Changeset 394


Ignore:
Timestamp:
Nov 14, 2008, 7:13:58 PM (11 years ago)
Author:
mnot@…
Message:

big stab at a major rewrite

File:
1 edited

Legend:

Unmodified
Added
Removed
  • draft-ietf-httpbis/latest-roy/p6-cache.xml

    r348 r394  
    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'/>">
     
    4345<?rfc-ext allow-markup-in-artwork="yes" ?>
    4446<?rfc-ext include-references-in-index="yes" ?>
    45 <rfc obsoletes="2616" category="std" x:maturity-level="draft"
    46      ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
    47      xmlns:x='http://purl.org/net/xml2rfc/ext'>
    48 <front>
    49 
    50   <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
    51 
    52   <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    53     <organization abbrev="Day Software">Day Software</organization>
    54     <address>
     47<?oxygen RNGSchema="../../rfc2629xslt/rfc2629-ext.rnc" type="compact"?>
     48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="full3978"
     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>
    5557      <postal>
    5658        <street>23 Corporate Plaza DR, Suite 280</street>
     
    6567      <uri>http://roy.gbiv.com/</uri>
    6668    </address>
    67   </author>
    68 
    69   <author initials="J." surname="Gettys" fullname="Jim Gettys">
    70     <organization>One Laptop per Child</organization>
    71     <address>
     69    </author>
     70
     71    <author fullname="Jim Gettys" initials="J." surname="Gettys">
     72      <organization>One Laptop per Child</organization>
     73      <address>
    7274      <postal>
    7375        <street>21 Oak Knoll Road</street>
     
    8082      <uri>http://www.laptop.org/</uri>
    8183    </address>
    82   </author>
    83  
    84   <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    85     <organization abbrev="HP">Hewlett-Packard Company</organization>
    86     <address>
     84    </author>
     85
     86    <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     87      <organization abbrev="HP">Hewlett-Packard Company</organization>
     88      <address>
    8789      <postal>
    8890        <street>HP Labs, Large Scale Systems Group</street>
     
    9597      <email>JeffMogul@acm.org</email>
    9698    </address>
    97   </author>
    98 
    99   <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    100     <organization abbrev="Microsoft">Microsoft Corporation</organization>
    101     <address>
     99    </author>
     100
     101    <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     102      <organization abbrev="Microsoft">Microsoft Corporation</organization>
     103      <address>
    102104      <postal>
    103105        <street>1 Microsoft Way</street>
     
    109111      <email>henrikn@microsoft.com</email>
    110112    </address>
    111   </author>
    112 
    113   <author initials="L." surname="Masinter" fullname="Larry Masinter">
    114     <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    115     <address>
     113    </author>
     114
     115    <author fullname="Larry Masinter" initials="L." surname="Masinter">
     116      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     117      <address>
    116118      <postal>
    117119        <street>345 Park Ave</street>
     
    124126      <uri>http://larry.masinter.net/</uri>
    125127    </address>
    126   </author>
    127  
    128   <author initials="P." surname="Leach" fullname="Paul J. Leach">
    129     <organization abbrev="Microsoft">Microsoft Corporation</organization>
    130     <address>
     128    </author>
     129
     130    <author fullname="Paul J. Leach" initials="P." surname="Leach">
     131      <organization abbrev="Microsoft">Microsoft Corporation</organization>
     132      <address>
    131133      <postal>
    132134        <street>1 Microsoft Way</street>
     
    137139      <email>paulle@microsoft.com</email>
    138140    </address>
    139   </author>
    140    
    141   <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    142     <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    143     <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>
    144146      <postal>
    145147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
     
    154156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
    155157    </address>
    156   </author>
    157 
    158   <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    159     <organization abbrev="W3C">World Wide Web Consortium</organization>
    160     <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>
    161163      <postal>
    162164        <street>W3C / ERCIM</street>
     
    170172      <uri>http://www.raubacapeu.net/people/yves/</uri>
    171173    </address>
    172   </author>
    173 
    174   <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    175     <organization abbrev="greenbytes">greenbytes GmbH</organization>
    176     <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>
    177179      <postal>
    178180        <street>Hafenweg 16</street>
     
    185187      <uri>http://greenbytes.de/tech/webdav/</uri>     
    186188    </address>
    187   </author>
    188 
    189   <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    190 
    191 <abstract>
    192 <t>
    193    The Hypertext Transfer Protocol (HTTP) is an application-level
    194    protocol for distributed, collaborative, hypermedia information
    195    systems.
    196    This document is Part 6 of the seven-part specification
    197    that defines the protocol referred to as "HTTP/1.1" and, taken together,
    198    obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
    199    and the associated header fields that control cache behavior or indicate
    200    cacheable response messages.
    201 </t>
    202 </abstract>
    203 
    204 <note title="Editorial Note (To be removed by RFC Editor)">
    205   <t>
    206     Discussion of this draft should take place on the HTTPBIS working group
    207     mailing list (ietf-http-wg@w3.org). The current issues list is
    208     at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
    209     and related documents (including fancy diffs) can be found at
    210     <eref target="http://tools.ietf.org/wg/httpbis/"/>.
    211   </t>
    212   <t>
    213     The changes in this draft are summarized in <xref target="changes.since.04"/>.
    214   </t>
    215 </note>
    216 </front>
    217 <middle>
    218 <section title="Introduction" anchor="caching">
    219 <t>
    220    HTTP is typically used for distributed information systems, where
    221    performance can be improved by the use of response caches.  This document
    222    defines aspects of HTTP/1.1 related to caching and reusing response messages.
    223 </t>
    224 
    225 <section title="Purpose" anchor="intro.purpose">
    226 <iref item="cache"/>
    227 <t>
    228    An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
    229    and the subsystem that controls its message storage, retrieval, and
    230    deletion. A cache stores cacheable responses in order to reduce the
    231    response time and network bandwidth consumption on future, equivalent
    232    requests. Any client or server may include a cache, though a cache
    233    cannot be used by a server that is acting as a tunnel.
    234 </t>
    235 <t>
    236    Caching would be useless if it did not significantly improve
    237    performance. The goal of caching in HTTP/1.1 is to reuse a prior response
    238    message to satisfy a current request.  In some cases, the existing response
    239    can be reused without the need for a network request, reducing latency and
    240    network round-trips; we use an "expiration" mechanism for this purpose
    241    (see <xref target="expiration.model"/>).  Even when a new request is required,
    242    it is often possible to reuse all or parts of the payload of a prior response
    243    to satisfy the request, thereby reducing network bandwidth usage; we use a
    244    "validation" mechanism for this purpose (see <xref target="validation.model"/>).
    245 </t>
    246 <iref item="semantically transparent"/>
    247 <t>
    248    A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
    249    respect to a particular response, when its use affects neither the
    250    requesting client nor the origin server, except to improve
    251    performance. When a cache is semantically transparent, the client
    252    receives exactly the same response status and payload
    253    that it would have received had its request been handled directly
    254    by the origin server.
    255 </t>
    256 <t>
    257    In an ideal world, all interactions with an HTTP cache would be
    258    semantically transparent.  However, for some resources, semantic
    259    transparency is not always necessary and can be effectively traded
    260    for the sake of bandwidth scaling, disconnected operation, and
    261    high availability.  HTTP/1.1 allows origin servers, caches,
    262    and clients to explicitly reduce transparency when necessary.
    263    However, because non-transparent operation may confuse non-expert
    264    users and might be incompatible with certain server applications
    265    (such as those for ordering merchandise), the protocol requires that
    266    transparency be relaxed
    267   <list style="symbols">
    268      <t>only by an explicit protocol-level request when relaxed by
    269         client or origin server</t>
    270 
    271      <t>only with an explicit warning to the end user when relaxed by
    272         cache or client</t>
    273   </list>
    274 </t>
    275 <t>
    276    Therefore, HTTP/1.1 provides these important elements:
    277   <list style="numbers">
    278       <t>Protocol features that provide full semantic transparency when
    279          this is required by all parties.</t>
    280 
    281       <t>Protocol features that allow an origin server or user agent to
    282          explicitly request and control non-transparent operation.</t>
    283 
    284       <t>Protocol features that allow a cache to attach warnings to
    285          responses that do not preserve the requested approximation of
    286          semantic transparency.</t>
    287   </list>
    288 </t>
    289 <t>
    290    A basic principle is that it must be possible for the clients to
    291    detect any potential relaxation of semantic transparency.
    292   <list><t>
    293       <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
    294       design decisions not explicitly discussed in this specification.
    295       If a decision might affect semantic transparency, the implementor
    296       ought to err on the side of maintaining transparency unless a
    297       careful and complete analysis shows significant benefits in
    298       breaking transparency.
    299     </t></list>
    300 </t>
    301 </section>
    302 
    303 <section title="Terminology" anchor="intro.terminology">
    304 <t>
    305    This specification uses a number of terms to refer to the roles
    306    played by participants in, and objects of, HTTP caching.
    307 </t>
    308 <t>
    309   <iref item="cacheable"/>
    310   <x:dfn>cacheable</x:dfn>
    311   <list>
    312     <t>
    313       A response is cacheable if a cache is allowed to store a copy of
    314       the response message for use in answering subsequent requests.
    315       Even when a response is cacheable, there may
    316       be additional constraints on whether a cache can use the cached
    317       copy for a particular request.
    318     </t>
    319   </list>
    320 </t>
    321 <t>
    322   <iref item="first-hand"/>
    323   <x:dfn>first-hand</x:dfn>
    324   <list>
    325     <t>
    326       A response is first-hand if it comes directly and without
    327       unnecessary delay from the origin server, perhaps via one or more
    328       proxies. A response is also first-hand if its validity has just
    329       been checked directly with the origin server.
    330     </t>
    331   </list>
    332 </t>
    333 <t>
    334   <iref item="explicit expiration time"/>
    335   <x:dfn>explicit expiration time</x:dfn>
    336   <list>
    337     <t>
    338       The time at which the origin server intends that an entity should
    339       no longer be returned by a cache without further validation.
    340     </t>
    341   </list>
    342 </t>
    343 <t>
    344   <iref item="heuristic expiration time"/>
    345   <x:dfn>heuristic expiration time</x:dfn>
    346   <list>
    347     <t>
    348       An expiration time assigned by a cache when no explicit expiration
    349       time is available.
    350     </t>
    351   </list>
    352 </t>
    353 <t>
    354   <iref item="age"/>
    355   <x:dfn>age</x:dfn>
    356   <list>
    357     <t>
    358       The age of a response is the time since it was sent by, or
    359       successfully validated with, the origin server.
    360     </t>
    361   </list>
    362 </t>
    363 <t>
    364   <iref item="freshness lifetime"/>
    365   <x:dfn>freshness lifetime</x:dfn>
    366   <list>
    367     <t>
    368       The length of time between the generation of a response and its
    369       expiration time.
    370     </t>
    371   </list>
    372 </t>
    373 <t>
    374   <iref item="fresh"/>
    375   <x:dfn>fresh</x:dfn>
    376   <list>
    377     <t>
    378       A response is fresh if its age has not yet exceeded its freshness
    379       lifetime.
    380     </t>
    381   </list>
    382 </t>
    383 <t>
    384   <iref item="stale"/>
    385   <x:dfn>stale</x:dfn>
    386   <list>
    387     <t>
    388       A response is stale if its age has passed its freshness lifetime.
    389     </t>
    390   </list>
    391 </t>
    392 <t>
    393   <iref item="validator"/>
    394   <x:dfn>validator</x:dfn>
    395   <list>
    396     <t>
    397       A protocol element (e.g., an entity tag or a Last-Modified time)
    398       that is used to find out whether a cache entry is an equivalent
    399       copy of an entity.
    400     </t>
    401   </list>
    402 </t>
    403 </section>
    404 
    405 <section title="Requirements" anchor="intro.requirements">
    406 <t>
    407    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    408    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    409    document are to be interpreted as described in <xref target="RFC2119"/>.
    410 </t>
    411 <t>
    412    An implementation is not compliant if it fails to satisfy one or more
    413    of the &MUST; or &REQUIRED; level requirements for the protocols it
    414    implements. An implementation that satisfies all the &MUST; or &REQUIRED;
    415    level and all the &SHOULD; level requirements for its protocols is said
    416    to be "unconditionally compliant"; one that satisfies all the &MUST;
    417    level requirements but not all the &SHOULD; level requirements for its
    418    protocols is said to be "conditionally compliant."
    419 </t>
    420 </section>
    421 </section>
    422 
    423 
    424 <section title="Notational Conventions and Generic Grammar" anchor="notation">
    425   <x:anchor-alias value="DIGIT"/>
    426   <x:anchor-alias value="DQUOTE"/>
    427   <x:anchor-alias value="quoted-string"/>
    428   <x:anchor-alias value="SP"/>
    429   <x:anchor-alias value="token"/>
    430 <t>
    431   This specification uses the ABNF syntax defined in &notation-abnf; and
    432   the core rules defined in &basic-rules;:
    433   <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
    434   <eref target="http://ietf.org/wg/httpbis/trac/ticket/36"/>.</cref>
    435 </t>
    436 <figure><artwork type="abnf2616">
     189    </author>
     190
     191    <date month="&ID-MONTH;" year="&ID-YEAR;" />
     192
     193    <abstract>
     194      <t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
     195        collaborative, hypermedia information systems. This document is Part 6 of the seven-part
     196        specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
     197        obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
     198        fields that control cache behavior or indicate cacheable response messages.</t>
     199    </abstract>
     200
     201    <note title="Editorial Note (To be removed by RFC Editor)">
     202      <t>Discussion of this draft should take place on the HTTPBIS working group mailing list
     203        (ietf-http-wg@w3.org). The current issues list is at <eref
     204          target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
     205        (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.</t>
     206      <t>The changes in this draft are summarized in <xref target="changes.since.04" />.</t>
     207    </note>
     208  </front>
     209  <middle>
     210    <section anchor="caching" title="Introduction">
     211      <t>HTTP is typically used for distributed information systems, where performance can be
     212        improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
     213        caching and reusing response messages.</t>
     214
     215      <section anchor="intro.purpose" title="Purpose">
     216        <iref item="cache" />
     217        <t>An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
     218          controls its message storage, retrieval, and deletion. A cache stores cacheable responses
     219          in order to reduce the response time and network bandwidth consumption on future,
     220          equivalent requests. Any client or server may include a cache, though a cache cannot be
     221          used by a server that is acting as a tunnel.</t>
     222        <t>Caching would be useless if it did not significantly improve performance. The goal of
     223          caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
     224          some cases, a stored response can be reused without the need for a network request,
     225          reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
     226          (see <xref target="expiration.model" />). Even when a new request is required, it is often
     227          possible to reuse all or parts of the payload of a prior response to satisfy the request,
     228          thereby reducing network bandwidth usage; a "validation" mechanism is used for this purpose
     229          (see <xref target="validation.model" />).</t>
     230      </section>
     231
     232      <section anchor="intro.terminology" title="Terminology">
     233        <t>This specification uses a number of terms to refer to the roles played by participants
     234          in, and objects of, HTTP caching.</t>
     235        <t>
     236          <iref item="cacheable" />
     237          <x:dfn>cacheable</x:dfn>
     238          <list>
     239            <t>A response is cacheable if a cache is allowed to store a copy of the response message
     240              for use in answering subsequent requests. Even when a response is cacheable, there may
     241              be additional constraints on whether a cache can use the cached copy to satisfy a
     242              particular request.</t>
     243          </list>
     244        </t>
     245        <t>
     246          <iref item="first-hand" />
     247          <x:dfn>first-hand</x:dfn>
     248          <list>
     249            <t>A response is first-hand if it comes from the origin server, perhaps via one or more
     250              proxies, but not from cache. A response is also first-hand if its validity has just
     251              been checked directly with the origin server.</t>
     252          </list>
     253        </t>
     254        <t>
     255          <iref item="explicit expiration time" />
     256          <x:dfn>explicit expiration time</x:dfn>
     257          <list>
     258            <t>The time at which the origin server intends that an entity should no longer be
     259              returned by a cache without further validation.</t>
     260          </list>
     261        </t>
     262        <t>
     263          <iref item="heuristic expiration time" />
     264          <x:dfn>heuristic expiration time</x:dfn>
     265          <list>
     266            <t>An expiration time assigned by a cache when no explicit expiration time is
     267            available.</t>
     268          </list>
     269        </t>
     270        <t>
     271          <iref item="age" />
     272          <x:dfn>age</x:dfn>
     273          <list>
     274            <t>The age of a response is the time since it was sent by, or successfully validated
     275              with, the origin server.</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.</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>
     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 that 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    </section>
     330
     331
     332    <section anchor="notation" title="Notational Conventions and Generic Grammar">
     333      <x:anchor-alias value="DIGIT" />
     334      <x:anchor-alias value="DQUOTE" />
     335      <x:anchor-alias value="quoted-string" />
     336      <x:anchor-alias value="SP" />
     337      <x:anchor-alias value="token" />
     338      <t>This specification uses the ABNF syntax defined in &notation-abnf; and the core rules
     339        defined in &basic-rules;: <cref anchor="abnf.dep">ABNF syntax and basic rules will be
     340          adopted from RFC 5234, see <eref target="http://ietf.org/wg/httpbis/trac/ticket/36"
     341         />.</cref>
     342      </t>
     343      <figure>
     344        <artwork type="abnf2616">
    437345  <x:ref>DIGIT</x:ref>         = &lt;DIGIT, defined in &basic-rules;&gt;
    438346  <x:ref>DQUOTE</x:ref>        = &lt;DQUOTE, defined in &basic-rules;&gt;
    439347  <x:ref>SP</x:ref>            = &lt;SP, defined in &basic-rules;&gt;
    440 </artwork></figure>
    441 <figure><artwork type="abnf2616">
     348</artwork>
     349      </figure>
     350      <figure>
     351        <artwork type="abnf2616">
    442352  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
    443353  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
    444 </artwork></figure>
    445 <t anchor="abnf.dependencies">
    446   <x:anchor-alias value="field-name"/>
    447   <x:anchor-alias value="HTTP-date"/>
    448   <x:anchor-alias value="port"/>
    449   <x:anchor-alias value="pseudonym"/>
    450   <x:anchor-alias value="uri-host"/>
    451   The ABNF rules below are defined in other parts:
    452 </t>
    453 <figure><!--Part1--><artwork type="abnf2616">
     354</artwork>
     355      </figure>
     356      <t anchor="abnf.dependencies">
     357        <x:anchor-alias value="field-name" />
     358        <x:anchor-alias value="HTTP-date" />
     359        <x:anchor-alias value="port" />
     360        <x:anchor-alias value="pseudonym" />
     361        <x:anchor-alias value="uri-host" /> The ABNF rules below are defined in other parts:</t>
     362      <figure>
     363        <!--Part1-->
     364        <artwork type="abnf2616">
    454365  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
    455366  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
     
    457368  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
    458369  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
    459 </artwork></figure>
    460 </section>
    461 
    462 <section title="Overview" anchor="caching.overview">
    463 <section title="Cache Correctness" anchor="cache.correctness">
    464 <t>
    465    A correct cache &MUST; respond to a request with the most up-to-date
    466    response held by the cache that is appropriate to the request (see
    467    Sections <xref target="disambiguating.expiration.values" format="counter"/>,
    468    <xref target="disambiguating.multiple.responses" format="counter"/>,
    469    and <xref target="cache.replacement" format="counter"/>) which meets one of the following
    470    conditions:
    471   <list style="numbers">
    472       <t>It has been checked for equivalence with what the origin server
    473          would have returned by revalidating the response with the
    474          origin server (<xref target="validation.model"/>);</t>
    475 
    476       <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
    477          this means it meets the least restrictive freshness requirement
    478          of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
    479          the origin server so specifies, it is the freshness requirement
    480          of the origin server alone.
    481 
    482          If a stored response is not "fresh enough" by the most
    483          restrictive freshness requirement of both the client and the
    484          origin server, in carefully considered circumstances the cache
    485          &MAY; still return the response with the appropriate Warning
    486          header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
    487          and <xref target="header.warning" format="counter"/>), unless such a response
    488          is prohibited (e.g., by a "no-store" cache-directive, or by a
    489          "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
    490 
    491       <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
    492          or error (4xx or 5xx) response message.</t>
    493   </list>
    494 </t>
    495 <t>
    496    If the cache can not communicate with the origin server, then a
    497    correct cache &SHOULD; respond as above if the response can be
    498    correctly served from the cache; if not it &MUST; return an error or
    499    warning indicating that there was a communication failure.
    500 </t>
    501 <t>
    502    If a cache receives a response (either an entire response, or a 304
    503    (Not Modified) response) that it would normally forward to the
    504    requesting client, and the received response is no longer fresh, the
    505    cache &SHOULD; forward it to the requesting client without adding a new
    506    Warning (but without removing any existing Warning headers). A cache
    507    &SHOULD-NOT;  attempt to revalidate a response simply because that
    508    response became stale in transit; this might lead to an infinite
    509    loop. A user agent that receives a stale response without a Warning
    510    &MAY; display a warning indication to the user.
    511 </t>
    512 </section>
    513 
    514 <section title="Warnings" anchor="warnings">
    515 <t>
    516    Whenever a cache returns a response that is neither first-hand nor
    517    "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
    518    &MUST; attach a warning to that effect, using a Warning general-header.
    519    The Warning header and the currently defined warnings are described
    520    in <xref target="header.warning"/>.
    521 </t>
    522 </section>
    523 
    524 <section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
    525 <t>
    526    The basic cache mechanisms in HTTP/1.1 (server-specified expiration
    527    times and validators) are implicit directives to caches. In some
    528    cases, a server or client might need to provide explicit directives
    529    to the HTTP caches. We use the Cache-Control header for this purpose.
    530 </t>
    531 <t>
    532    The Cache-Control header allows a client or server to transmit a
    533    variety of directives in either requests or responses. These
    534    directives typically override the default caching algorithms. As a
    535    general rule, if there is any apparent conflict between header
    536    values, the most restrictive interpretation is applied (that is, the
    537    one that is most likely to preserve semantic transparency). However,
    538    in some cases, cache-control directives are explicitly specified as
    539    weakening the approximation of semantic transparency (for example,
    540    "max-stale" or "public").
    541 </t>
    542 <t>
    543    The cache-control directives are described in detail in <xref target="header.cache-control"/>.
    544 </t>
    545 </section>
    546 
    547 <section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
    548 <t>
    549    Many user agents make it possible for users to override the basic
    550    caching mechanisms. For example, the user agent might allow the user
    551    to specify that cached entities (even explicitly stale ones) are
    552    never validated. Or the user agent might habitually add "Cache-Control:
    553    max-stale=3600" to every request. The user agent &SHOULD-NOT;
    554    default to either non-transparent behavior, or behavior that results
    555    in abnormally ineffective caching, but &MAY; be explicitly configured
    556    to do so by an explicit action of the user.
    557 </t>
    558 <t>
    559    If the user has overridden the basic caching mechanisms, the user
    560    agent &SHOULD; explicitly indicate to the user whenever this results in
    561    the display of information that might not meet the server's
    562    transparency requirements (in particular, if the displayed entity is
    563    known to be stale). Since the protocol normally allows the user agent
    564    to determine if responses are stale or not, this indication need only
    565    be displayed when this actually happens. The indication need not be a
    566    dialog box; it could be an icon (for example, a picture of a rotting
    567    fish) or some other indicator.
    568 </t>
    569 <t>
    570    If the user has overridden the caching mechanisms in a way that would
    571    abnormally reduce the effectiveness of caches, the user agent &SHOULD;
    572    continually indicate this state to the user (for example, by a
    573    display of a picture of currency in flames) so that the user does not
    574    inadvertently consume excess resources or suffer from excessive
    575    latency.
    576 </t>
    577 </section>
    578 
    579 <section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
    580 <t>
    581    In some cases, the operator of a cache &MAY; choose to configure it to
    582    return stale responses even when not requested by clients. This
    583    decision ought not be made lightly, but may be necessary for reasons
    584    of availability or performance, especially when the cache is poorly
    585    connected to the origin server. Whenever a cache returns a stale
    586    response, it &MUST; mark it as such (using a Warning header) enabling
    587    the client software to alert the user that there might be a potential
    588    problem.
    589 </t>
    590 <t>
    591    It also allows the user agent to take steps to obtain a first-hand or
    592    fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
    593    response if the client explicitly requests a first-hand or fresh one,
    594    unless it is impossible to comply for technical or policy reasons.
    595 </t>
    596 </section>
    597 
    598 <section title="Client-controlled Behavior" anchor="client-controlled.behavior">
    599 <t>
    600    While the origin server (and to a lesser extent, intermediate caches,
    601    by their contribution to the age of a response) are the primary
    602    source of expiration information, in some cases the client might need
    603    to control a cache's decision about whether to return a cached
    604    response without validating it. Clients do this using several
    605    directives of the Cache-Control header.
    606 </t>
    607 <t>
    608    A client's request &MAY; specify the maximum age it is willing to
    609    accept of an unvalidated response; specifying a value of zero forces
    610    the cache(s) to revalidate all responses. A client &MAY; also specify
    611    the minimum time remaining before a response expires. Both of these
    612    options increase constraints on the behavior of caches, and so cannot
    613    further relax the cache's approximation of semantic transparency.
    614 </t>
    615 <t>
    616    A client &MAY; also specify that it will accept stale responses, up to
    617    some maximum amount of staleness. This loosens the constraints on the
    618    caches, and so might violate the origin server's specified
    619    constraints on semantic transparency, but might be necessary to
    620    support disconnected operation, or high availability in the face of
    621    poor connectivity.
    622 </t>
    623 </section>
    624 </section>
    625 
    626 <section title="Expiration Model" anchor="expiration.model">
    627 
    628 <section title="Server-Specified Expiration" anchor="server-specified.expiration">
    629 <t>
    630    HTTP caching works best when caches can entirely avoid making
    631    requests to the origin server. The primary mechanism for avoiding
    632    requests is for an origin server to provide an explicit expiration
    633    time in the future, indicating that a response &MAY; be used to satisfy
    634    subsequent requests. In other words, a cache can return a fresh
    635    response without first contacting the server.
    636 </t>
    637 <t>
    638    Our expectation is that servers will assign future explicit
    639    expiration times to responses in the belief that the entity is not
    640    likely to change, in a semantically significant way, before the
    641    expiration time is reached. This normally preserves semantic
    642    transparency, as long as the server's expiration times are carefully
    643    chosen.
    644 </t>
    645 <t>
    646    The expiration mechanism applies only to responses taken from a cache
    647    and not to first-hand responses forwarded immediately to the
    648    requesting client.
    649 </t>
    650 <t>
    651    If an origin server wishes to force a semantically transparent cache
    652    to validate every request, it &MAY; assign an explicit expiration time
    653    in the past. This means that the response is always stale, and so the
    654    cache &SHOULD; validate it before using it for subsequent requests. See
    655    <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
    656 </t>
    657 <t>
    658    If an origin server wishes to force any HTTP/1.1 cache, no matter how
    659    it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
    660    cache-control directive (see <xref target="header.cache-control"/>).
    661 </t>
    662 <t>
    663    Servers specify explicit expiration times using either the Expires
    664    header, or the max-age directive of the Cache-Control header.
    665 </t>
    666 <t>
    667    An expiration time cannot be used to force a user agent to refresh
    668    its display or reload a resource; its semantics apply only to caching
    669    mechanisms, and such mechanisms need only check a resource's
    670    expiration status when a new request for that resource is initiated.
    671    See <xref target="history.lists"/> for an explanation of the difference between caches
    672    and history mechanisms.
    673 </t>
    674 </section>
    675 
    676 <section title="Heuristic Expiration" anchor="heuristic.expiration">
    677 <t>
    678    Since origin servers do not always provide explicit expiration times,
    679    HTTP caches typically assign heuristic expiration times, employing
    680    algorithms that use other header values (such as the Last-Modified
    681    time) to estimate a plausible expiration time. The HTTP/1.1
    682    specification does not provide specific algorithms, but does impose
    683    worst-case constraints on their results. Since heuristic expiration
    684    times might compromise semantic transparency, they ought to be used
    685    cautiously, and we encourage origin servers to provide explicit
    686    expiration times as much as possible.
    687 </t>
    688 </section>
    689 
    690 <section title="Age Calculations" anchor="age.calculations">
    691 <t>
    692    In order to know if a cached entry is fresh, a cache needs to know if
    693    its age exceeds its freshness lifetime. We discuss how to calculate
    694    the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
    695    the age of a response or cache entry.
    696 </t>
    697 <t>
    698    In this discussion, we use the term "now" to mean "the current value
    699    of the clock at the host performing the calculation." Hosts that use
    700    HTTP, but especially hosts running origin servers and caches, &SHOULD;
    701    use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
    702    a globally accurate time standard.
    703 </t>
    704 <t>
    705    HTTP/1.1 requires origin servers to send a Date header, if possible,
    706    with every response, giving the time at which the response was
    707    generated (see &header-date;). We use the term "date_value" to denote
    708    the value of the Date header, in a form appropriate for arithmetic
    709    operations.
    710 </t>
    711 <t>
    712    HTTP/1.1 uses the Age response-header to convey the estimated age of
    713    the response message when obtained from a cache. The Age field value
    714    is the cache's estimate of the amount of time since the response was
    715    generated or revalidated by the origin server.
    716 </t>
    717 <t>
    718    In essence, the Age value is the sum of the time that the response
    719    has been resident in each of the caches along the path from the
    720    origin server, plus the amount of time it has been in transit along
    721    network paths.
    722 </t>
    723 <t>
    724    We use the term "age_value" to denote the value of the Age header, in
    725    a form appropriate for arithmetic operations.
    726 </t>
    727 <t>
    728    A response's age can be calculated in two entirely independent ways:
    729   <list style="numbers">
    730       <t>now minus date_value, if the local clock is reasonably well
    731          synchronized to the origin server's clock. If the result is
    732          negative, the result is replaced by zero.</t>
    733 
    734       <t>age_value, if all of the caches along the response path
    735          implement HTTP/1.1.</t>
    736   </list>
    737 </t>
    738 <t>
    739    Given that we have two independent ways to compute the age of a
    740    response when it is received, we can combine these as
    741 </t>
    742 <figure><artwork type="code">
     370</artwork>
     371      </figure>
     372    </section>
     373
     374    <section anchor="caching.overview" title="Cache Operation">
     375
     376      <section anchor="cache.correctness" title="Cache Correctness">
     377        <iref item="cache.correctness" />
     378        <t>When a cache is "<x:dfn>correct</x:dfn>", the client receives exactly the same response
     379          status and payload that it would have received had its request been handled directly by
     380          the origin server.</t>
     381        <t>Ideally, all interactions with an HTTP cache would be correct. However, for some
     382          resources, complete correctness is not always necessary and can be effectively traded for
     383          the sake of bandwidth scaling, disconnected operation, and high availability. HTTP/1.1
     384          allows origin servers, caches, and clients to explicitly reduce correctness when
     385          necessary.</t>
     386        <t>However, because incorrect operation may confuse users and might be incompatible with
     387          server applications (such as those for ordering merchandise), caches MUST NOT relax
     388          correctness unless: <list style="symbols">
     389            <t>the client or origin server permits it with an explicit protocol-level element; see
     390                <xref target="header.cache-control" />, or</t>
     391            <t>the cache provides an explicit warning to the end user; see <xref
     392                target="header.warning" />.</t>
     393          </list>
     394        </t>
     395        <t>
     396          <cref>REVIEW: previous semantic transparency text didn't make a lot of sense; replacing
     397            with "correctness"</cref>
     398        </t>
     399        <t>
     400          <cref>TODO: align with intermediary semantic transparency in p1</cref>
     401        </t>
     402        <t>
     403          <cref>REVIEW: removed Explicit User Agent Warnings section</cref>
     404        </t>
     405      </section>
     406
     407      <section anchor="response.cacheability" title="Response Cacheability">
     408        <t>A cache MAY store a response to any request, provided that: <list style="symbols">
     409            <t>the "no-store" cache-control directive (see <xref target="header.cache-control" />)
     410              does not appear in request or response headers.</t>
     411            <t>the cache understands partial responses, if the response is partial or incomplete
     412              (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
     413          </list>
     414        </t>
     415        <t>Note that in normal operation, most caches will not store a response that has neither a
     416          cache validator nor an explicitly expiration time, as such responses are not usually
     417          useful to store. However, caches are not prohibited from storing such responses.</t>
     418      </section>
     419
     420      <section anchor="constructing.responses.from.caches"
     421        title="Constructing Responses from Caches">
     422        <t>For a given request, a non-shared cache &MAY; return a stored response, provided
     423          that: <list style="symbols">
     424            <t>the presented request-URI and that of the stored response match (see
     425              <cref>TBD</cref>), and</t>
     426            <t>selecting headers nominated by the stored response (if any) match (see <xref
     427                target="caching.negotiated.responses" />), and</t>
     428            <t>the stored response is either fresh (see <xref target="expiration.model" />) or
     429              allowed to be served stale (see <xref target="serving.stale.responses" />), and</t>
     430            <t>the presented request and stored response are free from directives that would prevent
     431              it (see <xref target="header.cache-control" /> and <xref target="header.pragma"
     432             />).</t>
     433          </list>
     434        </t>
     435        <t>
     436          <cref>ISSUE: This doesn't specify whether the request method is part of the cache key.</cref>
     437        </t>
     438        <t>A shared cache &MAY; return a stored response, provided that: <list style="symbols">
     439            <t>the criteria for non-shared caches above are met (including directives for shared
     440              caches; see <xref target="header.cache-control" />), and</t>
     441            <t>the stored response was not associated with an authenticated request (see
     442              &header-authorization;), unless explicitly allowed (see <xref
     443                target="header.cache-control" />).</t>
     444          </list>
     445        </t>
     446        <t>All request methods other than GET and HEAD &MUST; be written through the cache to
     447          the origin server. Note that such requests might invalidate already stored responses; see
     448            <xref target="invalidation.after.updates.or.deletions" />.</t>
     449        <t>Caches &SHOULD; use the most recent response (as determined by the Date header) when
     450          more than one applicable response is stored. They &MAY; also send a request with
     451          "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
     452          use.</t>
     453        <t>In the process of determining whether a stored response is fresh or not, a cache
     454          &MAY; validate that response (see <xref target="validation.model" />).</t>
     455      </section>
     456
     457
     458
     459
     460      <section anchor="expiration.model" title="Freshness Model">
     461
     462        <t>HTTP caching works best when caches can entirely avoid making requests to the origin
     463          server. When a response is "fresh" in the cache, it can be used to satisfy subsequent
     464          requests without contacting the origin server. This is also referred to as "expiration."</t>
     465        <t>Expiration applies only to responses taken from a cache and not to first-hand responses.
     466          It cannot be used to force a user agent to refresh its display or reload a resource; its
     467          semantics apply only to caches. See <xref target="history.lists" /> for an explanation of
     468          the difference between caches and history mechanisms.</t>
     469        <t>The primary mechanism for avoiding requests is for an origin server to provide an
     470          explicit expiration time in the future, using either the Expires header <xref
     471            target="header.expires" /> or the max-age directive of the Cache-Control header <xref
     472            target="header.cache-control" />. Generally, origin servers will assign future explicit
     473          expiration times to responses in the belief that the entity is not likely to change in a
     474          semantically significant way before the expiration time is reached. This normally
     475          preserves cache correctness, as long as the server's expiration times are carefully
     476          chosen.</t>
     477        <t>If an origin server wishes to force a cache to validate every request, it &MAY;
     478          assign an explicit expiration time in the past. This means that the response is always
     479          stale, and so the cache &SHOULD; validate it before using it for subsequent requests.</t>
     480        <t>Since origin servers do not always provide explicit expiration times, HTTP caches may
     481          assign heuristic expiration times when they are not specified, employing algorithms that
     482          use other header values (such as the Last-Modified time) to estimate a plausible
     483          expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
     484          impose worst-case constraints on their results.</t>
     485        <t>Additionally, in some cases the client might need to influence freshness calculation.
     486          Clients can do this using several directives of the Cache-Control header, with the effect
     487          of either increasing or loosening constraints on freshness.</t>
     488
     489        <t>The calculation to determine if a response has expired is:</t>
     490        <figure>
     491          <artwork type="code">
     492   response_is_fresh = (freshness_lifetime &gt; current_age)
     493</artwork>
     494        </figure>
     495
     496        <t>The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
     497          the current_age is defined in <xref target="age.calculations" />.</t>
     498
     499        <t>
     500          <cref>TODO: incorporate client-specified freshness controls.</cref>
     501        </t>
     502        <t>
     503          <cref>TODO: incorporate s-maxage</cref>
     504        </t>
     505
     506
     507        <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
     508          <t>"expires_value" denotes the value of the Expires header <xref target="header.expires"
     509             />. "max_age_value" denotes the number of seconds carried by the "max-age" directive of
     510            the Cache-Control header in a response (see <xref target="header.cache-control" />).</t>
     511          <t>The max-age directive takes priority over Expires, so if max-age is present in a
     512            response, the calculation:</t>
     513          <figure>
     514            <artwork type="code">
     515   freshness_lifetime = max_age_value
     516</artwork>
     517          </figure>
     518          <t>Otherwise, if Expires is present in the response, the calculation is:</t>
     519          <figure>
     520            <artwork type="code">
     521   freshness_lifetime = expires_value - date_value
     522</artwork>
     523          </figure>
     524          <t>Note that the calculations above are not vulnerable to clock skew, since all of the
     525            information comes from the origin server.</t>
     526        </section>
     527
     528        <section anchor="heuristic.freshness" title="Heuristic Freshness">
     529          <t>If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage appears in the
     530            response, and the response does not include other restrictions on caching, the cache
     531            &MAY; compute a freshness_lifetime using a heuristic, if the stored response's
     532            status code is one of 200, 203, 206, 300, 301 or 410. Heuristics &MUST-NOT; be used
     533            for other response status codes. When a heuristic is used to calculate
     534            freshness_lifetime, the cache &MUST; attach a Warning header with a 113 warn-code to the response if its
     535            current_age is more than 24 hours and such a warning is not already present.</t>
     536          <t>Also, if the response has a Last-Modified header &header-last-modified;, the
     537            heuristic expiration value &SHOULD; be no more than some fraction of the interval
     538            since that time. A typical setting of this fraction might be 10%.</t>
     539          <t>
     540            <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
     541          </t>
     542        </section>
     543
     544        <section anchor="age.calculations" title="Calculating Age">
     545          <t>HTTP/1.1 uses the Age response-header to convey the estimated age of the response
     546            message when obtained from a cache. The Age field value is the cache's estimate of the
     547            amount of time since the response was generated or validated by the origin server. In
     548            essence, the Age value is the sum of the time that the response has been resident in
     549            each of the caches along the path from the origin server, plus the amount of time it has
     550            been in transit along network paths.</t>
     551          <t>When a response is generated from a stored response, the cache &MUST; include a single
     552            Age header field in the response with a value equal to the stored response's current_age,
     553            calculated using the algorithm described in this section.</t>
     554          <t>The term "age_value" denotes the value of the Age header, in a form appropriate for
     555            arithmetic operations.</t>
     556          <t>HTTP/1.1 requires origin servers to send a Date header, if possible, with every
     557            response, giving the time at which the response was generated (see &header-date;).
     558            The term "date_value" denotes the value of the Date header, in a form appropriate for
     559            arithmetic operations.</t>
     560          <t>The term "now" means "the current value of the clock at the host performing the
     561            calculation." Hosts that use HTTP, but especially hosts running origin servers and
     562            caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
     563            synchronize their clocks to a globally accurate time standard.</t>
     564          <t>A response's age can be calculated in two entirely independent ways: <list
     565              style="numbers">
     566              <t>now minus date_value, if the local clock is reasonably well synchronized to the
     567                origin server's clock. If the result is negative, the result is replaced by zero.</t>
     568
     569              <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
     570            </list>
     571          </t>
     572          <t>These are combined as</t>
     573          <figure>
     574            <artwork type="code">
    743575    corrected_received_age = max(now - date_value, age_value)
    744 </artwork></figure>
    745 <t>
    746    and as long as we have either nearly synchronized clocks or all-HTTP/1.1
    747    paths, one gets a reliable (conservative) result.
    748 </t>
    749 <t>
    750    Because of network-imposed delays, some significant interval might
    751    pass between the time that a server generates a response and the time
    752    it is received at the next outbound cache or client. If uncorrected,
    753    this delay could result in improperly low ages.
    754 </t>
    755 <t>
    756    Because the request that resulted in the returned Age value must have
    757    been initiated prior to that Age value's generation, we can correct
    758    for delays imposed by the network by recording the time at which the
    759    request was initiated. Then, when an Age value is received, it &MUST;
    760    be interpreted relative to the time the request was initiated, not
    761    the time that the response was received. This algorithm results in
    762    conservative behavior no matter how much delay is experienced. So, we
    763    compute:
    764 </t>
    765 <figure><artwork type="code">
     576</artwork>
     577          </figure>
     578          <t>When an Age value is
     579            received, it &MUST; be interpreted relative to the time the request was initiated,
     580            not the time that the response was received.</t>
     581          <figure>
     582            <artwork type="code">
    766583   corrected_initial_age = corrected_received_age
    767584                         + (now - request_time)
    768 </artwork></figure>
    769 <t>
    770    where "request_time" is the time (according to the local clock) when
    771    the request that elicited this response was sent.
    772 </t>
    773 <t>
    774    Summary of age calculation algorithm, when a cache receives a
    775    response:
    776 </t>
    777 <figure><artwork type="code">
     585</artwork>
     586          </figure>
     587          <t>where "request_time" is the time (according to the local clock) when the request that
     588            elicited this response was sent.</t>
     589          <t>The current_age of a stored response can then be calculated by adding the amount of time
     590            (in seconds) since the stored response was last validated by the origin server to the
     591            corrected_initial_age.</t>
     592          <t>In summary:</t>
     593          <figure>
     594            <artwork type="code">
    778595   /*
    779596    * age_value
     
    784601    * request_time
    785602    *      is the (local) time when the cache made the request
    786     *              that resulted in this cached response
     603    *              that resulted in this stored response
    787604    * response_time
    788605    *      is the (local) time when the cache received the
     
    798615   resident_time = now - response_time;
    799616   current_age   = corrected_initial_age + resident_time;
    800 </artwork></figure>
    801 <t>
    802    The current_age of a cache entry is calculated by adding the amount
    803    of time (in seconds) since the cache entry was last validated by the
    804    origin server to the corrected_initial_age. When a response is
    805    generated from a cache entry, the cache &MUST; include a single Age
    806    header field in the response with a value equal to the cache entry's
    807    current_age.
    808 </t>
    809 <t>
    810    The presence of an Age header field in a response implies that a
    811    response is not first-hand. However, the converse is not true, since
    812    the lack of an Age header field in a response does not imply that the
    813    response is first-hand unless all caches along the request path are
    814    compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
    815    the Age header field).
    816 </t>
    817 </section>
    818 
    819 <section title="Expiration Calculations" anchor="expiration.calculations">
    820 <t>
    821    In order to decide whether a response is fresh or stale, we need to
    822    compare its freshness lifetime to its age. The age is calculated as
    823    described in <xref target="age.calculations"/>; this section describes how to calculate
    824    the freshness lifetime, and to determine if a response has expired.
    825    In the discussion below, the values can be represented in any form
    826    appropriate for arithmetic operations.
    827 </t>
    828 <t>
    829    We use the term "expires_value" to denote the value of the Expires
    830    header. We use the term "max_age_value" to denote an appropriate
    831    value of the number of seconds carried by the "max-age" directive of
    832    the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
    833 </t>
    834 <t>
    835    The max-age directive takes priority over Expires, so if max-age is
    836    present in a response, the calculation is simply:
    837 </t>
    838 <figure><artwork type="code">
    839    freshness_lifetime = max_age_value
    840 </artwork></figure>
    841 <t>
    842    Otherwise, if Expires is present in the response, the calculation is:
    843 </t>
    844 <figure><artwork type="code">
    845    freshness_lifetime = expires_value - date_value
    846 </artwork></figure>
    847 <t>
    848    Note that neither of these calculations is vulnerable to clock skew,
    849    since all of the information comes from the origin server.
    850 </t>
    851 <t>
    852    If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
    853    (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
    854    does not include other restrictions on caching, the cache &MAY; compute
    855    a freshness lifetime using a heuristic. The cache &MUST; attach Warning
    856    113 to any response whose age is more than 24 hours if such warning
    857    has not already been added.
    858 </t>
    859 <t>
    860    Also, if the response does have a Last-Modified time, the heuristic
    861    expiration value &SHOULD; be no more than some fraction of the interval
    862    since that time. A typical setting of this fraction might be 10%.
    863 </t>
    864 <t>
    865    The calculation to determine if a response has expired is quite
    866    simple:
    867 </t>
    868 <figure><artwork type="code">
    869    response_is_fresh = (freshness_lifetime &gt; current_age)
    870 </artwork></figure>
    871 </section>
    872 
    873 <section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
    874 <t>
    875    Because expiration values are assigned optimistically, it is possible
    876    for two caches to contain fresh values for the same resource that are
    877    different.
    878 </t>
    879 <t>
    880    If a client performing a retrieval receives a non-first-hand response
    881    for a request that was already fresh in its own cache, and the Date
    882    header in its existing cache entry is newer than the Date on the new
    883    response, then the client &MAY; ignore the response. If so, it &MAY;
    884    retry the request with a "Cache-Control: max-age=0" directive (see
    885    <xref target="header.cache-control"/>), to force a check with the origin server.
    886 </t>
    887 <t>
    888    If a cache has two fresh responses for the same representation with
    889    different validators, it &MUST; use the one with the more recent Date
    890    header. This situation might arise because the cache is pooling
    891    responses from other caches, or because a client has asked for a
    892    reload or a revalidation of an apparently fresh cache entry.
    893 </t>
    894 </section>
    895 
    896 <section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
    897 <t>
    898    Because a client might be receiving responses via multiple paths, so
    899    that some responses flow through one set of caches and other
    900    responses flow through a different set of caches, a client might
    901    receive responses in an order different from that in which the origin
    902    server sent them. We would like the client to use the most recently
    903    generated response, even if older responses are still apparently
    904    fresh.
    905 </t>
    906 <t>
    907    Neither the entity tag nor the expiration value can impose an
    908    ordering on responses, since it is possible that a later response
    909    intentionally carries an earlier expiration time. The Date values are
    910    ordered to a granularity of one second.
    911 </t>
    912 <t>
    913    When a client tries to revalidate a cache entry, and the response it
    914    receives contains a Date header that appears to be older than the one
    915    for the existing entry, then the client &SHOULD; repeat the request
    916    unconditionally, and include
    917 </t>
    918 <figure><artwork type="example">
    919     Cache-Control: max-age=0
    920 </artwork></figure>
    921 <t>
    922    to force any intermediate caches to validate their copies directly
    923    with the origin server, or
    924 </t>
    925 <figure><artwork type="example">
    926     Cache-Control: no-cache
    927 </artwork></figure>
    928 <t>
    929    to force any intermediate caches to obtain a new copy from the origin
    930    server.
    931 </t>
    932 <t>
    933    If the Date values are equal, then the client &MAY; use either response
    934    (or &MAY;, if it is being extremely prudent, request a new response).
    935    Servers &MUST-NOT; depend on clients being able to choose
    936    deterministically between responses generated during the same second,
    937    if their expiration times overlap.
    938 </t>
    939 </section>
    940 </section>
    941 
    942 <section title="Validation Model" anchor="validation.model">
    943 <t>
    944    When a cache has a stale entry that it would like to use as a
    945    response to a client's request, it first has to check with the origin
    946    server (or possibly an intermediate cache with a fresh response) to
    947    see if its cached entry is still usable. We call this "validating"
    948    the cache entry.
    949 </t>
    950 <t>
    951    HTTP's conditional request mechanism, defined in &conditional;, is
    952    used to avoid retransmitting the response payload when the cached entry
    953    is valid.  When a cached response includes one or more "cache validators,"
    954    such as the field values of an ETag or Last-Modified header field, then
    955    a validating GET request &SHOULD; be made conditional to those field values.
    956    The server checks the conditional request's validator against the current
    957    state of the requested resource and, if they match, the server responds
    958    with a 304 (Not Modified) status code to indicate that the cached response
    959    can be refreshed and reused without retransmitting the response payload.
    960    If the validator does not match the current state of the requested
    961    resource, then the server returns a full response, including payload,
    962    so that the request can be satisfied and the cache entry supplanted
    963    without the need for an additional network round-trip.
    964 </t>
    965 </section>
    966 
    967 <section title="Response Cacheability" anchor="response.cacheability">
    968 <t>
    969    Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
    970    directive, a caching system &MAY; always store a successful response
    971    (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
    972    if it is fresh, and &MAY; return it after successful validation. If
    973    there is neither a cache validator nor an explicit expiration time
    974    associated with a response, we do not expect it to be cached, but
    975    certain caches &MAY; violate this expectation (for example, when little
    976    or no network connectivity is available). A client can usually detect
    977    that such a response was taken from a cache by comparing the Date
    978    header to the current time.
    979   <list><t>
    980       <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
    981       without providing any Warning.
    982   </t></list>
    983 </t>
    984 <t>
    985    However, in some cases it might be inappropriate for a cache to
    986    retain an entity, or to return it in response to a subsequent
    987    request. This might be because absolute semantic transparency is
    988    deemed necessary by the service author, or because of security or
    989    privacy considerations. Certain cache-control directives are
    990    therefore provided so that the server can indicate that certain
    991    resource entities, or portions thereof, are not to be cached
    992    regardless of other considerations.
    993 </t>
    994 <t>
    995    Note that &header-authorization; normally prevents a shared cache from saving
    996    and returning a response to a previous request if that request
    997    included an Authorization header.
    998 </t>
    999 <t>
    1000    A response received with a status code of 200, 203, 206, 300, 301 or
    1001    410 &MAY; be stored by a cache and used in reply to a subsequent
    1002    request, subject to the expiration mechanism, unless a cache-control
    1003    directive prohibits caching. However, a cache that does not support
    1004    the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
    1005    Content) responses.
    1006 </t>
    1007 <t>
    1008    A response received with any other status code (e.g. status codes 302
    1009    and 307) &MUST-NOT; be returned in a reply to a subsequent request
    1010    unless there are cache-control directives or another header(s) that
    1011    explicitly allow it. For example, these include the following: an
    1012    Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
    1013    "proxy-revalidate", "public" or "private" cache-control
    1014    directive (<xref target="header.cache-control"/>).
    1015 </t>
    1016 </section>
    1017 
    1018 <section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
    1019 <t>
    1020    The purpose of an HTTP cache is to store information received in
    1021    response to requests for use in responding to future requests. In
    1022    many cases, a cache simply returns the appropriate parts of a
    1023    response to the requester. However, if the cache holds a cache entry
    1024    based on a previous response, it might have to combine parts of a new
    1025    response with what is held in the cache entry.
    1026 </t>
    1027 
    1028 <section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
    1029 <t>
    1030    For the purpose of defining the behavior of caches and non-caching
    1031    proxies, we divide HTTP headers into two categories:
    1032   <list style="symbols">
    1033       <t>End-to-end headers, which are  transmitted to the ultimate
    1034         recipient of a request or response. End-to-end headers in
    1035         responses &MUST; be stored as part of a cache entry and &MUST; be
    1036         transmitted in any response formed from a cache entry.</t>
    1037 
    1038       <t>Hop-by-hop headers, which are meaningful only for a single
    1039         transport-level connection, and are not stored by caches or
    1040         forwarded by proxies.</t>
    1041   </list>
    1042 </t>
    1043 <t>
    1044    The following HTTP/1.1 headers are hop-by-hop headers:
    1045   <list style="symbols">
    1046       <t>Connection</t>
    1047       <t>Keep-Alive</t>
    1048       <t>Proxy-Authenticate</t>
    1049       <t>Proxy-Authorization</t>
    1050       <t>TE</t>
    1051       <t>Trailer</t>
    1052       <t>Transfer-Encoding</t>
    1053       <t>Upgrade</t>
    1054   </list>
    1055 </t>
    1056 <t>
    1057    All other headers defined by HTTP/1.1 are end-to-end headers.
    1058 </t>
    1059 <t>
    1060    Other hop-by-hop headers &MUST; be listed in a Connection header
    1061    (&header-connection;).
    1062 </t>
    1063 </section>
    1064 
    1065 <section title="Non-modifiable Headers" anchor="non-modifiable.headers">
    1066 <t>
    1067    Some features of HTTP/1.1, such as Digest
    1068    Authentication, depend on the value of certain end-to-end headers. A
    1069    transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
    1070    definition of that header requires or specifically allows that.
    1071 </t>
    1072 <t>
    1073    A transparent proxy &MUST-NOT; modify any of the following fields in a
    1074    request or response, and it &MUST-NOT; add any of these fields if not
    1075    already present:
    1076   <list style="symbols">
    1077       <t>Content-Location</t>
    1078       <t>Content-MD5</t>
    1079       <t>ETag</t>
    1080       <t>Last-Modified</t>
    1081   </list>
    1082 </t>
    1083 <t>
    1084    A transparent proxy &MUST-NOT; modify any of the following fields in a
    1085    response:
    1086   <list style="symbols">
    1087     <t>Expires</t>
    1088   </list>
    1089 </t>
    1090 <t>
    1091    but it &MAY; add any of these fields if not already present. If an
    1092    Expires header is added, it &MUST; be given a field-value identical to
    1093    that of the Date header in that response.
    1094 </t>
    1095 <t>
    1096    A  proxy &MUST-NOT; modify or add any of the following fields in a
    1097    message that contains the no-transform cache-control directive, or in
    1098    any request:
    1099   <list style="symbols">
    1100     <t>Content-Encoding</t>
    1101     <t>Content-Range</t>
    1102     <t>Content-Type</t>
    1103   </list>
    1104 </t>
    1105 <t>
    1106    A non-transparent proxy &MAY; modify or add these fields to a message
    1107    that does not include no-transform, but if it does so, it &MUST; add a
    1108    Warning 214 (Transformation applied) if one does not already appear
    1109    in the message (see <xref target="header.warning"/>).
    1110   <list><t>
    1111       Warning: unnecessary modification of end-to-end headers might
    1112       cause authentication failures if stronger authentication
    1113       mechanisms are introduced in later versions of HTTP. Such
    1114       authentication mechanisms &MAY; rely on the values of header fields
    1115       not listed here.
    1116     </t></list>
    1117 </t>
    1118 <t>
    1119    The Content-Length field of a request or response is added or deleted
    1120    according to the rules in &message-length;. A transparent proxy &MUST;
    1121    preserve the entity-length (&entity-length;) of the entity-body,
    1122    although it &MAY; change the transfer-length (&message-length;).
    1123 </t>
    1124 </section>
    1125 
    1126 <section title="Combining Headers" anchor="combining.headers">
    1127 <t>
    1128    When a cache makes a validating request to a server, and the server
    1129    provides a 304 (Not Modified) response or a 206 (Partial Content)
    1130    response, the cache then constructs a response to send to the
    1131    requesting client.
    1132 </t>
    1133 <t>
    1134    If the status code is 304 (Not Modified), the cache uses the entity-body
    1135    stored in the cache entry as the entity-body of this outgoing
    1136    response. If the status code is 206 (Partial Content) and the ETag or
    1137    Last-Modified headers match exactly, the cache &MAY; combine the
    1138    contents stored in the cache entry with the new contents received in
    1139    the response and use the result as the entity-body of this outgoing
    1140    response, (see &combining-byte-ranges;).
    1141 </t>
    1142 <t>
    1143    The end-to-end headers stored in the cache entry are used for the
    1144    constructed response, except that
    1145   <list style="symbols">
    1146     <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
    1147       &MUST; be deleted from the cache entry and the forwarded response.</t>
    1148     <t>any stored Warning headers with warn-code 2xx &MUST; be retained
    1149         in the cache entry and the forwarded response.</t>
    1150     <t>any end-to-end headers provided in the 304 or 206 response &MUST;
    1151         replace the corresponding headers from the cache entry.</t>
    1152   </list>
    1153 </t>
    1154 <t>
    1155    Unless the cache decides to remove the cache entry, it &MUST; also
    1156    replace the end-to-end headers stored with the cache entry with
    1157    corresponding headers received in the incoming response, except for
    1158    Warning headers as described immediately above. If a header field-name
    1159    in the incoming response matches more than one header in the
    1160    cache entry, all such old headers &MUST; be replaced.
    1161 </t>
    1162 <t>
    1163    In other words, the set of end-to-end headers received in the
    1164    incoming response overrides all corresponding end-to-end headers
    1165    stored with the cache entry (except for stored Warning headers with
    1166    warn-code 1xx, which are deleted even if not overridden).
    1167   <list><t>
    1168       <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
    1169       Modified) or a 206 (Partial Content) response to update any header
    1170       associated with a previous response for the same entity or sub-ranges
    1171       thereof, although it might not always be meaningful or
    1172       correct to do so. This rule does not allow an origin server to use
    1173       a 304 (Not Modified) or a 206 (Partial Content) response to
    1174       entirely delete a header that it had provided with a previous
    1175       response.
    1176   </t></list>
    1177 </t>
    1178 </section>
    1179 
    1180 </section>
    1181 
    1182 <section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
    1183 <t>
    1184    Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
    1185    by the presence of a Vary header field in a response, alters the
    1186    conditions and procedure by which a cache can use the response for
    1187    subsequent requests. See <xref target="header.vary"/> for use of the Vary header
    1188    field by servers.
    1189 </t>
    1190 <t>
    1191    A server &SHOULD; use the Vary header field to inform a cache of what
    1192    request-header fields were used to select among multiple
    1193    representations of a cacheable response subject to server-driven
    1194    negotiation. The set of header fields named by the Vary field value
    1195    is known as the "selecting" request-headers.
    1196 </t>
    1197 <t>
    1198    When the cache receives a subsequent request whose Request-URI
    1199    specifies one or more cache entries including a Vary header field,
    1200    the cache &MUST-NOT; use such a cache entry to construct a response to
    1201    the new request unless all of the selecting request-headers present
    1202    in the new request match the corresponding stored request-headers in
    1203    the original request.
    1204 </t>
    1205 <t>
    1206    The selecting request-headers from two requests are defined to match
    1207    if and only if the selecting request-headers in the first request can
    1208    be transformed to the selecting request-headers in the second request
    1209    by adding or removing linear white space (LWS) at places where this
    1210    is allowed by the corresponding BNF, and/or combining multiple
    1211    message-header fields with the same field name following the rules
    1212    about message headers in &message-headers;.
    1213 </t>
    1214 <t>
    1215    A Vary header field-value of "*" always fails to match and subsequent
    1216    requests on that resource can only be properly interpreted by the
    1217    origin server.
    1218 </t>
    1219 <t>
    1220    If the selecting request header fields for the cached entry do not
    1221    match the selecting request header fields of the new request, then
    1222    the cache &MUST-NOT; use a cached entry to satisfy the request unless
    1223    it first relays the new request to the origin server in a conditional
    1224    request and the server responds with 304 (Not Modified), including an
    1225    entity tag or Content-Location that indicates the entity to be used.
    1226 </t>
    1227 <t>
    1228    If an entity tag was assigned to a cached representation, the
    1229    forwarded request &SHOULD; be conditional and include the entity tags
    1230    in an If-None-Match header field from all its cache entries for the
    1231    resource. This conveys to the server the set of entities currently
    1232    held by the cache, so that if any one of these entities matches the
    1233    requested entity, the server can use the ETag header field in its 304
    1234    (Not Modified) response to tell the cache which entry is appropriate.
    1235    If the entity-tag of the new response matches that of an existing
    1236    entry, the new response &SHOULD; be used to update the header fields of
    1237    the existing entry, and the result &MUST; be returned to the client.
    1238 </t>
    1239 <t>
    1240    If any of the existing cache entries contains only partial content
    1241    for the associated entity, its entity-tag &SHOULD-NOT;  be included in
    1242    the If-None-Match header field unless the request is for a range that
    1243    would be fully satisfied by that entry.
    1244 </t>
    1245 <t>
    1246    If a cache receives a successful response whose Content-Location
    1247    field matches that of an existing cache entry for the same Request-URI,
    1248    whose entity-tag differs from that of the existing entry, and
    1249    whose Date is more recent than that of the existing entry, the
    1250    existing entry &SHOULD-NOT;  be returned in response to future requests
    1251    and &SHOULD; be deleted from the cache.
    1252 </t>
    1253 </section>
    1254 
    1255 <section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
    1256 <t>
    1257    For reasons of security and privacy, it is necessary to make a
    1258    distinction between "shared" and "non-shared" caches. A non-shared
    1259    cache is one that is accessible only to a single user. Accessibility
    1260    in this case &SHOULD; be enforced by appropriate security mechanisms.
    1261    All other caches are considered to be "shared." Other sections of
    1262    this specification place certain constraints on the operation of
    1263    shared caches in order to prevent loss of privacy or failure of
    1264    access controls.
    1265 </t>
    1266 </section>
    1267 
    1268 <section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
    1269 <t>
    1270    A cache that receives an incomplete response (for example, with fewer
    1271    bytes of data than specified in a Content-Length header) &MAY; store
    1272    the response. However, the cache &MUST; treat this as a partial
    1273    response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
    1274    the result might be a full response or might still be
    1275    partial. A cache &MUST-NOT; return a partial response to a client
    1276    without explicitly marking it as such, using the 206 (Partial
    1277    Content) status code. A cache &MUST-NOT; return a partial response
    1278    using a status code of 200 (OK).
    1279 </t>
    1280 <t>
    1281    If a cache receives a 5xx response while attempting to revalidate an
    1282    entry, it &MAY; either forward this response to the requesting client,
    1283    or act as if the server failed to respond. In the latter case, it &MAY;
    1284    return a previously received response unless the cached entry
    1285    includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
    1286 </t>
    1287 </section>
    1288 
    1289 <section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
    1290 <t>
    1291    Unless the origin server explicitly prohibits the caching of their
    1292    responses, the application of GET and HEAD methods to any resources
    1293    &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
    1294    these responses are taken from a cache. They &MAY; still have side
    1295    effects, but a cache is not required to consider such side effects in
    1296    its caching decisions. Caches are always expected to observe an
    1297    origin server's explicit restrictions on caching.
    1298 </t>
    1299 <t>
    1300    We note one exception to this rule: since some applications have
    1301    traditionally used GET and HEAD requests with URLs containing a query part
    1302    to perform operations with significant side
    1303    effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
    1304    the server provides an explicit expiration time. This specifically
    1305    means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT;
    1306    be taken from a cache. See &safe-methods; for related information.
    1307 </t>
    1308 </section>
    1309 
    1310 <section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
    1311 <t>
    1312    The effect of certain methods performed on a resource at the origin
    1313    server might cause one or more existing cache entries to become non-transparently
    1314    invalid. That is, although they might continue to be
    1315    "fresh," they do not accurately reflect what the origin server would
    1316    return for a new request on that resource.
    1317 </t>
    1318 <t>
    1319    There is no way for HTTP to guarantee that all such
    1320    cache entries are marked invalid. For example, the request that
    1321    caused the change at the origin server might not have gone through
    1322    the proxy where a cache entry is stored. However, several rules help
    1323    reduce the likelihood of erroneous behavior.
    1324 </t>
    1325 <t>
    1326    In this section, the phrase "invalidate an entity" means that the
    1327    cache will either remove all instances of that entity from its
    1328    storage, or will mark these as "invalid" and in need of a mandatory
    1329    revalidation before they can be returned in response to a subsequent
    1330    request.
    1331 </t>
    1332 <t>
    1333    Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
    1334    either the entity referred to by the Request-URI, or by the Location
    1335    or Content-Location headers (if present). These methods are:
    1336   <list style="symbols">
    1337       <t>PUT</t>
    1338       <t>DELETE</t>
    1339       <t>POST</t>
    1340   </list>
    1341 </t> 
    1342 <t>
    1343    An invalidation based
    1344    on the URI in a Location or Content-Location header &MUST-NOT; be
    1345    performed if the host part of that URI differs from the host part
    1346    in the Request-URI. This helps prevent denial of service attacks.
    1347 </t>
    1348 <t>
    1349    A cache that passes through requests for methods it does not
    1350    understand &SHOULD; invalidate any entities referred to by the
    1351    Request-URI.
    1352 </t>
    1353 </section>
    1354 
    1355 <section title="Write-Through Mandatory" anchor="write-through.mandatory">
    1356 <t>
    1357    All methods that might be expected to cause modifications to the
    1358    origin server's resources &MUST; be written through to the origin
    1359    server. This currently includes all methods except for GET and HEAD.
    1360    A cache &MUST-NOT; reply to such a request from a client before having
    1361    transmitted the request to the inbound server, and having received a
    1362    corresponding response from the inbound server. This does not prevent
    1363    a proxy cache from sending a 100 (Continue) response before the
    1364    inbound server has sent its final reply.
    1365 </t>
    1366 <t>
    1367    The alternative (known as "write-back" or "copy-back" caching) is not
    1368    allowed in HTTP/1.1, due to the difficulty of providing consistent
    1369    updates and the problems arising from server, cache, or network
    1370    failure prior to write-back.
    1371 </t>
    1372 </section>
    1373 
    1374 <section title="Cache Replacement" anchor="cache.replacement">
    1375 <t>
    1376    If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
    1377    <xref target="disambiguating.expiration.values" format="counter"/>,
    1378    <xref target="disambiguating.multiple.responses" format="counter"/>
    1379    and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
    1380    response is received from a resource while any existing responses for
    1381    the same resource are cached, the cache &SHOULD; use the new response
    1382    to reply to the current request. It &MAY; insert it into cache storage
    1383    and &MAY;, if it meets all other requirements, use it to respond to any
    1384    future requests that would previously have caused the old response to
    1385    be returned. If it inserts the new response into cache storage  the
    1386    rules in <xref target="combining.headers"/> apply.
    1387   <list><t>
    1388       <x:h>Note:</x:h> a new response that has an older Date header value than
    1389       existing cached responses is not cacheable.
    1390   </t></list>
    1391 </t>
    1392 </section>
    1393 
    1394 <section title="History Lists" anchor="history.lists">
    1395 <t>
    1396    User agents often have history mechanisms, such as "Back" buttons and
    1397    history lists, which can be used to redisplay an entity retrieved
    1398    earlier in a session.
    1399 </t>
    1400 <t>
    1401    History mechanisms and caches are different. In particular history
    1402    mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
    1403    the current state of a resource. Rather, a history mechanism is meant
    1404    to show exactly what the user saw at the time when the resource was
    1405    retrieved.
    1406 </t>
    1407 <t>
    1408    By default, an expiration time does not apply to history mechanisms.
    1409    If the entity is still in storage, a history mechanism &SHOULD; display
    1410    it even if the entity has expired, unless the user has specifically
    1411    configured the agent to refresh expired history documents.
    1412 </t>
    1413 <t>
    1414    This is not to be construed to prohibit the history mechanism from
    1415    telling the user that a view might be stale.
    1416   <list><t>
    1417       <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
    1418       viewing stale resources, this will tend to force service authors
    1419       to avoid using HTTP expiration controls and cache controls when
    1420       they would otherwise like to. Service authors may consider it
    1421       important that users not be presented with error messages or
    1422       warning messages when they use navigation controls (such as BACK)
    1423       to view previously fetched resources. Even though sometimes such
    1424       resources ought not be cached, or ought to expire quickly, user
    1425       interface considerations may force service authors to resort to
    1426       other means of preventing caching (e.g. "once-only" URLs) in order
    1427       not to suffer the effects of improperly functioning history
    1428       mechanisms.
    1429   </t></list>
    1430 </t>
    1431 </section>
    1432 
    1433 <section title="Header Field Definitions" anchor="header.fields">
    1434 <t>
    1435    This section defines the syntax and semantics of HTTP/1.1 header fields
    1436    related to caching.
    1437 </t>
    1438 <t>
    1439    For entity-header fields, both sender and recipient refer to either the
    1440    client or the server, depending on who sends and who receives the entity.
    1441 </t>
    1442 
    1443 <section title="Age" anchor="header.age">
    1444   <iref primary="true" item="Age header" x:for-anchor=""/>
    1445   <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
    1446   <x:anchor-alias value="Age"/>
    1447   <x:anchor-alias value="age-value"/>
    1448 <t>
    1449       The Age response-header field conveys the sender's estimate of the
    1450       amount of time since the response (or its revalidation) was
    1451       generated at the origin server. A cached response is "fresh" if
    1452       its age does not exceed its freshness lifetime. Age values are
    1453       calculated as specified in <xref target="age.calculations"/>.
    1454 </t>
    1455 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
     617</artwork>
     618          </figure>
     619        </section>
     620
     621
     622        <section anchor="serving.stale.responses" title="Serving Stale Responses">
     623          <t>Caches &MAY; return a stale response, unless such a response is otherwise prohibited
     624            (e.g., by a "no-store" or "no-cache" cache-request-directive, or a "must-revalidate"
     625            cache-response-directive; see <xref target="header.cache-control" />). Such stale responses
     626            MUST have a Warning header with the 110 warn-code (see <xref format="counter"
     627              target="header.warning" />)</t>
     628          <t>A stale response &MUST; have freshness information available, either explicitly or
     629            heuristically. See <xref target="calculating.freshness.lifetime" /></t>
     630          <t>If a cache receives a first-hand response (either an entire response, or a 304 (Not
     631            Modified) response) that it would normally forward to the requesting client, and the
     632            received response is no longer fresh, the cache &SHOULD; forward it to the requesting
     633            client without adding a new Warning (but without removing any existing Warning headers). A
     634            cache &SHOULD-NOT; attempt to validate a response simply because that response
     635            became stale in transit.</t>
     636        </section>   
     637
     638      </section>
     639
     640
     641      <section anchor="validation.model" title="Validation Model">
     642        <t>When a cache has a stale response that it would like to use, it should first check with
     643          the origin server (or possibly an intermediate cache with a fresh response) to see if it
     644          is still usable. This is called "validating" or "revalidating" the stored response.</t>
     645        <t>HTTP's conditional request mechanism, defined in &conditional;, is used to avoid
     646          retransmitting the response payload when the stored response is valid. When a stored response
     647          includes one or more "cache validators," such as the field values of an ETag or
     648          Last-Modified header field, then a validating GET request &SHOULD; be made conditional
     649          to those field values. The server checks the conditional request's validator against the
     650          current state of the requested resource and, if they match, the server responds with a 304
     651          (Not Modified) status code to indicate that the stored response can be refreshed and
     652          reused without retransmitting the response payload. If the validator does not match the
     653          current state of the requested resource, then the server returns a full response,
     654          including payload, so that the request can be satisfied and the stored response supplanted
     655          without the need for an additional network round-trip.</t>
     656        <t>See <xref target="combining.headers" /> for information about combining cached headers
     657          with those in a 304 response.</t>
     658        <t>If a cache receives a 5xx response while attempting to validate a response, it &MAY;
     659          either forward this response to the requesting client, or act as if the server failed to
     660          respond. In the latter case, it &MAY; return a previously received response unless the
     661          stored response includes the "must-revalidate" cache-control directive (see <xref
     662            target="header.cache-control" />).</t>
     663        <t>
     664          <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in
     665            p1</cref>
     666        </t>
     667      </section>
     668
     669           
     670      <section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
     671        <t>Because unsafe methods &safe-methods; have the potential for changing state on the
     672          origin server, intervening caches have the opportunity to use them to keep their contents
     673          up-to-date.</t>
     674        <t>The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
     675          as the Location and Content-Location headers (if present): <list style="symbols">
     676            <t>PUT</t>
     677            <t>DELETE</t>
     678            <t>POST</t>
     679          </list>
     680        </t>
     681        <t>An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
     682          be performed if the host part of that URI differs from the host part in the Request-URI.
     683          This helps prevent denial of service attacks.</t>
     684        <t>
     685          <cref>TODO: "host part" needs to be specified better.</cref>
     686        </t>
     687        <t>A cache that passes through requests for methods it does not understand &SHOULD;
     688          invalidate the Request-URI.</t>
     689        <t>Here, "invalidate" means that the cache will either remove all stored responses related
     690          to the Request-URI, or will mark these as "invalid" and in need of a mandatory
     691          validation before they can be returned in response to a subsequent request.</t>
     692        <t>Note that this does not guarantee that all appropriate responses are invalidated. For
     693          example, the request that caused the change at the origin server might not have gone
     694          through the cache where a response is stored.</t>
     695        <t>
     696          <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
     697        </t>
     698      </section>
     699     
     700     
     701      <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
     702        <t>Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
     703          by the presence of a Vary header field <xref target="header.vary" /> in a response, alters
     704          the conditions and procedure by which a cache can use the response for subsequent
     705          requests.</t>
     706        <t>When the cache receives a subsequent request which may be satisfied by a stored responses
     707          that include a Vary header field, it &MUST-NOT; use it to satisfy the request unless
     708          all of the selecting request-headers present in the new request match the corresponding
     709          stored request-headers from the original request.</t>
     710        <t>The selecting request-headers from two requests are defined to match if and only if the
     711          selecting request-headers in the first request can be transformed to the selecting
     712          request-headers in the second request by adding or removing linear white space
     713          <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
     714          combining multiple message-header fields with the same field name following the rules
     715          about message headers in &message-headers;.</t>
     716        <t>A Vary header field-value of "*" always fails to match, and subsequent requests on that
     717          resource can only be properly interpreted by the origin server.</t>
     718        <t>If the selecting request header fields for the stored response do not match the selecting
     719          request header fields of the new request, then the cache &MUST-NOT; use the stored
     720          response to satisfy the request unless it first relays the new request to the origin
     721          server in a conditional request and the server responds with 304 (Not Modified), including
     722          an entity tag or Content-Location that indicates the entity to be used.</t>
     723        <t>If one or more applicable stored response has an entity tag, the forwarded request
     724          &SHOULD; be conditional and include all of these entity tags in an If-None-Match
     725          header field. This conveys to the server the set of entities currently stored by the
     726          cache, so that if any one of these entities matches the requested entity, the server can
     727          use the ETag header field in its 304 (Not Modified) response to tell the cache which
     728          stored response is appropriate. If the entity-tag of the new response matches that of an
     729          existing stored resopnse, the new response &SHOULD; be used to update its header
     730          fields, and the result &MUST; be returned to the client.</t>
     731        <t>If any of the existing stored responses contains only partial content for the associated
     732          entity, its entity-tag &SHOULD-NOT; be included in the If-None-Match header field
     733          unless the request is for a range that would be fully satisfied by that stored response.</t>
     734        <t>If a cache receives a successful response whose Content-Location field matches that of an
     735          existing stored response for the same Request-URI, whose entity-tag differs from that of the
     736          existing stored response, and whose Date is more recent than that of the existing response,
     737          the existing response &SHOULD-NOT; be returned in response to future requests and
     738          &SHOULD; be deleted from the cache.</t>
     739        <t>
     740          <cref>TODO: this is still really messed up.</cref>
     741        </t>
     742      </section>
     743     
     744     
     745      <section anchor="errors.or.incomplete.response.cache.behavior"
     746        title="Caching Incomplete Responses">
     747        <t>A cache that receives an incomplete response (for example, with fewer bytes of data than
     748          specified in a Content-Length header) &MAY; store the response. However, the cache
     749          &MUST; treat this as a partial response &partial;. Partial responses &MAY; be
     750          combined as described in &combining-byte-ranges;; the result might be a full response
     751          or might still be partial. A cache &MUST-NOT; return a partial response to a client
     752          without explicitly marking it as such using the 206 (Partial Content) status code.</t>
     753        <t>A cache that does not support the Range and Content-Range headers &MUST-NOT; cache
     754          incomplete or partial responses.</t>
     755      </section>
     756     
     757     
     758      <section anchor="combining.headers" title="Combining Responses">
     759        <t>When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
     760          it needs to update the stored response with the new one, so that the updated response can
     761          be sent to the client.</t>
     762        <t>If the status code is 304 (Not Modified), the cache SHOULD use the stored entity-body as
     763          the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
     764          Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
     765          the stored response with the updated entity-body received in the response and use the result
     766          as the updated entity-body (see &combining-byte-ranges;).</t>
     767        <t>The stored response headers are used for the updated response, except that <list
     768          style="symbols">
     769          <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
     770            &MUST; be deleted from the stored response and the forwarded response.</t>
     771          <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
     772            response and the forwarded response.</t>
     773          <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
     774            headers from the stored response.</t>
     775        </list>
     776        </t>
     777        <t>A cache &MUST; also replace stored headers with corresponding headers received in the
     778          incoming response, except for Warning headers as described immediately above. If a header
     779          field-name in the incoming response matches more than one header in the stored response, all
     780          such old headers &MUST; be replaced. it &MAY; store the combined entity-body.</t>
     781      </section>
     782             
     783    </section>
     784
     785
     786
     787
     788    <section anchor="header.fields" title="Header Field Definitions">
     789      <t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
     790      <t>For entity-header fields, both sender and recipient refer to either the client or the
     791        server, depending on who sends and who receives the entity.</t>
     792
     793      <section anchor="header.age" title="Age">
     794        <iref item="Age header" primary="true" x:for-anchor="" />
     795        <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
     796        <x:anchor-alias value="Age" />
     797        <x:anchor-alias value="age-value" />
     798        <t>The Age response-header field conveys the sender's estimate of the amount of time since
     799          the response (or its validation) was generated at the origin server. Age values are
     800          calculated as specified in <xref target="age.calculations" />.</t>
     801        <figure>
     802          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Age" /><iref item="Grammar" primary="true" subitem="age-value" />
    1456803  <x:ref>Age</x:ref> = "Age" ":" <x:ref>age-value</x:ref>
    1457804  <x:ref>age-value</x:ref> = <x:ref>delta-seconds</x:ref>
    1458 </artwork></figure>
    1459 <t anchor="rule.delta-seconds">
    1460   <x:anchor-alias value="delta-seconds"/>
    1461       Age values are non-negative decimal integers, representing time in
    1462       seconds.
    1463 </t>
    1464 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
     805</artwork>
     806        </figure>
     807        <t anchor="rule.delta-seconds">
     808          <x:anchor-alias value="delta-seconds" /> Age values are non-negative decimal integers,
     809          representing time in seconds.</t>
     810        <figure>
     811          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
    1465812  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
    1466 </artwork></figure>
    1467 <t>
    1468       If a cache receives a value larger than the largest positive
    1469       integer it can represent, or if any of its age calculations
    1470       overflows, it &MUST; transmit an Age header with a value of
    1471       2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache &MUST;
    1472       include an Age header field in every response generated from its
    1473       own cache. Caches &SHOULD; use an arithmetic type of at least 31
    1474       bits of range.
    1475 </t>
    1476 </section>
    1477 
    1478 <section title="Cache-Control" anchor="header.cache-control">
    1479   <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
    1480   <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
    1481   <x:anchor-alias value="Cache-Control"/>
    1482   <x:anchor-alias value="cache-directive"/>
    1483   <x:anchor-alias value="cache-extension"/>
    1484   <x:anchor-alias value="cache-request-directive"/>
    1485   <x:anchor-alias value="cache-response-directive"/>
    1486 <t>
    1487    The Cache-Control general-header field is used to specify directives
    1488    that &MUST; be obeyed by all caching mechanisms along the
    1489    request/response chain. The directives specify behavior intended to
    1490    prevent caches from adversely interfering with the request or
    1491    response. These directives typically override the default caching
    1492    algorithms. Cache directives are unidirectional in that the presence
    1493    of a directive in a request does not imply that the same directive is
    1494    to be given in the response.
    1495   <list><t>
    1496       Note that HTTP/1.0 caches might not implement Cache-Control and
    1497       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
    1498   </t></list>
    1499 </t>
    1500 <t>
    1501    Cache directives &MUST; be passed through by a proxy or gateway
    1502    application, regardless of their significance to that application,
    1503    since the directives might be applicable to all recipients along the
    1504    request/response chain. It is not possible to specify a cache-directive
    1505    for a specific cache.
    1506 </t>
    1507 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><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"/>
     813</artwork>
     814        </figure>
     815        <t>If a cache receives a value larger than the largest positive integer it can represent, or
     816          if any of its age calculations overflows, it &MUST; transmit an Age header with a
     817          value of 2147483648 (2<x:sup>31</x:sup>). An HTTP/1.1 server that includes a cache
     818          &MUST; include an Age header field in every response generated from its own cache.
     819          Caches &SHOULD; use an arithmetic type of at least 31 bits of range.</t>
     820        <t>The presence of an Age header field in a response implies that a response is not
     821          first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
     822          Age header field.</t>
     823      </section>
     824
     825      <section anchor="header.cache-control" title="Cache-Control">
     826        <iref item="Cache-Control header" primary="true" x:for-anchor="" />
     827        <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
     828        <x:anchor-alias value="Cache-Control" />
     829        <x:anchor-alias value="cache-directive" />
     830        <x:anchor-alias value="cache-extension" />
     831        <t>The Cache-Control general-header field is used to specify directives that &MUST; be
     832          obeyed by all caches along the request/response chain. The directives specify behavior
     833          intended to prevent caches from adversely interfering with the request or response. Cache
     834          directives are unidirectional in that the presence of a directive in a request does not
     835          imply that the same directive is to be given in the response. <list>
     836            <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
     837              Pragma: no-cache (see <xref target="header.pragma" />).</t>
     838          </list>
     839        </t>
     840        <t>Cache directives &MUST; be passed through by a proxy or gateway application,
     841          regardless of their significance to that application, since the directives might be
     842          applicable to all recipients along the request/response chain. It is not possible to
     843          target a cache-directive to a specific cache.</t>
     844        <figure>
     845          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Cache-Control" /><iref item="Grammar" primary="true" subitem="cache-directive" /><iref item="Grammar" primary="true" subitem="cache-extension" />
    1508846  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" 1#<x:ref>cache-directive</x:ref>
    1509847
     
    1511849     / <x:ref>cache-response-directive</x:ref>
    1512850
     851  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
     852</artwork>
     853        </figure>
     854
     855
     856        <section anchor="cache-request-directive" title="Request Cache-Control Directives">
     857          <x:anchor-alias value="cache-request-directive" />
     858
     859          <figure>
     860            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
    1513861  <x:ref>cache-request-directive</x:ref> =
    1514        "no-cache"                          ; <xref target="what.is.cacheable"/>
    1515      / "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
    1516      / "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"/>
    1517      / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    1518      / "min-fresh" "=" <x:ref>delta-seconds</x:ref>       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    1519      / "no-transform"                      ; <xref target="no-transform.directive"/>
    1520      / "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
    1521      / <x:ref>cache-extension</x:ref>                     ; <xref target="cache.control.extensions"/>
    1522 
     862       "no-cache"
     863     / "no-store"
     864     / "max-age" "=" <x:ref>delta-seconds</x:ref>
     865     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
     866     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
     867     / "no-transform"
     868     / "only-if-cached"
     869     / <x:ref>cache-extension</x:ref>
     870</artwork>
     871          </figure>
     872
     873
     874          <t>
     875            <iref item="Cache Directives" primary="true" subitem="no-cache" />
     876            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
     877              <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
     878                used to satisfy the request without successful validation on the origin server.
     879              </t>
     880            </list>
     881          </t>
     882
     883          <t>
     884            <iref item="Cache Directives" primary="true" subitem="no-store" />
     885            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
     886              <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
     887                of either this request or any response to it. This directive applies to both
     888                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
     889                cache &MUST-NOT; intentionally store the information in non-volatile storage,
     890                and &MUST; make a best-effort attempt to remove the information from volatile
     891                storage as promptly as possible after forwarding it.</t>
     892              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
     893                particular, malicious or compromised caches might not recognize or obey this
     894                directive, and communications networks may be vulnerable to eavesdropping.</t>
     895            </list>
     896          </t>
     897
     898          <t>
     899            <iref item="Cache Directives" primary="true" subitem="max-age" />
     900            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
     901              <t>The no-cache request directive indicates that the client is willing to accept a
     902                response whose age is no greater than the specified time in seconds. Unless
     903                max-stale directive is also included, the client is not willing to accept a stale
     904                response.</t>
     905            </list>
     906          </t>
     907          <t>
     908            <iref item="Cache Directives" primary="true" subitem="max-stale" />
     909            <iref item="max-stale" primary="true" subitem="Cache Directive" /> max-stale <list>
     910              <t>The max-stale request directive indicates that the client is willing to accept a
     911                response that has exceeded its expiration time. If max-stale is assigned a value,
     912                then the client is willing to accept a response that has exceeded its expiration
     913                time by no more than the specified number of seconds. If no value is assigned to
     914                max-stale, then the client is willing to accept a stale response of any age.</t>
     915            </list>
     916          </t>
     917          <t>
     918            <iref item="Cache Directives" primary="true" subitem="min-fresh" />
     919            <iref item="min-fresh" primary="true" subitem="Cache Directive" /> min-fresh <list>
     920              <t>The min-fresh request directive indicates that the client is willing to accept a
     921                response whose freshness lifetime is no less than its current age plus the specified
     922                time in seconds. That is, the client wants a response that will still be fresh for
     923                at least the specified number of seconds.</t>
     924            </list>
     925          </t>
     926          <t>
     927            <iref item="Cache Directives" primary="true" subitem="no-transform" />
     928            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
     929              <t>The no-transform request directive indicates that an intermediate cache or proxy
     930                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
     931                headers, nor the request entity-body.</t>
     932            </list>
     933          </t>
     934
     935          <t>
     936            <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
     937            <iref item="only-if-cached" primary="true" subitem="Cache Directive" /> only-if-cached <list>
     938              <t>The only-if-cached request directive indicates that the client only wishes to
     939                return a stored response. If it receives this directive, a cache &SHOULD; either
     940                respond using a stored response that is consistent with the other constraints of the
     941                request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
     942                being operated as a unified system with good internal connectivity, such a request
     943                &MAY; be forwarded within that group of caches.</t>
     944            </list>
     945          </t>
     946        </section>
     947
     948        <section anchor="cache-response-directive" title="Response Cache-Control Directives">
     949          <x:anchor-alias value="cache-response-directive" />
     950
     951          <figure>
     952            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
    1523953  <x:ref>cache-response-directive</x:ref> =
    1524        "public"                               ; <xref target="what.is.cacheable"/>
    1525      / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
    1526      / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ] ; <xref target="what.is.cacheable"/>
    1527      / "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
    1528      / "no-transform"                         ; <xref target="no-transform.directive"/>
    1529      / "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
    1530      / "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
    1531      / "max-age" "=" <x:ref>delta-seconds</x:ref>            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    1532      / "s-maxage" "=" <x:ref>delta-seconds</x:ref>           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    1533      / <x:ref>cache-extension</x:ref>                        ; <xref target="cache.control.extensions"/>
    1534 
    1535   <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
    1536 </artwork></figure>
    1537 <t>
    1538    When a directive appears without any 1#field-name parameter, the
    1539    directive applies to the entire request or response. When such a
    1540    directive appears with a 1#field-name parameter, it applies only to
    1541    the named field or fields, and not to the rest of the request or
    1542    response. This mechanism supports extensibility; implementations of
    1543    future versions of HTTP might apply these directives to
    1544    header fields not defined in HTTP/1.1.
    1545 </t>
    1546 <t>
    1547    The cache-control directives can be broken down into these general
    1548    categories:
    1549   <list style="symbols">
    1550      <t>Restrictions on what are cacheable; these may only be imposed by
    1551         the origin server.</t>
    1552 
    1553      <t>Restrictions on what may be stored by a cache; these may be
    1554         imposed by either the origin server or the user agent.</t>
    1555 
    1556      <t>Modifications of the basic expiration mechanism; these may be
    1557         imposed by either the origin server or the user agent.</t>
    1558 
    1559      <t>Controls over cache revalidation and reload; these may only be
    1560         imposed by a user agent.</t>
    1561 
    1562      <t>Control over transformation of entities.</t>
    1563 
    1564      <t>Extensions to the caching system.</t>
    1565   </list>
    1566 </t>
    1567 
    1568 <section title="What is Cacheable" anchor="what.is.cacheable">
    1569 <t>
    1570    By default, a response is cacheable if the requirements of the
    1571    request method, request header fields, and the response status
    1572    indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
    1573    for cacheability. The following Cache-Control response directives
    1574    allow an origin server to override the default cacheability of a
    1575    response:
    1576 </t>
    1577 <t>
    1578   <iref item="Cache Directives" subitem="public" primary="true"/>
    1579   <iref item="public" subitem="Cache Directive" primary="true"/>
    1580    public
    1581   <list><t>
    1582       Indicates that the response &MAY; be cached by any cache, even if it
    1583       would normally be non-cacheable or cacheable only within a non-shared
    1584       cache. (See also Authorization, &header-authorization;, for
    1585       additional details.)
    1586   </t></list>
    1587 </t>
    1588 <t>
    1589   <iref item="Cache Directives" subitem="private" primary="true"/>
    1590   <iref item="private" subitem="Cache Directive" primary="true"/>
    1591    private
    1592   <list><t>
    1593       Indicates that all or part of the response message is intended for
    1594       a single user and &MUST-NOT; be cached by a shared cache. This
    1595       allows an origin server to state that the specified parts of the
    1596       response are intended for only one user and are not a valid
    1597       response for requests by other users. A private (non-shared) cache
    1598       &MAY; cache the response.
    1599     </t><t>
    1600        <x:h>Note:</x:h> This usage of the word private only controls where the
    1601        response may be cached, and cannot ensure the privacy of the
    1602        message content.
    1603   </t></list>
    1604 </t>
    1605 <t>
    1606   <iref item="Cache Directives" subitem="no-cache" primary="true"/>
    1607   <iref item="no-cache" subitem="Cache Directive" primary="true"/>
    1608    no-cache
    1609   <list><t>
    1610        If the no-cache directive does not specify a field-name, then a
    1611       cache &MUST-NOT; use the response to satisfy a subsequent request
    1612       without successful revalidation with the origin server. This
    1613       allows an origin server to prevent caching even by caches that
    1614       have been configured to return stale responses to client requests.
    1615     </t><t>
    1616       If the no-cache directive does specify one or more field-names,
    1617       then a cache &MAY; use the response to satisfy a subsequent request,
    1618       subject to any other restrictions on caching. However, the
    1619       specified field-name(s) &MUST-NOT; be sent in the response to a
    1620       subsequent request without successful revalidation with the origin
    1621       server. This allows an origin server to prevent the re-use of
    1622       certain header fields in a response, while still allowing caching
    1623       of the rest of the response.
    1624     <list><t>
    1625        <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
    1626        directive.
    1627     </t></list>
    1628   </t></list>
    1629 </t>
    1630 </section>
    1631 
    1632 <section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
    1633 <t>
    1634   <iref item="Cache Directives" subitem="no-store" primary="true"/>
    1635   <iref item="no-store" subitem="Cache Directive" primary="true"/>
    1636    no-store
    1637   <list><t>   
    1638       The purpose of the no-store directive is to prevent the
    1639       inadvertent release or retention of sensitive information (for
    1640       example, on backup tapes). The no-store directive applies to the
    1641       entire message, and &MAY; be sent either in a response or in a
    1642       request. If sent in a request, a cache &MUST-NOT; store any part of
    1643       either this request or any response to it. If sent in a response,
    1644       a cache &MUST-NOT; store any part of either this response or the
    1645       request that elicited it. This directive applies to both non-shared
    1646       and shared caches. "&MUST-NOT; store" in this context means
    1647       that the cache &MUST-NOT; intentionally store the information in
    1648       non-volatile storage, and &MUST; make a best-effort attempt to
    1649       remove the information from volatile storage as promptly as
    1650       possible after forwarding it.
    1651   </t><t>
    1652       Even when this directive is associated with a response, users
    1653       might explicitly store such a response outside of the caching
    1654       system (e.g., with a "Save As" dialog). History buffers &MAY; store
    1655       such responses as part of their normal operation.
    1656   </t><t>
    1657       The purpose of this directive is to meet the stated requirements
    1658       of certain users and service authors who are concerned about
    1659       accidental releases of information via unanticipated accesses to
    1660       cache data structures. While the use of this directive might
    1661       improve privacy in some cases, we caution that it is NOT in any
    1662       way a reliable or sufficient mechanism for ensuring privacy. In
    1663       particular, malicious or compromised caches might not recognize or
    1664       obey this directive, and communications networks might be
    1665       vulnerable to eavesdropping.
    1666   </t></list>
    1667 </t>
    1668 </section>
    1669 
    1670 <section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
    1671 <t>
    1672    The expiration time of an entity &MAY; be specified by the origin
    1673    server using the Expires header (see <xref target="header.expires"/>). Alternatively,
    1674    it &MAY; be specified using the max-age directive in a response. When
    1675    the max-age cache-control directive is present in a cached response,
    1676    the response is stale if its current age is greater than the age
    1677    value given (in seconds) at the time of a new request for that
    1678    resource. The max-age directive on a response implies that the
    1679    response is cacheable (i.e., "public") unless some other, more
    1680    restrictive cache directive is also present.
    1681 </t>
    1682 <t>
    1683    If a response includes both an Expires header and a max-age
    1684    directive, the max-age directive overrides the Expires header, even
    1685    if the Expires header is more restrictive. This rule allows an origin
    1686    server to provide, for a given response, a longer expiration time to
    1687    an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
    1688    useful if certain HTTP/1.0 caches improperly calculate ages or
    1689    expiration times, perhaps due to desynchronized clocks.
    1690 </t>
    1691 <t>
    1692    Many HTTP/1.0 cache implementations will treat an Expires value that
    1693    is less than or equal to the response Date value as being equivalent
    1694    to the Cache-Control response directive "no-cache". If an HTTP/1.1
    1695    cache receives such a response, and the response does not include a
    1696    Cache-Control header field, it &SHOULD; consider the response to be
    1697    non-cacheable in order to retain compatibility with HTTP/1.0 servers.
    1698   <list><t>
    1699        <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
    1700        cache control feature, such as the "private" directive, on a
    1701        network including older caches that do not understand that
    1702        feature. The origin server will need to combine the new feature
    1703        with an Expires field whose value is less than or equal to the
    1704        Date value. This will prevent older caches from improperly
    1705        caching the response.
    1706   </t></list>
    1707 </t>
    1708 <t>
    1709   <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
    1710   <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
    1711    s-maxage
    1712   <list><t>
    1713        If a response includes an s-maxage directive, then for a shared
    1714        cache (but not for a private cache), the maximum age specified by
    1715        this directive overrides the maximum age specified by either the
    1716        max-age directive or the Expires header. The s-maxage directive
    1717        also implies the semantics of the proxy-revalidate directive (see
    1718        <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
    1719        entry after it becomes stale to respond to a subsequent request
    1720        without first revalidating it with the origin server. The s-maxage
    1721        directive is always ignored by a private cache.
    1722   </t></list>
    1723 </t>
    1724 <t>
    1725    Note that most older caches, not compliant with this specification,
    1726    do not implement any cache-control directives. An origin server
    1727    wishing to use a cache-control directive that restricts, but does not
    1728    prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
    1729    requirement that the max-age directive overrides the Expires header,
    1730    and the fact that pre-HTTP/1.1-compliant caches do not observe the
    1731    max-age directive.
    1732 </t>
    1733 <t>
    1734    Other directives allow a user agent to modify the basic expiration
    1735    mechanism. These directives &MAY; be specified on a request:
    1736 </t>
    1737 <t>
    1738   <iref item="Cache Directives" subitem="max-age" primary="true"/>
    1739   <iref item="max-age" subitem="Cache Directive" primary="true"/>
    1740    max-age
    1741   <list><t>
    1742       Indicates that the client is willing to accept a response whose
    1743       age is no greater than the specified time in seconds. Unless max-stale
    1744       directive is also included, the client is not willing to
    1745       accept a stale response.
    1746   </t></list>
    1747 </t>
    1748 <t>
    1749   <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
    1750   <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
    1751    min-fresh
    1752   <list><t>
    1753       Indicates that the client is willing to accept a response whose
    1754       freshness lifetime is no less than its current age plus the
    1755       specified time in seconds. That is, the client wants a response
    1756       that will still be fresh for at least the specified number of
    1757       seconds.
    1758   </t></list>
    1759 </t>
    1760 <t>
    1761   <iref item="Cache Directives" subitem="max-stale" primary="true"/>
    1762   <iref item="max-stale" subitem="Cache Directive" primary="true"/>
    1763    max-stale
    1764   <list><t>
    1765       Indicates that the client is willing to accept a response that has
    1766       exceeded its expiration time. If max-stale is assigned a value,
    1767       then the client is willing to accept a response that has exceeded
    1768       its expiration time by no more than the specified number of
    1769       seconds. If no value is assigned to max-stale, then the client is
    1770       willing to accept a stale response of any age.
    1771   </t></list>
    1772 </t>
    1773 <t>
    1774    If a cache returns a stale response, either because of a max-stale
    1775    directive on a request, or because the cache is configured to
    1776    override the expiration time of a response, the cache &MUST; attach a
    1777    Warning header to the stale response, using Warning 110 (Response is
    1778    stale).
    1779 </t>
    1780 <t>
    1781    A cache &MAY; be configured to return stale responses without
    1782    validation, but only if this does not conflict with any "MUST"-level
    1783    requirements concerning cache validation (e.g., a "must-revalidate"
    1784    cache-control directive).
    1785 </t>
    1786 <t>
    1787    If both the new request and the cached entry include "max-age"
    1788    directives, then the lesser of the two values is used for determining
    1789    the freshness of the cached entry for that request.
    1790 </t>
    1791 </section>
    1792 
    1793 <section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
    1794 <t>
    1795    Sometimes a user agent might want or need to insist that a cache
    1796    revalidate its cache entry with the origin server (and not just with
    1797    the next cache along the path to the origin server), or to reload its
    1798    cache entry from the origin server. End-to-end revalidation might be
    1799    necessary if either the cache or the origin server has overestimated
    1800    the expiration time of the cached response. End-to-end reload may be
    1801    necessary if the cache entry has become corrupted for some reason.
    1802 </t>
    1803 <t>
    1804    End-to-end revalidation may be requested either when the client does
    1805    not have its own local cached copy, in which case we call it
    1806    "unspecified end-to-end revalidation", or when the client does have a
    1807    local cached copy, in which case we call it "specific end-to-end
    1808    revalidation."
    1809 </t>
    1810 <t>
    1811    The client can specify these three kinds of action using Cache-Control
    1812    request directives:
    1813 </t>
    1814 <t>
    1815    End-to-end reload
    1816   <list><t>
    1817       The request includes a "no-cache" cache-control directive or, for
    1818       compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
    1819       names &MUST-NOT; be included with the no-cache directive in a
    1820       request. The server &MUST-NOT; use a cached copy when responding to
    1821       such a request.
    1822   </t></list>
    1823 </t>
    1824 <t>
    1825    Specific end-to-end revalidation
    1826   <list><t>
    1827       The request includes a "max-age=0" cache-control directive, which
    1828       forces each cache along the path to the origin server to
    1829       revalidate its own entry, if any, with the next cache or server.
    1830       The initial request includes a cache-validating conditional with
    1831       the client's current validator.
    1832   </t></list>
    1833 </t>
    1834 <t>
    1835    Unspecified end-to-end revalidation
    1836   <list><t>
    1837       The request includes "max-age=0" cache-control directive, which
    1838       forces each cache along the path to the origin server to
    1839       revalidate its own entry, if any, with the next cache or server.
    1840       The initial request does not include a cache-validating
    1841       conditional; the first cache along the path (if any) that holds a
    1842       cache entry for this resource includes a cache-validating
    1843       conditional with its current validator.
    1844   </t></list>
    1845 </t>
    1846 <t>
    1847   <iref item="Cache Directives" subitem="max-age" primary="true"/>
    1848   <iref item="max-age" subitem="Cache Directive" primary="true"/>
    1849    max-age
    1850   <list><t>
    1851       When an intermediate cache is forced, by means of a max-age=0
    1852       directive, to revalidate its own cache entry, and the client has
    1853       supplied its own validator in the request, the supplied validator
    1854       might differ from the validator currently stored with the cache
    1855       entry. In this case, the cache &MAY; use either validator in making
    1856       its own request without affecting semantic transparency.
    1857   </t><t>
    1858       However, the choice of validator might affect performance. The
    1859       best approach is for the intermediate cache to use its own
    1860       validator when making its request. If the server replies with 304
    1861       (Not Modified), then the cache can return its now validated copy
    1862       to the client with a 200 (OK) response. If the server replies with
    1863       a new entity and cache validator, however, the intermediate cache
    1864       can compare the returned validator with the one provided in the
    1865       client's request, using the strong comparison function. If the
    1866       client's validator is equal to the origin server's, then the
    1867       intermediate cache simply returns 304 (Not Modified). Otherwise,
    1868       it returns the new entity with a 200 (OK) response.
    1869   </t><t>
    1870       If a request includes the no-cache directive, it &SHOULD-NOT;
    1871       include min-fresh, max-stale, or max-age.
    1872   </t></list>
    1873 </t>
    1874 <t>
    1875   <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
    1876   <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
    1877    only-if-cached
    1878   <list><t>
    1879       In some cases, such as times of extremely poor network
    1880       connectivity, a client may want a cache to return only those
    1881       responses that it currently has stored, and not to reload or
    1882       revalidate with the origin server. To do this, the client may
    1883       include the only-if-cached directive in a request. If it receives
    1884       this directive, a cache &SHOULD; either respond using a cached entry
    1885       that is consistent with the other constraints of the request, or
    1886       respond with a 504 (Gateway Timeout) status. However, if a group
    1887       of caches is being operated as a unified system with good internal
    1888       connectivity, such a request &MAY; be forwarded within that group of
    1889       caches.
    1890   </t></list>
    1891 </t>
    1892 <t>
    1893   <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
    1894   <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
    1895    must-revalidate
    1896   <list><t>
    1897       Because a cache &MAY; be configured to ignore a server's specified
    1898       expiration time, and because a client request &MAY; include a max-stale
    1899       directive (which has a similar effect), the protocol also
    1900       includes a mechanism for the origin server to require revalidation
    1901       of a cache entry on any subsequent use. When the must-revalidate
    1902       directive is present in a response received by a cache, that cache
    1903       &MUST-NOT; use the entry after it becomes stale to respond to a
    1904       subsequent request without first revalidating it with the origin
    1905       server. (I.e., the cache &MUST; do an end-to-end revalidation every
    1906       time, if, based solely on the origin server's Expires or max-age
    1907       value, the cached response is stale.)
    1908   </t><t>
    1909       The must-revalidate directive is necessary to support reliable
    1910       operation for certain protocol features. In all circumstances an
    1911       HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
    1912       particular, if the cache cannot reach the origin server for any
    1913       reason, it &MUST; generate a 504 (Gateway Timeout) response.
    1914   </t><t>
    1915       Servers &SHOULD; send the must-revalidate directive if and only if
    1916       failure to revalidate a request on the entity could result in
    1917       incorrect operation, such as a silently unexecuted financial
    1918       transaction. Recipients &MUST-NOT; take any automated action that
    1919       violates this directive, and &MUST-NOT; automatically provide an
    1920       unvalidated copy of the entity if revalidation fails.
    1921   </t><t>
    1922       Although this is not recommended, user agents operating under
    1923       severe connectivity constraints &MAY; violate this directive but, if
    1924       so, &MUST; explicitly warn the user that an unvalidated response has
    1925       been provided. The warning &MUST; be provided on each unvalidated
    1926       access, and &SHOULD; require explicit user confirmation.
    1927   </t></list>
    1928 </t>
    1929 <t>
    1930   <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
    1931   <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
    1932    proxy-revalidate
    1933   <list><t>
    1934       The proxy-revalidate directive has the same meaning as the must-revalidate
    1935       directive, except that it does not apply to non-shared
    1936       user agent caches. It can be used on a response to an
    1937       authenticated request to permit the user's cache to store and
    1938       later return the response without needing to revalidate it (since
    1939       it has already been authenticated once by that user), while still
    1940       requiring proxies that service many users to revalidate each time
    1941       (in order to make sure that each user has been authenticated).
    1942       Note that such authenticated responses also need the public cache
    1943       control directive in order to allow them to be cached at all.
    1944   </t></list>
    1945 </t>
    1946 </section>
    1947 
    1948 <section title="No-Transform Directive" anchor="no-transform.directive">
    1949 <t>
    1950   <iref item="Cache Directives" subitem="no-transform" primary="true"/>
    1951   <iref item="no-transform" subitem="Cache Directive" primary="true"/>
    1952    no-transform
    1953   <list><t>
    1954       Implementors of intermediate caches (proxies) have found it useful
    1955       to convert the media type of certain entity bodies. A non-transparent
    1956       proxy might, for example, convert between image
    1957       formats in order to save cache space or to reduce the amount of
    1958       traffic on a slow link.
    1959   </t><t>
    1960       Serious operational problems occur, however, when these
    1961       transformations are applied to entity bodies intended for certain
    1962       kinds of applications. For example, applications for medical
    1963       imaging, scientific data analysis and those using end-to-end
    1964       authentication, all depend on receiving an entity body that is bit
    1965       for bit identical to the original entity-body.
    1966   </t><t>
    1967       Therefore, if a message includes the no-transform directive, an
    1968       intermediate cache or proxy &MUST-NOT; change those headers that are
    1969       listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
    1970       directive. This implies that the cache or proxy &MUST-NOT; change
    1971       any aspect of the entity-body that is specified by these headers,
    1972       including the value of the entity-body itself.
    1973   </t></list>
    1974 </t>
    1975 </section>
    1976 
    1977 <section title="Cache Control Extensions" anchor="cache.control.extensions">
    1978 <t>
    1979    The Cache-Control header field can be extended through the use of one
    1980    or more cache-extension tokens, each with an optional assigned value.
    1981    Informational extensions (those which do not require a change in
    1982    cache behavior) &MAY; be added without changing the semantics of other
    1983    directives. Behavioral extensions are designed to work by acting as
    1984    modifiers to the existing base of cache directives. Both the new
    1985    directive and the standard directive are supplied, such that
    1986    applications which do not understand the new directive will default
    1987    to the behavior specified by the standard directive, and those that
    1988    understand the new directive will recognize it as modifying the
    1989    requirements associated with the standard directive. In this way,
    1990    extensions to the cache-control directives can be made without
    1991    requiring changes to the base protocol.
    1992 </t>
    1993 <t>
    1994    This extension mechanism depends on an HTTP cache obeying all of the
    1995    cache-control directives defined for its native HTTP-version, obeying
    1996    certain extensions, and ignoring all directives that it does not
    1997    understand.
    1998 </t>
    1999 <t>
    2000    For example, consider a hypothetical new response directive called
    2001    community which acts as a modifier to the private directive. We
    2002    define this new directive to mean that, in addition to any non-shared
    2003    cache, any cache which is shared only by members of the community
    2004    named within its value may cache the response. An origin server
    2005    wishing to allow the UCI community to use an otherwise private
    2006    response in their shared cache(s) could do so by including
    2007 </t>
    2008 <figure><artwork type="example">
     954       "public"
     955     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
     956     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
     957     / "no-store"
     958     / "no-transform"
     959     / "must-revalidate"
     960     / "proxy-revalidate"
     961     / "max-age" "=" <x:ref>delta-seconds</x:ref>
     962     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
     963     / <x:ref>cache-extension</x:ref>
     964</artwork>
     965          </figure>
     966
     967          <t>
     968            <iref item="Cache Directives" primary="true" subitem="public" />
     969            <iref item="public" primary="true" subitem="Cache Directive" /> public <list>
     970              <t>The public response directive indicates that the response &MAY; be cached, even
     971                if it would normally be non-cacheable or cacheable only within a non-shared cache.
     972                (See also Authorization, &header-authorization;, for additional details.) </t>
     973            </list>
     974          </t>
     975
     976          <t>
     977            <iref item="Cache Directives" primary="true" subitem="private" />
     978            <iref item="private" primary="true" subitem="Cache Directive" /> private <list>
     979              <t>The private response directive indicates that the response message is intended for
     980                a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
     981                cache &MAY; store the response.</t>
     982              <t>If the private response directive specifies one or more field-names, this
     983                requirement is limited to the field-values associated with the listed response
     984                headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
     985                cache, whereas the remainder of the response message &MAY; be.</t>
     986              <t>
     987                <cref>ISSUE: What does this really mean? Is this a good idea?</cref>
     988              </t>
     989              <t>
     990                <x:h>Note:</x:h> This usage of the word private only controls where the response may
     991                be stored, and cannot ensure the privacy of the message content.</t>
     992            </list>
     993          </t>
     994
     995          <t>
     996            <iref item="Cache Directives" primary="true" subitem="no-cache" />
     997            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
     998              <t>The no-cache response directive indicates that a response &MUST-NOT; be used to
     999                satisfy a subseqent request without successful validation on the origin server.
     1000                This allows an origin server to prevent caching even by caches that have been
     1001                configured to return stale responses.</t>
     1002              <t>If the no-cache response directive specifies one or more field-names, this
     1003                requirement is limited to the field-values assosicated with the listed response
     1004                headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
     1005                to a subsequent request without successful validation on the origin server. This
     1006                allows an origin server to prevent the re-use of certain header fields in a
     1007                response, while still allowing caching of the rest of the response.</t>
     1008              <t>
     1009                <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
     1010              </t>
     1011            </list>
     1012          </t>
     1013
     1014          <t>
     1015            <iref item="Cache Directives" primary="true" subitem="no-store" />
     1016            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
     1017              <t>The no-store response directive indicates that a cache &MUST-NOT; store any
     1018                part of either the immediate request or response. This directive applies to both
     1019                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
     1020                cache &MUST-NOT; intentionally store the information in non-volatile storage,
     1021                and &MUST; make a best-effort attempt to remove the information from volatile
     1022                storage as promptly as possible after forwarding it.</t>
     1023              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
     1024                particular, malicious or compromised caches might not recognize or obey this
     1025                directive, and communications networks may be vulnerable to eavesdropping.</t>
     1026            </list>
     1027          </t>
     1028
     1029          <t>
     1030            <iref item="Cache Directives" primary="true" subitem="no-transform" />
     1031            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
     1032              <t>The no-transform response directive indicates that an intermediate cache or proxy
     1033                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
     1034                headers, nor the response entity-body.</t>
     1035            </list>
     1036          </t>
     1037
     1038          <t>
     1039            <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
     1040            <iref item="must-revalidate" primary="true" subitem="Cache Directive" /> must-revalidate <list>
     1041              <t>The must-revalidate response-directive indicates that validation is required before
     1042              the response is used by a cache to satisfy any request.</t>
     1043              <t>When the present, caches &MUST-NOT; use a stored after it becomes stale to respond to a subsequent
     1044                request without first validating it with the origin server.</t>
     1045              <t>The must-revalidate directive is necessary to support reliable operation for
     1046                certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
     1047                the must-revalidate directive; in particular, if the cache cannot reach the origin
     1048                server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
     1049              <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
     1050                validate a request on the entity could result in incorrect operation, such as a
     1051                silently unexecuted financial transaction. Recipients &MUST-NOT; take any
     1052                automated action that violates this directive, and &MUST-NOT; automatically
     1053                provide an unvalidated copy of the entity if validation fails.</t>
     1054              <t>Although this is not recommended, user agents operating under severe connectivity
     1055                constraints &MAY; violate this directive but, if so, &MUST; explicitly warn
     1056                the user that an unvalidated response has been provided. The warning &MUST; be
     1057                provided on each unvalidated access, and &SHOULD; require explicit user
     1058                confirmation.</t>
     1059              <t><cref>TODO: last two paragraphs seem nonsensical.</cref></t>
     1060            </list>
     1061          </t>
     1062
     1063          <t>
     1064            <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
     1065            <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
     1066            proxy-revalidate <list>
     1067              <t>The proxy-revalidate directive has the same meaning as the must-revalidate
     1068                directive, except that it does not apply to non-shared caches.</t>
     1069            </list>
     1070          </t>
     1071          <t>
     1072            <iref item="Cache Directives" primary="true" subitem="s-maxage" />
     1073            <iref item="s-maxage" primary="true" subitem="Cache Directive" /> s-maxage <list>
     1074              <t>The s-maxage response directive indicates that, in shared caches, the maximum age
     1075                specified by this directive overrides the maximum age specified by either the
     1076                max-age directive or the Expires header. The s-maxage directive also implies the
     1077                semantics of the proxy-revalidate response directive.</t>
     1078            </list>
     1079          </t>
     1080
     1081        </section>
     1082
     1083        <section anchor="cache.control.extensions" title="Cache Control Extensions">
     1084          <t>The Cache-Control header field can be extended through the use of one or more
     1085            cache-extension tokens, each with an optional value. Informational extensions (those
     1086            which do not require a change in cache behavior) &MAY; be added without changing the
     1087            semantics of other directives. Behavioral extensions are designed to work by acting as
     1088            modifiers to the existing base of cache directives. Both the new directive and the
     1089            standard directive are supplied, such that applications which do not understand the new
     1090            directive will default to the behavior specified by the standard directive, and those
     1091            that understand the new directive will recognize it as modifying the requirements
     1092            associated with the standard directive. In this way, extensions to the cache-control
     1093            directives can be made without requiring changes to the base protocol.</t>
     1094          <t>This extension mechanism depends on an HTTP cache obeying all of the cache-control
     1095            directives defined for its native HTTP-version, obeying certain extensions, and ignoring
     1096            all directives that it does not understand.</t>
     1097          <t>For example, consider a hypothetical new response directive called "community" which
     1098            acts as a modifier to the private directive. We define this new directive to mean that,
     1099            in addition to any non-shared cache, any cache which is shared only by members of the
     1100            community named within its value may cache the response. An origin server wishing to
     1101            allow the UCI community to use an otherwise private response in their shared cache(s)
     1102            could do so by including</t>
     1103          <figure>
     1104            <artwork type="example">
    20091105    Cache-Control: private, community="UCI"
    2010 </artwork></figure>
    2011 <t>
    2012    A cache seeing this header field will act correctly even if the cache
    2013    does not understand the community cache-extension, since it will also
    2014    see and understand the private directive and thus default to the safe
    2015    behavior.
    2016 </t>
    2017 <t>
    2018    Unrecognized cache-directives &MUST; be ignored; it is assumed that any
    2019    cache-directive likely to be unrecognized by an HTTP/1.1 cache will
    2020    be combined with standard directives (or the response's default
    2021    cacheability) such that the cache behavior will remain minimally
    2022    correct even if the cache does not understand the extension(s).
    2023 </t>
    2024 </section>
    2025 </section>
    2026 
    2027 <section title="Expires" anchor="header.expires">
    2028   <iref primary="true" item="Expires header" x:for-anchor=""/>
    2029   <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
    2030   <x:anchor-alias value="Expires"/>
    2031 <t>
    2032    The Expires entity-header field gives the date/time after which the
    2033    response is considered stale. A stale cache entry may not normally be
    2034    returned by a cache (either a proxy cache or a user agent cache)
    2035    unless it is first validated with the origin server (or with an
    2036    intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
    2037    for further discussion of the expiration model.
    2038 </t>
    2039 <t>
    2040    The presence of an Expires field does not imply that the original
    2041    resource will change or cease to exist at, before, or after that
    2042    time.
    2043 </t>
    2044 <t>
    2045    The format is an absolute date and time as defined by HTTP-date in
    2046    &full-date;; it &MUST; be sent in rfc1123-date format.
    2047 </t>
    2048 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
     1106</artwork>
     1107          </figure>
     1108          <t>A cache seeing this header field will act correctly even if the cache does not
     1109            understand the community cache-extension, since it will also see and understand the
     1110            private directive and thus default to the safe behavior.</t>
     1111          <t>Unrecognized cache-directives &MUST; be ignored; it is assumed that any
     1112            cache-directive likely to be unrecognized by an HTTP/1.1 cache will be combined with
     1113            standard directives (or the response's default cacheability) such that the cache
     1114            behavior will remain minimally correct even if the cache does not understand the
     1115            extension(s).</t>
     1116        </section>
     1117
     1118      </section>
     1119
     1120      <section anchor="header.expires" title="Expires">
     1121        <iref item="Expires header" primary="true" x:for-anchor="" />
     1122        <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
     1123        <x:anchor-alias value="Expires" />
     1124        <t>The Expires entity-header field gives the date/time after which the response is
     1125          considered stale. See <xref target="expiration.model" /> for further discussion of the
     1126          expiration model.</t>
     1127        <t>The presence of an Expires field does not imply that the original resource will change or
     1128          cease to exist at, before, or after that time.</t>
     1129        <t>The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
     1130          it &MUST; be sent in rfc1123-date format.</t>
     1131        <figure>
     1132          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Expires" />
    20491133  <x:ref>Expires</x:ref> = "Expires" ":" <x:ref>HTTP-date</x:ref>
    2050 </artwork></figure>
    2051 <t>
    2052    An example of its use is
    2053 </t>
    2054 <figure><artwork type="example">
     1134</artwork>
     1135        </figure>
     1136        <t>For example</t>
     1137        <figure>
     1138          <artwork type="example">
    20551139   Expires: Thu, 01 Dec 1994 16:00:00 GMT
    2056 </artwork></figure>
    2057 <t>
    2058   <list><t>
    2059       <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
    2060       directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
    2061       Expires field.
    2062   </t></list>
    2063 </t>
    2064 <t>
    2065    HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
    2066    especially including the value "0", as in the past (i.e., "already
    2067    expired").
    2068 </t>
    2069 <t>
    2070    To mark a response as "already expired," an origin server sends an
    2071    Expires date that is equal to the Date header value. (See the rules
    2072    for expiration calculations in <xref target="expiration.calculations"/>.)
    2073 </t>
    2074 <t>
    2075    To mark a response as "never expires," an origin server sends an
    2076    Expires date approximately one year from the time the response is
    2077    sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
    2078    year in the future.
    2079 </t>
    2080 <t>
    2081    The presence of an Expires header field with a date value of some
    2082    time in the future on a response that otherwise would by default be
    2083    non-cacheable indicates that the response is cacheable, unless
    2084    indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
    2085 </t>
    2086 </section>
    2087 
    2088 <section title="Pragma" anchor="header.pragma">
    2089   <iref primary="true" item="Pragma header" x:for-anchor=""/>
    2090   <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
    2091   <x:anchor-alias value="extension-pragma"/>
    2092   <x:anchor-alias value="Pragma"/>
    2093   <x:anchor-alias value="pragma-directive"/>
    2094 <t>
    2095    The Pragma general-header field is used to include implementation-specific
    2096    directives that might apply to any recipient along the
    2097    request/response chain. All pragma directives specify optional
    2098    behavior from the viewpoint of the protocol; however, some systems
    2099    &MAY; require that behavior be consistent with the directives.
    2100 </t>
    2101 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
     1140</artwork>
     1141        </figure>
     1142        <t>
     1143          <list>
     1144            <t>
     1145              <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
     1146              directive (see <xref target="header.cache-control" />), that directive overrides the
     1147              Expires field.</t>
     1148          </list>
     1149        </t>
     1150        <t>HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.</t>
     1151        <t>HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
     1152          including the value "0", as in the past (i.e., "already expired").</t>
     1153      </section>
     1154
     1155      <section anchor="header.pragma" title="Pragma">
     1156        <iref item="Pragma header" primary="true" x:for-anchor="" />
     1157        <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
     1158        <x:anchor-alias value="extension-pragma" />
     1159        <x:anchor-alias value="Pragma" />
     1160        <x:anchor-alias value="pragma-directive" />
     1161        <t>The Pragma general-header field is used to include implementation-specific directives
     1162          that might apply to any recipient along the request/response chain. All pragma directives
     1163          specify optional behavior from the viewpoint of the protocol; however, some systems
     1164          &MAY; require that behavior be consistent with the directives.</t>
     1165        <figure>
     1166          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Pragma" /><iref item="Grammar" primary="true" subitem="pragma-directive" /><iref item="Grammar" primary="true" subitem="extension-pragma" />
    21021167  <x:ref>Pragma</x:ref>            = "Pragma" ":" 1#<x:ref>pragma-directive</x:ref>
    21031168  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
    21041169  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
    2105 </artwork></figure>
    2106 <t>
    2107    When the no-cache directive is present in a request message, an
    2108    application &SHOULD; forward the request toward the origin server even
    2109    if it has a cached copy of what is being requested. This pragma
    2110    directive has the same semantics as the no-cache cache-directive (see
    2111    <xref target="header.cache-control"/>) and is defined here for backward compatibility with
    2112    HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
    2113    request is sent to a server not known to be HTTP/1.1 compliant.
    2114 </t>
    2115 <t>
    2116    Pragma directives &MUST; be passed through by a proxy or gateway
    2117    application, regardless of their significance to that application,
    2118    since the directives might be applicable to all recipients along the
    2119    request/response chain. It is not possible to specify a pragma for a
    2120    specific recipient; however, any pragma directive not relevant to a
    2121    recipient &SHOULD; be ignored by that recipient.
    2122 </t>
    2123 <t>
    2124    HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
    2125    sent "Cache-Control: no-cache". No new Pragma directives will be
    2126    defined in HTTP.
    2127   <list><t>
    2128       <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a
    2129       response-header field is not actually specified, it does not provide a
    2130       reliable replacement for "Cache-Control: no-cache" in a response.
    2131   </t></list>
    2132 </t>
    2133 </section>
    2134 
    2135 <section title="Vary" anchor="header.vary">
    2136   <iref primary="true" item="Vary header" x:for-anchor=""/>
    2137   <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
    2138   <x:anchor-alias value="Vary"/>
    2139 <t>
    2140    The Vary response-header field's value indicates the set of request-header
    2141    fields that fully determines, while the response is fresh, whether a cache
    2142    is permitted to use the response to reply to a subsequent request
    2143    without revalidation. For uncacheable or stale responses, the Vary
    2144    field value advises the user agent about the criteria that were used
    2145    to select the representation. A Vary field value of "*" implies that
    2146    a cache cannot determine from the request headers of a subsequent
    2147    request whether this response is the appropriate representation. See
    2148    <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
    2149 </t>
    2150 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
     1170</artwork>
     1171        </figure>
     1172        <t>When the no-cache directive is present in a request message, an application &SHOULD;
     1173          forward the request toward the origin server even if it has a cached copy of what is being
     1174          requested. This pragma directive has the same semantics as the no-cache cache-directive
     1175          (see <xref target="header.cache-control" />) and is defined here for backward
     1176          compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
     1177          no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
     1178          &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".</t>
     1179        <t>
     1180          <list>
     1181            <t>
     1182              <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
     1183              is not actually specified, it does not provide a reliable replacement for
     1184              "Cache-Control: no-cache" in a response.</t>
     1185          </list>
     1186        </t>
     1187        <t>This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</t>
     1188      </section>
     1189
     1190      <section anchor="header.vary" title="Vary">
     1191        <iref item="Vary header" primary="true" x:for-anchor="" />
     1192        <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
     1193        <x:anchor-alias value="Vary" />
     1194        <t>The Vary response-header field's value indicates the set of request-header fields that
     1195          fully determines, while the response is fresh, whether a cache is permitted to use the
     1196          response to reply to a subsequent request without validation. For uncacheable or stale
     1197          responses, the Vary field value advises the user agent about the criteria that were used
     1198          to select the representation. A Vary field value of "*" implies that a cache cannot
     1199          determine from the request headers of a subsequent request whether this response is the
     1200          appropriate representation. See <xref target="caching.negotiated.responses" /> for use of
     1201          the Vary header field by caches.</t>
     1202        <figure>
     1203          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Vary" />
    21511204  <x:ref>Vary</x:ref>  = "Vary" ":" ( "*" / 1#<x:ref>field-name</x:ref> )
    2152 </artwork></figure>
    2153 <t>
    2154    An HTTP/1.1 server &SHOULD; include a Vary header field with any
    2155    cacheable response that is subject to server-driven negotiation.
    2156    Doing so allows a cache to properly interpret future requests on that
    2157    resource and informs the user agent about the presence of negotiation
    2158    on that resource. A server &MAY; include a Vary header field with a
    2159    non-cacheable response that is subject to server-driven negotiation,
    2160    since this might provide the user agent with useful information about
    2161    the dimensions over which the response varies at the time of the
    2162    response.
    2163 </t>
    2164 <t>
    2165    A Vary field value consisting of a list of field-names signals that
    2166    the representation selected for the response is based on a selection
    2167    algorithm which considers ONLY the listed request-header field values
    2168    in selecting the most appropriate representation. A cache &MAY; assume
    2169    that the same selection will be made for future requests with the
    2170    same values for the listed field names, for the duration of time for
    2171    which the response is fresh.
    2172 </t>
    2173 <t>
    2174    The field-names given are not limited to the set of standard
    2175    request-header fields defined by this specification. Field names are
    2176    case-insensitive.
    2177 </t>
    2178 <t>
    2179    A Vary field value of "*" signals that unspecified parameters not
    2180    limited to the request-headers (e.g., the network address of the
    2181    client), play a role in the selection of the response representation.
    2182    The "*" value &MUST-NOT; be generated by a proxy server; it may only be
    2183    generated by an origin server.
    2184 </t>
    2185 </section>
    2186 
    2187 <section title="Warning" anchor="header.warning">
    2188   <iref primary="true" item="Warning header" x:for-anchor=""/>
    2189   <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
    2190   <x:anchor-alias value="Warning"/>
    2191   <x:anchor-alias value="warning-value"/>
    2192   <x:anchor-alias value="warn-agent"/>
    2193   <x:anchor-alias value="warn-code"/>
    2194   <x:anchor-alias value="warn-date"/>
    2195   <x:anchor-alias value="warn-text"/>
    2196 <t>
    2197    The Warning general-header field is used to carry additional
    2198    information about the status or transformation of a message which
    2199    might not be reflected in the message. This information is typically
    2200    used to warn about a possible lack of semantic transparency from
    2201    caching operations or transformations applied to the entity body of
    2202    the message.
    2203 </t>
    2204 <t>
    2205    Warnings MAY be used for other purposes, both cache-related and
    2206    otherwise.  The use of a warning, rather than an error status code,
    2207    distinguish these responses from true failures.
    2208 </t>
    2209 
    2210 <t>
    2211    Warning headers can in general be applied to any message, however
    2212    some warn-codes are specific to caches and can only be
    2213    applied to response messages.
    2214 </t>
    2215 
    2216 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
     1205</artwork>
     1206        </figure>
     1207        <t>The set of header fields named by the Vary field value is known as the "selecting"
     1208          request-headers.</t>
     1209        <t>Servers &SHOULD; include a Vary header field with any cacheable response
     1210          that is subject to server-driven negotiation. Doing so allows a cache to properly
     1211          interpret future requests on that resource and informs the user agent about the presence
     1212          of negotiation on that resource. A server &MAY; include a Vary header field with a
     1213          non-cacheable response that is subject to server-driven negotiation, since this might
     1214          provide the user agent with useful information about the dimensions over which the
     1215          response varies at the time of the response.</t>
     1216        <t>A Vary field value of "*" signals that unspecified parameters not limited to the
     1217          request-headers (e.g., the network address of the client), play a role in the selection of
     1218          the response representation. The "*" value &MUST-NOT; be generated by a proxy server;
     1219          it may only be generated by an origin server.</t>
     1220        <t>The field-names given are not limited to the set of standard request-header fields
     1221          defined by this specification. Field names are case-insensitive.</t>
     1222        <t>
     1223          <cref>ISSUE: Does 'server' here imply that non-origin servers can generate vary? note use of 'proxy'</cref></t>
     1224      </section>
     1225
     1226      <section anchor="header.warning" title="Warning">
     1227        <iref item="Warning header" primary="true" x:for-anchor="" />
     1228        <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
     1229        <x:anchor-alias value="Warning" />
     1230        <x:anchor-alias value="warning-value" />
     1231        <x:anchor-alias value="warn-agent" />
     1232        <x:anchor-alias value="warn-code" />
     1233        <x:anchor-alias value="warn-date" />
     1234        <x:anchor-alias value="warn-text" />
     1235        <t>The Warning general-header field is used to carry additional information about the status
     1236          or transformation of a message which might not be reflected in the message. This
     1237          information is typically used to warn about possible incorrectness introduced by caching
     1238          operations or transformations applied to the entity body of the message.</t>
     1239        <t>Warnings MAY be used for other purposes, both cache-related and otherwise. The use of a
     1240          warning, rather than an error status code, distinguish these responses from true failures.</t>
     1241
     1242        <t>Warning headers can in general be applied to any message, however some warn-codes are
     1243          specific to caches and can only be applied to response messages.</t>
     1244
     1245        <figure>
     1246          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="Warning" /><iref item="Grammar" primary="true" subitem="warning-value" /><iref item="Grammar" primary="true" subitem="warn-code" /><iref item="Grammar" primary="true" subitem="warn-agent" /><iref item="Grammar" primary="true" subitem="warn-text" /><iref item="Grammar" primary="true" subitem="warn-date" />
    22171247  <x:ref>Warning</x:ref>    = "Warning" ":" 1#<x:ref>warning-value</x:ref>
    22181248 
     
    22261256  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
    22271257  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
    2228 </artwork></figure>
    2229 
    2230 <t>
    2231    Multiple warnings &MAY; be attached to a response (either by the origin
    2232    server or by a cache), including multiple warnings with the same code
    2233    number.  For example, a server might provide the same warning with
    2234    texts in both English and Basque.
    2235 </t>
    2236 <t>
    2237    When this occurs, the user agent ought to inform the user of as many
    2238    of them as possible, in the order that they appear in the response. 
    2239    If it is not possible to inform the user of all of the warnings,
    2240    the user agent SHOULD follow these heuristics:
    2241   <list style="symbols">
    2242     <t>Warnings that appear early in the response take priority over
    2243         those appearing later in the response.</t>
    2244 
    2245     <t>Warnings in the user's preferred character set take priority
    2246         over warnings in other character sets but with identical warn-codes
    2247         and warn-agents.</t>
    2248   </list>
    2249 </t>
    2250 <t>
    2251    Systems that generate multiple Warning headers &SHOULD; order them with
    2252    this user agent behavior in mind. New Warning headers &SHOULD; be added
    2253   after any existing Warning headers.
    2254 </t>
    2255 <t>
    2256    Warnings are assigned three digit warn-codes.  The first digit
    2257    indicates whether the Warning is required to be deleted from a
    2258    stored cache entry after validation:
    2259    <list style="symbols">
    2260          <t>1xx  Warnings that describe the freshness or revalidation status of
    2261              the response, and so MUST be deleted by caches after validation.</t>
    2262          <t> 2xx  Warnings that describe some aspect of the entity body or entity
    2263              headers that is not rectified by a revalidation (for example, a
    2264              lossy compression of the entity bodies) and which MUST NOT be
    2265              deleted by caches after validation, unless a full response is
    2266              returned, in which case they MUST be.</t>
    2267    </list>
    2268 </t>
    2269 <t>
    2270    The warn-text &SHOULD; be in a natural language and character set that
    2271    is most likely to be intelligible to the human user receiving the
    2272    response. This decision &MAY; be based on any available knowledge, such
    2273    as the location of the cache or user, the Accept-Language field in a
    2274    request, the Content-Language field in a response, etc. The default
    2275    language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
    2276 </t>
    2277 <t>
    2278    If a character set other than ISO-8859-1 is used, it &MUST; be encoded
    2279    in the warn-text using the method described in <xref target="RFC2047"/>.
    2280 </t>
    2281 <t>
    2282    If an implementation sends a message with one or more Warning headers
    2283    to a receiver whose version is HTTP/1.0 or lower, then the sender &MUST;
    2284    include in each warning-value a warn-date that matches the Date header
    2285    in the message.
    2286 </t>
    2287 <t>
    2288    If an implementation receives a message with a warning-value that
    2289    includes a warn-date, and that warn-date is different from the Date
    2290    value in the response, then that warning-value &MUST; be deleted from
    2291    the message before storing, forwarding, or using it.  (This prevents
    2292    bad consequences of naive caching of Warning header fields.)  If all
    2293    of the warning-values are deleted for this reason, the Warning header
    2294    &MUST; be deleted as well.
    2295 </t>
    2296 <t>
    2297    The following warn-codes are defined by this specification, each with a
    2298    recommended warn-text in English, and a description of its meaning.
    2299 </t>
    2300 <t>
    2301    110 Response is stale
    2302   <list><t>
    2303      &MUST; be included whenever the returned response is stale.
    2304   </t></list>
    2305 </t>
    2306 <t>
    2307    111 Revalidation failed
    2308   <list><t>
    2309      &MUST; be included if a cache returns a stale response because an
    2310      attempt to revalidate the response failed, due to an inability to
    2311      reach the server.
    2312   </t></list>
    2313 </t>
    2314 <t>
    2315    112 Disconnected operation
    2316   <list><t>
    2317      &SHOULD; be included if the cache is intentionally disconnected from
    2318      the rest of the network for a period of time.
    2319   </t></list>
    2320 </t>
    2321 <t>
    2322    113 Heuristic expiration
    2323   <list><t>
    2324      &MUST; be included if the cache heuristically chose a freshness
    2325      lifetime greater than 24 hours and the response's age is greater
    2326      than 24 hours.
    2327   </t></list>
    2328 </t>
    2329 <t>
    2330    199 Miscellaneous warning
    2331   <list><t>
    2332      The warning text &MAY; include arbitrary information to be presented
    2333      to a human user, or logged. A system receiving this warning &MUST-NOT;
    2334      take any automated action, besides presenting the warning to
    2335      the user.
    2336   </t></list>
    2337 </t>
    2338 <t>
    2339    214 Transformation applied
    2340   <list><t>
    2341      &MUST; be added by an intermediate cache or proxy if it applies any
    2342      transformation changing the content-coding (as specified in the
    2343      Content-Encoding header) or media-type (as specified in the
    2344      Content-Type header) of the response, or the entity-body of the
    2345      response, unless this Warning code already appears in the response.
    2346   </t></list>
    2347 </t>
    2348 <t>
    2349    299 Miscellaneous persistent warning
    2350   <list><t>
    2351      The warning text &MAY; include arbitrary information to be presented
    2352      to a human user, or logged. A system receiving this warning &MUST-NOT;
    2353      take any automated action.
    2354   </t></list>
    2355 </t>
    2356 </section>
    2357 
    2358 </section>
    2359 
    2360 <section title="IANA Considerations" anchor="IANA.considerations">
    2361 <section title="Message Header Registration" anchor="message.header.registration">
    2362 <t>
    2363    The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
    2364    with the permanent registrations below (see <xref target="RFC3864"/>):
    2365 </t>
    2366 <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
    2367 <texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
    2368    <ttcol>Header Field Name</ttcol>
    2369    <ttcol>Protocol</ttcol>
    2370    <ttcol>Status</ttcol>
    2371    <ttcol>Reference</ttcol>
    2372 
    2373    <c>Age</c>
    2374    <c>http</c>
    2375    <c>standard</c>
    2376    <c>
    2377       <xref target="header.age"/>
    2378    </c>
    2379    <c>Cache-Control</c>
    2380    <c>http</c>
    2381    <c>standard</c>
    2382    <c>
    2383       <xref target="header.cache-control"/>
    2384    </c>
    2385    <c>Expires</c>
    2386    <c>http</c>
    2387    <c>standard</c>
    2388    <c>
    2389       <xref target="header.expires"/>
    2390    </c>
    2391    <c>Pragma</c>
    2392    <c>http</c>
    2393    <c>standard</c>
    2394    <c>
    2395       <xref target="header.pragma"/>
    2396    </c>
    2397    <c>Vary</c>
    2398    <c>http</c>
    2399    <c>standard</c>
    2400    <c>
    2401       <xref target="header.vary"/>
    2402    </c>
    2403    <c>Warning</c>
    2404    <c>http</c>
    2405    <c>standard</c>
    2406    <c>
    2407       <xref target="header.warning"/>
    2408    </c>
    2409 </texttable>
    2410 <!--(END)-->
    2411 <t>
    2412    The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
    2413 </t>
    2414 </section>
    2415 </section>
    2416 
    2417 <section title="Security Considerations" anchor="security.considerations">
    2418 <t>
    2419    Caching proxies provide additional potential vulnerabilities, since
    2420    the contents of the cache represent an attractive target for
    2421    malicious exploitation. Because cache contents persist after an HTTP
    2422    request is complete, an attack on the cache can reveal information
    2423    long after a user believes that the information has been removed from
    2424    the network. Therefore, cache contents should be protected as
    2425    sensitive information.
    2426 </t>
    2427 </section>
    2428 
    2429 <section title="Acknowledgments" anchor="ack">
    2430 <t>
    2431    Much of the content and presentation of the caching design is due to
    2432    suggestions and comments from individuals including: Shel Kaphan,
    2433    Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
    2434 </t>
    2435 </section>
    2436 </middle>
    2437 <back>
    2438 
    2439 <references title="Normative References">
    2440 
    2441 <reference anchor="ISO-8859-1">
    2442   <front>
    2443     <title>
    2444      Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
    2445     </title>
    2446     <author>
    2447       <organization>International Organization for Standardization</organization>
    2448     </author>
    2449     <date year="1998"/>
    2450   </front>
    2451   <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
    2452 </reference>
    2453 
    2454 <reference anchor="Part1">
    2455   <front>
    2456     <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
    2457     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2458       <organization abbrev="Day Software">Day Software</organization>
    2459       <address><email>fielding@gbiv.com</email></address>
    2460     </author>
    2461     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2462       <organization>One Laptop per Child</organization>
    2463       <address><email>jg@laptop.org</email></address>
    2464     </author>
    2465     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2466       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2467       <address><email>JeffMogul@acm.org</email></address>
    2468     </author>
    2469     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2470       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2471       <address><email>henrikn@microsoft.com</email></address>
    2472     </author>
    2473     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2474       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2475       <address><email>LMM@acm.org</email></address>
    2476     </author>
    2477     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2478       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2479       <address><email>paulle@microsoft.com</email></address>
    2480     </author>
    2481     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2482       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2483       <address><email>timbl@w3.org</email></address>
    2484     </author>
    2485     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2486       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2487       <address><email>ylafon@w3.org</email></address>
    2488     </author>
    2489     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2490       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2491       <address><email>julian.reschke@greenbytes.de</email></address>
    2492     </author>
    2493     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2494   </front>
    2495   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
    2496   <x:source href="p1-messaging.xml" basename="p1-messaging"/>
    2497 </reference>
    2498 
    2499 <reference anchor="Part2">
    2500   <front>
    2501     <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
    2502     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2503       <organization abbrev="Day Software">Day Software</organization>
    2504       <address><email>fielding@gbiv.com</email></address>
    2505     </author>
    2506     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2507       <organization>One Laptop per Child</organization>
    2508       <address><email>jg@laptop.org</email></address>
    2509     </author>
    2510     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2511       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2512       <address><email>JeffMogul@acm.org</email></address>
    2513     </author>
    2514     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2515       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2516       <address><email>henrikn@microsoft.com</email></address>
    2517     </author>
    2518     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2519       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2520       <address><email>LMM@acm.org</email></address>
    2521     </author>
    2522     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2523       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2524       <address><email>paulle@microsoft.com</email></address>
    2525     </author>
    2526     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2527       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2528       <address><email>timbl@w3.org</email></address>
    2529     </author>
    2530     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2531       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2532       <address><email>ylafon@w3.org</email></address>
    2533     </author>
    2534     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2535       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2536       <address><email>julian.reschke@greenbytes.de</email></address>
    2537     </author>
    2538     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2539   </front>
    2540   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
    2541   <x:source href="p2-semantics.xml" basename="p2-semantics"/>
    2542 </reference>
    2543 
    2544 <reference anchor="Part3">
    2545   <front>
    2546     <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
    2547     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2548       <organization abbrev="Day Software">Day Software</organization>
    2549       <address><email>fielding@gbiv.com</email></address>
    2550     </author>
    2551     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2552       <organization>One Laptop per Child</organization>
    2553       <address><email>jg@laptop.org</email></address>
    2554     </author>
    2555     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2556       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2557       <address><email>JeffMogul@acm.org</email></address>
    2558     </author>
    2559     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2560       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2561       <address><email>henrikn@microsoft.com</email></address>
    2562     </author>
    2563     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2564       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2565       <address><email>LMM@acm.org</email></address>
    2566     </author>
    2567     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2568       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2569       <address><email>paulle@microsoft.com</email></address>
    2570     </author>
    2571     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2572       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2573       <address><email>timbl@w3.org</email></address>
    2574     </author>
    2575     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2576       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2577       <address><email>ylafon@w3.org</email></address>
    2578     </author>
    2579     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2580       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2581       <address><email>julian.reschke@greenbytes.de</email></address>
    2582     </author>
    2583     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2584   </front>
    2585   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
    2586   <x:source href="p3-payload.xml" basename="p3-payload"/>
    2587 </reference>
    2588 
    2589 <reference anchor="Part4">
    2590   <front>
    2591     <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
    2592     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2593       <organization abbrev="Day Software">Day Software</organization>
    2594       <address><email>fielding@gbiv.com</email></address>
    2595     </author>
    2596     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2597       <organization>One Laptop per Child</organization>
    2598       <address><email>jg@laptop.org</email></address>
    2599     </author>
    2600     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2601       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2602       <address><email>JeffMogul@acm.org</email></address>
    2603     </author>
    2604     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2605       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2606       <address><email>henrikn@microsoft.com</email></address>
    2607     </author>
    2608     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2609       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2610       <address><email>LMM@acm.org</email></address>
    2611     </author>
    2612     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2613       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2614       <address><email>paulle@microsoft.com</email></address>
    2615     </author>
    2616     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2617       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2618       <address><email>timbl@w3.org</email></address>
    2619     </author>
    2620     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2621       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2622       <address><email>ylafon@w3.org</email></address>
    2623     </author>
    2624     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2625       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2626       <address><email>julian.reschke@greenbytes.de</email></address>
    2627     </author>
    2628     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2629   </front>
    2630   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
    2631   <x:source href="p4-conditional.xml" basename="p4-conditional"/>
    2632 </reference>
    2633 
    2634 <reference anchor="Part5">
    2635   <front>
    2636     <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
    2637     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2638       <organization abbrev="Day Software">Day Software</organization>
    2639       <address><email>fielding@gbiv.com</email></address>
    2640     </author>
    2641     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2642       <organization>One Laptop per Child</organization>
    2643       <address><email>jg@laptop.org</email></address>
    2644     </author>
    2645     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2646       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2647       <address><email>JeffMogul@acm.org</email></address>
    2648     </author>
    2649     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2650       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2651       <address><email>henrikn@microsoft.com</email></address>
    2652     </author>
    2653     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2654       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2655       <address><email>LMM@acm.org</email></address>
    2656     </author>
    2657     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2658       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2659       <address><email>paulle@microsoft.com</email></address>
    2660     </author>
    2661     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2662       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2663       <address><email>timbl@w3.org</email></address>
    2664     </author>
    2665     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2666       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2667       <address><email>ylafon@w3.org</email></address>
    2668     </author>
    2669     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2670       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2671       <address><email>julian.reschke@greenbytes.de</email></address>
    2672     </author>
    2673     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2674   </front>
    2675   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
    2676   <x:source href="p5-range.xml" basename="p5-range"/>
    2677 </reference>
    2678 
    2679 <reference anchor="Part7">
    2680   <front>
    2681     <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
    2682     <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    2683       <organization abbrev="Day Software">Day Software</organization>
    2684       <address><email>fielding@gbiv.com</email></address>
    2685     </author>
    2686     <author initials="J." surname="Gettys" fullname="Jim Gettys">
    2687       <organization>One Laptop per Child</organization>
    2688       <address><email>jg@laptop.org</email></address>
    2689     </author>
    2690     <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    2691       <organization abbrev="HP">Hewlett-Packard Company</organization>
    2692       <address><email>JeffMogul@acm.org</email></address>
    2693     </author>
    2694     <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    2695       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2696       <address><email>henrikn@microsoft.com</email></address>
    2697     </author>
    2698     <author initials="L." surname="Masinter" fullname="Larry Masinter">
    2699       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    2700       <address><email>LMM@acm.org</email></address>
    2701     </author>
    2702     <author initials="P." surname="Leach" fullname="Paul J. Leach">
    2703       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    2704       <address><email>paulle@microsoft.com</email></address>
    2705     </author>
    2706     <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    2707       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    2708       <address><email>timbl@w3.org</email></address>
    2709     </author>
    2710     <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    2711       <organization abbrev="W3C">World Wide Web Consortium</organization>
    2712       <address><email>ylafon@w3.org</email></address>
    2713     </author>
    2714     <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    2715       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    2716       <address><email>julian.reschke@greenbytes.de</email></address>
    2717     </author>
    2718     <date month="&ID-MONTH;" year="&ID-YEAR;"/>
    2719   </front>
    2720   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
    2721   <x:source href="p7-auth.xml" basename="p7-auth"/>
    2722 </reference>
    2723 
    2724 <reference anchor="RFC2047">
    2725   <front>
    2726     <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
    2727     <author initials="K." surname="Moore" fullname="Keith Moore">
    2728       <organization>University of Tennessee</organization>
    2729       <address><email>moore@cs.utk.edu</email></address>
    2730     </author>
    2731     <date month="November" year="1996"/>
    2732   </front>
    2733   <seriesInfo name="RFC" value="2047"/>
    2734 </reference>
    2735 
    2736 <reference anchor="RFC2119">
    2737   <front>
    2738     <title>Key words for use in RFCs to Indicate Requirement Levels</title>
    2739     <author initials="S." surname="Bradner" fullname="Scott Bradner">
    2740       <organization>Harvard University</organization>
    2741       <address><email>sob@harvard.edu</email></address>
    2742     </author>
    2743     <date month="March" year="1997"/>
    2744   </front>
    2745   <seriesInfo name="BCP" value="14"/>
    2746   <seriesInfo name="RFC" value="2119"/>
    2747 </reference>
    2748 
    2749 </references>
    2750 
    2751 <references title="Informative References">
    2752 
    2753 <reference anchor="RFC1305">
    2754   <front>
    2755     <title>Network Time Protocol (Version 3) Specification, Implementation</title>
    2756     <author initials="D." surname="Mills" fullname="David L. Mills">
    2757       <organization>University of Delaware, Electrical Engineering Department</organization>
    2758       <address><email>mills@udel.edu</email></address>
    2759     </author>
    2760     <date month="March" year="1992"/>
    2761   </front>
    2762   <seriesInfo name="RFC" value="1305"/>
    2763 </reference>
    2764 
    2765 <reference anchor="RFC2616">
    2766   <front>
    2767     <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
    2768     <author initials="R." surname="Fielding" fullname="R. Fielding">
    2769       <organization>University of California, Irvine</organization>
    2770       <address><email>fielding@ics.uci.edu</email></address>
    2771     </author>
    2772     <author initials="J." surname="Gettys" fullname="J. Gettys">
    2773       <organization>W3C</organization>
    2774       <address><email>jg@w3.org</email></address>
    2775     </author>
    2776     <author initials="J." surname="Mogul" fullname="J. Mogul">
    2777       <organization>Compaq Computer Corporation</organization>
    2778       <address><email>mogul@wrl.dec.com</email></address>
    2779     </author>
    2780     <author initials="H." surname="Frystyk" fullname="H. Frystyk">
    2781       <organization>MIT Laboratory for Computer Science</organization>
    2782       <address><email>frystyk@w3.org</email></address>
    2783     </author>
    2784     <author initials="L." surname="Masinter" fullname="L. Masinter">
    2785       <organization>Xerox Corporation</organization>
    2786       <address><email>masinter@parc.xerox.com</email></address>
    2787     </author>
    2788     <author initials="P." surname="Leach" fullname="P. Leach">
    2789       <organization>Microsoft Corporation</organization>
    2790       <address><email>paulle@microsoft.com</email></address>
    2791     </author>
    2792     <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
    2793       <organization>W3C</organization>
    2794       <address><email>timbl@w3.org</email></address>
    2795     </author>
    2796     <date month="June" year="1999"/>
    2797   </front>
    2798   <seriesInfo name="RFC" value="2616"/>
    2799 </reference>
    2800 
    2801 <reference anchor='RFC3864'>
    2802   <front>
    2803     <title>Registration Procedures for Message Header Fields</title>
    2804     <author initials='G.' surname='Klyne' fullname='G. Klyne'>
    2805       <organization>Nine by Nine</organization>
    2806       <address><email>GK-IETF@ninebynine.org</email></address>
    2807     </author>
    2808     <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
    2809       <organization>BEA Systems</organization>
    2810       <address><email>mnot@pobox.com</email></address>
    2811     </author>
    2812     <author initials='J.' surname='Mogul' fullname='J. Mogul'>
    2813       <organization>HP Labs</organization>
    2814       <address><email>JeffMogul@acm.org</email></address>
    2815     </author>
    2816     <date year='2004' month='September' />
    2817   </front>
    2818   <seriesInfo name='BCP' value='90' />
    2819   <seriesInfo name='RFC' value='3864' />
    2820 </reference>
    2821 
    2822 </references>
    2823 
    2824 <section title="Compatibility with Previous Versions" anchor="compatibility">
    2825 
    2826 <section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
    2827 <t>
    2828    A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
    2829    was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
    2830    <xref target="header.cache-control" format="counter"/>,
    2831    <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
    2832 </t>
    2833 <t>
    2834    Transfer-coding and message lengths all interact in ways that
    2835    required fixing exactly when chunked encoding is used (to allow for
    2836    transfer encoding that may not be self delimiting); it was important
    2837    to straighten out exactly how message lengths are computed.
    2838    (<xref target="non-modifiable.headers"/>,
    2839    see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
    2840 </t>
    2841 <t>
    2842    Proxies should be able to add Content-Length when appropriate.
    2843    (<xref target="non-modifiable.headers"/>)
    2844 </t>
    2845 <t>
    2846    Range request responses would become very verbose if all meta-data
    2847    were always returned; by allowing the server to only send needed
    2848    headers in a 206 response, this problem can be avoided.
    2849    (<xref target="combining.headers"/>)
    2850 </t>
    2851 <t>
    2852    The Cache-Control: max-age directive was not properly defined for
    2853    responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
    2854 </t>
    2855 <t>
    2856    Warnings could be cached incorrectly, or not updated appropriately.
    2857    (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
    2858    <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
    2859    and <xref target="header.warning" format="counter"/>) Warning
    2860    also needed to be a general header, as PUT or other methods may have
    2861    need for it in requests.
    2862 </t>
    2863 </section>
    2864 
    2865 <section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
    2866 <t>
    2867   Clarify denial of service attack avoidance requirement.
    2868   (<xref target="invalidation.after.updates.or.deletions"/>)
    2869 </t>
    2870 </section>
    2871 
    2872 </section>
    2873 
    2874 <section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
    2875 
    2876 <section title="Since RFC2616">
    2877 <t>
    2878   Extracted relevant partitions from <xref target="RFC2616"/>.
    2879 </t>
    2880 </section>
    2881 
    2882 <section title="Since draft-ietf-httpbis-p6-cache-00">
    2883 <t>
    2884   Closed issues:
    2885   <list style="symbols">
    2886     <t>
    2887       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
    2888       "Trailer"
    2889       (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
    2890     </t>
    2891     <t>
    2892       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
    2893       "Invalidation after Update or Delete"
    2894       (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
    2895     </t>
    2896     <t>
    2897       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
    2898       "Normative and Informative references"
    2899     </t>
    2900     <t>
    2901       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
    2902       "Date reference typo"
    2903     </t>
    2904     <t>
    2905       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
    2906       "Connection header text"
    2907     </t>
    2908     <t>
    2909       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
    2910       "Informative references"
    2911     </t>
    2912     <t>
    2913       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
    2914       "ISO-8859-1 Reference"
    2915     </t>
    2916     <t>
    2917       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
    2918       "Normative up-to-date references"
    2919     </t>
    2920     <t>
    2921       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
    2922       "typo in 13.2.2"
    2923     </t>
    2924   </list>
    2925 </t>
    2926 <t>
    2927   Other changes:
    2928   <list style="symbols">
    2929     <t>
    2930       Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
    2931     </t>
    2932   </list>
    2933 </t>
    2934 </section>
    2935 
    2936 <section title="Since draft-ietf-httpbis-p6-cache-01">
    2937 <t>
    2938   Closed issues:
    2939   <list style="symbols">
    2940     <t>
    2941       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
    2942       "rel_path not used"
    2943     </t>
    2944   </list>
    2945 </t>
    2946 <t>
    2947   Other changes:
    2948   <list style="symbols">
    2949     <t>
    2950        Get rid of duplicate BNF rule names ("host" -> "uri-host")
    2951        (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
    2952     </t>
    2953     <t>
    2954       Add explicit references to BNF syntax and rules imported from other parts of the specification.
    2955     </t>
    2956   </list>
    2957 </t>
    2958 </section>
    2959 
    2960 <section title="Since draft-ietf-httpbis-p6-cache-02" anchor="changes.since.02">
    2961 <t>
    2962   Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
    2963   <list style="symbols">
    2964     <t>
    2965       Reference RFC 3984, and update header registrations for headers defined
    2966       in this document.
    2967     </t>
    2968   </list>
    2969 </t>
    2970 </section>
    2971 
    2972 <section title="Since draft-ietf-httpbis-p6-cache-03" anchor="changes.since.03">
    2973 <t>
    2974   Closed issues:
    2975   <list style="symbols">
    2976     <t>
    2977       <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>:
    2978       "Vary header classification"
    2979     </t>
    2980   </list>
    2981 </t>
    2982 </section>
    2983 
    2984 <section title="Since draft-ietf-httpbis-p6-cache-04" anchor="changes.since.04">
    2985 <t>
    2986   Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
    2987   <list style="symbols">
    2988     <t>
    2989       Use "/" instead of "|" for alternatives.
    2990     </t>
    2991   </list>
    2992 </t>
    2993 </section>
    2994 
    2995 </section>
    2996 
    2997 </back>
     1258</artwork>
     1259        </figure>
     1260
     1261        <t>Multiple warnings &MAY; be attached to a response (either by the origin server or by
     1262          a cache), including multiple warnings with the same code number. For example, a server
     1263          might provide the same warning with texts in both English and Basque.</t>
     1264        <t>When this occurs, the user agent ought to inform the user of as many of them as possible,
     1265          in the order that they appear in the response. If it is not possible to inform the user of
     1266          all of the warnings, the user agent SHOULD follow these heuristics: <list style="symbols">
     1267            <t>Warnings that appear early in the response take priority over those appearing later
     1268              in the response.</t>
     1269
     1270            <t>Warnings in the user's preferred character set take priority over warnings in other
     1271              character sets but with identical warn-codes and warn-agents.</t>
     1272          </list>
     1273        </t>
     1274        <t>Systems that generate multiple Warning headers &SHOULD; order them with this user
     1275          agent behavior in mind. New Warning headers &SHOULD; be added after any existing
     1276          Warning headers.</t>
     1277        <t>Warnings are assigned three digit warn-codes. The first digit indicates whether the
     1278          Warning is required to be deleted from a stored response after validation: <list
     1279            style="symbols">
     1280            <t>1xx Warnings that describe the freshness or validation status of the response, and
     1281              so MUST be deleted by caches after validation. They MUST NOT be generated by a cache except when
     1282              validating a cached entry, and MUST NOT be generated by clients.</t>
     1283            <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
     1284              not rectified by a validation (for example, a lossy compression of the entity
     1285              bodies) and which MUST NOT be deleted by caches after validation, unless a full
     1286              response is returned, in which case they MUST be.</t>
     1287          </list>
     1288        </t>
     1289        <t>The warn-text &SHOULD; be in a natural language and character set that is most likely
     1290          to be intelligible to the human user receiving the response. This decision &MAY; be
     1291          based on any available knowledge, such as the location of the cache or user, the
     1292          Accept-Language field in a request, the Content-Language field in a response, etc. The
     1293          default language is English and the default character set is ISO-8859-1 (<xref
     1294            target="ISO-8859-1" />).</t>
     1295        <t>If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
     1296          warn-text using the method described in <xref target="RFC2047" />.</t>
     1297        <t>If an implementation sends a message with one or more Warning headers to a receiver whose
     1298          version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
     1299          warn-date that matches the Date header in the message.</t>
     1300        <t>If an implementation receives a message with a warning-value that includes a warn-date,
     1301          and that warn-date is different from the Date value in the response, then that
     1302          warning-value &MUST; be deleted from the message before storing, forwarding, or using
     1303          it. (This prevents bad consequences of naive caching of Warning header fields.) If all of
     1304          the warning-values are deleted for this reason, the Warning header &MUST; be deleted
     1305          as well.</t>
     1306        <t>The following warn-codes are defined by this specification, each with a recommended
     1307          warn-text in English, and a description of its meaning.</t>
     1308        <t>110 Response is stale <list>
     1309            <t>&MUST; be included whenever the returned response is stale.</t>
     1310          </list>
     1311        </t>
     1312        <t>111 Revalidation failed <list>
     1313            <t>&MUST; be included if a cache returns a stale response because an attempt to
     1314              validate the response failed, due to an inability to reach the server.</t>
     1315          </list>
     1316        </t>
     1317        <t>112 Disconnected operation <list>
     1318            <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
     1319              the network for a period of time.</t>
     1320          </list>
     1321        </t>
     1322        <t>113 Heuristic expiration <list>
     1323            <t>&MUST; be included if the cache heuristically chose a freshness lifetime greater
     1324              than 24 hours and the response's age is greater than 24 hours.</t>
     1325          </list>
     1326        </t>
     1327        <t>199 Miscellaneous warning <list>
     1328            <t>The warning text &MAY; include arbitrary information to be presented to a human
     1329              user, or logged. A system receiving this warning &MUST-NOT; take any automated
     1330              action, besides presenting the warning to the user.</t>
     1331          </list>
     1332        </t>
     1333        <t>214 Transformation applied <list>
     1334            <t>&MUST; be added by an intermediate cache or proxy if it applies any
     1335              transformation changing the content-coding (as specified in the Content-Encoding
     1336              header) or media-type (as specified in the Content-Type header) of the response, or
     1337              the entity-body of the response, unless this Warning code already appears in the
     1338              response.</t>
     1339          </list>
     1340        </t>
     1341        <t>299 Miscellaneous persistent warning <list>
     1342            <t>The warning text &MAY; include arbitrary information to be presented to a human
     1343              user, or logged. A system receiving this warning &MUST-NOT; take any automated
     1344              action.</t>
     1345          </list>
     1346        </t>
     1347      </section>
     1348
     1349    </section>
     1350
     1351
     1352    <section anchor="history.lists" title="History Lists">
     1353      <t>User agents often have history mechanisms, such as "Back" buttons and history lists, which
     1354        can be used to redisplay an entity retrieved earlier in a session.</t>
     1355      <t>History mechanisms and caches are different. In particular history mechanisms
     1356        &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
     1357        history mechanism is meant to show exactly what the user saw at the time when the resource
     1358        was retrieved.</t>
     1359      <t>By default, an expiration time does not apply to history mechanisms. If the entity is still
     1360        in storage, a history mechanism &SHOULD; display it even if the entity has expired,
     1361        unless the user has specifically configured the agent to refresh expired history documents.</t>
     1362      <t>This is not to be construed to prohibit the history mechanism from telling the user that a
     1363        view might be stale. <list>
     1364          <t>
     1365            <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
     1366            stale resources, this will tend to force service authors to avoid using HTTP expiration
     1367            controls and cache controls when they would otherwise like to. Service authors may
     1368            consider it important that users not be presented with error messages or warning
     1369            messages when they use navigation controls (such as BACK) to view previously fetched
     1370            resources. Even though sometimes such resources ought not be cached, or ought to expire
     1371            quickly, user interface considerations may force service authors to resort to other
     1372            means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
     1373            of improperly functioning history mechanisms.</t>
     1374        </list>
     1375      </t>
     1376    </section>
     1377
     1378
     1379    <section anchor="IANA.considerations" title="IANA Considerations">
     1380      <section anchor="message.header.registration" title="Message Header Registration">
     1381        <t>The Message Header Registry located at <eref
     1382            target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
     1383          should be updated with the permanent registrations below (see <xref target="RFC3864" />):</t>
     1384        <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
     1385        <texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
     1386          <ttcol>Header Field Name</ttcol>
     1387          <ttcol>Protocol</ttcol>
     1388          <ttcol>Status</ttcol>
     1389          <ttcol>Reference</ttcol>
     1390
     1391          <c>Age</c>
     1392          <c>http</c>
     1393          <c>standard</c>
     1394          <c>
     1395            <xref target="header.age" />
     1396          </c>
     1397          <c>Cache-Control</c>
     1398          <c>http</c>
     1399          <c>standard</c>
     1400          <c>
     1401            <xref target="header.cache-control" />
     1402          </c>
     1403          <c>Expires</c>
     1404          <c>http</c>
     1405          <c>standard</c>
     1406          <c>
     1407            <xref target="header.expires" />
     1408          </c>
     1409          <c>Pragma</c>
     1410          <c>http</c>
     1411          <c>standard</c>
     1412          <c>
     1413            <xref target="header.pragma" />
     1414          </c>
     1415          <c>Vary</c>
     1416          <c>http</c>
     1417          <c>standard</c>
     1418          <c>
     1419            <xref target="header.vary" />
     1420          </c>
     1421          <c>Warning</c>
     1422          <c>http</c>
     1423          <c>standard</c>
     1424          <c>
     1425            <xref target="header.warning" />
     1426          </c>
     1427        </texttable>
     1428        <!--(END)-->
     1429        <t>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</t>
     1430      </section>
     1431    </section>
     1432
     1433    <section anchor="security.considerations" title="Security Considerations">
     1434      <t>Caches expose additional potential vulnerabilities, since the contents of the
     1435        cache represent an attractive target for malicious exploitation. Because cache contents
     1436        persist after an HTTP request is complete, an attack on the cache can reveal information
     1437        long after a user believes that the information has been removed from the network.
     1438        Therefore, cache contents should be protected as sensitive information.</t>
     1439    </section>
     1440
     1441    <section anchor="ack" title="Acknowledgments">
     1442      <t>Much of the content and presentation of the caching design is due to suggestions and
     1443        comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
     1444        and Larry Masinter.</t>
     1445    </section>
     1446  </middle>
     1447
     1448  <back>
     1449    <references title="Normative References">
     1450
     1451      <reference anchor="ISO-8859-1">
     1452        <front>
     1453          <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
     1454            1: Latin alphabet No. 1 </title>
     1455          <author>
     1456            <organization>International Organization for Standardization</organization>
     1457          </author>
     1458          <date year="1998" />
     1459        </front>
     1460        <seriesInfo name="ISO/IEC" value="8859-1:1998" />
     1461      </reference>
     1462
     1463      <reference anchor="Part1">
     1464        <front>
     1465          <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
     1466          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1467            <organization abbrev="Day Software">Day Software</organization>
     1468            <address><email>fielding@gbiv.com</email></address>
     1469          </author>
     1470          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1471            <organization>One Laptop per Child</organization>
     1472            <address><email>jg@laptop.org</email></address>
     1473          </author>
     1474          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1475            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1476            <address><email>JeffMogul@acm.org</email></address>
     1477          </author>
     1478          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1479            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1480            <address><email>henrikn@microsoft.com</email></address>
     1481          </author>
     1482          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1483            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1484            <address><email>LMM@acm.org</email></address>
     1485          </author>
     1486          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1487            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1488            <address><email>paulle@microsoft.com</email></address>
     1489          </author>
     1490          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1491            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1492            <address><email>timbl@w3.org</email></address>
     1493          </author>
     1494          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1495            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1496            <address><email>ylafon@w3.org</email></address>
     1497          </author>
     1498          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1499            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1500            <address><email>julian.reschke@greenbytes.de</email></address>
     1501          </author>
     1502          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1503        </front>
     1504        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
     1505        <x:source basename="p1-messaging" href="p1-messaging.xml" />
     1506      </reference>
     1507
     1508      <reference anchor="Part2">
     1509        <front>
     1510          <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
     1511          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1512            <organization abbrev="Day Software">Day Software</organization>
     1513            <address><email>fielding@gbiv.com</email></address>
     1514          </author>
     1515          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1516            <organization>One Laptop per Child</organization>
     1517            <address><email>jg@laptop.org</email></address>
     1518          </author>
     1519          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1520            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1521            <address><email>JeffMogul@acm.org</email></address>
     1522          </author>
     1523          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1524            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1525            <address><email>henrikn@microsoft.com</email></address>
     1526          </author>
     1527          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1528            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1529            <address><email>LMM@acm.org</email></address>
     1530          </author>
     1531          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1532            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1533            <address><email>paulle@microsoft.com</email></address>
     1534          </author>
     1535          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1536            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1537            <address><email>timbl@w3.org</email></address>
     1538          </author>
     1539          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1540            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1541            <address><email>ylafon@w3.org</email></address>
     1542          </author>
     1543          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1544            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1545            <address><email>julian.reschke@greenbytes.de</email></address>
     1546          </author>
     1547          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1548        </front>
     1549        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
     1550        <x:source basename="p2-semantics" href="p2-semantics.xml" />
     1551      </reference>
     1552
     1553      <reference anchor="Part3">
     1554        <front>
     1555          <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
     1556          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1557            <organization abbrev="Day Software">Day Software</organization>
     1558            <address><email>fielding@gbiv.com</email></address>
     1559          </author>
     1560          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1561            <organization>One Laptop per Child</organization>
     1562            <address><email>jg@laptop.org</email></address>
     1563          </author>
     1564          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1565            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1566            <address><email>JeffMogul@acm.org</email></address>
     1567          </author>
     1568          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1569            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1570            <address><email>henrikn@microsoft.com</email></address>
     1571          </author>
     1572          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1573            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1574            <address><email>LMM@acm.org</email></address>
     1575          </author>
     1576          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1577            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1578            <address><email>paulle@microsoft.com</email></address>
     1579          </author>
     1580          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1581            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1582            <address><email>timbl@w3.org</email></address>
     1583          </author>
     1584          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1585            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1586            <address><email>ylafon@w3.org</email></address>
     1587          </author>
     1588          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1589            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1590            <address><email>julian.reschke@greenbytes.de</email></address>
     1591          </author>
     1592          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1593        </front>
     1594        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
     1595        <x:source basename="p3-payload" href="p3-payload.xml" />
     1596      </reference>
     1597
     1598      <reference anchor="Part4">
     1599        <front>
     1600          <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
     1601          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1602            <organization abbrev="Day Software">Day Software</organization>
     1603            <address><email>fielding@gbiv.com</email></address>
     1604          </author>
     1605          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1606            <organization>One Laptop per Child</organization>
     1607            <address><email>jg@laptop.org</email></address>
     1608          </author>
     1609          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1610            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1611            <address><email>JeffMogul@acm.org</email></address>
     1612          </author>
     1613          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1614            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1615            <address><email>henrikn@microsoft.com</email></address>
     1616          </author>
     1617          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1618            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1619            <address><email>LMM@acm.org</email></address>
     1620          </author>
     1621          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1622            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1623            <address><email>paulle@microsoft.com</email></address>
     1624          </author>
     1625          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1626            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1627            <address><email>timbl@w3.org</email></address>
     1628          </author>
     1629          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1630            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1631            <address><email>ylafon@w3.org</email></address>
     1632          </author>
     1633          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1634            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1635            <address><email>julian.reschke@greenbytes.de</email></address>
     1636          </author>
     1637          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1638        </front>
     1639        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
     1640        <x:source basename="p4-conditional" href="p4-conditional.xml" />
     1641      </reference>
     1642
     1643      <reference anchor="Part5">
     1644        <front>
     1645          <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
     1646          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1647            <organization abbrev="Day Software">Day Software</organization>
     1648            <address><email>fielding@gbiv.com</email></address>
     1649          </author>
     1650          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1651            <organization>One Laptop per Child</organization>
     1652            <address><email>jg@laptop.org</email></address>
     1653          </author>
     1654          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1655            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1656            <address><email>JeffMogul@acm.org</email></address>
     1657          </author>
     1658          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1659            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1660            <address><email>henrikn@microsoft.com</email></address>
     1661          </author>
     1662          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1663            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1664            <address><email>LMM@acm.org</email></address>
     1665          </author>
     1666          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1667            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1668            <address><email>paulle@microsoft.com</email></address>
     1669          </author>
     1670          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1671            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1672            <address><email>timbl@w3.org</email></address>
     1673          </author>
     1674          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1675            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1676            <address><email>ylafon@w3.org</email></address>
     1677          </author>
     1678          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1679            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1680            <address><email>julian.reschke@greenbytes.de</email></address>
     1681          </author>
     1682          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1683        </front>
     1684        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
     1685        <x:source basename="p5-range" href="p5-range.xml" />
     1686      </reference>
     1687
     1688      <reference anchor="Part7">
     1689        <front>
     1690          <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
     1691          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
     1692            <organization abbrev="Day Software">Day Software</organization>
     1693            <address><email>fielding@gbiv.com</email></address>
     1694          </author>
     1695          <author fullname="Jim Gettys" initials="J." surname="Gettys">
     1696            <organization>One Laptop per Child</organization>
     1697            <address><email>jg@laptop.org</email></address>
     1698          </author>
     1699          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     1700            <organization abbrev="HP">Hewlett-Packard Company</organization>
     1701            <address><email>JeffMogul@acm.org</email></address>
     1702          </author>
     1703          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     1704            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1705            <address><email>henrikn@microsoft.com</email></address>
     1706          </author>
     1707          <author fullname="Larry Masinter" initials="L." surname="Masinter">
     1708            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     1709            <address><email>LMM@acm.org</email></address>
     1710          </author>
     1711          <author fullname="Paul J. Leach" initials="P." surname="Leach">
     1712            <organization abbrev="Microsoft">Microsoft Corporation</organization>
     1713            <address><email>paulle@microsoft.com</email></address>
     1714          </author>
     1715          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
     1716            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     1717            <address><email>timbl@w3.org</email></address>
     1718          </author>
     1719          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
     1720            <organization abbrev="W3C">World Wide Web Consortium</organization>
     1721            <address><email>ylafon@w3.org</email></address>
     1722          </author>
     1723          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     1724            <organization abbrev="greenbytes">greenbytes GmbH</organization>
     1725            <address><email>julian.reschke@greenbytes.de</email></address>
     1726          </author>
     1727          <date month="&ID-MONTH;" year="&ID-YEAR;" />
     1728        </front>
     1729        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
     1730        <x:source basename="p7-auth" href="p7-auth.xml" />
     1731      </reference>
     1732
     1733      <reference anchor="RFC2047">
     1734        <front>
     1735          <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
     1736            Part Three: Message Header Extensions for Non-ASCII Text</title>
     1737          <author fullname="Keith Moore" initials="K." surname="Moore">
     1738            <organization>University of Tennessee</organization>
     1739            <address><email>moore@cs.utk.edu</email></address>
     1740          </author>
     1741          <date month="November" year="1996" />
     1742        </front>
     1743        <seriesInfo name="RFC" value="2047" />
     1744      </reference>
     1745
     1746      <reference anchor="RFC2119">
     1747        <front>
     1748          <title>Key words for use in RFCs to Indicate Requirement Levels</title>
     1749          <author fullname="Scott Bradner" initials="S." surname="Bradner">
     1750            <organization>Harvard University</organization>
     1751            <address><email>sob@harvard.edu</email></address>
     1752          </author>
     1753          <date month="March" year="1997" />
     1754        </front>
     1755        <seriesInfo name="BCP" value="14" />
     1756        <seriesInfo name="RFC" value="2119" />
     1757      </reference>
     1758
     1759    </references>
     1760
     1761    <references title="Informative References">
     1762
     1763      <reference anchor="RFC1305">
     1764        <front>
     1765          <title>Network Time Protocol (Version 3) Specification, Implementation</title>
     1766          <author fullname="David L. Mills" initials="D." surname="Mills">
     1767            <organization>University of Delaware, Electrical Engineering Department</organization>
     1768            <address><email>mills@udel.edu</email></address>
     1769          </author>
     1770          <date month="March" year="1992" />
     1771        </front>
     1772        <seriesInfo name="RFC" value="1305" />
     1773      </reference>
     1774
     1775      <reference anchor="RFC2616">
     1776        <front>
     1777          <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
     1778          <author fullname="R. Fielding" initials="R." surname="Fielding">
     1779            <organization>University of California, Irvine</organization>
     1780            <address><email>fielding@ics.uci.edu</email></address>
     1781          </author>
     1782          <author fullname="J. Gettys" initials="J." surname="Gettys">
     1783            <organization>W3C</organization>
     1784            <address><email>jg@w3.org</email></address>
     1785          </author>
     1786          <author fullname="J. Mogul" initials="J." surname="Mogul">
     1787            <organization>Compaq Computer Corporation</organization>
     1788            <address><email>mogul@wrl.dec.com</email></address>
     1789          </author>
     1790          <author fullname="H. Frystyk" initials="H." surname="Frystyk">
     1791            <organization>MIT Laboratory for Computer Science</organization>
     1792            <address><email>frystyk@w3.org</email></address>
     1793          </author>
     1794          <author fullname="L. Masinter" initials="L." surname="Masinter">
     1795            <organization>Xerox Corporation</organization>
     1796            <address><email>masinter@parc.xerox.com</email></address>
     1797          </author>
     1798          <author fullname="P. Leach" initials="P." surname="Leach">
     1799            <organization>Microsoft Corporation</organization>
     1800            <address><email>paulle@microsoft.com</email></address>
     1801          </author>
     1802          <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
     1803            <organization>W3C</organization>
     1804            <address><email>timbl@w3.org</email></address>
     1805          </author>
     1806          <date month="June" year="1999" />
     1807        </front>
     1808        <seriesInfo name="RFC" value="2616" />
     1809      </reference>
     1810
     1811      <reference anchor="RFC3864">
     1812        <front>
     1813          <title>Registration Procedures for Message Header Fields</title>
     1814          <author fullname="G. Klyne" initials="G." surname="Klyne">
     1815            <organization>Nine by Nine</organization>
     1816            <address><email>GK-IETF@ninebynine.org</email></address>
     1817          </author>
     1818          <author fullname="M. Nottingham" initials="M." surname="Nottingham">
     1819            <organization>BEA Systems</organization>
     1820            <address><email>mnot@pobox.com</email></address>
     1821          </author>
     1822          <author fullname="J. Mogul" initials="J." surname="Mogul">
     1823            <organization>HP Labs</organization>
     1824            <address><email>JeffMogul@acm.org</email></address>
     1825          </author>
     1826          <date month="September" year="2004" />
     1827        </front>
     1828        <seriesInfo name="BCP" value="90" />
     1829        <seriesInfo name="RFC" value="3864" />
     1830      </reference>
     1831
     1832    </references>
     1833
     1834    <section anchor="compatibility" title="Compatibility with Previous Versions">
     1835
     1836      <section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
     1837        <t>A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
     1838          this missing case. (Sections <xref format="counter" target="response.cacheability" />,
     1839            <xref format="counter" target="header.cache-control" />.</t>
     1840        <t>Transfer-coding and message lengths all interact in ways that required fixing exactly
     1841          when chunked encoding is used (to allow for transfer encoding that may not be self
     1842          delimiting); it was important to straighten out exactly how message lengths are computed.
     1843          (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)</t>
     1844        <t>Proxies should be able to add Content-Length when appropriate.</t>
     1845        <t>Range request responses would become very verbose if all meta-data were always returned;
     1846          by allowing the server to only send needed headers in a 206 response, this problem can be
     1847          avoided. (<xref target="combining.headers" />)</t>
     1848        <t>The Cache-Control: max-age directive was not properly defined for responses.</t>
     1849        <t>Warnings could be cached incorrectly, or not updated appropriately. <xref
     1850            format="counter" target="expiration.model" />, <xref format="counter"
     1851            target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
     1852          and <xref format="counter" target="header.warning" />) Warning also needed to be a general
     1853          header, as PUT or other methods may have need for it in requests.</t>
     1854      </section>
     1855
     1856      <section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
     1857        <t>Clarify denial of service attack avoidance requirement. (<xref
     1858            target="invalidation.after.updates.or.deletions" />)</t>
     1859      </section>
     1860
     1861    </section>
     1862
     1863    <section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
     1864
     1865      <section title="Since RFC2616">
     1866        <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
     1867      </section>
     1868
     1869      <section title="Since draft-ietf-httpbis-p6-cache-00">
     1870        <t>Closed issues: <list style="symbols">
     1871            <t>
     1872              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer"
     1873                (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
     1874            <t>
     1875              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation
     1876              after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
     1877            <t>
     1878              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and
     1879              Informative references"</t>
     1880            <t>
     1881              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date
     1882              reference typo"</t>
     1883            <t>
     1884              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection
     1885              header text"</t>
     1886            <t>
     1887              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative
     1888              references"</t>
     1889            <t>
     1890              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1
     1891              Reference"</t>
     1892            <t>
     1893              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative
     1894              up-to-date references"</t>
     1895            <t>
     1896              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in
     1897              13.2.2"</t>
     1898          </list>
     1899        </t>
     1900        <t>Other changes: <list style="symbols">
     1901            <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
     1902                target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
     1903          </list>
     1904        </t>
     1905      </section>
     1906
     1907      <section title="Since draft-ietf-httpbis-p6-cache-01">
     1908        <t>Closed issues: <list style="symbols">
     1909            <t>
     1910              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not
     1911              used"</t>
     1912          </list>
     1913        </t>
     1914        <t>Other changes: <list style="symbols">
     1915            <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
     1916              on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
     1917            <t>Add explicit references to BNF syntax and rules imported from other parts of the
     1918              specification.</t>
     1919          </list>
     1920        </t>
     1921      </section>
     1922
     1923      <section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
     1924        <t>Ongoing work on IANA Message Header Registration (<eref
     1925            target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40" />): <list style="symbols">
     1926            <t>Reference RFC 3984, and update header registrations for headers defined in this
     1927              document.</t>
     1928          </list>
     1929        </t>
     1930      </section>
     1931
     1932      <section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
     1933        <t>Closed issues: <list style="symbols">
     1934            <t>
     1935              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header
     1936              classification"</t>
     1937          </list>
     1938        </t>
     1939      </section>
     1940
     1941      <section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04"> </section>
     1942
     1943    </section>
     1944  </back>
    29981945</rfc>
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