Ignore:
Timestamp:
Mar 5, 2009, 5:51:26 AM (11 years ago)
Author:
julian.reschke@…
Message:

Move P6 from ./latest-roy to ./latest

File:
1 edited

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  • draft-ietf-httpbis/latest/p6-cache.xml

    r532 r538  
    1616  <!ENTITY ID-YEAR "2009">
    1717  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    18   <!ENTITY notation-abnf               "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    1918  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
     19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    2020  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    2121  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
     22  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    2223  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    2324  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
     
    2728  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    2829  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
     30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    2931  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    3032  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    3133  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    3234  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    33   <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
    3435]>
    3536<?rfc toc="yes" ?>
     
    4445<?rfc-ext allow-markup-in-artwork="yes" ?>
    4546<?rfc-ext include-references-in-index="yes" ?>
    46 <rfc obsoletes="2616" category="std" x:maturity-level="draft"
    47      ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
    48      xmlns:x='http://purl.org/net/xml2rfc/ext'>
    49 <front>
    50 
    51   <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
    52 
    53   <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    54     <organization abbrev="Day Software">Day Software</organization>
    55     <address>
     47<?oxygen RNGSchema="../../rfc2629xslt/rfc2629-ext.rnc" type="compact"?>
     48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
     49  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
     50  <front>
     51
     52    <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
     53
     54    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     55      <organization abbrev="Day Software">Day Software</organization>
     56      <address>
    5657      <postal>
    5758        <street>23 Corporate Plaza DR, Suite 280</street>
     
    6667      <uri>http://roy.gbiv.com/</uri>
    6768    </address>
    68   </author>
    69 
    70   <author initials="J." surname="Gettys" fullname="Jim Gettys">
    71     <organization>One Laptop per Child</organization>
    72     <address>
     69    </author>
     70
     71    <author fullname="Jim Gettys" initials="J." surname="Gettys">
     72      <organization>One Laptop per Child</organization>
     73      <address>
    7374      <postal>
    7475        <street>21 Oak Knoll Road</street>
     
    8182      <uri>http://www.laptop.org/</uri>
    8283    </address>
    83   </author>
    84  
    85   <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    86     <organization abbrev="HP">Hewlett-Packard Company</organization>
    87     <address>
     84    </author>
     85
     86    <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     87      <organization abbrev="HP">Hewlett-Packard Company</organization>
     88      <address>
    8889      <postal>
    8990        <street>HP Labs, Large Scale Systems Group</street>
     
    9697      <email>JeffMogul@acm.org</email>
    9798    </address>
    98   </author>
    99 
    100   <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    101     <organization abbrev="Microsoft">Microsoft Corporation</organization>
    102     <address>
     99    </author>
     100
     101    <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     102      <organization abbrev="Microsoft">Microsoft Corporation</organization>
     103      <address>
    103104      <postal>
    104105        <street>1 Microsoft Way</street>
     
    110111      <email>henrikn@microsoft.com</email>
    111112    </address>
    112   </author>
    113 
    114   <author initials="L." surname="Masinter" fullname="Larry Masinter">
    115     <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    116     <address>
     113    </author>
     114
     115    <author fullname="Larry Masinter" initials="L." surname="Masinter">
     116      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     117      <address>
    117118      <postal>
    118119        <street>345 Park Ave</street>
     
    125126      <uri>http://larry.masinter.net/</uri>
    126127    </address>
    127   </author>
    128  
    129   <author initials="P." surname="Leach" fullname="Paul J. Leach">
    130     <organization abbrev="Microsoft">Microsoft Corporation</organization>
    131     <address>
     128    </author>
     129
     130    <author fullname="Paul J. Leach" initials="P." surname="Leach">
     131      <organization abbrev="Microsoft">Microsoft Corporation</organization>
     132      <address>
    132133      <postal>
    133134        <street>1 Microsoft Way</street>
     
    138139      <email>paulle@microsoft.com</email>
    139140    </address>
    140   </author>
    141    
    142   <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    143     <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    144     <address>
     141    </author>
     142
     143    <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     144      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     145      <address>
    145146      <postal>
    146147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
     
    155156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
    156157    </address>
    157   </author>
    158 
    159   <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    160     <organization abbrev="W3C">World Wide Web Consortium</organization>
    161     <address>
     158    </author>
     159
     160    <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     161      <organization abbrev="W3C">World Wide Web Consortium</organization>
     162      <address>
    162163      <postal>
    163164        <street>W3C / ERCIM</street>
     
    171172      <uri>http://www.raubacapeu.net/people/yves/</uri>
    172173    </address>
    173   </author>
    174 
    175   <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    176     <organization abbrev="greenbytes">greenbytes GmbH</organization>
    177     <address>
     174    </author>
     175
     176    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     177      <organization abbrev="greenbytes">greenbytes GmbH</organization>
     178      <address>
    178179      <postal>
    179180        <street>Hafenweg 16</street>
     
    186187      <uri>http://greenbytes.de/tech/webdav/</uri>     
    187188    </address>
    188   </author>
    189 
    190   <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    191   <workgroup>HTTPbis Working Group</workgroup>
    192 
    193 <abstract>
    194 <t>
    195    The Hypertext Transfer Protocol (HTTP) is an application-level
    196    protocol for distributed, collaborative, hypermedia information
    197    systems. HTTP has been in use by the World Wide Web global information
    198    initiative since 1990. This document is Part 6 of the seven-part specification
    199    that defines the protocol referred to as "HTTP/1.1" and, taken together,
    200    obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
    201    and the associated header fields that control cache behavior or indicate
    202    cacheable response messages.
    203 </t>
    204 </abstract>
    205 
    206 <note title="Editorial Note (To be removed by RFC Editor)">
    207   <t>
    208     Discussion of this draft should take place on the HTTPBIS working group
    209     mailing list (ietf-http-wg@w3.org). The current issues list is
    210     at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
    211     and related documents (including fancy diffs) can be found at
    212     <eref target="http://tools.ietf.org/wg/httpbis/"/>.
    213   </t>
    214   <t>
    215     The changes in this draft are summarized in <xref target="changes.since.05"/>.
    216   </t>
    217 </note>
    218 </front>
    219 <middle>
    220 <section title="Introduction" anchor="caching">
    221 <t>
    222    HTTP is typically used for distributed information systems, where
    223    performance can be improved by the use of response caches, and includes
    224    a number of elements intended to make caching work as well as possible.
    225    Because these elements interact with each other, it is useful to describe
    226    the caching design of HTTP separately.  This document defines aspects of
    227    HTTP/1.1 related to caching and reusing response messages.
    228 </t>
    229 
    230 <section title="Purpose" anchor="intro.purpose">
    231 <iref item="cache"/>
    232 <t>
    233    An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
    234    and the subsystem that controls its message storage, retrieval, and
    235    deletion. A cache stores cacheable responses in order to reduce the
    236    response time and network bandwidth consumption on future, equivalent
    237    requests. Any client or server may include a cache, though a cache
    238    cannot be used by a server that is acting as a tunnel.
    239 </t>
    240 <t>
    241    Caching would be useless if it did not significantly improve
    242    performance. The goal of caching in HTTP/1.1 is to reuse a prior response
    243    message to satisfy a current request.  In some cases, the existing response
    244    can be reused without the need for a network request, reducing latency and
    245    network round-trips; we use an "expiration" mechanism for this purpose
    246    (see <xref target="expiration.model"/>).  Even when a new request is required,
    247    it is often possible to reuse all or parts of the payload of a prior response
    248    to satisfy the request, thereby reducing network bandwidth usage; we use a
    249    "validation" mechanism for this purpose (see <xref target="validation.model"/>).
    250 </t>
    251 <iref item="semantically transparent"/>
    252 <t>
    253    A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
    254    respect to a particular response, when its use affects neither the
    255    requesting client nor the origin server, except to improve
    256    performance. When a cache is semantically transparent, the client
    257    receives exactly the same response status and payload
    258    that it would have received had its request been handled directly
    259    by the origin server.
    260 </t>
    261 <t>
    262    In an ideal world, all interactions with an HTTP cache would be
    263    semantically transparent.  However, for some resources, semantic
    264    transparency is not always necessary and can be effectively traded
    265    for the sake of bandwidth scaling, disconnected operation, and
    266    high availability.  HTTP/1.1 allows origin servers, caches,
    267    and clients to explicitly reduce transparency when necessary.
    268    However, because non-transparent operation may confuse non-expert
    269    users and might be incompatible with certain server applications
    270    (such as those for ordering merchandise), the protocol requires that
    271    transparency be relaxed
    272   <list style="symbols">
    273      <t>only by an explicit protocol-level request when relaxed by
    274         client or origin server</t>
    275 
    276      <t>only with an explicit warning to the end user when relaxed by
    277         cache or client</t>
    278   </list>
    279 </t>
    280 <t>
    281    Therefore, HTTP/1.1 provides these important elements:
    282   <list style="numbers">
    283       <t>Protocol features that provide full semantic transparency when
    284          this is required by all parties.</t>
    285 
    286       <t>Protocol features that allow an origin server or user agent to
    287          explicitly request and control non-transparent operation.</t>
    288 
    289       <t>Protocol features that allow a cache to attach warnings to
    290          responses that do not preserve the requested approximation of
    291          semantic transparency.</t>
    292   </list>
    293 </t>
    294 <t>
    295    A basic principle is that it must be possible for the clients to
    296    detect any potential relaxation of semantic transparency.
    297   <list><t>
    298       <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
    299       design decisions not explicitly discussed in this specification.
    300       If a decision might affect semantic transparency, the implementor
    301       ought to err on the side of maintaining transparency unless a
    302       careful and complete analysis shows significant benefits in
    303       breaking transparency.
    304     </t></list>
    305 </t>
    306 </section>
    307 
    308 <section title="Terminology" anchor="intro.terminology">
    309 <t>
    310    This specification uses a number of terms to refer to the roles
    311    played by participants in, and objects of, HTTP caching.
    312 </t>
    313 <t>
    314   <iref item="cacheable"/>
    315   <x:dfn>cacheable</x:dfn>
    316   <list>
    317     <t>
    318       A response is cacheable if a cache is allowed to store a copy of
    319       the response message for use in answering subsequent requests.
    320       Even when a response is cacheable, there may
    321       be additional constraints on whether a cache can use the cached
    322       copy for a particular request.
    323     </t>
    324   </list>
    325 </t>
    326 <t>
    327   <iref item="first-hand"/>
    328   <x:dfn>first-hand</x:dfn>
    329   <list>
    330     <t>
    331       A response is first-hand if it comes directly and without
    332       unnecessary delay from the origin server, perhaps via one or more
    333       proxies. A response is also first-hand if its validity has just
    334       been checked directly with the origin server.
    335     </t>
    336   </list>
    337 </t>
    338 <t>
    339   <iref item="explicit expiration time"/>
    340   <x:dfn>explicit expiration time</x:dfn>
    341   <list>
    342     <t>
    343       The time at which the origin server intends that an entity should
    344       no longer be returned by a cache without further validation.
    345     </t>
    346   </list>
    347 </t>
    348 <t>
    349   <iref item="heuristic expiration time"/>
    350   <x:dfn>heuristic expiration time</x:dfn>
    351   <list>
    352     <t>
    353       An expiration time assigned by a cache when no explicit expiration
    354       time is available.
    355     </t>
    356   </list>
    357 </t>
    358 <t>
    359   <iref item="age"/>
    360   <x:dfn>age</x:dfn>
    361   <list>
    362     <t>
    363       The age of a response is the time since it was sent by, or
    364       successfully validated with, the origin server.
    365     </t>
    366   </list>
    367 </t>
    368 <t>
    369   <iref item="freshness lifetime"/>
    370   <x:dfn>freshness lifetime</x:dfn>
    371   <list>
    372     <t>
    373       The length of time between the generation of a response and its
    374       expiration time.
    375     </t>
    376   </list>
    377 </t>
    378 <t>
    379   <iref item="fresh"/>
    380   <x:dfn>fresh</x:dfn>
    381   <list>
    382     <t>
    383       A response is fresh if its age has not yet exceeded its freshness
    384       lifetime.
    385     </t>
    386   </list>
    387 </t>
    388 <t>
    389   <iref item="stale"/>
    390   <x:dfn>stale</x:dfn>
    391   <list>
    392     <t>
    393       A response is stale if its age has passed its freshness lifetime.
    394     </t>
    395   </list>
    396 </t>
    397 <t>
    398   <iref item="validator"/>
    399   <x:dfn>validator</x:dfn>
    400   <list>
    401     <t>
    402       A protocol element (e.g., an entity tag or a Last-Modified time)
    403       that is used to find out whether a cache entry is an equivalent
    404       copy of an entity.
    405     </t>
    406   </list>
    407 </t>
    408 </section>
    409 
    410 <section title="Requirements" anchor="intro.requirements">
    411 <t>
    412    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    413    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    414    document are to be interpreted as described in <xref target="RFC2119"/>.
    415 </t>
    416 <t>
    417    An implementation is not compliant if it fails to satisfy one or more
    418    of the &MUST; or &REQUIRED; level requirements for the protocols it
    419    implements. An implementation that satisfies all the &MUST; or &REQUIRED;
    420    level and all the &SHOULD; level requirements for its protocols is said
    421    to be "unconditionally compliant"; one that satisfies all the &MUST;
    422    level requirements but not all the &SHOULD; level requirements for its
    423    protocols is said to be "conditionally compliant."
    424 </t>
    425 </section>
     189    </author>
     190
     191    <date month="&ID-MONTH;" year="&ID-YEAR;" />
     192    <workgroup>HTTPbis Working Group</workgroup>
     193
     194    <abstract>
     195      <t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
     196        collaborative, hypermedia information systems. This document is Part 6 of the seven-part
     197        specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
     198        obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
     199        fields that control cache behavior or indicate cacheable response messages.</t>
     200    </abstract>
     201
     202    <note title="Editorial Note (To be removed by RFC Editor)">
     203      <t>Discussion of this draft should take place on the HTTPBIS working group mailing list
     204        (ietf-http-wg@w3.org). The current issues list is at <eref
     205          target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
     206        (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.</t>
     207      <t>The changes in this draft are summarized in <xref target="changes.since.05" />.</t>
     208    </note>
     209  </front>
     210  <middle>
     211    <section anchor="caching" title="Introduction">
     212      <t>HTTP is typically used for distributed information systems, where performance can be
     213        improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
     214        caching and reusing response messages.</t>
     215
     216      <section anchor="intro.purpose" title="Purpose">
     217        <iref item="cache" />
     218        <t>An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
     219          controls its message storage, retrieval, and deletion. A cache stores cacheable responses
     220          in order to reduce the response time and network bandwidth consumption on future,
     221          equivalent requests. Any client or server may include a cache, though a cache cannot be
     222          used by a server that is acting as a tunnel.</t>
     223        <t>Caching would be useless if it did not significantly improve performance. The goal of
     224          caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
     225          some cases, a stored response can be reused without the need for a network request,
     226          reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
     227          (see <xref target="expiration.model" />). Even when a new request is required, it is often
     228          possible to reuse all or parts of the payload of a prior response to satisfy the request,
     229          thereby reducing network bandwidth usage; a "validation" mechanism is used for this
     230          purpose (see <xref target="validation.model" />).</t>
     231      </section>
     232
     233      <section anchor="intro.terminology" title="Terminology">
     234        <t>This specification uses a number of terms to refer to the roles played by participants
     235          in, and objects of, HTTP caching.</t>
     236        <t>
     237          <iref item="cacheable" />
     238          <x:dfn>cacheable</x:dfn>
     239          <list>
     240            <t>A response is cacheable if a cache is allowed to store a copy of the response message
     241              for use in answering subsequent requests. Even when a response is cacheable, there may
     242              be additional constraints on whether a cache can use the cached copy to satisfy a
     243              particular request.</t>
     244          </list>
     245        </t>
     246        <t>
     247          <iref item="explicit expiration time" />
     248          <x:dfn>explicit expiration time</x:dfn>
     249          <list>
     250            <t>The time at which the origin server intends that an entity should no longer be
     251              returned by a cache without further validation.</t>
     252          </list>
     253        </t>
     254        <t>
     255          <iref item="heuristic expiration time" />
     256          <x:dfn>heuristic expiration time</x:dfn>
     257          <list>
     258            <t>An expiration time assigned by a cache when no explicit expiration time is
     259            available.</t>
     260          </list>
     261        </t>
     262        <t>
     263          <iref item="age" />
     264          <x:dfn>age</x:dfn>
     265          <list>
     266            <t>The age of a response is the time since it was sent by, or successfully validated
     267              with, the origin server.</t>
     268          </list>
     269        </t>
     270        <t>
     271          <iref item="first-hand" />
     272          <x:dfn>first-hand</x:dfn>
     273          <list>
     274            <t>A response is first-hand if the freshness model is not in use; i.e., its age is
     275            0.</t>
     276          </list>
     277        </t>
     278        <t>
     279          <iref item="freshness lifetime" />
     280          <x:dfn>freshness lifetime</x:dfn>
     281          <list>
     282            <t>The length of time between the generation of a response and its expiration time. </t>
     283          </list>
     284        </t>
     285        <t>
     286          <iref item="fresh" />
     287          <x:dfn>fresh</x:dfn>
     288          <list>
     289            <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
     290          </list>
     291        </t>
     292        <t>
     293          <iref item="stale" />
     294          <x:dfn>stale</x:dfn>
     295          <list>
     296            <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
     297          </list>
     298        </t>
     299        <t>
     300          <iref item="validator" />
     301          <x:dfn>validator</x:dfn>
     302          <list>
     303            <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
     304              out whether a stored response is an equivalent copy of an entity.</t>
     305          </list>
     306        </t>
     307        <t anchor="shared.and.non-shared.caches">
     308          <iref item="validator" />
     309          <x:dfn>shared cache</x:dfn>
     310          <list>
     311            <t>A cache that is accessible to more than one user. A non-shared cache is
     312              dedicated to a single user.</t>
     313          </list>
     314        </t>
     315      </section>
     316
     317
     318      <section anchor="intro.requirements" title="Requirements">
     319        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
     320          NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
     321          described in <xref target="RFC2119" />.</t>
     322        <t>An implementation is not compliant if it fails to satisfy one or more of the &MUST;
     323          or &REQUIRED; level requirements for the protocols it implements. An implementation
     324          that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
     325          requirements for its protocols is said to be "unconditionally compliant"; one that
     326          satisfies all the &MUST; level requirements but not all the &SHOULD; level
     327          requirements for its protocols is said to be "conditionally compliant."</t>
     328      </section>
     329
    426330
    427331<section title="Syntax Notation" anchor="notation">
     
    480384
    481385</section>
    482 
    483386</section>
    484387
    485 
    486 <section title="Overview" anchor="caching.overview">
    487 <section title="Cache Correctness" anchor="cache.correctness">
    488 <t>
    489    A correct cache &MUST; respond to a request with the most up-to-date
    490    response held by the cache that is appropriate to the request (see
    491    Sections <xref target="disambiguating.expiration.values" format="counter"/>,
    492    <xref target="disambiguating.multiple.responses" format="counter"/>,
    493    and <xref target="cache.replacement" format="counter"/>) which meets one of the following
    494    conditions:
    495   <list style="numbers">
    496       <t>It has been checked for equivalence with what the origin server
    497          would have returned by revalidating the response with the
    498          origin server (<xref target="validation.model"/>);</t>
    499 
    500       <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
    501          this means it meets the least restrictive freshness requirement
    502          of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
    503          the origin server so specifies, it is the freshness requirement
    504          of the origin server alone.
    505 
    506          If a stored response is not "fresh enough" by the most
    507          restrictive freshness requirement of both the client and the
    508          origin server, in carefully considered circumstances the cache
    509          &MAY; still return the response with the appropriate Warning
    510          header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
    511          and <xref target="header.warning" format="counter"/>), unless such a response
    512          is prohibited (e.g., by a "no-store" cache-directive, or by a
    513          "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
    514 
    515       <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
    516          or error (4xx or 5xx) response message.</t>
    517   </list>
    518 </t>
    519 <t>
    520    If the cache can not communicate with the origin server, then a
    521    correct cache &SHOULD; respond as above if the response can be
    522    correctly served from the cache; if not it &MUST; return an error or
    523    warning indicating that there was a communication failure.
    524 </t>
    525 <t>
    526    If a cache receives a response (either an entire response, or a 304
    527    (Not Modified) response) that it would normally forward to the
    528    requesting client, and the received response is no longer fresh, the
    529    cache &SHOULD; forward it to the requesting client without adding a new
    530    Warning (but without removing any existing Warning headers). A cache
    531    &SHOULD-NOT;  attempt to revalidate a response simply because that
    532    response became stale in transit; this might lead to an infinite
    533    loop. A user agent that receives a stale response without a Warning
    534    &MAY; display a warning indication to the user.
    535 </t>
    536 </section>
    537 
    538 <section title="Warnings" anchor="warnings">
    539 <t>
    540    Whenever a cache returns a response that is neither first-hand nor
    541    "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
    542    &MUST; attach a warning to that effect, using a Warning general-header.
    543    The Warning header and the currently defined warnings are described
    544    in <xref target="header.warning"/>. The warning allows clients to take appropriate
    545    action.
    546 </t>
    547 <t>
    548    Warnings &MAY; be used for other purposes, both cache-related and
    549    otherwise. The use of a warning, rather than an error status code,
    550    distinguish these responses from true failures.
    551 </t>
    552 <t>
    553    Warnings are assigned three digit warn-codes. The first digit
    554    indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
    555    stored cache entry after a successful revalidation:
    556 </t>
    557 <t>
    558   <list style="hanging">
    559     <t hangText="1xx">Warnings that describe the freshness or revalidation status of
    560      the response, and so &MUST; be deleted after a successful
    561      revalidation. 1xx warn-codes &MAY; be generated by a cache only when
    562      validating a cached entry. It &MUST-NOT; be generated by clients.</t>
    563 
    564     <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
    565      headers that is not rectified by a revalidation (for example, a
    566      lossy compression of the entity bodies) and which &MUST-NOT; be
    567      deleted after a successful revalidation.</t>
    568     </list>
    569 </t>
    570 <t>
    571    See <xref target="header.warning"/> for the definitions of the codes themselves.
    572 </t>
    573 <t>
    574    HTTP/1.0 caches will cache all Warnings in responses, without
    575    deleting the ones in the first category. Warnings in responses that
    576    are passed to HTTP/1.0 caches carry an extra warning-date field,
    577    which prevents a future HTTP/1.1 recipient from believing an
    578    erroneously cached Warning.
    579 </t>
    580 <t>
    581    Warnings also carry a warning text. The text &MAY; be in any
    582    appropriate natural language (perhaps based on the client's Accept
    583    headers), and include an &OPTIONAL; indication of what character set is
    584    used.
    585 </t>
    586 <t>
    587    Multiple warnings &MAY; be attached to a response (either by the origin
    588    server or by a cache), including multiple warnings with the same code
    589    number. For example, a server might provide the same warning with
    590    texts in both English and Basque.
    591 </t>
    592 <t>
    593    When multiple warnings are attached to a response, it might not be
    594    practical or reasonable to display all of them to the user. This
    595    version of HTTP does not specify strict priority rules for deciding
    596    which warnings to display and in what order, but does suggest some
    597    heuristics.
    598 </t>
    599 </section>
    600 
    601 <section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
    602 <t>
    603    The basic cache mechanisms in HTTP/1.1 (server-specified expiration
    604    times and validators) are implicit directives to caches. In some
    605    cases, a server or client might need to provide explicit directives
    606    to the HTTP caches. We use the Cache-Control header for this purpose.
    607 </t>
    608 <t>
    609    The Cache-Control header allows a client or server to transmit a
    610    variety of directives in either requests or responses. These
    611    directives typically override the default caching algorithms. As a
    612    general rule, if there is any apparent conflict between header
    613    values, the most restrictive interpretation is applied (that is, the
    614    one that is most likely to preserve semantic transparency). However,
    615    in some cases, cache-control directives are explicitly specified as
    616    weakening the approximation of semantic transparency (for example,
    617    "max-stale" or "public").
    618 </t>
    619 <t>
    620    The cache-control directives are described in detail in <xref target="header.cache-control"/>.
    621 </t>
    622 </section>
    623 
    624 <section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
    625 <t>
    626    Many user agents make it possible for users to override the basic
    627    caching mechanisms. For example, the user agent might allow the user
    628    to specify that cached entities (even explicitly stale ones) are
    629    never validated. Or the user agent might habitually add "Cache-Control:
    630    max-stale=3600" to every request. The user agent &SHOULD-NOT;
    631    default to either non-transparent behavior, or behavior that results
    632    in abnormally ineffective caching, but &MAY; be explicitly configured
    633    to do so by an explicit action of the user.
    634 </t>
    635 <t>
    636    If the user has overridden the basic caching mechanisms, the user
    637    agent &SHOULD; explicitly indicate to the user whenever this results in
    638    the display of information that might not meet the server's
    639    transparency requirements (in particular, if the displayed entity is
    640    known to be stale). Since the protocol normally allows the user agent
    641    to determine if responses are stale or not, this indication need only
    642    be displayed when this actually happens. The indication need not be a
    643    dialog box; it could be an icon (for example, a picture of a rotting
    644    fish) or some other indicator.
    645 </t>
    646 <t>
    647    If the user has overridden the caching mechanisms in a way that would
    648    abnormally reduce the effectiveness of caches, the user agent &SHOULD;
    649    continually indicate this state to the user (for example, by a
    650    display of a picture of currency in flames) so that the user does not
    651    inadvertently consume excess resources or suffer from excessive
    652    latency.
    653 </t>
    654 </section>
    655 
    656 <section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
    657 <t>
    658    In some cases, the operator of a cache &MAY; choose to configure it to
    659    return stale responses even when not requested by clients. This
    660    decision ought not be made lightly, but may be necessary for reasons
    661    of availability or performance, especially when the cache is poorly
    662    connected to the origin server. Whenever a cache returns a stale
    663    response, it &MUST; mark it as such (using a Warning header) enabling
    664    the client software to alert the user that there might be a potential
    665    problem.
    666 </t>
    667 <t>
    668    It also allows the user agent to take steps to obtain a first-hand or
    669    fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
    670    response if the client explicitly requests a first-hand or fresh one,
    671    unless it is impossible to comply for technical or policy reasons.
    672 </t>
    673 </section>
    674 
    675 <section title="Client-controlled Behavior" anchor="client-controlled.behavior">
    676 <t>
    677    While the origin server (and to a lesser extent, intermediate caches,
    678    by their contribution to the age of a response) are the primary
    679    source of expiration information, in some cases the client might need
    680    to control a cache's decision about whether to return a cached
    681    response without validating it. Clients do this using several
    682    directives of the Cache-Control header.
    683 </t>
    684 <t>
    685    A client's request &MAY; specify the maximum age it is willing to
    686    accept of an unvalidated response; specifying a value of zero forces
    687    the cache(s) to revalidate all responses. A client &MAY; also specify
    688    the minimum time remaining before a response expires. Both of these
    689    options increase constraints on the behavior of caches, and so cannot
    690    further relax the cache's approximation of semantic transparency.
    691 </t>
    692 <t>
    693    A client &MAY; also specify that it will accept stale responses, up to
    694    some maximum amount of staleness. This loosens the constraints on the
    695    caches, and so might violate the origin server's specified
    696    constraints on semantic transparency, but might be necessary to
    697    support disconnected operation, or high availability in the face of
    698    poor connectivity.
    699 </t>
    700 </section>
    701 </section>
    702 
    703 <section title="Expiration Model" anchor="expiration.model">
    704 
    705 <section title="Server-Specified Expiration" anchor="server-specified.expiration">
    706 <t>
    707    HTTP caching works best when caches can entirely avoid making
    708    requests to the origin server. The primary mechanism for avoiding
    709    requests is for an origin server to provide an explicit expiration
    710    time in the future, indicating that a response &MAY; be used to satisfy
    711    subsequent requests. In other words, a cache can return a fresh
    712    response without first contacting the server.
    713 </t>
    714 <t>
    715    Our expectation is that servers will assign future explicit
    716    expiration times to responses in the belief that the entity is not
    717    likely to change, in a semantically significant way, before the
    718    expiration time is reached. This normally preserves semantic
    719    transparency, as long as the server's expiration times are carefully
    720    chosen.
    721 </t>
    722 <t>
    723    The expiration mechanism applies only to responses taken from a cache
    724    and not to first-hand responses forwarded immediately to the
    725    requesting client.
    726 </t>
    727 <t>
    728    If an origin server wishes to force a semantically transparent cache
    729    to validate every request, it &MAY; assign an explicit expiration time
    730    in the past. This means that the response is always stale, and so the
    731    cache &SHOULD; validate it before using it for subsequent requests. See
    732    <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
    733 </t>
    734 <t>
    735    If an origin server wishes to force any HTTP/1.1 cache, no matter how
    736    it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
    737    cache-control directive (see <xref target="header.cache-control"/>).
    738 </t>
    739 <t>
    740    Servers specify explicit expiration times using either the Expires
    741    header, or the max-age directive of the Cache-Control header.
    742 </t>
    743 <t>
    744    An expiration time cannot be used to force a user agent to refresh
    745    its display or reload a resource; its semantics apply only to caching
    746    mechanisms, and such mechanisms need only check a resource's
    747    expiration status when a new request for that resource is initiated.
    748    See <xref target="history.lists"/> for an explanation of the difference between caches
    749    and history mechanisms.
    750 </t>
    751 </section>
    752 
    753 <section title="Heuristic Expiration" anchor="heuristic.expiration">
    754 <t>
    755    Since origin servers do not always provide explicit expiration times,
    756    HTTP caches typically assign heuristic expiration times, employing
    757    algorithms that use other header values (such as the Last-Modified
    758    time) to estimate a plausible expiration time. The HTTP/1.1
    759    specification does not provide specific algorithms, but does impose
    760    worst-case constraints on their results. Since heuristic expiration
    761    times might compromise semantic transparency, they ought to be used
    762    cautiously, and we encourage origin servers to provide explicit
    763    expiration times as much as possible.
    764 </t>
    765 </section>
    766 
    767 <section title="Age Calculations" anchor="age.calculations">
    768 <t>
    769    In order to know if a cached entry is fresh, a cache needs to know if
    770    its age exceeds its freshness lifetime. We discuss how to calculate
    771    the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
    772    the age of a response or cache entry.
    773 </t>
    774 <t>
    775    In this discussion, we use the term "now" to mean "the current value
    776    of the clock at the host performing the calculation." Hosts that use
    777    HTTP, but especially hosts running origin servers and caches, &SHOULD;
    778    use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
    779    a globally accurate time standard.
    780 </t>
    781 <t>
    782    HTTP/1.1 requires origin servers to send a Date header, if possible,
    783    with every response, giving the time at which the response was
    784    generated (see &header-date;). We use the term "date_value" to denote
    785    the value of the Date header, in a form appropriate for arithmetic
    786    operations.
    787 </t>
    788 <t>
    789    HTTP/1.1 uses the Age response-header to convey the estimated age of
    790    the response message when obtained from a cache. The Age field value
    791    is the cache's estimate of the amount of time since the response was
    792    generated or revalidated by the origin server.
    793 </t>
    794 <t>
    795    In essence, the Age value is the sum of the time that the response
    796    has been resident in each of the caches along the path from the
    797    origin server, plus the amount of time it has been in transit along
    798    network paths.
    799 </t>
    800 <t>
    801    We use the term "age_value" to denote the value of the Age header, in
    802    a form appropriate for arithmetic operations.
    803 </t>
    804 <t>
    805    A response's age can be calculated in two entirely independent ways:
    806   <list style="numbers">
    807       <t>now minus date_value, if the local clock is reasonably well
    808          synchronized to the origin server's clock. If the result is
    809          negative, the result is replaced by zero.</t>
    810 
    811       <t>age_value, if all of the caches along the response path
    812          implement HTTP/1.1.</t>
    813   </list>
    814 </t>
    815 <t>
    816    Given that we have two independent ways to compute the age of a
    817    response when it is received, we can combine these as
    818 </t>
    819 <figure><artwork type="code">
     388    <section anchor="caching.overview" title="Cache Operation">
     389
     390      <section anchor="response.cacheability" title="Response Cacheability">
     391        <t>A cache &MUST-NOT; store a response to any request, unless: <list style="symbols">
     392                <t>The request method is defined as being cacheable, and</t>
     393            <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
     394              appear in request or response headers, and</t>
     395            <t>the "private" cache response directive (see <xref target="header.cache-control" />
     396            does not appear in the response, if the cache is shared, and</t>
     397            <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
     398             the cache is shared (unless the "public" directive is present; see <xref
     399                target="header.cache-control" />), and</t>           
     400            <t>the cache understands partial responses, if the response is partial or incomplete
     401              (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
     402          </list>
     403        </t>
     404        <t>Note that in normal operation, most caches will not store a response that has neither a
     405          cache validator nor an explicit expiration time, as such responses are not usually
     406          useful to store. However, caches are not prohibited from storing such responses.</t>
     407
     408        <section anchor="errors.or.incomplete.response.cache.behavior"
     409          title="Storing Partial and Incomplete Responses">
     410          <t>A cache that receives an incomplete response (for example, with fewer bytes of data
     411            than specified in a Content-Length header) can store the response, but &MUST;
     412            treat it as a partial response &partial;. Partial responses
     413            can be combined as described in &combining-byte-ranges;; the result might be a
     414            full response or might still be partial. A cache &MUST-NOT; return a partial
     415            response to a client without explicitly marking it as such using the 206 (Partial
     416            Content) status code.</t>
     417          <t>A cache that does not support the Range and Content-Range headers &MUST-NOT; store
     418            incomplete or partial responses.</t>
     419        </section>
     420
     421      </section>
     422
     423
     424      <section anchor="constructing.responses.from.caches"
     425        title="Constructing Responses from Caches">
     426        <t>For a presented request, a cache &MUST-NOT; return a stored response, unless:
     427          <list style="symbols">
     428            <t>The presented Request-URI and that of the stored response match (see
     429              <cref>TBD</cref>), and</t>
     430            <t>the request method associated with the stored response allows it to be
     431              used for the presented request, and</t>
     432            <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
     433                target="caching.negotiated.responses" />), and</t>
     434            <t>the presented request and stored response are free from directives that would prevent
     435              its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"
     436             />), and</t>
     437            <t>the stored response is either:
     438            <list style="symbols">
     439                <t>fresh (see <xref target="expiration.model" />), or</t>
     440                <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
     441                <t>successfully validated (see <xref target="validation.model" />).</t>
     442            </list></t>
     443          </list>
     444        </t>
     445        <t><cref>TODO: define method cacheability for GET, HEAD and POST in p2-semantics.</cref></t>
     446        <t>When a stored response is used to satisfy a request, caches &MUST; include a
     447          single Age header field <xref target="header.age" /> in the response with a value equal to the stored response's
     448          current_age; see <xref target="age.calculations" />.
     449                <cref>DISCUSS: this currently includes successfully validated responses.</cref></t>
     450        <t>Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
     451          the origin server; i.e., A cache must not reply to such a request before having forwarded the request and having received a
     452   corresponding response.</t>
     453                <t>Also, note that unsafe requests might invalidate already stored responses; see
     454            <xref target="invalidation.after.updates.or.deletions" />.</t>
     455        <t>Caches &MUST; use the most recent response (as determined by the Date header) when
     456          more than one suitable response is stored. They can also forward a request with
     457          "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
     458          use.</t>
     459        <t>
     460          <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in
     461            p1</cref>
     462        </t>
     463      </section>
     464
     465
     466
     467
     468      <section anchor="expiration.model" title="Freshness Model">
     469
     470        <t>When a response is "fresh" in the cache, it can be used to satisfy subsequent
     471          requests without contacting the origin server, thereby improving efficiency.</t>
     472        <t>The primary mechanism for determining freshness is for an origin server to provide an
     473          explicit expiration time in the future, using either the Expires header (<xref
     474            target="header.expires" />) or the max-age response cache directive (<xref
     475            target="cache-response-directive" />). Generally, origin servers will assign future
     476          explicit expiration times to responses in the belief that the entity is not likely to
     477          change in a semantically significant way before the expiration time is reached.</t>
     478        <t>If an origin server wishes to force a cache to validate every request, it can
     479          assign an explicit expiration time in the past. This means that the response is always
     480          stale, so that caches should validate it before using it for subsequent requests. <cref>This wording may cause confusion, because the response may still be served stale.</cref></t>
     481        <t>Since origin servers do not always provide explicit expiration times, HTTP caches may
     482          also assign heuristic expiration times when they are not specified, employing algorithms that
     483          use other header values (such as the Last-Modified time) to estimate a plausible
     484          expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
     485          impose worst-case constraints on their results.</t>
     486        <t>The calculation to determine if a response is fresh is:</t>
     487        <figure>
     488          <artwork type="code">
     489   response_is_fresh = (freshness_lifetime &gt; current_age)
     490</artwork>
     491        </figure>
     492
     493        <t>The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
     494          the current_age is defined in <xref target="age.calculations" />.</t>
     495        <t>Additionally, clients may need to influence freshness calculation. They can do this using
     496          several request cache directives, with the effect of either increasing or loosening
     497          constraints on freshness. See <xref target="cache-request-directive" />.</t>
     498        <t>
     499          <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
     500            freshness.</cref>
     501        </t>
     502        <t>Note that freshness applies only to cache operation; it cannot be used to force a user agent
     503        to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
     504          the difference between caches and history mechanisms.</t>
     505
     506        <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
     507          <t>A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
     508            response by using the first match of: <list style="symbols">
     509              <t>If the cache is shared and the s-maxage response cache directive (<xref
     510                  target="cache-response-directive" />) is present, use its value, or</t>
     511              <t>If the max-age response cache directive (<xref target="cache-response-directive"
     512                 />) is present, use its value, or</t>
     513              <t>If the Expires response header (<xref target="header.expires" />) is present, use
     514                its value minus the value of the Date response header, or</t>
     515              <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
     516                may be used; see <xref target="heuristic.freshness" />.</t>
     517            </list>
     518          </t>
     519          <t>Note that this calculation is not vulnerable to clock skew, since all of the
     520            information comes from the origin server.</t>
     521
     522          <section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
     523            <t>If no explicit expiration time is present in a stored response that has a status code
     524              of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
     525              calculated. Heuristics &MUST-NOT; be used for other response status codes. </t>
     526            <t> When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
     527              attach a Warning header with a 113 warn-code to the response if its current_age is
     528              more than 24 hours and such a warning is not already present.</t>
     529            <t>Also, if the response has a Last-Modified header (&header-last-modified;), the
     530              heuristic expiration value &SHOULD; be no more than some fraction of the interval
     531              since that time. A typical setting of this fraction might be 10%.</t>
     532            <t>
     533              <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
     534            </t>
     535          </section>
     536        </section>
     537
     538        <section anchor="age.calculations" title="Calculating Age">
     539          <t>HTTP/1.1 uses the Age response-header to convey the estimated age of the response
     540            message when obtained from a cache. The Age field value is the cache's estimate of the
     541            amount of time since the response was generated or validated by the origin server. In
     542            essence, the Age value is the sum of the time that the response has been resident in
     543            each of the caches along the path from the origin server, plus the amount of time it has
     544            been in transit along network paths.</t>
     545          <t>The term "age_value" denotes the value of the Age header, in a form appropriate for
     546            arithmetic operations.</t>
     547          <t>HTTP/1.1 requires origin servers to send a Date header, if possible, with every
     548            response, giving the time at which the response was generated (see &header-date;).
     549            The term "date_value" denotes the value of the Date header, in a form appropriate for
     550            arithmetic operations.</t>
     551          <t>The term "now" means "the current value of the clock at the host performing the
     552            calculation." Hosts that use HTTP, but especially hosts running origin servers and
     553            caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
     554            synchronize their clocks to a globally accurate time standard.</t>
     555          <t>A response's age can be calculated in two entirely independent ways: <list
     556              style="numbers">
     557              <t>now minus date_value, if the local clock is reasonably well synchronized to the
     558                origin server's clock. If the result is negative, the result is replaced by zero.</t>
     559
     560              <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
     561            </list>
     562          </t>
     563          <t>These are combined as</t>
     564          <figure>
     565            <artwork type="code">
    820566    corrected_received_age = max(now - date_value, age_value)
    821 </artwork></figure>
    822 <t>
    823    and as long as we have either nearly synchronized clocks or all-HTTP/1.1
    824    paths, one gets a reliable (conservative) result.
    825 </t>
    826 <t>
    827    Because of network-imposed delays, some significant interval might
    828    pass between the time that a server generates a response and the time
    829    it is received at the next outbound cache or client. If uncorrected,
    830    this delay could result in improperly low ages.
    831 </t>
    832 <t>
    833    Because the request that resulted in the returned Age value must have
    834    been initiated prior to that Age value's generation, we can correct
    835    for delays imposed by the network by recording the time at which the
    836    request was initiated. Then, when an Age value is received, it &MUST;
    837    be interpreted relative to the time the request was initiated, not
    838    the time that the response was received. This algorithm results in
    839    conservative behavior no matter how much delay is experienced. So, we
    840    compute:
    841 </t>
    842 <figure><artwork type="code">
     567</artwork>
     568          </figure>
     569          <t>When an Age value is received, it &MUST; be interpreted relative to the time the
     570            request was initiated, not the time that the response was received.</t>
     571          <figure>
     572            <artwork type="code">
    843573   corrected_initial_age = corrected_received_age
    844574                         + (now - request_time)
    845 </artwork></figure>
    846 <t>
    847    where "request_time" is the time (according to the local clock) when
    848    the request that elicited this response was sent.
    849 </t>
    850 <t>
    851    Summary of age calculation algorithm, when a cache receives a
    852    response:
    853 </t>
    854 <figure><artwork type="code">
    855    /*
    856     * age_value
    857     *      is the value of Age: header received by the cache with
    858     *              this response.
    859     * date_value
    860     *      is the value of the origin server's Date: header
    861     * request_time
    862     *      is the (local) time when the cache made the request
    863     *              that resulted in this cached response
    864     * response_time
    865     *      is the (local) time when the cache received the
    866     *              response
    867     * now
    868     *      is the current (local) time
    869     */
     575</artwork>
     576          </figure>
     577          <t>where "request_time" is the time (according to the local clock) when the request that
     578            elicited this response was sent.</t>
     579          <t>The current_age of a stored response can then be calculated by adding the amount of
     580            time (in seconds) since the stored response was last validated by the origin server to
     581            the corrected_initial_age.</t>
     582          <t>In summary:</t>
     583          <figure>
     584            <artwork type="code">
     585   age_value     - Age header field-value received with the response
     586   date_value    - Date header field-value received with the response
     587   request_time  - local time when the cache made the request
     588                   resulting in the stored response
     589   response_time - local time when the cache received the response
     590   now           - current local time
    870591
    871592   apparent_age = max(0, response_time - date_value);
     
    875596   resident_time = now - response_time;
    876597   current_age   = corrected_initial_age + resident_time;
    877 </artwork></figure>
    878 <t>
    879    The current_age of a cache entry is calculated by adding the amount
    880    of time (in seconds) since the cache entry was last validated by the
    881    origin server to the corrected_initial_age. When a response is
    882    generated from a cache entry, the cache &MUST; include a single Age
    883    header field in the response with a value equal to the cache entry's
    884    current_age.
    885 </t>
    886 <t>
    887    The presence of an Age header field in a response implies that a
    888    response is not first-hand. However, the converse is not true, since
    889    the lack of an Age header field in a response does not imply that the
    890    response is first-hand unless all caches along the request path are
    891    compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
    892    the Age header field).
    893 </t>
    894 </section>
    895 
    896 <section title="Expiration Calculations" anchor="expiration.calculations">
    897 <t>
    898    In order to decide whether a response is fresh or stale, we need to
    899    compare its freshness lifetime to its age. The age is calculated as
    900    described in <xref target="age.calculations"/>; this section describes how to calculate
    901    the freshness lifetime, and to determine if a response has expired.
    902    In the discussion below, the values can be represented in any form
    903    appropriate for arithmetic operations.
    904 </t>
    905 <t>
    906    We use the term "expires_value" to denote the value of the Expires
    907    header. We use the term "max_age_value" to denote an appropriate
    908    value of the number of seconds carried by the "max-age" directive of
    909    the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
    910 </t>
    911 <t>
    912    The max-age directive takes priority over Expires, so if max-age is
    913    present in a response, the calculation is simply:
    914 </t>
    915 <figure><artwork type="code">
    916    freshness_lifetime = max_age_value
    917 </artwork></figure>
    918 <t>
    919    Otherwise, if Expires is present in the response, the calculation is:
    920 </t>
    921 <figure><artwork type="code">
    922    freshness_lifetime = expires_value - date_value
    923 </artwork></figure>
    924 <t>
    925    Note that neither of these calculations is vulnerable to clock skew,
    926    since all of the information comes from the origin server.
    927 </t>
    928 <t>
    929    If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
    930    (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
    931    does not include other restrictions on caching, the cache &MAY; compute
    932    a freshness lifetime using a heuristic. The cache &MUST; attach Warning
    933    113 to any response whose age is more than 24 hours if such warning
    934    has not already been added.
    935 </t>
    936 <t>
    937    Also, if the response does have a Last-Modified time, the heuristic
    938    expiration value &SHOULD; be no more than some fraction of the interval
    939    since that time. A typical setting of this fraction might be 10%.
    940 </t>
    941 <t>
    942    The calculation to determine if a response has expired is quite
    943    simple:
    944 </t>
    945 <figure><artwork type="code">
    946    response_is_fresh = (freshness_lifetime &gt; current_age)
    947 </artwork></figure>
    948 </section>
    949 
    950 <section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
    951 <t>
    952    Because expiration values are assigned optimistically, it is possible
    953    for two caches to contain fresh values for the same resource that are
    954    different.
    955 </t>
    956 <t>
    957    If a client performing a retrieval receives a non-first-hand response
    958    for a request that was already fresh in its own cache, and the Date
    959    header in its existing cache entry is newer than the Date on the new
    960    response, then the client &MAY; ignore the response. If so, it &MAY;
    961    retry the request with a "Cache-Control: max-age=0" directive (see
    962    <xref target="header.cache-control"/>), to force a check with the origin server.
    963 </t>
    964 <t>
    965    If a cache has two fresh responses for the same representation with
    966    different validators, it &MUST; use the one with the more recent Date
    967    header. This situation might arise because the cache is pooling
    968    responses from other caches, or because a client has asked for a
    969    reload or a revalidation of an apparently fresh cache entry.
    970 </t>
    971 </section>
    972 
    973 <section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
    974 <t>
    975    Because a client might be receiving responses via multiple paths, so
    976    that some responses flow through one set of caches and other
    977    responses flow through a different set of caches, a client might
    978    receive responses in an order different from that in which the origin
    979    server sent them. We would like the client to use the most recently
    980    generated response, even if older responses are still apparently
    981    fresh.
    982 </t>
    983 <t>
    984    Neither the entity tag nor the expiration value can impose an
    985    ordering on responses, since it is possible that a later response
    986    intentionally carries an earlier expiration time. The Date values are
    987    ordered to a granularity of one second.
    988 </t>
    989 <t>
    990    When a client tries to revalidate a cache entry, and the response it
    991    receives contains a Date header that appears to be older than the one
    992    for the existing entry, then the client &SHOULD; repeat the request
    993    unconditionally, and include
    994 </t>
    995 <figure><artwork type="example">
    996   Cache-Control: max-age=0
    997 </artwork></figure>
    998 <t>
    999    to force any intermediate caches to validate their copies directly
    1000    with the origin server, or
    1001 </t>
    1002 <figure><artwork type="example">
    1003   Cache-Control: no-cache
    1004 </artwork></figure>
    1005 <t>
    1006    to force any intermediate caches to obtain a new copy from the origin
    1007    server.
    1008 </t>
    1009 <t>
    1010    If the Date values are equal, then the client &MAY; use either response
    1011    (or &MAY;, if it is being extremely prudent, request a new response).
    1012    Servers &MUST-NOT; depend on clients being able to choose
    1013    deterministically between responses generated during the same second,
    1014    if their expiration times overlap.
    1015 </t>
    1016 </section>
    1017 </section>
    1018 
    1019 <section title="Validation Model" anchor="validation.model">
    1020 <t>
    1021    When a cache has a stale entry that it would like to use as a
    1022    response to a client's request, it first has to check with the origin
    1023    server (or possibly an intermediate cache with a fresh response) to
    1024    see if its cached entry is still usable. We call this "validating"
    1025    the cache entry.
    1026 </t>
    1027 <t>
    1028    HTTP's conditional request mechanism, defined in &conditional;, is
    1029    used to avoid retransmitting the response payload when the cached entry
    1030    is valid.  When a cached response includes one or more "cache validators,"
    1031    such as the field values of an ETag or Last-Modified header field, then
    1032    a validating GET request &SHOULD; be made conditional to those field values.
    1033    The server checks the conditional request's validator against the current
    1034    state of the requested resource and, if they match, the server responds
    1035    with a 304 (Not Modified) status code to indicate that the cached response
    1036    can be refreshed and reused without retransmitting the response payload.
    1037    If the validator does not match the current state of the requested
    1038    resource, then the server returns a full response, including payload,
    1039    so that the request can be satisfied and the cache entry supplanted
    1040    without the need for an additional network round-trip.
    1041 </t>
    1042 </section>
    1043 
    1044 <section title="Response Cacheability" anchor="response.cacheability">
    1045 <t>
    1046    Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
    1047    directive, a caching system &MAY; always store a successful response
    1048    (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
    1049    if it is fresh, and &MAY; return it after successful validation. If
    1050    there is neither a cache validator nor an explicit expiration time
    1051    associated with a response, we do not expect it to be cached, but
    1052    certain caches &MAY; violate this expectation (for example, when little
    1053    or no network connectivity is available). A client can usually detect
    1054    that such a response was taken from a cache by comparing the Date
    1055    header to the current time.
    1056   <list><t>
    1057       <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
    1058       without providing any Warning.
    1059   </t></list>
    1060 </t>
    1061 <t>
    1062    However, in some cases it might be inappropriate for a cache to
    1063    retain an entity, or to return it in response to a subsequent
    1064    request. This might be because absolute semantic transparency is
    1065    deemed necessary by the service author, or because of security or
    1066    privacy considerations. Certain cache-control directives are
    1067    therefore provided so that the server can indicate that certain
    1068    resource entities, or portions thereof, are not to be cached
    1069    regardless of other considerations.
    1070 </t>
    1071 <t>
    1072    Note that &header-authorization; normally prevents a shared cache from saving
    1073    and returning a response to a previous request if that request
    1074    included an Authorization header.
    1075 </t>
    1076 <t>
    1077    A response received with a status code of 200, 203, 206, 300, 301 or
    1078    410 &MAY; be stored by a cache and used in reply to a subsequent
    1079    request, subject to the expiration mechanism, unless a cache-control
    1080    directive prohibits caching. However, a cache that does not support
    1081    the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
    1082    Content) responses.
    1083 </t>
    1084 <t>
    1085    A response received with any other status code (e.g. status codes 302
    1086    and 307) &MUST-NOT; be returned in a reply to a subsequent request
    1087    unless there are cache-control directives or another header(s) that
    1088    explicitly allow it. For example, these include the following: an
    1089    Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
    1090    "proxy-revalidate", "public" or "private" cache-control
    1091    directive (<xref target="header.cache-control"/>).
    1092 </t>
    1093 </section>
    1094 
    1095 <section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
    1096 <t>
    1097    The purpose of an HTTP cache is to store information received in
    1098    response to requests for use in responding to future requests. In
    1099    many cases, a cache simply returns the appropriate parts of a
    1100    response to the requester. However, if the cache holds a cache entry
    1101    based on a previous response, it might have to combine parts of a new
    1102    response with what is held in the cache entry.
    1103 </t>
    1104 
    1105 <section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
    1106 <t>
    1107    For the purpose of defining the behavior of caches and non-caching
    1108    proxies, we divide HTTP headers into two categories:
    1109   <list style="symbols">
    1110       <t>End-to-end headers, which are  transmitted to the ultimate
    1111         recipient of a request or response. End-to-end headers in
    1112         responses &MUST; be stored as part of a cache entry and &MUST; be
    1113         transmitted in any response formed from a cache entry.</t>
    1114 
    1115       <t>Hop-by-hop headers, which are meaningful only for a single
    1116         transport-level connection, and are not stored by caches or
    1117         forwarded by proxies.</t>
    1118   </list>
    1119 </t>
    1120 <t>
    1121    The following HTTP/1.1 headers are hop-by-hop headers:
    1122   <list style="symbols">
    1123       <t>Connection</t>
    1124       <t>Keep-Alive</t>
    1125       <t>Proxy-Authenticate</t>
    1126       <t>Proxy-Authorization</t>
    1127       <t>TE</t>
    1128       <t>Trailer</t>
    1129       <t>Transfer-Encoding</t>
    1130       <t>Upgrade</t>
    1131   </list>
    1132 </t>
    1133 <t>
    1134    All other headers defined by HTTP/1.1 are end-to-end headers.
    1135 </t>
    1136 <t>
    1137    Other hop-by-hop headers &MUST; be listed in a Connection header
    1138    (&header-connection;).
    1139 </t>
    1140 </section>
    1141 
    1142 <section title="Non-modifiable Headers" anchor="non-modifiable.headers">
    1143 <t>
    1144    Some features of HTTP/1.1, such as Digest
    1145    Authentication, depend on the value of certain end-to-end headers. A
    1146    transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
    1147    definition of that header requires or specifically allows that.
    1148 </t>
    1149 <t>
    1150    A transparent proxy &MUST-NOT; modify any of the following fields in a
    1151    request or response, and it &MUST-NOT; add any of these fields if not
    1152    already present:
    1153   <list style="symbols">
    1154       <t>Content-Location</t>
    1155       <t>Content-MD5</t>
    1156       <t>ETag</t>
    1157       <t>Last-Modified</t>
    1158   </list>
    1159 </t>
    1160 <t>
    1161    A transparent proxy &MUST-NOT; modify any of the following fields in a
    1162    response:
    1163   <list style="symbols">
    1164     <t>Expires</t>
    1165   </list>
    1166 </t>
    1167 <t>
    1168    but it &MAY; add any of these fields if not already present. If an
    1169    Expires header is added, it &MUST; be given a field-value identical to
    1170    that of the Date header in that response.
    1171 </t>
    1172 <t>
    1173    A  proxy &MUST-NOT; modify or add any of the following fields in a
    1174    message that contains the no-transform cache-control directive, or in
    1175    any request:
    1176   <list style="symbols">
    1177     <t>Content-Encoding</t>
    1178     <t>Content-Range</t>
    1179     <t>Content-Type</t>
    1180   </list>
    1181 </t>
    1182 <t>
    1183    A non-transparent proxy &MAY; modify or add these fields to a message
    1184    that does not include no-transform, but if it does so, it &MUST; add a
    1185    Warning 214 (Transformation applied) if one does not already appear
    1186    in the message (see <xref target="header.warning"/>).
    1187   <list><t>
    1188       Warning: unnecessary modification of end-to-end headers might
    1189       cause authentication failures if stronger authentication
    1190       mechanisms are introduced in later versions of HTTP. Such
    1191       authentication mechanisms &MAY; rely on the values of header fields
    1192       not listed here.
    1193     </t></list>
    1194 </t>
    1195 <t>
    1196    The Content-Length field of a request or response is added or deleted
    1197    according to the rules in &message-length;. A transparent proxy &MUST;
    1198    preserve the entity-length (&entity-length;) of the entity-body,
    1199    although it &MAY; change the transfer-length (&message-length;).
    1200 </t>
    1201 </section>
    1202 
    1203 <section title="Combining Headers" anchor="combining.headers">
    1204 <t>
    1205    When a cache makes a validating request to a server, and the server
    1206    provides a 304 (Not Modified) response or a 206 (Partial Content)
    1207    response, the cache then constructs a response to send to the
    1208    requesting client.
    1209 </t>
    1210 <t>
    1211    If the status code is 304 (Not Modified), the cache uses the entity-body
    1212    stored in the cache entry as the entity-body of this outgoing
    1213    response. If the status code is 206 (Partial Content) and the ETag or
    1214    Last-Modified headers match exactly, the cache &MAY; combine the
    1215    contents stored in the cache entry with the new contents received in
    1216    the response and use the result as the entity-body of this outgoing
    1217    response, (see &combining-byte-ranges;).
    1218 </t>
    1219 <t>
    1220    The end-to-end headers stored in the cache entry are used for the
    1221    constructed response, except that
    1222   <list style="symbols">
    1223     <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
    1224       &MUST; be deleted from the cache entry and the forwarded response.</t>
    1225     <t>any stored Warning headers with warn-code 2xx &MUST; be retained
    1226         in the cache entry and the forwarded response.</t>
    1227     <t>any end-to-end headers provided in the 304 or 206 response &MUST;
    1228         replace the corresponding headers from the cache entry.</t>
    1229   </list>
    1230 </t>
    1231 <t>
    1232    Unless the cache decides to remove the cache entry, it &MUST; also
    1233    replace the end-to-end headers stored with the cache entry with
    1234    corresponding headers received in the incoming response, except for
    1235    Warning headers as described immediately above. If a header field-name
    1236    in the incoming response matches more than one header in the
    1237    cache entry, all such old headers &MUST; be replaced.
    1238 </t>
    1239 <t>
    1240    In other words, the set of end-to-end headers received in the
    1241    incoming response overrides all corresponding end-to-end headers
    1242    stored with the cache entry (except for stored Warning headers with
    1243    warn-code 1xx, which are deleted even if not overridden).
    1244   <list><t>
    1245       <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
    1246       Modified) or a 206 (Partial Content) response to update any header
    1247       associated with a previous response for the same entity or sub-ranges
    1248       thereof, although it might not always be meaningful or
    1249       correct to do so. This rule does not allow an origin server to use
    1250       a 304 (Not Modified) or a 206 (Partial Content) response to
    1251       entirely delete a header that it had provided with a previous
    1252       response.
    1253   </t></list>
    1254 </t>
    1255 </section>
    1256 
    1257 </section>
    1258 
    1259 <section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
    1260 <t>
    1261    Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
    1262    by the presence of a Vary header field in a response, alters the
    1263    conditions and procedure by which a cache can use the response for
    1264    subsequent requests. See <xref target="header.vary"/> for use of the Vary header
    1265    field by servers.
    1266 </t>
    1267 <t>
    1268    A server &SHOULD; use the Vary header field to inform a cache of what
    1269    request-header fields were used to select among multiple
    1270    representations of a cacheable response subject to server-driven
    1271    negotiation. The set of header fields named by the Vary field value
    1272    is known as the "selecting" request-headers.
    1273 </t>
    1274 <t>
    1275    When the cache receives a subsequent request whose request-target
    1276    specifies one or more cache entries including a Vary header field,
    1277    the cache &MUST-NOT; use such a cache entry to construct a response to
    1278    the new request unless all of the selecting request-headers present
    1279    in the new request match the corresponding stored request-headers in
    1280    the original request.
    1281 </t>
    1282 <t>
    1283    The selecting request-headers from two requests are defined to match
    1284    if and only if the selecting request-headers in the first request can
    1285    be transformed to the selecting request-headers in the second request
    1286    by adding or removing linear whitespace (LWS) at places where this
    1287    is allowed by the corresponding BNF, and/or combining multiple
    1288    message-header fields with the same field name following the rules
    1289    about message headers in &message-headers;.
    1290 </t>
    1291 <t>
    1292    A Vary header field-value of "*" always fails to match and subsequent
    1293    requests on that resource can only be properly interpreted by the
    1294    origin server.
    1295 </t>
    1296 <t>
    1297    If the selecting request header fields for the cached entry do not
    1298    match the selecting request header fields of the new request, then
    1299    the cache &MUST-NOT; use a cached entry to satisfy the request unless
    1300    it first relays the new request to the origin server in a conditional
    1301    request and the server responds with 304 (Not Modified), including an
    1302    entity tag or Content-Location that indicates the entity to be used.
    1303 </t>
    1304 <t>
    1305    If an entity tag was assigned to a cached representation, the
    1306    forwarded request &SHOULD; be conditional and include the entity tags
    1307    in an If-None-Match header field from all its cache entries for the
    1308    resource. This conveys to the server the set of entities currently
    1309    held by the cache, so that if any one of these entities matches the
    1310    requested entity, the server can use the ETag header field in its 304
    1311    (Not Modified) response to tell the cache which entry is appropriate.
    1312    If the entity-tag of the new response matches that of an existing
    1313    entry, the new response &SHOULD; be used to update the header fields of
    1314    the existing entry, and the result &MUST; be returned to the client.
    1315 </t>
    1316 <t>
    1317    If any of the existing cache entries contains only partial content
    1318    for the associated entity, its entity-tag &SHOULD-NOT;  be included in
    1319    the If-None-Match header field unless the request is for a range that
    1320    would be fully satisfied by that entry.
    1321 </t>
    1322 <t>
    1323    If a cache receives a successful response whose Content-Location
    1324    field matches that of an existing cache entry for the same request-target,
    1325    whose entity-tag differs from that of the existing entry, and
    1326    whose Date is more recent than that of the existing entry, the
    1327    existing entry &SHOULD-NOT;  be returned in response to future requests
    1328    and &SHOULD; be deleted from the cache.
    1329 </t>
    1330 </section>
    1331 
    1332 <section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
    1333 <t>
    1334    For reasons of security and privacy, it is necessary to make a
    1335    distinction between "shared" and "non-shared" caches. A non-shared
    1336    cache is one that is accessible only to a single user. Accessibility
    1337    in this case &SHOULD; be enforced by appropriate security mechanisms.
    1338    All other caches are considered to be "shared." Other sections of
    1339    this specification place certain constraints on the operation of
    1340    shared caches in order to prevent loss of privacy or failure of
    1341    access controls.
    1342 </t>
    1343 </section>
    1344 
    1345 <section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
    1346 <t>
    1347    A cache that receives an incomplete response (for example, with fewer
    1348    bytes of data than specified in a Content-Length header) &MAY; store
    1349    the response. However, the cache &MUST; treat this as a partial
    1350    response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
    1351    the result might be a full response or might still be
    1352    partial. A cache &MUST-NOT; return a partial response to a client
    1353    without explicitly marking it as such, using the 206 (Partial
    1354    Content) status code. A cache &MUST-NOT; return a partial response
    1355    using a status code of 200 (OK).
    1356 </t>
    1357 <t>
    1358    If a cache receives a 5xx response while attempting to revalidate an
    1359    entry, it &MAY; either forward this response to the requesting client,
    1360    or act as if the server failed to respond. In the latter case, it &MAY;
    1361    return a previously received response unless the cached entry
    1362    includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
    1363 </t>
    1364 </section>
    1365 
    1366 <section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
    1367 <t>
    1368    Unless the origin server explicitly prohibits the caching of their
    1369    responses, the application of GET and HEAD methods to any resources
    1370    &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
    1371    these responses are taken from a cache. They &MAY; still have side
    1372    effects, but a cache is not required to consider such side effects in
    1373    its caching decisions. Caches are always expected to observe an
    1374    origin server's explicit restrictions on caching.
    1375 </t>
    1376 <t>
    1377    We note one exception to this rule: since some applications have
    1378    traditionally used GET and HEAD requests with URLs containing a query part
    1379    to perform operations with significant side
    1380    effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
    1381    the server provides an explicit expiration time. This specifically
    1382    means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT;
    1383    be taken from a cache. See &safe-methods; for related information.
    1384 </t>
    1385 </section>
    1386 
    1387 <section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
    1388 <t>
    1389    The effect of certain methods performed on a resource at the origin
    1390    server might cause one or more existing cache entries to become non-transparently
    1391    invalid. That is, although they might continue to be
    1392    "fresh," they do not accurately reflect what the origin server would
    1393    return for a new request on that resource.
    1394 </t>
    1395 <t>
    1396    There is no way for HTTP to guarantee that all such
    1397    cache entries are marked invalid. For example, the request that
    1398    caused the change at the origin server might not have gone through
    1399    the proxy where a cache entry is stored. However, several rules help
    1400    reduce the likelihood of erroneous behavior.
    1401 </t>
    1402 <t>
    1403    In this section, the phrase "invalidate an entity" means that the
    1404    cache will either remove all instances of that entity from its
    1405    storage, or will mark these as "invalid" and in need of a mandatory
    1406    revalidation before they can be returned in response to a subsequent
    1407    request.
    1408 </t>
    1409 <t>
    1410    Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
    1411    either the entity referred to by the request-target, or by the Location
    1412    or Content-Location headers (if present). These methods are:
    1413   <list style="symbols">
    1414       <t>PUT</t>
    1415       <t>DELETE</t>
    1416       <t>POST</t>
    1417   </list>
    1418 </t> 
    1419 <t>
    1420    An invalidation based
    1421    on the URI in a Location or Content-Location header &MUST-NOT; be
    1422    performed if the host part of that URI differs from the host part
    1423    in the request-target. This helps prevent denial of service attacks.
    1424 </t>
    1425 <t>
    1426    A cache that passes through requests for methods it does not
    1427    understand &SHOULD; invalidate any entities referred to by the
    1428    request-target.
    1429 </t>
    1430 </section>
    1431 
    1432 <section title="Write-Through Mandatory" anchor="write-through.mandatory">
    1433 <t>
    1434    All methods that might be expected to cause modifications to the
    1435    origin server's resources &MUST; be written through to the origin
    1436    server. This currently includes all methods except for GET and HEAD.
    1437    A cache &MUST-NOT; reply to such a request from a client before having
    1438    transmitted the request to the inbound server, and having received a
    1439    corresponding response from the inbound server. This does not prevent
    1440    a proxy cache from sending a 100 (Continue) response before the
    1441    inbound server has sent its final reply.
    1442 </t>
    1443 <t>
    1444    The alternative (known as "write-back" or "copy-back" caching) is not
    1445    allowed in HTTP/1.1, due to the difficulty of providing consistent
    1446    updates and the problems arising from server, cache, or network
    1447    failure prior to write-back.
    1448 </t>
    1449 </section>
    1450 
    1451 <section title="Cache Replacement" anchor="cache.replacement">
    1452 <t>
    1453    If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
    1454    <xref target="disambiguating.expiration.values" format="counter"/>,
    1455    <xref target="disambiguating.multiple.responses" format="counter"/>
    1456    and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
    1457    response is received from a resource while any existing responses for
    1458    the same resource are cached, the cache &SHOULD; use the new response
    1459    to reply to the current request. It &MAY; insert it into cache storage
    1460    and &MAY;, if it meets all other requirements, use it to respond to any
    1461    future requests that would previously have caused the old response to
    1462    be returned. If it inserts the new response into cache storage  the
    1463    rules in <xref target="combining.headers"/> apply.
    1464   <list><t>
    1465       <x:h>Note:</x:h> a new response that has an older Date header value than
    1466       existing cached responses is not cacheable.
    1467   </t></list>
    1468 </t>
    1469 </section>
    1470 
    1471 <section title="History Lists" anchor="history.lists">
    1472 <t>
    1473    User agents often have history mechanisms, such as "Back" buttons and
    1474    history lists, which can be used to redisplay an entity retrieved
    1475    earlier in a session.
    1476 </t>
    1477 <t>
    1478    History mechanisms and caches are different. In particular history
    1479    mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
    1480    the current state of a resource. Rather, a history mechanism is meant
    1481    to show exactly what the user saw at the time when the resource was
    1482    retrieved.
    1483 </t>
    1484 <t>
    1485    By default, an expiration time does not apply to history mechanisms.
    1486    If the entity is still in storage, a history mechanism &SHOULD; display
    1487    it even if the entity has expired, unless the user has specifically
    1488    configured the agent to refresh expired history documents.
    1489 </t>
    1490 <t>
    1491    This is not to be construed to prohibit the history mechanism from
    1492    telling the user that a view might be stale.
    1493   <list><t>
    1494       <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
    1495       viewing stale resources, this will tend to force service authors
    1496       to avoid using HTTP expiration controls and cache controls when
    1497       they would otherwise like to. Service authors may consider it
    1498       important that users not be presented with error messages or
    1499       warning messages when they use navigation controls (such as BACK)
    1500       to view previously fetched resources. Even though sometimes such
    1501       resources ought not be cached, or ought to expire quickly, user
    1502       interface considerations may force service authors to resort to
    1503       other means of preventing caching (e.g. "once-only" URLs) in order
    1504       not to suffer the effects of improperly functioning history
    1505       mechanisms.
    1506   </t></list>
    1507 </t>
    1508 </section>
    1509 
    1510 <section title="Header Field Definitions" anchor="header.fields">
    1511 <t>
    1512    This section defines the syntax and semantics of HTTP/1.1 header fields
    1513    related to caching.
    1514 </t>
    1515 <t>
    1516    For entity-header fields, both sender and recipient refer to either the
    1517    client or the server, depending on who sends and who receives the entity.
    1518 </t>
    1519 
    1520 <section title="Age" anchor="header.age">
    1521   <iref primary="true" item="Age header" x:for-anchor=""/>
    1522   <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
    1523   <x:anchor-alias value="Age"/>
    1524   <x:anchor-alias value="Age-v"/>
    1525   <x:anchor-alias value="age-value"/>
    1526 <t>
    1527       The response-header field "Age" conveys the sender's estimate of the
    1528       amount of time since the response (or its revalidation) was
    1529       generated at the origin server. A cached response is "fresh" if
    1530       its age does not exceed its freshness lifetime. Age values are
    1531       calculated as specified in <xref target="age.calculations"/>.
    1532 </t>
     598</artwork>
     599          </figure>
     600        </section>
     601
     602        <section anchor="serving.stale.responses" title="Serving Stale Responses">
     603          <t>A "stale" response is one that either has explicit expiry information, or is allowed to
     604            have heuristic expiry calculated, but is not fresh according to the calculations in
     605              <xref target="expiration.model" />.</t>
     606          <t>Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
     607            in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
     608            "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
     609            "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"
     610             />). </t>
     611          <t>Caches &SHOULD-NOT; return stale responses unless they are
     612          disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
     613          or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />)..</t>
     614          <t>Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
     615              target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
     616              the cache is disconnected.</t>
     617          <t>If a cache receives a first-hand response (either an entire response, or a 304 (Not
     618            Modified) response) that it would normally forward to the requesting client, and the
     619            received response is no longer fresh, the cache &SHOULD; forward it to the
     620            requesting client without adding a new Warning (but without removing any existing
     621            Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
     622            that response became stale in transit.</t>
     623        </section>
     624      </section>
     625
     626
     627      <section anchor="validation.model" title="Validation Model">
     628        <t>Checking with the origin server to see if a stale or otherwise unusable cached response
     629        can be reused is called "validating" or "revalidating." Doing so potentially avoids
     630        the overhead of retransmitting the response body when the stored response is valid.</t>
     631        <t>HTTP's conditional request mechanism &conditional; is used for this purpose. When a stored
     632          response includes one or more validators, such as the field values of an ETag or
     633          Last-Modified header field, then a validating request &SHOULD; be made conditional
     634          to those field values.</t>
     635         <t>A 304 (Not Modified) response status code indicates that the stored
     636          response can be updated and reused; see <xref target="combining.headers"/>.</t>
     637        <t>If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the
     638        request and replace the stored response. <cref>Should there be a requirement here?</cref></t>
     639        <t>If a cache receives a 5xx response while attempting to validate a response, it &MAY;
     640          either forward this response to the requesting client, or act as if the server failed to
     641          respond. In the latter case, it &MAY; return a previously stored response (which &SHOULD; include the
     642          111 warn-code; see <xref target="header.warning"/>) unless the
     643          stored response includes the "must-revalidate" cache directive (see <xref
     644            target="serving.stale.responses" />).</t>
     645      </section>
     646
     647      <section anchor="invalidation.after.updates.or.deletions"
     648        title="Request Methods that Invalidate">
     649        <t>Because unsafe methods (&safe-methods;) have the potential for changing state on the
     650          origin server, intervening caches can use them to keep their contents
     651          up-to-date.</t>
     652        <t>The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
     653          as the Location and Content-Location headers (if present): <list style="symbols">
     654            <t>PUT</t>
     655            <t>DELETE</t>
     656            <t>POST</t>
     657          </list>
     658        </t>
     659        <t>An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
     660          be performed if the host part of that URI differs from the host part in the Request-URI.
     661          This helps prevent denial of service attacks.</t>
     662        <t>
     663          <cref>TODO: "host part" needs to be specified better.</cref>
     664        </t>
     665        <t>A cache that passes through requests for methods it does not understand &SHOULD;
     666          invalidate the Request-URI.</t>
     667        <t>Here, "invalidate" means that the cache will either remove all stored responses related
     668          to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
     669          before they can be returned in response to a subsequent request.</t>
     670        <t>Note that this does not guarantee that all appropriate responses are invalidated. For
     671          example, the request that caused the change at the origin server might not have gone
     672          through the cache where a response is stored.</t>
     673        <t>
     674          <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
     675        </t>
     676      </section>
     677
     678
     679
     680
     681
     682      <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
     683        <t>Use of server-driven content negotiation (&server-driven-negotiation;) alters
     684          the conditions under which a cache can use the response for subsequent
     685          requests.</t>
     686        <t>When a cache receives a request that can be satisfied by a stored response
     687          that includes a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that response unless
     688          all of the selecting request-headers in the presented request match the corresponding
     689          stored request-headers from the original request.</t>
     690        <t>The selecting request-headers from two requests are defined to match if and only if the
     691          selecting request-headers in the first request can be transformed to the selecting
     692          request-headers in the second request by adding or removing linear white space
     693          <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
     694          combining multiple message-header fields with the same field name following the rules
     695          about message headers in &message-headers;. <cref>DISCUSS: header-specific canonicalisation</cref></t>
     696        <t>A Vary header field-value of "*" always fails to match, and subsequent requests to that
     697          resource can only be properly interpreted by the origin server.</t>
     698        <t>If no stored response matches, the cache &MAY; forward the presented request to the origin
     699          server in a conditional request, and &SHOULD; include all ETags stored with
     700          potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
     701          includes an entity tag or Content-Location that indicates the entity to be used, that
     702          cached response &MUST; be used to satisfy the presented request, and &SHOULD;
     703          be used to update the corresponding stored response; see <xref target="combining.headers"/>.</t>
     704        <t>If any of the stored responses contains only partial content, its entity-tag &SHOULD-NOT;
     705          be included in the If-None-Match header field unless the request is for a range that would
     706          be fully satisfied by that stored response.</t>
     707        <t>If a cache receives a successful response whose Content-Location field matches that of an
     708          existing stored response for the same Request-URI, whose entity-tag differs from that of
     709          the existing stored response, and whose Date is more recent than that of the existing
     710          response, the existing response &SHOULD-NOT; be returned in response to future
     711          requests and &SHOULD; be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref></t>
     712      </section>
     713
     714
     715      <section anchor="combining.headers" title="Combining Responses">
     716        <t>When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
     717          it needs to update the stored response with the new one, so that the updated response can
     718          be sent to the client.</t>
     719        <t>If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
     720          the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
     721          Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
     722          the stored response with the updated entity-body received in the response and use the
     723          result as the updated entity-body (see &combining-byte-ranges;).</t>
     724        <t>The stored response headers are used for the updated response, except that <list
     725            style="symbols">
     726            <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
     727              &MUST; be deleted from the stored response and the forwarded response.</t>
     728            <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
     729              response and the forwarded response.</t>
     730            <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
     731              headers from the stored response.</t>
     732          </list>
     733        </t>
     734        <t>A cache &MUST; also replace any stored headers with corresponding headers received in the
     735          incoming response, except for Warning headers as described immediately above. If a header
     736          field-name in the incoming response matches more than one header in the stored response,
     737          all such old headers &MUST; be replaced. It &MAY; store the combined
     738        entity-body.</t>
     739        <t><cref>ISSUE: discuss how to handle HEAD updates</cref></t>
     740      </section>
     741
     742    </section>
     743
     744
     745
     746
     747    <section anchor="header.fields" title="Header Field Definitions">
     748      <t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
     749      <t>For entity-header fields, both sender and recipient refer to either the client or the
     750        server, depending on who sends and who receives the entity.</t>
     751
     752      <section anchor="header.age" title="Age">
     753        <iref item="Age header" primary="true" x:for-anchor="" />
     754        <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
     755        <x:anchor-alias value="Age"/>
     756        <x:anchor-alias value="Age-v"/>
     757        <x:anchor-alias value="age-value"/>
     758        <t>      The response-header field "Age" conveys the sender's estimate of the amount of time since
     759          the response (or its validation) was generated at the origin server. Age values are
     760          calculated as specified in <xref target="age.calculations" />.</t>
    1533761<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
    1534762  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
    1535763  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
    1536764</artwork></figure>
    1537 <t anchor="rule.delta-seconds">
    1538   <x:anchor-alias value="delta-seconds"/>
    1539       Age values are non-negative decimal integers, representing time in
    1540       seconds.
    1541 </t>
    1542 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
     765        <t anchor="rule.delta-seconds">
     766          <x:anchor-alias value="delta-seconds" /> Age field-values are non-negative decimal
     767          integers, representing time in seconds.</t>
     768        <figure>
     769          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
    1543770  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
    1544 </artwork></figure>
    1545 <t>
    1546       If a cache receives a value larger than the largest positive
    1547       integer it can represent, or if any of its age calculations
    1548       overflows, it &MUST; transmit an Age header with a value of
    1549       2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache &MUST;
    1550       include an Age header field in every response generated from its
    1551       own cache. Caches &SHOULD; use an arithmetic type of at least 31
    1552       bits of range.
    1553 </t>
    1554 </section>
    1555 
    1556 <section title="Cache-Control" anchor="header.cache-control">
    1557   <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
    1558   <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
    1559   <x:anchor-alias value="Cache-Control"/>
    1560   <x:anchor-alias value="Cache-Control-v"/>
    1561   <x:anchor-alias value="cache-directive"/>
    1562   <x:anchor-alias value="cache-extension"/>
    1563   <x:anchor-alias value="cache-request-directive"/>
    1564   <x:anchor-alias value="cache-response-directive"/>
    1565 <t>
    1566    The general-header field "Cache-Control" is used to specify directives
    1567    that &MUST; be obeyed by all caching mechanisms along the
    1568    request/response chain. The directives specify behavior intended to
    1569    prevent caches from adversely interfering with the request or
    1570    response. These directives typically override the default caching
    1571    algorithms. Cache directives are unidirectional in that the presence
    1572    of a directive in a request does not imply that the same directive is
    1573    to be given in the response.
    1574   <list><t>
    1575       Note that HTTP/1.0 caches might not implement Cache-Control and
    1576       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
    1577   </t></list>
    1578 </t>
    1579 <t>
    1580    Cache directives &MUST; be passed through by a proxy or gateway
    1581    application, regardless of their significance to that application,
    1582    since the directives might be applicable to all recipients along the
    1583    request/response chain. It is not possible to specify a cache-directive
    1584    for a specific cache.
    1585 </t>
    1586 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-directive"/><iref primary="true" item="Grammar" subitem="cache-request-directive"/><iref primary="true" item="Grammar" subitem="cache-response-directive"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
     771</artwork>
     772        </figure>
     773        <t>If a cache receives a value larger than the largest positive integer it can represent, or
     774          if any of its age calculations overflows, it &MUST; transmit an Age header with a
     775          field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
     776          of at least 31 bits of range.</t>
     777        <t>The presence of an Age header field in a response implies that a response is not
     778          first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
     779          Age header field.</t>
     780      </section>
     781
     782      <section anchor="header.cache-control" title="Cache-Control">
     783        <iref item="Cache-Control header" primary="true" x:for-anchor="" />
     784        <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
     785        <x:anchor-alias value="Cache-Control"/>
     786        <x:anchor-alias value="Cache-Control-v"/>
     787        <x:anchor-alias value="cache-directive"/>
     788        <x:anchor-alias value="cache-extension"/>
     789        <x:anchor-alias value="cache-request-directive"/>
     790        <x:anchor-alias value="cache-response-directive"/>
     791        <t>The general-header field "Cache-Control" is used to specify directives that &MUST; be
     792          obeyed by all caches along the request/response chain. The directives specify behavior
     793          intended to prevent caches from adversely interfering with the request or response. Cache
     794          directives are unidirectional in that the presence of a directive in a request does not
     795          imply that the same directive is to be given in the response. <list>
     796            <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
     797              Pragma: no-cache (see <xref target="header.pragma" />).</t>
     798          </list>
     799        </t>
     800        <t>Cache directives &MUST; be passed through by a proxy or gateway application,
     801          regardless of their significance to that application, since the directives might be
     802          applicable to all recipients along the request/response chain. It is not possible to
     803          target a directive to a specific cache.</t>
     804<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
    1587805  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
    1588806  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
     
    1591809     / <x:ref>cache-response-directive</x:ref>
    1592810
    1593   <x:ref>cache-request-directive</x:ref> =
    1594        "no-cache"                          ; <xref target="what.is.cacheable"/>
    1595      / "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
    1596      / "max-age" "=" <x:ref>delta-seconds</x:ref>         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
    1597      / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    1598      / "min-fresh" "=" <x:ref>delta-seconds</x:ref>       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    1599      / "no-transform"                      ; <xref target="no-transform.directive"/>
    1600      / "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
    1601      / <x:ref>cache-extension</x:ref>                     ; <xref target="cache.control.extensions"/>
    1602 
    1603   <x:ref>cache-response-directive</x:ref> =
    1604        "public"                               ; <xref target="what.is.cacheable"/>
    1605      / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
    1606      / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
    1607      / "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
    1608      / "no-transform"                         ; <xref target="no-transform.directive"/>
    1609      / "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
    1610      / "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
    1611      / "max-age" "=" <x:ref>delta-seconds</x:ref>            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    1612      / "s-maxage" "=" <x:ref>delta-seconds</x:ref>           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    1613      / <x:ref>cache-extension</x:ref>                        ; <xref target="cache.control.extensions"/>
    1614 
    1615811  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
    1616812</artwork></figure>
    1617 <t>
    1618    When a directive appears without any 1#field-name parameter, the
    1619    directive applies to the entire request or response. When such a
    1620    directive appears with a 1#field-name parameter, it applies only to
    1621    the named field or fields, and not to the rest of the request or
    1622    response. This mechanism supports extensibility; implementations of
    1623    future versions of HTTP might apply these directives to
    1624    header fields not defined in HTTP/1.1.
    1625 </t>
    1626 <t>
    1627    The cache-control directives can be broken down into these general
    1628    categories:
    1629   <list style="symbols">
    1630      <t>Restrictions on what are cacheable; these may only be imposed by
    1631         the origin server.</t>
    1632 
    1633      <t>Restrictions on what may be stored by a cache; these may be
    1634         imposed by either the origin server or the user agent.</t>
    1635 
    1636      <t>Modifications of the basic expiration mechanism; these may be
    1637         imposed by either the origin server or the user agent.</t>
    1638 
    1639      <t>Controls over cache revalidation and reload; these may only be
    1640         imposed by a user agent.</t>
    1641 
    1642      <t>Control over transformation of entities.</t>
    1643 
    1644      <t>Extensions to the caching system.</t>
    1645   </list>
    1646 </t>
    1647 
    1648 <section title="What is Cacheable" anchor="what.is.cacheable">
    1649 <t>
    1650    By default, a response is cacheable if the requirements of the
    1651    request method, request header fields, and the response status
    1652    indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
    1653    for cacheability. The following Cache-Control response directives
    1654    allow an origin server to override the default cacheability of a
    1655    response:
    1656 </t>
    1657 <t>
    1658   <iref item="Cache Directives" subitem="public" primary="true"/>
    1659   <iref item="public" subitem="Cache Directive" primary="true"/>
    1660    public
    1661   <list><t>
    1662       Indicates that the response &MAY; be cached by any cache, even if it
    1663       would normally be non-cacheable or cacheable only within a non-shared
    1664       cache. (See also Authorization, &header-authorization;, for
    1665       additional details.)
    1666   </t></list>
    1667 </t>
    1668 <t>
    1669   <iref item="Cache Directives" subitem="private" primary="true"/>
    1670   <iref item="private" subitem="Cache Directive" primary="true"/>
    1671    private
    1672   <list><t>
    1673       Indicates that all or part of the response message is intended for
    1674       a single user and &MUST-NOT; be cached by a shared cache. This
    1675       allows an origin server to state that the specified parts of the
    1676       response are intended for only one user and are not a valid
    1677       response for requests by other users. A private (non-shared) cache
    1678       &MAY; cache the response.
    1679     </t><t>
    1680        <x:h>Note:</x:h> This usage of the word private only controls where the
    1681        response may be cached, and cannot ensure the privacy of the
    1682        message content.
    1683   </t></list>
    1684 </t>
    1685 <t>
    1686   <iref item="Cache Directives" subitem="no-cache" primary="true"/>
    1687   <iref item="no-cache" subitem="Cache Directive" primary="true"/>
    1688    no-cache
    1689   <list><t>
    1690        If the no-cache directive does not specify a field-name, then a
    1691       cache &MUST-NOT; use the response to satisfy a subsequent request
    1692       without successful revalidation with the origin server. This
    1693       allows an origin server to prevent caching even by caches that
    1694       have been configured to return stale responses to client requests.
    1695     </t><t>
    1696       If the no-cache directive does specify one or more field-names,
    1697       then a cache &MAY; use the response to satisfy a subsequent request,
    1698       subject to any other restrictions on caching. However, the
    1699       specified field-name(s) &MUST-NOT; be sent in the response to a
    1700       subsequent request without successful revalidation with the origin
    1701       server. This allows an origin server to prevent the re-use of
    1702       certain header fields in a response, while still allowing caching
    1703       of the rest of the response.
    1704     <list><t>
    1705        <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
    1706        directive.
    1707     </t></list>
    1708   </t></list>
    1709 </t>
    1710 </section>
    1711 
    1712 <section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
    1713 <t>
    1714   <iref item="Cache Directives" subitem="no-store" primary="true"/>
    1715   <iref item="no-store" subitem="Cache Directive" primary="true"/>
    1716    no-store
    1717   <list><t>   
    1718       The purpose of the no-store directive is to prevent the
    1719       inadvertent release or retention of sensitive information (for
    1720       example, on backup tapes). The no-store directive applies to the
    1721       entire message, and &MAY; be sent either in a response or in a
    1722       request. If sent in a request, a cache &MUST-NOT; store any part of
    1723       either this request or any response to it. If sent in a response,
    1724       a cache &MUST-NOT; store any part of either this response or the
    1725       request that elicited it. This directive applies to both non-shared
    1726       and shared caches. "&MUST-NOT; store" in this context means
    1727       that the cache &MUST-NOT; intentionally store the information in
    1728       non-volatile storage, and &MUST; make a best-effort attempt to
    1729       remove the information from volatile storage as promptly as
    1730       possible after forwarding it.
    1731   </t><t>
    1732       Even when this directive is associated with a response, users
    1733       might explicitly store such a response outside of the caching
    1734       system (e.g., with a "Save As" dialog). History buffers &MAY; store
    1735       such responses as part of their normal operation.
    1736   </t><t>
    1737       The purpose of this directive is to meet the stated requirements
    1738       of certain users and service authors who are concerned about
    1739       accidental releases of information via unanticipated accesses to
    1740       cache data structures. While the use of this directive might
    1741       improve privacy in some cases, we caution that it is NOT in any
    1742       way a reliable or sufficient mechanism for ensuring privacy. In
    1743       particular, malicious or compromised caches might not recognize or
    1744       obey this directive, and communications networks might be
    1745       vulnerable to eavesdropping.
    1746   </t></list>
    1747 </t>
    1748 </section>
    1749 
    1750 <section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
    1751 <t>
    1752    The expiration time of an entity &MAY; be specified by the origin
    1753    server using the Expires header (see <xref target="header.expires"/>). Alternatively,
    1754    it &MAY; be specified using the max-age directive in a response. When
    1755    the max-age cache-control directive is present in a cached response,
    1756    the response is stale if its current age is greater than the age
    1757    value given (in seconds) at the time of a new request for that
    1758    resource. The max-age directive on a response implies that the
    1759    response is cacheable (i.e., "public") unless some other, more
    1760    restrictive cache directive is also present.
    1761 </t>
    1762 <t>
    1763    If a response includes both an Expires header and a max-age
    1764    directive, the max-age directive overrides the Expires header, even
    1765    if the Expires header is more restrictive. This rule allows an origin
    1766    server to provide, for a given response, a longer expiration time to
    1767    an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
    1768    useful if certain HTTP/1.0 caches improperly calculate ages or
    1769    expiration times, perhaps due to desynchronized clocks.
    1770 </t>
    1771 <t>
    1772    Many HTTP/1.0 cache implementations will treat an Expires value that
    1773    is less than or equal to the response Date value as being equivalent
    1774    to the Cache-Control response directive "no-cache". If an HTTP/1.1
    1775    cache receives such a response, and the response does not include a
    1776    Cache-Control header field, it &SHOULD; consider the response to be
    1777    non-cacheable in order to retain compatibility with HTTP/1.0 servers.
    1778   <list><t>
    1779        <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
    1780        cache control feature, such as the "private" directive, on a
    1781        network including older caches that do not understand that
    1782        feature. The origin server will need to combine the new feature
    1783        with an Expires field whose value is less than or equal to the
    1784        Date value. This will prevent older caches from improperly
    1785        caching the response.
    1786   </t></list>
    1787 </t>
    1788 <t>
    1789   <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
    1790   <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
    1791    s-maxage
    1792   <list><t>
    1793        If a response includes an s-maxage directive, then for a shared
    1794        cache (but not for a private cache), the maximum age specified by
    1795        this directive overrides the maximum age specified by either the
    1796        max-age directive or the Expires header. The s-maxage directive
    1797        also implies the semantics of the proxy-revalidate directive (see
    1798        <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
    1799        entry after it becomes stale to respond to a subsequent request
    1800        without first revalidating it with the origin server. The s-maxage
    1801        directive is always ignored by a private cache.
    1802   </t></list>
    1803 </t>
    1804 <t>
    1805    Note that most older caches, not compliant with this specification,
    1806    do not implement any cache-control directives. An origin server
    1807    wishing to use a cache-control directive that restricts, but does not
    1808    prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
    1809    requirement that the max-age directive overrides the Expires header,
    1810    and the fact that pre-HTTP/1.1-compliant caches do not observe the
    1811    max-age directive.
    1812 </t>
    1813 <t>
    1814    Other directives allow a user agent to modify the basic expiration
    1815    mechanism. These directives &MAY; be specified on a request:
    1816 </t>
    1817 <t>
    1818   <iref item="Cache Directives" subitem="max-age" primary="true"/>
    1819   <iref item="max-age" subitem="Cache Directive" primary="true"/>
    1820    max-age
    1821   <list><t>
    1822       Indicates that the client is willing to accept a response whose
    1823       age is no greater than the specified time in seconds. Unless max-stale
    1824       directive is also included, the client is not willing to
    1825       accept a stale response.
    1826   </t></list>
    1827 </t>
    1828 <t>
    1829   <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
    1830   <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
    1831    min-fresh
    1832   <list><t>
    1833       Indicates that the client is willing to accept a response whose
    1834       freshness lifetime is no less than its current age plus the
    1835       specified time in seconds. That is, the client wants a response
    1836       that will still be fresh for at least the specified number of
    1837       seconds.
    1838   </t></list>
    1839 </t>
    1840 <t>
    1841   <iref item="Cache Directives" subitem="max-stale" primary="true"/>
    1842   <iref item="max-stale" subitem="Cache Directive" primary="true"/>
    1843    max-stale
    1844   <list><t>
    1845       Indicates that the client is willing to accept a response that has
    1846       exceeded its expiration time. If max-stale is assigned a value,
    1847       then the client is willing to accept a response that has exceeded
    1848       its expiration time by no more than the specified number of
    1849       seconds. If no value is assigned to max-stale, then the client is
    1850       willing to accept a stale response of any age.
    1851   </t></list>
    1852 </t>
    1853 <t>
    1854    If a cache returns a stale response, either because of a max-stale
    1855    directive on a request, or because the cache is configured to
    1856    override the expiration time of a response, the cache &MUST; attach a
    1857    Warning header to the stale response, using Warning 110 (Response is
    1858    stale).
    1859 </t>
    1860 <t>
    1861    A cache &MAY; be configured to return stale responses without
    1862    validation, but only if this does not conflict with any "MUST"-level
    1863    requirements concerning cache validation (e.g., a "must-revalidate"
    1864    cache-control directive).
    1865 </t>
    1866 <t>
    1867    If both the new request and the cached entry include "max-age"
    1868    directives, then the lesser of the two values is used for determining
    1869    the freshness of the cached entry for that request.
    1870 </t>
    1871 </section>
    1872 
    1873 <section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
    1874 <t>
    1875    Sometimes a user agent might want or need to insist that a cache
    1876    revalidate its cache entry with the origin server (and not just with
    1877    the next cache along the path to the origin server), or to reload its
    1878    cache entry from the origin server. End-to-end revalidation might be
    1879    necessary if either the cache or the origin server has overestimated
    1880    the expiration time of the cached response. End-to-end reload may be
    1881    necessary if the cache entry has become corrupted for some reason.
    1882 </t>
    1883 <t>
    1884    End-to-end revalidation may be requested either when the client does
    1885    not have its own local cached copy, in which case we call it
    1886    "unspecified end-to-end revalidation", or when the client does have a
    1887    local cached copy, in which case we call it "specific end-to-end
    1888    revalidation."
    1889 </t>
    1890 <t>
    1891    The client can specify these three kinds of action using Cache-Control
    1892    request directives:
    1893 </t>
    1894 <t>
    1895    End-to-end reload
    1896   <list><t>
    1897       The request includes a "no-cache" cache-control directive or, for
    1898       compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
    1899       names &MUST-NOT; be included with the no-cache directive in a
    1900       request. The server &MUST-NOT; use a cached copy when responding to
    1901       such a request.
    1902   </t></list>
    1903 </t>
    1904 <t>
    1905    Specific end-to-end revalidation
    1906   <list><t>
    1907       The request includes a "max-age=0" cache-control directive, which
    1908       forces each cache along the path to the origin server to
    1909       revalidate its own entry, if any, with the next cache or server.
    1910       The initial request includes a cache-validating conditional with
    1911       the client's current validator.
    1912   </t></list>
    1913 </t>
    1914 <t>
    1915    Unspecified end-to-end revalidation
    1916   <list><t>
    1917       The request includes "max-age=0" cache-control directive, which
    1918       forces each cache along the path to the origin server to
    1919       revalidate its own entry, if any, with the next cache or server.
    1920       The initial request does not include a cache-validating
    1921       conditional; the first cache along the path (if any) that holds a
    1922       cache entry for this resource includes a cache-validating
    1923       conditional with its current validator.
    1924   </t></list>
    1925 </t>
    1926 <t>
    1927   <iref item="Cache Directives" subitem="max-age" primary="true"/>
    1928   <iref item="max-age" subitem="Cache Directive" primary="true"/>
    1929    max-age
    1930   <list><t>
    1931       When an intermediate cache is forced, by means of a max-age=0
    1932       directive, to revalidate its own cache entry, and the client has
    1933       supplied its own validator in the request, the supplied validator
    1934       might differ from the validator currently stored with the cache
    1935       entry. In this case, the cache &MAY; use either validator in making
    1936       its own request without affecting semantic transparency.
    1937   </t><t>
    1938       However, the choice of validator might affect performance. The
    1939       best approach is for the intermediate cache to use its own
    1940       validator when making its request. If the server replies with 304
    1941       (Not Modified), then the cache can return its now validated copy
    1942       to the client with a 200 (OK) response. If the server replies with
    1943       a new entity and cache validator, however, the intermediate cache
    1944       can compare the returned validator with the one provided in the
    1945       client's request, using the strong comparison function. If the
    1946       client's validator is equal to the origin server's, then the
    1947       intermediate cache simply returns 304 (Not Modified). Otherwise,
    1948       it returns the new entity with a 200 (OK) response.
    1949   </t><t>
    1950       If a request includes the no-cache directive, it &SHOULD-NOT;
    1951       include min-fresh, max-stale, or max-age.
    1952   </t></list>
    1953 </t>
    1954 <t>
    1955   <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
    1956   <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
    1957    only-if-cached
    1958   <list><t>
    1959       In some cases, such as times of extremely poor network
    1960       connectivity, a client may want a cache to return only those
    1961       responses that it currently has stored, and not to reload or
    1962       revalidate with the origin server. To do this, the client may
    1963       include the only-if-cached directive in a request. If it receives
    1964       this directive, a cache &SHOULD; either respond using a cached entry
    1965       that is consistent with the other constraints of the request, or
    1966       respond with a 504 (Gateway Timeout) status. However, if a group
    1967       of caches is being operated as a unified system with good internal
    1968       connectivity, such a request &MAY; be forwarded within that group of
    1969       caches.
    1970   </t></list>
    1971 </t>
    1972 <t>
    1973   <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
    1974   <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
    1975    must-revalidate
    1976   <list><t>
    1977       Because a cache &MAY; be configured to ignore a server's specified
    1978       expiration time, and because a client request &MAY; include a max-stale
    1979       directive (which has a similar effect), the protocol also
    1980       includes a mechanism for the origin server to require revalidation
    1981       of a cache entry on any subsequent use. When the must-revalidate
    1982       directive is present in a response received by a cache, that cache
    1983       &MUST-NOT; use the entry after it becomes stale to respond to a
    1984       subsequent request without first revalidating it with the origin
    1985       server. (I.e., the cache &MUST; do an end-to-end revalidation every
    1986       time, if, based solely on the origin server's Expires or max-age
    1987       value, the cached response is stale.)
    1988   </t><t>
    1989       The must-revalidate directive is necessary to support reliable
    1990       operation for certain protocol features. In all circumstances an
    1991       HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
    1992       particular, if the cache cannot reach the origin server for any
    1993       reason, it &MUST; generate a 504 (Gateway Timeout) response.
    1994   </t><t>
    1995       Servers &SHOULD; send the must-revalidate directive if and only if
    1996       failure to revalidate a request on the entity could result in
    1997       incorrect operation, such as a silently unexecuted financial
    1998       transaction. Recipients &MUST-NOT; take any automated action that
    1999       violates this directive, and &MUST-NOT; automatically provide an
    2000       unvalidated copy of the entity if revalidation fails.
    2001   </t><t>
    2002       Although this is not recommended, user agents operating under
    2003       severe connectivity constraints &MAY; violate this directive but, if
    2004       so, &MUST; explicitly warn the user that an unvalidated response has
    2005       been provided. The warning &MUST; be provided on each unvalidated
    2006       access, and &SHOULD; require explicit user confirmation.
    2007   </t></list>
    2008 </t>
    2009 <t>
    2010   <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
    2011   <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
    2012    proxy-revalidate
    2013   <list><t>
    2014       The proxy-revalidate directive has the same meaning as the must-revalidate
    2015       directive, except that it does not apply to non-shared
    2016       user agent caches. It can be used on a response to an
    2017       authenticated request to permit the user's cache to store and
    2018       later return the response without needing to revalidate it (since
    2019       it has already been authenticated once by that user), while still
    2020       requiring proxies that service many users to revalidate each time
    2021       (in order to make sure that each user has been authenticated).
    2022       Note that such authenticated responses also need the public cache
    2023       control directive in order to allow them to be cached at all.
    2024   </t></list>
    2025 </t>
    2026 </section>
    2027 
    2028 <section title="No-Transform Directive" anchor="no-transform.directive">
    2029 <t>
    2030   <iref item="Cache Directives" subitem="no-transform" primary="true"/>
    2031   <iref item="no-transform" subitem="Cache Directive" primary="true"/>
    2032    no-transform
    2033   <list><t>
    2034       Implementors of intermediate caches (proxies) have found it useful
    2035       to convert the media type of certain entity bodies. A non-transparent
    2036       proxy might, for example, convert between image
    2037       formats in order to save cache space or to reduce the amount of
    2038       traffic on a slow link.
    2039   </t><t>
    2040       Serious operational problems occur, however, when these
    2041       transformations are applied to entity bodies intended for certain
    2042       kinds of applications. For example, applications for medical
    2043       imaging, scientific data analysis and those using end-to-end
    2044       authentication, all depend on receiving an entity body that is bit
    2045       for bit identical to the original entity-body.
    2046   </t><t>
    2047       Therefore, if a message includes the no-transform directive, an
    2048       intermediate cache or proxy &MUST-NOT; change those headers that are
    2049       listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
    2050       directive. This implies that the cache or proxy &MUST-NOT; change
    2051       any aspect of the entity-body that is specified by these headers,
    2052       including the value of the entity-body itself.
    2053   </t></list>
    2054 </t>
    2055 </section>
    2056 
    2057 <section title="Cache Control Extensions" anchor="cache.control.extensions">
    2058 <t>
    2059    The Cache-Control header field can be extended through the use of one
    2060    or more cache-extension tokens, each with an optional assigned value.
    2061    Informational extensions (those which do not require a change in
    2062    cache behavior) &MAY; be added without changing the semantics of other
    2063    directives. Behavioral extensions are designed to work by acting as
    2064    modifiers to the existing base of cache directives. Both the new
    2065    directive and the standard directive are supplied, such that
    2066    applications which do not understand the new directive will default
    2067    to the behavior specified by the standard directive, and those that
    2068    understand the new directive will recognize it as modifying the
    2069    requirements associated with the standard directive. In this way,
    2070    extensions to the cache-control directives can be made without
    2071    requiring changes to the base protocol.
    2072 </t>
    2073 <t>
    2074    This extension mechanism depends on an HTTP cache obeying all of the
    2075    cache-control directives defined for its native HTTP-version, obeying
    2076    certain extensions, and ignoring all directives that it does not
    2077    understand.
    2078 </t>
    2079 <t>
    2080    For example, consider a hypothetical new response directive called
    2081    community which acts as a modifier to the private directive. We
    2082    define this new directive to mean that, in addition to any non-shared
    2083    cache, any cache which is shared only by members of the community
    2084    named within its value may cache the response. An origin server
    2085    wishing to allow the UCI community to use an otherwise private
    2086    response in their shared cache(s) could do so by including
    2087 </t>
    2088 <figure><artwork type="example">
     813
     814        <section anchor="cache-request-directive" title="Request Cache-Control Directives">
     815          <x:anchor-alias value="cache-request-directive" />
     816
     817          <figure>
     818            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
     819  <x:ref>cache-request-directive</x:ref> =
     820       "no-cache"
     821     / "no-store"
     822     / "max-age" "=" <x:ref>delta-seconds</x:ref>
     823     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
     824     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
     825     / "no-transform"
     826     / "only-if-cached"
     827     / <x:ref>cache-extension</x:ref>
     828</artwork>
     829          </figure>
     830
     831
     832          <t>
     833            <iref item="Cache Directives" primary="true" subitem="no-cache" />
     834            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
     835              <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
     836                used to satisfy the request without successful validation on the origin server. </t>
     837            </list>
     838          </t>
     839          <t>
     840            <iref item="Cache Directives" primary="true" subitem="no-store" />
     841            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
     842              <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
     843                of either this request or any response to it. This directive applies to both
     844                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
     845                cache &MUST-NOT; intentionally store the information in non-volatile storage,
     846                and &MUST; make a best-effort attempt to remove the information from volatile
     847                storage as promptly as possible after forwarding it.</t>
     848              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
     849                particular, malicious or compromised caches might not recognize or obey this
     850                directive, and communications networks may be vulnerable to eavesdropping.</t>
     851            </list>
     852          </t>
     853          <t>
     854            <iref item="Cache Directives" primary="true" subitem="max-age" />
     855            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
     856              <t>The max-age request directive indicates that the client is willing to accept a
     857                response whose age is no greater than the specified time in seconds. Unless
     858                max-stale directive is also included, the client is not willing to accept a stale
     859                response.</t>
     860            </list>
     861          </t>
     862          <t>
     863            <iref item="Cache Directives" primary="true" subitem="max-stale" />
     864            <iref item="max-stale" primary="true" subitem="Cache Directive" /> max-stale <list>
     865              <t>The max-stale request directive indicates that the client is willing to accept a
     866                response that has exceeded its expiration time. If max-stale is assigned a value,
     867                then the client is willing to accept a response that has exceeded its expiration
     868                time by no more than the specified number of seconds. If no value is assigned to
     869                max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
     870            </list>
     871          </t>
     872          <t>
     873            <iref item="Cache Directives" primary="true" subitem="min-fresh" />
     874            <iref item="min-fresh" primary="true" subitem="Cache Directive" /> min-fresh <list>
     875              <t>The min-fresh request directive indicates that the client is willing to accept a
     876                response whose freshness lifetime is no less than its current age plus the specified
     877                time in seconds. That is, the client wants a response that will still be fresh for
     878                at least the specified number of seconds.</t>
     879            </list>
     880          </t>
     881          <t>
     882            <iref item="Cache Directives" primary="true" subitem="no-transform" />
     883            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
     884              <t>The no-transform request directive indicates that an intermediate cache or proxy
     885                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
     886                headers, nor the request entity-body.</t>
     887            </list>
     888          </t>
     889
     890          <t>
     891            <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
     892            <iref item="only-if-cached" primary="true" subitem="Cache Directive" /> only-if-cached <list>
     893              <t>The only-if-cached request directive indicates that the client only wishes to
     894                return a stored response. If it receives this directive, a cache &SHOULD; either
     895                respond using a stored response that is consistent with the other constraints of the
     896                request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
     897                being operated as a unified system with good internal connectivity, such a request
     898                &MAY; be forwarded within that group of caches.</t>
     899            </list>
     900          </t>
     901        </section>
     902
     903        <section anchor="cache-response-directive" title="Response Cache-Control Directives">
     904          <x:anchor-alias value="cache-response-directive" />
     905
     906          <figure>
     907            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
     908  <x:ref>cache-response-directive</x:ref> =
     909       "public"
     910     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
     911     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
     912     / "no-store"
     913     / "no-transform"
     914     / "must-revalidate"
     915     / "proxy-revalidate"
     916     / "max-age" "=" <x:ref>delta-seconds</x:ref>
     917     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
     918     / <x:ref>cache-extension</x:ref>
     919</artwork>
     920          </figure>
     921
     922          <t>
     923            <iref item="Cache Directives" primary="true" subitem="public" />
     924            <iref item="public" primary="true" subitem="Cache Directive" /> public <list>
     925              <t>The public response directive indicates that the response &MAY; be cached, even
     926                if it would normally be non-cacheable or cacheable only within a non-shared cache.
     927                (See also Authorization, &header-authorization;, for additional details.) </t>
     928            </list>
     929          </t>
     930
     931          <t>
     932            <iref item="Cache Directives" primary="true" subitem="private" />
     933            <iref item="private" primary="true" subitem="Cache Directive" /> private <list>
     934              <t>The private response directive indicates that the response message is intended for
     935                a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
     936                cache &MAY; store the response.</t>
     937              <t>If the private response directive specifies one or more field-names, this
     938                requirement is limited to the field-values associated with the listed response
     939                headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
     940                cache, whereas the remainder of the response message &MAY; be.</t>
     941              <t>
     942                <x:h>Note:</x:h> This usage of the word private only controls where the response may
     943                be stored, and cannot ensure the privacy of the message content.</t>
     944            </list>
     945          </t>
     946          <t>
     947            <iref item="Cache Directives" primary="true" subitem="no-cache" />
     948            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
     949              <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
     950                satisfy a subsequent request without successful validation on the origin server.
     951                This allows an origin server to prevent caching even by caches that have been
     952                configured to return stale responses.</t>
     953              <t>If the no-cache response directive specifies one or more field-names, this
     954                requirement is limited to the field-values assosicated with the listed response
     955                headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
     956                to a subsequent request without successful validation on the origin server. This
     957                allows an origin server to prevent the re-use of certain header fields in a
     958                response, while still allowing caching of the rest of the response.</t>
     959              <t>
     960                <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
     961              </t>
     962            </list>
     963          </t>
     964
     965          <t>
     966            <iref item="Cache Directives" primary="true" subitem="no-store" />
     967            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
     968              <t>The no-store response directive indicates that a cache &MUST-NOT; store any
     969                part of either the immediate request or response. This directive applies to both
     970                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
     971                cache &MUST-NOT; intentionally store the information in non-volatile storage,
     972                and &MUST; make a best-effort attempt to remove the information from volatile
     973                storage as promptly as possible after forwarding it.</t>
     974              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
     975                particular, malicious or compromised caches might not recognize or obey this
     976                directive, and communications networks may be vulnerable to eavesdropping.</t>
     977            </list>
     978          </t>
     979          <t>
     980            <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
     981            <iref item="must-revalidate" primary="true" subitem="Cache Directive" /> must-revalidate <list>
     982              <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
     983               used to satisfy subsequent requests without successful validation on the origin server.</t>
     984              <t>The must-revalidate directive is necessary to support reliable operation for
     985                certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
     986                the must-revalidate directive; in particular, if the cache cannot reach the origin
     987                server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
     988              <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
     989                validate a request on the entity could result in incorrect operation, such as a
     990                silently unexecuted financial transaction.</t>
     991            </list>
     992          </t>
     993          <t>
     994            <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
     995            <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
     996            proxy-revalidate <list>
     997              <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
     998                response directive, except that it does not apply to non-shared caches.</t>
     999            </list>
     1000          </t>
     1001          <t>
     1002            <iref item="Cache Directives" primary="true" subitem="max-age" />
     1003            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
     1004              <t>The max-age response directive indicates that response is to be considered stale
     1005                after its age is greater than the specified number of seconds.</t>
     1006            </list>
     1007          </t>
     1008          <t>
     1009            <iref item="Cache Directives" primary="true" subitem="s-maxage" />
     1010            <iref item="s-maxage" primary="true" subitem="Cache Directive" /> s-maxage <list>
     1011              <t>The s-maxage response directive indicates that, in shared caches, the maximum age
     1012                specified by this directive overrides the maximum age specified by either the
     1013                max-age directive or the Expires header. The s-maxage directive also implies the
     1014                semantics of the proxy-revalidate response directive.</t>
     1015            </list>
     1016          </t>
     1017          <t>
     1018            <iref item="Cache Directives" primary="true" subitem="no-transform" />
     1019            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
     1020              <t>The no-transform response directive indicates that an intermediate cache or proxy
     1021                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
     1022                headers, nor the response entity-body.</t>
     1023            </list>
     1024          </t>
     1025
     1026        </section>
     1027
     1028        <section anchor="cache.control.extensions" title="Cache Control Extensions">
     1029          <t>The Cache-Control header field can be extended through the use of one or more
     1030            cache-extension tokens, each with an optional value. Informational extensions (those
     1031            that do not require a change in cache behavior) can be added without changing the
     1032            semantics of other directives. Behavioral extensions are designed to work by acting as
     1033            modifiers to the existing base of cache directives. Both the new directive and the
     1034            standard directive are supplied, such that applications that do not understand the new
     1035            directive will default to the behavior specified by the standard directive, and those
     1036            that understand the new directive will recognize it as modifying the requirements
     1037            associated with the standard directive. In this way, extensions to the cache-control
     1038            directives can be made without requiring changes to the base protocol.</t>
     1039          <t>This extension mechanism depends on an HTTP cache obeying all of the cache-control
     1040            directives defined for its native HTTP-version, obeying certain extensions, and ignoring
     1041            all directives that it does not understand.</t>
     1042          <t>For example, consider a hypothetical new response directive called "community" that
     1043            acts as a modifier to the private directive. We define this new directive to mean that,
     1044            in addition to any non-shared cache, any cache that is shared only by members of the
     1045            community named within its value may cache the response. An origin server wishing to
     1046            allow the UCI community to use an otherwise private response in their shared cache(s)
     1047            could do so by including</t>
     1048          <figure>
     1049            <artwork type="example">
    20891050  Cache-Control: private, community="UCI"
    2090 </artwork></figure>
    2091 <t>
    2092    A cache seeing this header field will act correctly even if the cache
    2093    does not understand the community cache-extension, since it will also
    2094    see and understand the private directive and thus default to the safe
    2095    behavior.
    2096 </t>
    2097 <t>
    2098    Unrecognized cache-directives &MUST; be ignored; it is assumed that any
    2099    cache-directive likely to be unrecognized by an HTTP/1.1 cache will
    2100    be combined with standard directives (or the response's default
    2101    cacheability) such that the cache behavior will remain minimally
    2102    correct even if the cache does not understand the extension(s).
    2103 </t>
    2104 </section>
    2105 </section>
    2106 
    2107 <section title="Expires" anchor="header.expires">
    2108   <iref primary="true" item="Expires header" x:for-anchor=""/>
    2109   <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
    2110   <x:anchor-alias value="Expires"/>
    2111   <x:anchor-alias value="Expires-v"/>
    2112 <t>
    2113    The entity-header field "Expires" gives the date/time after which the
    2114    response is considered stale. A stale cache entry may not normally be
    2115    returned by a cache (either a proxy cache or a user agent cache)
    2116    unless it is first validated with the origin server (or with an
    2117    intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
    2118    for further discussion of the expiration model.
    2119 </t>
    2120 <t>
    2121    The presence of an Expires field does not imply that the original
    2122    resource will change or cease to exist at, before, or after that
    2123    time.
    2124 </t>
    2125 <t>
    2126    The format is an absolute date and time as defined by HTTP-date in
    2127    &full-date;; it &MUST; be sent in rfc1123-date format.
    2128 </t>
     1051</artwork>
     1052          </figure>
     1053          <t>A cache seeing this header field will act correctly even if the cache does not
     1054            understand the community cache-extension, since it will also see and understand the
     1055            private directive and thus default to the safe behavior.</t>
     1056          <t>Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
     1057            directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
     1058            directives (or the response's default cacheability) such that the cache behavior will
     1059            remain minimally correct even if the cache does not understand the extension(s).</t>
     1060        </section>
     1061
     1062      </section>
     1063
     1064      <section anchor="header.expires" title="Expires">
     1065        <iref item="Expires header" primary="true" x:for-anchor="" />
     1066        <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
     1067        <x:anchor-alias value="Expires"/>
     1068        <x:anchor-alias value="Expires-v"/>
     1069        <t>The entity-header field "Expires" gives the date/time after which the response is
     1070          considered stale. See <xref target="expiration.model" /> for further discussion of the
     1071          freshness model.</t>
     1072        <t>The presence of an Expires field does not imply that the original resource will change or
     1073          cease to exist at, before, or after that time.</t>
     1074        <t>The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
     1075          it &MUST; be sent in rfc1123-date format.</t>
    21291076<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
    21301077  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
    21311078  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
    21321079</artwork></figure>
    2133 <t>
    2134    An example of its use is
    2135 </t>
    2136 <figure><artwork type="example">
     1080        <t>For example</t>
     1081        <figure>
     1082          <artwork type="example">
    21371083  Expires: Thu, 01 Dec 1994 16:00:00 GMT
    2138 </artwork></figure>
    2139 <t>
    2140   <list><t>
    2141       <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
    2142       directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
    2143       Expires field.
    2144   </t></list>
    2145 </t>
    2146 <t>
    2147    HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
    2148    especially including the value "0", as in the past (i.e., "already
    2149    expired").
    2150 </t>
    2151 <t>
    2152    To mark a response as "already expired," an origin server sends an
    2153    Expires date that is equal to the Date header value. (See the rules
    2154    for expiration calculations in <xref target="expiration.calculations"/>.)
    2155 </t>
    2156 <t>
    2157    To mark a response as "never expires," an origin server sends an
    2158    Expires date approximately one year from the time the response is
    2159    sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
    2160    year in the future.
    2161 </t>
    2162 <t>
    2163    The presence of an Expires header field with a date value of some
    2164    time in the future on a response that otherwise would by default be
    2165    non-cacheable indicates that the response is cacheable, unless
    2166    indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
    2167 </t>
    2168 </section>
    2169 
    2170 <section title="Pragma" anchor="header.pragma">
    2171   <iref primary="true" item="Pragma header" x:for-anchor=""/>
    2172   <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
    2173   <x:anchor-alias value="extension-pragma"/>
    2174   <x:anchor-alias value="Pragma"/>
    2175   <x:anchor-alias value="Pragma-v"/>
    2176   <x:anchor-alias value="pragma-directive"/>
    2177 <t>
    2178    The general-header field "Pragma" is used to include implementation-specific
    2179    directives that might apply to any recipient along the
    2180    request/response chain. All pragma directives specify optional
    2181    behavior from the viewpoint of the protocol; however, some systems
    2182    &MAY; require that behavior be consistent with the directives.
    2183 </t>
     1084</artwork>
     1085        </figure>
     1086        <t>
     1087          <list>
     1088            <t>
     1089              <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
     1090              directive (see <xref target="cache-response-directive" />), that directive overrides
     1091              the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.</t>
     1092          </list>
     1093        </t>
     1094        <t>HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.</t>
     1095        <t>HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
     1096          including the value "0", as in the past (i.e., "already expired").</t>
     1097      </section>
     1098
     1099      <section anchor="header.pragma" title="Pragma">
     1100        <iref item="Pragma header" primary="true" x:for-anchor="" />
     1101        <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
     1102        <x:anchor-alias value="extension-pragma"/>
     1103        <x:anchor-alias value="Pragma"/>
     1104        <x:anchor-alias value="Pragma-v"/>
     1105        <x:anchor-alias value="pragma-directive"/>
     1106        <t>The general-header field "Pragma" is used to include implementation-specific directives
     1107          that might apply to any recipient along the request/response chain. All pragma directives
     1108          specify optional behavior from the viewpoint of the protocol; however, some systems
     1109          &MAY; require that behavior be consistent with the directives.</t>
    21841110<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
    21851111  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
     
    21881114  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
    21891115</artwork></figure>
    2190 <t>
    2191    When the no-cache directive is present in a request message, an
    2192    application &SHOULD; forward the request toward the origin server even
    2193    if it has a cached copy of what is being requested. This pragma
    2194    directive has the same semantics as the no-cache cache-directive (see
    2195    <xref target="header.cache-control"/>) and is defined here for backward compatibility with
    2196    HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
    2197    request is sent to a server not known to be HTTP/1.1 compliant.
    2198 </t>
    2199 <t>
    2200    Pragma directives &MUST; be passed through by a proxy or gateway
    2201    application, regardless of their significance to that application,
    2202    since the directives might be applicable to all recipients along the
    2203    request/response chain. It is not possible to specify a pragma for a
    2204    specific recipient; however, any pragma directive not relevant to a
    2205    recipient &SHOULD; be ignored by that recipient.
    2206 </t>
    2207 <t>
    2208    HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
    2209    sent "Cache-Control: no-cache". No new Pragma directives will be
    2210    defined in HTTP.
    2211   <list><t>
    2212       <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
    2213       response-header field is not actually specified, it does not provide a
    2214       reliable replacement for "Cache-Control: no-cache" in a response.
    2215   </t></list>
    2216 </t>
    2217 </section>
    2218 
    2219 <section title="Vary" anchor="header.vary">
    2220   <iref primary="true" item="Vary header" x:for-anchor=""/>
    2221   <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
    2222   <x:anchor-alias value="Vary"/>
    2223   <x:anchor-alias value="Vary-v"/>
    2224 <t>
    2225    The "Vary" response-header field's value indicates the set of request-header
    2226    fields that fully determines, while the response is fresh, whether a cache
    2227    is permitted to use the response to reply to a subsequent request
    2228    without revalidation. For uncacheable or stale responses, the Vary
    2229    field value advises the user agent about the criteria that were used
    2230    to select the representation. A Vary field value of "*" implies that
    2231    a cache cannot determine from the request headers of a subsequent
    2232    request whether this response is the appropriate representation. See
    2233    <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
    2234 </t>
     1116        <t>When the no-cache directive is present in a request message, an application &SHOULD;
     1117          forward the request toward the origin server even if it has a cached copy of what is being
     1118          requested. This pragma directive has the same semantics as the no-cache response directive
     1119          (see <xref target="cache-response-directive" />) and is defined here for backward
     1120          compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
     1121          no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
     1122          &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".</t>
     1123        <t>
     1124          <list>
     1125            <t>
     1126              <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
     1127              is not actually specified, it does not provide a reliable replacement for
     1128              "Cache-Control: no-cache" in a response.</t>
     1129          </list>
     1130        </t>
     1131        <t>This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</t>
     1132      </section>
     1133
     1134      <section anchor="header.vary" title="Vary">
     1135        <iref item="Vary header" primary="true" x:for-anchor="" />
     1136        <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
     1137        <x:anchor-alias value="Vary"/>
     1138        <x:anchor-alias value="Vary-v"/>
     1139        <t>The "Vary" response-header field's value indicates the set of request-header fields that
     1140          determines, while the response is fresh, whether a cache is permitted to use the
     1141          response to reply to a subsequent request without validation; see <xref
     1142          target="caching.negotiated.responses" />.</t>
     1143        <t>In uncacheable or stale responses, the Vary field value advises the user agent about
     1144          the criteria that were used to select the representation.</t>
    22351145<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
    22361146  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
    22371147  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
    22381148</artwork></figure>
    2239 <t>
    2240    An HTTP/1.1 server &SHOULD; include a Vary header field with any
    2241    cacheable response that is subject to server-driven negotiation.
    2242    Doing so allows a cache to properly interpret future requests on that
    2243    resource and informs the user agent about the presence of negotiation
    2244    on that resource. A server &MAY; include a Vary header field with a
    2245    non-cacheable response that is subject to server-driven negotiation,
    2246    since this might provide the user agent with useful information about
    2247    the dimensions over which the response varies at the time of the
    2248    response.
    2249 </t>
    2250 <t>
    2251    A Vary field value consisting of a list of field-names signals that
    2252    the representation selected for the response is based on a selection
    2253    algorithm which considers ONLY the listed request-header field values
    2254    in selecting the most appropriate representation. A cache &MAY; assume
    2255    that the same selection will be made for future requests with the
    2256    same values for the listed field names, for the duration of time for
    2257    which the response is fresh.
    2258 </t>
    2259 <t>
    2260    The field-names given are not limited to the set of standard
    2261    request-header fields defined by this specification. Field names are
    2262    case-insensitive.
    2263 </t>
    2264 <t>
    2265    A Vary field value of "*" signals that unspecified parameters not
    2266    limited to the request-headers (e.g., the network address of the
    2267    client), play a role in the selection of the response representation.
    2268    The "*" value &MUST-NOT; be generated by a proxy server; it may only be
    2269    generated by an origin server.
    2270 </t>
    2271 </section>
    2272 
    2273 <section title="Warning" anchor="header.warning">
    2274   <iref primary="true" item="Warning header" x:for-anchor=""/>
    2275   <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
    2276   <x:anchor-alias value="Warning"/>
    2277   <x:anchor-alias value="Warning-v"/>
    2278   <x:anchor-alias value="warning-value"/>
    2279   <x:anchor-alias value="warn-agent"/>
    2280   <x:anchor-alias value="warn-code"/>
    2281   <x:anchor-alias value="warn-date"/>
    2282   <x:anchor-alias value="warn-text"/>
    2283 <t>
    2284    The general-header field "Warning" is used to carry additional
    2285    information about the status or transformation of a message which
    2286    might not be reflected in the message. This information is typically
    2287    used to warn about a possible lack of semantic transparency from
    2288    caching operations or transformations applied to the entity body of
    2289    the message.
    2290 </t>
    2291 <t>
    2292    Warning headers are sent with responses using:
    2293 </t>
     1149        <t>The set of header fields named by the Vary field value is known as the selecting
     1150          request-headers.</t>
     1151        <t>Servers &SHOULD; include a Vary header field with any cacheable response that is
     1152          subject to server-driven negotiation. Doing so allows a cache to properly interpret future
     1153          requests on that resource and informs the user agent about the presence of negotiation on
     1154          that resource. A server &MAY; include a Vary header field with a non-cacheable
     1155          response that is subject to server-driven negotiation, since this might provide the user
     1156          agent with useful information about the dimensions over which the response varies at the
     1157          time of the response.</t>
     1158        <t>A Vary field value of "*" signals that unspecified parameters not limited to the
     1159          request-headers (e.g., the network address of the client), play a role in the selection of
     1160          the response representation; therefore, a cache cannot determine whether this response is
     1161          appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
     1162          it may only be generated by an origin server.</t>
     1163        <t>The field-names given are not limited to the set of standard request-header fields
     1164          defined by this specification. Field names are case-insensitive.</t>
     1165      </section>
     1166
     1167      <section anchor="header.warning" title="Warning">
     1168        <iref item="Warning header" primary="true" x:for-anchor="" />
     1169        <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
     1170        <x:anchor-alias value="Warning"/>
     1171        <x:anchor-alias value="Warning-v"/>
     1172        <x:anchor-alias value="warning-value"/>
     1173        <x:anchor-alias value="warn-agent"/>
     1174        <x:anchor-alias value="warn-code"/>
     1175        <x:anchor-alias value="warn-date"/>
     1176        <x:anchor-alias value="warn-text"/>
     1177        <t>The general-header field "Warning" is used to carry additional information about the status
     1178          or transformation of a message that might not be reflected in the message. This
     1179          information is typically used to warn about possible incorrectness introduced by caching
     1180          operations or transformations applied to the entity body of the message.</t>
     1181        <t>Warnings can be used for other purposes, both cache-related and otherwise. The use of a
     1182          warning, rather than an error status code, distinguish these responses from true failures.</t>
     1183
     1184        <t>Warning headers can in general be applied to any message, however some warn-codes are
     1185          specific to caches and can only be applied to response messages.</t>
     1186
    22941187<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><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"/>
    22951188  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
     
    23061199  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
    23071200</artwork></figure>
    2308 <t>
    2309    A response &MAY; carry more than one Warning header.
    2310 </t>
    2311 <t>
    2312    The warn-text &SHOULD; be in a natural language and character set that
    2313    is most likely to be intelligible to the human user receiving the
    2314    response. This decision &MAY; be based on any available knowledge, such
    2315    as the location of the cache or user, the Accept-Language field in a
    2316    request, the Content-Language field in a response, etc. The default
    2317    language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
    2318 </t>
    2319 <t>
    2320    If a character set other than ISO-8859-1 is used, it &MUST; be encoded
    2321    in the warn-text using the method described in <xref target="RFC2047"/>.
    2322 </t>
    2323 <t>
    2324    Warning headers can in general be applied to any message, however
    2325    some specific warn-codes are specific to caches and can only be
    2326    applied to response messages. New Warning headers &SHOULD; be added
    2327    after any existing Warning headers. A cache &MUST-NOT; delete any
    2328    Warning header that it received with a message. However, if a cache
    2329    successfully validates a cache entry, it &SHOULD; remove any Warning
    2330    headers previously attached to that entry except as specified for
    2331    specific Warning codes. It &MUST; then add any Warning headers received
    2332    in the validating response. In other words, Warning headers are those
    2333    that would be attached to the most recent relevant response.
    2334 </t>
    2335 <t>
    2336    When multiple Warning headers are attached to a response, the user
    2337    agent ought to inform the user of as many of them as possible, in the
    2338    order that they appear in the response. If it is not possible to
    2339    inform the user of all of the warnings, the user agent &SHOULD; follow
    2340    these heuristics:
    2341   <list style="symbols">
    2342     <t>Warnings that appear early in the response take priority over
    2343         those appearing later in the response.</t>
    2344 
    2345     <t>Warnings in the user's preferred character set take priority
    2346         over warnings in other character sets but with identical warn-codes
    2347         and warn-agents.</t>
    2348   </list>
    2349 </t>
    2350 <t>
    2351    Systems that generate multiple Warning headers &SHOULD; order them with
    2352    this user agent behavior in mind.
    2353 </t>
    2354 <t>
    2355    Requirements for the behavior of caches with respect to Warnings are
    2356    stated in <xref target="warnings"/>.
    2357 </t>
    2358 <t>
    2359    This is a list of the currently-defined warn-codes, each with a
    2360    recommended warn-text in English, and a description of its meaning.
    2361 </t>
    2362 <t>
    2363    110 Response is stale
    2364   <list><t>
    2365      &MUST; be included whenever the returned response is stale.
    2366   </t></list>
    2367 </t>
    2368 <t>
    2369    111 Revalidation failed
    2370   <list><t>
    2371      &MUST; be included if a cache returns a stale response because an
    2372      attempt to revalidate the response failed, due to an inability to
    2373      reach the server.
    2374   </t></list>
    2375 </t>
    2376 <t>
    2377    112 Disconnected operation
    2378   <list><t>
    2379      &SHOULD; be included if the cache is intentionally disconnected from
    2380      the rest of the network for a period of time.
    2381   </t></list>
    2382 </t>
    2383 <t>
    2384    113 Heuristic expiration
    2385   <list><t>
    2386      &MUST; be included if the cache heuristically chose a freshness
    2387      lifetime greater than 24 hours and the response's age is greater
    2388      than 24 hours.
    2389   </t></list>
    2390 </t>
    2391 <t>
    2392    199 Miscellaneous warning
    2393   <list><t>
    2394      The warning text &MAY; include arbitrary information to be presented
    2395      to a human user, or logged. A system receiving this warning &MUST-NOT;
    2396      take any automated action, besides presenting the warning to
    2397      the user.
    2398   </t></list>
    2399 </t>
    2400 <t>
    2401    214 Transformation applied
    2402   <list><t>
    2403      &MUST; be added by an intermediate cache or proxy if it applies any
    2404      transformation changing the content-coding (as specified in the
    2405      Content-Encoding header) or media-type (as specified in the
    2406      Content-Type header) of the response, or the entity-body of the
    2407      response, unless this Warning code already appears in the response.
    2408   </t></list>
    2409 </t>
    2410 <t>
    2411    299 Miscellaneous persistent warning
    2412   <list><t>
    2413      The warning text &MAY; include arbitrary information to be presented
    2414      to a human user, or logged. A system receiving this warning &MUST-NOT;
    2415      take any automated action.
    2416   </t></list>
    2417 </t>
    2418 <t>
    2419    If an implementation sends a message with one or more Warning headers
    2420    whose version is HTTP/1.0 or lower, then the sender &MUST; include in
    2421    each warning-value a warn-date that matches the date in the response.
    2422 </t>
    2423 <t>
    2424    If an implementation receives a message with a warning-value that
    2425    includes a warn-date, and that warn-date is different from the Date
    2426    value in the response, then that warning-value &MUST; be deleted from
    2427    the message before storing, forwarding, or using it. (This prevents
    2428    bad consequences of naive caching of Warning header fields.) If all
    2429    of the warning-values are deleted for this reason, the Warning header
    2430    &MUST; be deleted as well.
    2431 </t>
    2432 </section>
    2433 
    2434 </section>
    2435 
    2436 <section title="IANA Considerations" anchor="IANA.considerations">
    2437 <section title="Message Header Registration" anchor="message.header.registration">
    2438 <t>
    2439    The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
    2440    with the permanent registrations below (see <xref target="RFC3864"/>):
    2441 </t>
    2442 <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
    2443 <texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
    2444    <ttcol>Header Field Name</ttcol>
    2445    <ttcol>Protocol</ttcol>
    2446    <ttcol>Status</ttcol>
    2447    <ttcol>Reference</ttcol>
    2448 
    2449    <c>Age</c>
    2450    <c>http</c>
    2451    <c>standard</c>
    2452    <c>
    2453       <xref target="header.age"/>
    2454    </c>
    2455    <c>Cache-Control</c>
    2456    <c>http</c>
    2457    <c>standard</c>
    2458    <c>
    2459       <xref target="header.cache-control"/>
    2460    </c>
    2461    <c>Expires</c>
    2462    <c>http</c>
    2463    <c>standard</c>
    2464    <c>
    2465       <xref target="header.expires"/>
    2466    </c>
    2467    <c>Pragma</c>
    2468    <c>http</c>
    2469    <c>standard</c>
    2470    <c>
    2471       <xref target="header.pragma"/>
    2472    </c>
    2473    <c>Vary</c>
    2474    <c>http</c>
    2475    <c>standard</c>
    2476    <c>
    2477       <xref target="header.vary"/>
    2478    </c>
    2479    <c>Warning</c>
    2480    <c>http</c>
    2481    <c>standard</c>
    2482    <c>
    2483       <xref target="header.warning"/>
    2484    </c>
    2485 </texttable>
    2486 <!--(END)-->
    2487 <t>
    2488    The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
    2489 </t>
    2490 </section>
    2491 </section>
    2492 
    2493 <section title="Security Considerations" anchor="security.considerations">
    2494 <t>
    2495    Caching proxies provide additional potential vulnerabilities, since
    2496    the contents of the cache represent an attractive target for
    2497    malicious exploitation. Because cache contents persist after an HTTP
    2498    request is complete, an attack on the cache can reveal information
    2499    long after a user believes that the information has been removed from
    2500    the network. Therefore, cache contents should be protected as
    2501    sensitive information.
    2502 </t>
    2503 </section>
    2504 
    2505 <section title="Acknowledgments" anchor="ack">
    2506 <t>
    2507    Much of the content and presentation of the caching design is due to
    2508    suggestions and comments from individuals including: Shel Kaphan,
    2509    Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
    2510 </t>
    2511 </section>
    2512 </middle>
    2513 <back>
    2514 
    2515 <references title="Normative References">
    2516 
    2517 <reference anchor="ISO-8859-1">
    2518   <front>
    2519     <title>
    2520      Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
    2521     </title>
    2522     <author>
    2523       <organization>International Organization for Standardization</organization>
    2524     </author>
    2525     <date year="1998"/>
    2526   </front>
    2527   <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
    2528 </reference>
    2529 
    2530 <reference anchor="Part1">
    2531   <front>
    2532     <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
    2533     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2534       <organization abbrev="Day Software">Day Software</organization>
    2535       <address><email>fielding@gbiv.com</email></address>
    2536     </author>
    2537     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2538       <organization>One Laptop per Child</organization>
    2539       <address><email>jg@laptop.org</email></address>
    2540     </author>
    2541     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2542       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2543       <address><email>JeffMogul@acm.org</email></address>
    2544     </author>
    2545     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2546       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2547       <address><email>henrikn@microsoft.com</email></address>
    2548     </author>
    2549     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2550       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2551       <address><email>LMM@acm.org</email></address>
    2552     </author>
    2553     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2554       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2555       <address><email>paulle@microsoft.com</email></address>
    2556     </author>
    2557     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2558       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2559       <address><email>timbl@w3.org</email></address>
    2560     </author>
    2561     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2562       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2563       <address><email>ylafon@w3.org</email></address>
    2564     </author>
    2565     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2566       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2567       <address><email>julian.reschke@greenbytes.de</email></address>
    2568     </author>
    2569     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2570   </front>
    2571   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
    2572   <x:source href="p1-messaging.xml" basename="p1-messaging"/>
    2573 </reference>
    2574 
    2575 <reference anchor="Part2">
    2576   <front>
    2577     <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
    2578     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2579       <organization abbrev="Day Software">Day Software</organization>
    2580       <address><email>fielding@gbiv.com</email></address>
    2581     </author>
    2582     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2583       <organization>One Laptop per Child</organization>
    2584       <address><email>jg@laptop.org</email></address>
    2585     </author>
    2586     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2587       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2588       <address><email>JeffMogul@acm.org</email></address>
    2589     </author>
    2590     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2591       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2592       <address><email>henrikn@microsoft.com</email></address>
    2593     </author>
    2594     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2595       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2596       <address><email>LMM@acm.org</email></address>
    2597     </author>
    2598     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2599       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2600       <address><email>paulle@microsoft.com</email></address>
    2601     </author>
    2602     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2603       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2604       <address><email>timbl@w3.org</email></address>
    2605     </author>
    2606     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2607       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2608       <address><email>ylafon@w3.org</email></address>
    2609     </author>
    2610     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2611       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2612       <address><email>julian.reschke@greenbytes.de</email></address>
    2613     </author>
    2614     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2615   </front>
    2616   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
    2617   <x:source href="p2-semantics.xml" basename="p2-semantics"/>
    2618 </reference>
    2619 
    2620 <reference anchor="Part3">
    2621   <front>
    2622     <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
    2623     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2624       <organization abbrev="Day Software">Day Software</organization>
    2625       <address><email>fielding@gbiv.com</email></address>
    2626     </author>
    2627     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2628       <organization>One Laptop per Child</organization>
    2629       <address><email>jg@laptop.org</email></address>
    2630     </author>
    2631     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2632       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2633       <address><email>JeffMogul@acm.org</email></address>
    2634     </author>
    2635     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2636       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2637       <address><email>henrikn@microsoft.com</email></address>
    2638     </author>
    2639     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2640       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2641       <address><email>LMM@acm.org</email></address>
    2642     </author>
    2643     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2644       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2645       <address><email>paulle@microsoft.com</email></address>
    2646     </author>
    2647     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2648       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2649       <address><email>timbl@w3.org</email></address>
    2650     </author>
    2651     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2652       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2653       <address><email>ylafon@w3.org</email></address>
    2654     </author>
    2655     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2656       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2657       <address><email>julian.reschke@greenbytes.de</email></address>
    2658     </author>
    2659     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2660   </front>
    2661   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
    2662   <x:source href="p3-payload.xml" basename="p3-payload"/>
    2663 </reference>
    2664 
    2665 <reference anchor="Part4">
    2666   <front>
    2667     <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
    2668     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2669       <organization abbrev="Day Software">Day Software</organization>
    2670       <address><email>fielding@gbiv.com</email></address>
    2671     </author>
    2672     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2673       <organization>One Laptop per Child</organization>
    2674       <address><email>jg@laptop.org</email></address>
    2675     </author>
    2676     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2677       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2678       <address><email>JeffMogul@acm.org</email></address>
    2679     </author>
    2680     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2681       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2682       <address><email>henrikn@microsoft.com</email></address>
    2683     </author>
    2684     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2685       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2686       <address><email>LMM@acm.org</email></address>
    2687     </author>
    2688     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2689       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2690       <address><email>paulle@microsoft.com</email></address>
    2691     </author>
    2692     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2693       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2694       <address><email>timbl@w3.org</email></address>
    2695     </author>
    2696     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2697       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2698       <address><email>ylafon@w3.org</email></address>
    2699     </author>
    2700     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2701       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2702       <address><email>julian.reschke@greenbytes.de</email></address>
    2703     </author>
    2704     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2705   </front>
    2706   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
    2707   <x:source href="p4-conditional.xml" basename="p4-conditional"/>
    2708 </reference>
    2709 
    2710 <reference anchor="Part5">
    2711   <front>
    2712     <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
    2713     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2714       <organization abbrev="Day Software">Day Software</organization>
    2715       <address><email>fielding@gbiv.com</email></address>
    2716     </author>
    2717     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2718       <organization>One Laptop per Child</organization>
    2719       <address><email>jg@laptop.org</email></address>
    2720     </author>
    2721     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2722       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2723       <address><email>JeffMogul@acm.org</email></address>
    2724     </author>
    2725     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2726       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2727       <address><email>henrikn@microsoft.com</email></address>
    2728     </author>
    2729     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2730       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2731       <address><email>LMM@acm.org</email></address>
    2732     </author>
    2733     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2734       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2735       <address><email>paulle@microsoft.com</email></address>
    2736     </author>
    2737     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2738       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2739       <address><email>timbl@w3.org</email></address>
    2740     </author>
    2741     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2742       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2743       <address><email>ylafon@w3.org</email></address>
    2744     </author>
    2745     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2746       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2747       <address><email>julian.reschke@greenbytes.de</email></address>
    2748     </author>
    2749     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2750   </front>
    2751   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
    2752   <x:source href="p5-range.xml" basename="p5-range"/>
    2753 </reference>
    2754 
    2755 <reference anchor="Part7">
    2756   <front>
    2757     <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
    2758     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2759       <organization abbrev="Day Software">Day Software</organization>
    2760       <address><email>fielding@gbiv.com</email></address>
    2761     </author>
    2762     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2763       <organization>One Laptop per Child</organization>
    2764       <address><email>jg@laptop.org</email></address>
    2765     </author>
    2766     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2767       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2768       <address><email>JeffMogul@acm.org</email></address>
    2769     </author>
    2770     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2771       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2772       <address><email>henrikn@microsoft.com</email></address>
    2773     </author>
    2774     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2775       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2776       <address><email>LMM@acm.org</email></address>
    2777     </author>
    2778     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2779       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2780       <address><email>paulle@microsoft.com</email></address>
    2781     </author>
    2782     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2783       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2784       <address><email>timbl@w3.org</email></address>
    2785     </author>
    2786     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2787       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2788       <address><email>ylafon@w3.org</email></address>
    2789     </author>
    2790     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2791       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2792       <address><email>julian.reschke@greenbytes.de</email></address>
    2793     </author>
    2794     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2795   </front>
    2796   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
    2797   <x:source href="p7-auth.xml" basename="p7-auth"/>
    2798 </reference>
    2799 
    2800 <reference anchor="RFC2047">
    2801   <front>
    2802     <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
    2803     <author initials="K." surname="Moore" fullname="Keith Moore">
    2804       <organization>University of Tennessee</organization>
    2805       <address><email>moore@cs.utk.edu</email></address>
    2806     </author>
    2807     <date month="November" year="1996"/>
    2808   </front>
    2809   <seriesInfo name="RFC" value="2047"/>
    2810 </reference>
    2811 
    2812 <reference anchor="RFC2119">
    2813   <front>
    2814     <title>Key words for use in RFCs to Indicate Requirement Levels</title>
    2815     <author initials="S." surname="Bradner" fullname="Scott Bradner">
    2816       <organization>Harvard University</organization>
    2817       <address><email>sob@harvard.edu</email></address>
    2818     </author>
    2819     <date month="March" year="1997"/>
    2820   </front>
    2821   <seriesInfo name="BCP" value="14"/>
    2822   <seriesInfo name="RFC" value="2119"/>
    2823 </reference>
    2824 
    2825 <reference anchor="RFC5234">
    2826   <front>
    2827     <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
    2828     <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
    2829       <organization>Brandenburg InternetWorking</organization>
    2830       <address>
    2831       <postal>
    2832       <street>675 Spruce Dr.</street>
    2833       <city>Sunnyvale</city>
    2834       <region>CA</region>
    2835       <code>94086</code>
    2836       <country>US</country></postal>
    2837       <phone>+1.408.246.8253</phone>
    2838       <email>dcrocker@bbiw.net</email></address> 
    2839     </author>
    2840     <author initials="P." surname="Overell" fullname="Paul Overell">
    2841       <organization>THUS plc.</organization>
    2842       <address>
    2843       <postal>
    2844       <street>1/2 Berkeley Square</street>
    2845       <street>99 Berkely Street</street>
    2846       <city>Glasgow</city>
    2847       <code>G3 7HR</code>
    2848       <country>UK</country></postal>
    2849       <email>paul.overell@thus.net</email></address>
    2850     </author>
    2851     <date month="January" year="2008"/>
    2852   </front>
    2853   <seriesInfo name="STD" value="68"/>
    2854   <seriesInfo name="RFC" value="5234"/>
    2855 </reference>
    2856 
    2857 </references>
    2858 
    2859 <references title="Informative References">
    2860 
    2861 <reference anchor="RFC1305">
    2862   <front>
    2863     <title>Network Time Protocol (Version 3) Specification, Implementation</title>
    2864     <author initials="D." surname="Mills" fullname="David L. Mills">
    2865       <organization>University of Delaware, Electrical Engineering Department</organization>
    2866       <address><email>mills@udel.edu</email></address>
    2867     </author>
    2868     <date month="March" year="1992"/>
    2869   </front>
    2870   <seriesInfo name="RFC" value="1305"/>
    2871 </reference>
    2872 
    2873 <reference anchor="RFC2616">
    2874   <front>
    2875     <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
    2876     <author initials="R." surname="Fielding" fullname="R. Fielding">
    2877       <organization>University of California, Irvine</organization>
    2878       <address><email>fielding@ics.uci.edu</email></address>
    2879     </author>
    2880     <author initials="J." surname="Gettys" fullname="J. Gettys">
    2881       <organization>W3C</organization>
    2882       <address><email>jg@w3.org</email></address>
    2883     </author>
    2884     <author initials="J." surname="Mogul" fullname="J. Mogul">
    2885       <organization>Compaq Computer Corporation</organization>
    2886       <address><email>mogul@wrl.dec.com</email></address>
    2887     </author>
    2888     <author initials="H." surname="Frystyk" fullname="H. Frystyk">
    2889       <organization>MIT Laboratory for Computer Science</organization>
    2890       <address><email>frystyk@w3.org</email></address>
    2891     </author>
    2892     <author initials="L." surname="Masinter" fullname="L. Masinter">
    2893       <organization>Xerox Corporation</organization>
    2894       <address><email>masinter@parc.xerox.com</email></address>
    2895     </author>
    2896     <author initials="P." surname="Leach" fullname="P. Leach">
    2897       <organization>Microsoft Corporation</organization>
    2898       <address><email>paulle@microsoft.com</email></address>
    2899     </author>
    2900     <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
    2901       <organization>W3C</organization>
    2902       <address><email>timbl@w3.org</email></address>
    2903     </author>
    2904     <date month="June" year="1999"/>
    2905   </front>
    2906   <seriesInfo name="RFC" value="2616"/>
    2907 </reference>
    2908 
    2909 <reference anchor='RFC3864'>
    2910   <front>
    2911     <title>Registration Procedures for Message Header Fields</title>
    2912     <author initials='G.' surname='Klyne' fullname='G. Klyne'>
    2913       <organization>Nine by Nine</organization>
    2914       <address><email>GK-IETF@ninebynine.org</email></address>
    2915     </author>
    2916     <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
    2917       <organization>BEA Systems</organization>
    2918       <address><email>mnot@pobox.com</email></address>
    2919     </author>
    2920     <author initials='J.' surname='Mogul' fullname='J. Mogul'>
    2921       <organization>HP Labs</organization>
    2922       <address><email>JeffMogul@acm.org</email></address>
    2923     </author>
    2924     <date year='2004' month='September' />
    2925   </front>
    2926   <seriesInfo name='BCP' value='90' />
    2927   <seriesInfo name='RFC' value='3864' />
    2928 </reference>
    2929 
    2930 </references>
    2931 
    2932 <section title="Compatibility with Previous Versions" anchor="compatibility">
    2933 
    2934 <section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
    2935 <t>
    2936    A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
    2937    was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
    2938    <xref target="header.cache-control" format="counter"/>,
    2939    <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
    2940 </t>
    2941 <t>
    2942    Transfer-coding and message lengths all interact in ways that
    2943    required fixing exactly when chunked encoding is used (to allow for
    2944    transfer encoding that may not be self delimiting); it was important
    2945    to straighten out exactly how message lengths are computed.
    2946    (<xref target="non-modifiable.headers"/>,
    2947    see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
    2948 </t>
    2949 <t>
    2950    Proxies should be able to add Content-Length when appropriate.
    2951    (<xref target="non-modifiable.headers"/>)
    2952 </t>
    2953 <t>
    2954    Range request responses would become very verbose if all meta-data
    2955    were always returned; by allowing the server to only send needed
    2956    headers in a 206 response, this problem can be avoided.
    2957    (<xref target="combining.headers"/>)
    2958 </t>
    2959 <t>
    2960    The Cache-Control: max-age directive was not properly defined for
    2961    responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
    2962 </t>
    2963 <t>
    2964    Warnings could be cached incorrectly, or not updated appropriately.
    2965    (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
    2966    <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
    2967    and <xref target="header.warning" format="counter"/>) Warning
    2968    also needed to be a general header, as PUT or other methods may have
    2969    need for it in requests.
    2970 </t>
    2971 </section>
    2972 
    2973 <section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
    2974 <t>
    2975   Clarify denial of service attack avoidance requirement.
    2976   (<xref target="invalidation.after.updates.or.deletions"/>)
    2977 </t>
    2978 </section>
    2979 
    2980 </section>
     1201
     1202        <t>Multiple warnings can be attached to a response (either by the origin server or by
     1203          a cache), including multiple warnings with the same code number. For example, a server
     1204          might provide the same warning with texts in both English and Basque.</t>
     1205        <t>When this occurs, the user agent &SHOULD; inform the user of as many of them as
     1206          possible, in the order that they appear in the response. If it is not possible to inform
     1207          the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
     1208            <list style="symbols">
     1209            <t>Warnings that appear early in the response take priority over those appearing later
     1210              in the response.</t>
     1211
     1212            <t>Warnings in the user's preferred character set take priority over warnings in other
     1213              character sets but with identical warn-codes and warn-agents.</t>
     1214          </list>
     1215        </t>
     1216        <t>Systems that generate multiple Warning headers &SHOULD; order them with this user
     1217          agent behavior in mind. New Warning headers &SHOULD; be added after any existing
     1218          Warning headers.</t>
     1219        <t>Warnings are assigned three digit warn-codes. The first digit indicates whether the
     1220          Warning is required to be deleted from a stored response after validation: <list
     1221            style="symbols">
     1222            <t>1xx Warnings that describe the freshness or validation status of the response, and so
     1223              &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
     1224              except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
     1225            <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
     1226              not rectified by a validation (for example, a lossy compression of the entity bodies)
     1227              and &MUST-NOT; be deleted by caches after validation, unless a full response is
     1228              returned, in which case they &MUST; be.</t>
     1229          </list>
     1230        </t>
     1231        <t>The warn-text &SHOULD; be in a natural language and character set that is most likely
     1232          to be intelligible to the human user receiving the response. This decision can be based on
     1233          any available knowledge, such as the location of the cache or user, the Accept-Language
     1234          field in a request, the Content-Language field in a response, etc. The default language is
     1235          English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).</t>
     1236        <t>If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
     1237          warn-text using the method described in <xref target="RFC2047" />.</t>
     1238        <t>If an implementation sends a message with one or more Warning headers to a receiver whose
     1239          version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
     1240          warn-date that matches the Date header in the message.</t>
     1241        <t>If an implementation receives a message with a warning-value that includes a warn-date,
     1242          and that warn-date is different from the Date value in the response, then that
     1243          warning-value &MUST; be deleted from the message before storing, forwarding, or using
     1244          it. (preventing the consequences of naive caching of Warning header fields.) If all of the
     1245          warning-values are deleted for this reason, the Warning header &MUST; be deleted as
     1246          well.</t>
     1247        <t>The following warn-codes are defined by this specification, each with a recommended
     1248          warn-text in English, and a description of its meaning.</t>
     1249        <t>110 Response is stale <list>
     1250            <t>&SHOULD; be included whenever the returned response is stale.</t>
     1251          </list>
     1252        </t>
     1253        <t>111 Revalidation failed <list>
     1254            <t>&SHOULD; be included if a cache returns a stale response because an attempt to
     1255              validate the response failed, due to an inability to reach the server.</t>
     1256          </list>
     1257        </t>
     1258        <t>112 Disconnected operation <list>
     1259            <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
     1260              the network for a period of time.</t>
     1261          </list>
     1262        </t>
     1263        <t>113 Heuristic expiration <list>
     1264            <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
     1265              greater than 24 hours and the response's age is greater than 24 hours.</t>
     1266          </list>
     1267        </t>
     1268        <t>199 Miscellaneous warning <list>
     1269            <t>The warning text can include arbitrary information to be presented to a human
     1270              user, or logged. A system receiving this warning &MUST-NOT; take any automated
     1271              action, besides presenting the warning to the user.</t>
     1272          </list>
     1273        </t>
     1274        <t>214 Transformation applied <list>
     1275            <t>&MUST; be added by an intermediate cache or proxy if it applies any
     1276              transformation changing the content-coding (as specified in the Content-Encoding
     1277              header) or media-type (as specified in the Content-Type header) of the response, or
     1278              the entity-body of the response, unless this Warning code already appears in the
     1279              response.</t>
     1280          </list>
     1281        </t>
     1282        <t>299 Miscellaneous persistent warning <list>
     1283            <t>The warning text can include arbitrary information to be presented to a human
     1284              user, or logged. A system receiving this warning &MUST-NOT; take any automated
     1285              action.</t>
     1286          </list>
     1287        </t>
     1288      </section>
     1289
     1290    </section>
     1291
     1292
     1293    <section anchor="history.lists" title="History Lists">
     1294      <t>User agents often have history mechanisms, such as "Back" buttons and history lists, that
     1295        can be used to redisplay an entity retrieved earlier in a session.</t>
     1296      <t>History mechanisms and caches are different. In particular history mechanisms
     1297        &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
     1298        history mechanism is meant to show exactly what the user saw at the time when the resource
     1299        was retrieved.</t>
     1300      <t>By default, an expiration time does not apply to history mechanisms. If the entity is still
     1301        in storage, a history mechanism &SHOULD; display it even if the entity has expired,
     1302        unless the user has specifically configured the agent to refresh expired history documents.</t>
     1303      <t>This is not to be construed to prohibit the history mechanism from telling the user that a
     1304        view might be stale. <list>
     1305          <t>
     1306            <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
     1307            stale resources, this will tend to force service authors to avoid using HTTP expiration
     1308            controls and cache controls when they would otherwise like to. Service authors may
     1309            consider it important that users not be presented with error messages or warning
     1310            messages when they use navigation controls (such as BACK) to view previously fetched
     1311            resources. Even though sometimes such resources ought not be cached, or ought to expire
     1312            quickly, user interface considerations may force service authors to resort to other
     1313            means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
     1314            of improperly functioning history mechanisms.</t>
     1315        </list>
     1316      </t>
     1317    </section>
     1318
     1319
     1320    <section anchor="IANA.considerations" title="IANA Considerations">
     1321      <section anchor="message.header.registration" title="Message Header Registration">
     1322        <t>The Message Header Registry located at <eref
     1323            target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
     1324          should be updated with the permanent registrations below (see <xref target="RFC3864" />):</t>
     1325        <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
     1326        <texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
     1327          <ttcol>Header Field Name</ttcol>
     1328          <ttcol>Protocol</ttcol>
     1329          <ttcol>Status</ttcol>
     1330          <ttcol>Reference</ttcol>
     1331
     1332          <c>Age</c>
     1333          <c>http</c>
     1334          <c>standard</c>
     1335          <c>
     1336            <xref target="header.age" />
     1337          </c>
     1338          <c>Cache-Control</c>
     1339          <c>http</c>
     1340          <c>standard</c>
     1341          <c>
     1342            <xref target="header.cache-control" />
     1343          </c>
     1344          <c>Expires</c>
     1345          <c>http</c>
     1346          <c>standard</c>
     1347          <c>
     1348            <xref target="header.expires" />
     1349          </c>
     1350          <c>Pragma</c>
     1351          <c>http</c>
     1352          <c>standard</c>
     1353          <c>
     1354            <xref target="header.pragma" />
     1355          </c>
     1356          <c>Vary</c>
     1357          <c>http</c>
     1358          <c>standard</c>
     1359          <c>
     1360            <xref target="header.vary" />
     1361          </c>
     1362          <c>Warning</c>
     1363          <c>http</c>
     1364          <c>standard</c>
     1365          <c>
     1366            <xref target="header.warning" />
     1367          </c>
     1368        </texttable>
     1369        <!--(END)-->
     1370        <t>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</t>
     1371      </section>
     1372    </section>
     1373
     1374    <section anchor="security.considerations" title="Security Considerations">
     1375      <t>Caches expose additional potential vulnerabilities, since the contents of the cache
     1376        represent an attractive target for malicious exploitation. Because cache contents persist
     1377        after an HTTP request is complete, an attack on the cache can reveal information long after
     1378        a user believes that the information has been removed from the network. Therefore, cache
     1379        contents should be protected as sensitive information.</t>
     1380    </section>
     1381
     1382    <section anchor="ack" title="Acknowledgments">
     1383      <t>Much of the content and presentation of the caching design is due to suggestions and
     1384        comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
     1385        and Larry Masinter.</t>
     1386    </section>
     1387  </middle>
     1388
     1389  <back>
     1390    <references title="Normative References">
     1391
     1392      <reference anchor="ISO-8859-1">
     1393        <front>
     1394          <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
     1395            1: Latin alphabet No. 1 </title>
     1396          <author>
     1397            <organization>International Organization for Standardization</organization>
     1398          </author>
     1399          <date year="1998" />
     1400        </front>
     1401        <seriesInfo name="ISO/IEC" value="8859-1:1998" />
     1402      </reference>
     1403
     1404      <reference anchor="Part1">
     1405        <front>
     1406          <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
     1407          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1408            <organization abbrev="Day Software">Day Software</organization>
     1409            <address><email>fielding@gbiv.com</email></address>
     1410          </author>
     1411          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1412            <organization>One Laptop per Child</organization>
     1413            <address><email>jg@laptop.org</email></address>
     1414          </author>
     1415          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1416            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1417            <address><email>JeffMogul@acm.org</email></address>
     1418          </author>
     1419          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1420            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1421            <address><email>henrikn@microsoft.com</email></address>
     1422          </author>
     1423          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1424            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1425            <address><email>LMM@acm.org</email></address>
     1426          </author>
     1427          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1428            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1429            <address><email>paulle@microsoft.com</email></address>
     1430          </author>
     1431          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1432            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1433            <address><email>timbl@w3.org</email></address>
     1434          </author>
     1435          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1436            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1437            <address><email>ylafon@w3.org</email></address>
     1438          </author>
     1439          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1440            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1441            <address><email>julian.reschke@greenbytes.de</email></address>
     1442          </author>
     1443          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1444        </front>
     1445        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
     1446        <x:source basename="p1-messaging" href="p1-messaging.xml" />
     1447      </reference>
     1448
     1449      <reference anchor="Part2">
     1450        <front>
     1451          <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
     1452          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1453            <organization abbrev="Day Software">Day Software</organization>
     1454            <address><email>fielding@gbiv.com</email></address>
     1455          </author>
     1456          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1457            <organization>One Laptop per Child</organization>
     1458            <address><email>jg@laptop.org</email></address>
     1459          </author>
     1460          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1461            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1462            <address><email>JeffMogul@acm.org</email></address>
     1463          </author>
     1464          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1465            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1466            <address><email>henrikn@microsoft.com</email></address>
     1467          </author>
     1468          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1469            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1470            <address><email>LMM@acm.org</email></address>
     1471          </author>
     1472          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1473            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1474            <address><email>paulle@microsoft.com</email></address>
     1475          </author>
     1476          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1477            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1478            <address><email>timbl@w3.org</email></address>
     1479          </author>
     1480          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1481            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1482            <address><email>ylafon@w3.org</email></address>
     1483          </author>
     1484          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1485            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1486            <address><email>julian.reschke@greenbytes.de</email></address>
     1487          </author>
     1488          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1489        </front>
     1490        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
     1491        <x:source basename="p2-semantics" href="p2-semantics.xml" />
     1492      </reference>
     1493
     1494      <reference anchor="Part3">
     1495        <front>
     1496          <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
     1497          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1498            <organization abbrev="Day Software">Day Software</organization>
     1499            <address><email>fielding@gbiv.com</email></address>
     1500          </author>
     1501          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1502            <organization>One Laptop per Child</organization>
     1503            <address><email>jg@laptop.org</email></address>
     1504          </author>
     1505          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1506            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1507            <address><email>JeffMogul@acm.org</email></address>
     1508          </author>
     1509          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1510            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1511            <address><email>henrikn@microsoft.com</email></address>
     1512          </author>
     1513          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1514            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1515            <address><email>LMM@acm.org</email></address>
     1516          </author>
     1517          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1518            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1519            <address><email>paulle@microsoft.com</email></address>
     1520          </author>
     1521          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1522            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1523            <address><email>timbl@w3.org</email></address>
     1524          </author>
     1525          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1526            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1527            <address><email>ylafon@w3.org</email></address>
     1528          </author>
     1529          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1530            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1531            <address><email>julian.reschke@greenbytes.de</email></address>
     1532          </author>
     1533          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1534        </front>
     1535        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
     1536        <x:source basename="p3-payload" href="p3-payload.xml" />
     1537      </reference>
     1538
     1539      <reference anchor="Part4">
     1540        <front>
     1541          <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
     1542          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1543            <organization abbrev="Day Software">Day Software</organization>
     1544            <address><email>fielding@gbiv.com</email></address>
     1545          </author>
     1546          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1547            <organization>One Laptop per Child</organization>
     1548            <address><email>jg@laptop.org</email></address>
     1549          </author>
     1550          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1551            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1552            <address><email>JeffMogul@acm.org</email></address>
     1553          </author>
     1554          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1555            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1556            <address><email>henrikn@microsoft.com</email></address>
     1557          </author>
     1558          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1559            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1560            <address><email>LMM@acm.org</email></address>
     1561          </author>
     1562          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1563            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1564            <address><email>paulle@microsoft.com</email></address>
     1565          </author>
     1566          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1567            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1568            <address><email>timbl@w3.org</email></address>
     1569          </author>
     1570          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1571            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1572            <address><email>ylafon@w3.org</email></address>
     1573          </author>
     1574          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1575            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1576            <address><email>julian.reschke@greenbytes.de</email></address>
     1577          </author>
     1578          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1579        </front>
     1580        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
     1581        <x:source basename="p4-conditional" href="p4-conditional.xml" />
     1582      </reference>
     1583
     1584      <reference anchor="Part5">
     1585        <front>
     1586          <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
     1587          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1588            <organization abbrev="Day Software">Day Software</organization>
     1589            <address><email>fielding@gbiv.com</email></address>
     1590          </author>
     1591          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1592            <organization>One Laptop per Child</organization>
     1593            <address><email>jg@laptop.org</email></address>
     1594          </author>
     1595          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1596            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1597            <address><email>JeffMogul@acm.org</email></address>
     1598          </author>
     1599          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1600            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1601            <address><email>henrikn@microsoft.com</email></address>
     1602          </author>
     1603          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1604            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1605            <address><email>LMM@acm.org</email></address>
     1606          </author>
     1607          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1608            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1609            <address><email>paulle@microsoft.com</email></address>
     1610          </author>
     1611          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1612            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1613            <address><email>timbl@w3.org</email></address>
     1614          </author>
     1615          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1616            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1617            <address><email>ylafon@w3.org</email></address>
     1618          </author>
     1619          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1620            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1621            <address><email>julian.reschke@greenbytes.de</email></address>
     1622          </author>
     1623          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1624        </front>
     1625        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
     1626        <x:source basename="p5-range" href="p5-range.xml" />
     1627      </reference>
     1628
     1629      <reference anchor="Part7">
     1630        <front>
     1631          <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
     1632          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1633            <organization abbrev="Day Software">Day Software</organization>
     1634            <address><email>fielding@gbiv.com</email></address>
     1635          </author>
     1636          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1637            <organization>One Laptop per Child</organization>
     1638            <address><email>jg@laptop.org</email></address>
     1639          </author>
     1640          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1641            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1642            <address><email>JeffMogul@acm.org</email></address>
     1643          </author>
     1644          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1645            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1646            <address><email>henrikn@microsoft.com</email></address>
     1647          </author>
     1648          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1649            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1650            <address><email>LMM@acm.org</email></address>
     1651          </author>
     1652          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1653            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1654            <address><email>paulle@microsoft.com</email></address>
     1655          </author>
     1656          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1657            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1658            <address><email>timbl@w3.org</email></address>
     1659          </author>
     1660          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1661            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1662            <address><email>ylafon@w3.org</email></address>
     1663          </author>
     1664          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1665            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1666            <address><email>julian.reschke@greenbytes.de</email></address>
     1667          </author>
     1668          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1669        </front>
     1670        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
     1671        <x:source basename="p7-auth" href="p7-auth.xml" />
     1672      </reference>
     1673
     1674      <reference anchor="RFC2047">
     1675        <front>
     1676          <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
     1677            Part Three: Message Header Extensions for Non-ASCII Text</title>
     1678          <author fullname="Keith Moore" initials="K." surname="Moore">
     1679            <organization>University of Tennessee</organization>
     1680            <address><email>moore@cs.utk.edu</email></address>
     1681          </author>
     1682          <date month="November" year="1996" />
     1683        </front>
     1684        <seriesInfo name="RFC" value="2047" />
     1685      </reference>
     1686
     1687      <reference anchor="RFC2119">
     1688        <front>
     1689          <title>Key words for use in RFCs to Indicate Requirement Levels</title>
     1690          <author fullname="Scott Bradner" initials="S." surname="Bradner">
     1691            <organization>Harvard University</organization>
     1692            <address><email>sob@harvard.edu</email></address>
     1693          </author>
     1694          <date month="March" year="1997" />
     1695        </front>
     1696        <seriesInfo name="BCP" value="14" />
     1697        <seriesInfo name="RFC" value="2119" />
     1698      </reference>
     1699
     1700      <reference anchor="RFC5234">
     1701        <front>
     1702          <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
     1703          <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
     1704            <organization>Brandenburg InternetWorking</organization>
     1705            <address>
     1706            <postal>
     1707            <street>675 Spruce Dr.</street>
     1708            <city>Sunnyvale</city>
     1709            <region>CA</region>
     1710            <code>94086</code>
     1711            <country>US</country></postal>
     1712            <phone>+1.408.246.8253</phone>
     1713            <email>dcrocker@bbiw.net</email></address> 
     1714          </author>
     1715          <author initials="P." surname="Overell" fullname="Paul Overell">
     1716            <organization>THUS plc.</organization>
     1717            <address>
     1718            <postal>
     1719            <street>1/2 Berkeley Square</street>
     1720            <street>99 Berkely Street</street>
     1721            <city>Glasgow</city>
     1722            <code>G3 7HR</code>
     1723            <country>UK</country></postal>
     1724            <email>paul.overell@thus.net</email></address>
     1725          </author>
     1726          <date month="January" year="2008"/>
     1727        </front>
     1728        <seriesInfo name="STD" value="68"/>
     1729        <seriesInfo name="RFC" value="5234"/>
     1730      </reference>
     1731     
     1732    </references>
     1733
     1734    <references title="Informative References">
     1735
     1736      <reference anchor="RFC1305">
     1737        <front>
     1738          <title>Network Time Protocol (Version 3) Specification, Implementation</title>
     1739          <author fullname="David L. Mills" initials="D." surname="Mills">
     1740            <organization>University of Delaware, Electrical Engineering Department</organization>
     1741            <address><email>mills@udel.edu</email></address>
     1742          </author>
     1743          <date month="March" year="1992" />
     1744        </front>
     1745        <seriesInfo name="RFC" value="1305" />
     1746      </reference>
     1747
     1748      <reference anchor="RFC2616">
     1749        <front>
     1750          <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
     1751          <author fullname="R. Fielding" initials="R." surname="Fielding">
     1752            <organization>University of California, Irvine</organization>
     1753            <address><email>fielding@ics.uci.edu</email></address>
     1754          </author>
     1755          <author fullname="J. Gettys" initials="J." surname="Gettys">
     1756            <organization>W3C</organization>
     1757            <address><email>jg@w3.org</email></address>
     1758          </author>
     1759          <author fullname="J. Mogul" initials="J." surname="Mogul">
     1760            <organization>Compaq Computer Corporation</organization>
     1761            <address><email>mogul@wrl.dec.com</email></address>
     1762          </author>
     1763          <author fullname="H. Frystyk" initials="H." surname="Frystyk">
     1764            <organization>MIT Laboratory for Computer Science</organization>
     1765            <address><email>frystyk@w3.org</email></address>
     1766          </author>
     1767          <author fullname="L. Masinter" initials="L." surname="Masinter">
     1768            <organization>Xerox Corporation</organization>
     1769            <address><email>masinter@parc.xerox.com</email></address>
     1770          </author>
     1771          <author fullname="P. Leach" initials="P." surname="Leach">
     1772            <organization>Microsoft Corporation</organization>
     1773            <address><email>paulle@microsoft.com</email></address>
     1774          </author>
     1775          <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
     1776            <organization>W3C</organization>
     1777            <address><email>timbl@w3.org</email></address>
     1778          </author>
     1779          <date month="June" year="1999" />
     1780        </front>
     1781        <seriesInfo name="RFC" value="2616" />
     1782      </reference>
     1783
     1784      <reference anchor="RFC3864">
     1785        <front>
     1786          <title>Registration Procedures for Message Header Fields</title>
     1787          <author fullname="G. Klyne" initials="G." surname="Klyne">
     1788            <organization>Nine by Nine</organization>
     1789            <address><email>GK-IETF@ninebynine.org</email></address>
     1790          </author>
     1791          <author fullname="M. Nottingham" initials="M." surname="Nottingham">
     1792            <organization>BEA Systems</organization>
     1793            <address><email>mnot@pobox.com</email></address>
     1794          </author>
     1795          <author fullname="J. Mogul" initials="J." surname="Mogul">
     1796            <organization>HP Labs</organization>
     1797            <address><email>JeffMogul@acm.org</email></address>
     1798          </author>
     1799          <date month="September" year="2004" />
     1800        </front>
     1801        <seriesInfo name="BCP" value="90" />
     1802        <seriesInfo name="RFC" value="3864" />
     1803      </reference>
     1804
     1805    </references>
     1806
     1807    <section anchor="compatibility" title="Compatibility with Previous Versions">
     1808
     1809      <section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
     1810        <t>A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
     1811          this missing case. (Sections <xref format="counter" target="response.cacheability" />,
     1812            <xref format="counter" target="header.cache-control" />).</t>
     1813        <t>Transfer-coding and message lengths all interact in ways that required fixing exactly
     1814          when chunked encoding is used (to allow for transfer encoding that may not be self
     1815          delimiting); it was important to straighten out exactly how message lengths are computed.
     1816          (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)
     1817          <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
     1818          </t>
     1819        <t>Proxies should be able to add Content-Length when appropriate.
     1820          <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
     1821        </t>
     1822        <t>Range request responses would become very verbose if all meta-data were always returned;
     1823          by allowing the server to only send needed headers in a 206 response, this problem can be
     1824          avoided. (<xref target="combining.headers" />)</t>
     1825        <t>The Cache-Control: max-age directive was not properly defined for responses. (<xref target="cache-response-directive"/>)</t>
     1826        <t>Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
     1827            format="counter" target="expiration.model" />, <xref format="counter"
     1828            target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
     1829          and <xref format="counter" target="header.warning" />) Warning also needed to be a general
     1830          header, as PUT or other methods may have need for it in requests.</t>
     1831      </section>
     1832
     1833      <section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
     1834        <t>Clarify denial of service attack avoidance requirement. (<xref
     1835            target="invalidation.after.updates.or.deletions" />)</t>
     1836      </section>
     1837
     1838    </section>
    29811839
    29821840<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
     
    30571915</artwork></figure></section>
    30581916
    3059 <section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
    3060 
    3061 <section title="Since RFC2616">
    3062 <t>
    3063   Extracted relevant partitions from <xref target="RFC2616"/>.
    3064 </t>
    3065 </section>
    3066 
    3067 <section title="Since draft-ietf-httpbis-p6-cache-00">
    3068 <t>
    3069   Closed issues:
    3070   <list style="symbols">
    3071     <t>
    3072       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
    3073       "Trailer"
    3074       (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
    3075     </t>
    3076     <t>
    3077       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
    3078       "Invalidation after Update or Delete"
    3079       (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
    3080     </t>
    3081     <t>
    3082       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
    3083       "Normative and Informative references"
    3084     </t>
    3085     <t>
    3086       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
    3087       "Date reference typo"
    3088     </t>
    3089     <t>
    3090       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
    3091       "Connection header text"
    3092     </t>
    3093     <t>
    3094       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
    3095       "Informative references"
    3096     </t>
    3097     <t>
    3098       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
    3099       "ISO-8859-1 Reference"
    3100     </t>
    3101     <t>
    3102       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
    3103       "Normative up-to-date references"
    3104     </t>
    3105     <t>
    3106       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
    3107       "typo in 13.2.2"
    3108     </t>
    3109   </list>
    3110 </t>
    3111 <t>
    3112   Other changes:
    3113   <list style="symbols">
    3114     <t>
    3115       Use names of RFC4234 core rule DQUOTE (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
    3116     </t>
    3117   </list>
    3118 </t>
    3119 </section>
    3120 
    3121 <section title="Since draft-ietf-httpbis-p6-cache-01">
    3122 <t>
    3123   Closed issues:
    3124   <list style="symbols">
    3125     <t>
    3126       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
    3127       "rel_path not used"
    3128     </t>
    3129   </list>
    3130 </t>
    3131 <t>
    3132   Other changes:
    3133   <list style="symbols">
    3134     <t>
    3135        Get rid of duplicate BNF rule names ("host" -> "uri-host")
    3136        (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
    3137     </t>
    3138     <t>
    3139       Add explicit references to BNF syntax and rules imported from other parts of the specification.
    3140     </t>
    3141   </list>
    3142 </t>
    3143 </section>
    3144 
    3145 <section title="Since draft-ietf-httpbis-p6-cache-02" anchor="changes.since.02">
    3146 <t>
    3147   Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
    3148   <list style="symbols">
    3149     <t>
    3150       Reference RFC 3984, and update header registrations for headers defined
    3151       in this document.
    3152     </t>
    3153   </list>
    3154 </t>
    3155 </section>
    3156 
    3157 <section title="Since draft-ietf-httpbis-p6-cache-03" anchor="changes.since.03">
    3158 <t>
    3159   Closed issues:
    3160   <list style="symbols">
    3161     <t>
    3162       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>:
    3163       "Vary header classification"
    3164     </t>
    3165   </list>
    3166 </t>
    3167 </section>
    3168 
    3169 <section title="Since draft-ietf-httpbis-p6-cache-04" anchor="changes.since.04">
    3170 <t>
    3171   Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
    3172   <list style="symbols">
    3173     <t>
    3174       Use "/" instead of "|" for alternatives.
    3175     </t>
    3176     <t>
    3177       Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
    3178       whitespace ("OWS") and required whitespace ("RWS").
    3179     </t>
    3180     <t>
    3181       Rewrite ABNFs to spell out whitespace rules, factor out
    3182       header value format definitions.
    3183     </t>
    3184   </list>
    3185 </t>
    3186 </section>
    3187 
    3188 <section title="Since draft-ietf-httpbis-p6-cache-05" anchor="changes.since.05">
    3189 <t>
    3190   Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
    3191   <list style="symbols">
    3192     <t>
    3193       Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
    3194     </t>
    3195   </list>
    3196 </t>
    3197 </section>
    3198 
    3199 </section>
    3200 
    3201 </back>
     1917    <section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
     1918
     1919      <section title="Since RFC2616">
     1920        <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
     1921      </section>
     1922
     1923      <section title="Since draft-ietf-httpbis-p6-cache-00">
     1924        <t>Closed issues: <list style="symbols">
     1925            <t>
     1926              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer"
     1927                (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
     1928            <t>
     1929              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation
     1930              after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
     1931            <t>
     1932              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and
     1933              Informative references"</t>
     1934            <t>
     1935              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date
     1936              reference typo"</t>
     1937            <t>
     1938              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection
     1939              header text"</t>
     1940            <t>
     1941              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative
     1942              references"</t>
     1943            <t>
     1944              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1
     1945              Reference"</t>
     1946            <t>
     1947              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative
     1948              up-to-date references"</t>
     1949            <t>
     1950              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in
     1951              13.2.2"</t>
     1952          </list>
     1953        </t>
     1954        <t>Other changes: <list style="symbols">
     1955            <t>Use names of RFC4234 core rule DQUOTE (work in progress on <eref
     1956                target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
     1957          </list>
     1958        </t>
     1959      </section>
     1960
     1961      <section title="Since draft-ietf-httpbis-p6-cache-01">
     1962        <t>Closed issues: <list style="symbols">
     1963            <t>
     1964              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not
     1965              used"</t>
     1966          </list>
     1967        </t>
     1968        <t>Other changes: <list style="symbols">
     1969            <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
     1970              on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
     1971            <t>Add explicit references to BNF syntax and rules imported from other parts of the
     1972              specification.</t>
     1973          </list>
     1974        </t>
     1975      </section>
     1976
     1977      <section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
     1978        <t>Ongoing work on IANA Message Header Registration (<eref
     1979            target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />): <list style="symbols">
     1980            <t>Reference RFC 3984, and update header registrations for headers defined in this
     1981              document.</t>
     1982          </list>
     1983        </t>
     1984      </section>
     1985
     1986      <section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
     1987        <t>Closed issues: <list style="symbols">
     1988            <t>
     1989              <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header
     1990              classification"</t>
     1991          </list>
     1992        </t>
     1993      </section>
     1994
     1995      <section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
     1996        <t>
     1997          Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
     1998          <list style="symbols">
     1999            <t>
     2000              Use "/" instead of "|" for alternatives.
     2001            </t>
     2002            <t>
     2003              Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
     2004              whitespace ("OWS") and required whitespace ("RWS").
     2005            </t>
     2006            <t>
     2007              Rewrite ABNFs to spell out whitespace rules, factor out
     2008              header value format definitions.
     2009            </t>
     2010          </list>
     2011        </t>
     2012      </section>
     2013
     2014      <section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
     2015        <t>
     2016          This is a total rewrite of this part of the specification.
     2017        </t>
     2018        <t>
     2019          In addition: Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
     2020          <list style="symbols">
     2021            <t>
     2022              Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
     2023            </t>
     2024          </list>
     2025        </t>
     2026      </section>
     2027
     2028    </section>
     2029  </back>
    32022030</rfc>
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