source: draft-ietf-httpbis/latest/p4-conditional.xml @ 965

Last change on this file since 965 was 965, checked in by fielding@…, 9 years ago

Addresses #109: Clarify entity / representation / variant terminology

Removed entity-header adjective and ABNF and clarified distinction
between payload and representation.

Uncapitalize the phrase effective request URI so that it doesn't
dominate the prose, and define the term "target resource" to be
used instead of the "resource identified by the effective request URI".

  • Property svn:eol-style set to native
File size: 69.8 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 representations of the same
315   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 might or might 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 &MAY; 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 code 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
728<section title="ETag" anchor="header.etag">
729  <iref primary="true" item="ETag header" x:for-anchor=""/>
730  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
731  <x:anchor-alias value="ETag"/>
732  <x:anchor-alias value="ETag-v"/>
733<t>
734   The "ETag" response-header field provides the current value of the
735   entity-tag (see <xref target="entity.tags"/>) for one representation of
736   the resource identified by the effective request URI.  An entity-tag
737   is intended for use as a resource-local identifier for differentiating
738   between representations of the same resource that vary over time or via
739   content negotiation (see <xref target="weak.and.strong.validators"/>).
740</t>
741<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
742  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
743  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
744</artwork></figure>
745<figure><preamble>
746  Examples:
747</preamble>
748<artwork type="example">
749  ETag: "xyzzy"
750  ETag: W/"xyzzy"
751  ETag: ""
752</artwork></figure>
753<t>
754   An entity-tag provides an "opaque" cache validator that allows for
755   more reliable validation than modification dates in situations where
756   it is inconvenient to store modification dates,
757   where the one-second resolution of HTTP date values is not
758   sufficient, or where the origin server wishes to avoid certain
759   paradoxes that might arise from the use of modification dates.
760</t>
761<t>
762   The principle behind entity-tags is that only the service author
763   knows the semantics of a resource well enough to select an
764   appropriate cache validation mechanism, and the specification of any
765   validator comparison function more complex than byte-equality would
766   open up a can of worms. Thus, comparisons of any other headers
767   (except Last-Modified, for compatibility with HTTP/1.0) are never
768   used for purposes of validating a cache entry.
769</t>
770</section>
771
772<section title="If-Match" anchor="header.if-match">
773  <iref primary="true" item="If-Match header" x:for-anchor=""/>
774  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
775  <x:anchor-alias value="If-Match"/>
776  <x:anchor-alias value="If-Match-v"/>
777<t>
778   The "If-Match" request-header field is used to make a request method
779   conditional. A client that has one or more representations previously
780   obtained from the resource can verify that one of those representations is
781   current by including a list of their associated entity-tags in the
782   If-Match header field.
783</t>
784<t>
785   This allows efficient updates of cached information with a minimum amount of
786   transaction overhead. It is also used when updating resources, to prevent
787   inadvertent modification of the wrong version of a resource. As a special
788   case, the value "*" matches any current representation of the resource.
789</t>
790<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
791  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
792  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
793</artwork></figure>
794<t>
795   If any of the entity-tags match the entity-tag of the representation that
796   would have been returned in the response to a similar GET request
797   (without the If-Match header) on that resource, or if "*" is given
798   and any current representation exists for that resource, then the server &MAY;
799   perform the requested method as if the If-Match header field did not
800   exist.
801</t>
802<t>
803   If none of the entity-tags match, or if "*" is given and no current
804   representation exists, the server &MUST-NOT; perform the requested method, and
805   &MUST; return a 412 (Precondition Failed) response. This behavior is
806   most useful when the client wants to prevent an updating method, such
807   as PUT, from modifying a resource that has changed since the client
808   last retrieved it.
809</t>
810<t>
811   If the request would, without the If-Match header field, result in
812   anything other than a 2xx or 412 status code, then the If-Match header
813   &MUST; be ignored.
814</t>
815<t>
816   The meaning of "If-Match: *" is that the method &SHOULD; be performed
817   if the representation selected by the origin server (or by a cache,
818   possibly using the Vary mechanism, see &header-vary;) exists, and
819   &MUST-NOT; be performed if the representation does not exist.
820</t>
821<t>
822   A request intended to update a resource (e.g., a PUT) &MAY; include an
823   If-Match header field to signal that the request method &MUST-NOT; be
824   applied if the representation corresponding to the If-Match value (a single
825   entity-tag) is no longer a representation of that resource. This
826   allows the user to indicate that they do not wish the request to be
827   successful if the resource has been changed without their knowledge.
828   Examples:
829</t>
830<figure><artwork type="example">
831  If-Match: "xyzzy"
832  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
833  If-Match: *
834</artwork></figure>
835<t>
836   The result of a request having both an If-Match header field and
837   either an If-None-Match or an If-Modified-Since header fields is
838   undefined by this specification.
839</t>
840</section>
841
842<section title="If-Modified-Since" anchor="header.if-modified-since">
843  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
844  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
845  <x:anchor-alias value="If-Modified-Since"/>
846  <x:anchor-alias value="If-Modified-Since-v"/>
847<t>
848   The "If-Modified-Since" request-header field is used to make a request
849   method conditional by date: if the representation that would have been
850   transferred in a 200 response to a GET request has not been modified since
851   the time specified in this field, then do not perform the method;
852   instead, respond as detailed below.
853</t>
854<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
855  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
856                        <x:ref>If-Modified-Since-v</x:ref>
857  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
858</artwork></figure>
859<t>
860   An example of the field is:
861</t>
862<figure><artwork type="example">
863  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
864</artwork></figure>
865<t>
866   A GET method with an If-Modified-Since header and no Range header
867   requests that the representation be transferred only if it has
868   been modified since the date given by the If-Modified-Since header.
869   The algorithm for determining this includes the following cases:
870  <list style="numbers">
871      <t>If the request would normally result in anything other than a
872         200 (OK) status code, or if the passed If-Modified-Since date is
873         invalid, the response is exactly the same as for a normal GET.
874         A date which is later than the server's current time is
875         invalid.</t>
876
877      <t>If the representation has been modified since the If-Modified-Since
878         date, the response is exactly the same as for a normal GET.</t>
879
880      <t>If the representation has not been modified since a valid
881         If-Modified-Since date, the server &SHOULD; return a
882         304 (Not Modified) response.</t>
883  </list>
884</t>
885<t>
886   The purpose of this feature is to allow efficient updates of cached
887   information with a minimum amount of transaction overhead.
888  <list><t>
889      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
890      see &header-range; for full details.
891    </t><t>
892      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
893      clock might not be synchronized with the client.
894    </t><t>
895      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
896      servers will use an exact date comparison function, rather than a
897      less-than function, for deciding whether to send a 304 (Not
898      Modified) response. To get best results when sending an If-Modified-Since
899      header field for cache validation, clients are
900      advised to use the exact date string received in a previous Last-Modified
901      header field whenever possible.
902    </t><t>
903      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
904      header instead of a date taken from the Last-Modified header for
905      the same request, the client should be aware of the fact that this
906      date is interpreted in the server's understanding of time. The
907      client should consider unsynchronized clocks and rounding problems
908      due to the different encodings of time between the client and
909      server. This includes the possibility of race conditions if the
910      document has changed between the time it was first requested and
911      the If-Modified-Since date of a subsequent request, and the
912      possibility of clock-skew-related problems if the If-Modified-Since
913      date is derived from the client's clock without correction
914      to the server's clock. Corrections for different time bases
915      between client and server are at best approximate due to network
916      latency.
917    </t>
918  </list>
919</t>
920<t>
921   The result of a request having both an If-Modified-Since header field
922   and either an If-Match or an If-Unmodified-Since header fields is
923   undefined by this specification.
924</t>
925</section>
926
927<section title="If-None-Match" anchor="header.if-none-match">
928  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
929  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
930  <x:anchor-alias value="If-None-Match"/>
931  <x:anchor-alias value="If-None-Match-v"/>
932<t>
933   The "If-None-Match" request-header field is used to make a request method
934   conditional. A client that has one or more representations previously
935   obtained from the resource can verify that none of those representations is
936   current by including a list of their associated entity-tags in the
937   If-None-Match header field.
938</t>
939<t>
940   This allows efficient updates of cached information with a minimum amount of
941   transaction overhead. It is also used to prevent a method (e.g., PUT)
942   from inadvertently modifying an existing resource when the client
943   believes that the resource does not exist.
944</t>
945<t>
946   As a special case, the value "*" matches any current representation of the
947   resource.
948</t>
949<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
950  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
951  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
952</artwork></figure>
953<t>
954   If any of the entity-tags match the entity-tag of the representation that
955   would have been returned in the response to a similar GET request
956   (without the If-None-Match header) on that resource, or if "*" is
957   given and any current representation exists for that resource, then the
958   server &MUST-NOT; perform the requested method, unless required to do
959   so because the resource's modification date fails to match that
960   supplied in an If-Modified-Since header field in the request.
961   Instead, if the request method was GET or HEAD, the server &SHOULD;
962   respond with a 304 (Not Modified) response, including the cache-related
963   header fields (particularly ETag) of one of the representations that
964   matched. For all other request methods, the server &MUST; respond with
965   a 412 (Precondition Failed) status code.
966</t>
967<t>
968   If none of the entity-tags match, then the server &MAY; perform the
969   requested method as if the If-None-Match header field did not exist,
970   but &MUST; also ignore any If-Modified-Since header field(s) in the
971   request. That is, if no entity-tags match, then the server &MUST-NOT;
972   return a 304 (Not Modified) response.
973</t>
974<t>
975   If the request would, without the If-None-Match header field, result
976   in anything other than a 2xx or 304 status code, then the If-None-Match
977   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
978   server behavior when both If-Modified-Since and If-None-Match appear
979   in the same request.)
980</t>
981<t>
982   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
983   performed if the representation selected by the origin server (or by
984   a cache, possibly using the Vary mechanism, see &header-vary;)
985   exists, and &SHOULD; be performed if the representation does not exist.
986   This feature is intended to be useful in preventing races between PUT
987   operations.
988</t>
989<t>
990   Examples:
991</t>
992<figure><artwork type="example">
993  If-None-Match: "xyzzy"
994  If-None-Match: W/"xyzzy"
995  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
996  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
997  If-None-Match: *
998</artwork></figure>
999<t>
1000   The result of a request having both an If-None-Match header field and
1001   either an If-Match or an If-Unmodified-Since header fields is
1002   undefined by this specification.
1003</t>
1004</section>
1005
1006<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
1007  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
1008  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
1009  <x:anchor-alias value="If-Unmodified-Since"/>
1010  <x:anchor-alias value="If-Unmodified-Since-v"/>
1011<t>
1012   The "If-Unmodified-Since" request-header field is used to make a request
1013   method conditional.  If the representation that would have been transferred
1014   in a 200 response to a GET request on the same resource has not been modified
1015   since the time specified in this field, the server &SHOULD; perform the
1016   requested operation as if the If-Unmodified-Since header were not
1017   present.
1018</t>
1019<t>
1020   If the representation has been modified since the specified time,
1021   the server &MUST-NOT; perform the requested operation, and &MUST; return
1022   a 412 (Precondition Failed).
1023</t>
1024<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
1025  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
1026                          <x:ref>If-Unmodified-Since-v</x:ref>
1027  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
1028</artwork></figure>
1029<t>
1030   An example of the field is:
1031</t>
1032<figure><artwork type="example">
1033  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1034</artwork></figure>
1035<t>
1036   If the request normally (i.e., without the If-Unmodified-Since
1037   header) would result in anything other than a 2xx or 412 status code,
1038   the If-Unmodified-Since header &SHOULD; be ignored.
1039</t>
1040<t>
1041   If the specified date is invalid, the header is ignored.
1042</t>
1043<t>
1044   The result of a request having both an If-Unmodified-Since header
1045   field and either an If-None-Match or an If-Modified-Since header
1046   fields is undefined by this specification.
1047</t>
1048</section>
1049
1050<section title="Last-Modified" anchor="header.last-modified">
1051  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
1052  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
1053  <x:anchor-alias value="Last-Modified"/>
1054  <x:anchor-alias value="Last-Modified-v"/>
1055<t>
1056   The "Last-Modified" header field indicates the date and time at
1057   which the origin server believes the representation was last modified.
1058</t>
1059<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
1060  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
1061  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
1062</artwork></figure>
1063<t>
1064   An example of its use is
1065</t>
1066<figure><artwork type="example">
1067  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1068</artwork></figure>
1069<t>
1070   The exact meaning of this header field depends on the implementation
1071   of the origin server and the nature of the original resource. For
1072   files, it might be just the file system last-modified time. For
1073   representations with dynamically included parts, it might be the most recent
1074   of the set of last-modify times for its component parts. For database
1075   gateways, it might be the last-update time stamp of the record. For
1076   virtual objects, it might be the last time the internal state changed.
1077</t>
1078<t>
1079   An origin server &MUST-NOT; send a Last-Modified date which is later
1080   than the server's time of message origination. In such cases, where
1081   the resource's last modification would indicate some time in the
1082   future, the server &MUST; replace that date with the message
1083   origination date.
1084</t>
1085<t>
1086   An origin server &SHOULD; obtain the Last-Modified value of the representation
1087   as close as possible to the time that it generates the Date value of
1088   its response. This allows a recipient to make an accurate assessment
1089   of the representation's modification time, especially if the representation changes
1090   near the time that the response is generated.
1091</t>
1092<t>
1093   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1094</t>
1095<t>
1096   The Last-Modified header field value is often used as a cache
1097   validator. In simple terms, a cache entry is considered to be valid
1098   if the representation has not been modified since the Last-Modified value.
1099</t>
1100</section>
1101
1102</section>
1103
1104<section title="IANA Considerations" anchor="IANA.considerations">
1105
1106<section title="Status Code Registration" anchor="status.code.registration">
1107<t>
1108   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1109   should be updated with the registrations below:
1110</t>
1111<?BEGININC p4-conditional.iana-status-codes ?>
1112<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1113<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1114   <ttcol>Value</ttcol>
1115   <ttcol>Description</ttcol>
1116   <ttcol>Reference</ttcol>
1117   <c>304</c>
1118   <c>Not Modified</c>
1119   <c>
1120      <xref target="status.304"/>
1121   </c>
1122   <c>412</c>
1123   <c>Precondition Failed</c>
1124   <c>
1125      <xref target="status.412"/>
1126   </c>
1127</texttable>
1128<!--(END)-->
1129<?ENDINC p4-conditional.iana-status-codes ?>
1130</section>
1131
1132<section title="Header Field Registration" anchor="header.field.registration">
1133<t>
1134   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1135   with the permanent registrations below (see <xref target="RFC3864"/>):
1136</t>
1137<?BEGININC p4-conditional.iana-headers ?>
1138<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1139<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1140   <ttcol>Header Field Name</ttcol>
1141   <ttcol>Protocol</ttcol>
1142   <ttcol>Status</ttcol>
1143   <ttcol>Reference</ttcol>
1144
1145   <c>ETag</c>
1146   <c>http</c>
1147   <c>standard</c>
1148   <c>
1149      <xref target="header.etag"/>
1150   </c>
1151   <c>If-Match</c>
1152   <c>http</c>
1153   <c>standard</c>
1154   <c>
1155      <xref target="header.if-match"/>
1156   </c>
1157   <c>If-Modified-Since</c>
1158   <c>http</c>
1159   <c>standard</c>
1160   <c>
1161      <xref target="header.if-modified-since"/>
1162   </c>
1163   <c>If-None-Match</c>
1164   <c>http</c>
1165   <c>standard</c>
1166   <c>
1167      <xref target="header.if-none-match"/>
1168   </c>
1169   <c>If-Unmodified-Since</c>
1170   <c>http</c>
1171   <c>standard</c>
1172   <c>
1173      <xref target="header.if-unmodified-since"/>
1174   </c>
1175   <c>Last-Modified</c>
1176   <c>http</c>
1177   <c>standard</c>
1178   <c>
1179      <xref target="header.last-modified"/>
1180   </c>
1181</texttable>
1182<!--(END)-->
1183<?ENDINC p4-conditional.iana-headers ?>
1184<t>
1185   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1186</t>
1187</section>
1188</section>
1189
1190<section title="Security Considerations" anchor="security.considerations">
1191<t>
1192   No additional security considerations have been identified beyond
1193   those applicable to HTTP in general &messaging;.
1194</t>
1195</section>
1196
1197<section title="Acknowledgments" anchor="ack">
1198</section>
1199</middle>
1200<back>
1201
1202<references title="Normative References">
1203
1204<reference anchor="Part1">
1205  <front>
1206    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1207    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1208      <organization abbrev="Day Software">Day Software</organization>
1209      <address><email>fielding@gbiv.com</email></address>
1210    </author>
1211    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1212      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1213      <address><email>jg@freedesktop.org</email></address>
1214    </author>
1215    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1216      <organization abbrev="HP">Hewlett-Packard Company</organization>
1217      <address><email>JeffMogul@acm.org</email></address>
1218    </author>
1219    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1220      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1221      <address><email>henrikn@microsoft.com</email></address>
1222    </author>
1223    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1224      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1225      <address><email>LMM@acm.org</email></address>
1226    </author>
1227    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1228      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1229      <address><email>paulle@microsoft.com</email></address>
1230    </author>
1231    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1232      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1233      <address><email>timbl@w3.org</email></address>
1234    </author>
1235    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1236      <organization abbrev="W3C">World Wide Web Consortium</organization>
1237      <address><email>ylafon@w3.org</email></address>
1238    </author>
1239    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1240      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1241      <address><email>julian.reschke@greenbytes.de</email></address>
1242    </author>
1243    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1244  </front>
1245  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1246  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1247</reference>
1248
1249<reference anchor="Part3">
1250  <front>
1251    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1252    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1253      <organization abbrev="Day Software">Day Software</organization>
1254      <address><email>fielding@gbiv.com</email></address>
1255    </author>
1256    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1257      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1258      <address><email>jg@freedesktop.org</email></address>
1259    </author>
1260    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1261      <organization abbrev="HP">Hewlett-Packard Company</organization>
1262      <address><email>JeffMogul@acm.org</email></address>
1263    </author>
1264    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1265      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1266      <address><email>henrikn@microsoft.com</email></address>
1267    </author>
1268    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1269      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1270      <address><email>LMM@acm.org</email></address>
1271    </author>
1272    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1273      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1274      <address><email>paulle@microsoft.com</email></address>
1275    </author>
1276    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1277      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1278      <address><email>timbl@w3.org</email></address>
1279    </author>
1280    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1281      <organization abbrev="W3C">World Wide Web Consortium</organization>
1282      <address><email>ylafon@w3.org</email></address>
1283    </author>
1284    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1285      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1286      <address><email>julian.reschke@greenbytes.de</email></address>
1287    </author>
1288    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1289  </front>
1290  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1291  <x:source href="p3-payload.xml" basename="p3-payload"/>
1292</reference>
1293
1294<reference anchor="Part5">
1295  <front>
1296    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1297    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1298      <organization abbrev="Day Software">Day Software</organization>
1299      <address><email>fielding@gbiv.com</email></address>
1300    </author>
1301    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1302      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1303      <address><email>jg@freedesktop.org</email></address>
1304    </author>
1305    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1306      <organization abbrev="HP">Hewlett-Packard Company</organization>
1307      <address><email>JeffMogul@acm.org</email></address>
1308    </author>
1309    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1310      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1311      <address><email>henrikn@microsoft.com</email></address>
1312    </author>
1313    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1314      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1315      <address><email>LMM@acm.org</email></address>
1316    </author>
1317    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1318      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1319      <address><email>paulle@microsoft.com</email></address>
1320    </author>
1321    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1322      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1323      <address><email>timbl@w3.org</email></address>
1324    </author>
1325    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1326      <organization abbrev="W3C">World Wide Web Consortium</organization>
1327      <address><email>ylafon@w3.org</email></address>
1328    </author>
1329    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1330      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1331      <address><email>julian.reschke@greenbytes.de</email></address>
1332    </author>
1333    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1334  </front>
1335  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1336  <x:source href="p5-range.xml" basename="p5-range"/>
1337</reference>
1338
1339<reference anchor="Part6">
1340  <front>
1341    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1342    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1343      <organization abbrev="Day Software">Day Software</organization>
1344      <address><email>fielding@gbiv.com</email></address>
1345    </author>
1346    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1347      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1348      <address><email>jg@freedesktop.org</email></address>
1349    </author>
1350    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1351      <organization abbrev="HP">Hewlett-Packard Company</organization>
1352      <address><email>JeffMogul@acm.org</email></address>
1353    </author>
1354    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1355      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1356      <address><email>henrikn@microsoft.com</email></address>
1357    </author>
1358    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1359      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1360      <address><email>LMM@acm.org</email></address>
1361    </author>
1362    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1363      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1364      <address><email>paulle@microsoft.com</email></address>
1365    </author>
1366    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1367      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1368      <address><email>timbl@w3.org</email></address>
1369    </author>
1370    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1371      <organization abbrev="W3C">World Wide Web Consortium</organization>
1372      <address><email>ylafon@w3.org</email></address>
1373    </author>
1374    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1375      <address><email>mnot@mnot.net</email></address>
1376    </author>
1377    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1378      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1379      <address><email>julian.reschke@greenbytes.de</email></address>
1380    </author>
1381    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1382  </front>
1383  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1384  <x:source href="p6-cache.xml" basename="p6-cache"/>
1385</reference>
1386
1387<reference anchor="RFC2119">
1388  <front>
1389    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1390    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1391      <organization>Harvard University</organization>
1392      <address><email>sob@harvard.edu</email></address>
1393    </author>
1394    <date month="March" year="1997"/>
1395  </front>
1396  <seriesInfo name="BCP" value="14"/>
1397  <seriesInfo name="RFC" value="2119"/>
1398</reference>
1399
1400<reference anchor="RFC5234">
1401  <front>
1402    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1403    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1404      <organization>Brandenburg InternetWorking</organization>
1405      <address>
1406        <email>dcrocker@bbiw.net</email>
1407      </address> 
1408    </author>
1409    <author initials="P." surname="Overell" fullname="Paul Overell">
1410      <organization>THUS plc.</organization>
1411      <address>
1412        <email>paul.overell@thus.net</email>
1413      </address>
1414    </author>
1415    <date month="January" year="2008"/>
1416  </front>
1417  <seriesInfo name="STD" value="68"/>
1418  <seriesInfo name="RFC" value="5234"/>
1419</reference>
1420
1421</references>
1422
1423<references title="Informative References">
1424
1425<reference anchor="RFC2616">
1426  <front>
1427    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1428    <author initials="R." surname="Fielding" fullname="R. Fielding">
1429      <organization>University of California, Irvine</organization>
1430      <address><email>fielding@ics.uci.edu</email></address>
1431    </author>
1432    <author initials="J." surname="Gettys" fullname="J. Gettys">
1433      <organization>W3C</organization>
1434      <address><email>jg@w3.org</email></address>
1435    </author>
1436    <author initials="J." surname="Mogul" fullname="J. Mogul">
1437      <organization>Compaq Computer Corporation</organization>
1438      <address><email>mogul@wrl.dec.com</email></address>
1439    </author>
1440    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1441      <organization>MIT Laboratory for Computer Science</organization>
1442      <address><email>frystyk@w3.org</email></address>
1443    </author>
1444    <author initials="L." surname="Masinter" fullname="L. Masinter">
1445      <organization>Xerox Corporation</organization>
1446      <address><email>masinter@parc.xerox.com</email></address>
1447    </author>
1448    <author initials="P." surname="Leach" fullname="P. Leach">
1449      <organization>Microsoft Corporation</organization>
1450      <address><email>paulle@microsoft.com</email></address>
1451    </author>
1452    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1453      <organization>W3C</organization>
1454      <address><email>timbl@w3.org</email></address>
1455    </author>
1456    <date month="June" year="1999"/>
1457  </front>
1458  <seriesInfo name="RFC" value="2616"/>
1459</reference>
1460
1461<reference anchor='RFC3864'>
1462  <front>
1463    <title>Registration Procedures for Message Header Fields</title>
1464    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1465      <organization>Nine by Nine</organization>
1466      <address><email>GK-IETF@ninebynine.org</email></address>
1467    </author>
1468    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1469      <organization>BEA Systems</organization>
1470      <address><email>mnot@pobox.com</email></address>
1471    </author>
1472    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1473      <organization>HP Labs</organization>
1474      <address><email>JeffMogul@acm.org</email></address>
1475    </author>
1476    <date year='2004' month='September' />
1477  </front>
1478  <seriesInfo name='BCP' value='90' />
1479  <seriesInfo name='RFC' value='3864' />
1480</reference>
1481
1482</references>
1483
1484<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1485<t>
1486  Allow weak entity-tags in all requests except range requests (Sections
1487  <xref target="weak.and.strong.validators" format="counter"/> and
1488  <xref target="header.if-none-match" format="counter"/>).
1489</t>
1490</section>
1491
1492<?BEGININC p4-conditional.abnf-appendix ?>
1493<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1494<figure>
1495<artwork type="abnf" name="p4-conditional.parsed-abnf">
1496<x:ref>ETag</x:ref> = "ETag:" OWS ETag-v
1497<x:ref>ETag-v</x:ref> = entity-tag
1498
1499<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1500
1501<x:ref>If-Match</x:ref> = "If-Match:" OWS If-Match-v
1502<x:ref>If-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1503 entity-tag ] ) )
1504<x:ref>If-Modified-Since</x:ref> = "If-Modified-Since:" OWS If-Modified-Since-v
1505<x:ref>If-Modified-Since-v</x:ref> = HTTP-date
1506<x:ref>If-None-Match</x:ref> = "If-None-Match:" OWS If-None-Match-v
1507<x:ref>If-None-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1508 entity-tag ] ) )
1509<x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since:" OWS
1510 If-Unmodified-Since-v
1511<x:ref>If-Unmodified-Since-v</x:ref> = HTTP-date
1512
1513<x:ref>Last-Modified</x:ref> = "Last-Modified:" OWS Last-Modified-v
1514<x:ref>Last-Modified-v</x:ref> = HTTP-date
1515
1516<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1517
1518<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1519
1520<x:ref>opaque-tag</x:ref> = quoted-string
1521
1522<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1523
1524<x:ref>weak</x:ref> = %x57.2F ; W/
1525</artwork>
1526</figure>
1527<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1528; ETag defined but not used
1529; If-Match defined but not used
1530; If-Modified-Since defined but not used
1531; If-None-Match defined but not used
1532; If-Unmodified-Since defined but not used
1533; Last-Modified defined but not used
1534</artwork></figure></section>
1535<?ENDINC p4-conditional.abnf-appendix ?>
1536
1537<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1538
1539<section title="Since RFC2616">
1540<t>
1541  Extracted relevant partitions from <xref target="RFC2616"/>.
1542</t>
1543</section>
1544
1545<section title="Since draft-ietf-httpbis-p4-conditional-00">
1546<t>
1547  Closed issues:
1548  <list style="symbols"> 
1549    <t>
1550      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1551      "Normative and Informative references"
1552    </t>
1553  </list>
1554</t>
1555<t>
1556  Other changes:
1557  <list style="symbols"> 
1558    <t>
1559      Move definitions of 304 and 412 condition codes from Part2.
1560    </t>
1561  </list>
1562</t>
1563</section>
1564
1565<section title="Since draft-ietf-httpbis-p4-conditional-01">
1566<t>
1567  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1568  <list style="symbols"> 
1569    <t>
1570      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1571    </t>
1572  </list>
1573</t>
1574</section>
1575
1576<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1577<t>
1578  Closed issues:
1579  <list style="symbols"> 
1580    <t>
1581      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1582      "Weak ETags on non-GET requests"
1583    </t>
1584  </list>
1585</t>
1586<t>
1587  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1588  <list style="symbols"> 
1589    <t>
1590      Reference RFC 3984, and update header registrations for headers defined
1591      in this document.
1592    </t>
1593  </list>
1594</t>
1595</section>
1596
1597<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1598<t>
1599  Closed issues:
1600  <list style="symbols"> 
1601    <t>
1602      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1603      "Examples for ETag matching"
1604    </t>
1605    <t>
1606      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1607      "'entity value' undefined"
1608    </t>
1609    <t>
1610      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1611      "bogus 2068 Date header reference"
1612    </t>
1613  </list>
1614</t>
1615</section>
1616
1617<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1618<t>
1619  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1620  <list style="symbols"> 
1621    <t>
1622      Use "/" instead of "|" for alternatives.
1623    </t>
1624    <t>
1625      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1626      whitespace ("OWS") and required whitespace ("RWS").
1627    </t>
1628    <t>
1629      Rewrite ABNFs to spell out whitespace rules, factor out
1630      header value format definitions.
1631    </t>
1632  </list>
1633</t>
1634</section>
1635
1636<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1637<t>
1638  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1639  <list style="symbols"> 
1640    <t>
1641      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1642    </t>
1643  </list>
1644</t>
1645</section>
1646
1647<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1648<t>
1649  Closed issues:
1650  <list style="symbols"> 
1651    <t>
1652      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1653      "case-sensitivity of etag weakness indicator"
1654    </t>
1655  </list>
1656</t>
1657</section>
1658
1659<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1660<t>
1661  Closed issues:
1662  <list style="symbols"> 
1663    <t>
1664      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1665      "Weak ETags on non-GET requests" (If-Match still was defined to require
1666      strong matching)
1667    </t>
1668    <t>
1669      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1670      "move IANA registrations for optional status codes"
1671    </t>
1672  </list>
1673</t>
1674</section>
1675
1676<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1677<t>
1678  No significant changes.
1679</t>
1680</section>
1681
1682<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1683<t>
1684  No significant changes.
1685</t>
1686</section>
1687
1688<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1689<t>
1690  Closed issues:
1691  <list style="symbols"> 
1692    <t>
1693      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1694      "Clarify 'Requested Variant'"
1695    </t>
1696    <t>
1697      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1698      "Clarify entity / representation / variant terminology"
1699    </t>
1700    <t>
1701      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1702      "consider removing the 'changes from 2068' sections"
1703    </t>
1704  </list>
1705</t>
1706</section>
1707
1708</section>
1709
1710</back>
1711</rfc>
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