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

    r1373 r1381  
    359359  }
    360360  @bottom-center {
    361        content: "Expires February 4, 2012";
     361       content: "Expires February 6, 2012";
    362362  }
    363363  @bottom-right {
     
    382382      <link rel="Index" href="#rfc.index">
    383383      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
    384       <link rel="Chapter" title="2 Resource State Metadata (Validators)" href="#rfc.section.2">
     384      <link rel="Chapter" title="2 Validators" href="#rfc.section.2">
    385385      <link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3">
    386386      <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4">
     
    404404      <meta name="dct.creator" content="Reschke, J. F.">
    405405      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest">
    406       <meta name="dct.issued" scheme="ISO8601" content="2011-08-03">
     406      <meta name="dct.issued" scheme="ISO8601" content="2011-08-05">
    407407      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
    408408      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.">
     
    430430            </tr>
    431431            <tr>
    432                <td class="left">Expires: February 4, 2012</td>
     432               <td class="left">Expires: February 6, 2012</td>
    433433               <td class="right">J. Mogul</td>
    434434            </tr>
     
    487487            <tr>
    488488               <td class="left"></td>
    489                <td class="right">August 3, 2011</td>
     489               <td class="right">August 5, 2011</td>
    490490            </tr>
    491491         </tbody>
     
    517517         in progress”.
    518518      </p>
    519       <p>This Internet-Draft will expire on February 4, 2012.</p>
     519      <p>This Internet-Draft will expire on February 6, 2012.</p>
    520520      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
    521521      <p>Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
     
    540540            </ul>
    541541         </li>
    542          <li>2.&nbsp;&nbsp;&nbsp;<a href="#resource.metadata">Resource State Metadata (Validators)</a><ul>
    543                <li>2.1&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
    544                      <li>2.1.1&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
    545                      <li>2.1.2&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
     542         <li>2.&nbsp;&nbsp;&nbsp;<a href="#validators">Validators</a><ul>
     543               <li>2.1&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
     544               <li>2.2&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
     545                     <li>2.2.1&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
     546                     <li>2.2.2&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
    546547                  </ul>
    547548               </li>
    548                <li>2.2&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
    549                      <li>2.2.1&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
    550                      <li>2.2.2&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
    551                      <li>2.2.3&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
    552                      <li>2.2.4&nbsp;&nbsp;&nbsp;<a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></li>
    553                      <li>2.2.5&nbsp;&nbsp;&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li>
     549               <li>2.3&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
     550                     <li>2.3.1&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
     551                     <li>2.3.2&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
     552                     <li>2.3.3&nbsp;&nbsp;&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li>
    554553                  </ul>
    555554               </li>
     555               <li>2.4&nbsp;&nbsp;&nbsp;<a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></li>
    556556            </ul>
    557557         </li>
     
    607607      </ul>
    608608      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
    609       <p id="rfc.section.1.p.1">This document defines the HTTP/1.1 conditional request mechanisms, including both response metadata that can be used to indicate
    610          or observe changes to resource state and request header fields that specify preconditions to be checked before performing
    611          the action given by the request method. Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem:
     609      <p id="rfc.section.1.p.1">This document defines the HTTP/1.1 conditional request mechanisms, including both metadata for indicating/observing changes
     610         in resource representations and request header fields that specify preconditions on that metadata be checked before performing
     611         the request method. Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem:
    612612         one client accidentally overwriting the work of another client that has been acting in parallel.
    613613      </p>
     
    646646</pre><div id="rfc.iref.m.1"></div>
    647647      <div id="rfc.iref.v.1"></div>
    648       <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="resource.metadata" href="#resource.metadata">Resource State Metadata (Validators)</a></h1>
     648      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="validators" href="#validators">Validators</a></h1>
    649649      <p id="rfc.section.2.p.1">This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions:
    650650         modification dates and opaque entity tags. Additional metadata that reflects resource state has been defined by various extensions
    651651         of HTTP, such as WebDAV <a href="#RFC4918" id="rfc.xref.RFC4918.1"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, that are beyond the scope of this specification. A resource metadata value is referred to as a "<dfn>validator</dfn>" when it is used within a precondition.
    652652      </p>
     653      <div id="rfc.iref.v.2"></div>
     654      <div id="rfc.iref.v.3"></div>
     655      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak versus Strong</a></h2>
     656      <p id="rfc.section.2.1.p.1">Validators come in two flavors: strong or weak. Weak validators are easy to generate but are far less useful for comparisons.
     657         Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently.
     658         Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator
     659         in use and imposes restrictions on when weak validators can be used as preconditions.
     660      </p>
     661      <p id="rfc.section.2.1.p.2">A "strong validator" is a representation metadata value that <em class="bcp14">MUST</em> be changed to a new, previously unused or guaranteed unique, value whenever a change occurs to the representation data such
     662         that a change would be observable in the payload body of a 200 response to GET. A strong validator <em class="bcp14">MAY</em> be changed for other reasons, such as when a semantically significant part of the representation metadata is changed (e.g.,
     663         Content-Type), but it is in the best interests of the origin server to only change the value when it is necessary to invalidate
     664         the stored responses held by remote caches and authoring tools. A strong validator <em class="bcp14">MUST</em> be unique across all representations of a given resource, such that no two representations of that resource share the same
     665         validator unless their payload body would be identical.
     666      </p>
     667      <p id="rfc.section.2.1.p.3">Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate
     668         an entry using a validator that it obtained in the distant past. A strong validator <em class="bcp14">MUST</em> be unique across all versions of all representations associated with a particular resource over time. However, there is no
     669         implication of uniqueness across representations of different resources (i.e., the same strong validator might be in use for
     670         representations of multiple resources at the same time and does not imply that those representations are equivalent).
     671      </p>
     672      <p id="rfc.section.2.1.p.4">There are a variety of strong validators used in practice. The best are based on strict revision control, wherein each change
     673         to a representation always results in a unique node name and revision identifier being assigned before the representation
     674         is made accessible to GET. A cryptographic hash function applied to the representation data is also sufficient if the data
     675         is available prior to the response header fields being sent and the digest does not need to be recalculated every time a validation
     676         request is received. However, if a resource has distinct representations that differ only in their metadata, such as might
     677         occur with content negotiation over media types that happen to share the same data format, then a server <em class="bcp14">SHOULD</em> incorporate additional information in the validator to distinguish those representations and avoid confusing cache behavior.
     678      </p>
     679      <p id="rfc.section.2.1.p.5">In contrast, a "weak validator" is a representation metadata value that might not be changed for every change to the representation
     680         data. This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to
     681         ensure uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations
     682         by some self-determined set of equivalency rather than unique sequences of data. A weak entity-tag <em class="bcp14">SHOULD</em> change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation.
     683         In other words, a weak entity-tag <em class="bcp14">SHOULD</em> change whenever the origin server wants caches to invalidate old responses.
     684      </p>
     685      <p id="rfc.section.2.1.p.6">For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might
     686         be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in
     687         order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server
     688         load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might
     689         be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between
     690         those modifications.
     691      </p>
     692      <p id="rfc.section.2.1.p.7">A "use" of a validator occurs when either a client generates a request and includes the validator in a precondition or when
     693         a server compares two validators. Weak validators are only usable in contexts that do not depend on exact equality of a representation's
     694         payload body. Strong validators are usable and preferred for all conditional requests, including cache validation, partial
     695         content ranges, and "lost update" avoidance.
     696      </p>
    653697      <div id="rfc.iref.l.1"></div>
    654698      <div id="rfc.iref.h.1"></div>
    655       <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2>
    656       <p id="rfc.section.2.1.p.1">The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation
     699      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2>
     700      <p id="rfc.section.2.2.p.1">The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation
    657701         was last modified.
    658702      </p>
    659703      <div id="rfc.figure.u.2"></div><pre class="inline"><span id="rfc.iref.g.1"></span>  <a href="#header.last-modified" class="smpl">Last-Modified</a> = <a href="#notation" class="smpl">HTTP-date</a>
    660 </pre><p id="rfc.section.2.1.p.3">An example of its use is</p>
     704</pre><p id="rfc.section.2.2.p.3">An example of its use is</p>
    661705      <div id="rfc.figure.u.3"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
    662 </pre><h3 id="rfc.section.2.1.1"><a href="#rfc.section.2.1.1">2.1.1</a>&nbsp;<a id="lastmod.generation" href="#lastmod.generation">Generation</a></h3>
    663       <p id="rfc.section.2.1.1.p.1">Origin servers <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined,
     706</pre><h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;<a id="lastmod.generation" href="#lastmod.generation">Generation</a></h3>
     707      <p id="rfc.section.2.2.1.p.1">Origin servers <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined,
    664708         since its use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service
    665709         scalability and reliability.
    666710      </p>
    667       <p id="rfc.section.2.1.1.p.2">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be
     711      <p id="rfc.section.2.2.1.p.2">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be
    668712         the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
    669713         detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
    670714         use its value to make conditional requests and test the validity of locally cached responses.
    671715      </p>
    672       <p id="rfc.section.2.1.1.p.3">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value
     716      <p id="rfc.section.2.2.1.p.3">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value
    673717         for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially
    674718         if the representation changes near the time that the response is generated.
    675719      </p>
    676       <p id="rfc.section.2.1.1.p.4">An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time
     720      <p id="rfc.section.2.2.1.p.4">An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time
    677721         is derived from implementation-specific metadata that evaluates to some time in the future, according to the origin server's
    678722         clock, then the origin server <em class="bcp14">MUST</em> replace that value with the message origination date. This prevents a future modification date from having an adverse impact
    679723         on cache validation.
    680724      </p>
    681       <h3 id="rfc.section.2.1.2"><a href="#rfc.section.2.1.2">2.1.2</a>&nbsp;<a id="lastmod.comparison" href="#lastmod.comparison">Comparison</a></h3>
    682       <p id="rfc.section.2.1.2.p.1">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is
     725      <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;<a id="lastmod.comparison" href="#lastmod.comparison">Comparison</a></h3>
     726      <p id="rfc.section.2.2.2.p.1">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is
    683727         strong, using the following rules:
    684728      </p>
     
    689733         </li>
    690734      </ul>
    691       <p id="rfc.section.2.1.2.p.2">or </p>
     735      <p id="rfc.section.2.2.2.p.2">or </p>
    692736      <ul>
    693737         <li>The validator is about to be used by a client in an If-Modified-Since, If-Unmodified-Since header field, because the client
     
    697741         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
    698742      </ul>
    699       <p id="rfc.section.2.1.2.p.3">or </p>
     743      <p id="rfc.section.2.2.2.p.3">or </p>
    700744      <ul>
    701745         <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
     
    705749         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
    706750      </ul>
    707       <p id="rfc.section.2.1.2.p.4">This method relies on the fact that if two different responses were sent by the origin server during the same second, but
     751      <p id="rfc.section.2.2.2.p.4">This method relies on the fact that if two different responses were sent by the origin server during the same second, but
    708752         both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified
    709753         time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from
     
    712756      <div id="rfc.iref.e.1"></div>
    713757      <div id="rfc.iref.h.2"></div>
    714       <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="header.etag" href="#header.etag">ETag</a></h2>
    715       <p id="rfc.section.2.2.p.1">The ETag header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator
     758      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="header.etag" href="#header.etag">ETag</a></h2>
     759      <p id="rfc.section.2.3.p.1">The ETag header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator
    716760         for differentiating between multiple representations of the same resource, regardless of whether those multiple representations
    717761         are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the
     
    723767  <a href="#header.etag" class="smpl">weak</a>       = %x57.2F ; "W/", case-sensitive
    724768  <a href="#header.etag" class="smpl">opaque-tag</a> = <a href="#notation" class="smpl">quoted-string</a>
    725 </pre><p id="rfc.section.2.2.p.3">An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store
     769</pre><p id="rfc.section.2.3.p.3">An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store
    726770         modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are
    727771         not consistently maintained.
     
    731775  ETag: W/"xyzzy"
    732776  ETag: ""
    733 </pre><h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;<a id="entity.tag.generation" href="#entity.tag.generation">Generation</a></h3>
    734       <p id="rfc.section.2.2.1.p.1">The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select
     777</pre><p id="rfc.section.2.3.p.5">An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag
     778         for a representation and the generation of that entity-tag does not satisfy the requirements for a strong validator (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section&nbsp;2.1</a>), then that entity-tag <em class="bcp14">MUST</em> be marked as weak by prefixing its opaque value with "W/" (case-sensitive).
     779      </p>
     780      <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a id="entity.tag.generation" href="#entity.tag.generation">Generation</a></h3>
     781      <p id="rfc.section.2.3.1.p.1">The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select
    735782         the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple
    736783         sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each
    737784         entity-tag is constructed.
    738785      </p>
    739       <p id="rfc.section.2.2.1.p.2">For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision
     786      <p id="rfc.section.2.3.1.p.2">For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision
    740787         number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations.
    741788         Other implementations might use a stored hash of representation content, a combination of various filesystem attributes, or
    742789         a modification timestamp that has sub-second resolution.
    743790      </p>
    744       <p id="rfc.section.2.2.1.p.3">Origin servers <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since
     791      <p id="rfc.section.2.3.1.p.3">Origin servers <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since
    745792         the entity-tag's use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability
    746793         and reliability.
    747794      </p>
    748       <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;<a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak versus Strong</a></h3>
    749       <p id="rfc.section.2.2.2.p.1">Since both origin servers and caches will compare two validators to decide if they indicate the same or different representations,
    750          one normally would expect that if the representation (including both representation header fields and representation body)
    751          changes in any way, then the associated validator would change as well. If this is true, then we call that validator a "strong
    752          validator". One example of a strong validator is an integer that is incremented in stable storage every time a representation
    753          is changed.
    754       </p>
    755       <p id="rfc.section.2.2.2.p.2">However, there might be cases when a server prefers to change the validator only when it desires cached representations to
    756          be invalidated. For example, the representation of a weather report that changes in content every second, based on dynamic
    757          measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) in order to
    758          allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load
    759          or weather quality). A validator that does not always change when the representation changes is a "weak validator".
    760       </p>
    761       <p id="rfc.section.2.2.2.p.3">One can think of a strong validator as part of an identifier for a specific representation, whereas a weak validator is part
    762          of an identifier for a set of equivalent representations (where this notion of equivalence is entirely governed by the origin
    763          server and beyond the scope of this specification).
    764       </p>
    765       <p id="rfc.section.2.2.2.p.4">An entity-tag is normally a strong validator, but the protocol provides a mechanism to tag an entity-tag as "weak". </p>
    766       <ul class="empty">
    767          <li>A representation's modification time, if defined with only one-second resolution, could be a weak validator, since it is possible
    768             that the representation might be modified twice during a single second.
    769          </li>
    770          <li>Support for weak validators is optional. However, weak validators allow for more efficient caching of equivalent objects;
    771             for example, a hit counter on a site is probably good enough if it is updated every few days or weeks, and any value during
    772             that period is likely "good enough" to be equivalent.
    773          </li>
    774       </ul>
    775       <p id="rfc.section.2.2.2.p.5">A strong entity-tag <em class="bcp14">MUST</em> change whenever the associated representation changes in any way. A weak entity-tag <em class="bcp14">SHOULD</em> change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation.
    776          In other words, a weak entity tag <em class="bcp14">SHOULD</em> change whenever the origin server wants caches to invalidate old responses.
    777       </p>
    778       <p id="rfc.section.2.2.2.p.6">A "strong entity-tag" <em class="bcp14">MAY</em> be shared by two representations of a resource only if they are equivalent by octet equality.
    779       </p>
    780       <p id="rfc.section.2.2.2.p.7">A "weak entity-tag", indicated by the "W/" prefix, <em class="bcp14">MAY</em> be shared by two representations of a resource. A weak entity-tag can only be used for weak comparison.
    781       </p>
    782       <p id="rfc.section.2.2.2.p.8">Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate
    783          an entry using a validator that it obtained in the distant past. A strong entity-tag <em class="bcp14">MUST</em> be unique across all versions of all representations associated with a particular resource over time. However, there is no
    784          implication of uniqueness across entity-tags of different resources (i.e., the same entity-tag value might be in use for representations
    785          of multiple resources at the same time and does not imply that those representations are equivalent).
    786       </p>
    787       <h3 id="rfc.section.2.2.3"><a href="#rfc.section.2.2.3">2.2.3</a>&nbsp;<a id="entity.tag.comparison" href="#entity.tag.comparison">Comparison</a></h3>
    788       <p id="rfc.section.2.2.3.p.1">There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators
     795      <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a id="entity.tag.comparison" href="#entity.tag.comparison">Comparison</a></h3>
     796      <p id="rfc.section.2.3.2.p.1">There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators
    789797         or not:
    790798      </p>
     
    795803         </li>
    796804      </ul>
    797       <p id="rfc.section.2.2.3.p.2">A "use" of a validator is either when a client generates a request and includes the validator in a precondition, or when a
    798          server compares two validators.
    799       </p>
    800       <p id="rfc.section.2.2.3.p.3">Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality
    801          of a representation. For example, either kind is usable for a normal conditional GET.
    802       </p>
    803       <p id="rfc.section.2.2.3.p.4">The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:</p>
     805      <p id="rfc.section.2.3.2.p.2">The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:</p>
    804806      <div id="rfc.table.u.1">
    805807         <table class="tt full left" cellpadding="3" cellspacing="0">
     
    840842         </table>
    841843      </div>
    842       <p id="rfc.section.2.2.3.p.5">An entity-tag is strong unless it is explicitly tagged as weak.</p>
    843       <h3 id="rfc.section.2.2.4"><a href="#rfc.section.2.2.4">2.2.4</a>&nbsp;<a id="rules.for.when.to.use.entity.tags.and.last-modified.dates" href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></h3>
    844       <p id="rfc.section.2.2.4.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types
    845          ought to be used, and for what purposes.
    846       </p>
    847       <p id="rfc.section.2.2.4.p.2">HTTP/1.1 origin servers: </p>
    848       <ul>
    849          <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
    850          </li>
    851          <li><em class="bcp14">MAY</em> send a weak entity-tag instead of a strong entity-tag, if performance considerations support the use of weak entity-tags,
    852             or if it is unfeasible to send a strong entity-tag.
    853          </li>
    854          <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one.
    855          </li>
    856       </ul>
    857       <p id="rfc.section.2.2.4.p.3">In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity-tag and a Last-Modified
    858          value.
    859       </p>
    860       <p id="rfc.section.2.2.4.p.4">HTTP/1.1 clients: </p>
    861       <ul>
    862          <li><em class="bcp14">MUST</em> use that entity-tag in any cache-conditional request (using If-Match or If-None-Match) if an entity-tag has been provided
    863             by the origin server.
    864          </li>
    865          <li><em class="bcp14">SHOULD</em> use the Last-Modified value in non-subrange cache-conditional requests (using If-Modified-Since) if only a Last-Modified value
    866             has been provided by the origin server.
    867          </li>
    868          <li><em class="bcp14">MAY</em> use the Last-Modified value in subrange cache-conditional requests (using If-Unmodified-Since) if only a Last-Modified value
    869             has been provided by an HTTP/1.0 origin server. The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty.
    870          </li>
    871          <li><em class="bcp14">SHOULD</em> use both validators in cache-conditional requests if both an entity-tag and a Last-Modified value have been provided by the
    872             origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
    873          </li>
    874       </ul>
    875       <p id="rfc.section.2.2.4.p.5">An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since
    876          or If-Unmodified-Since header field) and one or more entity-tags (e.g., in an If-Match, If-None-Match, or If-Range header
    877          field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status code of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields
    878          in the request.
    879       </p>
    880       <p id="rfc.section.2.2.4.p.6">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags
    881          as cache validators, <em class="bcp14">MUST NOT</em> return a locally cached response to the client unless that cached response is consistent with all of the conditional header
    882          fields in the request.
    883       </p>
    884       <ul class="empty">
    885          <li> <b>Note:</b> The general principle behind these rules is that HTTP/1.1 servers and clients ought to transmit as much non-redundant information
    886             as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative
    887             assumptions about the validators they receive.
    888          </li>
    889          <li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will
    890             support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare
    891             cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then
    892             HTTP/1.1 origin servers should not provide one.
    893          </li>
    894       </ul>
    895       <h3 id="rfc.section.2.2.5"><a href="#rfc.section.2.2.5">2.2.5</a>&nbsp;<a id="example.entity.tag.vs.conneg" href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h3>
    896       <p id="rfc.section.2.2.5.p.1">Consider a resource that is subject to content negotiation (<a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 5</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>), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (<a href="p3-payload.html#header.accept-encoding" title="Accept-Encoding">Section 6.3</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>):
     844      <h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a id="example.entity.tag.vs.conneg" href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h3>
     845      <p id="rfc.section.2.3.3.p.1">Consider a resource that is subject to content negotiation (<a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 5</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>), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (<a href="p3-payload.html#header.accept-encoding" title="Accept-Encoding">Section 6.3</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>):
    897846      </p>
    898847      <div id="rfc.figure.u.6"></div>
     
    901850Accept-Encoding: gzip
    902851
    903 </pre><p id="rfc.section.2.2.5.p.3">In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:</p>
     852</pre><p id="rfc.section.2.3.3.p.3">In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:</p>
    904853      <div id="rfc.figure.u.7"></div>
    905854      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
     
    915864Hello World!
    916865Hello World!
    917 </span></pre><p id="rfc.section.2.2.5.p.5">An alternative representation that does use gzip content coding would be:</p>
     866</span></pre><p id="rfc.section.2.3.3.p.5">An alternative representation that does use gzip content coding would be:</p>
    918867      <div id="rfc.figure.u.8"></div>
    919868      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
     
    925874Content-Encoding: gzip
    926875
    927 <em>...binary data...</em></pre><div class="note" id="rfc.section.2.2.5.p.7">
     876<em>...binary data...</em></pre><div class="note" id="rfc.section.2.3.3.p.7">
    928877         <p> <b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation must be distinct
    929878            from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings
     
    931880         </p>
    932881      </div>
     882      <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="rules.for.when.to.use.entity.tags.and.last-modified.dates" href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></h2>
     883      <p id="rfc.section.2.4.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types
     884         ought to be used, and for what purposes.
     885      </p>
     886      <p id="rfc.section.2.4.p.2">HTTP/1.1 origin servers: </p>
     887      <ul>
     888         <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
     889         </li>
     890         <li><em class="bcp14">MAY</em> send a weak entity-tag instead of a strong entity-tag, if performance considerations support the use of weak entity-tags,
     891            or if it is unfeasible to send a strong entity-tag.
     892         </li>
     893         <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one.
     894         </li>
     895      </ul>
     896      <p id="rfc.section.2.4.p.3">In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity-tag and a Last-Modified
     897         value.
     898      </p>
     899      <p id="rfc.section.2.4.p.4">HTTP/1.1 clients: </p>
     900      <ul>
     901         <li><em class="bcp14">MUST</em> use that entity-tag in any cache-conditional request (using If-Match or If-None-Match) if an entity-tag has been provided
     902            by the origin server.
     903         </li>
     904         <li><em class="bcp14">SHOULD</em> use the Last-Modified value in non-subrange cache-conditional requests (using If-Modified-Since) if only a Last-Modified value
     905            has been provided by the origin server.
     906         </li>
     907         <li><em class="bcp14">MAY</em> use the Last-Modified value in subrange cache-conditional requests (using If-Unmodified-Since) if only a Last-Modified value
     908            has been provided by an HTTP/1.0 origin server. The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty.
     909         </li>
     910         <li><em class="bcp14">SHOULD</em> use both validators in cache-conditional requests if both an entity-tag and a Last-Modified value have been provided by the
     911            origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
     912         </li>
     913      </ul>
     914      <p id="rfc.section.2.4.p.5">An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since
     915         or If-Unmodified-Since header field) and one or more entity-tags (e.g., in an If-Match, If-None-Match, or If-Range header
     916         field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status code of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields
     917         in the request.
     918      </p>
     919      <p id="rfc.section.2.4.p.6">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags
     920         as cache validators, <em class="bcp14">MUST NOT</em> return a locally cached response to the client unless that cached response is consistent with all of the conditional header
     921         fields in the request.
     922      </p>
     923      <ul class="empty">
     924         <li> <b>Note:</b> The general principle behind these rules is that HTTP/1.1 servers and clients ought to transmit as much non-redundant information
     925            as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative
     926            assumptions about the validators they receive.
     927         </li>
     928         <li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will
     929            support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare
     930            cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then
     931            HTTP/1.1 origin servers should not provide one.
     932         </li>
     933      </ul>
    933934      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="header.fields" href="#header.fields">Precondition Header Fields</a></h1>
    934935      <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests.</p>
     
    942943      </p>
    943944      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.6"></span>  <a href="#header.if-match" class="smpl">If-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
    944 </pre><p id="rfc.section.3.1.p.3">If any of the entity-tags listed in the If-Match field value match (as per <a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.2.3</a>) the entity-tag of the selected representation for the target resource, or if "*" is given and any current representation
     945</pre><p id="rfc.section.3.1.p.3">If any of the entity-tags listed in the If-Match field value match (as per <a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.3.2</a>) the entity-tag of the selected representation for the target resource, or if "*" is given and any current representation
    945946         exists for the target resource, then the server <em class="bcp14">MAY</em> perform the request method as if the If-Match header field was not present.
    946947      </p>
     
    972973      </p>
    973974      <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
    974 </pre><p id="rfc.section.3.2.p.4">If any of the entity-tags listed in the If-None-Match field-value match (as per <a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.2.3</a>) the entity-tag of the selected representation, or if "*" is given and any current representation exists for that resource,
     975</pre><p id="rfc.section.3.2.p.4">If any of the entity-tags listed in the If-None-Match field-value match (as per <a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.3.2</a>) the entity-tag of the selected representation, or if "*" is given and any current representation exists for that resource,
    975976         then the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, if the request method was GET or HEAD, the server <em class="bcp14">SHOULD</em> respond with a 304 (Not Modified) status code, including the cache-related header fields (particularly ETag) of the selected
    976977         representation that has a matching entity-tag. For all other request methods, the server <em class="bcp14">MUST</em> respond with a 412 (Precondition Failed) status code.
     
    979980      </p>
    980981      <p id="rfc.section.3.2.p.6">If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status code, then
    981          the If-None-Match header field <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;2.2.4</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)
     982         the If-None-Match header field <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;2.4</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)
    982983      </p>
    983984      <p id="rfc.section.3.2.p.7">Examples:</p>
     
    10601061      </p>
    10611062      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="status.code.definitions" href="#status.code.definitions">Status Code Definitions</a></h1>
    1062       <div id="rfc.iref.24"></div>
     1063      <div id="rfc.iref.26"></div>
    10631064      <div id="rfc.iref.s.2"></div>
    10641065      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="status.304" href="#status.304">304 Not Modified</a></h2>
     
    10821083      <p id="rfc.section.4.1.p.5">If a cache uses a received 304 response to update a cache entry, the cache <em class="bcp14">MUST</em> update the entry to reflect any new field values given in the response.
    10831084      </p>
    1084       <div id="rfc.iref.25"></div>
     1085      <div id="rfc.iref.27"></div>
    10851086      <div id="rfc.iref.s.3"></div>
    10861087      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="status.412" href="#status.412">412 Precondition Failed</a></h2>
     
    11381139                  <td class="left">http</td>
    11391140                  <td class="left">standard</td>
    1140                   <td class="left"> <a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;2.2</a>
     1141                  <td class="left"> <a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;2.3</a>
    11411142                  </td>
    11421143               </tr>
     
    11731174                  <td class="left">http</td>
    11741175                  <td class="left">standard</td>
    1175                   <td class="left"> <a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;2.1</a>
     1176                  <td class="left"> <a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;2.2</a>
    11761177                  </td>
    11771178               </tr>
     
    12601261      </div>
    12611262      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
    1262       <p id="rfc.section.A.p.1">Allow weak entity-tags in all requests except range requests (Sections <a href="#weak.and.strong.validators" title="Weak versus Strong">2.2.2</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>).
     1263      <p id="rfc.section.A.p.1">Allow weak entity-tags in all requests except range requests (Sections <a href="#weak.and.strong.validators" title="Weak versus Strong">2.1</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>).
    12631264      </p>
    12641265      <p id="rfc.section.A.p.2">Change ABNF productions for header fields to only define the field value. (<a href="#header.fields" title="Precondition Header Fields">Section&nbsp;3</a>)
     
    14141415         <ul class="ind">
    14151416            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
    1416                   <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.24"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li>
     1417                  <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.26"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li>
    14171418               </ul>
    14181419            </li>
    14191420            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
    1420                   <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.25"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li>
     1421                  <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.27"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li>
    14211422               </ul>
    14221423            </li>
    14231424            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
    1424                   <li>ETag header field&nbsp;&nbsp;<a href="#rfc.iref.e.1"><b>2.2</b></a>, <a href="#rfc.xref.header.etag.1">5.2</a></li>
     1425                  <li>ETag header field&nbsp;&nbsp;<a href="#rfc.iref.e.1"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.1">5.2</a></li>
    14251426               </ul>
    14261427            </li>
     
    14281429                  <li><tt>Grammar</tt>&nbsp;&nbsp;
    14291430                     <ul>
    1430                         <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2.2</b></a></li>
    1431                         <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2.2</b></a></li>
     1431                        <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2.3</b></a></li>
     1432                        <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2.3</b></a></li>
    14321433                        <li><tt>If-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>3.1</b></a></li>
    14331434                        <li><tt>If-Modified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>3.3</b></a></li>
    14341435                        <li><tt>If-None-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>3.2</b></a></li>
    14351436                        <li><tt>If-Unmodified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>3.4</b></a></li>
    1436                         <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2.1</b></a></li>
    1437                         <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>2.2</b></a></li>
    1438                         <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>2.2</b></a></li>
     1437                        <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2.2</b></a></li>
     1438                        <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>2.3</b></a></li>
     1439                        <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>2.3</b></a></li>
    14391440                     </ul>
    14401441                  </li>
     
    14441445                  <li>Header Fields&nbsp;&nbsp;
    14451446                     <ul>
    1446                         <li>ETag&nbsp;&nbsp;<a href="#rfc.iref.h.2"><b>2.2</b></a>, <a href="#rfc.xref.header.etag.1">5.2</a></li>
     1447                        <li>ETag&nbsp;&nbsp;<a href="#rfc.iref.h.2"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.1">5.2</a></li>
    14471448                        <li>If-Match&nbsp;&nbsp;<a href="#rfc.iref.h.3"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">5.2</a></li>
    14481449                        <li>If-Modified-Since&nbsp;&nbsp;<a href="#rfc.iref.h.5"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">5.2</a></li>
    14491450                        <li>If-None-Match&nbsp;&nbsp;<a href="#rfc.iref.h.4"><b>3.2</b></a>, <a href="#rfc.xref.header.if-none-match.1">5.2</a>, <a href="#rfc.xref.header.if-none-match.2">A</a></li>
    14501451                        <li>If-Unmodified-Since&nbsp;&nbsp;<a href="#rfc.iref.h.6"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">5.2</a></li>
    1451                         <li>Last-Modified&nbsp;&nbsp;<a href="#rfc.iref.h.1"><b>2.1</b></a>, <a href="#rfc.xref.header.last-modified.1">5.2</a></li>
     1452                        <li>Last-Modified&nbsp;&nbsp;<a href="#rfc.iref.h.1"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.1">5.2</a></li>
    14521453                     </ul>
    14531454                  </li>
     
    14621463            </li>
    14631464            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
    1464                   <li>Last-Modified header field&nbsp;&nbsp;<a href="#rfc.iref.l.1"><b>2.1</b></a>, <a href="#rfc.xref.header.last-modified.1">5.2</a></li>
     1465                  <li>Last-Modified header field&nbsp;&nbsp;<a href="#rfc.iref.l.1"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.1">5.2</a></li>
    14651466               </ul>
    14661467            </li>
     
    14701471            </li>
    14711472            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
    1472                   <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.2</a>, <a href="#rfc.xref.Part1.2">1.2</a>, <a href="#rfc.xref.Part1.3">1.2</a>, <a href="#rfc.xref.Part1.4">1.2</a>, <a href="#rfc.xref.Part1.5">2.2.5</a>, <a href="#rfc.xref.Part1.6">4.1</a>, <a href="#rfc.xref.Part1.7">4.1</a>, <a href="#rfc.xref.Part1.8">6</a>, <a href="#rfc.xref.Part1.9">7</a>, <a href="#Part1"><b>8.1</b></a><ul>
     1473                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.2</a>, <a href="#rfc.xref.Part1.2">1.2</a>, <a href="#rfc.xref.Part1.3">1.2</a>, <a href="#rfc.xref.Part1.4">1.2</a>, <a href="#rfc.xref.Part1.5">2.3.3</a>, <a href="#rfc.xref.Part1.6">4.1</a>, <a href="#rfc.xref.Part1.7">4.1</a>, <a href="#rfc.xref.Part1.8">6</a>, <a href="#rfc.xref.Part1.9">7</a>, <a href="#Part1"><b>8.1</b></a><ul>
    14731474                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.2</a></li>
    14741475                        <li><em>Section 1.2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.2</a>, <a href="#rfc.xref.Part1.3">1.2</a></li>
    14751476                        <li><em>Section 6.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">1.2</a></li>
    1476                         <li><em>Section 6.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">2.2.5</a></li>
     1477                        <li><em>Section 6.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">2.3.3</a></li>
    14771478                        <li><em>Section 9.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.6">4.1</a></li>
    14781479                        <li><em>Section 9.3.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.7">4.1</a></li>
     
    14801481                     </ul>
    14811482                  </li>
    1482                   <li><em>Part3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.2.5</a>, <a href="#rfc.xref.Part3.2">2.2.5</a>, <a href="#Part3"><b>8.1</b></a><ul>
    1483                         <li><em>Section 5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.2.5</a></li>
    1484                         <li><em>Section 6.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.2">2.2.5</a></li>
     1483                  <li><em>Part3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.3.3</a>, <a href="#rfc.xref.Part3.2">2.3.3</a>, <a href="#Part3"><b>8.1</b></a><ul>
     1484                        <li><em>Section 5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.3.3</a></li>
     1485                        <li><em>Section 6.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.2">2.3.3</a></li>
    14851486                     </ul>
    14861487                  </li>
     
    14901491                     </ul>
    14911492                  </li>
    1492                   <li><em>Part6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.1">1</a>, <a href="#rfc.xref.Part6.2">2.1.1</a>, <a href="#rfc.xref.Part6.3">2.2.1</a>, <a href="#Part6"><b>8.1</b></a></li>
     1493                  <li><em>Part6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.1">1</a>, <a href="#rfc.xref.Part6.2">2.2.1</a>, <a href="#rfc.xref.Part6.3">2.3.1</a>, <a href="#Part6"><b>8.1</b></a></li>
    14931494               </ul>
    14941495            </li>
     
    15151516            </li>
    15161517            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
    1517                   <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1"><b>2</b></a></li>
     1518                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1"><b>2</b></a><ul>
     1519                        <li>strong&nbsp;&nbsp;<a href="#rfc.iref.v.3"><b>2.1</b></a></li>
     1520                        <li>weak&nbsp;&nbsp;<a href="#rfc.iref.v.2"><b>2.1</b></a></li>
     1521                     </ul>
     1522                  </li>
    15181523               </ul>
    15191524            </li>
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