source: draft-ietf-httpbis/latest-roy/p6-cache.xml @ 473

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<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
  <front>

    <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>

    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
      <organization abbrev="Day Software">Day Software</organization>
      <address>
      <postal>
        <street>23 Corporate Plaza DR, Suite 280</street>
        <city>Newport Beach</city>
        <region>CA</region>
        <code>92660</code>
        <country>USA</country>
      </postal>
      <phone>+1-949-706-5300</phone>
      <facsimile>+1-949-706-5305</facsimile>
      <email>fielding@gbiv.com</email>
      <uri>http://roy.gbiv.com/</uri>
    </address>
    </author>

    <author fullname="Jim Gettys" initials="J." surname="Gettys">
      <organization>One Laptop per Child</organization>
      <address>
      <postal>
        <street>21 Oak Knoll Road</street>
        <city>Carlisle</city>
        <region>MA</region>
        <code>01741</code>
        <country>USA</country>
      </postal>
      <email>jg@laptop.org</email>
      <uri>http://www.laptop.org/</uri>
    </address>
    </author>

    <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address>
      <postal>
        <street>HP Labs, Large Scale Systems Group</street>
        <street>1501 Page Mill Road, MS 1177</street>
        <city>Palo Alto</city>
        <region>CA</region>
        <code>94304</code>
        <country>USA</country>
      </postal>
      <email>JeffMogul@acm.org</email>
    </address>
    </author>

    <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
        <country>USA</country>
      </postal>
      <email>henrikn@microsoft.com</email>
    </address>
    </author>

    <author fullname="Larry Masinter" initials="L." surname="Masinter">
      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
      <address>
      <postal>
        <street>345 Park Ave</street>
        <city>San Jose</city>
        <region>CA</region>
        <code>95110</code>
        <country>USA</country>
      </postal>
      <email>LMM@acm.org</email>
      <uri>http://larry.masinter.net/</uri>
    </address>
    </author>

    <author fullname="Paul J. Leach" initials="P." surname="Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
      </postal>
      <email>paulle@microsoft.com</email>
    </address>
    </author>

    <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address>
      <postal>
        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
        <street>The Stata Center, Building 32</street>
        <street>32 Vassar Street</street>
        <city>Cambridge</city>
        <region>MA</region>
        <code>02139</code>
        <country>USA</country>
      </postal>
      <email>timbl@w3.org</email>
      <uri>http://www.w3.org/People/Berners-Lee/</uri>
    </address>
    </author>

    <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address>
      <postal>
        <street>W3C / ERCIM</street>
        <street>2004, rte des Lucioles</street>
        <city>Sophia-Antipolis</city>
        <region>AM</region>
        <code>06902</code>
        <country>France</country>
      </postal>
      <email>ylafon@w3.org</email>
      <uri>http://www.raubacapeu.net/people/yves/</uri>
    </address>
    </author>

    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address>
      <postal>
        <street>Hafenweg 16</street>
        <city>Muenster</city><region>NW</region><code>48155</code>
        <country>Germany</country>
      </postal>
      <phone>+49 251 2807760</phone>    
      <facsimile>+49 251 2807761</facsimile>    
      <email>julian.reschke@greenbytes.de</email>       
      <uri>http://greenbytes.de/tech/webdav/</uri>      
    </address>
    </author>

    <date month="&ID-MONTH;" year="&ID-YEAR;" />
    <workgroup>HTTPbis Working Group</workgroup>

    <abstract>
      <t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
        collaborative, hypermedia information systems. This document is Part 6 of the seven-part
        specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
        obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
        fields that control cache behavior or indicate cacheable response messages.</t>
    </abstract>

    <note title="Editorial Note (To be removed by RFC Editor)">
      <t>Discussion of this draft should take place on the HTTPBIS working group mailing list
        (ietf-http-wg@w3.org). The current issues list is at <eref
          target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
        (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.</t>
      <t>The changes in this draft are summarized in <xref target="changes.since.04" />.</t>
    </note>
  </front>
  <middle>
    <section anchor="caching" title="Introduction">
      <t>HTTP is typically used for distributed information systems, where performance can be
        improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
        caching and reusing response messages.</t>

      <section anchor="intro.purpose" title="Purpose">
        <iref item="cache" />
        <t>An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
          controls its message storage, retrieval, and deletion. A cache stores cacheable responses
          in order to reduce the response time and network bandwidth consumption on future,
          equivalent requests. Any client or server may include a cache, though a cache cannot be
          used by a server that is acting as a tunnel.</t>
        <t>Caching would be useless if it did not significantly improve performance. The goal of
          caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
          some cases, a stored response can be reused without the need for a network request,
          reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
          (see <xref target="expiration.model" />). Even when a new request is required, it is often
          possible to reuse all or parts of the payload of a prior response to satisfy the request,
          thereby reducing network bandwidth usage; a "validation" mechanism is used for this
          purpose (see <xref target="validation.model" />).</t>
      </section>

      <section anchor="intro.terminology" title="Terminology">
        <t>This specification uses a number of terms to refer to the roles played by participants
          in, and objects of, HTTP caching.</t>
        <t>
          <iref item="cacheable" />
          <x:dfn>cacheable</x:dfn>
          <list>
            <t>A response is cacheable if a cache is allowed to store a copy of the response message
              for use in answering subsequent requests. Even when a response is cacheable, there may
              be additional constraints on whether a cache can use the cached copy to satisfy a
              particular request.</t>
          </list>
        </t>
        <t>
          <iref item="explicit expiration time" />
          <x:dfn>explicit expiration time</x:dfn>
          <list>
            <t>The time at which the origin server intends that an entity should no longer be
              returned by a cache without further validation.</t>
          </list>
        </t>
        <t>
          <iref item="heuristic expiration time" />
          <x:dfn>heuristic expiration time</x:dfn>
          <list>
            <t>An expiration time assigned by a cache when no explicit expiration time is
            available.</t>
          </list>
        </t>
        <t>
          <iref item="age" />
          <x:dfn>age</x:dfn>
          <list>
            <t>The age of a response is the time since it was sent by, or successfully validated
              with, the origin server.</t>
          </list>
        </t>
        <t>
          <iref item="first-hand" />
          <x:dfn>first-hand</x:dfn>
          <list>
            <t>A response is first-hand if the freshness model is not in use; i.e., its age is
            0.</t>
          </list>
        </t>
        <t>
          <iref item="freshness lifetime" />
          <x:dfn>freshness lifetime</x:dfn>
          <list>
            <t>The length of time between the generation of a response and its expiration time. </t>
          </list>
        </t>
        <t>
          <iref item="fresh" />
          <x:dfn>fresh</x:dfn>
          <list>
            <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
          </list>
        </t>
        <t>
          <iref item="stale" />
          <x:dfn>stale</x:dfn>
          <list>
            <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
          </list>
        </t>
        <t>
          <iref item="validator" />
          <x:dfn>validator</x:dfn>
          <list>
            <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
              out whether a stored response is an equivalent copy of an entity.</t>
          </list>
        </t>
        <t>
          <iref item="validator" />
          <x:dfn>shared cache</x:dfn>
          <list>
            <t>A cache that is accessible to more than one user. A non-shared cache is
              dedicated to a single user.</t>
          </list>
        </t>
      </section>


      <section anchor="intro.requirements" title="Requirements">
        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
          NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
          described in <xref target="RFC2119" />.</t>
        <t>An implementation is not compliant if it fails to satisfy one or more of the &MUST;
          or &REQUIRED; level requirements for the protocols it implements. An implementation
          that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
          requirements for its protocols is said to be "unconditionally compliant"; one that
          satisfies all the &MUST; level requirements but not all the &SHOULD; level
          requirements for its protocols is said to be "conditionally compliant."</t>
      </section>


<section title="Syntax Notation" anchor="notation">
  <x:anchor-alias value="ALPHA"/>
  <x:anchor-alias value="CR"/>
  <x:anchor-alias value="DIGIT"/>
  <x:anchor-alias value="DQUOTE"/>
  <x:anchor-alias value="LF"/>
  <x:anchor-alias value="OCTET"/>
  <x:anchor-alias value="SP"/>
  <x:anchor-alias value="VCHAR"/>
  <x:anchor-alias value="WSP"/>
<t>
  This specification uses the ABNF syntax defined in &notation;.
  The following core rules are included by
  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
  DIGIT (decimal 0-9), DQUOTE (double quote),
  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
  OCTET (any 8-bit sequence of data), SP (space),
  VCHAR (any visible USASCII character),
  and WSP (whitespace).
</t>

<section title="Core Rules" anchor="core.rules">
  <x:anchor-alias value="quoted-string"/>
  <x:anchor-alias value="token"/>
  <x:anchor-alias value="OWS"/>
<t>
  The core rules below are defined in &basic-rules;:
</t>
<figure><artwork type="abnf2616">
  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
</artwork></figure>
</section>

<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
  <x:anchor-alias value="field-name"/>
  <x:anchor-alias value="HTTP-date"/>
  <x:anchor-alias value="port"/>
  <x:anchor-alias value="pseudonym"/>
  <x:anchor-alias value="uri-host"/>
<t>
  The ABNF rules below are defined in other parts: 
</t>
<figure><!--Part1--><artwork type="abnf2616">
  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
</artwork></figure>
</section>

</section>
</section>

    <section anchor="caching.overview" title="Cache Operation">

      <section anchor="response.cacheability" title="Response Cacheability">
        <t>A cache &MAY; store a response to any request, provided that: <list style="symbols">
                <t>The request method is defined as being cacheable, and</t>
            <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
              appear in request or response headers, and</t>
            <t>the cache understands partial responses, if the response is partial or incomplete
              (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
          </list>
        </t>
        <t>Note that in normal operation, most caches will not store a response that has neither a
          cache validator nor an explicit expiration time, as such responses are not usually
          useful to store. However, caches are not prohibited from storing such responses.</t>

        <section anchor="errors.or.incomplete.response.cache.behavior"
          title="Storing Incomplete Responses">
          <t>A cache that receives an incomplete response (for example, with fewer bytes of data
            than specified in a Content-Length header) &MAY; store the response. However, the
            cache &MUST; treat this as a partial response &partial;. Partial responses
            &MAY; be combined as described in &combining-byte-ranges;; the result might be a
            full response or might still be partial. A cache &MUST-NOT; return a partial
            response to a client without explicitly marking it as such using the 206 (Partial
            Content) status code.</t>
          <t>A cache that does not support the Range and Content-Range headers &MUST-NOT; store
            incomplete or partial responses.</t>
        </section>

      </section>

      <section anchor="constructing.responses.from.caches"
        title="Constructing Responses from Caches">
        <t>For a given request, a non-shared cache &MAY; return a stored response, provided
          that: <list style="symbols">
            <t>the presented target-URI and that of the stored response match (see
              <cref>TBD</cref>), and</t>
            <t>the request method associated with the stored response allows it to be 
              used for the presented request, and</t>
            <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
                target="caching.negotiated.responses" />), and</t>
            <t>the stored response is either fresh (see <xref target="expiration.model" />) or
              allowed to be served stale (see <xref target="serving.stale.responses" />), and</t>
            <t>the presented request and stored response are free from directives that would prevent
              it (see <xref target="header.cache-control" /> and <xref target="header.pragma"
             />).</t>
          </list>
        </t>
        <t>A shared cache &MAY; return a stored response, provided that: <list style="symbols">
            <t>the criteria for non-shared caches above are met (taking into account directives specific to shared
              caches; see <xref target="header.cache-control" />), and</t>
            <t>the stored response was not associated with an authenticated request (see
              &header-authorization;), unless explicitly allowed (see <xref
                target="header.cache-control" />).</t>
          </list>
        </t>
        <t>All responses satisfied from cache &MUST; include an appropriate Age header field
            (<xref target="header.age" />).</t>
        <t>All request methods other than GET and HEAD &MUST; be written through the cache to
          the origin server. Note that such requests might invalidate already stored responses; see
            <xref target="invalidation.after.updates.or.deletions" />.</t>
        <t>Caches &SHOULD; use the most recent response (as determined by the Date header) when
          more than one applicable response is stored. They &MAY; also send a request with
          "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
          use.</t>
        <t>In the process of determining whether a stored response is fresh or not, a cache
          &MAY; validate that response (see <xref target="validation.model" />).</t>
        <t>
          <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in
            p1</cref>
        </t>
      </section>




      <section anchor="expiration.model" title="Freshness Model">

        <t>HTTP caching works best when caches can entirely avoid making requests to the origin
          server. When a response is "fresh" in the cache, it can be used to satisfy subsequent
          requests without contacting the origin server. This is also referred to as "expiration."<cref source="JRE">What exactly is called 'expiration'?</cref>.</t>
        <t>Expiration applies only to responses taken from a cache and not to first-hand responses.
          It cannot be used to force a user agent to refresh its display or reload a resource; its
          semantics apply only to caches. See <xref target="history.lists" /> for an explanation of
          the difference between caches and history mechanisms.</t>
        <t>The primary mechanism for avoiding requests is for an origin server to provide an
          explicit expiration time in the future, using either the Expires header (<xref
            target="header.expires" />) or the max-age response cache directive (<xref
            target="cache-response-directive" />). Generally, origin servers will assign future
          explicit expiration times to responses in the belief that the entity is not likely to
          change in a semantically significant way before the expiration time is reached. This
          normally preserves cache correctness, as long as the server's expiration times are
          carefully chosen.</t>
        <t>If an origin server wishes to force a cache to validate every request, it &MAY;
          assign an explicit expiration time in the past. This means that the response is always
          stale, so that caches should validate it before using it for subsequent requests.</t>
        <t>Since origin servers do not always provide explicit expiration times, HTTP caches may
          assign heuristic expiration times when they are not specified, employing algorithms that
          use other header values (such as the Last-Modified time) to estimate a plausible
          expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
          impose worst-case constraints on their results.</t>
        <t>The calculation to determine if a response has expired is:</t>
        <figure>
          <artwork type="code">
   response_is_fresh = (freshness_lifetime &gt; current_age)
</artwork>
        </figure>

        <t>The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
          the current_age is defined in <xref target="age.calculations" />.</t>
        <t>Additionally, clients may need to influence freshness calculation. They can do this using
          several request cache directives, with the effect of either increasing or loosening
          constraints on freshness. See <xref target="cache-request-directive" />.</t>
        <t>
          <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
            freshness.</cref>
        </t>

        <section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
          <t>A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
            response by using the first match of: <list style="symbols">
              <t>If the cache is shared and the s-maxage response cache directive (<xref
                  target="cache-response-directive" />) is present, use its value, or</t>
              <t>If the max-age response cache directive (<xref target="cache-response-directive"
                 />) is present, use its value, or</t>
              <t>If the Expires response header (<xref target="header.expires" />) is present, use
                its value minus the value of the Date response header, or</t>
              <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
                may be used; see <xref target="heuristic.freshness" />.</t>
            </list>
          </t>
          <t>Note that this calculation is not vulnerable to clock skew, since all of the
            information comes from the origin server.</t>

          <section anchor="heuristic.freshness" title="Using Heuristic Freshness">
            <t>If no explicit expiration time is present in a stored response that has a status code
              of 200, 203, 206, 300, 301 or 410, a heuristic expiration time &MAY; be
              calculated. Heuristics &MUST-NOT; be used for other response status codes. </t>
            <t> When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
              attach a Warning header with a 113 warn-code to the response if its current_age is
              more than 24 hours and such a warning is not already present.</t>
            <t>Also, if the response has a Last-Modified header (&header-last-modified;), the
              heuristic expiration value &SHOULD; be no more than some fraction of the interval
              since that time. A typical setting of this fraction might be 10%.</t>
            <t>
              <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
            </t>
          </section>
        </section>

        <section anchor="age.calculations" title="Calculating Age">
          <t>HTTP/1.1 uses the Age response-header to convey the estimated age of the response
            message when obtained from a cache. The Age field value is the cache's estimate of the
            amount of time since the response was generated or validated by the origin server. In
            essence, the Age value is the sum of the time that the response has been resident in
            each of the caches along the path from the origin server, plus the amount of time it has
            been in transit along network paths.</t>
          <t>When a response is generated from a stored response, the cache &MUST; include a
            single Age header field in the response with a value equal to the stored response's
            current_age, calculated using the algorithm described in this section.</t>
          <t>The term "age_value" denotes the value of the Age header, in a form appropriate for
            arithmetic operations.</t>
          <t>HTTP/1.1 requires origin servers to send a Date header, if possible, with every
            response, giving the time at which the response was generated (see &header-date;).
            The term "date_value" denotes the value of the Date header, in a form appropriate for
            arithmetic operations.</t>
          <t>The term "now" means "the current value of the clock at the host performing the
            calculation." Hosts that use HTTP, but especially hosts running origin servers and
            caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
            synchronize their clocks to a globally accurate time standard.</t>
          <t>A response's age can be calculated in two entirely independent ways: <list
              style="numbers">
              <t>now minus date_value, if the local clock is reasonably well synchronized to the
                origin server's clock. If the result is negative, the result is replaced by zero.</t>

              <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
            </list>
          </t>
          <t>These are combined as</t>
          <figure>
            <artwork type="code">
    corrected_received_age = max(now - date_value, age_value)
</artwork>
          </figure>
          <t>When an Age value is received, it &MUST; be interpreted relative to the time the
            request was initiated, not the time that the response was received.</t>
          <figure>
            <artwork type="code">
   corrected_initial_age = corrected_received_age
                         + (now - request_time)
</artwork>
          </figure>
          <t>where "request_time" is the time (according to the local clock) when the request that
            elicited this response was sent.</t>
          <t>The current_age of a stored response can then be calculated by adding the amount of
            time (in seconds) since the stored response was last validated by the origin server to
            the corrected_initial_age.</t>
          <t>In summary:</t>
          <figure>
            <artwork type="code">
   /*
    * age_value
    *      is the value of Age: header received by the cache with
    *              this response.
    * date_value
    *      is the value of the origin server's Date: header
    * request_time
    *      is the (local) time when the cache made the request
    *              that resulted in this stored response
    * response_time
    *      is the (local) time when the cache received the
    *              response
    * now
    *      is the current (local) time
    */

   apparent_age = max(0, response_time - date_value);
   corrected_received_age = max(apparent_age, age_value);
   response_delay = response_time - request_time;
   corrected_initial_age = corrected_received_age + response_delay;
   resident_time = now - response_time;
   current_age   = corrected_initial_age + resident_time;
</artwork>
          </figure>
        </section>

        <section anchor="serving.stale.responses" title="Serving Stale Responses">
          <t>A "stale" response is one that either has explicit expiry information, or is allowed to
            have heuristic expiry calculated, but is not fresh according to the calculations in
              <xref target="expiration.model" />. </t>
          <t>Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
            in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
            "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
            "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"
             />). </t>
          <t>Caches &MAY; return a stale response if disconnected or explicitly allowed (e.g.,
            the max-stale request directive; see <xref target="cache-request-directive" />).</t>
          <t>Otherwise, caches &SHOULD-NOT; return stale responses.</t>
          <t>Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
              target="header.warning" />).</t>
          <t>If a cache receives a first-hand response (either an entire response, or a 304 (Not
            Modified) response) that it would normally forward to the requesting client, and the
            received response is no longer fresh, the cache &SHOULD; forward it to the
            requesting client without adding a new Warning (but without removing any existing
            Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
            that response became stale in transit.</t>
        </section>
      </section>


      <section anchor="validation.model" title="Validation Model">
        <t>When a cache has a stale response that it would like to use, it &SHOULD; first check
          with the origin server (or possibly an intermediate cache with a fresh response) to see if
          it is still usable. This is called "validating" or "revalidating" the stored response.</t>
        <t>HTTP's conditional request mechanism, defined in &conditional;, is used to avoid
          retransmitting the response payload when the stored response is valid. When a stored
          response includes one or more "cache validators", such as the field values of an ETag or
          Last-Modified header field, then a validating GET request &SHOULD; be made conditional
          to those field values. The server checks the conditional request's validator against the
          current state of the requested resource and, if they match, the server responds with a 304
          (Not Modified) status code to indicate that the stored response can be refreshed and
          reused without retransmitting the response payload. If the validator does not match the
          current state of the requested resource, then the server returns a full response,
          including payload, so that the request can be satisfied and the stored response supplanted
          without the need for an additional network round-trip.</t>
        <t>See <xref target="combining.headers" /> regarding combining cached headers with those in
          a 304 response.</t>
        <t>If a cache receives a 5xx response while attempting to validate a response, it &MAY;
          either forward this response to the requesting client, or act as if the server failed to
          respond. In the latter case, it &MAY; return a previously received response unless the
          stored response includes the "must-revalidate" cache directive (see <xref
            target="cache-response-directive" />).</t>
      </section>

      <section anchor="invalidation.after.updates.or.deletions"
        title="Request Methods that Invalidate">
        <t>Because unsafe methods &safe-methods; have the potential for changing state on the
          origin server, intervening caches have the opportunity to use them to keep their contents
          up-to-date.</t>
        <t>The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
          as the Location and Content-Location headers (if present): <list style="symbols">
            <t>PUT</t>
            <t>DELETE</t>
            <t>POST</t>
          </list>
        </t>
        <t>An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
          be performed if the host part of that URI differs from the host part in the Request-URI.
          This helps prevent denial of service attacks.</t>
        <t>
          <cref>TODO: "host part" needs to be specified better.</cref>
        </t>
        <t>A cache that passes through requests for methods it does not understand &SHOULD;
          invalidate the Request-URI.</t>
        <t>Here, "invalidate" means that the cache will either remove all stored responses related
          to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
          before they can be returned in response to a subsequent request.</t>
        <t>Note that this does not guarantee that all appropriate responses are invalidated. For
          example, the request that caused the change at the origin server might not have gone
          through the cache where a response is stored.</t>
        <t>
          <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
        </t>
      </section>





      <section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
        <t>Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
          by the presence of a Vary header field (<xref target="header.vary" />) in a response, alters
          the conditions and procedure by which a cache can use the response for subsequent
          requests.</t>
        <t>When the cache receives a subsequent request which may be satisfied by a stored responses
          that include a Vary header field, it &MUST-NOT; use it to satisfy the request unless
          all of the selecting request-headers present in the new request match the corresponding
          stored request-headers from the original request.</t>
        <t>The selecting request-headers from two requests are defined to match if and only if the
          selecting request-headers in the first request can be transformed to the selecting
          request-headers in the second request by adding or removing linear white space
          <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
          combining multiple message-header fields with the same field name following the rules
          about message headers in &message-headers;.</t>
        <t>A Vary header field-value of "*" always fails to match, and subsequent requests on that
          resource can only be properly interpreted by the origin server.</t>
        <t>If the selecting request-headers for the stored response do not match the selecting
          request header fields of the new request, then the cache &MUST-NOT; use the stored
          response to satisfy the request unless it first relays the new request to the origin
          server in a conditional request and the server responds with 304 (Not Modified), including
          an entity tag or Content-Location that indicates the entity to be used.</t>
        <t>If one or more applicable stored response has an entity tag, the forwarded request
          &SHOULD; be conditional and include all of these entity tags in an If-None-Match
          header field. This conveys to the server the set of entities currently stored by the
          cache, so that if any one of these entities matches the requested entity, the server can
          use the ETag header field in its 304 (Not Modified) response to tell the cache which
          stored response is appropriate. If the entity-tag of the new response matches that of an
          existing stored resopnse, the new response &SHOULD; be used to update its header
          fields, and the result &MUST; be returned to the client.</t>
        <t>If any of the existing stored responses contains only partial content for the associated
          entity, its entity-tag &SHOULD-NOT; be included in the If-None-Match header field
          unless the request is for a range that would be fully satisfied by that stored response.</t>
        <t>If a cache receives a successful response whose Content-Location field matches that of an
          existing stored response for the same Request-URI, whose entity-tag differs from that of
          the existing stored response, and whose Date is more recent than that of the existing
          response, the existing response &SHOULD-NOT; be returned in response to future
          requests and &SHOULD; be deleted from the cache.</t>
        <t>
          <cref>TODO: this still needs work.</cref>
        </t>
      </section>


      <section anchor="combining.headers" title="Combining Responses">
        <t>When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
          it needs to update the stored response with the new one, so that the updated response can
          be sent to the client.</t>
        <t>If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
          the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
          Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
          the stored response with the updated entity-body received in the response and use the
          result as the updated entity-body (see &combining-byte-ranges;).</t>
        <t>The stored response headers are used for the updated response, except that <list
            style="symbols">
            <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
              &MUST; be deleted from the stored response and the forwarded response.</t>
            <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
              response and the forwarded response.</t>
            <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
              headers from the stored response.</t>
          </list>
        </t>
        <t>A cache &MUST; also replace stored headers with corresponding headers received in the
          incoming response, except for Warning headers as described immediately above. If a header
          field-name in the incoming response matches more than one header in the stored response,
          all such old headers &MUST; be replaced. it &MAY; store the combined
        entity-body.</t>
        <t><cref>ISSUE: discuss how to handle HEAD updates</cref></t>
      </section>

    </section>




    <section anchor="header.fields" title="Header Field Definitions">
      <t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
      <t>For entity-header fields, both sender and recipient refer to either the client or the
        server, depending on who sends and who receives the entity.</t>

      <section anchor="header.age" title="Age">
        <iref item="Age header" primary="true" x:for-anchor="" />
        <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
        <x:anchor-alias value="Age"/>
        <x:anchor-alias value="Age-v"/>
        <x:anchor-alias value="age-value"/>
        <t>      The response-header field "Age" conveys the sender's estimate of the amount of time since
          the response (or its validation) was generated at the origin server. Age values are
          calculated as specified in <xref target="age.calculations" />.</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
</artwork></figure>
        <t anchor="rule.delta-seconds">
          <x:anchor-alias value="delta-seconds" /> Age field-values are non-negative decimal
          integers, representing time in seconds.</t>
        <figure>
          <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
</artwork>
        </figure>
        <t>If a cache receives a value larger than the largest positive integer it can represent, or
          if any of its age calculations overflows, it &MUST; transmit an Age header with a
          field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
          of at least 31 bits of range.</t>
        <t>The presence of an Age header field in a response implies that a response is not
          first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
          Age header field.</t>
      </section>

      <section anchor="header.cache-control" title="Cache-Control">
        <iref item="Cache-Control header" primary="true" x:for-anchor="" />
        <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
        <x:anchor-alias value="Cache-Control"/>
        <x:anchor-alias value="Cache-Control-v"/>
        <x:anchor-alias value="cache-directive"/>
        <x:anchor-alias value="cache-extension"/>
        <x:anchor-alias value="cache-request-directive"/>
        <x:anchor-alias value="cache-response-directive"/>
        <t>The general-header field "Cache-Control" is used to specify directives that &MUST; be
          obeyed by all caches along the request/response chain. The directives specify behavior
          intended to prevent caches from adversely interfering with the request or response. Cache
          directives are unidirectional in that the presence of a directive in a request does not
          imply that the same directive is to be given in the response. <list>
            <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
              Pragma: no-cache (see <xref target="header.pragma" />).</t>
          </list>
        </t>
        <t>Cache directives &MUST; be passed through by a proxy or gateway application,
          regardless of their significance to that application, since the directives might be
          applicable to all recipients along the request/response chain. It is not possible to
          target a directive to a specific cache.</t>
<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"/>
  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>

  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
     / <x:ref>cache-response-directive</x:ref>

  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
</artwork></figure>

        <section anchor="cache-request-directive" title="Request Cache-Control Directives">
          <x:anchor-alias value="cache-request-directive" />

          <figure>
            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
  <x:ref>cache-request-directive</x:ref> =
       "no-cache"
     / "no-store"
     / "max-age" "=" <x:ref>delta-seconds</x:ref>
     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
     / "no-transform"
     / "only-if-cached"
     / <x:ref>cache-extension</x:ref>
</artwork>
          </figure>


          <t>
            <iref item="Cache Directives" primary="true" subitem="no-cache" />
            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
              <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
                used to satisfy the request without successful validation on the origin server. </t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="no-store" />
            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
              <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
                of either this request or any response to it. This directive applies to both
                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
                cache &MUST-NOT; intentionally store the information in non-volatile storage,
                and &MUST; make a best-effort attempt to remove the information from volatile
                storage as promptly as possible after forwarding it.</t>
              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
                particular, malicious or compromised caches might not recognize or obey this
                directive, and communications networks may be vulnerable to eavesdropping.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="max-age" />
            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
              <t>The max-age request directive indicates that the client is willing to accept a
                response whose age is no greater than the specified time in seconds. Unless
                max-stale directive is also included, the client is not willing to accept a stale
                response.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="max-stale" />
            <iref item="max-stale" primary="true" subitem="Cache Directive" /> max-stale <list>
              <t>The max-stale request directive indicates that the client is willing to accept a
                response that has exceeded its expiration time. If max-stale is assigned a value,
                then the client is willing to accept a response that has exceeded its expiration
                time by no more than the specified number of seconds. If no value is assigned to
                max-stale, then the client is willing to accept a stale response of any age.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="min-fresh" />
            <iref item="min-fresh" primary="true" subitem="Cache Directive" /> min-fresh <list>
              <t>The min-fresh request directive indicates that the client is willing to accept a
                response whose freshness lifetime is no less than its current age plus the specified
                time in seconds. That is, the client wants a response that will still be fresh for
                at least the specified number of seconds.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="no-transform" />
            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
              <t>The no-transform request directive indicates that an intermediate cache or proxy
                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
                headers, nor the request entity-body.</t>
            </list>
          </t>

          <t>
            <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
            <iref item="only-if-cached" primary="true" subitem="Cache Directive" /> only-if-cached <list>
              <t>The only-if-cached request directive indicates that the client only wishes to
                return a stored response. If it receives this directive, a cache &SHOULD; either
                respond using a stored response that is consistent with the other constraints of the
                request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
                being operated as a unified system with good internal connectivity, such a request
                &MAY; be forwarded within that group of caches.</t>
            </list>
          </t>
        </section>

        <section anchor="cache-response-directive" title="Response Cache-Control Directives">
          <x:anchor-alias value="cache-response-directive" />

          <figure>
            <artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
  <x:ref>cache-response-directive</x:ref> =
       "public"
     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
     / "no-store"
     / "no-transform"
     / "must-revalidate"
     / "proxy-revalidate"
     / "max-age" "=" <x:ref>delta-seconds</x:ref>
     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
     / <x:ref>cache-extension</x:ref>
</artwork>
          </figure>

          <t>
            <iref item="Cache Directives" primary="true" subitem="public" />
            <iref item="public" primary="true" subitem="Cache Directive" /> public <list>
              <t>The public response directive indicates that the response &MAY; be cached, even
                if it would normally be non-cacheable or cacheable only within a non-shared cache.
                (See also Authorization, &header-authorization;, for additional details.) </t>
            </list>
          </t>

          <t>
            <iref item="Cache Directives" primary="true" subitem="private" />
            <iref item="private" primary="true" subitem="Cache Directive" /> private <list>
              <t>The private response directive indicates that the response message is intended for
                a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
                cache &MAY; store the response.</t>
              <t>If the private response directive specifies one or more field-names, this
                requirement is limited to the field-values associated with the listed response
                headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
                cache, whereas the remainder of the response message &MAY; be.</t>
              <t>
                <x:h>Note:</x:h> This usage of the word private only controls where the response may
                be stored, and cannot ensure the privacy of the message content.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="no-cache" />
            <iref item="no-cache" primary="true" subitem="Cache Directive" /> no-cache <list>
              <t>The no-cache response directive indicates that a response &MUST-NOT; be used to
                satisfy a subsequent request without successful validation on the origin server.
                This allows an origin server to prevent caching even by caches that have been
                configured to return stale responses.</t>
              <t>If the no-cache response directive specifies one or more field-names, this
                requirement is limited to the field-values assosicated with the listed response
                headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
                to a subsequent request without successful validation on the origin server. This
                allows an origin server to prevent the re-use of certain header fields in a
                response, while still allowing caching of the rest of the response.</t>
              <t>
                <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
              </t>
            </list>
          </t>

          <t>
            <iref item="Cache Directives" primary="true" subitem="no-store" />
            <iref item="no-store" primary="true" subitem="Cache Directive" /> no-store <list>
              <t>The no-store response directive indicates that a cache &MUST-NOT; store any
                part of either the immediate request or response. This directive applies to both
                non-shared and shared caches. "&MUST-NOT; store" in this context means that the
                cache &MUST-NOT; intentionally store the information in non-volatile storage,
                and &MUST; make a best-effort attempt to remove the information from volatile
                storage as promptly as possible after forwarding it.</t>
              <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
                particular, malicious or compromised caches might not recognize or obey this
                directive, and communications networks may be vulnerable to eavesdropping.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
            <iref item="must-revalidate" primary="true" subitem="Cache Directive" /> must-revalidate <list>
              <t>The must-revalidate response directive indicates that validation is required before
                the response is used by a cache to satisfy any request.</t>
              <t>When the present, caches &MUST-NOT; use a stored after it becomes stale to
                respond to a subsequent request without first validating it with the origin server.</t>
              <t>The must-revalidate directive is necessary to support reliable operation for
                certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
                the must-revalidate directive; in particular, if the cache cannot reach the origin
                server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
              <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
                validate a request on the entity could result in incorrect operation, such as a
                silently unexecuted financial transaction.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
            <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
            proxy-revalidate <list>
              <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
                response directive, except that it does not apply to non-shared caches.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="max-age" />
            <iref item="max-age" primary="true" subitem="Cache Directive" /> max-age <list>
              <t>The max-age response directive indicates that response is to be considered stale
                after its age is greater than the specified number of seconds.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="s-maxage" />
            <iref item="s-maxage" primary="true" subitem="Cache Directive" /> s-maxage <list>
              <t>The s-maxage response directive indicates that, in shared caches, the maximum age
                specified by this directive overrides the maximum age specified by either the
                max-age directive or the Expires header. The s-maxage directive also implies the
                semantics of the proxy-revalidate response directive.</t>
            </list>
          </t>
          <t>
            <iref item="Cache Directives" primary="true" subitem="no-transform" />
            <iref item="no-transform" primary="true" subitem="Cache Directive" /> no-transform <list>
              <t>The no-transform response directive indicates that an intermediate cache or proxy
                &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
                headers, nor the response entity-body.</t>
            </list>
          </t>

        </section>

        <section anchor="cache.control.extensions" title="Cache Control Extensions">
          <t>The Cache-Control header field can be extended through the use of one or more
            cache-extension tokens, each with an optional value. Informational extensions (those
            which do not require a change in cache behavior) &MAY; be added without changing the
            semantics of other directives. Behavioral extensions are designed to work by acting as
            modifiers to the existing base of cache directives. Both the new directive and the
            standard directive are supplied, such that applications which do not understand the new
            directive will default to the behavior specified by the standard directive, and those
            that understand the new directive will recognize it as modifying the requirements
            associated with the standard directive. In this way, extensions to the cache-control
            directives can be made without requiring changes to the base protocol.</t>
          <t>This extension mechanism depends on an HTTP cache obeying all of the cache-control
            directives defined for its native HTTP-version, obeying certain extensions, and ignoring
            all directives that it does not understand.</t>
          <t>For example, consider a hypothetical new response directive called "community" which
            acts as a modifier to the private directive. We define this new directive to mean that,
            in addition to any non-shared cache, any cache which is shared only by members of the
            community named within its value may cache the response. An origin server wishing to
            allow the UCI community to use an otherwise private response in their shared cache(s)
            could do so by including</t>
          <figure>
            <artwork type="example">
  Cache-Control: private, community="UCI"
</artwork>
          </figure>
          <t>A cache seeing this header field will act correctly even if the cache does not
            understand the community cache-extension, since it will also see and understand the
            private directive and thus default to the safe behavior.</t>
          <t>Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
            directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
            directives (or the response's default cacheability) such that the cache behavior will
            remain minimally correct even if the cache does not understand the extension(s).</t>
        </section>

      </section>

      <section anchor="header.expires" title="Expires">
        <iref item="Expires header" primary="true" x:for-anchor="" />
        <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
        <x:anchor-alias value="Expires"/>
        <x:anchor-alias value="Expires-v"/>
        <t>The entity-header field "Expires" gives the date/time after which the response is
          considered stale. See <xref target="expiration.model" /> for further discussion of the
          expiration model.</t>
        <t>The presence of an Expires field does not imply that the original resource will change or
          cease to exist at, before, or after that time.</t>
        <t>The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
          it &MUST; be sent in rfc1123-date format.</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
</artwork></figure>
        <t>For example</t>
        <figure>
          <artwork type="example">
  Expires: Thu, 01 Dec 1994 16:00:00 GMT
</artwork>
        </figure>
        <t>
          <list>
            <t>
              <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
              directive (see <xref target="cache-response-directive" />), that directive overrides
              the Expires field.</t>
          </list>
        </t>
        <t>HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.</t>
        <t>HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
          including the value "0", as in the past (i.e., "already expired").</t>
      </section>

      <section anchor="header.pragma" title="Pragma">
        <iref item="Pragma header" primary="true" x:for-anchor="" />
        <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
        <x:anchor-alias value="extension-pragma"/>
        <x:anchor-alias value="Pragma"/>
        <x:anchor-alias value="Pragma-v"/>
        <x:anchor-alias value="pragma-directive"/>
        <t>The general-header field "Pragma" is used to include implementation-specific directives
          that might apply to any recipient along the request/response chain. All pragma directives
          specify optional behavior from the viewpoint of the protocol; however, some systems
          &MAY; require that behavior be consistent with the directives.</t>
<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"/>
  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
</artwork></figure>
        <t>When the no-cache directive is present in a request message, an application &SHOULD;
          forward the request toward the origin server even if it has a cached copy of what is being
          requested. This pragma directive has the same semantics as the no-cache response directive
          (see <xref target="cache-response-directive" />) and is defined here for backward
          compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
          no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
          &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".</t>
        <t>
          <list>
            <t>
              <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
              is not actually specified, it does not provide a reliable replacement for
              "Cache-Control: no-cache" in a response.</t>
          </list>
        </t>
        <t>This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</t>
      </section>

      <section anchor="header.vary" title="Vary">
        <iref item="Vary header" primary="true" x:for-anchor="" />
        <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
        <x:anchor-alias value="Vary"/>
        <x:anchor-alias value="Vary-v"/>
        <t>The "Vary" response-header field's value indicates the set of request-header fields that
          fully determines, while the response is fresh, whether a cache is permitted to use the
          response to reply to a subsequent request without validation. For uncacheable or stale
          responses, the Vary field value advises the user agent about the criteria that were used
          to select the representation. A Vary field value of "*" implies that a cache cannot
          determine from the request headers of a subsequent request whether this response is the
          appropriate representation. See <xref target="caching.negotiated.responses" /> for use of
          the Vary header field by caches.</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
</artwork></figure>
        <t>The set of header fields named by the Vary field value is known as the "selecting"
          request-headers.</t>
        <t>Servers &SHOULD; include a Vary header field with any cacheable response that is
          subject to server-driven negotiation. Doing so allows a cache to properly interpret future
          requests on that resource and informs the user agent about the presence of negotiation on
          that resource. A server &MAY; include a Vary header field with a non-cacheable
          response that is subject to server-driven negotiation, since this might provide the user
          agent with useful information about the dimensions over which the response varies at the
          time of the response.</t>
        <t>A Vary field value of "*" signals that unspecified parameters not limited to the
          request-headers (e.g., the network address of the client), play a role in the selection of
          the response representation. The "*" value &MUST-NOT; be generated by a proxy server;
          it may only be generated by an origin server.</t>
        <t>The field-names given are not limited to the set of standard request-header fields
          defined by this specification. Field names are case-insensitive.</t>
      </section>

      <section anchor="header.warning" title="Warning">
        <iref item="Warning header" primary="true" x:for-anchor="" />
        <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
        <x:anchor-alias value="Warning"/>
        <x:anchor-alias value="Warning-v"/>
        <x:anchor-alias value="warning-value"/>
        <x:anchor-alias value="warn-agent"/>
        <x:anchor-alias value="warn-code"/>
        <x:anchor-alias value="warn-date"/>
        <x:anchor-alias value="warn-text"/>
        <t>The general-header field "Warning" is used to carry additional information about the status
          or transformation of a message which might not be reflected in the message. This
          information is typically used to warn about possible incorrectness introduced by caching
          operations or transformations applied to the entity body of the message.</t>
        <t>Warnings can be used for other purposes, both cache-related and otherwise. The use of a
          warning, rather than an error status code, distinguish these responses from true failures.</t>

        <t>Warning headers can in general be applied to any message, however some warn-codes are
          specific to caches and can only be applied to response messages.</t>

<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"/>
  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
  
  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
  
  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
                  ; the name or pseudonym of the server adding
                  ; the Warning header, for use in debugging
  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
</artwork></figure>

        <t>Multiple warnings &MAY; be attached to a response (either by the origin server or by
          a cache), including multiple warnings with the same code number. For example, a server
          might provide the same warning with texts in both English and Basque.</t>
        <t>When this occurs, the user agent &SHOULD; inform the user of as many of them as
          possible, in the order that they appear in the response. If it is not possible to inform
          the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
            <list style="symbols">
            <t>Warnings that appear early in the response take priority over those appearing later
              in the response.</t>

            <t>Warnings in the user's preferred character set take priority over warnings in other
              character sets but with identical warn-codes and warn-agents.</t>
          </list>
        </t>
        <t>Systems that generate multiple Warning headers &SHOULD; order them with this user
          agent behavior in mind. New Warning headers &SHOULD; be added after any existing
          Warning headers.</t>
        <t>Warnings are assigned three digit warn-codes. The first digit indicates whether the
          Warning is required to be deleted from a stored response after validation: <list
            style="symbols">
            <t>1xx Warnings that describe the freshness or validation status of the response, and so
              &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
              except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
            <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
              not rectified by a validation (for example, a lossy compression of the entity bodies)
              and which &MUST-NOT; be deleted by caches after validation, unless a full response is
              returned, in which case they &MUST; be.</t>
          </list>
        </t>
        <t>The warn-text &SHOULD; be in a natural language and character set that is most likely
          to be intelligible to the human user receiving the response. This decision can be based on
          any available knowledge, such as the location of the cache or user, the Accept-Language
          field in a request, the Content-Language field in a response, etc. The default language is
          English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).</t>
        <t>If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
          warn-text using the method described in <xref target="RFC2047" />.</t>
        <t>If an implementation sends a message with one or more Warning headers to a receiver whose
          version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
          warn-date that matches the Date header in the message.</t>
        <t>If an implementation receives a message with a warning-value that includes a warn-date,
          and that warn-date is different from the Date value in the response, then that
          warning-value &MUST; be deleted from the message before storing, forwarding, or using
          it. (preventing the consequences of naive caching of Warning header fields.) If all of the
          warning-values are deleted for this reason, the Warning header &MUST; be deleted as
          well.</t>
        <t>The following warn-codes are defined by this specification, each with a recommended
          warn-text in English, and a description of its meaning.</t>
        <t>110 Response is stale <list>
            <t>&SHOULD; be included whenever the returned response is stale.</t>
          </list>
        </t>
        <t>111 Revalidation failed <list>
            <t>&SHOULD; be included if a cache returns a stale response because an attempt to
              validate the response failed, due to an inability to reach the server.</t>
          </list>
        </t>
        <t>112 Disconnected operation <list>
            <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
              the network for a period of time.</t>
          </list>
        </t>
        <t>113 Heuristic expiration <list>
            <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
              greater than 24 hours and the response's age is greater than 24 hours.</t>
          </list>
        </t>
        <t>199 Miscellaneous warning <list>
            <t>The warning text &MAY; include arbitrary information to be presented to a human
              user, or logged. A system receiving this warning &MUST-NOT; take any automated
              action, besides presenting the warning to the user.</t>
          </list>
        </t>
        <t>214 Transformation applied <list>
            <t>&MUST; be added by an intermediate cache or proxy if it applies any
              transformation changing the content-coding (as specified in the Content-Encoding
              header) or media-type (as specified in the Content-Type header) of the response, or
              the entity-body of the response, unless this Warning code already appears in the
              response.</t>
          </list>
        </t>
        <t>299 Miscellaneous persistent warning <list>
            <t>The warning text &MAY; include arbitrary information to be presented to a human
              user, or logged. A system receiving this warning &MUST-NOT; take any automated
              action.</t>
          </list>
        </t>
      </section>

    </section>


    <section anchor="history.lists" title="History Lists">
      <t>User agents often have history mechanisms, such as "Back" buttons and history lists, which
        can be used to redisplay an entity retrieved earlier in a session.</t>
      <t>History mechanisms and caches are different. In particular history mechanisms
        &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
        history mechanism is meant to show exactly what the user saw at the time when the resource
        was retrieved.</t>
      <t>By default, an expiration time does not apply to history mechanisms. If the entity is still
        in storage, a history mechanism &SHOULD; display it even if the entity has expired,
        unless the user has specifically configured the agent to refresh expired history documents.</t>
      <t>This is not to be construed to prohibit the history mechanism from telling the user that a
        view might be stale. <list>
          <t>
            <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
            stale resources, this will tend to force service authors to avoid using HTTP expiration
            controls and cache controls when they would otherwise like to. Service authors may
            consider it important that users not be presented with error messages or warning
            messages when they use navigation controls (such as BACK) to view previously fetched
            resources. Even though sometimes such resources ought not be cached, or ought to expire
            quickly, user interface considerations may force service authors to resort to other
            means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
            of improperly functioning history mechanisms.</t>
        </list>
      </t>
    </section>


    <section anchor="IANA.considerations" title="IANA Considerations">
      <section anchor="message.header.registration" title="Message Header Registration">
        <t>The Message Header Registry located at <eref
            target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
          should be updated with the permanent registrations below (see <xref target="RFC3864" />):</t>
        <!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
        <texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
          <ttcol>Header Field Name</ttcol>
          <ttcol>Protocol</ttcol>
          <ttcol>Status</ttcol>
          <ttcol>Reference</ttcol>

          <c>Age</c>
          <c>http</c>
          <c>standard</c>
          <c>
            <xref target="header.age" />
          </c>
          <c>Cache-Control</c>
          <c>http</c>
          <c>standard</c>
          <c>
            <xref target="header.cache-control" />
          </c>
          <c>Expires</c>
          <c>http</c>
          <c>standard</c>
          <c>
            <xref target="header.expires" />
          </c>
          <c>Pragma</c>
          <c>http</c>
          <c>standard</c>
          <c>
            <xref target="header.pragma" />
          </c>
          <c>Vary</c>
          <c>http</c>
          <c>standard</c>
          <c>
            <xref target="header.vary" />
          </c>
          <c>Warning</c>
          <c>http</c>
          <c>standard</c>
          <c>
            <xref target="header.warning" />
          </c>
        </texttable>
        <!--(END)-->
        <t>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</t>
      </section>
    </section>

    <section anchor="security.considerations" title="Security Considerations">
      <t>Caches expose additional potential vulnerabilities, since the contents of the cache
        represent an attractive target for malicious exploitation. Because cache contents persist
        after an HTTP request is complete, an attack on the cache can reveal information long after
        a user believes that the information has been removed from the network. Therefore, cache
        contents should be protected as sensitive information.</t>
    </section>

    <section anchor="ack" title="Acknowledgments">
      <t>Much of the content and presentation of the caching design is due to suggestions and
        comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
        and Larry Masinter.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">

      <reference anchor="ISO-8859-1">
        <front>
          <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
            1: Latin alphabet No. 1 </title>
          <author>
            <organization>International Organization for Standardization</organization>
          </author>
          <date year="1998" />
        </front>
        <seriesInfo name="ISO/IEC" value="8859-1:1998" />
      </reference>

      <reference anchor="Part1">
        <front>
          <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
            <organization abbrev="Day Software">Day Software</organization>
            <address><email>fielding@gbiv.com</email></address>
          </author>
          <author fullname="Jim Gettys" initials="J." surname="Gettys">
            <organization>One Laptop per Child</organization>
            <address><email>jg@laptop.org</email></address>
          </author>
          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
            <organization abbrev="HP">Hewlett-Packard Company</organization>
            <address><email>JeffMogul@acm.org</email></address>
          </author>
          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>henrikn@microsoft.com</email></address>
          </author>
          <author fullname="Larry Masinter" initials="L." surname="Masinter">
            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
            <address><email>LMM@acm.org</email></address>
          </author>
          <author fullname="Paul J. Leach" initials="P." surname="Leach">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>paulle@microsoft.com</email></address>
          </author>
          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
            <address><email>timbl@w3.org</email></address>
          </author>
          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
            <organization abbrev="W3C">World Wide Web Consortium</organization>
            <address><email>ylafon@w3.org</email></address>
          </author>
          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
            <organization abbrev="greenbytes">greenbytes GmbH</organization>
            <address><email>julian.reschke@greenbytes.de</email></address>
          </author>
          <date month="&ID-MONTH;" year="&ID-YEAR;" />
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
        <x:source basename="p1-messaging" href="p1-messaging.xml" />
      </reference>

      <reference anchor="Part2">
        <front>
          <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
            <organization abbrev="Day Software">Day Software</organization>
            <address><email>fielding@gbiv.com</email></address>
          </author>
          <author fullname="Jim Gettys" initials="J." surname="Gettys">
            <organization>One Laptop per Child</organization>
            <address><email>jg@laptop.org</email></address>
          </author>
          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
            <organization abbrev="HP">Hewlett-Packard Company</organization>
            <address><email>JeffMogul@acm.org</email></address>
          </author>
          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>henrikn@microsoft.com</email></address>
          </author>
          <author fullname="Larry Masinter" initials="L." surname="Masinter">
            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
            <address><email>LMM@acm.org</email></address>
          </author>
          <author fullname="Paul J. Leach" initials="P." surname="Leach">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>paulle@microsoft.com</email></address>
          </author>
          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
            <address><email>timbl@w3.org</email></address>
          </author>
          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
            <organization abbrev="W3C">World Wide Web Consortium</organization>
            <address><email>ylafon@w3.org</email></address>
          </author>
          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
            <organization abbrev="greenbytes">greenbytes GmbH</organization>
            <address><email>julian.reschke@greenbytes.de</email></address>
          </author>
          <date month="&ID-MONTH;" year="&ID-YEAR;" />
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
        <x:source basename="p2-semantics" href="p2-semantics.xml" />
      </reference>

      <reference anchor="Part3">
        <front>
          <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
            <organization abbrev="Day Software">Day Software</organization>
            <address><email>fielding@gbiv.com</email></address>
          </author>
          <author fullname="Jim Gettys" initials="J." surname="Gettys">
            <organization>One Laptop per Child</organization>
            <address><email>jg@laptop.org</email></address>
          </author>
          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
            <organization abbrev="HP">Hewlett-Packard Company</organization>
            <address><email>JeffMogul@acm.org</email></address>
          </author>
          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>henrikn@microsoft.com</email></address>
          </author>
          <author fullname="Larry Masinter" initials="L." surname="Masinter">
            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
            <address><email>LMM@acm.org</email></address>
          </author>
          <author fullname="Paul J. Leach" initials="P." surname="Leach">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>paulle@microsoft.com</email></address>
          </author>
          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
            <address><email>timbl@w3.org</email></address>
          </author>
          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
            <organization abbrev="W3C">World Wide Web Consortium</organization>
            <address><email>ylafon@w3.org</email></address>
          </author>
          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
            <organization abbrev="greenbytes">greenbytes GmbH</organization>
            <address><email>julian.reschke@greenbytes.de</email></address>
          </author>
          <date month="&ID-MONTH;" year="&ID-YEAR;" />
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
        <x:source basename="p3-payload" href="p3-payload.xml" />
      </reference>

      <reference anchor="Part4">
        <front>
          <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
            <organization abbrev="Day Software">Day Software</organization>
            <address><email>fielding@gbiv.com</email></address>
          </author>
          <author fullname="Jim Gettys" initials="J." surname="Gettys">
            <organization>One Laptop per Child</organization>
            <address><email>jg@laptop.org</email></address>
          </author>
          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
            <organization abbrev="HP">Hewlett-Packard Company</organization>
            <address><email>JeffMogul@acm.org</email></address>
          </author>
          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>henrikn@microsoft.com</email></address>
          </author>
          <author fullname="Larry Masinter" initials="L." surname="Masinter">
            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
            <address><email>LMM@acm.org</email></address>
          </author>
          <author fullname="Paul J. Leach" initials="P." surname="Leach">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>paulle@microsoft.com</email></address>
          </author>
          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
            <address><email>timbl@w3.org</email></address>
          </author>
          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
            <organization abbrev="W3C">World Wide Web Consortium</organization>
            <address><email>ylafon@w3.org</email></address>
          </author>
          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
            <organization abbrev="greenbytes">greenbytes GmbH</organization>
            <address><email>julian.reschke@greenbytes.de</email></address>
          </author>
          <date month="&ID-MONTH;" year="&ID-YEAR;" />
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
        <x:source basename="p4-conditional" href="p4-conditional.xml" />
      </reference>

      <reference anchor="Part5">
        <front>
          <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
            <organization abbrev="Day Software">Day Software</organization>
            <address><email>fielding@gbiv.com</email></address>
          </author>
          <author fullname="Jim Gettys" initials="J." surname="Gettys">
            <organization>One Laptop per Child</organization>
            <address><email>jg@laptop.org</email></address>
          </author>
          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
            <organization abbrev="HP">Hewlett-Packard Company</organization>
            <address><email>JeffMogul@acm.org</email></address>
          </author>
          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>henrikn@microsoft.com</email></address>
          </author>
          <author fullname="Larry Masinter" initials="L." surname="Masinter">
            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
            <address><email>LMM@acm.org</email></address>
          </author>
          <author fullname="Paul J. Leach" initials="P." surname="Leach">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>paulle@microsoft.com</email></address>
          </author>
          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
            <address><email>timbl@w3.org</email></address>
          </author>
          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
            <organization abbrev="W3C">World Wide Web Consortium</organization>
            <address><email>ylafon@w3.org</email></address>
          </author>
          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
            <organization abbrev="greenbytes">greenbytes GmbH</organization>
            <address><email>julian.reschke@greenbytes.de</email></address>
          </author>
          <date month="&ID-MONTH;" year="&ID-YEAR;" />
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
        <x:source basename="p5-range" href="p5-range.xml" />
      </reference>

      <reference anchor="Part7">
        <front>
          <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
          <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
            <organization abbrev="Day Software">Day Software</organization>
            <address><email>fielding@gbiv.com</email></address>
          </author>
          <author fullname="Jim Gettys" initials="J." surname="Gettys">
            <organization>One Laptop per Child</organization>
            <address><email>jg@laptop.org</email></address>
          </author>
          <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
            <organization abbrev="HP">Hewlett-Packard Company</organization>
            <address><email>JeffMogul@acm.org</email></address>
          </author>
          <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>henrikn@microsoft.com</email></address>
          </author>
          <author fullname="Larry Masinter" initials="L." surname="Masinter">
            <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
            <address><email>LMM@acm.org</email></address>
          </author>
          <author fullname="Paul J. Leach" initials="P." surname="Leach">
            <organization abbrev="Microsoft">Microsoft Corporation</organization>
            <address><email>paulle@microsoft.com</email></address>
          </author>
          <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
            <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
            <address><email>timbl@w3.org</email></address>
          </author>
          <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
            <organization abbrev="W3C">World Wide Web Consortium</organization>
            <address><email>ylafon@w3.org</email></address>
          </author>
          <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
            <organization abbrev="greenbytes">greenbytes GmbH</organization>
            <address><email>julian.reschke@greenbytes.de</email></address>
          </author>
          <date month="&ID-MONTH;" year="&ID-YEAR;" />
        </front>
        <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
        <x:source basename="p7-auth" href="p7-auth.xml" />
      </reference>

      <reference anchor="RFC2047">
        <front>
          <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
            Part Three: Message Header Extensions for Non-ASCII Text</title>
          <author fullname="Keith Moore" initials="K." surname="Moore">
            <organization>University of Tennessee</organization>
            <address><email>moore@cs.utk.edu</email></address>
          </author>
          <date month="November" year="1996" />
        </front>
        <seriesInfo name="RFC" value="2047" />
      </reference>

      <reference anchor="RFC2119">
        <front>
          <title>Key words for use in RFCs to Indicate Requirement Levels</title>
          <author fullname="Scott Bradner" initials="S." surname="Bradner">
            <organization>Harvard University</organization>
            <address><email>sob@harvard.edu</email></address>
          </author>
          <date month="March" year="1997" />
        </front>
        <seriesInfo name="BCP" value="14" />
        <seriesInfo name="RFC" value="2119" />
      </reference>

      <reference anchor="RFC5234">
        <front>
          <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
          <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
            <organization>Brandenburg InternetWorking</organization>
            <address>
            <postal>
            <street>675 Spruce Dr.</street>
            <city>Sunnyvale</city>
            <region>CA</region>
            <code>94086</code>
            <country>US</country></postal>
            <phone>+1.408.246.8253</phone>
            <email>dcrocker@bbiw.net</email></address>  
          </author>
          <author initials="P." surname="Overell" fullname="Paul Overell">
            <organization>THUS plc.</organization>
            <address>
            <postal>
            <street>1/2 Berkeley Square</street>
            <street>99 Berkely Street</street>
            <city>Glasgow</city>
            <code>G3 7HR</code>
            <country>UK</country></postal>
            <email>paul.overell@thus.net</email></address>
          </author>
          <date month="January" year="2008"/>
        </front>
        <seriesInfo name="STD" value="68"/>
        <seriesInfo name="RFC" value="5234"/>
      </reference>
      
    </references>

    <references title="Informative References">

      <reference anchor="RFC1305">
        <front>
          <title>Network Time Protocol (Version 3) Specification, Implementation</title>
          <author fullname="David L. Mills" initials="D." surname="Mills">
            <organization>University of Delaware, Electrical Engineering Department</organization>
            <address><email>mills@udel.edu</email></address>
          </author>
          <date month="March" year="1992" />
        </front>
        <seriesInfo name="RFC" value="1305" />
      </reference>

      <reference anchor="RFC2616">
        <front>
          <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
          <author fullname="R. Fielding" initials="R." surname="Fielding">
            <organization>University of California, Irvine</organization>
            <address><email>fielding@ics.uci.edu</email></address>
          </author>
          <author fullname="J. Gettys" initials="J." surname="Gettys">
            <organization>W3C</organization>
            <address><email>jg@w3.org</email></address>
          </author>
          <author fullname="J. Mogul" initials="J." surname="Mogul">
            <organization>Compaq Computer Corporation</organization>
            <address><email>mogul@wrl.dec.com</email></address>
          </author>
          <author fullname="H. Frystyk" initials="H." surname="Frystyk">
            <organization>MIT Laboratory for Computer Science</organization>
            <address><email>frystyk@w3.org</email></address>
          </author>
          <author fullname="L. Masinter" initials="L." surname="Masinter">
            <organization>Xerox Corporation</organization>
            <address><email>masinter@parc.xerox.com</email></address>
          </author>
          <author fullname="P. Leach" initials="P." surname="Leach">
            <organization>Microsoft Corporation</organization>
            <address><email>paulle@microsoft.com</email></address>
          </author>
          <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
            <organization>W3C</organization>
            <address><email>timbl@w3.org</email></address>
          </author>
          <date month="June" year="1999" />
        </front>
        <seriesInfo name="RFC" value="2616" />
      </reference>

      <reference anchor="RFC3864">
        <front>
          <title>Registration Procedures for Message Header Fields</title>
          <author fullname="G. Klyne" initials="G." surname="Klyne">
            <organization>Nine by Nine</organization>
            <address><email>GK-IETF@ninebynine.org</email></address>
          </author>
          <author fullname="M. Nottingham" initials="M." surname="Nottingham">
            <organization>BEA Systems</organization>
            <address><email>mnot@pobox.com</email></address>
          </author>
          <author fullname="J. Mogul" initials="J." surname="Mogul">
            <organization>HP Labs</organization>
            <address><email>JeffMogul@acm.org</email></address>
          </author>
          <date month="September" year="2004" />
        </front>
        <seriesInfo name="BCP" value="90" />
        <seriesInfo name="RFC" value="3864" />
      </reference>

    </references>

    <section anchor="compatibility" title="Compatibility with Previous Versions">

      <section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
        <t>A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
          this missing case. (Sections <xref format="counter" target="response.cacheability" />,
            <xref format="counter" target="header.cache-control" />).</t>
        <t>Transfer-coding and message lengths all interact in ways that required fixing exactly
          when chunked encoding is used (to allow for transfer encoding that may not be self
          delimiting); it was important to straighten out exactly how message lengths are computed.
          (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)</t>
        <t>Proxies should be able to add Content-Length when appropriate.</t>
        <t>Range request responses would become very verbose if all meta-data were always returned;
          by allowing the server to only send needed headers in a 206 response, this problem can be
          avoided. (<xref target="combining.headers" />)</t>
        <t>The Cache-Control: max-age directive was not properly defined for responses.</t>
        <t>Warnings could be cached incorrectly, or not updated appropriately. <xref
            format="counter" target="expiration.model" />, <xref format="counter"
            target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
          and <xref format="counter" target="header.warning" />) Warning also needed to be a general
          header, as PUT or other methods may have need for it in requests.</t>
      </section>

      <section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
        <t>Clarify denial of service attack avoidance requirement. (<xref
            target="invalidation.after.updates.or.deletions" />)</t>
      </section>

    </section>

<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
<figure>
<artwork type="abnf" name="p6-cache.parsed-abnf">
<x:ref>Age</x:ref> = "Age:" OWS Age-v
<x:ref>Age-v</x:ref> = delta-seconds

<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
 cache-directive ] )

<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
<x:ref>Expires-v</x:ref> = HTTP-date

<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;

<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;

<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
 pragma-directive ] )

<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
 ] ) )

<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
 ] )

<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
 cache-extension
<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
 ) / ( "s-maxage=" delta-seconds ) / cache-extension

<x:ref>delta-seconds</x:ref> = 1*DIGIT

<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]

<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;

<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;

<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;

<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;

<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;

<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
<x:ref>warn-code</x:ref> = 3DIGIT
<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
<x:ref>warn-text</x:ref> = quoted-string
<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
 ]


</artwork>
</figure>
<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">; Age defined but not used
; Cache-Control defined but not used
; Expires defined but not used
; Pragma defined but not used
; Vary defined but not used
; Warning defined but not used
</artwork></figure></section>

    <section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">

      <section title="Since RFC2616">
        <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
      </section>

      <section title="Since draft-ietf-httpbis-p6-cache-00">
        <t>Closed issues: <list style="symbols">
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer"
                (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation
              after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and
              Informative references"</t>
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date
              reference typo"</t>
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection
              header text"</t>
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative
              references"</t>
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1
              Reference"</t>
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative
              up-to-date references"</t>
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in
              13.2.2"</t>
          </list>
        </t>
        <t>Other changes: <list style="symbols">
            <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
                target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
          </list>
        </t>
      </section>

      <section title="Since draft-ietf-httpbis-p6-cache-01">
        <t>Closed issues: <list style="symbols">
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not
              used"</t>
          </list>
        </t>
        <t>Other changes: <list style="symbols">
            <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
              on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
            <t>Add explicit references to BNF syntax and rules imported from other parts of the
              specification.</t>
          </list>
        </t>
      </section>

      <section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
        <t>Ongoing work on IANA Message Header Registration (<eref
            target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40" />): <list style="symbols">
            <t>Reference RFC 3984, and update header registrations for headers defined in this
              document.</t>
          </list>
        </t>
      </section>

      <section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
        <t>Closed issues: <list style="symbols">
            <t>
              <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header
              classification"</t>
          </list>
        </t>
      </section>

      <section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
      <t>
      </t>
      </section>

      <section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
      <t>
        This is a total rewrite.
      </t>
      </section>

    </section>
  </back>
</rfc>