source: draft-ietf-httpbis/04/draft-ietf-httpbis-p4-conditional-04.xml @ 847

Last change on this file since 847 was 559, checked in by fielding@…, 11 years ago

remove executable and set eol-style for earlier drafts

  • Property svn:eol-style set to native
File size: 53.9 KB
Line 
1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<!DOCTYPE rfc
17  PUBLIC "" "rfc2629.dtd">
18<rfc obsoletes="2616" category="std" ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-04">
19<front>
20
21  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
22
23  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
24    <organization abbrev="Day Software">Day Software</organization>
25    <address>
26      <postal>
27        <street>23 Corporate Plaza DR, Suite 280</street>
28        <city>Newport Beach</city>
29        <region>CA</region>
30        <code>92660</code>
31        <country>USA</country>
32      </postal>
33      <phone>+1-949-706-5300</phone>
34      <facsimile>+1-949-706-5305</facsimile>
35      <email>fielding@gbiv.com</email>
36      <uri>http://roy.gbiv.com/</uri>
37    </address>
38  </author>
39
40  <author initials="J." surname="Gettys" fullname="Jim Gettys">
41    <organization>One Laptop per Child</organization>
42    <address>
43      <postal>
44        <street>21 Oak Knoll Road</street>
45        <city>Carlisle</city>
46        <region>MA</region>
47        <code>01741</code>
48        <country>USA</country>
49      </postal>
50      <email>jg@laptop.org</email>
51      <uri>http://www.laptop.org/</uri>
52    </address>
53  </author>
54 
55  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
56    <organization abbrev="HP">Hewlett-Packard Company</organization>
57    <address>
58      <postal>
59        <street>HP Labs, Large Scale Systems Group</street>
60        <street>1501 Page Mill Road, MS 1177</street>
61        <city>Palo Alto</city>
62        <region>CA</region>
63        <code>94304</code>
64        <country>USA</country>
65      </postal>
66      <email>JeffMogul@acm.org</email>
67    </address>
68  </author>
69
70  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
71    <organization abbrev="Microsoft">Microsoft Corporation</organization>
72    <address>
73      <postal>
74        <street>1 Microsoft Way</street>
75        <city>Redmond</city>
76        <region>WA</region>
77        <code>98052</code>
78        <country>USA</country>
79      </postal>
80      <email>henrikn@microsoft.com</email>
81    </address>
82  </author>
83
84  <author initials="L." surname="Masinter" fullname="Larry Masinter">
85    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
86    <address>
87      <postal>
88        <street>345 Park Ave</street>
89        <city>San Jose</city>
90        <region>CA</region>
91        <code>95110</code>
92        <country>USA</country>
93      </postal>
94      <email>LMM@acm.org</email>
95      <uri>http://larry.masinter.net/</uri>
96    </address>
97  </author>
98 
99  <author initials="P." surname="Leach" fullname="Paul J. Leach">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107      </postal>
108      <email>paulle@microsoft.com</email>
109    </address>
110  </author>
111   
112  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
113    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
114    <address>
115      <postal>
116        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
117        <street>The Stata Center, Building 32</street>
118        <street>32 Vassar Street</street>
119        <city>Cambridge</city>
120        <region>MA</region>
121        <code>02139</code>
122        <country>USA</country>
123      </postal>
124      <email>timbl@w3.org</email>
125      <uri>http://www.w3.org/People/Berners-Lee/</uri>
126    </address>
127  </author>
128
129  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
130    <organization abbrev="W3C">World Wide Web Consortium</organization>
131    <address>
132      <postal>
133        <street>W3C / ERCIM</street>
134        <street>2004, rte des Lucioles</street>
135        <city>Sophia-Antipolis</city>
136        <region>AM</region>
137        <code>06902</code>
138        <country>France</country>
139      </postal>
140      <email>ylafon@w3.org</email>
141      <uri>http://www.raubacapeu.net/people/yves/</uri>
142    </address>
143  </author>
144
145  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
146    <organization abbrev="greenbytes">greenbytes GmbH</organization>
147    <address>
148      <postal>
149        <street>Hafenweg 16</street>
150        <city>Muenster</city><region>NW</region><code>48155</code>
151        <country>Germany</country>
152      </postal>
153      <phone>+49 251 2807760</phone>   
154      <facsimile>+49 251 2807761</facsimile>   
155      <email>julian.reschke@greenbytes.de</email>       
156      <uri>http://greenbytes.de/tech/webdav/</uri>     
157    </address>
158  </author>
159
160  <date month="August" year="2008" day="29"/>
161
162<abstract>
163<t>
164   The Hypertext Transfer Protocol (HTTP) is an application-level
165   protocol for distributed, collaborative, hypermedia information
166   systems. HTTP has been in use by the World Wide Web global information
167   initiative since 1990. This document is Part 4 of the seven-part specification
168   that defines the protocol referred to as "HTTP/1.1" and, taken together,
169   obsoletes RFC 2616.  Part 4 defines request header fields for
170   indicating conditional requests and the rules for constructing responses
171   to those requests.
172</t>
173</abstract>
174
175<note title="Editorial Note (To be removed by RFC Editor)">
176  <t>
177    Discussion of this draft should take place on the HTTPBIS working group
178    mailing list (ietf-http-wg@w3.org). The current issues list is
179    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
180    and related documents (including fancy diffs) can be found at
181    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
182  </t>
183  <t>
184    The changes in this draft are summarized in <xref target="changes.since.02"/>.
185  </t>
186</note>
187</front>
188<middle>
189<section title="Introduction" anchor="introduction">
190<t>
191   This document defines HTTP/1.1 response metadata for indicating potential
192   changes to payload content, including modification time stamps and opaque
193   entity-tags, and the HTTP conditional request mechanisms that allow
194   preconditions to be placed on a request method.  Conditional GET requests
195   allow for efficient cache updates.  Other conditional request methods are
196   used to protect against overwriting or misunderstanding the state of a
197   resource that has been changed unbeknownst to the requesting client.
198</t>
199<t>
200   This document is currently disorganized in order to minimize the changes
201   between drafts and enable reviewers to see the smaller errata changes.
202   The next draft will reorganize the sections to better reflect the content.
203   In particular, the sections on resource metadata will be discussed first
204   and then followed by each conditional request-header, concluding with a
205   definition of precedence and the expectation of ordering strong validator
206   checks before weak validator checks.  It is likely that more content from
207   <xref target="Part6"/> will migrate to this part, where appropriate.
208   The current mess reflects how widely dispersed these topics and associated
209   requirements had become in <xref target="RFC2616"/>.
210</t>
211
212<section title="Requirements" anchor="intro.requirements">
213<t>
214   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
215   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
216   document are to be interpreted as described in <xref target="RFC2119"/>.
217</t>
218<t>
219   An implementation is not compliant if it fails to satisfy one or more
220   of the MUST or REQUIRED level requirements for the protocols it
221   implements. An implementation that satisfies all the MUST or REQUIRED
222   level and all the SHOULD level requirements for its protocols is said
223   to be "unconditionally compliant"; one that satisfies all the MUST
224   level requirements but not all the SHOULD level requirements for its
225   protocols is said to be "conditionally compliant."
226</t>
227</section>
228</section>
229
230<section title="Notational Conventions and Generic Grammar" anchor="notation">
231 
232<t>
233  This specification uses the ABNF syntax defined in Section 2.1 of <xref target="Part1"/> and
234  the core rules defined in Section 2.2 of <xref target="Part1"/>:
235  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
236  &lt;http://tools.ietf.org/wg/httpbis/trac/ticket/36&gt;.</cref>
237</t>
238<figure><artwork type="abnf2616"><![CDATA[
239  quoted-string = <quoted-string, defined in [Part1], Section 2.2>
240]]></artwork></figure>
241<t anchor="abnf.dependencies">
242 
243  The ABNF rules below are defined in other parts:
244</t>
245<figure><!--Part1--><artwork type="abnf2616"><![CDATA[
246  HTTP-date     = <HTTP-date, defined in [Part1], Section 3.3.1>
247]]></artwork></figure>
248</section>
249
250<section title="Entity Tags" anchor="entity.tags">
251 
252 
253 
254<t>
255   Entity tags are used for comparing two or more entities from the same
256   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
257   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
258   If-Range (Section 6.3 of <xref target="Part5"/>) header fields. The definition of how they
259   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
260   entity tag consists of an opaque quoted string, possibly prefixed by
261   a weakness indicator.
262</t>
263<figure><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/><artwork type="abnf2616"><![CDATA[
264  entity-tag = [ weak ] opaque-tag
265  weak       = "W/"
266  opaque-tag = quoted-string
267]]></artwork></figure>
268<t>
269   A "strong entity tag" MAY be shared by two entities of a resource
270   only if they are equivalent by octet equality.
271</t>
272<t>
273   A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
274   two entities of a resource only if the entities are equivalent and
275   could be substituted for each other with no significant change in
276   semantics. A weak entity tag can only be used for weak comparison.
277</t>
278<t>
279   An entity tag MUST be unique across all versions of all entities
280   associated with a particular resource. A given entity tag value MAY
281   be used for entities obtained by requests on different URIs. The use
282   of the same entity tag value in conjunction with entities obtained by
283   requests on different URIs does not imply the equivalence of those
284   entities.
285</t>
286</section>
287
288<section title="Status Code Definitions">
289<section title="304 Not Modified" anchor="status.304">
290  <iref primary="true" item="304 Not Modified (status code)"/>
291  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
292<t>
293   If the client has performed a conditional GET request and access is
294   allowed, but the document has not been modified, the server SHOULD
295   respond with this status code. The 304 response MUST NOT contain a
296   message-body, and thus is always terminated by the first empty line
297   after the header fields.
298</t>
299<t>
300   The response MUST include the following header fields:
301  <list style="symbols">
302    <t>Date, unless its omission is required by Section 8.3.1 of <xref target="Part1"/>.<vspace blankLines="1"/>
303         If a clockless origin server obeys these rules, and proxies and
304         clients add their own Date to any response received without one (as
305         already specified by Section 8.3 of <xref target="Part1"/>, caches will operate
306         correctly.</t>
307    <t>ETag and/or Content-Location, if the header would have been sent
308         in a 200 response to the same request.</t>
309    <t>Expires, Cache-Control, and/or Vary, if the field-value might
310         differ from that sent in any previous response for the same
311         variant.</t>
312  </list>
313</t>
314<t>
315   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
316   the response SHOULD NOT  include other entity-headers.
317   Otherwise (i.e., the conditional GET used a weak validator), the
318   response MUST NOT include other entity-headers; this prevents
319   inconsistencies between cached entity-bodies and updated headers.
320</t>
321<t>
322   If a 304 response indicates an entity not currently cached, then the
323   cache MUST disregard the response and repeat the request without the
324   conditional.
325</t>
326<t>
327   If a cache uses a received 304 response to update a cache entry, the
328   cache MUST update the entry to reflect any new field values given in
329   the response.
330</t>
331</section>
332
333<section title="412 Precondition Failed" anchor="status.412">
334  <iref primary="true" item="412 Precondition Failed (status code)"/>
335  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
336<t>
337   The precondition given in one or more of the request-header fields
338   evaluated to false when it was tested on the server. This response
339   code allows the client to place preconditions on the current resource
340   metainformation (header field data) and thus prevent the requested
341   method from being applied to a resource other than the one intended.
342</t>
343</section>
344</section>
345
346<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
347<t>
348   Since both origin servers and caches will compare two validators to
349   decide if they represent the same or different entities, one normally
350   would expect that if the entity (the entity-body or any entity-headers)
351   changes in any way, then the associated validator would
352   change as well. If this is true, then we call this validator a
353   "strong validator."
354</t>
355<t>
356   However, there might be cases when a server prefers to change the
357   validator only on semantically significant changes, and not when
358   insignificant aspects of the entity change. A validator that does not
359   always change when the resource changes is a "weak validator."
360</t>
361<t>
362   Entity tags are normally "strong validators," but the protocol
363   provides a mechanism to tag an entity tag as "weak." One can think of
364   a strong validator as one that changes whenever the bits of an entity
365   changes, while a weak value changes whenever the meaning of an entity
366   changes. Alternatively, one can think of a strong validator as part
367   of an identifier for a specific entity, while a weak validator is
368   part of an identifier for a set of semantically equivalent entities.
369  <list><t>
370      Note: One example of a strong validator is an integer that is
371      incremented in stable storage every time an entity is changed.
372    </t><t>
373      An entity's modification time, if represented with one-second
374      resolution, could be a weak validator, since it is possible that
375      the resource might be modified twice during a single second.
376    </t><t>
377      Support for weak validators is optional. However, weak validators
378      allow for more efficient caching of equivalent objects; for
379      example, a hit counter on a site is probably good enough if it is
380      updated every few days or weeks, and any value during that period
381      is likely "good enough" to be equivalent.
382    </t></list>
383</t>
384<t>
385   A "use" of a validator is either when a client generates a request
386   and includes the validator in a validating header field, or when a
387   server compares two validators.
388</t>
389<t>
390   Strong validators are usable in any context. Weak validators are only
391   usable in contexts that do not depend on exact equality of an entity.
392   For example, either kind is usable for a conditional GET of a full
393   entity. However, only a strong validator is usable for a sub-range
394   retrieval, since otherwise the client might end up with an internally
395   inconsistent entity.
396</t>
397<t>
398   Clients MUST NOT use weak validators in range requests (<xref target="Part5"/>).
399</t>
400<t>
401   The only function that HTTP/1.1 defines on validators is
402   comparison. There are two validator comparison functions, depending
403   on whether the comparison context allows the use of weak validators
404   or not:
405  <list style="symbols">
406     <t>The strong comparison function: in order to be considered equal,
407        both opaque-tags MUST be identical character-by-character, and both
408        MUST NOT be weak.</t>
409     <t>The weak comparison function: in order to be considered equal, both
410        opaque-tags MUST be identical character-by-character.</t>
411  </list>
412</t>
413<t>
414   The example below shows the results for a set of entity tag pairs,
415   and both the weak and strong comparison function results:
416</t>
417<texttable align="left">
418  <ttcol>ETag 1</ttcol>
419  <ttcol>ETag 2</ttcol>
420  <ttcol>Strong Comparison</ttcol>
421  <ttcol>Weak Comparison</ttcol>
422
423  <c>W/"1"</c>
424  <c>W/"1"</c>
425  <c>no match</c>
426  <c>match</c>
427 
428  <c>W/"1"</c>
429  <c>W/"2"</c>
430  <c>no match</c>
431  <c>no match</c>
432
433  <c>W/"1"</c>
434  <c>"1"</c>
435  <c>no match</c>
436  <c>match</c>
437
438  <c>"1"</c>
439  <c>"1"</c>
440  <c>match</c>
441  <c>match</c>
442</texttable>
443<t>
444   An entity tag is strong unless it is explicitly tagged as weak.
445   <xref target="entity.tags"/> gives the syntax for entity tags.
446</t>
447<t>
448   A Last-Modified time, when used as a validator in a request, is
449   implicitly weak unless it is possible to deduce that it is strong,
450   using the following rules:
451  <list style="symbols">
452     <t>The validator is being compared by an origin server to the
453        actual current validator for the entity and,</t>
454     <t>That origin server reliably knows that the associated entity did
455        not change twice during the second covered by the presented
456        validator.</t>
457  </list>
458</t>
459<t>
460   or
461  <list style="symbols">
462     <t>The validator is about to be used by a client in an If-Modified-Since
463        or If-Unmodified-Since header, because the client
464        has a cache entry for the associated entity, and</t>
465     <t>That cache entry includes a Date value, which gives the time
466        when the origin server sent the original response, and</t>
467     <t>The presented Last-Modified time is at least 60 seconds before
468        the Date value.</t>
469  </list>
470</t>
471<t>
472   or
473  <list style="symbols">
474     <t>The validator is being compared by an intermediate cache to the
475        validator stored in its cache entry for the entity, and</t>
476     <t>That cache entry includes a Date value, which gives the time
477        when the origin server sent the original response, and</t>
478     <t>The presented Last-Modified time is at least 60 seconds before
479        the Date value.</t>
480  </list>
481</t>
482<t>
483   This method relies on the fact that if two different responses were
484   sent by the origin server during the same second, but both had the
485   same Last-Modified time, then at least one of those responses would
486   have a Date value equal to its Last-Modified time. The arbitrary 60-second
487   limit guards against the possibility that the Date and Last-Modified
488   values are generated from different clocks, or at somewhat
489   different times during the preparation of the response. An
490   implementation MAY use a value larger than 60 seconds, if it is
491   believed that 60 seconds is too short.
492</t>
493<t>
494   If a client wishes to perform a sub-range retrieval on a value for
495   which it has only a Last-Modified time and no opaque validator, it
496   MAY do this only if the Last-Modified time is strong in the sense
497   described here.
498</t>
499<t>
500   A cache or origin server receiving a conditional range request
501   (<xref target="Part5"/>) MUST use the strong comparison function to
502   evaluate the condition.
503</t>
504<t>
505   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
506   retrievals on values that have been obtained from HTTP/1.0
507   servers.
508</t>
509</section>
510
511<section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="rules.for.when.to.use.entity.tags.and.last-modified.dates">
512<t>
513   We adopt a set of rules and recommendations for origin servers,
514   clients, and caches regarding when various validator types ought to
515   be used, and for what purposes.
516</t>
517<t>
518   HTTP/1.1 origin servers:
519  <list style="symbols">
520     <t>SHOULD send an entity tag validator unless it is not feasible to
521        generate one.</t>
522
523     <t>MAY send a weak entity tag instead of a strong entity tag, if
524        performance considerations support the use of weak entity tags,
525        or if it is unfeasible to send a strong entity tag.</t>
526
527     <t>SHOULD send a Last-Modified value if it is feasible to send one,
528        unless the risk of a breakdown in semantic transparency that
529        could result from using this date in an If-Modified-Since header
530        would lead to serious problems.</t>
531  </list>
532</t>
533<t>
534   In other words, the preferred behavior for an HTTP/1.1 origin server
535   is to send both a strong entity tag and a Last-Modified value.
536</t>
537<t>
538   In order to be legal, a strong entity tag MUST change whenever the
539   associated entity changes in any way. A weak entity tag SHOULD
540   change whenever the associated entity changes in a semantically
541   significant way.
542  <list><t>
543      Note: in order to provide semantically transparent caching, an
544      origin server must avoid reusing a specific strong entity tag
545      value for two different entities, or reusing a specific weak
546      entity tag value for two semantically different entities. Cache
547      entries might persist for arbitrarily long periods, regardless of
548      expiration times, so it might be inappropriate to expect that a
549      cache will never again attempt to validate an entry using a
550      validator that it obtained at some point in the past.
551  </t></list>
552</t>
553<t>
554   HTTP/1.1 clients:
555  <list style="symbols">
556     <t>If an entity tag has been provided by the origin server, MUST
557        use that entity tag in any cache-conditional request (using If-Match
558        or If-None-Match).</t>
559
560     <t>If only a Last-Modified value has been provided by the origin
561        server, SHOULD use that value in non-subrange cache-conditional
562        requests (using If-Modified-Since).</t>
563
564     <t>If only a Last-Modified value has been provided by an HTTP/1.0
565        origin server, MAY use that value in subrange cache-conditional
566        requests (using If-Unmodified-Since:). The user agent SHOULD
567        provide a way to disable this, in case of difficulty.</t>
568
569     <t>If both an entity tag and a Last-Modified value have been
570        provided by the origin server, SHOULD use both validators in
571        cache-conditional requests. This allows both HTTP/1.0 and
572        HTTP/1.1 caches to respond appropriately.</t>
573  </list>
574</t>
575<t>
576   An HTTP/1.1 origin server, upon receiving a conditional request that
577   includes both a Last-Modified date (e.g., in an If-Modified-Since or
578   If-Unmodified-Since header field) and one or more entity tags (e.g.,
579   in an If-Match, If-None-Match, or If-Range header field) as cache
580   validators, MUST NOT return a response status of 304 (Not Modified)
581   unless doing so is consistent with all of the conditional header
582   fields in the request.
583</t>
584<t>
585   An HTTP/1.1 caching proxy, upon receiving a conditional request that
586   includes both a Last-Modified date and one or more entity tags as
587   cache validators, MUST NOT return a locally cached response to the
588   client unless that cached response is consistent with all of the
589   conditional header fields in the request.
590  <list><t>
591      Note: The general principle behind these rules is that HTTP/1.1
592      servers and clients should transmit as much non-redundant
593      information as is available in their responses and requests.
594      HTTP/1.1 systems receiving this information will make the most
595      conservative assumptions about the validators they receive.
596  </t><t>
597      HTTP/1.0 clients and caches will ignore entity tags. Generally,
598      last-modified values received or used by these systems will
599      support transparent and efficient caching, and so HTTP/1.1 origin
600      servers should provide Last-Modified values. In those rare cases
601      where the use of a Last-Modified value as a validator by an
602      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
603      origin servers should not provide one.
604  </t></list>
605</t>
606</section>
607
608<section title="Header Field Definitions" anchor="header.fields">
609<t>
610   This section defines the syntax and semantics of HTTP/1.1 header fields
611   related to conditional requests.
612</t>
613<t>
614   For entity-header fields, both sender and recipient refer to either the
615   client or the server, depending on who sends and who receives the entity.
616</t>
617
618<section title="ETag" anchor="header.etag">
619  <iref primary="true" item="ETag header"/>
620  <iref primary="true" item="Headers" subitem="ETag"/>
621 
622<t>
623   The ETag response-header field provides the current value of the
624   entity tag for the requested variant. The headers used with entity
625   tags are described in Sections <xref target="header.if-match" format="counter"/>
626   and <xref target="header.if-none-match" format="counter"/> of this document,
627   and in Section 6.3 of <xref target="Part5"/>. The entity tag
628   MAY be used for comparison with other entities from the same resource
629   (see <xref target="weak.and.strong.validators"/>).
630</t>
631<figure><iref primary="true" item="Grammar" subitem="ETag"/><artwork type="abnf2616"><![CDATA[
632  ETag = "ETag" ":" entity-tag
633]]></artwork></figure>
634<figure><preamble>
635   Examples:
636</preamble>
637<artwork type="example"><![CDATA[
638   ETag: "xyzzy"
639   ETag: W/"xyzzy"
640   ETag: ""
641]]></artwork></figure>
642<t>
643   The ETag response-header field value, an entity tag, provides for an
644   "opaque" cache validator. This might allow more reliable validation
645   in situations where it is inconvenient to store modification dates,
646   where the one-second resolution of HTTP date values is not
647   sufficient, or where the origin server wishes to avoid certain
648   paradoxes that might arise from the use of modification dates.
649</t>
650<t>
651   The principle behind entity tags is that only the service author
652   knows the semantics of a resource well enough to select an
653   appropriate cache validation mechanism, and the specification of any
654   validator comparison function more complex than byte-equality would
655   open up a can of worms. Thus, comparisons of any other headers
656   (except Last-Modified, for compatibility with HTTP/1.0) are never
657   used for purposes of validating a cache entry.
658</t>
659</section>
660
661<section title="If-Match" anchor="header.if-match">
662  <iref primary="true" item="If-Match header"/>
663  <iref primary="true" item="Headers" subitem="If-Match"/>
664 
665<t>
666   The If-Match request-header field is used with a method to make it
667   conditional. A client that has one or more entities previously
668   obtained from the resource can verify that one of those entities is
669   current by including a list of their associated entity tags in the
670   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
671   purpose of this feature is to allow efficient updates of cached
672   information with a minimum amount of transaction overhead. It is also
673   used, on updating requests, to prevent inadvertent modification of
674   the wrong version of a resource. As a special case, the value "*"
675   matches any current entity of the resource.
676</t>
677<figure><iref primary="true" item="Grammar" subitem="If-Match"/><artwork type="abnf2616"><![CDATA[
678  If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
679]]></artwork></figure>
680<t>
681   If any of the entity tags match the entity tag of the entity that
682   would have been returned in the response to a similar GET request
683   (without the If-Match header) on that resource, or if "*" is given
684   and any current entity exists for that resource, then the server MAY
685   perform the requested method as if the If-Match header field did not
686   exist.
687</t>
688<t>
689   A server MUST use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
690   to compare the entity tags in If-Match.
691</t>
692<t>
693   If none of the entity tags match, or if "*" is given and no current
694   entity exists, the server MUST NOT perform the requested method, and
695   MUST return a 412 (Precondition Failed) response. This behavior is
696   most useful when the client wants to prevent an updating method, such
697   as PUT, from modifying a resource that has changed since the client
698   last retrieved it.
699</t>
700<t>
701   If the request would, without the If-Match header field, result in
702   anything other than a 2xx or 412 status, then the If-Match header
703   MUST be ignored.
704</t>
705<t>
706   The meaning of "If-Match: *" is that the method SHOULD be performed
707   if the representation selected by the origin server (or by a cache,
708   possibly using the Vary mechanism, see Section 16.5 of <xref target="Part6"/>) exists, and
709   MUST NOT be performed if the representation does not exist.
710</t>
711<t>
712   A request intended to update a resource (e.g., a PUT) MAY include an
713   If-Match header field to signal that the request method MUST NOT be
714   applied if the entity corresponding to the If-Match value (a single
715   entity tag) is no longer a representation of that resource. This
716   allows the user to indicate that they do not wish the request to be
717   successful if the resource has been changed without their knowledge.
718   Examples:
719</t>
720<figure><artwork type="example"><![CDATA[
721    If-Match: "xyzzy"
722    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
723    If-Match: *
724]]></artwork></figure>
725<t>
726   The result of a request having both an If-Match header field and
727   either an If-None-Match or an If-Modified-Since header fields is
728   undefined by this specification.
729</t>
730</section>
731
732<section title="If-Modified-Since" anchor="header.if-modified-since">
733  <iref primary="true" item="If-Modified-Since header"/>
734  <iref primary="true" item="Headers" subitem="If-Modified-Since"/>
735 
736<t>
737   The If-Modified-Since request-header field is used with a method to
738   make it conditional: if the requested variant has not been modified
739   since the time specified in this field, an entity will not be
740   returned from the server; instead, a 304 (Not Modified) response will
741   be returned without any message-body.
742</t>
743<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><artwork type="abnf2616"><![CDATA[
744  If-Modified-Since = "If-Modified-Since" ":" HTTP-date
745]]></artwork></figure>
746<t>
747   An example of the field is:
748</t>
749<figure><artwork type="example"><![CDATA[
750    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
751]]></artwork></figure>
752<t>
753   A GET method with an If-Modified-Since header and no Range header
754   requests that the identified entity be transferred only if it has
755   been modified since the date given by the If-Modified-Since header.
756   The algorithm for determining this includes the following cases:
757  <list style="numbers">
758      <t>If the request would normally result in anything other than a
759         200 (OK) status, or if the passed If-Modified-Since date is
760         invalid, the response is exactly the same as for a normal GET.
761         A date which is later than the server's current time is
762         invalid.</t>
763
764      <t>If the variant has been modified since the If-Modified-Since
765         date, the response is exactly the same as for a normal GET.</t>
766
767      <t>If the variant has not been modified since a valid If-Modified-Since
768         date, the server SHOULD return a 304 (Not
769         Modified) response.</t>
770  </list>
771</t>
772<t>
773   The purpose of this feature is to allow efficient updates of cached
774   information with a minimum amount of transaction overhead.
775  <list><t>
776      Note: The Range request-header field modifies the meaning of If-Modified-Since;
777      see Section 6.4 of <xref target="Part5"/> for full details.
778    </t><t>
779      Note: If-Modified-Since times are interpreted by the server, whose
780      clock might not be synchronized with the client.
781    </t><t>
782      Note: When handling an If-Modified-Since header field, some
783      servers will use an exact date comparison function, rather than a
784      less-than function, for deciding whether to send a 304 (Not
785      Modified) response. To get best results when sending an If-Modified-Since
786      header field for cache validation, clients are
787      advised to use the exact date string received in a previous Last-Modified
788      header field whenever possible.
789    </t><t>
790      Note: If a client uses an arbitrary date in the If-Modified-Since
791      header instead of a date taken from the Last-Modified header for
792      the same request, the client should be aware of the fact that this
793      date is interpreted in the server's understanding of time. The
794      client should consider unsynchronized clocks and rounding problems
795      due to the different encodings of time between the client and
796      server. This includes the possibility of race conditions if the
797      document has changed between the time it was first requested and
798      the If-Modified-Since date of a subsequent request, and the
799      possibility of clock-skew-related problems if the If-Modified-Since
800      date is derived from the client's clock without correction
801      to the server's clock. Corrections for different time bases
802      between client and server are at best approximate due to network
803      latency.
804    </t>
805  </list>
806</t>
807<t>
808   The result of a request having both an If-Modified-Since header field
809   and either an If-Match or an If-Unmodified-Since header fields is
810   undefined by this specification.
811</t>
812</section>
813
814<section title="If-None-Match" anchor="header.if-none-match">
815  <iref primary="true" item="If-None-Match header"/>
816  <iref primary="true" item="Headers" subitem="If-None-Match"/>
817 
818<t>
819   The If-None-Match request-header field is used with a method to make
820   it conditional. A client that has one or more entities previously
821   obtained from the resource can verify that none of those entities is
822   current by including a list of their associated entity tags in the
823   If-None-Match header field. The purpose of this feature is to allow
824   efficient updates of cached information with a minimum amount of
825   transaction overhead. It is also used to prevent a method (e.g. PUT)
826   from inadvertently modifying an existing resource when the client
827   believes that the resource does not exist.
828</t>
829<t>
830   As a special case, the value "*" matches any current entity of the
831   resource.
832</t>
833<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><artwork type="abnf2616"><![CDATA[
834  If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
835]]></artwork></figure>
836<t>
837   If any of the entity tags match the entity tag of the entity that
838   would have been returned in the response to a similar GET request
839   (without the If-None-Match header) on that resource, or if "*" is
840   given and any current entity exists for that resource, then the
841   server MUST NOT perform the requested method, unless required to do
842   so because the resource's modification date fails to match that
843   supplied in an If-Modified-Since header field in the request.
844   Instead, if the request method was GET or HEAD, the server SHOULD
845   respond with a 304 (Not Modified) response, including the cache-related
846   header fields (particularly ETag) of one of the entities that
847   matched. For all other request methods, the server MUST respond with
848   a status of 412 (Precondition Failed).
849</t>
850<t>
851   See <xref target="weak.and.strong.validators"/> for rules on how to
852   determine if two entity tags match.
853</t>
854<t>
855   If none of the entity tags match, then the server MAY perform the
856   requested method as if the If-None-Match header field did not exist,
857   but MUST also ignore any If-Modified-Since header field(s) in the
858   request. That is, if no entity tags match, then the server MUST NOT
859   return a 304 (Not Modified) response.
860</t>
861<t>
862   If the request would, without the If-None-Match header field, result
863   in anything other than a 2xx or 304 status, then the If-None-Match
864   header MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
865   server behavior when both If-Modified-Since and If-None-Match appear
866   in the same request.)
867</t>
868<t>
869   The meaning of "If-None-Match: *" is that the method MUST NOT be
870   performed if the representation selected by the origin server (or by
871   a cache, possibly using the Vary mechanism, see Section 16.5 of <xref target="Part6"/>)
872   exists, and SHOULD be performed if the representation does not exist.
873   This feature is intended to be useful in preventing races between PUT
874   operations.
875</t>
876<t>
877   Examples:
878</t>
879<figure><artwork type="example"><![CDATA[
880    If-None-Match: "xyzzy"
881    If-None-Match: W/"xyzzy"
882    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
883    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
884    If-None-Match: *
885]]></artwork></figure>
886<t>
887   The result of a request having both an If-None-Match header field and
888   either an If-Match or an If-Unmodified-Since header fields is
889   undefined by this specification.
890</t>
891</section>
892
893<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
894  <iref primary="true" item="If-Unmodified-Since header"/>
895  <iref primary="true" item="Headers" subitem="If-Unmodified-Since"/>
896 
897<t>
898   The If-Unmodified-Since request-header field is used with a method to
899   make it conditional. If the requested resource has not been modified
900   since the time specified in this field, the server SHOULD perform the
901   requested operation as if the If-Unmodified-Since header were not
902   present.
903</t>
904<t>
905   If the requested variant has been modified since the specified time,
906   the server MUST NOT perform the requested operation, and MUST return
907   a 412 (Precondition Failed).
908</t>
909<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><artwork type="abnf2616"><![CDATA[
910  If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
911]]></artwork></figure>
912<t>
913   An example of the field is:
914</t>
915<figure><artwork type="example"><![CDATA[
916    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
917]]></artwork></figure>
918<t>
919   If the request normally (i.e., without the If-Unmodified-Since
920   header) would result in anything other than a 2xx or 412 status, the
921   If-Unmodified-Since header SHOULD be ignored.
922</t>
923<t>
924   If the specified date is invalid, the header is ignored.
925</t>
926<t>
927   The result of a request having both an If-Unmodified-Since header
928   field and either an If-None-Match or an If-Modified-Since header
929   fields is undefined by this specification.
930</t>
931</section>
932
933<section title="Last-Modified" anchor="header.last-modified">
934  <iref primary="true" item="Last-Modified header"/>
935  <iref primary="true" item="Headers" subitem="Last-Modified"/>
936 
937<t>
938   The Last-Modified entity-header field indicates the date and time at
939   which the origin server believes the variant was last modified.
940</t>
941<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><artwork type="abnf2616"><![CDATA[
942  Last-Modified  = "Last-Modified" ":" HTTP-date
943]]></artwork></figure>
944<t>
945   An example of its use is
946</t>
947<figure><artwork type="example"><![CDATA[
948    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
949]]></artwork></figure>
950<t>
951   The exact meaning of this header field depends on the implementation
952   of the origin server and the nature of the original resource. For
953   files, it may be just the file system last-modified time. For
954   entities with dynamically included parts, it may be the most recent
955   of the set of last-modify times for its component parts. For database
956   gateways, it may be the last-update time stamp of the record. For
957   virtual objects, it may be the last time the internal state changed.
958</t>
959<t>
960   An origin server MUST NOT send a Last-Modified date which is later
961   than the server's time of message origination. In such cases, where
962   the resource's last modification would indicate some time in the
963   future, the server MUST replace that date with the message
964   origination date.
965</t>
966<t>
967   An origin server SHOULD obtain the Last-Modified value of the entity
968   as close as possible to the time that it generates the Date value of
969   its response. This allows a recipient to make an accurate assessment
970   of the entity's modification time, especially if the entity changes
971   near the time that the response is generated.
972</t>
973<t>
974   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
975</t>
976<t>
977   The Last-Modified entity-header field value is often used as a cache
978   validator. In simple terms, a cache entry is considered to be valid
979   if the entity has not been modified since the Last-Modified value.
980</t>
981</section>
982
983</section>
984
985<section title="IANA Considerations" anchor="IANA.considerations">
986<section title="Message Header Registration" anchor="message.header.registration">
987<t>
988   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
989   with the permanent registrations below (see <xref target="RFC3864"/>):
990</t>
991<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
992<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
993   <ttcol>Header Field Name</ttcol>
994   <ttcol>Protocol</ttcol>
995   <ttcol>Status</ttcol>
996   <ttcol>Reference</ttcol>
997
998   <c>ETag</c>
999   <c>http</c>
1000   <c>standard</c>
1001   <c>
1002      <xref target="header.etag"/>
1003   </c>
1004   <c>If-Match</c>
1005   <c>http</c>
1006   <c>standard</c>
1007   <c>
1008      <xref target="header.if-match"/>
1009   </c>
1010   <c>If-Modified-Since</c>
1011   <c>http</c>
1012   <c>standard</c>
1013   <c>
1014      <xref target="header.if-modified-since"/>
1015   </c>
1016   <c>If-None-Match</c>
1017   <c>http</c>
1018   <c>standard</c>
1019   <c>
1020      <xref target="header.if-none-match"/>
1021   </c>
1022   <c>If-Unmodified-Since</c>
1023   <c>http</c>
1024   <c>standard</c>
1025   <c>
1026      <xref target="header.if-unmodified-since"/>
1027   </c>
1028   <c>Last-Modified</c>
1029   <c>http</c>
1030   <c>standard</c>
1031   <c>
1032      <xref target="header.last-modified"/>
1033   </c>
1034</texttable>
1035<!--(END)-->
1036<t>
1037   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1038</t>
1039</section>
1040</section>
1041
1042<section title="Security Considerations" anchor="security.considerations">
1043<t>
1044   No additional security considerations have been identified beyond
1045   those applicable to HTTP in general <xref target="Part1"/>.
1046</t>
1047</section>
1048
1049<section title="Acknowledgments" anchor="ack">
1050</section>
1051</middle>
1052<back>
1053
1054<references title="Normative References">
1055
1056<reference anchor="Part1">
1057  <front>
1058    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1059    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1060      <organization abbrev="Day Software">Day Software</organization>
1061      <address><email>fielding@gbiv.com</email></address>
1062    </author>
1063    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1064      <organization>One Laptop per Child</organization>
1065      <address><email>jg@laptop.org</email></address>
1066    </author>
1067    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1068      <organization abbrev="HP">Hewlett-Packard Company</organization>
1069      <address><email>JeffMogul@acm.org</email></address>
1070    </author>
1071    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1072      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1073      <address><email>henrikn@microsoft.com</email></address>
1074    </author>
1075    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1076      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1077      <address><email>LMM@acm.org</email></address>
1078    </author>
1079    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1080      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1081      <address><email>paulle@microsoft.com</email></address>
1082    </author>
1083    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1084      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1085      <address><email>timbl@w3.org</email></address>
1086    </author>
1087    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1088      <organization abbrev="W3C">World Wide Web Consortium</organization>
1089      <address><email>ylafon@w3.org</email></address>
1090    </author>
1091    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1092      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1093      <address><email>julian.reschke@greenbytes.de</email></address>
1094    </author>
1095    <date month="August" year="2008"/>
1096  </front>
1097  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-04"/>
1098 
1099</reference>
1100
1101<reference anchor="Part5">
1102  <front>
1103    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1104    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1105      <organization abbrev="Day Software">Day Software</organization>
1106      <address><email>fielding@gbiv.com</email></address>
1107    </author>
1108    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1109      <organization>One Laptop per Child</organization>
1110      <address><email>jg@laptop.org</email></address>
1111    </author>
1112    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1113      <organization abbrev="HP">Hewlett-Packard Company</organization>
1114      <address><email>JeffMogul@acm.org</email></address>
1115    </author>
1116    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1117      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1118      <address><email>henrikn@microsoft.com</email></address>
1119    </author>
1120    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1121      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1122      <address><email>LMM@acm.org</email></address>
1123    </author>
1124    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1125      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1126      <address><email>paulle@microsoft.com</email></address>
1127    </author>
1128    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1129      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1130      <address><email>timbl@w3.org</email></address>
1131    </author>
1132    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1133      <organization abbrev="W3C">World Wide Web Consortium</organization>
1134      <address><email>ylafon@w3.org</email></address>
1135    </author>
1136    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1137      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1138      <address><email>julian.reschke@greenbytes.de</email></address>
1139    </author>
1140    <date month="August" year="2008"/>
1141  </front>
1142  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-04"/>
1143 
1144</reference>
1145
1146<reference anchor="Part6">
1147  <front>
1148    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1149    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1150      <organization abbrev="Day Software">Day Software</organization>
1151      <address><email>fielding@gbiv.com</email></address>
1152    </author>
1153    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1154      <organization>One Laptop per Child</organization>
1155      <address><email>jg@laptop.org</email></address>
1156    </author>
1157    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1158      <organization abbrev="HP">Hewlett-Packard Company</organization>
1159      <address><email>JeffMogul@acm.org</email></address>
1160    </author>
1161    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1162      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1163      <address><email>henrikn@microsoft.com</email></address>
1164    </author>
1165    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1166      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1167      <address><email>LMM@acm.org</email></address>
1168    </author>
1169    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1170      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1171      <address><email>paulle@microsoft.com</email></address>
1172    </author>
1173    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1174      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1175      <address><email>timbl@w3.org</email></address>
1176    </author>
1177    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1178      <organization abbrev="W3C">World Wide Web Consortium</organization>
1179      <address><email>ylafon@w3.org</email></address>
1180    </author>
1181    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1182      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1183      <address><email>julian.reschke@greenbytes.de</email></address>
1184    </author>
1185    <date month="August" year="2008"/>
1186  </front>
1187  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-04"/>
1188 
1189</reference>
1190
1191<reference anchor="RFC2119">
1192  <front>
1193    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1194    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1195      <organization>Harvard University</organization>
1196      <address><email>sob@harvard.edu</email></address>
1197    </author>
1198    <date month="March" year="1997"/>
1199  </front>
1200  <seriesInfo name="BCP" value="14"/>
1201  <seriesInfo name="RFC" value="2119"/>
1202</reference>
1203
1204</references>
1205
1206<references title="Informative References">
1207
1208<reference anchor="RFC2616">
1209  <front>
1210    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1211    <author initials="R." surname="Fielding" fullname="R. Fielding">
1212      <organization>University of California, Irvine</organization>
1213      <address><email>fielding@ics.uci.edu</email></address>
1214    </author>
1215    <author initials="J." surname="Gettys" fullname="J. Gettys">
1216      <organization>W3C</organization>
1217      <address><email>jg@w3.org</email></address>
1218    </author>
1219    <author initials="J." surname="Mogul" fullname="J. Mogul">
1220      <organization>Compaq Computer Corporation</organization>
1221      <address><email>mogul@wrl.dec.com</email></address>
1222    </author>
1223    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1224      <organization>MIT Laboratory for Computer Science</organization>
1225      <address><email>frystyk@w3.org</email></address>
1226    </author>
1227    <author initials="L." surname="Masinter" fullname="L. Masinter">
1228      <organization>Xerox Corporation</organization>
1229      <address><email>masinter@parc.xerox.com</email></address>
1230    </author>
1231    <author initials="P." surname="Leach" fullname="P. Leach">
1232      <organization>Microsoft Corporation</organization>
1233      <address><email>paulle@microsoft.com</email></address>
1234    </author>
1235    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1236      <organization>W3C</organization>
1237      <address><email>timbl@w3.org</email></address>
1238    </author>
1239    <date month="June" year="1999"/>
1240  </front>
1241  <seriesInfo name="RFC" value="2616"/>
1242</reference>
1243
1244<reference anchor="RFC3864">
1245  <front>
1246    <title>Registration Procedures for Message Header Fields</title>
1247    <author initials="G." surname="Klyne" fullname="G. Klyne">
1248      <organization>Nine by Nine</organization>
1249      <address><email>GK-IETF@ninebynine.org</email></address>
1250    </author>
1251    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
1252      <organization>BEA Systems</organization>
1253      <address><email>mnot@pobox.com</email></address>
1254    </author>
1255    <author initials="J." surname="Mogul" fullname="J. Mogul">
1256      <organization>HP Labs</organization>
1257      <address><email>JeffMogul@acm.org</email></address>
1258    </author>
1259    <date year="2004" month="September"/>
1260  </front>
1261  <seriesInfo name="BCP" value="90"/>
1262  <seriesInfo name="RFC" value="3864"/>
1263</reference>
1264
1265</references>
1266
1267<section title="Compatibility with Previous Versions" anchor="compatibility">
1268
1269<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1270<t>
1271  Allow weak entity tags in all requests except range requests (Sections
1272  <xref target="weak.and.strong.validators" format="counter"/> and
1273  <xref target="header.if-none-match" format="counter"/>).
1274</t>
1275</section>
1276
1277</section>
1278
1279<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1280
1281<section title="Since RFC2616">
1282<t>
1283  Extracted relevant partitions from <xref target="RFC2616"/>.
1284</t>
1285</section>
1286
1287<section title="Since draft-ietf-httpbis-p4-conditional-00">
1288<t>
1289  Closed issues:
1290  <list style="symbols"> 
1291    <t>
1292      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1293      "Normative and Informative references"
1294    </t>
1295  </list>
1296</t>
1297<t>
1298  Other changes:
1299  <list style="symbols"> 
1300    <t>
1301      Move definitions of 304 and 412 condition codes from Part2.
1302    </t>
1303  </list>
1304</t>
1305</section>
1306
1307<section title="Since draft-ietf-httpbis-p4-conditional-01">
1308<t>
1309  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1310  <list style="symbols"> 
1311    <t>
1312      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1313    </t>
1314  </list>
1315</t>
1316</section>
1317
1318<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1319<t>
1320  Closed issues:
1321  <list style="symbols"> 
1322    <t>
1323      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1324      "Weak ETags on non-GET requests"
1325    </t>
1326  </list>
1327</t>
1328<t>
1329  Ongoing work on IANA Message Header Registration (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1330  <list style="symbols"> 
1331    <t>
1332      Reference RFC 3984, and update header registrations for headers defined
1333      in this document.
1334    </t>
1335  </list>
1336</t>
1337</section>
1338
1339<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1340<t>
1341  Closed issues:
1342  <list style="symbols"> 
1343    <t>
1344      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1345      "Examples for ETag matching"
1346    </t>
1347    <t>
1348      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1349      "'entity value' undefined"
1350    </t>
1351    <t>
1352      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1353      "bogus 2068 Date header reference"
1354    </t>
1355  </list>
1356</t>
1357</section>
1358
1359</section>
1360
1361</back>
1362</rfc>
Note: See TracBrowser for help on using the repository browser.