Changeset 157 for draft-ietf-httpbis


Ignore:
Timestamp:
08/01/08 03:19:58 (15 years ago)
Author:
fielding@…
Message:

editorial: Move caching intro in overview to introduction along
with the prerequisite definitions. Move definition of delta-seconds
to where it is first used (Age). Add a couple glue sentences.
Replace redundant "the HTTP/1.1 protocol" with "HTTP/1.1".

Location:
draft-ietf-httpbis/latest
Files:
6 edited

Legend:

Unmodified
Added
Removed

  • draft-ietf-httpbis/latest/p1-messaging.html

    r155 r157  
    768768      <dl class="empty">
    769769         <dd>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
    770             The rules for determining the cacheability of HTTP responses are defined in <a href="p6-cache.html#caching" title="Caching in HTTP">Section 2</a> of <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
     770            The rules for determining the cacheability of HTTP responses are defined in <a href="p6-cache.html#caching" title="Introduction">Section 1</a> of <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
    771771            request.
    772772         </dd>
    … …  
    825825          &lt;--------- response chain
    826826</pre><p id="rfc.section.1.4.p.9">Not all responses are usefully cacheable, and some requests may contain modifiers which place special requirements on cache
    827          behavior. HTTP requirements for cache behavior and cacheable responses are defined in <a href="p6-cache.html#caching" title="Caching in HTTP">Section 2</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
     827         behavior. HTTP requirements for cache behavior and cacheable responses are defined in <a href="p6-cache.html#caching" title="Introduction">Section 1</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
    828828      </p>
    829829      <p id="rfc.section.1.4.p.10">In fact, there are a wide variety of architectures and configurations of caches and proxies currently being experimented with
    … …  
    12881288         to the entity being transferred. These header fields apply only to the message being transmitted.
    12891289      </p>
    1290       <div id="rfc.figure.u.26"></div><pre class="inline"><span id="rfc.iref.g.60"></span>  general-header = Cache-Control            ; <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 3.2</a>
     1290      <div id="rfc.figure.u.26"></div><pre class="inline"><span id="rfc.iref.g.60"></span>  general-header = Cache-Control            ; <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 15.2</a>
    12911291                 | Connection               ; <a href="#header.connection" id="rfc.xref.header.connection.1" title="Connection">Section&nbsp;8.1</a>
    12921292                 | Date                     ; <a href="#header.date" id="rfc.xref.header.date.1" title="Date">Section&nbsp;8.3</a>
    1293                  | Pragma                   ; <a href="#Part6" id="rfc.xref.Part6.4"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.pragma" title="Pragma">Section 3.4</a>
     1293                 | Pragma                   ; <a href="#Part6" id="rfc.xref.Part6.4"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.pragma" title="Pragma">Section 15.4</a>
    12941294                 | Trailer                  ; <a href="#header.trailer" id="rfc.xref.header.trailer.2" title="Trailer">Section&nbsp;8.6</a>
    12951295                 | Transfer-Encoding        ; <a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.4" title="Transfer-Encoding">Section&nbsp;8.7</a>
    12961296                 | Upgrade                  ; <a href="#header.upgrade" id="rfc.xref.header.upgrade.1" title="Upgrade">Section&nbsp;8.8</a>
    12971297                 | Via                      ; <a href="#header.via" id="rfc.xref.header.via.1" title="Via">Section&nbsp;8.9</a>
    1298                  | Warning                  ; <a href="#Part6" id="rfc.xref.Part6.5"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.warning" title="Warning">Section 3.6</a>
     1298                 | Warning                  ; <a href="#Part6" id="rfc.xref.Part6.5"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.warning" title="Warning">Section 15.6</a>
    12991299</pre><p id="rfc.section.4.5.p.3">General-header field names can be extended reliably only in combination with a change in the protocol version. However, new
    13001300         or experimental header fields may be given the semantics of general header fields if all parties in the communication recognize
    … …  
    26492649                  <li class="indline1"><em>Part5</em>&nbsp;&nbsp;<a class="iref" href="#Part5"><b>12.1</b></a>, <a class="iref" href="#rfc.xref.Part5.1">D.3</a></li>
    26502650                  <li class="indline1"><em>Part6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">1.3</a>, <a class="iref" href="#rfc.xref.Part6.2">1.4</a>, <a class="iref" href="#rfc.xref.Part6.3">4.5</a>, <a class="iref" href="#rfc.xref.Part6.4">4.5</a>, <a class="iref" href="#rfc.xref.Part6.5">4.5</a>, <a class="iref" href="#Part6"><b>12.1</b></a>, <a class="iref" href="#rfc.xref.Part6.6">D.3</a><ul class="ind">
    2651                         <li class="indline1"><em>Section 2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">1.3</a>, <a class="iref" href="#rfc.xref.Part6.2">1.4</a></li>
    2652                         <li class="indline1"><em>Section 3.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.3">4.5</a></li>
    2653                         <li class="indline1"><em>Section 3.4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.4">4.5</a></li>
    2654                         <li class="indline1"><em>Section 3.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.5">4.5</a></li>
     2651                        <li class="indline1"><em>Section 1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">1.3</a>, <a class="iref" href="#rfc.xref.Part6.2">1.4</a></li>
     2652                        <li class="indline1"><em>Section 15.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.3">4.5</a></li>
     2653                        <li class="indline1"><em>Section 15.4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.4">4.5</a></li>
     2654                        <li class="indline1"><em>Section 15.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.5">4.5</a></li>
    26552655                     </ul>
    26562656                  </li>

  • draft-ietf-httpbis/latest/p2-semantics.html

    r153 r157  
    745745      </p>
    746746      <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.9"></span>  response-header = Accept-Ranges           ; <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>, <a href="p5-range.html#header.accept-ranges" title="Accept-Ranges">Section 5.1</a>
    747                   | Age                     ; <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.age" title="Age">Section 3.1</a>
     747                  | Age                     ; <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.age" title="Age">Section 15.1</a>
    748748                  | ETag                    ; <a href="#Part4" id="rfc.xref.Part4.5"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>, <a href="p4-conditional.html#header.etag" title="ETag">Section 6.1</a>
    749749                  | Location                ; <a href="#header.location" id="rfc.xref.header.location.1" title="Location">Section&nbsp;10.4</a>
    … …  
    751751                  | Retry-After             ; <a href="#header.retry-after" id="rfc.xref.header.retry-after.1" title="Retry-After">Section&nbsp;10.7</a>
    752752                  | Server                  ; <a href="#header.server" id="rfc.xref.header.server.1" title="Server">Section&nbsp;10.8</a>
    753                   | Vary                    ; <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.vary" title="Vary">Section 3.5</a>
     753                  | Vary                    ; <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.vary" title="Vary">Section 15.5</a>
    754754                  | WWW-Authenticate        ; <a href="#Part7" id="rfc.xref.Part7.4"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>, <a href="p7-auth.html#header.www-authenticate" title="WWW-Authenticate">Section 3.4</a>
    755755</pre><p id="rfc.section.6.p.3">Response-header field names can be extended reliably only in combination with a change in the protocol version. However, new
    … …  
    19281928                  </li>
    19291929                  <li class="indline1"><em>Part6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">6</a>, <a class="iref" href="#rfc.xref.Part6.2">6</a>, <a class="iref" href="#rfc.xref.Part6.3">8.3</a>, <a class="iref" href="#Part6"><b>14.1</b></a><ul class="ind">
    1930                         <li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">6</a></li>
    1931                         <li class="indline1"><em>Section 3.5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.2">6</a></li>
     1930                        <li class="indline1"><em>Section 15.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">6</a></li>
     1931                        <li class="indline1"><em>Section 15.5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.2">6</a></li>
    19321932                     </ul>
    19331933                  </li>

  • draft-ietf-httpbis/latest/p3-payload.html

    r153 r157  
    759759                 | Content-Range            ; <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>, <a href="p5-range.html#header.content-range" title="Content-Range">Section 5.2</a>
    760760                 | Content-Type             ; <a href="#header.content-type" id="rfc.xref.header.content-type.2" title="Content-Type">Section&nbsp;5.9</a>
    761                  | Expires                  ; <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.expires" title="Expires">Section 3.3</a>
     761                 | Expires                  ; <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.expires" title="Expires">Section 15.3</a>
    762762                 | Last-Modified            ; <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>, <a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 6.6</a>
    763763                 | extension-header
    … …  
    835835         header fields not defined by this specification.
    836836      </p>
    837       <p id="rfc.section.4.1.p.5">The Vary header field (<a href="p6-cache.html#header.vary" title="Vary">Section 3.5</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) can be used to express the parameters the server uses to select a representation that is subject to server-driven negotiation.
     837      <p id="rfc.section.4.1.p.5">The Vary header field (<a href="p6-cache.html#header.vary" title="Vary">Section 15.5</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) can be used to express the parameters the server uses to select a representation that is subject to server-driven negotiation.
    838838      </p>
    839839      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="agent-driven.negotiation" href="#agent-driven.negotiation">Agent-driven Negotiation</a></h2>
    … …  
    11141114      <p id="rfc.section.5.7.p.5">A cache cannot assume that an entity with a Content-Location different from the URI used to retrieve it can be used to respond
    11151115         to later requests on that Content-Location URI. However, the Content-Location can be used to differentiate between multiple
    1116          entities retrieved from a single requested resource, as described in <a href="p6-cache.html#caching.negotiated.responses" title="Caching Negotiated Responses">Section 2.6</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
     1116         entities retrieved from a single requested resource, as described in <a href="p6-cache.html#caching.negotiated.responses" title="Caching Negotiated Responses">Section 7</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
    11171117      </p>
    11181118      <p id="rfc.section.5.7.p.6">If the Content-Location is a relative URI, the relative URI is interpreted relative to the Request-URI.</p>
    … …  
    16621662                  </li>
    16631663                  <li class="indline1"><em>Part6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">3.1</a>, <a class="iref" href="#rfc.xref.Part6.2">4.1</a>, <a class="iref" href="#rfc.xref.Part6.3">5.7</a>, <a class="iref" href="#Part6"><b>9.1</b></a>, <a class="iref" href="#rfc.xref.Part6.4">C.1</a><ul class="ind">
    1664                         <li class="indline1"><em>Section 2.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.3">5.7</a></li>
    1665                         <li class="indline1"><em>Section 3.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">3.1</a></li>
    1666                         <li class="indline1"><em>Section 3.5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.2">4.1</a></li>
     1664                        <li class="indline1"><em>Section 7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.3">5.7</a></li>
     1665                        <li class="indline1"><em>Section 15.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">3.1</a></li>
     1666                        <li class="indline1"><em>Section 15.5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.2">4.1</a></li>
    16671667                     </ul>
    16681668                  </li>

  • draft-ietf-httpbis/latest/p4-conditional.html

    r153 r157  
    756756         header <em class="bcp14">MUST</em> be ignored.
    757757      </p>
    758       <p id="rfc.section.6.2.p.7">The meaning of "If-Match: *" is that the method <em class="bcp14">SHOULD</em> be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see <a href="p6-cache.html#header.vary" title="Vary">Section 3.5</a> of <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) exists, and <em class="bcp14">MUST NOT</em> be performed if the representation does not exist.
     758      <p id="rfc.section.6.2.p.7">The meaning of "If-Match: *" is that the method <em class="bcp14">SHOULD</em> be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see <a href="p6-cache.html#header.vary" title="Vary">Section 15.5</a> of <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) exists, and <em class="bcp14">MUST NOT</em> be performed if the representation does not exist.
    759759      </p>
    760760      <p id="rfc.section.6.2.p.8">A request intended to update a resource (e.g., a PUT) <em class="bcp14">MAY</em> include an If-Match header field to signal that the request method <em class="bcp14">MUST NOT</em> be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource.
    … …  
    837837         If-None-Match header <em class="bcp14">MUST</em> be ignored. (See <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates" title="Rules for When to Use Entity Tags and Last-Modified Dates">Section&nbsp;5</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)
    838838      </p>
    839       <p id="rfc.section.6.4.p.8">The meaning of "If-None-Match: *" is that the method <em class="bcp14">MUST NOT</em> be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see <a href="p6-cache.html#header.vary" title="Vary">Section 3.5</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) exists, and <em class="bcp14">SHOULD</em> be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations.
     839      <p id="rfc.section.6.4.p.8">The meaning of "If-None-Match: *" is that the method <em class="bcp14">MUST NOT</em> be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see <a href="p6-cache.html#header.vary" title="Vary">Section 15.5</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) exists, and <em class="bcp14">SHOULD</em> be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations.
    840840      </p>
    841841      <p id="rfc.section.6.4.p.9">Examples:</p>
    … …  
    10651065                  </li>
    10661066                  <li class="indline1"><em>Part6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">6.2</a>, <a class="iref" href="#rfc.xref.Part6.2">6.4</a>, <a class="iref" href="#Part6"><b>10.1</b></a><ul class="ind">
    1067                         <li class="indline1"><em>Section 3.5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">6.2</a>, <a class="iref" href="#rfc.xref.Part6.2">6.4</a></li>
     1067                        <li class="indline1"><em>Section 15.5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">6.2</a>, <a class="iref" href="#rfc.xref.Part6.2">6.4</a></li>
    10681068                     </ul>
    10691069                  </li>

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

    r154 r157  
    333333      <link rel="Index" href="#rfc.index">
    334334      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
    335       <link rel="Chapter" title="2 Caching in HTTP" href="#rfc.section.2">
    336       <link rel="Chapter" title="3 Header Field Definitions" href="#rfc.section.3">
    337       <link rel="Chapter" title="4 IANA Considerations" href="#rfc.section.4">
    338       <link rel="Chapter" title="5 Security Considerations" href="#rfc.section.5">
    339       <link rel="Chapter" title="6 Acknowledgments" href="#rfc.section.6">
    340       <link rel="Chapter" href="#rfc.section.7" title="7 References">
     335      <link rel="Chapter" title="2 Overview" href="#rfc.section.2">
     336      <link rel="Chapter" title="3 Expiration Model" href="#rfc.section.3">
     337      <link rel="Chapter" title="4 Validation Model" href="#rfc.section.4">
     338      <link rel="Chapter" title="5 Response Cacheability" href="#rfc.section.5">
     339      <link rel="Chapter" title="6 Constructing Responses From Caches" href="#rfc.section.6">
     340      <link rel="Chapter" title="7 Caching Negotiated Responses" href="#rfc.section.7">
     341      <link rel="Chapter" title="8 Shared and Non-Shared Caches" href="#rfc.section.8">
     342      <link rel="Chapter" title="9 Errors or Incomplete Response Cache Behavior" href="#rfc.section.9">
     343      <link rel="Chapter" title="10 Side Effects of GET and HEAD" href="#rfc.section.10">
     344      <link rel="Chapter" title="11 Invalidation After Updates or Deletions" href="#rfc.section.11">
     345      <link rel="Chapter" title="12 Write-Through Mandatory" href="#rfc.section.12">
     346      <link rel="Chapter" title="13 Cache Replacement" href="#rfc.section.13">
     347      <link rel="Chapter" title="14 History Lists" href="#rfc.section.14">
     348      <link rel="Chapter" title="15 Header Field Definitions" href="#rfc.section.15">
     349      <link rel="Chapter" title="16 IANA Considerations" href="#rfc.section.16">
     350      <link rel="Chapter" title="17 Security Considerations" href="#rfc.section.17">
     351      <link rel="Chapter" title="18 Acknowledgments" href="#rfc.section.18">
     352      <link rel="Chapter" href="#rfc.section.19" title="19 References">
    341353      <link rel="Appendix" title="A Compatibility with Previous Versions" href="#rfc.section.A">
    342354      <link rel="Appendix" title="B Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.B">
    … …  
    476488      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
    477489      <ul class="toc">
    478          <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul class="toc">
    479                <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
     490         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul class="toc">
     491               <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
    480492               <li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
    481                <li class="tocline1">1.3&nbsp;&nbsp;&nbsp;<a href="#delta.seconds">Delta Seconds</a></li>
     493               <li class="tocline1">1.3&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
    482494            </ul>
    483495         </li>
    484          <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#caching">Caching in HTTP</a><ul class="toc">
    485                <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview</a><ul class="toc">
    486                      <li class="tocline1">2.1.1&nbsp;&nbsp;&nbsp;<a href="#cache.correctness">Cache Correctness</a></li>
    487                      <li class="tocline1">2.1.2&nbsp;&nbsp;&nbsp;<a href="#warnings">Warnings</a></li>
    488                      <li class="tocline1">2.1.3&nbsp;&nbsp;&nbsp;<a href="#cache-control.mechanisms">Cache-control Mechanisms</a></li>
    489                      <li class="tocline1">2.1.4&nbsp;&nbsp;&nbsp;<a href="#explicit.ua.warnings">Explicit User Agent Warnings</a></li>
    490                      <li class="tocline1">2.1.5&nbsp;&nbsp;&nbsp;<a href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></li>
    491                      <li class="tocline1">2.1.6&nbsp;&nbsp;&nbsp;<a href="#client-controlled.behavior">Client-controlled Behavior</a></li>
     496         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview</a><ul class="toc">
     497               <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#cache.correctness">Cache Correctness</a></li>
     498               <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#warnings">Warnings</a></li>
     499               <li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#cache-control.mechanisms">Cache-control Mechanisms</a></li>
     500               <li class="tocline1">2.4&nbsp;&nbsp;&nbsp;<a href="#explicit.ua.warnings">Explicit User Agent Warnings</a></li>
     501               <li class="tocline1">2.5&nbsp;&nbsp;&nbsp;<a href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></li>
     502               <li class="tocline1">2.6&nbsp;&nbsp;&nbsp;<a href="#client-controlled.behavior">Client-controlled Behavior</a></li>
     503            </ul>
     504         </li>
     505         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Expiration Model</a><ul class="toc">
     506               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#server-specified.expiration">Server-Specified Expiration</a></li>
     507               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#heuristic.expiration">Heuristic Expiration</a></li>
     508               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Age Calculations</a></li>
     509               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#expiration.calculations">Expiration Calculations</a></li>
     510               <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></li>
     511               <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></li>
     512            </ul>
     513         </li>
     514         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul class="toc">
     515               <li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#last-modified.dates">Last-Modified Dates</a></li>
     516               <li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></li>
     517               <li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#non-validating.conditionals">Non-validating Conditionals</a></li>
     518            </ul>
     519         </li>
     520         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a></li>
     521         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses From Caches</a><ul class="toc">
     522               <li class="tocline1">6.1&nbsp;&nbsp;&nbsp;<a href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></li>
     523               <li class="tocline1">6.2&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.headers">Non-modifiable Headers</a></li>
     524               <li class="tocline1">6.3&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Headers</a></li>
     525            </ul>
     526         </li>
     527         <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
     528         <li class="tocline0">8.&nbsp;&nbsp;&nbsp;<a href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></li>
     529         <li class="tocline0">9.&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></li>
     530         <li class="tocline0">10.&nbsp;&nbsp;&nbsp;<a href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></li>
     531         <li class="tocline0">11.&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></li>
     532         <li class="tocline0">12.&nbsp;&nbsp;&nbsp;<a href="#write-through.mandatory">Write-Through Mandatory</a></li>
     533         <li class="tocline0">13.&nbsp;&nbsp;&nbsp;<a href="#cache.replacement">Cache Replacement</a></li>
     534         <li class="tocline0">14.&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
     535         <li class="tocline0">15.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
     536               <li class="tocline1">15.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
     537               <li class="tocline1">15.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc">
     538                     <li class="tocline1">15.2.1&nbsp;&nbsp;&nbsp;<a href="#what.is.cacheable">What is Cacheable</a></li>
     539                     <li class="tocline1">15.2.2&nbsp;&nbsp;&nbsp;<a href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></li>
     540                     <li class="tocline1">15.2.3&nbsp;&nbsp;&nbsp;<a href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></li>
     541                     <li class="tocline1">15.2.4&nbsp;&nbsp;&nbsp;<a href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></li>
     542                     <li class="tocline1">15.2.5&nbsp;&nbsp;&nbsp;<a href="#no-transform.directive">No-Transform Directive</a></li>
     543                     <li class="tocline1">15.2.6&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
    492544                  </ul>
    493545               </li>
    494                <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Expiration Model</a><ul class="toc">
    495                      <li class="tocline1">2.2.1&nbsp;&nbsp;&nbsp;<a href="#server-specified.expiration">Server-Specified Expiration</a></li>
    496                      <li class="tocline1">2.2.2&nbsp;&nbsp;&nbsp;<a href="#heuristic.expiration">Heuristic Expiration</a></li>
    497                      <li class="tocline1">2.2.3&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Age Calculations</a></li>
    498                      <li class="tocline1">2.2.4&nbsp;&nbsp;&nbsp;<a href="#expiration.calculations">Expiration Calculations</a></li>
    499                      <li class="tocline1">2.2.5&nbsp;&nbsp;&nbsp;<a href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></li>
    500                      <li class="tocline1">2.2.6&nbsp;&nbsp;&nbsp;<a href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></li>
    501                   </ul>
    502                </li>
    503                <li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul class="toc">
    504                      <li class="tocline1">2.3.1&nbsp;&nbsp;&nbsp;<a href="#last-modified.dates">Last-Modified Dates</a></li>
    505                      <li class="tocline1">2.3.2&nbsp;&nbsp;&nbsp;<a href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></li>
    506                      <li class="tocline1">2.3.3&nbsp;&nbsp;&nbsp;<a href="#non-validating.conditionals">Non-validating Conditionals</a></li>
    507                   </ul>
    508                </li>
    509                <li class="tocline1">2.4&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a></li>
    510                <li class="tocline1">2.5&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses From Caches</a><ul class="toc">
    511                      <li class="tocline1">2.5.1&nbsp;&nbsp;&nbsp;<a href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></li>
    512                      <li class="tocline1">2.5.2&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.headers">Non-modifiable Headers</a></li>
    513                      <li class="tocline1">2.5.3&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Headers</a></li>
    514                   </ul>
    515                </li>
    516                <li class="tocline1">2.6&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
    517                <li class="tocline1">2.7&nbsp;&nbsp;&nbsp;<a href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></li>
    518                <li class="tocline1">2.8&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></li>
    519                <li class="tocline1">2.9&nbsp;&nbsp;&nbsp;<a href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></li>
    520                <li class="tocline1">2.10&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></li>
    521                <li class="tocline1">2.11&nbsp;&nbsp;&nbsp;<a href="#write-through.mandatory">Write-Through Mandatory</a></li>
    522                <li class="tocline1">2.12&nbsp;&nbsp;&nbsp;<a href="#cache.replacement">Cache Replacement</a></li>
    523                <li class="tocline1">2.13&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
     546               <li class="tocline1">15.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
     547               <li class="tocline1">15.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
     548               <li class="tocline1">15.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
     549               <li class="tocline1">15.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
    524550            </ul>
    525551         </li>
    526          <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
    527                <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
    528                <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc">
    529                      <li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#what.is.cacheable">What is Cacheable</a></li>
    530                      <li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></li>
    531                      <li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></li>
    532                      <li class="tocline1">3.2.4&nbsp;&nbsp;&nbsp;<a href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></li>
    533                      <li class="tocline1">3.2.5&nbsp;&nbsp;&nbsp;<a href="#no-transform.directive">No-Transform Directive</a></li>
    534                      <li class="tocline1">3.2.6&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
    535                   </ul>
    536                </li>
    537                <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
    538                <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
    539                <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
    540                <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
    541             </ul>
    542          </li>
    543          <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a></li>
    544          <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
    545          <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
    546          <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul class="toc">
    547                <li class="tocline1">7.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
    548                <li class="tocline1">7.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
     552         <li class="tocline0">16.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a></li>
     553         <li class="tocline0">17.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
     554         <li class="tocline0">18.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
     555         <li class="tocline0">19.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul class="toc">
     556               <li class="tocline1">19.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
     557               <li class="tocline1">19.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
    549558            </ul>
    550559         </li>
    … …  
    563572         <li class="tocline0"><a href="#rfc.index">Index</a></li>
    564573      </ul>
    565       <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
    566       <p id="rfc.section.1.p.1">This document will define aspects of HTTP related to caching response messages. Right now it only includes the extracted relevant
    567          sections of <cite title="Hypertext Transfer Protocol -- HTTP/1.1" id="rfc.xref.RFC2616.1">RFC 2616</cite> without edit.
    568       </p>
    569       <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2>
    570       <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
    571          in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
    572       </p>
    573       <p id="rfc.section.1.1.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."
    574       </p>
    575       <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="intro.terminology" href="#intro.terminology">Terminology</a></h2>
    576       <p id="rfc.section.1.2.p.1">This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication.</p>
    577       <p id="rfc.section.1.2.p.2"> <span id="rfc.iref.c.1"></span>  <dfn>cache</dfn> 
    578       </p>
    579       <dl class="empty">
    580          <dd>A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion.
    581             A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent
    582             requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
    583          </dd>
    584       </dl>
    585       <p id="rfc.section.1.2.p.3"> <span id="rfc.iref.c.2"></span>  <dfn>cacheable</dfn> 
    586       </p>
    587       <dl class="empty">
    588          <dd>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
    589             The rules for determining the cacheability of HTTP responses are defined in <a href="#caching" title="Caching in HTTP">Section&nbsp;2</a>. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
    590             request.
    591          </dd>
    592       </dl>
    593       <p id="rfc.section.1.2.p.4"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn> 
    594       </p>
    595       <dl class="empty">
    596          <dd>A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more
    597             proxies. A response is also first-hand if its validity has just been checked directly with the origin server.
    598          </dd>
    599       </dl>
    600       <p id="rfc.section.1.2.p.5"> <span id="rfc.iref.e.1"></span>  <dfn>explicit expiration time</dfn> 
    601       </p>
    602       <dl class="empty">
    603          <dd>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</dd>
    604       </dl>
    605       <p id="rfc.section.1.2.p.6"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn> 
    606       </p>
    607       <dl class="empty">
    608          <dd>An expiration time assigned by a cache when no explicit expiration time is available.</dd>
    609       </dl>
    610       <p id="rfc.section.1.2.p.7"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn> 
    611       </p>
    612       <dl class="empty">
    613          <dd>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</dd>
    614       </dl>
    615       <p id="rfc.section.1.2.p.8"> <span id="rfc.iref.f.2"></span>  <dfn>freshness lifetime</dfn> 
    616       </p>
    617       <dl class="empty">
    618          <dd>The length of time between the generation of a response and its expiration time.</dd>
    619       </dl>
    620       <p id="rfc.section.1.2.p.9"> <span id="rfc.iref.f.3"></span>  <dfn>fresh</dfn> 
    621       </p>
    622       <dl class="empty">
    623          <dd>A response is fresh if its age has not yet exceeded its freshness lifetime.</dd>
    624       </dl>
    625       <p id="rfc.section.1.2.p.10"> <span id="rfc.iref.s.1"></span>  <dfn>stale</dfn> 
    626       </p>
    627       <dl class="empty">
    628          <dd>A response is stale if its age has passed its freshness lifetime.</dd>
    629       </dl>
    630       <p id="rfc.section.1.2.p.11"> <span id="rfc.iref.s.2"></span>  <dfn>semantically transparent</dfn> 
    631       </p>
    632       <dl class="empty">
    633          <dd>A cache behaves in a "semantically transparent" manner, with respect to a particular response, when its use affects neither
    634             the requesting client nor the origin server, except to improve performance. When a cache is semantically transparent, the
    635             client receives exactly the same response (except for hop-by-hop headers) that it would have received had its request been
    636             handled directly by the origin server.
    637          </dd>
    638       </dl>
    639       <p id="rfc.section.1.2.p.12"> <span id="rfc.iref.v.1"></span>  <dfn>validator</dfn> 
    640       </p>
    641       <dl class="empty">
    642          <dd>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent
    643             copy of an entity.
    644          </dd>
    645       </dl>
    646       <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a id="delta.seconds" href="#delta.seconds">Delta Seconds</a></h2>
    647       <p id="rfc.section.1.3.p.1">Some HTTP header fields allow a time value to be specified as an integer number of seconds, represented in decimal, after
    648          the time that the message was received.
    649       </p>
    650       <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.g.1"></span>  delta-seconds  = 1*DIGIT
    651 </pre><h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching" href="#caching">Caching in HTTP</a></h1>
    652       <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="caching.overview" href="#caching.overview">Overview</a></h2>
    653       <p id="rfc.section.2.1.p.1">HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches.
     574      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="caching" href="#caching">Introduction</a></h1>
     575      <p id="rfc.section.1.p.1">HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches.
    654576         The HTTP/1.1 protocol includes a number of elements intended to make caching work as well as possible. Because these elements
    655577         are inextricable from other aspects of the protocol, and because they interact with each other, it is useful to describe the
    656          basic caching design of HTTP separately from the detailed descriptions of methods, headers, response codes, etc.
    657       </p>
    658       <p id="rfc.section.2.1.p.2">Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to eliminate
     578         basic caching design of HTTP separately from the detailed descriptions of methods, headers, response codes, etc. This document
     579         defines aspects of HTTP related to caching response messages.
     580      </p>
     581      <div id="rfc.iref.c.1"></div>
     582      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.purpose" href="#intro.purpose">Purpose</a></h2>
     583      <p id="rfc.section.1.1.p.1">An HTTP <dfn>cache</dfn> is a local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache
     584         stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests.
     585         Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
     586      </p>
     587      <p id="rfc.section.1.1.p.2">Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to eliminate
    659588         the need to send requests in many cases, and to eliminate the need to send full responses in many other cases. The former
    660589         reduces the number of network round-trips required for many operations; we use an "expiration" mechanism for this purpose
    661          (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;2.2</a>). The latter reduces network bandwidth requirements; we use a "validation" mechanism for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;2.3</a>).
    662       </p>
    663       <p id="rfc.section.2.1.p.3">Requirements for performance, availability, and disconnected operation require us to be able to relax the goal of semantic
    664          transparency. The HTTP/1.1 protocol allows origin servers, caches, and clients to explicitly reduce transparency when necessary.
    665          However, because non-transparent operation may confuse non-expert users, and might be incompatible with certain server applications
     590         (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;3</a>). The latter reduces network bandwidth requirements; we use a "validation" mechanism for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;4</a>).
     591      </p>
     592      <div id="rfc.iref.s.1"></div>
     593      <p id="rfc.section.1.1.p.3">A cache behaves in a "<dfn>semantically transparent</dfn>" manner, with respect to a particular response, when its use affects neither the requesting client nor the origin server,
     594         except to improve performance. When a cache is semantically transparent, the client receives exactly the same response (except
     595         for hop-by-hop headers) that it would have received had its request been handled directly by the origin server.
     596      </p>
     597      <p id="rfc.section.1.1.p.4">In an ideal world, all interactions with an HTTP cache would be semantically transparent. However, for some resources, semantic
     598         transparency is not always necessary and can be effectively traded for the sake of bandwidth scaling, disconnected operation,
     599         and high availability. HTTP/1.1 allows origin servers, caches, and clients to explicitly reduce transparency when necessary.
     600         However, because non-transparent operation may confuse non-expert users and might be incompatible with certain server applications
    666601         (such as those for ordering merchandise), the protocol requires that transparency be relaxed
    667602      </p>
    … …  
    670605         <li>only with an explicit warning to the end user when relaxed by cache or client</li>
    671606      </ul>
    672       <p id="rfc.section.2.1.p.4">Therefore, the HTTP/1.1 protocol provides these important elements: </p>
     607      <p id="rfc.section.1.1.p.5">Therefore, HTTP/1.1 provides these important elements: </p>
    673608      <ol>
    674609         <li>Protocol features that provide full semantic transparency when this is required by all parties.</li>
    … …  
    678613         </li>
    679614      </ol>
    680       <p id="rfc.section.2.1.p.5">A basic principle is that it must be possible for the clients to detect any potential relaxation of semantic transparency. </p>
     615      <p id="rfc.section.1.1.p.6">A basic principle is that it must be possible for the clients to detect any potential relaxation of semantic transparency. </p>
    681616      <dl class="empty">
    682617         <dd> <b>Note:</b> The server, cache, or client implementor might be faced with design decisions not explicitly discussed in this specification.
    … …  
    685620         </dd>
    686621      </dl>
    687       <h3 id="rfc.section.2.1.1"><a href="#rfc.section.2.1.1">2.1.1</a>&nbsp;<a id="cache.correctness" href="#cache.correctness">Cache Correctness</a></h3>
    688       <p id="rfc.section.2.1.1.p.1">A correct cache <em class="bcp14">MUST</em> respond to a request with the most up-to-date response held by the cache that is appropriate to the request (see Sections <a href="#disambiguating.expiration.values" title="Disambiguating Expiration Values">2.2.5</a>, <a href="#disambiguating.multiple.responses" title="Disambiguating Multiple Responses">2.2.6</a>, and <a href="#cache.replacement" title="Cache Replacement">2.12</a>) which meets one of the following conditions:
     622      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="intro.terminology" href="#intro.terminology">Terminology</a></h2>
     623      <p id="rfc.section.1.2.p.1">This specification uses a number of terms to refer to the roles played by participants in, and objects of, HTTP caching.</p>
     624      <p id="rfc.section.1.2.p.2"> <span id="rfc.iref.c.2"></span>  <dfn>cacheable</dfn> 
     625      </p>
     626      <dl class="empty">
     627         <dd>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
     628            Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
     629            request.
     630         </dd>
     631      </dl>
     632      <p id="rfc.section.1.2.p.3"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn> 
     633      </p>
     634      <dl class="empty">
     635         <dd>A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more
     636            proxies. A response is also first-hand if its validity has just been checked directly with the origin server.
     637         </dd>
     638      </dl>
     639      <p id="rfc.section.1.2.p.4"> <span id="rfc.iref.e.1"></span>  <dfn>explicit expiration time</dfn> 
     640      </p>
     641      <dl class="empty">
     642         <dd>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</dd>
     643      </dl>
     644      <p id="rfc.section.1.2.p.5"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn> 
     645      </p>
     646      <dl class="empty">
     647         <dd>An expiration time assigned by a cache when no explicit expiration time is available.</dd>
     648      </dl>
     649      <p id="rfc.section.1.2.p.6"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn> 
     650      </p>
     651      <dl class="empty">
     652         <dd>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</dd>
     653      </dl>
     654      <p id="rfc.section.1.2.p.7"> <span id="rfc.iref.f.2"></span>  <dfn>freshness lifetime</dfn> 
     655      </p>
     656      <dl class="empty">
     657         <dd>The length of time between the generation of a response and its expiration time.</dd>
     658      </dl>
     659      <p id="rfc.section.1.2.p.8"> <span id="rfc.iref.f.3"></span>  <dfn>fresh</dfn> 
     660      </p>
     661      <dl class="empty">
     662         <dd>A response is fresh if its age has not yet exceeded its freshness lifetime.</dd>
     663      </dl>
     664      <p id="rfc.section.1.2.p.9"> <span id="rfc.iref.s.2"></span>  <dfn>stale</dfn> 
     665      </p>
     666      <dl class="empty">
     667         <dd>A response is stale if its age has passed its freshness lifetime.</dd>
     668      </dl>
     669      <p id="rfc.section.1.2.p.10"> <span id="rfc.iref.v.1"></span>  <dfn>validator</dfn> 
     670      </p>
     671      <dl class="empty">
     672         <dd>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent
     673            copy of an entity.
     674         </dd>
     675      </dl>
     676      <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2>
     677      <p id="rfc.section.1.3.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
     678         in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
     679      </p>
     680      <p id="rfc.section.1.3.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."
     681      </p>
     682      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Overview</a></h1>
     683      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="cache.correctness" href="#cache.correctness">Cache Correctness</a></h2>
     684      <p id="rfc.section.2.1.p.1">A correct cache <em class="bcp14">MUST</em> respond to a request with the most up-to-date response held by the cache that is appropriate to the request (see Sections <a href="#disambiguating.expiration.values" title="Disambiguating Expiration Values">3.5</a>, <a href="#disambiguating.multiple.responses" title="Disambiguating Multiple Responses">3.6</a>, and <a href="#cache.replacement" title="Cache Replacement">13</a>) which meets one of the following conditions:
    689685      </p>
    690686      <ol>
    691687         <li>It has been checked for equivalence with what the origin server would have returned by revalidating the response with the
    692             origin server (<a href="#validation.model" title="Validation Model">Section&nbsp;2.3</a>);
    693          </li>
    694          <li>It is "fresh enough" (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;2.2</a>). In the default case, this means it meets the least restrictive freshness requirement of the client, origin server, and
    695             cache (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;3.2</a>); if the origin server so specifies, it is the freshness requirement of the origin server alone. If a stored response is
     688            origin server (<a href="#validation.model" title="Validation Model">Section&nbsp;4</a>);
     689         </li>
     690         <li>It is "fresh enough" (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;3</a>). In the default case, this means it meets the least restrictive freshness requirement of the client, origin server, and
     691            cache (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;15.2</a>); if the origin server so specifies, it is the freshness requirement of the origin server alone. If a stored response is
    696692            not "fresh enough" by the most restrictive freshness requirement of both the client and the origin server, in carefully considered
    697             circumstances the cache <em class="bcp14">MAY</em> still return the response with the appropriate Warning header (see Sections <a href="#exceptions.to.the.rules.and.warnings" title="Exceptions to the Rules and Warnings">2.1.5</a> and <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">3.6</a>), unless such a response is prohibited (e.g., by a "no-store" cache-directive, or by a "no-cache" cache-request-directive;
    698             see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;3.2</a>).
     693            circumstances the cache <em class="bcp14">MAY</em> still return the response with the appropriate Warning header (see Sections <a href="#exceptions.to.the.rules.and.warnings" title="Exceptions to the Rules and Warnings">2.5</a> and <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">15.6</a>), unless such a response is prohibited (e.g., by a "no-store" cache-directive, or by a "no-cache" cache-request-directive;
     694            see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;15.2</a>).
    699695         </li>
    700696         <li>It is an appropriate 304 (Not Modified), 305 (Use Proxy), or error (4xx or 5xx) response message.</li>
    701697      </ol>
    702       <p id="rfc.section.2.1.1.p.2">If the cache can not communicate with the origin server, then a correct cache <em class="bcp14">SHOULD</em> respond as above if the response can be correctly served from the cache; if not it <em class="bcp14">MUST</em> return an error or warning indicating that there was a communication failure.
    703       </p>
    704       <p id="rfc.section.2.1.1.p.3">If a cache receives a response (either an entire response, or a 304 (Not Modified) response) that it would normally forward
     698      <p id="rfc.section.2.1.p.2">If the cache can not communicate with the origin server, then a correct cache <em class="bcp14">SHOULD</em> respond as above if the response can be correctly served from the cache; if not it <em class="bcp14">MUST</em> return an error or warning indicating that there was a communication failure.
     699      </p>
     700      <p id="rfc.section.2.1.p.3">If a cache receives a response (either an entire response, or a 304 (Not Modified) response) that it would normally forward
    705701         to the requesting client, and the received response is no longer fresh, the cache <em class="bcp14">SHOULD</em> forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache <em class="bcp14">SHOULD NOT</em> attempt to revalidate a response simply because that response became stale in transit; this might lead to an infinite loop.
    706702         A user agent that receives a stale response without a Warning <em class="bcp14">MAY</em> display a warning indication to the user.
    707703      </p>
    708       <h3 id="rfc.section.2.1.2"><a href="#rfc.section.2.1.2">2.1.2</a>&nbsp;<a id="warnings" href="#warnings">Warnings</a></h3>
    709       <p id="rfc.section.2.1.2.p.1">Whenever a cache returns a response that is neither first-hand nor "fresh enough" (in the sense of condition 2 in <a href="#cache.correctness" title="Cache Correctness">Section&nbsp;2.1.1</a>), it <em class="bcp14">MUST</em> attach a warning to that effect, using a Warning general-header. The Warning header and the currently defined warnings are
    710          described in <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;3.6</a>. The warning allows clients to take appropriate action.
    711       </p>
    712       <p id="rfc.section.2.1.2.p.2">Warnings <em class="bcp14">MAY</em> be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguish
     704      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="warnings" href="#warnings">Warnings</a></h2>
     705      <p id="rfc.section.2.2.p.1">Whenever a cache returns a response that is neither first-hand nor "fresh enough" (in the sense of condition 2 in <a href="#cache.correctness" title="Cache Correctness">Section&nbsp;2.1</a>), it <em class="bcp14">MUST</em> attach a warning to that effect, using a Warning general-header. The Warning header and the currently defined warnings are
     706         described in <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;15.6</a>. The warning allows clients to take appropriate action.
     707      </p>
     708      <p id="rfc.section.2.2.p.2">Warnings <em class="bcp14">MAY</em> be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguish
    713709         these responses from true failures.
    714710      </p>
    715       <p id="rfc.section.2.1.2.p.3">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning <em class="bcp14">MUST</em> or <em class="bcp14">MUST NOT</em> be deleted from a stored cache entry after a successful revalidation:
    716       </p>
    717       <p id="rfc.section.2.1.2.p.4"> </p>
     711      <p id="rfc.section.2.2.p.3">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning <em class="bcp14">MUST</em> or <em class="bcp14">MUST NOT</em> be deleted from a stored cache entry after a successful revalidation:
     712      </p>
     713      <p id="rfc.section.2.2.p.4"> </p>
    718714      <dl>
    719715         <dt>1xx</dt>
    … …  
    725721         </dd>
    726722      </dl>
    727       <p id="rfc.section.2.1.2.p.5">See <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;3.6</a> for the definitions of the codes themselves.
    728       </p>
    729       <p id="rfc.section.2.1.2.p.6">HTTP/1.0 caches will cache all Warnings in responses, without deleting the ones in the first category. Warnings in responses
     723      <p id="rfc.section.2.2.p.5">See <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;15.6</a> for the definitions of the codes themselves.
     724      </p>
     725      <p id="rfc.section.2.2.p.6">HTTP/1.0 caches will cache all Warnings in responses, without deleting the ones in the first category. Warnings in responses
    730726         that are passed to HTTP/1.0 caches carry an extra warning-date field, which prevents a future HTTP/1.1 recipient from believing
    731727         an erroneously cached Warning.
    732728      </p>
    733       <p id="rfc.section.2.1.2.p.7">Warnings also carry a warning text. The text <em class="bcp14">MAY</em> be in any appropriate natural language (perhaps based on the client's Accept headers), and include an <em class="bcp14">OPTIONAL</em> indication of what character set is used.
    734       </p>
    735       <p id="rfc.section.2.1.2.p.8">Multiple warnings <em class="bcp14">MAY</em> be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number.
     729      <p id="rfc.section.2.2.p.7">Warnings also carry a warning text. The text <em class="bcp14">MAY</em> be in any appropriate natural language (perhaps based on the client's Accept headers), and include an <em class="bcp14">OPTIONAL</em> indication of what character set is used.
     730      </p>
     731      <p id="rfc.section.2.2.p.8">Multiple warnings <em class="bcp14">MAY</em> be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number.
    736732         For example, a server might provide the same warning with texts in both English and Basque.
    737733      </p>
    738       <p id="rfc.section.2.1.2.p.9">When multiple warnings are attached to a response, it might not be practical or reasonable to display all of them to the user.
     734      <p id="rfc.section.2.2.p.9">When multiple warnings are attached to a response, it might not be practical or reasonable to display all of them to the user.
    739735         This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but
    740736         does suggest some heuristics.
    741737      </p>
    742       <h3 id="rfc.section.2.1.3"><a href="#rfc.section.2.1.3">2.1.3</a>&nbsp;<a id="cache-control.mechanisms" href="#cache-control.mechanisms">Cache-control Mechanisms</a></h3>
    743       <p id="rfc.section.2.1.3.p.1">The basic cache mechanisms in HTTP/1.1 (server-specified expiration times and validators) are implicit directives to caches.
     738      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="cache-control.mechanisms" href="#cache-control.mechanisms">Cache-control Mechanisms</a></h2>
     739      <p id="rfc.section.2.3.p.1">The basic cache mechanisms in HTTP/1.1 (server-specified expiration times and validators) are implicit directives to caches.
    744740         In some cases, a server or client might need to provide explicit directives to the HTTP caches. We use the Cache-Control header
    745741         for this purpose.
    746742      </p>
    747       <p id="rfc.section.2.1.3.p.2">The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These
     743      <p id="rfc.section.2.3.p.2">The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These
    748744         directives typically override the default caching algorithms. As a general rule, if there is any apparent conflict between
    749745         header values, the most restrictive interpretation is applied (that is, the one that is most likely to preserve semantic transparency).
    … …  
    751747         (for example, "max-stale" or "public").
    752748      </p>
    753       <p id="rfc.section.2.1.3.p.3">The cache-control directives are described in detail in <a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;3.2</a>.
    754       </p>
    755       <h3 id="rfc.section.2.1.4"><a href="#rfc.section.2.1.4">2.1.4</a>&nbsp;<a id="explicit.ua.warnings" href="#explicit.ua.warnings">Explicit User Agent Warnings</a></h3>
    756       <p id="rfc.section.2.1.4.p.1">Many user agents make it possible for users to override the basic caching mechanisms. For example, the user agent might allow
     749      <p id="rfc.section.2.3.p.3">The cache-control directives are described in detail in <a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;15.2</a>.
     750      </p>
     751      <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="explicit.ua.warnings" href="#explicit.ua.warnings">Explicit User Agent Warnings</a></h2>
     752      <p id="rfc.section.2.4.p.1">Many user agents make it possible for users to override the basic caching mechanisms. For example, the user agent might allow
    757753         the user to specify that cached entities (even explicitly stale ones) are never validated. Or the user agent might habitually
    758754         add "Cache-Control: max-stale=3600" to every request. The user agent <em class="bcp14">SHOULD NOT</em> default to either non-transparent behavior, or behavior that results in abnormally ineffective caching, but <em class="bcp14">MAY</em> be explicitly configured to do so by an explicit action of the user.
    759755      </p>
    760       <p id="rfc.section.2.1.4.p.2">If the user has overridden the basic caching mechanisms, the user agent <em class="bcp14">SHOULD</em> explicitly indicate to the user whenever this results in the display of information that might not meet the server's transparency
     756      <p id="rfc.section.2.4.p.2">If the user has overridden the basic caching mechanisms, the user agent <em class="bcp14">SHOULD</em> explicitly indicate to the user whenever this results in the display of information that might not meet the server's transparency
    761757         requirements (in particular, if the displayed entity is known to be stale). Since the protocol normally allows the user agent
    762758         to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication
    763759         need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other indicator.
    764760      </p>
    765       <p id="rfc.section.2.1.4.p.3">If the user has overridden the caching mechanisms in a way that would abnormally reduce the effectiveness of caches, the user
     761      <p id="rfc.section.2.4.p.3">If the user has overridden the caching mechanisms in a way that would abnormally reduce the effectiveness of caches, the user
    766762         agent <em class="bcp14">SHOULD</em> continually indicate this state to the user (for example, by a display of a picture of currency in flames) so that the user
    767763         does not inadvertently consume excess resources or suffer from excessive latency.
    768764      </p>
    769       <h3 id="rfc.section.2.1.5"><a href="#rfc.section.2.1.5">2.1.5</a>&nbsp;<a id="exceptions.to.the.rules.and.warnings" href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></h3>
    770       <p id="rfc.section.2.1.5.p.1">In some cases, the operator of a cache <em class="bcp14">MAY</em> choose to configure it to return stale responses even when not requested by clients. This decision ought not be made lightly,
     765      <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a id="exceptions.to.the.rules.and.warnings" href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></h2>
     766      <p id="rfc.section.2.5.p.1">In some cases, the operator of a cache <em class="bcp14">MAY</em> choose to configure it to return stale responses even when not requested by clients. This decision ought not be made lightly,
    771767         but may be necessary for reasons of availability or performance, especially when the cache is poorly connected to the origin
    772768         server. Whenever a cache returns a stale response, it <em class="bcp14">MUST</em> mark it as such (using a Warning header) enabling the client software to alert the user that there might be a potential problem.
    773769      </p>
    774       <p id="rfc.section.2.1.5.p.2">It also allows the user agent to take steps to obtain a first-hand or fresh response. For this reason, a cache <em class="bcp14">SHOULD NOT</em> return a stale response if the client explicitly requests a first-hand or fresh one, unless it is impossible to comply for
     770      <p id="rfc.section.2.5.p.2">It also allows the user agent to take steps to obtain a first-hand or fresh response. For this reason, a cache <em class="bcp14">SHOULD NOT</em> return a stale response if the client explicitly requests a first-hand or fresh one, unless it is impossible to comply for
    775771         technical or policy reasons.
    776772      </p>
    777       <h3 id="rfc.section.2.1.6"><a href="#rfc.section.2.1.6">2.1.6</a>&nbsp;<a id="client-controlled.behavior" href="#client-controlled.behavior">Client-controlled Behavior</a></h3>
    778       <p id="rfc.section.2.1.6.p.1">While the origin server (and to a lesser extent, intermediate caches, by their contribution to the age of a response) are
     773      <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a id="client-controlled.behavior" href="#client-controlled.behavior">Client-controlled Behavior</a></h2>
     774      <p id="rfc.section.2.6.p.1">While the origin server (and to a lesser extent, intermediate caches, by their contribution to the age of a response) are
    779775         the primary source of expiration information, in some cases the client might need to control a cache's decision about whether
    780776         to return a cached response without validating it. Clients do this using several directives of the Cache-Control header.
    781777      </p>
    782       <p id="rfc.section.2.1.6.p.2">A client's request <em class="bcp14">MAY</em> specify the maximum age it is willing to accept of an unvalidated response; specifying a value of zero forces the cache(s)
     778      <p id="rfc.section.2.6.p.2">A client's request <em class="bcp14">MAY</em> specify the maximum age it is willing to accept of an unvalidated response; specifying a value of zero forces the cache(s)
    783779         to revalidate all responses. A client <em class="bcp14">MAY</em> also specify the minimum time remaining before a response expires. Both of these options increase constraints on the behavior
    784780         of caches, and so cannot further relax the cache's approximation of semantic transparency.
    785781      </p>
    786       <p id="rfc.section.2.1.6.p.3">A client <em class="bcp14">MAY</em> also specify that it will accept stale responses, up to some maximum amount of staleness. This loosens the constraints on
     782      <p id="rfc.section.2.6.p.3">A client <em class="bcp14">MAY</em> also specify that it will accept stale responses, up to some maximum amount of staleness. This loosens the constraints on
    787783         the caches, and so might violate the origin server's specified constraints on semantic transparency, but might be necessary
    788784         to support disconnected operation, or high availability in the face of poor connectivity.
    789785      </p>
    790       <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="expiration.model" href="#expiration.model">Expiration Model</a></h2>
    791       <h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;<a id="server-specified.expiration" href="#server-specified.expiration">Server-Specified Expiration</a></h3>
    792       <p id="rfc.section.2.2.1.p.1">HTTP caching works best when caches can entirely avoid making requests to the origin server. The primary mechanism for avoiding
     786      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="expiration.model" href="#expiration.model">Expiration Model</a></h1>
     787      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="server-specified.expiration" href="#server-specified.expiration">Server-Specified Expiration</a></h2>
     788      <p id="rfc.section.3.1.p.1">HTTP caching works best when caches can entirely avoid making requests to the origin server. The primary mechanism for avoiding
    793789         requests is for an origin server to provide an explicit expiration time in the future, indicating that a response <em class="bcp14">MAY</em> be used to satisfy subsequent requests. In other words, a cache can return a fresh response without first contacting the server.
    794790      </p>
    795       <p id="rfc.section.2.2.1.p.2">Our expectation is that servers will assign future explicit expiration times to responses in the belief that the entity is
     791      <p id="rfc.section.3.1.p.2">Our expectation is that servers will assign future explicit expiration times to responses in the belief that the entity is
    796792         not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic
    797793         transparency, as long as the server's expiration times are carefully chosen.
    798794      </p>
    799       <p id="rfc.section.2.2.1.p.3">The expiration mechanism applies only to responses taken from a cache and not to first-hand responses forwarded immediately
     795      <p id="rfc.section.3.1.p.3">The expiration mechanism applies only to responses taken from a cache and not to first-hand responses forwarded immediately
    800796         to the requesting client.
    801797      </p>
    802       <p id="rfc.section.2.2.1.p.4">If an origin server wishes to force a semantically transparent cache to validate every request, it <em class="bcp14">MAY</em> assign an explicit expiration time in the past. This means that the response is always stale, and so the cache <em class="bcp14">SHOULD</em> validate it before using it for subsequent requests. See <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;3.2.4</a> for a more restrictive way to force revalidation.
    803       </p>
    804       <p id="rfc.section.2.2.1.p.5">If an origin server wishes to force any HTTP/1.1 cache, no matter how it is configured, to validate every request, it <em class="bcp14">SHOULD</em> use the "must-revalidate" cache-control directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.4" title="Cache-Control">Section&nbsp;3.2</a>).
    805       </p>
    806       <p id="rfc.section.2.2.1.p.6">Servers specify explicit expiration times using either the Expires header, or the max-age directive of the Cache-Control header.</p>
    807       <p id="rfc.section.2.2.1.p.7">An expiration time cannot be used to force a user agent to refresh its display or reload a resource; its semantics apply only
     798      <p id="rfc.section.3.1.p.4">If an origin server wishes to force a semantically transparent cache to validate every request, it <em class="bcp14">MAY</em> assign an explicit expiration time in the past. This means that the response is always stale, and so the cache <em class="bcp14">SHOULD</em> validate it before using it for subsequent requests. See <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a> for a more restrictive way to force revalidation.
     799      </p>
     800      <p id="rfc.section.3.1.p.5">If an origin server wishes to force any HTTP/1.1 cache, no matter how it is configured, to validate every request, it <em class="bcp14">SHOULD</em> use the "must-revalidate" cache-control directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.4" title="Cache-Control">Section&nbsp;15.2</a>).
     801      </p>
     802      <p id="rfc.section.3.1.p.6">Servers specify explicit expiration times using either the Expires header, or the max-age directive of the Cache-Control header.</p>
     803      <p id="rfc.section.3.1.p.7">An expiration time cannot be used to force a user agent to refresh its display or reload a resource; its semantics apply only
    808804         to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource
    809          is initiated. See <a href="#history.lists" title="History Lists">Section&nbsp;2.13</a> for an explanation of the difference between caches and history mechanisms.
    810       </p>
    811       <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;<a id="heuristic.expiration" href="#heuristic.expiration">Heuristic Expiration</a></h3>
    812       <p id="rfc.section.2.2.2.p.1">Since origin servers do not always provide explicit expiration times, HTTP caches typically assign heuristic expiration times,
     805         is initiated. See <a href="#history.lists" title="History Lists">Section&nbsp;14</a> for an explanation of the difference between caches and history mechanisms.
     806      </p>
     807      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="heuristic.expiration" href="#heuristic.expiration">Heuristic Expiration</a></h2>
     808      <p id="rfc.section.3.2.p.1">Since origin servers do not always provide explicit expiration times, HTTP caches typically assign heuristic expiration times,
    813809         employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time.
    814810         The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results.
    … …  
    816812         origin servers to provide explicit expiration times as much as possible.
    817813      </p>
    818       <h3 id="rfc.section.2.2.3"><a href="#rfc.section.2.2.3">2.2.3</a>&nbsp;<a id="age.calculations" href="#age.calculations">Age Calculations</a></h3>
    819       <p id="rfc.section.2.2.3.p.1">In order to know if a cached entry is fresh, a cache needs to know if its age exceeds its freshness lifetime. We discuss how
    820          to calculate the latter in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;2.2.4</a>; this section describes how to calculate the age of a response or cache entry.
    821       </p>
    822       <p id="rfc.section.2.2.3.p.2">In this discussion, we use the term "now" to mean "the current value of the clock at the host performing the calculation."
     814      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="age.calculations" href="#age.calculations">Age Calculations</a></h2>
     815      <p id="rfc.section.3.3.p.1">In order to know if a cached entry is fresh, a cache needs to know if its age exceeds its freshness lifetime. We discuss how
     816         to calculate the latter in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;3.4</a>; this section describes how to calculate the age of a response or cache entry.
     817      </p>
     818      <p id="rfc.section.3.3.p.2">In this discussion, we use the term "now" to mean "the current value of the clock at the host performing the calculation."
    823819         Hosts that use HTTP, but especially hosts running origin servers and caches, <em class="bcp14">SHOULD</em> use NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a> or some similar protocol to synchronize their clocks to a globally accurate time standard.
    824820      </p>
    825       <p id="rfc.section.2.2.3.p.3">HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response
     821      <p id="rfc.section.3.3.p.3">HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response
    826822         was generated (see <a href="p1-messaging.html#header.date" title="Date">Section 8.3</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). We use the term "date_value" to denote the value of the Date header, in a form appropriate for arithmetic operations.
    827823      </p>
    828       <p id="rfc.section.2.2.3.p.4">HTTP/1.1 uses the Age response-header to convey the estimated age of the response message when obtained from a cache. The
     824      <p id="rfc.section.3.3.p.4">HTTP/1.1 uses the Age response-header to convey the estimated age of the response message when obtained from a cache. The
    829825         Age field value is the cache's estimate of the amount of time since the response was generated or revalidated by the origin
    830826         server.
    831827      </p>
    832       <p id="rfc.section.2.2.3.p.5">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
     828      <p id="rfc.section.3.3.p.5">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
    833829         from the origin server, plus the amount of time it has been in transit along network paths.
    834830      </p>
    835       <p id="rfc.section.2.2.3.p.6">We use the term "age_value" to denote the value of the Age header, in a form appropriate for arithmetic operations.</p>
    836       <p id="rfc.section.2.2.3.p.7">A response's age can be calculated in two entirely independent ways: </p>
     831      <p id="rfc.section.3.3.p.6">We use the term "age_value" to denote the value of the Age header, in a form appropriate for arithmetic operations.</p>
     832      <p id="rfc.section.3.3.p.7">A response's age can be calculated in two entirely independent ways: </p>
    837833      <ol>
    838834         <li>now minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative,
    … …  
    841837         <li>age_value, if all of the caches along the response path implement HTTP/1.1.</li>
    842838      </ol>
    843       <p id="rfc.section.2.2.3.p.8">Given that we have two independent ways to compute the age of a response when it is received, we can combine these as</p>
    844       <div id="rfc.figure.u.2"></div><pre class="text">    corrected_received_age = max(now - date_value, age_value)
    845 </pre><p id="rfc.section.2.2.3.p.10">and as long as we have either nearly synchronized clocks or all-HTTP/1.1 paths, one gets a reliable (conservative) result.</p>
    846       <p id="rfc.section.2.2.3.p.11">Because of network-imposed delays, some significant interval might pass between the time that a server generates a response
     839      <p id="rfc.section.3.3.p.8">Given that we have two independent ways to compute the age of a response when it is received, we can combine these as</p>
     840      <div id="rfc.figure.u.1"></div><pre class="text">    corrected_received_age = max(now - date_value, age_value)
     841</pre><p id="rfc.section.3.3.p.10">and as long as we have either nearly synchronized clocks or all-HTTP/1.1 paths, one gets a reliable (conservative) result.</p>
     842      <p id="rfc.section.3.3.p.11">Because of network-imposed delays, some significant interval might pass between the time that a server generates a response
    847843         and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low
    848844         ages.
    849845      </p>
    850       <p id="rfc.section.2.2.3.p.12">Because the request that resulted in the returned Age value must have been initiated prior to that Age value's generation,
     846      <p id="rfc.section.3.3.p.12">Because the request that resulted in the returned Age value must have been initiated prior to that Age value's generation,
    851847         we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age
    852848         value is received, it <em class="bcp14">MUST</em> be interpreted relative to the time the request was initiated, not the time that the response was received. This algorithm
    853849         results in conservative behavior no matter how much delay is experienced. So, we compute:
    854850      </p>
    855       <div id="rfc.figure.u.3"></div><pre class="text">   corrected_initial_age = corrected_received_age
     851      <div id="rfc.figure.u.2"></div><pre class="text">   corrected_initial_age = corrected_received_age
    856852                         + (now - request_time)
    857 </pre><p id="rfc.section.2.2.3.p.14">where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.</p>
    858       <p id="rfc.section.2.2.3.p.15">Summary of age calculation algorithm, when a cache receives a response:</p>
    859       <div id="rfc.figure.u.4"></div><pre class="text">   /*
     853</pre><p id="rfc.section.3.3.p.14">where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.</p>
     854      <p id="rfc.section.3.3.p.15">Summary of age calculation algorithm, when a cache receives a response:</p>
     855      <div id="rfc.figure.u.3"></div><pre class="text">   /*
    860856    * age_value
    861857    *      is the value of Age: header received by the cache with
    … …  
    879875   resident_time = now - response_time;
    880876   current_age   = corrected_initial_age + resident_time;
    881 </pre><p id="rfc.section.2.2.3.p.17">The current_age of a cache entry is calculated by adding the amount of time (in seconds) since the cache entry was last validated
     877</pre><p id="rfc.section.3.3.p.17">The current_age of a cache entry is calculated by adding the amount of time (in seconds) since the cache entry was last validated
    882878         by the origin server to the corrected_initial_age. When a response is generated from a cache entry, the cache <em class="bcp14">MUST</em> include a single Age header field in the response with a value equal to the cache entry's current_age.
    883879      </p>
    884       <p id="rfc.section.2.2.3.p.18">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
     880      <p id="rfc.section.3.3.p.18">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
    885881         true, since the lack of an Age header field in a response does not imply that the response is first-hand unless all caches
    886882         along the request path are compliant with HTTP/1.1 (i.e., older HTTP caches did not implement the Age header field).
    887883      </p>
    888       <h3 id="rfc.section.2.2.4"><a href="#rfc.section.2.2.4">2.2.4</a>&nbsp;<a id="expiration.calculations" href="#expiration.calculations">Expiration Calculations</a></h3>
    889       <p id="rfc.section.2.2.4.p.1">In order to decide whether a response is fresh or stale, we need to compare its freshness lifetime to its age. The age is
    890          calculated as described in <a href="#age.calculations" title="Age Calculations">Section&nbsp;2.2.3</a>; this section describes how to calculate the freshness lifetime, and to determine if a response has expired. In the discussion
     884      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="expiration.calculations" href="#expiration.calculations">Expiration Calculations</a></h2>
     885      <p id="rfc.section.3.4.p.1">In order to decide whether a response is fresh or stale, we need to compare its freshness lifetime to its age. The age is
     886         calculated as described in <a href="#age.calculations" title="Age Calculations">Section&nbsp;3.3</a>; this section describes how to calculate the freshness lifetime, and to determine if a response has expired. In the discussion
    891887         below, the values can be represented in any form appropriate for arithmetic operations.
    892888      </p>
    893       <p id="rfc.section.2.2.4.p.2">We use the term "expires_value" to denote the value of the Expires header. We use the term "max_age_value" to denote an appropriate
    894          value of the number of seconds carried by the "max-age" directive of the Cache-Control header in a response (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>).
    895       </p>
    896       <p id="rfc.section.2.2.4.p.3">The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply:</p>
    897       <div id="rfc.figure.u.5"></div><pre class="text">   freshness_lifetime = max_age_value
    898 </pre><p id="rfc.section.2.2.4.p.5">Otherwise, if Expires is present in the response, the calculation is:</p>
    899       <div id="rfc.figure.u.6"></div><pre class="text">   freshness_lifetime = expires_value - date_value
    900 </pre><p id="rfc.section.2.2.4.p.7">Note that neither of these calculations is vulnerable to clock skew, since all of the information comes from the origin server.</p>
    901       <p id="rfc.section.2.2.4.p.8">If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>) appears in the response, and the response does not include other restrictions on caching, the cache <em class="bcp14">MAY</em> compute a freshness lifetime using a heuristic. The cache <em class="bcp14">MUST</em> attach Warning 113 to any response whose age is more than 24 hours if such warning has not already been added.
    902       </p>
    903       <p id="rfc.section.2.2.4.p.9">Also, if the response does have a Last-Modified time, the heuristic expiration value <em class="bcp14">SHOULD</em> be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
    904       </p>
    905       <p id="rfc.section.2.2.4.p.10">The calculation to determine if a response has expired is quite simple:</p>
    906       <div id="rfc.figure.u.7"></div><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
    907 </pre><h3 id="rfc.section.2.2.5"><a href="#rfc.section.2.2.5">2.2.5</a>&nbsp;<a id="disambiguating.expiration.values" href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></h3>
    908       <p id="rfc.section.2.2.5.p.1">Because expiration values are assigned optimistically, it is possible for two caches to contain fresh values for the same
     889      <p id="rfc.section.3.4.p.2">We use the term "expires_value" to denote the value of the Expires header. We use the term "max_age_value" to denote an appropriate
     890         value of the number of seconds carried by the "max-age" directive of the Cache-Control header in a response (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>).
     891      </p>
     892      <p id="rfc.section.3.4.p.3">The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply:</p>
     893      <div id="rfc.figure.u.4"></div><pre class="text">   freshness_lifetime = max_age_value
     894</pre><p id="rfc.section.3.4.p.5">Otherwise, if Expires is present in the response, the calculation is:</p>
     895      <div id="rfc.figure.u.5"></div><pre class="text">   freshness_lifetime = expires_value - date_value
     896</pre><p id="rfc.section.3.4.p.7">Note that neither of these calculations is vulnerable to clock skew, since all of the information comes from the origin server.</p>
     897      <p id="rfc.section.3.4.p.8">If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>) appears in the response, and the response does not include other restrictions on caching, the cache <em class="bcp14">MAY</em> compute a freshness lifetime using a heuristic. The cache <em class="bcp14">MUST</em> attach Warning 113 to any response whose age is more than 24 hours if such warning has not already been added.
     898      </p>
     899      <p id="rfc.section.3.4.p.9">Also, if the response does have a Last-Modified time, the heuristic expiration value <em class="bcp14">SHOULD</em> be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
     900      </p>
     901      <p id="rfc.section.3.4.p.10">The calculation to determine if a response has expired is quite simple:</p>
     902      <div id="rfc.figure.u.6"></div><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
     903</pre><h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="disambiguating.expiration.values" href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></h2>
     904      <p id="rfc.section.3.5.p.1">Because expiration values are assigned optimistically, it is possible for two caches to contain fresh values for the same
    909905         resource that are different.
    910906      </p>
    911       <p id="rfc.section.2.2.5.p.2">If a client performing a retrieval receives a non-first-hand response for a request that was already fresh in its own cache,
    912          and the Date header in its existing cache entry is newer than the Date on the new response, then the client <em class="bcp14">MAY</em> ignore the response. If so, it <em class="bcp14">MAY</em> retry the request with a "Cache-Control: max-age=0" directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.5" title="Cache-Control">Section&nbsp;3.2</a>), to force a check with the origin server.
    913       </p>
    914       <p id="rfc.section.2.2.5.p.3">If a cache has two fresh responses for the same representation with different validators, it <em class="bcp14">MUST</em> use the one with the more recent Date header. This situation might arise because the cache is pooling responses from other
     907      <p id="rfc.section.3.5.p.2">If a client performing a retrieval receives a non-first-hand response for a request that was already fresh in its own cache,
     908         and the Date header in its existing cache entry is newer than the Date on the new response, then the client <em class="bcp14">MAY</em> ignore the response. If so, it <em class="bcp14">MAY</em> retry the request with a "Cache-Control: max-age=0" directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.5" title="Cache-Control">Section&nbsp;15.2</a>), to force a check with the origin server.
     909      </p>
     910      <p id="rfc.section.3.5.p.3">If a cache has two fresh responses for the same representation with different validators, it <em class="bcp14">MUST</em> use the one with the more recent Date header. This situation might arise because the cache is pooling responses from other
    915911         caches, or because a client has asked for a reload or a revalidation of an apparently fresh cache entry.
    916912      </p>
    917       <h3 id="rfc.section.2.2.6"><a href="#rfc.section.2.2.6">2.2.6</a>&nbsp;<a id="disambiguating.multiple.responses" href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></h3>
    918       <p id="rfc.section.2.2.6.p.1">Because a client might be receiving responses via multiple paths, so that some responses flow through one set of caches and
     913      <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="disambiguating.multiple.responses" href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></h2>
     914      <p id="rfc.section.3.6.p.1">Because a client might be receiving responses via multiple paths, so that some responses flow through one set of caches and
    919915         other responses flow through a different set of caches, a client might receive responses in an order different from that in
    920916         which the origin server sent them. We would like the client to use the most recently generated response, even if older responses
    921917         are still apparently fresh.
    922918      </p>
    923       <p id="rfc.section.2.2.6.p.2">Neither the entity tag nor the expiration value can impose an ordering on responses, since it is possible that a later response
     919      <p id="rfc.section.3.6.p.2">Neither the entity tag nor the expiration value can impose an ordering on responses, since it is possible that a later response
    924920         intentionally carries an earlier expiration time. The Date values are ordered to a granularity of one second.
    925921      </p>
    926       <p id="rfc.section.2.2.6.p.3">When a client tries to revalidate a cache entry, and the response it receives contains a Date header that appears to be older
     922      <p id="rfc.section.3.6.p.3">When a client tries to revalidate a cache entry, and the response it receives contains a Date header that appears to be older
    927923         than the one for the existing entry, then the client <em class="bcp14">SHOULD</em> repeat the request unconditionally, and include
    928924      </p>
    929       <div id="rfc.figure.u.8"></div><pre class="text">    Cache-Control: max-age=0
    930 </pre><p id="rfc.section.2.2.6.p.5">to force any intermediate caches to validate their copies directly with the origin server, or</p>
    931       <div id="rfc.figure.u.9"></div><pre class="text">    Cache-Control: no-cache
    932 </pre><p id="rfc.section.2.2.6.p.7">to force any intermediate caches to obtain a new copy from the origin server.</p>
    933       <p id="rfc.section.2.2.6.p.8">If the Date values are equal, then the client <em class="bcp14">MAY</em> use either response (or <em class="bcp14">MAY</em>, if it is being extremely prudent, request a new response). Servers <em class="bcp14">MUST NOT</em> depend on clients being able to choose deterministically between responses generated during the same second, if their expiration
     925      <div id="rfc.figure.u.7"></div><pre class="text">    Cache-Control: max-age=0
     926</pre><p id="rfc.section.3.6.p.5">to force any intermediate caches to validate their copies directly with the origin server, or</p>
     927      <div id="rfc.figure.u.8"></div><pre class="text">    Cache-Control: no-cache
     928</pre><p id="rfc.section.3.6.p.7">to force any intermediate caches to obtain a new copy from the origin server.</p>
     929      <p id="rfc.section.3.6.p.8">If the Date values are equal, then the client <em class="bcp14">MAY</em> use either response (or <em class="bcp14">MAY</em>, if it is being extremely prudent, request a new response). Servers <em class="bcp14">MUST NOT</em> depend on clients being able to choose deterministically between responses generated during the same second, if their expiration
    934930         times overlap.
    935931      </p>
    936       <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="validation.model" href="#validation.model">Validation Model</a></h2>
    937       <p id="rfc.section.2.3.p.1">When a cache has a stale entry that it would like to use as a response to a client's request, it first has to check with the
     932      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="validation.model" href="#validation.model">Validation Model</a></h1>
     933      <p id="rfc.section.4.p.1">When a cache has a stale entry that it would like to use as a response to a client's request, it first has to check with the
    938934         origin server (or possibly an intermediate cache with a fresh response) to see if its cached entry is still usable. We call
    939935         this "validating" the cache entry. Since we do not want to have to pay the overhead of retransmitting the full response if
    … …  
    941937         HTTP/1.1 protocol supports the use of conditional methods.
    942938      </p>
    943       <p id="rfc.section.2.3.p.2">The key protocol features for supporting conditional methods are those concerned with "cache validators." When an origin server
     939      <p id="rfc.section.4.p.2">The key protocol features for supporting conditional methods are those concerned with "cache validators." When an origin server
    944940         generates a full response, it attaches some sort of validator to it, which is kept with the cache entry. When a client (user
    945941         agent or proxy cache) makes a conditional request for a resource for which it has a cache entry, it includes the associated
    946942         validator in the request.
    947943      </p>
    948       <p id="rfc.section.2.3.p.3">The server then checks that validator against the current validator for the entity, and, if they match (see <a href="p4-conditional.html#weak.and.strong.validators" title="Weak and Strong Validators">Section 4</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), it responds with a special status code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it returns a full response
     944      <p id="rfc.section.4.p.3">The server then checks that validator against the current validator for the entity, and, if they match (see <a href="p4-conditional.html#weak.and.strong.validators" title="Weak and Strong Validators">Section 4</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), it responds with a special status code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it returns a full response
    949945         (including entity-body). Thus, we avoid transmitting the full response if the validator matches, and we avoid an extra round
    950946         trip if it does not match.
    951947      </p>
    952       <p id="rfc.section.2.3.p.4">In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries
     948      <p id="rfc.section.4.p.4">In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries
    953949         a special header (which includes the validator) that implicitly turns the method (usually, GET) into a conditional.
    954950      </p>
    955       <p id="rfc.section.2.3.p.5">The protocol includes both positive and negative senses of cache-validating conditions. That is, it is possible to request
     951      <p id="rfc.section.4.p.5">The protocol includes both positive and negative senses of cache-validating conditions. That is, it is possible to request
    956952         either that a method be performed if and only if a validator matches or if and only if no validators match.
    957953      </p>
    … …  
    962958         </dd>
    963959      </dl>
    964       <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a id="last-modified.dates" href="#last-modified.dates">Last-Modified Dates</a></h3>
    965       <p id="rfc.section.2.3.1.p.1">The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered
     960      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="last-modified.dates" href="#last-modified.dates">Last-Modified Dates</a></h2>
     961      <p id="rfc.section.4.1.p.1">The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered
    966962         to be valid if the entity has not been modified since the Last-Modified value.
    967963      </p>
    968       <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a id="entity.tag.cache.validators" href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></h3>
    969       <p id="rfc.section.2.3.2.p.1">The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable
     964      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="entity.tag.cache.validators" href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></h2>
     965      <p id="rfc.section.4.2.p.1">The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable
    970966         validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date
    971967         values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification
    972968         dates.
    973969      </p>
    974       <p id="rfc.section.2.3.2.p.2">Entity Tags are described in <a href="p4-conditional.html#entity.tags" title="Entity Tags">Section 2</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>. The headers used with entity tags are described in <a href="p4-conditional.html#header.fields" title="Header Field Definitions">Section 6</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>.
    975       </p>
    976       <h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a id="non-validating.conditionals" href="#non-validating.conditionals">Non-validating Conditionals</a></h3>
    977       <p id="rfc.section.2.3.3.p.1">The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an
     970      <p id="rfc.section.4.2.p.2">Entity Tags are described in <a href="p4-conditional.html#entity.tags" title="Entity Tags">Section 2</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>. The headers used with entity tags are described in <a href="p4-conditional.html#header.fields" title="Header Field Definitions">Section 6</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>.
     971      </p>
     972      <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="non-validating.conditionals" href="#non-validating.conditionals">Non-validating Conditionals</a></h2>
     973      <p id="rfc.section.4.3.p.1">The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an
    978974         appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality
    979975         would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0)
    980976         are never used for purposes of validating a cache entry.
    981977      </p>
    982       <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Response Cacheability</a></h2>
    983       <p id="rfc.section.2.4.p.1">Unless specifically constrained by a cache-control (<a href="#header.cache-control" id="rfc.xref.header.cache-control.6" title="Cache-Control">Section&nbsp;3.2</a>) directive, a caching system <em class="bcp14">MAY</em> always store a successful response (see <a href="#errors.or.incomplete.response.cache.behavior" title="Errors or Incomplete Response Cache Behavior">Section&nbsp;2.8</a>) as a cache entry, <em class="bcp14">MAY</em> return it without validation if it is fresh, and <em class="bcp14">MAY</em> return it after successful validation. If there is neither a cache validator nor an explicit expiration time associated with
     978      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Response Cacheability</a></h1>
     979      <p id="rfc.section.5.p.1">Unless specifically constrained by a cache-control (<a href="#header.cache-control" id="rfc.xref.header.cache-control.6" title="Cache-Control">Section&nbsp;15.2</a>) directive, a caching system <em class="bcp14">MAY</em> always store a successful response (see <a href="#errors.or.incomplete.response.cache.behavior" title="Errors or Incomplete Response Cache Behavior">Section&nbsp;9</a>) as a cache entry, <em class="bcp14">MAY</em> return it without validation if it is fresh, and <em class="bcp14">MAY</em> return it after successful validation. If there is neither a cache validator nor an explicit expiration time associated with
    984980         a response, we do not expect it to be cached, but certain caches <em class="bcp14">MAY</em> violate this expectation (for example, when little or no network connectivity is available). A client can usually detect that
    985981         such a response was taken from a cache by comparing the Date header to the current time.
    … …  
    989985         </dd>
    990986      </dl>
    991       <p id="rfc.section.2.4.p.2">However, in some cases it might be inappropriate for a cache to retain an entity, or to return it in response to a subsequent
     987      <p id="rfc.section.5.p.2">However, in some cases it might be inappropriate for a cache to retain an entity, or to return it in response to a subsequent
    992988         request. This might be because absolute semantic transparency is deemed necessary by the service author, or because of security
    993989         or privacy considerations. Certain cache-control directives are therefore provided so that the server can indicate that certain
    994990         resource entities, or portions thereof, are not to be cached regardless of other considerations.
    995991      </p>
    996       <p id="rfc.section.2.4.p.3">Note that <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.1"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a> normally prevents a shared cache from saving and returning a response to a previous request if that request included an Authorization
     992      <p id="rfc.section.5.p.3">Note that <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.1"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a> normally prevents a shared cache from saving and returning a response to a previous request if that request included an Authorization
    997993         header.
    998994      </p>
    999       <p id="rfc.section.2.4.p.4">A response received with a status code of 200, 203, 206, 300, 301 or 410 <em class="bcp14">MAY</em> be stored by a cache and used in reply to a subsequent request, subject to the expiration mechanism, unless a cache-control
     995      <p id="rfc.section.5.p.4">A response received with a status code of 200, 203, 206, 300, 301 or 410 <em class="bcp14">MAY</em> be stored by a cache and used in reply to a subsequent request, subject to the expiration mechanism, unless a cache-control
    1000996         directive prohibits caching. However, a cache that does not support the Range and Content-Range headers <em class="bcp14">MUST NOT</em> cache 206 (Partial Content) responses.
    1001997      </p>
    1002       <p id="rfc.section.2.4.p.5">A response received with any other status code (e.g. status codes 302 and 307) <em class="bcp14">MUST NOT</em> be returned in a reply to a subsequent request unless there are cache-control directives or another header(s) that explicitly
    1003          allow it. For example, these include the following: an Expires header (<a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;3.3</a>); a "max-age", "s-maxage", "must-revalidate", "proxy-revalidate", "public" or "private" cache-control directive (<a href="#header.cache-control" id="rfc.xref.header.cache-control.7" title="Cache-Control">Section&nbsp;3.2</a>).
    1004       </p>
    1005       <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a id="constructing.responses.from.caches" href="#constructing.responses.from.caches">Constructing Responses From Caches</a></h2>
    1006       <p id="rfc.section.2.5.p.1">The purpose of an HTTP cache is to store information received in response to requests for use in responding to future requests.
     998      <p id="rfc.section.5.p.5">A response received with any other status code (e.g. status codes 302 and 307) <em class="bcp14">MUST NOT</em> be returned in a reply to a subsequent request unless there are cache-control directives or another header(s) that explicitly
     999         allow it. For example, these include the following: an Expires header (<a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;15.3</a>); a "max-age", "s-maxage", "must-revalidate", "proxy-revalidate", "public" or "private" cache-control directive (<a href="#header.cache-control" id="rfc.xref.header.cache-control.7" title="Cache-Control">Section&nbsp;15.2</a>).
     1000      </p>
     1001      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="constructing.responses.from.caches" href="#constructing.responses.from.caches">Constructing Responses From Caches</a></h1>
     1002      <p id="rfc.section.6.p.1">The purpose of an HTTP cache is to store information received in response to requests for use in responding to future requests.
    10071003         In many cases, a cache simply returns the appropriate parts of a response to the requester. However, if the cache holds a
    10081004         cache entry based on a previous response, it might have to combine parts of a new response with what is held in the cache
    10091005         entry.
    10101006      </p>
    1011       <h3 id="rfc.section.2.5.1"><a href="#rfc.section.2.5.1">2.5.1</a>&nbsp;<a id="end-to-end.and.hop-by-hop.headers" href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></h3>
    1012       <p id="rfc.section.2.5.1.p.1">For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP headers into two categories: </p>
     1007      <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="end-to-end.and.hop-by-hop.headers" href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></h2>
     1008      <p id="rfc.section.6.1.p.1">For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP headers into two categories: </p>
    10131009      <ul>
    10141010         <li>End-to-end headers, which are transmitted to the ultimate recipient of a request or response. End-to-end headers in responses <em class="bcp14">MUST</em> be stored as part of a cache entry and <em class="bcp14">MUST</em> be transmitted in any response formed from a cache entry.
    … …  
    10181014         </li>
    10191015      </ul>
    1020       <p id="rfc.section.2.5.1.p.2">The following HTTP/1.1 headers are hop-by-hop headers: </p>
     1016      <p id="rfc.section.6.1.p.2">The following HTTP/1.1 headers are hop-by-hop headers: </p>
    10211017      <ul>
    10221018         <li>Connection</li>
    … …  
    10291025         <li>Upgrade</li>
    10301026      </ul>
    1031       <p id="rfc.section.2.5.1.p.3">All other headers defined by HTTP/1.1 are end-to-end headers.</p>
    1032       <p id="rfc.section.2.5.1.p.4">Other hop-by-hop headers <em class="bcp14">MUST</em> be listed in a Connection header (<a href="p1-messaging.html#header.connection" title="Connection">Section 8.1</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
    1033       </p>
    1034       <h3 id="rfc.section.2.5.2"><a href="#rfc.section.2.5.2">2.5.2</a>&nbsp;<a id="non-modifiable.headers" href="#non-modifiable.headers">Non-modifiable Headers</a></h3>
    1035       <p id="rfc.section.2.5.2.p.1">Some features of the HTTP/1.1 protocol, such as Digest Authentication, depend on the value of certain end-to-end headers.
     1027      <p id="rfc.section.6.1.p.3">All other headers defined by HTTP/1.1 are end-to-end headers.</p>
     1028      <p id="rfc.section.6.1.p.4">Other hop-by-hop headers <em class="bcp14">MUST</em> be listed in a Connection header (<a href="p1-messaging.html#header.connection" title="Connection">Section 8.1</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
     1029      </p>
     1030      <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a id="non-modifiable.headers" href="#non-modifiable.headers">Non-modifiable Headers</a></h2>
     1031      <p id="rfc.section.6.2.p.1">Some features of the HTTP/1.1 protocol, such as Digest Authentication, depend on the value of certain end-to-end headers.
    10361032         A transparent proxy <em class="bcp14">SHOULD NOT</em> modify an end-to-end header unless the definition of that header requires or specifically allows that.
    10371033      </p>
    1038       <p id="rfc.section.2.5.2.p.2">A transparent proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a request or response, and it <em class="bcp14">MUST NOT</em> add any of these fields if not already present:
     1034      <p id="rfc.section.6.2.p.2">A transparent proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a request or response, and it <em class="bcp14">MUST NOT</em> add any of these fields if not already present:
    10391035      </p>
    10401036      <ul>
    … …  
    10441040         <li>Last-Modified</li>
    10451041      </ul>
    1046       <p id="rfc.section.2.5.2.p.3">A transparent proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a response:
     1042      <p id="rfc.section.6.2.p.3">A transparent proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a response:
    10471043      </p>
    10481044      <ul>
    10491045         <li>Expires</li>
    10501046      </ul>
    1051       <p id="rfc.section.2.5.2.p.4">but it <em class="bcp14">MAY</em> add any of these fields if not already present. If an Expires header is added, it <em class="bcp14">MUST</em> be given a field-value identical to that of the Date header in that response.
    1052       </p>
    1053       <p id="rfc.section.2.5.2.p.5">A proxy <em class="bcp14">MUST NOT</em> modify or add any of the following fields in a message that contains the no-transform cache-control directive, or in any request:
     1047      <p id="rfc.section.6.2.p.4">but it <em class="bcp14">MAY</em> add any of these fields if not already present. If an Expires header is added, it <em class="bcp14">MUST</em> be given a field-value identical to that of the Date header in that response.
     1048      </p>
     1049      <p id="rfc.section.6.2.p.5">A proxy <em class="bcp14">MUST NOT</em> modify or add any of the following fields in a message that contains the no-transform cache-control directive, or in any request:
    10541050      </p>
    10551051      <ul>
    … …  
    10581054         <li>Content-Type</li>
    10591055      </ul>
    1060       <p id="rfc.section.2.5.2.p.6">A non-transparent proxy <em class="bcp14">MAY</em> modify or add these fields to a message that does not include no-transform, but if it does so, it <em class="bcp14">MUST</em> add a Warning 214 (Transformation applied) if one does not already appear in the message (see <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;3.6</a>).
     1056      <p id="rfc.section.6.2.p.6">A non-transparent proxy <em class="bcp14">MAY</em> modify or add these fields to a message that does not include no-transform, but if it does so, it <em class="bcp14">MUST</em> add a Warning 214 (Transformation applied) if one does not already appear in the message (see <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;15.6</a>).
    10611057      </p>
    10621058      <dl class="empty">
    … …  
    10651061         </dd>
    10661062      </dl>
    1067       <p id="rfc.section.2.5.2.p.7">The Content-Length field of a request or response is added or deleted according to the rules in <a href="p1-messaging.html#message.length" title="Message Length">Section 4.4</a> of <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. A transparent proxy <em class="bcp14">MUST</em> preserve the entity-length (<a href="p3-payload.html#entity.length" title="Entity Length">Section 3.2.2</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>) of the entity-body, although it <em class="bcp14">MAY</em> change the transfer-length (<a href="p1-messaging.html#message.length" title="Message Length">Section 4.4</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
    1068       </p>
    1069       <h3 id="rfc.section.2.5.3"><a href="#rfc.section.2.5.3">2.5.3</a>&nbsp;<a id="combining.headers" href="#combining.headers">Combining Headers</a></h3>
    1070       <p id="rfc.section.2.5.3.p.1">When a cache makes a validating request to a server, and the server provides a 304 (Not Modified) response or a 206 (Partial
     1063      <p id="rfc.section.6.2.p.7">The Content-Length field of a request or response is added or deleted according to the rules in <a href="p1-messaging.html#message.length" title="Message Length">Section 4.4</a> of <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. A transparent proxy <em class="bcp14">MUST</em> preserve the entity-length (<a href="p3-payload.html#entity.length" title="Entity Length">Section 3.2.2</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>) of the entity-body, although it <em class="bcp14">MAY</em> change the transfer-length (<a href="p1-messaging.html#message.length" title="Message Length">Section 4.4</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
     1064      </p>
     1065      <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a id="combining.headers" href="#combining.headers">Combining Headers</a></h2>
     1066      <p id="rfc.section.6.3.p.1">When a cache makes a validating request to a server, and the server provides a 304 (Not Modified) response or a 206 (Partial
    10711067         Content) response, the cache then constructs a response to send to the requesting client.
    10721068      </p>
    1073       <p id="rfc.section.2.5.3.p.2">If the status code is 304 (Not Modified), the cache uses the entity-body stored in the cache entry as the entity-body of this
     1069      <p id="rfc.section.6.3.p.2">If the status code is 304 (Not Modified), the cache uses the entity-body stored in the cache entry as the entity-body of this
    10741070         outgoing response. If the status code is 206 (Partial Content) and the ETag or Last-Modified headers match exactly, the cache <em class="bcp14">MAY</em> combine the contents stored in the cache entry with the new contents received in the response and use the result as the entity-body
    10751071         of this outgoing response, (see <a href="p5-range.html#combining.byte.ranges" title="Combining Byte Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>).
    10761072      </p>
    1077       <p id="rfc.section.2.5.3.p.3">The end-to-end headers stored in the cache entry are used for the constructed response, except that </p>
     1073      <p id="rfc.section.6.3.p.3">The end-to-end headers stored in the cache entry are used for the constructed response, except that </p>
    10781074      <ul>
    1079          <li>any stored Warning headers with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;3.6</a>) <em class="bcp14">MUST</em> be deleted from the cache entry and the forwarded response.
     1075         <li>any stored Warning headers with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;15.6</a>) <em class="bcp14">MUST</em> be deleted from the cache entry and the forwarded response.
    10801076         </li>
    10811077         <li>any stored Warning headers with warn-code 2xx <em class="bcp14">MUST</em> be retained in the cache entry and the forwarded response.
    … …  
    10841080         </li>
    10851081      </ul>
    1086       <p id="rfc.section.2.5.3.p.4">Unless the cache decides to remove the cache entry, it <em class="bcp14">MUST</em> also replace the end-to-end headers stored with the cache entry with corresponding headers received in the incoming response,
     1082      <p id="rfc.section.6.3.p.4">Unless the cache decides to remove the cache entry, it <em class="bcp14">MUST</em> also replace the end-to-end headers stored with the cache entry with corresponding headers received in the incoming response,
    10871083         except for Warning headers as described immediately above. If a header field-name in the incoming response matches more than
    10881084         one header in the cache entry, all such old headers <em class="bcp14">MUST</em> be replaced.
    10891085      </p>
    1090       <p id="rfc.section.2.5.3.p.5">In other words, the set of end-to-end headers received in the incoming response overrides all corresponding end-to-end headers
     1086      <p id="rfc.section.6.3.p.5">In other words, the set of end-to-end headers received in the incoming response overrides all corresponding end-to-end headers
    10911087         stored with the cache entry (except for stored Warning headers with warn-code 1xx, which are deleted even if not overridden).
    10921088      </p>
    … …  
    10981094         </dd>
    10991095      </dl>
    1100       <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a id="caching.negotiated.responses" href="#caching.negotiated.responses">Caching Negotiated Responses</a></h2>
    1101       <p id="rfc.section.2.6.p.1">Use of server-driven content negotiation (<a href="p3-payload.html#server-driven.negotiation" title="Server-driven Negotiation">Section 4.1</a> of <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), as indicated by the presence of a Vary header field in a response, alters the conditions and procedure by which a cache
    1102          can use the response for subsequent requests. See <a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;3.5</a> for use of the Vary header field by servers.
    1103       </p>
    1104       <p id="rfc.section.2.6.p.2">A server <em class="bcp14">SHOULD</em> use the Vary header field to inform a cache of what request-header fields were used to select among multiple representations
     1096      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="caching.negotiated.responses" href="#caching.negotiated.responses">Caching Negotiated Responses</a></h1>
     1097      <p id="rfc.section.7.p.1">Use of server-driven content negotiation (<a href="p3-payload.html#server-driven.negotiation" title="Server-driven Negotiation">Section 4.1</a> of <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), as indicated by the presence of a Vary header field in a response, alters the conditions and procedure by which a cache
     1098         can use the response for subsequent requests. See <a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;15.5</a> for use of the Vary header field by servers.
     1099      </p>
     1100      <p id="rfc.section.7.p.2">A server <em class="bcp14">SHOULD</em> use the Vary header field to inform a cache of what request-header fields were used to select among multiple representations
    11051101         of a cacheable response subject to server-driven negotiation. The set of header fields named by the Vary field value is known
    11061102         as the "selecting" request-headers.
    11071103      </p>
    1108       <p id="rfc.section.2.6.p.3">When the cache receives a subsequent request whose Request-URI specifies one or more cache entries including a Vary header
     1104      <p id="rfc.section.7.p.3">When the cache receives a subsequent request whose Request-URI specifies one or more cache entries including a Vary header
    11091105         field, the cache <em class="bcp14">MUST NOT</em> use such a cache entry to construct a response to the new request unless all of the selecting request-headers present in the
    11101106         new request match the corresponding stored request-headers in the original request.
    11111107      </p>
    1112       <p id="rfc.section.2.6.p.4">The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first
     1108      <p id="rfc.section.7.p.4">The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first
    11131109         request can be transformed to the selecting request-headers in the second request by adding or removing linear white space
    11141110         (LWS) at places where this is allowed by the corresponding BNF, and/or combining multiple message-header fields with the same
    11151111         field name following the rules about message headers in <a href="p1-messaging.html#message.headers" title="Message Headers">Section 4.2</a> of <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
    11161112      </p>
    1117       <p id="rfc.section.2.6.p.5">A Vary header field-value of "*" always fails to match and subsequent requests on that resource can only be properly interpreted
     1113      <p id="rfc.section.7.p.5">A Vary header field-value of "*" always fails to match and subsequent requests on that resource can only be properly interpreted
    11181114         by the origin server.
    11191115      </p>
    1120       <p id="rfc.section.2.6.p.6">If the selecting request header fields for the cached entry do not match the selecting request header fields of the new request,
     1116      <p id="rfc.section.7.p.6">If the selecting request header fields for the cached entry do not match the selecting request header fields of the new request,
    11211117         then the cache <em class="bcp14">MUST NOT</em> use a cached entry to satisfy the request unless it first relays the new request to the origin server in a conditional request
    11221118         and the server responds with 304 (Not Modified), including an entity tag or Content-Location that indicates the entity to
    11231119         be used.
    11241120      </p>
    1125       <p id="rfc.section.2.6.p.7">If an entity tag was assigned to a cached representation, the forwarded request <em class="bcp14">SHOULD</em> be conditional and include the entity tags in an If-None-Match header field from all its cache entries for the resource. This
     1121      <p id="rfc.section.7.p.7">If an entity tag was assigned to a cached representation, the forwarded request <em class="bcp14">SHOULD</em> be conditional and include the entity tags in an If-None-Match header field from all its cache entries for the resource. This
    11261122         conveys to the server the set of entities currently held by the cache, so that if any one of these entities matches the requested
    11271123         entity, the server can use the ETag header field in its 304 (Not Modified) response to tell the cache which entry is appropriate.
    11281124         If the entity-tag of the new response matches that of an existing entry, the new response <em class="bcp14">SHOULD</em> be used to update the header fields of the existing entry, and the result <em class="bcp14">MUST</em> be returned to the client.
    11291125      </p>
    1130       <p id="rfc.section.2.6.p.8">If any of the existing cache entries contains only partial content for the associated entity, its entity-tag <em class="bcp14">SHOULD NOT</em> be included in the If-None-Match header field unless the request is for a range that would be fully satisfied by that entry.
    1131       </p>
    1132       <p id="rfc.section.2.6.p.9">If a cache receives a successful response whose Content-Location field matches that of an existing cache entry for the same
     1126      <p id="rfc.section.7.p.8">If any of the existing cache entries contains only partial content for the associated entity, its entity-tag <em class="bcp14">SHOULD NOT</em> be included in the If-None-Match header field unless the request is for a range that would be fully satisfied by that entry.
     1127      </p>
     1128      <p id="rfc.section.7.p.9">If a cache receives a successful response whose Content-Location field matches that of an existing cache entry for the same
    11331129         Request-URI, whose entity-tag differs from that of the existing entry, and whose Date is more recent than that of the existing
    11341130         entry, the existing entry <em class="bcp14">SHOULD NOT</em> be returned in response to future requests and <em class="bcp14">SHOULD</em> be deleted from the cache.
    11351131      </p>
    1136       <h2 id="rfc.section.2.7"><a href="#rfc.section.2.7">2.7</a>&nbsp;<a id="shared.and.non-shared.caches" href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></h2>
    1137       <p id="rfc.section.2.7.p.1">For reasons of security and privacy, it is necessary to make a distinction between "shared" and "non-shared" caches. A non-shared
     1132      <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a id="shared.and.non-shared.caches" href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></h1>
     1133      <p id="rfc.section.8.p.1">For reasons of security and privacy, it is necessary to make a distinction between "shared" and "non-shared" caches. A non-shared
    11381134         cache is one that is accessible only to a single user. Accessibility in this case <em class="bcp14">SHOULD</em> be enforced by appropriate security mechanisms. All other caches are considered to be "shared." Other sections of this specification
    11391135         place certain constraints on the operation of shared caches in order to prevent loss of privacy or failure of access controls.
    11401136      </p>
    1141       <h2 id="rfc.section.2.8"><a href="#rfc.section.2.8">2.8</a>&nbsp;<a id="errors.or.incomplete.response.cache.behavior" href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></h2>
    1142       <p id="rfc.section.2.8.p.1">A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header) <em class="bcp14">MAY</em> store the response. However, the cache <em class="bcp14">MUST</em> treat this as a partial response. Partial responses <em class="bcp14">MAY</em> be combined as described in <a href="p5-range.html#combining.byte.ranges" title="Combining Byte Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>; the result might be a full response or might still be partial. A cache <em class="bcp14">MUST NOT</em> return a partial response to a client without explicitly marking it as such, using the 206 (Partial Content) status code.
     1137      <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a id="errors.or.incomplete.response.cache.behavior" href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></h1>
     1138      <p id="rfc.section.9.p.1">A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header) <em class="bcp14">MAY</em> store the response. However, the cache <em class="bcp14">MUST</em> treat this as a partial response. Partial responses <em class="bcp14">MAY</em> be combined as described in <a href="p5-range.html#combining.byte.ranges" title="Combining Byte Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>; the result might be a full response or might still be partial. A cache <em class="bcp14">MUST NOT</em> return a partial response to a client without explicitly marking it as such, using the 206 (Partial Content) status code.
    11431139         A cache <em class="bcp14">MUST NOT</em> return a partial response using a status code of 200 (OK).
    11441140      </p>
    1145       <p id="rfc.section.2.8.p.2">If a cache receives a 5xx response while attempting to revalidate an entry, it <em class="bcp14">MAY</em> either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it <em class="bcp14">MAY</em> return a previously received response unless the cached entry includes the "must-revalidate" cache-control directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.8" title="Cache-Control">Section&nbsp;3.2</a>).
    1146       </p>
    1147       <h2 id="rfc.section.2.9"><a href="#rfc.section.2.9">2.9</a>&nbsp;<a id="side.effects.of.get.and.head" href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></h2>
    1148       <p id="rfc.section.2.9.p.1">Unless the origin server explicitly prohibits the caching of their responses, the application of GET and HEAD methods to any
     1141      <p id="rfc.section.9.p.2">If a cache receives a 5xx response while attempting to revalidate an entry, it <em class="bcp14">MAY</em> either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it <em class="bcp14">MAY</em> return a previously received response unless the cached entry includes the "must-revalidate" cache-control directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.8" title="Cache-Control">Section&nbsp;15.2</a>).
     1142      </p>
     1143      <h1 id="rfc.section.10"><a href="#rfc.section.10">10.</a>&nbsp;<a id="side.effects.of.get.and.head" href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></h1>
     1144      <p id="rfc.section.10.p.1">Unless the origin server explicitly prohibits the caching of their responses, the application of GET and HEAD methods to any
    11491145         resources <em class="bcp14">SHOULD NOT</em> have side effects that would lead to erroneous behavior if these responses are taken from a cache. They <em class="bcp14">MAY</em> still have side effects, but a cache is not required to consider such side effects in its caching decisions. Caches are always
    11501146         expected to observe an origin server's explicit restrictions on caching.
    11511147      </p>
    1152       <p id="rfc.section.2.9.p.2">We note one exception to this rule: since some applications have traditionally used GETs and HEADs with query URLs (those
     1148      <p id="rfc.section.10.p.2">We note one exception to this rule: since some applications have traditionally used GETs and HEADs with query URLs (those
    11531149         containing a "?" in the rel_path part) to perform operations with significant side effects, caches <em class="bcp14">MUST NOT</em> treat responses to such URIs as fresh unless the server provides an explicit expiration time. This specifically means that
    11541150         responses from HTTP/1.0 servers for such URIs <em class="bcp14">SHOULD NOT</em> be taken from a cache. See <a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 8.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> for related information.
    11551151      </p>
    1156       <h2 id="rfc.section.2.10"><a href="#rfc.section.2.10">2.10</a>&nbsp;<a id="invalidation.after.updates.or.deletions" href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></h2>
    1157       <p id="rfc.section.2.10.p.1">The effect of certain methods performed on a resource at the origin server might cause one or more existing cache entries
     1152      <h1 id="rfc.section.11"><a href="#rfc.section.11">11.</a>&nbsp;<a id="invalidation.after.updates.or.deletions" href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></h1>
     1153      <p id="rfc.section.11.p.1">The effect of certain methods performed on a resource at the origin server might cause one or more existing cache entries
    11581154         to become non-transparently invalid. That is, although they might continue to be "fresh," they do not accurately reflect what
    11591155         the origin server would return for a new request on that resource.
    11601156      </p>
    1161       <p id="rfc.section.2.10.p.2">There is no way for the HTTP protocol to guarantee that all such cache entries are marked invalid. For example, the request
     1157      <p id="rfc.section.11.p.2">There is no way for the HTTP protocol to guarantee that all such cache entries are marked invalid. For example, the request
    11621158         that caused the change at the origin server might not have gone through the proxy where a cache entry is stored. However,
    11631159         several rules help reduce the likelihood of erroneous behavior.
    11641160      </p>
    1165       <p id="rfc.section.2.10.p.3">In this section, the phrase "invalidate an entity" means that the cache will either remove all instances of that entity from
     1161      <p id="rfc.section.11.p.3">In this section, the phrase "invalidate an entity" means that the cache will either remove all instances of that entity from
    11661162         its storage, or will mark these as "invalid" and in need of a mandatory revalidation before they can be returned in response
    11671163         to a subsequent request.
    11681164      </p>
    1169       <p id="rfc.section.2.10.p.4">Some HTTP methods <em class="bcp14">MUST</em> cause a cache to invalidate an entity. This is either the entity referred to by the Request-URI, or by the Location or Content-Location
     1165      <p id="rfc.section.11.p.4">Some HTTP methods <em class="bcp14">MUST</em> cause a cache to invalidate an entity. This is either the entity referred to by the Request-URI, or by the Location or Content-Location
    11701166         headers (if present). These methods are:
    11711167      </p>
    … …  
    11751171         <li>POST</li>
    11761172      </ul>
    1177       <p id="rfc.section.2.10.p.5">An invalidation based on the URI in a Location or Content-Location header <em class="bcp14">MUST NOT</em> be performed if the host part of that URI differs from the host part in the Request-URI. This helps prevent denial of service
     1173      <p id="rfc.section.11.p.5">An invalidation based on the URI in a Location or Content-Location header <em class="bcp14">MUST NOT</em> be performed if the host part of that URI differs from the host part in the Request-URI. This helps prevent denial of service
    11781174         attacks.
    11791175      </p>
    1180       <p id="rfc.section.2.10.p.6">A cache that passes through requests for methods it does not understand <em class="bcp14">SHOULD</em> invalidate any entities referred to by the Request-URI.
    1181       </p>
    1182       <h2 id="rfc.section.2.11"><a href="#rfc.section.2.11">2.11</a>&nbsp;<a id="write-through.mandatory" href="#write-through.mandatory">Write-Through Mandatory</a></h2>
    1183       <p id="rfc.section.2.11.p.1">All methods that might be expected to cause modifications to the origin server's resources <em class="bcp14">MUST</em> be written through to the origin server. This currently includes all methods except for GET and HEAD. A cache <em class="bcp14">MUST NOT</em> reply to such a request from a client before having transmitted the request to the inbound server, and having received a corresponding
     1176      <p id="rfc.section.11.p.6">A cache that passes through requests for methods it does not understand <em class="bcp14">SHOULD</em> invalidate any entities referred to by the Request-URI.
     1177      </p>
     1178      <h1 id="rfc.section.12"><a href="#rfc.section.12">12.</a>&nbsp;<a id="write-through.mandatory" href="#write-through.mandatory">Write-Through Mandatory</a></h1>
     1179      <p id="rfc.section.12.p.1">All methods that might be expected to cause modifications to the origin server's resources <em class="bcp14">MUST</em> be written through to the origin server. This currently includes all methods except for GET and HEAD. A cache <em class="bcp14">MUST NOT</em> reply to such a request from a client before having transmitted the request to the inbound server, and having received a corresponding
    11841180         response from the inbound server. This does not prevent a proxy cache from sending a 100 (Continue) response before the inbound
    11851181         server has sent its final reply.
    11861182      </p>
    1187       <p id="rfc.section.2.11.p.2">The alternative (known as "write-back" or "copy-back" caching) is not allowed in HTTP/1.1, due to the difficulty of providing
     1183      <p id="rfc.section.12.p.2">The alternative (known as "write-back" or "copy-back" caching) is not allowed in HTTP/1.1, due to the difficulty of providing
    11881184         consistent updates and the problems arising from server, cache, or network failure prior to write-back.
    11891185      </p>
    1190       <h2 id="rfc.section.2.12"><a href="#rfc.section.2.12">2.12</a>&nbsp;<a id="cache.replacement" href="#cache.replacement">Cache Replacement</a></h2>
    1191       <p id="rfc.section.2.12.p.1">If a new cacheable (see Sections <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">3.2.2</a>, <a href="#disambiguating.expiration.values" title="Disambiguating Expiration Values">2.2.5</a>, <a href="#disambiguating.multiple.responses" title="Disambiguating Multiple Responses">2.2.6</a> and <a href="#errors.or.incomplete.response.cache.behavior" title="Errors or Incomplete Response Cache Behavior">2.8</a>) response is received from a resource while any existing responses for the same resource are cached, the cache <em class="bcp14">SHOULD</em> use the new response to reply to the current request. It <em class="bcp14">MAY</em> insert it into cache storage and <em class="bcp14">MAY</em>, if it meets all other requirements, use it to respond to any future requests that would previously have caused the old response
    1192          to be returned. If it inserts the new response into cache storage the rules in <a href="#combining.headers" title="Combining Headers">Section&nbsp;2.5.3</a> apply.
     1186      <h1 id="rfc.section.13"><a href="#rfc.section.13">13.</a>&nbsp;<a id="cache.replacement" href="#cache.replacement">Cache Replacement</a></h1>
     1187      <p id="rfc.section.13.p.1">If a new cacheable (see Sections <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">15.2.2</a>, <a href="#disambiguating.expiration.values" title="Disambiguating Expiration Values">3.5</a>, <a href="#disambiguating.multiple.responses" title="Disambiguating Multiple Responses">3.6</a> and <a href="#errors.or.incomplete.response.cache.behavior" title="Errors or Incomplete Response Cache Behavior">9</a>) response is received from a resource while any existing responses for the same resource are cached, the cache <em class="bcp14">SHOULD</em> use the new response to reply to the current request. It <em class="bcp14">MAY</em> insert it into cache storage and <em class="bcp14">MAY</em>, if it meets all other requirements, use it to respond to any future requests that would previously have caused the old response
     1188         to be returned. If it inserts the new response into cache storage the rules in <a href="#combining.headers" title="Combining Headers">Section&nbsp;6.3</a> apply.
    11931189      </p>
    11941190      <dl class="empty">
    … …  
    11961192         </dd>
    11971193      </dl>
    1198       <h2 id="rfc.section.2.13"><a href="#rfc.section.2.13">2.13</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h2>
    1199       <p id="rfc.section.2.13.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, which can be used to redisplay an entity
     1194      <h1 id="rfc.section.14"><a href="#rfc.section.14">14.</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h1>
     1195      <p id="rfc.section.14.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, which can be used to redisplay an entity
    12001196         retrieved earlier in a session.
    12011197      </p>
    1202       <p id="rfc.section.2.13.p.2">History mechanisms and caches are different. In particular history mechanisms <em class="bcp14">SHOULD NOT</em> try to show a semantically transparent view of the current state of a resource. Rather, a history mechanism is meant to show
     1198      <p id="rfc.section.14.p.2">History mechanisms and caches are different. In particular history mechanisms <em class="bcp14">SHOULD NOT</em> try to show a semantically transparent view of the current state of a resource. Rather, a history mechanism is meant to show
    12031199         exactly what the user saw at the time when the resource was retrieved.
    12041200      </p>
    1205       <p id="rfc.section.2.13.p.3">By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism <em class="bcp14">SHOULD</em> display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history
     1201      <p id="rfc.section.14.p.3">By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism <em class="bcp14">SHOULD</em> display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history
    12061202         documents.
    12071203      </p>
    1208       <p id="rfc.section.2.13.p.4">This is not to be construed to prohibit the history mechanism from telling the user that a view might be stale. </p>
     1204      <p id="rfc.section.14.p.4">This is not to be construed to prohibit the history mechanism from telling the user that a view might be stale. </p>
    12091205      <dl class="empty">
    12101206         <dd> <b>Note:</b> if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors
    … …  
    12161212         </dd>
    12171213      </dl>
    1218       <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
    1219       <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
    1220       <p id="rfc.section.3.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who
     1214      <h1 id="rfc.section.15"><a href="#rfc.section.15">15.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
     1215      <p id="rfc.section.15.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
     1216      <p id="rfc.section.15.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who
    12211217         receives the entity.
    12221218      </p>
    12231219      <div id="rfc.iref.a.2"></div>
    12241220      <div id="rfc.iref.h.2"></div>
    1225       <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="header.age" href="#header.age">Age</a></h2>
    1226       <p id="rfc.section.3.1.p.1">The Age response-header field conveys the sender's estimate of the amount of time since the response (or its revalidation)
     1221      <h2 id="rfc.section.15.1"><a href="#rfc.section.15.1">15.1</a>&nbsp;<a id="header.age" href="#header.age">Age</a></h2>
     1222      <p id="rfc.section.15.1.p.1">The Age response-header field conveys the sender's estimate of the amount of time since the response (or its revalidation)
    12271223         was generated at the origin server. A cached response is "fresh" if its age does not exceed its freshness lifetime. Age values
    1228          are calculated as specified in <a href="#age.calculations" title="Age Calculations">Section&nbsp;2.2.3</a>.
    1229       </p>
    1230       <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span>  Age = "Age" ":" age-value
     1224         are calculated as specified in <a href="#age.calculations" title="Age Calculations">Section&nbsp;3.3</a>.
     1225      </p>
     1226      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span>  Age = "Age" ":" age-value
    12311227  age-value = delta-seconds
    1232 </pre><p id="rfc.section.3.1.p.3">Age values are non-negative decimal integers, representing time in seconds.</p>
    1233       <p id="rfc.section.3.1.p.4">If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows,
     1228</pre><p id="rfc.section.15.1.p.3">Age values are non-negative decimal integers, representing time in seconds.</p>
     1229      <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.3"></span>  delta-seconds  = 1*DIGIT
     1230</pre><p id="rfc.section.15.1.p.5">If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows,
    12341231         it <em class="bcp14">MUST</em> transmit an Age header with a value of 2147483648 (2^31). An HTTP/1.1 server that includes a cache <em class="bcp14">MUST</em> include an Age header field in every response generated from its own cache. Caches <em class="bcp14">SHOULD</em> use an arithmetic type of at least 31 bits of range.
    12351232      </p>
    12361233      <div id="rfc.iref.c.3"></div>
    12371234      <div id="rfc.iref.h.3"></div>
    1238       <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="header.cache-control" href="#header.cache-control">Cache-Control</a></h2>
    1239       <p id="rfc.section.3.2.p.1">The Cache-Control general-header field is used to specify directives that <em class="bcp14">MUST</em> be obeyed by all caching mechanisms along the request/response chain. The directives specify behavior intended to prevent
     1235      <h2 id="rfc.section.15.2"><a href="#rfc.section.15.2">15.2</a>&nbsp;<a id="header.cache-control" href="#header.cache-control">Cache-Control</a></h2>
     1236      <p id="rfc.section.15.2.p.1">The Cache-Control general-header field is used to specify directives that <em class="bcp14">MUST</em> be obeyed by all caching mechanisms along the request/response chain. The directives specify behavior intended to prevent
    12401237         caches from adversely interfering with the request or response. These directives typically override the default caching algorithms.
    12411238         Cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive
    … …  
    12431240      </p>
    12441241      <dl class="empty">
    1245          <dd>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see <a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;3.4</a>).
    1246          </dd>
    1247       </dl>
    1248       <p id="rfc.section.3.2.p.2">Cache directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
     1242         <dd>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see <a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;15.4</a>).
     1243         </dd>
     1244      </dl>
     1245      <p id="rfc.section.15.2.p.2">Cache directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
    12491246         might be applicable to all recipients along the request/response chain. It is not possible to specify a cache-directive for
    12501247         a specific cache.
    … …  
    12561253
    12571254  cache-request-directive =
    1258          "no-cache"                          ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;3.2.1</a>
    1259        | "no-store"                          ; <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">Section&nbsp;3.2.2</a>
    1260        | "max-age" "=" delta-seconds         ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>, <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">3.2.4</a>
    1261        | "max-stale" [ "=" delta-seconds ]   ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>
    1262        | "min-fresh" "=" delta-seconds       ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>
    1263        | "no-transform"                      ; <a href="#no-transform.directive" title="No-Transform Directive">Section&nbsp;3.2.5</a>
    1264        | "only-if-cached"                    ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;3.2.4</a>
    1265        | cache-extension                     ; <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;3.2.6</a>
     1255         "no-cache"                          ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;15.2.1</a>
     1256       | "no-store"                          ; <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">Section&nbsp;15.2.2</a>
     1257       | "max-age" "=" delta-seconds         ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>, <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">15.2.4</a>
     1258       | "max-stale" [ "=" delta-seconds ]   ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>
     1259       | "min-fresh" "=" delta-seconds       ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>
     1260       | "no-transform"                      ; <a href="#no-transform.directive" title="No-Transform Directive">Section&nbsp;15.2.5</a>
     1261       | "only-if-cached"                    ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a>
     1262       | cache-extension                     ; <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;15.2.6</a>
    12661263
    12671264  cache-response-directive =
    1268          "public"                               ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;3.2.1</a>
    1269        | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;3.2.1</a>
    1270        | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;3.2.1</a>
    1271        | "no-store"                             ; <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">Section&nbsp;3.2.2</a>
    1272        | "no-transform"                         ; <a href="#no-transform.directive" title="No-Transform Directive">Section&nbsp;3.2.5</a>
    1273        | "must-revalidate"                      ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;3.2.4</a>
    1274        | "proxy-revalidate"                     ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;3.2.4</a>
    1275        | "max-age" "=" delta-seconds            ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>
    1276        | "s-maxage" "=" delta-seconds           ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>
    1277        | cache-extension                        ; <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;3.2.6</a>
     1265         "public"                               ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;15.2.1</a>
     1266       | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;15.2.1</a>
     1267       | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;15.2.1</a>
     1268       | "no-store"                             ; <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">Section&nbsp;15.2.2</a>
     1269       | "no-transform"                         ; <a href="#no-transform.directive" title="No-Transform Directive">Section&nbsp;15.2.5</a>
     1270       | "must-revalidate"                      ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a>
     1271       | "proxy-revalidate"                     ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a>
     1272       | "max-age" "=" delta-seconds            ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>
     1273       | "s-maxage" "=" delta-seconds           ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>
     1274       | cache-extension                        ; <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;15.2.6</a>
    12781275
    12791276  cache-extension = token [ "=" ( token | quoted-string ) ]
    1280 </pre><p id="rfc.section.3.2.p.4">When a directive appears without any 1#field-name parameter, the directive applies to the entire request or response. When
     1277</pre><p id="rfc.section.15.2.p.4">When a directive appears without any 1#field-name parameter, the directive applies to the entire request or response. When
    12811278         such a directive appears with a 1#field-name parameter, it applies only to the named field or fields, and not to the rest
    12821279         of the request or response. This mechanism supports extensibility; implementations of future versions of the HTTP protocol
    12831280         might apply these directives to header fields not defined in HTTP/1.1.
    12841281      </p>
    1285       <p id="rfc.section.3.2.p.5">The cache-control directives can be broken down into these general categories: </p>
     1282      <p id="rfc.section.15.2.p.5">The cache-control directives can be broken down into these general categories: </p>
    12861283      <ul>
    12871284         <li>Restrictions on what are cacheable; these may only be imposed by the origin server.</li>
    … …  
    12921289         <li>Extensions to the caching system.</li>
    12931290      </ul>
    1294       <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="what.is.cacheable" href="#what.is.cacheable">What is Cacheable</a></h3>
    1295       <p id="rfc.section.3.2.1.p.1">By default, a response is cacheable if the requirements of the request method, request header fields, and the response status
    1296          indicate that it is cacheable. <a href="#response.cacheability" title="Response Cacheability">Section&nbsp;2.4</a> summarizes these defaults for cacheability. The following Cache-Control response directives allow an origin server to override
     1291      <h3 id="rfc.section.15.2.1"><a href="#rfc.section.15.2.1">15.2.1</a>&nbsp;<a id="what.is.cacheable" href="#what.is.cacheable">What is Cacheable</a></h3>
     1292      <p id="rfc.section.15.2.1.p.1">By default, a response is cacheable if the requirements of the request method, request header fields, and the response status
     1293         indicate that it is cacheable. <a href="#response.cacheability" title="Response Cacheability">Section&nbsp;5</a> summarizes these defaults for cacheability. The following Cache-Control response directives allow an origin server to override
    12971294         the default cacheability of a response:
    12981295      </p>
    1299       <p id="rfc.section.3.2.1.p.2"> <span id="rfc.iref.c.4"></span>  <span id="rfc.iref.p.1"></span> public
     1296      <p id="rfc.section.15.2.1.p.2"> <span id="rfc.iref.c.4"></span>  <span id="rfc.iref.p.1"></span> public
    13001297      </p>
    13011298      <dl class="empty">
    … …  
    13041301         </dd>
    13051302      </dl>
    1306       <p id="rfc.section.3.2.1.p.3"> <span id="rfc.iref.c.5"></span>  <span id="rfc.iref.p.2"></span> private
     1303      <p id="rfc.section.15.2.1.p.3"> <span id="rfc.iref.c.5"></span>  <span id="rfc.iref.p.2"></span> private
    13071304      </p>
    13081305      <dl class="empty">
    … …  
    13141311         </dd>
    13151312      </dl>
    1316       <p id="rfc.section.3.2.1.p.4"> <span id="rfc.iref.c.6"></span>  <span id="rfc.iref.n.1"></span> no-cache
     1313      <p id="rfc.section.15.2.1.p.4"> <span id="rfc.iref.c.6"></span>  <span id="rfc.iref.n.1"></span> no-cache
    13171314      </p>
    13181315      <dl class="empty">
    … …  
    13281325         </dd>
    13291326      </dl>
    1330       <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="what.may.be.stored.by.caches" href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></h3>
    1331       <p id="rfc.section.3.2.2.p.1"> <span id="rfc.iref.c.7"></span>  <span id="rfc.iref.n.2"></span> no-store
     1327      <h3 id="rfc.section.15.2.2"><a href="#rfc.section.15.2.2">15.2.2</a>&nbsp;<a id="what.may.be.stored.by.caches" href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></h3>
     1328      <p id="rfc.section.15.2.2.p.1"> <span id="rfc.iref.c.7"></span>  <span id="rfc.iref.n.2"></span> no-store
    13321329      </p>
    13331330      <dl class="empty">
    … …  
    13461343         </dd>
    13471344      </dl>
    1348       <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a id="modifications.of.the.basic.expiration.mechanism" href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></h3>
    1349       <p id="rfc.section.3.2.3.p.1">The expiration time of an entity <em class="bcp14">MAY</em> be specified by the origin server using the Expires header (see <a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;3.3</a>). Alternatively, it <em class="bcp14">MAY</em> be specified using the max-age directive in a response. When the max-age cache-control directive is present in a cached response,
     1345      <h3 id="rfc.section.15.2.3"><a href="#rfc.section.15.2.3">15.2.3</a>&nbsp;<a id="modifications.of.the.basic.expiration.mechanism" href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></h3>
     1346      <p id="rfc.section.15.2.3.p.1">The expiration time of an entity <em class="bcp14">MAY</em> be specified by the origin server using the Expires header (see <a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;15.3</a>). Alternatively, it <em class="bcp14">MAY</em> be specified using the max-age directive in a response. When the max-age cache-control directive is present in a cached response,
    13501347         the response is stale if its current age is greater than the age value given (in seconds) at the time of a new request for
    13511348         that resource. The max-age directive on a response implies that the response is cacheable (i.e., "public") unless some other,
    13521349         more restrictive cache directive is also present.
    13531350      </p>
    1354       <p id="rfc.section.3.2.3.p.2">If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header,
     1351      <p id="rfc.section.15.2.3.p.2">If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header,
    13551352         even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer
    13561353         expiration time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be useful if certain HTTP/1.0 caches
    13571354         improperly calculate ages or expiration times, perhaps due to desynchronized clocks.
    13581355      </p>
    1359       <p id="rfc.section.3.2.3.p.3">Many HTTP/1.0 cache implementations will treat an Expires value that is less than or equal to the response Date value as being
     1356      <p id="rfc.section.15.2.3.p.3">Many HTTP/1.0 cache implementations will treat an Expires value that is less than or equal to the response Date value as being
    13601357         equivalent to the Cache-Control response directive "no-cache". If an HTTP/1.1 cache receives such a response, and the response
    13611358         does not include a Cache-Control header field, it <em class="bcp14">SHOULD</em> consider the response to be non-cacheable in order to retain compatibility with HTTP/1.0 servers.
    … …  
    13681365         </dd>
    13691366      </dl>
    1370       <p id="rfc.section.3.2.3.p.4"> <span id="rfc.iref.c.8"></span>  <span id="rfc.iref.s.3"></span> s-maxage
     1367      <p id="rfc.section.15.2.3.p.4"> <span id="rfc.iref.c.8"></span>  <span id="rfc.iref.s.3"></span> s-maxage
    13711368      </p>
    13721369      <dl class="empty">
    13731370         <dd>If a response includes an s-maxage directive, then for a shared cache (but not for a private cache), the maximum age specified
    13741371            by this directive overrides the maximum age specified by either the max-age directive or the Expires header. The s-maxage
    1375             directive also implies the semantics of the proxy-revalidate directive (see <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;3.2.4</a>), i.e., that the shared cache must not use the entry after it becomes stale to respond to a subsequent request without first
     1372            directive also implies the semantics of the proxy-revalidate directive (see <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a>), i.e., that the shared cache must not use the entry after it becomes stale to respond to a subsequent request without first
    13761373            revalidating it with the origin server. The s-maxage directive is always ignored by a private cache.
    13771374         </dd>
    13781375      </dl>
    1379       <p id="rfc.section.3.2.3.p.5">Note that most older caches, not compliant with this specification, do not implement any cache-control directives. An origin
     1376      <p id="rfc.section.15.2.3.p.5">Note that most older caches, not compliant with this specification, do not implement any cache-control directives. An origin
    13801377         server wishing to use a cache-control directive that restricts, but does not prevent, caching by an HTTP/1.1-compliant cache <em class="bcp14">MAY</em> exploit the requirement that the max-age directive overrides the Expires header, and the fact that pre-HTTP/1.1-compliant
    13811378         caches do not observe the max-age directive.
    13821379      </p>
    1383       <p id="rfc.section.3.2.3.p.6">Other directives allow a user agent to modify the basic expiration mechanism. These directives <em class="bcp14">MAY</em> be specified on a request:
    1384       </p>
    1385       <p id="rfc.section.3.2.3.p.7"> <span id="rfc.iref.c.9"></span>  <span id="rfc.iref.m.1"></span> max-age
     1380      <p id="rfc.section.15.2.3.p.6">Other directives allow a user agent to modify the basic expiration mechanism. These directives <em class="bcp14">MAY</em> be specified on a request:
     1381      </p>
     1382      <p id="rfc.section.15.2.3.p.7"> <span id="rfc.iref.c.9"></span>  <span id="rfc.iref.m.1"></span> max-age
    13861383      </p>
    13871384      <dl class="empty">
    … …  
    13901387         </dd>
    13911388      </dl>
    1392       <p id="rfc.section.3.2.3.p.8"> <span id="rfc.iref.c.10"></span>  <span id="rfc.iref.m.2"></span> min-fresh
     1389      <p id="rfc.section.15.2.3.p.8"> <span id="rfc.iref.c.10"></span>  <span id="rfc.iref.m.2"></span> min-fresh
    13931390      </p>
    13941391      <dl class="empty">
    … …  
    13981395         </dd>
    13991396      </dl>
    1400       <p id="rfc.section.3.2.3.p.9"> <span id="rfc.iref.c.11"></span>  <span id="rfc.iref.m.3"></span> max-stale
     1397      <p id="rfc.section.15.2.3.p.9"> <span id="rfc.iref.c.11"></span>  <span id="rfc.iref.m.3"></span> max-stale
    14011398      </p>
    14021399      <dl class="empty">
    … …  
    14061403         </dd>
    14071404      </dl>
    1408       <p id="rfc.section.3.2.3.p.10">If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured
     1405      <p id="rfc.section.15.2.3.p.10">If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured
    14091406         to override the expiration time of a response, the cache <em class="bcp14">MUST</em> attach a Warning header to the stale response, using Warning 110 (Response is stale).
    14101407      </p>
    1411       <p id="rfc.section.3.2.3.p.11">A cache <em class="bcp14">MAY</em> be configured to return stale responses without validation, but only if this does not conflict with any "MUST"-level requirements
     1408      <p id="rfc.section.15.2.3.p.11">A cache <em class="bcp14">MAY</em> be configured to return stale responses without validation, but only if this does not conflict with any "MUST"-level requirements
    14121409         concerning cache validation (e.g., a "must-revalidate" cache-control directive).
    14131410      </p>
    1414       <p id="rfc.section.3.2.3.p.12">If both the new request and the cached entry include "max-age" directives, then the lesser of the two values is used for determining
     1411      <p id="rfc.section.15.2.3.p.12">If both the new request and the cached entry include "max-age" directives, then the lesser of the two values is used for determining
    14151412         the freshness of the cached entry for that request.
    14161413      </p>
    1417       <h3 id="rfc.section.3.2.4"><a href="#rfc.section.3.2.4">3.2.4</a>&nbsp;<a id="cache.revalidation.and.reload.controls" href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></h3>
    1418       <p id="rfc.section.3.2.4.p.1">Sometimes a user agent might want or need to insist that a cache revalidate its cache entry with the origin server (and not
     1414      <h3 id="rfc.section.15.2.4"><a href="#rfc.section.15.2.4">15.2.4</a>&nbsp;<a id="cache.revalidation.and.reload.controls" href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></h3>
     1415      <p id="rfc.section.15.2.4.p.1">Sometimes a user agent might want or need to insist that a cache revalidate its cache entry with the origin server (and not
    14191416         just with the next cache along the path to the origin server), or to reload its cache entry from the origin server. End-to-end
    14201417         revalidation might be necessary if either the cache or the origin server has overestimated the expiration time of the cached
    14211418         response. End-to-end reload may be necessary if the cache entry has become corrupted for some reason.
    14221419      </p>
    1423       <p id="rfc.section.3.2.4.p.2">End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we
     1420      <p id="rfc.section.15.2.4.p.2">End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we
    14241421         call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it
    14251422         "specific end-to-end revalidation."
    14261423      </p>
    1427       <p id="rfc.section.3.2.4.p.3">The client can specify these three kinds of action using Cache-Control request directives:</p>
    1428       <p id="rfc.section.3.2.4.p.4">End-to-end reload </p>
     1424      <p id="rfc.section.15.2.4.p.3">The client can specify these three kinds of action using Cache-Control request directives:</p>
     1425      <p id="rfc.section.15.2.4.p.4">End-to-end reload </p>
    14291426      <dl class="empty">
    14301427         <dd>The request includes a "no-cache" cache-control directive or, for compatibility with HTTP/1.0 clients, "Pragma: no-cache".
    … …  
    14321429         </dd>
    14331430      </dl>
    1434       <p id="rfc.section.3.2.4.p.5">Specific end-to-end revalidation </p>
     1431      <p id="rfc.section.15.2.4.p.5">Specific end-to-end revalidation </p>
    14351432      <dl class="empty">
    14361433         <dd>The request includes a "max-age=0" cache-control directive, which forces each cache along the path to the origin server to
    … …  
    14391436         </dd>
    14401437      </dl>
    1441       <p id="rfc.section.3.2.4.p.6">Unspecified end-to-end revalidation </p>
     1438      <p id="rfc.section.15.2.4.p.6">Unspecified end-to-end revalidation </p>
    14421439      <dl class="empty">
    14431440         <dd>The request includes "max-age=0" cache-control directive, which forces each cache along the path to the origin server to revalidate
    … …  
    14471444         </dd>
    14481445      </dl>
    1449       <p id="rfc.section.3.2.4.p.7"> <span id="rfc.iref.c.12"></span>  <span id="rfc.iref.m.4"></span> max-age
     1446      <p id="rfc.section.15.2.4.p.7"> <span id="rfc.iref.c.12"></span>  <span id="rfc.iref.m.4"></span> max-age
    14501447      </p>
    14511448      <dl class="empty">
    … …  
    14641461         </dd>
    14651462      </dl>
    1466       <p id="rfc.section.3.2.4.p.8"> <span id="rfc.iref.c.13"></span>  <span id="rfc.iref.o.1"></span> only-if-cached
     1463      <p id="rfc.section.15.2.4.p.8"> <span id="rfc.iref.c.13"></span>  <span id="rfc.iref.o.1"></span> only-if-cached
    14671464      </p>
    14681465      <dl class="empty">
    … …  
    14741471         </dd>
    14751472      </dl>
    1476       <p id="rfc.section.3.2.4.p.9"> <span id="rfc.iref.c.14"></span>  <span id="rfc.iref.m.5"></span> must-revalidate
     1473      <p id="rfc.section.15.2.4.p.9"> <span id="rfc.iref.c.14"></span>  <span id="rfc.iref.m.5"></span> must-revalidate
    14771474      </p>
    14781475      <dl class="empty">
    … …  
    14921489         </dd>
    14931490      </dl>
    1494       <p id="rfc.section.3.2.4.p.10"> <span id="rfc.iref.c.15"></span>  <span id="rfc.iref.p.3"></span> proxy-revalidate
     1491      <p id="rfc.section.15.2.4.p.10"> <span id="rfc.iref.c.15"></span>  <span id="rfc.iref.p.3"></span> proxy-revalidate
    14951492      </p>
    14961493      <dl class="empty">
    … …  
    15031500         </dd>
    15041501      </dl>
    1505       <h3 id="rfc.section.3.2.5"><a href="#rfc.section.3.2.5">3.2.5</a>&nbsp;<a id="no-transform.directive" href="#no-transform.directive">No-Transform Directive</a></h3>
    1506       <p id="rfc.section.3.2.5.p.1"> <span id="rfc.iref.c.16"></span>  <span id="rfc.iref.n.3"></span> no-transform
     1502      <h3 id="rfc.section.15.2.5"><a href="#rfc.section.15.2.5">15.2.5</a>&nbsp;<a id="no-transform.directive" href="#no-transform.directive">No-Transform Directive</a></h3>
     1503      <p id="rfc.section.15.2.5.p.1"> <span id="rfc.iref.c.16"></span>  <span id="rfc.iref.n.3"></span> no-transform
    15071504      </p>
    15081505      <dl class="empty">
    … …  
    15151512            authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body.
    15161513         </dd>
    1517          <dd>Therefore, if a message includes the no-transform directive, an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change those headers that are listed in <a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;2.5.2</a> as being subject to the no-transform directive. This implies that the cache or proxy <em class="bcp14">MUST NOT</em> change any aspect of the entity-body that is specified by these headers, including the value of the entity-body itself.
    1518          </dd>
    1519       </dl>
    1520       <h3 id="rfc.section.3.2.6"><a href="#rfc.section.3.2.6">3.2.6</a>&nbsp;<a id="cache.control.extensions" href="#cache.control.extensions">Cache Control Extensions</a></h3>
    1521       <p id="rfc.section.3.2.6.p.1">The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional
     1514         <dd>Therefore, if a message includes the no-transform directive, an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change those headers that are listed in <a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;6.2</a> as being subject to the no-transform directive. This implies that the cache or proxy <em class="bcp14">MUST NOT</em> change any aspect of the entity-body that is specified by these headers, including the value of the entity-body itself.
     1515         </dd>
     1516      </dl>
     1517      <h3 id="rfc.section.15.2.6"><a href="#rfc.section.15.2.6">15.2.6</a>&nbsp;<a id="cache.control.extensions" href="#cache.control.extensions">Cache Control Extensions</a></h3>
     1518      <p id="rfc.section.15.2.6.p.1">The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional
    15221519         assigned value. Informational extensions (those which do not require a change in cache behavior) <em class="bcp14">MAY</em> be added without changing the semantics of other directives. Behavioral extensions are designed to work by acting as modifiers
    15231520         to the existing base of cache directives. Both the new directive and the standard directive are supplied, such that applications
    … …  
    15261523         way, extensions to the cache-control directives can be made without requiring changes to the base protocol.
    15271524      </p>
    1528       <p id="rfc.section.3.2.6.p.2">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
     1525      <p id="rfc.section.15.2.6.p.2">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
    15291526         obeying certain extensions, and ignoring all directives that it does not understand.
    15301527      </p>
    1531       <p id="rfc.section.3.2.6.p.3">For example, consider a hypothetical new response directive called community which acts as a modifier to the private directive.
     1528      <p id="rfc.section.15.2.6.p.3">For example, consider a hypothetical new response directive called community which acts as a modifier to the private directive.
    15321529         We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members
    15331530         of the community named within its value may cache the response. An origin server wishing to allow the UCI community to use
    … …  
    15351532      </p>
    15361533      <div id="rfc.figure.u.12"></div><pre class="text">    Cache-Control: private, community="UCI"
    1537 </pre><p id="rfc.section.3.2.6.p.5">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
     1534</pre><p id="rfc.section.15.2.6.p.5">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
    15381535         it will also see and understand the private directive and thus default to the safe behavior.
    15391536      </p>
    1540       <p id="rfc.section.3.2.6.p.6">Unrecognized cache-directives <em class="bcp14">MUST</em> be ignored; it is assumed that any cache-directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
     1537      <p id="rfc.section.15.2.6.p.6">Unrecognized cache-directives <em class="bcp14">MUST</em> be ignored; it is assumed that any cache-directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
    15411538         directives (or the response's default cacheability) such that the cache behavior will remain minimally correct even if the
    15421539         cache does not understand the extension(s).
    … …  
    15441541      <div id="rfc.iref.e.2"></div>
    15451542      <div id="rfc.iref.h.4"></div>
    1546       <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="header.expires" href="#header.expires">Expires</a></h2>
    1547       <p id="rfc.section.3.3.p.1">The Expires entity-header field gives the date/time after which the response is considered stale. A stale cache entry may
     1543      <h2 id="rfc.section.15.3"><a href="#rfc.section.15.3">15.3</a>&nbsp;<a id="header.expires" href="#header.expires">Expires</a></h2>
     1544      <p id="rfc.section.15.3.p.1">The Expires entity-header field gives the date/time after which the response is considered stale. A stale cache entry may
    15481545         not normally be returned by a cache (either a proxy cache or a user agent cache) unless it is first validated with the origin
    1549          server (or with an intermediate cache that has a fresh copy of the entity). See <a href="#expiration.model" title="Expiration Model">Section&nbsp;2.2</a> for further discussion of the expiration model.
    1550       </p>
    1551       <p id="rfc.section.3.3.p.2">The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after
     1546         server (or with an intermediate cache that has a fresh copy of the entity). See <a href="#expiration.model" title="Expiration Model">Section&nbsp;3</a> for further discussion of the expiration model.
     1547      </p>
     1548      <p id="rfc.section.15.3.p.2">The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after
    15521549         that time.
    15531550      </p>
    1554       <p id="rfc.section.3.3.p.3">The format is an absolute date and time as defined by HTTP-date in <a href="p1-messaging.html#full.date" title="Full Date">Section 3.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>; it <em class="bcp14">MUST</em> be sent in rfc1123-date format.
     1551      <p id="rfc.section.15.3.p.3">The format is an absolute date and time as defined by HTTP-date in <a href="p1-messaging.html#full.date" title="Full Date">Section 3.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>; it <em class="bcp14">MUST</em> be sent in rfc1123-date format.
    15551552      </p>
    15561553      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span>  Expires = "Expires" ":" HTTP-date
    1557 </pre><p id="rfc.section.3.3.p.5">An example of its use is</p>
     1554</pre><p id="rfc.section.15.3.p.5">An example of its use is</p>
    15581555      <div id="rfc.figure.u.14"></div><pre class="text">   Expires: Thu, 01 Dec 1994 16:00:00 GMT
    1559 </pre><p id="rfc.section.3.3.p.7"> </p>
    1560       <dl class="empty">
    1561          <dd> <b>Note:</b> if a response includes a Cache-Control field with the max-age directive (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>), that directive overrides the Expires field.
    1562          </dd>
    1563       </dl>
    1564       <p id="rfc.section.3.3.p.8">HTTP/1.1 clients and caches <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
    1565       </p>
    1566       <p id="rfc.section.3.3.p.9">To mark a response as "already expired," an origin server sends an Expires date that is equal to the Date header value. (See
    1567          the rules for expiration calculations in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;2.2.4</a>.)
    1568       </p>
    1569       <p id="rfc.section.3.3.p.10">To mark a response as "never expires," an origin server sends an Expires date approximately one year from the time the response
     1556</pre><p id="rfc.section.15.3.p.7"> </p>
     1557      <dl class="empty">
     1558         <dd> <b>Note:</b> if a response includes a Cache-Control field with the max-age directive (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>), that directive overrides the Expires field.
     1559         </dd>
     1560      </dl>
     1561      <p id="rfc.section.15.3.p.8">HTTP/1.1 clients and caches <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
     1562      </p>
     1563      <p id="rfc.section.15.3.p.9">To mark a response as "already expired," an origin server sends an Expires date that is equal to the Date header value. (See
     1564         the rules for expiration calculations in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;3.4</a>.)
     1565      </p>
     1566      <p id="rfc.section.15.3.p.10">To mark a response as "never expires," an origin server sends an Expires date approximately one year from the time the response
    15701567         is sent. HTTP/1.1 servers <em class="bcp14">SHOULD NOT</em> send Expires dates more than one year in the future.
    15711568      </p>
    1572       <p id="rfc.section.3.3.p.11">The presence of an Expires header field with a date value of some time in the future on a response that otherwise would by
     1569      <p id="rfc.section.15.3.p.11">The presence of an Expires header field with a date value of some time in the future on a response that otherwise would by
    15731570         default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a Cache-Control header field
    1574          (<a href="#header.cache-control" id="rfc.xref.header.cache-control.9" title="Cache-Control">Section&nbsp;3.2</a>).
     1571         (<a href="#header.cache-control" id="rfc.xref.header.cache-control.9" title="Cache-Control">Section&nbsp;15.2</a>).
    15751572      </p>
    15761573      <div id="rfc.iref.p.4"></div>
    15771574      <div id="rfc.iref.h.5"></div>
    1578       <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="header.pragma" href="#header.pragma">Pragma</a></h2>
    1579       <p id="rfc.section.3.4.p.1">The Pragma general-header field is used to include implementation-specific directives that might apply to any recipient along
     1575      <h2 id="rfc.section.15.4"><a href="#rfc.section.15.4">15.4</a>&nbsp;<a id="header.pragma" href="#header.pragma">Pragma</a></h2>
     1576      <p id="rfc.section.15.4.p.1">The Pragma general-header field is used to include implementation-specific directives that might apply to any recipient along
    15801577         the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some
    15811578         systems <em class="bcp14">MAY</em> require that behavior be consistent with the directives.
    … …  
    15841581  pragma-directive  = "no-cache" | extension-pragma
    15851582  extension-pragma  = token [ "=" ( token | quoted-string ) ]
    1586 </pre><p id="rfc.section.3.4.p.3">When the no-cache directive is present in a request message, an application <em class="bcp14">SHOULD</em> forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive
    1587          has the same semantics as the no-cache cache-directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.10" title="Cache-Control">Section&nbsp;3.2</a>) and is defined here for backward compatibility with HTTP/1.0. Clients <em class="bcp14">SHOULD</em> include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant.
    1588       </p>
    1589       <p id="rfc.section.3.4.p.4">Pragma directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
     1583</pre><p id="rfc.section.15.4.p.3">When the no-cache directive is present in a request message, an application <em class="bcp14">SHOULD</em> forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive
     1584         has the same semantics as the no-cache cache-directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.10" title="Cache-Control">Section&nbsp;15.2</a>) and is defined here for backward compatibility with HTTP/1.0. Clients <em class="bcp14">SHOULD</em> include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant.
     1585      </p>
     1586      <p id="rfc.section.15.4.p.4">Pragma directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
    15901587         might be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific
    15911588         recipient; however, any pragma directive not relevant to a recipient <em class="bcp14">SHOULD</em> be ignored by that recipient.
    15921589      </p>
    1593       <p id="rfc.section.3.4.p.5">HTTP/1.1 caches <em class="bcp14">SHOULD</em> treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". No new Pragma directives will be defined in
     1590      <p id="rfc.section.15.4.p.5">HTTP/1.1 caches <em class="bcp14">SHOULD</em> treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". No new Pragma directives will be defined in
    15941591         HTTP.
    15951592      </p>
    … …  
    16011598      <div id="rfc.iref.v.2"></div>
    16021599      <div id="rfc.iref.h.6"></div>
    1603       <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="header.vary" href="#header.vary">Vary</a></h2>
    1604       <p id="rfc.section.3.5.p.1">The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether
     1600      <h2 id="rfc.section.15.5"><a href="#rfc.section.15.5">15.5</a>&nbsp;<a id="header.vary" href="#header.vary">Vary</a></h2>
     1601      <p id="rfc.section.15.5.p.1">The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether
    16051602         a cache is permitted to use the response to reply to a subsequent request without revalidation. For uncacheable or stale responses,
    16061603         the Vary field value advises the user agent about the criteria that were used to select the representation. A Vary field value
    16071604         of "*" implies that a cache cannot determine from the request headers of a subsequent request whether this response is the
    1608          appropriate representation. See <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.6</a> for use of the Vary header field by caches.
     1605         appropriate representation. See <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;7</a> for use of the Vary header field by caches.
    16091606      </p>
    16101607      <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.13"></span>  Vary  = "Vary" ":" ( "*" | 1#field-name )
    1611 </pre><p id="rfc.section.3.5.p.3">An HTTP/1.1 server <em class="bcp14">SHOULD</em> include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache
     1608</pre><p id="rfc.section.15.5.p.3">An HTTP/1.1 server <em class="bcp14">SHOULD</em> include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache
    16121609         to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that
    16131610         resource. A server <em class="bcp14">MAY</em> include a Vary header field with a non-cacheable response that is subject to server-driven negotiation, since this might provide
    16141611         the user agent with useful information about the dimensions over which the response varies at the time of the response.
    16151612      </p>
    1616       <p id="rfc.section.3.5.p.4">A Vary field value consisting of a list of field-names signals that the representation selected for the response is based
     1613      <p id="rfc.section.15.5.p.4">A Vary field value consisting of a list of field-names signals that the representation selected for the response is based
    16171614         on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation.
    16181615         A cache <em class="bcp14">MAY</em> assume that the same selection will be made for future requests with the same values for the listed field names, for the duration
    16191616         of time for which the response is fresh.
    16201617      </p>
    1621       <p id="rfc.section.3.5.p.5">The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names
     1618      <p id="rfc.section.15.5.p.5">The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names
    16221619         are case-insensitive.
    16231620      </p>
    1624       <p id="rfc.section.3.5.p.6">A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address
     1621      <p id="rfc.section.15.5.p.6">A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address
    16251622         of the client), play a role in the selection of the response representation. The "*" value <em class="bcp14">MUST NOT</em> be generated by a proxy server; it may only be generated by an origin server.
    16261623      </p>
    16271624      <div id="rfc.iref.w.1"></div>
    16281625      <div id="rfc.iref.h.7"></div>
    1629       <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="header.warning" href="#header.warning">Warning</a></h2>
    1630       <p id="rfc.section.3.6.p.1">The Warning general-header field is used to carry additional information about the status or transformation of a message which
     1626      <h2 id="rfc.section.15.6"><a href="#rfc.section.15.6">15.6</a>&nbsp;<a id="header.warning" href="#header.warning">Warning</a></h2>
     1627      <p id="rfc.section.15.6.p.1">The Warning general-header field is used to carry additional information about the status or transformation of a message which
    16311628         might not be reflected in the message. This information is typically used to warn about a possible lack of semantic transparency
    16321629         from caching operations or transformations applied to the entity body of the message.
    16331630      </p>
    1634       <p id="rfc.section.3.6.p.2">Warning headers are sent with responses using:</p>
     1631      <p id="rfc.section.15.6.p.2">Warning headers are sent with responses using:</p>
    16351632      <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span>  Warning    = "Warning" ":" 1#warning-value
    16361633 
    … …  
    16441641  warn-text  = quoted-string
    16451642  warn-date  = DQUOTE HTTP-date DQUOTE
    1646 </pre><p id="rfc.section.3.6.p.4">A response <em class="bcp14">MAY</em> carry more than one Warning header.
    1647       </p>
    1648       <p id="rfc.section.3.6.p.5">The warn-text <em class="bcp14">SHOULD</em> be in a natural language and character set that is most likely to be intelligible to the human user receiving the response.
     1643</pre><p id="rfc.section.15.6.p.4">A response <em class="bcp14">MAY</em> carry more than one Warning header.
     1644      </p>
     1645      <p id="rfc.section.15.6.p.5">The warn-text <em class="bcp14">SHOULD</em> be in a natural language and character set that is most likely to be intelligible to the human user receiving the response.
    16491646         This decision <em class="bcp14">MAY</em> be based on any available knowledge, such as the location of the cache or user, the Accept-Language field in a request, the
    16501647         Content-Language field in a response, etc. The default language is English and the default character set is ISO-8859-1 (<a href="#ISO-8859-1" id="rfc.xref.ISO-8859-1.1"><cite title="Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1">[ISO-8859-1]</cite></a>).
    16511648      </p>
    1652       <p id="rfc.section.3.6.p.6">If a character set other than ISO-8859-1 is used, it <em class="bcp14">MUST</em> be encoded in the warn-text using the method described in <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a>.
    1653       </p>
    1654       <p id="rfc.section.3.6.p.7">Warning headers can in general be applied to any message, however some specific warn-codes are specific to caches and can
     1649      <p id="rfc.section.15.6.p.6">If a character set other than ISO-8859-1 is used, it <em class="bcp14">MUST</em> be encoded in the warn-text using the method described in <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a>.
     1650      </p>
     1651      <p id="rfc.section.15.6.p.7">Warning headers can in general be applied to any message, however some specific warn-codes are specific to caches and can
    16551652         only be applied to response messages. New Warning headers <em class="bcp14">SHOULD</em> be added after any existing Warning headers. A cache <em class="bcp14">MUST NOT</em> delete any Warning header that it received with a message. However, if a cache successfully validates a cache entry, it <em class="bcp14">SHOULD</em> remove any Warning headers previously attached to that entry except as specified for specific Warning codes. It <em class="bcp14">MUST</em> then add any Warning headers received in the validating response. In other words, Warning headers are those that would be
    16561653         attached to the most recent relevant response.
    16571654      </p>
    1658       <p id="rfc.section.3.6.p.8">When multiple Warning headers are attached to a response, the user agent ought to inform the user of as many of them as possible,
     1655      <p id="rfc.section.15.6.p.8">When multiple Warning headers are attached to a response, the user agent ought to inform the user of as many of them as possible,
    16591656         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 <em class="bcp14">SHOULD</em> follow these heuristics:
    16601657      </p>
    … …  
    16651662         </li>
    16661663      </ul>
    1667       <p id="rfc.section.3.6.p.9">Systems that generate multiple Warning headers <em class="bcp14">SHOULD</em> order them with this user agent behavior in mind.
    1668       </p>
    1669       <p id="rfc.section.3.6.p.10">Requirements for the behavior of caches with respect to Warnings are stated in <a href="#warnings" title="Warnings">Section&nbsp;2.1.2</a>.
    1670       </p>
    1671       <p id="rfc.section.3.6.p.11">This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its
     1664      <p id="rfc.section.15.6.p.9">Systems that generate multiple Warning headers <em class="bcp14">SHOULD</em> order them with this user agent behavior in mind.
     1665      </p>
     1666      <p id="rfc.section.15.6.p.10">Requirements for the behavior of caches with respect to Warnings are stated in <a href="#warnings" title="Warnings">Section&nbsp;2.2</a>.
     1667      </p>
     1668      <p id="rfc.section.15.6.p.11">This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its
    16721669         meaning.
    16731670      </p>
    1674       <p id="rfc.section.3.6.p.12">110 Response is stale </p>
     1671      <p id="rfc.section.15.6.p.12">110 Response is stale </p>
    16751672      <dl class="empty">
    16761673         <dd> <em class="bcp14">MUST</em> be included whenever the returned response is stale.
    16771674         </dd>
    16781675      </dl>
    1679       <p id="rfc.section.3.6.p.13">111 Revalidation failed </p>
     1676      <p id="rfc.section.15.6.p.13">111 Revalidation failed </p>
    16801677      <dl class="empty">
    16811678         <dd> <em class="bcp14">MUST</em> be included if a cache returns a stale response because an attempt to revalidate the response failed, due to an inability
    … …  
    16831680         </dd>
    16841681      </dl>
    1685       <p id="rfc.section.3.6.p.14">112 Disconnected operation </p>
     1682      <p id="rfc.section.15.6.p.14">112 Disconnected operation </p>
    16861683      <dl class="empty">
    16871684         <dd> <em class="bcp14">SHOULD</em> be included if the cache is intentionally disconnected from the rest of the network for a period of time.
    16881685         </dd>
    16891686      </dl>
    1690       <p id="rfc.section.3.6.p.15">113 Heuristic expiration </p>
     1687      <p id="rfc.section.15.6.p.15">113 Heuristic expiration </p>
    16911688      <dl class="empty">
    16921689         <dd> <em class="bcp14">MUST</em> be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater
    … …  
    16941691         </dd>
    16951692      </dl>
    1696       <p id="rfc.section.3.6.p.16">199 Miscellaneous warning </p>
     1693      <p id="rfc.section.15.6.p.16">199 Miscellaneous warning </p>
    16971694      <dl class="empty">
    16981695         <dd>The warning text <em class="bcp14">MAY</em> include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action, besides presenting the warning to the user.
    16991696         </dd>
    17001697      </dl>
    1701       <p id="rfc.section.3.6.p.17">214 Transformation applied </p>
     1698      <p id="rfc.section.15.6.p.17">214 Transformation applied </p>
    17021699      <dl class="empty">
    17031700         <dd> <em class="bcp14">MUST</em> be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the
    … …  
    17061703         </dd>
    17071704      </dl>
    1708       <p id="rfc.section.3.6.p.18">299 Miscellaneous persistent warning </p>
     1705      <p id="rfc.section.15.6.p.18">299 Miscellaneous persistent warning </p>
    17091706      <dl class="empty">
    17101707         <dd>The warning text <em class="bcp14">MAY</em> include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action.
    17111708         </dd>
    17121709      </dl>
    1713       <p id="rfc.section.3.6.p.19">If an implementation sends a message with one or more Warning headers whose version is HTTP/1.0 or lower, then the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the date in the response.
    1714       </p>
    1715       <p id="rfc.section.3.6.p.20">If an implementation receives a message with a warning-value that includes a warn-date, and that warn-date is different from
     1710      <p id="rfc.section.15.6.p.19">If an implementation sends a message with one or more Warning headers whose version is HTTP/1.0 or lower, then the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the date in the response.
     1711      </p>
     1712      <p id="rfc.section.15.6.p.20">If an implementation receives a message with a warning-value that includes a warn-date, and that warn-date is different from
    17161713         the Date value in the response, then that warning-value <em class="bcp14">MUST</em> be deleted from the message before storing, forwarding, or using it. (This prevents bad consequences of naive caching of Warning
    17171714         header fields.) If all of the warning-values are deleted for this reason, the Warning header <em class="bcp14">MUST</em> be deleted as well.
    17181715      </p>
    1719       <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
    1720       <p id="rfc.section.4.p.1">TBD.</p>
    1721       <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
    1722       <p id="rfc.section.5.p.1">Caching proxies provide additional potential vulnerabilities, since the contents of the cache represent an attractive target
     1716      <h1 id="rfc.section.16"><a href="#rfc.section.16">16.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
     1717      <p id="rfc.section.16.p.1">TBD.</p>
     1718      <h1 id="rfc.section.17"><a href="#rfc.section.17">17.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
     1719      <p id="rfc.section.17.p.1">Caching proxies provide additional potential vulnerabilities, since the contents of the cache represent an attractive target
    17231720         for malicious exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal
    17241721         information long after a user believes that the information has been removed from the network. Therefore, cache contents should
    17251722         be protected as sensitive information.
    17261723      </p>
    1727       <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
    1728       <p id="rfc.section.6.p.1">Much of the content and presentation of the caching design is due to suggestions and comments from individuals including:
     1724      <h1 id="rfc.section.18"><a href="#rfc.section.18">18.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
     1725      <p id="rfc.section.18.p.1">Much of the content and presentation of the caching design is due to suggestions and comments from individuals including:
    17291726         Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
    17301727      </p>
    1731       <h1 id="rfc.references"><a id="rfc.section.7" href="#rfc.section.7">7.</a> References
     1728      <h1 id="rfc.references"><a id="rfc.section.19" href="#rfc.section.19">19.</a> References
    17321729      </h1>
    1733       <h2 id="rfc.references.1"><a href="#rfc.section.7.1" id="rfc.section.7.1">7.1</a> Normative References
     1730      <h2 id="rfc.references.1"><a href="#rfc.section.19.1" id="rfc.section.19.1">19.1</a> Normative References
    17341731      </h2>
    17351732      <table summary="Normative References">                 
    … …  
    17821779         </tr>
    17831780      </table>
    1784       <h2 id="rfc.references.2"><a href="#rfc.section.7.2" id="rfc.section.7.2">7.2</a> Informative References
     1781      <h2 id="rfc.references.2"><a href="#rfc.section.19.2" id="rfc.section.19.2">19.2</a> Informative References
    17851782      </h2>
    17861783      <table summary="Informative References">   
    … …  
    18141811      <h1 id="rfc.section.A"><a href="#rfc.section.A">A.</a>&nbsp;<a id="compatibility" href="#compatibility">Compatibility with Previous Versions</a></h1>
    18151812      <h2 id="rfc.section.A.1"><a href="#rfc.section.A.1">A.1</a>&nbsp;<a id="changes.from.rfc.2068" href="#changes.from.rfc.2068">Changes from RFC 2068</a></h2>
    1816       <p id="rfc.section.A.1.p.1">A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add this missing case. (Sections <a href="#response.cacheability" title="Response Cacheability">2.4</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.11" title="Cache-Control">3.2</a>, <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">3.2.3</a>)
     1813      <p id="rfc.section.A.1.p.1">A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add this missing case. (Sections <a href="#response.cacheability" title="Response Cacheability">5</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.11" title="Cache-Control">15.2</a>, <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">15.2.3</a>)
    18171814      </p>
    18181815      <p id="rfc.section.A.1.p.2">Transfer-coding and message lengths all interact in ways that required fixing exactly when chunked encoding is used (to allow
    18191816         for transfer encoding that may not be self delimiting); it was important to straighten out exactly how message lengths are
    1820          computed. (<a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;2.5.2</a>, see also <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="#Part3" id="rfc.xref.Part3.3"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a> and <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>)
    1821       </p>
    1822       <p id="rfc.section.A.1.p.3">Proxies should be able to add Content-Length when appropriate. (<a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;2.5.2</a>)
     1817         computed. (<a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;6.2</a>, see also <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="#Part3" id="rfc.xref.Part3.3"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a> and <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>)
     1818      </p>
     1819      <p id="rfc.section.A.1.p.3">Proxies should be able to add Content-Length when appropriate. (<a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;6.2</a>)
    18231820      </p>
    18241821      <p id="rfc.section.A.1.p.4">Range request responses would become very verbose if all meta-data were always returned; by allowing the server to only send
    1825          needed headers in a 206 response, this problem can be avoided. (<a href="#combining.headers" title="Combining Headers">Section&nbsp;2.5.3</a>)
    1826       </p>
    1827       <p id="rfc.section.A.1.p.5">The Cache-Control: max-age directive was not properly defined for responses. (<a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;3.2.3</a>)
    1828       </p>
    1829       <p id="rfc.section.A.1.p.6">Warnings could be cached incorrectly, or not updated appropriately. (Section <a href="#warnings" title="Warnings">2.1.2</a>, <a href="#expiration.calculations" title="Expiration Calculations">2.2.4</a>, <a href="#non-modifiable.headers" title="Non-modifiable Headers">2.5.2</a>, <a href="#combining.headers" title="Combining Headers">2.5.3</a>, <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">3.2.3</a>, and <a href="#header.warning" id="rfc.xref.header.warning.6" title="Warning">3.6</a>) Warning also needed to be a general header, as PUT or other methods may have need for it in requests.
     1822         needed headers in a 206 response, this problem can be avoided. (<a href="#combining.headers" title="Combining Headers">Section&nbsp;6.3</a>)
     1823      </p>
     1824      <p id="rfc.section.A.1.p.5">The Cache-Control: max-age directive was not properly defined for responses. (<a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>)
     1825      </p>
     1826      <p id="rfc.section.A.1.p.6">Warnings could be cached incorrectly, or not updated appropriately. (Section <a href="#warnings" title="Warnings">2.2</a>, <a href="#expiration.calculations" title="Expiration Calculations">3.4</a>, <a href="#non-modifiable.headers" title="Non-modifiable Headers">6.2</a>, <a href="#combining.headers" title="Combining Headers">6.3</a>, <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">15.2.3</a>, and <a href="#header.warning" id="rfc.xref.header.warning.6" title="Warning">15.6</a>) Warning also needed to be a general header, as PUT or other methods may have need for it in requests.
    18301827      </p>
    18311828      <h2 id="rfc.section.A.2"><a href="#rfc.section.A.2">A.2</a>&nbsp;<a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h2>
    1832       <p id="rfc.section.A.2.p.1">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Invalidation After Updates or Deletions">Section&nbsp;2.10</a>)
     1829      <p id="rfc.section.A.2.p.1">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Invalidation After Updates or Deletions">Section&nbsp;11</a>)
    18331830      </p>
    18341831      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;Change Log (to be removed by RFC Editor before publication)
    … …  
    18361833      <h2 id="rfc.section.B.1"><a href="#rfc.section.B.1">B.1</a>&nbsp;Since RFC2616
    18371834      </h2>
    1838       <p id="rfc.section.B.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
     1835      <p id="rfc.section.B.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
    18391836      </p>
    18401837      <h2 id="rfc.section.B.2"><a href="#rfc.section.B.2">B.2</a>&nbsp;Since draft-ietf-httpbis-p6-cache-00
    … …  
    19001897            <li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind">
    19011898                  <li class="indline1">age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1">1.2</a></li>
    1902                   <li class="indline1">Age header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.2"><b>3.1</b></a></li>
     1899                  <li class="indline1">Age header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.2"><b>15.1</b></a></li>
    19031900               </ul>
    19041901            </li>
    19051902            <li class="indline0"><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul class="ind">
    1906                   <li class="indline1">cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.1">1.2</a></li>
     1903                  <li class="indline1">cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.1">1.1</a></li>
    19071904                  <li class="indline1">Cache Directives&nbsp;&nbsp;
    19081905                     <ul class="ind">
    1909                         <li class="indline1">max-age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.9"><b>3.2.3</b></a>, <a class="iref" href="#rfc.iref.c.12"><b>3.2.4</b></a></li>
    1910                         <li class="indline1">max-stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.11"><b>3.2.3</b></a></li>
    1911                         <li class="indline1">min-fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.10"><b>3.2.3</b></a></li>
    1912                         <li class="indline1">must-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.14"><b>3.2.4</b></a></li>
    1913                         <li class="indline1">no-cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.6"><b>3.2.1</b></a></li>
    1914                         <li class="indline1">no-store&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>3.2.2</b></a></li>
    1915                         <li class="indline1">no-transform&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.16"><b>3.2.5</b></a></li>
    1916                         <li class="indline1">only-if-cached&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.13"><b>3.2.4</b></a></li>
    1917                         <li class="indline1">private&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.5"><b>3.2.1</b></a></li>
    1918                         <li class="indline1">proxy-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.15"><b>3.2.4</b></a></li>
    1919                         <li class="indline1">public&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.4"><b>3.2.1</b></a></li>
    1920                         <li class="indline1">s-maxage&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>3.2.3</b></a></li>
     1906                        <li class="indline1">max-age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.9"><b>15.2.3</b></a>, <a class="iref" href="#rfc.iref.c.12"><b>15.2.4</b></a></li>
     1907                        <li class="indline1">max-stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.11"><b>15.2.3</b></a></li>
     1908                        <li class="indline1">min-fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.10"><b>15.2.3</b></a></li>
     1909                        <li class="indline1">must-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.14"><b>15.2.4</b></a></li>
     1910                        <li class="indline1">no-cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.6"><b>15.2.1</b></a></li>
     1911                        <li class="indline1">no-store&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>15.2.2</b></a></li>
     1912                        <li class="indline1">no-transform&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.16"><b>15.2.5</b></a></li>
     1913                        <li class="indline1">only-if-cached&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.13"><b>15.2.4</b></a></li>
     1914                        <li class="indline1">private&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.5"><b>15.2.1</b></a></li>
     1915                        <li class="indline1">proxy-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.15"><b>15.2.4</b></a></li>
     1916                        <li class="indline1">public&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.4"><b>15.2.1</b></a></li>
     1917                        <li class="indline1">s-maxage&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>15.2.3</b></a></li>
    19211918                     </ul>
    19221919                  </li>
    1923                   <li class="indline1">Cache-Control header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.1.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">2.2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.5">2.2.5</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">2.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">2.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.8">2.8</a>, <a class="iref" href="#rfc.iref.c.3"><b>3.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.9">3.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.10">3.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.11">A.1</a></li>
     1920                  <li class="indline1">Cache-Control header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">3.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.5">3.5</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">5</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">5</a>, <a class="iref" href="#rfc.xref.header.cache-control.8">9</a>, <a class="iref" href="#rfc.iref.c.3"><b>15.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.9">15.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.10">15.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.11">A.1</a></li>
    19241921                  <li class="indline1">cacheable&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.2">1.2</a></li>
    19251922               </ul>
    19261923            </li>
    19271924            <li class="indline0"><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul class="ind">
    1928                   <li class="indline1">Expires header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">2.4</a>, <a class="iref" href="#rfc.xref.header.expires.2">3.2.3</a>, <a class="iref" href="#rfc.iref.e.2"><b>3.3</b></a></li>
     1925                  <li class="indline1">Expires header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">5</a>, <a class="iref" href="#rfc.xref.header.expires.2">15.2.3</a>, <a class="iref" href="#rfc.iref.e.2"><b>15.3</b></a></li>
    19291926                  <li class="indline1">explicit expiration time&nbsp;&nbsp;<a class="iref" href="#rfc.iref.e.1">1.2</a></li>
    19301927               </ul>
    … …  
    19391936                  <li class="indline1"><tt>Grammar</tt>&nbsp;&nbsp;
    19401937                     <ul class="ind">
    1941                         <li class="indline1"><tt>Age</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.2"><b>3.1</b></a></li>
    1942                         <li class="indline1"><tt>age-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.3"><b>3.1</b></a></li>
    1943                         <li class="indline1"><tt>Cache-Control</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.4"><b>3.2</b></a></li>
    1944                         <li class="indline1"><tt>cache-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.5"><b>3.2</b></a></li>
    1945                         <li class="indline1"><tt>cache-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.8"><b>3.2</b></a></li>
    1946                         <li class="indline1"><tt>cache-request-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.6"><b>3.2</b></a></li>
    1947                         <li class="indline1"><tt>cache-response-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.7"><b>3.2</b></a></li>
    1948                         <li class="indline1"><tt>delta-seconds</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.1"><b>1.3</b></a></li>
    1949                         <li class="indline1"><tt>Expires</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.9"><b>3.3</b></a></li>
    1950                         <li class="indline1"><tt>extension-pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.12"><b>3.4</b></a></li>
    1951                         <li class="indline1"><tt>Pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.10"><b>3.4</b></a></li>
    1952                         <li class="indline1"><tt>pragma-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.11"><b>3.4</b></a></li>
    1953                         <li class="indline1"><tt>Vary</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.13"><b>3.5</b></a></li>
    1954                         <li class="indline1"><tt>warn-agent</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.17"><b>3.6</b></a></li>
    1955                         <li class="indline1"><tt>warn-code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.16"><b>3.6</b></a></li>
    1956                         <li class="indline1"><tt>warn-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.19"><b>3.6</b></a></li>
    1957                         <li class="indline1"><tt>warn-text</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.18"><b>3.6</b></a></li>
    1958                         <li class="indline1"><tt>Warning</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.14"><b>3.6</b></a></li>
    1959                         <li class="indline1"><tt>warning-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.15"><b>3.6</b></a></li>
     1938                        <li class="indline1"><tt>Age</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.1"><b>15.1</b></a></li>
     1939                        <li class="indline1"><tt>age-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.2"><b>15.1</b></a></li>
     1940                        <li class="indline1"><tt>Cache-Control</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.4"><b>15.2</b></a></li>
     1941                        <li class="indline1"><tt>cache-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.5"><b>15.2</b></a></li>
     1942                        <li class="indline1"><tt>cache-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.8"><b>15.2</b></a></li>
     1943                        <li class="indline1"><tt>cache-request-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.6"><b>15.2</b></a></li>
     1944                        <li class="indline1"><tt>cache-response-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.7"><b>15.2</b></a></li>
     1945                        <li class="indline1"><tt>delta-seconds</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.3"><b>15.1</b></a></li>
     1946                        <li class="indline1"><tt>Expires</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.9"><b>15.3</b></a></li>
     1947                        <li class="indline1"><tt>extension-pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.12"><b>15.4</b></a></li>
     1948                        <li class="indline1"><tt>Pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.10"><b>15.4</b></a></li>
     1949                        <li class="indline1"><tt>pragma-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.11"><b>15.4</b></a></li>
     1950                        <li class="indline1"><tt>Vary</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.13"><b>15.5</b></a></li>
     1951                        <li class="indline1"><tt>warn-agent</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.17"><b>15.6</b></a></li>
     1952                        <li class="indline1"><tt>warn-code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.16"><b>15.6</b></a></li>
     1953                        <li class="indline1"><tt>warn-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.19"><b>15.6</b></a></li>
     1954                        <li class="indline1"><tt>warn-text</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.18"><b>15.6</b></a></li>
     1955                        <li class="indline1"><tt>Warning</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.14"><b>15.6</b></a></li>
     1956                        <li class="indline1"><tt>warning-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.15"><b>15.6</b></a></li>
    19601957                     </ul>
    19611958                  </li>
    … …  
    19651962                  <li class="indline1">Headers&nbsp;&nbsp;
    19661963                     <ul class="ind">
    1967                         <li class="indline1">Age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.2"><b>3.1</b></a></li>
    1968                         <li class="indline1">Cache-Control&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.1.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">2.2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.5">2.2.5</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">2.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">2.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.8">2.8</a>, <a class="iref" href="#rfc.iref.h.3"><b>3.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.9">3.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.10">3.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.11">A.1</a></li>
    1969                         <li class="indline1">Expires&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">2.4</a>, <a class="iref" href="#rfc.xref.header.expires.2">3.2.3</a>, <a class="iref" href="#rfc.iref.h.4"><b>3.3</b></a></li>
    1970                         <li class="indline1">Pragma&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">3.2</a>, <a class="iref" href="#rfc.iref.h.5"><b>3.4</b></a></li>
    1971                         <li class="indline1">Vary&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">2.6</a>, <a class="iref" href="#rfc.iref.h.6"><b>3.5</b></a></li>
    1972                         <li class="indline1">Warning&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.1.1</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.1.2</a>, <a class="iref" href="#rfc.xref.header.warning.3">2.1.2</a>, <a class="iref" href="#rfc.xref.header.warning.4">2.5.2</a>, <a class="iref" href="#rfc.xref.header.warning.5">2.5.3</a>, <a class="iref" href="#rfc.iref.h.7"><b>3.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.6">A.1</a></li>
     1964                        <li class="indline1">Age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.2"><b>15.1</b></a></li>
     1965                        <li class="indline1">Cache-Control&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">3.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.5">3.5</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">5</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">5</a>, <a class="iref" href="#rfc.xref.header.cache-control.8">9</a>, <a class="iref" href="#rfc.iref.h.3"><b>15.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.9">15.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.10">15.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.11">A.1</a></li>
     1966                        <li class="indline1">Expires&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">5</a>, <a class="iref" href="#rfc.xref.header.expires.2">15.2.3</a>, <a class="iref" href="#rfc.iref.h.4"><b>15.3</b></a></li>
     1967                        <li class="indline1">Pragma&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">15.2</a>, <a class="iref" href="#rfc.iref.h.5"><b>15.4</b></a></li>
     1968                        <li class="indline1">Vary&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">7</a>, <a class="iref" href="#rfc.iref.h.6"><b>15.5</b></a></li>
     1969                        <li class="indline1">Warning&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.1</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.2</a>, <a class="iref" href="#rfc.xref.header.warning.3">2.2</a>, <a class="iref" href="#rfc.xref.header.warning.4">6.2</a>, <a class="iref" href="#rfc.xref.header.warning.5">6.3</a>, <a class="iref" href="#rfc.iref.h.7"><b>15.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.6">A.1</a></li>
    19731970                     </ul>
    19741971                  </li>
    … …  
    19771974            </li>
    19781975            <li class="indline0"><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul class="ind">
    1979                   <li class="indline1"><em>ISO-8859-1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ISO-8859-1.1">3.6</a>, <a class="iref" href="#ISO-8859-1"><b>7.1</b></a></li>
     1976                  <li class="indline1"><em>ISO-8859-1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ISO-8859-1.1">15.6</a>, <a class="iref" href="#ISO-8859-1"><b>19.1</b></a></li>
    19801977               </ul>
    19811978            </li>
    … …  
    19831980                  <li class="indline1">max-age&nbsp;&nbsp;
    19841981                     <ul class="ind">
    1985                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>3.2.3</b></a>, <a class="iref" href="#rfc.iref.m.4"><b>3.2.4</b></a></li>
     1982                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>15.2.3</b></a>, <a class="iref" href="#rfc.iref.m.4"><b>15.2.4</b></a></li>
    19861983                     </ul>
    19871984                  </li>
    19881985                  <li class="indline1">max-stale&nbsp;&nbsp;
    19891986                     <ul class="ind">
    1990                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.3"><b>3.2.3</b></a></li>
     1987                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.3"><b>15.2.3</b></a></li>
    19911988                     </ul>
    19921989                  </li>
    19931990                  <li class="indline1">min-fresh&nbsp;&nbsp;
    19941991                     <ul class="ind">
    1995                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.2"><b>3.2.3</b></a></li>
     1992                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.2"><b>15.2.3</b></a></li>
    19961993                     </ul>
    19971994                  </li>
    19981995                  <li class="indline1">must-revalidate&nbsp;&nbsp;
    19991996                     <ul class="ind">
    2000                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.5"><b>3.2.4</b></a></li>
     1997                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.5"><b>15.2.4</b></a></li>
    20011998                     </ul>
    20021999                  </li>
    … …  
    20062003                  <li class="indline1">no-cache&nbsp;&nbsp;
    20072004                     <ul class="ind">
    2008                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>3.2.1</b></a></li>
     2005                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>15.2.1</b></a></li>
    20092006                     </ul>
    20102007                  </li>
    20112008                  <li class="indline1">no-store&nbsp;&nbsp;
    20122009                     <ul class="ind">
    2013                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>3.2.2</b></a></li>
     2010                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>15.2.2</b></a></li>
    20142011                     </ul>
    20152012                  </li>
    20162013                  <li class="indline1">no-transform&nbsp;&nbsp;
    20172014                     <ul class="ind">
    2018                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>3.2.5</b></a></li>
     2015                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>15.2.5</b></a></li>
    20192016                     </ul>
    20202017                  </li>
    … …  
    20242021                  <li class="indline1">only-if-cached&nbsp;&nbsp;
    20252022                     <ul class="ind">
    2026                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>3.2.4</b></a></li>
     2023                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>15.2.4</b></a></li>
    20272024                     </ul>
    20282025                  </li>
    … …  
    20302027            </li>
    20312028            <li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind">
    2032                   <li class="indline1"><em>Part1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">2.2.3</a>, <a class="iref" href="#rfc.xref.Part1.2">2.5.1</a>, <a class="iref" href="#rfc.xref.Part1.3">2.5.2</a>, <a class="iref" href="#rfc.xref.Part1.4">2.5.2</a>, <a class="iref" href="#rfc.xref.Part1.5">2.6</a>, <a class="iref" href="#rfc.xref.Part1.6">3.3</a>, <a class="iref" href="#Part1"><b>7.1</b></a>, <a class="iref" href="#rfc.xref.Part1.7">A.1</a><ul class="ind">
    2033                         <li class="indline1"><em>Section 3.3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.6">3.3</a></li>
    2034                         <li class="indline1"><em>Section 4.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.5">2.6</a></li>
    2035                         <li class="indline1"><em>Section 4.4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.3">2.5.2</a>, <a class="iref" href="#rfc.xref.Part1.4">2.5.2</a></li>
    2036                         <li class="indline1"><em>Section 8.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.2">2.5.1</a></li>
    2037                         <li class="indline1"><em>Section 8.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">2.2.3</a></li>
     2029                  <li class="indline1"><em>Part1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">3.3</a>, <a class="iref" href="#rfc.xref.Part1.2">6.1</a>, <a class="iref" href="#rfc.xref.Part1.3">6.2</a>, <a class="iref" href="#rfc.xref.Part1.4">6.2</a>, <a class="iref" href="#rfc.xref.Part1.5">7</a>, <a class="iref" href="#rfc.xref.Part1.6">15.3</a>, <a class="iref" href="#Part1"><b>19.1</b></a>, <a class="iref" href="#rfc.xref.Part1.7">A.1</a><ul class="ind">
     2030                        <li class="indline1"><em>Section 3.3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.6">15.3</a></li>
     2031                        <li class="indline1"><em>Section 4.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.5">7</a></li>
     2032                        <li class="indline1"><em>Section 4.4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.3">6.2</a>, <a class="iref" href="#rfc.xref.Part1.4">6.2</a></li>
     2033                        <li class="indline1"><em>Section 8.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.2">6.1</a></li>
     2034                        <li class="indline1"><em>Section 8.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">3.3</a></li>
    20382035                     </ul>
    20392036                  </li>
    2040                   <li class="indline1"><em>Part2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">2.9</a>, <a class="iref" href="#Part2"><b>7.1</b></a><ul class="ind">
    2041                         <li class="indline1"><em>Section 8.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">2.9</a></li>
     2037                  <li class="indline1"><em>Part2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">10</a>, <a class="iref" href="#Part2"><b>19.1</b></a><ul class="ind">
     2038                        <li class="indline1"><em>Section 8.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">10</a></li>
    20422039                     </ul>
    20432040                  </li>
    2044                   <li class="indline1"><em>Part3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">2.5.2</a>, <a class="iref" href="#rfc.xref.Part3.2">2.6</a>, <a class="iref" href="#Part3"><b>7.1</b></a>, <a class="iref" href="#rfc.xref.Part3.3">A.1</a><ul class="ind">
    2045                         <li class="indline1"><em>Section 3.2.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">2.5.2</a></li>
    2046                         <li class="indline1"><em>Section 4.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.2">2.6</a></li>
     2041                  <li class="indline1"><em>Part3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">6.2</a>, <a class="iref" href="#rfc.xref.Part3.2">7</a>, <a class="iref" href="#Part3"><b>19.1</b></a>, <a class="iref" href="#rfc.xref.Part3.3">A.1</a><ul class="ind">
     2042                        <li class="indline1"><em>Section 3.2.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">6.2</a></li>
     2043                        <li class="indline1"><em>Section 4.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.2">7</a></li>
    20472044                     </ul>
    20482045                  </li>
    2049                   <li class="indline1"><em>Part4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">2.3</a>, <a class="iref" href="#rfc.xref.Part4.2">2.3.2</a>, <a class="iref" href="#rfc.xref.Part4.3">2.3.2</a>, <a class="iref" href="#Part4"><b>7.1</b></a><ul class="ind">
    2050                         <li class="indline1"><em>Section 2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.2">2.3.2</a></li>
    2051                         <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">2.3</a></li>
    2052                         <li class="indline1"><em>Section 6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.3">2.3.2</a></li>
     2046                  <li class="indline1"><em>Part4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">4</a>, <a class="iref" href="#rfc.xref.Part4.2">4.2</a>, <a class="iref" href="#rfc.xref.Part4.3">4.2</a>, <a class="iref" href="#Part4"><b>19.1</b></a><ul class="ind">
     2047                        <li class="indline1"><em>Section 2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.2">4.2</a></li>
     2048                        <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">4</a></li>
     2049                        <li class="indline1"><em>Section 6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.3">4.2</a></li>
    20532050                     </ul>
    20542051                  </li>
    2055                   <li class="indline1"><em>Part5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">2.5.3</a>, <a class="iref" href="#rfc.xref.Part5.2">2.8</a>, <a class="iref" href="#Part5"><b>7.1</b></a>, <a class="iref" href="#rfc.xref.Part5.3">A.1</a><ul class="ind">
    2056                         <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">2.5.3</a>, <a class="iref" href="#rfc.xref.Part5.2">2.8</a></li>
     2052                  <li class="indline1"><em>Part5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">6.3</a>, <a class="iref" href="#rfc.xref.Part5.2">9</a>, <a class="iref" href="#Part5"><b>19.1</b></a>, <a class="iref" href="#rfc.xref.Part5.3">A.1</a><ul class="ind">
     2053                        <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">6.3</a>, <a class="iref" href="#rfc.xref.Part5.2">9</a></li>
    20572054                     </ul>
    20582055                  </li>
    2059                   <li class="indline1"><em>Part7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">2.4</a>, <a class="iref" href="#rfc.xref.Part7.2">3.2.1</a>, <a class="iref" href="#Part7"><b>7.1</b></a><ul class="ind">
    2060                         <li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">2.4</a>, <a class="iref" href="#rfc.xref.Part7.2">3.2.1</a></li>
     2056                  <li class="indline1"><em>Part7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">5</a>, <a class="iref" href="#rfc.xref.Part7.2">15.2.1</a>, <a class="iref" href="#Part7"><b>19.1</b></a><ul class="ind">
     2057                        <li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">5</a>, <a class="iref" href="#rfc.xref.Part7.2">15.2.1</a></li>
    20612058                     </ul>
    20622059                  </li>
    2063                   <li class="indline1">Pragma header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">3.2</a>, <a class="iref" href="#rfc.iref.p.4"><b>3.4</b></a></li>
     2060                  <li class="indline1">Pragma header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">15.2</a>, <a class="iref" href="#rfc.iref.p.4"><b>15.4</b></a></li>
    20642061                  <li class="indline1">private&nbsp;&nbsp;
    20652062                     <ul class="ind">
    2066                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.2"><b>3.2.1</b></a></li>
     2063                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.2"><b>15.2.1</b></a></li>
    20672064                     </ul>
    20682065                  </li>
    20692066                  <li class="indline1">proxy-revalidate&nbsp;&nbsp;
    20702067                     <ul class="ind">
    2071                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.3"><b>3.2.4</b></a></li>
     2068                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.3"><b>15.2.4</b></a></li>
    20722069                     </ul>
    20732070                  </li>
    20742071                  <li class="indline1">public&nbsp;&nbsp;
    20752072                     <ul class="ind">
    2076                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>3.2.1</b></a></li>
     2073                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>15.2.1</b></a></li>
    20772074                     </ul>
    20782075                  </li>
    … …  
    20802077            </li>
    20812078            <li class="indline0"><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul class="ind">
    2082                   <li class="indline1"><em>RFC1305</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1305.1">2.2.3</a>, <a class="iref" href="#RFC1305"><b>7.2</b></a></li>
    2083                   <li class="indline1"><em>RFC2047</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2047.1">3.6</a>, <a class="iref" href="#RFC2047"><b>7.1</b></a></li>
    2084                   <li class="indline1"><em>RFC2119</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2119.1">1.1</a>, <a class="iref" href="#RFC2119"><b>7.1</b></a></li>
    2085                   <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.1">1</a>, <a class="iref" href="#RFC2616"><b>7.2</b></a>, <a class="iref" href="#rfc.xref.RFC2616.2">B.1</a></li>
     2079                  <li class="indline1"><em>RFC1305</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1305.1">3.3</a>, <a class="iref" href="#RFC1305"><b>19.2</b></a></li>
     2080                  <li class="indline1"><em>RFC2047</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2047.1">15.6</a>, <a class="iref" href="#RFC2047"><b>19.1</b></a></li>
     2081                  <li class="indline1"><em>RFC2119</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2119.1">1.3</a>, <a class="iref" href="#RFC2119"><b>19.1</b></a></li>
     2082                  <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#RFC2616"><b>19.2</b></a>, <a class="iref" href="#rfc.xref.RFC2616.1">B.1</a></li>
    20862083               </ul>
    20872084            </li>
    … …  
    20892086                  <li class="indline1">s-maxage&nbsp;&nbsp;
    20902087                     <ul class="ind">
    2091                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.3"><b>3.2.3</b></a></li>
     2088                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.3"><b>15.2.3</b></a></li>
    20922089                     </ul>
    20932090                  </li>
    2094                   <li class="indline1">semantically transparent&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.2">1.2</a></li>
    2095                   <li class="indline1">stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1">1.2</a></li>
     2091                  <li class="indline1">semantically transparent&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1">1.1</a></li>
     2092                  <li class="indline1">stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.2">1.2</a></li>
    20962093               </ul>
    20972094            </li>
    20982095            <li class="indline0"><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul class="ind">
    20992096                  <li class="indline1">validator&nbsp;&nbsp;<a class="iref" href="#rfc.iref.v.1">1.2</a></li>
    2100                   <li class="indline1">Vary header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">2.6</a>, <a class="iref" href="#rfc.iref.v.2"><b>3.5</b></a></li>
     2097                  <li class="indline1">Vary header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">7</a>, <a class="iref" href="#rfc.iref.v.2"><b>15.5</b></a></li>
    21012098               </ul>
    21022099            </li>
    21032100            <li class="indline0"><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul class="ind">
    2104                   <li class="indline1">Warning header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.1.1</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.1.2</a>, <a class="iref" href="#rfc.xref.header.warning.3">2.1.2</a>, <a class="iref" href="#rfc.xref.header.warning.4">2.5.2</a>, <a class="iref" href="#rfc.xref.header.warning.5">2.5.3</a>, <a class="iref" href="#rfc.iref.w.1"><b>3.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.6">A.1</a></li>
     2101                  <li class="indline1">Warning header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.1</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.2</a>, <a class="iref" href="#rfc.xref.header.warning.3">2.2</a>, <a class="iref" href="#rfc.xref.header.warning.4">6.2</a>, <a class="iref" href="#rfc.xref.header.warning.5">6.3</a>, <a class="iref" href="#rfc.iref.w.1"><b>15.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.6">A.1</a></li>
    21052102               </ul>
    21062103            </li>

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

    r154 r157  
    215215</front>
    216216<middle>
    217 <section title="Introduction" anchor="introduction">
    218 <t>
    219    This document will define aspects of HTTP related to caching response
    220    messages.  Right now it only includes the extracted relevant sections
    221    of <xref target="RFC2616" x:fmt="none">RFC 2616</xref> without edit.
    222 </t>
    223 
    224 <section title="Requirements" anchor="intro.requirements">
    225 <t>
    226    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    227    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    228    document are to be interpreted as described in <xref target="RFC2119"/>.
    229 </t>
    230 <t>
    231    An implementation is not compliant if it fails to satisfy one or more
    232    of the &MUST; or &REQUIRED; level requirements for the protocols it
    233    implements. An implementation that satisfies all the &MUST; or &REQUIRED;
    234    level and all the &SHOULD; level requirements for its protocols is said
    235    to be "unconditionally compliant"; one that satisfies all the &MUST;
    236    level requirements but not all the &SHOULD; level requirements for its
    237    protocols is said to be "conditionally compliant."
     217<section title="Introduction" anchor="caching">
     218<t>
     219   HTTP is typically used for distributed information systems, where
     220   performance can be improved by the use of response caches. The
     221   HTTP/1.1 protocol includes a number of elements intended to make
     222   caching work as well as possible. Because these elements are
     223   inextricable from other aspects of the protocol, and because they
     224   interact with each other, it is useful to describe the basic caching
     225   design of HTTP separately from the detailed descriptions of methods,
     226   headers, response codes, etc.  This document defines aspects of HTTP
     227   related to caching response messages.
     228</t>
     229
     230<section title="Purpose" anchor="intro.purpose">
     231<iref item="cache"/>
     232<t>
     233   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages
     234   and the subsystem that controls its message storage, retrieval, and
     235   deletion. A cache stores cacheable responses in order to reduce the
     236   response time and network bandwidth consumption on future, equivalent
     237   requests. Any client or server may include a cache, though a cache
     238   cannot be used by a server that is acting as a tunnel.
     239</t>
     240<t>
     241   Caching would be useless if it did not significantly improve
     242   performance. The goal of caching in HTTP/1.1 is to eliminate the need
     243   to send requests in many cases, and to eliminate the need to send
     244   full responses in many other cases. The former reduces the number of
     245   network round-trips required for many operations; we use an
     246   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
     247   latter reduces network bandwidth requirements; we use a "validation"
     248   mechanism for this purpose (see <xref target="validation.model"/>).
     249</t>
     250<iref item="semantically transparent"/>
     251<t>
     252   A cache behaves in a "<x:dfn>semantically transparent</x:dfn>" manner, with
     253   respect to a particular response, when its use affects neither the
     254   requesting client nor the origin server, except to improve
     255   performance. When a cache is semantically transparent, the client
     256   receives exactly the same response (except for hop-by-hop headers)
     257   that it would have received had its request been handled directly
     258   by the origin server.
     259</t>
     260<t>
     261   In an ideal world, all interactions with an HTTP cache would be
     262   semantically transparent.  However, for some resources, semantic
     263   transparency is not always necessary and can be effectively traded
     264   for the sake of bandwidth scaling, disconnected operation, and
     265   high availability.  HTTP/1.1 allows origin servers, caches,
     266   and clients to explicitly reduce transparency when necessary.
     267   However, because non-transparent operation may confuse non-expert
     268   users and might be incompatible with certain server applications
     269   (such as those for ordering merchandise), the protocol requires that
     270   transparency be relaxed
     271  <list style="symbols">
     272     <t>only by an explicit protocol-level request when relaxed by
     273        client or origin server</t>
     274
     275     <t>only with an explicit warning to the end user when relaxed by
     276        cache or client</t>
     277  </list>
     278</t>
     279<t>
     280   Therefore, HTTP/1.1 provides these important elements:
     281  <list style="numbers">
     282      <t>Protocol features that provide full semantic transparency when
     283         this is required by all parties.</t>
     284
     285      <t>Protocol features that allow an origin server or user agent to
     286         explicitly request and control non-transparent operation.</t>
     287
     288      <t>Protocol features that allow a cache to attach warnings to
     289         responses that do not preserve the requested approximation of
     290         semantic transparency.</t>
     291  </list>
     292</t>
     293<t>
     294   A basic principle is that it must be possible for the clients to
     295   detect any potential relaxation of semantic transparency.
     296  <list><t>
     297      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
     298      design decisions not explicitly discussed in this specification.
     299      If a decision might affect semantic transparency, the implementor
     300      ought to err on the side of maintaining transparency unless a
     301      careful and complete analysis shows significant benefits in
     302      breaking transparency.
     303    </t></list>
    238304</t>
    239305</section>
    … …  
    242308<t>
    243309   This specification uses a number of terms to refer to the roles
    244    played by participants in, and objects of, the HTTP communication.
    245 </t>
    246 <t>
    247   <iref item="cache"/>
    248   <x:dfn>cache</x:dfn>
    249   <list>
    250     <t>
    251       A program's local store of response messages and the subsystem
    252       that controls its message storage, retrieval, and deletion. A
    253       cache stores cacheable responses in order to reduce the response
    254       time and network bandwidth consumption on future, equivalent
    255       requests. Any client or server may include a cache, though a cache
    256       cannot be used by a server that is acting as a tunnel.
    257     </t>
    258   </list>
     310   played by participants in, and objects of, HTTP caching.
    259311</t>
    260312<t>
    … …  
    264316    <t>
    265317      A response is cacheable if a cache is allowed to store a copy of
    266       the response message for use in answering subsequent requests. The
    267       rules for determining the cacheability of HTTP responses are
    268       defined in <xref target="caching"/>. Even if a resource is cacheable, there may
     318      the response message for use in answering subsequent requests.
     319      Even if a resource is cacheable, there may
    269320      be additional constraints on whether a cache can use the cached
    270321      copy for a particular request.
    … …  
    344395</t>
    345396<t>
    346   <iref item="semantically transparent"/>
    347   <x:dfn>semantically transparent</x:dfn>
    348   <list>
    349     <t>
    350       A cache behaves in a "semantically transparent" manner, with
    351       respect to a particular response, when its use affects neither the
    352       requesting client nor the origin server, except to improve
    353       performance. When a cache is semantically transparent, the client
    354       receives exactly the same response (except for hop-by-hop headers)
    355       that it would have received had its request been handled directly
    356       by the origin server.
    357     </t>
    358   </list>
    359 </t>
    360 <t>
    361397  <iref item="validator"/>
    362398  <x:dfn>validator</x:dfn>
    … …  
    371407</section>
    372408
    373 <section title="Delta Seconds" anchor="delta.seconds">
    374 <t>
    375    Some HTTP header fields allow a time value to be specified as an
    376    integer number of seconds, represented in decimal, after the time
    377    that the message was received.
    378 </t>
    379 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
    380   delta-seconds  = 1*DIGIT
    381 </artwork></figure>
    382 </section>
    383 </section>
    384 
    385 <section title="Caching in HTTP" anchor="caching">
     409<section title="Requirements" anchor="intro.requirements">
     410<t>
     411   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
     412   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
     413   document are to be interpreted as described in <xref target="RFC2119"/>.
     414</t>
     415<t>
     416   An implementation is not compliant if it fails to satisfy one or more
     417   of the &MUST; or &REQUIRED; level requirements for the protocols it
     418   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
     419   level and all the &SHOULD; level requirements for its protocols is said
     420   to be "unconditionally compliant"; one that satisfies all the &MUST;
     421   level requirements but not all the &SHOULD; level requirements for its
     422   protocols is said to be "conditionally compliant."
     423</t>
     424</section>
     425</section>
     426
    386427<section title="Overview" anchor="caching.overview">
    387 <t>
    388    HTTP is typically used for distributed information systems, where
    389    performance can be improved by the use of response caches. The
    390    HTTP/1.1 protocol includes a number of elements intended to make
    391    caching work as well as possible. Because these elements are
    392    inextricable from other aspects of the protocol, and because they
    393    interact with each other, it is useful to describe the basic caching
    394    design of HTTP separately from the detailed descriptions of methods,
    395    headers, response codes, etc.
    396 </t>
    397 <t>
    398    Caching would be useless if it did not significantly improve
    399    performance. The goal of caching in HTTP/1.1 is to eliminate the need
    400    to send requests in many cases, and to eliminate the need to send
    401    full responses in many other cases. The former reduces the number of
    402    network round-trips required for many operations; we use an
    403    "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
    404    latter reduces network bandwidth requirements; we use a "validation"
    405    mechanism for this purpose (see <xref target="validation.model"/>).
    406 </t>
    407 <t>
    408    Requirements for performance, availability, and disconnected
    409    operation require us to be able to relax the goal of semantic
    410    transparency. The HTTP/1.1 protocol allows origin servers, caches,
    411    and clients to explicitly reduce transparency when necessary.
    412    However, because non-transparent operation may confuse non-expert
    413    users, and might be incompatible with certain server applications
    414    (such as those for ordering merchandise), the protocol requires that
    415    transparency be relaxed
    416   <list style="symbols">
    417      <t>only by an explicit protocol-level request when relaxed by
    418         client or origin server</t>
    419 
    420      <t>only with an explicit warning to the end user when relaxed by
    421         cache or client</t>
    422   </list>
    423 </t>
    424 <t>
    425    Therefore, the HTTP/1.1 protocol provides these important elements:
    426   <list style="numbers">
    427       <t>Protocol features that provide full semantic transparency when
    428          this is required by all parties.</t>
    429 
    430       <t>Protocol features that allow an origin server or user agent to
    431          explicitly request and control non-transparent operation.</t>
    432 
    433       <t>Protocol features that allow a cache to attach warnings to
    434          responses that do not preserve the requested approximation of
    435          semantic transparency.</t>
    436   </list>
    437 </t>
    438 <t>
    439    A basic principle is that it must be possible for the clients to
    440    detect any potential relaxation of semantic transparency.
    441   <list><t>
    442       <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
    443       design decisions not explicitly discussed in this specification.
    444       If a decision might affect semantic transparency, the implementor
    445       ought to err on the side of maintaining transparency unless a
    446       careful and complete analysis shows significant benefits in
    447       breaking transparency.
    448     </t></list>
    449 </t>
    450 
    451428<section title="Cache Correctness" anchor="cache.correctness">
    452429<t>
    … …  
    15321509</t>
    15331510</section>
    1534 </section>
    15351511
    15361512<section title="Header Field Definitions" anchor="header.fields">
    … …  
    15621538      seconds.
    15631539</t>
     1540<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
     1541  delta-seconds  = 1*DIGIT
     1542</artwork></figure>
    15641543<t>
    15651544      If a cache receives a value larger than the largest positive
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