] )
[1231]2506<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2507 ) )
[1231]2509<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2510 )
[427]2512<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2513<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2514<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
[425]2515 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2516 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2517 cache-extension
[427]2518<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
[425]2519 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2520 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2521 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2522 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2523 ) / ( "s-maxage=" delta-seconds ) / cache-extension
[427]2525<x:ref>delta-seconds</x:ref> = 1*DIGIT
2527<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
[678]2529<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
[1318]2531<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
[427]2532<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
[1436]2533<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.8&gt;
[1396]2535<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
[1396]2537<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.3&gt;
[1318]2539<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2541<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2542<x:ref>warn-code</x:ref> = 3DIGIT
2543<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2544<x:ref>warn-text</x:ref> = quoted-string
2545<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
[425]2546 ]
[532]2549<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2550; Age defined but not used
[425]2551; Cache-Control defined but not used
2552; Expires defined but not used
2553; Pragma defined but not used
2554; Vary defined but not used
2555; Warning defined but not used
[680]2557<?ENDINC p6-cache.abnf-appendix ?>
[540]2559<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
[1002]2561<section title="Since RFC 2616">
[540]2562  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
[540]2565<section title="Since draft-ietf-httpbis-p6-cache-00">
2567  Closed issues:
2568  <list style="symbols">
2569    <t>
[542]2570      <eref target="" />: "Trailer" (<eref target="" />)</t>
[540]2571    <t>
2572      <eref target="" />: "Invalidation after Update or Delete" (<eref target="" />)</t>
2573    <t>
2574      <eref target="" />: "Normative and Informative references"</t>
2575    <t>
2576      <eref target="" />: "Date reference typo"</t>
2577    <t>
2578      <eref target="" />: "Connection header text"</t>
2579    <t>
2580      <eref target="" />: "Informative references"</t>
2581    <t>
2582      <eref target="" />: "ISO-8859-1 Reference"</t>
2583    <t>
2584      <eref target="" />: "Normative up-to-date references"</t>
2585    <t>
2586      <eref target="" />: "typo in 13.2.2"</t>
2587  </list>
2590  Other changes:
2591  <list style="symbols">
2592    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2593        target="" />)</t>
2594  </list>
[540]2598<section title="Since draft-ietf-httpbis-p6-cache-01">
2600  Closed issues:
2601  <list style="symbols">
2602    <t>
2603      <eref target="" />: "rel_path not used"</t>
2604  </list>
2607  Other changes:
2608  <list style="symbols">
2609    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2610      on <eref target="" />)</t>
2611    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2612      specification.</t>
2613  </list>
[540]2617<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
[994]2619  Ongoing work on IANA Message Header Field Registration (<eref target="" />):
[540]2620  <list style="symbols">
[994]2621    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
[540]2622      document.</t>
2623  </list>
[540]2627<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2629  Closed issues:
2630  <list style="symbols">
2631    <t>
2632      <eref target="" />: "Vary header classification"</t>
2633  </list>
[540]2637<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2639  Ongoing work on ABNF conversion (<eref target=""/>):
2640  <list style="symbols"> 
2641    <t>
2642      Use "/" instead of "|" for alternatives.
2643    </t>
2644    <t>
2645      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2646      whitespace ("OWS") and required whitespace ("RWS").
2647    </t>
2648    <t>
2649      Rewrite ABNFs to spell out whitespace rules, factor out
[994]2650      header field value format definitions.
[540]2651    </t>
2652  </list>
[540]2656<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2658  This is a total rewrite of this part of the specification.
2661  Affected issues:
2662  <list style="symbols">
2663    <t>
2664      <eref target="" />: "Definition of 1xx Warn-Codes"</t>
2665    <t>
[839]2666      <eref target="" />: "Placement of 13.5.1 and 13.5.2"</t>
[542]2667    <t>
[839]2668      <eref target="" />: "The role of Warning and Semantic Transparency in Caching"</t>
[542]2669    <t>
[839]2670      <eref target="" />: "Methods and Caching"</t>
[542]2671  </list>
[543]2674  In addition: Final work on ABNF conversion (<eref target=""/>):
[542]2675  <list style="symbols"> 
2676    <t>
2677      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2678    </t>
2679  </list>
[547]2683<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
[576]2685  Closed issues:
2686  <list style="symbols"> 
2687    <t>
2688      <eref target=""/>:
2689      "base for numeric protocol elements"
2690    </t>
2691  </list>
2694  Affected issues:
2695  <list style="symbols">
2696    <t>
2697      <eref target=""/>:
[1166]2698      "Vary and non-existant headers"
[607]2699    </t>
2700  </list>
[604]2704<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
[669]2706  Closed issues:
2707  <list style="symbols"> 
2708    <t>
[711]2709      <eref target="" />:
2710      "Definition of 1xx Warn-Codes"
2711    </t>
2712    <t>
[691]2713      <eref target=""/>:
2714      "Content-Location on 304 responses"
2715    </t>
2716    <t>
[711]2717      <eref target="" />:
2718      "private and no-cache CC directives with headers"
2719    </t>
2720    <t>
[669]2721      <eref target=""/>:
2722      "RFC2047 and warn-text"
2723    </t>
2724  </list>
[720]2728<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
[766]2730  Closed issues:
2731  <list style="symbols"> 
2732    <t>
[771]2733      <eref target="" />:
2734      "serving negotiated responses from cache: header-specific canonicalization"
2735    </t>
2736    <t>
[766]2737      <eref target="" />:
2738      "Effect of CC directives on history lists"
2739    </t>
[1291]2740    <t>
[1292]2741      <eref target="" />:
2742      "Cache Extensions can override no-store, etc."
2743    </t>
[766]2744  </list>
[722]2747  Affected issues:
2748  <list style="symbols">
2749    <t>
2750      <eref target=""/>:
2751      Status codes and caching
2752    </t>
2753  </list>
2756  Partly resolved issues:
2757  <list style="symbols"> 
2758    <t>
2759      <eref target=""/>:
2760      "Placement of 13.5.1 and 13.5.2"
2761    </t>
2762  </list>
[773]2766<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
[792]2768  Closed issues:
2769  <list style="symbols"> 
2770    <t>
2771      <eref target="" />:
2772      "Age calculation"
2773    </t>
[817]2774    <t>
[827]2775      <eref target="" />:
2776      "Clarify differences between / requirements for request and response CC directives"
2777    </t>
2778    <t>
[1287]2779      <eref target="" />:
2780      "Caching authenticated responses"
2781    </t>
2782    <t>
[817]2783      <eref target="" />:
2784      "IANA registry for cache-control directives"
2785    </t>
[828]2786    <t>
2787      <eref target="" />:
2788      "Heuristic caching of URLs with query components"
2789    </t>
[792]2790  </list>
2793  Partly resolved issues:
2794  <list style="symbols"> 
2795    <t>
2796      <eref target=""/>:
2797      "Term for the requested resource's URI"
2798    </t>
2799  </list>
[841]2803<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
[911]2805  Closed issues:
2806  <list style="symbols"> 
2807    <t>
[965]2808      <eref target=""/>:
2809      "Clarify entity / representation / variant terminology"
2810    </t>
2811    <t>
[911]2812      <eref target=""/>:
2813      "consider removing the 'changes from 2068' sections"
2814    </t>
[916]2815    <t>
2816      <eref target=""/>:
2817      "Allowing heuristic caching for new status codes"
2818    </t>
[944]2819    <t>
2820      Clean up TODOs and prose in "Combining Responses."
2821    </t>
[911]2822  </list>
[973]2826<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
[1045]2828  Closed issues:
2829  <list style="symbols"> 
2830    <t>
2831      <eref target=""/>:
2832      "Text about clock requirement for caches belongs in p6"
2833    </t>
2834  </list>
[1052]2838<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
[1130]2840  Closed issues:
2841  <list style="symbols"> 
2842    <t>
[1165]2843      <eref target=""/>:
2844      "Header Classification"
2845    </t>
2846    <t>
[1130]2847      <eref target=""/>:
2848      "Clarify 'public'"
2849    </t>
2850  </list>
[1180]2854<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
[1231]2856  Closed issues:
2857  <list style="symbols">
2858    <t>
2859      <eref target=""/>:
2860      "untangle ABNFs for header fields"
2861    </t>
2862  </list>
[1271]2866<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
[1284]2868  Closed issues:
[1287]2869  <list style="symbols">
2870    <t>
[1292]2871      <eref target=""/>:
2872      "Mismatch Vary"
2873    </t>
2874    <t>
[1319]2875      <eref target=""/>:
2876      "Cache Invalidation only happens upon successful responses"
2877    </t>
2878    <t>
[1323]2879      <eref target=""/>:
2880      "Recommend minimum sizes for protocol elements"
2881    </t>
2882    <t>
[1292]2883      <eref target=""/>:
2884      "Proxies don't 'understand' methods"
2885    </t>
[1314]2886    <t>
[1333]2887      <eref target=""/>:
2888      "Cache Extensions can override no-store, etc."
2889    </t>
2890    <t>
[1319]2891      <eref target=""/>:
2892      "Pragma"
[1315]2893    </t>
[1287]2894  </list>
[1326]2898<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
[1400]2900  Closed issues:
2901  <list style="symbols">
2902    <t>
2903      <eref target=""/>:
2904      "Motivate one-year limit for Expires"
2905    </t>
2906  </list>
[1401]2910<section title="Since draft-ietf-httpbis-p6-cache-16" anchor="changes.since.16">
[1449]2912  Closed issues:
2913  <list style="symbols">
2914    <t>
[1452]2915      <eref target=""/>:
2916      "Document HTTP's error-handling philosophy"
2917    </t>
2918    <t>
[1449]2919      <eref target=""/>:
2920      "Cache-Control directive case sensitivity"
2921    </t>
2922  </list>
[538]2927  </back>
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