Changeset 945 for draft-ietf-httpbis/10


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Timestamp:
Jul 27, 2010, 7:38:11 AM (9 years ago)
Author:
mnot@…
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reformat xml

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1 edited

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

    r848 r945  
    4646<?rfc-ext allow-markup-in-artwork="yes" ?>
    4747<?rfc-ext include-references-in-index="yes" ?>
    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">
     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">
    5053<front>
    5154
    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>
     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>
    200207
    201208<abstract>
    202209<t>
    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.
     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.
    208216</t>
    209217</abstract>
    210218
    211219<note title="Editorial Note (To be removed by RFC Editor)">
    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>
     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>
    222231</note>
    223232
     
    227236<section anchor="caching" title="Introduction">
    228237<t>
    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.
     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.
    232242</t>
    233243
     
    235245<iref item="cache" />
    236246<t>
    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" />).
     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" />).
    252265</t>
    253266</section>
     
    255268<section anchor="intro.terminology" title="Terminology">
    256269<t>
    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>
     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>
    315330</t>
    316331<t>
     
    318333  <x:dfn>stale</x:dfn>
    319334  <list>
    320     <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
     335      <t>A response is stale if its age has passed its freshness lifetime
     336      (either explicit or heuristic).</t>
    321337  </list>
    322338</t>
    323339<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>
     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>
    330347</t>
    331348<t anchor="shared.and.non-shared.caches">
    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>
     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>
    338355</t>
    339356</section>
     
    346363</t>
    347364<t>
    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
     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
    350367   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
    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".
     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".
    355372</t>
    356373</section>
    357374
    358375<section title="Syntax Notation" anchor="notation">
    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).
     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).
    383398</t>
    384399
    385400<section title="Core Rules" anchor="core.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;:
     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;:
    391406</t>
    392407<figure><artwork type="abnf2616">
     
    397412</section>
    398413
    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:
     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:
    407423</t>
    408424<figure><!--Part1--><artwork type="abnf2616">
     
    422438<section anchor="response.cacheability" title="Response Cacheability">
    423439<t>
    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>
     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>
    445470  </list>
    446471</t>
    447472<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.
     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.
    523559</t>
    524560</section>
     
    526562<section anchor="expiration.model" title="Freshness Model">
    527563<t>
    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.
     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.
    551591</t>
    552592<figure>
     
    559599</figure>
    560600<t>
    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
     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
    585632      target="cache-response-directive" />) is present, use its value, or</t>
    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>
     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>
    592639  </list>
    593640</t>
    594641<t>
    595   Note that this calculation is not vulnerable to clock skew, since all of the
    596   information comes from the origin server.
     642   Note that this calculation is not vulnerable to clock skew, since all of
     643   the information comes from the origin server.
    597644</t>
    598645
    599646<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
    600647<t>
    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%.
     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%.
    614664</t>
    615665<x:note>
    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>
     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>
    624674</x:note>
    625675</section>
     
    628678<section anchor="age.calculations" title="Calculating Age">
    629679<t>
    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>
     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>
    647712  </list>
    648713</t>
    649714<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>
     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>
    701755</t>
    702756<figure>
     
    714768</artwork></figure>
    715769<t>
    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.
     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.
    719773</t>
    720774<figure><artwork type="code">
     
    726780<section anchor="serving.stale.responses" title="Serving Stale Responses">
    727781<t>
    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.
     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.
    755812</t>
    756813</section>
     
    759816<section anchor="validation.model" title="Validation Model">
    760817<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>
     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>
    865948      adding or removing whitespace, where allowed in the header's syntax
    866     </t>
    867     <t>
     949      </t>
     950      <t>
    868951      combining multiple message-header fields with the same field name (see
    869952      &header-fields;)
    870     </t>
    871     <t>
     953      </t>
     954      <t>
    872955      normalizing both header values in a way that is known to have identical
    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"/>.
     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"/>.
    894980</t>
    895981</section>
     
    897983<section anchor="combining.headers" title="Combining Responses">
    898984<t>
    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>
     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>
    9361026</t>
    9371027</section>
     
    9411031<section anchor="header.fields" title="Header Field Definitions">
    9421032<t>
    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.
     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.
    9491039</t>
    9501040
    9511041<section anchor="header.age" title="Age">
    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" />.
     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" />.
    9621052</t>
    9631053<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
     
    9661056</artwork></figure>
    9671057<t anchor="rule.delta-seconds">
    968   <x:anchor-alias value="delta-seconds" />
    969   Age field-values are non-negative integers, representing time in seconds.
     1058   <x:anchor-alias value="delta-seconds" />
     1059   Age field-values are non-negative integers, representing time in seconds.
    9701060</t>
    9711061<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
     
    9731063</artwork></figure>
    9741064<t>
    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.
     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.
    9841074</t>
    9851075</section>
    9861076
    9871077<section anchor="header.cache-control" title="Cache-Control">
    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.
     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.
    10061098</t>
    10071099<x:note>
    10081100  <t>
    10091101    <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
    1010     might only implement Pragma: no-cache (see <xref target="header.pragma" />).
     1102    might only implement Pragma: no-cache (see <xref target="header.pragma"
     1103    />).
    10111104  </t>
    10121105</x:note>
    10131106<t>
    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.
     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.
    10181112</t>
    10191113<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"/>
     
    10271121</artwork></figure>
    10281122
    1029 <section anchor="cache-request-directive" title="Request Cache-Control Directives">
     1123<section anchor="cache-request-directive"
     1124   title="Request Cache-Control Directives">
    10301125  <x:anchor-alias value="cache-request-directive" />
    10311126
    1032 <figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
     1127<figure><artwork type="abnf2616"><iref item="Grammar" primary="true"
     1128   subitem="cache-request-directive" />
    10331129  <x:ref>cache-request-directive</x:ref> =
    10341130       "no-cache"
     
    10431139
    10441140<t>
    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>
     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>
    10511166  </list>
    10521167</t>
    10531168<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" />
     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" />
    11301234
    11311235<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
     
    11441248
    11451249<t>
    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>
     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>
    11531279  </list>
    11541280</t>
    11551281<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>
     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>
    11731303  </list>
    11741304</t>
    11751305<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>
     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>
    12681382</t>
    12691383
     
    12721386<section anchor="cache.control.extensions" title="Cache Control Extensions">
    12731387<t>
    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
     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
    12971415</t>
    12981416<figure><artwork type="example">
     
    13001418</artwork></figure>
    13011419<t>
    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"/>.
     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"/>.
    13291449</t>
    13301450</section>
     
    13381458  <x:anchor-alias value="Expires-v"/>
    13391459<t>
    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.
     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.
    13511471</t>
    13521472<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
     
    13601480</artwork></figure>
    13611481<x:note>
    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>
     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>
    13671488</x:note>
    13681489<t>
    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").
     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").
    13741497</t>
    13751498</section>
     
    13831506  <x:anchor-alias value="pragma-directive"/>
    13841507<t>
    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.
     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.
    13891513</t>
    13901514<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"/>
     
    13951519</artwork></figure>
    13961520<t>
    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".
     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".
    14041530</t>
    14051531<x:note>
    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>
     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>
    14111537</x:note>
    14121538<t>
    1413   This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
     1539   This mechanism is deprecated; no new Pragma directives will be defined in
     1540   HTTP.
    14141541</t>
    14151542</section>
     
    14211548  <x:anchor-alias value="Vary-v"/>
    14221549<t>
    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.
     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.
    14371564</t>
    14381565<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
     
    14411568</artwork></figure>
    14421569<t>
    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.
     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.
    14651594</t>
    14661595</section>
     
    14771606  <x:anchor-alias value="warn-text"/>
    14781607<t>
    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.
     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.
    14911623</t>
    14921624<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"/>
     
    15051637</artwork></figure>
    15061638<t>
    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>
     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.
     1684</t>
     1685<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>
     1690</t>
     1691<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>
    15311697  </list>
    15321698</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.
    1549 </t>
    15501699<t><?rfc needLines="4"?>
    1551   110 Response is stale
    1552   <list>
    1553     <t>&SHOULD; be included whenever the returned response is stale.</t>
    1554   </list>
     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>
    15551705</t>
    15561706<t><?rfc needLines="4"?>
    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>
    1561   </list>
     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>
    15621713</t>
    15631714<t><?rfc needLines="4"?>
    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>
     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>
    15691721</t>
    15701722<t><?rfc needLines="4"?>
    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>
     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>
    15761731</t>
    15771732<t><?rfc needLines="4"?>
    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>
    1584 </t>
    1585 <t><?rfc needLines="4"?>
    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>
    1594 </t>
    1595 <t><?rfc needLines="4"?>
    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>
     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>
    16021739</t>
    16031740</section>
     
    16071744<section anchor="history.lists" title="History Lists">
    16081745<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>
     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>
    16201760</section>
    16211761
     
    16231763<section anchor="IANA.considerations" title="IANA Considerations">
    16241764
    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:
     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:
    16331775</t>
    16341776<?BEGININC p6-cache.cache-directives ?>
     
    17541896<?ENDINC p6-cache.iana-headers ?>
    17551897<t>
    1756   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
     1898   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
     1899   Force".
    17571900</t>
    17581901</section>
     
    17621905<section anchor="security.considerations" title="Security Considerations">
    17631906<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>
     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>
    17701913</section>
    17711914
    17721915<section anchor="ack" title="Acknowledgments">
    17731916<t>
    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.
     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.
    17771920</t>
    17781921</section>
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