source: draft-ietf-httpbis/latest/p4-conditional.html @ 1350

Last change on this file since 1350 was 1350, checked in by fielding@…, 8 years ago

update generated dates

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/html;charset=utf-8
File size: 106.9 KB
Line 
1<!DOCTYPE html
2  PUBLIC "-//W3C//DTD HTML 4.01//EN">
3<html lang="en">
4   <head profile="http://www.w3.org/2006/03/hcard 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: verdana, helvetica, arial, sans-serif;
27  font-size: 10pt;
28}
29cite {
30  font-style: normal;
31}
32div.note {
33  margin-left: 2em;
34}
35dd {
36  margin-right: 2em;
37}
38dl {
39  margin-left: 2em;
40}
41
42ul.empty {
43  list-style-type: none;
44}
45ul.empty li {
46  margin-top: .5em;
47}
48dl p {
49  margin-left: 0em;
50}
51dt {
52  margin-top: .5em;
53}
54h1 {
55  font-size: 14pt;
56  line-height: 21pt;
57  page-break-after: avoid;
58}
59h1.np {
60  page-break-before: always;
61}
62h1 a {
63  color: #333333;
64}
65h2 {
66  font-size: 12pt;
67  line-height: 15pt;
68  page-break-after: avoid;
69}
70h3, h4, h5, h6 {
71  font-size: 10pt;
72  page-break-after: avoid;
73}
74h2 a, h3 a, h4 a, h5 a, h6 a {
75  color: black;
76}
77img {
78  margin-left: 3em;
79}
80li {
81  margin-left: 2em;
82  margin-right: 2em;
83}
84ol {
85  margin-left: 2em;
86  margin-right: 2em;
87}
88ol.la {
89  list-style-type: lower-alpha;
90}
91ol.ua {
92  list-style-type: upper-alpha;
93}
94ol p {
95  margin-left: 0em;
96}
97p {
98  margin-left: 2em;
99  margin-right: 2em;
100}
101pre {
102  margin-left: 3em;
103  background-color: lightyellow;
104  padding: .25em;
105}
106pre.text2 {
107  border-style: dotted;
108  border-width: 1px;
109  background-color: #f0f0f0;
110  width: 69em;
111}
112pre.inline {
113  background-color: white;
114  padding: 0em;
115}
116pre.text {
117  border-style: dotted;
118  border-width: 1px;
119  background-color: #f8f8f8;
120  width: 69em;
121}
122pre.drawing {
123  border-style: solid;
124  border-width: 1px;
125  background-color: #f8f8f8;
126  padding: 2em;
127}
128table {
129  margin-left: 2em;
130}
131table.tt {
132  vertical-align: top;
133}
134table.full {
135  border-style: outset;
136  border-width: 1px;
137}
138table.headers {
139  border-style: outset;
140  border-width: 1px;
141}
142table.tt td {
143  vertical-align: top;
144}
145table.full td {
146  border-style: inset;
147  border-width: 1px;
148}
149table.tt th {
150  vertical-align: top;
151}
152table.full th {
153  border-style: inset;
154  border-width: 1px;
155}
156table.headers th {
157  border-style: none none inset none;
158  border-width: 1px;
159}
160table.left {
161  margin-right: auto;
162}
163table.right {
164  margin-left: auto;
165}
166table.center {
167  margin-left: auto;
168  margin-right: auto;
169}
170caption {
171  caption-side: bottom;
172  font-weight: bold;
173  font-size: 9pt;
174  margin-top: .5em;
175}
176
177table.header {
178  border-spacing: 1px;
179  width: 95%;
180  font-size: 10pt;
181  color: white;
182}
183td.top {
184  vertical-align: top;
185}
186td.topnowrap {
187  vertical-align: top;
188  white-space: nowrap; 
189}
190table.header td {
191  background-color: gray;
192  width: 50%;
193}
194table.header a {
195  color: white;
196}
197td.reference {
198  vertical-align: top;
199  white-space: nowrap;
200  padding-right: 1em;
201}
202thead {
203  display:table-header-group;
204}
205ul.toc, ul.toc ul {
206  list-style: none;
207  margin-left: 1.5em;
208  margin-right: 0em;
209  padding-left: 0em;
210}
211ul.toc li {
212  line-height: 150%;
213  font-weight: bold;
214  font-size: 10pt;
215  margin-left: 0em;
216  margin-right: 0em;
217}
218ul.toc li li {
219  line-height: normal;
220  font-weight: normal;
221  font-size: 9pt;
222  margin-left: 0em;
223  margin-right: 0em;
224}
225li.excluded {
226  font-size: 0pt;
227}
228ul p {
229  margin-left: 0em;
230}
231ul.ind, ul.ind ul {
232  list-style: none;
233  margin-left: 1.5em;
234  margin-right: 0em;
235  padding-left: 0em;
236  page-break-before: avoid;
237}
238ul.ind li {
239  font-weight: bold;
240  line-height: 200%;
241  margin-left: 0em;
242  margin-right: 0em;
243}
244ul.ind li li {
245  font-weight: normal;
246  line-height: 150%;
247  margin-left: 0em;
248  margin-right: 0em;
249}
250.avoidbreak {
251  page-break-inside: avoid;
252}
253.bcp14 {
254  font-style: normal;
255  text-transform: lowercase;
256  font-variant: small-caps;
257}
258.comment {
259  background-color: yellow;
260}
261.center {
262  text-align: center;
263}
264.error {
265  color: red;
266  font-style: italic;
267  font-weight: bold;
268}
269.figure {
270  font-weight: bold;
271  text-align: center;
272  font-size: 9pt;
273}
274.filename {
275  color: #333333;
276  font-weight: bold;
277  font-size: 12pt;
278  line-height: 21pt;
279  text-align: center;
280}
281.fn {
282  font-weight: bold;
283}
284.hidden {
285  display: none;
286}
287.left {
288  text-align: left;
289}
290.right {
291  text-align: right;
292}
293.title {
294  color: #990000;
295  font-size: 18pt;
296  line-height: 18pt;
297  font-weight: bold;
298  text-align: center;
299  margin-top: 36pt;
300}
301.vcardline {
302  display: block;
303}
304.warning {
305  font-size: 14pt;
306  background-color: yellow;
307}
308
309
310@media print {
311  .noprint {
312    display: none;
313  }
314 
315  a {
316    color: black;
317    text-decoration: none;
318  }
319
320  table.header {
321    width: 90%;
322  }
323
324  td.header {
325    width: 50%;
326    color: black;
327    background-color: white;
328    vertical-align: top;
329    font-size: 12pt;
330  }
331
332  ul.toc a::after {
333    content: leader('.') target-counter(attr(href), page);
334  }
335 
336  ul.ind li li a {
337    content: target-counter(attr(href), page);
338  }
339 
340  .print2col {
341    column-count: 2;
342    -moz-column-count: 2;
343    column-fill: auto;
344  }
345}
346
347@page {
348  @top-left {
349       content: "Internet-Draft"; 
350  } 
351  @top-right {
352       content: "July 2011"; 
353  } 
354  @top-center {
355       content: "HTTP/1.1, Part 4"; 
356  } 
357  @bottom-left {
358       content: "Fielding, et al."; 
359  } 
360  @bottom-center {
361       content: "Expires January 26, 2012"; 
362  } 
363  @bottom-right {
364       content: "[Page " counter(page) "]"; 
365  } 
366}
367
368@page:first { 
369    @top-left {
370      content: normal;
371    }
372    @top-right {
373      content: normal;
374    }
375    @top-center {
376      content: normal;
377    }
378}
379</style><link rel="Contents" href="#rfc.toc">
380      <link rel="Author" href="#rfc.authors">
381      <link rel="Copyright" href="#rfc.copyrightnotice">
382      <link rel="Index" href="#rfc.index">
383      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
384      <link rel="Chapter" title="2 Resource State Metadata (Validators)" href="#rfc.section.2">
385      <link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3">
386      <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4">
387      <link rel="Chapter" title="5 IANA Considerations" href="#rfc.section.5">
388      <link rel="Chapter" title="6 Security Considerations" href="#rfc.section.6">
389      <link rel="Chapter" title="7 Acknowledgments" href="#rfc.section.7">
390      <link rel="Chapter" href="#rfc.section.8" title="8 References">
391      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
392      <link rel="Appendix" title="B Collected ABNF" href="#rfc.section.B">
393      <link rel="Appendix" title="C Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.C">
394      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.550, 2011-05-30 14:02:12, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
395      <link rel="schema.dct" href="http://purl.org/dc/terms/">
396      <meta name="dct.creator" content="Fielding, R.">
397      <meta name="dct.creator" content="Gettys, J.">
398      <meta name="dct.creator" content="Mogul, J.">
399      <meta name="dct.creator" content="Frystyk, H.">
400      <meta name="dct.creator" content="Masinter, L.">
401      <meta name="dct.creator" content="Leach, P.">
402      <meta name="dct.creator" content="Berners-Lee, T.">
403      <meta name="dct.creator" content="Lafon, Y.">
404      <meta name="dct.creator" content="Reschke, J. F.">
405      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest">
406      <meta name="dct.issued" scheme="ISO8601" content="2011-07-25">
407      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
408      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia 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.">
409      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia 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.">
410   </head>
411   <body>
412      <table class="header">
413         <tbody>
414            <tr>
415               <td class="left">HTTPbis Working Group</td>
416               <td class="right">R. Fielding, Editor</td>
417            </tr>
418            <tr>
419               <td class="left">Internet-Draft</td>
420               <td class="right">Adobe</td>
421            </tr>
422            <tr>
423               <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
424               </td>
425               <td class="right">J. Gettys</td>
426            </tr>
427            <tr>
428               <td class="left">Intended status: Standards Track</td>
429               <td class="right">Alcatel-Lucent</td>
430            </tr>
431            <tr>
432               <td class="left">Expires: January 26, 2012</td>
433               <td class="right">J. Mogul</td>
434            </tr>
435            <tr>
436               <td class="left"></td>
437               <td class="right">HP</td>
438            </tr>
439            <tr>
440               <td class="left"></td>
441               <td class="right">H. Frystyk</td>
442            </tr>
443            <tr>
444               <td class="left"></td>
445               <td class="right">Microsoft</td>
446            </tr>
447            <tr>
448               <td class="left"></td>
449               <td class="right">L. Masinter</td>
450            </tr>
451            <tr>
452               <td class="left"></td>
453               <td class="right">Adobe</td>
454            </tr>
455            <tr>
456               <td class="left"></td>
457               <td class="right">P. Leach</td>
458            </tr>
459            <tr>
460               <td class="left"></td>
461               <td class="right">Microsoft</td>
462            </tr>
463            <tr>
464               <td class="left"></td>
465               <td class="right">T. Berners-Lee</td>
466            </tr>
467            <tr>
468               <td class="left"></td>
469               <td class="right">W3C/MIT</td>
470            </tr>
471            <tr>
472               <td class="left"></td>
473               <td class="right">Y. Lafon, Editor</td>
474            </tr>
475            <tr>
476               <td class="left"></td>
477               <td class="right">W3C</td>
478            </tr>
479            <tr>
480               <td class="left"></td>
481               <td class="right">J. Reschke, Editor</td>
482            </tr>
483            <tr>
484               <td class="left"></td>
485               <td class="right">greenbytes</td>
486            </tr>
487            <tr>
488               <td class="left"></td>
489               <td class="right">July 25, 2011</td>
490            </tr>
491         </tbody>
492      </table>
493      <p class="title">HTTP/1.1, part 4: Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-latest</span></p>
494      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
495      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
496         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the
497         seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part
498         4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.
499      </p> 
500      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
501      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived
502         at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
503      </p> 
504      <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;.
505      </p> 
506      <p>The changes in this draft are summarized in <a href="#changes.since.15" title="Since draft-ietf-httpbis-p4-conditional-15">Appendix&nbsp;C.17</a>.
507      </p> 
508      <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
509      <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
510      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
511         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>.
512      </p>
513      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
514         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
515         in progress”.
516      </p>
517      <p>This Internet-Draft will expire on January 26, 2012.</p>
518      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
519      <p>Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
520      <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
521         and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
522         text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
523         BSD License.
524      </p>
525      <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
526         10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
527         allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
528         controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
529         works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
530         it into languages other than English.
531      </p>
532      <hr class="noprint">
533      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
534      <ul class="toc">
535         <li>1.&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
536               <li>1.1&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
537               <li>1.2&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
538            </ul>
539         </li>
540         <li>2.&nbsp;&nbsp;&nbsp;<a href="#resource.metadata">Resource State Metadata (Validators)</a><ul>
541               <li>2.1&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
542                     <li>2.1.1&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
543                     <li>2.1.2&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
544                  </ul>
545               </li>
546               <li>2.2&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
547                     <li>2.2.1&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
548                     <li>2.2.2&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
549                     <li>2.2.3&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
550                     <li>2.2.4&nbsp;&nbsp;&nbsp;<a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></li>
551                     <li>2.2.5&nbsp;&nbsp;&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li>
552                  </ul>
553               </li>
554            </ul>
555         </li>
556         <li>3.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Precondition Header Fields</a><ul>
557               <li>3.1&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
558               <li>3.2&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
559               <li>3.3&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
560               <li>3.4&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
561               <li>3.5&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li>
562            </ul>
563         </li>
564         <li>4.&nbsp;&nbsp;&nbsp;<a href="#status.code.definitions">Status Code Definitions</a><ul>
565               <li>4.1&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
566               <li>4.2&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
567            </ul>
568         </li>
569         <li>5.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
570               <li>5.1&nbsp;&nbsp;&nbsp;<a href="#status.code.registration">Status Code Registration</a></li>
571               <li>5.2&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
572            </ul>
573         </li>
574         <li>6.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
575         <li>7.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
576         <li>8.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
577               <li>8.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
578               <li>8.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
579            </ul>
580         </li>
581         <li><a href="#rfc.authors">Authors' Addresses</a></li>
582         <li>A.&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
583         <li>B.&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
584         <li>C.&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
585               <li>C.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC 2616</a></li>
586               <li>C.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p4-conditional-00</a></li>
587               <li>C.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p4-conditional-01</a></li>
588               <li>C.4&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></li>
589               <li>C.5&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></li>
590               <li>C.6&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></li>
591               <li>C.7&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></li>
592               <li>C.8&nbsp;&nbsp;&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></li>
593               <li>C.9&nbsp;&nbsp;&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></li>
594               <li>C.10&nbsp;&nbsp;&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></li>
595               <li>C.11&nbsp;&nbsp;&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></li>
596               <li>C.12&nbsp;&nbsp;&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></li>
597               <li>C.13&nbsp;&nbsp;&nbsp;<a href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></li>
598               <li>C.14&nbsp;&nbsp;&nbsp;<a href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></li>
599               <li>C.15&nbsp;&nbsp;&nbsp;<a href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></li>
600               <li>C.16&nbsp;&nbsp;&nbsp;<a href="#changes.since.14">Since draft-ietf-httpbis-p4-conditional-14</a></li>
601               <li>C.17&nbsp;&nbsp;&nbsp;<a href="#changes.since.15">Since draft-ietf-httpbis-p4-conditional-15</a></li>
602            </ul>
603         </li>
604         <li><a href="#rfc.index">Index</a></li>
605      </ul>
606      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
607      <p id="rfc.section.1.p.1">This document defines the HTTP/1.1 conditional request mechanisms, including both response metadata that can be used to indicate
608         or observe changes to resource state and request header fields that specify preconditions to be checked before performing
609         the action given by the request method. Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem:
610         one client accidentally overwriting the work of another client that has been acting in parallel.
611      </p>
612      <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
613         state as observed in a previously obtained representation (one value in that set). A resource might have multiple current
614         representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests
615         to corresponding representations will be consistent over time if the server intends to take advantage of conditionals. Regardless,
616         if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result
617         when the precondition evaluates to false.
618      </p>
619      <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
620         the same request had used the method GET and had excluded all of the conditional request header fields. The conditional request
621         preconditions are evaluated by comparing the values provided in the request header fields to the current metadata for the
622         selected representation.
623      </p>
624      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2>
625      <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"
626         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>.
627      </p>
628      <p id="rfc.section.1.1.p.2">An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the
629         protocols it implements. An implementation that satisfies all the "MUST" or "REQUIRED" level and all the "SHOULD" level requirements
630         for its protocols is said to be "unconditionally compliant"; one that satisfies all the "MUST" level requirements but not
631         all the "SHOULD" level requirements for its protocols is said to be "conditionally compliant".
632      </p>
633      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="notation" href="#notation">Syntax Notation</a></h2>
634      <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.Part1.1"><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.
635      </p>
636      <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="http://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
637         (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character),
638         and WSP (whitespace).
639      </p>
640      <p id="rfc.section.1.2.p.3">The ABNF rules below are defined in other parts:</p>
641      <div id="rfc.figure.u.1"></div><pre class="inline">  <a href="#notation" class="smpl">quoted-string</a> = &lt;quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.2"><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;
642  <a href="#notation" class="smpl">OWS</a>           = &lt;OWS, 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>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
643  <a href="#notation" class="smpl">HTTP-date</a>     = &lt;HTTP-date, 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#date.time.formats.full.date" title="Date/Time Formats: Full Date">Section 6.1</a>&gt;
644</pre><div id="rfc.iref.m.1"></div>
645      <div id="rfc.iref.v.1"></div>
646      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="resource.metadata" href="#resource.metadata">Resource State Metadata (Validators)</a></h1>
647      <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:
648         modification dates and opaque entity tags. Additional metadata that reflects resource state has been defined by various extensions
649         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.
650      </p>
651      <div id="rfc.iref.l.1"></div>
652      <div id="rfc.iref.h.1"></div>
653      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2>
654      <p id="rfc.section.2.1.p.1">The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation
655         was last modified.
656      </p>
657      <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>
658</pre><p id="rfc.section.2.1.p.3">An example of its use is</p>
659      <div id="rfc.figure.u.3"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
660</pre><h3 id="rfc.section.2.1.1"><a href="#rfc.section.2.1.1">2.1.1</a>&nbsp;<a id="lastmod.generation" href="#lastmod.generation">Generation</a></h3>
661      <p id="rfc.section.2.1.1.p.1">Origin servers <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined,
662         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
663         scalability and reliability.
664      </p>
665      <p id="rfc.section.2.1.1.p.2">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be
666         the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
667         detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
668         use its value to make conditional requests and test the validity of locally cached responses.
669      </p>
670      <p id="rfc.section.2.1.1.p.3">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value
671         for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially
672         if the representation changes near the time that the response is generated.
673      </p>
674      <p id="rfc.section.2.1.1.p.4">An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time
675         is derived from implementation-specific metadata that evaluates to some time in the future, according to the origin server's
676         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
677         on cache validation.
678      </p>
679      <h3 id="rfc.section.2.1.2"><a href="#rfc.section.2.1.2">2.1.2</a>&nbsp;<a id="lastmod.comparison" href="#lastmod.comparison">Comparison</a></h3>
680      <p id="rfc.section.2.1.2.p.1">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is
681         strong, using the following rules:
682      </p>
683      <ul>
684         <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li>
685         <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the
686            presented validator.
687         </li>
688      </ul>
689      <p id="rfc.section.2.1.2.p.2">or </p>
690      <ul>
691         <li>The validator is about to be used by a client in an If-Modified-Since, If-Unmodified-Since header field, because the client
692            has a cache entry, or If-Range for the associated representation, and
693         </li>
694         <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
695         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
696      </ul>
697      <p id="rfc.section.2.1.2.p.3">or </p>
698      <ul>
699         <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
700            and
701         </li>
702         <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
703         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
704      </ul>
705      <p id="rfc.section.2.1.2.p.4">This method relies on the fact that if two different responses were sent by the origin server during the same second, but
706         both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified
707         time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from
708         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.
709      </p>
710      <div id="rfc.iref.e.1"></div>
711      <div id="rfc.iref.h.2"></div>
712      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="header.etag" href="#header.etag">ETag</a></h2>
713      <p id="rfc.section.2.2.p.1">The ETag header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator
714         for differentiating between multiple representations of the same resource, regardless of whether those multiple representations
715         are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the
716         same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.
717      </p>
718      <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>
719
720  <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>
721  <a href="#header.etag" class="smpl">weak</a>       = %x57.2F ; "W/", case-sensitive
722  <a href="#header.etag" class="smpl">opaque-tag</a> = <a href="#notation" class="smpl">quoted-string</a>
723</pre><p id="rfc.section.2.2.p.3">An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store
724         modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are
725         not consistently maintained.
726      </p>
727      <div id="rfc.figure.u.5"></div>
728      <p>Examples:</p>  <pre class="text">  ETag: "xyzzy"
729  ETag: W/"xyzzy"
730  ETag: ""
731</pre><h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;<a id="entity.tag.generation" href="#entity.tag.generation">Generation</a></h3>
732      <p id="rfc.section.2.2.1.p.1">The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select
733         the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple
734         sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each
735         entity-tag is constructed.
736      </p>
737      <p id="rfc.section.2.2.1.p.2">For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision
738         number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations.
739         Other implementations might use a stored hash of representation content, a combination of various filesystem attributes, or
740         a modification timestamp that has sub-second resolution.
741      </p>
742      <p id="rfc.section.2.2.1.p.3">Origin servers <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since
743         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
744         and reliability.
745      </p>
746      <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;<a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak versus Strong</a></h3>
747      <p id="rfc.section.2.2.2.p.1">Since both origin servers and caches will compare two validators to decide if they indicate the same or different representations,
748         one normally would expect that if the representation (including both representation header fields and representation body)
749         changes in any way, then the associated validator would change as well. If this is true, then we call that validator a "strong
750         validator". One example of a strong validator is an integer that is incremented in stable storage every time a representation
751         is changed.
752      </p>
753      <p id="rfc.section.2.2.2.p.2">However, there might be cases when a server prefers to change the validator only when it desires cached representations to
754         be invalidated. For example, the representation of a weather report that changes in content every second, based on dynamic
755         measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) in order to
756         allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load
757         or weather quality). A validator that does not always change when the representation changes is a "weak validator".
758      </p>
759      <p id="rfc.section.2.2.2.p.3">One can think of a strong validator as part of an identifier for a specific representation, whereas a weak validator is part
760         of an identifier for a set of equivalent representations (where this notion of equivalence is entirely governed by the origin
761         server and beyond the scope of this specification).
762      </p>
763      <p id="rfc.section.2.2.2.p.4">An entity-tag is normally a strong validator, but the protocol provides a mechanism to tag an entity-tag as "weak". </p>
764      <ul class="empty">
765         <li>A representation's modification time, if defined with only one-second resolution, could be a weak validator, since it is possible
766            that the representation might be modified twice during a single second.
767         </li>
768         <li>Support for weak validators is optional. However, weak validators allow for more efficient caching of equivalent objects;
769            for example, a hit counter on a site is probably good enough if it is updated every few days or weeks, and any value during
770            that period is likely "good enough" to be equivalent.
771         </li>
772      </ul>
773      <p id="rfc.section.2.2.2.p.5">A strong entity-tag <em class="bcp14">MUST</em> change whenever the associated representation changes in any way. A weak entity-tag <em class="bcp14">SHOULD</em> change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation.
774         In other words, a weak entity tag <em class="bcp14">SHOULD</em> change whenever the origin server wants caches to invalidate old responses.
775      </p>
776      <p id="rfc.section.2.2.2.p.6">A "strong entity-tag" <em class="bcp14">MAY</em> be shared by two representations of a resource only if they are equivalent by octet equality.
777      </p>
778      <p id="rfc.section.2.2.2.p.7">A "weak entity-tag", indicated by the "W/" prefix, <em class="bcp14">MAY</em> be shared by two representations of a resource. A weak entity-tag can only be used for weak comparison.
779      </p>
780      <p id="rfc.section.2.2.2.p.8">Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate
781         an entry using a validator that it obtained in the distant past. A strong entity-tag <em class="bcp14">MUST</em> be unique across all versions of all representations associated with a particular resource over time. However, there is no
782         implication of uniqueness across entity-tags of different resources (i.e., the same entity-tag value might be in use for representations
783         of multiple resources at the same time and does not imply that those representations are equivalent).
784      </p>
785      <h3 id="rfc.section.2.2.3"><a href="#rfc.section.2.2.3">2.2.3</a>&nbsp;<a id="entity.tag.comparison" href="#entity.tag.comparison">Comparison</a></h3>
786      <p id="rfc.section.2.2.3.p.1">There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators
787         or not:
788      </p>
789      <ul>
790         <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.
791         </li>
792         <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.
793         </li>
794      </ul>
795      <p id="rfc.section.2.2.3.p.2">A "use" of a validator is either when a client generates a request and includes the validator in a precondition, or when a
796         server compares two validators.
797      </p>
798      <p id="rfc.section.2.2.3.p.3">Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality
799         of a representation. For example, either kind is usable for a normal conditional GET.
800      </p>
801      <p id="rfc.section.2.2.3.p.4">The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:</p>
802      <div id="rfc.table.u.1">
803         <table class="tt full left" cellpadding="3" cellspacing="0">
804            <thead>
805               <tr>
806                  <th>ETag 1</th>
807                  <th>ETag 2</th>
808                  <th>Strong Comparison</th>
809                  <th>Weak Comparison</th>
810               </tr>
811            </thead>
812            <tbody>
813               <tr>
814                  <td class="left">W/"1"</td>
815                  <td class="left">W/"1"</td>
816                  <td class="left">no match</td>
817                  <td class="left">match</td>
818               </tr>
819               <tr>
820                  <td class="left">W/"1"</td>
821                  <td class="left">W/"2"</td>
822                  <td class="left">no match</td>
823                  <td class="left">no match</td>
824               </tr>
825               <tr>
826                  <td class="left">W/"1"</td>
827                  <td class="left">"1"</td>
828                  <td class="left">no match</td>
829                  <td class="left">match</td>
830               </tr>
831               <tr>
832                  <td class="left">"1"</td>
833                  <td class="left">"1"</td>
834                  <td class="left">match</td>
835                  <td class="left">match</td>
836               </tr>
837            </tbody>
838         </table>
839      </div>
840      <p id="rfc.section.2.2.3.p.5">An entity-tag is strong unless it is explicitly tagged as weak.</p>
841      <h3 id="rfc.section.2.2.4"><a href="#rfc.section.2.2.4">2.2.4</a>&nbsp;<a id="rules.for.when.to.use.entity.tags.and.last-modified.dates" href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></h3>
842      <p id="rfc.section.2.2.4.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types
843         ought to be used, and for what purposes.
844      </p>
845      <p id="rfc.section.2.2.4.p.2">HTTP/1.1 origin servers: </p>
846      <ul>
847         <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
848         </li>
849         <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,
850            or if it is unfeasible to send a strong entity-tag.
851         </li>
852         <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one.
853         </li>
854      </ul>
855      <p id="rfc.section.2.2.4.p.3">In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity-tag and a Last-Modified
856         value.
857      </p>
858      <p id="rfc.section.2.2.4.p.4">HTTP/1.1 clients: </p>
859      <ul>
860         <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
861            by the origin server.
862         </li>
863         <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
864            has been provided by the origin server.
865         </li>
866         <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
867            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.
868         </li>
869         <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
870            origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
871         </li>
872      </ul>
873      <p id="rfc.section.2.2.4.p.5">An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since
874         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
875         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
876         in the request.
877      </p>
878      <p id="rfc.section.2.2.4.p.6">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags
879         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
880         fields in the request.
881      </p>
882      <ul class="empty">
883         <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
884            as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative
885            assumptions about the validators they receive.
886         </li>
887         <li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will
888            support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare
889            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
890            HTTP/1.1 origin servers should not provide one.
891         </li>
892      </ul>
893      <h3 id="rfc.section.2.2.5"><a href="#rfc.section.2.2.5">2.2.5</a>&nbsp;<a id="example.entity.tag.vs.conneg" href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h3>
894      <p id="rfc.section.2.2.5.p.1">Consider a resource that is subject to content negotiation (<a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 5</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (<a href="p3-payload.html#header.accept-encoding" title="Accept-Encoding">Section 6.3</a> of <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>):
895      </p>
896      <div id="rfc.figure.u.6"></div>
897      <p>&gt;&gt; Request:</p><pre class="text2">GET /index HTTP/1.1
898Host: www.example.com
899Accept-Encoding: gzip
900
901</pre><p id="rfc.section.2.2.5.p.3">In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:</p>
902      <div id="rfc.figure.u.7"></div>
903      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
904Date: Thu, 26 Mar 2010 00:05:00 GMT
905ETag: "123-a"
906Content-Length: 70
907Vary: Accept-Encoding
908Content-Type: text/plain
909
910<span id="exbody">Hello World!
911Hello World!
912Hello World!
913Hello World!
914Hello World!
915</span></pre><p id="rfc.section.2.2.5.p.5">An alternative representation that does use gzip content coding would be:</p>
916      <div id="rfc.figure.u.8"></div>
917      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
918Date: Thu, 26 Mar 2010 00:05:00 GMT
919ETag: "123-b"
920Content-Length: 43
921Vary: Accept-Encoding
922Content-Type: text/plain
923Content-Encoding: gzip
924
925<em>...binary data...</em></pre><div class="note" id="rfc.section.2.2.5.p.7"> 
926         <p> <b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation must be distinct
927            from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings
928            (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 6.2</a> of <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) apply only during message transfer and do not require distinct entity-tags.
929         </p> 
930      </div>
931      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="header.fields" href="#header.fields">Precondition Header Fields</a></h1>
932      <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>
933      <div id="rfc.iref.i.1"></div>
934      <div id="rfc.iref.h.3"></div>
935      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="header.if-match" href="#header.if-match">If-Match</a></h2>
936      <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
937         of the target resource. If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting
938         against accidental overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem).
939         An If-Match field-value of "*" places the precondition on the existence of any current representation for the target resource.
940      </p>
941      <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>
942</pre><p id="rfc.section.3.1.p.3">If any of the entity-tags listed in the If-Match field value match (as per <a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.2.3</a>) the entity-tag of the selected representation for the target resource, or if "*" is given and any current representation
943         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.
944      </p>
945      <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.
946      </p>
947      <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
948         If-Match header field <em class="bcp14">MUST</em> be ignored.
949      </p>
950      <p id="rfc.section.3.1.p.6">Examples:</p>
951      <div id="rfc.figure.u.10"></div><pre class="text">  If-Match: "xyzzy"
952  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
953  If-Match: *
954</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
955         is undefined by this specification.
956      </p>
957      <div id="rfc.iref.i.2"></div>
958      <div id="rfc.iref.h.4"></div>
959      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="header.if-none-match" href="#header.if-none-match">If-None-Match</a></h2>
960      <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
961         target resource. If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information
962         with a minimum amount of transaction overhead. A client that has one or more representations previously obtained from the
963         target resource can send If-None-Match with a list of the associated entity-tags in the hope of receiving a 304 response if
964         at least one of those representations matches the selected representation.
965      </p>
966      <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
967         an existing representation of the target resource when the client believes that the resource does not have a current representation.
968         This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation
969         for the target resource.
970      </p>
971      <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>
972</pre><p id="rfc.section.3.2.p.4">If any of the entity-tags listed in the If-None-Match field-value match (as per <a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.2.3</a>) the entity-tag of the selected representation, or if "*" is given and any current representation exists for that resource,
973         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
974         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.
975      </p>
976      <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.
977      </p>
978      <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
979         the If-None-Match header field <em class="bcp14">MUST</em> be ignored. (See <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates" title="Rules for When to Use Entity-tags and Last-Modified Dates">Section&nbsp;2.2.4</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)
980      </p>
981      <p id="rfc.section.3.2.p.7">Examples:</p>
982      <div id="rfc.figure.u.12"></div><pre class="text">  If-None-Match: "xyzzy"
983  If-None-Match: W/"xyzzy"
984  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
985  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
986  If-None-Match: *
987</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
988         fields is undefined by this specification.
989      </p>
990      <div id="rfc.iref.i.3"></div>
991      <div id="rfc.iref.h.5"></div>
992      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="header.if-modified-since" href="#header.if-modified-since">If-Modified-Since</a></h2>
993      <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
994         the time specified in this field, then do not perform the request method; instead, respond as detailed below.
995      </p>
996      <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>
997</pre><p id="rfc.section.3.3.p.3">An example of the field is:</p>
998      <div id="rfc.figure.u.14"></div><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
999</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
1000         transferred only if it has been modified since the date given by the If-Modified-Since header field. The algorithm for determining
1001         this includes the following cases:
1002      </p>
1003      <ol>
1004         <li>If the request would normally result in anything other than a 200 (OK) status code, or if the passed If-Modified-Since date
1005            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
1006            invalid.
1007         </li>
1008         <li>If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for
1009            a normal GET.
1010         </li>
1011         <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.
1012         </li>
1013      </ol>
1014      <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>
1015      <ul class="empty">
1016         <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.
1017         </li>
1018         <li> <b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client.
1019         </li>
1020         <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
1021            function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since
1022            header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header
1023            field whenever possible.
1024         </li>
1025         <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
1026            field for the same request, the client needs to be aware that this date is interpreted in the server's understanding of time.
1027            Unsynchronized clocks and rounding problems, due to the different encodings of time between the client and server, are concerns.
1028            This includes the possibility of race conditions if the document has changed between the time it was first requested and the
1029            If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since
1030            date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between
1031            client and server are at best approximate due to network latency.
1032         </li>
1033      </ul>
1034      <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
1035         fields is undefined by this specification.
1036      </p>
1037      <div id="rfc.iref.i.4"></div>
1038      <div id="rfc.iref.h.6"></div>
1039      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="header.if-unmodified-since" href="#header.if-unmodified-since">If-Unmodified-Since</a></h2>
1040      <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
1041         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
1042         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.
1043      </p>
1044      <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>
1045</pre><p id="rfc.section.3.4.p.3">An example of the field is:</p>
1046      <div id="rfc.figure.u.16"></div><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1047</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
1048         412 status code, the If-Unmodified-Since header field <em class="bcp14">SHOULD</em> be ignored.
1049      </p>
1050      <p id="rfc.section.3.4.p.6">If the specified date is invalid, the header field <em class="bcp14">MUST</em> be ignored.
1051      </p>
1052      <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
1053         header fields is undefined by this specification.
1054      </p>
1055      <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="header.if-range" href="#header.if-range">If-Range</a></h2>
1056      <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
1057         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>.
1058      </p>
1059      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="status.code.definitions" href="#status.code.definitions">Status Code Definitions</a></h1>
1060      <div id="rfc.iref.24"></div>
1061      <div id="rfc.iref.s.2"></div>
1062      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="status.304" href="#status.304">304 Not Modified</a></h2>
1063      <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
1064         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
1065         transfer a representation of the target resource because the client's request indicates that it already has a valid representation,
1066         as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation
1067         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.
1068      </p>
1069      <p id="rfc.section.4.1.p.2">A 304 response <em class="bcp14">MUST</em> include a Date header field (<a href="p1-messaging.html#header.date" title="Date">Section 9.3</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>) unless its omission is required by <a href="p1-messaging.html#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section 9.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. If a 200 response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag,
1070         Expires, Last-Modified, or Vary, then those same header fields <em class="bcp14">MUST</em> be sent in a 304 response.
1071      </p>
1072      <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,
1073         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
1074         cache updates (e.g., future HTTP extensions).
1075      </p>
1076      <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
1077         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
1078         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.
1079      </p>
1080      <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.
1081      </p>
1082      <div id="rfc.iref.25"></div>
1083      <div id="rfc.iref.s.3"></div>
1084      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="status.412" href="#status.412">412 Precondition Failed</a></h2>
1085      <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
1086         on the server. This response code allows the client to place preconditions on the current resource state (its current representations
1087         and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.
1088      </p>
1089      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
1090      <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="status.code.registration" href="#status.code.registration">Status Code Registration</a></h2>
1091      <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:
1092      </p>
1093      <div id="rfc.table.1">
1094         <div id="iana.status.code.registration.table"></div>
1095         <table class="tt full left" cellpadding="3" cellspacing="0">
1096            <thead>
1097               <tr>
1098                  <th>Value</th>
1099                  <th>Description</th>
1100                  <th>Reference</th>
1101               </tr>
1102            </thead>
1103            <tbody>
1104               <tr>
1105                  <td class="left">304</td>
1106                  <td class="left">Not Modified</td>
1107                  <td class="left"> <a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section&nbsp;4.1</a> 
1108                  </td>
1109               </tr>
1110               <tr>
1111                  <td class="left">412</td>
1112                  <td class="left">Precondition Failed</td>
1113                  <td class="left"> <a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section&nbsp;4.2</a> 
1114                  </td>
1115               </tr>
1116            </tbody>
1117         </table>
1118      </div>
1119      <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
1120      <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>):
1121      </p>
1122      <div id="rfc.table.2">
1123         <div id="iana.header.registration.table"></div>
1124         <table class="tt full left" cellpadding="3" cellspacing="0">
1125            <thead>
1126               <tr>
1127                  <th>Header Field Name</th>
1128                  <th>Protocol</th>
1129                  <th>Status</th>
1130                  <th>Reference</th>
1131               </tr>
1132            </thead>
1133            <tbody>
1134               <tr>
1135                  <td class="left">ETag</td>
1136                  <td class="left">http</td>
1137                  <td class="left">standard</td>
1138                  <td class="left"> <a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;2.2</a> 
1139                  </td>
1140               </tr>
1141               <tr>
1142                  <td class="left">If-Match</td>
1143                  <td class="left">http</td>
1144                  <td class="left">standard</td>
1145                  <td class="left"> <a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section&nbsp;3.1</a> 
1146                  </td>
1147               </tr>
1148               <tr>
1149                  <td class="left">If-Modified-Since</td>
1150                  <td class="left">http</td>
1151                  <td class="left">standard</td>
1152                  <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> 
1153                  </td>
1154               </tr>
1155               <tr>
1156                  <td class="left">If-None-Match</td>
1157                  <td class="left">http</td>
1158                  <td class="left">standard</td>
1159                  <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> 
1160                  </td>
1161               </tr>
1162               <tr>
1163                  <td class="left">If-Unmodified-Since</td>
1164                  <td class="left">http</td>
1165                  <td class="left">standard</td>
1166                  <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> 
1167                  </td>
1168               </tr>
1169               <tr>
1170                  <td class="left">Last-Modified</td>
1171                  <td class="left">http</td>
1172                  <td class="left">standard</td>
1173                  <td class="left"> <a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;2.1</a> 
1174                  </td>
1175               </tr>
1176            </tbody>
1177         </table>
1178      </div>
1179      <p id="rfc.section.5.2.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1180      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1181      <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.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
1182      </p>
1183      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
1184      <h1 id="rfc.references"><a id="rfc.section.8" href="#rfc.section.8">8.</a> References
1185      </h1>
1186      <h2 id="rfc.references.1"><a href="#rfc.section.8.1" id="rfc.section.8.1">8.1</a> Normative References
1187      </h2>
1188      <table>           
1189         <tr>
1190            <td class="reference"><b id="Part1">[Part1]</b></td>
1191            <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="http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), July&nbsp;2011.
1192            </td>
1193         </tr>
1194         <tr>
1195            <td class="reference"><b id="Part3">[Part3]</b></td>
1196            <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="http://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-latest">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p3-payload-latest (work in progress), July&nbsp;2011.
1197            </td>
1198         </tr>
1199         <tr>
1200            <td class="reference"><b id="Part5">[Part5]</b></td>
1201            <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="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), July&nbsp;2011.
1202            </td>
1203         </tr>
1204         <tr>
1205            <td class="reference"><b id="Part6">[Part6]</b></td>
1206            <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">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">HTTP/1.1, part 6: Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-latest (work in progress), July&nbsp;2011.
1207            </td>
1208         </tr>
1209         <tr>
1210            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1211            <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="http://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.
1212            </td>
1213         </tr>
1214         <tr>
1215            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1216            <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="http://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
1217            </td>
1218         </tr>
1219      </table>
1220      <h2 id="rfc.references.2"><a href="#rfc.section.8.2" id="rfc.section.8.2">8.2</a> Informative References
1221      </h2>
1222      <table>     
1223         <tr>
1224            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1225            <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="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
1226            </td>
1227         </tr>
1228         <tr>
1229            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1230            <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="http://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
1231            </td>
1232         </tr>
1233         <tr>
1234            <td class="reference"><b id="RFC4918">[RFC4918]</b></td>
1235            <td class="top"><a href="mailto:ldusseault@commerce.net" title="CommerceNet">Dusseault, L., Ed.</a>, “<a href="http://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC&nbsp;4918, June&nbsp;2007.
1236            </td>
1237         </tr>
1238      </table>
1239      <div class="avoidbreak">
1240         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1241         <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
1242               (editor)
1243               <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:fielding@gbiv.com"><span class="email">fielding@gbiv.com</span></a></span><span class="vcardline">URI: <a href="http://roy.gbiv.com/" class="url">http://roy.gbiv.com/</a></span></address>
1244         <address class="vcard"><span class="vcardline"><span class="fn">Jim Gettys</span><span class="n hidden"><span class="family-name">Gettys</span><span class="given-name">Jim</span></span></span><span class="org vcardline">Alcatel-Lucent Bell Labs</span><span class="adr"><span class="street-address vcardline">21 Oak Knoll Road</span><span class="vcardline"><span class="locality">Carlisle</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01741</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:jg@freedesktop.org"><span class="email">jg@freedesktop.org</span></a></span><span class="vcardline">URI: <a href="http://gettys.wordpress.com/" class="url">http://gettys.wordpress.com/</a></span></address>
1245         <address class="vcard"><span class="vcardline"><span class="fn">Jeffrey C. Mogul</span><span class="n hidden"><span class="family-name">Mogul</span><span class="given-name">Jeffrey C.</span></span></span><span class="org vcardline">Hewlett-Packard Company</span><span class="adr"><span class="street-address vcardline">HP Labs, Large Scale Systems Group</span><span class="street-address vcardline">1501 Page Mill Road, MS 1177</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">94304</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:JeffMogul@acm.org"><span class="email">JeffMogul@acm.org</span></a></span></address>
1246         <address class="vcard"><span class="vcardline"><span class="fn">Henrik Frystyk Nielsen</span><span class="n hidden"><span class="family-name">Frystyk</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:henrikn@microsoft.com"><span class="email">henrikn@microsoft.com</span></a></span></address>
1247         <address class="vcard"><span class="vcardline"><span class="fn">Larry Masinter</span><span class="n hidden"><span class="family-name">Masinter</span><span class="given-name">Larry</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:LMM@acm.org"><span class="email">LMM@acm.org</span></a></span><span class="vcardline">URI: <a href="http://larry.masinter.net/" class="url">http://larry.masinter.net/</a></span></address>
1248         <address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span></span><span class="vcardline">Email: <a href="mailto:paulle@microsoft.com"><span class="email">paulle@microsoft.com</span></a></span></address>
1249         <address class="vcard"><span class="vcardline"><span class="fn">Tim Berners-Lee</span><span class="n hidden"><span class="family-name">Berners-Lee</span><span class="given-name">Tim</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">MIT Computer Science and Artificial Intelligence Laboratory</span><span class="street-address vcardline">The Stata Center, Building 32</span><span class="street-address vcardline">32 Vassar Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:timbl@w3.org"><span class="email">timbl@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.w3.org/People/Berners-Lee/" class="url">http://www.w3.org/People/Berners-Lee/</a></span></address>
1250         <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
1251               (editor)
1252               <span class="n hidden"><span class="family-name">Lafon</span><span class="given-name">Yves</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">W3C / ERCIM</span><span class="street-address vcardline">2004, rte des Lucioles</span><span class="vcardline"><span class="locality">Sophia-Antipolis</span>, <span class="region">AM</span>&nbsp;<span class="postal-code">06902</span></span><span class="country-name vcardline">France</span></span><span class="vcardline">Email: <a href="mailto:ylafon@w3.org"><span class="email">ylafon@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.raubacapeu.net/people/yves/" class="url">http://www.raubacapeu.net/people/yves/</a></span></address>
1253         <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
1254               (editor)
1255               <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span>&nbsp;<span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline tel">Phone: <a href="tel:+492512807760"><span class="value">+49 251 2807760</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+492512807761"><span class="value">+49 251 2807761</span></a></span><span class="vcardline">Email: <a href="mailto:julian.reschke@greenbytes.de"><span class="email">julian.reschke@greenbytes.de</span></a></span><span class="vcardline">URI: <a href="http://greenbytes.de/tech/webdav/" class="url">http://greenbytes.de/tech/webdav/</a></span></address>
1256      </div>
1257      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
1258      <p id="rfc.section.A.p.1">Allow weak entity-tags in all requests except range requests (Sections <a href="#weak.and.strong.validators" title="Weak versus Strong">2.2.2</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>).
1259      </p>
1260      <p id="rfc.section.A.p.2">Change ABNF productions for header fields to only define the field value. (<a href="#header.fields" title="Precondition Header Fields">Section&nbsp;3</a>)
1261      </p>
1262      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
1263      <div id="rfc.figure.u.17"></div> <pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag
1264
1265<a href="#notation" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1266
1267<a href="#header.if-match" class="smpl">If-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1268 entity-tag ] ) )
1269<a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = HTTP-date
1270<a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1271 entity-tag ] ) )
1272<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = HTTP-date
1273
1274<a href="#header.last-modified" class="smpl">Last-Modified</a> = HTTP-date
1275
1276<a href="#notation" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1277
1278<a href="#header.etag" class="smpl">entity-tag</a> = [ weak ] opaque-tag
1279
1280<a href="#header.etag" class="smpl">opaque-tag</a> = quoted-string
1281
1282<a href="#notation" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1283
1284<a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/
1285</pre> <div id="rfc.figure.u.18"></div>
1286      <p>ABNF diagnostics:</p><pre class="inline">; ETag defined but not used
1287; If-Match defined but not used
1288; If-Modified-Since defined but not used
1289; If-None-Match defined but not used
1290; If-Unmodified-Since defined but not used
1291; Last-Modified defined but not used
1292</pre><h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
1293      <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a>&nbsp;Since RFC 2616
1294      </h2>
1295      <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>.
1296      </p>
1297      <h2 id="rfc.section.C.2"><a href="#rfc.section.C.2">C.2</a>&nbsp;Since draft-ietf-httpbis-p4-conditional-00
1298      </h2>
1299      <p id="rfc.section.C.2.p.1">Closed issues: </p>
1300      <ul>
1301         <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"
1302         </li>
1303      </ul>
1304      <p id="rfc.section.C.2.p.2">Other changes: </p>
1305      <ul>
1306         <li>Move definitions of 304 and 412 condition codes from Part2.</li>
1307      </ul>
1308      <h2 id="rfc.section.C.3"><a href="#rfc.section.C.3">C.3</a>&nbsp;Since draft-ietf-httpbis-p4-conditional-01
1309      </h2>
1310      <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;):
1311      </p>
1312      <ul>
1313         <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li>
1314      </ul>
1315      <h2 id="rfc.section.C.4"><a href="#rfc.section.C.4">C.4</a>&nbsp;<a id="changes.since.02" href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></h2>
1316      <p id="rfc.section.C.4.p.1">Closed issues: </p>
1317      <ul>
1318         <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"
1319         </li>
1320      </ul>
1321      <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;):
1322      </p>
1323      <ul>
1324         <li>Reference RFC 3984, and update header field registrations for header fields defined in this document.</li>
1325      </ul>
1326      <h2 id="rfc.section.C.5"><a href="#rfc.section.C.5">C.5</a>&nbsp;<a id="changes.since.03" href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></h2>
1327      <p id="rfc.section.C.5.p.1">Closed issues: </p>
1328      <ul>
1329         <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"
1330         </li>
1331         <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"
1332         </li>
1333         <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"
1334         </li>
1335      </ul>
1336      <h2 id="rfc.section.C.6"><a href="#rfc.section.C.6">C.6</a>&nbsp;<a id="changes.since.04" href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></h2>
1337      <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;):
1338      </p>
1339      <ul>
1340         <li>Use "/" instead of "|" for alternatives.</li>
1341         <li>Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").</li>
1342         <li>Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.</li>
1343      </ul>
1344      <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a>&nbsp;<a id="changes.since.05" href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></h2>
1345      <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;):
1346      </p>
1347      <ul>
1348         <li>Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.</li>
1349      </ul>
1350      <h2 id="rfc.section.C.8"><a href="#rfc.section.C.8">C.8</a>&nbsp;<a id="changes.since.06" href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></h2>
1351      <p id="rfc.section.C.8.p.1">Closed issues: </p>
1352      <ul>
1353         <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"
1354         </li>
1355      </ul>
1356      <h2 id="rfc.section.C.9"><a href="#rfc.section.C.9">C.9</a>&nbsp;<a id="changes.since.07" href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></h2>
1357      <p id="rfc.section.C.9.p.1">Closed issues: </p>
1358      <ul>
1359         <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)
1360         </li>
1361         <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"
1362         </li>
1363      </ul>
1364      <h2 id="rfc.section.C.10"><a href="#rfc.section.C.10">C.10</a>&nbsp;<a id="changes.since.08" href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></h2>
1365      <p id="rfc.section.C.10.p.1">No significant changes.</p>
1366      <h2 id="rfc.section.C.11"><a href="#rfc.section.C.11">C.11</a>&nbsp;<a id="changes.since.09" href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></h2>
1367      <p id="rfc.section.C.11.p.1">No significant changes.</p>
1368      <h2 id="rfc.section.C.12"><a href="#rfc.section.C.12">C.12</a>&nbsp;<a id="changes.since.10" href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></h2>
1369      <p id="rfc.section.C.12.p.1">Closed issues: </p>
1370      <ul>
1371         <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'"
1372         </li>
1373         <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"
1374         </li>
1375         <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"
1376         </li>
1377      </ul>
1378      <h2 id="rfc.section.C.13"><a href="#rfc.section.C.13">C.13</a>&nbsp;<a id="changes.since.11" href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></h2>
1379      <p id="rfc.section.C.13.p.1">None.</p>
1380      <h2 id="rfc.section.C.14"><a href="#rfc.section.C.14">C.14</a>&nbsp;<a id="changes.since.12" href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></h2>
1381      <p id="rfc.section.C.14.p.1">Closed issues: </p>
1382      <ul>
1383         <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"
1384         </li>
1385      </ul>
1386      <h2 id="rfc.section.C.15"><a href="#rfc.section.C.15">C.15</a>&nbsp;<a id="changes.since.13" href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></h2>
1387      <p id="rfc.section.C.15.p.1">Closed issues: </p>
1388      <ul>
1389         <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"
1390         </li>
1391         <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"
1392         </li>
1393         <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"
1394         </li>
1395         <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"
1396         </li>
1397      </ul>
1398      <h2 id="rfc.section.C.16"><a href="#rfc.section.C.16">C.16</a>&nbsp;<a id="changes.since.14" href="#changes.since.14">Since draft-ietf-httpbis-p4-conditional-14</a></h2>
1399      <p id="rfc.section.C.16.p.1">None.</p>
1400      <h2 id="rfc.section.C.17"><a href="#rfc.section.C.17">C.17</a>&nbsp;<a id="changes.since.15" href="#changes.since.15">Since draft-ietf-httpbis-p4-conditional-15</a></h2>
1401      <p id="rfc.section.C.17.p.1">Closed issues: </p>
1402      <ul>
1403         <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"
1404         </li>
1405      </ul>
1406      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1407      <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> 
1408      </p>
1409      <div class="print2col">
1410         <ul class="ind">
1411            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
1412                  <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.24"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li>
1413               </ul>
1414            </li>
1415            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
1416                  <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.25"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li>
1417               </ul>
1418            </li>
1419            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
1420                  <li>ETag header field&nbsp;&nbsp;<a href="#rfc.iref.e.1"><b>2.2</b></a>, <a href="#rfc.xref.header.etag.1">5.2</a></li>
1421               </ul>
1422            </li>
1423            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
1424                  <li><tt>Grammar</tt>&nbsp;&nbsp;
1425                     <ul>
1426                        <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2.2</b></a></li>
1427                        <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2.2</b></a></li>
1428                        <li><tt>If-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>3.1</b></a></li>
1429                        <li><tt>If-Modified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>3.3</b></a></li>
1430                        <li><tt>If-None-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>3.2</b></a></li>
1431                        <li><tt>If-Unmodified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>3.4</b></a></li>
1432                        <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2.1</b></a></li>
1433                        <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>2.2</b></a></li>
1434                        <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>2.2</b></a></li>
1435                     </ul>
1436                  </li>
1437               </ul>
1438            </li>
1439            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
1440                  <li>Header Fields&nbsp;&nbsp;
1441                     <ul>
1442                        <li>ETag&nbsp;&nbsp;<a href="#rfc.iref.h.2"><b>2.2</b></a>, <a href="#rfc.xref.header.etag.1">5.2</a></li>
1443                        <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>
1444                        <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>
1445                        <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>
1446                        <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>
1447                        <li>Last-Modified&nbsp;&nbsp;<a href="#rfc.iref.h.1"><b>2.1</b></a>, <a href="#rfc.xref.header.last-modified.1">5.2</a></li>
1448                     </ul>
1449                  </li>
1450               </ul>
1451            </li>
1452            <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
1453                  <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>
1454                  <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>
1455                  <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>
1456                  <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>
1457               </ul>
1458            </li>
1459            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
1460                  <li>Last-Modified header field&nbsp;&nbsp;<a href="#rfc.iref.l.1"><b>2.1</b></a>, <a href="#rfc.xref.header.last-modified.1">5.2</a></li>
1461               </ul>
1462            </li>
1463            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
1464                  <li>metadata&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b>2</b></a></li>
1465               </ul>
1466            </li>
1467            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
1468                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.2</a>, <a href="#rfc.xref.Part1.2">1.2</a>, <a href="#rfc.xref.Part1.3">1.2</a>, <a href="#rfc.xref.Part1.4">1.2</a>, <a href="#rfc.xref.Part1.5">2.2.5</a>, <a href="#rfc.xref.Part1.6">4.1</a>, <a href="#rfc.xref.Part1.7">4.1</a>, <a href="#rfc.xref.Part1.8">6</a>, <a href="#Part1"><b>8.1</b></a><ul>
1469                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.2</a></li>
1470                        <li><em>Section 1.2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.2</a>, <a href="#rfc.xref.Part1.3">1.2</a></li>
1471                        <li><em>Section 6.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">1.2</a></li>
1472                        <li><em>Section 6.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">2.2.5</a></li>
1473                        <li><em>Section 9.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.6">4.1</a></li>
1474                        <li><em>Section 9.3.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.7">4.1</a></li>
1475                     </ul>
1476                  </li>
1477                  <li><em>Part3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.2.5</a>, <a href="#rfc.xref.Part3.2">2.2.5</a>, <a href="#Part3"><b>8.1</b></a><ul>
1478                        <li><em>Section 5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.2.5</a></li>
1479                        <li><em>Section 6.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.2">2.2.5</a></li>
1480                     </ul>
1481                  </li>
1482                  <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>
1483                        <li><em>Section 5.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.2">3.5</a></li>
1484                        <li><em>Section 5.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.3</a></li>
1485                     </ul>
1486                  </li>
1487                  <li><em>Part6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.1">1</a>, <a href="#rfc.xref.Part6.2">2.1.1</a>, <a href="#rfc.xref.Part6.3">2.2.1</a>, <a href="#Part6"><b>8.1</b></a></li>
1488               </ul>
1489            </li>
1490            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
1491                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>8.1</b></a></li>
1492                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#RFC2616"><b>8.2</b></a>, <a href="#rfc.xref.RFC2616.1">C.1</a></li>
1493                  <li><em>RFC3864</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC3864.1">5.2</a>, <a href="#RFC3864"><b>8.2</b></a></li>
1494                  <li><em>RFC4918</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC4918.1">2</a>, <a href="#RFC4918"><b>8.2</b></a></li>
1495                  <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>
1496                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">1.2</a></li>
1497                     </ul>
1498                  </li>
1499               </ul>
1500            </li>
1501            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
1502                  <li>selected representation&nbsp;&nbsp;<a href="#rfc.iref.s.1"><b>1</b></a></li>
1503                  <li>Status Codes&nbsp;&nbsp;
1504                     <ul>
1505                        <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>
1506                        <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>
1507                     </ul>
1508                  </li>
1509               </ul>
1510            </li>
1511            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
1512                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1"><b>2</b></a></li>
1513               </ul>
1514            </li>
1515         </ul>
1516      </div>
1517   </body>
1518</html>
Note: See TracBrowser for help on using the repository browser.