Changeset 949


Ignore:
Timestamp:
Jul 27, 2010, 8:24:54 AM (9 years ago)
Author:
mnot@…
Message:

reformat xml (in hopefully the right place)

File:
1 edited

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

    r944 r949  
    202202<abstract>
    203203<t>
    204   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
    205   collaborative, hypermedia information systems. This document is Part 6 of the seven-part
    206   specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
    207   obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
    208   fields that control cache behavior or indicate cacheable response messages.
     204   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
     205   distributed, collaborative, hypermedia information systems. This document
     206   is Part 6 of the seven-part specification that defines the protocol
     207   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6
     208   defines requirements on HTTP caches and the associated header fields that
     209   control cache behavior or indicate cacheable response messages.
    209210</t>
    210211</abstract>
    211212
    212213<note title="Editorial Note (To be removed by RFC Editor)">
    213   <t>
    214     Discussion of this draft should take place on the HTTPBIS working group
    215     mailing list (ietf-http-wg@w3.org). The current issues list is
    216     at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
    217     and related documents (including fancy diffs) can be found at
    218     <eref target="http://tools.ietf.org/wg/httpbis/"/>.
    219   </t>
    220   <t>
    221     The changes in this draft are summarized in <xref target="changes.since.10"/>.
    222   </t>
     214   <t>
     215      Discussion of this draft should take place on the HTTPBIS working group
     216      mailing list (ietf-http-wg@w3.org). The current issues list is at <eref
     217      target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
     218      documents (including fancy diffs) can be found at <eref
     219      target="http://tools.ietf.org/wg/httpbis/"/>.
     220   </t>
     221   <t>
     222      The changes in this draft are summarized in <xref
     223      target="changes.since.10"/>.
     224   </t>
    223225</note>
    224226
    225   </front>
    226   <middle>
     227   </front>
     228   <middle>
    227229
    228230<section anchor="caching" title="Introduction">
    229231<t>
    230   HTTP is typically used for distributed information systems, where performance can be
    231   improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
    232   caching and reusing response messages.
     232   HTTP is typically used for distributed information systems, where
     233   performance can be improved by the use of response caches. This document
     234   defines aspects of HTTP/1.1 related to caching and reusing response
     235   messages.
    233236</t>
    234237
     
    236239<iref item="cache" />
    237240<t>
    238   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
    239   controls its message storage, retrieval, and deletion. A cache stores cacheable responses
    240   in order to reduce the response time and network bandwidth consumption on future,
    241   equivalent requests. Any client or server &MAY; employ a cache, though a cache cannot be
    242   used by a server that is acting as a tunnel.
    243 </t>
    244 <t>
    245   Caching would be useless if it did not significantly improve performance. The goal of
    246   caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
    247   some cases, a stored response can be reused without the need for a network request,
    248   reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
    249   (see <xref target="expiration.model" />). Even when a new request is required, it is often
    250   possible to reuse all or parts of the payload of a prior response to satisfy the request,
    251   thereby reducing network bandwidth usage; a "validation" mechanism is used for this
    252   purpose (see <xref target="validation.model" />).
     241   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
     242   subsystem that controls its message storage, retrieval, and deletion. A
     243   cache stores cacheable responses in order to reduce the response time and
     244   network bandwidth consumption on future, equivalent requests. Any client or
     245   server &MAY; employ a cache, though a cache cannot be used by a server that
     246   is acting as a tunnel.
     247</t>
     248<t>
     249   Caching would be useless if it did not significantly improve performance.
     250   The goal of caching in HTTP/1.1 is to reuse a prior response message to
     251   satisfy a current request. In some cases, a stored response can be reused
     252   without the need for a network request, reducing latency and network
     253   round-trips; a "freshness" mechanism is used for this purpose (see <xref
     254   target="expiration.model" />). Even when a new request is required, it is
     255   often possible to reuse all or parts of the payload of a prior response to
     256   satisfy the request, thereby reducing network bandwidth usage; a
     257   "validation" mechanism is used for this purpose (see <xref
     258   target="validation.model" />).
    253259</t>
    254260</section>
     
    256262<section anchor="intro.terminology" title="Terminology">
    257263<t>
    258   This specification uses a number of terms to refer to the roles played by participants
    259   in, and objects of, HTTP caching.
    260 </t>
    261 <t>
    262   <iref item="cacheable" />
    263   <x:dfn>cacheable</x:dfn>
    264   <list>
    265     <t>A response is cacheable if a cache is allowed to store a copy of the response message
    266       for use in answering subsequent requests. Even when a response is cacheable, there might
    267       be additional constraints on whether a cache can use the cached copy to satisfy a
    268       particular request.</t>
    269   </list>
    270 </t>
    271 <t>
    272   <iref item="explicit expiration time" />
    273   <x:dfn>explicit expiration time</x:dfn>
    274   <list>
    275     <t>The time at which the origin server intends that a representation should no longer be
    276       returned by a cache without further validation.</t>
    277   </list>
    278 </t>
    279 <t>
    280   <iref item="heuristic expiration time" />
    281   <x:dfn>heuristic expiration time</x:dfn>
    282   <list>
    283     <t>An expiration time assigned by a cache when no explicit expiration time is
    284     available.</t>
    285   </list>
    286 </t>
    287 <t>
    288   <iref item="age" />
    289   <x:dfn>age</x:dfn>
    290   <list>
    291     <t>The age of a response is the time since it was sent by, or successfully validated
    292       with, the origin server.</t>
    293   </list>
    294 </t>
    295 <t>
    296   <iref item="first-hand" />
    297   <x:dfn>first-hand</x:dfn>
    298   <list>
    299     <t>A response is first-hand if the freshness model is not in use; i.e., its age is
    300     0.</t>
    301   </list>
    302 </t>
    303 <t>
    304   <iref item="freshness lifetime" />
    305   <x:dfn>freshness lifetime</x:dfn>
    306   <list>
    307     <t>The length of time between the generation of a response and its expiration time. </t>
    308   </list>
    309 </t>
    310 <t>
    311   <iref item="fresh" />
    312   <x:dfn>fresh</x:dfn>
    313   <list>
    314     <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
    315   </list>
    316 </t>
    317 <t>
    318   <iref item="stale" />
    319   <x:dfn>stale</x:dfn>
    320   <list>
    321     <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
    322   </list>
    323 </t>
    324 <t>
    325   <iref item="validator" />
    326   <x:dfn>validator</x:dfn>
    327   <list>
    328     <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that is used to find
    329       out whether a stored response has an equivalent copy of a representation.</t>
    330   </list>
     264   This specification uses a number of terms to refer to the roles played by
     265   participants in, and objects of, HTTP caching.
     266</t>
     267<t>
     268   <iref item="cacheable" />
     269   <x:dfn>cacheable</x:dfn>
     270   <list>
     271      <t>A response is cacheable if a cache is allowed to store a copy of the
     272      response message for use in answering subsequent requests. Even when a
     273      response is cacheable, there might be additional constraints on whether
     274      a cache can use the cached copy to satisfy a particular request.</t>
     275   </list>
     276</t>
     277<t>
     278   <iref item="explicit expiration time" />
     279   <x:dfn>explicit expiration time</x:dfn>
     280   <list>
     281      <t>The time at which the origin server intends that a representation
     282      should no longer be returned by a cache without further validation.</t>
     283   </list>
     284</t>
     285<t>
     286   <iref item="heuristic expiration time" />
     287   <x:dfn>heuristic expiration time</x:dfn>
     288   <list>
     289      <t>An expiration time assigned by a cache when no explicit expiration
     290      time is available.</t>
     291   </list>
     292</t>
     293<t>
     294   <iref item="age" />
     295   <x:dfn>age</x:dfn>
     296   <list>
     297      <t>The age of a response is the time since it was sent by, or
     298      successfully validated with, the origin server.</t>
     299   </list>
     300</t>
     301<t>
     302   <iref item="first-hand" />
     303   <x:dfn>first-hand</x:dfn>
     304   <list>
     305      <t>A response is first-hand if the freshness model is not in use; i.e.,
     306      its age is 0.</t>
     307   </list>
     308</t>
     309<t>
     310   <iref item="freshness lifetime" />
     311   <x:dfn>freshness lifetime</x:dfn>
     312   <list>
     313      <t>The length of time between the generation of a response and its
     314      expiration time.</t>
     315   </list>
     316</t>
     317<t>
     318   <iref item="fresh" />
     319   <x:dfn>fresh</x:dfn>
     320   <list>
     321      <t>A response is fresh if its age has not yet exceeded its freshness
     322      lifetime.</t>
     323   </list>
     324</t>
     325<t>
     326   <iref item="stale" />
     327   <x:dfn>stale</x:dfn>
     328   <list>
     329      <t>A response is stale if its age has passed its freshness lifetime
     330      (either explicit or heuristic).</t>
     331   </list>
     332</t>
     333<t>
     334   <iref item="validator" />
     335   <x:dfn>validator</x:dfn>
     336   <list>
     337      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
     338      is used to find out whether a stored response has an equivalent copy of
     339      a representation.</t>
     340   </list>
    331341</t>
    332342<t anchor="shared.and.non-shared.caches">
    333   <iref item="validator" />
    334   <x:dfn>shared cache</x:dfn>
    335   <list>
    336     <t>A cache that is accessible to more than one user. A non-shared cache is
    337       dedicated to a single user.</t>
    338   </list>
     343   <iref item="validator" />
     344   <x:dfn>shared cache</x:dfn>
     345   <list>
     346      <t>A cache that is accessible to more than one user. A non-shared cache
     347      is dedicated to a single user.</t>
     348   </list>
    339349</t>
    340350</section>
     
    347357</t>
    348358<t>
    349    An implementation is not compliant if it fails to satisfy one or more
    350    of the "MUST" or "REQUIRED" level requirements for the protocols it
     359   An implementation is not compliant if it fails to satisfy one or more of
     360   the "MUST" or "REQUIRED" level requirements for the protocols it
    351361   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
    352    level and all the "SHOULD" level requirements for its protocols is said
    353    to be "unconditionally compliant"; one that satisfies all the "MUST"
    354    level requirements but not all the "SHOULD" level requirements for its
    355    protocols is said to be "conditionally compliant".
     362   level and all the "SHOULD" level requirements for its protocols is said to
     363   be "unconditionally compliant"; one that satisfies all the "MUST" level
     364   requirements but not all the "SHOULD" level requirements for its protocols
     365   is said to be "conditionally compliant".
    356366</t>
    357367</section>
    358368
    359369<section title="Syntax Notation" anchor="notation">
    360   <x:anchor-alias value="ALPHA"/>
    361   <x:anchor-alias value="CR"/>
    362   <x:anchor-alias value="DIGIT"/>
    363   <x:anchor-alias value="DQUOTE"/>
    364   <x:anchor-alias value="LF"/>
    365   <x:anchor-alias value="OCTET"/>
    366   <x:anchor-alias value="SP"/>
    367   <x:anchor-alias value="VCHAR"/>
    368   <x:anchor-alias value="WSP"/>
    369 <t>
    370   This specification uses the ABNF syntax defined in &notation; (which
    371   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
    372   <xref target="collected.abnf"/> shows the collected ABNF, with the list
    373   rule expanded.
    374 </t>
    375 <t>
    376   The following core rules are included by
    377   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
    378   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
    379   DIGIT (decimal 0-9), DQUOTE (double quote),
    380   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
    381   OCTET (any 8-bit sequence of data), SP (space),
    382   VCHAR (any visible USASCII character),
    383   and WSP (whitespace).
     370   <x:anchor-alias value="ALPHA"/>
     371   <x:anchor-alias value="CR"/>
     372   <x:anchor-alias value="DIGIT"/>
     373   <x:anchor-alias value="DQUOTE"/>
     374   <x:anchor-alias value="LF"/>
     375   <x:anchor-alias value="OCTET"/>
     376   <x:anchor-alias value="SP"/>
     377   <x:anchor-alias value="VCHAR"/>
     378   <x:anchor-alias value="WSP"/>
     379<t>
     380   This specification uses the ABNF syntax defined in &notation; (which
     381   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
     382   <xref target="collected.abnf"/> shows the collected ABNF, with the list
     383   rule expanded.
     384</t>
     385<t>
     386   The following core rules are included by reference, as defined in <xref
     387   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
     388   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
     389   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
     390   sequence of data), SP (space), VCHAR (any visible USASCII character), and
     391   WSP (whitespace).
    384392</t>
    385393
    386394<section title="Core Rules" anchor="core.rules">
    387   <x:anchor-alias value="quoted-string"/>
    388   <x:anchor-alias value="token"/>
    389   <x:anchor-alias value="OWS"/>
    390 <t>
    391   The core rules below are defined in &basic-rules;:
     395   <x:anchor-alias value="quoted-string"/>
     396   <x:anchor-alias value="token"/>
     397   <x:anchor-alias value="OWS"/>
     398<t>
     399   The core rules below are defined in &basic-rules;:
    392400</t>
    393401<figure><artwork type="abnf2616">
     
    398406</section>
    399407
    400 <section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
    401   <x:anchor-alias value="field-name"/>
    402   <x:anchor-alias value="HTTP-date"/>
    403   <x:anchor-alias value="port"/>
    404   <x:anchor-alias value="pseudonym"/>
    405   <x:anchor-alias value="uri-host"/>
    406 <t>
    407   The ABNF rules below are defined in other parts:
     408<section title="ABNF Rules defined in other Parts of the Specification"
     409    anchor="abnf.dependencies">
     410   <x:anchor-alias value="field-name"/>
     411   <x:anchor-alias value="HTTP-date"/>
     412   <x:anchor-alias value="port"/>
     413   <x:anchor-alias value="pseudonym"/>
     414   <x:anchor-alias value="uri-host"/>
     415<t>
     416   The ABNF rules below are defined in other parts:
    408417</t>
    409418<figure><!--Part1--><artwork type="abnf2616">
     
    423432<section anchor="response.cacheability" title="Response Cacheability">
    424433<t>
    425   A cache &MUST-NOT; store a response to any request, unless:
    426   <list style="symbols">
    427     <t>The request method is understood by the cache and defined as being cacheable, and</t>
    428     <t>the response status code is understood by the cache, and</t>
    429     <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
    430        appear in request or response headers, and</t>
    431     <t>the "private" cache response directive (see <xref target="cache-response-directive" />
    432        does not appear in the response, if the cache is shared, and</t>
    433     <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
    434        the cache is shared, unless the response explicitly allows it (see <xref
    435        target="caching.authenticated.responses" />), and</t>
    436     <t>the response either:
    437       <list style="symbols">
    438         <t>contains an Expires header (see <xref target="header.expires" />), or</t>
    439         <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
    440         <t>contains a s-maxage response cache directive and the cache is shared, or</t>
    441         <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
    442         <t>has a status code that can be served with heuristic freshness (see <xref
    443            target="heuristic.freshness" />).</t>
    444       </list>
    445     </t>
     434   A cache &MUST-NOT; store a response to any request, unless:
     435   <list style="symbols">
     436      <t>The request method is understood by the cache and defined as being
     437      cacheable, and</t>
     438      <t>the response status code is understood by the cache, and</t>
     439      <t>the "no-store" cache directive (see <xref
     440      target="header.cache-control" />) does not appear in request or response
     441      headers, and</t>
     442      <t>the "private" cache response directive (see <xref
     443      target="cache-response-directive" /> does not appear in the response, if
     444      the cache is shared, and</t>
     445      <t>the "Authorization" header (see &header-authorization;) does not
     446      appear in the request, if the cache is shared, unless the response
     447      explicitly allows it (see <xref target="caching.authenticated.responses"
     448      />), and</t>
     449      <t>the response either:
     450         <list style="symbols">
     451            <t>contains an Expires header (see <xref target="header.expires"
     452            />), or</t>
     453            <t>contains a max-age response cache directive (see <xref
     454            target="cache-response-directive" />), or</t>
     455            <t>contains a s-maxage response cache directive and the cache is
     456            shared, or</t>
     457            <t>contains a Cache Control Extension (see <xref
     458            target="cache.control.extensions" />) that allows it to be cached,
     459            or</t>
     460            <t>has a status code that can be served with heuristic freshness
     461            (see <xref target="heuristic.freshness" />).</t>
     462         </list>
     463      </t>
     464   </list>
     465</t>
     466<t>
     467   In this context, a cache has "understood" a request method or a response
     468   status code if it recognises it and implements any cache-specific
     469   behaviour. In particular, 206 Partial Content responses cannot be cached by
     470   an implementation that does not handle partial content (see <xref
     471   target="errors.or.incomplete.response.cache.behavior" />).
     472</t>
     473<t>
     474   Note that in normal operation, most caches will not store a response that
     475   has neither a cache validator nor an explicit expiration time, as such
     476   responses are not usually useful to store. However, caches are not
     477   prohibited from storing such responses.
     478</t>
     479
     480<section anchor="errors.or.incomplete.response.cache.behavior"
     481   title="Storing Partial and Incomplete Responses">
     482<t>
     483   A cache that receives an incomplete response (for example, with fewer bytes
     484   of data than specified in a Content-Length header) can store the response,
     485   but &MUST; treat it as a partial response &partial;. Partial responses can
     486   be combined as described in &combining-byte-ranges;; the result might be a
     487   full response or might still be partial. A cache &MUST-NOT; return a
     488   partial response to a client without explicitly marking it as such using
     489   the 206 (Partial Content) status code.
     490</t>
     491<t>
     492   A cache that does not support the Range and Content-Range headers
     493   &MUST-NOT; store incomplete or partial responses.
     494</t>
     495</section>
     496
     497</section>
     498
     499
     500<section anchor="constructing.responses.from.caches"
     501   title="Constructing Responses from Caches">
     502<t>
     503   For a presented request, a cache &MUST-NOT; return a stored response,
     504   unless:
     505   <list style="symbols">
     506      <t>The presented Effective Request URI (&effective-request-uri;) and
     507      that of the stored response match, and</t>
     508      <t>the request method associated with the stored response allows it to
     509      be used for the presented request, and</t>
     510      <t>selecting request-headers nominated by the stored response (if any)
     511      match those presented (see <xref target="caching.negotiated.responses"
     512      />), and</t>
     513      <t>the presented request and stored response are free from directives
     514      that would prevent its use (see <xref target="header.cache-control" />
     515      and <xref target="header.pragma"/>), and</t>
     516      <t>the stored response is either:
     517         <list style="symbols">
     518            <t>fresh (see <xref target="expiration.model" />), or</t>
     519            <t>allowed to be served stale (see <xref
     520            target="serving.stale.responses" />), or</t>
     521            <t>successfully validated (see <xref target="validation.model"
     522            />).</t>
     523         </list>
     524      </t>
    446525  </list>
    447526</t>
    448527<t>
    449   In this context, a cache has "understood" a request method or a response status
    450   code if it recognises it and implements any cache-specific behaviour. In
    451   particular, 206 Partial Content responses cannot be cached by an
    452   implementation that does not handle partial content
    453   (see <xref target="errors.or.incomplete.response.cache.behavior" />).
    454 </t>
    455 <t>
    456   Note that in normal operation, most caches will not store a response that has neither a
    457   cache validator nor an explicit expiration time, as such responses are not usually
    458   useful to store. However, caches are not prohibited from storing such responses.
    459 </t>
    460 
    461 <section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
    462 <t>
    463   A cache that receives an incomplete response (for example, with fewer bytes of data
    464   than specified in a Content-Length header) can store the response, but &MUST;
    465   treat it as a partial response &partial;. Partial responses
    466   can be combined as described in &combining-byte-ranges;; the result might be a
    467   full response or might still be partial. A cache &MUST-NOT; return a partial
    468   response to a client without explicitly marking it as such using the 206 (Partial
    469   Content) status code.
    470 </t>
    471 <t>
    472   A cache that does not support the Range and Content-Range headers &MUST-NOT; store
    473   incomplete or partial responses.
    474 </t>
    475 </section>
    476 
    477 </section>
    478 
    479 
    480 <section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
    481 <t>
    482   For a presented request, a cache &MUST-NOT; return a stored response, unless:
    483   <list style="symbols">
    484     <t>The presented Effective Request URI (&effective-request-uri;) and that of the stored response match, and</t>
    485     <t>the request method associated with the stored response allows it to be
    486       used for the presented request, and</t>
    487     <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
    488       target="caching.negotiated.responses" />), and</t>
    489     <t>the presented request and stored response are free from directives that would prevent
    490       its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
    491       and</t>
    492     <t>the stored response is either:
    493       <list style="symbols">
    494         <t>fresh (see <xref target="expiration.model" />), or</t>
    495         <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
    496         <t>successfully validated (see <xref target="validation.model" />).</t>
    497       </list>
    498     </t>
    499   </list>
    500 </t>
    501 <t>
    502   When a stored response is used to satisfy a request without validation, caches &MUST; include a
    503   single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
    504   current_age; see <xref target="age.calculations" />.
    505 </t>
    506 <t>
    507   Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
    508   the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
    509   corresponding response.
    510 </t>
    511 <t>
    512   Also, note that unsafe requests might invalidate already stored responses; see
    513   <xref target="invalidation.after.updates.or.deletions" />.
    514 </t>
    515 <t>
    516   Caches &MUST; use the most recent response (as determined by the Date header) when
    517   more than one suitable response is stored. They can also forward a request with
    518   "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
    519   use.
     528   When a stored response is used to satisfy a request without validation,
     529   caches &MUST; include a single Age header field (<xref target="header.age"
     530   />) in the response with a value equal to the stored response's
     531   current_age; see <xref target="age.calculations" />.
     532</t>
     533<t>
     534   Requests with methods that are unsafe (&safe-methods;) &MUST; be written
     535   through the cache to the origin server; i.e., a cache must not reply to
     536   such a request before having forwarded the request and having received a
     537   corresponding response.
     538</t>
     539<t>
     540   Also, note that unsafe requests might invalidate already stored responses;
     541   see <xref target="invalidation.after.updates.or.deletions" />.
     542</t>
     543<t>
     544   Caches &MUST; use the most recent response (as determined by the Date
     545   header) when more than one suitable response is stored. They can also
     546   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
     547   no-cache" to disambiguate which response to use.
    520548</t>
    521549</section>
     
    523551<section anchor="expiration.model" title="Freshness Model">
    524552<t>
    525   When a response is "fresh" in the cache, it can be used to satisfy subsequent
    526   requests without contacting the origin server, thereby improving efficiency.
    527 </t>
    528 <t>
    529   The primary mechanism for determining freshness is for an origin server to provide an
    530   explicit expiration time in the future, using either the Expires header (<xref
    531   target="header.expires" />) or the max-age response cache directive (<xref
    532   target="cache-response-directive" />). Generally, origin servers will assign future
    533   explicit expiration times to responses in the belief that the representation is not likely to
    534   change in a semantically significant way before the expiration time is reached.
    535 </t>
    536 <t>
    537   If an origin server wishes to force a cache to validate every request, it can
    538   assign an explicit expiration time in the past. This means that the response is always
    539   stale, so that caches should validate it before using it for subsequent requests.
    540   <cref anchor="TODO-response-stale">This wording might cause confusion, because the response might still be served stale.</cref>
    541 </t>
    542 <t>
    543   Since origin servers do not always provide explicit expiration times, HTTP caches &MAY;
    544   assign heuristic expiration times when explicit times are not specified, employing algorithms that
    545   use other header values (such as the Last-Modified time) to estimate a plausible
    546   expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
    547   impose worst-case constraints on their results.
     553   When a response is "fresh" in the cache, it can be used to satisfy
     554   subsequent requests without contacting the origin server, thereby improving
     555   efficiency.
     556</t>
     557<t>
     558   The primary mechanism for determining freshness is for an origin server to
     559   provide an explicit expiration time in the future, using either the Expires
     560   header (<xref target="header.expires" />) or the max-age response cache
     561   directive (<xref target="cache-response-directive" />). Generally, origin
     562   servers will assign future explicit expiration times to responses in the
     563   belief that the representation is not likely to change in a semantically
     564   significant way before the expiration time is reached.
     565</t>
     566<t>
     567   If an origin server wishes to force a cache to validate every request, it
     568   can assign an explicit expiration time in the past. This means that the
     569   response is always stale, so that caches should validate it before using it
     570   for subsequent requests. <cref anchor="TODO-response-stale">This wording
     571   might cause confusion, because the response might still be served
     572   stale.</cref>
     573</t>
     574<t>
     575   Since origin servers do not always provide explicit expiration times, HTTP
     576   caches &MAY; assign heuristic expiration times when explicit times are not
     577   specified, employing algorithms that use other header values (such as the
     578   Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1
     579   specification does not provide specific algorithms, but does impose
     580   worst-case constraints on their results.
    548581</t>
    549582<figure>
     
    556589</figure>
    557590<t>
    558   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
    559   the current_age is defined in <xref target="age.calculations" />.
    560 </t>
    561 <t>
    562   Additionally, clients might need to influence freshness calculation. They can do this using
    563   several request cache directives, with the effect of either increasing or loosening
    564   constraints on freshness. See <xref target="cache-request-directive" />.
    565 </t>
    566 <t>
    567   <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
    568   freshness.</cref>
    569 </t>
    570 <t>
    571   Note that freshness applies only to cache operation; it cannot be used to force a user agent
    572   to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
    573   the difference between caches and history mechanisms.
    574 </t>
    575 
    576 <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
    577 <t>
    578   A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
    579   response by using the first match of:
    580   <list style="symbols">
    581     <t>If the cache is shared and the s-maxage response cache directive (<xref
     591   The freshness_lifetime is defined in <xref
     592   target="calculating.freshness.lifetime" />; the current_age is defined in
     593   <xref target="age.calculations" />.
     594</t>
     595<t>
     596   Additionally, clients might need to influence freshness calculation. They
     597   can do this using several request cache directives, with the effect of
     598   either increasing or loosening constraints on freshness. See <xref
     599   target="cache-request-directive" />.
     600</t>
     601<t>
     602   <cref anchor="ISSUE-no-req-for-directives">there are not requirements
     603   directly applying to cache-request-directives and freshness.</cref>
     604</t>
     605<t>
     606   Note that freshness applies only to cache operation; it cannot be used to
     607   force a user agent to refresh its display or reload a resource. See <xref
     608   target="history.lists" /> for an explanation of the difference between
     609   caches and history mechanisms.
     610</t>
     611
     612<section anchor="calculating.freshness.lifetime"
     613   title="Calculating Freshness Lifetime">
     614<t>
     615   A cache can calculate the freshness lifetime (denoted as
     616   freshness_lifetime) of a response by using the first match of:
     617   <list style="symbols">
     618      <t>If the cache is shared and the s-maxage response cache directive
     619      (<xref target="cache-response-directive" />) is present, use its value,
     620      or</t>
     621      <t>If the max-age response cache directive (<xref
    582622      target="cache-response-directive" />) is present, use its value, or</t>
    583     <t>If the max-age response cache directive (<xref target="cache-response-directive"
    584       />) is present, use its value, or</t>
    585     <t>If the Expires response header (<xref target="header.expires" />) is present, use
    586       its value minus the value of the Date response header, or</t>
    587     <t>Otherwise, no explicit expiration time is present in the response. A heuristic
    588       freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
    589   </list>
    590 </t>
    591 <t>
    592   Note that this calculation is not vulnerable to clock skew, since all of the
    593   information comes from the origin server.
     623      <t>If the Expires response header (<xref target="header.expires" />) is
     624      present, use its value minus the value of the Date response header,
     625      or</t>
     626      <t>Otherwise, no explicit expiration time is present in the response. A
     627      heuristic freshness lifetime might be applicable; see <xref
     628      target="heuristic.freshness" />.</t>
     629   </list>
     630</t>
     631<t>
     632   Note that this calculation is not vulnerable to clock skew, since all of
     633   the information comes from the origin server.
    594634</t>
    595635
    596636<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
    597637<t>
    598   If no explicit expiration time is present in a stored response that has a status code
    599   whose definition allows heuristic freshness to be used (including the following
    600   in &status-codes;: 200, 203, 206, 300, 301 and 410), a heuristic expiration time &MAY; be
    601   calculated. Heuristics &MUST-NOT; be used for response status codes that do not explicitly
    602   allow it.
    603 </t>
    604 <t>
    605   When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
    606   attach a Warning header with a 113 warn-code to the response if its current_age is
    607   more than 24 hours and such a warning is not already present.
    608 </t>
    609 <t>
    610   Also, if the response has a Last-Modified header (&header-last-modified;), the
    611   heuristic expiration value &SHOULD; be no more than some fraction of the interval
    612   since that time. A typical setting of this fraction might be 10%.
     638   If no explicit expiration time is present in a stored response that has a
     639   status code whose definition allows heuristic freshness to be used
     640   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
     641   410), a heuristic expiration time &MAY; be calculated. Heuristics
     642   &MUST-NOT; be used for response status codes that do not explicitly allow
     643   it.
     644</t>
     645<t>
     646   When a heuristic is used to calculate freshness lifetime, the cache
     647   &SHOULD; attach a Warning header with a 113 warn-code to the response if
     648   its current_age is more than 24 hours and such a warning is not already
     649   present.
     650</t>
     651<t>
     652   Also, if the response has a Last-Modified header (&header-last-modified;),
     653   the heuristic expiration value &SHOULD; be no more than some fraction of
     654   the interval since that time. A typical setting of this fraction might be
     655   10%.
    613656</t>
    614657<x:note>
    615   <t>
    616     <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt="," x:sec="13.9"/>)
    617     required that caches do not calculate heuristic freshness for URLs with
    618     query components (i.e., those containing '?'). In practice, this has not
    619     been widely implemented. Therefore, servers are encouraged to send explicit
    620     directives (e.g., Cache-Control: no-cache) if they wish to preclude
    621     caching.
    622   </t>
     658   <t>
     659      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
     660      x:sec="13.9"/>) required that caches do not calculate heuristic
     661      freshness for URLs with query components (i.e., those containing '?').
     662      In practice, this has not been widely implemented. Therefore, servers
     663      are encouraged to send explicit directives (e.g., Cache-Control:
     664      no-cache) if they wish to preclude caching.
     665   </t>
    623666</x:note>
    624667</section>
     
    627670<section anchor="age.calculations" title="Calculating Age">
    628671<t>
    629   HTTP/1.1 uses the Age response-header to convey the estimated age of the response
    630   message when obtained from a cache. The Age field value is the cache's estimate of the
    631   amount of time since the response was generated or validated by the origin server. In
    632   essence, the Age value is the sum of the time that the response has been resident in
    633   each of the caches along the path from the origin server, plus the amount of time it has
    634   been in transit along network paths.
    635 </t>
    636 <t>
    637   The following data is used for the age calculation:
    638 </t>
    639 <t>
    640   <x:dfn>age_value</x:dfn>
    641   <list>
    642     <t>
    643       The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
    644       in a form appropriate for arithmetic operation; or 0, if not available.
    645     </t>
    646   </list>
    647 </t>
    648 <t>
    649   <x:dfn>date_value</x:dfn>
    650   <list>
    651     <t>
    652       HTTP/1.1 requires origin servers to send a Date header, if possible,
    653       with every response, giving the time at which the response was generated.
    654       The term "date_value" denotes the value of the Date header, in a form
    655       appropriate for arithmetic operations. See &header-date; for the definition
    656       of the Date header, and for requirements regarding responses without a
    657       Date response header.
    658     </t>
    659   </list>
    660 </t>
    661 <t>
    662   <x:dfn>now</x:dfn>
    663   <list>
    664     <t>
    665       The term "now" means "the current value of the clock at the host
    666       performing the calculation". Hosts that use HTTP, but especially hosts
    667       running origin servers and caches, &SHOULD; use NTP
    668       (<xref target="RFC1305"/>) or some similar protocol to synchronize their
    669       clocks to a globally accurate time standard.
    670     </t>
    671   </list>
    672 </t>
    673 <t>
    674   <x:dfn>request_time</x:dfn>
    675   <list>
    676     <t>
    677       The current value of the clock at the host at the time the request
    678       resulting in the stored response was made.
    679     </t>
    680   </list>
    681 </t>
    682 <t>
    683   <x:dfn>response_time</x:dfn>
    684   <list>
    685     <t>
    686       The current value of the clock at the host at the time the response was
    687       received.
    688     </t>
    689   </list>
    690 </t>
    691 <t>
    692   A response's age can be calculated in two entirely independent ways:
    693   <list style="numbers">
    694     <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
    695       origin server's clock. If the result is negative, the result is replaced by zero.</t>
    696     <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
    697       note this value &MUST; be interpreted relative to the time the
    698       request was initiated, not the time that the response was received.</t>
    699   </list>
     672   HTTP/1.1 uses the Age response-header to convey the estimated age of the
     673   response message when obtained from a cache. The Age field value is the
     674   cache's estimate of the amount of time since the response was generated or
     675   validated by the origin server. In essence, the Age value is the sum of the
     676   time that the response has been resident in each of the caches along the
     677   path from the origin server, plus the amount of time it has been in transit
     678   along network paths.
     679</t>
     680<t>
     681   The following data is used for the age calculation:
     682</t>
     683<t>
     684   <x:dfn>age_value</x:dfn>
     685   <list>
     686      <t>
     687         The term "age_value" denotes the value of the Age header (<xref
     688         target="header.age"/>), in a form appropriate for arithmetic
     689         operation; or 0, if not available.
     690      </t>
     691   </list>
     692</t>
     693<t>
     694   <x:dfn>date_value</x:dfn>
     695   <list>
     696      <t>
     697         HTTP/1.1 requires origin servers to send a Date header, if possible,
     698         with every response, giving the time at which the response was
     699         generated. The term "date_value" denotes the value of the Date
     700         header, in a form appropriate for arithmetic operations. See
     701         &header-date; for the definition of the Date header, and for
     702         requirements regarding responses without a Date response header.
     703      </t>
     704   </list>
     705</t>
     706<t>
     707   <x:dfn>now</x:dfn>
     708   <list>
     709      <t>
     710         The term "now" means "the current value of the clock at the host
     711         performing the calculation". Hosts that use HTTP, but especially
     712         hosts running origin servers and caches, &SHOULD; use NTP (<xref
     713         target="RFC1305"/>) or some similar protocol to synchronize their
     714         clocks to a globally accurate time standard.
     715      </t>
     716   </list>
     717</t>
     718<t>
     719   <x:dfn>request_time</x:dfn>
     720   <list>
     721      <t>
     722         The current value of the clock at the host at the time the request
     723         resulting in the stored response was made.
     724      </t>
     725   </list>
     726</t>
     727<t>
     728   <x:dfn>response_time</x:dfn>
     729   <list>
     730      <t>
     731         The current value of the clock at the host at the time the response
     732         was received.
     733      </t>
     734   </list>
     735</t>
     736<t>
     737   A response's age can be calculated in two entirely independent ways:
     738   <list style="numbers">
     739      <t>the "apparent_age": response_time minus date_value, if the local
     740      clock is reasonably well synchronized to the origin server's clock. If
     741      the result is negative, the result is replaced by zero.</t>
     742      <t>the "corrected_age_value", if all of the caches along the response
     743      path implement HTTP/1.1; note this value &MUST; be interpreted relative
     744      to the time the request was initiated, not the time that the response
     745      was received.</t>
     746   </list>
    700747</t>
    701748<figure>
     
    713760</artwork></figure>
    714761<t>
    715   The current_age of a stored response can then be calculated by adding the amount of
    716   time (in seconds) since the stored response was last validated by the origin server to
    717   the corrected_initial_age.
     762   The current_age of a stored response can then be calculated by adding the
     763   amount of time (in seconds) since the stored response was last validated by
     764   the origin server to the corrected_initial_age.
    718765</t>
    719766<figure><artwork type="code">
     
    725772<section anchor="serving.stale.responses" title="Serving Stale Responses">
    726773<t>
    727   A "stale" response is one that either has explicit expiry information or is allowed to
    728   have heuristic expiry calculated, but is not fresh according to the calculations in
    729   <xref target="expiration.model" />.
    730 </t>
    731 <t>
    732   Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
    733   in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
    734   "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
    735   "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
    736 </t>
    737 <t>
    738   Caches &SHOULD-NOT; return stale responses unless they are
    739   disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
    740   or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
    741 </t>
    742 <t>
    743   Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
    744   target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
    745   the cache is disconnected.
    746 </t>
    747 <t>
    748   If a cache receives a first-hand response (either an entire response, or a 304 (Not
    749   Modified) response) that it would normally forward to the requesting client, and the
    750   received response is no longer fresh, the cache &SHOULD; forward it to the
    751   requesting client without adding a new Warning (but without removing any existing
    752   Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
    753   that response became stale in transit.
     774   A "stale" response is one that either has explicit expiry information or is
     775   allowed to have heuristic expiry calculated, but is not fresh according to
     776   the calculations in <xref target="expiration.model" />.
     777</t>
     778<t>
     779   Caches &MUST-NOT; return a stale response if it is prohibited by an
     780   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
     781   directive, a "must-revalidate" cache-response-directive, or an applicable
     782   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
     783   target="cache-response-directive"/>).
     784</t>
     785<t>
     786   Caches &SHOULD-NOT; return stale responses unless they are disconnected
     787   (i.e., it cannot contact the origin server or otherwise find a forward
     788   path) or otherwise explicitly allowed (e.g., the max-stale request
     789   directive; see <xref target="cache-request-directive" />).
     790</t>
     791<t>
     792   Stale responses &SHOULD; have a Warning header with the 110 warn-code (see
     793   <xref target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be
     794   sent on stale responses if the cache is disconnected.
     795</t>
     796<t>
     797   If a cache receives a first-hand response (either an entire response, or a
     798   304 (Not Modified) response) that it would normally forward to the
     799   requesting client, and the received response is no longer fresh, the cache
     800   &SHOULD; forward it to the requesting client without adding a new Warning
     801   (but without removing any existing Warning headers). A cache &SHOULD-NOT;
     802   attempt to validate a response simply because that response became stale in
     803   transit.
    754804</t>
    755805</section>
     
    758808<section anchor="validation.model" title="Validation Model">
    759809<t>
    760   When a cache has one or more stored responses for a requested URI, but cannot 
    761   serve any of them (e.g., because they are not fresh, or one cannot be selected;
    762   see <xref target="caching.negotiated.responses"/>),
    763   it can use the conditional request mechanism &conditional; in the forwarded
    764   request to give the origin server an opportunity to both select a valid stored
    765   response to be used, and to update it. This process is known as "validating"
    766   or "revalidating" the stored response.
    767 </t>
    768 <t>
    769   When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
    770   header whose value is that of the Last-Modified header from the selected
    771   (see <xref target="caching.negotiated.responses"/>) stored response, if available.
    772 </t>
    773 <t>
    774   Additionally, the cache &SHOULD; add an If-None-Match header whose value 
    775   is that of the ETag header(s) from all responses stored for the requested URI,
    776   if present. However, if any of the stored responses contains only partial
    777   content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
    778   field unless the request is for a range that would be fully satisfied by
    779   that stored response.
    780 </t>
    781 <t>
    782   A 304 (Not Modified) response status code indicates that the stored
    783   response can be updated and reused; see <xref target="combining.headers"/>.
    784 </t>
    785 <t>
    786   A full response (i.e., one with a response body) indicates that none 
    787   of the stored responses nominated in the conditional request is
    788   suitable. Instead, the full response &SHOULD; be used to satisfy the
    789   request and &MAY; replace the stored response.
    790 </t>
    791 <t>
    792   If a cache receives a 5xx response while attempting to validate a response, it &MAY;
    793   either forward this response to the requesting client, or act as if the server failed to
    794   respond. In the latter case, it &MAY; return a previously stored response (see <xref
    795   target="serving.stale.responses" />).
    796 </t>
    797 </section>
    798 
    799 <section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
    800 <t>
    801   Because unsafe methods (&safe-methods;) have the potential for changing state on the
    802   origin server, intervening caches can use them to keep their contents
    803   up-to-date.
    804 </t>
    805 <t>
     810   When a cache has one or more stored responses for a requested URI, but
     811   cannot serve any of them (e.g., because they are not fresh, or one cannot
     812   be selected; see <xref target="caching.negotiated.responses"/>), it can use
     813   the conditional request mechanism &conditional; in the forwarded request to
     814   give the origin server an opportunity to both select a valid stored
     815   response to be used, and to update it. This process is known as
     816   "validating" or "revalidating" the stored response.
     817</t>
     818<t>
     819   When sending such a conditional request, the cache &SHOULD; add an
     820   If-Modified-Since header whose value is that of the Last-Modified header
     821   from the selected (see <xref target="caching.negotiated.responses"/>)
     822   stored response, if available.
     823</t>
     824<t>
     825   Additionally, the cache &SHOULD; add an If-None-Match header whose value is
     826   that of the ETag header(s) from all responses stored for the requested URI,
     827   if present. However, if any of the stored responses contains only partial
     828   content, its entity-tag &SHOULD-NOT; be included in the If-None-Match
     829   header field unless the request is for a range that would be fully
     830   satisfied by that stored response.
     831</t>
     832<t>
     833   A 304 (Not Modified) response status code indicates that the stored
     834   response can be updated and reused; see <xref target="combining.headers"/>.
     835</t>
     836<t>
     837   A full response (i.e., one with a response body) indicates that none of the
     838   stored responses nominated in the conditional request is suitable. Instead,
     839   the full response &SHOULD; be used to satisfy the request and &MAY; replace
     840   the stored response.
     841</t>
     842<t>
     843   If a cache receives a 5xx response while attempting to validate a response,
     844   it &MAY; either forward this response to the requesting client, or act as
     845   if the server failed to respond. In the latter case, it &MAY; return a
     846   previously stored response (see <xref target="serving.stale.responses" />).
     847</t>
     848</section>
     849
     850<section anchor="invalidation.after.updates.or.deletions"
     851   title="Request Methods that Invalidate">
     852<t>
     853   Because unsafe methods (&safe-methods;) have the potential for changing
     854   state on the origin server, intervening caches can use them to keep their
     855   contents up-to-date.
     856</t>
     857<t>
     858<<<<<<< .mine
     859   The following HTTP methods &MUST; cause a cache to invalidate the Effective
     860   Request URI (&effective-request-uri;) as well as the URI(s) in the Location
     861   and Content-Location headers (if present):
     862   <list style="symbols">
     863      <t>PUT</t>
     864      <t>DELETE</t>
     865      <t>POST</t>
     866   </list>
     867=======
    806868  In a succesful exchange (&status.2xx;), the following HTTP methods &MUST; cause a cache
    807869  to invalidate the Effective Request URI (&effective-request-uri;) as well
     
    812874    <t>POST</t>
    813875  </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   A cache that passes through requests for methods it does not understand &SHOULD;
    822   invalidate the Effective Request URI (&effective-request-uri;).
    823 </t>
    824 <t>
    825   Here, "invalidate" means that the cache will either remove all stored responses related
    826   to the Effective Request URI, or will mark these as "invalid" and in need of a mandatory validation
    827   before they can be returned in response to a subsequent request.
    828 </t>
    829 <t>
    830   Note that this does not guarantee that all appropriate responses are invalidated. For
    831   example, the request that caused the change at the origin server might not have gone
    832   through the cache where a response is stored.
    833 </t>
    834 </section>
    835 
    836 <section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
    837 
    838 <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>
    839 
    840 <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>
    841 
    842 <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>
    843 </section>
    844 
    845 <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
    846 <t>
    847   When a cache receives a request that can be satisfied by a stored response
    848   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
    849   response unless all of the selecting request-headers nominated by the Vary header match
    850   in both the original request (i.e., that associated with the stored response),
    851   and the presented request.
    852 </t>
    853 <t>
    854   The selecting request-headers from two requests are defined to match
    855   if and only if those in the first request can be transformed to those in the
    856   second request by applying any of the following:
    857   <list style="symbols">
    858     <t>
    859       adding or removing whitespace, where allowed in the header's syntax
    860     </t>
    861     <t>
    862       combining multiple message-header fields with the same field name (see
    863       &header-fields;)
    864     </t>
    865     <t>
    866       normalizing both header values in a way that is known to have identical
    867       semantics, according to the header's specification (e.g., re-ordering field values
    868       when order is not significant; case-normalization, where values are defined to be
    869       case-insensitive)   
    870     </t>
     876>>>>>>> .r948
     877</t>
     878<t>
     879   An invalidation based on a URI from a Location or Content-Location header
     880   &MUST-NOT; be performed if the host part of that URI differs from the host
     881   part in the Effective Request URI (&effective-request-uri;). This helps
     882   prevent denial of service attacks.
     883</t>
     884<t>
     885   A cache that passes through requests for methods it does not understand
     886   &SHOULD; invalidate the Effective Request URI (&effective-request-uri;).
     887</t>
     888<t>
     889   Here, "invalidate" means that the cache will either remove all stored
     890   responses related to the Effective Request URI, or will mark these as
     891   "invalid" and in need of a mandatory validation before they can be returned
     892   in response to a subsequent request.
     893</t>
     894<t>
     895   Note that this does not guarantee that all appropriate responses are
     896   invalidated. For example, the request that caused the change at the origin
     897   server might not have gone through the cache where a response is stored.
     898</t>
     899</section>
     900
     901<section anchor="caching.authenticated.responses"
     902   title="Shared Caching of Authenticated Responses">
     903
     904<t>
     905   Shared caches &MUST-NOT; use a cached response to a request with an
     906   Authorization header (&header-authorization;) to satisfy any subsequent
     907   request unless a cache directive that allows such responses to be stored is
     908   present in the response.
     909</t>
     910
     911<t>
     912   In this specification, the following Cache-Control response directives
     913   (<xref target="cache-response-directive"/>) have such an effect:
     914   must-revalidate, public, s-maxage.
     915</t>
     916
     917<t>
     918   Note that cached responses that contain the "must-revalidate" and/or
     919   "s-maxage" response directives are not allowed to be served stale (<xref
     920   target="serving.stale.responses"/>) by shared caches. In particular, a
     921   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
     922   used to satisfy a subsequent request without revalidating it on the origin
     923   server.
     924</t>
     925</section>
     926
     927<section anchor="caching.negotiated.responses"
     928   title="Caching Negotiated Responses">
     929<t>
     930   When a cache receives a request that can be satisfied by a stored response
     931   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
     932   use that response unless all of the selecting request-headers nominated by
     933   the Vary header match in both the original request (i.e., that associated
     934   with the stored response), and the presented request.
     935</t>
     936<t>
     937   The selecting request-headers from two requests are defined to match if and
     938   only if those in the first request can be transformed to those in the
     939   second request by applying any of the following:
     940   <list style="symbols">
     941      <t>
     942         adding or removing whitespace, where allowed in the header's syntax
     943      </t>
     944      <t>
     945         combining multiple message-header fields with the same field name
     946         (see &header-fields;)
     947      </t>
     948      <t>
     949         normalizing both header values in a way that is known to have
     950         identical semantics, according to the header's specification (e.g.,
     951         re-ordering field values when order is not significant;
     952         case-normalization, where values are defined to be case-insensitive)
     953      </t>
    871954  </list>
    872955</t>
    873956<t>
    874   If (after any normalization that might take place) a header field is absent
    875   from a request, it can only match another request if it is also absent there.
    876 </t>
    877 <t>
    878   A Vary header field-value of "*" always fails to match, and subsequent requests to that
    879   resource can only be properly interpreted by the origin server.
    880 </t>
    881 <t>
    882   The stored response with matching selecting request-headers is known as the
    883   selected response.
    884 </t>
    885 <t>
    886   If no selected response is available, the cache &MAY; forward the presented
    887   request to the origin server in a conditional request; see <xref target="validation.model"/>.
     957   If (after any normalization that might take place) a header field is absent
     958   from a request, it can only match another request if it is also absent
     959   there.
     960</t>
     961<t>
     962   A Vary header field-value of "*" always fails to match, and subsequent
     963   requests to that resource can only be properly interpreted by the origin
     964   server.
     965</t>
     966<t>
     967   The stored response with matching selecting request-headers is known as the
     968   selected response.
     969</t>
     970<t>
     971   If no selected response is available, the cache &MAY; forward the presented
     972   request to the origin server in a conditional request; see <xref
     973   target="validation.model"/>.
    888974</t>
    889975</section>
     
    891977<section anchor="combining.headers" title="Combining Responses">
    892978<t>
    893   When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
    894   (in this section, the "new" response"), it needs to created an updated response by combining
    895   the stored response with the new one, so that the updated response can be used to satisfy
    896   the request, and potentially update the cached response.
    897 </t>
    898 <t>
    899   If the new response contains an ETag, it identifies the stored 
    900   response to use. <cref anchor="TODO-mention-CL">might need language about Content-Location 
    901   here</cref><cref anchor="TODO-select-for-combine">Shouldn't this be the selected response?</cref>
    902 </t>
    903 <t>
    904   If the new response's status code is 206 (partial content), both the stored and new 
    905   responses &MUST; have validators, and those validators &MUST; match using the strong 
    906   comparison function (see &weak-and-strong;). Otherwise, the 
    907   responses &MUST-NOT; be combined.
    908 </t>
    909 <t>
    910   The stored response headers are used as those of the updated response, except that
    911   <list style="symbols">
    912     <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
    913       &MUST; be deleted.</t>
    914     <t>any stored Warning headers with warn-code 2xx &MUST; be retained.</t>
    915     <t>any other headers provided in the new response &MUST; replace all instances of
    916           the corresponding headers from the stored response.</t>
    917   </list>
    918 </t>
    919 <t>
    920   The updated response headers &MUST; be used to replace those of the 
    921   stored response in cache (unless the stored response is removed from cache). In the
    922   case of a 206 response, the combined representation &MAY; be stored.
     979   When a cache receives a 304 (Not Modified) response or a 206 (Partial
     980   Content) response (in this section, the "new" response"), it needs to
     981   created an updated response by combining the stored response with the new
     982   one, so that the updated response can be used to satisfy the request, and
     983   potentially update the cached response.
     984</t>
     985<t>
     986   If the new response contains an ETag, it identifies the stored response to
     987   use. <cref anchor="TODO-mention-CL">might need language about
     988   Content-Location here</cref><cref
     989   anchor="TODO-select-for-combine">Shouldn't this be the selected
     990   response?</cref>
     991</t>
     992<t>
     993   If the new response's status code is 206 (partial content), both the stored
     994   and new responses &MUST; have validators, and those validators &MUST; match
     995   using the strong comparison function (see &weak-and-strong;). Otherwise,
     996   the responses &MUST-NOT; be combined.
     997</t>
     998<t>
     999   The stored response headers are used as those of the updated response,
     1000   except that
     1001   <list style="symbols">
     1002      <t>any stored Warning headers with warn-code 1xx (see <xref
     1003      target="header.warning" />) &MUST; be deleted.</t>
     1004      <t>any stored Warning headers with warn-code 2xx &MUST; be retained.</t>
     1005      <t>any other headers provided in the new response &MUST; replace all
     1006      instances of the corresponding headers from the stored response.</t>
     1007   </list>
     1008</t>
     1009<t>
     1010   The updated response headers &MUST; be used to replace those of the stored
     1011   response in cache (unless the stored response is removed from cache). In
     1012   the case of a 206 response, the combined representation &MAY; be stored.
    9231013</t>
    9241014</section>
     
    9281018<section anchor="header.fields" title="Header Field Definitions">
    9291019<t>
    930   This section defines the syntax and semantics of HTTP/1.1 header fields
    931   related to caching.
    932 </t>
    933 <t>
    934   For entity-header fields, both sender and recipient refer to either the client or the
    935   server, depending on who sends and who receives the message.
     1020   This section defines the syntax and semantics of HTTP/1.1 header fields
     1021   related to caching.
     1022</t>
     1023<t>
     1024   For entity-header fields, both sender and recipient refer to either the
     1025   client or the server, depending on who sends and who receives the message.
    9361026</t>
    9371027
    9381028<section anchor="header.age" title="Age">
    939   <iref item="Age header" primary="true" x:for-anchor="" />
    940   <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
    941   <x:anchor-alias value="Age"/>
    942   <x:anchor-alias value="Age-v"/>
    943   <x:anchor-alias value="age-value"/>
    944 <t>
    945   The "Age" response-header field conveys the sender's estimate of the amount
    946   of time since the response was generated or successfully validated at the
    947   origin server. Age values are calculated as specified in
    948   <xref target="age.calculations" />.
     1029   <iref item="Age header" primary="true" x:for-anchor="" />
     1030   <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
     1031   <x:anchor-alias value="Age"/>
     1032   <x:anchor-alias value="Age-v"/>
     1033   <x:anchor-alias value="age-value"/>
     1034<t>
     1035   The "Age" response-header field conveys the sender's estimate of the amount
     1036   of time since the response was generated or successfully validated at the
     1037   origin server. Age values are calculated as specified in <xref
     1038  target="age.calculations" />.
    9491039</t>
    9501040<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
     
    9601050</artwork></figure>
    9611051<t>
    962   If a cache receives a value larger than the largest positive integer it can represent, or
    963   if any of its age calculations overflows, it &MUST; transmit an Age header with a
    964   field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
    965   of at least 31 bits of range.
    966 </t>
    967 <t>
    968   The presence of an Age header field in a response implies that a response is not
    969   first-hand. However, the converse is not true, since HTTP/1.0 caches might not implement the
    970   Age header field.
     1052   If a cache receives a value larger than the largest positive integer it can
     1053   represent, or if any of its age calculations overflows, it &MUST; transmit
     1054   an Age header with a field-value of 2147483648 (2<x:sup>31</x:sup>). Caches
     1055   &SHOULD; use an arithmetic type of at least 31 bits of range.
     1056</t>
     1057<t>
     1058   The presence of an Age header field in a response implies that a response
     1059   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
     1060   might not implement the Age header field.
    9711061</t>
    9721062</section>
    9731063
    9741064<section anchor="header.cache-control" title="Cache-Control">
    975   <iref item="Cache-Control header" primary="true" x:for-anchor="" />
    976   <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
    977   <x:anchor-alias value="Cache-Control"/>
    978   <x:anchor-alias value="Cache-Control-v"/>
    979   <x:anchor-alias value="cache-directive"/>
    980   <x:anchor-alias value="cache-extension"/>
    981   <x:anchor-alias value="cache-request-directive"/>
    982   <x:anchor-alias value="cache-response-directive"/>
    983 <t>
    984   The "Cache-Control" general-header field is used to specify directives for
    985   caches along the request/response chain. Such cache directives are
    986   unidirectional in that the presence of a directive in a request does not
    987   imply that the same directive is to be given in the response.
    988 </t>
    989 <t>
    990   HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control directives
    991   defined in this section. See <xref target="cache.control.extensions"/> for
    992   information about how Cache-Control directives defined elsewhere are handled.
     1065   <iref item="Cache-Control header" primary="true" x:for-anchor="" />
     1066   <iref item="Headers" primary="true" subitem="Cache-Control"
     1067      x:for-anchor="" />
     1068   <x:anchor-alias value="Cache-Control"/>
     1069   <x:anchor-alias value="Cache-Control-v"/>
     1070   <x:anchor-alias value="cache-directive"/>
     1071   <x:anchor-alias value="cache-extension"/>
     1072   <x:anchor-alias value="cache-request-directive"/>
     1073   <x:anchor-alias value="cache-response-directive"/>
     1074<t>
     1075   The "Cache-Control" general-header field is used to specify directives for
     1076   caches along the request/response chain. Such cache directives are
     1077   unidirectional in that the presence of a directive in a request does not
     1078   imply that the same directive is to be given in the response.
     1079</t>
     1080<t>
     1081   HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control
     1082   directives defined in this section. See <xref
     1083   target="cache.control.extensions"/> for information about how Cache-Control
     1084   directives defined elsewhere are handled.
    9931085</t>
    9941086<x:note>
    995   <t>
    996     <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
    997     might only implement Pragma: no-cache (see <xref target="header.pragma" />).
    998   </t>
     1087   <t>
     1088       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
     1089       might only implement Pragma: no-cache (see <xref target="header.pragma"
     1090       />).
     1091   </t>
    9991092</x:note>
    10001093<t>
    1001   Cache directives &MUST; be passed through by a proxy or gateway application,
    1002   regardless of their significance to that application, since the directives might be
    1003   applicable to all recipients along the request/response chain. It is not possible to
    1004   target a directive to a specific cache.
     1094   Cache directives &MUST; be passed through by a proxy or gateway
     1095   application, regardless of their significance to that application, since
     1096   the directives might be applicable to all recipients along the
     1097   request/response chain. It is not possible to target a directive to a
     1098   specific cache.
    10051099</t>
    10061100<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"/>
     
    10141108</artwork></figure>
    10151109
    1016 <section anchor="cache-request-directive" title="Request Cache-Control Directives">
    1017   <x:anchor-alias value="cache-request-directive" />
    1018 
    1019 <figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
     1110<section anchor="cache-request-directive"
     1111   title="Request Cache-Control Directives">
     1112   <x:anchor-alias value="cache-request-directive" />
     1113
     1114<figure><artwork type="abnf2616"><iref item="Grammar" primary="true"
     1115   subitem="cache-request-directive" />
    10201116  <x:ref>cache-request-directive</x:ref> =
    10211117       "no-cache"
     
    10301126
    10311127<t>
    1032   <x:dfn>no-cache</x:dfn>
    1033   <iref item="Cache Directives" primary="true" subitem="no-cache" />
    1034   <iref item="no-cache" primary="true" subitem="Cache Directive" />
    1035   <list>
    1036     <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
    1037       used to satisfy the request without successful validation on the origin server.</t>
    1038   </list>
    1039 </t>
    1040 <t>
    1041   <x:dfn>no-store</x:dfn>
    1042   <iref item="Cache Directives" primary="true" subitem="no-store" />
    1043   <iref item="no-store" primary="true" subitem="Cache Directive" />
    1044   <list>
    1045     <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
    1046       of either this request or any response to it. This directive applies to both
    1047       non-shared and shared caches. "&MUST-NOT; store" in this context means that the
    1048       cache &MUST-NOT; intentionally store the information in non-volatile storage,
    1049       and &MUST; make a best-effort attempt to remove the information from volatile
    1050       storage as promptly as possible after forwarding it.</t>
    1051     <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
    1052       particular, malicious or compromised caches might not recognize or obey this
    1053       directive, and communications networks might be vulnerable to eavesdropping.</t>
    1054   </list>
    1055 </t>
    1056 <t>
    1057   <x:dfn>max-age</x:dfn>
    1058   <iref item="Cache Directives" primary="true" subitem="max-age" />
    1059   <iref item="max-age" primary="true" subitem="Cache Directive" />
    1060   <list>
    1061     <t>The max-age request directive indicates that the client is willing to accept a
    1062       response whose age is no greater than the specified time in seconds. Unless
    1063       the max-stale request directive is also present, the client is not willing to accept a stale
    1064       response.</t>
    1065   </list>
    1066 </t>
    1067 <t>
    1068   <x:dfn>max-stale</x:dfn>
    1069   <iref item="Cache Directives" primary="true" subitem="max-stale" />
    1070   <iref item="max-stale" primary="true" subitem="Cache Directive" />
    1071   <list>
    1072     <t>The max-stale request directive indicates that the client is willing to accept a
    1073       response that has exceeded its expiration time. If max-stale is assigned a value,
    1074       then the client is willing to accept a response that has exceeded its expiration
    1075       time by no more than the specified number of seconds. If no value is assigned to
    1076       max-stale, then the client is willing to accept a stale response of any age.</t>
    1077   </list>
    1078 </t>
    1079 <t>
    1080   <x:dfn>min-fresh</x:dfn>
    1081   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
    1082   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
    1083   <list>
    1084     <t>The min-fresh request directive indicates that the client is willing to accept a
    1085       response whose freshness lifetime is no less than its current age plus the specified
    1086       time in seconds. That is, the client wants a response that will still be fresh for
    1087       at least the specified number of seconds.</t>
    1088   </list>
    1089 </t>
    1090 <t>
    1091   <x:dfn>no-transform</x:dfn>
    1092   <iref item="Cache Directives" primary="true" subitem="no-transform" />
    1093   <iref item="no-transform" primary="true" subitem="Cache Directive" />
    1094   <list>
    1095     <t>The no-transform request directive indicates that an intermediate cache or proxy
    1096       &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
    1097       headers, nor the request representation.</t>
    1098   </list>
    1099 </t>
    1100 <t>
    1101   <x:dfn>only-if-cached</x:dfn>
    1102   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
    1103   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
    1104   <list>
    1105     <t>The only-if-cached request directive indicates that the client only wishes to
    1106       return a stored response. If it receives this directive, a cache &SHOULD; either
    1107       respond using a stored response that is consistent with the other constraints of the
    1108       request, or respond with a 504 (Gateway Timeout) status code. If a group of caches is
    1109       being operated as a unified system with good internal connectivity, such a request
    1110       &MAY; be forwarded within that group of caches.</t>
    1111   </list>
    1112 </t>
    1113 </section>
    1114 
    1115 <section anchor="cache-response-directive" title="Response Cache-Control Directives">
    1116   <x:anchor-alias value="cache-response-directive" />
    1117 
    1118 <figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
     1128   <x:dfn>no-cache</x:dfn>
     1129   <iref item="Cache Directives" primary="true" subitem="no-cache" />
     1130   <iref item="no-cache" primary="true" subitem="Cache Directive" />
     1131   <list>
     1132      <t>The no-cache request directive indicates that a stored response
     1133      &MUST-NOT; be used to satisfy the request without successful validation
     1134      on the origin server.</t>
     1135   </list>
     1136</t>
     1137<t>
     1138   <x:dfn>no-store</x:dfn>
     1139   <iref item="Cache Directives" primary="true" subitem="no-store" />
     1140   <iref item="no-store" primary="true" subitem="Cache Directive" />
     1141   <list>
     1142      <t>The no-store request directive indicates that a cache &MUST-NOT;
     1143      store any part of either this request or any response to it. This
     1144      directive applies to both non-shared and shared caches. "&MUST-NOT;
     1145      store" in this context means that the cache &MUST-NOT; intentionally
     1146      store the information in non-volatile storage, and &MUST; make a
     1147      best-effort attempt to remove the information from volatile storage as
     1148      promptly as possible after forwarding it.</t>
     1149      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
     1150      privacy. In particular, malicious or compromised caches might not
     1151      recognize or obey this directive, and communications networks might be
     1152      vulnerable to eavesdropping.</t>
     1153   </list>
     1154</t>
     1155<t>
     1156   <x:dfn>max-age</x:dfn>
     1157   <iref item="Cache Directives" primary="true" subitem="max-age" />
     1158   <iref item="max-age" primary="true" subitem="Cache Directive" />
     1159   <list>
     1160      <t>The max-age request directive indicates that the client is willing to
     1161      accept a response whose age is no greater than the specified time in
     1162      seconds. Unless the max-stale request directive is also present, the
     1163      client is not willing to accept a stale response.</t>
     1164   </list>
     1165</t>
     1166<t>
     1167   <x:dfn>max-stale</x:dfn>
     1168   <iref item="Cache Directives" primary="true" subitem="max-stale" />
     1169   <iref item="max-stale" primary="true" subitem="Cache Directive" />
     1170   <list>
     1171      <t>The max-stale request directive indicates that the client is willing
     1172      to accept a response that has exceeded its expiration time. If max-stale
     1173      is assigned a value, then the client is willing to accept a response
     1174      that has exceeded its expiration time by no more than the specified
     1175      number of seconds. If no value is assigned to max-stale, then the client
     1176      is willing to accept a stale response of any age.</t>
     1177   </list>
     1178</t>
     1179<t>
     1180   <x:dfn>min-fresh</x:dfn>
     1181   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
     1182   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
     1183   <list>
     1184      <t>The min-fresh request directive indicates that the client is willing
     1185      to accept a response whose freshness lifetime is no less than its
     1186      current age plus the specified time in seconds. That is, the client
     1187      wants a response that will still be fresh for at least the specified
     1188      number of seconds.</t>
     1189   </list>
     1190</t>
     1191<t>
     1192   <x:dfn>no-transform</x:dfn>
     1193   <iref item="Cache Directives" primary="true" subitem="no-transform" />
     1194   <iref item="no-transform" primary="true" subitem="Cache Directive" />
     1195   <list>
     1196      <t>The no-transform request directive indicates that an intermediate
     1197      cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
     1198      Content-Type request headers, nor the request representation.</t>
     1199   </list>
     1200</t>
     1201<t>
     1202   <x:dfn>only-if-cached</x:dfn>
     1203   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
     1204   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
     1205   <list>
     1206      <t>The only-if-cached request directive indicates that the client only
     1207      wishes to return a stored response. If it receives this directive, a
     1208      cache &SHOULD; either respond using a stored response that is consistent
     1209      with the other constraints of the request, or respond with a 504
     1210      (Gateway Timeout) status code. If a group of caches is being operated as
     1211      a unified system with good internal connectivity, such a request &MAY;
     1212      be forwarded within that group of caches.</t>
     1213   </list>
     1214</t>
     1215</section>
     1216
     1217<section anchor="cache-response-directive"
     1218   title="Response Cache-Control Directives">
     1219   <x:anchor-alias value="cache-response-directive" />
     1220
     1221<figure><artwork type="abnf2616"><iref item="Grammar" primary="true"
     1222   subitem="cache-response-directive" />
    11191223  <x:ref>cache-response-directive</x:ref> =
    11201224       "public"
     
    11311235
    11321236<t>
    1133   <x:dfn>public</x:dfn>
    1134   <iref item="Cache Directives" primary="true" subitem="public" />
    1135   <iref item="public" primary="true" subitem="Cache Directive" />
    1136   <list>
    1137     <t>The public response directive indicates that the response &MAY; be cached, even
    1138       if it would normally be non-cacheable or cacheable only within a non-shared cache.
    1139       (See also Authorization, &header-authorization;, for additional details.) </t>
     1237   <x:dfn>public</x:dfn>
     1238   <iref item="Cache Directives" primary="true" subitem="public" />
     1239   <iref item="public" primary="true" subitem="Cache Directive" />
     1240   <list>
     1241      <t>The public response directive indicates that the response &MAY; be
     1242      cached, even if it would normally be non-cacheable or cacheable only
     1243      within a non-shared cache. (See also Authorization,
     1244      &header-authorization;, for additional details.) </t>
    11401245  </list>
    11411246</t>
    11421247<t>
    1143   <x:dfn>private</x:dfn>
    1144   <iref item="Cache Directives" primary="true" subitem="private" />
    1145   <iref item="private" primary="true" subitem="Cache Directive" />
    1146   <list>
    1147     <t>The private response directive indicates that the response message is intended for
    1148       a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
    1149       cache &MAY; store the response.</t>
    1150     <t>If the private response directive specifies one or more field-names, this
    1151       requirement is limited to the field-values associated with the listed response
    1152       headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
    1153       cache, whereas the remainder of the response message &MAY; be.</t>
    1154     <t>
    1155       <x:h>Note:</x:h> This usage of the word private only controls where the response can
    1156       be stored; it cannot ensure the privacy of the message content.
    1157       Also, private response directives with field-names are often handled by
    1158       implementations as if an unqualified private directive was received; i.e.,
    1159       the special handling for the qualified form is not widely implemented.</t>
    1160   </list>
    1161 </t>
    1162 <t>
    1163   <x:dfn>no-cache</x:dfn>
    1164   <iref item="Cache Directives" primary="true" subitem="no-cache" />
    1165   <iref item="no-cache" primary="true" subitem="Cache Directive" />
    1166   <list>
    1167     <t>The no-cache response directive indicates that the response MUST NOT be used
    1168           to satisfy a subsequent request without successful validation on the origin
    1169           server. This allows an origin server to prevent a cache from using it to satisfy
    1170           a request without contacting it, even by caches that have been configured to
    1171           return stale responses.</t>
    1172     <t>If the no-cache response directive specifies one or more field-names, this
    1173       requirement is limited to the field-values associated with the listed response
    1174       headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
    1175       to a subsequent request without successful validation on the origin server. This
    1176       allows an origin server to prevent the re-use of certain header fields in a
    1177       response, while still allowing caching of the rest of the response.</t>
    1178     <t>
    1179       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
    1180       Also, no-cache response directives with field-names are often handled by
    1181       implementations as if an unqualified no-cache directive was received; i.e.,
    1182       the special handling for the qualified form is not widely implemented.
    1183     </t>
    1184   </list>
    1185 </t>
    1186 <t>
    1187   <x:dfn>no-store</x:dfn>
    1188   <iref item="Cache Directives" primary="true" subitem="no-store" />
    1189   <iref item="no-store" primary="true" subitem="Cache Directive" />
    1190   <list>
    1191     <t>The no-store response directive indicates that a cache &MUST-NOT; store any
    1192       part of either the immediate request or response. This directive applies to both
    1193       non-shared and shared caches. "&MUST-NOT; store" in this context means that the
    1194       cache &MUST-NOT; intentionally store the information in non-volatile storage,
    1195       and &MUST; make a best-effort attempt to remove the information from volatile
    1196       storage as promptly as possible after forwarding it.</t>
    1197     <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
    1198       particular, malicious or compromised caches might not recognize or obey this
    1199       directive, and communications networks might be vulnerable to eavesdropping.</t>
    1200   </list>
    1201 </t>
    1202 <t>
    1203   <x:dfn>must-revalidate</x:dfn>
    1204   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
    1205   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
    1206   <list>
    1207     <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
    1208      used to satisfy subsequent requests without successful validation on the origin server.</t>
    1209     <t>The must-revalidate directive is necessary to support reliable operation for
    1210       certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
    1211       the must-revalidate directive; in particular, if the cache cannot reach the origin
    1212       server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
    1213     <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
    1214       validate a request on the representation could result in incorrect operation, such as a
    1215       silently unexecuted financial transaction.</t>
    1216   </list>
    1217 </t>
    1218 <t>
    1219   <x:dfn>proxy-revalidate</x:dfn>
    1220   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
    1221   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
    1222   <list>
    1223     <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
    1224       response directive, except that it does not apply to non-shared caches.</t>
    1225   </list>
    1226 </t>
    1227 <t>
    1228   <x:dfn>max-age</x:dfn>
    1229   <iref item="Cache Directives" primary="true" subitem="max-age" />
    1230   <iref item="max-age" primary="true" subitem="Cache Directive" />
    1231   <list>
    1232     <t>The max-age response directive indicates that response is to be considered stale
    1233       after its age is greater than the specified number of seconds.</t>
    1234   </list>
    1235 </t>
    1236 <t>
    1237   <x:dfn>s-maxage</x:dfn>
    1238   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
    1239   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
    1240   <list>
    1241     <t>The s-maxage response directive indicates that, in shared caches, the maximum age
    1242       specified by this directive overrides the maximum age specified by either the
    1243       max-age directive or the Expires header. The s-maxage directive also implies the
    1244       semantics of the proxy-revalidate response directive.</t>
    1245   </list>
    1246 </t>
    1247 <t>
    1248   <x:dfn>no-transform</x:dfn>
    1249   <iref item="Cache Directives" primary="true" subitem="no-transform" />
    1250   <iref item="no-transform" primary="true" subitem="Cache Directive" />
    1251   <list>
    1252     <t>The no-transform response directive indicates that an intermediate cache or proxy
    1253       &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
    1254       headers, nor the response representation.</t>
    1255   </list>
     1248   <x:dfn>private</x:dfn>
     1249   <iref item="Cache Directives" primary="true" subitem="private" />
     1250   <iref item="private" primary="true" subitem="Cache Directive" />
     1251   <list>
     1252      <t>The private response directive indicates that the response message is
     1253      intended for a single user and &MUST-NOT; be stored by a shared cache. A
     1254      private (non-shared) cache &MAY; store the response.</t>
     1255      <t>If the private response directive specifies one or more field-names,
     1256      this requirement is limited to the field-values associated with the
     1257      listed response headers. That is, the specified field-names(s)
     1258      &MUST-NOT; be stored by a shared cache, whereas the remainder of the
     1259      response message &MAY; be.</t>
     1260      <t> <x:h>Note:</x:h> This usage of the word private only controls where
     1261      the response can be stored; it cannot ensure the privacy of the message
     1262      content. Also, private response directives with field-names are often
     1263      handled by implementations as if an unqualified private directive was
     1264      received; i.e., the special handling for the qualified form is not
     1265      widely implemented.</t>
     1266   </list>
     1267</t>
     1268<t>
     1269   <x:dfn>no-cache</x:dfn>
     1270   <iref item="Cache Directives" primary="true" subitem="no-cache" />
     1271   <iref item="no-cache" primary="true" subitem="Cache Directive" />
     1272   <list>
     1273      <t>The no-cache response directive indicates that the response MUST NOT
     1274      be used to satisfy a subsequent request without successful validation on
     1275      the origin server. This allows an origin server to prevent a cache from
     1276      using it to satisfy a request without contacting it, even by caches that
     1277      have been configured to return stale responses.</t>
     1278      <t>If the no-cache response directive specifies one or more field-names,
     1279      this requirement is limited to the field-values associated with the
     1280      listed response headers. That is, the specified field-name(s) &MUST-NOT;
     1281      be sent in the response to a subsequent request without successful
     1282      validation on the origin server. This allows an origin server to prevent
     1283      the re-use of certain header fields in a response, while still allowing
     1284      caching of the rest of the response.</t>
     1285      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
     1286      this directive. Also, no-cache response directives with field-names are
     1287      often handled by implementations as if an unqualified no-cache directive
     1288      was received; i.e., the special handling for the qualified form is not
     1289      widely implemented. </t>
     1290   </list>
     1291</t>
     1292<t>
     1293   <x:dfn>no-store</x:dfn>
     1294   <iref item="Cache Directives" primary="true" subitem="no-store" />
     1295   <iref item="no-store" primary="true" subitem="Cache Directive" />
     1296   <list>
     1297      <t>The no-store response directive indicates that a cache &MUST-NOT;
     1298      store any part of either the immediate request or response. This
     1299      directive applies to both non-shared and shared caches. "&MUST-NOT;
     1300      store" in this context means that the cache &MUST-NOT; intentionally
     1301      store the information in non-volatile storage, and &MUST; make a
     1302      best-effort attempt to remove the information from volatile storage as
     1303      promptly as possible after forwarding it.</t>
     1304      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
     1305      privacy. In particular, malicious or compromised caches might not
     1306      recognize or obey this directive, and communications networks might be
     1307      vulnerable to eavesdropping.</t>
     1308   </list>
     1309</t>
     1310<t>
     1311   <x:dfn>must-revalidate</x:dfn>
     1312   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
     1313   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
     1314   <list>
     1315      <t>The must-revalidate response directive indicates that once it has
     1316      become stale, the response &MUST-NOT; be used to satisfy subsequent
     1317      requests without successful validation on the origin server.</t>
     1318      <t>The must-revalidate directive is necessary to support reliable
     1319      operation for certain protocol features. In all circumstances an
     1320      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in particular,
     1321      if the cache cannot reach the origin server for any reason, it &MUST;
     1322      generate a 504 (Gateway Timeout) response.</t>
     1323      <t>Servers &SHOULD; send the must-revalidate directive if and only if
     1324      failure to validate a request on the representation could result in
     1325      incorrect operation, such as a silently unexecuted financial
     1326      transaction.</t>
     1327   </list>
     1328</t>
     1329<t>
     1330   <x:dfn>proxy-revalidate</x:dfn>
     1331   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
     1332   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
     1333   <list>
     1334      <t>The proxy-revalidate response directive has the same meaning as the
     1335      must-revalidate response directive, except that it does not apply to
     1336      non-shared caches.</t>
     1337   </list>
     1338</t>
     1339<t>
     1340   <x:dfn>max-age</x:dfn>
     1341   <iref item="Cache Directives" primary="true" subitem="max-age" />
     1342   <iref item="max-age" primary="true" subitem="Cache Directive" />
     1343   <list>
     1344      <t>The max-age response directive indicates that response is to be
     1345      considered stale after its age is greater than the specified number of
     1346      seconds.</t>
     1347   </list>
     1348</t>
     1349<t>
     1350   <x:dfn>s-maxage</x:dfn>
     1351   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
     1352   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
     1353   <list>
     1354      <t>The s-maxage response directive indicates that, in shared caches, the
     1355      maximum age specified by this directive overrides the maximum age
     1356      specified by either the max-age directive or the Expires header. The
     1357      s-maxage directive also implies the semantics of the proxy-revalidate
     1358      response directive.</t>
     1359   </list>
     1360</t>
     1361<t>
     1362   <x:dfn>no-transform</x:dfn>
     1363   <iref item="Cache Directives" primary="true" subitem="no-transform" />
     1364   <iref item="no-transform" primary="true" subitem="Cache Directive" />
     1365   <list>
     1366      <t>The no-transform response directive indicates that an intermediate
     1367      cache or proxy &MUST-NOT; change the Content-Encoding, Content-Range or
     1368      Content-Type response headers, nor the response representation.</t>
     1369   </list>
    12561370</t>
    12571371
     
    12601374<section anchor="cache.control.extensions" title="Cache Control Extensions">
    12611375<t>
    1262   The Cache-Control header field can be extended through the use of one or more
    1263   cache-extension tokens, each with an optional value. Informational extensions (those
    1264   that do not require a change in cache behavior) can be added without changing the
    1265   semantics of other directives. Behavioral extensions are designed to work by acting as
    1266   modifiers to the existing base of cache directives. Both the new directive and the
    1267   standard directive are supplied, such that applications that do not understand the new
    1268   directive will default to the behavior specified by the standard directive, and those
    1269   that understand the new directive will recognize it as modifying the requirements
    1270   associated with the standard directive. In this way, extensions to the cache-control
    1271   directives can be made without requiring changes to the base protocol.
    1272 </t>
    1273 <t>
    1274   This extension mechanism depends on an HTTP cache obeying all of the cache-control
    1275   directives defined for its native HTTP-version, obeying certain extensions, and ignoring
    1276   all directives that it does not understand.
    1277 </t>
    1278 <t>
    1279   For example, consider a hypothetical new response directive called "community" that
    1280   acts as a modifier to the private directive. We define this new directive to mean that,
    1281   in addition to any non-shared cache, any cache that is shared only by members of the
    1282   community named within its value may cache the response. An origin server wishing to
    1283   allow the UCI community to use an otherwise private response in their shared cache(s)
    1284   could do so by including
     1376   The Cache-Control header field can be extended through the use of one or
     1377   more cache-extension tokens, each with an optional value. Informational
     1378   extensions (those that do not require a change in cache behavior) can be
     1379   added without changing the semantics of other directives. Behavioral
     1380   extensions are designed to work by acting as modifiers to the existing base
     1381   of cache directives. Both the new directive and the standard directive are
     1382   supplied, such that applications that do not understand the new directive
     1383   will default to the behavior specified by the standard directive, and those
     1384   that understand the new directive will recognize it as modifying the
     1385   requirements associated with the standard directive. In this way,
     1386   extensions to the cache-control directives can be made without requiring
     1387   changes to the base protocol.
     1388</t>
     1389<t>
     1390   This extension mechanism depends on an HTTP cache obeying all of the
     1391   cache-control directives defined for its native HTTP-version, obeying
     1392   certain extensions, and ignoring all directives that it does not
     1393   understand.
     1394</t>
     1395<t>
     1396   For example, consider a hypothetical new response directive called
     1397   "community" that acts as a modifier to the private directive. We define
     1398   this new directive to mean that, in addition to any non-shared cache, any
     1399   cache that is shared only by members of the community named within its
     1400   value may cache the response. An origin server wishing to allow the UCI
     1401   community to use an otherwise private response in their shared cache(s)
     1402   could do so by including
    12851403</t>
    12861404<figure><artwork type="example">
     
    12881406</artwork></figure>
    12891407<t>
    1290   A cache seeing this header field will act correctly even if the cache does not
    1291   understand the community cache-extension, since it will also see and understand the
    1292   private directive and thus default to the safe behavior.
    1293 </t>
    1294 <t>
    1295   Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
    1296   directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
    1297   directives (or the response's default cacheability) such that the cache behavior will
    1298   remain minimally correct even if the cache does not understand the extension(s).
    1299 </t>
    1300 <t>
    1301   The HTTP Cache Directive Registry defines the name space for the cache
    1302   directives.
    1303 </t>
    1304 <t>
    1305   Registrations &MUST; include the following fields:
    1306   <list style="symbols">
    1307     <t>Cache Directive Name</t>
    1308     <t>Pointer to specification text</t>
    1309   </list>
    1310 </t>
    1311 <t>
    1312   Values to be added to this name space are subject to IETF review
    1313   (<xref target="RFC5226" x:fmt="," x:sec="4.1"/>).
    1314 </t>
    1315 <t>
    1316   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
     1408   A cache seeing this header field will act correctly even if the cache does
     1409   not understand the community cache-extension, since it will also see and
     1410   understand the private directive and thus default to the safe behavior.
     1411</t>
     1412<t>
     1413   Unrecognized cache directives &MUST; be ignored; it is assumed that any
     1414   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
     1415   combined with standard directives (or the response's default cacheability)
     1416   such that the cache behavior will remain minimally correct even if the
     1417   cache does not understand the extension(s).
     1418</t>
     1419<t>
     1420   The HTTP Cache Directive Registry defines the name space for the cache
     1421   directives.
     1422</t>
     1423<t>
     1424   Registrations &MUST; include the following fields:
     1425   <list style="symbols">
     1426      <t>Cache Directive Name</t>
     1427      <t>Pointer to specification text</t>
     1428   </list>
     1429</t>
     1430<t>
     1431   Values to be added to this name space are subject to IETF review (<xref
     1432   target="RFC5226" x:fmt="," x:sec="4.1"/>).
     1433</t>
     1434<t>
     1435   The registry itself is maintained at <eref
     1436   target="http://www.iana.org/assignments/http-cache-directives"/>.
    13171437</t>
    13181438</section>
     
    13211441
    13221442<section anchor="header.expires" title="Expires">
    1323   <iref item="Expires header" primary="true" x:for-anchor="" />
    1324   <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
    1325   <x:anchor-alias value="Expires"/>
    1326   <x:anchor-alias value="Expires-v"/>
    1327 <t>
    1328   The "Expires" entity-header field gives the date/time after which the response is
    1329   considered stale. See <xref target="expiration.model" /> for further discussion of the
    1330   freshness model.
    1331 </t>
    1332 <t>
    1333   The presence of an Expires field does not imply that the original resource will change or
    1334   cease to exist at, before, or after that time.
    1335 </t>
    1336 <t>
    1337   The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
    1338   it &MUST; be sent in rfc1123-date format.
     1443   <iref item="Expires header" primary="true" x:for-anchor="" />
     1444   <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
     1445   <x:anchor-alias value="Expires"/>
     1446   <x:anchor-alias value="Expires-v"/>
     1447<t>
     1448   The "Expires" entity-header field gives the date/time after which the
     1449   response is considered stale. See <xref target="expiration.model" /> for
     1450   further discussion of the freshness model.
     1451</t>
     1452<t>
     1453   The presence of an Expires field does not imply that the original resource
     1454   will change or cease to exist at, before, or after that time.
     1455</t>
     1456<t>
     1457   The field-value is an absolute date and time as defined by HTTP-date in
     1458   &full-date;; it &MUST; be sent in rfc1123-date format.
    13391459</t>
    13401460<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
     
    13481468</artwork></figure>
    13491469<x:note>
    1350   <t>
    1351     <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
    1352     directive (see <xref target="cache-response-directive" />), that directive overrides
    1353     the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
    1354   </t>
     1470   <t>
     1471       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
     1472       max-age directive (see <xref target="cache-response-directive" />),
     1473       that directive overrides the Expires field. Likewise, the s-maxage
     1474       directive overrides Expires in shared caches.
     1475   </t>
    13551476</x:note>
    13561477<t>
    1357   HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
    1358 </t>
    1359 <t>
    1360   HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
    1361   including the value "0", as in the past (i.e., "already expired").
     1478   HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the
     1479   future.
     1480</t>
     1481<t>
     1482   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
     1483   especially including the value "0", as in the past (i.e., "already
     1484   expired").
    13621485</t>
    13631486</section>
    13641487
    13651488<section anchor="header.pragma" title="Pragma">
    1366   <iref item="Pragma header" primary="true" x:for-anchor="" />
    1367   <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
    1368   <x:anchor-alias value="extension-pragma"/>
    1369   <x:anchor-alias value="Pragma"/>
    1370   <x:anchor-alias value="Pragma-v"/>
    1371   <x:anchor-alias value="pragma-directive"/>
    1372 <t>
    1373   The "Pragma" general-header field is used to include implementation-specific directives
    1374   that might apply to any recipient along the request/response chain. All pragma directives
    1375   specify optional behavior from the viewpoint of the protocol; however, some systems
    1376   &MAY; require that behavior be consistent with the directives.
     1489   <iref item="Pragma header" primary="true" x:for-anchor="" />
     1490   <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
     1491   <x:anchor-alias value="extension-pragma"/>
     1492   <x:anchor-alias value="Pragma"/>
     1493   <x:anchor-alias value="Pragma-v"/>
     1494   <x:anchor-alias value="pragma-directive"/>
     1495<t>
     1496   The "Pragma" general-header field is used to include
     1497   implementation-specific directives that might apply to any recipient along
     1498   the request/response chain. All pragma directives specify optional behavior
     1499   from the viewpoint of the protocol; however, some systems &MAY; require
     1500   that behavior be consistent with the directives.
    13771501</t>
    13781502<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"/>
     
    13831507</artwork></figure>
    13841508<t>
    1385   When the no-cache directive is present in a request message, an application &SHOULD;
    1386   forward the request toward the origin server even if it has a cached copy of what is being
    1387   requested. This pragma directive has the same semantics as the no-cache response directive
    1388   (see <xref target="cache-response-directive" />) and is defined here for backward
    1389   compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
    1390   no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
    1391   &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
     1509   When the no-cache directive is present in a request message, an application
     1510   &SHOULD; forward the request toward the origin server even if it has a
     1511   cached copy of what is being requested. This pragma directive has the same
     1512   semantics as the no-cache response directive (see <xref
     1513   target="cache-response-directive" />) and is defined here for backward
     1514   compatibility with HTTP/1.0. Clients &SHOULD; include both header fields
     1515   when a no-cache request is sent to a server not known to be HTTP/1.1
     1516   compliant. HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the
     1517   client had sent "Cache-Control: no-cache".
    13921518</t>
    13931519<x:note>
    1394   <t>
    1395     <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
    1396     is not actually specified, it does not provide a reliable replacement for
    1397     "Cache-Control: no-cache" in a response.
    1398   </t>
     1520   <t>
     1521      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a
     1522      response-header field is not actually specified, it does not provide a
     1523      reliable replacement for "Cache-Control: no-cache" in a response.
     1524   </t>
    13991525</x:note>
    14001526<t>
    1401   This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
     1527   This mechanism is deprecated; no new Pragma directives will be defined in
     1528   HTTP.
    14021529</t>
    14031530</section>
    14041531
    14051532<section anchor="header.vary" title="Vary">
    1406   <iref item="Vary header" primary="true" x:for-anchor="" />
    1407   <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
    1408   <x:anchor-alias value="Vary"/>
    1409   <x:anchor-alias value="Vary-v"/>
    1410 <t>
    1411   The "Vary" response-header field conveys the set of request-header fields
    1412   that were used to select the representation.
    1413 </t>
    1414 <t>
    1415   Caches use this information, in part, to determine whether a stored response
    1416   can be used to satisfy a given request; see
    1417   <xref target="caching.negotiated.responses" />.
    1418   determines, while the response is fresh, whether a cache is permitted to use the
    1419   response to reply to a subsequent request without validation; see <xref
    1420   target="caching.negotiated.responses" />.
    1421 </t>
    1422 <t>
    1423   In uncacheable or stale responses, the Vary field value advises the user agent about
    1424   the criteria that were used to select the representation.
     1533   <iref item="Vary header" primary="true" x:for-anchor="" />
     1534   <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
     1535   <x:anchor-alias value="Vary"/>
     1536   <x:anchor-alias value="Vary-v"/>
     1537<t>
     1538   The "Vary" response-header field conveys the set of request-header fields
     1539   that were used to select the representation.
     1540</t>
     1541<t>
     1542   Caches use this information, in part, to determine whether a stored
     1543   response can be used to satisfy a given request; see <xref
     1544   target="caching.negotiated.responses" />. determines, while the response is
     1545   fresh, whether a cache is permitted to use the response to reply to a
     1546   subsequent request without validation; see <xref
     1547   target="caching.negotiated.responses" />.
     1548</t>
     1549<t>
     1550   In uncacheable or stale responses, the Vary field value advises the user
     1551   agent about the criteria that were used to select the representation.
    14251552</t>
    14261553<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
     
    14291556</artwork></figure>
    14301557<t>
    1431   The set of header fields named by the Vary field value is known as the selecting
    1432   request-headers.
    1433 </t>
    1434 <t>
    1435   Servers &SHOULD; include a Vary header field with any cacheable response that is
    1436   subject to server-driven negotiation. Doing so allows a cache to properly interpret future
    1437   requests on that resource and informs the user agent about the presence of negotiation on
    1438   that resource. A server &MAY; include a Vary header field with a non-cacheable
    1439   response that is subject to server-driven negotiation, since this might provide the user
    1440   agent with useful information about the dimensions over which the response varies at the
    1441   time of the response.
    1442 </t>
    1443 <t>
    1444   A Vary field value of "*" signals that unspecified parameters not limited to the
    1445   request-headers (e.g., the network address of the client), play a role in the selection of
    1446   the response representation; therefore, a cache cannot determine whether this response is
    1447   appropriate. The "*" value &MUST-NOT; be generated by a proxy server.
    1448 </t>
    1449 <t>
    1450   The field-names given are not limited to the set of standard request-header fields
    1451   defined by this specification. Field names are case-insensitive.
     1558   The set of header fields named by the Vary field value is known as the
     1559   selecting request-headers.
     1560</t>
     1561<t>
     1562   Servers &SHOULD; include a Vary header field with any cacheable response
     1563   that is subject to server-driven negotiation. Doing so allows a cache to
     1564   properly interpret future requests on that resource and informs the user
     1565   agent about the presence of negotiation on that resource. A server &MAY;
     1566   include a Vary header field with a non-cacheable response that is subject
     1567   to server-driven negotiation, since this might provide the user agent with
     1568   useful information about the dimensions over which the response varies at
     1569   the time of the response.
     1570</t>
     1571<t>
     1572   A Vary field value of "*" signals that unspecified parameters not limited
     1573   to the request-headers (e.g., the network address of the client), play a
     1574   role in the selection of the response representation; therefore, a cache
     1575   cannot determine whether this response is appropriate. The "*" value
     1576   &MUST-NOT; be generated by a proxy server.
     1577</t>
     1578<t>
     1579   The field-names given are not limited to the set of standard request-header
     1580   fields defined by this specification. Field names are case-insensitive.
    14521581</t>
    14531582</section>
    14541583
    14551584<section anchor="header.warning" title="Warning">
    1456   <iref item="Warning header" primary="true" x:for-anchor="" />
    1457   <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
    1458   <x:anchor-alias value="Warning"/>
    1459   <x:anchor-alias value="Warning-v"/>
    1460   <x:anchor-alias value="warning-value"/>
    1461   <x:anchor-alias value="warn-agent"/>
    1462   <x:anchor-alias value="warn-code"/>
    1463   <x:anchor-alias value="warn-date"/>
    1464   <x:anchor-alias value="warn-text"/>
    1465 <t>
    1466   The "Warning" general-header field is used to carry additional information about the status
    1467   or transformation of a message that might not be reflected in the message. This
    1468   information is typically used to warn about possible incorrectness introduced by caching
    1469   operations or transformations applied to the payload of the message.
    1470 </t>
    1471 <t>
    1472   Warnings can be used for other purposes, both cache-related and otherwise. The use of a
    1473   warning, rather than an error status code, distinguishes these responses from true failures.
    1474 </t>
    1475 <t>
    1476   Warning headers can in general be applied to any message, however some warn-codes are
    1477   specific to caches and can only be applied to response messages.
     1585   <iref item="Warning header" primary="true" x:for-anchor="" />
     1586   <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
     1587   <x:anchor-alias value="Warning"/>
     1588   <x:anchor-alias value="Warning-v"/>
     1589   <x:anchor-alias value="warning-value"/>
     1590   <x:anchor-alias value="warn-agent"/>
     1591   <x:anchor-alias value="warn-code"/>
     1592   <x:anchor-alias value="warn-date"/>
     1593   <x:anchor-alias value="warn-text"/>
     1594<t>
     1595   The "Warning" general-header field is used to carry additional information
     1596   about the status or transformation of a message that might not be reflected
     1597   in the message. This information is typically used to warn about possible
     1598   incorrectness introduced by caching operations or transformations applied
     1599   to the payload of the message.
     1600</t>
     1601<t>
     1602   Warnings can be used for other purposes, both cache-related and otherwise.
     1603   The use of a warning, rather than an error status code, distinguishes these
     1604   responses from true failures.
     1605</t>
     1606<t>
     1607   Warning headers can in general be applied to any message, however some
     1608   warn-codes are specific to caches and can only be applied to response
     1609   messages.
    14781610</t>
    14791611<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"/>
     
    14921624</artwork></figure>
    14931625<t>
    1494   Multiple warnings can be attached to a response (either by the origin server or by
    1495   a cache), including multiple warnings with the same code number, only differing
    1496   in warn-text.
    1497 </t>
    1498 <t>
    1499   When this occurs, the user agent &SHOULD; inform the user of as many of them as
    1500   possible, in the order that they appear in the response.
    1501 </t>
    1502 <t>
    1503   Systems that generate multiple Warning headers &SHOULD; order them with this user
    1504   agent behavior in mind. New Warning headers &SHOULD; be added after any existing
    1505   Warning headers.
    1506 </t>
    1507 <t>
    1508   Warnings are assigned three digit warn-codes. The first digit indicates whether the
    1509   Warning is required to be deleted from a stored response after validation:
    1510   <list style="symbols">
    1511     <t>1xx Warnings describe the freshness or validation status of the response, and so
    1512       &MUST; be deleted by caches after validation. They can only be generated by a cache
    1513       when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
    1514     <t>2xx Warnings describe some aspect of the representation that is
    1515       not rectified by a validation (for example, a lossy compression of the representation)
    1516       and &MUST-NOT; be deleted by caches after validation, unless a full response is
    1517       returned, in which case they &MUST; be.</t>
    1518   </list>
    1519 </t>
    1520 <t>
    1521   If an implementation sends a message with one or more Warning headers to a receiver whose
    1522   version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
    1523   warn-date that matches the Date header in the message.
    1524 </t>
    1525 <t>
    1526   If an implementation receives a message with a warning-value that includes a warn-date,
    1527   and that warn-date is different from the Date value in the response, then that
    1528   warning-value &MUST; be deleted from the message before storing, forwarding, or using
    1529   it. (preventing the consequences of naive caching of Warning header fields.) If all of the
    1530   warning-values are deleted for this reason, the Warning header &MUST; be deleted as
    1531   well.
    1532 </t>
    1533 <t>
    1534   The following warn-codes are defined by this specification, each with a recommended
    1535   warn-text in English, and a description of its meaning.
     1626   Multiple warnings can be attached to a response (either by the origin
     1627   server or by a cache), including multiple warnings with the same code
     1628   number, only differing in warn-text.
     1629</t>
     1630<t>
     1631   When this occurs, the user agent &SHOULD; inform the user of as many of
     1632   them as possible, in the order that they appear in the response.
     1633</t>
     1634<t>
     1635   Systems that generate multiple Warning headers &SHOULD; order them with
     1636   this user agent behavior in mind. New Warning headers &SHOULD; be added
     1637   after any existing Warning headers.
     1638</t>
     1639<t>
     1640   Warnings are assigned three digit warn-codes. The first digit indicates
     1641   whether the Warning is required to be deleted from a stored response after
     1642   validation:
     1643   <list style="symbols">
     1644      <t>1xx Warnings describe the freshness or validation status of the
     1645      response, and so &MUST; be deleted by caches after validation. They can
     1646      only be generated by a cache when validating a cached entry, and
     1647      &MUST-NOT; be generated in any other situation.</t>
     1648      <t>2xx Warnings describe some aspect of the representation that is not
     1649      rectified by a validation (for example, a lossy compression of the
     1650      representation) and &MUST-NOT; be deleted by caches after validation,
     1651      unless a full response is returned, in which case they &MUST; be.</t>
     1652   </list>
     1653</t>
     1654<t>
     1655   If an implementation sends a message with one or more Warning headers to a
     1656   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
     1657   in each warning-value a warn-date that matches the Date header in the
     1658   message.
     1659</t>
     1660<t>
     1661   If an implementation receives a message with a warning-value that includes
     1662   a warn-date, and that warn-date is different from the Date value in the
     1663   response, then that warning-value &MUST; be deleted from the message before
     1664   storing, forwarding, or using it. (preventing the consequences of naive
     1665   caching of Warning header fields.) If all of the warning-values are deleted
     1666   for this reason, the Warning header &MUST; be deleted as well.
     1667</t>
     1668<t>
     1669   The following warn-codes are defined by this specification, each with a
     1670   recommended warn-text in English, and a description of its meaning.
    15361671</t>
    15371672<t><?rfc needLines="4"?>
    1538   110 Response is stale
    1539   <list>
    1540     <t>&SHOULD; be included whenever the returned response is stale.</t>
    1541   </list>
     1673   110 Response is stale
     1674   <list>
     1675      <t>&SHOULD; be included whenever the returned response is stale.</t>
     1676   </list>
    15421677</t>
    15431678<t><?rfc needLines="4"?>
    1544   111 Revalidation failed
    1545   <list>
    1546     <t>&SHOULD; be included if a cache returns a stale response because an attempt to
    1547       validate the response failed, due to an inability to reach the server.</t>
    1548   </list>
     1679   111 Revalidation failed
     1680   <list>
     1681      <t>&SHOULD; be included if a cache returns a stale response because an
     1682      attempt to validate the response failed, due to an inability to reach
     1683      the server.</t>
     1684   </list>
    15491685</t>
    15501686<t><?rfc needLines="4"?>
    1551   112 Disconnected operation
    1552   <list>
    1553     <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
    1554       the network for a period of time.</t>
    1555   </list>
     1687   112 Disconnected operation
     1688   <list>
     1689      <t>&SHOULD; be included if the cache is intentionally disconnected from
     1690      the rest of the network for a period of time.</t>
     1691   </list>
    15561692</t>
    15571693<t><?rfc needLines="4"?>
    1558   113 Heuristic expiration
    1559   <list>
    1560     <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
    1561       greater than 24 hours and the response's age is greater than 24 hours.</t>
    1562   </list>
     1694   113 Heuristic expiration
     1695   <list>
     1696      <t>&SHOULD; be included if the cache heuristically chose a freshness
     1697      lifetime greater than 24 hours and the response's age is greater than 24
     1698      hours.</t>
     1699   </list>
    15631700</t>
    15641701<t><?rfc needLines="4"?>
    1565   199 Miscellaneous warning
    1566   <list>
    1567     <t>The warning text can include arbitrary information to be presented to a human
    1568       user, or logged. A system receiving this warning &MUST-NOT; take any automated
    1569       action, besides presenting the warning to the user.</t>
    1570   </list>
     1702   199 Miscellaneous warning
     1703   <list>
     1704      <t>The warning text can include arbitrary information to be presented to
     1705      a human user, or logged. A system receiving this warning &MUST-NOT; take
     1706      any automated action, besides presenting the warning to the user.</t>
     1707   </list>
    15711708</t>
    15721709<t><?rfc needLines="4"?>
    1573   214 Transformation applied
    1574   <list>
    1575     <t>&MUST; be added by an intermediate proxy if it applies any
    1576       transformation to the representation, such as changing the content-coding,
    1577       media-type, or modifying the representation data, unless this Warning code
    1578       already appears in the response.</t>
    1579   </list>
     1710   214 Transformation applied
     1711   <list>
     1712      <t>&MUST; be added by an intermediate proxy if it applies any
     1713      transformation to the representation, such as changing the
     1714      content-coding, media-type, or modifying the representation data, unless
     1715      this Warning code already appears in the response.</t>
     1716   </list>
    15801717</t>
    15811718<t><?rfc needLines="4"?>
    1582   299 Miscellaneous persistent warning
    1583   <list>
    1584     <t>The warning text can include arbitrary information to be presented to a human
    1585       user, or logged. A system receiving this warning &MUST-NOT; take any automated
    1586       action.</t>
    1587   </list>
     1719   299 Miscellaneous persistent warning
     1720   <list>
     1721      <t>The warning text can include arbitrary information to be presented to
     1722      a human user, or logged. A system receiving this warning &MUST-NOT; take
     1723      any automated action.</t>
     1724   </list>
    15881725</t>
    15891726</section>
     
    15931730<section anchor="history.lists" title="History Lists">
    15941731<t>
    1595   User agents often have history mechanisms, such as "Back" buttons and history lists, that
    1596   can be used to redisplay a representation retrieved earlier in a session.
    1597 </t>
    1598 <t>
    1599   The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
    1600   a history mechanism can display a previous representation even if it has expired.
    1601 </t>
    1602   <t>
    1603   This does not prohibit the history mechanism from telling the user that a
    1604   view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
    1605   </t>
     1732   User agents often have history mechanisms, such as "Back" buttons and
     1733   history lists, that can be used to redisplay a representation retrieved
     1734   earlier in a session.
     1735</t>
     1736<t>
     1737   The freshness model (<xref target="expiration.model"/>) does not
     1738   necessarily apply to history mechanisms. I.e., a history mechanism can
     1739   display a previous representation even if it has expired.
     1740</t>
     1741<t>
     1742   This does not prohibit the history mechanism from telling the user that a
     1743   view might be stale, or from honoring cache directives (e.g.,
     1744   Cache-Control: no-store).
     1745</t>
    16061746</section>
    16071747
     
    16091749<section anchor="IANA.considerations" title="IANA Considerations">
    16101750
    1611 <section title="Cache Directive Registry" anchor="cache.directive.registration">
    1612 <t>
    1613   The registration procedure for HTTP Cache Directives is defined by
    1614   <xref target="cache.control.extensions"/> of this document.
    1615 </t>
    1616 <t>
    1617    The HTTP Cache Directive Registry should be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/>
    1618    and be populated with the registrations below:
     1751<section title="Cache Directive Registry"
     1752   anchor="cache.directive.registration">
     1753<t>
     1754   The registration procedure for HTTP Cache Directives is defined by <xref
     1755   target="cache.control.extensions"/> of this document.
     1756</t>
     1757<t>
     1758   The HTTP Cache Directive Registry should be created at <eref
     1759   target="http://www.iana.org/assignments/http-cache-directives"/> and be
     1760   populated with the registrations below:
    16191761</t>
    16201762<?BEGININC p6-cache.cache-directives ?>
     
    16881830<section title="Header Field Registration" anchor="header.field.registration">
    16891831<t>
    1690   The Message Header Field Registry located at <eref
     1832  The Message Header Field Registry located at <eref 
    16911833  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
    16921834  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
     
    17401882<?ENDINC p6-cache.iana-headers ?>
    17411883<t>
    1742   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
     1884   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
     1885   Force".
    17431886</t>
    17441887</section>
     
    17481891<section anchor="security.considerations" title="Security Considerations">
    17491892<t>
    1750   Caches expose additional potential vulnerabilities, since the contents of the cache
    1751   represent an attractive target for malicious exploitation. Because cache contents persist
    1752   after an HTTP request is complete, an attack on the cache can reveal information long after
    1753   a user believes that the information has been removed from the network. Therefore, cache
    1754   contents should be protected as sensitive information.
     1893   Caches expose additional potential vulnerabilities, since the contents of
     1894   the cache represent an attractive target for malicious exploitation.
     1895   Because cache contents persist after an HTTP request is complete, an attack
     1896   on the cache can reveal information long after a user believes that the
     1897   information has been removed from the network. Therefore, cache contents
     1898   should be protected as sensitive information.
    17551899</t>
    17561900</section>
     
    17581902<section anchor="ack" title="Acknowledgments">
    17591903<t>
    1760   Much of the content and presentation of the caching design is due to suggestions and
    1761   comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
    1762   and Larry Masinter.
     1904   Much of the content and presentation of the caching design is due to
     1905   suggestions and comments from individuals including: Shel Kaphan, Paul
     1906   Leach, Koen Holtman, David Morris, and Larry Masinter.
    17631907</t>
    17641908</section>
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