source: draft-ietf-httpbis/03/p4-conditional.xml @ 872

Last change on this file since 872 was 264, checked in by julian.reschke@…, 12 years ago

Prepare for release of draft 03 on Tuesday, 2008-06-17.

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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 "03">
15  <!ENTITY ID-MONTH "June">
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;" day="17"/>
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    <t>Date, unless its omission is required by &clockless;</t>
325  </list>
326</t>
327<t>
328   If a clockless origin server obeys these rules, and proxies and
329   clients add their own Date to any response received without one (as
330   already specified by <xref target="RFC2068" x:sec="14.19" x:fmt=","/>), caches will operate
331   correctly.
332  <list style="symbols">
333    <t>ETag and/or Content-Location, if the header would have been sent
334        in a 200 response to the same request</t>
335    <t>Expires, Cache-Control, and/or Vary, if the field-value might
336        differ from that sent in any previous response for the same
337        variant</t>
338  </list>
339</t>
340<t>
341   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
342   the response &SHOULD-NOT;  include other entity-headers.
343   Otherwise (i.e., the conditional GET used a weak validator), the
344   response &MUST-NOT; include other entity-headers; this prevents
345   inconsistencies between cached entity-bodies and updated headers.
346</t>
347<t>
348   If a 304 response indicates an entity not currently cached, then the
349   cache &MUST; disregard the response and repeat the request without the
350   conditional.
351</t>
352<t>
353   If a cache uses a received 304 response to update a cache entry, the
354   cache &MUST; update the entry to reflect any new field values given in
355   the response.
356</t>
357</section>
358
359<section title="412 Precondition Failed" anchor="status.412">
360  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
361  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
362<t>
363   The precondition given in one or more of the request-header fields
364   evaluated to false when it was tested on the server. This response
365   code allows the client to place preconditions on the current resource
366   metainformation (header field data) and thus prevent the requested
367   method from being applied to a resource other than the one intended.
368</t>
369</section>
370</section>
371
372<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
373<t>
374   Since both origin servers and caches will compare two validators to
375   decide if they represent the same or different entities, one normally
376   would expect that if the entity (the entity-body or any entity-headers)
377   changes in any way, then the associated validator would
378   change as well. If this is true, then we call this validator a
379   "strong validator."
380</t>
381<t>
382   However, there might be cases when a server prefers to change the
383   validator only on semantically significant changes, and not when
384   insignificant aspects of the entity change. A validator that does not
385   always change when the resource changes is a "weak validator."
386</t>
387<t>
388   Entity tags are normally "strong validators," but the protocol
389   provides a mechanism to tag an entity tag as "weak." One can think of
390   a strong validator as one that changes whenever the bits of an entity
391   changes, while a weak value changes whenever the meaning of an entity
392   changes. Alternatively, one can think of a strong validator as part
393   of an identifier for a specific entity, while a weak validator is
394   part of an identifier for a set of semantically equivalent entities.
395  <list><t>
396      <x:h>Note:</x:h> One example of a strong validator is an integer that is
397      incremented in stable storage every time an entity is changed.
398    </t><t>
399      An entity's modification time, if represented with one-second
400      resolution, could be a weak validator, since it is possible that
401      the resource might be modified twice during a single second.
402    </t><t>
403      Support for weak validators is optional. However, weak validators
404      allow for more efficient caching of equivalent objects; for
405      example, a hit counter on a site is probably good enough if it is
406      updated every few days or weeks, and any value during that period
407      is likely "good enough" to be equivalent.
408    </t></list>
409</t>
410<t>
411   A "use" of a validator is either when a client generates a request
412   and includes the validator in a validating header field, or when a
413   server compares two validators.
414</t>
415<t>
416   Strong validators are usable in any context. Weak validators are only
417   usable in contexts that do not depend on exact equality of an entity.
418   For example, either kind is usable for a conditional GET of a full
419   entity. However, only a strong validator is usable for a sub-range
420   retrieval, since otherwise the client might end up with an internally
421   inconsistent entity.
422</t>
423<t>
424   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
425</t>
426<t>
427   The only function that HTTP/1.1 defines on validators is
428   comparison. There are two validator comparison functions, depending
429   on whether the comparison context allows the use of weak validators
430   or not:
431  <list style="symbols">
432     <t>The strong comparison function: in order to be considered equal,
433        both validators &MUST; be identical in every way, and both &MUST-NOT;
434        be weak.</t>
435     <t>The weak comparison function: in order to be considered equal,
436        both validators &MUST; be identical in every way, but either or
437        both of them &MAY; be tagged as "weak" without affecting the
438        result.</t>
439  </list>
440</t>
441<t>
442   An entity tag is strong unless it is explicitly tagged as weak.
443   <xref target="entity.tags"/> gives the syntax for entity tags.
444</t>
445<t>
446   A Last-Modified time, when used as a validator in a request, is
447   implicitly weak unless it is possible to deduce that it is strong,
448   using the following rules:
449  <list style="symbols">
450     <t>The validator is being compared by an origin server to the
451        actual current validator for the entity and,</t>
452     <t>That origin server reliably knows that the associated entity did
453        not change twice during the second covered by the presented
454        validator.</t>
455  </list>
456</t>
457<t>
458   or
459  <list style="symbols">
460     <t>The validator is about to be used by a client in an If-Modified-Since
461        or If-Unmodified-Since header, because the client
462        has a cache entry for the associated entity, and</t>
463     <t>That cache entry includes a Date value, which gives the time
464        when the origin server sent the original response, and</t>
465     <t>The presented Last-Modified time is at least 60 seconds before
466        the Date value.</t>
467  </list>
468</t>
469<t>
470   or
471  <list style="symbols">
472     <t>The validator is being compared by an intermediate cache to the
473        validator stored in its cache entry for the entity, and</t>
474     <t>That cache entry includes a Date value, which gives the time
475        when the origin server sent the original response, and</t>
476     <t>The presented Last-Modified time is at least 60 seconds before
477        the Date value.</t>
478  </list>
479</t>
480<t>
481   This method relies on the fact that if two different responses were
482   sent by the origin server during the same second, but both had the
483   same Last-Modified time, then at least one of those responses would
484   have a Date value equal to its Last-Modified time. The arbitrary 60-second
485   limit guards against the possibility that the Date and Last-Modified
486   values are generated from different clocks, or at somewhat
487   different times during the preparation of the response. An
488   implementation &MAY; use a value larger than 60 seconds, if it is
489   believed that 60 seconds is too short.
490</t>
491<t>
492   If a client wishes to perform a sub-range retrieval on a value for
493   which it has only a Last-Modified time and no opaque validator, it
494   &MAY; do this only if the Last-Modified time is strong in the sense
495   described here.
496</t>
497<t>
498   A cache or origin server receiving a conditional range request
499   (<xref target="Part5"/>) &MUST; use the strong comparison function to
500   evaluate the condition.
501</t>
502<t>
503   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
504   retrievals on values that have been obtained from HTTP/1.0
505   servers.
506</t>
507</section>
508
509<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">
510<t>
511   We adopt a set of rules and recommendations for origin servers,
512   clients, and caches regarding when various validator types ought to
513   be used, and for what purposes.
514</t>
515<t>
516   HTTP/1.1 origin servers:
517  <list style="symbols">
518     <t>&SHOULD; send an entity tag validator unless it is not feasible to
519        generate one.</t>
520
521     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
522        performance considerations support the use of weak entity tags,
523        or if it is unfeasible to send a strong entity tag.</t>
524
525     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
526        unless the risk of a breakdown in semantic transparency that
527        could result from using this date in an If-Modified-Since header
528        would lead to serious problems.</t>
529  </list>
530</t>
531<t>
532   In other words, the preferred behavior for an HTTP/1.1 origin server
533   is to send both a strong entity tag and a Last-Modified value.
534</t>
535<t>
536   In order to be legal, a strong entity tag &MUST; change whenever the
537   associated entity value changes in any way. A weak entity tag &SHOULD;
538   change whenever the associated entity changes in a semantically
539   significant way.
540  <list><t>
541      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
542      origin server must avoid reusing a specific strong entity tag
543      value for two different entities, or reusing a specific weak
544      entity tag value for two semantically different entities. Cache
545      entries might persist for arbitrarily long periods, regardless of
546      expiration times, so it might be inappropriate to expect that a
547      cache will never again attempt to validate an entry using a
548      validator that it obtained at some point in the past.
549  </t></list>
550</t>
551<t>
552   HTTP/1.1 clients:
553  <list style="symbols">
554     <t>If an entity tag has been provided by the origin server, &MUST;
555        use that entity tag in any cache-conditional request (using If-Match
556        or If-None-Match).</t>
557
558     <t>If only a Last-Modified value has been provided by the origin
559        server, &SHOULD; use that value in non-subrange cache-conditional
560        requests (using If-Modified-Since).</t>
561
562     <t>If only a Last-Modified value has been provided by an HTTP/1.0
563        origin server, &MAY; use that value in subrange cache-conditional
564        requests (using If-Unmodified-Since:). The user agent &SHOULD;
565        provide a way to disable this, in case of difficulty.</t>
566
567     <t>If both an entity tag and a Last-Modified value have been
568        provided by the origin server, &SHOULD; use both validators in
569        cache-conditional requests. This allows both HTTP/1.0 and
570        HTTP/1.1 caches to respond appropriately.</t>
571  </list>
572</t>
573<t>
574   An HTTP/1.1 origin server, upon receiving a conditional request that
575   includes both a Last-Modified date (e.g., in an If-Modified-Since or
576   If-Unmodified-Since header field) and one or more entity tags (e.g.,
577   in an If-Match, If-None-Match, or If-Range header field) as cache
578   validators, &MUST-NOT; return a response status of 304 (Not Modified)
579   unless doing so is consistent with all of the conditional header
580   fields in the request.
581</t>
582<t>
583   An HTTP/1.1 caching proxy, upon receiving a conditional request that
584   includes both a Last-Modified date and one or more entity tags as
585   cache validators, &MUST-NOT; return a locally cached response to the
586   client unless that cached response is consistent with all of the
587   conditional header fields in the request.
588  <list><t>
589      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
590      servers and clients should transmit as much non-redundant
591      information as is available in their responses and requests.
592      HTTP/1.1 systems receiving this information will make the most
593      conservative assumptions about the validators they receive.
594  </t><t>
595      HTTP/1.0 clients and caches will ignore entity tags. Generally,
596      last-modified values received or used by these systems will
597      support transparent and efficient caching, and so HTTP/1.1 origin
598      servers should provide Last-Modified values. In those rare cases
599      where the use of a Last-Modified value as a validator by an
600      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
601      origin servers should not provide one.
602  </t></list>
603</t>
604</section>
605
606<section title="Header Field Definitions" anchor="header.fields">
607<t>
608   This section defines the syntax and semantics of HTTP/1.1 header fields
609   related to conditional requests.
610</t>
611<t>
612   For entity-header fields, both sender and recipient refer to either the
613   client or the server, depending on who sends and who receives the entity.
614</t>
615
616<section title="ETag" anchor="header.etag">
617  <iref primary="true" item="ETag header" x:for-anchor=""/>
618  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
619  <x:anchor-alias value="ETag"/>
620<t>
621   The ETag response-header field provides the current value of the
622   entity tag for the requested variant. The headers used with entity
623   tags are described in Sections <xref target="header.if-match" format="counter"/>
624   and <xref target="header.if-none-match" format="counter"/> of this document,
625   and in &header-if-range;. The entity tag
626   &MAY; be used for comparison with other entities from the same resource
627   (see <xref target="weak.and.strong.validators"/>).
628</t>
629<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
630  <x:ref>ETag</x:ref> = "ETag" ":" <x:ref>entity-tag</x:ref>
631</artwork></figure>
632<figure><preamble>
633   Examples:
634</preamble>
635<artwork type="example">
636   ETag: "xyzzy"
637   ETag: W/"xyzzy"
638   ETag: ""
639</artwork></figure>
640<t>
641   The ETag response-header field value, an entity tag, provides for an
642   "opaque" cache validator. This might allow more reliable validation
643   in situations where it is inconvenient to store modification dates,
644   where the one-second resolution of HTTP date values is not
645   sufficient, or where the origin server wishes to avoid certain
646   paradoxes that might arise from the use of modification dates.
647</t>
648<t>
649   The principle behind entity tags is that only the service author
650   knows the semantics of a resource well enough to select an
651   appropriate cache validation mechanism, and the specification of any
652   validator comparison function more complex than byte-equality would
653   open up a can of worms. Thus, comparisons of any other headers
654   (except Last-Modified, for compatibility with HTTP/1.0) are never
655   used for purposes of validating a cache entry.
656</t>
657</section>
658
659<section title="If-Match" anchor="header.if-match">
660  <iref primary="true" item="If-Match header" x:for-anchor=""/>
661  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
662  <x:anchor-alias value="If-Match"/>
663<t>
664   The If-Match request-header field is used with a method to make it
665   conditional. A client that has one or more entities previously
666   obtained from the resource can verify that one of those entities is
667   current by including a list of their associated entity tags in the
668   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
669   purpose of this feature is to allow efficient updates of cached
670   information with a minimum amount of transaction overhead. It is also
671   used, on updating requests, to prevent inadvertent modification of
672   the wrong version of a resource. As a special case, the value "*"
673   matches any current entity of the resource.
674</t>
675<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
676  <x:ref>If-Match</x:ref> = "If-Match" ":" ( "*" | 1#<x:ref>entity-tag</x:ref> )
677</artwork></figure>
678<t>
679   If any of the entity tags match the entity tag of the entity that
680   would have been returned in the response to a similar GET request
681   (without the If-Match header) on that resource, or if "*" is given
682   and any current entity exists for that resource, then the server &MAY;
683   perform the requested method as if the If-Match header field did not
684   exist.
685</t>
686<t>
687   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
688   to compare the entity tags in If-Match.
689</t>
690<t>
691   If none of the entity tags match, or if "*" is given and no current
692   entity exists, the server &MUST-NOT; perform the requested method, and
693   &MUST; return a 412 (Precondition Failed) response. This behavior is
694   most useful when the client wants to prevent an updating method, such
695   as PUT, from modifying a resource that has changed since the client
696   last retrieved it.
697</t>
698<t>
699   If the request would, without the If-Match header field, result in
700   anything other than a 2xx or 412 status, then the If-Match header
701   &MUST; be ignored.
702</t>
703<t>
704   The meaning of "If-Match: *" is that the method &SHOULD; be performed
705   if the representation selected by the origin server (or by a cache,
706   possibly using the Vary mechanism, see &header-vary;) exists, and
707   &MUST-NOT; be performed if the representation does not exist.
708</t>
709<t>
710   A request intended to update a resource (e.g., a PUT) &MAY; include an
711   If-Match header field to signal that the request method &MUST-NOT; be
712   applied if the entity corresponding to the If-Match value (a single
713   entity tag) is no longer a representation of that resource. This
714   allows the user to indicate that they do not wish the request to be
715   successful if the resource has been changed without their knowledge.
716   Examples:
717</t>
718<figure><artwork type="example">
719    If-Match: "xyzzy"
720    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
721    If-Match: *
722</artwork></figure>
723<t>
724   The result of a request having both an If-Match header field and
725   either an If-None-Match or an If-Modified-Since header fields is
726   undefined by this specification.
727</t>
728</section>
729
730<section title="If-Modified-Since" anchor="header.if-modified-since">
731  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
732  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
733  <x:anchor-alias value="If-Modified-Since"/>
734<t>
735   The If-Modified-Since request-header field is used with a method to
736   make it conditional: if the requested variant has not been modified
737   since the time specified in this field, an entity will not be
738   returned from the server; instead, a 304 (Not Modified) response will
739   be returned without any message-body.
740</t>
741<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
742  <x:ref>If-Modified-Since</x:ref> = "If-Modified-Since" ":" <x:ref>HTTP-date</x:ref>
743</artwork></figure>
744<t>
745   An example of the field is:
746</t>
747<figure><artwork type="example">
748    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
749</artwork></figure>
750<t>
751   A GET method with an If-Modified-Since header and no Range header
752   requests that the identified entity be transferred only if it has
753   been modified since the date given by the If-Modified-Since header.
754   The algorithm for determining this includes the following cases:
755  <list style="numbers">
756      <t>If the request would normally result in anything other than a
757         200 (OK) status, or if the passed If-Modified-Since date is
758         invalid, the response is exactly the same as for a normal GET.
759         A date which is later than the server's current time is
760         invalid.</t>
761
762      <t>If the variant has been modified since the If-Modified-Since
763         date, the response is exactly the same as for a normal GET.</t>
764
765      <t>If the variant has not been modified since a valid If-Modified-Since
766         date, the server &SHOULD; return a 304 (Not
767         Modified) response.</t>
768  </list>
769</t>
770<t>
771   The purpose of this feature is to allow efficient updates of cached
772   information with a minimum amount of transaction overhead.
773  <list><t>
774      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
775      see &header-range; for full details.
776    </t><t>
777      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
778      clock might not be synchronized with the client.
779    </t><t>
780      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
781      servers will use an exact date comparison function, rather than a
782      less-than function, for deciding whether to send a 304 (Not
783      Modified) response. To get best results when sending an If-Modified-Since
784      header field for cache validation, clients are
785      advised to use the exact date string received in a previous Last-Modified
786      header field whenever possible.
787    </t><t>
788      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
789      header instead of a date taken from the Last-Modified header for
790      the same request, the client should be aware of the fact that this
791      date is interpreted in the server's understanding of time. The
792      client should consider unsynchronized clocks and rounding problems
793      due to the different encodings of time between the client and
794      server. This includes the possibility of race conditions if the
795      document has changed between the time it was first requested and
796      the If-Modified-Since date of a subsequent request, and the
797      possibility of clock-skew-related problems if the If-Modified-Since
798      date is derived from the client's clock without correction
799      to the server's clock. Corrections for different time bases
800      between client and server are at best approximate due to network
801      latency.
802    </t>
803  </list>
804</t>
805<t>
806   The result of a request having both an If-Modified-Since header field
807   and either an If-Match or an If-Unmodified-Since header fields is
808   undefined by this specification.
809</t>
810</section>
811
812<section title="If-None-Match" anchor="header.if-none-match">
813  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
814  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
815  <x:anchor-alias value="If-None-Match"/>
816<t>
817   The If-None-Match request-header field is used with a method to make
818   it conditional. A client that has one or more entities previously
819   obtained from the resource can verify that none of those entities is
820   current by including a list of their associated entity tags in the
821   If-None-Match header field. The purpose of this feature is to allow
822   efficient updates of cached information with a minimum amount of
823   transaction overhead. It is also used to prevent a method (e.g. PUT)
824   from inadvertently modifying an existing resource when the client
825   believes that the resource does not exist.
826</t>
827<t>
828   As a special case, the value "*" matches any current entity of the
829   resource.
830</t>
831<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
832  <x:ref>If-None-Match</x:ref> = "If-None-Match" ":" ( "*" | 1#<x:ref>entity-tag</x:ref> )
833</artwork></figure>
834<t>
835   If any of the entity tags match the entity tag of the entity that
836   would have been returned in the response to a similar GET request
837   (without the If-None-Match header) on that resource, or if "*" is
838   given and any current entity exists for that resource, then the
839   server &MUST-NOT; perform the requested method, unless required to do
840   so because the resource's modification date fails to match that
841   supplied in an If-Modified-Since header field in the request.
842   Instead, if the request method was GET or HEAD, the server &SHOULD;
843   respond with a 304 (Not Modified) response, including the cache-related
844   header fields (particularly ETag) of one of the entities that
845   matched. For all other request methods, the server &MUST; respond with
846   a status of 412 (Precondition Failed).
847</t>
848<t>
849   See <xref target="weak.and.strong.validators"/> for rules on how to
850   determine if two entity tags match.
851</t>
852<t>
853   If none of the entity tags match, then the server &MAY; perform the
854   requested method as if the If-None-Match header field did not exist,
855   but &MUST; also ignore any If-Modified-Since header field(s) in the
856   request. That is, if no entity tags match, then the server &MUST-NOT;
857   return a 304 (Not Modified) response.
858</t>
859<t>
860   If the request would, without the If-None-Match header field, result
861   in anything other than a 2xx or 304 status, then the If-None-Match
862   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
863   server behavior when both If-Modified-Since and If-None-Match appear
864   in the same request.)
865</t>
866<t>
867   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
868   performed if the representation selected by the origin server (or by
869   a cache, possibly using the Vary mechanism, see &header-vary;)
870   exists, and &SHOULD; be performed if the representation does not exist.
871   This feature is intended to be useful in preventing races between PUT
872   operations.
873</t>
874<t>
875   Examples:
876</t>
877<figure><artwork type="example">
878    If-None-Match: "xyzzy"
879    If-None-Match: W/"xyzzy"
880    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
881    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
882    If-None-Match: *
883</artwork></figure>
884<t>
885   The result of a request having both an If-None-Match header field and
886   either an If-Match or an If-Unmodified-Since header fields is
887   undefined by this specification.
888</t>
889</section>
890
891<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
892  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
893  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
894  <x:anchor-alias value="If-Unmodified-Since"/>
895<t>
896   The If-Unmodified-Since request-header field is used with a method to
897   make it conditional. If the requested resource has not been modified
898   since the time specified in this field, the server &SHOULD; perform the
899   requested operation as if the If-Unmodified-Since header were not
900   present.
901</t>
902<t>
903   If the requested variant has been modified since the specified time,
904   the server &MUST-NOT; perform the requested operation, and &MUST; return
905   a 412 (Precondition Failed).
906</t>
907<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
908  <x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since" ":" <x:ref>HTTP-date</x:ref>
909</artwork></figure>
910<t>
911   An example of the field is:
912</t>
913<figure><artwork type="example">
914    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
915</artwork></figure>
916<t>
917   If the request normally (i.e., without the If-Unmodified-Since
918   header) would result in anything other than a 2xx or 412 status, the
919   If-Unmodified-Since header &SHOULD; be ignored.
920</t>
921<t>
922   If the specified date is invalid, the header is ignored.
923</t>
924<t>
925   The result of a request having both an If-Unmodified-Since header
926   field and either an If-None-Match or an If-Modified-Since header
927   fields is undefined by this specification.
928</t>
929</section>
930
931<section title="Last-Modified" anchor="header.last-modified">
932  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
933  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
934  <x:anchor-alias value="Last-Modified"/>
935<t>
936   The Last-Modified entity-header field indicates the date and time at
937   which the origin server believes the variant was last modified.
938</t>
939<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
940  <x:ref>Last-Modified</x:ref>  = "Last-Modified" ":" <x:ref>HTTP-date</x:ref>
941</artwork></figure>
942<t>
943   An example of its use is
944</t>
945<figure><artwork type="example">
946    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
947</artwork></figure>
948<t>
949   The exact meaning of this header field depends on the implementation
950   of the origin server and the nature of the original resource. For
951   files, it may be just the file system last-modified time. For
952   entities with dynamically included parts, it may be the most recent
953   of the set of last-modify times for its component parts. For database
954   gateways, it may be the last-update time stamp of the record. For
955   virtual objects, it may be the last time the internal state changed.
956</t>
957<t>
958   An origin server &MUST-NOT; send a Last-Modified date which is later
959   than the server's time of message origination. In such cases, where
960   the resource's last modification would indicate some time in the
961   future, the server &MUST; replace that date with the message
962   origination date.
963</t>
964<t>
965   An origin server &SHOULD; obtain the Last-Modified value of the entity
966   as close as possible to the time that it generates the Date value of
967   its response. This allows a recipient to make an accurate assessment
968   of the entity's modification time, especially if the entity changes
969   near the time that the response is generated.
970</t>
971<t>
972   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
973</t>
974<t>
975   The Last-Modified entity-header field value is often used as a cache
976   validator. In simple terms, a cache entry is considered to be valid
977   if the entity has not been modified since the Last-Modified value.
978</t>
979</section>
980
981</section>
982
983<section title="IANA Considerations" anchor="IANA.considerations">
984<section title="Message Header Registration" anchor="message.header.registration">
985<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
986<!--(START)-->
987<t>
988    The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
989    with the permanent registrations below (see <xref target="RFC3864"/>):
990  </t>
991<texttable>
992   <ttcol>Header Field Name</ttcol>
993   <ttcol>Protocol</ttcol>
994   <ttcol>Status</ttcol>
995   <ttcol>Reference</ttcol>
996
997   <c>ETag</c>
998   <c>http</c>
999   <c>standard</c>
1000   <c>
1001      <xref target="header.etag"/>
1002   </c>
1003
1004   <c>If-Match</c>
1005   <c>http</c>
1006   <c>standard</c>
1007   <c>
1008      <xref target="header.if-match"/>
1009   </c>
1010
1011   <c>If-Modified-Since</c>
1012   <c>http</c>
1013   <c>standard</c>
1014   <c>
1015      <xref target="header.if-modified-since"/>
1016   </c>
1017
1018   <c>If-None-Match</c>
1019   <c>http</c>
1020   <c>standard</c>
1021   <c>
1022      <xref target="header.if-none-match"/>
1023   </c>
1024
1025   <c>If-Unmodified-Since</c>
1026   <c>http</c>
1027   <c>standard</c>
1028   <c>
1029      <xref target="header.if-unmodified-since"/>
1030   </c>
1031
1032   <c>Last-Modified</c>
1033   <c>http</c>
1034   <c>standard</c>
1035   <c>
1036      <xref target="header.last-modified"/>
1037   </c>
1038</texttable>
1039<t>
1040    The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1041  </t>
1042<!--(END)-->
1043</section>
1044</section>
1045
1046<section title="Security Considerations" anchor="security.considerations">
1047<t>
1048   No additional security considerations have been identified beyond
1049   those applicable to HTTP in general &messaging;.
1050</t>
1051</section>
1052
1053<section title="Acknowledgments" anchor="ack">
1054</section>
1055</middle>
1056<back>
1057
1058<references title="Normative References">
1059
1060<reference anchor="Part1">
1061  <front>
1062    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1063    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1064      <organization abbrev="Day Software">Day Software</organization>
1065      <address><email>fielding@gbiv.com</email></address>
1066    </author>
1067    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1068      <organization>One Laptop per Child</organization>
1069      <address><email>jg@laptop.org</email></address>
1070    </author>
1071    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1072      <organization abbrev="HP">Hewlett-Packard Company</organization>
1073      <address><email>JeffMogul@acm.org</email></address>
1074    </author>
1075    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1076      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1077      <address><email>henrikn@microsoft.com</email></address>
1078    </author>
1079    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1080      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1081      <address><email>LMM@acm.org</email></address>
1082    </author>
1083    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1084      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1085      <address><email>paulle@microsoft.com</email></address>
1086    </author>
1087    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1088      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1089      <address><email>timbl@w3.org</email></address>
1090    </author>
1091    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1092      <organization abbrev="W3C">World Wide Web Consortium</organization>
1093      <address><email>ylafon@w3.org</email></address>
1094    </author>
1095    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1096      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1097      <address><email>julian.reschke@greenbytes.de</email></address>
1098    </author>
1099    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1100  </front>
1101  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1102  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1103</reference>
1104
1105<reference anchor="Part5">
1106  <front>
1107    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1108    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1109      <organization abbrev="Day Software">Day Software</organization>
1110      <address><email>fielding@gbiv.com</email></address>
1111    </author>
1112    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1113      <organization>One Laptop per Child</organization>
1114      <address><email>jg@laptop.org</email></address>
1115    </author>
1116    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1117      <organization abbrev="HP">Hewlett-Packard Company</organization>
1118      <address><email>JeffMogul@acm.org</email></address>
1119    </author>
1120    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1121      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1122      <address><email>henrikn@microsoft.com</email></address>
1123    </author>
1124    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1125      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1126      <address><email>LMM@acm.org</email></address>
1127    </author>
1128    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1129      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1130      <address><email>paulle@microsoft.com</email></address>
1131    </author>
1132    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1133      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1134      <address><email>timbl@w3.org</email></address>
1135    </author>
1136    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1137      <organization abbrev="W3C">World Wide Web Consortium</organization>
1138      <address><email>ylafon@w3.org</email></address>
1139    </author>
1140    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1141      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1142      <address><email>julian.reschke@greenbytes.de</email></address>
1143    </author>
1144    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1145  </front>
1146  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1147  <x:source href="p5-range.xml" basename="p5-range"/>
1148</reference>
1149
1150<reference anchor="Part6">
1151  <front>
1152    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1153    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1154      <organization abbrev="Day Software">Day Software</organization>
1155      <address><email>fielding@gbiv.com</email></address>
1156    </author>
1157    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1158      <organization>One Laptop per Child</organization>
1159      <address><email>jg@laptop.org</email></address>
1160    </author>
1161    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1162      <organization abbrev="HP">Hewlett-Packard Company</organization>
1163      <address><email>JeffMogul@acm.org</email></address>
1164    </author>
1165    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1166      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1167      <address><email>henrikn@microsoft.com</email></address>
1168    </author>
1169    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1170      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1171      <address><email>LMM@acm.org</email></address>
1172    </author>
1173    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1174      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1175      <address><email>paulle@microsoft.com</email></address>
1176    </author>
1177    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1178      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1179      <address><email>timbl@w3.org</email></address>
1180    </author>
1181    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1182      <organization abbrev="W3C">World Wide Web Consortium</organization>
1183      <address><email>ylafon@w3.org</email></address>
1184    </author>
1185    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1186      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1187      <address><email>julian.reschke@greenbytes.de</email></address>
1188    </author>
1189    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1190  </front>
1191  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1192  <x:source href="p6-cache.xml" basename="p6-cache"/>
1193</reference>
1194
1195<reference anchor="RFC2119">
1196  <front>
1197    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1198    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1199      <organization>Harvard University</organization>
1200      <address><email>sob@harvard.edu</email></address>
1201    </author>
1202    <date month="March" year="1997"/>
1203  </front>
1204  <seriesInfo name="BCP" value="14"/>
1205  <seriesInfo name="RFC" value="2119"/>
1206</reference>
1207
1208</references>
1209
1210<references title="Informative References">
1211
1212<reference anchor="RFC2068">
1213  <front>
1214    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1215    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1216      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1217      <address><email>fielding@ics.uci.edu</email></address>
1218    </author>
1219    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1220      <organization>MIT Laboratory for Computer Science</organization>
1221      <address><email>jg@w3.org</email></address>
1222    </author>
1223    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1224      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1225      <address><email>mogul@wrl.dec.com</email></address>
1226    </author>
1227    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1228      <organization>MIT Laboratory for Computer Science</organization>
1229      <address><email>frystyk@w3.org</email></address>
1230    </author>
1231    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1232      <organization>MIT Laboratory for Computer Science</organization>
1233      <address><email>timbl@w3.org</email></address>
1234    </author>
1235    <date month="January" year="1997"/>
1236  </front>
1237  <seriesInfo name="RFC" value="2068"/>
1238</reference>
1239
1240<reference anchor="RFC2616">
1241  <front>
1242    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1243    <author initials="R." surname="Fielding" fullname="R. Fielding">
1244      <organization>University of California, Irvine</organization>
1245      <address><email>fielding@ics.uci.edu</email></address>
1246    </author>
1247    <author initials="J." surname="Gettys" fullname="J. Gettys">
1248      <organization>W3C</organization>
1249      <address><email>jg@w3.org</email></address>
1250    </author>
1251    <author initials="J." surname="Mogul" fullname="J. Mogul">
1252      <organization>Compaq Computer Corporation</organization>
1253      <address><email>mogul@wrl.dec.com</email></address>
1254    </author>
1255    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1256      <organization>MIT Laboratory for Computer Science</organization>
1257      <address><email>frystyk@w3.org</email></address>
1258    </author>
1259    <author initials="L." surname="Masinter" fullname="L. Masinter">
1260      <organization>Xerox Corporation</organization>
1261      <address><email>masinter@parc.xerox.com</email></address>
1262    </author>
1263    <author initials="P." surname="Leach" fullname="P. Leach">
1264      <organization>Microsoft Corporation</organization>
1265      <address><email>paulle@microsoft.com</email></address>
1266    </author>
1267    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1268      <organization>W3C</organization>
1269      <address><email>timbl@w3.org</email></address>
1270    </author>
1271    <date month="June" year="1999"/>
1272  </front>
1273  <seriesInfo name="RFC" value="2616"/>
1274</reference>
1275
1276<reference anchor='RFC3864'>
1277  <front>
1278    <title>Registration Procedures for Message Header Fields</title>
1279    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1280      <organization>Nine by Nine</organization>
1281      <address><email>GK-IETF@ninebynine.org</email></address>
1282    </author>
1283    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1284      <organization>BEA Systems</organization>
1285      <address><email>mnot@pobox.com</email></address>
1286    </author>
1287    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1288      <organization>HP Labs</organization>
1289      <address><email>JeffMogul@acm.org</email></address>
1290    </author>
1291    <date year='2004' month='September' />
1292  </front>
1293  <seriesInfo name='BCP' value='90' />
1294  <seriesInfo name='RFC' value='3864' />
1295</reference>
1296
1297</references>
1298
1299<section title="Compatibility with Previous Versions" anchor="compatibility">
1300
1301<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1302<t>
1303  Allow weak entity tags in all requests except range requests (Sections
1304  <xref target="weak.and.strong.validators" format="counter"/> and
1305  <xref target="header.if-none-match" format="counter"/>).
1306</t>
1307</section>
1308
1309</section>
1310
1311<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1312
1313<section title="Since RFC2616">
1314<t>
1315  Extracted relevant partitions from <xref target="RFC2616"/>.
1316</t>
1317</section>
1318
1319<section title="Since draft-ietf-httpbis-p4-conditional-00">
1320<t>
1321  Closed issues:
1322  <list style="symbols"> 
1323    <t>
1324      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1325      "Normative and Informative references"
1326    </t>
1327  </list>
1328</t>
1329<t>
1330  Other changes:
1331  <list style="symbols"> 
1332    <t>
1333      Move definitions of 304 and 412 condition codes from Part2.
1334    </t>
1335  </list>
1336</t>
1337</section>
1338
1339<section title="Since draft-ietf-httpbis-p4-conditional-01">
1340<t>
1341  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1342  <list style="symbols"> 
1343    <t>
1344      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1345    </t>
1346  </list>
1347</t>
1348</section>
1349
1350<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1351<t>
1352  Closed issues:
1353  <list style="symbols"> 
1354    <t>
1355      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1356      "Weak ETags on non-GET requests"
1357    </t>
1358  </list>
1359</t>
1360<t>
1361  Ongoing work on IANA Message Header Registration (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1362  <list style="symbols"> 
1363    <t>
1364      Reference RFC 3984, and update header registrations for headers defined
1365      in this document.
1366    </t>
1367  </list>
1368</t>
1369</section>
1370
1371</section>
1372
1373</back>
1374</rfc>
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