source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1364

Last change on this file since 1364 was 1364, checked in by julian.reschke@…, 8 years ago

remove historical Acks (see #219)

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "July">
16  <!ENTITY ID-YEAR "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
193   protocol for distributed, collaborative, hypermedia information
194   systems. HTTP has been in use by the World Wide Web global information
195   initiative since 1990. This document is Part 4 of the seven-part specification
196   that defines the protocol referred to as "HTTP/1.1" and, taken together,
197   obsoletes RFC 2616.  Part 4 defines request header fields for
198   indicating conditional requests and the rules for constructing responses
199   to those requests.
200</t>
201</abstract>
202
203<note title="Editorial Note (To be removed by RFC Editor)">
204  <t>
205    Discussion of this draft should take place on the HTTPBIS working group
206    mailing list (ietf-http-wg@w3.org), which is archived at
207    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
208  </t>
209  <t>
210    The current issues list is at
211    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
212    documents (including fancy diffs) can be found at
213    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
214  </t>
215  <t>
216    The changes in this draft are summarized in <xref target="changes.since.15"/>.
217  </t>
218</note>
219</front>
220<middle>
221<section title="Introduction" anchor="introduction">
222<t>
223   This document defines the HTTP/1.1 conditional request mechanisms,
224   including both response metadata that can be used to indicate or
225   observe changes to resource state and request header fields that
226   specify preconditions to be checked before performing the action
227   given by the request method.  Conditional GET requests are the most
228   efficient mechanism for HTTP cache updates &caching;.  Conditionals
229   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="Resource State Metadata (Validators)" anchor="resource.metadata">
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="Last-Modified" anchor="header.last-modified">
326  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
327  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
328  <x:anchor-alias value="Last-Modified"/>
329<t>
330   The "Last-Modified" header field indicates the date and time at
331   which the origin server believes the selected representation was
332   last modified.
333</t>
334<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
335  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
336</artwork></figure>
337<t>
338   An example of its use is
339</t>
340<figure><artwork type="example">
341  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
342</artwork></figure>
343
344<section title="Generation" anchor="lastmod.generation">
345<t>
346   Origin servers &SHOULD; send Last-Modified for any selected
347   representation for which a last modification date can be reasonably
348   and consistently determined, since its use in conditional requests
349   and evaluating cache freshness (&caching;) results in a substantial
350   reduction of HTTP traffic on the Internet and can be a significant
351   factor in improving service scalability and reliability.
352</t>
353<t>
354   A representation is typically the sum of many parts behind the
355   resource interface.  The last-modified time would usually be
356   the most recent time that any of those parts were changed.
357   How that value is determined for any given resource is an
358   implementation detail beyond the scope of this specification.
359   What matters to HTTP is how recipients of the Last-Modified
360   header field can use its value to make conditional requests
361   and test the validity of locally cached responses.
362</t>
363<t>
364   An origin server &SHOULD; obtain the Last-Modified value of the
365   representation as close as possible to the time that it generates
366   the Date field-value for its response. This allows a recipient to
367   make an accurate assessment of the representation's modification time,
368   especially if the representation changes near the time that the
369   response is generated.
370</t>
371<t>
372   An origin server with a clock &MUST-NOT; send a Last-Modified date
373   that is later than the server's time of message origination (Date).
374   If the last modification time is derived from implementation-specific
375   metadata that evaluates to some time in the future, according to the
376   origin server's clock, then the origin server &MUST; replace that
377   value with the message origination date. This prevents a future
378   modification date from having an adverse impact on cache validation.
379</t>
380</section>
381
382<section title="Comparison" anchor="lastmod.comparison">
383<t>
384   A Last-Modified time, when used as a validator in a request, is
385   implicitly weak unless it is possible to deduce that it is strong,
386   using the following rules:
387  <list style="symbols">
388     <t>The validator is being compared by an origin server to the
389        actual current validator for the representation and,</t>
390     <t>That origin server reliably knows that the associated representation did
391        not change twice during the second covered by the presented
392        validator.</t>
393  </list>
394</t>
395<t>
396   or
397  <list style="symbols">
398     <t>The validator is about to be used by a client in an If-Modified-Since,
399        If-Unmodified-Since header field, because the client has a cache entry,
400        or If-Range for the associated representation, and</t>
401     <t>That cache entry includes a Date value, which gives the time
402        when the origin server sent the original response, and</t>
403     <t>The presented Last-Modified time is at least 60 seconds before
404        the Date value.</t>
405  </list>
406</t>
407<t>
408   or
409  <list style="symbols">
410     <t>The validator is being compared by an intermediate cache to the
411        validator stored in its cache entry for the representation, and</t>
412     <t>That cache entry includes a Date value, which gives the time
413        when the origin server sent the original response, and</t>
414     <t>The presented Last-Modified time is at least 60 seconds before
415        the Date value.</t>
416  </list>
417</t>
418<t>
419   This method relies on the fact that if two different responses were
420   sent by the origin server during the same second, but both had the
421   same Last-Modified time, then at least one of those responses would
422   have a Date value equal to its Last-Modified time. The arbitrary 60-second
423   limit guards against the possibility that the Date and Last-Modified
424   values are generated from different clocks, or at somewhat
425   different times during the preparation of the response. An
426   implementation &MAY; use a value larger than 60 seconds, if it is
427   believed that 60 seconds is too short.
428</t>
429</section>
430</section>
431
432<section title="ETag" anchor="header.etag">
433  <iref primary="true" item="ETag header field" x:for-anchor=""/>
434  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
435  <x:anchor-alias value="ETag"/>
436  <x:anchor-alias value="entity-tag"/>
437  <x:anchor-alias value="entity.tags"/>
438  <x:anchor-alias value="opaque-tag"/>
439  <x:anchor-alias value="weak"/>
440<t>
441   The ETag header field provides the current entity-tag for the
442   selected representation.
443   An entity-tag is an opaque validator for differentiating between
444   multiple representations of the same resource, regardless of whether
445   those multiple representations are due to resource state changes over
446   time, content negotiation resulting in multiple representations being
447   valid at the same time, or both. An entity-tag consists of an opaque
448   quoted string, possibly prefixed by a weakness indicator.
449</t>
450<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"/>
451  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
452
453  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
454  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
455  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
456</artwork></figure>
457<t>
458   An entity-tag can be more reliable for validation than a modification
459   date in situations where it is inconvenient to store modification
460   dates, where the one-second resolution of HTTP date values is not
461   sufficient, or where modification dates are not consistently maintained.
462</t>
463<figure><preamble>
464  Examples:
465</preamble>
466<artwork type="example">
467  ETag: "xyzzy"
468  ETag: W/"xyzzy"
469  ETag: ""
470</artwork></figure>
471
472<section title="Generation" anchor="entity.tag.generation">
473<t>
474   The principle behind entity-tags is that only the service author
475   knows the implementation of a resource well enough to select the
476   most accurate and efficient validation mechanism for that resource,
477   and that any such mechanism can be mapped to a simple sequence of
478   octets for easy comparison.  Since the value is opaque, there is no
479   need for the client to be aware of how each entity-tag is constructed.
480</t>
481<t>
482   For example, a resource that has implementation-specific versioning
483   applied to all changes might use an internal revision number, perhaps
484   combined with a variance identifier for content negotiation, to
485   accurately differentiate between representations.
486   Other implementations might use a stored hash of representation content,
487   a combination of various filesystem attributes, or a modification
488   timestamp that has sub-second resolution.
489</t>
490<t>
491   Origin servers &SHOULD; send ETag for any selected representation
492   for which detection of changes can be reasonably and consistently
493   determined, since the entity-tag's use in conditional requests and
494   evaluating cache freshness (&caching;) can result in a substantial
495   reduction of HTTP network traffic and can be a significant factor in
496   improving service scalability and reliability.
497</t>
498</section>
499
500<section title="Weak versus Strong" anchor="weak.and.strong.validators">
501<t>
502   Since both origin servers and caches will compare two validators to
503   decide if they indicate the same or different representations, one
504   normally would expect that if the representation (including both
505   representation header fields and representation body) changes in any
506   way, then the associated validator would change as well. If this is
507   true, then we call that validator a "strong validator".  One example
508   of a strong validator is an integer that is incremented in stable
509   storage every time a representation is changed.
510</t>
511<t>
512   However, there might be cases when a server prefers to change the
513   validator only when it desires cached representations to be invalidated.
514   For example, the representation of a weather report that changes in
515   content every second, based on dynamic measurements, might be grouped
516   into sets of equivalent representations (from the origin server's
517   perspective) in order to allow cached representations to be valid
518   for a reasonable period of time (perhaps adjusted dynamically based
519   on server load or weather quality).
520   A validator that does not always change when the representation
521   changes is a "weak validator".
522</t>
523<t>
524   One can think of a strong validator as part of an identifier for a
525   specific representation, whereas a weak validator is part of an
526   identifier for a set of equivalent representations (where this notion
527   of equivalence is entirely governed by the origin server and beyond
528   the scope of this specification).
529</t>
530<t>
531   An entity-tag is normally a strong validator, but the protocol
532   provides a mechanism to tag an entity-tag as "weak".
533  <list><t>
534      A representation's modification time, if defined with only one-second
535      resolution, could be a weak validator, since it is possible that
536      the representation might be modified twice during a single second.
537    </t><t>
538      Support for weak validators is optional. However, weak validators
539      allow for more efficient caching of equivalent objects; for
540      example, a hit counter on a site is probably good enough if it is
541      updated every few days or weeks, and any value during that period
542      is likely "good enough" to be equivalent.
543    </t></list>
544</t>
545<t>
546   A strong entity-tag &MUST; change whenever the associated representation
547   changes in any way. A weak entity-tag &SHOULD; change whenever the origin
548   server considers prior representations to be unacceptable as a substitute
549   for the current representation. In other words, a weak entity tag &SHOULD;
550   change whenever the origin server wants caches to invalidate old responses.
551</t>
552<t>
553   A "strong entity-tag" &MAY; be shared by two representations of a resource
554   only if they are equivalent by octet equality.
555</t>
556<t>
557   A "weak entity-tag", indicated by the "W/" prefix, &MAY; be shared by
558   two representations of a resource. A weak entity-tag can only be used
559   for weak comparison.
560</t>
561<t>
562   Cache entries might persist for arbitrarily long periods, regardless
563   of expiration times.  Thus, a cache might attempt to validate an
564   entry using a validator that it obtained in the distant past.
565   A strong entity-tag &MUST; be unique across all versions of all
566   representations associated with a particular resource over time.
567   However, there is no implication of uniqueness across entity-tags
568   of different resources (i.e., the same entity-tag value might be
569   in use for representations of multiple resources at the same time
570   and does not imply that those representations are equivalent).
571</t>
572</section>
573
574<section title="Comparison" anchor="entity.tag.comparison">
575  <x:anchor-alias value="validator.comparison"/>
576<t>
577   There are two entity-tag comparison functions, depending
578   on whether the comparison context allows the use of weak validators
579   or not:
580  <list style="symbols">
581     <t>The strong comparison function: in order to be considered equal,
582        both opaque-tags &MUST; be identical character-by-character, and both
583        &MUST-NOT; be weak.</t>
584     <t>The weak comparison function: in order to be considered equal, both
585        opaque-tags &MUST; be identical character-by-character, but
586        either or both of them &MAY; be tagged as "weak" without affecting
587        the result.</t>
588  </list>
589</t>
590<t>
591   A "use" of a validator is either when a client generates a request
592   and includes the validator in a precondition, or when a server
593   compares two validators.
594</t>
595<t>
596   Strong validators are usable in any context. Weak validators are only
597   usable in contexts that do not depend on exact equality of a representation.
598   For example, either kind is usable for a normal conditional GET.
599</t>
600<t>
601   The example below shows the results for a set of entity-tag pairs,
602   and both the weak and strong comparison function results:
603</t>
604<texttable align="left">
605  <ttcol>ETag 1</ttcol>
606  <ttcol>ETag 2</ttcol>
607  <ttcol>Strong Comparison</ttcol>
608  <ttcol>Weak Comparison</ttcol>
609
610  <c>W/"1"</c>
611  <c>W/"1"</c>
612  <c>no match</c>
613  <c>match</c>
614 
615  <c>W/"1"</c>
616  <c>W/"2"</c>
617  <c>no match</c>
618  <c>no match</c>
619
620  <c>W/"1"</c>
621  <c>"1"</c>
622  <c>no match</c>
623  <c>match</c>
624
625  <c>"1"</c>
626  <c>"1"</c>
627  <c>match</c>
628  <c>match</c>
629</texttable>
630<t>
631   An entity-tag is strong unless it is explicitly tagged as weak.
632</t>
633</section>
634
635<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">
636<t>
637   We adopt a set of rules and recommendations for origin servers,
638   clients, and caches regarding when various validator types ought to
639   be used, and for what purposes.
640</t>
641<t>
642   HTTP/1.1 origin servers:
643  <list style="symbols">
644     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
645        generate one.</t>
646
647     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
648        performance considerations support the use of weak entity-tags,
649        or if it is unfeasible to send a strong entity-tag.</t>
650
651     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
652  </list>
653</t>
654<t>
655   In other words, the preferred behavior for an HTTP/1.1 origin server
656   is to send both a strong entity-tag and a Last-Modified value.
657</t>
658<t>
659   HTTP/1.1 clients:
660  <list style="symbols">
661     <t>&MUST; use that entity-tag in any cache-conditional request (using
662        If-Match or If-None-Match) if an entity-tag has been provided by the
663        origin server.</t>
664
665     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
666        requests (using If-Modified-Since) if only a Last-Modified value has
667        been provided by the origin server. </t>
668
669     <t>&MAY; use the Last-Modified value in subrange cache-conditional
670        requests (using If-Unmodified-Since) if only a Last-Modified value has
671        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
672        provide a way to disable this, in case of difficulty.</t>
673
674     <t>&SHOULD; use both validators in cache-conditional requests if both an
675        entity-tag and a Last-Modified value have been provided by the origin
676        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
677        appropriately.</t>
678  </list>
679</t>
680<t>
681   An HTTP/1.1 origin server, upon receiving a conditional request that
682   includes both a Last-Modified date (e.g., in an If-Modified-Since or
683   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
684   in an If-Match, If-None-Match, or If-Range header field) as cache
685   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
686   unless doing so is consistent with all of the conditional header
687   fields in the request.
688</t>
689<t>
690   An HTTP/1.1 caching proxy, upon receiving a conditional request that
691   includes both a Last-Modified date and one or more entity-tags as
692   cache validators, &MUST-NOT; return a locally cached response to the
693   client unless that cached response is consistent with all of the
694   conditional header fields in the request.
695  <list><t>
696      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
697      servers and clients ought to transmit as much non-redundant
698      information as is available in their responses and requests.
699      HTTP/1.1 systems receiving this information will make the most
700      conservative assumptions about the validators they receive.
701  </t><t>
702      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
703      last-modified values received or used by these systems will
704      support transparent and efficient caching, and so HTTP/1.1 origin
705      servers should provide Last-Modified values. In those rare cases
706      where the use of a Last-Modified value as a validator by an
707      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
708      origin servers should not provide one.
709  </t></list>
710</t>
711</section>
712
713<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
714<t>
715   Consider a resource that is subject to content negotiation (&content-negotiation;),
716   and where the representations returned upon a GET request vary based on
717   the Accept-Encoding request header field (&header-accept-encoding;):
718</t>
719<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
720GET /index HTTP/1.1
721Host: www.example.com
722Accept-Encoding: gzip
723
724</artwork></figure>
725<t>
726   In this case, the response might or might not use the gzip content coding.
727   If it does not, the response might look like:
728</t>
729<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
730HTTP/1.1 200 OK
731Date: Thu, 26 Mar 2010 00:05:00 GMT
732ETag: "123-a"
733Content-Length: <x:length-of target="exbody"/>
734Vary: Accept-Encoding
735Content-Type: text/plain
736
737<x:span anchor="exbody">Hello World!
738Hello World!
739Hello World!
740Hello World!
741Hello World!
742</x:span></artwork></figure>
743<t>
744   An alternative representation that does use gzip content coding would be:
745</t>
746<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
747HTTP/1.1 200 OK
748Date: Thu, 26 Mar 2010 00:05:00 GMT
749ETag: "123-b"
750Content-Length: 43
751Vary: Accept-Encoding
752Content-Type: text/plain
753Content-Encoding: gzip
754
755<spanx>...binary data...</spanx></artwork></figure>
756<x:note>
757  <t>
758    <x:h>Note:</x:h> Content codings are a property of the representation,
759    so therefore an entity-tag of an encoded representation must be distinct
760    from an unencoded representation to prevent conflicts during cache updates
761    and range requests.  In contrast, transfer codings (&transfer-codings;)
762    apply only during message transfer and do not require distinct entity-tags.
763  </t>
764</x:note>
765</section>
766</section>
767</section>
768
769<section title="Precondition Header Fields" anchor="header.fields">
770<t>
771   This section defines the syntax and semantics of HTTP/1.1 header fields
772   for applying preconditions on requests.
773</t>
774
775<section title="If-Match" anchor="header.if-match">
776  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
777  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
778  <x:anchor-alias value="If-Match"/>
779<t>
780   The "If-Match" header field &MAY; be used to make a request method
781   conditional on the current existence or value of an entity-tag for
782   one or more representations of the target resource.  If-Match is
783   generally useful for resource update requests, such as PUT requests,
784   as a means for protecting against accidental overwrites when multiple
785   clients are acting in parallel on the same resource (i.e., the
786   "lost update" problem).  An If-Match field-value of "*" places the
787   precondition on the existence of any current representation for the
788   target resource.
789</t>
790<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
791  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
792</artwork></figure>
793<t>
794   If any of the entity-tags listed in the If-Match field value match
795   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
796   selected representation for the target resource,
797   or if "*" is given and any current representation exists for the
798   target resource, then the server &MAY; perform the request method
799   as if the If-Match header field was not present.
800</t>
801<t>
802   If none of the entity-tags match, or if "*" is given and no current
803   representation exists, the server &MUST-NOT; perform the requested method.
804   Instead, the server &MUST; respond with the 412 (Precondition Failed)
805   status code.
806</t>
807<t>
808   If the request would, without the If-Match header field, result in
809   anything other than a 2xx or 412 status code, then the If-Match header field
810   &MUST; be ignored.
811</t>
812<t>
813   Examples:
814</t>
815<figure><artwork type="example">
816  If-Match: "xyzzy"
817  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
818  If-Match: *
819</artwork></figure>
820<t>
821   The result of a request having both an If-Match header field and
822   either an If-None-Match or an If-Modified-Since header fields is
823   undefined by this specification.
824</t>
825</section>
826
827<section title="If-None-Match" anchor="header.if-none-match">
828  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
829  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
830  <x:anchor-alias value="If-None-Match"/>
831<t>
832   The "If-None-Match" header field &MAY; be used to make a request method
833   conditional on not matching any of the current entity-tag values for
834   representations of the target resource.  If-None-Match is primarily
835   used in conditional GET requests to enable efficient updates of cached
836   information with a minimum amount of transaction overhead.  A client
837   that has one or more representations previously obtained from the
838   target resource can send If-None-Match with a list of the associated
839   entity-tags in the hope of receiving a 304 response if at least one
840   of those representations matches the selected representation.
841</t>
842<t>
843   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
844   request method (e.g., PUT) from inadvertently modifying an existing
845   representation of the target resource when the client believes that
846   the resource does not have a current representation.  This is a variation
847   on the "lost update" problem that might arise if more than one client
848   attempts to create an initial representation for the target resource.
849</t>
850<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
851  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
852</artwork></figure>
853<t>
854   If any of the entity-tags listed in the If-None-Match field-value match
855   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
856   selected representation, or if "*" is
857   given and any current representation exists for that resource, then the
858   server &MUST-NOT; perform the requested method.
859   Instead, if the request method was GET or HEAD, the server &SHOULD;
860   respond with a 304 (Not Modified) status code, including the cache-related
861   header fields (particularly ETag) of the selected representation that has
862   a matching entity-tag.  For all other request methods, the server &MUST;
863   respond with a 412 (Precondition Failed) status code.
864</t>
865<t>
866   If none of the entity-tags match, then the server &MAY; perform the
867   requested method as if the If-None-Match header field did not exist,
868   but &MUST; also ignore any If-Modified-Since header field(s) in the
869   request. That is, if no entity-tags match, then the server &MUST-NOT;
870   return a 304 (Not Modified) response.
871</t>
872<t>
873   If the request would, without the If-None-Match header field, result
874   in anything other than a 2xx or 304 status code, then the If-None-Match
875   header field &MUST; be ignored. (See <xref
876   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
877   a discussion of server behavior when both If-Modified-Since and
878   If-None-Match appear in the same request.)
879</t>
880<t>
881   Examples:
882</t>
883<figure><artwork type="example">
884  If-None-Match: "xyzzy"
885  If-None-Match: W/"xyzzy"
886  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
887  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
888  If-None-Match: *
889</artwork></figure>
890<t>
891   The result of a request having both an If-None-Match header field and
892   either an If-Match or an If-Unmodified-Since header fields is
893   undefined by this specification.
894</t>
895</section>
896
897<section title="If-Modified-Since" anchor="header.if-modified-since">
898  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
899  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
900  <x:anchor-alias value="If-Modified-Since"/>
901<t>
902   The "If-Modified-Since" header field &MAY; be used to make a request
903   method conditional by modification date: if the selected representation
904   has not been modified since the time specified in this field, then
905   do not perform the request method; instead, respond as detailed below.
906</t>
907<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
908  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
909</artwork></figure>
910<t>
911   An example of the field is:
912</t>
913<figure><artwork type="example">
914  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
915</artwork></figure>
916<t>
917   A GET method with an If-Modified-Since header field and no Range header
918   field requests that the selected representation be transferred only if
919   it has been modified since the date given by the If-Modified-Since
920   header field.
921   The algorithm for determining this includes the following cases:
922  <list style="numbers">
923      <t>If the request would normally result in anything other than a
924         200 (OK) status code, or if the passed If-Modified-Since date is
925         invalid, the response is exactly the same as for a normal GET.
926         A date which is later than the server's current time is
927         invalid.</t>
928
929      <t>If the selected representation has been modified since the
930         If-Modified-Since date, the response is exactly the same as for
931         a normal GET.</t>
932
933      <t>If the selected representation has not been modified since a valid
934         If-Modified-Since date, the server &SHOULD; return a
935         304 (Not Modified) response.</t>
936  </list>
937</t>
938<t>
939   The purpose of this feature is to allow efficient updates of cached
940   information with a minimum amount of transaction overhead.
941  <list><t>
942      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
943      see &header-range; for full details.
944    </t><t>
945      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
946      clock might not be synchronized with the client.
947    </t><t>
948      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
949      servers will use an exact date comparison function, rather than a
950      less-than function, for deciding whether to send a 304 (Not
951      Modified) response. To get best results when sending an If-Modified-Since
952      header field for cache validation, clients are
953      advised to use the exact date string received in a previous Last-Modified
954      header field whenever possible.
955    </t><t>
956      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
957      header field instead of a date taken from the Last-Modified header field for
958      the same request, the client needs to be aware that this
959      date is interpreted in the server's understanding of time.
960      Unsynchronized clocks and rounding problems, due to the different
961      encodings of time between the client and server, are concerns.
962      This includes the possibility of race conditions if the
963      document has changed between the time it was first requested and
964      the If-Modified-Since date of a subsequent request, and the
965      possibility of clock-skew-related problems if the If-Modified-Since
966      date is derived from the client's clock without correction
967      to the server's clock. Corrections for different time bases
968      between client and server are at best approximate due to network
969      latency.
970    </t>
971  </list>
972</t>
973<t>
974   The result of a request having both an If-Modified-Since header field
975   and either an If-Match or an If-Unmodified-Since header fields is
976   undefined by this specification.
977</t>
978</section>
979
980<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
981  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
982  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
983  <x:anchor-alias value="If-Unmodified-Since"/>
984<t>
985   The "If-Unmodified-Since" header field &MAY; be used to make a request
986   method conditional by modification date: if the selected representation
987   has been modified since the time specified in this field, then the
988   server &MUST-NOT; perform the requested operation and &MUST; instead
989   respond with the 412 (Precondition Failed) status code.
990   If the selected representation has not been modified since the time
991   specified in this field, the server &SHOULD; perform the request
992   method as if the If-Unmodified-Since header field were not present.
993</t>
994<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
995  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
996</artwork></figure>
997<t>
998   An example of the field is:
999</t>
1000<figure><artwork type="example">
1001  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1002</artwork></figure>
1003<t>
1004   If the request normally (i.e., without the If-Unmodified-Since
1005   header field) would result in anything other than a 2xx or 412 status code,
1006   the If-Unmodified-Since header field &SHOULD; be ignored.
1007</t>
1008<t>
1009   If the specified date is invalid, the header field &MUST; be ignored.
1010</t>
1011<t>
1012   The result of a request having both an If-Unmodified-Since header
1013   field and either an If-None-Match or an If-Modified-Since header
1014   fields is undefined by this specification.
1015</t>
1016</section>
1017
1018<section title="If-Range" anchor="header.if-range">
1019<t>
1020   The If-Range header field provides a special conditional request
1021   mechanism that is similar to If-Match and If-Unmodified-Since but
1022   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1023</t>
1024</section>
1025
1026</section>
1027
1028<section title="Status Code Definitions" anchor="status.code.definitions">
1029<section title="304 Not Modified" anchor="status.304">
1030  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1031  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1032<t>
1033   The 304 status code indicates that a conditional GET request has been
1034   received and would have resulted in a 200 (OK) response if it were not
1035   for the fact that the condition has evaluated to false.  In other words,
1036   there is no need for the server to transfer a representation of the
1037   target resource because the client's request indicates that it already
1038   has a valid representation, as indicated by the 304 response header
1039   fields, and is therefore redirecting the client to make use of that
1040   stored representation as if it were the payload of a 200 response.
1041   The 304 response &MUST-NOT; contain a message-body, and thus is always
1042   terminated by the first empty line after the header fields.
1043</t>
1044<t>
1045   A 304 response &MUST; include a Date header field (&header-date;)
1046   unless its omission is required by &clockless;.  If a 200 response
1047   to the same request would have included any of the header fields
1048   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1049   Vary, then those same header fields &MUST; be sent in a 304 response.
1050</t>
1051<t>
1052   Since the goal of a 304 response is to minimize information transfer
1053   when the recipient already has one or more cached representations,
1054   the response &SHOULD-NOT; include representation metadata other
1055   than the above listed fields unless said metadata exists for the
1056   purpose of guiding cache updates (e.g., future HTTP extensions).
1057</t>
1058<t>
1059   If the recipient of a 304 response does not have a cached representation
1060   corresponding to the entity-tag indicated by the 304 response, then the
1061   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1062   conditional request originated with an outbound client, such as a
1063   user agent with its own cache sending a conditional GET to a shared
1064   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1065   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1066   the request without any preconditions.
1067</t>
1068<t>
1069   If a cache uses a received 304 response to update a cache entry, the
1070   cache &MUST; update the entry to reflect any new field values given in
1071   the response.
1072</t>
1073</section>
1074
1075<section title="412 Precondition Failed" anchor="status.412">
1076  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1077  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1078<t>
1079   The 412 status code indicates that one or more preconditions given in
1080   the request header fields evaluated to false when tested on the server.
1081   This response code allows the client to place preconditions on the
1082   current resource state (its current representations and metadata)
1083   and thus prevent the request method from being applied if the target
1084   resource is in an unexpected state.
1085</t>
1086</section>
1087</section>
1088
1089<section title="IANA Considerations" anchor="IANA.considerations">
1090
1091<section title="Status Code Registration" anchor="status.code.registration">
1092<t>
1093   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1094   shall be updated with the registrations below:
1095</t>
1096<?BEGININC p4-conditional.iana-status-codes ?>
1097<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1098<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1099   <ttcol>Value</ttcol>
1100   <ttcol>Description</ttcol>
1101   <ttcol>Reference</ttcol>
1102   <c>304</c>
1103   <c>Not Modified</c>
1104   <c>
1105      <xref target="status.304"/>
1106   </c>
1107   <c>412</c>
1108   <c>Precondition Failed</c>
1109   <c>
1110      <xref target="status.412"/>
1111   </c>
1112</texttable>
1113<!--(END)-->
1114<?ENDINC p4-conditional.iana-status-codes ?>
1115</section>
1116
1117<section title="Header Field Registration" anchor="header.field.registration">
1118<t>
1119   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1120   with the permanent registrations below (see <xref target="RFC3864"/>):
1121</t>
1122<?BEGININC p4-conditional.iana-headers ?>
1123<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1124<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1125   <ttcol>Header Field Name</ttcol>
1126   <ttcol>Protocol</ttcol>
1127   <ttcol>Status</ttcol>
1128   <ttcol>Reference</ttcol>
1129
1130   <c>ETag</c>
1131   <c>http</c>
1132   <c>standard</c>
1133   <c>
1134      <xref target="header.etag"/>
1135   </c>
1136   <c>If-Match</c>
1137   <c>http</c>
1138   <c>standard</c>
1139   <c>
1140      <xref target="header.if-match"/>
1141   </c>
1142   <c>If-Modified-Since</c>
1143   <c>http</c>
1144   <c>standard</c>
1145   <c>
1146      <xref target="header.if-modified-since"/>
1147   </c>
1148   <c>If-None-Match</c>
1149   <c>http</c>
1150   <c>standard</c>
1151   <c>
1152      <xref target="header.if-none-match"/>
1153   </c>
1154   <c>If-Unmodified-Since</c>
1155   <c>http</c>
1156   <c>standard</c>
1157   <c>
1158      <xref target="header.if-unmodified-since"/>
1159   </c>
1160   <c>Last-Modified</c>
1161   <c>http</c>
1162   <c>standard</c>
1163   <c>
1164      <xref target="header.last-modified"/>
1165   </c>
1166</texttable>
1167<!--(END)-->
1168<?ENDINC p4-conditional.iana-headers ?>
1169<t>
1170   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1171</t>
1172</section>
1173</section>
1174
1175<section title="Security Considerations" anchor="security.considerations">
1176<t>
1177   No additional security considerations have been identified beyond
1178   those applicable to HTTP in general &messaging;.
1179</t>
1180</section>
1181
1182<section title="Acknowledgments" anchor="acks">
1183<t>
1184  See &acks;.
1185</t>
1186</section>
1187</middle>
1188<back>
1189
1190<references title="Normative References">
1191
1192<reference anchor="Part1">
1193  <front>
1194    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1195    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1196      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1197      <address><email>fielding@gbiv.com</email></address>
1198    </author>
1199    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1200      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1201      <address><email>jg@freedesktop.org</email></address>
1202    </author>
1203    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1204      <organization abbrev="HP">Hewlett-Packard Company</organization>
1205      <address><email>JeffMogul@acm.org</email></address>
1206    </author>
1207    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1208      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1209      <address><email>henrikn@microsoft.com</email></address>
1210    </author>
1211    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1212      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1213      <address><email>LMM@acm.org</email></address>
1214    </author>
1215    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1216      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1217      <address><email>paulle@microsoft.com</email></address>
1218    </author>
1219    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1220      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1221      <address><email>timbl@w3.org</email></address>
1222    </author>
1223    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1224      <organization abbrev="W3C">World Wide Web Consortium</organization>
1225      <address><email>ylafon@w3.org</email></address>
1226    </author>
1227    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1228      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1229      <address><email>julian.reschke@greenbytes.de</email></address>
1230    </author>
1231    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1232  </front>
1233  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1234  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1235</reference>
1236
1237<reference anchor="Part3">
1238  <front>
1239    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1240    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1241      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1242      <address><email>fielding@gbiv.com</email></address>
1243    </author>
1244    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1245      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1246      <address><email>jg@freedesktop.org</email></address>
1247    </author>
1248    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1249      <organization abbrev="HP">Hewlett-Packard Company</organization>
1250      <address><email>JeffMogul@acm.org</email></address>
1251    </author>
1252    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1253      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1254      <address><email>henrikn@microsoft.com</email></address>
1255    </author>
1256    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1257      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1258      <address><email>LMM@acm.org</email></address>
1259    </author>
1260    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1261      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1262      <address><email>paulle@microsoft.com</email></address>
1263    </author>
1264    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1265      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1266      <address><email>timbl@w3.org</email></address>
1267    </author>
1268    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1269      <organization abbrev="W3C">World Wide Web Consortium</organization>
1270      <address><email>ylafon@w3.org</email></address>
1271    </author>
1272    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1273      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1274      <address><email>julian.reschke@greenbytes.de</email></address>
1275    </author>
1276    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1277  </front>
1278  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1279  <x:source href="p3-payload.xml" basename="p3-payload"/>
1280</reference>
1281
1282<reference anchor="Part5">
1283  <front>
1284    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1285    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1286      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1287      <address><email>fielding@gbiv.com</email></address>
1288    </author>
1289    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1290      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1291      <address><email>jg@freedesktop.org</email></address>
1292    </author>
1293    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1294      <organization abbrev="HP">Hewlett-Packard Company</organization>
1295      <address><email>JeffMogul@acm.org</email></address>
1296    </author>
1297    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1298      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1299      <address><email>henrikn@microsoft.com</email></address>
1300    </author>
1301    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1302      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1303      <address><email>LMM@acm.org</email></address>
1304    </author>
1305    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1306      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1307      <address><email>paulle@microsoft.com</email></address>
1308    </author>
1309    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1310      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1311      <address><email>timbl@w3.org</email></address>
1312    </author>
1313    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1314      <organization abbrev="W3C">World Wide Web Consortium</organization>
1315      <address><email>ylafon@w3.org</email></address>
1316    </author>
1317    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1318      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1319      <address><email>julian.reschke@greenbytes.de</email></address>
1320    </author>
1321    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1322  </front>
1323  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1324  <x:source href="p5-range.xml" basename="p5-range"/>
1325</reference>
1326
1327<reference anchor="Part6">
1328  <front>
1329    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1330    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1331      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1332      <address><email>fielding@gbiv.com</email></address>
1333    </author>
1334    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1335      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1336      <address><email>jg@freedesktop.org</email></address>
1337    </author>
1338    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1339      <organization abbrev="HP">Hewlett-Packard Company</organization>
1340      <address><email>JeffMogul@acm.org</email></address>
1341    </author>
1342    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1343      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1344      <address><email>henrikn@microsoft.com</email></address>
1345    </author>
1346    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1347      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1348      <address><email>LMM@acm.org</email></address>
1349    </author>
1350    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1351      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1352      <address><email>paulle@microsoft.com</email></address>
1353    </author>
1354    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1355      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1356      <address><email>timbl@w3.org</email></address>
1357    </author>
1358    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1359      <organization abbrev="W3C">World Wide Web Consortium</organization>
1360      <address><email>ylafon@w3.org</email></address>
1361    </author>
1362    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1363      <address><email>mnot@mnot.net</email></address>
1364    </author>
1365    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1366      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1367      <address><email>julian.reschke@greenbytes.de</email></address>
1368    </author>
1369    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1370  </front>
1371  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1372  <x:source href="p6-cache.xml" basename="p6-cache"/>
1373</reference>
1374
1375<reference anchor="RFC2119">
1376  <front>
1377    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1378    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1379      <organization>Harvard University</organization>
1380      <address><email>sob@harvard.edu</email></address>
1381    </author>
1382    <date month="March" year="1997"/>
1383  </front>
1384  <seriesInfo name="BCP" value="14"/>
1385  <seriesInfo name="RFC" value="2119"/>
1386</reference>
1387
1388<reference anchor="RFC5234">
1389  <front>
1390    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1391    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1392      <organization>Brandenburg InternetWorking</organization>
1393      <address>
1394        <email>dcrocker@bbiw.net</email>
1395      </address> 
1396    </author>
1397    <author initials="P." surname="Overell" fullname="Paul Overell">
1398      <organization>THUS plc.</organization>
1399      <address>
1400        <email>paul.overell@thus.net</email>
1401      </address>
1402    </author>
1403    <date month="January" year="2008"/>
1404  </front>
1405  <seriesInfo name="STD" value="68"/>
1406  <seriesInfo name="RFC" value="5234"/>
1407</reference>
1408
1409</references>
1410
1411<references title="Informative References">
1412
1413<reference anchor="RFC2616">
1414  <front>
1415    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1416    <author initials="R." surname="Fielding" fullname="R. Fielding">
1417      <organization>University of California, Irvine</organization>
1418      <address><email>fielding@ics.uci.edu</email></address>
1419    </author>
1420    <author initials="J." surname="Gettys" fullname="J. Gettys">
1421      <organization>W3C</organization>
1422      <address><email>jg@w3.org</email></address>
1423    </author>
1424    <author initials="J." surname="Mogul" fullname="J. Mogul">
1425      <organization>Compaq Computer Corporation</organization>
1426      <address><email>mogul@wrl.dec.com</email></address>
1427    </author>
1428    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1429      <organization>MIT Laboratory for Computer Science</organization>
1430      <address><email>frystyk@w3.org</email></address>
1431    </author>
1432    <author initials="L." surname="Masinter" fullname="L. Masinter">
1433      <organization>Xerox Corporation</organization>
1434      <address><email>masinter@parc.xerox.com</email></address>
1435    </author>
1436    <author initials="P." surname="Leach" fullname="P. Leach">
1437      <organization>Microsoft Corporation</organization>
1438      <address><email>paulle@microsoft.com</email></address>
1439    </author>
1440    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1441      <organization>W3C</organization>
1442      <address><email>timbl@w3.org</email></address>
1443    </author>
1444    <date month="June" year="1999"/>
1445  </front>
1446  <seriesInfo name="RFC" value="2616"/>
1447</reference>
1448
1449<reference anchor='RFC3864'>
1450  <front>
1451    <title>Registration Procedures for Message Header Fields</title>
1452    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1453      <organization>Nine by Nine</organization>
1454      <address><email>GK-IETF@ninebynine.org</email></address>
1455    </author>
1456    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1457      <organization>BEA Systems</organization>
1458      <address><email>mnot@pobox.com</email></address>
1459    </author>
1460    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1461      <organization>HP Labs</organization>
1462      <address><email>JeffMogul@acm.org</email></address>
1463    </author>
1464    <date year='2004' month='September' />
1465  </front>
1466  <seriesInfo name='BCP' value='90' />
1467  <seriesInfo name='RFC' value='3864' />
1468</reference>
1469
1470<reference anchor='RFC4918'>
1471  <front>
1472    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1473    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1474      <organization abbrev="CommerceNet">CommerceNet</organization>
1475      <address><email>ldusseault@commerce.net</email></address>
1476    </author>
1477    <date month="June" year="2007" />
1478  </front>
1479  <seriesInfo name='RFC' value='4918' />
1480</reference>
1481</references>
1482
1483<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1484<t>
1485  Allow weak entity-tags in all requests except range requests (Sections
1486  <xref target="weak.and.strong.validators" format="counter"/> and
1487  <xref target="header.if-none-match" format="counter"/>).
1488</t>
1489<t>
1490  Change ABNF productions for header fields to only define the field value.
1491  (<xref target="header.fields"/>)
1492</t>
1493</section>
1494
1495<?BEGININC p4-conditional.abnf-appendix ?>
1496<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1497<figure>
1498<artwork type="abnf" name="p4-conditional.parsed-abnf">
1499<x:ref>ETag</x:ref> = entity-tag
1500
1501<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1502
1503<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1504 entity-tag ] ) )
1505<x:ref>If-Modified-Since</x:ref> = HTTP-date
1506<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1507 entity-tag ] ) )
1508<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1509
1510<x:ref>Last-Modified</x:ref> = HTTP-date
1511
1512<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1513
1514<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1515
1516<x:ref>opaque-tag</x:ref> = quoted-string
1517
1518<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1519
1520<x:ref>weak</x:ref> = %x57.2F ; W/
1521</artwork>
1522</figure>
1523<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1524; ETag defined but not used
1525; If-Match defined but not used
1526; If-Modified-Since defined but not used
1527; If-None-Match defined but not used
1528; If-Unmodified-Since defined but not used
1529; Last-Modified defined but not used
1530</artwork></figure></section>
1531<?ENDINC p4-conditional.abnf-appendix ?>
1532
1533<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1534
1535<section title="Since RFC 2616">
1536<t>
1537  Extracted relevant partitions from <xref target="RFC2616"/>.
1538</t>
1539</section>
1540
1541<section title="Since draft-ietf-httpbis-p4-conditional-00">
1542<t>
1543  Closed issues:
1544  <list style="symbols"> 
1545    <t>
1546      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1547      "Normative and Informative references"
1548    </t>
1549  </list>
1550</t>
1551<t>
1552  Other changes:
1553  <list style="symbols"> 
1554    <t>
1555      Move definitions of 304 and 412 condition codes from Part2.
1556    </t>
1557  </list>
1558</t>
1559</section>
1560
1561<section title="Since draft-ietf-httpbis-p4-conditional-01">
1562<t>
1563  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1564  <list style="symbols"> 
1565    <t>
1566      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1567    </t>
1568  </list>
1569</t>
1570</section>
1571
1572<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1573<t>
1574  Closed issues:
1575  <list style="symbols"> 
1576    <t>
1577      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1578      "Weak ETags on non-GET requests"
1579    </t>
1580  </list>
1581</t>
1582<t>
1583  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1584  <list style="symbols"> 
1585    <t>
1586      Reference RFC 3984, and update header field registrations for header fields defined
1587      in this document.
1588    </t>
1589  </list>
1590</t>
1591</section>
1592
1593<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1594<t>
1595  Closed issues:
1596  <list style="symbols"> 
1597    <t>
1598      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1599      "Examples for ETag matching"
1600    </t>
1601    <t>
1602      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1603      "'entity value' undefined"
1604    </t>
1605    <t>
1606      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1607      "bogus 2068 Date header reference"
1608    </t>
1609  </list>
1610</t>
1611</section>
1612
1613<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1614<t>
1615  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1616  <list style="symbols"> 
1617    <t>
1618      Use "/" instead of "|" for alternatives.
1619    </t>
1620    <t>
1621      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1622      whitespace ("OWS") and required whitespace ("RWS").
1623    </t>
1624    <t>
1625      Rewrite ABNFs to spell out whitespace rules, factor out
1626      header field value format definitions.
1627    </t>
1628  </list>
1629</t>
1630</section>
1631
1632<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1633<t>
1634  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1635  <list style="symbols"> 
1636    <t>
1637      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1638    </t>
1639  </list>
1640</t>
1641</section>
1642
1643<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1644<t>
1645  Closed issues:
1646  <list style="symbols"> 
1647    <t>
1648      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1649      "case-sensitivity of etag weakness indicator"
1650    </t>
1651  </list>
1652</t>
1653</section>
1654
1655<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1656<t>
1657  Closed issues:
1658  <list style="symbols"> 
1659    <t>
1660      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1661      "Weak ETags on non-GET requests" (If-Match still was defined to require
1662      strong matching)
1663    </t>
1664    <t>
1665      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1666      "move IANA registrations for optional status codes"
1667    </t>
1668  </list>
1669</t>
1670</section>
1671
1672<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1673<t>
1674  No significant changes.
1675</t>
1676</section>
1677
1678<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1679<t>
1680  No significant changes.
1681</t>
1682</section>
1683
1684<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1685<t>
1686  Closed issues:
1687  <list style="symbols"> 
1688    <t>
1689      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1690      "Clarify 'Requested Variant'"
1691    </t>
1692    <t>
1693      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1694      "Clarify entity / representation / variant terminology"
1695    </t>
1696    <t>
1697      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1698      "consider removing the 'changes from 2068' sections"
1699    </t>
1700  </list>
1701</t>
1702</section>
1703
1704<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1705<t>
1706  None.
1707</t>
1708</section>
1709
1710<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1711<t>
1712  Closed issues:
1713  <list style="symbols"> 
1714    <t>
1715      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1716      "Header Classification"
1717    </t>
1718  </list>
1719</t>
1720</section>
1721
1722<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1723<t>
1724  Closed issues:
1725  <list style="symbols"> 
1726    <t>
1727      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1728      "If-* and entities"
1729    </t>
1730    <t>
1731      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1732      "Definition of validator weakness"
1733    </t>
1734    <t>
1735      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1736      "untangle ABNFs for header fields"
1737    </t>
1738    <t>
1739      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1740      "ETags and Quotes"
1741    </t>
1742  </list>
1743</t>
1744</section>
1745
1746<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1747<t>
1748  None.
1749</t>
1750</section>
1751
1752<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1753<t>
1754  Closed issues:
1755  <list style="symbols"> 
1756    <t>
1757      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1758      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1759    </t>
1760  </list>
1761</t>
1762</section>
1763
1764</section>
1765
1766</back>
1767</rfc>
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