source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1379

Last change on this file since 1379 was 1379, checked in by fielding@…, 8 years ago

Editorial. Move sections around without changing (much) the
text so that I can reduce more redundancy in a later commit.

  • Property svn:eol-style set to native
File size: 70.9 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "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 response metadata that can be used to indicate or
226   observe changes to resource state and request header fields that
227   specify preconditions to be checked before performing the action
228   given by the request method.  Conditional GET requests are the most
229   efficient mechanism for HTTP cache updates &caching;.  Conditionals
230   can also be
231   applied to state-changing methods, such as PUT and DELETE, to prevent
232   the "lost update" problem: one client accidentally overwriting
233   the work of another client that has been acting in parallel.
234</t>
235<t>
236   Conditional request preconditions are based on the state of the target
237   resource as a whole (its current value set) or the state as observed
238   in a previously obtained representation (one value in that set).
239   A resource might have multiple current representations, each with its
240   own observable state.  The conditional request mechanisms assume that
241   the mapping of requests to corresponding representations will be
242   consistent over time if the server intends to take advantage of
243   conditionals.  Regardless, if the mapping is inconsistent and
244   the server is unable to select the appropriate representation, then
245   no harm will result when the precondition evaluates to false.
246</t>
247<t><iref primary="true" item="selected representation"/>
248   We use the term "<x:dfn>selected representation</x:dfn>" to refer to
249   the current representation of the target resource that would have been
250   selected in a successful response if the same request had used the method
251   GET and had excluded all of the conditional request header fields.
252   The conditional request preconditions are evaluated by comparing the
253   values provided in the request header fields to the current metadata
254   for the selected representation.
255</t>
256
257<section title="Requirements" anchor="intro.requirements">
258<t>
259   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
260   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
261   document are to be interpreted as described in <xref target="RFC2119"/>.
262</t>
263<t>
264   An implementation is not compliant if it fails to satisfy one or more
265   of the "MUST" or "REQUIRED" level requirements for the protocols it
266   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
267   level and all the "SHOULD" level requirements for its protocols is said
268   to be "unconditionally compliant"; one that satisfies all the "MUST"
269   level requirements but not all the "SHOULD" level requirements for its
270   protocols is said to be "conditionally compliant".
271</t>
272</section>
273
274<section title="Syntax Notation" anchor="notation">
275  <x:anchor-alias value="ALPHA"/>
276  <x:anchor-alias value="CR"/>
277  <x:anchor-alias value="DIGIT"/>
278  <x:anchor-alias value="LF"/>
279  <x:anchor-alias value="OCTET"/>
280  <x:anchor-alias value="VCHAR"/>
281  <x:anchor-alias value="WSP"/>
282  <x:anchor-alias value="core.rules"/>
283  <x:anchor-alias value="quoted-string"/>
284  <x:anchor-alias value="OWS"/>
285  <x:anchor-alias value="HTTP-date"/>
286<t>
287  This specification uses the ABNF syntax defined in &notation; (which
288  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
289  <xref target="collected.abnf"/> shows the collected ABNF, with the list
290  rule expanded.
291</t>
292<t>
293  The following core rules are included by
294  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
295  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
296  DIGIT (decimal 0-9), DQUOTE (double quote),
297  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
298  OCTET (any 8-bit sequence of data), SP (space),
299  VCHAR (any visible USASCII character),
300  and WSP (whitespace).
301</t>
302<t>
303  The ABNF rules below are defined in other parts:
304</t>
305<figure><artwork type="abnf2616">
306  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
307  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
308  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
309</artwork></figure>
310</section>
311</section>
312
313<section title="Resource State Metadata (Validators)" anchor="resource.metadata">
314   <iref primary="true" item="metadata"/>
315   <iref primary="true" item="validator"/>
316<t>
317   This specification defines two forms of metadata that are commonly used
318   to observe resource state and test for preconditions: modification dates
319   and opaque entity tags.  Additional metadata that reflects resource state
320   has been defined by various extensions of HTTP, such as WebDAV
321   <xref target="RFC4918"/>, that are beyond the scope of this specification.
322   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
323   when it is used within a precondition.
324</t>
325
326<section title="Weak versus Strong" anchor="weak.and.strong.validators">
327<t>
328   Since both origin servers and caches will compare two validators to
329   decide if they indicate the same or different representations, one
330   normally would expect that if the representation (including both
331   representation header fields and representation body) changes in any
332   way, then the associated validator would change as well. If this is
333   true, then we call that validator a "strong validator".  One example
334   of a strong validator is an integer that is incremented in stable
335   storage every time a representation is changed.
336</t>
337<t>
338   However, there might be cases when a server prefers to change the
339   validator only when it desires cached representations to be invalidated.
340   For example, the representation of a weather report that changes in
341   content every second, based on dynamic measurements, might be grouped
342   into sets of equivalent representations (from the origin server's
343   perspective) in order to allow cached representations to be valid
344   for a reasonable period of time (perhaps adjusted dynamically based
345   on server load or weather quality).
346   A validator that does not always change when the representation
347   changes is a "weak validator".
348</t>
349<t>
350   One can think of a strong validator as part of an identifier for a
351   specific representation, whereas a weak validator is part of an
352   identifier for a set of equivalent representations (where this notion
353   of equivalence is entirely governed by the origin server and beyond
354   the scope of this specification).
355</t>
356<t>
357      A representation's modification time, if defined with only one-second
358      resolution, could be a weak validator, since it is possible that
359      the representation might be modified twice during a single second.
360</t><t>
361      Support for weak validators is optional. However, weak validators
362      allow for more efficient caching of equivalent objects; for
363      example, a hit counter on a site is probably good enough if it is
364      updated every few days or weeks, and any value during that period
365      is likely "good enough" to be equivalent.
366</t>
367<t>
368   A strong entity-tag &MUST; change whenever the associated representation
369   changes in any way. A weak entity-tag &SHOULD; change whenever the origin
370   server considers prior representations to be unacceptable as a substitute
371   for the current representation. In other words, a weak entity tag &SHOULD;
372   change whenever the origin server wants caches to invalidate old responses.
373</t>
374<t>
375   A "strong entity-tag" &MAY; be shared by two representations of a resource
376   only if they are equivalent by octet equality.
377</t>
378<t>
379   A "weak entity-tag", indicated by the "W/" prefix, &MAY; be shared by
380   two representations of a resource. A weak entity-tag can only be used
381   for weak comparison.
382</t>
383<t>
384   Cache entries might persist for arbitrarily long periods, regardless
385   of expiration times.  Thus, a cache might attempt to validate an
386   entry using a validator that it obtained in the distant past.
387   A strong entity-tag &MUST; be unique across all versions of all
388   representations associated with a particular resource over time.
389   However, there is no implication of uniqueness across entity-tags
390   of different resources (i.e., the same entity-tag value might be
391   in use for representations of multiple resources at the same time
392   and does not imply that those representations are equivalent).
393</t>
394<t>
395   A "use" of a validator is either when a client generates a request
396   and includes the validator in a precondition, or when a server
397   compares two validators.
398</t>
399<t>
400   Strong validators are usable in any context. Weak validators are only
401   usable in contexts that do not depend on exact equality of a representation.
402   For example, either kind is usable for a normal conditional GET.
403</t>
404</section>
405
406<section title="Last-Modified" anchor="header.last-modified">
407  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
408  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
409  <x:anchor-alias value="Last-Modified"/>
410<t>
411   The "Last-Modified" header field indicates the date and time at
412   which the origin server believes the selected representation was
413   last modified.
414</t>
415<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
416  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
417</artwork></figure>
418<t>
419   An example of its use is
420</t>
421<figure><artwork type="example">
422  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
423</artwork></figure>
424
425<section title="Generation" anchor="lastmod.generation">
426<t>
427   Origin servers &SHOULD; send Last-Modified for any selected
428   representation for which a last modification date can be reasonably
429   and consistently determined, since its use in conditional requests
430   and evaluating cache freshness (&caching;) results in a substantial
431   reduction of HTTP traffic on the Internet and can be a significant
432   factor in improving service scalability and reliability.
433</t>
434<t>
435   A representation is typically the sum of many parts behind the
436   resource interface.  The last-modified time would usually be
437   the most recent time that any of those parts were changed.
438   How that value is determined for any given resource is an
439   implementation detail beyond the scope of this specification.
440   What matters to HTTP is how recipients of the Last-Modified
441   header field can use its value to make conditional requests
442   and test the validity of locally cached responses.
443</t>
444<t>
445   An origin server &SHOULD; obtain the Last-Modified value of the
446   representation as close as possible to the time that it generates
447   the Date field-value for its response. This allows a recipient to
448   make an accurate assessment of the representation's modification time,
449   especially if the representation changes near the time that the
450   response is generated.
451</t>
452<t>
453   An origin server with a clock &MUST-NOT; send a Last-Modified date
454   that is later than the server's time of message origination (Date).
455   If the last modification time is derived from implementation-specific
456   metadata that evaluates to some time in the future, according to the
457   origin server's clock, then the origin server &MUST; replace that
458   value with the message origination date. This prevents a future
459   modification date from having an adverse impact on cache validation.
460</t>
461</section>
462
463<section title="Comparison" anchor="lastmod.comparison">
464<t>
465   A Last-Modified time, when used as a validator in a request, is
466   implicitly weak unless it is possible to deduce that it is strong,
467   using the following rules:
468  <list style="symbols">
469     <t>The validator is being compared by an origin server to the
470        actual current validator for the representation and,</t>
471     <t>That origin server reliably knows that the associated representation did
472        not change twice during the second covered by the presented
473        validator.</t>
474  </list>
475</t>
476<t>
477   or
478  <list style="symbols">
479     <t>The validator is about to be used by a client in an If-Modified-Since,
480        If-Unmodified-Since header field, because the client has a cache entry,
481        or If-Range for the associated representation, and</t>
482     <t>That cache entry includes a Date value, which gives the time
483        when the origin server sent the original response, and</t>
484     <t>The presented Last-Modified time is at least 60 seconds before
485        the Date value.</t>
486  </list>
487</t>
488<t>
489   or
490  <list style="symbols">
491     <t>The validator is being compared by an intermediate cache to the
492        validator stored in its cache entry for the representation, and</t>
493     <t>That cache entry includes a Date value, which gives the time
494        when the origin server sent the original response, and</t>
495     <t>The presented Last-Modified time is at least 60 seconds before
496        the Date value.</t>
497  </list>
498</t>
499<t>
500   This method relies on the fact that if two different responses were
501   sent by the origin server during the same second, but both had the
502   same Last-Modified time, then at least one of those responses would
503   have a Date value equal to its Last-Modified time. The arbitrary 60-second
504   limit guards against the possibility that the Date and Last-Modified
505   values are generated from different clocks, or at somewhat
506   different times during the preparation of the response. An
507   implementation &MAY; use a value larger than 60 seconds, if it is
508   believed that 60 seconds is too short.
509</t>
510</section>
511</section>
512
513<section title="ETag" anchor="header.etag">
514  <iref primary="true" item="ETag header field" x:for-anchor=""/>
515  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
516  <x:anchor-alias value="ETag"/>
517  <x:anchor-alias value="entity-tag"/>
518  <x:anchor-alias value="entity.tags"/>
519  <x:anchor-alias value="opaque-tag"/>
520  <x:anchor-alias value="weak"/>
521<t>
522   The ETag header field provides the current entity-tag for the
523   selected representation.
524   An entity-tag is an opaque validator for differentiating between
525   multiple representations of the same resource, regardless of whether
526   those multiple representations are due to resource state changes over
527   time, content negotiation resulting in multiple representations being
528   valid at the same time, or both. An entity-tag consists of an opaque
529   quoted string, possibly prefixed by a weakness indicator.
530</t>
531<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"/>
532  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
533
534  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
535  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
536  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
537</artwork></figure>
538<t>
539   An entity-tag can be more reliable for validation than a modification
540   date in situations where it is inconvenient to store modification
541   dates, where the one-second resolution of HTTP date values is not
542   sufficient, or where modification dates are not consistently maintained.
543</t>
544<figure><preamble>
545  Examples:
546</preamble>
547<artwork type="example">
548  ETag: "xyzzy"
549  ETag: W/"xyzzy"
550  ETag: ""
551</artwork></figure>
552<t>
553   An entity-tag can be either a weak or strong validator, with
554   strong being the default.  If an origin server provides an entity-tag
555   for a representation and the generation of that entity-tag does not satisfy
556   the requirements for a strong validator
557   (<xref target="weak.and.strong.validators"/>), then that
558   entity-tag &MUST; be marked as weak by prefixing its opaque value
559   with "W/" (case-sensitive).
560</t>
561
562<section title="Generation" anchor="entity.tag.generation">
563<t>
564   The principle behind entity-tags is that only the service author
565   knows the implementation of a resource well enough to select the
566   most accurate and efficient validation mechanism for that resource,
567   and that any such mechanism can be mapped to a simple sequence of
568   octets for easy comparison.  Since the value is opaque, there is no
569   need for the client to be aware of how each entity-tag is constructed.
570</t>
571<t>
572   For example, a resource that has implementation-specific versioning
573   applied to all changes might use an internal revision number, perhaps
574   combined with a variance identifier for content negotiation, to
575   accurately differentiate between representations.
576   Other implementations might use a stored hash of representation content,
577   a combination of various filesystem attributes, or a modification
578   timestamp that has sub-second resolution.
579</t>
580<t>
581   Origin servers &SHOULD; send ETag for any selected representation
582   for which detection of changes can be reasonably and consistently
583   determined, since the entity-tag's use in conditional requests and
584   evaluating cache freshness (&caching;) can result in a substantial
585   reduction of HTTP network traffic and can be a significant factor in
586   improving service scalability and reliability.
587</t>
588</section>
589
590<section title="Comparison" anchor="entity.tag.comparison">
591  <x:anchor-alias value="validator.comparison"/>
592<t>
593   There are two entity-tag comparison functions, depending
594   on whether the comparison context allows the use of weak validators
595   or not:
596  <list style="symbols">
597     <t>The strong comparison function: in order to be considered equal,
598        both opaque-tags &MUST; be identical character-by-character, and both
599        &MUST-NOT; be weak.</t>
600     <t>The weak comparison function: in order to be considered equal, both
601        opaque-tags &MUST; be identical character-by-character, but
602        either or both of them &MAY; be tagged as "weak" without affecting
603        the result.</t>
604  </list>
605</t>
606<t>
607   The example below shows the results for a set of entity-tag pairs,
608   and both the weak and strong comparison function results:
609</t>
610<texttable align="left">
611  <ttcol>ETag 1</ttcol>
612  <ttcol>ETag 2</ttcol>
613  <ttcol>Strong Comparison</ttcol>
614  <ttcol>Weak Comparison</ttcol>
615
616  <c>W/"1"</c>
617  <c>W/"1"</c>
618  <c>no match</c>
619  <c>match</c>
620 
621  <c>W/"1"</c>
622  <c>W/"2"</c>
623  <c>no match</c>
624  <c>no match</c>
625
626  <c>W/"1"</c>
627  <c>"1"</c>
628  <c>no match</c>
629  <c>match</c>
630
631  <c>"1"</c>
632  <c>"1"</c>
633  <c>match</c>
634  <c>match</c>
635</texttable>
636</section>
637
638<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
639<t>
640   Consider a resource that is subject to content negotiation (&content-negotiation;),
641   and where the representations returned upon a GET request vary based on
642   the Accept-Encoding request header field (&header-accept-encoding;):
643</t>
644<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
645GET /index HTTP/1.1
646Host: www.example.com
647Accept-Encoding: gzip
648
649</artwork></figure>
650<t>
651   In this case, the response might or might not use the gzip content coding.
652   If it does not, the response might look like:
653</t>
654<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
655HTTP/1.1 200 OK
656Date: Thu, 26 Mar 2010 00:05:00 GMT
657ETag: "123-a"
658Content-Length: <x:length-of target="exbody"/>
659Vary: Accept-Encoding
660Content-Type: text/plain
661
662<x:span anchor="exbody">Hello World!
663Hello World!
664Hello World!
665Hello World!
666Hello World!
667</x:span></artwork></figure>
668<t>
669   An alternative representation that does use gzip content coding would be:
670</t>
671<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
672HTTP/1.1 200 OK
673Date: Thu, 26 Mar 2010 00:05:00 GMT
674ETag: "123-b"
675Content-Length: 43
676Vary: Accept-Encoding
677Content-Type: text/plain
678Content-Encoding: gzip
679
680<spanx>...binary data...</spanx></artwork></figure>
681<x:note>
682  <t>
683    <x:h>Note:</x:h> Content codings are a property of the representation,
684    so therefore an entity-tag of an encoded representation must be distinct
685    from an unencoded representation to prevent conflicts during cache updates
686    and range requests.  In contrast, transfer codings (&transfer-codings;)
687    apply only during message transfer and do not require distinct entity-tags.
688  </t>
689</x:note>
690</section>
691</section>
692
693<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">
694<t>
695   We adopt a set of rules and recommendations for origin servers,
696   clients, and caches regarding when various validator types ought to
697   be used, and for what purposes.
698</t>
699<t>
700   HTTP/1.1 origin servers:
701  <list style="symbols">
702     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
703        generate one.</t>
704
705     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
706        performance considerations support the use of weak entity-tags,
707        or if it is unfeasible to send a strong entity-tag.</t>
708
709     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
710  </list>
711</t>
712<t>
713   In other words, the preferred behavior for an HTTP/1.1 origin server
714   is to send both a strong entity-tag and a Last-Modified value.
715</t>
716<t>
717   HTTP/1.1 clients:
718  <list style="symbols">
719     <t>&MUST; use that entity-tag in any cache-conditional request (using
720        If-Match or If-None-Match) if an entity-tag has been provided by the
721        origin server.</t>
722
723     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
724        requests (using If-Modified-Since) if only a Last-Modified value has
725        been provided by the origin server. </t>
726
727     <t>&MAY; use the Last-Modified value in subrange cache-conditional
728        requests (using If-Unmodified-Since) if only a Last-Modified value has
729        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
730        provide a way to disable this, in case of difficulty.</t>
731
732     <t>&SHOULD; use both validators in cache-conditional requests if both an
733        entity-tag and a Last-Modified value have been provided by the origin
734        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
735        appropriately.</t>
736  </list>
737</t>
738<t>
739   An HTTP/1.1 origin server, upon receiving a conditional request that
740   includes both a Last-Modified date (e.g., in an If-Modified-Since or
741   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
742   in an If-Match, If-None-Match, or If-Range header field) as cache
743   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
744   unless doing so is consistent with all of the conditional header
745   fields in the request.
746</t>
747<t>
748   An HTTP/1.1 caching proxy, upon receiving a conditional request that
749   includes both a Last-Modified date and one or more entity-tags as
750   cache validators, &MUST-NOT; return a locally cached response to the
751   client unless that cached response is consistent with all of the
752   conditional header fields in the request.
753  <list><t>
754      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
755      servers and clients ought to transmit as much non-redundant
756      information as is available in their responses and requests.
757      HTTP/1.1 systems receiving this information will make the most
758      conservative assumptions about the validators they receive.
759  </t><t>
760      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
761      last-modified values received or used by these systems will
762      support transparent and efficient caching, and so HTTP/1.1 origin
763      servers should provide Last-Modified values. In those rare cases
764      where the use of a Last-Modified value as a validator by an
765      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
766      origin servers should not provide one.
767  </t></list>
768</t>
769</section>
770
771</section>
772
773<section title="Precondition Header Fields" anchor="header.fields">
774<t>
775   This section defines the syntax and semantics of HTTP/1.1 header fields
776   for applying preconditions on requests.
777</t>
778
779<section title="If-Match" anchor="header.if-match">
780  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
781  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
782  <x:anchor-alias value="If-Match"/>
783<t>
784   The "If-Match" header field &MAY; be used to make a request method
785   conditional on the current existence or value of an entity-tag for
786   one or more representations of the target resource.  If-Match is
787   generally useful for resource update requests, such as PUT requests,
788   as a means for protecting against accidental overwrites when multiple
789   clients are acting in parallel on the same resource (i.e., the
790   "lost update" problem).  An If-Match field-value of "*" places the
791   precondition on the existence of any current representation for the
792   target resource.
793</t>
794<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
795  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
796</artwork></figure>
797<t>
798   If any of the entity-tags listed in the If-Match field value match
799   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
800   selected representation for the target resource,
801   or if "*" is given and any current representation exists for the
802   target resource, then the server &MAY; perform the request method
803   as if the If-Match header field was not present.
804</t>
805<t>
806   If none of the entity-tags match, or if "*" is given and no current
807   representation exists, the server &MUST-NOT; perform the requested method.
808   Instead, the server &MUST; respond with the 412 (Precondition Failed)
809   status code.
810</t>
811<t>
812   If the request would, without the If-Match header field, result in
813   anything other than a 2xx or 412 status code, then the If-Match header field
814   &MUST; be ignored.
815</t>
816<t>
817   Examples:
818</t>
819<figure><artwork type="example">
820  If-Match: "xyzzy"
821  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
822  If-Match: *
823</artwork></figure>
824<t>
825   The result of a request having both an If-Match header field and
826   either an If-None-Match or an If-Modified-Since header fields is
827   undefined by this specification.
828</t>
829</section>
830
831<section title="If-None-Match" anchor="header.if-none-match">
832  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
833  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
834  <x:anchor-alias value="If-None-Match"/>
835<t>
836   The "If-None-Match" header field &MAY; be used to make a request method
837   conditional on not matching any of the current entity-tag values for
838   representations of the target resource.  If-None-Match is primarily
839   used in conditional GET requests to enable efficient updates of cached
840   information with a minimum amount of transaction overhead.  A client
841   that has one or more representations previously obtained from the
842   target resource can send If-None-Match with a list of the associated
843   entity-tags in the hope of receiving a 304 response if at least one
844   of those representations matches the selected representation.
845</t>
846<t>
847   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
848   request method (e.g., PUT) from inadvertently modifying an existing
849   representation of the target resource when the client believes that
850   the resource does not have a current representation.  This is a variation
851   on the "lost update" problem that might arise if more than one client
852   attempts to create an initial representation for the target resource.
853</t>
854<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
855  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
856</artwork></figure>
857<t>
858   If any of the entity-tags listed in the If-None-Match field-value match
859   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
860   selected representation, or if "*" is
861   given and any current representation exists for that resource, then the
862   server &MUST-NOT; perform the requested method.
863   Instead, if the request method was GET or HEAD, the server &SHOULD;
864   respond with a 304 (Not Modified) status code, including the cache-related
865   header fields (particularly ETag) of the selected representation that has
866   a matching entity-tag.  For all other request methods, the server &MUST;
867   respond with a 412 (Precondition Failed) status code.
868</t>
869<t>
870   If none of the entity-tags match, then the server &MAY; perform the
871   requested method as if the If-None-Match header field did not exist,
872   but &MUST; also ignore any If-Modified-Since header field(s) in the
873   request. That is, if no entity-tags match, then the server &MUST-NOT;
874   return a 304 (Not Modified) response.
875</t>
876<t>
877   If the request would, without the If-None-Match header field, result
878   in anything other than a 2xx or 304 status code, then the If-None-Match
879   header field &MUST; be ignored. (See <xref
880   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
881   a discussion of server behavior when both If-Modified-Since and
882   If-None-Match appear in the same request.)
883</t>
884<t>
885   Examples:
886</t>
887<figure><artwork type="example">
888  If-None-Match: "xyzzy"
889  If-None-Match: W/"xyzzy"
890  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
891  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
892  If-None-Match: *
893</artwork></figure>
894<t>
895   The result of a request having both an If-None-Match header field and
896   either an If-Match or an If-Unmodified-Since header fields is
897   undefined by this specification.
898</t>
899</section>
900
901<section title="If-Modified-Since" anchor="header.if-modified-since">
902  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
903  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
904  <x:anchor-alias value="If-Modified-Since"/>
905<t>
906   The "If-Modified-Since" header field &MAY; be used to make a request
907   method conditional by modification date: if the selected representation
908   has not been modified since the time specified in this field, then
909   do not perform the request method; instead, respond as detailed below.
910</t>
911<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
912  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
913</artwork></figure>
914<t>
915   An example of the field is:
916</t>
917<figure><artwork type="example">
918  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
919</artwork></figure>
920<t>
921   A GET method with an If-Modified-Since header field and no Range header
922   field requests that the selected representation be transferred only if
923   it has been modified since the date given by the If-Modified-Since
924   header field.
925   The algorithm for determining this includes the following cases:
926  <list style="numbers">
927      <t>If the request would normally result in anything other than a
928         200 (OK) status code, or if the passed If-Modified-Since date is
929         invalid, the response is exactly the same as for a normal GET.
930         A date which is later than the server's current time is
931         invalid.</t>
932
933      <t>If the selected representation has been modified since the
934         If-Modified-Since date, the response is exactly the same as for
935         a normal GET.</t>
936
937      <t>If the selected representation has not been modified since a valid
938         If-Modified-Since date, the server &SHOULD; return a
939         304 (Not Modified) response.</t>
940  </list>
941</t>
942<t>
943   The purpose of this feature is to allow efficient updates of cached
944   information with a minimum amount of transaction overhead.
945  <list><t>
946      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
947      see &header-range; for full details.
948    </t><t>
949      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
950      clock might not be synchronized with the client.
951    </t><t>
952      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
953      servers will use an exact date comparison function, rather than a
954      less-than function, for deciding whether to send a 304 (Not
955      Modified) response. To get best results when sending an If-Modified-Since
956      header field for cache validation, clients are
957      advised to use the exact date string received in a previous Last-Modified
958      header field whenever possible.
959    </t><t>
960      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
961      header field instead of a date taken from the Last-Modified header field for
962      the same request, the client needs to be aware that this
963      date is interpreted in the server's understanding of time.
964      Unsynchronized clocks and rounding problems, due to the different
965      encodings of time between the client and server, are concerns.
966      This includes the possibility of race conditions if the
967      document has changed between the time it was first requested and
968      the If-Modified-Since date of a subsequent request, and the
969      possibility of clock-skew-related problems if the If-Modified-Since
970      date is derived from the client's clock without correction
971      to the server's clock. Corrections for different time bases
972      between client and server are at best approximate due to network
973      latency.
974    </t>
975  </list>
976</t>
977<t>
978   The result of a request having both an If-Modified-Since header field
979   and either an If-Match or an If-Unmodified-Since header fields is
980   undefined by this specification.
981</t>
982</section>
983
984<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
985  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
986  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
987  <x:anchor-alias value="If-Unmodified-Since"/>
988<t>
989   The "If-Unmodified-Since" header field &MAY; be used to make a request
990   method conditional by modification date: if the selected representation
991   has been modified since the time specified in this field, then the
992   server &MUST-NOT; perform the requested operation and &MUST; instead
993   respond with the 412 (Precondition Failed) status code.
994   If the selected representation has not been modified since the time
995   specified in this field, the server &SHOULD; perform the request
996   method as if the If-Unmodified-Since header field were not present.
997</t>
998<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
999  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
1000</artwork></figure>
1001<t>
1002   An example of the field is:
1003</t>
1004<figure><artwork type="example">
1005  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1006</artwork></figure>
1007<t>
1008   If the request normally (i.e., without the If-Unmodified-Since
1009   header field) would result in anything other than a 2xx or 412 status code,
1010   the If-Unmodified-Since header field &SHOULD; be ignored.
1011</t>
1012<t>
1013   If the specified date is invalid, the header field &MUST; be ignored.
1014</t>
1015<t>
1016   The result of a request having both an If-Unmodified-Since header
1017   field and either an If-None-Match or an If-Modified-Since header
1018   fields is undefined by this specification.
1019</t>
1020</section>
1021
1022<section title="If-Range" anchor="header.if-range">
1023<t>
1024   The If-Range header field provides a special conditional request
1025   mechanism that is similar to If-Match and If-Unmodified-Since but
1026   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1027</t>
1028</section>
1029
1030</section>
1031
1032<section title="Status Code Definitions" anchor="status.code.definitions">
1033<section title="304 Not Modified" anchor="status.304">
1034  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1035  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1036<t>
1037   The 304 status code indicates that a conditional GET request has been
1038   received and would have resulted in a 200 (OK) response if it were not
1039   for the fact that the condition has evaluated to false.  In other words,
1040   there is no need for the server to transfer a representation of the
1041   target resource because the client's request indicates that it already
1042   has a valid representation, as indicated by the 304 response header
1043   fields, and is therefore redirecting the client to make use of that
1044   stored representation as if it were the payload of a 200 response.
1045   The 304 response &MUST-NOT; contain a message-body, and thus is always
1046   terminated by the first empty line after the header fields.
1047</t>
1048<t>
1049   A 304 response &MUST; include a Date header field (&header-date;)
1050   unless its omission is required by &clockless;.  If a 200 response
1051   to the same request would have included any of the header fields
1052   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1053   Vary, then those same header fields &MUST; be sent in a 304 response.
1054</t>
1055<t>
1056   Since the goal of a 304 response is to minimize information transfer
1057   when the recipient already has one or more cached representations,
1058   the response &SHOULD-NOT; include representation metadata other
1059   than the above listed fields unless said metadata exists for the
1060   purpose of guiding cache updates (e.g., future HTTP extensions).
1061</t>
1062<t>
1063   If the recipient of a 304 response does not have a cached representation
1064   corresponding to the entity-tag indicated by the 304 response, then the
1065   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1066   conditional request originated with an outbound client, such as a
1067   user agent with its own cache sending a conditional GET to a shared
1068   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1069   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1070   the request without any preconditions.
1071</t>
1072<t>
1073   If a cache uses a received 304 response to update a cache entry, the
1074   cache &MUST; update the entry to reflect any new field values given in
1075   the response.
1076</t>
1077</section>
1078
1079<section title="412 Precondition Failed" anchor="status.412">
1080  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1081  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1082<t>
1083   The 412 status code indicates that one or more preconditions given in
1084   the request header fields evaluated to false when tested on the server.
1085   This response code allows the client to place preconditions on the
1086   current resource state (its current representations and metadata)
1087   and thus prevent the request method from being applied if the target
1088   resource is in an unexpected state.
1089</t>
1090</section>
1091</section>
1092
1093<section title="IANA Considerations" anchor="IANA.considerations">
1094
1095<section title="Status Code Registration" anchor="status.code.registration">
1096<t>
1097   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1098   shall be updated with the registrations below:
1099</t>
1100<?BEGININC p4-conditional.iana-status-codes ?>
1101<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1102<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1103   <ttcol>Value</ttcol>
1104   <ttcol>Description</ttcol>
1105   <ttcol>Reference</ttcol>
1106   <c>304</c>
1107   <c>Not Modified</c>
1108   <c>
1109      <xref target="status.304"/>
1110   </c>
1111   <c>412</c>
1112   <c>Precondition Failed</c>
1113   <c>
1114      <xref target="status.412"/>
1115   </c>
1116</texttable>
1117<!--(END)-->
1118<?ENDINC p4-conditional.iana-status-codes ?>
1119</section>
1120
1121<section title="Header Field Registration" anchor="header.field.registration">
1122<t>
1123   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1124   with the permanent registrations below (see <xref target="RFC3864"/>):
1125</t>
1126<?BEGININC p4-conditional.iana-headers ?>
1127<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1128<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1129   <ttcol>Header Field Name</ttcol>
1130   <ttcol>Protocol</ttcol>
1131   <ttcol>Status</ttcol>
1132   <ttcol>Reference</ttcol>
1133
1134   <c>ETag</c>
1135   <c>http</c>
1136   <c>standard</c>
1137   <c>
1138      <xref target="header.etag"/>
1139   </c>
1140   <c>If-Match</c>
1141   <c>http</c>
1142   <c>standard</c>
1143   <c>
1144      <xref target="header.if-match"/>
1145   </c>
1146   <c>If-Modified-Since</c>
1147   <c>http</c>
1148   <c>standard</c>
1149   <c>
1150      <xref target="header.if-modified-since"/>
1151   </c>
1152   <c>If-None-Match</c>
1153   <c>http</c>
1154   <c>standard</c>
1155   <c>
1156      <xref target="header.if-none-match"/>
1157   </c>
1158   <c>If-Unmodified-Since</c>
1159   <c>http</c>
1160   <c>standard</c>
1161   <c>
1162      <xref target="header.if-unmodified-since"/>
1163   </c>
1164   <c>Last-Modified</c>
1165   <c>http</c>
1166   <c>standard</c>
1167   <c>
1168      <xref target="header.last-modified"/>
1169   </c>
1170</texttable>
1171<!--(END)-->
1172<?ENDINC p4-conditional.iana-headers ?>
1173<t>
1174   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1175</t>
1176</section>
1177</section>
1178
1179<section title="Security Considerations" anchor="security.considerations">
1180<t>
1181   No additional security considerations have been identified beyond
1182   those applicable to HTTP in general &messaging;.
1183</t>
1184</section>
1185
1186<section title="Acknowledgments" anchor="acks">
1187<t>
1188  See &acks;.
1189</t>
1190</section>
1191</middle>
1192<back>
1193
1194<references title="Normative References">
1195
1196<reference anchor="Part1">
1197  <front>
1198    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1199    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1200      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1201      <address><email>fielding@gbiv.com</email></address>
1202    </author>
1203    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1204      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1205      <address><email>jg@freedesktop.org</email></address>
1206    </author>
1207    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1208      <organization abbrev="HP">Hewlett-Packard Company</organization>
1209      <address><email>JeffMogul@acm.org</email></address>
1210    </author>
1211    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1212      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1213      <address><email>henrikn@microsoft.com</email></address>
1214    </author>
1215    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1216      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1217      <address><email>LMM@acm.org</email></address>
1218    </author>
1219    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1220      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1221      <address><email>paulle@microsoft.com</email></address>
1222    </author>
1223    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1224      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1225      <address><email>timbl@w3.org</email></address>
1226    </author>
1227    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1228      <organization abbrev="W3C">World Wide Web Consortium</organization>
1229      <address><email>ylafon@w3.org</email></address>
1230    </author>
1231    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1232      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1233      <address><email>julian.reschke@greenbytes.de</email></address>
1234    </author>
1235    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1236  </front>
1237  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1238  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1239</reference>
1240
1241<reference anchor="Part3">
1242  <front>
1243    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1244    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1245      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1246      <address><email>fielding@gbiv.com</email></address>
1247    </author>
1248    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1249      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1250      <address><email>jg@freedesktop.org</email></address>
1251    </author>
1252    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1253      <organization abbrev="HP">Hewlett-Packard Company</organization>
1254      <address><email>JeffMogul@acm.org</email></address>
1255    </author>
1256    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1257      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1258      <address><email>henrikn@microsoft.com</email></address>
1259    </author>
1260    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1261      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1262      <address><email>LMM@acm.org</email></address>
1263    </author>
1264    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1265      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1266      <address><email>paulle@microsoft.com</email></address>
1267    </author>
1268    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1269      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1270      <address><email>timbl@w3.org</email></address>
1271    </author>
1272    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1273      <organization abbrev="W3C">World Wide Web Consortium</organization>
1274      <address><email>ylafon@w3.org</email></address>
1275    </author>
1276    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1277      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1278      <address><email>julian.reschke@greenbytes.de</email></address>
1279    </author>
1280    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1281  </front>
1282  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1283  <x:source href="p3-payload.xml" basename="p3-payload"/>
1284</reference>
1285
1286<reference anchor="Part5">
1287  <front>
1288    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1289    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1290      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1291      <address><email>fielding@gbiv.com</email></address>
1292    </author>
1293    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1294      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1295      <address><email>jg@freedesktop.org</email></address>
1296    </author>
1297    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1298      <organization abbrev="HP">Hewlett-Packard Company</organization>
1299      <address><email>JeffMogul@acm.org</email></address>
1300    </author>
1301    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1302      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1303      <address><email>henrikn@microsoft.com</email></address>
1304    </author>
1305    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1306      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1307      <address><email>LMM@acm.org</email></address>
1308    </author>
1309    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1310      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1311      <address><email>paulle@microsoft.com</email></address>
1312    </author>
1313    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1314      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1315      <address><email>timbl@w3.org</email></address>
1316    </author>
1317    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1318      <organization abbrev="W3C">World Wide Web Consortium</organization>
1319      <address><email>ylafon@w3.org</email></address>
1320    </author>
1321    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1322      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1323      <address><email>julian.reschke@greenbytes.de</email></address>
1324    </author>
1325    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1326  </front>
1327  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1328  <x:source href="p5-range.xml" basename="p5-range"/>
1329</reference>
1330
1331<reference anchor="Part6">
1332  <front>
1333    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1334    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1335      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1336      <address><email>fielding@gbiv.com</email></address>
1337    </author>
1338    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1339      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1340      <address><email>jg@freedesktop.org</email></address>
1341    </author>
1342    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1343      <organization abbrev="HP">Hewlett-Packard Company</organization>
1344      <address><email>JeffMogul@acm.org</email></address>
1345    </author>
1346    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1347      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1348      <address><email>henrikn@microsoft.com</email></address>
1349    </author>
1350    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1351      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1352      <address><email>LMM@acm.org</email></address>
1353    </author>
1354    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1355      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1356      <address><email>paulle@microsoft.com</email></address>
1357    </author>
1358    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1359      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1360      <address><email>timbl@w3.org</email></address>
1361    </author>
1362    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1363      <organization abbrev="W3C">World Wide Web Consortium</organization>
1364      <address><email>ylafon@w3.org</email></address>
1365    </author>
1366    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1367      <address><email>mnot@mnot.net</email></address>
1368    </author>
1369    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1370      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1371      <address><email>julian.reschke@greenbytes.de</email></address>
1372    </author>
1373    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1374  </front>
1375  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1376  <x:source href="p6-cache.xml" basename="p6-cache"/>
1377</reference>
1378
1379<reference anchor="RFC2119">
1380  <front>
1381    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1382    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1383      <organization>Harvard University</organization>
1384      <address><email>sob@harvard.edu</email></address>
1385    </author>
1386    <date month="March" year="1997"/>
1387  </front>
1388  <seriesInfo name="BCP" value="14"/>
1389  <seriesInfo name="RFC" value="2119"/>
1390</reference>
1391
1392<reference anchor="RFC5234">
1393  <front>
1394    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1395    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1396      <organization>Brandenburg InternetWorking</organization>
1397      <address>
1398        <email>dcrocker@bbiw.net</email>
1399      </address> 
1400    </author>
1401    <author initials="P." surname="Overell" fullname="Paul Overell">
1402      <organization>THUS plc.</organization>
1403      <address>
1404        <email>paul.overell@thus.net</email>
1405      </address>
1406    </author>
1407    <date month="January" year="2008"/>
1408  </front>
1409  <seriesInfo name="STD" value="68"/>
1410  <seriesInfo name="RFC" value="5234"/>
1411</reference>
1412
1413</references>
1414
1415<references title="Informative References">
1416
1417<reference anchor="RFC2616">
1418  <front>
1419    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1420    <author initials="R." surname="Fielding" fullname="R. Fielding">
1421      <organization>University of California, Irvine</organization>
1422      <address><email>fielding@ics.uci.edu</email></address>
1423    </author>
1424    <author initials="J." surname="Gettys" fullname="J. Gettys">
1425      <organization>W3C</organization>
1426      <address><email>jg@w3.org</email></address>
1427    </author>
1428    <author initials="J." surname="Mogul" fullname="J. Mogul">
1429      <organization>Compaq Computer Corporation</organization>
1430      <address><email>mogul@wrl.dec.com</email></address>
1431    </author>
1432    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1433      <organization>MIT Laboratory for Computer Science</organization>
1434      <address><email>frystyk@w3.org</email></address>
1435    </author>
1436    <author initials="L." surname="Masinter" fullname="L. Masinter">
1437      <organization>Xerox Corporation</organization>
1438      <address><email>masinter@parc.xerox.com</email></address>
1439    </author>
1440    <author initials="P." surname="Leach" fullname="P. Leach">
1441      <organization>Microsoft Corporation</organization>
1442      <address><email>paulle@microsoft.com</email></address>
1443    </author>
1444    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1445      <organization>W3C</organization>
1446      <address><email>timbl@w3.org</email></address>
1447    </author>
1448    <date month="June" year="1999"/>
1449  </front>
1450  <seriesInfo name="RFC" value="2616"/>
1451</reference>
1452
1453<reference anchor='RFC3864'>
1454  <front>
1455    <title>Registration Procedures for Message Header Fields</title>
1456    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1457      <organization>Nine by Nine</organization>
1458      <address><email>GK-IETF@ninebynine.org</email></address>
1459    </author>
1460    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1461      <organization>BEA Systems</organization>
1462      <address><email>mnot@pobox.com</email></address>
1463    </author>
1464    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1465      <organization>HP Labs</organization>
1466      <address><email>JeffMogul@acm.org</email></address>
1467    </author>
1468    <date year='2004' month='September' />
1469  </front>
1470  <seriesInfo name='BCP' value='90' />
1471  <seriesInfo name='RFC' value='3864' />
1472</reference>
1473
1474<reference anchor='RFC4918'>
1475  <front>
1476    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1477    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1478      <organization abbrev="CommerceNet">CommerceNet</organization>
1479      <address><email>ldusseault@commerce.net</email></address>
1480    </author>
1481    <date month="June" year="2007" />
1482  </front>
1483  <seriesInfo name='RFC' value='4918' />
1484</reference>
1485</references>
1486
1487<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1488<t>
1489  Allow weak entity-tags in all requests except range requests (Sections
1490  <xref target="weak.and.strong.validators" format="counter"/> and
1491  <xref target="header.if-none-match" format="counter"/>).
1492</t>
1493<t>
1494  Change ABNF productions for header fields to only define the field value.
1495  (<xref target="header.fields"/>)
1496</t>
1497</section>
1498
1499<?BEGININC p4-conditional.abnf-appendix ?>
1500<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1501<figure>
1502<artwork type="abnf" name="p4-conditional.parsed-abnf">
1503<x:ref>ETag</x:ref> = entity-tag
1504
1505<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1506
1507<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1508 entity-tag ] ) )
1509<x:ref>If-Modified-Since</x:ref> = HTTP-date
1510<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1511 entity-tag ] ) )
1512<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1513
1514<x:ref>Last-Modified</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 RFC 2616">
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 Field 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 field registrations for header fields 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 field 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 title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1709<t>
1710  None.
1711</t>
1712</section>
1713
1714<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1715<t>
1716  Closed issues:
1717  <list style="symbols"> 
1718    <t>
1719      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1720      "Header Classification"
1721    </t>
1722  </list>
1723</t>
1724</section>
1725
1726<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1727<t>
1728  Closed issues:
1729  <list style="symbols"> 
1730    <t>
1731      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1732      "If-* and entities"
1733    </t>
1734    <t>
1735      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1736      "Definition of validator weakness"
1737    </t>
1738    <t>
1739      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1740      "untangle ABNFs for header fields"
1741    </t>
1742    <t>
1743      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1744      "ETags and Quotes"
1745    </t>
1746  </list>
1747</t>
1748</section>
1749
1750<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1751<t>
1752  None.
1753</t>
1754</section>
1755
1756<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1757<t>
1758  Closed issues:
1759  <list style="symbols"> 
1760    <t>
1761      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1762      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1763    </t>
1764  </list>
1765</t>
1766</section>
1767
1768</section>
1769
1770</back>
1771</rfc>
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