source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1391

Last change on this file since 1391 was 1380, checked in by fielding@…, 8 years ago

Editorial. Rewrite the requirements on weak and strong validators
so that they are all in one section.

  • Property svn:eol-style set to native
File size: 72.0 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 "August">
16  <!ENTITY ID-YEAR "2011">
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 acks                       "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY header-date                "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-accept-encoding     "<xref target='Part3' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY full-date                  "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY content-negotiation        "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32]>
33<?rfc toc="yes" ?>
34<?rfc symrefs="yes" ?>
35<?rfc sortrefs="yes" ?>
36<?rfc compact="yes"?>
37<?rfc subcompact="no" ?>
38<?rfc linkmailto="no" ?>
39<?rfc editing="no" ?>
40<?rfc comments="yes"?>
41<?rfc inline="yes"?>
42<?rfc rfcedstyle="yes"?>
43<?rfc-ext allow-markup-in-artwork="yes" ?>
44<?rfc-ext include-references-in-index="yes" ?>
45<rfc obsoletes="2616" category="std" x:maturity-level="draft"
46     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
47     xmlns:x='http://purl.org/net/xml2rfc/ext'>
48<front>
49
50  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
51
52  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
53    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
54    <address>
55      <postal>
56        <street>345 Park Ave</street>
57        <city>San Jose</city>
58        <region>CA</region>
59        <code>95110</code>
60        <country>USA</country>
61      </postal>
62      <email>fielding@gbiv.com</email>
63      <uri>http://roy.gbiv.com/</uri>
64    </address>
65  </author>
66
67  <author initials="J." surname="Gettys" fullname="Jim Gettys">
68    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
69    <address>
70      <postal>
71        <street>21 Oak Knoll Road</street>
72        <city>Carlisle</city>
73        <region>MA</region>
74        <code>01741</code>
75        <country>USA</country>
76      </postal>
77      <email>jg@freedesktop.org</email>
78      <uri>http://gettys.wordpress.com/</uri>
79    </address>
80  </author>
81 
82  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
83    <organization abbrev="HP">Hewlett-Packard Company</organization>
84    <address>
85      <postal>
86        <street>HP Labs, Large Scale Systems Group</street>
87        <street>1501 Page Mill Road, MS 1177</street>
88        <city>Palo Alto</city>
89        <region>CA</region>
90        <code>94304</code>
91        <country>USA</country>
92      </postal>
93      <email>JeffMogul@acm.org</email>
94    </address>
95  </author>
96
97  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
98    <organization abbrev="Microsoft">Microsoft Corporation</organization>
99    <address>
100      <postal>
101        <street>1 Microsoft Way</street>
102        <city>Redmond</city>
103        <region>WA</region>
104        <code>98052</code>
105        <country>USA</country>
106      </postal>
107      <email>henrikn@microsoft.com</email>
108    </address>
109  </author>
110
111  <author initials="L." surname="Masinter" fullname="Larry Masinter">
112    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
113    <address>
114      <postal>
115        <street>345 Park Ave</street>
116        <city>San Jose</city>
117        <region>CA</region>
118        <code>95110</code>
119        <country>USA</country>
120      </postal>
121      <email>LMM@acm.org</email>
122      <uri>http://larry.masinter.net/</uri>
123    </address>
124  </author>
125 
126  <author initials="P." surname="Leach" fullname="Paul J. Leach">
127    <organization abbrev="Microsoft">Microsoft Corporation</organization>
128    <address>
129      <postal>
130        <street>1 Microsoft Way</street>
131        <city>Redmond</city>
132        <region>WA</region>
133        <code>98052</code>
134      </postal>
135      <email>paulle@microsoft.com</email>
136    </address>
137  </author>
138   
139  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
140    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
141    <address>
142      <postal>
143        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
144        <street>The Stata Center, Building 32</street>
145        <street>32 Vassar Street</street>
146        <city>Cambridge</city>
147        <region>MA</region>
148        <code>02139</code>
149        <country>USA</country>
150      </postal>
151      <email>timbl@w3.org</email>
152      <uri>http://www.w3.org/People/Berners-Lee/</uri>
153    </address>
154  </author>
155
156  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
157    <organization abbrev="W3C">World Wide Web Consortium</organization>
158    <address>
159      <postal>
160        <street>W3C / ERCIM</street>
161        <street>2004, rte des Lucioles</street>
162        <city>Sophia-Antipolis</city>
163        <region>AM</region>
164        <code>06902</code>
165        <country>France</country>
166      </postal>
167      <email>ylafon@w3.org</email>
168      <uri>http://www.raubacapeu.net/people/yves/</uri>
169    </address>
170  </author>
171
172  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
173    <organization abbrev="greenbytes">greenbytes GmbH</organization>
174    <address>
175      <postal>
176        <street>Hafenweg 16</street>
177        <city>Muenster</city><region>NW</region><code>48155</code>
178        <country>Germany</country>
179      </postal>
180      <phone>+49 251 2807760</phone>
181      <facsimile>+49 251 2807761</facsimile>
182      <email>julian.reschke@greenbytes.de</email>
183      <uri>http://greenbytes.de/tech/webdav/</uri>
184    </address>
185  </author>
186
187  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
188  <workgroup>HTTPbis Working Group</workgroup>
189
190<abstract>
191<t>
192   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
193   distributed, collaborative, hypertext information systems. HTTP has been in
194   use by the World Wide Web global information initiative since 1990. This
195   document is Part 4 of the seven-part specification that defines the protocol
196   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
197</t>
198<t>
199   Part 4 defines request header fields for indicating conditional requests and
200   the rules for constructing responses 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), which is archived at
208    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
209  </t>
210  <t>
211    The current issues list is at
212    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
213    documents (including fancy diffs) can be found at
214    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
215  </t>
216  <t>
217    The changes in this draft are summarized in <xref target="changes.since.15"/>.
218  </t>
219</note>
220</front>
221<middle>
222<section title="Introduction" anchor="introduction">
223<t>
224   This document defines the HTTP/1.1 conditional request mechanisms,
225   including both metadata for indicating/observing changes in resource
226   representations and request header fields that specify preconditions
227   on that metadata be checked before performing the request method.
228   Conditional GET requests are the most efficient mechanism for HTTP
229   cache updates &caching;.  Conditionals can also be
230   applied to state-changing methods, such as PUT and DELETE, to prevent
231   the "lost update" problem: one client accidentally overwriting
232   the work of another client that has been acting in parallel.
233</t>
234<t>
235   Conditional request preconditions are based on the state of the target
236   resource as a whole (its current value set) or the state as observed
237   in a previously obtained representation (one value in that set).
238   A resource might have multiple current representations, each with its
239   own observable state.  The conditional request mechanisms assume that
240   the mapping of requests to corresponding representations will be
241   consistent over time if the server intends to take advantage of
242   conditionals.  Regardless, if the mapping is inconsistent and
243   the server is unable to select the appropriate representation, then
244   no harm will result when the precondition evaluates to false.
245</t>
246<t><iref primary="true" item="selected representation"/>
247   We use the term "<x:dfn>selected representation</x:dfn>" to refer to
248   the current representation of the target resource that would have been
249   selected in a successful response if the same request had used the method
250   GET and had excluded all of the conditional request header fields.
251   The conditional request preconditions are evaluated by comparing the
252   values provided in the request header fields to the current metadata
253   for the selected representation.
254</t>
255
256<section title="Requirements" anchor="intro.requirements">
257<t>
258   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
259   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
260   document are to be interpreted as described in <xref target="RFC2119"/>.
261</t>
262<t>
263   An implementation is not compliant if it fails to satisfy one or more
264   of the "MUST" or "REQUIRED" level requirements for the protocols it
265   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
266   level and all the "SHOULD" level requirements for its protocols is said
267   to be "unconditionally compliant"; one that satisfies all the "MUST"
268   level requirements but not all the "SHOULD" level requirements for its
269   protocols is said to be "conditionally compliant".
270</t>
271</section>
272
273<section title="Syntax Notation" anchor="notation">
274  <x:anchor-alias value="ALPHA"/>
275  <x:anchor-alias value="CR"/>
276  <x:anchor-alias value="DIGIT"/>
277  <x:anchor-alias value="LF"/>
278  <x:anchor-alias value="OCTET"/>
279  <x:anchor-alias value="VCHAR"/>
280  <x:anchor-alias value="WSP"/>
281  <x:anchor-alias value="core.rules"/>
282  <x:anchor-alias value="quoted-string"/>
283  <x:anchor-alias value="OWS"/>
284  <x:anchor-alias value="HTTP-date"/>
285<t>
286  This specification uses the ABNF syntax defined in &notation; (which
287  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
288  <xref target="collected.abnf"/> shows the collected ABNF, with the list
289  rule expanded.
290</t>
291<t>
292  The following core rules are included by
293  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
294  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
295  DIGIT (decimal 0-9), DQUOTE (double quote),
296  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
297  OCTET (any 8-bit sequence of data), SP (space),
298  VCHAR (any visible USASCII character),
299  and WSP (whitespace).
300</t>
301<t>
302  The ABNF rules below are defined in other parts:
303</t>
304<figure><artwork type="abnf2616">
305  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
306  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
307  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
308</artwork></figure>
309</section>
310</section>
311
312<section title="Validators" anchor="validators">
313   <iref primary="true" item="metadata"/>
314   <iref primary="true" item="validator"/>
315<t>
316   This specification defines two forms of metadata that are commonly used
317   to observe resource state and test for preconditions: modification dates
318   and opaque entity tags.  Additional metadata that reflects resource state
319   has been defined by various extensions of HTTP, such as WebDAV
320   <xref target="RFC4918"/>, that are beyond the scope of this specification.
321   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
322   when it is used within a precondition.
323</t>
324
325<section title="Weak versus Strong" anchor="weak.and.strong.validators">
326   <iref primary="true" item="validator" subitem="weak"/>
327   <iref primary="true" item="validator" subitem="strong"/>
328<t>
329   Validators come in two flavors: strong or weak.  Weak validators are easy
330   to generate but are far less useful for comparisons.  Strong validators
331   are ideal for comparisons but can be very difficult (and occasionally
332   impossible) to generate efficiently.  Rather than impose that all forms
333   of resource adhere to the same strength of validator, HTTP exposes the
334   type of validator in use and imposes restrictions on when weak validators
335   can be used as preconditions.
336</t>
337<t>
338   A "strong validator" is a representation metadata value that &MUST; be
339   changed to a new, previously unused or guaranteed unique, value whenever
340   a change occurs to the representation data such that a change would be
341   observable in the payload body of a 200 response to GET.  A strong
342   validator &MAY; be changed for other reasons, such as when a semantically
343   significant part of the representation metadata is changed (e.g.,
344   Content-Type), but it is in the best interests of the origin server to only
345   change the value when it is necessary to invalidate the stored responses
346   held by remote caches and authoring tools.  A strong validator &MUST; be
347   unique across all representations of a given resource, such that no two
348   representations of that resource share the same validator unless
349   their payload body would be identical.
350</t>
351<t>
352   Cache entries might persist for arbitrarily long periods, regardless
353   of expiration times.  Thus, a cache might attempt to validate an
354   entry using a validator that it obtained in the distant past.
355   A strong validator &MUST; be unique across all versions of all
356   representations associated with a particular resource over time.
357   However, there is no implication of uniqueness across representations
358   of different resources (i.e., the same strong validator might be
359   in use for representations of multiple resources at the same time
360   and does not imply that those representations are equivalent).
361</t>
362<t>
363   There are a variety of strong validators used in practice.  The best are
364   based on strict revision control, wherein each change to a representation
365   always results in a unique node name and revision identifier being assigned
366   before the representation is made accessible to GET.  A cryptographic hash
367   function applied to the representation data is also sufficient if the data
368   is available prior to the response header fields being sent and the digest
369   does not need to be recalculated every time a validation request is
370   received.  However, if a resource has distinct representations that differ
371   only in their metadata, such as might occur with content negotiation over
372   media types that happen to share the same data format, then a server
373   &SHOULD; incorporate additional information in the validator to
374   distinguish those representations and avoid confusing cache behavior.
375</t>
376<t>
377   In contrast, a "weak validator" is a representation metadata value that
378   might not be changed for every change to the representation data.  This
379   weakness might be due to limitations in how the value is calculated, such
380   as clock resolution or an inability to ensure uniqueness for all possible
381   representations of the resource, or due to a desire by the resource owner
382   to group representations by some self-determined set of equivalency
383   rather than unique sequences of data.  A weak entity-tag &SHOULD; change
384   whenever the origin server considers prior representations to be
385   unacceptable as a substitute for the current representation. In other
386   words, a weak entity-tag &SHOULD; change whenever the origin server wants
387   caches to invalidate old responses.
388</t>
389<t>
390   For example, the representation of a weather report that changes in
391   content every second, based on dynamic measurements, might be grouped
392   into sets of equivalent representations (from the origin server's
393   perspective) with the same weak validator in order to allow cached
394   representations to be valid for a reasonable period of time (perhaps
395   adjusted dynamically based on server load or weather quality).
396   Likewise, a representation's modification time, if defined with only
397   one-second resolution, might be a weak validator if it is possible
398   for the representation to be modified twice during a single second and
399   retrieved between those modifications.
400</t>
401<t>
402   A "use" of a validator occurs when either a client generates a request
403   and includes the validator in a precondition or when a server
404   compares two validators.
405   Weak validators are only usable in contexts that do not depend on exact
406   equality of a representation's payload body.
407   Strong validators are usable and preferred for all conditional requests,
408   including cache validation, partial content ranges, and "lost update"
409   avoidance.
410</t>
411</section>
412
413<section title="Last-Modified" anchor="header.last-modified">
414  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
415  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
416  <x:anchor-alias value="Last-Modified"/>
417<t>
418   The "Last-Modified" header field indicates the date and time at
419   which the origin server believes the selected representation was
420   last modified.
421</t>
422<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
423  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
424</artwork></figure>
425<t>
426   An example of its use is
427</t>
428<figure><artwork type="example">
429  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
430</artwork></figure>
431
432<section title="Generation" anchor="lastmod.generation">
433<t>
434   Origin servers &SHOULD; send Last-Modified for any selected
435   representation for which a last modification date can be reasonably
436   and consistently determined, since its use in conditional requests
437   and evaluating cache freshness (&caching;) results in a substantial
438   reduction of HTTP traffic on the Internet and can be a significant
439   factor in improving service scalability and reliability.
440</t>
441<t>
442   A representation is typically the sum of many parts behind the
443   resource interface.  The last-modified time would usually be
444   the most recent time that any of those parts were changed.
445   How that value is determined for any given resource is an
446   implementation detail beyond the scope of this specification.
447   What matters to HTTP is how recipients of the Last-Modified
448   header field can use its value to make conditional requests
449   and test the validity of locally cached responses.
450</t>
451<t>
452   An origin server &SHOULD; obtain the Last-Modified value of the
453   representation as close as possible to the time that it generates
454   the Date field-value for its response. This allows a recipient to
455   make an accurate assessment of the representation's modification time,
456   especially if the representation changes near the time that the
457   response is generated.
458</t>
459<t>
460   An origin server with a clock &MUST-NOT; send a Last-Modified date
461   that is later than the server's time of message origination (Date).
462   If the last modification time is derived from implementation-specific
463   metadata that evaluates to some time in the future, according to the
464   origin server's clock, then the origin server &MUST; replace that
465   value with the message origination date. This prevents a future
466   modification date from having an adverse impact on cache validation.
467</t>
468</section>
469
470<section title="Comparison" anchor="lastmod.comparison">
471<t>
472   A Last-Modified time, when used as a validator in a request, is
473   implicitly weak unless it is possible to deduce that it is strong,
474   using the following rules:
475  <list style="symbols">
476     <t>The validator is being compared by an origin server to the
477        actual current validator for the representation and,</t>
478     <t>That origin server reliably knows that the associated representation did
479        not change twice during the second covered by the presented
480        validator.</t>
481  </list>
482</t>
483<t>
484   or
485  <list style="symbols">
486     <t>The validator is about to be used by a client in an If-Modified-Since,
487        If-Unmodified-Since header field, because the client has a cache entry,
488        or If-Range for the associated representation, and</t>
489     <t>That cache entry includes a Date value, which gives the time
490        when the origin server sent the original response, and</t>
491     <t>The presented Last-Modified time is at least 60 seconds before
492        the Date value.</t>
493  </list>
494</t>
495<t>
496   or
497  <list style="symbols">
498     <t>The validator is being compared by an intermediate cache to the
499        validator stored in its cache entry for the representation, and</t>
500     <t>That cache entry includes a Date value, which gives the time
501        when the origin server sent the original response, and</t>
502     <t>The presented Last-Modified time is at least 60 seconds before
503        the Date value.</t>
504  </list>
505</t>
506<t>
507   This method relies on the fact that if two different responses were
508   sent by the origin server during the same second, but both had the
509   same Last-Modified time, then at least one of those responses would
510   have a Date value equal to its Last-Modified time. The arbitrary 60-second
511   limit guards against the possibility that the Date and Last-Modified
512   values are generated from different clocks, or at somewhat
513   different times during the preparation of the response. An
514   implementation &MAY; use a value larger than 60 seconds, if it is
515   believed that 60 seconds is too short.
516</t>
517</section>
518</section>
519
520<section title="ETag" anchor="header.etag">
521  <iref primary="true" item="ETag header field" x:for-anchor=""/>
522  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
523  <x:anchor-alias value="ETag"/>
524  <x:anchor-alias value="entity-tag"/>
525  <x:anchor-alias value="entity.tags"/>
526  <x:anchor-alias value="opaque-tag"/>
527  <x:anchor-alias value="weak"/>
528<t>
529   The ETag header field provides the current entity-tag for the
530   selected representation.
531   An entity-tag is an opaque validator for differentiating between
532   multiple representations of the same resource, regardless of whether
533   those multiple representations are due to resource state changes over
534   time, content negotiation resulting in multiple representations being
535   valid at the same time, or both. An entity-tag consists of an opaque
536   quoted string, possibly prefixed by a weakness indicator.
537</t>
538<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/>
539  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
540
541  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
542  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
543  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
544</artwork></figure>
545<t>
546   An entity-tag can be more reliable for validation than a modification
547   date in situations where it is inconvenient to store modification
548   dates, where the one-second resolution of HTTP date values is not
549   sufficient, or where modification dates are not consistently maintained.
550</t>
551<figure><preamble>
552  Examples:
553</preamble>
554<artwork type="example">
555  ETag: "xyzzy"
556  ETag: W/"xyzzy"
557  ETag: ""
558</artwork></figure>
559<t>
560   An entity-tag can be either a weak or strong validator, with
561   strong being the default.  If an origin server provides an entity-tag
562   for a representation and the generation of that entity-tag does not satisfy
563   the requirements for a strong validator
564   (<xref target="weak.and.strong.validators"/>), then that
565   entity-tag &MUST; be marked as weak by prefixing its opaque value
566   with "W/" (case-sensitive).
567</t>
568
569<section title="Generation" anchor="entity.tag.generation">
570<t>
571   The principle behind entity-tags is that only the service author
572   knows the implementation of a resource well enough to select the
573   most accurate and efficient validation mechanism for that resource,
574   and that any such mechanism can be mapped to a simple sequence of
575   octets for easy comparison.  Since the value is opaque, there is no
576   need for the client to be aware of how each entity-tag is constructed.
577</t>
578<t>
579   For example, a resource that has implementation-specific versioning
580   applied to all changes might use an internal revision number, perhaps
581   combined with a variance identifier for content negotiation, to
582   accurately differentiate between representations.
583   Other implementations might use a stored hash of representation content,
584   a combination of various filesystem attributes, or a modification
585   timestamp that has sub-second resolution.
586</t>
587<t>
588   Origin servers &SHOULD; send ETag for any selected representation
589   for which detection of changes can be reasonably and consistently
590   determined, since the entity-tag's use in conditional requests and
591   evaluating cache freshness (&caching;) can result in a substantial
592   reduction of HTTP network traffic and can be a significant factor in
593   improving service scalability and reliability.
594</t>
595</section>
596
597<section title="Comparison" anchor="entity.tag.comparison">
598  <x:anchor-alias value="validator.comparison"/>
599<t>
600   There are two entity-tag comparison functions, depending
601   on whether the comparison context allows the use of weak validators
602   or not:
603  <list style="symbols">
604     <t>The strong comparison function: in order to be considered equal,
605        both opaque-tags &MUST; be identical character-by-character, and both
606        &MUST-NOT; be weak.</t>
607     <t>The weak comparison function: in order to be considered equal, both
608        opaque-tags &MUST; be identical character-by-character, but
609        either or both of them &MAY; be tagged as "weak" without affecting
610        the result.</t>
611  </list>
612</t>
613<t>
614   The example below shows the results for a set of entity-tag pairs,
615   and both the weak and strong comparison function results:
616</t>
617<texttable align="left">
618  <ttcol>ETag 1</ttcol>
619  <ttcol>ETag 2</ttcol>
620  <ttcol>Strong Comparison</ttcol>
621  <ttcol>Weak Comparison</ttcol>
622
623  <c>W/"1"</c>
624  <c>W/"1"</c>
625  <c>no match</c>
626  <c>match</c>
627 
628  <c>W/"1"</c>
629  <c>W/"2"</c>
630  <c>no match</c>
631  <c>no match</c>
632
633  <c>W/"1"</c>
634  <c>"1"</c>
635  <c>no match</c>
636  <c>match</c>
637
638  <c>"1"</c>
639  <c>"1"</c>
640  <c>match</c>
641  <c>match</c>
642</texttable>
643</section>
644
645<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
646<t>
647   Consider a resource that is subject to content negotiation (&content-negotiation;),
648   and where the representations returned upon a GET request vary based on
649   the Accept-Encoding request header field (&header-accept-encoding;):
650</t>
651<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
652GET /index HTTP/1.1
653Host: www.example.com
654Accept-Encoding: gzip
655
656</artwork></figure>
657<t>
658   In this case, the response might or might not use the gzip content coding.
659   If it does not, the response might look like:
660</t>
661<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
662HTTP/1.1 200 OK
663Date: Thu, 26 Mar 2010 00:05:00 GMT
664ETag: "123-a"
665Content-Length: <x:length-of target="exbody"/>
666Vary: Accept-Encoding
667Content-Type: text/plain
668
669<x:span anchor="exbody">Hello World!
670Hello World!
671Hello World!
672Hello World!
673Hello World!
674</x:span></artwork></figure>
675<t>
676   An alternative representation that does use gzip content coding would be:
677</t>
678<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
679HTTP/1.1 200 OK
680Date: Thu, 26 Mar 2010 00:05:00 GMT
681ETag: "123-b"
682Content-Length: 43
683Vary: Accept-Encoding
684Content-Type: text/plain
685Content-Encoding: gzip
686
687<spanx>...binary data...</spanx></artwork></figure>
688<x:note>
689  <t>
690    <x:h>Note:</x:h> Content codings are a property of the representation,
691    so therefore an entity-tag of an encoded representation must be distinct
692    from an unencoded representation to prevent conflicts during cache updates
693    and range requests.  In contrast, transfer codings (&transfer-codings;)
694    apply only during message transfer and do not require distinct entity-tags.
695  </t>
696</x:note>
697</section>
698</section>
699
700<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">
701<t>
702   We adopt a set of rules and recommendations for origin servers,
703   clients, and caches regarding when various validator types ought to
704   be used, and for what purposes.
705</t>
706<t>
707   HTTP/1.1 origin servers:
708  <list style="symbols">
709     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
710        generate one.</t>
711
712     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
713        performance considerations support the use of weak entity-tags,
714        or if it is unfeasible to send a strong entity-tag.</t>
715
716     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
717  </list>
718</t>
719<t>
720   In other words, the preferred behavior for an HTTP/1.1 origin server
721   is to send both a strong entity-tag and a Last-Modified value.
722</t>
723<t>
724   HTTP/1.1 clients:
725  <list style="symbols">
726     <t>&MUST; use that entity-tag in any cache-conditional request (using
727        If-Match or If-None-Match) if an entity-tag has been provided by the
728        origin server.</t>
729
730     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
731        requests (using If-Modified-Since) if only a Last-Modified value has
732        been provided by the origin server. </t>
733
734     <t>&MAY; use the Last-Modified value in subrange cache-conditional
735        requests (using If-Unmodified-Since) if only a Last-Modified value has
736        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
737        provide a way to disable this, in case of difficulty.</t>
738
739     <t>&SHOULD; use both validators in cache-conditional requests if both an
740        entity-tag and a Last-Modified value have been provided by the origin
741        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
742        appropriately.</t>
743  </list>
744</t>
745<t>
746   An HTTP/1.1 origin server, upon receiving a conditional request that
747   includes both a Last-Modified date (e.g., in an If-Modified-Since or
748   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
749   in an If-Match, If-None-Match, or If-Range header field) as cache
750   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
751   unless doing so is consistent with all of the conditional header
752   fields in the request.
753</t>
754<t>
755   An HTTP/1.1 caching proxy, upon receiving a conditional request that
756   includes both a Last-Modified date and one or more entity-tags as
757   cache validators, &MUST-NOT; return a locally cached response to the
758   client unless that cached response is consistent with all of the
759   conditional header fields in the request.
760  <list><t>
761      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
762      servers and clients ought to transmit as much non-redundant
763      information as is available in their responses and requests.
764      HTTP/1.1 systems receiving this information will make the most
765      conservative assumptions about the validators they receive.
766  </t><t>
767      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
768      last-modified values received or used by these systems will
769      support transparent and efficient caching, and so HTTP/1.1 origin
770      servers should provide Last-Modified values. In those rare cases
771      where the use of a Last-Modified value as a validator by an
772      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
773      origin servers should not provide one.
774  </t></list>
775</t>
776</section>
777</section>
778
779<section title="Precondition Header Fields" anchor="header.fields">
780<t>
781   This section defines the syntax and semantics of HTTP/1.1 header fields
782   for applying preconditions on requests.
783</t>
784
785<section title="If-Match" anchor="header.if-match">
786  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
787  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
788  <x:anchor-alias value="If-Match"/>
789<t>
790   The "If-Match" header field &MAY; be used to make a request method
791   conditional on the current existence or value of an entity-tag for
792   one or more representations of the target resource.  If-Match is
793   generally useful for resource update requests, such as PUT requests,
794   as a means for protecting against accidental overwrites when multiple
795   clients are acting in parallel on the same resource (i.e., the
796   "lost update" problem).  An If-Match field-value of "*" places the
797   precondition on the existence of any current representation for the
798   target resource.
799</t>
800<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
801  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
802</artwork></figure>
803<t>
804   If any of the entity-tags listed in the If-Match field value match
805   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
806   selected representation for the target resource,
807   or if "*" is given and any current representation exists for the
808   target resource, then the server &MAY; perform the request method
809   as if the If-Match header field was not present.
810</t>
811<t>
812   If none of the entity-tags match, or if "*" is given and no current
813   representation exists, the server &MUST-NOT; perform the requested method.
814   Instead, the server &MUST; respond with the 412 (Precondition Failed)
815   status code.
816</t>
817<t>
818   If the request would, without the If-Match header field, result in
819   anything other than a 2xx or 412 status code, then the If-Match header field
820   &MUST; be ignored.
821</t>
822<t>
823   Examples:
824</t>
825<figure><artwork type="example">
826  If-Match: "xyzzy"
827  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
828  If-Match: *
829</artwork></figure>
830<t>
831   The result of a request having both an If-Match header field and
832   either an If-None-Match or an If-Modified-Since header fields is
833   undefined by this specification.
834</t>
835</section>
836
837<section title="If-None-Match" anchor="header.if-none-match">
838  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
839  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
840  <x:anchor-alias value="If-None-Match"/>
841<t>
842   The "If-None-Match" header field &MAY; be used to make a request method
843   conditional on not matching any of the current entity-tag values for
844   representations of the target resource.  If-None-Match is primarily
845   used in conditional GET requests to enable efficient updates of cached
846   information with a minimum amount of transaction overhead.  A client
847   that has one or more representations previously obtained from the
848   target resource can send If-None-Match with a list of the associated
849   entity-tags in the hope of receiving a 304 response if at least one
850   of those representations matches the selected representation.
851</t>
852<t>
853   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
854   request method (e.g., PUT) from inadvertently modifying an existing
855   representation of the target resource when the client believes that
856   the resource does not have a current representation.  This is a variation
857   on the "lost update" problem that might arise if more than one client
858   attempts to create an initial representation for the target resource.
859</t>
860<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
861  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
862</artwork></figure>
863<t>
864   If any of the entity-tags listed in the If-None-Match field-value match
865   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
866   selected representation, or if "*" is
867   given and any current representation exists for that resource, then the
868   server &MUST-NOT; perform the requested method.
869   Instead, if the request method was GET or HEAD, the server &SHOULD;
870   respond with a 304 (Not Modified) status code, including the cache-related
871   header fields (particularly ETag) of the selected representation that has
872   a matching entity-tag.  For all other request methods, the server &MUST;
873   respond with a 412 (Precondition Failed) status code.
874</t>
875<t>
876   If none of the entity-tags match, then the server &MAY; perform the
877   requested method as if the If-None-Match header field did not exist,
878   but &MUST; also ignore any If-Modified-Since header field(s) in the
879   request. That is, if no entity-tags match, then the server &MUST-NOT;
880   return a 304 (Not Modified) response.
881</t>
882<t>
883   If the request would, without the If-None-Match header field, result
884   in anything other than a 2xx or 304 status code, then the If-None-Match
885   header field &MUST; be ignored. (See <xref
886   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
887   a discussion of server behavior when both If-Modified-Since and
888   If-None-Match appear in the same request.)
889</t>
890<t>
891   Examples:
892</t>
893<figure><artwork type="example">
894  If-None-Match: "xyzzy"
895  If-None-Match: W/"xyzzy"
896  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
897  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
898  If-None-Match: *
899</artwork></figure>
900<t>
901   The result of a request having both an If-None-Match header field and
902   either an If-Match or an If-Unmodified-Since header fields is
903   undefined by this specification.
904</t>
905</section>
906
907<section title="If-Modified-Since" anchor="header.if-modified-since">
908  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
909  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
910  <x:anchor-alias value="If-Modified-Since"/>
911<t>
912   The "If-Modified-Since" header field &MAY; be used to make a request
913   method conditional by modification date: if the selected representation
914   has not been modified since the time specified in this field, then
915   do not perform the request method; instead, respond as detailed below.
916</t>
917<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
918  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
919</artwork></figure>
920<t>
921   An example of the field is:
922</t>
923<figure><artwork type="example">
924  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
925</artwork></figure>
926<t>
927   A GET method with an If-Modified-Since header field and no Range header
928   field requests that the selected representation be transferred only if
929   it has been modified since the date given by the If-Modified-Since
930   header field.
931   The algorithm for determining this includes the following cases:
932  <list style="numbers">
933      <t>If the request would normally result in anything other than a
934         200 (OK) status code, or if the passed If-Modified-Since date is
935         invalid, the response is exactly the same as for a normal GET.
936         A date which is later than the server's current time is
937         invalid.</t>
938
939      <t>If the selected representation has been modified since the
940         If-Modified-Since date, the response is exactly the same as for
941         a normal GET.</t>
942
943      <t>If the selected representation has not been modified since a valid
944         If-Modified-Since date, the server &SHOULD; return a
945         304 (Not Modified) response.</t>
946  </list>
947</t>
948<t>
949   The purpose of this feature is to allow efficient updates of cached
950   information with a minimum amount of transaction overhead.
951  <list><t>
952      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
953      see &header-range; for full details.
954    </t><t>
955      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
956      clock might not be synchronized with the client.
957    </t><t>
958      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
959      servers will use an exact date comparison function, rather than a
960      less-than function, for deciding whether to send a 304 (Not
961      Modified) response. To get best results when sending an If-Modified-Since
962      header field for cache validation, clients are
963      advised to use the exact date string received in a previous Last-Modified
964      header field whenever possible.
965    </t><t>
966      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
967      header field instead of a date taken from the Last-Modified header field for
968      the same request, the client needs to be aware that this
969      date is interpreted in the server's understanding of time.
970      Unsynchronized clocks and rounding problems, due to the different
971      encodings of time between the client and server, are concerns.
972      This includes the possibility of race conditions if the
973      document has changed between the time it was first requested and
974      the If-Modified-Since date of a subsequent request, and the
975      possibility of clock-skew-related problems if the If-Modified-Since
976      date is derived from the client's clock without correction
977      to the server's clock. Corrections for different time bases
978      between client and server are at best approximate due to network
979      latency.
980    </t>
981  </list>
982</t>
983<t>
984   The result of a request having both an If-Modified-Since header field
985   and either an If-Match or an If-Unmodified-Since header fields is
986   undefined by this specification.
987</t>
988</section>
989
990<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
991  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
992  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
993  <x:anchor-alias value="If-Unmodified-Since"/>
994<t>
995   The "If-Unmodified-Since" header field &MAY; be used to make a request
996   method conditional by modification date: if the selected representation
997   has been modified since the time specified in this field, then the
998   server &MUST-NOT; perform the requested operation and &MUST; instead
999   respond with the 412 (Precondition Failed) status code.
1000   If the selected representation has not been modified since the time
1001   specified in this field, the server &SHOULD; perform the request
1002   method as if the If-Unmodified-Since header field were not present.
1003</t>
1004<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
1005  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
1006</artwork></figure>
1007<t>
1008   An example of the field is:
1009</t>
1010<figure><artwork type="example">
1011  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1012</artwork></figure>
1013<t>
1014   If the request normally (i.e., without the If-Unmodified-Since
1015   header field) would result in anything other than a 2xx or 412 status code,
1016   the If-Unmodified-Since header field &SHOULD; be ignored.
1017</t>
1018<t>
1019   If the specified date is invalid, the header field &MUST; be ignored.
1020</t>
1021<t>
1022   The result of a request having both an If-Unmodified-Since header
1023   field and either an If-None-Match or an If-Modified-Since header
1024   fields is undefined by this specification.
1025</t>
1026</section>
1027
1028<section title="If-Range" anchor="header.if-range">
1029<t>
1030   The If-Range header field provides a special conditional request
1031   mechanism that is similar to If-Match and If-Unmodified-Since but
1032   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1033</t>
1034</section>
1035
1036</section>
1037
1038<section title="Status Code Definitions" anchor="status.code.definitions">
1039<section title="304 Not Modified" anchor="status.304">
1040  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1041  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1042<t>
1043   The 304 status code indicates that a conditional GET request has been
1044   received and would have resulted in a 200 (OK) response if it were not
1045   for the fact that the condition has evaluated to false.  In other words,
1046   there is no need for the server to transfer a representation of the
1047   target resource because the client's request indicates that it already
1048   has a valid representation, as indicated by the 304 response header
1049   fields, and is therefore redirecting the client to make use of that
1050   stored representation as if it were the payload of a 200 response.
1051   The 304 response &MUST-NOT; contain a message-body, and thus is always
1052   terminated by the first empty line after the header fields.
1053</t>
1054<t>
1055   A 304 response &MUST; include a Date header field (&header-date;)
1056   unless its omission is required by &clockless;.  If a 200 response
1057   to the same request would have included any of the header fields
1058   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1059   Vary, then those same header fields &MUST; be sent in a 304 response.
1060</t>
1061<t>
1062   Since the goal of a 304 response is to minimize information transfer
1063   when the recipient already has one or more cached representations,
1064   the response &SHOULD-NOT; include representation metadata other
1065   than the above listed fields unless said metadata exists for the
1066   purpose of guiding cache updates (e.g., future HTTP extensions).
1067</t>
1068<t>
1069   If the recipient of a 304 response does not have a cached representation
1070   corresponding to the entity-tag indicated by the 304 response, then the
1071   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1072   conditional request originated with an outbound client, such as a
1073   user agent with its own cache sending a conditional GET to a shared
1074   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1075   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1076   the request without any preconditions.
1077</t>
1078<t>
1079   If a cache uses a received 304 response to update a cache entry, the
1080   cache &MUST; update the entry to reflect any new field values given in
1081   the response.
1082</t>
1083</section>
1084
1085<section title="412 Precondition Failed" anchor="status.412">
1086  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1087  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1088<t>
1089   The 412 status code indicates that one or more preconditions given in
1090   the request header fields evaluated to false when tested on the server.
1091   This response code allows the client to place preconditions on the
1092   current resource state (its current representations and metadata)
1093   and thus prevent the request method from being applied if the target
1094   resource is in an unexpected state.
1095</t>
1096</section>
1097</section>
1098
1099<section title="IANA Considerations" anchor="IANA.considerations">
1100
1101<section title="Status Code Registration" anchor="status.code.registration">
1102<t>
1103   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1104   shall be updated with the registrations below:
1105</t>
1106<?BEGININC p4-conditional.iana-status-codes ?>
1107<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1108<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1109   <ttcol>Value</ttcol>
1110   <ttcol>Description</ttcol>
1111   <ttcol>Reference</ttcol>
1112   <c>304</c>
1113   <c>Not Modified</c>
1114   <c>
1115      <xref target="status.304"/>
1116   </c>
1117   <c>412</c>
1118   <c>Precondition Failed</c>
1119   <c>
1120      <xref target="status.412"/>
1121   </c>
1122</texttable>
1123<!--(END)-->
1124<?ENDINC p4-conditional.iana-status-codes ?>
1125</section>
1126
1127<section title="Header Field Registration" anchor="header.field.registration">
1128<t>
1129   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1130   with the permanent registrations below (see <xref target="RFC3864"/>):
1131</t>
1132<?BEGININC p4-conditional.iana-headers ?>
1133<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1134<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1135   <ttcol>Header Field Name</ttcol>
1136   <ttcol>Protocol</ttcol>
1137   <ttcol>Status</ttcol>
1138   <ttcol>Reference</ttcol>
1139
1140   <c>ETag</c>
1141   <c>http</c>
1142   <c>standard</c>
1143   <c>
1144      <xref target="header.etag"/>
1145   </c>
1146   <c>If-Match</c>
1147   <c>http</c>
1148   <c>standard</c>
1149   <c>
1150      <xref target="header.if-match"/>
1151   </c>
1152   <c>If-Modified-Since</c>
1153   <c>http</c>
1154   <c>standard</c>
1155   <c>
1156      <xref target="header.if-modified-since"/>
1157   </c>
1158   <c>If-None-Match</c>
1159   <c>http</c>
1160   <c>standard</c>
1161   <c>
1162      <xref target="header.if-none-match"/>
1163   </c>
1164   <c>If-Unmodified-Since</c>
1165   <c>http</c>
1166   <c>standard</c>
1167   <c>
1168      <xref target="header.if-unmodified-since"/>
1169   </c>
1170   <c>Last-Modified</c>
1171   <c>http</c>
1172   <c>standard</c>
1173   <c>
1174      <xref target="header.last-modified"/>
1175   </c>
1176</texttable>
1177<!--(END)-->
1178<?ENDINC p4-conditional.iana-headers ?>
1179<t>
1180   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1181</t>
1182</section>
1183</section>
1184
1185<section title="Security Considerations" anchor="security.considerations">
1186<t>
1187   No additional security considerations have been identified beyond
1188   those applicable to HTTP in general &messaging;.
1189</t>
1190</section>
1191
1192<section title="Acknowledgments" anchor="acks">
1193<t>
1194  See &acks;.
1195</t>
1196</section>
1197</middle>
1198<back>
1199
1200<references title="Normative References">
1201
1202<reference anchor="Part1">
1203  <front>
1204    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1205    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1206      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1207      <address><email>fielding@gbiv.com</email></address>
1208    </author>
1209    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1210      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1211      <address><email>jg@freedesktop.org</email></address>
1212    </author>
1213    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1214      <organization abbrev="HP">Hewlett-Packard Company</organization>
1215      <address><email>JeffMogul@acm.org</email></address>
1216    </author>
1217    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1218      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1219      <address><email>henrikn@microsoft.com</email></address>
1220    </author>
1221    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1222      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1223      <address><email>LMM@acm.org</email></address>
1224    </author>
1225    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1226      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1227      <address><email>paulle@microsoft.com</email></address>
1228    </author>
1229    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1230      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1231      <address><email>timbl@w3.org</email></address>
1232    </author>
1233    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1234      <organization abbrev="W3C">World Wide Web Consortium</organization>
1235      <address><email>ylafon@w3.org</email></address>
1236    </author>
1237    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1238      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1239      <address><email>julian.reschke@greenbytes.de</email></address>
1240    </author>
1241    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1242  </front>
1243  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1244  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1245</reference>
1246
1247<reference anchor="Part3">
1248  <front>
1249    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1250    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1251      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1252      <address><email>fielding@gbiv.com</email></address>
1253    </author>
1254    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1255      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1256      <address><email>jg@freedesktop.org</email></address>
1257    </author>
1258    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1259      <organization abbrev="HP">Hewlett-Packard Company</organization>
1260      <address><email>JeffMogul@acm.org</email></address>
1261    </author>
1262    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1263      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1264      <address><email>henrikn@microsoft.com</email></address>
1265    </author>
1266    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1267      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1268      <address><email>LMM@acm.org</email></address>
1269    </author>
1270    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1271      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1272      <address><email>paulle@microsoft.com</email></address>
1273    </author>
1274    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1275      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1276      <address><email>timbl@w3.org</email></address>
1277    </author>
1278    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1279      <organization abbrev="W3C">World Wide Web Consortium</organization>
1280      <address><email>ylafon@w3.org</email></address>
1281    </author>
1282    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1283      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1284      <address><email>julian.reschke@greenbytes.de</email></address>
1285    </author>
1286    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1287  </front>
1288  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1289  <x:source href="p3-payload.xml" basename="p3-payload"/>
1290</reference>
1291
1292<reference anchor="Part5">
1293  <front>
1294    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1295    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1296      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1297      <address><email>fielding@gbiv.com</email></address>
1298    </author>
1299    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1300      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1301      <address><email>jg@freedesktop.org</email></address>
1302    </author>
1303    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1304      <organization abbrev="HP">Hewlett-Packard Company</organization>
1305      <address><email>JeffMogul@acm.org</email></address>
1306    </author>
1307    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1308      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1309      <address><email>henrikn@microsoft.com</email></address>
1310    </author>
1311    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1312      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1313      <address><email>LMM@acm.org</email></address>
1314    </author>
1315    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1316      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1317      <address><email>paulle@microsoft.com</email></address>
1318    </author>
1319    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1320      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1321      <address><email>timbl@w3.org</email></address>
1322    </author>
1323    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1324      <organization abbrev="W3C">World Wide Web Consortium</organization>
1325      <address><email>ylafon@w3.org</email></address>
1326    </author>
1327    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1328      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1329      <address><email>julian.reschke@greenbytes.de</email></address>
1330    </author>
1331    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1332  </front>
1333  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1334  <x:source href="p5-range.xml" basename="p5-range"/>
1335</reference>
1336
1337<reference anchor="Part6">
1338  <front>
1339    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1340    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1341      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1342      <address><email>fielding@gbiv.com</email></address>
1343    </author>
1344    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1345      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1346      <address><email>jg@freedesktop.org</email></address>
1347    </author>
1348    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1349      <organization abbrev="HP">Hewlett-Packard Company</organization>
1350      <address><email>JeffMogul@acm.org</email></address>
1351    </author>
1352    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1353      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1354      <address><email>henrikn@microsoft.com</email></address>
1355    </author>
1356    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1357      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1358      <address><email>LMM@acm.org</email></address>
1359    </author>
1360    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1361      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1362      <address><email>paulle@microsoft.com</email></address>
1363    </author>
1364    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1365      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1366      <address><email>timbl@w3.org</email></address>
1367    </author>
1368    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1369      <organization abbrev="W3C">World Wide Web Consortium</organization>
1370      <address><email>ylafon@w3.org</email></address>
1371    </author>
1372    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1373      <address><email>mnot@mnot.net</email></address>
1374    </author>
1375    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1376      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1377      <address><email>julian.reschke@greenbytes.de</email></address>
1378    </author>
1379    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1380  </front>
1381  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1382  <x:source href="p6-cache.xml" basename="p6-cache"/>
1383</reference>
1384
1385<reference anchor="RFC2119">
1386  <front>
1387    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1388    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1389      <organization>Harvard University</organization>
1390      <address><email>sob@harvard.edu</email></address>
1391    </author>
1392    <date month="March" year="1997"/>
1393  </front>
1394  <seriesInfo name="BCP" value="14"/>
1395  <seriesInfo name="RFC" value="2119"/>
1396</reference>
1397
1398<reference anchor="RFC5234">
1399  <front>
1400    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1401    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1402      <organization>Brandenburg InternetWorking</organization>
1403      <address>
1404        <email>dcrocker@bbiw.net</email>
1405      </address> 
1406    </author>
1407    <author initials="P." surname="Overell" fullname="Paul Overell">
1408      <organization>THUS plc.</organization>
1409      <address>
1410        <email>paul.overell@thus.net</email>
1411      </address>
1412    </author>
1413    <date month="January" year="2008"/>
1414  </front>
1415  <seriesInfo name="STD" value="68"/>
1416  <seriesInfo name="RFC" value="5234"/>
1417</reference>
1418
1419</references>
1420
1421<references title="Informative References">
1422
1423<reference anchor="RFC2616">
1424  <front>
1425    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1426    <author initials="R." surname="Fielding" fullname="R. Fielding">
1427      <organization>University of California, Irvine</organization>
1428      <address><email>fielding@ics.uci.edu</email></address>
1429    </author>
1430    <author initials="J." surname="Gettys" fullname="J. Gettys">
1431      <organization>W3C</organization>
1432      <address><email>jg@w3.org</email></address>
1433    </author>
1434    <author initials="J." surname="Mogul" fullname="J. Mogul">
1435      <organization>Compaq Computer Corporation</organization>
1436      <address><email>mogul@wrl.dec.com</email></address>
1437    </author>
1438    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1439      <organization>MIT Laboratory for Computer Science</organization>
1440      <address><email>frystyk@w3.org</email></address>
1441    </author>
1442    <author initials="L." surname="Masinter" fullname="L. Masinter">
1443      <organization>Xerox Corporation</organization>
1444      <address><email>masinter@parc.xerox.com</email></address>
1445    </author>
1446    <author initials="P." surname="Leach" fullname="P. Leach">
1447      <organization>Microsoft Corporation</organization>
1448      <address><email>paulle@microsoft.com</email></address>
1449    </author>
1450    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1451      <organization>W3C</organization>
1452      <address><email>timbl@w3.org</email></address>
1453    </author>
1454    <date month="June" year="1999"/>
1455  </front>
1456  <seriesInfo name="RFC" value="2616"/>
1457</reference>
1458
1459<reference anchor='RFC3864'>
1460  <front>
1461    <title>Registration Procedures for Message Header Fields</title>
1462    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1463      <organization>Nine by Nine</organization>
1464      <address><email>GK-IETF@ninebynine.org</email></address>
1465    </author>
1466    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1467      <organization>BEA Systems</organization>
1468      <address><email>mnot@pobox.com</email></address>
1469    </author>
1470    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1471      <organization>HP Labs</organization>
1472      <address><email>JeffMogul@acm.org</email></address>
1473    </author>
1474    <date year='2004' month='September' />
1475  </front>
1476  <seriesInfo name='BCP' value='90' />
1477  <seriesInfo name='RFC' value='3864' />
1478</reference>
1479
1480<reference anchor='RFC4918'>
1481  <front>
1482    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1483    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1484      <organization abbrev="CommerceNet">CommerceNet</organization>
1485      <address><email>ldusseault@commerce.net</email></address>
1486    </author>
1487    <date month="June" year="2007" />
1488  </front>
1489  <seriesInfo name='RFC' value='4918' />
1490</reference>
1491</references>
1492
1493<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1494<t>
1495  Allow weak entity-tags in all requests except range requests (Sections
1496  <xref target="weak.and.strong.validators" format="counter"/> and
1497  <xref target="header.if-none-match" format="counter"/>).
1498</t>
1499<t>
1500  Change ABNF productions for header fields to only define the field value.
1501  (<xref target="header.fields"/>)
1502</t>
1503</section>
1504
1505<?BEGININC p4-conditional.abnf-appendix ?>
1506<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1507<figure>
1508<artwork type="abnf" name="p4-conditional.parsed-abnf">
1509<x:ref>ETag</x:ref> = entity-tag
1510
1511<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1512
1513<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1514 entity-tag ] ) )
1515<x:ref>If-Modified-Since</x:ref> = HTTP-date
1516<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1517 entity-tag ] ) )
1518<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1519
1520<x:ref>Last-Modified</x:ref> = HTTP-date
1521
1522<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1523
1524<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1525
1526<x:ref>opaque-tag</x:ref> = quoted-string
1527
1528<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1529
1530<x:ref>weak</x:ref> = %x57.2F ; W/
1531</artwork>
1532</figure>
1533<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1534; ETag defined but not used
1535; If-Match defined but not used
1536; If-Modified-Since defined but not used
1537; If-None-Match defined but not used
1538; If-Unmodified-Since defined but not used
1539; Last-Modified defined but not used
1540</artwork></figure></section>
1541<?ENDINC p4-conditional.abnf-appendix ?>
1542
1543<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1544
1545<section title="Since RFC 2616">
1546<t>
1547  Extracted relevant partitions from <xref target="RFC2616"/>.
1548</t>
1549</section>
1550
1551<section title="Since draft-ietf-httpbis-p4-conditional-00">
1552<t>
1553  Closed issues:
1554  <list style="symbols"> 
1555    <t>
1556      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1557      "Normative and Informative references"
1558    </t>
1559  </list>
1560</t>
1561<t>
1562  Other changes:
1563  <list style="symbols"> 
1564    <t>
1565      Move definitions of 304 and 412 condition codes from Part2.
1566    </t>
1567  </list>
1568</t>
1569</section>
1570
1571<section title="Since draft-ietf-httpbis-p4-conditional-01">
1572<t>
1573  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1574  <list style="symbols"> 
1575    <t>
1576      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1577    </t>
1578  </list>
1579</t>
1580</section>
1581
1582<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1583<t>
1584  Closed issues:
1585  <list style="symbols"> 
1586    <t>
1587      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1588      "Weak ETags on non-GET requests"
1589    </t>
1590  </list>
1591</t>
1592<t>
1593  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1594  <list style="symbols"> 
1595    <t>
1596      Reference RFC 3984, and update header field registrations for header fields defined
1597      in this document.
1598    </t>
1599  </list>
1600</t>
1601</section>
1602
1603<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1604<t>
1605  Closed issues:
1606  <list style="symbols"> 
1607    <t>
1608      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1609      "Examples for ETag matching"
1610    </t>
1611    <t>
1612      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1613      "'entity value' undefined"
1614    </t>
1615    <t>
1616      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1617      "bogus 2068 Date header reference"
1618    </t>
1619  </list>
1620</t>
1621</section>
1622
1623<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1624<t>
1625  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1626  <list style="symbols"> 
1627    <t>
1628      Use "/" instead of "|" for alternatives.
1629    </t>
1630    <t>
1631      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1632      whitespace ("OWS") and required whitespace ("RWS").
1633    </t>
1634    <t>
1635      Rewrite ABNFs to spell out whitespace rules, factor out
1636      header field value format definitions.
1637    </t>
1638  </list>
1639</t>
1640</section>
1641
1642<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1643<t>
1644  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1645  <list style="symbols"> 
1646    <t>
1647      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1648    </t>
1649  </list>
1650</t>
1651</section>
1652
1653<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1654<t>
1655  Closed issues:
1656  <list style="symbols"> 
1657    <t>
1658      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1659      "case-sensitivity of etag weakness indicator"
1660    </t>
1661  </list>
1662</t>
1663</section>
1664
1665<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1666<t>
1667  Closed issues:
1668  <list style="symbols"> 
1669    <t>
1670      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1671      "Weak ETags on non-GET requests" (If-Match still was defined to require
1672      strong matching)
1673    </t>
1674    <t>
1675      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1676      "move IANA registrations for optional status codes"
1677    </t>
1678  </list>
1679</t>
1680</section>
1681
1682<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1683<t>
1684  No significant changes.
1685</t>
1686</section>
1687
1688<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1689<t>
1690  No significant changes.
1691</t>
1692</section>
1693
1694<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1695<t>
1696  Closed issues:
1697  <list style="symbols"> 
1698    <t>
1699      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1700      "Clarify 'Requested Variant'"
1701    </t>
1702    <t>
1703      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1704      "Clarify entity / representation / variant terminology"
1705    </t>
1706    <t>
1707      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1708      "consider removing the 'changes from 2068' sections"
1709    </t>
1710  </list>
1711</t>
1712</section>
1713
1714<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1715<t>
1716  None.
1717</t>
1718</section>
1719
1720<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1721<t>
1722  Closed issues:
1723  <list style="symbols"> 
1724    <t>
1725      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1726      "Header Classification"
1727    </t>
1728  </list>
1729</t>
1730</section>
1731
1732<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1733<t>
1734  Closed issues:
1735  <list style="symbols"> 
1736    <t>
1737      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1738      "If-* and entities"
1739    </t>
1740    <t>
1741      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1742      "Definition of validator weakness"
1743    </t>
1744    <t>
1745      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1746      "untangle ABNFs for header fields"
1747    </t>
1748    <t>
1749      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1750      "ETags and Quotes"
1751    </t>
1752  </list>
1753</t>
1754</section>
1755
1756<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1757<t>
1758  None.
1759</t>
1760</section>
1761
1762<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1763<t>
1764  Closed issues:
1765  <list style="symbols"> 
1766    <t>
1767      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1768      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1769    </t>
1770  </list>
1771</t>
1772</section>
1773
1774</section>
1775
1776</back>
1777</rfc>
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