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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-05">
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="November" year="2008" day="16"/>
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://tools.ietf.org/wg/httpbis/trac/report/11"/>
180    and related documents (including fancy diffs) can be found at
181    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
182  </t>
183  <t>
184    The changes in this draft are summarized in <xref target="changes.since.04"/>.
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 
233<t>
234  This specification uses the ABNF syntax defined in Section 2.1 of <xref target="Part1"/> and
235  the core rules defined in Section 2.2 of <xref target="Part1"/>:
236</t>
237<figure><artwork type="abnf2616"><![CDATA[
238  quoted-string = <quoted-string, defined in [Part1], Section 2.2>
239  OWS           = <OWS, 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 
623<t>
624   The response-header field "ETag" provides the current value of the
625   entity tag (see <xref target="entity.tags"/>) for the requested variant.
626   The headers used with entity
627   tags are described in Sections <xref target="header.if-match" format="counter"/>
628   and <xref target="header.if-none-match" format="counter"/> of this document,
629   and in Section 6.3 of <xref target="Part5"/>. The entity tag
630   MAY be used for comparison with other entities from the same resource
631   (see <xref target="weak.and.strong.validators"/>).
632</t>
633<figure><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/><artwork type="abnf2616"><![CDATA[
634  ETag   = "ETag" ":" OWS ETag-v
635  ETag-v = entity-tag
636]]></artwork></figure>
637<figure><preamble>
638  Examples:
639</preamble>
640<artwork type="example"><![CDATA[
641  ETag: "xyzzy"
642  ETag: W/"xyzzy"
643  ETag: ""
644]]></artwork></figure>
645<t>
646   The ETag response-header field value, an entity tag, provides for an
647   "opaque" cache validator. This might allow more reliable validation
648   in situations where it is inconvenient to store modification dates,
649   where the one-second resolution of HTTP date values is not
650   sufficient, or where the origin server wishes to avoid certain
651   paradoxes that might arise from the use of modification dates.
652</t>
653<t>
654   The principle behind entity tags is that only the service author
655   knows the semantics of a resource well enough to select an
656   appropriate cache validation mechanism, and the specification of any
657   validator comparison function more complex than byte-equality would
658   open up a can of worms. Thus, comparisons of any other headers
659   (except Last-Modified, for compatibility with HTTP/1.0) are never
660   used for purposes of validating a cache entry.
661</t>
662</section>
663
664<section title="If-Match" anchor="header.if-match">
665  <iref primary="true" item="If-Match header"/>
666  <iref primary="true" item="Headers" subitem="If-Match"/>
667 
668 
669<t>
670   The request-header field "If-Match" is used with a method to make it
671   conditional. A client that has one or more entities previously
672   obtained from the resource can verify that one of those entities is
673   current by including a list of their associated entity tags in the
674   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
675   purpose of this feature is to allow efficient updates of cached
676   information with a minimum amount of transaction overhead. It is also
677   used, on updating requests, to prevent inadvertent modification of
678   the wrong version of a resource. As a special case, the value "*"
679   matches any current entity of the resource.
680</t>
681<figure><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/><artwork type="abnf2616"><![CDATA[
682  If-Match   = "If-Match" ":" OWS If-Match-v
683  If-Match-v = "*" / 1#entity-tag
684]]></artwork></figure>
685<t>
686   If any of the entity tags match the entity tag of the entity that
687   would have been returned in the response to a similar GET request
688   (without the If-Match header) on that resource, or if "*" is given
689   and any current entity exists for that resource, then the server MAY
690   perform the requested method as if the If-Match header field did not
691   exist.
692</t>
693<t>
694   A server MUST use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
695   to compare the entity tags in If-Match.
696</t>
697<t>
698   If none of the entity tags match, or if "*" is given and no current
699   entity exists, the server MUST NOT perform the requested method, and
700   MUST return a 412 (Precondition Failed) response. This behavior is
701   most useful when the client wants to prevent an updating method, such
702   as PUT, from modifying a resource that has changed since the client
703   last retrieved it.
704</t>
705<t>
706   If the request would, without the If-Match header field, result in
707   anything other than a 2xx or 412 status, then the If-Match header
708   MUST be ignored.
709</t>
710<t>
711   The meaning of "If-Match: *" is that the method SHOULD be performed
712   if the representation selected by the origin server (or by a cache,
713   possibly using the Vary mechanism, see Section 16.5 of <xref target="Part6"/>) exists, and
714   MUST NOT be performed if the representation does not exist.
715</t>
716<t>
717   A request intended to update a resource (e.g., a PUT) MAY include an
718   If-Match header field to signal that the request method MUST NOT be
719   applied if the entity corresponding to the If-Match value (a single
720   entity tag) is no longer a representation of that resource. This
721   allows the user to indicate that they do not wish the request to be
722   successful if the resource has been changed without their knowledge.
723   Examples:
724</t>
725<figure><artwork type="example"><![CDATA[
726  If-Match: "xyzzy"
727  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
728  If-Match: *
729]]></artwork></figure>
730<t>
731   The result of a request having both an If-Match header field and
732   either an If-None-Match or an If-Modified-Since header fields is
733   undefined by this specification.
734</t>
735</section>
736
737<section title="If-Modified-Since" anchor="header.if-modified-since">
738  <iref primary="true" item="If-Modified-Since header"/>
739  <iref primary="true" item="Headers" subitem="If-Modified-Since"/>
740 
741 
742<t>
743   The request-header field "If-Modified-Since" is used with a method to
744   make it conditional: if the requested variant has not been modified
745   since the time specified in this field, an entity will not be
746   returned from the server; instead, a 304 (Not Modified) response will
747   be returned without any message-body.
748</t>
749<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/><artwork type="abnf2616"><![CDATA[
750  If-Modified-Since   = "If-Modified-Since" ":" OWS
751                        If-Modified-Since-v
752  If-Modified-Since-v = HTTP-date
753]]></artwork></figure>
754<t>
755   An example of the field is:
756</t>
757<figure><artwork type="example"><![CDATA[
758  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
759]]></artwork></figure>
760<t>
761   A GET method with an If-Modified-Since header and no Range header
762   requests that the identified entity be transferred only if it has
763   been modified since the date given by the If-Modified-Since header.
764   The algorithm for determining this includes the following cases:
765  <list style="numbers">
766      <t>If the request would normally result in anything other than a
767         200 (OK) status, or if the passed If-Modified-Since date is
768         invalid, the response is exactly the same as for a normal GET.
769         A date which is later than the server's current time is
770         invalid.</t>
771
772      <t>If the variant has been modified since the If-Modified-Since
773         date, the response is exactly the same as for a normal GET.</t>
774
775      <t>If the variant has not been modified since a valid If-Modified-Since
776         date, the server SHOULD return a 304 (Not
777         Modified) response.</t>
778  </list>
779</t>
780<t>
781   The purpose of this feature is to allow efficient updates of cached
782   information with a minimum amount of transaction overhead.
783  <list><t>
784      Note: The Range request-header field modifies the meaning of If-Modified-Since;
785      see Section 6.4 of <xref target="Part5"/> for full details.
786    </t><t>
787      Note: If-Modified-Since times are interpreted by the server, whose
788      clock might not be synchronized with the client.
789    </t><t>
790      Note: When handling an If-Modified-Since header field, some
791      servers will use an exact date comparison function, rather than a
792      less-than function, for deciding whether to send a 304 (Not
793      Modified) response. To get best results when sending an If-Modified-Since
794      header field for cache validation, clients are
795      advised to use the exact date string received in a previous Last-Modified
796      header field whenever possible.
797    </t><t>
798      Note: If a client uses an arbitrary date in the If-Modified-Since
799      header instead of a date taken from the Last-Modified header for
800      the same request, the client should be aware of the fact that this
801      date is interpreted in the server's understanding of time. The
802      client should consider unsynchronized clocks and rounding problems
803      due to the different encodings of time between the client and
804      server. This includes the possibility of race conditions if the
805      document has changed between the time it was first requested and
806      the If-Modified-Since date of a subsequent request, and the
807      possibility of clock-skew-related problems if the If-Modified-Since
808      date is derived from the client's clock without correction
809      to the server's clock. Corrections for different time bases
810      between client and server are at best approximate due to network
811      latency.
812    </t>
813  </list>
814</t>
815<t>
816   The result of a request having both an If-Modified-Since header field
817   and either an If-Match or an If-Unmodified-Since header fields is
818   undefined by this specification.
819</t>
820</section>
821
822<section title="If-None-Match" anchor="header.if-none-match">
823  <iref primary="true" item="If-None-Match header"/>
824  <iref primary="true" item="Headers" subitem="If-None-Match"/>
825 
826 
827<t>
828   The request-header field "If-None-Match" is used with a method to make
829   it conditional. A client that has one or more entities previously
830   obtained from the resource can verify that none of those entities is
831   current by including a list of their associated entity tags in the
832   If-None-Match header field. The purpose of this feature is to allow
833   efficient updates of cached information with a minimum amount of
834   transaction overhead. It is also used to prevent a method (e.g. PUT)
835   from inadvertently modifying an existing resource when the client
836   believes that the resource does not exist.
837</t>
838<t>
839   As a special case, the value "*" matches any current entity of the
840   resource.
841</t>
842<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/><artwork type="abnf2616"><![CDATA[
843  If-None-Match   = "If-None-Match" ":" OWS If-None-Match-v
844  If-None-Match-v = "*" / 1#entity-tag
845]]></artwork></figure>
846<t>
847   If any of the entity tags match the entity tag of the entity that
848   would have been returned in the response to a similar GET request
849   (without the If-None-Match header) on that resource, or if "*" is
850   given and any current entity exists for that resource, then the
851   server MUST NOT perform the requested method, unless required to do
852   so because the resource's modification date fails to match that
853   supplied in an If-Modified-Since header field in the request.
854   Instead, if the request method was GET or HEAD, the server SHOULD
855   respond with a 304 (Not Modified) response, including the cache-related
856   header fields (particularly ETag) of one of the entities that
857   matched. For all other request methods, the server MUST respond with
858   a status of 412 (Precondition Failed).
859</t>
860<t>
861   See <xref target="weak.and.strong.validators"/> for rules on how to
862   determine if two entity tags match.
863</t>
864<t>
865   If none of the entity tags match, then the server MAY perform the
866   requested method as if the If-None-Match header field did not exist,
867   but MUST also ignore any If-Modified-Since header field(s) in the
868   request. That is, if no entity tags match, then the server MUST NOT
869   return a 304 (Not Modified) response.
870</t>
871<t>
872   If the request would, without the If-None-Match header field, result
873   in anything other than a 2xx or 304 status, then the If-None-Match
874   header MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
875   server behavior when both If-Modified-Since and If-None-Match appear
876   in the same request.)
877</t>
878<t>
879   The meaning of "If-None-Match: *" is that the method MUST NOT be
880   performed if the representation selected by the origin server (or by
881   a cache, possibly using the Vary mechanism, see Section 16.5 of <xref target="Part6"/>)
882   exists, and SHOULD be performed if the representation does not exist.
883   This feature is intended to be useful in preventing races between PUT
884   operations.
885</t>
886<t>
887   Examples:
888</t>
889<figure><artwork type="example"><![CDATA[
890  If-None-Match: "xyzzy"
891  If-None-Match: W/"xyzzy"
892  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
893  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
894  If-None-Match: *
895]]></artwork></figure>
896<t>
897   The result of a request having both an If-None-Match header field and
898   either an If-Match or an If-Unmodified-Since header fields is
899   undefined by this specification.
900</t>
901</section>
902
903<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
904  <iref primary="true" item="If-Unmodified-Since header"/>
905  <iref primary="true" item="Headers" subitem="If-Unmodified-Since"/>
906 
907 
908<t>
909   The request-header field "If-Unmodified-Since" is used with a method to
910   make it conditional. If the requested resource has not been modified
911   since the time specified in this field, the server SHOULD perform the
912   requested operation as if the If-Unmodified-Since header were not
913   present.
914</t>
915<t>
916   If the requested variant has been modified since the specified time,
917   the server MUST NOT perform the requested operation, and MUST return
918   a 412 (Precondition Failed).
919</t>
920<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/><artwork type="abnf2616"><![CDATA[
921  If-Unmodified-Since   = "If-Unmodified-Since" ":" OWS
922                          If-Unmodified-Since-v
923  If-Unmodified-Since-v = HTTP-date
924]]></artwork></figure>
925<t>
926   An example of the field is:
927</t>
928<figure><artwork type="example"><![CDATA[
929  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
930]]></artwork></figure>
931<t>
932   If the request normally (i.e., without the If-Unmodified-Since
933   header) would result in anything other than a 2xx or 412 status, the
934   If-Unmodified-Since header SHOULD be ignored.
935</t>
936<t>
937   If the specified date is invalid, the header is ignored.
938</t>
939<t>
940   The result of a request having both an If-Unmodified-Since header
941   field and either an If-None-Match or an If-Modified-Since header
942   fields is undefined by this specification.
943</t>
944</section>
945
946<section title="Last-Modified" anchor="header.last-modified">
947  <iref primary="true" item="Last-Modified header"/>
948  <iref primary="true" item="Headers" subitem="Last-Modified"/>
949 
950 
951<t>
952   The entity-header field "Last-Modified" indicates the date and time at
953   which the origin server believes the variant was last modified.
954</t>
955<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/><artwork type="abnf2616"><![CDATA[
956  Last-Modified   = "Last-Modified" ":" OWS Last-Modified-v
957  Last-Modified-v = HTTP-date
958]]></artwork></figure>
959<t>
960   An example of its use is
961</t>
962<figure><artwork type="example"><![CDATA[
963  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
964]]></artwork></figure>
965<t>
966   The exact meaning of this header field depends on the implementation
967   of the origin server and the nature of the original resource. For
968   files, it may be just the file system last-modified time. For
969   entities with dynamically included parts, it may be the most recent
970   of the set of last-modify times for its component parts. For database
971   gateways, it may be the last-update time stamp of the record. For
972   virtual objects, it may be the last time the internal state changed.
973</t>
974<t>
975   An origin server MUST NOT send a Last-Modified date which is later
976   than the server's time of message origination. In such cases, where
977   the resource's last modification would indicate some time in the
978   future, the server MUST replace that date with the message
979   origination date.
980</t>
981<t>
982   An origin server SHOULD obtain the Last-Modified value of the entity
983   as close as possible to the time that it generates the Date value of
984   its response. This allows a recipient to make an accurate assessment
985   of the entity's modification time, especially if the entity changes
986   near the time that the response is generated.
987</t>
988<t>
989   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
990</t>
991<t>
992   The Last-Modified entity-header field value is often used as a cache
993   validator. In simple terms, a cache entry is considered to be valid
994   if the entity has not been modified since the Last-Modified value.
995</t>
996</section>
997
998</section>
999
1000<section title="IANA Considerations" anchor="IANA.considerations">
1001<section title="Message Header Registration" anchor="message.header.registration">
1002<t>
1003   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1004   with the permanent registrations below (see <xref target="RFC3864"/>):
1005</t>
1006<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1007<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1008   <ttcol>Header Field Name</ttcol>
1009   <ttcol>Protocol</ttcol>
1010   <ttcol>Status</ttcol>
1011   <ttcol>Reference</ttcol>
1012
1013   <c>ETag</c>
1014   <c>http</c>
1015   <c>standard</c>
1016   <c>
1017      <xref target="header.etag"/>
1018   </c>
1019   <c>If-Match</c>
1020   <c>http</c>
1021   <c>standard</c>
1022   <c>
1023      <xref target="header.if-match"/>
1024   </c>
1025   <c>If-Modified-Since</c>
1026   <c>http</c>
1027   <c>standard</c>
1028   <c>
1029      <xref target="header.if-modified-since"/>
1030   </c>
1031   <c>If-None-Match</c>
1032   <c>http</c>
1033   <c>standard</c>
1034   <c>
1035      <xref target="header.if-none-match"/>
1036   </c>
1037   <c>If-Unmodified-Since</c>
1038   <c>http</c>
1039   <c>standard</c>
1040   <c>
1041      <xref target="header.if-unmodified-since"/>
1042   </c>
1043   <c>Last-Modified</c>
1044   <c>http</c>
1045   <c>standard</c>
1046   <c>
1047      <xref target="header.last-modified"/>
1048   </c>
1049</texttable>
1050<!--(END)-->
1051<t>
1052   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1053</t>
1054</section>
1055</section>
1056
1057<section title="Security Considerations" anchor="security.considerations">
1058<t>
1059   No additional security considerations have been identified beyond
1060   those applicable to HTTP in general <xref target="Part1"/>.
1061</t>
1062</section>
1063
1064<section title="Acknowledgments" anchor="ack">
1065</section>
1066</middle>
1067<back>
1068
1069<references title="Normative References">
1070
1071<reference anchor="Part1">
1072  <front>
1073    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1074    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1075      <organization abbrev="Day Software">Day Software</organization>
1076      <address><email>fielding@gbiv.com</email></address>
1077    </author>
1078    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1079      <organization>One Laptop per Child</organization>
1080      <address><email>jg@laptop.org</email></address>
1081    </author>
1082    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1083      <organization abbrev="HP">Hewlett-Packard Company</organization>
1084      <address><email>JeffMogul@acm.org</email></address>
1085    </author>
1086    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1087      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1088      <address><email>henrikn@microsoft.com</email></address>
1089    </author>
1090    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1091      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1092      <address><email>LMM@acm.org</email></address>
1093    </author>
1094    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1095      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1096      <address><email>paulle@microsoft.com</email></address>
1097    </author>
1098    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1099      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1100      <address><email>timbl@w3.org</email></address>
1101    </author>
1102    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1103      <organization abbrev="W3C">World Wide Web Consortium</organization>
1104      <address><email>ylafon@w3.org</email></address>
1105    </author>
1106    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1107      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1108      <address><email>julian.reschke@greenbytes.de</email></address>
1109    </author>
1110    <date month="November" year="2008"/>
1111  </front>
1112  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-05"/>
1113 
1114</reference>
1115
1116<reference anchor="Part5">
1117  <front>
1118    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1119    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1120      <organization abbrev="Day Software">Day Software</organization>
1121      <address><email>fielding@gbiv.com</email></address>
1122    </author>
1123    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1124      <organization>One Laptop per Child</organization>
1125      <address><email>jg@laptop.org</email></address>
1126    </author>
1127    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1128      <organization abbrev="HP">Hewlett-Packard Company</organization>
1129      <address><email>JeffMogul@acm.org</email></address>
1130    </author>
1131    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1132      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1133      <address><email>henrikn@microsoft.com</email></address>
1134    </author>
1135    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1136      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1137      <address><email>LMM@acm.org</email></address>
1138    </author>
1139    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1140      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1141      <address><email>paulle@microsoft.com</email></address>
1142    </author>
1143    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1144      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1145      <address><email>timbl@w3.org</email></address>
1146    </author>
1147    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1148      <organization abbrev="W3C">World Wide Web Consortium</organization>
1149      <address><email>ylafon@w3.org</email></address>
1150    </author>
1151    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1152      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1153      <address><email>julian.reschke@greenbytes.de</email></address>
1154    </author>
1155    <date month="November" year="2008"/>
1156  </front>
1157  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-05"/>
1158 
1159</reference>
1160
1161<reference anchor="Part6">
1162  <front>
1163    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1164    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1165      <organization abbrev="Day Software">Day Software</organization>
1166      <address><email>fielding@gbiv.com</email></address>
1167    </author>
1168    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1169      <organization>One Laptop per Child</organization>
1170      <address><email>jg@laptop.org</email></address>
1171    </author>
1172    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1173      <organization abbrev="HP">Hewlett-Packard Company</organization>
1174      <address><email>JeffMogul@acm.org</email></address>
1175    </author>
1176    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1177      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1178      <address><email>henrikn@microsoft.com</email></address>
1179    </author>
1180    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1181      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1182      <address><email>LMM@acm.org</email></address>
1183    </author>
1184    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1185      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1186      <address><email>paulle@microsoft.com</email></address>
1187    </author>
1188    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1189      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1190      <address><email>timbl@w3.org</email></address>
1191    </author>
1192    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1193      <organization abbrev="W3C">World Wide Web Consortium</organization>
1194      <address><email>ylafon@w3.org</email></address>
1195    </author>
1196    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1197      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1198      <address><email>julian.reschke@greenbytes.de</email></address>
1199    </author>
1200    <date month="November" year="2008"/>
1201  </front>
1202  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-05"/>
1203 
1204</reference>
1205
1206<reference anchor="RFC2119">
1207  <front>
1208    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1209    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1210      <organization>Harvard University</organization>
1211      <address><email>sob@harvard.edu</email></address>
1212    </author>
1213    <date month="March" year="1997"/>
1214  </front>
1215  <seriesInfo name="BCP" value="14"/>
1216  <seriesInfo name="RFC" value="2119"/>
1217</reference>
1218
1219</references>
1220
1221<references title="Informative References">
1222
1223<reference anchor="RFC2616">
1224  <front>
1225    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1226    <author initials="R." surname="Fielding" fullname="R. Fielding">
1227      <organization>University of California, Irvine</organization>
1228      <address><email>fielding@ics.uci.edu</email></address>
1229    </author>
1230    <author initials="J." surname="Gettys" fullname="J. Gettys">
1231      <organization>W3C</organization>
1232      <address><email>jg@w3.org</email></address>
1233    </author>
1234    <author initials="J." surname="Mogul" fullname="J. Mogul">
1235      <organization>Compaq Computer Corporation</organization>
1236      <address><email>mogul@wrl.dec.com</email></address>
1237    </author>
1238    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1239      <organization>MIT Laboratory for Computer Science</organization>
1240      <address><email>frystyk@w3.org</email></address>
1241    </author>
1242    <author initials="L." surname="Masinter" fullname="L. Masinter">
1243      <organization>Xerox Corporation</organization>
1244      <address><email>masinter@parc.xerox.com</email></address>
1245    </author>
1246    <author initials="P." surname="Leach" fullname="P. Leach">
1247      <organization>Microsoft Corporation</organization>
1248      <address><email>paulle@microsoft.com</email></address>
1249    </author>
1250    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1251      <organization>W3C</organization>
1252      <address><email>timbl@w3.org</email></address>
1253    </author>
1254    <date month="June" year="1999"/>
1255  </front>
1256  <seriesInfo name="RFC" value="2616"/>
1257</reference>
1258
1259<reference anchor="RFC3864">
1260  <front>
1261    <title>Registration Procedures for Message Header Fields</title>
1262    <author initials="G." surname="Klyne" fullname="G. Klyne">
1263      <organization>Nine by Nine</organization>
1264      <address><email>GK-IETF@ninebynine.org</email></address>
1265    </author>
1266    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
1267      <organization>BEA Systems</organization>
1268      <address><email>mnot@pobox.com</email></address>
1269    </author>
1270    <author initials="J." surname="Mogul" fullname="J. Mogul">
1271      <organization>HP Labs</organization>
1272      <address><email>JeffMogul@acm.org</email></address>
1273    </author>
1274    <date year="2004" month="September"/>
1275  </front>
1276  <seriesInfo name="BCP" value="90"/>
1277  <seriesInfo name="RFC" value="3864"/>
1278</reference>
1279
1280</references>
1281
1282<section title="Compatibility with Previous Versions" anchor="compatibility">
1283
1284<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1285<t>
1286  Allow weak entity tags in all requests except range requests (Sections
1287  <xref target="weak.and.strong.validators" format="counter"/> and
1288  <xref target="header.if-none-match" format="counter"/>).
1289</t>
1290</section>
1291
1292</section>
1293
1294<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1295
1296<section title="Since RFC2616">
1297<t>
1298  Extracted relevant partitions from <xref target="RFC2616"/>.
1299</t>
1300</section>
1301
1302<section title="Since draft-ietf-httpbis-p4-conditional-00">
1303<t>
1304  Closed issues:
1305  <list style="symbols"> 
1306    <t>
1307      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1308      "Normative and Informative references"
1309    </t>
1310  </list>
1311</t>
1312<t>
1313  Other changes:
1314  <list style="symbols"> 
1315    <t>
1316      Move definitions of 304 and 412 condition codes from Part2.
1317    </t>
1318  </list>
1319</t>
1320</section>
1321
1322<section title="Since draft-ietf-httpbis-p4-conditional-01">
1323<t>
1324  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1325  <list style="symbols"> 
1326    <t>
1327      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1328    </t>
1329  </list>
1330</t>
1331</section>
1332
1333<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1334<t>
1335  Closed issues:
1336  <list style="symbols"> 
1337    <t>
1338      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1339      "Weak ETags on non-GET requests"
1340    </t>
1341  </list>
1342</t>
1343<t>
1344  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1345  <list style="symbols"> 
1346    <t>
1347      Reference RFC 3984, and update header registrations for headers defined
1348      in this document.
1349    </t>
1350  </list>
1351</t>
1352</section>
1353
1354<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1355<t>
1356  Closed issues:
1357  <list style="symbols"> 
1358    <t>
1359      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1360      "Examples for ETag matching"
1361    </t>
1362    <t>
1363      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1364      "'entity value' undefined"
1365    </t>
1366    <t>
1367      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1368      "bogus 2068 Date header reference"
1369    </t>
1370  </list>
1371</t>
1372</section>
1373
1374<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1375<t>
1376  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1377  <list style="symbols"> 
1378    <t>
1379      Use "/" instead of "|" for alternatives.
1380    </t>
1381    <t>
1382      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1383      whitespace ("OWS") and required whitespace ("RWS").
1384    </t>
1385    <t>
1386      Rewrite ABNFs to spell out whitespace rules, factor out
1387      header value format definitions.
1388    </t>
1389  </list>
1390</t>
1391</section>
1392
1393</section>
1394
1395</back>
1396</rfc>
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