source: draft-ietf-httpbis/latest/p4-conditional.xml @ 886

Last change on this file since 886 was 879, checked in by julian.reschke@…, 9 years ago

change citing style to match RFC Editor expectations (see <http://www.rfc-editor.org/rfc-style-guide/rfc-style>, Section 3.3)

  • Property svn:eol-style set to native
File size: 69.9 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "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 representations 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 representations 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 representations of a resource only if the representations 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 representations
339   associated with a particular resource. A given entity-tag value &MAY;
340   be used for representations obtained by requests on different URIs. The use
341   of the same entity-tag value in conjunction with representations obtained by
342   requests on different URIs does not imply the equivalence of those
343   representations.
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   metadata (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 representations, one normally
459   would expect that if the representation (including both representation
460   header fields and representation body) changes in any way, then the
461   associated validator would change as well. If this is true, then we
462   call this validator a "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 representation change. A validator that does not
468   always change when the representation changes is a "weak validator".
469</t>
470<t>
471   An entity-tag is normally a strong validator, 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 sequence of bits
474   in a representation changes, while a weak value changes whenever the
475   meaning of a representation changes. Alternatively, one can think of
476   a strong validator as part of an identifier for a specific representation,
477   whereas a weak validator is part of an identifier for a set of semantically
478   equivalent representations.
479  <list><t>
480      <x:h>Note:</x:h> One example of a strong validator is an integer that is
481      incremented in stable storage every time a representation is changed.
482    </t><t>
483      A representation's modification time, if defined with only one-second
484      resolution, could be a weak validator, since it is possible that
485      the representation might be modified twice during a single second.
486    </t><t>
487      Support for weak validators is optional. However, weak validators
488      allow for more efficient caching of equivalent objects; for
489      example, a hit counter on a site is probably good enough if it is
490      updated every few days or weeks, and any value during that period
491      is likely "good enough" to be equivalent.
492    </t></list>
493</t>
494<t>
495   A "use" of a validator is either when a client generates a request
496   and includes the validator in a validating header field, or when a
497   server compares two validators.
498</t>
499<t>
500   Strong validators are usable in any context. Weak validators are only
501   usable in contexts that do not depend on exact equality of a representation.
502   For example, either kind is usable for a normal conditional GET.
503   However, only a strong validator is usable for a sub-range
504   retrieval, since otherwise the client might end up with an internally
505   inconsistent representation.
506</t>
507<t>
508   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
509</t>
510<t>
511   The only function that HTTP/1.1 defines on validators is
512   comparison. There are two validator comparison functions, depending
513   on whether the comparison context allows the use of weak validators
514   or not:
515  <list style="symbols">
516     <t>The strong comparison function: in order to be considered equal,
517        both opaque-tags &MUST; be identical character-by-character, and both
518        &MUST-NOT; be weak.</t>
519     <t>The weak comparison function: in order to be considered equal, both
520        opaque-tags &MUST; be identical character-by-character, but
521        either or both of them &MAY; be tagged as "weak" without affecting
522        the result.</t>
523  </list>
524</t>
525<t>
526   The example below shows the results for a set of entity-tag pairs,
527   and both the weak and strong comparison function results:
528</t>
529<texttable align="left">
530  <ttcol>ETag 1</ttcol>
531  <ttcol>ETag 2</ttcol>
532  <ttcol>Strong Comparison</ttcol>
533  <ttcol>Weak Comparison</ttcol>
534
535  <c>W/"1"</c>
536  <c>W/"1"</c>
537  <c>no match</c>
538  <c>match</c>
539 
540  <c>W/"1"</c>
541  <c>W/"2"</c>
542  <c>no match</c>
543  <c>no match</c>
544
545  <c>W/"1"</c>
546  <c>"1"</c>
547  <c>no match</c>
548  <c>match</c>
549
550  <c>"1"</c>
551  <c>"1"</c>
552  <c>match</c>
553  <c>match</c>
554</texttable>
555<t>
556   An entity-tag is strong unless it is explicitly tagged as weak.
557   <xref target="entity.tags"/> gives the syntax for entity-tags.
558</t>
559<t>
560   A Last-Modified time, when used as a validator in a request, is
561   implicitly weak unless it is possible to deduce that it is strong,
562   using the following rules:
563  <list style="symbols">
564     <t>The validator is being compared by an origin server to the
565        actual current validator for the representation and,</t>
566     <t>That origin server reliably knows that the associated representation did
567        not change twice during the second covered by the presented
568        validator.</t>
569  </list>
570</t>
571<t>
572   or
573  <list style="symbols">
574     <t>The validator is about to be used by a client in an If-Modified-Since
575        or If-Unmodified-Since header, because the client
576        has a cache entry for the associated representation, and</t>
577     <t>That cache entry includes a Date value, which gives the time
578        when the origin server sent the original response, and</t>
579     <t>The presented Last-Modified time is at least 60 seconds before
580        the Date value.</t>
581  </list>
582</t>
583<t>
584   or
585  <list style="symbols">
586     <t>The validator is being compared by an intermediate cache to the
587        validator stored in its cache entry for the representation, and</t>
588     <t>That cache entry includes a Date value, which gives the time
589        when the origin server sent the original response, and</t>
590     <t>The presented Last-Modified time is at least 60 seconds before
591        the Date value.</t>
592  </list>
593</t>
594<t>
595   This method relies on the fact that if two different responses were
596   sent by the origin server during the same second, but both had the
597   same Last-Modified time, then at least one of those responses would
598   have a Date value equal to its Last-Modified time. The arbitrary 60-second
599   limit guards against the possibility that the Date and Last-Modified
600   values are generated from different clocks, or at somewhat
601   different times during the preparation of the response. An
602   implementation &MAY; use a value larger than 60 seconds, if it is
603   believed that 60 seconds is too short.
604</t>
605<t>
606   If a client wishes to perform a sub-range retrieval on a value for
607   which it has only a Last-Modified time and no opaque validator, it
608   &MAY; do this only if the Last-Modified time is strong in the sense
609   described here.
610</t>
611<t>
612   A cache or origin server receiving a conditional range request
613   (<xref target="Part5"/>) &MUST; use the strong comparison function to
614   evaluate the condition.
615</t>
616<t>
617   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
618   retrievals on values that have been obtained from HTTP/1.0
619   servers.
620</t>
621</section>
622
623<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">
624<t>
625   We adopt a set of rules and recommendations for origin servers,
626   clients, and caches regarding when various validator types ought to
627   be used, and for what purposes.
628</t>
629<t>
630   HTTP/1.1 origin servers:
631  <list style="symbols">
632     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
633        generate one.</t>
634
635     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
636        performance considerations support the use of weak entity-tags,
637        or if it is unfeasible to send a strong entity-tag.</t>
638
639     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
640        unless the risk of a breakdown in semantic transparency that
641        could result from using this date in an If-Modified-Since header
642        would lead to serious problems.</t>
643  </list>
644</t>
645<t>
646   In other words, the preferred behavior for an HTTP/1.1 origin server
647   is to send both a strong entity-tag and a Last-Modified value.
648</t>
649<t>
650   In order to be legal, a strong entity-tag &MUST; change whenever the
651   associated representation changes in any way. A weak entity-tag &SHOULD;
652   change whenever the associated representation changes in a semantically
653   significant way.
654</t>
655<x:note>
656  <t>
657    <x:h>Note:</x:h> In order to provide semantically transparent caching, an
658    origin server must avoid reusing a specific strong entity-tag
659    value for two different representations, or reusing a specific weak
660    entity-tag value for two semantically different representations. Cache
661    entries might persist for arbitrarily long periods, regardless of
662    expiration times, so it might be inappropriate to expect that a
663    cache will never again attempt to validate an entry using a
664    validator that it obtained at some point in the past.
665  </t>
666</x:note>
667<t>
668   HTTP/1.1 clients:
669  <list style="symbols">
670     <t>&MUST; use that entity-tag in any cache-conditional request (using
671        If-Match or If-None-Match) if an entity-tag has been provided by the
672        origin server.</t>
673
674     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
675        requests (using If-Modified-Since) if only a Last-Modified value has
676        been provided by the origin server. </t>
677
678     <t>&MAY; use the Last-Modified value in subrange cache-conditional
679        requests (using If-Unmodified-Since) if only a Last-Modified value has
680        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
681        provide a way to disable this, in case of difficulty.</t>
682
683     <t>&SHOULD; use both validators in cache-conditional requests if both an
684        entity-tag and a Last-Modified value have been provided by the origin
685        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
686        appropriately.</t>
687  </list>
688</t>
689<t>
690   An HTTP/1.1 origin server, upon receiving a conditional request that
691   includes both a Last-Modified date (e.g., in an If-Modified-Since or
692   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
693   in an If-Match, If-None-Match, or If-Range header field) as cache
694   validators, &MUST-NOT; return a response status of 304 (Not Modified)
695   unless doing so is consistent with all of the conditional header
696   fields in the request.
697</t>
698<t>
699   An HTTP/1.1 caching proxy, upon receiving a conditional request that
700   includes both a Last-Modified date and one or more entity-tags as
701   cache validators, &MUST-NOT; return a locally cached response to the
702   client unless that cached response is consistent with all of the
703   conditional header fields in the request.
704  <list><t>
705      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
706      servers and clients should transmit as much non-redundant
707      information as is available in their responses and requests.
708      HTTP/1.1 systems receiving this information will make the most
709      conservative assumptions about the validators they receive.
710  </t><t>
711      HTTP/1.0 clients and caches will ignore entity-tags. Generally,
712      last-modified values received or used by these systems will
713      support transparent and efficient caching, and so HTTP/1.1 origin
714      servers should provide Last-Modified values. In those rare cases
715      where the use of a Last-Modified value as a validator by an
716      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
717      origin servers should not provide one.
718  </t></list>
719</t>
720</section>
721
722<section title="Header Field Definitions" anchor="header.fields">
723<t>
724   This section defines the syntax and semantics of HTTP/1.1 header fields
725   related to conditional requests.
726</t>
727<t>
728   For entity-header fields, both sender and recipient refer to either the
729   client or the server, depending on who sends and who receives the message.
730</t>
731
732<section title="ETag" anchor="header.etag">
733  <iref primary="true" item="ETag header" x:for-anchor=""/>
734  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
735  <x:anchor-alias value="ETag"/>
736  <x:anchor-alias value="ETag-v"/>
737<t>
738   The "ETag" response-header field provides the current value of the
739   entity-tag (see <xref target="entity.tags"/>) for one representation of
740   the resource identified by the Effective Request URI.  An entity-tag
741   is intended for use as a resource-local identifier for differentiating
742   between representations of the same resource that vary over time or via
743   content negotiation (see <xref target="weak.and.strong.validators"/>).
744</t>
745<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
746  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
747  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
748</artwork></figure>
749<figure><preamble>
750  Examples:
751</preamble>
752<artwork type="example">
753  ETag: "xyzzy"
754  ETag: W/"xyzzy"
755  ETag: ""
756</artwork></figure>
757<t>
758   An entity-tag provides an "opaque" cache validator that allows for
759   more reliable validation than modification dates in situations where
760   it is inconvenient to store modification dates,
761   where the one-second resolution of HTTP date values is not
762   sufficient, or where the origin server wishes to avoid certain
763   paradoxes that might arise from the use of modification dates.
764</t>
765<t>
766   The principle behind entity-tags is that only the service author
767   knows the semantics of a resource well enough to select an
768   appropriate cache validation mechanism, and the specification of any
769   validator comparison function more complex than byte-equality would
770   open up a can of worms. Thus, comparisons of any other headers
771   (except Last-Modified, for compatibility with HTTP/1.0) are never
772   used for purposes of validating a cache entry.
773</t>
774</section>
775
776<section title="If-Match" anchor="header.if-match">
777  <iref primary="true" item="If-Match header" x:for-anchor=""/>
778  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
779  <x:anchor-alias value="If-Match"/>
780  <x:anchor-alias value="If-Match-v"/>
781<t>
782   The "If-Match" request-header field is used to make a request method
783   conditional. A client that has one or more representations previously
784   obtained from the resource can verify that one of those representations is
785   current by including a list of their associated entity-tags in the
786   If-Match header field.
787</t>
788<t>
789   This allows efficient updates of cached information with a minimum amount of
790   transaction overhead. It is also used when updating resources, to prevent
791   inadvertent modification of the wrong version of a resource. As a special
792   case, the value "*" matches any current representation of the resource.
793</t>
794<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
795  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
796  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
797</artwork></figure>
798<t>
799   If any of the entity-tags match the entity-tag of the representation that
800   would have been returned in the response to a similar GET request
801   (without the If-Match header) on that resource, or if "*" is given
802   and any current representation exists for that resource, then the server &MAY;
803   perform the requested method as if the If-Match header field did not
804   exist.
805</t>
806<t>
807   If none of the entity-tags match, or if "*" is given and no current
808   representation exists, the server &MUST-NOT; perform the requested method, and
809   &MUST; return a 412 (Precondition Failed) response. This behavior is
810   most useful when the client wants to prevent an updating method, such
811   as PUT, from modifying a resource that has changed since the client
812   last retrieved it.
813</t>
814<t>
815   If the request would, without the If-Match header field, result in
816   anything other than a 2xx or 412 status, then the If-Match header
817   &MUST; be ignored.
818</t>
819<t>
820   The meaning of "If-Match: *" is that the method &SHOULD; be performed
821   if the representation selected by the origin server (or by a cache,
822   possibly using the Vary mechanism, see &header-vary;) exists, and
823   &MUST-NOT; be performed if the representation does not exist.
824</t>
825<t>
826   A request intended to update a resource (e.g., a PUT) &MAY; include an
827   If-Match header field to signal that the request method &MUST-NOT; be
828   applied if the representation corresponding to the If-Match value (a single
829   entity-tag) is no longer a representation of that resource. This
830   allows the user to indicate that they do not wish the request to be
831   successful if the resource has been changed without their knowledge.
832   Examples:
833</t>
834<figure><artwork type="example">
835  If-Match: "xyzzy"
836  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
837  If-Match: *
838</artwork></figure>
839<t>
840   The result of a request having both an If-Match header field and
841   either an If-None-Match or an If-Modified-Since header fields is
842   undefined by this specification.
843</t>
844</section>
845
846<section title="If-Modified-Since" anchor="header.if-modified-since">
847  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
848  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
849  <x:anchor-alias value="If-Modified-Since"/>
850  <x:anchor-alias value="If-Modified-Since-v"/>
851<t>
852   The "If-Modified-Since" request-header field is used to make a request
853   method conditional by date: if the representation that would have been
854   transferred in a 200 response to a GET request has not been modified since
855   the time specified in this field, then do not perform the method;
856   instead, respond as detailed below.
857</t>
858<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
859  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
860                        <x:ref>If-Modified-Since-v</x:ref>
861  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
862</artwork></figure>
863<t>
864   An example of the field is:
865</t>
866<figure><artwork type="example">
867  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
868</artwork></figure>
869<t>
870   A GET method with an If-Modified-Since header and no Range header
871   requests that the representation be transferred only if it has
872   been modified since the date given by the If-Modified-Since header.
873   The algorithm for determining this includes the following cases:
874  <list style="numbers">
875      <t>If the request would normally result in anything other than a
876         200 (OK) status, or if the passed If-Modified-Since date is
877         invalid, the response is exactly the same as for a normal GET.
878         A date which is later than the server's current time is
879         invalid.</t>
880
881      <t>If the representation has been modified since the If-Modified-Since
882         date, the response is exactly the same as for a normal GET.</t>
883
884      <t>If the representation has not been modified since a valid
885         If-Modified-Since date, the server &SHOULD; return a
886         304 (Not Modified) response.</t>
887  </list>
888</t>
889<t>
890   The purpose of this feature is to allow efficient updates of cached
891   information with a minimum amount of transaction overhead.
892  <list><t>
893      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
894      see &header-range; for full details.
895    </t><t>
896      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
897      clock might not be synchronized with the client.
898    </t><t>
899      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
900      servers will use an exact date comparison function, rather than a
901      less-than function, for deciding whether to send a 304 (Not
902      Modified) response. To get best results when sending an If-Modified-Since
903      header field for cache validation, clients are
904      advised to use the exact date string received in a previous Last-Modified
905      header field whenever possible.
906    </t><t>
907      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
908      header instead of a date taken from the Last-Modified header for
909      the same request, the client should be aware of the fact that this
910      date is interpreted in the server's understanding of time. The
911      client should consider unsynchronized clocks and rounding problems
912      due to the different encodings of time between the client and
913      server. This includes the possibility of race conditions if the
914      document has changed between the time it was first requested and
915      the If-Modified-Since date of a subsequent request, and the
916      possibility of clock-skew-related problems if the If-Modified-Since
917      date is derived from the client's clock without correction
918      to the server's clock. Corrections for different time bases
919      between client and server are at best approximate due to network
920      latency.
921    </t>
922  </list>
923</t>
924<t>
925   The result of a request having both an If-Modified-Since header field
926   and either an If-Match or an If-Unmodified-Since header fields is
927   undefined by this specification.
928</t>
929</section>
930
931<section title="If-None-Match" anchor="header.if-none-match">
932  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
933  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
934  <x:anchor-alias value="If-None-Match"/>
935  <x:anchor-alias value="If-None-Match-v"/>
936<t>
937   The "If-None-Match" request-header field is used to make a request method
938   conditional. A client that has one or more representations previously
939   obtained from the resource can verify that none of those representations is
940   current by including a list of their associated entity-tags in the
941   If-None-Match header field.
942</t>
943<t>
944   This allows efficient updates of cached information with a minimum amount of
945   transaction overhead. It is also used to prevent a method (e.g., PUT)
946   from inadvertently modifying an existing resource when the client
947   believes that the resource does not exist.
948</t>
949<t>
950   As a special case, the value "*" matches any current representation of the
951   resource.
952</t>
953<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
954  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
955  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
956</artwork></figure>
957<t>
958   If any of the entity-tags match the entity-tag of the representation that
959   would have been returned in the response to a similar GET request
960   (without the If-None-Match header) on that resource, or if "*" is
961   given and any current representation exists for that resource, then the
962   server &MUST-NOT; perform the requested method, unless required to do
963   so because the resource's modification date fails to match that
964   supplied in an If-Modified-Since header field in the request.
965   Instead, if the request method was GET or HEAD, the server &SHOULD;
966   respond with a 304 (Not Modified) response, including the cache-related
967   header fields (particularly ETag) of one of the representations that
968   matched. For all other request methods, the server &MUST; respond with
969   a status of 412 (Precondition Failed).
970</t>
971<t>
972   If none of the entity-tags match, then the server &MAY; perform the
973   requested method as if the If-None-Match header field did not exist,
974   but &MUST; also ignore any If-Modified-Since header field(s) in the
975   request. That is, if no entity-tags match, then the server &MUST-NOT;
976   return a 304 (Not Modified) response.
977</t>
978<t>
979   If the request would, without the If-None-Match header field, result
980   in anything other than a 2xx or 304 status, then the If-None-Match
981   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
982   server behavior when both If-Modified-Since and If-None-Match appear
983   in the same request.)
984</t>
985<t>
986   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
987   performed if the representation selected by the origin server (or by
988   a cache, possibly using the Vary mechanism, see &header-vary;)
989   exists, and &SHOULD; be performed if the representation does not exist.
990   This feature is intended to be useful in preventing races between PUT
991   operations.
992</t>
993<t>
994   Examples:
995</t>
996<figure><artwork type="example">
997  If-None-Match: "xyzzy"
998  If-None-Match: W/"xyzzy"
999  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
1000  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
1001  If-None-Match: *
1002</artwork></figure>
1003<t>
1004   The result of a request having both an If-None-Match header field and
1005   either an If-Match or an If-Unmodified-Since header fields is
1006   undefined by this specification.
1007</t>
1008</section>
1009
1010<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
1011  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
1012  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
1013  <x:anchor-alias value="If-Unmodified-Since"/>
1014  <x:anchor-alias value="If-Unmodified-Since-v"/>
1015<t>
1016   The "If-Unmodified-Since" request-header field is used to make a request
1017   method conditional.  If the representation that would have been transferred
1018   in a 200 response to a GET request on the same resource has not been modified
1019   since the time specified in this field, the server &SHOULD; perform the
1020   requested operation as if the If-Unmodified-Since header were not
1021   present.
1022</t>
1023<t>
1024   If the representation has been modified since the specified time,
1025   the server &MUST-NOT; perform the requested operation, and &MUST; return
1026   a 412 (Precondition Failed).
1027</t>
1028<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
1029  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
1030                          <x:ref>If-Unmodified-Since-v</x:ref>
1031  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
1032</artwork></figure>
1033<t>
1034   An example of the field is:
1035</t>
1036<figure><artwork type="example">
1037  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1038</artwork></figure>
1039<t>
1040   If the request normally (i.e., without the If-Unmodified-Since
1041   header) would result in anything other than a 2xx or 412 status, the
1042   If-Unmodified-Since header &SHOULD; be ignored.
1043</t>
1044<t>
1045   If the specified date is invalid, the header is ignored.
1046</t>
1047<t>
1048   The result of a request having both an If-Unmodified-Since header
1049   field and either an If-None-Match or an If-Modified-Since header
1050   fields is undefined by this specification.
1051</t>
1052</section>
1053
1054<section title="Last-Modified" anchor="header.last-modified">
1055  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
1056  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
1057  <x:anchor-alias value="Last-Modified"/>
1058  <x:anchor-alias value="Last-Modified-v"/>
1059<t>
1060   The "Last-Modified" entity-header field indicates the date and time at
1061   which the origin server believes the representation was last modified.
1062</t>
1063<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
1064  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
1065  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
1066</artwork></figure>
1067<t>
1068   An example of its use is
1069</t>
1070<figure><artwork type="example">
1071  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1072</artwork></figure>
1073<t>
1074   The exact meaning of this header field depends on the implementation
1075   of the origin server and the nature of the original resource. For
1076   files, it may be just the file system last-modified time. For
1077   representations with dynamically included parts, it may be the most recent
1078   of the set of last-modify times for its component parts. For database
1079   gateways, it may be the last-update time stamp of the record. For
1080   virtual objects, it may be the last time the internal state changed.
1081</t>
1082<t>
1083   An origin server &MUST-NOT; send a Last-Modified date which is later
1084   than the server's time of message origination. In such cases, where
1085   the resource's last modification would indicate some time in the
1086   future, the server &MUST; replace that date with the message
1087   origination date.
1088</t>
1089<t>
1090   An origin server &SHOULD; obtain the Last-Modified value of the representation
1091   as close as possible to the time that it generates the Date value of
1092   its response. This allows a recipient to make an accurate assessment
1093   of the representation's modification time, especially if the representation changes
1094   near the time that the response is generated.
1095</t>
1096<t>
1097   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1098</t>
1099<t>
1100   The Last-Modified entity-header field value is often used as a cache
1101   validator. In simple terms, a cache entry is considered to be valid
1102   if the representation has not been modified since the Last-Modified value.
1103</t>
1104</section>
1105
1106</section>
1107
1108<section title="IANA Considerations" anchor="IANA.considerations">
1109
1110<section title="Status Code Registration" anchor="status.code.registration">
1111<t>
1112   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1113   should be updated with the registrations below:
1114</t>
1115<?BEGININC p4-conditional.iana-status-codes ?>
1116<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1117<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1118   <ttcol>Value</ttcol>
1119   <ttcol>Description</ttcol>
1120   <ttcol>Reference</ttcol>
1121   <c>304</c>
1122   <c>Not Modified</c>
1123   <c>
1124      <xref target="status.304"/>
1125   </c>
1126   <c>412</c>
1127   <c>Precondition Failed</c>
1128   <c>
1129      <xref target="status.412"/>
1130   </c>
1131</texttable>
1132<!--(END)-->
1133<?ENDINC p4-conditional.iana-status-codes ?>
1134</section>
1135
1136<section title="Message Header Registration" anchor="message.header.registration">
1137<t>
1138   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1139   with the permanent registrations below (see <xref target="RFC3864"/>):
1140</t>
1141<?BEGININC p4-conditional.iana-headers ?>
1142<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1143<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1144   <ttcol>Header Field Name</ttcol>
1145   <ttcol>Protocol</ttcol>
1146   <ttcol>Status</ttcol>
1147   <ttcol>Reference</ttcol>
1148
1149   <c>ETag</c>
1150   <c>http</c>
1151   <c>standard</c>
1152   <c>
1153      <xref target="header.etag"/>
1154   </c>
1155   <c>If-Match</c>
1156   <c>http</c>
1157   <c>standard</c>
1158   <c>
1159      <xref target="header.if-match"/>
1160   </c>
1161   <c>If-Modified-Since</c>
1162   <c>http</c>
1163   <c>standard</c>
1164   <c>
1165      <xref target="header.if-modified-since"/>
1166   </c>
1167   <c>If-None-Match</c>
1168   <c>http</c>
1169   <c>standard</c>
1170   <c>
1171      <xref target="header.if-none-match"/>
1172   </c>
1173   <c>If-Unmodified-Since</c>
1174   <c>http</c>
1175   <c>standard</c>
1176   <c>
1177      <xref target="header.if-unmodified-since"/>
1178   </c>
1179   <c>Last-Modified</c>
1180   <c>http</c>
1181   <c>standard</c>
1182   <c>
1183      <xref target="header.last-modified"/>
1184   </c>
1185</texttable>
1186<!--(END)-->
1187<?ENDINC p4-conditional.iana-headers ?>
1188<t>
1189   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1190</t>
1191</section>
1192</section>
1193
1194<section title="Security Considerations" anchor="security.considerations">
1195<t>
1196   No additional security considerations have been identified beyond
1197   those applicable to HTTP in general &messaging;.
1198</t>
1199</section>
1200
1201<section title="Acknowledgments" anchor="ack">
1202</section>
1203</middle>
1204<back>
1205
1206<references title="Normative References">
1207
1208<reference anchor="Part1">
1209  <front>
1210    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1211    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1212      <organization abbrev="Day Software">Day Software</organization>
1213      <address><email>fielding@gbiv.com</email></address>
1214    </author>
1215    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1216      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1217      <address><email>jg@freedesktop.org</email></address>
1218    </author>
1219    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1220      <organization abbrev="HP">Hewlett-Packard Company</organization>
1221      <address><email>JeffMogul@acm.org</email></address>
1222    </author>
1223    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1224      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1225      <address><email>henrikn@microsoft.com</email></address>
1226    </author>
1227    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1228      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1229      <address><email>LMM@acm.org</email></address>
1230    </author>
1231    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1232      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1233      <address><email>paulle@microsoft.com</email></address>
1234    </author>
1235    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1236      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1237      <address><email>timbl@w3.org</email></address>
1238    </author>
1239    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1240      <organization abbrev="W3C">World Wide Web Consortium</organization>
1241      <address><email>ylafon@w3.org</email></address>
1242    </author>
1243    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1244      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1245      <address><email>julian.reschke@greenbytes.de</email></address>
1246    </author>
1247    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1248  </front>
1249  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1250  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1251</reference>
1252
1253<reference anchor="Part3">
1254  <front>
1255    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1256    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1257      <organization abbrev="Day Software">Day Software</organization>
1258      <address><email>fielding@gbiv.com</email></address>
1259    </author>
1260    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1261      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1262      <address><email>jg@freedesktop.org</email></address>
1263    </author>
1264    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1265      <organization abbrev="HP">Hewlett-Packard Company</organization>
1266      <address><email>JeffMogul@acm.org</email></address>
1267    </author>
1268    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1269      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1270      <address><email>henrikn@microsoft.com</email></address>
1271    </author>
1272    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1273      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1274      <address><email>LMM@acm.org</email></address>
1275    </author>
1276    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1277      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1278      <address><email>paulle@microsoft.com</email></address>
1279    </author>
1280    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1281      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1282      <address><email>timbl@w3.org</email></address>
1283    </author>
1284    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1285      <organization abbrev="W3C">World Wide Web Consortium</organization>
1286      <address><email>ylafon@w3.org</email></address>
1287    </author>
1288    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1289      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1290      <address><email>julian.reschke@greenbytes.de</email></address>
1291    </author>
1292    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1293  </front>
1294  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1295  <x:source href="p3-payload.xml" basename="p3-payload"/>
1296</reference>
1297
1298<reference anchor="Part5">
1299  <front>
1300    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1301    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1302      <organization abbrev="Day Software">Day Software</organization>
1303      <address><email>fielding@gbiv.com</email></address>
1304    </author>
1305    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1306      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1307      <address><email>jg@freedesktop.org</email></address>
1308    </author>
1309    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1310      <organization abbrev="HP">Hewlett-Packard Company</organization>
1311      <address><email>JeffMogul@acm.org</email></address>
1312    </author>
1313    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1314      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1315      <address><email>henrikn@microsoft.com</email></address>
1316    </author>
1317    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1318      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1319      <address><email>LMM@acm.org</email></address>
1320    </author>
1321    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1322      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1323      <address><email>paulle@microsoft.com</email></address>
1324    </author>
1325    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1326      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1327      <address><email>timbl@w3.org</email></address>
1328    </author>
1329    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1330      <organization abbrev="W3C">World Wide Web Consortium</organization>
1331      <address><email>ylafon@w3.org</email></address>
1332    </author>
1333    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1334      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1335      <address><email>julian.reschke@greenbytes.de</email></address>
1336    </author>
1337    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1338  </front>
1339  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1340  <x:source href="p5-range.xml" basename="p5-range"/>
1341</reference>
1342
1343<reference anchor="Part6">
1344  <front>
1345    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1346    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1347      <organization abbrev="Day Software">Day Software</organization>
1348      <address><email>fielding@gbiv.com</email></address>
1349    </author>
1350    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1351      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1352      <address><email>jg@freedesktop.org</email></address>
1353    </author>
1354    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1355      <organization abbrev="HP">Hewlett-Packard Company</organization>
1356      <address><email>JeffMogul@acm.org</email></address>
1357    </author>
1358    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1359      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1360      <address><email>henrikn@microsoft.com</email></address>
1361    </author>
1362    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1363      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1364      <address><email>LMM@acm.org</email></address>
1365    </author>
1366    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1367      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1368      <address><email>paulle@microsoft.com</email></address>
1369    </author>
1370    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1371      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1372      <address><email>timbl@w3.org</email></address>
1373    </author>
1374    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1375      <organization abbrev="W3C">World Wide Web Consortium</organization>
1376      <address><email>ylafon@w3.org</email></address>
1377    </author>
1378    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1379      <address><email>mnot@mnot.net</email></address>
1380    </author>
1381    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1382      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1383      <address><email>julian.reschke@greenbytes.de</email></address>
1384    </author>
1385    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1386  </front>
1387  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1388  <x:source href="p6-cache.xml" basename="p6-cache"/>
1389</reference>
1390
1391<reference anchor="RFC2119">
1392  <front>
1393    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1394    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1395      <organization>Harvard University</organization>
1396      <address><email>sob@harvard.edu</email></address>
1397    </author>
1398    <date month="March" year="1997"/>
1399  </front>
1400  <seriesInfo name="BCP" value="14"/>
1401  <seriesInfo name="RFC" value="2119"/>
1402</reference>
1403
1404<reference anchor="RFC5234">
1405  <front>
1406    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1407    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1408      <organization>Brandenburg InternetWorking</organization>
1409      <address>
1410        <email>dcrocker@bbiw.net</email>
1411      </address> 
1412    </author>
1413    <author initials="P." surname="Overell" fullname="Paul Overell">
1414      <organization>THUS plc.</organization>
1415      <address>
1416        <email>paul.overell@thus.net</email>
1417      </address>
1418    </author>
1419    <date month="January" year="2008"/>
1420  </front>
1421  <seriesInfo name="STD" value="68"/>
1422  <seriesInfo name="RFC" value="5234"/>
1423</reference>
1424
1425</references>
1426
1427<references title="Informative References">
1428
1429<reference anchor="RFC2616">
1430  <front>
1431    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1432    <author initials="R." surname="Fielding" fullname="R. Fielding">
1433      <organization>University of California, Irvine</organization>
1434      <address><email>fielding@ics.uci.edu</email></address>
1435    </author>
1436    <author initials="J." surname="Gettys" fullname="J. Gettys">
1437      <organization>W3C</organization>
1438      <address><email>jg@w3.org</email></address>
1439    </author>
1440    <author initials="J." surname="Mogul" fullname="J. Mogul">
1441      <organization>Compaq Computer Corporation</organization>
1442      <address><email>mogul@wrl.dec.com</email></address>
1443    </author>
1444    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1445      <organization>MIT Laboratory for Computer Science</organization>
1446      <address><email>frystyk@w3.org</email></address>
1447    </author>
1448    <author initials="L." surname="Masinter" fullname="L. Masinter">
1449      <organization>Xerox Corporation</organization>
1450      <address><email>masinter@parc.xerox.com</email></address>
1451    </author>
1452    <author initials="P." surname="Leach" fullname="P. Leach">
1453      <organization>Microsoft Corporation</organization>
1454      <address><email>paulle@microsoft.com</email></address>
1455    </author>
1456    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1457      <organization>W3C</organization>
1458      <address><email>timbl@w3.org</email></address>
1459    </author>
1460    <date month="June" year="1999"/>
1461  </front>
1462  <seriesInfo name="RFC" value="2616"/>
1463</reference>
1464
1465<reference anchor='RFC3864'>
1466  <front>
1467    <title>Registration Procedures for Message Header Fields</title>
1468    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1469      <organization>Nine by Nine</organization>
1470      <address><email>GK-IETF@ninebynine.org</email></address>
1471    </author>
1472    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1473      <organization>BEA Systems</organization>
1474      <address><email>mnot@pobox.com</email></address>
1475    </author>
1476    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1477      <organization>HP Labs</organization>
1478      <address><email>JeffMogul@acm.org</email></address>
1479    </author>
1480    <date year='2004' month='September' />
1481  </front>
1482  <seriesInfo name='BCP' value='90' />
1483  <seriesInfo name='RFC' value='3864' />
1484</reference>
1485
1486</references>
1487
1488<section title="Compatibility with Previous Versions" anchor="compatibility">
1489
1490<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1491<t>
1492  Allow weak entity-tags in all requests except range requests (Sections
1493  <xref target="weak.and.strong.validators" format="counter"/> and
1494  <xref target="header.if-none-match" format="counter"/>).
1495</t>
1496</section>
1497
1498</section>
1499
1500<?BEGININC p4-conditional.abnf-appendix ?>
1501<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1502<figure>
1503<artwork type="abnf" name="p4-conditional.parsed-abnf">
1504<x:ref>ETag</x:ref> = "ETag:" OWS ETag-v
1505<x:ref>ETag-v</x:ref> = entity-tag
1506
1507<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1508
1509<x:ref>If-Match</x:ref> = "If-Match:" OWS If-Match-v
1510<x:ref>If-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1511 entity-tag ] ) )
1512<x:ref>If-Modified-Since</x:ref> = "If-Modified-Since:" OWS If-Modified-Since-v
1513<x:ref>If-Modified-Since-v</x:ref> = HTTP-date
1514<x:ref>If-None-Match</x:ref> = "If-None-Match:" OWS If-None-Match-v
1515<x:ref>If-None-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1516 entity-tag ] ) )
1517<x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since:" OWS
1518 If-Unmodified-Since-v
1519<x:ref>If-Unmodified-Since-v</x:ref> = HTTP-date
1520
1521<x:ref>Last-Modified</x:ref> = "Last-Modified:" OWS Last-Modified-v
1522<x:ref>Last-Modified-v</x:ref> = HTTP-date
1523
1524<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1525
1526<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1527
1528<x:ref>opaque-tag</x:ref> = quoted-string
1529
1530<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1531
1532<x:ref>weak</x:ref> = %x57.2F ; W/
1533</artwork>
1534</figure>
1535<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1536; ETag defined but not used
1537; If-Match defined but not used
1538; If-Modified-Since defined but not used
1539; If-None-Match defined but not used
1540; If-Unmodified-Since defined but not used
1541; Last-Modified defined but not used
1542</artwork></figure></section>
1543<?ENDINC p4-conditional.abnf-appendix ?>
1544
1545<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1546
1547<section title="Since RFC2616">
1548<t>
1549  Extracted relevant partitions from <xref target="RFC2616"/>.
1550</t>
1551</section>
1552
1553<section title="Since draft-ietf-httpbis-p4-conditional-00">
1554<t>
1555  Closed issues:
1556  <list style="symbols"> 
1557    <t>
1558      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1559      "Normative and Informative references"
1560    </t>
1561  </list>
1562</t>
1563<t>
1564  Other changes:
1565  <list style="symbols"> 
1566    <t>
1567      Move definitions of 304 and 412 condition codes from Part2.
1568    </t>
1569  </list>
1570</t>
1571</section>
1572
1573<section title="Since draft-ietf-httpbis-p4-conditional-01">
1574<t>
1575  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1576  <list style="symbols"> 
1577    <t>
1578      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1579    </t>
1580  </list>
1581</t>
1582</section>
1583
1584<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1585<t>
1586  Closed issues:
1587  <list style="symbols"> 
1588    <t>
1589      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1590      "Weak ETags on non-GET requests"
1591    </t>
1592  </list>
1593</t>
1594<t>
1595  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1596  <list style="symbols"> 
1597    <t>
1598      Reference RFC 3984, and update header registrations for headers defined
1599      in this document.
1600    </t>
1601  </list>
1602</t>
1603</section>
1604
1605<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1606<t>
1607  Closed issues:
1608  <list style="symbols"> 
1609    <t>
1610      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1611      "Examples for ETag matching"
1612    </t>
1613    <t>
1614      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1615      "'entity value' undefined"
1616    </t>
1617    <t>
1618      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1619      "bogus 2068 Date header reference"
1620    </t>
1621  </list>
1622</t>
1623</section>
1624
1625<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1626<t>
1627  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1628  <list style="symbols"> 
1629    <t>
1630      Use "/" instead of "|" for alternatives.
1631    </t>
1632    <t>
1633      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1634      whitespace ("OWS") and required whitespace ("RWS").
1635    </t>
1636    <t>
1637      Rewrite ABNFs to spell out whitespace rules, factor out
1638      header value format definitions.
1639    </t>
1640  </list>
1641</t>
1642</section>
1643
1644<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1645<t>
1646  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1647  <list style="symbols"> 
1648    <t>
1649      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1650    </t>
1651  </list>
1652</t>
1653</section>
1654
1655<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1656<t>
1657  Closed issues:
1658  <list style="symbols"> 
1659    <t>
1660      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1661      "case-sensitivity of etag weakness indicator"
1662    </t>
1663  </list>
1664</t>
1665</section>
1666
1667<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1668<t>
1669  Closed issues:
1670  <list style="symbols"> 
1671    <t>
1672      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1673      "Weak ETags on non-GET requests" (If-Match still was defined to require
1674      strong matching)
1675    </t>
1676    <t>
1677      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1678      "move IANA registrations for optional status codes"
1679    </t>
1680  </list>
1681</t>
1682</section>
1683
1684<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1685<t>
1686  No significant changes.
1687</t>
1688</section>
1689
1690<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1691<t>
1692  No significant changes.
1693</t>
1694</section>
1695
1696<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1697<t>
1698  Closed issues:
1699  <list style="symbols"> 
1700    <t>
1701      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1702      "Clarify 'Requested Variant'"
1703    </t>
1704    <t>
1705      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1706      "Clarify entity / representation / variant terminology"
1707    </t>
1708  </list>
1709</t>
1710</section>
1711
1712</section>
1713
1714</back>
1715</rfc>
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