source: draft-ietf-httpbis/11/draft-ietf-httpbis-p4-conditional-11.xml @ 1500

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fix mime types

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  • Property svn:mime-type set to text/xml
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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<?rfc rfcedstyle="yes"?>
17<!DOCTYPE rfc
18  PUBLIC "" "rfc2629.dtd">
19<rfc obsoletes="2616" category="std" ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-11">
20<front>
21
22  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
23
24  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
25    <organization abbrev="Day Software">Day Software</organization>
26    <address>
27      <postal>
28        <street>23 Corporate Plaza DR, Suite 280</street>
29        <city>Newport Beach</city>
30        <region>CA</region>
31        <code>92660</code>
32        <country>USA</country>
33      </postal>
34      <phone>+1-949-706-5300</phone>
35      <facsimile>+1-949-706-5305</facsimile>
36      <email>fielding@gbiv.com</email>
37      <uri>http://roy.gbiv.com/</uri>
38    </address>
39  </author>
40
41  <author initials="J." surname="Gettys" fullname="Jim Gettys">
42    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
43    <address>
44      <postal>
45        <street>21 Oak Knoll Road</street>
46        <city>Carlisle</city>
47        <region>MA</region>
48        <code>01741</code>
49        <country>USA</country>
50      </postal>
51      <email>jg@freedesktop.org</email>
52      <uri>http://gettys.wordpress.com/</uri>
53    </address>
54  </author>
55 
56  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
57    <organization abbrev="HP">Hewlett-Packard Company</organization>
58    <address>
59      <postal>
60        <street>HP Labs, Large Scale Systems Group</street>
61        <street>1501 Page Mill Road, MS 1177</street>
62        <city>Palo Alto</city>
63        <region>CA</region>
64        <code>94304</code>
65        <country>USA</country>
66      </postal>
67      <email>JeffMogul@acm.org</email>
68    </address>
69  </author>
70
71  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
72    <organization abbrev="Microsoft">Microsoft Corporation</organization>
73    <address>
74      <postal>
75        <street>1 Microsoft Way</street>
76        <city>Redmond</city>
77        <region>WA</region>
78        <code>98052</code>
79        <country>USA</country>
80      </postal>
81      <email>henrikn@microsoft.com</email>
82    </address>
83  </author>
84
85  <author initials="L." surname="Masinter" fullname="Larry Masinter">
86    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
87    <address>
88      <postal>
89        <street>345 Park Ave</street>
90        <city>San Jose</city>
91        <region>CA</region>
92        <code>95110</code>
93        <country>USA</country>
94      </postal>
95      <email>LMM@acm.org</email>
96      <uri>http://larry.masinter.net/</uri>
97    </address>
98  </author>
99 
100  <author initials="P." surname="Leach" fullname="Paul J. Leach">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108      </postal>
109      <email>paulle@microsoft.com</email>
110    </address>
111  </author>
112   
113  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
114    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
115    <address>
116      <postal>
117        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
118        <street>The Stata Center, Building 32</street>
119        <street>32 Vassar Street</street>
120        <city>Cambridge</city>
121        <region>MA</region>
122        <code>02139</code>
123        <country>USA</country>
124      </postal>
125      <email>timbl@w3.org</email>
126      <uri>http://www.w3.org/People/Berners-Lee/</uri>
127    </address>
128  </author>
129
130  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
131    <organization abbrev="W3C">World Wide Web Consortium</organization>
132    <address>
133      <postal>
134        <street>W3C / ERCIM</street>
135        <street>2004, rte des Lucioles</street>
136        <city>Sophia-Antipolis</city>
137        <region>AM</region>
138        <code>06902</code>
139        <country>France</country>
140      </postal>
141      <email>ylafon@w3.org</email>
142      <uri>http://www.raubacapeu.net/people/yves/</uri>
143    </address>
144  </author>
145
146  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
147    <organization abbrev="greenbytes">greenbytes GmbH</organization>
148    <address>
149      <postal>
150        <street>Hafenweg 16</street>
151        <city>Muenster</city><region>NW</region><code>48155</code>
152        <country>Germany</country>
153      </postal>
154      <phone>+49 251 2807760</phone>
155      <facsimile>+49 251 2807761</facsimile>
156      <email>julian.reschke@greenbytes.de</email>
157      <uri>http://greenbytes.de/tech/webdav/</uri>
158    </address>
159  </author>
160
161  <date month="August" year="2010" day="4"/>
162  <workgroup>HTTPbis Working Group</workgroup>
163
164<abstract>
165<t>
166   The Hypertext Transfer Protocol (HTTP) is an application-level
167   protocol for distributed, collaborative, hypermedia information
168   systems. HTTP has been in use by the World Wide Web global information
169   initiative since 1990. This document is Part 4 of the seven-part specification
170   that defines the protocol referred to as "HTTP/1.1" and, taken together,
171   obsoletes RFC 2616.  Part 4 defines request header fields for
172   indicating conditional requests and the rules for constructing responses
173   to those requests.
174</t>
175</abstract>
176
177<note title="Editorial Note (To be removed by RFC Editor)">
178  <t>
179    Discussion of this draft should take place on the HTTPBIS working group
180    mailing list (ietf-http-wg@w3.org). The current issues list is
181    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
182    and related documents (including fancy diffs) can be found at
183    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
184  </t>
185  <t>
186    The changes in this draft are summarized in <xref target="changes.since.10"/>.
187  </t>
188</note>
189</front>
190<middle>
191<section title="Introduction" anchor="introduction">
192<t>
193   This document defines HTTP/1.1 response metadata for indicating potential
194   changes to payload content, including modification time stamps and opaque
195   entity-tags, and the HTTP conditional request mechanisms that allow
196   preconditions to be placed on a request method.  Conditional GET requests
197   allow for efficient cache updates.  Other conditional request methods are
198   used to protect against overwriting or misunderstanding the state of a
199   resource that has been changed unbeknownst to the requesting client.
200</t>
201<t>
202   This document is currently disorganized in order to minimize the changes
203   between drafts and enable reviewers to see the smaller errata changes.
204   The next draft will reorganize the sections to better reflect the content.
205   In particular, the sections on resource metadata will be discussed first
206   and then followed by each conditional request-header, concluding with a
207   definition of precedence and the expectation of ordering strong validator
208   checks before weak validator checks.  It is likely that more content from
209   <xref target="Part6"/> will migrate to this part, where appropriate.
210   The current mess reflects how widely dispersed these topics and associated
211   requirements had become in <xref target="RFC2616"/>.
212</t>
213
214<section title="Requirements" anchor="intro.requirements">
215<t>
216   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
217   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
218   document are to be interpreted as described in <xref target="RFC2119"/>.
219</t>
220<t>
221   An implementation is not compliant if it fails to satisfy one or more
222   of the "MUST" or "REQUIRED" level requirements for the protocols it
223   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
224   level and all the "SHOULD" level requirements for its protocols is said
225   to be "unconditionally compliant"; one that satisfies all the "MUST"
226   level requirements but not all the "SHOULD" level requirements for its
227   protocols is said to be "conditionally compliant".
228</t>
229</section>
230
231<section title="Syntax Notation" anchor="notation">
232 
233 
234 
235 
236 
237 
238 
239<t>
240  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
241  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
242  <xref target="collected.abnf"/> shows the collected ABNF, with the list
243  rule expanded.
244</t>
245<t>
246  The following core rules are included by
247  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
248  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
249  DIGIT (decimal 0-9), DQUOTE (double quote),
250  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
251  OCTET (any 8-bit sequence of data), SP (space),
252  VCHAR (any visible USASCII character),
253  and WSP (whitespace).
254</t>
255
256<section title="Core Rules" anchor="core.rules">
257 
258 
259<t>
260  The core rules below are defined in Section 1.2.2 of <xref target="Part1"/>:
261</t>
262<figure><artwork type="abnf2616"><![CDATA[
263  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
264  OWS           = <OWS, defined in [Part1], Section 1.2.2>
265]]></artwork></figure>
266</section>
267
268<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
269 
270<t>
271  The ABNF rules below are defined in other parts:
272</t>
273<figure><artwork type="abnf2616"><![CDATA[
274  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
275]]></artwork></figure>
276</section>
277
278</section>
279
280</section>
281
282<section title="Entity-Tags" anchor="entity.tags">
283 
284 
285 
286<t>
287   Entity-tags are used for comparing two or more representations of the same
288   resource. HTTP/1.1 uses entity-tags in the ETag (<xref target="header.etag"/>),
289   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
290   If-Range (Section 5.3 of <xref target="Part5"/>) header fields. The definition of how they
291   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
292   entity-tag consists of an opaque quoted string, possibly prefixed by
293   a weakness indicator.
294</t>
295<figure><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/><artwork type="abnf2616"><![CDATA[
296  entity-tag = [ weak ] opaque-tag
297  weak       = %x57.2F ; "W/", case-sensitive
298  opaque-tag = quoted-string
299]]></artwork></figure>
300<t>
301   A "strong entity-tag" MAY be shared by two representations of a resource
302   only if they are equivalent by octet equality.
303</t>
304<t>
305   A "weak entity-tag", indicated by the "W/" prefix, MAY be shared by
306   two representations of a resource only if the representations are equivalent and
307   could be substituted for each other with no significant change in
308   semantics. A weak entity-tag can only be used for weak comparison.
309</t>
310<t>
311   An entity-tag MUST be unique across all versions of all representations
312   associated with a particular resource. A given entity-tag value MAY
313   be used for representations obtained by requests on different URIs. The use
314   of the same entity-tag value in conjunction with representations obtained by
315   requests on different URIs does not imply the equivalence of those
316   representations.
317</t>
318
319<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
320<t>
321   Consider a resource that is subject to content negotiation (Section 5 of <xref target="Part3"/>),
322   and where the representations returned upon a GET request vary based on
323   the Accept-Encoding request header field (Section 6.3 of <xref target="Part3"/>):
324</t>
325<figure><preamble>&gt;&gt; Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
326  GET /index HTTP/1.1
327  Host: www.example.com
328  Accept-Encoding: gzip
329 
330  ]]></artwork></figure>
331<t>
332   In this case, the response might or might not use the gzip content coding.
333   If it does not, the response might look like:
334</t>
335<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
336  HTTP/1.1 200 OK
337  Date: Thu, 26 Mar 2010 00:05:00 GMT
338  ETag: "123-a"
339  Content-Length: 70
340  Vary: Accept-Encoding
341  Content-Type: text/plain
342 
343  Hello World!
344  Hello World!
345  Hello World!
346  Hello World!
347  Hello World!
348  ]]></artwork></figure>
349<t>
350   An alternative representation that does use gzip content coding would be:
351</t>
352<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
353  HTTP/1.1 200 OK
354  Date: Thu, 26 Mar 2010 00:05:00 GMT
355  ETag: "123-b"
356  Content-Length: 43
357  Vary: Accept-Encoding
358  Content-Type: text/plain
359  Content-Encoding: gzip
360 
361  ...binary data...]]></artwork></figure>
362<t><list>
363  <t>
364    Note: Content codings are a property of the representation,
365    so therefore an entity-tag of an encoded representation must be distinct
366    from an unencoded representation to prevent conflicts during cache updates
367    and range requests.  In contrast, transfer codings (Section 6.2 of <xref target="Part1"/>)
368    apply only during message transfer and do not require distinct entity-tags.
369  </t>
370</list></t>
371</section>
372</section>
373
374<section title="Status Code Definitions" anchor="status.code.definitions">
375<section title="304 Not Modified" anchor="status.304">
376  <iref primary="true" item="304 Not Modified (status code)"/>
377  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
378<t>
379   If the client has performed a conditional GET request and access is
380   allowed, but the document has not been modified, the server SHOULD
381   respond with this status code. The 304 response MUST NOT contain a
382   message-body, and thus is always terminated by the first empty line
383   after the header fields.
384</t>
385<t>
386   A 304 response MUST include a Date header field (Section 9.3 of <xref target="Part1"/>)
387   unless its omission is required by Section 9.3.1 of <xref target="Part1"/>.  If a 200 response
388   to the same request would have included any of the header fields
389   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
390   Vary, then those same header fields MUST be sent in a 304 response.
391</t>
392<t>
393   Since the goal of a 304 response is to minimize information transfer
394   when the recipient already has one or more cached representations,
395   the response SHOULD NOT include representation metadata other
396   than the above listed fields unless said metadata exists for the
397   purpose of guiding cache updates (e.g., future HTTP extensions).
398</t>
399<t>
400   If a 304 response includes an entity-tag that indicates a
401   representation not currently cached, then the recipient MUST NOT
402   use the 304 to update its own cache.  If that conditional request originated
403   with an outbound client, such as a user agent with its own cache sending a
404   conditional GET to a shared proxy, then the 304 response MAY be
405   forwarded to the outbound client.  Otherwise, disregard the response
406   and repeat the request without the conditional.
407</t>
408<t>
409   If a cache uses a received 304 response to update a cache entry, the
410   cache MUST update the entry to reflect any new field values given in
411   the response.
412</t>
413</section>
414
415<section title="412 Precondition Failed" anchor="status.412">
416  <iref primary="true" item="412 Precondition Failed (status code)"/>
417  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
418<t>
419   The precondition given in one or more of the request-header fields
420   evaluated to false when it was tested on the server. This response
421   code allows the client to place preconditions on the current resource
422   metadata (header field data) and thus prevent the requested
423   method from being applied to a resource other than the one intended.
424</t>
425</section>
426</section>
427
428<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
429<t>
430   Since both origin servers and caches will compare two validators to
431   decide if they represent the same or different representations, one normally
432   would expect that if the representation (including both representation
433   header fields and representation body) changes in any way, then the
434   associated validator would change as well. If this is true, then we
435   call this validator a "strong validator".
436</t>
437<t>
438   However, there might be cases when a server prefers to change the
439   validator only on semantically significant changes, and not when
440   insignificant aspects of the representation change. A validator that does not
441   always change when the representation changes is a "weak validator".
442</t>
443<t>
444   An entity-tag is normally a strong validator, but the protocol
445   provides a mechanism to tag an entity-tag as "weak". One can think of
446   a strong validator as one that changes whenever the sequence of bits
447   in a representation changes, while a weak value changes whenever the
448   meaning of a representation changes. Alternatively, one can think of
449   a strong validator as part of an identifier for a specific representation,
450   whereas a weak validator is part of an identifier for a set of semantically
451   equivalent representations.
452  <list><t>
453      Note: One example of a strong validator is an integer that is
454      incremented in stable storage every time a representation is changed.
455    </t><t>
456      A representation's modification time, if defined with only one-second
457      resolution, could be a weak validator, since it is possible that
458      the representation might be modified twice during a single second.
459    </t><t>
460      Support for weak validators is optional. However, weak validators
461      allow for more efficient caching of equivalent objects; for
462      example, a hit counter on a site is probably good enough if it is
463      updated every few days or weeks, and any value during that period
464      is likely "good enough" to be equivalent.
465    </t></list>
466</t>
467<t>
468   A "use" of a validator is either when a client generates a request
469   and includes the validator in a validating header field, or when a
470   server compares two validators.
471</t>
472<t>
473   Strong validators are usable in any context. Weak validators are only
474   usable in contexts that do not depend on exact equality of a representation.
475   For example, either kind is usable for a normal conditional GET.
476   However, only a strong validator is usable for a sub-range
477   retrieval, since otherwise the client might end up with an internally
478   inconsistent representation.
479</t>
480<t>
481   Clients MUST NOT use weak validators in range requests (<xref target="Part5"/>).
482</t>
483<t>
484   The only function that HTTP/1.1 defines on validators is
485   comparison. There are two validator comparison functions, depending
486   on whether the comparison context allows the use of weak validators
487   or not:
488  <list style="symbols">
489     <t>The strong comparison function: in order to be considered equal,
490        both opaque-tags MUST be identical character-by-character, and both
491        MUST NOT be weak.</t>
492     <t>The weak comparison function: in order to be considered equal, both
493        opaque-tags MUST be identical character-by-character, but
494        either or both of them MAY be tagged as "weak" without affecting
495        the result.</t>
496  </list>
497</t>
498<t>
499   The example below shows the results for a set of entity-tag pairs,
500   and both the weak and strong comparison function results:
501</t>
502<texttable align="left">
503  <ttcol>ETag 1</ttcol>
504  <ttcol>ETag 2</ttcol>
505  <ttcol>Strong Comparison</ttcol>
506  <ttcol>Weak Comparison</ttcol>
507
508  <c>W/"1"</c>
509  <c>W/"1"</c>
510  <c>no match</c>
511  <c>match</c>
512 
513  <c>W/"1"</c>
514  <c>W/"2"</c>
515  <c>no match</c>
516  <c>no match</c>
517
518  <c>W/"1"</c>
519  <c>"1"</c>
520  <c>no match</c>
521  <c>match</c>
522
523  <c>"1"</c>
524  <c>"1"</c>
525  <c>match</c>
526  <c>match</c>
527</texttable>
528<t>
529   An entity-tag is strong unless it is explicitly tagged as weak.
530   <xref target="entity.tags"/> gives the syntax for entity-tags.
531</t>
532<t>
533   A Last-Modified time, when used as a validator in a request, is
534   implicitly weak unless it is possible to deduce that it is strong,
535   using the following rules:
536  <list style="symbols">
537     <t>The validator is being compared by an origin server to the
538        actual current validator for the representation and,</t>
539     <t>That origin server reliably knows that the associated representation did
540        not change twice during the second covered by the presented
541        validator.</t>
542  </list>
543</t>
544<t>
545   or
546  <list style="symbols">
547     <t>The validator is about to be used by a client in an If-Modified-Since
548        or If-Unmodified-Since header, because the client
549        has a cache entry for the associated representation, and</t>
550     <t>That cache entry includes a Date value, which gives the time
551        when the origin server sent the original response, and</t>
552     <t>The presented Last-Modified time is at least 60 seconds before
553        the Date value.</t>
554  </list>
555</t>
556<t>
557   or
558  <list style="symbols">
559     <t>The validator is being compared by an intermediate cache to the
560        validator stored in its cache entry for the representation, and</t>
561     <t>That cache entry includes a Date value, which gives the time
562        when the origin server sent the original response, and</t>
563     <t>The presented Last-Modified time is at least 60 seconds before
564        the Date value.</t>
565  </list>
566</t>
567<t>
568   This method relies on the fact that if two different responses were
569   sent by the origin server during the same second, but both had the
570   same Last-Modified time, then at least one of those responses would
571   have a Date value equal to its Last-Modified time. The arbitrary 60-second
572   limit guards against the possibility that the Date and Last-Modified
573   values are generated from different clocks, or at somewhat
574   different times during the preparation of the response. An
575   implementation MAY use a value larger than 60 seconds, if it is
576   believed that 60 seconds is too short.
577</t>
578<t>
579   If a client wishes to perform a sub-range retrieval on a value for
580   which it has only a Last-Modified time and no opaque validator, it
581   MAY do this only if the Last-Modified time is strong in the sense
582   described here.
583</t>
584<t>
585   A cache or origin server receiving a conditional range request
586   (<xref target="Part5"/>) MUST use the strong comparison function to
587   evaluate the condition.
588</t>
589<t>
590   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
591   retrievals on values that have been obtained from HTTP/1.0
592   servers.
593</t>
594</section>
595
596<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">
597<t>
598   We adopt a set of rules and recommendations for origin servers,
599   clients, and caches regarding when various validator types ought to
600   be used, and for what purposes.
601</t>
602<t>
603   HTTP/1.1 origin servers:
604  <list style="symbols">
605     <t>SHOULD send an entity-tag validator unless it is not feasible to
606        generate one.</t>
607
608     <t>MAY send a weak entity-tag instead of a strong entity-tag, if
609        performance considerations support the use of weak entity-tags,
610        or if it is unfeasible to send a strong entity-tag.</t>
611
612     <t>SHOULD send a Last-Modified value if it is feasible to send one,
613        unless the risk of a breakdown in semantic transparency that
614        could result from using this date in an If-Modified-Since header
615        would lead to serious problems.</t>
616  </list>
617</t>
618<t>
619   In other words, the preferred behavior for an HTTP/1.1 origin server
620   is to send both a strong entity-tag and a Last-Modified value.
621</t>
622<t>
623   In order to be legal, a strong entity-tag MUST change whenever the
624   associated representation changes in any way. A weak entity-tag SHOULD
625   change whenever the associated representation changes in a semantically
626   significant way.
627</t>
628<t><list>
629  <t>
630    Note: In order to provide semantically transparent caching, an
631    origin server must avoid reusing a specific strong entity-tag
632    value for two different representations, or reusing a specific weak
633    entity-tag value for two semantically different representations. Cache
634    entries might persist for arbitrarily long periods, regardless of
635    expiration times, so it might be inappropriate to expect that a
636    cache will never again attempt to validate an entry using a
637    validator that it obtained at some point in the past.
638  </t>
639</list></t>
640<t>
641   HTTP/1.1 clients:
642  <list style="symbols">
643     <t>MUST use that entity-tag in any cache-conditional request (using
644        If-Match or If-None-Match) if an entity-tag has been provided by the
645        origin server.</t>
646
647     <t>SHOULD use the Last-Modified value in non-subrange cache-conditional
648        requests (using If-Modified-Since) if only a Last-Modified value has
649        been provided by the origin server. </t>
650
651     <t>MAY use the Last-Modified value in subrange cache-conditional
652        requests (using If-Unmodified-Since) if only a Last-Modified value has
653        been provided by an HTTP/1.0 origin server. The user agent SHOULD
654        provide a way to disable this, in case of difficulty.</t>
655
656     <t>SHOULD use both validators in cache-conditional requests if both an
657        entity-tag and a Last-Modified value have been provided by the origin
658        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
659        appropriately.</t>
660  </list>
661</t>
662<t>
663   An HTTP/1.1 origin server, upon receiving a conditional request that
664   includes both a Last-Modified date (e.g., in an If-Modified-Since or
665   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
666   in an If-Match, If-None-Match, or If-Range header field) as cache
667   validators, MUST NOT return a response status code of 304 (Not Modified)
668   unless doing so is consistent with all of the conditional header
669   fields in the request.
670</t>
671<t>
672   An HTTP/1.1 caching proxy, upon receiving a conditional request that
673   includes both a Last-Modified date and one or more entity-tags as
674   cache validators, MUST NOT return a locally cached response to the
675   client unless that cached response is consistent with all of the
676   conditional header fields in the request.
677  <list><t>
678      Note: The general principle behind these rules is that HTTP/1.1
679      servers and clients ought to transmit as much non-redundant
680      information as is available in their responses and requests.
681      HTTP/1.1 systems receiving this information will make the most
682      conservative assumptions about the validators they receive.
683  </t><t>
684      HTTP/1.0 clients and caches will ignore entity-tags. Generally,
685      last-modified values received or used by these systems will
686      support transparent and efficient caching, and so HTTP/1.1 origin
687      servers should provide Last-Modified values. In those rare cases
688      where the use of a Last-Modified value as a validator by an
689      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
690      origin servers should not provide one.
691  </t></list>
692</t>
693</section>
694
695<section title="Header Field Definitions" anchor="header.fields">
696<t>
697   This section defines the syntax and semantics of HTTP/1.1 header fields
698   related to conditional requests.
699</t>
700
701<section title="ETag" anchor="header.etag">
702  <iref primary="true" item="ETag header"/>
703  <iref primary="true" item="Headers" subitem="ETag"/>
704 
705 
706<t>
707   The "ETag" response-header field provides the current value of the
708   entity-tag (see <xref target="entity.tags"/>) for one representation of
709   the target resource.  An entity-tag
710   is intended for use as a resource-local identifier for differentiating
711   between representations of the same resource that vary over time or via
712   content negotiation (see <xref target="weak.and.strong.validators"/>).
713</t>
714<figure><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/><artwork type="abnf2616"><![CDATA[
715  ETag   = "ETag" ":" OWS ETag-v
716  ETag-v = entity-tag
717]]></artwork></figure>
718<figure><preamble>
719  Examples:
720</preamble>
721<artwork type="example"><![CDATA[
722  ETag: "xyzzy"
723  ETag: W/"xyzzy"
724  ETag: ""
725]]></artwork></figure>
726<t>
727   An entity-tag provides an "opaque" cache validator that allows for
728   more reliable validation than modification dates in situations where
729   it is inconvenient to store modification dates,
730   where the one-second resolution of HTTP date values is not
731   sufficient, or where the origin server wishes to avoid certain
732   paradoxes that might arise from the use of modification dates.
733</t>
734<t>
735   The principle behind entity-tags is that only the service author
736   knows the semantics of a resource well enough to select an
737   appropriate cache validation mechanism, and the specification of any
738   validator comparison function more complex than byte-equality would
739   open up a can of worms. Thus, comparisons of any other headers
740   (except Last-Modified, for compatibility with HTTP/1.0) are never
741   used for purposes of validating a cache entry.
742</t>
743</section>
744
745<section title="If-Match" anchor="header.if-match">
746  <iref primary="true" item="If-Match header"/>
747  <iref primary="true" item="Headers" subitem="If-Match"/>
748 
749 
750<t>
751   The "If-Match" request-header field is used to make a request method
752   conditional. A client that has one or more representations previously
753   obtained from the resource can verify that one of those representations is
754   current by including a list of their associated entity-tags in the
755   If-Match header field.
756</t>
757<t>
758   This allows efficient updates of cached information with a minimum amount of
759   transaction overhead. It is also used when updating resources, to prevent
760   inadvertent modification of the wrong version of a resource. As a special
761   case, the value "*" matches any current representation of the resource.
762</t>
763<figure><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/><artwork type="abnf2616"><![CDATA[
764  If-Match   = "If-Match" ":" OWS If-Match-v
765  If-Match-v = "*" / 1#entity-tag
766]]></artwork></figure>
767<t>
768   If any of the entity-tags match the entity-tag of the representation that
769   would have been returned in the response to a similar GET request
770   (without the If-Match header) on that resource, or if "*" is given
771   and any current representation exists for that resource, then the server MAY
772   perform the requested method as if the If-Match header field did not
773   exist.
774</t>
775<t>
776   If none of the entity-tags match, or if "*" is given and no current
777   representation exists, the server MUST NOT perform the requested method, and
778   MUST return a 412 (Precondition Failed) response. This behavior is
779   most useful when the client wants to prevent an updating method, such
780   as PUT, from modifying a resource that has changed since the client
781   last retrieved it.
782</t>
783<t>
784   If the request would, without the If-Match header field, result in
785   anything other than a 2xx or 412 status code, then the If-Match header
786   MUST be ignored.
787</t>
788<t>
789   The meaning of "If-Match: *" is that the method SHOULD be performed
790   if the representation selected by the origin server (or by a cache,
791   possibly using the Vary mechanism, see Section 3.5 of <xref target="Part6"/>) exists, and
792   MUST NOT be performed if the representation does not exist.
793</t>
794<t>
795   A request intended to update a resource (e.g., a PUT) MAY include an
796   If-Match header field to signal that the request method MUST NOT be
797   applied if the representation corresponding to the If-Match value (a single
798   entity-tag) is no longer a representation of that resource. This
799   allows the user to indicate that they do not wish the request to be
800   successful if the resource has been changed without their knowledge.
801   Examples:
802</t>
803<figure><artwork type="example"><![CDATA[
804  If-Match: "xyzzy"
805  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
806  If-Match: *
807]]></artwork></figure>
808<t>
809   The result of a request having both an If-Match header field and
810   either an If-None-Match or an If-Modified-Since header fields is
811   undefined by this specification.
812</t>
813</section>
814
815<section title="If-Modified-Since" anchor="header.if-modified-since">
816  <iref primary="true" item="If-Modified-Since header"/>
817  <iref primary="true" item="Headers" subitem="If-Modified-Since"/>
818 
819 
820<t>
821   The "If-Modified-Since" request-header field is used to make a request
822   method conditional by date: if the representation that would have been
823   transferred in a 200 response to a GET request has not been modified since
824   the time specified in this field, then do not perform the method;
825   instead, respond as detailed below.
826</t>
827<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/><artwork type="abnf2616"><![CDATA[
828  If-Modified-Since   = "If-Modified-Since" ":" OWS
829                        If-Modified-Since-v
830  If-Modified-Since-v = HTTP-date
831]]></artwork></figure>
832<t>
833   An example of the field is:
834</t>
835<figure><artwork type="example"><![CDATA[
836  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
837]]></artwork></figure>
838<t>
839   A GET method with an If-Modified-Since header and no Range header
840   requests that the representation be transferred only if it has
841   been modified since the date given by the If-Modified-Since header.
842   The algorithm for determining this includes the following cases:
843  <list style="numbers">
844      <t>If the request would normally result in anything other than a
845         200 (OK) status code, or if the passed If-Modified-Since date is
846         invalid, the response is exactly the same as for a normal GET.
847         A date which is later than the server's current time is
848         invalid.</t>
849
850      <t>If the representation has been modified since the If-Modified-Since
851         date, the response is exactly the same as for a normal GET.</t>
852
853      <t>If the representation has not been modified since a valid
854         If-Modified-Since date, the server SHOULD return a
855         304 (Not Modified) response.</t>
856  </list>
857</t>
858<t>
859   The purpose of this feature is to allow efficient updates of cached
860   information with a minimum amount of transaction overhead.
861  <list><t>
862      Note: The Range request-header field modifies the meaning of If-Modified-Since;
863      see Section 5.4 of <xref target="Part5"/> for full details.
864    </t><t>
865      Note: If-Modified-Since times are interpreted by the server, whose
866      clock might not be synchronized with the client.
867    </t><t>
868      Note: When handling an If-Modified-Since header field, some
869      servers will use an exact date comparison function, rather than a
870      less-than function, for deciding whether to send a 304 (Not
871      Modified) response. To get best results when sending an If-Modified-Since
872      header field for cache validation, clients are
873      advised to use the exact date string received in a previous Last-Modified
874      header field whenever possible.
875    </t><t>
876      Note: If a client uses an arbitrary date in the If-Modified-Since
877      header instead of a date taken from the Last-Modified header for
878      the same request, the client needs to be aware that this
879      date is interpreted in the server's understanding of time.
880      Unsynchronized clocks and rounding problems, due to the different
881      encodings of time between the client and server, are concerns.
882      This includes the possibility of race conditions if the
883      document has changed between the time it was first requested and
884      the If-Modified-Since date of a subsequent request, and the
885      possibility of clock-skew-related problems if the If-Modified-Since
886      date is derived from the client's clock without correction
887      to the server's clock. Corrections for different time bases
888      between client and server are at best approximate due to network
889      latency.
890    </t>
891  </list>
892</t>
893<t>
894   The result of a request having both an If-Modified-Since header field
895   and either an If-Match or an If-Unmodified-Since header fields is
896   undefined by this specification.
897</t>
898</section>
899
900<section title="If-None-Match" anchor="header.if-none-match">
901  <iref primary="true" item="If-None-Match header"/>
902  <iref primary="true" item="Headers" subitem="If-None-Match"/>
903 
904 
905<t>
906   The "If-None-Match" request-header field is used to make a request method
907   conditional. A client that has one or more representations previously
908   obtained from the resource can verify that none of those representations is
909   current by including a list of their associated entity-tags in the
910   If-None-Match header field.
911</t>
912<t>
913   This allows efficient updates of cached information with a minimum amount of
914   transaction overhead. It is also used to prevent a method (e.g., PUT)
915   from inadvertently modifying an existing resource when the client
916   believes that the resource does not exist.
917</t>
918<t>
919   As a special case, the value "*" matches any current representation of the
920   resource.
921</t>
922<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/><artwork type="abnf2616"><![CDATA[
923  If-None-Match   = "If-None-Match" ":" OWS If-None-Match-v
924  If-None-Match-v = "*" / 1#entity-tag
925]]></artwork></figure>
926<t>
927   If any of the entity-tags match the entity-tag of the representation that
928   would have been returned in the response to a similar GET request
929   (without the If-None-Match header) on that resource, or if "*" is
930   given and any current representation exists for that resource, then the
931   server MUST NOT perform the requested method, unless required to do
932   so because the resource's modification date fails to match that
933   supplied in an If-Modified-Since header field in the request.
934   Instead, if the request method was GET or HEAD, the server SHOULD
935   respond with a 304 (Not Modified) response, including the cache-related
936   header fields (particularly ETag) of one of the representations that
937   matched. For all other request methods, the server MUST respond with
938   a 412 (Precondition Failed) status code.
939</t>
940<t>
941   If none of the entity-tags match, then the server MAY perform the
942   requested method as if the If-None-Match header field did not exist,
943   but MUST also ignore any If-Modified-Since header field(s) in the
944   request. That is, if no entity-tags match, then the server MUST NOT
945   return a 304 (Not Modified) response.
946</t>
947<t>
948   If the request would, without the If-None-Match header field, result
949   in anything other than a 2xx or 304 status code, then the If-None-Match
950   header MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
951   server behavior when both If-Modified-Since and If-None-Match appear
952   in the same request.)
953</t>
954<t>
955   The meaning of "If-None-Match: *" is that the method MUST NOT be
956   performed if the representation selected by the origin server (or by
957   a cache, possibly using the Vary mechanism, see Section 3.5 of <xref target="Part6"/>)
958   exists, and SHOULD be performed if the representation does not exist.
959   This feature is intended to be useful in preventing races between PUT
960   operations.
961</t>
962<t>
963   Examples:
964</t>
965<figure><artwork type="example"><![CDATA[
966  If-None-Match: "xyzzy"
967  If-None-Match: W/"xyzzy"
968  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
969  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
970  If-None-Match: *
971]]></artwork></figure>
972<t>
973   The result of a request having both an If-None-Match header field and
974   either an If-Match or an If-Unmodified-Since header fields is
975   undefined by this specification.
976</t>
977</section>
978
979<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
980  <iref primary="true" item="If-Unmodified-Since header"/>
981  <iref primary="true" item="Headers" subitem="If-Unmodified-Since"/>
982 
983 
984<t>
985   The "If-Unmodified-Since" request-header field is used to make a request
986   method conditional.  If the representation that would have been transferred
987   in a 200 response to a GET request on the same resource has not been modified
988   since the time specified in this field, the server SHOULD perform the
989   requested operation as if the If-Unmodified-Since header were not
990   present.
991</t>
992<t>
993   If the representation has been modified since the specified time,
994   the server MUST NOT perform the requested operation, and MUST return
995   a 412 (Precondition Failed).
996</t>
997<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/><artwork type="abnf2616"><![CDATA[
998  If-Unmodified-Since   = "If-Unmodified-Since" ":" OWS
999                          If-Unmodified-Since-v
1000  If-Unmodified-Since-v = HTTP-date
1001]]></artwork></figure>
1002<t>
1003   An example of the field is:
1004</t>
1005<figure><artwork type="example"><![CDATA[
1006  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1007]]></artwork></figure>
1008<t>
1009   If the request normally (i.e., without the If-Unmodified-Since
1010   header) would result in anything other than a 2xx or 412 status code,
1011   the If-Unmodified-Since header SHOULD be ignored.
1012</t>
1013<t>
1014   If the specified date is invalid, the header is ignored.
1015</t>
1016<t>
1017   The result of a request having both an If-Unmodified-Since header
1018   field and either an If-None-Match or an If-Modified-Since header
1019   fields is undefined by this specification.
1020</t>
1021</section>
1022
1023<section title="Last-Modified" anchor="header.last-modified">
1024  <iref primary="true" item="Last-Modified header"/>
1025  <iref primary="true" item="Headers" subitem="Last-Modified"/>
1026 
1027 
1028<t>
1029   The "Last-Modified" header field indicates the date and time at
1030   which the origin server believes the representation was last modified.
1031</t>
1032<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/><artwork type="abnf2616"><![CDATA[
1033  Last-Modified   = "Last-Modified" ":" OWS Last-Modified-v
1034  Last-Modified-v = HTTP-date
1035]]></artwork></figure>
1036<t>
1037   An example of its use is
1038</t>
1039<figure><artwork type="example"><![CDATA[
1040  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1041]]></artwork></figure>
1042<t>
1043   The exact meaning of this header field depends on the implementation
1044   of the origin server and the nature of the original resource. For
1045   files, it might be just the file system last-modified time. For
1046   representations with dynamically included parts, it might be the most recent
1047   of the set of last-modify times for its component parts. For database
1048   gateways, it might be the last-update time stamp of the record. For
1049   virtual objects, it might be the last time the internal state changed.
1050</t>
1051<t>
1052   An origin server MUST NOT send a Last-Modified date which is later
1053   than the server's time of message origination. In such cases, where
1054   the resource's last modification would indicate some time in the
1055   future, the server MUST replace that date with the message
1056   origination date.
1057</t>
1058<t>
1059   An origin server SHOULD obtain the Last-Modified value of the representation
1060   as close as possible to the time that it generates the Date value of
1061   its response. This allows a recipient to make an accurate assessment
1062   of the representation's modification time, especially if the representation changes
1063   near the time that the response is generated.
1064</t>
1065<t>
1066   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
1067</t>
1068<t>
1069   The Last-Modified header field value is often used as a cache
1070   validator. In simple terms, a cache entry is considered to be valid
1071   if the representation has not been modified since the Last-Modified value.
1072</t>
1073</section>
1074
1075</section>
1076
1077<section title="IANA Considerations" anchor="IANA.considerations">
1078
1079<section title="Status Code Registration" anchor="status.code.registration">
1080<t>
1081   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1082   shall be updated with the registrations below:
1083</t>
1084
1085<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1086<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1087   <ttcol>Value</ttcol>
1088   <ttcol>Description</ttcol>
1089   <ttcol>Reference</ttcol>
1090   <c>304</c>
1091   <c>Not Modified</c>
1092   <c>
1093      <xref target="status.304"/>
1094   </c>
1095   <c>412</c>
1096   <c>Precondition Failed</c>
1097   <c>
1098      <xref target="status.412"/>
1099   </c>
1100</texttable>
1101<!--(END)-->
1102
1103</section>
1104
1105<section title="Header Field Registration" anchor="header.field.registration">
1106<t>
1107   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1108   with the permanent registrations below (see <xref target="RFC3864"/>):
1109</t>
1110
1111<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1112<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1113   <ttcol>Header Field Name</ttcol>
1114   <ttcol>Protocol</ttcol>
1115   <ttcol>Status</ttcol>
1116   <ttcol>Reference</ttcol>
1117
1118   <c>ETag</c>
1119   <c>http</c>
1120   <c>standard</c>
1121   <c>
1122      <xref target="header.etag"/>
1123   </c>
1124   <c>If-Match</c>
1125   <c>http</c>
1126   <c>standard</c>
1127   <c>
1128      <xref target="header.if-match"/>
1129   </c>
1130   <c>If-Modified-Since</c>
1131   <c>http</c>
1132   <c>standard</c>
1133   <c>
1134      <xref target="header.if-modified-since"/>
1135   </c>
1136   <c>If-None-Match</c>
1137   <c>http</c>
1138   <c>standard</c>
1139   <c>
1140      <xref target="header.if-none-match"/>
1141   </c>
1142   <c>If-Unmodified-Since</c>
1143   <c>http</c>
1144   <c>standard</c>
1145   <c>
1146      <xref target="header.if-unmodified-since"/>
1147   </c>
1148   <c>Last-Modified</c>
1149   <c>http</c>
1150   <c>standard</c>
1151   <c>
1152      <xref target="header.last-modified"/>
1153   </c>
1154</texttable>
1155<!--(END)-->
1156
1157<t>
1158   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1159</t>
1160</section>
1161</section>
1162
1163<section title="Security Considerations" anchor="security.considerations">
1164<t>
1165   No additional security considerations have been identified beyond
1166   those applicable to HTTP in general <xref target="Part1"/>.
1167</t>
1168</section>
1169
1170<section title="Acknowledgments" anchor="ack">
1171</section>
1172</middle>
1173<back>
1174
1175<references title="Normative References">
1176
1177<reference anchor="Part1">
1178  <front>
1179    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1180    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1181      <organization abbrev="Day Software">Day Software</organization>
1182      <address><email>fielding@gbiv.com</email></address>
1183    </author>
1184    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1185      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1186      <address><email>jg@freedesktop.org</email></address>
1187    </author>
1188    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1189      <organization abbrev="HP">Hewlett-Packard Company</organization>
1190      <address><email>JeffMogul@acm.org</email></address>
1191    </author>
1192    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1193      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1194      <address><email>henrikn@microsoft.com</email></address>
1195    </author>
1196    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1197      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1198      <address><email>LMM@acm.org</email></address>
1199    </author>
1200    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1201      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1202      <address><email>paulle@microsoft.com</email></address>
1203    </author>
1204    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1205      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1206      <address><email>timbl@w3.org</email></address>
1207    </author>
1208    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1209      <organization abbrev="W3C">World Wide Web Consortium</organization>
1210      <address><email>ylafon@w3.org</email></address>
1211    </author>
1212    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1213      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1214      <address><email>julian.reschke@greenbytes.de</email></address>
1215    </author>
1216    <date month="August" year="2010"/>
1217  </front>
1218  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-11"/>
1219 
1220</reference>
1221
1222<reference anchor="Part3">
1223  <front>
1224    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1225    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1226      <organization abbrev="Day Software">Day Software</organization>
1227      <address><email>fielding@gbiv.com</email></address>
1228    </author>
1229    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1230      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1231      <address><email>jg@freedesktop.org</email></address>
1232    </author>
1233    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1234      <organization abbrev="HP">Hewlett-Packard Company</organization>
1235      <address><email>JeffMogul@acm.org</email></address>
1236    </author>
1237    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1238      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1239      <address><email>henrikn@microsoft.com</email></address>
1240    </author>
1241    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1242      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1243      <address><email>LMM@acm.org</email></address>
1244    </author>
1245    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1246      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1247      <address><email>paulle@microsoft.com</email></address>
1248    </author>
1249    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1250      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1251      <address><email>timbl@w3.org</email></address>
1252    </author>
1253    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1254      <organization abbrev="W3C">World Wide Web Consortium</organization>
1255      <address><email>ylafon@w3.org</email></address>
1256    </author>
1257    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1258      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1259      <address><email>julian.reschke@greenbytes.de</email></address>
1260    </author>
1261    <date month="August" year="2010"/>
1262  </front>
1263  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-11"/>
1264 
1265</reference>
1266
1267<reference anchor="Part5">
1268  <front>
1269    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1270    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1271      <organization abbrev="Day Software">Day Software</organization>
1272      <address><email>fielding@gbiv.com</email></address>
1273    </author>
1274    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1275      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1276      <address><email>jg@freedesktop.org</email></address>
1277    </author>
1278    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1279      <organization abbrev="HP">Hewlett-Packard Company</organization>
1280      <address><email>JeffMogul@acm.org</email></address>
1281    </author>
1282    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1283      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1284      <address><email>henrikn@microsoft.com</email></address>
1285    </author>
1286    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1287      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1288      <address><email>LMM@acm.org</email></address>
1289    </author>
1290    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1291      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1292      <address><email>paulle@microsoft.com</email></address>
1293    </author>
1294    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1295      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1296      <address><email>timbl@w3.org</email></address>
1297    </author>
1298    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1299      <organization abbrev="W3C">World Wide Web Consortium</organization>
1300      <address><email>ylafon@w3.org</email></address>
1301    </author>
1302    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1303      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1304      <address><email>julian.reschke@greenbytes.de</email></address>
1305    </author>
1306    <date month="August" year="2010"/>
1307  </front>
1308  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-11"/>
1309 
1310</reference>
1311
1312<reference anchor="Part6">
1313  <front>
1314    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1315    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1316      <organization abbrev="Day Software">Day Software</organization>
1317      <address><email>fielding@gbiv.com</email></address>
1318    </author>
1319    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1320      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1321      <address><email>jg@freedesktop.org</email></address>
1322    </author>
1323    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1324      <organization abbrev="HP">Hewlett-Packard Company</organization>
1325      <address><email>JeffMogul@acm.org</email></address>
1326    </author>
1327    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1328      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1329      <address><email>henrikn@microsoft.com</email></address>
1330    </author>
1331    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1332      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1333      <address><email>LMM@acm.org</email></address>
1334    </author>
1335    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1336      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1337      <address><email>paulle@microsoft.com</email></address>
1338    </author>
1339    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1340      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1341      <address><email>timbl@w3.org</email></address>
1342    </author>
1343    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1344      <organization abbrev="W3C">World Wide Web Consortium</organization>
1345      <address><email>ylafon@w3.org</email></address>
1346    </author>
1347    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1348      <address><email>mnot@mnot.net</email></address>
1349    </author>
1350    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1351      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1352      <address><email>julian.reschke@greenbytes.de</email></address>
1353    </author>
1354    <date month="August" year="2010"/>
1355  </front>
1356  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-11"/>
1357 
1358</reference>
1359
1360<reference anchor="RFC2119">
1361  <front>
1362    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1363    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1364      <organization>Harvard University</organization>
1365      <address><email>sob@harvard.edu</email></address>
1366    </author>
1367    <date month="March" year="1997"/>
1368  </front>
1369  <seriesInfo name="BCP" value="14"/>
1370  <seriesInfo name="RFC" value="2119"/>
1371</reference>
1372
1373<reference anchor="RFC5234">
1374  <front>
1375    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1376    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1377      <organization>Brandenburg InternetWorking</organization>
1378      <address>
1379        <email>dcrocker@bbiw.net</email>
1380      </address> 
1381    </author>
1382    <author initials="P." surname="Overell" fullname="Paul Overell">
1383      <organization>THUS plc.</organization>
1384      <address>
1385        <email>paul.overell@thus.net</email>
1386      </address>
1387    </author>
1388    <date month="January" year="2008"/>
1389  </front>
1390  <seriesInfo name="STD" value="68"/>
1391  <seriesInfo name="RFC" value="5234"/>
1392</reference>
1393
1394</references>
1395
1396<references title="Informative References">
1397
1398<reference anchor="RFC2616">
1399  <front>
1400    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1401    <author initials="R." surname="Fielding" fullname="R. Fielding">
1402      <organization>University of California, Irvine</organization>
1403      <address><email>fielding@ics.uci.edu</email></address>
1404    </author>
1405    <author initials="J." surname="Gettys" fullname="J. Gettys">
1406      <organization>W3C</organization>
1407      <address><email>jg@w3.org</email></address>
1408    </author>
1409    <author initials="J." surname="Mogul" fullname="J. Mogul">
1410      <organization>Compaq Computer Corporation</organization>
1411      <address><email>mogul@wrl.dec.com</email></address>
1412    </author>
1413    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1414      <organization>MIT Laboratory for Computer Science</organization>
1415      <address><email>frystyk@w3.org</email></address>
1416    </author>
1417    <author initials="L." surname="Masinter" fullname="L. Masinter">
1418      <organization>Xerox Corporation</organization>
1419      <address><email>masinter@parc.xerox.com</email></address>
1420    </author>
1421    <author initials="P." surname="Leach" fullname="P. Leach">
1422      <organization>Microsoft Corporation</organization>
1423      <address><email>paulle@microsoft.com</email></address>
1424    </author>
1425    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1426      <organization>W3C</organization>
1427      <address><email>timbl@w3.org</email></address>
1428    </author>
1429    <date month="June" year="1999"/>
1430  </front>
1431  <seriesInfo name="RFC" value="2616"/>
1432</reference>
1433
1434<reference anchor="RFC3864">
1435  <front>
1436    <title>Registration Procedures for Message Header Fields</title>
1437    <author initials="G." surname="Klyne" fullname="G. Klyne">
1438      <organization>Nine by Nine</organization>
1439      <address><email>GK-IETF@ninebynine.org</email></address>
1440    </author>
1441    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
1442      <organization>BEA Systems</organization>
1443      <address><email>mnot@pobox.com</email></address>
1444    </author>
1445    <author initials="J." surname="Mogul" fullname="J. Mogul">
1446      <organization>HP Labs</organization>
1447      <address><email>JeffMogul@acm.org</email></address>
1448    </author>
1449    <date year="2004" month="September"/>
1450  </front>
1451  <seriesInfo name="BCP" value="90"/>
1452  <seriesInfo name="RFC" value="3864"/>
1453</reference>
1454
1455</references>
1456
1457<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1458<t>
1459  Allow weak entity-tags in all requests except range requests (Sections
1460  <xref target="weak.and.strong.validators" format="counter"/> and
1461  <xref target="header.if-none-match" format="counter"/>).
1462</t>
1463</section>
1464
1465
1466<section title="Collected ABNF" anchor="collected.abnf">
1467<figure>
1468<artwork type="abnf" name="p4-conditional.parsed-abnf"><![CDATA[
1469ETag = "ETag:" OWS ETag-v
1470ETag-v = entity-tag
1471
1472HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1473
1474If-Match = "If-Match:" OWS If-Match-v
1475If-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1476 entity-tag ] ) )
1477If-Modified-Since = "If-Modified-Since:" OWS If-Modified-Since-v
1478If-Modified-Since-v = HTTP-date
1479If-None-Match = "If-None-Match:" OWS If-None-Match-v
1480If-None-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1481 entity-tag ] ) )
1482If-Unmodified-Since = "If-Unmodified-Since:" OWS
1483 If-Unmodified-Since-v
1484If-Unmodified-Since-v = HTTP-date
1485
1486Last-Modified = "Last-Modified:" OWS Last-Modified-v
1487Last-Modified-v = HTTP-date
1488
1489OWS = <OWS, defined in [Part1], Section 1.2.2>
1490
1491entity-tag = [ weak ] opaque-tag
1492
1493opaque-tag = quoted-string
1494
1495quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1496
1497weak = %x57.2F ; W/
1498]]></artwork>
1499</figure>
1500<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
1501; ETag defined but not used
1502; If-Match defined but not used
1503; If-Modified-Since defined but not used
1504; If-None-Match defined but not used
1505; If-Unmodified-Since defined but not used
1506; Last-Modified defined but not used
1507]]></artwork></figure></section>
1508
1509
1510<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1511
1512<section title="Since RFC2616">
1513<t>
1514  Extracted relevant partitions from <xref target="RFC2616"/>.
1515</t>
1516</section>
1517
1518<section title="Since draft-ietf-httpbis-p4-conditional-00">
1519<t>
1520  Closed issues:
1521  <list style="symbols"> 
1522    <t>
1523      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1524      "Normative and Informative references"
1525    </t>
1526  </list>
1527</t>
1528<t>
1529  Other changes:
1530  <list style="symbols"> 
1531    <t>
1532      Move definitions of 304 and 412 condition codes from Part2.
1533    </t>
1534  </list>
1535</t>
1536</section>
1537
1538<section title="Since draft-ietf-httpbis-p4-conditional-01">
1539<t>
1540  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1541  <list style="symbols"> 
1542    <t>
1543      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1544    </t>
1545  </list>
1546</t>
1547</section>
1548
1549<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1550<t>
1551  Closed issues:
1552  <list style="symbols"> 
1553    <t>
1554      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1555      "Weak ETags on non-GET requests"
1556    </t>
1557  </list>
1558</t>
1559<t>
1560  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1561  <list style="symbols"> 
1562    <t>
1563      Reference RFC 3984, and update header registrations for headers defined
1564      in this document.
1565    </t>
1566  </list>
1567</t>
1568</section>
1569
1570<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1571<t>
1572  Closed issues:
1573  <list style="symbols"> 
1574    <t>
1575      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1576      "Examples for ETag matching"
1577    </t>
1578    <t>
1579      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1580      "'entity value' undefined"
1581    </t>
1582    <t>
1583      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1584      "bogus 2068 Date header reference"
1585    </t>
1586  </list>
1587</t>
1588</section>
1589
1590<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1591<t>
1592  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1593  <list style="symbols"> 
1594    <t>
1595      Use "/" instead of "|" for alternatives.
1596    </t>
1597    <t>
1598      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1599      whitespace ("OWS") and required whitespace ("RWS").
1600    </t>
1601    <t>
1602      Rewrite ABNFs to spell out whitespace rules, factor out
1603      header value format definitions.
1604    </t>
1605  </list>
1606</t>
1607</section>
1608
1609<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1610<t>
1611  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1612  <list style="symbols"> 
1613    <t>
1614      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1615    </t>
1616  </list>
1617</t>
1618</section>
1619
1620<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1621<t>
1622  Closed issues:
1623  <list style="symbols"> 
1624    <t>
1625      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1626      "case-sensitivity of etag weakness indicator"
1627    </t>
1628  </list>
1629</t>
1630</section>
1631
1632<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1633<t>
1634  Closed issues:
1635  <list style="symbols"> 
1636    <t>
1637      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1638      "Weak ETags on non-GET requests" (If-Match still was defined to require
1639      strong matching)
1640    </t>
1641    <t>
1642      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1643      "move IANA registrations for optional status codes"
1644    </t>
1645  </list>
1646</t>
1647</section>
1648
1649<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1650<t>
1651  No significant changes.
1652</t>
1653</section>
1654
1655<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1656<t>
1657  No significant changes.
1658</t>
1659</section>
1660
1661<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1662<t>
1663  Closed issues:
1664  <list style="symbols"> 
1665    <t>
1666      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1667      "Clarify 'Requested Variant'"
1668    </t>
1669    <t>
1670      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1671      "Clarify entity / representation / variant terminology"
1672    </t>
1673    <t>
1674      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1675      "consider removing the 'changes from 2068' sections"
1676    </t>
1677  </list>
1678</t>
1679</section>
1680
1681</section>
1682
1683</back>
1684</rfc>
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