source: draft-ietf-httpbis/latest/p4-conditional.xml @ 376

Last change on this file since 376 was 376, checked in by julian.reschke@…, 11 years ago

fix width problems in ABNF (related to #36)

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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  <x:anchor-alias value="OWS"/>
256<t>
257  This specification uses the ABNF syntax defined in &notation-abnf; and
258  the core rules defined in &basic-rules;:
259  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
260  <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>.</cref>
261</t>
262<figure><artwork type="abnf2616">
263  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
264  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
265</artwork></figure>
266<t anchor="abnf.dependencies">
267  <x:anchor-alias value="HTTP-date"/>
268  The ABNF rules below are defined in other parts:
269</t>
270<figure><!--Part1--><artwork type="abnf2616">
271  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
272</artwork></figure>
273</section>
274
275<section title="Entity Tags" anchor="entity.tags">
276  <x:anchor-alias value="entity-tag"/>
277  <x:anchor-alias value="opaque-tag"/>
278  <x:anchor-alias value="weak"/>
279<t>
280   Entity tags are used for comparing two or more entities from the same
281   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
282   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
283   If-Range (&header-if-range;) header fields. The definition of how they
284   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
285   entity tag consists of an opaque quoted string, possibly prefixed by
286   a weakness indicator.
287</t>
288<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"/>
289  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
290  <x:ref>weak</x:ref>       = "W/"
291  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
292</artwork></figure>
293<t>
294   A "strong entity tag" &MAY; be shared by two entities of a resource
295   only if they are equivalent by octet equality.
296</t>
297<t>
298   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
299   two entities of a resource only if the entities are equivalent and
300   could be substituted for each other with no significant change in
301   semantics. A weak entity tag can only be used for weak comparison.
302</t>
303<t>
304   An entity tag &MUST; be unique across all versions of all entities
305   associated with a particular resource. A given entity tag value &MAY;
306   be used for entities obtained by requests on different URIs. The use
307   of the same entity tag value in conjunction with entities obtained by
308   requests on different URIs does not imply the equivalence of those
309   entities.
310</t>
311</section>
312
313<section title="Status Code Definitions">
314<section title="304 Not Modified" anchor="status.304">
315  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
316  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
317<t>
318   If the client has performed a conditional GET request and access is
319   allowed, but the document has not been modified, the server &SHOULD;
320   respond with this status code. The 304 response &MUST-NOT; contain a
321   message-body, and thus is always terminated by the first empty line
322   after the header fields.
323</t>
324<t>
325   The response &MUST; include the following header fields:
326  <list style="symbols">
327    <x:lt>
328      <t>Date, unless its omission is required by &clockless;.</t>
329      <t>
330         If a clockless origin server obeys these rules, and proxies and
331         clients add their own Date to any response received without one (as
332         already specified by &header-date;, caches will operate
333         correctly.</t>
334    </x:lt>
335    <x:lt>
336      <t>ETag and/or Content-Location, if the header would have been sent
337         in a 200 response to the same request.</t>
338    </x:lt>
339    <x:lt>
340      <t>Expires, Cache-Control, and/or Vary, if the field-value might
341         differ from that sent in any previous response for the same
342         variant.</t>
343    </x:lt>
344  </list>
345</t>
346<t>
347   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
348   the response &SHOULD-NOT;  include other entity-headers.
349   Otherwise (i.e., the conditional GET used a weak validator), the
350   response &MUST-NOT; include other entity-headers; this prevents
351   inconsistencies between cached entity-bodies and updated headers.
352</t>
353<t>
354   If a 304 response indicates an entity not currently cached, then the
355   cache &MUST; disregard the response and repeat the request without the
356   conditional.
357</t>
358<t>
359   If a cache uses a received 304 response to update a cache entry, the
360   cache &MUST; update the entry to reflect any new field values given in
361   the response.
362</t>
363</section>
364
365<section title="412 Precondition Failed" anchor="status.412">
366  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
367  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
368<t>
369   The precondition given in one or more of the request-header fields
370   evaluated to false when it was tested on the server. This response
371   code allows the client to place preconditions on the current resource
372   metainformation (header field data) and thus prevent the requested
373   method from being applied to a resource other than the one intended.
374</t>
375</section>
376</section>
377
378<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
379<t>
380   Since both origin servers and caches will compare two validators to
381   decide if they represent the same or different entities, one normally
382   would expect that if the entity (the entity-body or any entity-headers)
383   changes in any way, then the associated validator would
384   change as well. If this is true, then we call this validator a
385   "strong validator."
386</t>
387<t>
388   However, there might be cases when a server prefers to change the
389   validator only on semantically significant changes, and not when
390   insignificant aspects of the entity change. A validator that does not
391   always change when the resource changes is a "weak validator."
392</t>
393<t>
394   Entity tags are normally "strong validators," but the protocol
395   provides a mechanism to tag an entity tag as "weak." One can think of
396   a strong validator as one that changes whenever the bits of an entity
397   changes, while a weak value changes whenever the meaning of an entity
398   changes. Alternatively, one can think of a strong validator as part
399   of an identifier for a specific entity, while a weak validator is
400   part of an identifier for a set of semantically equivalent entities.
401  <list><t>
402      <x:h>Note:</x:h> One example of a strong validator is an integer that is
403      incremented in stable storage every time an entity is changed.
404    </t><t>
405      An entity's modification time, if represented with one-second
406      resolution, could be a weak validator, since it is possible that
407      the resource might be modified twice during a single second.
408    </t><t>
409      Support for weak validators is optional. However, weak validators
410      allow for more efficient caching of equivalent objects; for
411      example, a hit counter on a site is probably good enough if it is
412      updated every few days or weeks, and any value during that period
413      is likely "good enough" to be equivalent.
414    </t></list>
415</t>
416<t>
417   A "use" of a validator is either when a client generates a request
418   and includes the validator in a validating header field, or when a
419   server compares two validators.
420</t>
421<t>
422   Strong validators are usable in any context. Weak validators are only
423   usable in contexts that do not depend on exact equality of an entity.
424   For example, either kind is usable for a conditional GET of a full
425   entity. However, only a strong validator is usable for a sub-range
426   retrieval, since otherwise the client might end up with an internally
427   inconsistent entity.
428</t>
429<t>
430   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
431</t>
432<t>
433   The only function that HTTP/1.1 defines on validators is
434   comparison. There are two validator comparison functions, depending
435   on whether the comparison context allows the use of weak validators
436   or not:
437  <list style="symbols">
438     <t>The strong comparison function: in order to be considered equal,
439        both opaque-tags &MUST; be identical character-by-character, and both
440        &MUST-NOT; be weak.</t>
441     <t>The weak comparison function: in order to be considered equal, both
442        opaque-tags &MUST; be identical character-by-character.</t>
443  </list>
444</t>
445<t>
446   The example below shows the results for a set of entity tag pairs,
447   and both the weak and strong comparison function results:
448</t>
449<texttable align="left">
450  <ttcol>ETag 1</ttcol>
451  <ttcol>ETag 2</ttcol>
452  <ttcol>Strong Comparison</ttcol>
453  <ttcol>Weak Comparison</ttcol>
454
455  <c>W/"1"</c>
456  <c>W/"1"</c>
457  <c>no match</c>
458  <c>match</c>
459 
460  <c>W/"1"</c>
461  <c>W/"2"</c>
462  <c>no match</c>
463  <c>no match</c>
464
465  <c>W/"1"</c>
466  <c>"1"</c>
467  <c>no match</c>
468  <c>match</c>
469
470  <c>"1"</c>
471  <c>"1"</c>
472  <c>match</c>
473  <c>match</c>
474</texttable>
475<t>
476   An entity tag is strong unless it is explicitly tagged as weak.
477   <xref target="entity.tags"/> gives the syntax for entity tags.
478</t>
479<t>
480   A Last-Modified time, when used as a validator in a request, is
481   implicitly weak unless it is possible to deduce that it is strong,
482   using the following rules:
483  <list style="symbols">
484     <t>The validator is being compared by an origin server to the
485        actual current validator for the entity and,</t>
486     <t>That origin server reliably knows that the associated entity did
487        not change twice during the second covered by the presented
488        validator.</t>
489  </list>
490</t>
491<t>
492   or
493  <list style="symbols">
494     <t>The validator is about to be used by a client in an If-Modified-Since
495        or If-Unmodified-Since header, because the client
496        has a cache entry for the associated entity, and</t>
497     <t>That cache entry includes a Date value, which gives the time
498        when the origin server sent the original response, and</t>
499     <t>The presented Last-Modified time is at least 60 seconds before
500        the Date value.</t>
501  </list>
502</t>
503<t>
504   or
505  <list style="symbols">
506     <t>The validator is being compared by an intermediate cache to the
507        validator stored in its cache entry for the entity, and</t>
508     <t>That cache entry includes a Date value, which gives the time
509        when the origin server sent the original response, and</t>
510     <t>The presented Last-Modified time is at least 60 seconds before
511        the Date value.</t>
512  </list>
513</t>
514<t>
515   This method relies on the fact that if two different responses were
516   sent by the origin server during the same second, but both had the
517   same Last-Modified time, then at least one of those responses would
518   have a Date value equal to its Last-Modified time. The arbitrary 60-second
519   limit guards against the possibility that the Date and Last-Modified
520   values are generated from different clocks, or at somewhat
521   different times during the preparation of the response. An
522   implementation &MAY; use a value larger than 60 seconds, if it is
523   believed that 60 seconds is too short.
524</t>
525<t>
526   If a client wishes to perform a sub-range retrieval on a value for
527   which it has only a Last-Modified time and no opaque validator, it
528   &MAY; do this only if the Last-Modified time is strong in the sense
529   described here.
530</t>
531<t>
532   A cache or origin server receiving a conditional range request
533   (<xref target="Part5"/>) &MUST; use the strong comparison function to
534   evaluate the condition.
535</t>
536<t>
537   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
538   retrievals on values that have been obtained from HTTP/1.0
539   servers.
540</t>
541</section>
542
543<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">
544<t>
545   We adopt a set of rules and recommendations for origin servers,
546   clients, and caches regarding when various validator types ought to
547   be used, and for what purposes.
548</t>
549<t>
550   HTTP/1.1 origin servers:
551  <list style="symbols">
552     <t>&SHOULD; send an entity tag validator unless it is not feasible to
553        generate one.</t>
554
555     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
556        performance considerations support the use of weak entity tags,
557        or if it is unfeasible to send a strong entity tag.</t>
558
559     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
560        unless the risk of a breakdown in semantic transparency that
561        could result from using this date in an If-Modified-Since header
562        would lead to serious problems.</t>
563  </list>
564</t>
565<t>
566   In other words, the preferred behavior for an HTTP/1.1 origin server
567   is to send both a strong entity tag and a Last-Modified value.
568</t>
569<t>
570   In order to be legal, a strong entity tag &MUST; change whenever the
571   associated entity changes in any way. A weak entity tag &SHOULD;
572   change whenever the associated entity changes in a semantically
573   significant way.
574  <list><t>
575      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
576      origin server must avoid reusing a specific strong entity tag
577      value for two different entities, or reusing a specific weak
578      entity tag value for two semantically different entities. Cache
579      entries might persist for arbitrarily long periods, regardless of
580      expiration times, so it might be inappropriate to expect that a
581      cache will never again attempt to validate an entry using a
582      validator that it obtained at some point in the past.
583  </t></list>
584</t>
585<t>
586   HTTP/1.1 clients:
587  <list style="symbols">
588     <t>If an entity tag has been provided by the origin server, &MUST;
589        use that entity tag in any cache-conditional request (using If-Match
590        or If-None-Match).</t>
591
592     <t>If only a Last-Modified value has been provided by the origin
593        server, &SHOULD; use that value in non-subrange cache-conditional
594        requests (using If-Modified-Since).</t>
595
596     <t>If only a Last-Modified value has been provided by an HTTP/1.0
597        origin server, &MAY; use that value in subrange cache-conditional
598        requests (using If-Unmodified-Since:). The user agent &SHOULD;
599        provide a way to disable this, in case of difficulty.</t>
600
601     <t>If both an entity tag and a Last-Modified value have been
602        provided by the origin server, &SHOULD; use both validators in
603        cache-conditional requests. This allows both HTTP/1.0 and
604        HTTP/1.1 caches to respond appropriately.</t>
605  </list>
606</t>
607<t>
608   An HTTP/1.1 origin server, upon receiving a conditional request that
609   includes both a Last-Modified date (e.g., in an If-Modified-Since or
610   If-Unmodified-Since header field) and one or more entity tags (e.g.,
611   in an If-Match, If-None-Match, or If-Range header field) as cache
612   validators, &MUST-NOT; return a response status of 304 (Not Modified)
613   unless doing so is consistent with all of the conditional header
614   fields in the request.
615</t>
616<t>
617   An HTTP/1.1 caching proxy, upon receiving a conditional request that
618   includes both a Last-Modified date and one or more entity tags as
619   cache validators, &MUST-NOT; return a locally cached response to the
620   client unless that cached response is consistent with all of the
621   conditional header fields in the request.
622  <list><t>
623      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
624      servers and clients should transmit as much non-redundant
625      information as is available in their responses and requests.
626      HTTP/1.1 systems receiving this information will make the most
627      conservative assumptions about the validators they receive.
628  </t><t>
629      HTTP/1.0 clients and caches will ignore entity tags. Generally,
630      last-modified values received or used by these systems will
631      support transparent and efficient caching, and so HTTP/1.1 origin
632      servers should provide Last-Modified values. In those rare cases
633      where the use of a Last-Modified value as a validator by an
634      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
635      origin servers should not provide one.
636  </t></list>
637</t>
638</section>
639
640<section title="Header Field Definitions" anchor="header.fields">
641<t>
642   This section defines the syntax and semantics of HTTP/1.1 header fields
643   related to conditional requests.
644</t>
645<t>
646   For entity-header fields, both sender and recipient refer to either the
647   client or the server, depending on who sends and who receives the entity.
648</t>
649
650<section title="ETag" anchor="header.etag">
651  <iref primary="true" item="ETag header" x:for-anchor=""/>
652  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
653  <x:anchor-alias value="ETag"/>
654  <x:anchor-alias value="ETag-v"/>
655<t>
656   The response-header field "ETag" provides the current value of the
657   entity tag (see <xref target="entity.tags"/>) for the requested variant.
658   The headers used with entity
659   tags are described in Sections <xref target="header.if-match" format="counter"/>
660   and <xref target="header.if-none-match" format="counter"/> of this document,
661   and in &header-if-range;. The entity tag
662   &MAY; be used for comparison with other entities from the same resource
663   (see <xref target="weak.and.strong.validators"/>).
664</t>
665<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
666  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
667  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
668</artwork></figure>
669<figure><preamble>
670  Examples:
671</preamble>
672<artwork type="example">
673  ETag: "xyzzy"
674  ETag: W/"xyzzy"
675  ETag: ""
676</artwork></figure>
677<t>
678   The ETag response-header field value, an entity tag, provides for an
679   "opaque" cache validator. This might allow more reliable validation
680   in situations where it is inconvenient to store modification dates,
681   where the one-second resolution of HTTP date values is not
682   sufficient, or where the origin server wishes to avoid certain
683   paradoxes that might arise from the use of modification dates.
684</t>
685<t>
686   The principle behind entity tags is that only the service author
687   knows the semantics of a resource well enough to select an
688   appropriate cache validation mechanism, and the specification of any
689   validator comparison function more complex than byte-equality would
690   open up a can of worms. Thus, comparisons of any other headers
691   (except Last-Modified, for compatibility with HTTP/1.0) are never
692   used for purposes of validating a cache entry.
693</t>
694</section>
695
696<section title="If-Match" anchor="header.if-match">
697  <iref primary="true" item="If-Match header" x:for-anchor=""/>
698  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
699  <x:anchor-alias value="If-Match"/>
700  <x:anchor-alias value="If-Match-v"/>
701<t>
702   The request-header field "If-Match" is used with a method to make it
703   conditional. A client that has one or more entities previously
704   obtained from the resource can verify that one of those entities is
705   current by including a list of their associated entity tags in the
706   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
707   purpose of this feature is to allow efficient updates of cached
708   information with a minimum amount of transaction overhead. It is also
709   used, on updating requests, to prevent inadvertent modification of
710   the wrong version of a resource. As a special case, the value "*"
711   matches any current entity of the resource.
712</t>
713<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
714  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
715  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
716</artwork></figure>
717<t>
718   If any of the entity tags match the entity tag of the entity that
719   would have been returned in the response to a similar GET request
720   (without the If-Match header) on that resource, or if "*" is given
721   and any current entity exists for that resource, then the server &MAY;
722   perform the requested method as if the If-Match header field did not
723   exist.
724</t>
725<t>
726   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
727   to compare the entity tags in If-Match.
728</t>
729<t>
730   If none of the entity tags match, or if "*" is given and no current
731   entity exists, the server &MUST-NOT; perform the requested method, and
732   &MUST; return a 412 (Precondition Failed) response. This behavior is
733   most useful when the client wants to prevent an updating method, such
734   as PUT, from modifying a resource that has changed since the client
735   last retrieved it.
736</t>
737<t>
738   If the request would, without the If-Match header field, result in
739   anything other than a 2xx or 412 status, then the If-Match header
740   &MUST; be ignored.
741</t>
742<t>
743   The meaning of "If-Match: *" is that the method &SHOULD; be performed
744   if the representation selected by the origin server (or by a cache,
745   possibly using the Vary mechanism, see &header-vary;) exists, and
746   &MUST-NOT; be performed if the representation does not exist.
747</t>
748<t>
749   A request intended to update a resource (e.g., a PUT) &MAY; include an
750   If-Match header field to signal that the request method &MUST-NOT; be
751   applied if the entity corresponding to the If-Match value (a single
752   entity tag) is no longer a representation of that resource. This
753   allows the user to indicate that they do not wish the request to be
754   successful if the resource has been changed without their knowledge.
755   Examples:
756</t>
757<figure><artwork type="example">
758  If-Match: "xyzzy"
759  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
760  If-Match: *
761</artwork></figure>
762<t>
763   The result of a request having both an If-Match header field and
764   either an If-None-Match or an If-Modified-Since header fields is
765   undefined by this specification.
766</t>
767</section>
768
769<section title="If-Modified-Since" anchor="header.if-modified-since">
770  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
771  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
772  <x:anchor-alias value="If-Modified-Since"/>
773  <x:anchor-alias value="If-Modified-Since-v"/>
774<t>
775   The request-header field "If-Modified-Since" is used with a method to
776   make it conditional: if the requested variant has not been modified
777   since the time specified in this field, an entity will not be
778   returned from the server; instead, a 304 (Not Modified) response will
779   be returned without any message-body.
780</t>
781<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
782  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
783                        <x:ref>If-Modified-Since-v</x:ref>
784  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
785</artwork></figure>
786<t>
787   An example of the field is:
788</t>
789<figure><artwork type="example">
790  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
791</artwork></figure>
792<t>
793   A GET method with an If-Modified-Since header and no Range header
794   requests that the identified entity be transferred only if it has
795   been modified since the date given by the If-Modified-Since header.
796   The algorithm for determining this includes the following cases:
797  <list style="numbers">
798      <t>If the request would normally result in anything other than a
799         200 (OK) status, or if the passed If-Modified-Since date is
800         invalid, the response is exactly the same as for a normal GET.
801         A date which is later than the server's current time is
802         invalid.</t>
803
804      <t>If the variant has been modified since the If-Modified-Since
805         date, the response is exactly the same as for a normal GET.</t>
806
807      <t>If the variant has not been modified since a valid If-Modified-Since
808         date, the server &SHOULD; return a 304 (Not
809         Modified) response.</t>
810  </list>
811</t>
812<t>
813   The purpose of this feature is to allow efficient updates of cached
814   information with a minimum amount of transaction overhead.
815  <list><t>
816      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
817      see &header-range; for full details.
818    </t><t>
819      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
820      clock might not be synchronized with the client.
821    </t><t>
822      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
823      servers will use an exact date comparison function, rather than a
824      less-than function, for deciding whether to send a 304 (Not
825      Modified) response. To get best results when sending an If-Modified-Since
826      header field for cache validation, clients are
827      advised to use the exact date string received in a previous Last-Modified
828      header field whenever possible.
829    </t><t>
830      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
831      header instead of a date taken from the Last-Modified header for
832      the same request, the client should be aware of the fact that this
833      date is interpreted in the server's understanding of time. The
834      client should consider unsynchronized clocks and rounding problems
835      due to the different encodings of time between the client and
836      server. This includes the possibility of race conditions if the
837      document has changed between the time it was first requested and
838      the If-Modified-Since date of a subsequent request, and the
839      possibility of clock-skew-related problems if the If-Modified-Since
840      date is derived from the client's clock without correction
841      to the server's clock. Corrections for different time bases
842      between client and server are at best approximate due to network
843      latency.
844    </t>
845  </list>
846</t>
847<t>
848   The result of a request having both an If-Modified-Since header field
849   and either an If-Match or an If-Unmodified-Since header fields is
850   undefined by this specification.
851</t>
852</section>
853
854<section title="If-None-Match" anchor="header.if-none-match">
855  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
856  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
857  <x:anchor-alias value="If-None-Match"/>
858  <x:anchor-alias value="If-None-Match-v"/>
859<t>
860   The request-header field "If-None-Match" is used with a method to make
861   it conditional. A client that has one or more entities previously
862   obtained from the resource can verify that none of those entities is
863   current by including a list of their associated entity tags in the
864   If-None-Match header field. The purpose of this feature is to allow
865   efficient updates of cached information with a minimum amount of
866   transaction overhead. It is also used to prevent a method (e.g. PUT)
867   from inadvertently modifying an existing resource when the client
868   believes that the resource does not exist.
869</t>
870<t>
871   As a special case, the value "*" matches any current entity of the
872   resource.
873</t>
874<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
875  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
876  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
877</artwork></figure>
878<t>
879   If any of the entity tags match the entity tag of the entity that
880   would have been returned in the response to a similar GET request
881   (without the If-None-Match header) on that resource, or if "*" is
882   given and any current entity exists for that resource, then the
883   server &MUST-NOT; perform the requested method, unless required to do
884   so because the resource's modification date fails to match that
885   supplied in an If-Modified-Since header field in the request.
886   Instead, if the request method was GET or HEAD, the server &SHOULD;
887   respond with a 304 (Not Modified) response, including the cache-related
888   header fields (particularly ETag) of one of the entities that
889   matched. For all other request methods, the server &MUST; respond with
890   a status of 412 (Precondition Failed).
891</t>
892<t>
893   See <xref target="weak.and.strong.validators"/> for rules on how to
894   determine if two entity tags match.
895</t>
896<t>
897   If none of the entity tags match, then the server &MAY; perform the
898   requested method as if the If-None-Match header field did not exist,
899   but &MUST; also ignore any If-Modified-Since header field(s) in the
900   request. That is, if no entity tags match, then the server &MUST-NOT;
901   return a 304 (Not Modified) response.
902</t>
903<t>
904   If the request would, without the If-None-Match header field, result
905   in anything other than a 2xx or 304 status, then the If-None-Match
906   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
907   server behavior when both If-Modified-Since and If-None-Match appear
908   in the same request.)
909</t>
910<t>
911   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
912   performed if the representation selected by the origin server (or by
913   a cache, possibly using the Vary mechanism, see &header-vary;)
914   exists, and &SHOULD; be performed if the representation does not exist.
915   This feature is intended to be useful in preventing races between PUT
916   operations.
917</t>
918<t>
919   Examples:
920</t>
921<figure><artwork type="example">
922  If-None-Match: "xyzzy"
923  If-None-Match: W/"xyzzy"
924  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
925  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
926  If-None-Match: *
927</artwork></figure>
928<t>
929   The result of a request having both an If-None-Match header field and
930   either an If-Match or an If-Unmodified-Since header fields is
931   undefined by this specification.
932</t>
933</section>
934
935<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
936  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
937  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
938  <x:anchor-alias value="If-Unmodified-Since"/>
939  <x:anchor-alias value="If-Unmodified-Since-v"/>
940<t>
941   The request-header field "If-Unmodified-Since" is used with a method to
942   make it conditional. If the requested resource has not been modified
943   since the time specified in this field, the server &SHOULD; perform the
944   requested operation as if the If-Unmodified-Since header were not
945   present.
946</t>
947<t>
948   If the requested variant has been modified since the specified time,
949   the server &MUST-NOT; perform the requested operation, and &MUST; return
950   a 412 (Precondition Failed).
951</t>
952<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
953  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
954                          <x:ref>If-Unmodified-Since-v</x:ref>
955  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
956</artwork></figure>
957<t>
958   An example of the field is:
959</t>
960<figure><artwork type="example">
961  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
962</artwork></figure>
963<t>
964   If the request normally (i.e., without the If-Unmodified-Since
965   header) would result in anything other than a 2xx or 412 status, the
966   If-Unmodified-Since header &SHOULD; be ignored.
967</t>
968<t>
969   If the specified date is invalid, the header is ignored.
970</t>
971<t>
972   The result of a request having both an If-Unmodified-Since header
973   field and either an If-None-Match or an If-Modified-Since header
974   fields is undefined by this specification.
975</t>
976</section>
977
978<section title="Last-Modified" anchor="header.last-modified">
979  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
980  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
981  <x:anchor-alias value="Last-Modified"/>
982  <x:anchor-alias value="Last-Modified-v"/>
983<t>
984   The entity-header field "Last-Modified" indicates the date and time at
985   which the origin server believes the variant was last modified.
986</t>
987<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
988  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
989  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
990</artwork></figure>
991<t>
992   An example of its use is
993</t>
994<figure><artwork type="example">
995  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
996</artwork></figure>
997<t>
998   The exact meaning of this header field depends on the implementation
999   of the origin server and the nature of the original resource. For
1000   files, it may be just the file system last-modified time. For
1001   entities with dynamically included parts, it may be the most recent
1002   of the set of last-modify times for its component parts. For database
1003   gateways, it may be the last-update time stamp of the record. For
1004   virtual objects, it may be the last time the internal state changed.
1005</t>
1006<t>
1007   An origin server &MUST-NOT; send a Last-Modified date which is later
1008   than the server's time of message origination. In such cases, where
1009   the resource's last modification would indicate some time in the
1010   future, the server &MUST; replace that date with the message
1011   origination date.
1012</t>
1013<t>
1014   An origin server &SHOULD; obtain the Last-Modified value of the entity
1015   as close as possible to the time that it generates the Date value of
1016   its response. This allows a recipient to make an accurate assessment
1017   of the entity's modification time, especially if the entity changes
1018   near the time that the response is generated.
1019</t>
1020<t>
1021   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1022</t>
1023<t>
1024   The Last-Modified entity-header field value is often used as a cache
1025   validator. In simple terms, a cache entry is considered to be valid
1026   if the entity has not been modified since the Last-Modified value.
1027</t>
1028</section>
1029
1030</section>
1031
1032<section title="IANA Considerations" anchor="IANA.considerations">
1033<section title="Message Header Registration" anchor="message.header.registration">
1034<t>
1035   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1036   with the permanent registrations below (see <xref target="RFC3864"/>):
1037</t>
1038<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1039<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1040   <ttcol>Header Field Name</ttcol>
1041   <ttcol>Protocol</ttcol>
1042   <ttcol>Status</ttcol>
1043   <ttcol>Reference</ttcol>
1044
1045   <c>ETag</c>
1046   <c>http</c>
1047   <c>standard</c>
1048   <c>
1049      <xref target="header.etag"/>
1050   </c>
1051   <c>If-Match</c>
1052   <c>http</c>
1053   <c>standard</c>
1054   <c>
1055      <xref target="header.if-match"/>
1056   </c>
1057   <c>If-Modified-Since</c>
1058   <c>http</c>
1059   <c>standard</c>
1060   <c>
1061      <xref target="header.if-modified-since"/>
1062   </c>
1063   <c>If-None-Match</c>
1064   <c>http</c>
1065   <c>standard</c>
1066   <c>
1067      <xref target="header.if-none-match"/>
1068   </c>
1069   <c>If-Unmodified-Since</c>
1070   <c>http</c>
1071   <c>standard</c>
1072   <c>
1073      <xref target="header.if-unmodified-since"/>
1074   </c>
1075   <c>Last-Modified</c>
1076   <c>http</c>
1077   <c>standard</c>
1078   <c>
1079      <xref target="header.last-modified"/>
1080   </c>
1081</texttable>
1082<!--(END)-->
1083<t>
1084   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1085</t>
1086</section>
1087</section>
1088
1089<section title="Security Considerations" anchor="security.considerations">
1090<t>
1091   No additional security considerations have been identified beyond
1092   those applicable to HTTP in general &messaging;.
1093</t>
1094</section>
1095
1096<section title="Acknowledgments" anchor="ack">
1097</section>
1098</middle>
1099<back>
1100
1101<references title="Normative References">
1102
1103<reference anchor="Part1">
1104  <front>
1105    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1106    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1107      <organization abbrev="Day Software">Day Software</organization>
1108      <address><email>fielding@gbiv.com</email></address>
1109    </author>
1110    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1111      <organization>One Laptop per Child</organization>
1112      <address><email>jg@laptop.org</email></address>
1113    </author>
1114    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1115      <organization abbrev="HP">Hewlett-Packard Company</organization>
1116      <address><email>JeffMogul@acm.org</email></address>
1117    </author>
1118    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1119      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1120      <address><email>henrikn@microsoft.com</email></address>
1121    </author>
1122    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1123      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1124      <address><email>LMM@acm.org</email></address>
1125    </author>
1126    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1127      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1128      <address><email>paulle@microsoft.com</email></address>
1129    </author>
1130    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1131      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1132      <address><email>timbl@w3.org</email></address>
1133    </author>
1134    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1135      <organization abbrev="W3C">World Wide Web Consortium</organization>
1136      <address><email>ylafon@w3.org</email></address>
1137    </author>
1138    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1139      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1140      <address><email>julian.reschke@greenbytes.de</email></address>
1141    </author>
1142    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1143  </front>
1144  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1145  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1146</reference>
1147
1148<reference anchor="Part5">
1149  <front>
1150    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1151    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1152      <organization abbrev="Day Software">Day Software</organization>
1153      <address><email>fielding@gbiv.com</email></address>
1154    </author>
1155    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1156      <organization>One Laptop per Child</organization>
1157      <address><email>jg@laptop.org</email></address>
1158    </author>
1159    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1160      <organization abbrev="HP">Hewlett-Packard Company</organization>
1161      <address><email>JeffMogul@acm.org</email></address>
1162    </author>
1163    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1164      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1165      <address><email>henrikn@microsoft.com</email></address>
1166    </author>
1167    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1168      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1169      <address><email>LMM@acm.org</email></address>
1170    </author>
1171    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1172      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1173      <address><email>paulle@microsoft.com</email></address>
1174    </author>
1175    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1176      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1177      <address><email>timbl@w3.org</email></address>
1178    </author>
1179    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1180      <organization abbrev="W3C">World Wide Web Consortium</organization>
1181      <address><email>ylafon@w3.org</email></address>
1182    </author>
1183    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1184      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1185      <address><email>julian.reschke@greenbytes.de</email></address>
1186    </author>
1187    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1188  </front>
1189  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1190  <x:source href="p5-range.xml" basename="p5-range"/>
1191</reference>
1192
1193<reference anchor="Part6">
1194  <front>
1195    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1196    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1197      <organization abbrev="Day Software">Day Software</organization>
1198      <address><email>fielding@gbiv.com</email></address>
1199    </author>
1200    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1201      <organization>One Laptop per Child</organization>
1202      <address><email>jg@laptop.org</email></address>
1203    </author>
1204    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1205      <organization abbrev="HP">Hewlett-Packard Company</organization>
1206      <address><email>JeffMogul@acm.org</email></address>
1207    </author>
1208    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1209      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1210      <address><email>henrikn@microsoft.com</email></address>
1211    </author>
1212    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1213      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1214      <address><email>LMM@acm.org</email></address>
1215    </author>
1216    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1217      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1218      <address><email>paulle@microsoft.com</email></address>
1219    </author>
1220    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1221      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1222      <address><email>timbl@w3.org</email></address>
1223    </author>
1224    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1225      <organization abbrev="W3C">World Wide Web Consortium</organization>
1226      <address><email>ylafon@w3.org</email></address>
1227    </author>
1228    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1229      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1230      <address><email>julian.reschke@greenbytes.de</email></address>
1231    </author>
1232    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1233  </front>
1234  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1235  <x:source href="p6-cache.xml" basename="p6-cache"/>
1236</reference>
1237
1238<reference anchor="RFC2119">
1239  <front>
1240    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1241    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1242      <organization>Harvard University</organization>
1243      <address><email>sob@harvard.edu</email></address>
1244    </author>
1245    <date month="March" year="1997"/>
1246  </front>
1247  <seriesInfo name="BCP" value="14"/>
1248  <seriesInfo name="RFC" value="2119"/>
1249</reference>
1250
1251</references>
1252
1253<references title="Informative References">
1254
1255<reference anchor="RFC2616">
1256  <front>
1257    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1258    <author initials="R." surname="Fielding" fullname="R. Fielding">
1259      <organization>University of California, Irvine</organization>
1260      <address><email>fielding@ics.uci.edu</email></address>
1261    </author>
1262    <author initials="J." surname="Gettys" fullname="J. Gettys">
1263      <organization>W3C</organization>
1264      <address><email>jg@w3.org</email></address>
1265    </author>
1266    <author initials="J." surname="Mogul" fullname="J. Mogul">
1267      <organization>Compaq Computer Corporation</organization>
1268      <address><email>mogul@wrl.dec.com</email></address>
1269    </author>
1270    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1271      <organization>MIT Laboratory for Computer Science</organization>
1272      <address><email>frystyk@w3.org</email></address>
1273    </author>
1274    <author initials="L." surname="Masinter" fullname="L. Masinter">
1275      <organization>Xerox Corporation</organization>
1276      <address><email>masinter@parc.xerox.com</email></address>
1277    </author>
1278    <author initials="P." surname="Leach" fullname="P. Leach">
1279      <organization>Microsoft Corporation</organization>
1280      <address><email>paulle@microsoft.com</email></address>
1281    </author>
1282    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1283      <organization>W3C</organization>
1284      <address><email>timbl@w3.org</email></address>
1285    </author>
1286    <date month="June" year="1999"/>
1287  </front>
1288  <seriesInfo name="RFC" value="2616"/>
1289</reference>
1290
1291<reference anchor='RFC3864'>
1292  <front>
1293    <title>Registration Procedures for Message Header Fields</title>
1294    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1295      <organization>Nine by Nine</organization>
1296      <address><email>GK-IETF@ninebynine.org</email></address>
1297    </author>
1298    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1299      <organization>BEA Systems</organization>
1300      <address><email>mnot@pobox.com</email></address>
1301    </author>
1302    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1303      <organization>HP Labs</organization>
1304      <address><email>JeffMogul@acm.org</email></address>
1305    </author>
1306    <date year='2004' month='September' />
1307  </front>
1308  <seriesInfo name='BCP' value='90' />
1309  <seriesInfo name='RFC' value='3864' />
1310</reference>
1311
1312</references>
1313
1314<section title="Compatibility with Previous Versions" anchor="compatibility">
1315
1316<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1317<t>
1318  Allow weak entity tags in all requests except range requests (Sections
1319  <xref target="weak.and.strong.validators" format="counter"/> and
1320  <xref target="header.if-none-match" format="counter"/>).
1321</t>
1322</section>
1323
1324</section>
1325
1326<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1327
1328<section title="Since RFC2616">
1329<t>
1330  Extracted relevant partitions from <xref target="RFC2616"/>.
1331</t>
1332</section>
1333
1334<section title="Since draft-ietf-httpbis-p4-conditional-00">
1335<t>
1336  Closed issues:
1337  <list style="symbols"> 
1338    <t>
1339      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1340      "Normative and Informative references"
1341    </t>
1342  </list>
1343</t>
1344<t>
1345  Other changes:
1346  <list style="symbols"> 
1347    <t>
1348      Move definitions of 304 and 412 condition codes from Part2.
1349    </t>
1350  </list>
1351</t>
1352</section>
1353
1354<section title="Since draft-ietf-httpbis-p4-conditional-01">
1355<t>
1356  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1357  <list style="symbols"> 
1358    <t>
1359      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1360    </t>
1361  </list>
1362</t>
1363</section>
1364
1365<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1366<t>
1367  Closed issues:
1368  <list style="symbols"> 
1369    <t>
1370      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1371      "Weak ETags on non-GET requests"
1372    </t>
1373  </list>
1374</t>
1375<t>
1376  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1377  <list style="symbols"> 
1378    <t>
1379      Reference RFC 3984, and update header registrations for headers defined
1380      in this document.
1381    </t>
1382  </list>
1383</t>
1384</section>
1385
1386<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1387<t>
1388  Closed issues:
1389  <list style="symbols"> 
1390    <t>
1391      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1392      "Examples for ETag matching"
1393    </t>
1394    <t>
1395      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1396      "'entity value' undefined"
1397    </t>
1398    <t>
1399      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1400      "bogus 2068 Date header reference"
1401    </t>
1402  </list>
1403</t>
1404</section>
1405
1406<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1407<t>
1408  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1409  <list style="symbols"> 
1410    <t>
1411      Use "/" instead of "|" for alternatives.
1412    </t>
1413    <t>
1414      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1415      whitespace ("OWS") and required whitespace ("RWS").
1416    </t>
1417    <t>
1418      Rewrite ABNFs to spell out whitespace rules, factor out
1419      header value format definitions.
1420    </t>
1421  </list>
1422</t>
1423</section>
1424
1425</section>
1426
1427</back>
1428</rfc>
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