source: draft-ietf-httpbis/latest/p4-conditional.xml @ 345

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "November">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY header-date                "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY full-date                  "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27]>
28<?rfc toc="yes" ?>
29<?rfc symrefs="yes" ?>
30<?rfc sortrefs="yes" ?>
31<?rfc compact="yes"?>
32<?rfc subcompact="no" ?>
33<?rfc linkmailto="no" ?>
34<?rfc editing="no" ?>
35<?rfc comments="yes"?>
36<?rfc inline="yes"?>
37<?rfc-ext allow-markup-in-artwork="yes" ?>
38<?rfc-ext include-references-in-index="yes" ?>
39<rfc obsoletes="2616" category="std" x:maturity-level="draft"
40     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
41     xmlns:x='http://purl.org/net/xml2rfc/ext'>
42<front>
43
44  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
45
46  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
47    <organization abbrev="Day Software">Day Software</organization>
48    <address>
49      <postal>
50        <street>23 Corporate Plaza DR, Suite 280</street>
51        <city>Newport Beach</city>
52        <region>CA</region>
53        <code>92660</code>
54        <country>USA</country>
55      </postal>
56      <phone>+1-949-706-5300</phone>
57      <facsimile>+1-949-706-5305</facsimile>
58      <email>fielding@gbiv.com</email>
59      <uri>http://roy.gbiv.com/</uri>
60    </address>
61  </author>
62
63  <author initials="J." surname="Gettys" fullname="Jim Gettys">
64    <organization>One Laptop per Child</organization>
65    <address>
66      <postal>
67        <street>21 Oak Knoll Road</street>
68        <city>Carlisle</city>
69        <region>MA</region>
70        <code>01741</code>
71        <country>USA</country>
72      </postal>
73      <email>jg@laptop.org</email>
74      <uri>http://www.laptop.org/</uri>
75    </address>
76  </author>
77 
78  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
79    <organization abbrev="HP">Hewlett-Packard Company</organization>
80    <address>
81      <postal>
82        <street>HP Labs, Large Scale Systems Group</street>
83        <street>1501 Page Mill Road, MS 1177</street>
84        <city>Palo Alto</city>
85        <region>CA</region>
86        <code>94304</code>
87        <country>USA</country>
88      </postal>
89      <email>JeffMogul@acm.org</email>
90    </address>
91  </author>
92
93  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
94    <organization abbrev="Microsoft">Microsoft Corporation</organization>
95    <address>
96      <postal>
97        <street>1 Microsoft Way</street>
98        <city>Redmond</city>
99        <region>WA</region>
100        <code>98052</code>
101        <country>USA</country>
102      </postal>
103      <email>henrikn@microsoft.com</email>
104    </address>
105  </author>
106
107  <author initials="L." surname="Masinter" fullname="Larry Masinter">
108    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
109    <address>
110      <postal>
111        <street>345 Park Ave</street>
112        <city>San Jose</city>
113        <region>CA</region>
114        <code>95110</code>
115        <country>USA</country>
116      </postal>
117      <email>LMM@acm.org</email>
118      <uri>http://larry.masinter.net/</uri>
119    </address>
120  </author>
121 
122  <author initials="P." surname="Leach" fullname="Paul J. Leach">
123    <organization abbrev="Microsoft">Microsoft Corporation</organization>
124    <address>
125      <postal>
126        <street>1 Microsoft Way</street>
127        <city>Redmond</city>
128        <region>WA</region>
129        <code>98052</code>
130      </postal>
131      <email>paulle@microsoft.com</email>
132    </address>
133  </author>
134   
135  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
136    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
137    <address>
138      <postal>
139        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
140        <street>The Stata Center, Building 32</street>
141        <street>32 Vassar Street</street>
142        <city>Cambridge</city>
143        <region>MA</region>
144        <code>02139</code>
145        <country>USA</country>
146      </postal>
147      <email>timbl@w3.org</email>
148      <uri>http://www.w3.org/People/Berners-Lee/</uri>
149    </address>
150  </author>
151
152  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
153    <organization abbrev="W3C">World Wide Web Consortium</organization>
154    <address>
155      <postal>
156        <street>W3C / ERCIM</street>
157        <street>2004, rte des Lucioles</street>
158        <city>Sophia-Antipolis</city>
159        <region>AM</region>
160        <code>06902</code>
161        <country>France</country>
162      </postal>
163      <email>ylafon@w3.org</email>
164      <uri>http://www.raubacapeu.net/people/yves/</uri>
165    </address>
166  </author>
167
168  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
169    <organization abbrev="greenbytes">greenbytes GmbH</organization>
170    <address>
171      <postal>
172        <street>Hafenweg 16</street>
173        <city>Muenster</city><region>NW</region><code>48155</code>
174        <country>Germany</country>
175      </postal>
176      <phone>+49 251 2807760</phone>   
177      <facsimile>+49 251 2807761</facsimile>   
178      <email>julian.reschke@greenbytes.de</email>       
179      <uri>http://greenbytes.de/tech/webdav/</uri>     
180    </address>
181  </author>
182
183  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
184
185<abstract>
186<t>
187   The Hypertext Transfer Protocol (HTTP) is an application-level
188   protocol for distributed, collaborative, hypermedia information
189   systems. HTTP has been in use by the World Wide Web global information
190   initiative since 1990. This document is Part 4 of the seven-part specification
191   that defines the protocol referred to as "HTTP/1.1" and, taken together,
192   obsoletes RFC 2616.  Part 4 defines request header fields for
193   indicating conditional requests and the rules for constructing responses
194   to those requests.
195</t>
196</abstract>
197
198<note title="Editorial Note (To be removed by RFC Editor)">
199  <t>
200    Discussion of this draft should take place on the HTTPBIS working group
201    mailing list (ietf-http-wg@w3.org). The current issues list is
202    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
203    and related documents (including fancy diffs) can be found at
204    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
205  </t>
206  <t>
207    The changes in this draft are summarized in <xref target="changes.since.04"/>.
208  </t>
209</note>
210</front>
211<middle>
212<section title="Introduction" anchor="introduction">
213<t>
214   This document defines HTTP/1.1 response metadata for indicating potential
215   changes to payload content, including modification time stamps and opaque
216   entity-tags, and the HTTP conditional request mechanisms that allow
217   preconditions to be placed on a request method.  Conditional GET requests
218   allow for efficient cache updates.  Other conditional request methods are
219   used to protect against overwriting or misunderstanding the state of a
220   resource that has been changed unbeknownst to the requesting client.
221</t>
222<t>
223   This document is currently disorganized in order to minimize the changes
224   between drafts and enable reviewers to see the smaller errata changes.
225   The next draft will reorganize the sections to better reflect the content.
226   In particular, the sections on resource metadata will be discussed first
227   and then followed by each conditional request-header, concluding with a
228   definition of precedence and the expectation of ordering strong validator
229   checks before weak validator checks.  It is likely that more content from
230   &caching; will migrate to this part, where appropriate.
231   The current mess reflects how widely dispersed these topics and associated
232   requirements had become in <xref target="RFC2616"/>.
233</t>
234
235<section title="Requirements" anchor="intro.requirements">
236<t>
237   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
238   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
239   document are to be interpreted as described in <xref target="RFC2119"/>.
240</t>
241<t>
242   An implementation is not compliant if it fails to satisfy one or more
243   of the &MUST; or &REQUIRED; level requirements for the protocols it
244   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
245   level and all the &SHOULD; level requirements for its protocols is said
246   to be "unconditionally compliant"; one that satisfies all the &MUST;
247   level requirements but not all the &SHOULD; level requirements for its
248   protocols is said to be "conditionally compliant."
249</t>
250</section>
251</section>
252
253<section title="Notational Conventions and Generic Grammar" anchor="notation">
254  <x:anchor-alias value="quoted-string"/>
255<t>
256  This specification uses the ABNF syntax defined in &notation-abnf; and
257  the core rules defined in &basic-rules;:
258  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
259  <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>.</cref>
260</t>
261<figure><artwork type="abnf2616">
262  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
263</artwork></figure>
264<t anchor="abnf.dependencies">
265  <x:anchor-alias value="HTTP-date"/>
266  The ABNF rules below are defined in other parts:
267</t>
268<figure><!--Part1--><artwork type="abnf2616">
269  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
270</artwork></figure>
271</section>
272
273<section title="Entity Tags" anchor="entity.tags">
274  <x:anchor-alias value="entity-tag"/>
275  <x:anchor-alias value="opaque-tag"/>
276  <x:anchor-alias value="weak"/>
277<t>
278   Entity tags are used for comparing two or more entities from the same
279   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
280   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
281   If-Range (&header-if-range;) header fields. The definition of how they
282   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
283   entity tag consists of an opaque quoted string, possibly prefixed by
284   a weakness indicator.
285</t>
286<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/>
287  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
288  <x:ref>weak</x:ref>       = "W/"
289  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
290</artwork></figure>
291<t>
292   A "strong entity tag" &MAY; be shared by two entities of a resource
293   only if they are equivalent by octet equality.
294</t>
295<t>
296   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
297   two entities of a resource only if the entities are equivalent and
298   could be substituted for each other with no significant change in
299   semantics. A weak entity tag can only be used for weak comparison.
300</t>
301<t>
302   An entity tag &MUST; be unique across all versions of all entities
303   associated with a particular resource. A given entity tag value &MAY;
304   be used for entities obtained by requests on different URIs. The use
305   of the same entity tag value in conjunction with entities obtained by
306   requests on different URIs does not imply the equivalence of those
307   entities.
308</t>
309</section>
310
311<section title="Status Code Definitions">
312<section title="304 Not Modified" anchor="status.304">
313  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
314  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
315<t>
316   If the client has performed a conditional GET request and access is
317   allowed, but the document has not been modified, the server &SHOULD;
318   respond with this status code. The 304 response &MUST-NOT; contain a
319   message-body, and thus is always terminated by the first empty line
320   after the header fields.
321</t>
322<t>
323   The response &MUST; include the following header fields:
324  <list style="symbols">
325    <x:lt>
326      <t>Date, unless its omission is required by &clockless;.</t>
327      <t>
328         If a clockless origin server obeys these rules, and proxies and
329         clients add their own Date to any response received without one (as
330         already specified by &header-date;, caches will operate
331         correctly.</t>
332    </x:lt>
333    <x:lt>
334      <t>ETag and/or Content-Location, if the header would have been sent
335         in a 200 response to the same request.</t>
336    </x:lt>
337    <x:lt>
338      <t>Expires, Cache-Control, and/or Vary, if the field-value might
339         differ from that sent in any previous response for the same
340         variant.</t>
341    </x:lt>
342  </list>
343</t>
344<t>
345   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
346   the response &SHOULD-NOT;  include other entity-headers.
347   Otherwise (i.e., the conditional GET used a weak validator), the
348   response &MUST-NOT; include other entity-headers; this prevents
349   inconsistencies between cached entity-bodies and updated headers.
350</t>
351<t>
352   If a 304 response indicates an entity not currently cached, then the
353   cache &MUST; disregard the response and repeat the request without the
354   conditional.
355</t>
356<t>
357   If a cache uses a received 304 response to update a cache entry, the
358   cache &MUST; update the entry to reflect any new field values given in
359   the response.
360</t>
361</section>
362
363<section title="412 Precondition Failed" anchor="status.412">
364  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
365  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
366<t>
367   The precondition given in one or more of the request-header fields
368   evaluated to false when it was tested on the server. This response
369   code allows the client to place preconditions on the current resource
370   metainformation (header field data) and thus prevent the requested
371   method from being applied to a resource other than the one intended.
372</t>
373</section>
374</section>
375
376<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
377<t>
378   Since both origin servers and caches will compare two validators to
379   decide if they represent the same or different entities, one normally
380   would expect that if the entity (the entity-body or any entity-headers)
381   changes in any way, then the associated validator would
382   change as well. If this is true, then we call this validator a
383   "strong validator."
384</t>
385<t>
386   However, there might be cases when a server prefers to change the
387   validator only on semantically significant changes, and not when
388   insignificant aspects of the entity change. A validator that does not
389   always change when the resource changes is a "weak validator."
390</t>
391<t>
392   Entity tags are normally "strong validators," but the protocol
393   provides a mechanism to tag an entity tag as "weak." One can think of
394   a strong validator as one that changes whenever the bits of an entity
395   changes, while a weak value changes whenever the meaning of an entity
396   changes. Alternatively, one can think of a strong validator as part
397   of an identifier for a specific entity, while a weak validator is
398   part of an identifier for a set of semantically equivalent entities.
399  <list><t>
400      <x:h>Note:</x:h> One example of a strong validator is an integer that is
401      incremented in stable storage every time an entity is changed.
402    </t><t>
403      An entity's modification time, if represented with one-second
404      resolution, could be a weak validator, since it is possible that
405      the resource might be modified twice during a single second.
406    </t><t>
407      Support for weak validators is optional. However, weak validators
408      allow for more efficient caching of equivalent objects; for
409      example, a hit counter on a site is probably good enough if it is
410      updated every few days or weeks, and any value during that period
411      is likely "good enough" to be equivalent.
412    </t></list>
413</t>
414<t>
415   A "use" of a validator is either when a client generates a request
416   and includes the validator in a validating header field, or when a
417   server compares two validators.
418</t>
419<t>
420   Strong validators are usable in any context. Weak validators are only
421   usable in contexts that do not depend on exact equality of an entity.
422   For example, either kind is usable for a conditional GET of a full
423   entity. However, only a strong validator is usable for a sub-range
424   retrieval, since otherwise the client might end up with an internally
425   inconsistent entity.
426</t>
427<t>
428   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
429</t>
430<t>
431   The only function that HTTP/1.1 defines on validators is
432   comparison. There are two validator comparison functions, depending
433   on whether the comparison context allows the use of weak validators
434   or not:
435  <list style="symbols">
436     <t>The strong comparison function: in order to be considered equal,
437        both opaque-tags &MUST; be identical character-by-character, and both
438        &MUST-NOT; be weak.</t>
439     <t>The weak comparison function: in order to be considered equal, both
440        opaque-tags &MUST; be identical character-by-character.</t>
441  </list>
442</t>
443<t>
444   The example below shows the results for a set of entity tag pairs,
445   and both the weak and strong comparison function results:
446</t>
447<texttable align="left">
448  <ttcol>ETag 1</ttcol>
449  <ttcol>ETag 2</ttcol>
450  <ttcol>Strong Comparison</ttcol>
451  <ttcol>Weak Comparison</ttcol>
452
453  <c>W/"1"</c>
454  <c>W/"1"</c>
455  <c>no match</c>
456  <c>match</c>
457 
458  <c>W/"1"</c>
459  <c>W/"2"</c>
460  <c>no match</c>
461  <c>no match</c>
462
463  <c>W/"1"</c>
464  <c>"1"</c>
465  <c>no match</c>
466  <c>match</c>
467
468  <c>"1"</c>
469  <c>"1"</c>
470  <c>match</c>
471  <c>match</c>
472</texttable>
473<t>
474   An entity tag is strong unless it is explicitly tagged as weak.
475   <xref target="entity.tags"/> gives the syntax for entity tags.
476</t>
477<t>
478   A Last-Modified time, when used as a validator in a request, is
479   implicitly weak unless it is possible to deduce that it is strong,
480   using the following rules:
481  <list style="symbols">
482     <t>The validator is being compared by an origin server to the
483        actual current validator for the entity and,</t>
484     <t>That origin server reliably knows that the associated entity did
485        not change twice during the second covered by the presented
486        validator.</t>
487  </list>
488</t>
489<t>
490   or
491  <list style="symbols">
492     <t>The validator is about to be used by a client in an If-Modified-Since
493        or If-Unmodified-Since header, because the client
494        has a cache entry for the associated entity, and</t>
495     <t>That cache entry includes a Date value, which gives the time
496        when the origin server sent the original response, and</t>
497     <t>The presented Last-Modified time is at least 60 seconds before
498        the Date value.</t>
499  </list>
500</t>
501<t>
502   or
503  <list style="symbols">
504     <t>The validator is being compared by an intermediate cache to the
505        validator stored in its cache entry for the entity, and</t>
506     <t>That cache entry includes a Date value, which gives the time
507        when the origin server sent the original response, and</t>
508     <t>The presented Last-Modified time is at least 60 seconds before
509        the Date value.</t>
510  </list>
511</t>
512<t>
513   This method relies on the fact that if two different responses were
514   sent by the origin server during the same second, but both had the
515   same Last-Modified time, then at least one of those responses would
516   have a Date value equal to its Last-Modified time. The arbitrary 60-second
517   limit guards against the possibility that the Date and Last-Modified
518   values are generated from different clocks, or at somewhat
519   different times during the preparation of the response. An
520   implementation &MAY; use a value larger than 60 seconds, if it is
521   believed that 60 seconds is too short.
522</t>
523<t>
524   If a client wishes to perform a sub-range retrieval on a value for
525   which it has only a Last-Modified time and no opaque validator, it
526   &MAY; do this only if the Last-Modified time is strong in the sense
527   described here.
528</t>
529<t>
530   A cache or origin server receiving a conditional range request
531   (<xref target="Part5"/>) &MUST; use the strong comparison function to
532   evaluate the condition.
533</t>
534<t>
535   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
536   retrievals on values that have been obtained from HTTP/1.0
537   servers.
538</t>
539</section>
540
541<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">
542<t>
543   We adopt a set of rules and recommendations for origin servers,
544   clients, and caches regarding when various validator types ought to
545   be used, and for what purposes.
546</t>
547<t>
548   HTTP/1.1 origin servers:
549  <list style="symbols">
550     <t>&SHOULD; send an entity tag validator unless it is not feasible to
551        generate one.</t>
552
553     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
554        performance considerations support the use of weak entity tags,
555        or if it is unfeasible to send a strong entity tag.</t>
556
557     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
558        unless the risk of a breakdown in semantic transparency that
559        could result from using this date in an If-Modified-Since header
560        would lead to serious problems.</t>
561  </list>
562</t>
563<t>
564   In other words, the preferred behavior for an HTTP/1.1 origin server
565   is to send both a strong entity tag and a Last-Modified value.
566</t>
567<t>
568   In order to be legal, a strong entity tag &MUST; change whenever the
569   associated entity changes in any way. A weak entity tag &SHOULD;
570   change whenever the associated entity changes in a semantically
571   significant way.
572  <list><t>
573      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
574      origin server must avoid reusing a specific strong entity tag
575      value for two different entities, or reusing a specific weak
576      entity tag value for two semantically different entities. Cache
577      entries might persist for arbitrarily long periods, regardless of
578      expiration times, so it might be inappropriate to expect that a
579      cache will never again attempt to validate an entry using a
580      validator that it obtained at some point in the past.
581  </t></list>
582</t>
583<t>
584   HTTP/1.1 clients:
585  <list style="symbols">
586     <t>If an entity tag has been provided by the origin server, &MUST;
587        use that entity tag in any cache-conditional request (using If-Match
588        or If-None-Match).</t>
589
590     <t>If only a Last-Modified value has been provided by the origin
591        server, &SHOULD; use that value in non-subrange cache-conditional
592        requests (using If-Modified-Since).</t>
593
594     <t>If only a Last-Modified value has been provided by an HTTP/1.0
595        origin server, &MAY; use that value in subrange cache-conditional
596        requests (using If-Unmodified-Since:). The user agent &SHOULD;
597        provide a way to disable this, in case of difficulty.</t>
598
599     <t>If both an entity tag and a Last-Modified value have been
600        provided by the origin server, &SHOULD; use both validators in
601        cache-conditional requests. This allows both HTTP/1.0 and
602        HTTP/1.1 caches to respond appropriately.</t>
603  </list>
604</t>
605<t>
606   An HTTP/1.1 origin server, upon receiving a conditional request that
607   includes both a Last-Modified date (e.g., in an If-Modified-Since or
608   If-Unmodified-Since header field) and one or more entity tags (e.g.,
609   in an If-Match, If-None-Match, or If-Range header field) as cache
610   validators, &MUST-NOT; return a response status of 304 (Not Modified)
611   unless doing so is consistent with all of the conditional header
612   fields in the request.
613</t>
614<t>
615   An HTTP/1.1 caching proxy, upon receiving a conditional request that
616   includes both a Last-Modified date and one or more entity tags as
617   cache validators, &MUST-NOT; return a locally cached response to the
618   client unless that cached response is consistent with all of the
619   conditional header fields in the request.
620  <list><t>
621      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
622      servers and clients should transmit as much non-redundant
623      information as is available in their responses and requests.
624      HTTP/1.1 systems receiving this information will make the most
625      conservative assumptions about the validators they receive.
626  </t><t>
627      HTTP/1.0 clients and caches will ignore entity tags. Generally,
628      last-modified values received or used by these systems will
629      support transparent and efficient caching, and so HTTP/1.1 origin
630      servers should provide Last-Modified values. In those rare cases
631      where the use of a Last-Modified value as a validator by an
632      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
633      origin servers should not provide one.
634  </t></list>
635</t>
636</section>
637
638<section title="Header Field Definitions" anchor="header.fields">
639<t>
640   This section defines the syntax and semantics of HTTP/1.1 header fields
641   related to conditional requests.
642</t>
643<t>
644   For entity-header fields, both sender and recipient refer to either the
645   client or the server, depending on who sends and who receives the entity.
646</t>
647
648<section title="ETag" anchor="header.etag">
649  <iref primary="true" item="ETag header" x:for-anchor=""/>
650  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
651  <x:anchor-alias value="ETag"/>
652<t>
653   The ETag response-header field provides the current value of the
654   entity tag (see <xref target="entity.tags"/>) for the requested variant.
655   The headers used with entity
656   tags are described in Sections <xref target="header.if-match" format="counter"/>
657   and <xref target="header.if-none-match" format="counter"/> of this document,
658   and in &header-if-range;. The entity tag
659   &MAY; be used for comparison with other entities from the same resource
660   (see <xref target="weak.and.strong.validators"/>).
661</t>
662<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
663  <x:ref>ETag</x:ref> = "ETag" ":" <x:ref>entity-tag</x:ref>
664</artwork></figure>
665<figure><preamble>
666   Examples:
667</preamble>
668<artwork type="example">
669   ETag: "xyzzy"
670   ETag: W/"xyzzy"
671   ETag: ""
672</artwork></figure>
673<t>
674   The ETag response-header field value, an entity tag, provides for an
675   "opaque" cache validator. This might allow more reliable validation
676   in situations where it is inconvenient to store modification dates,
677   where the one-second resolution of HTTP date values is not
678   sufficient, or where the origin server wishes to avoid certain
679   paradoxes that might arise from the use of modification dates.
680</t>
681<t>
682   The principle behind entity tags is that only the service author
683   knows the semantics of a resource well enough to select an
684   appropriate cache validation mechanism, and the specification of any
685   validator comparison function more complex than byte-equality would
686   open up a can of worms. Thus, comparisons of any other headers
687   (except Last-Modified, for compatibility with HTTP/1.0) are never
688   used for purposes of validating a cache entry.
689</t>
690</section>
691
692<section title="If-Match" anchor="header.if-match">
693  <iref primary="true" item="If-Match header" x:for-anchor=""/>
694  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
695  <x:anchor-alias value="If-Match"/>
696<t>
697   The If-Match request-header field is used with a method to make it
698   conditional. A client that has one or more entities previously
699   obtained from the resource can verify that one of those entities is
700   current by including a list of their associated entity tags in the
701   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
702   purpose of this feature is to allow efficient updates of cached
703   information with a minimum amount of transaction overhead. It is also
704   used, on updating requests, to prevent inadvertent modification of
705   the wrong version of a resource. As a special case, the value "*"
706   matches any current entity of the resource.
707</t>
708<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
709  <x:ref>If-Match</x:ref> = "If-Match" ":" ( "*" / 1#<x:ref>entity-tag</x:ref> )
710</artwork></figure>
711<t>
712   If any of the entity tags match the entity tag of the entity that
713   would have been returned in the response to a similar GET request
714   (without the If-Match header) on that resource, or if "*" is given
715   and any current entity exists for that resource, then the server &MAY;
716   perform the requested method as if the If-Match header field did not
717   exist.
718</t>
719<t>
720   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
721   to compare the entity tags in If-Match.
722</t>
723<t>
724   If none of the entity tags match, or if "*" is given and no current
725   entity exists, the server &MUST-NOT; perform the requested method, and
726   &MUST; return a 412 (Precondition Failed) response. This behavior is
727   most useful when the client wants to prevent an updating method, such
728   as PUT, from modifying a resource that has changed since the client
729   last retrieved it.
730</t>
731<t>
732   If the request would, without the If-Match header field, result in
733   anything other than a 2xx or 412 status, then the If-Match header
734   &MUST; be ignored.
735</t>
736<t>
737   The meaning of "If-Match: *" is that the method &SHOULD; be performed
738   if the representation selected by the origin server (or by a cache,
739   possibly using the Vary mechanism, see &header-vary;) exists, and
740   &MUST-NOT; be performed if the representation does not exist.
741</t>
742<t>
743   A request intended to update a resource (e.g., a PUT) &MAY; include an
744   If-Match header field to signal that the request method &MUST-NOT; be
745   applied if the entity corresponding to the If-Match value (a single
746   entity tag) is no longer a representation of that resource. This
747   allows the user to indicate that they do not wish the request to be
748   successful if the resource has been changed without their knowledge.
749   Examples:
750</t>
751<figure><artwork type="example">
752    If-Match: "xyzzy"
753    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
754    If-Match: *
755</artwork></figure>
756<t>
757   The result of a request having both an If-Match header field and
758   either an If-None-Match or an If-Modified-Since header fields is
759   undefined by this specification.
760</t>
761</section>
762
763<section title="If-Modified-Since" anchor="header.if-modified-since">
764  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
765  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
766  <x:anchor-alias value="If-Modified-Since"/>
767<t>
768   The If-Modified-Since request-header field is used with a method to
769   make it conditional: if the requested variant has not been modified
770   since the time specified in this field, an entity will not be
771   returned from the server; instead, a 304 (Not Modified) response will
772   be returned without any message-body.
773</t>
774<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
775  <x:ref>If-Modified-Since</x:ref> = "If-Modified-Since" ":" <x:ref>HTTP-date</x:ref>
776</artwork></figure>
777<t>
778   An example of the field is:
779</t>
780<figure><artwork type="example">
781    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
782</artwork></figure>
783<t>
784   A GET method with an If-Modified-Since header and no Range header
785   requests that the identified entity be transferred only if it has
786   been modified since the date given by the If-Modified-Since header.
787   The algorithm for determining this includes the following cases:
788  <list style="numbers">
789      <t>If the request would normally result in anything other than a
790         200 (OK) status, or if the passed If-Modified-Since date is
791         invalid, the response is exactly the same as for a normal GET.
792         A date which is later than the server's current time is
793         invalid.</t>
794
795      <t>If the variant has been modified since the If-Modified-Since
796         date, the response is exactly the same as for a normal GET.</t>
797
798      <t>If the variant has not been modified since a valid If-Modified-Since
799         date, the server &SHOULD; return a 304 (Not
800         Modified) response.</t>
801  </list>
802</t>
803<t>
804   The purpose of this feature is to allow efficient updates of cached
805   information with a minimum amount of transaction overhead.
806  <list><t>
807      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
808      see &header-range; for full details.
809    </t><t>
810      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
811      clock might not be synchronized with the client.
812    </t><t>
813      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
814      servers will use an exact date comparison function, rather than a
815      less-than function, for deciding whether to send a 304 (Not
816      Modified) response. To get best results when sending an If-Modified-Since
817      header field for cache validation, clients are
818      advised to use the exact date string received in a previous Last-Modified
819      header field whenever possible.
820    </t><t>
821      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
822      header instead of a date taken from the Last-Modified header for
823      the same request, the client should be aware of the fact that this
824      date is interpreted in the server's understanding of time. The
825      client should consider unsynchronized clocks and rounding problems
826      due to the different encodings of time between the client and
827      server. This includes the possibility of race conditions if the
828      document has changed between the time it was first requested and
829      the If-Modified-Since date of a subsequent request, and the
830      possibility of clock-skew-related problems if the If-Modified-Since
831      date is derived from the client's clock without correction
832      to the server's clock. Corrections for different time bases
833      between client and server are at best approximate due to network
834      latency.
835    </t>
836  </list>
837</t>
838<t>
839   The result of a request having both an If-Modified-Since header field
840   and either an If-Match or an If-Unmodified-Since header fields is
841   undefined by this specification.
842</t>
843</section>
844
845<section title="If-None-Match" anchor="header.if-none-match">
846  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
847  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
848  <x:anchor-alias value="If-None-Match"/>
849<t>
850   The If-None-Match request-header field is used with a method to make
851   it conditional. A client that has one or more entities previously
852   obtained from the resource can verify that none of those entities is
853   current by including a list of their associated entity tags in the
854   If-None-Match header field. The purpose of this feature is to allow
855   efficient updates of cached information with a minimum amount of
856   transaction overhead. It is also used to prevent a method (e.g. PUT)
857   from inadvertently modifying an existing resource when the client
858   believes that the resource does not exist.
859</t>
860<t>
861   As a special case, the value "*" matches any current entity of the
862   resource.
863</t>
864<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
865  <x:ref>If-None-Match</x:ref> = "If-None-Match" ":" ( "*" / 1#<x:ref>entity-tag</x:ref> )
866</artwork></figure>
867<t>
868   If any of the entity tags match the entity tag of the entity that
869   would have been returned in the response to a similar GET request
870   (without the If-None-Match header) on that resource, or if "*" is
871   given and any current entity exists for that resource, then the
872   server &MUST-NOT; perform the requested method, unless required to do
873   so because the resource's modification date fails to match that
874   supplied in an If-Modified-Since header field in the request.
875   Instead, if the request method was GET or HEAD, the server &SHOULD;
876   respond with a 304 (Not Modified) response, including the cache-related
877   header fields (particularly ETag) of one of the entities that
878   matched. For all other request methods, the server &MUST; respond with
879   a status of 412 (Precondition Failed).
880</t>
881<t>
882   See <xref target="weak.and.strong.validators"/> for rules on how to
883   determine if two entity tags match.
884</t>
885<t>
886   If none of the entity tags match, then the server &MAY; perform the
887   requested method as if the If-None-Match header field did not exist,
888   but &MUST; also ignore any If-Modified-Since header field(s) in the
889   request. That is, if no entity tags match, then the server &MUST-NOT;
890   return a 304 (Not Modified) response.
891</t>
892<t>
893   If the request would, without the If-None-Match header field, result
894   in anything other than a 2xx or 304 status, then the If-None-Match
895   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
896   server behavior when both If-Modified-Since and If-None-Match appear
897   in the same request.)
898</t>
899<t>
900   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
901   performed if the representation selected by the origin server (or by
902   a cache, possibly using the Vary mechanism, see &header-vary;)
903   exists, and &SHOULD; be performed if the representation does not exist.
904   This feature is intended to be useful in preventing races between PUT
905   operations.
906</t>
907<t>
908   Examples:
909</t>
910<figure><artwork type="example">
911    If-None-Match: "xyzzy"
912    If-None-Match: W/"xyzzy"
913    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
914    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
915    If-None-Match: *
916</artwork></figure>
917<t>
918   The result of a request having both an If-None-Match header field and
919   either an If-Match or an If-Unmodified-Since header fields is
920   undefined by this specification.
921</t>
922</section>
923
924<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
925  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
926  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
927  <x:anchor-alias value="If-Unmodified-Since"/>
928<t>
929   The If-Unmodified-Since request-header field is used with a method to
930   make it conditional. If the requested resource has not been modified
931   since the time specified in this field, the server &SHOULD; perform the
932   requested operation as if the If-Unmodified-Since header were not
933   present.
934</t>
935<t>
936   If the requested variant has been modified since the specified time,
937   the server &MUST-NOT; perform the requested operation, and &MUST; return
938   a 412 (Precondition Failed).
939</t>
940<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
941  <x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since" ":" <x:ref>HTTP-date</x:ref>
942</artwork></figure>
943<t>
944   An example of the field is:
945</t>
946<figure><artwork type="example">
947    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
948</artwork></figure>
949<t>
950   If the request normally (i.e., without the If-Unmodified-Since
951   header) would result in anything other than a 2xx or 412 status, the
952   If-Unmodified-Since header &SHOULD; be ignored.
953</t>
954<t>
955   If the specified date is invalid, the header is ignored.
956</t>
957<t>
958   The result of a request having both an If-Unmodified-Since header
959   field and either an If-None-Match or an If-Modified-Since header
960   fields is undefined by this specification.
961</t>
962</section>
963
964<section title="Last-Modified" anchor="header.last-modified">
965  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
966  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
967  <x:anchor-alias value="Last-Modified"/>
968<t>
969   The Last-Modified entity-header field indicates the date and time at
970   which the origin server believes the variant was last modified.
971</t>
972<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
973  <x:ref>Last-Modified</x:ref>  = "Last-Modified" ":" <x:ref>HTTP-date</x:ref>
974</artwork></figure>
975<t>
976   An example of its use is
977</t>
978<figure><artwork type="example">
979    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
980</artwork></figure>
981<t>
982   The exact meaning of this header field depends on the implementation
983   of the origin server and the nature of the original resource. For
984   files, it may be just the file system last-modified time. For
985   entities with dynamically included parts, it may be the most recent
986   of the set of last-modify times for its component parts. For database
987   gateways, it may be the last-update time stamp of the record. For
988   virtual objects, it may be the last time the internal state changed.
989</t>
990<t>
991   An origin server &MUST-NOT; send a Last-Modified date which is later
992   than the server's time of message origination. In such cases, where
993   the resource's last modification would indicate some time in the
994   future, the server &MUST; replace that date with the message
995   origination date.
996</t>
997<t>
998   An origin server &SHOULD; obtain the Last-Modified value of the entity
999   as close as possible to the time that it generates the Date value of
1000   its response. This allows a recipient to make an accurate assessment
1001   of the entity's modification time, especially if the entity changes
1002   near the time that the response is generated.
1003</t>
1004<t>
1005   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1006</t>
1007<t>
1008   The Last-Modified entity-header field value is often used as a cache
1009   validator. In simple terms, a cache entry is considered to be valid
1010   if the entity has not been modified since the Last-Modified value.
1011</t>
1012</section>
1013
1014</section>
1015
1016<section title="IANA Considerations" anchor="IANA.considerations">
1017<section title="Message Header Registration" anchor="message.header.registration">
1018<t>
1019   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1020   with the permanent registrations below (see <xref target="RFC3864"/>):
1021</t>
1022<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1023<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1024   <ttcol>Header Field Name</ttcol>
1025   <ttcol>Protocol</ttcol>
1026   <ttcol>Status</ttcol>
1027   <ttcol>Reference</ttcol>
1028
1029   <c>ETag</c>
1030   <c>http</c>
1031   <c>standard</c>
1032   <c>
1033      <xref target="header.etag"/>
1034   </c>
1035   <c>If-Match</c>
1036   <c>http</c>
1037   <c>standard</c>
1038   <c>
1039      <xref target="header.if-match"/>
1040   </c>
1041   <c>If-Modified-Since</c>
1042   <c>http</c>
1043   <c>standard</c>
1044   <c>
1045      <xref target="header.if-modified-since"/>
1046   </c>
1047   <c>If-None-Match</c>
1048   <c>http</c>
1049   <c>standard</c>
1050   <c>
1051      <xref target="header.if-none-match"/>
1052   </c>
1053   <c>If-Unmodified-Since</c>
1054   <c>http</c>
1055   <c>standard</c>
1056   <c>
1057      <xref target="header.if-unmodified-since"/>
1058   </c>
1059   <c>Last-Modified</c>
1060   <c>http</c>
1061   <c>standard</c>
1062   <c>
1063      <xref target="header.last-modified"/>
1064   </c>
1065</texttable>
1066<!--(END)-->
1067<t>
1068   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1069</t>
1070</section>
1071</section>
1072
1073<section title="Security Considerations" anchor="security.considerations">
1074<t>
1075   No additional security considerations have been identified beyond
1076   those applicable to HTTP in general &messaging;.
1077</t>
1078</section>
1079
1080<section title="Acknowledgments" anchor="ack">
1081</section>
1082</middle>
1083<back>
1084
1085<references title="Normative References">
1086
1087<reference anchor="Part1">
1088  <front>
1089    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1090    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1091      <organization abbrev="Day Software">Day Software</organization>
1092      <address><email>fielding@gbiv.com</email></address>
1093    </author>
1094    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1095      <organization>One Laptop per Child</organization>
1096      <address><email>jg@laptop.org</email></address>
1097    </author>
1098    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1099      <organization abbrev="HP">Hewlett-Packard Company</organization>
1100      <address><email>JeffMogul@acm.org</email></address>
1101    </author>
1102    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1103      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1104      <address><email>henrikn@microsoft.com</email></address>
1105    </author>
1106    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1107      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1108      <address><email>LMM@acm.org</email></address>
1109    </author>
1110    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1111      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1112      <address><email>paulle@microsoft.com</email></address>
1113    </author>
1114    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1115      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1116      <address><email>timbl@w3.org</email></address>
1117    </author>
1118    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1119      <organization abbrev="W3C">World Wide Web Consortium</organization>
1120      <address><email>ylafon@w3.org</email></address>
1121    </author>
1122    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1123      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1124      <address><email>julian.reschke@greenbytes.de</email></address>
1125    </author>
1126    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1127  </front>
1128  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1129  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1130</reference>
1131
1132<reference anchor="Part5">
1133  <front>
1134    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1135    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1136      <organization abbrev="Day Software">Day Software</organization>
1137      <address><email>fielding@gbiv.com</email></address>
1138    </author>
1139    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1140      <organization>One Laptop per Child</organization>
1141      <address><email>jg@laptop.org</email></address>
1142    </author>
1143    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1144      <organization abbrev="HP">Hewlett-Packard Company</organization>
1145      <address><email>JeffMogul@acm.org</email></address>
1146    </author>
1147    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1148      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1149      <address><email>henrikn@microsoft.com</email></address>
1150    </author>
1151    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1152      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1153      <address><email>LMM@acm.org</email></address>
1154    </author>
1155    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1156      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1157      <address><email>paulle@microsoft.com</email></address>
1158    </author>
1159    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1160      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1161      <address><email>timbl@w3.org</email></address>
1162    </author>
1163    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1164      <organization abbrev="W3C">World Wide Web Consortium</organization>
1165      <address><email>ylafon@w3.org</email></address>
1166    </author>
1167    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1168      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1169      <address><email>julian.reschke@greenbytes.de</email></address>
1170    </author>
1171    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1172  </front>
1173  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1174  <x:source href="p5-range.xml" basename="p5-range"/>
1175</reference>
1176
1177<reference anchor="Part6">
1178  <front>
1179    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1180    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1181      <organization abbrev="Day Software">Day Software</organization>
1182      <address><email>fielding@gbiv.com</email></address>
1183    </author>
1184    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1185      <organization>One Laptop per Child</organization>
1186      <address><email>jg@laptop.org</email></address>
1187    </author>
1188    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1189      <organization abbrev="HP">Hewlett-Packard Company</organization>
1190      <address><email>JeffMogul@acm.org</email></address>
1191    </author>
1192    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1193      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1194      <address><email>henrikn@microsoft.com</email></address>
1195    </author>
1196    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1197      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1198      <address><email>LMM@acm.org</email></address>
1199    </author>
1200    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1201      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1202      <address><email>paulle@microsoft.com</email></address>
1203    </author>
1204    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1205      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1206      <address><email>timbl@w3.org</email></address>
1207    </author>
1208    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1209      <organization abbrev="W3C">World Wide Web Consortium</organization>
1210      <address><email>ylafon@w3.org</email></address>
1211    </author>
1212    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1213      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1214      <address><email>julian.reschke@greenbytes.de</email></address>
1215    </author>
1216    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1217  </front>
1218  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1219  <x:source href="p6-cache.xml" basename="p6-cache"/>
1220</reference>
1221
1222<reference anchor="RFC2119">
1223  <front>
1224    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1225    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1226      <organization>Harvard University</organization>
1227      <address><email>sob@harvard.edu</email></address>
1228    </author>
1229    <date month="March" year="1997"/>
1230  </front>
1231  <seriesInfo name="BCP" value="14"/>
1232  <seriesInfo name="RFC" value="2119"/>
1233</reference>
1234
1235</references>
1236
1237<references title="Informative References">
1238
1239<reference anchor="RFC2616">
1240  <front>
1241    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1242    <author initials="R." surname="Fielding" fullname="R. Fielding">
1243      <organization>University of California, Irvine</organization>
1244      <address><email>fielding@ics.uci.edu</email></address>
1245    </author>
1246    <author initials="J." surname="Gettys" fullname="J. Gettys">
1247      <organization>W3C</organization>
1248      <address><email>jg@w3.org</email></address>
1249    </author>
1250    <author initials="J." surname="Mogul" fullname="J. Mogul">
1251      <organization>Compaq Computer Corporation</organization>
1252      <address><email>mogul@wrl.dec.com</email></address>
1253    </author>
1254    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1255      <organization>MIT Laboratory for Computer Science</organization>
1256      <address><email>frystyk@w3.org</email></address>
1257    </author>
1258    <author initials="L." surname="Masinter" fullname="L. Masinter">
1259      <organization>Xerox Corporation</organization>
1260      <address><email>masinter@parc.xerox.com</email></address>
1261    </author>
1262    <author initials="P." surname="Leach" fullname="P. Leach">
1263      <organization>Microsoft Corporation</organization>
1264      <address><email>paulle@microsoft.com</email></address>
1265    </author>
1266    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1267      <organization>W3C</organization>
1268      <address><email>timbl@w3.org</email></address>
1269    </author>
1270    <date month="June" year="1999"/>
1271  </front>
1272  <seriesInfo name="RFC" value="2616"/>
1273</reference>
1274
1275<reference anchor='RFC3864'>
1276  <front>
1277    <title>Registration Procedures for Message Header Fields</title>
1278    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1279      <organization>Nine by Nine</organization>
1280      <address><email>GK-IETF@ninebynine.org</email></address>
1281    </author>
1282    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1283      <organization>BEA Systems</organization>
1284      <address><email>mnot@pobox.com</email></address>
1285    </author>
1286    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1287      <organization>HP Labs</organization>
1288      <address><email>JeffMogul@acm.org</email></address>
1289    </author>
1290    <date year='2004' month='September' />
1291  </front>
1292  <seriesInfo name='BCP' value='90' />
1293  <seriesInfo name='RFC' value='3864' />
1294</reference>
1295
1296</references>
1297
1298<section title="Compatibility with Previous Versions" anchor="compatibility">
1299
1300<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1301<t>
1302  Allow weak entity tags in all requests except range requests (Sections
1303  <xref target="weak.and.strong.validators" format="counter"/> and
1304  <xref target="header.if-none-match" format="counter"/>).
1305</t>
1306</section>
1307
1308</section>
1309
1310<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1311
1312<section title="Since RFC2616">
1313<t>
1314  Extracted relevant partitions from <xref target="RFC2616"/>.
1315</t>
1316</section>
1317
1318<section title="Since draft-ietf-httpbis-p4-conditional-00">
1319<t>
1320  Closed issues:
1321  <list style="symbols"> 
1322    <t>
1323      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1324      "Normative and Informative references"
1325    </t>
1326  </list>
1327</t>
1328<t>
1329  Other changes:
1330  <list style="symbols"> 
1331    <t>
1332      Move definitions of 304 and 412 condition codes from Part2.
1333    </t>
1334  </list>
1335</t>
1336</section>
1337
1338<section title="Since draft-ietf-httpbis-p4-conditional-01">
1339<t>
1340  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1341  <list style="symbols"> 
1342    <t>
1343      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1344    </t>
1345  </list>
1346</t>
1347</section>
1348
1349<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1350<t>
1351  Closed issues:
1352  <list style="symbols"> 
1353    <t>
1354      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1355      "Weak ETags on non-GET requests"
1356    </t>
1357  </list>
1358</t>
1359<t>
1360  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1361  <list style="symbols"> 
1362    <t>
1363      Reference RFC 3984, and update header registrations for headers defined
1364      in this document.
1365    </t>
1366  </list>
1367</t>
1368</section>
1369
1370<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1371<t>
1372  Closed issues:
1373  <list style="symbols"> 
1374    <t>
1375      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1376      "Examples for ETag matching"
1377    </t>
1378    <t>
1379      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1380      "'entity value' undefined"
1381    </t>
1382    <t>
1383      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1384      "bogus 2068 Date header reference"
1385    </t>
1386  </list>
1387</t>
1388</section>
1389
1390<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1391<t>
1392  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1393  <list style="symbols"> 
1394    <t>
1395      Use "/" instead of "|" for alternatives.
1396    </t>
1397  </list>
1398</t>
1399</section>
1400
1401</section>
1402
1403</back>
1404</rfc>
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