source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1465

Last change on this file since 1465 was 1452, checked in by julian.reschke@…, 8 years ago

Rephrase description of conformance; explain how the spec handles error handling (see #186)

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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 "October">
16  <!ENTITY ID-YEAR "2011">
17  <!ENTITY architecture               "<xref target='Part1' x:rel='#architecture' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY notation                   "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY acks                       "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY field-rules                "<xref target='Part1' x:rel='#field.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-date                "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-accept-encoding     "<xref target='Part3' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY http-date                  "<xref target='Part2' x:rel='#http.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="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
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   This document defines conformance criteria for several roles in HTTP
265   communication, including Senders, Recipients, Clients, Servers, User-Agents,
266   Origin Servers, Intermediaries, Proxies and Gateways. See &architecture;
267   for definitions of these terms.
268</t>
269<t>
270   An implementation is considered conformant if it complies with all of the
271   requirements associated with its role(s). Note that SHOULD-level requirements
272   are relevant here, unless one of the documented exceptions is applicable.
273</t>
274<t>
275   This document also uses ABNF to define valid protocol elements
276   (<xref target="notation"/>). In addition to the prose requirements placed
277   upon them, Senders &MUST-NOT; generate protocol elements that are invalid.
278</t>
279<t>
280   Unless noted otherwise, Recipients &MAY; take steps to recover a usable
281   protocol element from an invalid construct. However, HTTP does not define
282   specific error handling mechanisms, except in cases where it has direct
283   impact on security. This is because different uses of the protocol require
284   different error handling strategies; for example, a Web browser may wish to
285   transparently recover from a response where the Location header field
286   doesn't parse according to the ABNF, whereby in a systems control protocol
287   using HTTP, this type of error recovery could lead to dangerous consequences.
288</t>
289</section>
290
291<section title="Syntax Notation" anchor="notation">
292  <x:anchor-alias value="ALPHA"/>
293  <x:anchor-alias value="CR"/>
294  <x:anchor-alias value="DIGIT"/>
295  <x:anchor-alias value="LF"/>
296  <x:anchor-alias value="OCTET"/>
297  <x:anchor-alias value="VCHAR"/>
298  <x:anchor-alias value="core.rules"/>
299  <x:anchor-alias value="quoted-string"/>
300  <x:anchor-alias value="OWS"/>
301  <x:anchor-alias value="HTTP-date"/>
302<t>
303  This specification uses the ABNF syntax defined in &notation; (which
304  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
305  <xref target="collected.abnf"/> shows the collected ABNF, with the list
306  rule expanded.
307</t>
308<t>
309  The following core rules are included by
310  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
311  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
312  DIGIT (decimal 0-9), DQUOTE (double quote),
313  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
314  OCTET (any 8-bit sequence of data), SP (space), and
315  VCHAR (any visible US-ASCII character).
316</t>
317<t>
318  The ABNF rules below are defined in <xref target="Part1"/> and
319  <xref target="Part2"/>:
320</t>
321<figure><artwork type="abnf2616">
322  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
323  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-rules;&gt;
324  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
325</artwork></figure>
326</section>
327</section>
328
329<section title="Validators" anchor="validators">
330   <iref primary="true" item="metadata"/>
331   <iref primary="true" item="validator"/>
332<t>
333   This specification defines two forms of metadata that are commonly used
334   to observe resource state and test for preconditions: modification dates
335   and opaque entity tags.  Additional metadata that reflects resource state
336   has been defined by various extensions of HTTP, such as WebDAV
337   <xref target="RFC4918"/>, that are beyond the scope of this specification.
338   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
339   when it is used within a precondition.
340</t>
341
342<section title="Weak versus Strong" anchor="weak.and.strong.validators">
343   <iref primary="true" item="validator" subitem="weak"/>
344   <iref primary="true" item="validator" subitem="strong"/>
345<t>
346   Validators come in two flavors: strong or weak.  Weak validators are easy
347   to generate but are far less useful for comparisons.  Strong validators
348   are ideal for comparisons but can be very difficult (and occasionally
349   impossible) to generate efficiently.  Rather than impose that all forms
350   of resource adhere to the same strength of validator, HTTP exposes the
351   type of validator in use and imposes restrictions on when weak validators
352   can be used as preconditions.
353</t>
354<t>
355   A "strong validator" is a representation metadata value that &MUST; be
356   changed to a new, previously unused or guaranteed unique, value whenever
357   a change occurs to the representation data such that a change would be
358   observable in the payload body of a 200 response to GET.  A strong
359   validator &MAY; be changed for other reasons, such as when a semantically
360   significant part of the representation metadata is changed (e.g.,
361   Content-Type), but it is in the best interests of the origin server to only
362   change the value when it is necessary to invalidate the stored responses
363   held by remote caches and authoring tools.  A strong validator &MUST; be
364   unique across all representations of a given resource, such that no two
365   representations of that resource share the same validator unless
366   their payload body would be identical.
367</t>
368<t>
369   Cache entries might persist for arbitrarily long periods, regardless
370   of expiration times.  Thus, a cache might attempt to validate an
371   entry using a validator that it obtained in the distant past.
372   A strong validator &MUST; be unique across all versions of all
373   representations associated with a particular resource over time.
374   However, there is no implication of uniqueness across representations
375   of different resources (i.e., the same strong validator might be
376   in use for representations of multiple resources at the same time
377   and does not imply that those representations are equivalent).
378</t>
379<t>
380   There are a variety of strong validators used in practice.  The best are
381   based on strict revision control, wherein each change to a representation
382   always results in a unique node name and revision identifier being assigned
383   before the representation is made accessible to GET.  A cryptographic hash
384   function applied to the representation data is also sufficient if the data
385   is available prior to the response header fields being sent and the digest
386   does not need to be recalculated every time a validation request is
387   received.  However, if a resource has distinct representations that differ
388   only in their metadata, such as might occur with content negotiation over
389   media types that happen to share the same data format, then a server
390   &SHOULD; incorporate additional information in the validator to
391   distinguish those representations and avoid confusing cache behavior.
392</t>
393<t>
394   In contrast, a "weak validator" is a representation metadata value that
395   might not be changed for every change to the representation data.  This
396   weakness might be due to limitations in how the value is calculated, such
397   as clock resolution or an inability to ensure uniqueness for all possible
398   representations of the resource, or due to a desire by the resource owner
399   to group representations by some self-determined set of equivalency
400   rather than unique sequences of data.  A weak entity-tag &SHOULD; change
401   whenever the origin server considers prior representations to be
402   unacceptable as a substitute for the current representation. In other
403   words, a weak entity-tag &SHOULD; change whenever the origin server wants
404   caches to invalidate old responses.
405</t>
406<t>
407   For example, the representation of a weather report that changes in
408   content every second, based on dynamic measurements, might be grouped
409   into sets of equivalent representations (from the origin server's
410   perspective) with the same weak validator in order to allow cached
411   representations to be valid for a reasonable period of time (perhaps
412   adjusted dynamically based on server load or weather quality).
413   Likewise, a representation's modification time, if defined with only
414   one-second resolution, might be a weak validator if it is possible
415   for the representation to be modified twice during a single second and
416   retrieved between those modifications.
417</t>
418<t>
419   A "use" of a validator occurs when either a client generates a request
420   and includes the validator in a precondition or when a server
421   compares two validators.
422   Weak validators are only usable in contexts that do not depend on exact
423   equality of a representation's payload body.
424   Strong validators are usable and preferred for all conditional requests,
425   including cache validation, partial content ranges, and "lost update"
426   avoidance.
427</t>
428</section>
429
430<section title="Last-Modified" anchor="header.last-modified">
431  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
432  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
433  <x:anchor-alias value="Last-Modified"/>
434<t>
435   The "Last-Modified" header field indicates the date and time at
436   which the origin server believes the selected representation was
437   last modified.
438</t>
439<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
440  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
441</artwork></figure>
442<t>
443   An example of its use is
444</t>
445<figure><artwork type="example">
446  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
447</artwork></figure>
448
449<section title="Generation" anchor="lastmod.generation">
450<t>
451   Origin servers &SHOULD; send Last-Modified for any selected
452   representation for which a last modification date can be reasonably
453   and consistently determined, since its use in conditional requests
454   and evaluating cache freshness (&caching;) results in a substantial
455   reduction of HTTP traffic on the Internet and can be a significant
456   factor in improving service scalability and reliability.
457</t>
458<t>
459   A representation is typically the sum of many parts behind the
460   resource interface.  The last-modified time would usually be
461   the most recent time that any of those parts were changed.
462   How that value is determined for any given resource is an
463   implementation detail beyond the scope of this specification.
464   What matters to HTTP is how recipients of the Last-Modified
465   header field can use its value to make conditional requests
466   and test the validity of locally cached responses.
467</t>
468<t>
469   An origin server &SHOULD; obtain the Last-Modified value of the
470   representation as close as possible to the time that it generates
471   the Date field-value for its response. This allows a recipient to
472   make an accurate assessment of the representation's modification time,
473   especially if the representation changes near the time that the
474   response is generated.
475</t>
476<t>
477   An origin server with a clock &MUST-NOT; send a Last-Modified date
478   that is later than the server's time of message origination (Date).
479   If the last modification time is derived from implementation-specific
480   metadata that evaluates to some time in the future, according to the
481   origin server's clock, then the origin server &MUST; replace that
482   value with the message origination date. This prevents a future
483   modification date from having an adverse impact on cache validation.
484</t>
485<t>
486   An origin server without a clock &MUST-NOT; assign Last-Modified
487   values to a response unless these values were associated
488   with the resource by some other system or user with a reliable clock.
489</t>
490</section>
491
492<section title="Comparison" anchor="lastmod.comparison">
493<t>
494   A Last-Modified time, when used as a validator in a request, is
495   implicitly weak unless it is possible to deduce that it is strong,
496   using the following rules:
497  <list style="symbols">
498     <t>The validator is being compared by an origin server to the
499        actual current validator for the representation and,</t>
500     <t>That origin server reliably knows that the associated representation did
501        not change twice during the second covered by the presented
502        validator.</t>
503  </list>
504</t>
505<t>
506   or
507  <list style="symbols">
508     <t>The validator is about to be used by a client in an If-Modified-Since,
509        If-Unmodified-Since header field, because the client has a cache entry,
510        or If-Range for the associated representation, and</t>
511     <t>That cache entry includes a Date value, which gives the time
512        when the origin server sent the original response, and</t>
513     <t>The presented Last-Modified time is at least 60 seconds before
514        the Date value.</t>
515  </list>
516</t>
517<t>
518   or
519  <list style="symbols">
520     <t>The validator is being compared by an intermediate cache to the
521        validator stored in its cache entry for the representation, and</t>
522     <t>That cache entry includes a Date value, which gives the time
523        when the origin server sent the original response, and</t>
524     <t>The presented Last-Modified time is at least 60 seconds before
525        the Date value.</t>
526  </list>
527</t>
528<t>
529   This method relies on the fact that if two different responses were
530   sent by the origin server during the same second, but both had the
531   same Last-Modified time, then at least one of those responses would
532   have a Date value equal to its Last-Modified time. The arbitrary 60-second
533   limit guards against the possibility that the Date and Last-Modified
534   values are generated from different clocks, or at somewhat
535   different times during the preparation of the response. An
536   implementation &MAY; use a value larger than 60 seconds, if it is
537   believed that 60 seconds is too short.
538</t>
539</section>
540</section>
541
542<section title="ETag" anchor="header.etag">
543  <iref primary="true" item="ETag header field" x:for-anchor=""/>
544  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
545  <x:anchor-alias value="ETag"/>
546  <x:anchor-alias value="entity-tag"/>
547  <x:anchor-alias value="entity.tags"/>
548  <x:anchor-alias value="opaque-tag"/>
549  <x:anchor-alias value="weak"/>
550<t>
551   The ETag header field provides the current entity-tag for the
552   selected representation.
553   An entity-tag is an opaque validator for differentiating between
554   multiple representations of the same resource, regardless of whether
555   those multiple representations are due to resource state changes over
556   time, content negotiation resulting in multiple representations being
557   valid at the same time, or both. An entity-tag consists of an opaque
558   quoted string, possibly prefixed by a weakness indicator.
559</t>
560<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"/>
561  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
562
563  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
564  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
565  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
566</artwork></figure>
567<t>
568   An entity-tag can be more reliable for validation than a modification
569   date in situations where it is inconvenient to store modification
570   dates, where the one-second resolution of HTTP date values is not
571   sufficient, or where modification dates are not consistently maintained.
572</t>
573<figure><preamble>
574  Examples:
575</preamble>
576<artwork type="example">
577  ETag: "xyzzy"
578  ETag: W/"xyzzy"
579  ETag: ""
580</artwork></figure>
581<t>
582   An entity-tag can be either a weak or strong validator, with
583   strong being the default.  If an origin server provides an entity-tag
584   for a representation and the generation of that entity-tag does not satisfy
585   the requirements for a strong validator
586   (<xref target="weak.and.strong.validators"/>), then that
587   entity-tag &MUST; be marked as weak by prefixing its opaque value
588   with "W/" (case-sensitive).
589</t>
590
591<section title="Generation" anchor="entity.tag.generation">
592<t>
593   The principle behind entity-tags is that only the service author
594   knows the implementation of a resource well enough to select the
595   most accurate and efficient validation mechanism for that resource,
596   and that any such mechanism can be mapped to a simple sequence of
597   octets for easy comparison.  Since the value is opaque, there is no
598   need for the client to be aware of how each entity-tag is constructed.
599</t>
600<t>
601   For example, a resource that has implementation-specific versioning
602   applied to all changes might use an internal revision number, perhaps
603   combined with a variance identifier for content negotiation, to
604   accurately differentiate between representations.
605   Other implementations might use a stored hash of representation content,
606   a combination of various filesystem attributes, or a modification
607   timestamp that has sub-second resolution.
608</t>
609<t>
610   Origin servers &SHOULD; send ETag for any selected representation
611   for which detection of changes can be reasonably and consistently
612   determined, since the entity-tag's use in conditional requests and
613   evaluating cache freshness (&caching;) can result in a substantial
614   reduction of HTTP network traffic and can be a significant factor in
615   improving service scalability and reliability.
616</t>
617</section>
618
619<section title="Comparison" anchor="entity.tag.comparison">
620  <x:anchor-alias value="validator.comparison"/>
621<t>
622   There are two entity-tag comparison functions, depending
623   on whether the comparison context allows the use of weak validators
624   or not:
625  <list style="symbols">
626     <t>The strong comparison function: in order to be considered equal,
627        both opaque-tags &MUST; be identical character-by-character, and both
628        &MUST-NOT; be weak.</t>
629     <t>The weak comparison function: in order to be considered equal, both
630        opaque-tags &MUST; be identical character-by-character, but
631        either or both of them &MAY; be tagged as "weak" without affecting
632        the result.</t>
633  </list>
634</t>
635<t>
636   The example below shows the results for a set of entity-tag pairs,
637   and both the weak and strong comparison function results:
638</t>
639<texttable align="left">
640  <ttcol>ETag 1</ttcol>
641  <ttcol>ETag 2</ttcol>
642  <ttcol>Strong Comparison</ttcol>
643  <ttcol>Weak Comparison</ttcol>
644
645  <c>W/"1"</c>
646  <c>W/"1"</c>
647  <c>no match</c>
648  <c>match</c>
649 
650  <c>W/"1"</c>
651  <c>W/"2"</c>
652  <c>no match</c>
653  <c>no match</c>
654
655  <c>W/"1"</c>
656  <c>"1"</c>
657  <c>no match</c>
658  <c>match</c>
659
660  <c>"1"</c>
661  <c>"1"</c>
662  <c>match</c>
663  <c>match</c>
664</texttable>
665</section>
666
667<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
668<t>
669   Consider a resource that is subject to content negotiation (&content-negotiation;),
670   and where the representations returned upon a GET request vary based on
671   the Accept-Encoding request header field (&header-accept-encoding;):
672</t>
673<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
674GET /index HTTP/1.1
675Host: www.example.com
676Accept-Encoding: gzip
677
678</artwork></figure>
679<t>
680   In this case, the response might or might not use the gzip content coding.
681   If it does not, the response might look like:
682</t>
683<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
684HTTP/1.1 200 OK
685Date: Thu, 26 Mar 2010 00:05:00 GMT
686ETag: "123-a"
687Content-Length: <x:length-of target="exbody"/>
688Vary: Accept-Encoding
689Content-Type: text/plain
690
691<x:span anchor="exbody">Hello World!
692Hello World!
693Hello World!
694Hello World!
695Hello World!
696</x:span></artwork></figure>
697<t>
698   An alternative representation that does use gzip content coding would be:
699</t>
700<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
701HTTP/1.1 200 OK
702Date: Thu, 26 Mar 2010 00:05:00 GMT
703ETag: "123-b"
704Content-Length: 43
705Vary: Accept-Encoding
706Content-Type: text/plain
707Content-Encoding: gzip
708
709<spanx>...binary data...</spanx></artwork></figure>
710<x:note>
711  <t>
712    <x:h>Note:</x:h> Content codings are a property of the representation,
713    so therefore an entity-tag of an encoded representation must be distinct
714    from an unencoded representation to prevent conflicts during cache updates
715    and range requests.  In contrast, transfer codings (&transfer-codings;)
716    apply only during message transfer and do not require distinct entity-tags.
717  </t>
718</x:note>
719</section>
720</section>
721
722<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">
723<t>
724   We adopt a set of rules and recommendations for origin servers,
725   clients, and caches regarding when various validator types ought to
726   be used, and for what purposes.
727</t>
728<t>
729   HTTP/1.1 origin servers:
730  <list style="symbols">
731     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
732        generate one.</t>
733
734     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
735        performance considerations support the use of weak entity-tags,
736        or if it is unfeasible to send a strong entity-tag.</t>
737
738     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
739  </list>
740</t>
741<t>
742   In other words, the preferred behavior for an HTTP/1.1 origin server
743   is to send both a strong entity-tag and a Last-Modified value.
744</t>
745<t>
746   HTTP/1.1 clients:
747  <list style="symbols">
748     <t>&MUST; use that entity-tag in any cache-conditional request (using
749        If-Match or If-None-Match) if an entity-tag has been provided by the
750        origin server.</t>
751
752     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
753        requests (using If-Modified-Since) if only a Last-Modified value has
754        been provided by the origin server. </t>
755
756     <t>&MAY; use the Last-Modified value in subrange cache-conditional
757        requests (using If-Unmodified-Since) if only a Last-Modified value has
758        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
759        provide a way to disable this, in case of difficulty.</t>
760
761     <t>&SHOULD; use both validators in cache-conditional requests if both an
762        entity-tag and a Last-Modified value have been provided by the origin
763        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
764        appropriately.</t>
765  </list>
766</t>
767<t>
768   An HTTP/1.1 origin server, upon receiving a conditional request that
769   includes both a Last-Modified date (e.g., in an If-Modified-Since or
770   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
771   in an If-Match, If-None-Match, or If-Range header field) as cache
772   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
773   unless doing so is consistent with all of the conditional header
774   fields in the request.
775</t>
776<t>
777   An HTTP/1.1 caching proxy, upon receiving a conditional request that
778   includes both a Last-Modified date and one or more entity-tags as
779   cache validators, &MUST-NOT; return a locally cached response to the
780   client unless that cached response is consistent with all of the
781   conditional header fields in the request.
782  <list><t>
783      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
784      servers and clients ought to transmit as much non-redundant
785      information as is available in their responses and requests.
786      HTTP/1.1 systems receiving this information will make the most
787      conservative assumptions about the validators they receive.
788  </t><t>
789      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
790      last-modified values received or used by these systems will
791      support transparent and efficient caching, and so HTTP/1.1 origin
792      servers should provide Last-Modified values. In those rare cases
793      where the use of a Last-Modified value as a validator by an
794      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
795      origin servers should not provide one.
796  </t></list>
797</t>
798</section>
799</section>
800
801<section title="Precondition Header Fields" anchor="header.field.definitions">
802<t>
803   This section defines the syntax and semantics of HTTP/1.1 header fields
804   for applying preconditions on requests.
805</t>
806
807<section title="If-Match" anchor="header.if-match">
808  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
809  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
810  <x:anchor-alias value="If-Match"/>
811<t>
812   The "If-Match" header field &MAY; be used to make a request method
813   conditional on the current existence or value of an entity-tag for
814   one or more representations of the target resource.  If-Match is
815   generally useful for resource update requests, such as PUT requests,
816   as a means for protecting against accidental overwrites when multiple
817   clients are acting in parallel on the same resource (i.e., the
818   "lost update" problem).  An If-Match field-value of "*" places the
819   precondition on the existence of any current representation for the
820   target resource.
821</t>
822<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
823  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
824</artwork></figure>
825<t>
826   If any of the entity-tags listed in the If-Match field value match
827   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
828   selected representation for the target resource,
829   or if "*" is given and any current representation exists for the
830   target resource, then the server &MAY; perform the request method
831   as if the If-Match header field was not present.
832</t>
833<t>
834   If none of the entity-tags match, or if "*" is given and no current
835   representation exists, the server &MUST-NOT; perform the requested method.
836   Instead, the server &MUST; respond with the 412 (Precondition Failed)
837   status code.
838</t>
839<t>
840   If the request would, without the If-Match header field, result in
841   anything other than a 2xx or 412 status code, then the If-Match header field
842   &MUST; be ignored.
843</t>
844<t>
845   Examples:
846</t>
847<figure><artwork type="example">
848  If-Match: "xyzzy"
849  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
850  If-Match: *
851</artwork></figure>
852<t>
853   The result of a request having both an If-Match header field and
854   either an If-None-Match or an If-Modified-Since header fields is
855   undefined by this specification.
856</t>
857</section>
858
859<section title="If-None-Match" anchor="header.if-none-match">
860  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
861  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
862  <x:anchor-alias value="If-None-Match"/>
863<t>
864   The "If-None-Match" header field &MAY; be used to make a request method
865   conditional on not matching any of the current entity-tag values for
866   representations of the target resource.  If-None-Match is primarily
867   used in conditional GET requests to enable efficient updates of cached
868   information with a minimum amount of transaction overhead.  A client
869   that has one or more representations previously obtained from the
870   target resource can send If-None-Match with a list of the associated
871   entity-tags in the hope of receiving a 304 response if at least one
872   of those representations matches the selected representation.
873</t>
874<t>
875   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
876   request method (e.g., PUT) from inadvertently modifying an existing
877   representation of the target resource when the client believes that
878   the resource does not have a current representation.  This is a variation
879   on the "lost update" problem that might arise if more than one client
880   attempts to create an initial representation for the target resource.
881</t>
882<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
883  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
884</artwork></figure>
885<t>
886   If any of the entity-tags listed in the If-None-Match field-value match
887   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
888   selected representation, or if "*" is
889   given and any current representation exists for that resource, then the
890   server &MUST-NOT; perform the requested method.
891   Instead, if the request method was GET or HEAD, the server &SHOULD;
892   respond with a 304 (Not Modified) status code, including the cache-related
893   header fields (particularly ETag) of the selected representation that has
894   a matching entity-tag.  For all other request methods, the server &MUST;
895   respond with a 412 (Precondition Failed) status code.
896</t>
897<t>
898   If none of the entity-tags match, then the server &MAY; perform the
899   requested method as if the If-None-Match header field did not exist,
900   but &MUST; also ignore any If-Modified-Since header field(s) in the
901   request. That is, if no entity-tags match, then the server &MUST-NOT;
902   return a 304 (Not Modified) response.
903</t>
904<t>
905   If the request would, without the If-None-Match header field, result
906   in anything other than a 2xx or 304 status code, then the If-None-Match
907   header field &MUST; be ignored. (See <xref
908   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
909   a discussion of server behavior when both If-Modified-Since and
910   If-None-Match appear in the same request.)
911</t>
912<t>
913   Examples:
914</t>
915<figure><artwork type="example">
916  If-None-Match: "xyzzy"
917  If-None-Match: W/"xyzzy"
918  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
919  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
920  If-None-Match: *
921</artwork></figure>
922<t>
923   The result of a request having both an If-None-Match header field and
924   either an If-Match or an If-Unmodified-Since header fields is
925   undefined by this specification.
926</t>
927</section>
928
929<section title="If-Modified-Since" anchor="header.if-modified-since">
930  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
931  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
932  <x:anchor-alias value="If-Modified-Since"/>
933<t>
934   The "If-Modified-Since" header field &MAY; be used to make a request
935   method conditional by modification date: if the selected representation
936   has not been modified since the time specified in this field, then
937   do not perform the request method; instead, respond as detailed below.
938</t>
939<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
940  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
941</artwork></figure>
942<t>
943   An example of the field is:
944</t>
945<figure><artwork type="example">
946  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
947</artwork></figure>
948<t>
949   A GET method with an If-Modified-Since header field and no Range header
950   field requests that the selected representation be transferred only if
951   it has been modified since the date given by the If-Modified-Since
952   header field.
953   The algorithm for determining this includes the following cases:
954  <list style="numbers">
955      <t>If the request would normally result in anything other than a
956         200 (OK) status code, or if the passed If-Modified-Since date is
957         invalid, the response is exactly the same as for a normal GET.
958         A date which is later than the server's current time is
959         invalid.</t>
960
961      <t>If the selected representation has been modified since the
962         If-Modified-Since date, the response is exactly the same as for
963         a normal GET.</t>
964
965      <t>If the selected representation has not been modified since a valid
966         If-Modified-Since date, the server &SHOULD; return a
967         304 (Not Modified) response.</t>
968  </list>
969</t>
970<t>
971   The purpose of this feature is to allow efficient updates of cached
972   information with a minimum amount of transaction overhead.
973  <list><t>
974      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
975      see &header-range; for full details.
976    </t><t>
977      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
978      clock might not be synchronized with the client.
979    </t><t>
980      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
981      servers will use an exact date comparison function, rather than a
982      less-than function, for deciding whether to send a 304 (Not
983      Modified) response. To get best results when sending an If-Modified-Since
984      header field for cache validation, clients are
985      advised to use the exact date string received in a previous Last-Modified
986      header field whenever possible.
987    </t><t>
988      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
989      header field instead of a date taken from the Last-Modified header field for
990      the same request, the client needs to be aware that this
991      date is interpreted in the server's understanding of time.
992      Unsynchronized clocks and rounding problems, due to the different
993      encodings of time between the client and server, are concerns.
994      This includes the possibility of race conditions if the
995      document has changed between the time it was first requested and
996      the If-Modified-Since date of a subsequent request, and the
997      possibility of clock-skew-related problems if the If-Modified-Since
998      date is derived from the client's clock without correction
999      to the server's clock. Corrections for different time bases
1000      between client and server are at best approximate due to network
1001      latency.
1002    </t>
1003  </list>
1004</t>
1005<t>
1006   The result of a request having both an If-Modified-Since header field
1007   and either an If-Match or an If-Unmodified-Since header fields is
1008   undefined by this specification.
1009</t>
1010</section>
1011
1012<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
1013  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
1014  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
1015  <x:anchor-alias value="If-Unmodified-Since"/>
1016<t>
1017   The "If-Unmodified-Since" header field &MAY; be used to make a request
1018   method conditional by modification date: if the selected representation
1019   has been modified since the time specified in this field, then the
1020   server &MUST-NOT; perform the requested operation and &MUST; instead
1021   respond with the 412 (Precondition Failed) status code.
1022   If the selected representation has not been modified since the time
1023   specified in this field, the server &SHOULD; perform the request
1024   method as if the If-Unmodified-Since header field were not present.
1025</t>
1026<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
1027  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
1028</artwork></figure>
1029<t>
1030   An example of the field is:
1031</t>
1032<figure><artwork type="example">
1033  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1034</artwork></figure>
1035<t>
1036   If the request normally (i.e., without the If-Unmodified-Since
1037   header field) would result in anything other than a 2xx or 412 status code,
1038   the If-Unmodified-Since header field &SHOULD; be ignored.
1039</t>
1040<t>
1041   If the specified date is invalid, the header field &MUST; be ignored.
1042</t>
1043<t>
1044   The result of a request having both an If-Unmodified-Since header
1045   field and either an If-None-Match or an If-Modified-Since header
1046   fields is undefined by this specification.
1047</t>
1048</section>
1049
1050<section title="If-Range" anchor="header.if-range">
1051<t>
1052   The If-Range header field provides a special conditional request
1053   mechanism that is similar to If-Match and If-Unmodified-Since but
1054   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1055</t>
1056</section>
1057
1058</section>
1059
1060<section title="Status Code Definitions" anchor="status.code.definitions">
1061<section title="304 Not Modified" anchor="status.304">
1062  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1063  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1064<t>
1065   The 304 status code indicates that a conditional GET request has been
1066   received and would have resulted in a 200 (OK) response if it were not
1067   for the fact that the condition has evaluated to false.  In other words,
1068   there is no need for the server to transfer a representation of the
1069   target resource because the client's request indicates that it already
1070   has a valid representation, as indicated by the 304 response header
1071   fields, and is therefore redirecting the client to make use of that
1072   stored representation as if it were the payload of a 200 response.
1073   The 304 response &MUST-NOT; contain a message-body, and thus is always
1074   terminated by the first empty line after the header fields.
1075</t>
1076<t>
1077   A 304 response &MUST; include a Date header field (&header-date;)
1078   unless the origin server does not have a clock that can provide a
1079   reasonable approximation of the current time.  If a 200 response
1080   to the same request would have included any of the header fields
1081   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1082   Vary, then those same header fields &MUST; be sent in a 304 response.
1083</t>
1084<t>
1085   Since the goal of a 304 response is to minimize information transfer
1086   when the recipient already has one or more cached representations,
1087   the response &SHOULD-NOT; include representation metadata other
1088   than the above listed fields unless said metadata exists for the
1089   purpose of guiding cache updates (e.g., future HTTP extensions).
1090</t>
1091<t>
1092   If the recipient of a 304 response does not have a cached representation
1093   corresponding to the entity-tag indicated by the 304 response, then the
1094   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1095   conditional request originated with an outbound client, such as a
1096   user agent with its own cache sending a conditional GET to a shared
1097   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1098   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1099   the request without any preconditions.
1100</t>
1101<t>
1102   If a cache uses a received 304 response to update a cache entry, the
1103   cache &MUST; update the entry to reflect any new field values given in
1104   the response.
1105</t>
1106</section>
1107
1108<section title="412 Precondition Failed" anchor="status.412">
1109  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1110  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1111<t>
1112   The 412 status code indicates that one or more preconditions given in
1113   the request header fields evaluated to false when tested on the server.
1114   This response code allows the client to place preconditions on the
1115   current resource state (its current representations and metadata)
1116   and thus prevent the request method from being applied if the target
1117   resource is in an unexpected state.
1118</t>
1119</section>
1120</section>
1121
1122<section title="IANA Considerations" anchor="IANA.considerations">
1123
1124<section title="Status Code Registration" anchor="status.code.registration">
1125<t>
1126   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1127   shall be updated with the registrations below:
1128</t>
1129<?BEGININC p4-conditional.iana-status-codes ?>
1130<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1131<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1132   <ttcol>Value</ttcol>
1133   <ttcol>Description</ttcol>
1134   <ttcol>Reference</ttcol>
1135   <c>304</c>
1136   <c>Not Modified</c>
1137   <c>
1138      <xref target="status.304"/>
1139   </c>
1140   <c>412</c>
1141   <c>Precondition Failed</c>
1142   <c>
1143      <xref target="status.412"/>
1144   </c>
1145</texttable>
1146<!--(END)-->
1147<?ENDINC p4-conditional.iana-status-codes ?>
1148</section>
1149
1150<section title="Header Field Registration" anchor="header.field.registration">
1151<t>
1152   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1153   with the permanent registrations below (see <xref target="RFC3864"/>):
1154</t>
1155<?BEGININC p4-conditional.iana-headers ?>
1156<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1157<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1158   <ttcol>Header Field Name</ttcol>
1159   <ttcol>Protocol</ttcol>
1160   <ttcol>Status</ttcol>
1161   <ttcol>Reference</ttcol>
1162
1163   <c>ETag</c>
1164   <c>http</c>
1165   <c>standard</c>
1166   <c>
1167      <xref target="header.etag"/>
1168   </c>
1169   <c>If-Match</c>
1170   <c>http</c>
1171   <c>standard</c>
1172   <c>
1173      <xref target="header.if-match"/>
1174   </c>
1175   <c>If-Modified-Since</c>
1176   <c>http</c>
1177   <c>standard</c>
1178   <c>
1179      <xref target="header.if-modified-since"/>
1180   </c>
1181   <c>If-None-Match</c>
1182   <c>http</c>
1183   <c>standard</c>
1184   <c>
1185      <xref target="header.if-none-match"/>
1186   </c>
1187   <c>If-Unmodified-Since</c>
1188   <c>http</c>
1189   <c>standard</c>
1190   <c>
1191      <xref target="header.if-unmodified-since"/>
1192   </c>
1193   <c>Last-Modified</c>
1194   <c>http</c>
1195   <c>standard</c>
1196   <c>
1197      <xref target="header.last-modified"/>
1198   </c>
1199</texttable>
1200<!--(END)-->
1201<?ENDINC p4-conditional.iana-headers ?>
1202<t>
1203   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1204</t>
1205</section>
1206</section>
1207
1208<section title="Security Considerations" anchor="security.considerations">
1209<t>
1210   No additional security considerations have been identified beyond
1211   those applicable to HTTP in general &messaging;.
1212</t>
1213</section>
1214
1215<section title="Acknowledgments" anchor="acks">
1216<t>
1217  See &acks;.
1218</t>
1219</section>
1220</middle>
1221<back>
1222
1223<references title="Normative References">
1224
1225<reference anchor="Part1">
1226  <front>
1227    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1228    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1229      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1230      <address><email>fielding@gbiv.com</email></address>
1231    </author>
1232    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1233      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1234      <address><email>jg@freedesktop.org</email></address>
1235    </author>
1236    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1237      <organization abbrev="HP">Hewlett-Packard Company</organization>
1238      <address><email>JeffMogul@acm.org</email></address>
1239    </author>
1240    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1241      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1242      <address><email>henrikn@microsoft.com</email></address>
1243    </author>
1244    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1245      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1246      <address><email>LMM@acm.org</email></address>
1247    </author>
1248    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1249      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1250      <address><email>paulle@microsoft.com</email></address>
1251    </author>
1252    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1253      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1254      <address><email>timbl@w3.org</email></address>
1255    </author>
1256    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1257      <organization abbrev="W3C">World Wide Web Consortium</organization>
1258      <address><email>ylafon@w3.org</email></address>
1259    </author>
1260    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1261      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1262      <address><email>julian.reschke@greenbytes.de</email></address>
1263    </author>
1264    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1265  </front>
1266  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1267  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1268</reference>
1269
1270<reference anchor="Part2">
1271  <front>
1272    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1273    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1274      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1275      <address><email>fielding@gbiv.com</email></address>
1276    </author>
1277    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1278      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1279      <address><email>jg@freedesktop.org</email></address>
1280    </author>
1281    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1282      <organization abbrev="HP">Hewlett-Packard Company</organization>
1283      <address><email>JeffMogul@acm.org</email></address>
1284    </author>
1285    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1286      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1287      <address><email>henrikn@microsoft.com</email></address>
1288    </author>
1289    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1290      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1291      <address><email>LMM@acm.org</email></address>
1292    </author>
1293    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1294      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1295      <address><email>paulle@microsoft.com</email></address>
1296    </author>
1297    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1298      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1299      <address><email>timbl@w3.org</email></address>
1300    </author>
1301    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1302      <organization abbrev="W3C">World Wide Web Consortium</organization>
1303      <address><email>ylafon@w3.org</email></address>
1304    </author>
1305    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1306      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1307      <address><email>julian.reschke@greenbytes.de</email></address>
1308    </author>
1309    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1310  </front>
1311  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1312  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
1313</reference>
1314
1315<reference anchor="Part3">
1316  <front>
1317    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1318    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1319      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1320      <address><email>fielding@gbiv.com</email></address>
1321    </author>
1322    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1323      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1324      <address><email>jg@freedesktop.org</email></address>
1325    </author>
1326    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1327      <organization abbrev="HP">Hewlett-Packard Company</organization>
1328      <address><email>JeffMogul@acm.org</email></address>
1329    </author>
1330    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1331      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1332      <address><email>henrikn@microsoft.com</email></address>
1333    </author>
1334    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1335      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1336      <address><email>LMM@acm.org</email></address>
1337    </author>
1338    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1339      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1340      <address><email>paulle@microsoft.com</email></address>
1341    </author>
1342    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1343      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1344      <address><email>timbl@w3.org</email></address>
1345    </author>
1346    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1347      <organization abbrev="W3C">World Wide Web Consortium</organization>
1348      <address><email>ylafon@w3.org</email></address>
1349    </author>
1350    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1351      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1352      <address><email>julian.reschke@greenbytes.de</email></address>
1353    </author>
1354    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1355  </front>
1356  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1357  <x:source href="p3-payload.xml" basename="p3-payload"/>
1358</reference>
1359
1360<reference anchor="Part5">
1361  <front>
1362    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1363    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1364      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1365      <address><email>fielding@gbiv.com</email></address>
1366    </author>
1367    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1368      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1369      <address><email>jg@freedesktop.org</email></address>
1370    </author>
1371    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1372      <organization abbrev="HP">Hewlett-Packard Company</organization>
1373      <address><email>JeffMogul@acm.org</email></address>
1374    </author>
1375    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1376      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1377      <address><email>henrikn@microsoft.com</email></address>
1378    </author>
1379    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1380      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1381      <address><email>LMM@acm.org</email></address>
1382    </author>
1383    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1384      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1385      <address><email>paulle@microsoft.com</email></address>
1386    </author>
1387    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1388      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1389      <address><email>timbl@w3.org</email></address>
1390    </author>
1391    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1392      <organization abbrev="W3C">World Wide Web Consortium</organization>
1393      <address><email>ylafon@w3.org</email></address>
1394    </author>
1395    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1396      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1397      <address><email>julian.reschke@greenbytes.de</email></address>
1398    </author>
1399    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1400  </front>
1401  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1402  <x:source href="p5-range.xml" basename="p5-range"/>
1403</reference>
1404
1405<reference anchor="Part6">
1406  <front>
1407    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1408    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1409      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1410      <address><email>fielding@gbiv.com</email></address>
1411    </author>
1412    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1413      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1414      <address><email>jg@freedesktop.org</email></address>
1415    </author>
1416    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1417      <organization abbrev="HP">Hewlett-Packard Company</organization>
1418      <address><email>JeffMogul@acm.org</email></address>
1419    </author>
1420    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1421      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1422      <address><email>henrikn@microsoft.com</email></address>
1423    </author>
1424    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1425      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1426      <address><email>LMM@acm.org</email></address>
1427    </author>
1428    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1429      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1430      <address><email>paulle@microsoft.com</email></address>
1431    </author>
1432    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1433      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1434      <address><email>timbl@w3.org</email></address>
1435    </author>
1436    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1437      <organization abbrev="W3C">World Wide Web Consortium</organization>
1438      <address><email>ylafon@w3.org</email></address>
1439    </author>
1440    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1441      <organization>Rackspace</organization>
1442      <address><email>mnot@mnot.net</email></address>
1443    </author>
1444    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1445      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1446      <address><email>julian.reschke@greenbytes.de</email></address>
1447    </author>
1448    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1449  </front>
1450  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1451  <x:source href="p6-cache.xml" basename="p6-cache"/>
1452</reference>
1453
1454<reference anchor="RFC2119">
1455  <front>
1456    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1457    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1458      <organization>Harvard University</organization>
1459      <address><email>sob@harvard.edu</email></address>
1460    </author>
1461    <date month="March" year="1997"/>
1462  </front>
1463  <seriesInfo name="BCP" value="14"/>
1464  <seriesInfo name="RFC" value="2119"/>
1465</reference>
1466
1467<reference anchor="RFC5234">
1468  <front>
1469    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1470    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1471      <organization>Brandenburg InternetWorking</organization>
1472      <address>
1473        <email>dcrocker@bbiw.net</email>
1474      </address> 
1475    </author>
1476    <author initials="P." surname="Overell" fullname="Paul Overell">
1477      <organization>THUS plc.</organization>
1478      <address>
1479        <email>paul.overell@thus.net</email>
1480      </address>
1481    </author>
1482    <date month="January" year="2008"/>
1483  </front>
1484  <seriesInfo name="STD" value="68"/>
1485  <seriesInfo name="RFC" value="5234"/>
1486</reference>
1487
1488</references>
1489
1490<references title="Informative References">
1491
1492<reference anchor="RFC2616">
1493  <front>
1494    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1495    <author initials="R." surname="Fielding" fullname="R. Fielding">
1496      <organization>University of California, Irvine</organization>
1497      <address><email>fielding@ics.uci.edu</email></address>
1498    </author>
1499    <author initials="J." surname="Gettys" fullname="J. Gettys">
1500      <organization>W3C</organization>
1501      <address><email>jg@w3.org</email></address>
1502    </author>
1503    <author initials="J." surname="Mogul" fullname="J. Mogul">
1504      <organization>Compaq Computer Corporation</organization>
1505      <address><email>mogul@wrl.dec.com</email></address>
1506    </author>
1507    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1508      <organization>MIT Laboratory for Computer Science</organization>
1509      <address><email>frystyk@w3.org</email></address>
1510    </author>
1511    <author initials="L." surname="Masinter" fullname="L. Masinter">
1512      <organization>Xerox Corporation</organization>
1513      <address><email>masinter@parc.xerox.com</email></address>
1514    </author>
1515    <author initials="P." surname="Leach" fullname="P. Leach">
1516      <organization>Microsoft Corporation</organization>
1517      <address><email>paulle@microsoft.com</email></address>
1518    </author>
1519    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1520      <organization>W3C</organization>
1521      <address><email>timbl@w3.org</email></address>
1522    </author>
1523    <date month="June" year="1999"/>
1524  </front>
1525  <seriesInfo name="RFC" value="2616"/>
1526</reference>
1527
1528<reference anchor='RFC3864'>
1529  <front>
1530    <title>Registration Procedures for Message Header Fields</title>
1531    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1532      <organization>Nine by Nine</organization>
1533      <address><email>GK-IETF@ninebynine.org</email></address>
1534    </author>
1535    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1536      <organization>BEA Systems</organization>
1537      <address><email>mnot@pobox.com</email></address>
1538    </author>
1539    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1540      <organization>HP Labs</organization>
1541      <address><email>JeffMogul@acm.org</email></address>
1542    </author>
1543    <date year='2004' month='September' />
1544  </front>
1545  <seriesInfo name='BCP' value='90' />
1546  <seriesInfo name='RFC' value='3864' />
1547</reference>
1548
1549<reference anchor='RFC4918'>
1550  <front>
1551    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1552    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1553      <organization abbrev="CommerceNet">CommerceNet</organization>
1554      <address><email>ldusseault@commerce.net</email></address>
1555    </author>
1556    <date month="June" year="2007" />
1557  </front>
1558  <seriesInfo name='RFC' value='4918' />
1559</reference>
1560</references>
1561
1562<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1563<t>
1564  Allow weak entity-tags in all requests except range requests (Sections
1565  <xref target="weak.and.strong.validators" format="counter"/> and
1566  <xref target="header.if-none-match" format="counter"/>).
1567</t>
1568<t>
1569  Change ABNF productions for header fields to only define the field value.
1570  (<xref target="header.field.definitions"/>)
1571</t>
1572</section>
1573
1574<?BEGININC p4-conditional.abnf-appendix ?>
1575<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1576<figure>
1577<artwork type="abnf" name="p4-conditional.parsed-abnf">
1578<x:ref>ETag</x:ref> = entity-tag
1579
1580<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
1581
1582<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1583 entity-tag ] ) )
1584<x:ref>If-Modified-Since</x:ref> = HTTP-date
1585<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1586 entity-tag ] ) )
1587<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1588
1589<x:ref>Last-Modified</x:ref> = HTTP-date
1590
1591<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1592
1593<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1594
1595<x:ref>opaque-tag</x:ref> = quoted-string
1596
1597<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
1598
1599<x:ref>weak</x:ref> = %x57.2F ; W/
1600</artwork>
1601</figure>
1602<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1603; ETag defined but not used
1604; If-Match defined but not used
1605; If-Modified-Since defined but not used
1606; If-None-Match defined but not used
1607; If-Unmodified-Since defined but not used
1608; Last-Modified defined but not used
1609</artwork></figure></section>
1610<?ENDINC p4-conditional.abnf-appendix ?>
1611
1612<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1613
1614<section title="Since RFC 2616">
1615<t>
1616  Extracted relevant partitions from <xref target="RFC2616"/>.
1617</t>
1618</section>
1619
1620<section title="Since draft-ietf-httpbis-p4-conditional-00">
1621<t>
1622  Closed issues:
1623  <list style="symbols"> 
1624    <t>
1625      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1626      "Normative and Informative references"
1627    </t>
1628  </list>
1629</t>
1630<t>
1631  Other changes:
1632  <list style="symbols"> 
1633    <t>
1634      Move definitions of 304 and 412 condition codes from Part2.
1635    </t>
1636  </list>
1637</t>
1638</section>
1639
1640<section title="Since draft-ietf-httpbis-p4-conditional-01">
1641<t>
1642  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1643  <list style="symbols"> 
1644    <t>
1645      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1646    </t>
1647  </list>
1648</t>
1649</section>
1650
1651<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1652<t>
1653  Closed issues:
1654  <list style="symbols"> 
1655    <t>
1656      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1657      "Weak ETags on non-GET requests"
1658    </t>
1659  </list>
1660</t>
1661<t>
1662  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1663  <list style="symbols"> 
1664    <t>
1665      Reference RFC 3984, and update header field registrations for header fields defined
1666      in this document.
1667    </t>
1668  </list>
1669</t>
1670</section>
1671
1672<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1673<t>
1674  Closed issues:
1675  <list style="symbols"> 
1676    <t>
1677      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1678      "Examples for ETag matching"
1679    </t>
1680    <t>
1681      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1682      "'entity value' undefined"
1683    </t>
1684    <t>
1685      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1686      "bogus 2068 Date header reference"
1687    </t>
1688  </list>
1689</t>
1690</section>
1691
1692<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1693<t>
1694  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1695  <list style="symbols"> 
1696    <t>
1697      Use "/" instead of "|" for alternatives.
1698    </t>
1699    <t>
1700      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1701      whitespace ("OWS") and required whitespace ("RWS").
1702    </t>
1703    <t>
1704      Rewrite ABNFs to spell out whitespace rules, factor out
1705      header field value format definitions.
1706    </t>
1707  </list>
1708</t>
1709</section>
1710
1711<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1712<t>
1713  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1714  <list style="symbols"> 
1715    <t>
1716      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1717    </t>
1718  </list>
1719</t>
1720</section>
1721
1722<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1723<t>
1724  Closed issues:
1725  <list style="symbols"> 
1726    <t>
1727      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1728      "case-sensitivity of etag weakness indicator"
1729    </t>
1730  </list>
1731</t>
1732</section>
1733
1734<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1735<t>
1736  Closed issues:
1737  <list style="symbols"> 
1738    <t>
1739      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1740      "Weak ETags on non-GET requests" (If-Match still was defined to require
1741      strong matching)
1742    </t>
1743    <t>
1744      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1745      "move IANA registrations for optional status codes"
1746    </t>
1747  </list>
1748</t>
1749</section>
1750
1751<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1752<t>
1753  No significant changes.
1754</t>
1755</section>
1756
1757<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1758<t>
1759  No significant changes.
1760</t>
1761</section>
1762
1763<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1764<t>
1765  Closed issues:
1766  <list style="symbols"> 
1767    <t>
1768      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1769      "Clarify 'Requested Variant'"
1770    </t>
1771    <t>
1772      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1773      "Clarify entity / representation / variant terminology"
1774    </t>
1775    <t>
1776      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1777      "consider removing the 'changes from 2068' sections"
1778    </t>
1779  </list>
1780</t>
1781</section>
1782
1783<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1784<t>
1785  None.
1786</t>
1787</section>
1788
1789<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1790<t>
1791  Closed issues:
1792  <list style="symbols"> 
1793    <t>
1794      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1795      "Header Classification"
1796    </t>
1797  </list>
1798</t>
1799</section>
1800
1801<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1802<t>
1803  Closed issues:
1804  <list style="symbols"> 
1805    <t>
1806      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1807      "If-* and entities"
1808    </t>
1809    <t>
1810      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1811      "Definition of validator weakness"
1812    </t>
1813    <t>
1814      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1815      "untangle ABNFs for header fields"
1816    </t>
1817    <t>
1818      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1819      "ETags and Quotes"
1820    </t>
1821  </list>
1822</t>
1823</section>
1824
1825<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1826<t>
1827  None.
1828</t>
1829</section>
1830
1831<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1832<t>
1833  Closed issues:
1834  <list style="symbols"> 
1835    <t>
1836      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1837      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1838    </t>
1839  </list>
1840</t>
1841</section>
1842
1843<section title="Since draft-ietf-httpbis-p4-conditional-16" anchor="changes.since.16">
1844<t>
1845  Closed issues:
1846  <list style="symbols"> 
1847    <t>
1848      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
1849      "Document HTTP's error-handling philosophy"
1850    </t>
1851  </list>
1852</t>
1853</section>
1854
1855</section>
1856
1857</back>
1858</rfc>
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