source: draft-ietf-httpbis/17/p4-conditional.html

Last change on this file was 2726, checked in by julian.reschke@…, 7 years ago

update to latest version of rfc2629.xslt, regen all HTML

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/html;charset=utf-8
File size: 114.6 KB
Line 
1<!DOCTYPE html
2  PUBLIC "-//W3C//DTD HTML 4.01//EN">
3<html lang="en">
4   <head profile="http://dublincore.org/documents/2008/08/04/dc-html/">
5      <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
6      <title>HTTP/1.1, part 4: Conditional Requests</title><style type="text/css" title="Xml2Rfc (sans serif)">
7a {
8  text-decoration: none;
9}
10a.smpl {
11  color: black;
12}
13a:hover {
14  text-decoration: underline;
15}
16a:active {
17  text-decoration: underline;
18}
19address {
20  margin-top: 1em;
21  margin-left: 2em;
22  font-style: normal;
23}
24body {
25  color: black;
26  font-family: cambria, helvetica, arial, sans-serif;
27  font-size: 11pt;
28  margin-right: 2em;
29}
30cite {
31  font-style: normal;
32}
33div.note {
34  margin-left: 2em;
35}
36dl {
37  margin-left: 2em;
38}
39ul.empty {
40  list-style-type: none;
41}
42ul.empty li {
43  margin-top: .5em;
44}
45dl p {
46  margin-left: 0em;
47}
48dt {
49  margin-top: .5em;
50}
51h1 {
52  font-size: 130%;
53  line-height: 21pt;
54  page-break-after: avoid;
55}
56h1.np {
57  page-break-before: always;
58}
59h2 {
60  font-size: 120%;
61  line-height: 15pt;
62  page-break-after: avoid;
63}
64h3 {
65  font-size: 110%;
66  page-break-after: avoid;
67}
68h4, h5, h6 {
69  page-break-after: avoid;
70}
71h1 a, h2 a, h3 a, h4 a, h5 a, h6 a {
72  color: black;
73}
74img {
75  margin-left: 3em;
76}
77li {
78  margin-left: 2em;
79}
80ol {
81  margin-left: 2em;
82}
83ol.la {
84  list-style-type: lower-alpha;
85}
86ol.ua {
87  list-style-type: upper-alpha;
88}
89ol p {
90  margin-left: 0em;
91}
92p {
93  margin-left: 2em;
94}
95pre {
96  margin-left: 3em;
97  background-color: lightyellow;
98  padding: .25em;
99  page-break-inside: avoid;
100}
101pre.text2 {
102  border-style: dotted;
103  border-width: 1px;
104  background-color: #f0f0f0;
105  width: 69em;
106}
107pre.inline {
108  background-color: white;
109  padding: 0em;
110}
111pre.text {
112  border-style: dotted;
113  border-width: 1px;
114  background-color: #f8f8f8;
115  width: 69em;
116}
117pre.drawing {
118  border-style: solid;
119  border-width: 1px;
120  background-color: #f8f8f8;
121  padding: 2em;
122}
123table {
124  margin-left: 2em;
125}
126table.tt {
127  vertical-align: top;
128  border-color: gray;
129}
130table.tt th {
131  border-color: gray;
132}
133table.tt td {
134  border-color: gray;
135}
136table.all {
137  border-style: solid;
138  border-width: 2px;
139}
140table.full {
141  border-style: solid;
142  border-width: 2px;
143}
144table.tt td {
145  vertical-align: top;
146}
147table.all td {
148  border-style: solid;
149  border-width: 1px;
150}
151table.full td {
152  border-style: none solid;
153  border-width: 1px;
154}
155table.tt th {
156  vertical-align: top;
157}
158table.all th {
159  border-style: solid;
160  border-width: 1px;
161}
162table.full th {
163  border-style: solid;
164  border-width: 1px 1px 2px 1px;
165}
166table.headers th {
167  border-style: none none solid none;
168  border-width: 2px;
169}
170table.left {
171  margin-right: auto;
172}
173table.right {
174  margin-left: auto;
175}
176table.center {
177  margin-left: auto;
178  margin-right: auto;
179}
180caption {
181  caption-side: bottom;
182  font-weight: bold;
183  font-size: 10pt;
184  margin-top: .5em;
185}
186
187table.header {
188  border-spacing: 1px;
189  width: 95%;
190  font-size: 11pt;
191  color: white;
192}
193td.top {
194  vertical-align: top;
195}
196td.topnowrap {
197  vertical-align: top;
198  white-space: nowrap;
199}
200table.header td {
201  background-color: gray;
202  width: 50%;
203}
204table.header a {
205  color: white;
206}
207td.reference {
208  vertical-align: top;
209  white-space: nowrap;
210  padding-right: 1em;
211}
212thead {
213  display:table-header-group;
214}
215ul.toc, ul.toc ul {
216  list-style: none;
217  margin-left: 1.5em;
218  padding-left: 0em;
219}
220ul.toc li {
221  line-height: 150%;
222  font-weight: bold;
223  margin-left: 0em;
224}
225ul.toc li li {
226  line-height: normal;
227  font-weight: normal;
228  font-size: 10pt;
229  margin-left: 0em;
230}
231li.excluded {
232  font-size: 0pt;
233}
234ul p {
235  margin-left: 0em;
236}
237.title, .filename, h1, h2, h3, h4 {
238  font-family: candara, helvetica, arial, sans-serif;
239}
240samp, tt, code, pre {
241  font: consolas, monospace;
242}
243ul.ind, ul.ind ul {
244  list-style: none;
245  margin-left: 1.5em;
246  padding-left: 0em;
247  page-break-before: avoid;
248}
249ul.ind li {
250  font-weight: bold;
251  line-height: 200%;
252  margin-left: 0em;
253}
254ul.ind li li {
255  font-weight: normal;
256  line-height: 150%;
257  margin-left: 0em;
258}
259.avoidbreak {
260  page-break-inside: avoid;
261}
262.bcp14 {
263  font-style: normal;
264  text-transform: lowercase;
265  font-variant: small-caps;
266}
267.comment {
268  background-color: yellow;
269}
270.center {
271  text-align: center;
272}
273.error {
274  color: red;
275  font-style: italic;
276  font-weight: bold;
277}
278.figure {
279  font-weight: bold;
280  text-align: center;
281  font-size: 10pt;
282}
283.filename {
284  color: #333333;
285  font-size: 75%;
286  font-weight: bold;
287  line-height: 21pt;
288  text-align: center;
289}
290.fn {
291  font-weight: bold;
292}
293.left {
294  text-align: left;
295}
296.right {
297  text-align: right;
298}
299.title {
300  color: green;
301  font-size: 150%;
302  line-height: 18pt;
303  font-weight: bold;
304  text-align: center;
305  margin-top: 36pt;
306}
307.warning {
308  font-size: 130%;
309  background-color: yellow;
310}
311
312
313@media print {
314  .noprint {
315    display: none;
316  }
317
318  a {
319    color: black;
320    text-decoration: none;
321  }
322
323  table.header {
324    width: 90%;
325  }
326
327  td.header {
328    width: 50%;
329    color: black;
330    background-color: white;
331    vertical-align: top;
332    font-size: 110%;
333  }
334
335  ul.toc a:nth-child(2)::after {
336    content: leader('.') target-counter(attr(href), page);
337  }
338
339  ul.ind li li a {
340    content: target-counter(attr(href), page);
341  }
342
343  .print2col {
344    column-count: 2;
345    -moz-column-count: 2;
346    column-fill: auto;
347  }
348}
349
350@page {
351  @top-left {
352       content: "Internet-Draft";
353  }
354  @top-right {
355       content: "October 2011";
356  }
357  @top-center {
358       content: "HTTP/1.1, Part 4";
359  }
360  @bottom-left {
361       content: "Fielding, et al.";
362  }
363  @bottom-center {
364       content: "Expires May 3, 2012";
365  }
366  @bottom-right {
367       content: "[Page " counter(page) "]";
368  }
369}
370
371@page:first {
372    @top-left {
373      content: normal;
374    }
375    @top-right {
376      content: normal;
377    }
378    @top-center {
379      content: normal;
380    }
381}
382</style><link rel="Contents" href="#rfc.toc">
383      <link rel="Author" href="#rfc.authors">
384      <link rel="Copyright" href="#rfc.copyrightnotice">
385      <link rel="Index" href="#rfc.index">
386      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
387      <link rel="Chapter" title="2 Validators" href="#rfc.section.2">
388      <link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3">
389      <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4">
390      <link rel="Chapter" title="5 IANA Considerations" href="#rfc.section.5">
391      <link rel="Chapter" title="6 Security Considerations" href="#rfc.section.6">
392      <link rel="Chapter" title="7 Acknowledgments" href="#rfc.section.7">
393      <link rel="Chapter" href="#rfc.section.8" title="8 References">
394      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
395      <link rel="Appendix" title="B Collected ABNF" href="#rfc.section.B">
396      <link rel="Appendix" title="C Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.C">
397      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.640, 2014/06/13 12:42:58, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
398      <link rel="schema.dct" href="http://purl.org/dc/terms/">
399      <meta name="dct.creator" content="Fielding, R.">
400      <meta name="dct.creator" content="Gettys, J.">
401      <meta name="dct.creator" content="Mogul, J.">
402      <meta name="dct.creator" content="Frystyk, H.">
403      <meta name="dct.creator" content="Masinter, L.">
404      <meta name="dct.creator" content="Leach, P.">
405      <meta name="dct.creator" content="Berners-Lee, T.">
406      <meta name="dct.creator" content="Lafon, Y.">
407      <meta name="dct.creator" content="Reschke, J. F.">
408      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-17">
409      <meta name="dct.issued" scheme="ISO8601" content="2011-10-31">
410      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
411      <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.">
412      <meta name="description" 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.">
413   </head>
414   <body>
415      <table class="header">
416         <tbody>
417            <tr>
418               <td class="left">HTTPbis Working Group</td>
419               <td class="right">R. Fielding, Editor</td>
420            </tr>
421            <tr>
422               <td class="left">Internet-Draft</td>
423               <td class="right">Adobe</td>
424            </tr>
425            <tr>
426               <td class="left">Obsoletes: <a href="https://tools.ietf.org/html/rfc2616">2616</a> (if approved)
427               </td>
428               <td class="right">J. Gettys</td>
429            </tr>
430            <tr>
431               <td class="left">Intended status: Standards Track</td>
432               <td class="right">Alcatel-Lucent</td>
433            </tr>
434            <tr>
435               <td class="left">Expires: May 3, 2012</td>
436               <td class="right">J. Mogul</td>
437            </tr>
438            <tr>
439               <td class="left"></td>
440               <td class="right">HP</td>
441            </tr>
442            <tr>
443               <td class="left"></td>
444               <td class="right">H. Frystyk</td>
445            </tr>
446            <tr>
447               <td class="left"></td>
448               <td class="right">Microsoft</td>
449            </tr>
450            <tr>
451               <td class="left"></td>
452               <td class="right">L. Masinter</td>
453            </tr>
454            <tr>
455               <td class="left"></td>
456               <td class="right">Adobe</td>
457            </tr>
458            <tr>
459               <td class="left"></td>
460               <td class="right">P. Leach</td>
461            </tr>
462            <tr>
463               <td class="left"></td>
464               <td class="right">Microsoft</td>
465            </tr>
466            <tr>
467               <td class="left"></td>
468               <td class="right">T. Berners-Lee</td>
469            </tr>
470            <tr>
471               <td class="left"></td>
472               <td class="right">W3C/MIT</td>
473            </tr>
474            <tr>
475               <td class="left"></td>
476               <td class="right">Y. Lafon, Editor</td>
477            </tr>
478            <tr>
479               <td class="left"></td>
480               <td class="right">W3C</td>
481            </tr>
482            <tr>
483               <td class="left"></td>
484               <td class="right">J. Reschke, Editor</td>
485            </tr>
486            <tr>
487               <td class="left"></td>
488               <td class="right">greenbytes</td>
489            </tr>
490            <tr>
491               <td class="left"></td>
492               <td class="right">October 31, 2011</td>
493            </tr>
494         </tbody>
495      </table>
496      <p class="title">HTTP/1.1, part 4: Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-17</span></p>
497      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
498      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
499         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the
500         seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
501      </p>
502      <p>Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those
503         requests.
504      </p>
505      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
506      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived
507         at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
508      </p>
509      <p>The current issues list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
510      </p>
511      <p>The changes in this draft are summarized in <a href="#changes.since.16" title="Since draft-ietf-httpbis-p4-conditional-16">Appendix&nbsp;C.18</a>.
512      </p>
513      <div id="rfc.status">
514         <h1><a href="#rfc.status">Status of This Memo</a></h1>
515         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
516         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
517            working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
518         </p>
519         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
520            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
521            in progress”.
522         </p>
523         <p>This Internet-Draft will expire on May 3, 2012.</p>
524      </div>
525      <div id="rfc.copyrightnotice">
526         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
527         <p>Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
528         <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
529            and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
530            text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
531            BSD License.
532         </p>
533         <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
534            10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
535            allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
536            controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
537            works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
538            it into languages other than English.
539         </p>
540      </div>
541      <hr class="noprint">
542      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
543      <ul class="toc">
544         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
545               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></li>
546               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
547            </ul>
548         </li>
549         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#validators">Validators</a><ul>
550               <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
551               <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
552                     <li><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
553                     <li><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
554                  </ul>
555               </li>
556               <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
557                     <li><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
558                     <li><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
559                     <li><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;&nbsp;&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li>
560                  </ul>
561               </li>
562               <li><a href="#rfc.section.2.4">2.4</a>&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>
563            </ul>
564         </li>
565         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.definitions">Precondition Header Fields</a><ul>
566               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
567               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
568               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
569               <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
570               <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li>
571            </ul>
572         </li>
573         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.definitions">Status Code Definitions</a><ul>
574               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
575               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
576            </ul>
577         </li>
578         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
579               <li><a href="#rfc.section.5.1">5.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.registration">Status Code Registration</a></li>
580               <li><a href="#rfc.section.5.2">5.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
581            </ul>
582         </li>
583         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
584         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
585         <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
586               <li><a href="#rfc.section.8.1">8.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
587               <li><a href="#rfc.section.8.2">8.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
588            </ul>
589         </li>
590         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
591         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
592         <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
593               <li><a href="#rfc.section.C.1">C.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC 2616</a></li>
594               <li><a href="#rfc.section.C.2">C.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p4-conditional-00</a></li>
595               <li><a href="#rfc.section.C.3">C.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p4-conditional-01</a></li>
596               <li><a href="#rfc.section.C.4">C.4</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></li>
597               <li><a href="#rfc.section.C.5">C.5</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></li>
598               <li><a href="#rfc.section.C.6">C.6</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></li>
599               <li><a href="#rfc.section.C.7">C.7</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></li>
600               <li><a href="#rfc.section.C.8">C.8</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></li>
601               <li><a href="#rfc.section.C.9">C.9</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></li>
602               <li><a href="#rfc.section.C.10">C.10</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></li>
603               <li><a href="#rfc.section.C.11">C.11</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></li>
604               <li><a href="#rfc.section.C.12">C.12</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></li>
605               <li><a href="#rfc.section.C.13">C.13</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></li>
606               <li><a href="#rfc.section.C.14">C.14</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></li>
607               <li><a href="#rfc.section.C.15">C.15</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></li>
608               <li><a href="#rfc.section.C.16">C.16</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.14">Since draft-ietf-httpbis-p4-conditional-14</a></li>
609               <li><a href="#rfc.section.C.17">C.17</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.15">Since draft-ietf-httpbis-p4-conditional-15</a></li>
610               <li><a href="#rfc.section.C.18">C.18</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.16">Since draft-ietf-httpbis-p4-conditional-16</a></li>
611            </ul>
612         </li>
613         <li><a href="#rfc.index">Index</a></li>
614         <li><a href="#rfc.authors">Authors' Addresses</a></li>
615      </ul>
616      <div id="introduction">
617         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#introduction">Introduction</a></h1>
618         <p id="rfc.section.1.p.1">This document defines the HTTP/1.1 conditional request mechanisms, including both metadata for indicating/observing changes
619            in resource representations and request header fields that specify preconditions on that metadata be checked before performing
620            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:
621            one client accidentally overwriting the work of another client that has been acting in parallel.
622         </p>
623         <p id="rfc.section.1.p.2">Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the
624            state as observed in a previously obtained representation (one value in that set). A resource might have multiple current
625            representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests
626            to corresponding representations will be consistent over time if the server intends to take advantage of conditionals. Regardless,
627            if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result
628            when the precondition evaluates to false.
629         </p>
630         <p id="rfc.section.1.p.3"><span id="rfc.iref.s.1"></span> We use the term "<dfn>selected representation</dfn>" to refer to the current representation of the target resource that would have been selected in a successful response if
631            the same request had used the method GET and had excluded all of the conditional request header fields. The conditional request
632            preconditions are evaluated by comparing the values provided in the request header fields to the current metadata for the
633            selected representation.
634         </p>
635         <div id="intro.conformance.and.error.handling">
636            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></h2>
637            <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"
638               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>.
639            </p>
640            <p id="rfc.section.1.1.p.2">This document defines conformance criteria for several roles in HTTP communication, including Senders, Recipients, Clients,
641               Servers, User-Agents, Origin Servers, Intermediaries, Proxies and Gateways. See <a href="p1-messaging.html#architecture" title="Architecture">Section 2</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> for definitions of these terms.
642            </p>
643            <p id="rfc.section.1.1.p.3">An implementation is considered conformant if it complies with all of the requirements associated with its role(s). Note that
644               SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.
645            </p>
646            <p id="rfc.section.1.1.p.4">This document also uses ABNF to define valid protocol elements (<a href="#notation" title="Syntax Notation">Section&nbsp;1.2</a>). In addition to the prose requirements placed upon them, Senders <em class="bcp14">MUST NOT</em> generate protocol elements that are invalid.
647            </p>
648            <p id="rfc.section.1.1.p.5">Unless noted otherwise, Recipients <em class="bcp14">MAY</em> take steps to recover a usable protocol element from an invalid construct. However, HTTP does not define specific error handling
649               mechanisms, except in cases where it has direct impact on security. This is because different uses of the protocol require
650               different error handling strategies; for example, a Web browser may wish to transparently recover from a response where the
651               Location header field doesn't parse according to the ABNF, whereby in a systems control protocol using HTTP, this type of
652               error recovery could lead to dangerous consequences.
653            </p>
654         </div>
655         <div id="notation">
656            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
657            <p id="rfc.section.1.2.p.1">This specification uses the ABNF syntax defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</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> (which extends the syntax defined in <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with a list rule). <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF, with the list rule expanded.
658            </p>
659            <p id="rfc.section.1.2.p.2">The following core rules are included by reference, as defined in <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>, <a href="https://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
660               (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
661               character).
662            </p>
663            <p id="rfc.section.1.2.p.3">The ABNF rules below are defined in <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a> and <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>:
664            </p>
665            <div id="rfc.figure.u.1"></div><pre class="inline">  <a href="#notation" class="smpl">OWS</a>           = &lt;OWS, defined in <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
666  <a href="#notation" class="smpl">quoted-string</a> = &lt;quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#field.rules" title="Common Field ABNF Rules">Section 3.2.3</a>&gt;
667  <a href="#notation" class="smpl">HTTP-date</a>     = &lt;HTTP-date, defined in <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 8</a>&gt;
668</pre></div>
669      </div>
670      <div id="validators">
671         <div id="rfc.iref.m.1"></div>
672         <div id="rfc.iref.v.1"></div>
673         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#validators">Validators</a></h1>
674         <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:
675            modification dates and opaque entity tags. Additional metadata that reflects resource state has been defined by various extensions
676            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.
677         </p>
678         <div id="weak.and.strong.validators">
679            <div id="rfc.iref.v.2"></div>
680            <div id="rfc.iref.v.3"></div>
681            <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></h2>
682            <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.
683               Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently.
684               Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator
685               in use and imposes restrictions on when weak validators can be used as preconditions.
686            </p>
687            <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
688               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.,
689               Content-Type), but it is in the best interests of the origin server to only change the value when it is necessary to invalidate
690               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
691               validator unless their payload body would be identical.
692            </p>
693            <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
694               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
695               implication of uniqueness across representations of different resources (i.e., the same strong validator might be in use for
696               representations of multiple resources at the same time and does not imply that those representations are equivalent).
697            </p>
698            <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
699               to a representation always results in a unique node name and revision identifier being assigned before the representation
700               is made accessible to GET. A cryptographic hash function applied to the representation data is also sufficient if the data
701               is available prior to the response header fields being sent and the digest does not need to be recalculated every time a validation
702               request is received. However, if a resource has distinct representations that differ only in their metadata, such as might
703               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.
704            </p>
705            <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
706               data. This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to
707               ensure uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations
708               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.
709               In other words, a weak entity-tag <em class="bcp14">SHOULD</em> change whenever the origin server wants caches to invalidate old responses.
710            </p>
711            <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
712               be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in
713               order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server
714               load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might
715               be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between
716               those modifications.
717            </p>
718            <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
719               a server compares two validators. Weak validators are only usable in contexts that do not depend on exact equality of a representation's
720               payload body. Strong validators are usable and preferred for all conditional requests, including cache validation, partial
721               content ranges, and "lost update" avoidance.
722            </p>
723         </div>
724         <div id="header.last-modified">
725            <div id="rfc.iref.l.1"></div>
726            <div id="rfc.iref.h.1"></div>
727            <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a href="#header.last-modified">Last-Modified</a></h2>
728            <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
729               was last modified.
730            </p>
731            <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>
732</pre><p id="rfc.section.2.2.p.3">An example of its use is</p>
733            <div id="rfc.figure.u.3"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
734</pre><div id="lastmod.generation">
735               <h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;<a href="#lastmod.generation">Generation</a></h3>
736               <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,
737                  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
738                  scalability and reliability.
739               </p>
740               <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
741                  the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
742                  detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
743                  use its value to make conditional requests and test the validity of locally cached responses.
744               </p>
745               <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
746                  for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially
747                  if the representation changes near the time that the response is generated.
748               </p>
749               <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
750                  is derived from implementation-specific metadata that evaluates to some time in the future, according to the origin server's
751                  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
752                  on cache validation.
753               </p>
754               <p id="rfc.section.2.2.1.p.5">An origin server without a clock <em class="bcp14">MUST NOT</em> assign Last-Modified values to a response unless these values were associated with the resource by some other system or user
755                  with a reliable clock.
756               </p>
757            </div>
758            <div id="lastmod.comparison">
759               <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;<a href="#lastmod.comparison">Comparison</a></h3>
760               <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
761                  strong, using the following rules:
762               </p>
763               <ul>
764                  <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li>
765                  <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the
766                     presented validator.
767                  </li>
768               </ul>
769               <p id="rfc.section.2.2.2.p.2">or </p>
770               <ul>
771                  <li>The validator is about to be used by a client in an If-Modified-Since, If-Unmodified-Since header field, because the client
772                     has a cache entry, or If-Range for the associated representation, and
773                  </li>
774                  <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
775                  <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
776               </ul>
777               <p id="rfc.section.2.2.2.p.3">or </p>
778               <ul>
779                  <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
780                     and
781                  </li>
782                  <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
783                  <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
784               </ul>
785               <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
786                  both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified
787                  time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from
788                  different clocks, or at somewhat different times during the preparation of the response. An implementation <em class="bcp14">MAY</em> use a value larger than 60 seconds, if it is believed that 60 seconds is too short.
789               </p>
790            </div>
791         </div>
792         <div id="header.etag">
793            <div id="rfc.iref.e.1"></div>
794            <div id="rfc.iref.h.2"></div>
795            <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a href="#header.etag">ETag</a></h2>
796            <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
797               for differentiating between multiple representations of the same resource, regardless of whether those multiple representations
798               are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the
799               same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.
800            </p>
801            <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span>  <a href="#header.etag" class="smpl">ETag</a>       = <a href="#header.etag" class="smpl">entity-tag</a>
802
803  <a href="#header.etag" class="smpl">entity-tag</a> = [ <a href="#header.etag" class="smpl">weak</a> ] <a href="#header.etag" class="smpl">opaque-tag</a>
804  <a href="#header.etag" class="smpl">weak</a>       = %x57.2F ; "W/", case-sensitive
805  <a href="#header.etag" class="smpl">opaque-tag</a> = <a href="#notation" class="smpl">quoted-string</a>
806</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
807               modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are
808               not consistently maintained.
809            </p>
810            <div id="rfc.figure.u.5"></div>
811            <p>Examples:</p><pre class="text">  ETag: "xyzzy"
812  ETag: W/"xyzzy"
813  ETag: ""
814</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
815               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).
816            </p>
817            <div id="entity.tag.generation">
818               <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a href="#entity.tag.generation">Generation</a></h3>
819               <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
820                  the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple
821                  sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each
822                  entity-tag is constructed.
823               </p>
824               <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
825                  number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations.
826                  Other implementations might use a stored hash of representation content, a combination of various filesystem attributes, or
827                  a modification timestamp that has sub-second resolution.
828               </p>
829               <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
830                  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
831                  and reliability.
832               </p>
833            </div>
834            <div id="entity.tag.comparison">
835               <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a href="#entity.tag.comparison">Comparison</a></h3>
836               <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
837                  or not:
838               </p>
839               <ul>
840                  <li>The strong comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, and both <em class="bcp14">MUST NOT</em> be weak.
841                  </li>
842                  <li>The weak comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, but either or both of them <em class="bcp14">MAY</em> be tagged as "weak" without affecting the result.
843                  </li>
844               </ul>
845               <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>
846               <div id="rfc.table.u.1">
847                  <table class="tt full left" cellpadding="3" cellspacing="0">
848                     <thead>
849                        <tr>
850                           <th>ETag 1</th>
851                           <th>ETag 2</th>
852                           <th>Strong Comparison</th>
853                           <th>Weak Comparison</th>
854                        </tr>
855                     </thead>
856                     <tbody>
857                        <tr>
858                           <td class="left">W/"1"</td>
859                           <td class="left">W/"1"</td>
860                           <td class="left">no match</td>
861                           <td class="left">match</td>
862                        </tr>
863                        <tr>
864                           <td class="left">W/"1"</td>
865                           <td class="left">W/"2"</td>
866                           <td class="left">no match</td>
867                           <td class="left">no match</td>
868                        </tr>
869                        <tr>
870                           <td class="left">W/"1"</td>
871                           <td class="left">"1"</td>
872                           <td class="left">no match</td>
873                           <td class="left">match</td>
874                        </tr>
875                        <tr>
876                           <td class="left">"1"</td>
877                           <td class="left">"1"</td>
878                           <td class="left">match</td>
879                           <td class="left">match</td>
880                        </tr>
881                     </tbody>
882                  </table>
883               </div>
884            </div>
885            <div id="example.entity.tag.vs.conneg">
886               <h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h3>
887               <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>):
888               </p>
889               <div id="rfc.figure.u.6"></div>
890               <p>&gt;&gt; Request:</p><pre class="text2">GET /index HTTP/1.1
891Host: www.example.com
892Accept-Encoding: gzip
893
894</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>
895               <div id="rfc.figure.u.7"></div>
896               <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
897Date: Thu, 26 Mar 2010 00:05:00 GMT
898ETag: "123-a"
899Content-Length: 70
900Vary: Accept-Encoding
901Content-Type: text/plain
902
903<span id="exbody">Hello World!
904Hello World!
905Hello World!
906Hello World!
907Hello World!
908</span></pre><p id="rfc.section.2.3.3.p.5">An alternative representation that does use gzip content coding would be:</p>
909               <div id="rfc.figure.u.8"></div>
910               <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
911Date: Thu, 26 Mar 2010 00:05:00 GMT
912ETag: "123-b"
913Content-Length: 43
914Vary: Accept-Encoding
915Content-Type: text/plain
916Content-Encoding: gzip
917
918<em>...binary data...</em></pre><div class="note" id="rfc.section.2.3.3.p.7">
919                  <p><b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation must be distinct
920                     from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings
921                     (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 5.1</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>) apply only during message transfer and do not require distinct entity-tags.
922                  </p>
923               </div>
924            </div>
925         </div>
926         <div id="rules.for.when.to.use.entity.tags.and.last-modified.dates">
927            <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&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></h2>
928            <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
929               ought to be used, and for what purposes.
930            </p>
931            <p id="rfc.section.2.4.p.2">HTTP/1.1 origin servers: </p>
932            <ul>
933               <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
934               </li>
935               <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,
936                  or if it is unfeasible to send a strong entity-tag.
937               </li>
938               <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one.
939               </li>
940            </ul>
941            <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
942               value.
943            </p>
944            <p id="rfc.section.2.4.p.4">HTTP/1.1 clients: </p>
945            <ul>
946               <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
947                  by the origin server.
948               </li>
949               <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
950                  has been provided by the origin server.
951               </li>
952               <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
953                  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.
954               </li>
955               <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
956                  origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
957               </li>
958            </ul>
959            <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
960               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
961               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
962               in the request.
963            </p>
964            <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
965               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
966               fields in the request.
967            </p>
968            <ul class="empty">
969               <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
970                  as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative
971                  assumptions about the validators they receive.
972               </li>
973               <li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will
974                  support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare
975                  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
976                  HTTP/1.1 origin servers should not provide one.
977               </li>
978            </ul>
979         </div>
980      </div>
981      <div id="header.field.definitions">
982         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#header.field.definitions">Precondition Header Fields</a></h1>
983         <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>
984         <div id="header.if-match">
985            <div id="rfc.iref.i.1"></div>
986            <div id="rfc.iref.h.3"></div>
987            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#header.if-match">If-Match</a></h2>
988            <p id="rfc.section.3.1.p.1">The "If-Match" header field <em class="bcp14">MAY</em> be used to make a request method conditional on the current existence or value of an entity-tag for one or more representations
989               of the target resource. If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting
990               against accidental overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem).
991               An If-Match field-value of "*" places the precondition on the existence of any current representation for the target resource.
992            </p>
993            <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>
994</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
995               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.
996            </p>
997            <p id="rfc.section.3.1.p.4">If none of the entity-tags match, or if "*" is given and no current representation exists, the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, the server <em class="bcp14">MUST</em> respond with the 412 (Precondition Failed) status code.
998            </p>
999            <p id="rfc.section.3.1.p.5">If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status code, then the
1000               If-Match header field <em class="bcp14">MUST</em> be ignored.
1001            </p>
1002            <p id="rfc.section.3.1.p.6">Examples:</p>
1003            <div id="rfc.figure.u.10"></div><pre class="text">  If-Match: "xyzzy"
1004  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
1005  If-Match: *
1006</pre><p id="rfc.section.3.1.p.8">The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields
1007               is undefined by this specification.
1008            </p>
1009         </div>
1010         <div id="header.if-none-match">
1011            <div id="rfc.iref.i.2"></div>
1012            <div id="rfc.iref.h.4"></div>
1013            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#header.if-none-match">If-None-Match</a></h2>
1014            <p id="rfc.section.3.2.p.1">The "If-None-Match" header field <em class="bcp14">MAY</em> be used to make a request method conditional on not matching any of the current entity-tag values for representations of the
1015               target resource. If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information
1016               with a minimum amount of transaction overhead. A client that has one or more representations previously obtained from the
1017               target resource can send If-None-Match with a list of the associated entity-tags in the hope of receiving a 304 response if
1018               at least one of those representations matches the selected representation.
1019            </p>
1020            <p id="rfc.section.3.2.p.2">If-None-Match MAY also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying
1021               an existing representation of the target resource when the client believes that the resource does not have a current representation.
1022               This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation
1023               for the target resource.
1024            </p>
1025            <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>
1026</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,
1027               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
1028               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.
1029            </p>
1030            <p id="rfc.section.3.2.p.5">If none of the entity-tags match, then the server <em class="bcp14">MAY</em> perform the requested method as if the If-None-Match header field did not exist, but <em class="bcp14">MUST</em> also ignore any If-Modified-Since header field(s) in the request. That is, if no entity-tags match, then the server <em class="bcp14">MUST NOT</em> return a 304 (Not Modified) response.
1031            </p>
1032            <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
1033               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.)
1034            </p>
1035            <p id="rfc.section.3.2.p.7">Examples:</p>
1036            <div id="rfc.figure.u.12"></div><pre class="text">  If-None-Match: "xyzzy"
1037  If-None-Match: W/"xyzzy"
1038  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
1039  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
1040  If-None-Match: *
1041</pre><p id="rfc.section.3.2.p.9">The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header
1042               fields is undefined by this specification.
1043            </p>
1044         </div>
1045         <div id="header.if-modified-since">
1046            <div id="rfc.iref.i.3"></div>
1047            <div id="rfc.iref.h.5"></div>
1048            <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></h2>
1049            <p id="rfc.section.3.3.p.1">The "If-Modified-Since" header field <em class="bcp14">MAY</em> be used to make a request method conditional by modification date: if the selected representation has not been modified since
1050               the time specified in this field, then do not perform the request method; instead, respond as detailed below.
1051            </p>
1052            <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = <a href="#notation" class="smpl">HTTP-date</a>
1053</pre><p id="rfc.section.3.3.p.3">An example of the field is:</p>
1054            <div id="rfc.figure.u.14"></div><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1055</pre><p id="rfc.section.3.3.p.5">A GET method with an If-Modified-Since header field and no Range header field requests that the selected representation be
1056               transferred only if it has been modified since the date given by the If-Modified-Since header field. The algorithm for determining
1057               this includes the following cases:
1058            </p>
1059            <ol>
1060               <li>If the request would normally result in anything other than a 200 (OK) status code, or if the passed If-Modified-Since date
1061                  is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is
1062                  invalid.
1063               </li>
1064               <li>If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for
1065                  a normal GET.
1066               </li>
1067               <li>If the selected representation has not been modified since a valid If-Modified-Since date, the server <em class="bcp14">SHOULD</em> return a 304 (Not Modified) response.
1068               </li>
1069            </ol>
1070            <p id="rfc.section.3.3.p.6">The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. </p>
1071            <ul class="empty">
1072               <li><b>Note:</b> The Range header field modifies the meaning of If-Modified-Since; see <a href="p5-range.html#header.range" title="Range">Section 5.4</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> for full details.
1073               </li>
1074               <li><b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client.
1075               </li>
1076               <li><b>Note:</b> When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than
1077                  function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since
1078                  header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header
1079                  field whenever possible.
1080               </li>
1081               <li><b>Note:</b> If a client uses an arbitrary date in the If-Modified-Since header field instead of a date taken from the Last-Modified header
1082                  field for the same request, the client needs to be aware that this date is interpreted in the server's understanding of time.
1083                  Unsynchronized clocks and rounding problems, due to the different encodings of time between the client and server, are concerns.
1084                  This includes the possibility of race conditions if the document has changed between the time it was first requested and the
1085                  If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since
1086                  date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between
1087                  client and server are at best approximate due to network latency.
1088               </li>
1089            </ul>
1090            <p id="rfc.section.3.3.p.7">The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header
1091               fields is undefined by this specification.
1092            </p>
1093         </div>
1094         <div id="header.if-unmodified-since">
1095            <div id="rfc.iref.i.4"></div>
1096            <div id="rfc.iref.h.6"></div>
1097            <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></h2>
1098            <p id="rfc.section.3.4.p.1">The "If-Unmodified-Since" header field <em class="bcp14">MAY</em> be used to make a request method conditional by modification date: if the selected representation has been modified since
1099               the time specified in this field, then the server <em class="bcp14">MUST NOT</em> perform the requested operation and <em class="bcp14">MUST</em> instead respond with the 412 (Precondition Failed) status code. If the selected representation has not been modified since
1100               the time specified in this field, the server <em class="bcp14">SHOULD</em> perform the request method as if the If-Unmodified-Since header field were not present.
1101            </p>
1102            <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.9"></span>  <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = <a href="#notation" class="smpl">HTTP-date</a>
1103</pre><p id="rfc.section.3.4.p.3">An example of the field is:</p>
1104            <div id="rfc.figure.u.16"></div><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1105</pre><p id="rfc.section.3.4.p.5">If the request normally (i.e., without the If-Unmodified-Since header field) would result in anything other than a 2xx or
1106               412 status code, the If-Unmodified-Since header field <em class="bcp14">SHOULD</em> be ignored.
1107            </p>
1108            <p id="rfc.section.3.4.p.6">If the specified date is invalid, the header field <em class="bcp14">MUST</em> be ignored.
1109            </p>
1110            <p id="rfc.section.3.4.p.7">The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since
1111               header fields is undefined by this specification.
1112            </p>
1113         </div>
1114         <div id="header.if-range">
1115            <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a href="#header.if-range">If-Range</a></h2>
1116            <p id="rfc.section.3.5.p.1">The If-Range header field provides a special conditional request mechanism that is similar to If-Match and If-Unmodified-Since
1117               but specific to HTTP range requests. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 5.3</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>.
1118            </p>
1119         </div>
1120      </div>
1121      <div id="status.code.definitions">
1122         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#status.code.definitions">Status Code Definitions</a></h1>
1123         <div id="status.304">
1124            <div id="rfc.iref.3.1"></div>
1125            <div id="rfc.iref.s.2"></div>
1126            <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a href="#status.304">304 Not Modified</a></h2>
1127            <p id="rfc.section.4.1.p.1">The 304 status code indicates that a conditional GET request has been received and would have resulted in a 200 (OK) response
1128               if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server to
1129               transfer a representation of the target resource because the client's request indicates that it already has a valid representation,
1130               as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation
1131               as if it were the payload of a 200 response. The 304 response <em class="bcp14">MUST NOT</em> contain a message-body, and thus is always terminated by the first empty line after the header fields.
1132            </p>
1133            <p id="rfc.section.4.1.p.2">A 304 response <em class="bcp14">MUST</em> include a Date header field (<a href="p2-semantics.html#header.date" title="Date">Section 9.2</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) unless the origin server does not have a clock that can provide a reasonable approximation of the current time. If a 200
1134               response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag, Expires,
1135               Last-Modified, or Vary, then those same header fields <em class="bcp14">MUST</em> be sent in a 304 response.
1136            </p>
1137            <p id="rfc.section.4.1.p.3">Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations,
1138               the response <em class="bcp14">SHOULD NOT</em> include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding
1139               cache updates (e.g., future HTTP extensions).
1140            </p>
1141            <p id="rfc.section.4.1.p.4">If the recipient of a 304 response does not have a cached representation corresponding to the entity-tag indicated by the
1142               304 response, then the recipient <em class="bcp14">MUST NOT</em> use the 304 to update its own cache. If this conditional request originated with an outbound client, such as a user agent
1143               with its own cache sending a conditional GET to a shared proxy, then the 304 response <em class="bcp14">MAY</em> be forwarded to the outbound client. Otherwise, the recipient <em class="bcp14">MUST</em> disregard the 304 response and repeat the request without any preconditions.
1144            </p>
1145            <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.
1146            </p>
1147         </div>
1148         <div id="status.412">
1149            <div id="rfc.iref.4.1"></div>
1150            <div id="rfc.iref.s.3"></div>
1151            <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a href="#status.412">412 Precondition Failed</a></h2>
1152            <p id="rfc.section.4.2.p.1">The 412 status code indicates that one or more preconditions given in the request header fields evaluated to false when tested
1153               on the server. This response code allows the client to place preconditions on the current resource state (its current representations
1154               and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.
1155            </p>
1156         </div>
1157      </div>
1158      <div id="IANA.considerations">
1159         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#IANA.considerations">IANA Considerations</a></h1>
1160         <div id="status.code.registration">
1161            <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a href="#status.code.registration">Status Code Registration</a></h2>
1162            <p id="rfc.section.5.1.p.1">The HTTP Status Code Registry located at &lt;<a href="http://www.iana.org/assignments/http-status-codes">http://www.iana.org/assignments/http-status-codes</a>&gt; shall be updated with the registrations below:
1163            </p>
1164            <div id="rfc.table.1">
1165               <div id="iana.status.code.registration.table"></div>
1166               <table class="tt full left" cellpadding="3" cellspacing="0">
1167                  <thead>
1168                     <tr>
1169                        <th>Value</th>
1170                        <th>Description</th>
1171                        <th>Reference</th>
1172                     </tr>
1173                  </thead>
1174                  <tbody>
1175                     <tr>
1176                        <td class="left">304</td>
1177                        <td class="left">Not Modified</td>
1178                        <td class="left"><a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section&nbsp;4.1</a>
1179                        </td>
1180                     </tr>
1181                     <tr>
1182                        <td class="left">412</td>
1183                        <td class="left">Precondition Failed</td>
1184                        <td class="left"><a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section&nbsp;4.2</a>
1185                        </td>
1186                     </tr>
1187                  </tbody>
1188               </table>
1189            </div>
1190         </div>
1191         <div id="header.field.registration">
1192            <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a href="#header.field.registration">Header Field Registration</a></h2>
1193            <p id="rfc.section.5.2.p.1">The Message Header Field Registry located at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt; shall be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
1194            </p>
1195            <div id="rfc.table.2">
1196               <div id="iana.header.registration.table"></div>
1197               <table class="tt full left" cellpadding="3" cellspacing="0">
1198                  <thead>
1199                     <tr>
1200                        <th>Header Field Name</th>
1201                        <th>Protocol</th>
1202                        <th>Status</th>
1203                        <th>Reference</th>
1204                     </tr>
1205                  </thead>
1206                  <tbody>
1207                     <tr>
1208                        <td class="left">ETag</td>
1209                        <td class="left">http</td>
1210                        <td class="left">standard</td>
1211                        <td class="left"><a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;2.3</a>
1212                        </td>
1213                     </tr>
1214                     <tr>
1215                        <td class="left">If-Match</td>
1216                        <td class="left">http</td>
1217                        <td class="left">standard</td>
1218                        <td class="left"><a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section&nbsp;3.1</a>
1219                        </td>
1220                     </tr>
1221                     <tr>
1222                        <td class="left">If-Modified-Since</td>
1223                        <td class="left">http</td>
1224                        <td class="left">standard</td>
1225                        <td class="left"><a href="#header.if-modified-since" id="rfc.xref.header.if-modified-since.1" title="If-Modified-Since">Section&nbsp;3.3</a>
1226                        </td>
1227                     </tr>
1228                     <tr>
1229                        <td class="left">If-None-Match</td>
1230                        <td class="left">http</td>
1231                        <td class="left">standard</td>
1232                        <td class="left"><a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section&nbsp;3.2</a>
1233                        </td>
1234                     </tr>
1235                     <tr>
1236                        <td class="left">If-Unmodified-Since</td>
1237                        <td class="left">http</td>
1238                        <td class="left">standard</td>
1239                        <td class="left"><a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section&nbsp;3.4</a>
1240                        </td>
1241                     </tr>
1242                     <tr>
1243                        <td class="left">Last-Modified</td>
1244                        <td class="left">http</td>
1245                        <td class="left">standard</td>
1246                        <td class="left"><a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;2.2</a>
1247                        </td>
1248                     </tr>
1249                  </tbody>
1250               </table>
1251            </div>
1252            <p id="rfc.section.5.2.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1253         </div>
1254      </div>
1255      <div id="security.considerations">
1256         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h1>
1257         <p id="rfc.section.6.p.1">No additional security considerations have been identified beyond those applicable to HTTP in general <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
1258         </p>
1259      </div>
1260      <div id="acks">
1261         <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a href="#acks">Acknowledgments</a></h1>
1262         <p id="rfc.section.7.p.1">See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 11</a> of <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
1263         </p>
1264      </div>
1265      <h1 id="rfc.references"><a id="rfc.section.8" href="#rfc.section.8">8.</a> References
1266      </h1>
1267      <h2 id="rfc.references.1"><a href="#rfc.section.8.1" id="rfc.section.8.1">8.1</a> Normative References
1268      </h2>
1269      <table>
1270         <tr>
1271            <td class="reference"><b id="Part1">[Part1]</b></td>
1272            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-17">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-17 (work in progress), October&nbsp;2011.
1273            </td>
1274         </tr>
1275         <tr>
1276            <td class="reference"><b id="Part2">[Part2]</b></td>
1277            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-17">HTTP/1.1, part 2: Message Semantics</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-17 (work in progress), October&nbsp;2011.
1278            </td>
1279         </tr>
1280         <tr>
1281            <td class="reference"><b id="Part3">[Part3]</b></td>
1282            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-17">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p3-payload-17 (work in progress), October&nbsp;2011.
1283            </td>
1284         </tr>
1285         <tr>
1286            <td class="reference"><b id="Part5">[Part5]</b></td>
1287            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-17">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-17 (work in progress), October&nbsp;2011.
1288            </td>
1289         </tr>
1290         <tr>
1291            <td class="reference"><b id="Part6">[Part6]</b></td>
1292            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, <a href="mailto:mnot@mnot.net" title="Rackspace">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-17">HTTP/1.1, part 6: Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-17 (work in progress), October&nbsp;2011.
1293            </td>
1294         </tr>
1295         <tr>
1296            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1297            <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
1298            </td>
1299         </tr>
1300         <tr>
1301            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1302            <td class="top"><a href="mailto:dcrocker@bbiw.net" title="Brandenburg InternetWorking">Crocker, D., Ed.</a> and <a href="mailto:paul.overell@thus.net" title="THUS plc.">P. Overell</a>, “<a href="https://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
1303            </td>
1304         </tr>
1305      </table>
1306      <h2 id="rfc.references.2"><a href="#rfc.section.8.2" id="rfc.section.8.2">8.2</a> Informative References
1307      </h2>
1308      <table>
1309         <tr>
1310            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1311            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
1312            </td>
1313         </tr>
1314         <tr>
1315            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1316            <td class="top"><a href="mailto:GK-IETF@ninebynine.org" title="Nine by Nine">Klyne, G.</a>, <a href="mailto:mnot@pobox.com" title="BEA Systems">Nottingham, M.</a>, and <a href="mailto:JeffMogul@acm.org" title="HP Labs">J. Mogul</a>, “<a href="https://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
1317            </td>
1318         </tr>
1319         <tr>
1320            <td class="reference"><b id="RFC4918">[RFC4918]</b></td>
1321            <td class="top"><a href="mailto:ldusseault@commerce.net" title="CommerceNet">Dusseault, L., Ed.</a>, “<a href="https://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC&nbsp;4918, June&nbsp;2007.
1322            </td>
1323         </tr>
1324      </table>
1325      <div id="changes.from.rfc.2616">
1326         <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
1327         <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>).
1328         </p>
1329         <p id="rfc.section.A.p.2">Change ABNF productions for header fields to only define the field value. (<a href="#header.field.definitions" title="Precondition Header Fields">Section&nbsp;3</a>)
1330         </p>
1331      </div>
1332      <div id="collected.abnf">
1333         <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a href="#collected.abnf">Collected ABNF</a></h1>
1334         <div id="rfc.figure.u.17"></div><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag
1335
1336<a href="#notation" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
1337
1338<a href="#header.if-match" class="smpl">If-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1339 entity-tag ] ) )
1340<a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = HTTP-date
1341<a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1342 entity-tag ] ) )
1343<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = HTTP-date
1344
1345<a href="#header.last-modified" class="smpl">Last-Modified</a> = HTTP-date
1346
1347<a href="#notation" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1348
1349<a href="#header.etag" class="smpl">entity-tag</a> = [ weak ] opaque-tag
1350
1351<a href="#header.etag" class="smpl">opaque-tag</a> = quoted-string
1352
1353<a href="#notation" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
1354
1355<a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/
1356</pre><div id="rfc.figure.u.18"></div>
1357         <p>ABNF diagnostics:</p><pre class="inline">; ETag defined but not used
1358; If-Match defined but not used
1359; If-Modified-Since defined but not used
1360; If-None-Match defined but not used
1361; If-Unmodified-Since defined but not used
1362; Last-Modified defined but not used
1363</pre></div>
1364      <div id="change.log">
1365         <h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
1366         <div>
1367            <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a>&nbsp;Since RFC 2616
1368            </h2>
1369            <p id="rfc.section.C.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>.
1370            </p>
1371         </div>
1372         <div>
1373            <h2 id="rfc.section.C.2"><a href="#rfc.section.C.2">C.2</a>&nbsp;Since draft-ietf-httpbis-p4-conditional-00
1374            </h2>
1375            <p id="rfc.section.C.2.p.1">Closed issues: </p>
1376            <ul>
1377               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/35">http://tools.ietf.org/wg/httpbis/trac/ticket/35</a>&gt;: "Normative and Informative references"
1378               </li>
1379            </ul>
1380            <p id="rfc.section.C.2.p.2">Other changes: </p>
1381            <ul>
1382               <li>Move definitions of 304 and 412 condition codes from Part2.</li>
1383            </ul>
1384         </div>
1385         <div>
1386            <h2 id="rfc.section.C.3"><a href="#rfc.section.C.3">C.3</a>&nbsp;Since draft-ietf-httpbis-p4-conditional-01
1387            </h2>
1388            <p id="rfc.section.C.3.p.1">Ongoing work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
1389            </p>
1390            <ul>
1391               <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li>
1392            </ul>
1393         </div>
1394         <div id="changes.since.02">
1395            <h2 id="rfc.section.C.4"><a href="#rfc.section.C.4">C.4</a>&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></h2>
1396            <p id="rfc.section.C.4.p.1">Closed issues: </p>
1397            <ul>
1398               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/116">http://tools.ietf.org/wg/httpbis/trac/ticket/116</a>&gt;: "Weak ETags on non-GET requests"
1399               </li>
1400            </ul>
1401            <p id="rfc.section.C.4.p.2">Ongoing work on IANA Message Header Field Registration (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/40">http://tools.ietf.org/wg/httpbis/trac/ticket/40</a>&gt;):
1402            </p>
1403            <ul>
1404               <li>Reference RFC 3984, and update header field registrations for header fields defined in this document.</li>
1405            </ul>
1406         </div>
1407         <div id="changes.since.03">
1408            <h2 id="rfc.section.C.5"><a href="#rfc.section.C.5">C.5</a>&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></h2>
1409            <p id="rfc.section.C.5.p.1">Closed issues: </p>
1410            <ul>
1411               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/71">http://tools.ietf.org/wg/httpbis/trac/ticket/71</a>&gt;: "Examples for ETag matching"
1412               </li>
1413               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/124">http://tools.ietf.org/wg/httpbis/trac/ticket/124</a>&gt;: "'entity value' undefined"
1414               </li>
1415               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/126">http://tools.ietf.org/wg/httpbis/trac/ticket/126</a>&gt;: "bogus 2068 Date header reference"
1416               </li>
1417            </ul>
1418         </div>
1419         <div id="changes.since.04">
1420            <h2 id="rfc.section.C.6"><a href="#rfc.section.C.6">C.6</a>&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></h2>
1421            <p id="rfc.section.C.6.p.1">Ongoing work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
1422            </p>
1423            <ul>
1424               <li>Use "/" instead of "|" for alternatives.</li>
1425               <li>Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").</li>
1426               <li>Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.</li>
1427            </ul>
1428         </div>
1429         <div id="changes.since.05">
1430            <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a>&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></h2>
1431            <p id="rfc.section.C.7.p.1">Final work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
1432            </p>
1433            <ul>
1434               <li>Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.</li>
1435            </ul>
1436         </div>
1437         <div id="changes.since.06">
1438            <h2 id="rfc.section.C.8"><a href="#rfc.section.C.8">C.8</a>&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></h2>
1439            <p id="rfc.section.C.8.p.1">Closed issues: </p>
1440            <ul>
1441               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/153">http://tools.ietf.org/wg/httpbis/trac/ticket/153</a>&gt;: "case-sensitivity of etag weakness indicator"
1442               </li>
1443            </ul>
1444         </div>
1445         <div id="changes.since.07">
1446            <h2 id="rfc.section.C.9"><a href="#rfc.section.C.9">C.9</a>&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></h2>
1447            <p id="rfc.section.C.9.p.1">Closed issues: </p>
1448            <ul>
1449               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/116">http://tools.ietf.org/wg/httpbis/trac/ticket/116</a>&gt;: "Weak ETags on non-GET requests" (If-Match still was defined to require strong matching)
1450               </li>
1451               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/198">http://tools.ietf.org/wg/httpbis/trac/ticket/198</a>&gt;: "move IANA registrations for optional status codes"
1452               </li>
1453            </ul>
1454         </div>
1455         <div id="changes.since.08">
1456            <h2 id="rfc.section.C.10"><a href="#rfc.section.C.10">C.10</a>&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></h2>
1457            <p id="rfc.section.C.10.p.1">No significant changes.</p>
1458         </div>
1459         <div id="changes.since.09">
1460            <h2 id="rfc.section.C.11"><a href="#rfc.section.C.11">C.11</a>&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></h2>
1461            <p id="rfc.section.C.11.p.1">No significant changes.</p>
1462         </div>
1463         <div id="changes.since.10">
1464            <h2 id="rfc.section.C.12"><a href="#rfc.section.C.12">C.12</a>&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></h2>
1465            <p id="rfc.section.C.12.p.1">Closed issues: </p>
1466            <ul>
1467               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/69">http://tools.ietf.org/wg/httpbis/trac/ticket/69</a>&gt;: "Clarify 'Requested Variant'"
1468               </li>
1469               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/109">http://tools.ietf.org/wg/httpbis/trac/ticket/109</a>&gt;: "Clarify entity / representation / variant terminology"
1470               </li>
1471               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/220">http://tools.ietf.org/wg/httpbis/trac/ticket/220</a>&gt;: "consider removing the 'changes from 2068' sections"
1472               </li>
1473            </ul>
1474         </div>
1475         <div id="changes.since.11">
1476            <h2 id="rfc.section.C.13"><a href="#rfc.section.C.13">C.13</a>&nbsp;<a href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></h2>
1477            <p id="rfc.section.C.13.p.1">None.</p>
1478         </div>
1479         <div id="changes.since.12">
1480            <h2 id="rfc.section.C.14"><a href="#rfc.section.C.14">C.14</a>&nbsp;<a href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></h2>
1481            <p id="rfc.section.C.14.p.1">Closed issues: </p>
1482            <ul>
1483               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/224">http://tools.ietf.org/wg/httpbis/trac/ticket/224</a>&gt;: "Header Classification"
1484               </li>
1485            </ul>
1486         </div>
1487         <div id="changes.since.13">
1488            <h2 id="rfc.section.C.15"><a href="#rfc.section.C.15">C.15</a>&nbsp;<a href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></h2>
1489            <p id="rfc.section.C.15.p.1">Closed issues: </p>
1490            <ul>
1491               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/89">http://tools.ietf.org/wg/httpbis/trac/ticket/89</a>&gt;: "If-* and entities"
1492               </li>
1493               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/101">http://tools.ietf.org/wg/httpbis/trac/ticket/101</a>&gt;: "Definition of validator weakness"
1494               </li>
1495               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/276">http://tools.ietf.org/wg/httpbis/trac/ticket/276</a>&gt;: "untangle ABNFs for header fields"
1496               </li>
1497               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/269">http://tools.ietf.org/wg/httpbis/trac/ticket/269</a>&gt;: "ETags and Quotes"
1498               </li>
1499            </ul>
1500         </div>
1501         <div id="changes.since.14">
1502            <h2 id="rfc.section.C.16"><a href="#rfc.section.C.16">C.16</a>&nbsp;<a href="#changes.since.14">Since draft-ietf-httpbis-p4-conditional-14</a></h2>
1503            <p id="rfc.section.C.16.p.1">None.</p>
1504         </div>
1505         <div id="changes.since.15">
1506            <h2 id="rfc.section.C.17"><a href="#rfc.section.C.17">C.17</a>&nbsp;<a href="#changes.since.15">Since draft-ietf-httpbis-p4-conditional-15</a></h2>
1507            <p id="rfc.section.C.17.p.1">Closed issues: </p>
1508            <ul>
1509               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/304">http://tools.ietf.org/wg/httpbis/trac/ticket/304</a>&gt;: "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1510               </li>
1511            </ul>
1512         </div>
1513         <div id="changes.since.16">
1514            <h2 id="rfc.section.C.18"><a href="#rfc.section.C.18">C.18</a>&nbsp;<a href="#changes.since.16">Since draft-ietf-httpbis-p4-conditional-16</a></h2>
1515            <p id="rfc.section.C.18.p.1">Closed issues: </p>
1516            <ul>
1517               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/186">http://tools.ietf.org/wg/httpbis/trac/ticket/186</a>&gt;: "Document HTTP's error-handling philosophy"
1518               </li>
1519            </ul>
1520         </div>
1521      </div>
1522      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1523      <p class="noprint"><a href="#rfc.index.3">3</a> <a href="#rfc.index.4">4</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.I">I</a> <a href="#rfc.index.L">L</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a>
1524      </p>
1525      <div class="print2col">
1526         <ul class="ind">
1527            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
1528                  <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.3.1"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li>
1529               </ul>
1530            </li>
1531            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
1532                  <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.4.1"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li>
1533               </ul>
1534            </li>
1535            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
1536                  <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>
1537               </ul>
1538            </li>
1539            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
1540                  <li><tt>Grammar</tt>&nbsp;&nbsp;
1541                     <ul>
1542                        <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2.3</b></a></li>
1543                        <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2.3</b></a></li>
1544                        <li><tt>If-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>3.1</b></a></li>
1545                        <li><tt>If-Modified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>3.3</b></a></li>
1546                        <li><tt>If-None-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>3.2</b></a></li>
1547                        <li><tt>If-Unmodified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>3.4</b></a></li>
1548                        <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2.2</b></a></li>
1549                        <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>2.3</b></a></li>
1550                        <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>2.3</b></a></li>
1551                     </ul>
1552                  </li>
1553               </ul>
1554            </li>
1555            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
1556                  <li>Header Fields&nbsp;&nbsp;
1557                     <ul>
1558                        <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>
1559                        <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>
1560                        <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>
1561                        <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>
1562                        <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>
1563                        <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>
1564                     </ul>
1565                  </li>
1566               </ul>
1567            </li>
1568            <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
1569                  <li>If-Match header field&nbsp;&nbsp;<a href="#rfc.iref.i.1"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">5.2</a></li>
1570                  <li>If-Modified-Since header field&nbsp;&nbsp;<a href="#rfc.iref.i.3"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">5.2</a></li>
1571                  <li>If-None-Match header field&nbsp;&nbsp;<a href="#rfc.iref.i.2"><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>
1572                  <li>If-Unmodified-Since header field&nbsp;&nbsp;<a href="#rfc.iref.i.4"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">5.2</a></li>
1573               </ul>
1574            </li>
1575            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
1576                  <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>
1577               </ul>
1578            </li>
1579            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
1580                  <li>metadata&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b>2</b></a></li>
1581               </ul>
1582            </li>
1583            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
1584                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.1</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">1.2</a>, <a href="#rfc.xref.Part1.6">2.3.3</a>, <a href="#rfc.xref.Part1.7">6</a>, <a href="#rfc.xref.Part1.8">7</a>, <a href="#Part1"><b>8.1</b></a><ul>
1585                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.2</a></li>
1586                        <li><em>Section 1.2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">1.2</a></li>
1587                        <li><em>Section 2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.1</a></li>
1588                        <li><em>Section 3.2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">1.2</a></li>
1589                        <li><em>Section 5.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.6">2.3.3</a></li>
1590                        <li><em>Section 11</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.8">7</a></li>
1591                     </ul>
1592                  </li>
1593                  <li><em>Part2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.1">1.2</a>, <a href="#rfc.xref.Part2.2">1.2</a>, <a href="#rfc.xref.Part2.3">4.1</a>, <a href="#Part2"><b>8.1</b></a><ul>
1594                        <li><em>Section 8</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.2">1.2</a></li>
1595                        <li><em>Section 9.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.3">4.1</a></li>
1596                     </ul>
1597                  </li>
1598                  <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>
1599                        <li><em>Section 5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.3.3</a></li>
1600                        <li><em>Section 6.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.2">2.3.3</a></li>
1601                     </ul>
1602                  </li>
1603                  <li><em>Part5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.3</a>, <a href="#rfc.xref.Part5.2">3.5</a>, <a href="#Part5"><b>8.1</b></a><ul>
1604                        <li><em>Section 5.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.2">3.5</a></li>
1605                        <li><em>Section 5.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.3</a></li>
1606                     </ul>
1607                  </li>
1608                  <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>
1609               </ul>
1610            </li>
1611            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
1612                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>8.1</b></a></li>
1613                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#RFC2616"><b>8.2</b></a>, <a href="#rfc.xref.RFC2616.1">C.1</a></li>
1614                  <li><em>RFC3864</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC3864.1">5.2</a>, <a href="#RFC3864"><b>8.2</b></a></li>
1615                  <li><em>RFC4918</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC4918.1">2</a>, <a href="#RFC4918"><b>8.2</b></a></li>
1616                  <li><em>RFC5234</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.1">1.2</a>, <a href="#rfc.xref.RFC5234.2">1.2</a>, <a href="#RFC5234"><b>8.1</b></a><ul>
1617                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">1.2</a></li>
1618                     </ul>
1619                  </li>
1620               </ul>
1621            </li>
1622            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
1623                  <li>selected representation&nbsp;&nbsp;<a href="#rfc.iref.s.1"><b>1</b></a></li>
1624                  <li>Status Codes&nbsp;&nbsp;
1625                     <ul>
1626                        <li>304 Not Modified&nbsp;&nbsp;<a href="#rfc.iref.s.2"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li>
1627                        <li>412 Precondition Failed&nbsp;&nbsp;<a href="#rfc.iref.s.3"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li>
1628                     </ul>
1629                  </li>
1630               </ul>
1631            </li>
1632            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
1633                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1"><b>2</b></a><ul>
1634                        <li>strong&nbsp;&nbsp;<a href="#rfc.iref.v.3"><b>2.1</b></a></li>
1635                        <li>weak&nbsp;&nbsp;<a href="#rfc.iref.v.2"><b>2.1</b></a></li>
1636                     </ul>
1637                  </li>
1638               </ul>
1639            </li>
1640         </ul>
1641      </div>
1642      <div class="avoidbreak">
1643         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1644         <p><b>Roy T. Fielding</b>
1645            (editor)
1646            <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: <a href="mailto:fielding@gbiv.com">fielding@gbiv.com</a><br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p>
1647         <p><b>Jim Gettys</b><br>Alcatel-Lucent Bell Labs<br>21 Oak Knoll Road<br>Carlisle, MA&nbsp;01741<br>USA<br>Email: <a href="mailto:jg@freedesktop.org">jg@freedesktop.org</a><br>URI: <a href="http://gettys.wordpress.com/">http://gettys.wordpress.com/</a></p>
1648         <p><b>Jeffrey C. Mogul</b><br>Hewlett-Packard Company<br>HP Labs, Large Scale Systems Group<br>1501 Page Mill Road, MS 1177<br>Palo Alto, CA&nbsp;94304<br>USA<br>Email: <a href="mailto:JeffMogul@acm.org">JeffMogul@acm.org</a></p>
1649         <p><b>Henrik Frystyk Nielsen</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>USA<br>Email: <a href="mailto:henrikn@microsoft.com">henrikn@microsoft.com</a></p>
1650         <p><b>Larry Masinter</b><br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: <a href="mailto:LMM@acm.org">LMM@acm.org</a><br>URI: <a href="http://larry.masinter.net/">http://larry.masinter.net/</a></p>
1651         <p><b>Paul J. Leach</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>Email: <a href="mailto:paulle@microsoft.com">paulle@microsoft.com</a></p>
1652         <p><b>Tim Berners-Lee</b><br>World Wide Web Consortium<br>MIT Computer Science and Artificial Intelligence Laboratory<br>The Stata Center, Building 32<br>32 Vassar Street<br>Cambridge, MA&nbsp;02139<br>USA<br>Email: <a href="mailto:timbl@w3.org">timbl@w3.org</a><br>URI: <a href="http://www.w3.org/People/Berners-Lee/">http://www.w3.org/People/Berners-Lee/</a></p>
1653         <p><b>Yves Lafon</b>
1654            (editor)
1655            <br>World Wide Web Consortium<br>W3C / ERCIM<br>2004, rte des Lucioles<br>Sophia-Antipolis, AM&nbsp;06902<br>France<br>Email: <a href="mailto:ylafon@w3.org">ylafon@w3.org</a><br>URI: <a href="http://www.raubacapeu.net/people/yves/">http://www.raubacapeu.net/people/yves/</a></p>
1656         <p><b>Julian F. Reschke</b>
1657            (editor)
1658            <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Phone: <a href="tel:+492512807760">+49 251 2807760</a><br>Fax: <a href="fax:+492512807761">+49 251 2807761</a><br>Email: <a href="mailto:julian.reschke@greenbytes.de">julian.reschke@greenbytes.de</a><br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p>
1659      </div>
1660   </body>
1661</html>
Note: See TracBrowser for help on using the repository browser.