source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1415

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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 field-rules                "<xref target='Part1' x:rel='#field.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-date                "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-accept-encoding     "<xref target='Part3' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY full-date                  "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY content-negotiation        "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33]>
34<?rfc toc="yes" ?>
35<?rfc symrefs="yes" ?>
36<?rfc sortrefs="yes" ?>
37<?rfc compact="yes"?>
38<?rfc subcompact="no" ?>
39<?rfc linkmailto="no" ?>
40<?rfc editing="no" ?>
41<?rfc comments="yes"?>
42<?rfc inline="yes"?>
43<?rfc rfcedstyle="yes"?>
44<?rfc-ext allow-markup-in-artwork="yes" ?>
45<?rfc-ext include-references-in-index="yes" ?>
46<rfc obsoletes="2616" category="std" x:maturity-level="draft"
47     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
48     xmlns:x='http://purl.org/net/xml2rfc/ext'>
49<front>
50
51  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
52
53  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
54    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
55    <address>
56      <postal>
57        <street>345 Park Ave</street>
58        <city>San Jose</city>
59        <region>CA</region>
60        <code>95110</code>
61        <country>USA</country>
62      </postal>
63      <email>fielding@gbiv.com</email>
64      <uri>http://roy.gbiv.com/</uri>
65    </address>
66  </author>
67
68  <author initials="J." surname="Gettys" fullname="Jim Gettys">
69    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
70    <address>
71      <postal>
72        <street>21 Oak Knoll Road</street>
73        <city>Carlisle</city>
74        <region>MA</region>
75        <code>01741</code>
76        <country>USA</country>
77      </postal>
78      <email>jg@freedesktop.org</email>
79      <uri>http://gettys.wordpress.com/</uri>
80    </address>
81  </author>
82 
83  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
84    <organization abbrev="HP">Hewlett-Packard Company</organization>
85    <address>
86      <postal>
87        <street>HP Labs, Large Scale Systems Group</street>
88        <street>1501 Page Mill Road, MS 1177</street>
89        <city>Palo Alto</city>
90        <region>CA</region>
91        <code>94304</code>
92        <country>USA</country>
93      </postal>
94      <email>JeffMogul@acm.org</email>
95    </address>
96  </author>
97
98  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
100    <address>
101      <postal>
102        <street>1 Microsoft Way</street>
103        <city>Redmond</city>
104        <region>WA</region>
105        <code>98052</code>
106        <country>USA</country>
107      </postal>
108      <email>henrikn@microsoft.com</email>
109    </address>
110  </author>
111
112  <author initials="L." surname="Masinter" fullname="Larry Masinter">
113    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
114    <address>
115      <postal>
116        <street>345 Park Ave</street>
117        <city>San Jose</city>
118        <region>CA</region>
119        <code>95110</code>
120        <country>USA</country>
121      </postal>
122      <email>LMM@acm.org</email>
123      <uri>http://larry.masinter.net/</uri>
124    </address>
125  </author>
126 
127  <author initials="P." surname="Leach" fullname="Paul J. Leach">
128    <organization abbrev="Microsoft">Microsoft Corporation</organization>
129    <address>
130      <postal>
131        <street>1 Microsoft Way</street>
132        <city>Redmond</city>
133        <region>WA</region>
134        <code>98052</code>
135      </postal>
136      <email>paulle@microsoft.com</email>
137    </address>
138  </author>
139   
140  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
141    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
142    <address>
143      <postal>
144        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
145        <street>The Stata Center, Building 32</street>
146        <street>32 Vassar Street</street>
147        <city>Cambridge</city>
148        <region>MA</region>
149        <code>02139</code>
150        <country>USA</country>
151      </postal>
152      <email>timbl@w3.org</email>
153      <uri>http://www.w3.org/People/Berners-Lee/</uri>
154    </address>
155  </author>
156
157  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
158    <organization abbrev="W3C">World Wide Web Consortium</organization>
159    <address>
160      <postal>
161        <street>W3C / ERCIM</street>
162        <street>2004, rte des Lucioles</street>
163        <city>Sophia-Antipolis</city>
164        <region>AM</region>
165        <code>06902</code>
166        <country>France</country>
167      </postal>
168      <email>ylafon@w3.org</email>
169      <uri>http://www.raubacapeu.net/people/yves/</uri>
170    </address>
171  </author>
172
173  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
174    <organization abbrev="greenbytes">greenbytes GmbH</organization>
175    <address>
176      <postal>
177        <street>Hafenweg 16</street>
178        <city>Muenster</city><region>NW</region><code>48155</code>
179        <country>Germany</country>
180      </postal>
181      <phone>+49 251 2807760</phone>
182      <facsimile>+49 251 2807761</facsimile>
183      <email>julian.reschke@greenbytes.de</email>
184      <uri>http://greenbytes.de/tech/webdav/</uri>
185    </address>
186  </author>
187
188  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
189  <workgroup>HTTPbis Working Group</workgroup>
190
191<abstract>
192<t>
193   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
194   distributed, collaborative, hypertext information systems. HTTP has been in
195   use by the World Wide Web global information initiative since 1990. This
196   document is Part 4 of the seven-part specification that defines the protocol
197   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
198</t>
199<t>
200   Part 4 defines request header fields for indicating conditional requests and
201   the rules for constructing responses to those requests.
202</t>
203</abstract>
204
205<note title="Editorial Note (To be removed by RFC Editor)">
206  <t>
207    Discussion of this draft should take place on the HTTPBIS working group
208    mailing list (ietf-http-wg@w3.org), which is archived at
209    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
210  </t>
211  <t>
212    The current issues list is at
213    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
214    documents (including fancy diffs) can be found at
215    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
216  </t>
217  <t>
218    The changes in this draft are summarized in <xref target="changes.since.16"/>.
219  </t>
220</note>
221</front>
222<middle>
223<section title="Introduction" anchor="introduction">
224<t>
225   This document defines the HTTP/1.1 conditional request mechanisms,
226   including both metadata for indicating/observing changes in resource
227   representations and request header fields that specify preconditions
228   on that metadata be checked before performing the request method.
229   Conditional GET requests are the most efficient mechanism for HTTP
230   cache updates &caching;.  Conditionals 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 <xref target="Part1"/>:
304</t>
305<figure><artwork type="abnf2616">
306  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
307  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-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="Validators" anchor="validators">
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   <iref primary="true" item="validator" subitem="weak"/>
328   <iref primary="true" item="validator" subitem="strong"/>
329<t>
330   Validators come in two flavors: strong or weak.  Weak validators are easy
331   to generate but are far less useful for comparisons.  Strong validators
332   are ideal for comparisons but can be very difficult (and occasionally
333   impossible) to generate efficiently.  Rather than impose that all forms
334   of resource adhere to the same strength of validator, HTTP exposes the
335   type of validator in use and imposes restrictions on when weak validators
336   can be used as preconditions.
337</t>
338<t>
339   A "strong validator" is a representation metadata value that &MUST; be
340   changed to a new, previously unused or guaranteed unique, value whenever
341   a change occurs to the representation data such that a change would be
342   observable in the payload body of a 200 response to GET.  A strong
343   validator &MAY; be changed for other reasons, such as when a semantically
344   significant part of the representation metadata is changed (e.g.,
345   Content-Type), but it is in the best interests of the origin server to only
346   change the value when it is necessary to invalidate the stored responses
347   held by remote caches and authoring tools.  A strong validator &MUST; be
348   unique across all representations of a given resource, such that no two
349   representations of that resource share the same validator unless
350   their payload body would be identical.
351</t>
352<t>
353   Cache entries might persist for arbitrarily long periods, regardless
354   of expiration times.  Thus, a cache might attempt to validate an
355   entry using a validator that it obtained in the distant past.
356   A strong validator &MUST; be unique across all versions of all
357   representations associated with a particular resource over time.
358   However, there is no implication of uniqueness across representations
359   of different resources (i.e., the same strong validator might be
360   in use for representations of multiple resources at the same time
361   and does not imply that those representations are equivalent).
362</t>
363<t>
364   There are a variety of strong validators used in practice.  The best are
365   based on strict revision control, wherein each change to a representation
366   always results in a unique node name and revision identifier being assigned
367   before the representation is made accessible to GET.  A cryptographic hash
368   function applied to the representation data is also sufficient if the data
369   is available prior to the response header fields being sent and the digest
370   does not need to be recalculated every time a validation request is
371   received.  However, if a resource has distinct representations that differ
372   only in their metadata, such as might occur with content negotiation over
373   media types that happen to share the same data format, then a server
374   &SHOULD; incorporate additional information in the validator to
375   distinguish those representations and avoid confusing cache behavior.
376</t>
377<t>
378   In contrast, a "weak validator" is a representation metadata value that
379   might not be changed for every change to the representation data.  This
380   weakness might be due to limitations in how the value is calculated, such
381   as clock resolution or an inability to ensure uniqueness for all possible
382   representations of the resource, or due to a desire by the resource owner
383   to group representations by some self-determined set of equivalency
384   rather than unique sequences of data.  A weak entity-tag &SHOULD; change
385   whenever the origin server considers prior representations to be
386   unacceptable as a substitute for the current representation. In other
387   words, a weak entity-tag &SHOULD; change whenever the origin server wants
388   caches to invalidate old responses.
389</t>
390<t>
391   For example, the representation of a weather report that changes in
392   content every second, based on dynamic measurements, might be grouped
393   into sets of equivalent representations (from the origin server's
394   perspective) with the same weak validator in order to allow cached
395   representations to be valid for a reasonable period of time (perhaps
396   adjusted dynamically based on server load or weather quality).
397   Likewise, a representation's modification time, if defined with only
398   one-second resolution, might be a weak validator if it is possible
399   for the representation to be modified twice during a single second and
400   retrieved between those modifications.
401</t>
402<t>
403   A "use" of a validator occurs when either a client generates a request
404   and includes the validator in a precondition or when a server
405   compares two validators.
406   Weak validators are only usable in contexts that do not depend on exact
407   equality of a representation's payload body.
408   Strong validators are usable and preferred for all conditional requests,
409   including cache validation, partial content ranges, and "lost update"
410   avoidance.
411</t>
412</section>
413
414<section title="Last-Modified" anchor="header.last-modified">
415  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
416  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
417  <x:anchor-alias value="Last-Modified"/>
418<t>
419   The "Last-Modified" header field indicates the date and time at
420   which the origin server believes the selected representation was
421   last modified.
422</t>
423<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
424  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
425</artwork></figure>
426<t>
427   An example of its use is
428</t>
429<figure><artwork type="example">
430  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
431</artwork></figure>
432
433<section title="Generation" anchor="lastmod.generation">
434<t>
435   Origin servers &SHOULD; send Last-Modified for any selected
436   representation for which a last modification date can be reasonably
437   and consistently determined, since its use in conditional requests
438   and evaluating cache freshness (&caching;) results in a substantial
439   reduction of HTTP traffic on the Internet and can be a significant
440   factor in improving service scalability and reliability.
441</t>
442<t>
443   A representation is typically the sum of many parts behind the
444   resource interface.  The last-modified time would usually be
445   the most recent time that any of those parts were changed.
446   How that value is determined for any given resource is an
447   implementation detail beyond the scope of this specification.
448   What matters to HTTP is how recipients of the Last-Modified
449   header field can use its value to make conditional requests
450   and test the validity of locally cached responses.
451</t>
452<t>
453   An origin server &SHOULD; obtain the Last-Modified value of the
454   representation as close as possible to the time that it generates
455   the Date field-value for its response. This allows a recipient to
456   make an accurate assessment of the representation's modification time,
457   especially if the representation changes near the time that the
458   response is generated.
459</t>
460<t>
461   An origin server with a clock &MUST-NOT; send a Last-Modified date
462   that is later than the server's time of message origination (Date).
463   If the last modification time is derived from implementation-specific
464   metadata that evaluates to some time in the future, according to the
465   origin server's clock, then the origin server &MUST; replace that
466   value with the message origination date. This prevents a future
467   modification date from having an adverse impact on cache validation.
468</t>
469</section>
470
471<section title="Comparison" anchor="lastmod.comparison">
472<t>
473   A Last-Modified time, when used as a validator in a request, is
474   implicitly weak unless it is possible to deduce that it is strong,
475   using the following rules:
476  <list style="symbols">
477     <t>The validator is being compared by an origin server to the
478        actual current validator for the representation and,</t>
479     <t>That origin server reliably knows that the associated representation did
480        not change twice during the second covered by the presented
481        validator.</t>
482  </list>
483</t>
484<t>
485   or
486  <list style="symbols">
487     <t>The validator is about to be used by a client in an If-Modified-Since,
488        If-Unmodified-Since header field, because the client has a cache entry,
489        or If-Range for the associated representation, and</t>
490     <t>That cache entry includes a Date value, which gives the time
491        when the origin server sent the original response, and</t>
492     <t>The presented Last-Modified time is at least 60 seconds before
493        the Date value.</t>
494  </list>
495</t>
496<t>
497   or
498  <list style="symbols">
499     <t>The validator is being compared by an intermediate cache to the
500        validator stored in its cache entry for the representation, and</t>
501     <t>That cache entry includes a Date value, which gives the time
502        when the origin server sent the original response, and</t>
503     <t>The presented Last-Modified time is at least 60 seconds before
504        the Date value.</t>
505  </list>
506</t>
507<t>
508   This method relies on the fact that if two different responses were
509   sent by the origin server during the same second, but both had the
510   same Last-Modified time, then at least one of those responses would
511   have a Date value equal to its Last-Modified time. The arbitrary 60-second
512   limit guards against the possibility that the Date and Last-Modified
513   values are generated from different clocks, or at somewhat
514   different times during the preparation of the response. An
515   implementation &MAY; use a value larger than 60 seconds, if it is
516   believed that 60 seconds is too short.
517</t>
518</section>
519</section>
520
521<section title="ETag" anchor="header.etag">
522  <iref primary="true" item="ETag header field" x:for-anchor=""/>
523  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
524  <x:anchor-alias value="ETag"/>
525  <x:anchor-alias value="entity-tag"/>
526  <x:anchor-alias value="entity.tags"/>
527  <x:anchor-alias value="opaque-tag"/>
528  <x:anchor-alias value="weak"/>
529<t>
530   The ETag header field provides the current entity-tag for the
531   selected representation.
532   An entity-tag is an opaque validator for differentiating between
533   multiple representations of the same resource, regardless of whether
534   those multiple representations are due to resource state changes over
535   time, content negotiation resulting in multiple representations being
536   valid at the same time, or both. An entity-tag consists of an opaque
537   quoted string, possibly prefixed by a weakness indicator.
538</t>
539<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"/>
540  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
541
542  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
543  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
544  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
545</artwork></figure>
546<t>
547   An entity-tag can be more reliable for validation than a modification
548   date in situations where it is inconvenient to store modification
549   dates, where the one-second resolution of HTTP date values is not
550   sufficient, or where modification dates are not consistently maintained.
551</t>
552<figure><preamble>
553  Examples:
554</preamble>
555<artwork type="example">
556  ETag: "xyzzy"
557  ETag: W/"xyzzy"
558  ETag: ""
559</artwork></figure>
560<t>
561   An entity-tag can be either a weak or strong validator, with
562   strong being the default.  If an origin server provides an entity-tag
563   for a representation and the generation of that entity-tag does not satisfy
564   the requirements for a strong validator
565   (<xref target="weak.and.strong.validators"/>), then that
566   entity-tag &MUST; be marked as weak by prefixing its opaque value
567   with "W/" (case-sensitive).
568</t>
569
570<section title="Generation" anchor="entity.tag.generation">
571<t>
572   The principle behind entity-tags is that only the service author
573   knows the implementation of a resource well enough to select the
574   most accurate and efficient validation mechanism for that resource,
575   and that any such mechanism can be mapped to a simple sequence of
576   octets for easy comparison.  Since the value is opaque, there is no
577   need for the client to be aware of how each entity-tag is constructed.
578</t>
579<t>
580   For example, a resource that has implementation-specific versioning
581   applied to all changes might use an internal revision number, perhaps
582   combined with a variance identifier for content negotiation, to
583   accurately differentiate between representations.
584   Other implementations might use a stored hash of representation content,
585   a combination of various filesystem attributes, or a modification
586   timestamp that has sub-second resolution.
587</t>
588<t>
589   Origin servers &SHOULD; send ETag for any selected representation
590   for which detection of changes can be reasonably and consistently
591   determined, since the entity-tag's use in conditional requests and
592   evaluating cache freshness (&caching;) can result in a substantial
593   reduction of HTTP network traffic and can be a significant factor in
594   improving service scalability and reliability.
595</t>
596</section>
597
598<section title="Comparison" anchor="entity.tag.comparison">
599  <x:anchor-alias value="validator.comparison"/>
600<t>
601   There are two entity-tag comparison functions, depending
602   on whether the comparison context allows the use of weak validators
603   or not:
604  <list style="symbols">
605     <t>The strong comparison function: in order to be considered equal,
606        both opaque-tags &MUST; be identical character-by-character, and both
607        &MUST-NOT; be weak.</t>
608     <t>The weak comparison function: in order to be considered equal, both
609        opaque-tags &MUST; be identical character-by-character, but
610        either or both of them &MAY; be tagged as "weak" without affecting
611        the result.</t>
612  </list>
613</t>
614<t>
615   The example below shows the results for a set of entity-tag pairs,
616   and both the weak and strong comparison function results:
617</t>
618<texttable align="left">
619  <ttcol>ETag 1</ttcol>
620  <ttcol>ETag 2</ttcol>
621  <ttcol>Strong Comparison</ttcol>
622  <ttcol>Weak Comparison</ttcol>
623
624  <c>W/"1"</c>
625  <c>W/"1"</c>
626  <c>no match</c>
627  <c>match</c>
628 
629  <c>W/"1"</c>
630  <c>W/"2"</c>
631  <c>no match</c>
632  <c>no match</c>
633
634  <c>W/"1"</c>
635  <c>"1"</c>
636  <c>no match</c>
637  <c>match</c>
638
639  <c>"1"</c>
640  <c>"1"</c>
641  <c>match</c>
642  <c>match</c>
643</texttable>
644</section>
645
646<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
647<t>
648   Consider a resource that is subject to content negotiation (&content-negotiation;),
649   and where the representations returned upon a GET request vary based on
650   the Accept-Encoding request header field (&header-accept-encoding;):
651</t>
652<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
653GET /index HTTP/1.1
654Host: www.example.com
655Accept-Encoding: gzip
656
657</artwork></figure>
658<t>
659   In this case, the response might or might not use the gzip content coding.
660   If it does not, the response might look like:
661</t>
662<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
663HTTP/1.1 200 OK
664Date: Thu, 26 Mar 2010 00:05:00 GMT
665ETag: "123-a"
666Content-Length: <x:length-of target="exbody"/>
667Vary: Accept-Encoding
668Content-Type: text/plain
669
670<x:span anchor="exbody">Hello World!
671Hello World!
672Hello World!
673Hello World!
674Hello World!
675</x:span></artwork></figure>
676<t>
677   An alternative representation that does use gzip content coding would be:
678</t>
679<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
680HTTP/1.1 200 OK
681Date: Thu, 26 Mar 2010 00:05:00 GMT
682ETag: "123-b"
683Content-Length: 43
684Vary: Accept-Encoding
685Content-Type: text/plain
686Content-Encoding: gzip
687
688<spanx>...binary data...</spanx></artwork></figure>
689<x:note>
690  <t>
691    <x:h>Note:</x:h> Content codings are a property of the representation,
692    so therefore an entity-tag of an encoded representation must be distinct
693    from an unencoded representation to prevent conflicts during cache updates
694    and range requests.  In contrast, transfer codings (&transfer-codings;)
695    apply only during message transfer and do not require distinct entity-tags.
696  </t>
697</x:note>
698</section>
699</section>
700
701<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">
702<t>
703   We adopt a set of rules and recommendations for origin servers,
704   clients, and caches regarding when various validator types ought to
705   be used, and for what purposes.
706</t>
707<t>
708   HTTP/1.1 origin servers:
709  <list style="symbols">
710     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
711        generate one.</t>
712
713     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
714        performance considerations support the use of weak entity-tags,
715        or if it is unfeasible to send a strong entity-tag.</t>
716
717     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
718  </list>
719</t>
720<t>
721   In other words, the preferred behavior for an HTTP/1.1 origin server
722   is to send both a strong entity-tag and a Last-Modified value.
723</t>
724<t>
725   HTTP/1.1 clients:
726  <list style="symbols">
727     <t>&MUST; use that entity-tag in any cache-conditional request (using
728        If-Match or If-None-Match) if an entity-tag has been provided by the
729        origin server.</t>
730
731     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
732        requests (using If-Modified-Since) if only a Last-Modified value has
733        been provided by the origin server. </t>
734
735     <t>&MAY; use the Last-Modified value in subrange cache-conditional
736        requests (using If-Unmodified-Since) if only a Last-Modified value has
737        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
738        provide a way to disable this, in case of difficulty.</t>
739
740     <t>&SHOULD; use both validators in cache-conditional requests if both an
741        entity-tag and a Last-Modified value have been provided by the origin
742        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
743        appropriately.</t>
744  </list>
745</t>
746<t>
747   An HTTP/1.1 origin server, upon receiving a conditional request that
748   includes both a Last-Modified date (e.g., in an If-Modified-Since or
749   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
750   in an If-Match, If-None-Match, or If-Range header field) as cache
751   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
752   unless doing so is consistent with all of the conditional header
753   fields in the request.
754</t>
755<t>
756   An HTTP/1.1 caching proxy, upon receiving a conditional request that
757   includes both a Last-Modified date and one or more entity-tags as
758   cache validators, &MUST-NOT; return a locally cached response to the
759   client unless that cached response is consistent with all of the
760   conditional header fields in the request.
761  <list><t>
762      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
763      servers and clients ought to transmit as much non-redundant
764      information as is available in their responses and requests.
765      HTTP/1.1 systems receiving this information will make the most
766      conservative assumptions about the validators they receive.
767  </t><t>
768      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
769      last-modified values received or used by these systems will
770      support transparent and efficient caching, and so HTTP/1.1 origin
771      servers should provide Last-Modified values. In those rare cases
772      where the use of a Last-Modified value as a validator by an
773      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
774      origin servers should not provide one.
775  </t></list>
776</t>
777</section>
778</section>
779
780<section title="Precondition Header Fields" anchor="header.field.definitions">
781<t>
782   This section defines the syntax and semantics of HTTP/1.1 header fields
783   for applying preconditions on requests.
784</t>
785
786<section title="If-Match" anchor="header.if-match">
787  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
788  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
789  <x:anchor-alias value="If-Match"/>
790<t>
791   The "If-Match" header field &MAY; be used to make a request method
792   conditional on the current existence or value of an entity-tag for
793   one or more representations of the target resource.  If-Match is
794   generally useful for resource update requests, such as PUT requests,
795   as a means for protecting against accidental overwrites when multiple
796   clients are acting in parallel on the same resource (i.e., the
797   "lost update" problem).  An If-Match field-value of "*" places the
798   precondition on the existence of any current representation for the
799   target resource.
800</t>
801<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
802  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
803</artwork></figure>
804<t>
805   If any of the entity-tags listed in the If-Match field value match
806   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
807   selected representation for the target resource,
808   or if "*" is given and any current representation exists for the
809   target resource, then the server &MAY; perform the request method
810   as if the If-Match header field was not present.
811</t>
812<t>
813   If none of the entity-tags match, or if "*" is given and no current
814   representation exists, the server &MUST-NOT; perform the requested method.
815   Instead, the server &MUST; respond with the 412 (Precondition Failed)
816   status code.
817</t>
818<t>
819   If the request would, without the If-Match header field, result in
820   anything other than a 2xx or 412 status code, then the If-Match header field
821   &MUST; be ignored.
822</t>
823<t>
824   Examples:
825</t>
826<figure><artwork type="example">
827  If-Match: "xyzzy"
828  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
829  If-Match: *
830</artwork></figure>
831<t>
832   The result of a request having both an If-Match header field and
833   either an If-None-Match or an If-Modified-Since header fields is
834   undefined by this specification.
835</t>
836</section>
837
838<section title="If-None-Match" anchor="header.if-none-match">
839  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
840  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
841  <x:anchor-alias value="If-None-Match"/>
842<t>
843   The "If-None-Match" header field &MAY; be used to make a request method
844   conditional on not matching any of the current entity-tag values for
845   representations of the target resource.  If-None-Match is primarily
846   used in conditional GET requests to enable efficient updates of cached
847   information with a minimum amount of transaction overhead.  A client
848   that has one or more representations previously obtained from the
849   target resource can send If-None-Match with a list of the associated
850   entity-tags in the hope of receiving a 304 response if at least one
851   of those representations matches the selected representation.
852</t>
853<t>
854   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
855   request method (e.g., PUT) from inadvertently modifying an existing
856   representation of the target resource when the client believes that
857   the resource does not have a current representation.  This is a variation
858   on the "lost update" problem that might arise if more than one client
859   attempts to create an initial representation for the target resource.
860</t>
861<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
862  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
863</artwork></figure>
864<t>
865   If any of the entity-tags listed in the If-None-Match field-value match
866   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
867   selected representation, or if "*" is
868   given and any current representation exists for that resource, then the
869   server &MUST-NOT; perform the requested method.
870   Instead, if the request method was GET or HEAD, the server &SHOULD;
871   respond with a 304 (Not Modified) status code, including the cache-related
872   header fields (particularly ETag) of the selected representation that has
873   a matching entity-tag.  For all other request methods, the server &MUST;
874   respond with a 412 (Precondition Failed) status code.
875</t>
876<t>
877   If none of the entity-tags match, then the server &MAY; perform the
878   requested method as if the If-None-Match header field did not exist,
879   but &MUST; also ignore any If-Modified-Since header field(s) in the
880   request. That is, if no entity-tags match, then the server &MUST-NOT;
881   return a 304 (Not Modified) response.
882</t>
883<t>
884   If the request would, without the If-None-Match header field, result
885   in anything other than a 2xx or 304 status code, then the If-None-Match
886   header field &MUST; be ignored. (See <xref
887   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
888   a discussion of server behavior when both If-Modified-Since and
889   If-None-Match appear in the same request.)
890</t>
891<t>
892   Examples:
893</t>
894<figure><artwork type="example">
895  If-None-Match: "xyzzy"
896  If-None-Match: W/"xyzzy"
897  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
898  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
899  If-None-Match: *
900</artwork></figure>
901<t>
902   The result of a request having both an If-None-Match header field and
903   either an If-Match or an If-Unmodified-Since header fields is
904   undefined by this specification.
905</t>
906</section>
907
908<section title="If-Modified-Since" anchor="header.if-modified-since">
909  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
910  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
911  <x:anchor-alias value="If-Modified-Since"/>
912<t>
913   The "If-Modified-Since" header field &MAY; be used to make a request
914   method conditional by modification date: if the selected representation
915   has not been modified since the time specified in this field, then
916   do not perform the request method; instead, respond as detailed below.
917</t>
918<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
919  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
920</artwork></figure>
921<t>
922   An example of the field is:
923</t>
924<figure><artwork type="example">
925  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
926</artwork></figure>
927<t>
928   A GET method with an If-Modified-Since header field and no Range header
929   field requests that the selected representation be transferred only if
930   it has been modified since the date given by the If-Modified-Since
931   header field.
932   The algorithm for determining this includes the following cases:
933  <list style="numbers">
934      <t>If the request would normally result in anything other than a
935         200 (OK) status code, or if the passed If-Modified-Since date is
936         invalid, the response is exactly the same as for a normal GET.
937         A date which is later than the server's current time is
938         invalid.</t>
939
940      <t>If the selected representation has been modified since the
941         If-Modified-Since date, the response is exactly the same as for
942         a normal GET.</t>
943
944      <t>If the selected representation has not been modified since a valid
945         If-Modified-Since date, the server &SHOULD; return a
946         304 (Not Modified) response.</t>
947  </list>
948</t>
949<t>
950   The purpose of this feature is to allow efficient updates of cached
951   information with a minimum amount of transaction overhead.
952  <list><t>
953      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
954      see &header-range; for full details.
955    </t><t>
956      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
957      clock might not be synchronized with the client.
958    </t><t>
959      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
960      servers will use an exact date comparison function, rather than a
961      less-than function, for deciding whether to send a 304 (Not
962      Modified) response. To get best results when sending an If-Modified-Since
963      header field for cache validation, clients are
964      advised to use the exact date string received in a previous Last-Modified
965      header field whenever possible.
966    </t><t>
967      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
968      header field instead of a date taken from the Last-Modified header field for
969      the same request, the client needs to be aware that this
970      date is interpreted in the server's understanding of time.
971      Unsynchronized clocks and rounding problems, due to the different
972      encodings of time between the client and server, are concerns.
973      This includes the possibility of race conditions if the
974      document has changed between the time it was first requested and
975      the If-Modified-Since date of a subsequent request, and the
976      possibility of clock-skew-related problems if the If-Modified-Since
977      date is derived from the client's clock without correction
978      to the server's clock. Corrections for different time bases
979      between client and server are at best approximate due to network
980      latency.
981    </t>
982  </list>
983</t>
984<t>
985   The result of a request having both an If-Modified-Since header field
986   and either an If-Match or an If-Unmodified-Since header fields is
987   undefined by this specification.
988</t>
989</section>
990
991<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
992  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
993  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
994  <x:anchor-alias value="If-Unmodified-Since"/>
995<t>
996   The "If-Unmodified-Since" header field &MAY; be used to make a request
997   method conditional by modification date: if the selected representation
998   has been modified since the time specified in this field, then the
999   server &MUST-NOT; perform the requested operation and &MUST; instead
1000   respond with the 412 (Precondition Failed) status code.
1001   If the selected representation has not been modified since the time
1002   specified in this field, the server &SHOULD; perform the request
1003   method as if the If-Unmodified-Since header field were not present.
1004</t>
1005<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
1006  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
1007</artwork></figure>
1008<t>
1009   An example of the field is:
1010</t>
1011<figure><artwork type="example">
1012  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1013</artwork></figure>
1014<t>
1015   If the request normally (i.e., without the If-Unmodified-Since
1016   header field) would result in anything other than a 2xx or 412 status code,
1017   the If-Unmodified-Since header field &SHOULD; be ignored.
1018</t>
1019<t>
1020   If the specified date is invalid, the header field &MUST; be ignored.
1021</t>
1022<t>
1023   The result of a request having both an If-Unmodified-Since header
1024   field and either an If-None-Match or an If-Modified-Since header
1025   fields is undefined by this specification.
1026</t>
1027</section>
1028
1029<section title="If-Range" anchor="header.if-range">
1030<t>
1031   The If-Range header field provides a special conditional request
1032   mechanism that is similar to If-Match and If-Unmodified-Since but
1033   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1034</t>
1035</section>
1036
1037</section>
1038
1039<section title="Status Code Definitions" anchor="status.code.definitions">
1040<section title="304 Not Modified" anchor="status.304">
1041  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1042  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1043<t>
1044   The 304 status code indicates that a conditional GET request has been
1045   received and would have resulted in a 200 (OK) response if it were not
1046   for the fact that the condition has evaluated to false.  In other words,
1047   there is no need for the server to transfer a representation of the
1048   target resource because the client's request indicates that it already
1049   has a valid representation, as indicated by the 304 response header
1050   fields, and is therefore redirecting the client to make use of that
1051   stored representation as if it were the payload of a 200 response.
1052   The 304 response &MUST-NOT; contain a message-body, and thus is always
1053   terminated by the first empty line after the header fields.
1054</t>
1055<t>
1056   A 304 response &MUST; include a Date header field (&header-date;)
1057   unless its omission is required by &clockless;.  If a 200 response
1058   to the same request would have included any of the header fields
1059   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1060   Vary, then those same header fields &MUST; be sent in a 304 response.
1061</t>
1062<t>
1063   Since the goal of a 304 response is to minimize information transfer
1064   when the recipient already has one or more cached representations,
1065   the response &SHOULD-NOT; include representation metadata other
1066   than the above listed fields unless said metadata exists for the
1067   purpose of guiding cache updates (e.g., future HTTP extensions).
1068</t>
1069<t>
1070   If the recipient of a 304 response does not have a cached representation
1071   corresponding to the entity-tag indicated by the 304 response, then the
1072   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1073   conditional request originated with an outbound client, such as a
1074   user agent with its own cache sending a conditional GET to a shared
1075   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1076   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1077   the request without any preconditions.
1078</t>
1079<t>
1080   If a cache uses a received 304 response to update a cache entry, the
1081   cache &MUST; update the entry to reflect any new field values given in
1082   the response.
1083</t>
1084</section>
1085
1086<section title="412 Precondition Failed" anchor="status.412">
1087  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1088  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1089<t>
1090   The 412 status code indicates that one or more preconditions given in
1091   the request header fields evaluated to false when tested on the server.
1092   This response code allows the client to place preconditions on the
1093   current resource state (its current representations and metadata)
1094   and thus prevent the request method from being applied if the target
1095   resource is in an unexpected state.
1096</t>
1097</section>
1098</section>
1099
1100<section title="IANA Considerations" anchor="IANA.considerations">
1101
1102<section title="Status Code Registration" anchor="status.code.registration">
1103<t>
1104   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1105   shall be updated with the registrations below:
1106</t>
1107<?BEGININC p4-conditional.iana-status-codes ?>
1108<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1109<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1110   <ttcol>Value</ttcol>
1111   <ttcol>Description</ttcol>
1112   <ttcol>Reference</ttcol>
1113   <c>304</c>
1114   <c>Not Modified</c>
1115   <c>
1116      <xref target="status.304"/>
1117   </c>
1118   <c>412</c>
1119   <c>Precondition Failed</c>
1120   <c>
1121      <xref target="status.412"/>
1122   </c>
1123</texttable>
1124<!--(END)-->
1125<?ENDINC p4-conditional.iana-status-codes ?>
1126</section>
1127
1128<section title="Header Field Registration" anchor="header.field.registration">
1129<t>
1130   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1131   with the permanent registrations below (see <xref target="RFC3864"/>):
1132</t>
1133<?BEGININC p4-conditional.iana-headers ?>
1134<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1135<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1136   <ttcol>Header Field Name</ttcol>
1137   <ttcol>Protocol</ttcol>
1138   <ttcol>Status</ttcol>
1139   <ttcol>Reference</ttcol>
1140
1141   <c>ETag</c>
1142   <c>http</c>
1143   <c>standard</c>
1144   <c>
1145      <xref target="header.etag"/>
1146   </c>
1147   <c>If-Match</c>
1148   <c>http</c>
1149   <c>standard</c>
1150   <c>
1151      <xref target="header.if-match"/>
1152   </c>
1153   <c>If-Modified-Since</c>
1154   <c>http</c>
1155   <c>standard</c>
1156   <c>
1157      <xref target="header.if-modified-since"/>
1158   </c>
1159   <c>If-None-Match</c>
1160   <c>http</c>
1161   <c>standard</c>
1162   <c>
1163      <xref target="header.if-none-match"/>
1164   </c>
1165   <c>If-Unmodified-Since</c>
1166   <c>http</c>
1167   <c>standard</c>
1168   <c>
1169      <xref target="header.if-unmodified-since"/>
1170   </c>
1171   <c>Last-Modified</c>
1172   <c>http</c>
1173   <c>standard</c>
1174   <c>
1175      <xref target="header.last-modified"/>
1176   </c>
1177</texttable>
1178<!--(END)-->
1179<?ENDINC p4-conditional.iana-headers ?>
1180<t>
1181   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1182</t>
1183</section>
1184</section>
1185
1186<section title="Security Considerations" anchor="security.considerations">
1187<t>
1188   No additional security considerations have been identified beyond
1189   those applicable to HTTP in general &messaging;.
1190</t>
1191</section>
1192
1193<section title="Acknowledgments" anchor="acks">
1194<t>
1195  See &acks;.
1196</t>
1197</section>
1198</middle>
1199<back>
1200
1201<references title="Normative References">
1202
1203<reference anchor="Part1">
1204  <front>
1205    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1206    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1207      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1208      <address><email>fielding@gbiv.com</email></address>
1209    </author>
1210    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1211      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1212      <address><email>jg@freedesktop.org</email></address>
1213    </author>
1214    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1215      <organization abbrev="HP">Hewlett-Packard Company</organization>
1216      <address><email>JeffMogul@acm.org</email></address>
1217    </author>
1218    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1219      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1220      <address><email>henrikn@microsoft.com</email></address>
1221    </author>
1222    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1223      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1224      <address><email>LMM@acm.org</email></address>
1225    </author>
1226    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1227      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1228      <address><email>paulle@microsoft.com</email></address>
1229    </author>
1230    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1231      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1232      <address><email>timbl@w3.org</email></address>
1233    </author>
1234    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1235      <organization abbrev="W3C">World Wide Web Consortium</organization>
1236      <address><email>ylafon@w3.org</email></address>
1237    </author>
1238    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1239      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1240      <address><email>julian.reschke@greenbytes.de</email></address>
1241    </author>
1242    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1243  </front>
1244  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1245  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1246</reference>
1247
1248<reference anchor="Part3">
1249  <front>
1250    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1251    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1252      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1253      <address><email>fielding@gbiv.com</email></address>
1254    </author>
1255    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1256      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1257      <address><email>jg@freedesktop.org</email></address>
1258    </author>
1259    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1260      <organization abbrev="HP">Hewlett-Packard Company</organization>
1261      <address><email>JeffMogul@acm.org</email></address>
1262    </author>
1263    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1264      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1265      <address><email>henrikn@microsoft.com</email></address>
1266    </author>
1267    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1268      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1269      <address><email>LMM@acm.org</email></address>
1270    </author>
1271    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1272      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1273      <address><email>paulle@microsoft.com</email></address>
1274    </author>
1275    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1276      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1277      <address><email>timbl@w3.org</email></address>
1278    </author>
1279    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1280      <organization abbrev="W3C">World Wide Web Consortium</organization>
1281      <address><email>ylafon@w3.org</email></address>
1282    </author>
1283    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1284      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1285      <address><email>julian.reschke@greenbytes.de</email></address>
1286    </author>
1287    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1288  </front>
1289  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1290  <x:source href="p3-payload.xml" basename="p3-payload"/>
1291</reference>
1292
1293<reference anchor="Part5">
1294  <front>
1295    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1296    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1297      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1298      <address><email>fielding@gbiv.com</email></address>
1299    </author>
1300    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1301      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1302      <address><email>jg@freedesktop.org</email></address>
1303    </author>
1304    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1305      <organization abbrev="HP">Hewlett-Packard Company</organization>
1306      <address><email>JeffMogul@acm.org</email></address>
1307    </author>
1308    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1309      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1310      <address><email>henrikn@microsoft.com</email></address>
1311    </author>
1312    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1313      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1314      <address><email>LMM@acm.org</email></address>
1315    </author>
1316    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1317      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1318      <address><email>paulle@microsoft.com</email></address>
1319    </author>
1320    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1321      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1322      <address><email>timbl@w3.org</email></address>
1323    </author>
1324    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1325      <organization abbrev="W3C">World Wide Web Consortium</organization>
1326      <address><email>ylafon@w3.org</email></address>
1327    </author>
1328    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1329      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1330      <address><email>julian.reschke@greenbytes.de</email></address>
1331    </author>
1332    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1333  </front>
1334  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1335  <x:source href="p5-range.xml" basename="p5-range"/>
1336</reference>
1337
1338<reference anchor="Part6">
1339  <front>
1340    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1341    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1342      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1343      <address><email>fielding@gbiv.com</email></address>
1344    </author>
1345    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1346      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1347      <address><email>jg@freedesktop.org</email></address>
1348    </author>
1349    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1350      <organization abbrev="HP">Hewlett-Packard Company</organization>
1351      <address><email>JeffMogul@acm.org</email></address>
1352    </author>
1353    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1354      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1355      <address><email>henrikn@microsoft.com</email></address>
1356    </author>
1357    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1358      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1359      <address><email>LMM@acm.org</email></address>
1360    </author>
1361    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1362      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1363      <address><email>paulle@microsoft.com</email></address>
1364    </author>
1365    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1366      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1367      <address><email>timbl@w3.org</email></address>
1368    </author>
1369    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1370      <organization abbrev="W3C">World Wide Web Consortium</organization>
1371      <address><email>ylafon@w3.org</email></address>
1372    </author>
1373    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1374      <address><email>mnot@mnot.net</email></address>
1375    </author>
1376    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1377      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1378      <address><email>julian.reschke@greenbytes.de</email></address>
1379    </author>
1380    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1381  </front>
1382  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1383  <x:source href="p6-cache.xml" basename="p6-cache"/>
1384</reference>
1385
1386<reference anchor="RFC2119">
1387  <front>
1388    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1389    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1390      <organization>Harvard University</organization>
1391      <address><email>sob@harvard.edu</email></address>
1392    </author>
1393    <date month="March" year="1997"/>
1394  </front>
1395  <seriesInfo name="BCP" value="14"/>
1396  <seriesInfo name="RFC" value="2119"/>
1397</reference>
1398
1399<reference anchor="RFC5234">
1400  <front>
1401    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1402    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1403      <organization>Brandenburg InternetWorking</organization>
1404      <address>
1405        <email>dcrocker@bbiw.net</email>
1406      </address> 
1407    </author>
1408    <author initials="P." surname="Overell" fullname="Paul Overell">
1409      <organization>THUS plc.</organization>
1410      <address>
1411        <email>paul.overell@thus.net</email>
1412      </address>
1413    </author>
1414    <date month="January" year="2008"/>
1415  </front>
1416  <seriesInfo name="STD" value="68"/>
1417  <seriesInfo name="RFC" value="5234"/>
1418</reference>
1419
1420</references>
1421
1422<references title="Informative References">
1423
1424<reference anchor="RFC2616">
1425  <front>
1426    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1427    <author initials="R." surname="Fielding" fullname="R. Fielding">
1428      <organization>University of California, Irvine</organization>
1429      <address><email>fielding@ics.uci.edu</email></address>
1430    </author>
1431    <author initials="J." surname="Gettys" fullname="J. Gettys">
1432      <organization>W3C</organization>
1433      <address><email>jg@w3.org</email></address>
1434    </author>
1435    <author initials="J." surname="Mogul" fullname="J. Mogul">
1436      <organization>Compaq Computer Corporation</organization>
1437      <address><email>mogul@wrl.dec.com</email></address>
1438    </author>
1439    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1440      <organization>MIT Laboratory for Computer Science</organization>
1441      <address><email>frystyk@w3.org</email></address>
1442    </author>
1443    <author initials="L." surname="Masinter" fullname="L. Masinter">
1444      <organization>Xerox Corporation</organization>
1445      <address><email>masinter@parc.xerox.com</email></address>
1446    </author>
1447    <author initials="P." surname="Leach" fullname="P. Leach">
1448      <organization>Microsoft Corporation</organization>
1449      <address><email>paulle@microsoft.com</email></address>
1450    </author>
1451    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1452      <organization>W3C</organization>
1453      <address><email>timbl@w3.org</email></address>
1454    </author>
1455    <date month="June" year="1999"/>
1456  </front>
1457  <seriesInfo name="RFC" value="2616"/>
1458</reference>
1459
1460<reference anchor='RFC3864'>
1461  <front>
1462    <title>Registration Procedures for Message Header Fields</title>
1463    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1464      <organization>Nine by Nine</organization>
1465      <address><email>GK-IETF@ninebynine.org</email></address>
1466    </author>
1467    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1468      <organization>BEA Systems</organization>
1469      <address><email>mnot@pobox.com</email></address>
1470    </author>
1471    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1472      <organization>HP Labs</organization>
1473      <address><email>JeffMogul@acm.org</email></address>
1474    </author>
1475    <date year='2004' month='September' />
1476  </front>
1477  <seriesInfo name='BCP' value='90' />
1478  <seriesInfo name='RFC' value='3864' />
1479</reference>
1480
1481<reference anchor='RFC4918'>
1482  <front>
1483    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1484    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1485      <organization abbrev="CommerceNet">CommerceNet</organization>
1486      <address><email>ldusseault@commerce.net</email></address>
1487    </author>
1488    <date month="June" year="2007" />
1489  </front>
1490  <seriesInfo name='RFC' value='4918' />
1491</reference>
1492</references>
1493
1494<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1495<t>
1496  Allow weak entity-tags in all requests except range requests (Sections
1497  <xref target="weak.and.strong.validators" format="counter"/> and
1498  <xref target="header.if-none-match" format="counter"/>).
1499</t>
1500<t>
1501  Change ABNF productions for header fields to only define the field value.
1502  (<xref target="header.field.definitions"/>)
1503</t>
1504</section>
1505
1506<?BEGININC p4-conditional.abnf-appendix ?>
1507<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1508<figure>
1509<artwork type="abnf" name="p4-conditional.parsed-abnf">
1510<x:ref>ETag</x:ref> = entity-tag
1511
1512<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1513
1514<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1515 entity-tag ] ) )
1516<x:ref>If-Modified-Since</x:ref> = HTTP-date
1517<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1518 entity-tag ] ) )
1519<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1520
1521<x:ref>Last-Modified</x:ref> = HTTP-date
1522
1523<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1524
1525<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1526
1527<x:ref>opaque-tag</x:ref> = quoted-string
1528
1529<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
1530
1531<x:ref>weak</x:ref> = %x57.2F ; W/
1532</artwork>
1533</figure>
1534<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1535; ETag defined but not used
1536; If-Match defined but not used
1537; If-Modified-Since defined but not used
1538; If-None-Match defined but not used
1539; If-Unmodified-Since defined but not used
1540; Last-Modified defined but not used
1541</artwork></figure></section>
1542<?ENDINC p4-conditional.abnf-appendix ?>
1543
1544<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1545
1546<section title="Since RFC 2616">
1547<t>
1548  Extracted relevant partitions from <xref target="RFC2616"/>.
1549</t>
1550</section>
1551
1552<section title="Since draft-ietf-httpbis-p4-conditional-00">
1553<t>
1554  Closed issues:
1555  <list style="symbols"> 
1556    <t>
1557      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1558      "Normative and Informative references"
1559    </t>
1560  </list>
1561</t>
1562<t>
1563  Other changes:
1564  <list style="symbols"> 
1565    <t>
1566      Move definitions of 304 and 412 condition codes from Part2.
1567    </t>
1568  </list>
1569</t>
1570</section>
1571
1572<section title="Since draft-ietf-httpbis-p4-conditional-01">
1573<t>
1574  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1575  <list style="symbols"> 
1576    <t>
1577      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1578    </t>
1579  </list>
1580</t>
1581</section>
1582
1583<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1584<t>
1585  Closed issues:
1586  <list style="symbols"> 
1587    <t>
1588      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1589      "Weak ETags on non-GET requests"
1590    </t>
1591  </list>
1592</t>
1593<t>
1594  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1595  <list style="symbols"> 
1596    <t>
1597      Reference RFC 3984, and update header field registrations for header fields defined
1598      in this document.
1599    </t>
1600  </list>
1601</t>
1602</section>
1603
1604<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1605<t>
1606  Closed issues:
1607  <list style="symbols"> 
1608    <t>
1609      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1610      "Examples for ETag matching"
1611    </t>
1612    <t>
1613      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1614      "'entity value' undefined"
1615    </t>
1616    <t>
1617      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1618      "bogus 2068 Date header reference"
1619    </t>
1620  </list>
1621</t>
1622</section>
1623
1624<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1625<t>
1626  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1627  <list style="symbols"> 
1628    <t>
1629      Use "/" instead of "|" for alternatives.
1630    </t>
1631    <t>
1632      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1633      whitespace ("OWS") and required whitespace ("RWS").
1634    </t>
1635    <t>
1636      Rewrite ABNFs to spell out whitespace rules, factor out
1637      header field value format definitions.
1638    </t>
1639  </list>
1640</t>
1641</section>
1642
1643<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1644<t>
1645  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1646  <list style="symbols"> 
1647    <t>
1648      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1649    </t>
1650  </list>
1651</t>
1652</section>
1653
1654<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1655<t>
1656  Closed issues:
1657  <list style="symbols"> 
1658    <t>
1659      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1660      "case-sensitivity of etag weakness indicator"
1661    </t>
1662  </list>
1663</t>
1664</section>
1665
1666<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1667<t>
1668  Closed issues:
1669  <list style="symbols"> 
1670    <t>
1671      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1672      "Weak ETags on non-GET requests" (If-Match still was defined to require
1673      strong matching)
1674    </t>
1675    <t>
1676      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1677      "move IANA registrations for optional status codes"
1678    </t>
1679  </list>
1680</t>
1681</section>
1682
1683<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1684<t>
1685  No significant changes.
1686</t>
1687</section>
1688
1689<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1690<t>
1691  No significant changes.
1692</t>
1693</section>
1694
1695<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1696<t>
1697  Closed issues:
1698  <list style="symbols"> 
1699    <t>
1700      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1701      "Clarify 'Requested Variant'"
1702    </t>
1703    <t>
1704      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1705      "Clarify entity / representation / variant terminology"
1706    </t>
1707    <t>
1708      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1709      "consider removing the 'changes from 2068' sections"
1710    </t>
1711  </list>
1712</t>
1713</section>
1714
1715<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1716<t>
1717  None.
1718</t>
1719</section>
1720
1721<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1722<t>
1723  Closed issues:
1724  <list style="symbols"> 
1725    <t>
1726      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1727      "Header Classification"
1728    </t>
1729  </list>
1730</t>
1731</section>
1732
1733<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1734<t>
1735  Closed issues:
1736  <list style="symbols"> 
1737    <t>
1738      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1739      "If-* and entities"
1740    </t>
1741    <t>
1742      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1743      "Definition of validator weakness"
1744    </t>
1745    <t>
1746      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1747      "untangle ABNFs for header fields"
1748    </t>
1749    <t>
1750      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1751      "ETags and Quotes"
1752    </t>
1753  </list>
1754</t>
1755</section>
1756
1757<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1758<t>
1759  None.
1760</t>
1761</section>
1762
1763<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1764<t>
1765  Closed issues:
1766  <list style="symbols"> 
1767    <t>
1768      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1769      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1770    </t>
1771  </list>
1772</t>
1773</section>
1774
1775<section title="Since draft-ietf-httpbis-p4-conditional-16" anchor="changes.since.16">
1776<t>
1777  None yet.
1778</t>
1779</section>
1780
1781</section>
1782
1783</back>
1784</rfc>
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