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Changeset 2106 for draft-ietf-httpbis/latest
- Timestamp:
- 11/01/13 06:03:59 (10 years ago)
- Location:
- draft-ietf-httpbis/latest
- Files:
-
- 2 edited
-
p2-semantics.html (modified) (11 diffs)
-
p2-semantics.xml (modified) (4 diffs)
Legend:
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- Added
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draft-ietf-httpbis/latest/p2-semantics.html
r2105 r2106 725 725 <li><a href="#rfc.section.7.1.2">7.1.2</a> <a href="#header.location">Location</a></li> 726 726 <li><a href="#rfc.section.7.1.3">7.1.3</a> <a href="#header.retry-after">Retry-After</a></li> 727 <li><a href="#rfc.section.7.1.4">7.1.4</a> <a href="#header.vary">Vary</a></li> 727 728 </ul> 728 729 </li> 729 <li><a href="#rfc.section.7.2">7.2</a> <a href="#selected.representation">Selected Representation Header Fields</a><ul> 730 <li><a href="#rfc.section.7.2.1">7.2.1</a> <a href="#header.vary">Vary</a></li> 731 </ul> 732 </li> 730 <li><a href="#rfc.section.7.2">7.2</a> <a href="#selected.representation">Selected Representation Header Fields</a></li> 733 731 <li><a href="#rfc.section.7.3">7.3</a> <a href="#response.auth">Authentication Challenges</a></li> 734 732 <li><a href="#rfc.section.7.4">7.4</a> <a href="#response.context">Response Context</a><ul> … … 1224 1222 the user agent's preferences is better than sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response. 1225 1223 </p> 1226 <p id="rfc.section.3.4.1.p.5">An origin server <em class="bcp14">MAY</em> generate a <a href="#header.vary" class="smpl">Vary</a> header field (<a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section 7. 2.1</a>) in responses that are subject to proactive negotiation to indicate what parameters of request information might be used1224 <p id="rfc.section.3.4.1.p.5">An origin server <em class="bcp14">MAY</em> generate a <a href="#header.vary" class="smpl">Vary</a> header field (<a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section 7.1.4</a>) in responses that are subject to proactive negotiation to indicate what parameters of request information might be used 1227 1225 in its selection algorithm, thereby providing a means for recipients to determine the reusability of that same response for 1228 1226 user agents with differing request information. … … 2447 2445 primary resource created by the request is identified by either a <a href="#header.location" class="smpl">Location</a> header field in the response or, if no <a href="#header.location" class="smpl">Location</a> field is received, by the effective request URI. 2448 2446 </p> 2449 <p id="rfc.section.6.3.2.p.2">The 201 response payload typically describes and links to the resource(s) created. See <a href="#selected.representation" title="Selected Representation Header Fields">Section 7.2</a> for discussion of the meaning and purpose of selected representation header fields, such as <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> and Last-Modified, in a 201 response.2447 <p id="rfc.section.6.3.2.p.2">The 201 response payload typically describes and links to the resource(s) created. See <a href="#selected.representation" title="Selected Representation Header Fields">Section 7.2</a> for discussion of the meaning and purpose of selected representation header fields, such as <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> and <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a>, in a 201 response. 2450 2448 </p> 2451 2449 <div id="rfc.iref.72"></div> … … 2824 2822 <td class="left">Retry-After</td> 2825 2823 <td class="left"><a href="#header.retry-after" id="rfc.xref.header.retry-after.2" title="Retry-After">Section 7.1.3</a></td> 2824 </tr> 2825 <tr> 2826 <td class="left">Vary</td> 2827 <td class="left"><a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section 7.1.4</a></td> 2826 2828 </tr> 2827 2829 <tr> … … 2997 2999 Retry-After: 120 2998 3000 </pre><p id="rfc.section.7.1.3.p.8">In the latter example, the delay is 2 minutes.</p> 3001 <div id="rfc.iref.v.1"></div> 3002 <h3 id="rfc.section.7.1.4"><a href="#rfc.section.7.1.4">7.1.4</a> <a id="header.vary" href="#header.vary">Vary</a></h3> 3003 <p id="rfc.section.7.1.4.p.1">The "Vary" header field describes what parts of a request message, aside from the method and request target, might influence 3004 the origin server's process for selecting and representing the response. The value consists of either a single asterisk ("*") 3005 or a list of header field names (case-insensitive). 3006 </p> 3007 <div id="rfc.figure.u.57"></div><pre class="inline"><span id="rfc.iref.g.59"></span> <a href="#header.vary" class="smpl">Vary</a> = "*" / 1#<a href="#imported.abnf" class="smpl">field-name</a> 3008 </pre><p id="rfc.section.7.1.4.p.3">A Vary field value of "*" signals that anything about the request might play a role in selecting the response representation, 3009 possibly including elements outside the message syntax (e.g., the client's network address), and thus a recipient will not 3010 be able to determine whether this response is appropriate for a later request without forwarding the request to the origin 3011 server. A proxy <em class="bcp14">MUST NOT</em> generate a Vary field with a "*" value. 3012 </p> 3013 <p id="rfc.section.7.1.4.p.4">A Vary field value consisting of a comma-separated list of names indicates that the named request header fields, known as 3014 the selecting header fields, might have a role in selecting the representation. The potential selecting header fields are 3015 not limited to those defined by this specification. 3016 </p> 3017 <div id="rfc.figure.u.58"></div> 3018 <p>For example, a response that contains</p><pre class="text"> Vary: accept-encoding, accept-language 3019 </pre><p>indicates that the origin server might have used the request's <a href="#header.accept-encoding" class="smpl">Accept-Encoding</a> and <a href="#header.accept-language" class="smpl">Accept-Language</a> fields (or lack thereof) as determining factors while choosing this <a href="#selected.representation" class="smpl">selected representation</a>. 3020 </p> 3021 <p id="rfc.section.7.1.4.p.6">An origin server might send Vary with a list of fields for two purposes: </p> 3022 <ol> 3023 <li> 3024 <p>To inform cache recipients that they <em class="bcp14">MUST NOT</em> use this response to satisfy a later request unless the later request has the same values for the listed fields as the original 3025 request (<a href="p6-cache.html#caching.negotiated.responses" title="Using Negotiated Responses">Section 4.3</a> of <a href="#Part6" id="rfc.xref.Part6.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). In other words, Vary expands the cache key required to match a new request to the stored cache entry. 3026 </p> 3027 </li> 3028 <li> 3029 <p>To inform user agent recipients that this response is subject to content negotiation (<a href="#request.conneg" title="Content Negotiation">Section 5.3</a>) and that a different representation might be sent in a subsequent request if additional parameters are provided in the listed 3030 header fields (<a href="#proactive.negotiation" class="smpl">proactive negotiation</a>). 3031 </p> 3032 </li> 3033 </ol> 3034 <p id="rfc.section.7.1.4.p.7">An origin server <em class="bcp14">SHOULD</em> send a Vary header field when its algorithm for selecting a representation varies based on aspects of the request message 3035 other than the method and request target, unless the variance cannot be crossed or the origin server has been deliberately 3036 configured to prevent cache transparency. For example, there is no need to send the Authorization field name (<a href="p7-auth.html#header.authorization" title="Authorization">Section 4.1</a> of <a href="#Part7" id="rfc.xref.Part7.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a>) in Vary because reuse across users is constrained by the field definition. Likewise, Cache-Control directives (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 7.2</a> of <a href="#Part6" id="rfc.xref.Part6.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>) might be used to supplant the need for Vary, particularly when the variance is considered less significant than the performance 3037 cost of Vary's impact on caching. 3038 </p> 2999 3039 <div id="rfc.iref.s.7"></div> 3000 3040 <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a> <a id="selected.representation" href="#selected.representation">Selected Representation Header Fields</a></h2> … … 3024 3064 <td class="left"><a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 2.2</a> of <a href="#Part4" id="rfc.xref.Part4.12"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td> 3025 3065 </tr> 3026 <tr>3027 <td class="left">Vary</td>3028 <td class="left"><a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section 7.2.1</a></td>3029 </tr>3030 3066 </tbody> 3031 3067 </table> 3032 3068 </div> 3033 <div id="rfc.iref.v.1"></div>3034 <h3 id="rfc.section.7.2.1"><a href="#rfc.section.7.2.1">7.2.1</a> <a id="header.vary" href="#header.vary">Vary</a></h3>3035 <p id="rfc.section.7.2.1.p.1">The "Vary" header field describes what parts of a request message, aside from the method and request target, might influence3036 the origin server's process for selecting and representing the response. The value consists of either a single asterisk ("*")3037 or a list of header field names (case-insensitive).3038 </p>3039 <div id="rfc.figure.u.57"></div><pre class="inline"><span id="rfc.iref.g.59"></span> <a href="#header.vary" class="smpl">Vary</a> = "*" / 1#<a href="#imported.abnf" class="smpl">field-name</a>3040 </pre><p id="rfc.section.7.2.1.p.3">A Vary field value of "*" signals that anything about the request might play a role in selecting the response representation,3041 possibly including elements outside the message syntax (e.g., the client's network address), and thus a recipient will not3042 be able to determine whether this response is appropriate for a later request without forwarding the request to the origin3043 server. A proxy <em class="bcp14">MUST NOT</em> generate a Vary field with a "*" value.3044 </p>3045 <p id="rfc.section.7.2.1.p.4">A Vary field value consisting of a comma-separated list of names indicates that the named request header fields, known as3046 the selecting header fields, might have a role in selecting the representation. The potential selecting header fields are3047 not limited to those defined by this specification.3048 </p>3049 <div id="rfc.figure.u.58"></div>3050 <p>For example, a response that contains</p><pre class="text"> Vary: accept-encoding, accept-language3051 </pre><p>indicates that the origin server might have used the request's <a href="#header.accept-encoding" class="smpl">Accept-Encoding</a> and <a href="#header.accept-language" class="smpl">Accept-Language</a> fields (or lack thereof) as determining factors while choosing this <a href="#selected.representation" class="smpl">selected representation</a>.3052 </p>3053 <p id="rfc.section.7.2.1.p.6">An origin server might send Vary with a list of fields for two purposes: </p>3054 <ol>3055 <li>3056 <p>To inform cache recipients that they <em class="bcp14">MUST NOT</em> use this response to satisfy a later request unless the later request has the same values for the listed fields as the original3057 request (<a href="p6-cache.html#caching.negotiated.responses" title="Using Negotiated Responses">Section 4.3</a> of <a href="#Part6" id="rfc.xref.Part6.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). In other words, Vary expands the cache key required to match a new request to the stored cache entry.3058 </p>3059 </li>3060 <li>3061 <p>To inform user agent recipients that this response is subject to content negotiation (<a href="#request.conneg" title="Content Negotiation">Section 5.3</a>) and that a different representation might be sent in a subsequent request if additional parameters are provided in the listed3062 header fields (<a href="#proactive.negotiation" class="smpl">proactive negotiation</a>).3063 </p>3064 </li>3065 </ol>3066 <p id="rfc.section.7.2.1.p.7">An origin server <em class="bcp14">SHOULD</em> send a Vary header field when its algorithm for selecting a representation varies based on aspects of the request message3067 other than the method and request target, unless the variance cannot be crossed or the origin server has been deliberately3068 configured to prevent cache transparency. For example, there is no need to send the Authorization field name (<a href="p7-auth.html#header.authorization" title="Authorization">Section 4.1</a> of <a href="#Part7" id="rfc.xref.Part7.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a>) in Vary because reuse across users is constrained by the field definition. Likewise, Cache-Control directives (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 7.2</a> of <a href="#Part6" id="rfc.xref.Part6.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>) might be used to supplant the need for Vary, particularly when the variance is considered less significant than the performance3069 cost of Vary's impact on caching.3070 </p>3071 3069 <h2 id="rfc.section.7.3"><a href="#rfc.section.7.3">7.3</a> <a id="response.auth" href="#response.auth">Authentication Challenges</a></h2> 3072 3070 <p id="rfc.section.7.3.p.1">Authentication challenges indicate what mechanisms are available for the client to provide authentication credentials in future … … 3740 3738 <td class="left">http</td> 3741 3739 <td class="left">standard</td> 3742 <td class="left"> <a href="#header.vary" id="rfc.xref.header.vary.3" title="Vary">Section 7. 2.1</a>3740 <td class="left"> <a href="#header.vary" id="rfc.xref.header.vary.3" title="Vary">Section 7.1.4</a> 3743 3741 </td> 3744 3742 </tr> … … 4558 4556 <li><tt>User-Agent</tt> <a href="#rfc.iref.g.36"><b>5.5.3</b></a></li> 4559 4557 <li><tt>value</tt> <a href="#rfc.iref.g.6"><b>3.1.1.1</b></a></li> 4560 <li><tt>Vary</tt> <a href="#rfc.iref.g.59"><b>7. 2.1</b></a></li>4558 <li><tt>Vary</tt> <a href="#rfc.iref.g.59"><b>7.1.4</b></a></li> 4561 4559 <li><tt>weight</tt> <a href="#rfc.iref.g.23"><b>5.3.1</b></a></li> 4562 4560 <li><tt>year</tt> <a href="#rfc.iref.g.50"><b>7.1.1.1</b></a></li> … … 4641 4639 </ul> 4642 4640 </li> 4643 <li><em>Part6</em> <a href="#rfc.xref.Part6.1">4.2.3</a>, <a href="#rfc.xref.Part6.2">4.3.1</a>, <a href="#rfc.xref.Part6.3">4.3.2</a>, <a href="#rfc.xref.Part6.4">4.3.2</a>, <a href="#rfc.xref.Part6.5">4.3.3</a>, <a href="#rfc.xref.Part6.6">4.3.4</a>, <a href="#rfc.xref.Part6.7">4.3.5</a>, <a href="#rfc.xref.Part6.8">5.1</a>, <a href="#rfc.xref.Part6.9">5.1</a>, <a href="#rfc.xref.Part6.10">6.3.1</a>, <a href="#rfc.xref.Part6.11">6.3.4</a>, <a href="#rfc.xref.Part6.12">6.4.1</a>, <a href="#rfc.xref.Part6.13">6.4.2</a>, <a href="#rfc.xref.Part6.14">6.5.9</a>, <a href="#rfc.xref.Part6.15">7.1</a>, <a href="#rfc.xref.Part6.16">7.1</a>, <a href="#rfc.xref.Part6.17">7.1</a>, <a href="#rfc.xref.Part6.18">7.1</a>, <a href="#rfc.xref.Part6.19">7. 2.1</a>, <a href="#rfc.xref.Part6.20">7.2.1</a>, <a href="#rfc.xref.Part6.21">8.2.2</a>, <a href="#rfc.xref.Part6.22">8.3.1</a>, <a href="#Part6"><b>11.1</b></a><ul>4641 <li><em>Part6</em> <a href="#rfc.xref.Part6.1">4.2.3</a>, <a href="#rfc.xref.Part6.2">4.3.1</a>, <a href="#rfc.xref.Part6.3">4.3.2</a>, <a href="#rfc.xref.Part6.4">4.3.2</a>, <a href="#rfc.xref.Part6.5">4.3.3</a>, <a href="#rfc.xref.Part6.6">4.3.4</a>, <a href="#rfc.xref.Part6.7">4.3.5</a>, <a href="#rfc.xref.Part6.8">5.1</a>, <a href="#rfc.xref.Part6.9">5.1</a>, <a href="#rfc.xref.Part6.10">6.3.1</a>, <a href="#rfc.xref.Part6.11">6.3.4</a>, <a href="#rfc.xref.Part6.12">6.4.1</a>, <a href="#rfc.xref.Part6.13">6.4.2</a>, <a href="#rfc.xref.Part6.14">6.5.9</a>, <a href="#rfc.xref.Part6.15">7.1</a>, <a href="#rfc.xref.Part6.16">7.1</a>, <a href="#rfc.xref.Part6.17">7.1</a>, <a href="#rfc.xref.Part6.18">7.1</a>, <a href="#rfc.xref.Part6.19">7.1.4</a>, <a href="#rfc.xref.Part6.20">7.1.4</a>, <a href="#rfc.xref.Part6.21">8.2.2</a>, <a href="#rfc.xref.Part6.22">8.3.1</a>, <a href="#Part6"><b>11.1</b></a><ul> 4644 4642 <li><em>Section 4.1.1</em> <a href="#rfc.xref.Part6.5">4.3.3</a></li> 4645 4643 <li><em>Section 4.1.2</em> <a href="#rfc.xref.Part6.10">6.3.1</a>, <a href="#rfc.xref.Part6.12">6.4.1</a>, <a href="#rfc.xref.Part6.13">6.4.2</a>, <a href="#rfc.xref.Part6.14">6.5.9</a></li> 4646 <li><em>Section 4.3</em> <a href="#rfc.xref.Part6.19">7. 2.1</a></li>4644 <li><em>Section 4.3</em> <a href="#rfc.xref.Part6.19">7.1.4</a></li> 4647 4645 <li><em>Section 5</em> <a href="#rfc.xref.Part6.4">4.3.2</a></li> 4648 4646 <li><em>Section 6</em> <a href="#rfc.xref.Part6.6">4.3.4</a>, <a href="#rfc.xref.Part6.7">4.3.5</a></li> 4649 4647 <li><em>Section 7.1</em> <a href="#rfc.xref.Part6.15">7.1</a></li> 4650 <li><em>Section 7.2</em> <a href="#rfc.xref.Part6.2">4.3.1</a>, <a href="#rfc.xref.Part6.3">4.3.2</a>, <a href="#rfc.xref.Part6.8">5.1</a>, <a href="#rfc.xref.Part6.16">7.1</a>, <a href="#rfc.xref.Part6.20">7. 2.1</a></li>4648 <li><em>Section 7.2</em> <a href="#rfc.xref.Part6.2">4.3.1</a>, <a href="#rfc.xref.Part6.3">4.3.2</a>, <a href="#rfc.xref.Part6.8">5.1</a>, <a href="#rfc.xref.Part6.16">7.1</a>, <a href="#rfc.xref.Part6.20">7.1.4</a></li> 4651 4649 <li><em>Section 7.3</em> <a href="#rfc.xref.Part6.17">7.1</a></li> 4652 4650 <li><em>Section 7.4</em> <a href="#rfc.xref.Part6.9">5.1</a></li> … … 4654 4652 </ul> 4655 4653 </li> 4656 <li><em>Part7</em> <a href="#rfc.xref.Part7.1">4.3.8</a>, <a href="#rfc.xref.Part7.2">5.4</a>, <a href="#rfc.xref.Part7.3">5.4</a>, <a href="#rfc.xref.Part7.4">6.1</a>, <a href="#rfc.xref.Part7.5">6.1</a>, <a href="#rfc.xref.Part7.6">6.1</a>, <a href="#rfc.xref.Part7.7">7. 2.1</a>, <a href="#rfc.xref.Part7.8">7.3</a>, <a href="#rfc.xref.Part7.9">7.3</a>, <a href="#Part7"><b>11.1</b></a><ul>4654 <li><em>Part7</em> <a href="#rfc.xref.Part7.1">4.3.8</a>, <a href="#rfc.xref.Part7.2">5.4</a>, <a href="#rfc.xref.Part7.3">5.4</a>, <a href="#rfc.xref.Part7.4">6.1</a>, <a href="#rfc.xref.Part7.5">6.1</a>, <a href="#rfc.xref.Part7.6">6.1</a>, <a href="#rfc.xref.Part7.7">7.1.4</a>, <a href="#rfc.xref.Part7.8">7.3</a>, <a href="#rfc.xref.Part7.9">7.3</a>, <a href="#Part7"><b>11.1</b></a><ul> 4657 4655 <li><em>Section 3</em> <a href="#rfc.xref.Part7.4">6.1</a></li> 4658 4656 <li><em>Section 3.1</em> <a href="#rfc.xref.Part7.5">6.1</a></li> 4659 4657 <li><em>Section 3.2</em> <a href="#rfc.xref.Part7.6">6.1</a></li> 4660 <li><em>Section 4.1</em> <a href="#rfc.xref.Part7.2">5.4</a>, <a href="#rfc.xref.Part7.7">7. 2.1</a></li>4658 <li><em>Section 4.1</em> <a href="#rfc.xref.Part7.2">5.4</a>, <a href="#rfc.xref.Part7.7">7.1.4</a></li> 4661 4659 <li><em>Section 4.2</em> <a href="#rfc.xref.Part7.9">7.3</a></li> 4662 4660 <li><em>Section 4.3</em> <a href="#rfc.xref.Part7.3">5.4</a></li> … … 4775 4773 </li> 4776 4774 <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul> 4777 <li>Vary header field <a href="#rfc.xref.header.vary.1">3.4.1</a>, <a href="#rfc.xref.header.vary.2">7. 2</a>, <a href="#rfc.iref.v.1"><b>7.2.1</b></a>, <a href="#rfc.xref.header.vary.3">8.3.2</a></li>4775 <li>Vary header field <a href="#rfc.xref.header.vary.1">3.4.1</a>, <a href="#rfc.xref.header.vary.2">7.1</a>, <a href="#rfc.iref.v.1"><b>7.1.4</b></a>, <a href="#rfc.xref.header.vary.3">8.3.2</a></li> 4778 4776 </ul> 4779 4777 </li> -
draft-ietf-httpbis/latest/p2-semantics.xml
r2105 r2106 3547 3547 <c>Location</c> <c><xref target="header.location"/></c> 3548 3548 <c>Retry-After</c> <c><xref target="header.retry-after"/></c> 3549 <c>Vary</c> <c><xref target="header.vary"/></c> 3549 3550 <c>Warning</c> <c>&header-warning;</c> 3550 3551 </texttable> … … 3865 3866 </t> 3866 3867 </section> 3867 </section>3868 3869 <section title="Selected Representation Header Fields" anchor="selected.representation">3870 <iref primary="true" item="selected representation"/>3871 <x:anchor-alias value="selected representation"/>3872 <t>3873 We use the term "<x:dfn>selected representation</x:dfn>" to refer to the3874 representation of the <x:ref>target resource</x:ref> in a3875 <x:ref>200 (OK)</x:ref> response to <x:ref>GET</x:ref>, or the3876 representation that would have been selected in a successful response if3877 the request had used the method <x:ref>GET</x:ref> and excluded any3878 conditional request header fields.3879 </t>3880 <t>3881 The following header fields define metadata about the selected3882 representation at the conclusion of handling the request. In other words,3883 for a successful response to a state-changing method, these fields describe3884 the new representation that has replaced the prior selected representation.3885 For example, an ETag header field in a 201 response communicates the3886 entity-tag of the newly created resource's representation, so that it can3887 be used in later conditional requests to prevent the "lost update"3888 problem <xref target="Part4"/>.3889 </t>3890 <texttable align="left">3891 <ttcol>Header Field Name</ttcol>3892 <ttcol>Defined in...</ttcol>3893 3894 <c>ETag</c> <c>&header-etag;</c>3895 <c>Last-Modified</c> <c>&header-last-modified;</c>3896 <c>Vary</c> <c><xref target="header.vary"/></c>3897 </texttable>3898 3868 3899 3869 <section anchor="header.vary" title="Vary"> … … 3967 3937 </t> 3968 3938 </section> 3939 </section> 3940 3941 <section title="Selected Representation Header Fields" anchor="selected.representation"> 3942 <iref primary="true" item="selected representation"/> 3943 <x:anchor-alias value="selected representation"/> 3944 <t> 3945 We use the term "<x:dfn>selected representation</x:dfn>" to refer to the 3946 representation of the <x:ref>target resource</x:ref> in a 3947 <x:ref>200 (OK)</x:ref> response to <x:ref>GET</x:ref>, or the 3948 representation that would have been selected in a successful response if 3949 the request had used the method <x:ref>GET</x:ref> and excluded any 3950 conditional request header fields. 3951 </t> 3952 <t> 3953 The following header fields define metadata about the selected 3954 representation at the conclusion of handling the request. In other words, 3955 for a successful response to a state-changing method, these fields describe 3956 the new representation that has replaced the prior selected representation. 3957 For example, an ETag header field in a 201 response communicates the 3958 entity-tag of the newly created resource's representation, so that it can 3959 be used in later conditional requests to prevent the "lost update" 3960 problem <xref target="Part4"/>. 3961 </t> 3962 <texttable align="left"> 3963 <ttcol>Header Field Name</ttcol> 3964 <ttcol>Defined in...</ttcol> 3965 3966 <c>ETag</c> <c>&header-etag;</c> 3967 <c>Last-Modified</c> <c>&header-last-modified;</c> 3968 </texttable> 3969 3969 3970 </section> 3970 3971 … … 4965 4966 <x:defines>If-None-Match</x:defines> 4966 4967 <x:defines>If-Unmodified-Since</x:defines> 4968 <x:defines>Last-Modified</x:defines> 4967 4969 </x:source> 4968 4970 </reference>
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