Ignore:
Timestamp:
27/07/10 14:45:08 (10 years ago)
Author:
julian.reschke@…
Message:

undo [945]

File:
1 edited

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  • draft-ietf-httpbis/10/p6-cache.xml

    r945 r947  
    4646<?rfc-ext allow-markup-in-artwork="yes" ?>
    4747<?rfc-ext include-references-in-index="yes" ?>
    48 <rfc category="std"
    49    docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
    50    ipr="pre5378Trust200902"
    51    obsoletes="2616" x:maturity-level="draft"
    52    xmlns:x="http://purl.org/net/xml2rfc/ext">
     48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
     49  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
    5350<front>
    5451
    55    <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
    56 
    57    <author fullname="Roy T. Fielding" initials="R."
    58      role="editor" surname="Fielding">
    59       <organization abbrev="Day Software">Day Software</organization>
    60       <address>
    61          <postal>
    62             <street>23 Corporate Plaza DR, Suite 280</street>
    63             <city>Newport Beach</city>
    64             <region>CA</region>
    65             <code>92660</code>
    66             <country>USA</country>
    67          </postal>
    68          <phone>+1-949-706-5300</phone>
    69          <facsimile>+1-949-706-5305</facsimile>
    70          <email>fielding@gbiv.com</email>
    71          <uri>http://roy.gbiv.com/</uri>
    72       </address>
    73    </author>
    74 
    75    <author initials="J." surname="Gettys" fullname="Jim Gettys">
    76       <organization
    77          abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
    78       <address>
    79          <postal>
    80             <street>21 Oak Knoll Road</street>
    81             <city>Carlisle</city>
    82             <region>MA</region>
    83             <code>01741</code>
    84             <country>USA</country>
    85          </postal>
    86          <email>jg@freedesktop.org</email>
    87          <uri>http://gettys.wordpress.com/</uri>
    88       </address>
    89    </author>
    90 
    91    <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
    92       <organization abbrev="HP">Hewlett-Packard Company</organization>
    93       <address>
    94          <postal>
    95             <street>HP Labs, Large Scale Systems Group</street>
    96             <street>1501 Page Mill Road, MS 1177</street>
    97             <city>Palo Alto</city>
    98             <region>CA</region>
    99             <code>94304</code>
    100             <country>USA</country>
    101          </postal>
    102          <email>JeffMogul@acm.org</email>
    103       </address>
    104    </author>
    105 
    106    <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
    107       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    108       <address>
    109          <postal>
    110             <street>1 Microsoft Way</street>
    111             <city>Redmond</city>
    112             <region>WA</region>
    113             <code>98052</code>
    114             <country>USA</country>
    115          </postal>
    116          <email>henrikn@microsoft.com</email>
    117       </address>
    118    </author>
    119 
    120    <author fullname="Larry Masinter" initials="L." surname="Masinter">
    121       <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    122       <address>
    123          <postal>
    124             <street>345 Park Ave</street>
    125             <city>San Jose</city>
    126             <region>CA</region>
    127             <code>95110</code>
    128             <country>USA</country>
    129          </postal>
    130          <email>LMM@acm.org</email>
    131          <uri>http://larry.masinter.net/</uri>
    132       </address>
    133    </author>
    134 
    135    <author fullname="Paul J. Leach" initials="P." surname="Leach">
    136       <organization abbrev="Microsoft">Microsoft Corporation</organization>
    137       <address>
    138          <postal>
    139             <street>1 Microsoft Way</street>
    140             <city>Redmond</city>
    141             <region>WA</region>
    142             <code>98052</code>
    143          </postal>
    144          <email>paulle@microsoft.com</email>
    145       </address>
    146    </author>
    147 
    148    <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
    149       <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    150       <address>
    151          <postal>
    152             <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
    153             <street>The Stata Center, Building 32</street>
    154             <street>32 Vassar Street</street>
    155             <city>Cambridge</city>
    156             <region>MA</region>
    157             <code>02139</code>
    158             <country>USA</country>
    159          </postal>
    160          <email>timbl@w3.org</email>
    161          <uri>http://www.w3.org/People/Berners-Lee/</uri>
    162       </address>
    163    </author>
    164 
    165    <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
    166       <organization abbrev="W3C">World Wide Web Consortium</organization>
    167       <address>
    168          <postal>
    169             <street>W3C / ERCIM</street>
    170             <street>2004, rte des Lucioles</street>
    171             <city>Sophia-Antipolis</city>
    172             <region>AM</region>
    173             <code>06902</code>
    174             <country>France</country>
    175          </postal>
    176          <email>ylafon@w3.org</email>
    177          <uri>http://www.raubacapeu.net/people/yves/</uri>
    178       </address>
    179    </author>
    180 
    181    <author fullname="Mark Nottingham" initials="M." role="editor"
    182       surname="Nottingham">
    183       <address>
    184          <email>mnot@mnot.net</email>
    185          <uri>http://www.mnot.net/</uri>
    186       </address>
    187    </author>
    188 
    189    <author fullname="Julian F. Reschke" initials="J. F." role="editor"
    190       surname="Reschke">
    191       <organization abbrev="greenbytes">greenbytes GmbH</organization>
    192       <address>
    193          <postal>
    194             <street>Hafenweg 16</street>
    195             <city>Muenster</city><region>NW</region><code>48155</code>
    196             <country>Germany</country>
    197          </postal>
    198          <phone>+49 251 2807760</phone>
    199          <facsimile>+49 251 2807761</facsimile>
    200          <email>julian.reschke@greenbytes.de</email>
    201          <uri>http://greenbytes.de/tech/webdav/</uri>
    202       </address>
    203    </author>
    204 
    205    <date month="&ID-MONTH;" year="&ID-YEAR;" day="12"/>
    206    <workgroup>HTTPbis Working Group</workgroup>
     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>
     57      <postal>
     58        <street>23 Corporate Plaza DR, Suite 280</street>
     59        <city>Newport Beach</city>
     60        <region>CA</region>
     61        <code>92660</code>
     62        <country>USA</country>
     63      </postal>
     64      <phone>+1-949-706-5300</phone>
     65      <facsimile>+1-949-706-5305</facsimile>
     66      <email>fielding@gbiv.com</email>
     67      <uri>http://roy.gbiv.com/</uri>
     68    </address>
     69  </author>
     70
     71  <author initials="J." surname="Gettys" fullname="Jim Gettys">
     72    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
     73    <address>
     74      <postal>
     75        <street>21 Oak Knoll Road</street>
     76        <city>Carlisle</city>
     77        <region>MA</region>
     78        <code>01741</code>
     79        <country>USA</country>
     80      </postal>
     81      <email>jg@freedesktop.org</email>
     82      <uri>http://gettys.wordpress.com/</uri>
     83    </address>
     84  </author>
     85
     86  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
     87    <organization abbrev="HP">Hewlett-Packard Company</organization>
     88    <address>
     89      <postal>
     90        <street>HP Labs, Large Scale Systems Group</street>
     91        <street>1501 Page Mill Road, MS 1177</street>
     92        <city>Palo Alto</city>
     93        <region>CA</region>
     94        <code>94304</code>
     95        <country>USA</country>
     96      </postal>
     97      <email>JeffMogul@acm.org</email>
     98    </address>
     99  </author>
     100
     101  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
     102    <organization abbrev="Microsoft">Microsoft Corporation</organization>
     103    <address>
     104      <postal>
     105        <street>1 Microsoft Way</street>
     106        <city>Redmond</city>
     107        <region>WA</region>
     108        <code>98052</code>
     109        <country>USA</country>
     110      </postal>
     111      <email>henrikn@microsoft.com</email>
     112    </address>
     113  </author>
     114
     115  <author fullname="Larry Masinter" initials="L." surname="Masinter">
     116    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
     117    <address>
     118      <postal>
     119        <street>345 Park Ave</street>
     120        <city>San Jose</city>
     121        <region>CA</region>
     122        <code>95110</code>
     123        <country>USA</country>
     124      </postal>
     125      <email>LMM@acm.org</email>
     126      <uri>http://larry.masinter.net/</uri>
     127    </address>
     128  </author>
     129
     130  <author fullname="Paul J. Leach" initials="P." surname="Leach">
     131    <organization abbrev="Microsoft">Microsoft Corporation</organization>
     132    <address>
     133      <postal>
     134        <street>1 Microsoft Way</street>
     135        <city>Redmond</city>
     136        <region>WA</region>
     137        <code>98052</code>
     138      </postal>
     139      <email>paulle@microsoft.com</email>
     140    </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>
     146      <postal>
     147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
     148        <street>The Stata Center, Building 32</street>
     149        <street>32 Vassar Street</street>
     150        <city>Cambridge</city>
     151        <region>MA</region>
     152        <code>02139</code>
     153        <country>USA</country>
     154      </postal>
     155      <email>timbl@w3.org</email>
     156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
     157    </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>
     163      <postal>
     164        <street>W3C / ERCIM</street>
     165        <street>2004, rte des Lucioles</street>
     166        <city>Sophia-Antipolis</city>
     167        <region>AM</region>
     168        <code>06902</code>
     169        <country>France</country>
     170      </postal>
     171      <email>ylafon@w3.org</email>
     172      <uri>http://www.raubacapeu.net/people/yves/</uri>
     173    </address>
     174  </author>
     175
     176  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
     177    <address>
     178      <email>mnot@mnot.net</email>
     179      <uri>http://www.mnot.net/</uri>
     180    </address>
     181  </author>
     182
     183  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
     184    <organization abbrev="greenbytes">greenbytes GmbH</organization>
     185    <address>
     186      <postal>
     187        <street>Hafenweg 16</street>
     188        <city>Muenster</city><region>NW</region><code>48155</code>
     189        <country>Germany</country>
     190      </postal>
     191      <phone>+49 251 2807760</phone>
     192      <facsimile>+49 251 2807761</facsimile>
     193      <email>julian.reschke@greenbytes.de</email>
     194      <uri>http://greenbytes.de/tech/webdav/</uri>
     195    </address>
     196  </author>
     197
     198  <date month="&ID-MONTH;" year="&ID-YEAR;" day="12"/>
     199  <workgroup>HTTPbis Working Group</workgroup>
    207200
    208201<abstract>
    209202<t>
    210    The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
    211    distributed, collaborative, hypermedia information systems. This document
    212    is Part 6 of the seven-part specification that defines the protocol
    213    referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6
    214    defines requirements on HTTP caches and the associated header fields that
    215    control cache behavior or indicate cacheable response messages.
     203  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
     204  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
     205  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
     206  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
     207  fields that control cache behavior or indicate cacheable response messages.
    216208</t>
    217209</abstract>
    218210
    219211<note title="Editorial Note (To be removed by RFC Editor)">
    220    <t>
    221        Discussion of this draft should take place on the HTTPBIS working group
    222        mailing list (ietf-http-wg@w3.org). The current issues list is at <eref
    223        target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
    224        documents (including fancy diffs) can be found at <eref
    225        target="http://tools.ietf.org/wg/httpbis/"/>.
    226    </t>
    227    <t>
    228        The changes in this draft are summarized in <xref
    229        target="changes.since.09"/>.
    230    </t>
     212  <t>
     213    Discussion of this draft should take place on the HTTPBIS working group
     214    mailing list (ietf-http-wg@w3.org). The current issues list is
     215    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
     216    and related documents (including fancy diffs) can be found at
     217    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
     218  </t>
     219  <t>
     220    The changes in this draft are summarized in <xref target="changes.since.09"/>.
     221  </t>
    231222</note>
    232223
    … …  
    236227<section anchor="caching" title="Introduction">
    237228<t>
    238    HTTP is typically used for distributed information systems, where
    239    performance can be improved by the use of response caches. This document
    240    defines aspects of HTTP/1.1 related to caching and reusing response
    241    messages.
     229  HTTP is typically used for distributed information systems, where performance can be
     230  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
     231  caching and reusing response messages.
    242232</t>
    243233
    … …  
    245235<iref item="cache" />
    246236<t>
    247    An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
    248    subsystem that controls its message storage, retrieval, and deletion. A
    249    cache stores cacheable responses in order to reduce the response time and
    250    network bandwidth consumption on future, equivalent requests. Any client or
    251    server may include a cache, though a cache cannot be used by a server that
    252    is acting as a tunnel.
    253 </t>
    254 <t>
    255    Caching would be useless if it did not significantly improve performance.
    256    The goal of caching in HTTP/1.1 is to reuse a prior response message to
    257    satisfy a current request. In some cases, a stored response can be reused
    258    without the need for a network request, reducing latency and network
    259    round-trips; a "freshness" mechanism is used for this purpose (see <xref
    260    target="expiration.model" />). Even when a new request is required, it is
    261    often possible to reuse all or parts of the payload of a prior response to
    262    satisfy the request, thereby reducing network bandwidth usage; a
    263    "validation" mechanism is used for this purpose (see <xref
    264    target="validation.model" />).
     237  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
     238  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
     239  in order to reduce the response time and network bandwidth consumption on future,
     240  equivalent requests. Any client or server may include a cache, though a cache cannot be
     241  used by a server that is acting as a tunnel.
     242</t>
     243<t>
     244  Caching would be useless if it did not significantly improve performance. The goal of
     245  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
     246  some cases, a stored response can be reused without the need for a network request,
     247  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
     248  (see <xref target="expiration.model" />). Even when a new request is required, it is often
     249  possible to reuse all or parts of the payload of a prior response to satisfy the request,
     250  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
     251  purpose (see <xref target="validation.model" />).
    265252</t>
    266253</section>
    … …  
    268255<section anchor="intro.terminology" title="Terminology">
    269256<t>
    270    This specification uses a number of terms to refer to the roles played by
    271    participants in, and objects of, HTTP caching.
    272 </t>
    273 <t>
    274    <iref item="cacheable" />
    275    <x:dfn>cacheable</x:dfn>
    276    <list>
    277       <t>A response is cacheable if a cache is allowed to store a copy of the
    278       response message for use in answering subsequent requests. Even when a
    279       response is cacheable, there may be additional constraints on whether a
    280       cache can use the cached copy to satisfy a particular request.</t>
    281    </list>
    282 </t>
    283 <t>
    284    <iref item="explicit expiration time" />
    285    <x:dfn>explicit expiration time</x:dfn>
    286    <list>
    287       <t>The time at which the origin server intends that an entity should no
    288       longer be returned by a cache without further validation.</t>
    289    </list>
    290 </t>
    291 <t>
    292    <iref item="heuristic expiration time" />
    293    <x:dfn>heuristic expiration time</x:dfn>
    294    <list>
    295        <t>An expiration time assigned by a cache when no explicit expiration
    296        time is available.</t>
    297    </list>
    298 </t>
    299 <t>
    300    <iref item="age" />
    301    <x:dfn>age</x:dfn>
    302    <list>
    303       <t>The age of a response is the time since it was sent by, or
    304       successfully validated with, the origin server.</t>
    305    </list>
    306 </t>
    307 <t>
    308    <iref item="first-hand" />
    309    <x:dfn>first-hand</x:dfn>
    310    <list>
    311        <t>A response is first-hand if the freshness model is not in use; i.e.,
    312        its age is 0.</t>
    313    </list>
    314 </t>
    315 <t>
    316    <iref item="freshness lifetime" />
    317    <x:dfn>freshness lifetime</x:dfn>
    318    <list>
    319        <t>The length of time between the generation of a response and its
    320        expiration time.</t>
    321    </list>
    322 </t>
    323 <t>
    324    <iref item="fresh" />
    325    <x:dfn>fresh</x:dfn>
    326    <list>
    327       <t>A response is fresh if its age has not yet exceeded its freshness
    328       lifetime.</t>
    329    </list>
     257  This specification uses a number of terms to refer to the roles played by participants
     258  in, and objects of, HTTP caching.
     259</t>
     260<t>
     261  <iref item="cacheable" />
     262  <x:dfn>cacheable</x:dfn>
     263  <list>
     264    <t>A response is cacheable if a cache is allowed to store a copy of the response message
     265      for use in answering subsequent requests. Even when a response is cacheable, there may
     266      be additional constraints on whether a cache can use the cached copy to satisfy a
     267      particular request.</t>
     268  </list>
     269</t>
     270<t>
     271  <iref item="explicit expiration time" />
     272  <x:dfn>explicit expiration time</x:dfn>
     273  <list>
     274    <t>The time at which the origin server intends that an entity should no longer be
     275      returned by a cache without further validation.</t>
     276  </list>
     277</t>
     278<t>
     279  <iref item="heuristic expiration time" />
     280  <x:dfn>heuristic expiration time</x:dfn>
     281  <list>
     282    <t>An expiration time assigned by a cache when no explicit expiration time is
     283    available.</t>
     284  </list>
     285</t>
     286<t>
     287  <iref item="age" />
     288  <x:dfn>age</x:dfn>
     289  <list>
     290    <t>The age of a response is the time since it was sent by, or successfully validated
     291      with, the origin server.</t>
     292  </list>
     293</t>
     294<t>
     295  <iref item="first-hand" />
     296  <x:dfn>first-hand</x:dfn>
     297  <list>
     298    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
     299    0.</t>
     300  </list>
     301</t>
     302<t>
     303  <iref item="freshness lifetime" />
     304  <x:dfn>freshness lifetime</x:dfn>
     305  <list>
     306    <t>The length of time between the generation of a response and its expiration time. </t>
     307  </list>
     308</t>
     309<t>
     310  <iref item="fresh" />
     311  <x:dfn>fresh</x:dfn>
     312  <list>
     313    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
     314  </list>
    330315</t>
    331316<t>
    … …  
    333318  <x:dfn>stale</x:dfn>
    334319  <list>
    335       <t>A response is stale if its age has passed its freshness lifetime
    336       (either explicit or heuristic).</t>
    337   </list>
    338 </t>
    339 <t>
    340    <iref item="validator" />
    341    <x:dfn>validator</x:dfn>
    342    <list>
    343       <t>A protocol element (e.g., an entity tag or a Last-Modified time) that
    344       is used to find out whether a stored response is an equivalent copy of
    345       an entity.</t>
    346    </list>
     320    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
     321  </list>
     322</t>
     323<t>
     324  <iref item="validator" />
     325  <x:dfn>validator</x:dfn>
     326  <list>
     327    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
     328      out whether a stored response is an equivalent copy of an entity.</t>
     329  </list>
    347330</t>
    348331<t anchor="shared.and.non-shared.caches">
    349    <iref item="validator" />
    350    <x:dfn>shared cache</x:dfn>
    351    <list>
    352       <t>A cache that is accessible to more than one user. A non-shared cache
    353       is dedicated to a single user.</t>
    354    </list>
     332  <iref item="validator" />
     333  <x:dfn>shared cache</x:dfn>
     334  <list>
     335    <t>A cache that is accessible to more than one user. A non-shared cache is
     336      dedicated to a single user.</t>
     337  </list>
    355338</t>
    356339</section>
    … …  
    363346</t>
    364347<t>
    365    An implementation is not compliant if it fails to satisfy one or more of
    366    the "MUST" or "REQUIRED" level requirements for the protocols it
     348   An implementation is not compliant if it fails to satisfy one or more
     349   of the "MUST" or "REQUIRED" level requirements for the protocols it
    367350   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
    368    level and all the "SHOULD" level requirements for its protocols is said to
    369    be "unconditionally compliant"; one that satisfies all the "MUST" level
    370    requirements but not all the "SHOULD" level requirements for its protocols
    371    is said to be "conditionally compliant".
     351   level and all the "SHOULD" level requirements for its protocols is said
     352   to be "unconditionally compliant"; one that satisfies all the "MUST"
     353   level requirements but not all the "SHOULD" level requirements for its
     354   protocols is said to be "conditionally compliant".
    372355</t>
    373356</section>
    374357
    375358<section title="Syntax Notation" anchor="notation">
    376    <x:anchor-alias value="ALPHA"/>
    377    <x:anchor-alias value="CR"/>
    378    <x:anchor-alias value="DIGIT"/>
    379    <x:anchor-alias value="DQUOTE"/>
    380    <x:anchor-alias value="LF"/>
    381    <x:anchor-alias value="OCTET"/>
    382    <x:anchor-alias value="SP"/>
    383    <x:anchor-alias value="VCHAR"/>
    384    <x:anchor-alias value="WSP"/>
    385 <t>
    386    This specification uses the ABNF syntax defined in &notation; (which
    387    extends the syntax defined in <xref target="RFC5234"/> with a list rule).
    388    <xref target="collected.abnf"/> shows the collected ABNF, with the list
    389    rule expanded.
    390 </t>
    391 <t>
    392    The following core rules are included by reference, as defined in <xref
    393    target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
    394    return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
    395    quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
    396    sequence of data), SP (space), VCHAR (any visible USASCII character), and
    397    WSP (whitespace).
     359  <x:anchor-alias value="ALPHA"/>
     360  <x:anchor-alias value="CR"/>
     361  <x:anchor-alias value="DIGIT"/>
     362  <x:anchor-alias value="DQUOTE"/>
     363  <x:anchor-alias value="LF"/>
     364  <x:anchor-alias value="OCTET"/>
     365  <x:anchor-alias value="SP"/>
     366  <x:anchor-alias value="VCHAR"/>
     367  <x:anchor-alias value="WSP"/>
     368<t>
     369  This specification uses the ABNF syntax defined in &notation; (which
     370  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
     371  <xref target="collected.abnf"/> shows the collected ABNF, with the list
     372  rule expanded.
     373</t>
     374<t>
     375  The following core rules are included by
     376  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
     377  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
     378  DIGIT (decimal 0-9), DQUOTE (double quote),
     379  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
     380  OCTET (any 8-bit sequence of data), SP (space),
     381  VCHAR (any visible USASCII character),
     382  and WSP (whitespace).
    398383</t>
    399384
    400385<section title="Core Rules" anchor="core.rules">
    401    <x:anchor-alias value="quoted-string"/>
    402    <x:anchor-alias value="token"/>
    403    <x:anchor-alias value="OWS"/>
    404 <t>
    405    The core rules below are defined in &basic-rules;:
     386  <x:anchor-alias value="quoted-string"/>
     387  <x:anchor-alias value="token"/>
     388  <x:anchor-alias value="OWS"/>
     389<t>
     390  The core rules below are defined in &basic-rules;:
    406391</t>
    407392<figure><artwork type="abnf2616">
    … …  
    412397</section>
    413398
    414 <section title="ABNF Rules defined in other Parts of the Specification"
    415    anchor="abnf.dependencies">
    416    <x:anchor-alias value="field-name"/>
    417    <x:anchor-alias value="HTTP-date"/>
    418    <x:anchor-alias value="port"/>
    419    <x:anchor-alias value="pseudonym"/>
    420    <x:anchor-alias value="uri-host"/>
    421 <t>
    422    The ABNF rules below are defined in other parts:
     399<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
     400  <x:anchor-alias value="field-name"/>
     401  <x:anchor-alias value="HTTP-date"/>
     402  <x:anchor-alias value="port"/>
     403  <x:anchor-alias value="pseudonym"/>
     404  <x:anchor-alias value="uri-host"/>
     405<t>
     406  The ABNF rules below are defined in other parts:
    423407</t>
    424408<figure><!--Part1--><artwork type="abnf2616">
    … …  
    438422<section anchor="response.cacheability" title="Response Cacheability">
    439423<t>
    440    A cache &MUST-NOT; store a response to any request, unless:
    441    <list style="symbols">
    442       <t>The request method is understood by the cache and defined as being
    443       cacheable, and</t>
    444       <t>the response status code is understood by the cache, and</t>
    445       <t>the "no-store" cache directive (see <xref
    446       target="header.cache-control" />) does not appear in request or response
    447       headers, and</t>
    448       <t>the "private" cache response directive (see <xref
    449       target="cache-response-directive" /> does not appear in the response, if
    450       the cache is shared, and</t>
    451       <t>the "Authorization" header (see &header-authorization;) does not
    452       appear in the request, if the cache is shared, unless the response
    453       explicitly allows it (see <xref target="caching.authenticated.responses"
    454       />), and</t>
    455       <t>the response either:
    456          <list style="symbols">
    457             <t>contains an Expires header (see <xref target="header.expires"
    458             />), or</t>
    459             <t>contains a max-age response cache directive (see <xref
    460             target="cache-response-directive" />), or</t>
    461             <t>contains a s-maxage response cache directive and the cache is
    462             shared, or</t>
    463             <t>contains a Cache Control Extension (see <xref
    464             target="cache.control.extensions" />) that allows it to be cached,
    465             or</t>
    466             <t>has a status code that can be served with heuristic freshness
    467             (see <xref target="heuristic.freshness" />).</t>
    468          </list>
    469       </t>
    470   </list>
    471 </t>
    472 <t>
    473    In this context, a cache has "understood" a request method or a response
    474    status code if it recognises it and implements any cache-specific
    475    behaviour. In particular, 206 Partial Content responses cannot be cached by
    476    an implementation that does not handle partial content (see <xref
    477    target="errors.or.incomplete.response.cache.behavior" />).
    478 </t>
    479 <t>
    480    Note that in normal operation, most caches will not store a response that
    481    has neither a cache validator nor an explicit expiration time, as such
    482    responses are not usually useful to store. However, caches are not
    483    prohibited from storing such responses.
    484 </t>
    485 
    486 <section anchor="errors.or.incomplete.response.cache.behavior"
    487    title="Storing Partial and Incomplete Responses">
    488 <t>
    489    A cache that receives an incomplete response (for example, with fewer bytes
    490    of data than specified in a Content-Length header) can store the response,
    491    but &MUST; treat it as a partial response &partial;. Partial responses can
    492    be combined as described in &combining-byte-ranges;; the result might be a
    493    full response or might still be partial. A cache &MUST-NOT; return a
    494    partial response to a client without explicitly marking it as such using
    495    the 206 (Partial Content) status code.
    496 </t>
    497 <t>
    498    A cache that does not support the Range and Content-Range headers
    499    &MUST-NOT; store incomplete or partial responses.
    500 </t>
    501 </section>
    502 
    503 </section>
    504 
    505 
    506 <section anchor="constructing.responses.from.caches"
    507    title="Constructing Responses from Caches">
    508 <t>
    509    For a presented request, a cache &MUST-NOT; return a stored response,
    510    unless:
    511    <list style="symbols">
    512       <t>The presented Effective Request URI (&effective-request-uri;) and
    513       that of the stored response match, and</t>
    514       <t>the request method associated with the stored response allows it to
    515       be used for the presented request, and</t>
    516       <t>selecting request-headers nominated by the stored response (if any)
    517       match those presented (see <xref target="caching.negotiated.responses"
    518       />), and</t>
    519       <t>the presented request and stored response are free from directives
    520       that would prevent its use (see <xref target="header.cache-control" />
    521       and <xref target="header.pragma"/>), and</t>
    522       <t>the stored response is either:
    523          <list style="symbols">
    524             <t>fresh (see <xref target="expiration.model" />), or</t>
    525             <t>allowed to be served stale (see <xref
    526             target="serving.stale.responses" />), or</t>
    527             <t>successfully validated (see <xref target="validation.model"
    528             />).</t>
    529          </list>
    530       </t>
    531    </list>
    532 </t>
    533 <t>
    534    <cref anchor="TODO-method-cacheability">define method cacheability for GET,
    535    HEAD and POST in p2-semantics.</cref>
    536 </t>
    537 <t>
    538    When a stored response is used to satisfy a request, caches &MUST; include
    539    a single Age header field (<xref target="header.age" />) in the response
    540    with a value equal to the stored response's current_age; see <xref
    541    target="age.calculations" />.
    542    <cref anchor="DISCUSS-includes-validated">this currently includes
    543    successfully validated responses.</cref>
    544 </t>
    545 <t>
    546    Requests with methods that are unsafe (&safe-methods;) &MUST; be written
    547    through the cache to the origin server; i.e., a cache must not reply to
    548    such a request before having forwarded the request and having received a
    549    corresponding response.</t>
    550 <t>
    551    Also, note that unsafe requests might invalidate already stored responses;
    552    see <xref target="invalidation.after.updates.or.deletions" />.
    553 </t>
    554 <t>
    555    Caches &MUST; use the most recent response (as determined by the Date
    556    header) when more than one suitable response is stored. They can also
    557    forward a request with "Cache-Control: max-age=0" or "Cache-Control:
    558    no-cache" to disambiguate which response to use.
     424  A cache &MUST-NOT; store a response to any request, unless:
     425  <list style="symbols">
     426    <t>The request method is understood by the cache and defined as being cacheable, and</t>
     427    <t>the response status code is understood by the cache, and</t>
     428    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
     429       appear in request or response headers, and</t>
     430    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
     431       does not appear in the response, if the cache is shared, and</t>
     432    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
     433       the cache is shared, unless the response explicitly allows it (see <xref
     434       target="caching.authenticated.responses" />), and</t>
     435    <t>the response either:
     436      <list style="symbols">
     437        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
     438        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
     439        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
     440        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
     441        <t>has a status code that can be served with heuristic freshness (see <xref
     442           target="heuristic.freshness" />).</t>
     443      </list>
     444    </t>
     445  </list>
     446</t>
     447<t>
     448  In this context, a cache has "understood" a request method or a response status
     449  code if it recognises it and implements any cache-specific behaviour. In
     450  particular, 206 Partial Content responses cannot be cached by an
     451  implementation that does not handle partial content
     452  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
     453</t>
     454<t>
     455  Note that in normal operation, most caches will not store a response that has neither a
     456  cache validator nor an explicit expiration time, as such responses are not usually
     457  useful to store. However, caches are not prohibited from storing such responses.
     458</t>
     459
     460<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
     461<t>
     462  A cache that receives an incomplete response (for example, with fewer bytes of data
     463  than specified in a Content-Length header) can store the response, but &MUST;
     464  treat it as a partial response &partial;. Partial responses
     465  can be combined as described in &combining-byte-ranges;; the result might be a
     466  full response or might still be partial. A cache &MUST-NOT; return a partial
     467  response to a client without explicitly marking it as such using the 206 (Partial
     468  Content) status code.
     469</t>
     470<t>
     471  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
     472  incomplete or partial responses.
     473</t>
     474</section>
     475
     476</section>
     477
     478
     479<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
     480<t>
     481  For a presented request, a cache &MUST-NOT; return a stored response, unless:
     482  <list style="symbols">
     483    <t>The presented Effective Request URI (&effective-request-uri;) and that of the stored response match, and</t>
     484    <t>the request method associated with the stored response allows it to be
     485      used for the presented request, and</t>
     486    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
     487      target="caching.negotiated.responses" />), and</t>
     488    <t>the presented request and stored response are free from directives that would prevent
     489      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
     490      and</t>
     491    <t>the stored response is either:
     492      <list style="symbols">
     493        <t>fresh (see <xref target="expiration.model" />), or</t>
     494        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
     495        <t>successfully validated (see <xref target="validation.model" />).</t>
     496      </list>
     497    </t>
     498  </list>
     499</t>
     500<t>
     501  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
     502</t>
     503<t>
     504  When a stored response is used to satisfy a request, caches &MUST; include a
     505  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
     506  current_age; see <xref target="age.calculations" />.
     507  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
     508</t>
     509<t>
     510  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
     511  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
     512  corresponding response.
     513</t>
     514<t>
     515  Also, note that unsafe requests might invalidate already stored responses; see
     516  <xref target="invalidation.after.updates.or.deletions" />.
     517</t>
     518<t>
     519  Caches &MUST; use the most recent response (as determined by the Date header) when
     520  more than one suitable response is stored. They can also forward a request with
     521  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
     522  use.
    559523</t>
    560524</section>
    … …  
    562526<section anchor="expiration.model" title="Freshness Model">
    563527<t>
    564    When a response is "fresh" in the cache, it can be used to satisfy
    565    subsequent requests without contacting the origin server, thereby improving
    566    efficiency.
    567 </t>
    568 <t>
    569    The primary mechanism for determining freshness is for an origin server to
    570    provide an explicit expiration time in the future, using either the Expires
    571    header (<xref target="header.expires" />) or the max-age response cache
    572    directive (<xref target="cache-response-directive" />). Generally, origin
    573    servers will assign future explicit expiration times to responses in the
    574    belief that the entity is not likely to change in a semantically
    575    significant way before the expiration time is reached.
    576 </t>
    577 <t>
    578    If an origin server wishes to force a cache to validate every request, it
    579    can assign an explicit expiration time in the past. This means that the
    580    response is always stale, so that caches should validate it before using it
    581    for subsequent requests. <cref anchor="TODO-response-stale">This wording
    582    may cause confusion, because the response may still be served stale.</cref>
    583 </t>
    584 <t>
    585    Since origin servers do not always provide explicit expiration times, HTTP
    586    caches may also assign heuristic expiration times when they are not
    587    specified, employing algorithms that use other header values (such as the
    588    Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1
    589    specification does not provide specific algorithms, but does impose
    590    worst-case constraints on their results.
     528  When a response is "fresh" in the cache, it can be used to satisfy subsequent
     529  requests without contacting the origin server, thereby improving efficiency.
     530</t>
     531<t>
     532  The primary mechanism for determining freshness is for an origin server to provide an
     533  explicit expiration time in the future, using either the Expires header (<xref
     534  target="header.expires" />) or the max-age response cache directive (<xref
     535  target="cache-response-directive" />). Generally, origin servers will assign future
     536  explicit expiration times to responses in the belief that the entity is not likely to
     537  change in a semantically significant way before the expiration time is reached.
     538</t>
     539<t>
     540  If an origin server wishes to force a cache to validate every request, it can
     541  assign an explicit expiration time in the past. This means that the response is always
     542  stale, so that caches should validate it before using it for subsequent requests.
     543  <cref anchor="TODO-response-stale">This wording may cause confusion, because the response may still be served stale.</cref>
     544</t>
     545<t>
     546  Since origin servers do not always provide explicit expiration times, HTTP caches may
     547  also assign heuristic expiration times when they are not specified, employing algorithms that
     548  use other header values (such as the Last-Modified time) to estimate a plausible
     549  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
     550  impose worst-case constraints on their results.
    591551</t>
    592552<figure>
    … …  
    599559</figure>
    600560<t>
    601    The freshness_lifetime is defined in <xref
    602    target="calculating.freshness.lifetime" />; the current_age is defined in
    603    <xref target="age.calculations" />.
    604 </t>
    605 <t>
    606    Additionally, clients may need to influence freshness calculation. They can
    607    do this using several request cache directives, with the effect of either
    608    increasing or loosening constraints on freshness. See <xref
    609    target="cache-request-directive" />.
    610 </t>
    611 <t>
    612    <cref anchor="ISSUE-no-req-for-directives">there are not requirements
    613    directly applying to cache-request-directives and freshness.</cref>
    614 </t>
    615 <t>
    616    Note that freshness applies only to cache operation; it cannot be used to
    617    force a user agent to refresh its display or reload a resource. See <xref
    618    target="history.lists" /> for an explanation of the difference between
    619    caches and history mechanisms.
    620 </t>
    621 
    622 <section anchor="calculating.freshness.lifetime"
    623    title="Calculating Freshness Lifetime">
    624 <t>
    625    A cache can calculate the freshness lifetime (denoted as
    626    freshness_lifetime) of a response by using the first match of:
    627    <list style="symbols">
    628       <t>If the cache is shared and the s-maxage response cache directive
    629       (<xref target="cache-response-directive" />) is present, use its value,
    630       or</t>
    631       <t>If the max-age response cache directive (<xref
     561  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
     562  the current_age is defined in <xref target="age.calculations" />.
     563</t>
     564<t>
     565  Additionally, clients may need to influence freshness calculation. They can do this using
     566  several request cache directives, with the effect of either increasing or loosening
     567  constraints on freshness. See <xref target="cache-request-directive" />.
     568</t>
     569<t>
     570  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
     571  freshness.</cref>
     572</t>
     573<t>
     574  Note that freshness applies only to cache operation; it cannot be used to force a user agent
     575  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
     576  the difference between caches and history mechanisms.
     577</t>
     578
     579<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
     580<t>
     581  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
     582  response by using the first match of:
     583  <list style="symbols">
     584    <t>If the cache is shared and the s-maxage response cache directive (<xref
    632585      target="cache-response-directive" />) is present, use its value, or</t>
    633       <t>If the Expires response header (<xref target="header.expires" />) is
    634       present, use its value minus the value of the Date response header,
    635       or</t>
    636       <t>Otherwise, no explicit expiration time is present in the response. A
    637       heuristic freshness lifetime might be applicable; see <xref
    638       target="heuristic.freshness" />.</t>
    639   </list>
    640 </t>
    641 <t>
    642    Note that this calculation is not vulnerable to clock skew, since all of
    643    the information comes from the origin server.
     586    <t>If the max-age response cache directive (<xref target="cache-response-directive"
     587      />) is present, use its value, or</t>
     588    <t>If the Expires response header (<xref target="header.expires" />) is present, use
     589      its value minus the value of the Date response header, or</t>
     590    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
     591      freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
     592  </list>
     593</t>
     594<t>
     595  Note that this calculation is not vulnerable to clock skew, since all of the
     596  information comes from the origin server.
    644597</t>
    645598
    646599<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
    647600<t>
    648    If no explicit expiration time is present in a stored response that has a
    649    status code of 200, 203, 206, 300, 301 or 410, a heuristic expiration time
    650    can be calculated. Heuristics &MUST-NOT; be used for other response status
    651    codes.
    652 </t>
    653 <t>
    654    When a heuristic is used to calculate freshness lifetime, the cache
    655    &SHOULD; attach a Warning header with a 113 warn-code to the response if
    656    its current_age is more than 24 hours and such a warning is not already
    657    present.
    658 </t>
    659 <t>
    660    Also, if the response has a Last-Modified header (&header-last-modified;),
    661    the heuristic expiration value &SHOULD; be no more than some fraction of
    662    the interval since that time. A typical setting of this fraction might be
    663    10%.
     601  If no explicit expiration time is present in a stored response that has a status code
     602  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
     603  calculated. Heuristics &MUST-NOT; be used for other response status codes.
     604</t>
     605<t>
     606  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
     607  attach a Warning header with a 113 warn-code to the response if its current_age is
     608  more than 24 hours and such a warning is not already present.
     609</t>
     610<t>
     611  Also, if the response has a Last-Modified header (&header-last-modified;), the
     612  heuristic expiration value &SHOULD; be no more than some fraction of the interval
     613  since that time. A typical setting of this fraction might be 10%.
    664614</t>
    665615<x:note>
    666    <t>
    667       <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
    668       x:sec="13.9"/>) required that caches do not calculate heuristic
    669       freshness for URLs with query components (i.e., those containing '?').
    670       In practice, this has not been widely implemented. Therefore, servers
    671       are encouraged to send explicit directives (e.g., Cache-Control:
    672       no-cache) if they wish to preclude caching.
    673    </t>
     616  <t>
     617    <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt="," x:sec="13.9"/>)
     618    required that caches do not calculate heuristic freshness for URLs with
     619    query components (i.e., those containing '?'). In practice, this has not
     620    been widely implemented. Therefore, servers are encouraged to send explicit
     621    directives (e.g., Cache-Control: no-cache) if they wish to preclude
     622    caching.
     623  </t>
    674624</x:note>
    675625</section>
    … …  
    678628<section anchor="age.calculations" title="Calculating Age">
    679629<t>
    680    HTTP/1.1 uses the Age response-header to convey the estimated age of the
    681    response message when obtained from a cache. The Age field value is the
    682    cache's estimate of the amount of time since the response was generated or
    683    validated by the origin server. In essence, the Age value is the sum of the
    684    time that the response has been resident in each of the caches along the
    685    path from the origin server, plus the amount of time it has been in transit
    686    along network paths.
    687 </t>
    688 <t>
    689    The following data is used for the age calculation:
    690 </t>
    691 <t>
    692    <x:dfn>age_value</x:dfn>
    693    <list>
    694       <t>
    695          The term "age_value" denotes the value of the Age header (<xref
    696          target="header.age"/>), in a form appropriate for arithmetic
    697          operation; or 0, if not available.
    698       </t>
    699    </list>
    700 </t>
    701 <t>
    702    <x:dfn>date_value</x:dfn>
    703    <list>
    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. The term "date_value" denotes the value of the Date
    708          header, in a form appropriate for arithmetic operations. See
    709          &header-date; for the definition of the Date header, and for
    710          requirements regarding responses without a Date response header.
    711       </t>
    712   </list>
    713 </t>
    714 <t>
    715    <x:dfn>now</x:dfn>
    716    <list>
    717       <t>
    718          The term "now" means "the current value of the clock at the host
    719          performing the calculation". Hosts that use HTTP, but especially
    720          hosts running origin servers and caches, &SHOULD; use NTP (<xref
    721          target="RFC1305"/>) or some similar protocol to synchronize their
    722          clocks to a globally accurate time standard.
    723       </t>
    724    </list>
    725 </t>
    726 <t>
    727    <x:dfn>request_time</x:dfn>
    728    <list>
    729       <t>
    730          The current value of the clock at the host at the time the request
    731          resulting in the stored response was made.
    732       </t>
    733    </list>
    734 </t>
    735 <t>
    736    <x:dfn>response_time</x:dfn>
    737    <list>
    738       <t>
    739          The current value of the clock at the host at the time the response
    740          was received.
    741       </t>
    742    </list>
    743 </t>
    744 <t>
    745    A response's age can be calculated in two entirely independent ways:
    746    <list style="numbers">
    747       <t>the "apparent_age": response_time minus date_value, if the local
    748       clock is reasonably well synchronized to the origin server's clock. If
    749       the result is negative, the result is replaced by zero.</t>
    750       <t>the "corrected_age_value", if all of the caches along the response
    751       path implement HTTP/1.1; note this value &MUST; be interpreted relative
    752       to the time the request was initiated, not the time that the response
    753       was received.</t>
    754    </list>
     630  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
     631  message when obtained from a cache. The Age field value is the cache's estimate of the
     632  amount of time since the response was generated or validated by the origin server. In
     633  essence, the Age value is the sum of the time that the response has been resident in
     634  each of the caches along the path from the origin server, plus the amount of time it has
     635  been in transit along network paths.
     636</t>
     637<t>
     638  The following data is used for the age calculation:
     639</t>
     640<t>
     641  <x:dfn>age_value</x:dfn>
     642  <list>
     643    <t>
     644      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
     645      in a form appropriate for arithmetic operation; or 0, if not available.
     646    </t>
     647  </list>
     648</t>
     649<t>
     650  <x:dfn>date_value</x:dfn>
     651  <list>
     652    <t>
     653      HTTP/1.1 requires origin servers to send a Date header, if possible,
     654      with every response, giving the time at which the response was generated.
     655      The term "date_value" denotes the value of the Date header, in a form
     656      appropriate for arithmetic operations. See &header-date; for the definition
     657      of the Date header, and for requirements regarding responses without a
     658      Date response header.
     659    </t>
     660  </list>
     661</t>
     662<t>
     663  <x:dfn>now</x:dfn>
     664  <list>
     665    <t>
     666      The term "now" means "the current value of the clock at the host
     667      performing the calculation". Hosts that use HTTP, but especially hosts
     668      running origin servers and caches, &SHOULD; use NTP
     669      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
     670      clocks to a globally accurate time standard.
     671    </t>
     672  </list>
     673</t>
     674<t>
     675  <x:dfn>request_time</x:dfn>
     676  <list>
     677    <t>
     678      The current value of the clock at the host at the time the request
     679      resulting in the stored response was made.
     680    </t>
     681  </list>
     682</t>
     683<t>
     684  <x:dfn>response_time</x:dfn>
     685  <list>
     686    <t>
     687      The current value of the clock at the host at the time the response was
     688      received.
     689    </t>
     690  </list>
     691</t>
     692<t>
     693  A response's age can be calculated in two entirely independent ways:
     694  <list style="numbers">
     695    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
     696      origin server's clock. If the result is negative, the result is replaced by zero.</t>
     697    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
     698      note this value &MUST; be interpreted relative to the time the
     699      request was initiated, not the time that the response was received.</t>
     700  </list>
    755701</t>
    756702<figure>
    … …  
    768714</artwork></figure>
    769715<t>
    770    The current_age of a stored response can then be calculated by adding the
    771    amount of time (in seconds) since the stored response was last validated by
    772    the origin server to the corrected_initial_age.
     716  The current_age of a stored response can then be calculated by adding the amount of
     717  time (in seconds) since the stored response was last validated by the origin server to
     718  the corrected_initial_age.
    773719</t>
    774720<figure><artwork type="code">
    … …  
    780726<section anchor="serving.stale.responses" title="Serving Stale Responses">
    781727<t>
    782    A "stale" response is one that either has explicit expiry information or is
    783    allowed to have heuristic expiry calculated, but is not fresh according to
    784    the calculations in <xref target="expiration.model" />.
    785 </t>
    786 <t>
    787    Caches &MUST-NOT; return a stale response if it is prohibited by an
    788    explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
    789    directive, a "must-revalidate" cache-response-directive, or an applicable
    790    "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
    791    target="cache-response-directive"/>).
    792 </t>
    793 <t>
    794    Caches &SHOULD-NOT; return stale responses unless they are disconnected
    795    (i.e., it cannot contact the origin server or otherwise find a forward
    796    path) or otherwise explicitly allowed (e.g., the max-stale request
    797    directive; see <xref target="cache-request-directive" />).
    798 </t>
    799 <t>
    800    Stale responses &SHOULD; have a Warning header with the 110 warn-code (see
    801    <xref target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be
    802    sent on stale responses if the cache is disconnected.
    803 </t>
    804 <t>
    805    If a cache receives a first-hand response (either an entire response, or a
    806    304 (Not Modified) response) that it would normally forward to the
    807    requesting client, and the received response is no longer fresh, the cache
    808    &SHOULD; forward it to the requesting client without adding a new Warning
    809    (but without removing any existing Warning headers). A cache &SHOULD-NOT;
    810    attempt to validate a response simply because that response became stale in
    811    transit.
     728  A "stale" response is one that either has explicit expiry information or is allowed to
     729  have heuristic expiry calculated, but is not fresh according to the calculations in
     730  <xref target="expiration.model" />.
     731</t>
     732<t>
     733  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
     734  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
     735  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
     736  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
     737</t>
     738<t>
     739  Caches &SHOULD-NOT; return stale responses unless they are
     740  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
     741  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
     742</t>
     743<t>
     744  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
     745  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
     746  the cache is disconnected.
     747</t>
     748<t>
     749  If a cache receives a first-hand response (either an entire response, or a 304 (Not
     750  Modified) response) that it would normally forward to the requesting client, and the
     751  received response is no longer fresh, the cache &SHOULD; forward it to the
     752  requesting client without adding a new Warning (but without removing any existing
     753  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
     754  that response became stale in transit.
    812755</t>
    813756</section>
    … …  
    816759<section anchor="validation.model" title="Validation Model">
    817760<t>
    818    When a cache has one or more stored responses for a requested URI, but
    819    cannot serve any of them (e.g., because they are not fresh, or one cannot
    820    be selected; see <xref target="caching.negotiated.responses"/>), it can use
    821    the conditional request mechanism &conditional; in the forwarded request to
    822    give the origin server an opportunity to both select a valid stored
    823    response to be used, and to update it. This process is known as
    824    "validating" or "revalidating" the stored response.
    825 </t>
    826 <t>
    827    When sending such a conditional request, the cache &SHOULD; add an
    828    If-Modified-Since header whose value is that of the Last-Modified header
    829    from the selected (see <xref target="caching.negotiated.responses"/>)
    830    stored response, if available.
    831 </t>
    832 <t>
    833    Additionally, the cache &SHOULD; add an If-None-Match header whose value is
    834    that of the ETag header(s) from all responses stored for the requested URI,
    835    if present. However, if any of the stored responses contains only partial
    836    content, its entity-tag &SHOULD-NOT; be included in the If-None-Match
    837    header field unless the request is for a range that would be fully
    838    satisfied by that stored response.
    839 </t>
    840 <t>
    841    A 304 (Not Modified) response status code indicates that the stored
    842    response can be updated and reused; see <xref target="combining.headers"/>.
    843 </t>
    844 <t>
    845    A full response (i.e., one with a response body) indicates that none of the
    846    stored responses nominated in the conditional request is suitable. Instead,
    847    the full response is used both to satisfy the request and replace the
    848    stored response. <cref anchor="TODO-req-missing">Should there be a
    849    requirement here?</cref>
    850 </t>
    851 <t>
    852    If a cache receives a 5xx response while attempting to validate a response,
    853    it &MAY; either forward this response to the requesting client, or act as
    854    if the server failed to respond. In the latter case, it &MAY; return a
    855    previously stored response (see <xref target="serving.stale.responses" />).
    856 </t>
    857 </section>
    858 
    859 <section anchor="invalidation.after.updates.or.deletions"
    860    title="Request Methods that Invalidate">
    861 <t>
    862    Because unsafe methods (&safe-methods;) have the potential for changing
    863    state on the origin server, intervening caches can use them to keep their
    864    contents up-to-date.
    865 </t>
    866 <t>
    867    The following HTTP methods &MUST; cause a cache to invalidate the Effective
    868    Request URI (&effective-request-uri;) as well as the URI(s) in the Location
    869    and Content-Location headers (if present):
    870    <list style="symbols">
    871       <t>PUT</t>
    872       <t>DELETE</t>
    873       <t>POST</t>
    874    </list>
    875 </t>
    876 <t>
    877    An invalidation based on a URI from a Location or Content-Location header
    878    &MUST-NOT; be performed if the host part of that URI differs from the host
    879    part in the Effective Request URI (&effective-request-uri;). This helps
    880    prevent denial of service attacks.
    881 </t>
    882 <t>
    883    <cref anchor="TODO-def-host-part">"host part" needs to be specified
    884    better.</cref>
    885 </t>
    886 <t>
    887    A cache that passes through requests for methods it does not understand
    888    &SHOULD; invalidate the Effective Request URI (&effective-request-uri;).
    889 </t>
    890 <t>
    891    Here, "invalidate" means that the cache will either remove all stored
    892    responses related to the Effective Request URI, or will mark these as
    893    "invalid" and in need of a mandatory validation before they can be returned
    894    in response to a subsequent request.
    895 </t>
    896 <t>
    897    Note that this does not guarantee that all appropriate responses are
    898    invalidated. For example, the request that caused the change at the origin
    899    server might not have gone through the cache where a response is stored.
    900 </t>
    901 <t>
    902    <cref anchor="TODO-spec-success-invalidate">specify that only successful
    903    (2xx, 3xx?) responses invalidate.</cref>
    904 </t>
    905 </section>
    906 
    907 <section anchor="caching.authenticated.responses"
    908    title="Shared Caching of Authenticated Responses">
    909 
    910 <t>
    911    Shared caches &MUST-NOT; use a cached response to a request with an
    912    Authorization header (&header-authorization;) to satisfy any subsequent
    913    request unless a cache directive that allows such responses to be stored is
    914    present in the response.
    915 </t>
    916 
    917 <t>
    918    In this specification, the following Cache-Control response directives
    919    (<xref target="cache-response-directive"/>) have such an effect:
    920    must-revalidate, public, s-maxage.
    921 </t>
    922 
    923 <t>
    924    Note that cached responses that contain the "must-revalidate" and/or
    925    "s-maxage" response directives are not allowed to be served stale (<xref
    926    target="serving.stale.responses"/>) by shared caches. In particular, a
    927    response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
    928    used to satisfy a subsequent request without revalidating it on the origin
    929    server.
    930 </t>
    931 </section>
    932 
    933 <section anchor="caching.negotiated.responses"
    934    title="Caching Negotiated Responses">
    935 <t>
    936    When a cache receives a request that can be satisfied by a stored response
    937    that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
    938    use that response unless all of the selecting request-headers nominated by
    939    the Vary header match in both the original request (i.e., that associated
    940    with the stored response), and the presented request.
    941 </t>
    942 <t>
    943    The selecting request-headers from two requests are defined to match if and
    944    only if those in the first request can be transformed to those in the
    945    second request by applying any of the following:
    946    <list style="symbols">
    947       <t>
     761  When a cache has one or more stored responses for a requested URI, but cannot 
     762  serve any of them (e.g., because they are not fresh, or one cannot be selected;
     763  see <xref target="caching.negotiated.responses"/>),
     764  it can use the conditional request mechanism &conditional; in the forwarded
     765  request to give the origin server an opportunity to both select a valid stored
     766  response to be used, and to update it. This process is known as "validating"
     767  or "revalidating" the stored response.
     768</t>
     769<t>
     770  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
     771  header whose value is that of the Last-Modified header from the selected
     772  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
     773</t>
     774<t>
     775  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
     776  is that of the ETag header(s) from all responses stored for the requested URI,
     777  if present. However, if any of the stored responses contains only partial
     778  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
     779  field unless the request is for a range that would be fully satisfied by
     780  that stored response.
     781</t>
     782<t>
     783  A 304 (Not Modified) response status code indicates that the stored
     784  response can be updated and reused; see <xref target="combining.headers"/>.
     785</t>
     786<t>
     787  A full response (i.e., one with a response body) indicates that none 
     788  of the stored responses nominated in the conditional request is
     789  suitable. Instead, the full response is used both to satisfy the
     790  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
     791</t>
     792<t>
     793  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
     794  either forward this response to the requesting client, or act as if the server failed to
     795  respond. In the latter case, it &MAY; return a previously stored response (see <xref
     796  target="serving.stale.responses" />).
     797</t>
     798</section>
     799
     800<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
     801<t>
     802  Because unsafe methods (&safe-methods;) have the potential for changing state on the
     803  origin server, intervening caches can use them to keep their contents
     804  up-to-date.
     805</t>
     806<t>
     807  The following HTTP methods &MUST; cause a cache to invalidate the Effective Request URI (&effective-request-uri;) as well
     808  as the URI(s) in the Location and Content-Location headers (if present):
     809  <list style="symbols">
     810    <t>PUT</t>
     811    <t>DELETE</t>
     812    <t>POST</t>
     813  </list>
     814</t>
     815<t>
     816  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
     817  be performed if the host part of that URI differs from the host part in the Effective Request URI (&effective-request-uri;).
     818  This helps prevent denial of service attacks.
     819</t>
     820<t>
     821  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
     822</t>
     823<t>
     824  A cache that passes through requests for methods it does not understand &SHOULD;
     825  invalidate the Effective Request URI (&effective-request-uri;).
     826</t>
     827<t>
     828  Here, "invalidate" means that the cache will either remove all stored responses related
     829  to the Effective Request URI, or will mark these as "invalid" and in need of a mandatory validation
     830  before they can be returned in response to a subsequent request.
     831</t>
     832<t>
     833  Note that this does not guarantee that all appropriate responses are invalidated. For
     834  example, the request that caused the change at the origin server might not have gone
     835  through the cache where a response is stored.
     836</t>
     837<t>
     838  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
     839</t>
     840</section>
     841
     842<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
     843
     844<t>Shared caches &MUST-NOT; use a cached response to a request with an Authorization header (&header-authorization;) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.</t>
     845
     846<t>In this specification, the following Cache-Control response directives (<xref target="cache-response-directive"/>) have such an effect: must-revalidate, public, s-maxage.</t>
     847
     848<t>Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating it on the origin server.</t>
     849</section>
     850
     851<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
     852<t>
     853  When a cache receives a request that can be satisfied by a stored response
     854  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
     855  response unless all of the selecting request-headers nominated by the Vary header match
     856  in both the original request (i.e., that associated with the stored response),
     857  and the presented request.
     858</t>
     859<t>
     860  The selecting request-headers from two requests are defined to match
     861  if and only if those in the first request can be transformed to those in the
     862  second request by applying any of the following:
     863  <list style="symbols">
     864    <t>
    948865      adding or removing whitespace, where allowed in the header's syntax
    949       </t>
    950       <t>
     866    </t>
     867    <t>
    951868      combining multiple message-header fields with the same field name (see
    952869      &header-fields;)
    953       </t>
    954       <t>
     870    </t>
     871    <t>
    955872      normalizing both header values in a way that is known to have identical
    956       semantics, according to the header's specification (e.g., re-ordering
    957       field values when order is not significant; case-normalization, where
    958       values are defined to be case-insensitive)
    959       </t>
    960    </list>
    961 </t>
    962 <t>
    963    If (after any normalisation that may take place) a header field is absent
    964    from a request, it can only match another request if it is also absent
    965    there.
    966 </t>
    967 <t>
    968    A Vary header field-value of "*" always fails to match, and subsequent
    969    requests to that resource can only be properly interpreted by the origin
    970    server.
    971 </t>
    972 <t>
    973    The stored response with matching selecting request-headers is known as the
    974    selected response.
    975 </t>
    976 <t>
    977    If no selected response is available, the cache &MAY; forward the presented
    978    request to the origin server in a conditional request; see <xref
    979    target="validation.model"/>.
     873      semantics, according to the header's specification (e.g., re-ordering field values
     874      when order is not significant; case-normalization, where values are defined to be
     875      case-insensitive)   
     876    </t>
     877  </list>
     878</t>
     879<t>
     880  If (after any normalisation that may take place) a header field is absent
     881  from a request, it can only match another request if it is also absent there.
     882</t>
     883<t>
     884  A Vary header field-value of "*" always fails to match, and subsequent requests to that
     885  resource can only be properly interpreted by the origin server.
     886</t>
     887<t>
     888  The stored response with matching selecting request-headers is known as the
     889  selected response.
     890</t>
     891<t>
     892  If no selected response is available, the cache &MAY; forward the presented
     893  request to the origin server in a conditional request; see <xref target="validation.model"/>.
    980894</t>
    981895</section>
    … …  
    983897<section anchor="combining.headers" title="Combining Responses">
    984898<t>
    985    When a cache receives a 304 (Not Modified) response or a 206 (Partial
    986    Content) response (in this section, the "new" response"), it needs to
    987    created an updated response by combining the stored response with the new
    988    one, so that the updated response can be used to satisfy the request.
    989 </t>
    990 <t>
    991    If the new response contains an ETag, it identifies the stored response to
    992    use. <cref anchor="TODO-mention-CL">may need language about
    993    Content-Location here</cref><cref anchor="TODO-inm-mult-etags">cover case
    994    where INM with multiple etags was sent</cref>
    995 </t>
    996 <t>
    997    If the status code is 206 (partial content), both the stored and new
    998    responses &MUST; have validators, and those validators &MUST; match using
    999    the strong comparison function (see &weak-and-strong;). Otherwise, the
    1000    responses &MUST-NOT; be combined.
    1001 </t>
    1002 <t>
    1003    The stored response headers are used as those of the updated response,
    1004    except that
    1005    <list style="symbols">
    1006       <t>any stored Warning headers with warn-code 1xx (see <xref
    1007       target="header.warning" />) &MUST; be deleted from the stored response
    1008       and the updated response.</t>
    1009       <t>any stored Warning headers with warn-code 2xx &MUST; be retained in
    1010       the stored response and the updated response.</t>
    1011       <t>any headers provided in the new response &MUST; replace the
    1012       corresponding headers from the stored response.</t>
    1013    </list>
    1014 </t>
    1015 <t>
    1016    If a header field-name in the new response matches more than one header in
    1017    the stored response, all such stored headers &MUST; be replaced.
    1018 </t>
    1019 <t>
    1020    The updated response can <cref anchor="TODO-is-req">requirement?</cref> be
    1021    used to replace the stored response in cache. In the case of a 206
    1022    response, the combined entity-body &MAY; be stored.
    1023 </t>
    1024 <t>
    1025    <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
     899  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
     900  (in this section, the "new" response"), it needs to created an updated response by combining
     901  the stored response with the new one, so that the updated response can be used to satisfy the request.
     902</t>
     903<t>
     904  If the new response contains an ETag, it identifies the stored 
     905  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
     906  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
     907</t>
     908<t>
     909  If the status code is 206 (partial content), both the stored and new 
     910  responses &MUST; have validators, and those validators &MUST; match using the strong 
     911  comparison function (see &weak-and-strong;). Otherwise, the 
     912  responses &MUST-NOT; be combined.
     913</t>
     914<t>
     915  The stored response headers are used as those of the updated response, except that
     916  <list style="symbols">
     917    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
     918      &MUST; be deleted from the stored response and the updated response.</t>
     919    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
     920      response and the updated response.</t>
     921    <t>any headers provided in the new response &MUST; replace the corresponding
     922      headers from the stored response.</t>
     923  </list>
     924</t>
     925<t>
     926  If a header field-name in the new response matches more than one 
     927  header in the stored response, all such stored headers &MUST; be replaced.
     928</t>
     929<t>
     930  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
     931  stored response in cache. In the case of a 206 response, the combined 
     932  entity-body &MAY; be stored.
     933</t>
     934<t>
     935  <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
    1026936</t>
    1027937</section>
    … …  
    1031941<section anchor="header.fields" title="Header Field Definitions">
    1032942<t>
    1033    This section defines the syntax and semantics of HTTP/1.1 header fields
    1034    related to caching.
    1035 </t>
    1036 <t>
    1037    For entity-header fields, both sender and recipient refer to either the
    1038    client or the server, depending on who sends and who receives the entity.
     943  This section defines the syntax and semantics of HTTP/1.1 header fields
     944  related to caching.
     945</t>
     946<t>
     947  For entity-header fields, both sender and recipient refer to either the client or the
     948  server, depending on who sends and who receives the entity.
    1039949</t>
    1040950
    1041951<section anchor="header.age" title="Age">
    1042    <iref item="Age header" primary="true" x:for-anchor="" />
    1043    <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
    1044    <x:anchor-alias value="Age"/>
    1045    <x:anchor-alias value="Age-v"/>
    1046    <x:anchor-alias value="age-value"/>
    1047 <t>
    1048    The "Age" response-header field conveys the sender's estimate of the amount
    1049    of time since the response was generated or successfully validated at the
    1050    origin server. Age values are calculated as specified in <xref
    1051   target="age.calculations" />.
     952  <iref item="Age header" primary="true" x:for-anchor="" />
     953  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
     954  <x:anchor-alias value="Age"/>
     955  <x:anchor-alias value="Age-v"/>
     956  <x:anchor-alias value="age-value"/>
     957<t>
     958  The "Age" response-header field conveys the sender's estimate of the amount
     959  of time since the response was generated or successfully validated at the
     960  origin server. Age values are calculated as specified in
     961  <xref target="age.calculations" />.
    1052962</t>
    1053963<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
    … …  
    1056966</artwork></figure>
    1057967<t anchor="rule.delta-seconds">
    1058    <x:anchor-alias value="delta-seconds" />
    1059    Age field-values are non-negative integers, representing time in seconds.
     968  <x:anchor-alias value="delta-seconds" />
     969  Age field-values are non-negative integers, representing time in seconds.
    1060970</t>
    1061971<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
    … …  
    1063973</artwork></figure>
    1064974<t>
    1065    If a cache receives a value larger than the largest positive integer it can
    1066    represent, or if any of its age calculations overflows, it &MUST; transmit
    1067    an Age header with a field-value of 2147483648 (2<x:sup>31</x:sup>). Caches
    1068    &SHOULD; use an arithmetic type of at least 31 bits of range.
    1069 </t>
    1070 <t>
    1071    The presence of an Age header field in a response implies that a response
    1072    is not first-hand. However, the converse is not true, since HTTP/1.0 caches
    1073    may not implement the Age header field.
     975  If a cache receives a value larger than the largest positive integer it can represent, or
     976  if any of its age calculations overflows, it &MUST; transmit an Age header with a
     977  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
     978  of at least 31 bits of range.
     979</t>
     980<t>
     981  The presence of an Age header field in a response implies that a response is not
     982  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
     983  Age header field.
    1074984</t>
    1075985</section>
    1076986
    1077987<section anchor="header.cache-control" title="Cache-Control">
    1078    <iref item="Cache-Control header" primary="true" x:for-anchor="" />
    1079    <iref item="Headers" primary="true" subitem="Cache-Control"
    1080       x:for-anchor="" />
    1081    <x:anchor-alias value="Cache-Control"/>
    1082    <x:anchor-alias value="Cache-Control-v"/>
    1083    <x:anchor-alias value="cache-directive"/>
    1084    <x:anchor-alias value="cache-extension"/>
    1085    <x:anchor-alias value="cache-request-directive"/>
    1086    <x:anchor-alias value="cache-response-directive"/>
    1087 <t>
    1088    The "Cache-Control" general-header field is used to specify directives for
    1089    caches along the request/response chain. Such cache directives are
    1090    unidirectional in that the presence of a directive in a request does not
    1091    imply that the same directive is to be given in the response.
    1092 </t>
    1093 <t>
    1094    HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control
    1095    directives defined in this section. See <xref
    1096    target="cache.control.extensions"/> for information about how Cache-Control
    1097    directives defined elsewhere are handled.
     988  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
     989  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
     990  <x:anchor-alias value="Cache-Control"/>
     991  <x:anchor-alias value="Cache-Control-v"/>
     992  <x:anchor-alias value="cache-directive"/>
     993  <x:anchor-alias value="cache-extension"/>
     994  <x:anchor-alias value="cache-request-directive"/>
     995  <x:anchor-alias value="cache-response-directive"/>
     996<t>
     997  The "Cache-Control" general-header field is used to specify directives for
     998  caches along the request/response chain. Such cache directives are
     999  unidirectional in that the presence of a directive in a request does not
     1000  imply that the same directive is to be given in the response.
     1001</t>
     1002<t>
     1003  HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control directives
     1004  defined in this section. See <xref target="cache.control.extensions"/> for
     1005  information about how Cache-Control directives defined elsewhere are handled.
    10981006</t>
    10991007<x:note>
    11001008  <t>
    11011009    <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
    1102     might only implement Pragma: no-cache (see <xref target="header.pragma"
    1103     />).
     1010    might only implement Pragma: no-cache (see <xref target="header.pragma" />).
    11041011  </t>
    11051012</x:note>
    11061013<t>
    1107    Cache directives &MUST; be passed through by a proxy or gateway
    1108    application, regardless of their significance to that application, since
    1109    the directives might be applicable to all recipients along the
    1110    request/response chain. It is not possible to target a directive to a
    1111    specific cache.
     1014  Cache directives &MUST; be passed through by a proxy or gateway application,
     1015  regardless of their significance to that application, since the directives might be
     1016  applicable to all recipients along the request/response chain. It is not possible to
     1017  target a directive to a specific cache.
    11121018</t>
    11131019<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
    … …  
    11211027</artwork></figure>
    11221028
    1123 <section anchor="cache-request-directive"
    1124    title="Request Cache-Control Directives">
     1029<section anchor="cache-request-directive" title="Request Cache-Control Directives">
    11251030  <x:anchor-alias value="cache-request-directive" />
    11261031
    1127 <figure><artwork type="abnf2616"><iref item="Grammar" primary="true"
    1128    subitem="cache-request-directive" />
     1032<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
    11291033  <x:ref>cache-request-directive</x:ref> =
    11301034       "no-cache"
    … …  
    11391043
    11401044<t>
    1141    <x:dfn>no-cache</x:dfn>
    1142    <iref item="Cache Directives" primary="true" subitem="no-cache" />
    1143    <iref item="no-cache" primary="true" subitem="Cache Directive" />
    1144    <list>
    1145       <t>The no-cache request directive indicates that a stored response
    1146       &MUST-NOT; be used to satisfy the request without successful validation
    1147       on the origin server.</t>
    1148    </list>
    1149 </t>
    1150 <t>
    1151    <x:dfn>no-store</x:dfn>
    1152    <iref item="Cache Directives" primary="true" subitem="no-store" />
    1153    <iref item="no-store" primary="true" subitem="Cache Directive" />
    1154    <list>
    1155       <t>The no-store request directive indicates that a cache &MUST-NOT;
    1156       store any part of either this request or any response to it. This
    1157       directive applies to both non-shared and shared caches. "&MUST-NOT;
    1158       store" in this context means that the cache &MUST-NOT; intentionally
    1159       store the information in non-volatile storage, and &MUST; make a
    1160       best-effort attempt to remove the information from volatile storage as
    1161       promptly as possible after forwarding it.</t>
    1162       <t>This directive is NOT a reliable or sufficient mechanism for ensuring
    1163       privacy. In particular, malicious or compromised caches might not
    1164       recognize or obey this directive, and communications networks may be
    1165       vulnerable to eavesdropping.</t>
    1166   </list>
    1167 </t>
    1168 <t>
    1169    <x:dfn>max-age</x:dfn>
    1170    <iref item="Cache Directives" primary="true" subitem="max-age" />
    1171    <iref item="max-age" primary="true" subitem="Cache Directive" />
    1172    <list>
    1173       <t>The max-age request directive indicates that the client is willing to
    1174       accept a response whose age is no greater than the specified time in
    1175       seconds. Unless the max-stale request directive is also present, the
    1176       client is not willing to accept a stale response.</t>
    1177    </list>
    1178 </t>
    1179 <t>
    1180    <x:dfn>max-stale</x:dfn>
    1181    <iref item="Cache Directives" primary="true" subitem="max-stale" />
    1182    <iref item="max-stale" primary="true" subitem="Cache Directive" />
    1183    <list>
    1184       <t>The max-stale request directive indicates that the client is willing
    1185       to accept a response that has exceeded its expiration time. If max-stale
    1186       is assigned a value, then the client is willing to accept a response
    1187       that has exceeded its expiration time by no more than the specified
    1188       number of seconds. If no value is assigned to max-stale, then the client
    1189       is willing to accept a stale response of any age. <cref
    1190       anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
    1191    </list>
    1192 </t>
    1193 <t>
    1194    <x:dfn>min-fresh</x:dfn>
    1195    <iref item="Cache Directives" primary="true" subitem="min-fresh" />
    1196    <iref item="min-fresh" primary="true" subitem="Cache Directive" />
    1197    <list>
    1198       <t>The min-fresh request directive indicates that the client is willing
    1199       to accept a response whose freshness lifetime is no less than its
    1200       current age plus the specified time in seconds. That is, the client
    1201       wants a response that will still be fresh for at least the specified
    1202       number of seconds.</t>
    1203    </list>
    1204 </t>
    1205 <t>
    1206    <x:dfn>no-transform</x:dfn>
    1207    <iref item="Cache Directives" primary="true" subitem="no-transform" />
    1208    <iref item="no-transform" primary="true" subitem="Cache Directive" />
    1209    <list>
    1210       <t>The no-transform request directive indicates that an intermediate
    1211       cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
    1212       Content-Type request headers, nor the request entity-body.</t>
    1213    </list>
    1214 </t>
    1215 <t>
    1216    <x:dfn>only-if-cached</x:dfn>
    1217    <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
    1218    <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
    1219    <list>
    1220       <t>The only-if-cached request directive indicates that the client only
    1221       wishes to return a stored response. If it receives this directive, a
    1222       cache &SHOULD; either respond using a stored response that is consistent
    1223       with the other constraints of the request, or respond with a 504
    1224       (Gateway Timeout) status. If a group of caches is being operated as a
    1225       unified system with good internal connectivity, such a request &MAY; be
    1226       forwarded within that group of caches.</t>
    1227    </list>
    1228 </t>
    1229 </section>
    1230 
    1231 <section anchor="cache-response-directive"
    1232    title="Response Cache-Control Directives">
    1233    <x:anchor-alias value="cache-response-directive" />
     1045  <x:dfn>no-cache</x:dfn>
     1046  <iref item="Cache Directives" primary="true" subitem="no-cache" />
     1047  <iref item="no-cache" primary="true" subitem="Cache Directive" />
     1048  <list>
     1049    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
     1050      used to satisfy the request without successful validation on the origin server.</t>
     1051  </list>
     1052</t>
     1053<t>
     1054  <x:dfn>no-store</x:dfn>
     1055  <iref item="Cache Directives" primary="true" subitem="no-store" />
     1056  <iref item="no-store" primary="true" subitem="Cache Directive" />
     1057  <list>
     1058    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
     1059      of either this request or any response to it. This directive applies to both
     1060      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
     1061      cache &MUST-NOT; intentionally store the information in non-volatile storage,
     1062      and &MUST; make a best-effort attempt to remove the information from volatile
     1063      storage as promptly as possible after forwarding it.</t>
     1064    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
     1065      particular, malicious or compromised caches might not recognize or obey this
     1066      directive, and communications networks may be vulnerable to eavesdropping.</t>
     1067  </list>
     1068</t>
     1069<t>
     1070  <x:dfn>max-age</x:dfn>
     1071  <iref item="Cache Directives" primary="true" subitem="max-age" />
     1072  <iref item="max-age" primary="true" subitem="Cache Directive" />
     1073  <list>
     1074    <t>The max-age request directive indicates that the client is willing to accept a
     1075      response whose age is no greater than the specified time in seconds. Unless
     1076      the max-stale request directive is also present, the client is not willing to accept a stale
     1077      response.</t>
     1078  </list>
     1079</t>
     1080<t>
     1081  <x:dfn>max-stale</x:dfn>
     1082  <iref item="Cache Directives" primary="true" subitem="max-stale" />
     1083  <iref item="max-stale" primary="true" subitem="Cache Directive" />
     1084  <list>
     1085    <t>The max-stale request directive indicates that the client is willing to accept a
     1086      response that has exceeded its expiration time. If max-stale is assigned a value,
     1087      then the client is willing to accept a response that has exceeded its expiration
     1088      time by no more than the specified number of seconds. If no value is assigned to
     1089      max-stale, then the client is willing to accept a stale response of any age. <cref anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
     1090  </list>
     1091</t>
     1092<t>
     1093  <x:dfn>min-fresh</x:dfn>
     1094  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
     1095  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
     1096  <list>
     1097    <t>The min-fresh request directive indicates that the client is willing to accept a
     1098      response whose freshness lifetime is no less than its current age plus the specified
     1099      time in seconds. That is, the client wants a response that will still be fresh for
     1100      at least the specified number of seconds.</t>
     1101  </list>
     1102</t>
     1103<t>
     1104  <x:dfn>no-transform</x:dfn>
     1105  <iref item="Cache Directives" primary="true" subitem="no-transform" />
     1106  <iref item="no-transform" primary="true" subitem="Cache Directive" />
     1107  <list>
     1108    <t>The no-transform request directive indicates that an intermediate cache or proxy
     1109      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
     1110      headers, nor the request entity-body.</t>
     1111  </list>
     1112</t>
     1113<t>
     1114  <x:dfn>only-if-cached</x:dfn>
     1115  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
     1116  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
     1117  <list>
     1118    <t>The only-if-cached request directive indicates that the client only wishes to
     1119      return a stored response. If it receives this directive, a cache &SHOULD; either
     1120      respond using a stored response that is consistent with the other constraints of the
     1121      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
     1122      being operated as a unified system with good internal connectivity, such a request
     1123      &MAY; be forwarded within that group of caches.</t>
     1124  </list>
     1125</t>
     1126</section>
     1127
     1128<section anchor="cache-response-directive" title="Response Cache-Control Directives">
     1129  <x:anchor-alias value="cache-response-directive" />
    12341130
    12351131<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
    … …  
    12481144
    12491145<t>
    1250    <x:dfn>public</x:dfn>
    1251    <iref item="Cache Directives" primary="true" subitem="public" />
    1252    <iref item="public" primary="true" subitem="Cache Directive" />
    1253    <list>
    1254       <t>The public response directive indicates that the response &MAY; be
    1255       cached, even if it would normally be non-cacheable or cacheable only
    1256       within a non-shared cache. (See also Authorization,
    1257       &header-authorization;, for additional details.) </t>
    1258    </list>
    1259 </t>
    1260 <t>
    1261    <x:dfn>private</x:dfn>
    1262    <iref item="Cache Directives" primary="true" subitem="private" />
    1263    <iref item="private" primary="true" subitem="Cache Directive" />
    1264    <list>
    1265       <t>The private response directive indicates that the response message is
    1266       intended for a single user and &MUST-NOT; be stored by a shared cache. A
    1267       private (non-shared) cache &MAY; store the response.</t>
    1268       <t>If the private response directive specifies one or more field-names,
    1269       this requirement is limited to the field-values associated with the
    1270       listed response headers. That is, the specified field-names(s)
    1271       &MUST-NOT; be stored by a shared cache, whereas the remainder of the
    1272       response message &MAY; be.</t>
    1273       <t><x:h>Note:</x:h> This usage of the word private only controls where
    1274       the response may be stored, and cannot ensure the privacy of the message
    1275       content. Also, private response directives with field-names are often
    1276       handled by implementations as if an unqualified private directive was
    1277       received; i.e., the special handling for the qualified form is not
    1278       widely implemented.</t>
    1279   </list>
    1280 </t>
    1281 <t>
    1282    <x:dfn>no-cache</x:dfn>
    1283    <iref item="Cache Directives" primary="true" subitem="no-cache" />
    1284    <iref item="no-cache" primary="true" subitem="Cache Directive" />
    1285    <list>
    1286       <t>The no-cache response directive indicates that the response
    1287       &MUST-NOT; be used to satisfy a subsequent request without successful
    1288       validation on the origin server. This allows an origin server to prevent
    1289       caching even by caches that have been configured to return stale
    1290       responses.</t>
    1291       <t>If the no-cache response directive specifies one or more field-names,
    1292       this requirement is limited to the field-values associated with the
    1293       listed response headers. That is, the specified field-name(s) &MUST-NOT;
    1294       be sent in the response to a subsequent request without successful
    1295       validation on the origin server. This allows an origin server to prevent
    1296       the re-use of certain header fields in a response, while still allowing
    1297       caching of the rest of the response.</t>
    1298       <t><x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
    1299       directive. Also, no-cache response directives with field-names are often
    1300       handled by implementations as if an unqualified no-cache directive was
    1301       received; i.e., the special handling for the qualified form is not
    1302       widely implemented.</t>
    1303   </list>
    1304 </t>
    1305 <t>
    1306    <x:dfn>no-store</x:dfn>
    1307    <iref item="Cache Directives" primary="true" subitem="no-store" />
    1308    <iref item="no-store" primary="true" subitem="Cache Directive" />
    1309    <list>
    1310       <t>The no-store response directive indicates that a cache &MUST-NOT;
    1311       store any part of either the immediate request or response. This
    1312       directive applies to both non-shared and shared caches. "&MUST-NOT;
    1313       store" in this context means that the cache &MUST-NOT; intentionally
    1314       store the information in non-volatile storage, and &MUST; make a
    1315       best-effort attempt to remove the information from volatile storage as
    1316       promptly as possible after forwarding it.</t>
    1317       <t>This directive is NOT a reliable or sufficient mechanism for ensuring
    1318       privacy. In particular, malicious or compromised caches might not
    1319       recognize or obey this directive, and communications networks may be
    1320       vulnerable to eavesdropping.</t>
    1321    </list>
    1322 </t>
    1323 <t>
    1324    <x:dfn>must-revalidate</x:dfn>
    1325    <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
    1326    <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
    1327    <list>
    1328       <t>The must-revalidate response directive indicates that once it has
    1329       become stale, the response &MUST-NOT; be used to satisfy subsequent
    1330       requests without successful validation on the origin server.</t>
    1331       <t>The must-revalidate directive is necessary to support reliable
    1332       operation for certain protocol features. In all circumstances an
    1333       HTTP/1.1 cache &MUST; obey the must-revalidate directive; in particular,
    1334       if the cache cannot reach the origin server for any reason, it &MUST;
    1335       generate a 504 (Gateway Timeout) response.</t>
    1336       <t>Servers &SHOULD; send the must-revalidate directive if and only if
    1337       failure to validate a request on the entity could result in incorrect
    1338       operation, such as a silently unexecuted financial transaction.</t>
    1339    </list>
    1340 </t>
    1341 <t>
    1342    <x:dfn>proxy-revalidate</x:dfn>
    1343    <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
    1344    <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
    1345    <list>
    1346       <t>The proxy-revalidate response directive has the same meaning as the
    1347       must-revalidate response directive, except that it does not apply to
    1348       non-shared caches.</t>
    1349    </list>
    1350 </t>
    1351 <t>
    1352    <x:dfn>max-age</x:dfn>
    1353    <iref item="Cache Directives" primary="true" subitem="max-age" />
    1354    <iref item="max-age" primary="true" subitem="Cache Directive" />
    1355    <list>
    1356       <t>The max-age response directive indicates that response is to be
    1357       considered stale after its age is greater than the specified number of
    1358       seconds.</t>
    1359    </list>
    1360 </t>
    1361 <t>
    1362    <x:dfn>s-maxage</x:dfn>
    1363    <iref item="Cache Directives" primary="true" subitem="s-maxage" />
    1364    <iref item="s-maxage" primary="true" subitem="Cache Directive" />
    1365    <list>
    1366       <t>The s-maxage response directive indicates that, in shared caches, the
    1367       maximum age specified by this directive overrides the maximum age
    1368       specified by either the max-age directive or the Expires header. The
    1369       s-maxage directive also implies the semantics of the proxy-revalidate
    1370       response directive.</t>
    1371    </list>
    1372 </t>
    1373 <t>
    1374    <x:dfn>no-transform</x:dfn>
    1375    <iref item="Cache Directives" primary="true" subitem="no-transform" />
    1376    <iref item="no-transform" primary="true" subitem="Cache Directive" />
    1377    <list>
    1378       <t>The no-transform response directive indicates that an intermediate
    1379       cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
    1380       Content-Type response headers, nor the response entity-body.</t>
    1381    </list>
     1146  <x:dfn>public</x:dfn>
     1147  <iref item="Cache Directives" primary="true" subitem="public" />
     1148  <iref item="public" primary="true" subitem="Cache Directive" />
     1149  <list>
     1150    <t>The public response directive indicates that the response &MAY; be cached, even
     1151      if it would normally be non-cacheable or cacheable only within a non-shared cache.
     1152      (See also Authorization, &header-authorization;, for additional details.) </t>
     1153  </list>
     1154</t>
     1155<t>
     1156  <x:dfn>private</x:dfn>
     1157  <iref item="Cache Directives" primary="true" subitem="private" />
     1158  <iref item="private" primary="true" subitem="Cache Directive" />
     1159  <list>
     1160    <t>The private response directive indicates that the response message is intended for
     1161      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
     1162      cache &MAY; store the response.</t>
     1163    <t>If the private response directive specifies one or more field-names, this
     1164      requirement is limited to the field-values associated with the listed response
     1165      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
     1166      cache, whereas the remainder of the response message &MAY; be.</t>
     1167    <t>
     1168      <x:h>Note:</x:h> This usage of the word private only controls where the response may
     1169      be stored, and cannot ensure the privacy of the message content.
     1170      Also, private response directives with field-names are often handled by
     1171      implementations as if an unqualified private directive was received; i.e.,
     1172      the special handling for the qualified form is not widely implemented.</t>
     1173  </list>
     1174</t>
     1175<t>
     1176  <x:dfn>no-cache</x:dfn>
     1177  <iref item="Cache Directives" primary="true" subitem="no-cache" />
     1178  <iref item="no-cache" primary="true" subitem="Cache Directive" />
     1179  <list>
     1180    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
     1181      satisfy a subsequent request without successful validation on the origin server.
     1182      This allows an origin server to prevent caching even by caches that have been
     1183      configured to return stale responses.</t>
     1184    <t>If the no-cache response directive specifies one or more field-names, this
     1185      requirement is limited to the field-values associated with the listed response
     1186      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
     1187      to a subsequent request without successful validation on the origin server. This
     1188      allows an origin server to prevent the re-use of certain header fields in a
     1189      response, while still allowing caching of the rest of the response.</t>
     1190    <t>
     1191      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
     1192      Also, no-cache response directives with field-names are often handled by
     1193      implementations as if an unqualified no-cache directive was received; i.e.,
     1194      the special handling for the qualified form is not widely implemented.
     1195    </t>
     1196  </list>
     1197</t>
     1198<t>
     1199  <x:dfn>no-store</x:dfn>
     1200  <iref item="Cache Directives" primary="true" subitem="no-store" />
     1201  <iref item="no-store" primary="true" subitem="Cache Directive" />
     1202  <list>
     1203    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
     1204      part of either the immediate request or response. This directive applies to both
     1205      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
     1206      cache &MUST-NOT; intentionally store the information in non-volatile storage,
     1207      and &MUST; make a best-effort attempt to remove the information from volatile
     1208      storage as promptly as possible after forwarding it.</t>
     1209    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
     1210      particular, malicious or compromised caches might not recognize or obey this
     1211      directive, and communications networks may be vulnerable to eavesdropping.</t>
     1212  </list>
     1213</t>
     1214<t>
     1215  <x:dfn>must-revalidate</x:dfn>
     1216  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
     1217  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
     1218  <list>
     1219    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
     1220     used to satisfy subsequent requests without successful validation on the origin server.</t>
     1221    <t>The must-revalidate directive is necessary to support reliable operation for
     1222      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
     1223      the must-revalidate directive; in particular, if the cache cannot reach the origin
     1224      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
     1225    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
     1226      validate a request on the entity could result in incorrect operation, such as a
     1227      silently unexecuted financial transaction.</t>
     1228  </list>
     1229</t>
     1230<t>
     1231  <x:dfn>proxy-revalidate</x:dfn>
     1232  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
     1233  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
     1234  <list>
     1235    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
     1236      response directive, except that it does not apply to non-shared caches.</t>
     1237  </list>
     1238</t>
     1239<t>
     1240  <x:dfn>max-age</x:dfn>
     1241  <iref item="Cache Directives" primary="true" subitem="max-age" />
     1242  <iref item="max-age" primary="true" subitem="Cache Directive" />
     1243  <list>
     1244    <t>The max-age response directive indicates that response is to be considered stale
     1245      after its age is greater than the specified number of seconds.</t>
     1246  </list>
     1247</t>
     1248<t>
     1249  <x:dfn>s-maxage</x:dfn>
     1250  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
     1251  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
     1252  <list>
     1253    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
     1254      specified by this directive overrides the maximum age specified by either the
     1255      max-age directive or the Expires header. The s-maxage directive also implies the
     1256      semantics of the proxy-revalidate response directive.</t>
     1257  </list>
     1258</t>
     1259<t>
     1260  <x:dfn>no-transform</x:dfn>
     1261  <iref item="Cache Directives" primary="true" subitem="no-transform" />
     1262  <iref item="no-transform" primary="true" subitem="Cache Directive" />
     1263  <list>
     1264    <t>The no-transform response directive indicates that an intermediate cache or proxy
     1265      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
     1266      headers, nor the response entity-body.</t>
     1267  </list>
    13821268</t>
    13831269
    … …  
    13861272<section anchor="cache.control.extensions" title="Cache Control Extensions">
    13871273<t>
    1388    The Cache-Control header field can be extended through the use of one or
    1389    more cache-extension tokens, each with an optional value. Informational
    1390    extensions (those that do not require a change in cache behavior) can be
    1391    added without changing the semantics of other directives. Behavioral
    1392    extensions are designed to work by acting as modifiers to the existing base
    1393    of cache directives. Both the new directive and the standard directive are
    1394    supplied, such that applications that do not understand the new directive
    1395    will default to the behavior specified by the standard directive, and those
    1396    that understand the new directive will recognize it as modifying the
    1397    requirements associated with the standard directive. In this way,
    1398    extensions to the cache-control directives can be made without requiring
    1399    changes to the base protocol.
    1400 </t>
    1401 <t>
    1402    This extension mechanism depends on an HTTP cache obeying all of the
    1403    cache-control directives defined for its native HTTP-version, obeying
    1404    certain extensions, and ignoring all directives that it does not
    1405    understand.
    1406 </t>
    1407 <t>
    1408    For example, consider a hypothetical new response directive called
    1409    "community" that acts as a modifier to the private directive. We define
    1410    this new directive to mean that, in addition to any non-shared cache, any
    1411    cache that is shared only by members of the community named within its
    1412    value may cache the response. An origin server wishing to allow the UCI
    1413    community to use an otherwise private response in their shared cache(s)
    1414    could do so by including
     1274  The Cache-Control header field can be extended through the use of one or more
     1275  cache-extension tokens, each with an optional value. Informational extensions (those
     1276  that do not require a change in cache behavior) can be added without changing the
     1277  semantics of other directives. Behavioral extensions are designed to work by acting as
     1278  modifiers to the existing base of cache directives. Both the new directive and the
     1279  standard directive are supplied, such that applications that do not understand the new
     1280  directive will default to the behavior specified by the standard directive, and those
     1281  that understand the new directive will recognize it as modifying the requirements
     1282  associated with the standard directive. In this way, extensions to the cache-control
     1283  directives can be made without requiring changes to the base protocol.
     1284</t>
     1285<t>
     1286  This extension mechanism depends on an HTTP cache obeying all of the cache-control
     1287  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
     1288  all directives that it does not understand.
     1289</t>
     1290<t>
     1291  For example, consider a hypothetical new response directive called "community" that
     1292  acts as a modifier to the private directive. We define this new directive to mean that,
     1293  in addition to any non-shared cache, any cache that is shared only by members of the
     1294  community named within its value may cache the response. An origin server wishing to
     1295  allow the UCI community to use an otherwise private response in their shared cache(s)
     1296  could do so by including
    14151297</t>
    14161298<figure><artwork type="example">
    … …  
    14181300</artwork></figure>
    14191301<t>
    1420    A cache seeing this header field will act correctly even if the cache does
    1421    not understand the community cache-extension, since it will also see and
    1422    understand the private directive and thus default to the safe behavior.
    1423 </t>
    1424 <t>
    1425    Unrecognized cache directives &MUST; be ignored; it is assumed that any
    1426    cache directive likely to be unrecognized by an HTTP/1.1 cache will be
    1427    combined with standard directives (or the response's default cacheability)
    1428    such that the cache behavior will remain minimally correct even if the
    1429    cache does not understand the extension(s).
    1430 </t>
    1431 <t>
    1432    The HTTP Cache Directive Registry defines the name space for the cache
    1433    directives.
    1434 </t>
    1435 <t>
    1436    Registrations &MUST; include the following fields:
    1437    <list style="symbols">
    1438       <t>Cache Directive Name</t>
    1439       <t>Pointer to specification text</t>
    1440    </list>
    1441 </t>
    1442 <t>
    1443    Values to be added to this name space are subject to IETF review (<xref
    1444    target="RFC5226" x:fmt="," x:sec="4.1"/>).
    1445 </t>
    1446 <t>
    1447    The registry itself is maintained at <eref
    1448    target="http://www.iana.org/assignments/http-cache-directives"/>.
     1302  A cache seeing this header field will act correctly even if the cache does not
     1303  understand the community cache-extension, since it will also see and understand the
     1304  private directive and thus default to the safe behavior.
     1305</t>
     1306<t>
     1307  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
     1308  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
     1309  directives (or the response's default cacheability) such that the cache behavior will
     1310  remain minimally correct even if the cache does not understand the extension(s).
     1311</t>
     1312<t>
     1313  The HTTP Cache Directive Registry defines the name space for the cache
     1314  directives.
     1315</t>
     1316<t>
     1317  Registrations &MUST; include the following fields:
     1318  <list style="symbols">
     1319    <t>Cache Directive Name</t>
     1320    <t>Pointer to specification text</t>
     1321  </list>
     1322</t>
     1323<t>
     1324  Values to be added to this name space are subject to IETF review
     1325  (<xref target="RFC5226" x:fmt="," x:sec="4.1"/>).
     1326</t>
     1327<t>
     1328  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
    14491329</t>
    14501330</section>
    … …  
    14581338  <x:anchor-alias value="Expires-v"/>
    14591339<t>
    1460    The "Expires" entity-header field gives the date/time after which the
    1461    response is considered stale. See <xref target="expiration.model" /> for
    1462    further discussion of the freshness model.
    1463 </t>
    1464 <t>
    1465    The presence of an Expires field does not imply that the original resource
    1466    will change or cease to exist at, before, or after that time.
    1467 </t>
    1468 <t>
    1469    The field-value is an absolute date and time as defined by HTTP-date in
    1470    &full-date;; it &MUST; be sent in rfc1123-date format.
     1340  The "Expires" entity-header field gives the date/time after which the response is
     1341  considered stale. See <xref target="expiration.model" /> for further discussion of the
     1342  freshness model.
     1343</t>
     1344<t>
     1345  The presence of an Expires field does not imply that the original resource will change or
     1346  cease to exist at, before, or after that time.
     1347</t>
     1348<t>
     1349  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
     1350  it &MUST; be sent in rfc1123-date format.
    14711351</t>
    14721352<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
    … …  
    14801360</artwork></figure>
    14811361<x:note>
    1482    <t>
    1483        <x:h>Note:</x:h> If a response includes a Cache-Control field with the
    1484        max-age directive (see <xref target="cache-response-directive" />),
    1485        that directive overrides the Expires field. Likewise, the s-maxage
    1486        directive overrides Expires in shared caches.
    1487    </t>
     1362  <t>
     1363    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
     1364    directive (see <xref target="cache-response-directive" />), that directive overrides
     1365    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
     1366  </t>
    14881367</x:note>
    14891368<t>
    1490    HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the
    1491    future.
    1492 </t>
    1493 <t>
    1494    HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
    1495    especially including the value "0", as in the past (i.e., "already
    1496    expired").
     1369  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
     1370</t>
     1371<t>
     1372  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
     1373  including the value "0", as in the past (i.e., "already expired").
    14971374</t>
    14981375</section>
    … …  
    15061383  <x:anchor-alias value="pragma-directive"/>
    15071384<t>
    1508    The "Pragma" general-header field is used to include
    1509    implementation-specific directives that might apply to any recipient along
    1510    the request/response chain. All pragma directives specify optional behavior
    1511    from the viewpoint of the protocol; however, some systems &MAY; require
    1512    that behavior be consistent with the directives.
     1385  The "Pragma" general-header field is used to include implementation-specific directives
     1386  that might apply to any recipient along the request/response chain. All pragma directives
     1387  specify optional behavior from the viewpoint of the protocol; however, some systems
     1388  &MAY; require that behavior be consistent with the directives.
    15131389</t>
    15141390<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
    … …  
    15191395</artwork></figure>
    15201396<t>
    1521    When the no-cache directive is present in a request message, an application
    1522    &SHOULD; forward the request toward the origin server even if it has a
    1523    cached copy of what is being requested. This pragma directive has the same
    1524    semantics as the no-cache response directive (see <xref
    1525    target="cache-response-directive" />) and is defined here for backward
    1526    compatibility with HTTP/1.0. Clients &SHOULD; include both header fields
    1527    when a no-cache request is sent to a server not known to be HTTP/1.1
    1528    compliant. HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the
    1529    client had sent "Cache-Control: no-cache".
     1397  When the no-cache directive is present in a request message, an application &SHOULD;
     1398  forward the request toward the origin server even if it has a cached copy of what is being
     1399  requested. This pragma directive has the same semantics as the no-cache response directive
     1400  (see <xref target="cache-response-directive" />) and is defined here for backward
     1401  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
     1402  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
     1403  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
    15301404</t>
    15311405<x:note>
    1532    <t>
    1533       <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a
    1534       response-header field is not actually specified, it does not provide a
    1535       reliable replacement for "Cache-Control: no-cache" in a response.
    1536    </t>
     1406  <t>
     1407    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
     1408    is not actually specified, it does not provide a reliable replacement for
     1409    "Cache-Control: no-cache" in a response.
     1410  </t>
    15371411</x:note>
    15381412<t>
    1539    This mechanism is deprecated; no new Pragma directives will be defined in
    1540    HTTP.
     1413  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
    15411414</t>
    15421415</section>
    … …  
    15481421  <x:anchor-alias value="Vary-v"/>
    15491422<t>
    1550    The "Vary" response-header field conveys the set of request-header fields
    1551    that were used to select the representation.
    1552 </t>
    1553 <t>
    1554    Caches use this information, in part, to determine whether a stored
    1555    response can be used to satisfy a given request; see <xref
    1556    target="caching.negotiated.responses" />. determines, while the response is
    1557    fresh, whether a cache is permitted to use the response to reply to a
    1558    subsequent request without validation; see <xref
    1559    target="caching.negotiated.responses" />.
    1560 </t>
    1561 <t>
    1562    In uncacheable or stale responses, the Vary field value advises the user
    1563    agent about the criteria that were used to select the representation.
     1423  The "Vary" response-header field conveys the set of request-header fields
     1424  that were used to select the representation.
     1425</t>
     1426<t>
     1427  Caches use this information, in part, to determine whether a stored response
     1428  can be used to satisfy a given request; see
     1429  <xref target="caching.negotiated.responses" />.
     1430  determines, while the response is fresh, whether a cache is permitted to use the
     1431  response to reply to a subsequent request without validation; see <xref
     1432  target="caching.negotiated.responses" />.
     1433</t>
     1434<t>
     1435  In uncacheable or stale responses, the Vary field value advises the user agent about
     1436  the criteria that were used to select the representation.
    15641437</t>
    15651438<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
    … …  
    15681441</artwork></figure>
    15691442<t>
    1570    The set of header fields named by the Vary field value is known as the
    1571    selecting request-headers.
    1572 </t>
    1573 <t>
    1574    Servers &SHOULD; include a Vary header field with any cacheable response
    1575    that is subject to server-driven negotiation. Doing so allows a cache to
    1576    properly interpret future requests on that resource and informs the user
    1577    agent about the presence of negotiation on that resource. A server &MAY;
    1578    include a Vary header field with a non-cacheable response that is subject
    1579    to server-driven negotiation, since this might provide the user agent with
    1580    useful information about the dimensions over which the response varies at
    1581    the time of the response.
    1582 </t>
    1583 <t>
    1584    A Vary field value of "*" signals that unspecified parameters not limited
    1585    to the request-headers (e.g., the network address of the client), play a
    1586    role in the selection of the response representation; therefore, a cache
    1587    cannot determine whether this response is appropriate. The "*" value
    1588    &MUST-NOT; be generated by a proxy server; it may only be generated by an
    1589    origin server.
    1590 </t>
    1591 <t>
    1592    The field-names given are not limited to the set of standard request-header
    1593    fields defined by this specification. Field names are case-insensitive.
     1443  The set of header fields named by the Vary field value is known as the selecting
     1444  request-headers.
     1445</t>
     1446<t>
     1447  Servers &SHOULD; include a Vary header field with any cacheable response that is
     1448  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
     1449  requests on that resource and informs the user agent about the presence of negotiation on
     1450  that resource. A server &MAY; include a Vary header field with a non-cacheable
     1451  response that is subject to server-driven negotiation, since this might provide the user
     1452  agent with useful information about the dimensions over which the response varies at the
     1453  time of the response.
     1454</t>
     1455<t>
     1456  A Vary field value of "*" signals that unspecified parameters not limited to the
     1457  request-headers (e.g., the network address of the client), play a role in the selection of
     1458  the response representation; therefore, a cache cannot determine whether this response is
     1459  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
     1460  it may only be generated by an origin server.
     1461</t>
     1462<t>
     1463  The field-names given are not limited to the set of standard request-header fields
     1464  defined by this specification. Field names are case-insensitive.
    15941465</t>
    15951466</section>
    … …  
    16061477  <x:anchor-alias value="warn-text"/>
    16071478<t>
    1608    The "Warning" general-header field is used to carry additional information
    1609    about the status or transformation of a message that might not be reflected
    1610    in the message. This information is typically used to warn about possible
    1611    incorrectness introduced by caching operations or transformations applied
    1612    to the entity body of the message.
    1613 </t>
    1614 <t>
    1615    Warnings can be used for other purposes, both cache-related and otherwise.
    1616    The use of a warning, rather than an error status code, distinguishes these
    1617    responses from true failures.
    1618 </t>
    1619 <t>
    1620    Warning headers can in general be applied to any message, however some
    1621    warn-codes are specific to caches and can only be applied to response
    1622    messages.
     1479  The "Warning" general-header field is used to carry additional information about the status
     1480  or transformation of a message that might not be reflected in the message. This
     1481  information is typically used to warn about possible incorrectness introduced by caching
     1482  operations or transformations applied to the entity body of the message.
     1483</t>
     1484<t>
     1485  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
     1486  warning, rather than an error status code, distinguishes these responses from true failures.
     1487</t>
     1488<t>
     1489  Warning headers can in general be applied to any message, however some warn-codes are
     1490  specific to caches and can only be applied to response messages.
    16231491</t>
    16241492<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
    … …  
    16371505</artwork></figure>
    16381506<t>
    1639    Multiple warnings can be attached to a response (either by the origin
    1640    server or by a cache), including multiple warnings with the same code
    1641    number, only differing in warn-text.
    1642 </t>
    1643 <t>
    1644    When this occurs, the user agent &SHOULD; inform the user of as many of
    1645    them as possible, in the order that they appear in the response.
    1646 </t>
    1647 <t>
    1648    Systems that generate multiple Warning headers &SHOULD; order them with
    1649    this user agent behavior in mind. New Warning headers &SHOULD; be added
    1650    after any existing Warning headers.
    1651 </t>
    1652 <t>
    1653    Warnings are assigned three digit warn-codes. The first digit indicates
    1654    whether the Warning is required to be deleted from a stored response after
    1655    validation:
    1656    <list style="symbols">
    1657       <t>1xx Warnings describe the freshness or validation status of the
    1658       response, and so &MUST; be deleted by caches after validation. They can
    1659       only be generated by a cache when validating a cached entry, and
    1660       &MUST-NOT; be generated in any other situation.</t> <t>2xx Warnings
    1661       describe some aspect of the entity body or entity headers that is not
    1662       rectified by a validation (for example, a lossy compression of the
    1663       entity bodies) and &MUST-NOT; be deleted by caches after validation,
    1664       unless a full response is returned, in which case they &MUST; be.</t>
    1665    </list>
    1666 </t>
    1667 <t>
    1668    If an implementation sends a message with one or more Warning headers to a
    1669    receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
    1670    in each warning-value a warn-date that matches the Date header in the
    1671    message.
    1672 </t>
    1673 <t>
    1674    If an implementation receives a message with a warning-value that includes
    1675    a warn-date, and that warn-date is different from the Date value in the
    1676    response, then that warning-value &MUST; be deleted from the message before
    1677    storing, forwarding, or using it. (preventing the consequences of naive
    1678    caching of Warning header fields.) If all of the warning-values are deleted
    1679    for this reason, the Warning header &MUST; be deleted as well.
    1680 </t>
    1681 <t>
    1682    The following warn-codes are defined by this specification, each with a
    1683    recommended warn-text in English, and a description of its meaning.
     1507  Multiple warnings can be attached to a response (either by the origin server or by
     1508  a cache), including multiple warnings with the same code number, only differing
     1509  in warn-text.
     1510</t>
     1511<t>
     1512  When this occurs, the user agent &SHOULD; inform the user of as many of them as
     1513  possible, in the order that they appear in the response.
     1514</t>
     1515<t>
     1516  Systems that generate multiple Warning headers &SHOULD; order them with this user
     1517  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
     1518  Warning headers.
     1519</t>
     1520<t>
     1521  Warnings are assigned three digit warn-codes. The first digit indicates whether the
     1522  Warning is required to be deleted from a stored response after validation:
     1523  <list style="symbols">
     1524    <t>1xx Warnings describe the freshness or validation status of the response, and so
     1525      &MUST; be deleted by caches after validation. They can only be generated by a cache
     1526      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
     1527    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
     1528      not rectified by a validation (for example, a lossy compression of the entity bodies)
     1529      and &MUST-NOT; be deleted by caches after validation, unless a full response is
     1530      returned, in which case they &MUST; be.</t>
     1531  </list>
     1532</t>
     1533<t>
     1534  If an implementation sends a message with one or more Warning headers to a receiver whose
     1535  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
     1536  warn-date that matches the Date header in the message.
     1537</t>
     1538<t>
     1539  If an implementation receives a message with a warning-value that includes a warn-date,
     1540  and that warn-date is different from the Date value in the response, then that
     1541  warning-value &MUST; be deleted from the message before storing, forwarding, or using
     1542  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
     1543  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
     1544  well.
     1545</t>
     1546<t>
     1547  The following warn-codes are defined by this specification, each with a recommended
     1548  warn-text in English, and a description of its meaning.
    16841549</t>
    16851550<t><?rfc needLines="4"?>
    1686    110 Response is stale
    1687    <list>
    1688       <t>&SHOULD; be included whenever the returned response is stale.</t>
    1689    </list>
     1551  110 Response is stale
     1552  <list>
     1553    <t>&SHOULD; be included whenever the returned response is stale.</t>
     1554  </list>
    16901555</t>
    16911556<t><?rfc needLines="4"?>
    1692    111 Revalidation failed
    1693    <list>
    1694       <t>&SHOULD; be included if a cache returns a stale response because an
    1695       attempt to validate the response failed, due to an inability to reach
    1696       the server.</t>
     1557  111 Revalidation failed
     1558  <list>
     1559    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
     1560      validate the response failed, due to an inability to reach the server.</t>
    16971561  </list>
    16981562</t>
    16991563<t><?rfc needLines="4"?>
    1700    112 Disconnected operation
    1701    <list>
    1702       <t>&SHOULD; be included if the cache is intentionally disconnected from
    1703       the rest of the network for a period of time.</t>
    1704    </list>
     1564  112 Disconnected operation
     1565  <list>
     1566    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
     1567      the network for a period of time.</t>
     1568  </list>
    17051569</t>
    17061570<t><?rfc needLines="4"?>
    1707    113 Heuristic expiration
    1708    <list>
    1709       <t>&SHOULD; be included if the cache heuristically chose a freshness
    1710       lifetime greater than 24 hours and the response's age is greater than 24
    1711       hours.</t>
    1712    </list>
     1571  113 Heuristic expiration
     1572  <list>
     1573    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
     1574      greater than 24 hours and the response's age is greater than 24 hours.</t>
     1575  </list>
    17131576</t>
    17141577<t><?rfc needLines="4"?>
    1715    199 Miscellaneous warning
    1716    <list>
    1717       <t>The warning text can include arbitrary information to be presented to
    1718       a human user, or logged. A system receiving this warning &MUST-NOT; take
    1719       any automated action, besides presenting the warning to the user.</t>
    1720    </list>
     1578  199 Miscellaneous warning
     1579  <list>
     1580    <t>The warning text can include arbitrary information to be presented to a human
     1581      user, or logged. A system receiving this warning &MUST-NOT; take any automated
     1582      action, besides presenting the warning to the user.</t>
     1583  </list>
    17211584</t>
    17221585<t><?rfc needLines="4"?>
    1723    214 Transformation applied
    1724    <list>
    1725       <t>&MUST; be added by an intermediate cache or proxy if it applies any
    1726       transformation changing the content-coding (as specified in the
    1727       Content-Encoding header) or media-type (as specified in the Content-Type
    1728       header) of the response, or the entity-body of the response, unless this
    1729       Warning code already appears in the response.</t>
    1730    </list>
     1586  214 Transformation applied
     1587  <list>
     1588    <t>&MUST; be added by an intermediate cache or proxy if it applies any
     1589      transformation changing the content-coding (as specified in the Content-Encoding
     1590      header) or media-type (as specified in the Content-Type header) of the response, or
     1591      the entity-body of the response, unless this Warning code already appears in the
     1592      response.</t>
     1593  </list>
    17311594</t>
    17321595<t><?rfc needLines="4"?>
    1733    299 Miscellaneous persistent warning
    1734    <list>
    1735        <t>The warning text can include arbitrary information to be presented
    1736        to a human user, or logged. A system receiving this warning &MUST-NOT;
    1737        take any automated action.</t>
    1738    </list>
     1596  299 Miscellaneous persistent warning
     1597  <list>
     1598    <t>The warning text can include arbitrary information to be presented to a human
     1599      user, or logged. A system receiving this warning &MUST-NOT; take any automated
     1600      action.</t>
     1601  </list>
    17391602</t>
    17401603</section>
    … …  
    17441607<section anchor="history.lists" title="History Lists">
    17451608<t>
    1746    User agents often have history mechanisms, such as "Back" buttons and
    1747    history lists, that can be used to redisplay an entity retrieved earlier in
    1748    a session.
    1749 </t>
    1750 <t>
    1751    The freshness model (<xref target="expiration.model"/>) does not
    1752    necessarily apply to history mechanisms. I.e., a history mechanism can
    1753    display a previous representation even if it has expired.
    1754 </t>
    1755 <t>
    1756    This does not prohibit the history mechanism from telling the user that a
    1757    view might be stale, or from honoring cache directives (e.g.,
    1758    Cache-Control: no-store).
    1759 </t>
     1609  User agents often have history mechanisms, such as "Back" buttons and history lists, that
     1610  can be used to redisplay an entity retrieved earlier in a session.
     1611</t>
     1612<t>
     1613  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
     1614  a history mechanism can display a previous representation even if it has expired.
     1615</t>
     1616  <t>
     1617  This does not prohibit the history mechanism from telling the user that a
     1618  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
     1619  </t>
    17601620</section>
    17611621
    … …  
    17631623<section anchor="IANA.considerations" title="IANA Considerations">
    17641624
    1765 <section title="Cache Directive Registry"
    1766    anchor="cache.directive.registration">
    1767 <t>
    1768    The registration procedure for HTTP Cache Directives is defined by <xref
    1769    target="cache.control.extensions"/> of this document.
    1770 </t>
    1771 <t>
    1772    The HTTP Cache Directive Registry should be created at <eref
    1773    target="http://www.iana.org/assignments/http-cache-directives"/> and be
    1774    populated with the registrations below:
     1625<section title="Cache Directive Registry" anchor="cache.directive.registration">
     1626<t>
     1627  The registration procedure for HTTP Cache Directives is defined by
     1628  <xref target="cache.control.extensions"/> of this document.
     1629</t>
     1630<t>
     1631   The HTTP Cache Directive Registry should be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/>
     1632   and be populated with the registrations below:
    17751633</t>
    17761634<?BEGININC p6-cache.cache-directives ?>
    … …  
    18961754<?ENDINC p6-cache.iana-headers ?>
    18971755<t>
    1898    The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
    1899    Force".
     1756  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
    19001757</t>
    19011758</section>
    … …  
    19051762<section anchor="security.considerations" title="Security Considerations">
    19061763<t>
    1907    Caches expose additional potential vulnerabilities, since the contents of
    1908    the cache represent an attractive target for malicious exploitation.
    1909    Because cache contents persist after an HTTP request is complete, an attack
    1910    on the cache can reveal information long after a user believes that the
    1911    information has been removed from the network. Therefore, cache contents
    1912    should be protected as sensitive information. </t>
     1764  Caches expose additional potential vulnerabilities, since the contents of the cache
     1765  represent an attractive target for malicious exploitation. Because cache contents persist
     1766  after an HTTP request is complete, an attack on the cache can reveal information long after
     1767  a user believes that the information has been removed from the network. Therefore, cache
     1768  contents should be protected as sensitive information.
     1769</t>
    19131770</section>
    19141771
    19151772<section anchor="ack" title="Acknowledgments">
    19161773<t>
    1917    Much of the content and presentation of the caching design is due to
    1918    suggestions and comments from individuals including: Shel Kaphan, Paul
    1919    Leach, Koen Holtman, David Morris, and Larry Masinter.
     1774  Much of the content and presentation of the caching design is due to suggestions and
     1775  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
     1776  and Larry Masinter.
    19201777</t>
    19211778</section>
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