source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1437

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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 "September">
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='Part2' 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 http-date                  "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY content-negotiation        "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32]>
33<?rfc toc="yes" ?>
34<?rfc symrefs="yes" ?>
35<?rfc sortrefs="yes" ?>
36<?rfc compact="yes"?>
37<?rfc subcompact="no" ?>
38<?rfc linkmailto="no" ?>
39<?rfc editing="no" ?>
40<?rfc comments="yes"?>
41<?rfc inline="yes"?>
42<?rfc rfcedstyle="yes"?>
43<?rfc-ext allow-markup-in-artwork="yes" ?>
44<?rfc-ext include-references-in-index="yes" ?>
45<rfc obsoletes="2616" category="std" x:maturity-level="draft"
46     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
47     xmlns:x='http://purl.org/net/xml2rfc/ext'>
48<front>
49
50  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
51
52  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
53    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
54    <address>
55      <postal>
56        <street>345 Park Ave</street>
57        <city>San Jose</city>
58        <region>CA</region>
59        <code>95110</code>
60        <country>USA</country>
61      </postal>
62      <email>fielding@gbiv.com</email>
63      <uri>http://roy.gbiv.com/</uri>
64    </address>
65  </author>
66
67  <author initials="J." surname="Gettys" fullname="Jim Gettys">
68    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
69    <address>
70      <postal>
71        <street>21 Oak Knoll Road</street>
72        <city>Carlisle</city>
73        <region>MA</region>
74        <code>01741</code>
75        <country>USA</country>
76      </postal>
77      <email>jg@freedesktop.org</email>
78      <uri>http://gettys.wordpress.com/</uri>
79    </address>
80  </author>
81 
82  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
83    <organization abbrev="HP">Hewlett-Packard Company</organization>
84    <address>
85      <postal>
86        <street>HP Labs, Large Scale Systems Group</street>
87        <street>1501 Page Mill Road, MS 1177</street>
88        <city>Palo Alto</city>
89        <region>CA</region>
90        <code>94304</code>
91        <country>USA</country>
92      </postal>
93      <email>JeffMogul@acm.org</email>
94    </address>
95  </author>
96
97  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
98    <organization abbrev="Microsoft">Microsoft Corporation</organization>
99    <address>
100      <postal>
101        <street>1 Microsoft Way</street>
102        <city>Redmond</city>
103        <region>WA</region>
104        <code>98052</code>
105        <country>USA</country>
106      </postal>
107      <email>henrikn@microsoft.com</email>
108    </address>
109  </author>
110
111  <author initials="L." surname="Masinter" fullname="Larry Masinter">
112    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
113    <address>
114      <postal>
115        <street>345 Park Ave</street>
116        <city>San Jose</city>
117        <region>CA</region>
118        <code>95110</code>
119        <country>USA</country>
120      </postal>
121      <email>LMM@acm.org</email>
122      <uri>http://larry.masinter.net/</uri>
123    </address>
124  </author>
125 
126  <author initials="P." surname="Leach" fullname="Paul J. Leach">
127    <organization abbrev="Microsoft">Microsoft Corporation</organization>
128    <address>
129      <postal>
130        <street>1 Microsoft Way</street>
131        <city>Redmond</city>
132        <region>WA</region>
133        <code>98052</code>
134      </postal>
135      <email>paulle@microsoft.com</email>
136    </address>
137  </author>
138   
139  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
140    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
141    <address>
142      <postal>
143        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
144        <street>The Stata Center, Building 32</street>
145        <street>32 Vassar Street</street>
146        <city>Cambridge</city>
147        <region>MA</region>
148        <code>02139</code>
149        <country>USA</country>
150      </postal>
151      <email>timbl@w3.org</email>
152      <uri>http://www.w3.org/People/Berners-Lee/</uri>
153    </address>
154  </author>
155
156  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
157    <organization abbrev="W3C">World Wide Web Consortium</organization>
158    <address>
159      <postal>
160        <street>W3C / ERCIM</street>
161        <street>2004, rte des Lucioles</street>
162        <city>Sophia-Antipolis</city>
163        <region>AM</region>
164        <code>06902</code>
165        <country>France</country>
166      </postal>
167      <email>ylafon@w3.org</email>
168      <uri>http://www.raubacapeu.net/people/yves/</uri>
169    </address>
170  </author>
171
172  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
173    <organization abbrev="greenbytes">greenbytes GmbH</organization>
174    <address>
175      <postal>
176        <street>Hafenweg 16</street>
177        <city>Muenster</city><region>NW</region><code>48155</code>
178        <country>Germany</country>
179      </postal>
180      <phone>+49 251 2807760</phone>
181      <facsimile>+49 251 2807761</facsimile>
182      <email>julian.reschke@greenbytes.de</email>
183      <uri>http://greenbytes.de/tech/webdav/</uri>
184    </address>
185  </author>
186
187  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
188  <workgroup>HTTPbis Working Group</workgroup>
189
190<abstract>
191<t>
192   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
193   distributed, collaborative, hypertext information systems. HTTP has been in
194   use by the World Wide Web global information initiative since 1990. This
195   document is Part 4 of the seven-part specification that defines the protocol
196   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
197</t>
198<t>
199   Part 4 defines request header fields for indicating conditional requests and
200   the rules for constructing responses to those requests.
201</t>
202</abstract>
203
204<note title="Editorial Note (To be removed by RFC Editor)">
205  <t>
206    Discussion of this draft should take place on the HTTPBIS working group
207    mailing list (ietf-http-wg@w3.org), which is archived at
208    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
209  </t>
210  <t>
211    The current issues list is at
212    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
213    documents (including fancy diffs) can be found at
214    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
215  </t>
216  <t>
217    The changes in this draft are summarized in <xref target="changes.since.16"/>.
218  </t>
219</note>
220</front>
221<middle>
222<section title="Introduction" anchor="introduction">
223<t>
224   This document defines the HTTP/1.1 conditional request mechanisms,
225   including both metadata for indicating/observing changes in resource
226   representations and request header fields that specify preconditions
227   on that metadata be checked before performing the request method.
228   Conditional GET requests are the most efficient mechanism for HTTP
229   cache updates &caching;.  Conditionals can also be
230   applied to state-changing methods, such as PUT and DELETE, to prevent
231   the "lost update" problem: one client accidentally overwriting
232   the work of another client that has been acting in parallel.
233</t>
234<t>
235   Conditional request preconditions are based on the state of the target
236   resource as a whole (its current value set) or the state as observed
237   in a previously obtained representation (one value in that set).
238   A resource might have multiple current representations, each with its
239   own observable state.  The conditional request mechanisms assume that
240   the mapping of requests to corresponding representations will be
241   consistent over time if the server intends to take advantage of
242   conditionals.  Regardless, if the mapping is inconsistent and
243   the server is unable to select the appropriate representation, then
244   no harm will result when the precondition evaluates to false.
245</t>
246<t><iref primary="true" item="selected representation"/>
247   We use the term "<x:dfn>selected representation</x:dfn>" to refer to
248   the current representation of the target resource that would have been
249   selected in a successful response if the same request had used the method
250   GET and had excluded all of the conditional request header fields.
251   The conditional request preconditions are evaluated by comparing the
252   values provided in the request header fields to the current metadata
253   for the selected representation.
254</t>
255
256<section title="Requirements" anchor="intro.requirements">
257<t>
258   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
259   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
260   document are to be interpreted as described in <xref target="RFC2119"/>.
261</t>
262<t>
263   An implementation is not compliant if it fails to satisfy one or more
264   of the "MUST" or "REQUIRED" level requirements for the protocols it
265   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
266   level and all the "SHOULD" level requirements for its protocols is said
267   to be "unconditionally compliant"; one that satisfies all the "MUST"
268   level requirements but not all the "SHOULD" level requirements for its
269   protocols is said to be "conditionally compliant".
270</t>
271</section>
272
273<section title="Syntax Notation" anchor="notation">
274  <x:anchor-alias value="ALPHA"/>
275  <x:anchor-alias value="CR"/>
276  <x:anchor-alias value="DIGIT"/>
277  <x:anchor-alias value="LF"/>
278  <x:anchor-alias value="OCTET"/>
279  <x:anchor-alias value="VCHAR"/>
280  <x:anchor-alias value="core.rules"/>
281  <x:anchor-alias value="quoted-string"/>
282  <x:anchor-alias value="OWS"/>
283  <x:anchor-alias value="HTTP-date"/>
284<t>
285  This specification uses the ABNF syntax defined in &notation; (which
286  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
287  <xref target="collected.abnf"/> shows the collected ABNF, with the list
288  rule expanded.
289</t>
290<t>
291  The following core rules are included by
292  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
293  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
294  DIGIT (decimal 0-9), DQUOTE (double quote),
295  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
296  OCTET (any 8-bit sequence of data), SP (space), and
297  VCHAR (any visible US-ASCII character).
298</t>
299<t>
300  The ABNF rules below are defined in <xref target="Part1"/> and
301  <xref target="Part2"/>:
302</t>
303<figure><artwork type="abnf2616">
304  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
305  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-rules;&gt;
306  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
307</artwork></figure>
308</section>
309</section>
310
311<section title="Validators" anchor="validators">
312   <iref primary="true" item="metadata"/>
313   <iref primary="true" item="validator"/>
314<t>
315   This specification defines two forms of metadata that are commonly used
316   to observe resource state and test for preconditions: modification dates
317   and opaque entity tags.  Additional metadata that reflects resource state
318   has been defined by various extensions of HTTP, such as WebDAV
319   <xref target="RFC4918"/>, that are beyond the scope of this specification.
320   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
321   when it is used within a precondition.
322</t>
323
324<section title="Weak versus Strong" anchor="weak.and.strong.validators">
325   <iref primary="true" item="validator" subitem="weak"/>
326   <iref primary="true" item="validator" subitem="strong"/>
327<t>
328   Validators come in two flavors: strong or weak.  Weak validators are easy
329   to generate but are far less useful for comparisons.  Strong validators
330   are ideal for comparisons but can be very difficult (and occasionally
331   impossible) to generate efficiently.  Rather than impose that all forms
332   of resource adhere to the same strength of validator, HTTP exposes the
333   type of validator in use and imposes restrictions on when weak validators
334   can be used as preconditions.
335</t>
336<t>
337   A "strong validator" is a representation metadata value that &MUST; be
338   changed to a new, previously unused or guaranteed unique, value whenever
339   a change occurs to the representation data such that a change would be
340   observable in the payload body of a 200 response to GET.  A strong
341   validator &MAY; be changed for other reasons, such as when a semantically
342   significant part of the representation metadata is changed (e.g.,
343   Content-Type), but it is in the best interests of the origin server to only
344   change the value when it is necessary to invalidate the stored responses
345   held by remote caches and authoring tools.  A strong validator &MUST; be
346   unique across all representations of a given resource, such that no two
347   representations of that resource share the same validator unless
348   their payload body would be identical.
349</t>
350<t>
351   Cache entries might persist for arbitrarily long periods, regardless
352   of expiration times.  Thus, a cache might attempt to validate an
353   entry using a validator that it obtained in the distant past.
354   A strong validator &MUST; be unique across all versions of all
355   representations associated with a particular resource over time.
356   However, there is no implication of uniqueness across representations
357   of different resources (i.e., the same strong validator might be
358   in use for representations of multiple resources at the same time
359   and does not imply that those representations are equivalent).
360</t>
361<t>
362   There are a variety of strong validators used in practice.  The best are
363   based on strict revision control, wherein each change to a representation
364   always results in a unique node name and revision identifier being assigned
365   before the representation is made accessible to GET.  A cryptographic hash
366   function applied to the representation data is also sufficient if the data
367   is available prior to the response header fields being sent and the digest
368   does not need to be recalculated every time a validation request is
369   received.  However, if a resource has distinct representations that differ
370   only in their metadata, such as might occur with content negotiation over
371   media types that happen to share the same data format, then a server
372   &SHOULD; incorporate additional information in the validator to
373   distinguish those representations and avoid confusing cache behavior.
374</t>
375<t>
376   In contrast, a "weak validator" is a representation metadata value that
377   might not be changed for every change to the representation data.  This
378   weakness might be due to limitations in how the value is calculated, such
379   as clock resolution or an inability to ensure uniqueness for all possible
380   representations of the resource, or due to a desire by the resource owner
381   to group representations by some self-determined set of equivalency
382   rather than unique sequences of data.  A weak entity-tag &SHOULD; change
383   whenever the origin server considers prior representations to be
384   unacceptable as a substitute for the current representation. In other
385   words, a weak entity-tag &SHOULD; change whenever the origin server wants
386   caches to invalidate old responses.
387</t>
388<t>
389   For example, the representation of a weather report that changes in
390   content every second, based on dynamic measurements, might be grouped
391   into sets of equivalent representations (from the origin server's
392   perspective) with the same weak validator in order to allow cached
393   representations to be valid for a reasonable period of time (perhaps
394   adjusted dynamically based on server load or weather quality).
395   Likewise, a representation's modification time, if defined with only
396   one-second resolution, might be a weak validator if it is possible
397   for the representation to be modified twice during a single second and
398   retrieved between those modifications.
399</t>
400<t>
401   A "use" of a validator occurs when either a client generates a request
402   and includes the validator in a precondition or when a server
403   compares two validators.
404   Weak validators are only usable in contexts that do not depend on exact
405   equality of a representation's payload body.
406   Strong validators are usable and preferred for all conditional requests,
407   including cache validation, partial content ranges, and "lost update"
408   avoidance.
409</t>
410</section>
411
412<section title="Last-Modified" anchor="header.last-modified">
413  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
414  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
415  <x:anchor-alias value="Last-Modified"/>
416<t>
417   The "Last-Modified" header field indicates the date and time at
418   which the origin server believes the selected representation was
419   last modified.
420</t>
421<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
422  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
423</artwork></figure>
424<t>
425   An example of its use is
426</t>
427<figure><artwork type="example">
428  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
429</artwork></figure>
430
431<section title="Generation" anchor="lastmod.generation">
432<t>
433   Origin servers &SHOULD; send Last-Modified for any selected
434   representation for which a last modification date can be reasonably
435   and consistently determined, since its use in conditional requests
436   and evaluating cache freshness (&caching;) results in a substantial
437   reduction of HTTP traffic on the Internet and can be a significant
438   factor in improving service scalability and reliability.
439</t>
440<t>
441   A representation is typically the sum of many parts behind the
442   resource interface.  The last-modified time would usually be
443   the most recent time that any of those parts were changed.
444   How that value is determined for any given resource is an
445   implementation detail beyond the scope of this specification.
446   What matters to HTTP is how recipients of the Last-Modified
447   header field can use its value to make conditional requests
448   and test the validity of locally cached responses.
449</t>
450<t>
451   An origin server &SHOULD; obtain the Last-Modified value of the
452   representation as close as possible to the time that it generates
453   the Date field-value for its response. This allows a recipient to
454   make an accurate assessment of the representation's modification time,
455   especially if the representation changes near the time that the
456   response is generated.
457</t>
458<t>
459   An origin server with a clock &MUST-NOT; send a Last-Modified date
460   that is later than the server's time of message origination (Date).
461   If the last modification time is derived from implementation-specific
462   metadata that evaluates to some time in the future, according to the
463   origin server's clock, then the origin server &MUST; replace that
464   value with the message origination date. This prevents a future
465   modification date from having an adverse impact on cache validation.
466</t>
467<t>
468   An origin server without a clock &MUST-NOT; assign Last-Modified
469   values to a response unless these values were associated
470   with the resource by some other system or user with a reliable clock.
471</t>
472</section>
473
474<section title="Comparison" anchor="lastmod.comparison">
475<t>
476   A Last-Modified time, when used as a validator in a request, is
477   implicitly weak unless it is possible to deduce that it is strong,
478   using the following rules:
479  <list style="symbols">
480     <t>The validator is being compared by an origin server to the
481        actual current validator for the representation and,</t>
482     <t>That origin server reliably knows that the associated representation did
483        not change twice during the second covered by the presented
484        validator.</t>
485  </list>
486</t>
487<t>
488   or
489  <list style="symbols">
490     <t>The validator is about to be used by a client in an If-Modified-Since,
491        If-Unmodified-Since header field, because the client has a cache entry,
492        or If-Range for the associated representation, and</t>
493     <t>That cache entry includes a Date value, which gives the time
494        when the origin server sent the original response, and</t>
495     <t>The presented Last-Modified time is at least 60 seconds before
496        the Date value.</t>
497  </list>
498</t>
499<t>
500   or
501  <list style="symbols">
502     <t>The validator is being compared by an intermediate cache to the
503        validator stored in its cache entry for the representation, and</t>
504     <t>That cache entry includes a Date value, which gives the time
505        when the origin server sent the original response, and</t>
506     <t>The presented Last-Modified time is at least 60 seconds before
507        the Date value.</t>
508  </list>
509</t>
510<t>
511   This method relies on the fact that if two different responses were
512   sent by the origin server during the same second, but both had the
513   same Last-Modified time, then at least one of those responses would
514   have a Date value equal to its Last-Modified time. The arbitrary 60-second
515   limit guards against the possibility that the Date and Last-Modified
516   values are generated from different clocks, or at somewhat
517   different times during the preparation of the response. An
518   implementation &MAY; use a value larger than 60 seconds, if it is
519   believed that 60 seconds is too short.
520</t>
521</section>
522</section>
523
524<section title="ETag" anchor="header.etag">
525  <iref primary="true" item="ETag header field" x:for-anchor=""/>
526  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
527  <x:anchor-alias value="ETag"/>
528  <x:anchor-alias value="entity-tag"/>
529  <x:anchor-alias value="entity.tags"/>
530  <x:anchor-alias value="opaque-tag"/>
531  <x:anchor-alias value="weak"/>
532<t>
533   The ETag header field provides the current entity-tag for the
534   selected representation.
535   An entity-tag is an opaque validator for differentiating between
536   multiple representations of the same resource, regardless of whether
537   those multiple representations are due to resource state changes over
538   time, content negotiation resulting in multiple representations being
539   valid at the same time, or both. An entity-tag consists of an opaque
540   quoted string, possibly prefixed by a weakness indicator.
541</t>
542<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"/>
543  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
544
545  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
546  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
547  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
548</artwork></figure>
549<t>
550   An entity-tag can be more reliable for validation than a modification
551   date in situations where it is inconvenient to store modification
552   dates, where the one-second resolution of HTTP date values is not
553   sufficient, or where modification dates are not consistently maintained.
554</t>
555<figure><preamble>
556  Examples:
557</preamble>
558<artwork type="example">
559  ETag: "xyzzy"
560  ETag: W/"xyzzy"
561  ETag: ""
562</artwork></figure>
563<t>
564   An entity-tag can be either a weak or strong validator, with
565   strong being the default.  If an origin server provides an entity-tag
566   for a representation and the generation of that entity-tag does not satisfy
567   the requirements for a strong validator
568   (<xref target="weak.and.strong.validators"/>), then that
569   entity-tag &MUST; be marked as weak by prefixing its opaque value
570   with "W/" (case-sensitive).
571</t>
572
573<section title="Generation" anchor="entity.tag.generation">
574<t>
575   The principle behind entity-tags is that only the service author
576   knows the implementation of a resource well enough to select the
577   most accurate and efficient validation mechanism for that resource,
578   and that any such mechanism can be mapped to a simple sequence of
579   octets for easy comparison.  Since the value is opaque, there is no
580   need for the client to be aware of how each entity-tag is constructed.
581</t>
582<t>
583   For example, a resource that has implementation-specific versioning
584   applied to all changes might use an internal revision number, perhaps
585   combined with a variance identifier for content negotiation, to
586   accurately differentiate between representations.
587   Other implementations might use a stored hash of representation content,
588   a combination of various filesystem attributes, or a modification
589   timestamp that has sub-second resolution.
590</t>
591<t>
592   Origin servers &SHOULD; send ETag for any selected representation
593   for which detection of changes can be reasonably and consistently
594   determined, since the entity-tag's use in conditional requests and
595   evaluating cache freshness (&caching;) can result in a substantial
596   reduction of HTTP network traffic and can be a significant factor in
597   improving service scalability and reliability.
598</t>
599</section>
600
601<section title="Comparison" anchor="entity.tag.comparison">
602  <x:anchor-alias value="validator.comparison"/>
603<t>
604   There are two entity-tag comparison functions, depending
605   on whether the comparison context allows the use of weak validators
606   or not:
607  <list style="symbols">
608     <t>The strong comparison function: in order to be considered equal,
609        both opaque-tags &MUST; be identical character-by-character, and both
610        &MUST-NOT; be weak.</t>
611     <t>The weak comparison function: in order to be considered equal, both
612        opaque-tags &MUST; be identical character-by-character, but
613        either or both of them &MAY; be tagged as "weak" without affecting
614        the result.</t>
615  </list>
616</t>
617<t>
618   The example below shows the results for a set of entity-tag pairs,
619   and both the weak and strong comparison function results:
620</t>
621<texttable align="left">
622  <ttcol>ETag 1</ttcol>
623  <ttcol>ETag 2</ttcol>
624  <ttcol>Strong Comparison</ttcol>
625  <ttcol>Weak Comparison</ttcol>
626
627  <c>W/"1"</c>
628  <c>W/"1"</c>
629  <c>no match</c>
630  <c>match</c>
631 
632  <c>W/"1"</c>
633  <c>W/"2"</c>
634  <c>no match</c>
635  <c>no match</c>
636
637  <c>W/"1"</c>
638  <c>"1"</c>
639  <c>no match</c>
640  <c>match</c>
641
642  <c>"1"</c>
643  <c>"1"</c>
644  <c>match</c>
645  <c>match</c>
646</texttable>
647</section>
648
649<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
650<t>
651   Consider a resource that is subject to content negotiation (&content-negotiation;),
652   and where the representations returned upon a GET request vary based on
653   the Accept-Encoding request header field (&header-accept-encoding;):
654</t>
655<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
656GET /index HTTP/1.1
657Host: www.example.com
658Accept-Encoding: gzip
659
660</artwork></figure>
661<t>
662   In this case, the response might or might not use the gzip content coding.
663   If it does not, the response might look like:
664</t>
665<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
666HTTP/1.1 200 OK
667Date: Thu, 26 Mar 2010 00:05:00 GMT
668ETag: "123-a"
669Content-Length: <x:length-of target="exbody"/>
670Vary: Accept-Encoding
671Content-Type: text/plain
672
673<x:span anchor="exbody">Hello World!
674Hello World!
675Hello World!
676Hello World!
677Hello World!
678</x:span></artwork></figure>
679<t>
680   An alternative representation that does use gzip content coding would be:
681</t>
682<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
683HTTP/1.1 200 OK
684Date: Thu, 26 Mar 2010 00:05:00 GMT
685ETag: "123-b"
686Content-Length: 43
687Vary: Accept-Encoding
688Content-Type: text/plain
689Content-Encoding: gzip
690
691<spanx>...binary data...</spanx></artwork></figure>
692<x:note>
693  <t>
694    <x:h>Note:</x:h> Content codings are a property of the representation,
695    so therefore an entity-tag of an encoded representation must be distinct
696    from an unencoded representation to prevent conflicts during cache updates
697    and range requests.  In contrast, transfer codings (&transfer-codings;)
698    apply only during message transfer and do not require distinct entity-tags.
699  </t>
700</x:note>
701</section>
702</section>
703
704<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">
705<t>
706   We adopt a set of rules and recommendations for origin servers,
707   clients, and caches regarding when various validator types ought to
708   be used, and for what purposes.
709</t>
710<t>
711   HTTP/1.1 origin servers:
712  <list style="symbols">
713     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
714        generate one.</t>
715
716     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
717        performance considerations support the use of weak entity-tags,
718        or if it is unfeasible to send a strong entity-tag.</t>
719
720     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
721  </list>
722</t>
723<t>
724   In other words, the preferred behavior for an HTTP/1.1 origin server
725   is to send both a strong entity-tag and a Last-Modified value.
726</t>
727<t>
728   HTTP/1.1 clients:
729  <list style="symbols">
730     <t>&MUST; use that entity-tag in any cache-conditional request (using
731        If-Match or If-None-Match) if an entity-tag has been provided by the
732        origin server.</t>
733
734     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
735        requests (using If-Modified-Since) if only a Last-Modified value has
736        been provided by the origin server. </t>
737
738     <t>&MAY; use the Last-Modified value in subrange cache-conditional
739        requests (using If-Unmodified-Since) if only a Last-Modified value has
740        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
741        provide a way to disable this, in case of difficulty.</t>
742
743     <t>&SHOULD; use both validators in cache-conditional requests if both an
744        entity-tag and a Last-Modified value have been provided by the origin
745        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
746        appropriately.</t>
747  </list>
748</t>
749<t>
750   An HTTP/1.1 origin server, upon receiving a conditional request that
751   includes both a Last-Modified date (e.g., in an If-Modified-Since or
752   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
753   in an If-Match, If-None-Match, or If-Range header field) as cache
754   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
755   unless doing so is consistent with all of the conditional header
756   fields in the request.
757</t>
758<t>
759   An HTTP/1.1 caching proxy, upon receiving a conditional request that
760   includes both a Last-Modified date and one or more entity-tags as
761   cache validators, &MUST-NOT; return a locally cached response to the
762   client unless that cached response is consistent with all of the
763   conditional header fields in the request.
764  <list><t>
765      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
766      servers and clients ought to transmit as much non-redundant
767      information as is available in their responses and requests.
768      HTTP/1.1 systems receiving this information will make the most
769      conservative assumptions about the validators they receive.
770  </t><t>
771      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
772      last-modified values received or used by these systems will
773      support transparent and efficient caching, and so HTTP/1.1 origin
774      servers should provide Last-Modified values. In those rare cases
775      where the use of a Last-Modified value as a validator by an
776      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
777      origin servers should not provide one.
778  </t></list>
779</t>
780</section>
781</section>
782
783<section title="Precondition Header Fields" anchor="header.field.definitions">
784<t>
785   This section defines the syntax and semantics of HTTP/1.1 header fields
786   for applying preconditions on requests.
787</t>
788
789<section title="If-Match" anchor="header.if-match">
790  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
791  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
792  <x:anchor-alias value="If-Match"/>
793<t>
794   The "If-Match" header field &MAY; be used to make a request method
795   conditional on the current existence or value of an entity-tag for
796   one or more representations of the target resource.  If-Match is
797   generally useful for resource update requests, such as PUT requests,
798   as a means for protecting against accidental overwrites when multiple
799   clients are acting in parallel on the same resource (i.e., the
800   "lost update" problem).  An If-Match field-value of "*" places the
801   precondition on the existence of any current representation for the
802   target resource.
803</t>
804<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
805  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
806</artwork></figure>
807<t>
808   If any of the entity-tags listed in the If-Match field value match
809   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
810   selected representation for the target resource,
811   or if "*" is given and any current representation exists for the
812   target resource, then the server &MAY; perform the request method
813   as if the If-Match header field was not present.
814</t>
815<t>
816   If none of the entity-tags match, or if "*" is given and no current
817   representation exists, the server &MUST-NOT; perform the requested method.
818   Instead, the server &MUST; respond with the 412 (Precondition Failed)
819   status code.
820</t>
821<t>
822   If the request would, without the If-Match header field, result in
823   anything other than a 2xx or 412 status code, then the If-Match header field
824   &MUST; be ignored.
825</t>
826<t>
827   Examples:
828</t>
829<figure><artwork type="example">
830  If-Match: "xyzzy"
831  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
832  If-Match: *
833</artwork></figure>
834<t>
835   The result of a request having both an If-Match header field and
836   either an If-None-Match or an If-Modified-Since header fields is
837   undefined by this specification.
838</t>
839</section>
840
841<section title="If-None-Match" anchor="header.if-none-match">
842  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
843  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
844  <x:anchor-alias value="If-None-Match"/>
845<t>
846   The "If-None-Match" header field &MAY; be used to make a request method
847   conditional on not matching any of the current entity-tag values for
848   representations of the target resource.  If-None-Match is primarily
849   used in conditional GET requests to enable efficient updates of cached
850   information with a minimum amount of transaction overhead.  A client
851   that has one or more representations previously obtained from the
852   target resource can send If-None-Match with a list of the associated
853   entity-tags in the hope of receiving a 304 response if at least one
854   of those representations matches the selected representation.
855</t>
856<t>
857   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
858   request method (e.g., PUT) from inadvertently modifying an existing
859   representation of the target resource when the client believes that
860   the resource does not have a current representation.  This is a variation
861   on the "lost update" problem that might arise if more than one client
862   attempts to create an initial representation for the target resource.
863</t>
864<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
865  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
866</artwork></figure>
867<t>
868   If any of the entity-tags listed in the If-None-Match field-value match
869   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
870   selected representation, or if "*" is
871   given and any current representation exists for that resource, then the
872   server &MUST-NOT; perform the requested method.
873   Instead, if the request method was GET or HEAD, the server &SHOULD;
874   respond with a 304 (Not Modified) status code, including the cache-related
875   header fields (particularly ETag) of the selected representation that has
876   a matching entity-tag.  For all other request methods, the server &MUST;
877   respond with a 412 (Precondition Failed) status code.
878</t>
879<t>
880   If none of the entity-tags match, then the server &MAY; perform the
881   requested method as if the If-None-Match header field did not exist,
882   but &MUST; also ignore any If-Modified-Since header field(s) in the
883   request. That is, if no entity-tags match, then the server &MUST-NOT;
884   return a 304 (Not Modified) response.
885</t>
886<t>
887   If the request would, without the If-None-Match header field, result
888   in anything other than a 2xx or 304 status code, then the If-None-Match
889   header field &MUST; be ignored. (See <xref
890   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
891   a discussion of server behavior when both If-Modified-Since and
892   If-None-Match appear in the same request.)
893</t>
894<t>
895   Examples:
896</t>
897<figure><artwork type="example">
898  If-None-Match: "xyzzy"
899  If-None-Match: W/"xyzzy"
900  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
901  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
902  If-None-Match: *
903</artwork></figure>
904<t>
905   The result of a request having both an If-None-Match header field and
906   either an If-Match or an If-Unmodified-Since header fields is
907   undefined by this specification.
908</t>
909</section>
910
911<section title="If-Modified-Since" anchor="header.if-modified-since">
912  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
913  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
914  <x:anchor-alias value="If-Modified-Since"/>
915<t>
916   The "If-Modified-Since" header field &MAY; be used to make a request
917   method conditional by modification date: if the selected representation
918   has not been modified since the time specified in this field, then
919   do not perform the request method; instead, respond as detailed below.
920</t>
921<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
922  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
923</artwork></figure>
924<t>
925   An example of the field is:
926</t>
927<figure><artwork type="example">
928  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
929</artwork></figure>
930<t>
931   A GET method with an If-Modified-Since header field and no Range header
932   field requests that the selected representation be transferred only if
933   it has been modified since the date given by the If-Modified-Since
934   header field.
935   The algorithm for determining this includes the following cases:
936  <list style="numbers">
937      <t>If the request would normally result in anything other than a
938         200 (OK) status code, or if the passed If-Modified-Since date is
939         invalid, the response is exactly the same as for a normal GET.
940         A date which is later than the server's current time is
941         invalid.</t>
942
943      <t>If the selected representation has been modified since the
944         If-Modified-Since date, the response is exactly the same as for
945         a normal GET.</t>
946
947      <t>If the selected representation has not been modified since a valid
948         If-Modified-Since date, the server &SHOULD; return a
949         304 (Not Modified) response.</t>
950  </list>
951</t>
952<t>
953   The purpose of this feature is to allow efficient updates of cached
954   information with a minimum amount of transaction overhead.
955  <list><t>
956      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
957      see &header-range; for full details.
958    </t><t>
959      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
960      clock might not be synchronized with the client.
961    </t><t>
962      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
963      servers will use an exact date comparison function, rather than a
964      less-than function, for deciding whether to send a 304 (Not
965      Modified) response. To get best results when sending an If-Modified-Since
966      header field for cache validation, clients are
967      advised to use the exact date string received in a previous Last-Modified
968      header field whenever possible.
969    </t><t>
970      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
971      header field instead of a date taken from the Last-Modified header field for
972      the same request, the client needs to be aware that this
973      date is interpreted in the server's understanding of time.
974      Unsynchronized clocks and rounding problems, due to the different
975      encodings of time between the client and server, are concerns.
976      This includes the possibility of race conditions if the
977      document has changed between the time it was first requested and
978      the If-Modified-Since date of a subsequent request, and the
979      possibility of clock-skew-related problems if the If-Modified-Since
980      date is derived from the client's clock without correction
981      to the server's clock. Corrections for different time bases
982      between client and server are at best approximate due to network
983      latency.
984    </t>
985  </list>
986</t>
987<t>
988   The result of a request having both an If-Modified-Since header field
989   and either an If-Match or an If-Unmodified-Since header fields is
990   undefined by this specification.
991</t>
992</section>
993
994<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
995  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
996  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
997  <x:anchor-alias value="If-Unmodified-Since"/>
998<t>
999   The "If-Unmodified-Since" header field &MAY; be used to make a request
1000   method conditional by modification date: if the selected representation
1001   has been modified since the time specified in this field, then the
1002   server &MUST-NOT; perform the requested operation and &MUST; instead
1003   respond with the 412 (Precondition Failed) status code.
1004   If the selected representation has not been modified since the time
1005   specified in this field, the server &SHOULD; perform the request
1006   method as if the If-Unmodified-Since header field were not present.
1007</t>
1008<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
1009  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
1010</artwork></figure>
1011<t>
1012   An example of the field is:
1013</t>
1014<figure><artwork type="example">
1015  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1016</artwork></figure>
1017<t>
1018   If the request normally (i.e., without the If-Unmodified-Since
1019   header field) would result in anything other than a 2xx or 412 status code,
1020   the If-Unmodified-Since header field &SHOULD; be ignored.
1021</t>
1022<t>
1023   If the specified date is invalid, the header field &MUST; be ignored.
1024</t>
1025<t>
1026   The result of a request having both an If-Unmodified-Since header
1027   field and either an If-None-Match or an If-Modified-Since header
1028   fields is undefined by this specification.
1029</t>
1030</section>
1031
1032<section title="If-Range" anchor="header.if-range">
1033<t>
1034   The If-Range header field provides a special conditional request
1035   mechanism that is similar to If-Match and If-Unmodified-Since but
1036   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1037</t>
1038</section>
1039
1040</section>
1041
1042<section title="Status Code Definitions" anchor="status.code.definitions">
1043<section title="304 Not Modified" anchor="status.304">
1044  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1045  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1046<t>
1047   The 304 status code indicates that a conditional GET request has been
1048   received and would have resulted in a 200 (OK) response if it were not
1049   for the fact that the condition has evaluated to false.  In other words,
1050   there is no need for the server to transfer a representation of the
1051   target resource because the client's request indicates that it already
1052   has a valid representation, as indicated by the 304 response header
1053   fields, and is therefore redirecting the client to make use of that
1054   stored representation as if it were the payload of a 200 response.
1055   The 304 response &MUST-NOT; contain a message-body, and thus is always
1056   terminated by the first empty line after the header fields.
1057</t>
1058<t>
1059   A 304 response &MUST; include a Date header field (&header-date;)
1060   unless the origin server does not have a clock that can provide a
1061   reasonable approximation of the current time.  If a 200 response
1062   to the same request would have included any of the header fields
1063   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1064   Vary, then those same header fields &MUST; be sent in a 304 response.
1065</t>
1066<t>
1067   Since the goal of a 304 response is to minimize information transfer
1068   when the recipient already has one or more cached representations,
1069   the response &SHOULD-NOT; include representation metadata other
1070   than the above listed fields unless said metadata exists for the
1071   purpose of guiding cache updates (e.g., future HTTP extensions).
1072</t>
1073<t>
1074   If the recipient of a 304 response does not have a cached representation
1075   corresponding to the entity-tag indicated by the 304 response, then the
1076   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1077   conditional request originated with an outbound client, such as a
1078   user agent with its own cache sending a conditional GET to a shared
1079   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1080   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1081   the request without any preconditions.
1082</t>
1083<t>
1084   If a cache uses a received 304 response to update a cache entry, the
1085   cache &MUST; update the entry to reflect any new field values given in
1086   the response.
1087</t>
1088</section>
1089
1090<section title="412 Precondition Failed" anchor="status.412">
1091  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1092  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1093<t>
1094   The 412 status code indicates that one or more preconditions given in
1095   the request header fields evaluated to false when tested on the server.
1096   This response code allows the client to place preconditions on the
1097   current resource state (its current representations and metadata)
1098   and thus prevent the request method from being applied if the target
1099   resource is in an unexpected state.
1100</t>
1101</section>
1102</section>
1103
1104<section title="IANA Considerations" anchor="IANA.considerations">
1105
1106<section title="Status Code Registration" anchor="status.code.registration">
1107<t>
1108   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1109   shall be updated with the registrations below:
1110</t>
1111<?BEGININC p4-conditional.iana-status-codes ?>
1112<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1113<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1114   <ttcol>Value</ttcol>
1115   <ttcol>Description</ttcol>
1116   <ttcol>Reference</ttcol>
1117   <c>304</c>
1118   <c>Not Modified</c>
1119   <c>
1120      <xref target="status.304"/>
1121   </c>
1122   <c>412</c>
1123   <c>Precondition Failed</c>
1124   <c>
1125      <xref target="status.412"/>
1126   </c>
1127</texttable>
1128<!--(END)-->
1129<?ENDINC p4-conditional.iana-status-codes ?>
1130</section>
1131
1132<section title="Header Field Registration" anchor="header.field.registration">
1133<t>
1134   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1135   with the permanent registrations below (see <xref target="RFC3864"/>):
1136</t>
1137<?BEGININC p4-conditional.iana-headers ?>
1138<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1139<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1140   <ttcol>Header Field Name</ttcol>
1141   <ttcol>Protocol</ttcol>
1142   <ttcol>Status</ttcol>
1143   <ttcol>Reference</ttcol>
1144
1145   <c>ETag</c>
1146   <c>http</c>
1147   <c>standard</c>
1148   <c>
1149      <xref target="header.etag"/>
1150   </c>
1151   <c>If-Match</c>
1152   <c>http</c>
1153   <c>standard</c>
1154   <c>
1155      <xref target="header.if-match"/>
1156   </c>
1157   <c>If-Modified-Since</c>
1158   <c>http</c>
1159   <c>standard</c>
1160   <c>
1161      <xref target="header.if-modified-since"/>
1162   </c>
1163   <c>If-None-Match</c>
1164   <c>http</c>
1165   <c>standard</c>
1166   <c>
1167      <xref target="header.if-none-match"/>
1168   </c>
1169   <c>If-Unmodified-Since</c>
1170   <c>http</c>
1171   <c>standard</c>
1172   <c>
1173      <xref target="header.if-unmodified-since"/>
1174   </c>
1175   <c>Last-Modified</c>
1176   <c>http</c>
1177   <c>standard</c>
1178   <c>
1179      <xref target="header.last-modified"/>
1180   </c>
1181</texttable>
1182<!--(END)-->
1183<?ENDINC p4-conditional.iana-headers ?>
1184<t>
1185   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1186</t>
1187</section>
1188</section>
1189
1190<section title="Security Considerations" anchor="security.considerations">
1191<t>
1192   No additional security considerations have been identified beyond
1193   those applicable to HTTP in general &messaging;.
1194</t>
1195</section>
1196
1197<section title="Acknowledgments" anchor="acks">
1198<t>
1199  See &acks;.
1200</t>
1201</section>
1202</middle>
1203<back>
1204
1205<references title="Normative References">
1206
1207<reference anchor="Part1">
1208  <front>
1209    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1210    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1211      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1212      <address><email>fielding@gbiv.com</email></address>
1213    </author>
1214    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1215      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1216      <address><email>jg@freedesktop.org</email></address>
1217    </author>
1218    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1219      <organization abbrev="HP">Hewlett-Packard Company</organization>
1220      <address><email>JeffMogul@acm.org</email></address>
1221    </author>
1222    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1223      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1224      <address><email>henrikn@microsoft.com</email></address>
1225    </author>
1226    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1227      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1228      <address><email>LMM@acm.org</email></address>
1229    </author>
1230    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1231      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1232      <address><email>paulle@microsoft.com</email></address>
1233    </author>
1234    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1235      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1236      <address><email>timbl@w3.org</email></address>
1237    </author>
1238    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1239      <organization abbrev="W3C">World Wide Web Consortium</organization>
1240      <address><email>ylafon@w3.org</email></address>
1241    </author>
1242    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1243      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1244      <address><email>julian.reschke@greenbytes.de</email></address>
1245    </author>
1246    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1247  </front>
1248  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1249  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1250</reference>
1251
1252<reference anchor="Part2">
1253  <front>
1254    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1255    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1256      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1257      <address><email>fielding@gbiv.com</email></address>
1258    </author>
1259    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1260      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1261      <address><email>jg@freedesktop.org</email></address>
1262    </author>
1263    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1264      <organization abbrev="HP">Hewlett-Packard Company</organization>
1265      <address><email>JeffMogul@acm.org</email></address>
1266    </author>
1267    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1268      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1269      <address><email>henrikn@microsoft.com</email></address>
1270    </author>
1271    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1272      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1273      <address><email>LMM@acm.org</email></address>
1274    </author>
1275    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1276      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1277      <address><email>paulle@microsoft.com</email></address>
1278    </author>
1279    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1280      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1281      <address><email>timbl@w3.org</email></address>
1282    </author>
1283    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1284      <organization abbrev="W3C">World Wide Web Consortium</organization>
1285      <address><email>ylafon@w3.org</email></address>
1286    </author>
1287    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1288      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1289      <address><email>julian.reschke@greenbytes.de</email></address>
1290    </author>
1291    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1292  </front>
1293  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1294  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
1295</reference>
1296
1297<reference anchor="Part3">
1298  <front>
1299    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1300    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1301      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1302      <address><email>fielding@gbiv.com</email></address>
1303    </author>
1304    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1305      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1306      <address><email>jg@freedesktop.org</email></address>
1307    </author>
1308    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1309      <organization abbrev="HP">Hewlett-Packard Company</organization>
1310      <address><email>JeffMogul@acm.org</email></address>
1311    </author>
1312    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1313      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1314      <address><email>henrikn@microsoft.com</email></address>
1315    </author>
1316    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1317      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1318      <address><email>LMM@acm.org</email></address>
1319    </author>
1320    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1321      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1322      <address><email>paulle@microsoft.com</email></address>
1323    </author>
1324    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1325      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1326      <address><email>timbl@w3.org</email></address>
1327    </author>
1328    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1329      <organization abbrev="W3C">World Wide Web Consortium</organization>
1330      <address><email>ylafon@w3.org</email></address>
1331    </author>
1332    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1333      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1334      <address><email>julian.reschke@greenbytes.de</email></address>
1335    </author>
1336    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1337  </front>
1338  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1339  <x:source href="p3-payload.xml" basename="p3-payload"/>
1340</reference>
1341
1342<reference anchor="Part5">
1343  <front>
1344    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1345    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1346      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1347      <address><email>fielding@gbiv.com</email></address>
1348    </author>
1349    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1350      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1351      <address><email>jg@freedesktop.org</email></address>
1352    </author>
1353    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1354      <organization abbrev="HP">Hewlett-Packard Company</organization>
1355      <address><email>JeffMogul@acm.org</email></address>
1356    </author>
1357    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1358      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1359      <address><email>henrikn@microsoft.com</email></address>
1360    </author>
1361    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1362      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1363      <address><email>LMM@acm.org</email></address>
1364    </author>
1365    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1366      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1367      <address><email>paulle@microsoft.com</email></address>
1368    </author>
1369    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1370      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1371      <address><email>timbl@w3.org</email></address>
1372    </author>
1373    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1374      <organization abbrev="W3C">World Wide Web Consortium</organization>
1375      <address><email>ylafon@w3.org</email></address>
1376    </author>
1377    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1378      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1379      <address><email>julian.reschke@greenbytes.de</email></address>
1380    </author>
1381    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1382  </front>
1383  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1384  <x:source href="p5-range.xml" basename="p5-range"/>
1385</reference>
1386
1387<reference anchor="Part6">
1388  <front>
1389    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1390    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1391      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1392      <address><email>fielding@gbiv.com</email></address>
1393    </author>
1394    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1395      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1396      <address><email>jg@freedesktop.org</email></address>
1397    </author>
1398    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1399      <organization abbrev="HP">Hewlett-Packard Company</organization>
1400      <address><email>JeffMogul@acm.org</email></address>
1401    </author>
1402    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1403      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1404      <address><email>henrikn@microsoft.com</email></address>
1405    </author>
1406    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1407      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1408      <address><email>LMM@acm.org</email></address>
1409    </author>
1410    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1411      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1412      <address><email>paulle@microsoft.com</email></address>
1413    </author>
1414    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1415      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1416      <address><email>timbl@w3.org</email></address>
1417    </author>
1418    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1419      <organization abbrev="W3C">World Wide Web Consortium</organization>
1420      <address><email>ylafon@w3.org</email></address>
1421    </author>
1422    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1423      <address><email>mnot@mnot.net</email></address>
1424    </author>
1425    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1426      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1427      <address><email>julian.reschke@greenbytes.de</email></address>
1428    </author>
1429    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1430  </front>
1431  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1432  <x:source href="p6-cache.xml" basename="p6-cache"/>
1433</reference>
1434
1435<reference anchor="RFC2119">
1436  <front>
1437    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1438    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1439      <organization>Harvard University</organization>
1440      <address><email>sob@harvard.edu</email></address>
1441    </author>
1442    <date month="March" year="1997"/>
1443  </front>
1444  <seriesInfo name="BCP" value="14"/>
1445  <seriesInfo name="RFC" value="2119"/>
1446</reference>
1447
1448<reference anchor="RFC5234">
1449  <front>
1450    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1451    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1452      <organization>Brandenburg InternetWorking</organization>
1453      <address>
1454        <email>dcrocker@bbiw.net</email>
1455      </address> 
1456    </author>
1457    <author initials="P." surname="Overell" fullname="Paul Overell">
1458      <organization>THUS plc.</organization>
1459      <address>
1460        <email>paul.overell@thus.net</email>
1461      </address>
1462    </author>
1463    <date month="January" year="2008"/>
1464  </front>
1465  <seriesInfo name="STD" value="68"/>
1466  <seriesInfo name="RFC" value="5234"/>
1467</reference>
1468
1469</references>
1470
1471<references title="Informative References">
1472
1473<reference anchor="RFC2616">
1474  <front>
1475    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1476    <author initials="R." surname="Fielding" fullname="R. Fielding">
1477      <organization>University of California, Irvine</organization>
1478      <address><email>fielding@ics.uci.edu</email></address>
1479    </author>
1480    <author initials="J." surname="Gettys" fullname="J. Gettys">
1481      <organization>W3C</organization>
1482      <address><email>jg@w3.org</email></address>
1483    </author>
1484    <author initials="J." surname="Mogul" fullname="J. Mogul">
1485      <organization>Compaq Computer Corporation</organization>
1486      <address><email>mogul@wrl.dec.com</email></address>
1487    </author>
1488    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1489      <organization>MIT Laboratory for Computer Science</organization>
1490      <address><email>frystyk@w3.org</email></address>
1491    </author>
1492    <author initials="L." surname="Masinter" fullname="L. Masinter">
1493      <organization>Xerox Corporation</organization>
1494      <address><email>masinter@parc.xerox.com</email></address>
1495    </author>
1496    <author initials="P." surname="Leach" fullname="P. Leach">
1497      <organization>Microsoft Corporation</organization>
1498      <address><email>paulle@microsoft.com</email></address>
1499    </author>
1500    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1501      <organization>W3C</organization>
1502      <address><email>timbl@w3.org</email></address>
1503    </author>
1504    <date month="June" year="1999"/>
1505  </front>
1506  <seriesInfo name="RFC" value="2616"/>
1507</reference>
1508
1509<reference anchor='RFC3864'>
1510  <front>
1511    <title>Registration Procedures for Message Header Fields</title>
1512    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1513      <organization>Nine by Nine</organization>
1514      <address><email>GK-IETF@ninebynine.org</email></address>
1515    </author>
1516    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1517      <organization>BEA Systems</organization>
1518      <address><email>mnot@pobox.com</email></address>
1519    </author>
1520    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1521      <organization>HP Labs</organization>
1522      <address><email>JeffMogul@acm.org</email></address>
1523    </author>
1524    <date year='2004' month='September' />
1525  </front>
1526  <seriesInfo name='BCP' value='90' />
1527  <seriesInfo name='RFC' value='3864' />
1528</reference>
1529
1530<reference anchor='RFC4918'>
1531  <front>
1532    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1533    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1534      <organization abbrev="CommerceNet">CommerceNet</organization>
1535      <address><email>ldusseault@commerce.net</email></address>
1536    </author>
1537    <date month="June" year="2007" />
1538  </front>
1539  <seriesInfo name='RFC' value='4918' />
1540</reference>
1541</references>
1542
1543<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1544<t>
1545  Allow weak entity-tags in all requests except range requests (Sections
1546  <xref target="weak.and.strong.validators" format="counter"/> and
1547  <xref target="header.if-none-match" format="counter"/>).
1548</t>
1549<t>
1550  Change ABNF productions for header fields to only define the field value.
1551  (<xref target="header.field.definitions"/>)
1552</t>
1553</section>
1554
1555<?BEGININC p4-conditional.abnf-appendix ?>
1556<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1557<figure>
1558<artwork type="abnf" name="p4-conditional.parsed-abnf">
1559<x:ref>ETag</x:ref> = entity-tag
1560
1561<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
1562
1563<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1564 entity-tag ] ) )
1565<x:ref>If-Modified-Since</x:ref> = HTTP-date
1566<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1567 entity-tag ] ) )
1568<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1569
1570<x:ref>Last-Modified</x:ref> = HTTP-date
1571
1572<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1573
1574<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1575
1576<x:ref>opaque-tag</x:ref> = quoted-string
1577
1578<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
1579
1580<x:ref>weak</x:ref> = %x57.2F ; W/
1581</artwork>
1582</figure>
1583<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1584; ETag defined but not used
1585; If-Match defined but not used
1586; If-Modified-Since defined but not used
1587; If-None-Match defined but not used
1588; If-Unmodified-Since defined but not used
1589; Last-Modified defined but not used
1590</artwork></figure></section>
1591<?ENDINC p4-conditional.abnf-appendix ?>
1592
1593<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1594
1595<section title="Since RFC 2616">
1596<t>
1597  Extracted relevant partitions from <xref target="RFC2616"/>.
1598</t>
1599</section>
1600
1601<section title="Since draft-ietf-httpbis-p4-conditional-00">
1602<t>
1603  Closed issues:
1604  <list style="symbols"> 
1605    <t>
1606      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1607      "Normative and Informative references"
1608    </t>
1609  </list>
1610</t>
1611<t>
1612  Other changes:
1613  <list style="symbols"> 
1614    <t>
1615      Move definitions of 304 and 412 condition codes from Part2.
1616    </t>
1617  </list>
1618</t>
1619</section>
1620
1621<section title="Since draft-ietf-httpbis-p4-conditional-01">
1622<t>
1623  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1624  <list style="symbols"> 
1625    <t>
1626      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1627    </t>
1628  </list>
1629</t>
1630</section>
1631
1632<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1633<t>
1634  Closed issues:
1635  <list style="symbols"> 
1636    <t>
1637      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1638      "Weak ETags on non-GET requests"
1639    </t>
1640  </list>
1641</t>
1642<t>
1643  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1644  <list style="symbols"> 
1645    <t>
1646      Reference RFC 3984, and update header field registrations for header fields defined
1647      in this document.
1648    </t>
1649  </list>
1650</t>
1651</section>
1652
1653<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1654<t>
1655  Closed issues:
1656  <list style="symbols"> 
1657    <t>
1658      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1659      "Examples for ETag matching"
1660    </t>
1661    <t>
1662      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1663      "'entity value' undefined"
1664    </t>
1665    <t>
1666      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1667      "bogus 2068 Date header reference"
1668    </t>
1669  </list>
1670</t>
1671</section>
1672
1673<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1674<t>
1675  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1676  <list style="symbols"> 
1677    <t>
1678      Use "/" instead of "|" for alternatives.
1679    </t>
1680    <t>
1681      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1682      whitespace ("OWS") and required whitespace ("RWS").
1683    </t>
1684    <t>
1685      Rewrite ABNFs to spell out whitespace rules, factor out
1686      header field value format definitions.
1687    </t>
1688  </list>
1689</t>
1690</section>
1691
1692<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1693<t>
1694  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1695  <list style="symbols"> 
1696    <t>
1697      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1698    </t>
1699  </list>
1700</t>
1701</section>
1702
1703<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1704<t>
1705  Closed issues:
1706  <list style="symbols"> 
1707    <t>
1708      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1709      "case-sensitivity of etag weakness indicator"
1710    </t>
1711  </list>
1712</t>
1713</section>
1714
1715<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1716<t>
1717  Closed issues:
1718  <list style="symbols"> 
1719    <t>
1720      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1721      "Weak ETags on non-GET requests" (If-Match still was defined to require
1722      strong matching)
1723    </t>
1724    <t>
1725      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1726      "move IANA registrations for optional status codes"
1727    </t>
1728  </list>
1729</t>
1730</section>
1731
1732<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1733<t>
1734  No significant changes.
1735</t>
1736</section>
1737
1738<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1739<t>
1740  No significant changes.
1741</t>
1742</section>
1743
1744<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1745<t>
1746  Closed issues:
1747  <list style="symbols"> 
1748    <t>
1749      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1750      "Clarify 'Requested Variant'"
1751    </t>
1752    <t>
1753      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1754      "Clarify entity / representation / variant terminology"
1755    </t>
1756    <t>
1757      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1758      "consider removing the 'changes from 2068' sections"
1759    </t>
1760  </list>
1761</t>
1762</section>
1763
1764<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1765<t>
1766  None.
1767</t>
1768</section>
1769
1770<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1771<t>
1772  Closed issues:
1773  <list style="symbols"> 
1774    <t>
1775      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1776      "Header Classification"
1777    </t>
1778  </list>
1779</t>
1780</section>
1781
1782<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1783<t>
1784  Closed issues:
1785  <list style="symbols"> 
1786    <t>
1787      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1788      "If-* and entities"
1789    </t>
1790    <t>
1791      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1792      "Definition of validator weakness"
1793    </t>
1794    <t>
1795      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1796      "untangle ABNFs for header fields"
1797    </t>
1798    <t>
1799      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1800      "ETags and Quotes"
1801    </t>
1802  </list>
1803</t>
1804</section>
1805
1806<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1807<t>
1808  None.
1809</t>
1810</section>
1811
1812<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1813<t>
1814  Closed issues:
1815  <list style="symbols"> 
1816    <t>
1817      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1818      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1819    </t>
1820  </list>
1821</t>
1822</section>
1823
1824<section title="Since draft-ietf-httpbis-p4-conditional-16" anchor="changes.since.16">
1825<t>
1826  None yet.
1827</t>
1828</section>
1829
1830</section>
1831
1832</back>
1833</rfc>
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