source: draft-ietf-httpbis/latest/p4-conditional.xml @ 305

Last change on this file since 305 was 305, checked in by julian.reschke@…, 11 years ago

Outdent comment about Date handling (this gets the list format closer to the intended format, see <http://www.w3.org/Protocols/HTTP/1.1/rfc2616.pdf>)

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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 "August">
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 messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                  "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26]>
27<?rfc toc="yes" ?>
28<?rfc symrefs="yes" ?>
29<?rfc sortrefs="yes" ?>
30<?rfc compact="yes"?>
31<?rfc subcompact="no" ?>
32<?rfc linkmailto="no" ?>
33<?rfc editing="no" ?>
34<?rfc comments="yes"?>
35<?rfc inline="yes"?>
36<?rfc-ext allow-markup-in-artwork="yes" ?>
37<?rfc-ext include-references-in-index="yes" ?>
38<rfc obsoletes="2616" category="std"
39     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
40     xmlns:x='http://purl.org/net/xml2rfc/ext'>
41<front>
42
43  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
44
45  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
46    <organization abbrev="Day Software">Day Software</organization>
47    <address>
48      <postal>
49        <street>23 Corporate Plaza DR, Suite 280</street>
50        <city>Newport Beach</city>
51        <region>CA</region>
52        <code>92660</code>
53        <country>USA</country>
54      </postal>
55      <phone>+1-949-706-5300</phone>
56      <facsimile>+1-949-706-5305</facsimile>
57      <email>fielding@gbiv.com</email>
58      <uri>http://roy.gbiv.com/</uri>
59    </address>
60  </author>
61
62  <author initials="J." surname="Gettys" fullname="Jim Gettys">
63    <organization>One Laptop per Child</organization>
64    <address>
65      <postal>
66        <street>21 Oak Knoll Road</street>
67        <city>Carlisle</city>
68        <region>MA</region>
69        <code>01741</code>
70        <country>USA</country>
71      </postal>
72      <email>jg@laptop.org</email>
73      <uri>http://www.laptop.org/</uri>
74    </address>
75  </author>
76 
77  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
78    <organization abbrev="HP">Hewlett-Packard Company</organization>
79    <address>
80      <postal>
81        <street>HP Labs, Large Scale Systems Group</street>
82        <street>1501 Page Mill Road, MS 1177</street>
83        <city>Palo Alto</city>
84        <region>CA</region>
85        <code>94304</code>
86        <country>USA</country>
87      </postal>
88      <email>JeffMogul@acm.org</email>
89    </address>
90  </author>
91
92  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
93    <organization abbrev="Microsoft">Microsoft Corporation</organization>
94    <address>
95      <postal>
96        <street>1 Microsoft Way</street>
97        <city>Redmond</city>
98        <region>WA</region>
99        <code>98052</code>
100        <country>USA</country>
101      </postal>
102      <email>henrikn@microsoft.com</email>
103    </address>
104  </author>
105
106  <author initials="L." surname="Masinter" fullname="Larry Masinter">
107    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
108    <address>
109      <postal>
110        <street>345 Park Ave</street>
111        <city>San Jose</city>
112        <region>CA</region>
113        <code>95110</code>
114        <country>USA</country>
115      </postal>
116      <email>LMM@acm.org</email>
117      <uri>http://larry.masinter.net/</uri>
118    </address>
119  </author>
120 
121  <author initials="P." surname="Leach" fullname="Paul J. Leach">
122    <organization abbrev="Microsoft">Microsoft Corporation</organization>
123    <address>
124      <postal>
125        <street>1 Microsoft Way</street>
126        <city>Redmond</city>
127        <region>WA</region>
128        <code>98052</code>
129      </postal>
130      <email>paulle@microsoft.com</email>
131    </address>
132  </author>
133   
134  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
135    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
136    <address>
137      <postal>
138        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
139        <street>The Stata Center, Building 32</street>
140        <street>32 Vassar Street</street>
141        <city>Cambridge</city>
142        <region>MA</region>
143        <code>02139</code>
144        <country>USA</country>
145      </postal>
146      <email>timbl@w3.org</email>
147      <uri>http://www.w3.org/People/Berners-Lee/</uri>
148    </address>
149  </author>
150
151  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
152    <organization abbrev="W3C">World Wide Web Consortium</organization>
153    <address>
154      <postal>
155        <street>W3C / ERCIM</street>
156        <street>2004, rte des Lucioles</street>
157        <city>Sophia-Antipolis</city>
158        <region>AM</region>
159        <code>06902</code>
160        <country>France</country>
161      </postal>
162      <email>ylafon@w3.org</email>
163      <uri>http://www.raubacapeu.net/people/yves/</uri>
164    </address>
165  </author>
166
167  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
168    <organization abbrev="greenbytes">greenbytes GmbH</organization>
169    <address>
170      <postal>
171        <street>Hafenweg 16</street>
172        <city>Muenster</city><region>NW</region><code>48155</code>
173        <country>Germany</country>
174      </postal>
175      <phone>+49 251 2807760</phone>   
176      <facsimile>+49 251 2807761</facsimile>   
177      <email>julian.reschke@greenbytes.de</email>       
178      <uri>http://greenbytes.de/tech/webdav/</uri>     
179    </address>
180  </author>
181
182  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
183
184<abstract>
185<t>
186   The Hypertext Transfer Protocol (HTTP) is an application-level
187   protocol for distributed, collaborative, hypermedia information
188   systems. HTTP has been in use by the World Wide Web global information
189   initiative since 1990. This document is Part 4 of the seven-part specification
190   that defines the protocol referred to as "HTTP/1.1" and, taken together,
191   obsoletes RFC 2616.  Part 4 defines request header fields for
192   indicating conditional requests and the rules for constructing responses
193   to those requests.
194</t>
195</abstract>
196
197<note title="Editorial Note (To be removed by RFC Editor)">
198  <t>
199    Discussion of this draft should take place on the HTTPBIS working group
200    mailing list (ietf-http-wg@w3.org). The current issues list is
201    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
202    and related documents (including fancy diffs) can be found at
203    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
204  </t>
205  <t>
206    The changes in this draft are summarized in <xref target="changes.since.02"/>.
207  </t>
208</note>
209</front>
210<middle>
211<section title="Introduction" anchor="introduction">
212<t>
213   This document defines HTTP/1.1 response metadata for indicating potential
214   changes to payload content, including modification time stamps and opaque
215   entity-tags, and the HTTP conditional request mechanisms that allow
216   preconditions to be placed on a request method.  Conditional GET requests
217   allow for efficient cache updates.  Other conditional request methods are
218   used to protect against overwriting or misunderstanding the state of a
219   resource that has been changed unbeknownst to the requesting client.
220</t>
221<t>
222   This document is currently disorganized in order to minimize the changes
223   between drafts and enable reviewers to see the smaller errata changes.
224   The next draft will reorganize the sections to better reflect the content.
225   In particular, the sections on resource metadata will be discussed first
226   and then followed by each conditional request-header, concluding with a
227   definition of precedence and the expectation of ordering strong validator
228   checks before weak validator checks.  It is likely that more content from
229   &caching; will migrate to this part, where appropriate.
230   The current mess reflects how widely dispersed these topics and associated
231   requirements had become in <xref target="RFC2616"/>.
232</t>
233
234<section title="Requirements" anchor="intro.requirements">
235<t>
236   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
237   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
238   document are to be interpreted as described in <xref target="RFC2119"/>.
239</t>
240<t>
241   An implementation is not compliant if it fails to satisfy one or more
242   of the &MUST; or &REQUIRED; level requirements for the protocols it
243   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
244   level and all the &SHOULD; level requirements for its protocols is said
245   to be "unconditionally compliant"; one that satisfies all the &MUST;
246   level requirements but not all the &SHOULD; level requirements for its
247   protocols is said to be "conditionally compliant."
248</t>
249</section>
250</section>
251
252<section title="Notational Conventions and Generic Grammar" anchor="notation">
253  <x:anchor-alias value="quoted-string"/>
254<t>
255  This specification uses the ABNF syntax defined in &notation-abnf; and
256  the core rules defined in &basic-rules;:
257  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
258  <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>.</cref>
259</t>
260<figure><artwork type="abnf2616">
261  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
262</artwork></figure>
263<t anchor="abnf.dependencies">
264  <x:anchor-alias value="HTTP-date"/>
265  The ABNF rules below are defined in other parts:
266</t>
267<figure><!--Part1--><artwork type="abnf2616">
268  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
269</artwork></figure>
270</section>
271
272<section title="Entity Tags" anchor="entity.tags">
273  <x:anchor-alias value="entity-tag"/>
274  <x:anchor-alias value="opaque-tag"/>
275  <x:anchor-alias value="weak"/>
276<t>
277   Entity tags are used for comparing two or more entities from the same
278   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
279   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
280   If-Range (&header-if-range;) header fields. The definition of how they
281   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
282   entity tag consists of an opaque quoted string, possibly prefixed by
283   a weakness indicator.
284</t>
285<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"/>
286  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
287  <x:ref>weak</x:ref>       = "W/"
288  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
289</artwork></figure>
290<t>
291   A "strong entity tag" &MAY; be shared by two entities of a resource
292   only if they are equivalent by octet equality.
293</t>
294<t>
295   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
296   two entities of a resource only if the entities are equivalent and
297   could be substituted for each other with no significant change in
298   semantics. A weak entity tag can only be used for weak comparison.
299</t>
300<t>
301   An entity tag &MUST; be unique across all versions of all entities
302   associated with a particular resource. A given entity tag value &MAY;
303   be used for entities obtained by requests on different URIs. The use
304   of the same entity tag value in conjunction with entities obtained by
305   requests on different URIs does not imply the equivalence of those
306   entities.
307</t>
308</section>
309
310<section title="Status Code Definitions">
311<section title="304 Not Modified" anchor="status.304">
312  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
313  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
314<t>
315   If the client has performed a conditional GET request and access is
316   allowed, but the document has not been modified, the server &SHOULD;
317   respond with this status code. The 304 response &MUST-NOT; contain a
318   message-body, and thus is always terminated by the first empty line
319   after the header fields.
320</t>
321<t>
322   The response &MUST; include the following header fields:
323  <list style="symbols">
324    <x:lt>
325      <t>Date, unless its omission is required by &clockless;</t>
326      <t>If a clockless origin server obeys these rules, and proxies and
327         clients add their own Date to any response received without one (as
328         already specified by <xref target="RFC2068" x:sec="14.19" x:fmt=","/>), caches will operate
329         correctly.</t>
330    </x:lt>
331    <x:lt>
332      <t>ETag and/or Content-Location, if the header would have been sent
333          in a 200 response to the same request</t>
334    </x:lt>
335    <x:lt>
336      <t>Expires, Cache-Control, and/or Vary, if the field-value might
337          differ from that sent in any previous response for the same
338          variant</t>
339    </x:lt>
340  </list>
341</t>
342<t>
343   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
344   the response &SHOULD-NOT;  include other entity-headers.
345   Otherwise (i.e., the conditional GET used a weak validator), the
346   response &MUST-NOT; include other entity-headers; this prevents
347   inconsistencies between cached entity-bodies and updated headers.
348</t>
349<t>
350   If a 304 response indicates an entity not currently cached, then the
351   cache &MUST; disregard the response and repeat the request without the
352   conditional.
353</t>
354<t>
355   If a cache uses a received 304 response to update a cache entry, the
356   cache &MUST; update the entry to reflect any new field values given in
357   the response.
358</t>
359</section>
360
361<section title="412 Precondition Failed" anchor="status.412">
362  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
363  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
364<t>
365   The precondition given in one or more of the request-header fields
366   evaluated to false when it was tested on the server. This response
367   code allows the client to place preconditions on the current resource
368   metainformation (header field data) and thus prevent the requested
369   method from being applied to a resource other than the one intended.
370</t>
371</section>
372</section>
373
374<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
375<t>
376   Since both origin servers and caches will compare two validators to
377   decide if they represent the same or different entities, one normally
378   would expect that if the entity (the entity-body or any entity-headers)
379   changes in any way, then the associated validator would
380   change as well. If this is true, then we call this validator a
381   "strong validator."
382</t>
383<t>
384   However, there might be cases when a server prefers to change the
385   validator only on semantically significant changes, and not when
386   insignificant aspects of the entity change. A validator that does not
387   always change when the resource changes is a "weak validator."
388</t>
389<t>
390   Entity tags are normally "strong validators," but the protocol
391   provides a mechanism to tag an entity tag as "weak." One can think of
392   a strong validator as one that changes whenever the bits of an entity
393   changes, while a weak value changes whenever the meaning of an entity
394   changes. Alternatively, one can think of a strong validator as part
395   of an identifier for a specific entity, while a weak validator is
396   part of an identifier for a set of semantically equivalent entities.
397  <list><t>
398      <x:h>Note:</x:h> One example of a strong validator is an integer that is
399      incremented in stable storage every time an entity is changed.
400    </t><t>
401      An entity's modification time, if represented with one-second
402      resolution, could be a weak validator, since it is possible that
403      the resource might be modified twice during a single second.
404    </t><t>
405      Support for weak validators is optional. However, weak validators
406      allow for more efficient caching of equivalent objects; for
407      example, a hit counter on a site is probably good enough if it is
408      updated every few days or weeks, and any value during that period
409      is likely "good enough" to be equivalent.
410    </t></list>
411</t>
412<t>
413   A "use" of a validator is either when a client generates a request
414   and includes the validator in a validating header field, or when a
415   server compares two validators.
416</t>
417<t>
418   Strong validators are usable in any context. Weak validators are only
419   usable in contexts that do not depend on exact equality of an entity.
420   For example, either kind is usable for a conditional GET of a full
421   entity. However, only a strong validator is usable for a sub-range
422   retrieval, since otherwise the client might end up with an internally
423   inconsistent entity.
424</t>
425<t>
426   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
427</t>
428<t>
429   The only function that HTTP/1.1 defines on validators is
430   comparison. There are two validator comparison functions, depending
431   on whether the comparison context allows the use of weak validators
432   or not:
433  <list style="symbols">
434     <t>The strong comparison function: in order to be considered equal,
435        both opaque-tags &MUST; be identical character-by-character, and both
436        &MUST-NOT; be weak.</t>
437     <t>The weak comparison function: in order to be considered equal, both
438        opaque-tags &MUST; be identical character-by-character.</t>
439  </list>
440</t>
441<t>
442   The example below shows the results for a set of entity tag pairs,
443   and both the weak and strong comparison function results:
444</t>
445<texttable align="left">
446  <ttcol>ETag 1</ttcol>
447  <ttcol>ETag 2</ttcol>
448  <ttcol>Strong Comparison</ttcol>
449  <ttcol>Weak Comparison</ttcol>
450
451  <c>W/"1"</c>
452  <c>W/"1"</c>
453  <c>no match</c>
454  <c>match</c>
455 
456  <c>W/"1"</c>
457  <c>W/"2"</c>
458  <c>no match</c>
459  <c>no match</c>
460
461  <c>W/"1"</c>
462  <c>"1"</c>
463  <c>no match</c>
464  <c>match</c>
465
466  <c>"1"</c>
467  <c>"1"</c>
468  <c>match</c>
469  <c>match</c>
470</texttable>
471<t>
472   An entity tag is strong unless it is explicitly tagged as weak.
473   <xref target="entity.tags"/> gives the syntax for entity tags.
474</t>
475<t>
476   A Last-Modified time, when used as a validator in a request, is
477   implicitly weak unless it is possible to deduce that it is strong,
478   using the following rules:
479  <list style="symbols">
480     <t>The validator is being compared by an origin server to the
481        actual current validator for the entity and,</t>
482     <t>That origin server reliably knows that the associated entity did
483        not change twice during the second covered by the presented
484        validator.</t>
485  </list>
486</t>
487<t>
488   or
489  <list style="symbols">
490     <t>The validator is about to be used by a client in an If-Modified-Since
491        or If-Unmodified-Since header, because the client
492        has a cache entry for the associated entity, and</t>
493     <t>That cache entry includes a Date value, which gives the time
494        when the origin server sent the original response, and</t>
495     <t>The presented Last-Modified time is at least 60 seconds before
496        the Date value.</t>
497  </list>
498</t>
499<t>
500   or
501  <list style="symbols">
502     <t>The validator is being compared by an intermediate cache to the
503        validator stored in its cache entry for the entity, and</t>
504     <t>That cache entry includes a Date value, which gives the time
505        when the origin server sent the original response, and</t>
506     <t>The presented Last-Modified time is at least 60 seconds before
507        the Date value.</t>
508  </list>
509</t>
510<t>
511   This method relies on the fact that if two different responses were
512   sent by the origin server during the same second, but both had the
513   same Last-Modified time, then at least one of those responses would
514   have a Date value equal to its Last-Modified time. The arbitrary 60-second
515   limit guards against the possibility that the Date and Last-Modified
516   values are generated from different clocks, or at somewhat
517   different times during the preparation of the response. An
518   implementation &MAY; use a value larger than 60 seconds, if it is
519   believed that 60 seconds is too short.
520</t>
521<t>
522   If a client wishes to perform a sub-range retrieval on a value for
523   which it has only a Last-Modified time and no opaque validator, it
524   &MAY; do this only if the Last-Modified time is strong in the sense
525   described here.
526</t>
527<t>
528   A cache or origin server receiving a conditional range request
529   (<xref target="Part5"/>) &MUST; use the strong comparison function to
530   evaluate the condition.
531</t>
532<t>
533   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
534   retrievals on values that have been obtained from HTTP/1.0
535   servers.
536</t>
537</section>
538
539<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">
540<t>
541   We adopt a set of rules and recommendations for origin servers,
542   clients, and caches regarding when various validator types ought to
543   be used, and for what purposes.
544</t>
545<t>
546   HTTP/1.1 origin servers:
547  <list style="symbols">
548     <t>&SHOULD; send an entity tag validator unless it is not feasible to
549        generate one.</t>
550
551     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
552        performance considerations support the use of weak entity tags,
553        or if it is unfeasible to send a strong entity tag.</t>
554
555     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
556        unless the risk of a breakdown in semantic transparency that
557        could result from using this date in an If-Modified-Since header
558        would lead to serious problems.</t>
559  </list>
560</t>
561<t>
562   In other words, the preferred behavior for an HTTP/1.1 origin server
563   is to send both a strong entity tag and a Last-Modified value.
564</t>
565<t>
566   In order to be legal, a strong entity tag &MUST; change whenever the
567   associated entity changes in any way. A weak entity tag &SHOULD;
568   change whenever the associated entity changes in a semantically
569   significant way.
570  <list><t>
571      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
572      origin server must avoid reusing a specific strong entity tag
573      value for two different entities, or reusing a specific weak
574      entity tag value for two semantically different entities. Cache
575      entries might persist for arbitrarily long periods, regardless of
576      expiration times, so it might be inappropriate to expect that a
577      cache will never again attempt to validate an entry using a
578      validator that it obtained at some point in the past.
579  </t></list>
580</t>
581<t>
582   HTTP/1.1 clients:
583  <list style="symbols">
584     <t>If an entity tag has been provided by the origin server, &MUST;
585        use that entity tag in any cache-conditional request (using If-Match
586        or If-None-Match).</t>
587
588     <t>If only a Last-Modified value has been provided by the origin
589        server, &SHOULD; use that value in non-subrange cache-conditional
590        requests (using If-Modified-Since).</t>
591
592     <t>If only a Last-Modified value has been provided by an HTTP/1.0
593        origin server, &MAY; use that value in subrange cache-conditional
594        requests (using If-Unmodified-Since:). The user agent &SHOULD;
595        provide a way to disable this, in case of difficulty.</t>
596
597     <t>If both an entity tag and a Last-Modified value have been
598        provided by the origin server, &SHOULD; use both validators in
599        cache-conditional requests. This allows both HTTP/1.0 and
600        HTTP/1.1 caches to respond appropriately.</t>
601  </list>
602</t>
603<t>
604   An HTTP/1.1 origin server, upon receiving a conditional request that
605   includes both a Last-Modified date (e.g., in an If-Modified-Since or
606   If-Unmodified-Since header field) and one or more entity tags (e.g.,
607   in an If-Match, If-None-Match, or If-Range header field) as cache
608   validators, &MUST-NOT; return a response status of 304 (Not Modified)
609   unless doing so is consistent with all of the conditional header
610   fields in the request.
611</t>
612<t>
613   An HTTP/1.1 caching proxy, upon receiving a conditional request that
614   includes both a Last-Modified date and one or more entity tags as
615   cache validators, &MUST-NOT; return a locally cached response to the
616   client unless that cached response is consistent with all of the
617   conditional header fields in the request.
618  <list><t>
619      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
620      servers and clients should transmit as much non-redundant
621      information as is available in their responses and requests.
622      HTTP/1.1 systems receiving this information will make the most
623      conservative assumptions about the validators they receive.
624  </t><t>
625      HTTP/1.0 clients and caches will ignore entity tags. Generally,
626      last-modified values received or used by these systems will
627      support transparent and efficient caching, and so HTTP/1.1 origin
628      servers should provide Last-Modified values. In those rare cases
629      where the use of a Last-Modified value as a validator by an
630      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
631      origin servers should not provide one.
632  </t></list>
633</t>
634</section>
635
636<section title="Header Field Definitions" anchor="header.fields">
637<t>
638   This section defines the syntax and semantics of HTTP/1.1 header fields
639   related to conditional requests.
640</t>
641<t>
642   For entity-header fields, both sender and recipient refer to either the
643   client or the server, depending on who sends and who receives the entity.
644</t>
645
646<section title="ETag" anchor="header.etag">
647  <iref primary="true" item="ETag header" x:for-anchor=""/>
648  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
649  <x:anchor-alias value="ETag"/>
650<t>
651   The ETag response-header field provides the current value of the
652   entity tag for the requested variant. The headers used with entity
653   tags are described in Sections <xref target="header.if-match" format="counter"/>
654   and <xref target="header.if-none-match" format="counter"/> of this document,
655   and in &header-if-range;. The entity tag
656   &MAY; be used for comparison with other entities from the same resource
657   (see <xref target="weak.and.strong.validators"/>).
658</t>
659<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
660  <x:ref>ETag</x:ref> = "ETag" ":" <x:ref>entity-tag</x:ref>
661</artwork></figure>
662<figure><preamble>
663   Examples:
664</preamble>
665<artwork type="example">
666   ETag: "xyzzy"
667   ETag: W/"xyzzy"
668   ETag: ""
669</artwork></figure>
670<t>
671   The ETag response-header field value, an entity tag, provides for an
672   "opaque" cache validator. This might allow more reliable validation
673   in situations where it is inconvenient to store modification dates,
674   where the one-second resolution of HTTP date values is not
675   sufficient, or where the origin server wishes to avoid certain
676   paradoxes that might arise from the use of modification dates.
677</t>
678<t>
679   The principle behind entity tags is that only the service author
680   knows the semantics of a resource well enough to select an
681   appropriate cache validation mechanism, and the specification of any
682   validator comparison function more complex than byte-equality would
683   open up a can of worms. Thus, comparisons of any other headers
684   (except Last-Modified, for compatibility with HTTP/1.0) are never
685   used for purposes of validating a cache entry.
686</t>
687</section>
688
689<section title="If-Match" anchor="header.if-match">
690  <iref primary="true" item="If-Match header" x:for-anchor=""/>
691  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
692  <x:anchor-alias value="If-Match"/>
693<t>
694   The If-Match request-header field is used with a method to make it
695   conditional. A client that has one or more entities previously
696   obtained from the resource can verify that one of those entities is
697   current by including a list of their associated entity tags in the
698   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
699   purpose of this feature is to allow efficient updates of cached
700   information with a minimum amount of transaction overhead. It is also
701   used, on updating requests, to prevent inadvertent modification of
702   the wrong version of a resource. As a special case, the value "*"
703   matches any current entity of the resource.
704</t>
705<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
706  <x:ref>If-Match</x:ref> = "If-Match" ":" ( "*" | 1#<x:ref>entity-tag</x:ref> )
707</artwork></figure>
708<t>
709   If any of the entity tags match the entity tag of the entity that
710   would have been returned in the response to a similar GET request
711   (without the If-Match header) on that resource, or if "*" is given
712   and any current entity exists for that resource, then the server &MAY;
713   perform the requested method as if the If-Match header field did not
714   exist.
715</t>
716<t>
717   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
718   to compare the entity tags in If-Match.
719</t>
720<t>
721   If none of the entity tags match, or if "*" is given and no current
722   entity exists, the server &MUST-NOT; perform the requested method, and
723   &MUST; return a 412 (Precondition Failed) response. This behavior is
724   most useful when the client wants to prevent an updating method, such
725   as PUT, from modifying a resource that has changed since the client
726   last retrieved it.
727</t>
728<t>
729   If the request would, without the If-Match header field, result in
730   anything other than a 2xx or 412 status, then the If-Match header
731   &MUST; be ignored.
732</t>
733<t>
734   The meaning of "If-Match: *" is that the method &SHOULD; be performed
735   if the representation selected by the origin server (or by a cache,
736   possibly using the Vary mechanism, see &header-vary;) exists, and
737   &MUST-NOT; be performed if the representation does not exist.
738</t>
739<t>
740   A request intended to update a resource (e.g., a PUT) &MAY; include an
741   If-Match header field to signal that the request method &MUST-NOT; be
742   applied if the entity corresponding to the If-Match value (a single
743   entity tag) is no longer a representation of that resource. This
744   allows the user to indicate that they do not wish the request to be
745   successful if the resource has been changed without their knowledge.
746   Examples:
747</t>
748<figure><artwork type="example">
749    If-Match: "xyzzy"
750    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
751    If-Match: *
752</artwork></figure>
753<t>
754   The result of a request having both an If-Match header field and
755   either an If-None-Match or an If-Modified-Since header fields is
756   undefined by this specification.
757</t>
758</section>
759
760<section title="If-Modified-Since" anchor="header.if-modified-since">
761  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
762  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
763  <x:anchor-alias value="If-Modified-Since"/>
764<t>
765   The If-Modified-Since request-header field is used with a method to
766   make it conditional: if the requested variant has not been modified
767   since the time specified in this field, an entity will not be
768   returned from the server; instead, a 304 (Not Modified) response will
769   be returned without any message-body.
770</t>
771<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
772  <x:ref>If-Modified-Since</x:ref> = "If-Modified-Since" ":" <x:ref>HTTP-date</x:ref>
773</artwork></figure>
774<t>
775   An example of the field is:
776</t>
777<figure><artwork type="example">
778    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
779</artwork></figure>
780<t>
781   A GET method with an If-Modified-Since header and no Range header
782   requests that the identified entity be transferred only if it has
783   been modified since the date given by the If-Modified-Since header.
784   The algorithm for determining this includes the following cases:
785  <list style="numbers">
786      <t>If the request would normally result in anything other than a
787         200 (OK) status, or if the passed If-Modified-Since date is
788         invalid, the response is exactly the same as for a normal GET.
789         A date which is later than the server's current time is
790         invalid.</t>
791
792      <t>If the variant has been modified since the If-Modified-Since
793         date, the response is exactly the same as for a normal GET.</t>
794
795      <t>If the variant has not been modified since a valid If-Modified-Since
796         date, the server &SHOULD; return a 304 (Not
797         Modified) response.</t>
798  </list>
799</t>
800<t>
801   The purpose of this feature is to allow efficient updates of cached
802   information with a minimum amount of transaction overhead.
803  <list><t>
804      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
805      see &header-range; for full details.
806    </t><t>
807      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
808      clock might not be synchronized with the client.
809    </t><t>
810      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
811      servers will use an exact date comparison function, rather than a
812      less-than function, for deciding whether to send a 304 (Not
813      Modified) response. To get best results when sending an If-Modified-Since
814      header field for cache validation, clients are
815      advised to use the exact date string received in a previous Last-Modified
816      header field whenever possible.
817    </t><t>
818      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
819      header instead of a date taken from the Last-Modified header for
820      the same request, the client should be aware of the fact that this
821      date is interpreted in the server's understanding of time. The
822      client should consider unsynchronized clocks and rounding problems
823      due to the different encodings of time between the client and
824      server. This includes the possibility of race conditions if the
825      document has changed between the time it was first requested and
826      the If-Modified-Since date of a subsequent request, and the
827      possibility of clock-skew-related problems if the If-Modified-Since
828      date is derived from the client's clock without correction
829      to the server's clock. Corrections for different time bases
830      between client and server are at best approximate due to network
831      latency.
832    </t>
833  </list>
834</t>
835<t>
836   The result of a request having both an If-Modified-Since header field
837   and either an If-Match or an If-Unmodified-Since header fields is
838   undefined by this specification.
839</t>
840</section>
841
842<section title="If-None-Match" anchor="header.if-none-match">
843  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
844  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
845  <x:anchor-alias value="If-None-Match"/>
846<t>
847   The If-None-Match request-header field is used with a method to make
848   it conditional. A client that has one or more entities previously
849   obtained from the resource can verify that none of those entities is
850   current by including a list of their associated entity tags in the
851   If-None-Match header field. The purpose of this feature is to allow
852   efficient updates of cached information with a minimum amount of
853   transaction overhead. It is also used to prevent a method (e.g. PUT)
854   from inadvertently modifying an existing resource when the client
855   believes that the resource does not exist.
856</t>
857<t>
858   As a special case, the value "*" matches any current entity of the
859   resource.
860</t>
861<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
862  <x:ref>If-None-Match</x:ref> = "If-None-Match" ":" ( "*" | 1#<x:ref>entity-tag</x:ref> )
863</artwork></figure>
864<t>
865   If any of the entity tags match the entity tag of the entity that
866   would have been returned in the response to a similar GET request
867   (without the If-None-Match header) on that resource, or if "*" is
868   given and any current entity exists for that resource, then the
869   server &MUST-NOT; perform the requested method, unless required to do
870   so because the resource's modification date fails to match that
871   supplied in an If-Modified-Since header field in the request.
872   Instead, if the request method was GET or HEAD, the server &SHOULD;
873   respond with a 304 (Not Modified) response, including the cache-related
874   header fields (particularly ETag) of one of the entities that
875   matched. For all other request methods, the server &MUST; respond with
876   a status of 412 (Precondition Failed).
877</t>
878<t>
879   See <xref target="weak.and.strong.validators"/> for rules on how to
880   determine if two entity tags match.
881</t>
882<t>
883   If none of the entity tags match, then the server &MAY; perform the
884   requested method as if the If-None-Match header field did not exist,
885   but &MUST; also ignore any If-Modified-Since header field(s) in the
886   request. That is, if no entity tags match, then the server &MUST-NOT;
887   return a 304 (Not Modified) response.
888</t>
889<t>
890   If the request would, without the If-None-Match header field, result
891   in anything other than a 2xx or 304 status, then the If-None-Match
892   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
893   server behavior when both If-Modified-Since and If-None-Match appear
894   in the same request.)
895</t>
896<t>
897   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
898   performed if the representation selected by the origin server (or by
899   a cache, possibly using the Vary mechanism, see &header-vary;)
900   exists, and &SHOULD; be performed if the representation does not exist.
901   This feature is intended to be useful in preventing races between PUT
902   operations.
903</t>
904<t>
905   Examples:
906</t>
907<figure><artwork type="example">
908    If-None-Match: "xyzzy"
909    If-None-Match: W/"xyzzy"
910    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
911    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
912    If-None-Match: *
913</artwork></figure>
914<t>
915   The result of a request having both an If-None-Match header field and
916   either an If-Match or an If-Unmodified-Since header fields is
917   undefined by this specification.
918</t>
919</section>
920
921<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
922  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
923  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
924  <x:anchor-alias value="If-Unmodified-Since"/>
925<t>
926   The If-Unmodified-Since request-header field is used with a method to
927   make it conditional. If the requested resource has not been modified
928   since the time specified in this field, the server &SHOULD; perform the
929   requested operation as if the If-Unmodified-Since header were not
930   present.
931</t>
932<t>
933   If the requested variant has been modified since the specified time,
934   the server &MUST-NOT; perform the requested operation, and &MUST; return
935   a 412 (Precondition Failed).
936</t>
937<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
938  <x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since" ":" <x:ref>HTTP-date</x:ref>
939</artwork></figure>
940<t>
941   An example of the field is:
942</t>
943<figure><artwork type="example">
944    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
945</artwork></figure>
946<t>
947   If the request normally (i.e., without the If-Unmodified-Since
948   header) would result in anything other than a 2xx or 412 status, the
949   If-Unmodified-Since header &SHOULD; be ignored.
950</t>
951<t>
952   If the specified date is invalid, the header is ignored.
953</t>
954<t>
955   The result of a request having both an If-Unmodified-Since header
956   field and either an If-None-Match or an If-Modified-Since header
957   fields is undefined by this specification.
958</t>
959</section>
960
961<section title="Last-Modified" anchor="header.last-modified">
962  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
963  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
964  <x:anchor-alias value="Last-Modified"/>
965<t>
966   The Last-Modified entity-header field indicates the date and time at
967   which the origin server believes the variant was last modified.
968</t>
969<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
970  <x:ref>Last-Modified</x:ref>  = "Last-Modified" ":" <x:ref>HTTP-date</x:ref>
971</artwork></figure>
972<t>
973   An example of its use is
974</t>
975<figure><artwork type="example">
976    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
977</artwork></figure>
978<t>
979   The exact meaning of this header field depends on the implementation
980   of the origin server and the nature of the original resource. For
981   files, it may be just the file system last-modified time. For
982   entities with dynamically included parts, it may be the most recent
983   of the set of last-modify times for its component parts. For database
984   gateways, it may be the last-update time stamp of the record. For
985   virtual objects, it may be the last time the internal state changed.
986</t>
987<t>
988   An origin server &MUST-NOT; send a Last-Modified date which is later
989   than the server's time of message origination. In such cases, where
990   the resource's last modification would indicate some time in the
991   future, the server &MUST; replace that date with the message
992   origination date.
993</t>
994<t>
995   An origin server &SHOULD; obtain the Last-Modified value of the entity
996   as close as possible to the time that it generates the Date value of
997   its response. This allows a recipient to make an accurate assessment
998   of the entity's modification time, especially if the entity changes
999   near the time that the response is generated.
1000</t>
1001<t>
1002   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1003</t>
1004<t>
1005   The Last-Modified entity-header field value is often used as a cache
1006   validator. In simple terms, a cache entry is considered to be valid
1007   if the entity has not been modified since the Last-Modified value.
1008</t>
1009</section>
1010
1011</section>
1012
1013<section title="IANA Considerations" anchor="IANA.considerations">
1014<section title="Message Header Registration" anchor="message.header.registration">
1015<t>
1016   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1017   with the permanent registrations below (see <xref target="RFC3864"/>):
1018</t>
1019<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1020<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1021   <ttcol>Header Field Name</ttcol>
1022   <ttcol>Protocol</ttcol>
1023   <ttcol>Status</ttcol>
1024   <ttcol>Reference</ttcol>
1025
1026   <c>ETag</c>
1027   <c>http</c>
1028   <c>standard</c>
1029   <c>
1030      <xref target="header.etag"/>
1031   </c>
1032   <c>If-Match</c>
1033   <c>http</c>
1034   <c>standard</c>
1035   <c>
1036      <xref target="header.if-match"/>
1037   </c>
1038   <c>If-Modified-Since</c>
1039   <c>http</c>
1040   <c>standard</c>
1041   <c>
1042      <xref target="header.if-modified-since"/>
1043   </c>
1044   <c>If-None-Match</c>
1045   <c>http</c>
1046   <c>standard</c>
1047   <c>
1048      <xref target="header.if-none-match"/>
1049   </c>
1050   <c>If-Unmodified-Since</c>
1051   <c>http</c>
1052   <c>standard</c>
1053   <c>
1054      <xref target="header.if-unmodified-since"/>
1055   </c>
1056   <c>Last-Modified</c>
1057   <c>http</c>
1058   <c>standard</c>
1059   <c>
1060      <xref target="header.last-modified"/>
1061   </c>
1062</texttable>
1063<!--(END)-->
1064<t>
1065   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1066</t>
1067</section>
1068</section>
1069
1070<section title="Security Considerations" anchor="security.considerations">
1071<t>
1072   No additional security considerations have been identified beyond
1073   those applicable to HTTP in general &messaging;.
1074</t>
1075</section>
1076
1077<section title="Acknowledgments" anchor="ack">
1078</section>
1079</middle>
1080<back>
1081
1082<references title="Normative References">
1083
1084<reference anchor="Part1">
1085  <front>
1086    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1087    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1088      <organization abbrev="Day Software">Day Software</organization>
1089      <address><email>fielding@gbiv.com</email></address>
1090    </author>
1091    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1092      <organization>One Laptop per Child</organization>
1093      <address><email>jg@laptop.org</email></address>
1094    </author>
1095    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1096      <organization abbrev="HP">Hewlett-Packard Company</organization>
1097      <address><email>JeffMogul@acm.org</email></address>
1098    </author>
1099    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1100      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1101      <address><email>henrikn@microsoft.com</email></address>
1102    </author>
1103    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1104      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1105      <address><email>LMM@acm.org</email></address>
1106    </author>
1107    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1108      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1109      <address><email>paulle@microsoft.com</email></address>
1110    </author>
1111    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1112      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1113      <address><email>timbl@w3.org</email></address>
1114    </author>
1115    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1116      <organization abbrev="W3C">World Wide Web Consortium</organization>
1117      <address><email>ylafon@w3.org</email></address>
1118    </author>
1119    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1120      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1121      <address><email>julian.reschke@greenbytes.de</email></address>
1122    </author>
1123    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1124  </front>
1125  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1126  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1127</reference>
1128
1129<reference anchor="Part5">
1130  <front>
1131    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1132    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1133      <organization abbrev="Day Software">Day Software</organization>
1134      <address><email>fielding@gbiv.com</email></address>
1135    </author>
1136    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1137      <organization>One Laptop per Child</organization>
1138      <address><email>jg@laptop.org</email></address>
1139    </author>
1140    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1141      <organization abbrev="HP">Hewlett-Packard Company</organization>
1142      <address><email>JeffMogul@acm.org</email></address>
1143    </author>
1144    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1145      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1146      <address><email>henrikn@microsoft.com</email></address>
1147    </author>
1148    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1149      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1150      <address><email>LMM@acm.org</email></address>
1151    </author>
1152    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1153      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1154      <address><email>paulle@microsoft.com</email></address>
1155    </author>
1156    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1157      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1158      <address><email>timbl@w3.org</email></address>
1159    </author>
1160    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1161      <organization abbrev="W3C">World Wide Web Consortium</organization>
1162      <address><email>ylafon@w3.org</email></address>
1163    </author>
1164    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1165      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1166      <address><email>julian.reschke@greenbytes.de</email></address>
1167    </author>
1168    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1169  </front>
1170  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1171  <x:source href="p5-range.xml" basename="p5-range"/>
1172</reference>
1173
1174<reference anchor="Part6">
1175  <front>
1176    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1177    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1178      <organization abbrev="Day Software">Day Software</organization>
1179      <address><email>fielding@gbiv.com</email></address>
1180    </author>
1181    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1182      <organization>One Laptop per Child</organization>
1183      <address><email>jg@laptop.org</email></address>
1184    </author>
1185    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1186      <organization abbrev="HP">Hewlett-Packard Company</organization>
1187      <address><email>JeffMogul@acm.org</email></address>
1188    </author>
1189    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1190      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1191      <address><email>henrikn@microsoft.com</email></address>
1192    </author>
1193    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1194      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1195      <address><email>LMM@acm.org</email></address>
1196    </author>
1197    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1198      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1199      <address><email>paulle@microsoft.com</email></address>
1200    </author>
1201    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1202      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1203      <address><email>timbl@w3.org</email></address>
1204    </author>
1205    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1206      <organization abbrev="W3C">World Wide Web Consortium</organization>
1207      <address><email>ylafon@w3.org</email></address>
1208    </author>
1209    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1210      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1211      <address><email>julian.reschke@greenbytes.de</email></address>
1212    </author>
1213    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1214  </front>
1215  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1216  <x:source href="p6-cache.xml" basename="p6-cache"/>
1217</reference>
1218
1219<reference anchor="RFC2119">
1220  <front>
1221    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1222    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1223      <organization>Harvard University</organization>
1224      <address><email>sob@harvard.edu</email></address>
1225    </author>
1226    <date month="March" year="1997"/>
1227  </front>
1228  <seriesInfo name="BCP" value="14"/>
1229  <seriesInfo name="RFC" value="2119"/>
1230</reference>
1231
1232</references>
1233
1234<references title="Informative References">
1235
1236<reference anchor="RFC2068">
1237  <front>
1238    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1239    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1240      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1241      <address><email>fielding@ics.uci.edu</email></address>
1242    </author>
1243    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1244      <organization>MIT Laboratory for Computer Science</organization>
1245      <address><email>jg@w3.org</email></address>
1246    </author>
1247    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1248      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1249      <address><email>mogul@wrl.dec.com</email></address>
1250    </author>
1251    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1252      <organization>MIT Laboratory for Computer Science</organization>
1253      <address><email>frystyk@w3.org</email></address>
1254    </author>
1255    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1256      <organization>MIT Laboratory for Computer Science</organization>
1257      <address><email>timbl@w3.org</email></address>
1258    </author>
1259    <date month="January" year="1997"/>
1260  </front>
1261  <seriesInfo name="RFC" value="2068"/>
1262</reference>
1263
1264<reference anchor="RFC2616">
1265  <front>
1266    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1267    <author initials="R." surname="Fielding" fullname="R. Fielding">
1268      <organization>University of California, Irvine</organization>
1269      <address><email>fielding@ics.uci.edu</email></address>
1270    </author>
1271    <author initials="J." surname="Gettys" fullname="J. Gettys">
1272      <organization>W3C</organization>
1273      <address><email>jg@w3.org</email></address>
1274    </author>
1275    <author initials="J." surname="Mogul" fullname="J. Mogul">
1276      <organization>Compaq Computer Corporation</organization>
1277      <address><email>mogul@wrl.dec.com</email></address>
1278    </author>
1279    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1280      <organization>MIT Laboratory for Computer Science</organization>
1281      <address><email>frystyk@w3.org</email></address>
1282    </author>
1283    <author initials="L." surname="Masinter" fullname="L. Masinter">
1284      <organization>Xerox Corporation</organization>
1285      <address><email>masinter@parc.xerox.com</email></address>
1286    </author>
1287    <author initials="P." surname="Leach" fullname="P. Leach">
1288      <organization>Microsoft Corporation</organization>
1289      <address><email>paulle@microsoft.com</email></address>
1290    </author>
1291    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1292      <organization>W3C</organization>
1293      <address><email>timbl@w3.org</email></address>
1294    </author>
1295    <date month="June" year="1999"/>
1296  </front>
1297  <seriesInfo name="RFC" value="2616"/>
1298</reference>
1299
1300<reference anchor='RFC3864'>
1301  <front>
1302    <title>Registration Procedures for Message Header Fields</title>
1303    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1304      <organization>Nine by Nine</organization>
1305      <address><email>GK-IETF@ninebynine.org</email></address>
1306    </author>
1307    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1308      <organization>BEA Systems</organization>
1309      <address><email>mnot@pobox.com</email></address>
1310    </author>
1311    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1312      <organization>HP Labs</organization>
1313      <address><email>JeffMogul@acm.org</email></address>
1314    </author>
1315    <date year='2004' month='September' />
1316  </front>
1317  <seriesInfo name='BCP' value='90' />
1318  <seriesInfo name='RFC' value='3864' />
1319</reference>
1320
1321</references>
1322
1323<section title="Compatibility with Previous Versions" anchor="compatibility">
1324
1325<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1326<t>
1327  Allow weak entity tags in all requests except range requests (Sections
1328  <xref target="weak.and.strong.validators" format="counter"/> and
1329  <xref target="header.if-none-match" format="counter"/>).
1330</t>
1331</section>
1332
1333</section>
1334
1335<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1336
1337<section title="Since RFC2616">
1338<t>
1339  Extracted relevant partitions from <xref target="RFC2616"/>.
1340</t>
1341</section>
1342
1343<section title="Since draft-ietf-httpbis-p4-conditional-00">
1344<t>
1345  Closed issues:
1346  <list style="symbols"> 
1347    <t>
1348      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1349      "Normative and Informative references"
1350    </t>
1351  </list>
1352</t>
1353<t>
1354  Other changes:
1355  <list style="symbols"> 
1356    <t>
1357      Move definitions of 304 and 412 condition codes from Part2.
1358    </t>
1359  </list>
1360</t>
1361</section>
1362
1363<section title="Since draft-ietf-httpbis-p4-conditional-01">
1364<t>
1365  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1366  <list style="symbols"> 
1367    <t>
1368      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1369    </t>
1370  </list>
1371</t>
1372</section>
1373
1374<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1375<t>
1376  Closed issues:
1377  <list style="symbols"> 
1378    <t>
1379      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1380      "Weak ETags on non-GET requests"
1381    </t>
1382  </list>
1383</t>
1384<t>
1385  Ongoing work on IANA Message Header Registration (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1386  <list style="symbols"> 
1387    <t>
1388      Reference RFC 3984, and update header registrations for headers defined
1389      in this document.
1390    </t>
1391  </list>
1392</t>
1393</section>
1394
1395<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1396<t>
1397  Closed issues:
1398  <list style="symbols"> 
1399    <t>
1400      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1401      "Examples for ETag matching"
1402    </t>
1403    <t>
1404      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1405      "'entity value' undefined"
1406    </t>
1407  </list>
1408</t>
1409</section>
1410
1411</section>
1412
1413</back>
1414</rfc>
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