source: draft-ietf-httpbis/latest/p4-conditional.xml @ 860

Last change on this file since 860 was 860, checked in by fielding@…, 9 years ago

Addresses #69: Clarify "Requested Variant"

Eliminated that phrase.

Addresses #109: Clarify entity / representation / variant terminology

Replaced variant with representation.
Cleaned up p4 description of 304 and 206 status code

  • Property svn:eol-style set to native
File size: 69.2 KB
Line 
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "July">
16  <!ENTITY ID-YEAR "2010">
17  <!ENTITY notation                   "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY header-date                "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-accept-encoding     "<xref target='Part3' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY full-date                  "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY content-negotiation        "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31]>
32<?rfc toc="yes" ?>
33<?rfc symrefs="yes" ?>
34<?rfc sortrefs="yes" ?>
35<?rfc compact="yes"?>
36<?rfc subcompact="no" ?>
37<?rfc linkmailto="no" ?>
38<?rfc editing="no" ?>
39<?rfc comments="yes"?>
40<?rfc inline="yes"?>
41<?rfc rfcedstyle="yes"?>
42<?rfc-ext allow-markup-in-artwork="yes" ?>
43<?rfc-ext include-references-in-index="yes" ?>
44<rfc obsoletes="2616" category="std" x:maturity-level="draft"
45     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
46     xmlns:x='http://purl.org/net/xml2rfc/ext'>
47<front>
48
49  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
50
51  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
52    <organization abbrev="Day Software">Day Software</organization>
53    <address>
54      <postal>
55        <street>23 Corporate Plaza DR, Suite 280</street>
56        <city>Newport Beach</city>
57        <region>CA</region>
58        <code>92660</code>
59        <country>USA</country>
60      </postal>
61      <phone>+1-949-706-5300</phone>
62      <facsimile>+1-949-706-5305</facsimile>
63      <email>fielding@gbiv.com</email>
64      <uri>http://roy.gbiv.com/</uri>
65    </address>
66  </author>
67
68  <author initials="J." surname="Gettys" fullname="Jim Gettys">
69    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
70    <address>
71      <postal>
72        <street>21 Oak Knoll Road</street>
73        <city>Carlisle</city>
74        <region>MA</region>
75        <code>01741</code>
76        <country>USA</country>
77      </postal>
78      <email>jg@freedesktop.org</email>
79      <uri>http://gettys.wordpress.com/</uri>
80    </address>
81  </author>
82 
83  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
84    <organization abbrev="HP">Hewlett-Packard Company</organization>
85    <address>
86      <postal>
87        <street>HP Labs, Large Scale Systems Group</street>
88        <street>1501 Page Mill Road, MS 1177</street>
89        <city>Palo Alto</city>
90        <region>CA</region>
91        <code>94304</code>
92        <country>USA</country>
93      </postal>
94      <email>JeffMogul@acm.org</email>
95    </address>
96  </author>
97
98  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
100    <address>
101      <postal>
102        <street>1 Microsoft Way</street>
103        <city>Redmond</city>
104        <region>WA</region>
105        <code>98052</code>
106        <country>USA</country>
107      </postal>
108      <email>henrikn@microsoft.com</email>
109    </address>
110  </author>
111
112  <author initials="L." surname="Masinter" fullname="Larry Masinter">
113    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
114    <address>
115      <postal>
116        <street>345 Park Ave</street>
117        <city>San Jose</city>
118        <region>CA</region>
119        <code>95110</code>
120        <country>USA</country>
121      </postal>
122      <email>LMM@acm.org</email>
123      <uri>http://larry.masinter.net/</uri>
124    </address>
125  </author>
126 
127  <author initials="P." surname="Leach" fullname="Paul J. Leach">
128    <organization abbrev="Microsoft">Microsoft Corporation</organization>
129    <address>
130      <postal>
131        <street>1 Microsoft Way</street>
132        <city>Redmond</city>
133        <region>WA</region>
134        <code>98052</code>
135      </postal>
136      <email>paulle@microsoft.com</email>
137    </address>
138  </author>
139   
140  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
141    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
142    <address>
143      <postal>
144        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
145        <street>The Stata Center, Building 32</street>
146        <street>32 Vassar Street</street>
147        <city>Cambridge</city>
148        <region>MA</region>
149        <code>02139</code>
150        <country>USA</country>
151      </postal>
152      <email>timbl@w3.org</email>
153      <uri>http://www.w3.org/People/Berners-Lee/</uri>
154    </address>
155  </author>
156
157  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
158    <organization abbrev="W3C">World Wide Web Consortium</organization>
159    <address>
160      <postal>
161        <street>W3C / ERCIM</street>
162        <street>2004, rte des Lucioles</street>
163        <city>Sophia-Antipolis</city>
164        <region>AM</region>
165        <code>06902</code>
166        <country>France</country>
167      </postal>
168      <email>ylafon@w3.org</email>
169      <uri>http://www.raubacapeu.net/people/yves/</uri>
170    </address>
171  </author>
172
173  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
174    <organization abbrev="greenbytes">greenbytes GmbH</organization>
175    <address>
176      <postal>
177        <street>Hafenweg 16</street>
178        <city>Muenster</city><region>NW</region><code>48155</code>
179        <country>Germany</country>
180      </postal>
181      <phone>+49 251 2807760</phone>
182      <facsimile>+49 251 2807761</facsimile>
183      <email>julian.reschke@greenbytes.de</email>
184      <uri>http://greenbytes.de/tech/webdav/</uri>
185    </address>
186  </author>
187
188  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
189  <workgroup>HTTPbis Working Group</workgroup>
190
191<abstract>
192<t>
193   The Hypertext Transfer Protocol (HTTP) is an application-level
194   protocol for distributed, collaborative, hypermedia information
195   systems. HTTP has been in use by the World Wide Web global information
196   initiative since 1990. This document is Part 4 of the seven-part specification
197   that defines the protocol referred to as "HTTP/1.1" and, taken together,
198   obsoletes RFC 2616.  Part 4 defines request header fields for
199   indicating conditional requests and the rules for constructing responses
200   to those requests.
201</t>
202</abstract>
203
204<note title="Editorial Note (To be removed by RFC Editor)">
205  <t>
206    Discussion of this draft should take place on the HTTPBIS working group
207    mailing list (ietf-http-wg@w3.org). The current issues list is
208    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
209    and related documents (including fancy diffs) can be found at
210    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
211  </t>
212  <t>
213    The changes in this draft are summarized in <xref target="changes.since.10"/>.
214  </t>
215</note>
216</front>
217<middle>
218<section title="Introduction" anchor="introduction">
219<t>
220   This document defines HTTP/1.1 response metadata for indicating potential
221   changes to payload content, including modification time stamps and opaque
222   entity-tags, and the HTTP conditional request mechanisms that allow
223   preconditions to be placed on a request method.  Conditional GET requests
224   allow for efficient cache updates.  Other conditional request methods are
225   used to protect against overwriting or misunderstanding the state of a
226   resource that has been changed unbeknownst to the requesting client.
227</t>
228<t>
229   This document is currently disorganized in order to minimize the changes
230   between drafts and enable reviewers to see the smaller errata changes.
231   The next draft will reorganize the sections to better reflect the content.
232   In particular, the sections on resource metadata will be discussed first
233   and then followed by each conditional request-header, concluding with a
234   definition of precedence and the expectation of ordering strong validator
235   checks before weak validator checks.  It is likely that more content from
236   &caching; will migrate to this part, where appropriate.
237   The current mess reflects how widely dispersed these topics and associated
238   requirements had become in <xref target="RFC2616"/>.
239</t>
240
241<section title="Requirements" anchor="intro.requirements">
242<t>
243   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
244   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
245   document are to be interpreted as described in <xref target="RFC2119"/>.
246</t>
247<t>
248   An implementation is not compliant if it fails to satisfy one or more
249   of the "MUST" or "REQUIRED" level requirements for the protocols it
250   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
251   level and all the "SHOULD" level requirements for its protocols is said
252   to be "unconditionally compliant"; one that satisfies all the "MUST"
253   level requirements but not all the "SHOULD" level requirements for its
254   protocols is said to be "conditionally compliant".
255</t>
256</section>
257
258<section title="Syntax Notation" anchor="notation">
259  <x:anchor-alias value="ALPHA"/>
260  <x:anchor-alias value="CR"/>
261  <x:anchor-alias value="DIGIT"/>
262  <x:anchor-alias value="LF"/>
263  <x:anchor-alias value="OCTET"/>
264  <x:anchor-alias value="VCHAR"/>
265  <x:anchor-alias value="WSP"/>
266<t>
267  This specification uses the ABNF syntax defined in &notation; (which
268  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
269  <xref target="collected.abnf"/> shows the collected ABNF, with the list
270  rule expanded.
271</t>
272<t>
273  The following core rules are included by
274  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
275  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
276  DIGIT (decimal 0-9), DQUOTE (double quote),
277  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
278  OCTET (any 8-bit sequence of data), SP (space),
279  VCHAR (any visible USASCII character),
280  and WSP (whitespace).
281</t>
282
283<section title="Core Rules" anchor="core.rules">
284  <x:anchor-alias value="quoted-string"/>
285  <x:anchor-alias value="OWS"/>
286<t>
287  The core rules below are defined in &basic-rules;:
288</t>
289<figure><artwork type="abnf2616">
290  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
291  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
292</artwork></figure>
293</section>
294
295<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
296  <x:anchor-alias value="HTTP-date"/>
297<t>
298  The ABNF rules below are defined in other parts:
299</t>
300<figure><!--Part1--><artwork type="abnf2616">
301  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
302</artwork></figure>
303</section>
304
305</section>
306
307</section>
308
309<section title="Entity Tags" anchor="entity.tags">
310  <x:anchor-alias value="entity-tag"/>
311  <x:anchor-alias value="opaque-tag"/>
312  <x:anchor-alias value="weak"/>
313<t>
314   Entity tags are used for comparing two or more entities from the same
315   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
316   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
317   If-Range (&header-if-range;) header fields. The definition of how they
318   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
319   entity tag consists of an opaque quoted string, possibly prefixed by
320   a weakness indicator.
321</t>
322<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"/>
323  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
324  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
325  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
326</artwork></figure>
327<t>
328   A "strong entity tag" &MAY; be shared by two entities of a resource
329   only if they are equivalent by octet equality.
330</t>
331<t>
332   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
333   two entities of a resource only if the entities are equivalent and
334   could be substituted for each other with no significant change in
335   semantics. A weak entity tag can only be used for weak comparison.
336</t>
337<t>
338   An entity tag &MUST; be unique across all versions of all entities
339   associated with a particular resource. A given entity tag value &MAY;
340   be used for entities obtained by requests on different URIs. The use
341   of the same entity tag value in conjunction with entities obtained by
342   requests on different URIs does not imply the equivalence of those
343   entities.
344</t>
345
346<section title="Example: Entity Tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
347<t>
348   Consider a resource that is subject to content negotiation (&content-negotiation;),
349   and where the representations returned upon a GET request vary based on
350   the Accept-Encoding request header field (&header-accept-encoding;):
351</t>
352<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
353GET /index HTTP/1.1
354Host: www.example.com
355Accept-Encoding: gzip
356
357</artwork></figure>
358<t>
359   In this case, the response may or may not use the gzip content coding.
360   If it does not, the response might look like:
361</t>
362<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
363HTTP/1.1 200 OK
364Date: Thu, 26 Mar 2010 00:05:00 GMT
365ETag: "123-a"
366Content-Length: <x:length-of target="exbody"/>
367Vary: Accept-Encoding
368Content-Type: text/plain
369
370<x:span anchor="exbody">Hello World!
371Hello World!
372Hello World!
373Hello World!
374Hello World!
375</x:span></artwork></figure>
376<t>
377   An alternative representation that does use gzip content coding would be:
378</t>
379<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
380HTTP/1.1 200 OK
381Date: Thu, 26 Mar 2010 00:05:00 GMT
382ETag: "123-b"
383Content-Length: 43
384Vary: Accept-Encoding
385Content-Type: text/plain
386Content-Encoding: gzip
387
388<spanx>...binary data...</spanx></artwork></figure>
389<x:note>
390  <t>
391    <x:h>Note:</x:h> Content codings are a property of the representation,
392    so therefore an entity tag of an encoded representation must be distinct
393    from an unencoded representation to prevent conflicts during cache updates
394    and range requests.  In contrast, transfer codings (&transfer-codings;)
395    apply only during message transfer and do not require distinct entity tags.
396  </t>
397</x:note>
398</section>
399</section>
400
401<section title="Status Code Definitions" anchor="status.code.definitions">
402<section title="304 Not Modified" anchor="status.304">
403  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
404  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
405<t>
406   If the client has performed a conditional GET request and access is
407   allowed, but the document has not been modified, the server &SHOULD;
408   respond with this status code. The 304 response &MUST-NOT; contain a
409   message-body, and thus is always terminated by the first empty line
410   after the header fields.
411</t>
412<t>
413   A 304 response &MUST; include a Date header field (&header-date;)
414   unless its omission is required by &clockless;.  If a 200 response
415   to the same request would have included any of the header fields
416   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
417   Vary, then those same header fields &MUST; be sent in a 304 response.
418</t>
419<t>
420   Since the goal of a 304 response is to minimize information transfer
421   when the recipient already has one or more cached representations,
422   the response &SHOULD-NOT; include representation metadata other
423   than the above listed fields unless said metadata exists for the
424   purpose of guiding cache updates (e.g., future HTTP extensions).
425</t>
426<t>
427   If a 304 response includes an entity-tag that indicates a
428   representation not currently cached, then the recipient &MUST-NOT;
429   use the 304 to update its own cache.  If that conditional request originated
430   with an outbound client, such as a user agent with its own cache sending a
431   conditional GET to a shared proxy, then the 304 response &MUST; be
432   forwarded to the outbound client.  Otherwise, disregard the response
433   and repeat the request without the conditional.
434</t>
435<t>
436   If a cache uses a received 304 response to update a cache entry, the
437   cache &MUST; update the entry to reflect any new field values given in
438   the response.
439</t>
440</section>
441
442<section title="412 Precondition Failed" anchor="status.412">
443  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
444  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
445<t>
446   The precondition given in one or more of the request-header fields
447   evaluated to false when it was tested on the server. This response
448   code allows the client to place preconditions on the current resource
449   metainformation (header field data) and thus prevent the requested
450   method from being applied to a resource other than the one intended.
451</t>
452</section>
453</section>
454
455<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
456<t>
457   Since both origin servers and caches will compare two validators to
458   decide if they represent the same or different entities, one normally
459   would expect that if the entity (the entity-body or any entity-headers)
460   changes in any way, then the associated validator would
461   change as well. If this is true, then we call this validator a
462   "strong validator."
463</t>
464<t>
465   However, there might be cases when a server prefers to change the
466   validator only on semantically significant changes, and not when
467   insignificant aspects of the entity change. A validator that does not
468   always change when the resource changes is a "weak validator."
469</t>
470<t>
471   Entity tags are normally "strong validators," but the protocol
472   provides a mechanism to tag an entity tag as "weak." One can think of
473   a strong validator as one that changes whenever the bits of an entity
474   changes, while a weak value changes whenever the meaning of an entity
475   changes. Alternatively, one can think of a strong validator as part
476   of an identifier for a specific entity, while a weak validator is
477   part of an identifier for a set of semantically equivalent entities.
478  <list><t>
479      <x:h>Note:</x:h> One example of a strong validator is an integer that is
480      incremented in stable storage every time an entity is changed.
481    </t><t>
482      An entity's modification time, if represented with one-second
483      resolution, could be a weak validator, since it is possible that
484      the resource might be modified twice during a single second.
485    </t><t>
486      Support for weak validators is optional. However, weak validators
487      allow for more efficient caching of equivalent objects; for
488      example, a hit counter on a site is probably good enough if it is
489      updated every few days or weeks, and any value during that period
490      is likely "good enough" to be equivalent.
491    </t></list>
492</t>
493<t>
494   A "use" of a validator is either when a client generates a request
495   and includes the validator in a validating header field, or when a
496   server compares two validators.
497</t>
498<t>
499   Strong validators are usable in any context. Weak validators are only
500   usable in contexts that do not depend on exact equality of an entity.
501   For example, either kind is usable for a conditional GET of a full
502   entity. However, only a strong validator is usable for a sub-range
503   retrieval, since otherwise the client might end up with an internally
504   inconsistent entity.
505</t>
506<t>
507   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
508</t>
509<t>
510   The only function that HTTP/1.1 defines on validators is
511   comparison. There are two validator comparison functions, depending
512   on whether the comparison context allows the use of weak validators
513   or not:
514  <list style="symbols">
515     <t>The strong comparison function: in order to be considered equal,
516        both opaque-tags &MUST; be identical character-by-character, and both
517        &MUST-NOT; be weak.</t>
518     <t>The weak comparison function: in order to be considered equal, both
519        opaque-tags &MUST; be identical character-by-character, but
520        either or both of them &MAY; be tagged as "weak" without affecting
521        the result.</t>
522  </list>
523</t>
524<t>
525   The example below shows the results for a set of entity tag pairs,
526   and both the weak and strong comparison function results:
527</t>
528<texttable align="left">
529  <ttcol>ETag 1</ttcol>
530  <ttcol>ETag 2</ttcol>
531  <ttcol>Strong Comparison</ttcol>
532  <ttcol>Weak Comparison</ttcol>
533
534  <c>W/"1"</c>
535  <c>W/"1"</c>
536  <c>no match</c>
537  <c>match</c>
538 
539  <c>W/"1"</c>
540  <c>W/"2"</c>
541  <c>no match</c>
542  <c>no match</c>
543
544  <c>W/"1"</c>
545  <c>"1"</c>
546  <c>no match</c>
547  <c>match</c>
548
549  <c>"1"</c>
550  <c>"1"</c>
551  <c>match</c>
552  <c>match</c>
553</texttable>
554<t>
555   An entity tag is strong unless it is explicitly tagged as weak.
556   <xref target="entity.tags"/> gives the syntax for entity tags.
557</t>
558<t>
559   A Last-Modified time, when used as a validator in a request, is
560   implicitly weak unless it is possible to deduce that it is strong,
561   using the following rules:
562  <list style="symbols">
563     <t>The validator is being compared by an origin server to the
564        actual current validator for the entity and,</t>
565     <t>That origin server reliably knows that the associated entity did
566        not change twice during the second covered by the presented
567        validator.</t>
568  </list>
569</t>
570<t>
571   or
572  <list style="symbols">
573     <t>The validator is about to be used by a client in an If-Modified-Since
574        or If-Unmodified-Since header, because the client
575        has a cache entry for the associated entity, and</t>
576     <t>That cache entry includes a Date value, which gives the time
577        when the origin server sent the original response, and</t>
578     <t>The presented Last-Modified time is at least 60 seconds before
579        the Date value.</t>
580  </list>
581</t>
582<t>
583   or
584  <list style="symbols">
585     <t>The validator is being compared by an intermediate cache to the
586        validator stored in its cache entry for the entity, and</t>
587     <t>That cache entry includes a Date value, which gives the time
588        when the origin server sent the original response, and</t>
589     <t>The presented Last-Modified time is at least 60 seconds before
590        the Date value.</t>
591  </list>
592</t>
593<t>
594   This method relies on the fact that if two different responses were
595   sent by the origin server during the same second, but both had the
596   same Last-Modified time, then at least one of those responses would
597   have a Date value equal to its Last-Modified time. The arbitrary 60-second
598   limit guards against the possibility that the Date and Last-Modified
599   values are generated from different clocks, or at somewhat
600   different times during the preparation of the response. An
601   implementation &MAY; use a value larger than 60 seconds, if it is
602   believed that 60 seconds is too short.
603</t>
604<t>
605   If a client wishes to perform a sub-range retrieval on a value for
606   which it has only a Last-Modified time and no opaque validator, it
607   &MAY; do this only if the Last-Modified time is strong in the sense
608   described here.
609</t>
610<t>
611   A cache or origin server receiving a conditional range request
612   (<xref target="Part5"/>) &MUST; use the strong comparison function to
613   evaluate the condition.
614</t>
615<t>
616   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
617   retrievals on values that have been obtained from HTTP/1.0
618   servers.
619</t>
620</section>
621
622<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">
623<t>
624   We adopt a set of rules and recommendations for origin servers,
625   clients, and caches regarding when various validator types ought to
626   be used, and for what purposes.
627</t>
628<t>
629   HTTP/1.1 origin servers:
630  <list style="symbols">
631     <t>&SHOULD; send an entity tag validator unless it is not feasible to
632        generate one.</t>
633
634     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
635        performance considerations support the use of weak entity tags,
636        or if it is unfeasible to send a strong entity tag.</t>
637
638     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
639        unless the risk of a breakdown in semantic transparency that
640        could result from using this date in an If-Modified-Since header
641        would lead to serious problems.</t>
642  </list>
643</t>
644<t>
645   In other words, the preferred behavior for an HTTP/1.1 origin server
646   is to send both a strong entity tag and a Last-Modified value.
647</t>
648<t>
649   In order to be legal, a strong entity tag &MUST; change whenever the
650   associated entity changes in any way. A weak entity tag &SHOULD;
651   change whenever the associated entity changes in a semantically
652   significant way.
653</t>
654<x:note>
655  <t>
656    <x:h>Note:</x:h> In order to provide semantically transparent caching, an
657    origin server must avoid reusing a specific strong entity tag
658    value for two different entities, or reusing a specific weak
659    entity tag value for two semantically different entities. Cache
660    entries might persist for arbitrarily long periods, regardless of
661    expiration times, so it might be inappropriate to expect that a
662    cache will never again attempt to validate an entry using a
663    validator that it obtained at some point in the past.
664  </t>
665</x:note>
666<t>
667   HTTP/1.1 clients:
668  <list style="symbols">
669     <t>&MUST; use that entity tag in any cache-conditional request (using
670        If-Match or If-None-Match) if an entity tag has been provided by the
671        origin server.</t>
672
673     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
674        requests (using If-Modified-Since) if only a Last-Modified value has
675        been provided by the origin server. </t>
676
677     <t>&MAY; use the Last-Modified value in subrange cache-conditional
678        requests (using If-Unmodified-Since) if only a Last-Modified value has
679        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
680        provide a way to disable this, in case of difficulty.</t>
681
682     <t>&SHOULD; use both validators in cache-conditional requests if both an
683        entity tag and a Last-Modified value have been provided by the origin
684        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
685        appropriately.</t>
686  </list>
687</t>
688<t>
689   An HTTP/1.1 origin server, upon receiving a conditional request that
690   includes both a Last-Modified date (e.g., in an If-Modified-Since or
691   If-Unmodified-Since header field) and one or more entity tags (e.g.,
692   in an If-Match, If-None-Match, or If-Range header field) as cache
693   validators, &MUST-NOT; return a response status of 304 (Not Modified)
694   unless doing so is consistent with all of the conditional header
695   fields in the request.
696</t>
697<t>
698   An HTTP/1.1 caching proxy, upon receiving a conditional request that
699   includes both a Last-Modified date and one or more entity tags as
700   cache validators, &MUST-NOT; return a locally cached response to the
701   client unless that cached response is consistent with all of the
702   conditional header fields in the request.
703  <list><t>
704      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
705      servers and clients should transmit as much non-redundant
706      information as is available in their responses and requests.
707      HTTP/1.1 systems receiving this information will make the most
708      conservative assumptions about the validators they receive.
709  </t><t>
710      HTTP/1.0 clients and caches will ignore entity tags. Generally,
711      last-modified values received or used by these systems will
712      support transparent and efficient caching, and so HTTP/1.1 origin
713      servers should provide Last-Modified values. In those rare cases
714      where the use of a Last-Modified value as a validator by an
715      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
716      origin servers should not provide one.
717  </t></list>
718</t>
719</section>
720
721<section title="Header Field Definitions" anchor="header.fields">
722<t>
723   This section defines the syntax and semantics of HTTP/1.1 header fields
724   related to conditional requests.
725</t>
726<t>
727   For entity-header fields, both sender and recipient refer to either the
728   client or the server, depending on who sends and who receives the entity.
729</t>
730
731<section title="ETag" anchor="header.etag">
732  <iref primary="true" item="ETag header" x:for-anchor=""/>
733  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
734  <x:anchor-alias value="ETag"/>
735  <x:anchor-alias value="ETag-v"/>
736<t>
737   The "ETag" response-header field provides the current value of the
738   entity tag (see <xref target="entity.tags"/>) for one representation of
739   the resource identified by the Effective Request URI.  An entity tag
740   is intended for use as a resource-local identifier for differentiating
741   between representations of the same resource that vary over time or via
742   content negotiation (see <xref target="weak.and.strong.validators"/>).
743</t>
744<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
745  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
746  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
747</artwork></figure>
748<figure><preamble>
749  Examples:
750</preamble>
751<artwork type="example">
752  ETag: "xyzzy"
753  ETag: W/"xyzzy"
754  ETag: ""
755</artwork></figure>
756<t>
757   An entity tag provides an "opaque" cache validator that allows for
758   more reliable validation than modification dates in situations where
759   it is inconvenient to store modification dates,
760   where the one-second resolution of HTTP date values is not
761   sufficient, or where the origin server wishes to avoid certain
762   paradoxes that might arise from the use of modification dates.
763</t>
764<t>
765   The principle behind entity tags is that only the service author
766   knows the semantics of a resource well enough to select an
767   appropriate cache validation mechanism, and the specification of any
768   validator comparison function more complex than byte-equality would
769   open up a can of worms. Thus, comparisons of any other headers
770   (except Last-Modified, for compatibility with HTTP/1.0) are never
771   used for purposes of validating a cache entry.
772</t>
773</section>
774
775<section title="If-Match" anchor="header.if-match">
776  <iref primary="true" item="If-Match header" x:for-anchor=""/>
777  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
778  <x:anchor-alias value="If-Match"/>
779  <x:anchor-alias value="If-Match-v"/>
780<t>
781   The "If-Match" request-header field is used to make a request method
782   conditional. A client that has one or more entities previously
783   obtained from the resource can verify that one of those entities is
784   current by including a list of their associated entity tags in the
785   If-Match header field.
786</t>
787<t>
788   This allows efficient updates of cached information with a minimum amount of
789   transaction overhead. It is also used when updating resources, to prevent
790   inadvertent modification of the wrong version of a resource. As a special
791   case, the value "*" matches any current entity of the resource.
792</t>
793<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
794  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
795  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
796</artwork></figure>
797<t>
798   If any of the entity tags match the entity tag of the entity that
799   would have been returned in the response to a similar GET request
800   (without the If-Match header) on that resource, or if "*" is given
801   and any current entity exists for that resource, then the server &MAY;
802   perform the requested method as if the If-Match header field did not
803   exist.
804</t>
805<t>
806   If none of the entity tags match, or if "*" is given and no current
807   entity exists, the server &MUST-NOT; perform the requested method, and
808   &MUST; return a 412 (Precondition Failed) response. This behavior is
809   most useful when the client wants to prevent an updating method, such
810   as PUT, from modifying a resource that has changed since the client
811   last retrieved it.
812</t>
813<t>
814   If the request would, without the If-Match header field, result in
815   anything other than a 2xx or 412 status, then the If-Match header
816   &MUST; be ignored.
817</t>
818<t>
819   The meaning of "If-Match: *" is that the method &SHOULD; be performed
820   if the representation selected by the origin server (or by a cache,
821   possibly using the Vary mechanism, see &header-vary;) exists, and
822   &MUST-NOT; be performed if the representation does not exist.
823</t>
824<t>
825   A request intended to update a resource (e.g., a PUT) &MAY; include an
826   If-Match header field to signal that the request method &MUST-NOT; be
827   applied if the entity corresponding to the If-Match value (a single
828   entity tag) is no longer a representation of that resource. This
829   allows the user to indicate that they do not wish the request to be
830   successful if the resource has been changed without their knowledge.
831   Examples:
832</t>
833<figure><artwork type="example">
834  If-Match: "xyzzy"
835  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
836  If-Match: *
837</artwork></figure>
838<t>
839   The result of a request having both an If-Match header field and
840   either an If-None-Match or an If-Modified-Since header fields is
841   undefined by this specification.
842</t>
843</section>
844
845<section title="If-Modified-Since" anchor="header.if-modified-since">
846  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
847  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
848  <x:anchor-alias value="If-Modified-Since"/>
849  <x:anchor-alias value="If-Modified-Since-v"/>
850<t>
851   The "If-Modified-Since" request-header field is used to make a request
852   method conditional by date: if the representation that would have been
853   transferred in a 200 response to a GET request has not been modified since
854   the time specified in this field, then do not perform the method;
855   instead, respond as detailed below.
856</t>
857<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
858  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
859                        <x:ref>If-Modified-Since-v</x:ref>
860  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
861</artwork></figure>
862<t>
863   An example of the field is:
864</t>
865<figure><artwork type="example">
866  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
867</artwork></figure>
868<t>
869   A GET method with an If-Modified-Since header and no Range header
870   requests that the representation be transferred only if it has
871   been modified since the date given by the If-Modified-Since header.
872   The algorithm for determining this includes the following cases:
873  <list style="numbers">
874      <t>If the request would normally result in anything other than a
875         200 (OK) status, or if the passed If-Modified-Since date is
876         invalid, the response is exactly the same as for a normal GET.
877         A date which is later than the server's current time is
878         invalid.</t>
879
880      <t>If the representation has been modified since the If-Modified-Since
881         date, the response is exactly the same as for a normal GET.</t>
882
883      <t>If the representation has not been modified since a valid
884         If-Modified-Since date, the server &SHOULD; return a
885         304 (Not Modified) response.</t>
886  </list>
887</t>
888<t>
889   The purpose of this feature is to allow efficient updates of cached
890   information with a minimum amount of transaction overhead.
891  <list><t>
892      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
893      see &header-range; for full details.
894    </t><t>
895      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
896      clock might not be synchronized with the client.
897    </t><t>
898      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
899      servers will use an exact date comparison function, rather than a
900      less-than function, for deciding whether to send a 304 (Not
901      Modified) response. To get best results when sending an If-Modified-Since
902      header field for cache validation, clients are
903      advised to use the exact date string received in a previous Last-Modified
904      header field whenever possible.
905    </t><t>
906      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
907      header instead of a date taken from the Last-Modified header for
908      the same request, the client should be aware of the fact that this
909      date is interpreted in the server's understanding of time. The
910      client should consider unsynchronized clocks and rounding problems
911      due to the different encodings of time between the client and
912      server. This includes the possibility of race conditions if the
913      document has changed between the time it was first requested and
914      the If-Modified-Since date of a subsequent request, and the
915      possibility of clock-skew-related problems if the If-Modified-Since
916      date is derived from the client's clock without correction
917      to the server's clock. Corrections for different time bases
918      between client and server are at best approximate due to network
919      latency.
920    </t>
921  </list>
922</t>
923<t>
924   The result of a request having both an If-Modified-Since header field
925   and either an If-Match or an If-Unmodified-Since header fields is
926   undefined by this specification.
927</t>
928</section>
929
930<section title="If-None-Match" anchor="header.if-none-match">
931  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
932  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
933  <x:anchor-alias value="If-None-Match"/>
934  <x:anchor-alias value="If-None-Match-v"/>
935<t>
936   The "If-None-Match" request-header field is used to make a request method
937   conditional. A client that has one or more entities previously
938   obtained from the resource can verify that none of those entities is
939   current by including a list of their associated entity tags in the
940   If-None-Match header field.
941</t>
942<t>
943   This allows efficient updates of cached information with a minimum amount of
944   transaction overhead. It is also used to prevent a method (e.g., PUT)
945   from inadvertently modifying an existing resource when the client
946   believes that the resource does not exist.
947</t>
948<t>
949   As a special case, the value "*" matches any current entity of the
950   resource.
951</t>
952<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
953  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
954  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
955</artwork></figure>
956<t>
957   If any of the entity tags match the entity tag of the entity that
958   would have been returned in the response to a similar GET request
959   (without the If-None-Match header) on that resource, or if "*" is
960   given and any current entity exists for that resource, then the
961   server &MUST-NOT; perform the requested method, unless required to do
962   so because the resource's modification date fails to match that
963   supplied in an If-Modified-Since header field in the request.
964   Instead, if the request method was GET or HEAD, the server &SHOULD;
965   respond with a 304 (Not Modified) response, including the cache-related
966   header fields (particularly ETag) of one of the entities that
967   matched. For all other request methods, the server &MUST; respond with
968   a status of 412 (Precondition Failed).
969</t>
970<t>
971   If none of the entity tags match, then the server &MAY; perform the
972   requested method as if the If-None-Match header field did not exist,
973   but &MUST; also ignore any If-Modified-Since header field(s) in the
974   request. That is, if no entity tags match, then the server &MUST-NOT;
975   return a 304 (Not Modified) response.
976</t>
977<t>
978   If the request would, without the If-None-Match header field, result
979   in anything other than a 2xx or 304 status, then the If-None-Match
980   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
981   server behavior when both If-Modified-Since and If-None-Match appear
982   in the same request.)
983</t>
984<t>
985   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
986   performed if the representation selected by the origin server (or by
987   a cache, possibly using the Vary mechanism, see &header-vary;)
988   exists, and &SHOULD; be performed if the representation does not exist.
989   This feature is intended to be useful in preventing races between PUT
990   operations.
991</t>
992<t>
993   Examples:
994</t>
995<figure><artwork type="example">
996  If-None-Match: "xyzzy"
997  If-None-Match: W/"xyzzy"
998  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
999  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
1000  If-None-Match: *
1001</artwork></figure>
1002<t>
1003   The result of a request having both an If-None-Match header field and
1004   either an If-Match or an If-Unmodified-Since header fields is
1005   undefined by this specification.
1006</t>
1007</section>
1008
1009<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
1010  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
1011  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
1012  <x:anchor-alias value="If-Unmodified-Since"/>
1013  <x:anchor-alias value="If-Unmodified-Since-v"/>
1014<t>
1015   The "If-Unmodified-Since" request-header field is used to make a request
1016   method conditional.  If the representation that would have been transferred
1017   in a 200 response to a GET request on the same resource has not been modified
1018   since the time specified in this field, the server &SHOULD; perform the
1019   requested operation as if the If-Unmodified-Since header were not
1020   present.
1021</t>
1022<t>
1023   If the representation has been modified since the specified time,
1024   the server &MUST-NOT; perform the requested operation, and &MUST; return
1025   a 412 (Precondition Failed).
1026</t>
1027<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
1028  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
1029                          <x:ref>If-Unmodified-Since-v</x:ref>
1030  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
1031</artwork></figure>
1032<t>
1033   An example of the field is:
1034</t>
1035<figure><artwork type="example">
1036  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1037</artwork></figure>
1038<t>
1039   If the request normally (i.e., without the If-Unmodified-Since
1040   header) would result in anything other than a 2xx or 412 status, the
1041   If-Unmodified-Since header &SHOULD; be ignored.
1042</t>
1043<t>
1044   If the specified date is invalid, the header is ignored.
1045</t>
1046<t>
1047   The result of a request having both an If-Unmodified-Since header
1048   field and either an If-None-Match or an If-Modified-Since header
1049   fields is undefined by this specification.
1050</t>
1051</section>
1052
1053<section title="Last-Modified" anchor="header.last-modified">
1054  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
1055  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
1056  <x:anchor-alias value="Last-Modified"/>
1057  <x:anchor-alias value="Last-Modified-v"/>
1058<t>
1059   The "Last-Modified" entity-header field indicates the date and time at
1060   which the origin server believes the representation was last modified.
1061</t>
1062<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
1063  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
1064  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
1065</artwork></figure>
1066<t>
1067   An example of its use is
1068</t>
1069<figure><artwork type="example">
1070  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1071</artwork></figure>
1072<t>
1073   The exact meaning of this header field depends on the implementation
1074   of the origin server and the nature of the original resource. For
1075   files, it may be just the file system last-modified time. For
1076   entities with dynamically included parts, it may be the most recent
1077   of the set of last-modify times for its component parts. For database
1078   gateways, it may be the last-update time stamp of the record. For
1079   virtual objects, it may be the last time the internal state changed.
1080</t>
1081<t>
1082   An origin server &MUST-NOT; send a Last-Modified date which is later
1083   than the server's time of message origination. In such cases, where
1084   the resource's last modification would indicate some time in the
1085   future, the server &MUST; replace that date with the message
1086   origination date.
1087</t>
1088<t>
1089   An origin server &SHOULD; obtain the Last-Modified value of the entity
1090   as close as possible to the time that it generates the Date value of
1091   its response. This allows a recipient to make an accurate assessment
1092   of the entity's modification time, especially if the entity changes
1093   near the time that the response is generated.
1094</t>
1095<t>
1096   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1097</t>
1098<t>
1099   The Last-Modified entity-header field value is often used as a cache
1100   validator. In simple terms, a cache entry is considered to be valid
1101   if the representation has not been modified since the Last-Modified value.
1102</t>
1103</section>
1104
1105</section>
1106
1107<section title="IANA Considerations" anchor="IANA.considerations">
1108
1109<section title="Status Code Registration" anchor="status.code.registration">
1110<t>
1111   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1112   should be updated with the registrations below:
1113</t>
1114<?BEGININC p4-conditional.iana-status-codes ?>
1115<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1116<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1117   <ttcol>Value</ttcol>
1118   <ttcol>Description</ttcol>
1119   <ttcol>Reference</ttcol>
1120   <c>304</c>
1121   <c>Not Modified</c>
1122   <c>
1123      <xref target="status.304"/>
1124   </c>
1125   <c>412</c>
1126   <c>Precondition Failed</c>
1127   <c>
1128      <xref target="status.412"/>
1129   </c>
1130</texttable>
1131<!--(END)-->
1132<?ENDINC p4-conditional.iana-status-codes ?>
1133</section>
1134
1135<section title="Message Header Registration" anchor="message.header.registration">
1136<t>
1137   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1138   with the permanent registrations below (see <xref target="RFC3864"/>):
1139</t>
1140<?BEGININC p4-conditional.iana-headers ?>
1141<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1142<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1143   <ttcol>Header Field Name</ttcol>
1144   <ttcol>Protocol</ttcol>
1145   <ttcol>Status</ttcol>
1146   <ttcol>Reference</ttcol>
1147
1148   <c>ETag</c>
1149   <c>http</c>
1150   <c>standard</c>
1151   <c>
1152      <xref target="header.etag"/>
1153   </c>
1154   <c>If-Match</c>
1155   <c>http</c>
1156   <c>standard</c>
1157   <c>
1158      <xref target="header.if-match"/>
1159   </c>
1160   <c>If-Modified-Since</c>
1161   <c>http</c>
1162   <c>standard</c>
1163   <c>
1164      <xref target="header.if-modified-since"/>
1165   </c>
1166   <c>If-None-Match</c>
1167   <c>http</c>
1168   <c>standard</c>
1169   <c>
1170      <xref target="header.if-none-match"/>
1171   </c>
1172   <c>If-Unmodified-Since</c>
1173   <c>http</c>
1174   <c>standard</c>
1175   <c>
1176      <xref target="header.if-unmodified-since"/>
1177   </c>
1178   <c>Last-Modified</c>
1179   <c>http</c>
1180   <c>standard</c>
1181   <c>
1182      <xref target="header.last-modified"/>
1183   </c>
1184</texttable>
1185<!--(END)-->
1186<?ENDINC p4-conditional.iana-headers ?>
1187<t>
1188   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1189</t>
1190</section>
1191</section>
1192
1193<section title="Security Considerations" anchor="security.considerations">
1194<t>
1195   No additional security considerations have been identified beyond
1196   those applicable to HTTP in general &messaging;.
1197</t>
1198</section>
1199
1200<section title="Acknowledgments" anchor="ack">
1201</section>
1202</middle>
1203<back>
1204
1205<references title="Normative References">
1206
1207<reference anchor="Part1">
1208  <front>
1209    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1210    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1211      <organization abbrev="Day Software">Day Software</organization>
1212      <address><email>fielding@gbiv.com</email></address>
1213    </author>
1214    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1215      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1216      <address><email>jg@freedesktop.org</email></address>
1217    </author>
1218    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1219      <organization abbrev="HP">Hewlett-Packard Company</organization>
1220      <address><email>JeffMogul@acm.org</email></address>
1221    </author>
1222    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1223      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1224      <address><email>henrikn@microsoft.com</email></address>
1225    </author>
1226    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1227      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1228      <address><email>LMM@acm.org</email></address>
1229    </author>
1230    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1231      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1232      <address><email>paulle@microsoft.com</email></address>
1233    </author>
1234    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1235      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1236      <address><email>timbl@w3.org</email></address>
1237    </author>
1238    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1239      <organization abbrev="W3C">World Wide Web Consortium</organization>
1240      <address><email>ylafon@w3.org</email></address>
1241    </author>
1242    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1243      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1244      <address><email>julian.reschke@greenbytes.de</email></address>
1245    </author>
1246    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1247  </front>
1248  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1249  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1250</reference>
1251
1252<reference anchor="Part3">
1253  <front>
1254    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1255    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1256      <organization abbrev="Day Software">Day Software</organization>
1257      <address><email>fielding@gbiv.com</email></address>
1258    </author>
1259    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1260      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1261      <address><email>jg@freedesktop.org</email></address>
1262    </author>
1263    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1264      <organization abbrev="HP">Hewlett-Packard Company</organization>
1265      <address><email>JeffMogul@acm.org</email></address>
1266    </author>
1267    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1268      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1269      <address><email>henrikn@microsoft.com</email></address>
1270    </author>
1271    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1272      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1273      <address><email>LMM@acm.org</email></address>
1274    </author>
1275    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1276      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1277      <address><email>paulle@microsoft.com</email></address>
1278    </author>
1279    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1280      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1281      <address><email>timbl@w3.org</email></address>
1282    </author>
1283    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1284      <organization abbrev="W3C">World Wide Web Consortium</organization>
1285      <address><email>ylafon@w3.org</email></address>
1286    </author>
1287    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1288      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1289      <address><email>julian.reschke@greenbytes.de</email></address>
1290    </author>
1291    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1292  </front>
1293  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1294  <x:source href="p3-payload.xml" basename="p3-payload"/>
1295</reference>
1296
1297<reference anchor="Part5">
1298  <front>
1299    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1300    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1301      <organization abbrev="Day Software">Day Software</organization>
1302      <address><email>fielding@gbiv.com</email></address>
1303    </author>
1304    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1305      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1306      <address><email>jg@freedesktop.org</email></address>
1307    </author>
1308    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1309      <organization abbrev="HP">Hewlett-Packard Company</organization>
1310      <address><email>JeffMogul@acm.org</email></address>
1311    </author>
1312    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1313      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1314      <address><email>henrikn@microsoft.com</email></address>
1315    </author>
1316    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1317      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1318      <address><email>LMM@acm.org</email></address>
1319    </author>
1320    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1321      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1322      <address><email>paulle@microsoft.com</email></address>
1323    </author>
1324    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1325      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1326      <address><email>timbl@w3.org</email></address>
1327    </author>
1328    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1329      <organization abbrev="W3C">World Wide Web Consortium</organization>
1330      <address><email>ylafon@w3.org</email></address>
1331    </author>
1332    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1333      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1334      <address><email>julian.reschke@greenbytes.de</email></address>
1335    </author>
1336    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1337  </front>
1338  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1339  <x:source href="p5-range.xml" basename="p5-range"/>
1340</reference>
1341
1342<reference anchor="Part6">
1343  <front>
1344    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1345    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1346      <organization abbrev="Day Software">Day Software</organization>
1347      <address><email>fielding@gbiv.com</email></address>
1348    </author>
1349    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1350      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1351      <address><email>jg@freedesktop.org</email></address>
1352    </author>
1353    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1354      <organization abbrev="HP">Hewlett-Packard Company</organization>
1355      <address><email>JeffMogul@acm.org</email></address>
1356    </author>
1357    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1358      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1359      <address><email>henrikn@microsoft.com</email></address>
1360    </author>
1361    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1362      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1363      <address><email>LMM@acm.org</email></address>
1364    </author>
1365    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1366      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1367      <address><email>paulle@microsoft.com</email></address>
1368    </author>
1369    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1370      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1371      <address><email>timbl@w3.org</email></address>
1372    </author>
1373    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1374      <organization abbrev="W3C">World Wide Web Consortium</organization>
1375      <address><email>ylafon@w3.org</email></address>
1376    </author>
1377    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1378      <address><email>mnot@mnot.net</email></address>
1379    </author>
1380    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1381      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1382      <address><email>julian.reschke@greenbytes.de</email></address>
1383    </author>
1384    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1385  </front>
1386  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1387  <x:source href="p6-cache.xml" basename="p6-cache"/>
1388</reference>
1389
1390<reference anchor="RFC2119">
1391  <front>
1392    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1393    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1394      <organization>Harvard University</organization>
1395      <address><email>sob@harvard.edu</email></address>
1396    </author>
1397    <date month="March" year="1997"/>
1398  </front>
1399  <seriesInfo name="BCP" value="14"/>
1400  <seriesInfo name="RFC" value="2119"/>
1401</reference>
1402
1403<reference anchor="RFC5234">
1404  <front>
1405    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1406    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1407      <organization>Brandenburg InternetWorking</organization>
1408      <address>
1409        <email>dcrocker@bbiw.net</email>
1410      </address> 
1411    </author>
1412    <author initials="P." surname="Overell" fullname="Paul Overell">
1413      <organization>THUS plc.</organization>
1414      <address>
1415        <email>paul.overell@thus.net</email>
1416      </address>
1417    </author>
1418    <date month="January" year="2008"/>
1419  </front>
1420  <seriesInfo name="STD" value="68"/>
1421  <seriesInfo name="RFC" value="5234"/>
1422</reference>
1423
1424</references>
1425
1426<references title="Informative References">
1427
1428<reference anchor="RFC2616">
1429  <front>
1430    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1431    <author initials="R." surname="Fielding" fullname="R. Fielding">
1432      <organization>University of California, Irvine</organization>
1433      <address><email>fielding@ics.uci.edu</email></address>
1434    </author>
1435    <author initials="J." surname="Gettys" fullname="J. Gettys">
1436      <organization>W3C</organization>
1437      <address><email>jg@w3.org</email></address>
1438    </author>
1439    <author initials="J." surname="Mogul" fullname="J. Mogul">
1440      <organization>Compaq Computer Corporation</organization>
1441      <address><email>mogul@wrl.dec.com</email></address>
1442    </author>
1443    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1444      <organization>MIT Laboratory for Computer Science</organization>
1445      <address><email>frystyk@w3.org</email></address>
1446    </author>
1447    <author initials="L." surname="Masinter" fullname="L. Masinter">
1448      <organization>Xerox Corporation</organization>
1449      <address><email>masinter@parc.xerox.com</email></address>
1450    </author>
1451    <author initials="P." surname="Leach" fullname="P. Leach">
1452      <organization>Microsoft Corporation</organization>
1453      <address><email>paulle@microsoft.com</email></address>
1454    </author>
1455    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1456      <organization>W3C</organization>
1457      <address><email>timbl@w3.org</email></address>
1458    </author>
1459    <date month="June" year="1999"/>
1460  </front>
1461  <seriesInfo name="RFC" value="2616"/>
1462</reference>
1463
1464<reference anchor='RFC3864'>
1465  <front>
1466    <title>Registration Procedures for Message Header Fields</title>
1467    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1468      <organization>Nine by Nine</organization>
1469      <address><email>GK-IETF@ninebynine.org</email></address>
1470    </author>
1471    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1472      <organization>BEA Systems</organization>
1473      <address><email>mnot@pobox.com</email></address>
1474    </author>
1475    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1476      <organization>HP Labs</organization>
1477      <address><email>JeffMogul@acm.org</email></address>
1478    </author>
1479    <date year='2004' month='September' />
1480  </front>
1481  <seriesInfo name='BCP' value='90' />
1482  <seriesInfo name='RFC' value='3864' />
1483</reference>
1484
1485</references>
1486
1487<section title="Compatibility with Previous Versions" anchor="compatibility">
1488
1489<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1490<t>
1491  Allow weak entity tags in all requests except range requests (Sections
1492  <xref target="weak.and.strong.validators" format="counter"/> and
1493  <xref target="header.if-none-match" format="counter"/>).
1494</t>
1495</section>
1496
1497</section>
1498
1499<?BEGININC p4-conditional.abnf-appendix ?>
1500<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1501<figure>
1502<artwork type="abnf" name="p4-conditional.parsed-abnf">
1503<x:ref>ETag</x:ref> = "ETag:" OWS ETag-v
1504<x:ref>ETag-v</x:ref> = entity-tag
1505
1506<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1507
1508<x:ref>If-Match</x:ref> = "If-Match:" OWS If-Match-v
1509<x:ref>If-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1510 entity-tag ] ) )
1511<x:ref>If-Modified-Since</x:ref> = "If-Modified-Since:" OWS If-Modified-Since-v
1512<x:ref>If-Modified-Since-v</x:ref> = HTTP-date
1513<x:ref>If-None-Match</x:ref> = "If-None-Match:" OWS If-None-Match-v
1514<x:ref>If-None-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1515 entity-tag ] ) )
1516<x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since:" OWS
1517 If-Unmodified-Since-v
1518<x:ref>If-Unmodified-Since-v</x:ref> = HTTP-date
1519
1520<x:ref>Last-Modified</x:ref> = "Last-Modified:" OWS Last-Modified-v
1521<x:ref>Last-Modified-v</x:ref> = HTTP-date
1522
1523<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1524
1525<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1526
1527<x:ref>opaque-tag</x:ref> = quoted-string
1528
1529<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1530
1531<x:ref>weak</x:ref> = %x57.2F ; W/
1532</artwork>
1533</figure>
1534<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1535; ETag defined but not used
1536; If-Match defined but not used
1537; If-Modified-Since defined but not used
1538; If-None-Match defined but not used
1539; If-Unmodified-Since defined but not used
1540; Last-Modified defined but not used
1541</artwork></figure></section>
1542<?ENDINC p4-conditional.abnf-appendix ?>
1543
1544<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1545
1546<section title="Since RFC2616">
1547<t>
1548  Extracted relevant partitions from <xref target="RFC2616"/>.
1549</t>
1550</section>
1551
1552<section title="Since draft-ietf-httpbis-p4-conditional-00">
1553<t>
1554  Closed issues:
1555  <list style="symbols"> 
1556    <t>
1557      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1558      "Normative and Informative references"
1559    </t>
1560  </list>
1561</t>
1562<t>
1563  Other changes:
1564  <list style="symbols"> 
1565    <t>
1566      Move definitions of 304 and 412 condition codes from Part2.
1567    </t>
1568  </list>
1569</t>
1570</section>
1571
1572<section title="Since draft-ietf-httpbis-p4-conditional-01">
1573<t>
1574  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1575  <list style="symbols"> 
1576    <t>
1577      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1578    </t>
1579  </list>
1580</t>
1581</section>
1582
1583<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1584<t>
1585  Closed issues:
1586  <list style="symbols"> 
1587    <t>
1588      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1589      "Weak ETags on non-GET requests"
1590    </t>
1591  </list>
1592</t>
1593<t>
1594  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1595  <list style="symbols"> 
1596    <t>
1597      Reference RFC 3984, and update header registrations for headers defined
1598      in this document.
1599    </t>
1600  </list>
1601</t>
1602</section>
1603
1604<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1605<t>
1606  Closed issues:
1607  <list style="symbols"> 
1608    <t>
1609      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1610      "Examples for ETag matching"
1611    </t>
1612    <t>
1613      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1614      "'entity value' undefined"
1615    </t>
1616    <t>
1617      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1618      "bogus 2068 Date header reference"
1619    </t>
1620  </list>
1621</t>
1622</section>
1623
1624<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1625<t>
1626  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1627  <list style="symbols"> 
1628    <t>
1629      Use "/" instead of "|" for alternatives.
1630    </t>
1631    <t>
1632      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1633      whitespace ("OWS") and required whitespace ("RWS").
1634    </t>
1635    <t>
1636      Rewrite ABNFs to spell out whitespace rules, factor out
1637      header value format definitions.
1638    </t>
1639  </list>
1640</t>
1641</section>
1642
1643<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1644<t>
1645  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1646  <list style="symbols"> 
1647    <t>
1648      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1649    </t>
1650  </list>
1651</t>
1652</section>
1653
1654<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1655<t>
1656  Closed issues:
1657  <list style="symbols"> 
1658    <t>
1659      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1660      "case-sensitivity of etag weakness indicator"
1661    </t>
1662  </list>
1663</t>
1664</section>
1665
1666<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1667<t>
1668  Closed issues:
1669  <list style="symbols"> 
1670    <t>
1671      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1672      "Weak ETags on non-GET requests" (If-Match still was defined to require
1673      strong matching)
1674    </t>
1675    <t>
1676      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1677      "move IANA registrations for optional status codes"
1678    </t>
1679  </list>
1680</t>
1681</section>
1682
1683<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1684<t>
1685  No significant changes.
1686</t>
1687</section>
1688
1689<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1690<t>
1691  No significant changes.
1692</t>
1693</section>
1694
1695<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1696<t>
1697  None yet.
1698</t>
1699</section>
1700
1701</section>
1702
1703</back>
1704</rfc>
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