source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1470

Last change on this file since 1470 was 1470, checked in by julian.reschke@…, 8 years ago

Change ETag header field ABNF not to use quoted-string, thus avoiding escaping issues (see #306).

  • Property svn:eol-style set to native
File size: 76.9 KB
Line 
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 "November">
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.17"/>.
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="DQUOTE"/>
296  <x:anchor-alias value="LF"/>
297  <x:anchor-alias value="OCTET"/>
298  <x:anchor-alias value="VCHAR"/>
299  <x:anchor-alias value="core.rules"/>
300  <x:anchor-alias value="obs-text"/>
301  <x:anchor-alias value="OWS"/>
302  <x:anchor-alias value="HTTP-date"/>
303<t>
304  This specification uses the ABNF syntax defined in &notation; (which
305  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
306  <xref target="collected.abnf"/> shows the collected ABNF, with the list
307  rule expanded.
308</t>
309<t>
310  The following core rules are included by
311  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
312  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
313  DIGIT (decimal 0-9), DQUOTE (double quote),
314  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
315  OCTET (any 8-bit sequence of data), SP (space), and
316  VCHAR (any visible US-ASCII character).
317</t>
318<t>
319  The ABNF rules below are defined in <xref target="Part1"/> and
320  <xref target="Part2"/>:
321</t>
322<figure><artwork type="abnf2616">
323  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
324  <x:ref>obs-text</x:ref>      = &lt;obs-text, defined in &field-rules;&gt;
325  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
326</artwork></figure>
327</section>
328</section>
329
330<section title="Validators" anchor="validators">
331   <iref primary="true" item="metadata"/>
332   <iref primary="true" item="validator"/>
333<t>
334   This specification defines two forms of metadata that are commonly used
335   to observe resource state and test for preconditions: modification dates
336   and opaque entity tags.  Additional metadata that reflects resource state
337   has been defined by various extensions of HTTP, such as WebDAV
338   <xref target="RFC4918"/>, that are beyond the scope of this specification.
339   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
340   when it is used within a precondition.
341</t>
342
343<section title="Weak versus Strong" anchor="weak.and.strong.validators">
344   <iref primary="true" item="validator" subitem="weak"/>
345   <iref primary="true" item="validator" subitem="strong"/>
346<t>
347   Validators come in two flavors: strong or weak.  Weak validators are easy
348   to generate but are far less useful for comparisons.  Strong validators
349   are ideal for comparisons but can be very difficult (and occasionally
350   impossible) to generate efficiently.  Rather than impose that all forms
351   of resource adhere to the same strength of validator, HTTP exposes the
352   type of validator in use and imposes restrictions on when weak validators
353   can be used as preconditions.
354</t>
355<t>
356   A "strong validator" is a representation metadata value that &MUST; be
357   changed to a new, previously unused or guaranteed unique, value whenever
358   a change occurs to the representation data such that a change would be
359   observable in the payload body of a 200 response to GET.  A strong
360   validator &MAY; be changed for other reasons, such as when a semantically
361   significant part of the representation metadata is changed (e.g.,
362   Content-Type), but it is in the best interests of the origin server to only
363   change the value when it is necessary to invalidate the stored responses
364   held by remote caches and authoring tools.  A strong validator &MUST; be
365   unique across all representations of a given resource, such that no two
366   representations of that resource share the same validator unless
367   their payload body would be identical.
368</t>
369<t>
370   Cache entries might persist for arbitrarily long periods, regardless
371   of expiration times.  Thus, a cache might attempt to validate an
372   entry using a validator that it obtained in the distant past.
373   A strong validator &MUST; be unique across all versions of all
374   representations associated with a particular resource over time.
375   However, there is no implication of uniqueness across representations
376   of different resources (i.e., the same strong validator might be
377   in use for representations of multiple resources at the same time
378   and does not imply that those representations are equivalent).
379</t>
380<t>
381   There are a variety of strong validators used in practice.  The best are
382   based on strict revision control, wherein each change to a representation
383   always results in a unique node name and revision identifier being assigned
384   before the representation is made accessible to GET.  A cryptographic hash
385   function applied to the representation data is also sufficient if the data
386   is available prior to the response header fields being sent and the digest
387   does not need to be recalculated every time a validation request is
388   received.  However, if a resource has distinct representations that differ
389   only in their metadata, such as might occur with content negotiation over
390   media types that happen to share the same data format, then a server
391   &SHOULD; incorporate additional information in the validator to
392   distinguish those representations and avoid confusing cache behavior.
393</t>
394<t>
395   In contrast, a "weak validator" is a representation metadata value that
396   might not be changed for every change to the representation data.  This
397   weakness might be due to limitations in how the value is calculated, such
398   as clock resolution or an inability to ensure uniqueness for all possible
399   representations of the resource, or due to a desire by the resource owner
400   to group representations by some self-determined set of equivalency
401   rather than unique sequences of data.  A weak entity-tag &SHOULD; change
402   whenever the origin server considers prior representations to be
403   unacceptable as a substitute for the current representation. In other
404   words, a weak entity-tag &SHOULD; change whenever the origin server wants
405   caches to invalidate old responses.
406</t>
407<t>
408   For example, the representation of a weather report that changes in
409   content every second, based on dynamic measurements, might be grouped
410   into sets of equivalent representations (from the origin server's
411   perspective) with the same weak validator in order to allow cached
412   representations to be valid for a reasonable period of time (perhaps
413   adjusted dynamically based on server load or weather quality).
414   Likewise, a representation's modification time, if defined with only
415   one-second resolution, might be a weak validator if it is possible
416   for the representation to be modified twice during a single second and
417   retrieved between those modifications.
418</t>
419<t>
420   A "use" of a validator occurs when either a client generates a request
421   and includes the validator in a precondition or when a server
422   compares two validators.
423   Weak validators are only usable in contexts that do not depend on exact
424   equality of a representation's payload body.
425   Strong validators are usable and preferred for all conditional requests,
426   including cache validation, partial content ranges, and "lost update"
427   avoidance.
428</t>
429</section>
430
431<section title="Last-Modified" anchor="header.last-modified">
432  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
433  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
434  <x:anchor-alias value="Last-Modified"/>
435<t>
436   The "Last-Modified" header field indicates the date and time at
437   which the origin server believes the selected representation was
438   last modified.
439</t>
440<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
441  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
442</artwork></figure>
443<t>
444   An example of its use is
445</t>
446<figure><artwork type="example">
447  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
448</artwork></figure>
449
450<section title="Generation" anchor="lastmod.generation">
451<t>
452   Origin servers &SHOULD; send Last-Modified for any selected
453   representation for which a last modification date can be reasonably
454   and consistently determined, since its use in conditional requests
455   and evaluating cache freshness (&caching;) results in a substantial
456   reduction of HTTP traffic on the Internet and can be a significant
457   factor in improving service scalability and reliability.
458</t>
459<t>
460   A representation is typically the sum of many parts behind the
461   resource interface.  The last-modified time would usually be
462   the most recent time that any of those parts were changed.
463   How that value is determined for any given resource is an
464   implementation detail beyond the scope of this specification.
465   What matters to HTTP is how recipients of the Last-Modified
466   header field can use its value to make conditional requests
467   and test the validity of locally cached responses.
468</t>
469<t>
470   An origin server &SHOULD; obtain the Last-Modified value of the
471   representation as close as possible to the time that it generates
472   the Date field-value for its response. This allows a recipient to
473   make an accurate assessment of the representation's modification time,
474   especially if the representation changes near the time that the
475   response is generated.
476</t>
477<t>
478   An origin server with a clock &MUST-NOT; send a Last-Modified date
479   that is later than the server's time of message origination (Date).
480   If the last modification time is derived from implementation-specific
481   metadata that evaluates to some time in the future, according to the
482   origin server's clock, then the origin server &MUST; replace that
483   value with the message origination date. This prevents a future
484   modification date from having an adverse impact on cache validation.
485</t>
486<t>
487   An origin server without a clock &MUST-NOT; assign Last-Modified
488   values to a response unless these values were associated
489   with the resource by some other system or user with a reliable clock.
490</t>
491</section>
492
493<section title="Comparison" anchor="lastmod.comparison">
494<t>
495   A Last-Modified time, when used as a validator in a request, is
496   implicitly weak unless it is possible to deduce that it is strong,
497   using the following rules:
498  <list style="symbols">
499     <t>The validator is being compared by an origin server to the
500        actual current validator for the representation and,</t>
501     <t>That origin server reliably knows that the associated representation did
502        not change twice during the second covered by the presented
503        validator.</t>
504  </list>
505</t>
506<t>
507   or
508  <list style="symbols">
509     <t>The validator is about to be used by a client in an If-Modified-Since,
510        If-Unmodified-Since header field, because the client has a cache entry,
511        or If-Range for the associated representation, and</t>
512     <t>That cache entry includes a Date value, which gives the time
513        when the origin server sent the original response, and</t>
514     <t>The presented Last-Modified time is at least 60 seconds before
515        the Date value.</t>
516  </list>
517</t>
518<t>
519   or
520  <list style="symbols">
521     <t>The validator is being compared by an intermediate cache to the
522        validator stored in its cache entry for the representation, and</t>
523     <t>That cache entry includes a Date value, which gives the time
524        when the origin server sent the original response, and</t>
525     <t>The presented Last-Modified time is at least 60 seconds before
526        the Date value.</t>
527  </list>
528</t>
529<t>
530   This method relies on the fact that if two different responses were
531   sent by the origin server during the same second, but both had the
532   same Last-Modified time, then at least one of those responses would
533   have a Date value equal to its Last-Modified time. The arbitrary 60-second
534   limit guards against the possibility that the Date and Last-Modified
535   values are generated from different clocks, or at somewhat
536   different times during the preparation of the response. An
537   implementation &MAY; use a value larger than 60 seconds, if it is
538   believed that 60 seconds is too short.
539</t>
540</section>
541</section>
542
543<section title="ETag" anchor="header.etag">
544  <iref primary="true" item="ETag header field" x:for-anchor=""/>
545  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
546  <x:anchor-alias value="ETag"/>
547  <x:anchor-alias value="entity-tag"/>
548  <x:anchor-alias value="entity.tags"/>
549  <x:anchor-alias value="opaque-tag"/>
550  <x:anchor-alias value="weak"/>
551  <x:anchor-alias value="etagc"/>
552<t>
553   The ETag header field provides the current entity-tag for the
554   selected representation.
555   An entity-tag is an opaque validator for differentiating between
556   multiple representations of the same resource, regardless of whether
557   those multiple representations are due to resource state changes over
558   time, content negotiation resulting in multiple representations being
559   valid at the same time, or both. An entity-tag consists of an opaque
560   quoted string, possibly prefixed by a weakness indicator.
561</t>
562<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"/><iref primary="true" item="Grammar" subitem="etagc"/>
563  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
564
565  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
566  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
567  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
568  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
569             ; <xref>VCHAR</xref> except double quotes, plus obs-text
570</artwork></figure>
571<x:note>
572  <t>
573    <x:h>Note:</x:h> Previously, opaque-tag was defined to be a quoted-string
574    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
575    might perform backslash unescaping. Servers therefore ought to avoid
576    backslash characters in entity tags.
577  </t>
578</x:note>
579<t>
580   An entity-tag can be more reliable for validation than a modification
581   date in situations where it is inconvenient to store modification
582   dates, where the one-second resolution of HTTP date values is not
583   sufficient, or where modification dates are not consistently maintained.
584</t>
585<figure><preamble>
586  Examples:
587</preamble>
588<artwork type="example">
589  ETag: "xyzzy"
590  ETag: W/"xyzzy"
591  ETag: ""
592</artwork></figure>
593<t>
594   An entity-tag can be either a weak or strong validator, with
595   strong being the default.  If an origin server provides an entity-tag
596   for a representation and the generation of that entity-tag does not satisfy
597   the requirements for a strong validator
598   (<xref target="weak.and.strong.validators"/>), then that
599   entity-tag &MUST; be marked as weak by prefixing its opaque value
600   with "W/" (case-sensitive).
601</t>
602
603<section title="Generation" anchor="entity.tag.generation">
604<t>
605   The principle behind entity-tags is that only the service author
606   knows the implementation of a resource well enough to select the
607   most accurate and efficient validation mechanism for that resource,
608   and that any such mechanism can be mapped to a simple sequence of
609   octets for easy comparison.  Since the value is opaque, there is no
610   need for the client to be aware of how each entity-tag is constructed.
611</t>
612<t>
613   For example, a resource that has implementation-specific versioning
614   applied to all changes might use an internal revision number, perhaps
615   combined with a variance identifier for content negotiation, to
616   accurately differentiate between representations.
617   Other implementations might use a stored hash of representation content,
618   a combination of various filesystem attributes, or a modification
619   timestamp that has sub-second resolution.
620</t>
621<t>
622   Origin servers &SHOULD; send ETag for any selected representation
623   for which detection of changes can be reasonably and consistently
624   determined, since the entity-tag's use in conditional requests and
625   evaluating cache freshness (&caching;) can result in a substantial
626   reduction of HTTP network traffic and can be a significant factor in
627   improving service scalability and reliability.
628</t>
629</section>
630
631<section title="Comparison" anchor="entity.tag.comparison">
632  <x:anchor-alias value="validator.comparison"/>
633<t>
634   There are two entity-tag comparison functions, depending
635   on whether the comparison context allows the use of weak validators
636   or not:
637  <list style="symbols">
638     <t>The strong comparison function: in order to be considered equal,
639        both opaque-tags &MUST; be identical character-by-character, and both
640        &MUST-NOT; be weak.</t>
641     <t>The weak comparison function: in order to be considered equal, both
642        opaque-tags &MUST; be identical character-by-character, but
643        either or both of them &MAY; be tagged as "weak" without affecting
644        the result.</t>
645  </list>
646</t>
647<t>
648   The example below shows the results for a set of entity-tag pairs,
649   and both the weak and strong comparison function results:
650</t>
651<texttable align="left">
652  <ttcol>ETag 1</ttcol>
653  <ttcol>ETag 2</ttcol>
654  <ttcol>Strong Comparison</ttcol>
655  <ttcol>Weak Comparison</ttcol>
656
657  <c>W/"1"</c>
658  <c>W/"1"</c>
659  <c>no match</c>
660  <c>match</c>
661 
662  <c>W/"1"</c>
663  <c>W/"2"</c>
664  <c>no match</c>
665  <c>no match</c>
666
667  <c>W/"1"</c>
668  <c>"1"</c>
669  <c>no match</c>
670  <c>match</c>
671
672  <c>"1"</c>
673  <c>"1"</c>
674  <c>match</c>
675  <c>match</c>
676</texttable>
677</section>
678
679<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
680<t>
681   Consider a resource that is subject to content negotiation (&content-negotiation;),
682   and where the representations returned upon a GET request vary based on
683   the Accept-Encoding request header field (&header-accept-encoding;):
684</t>
685<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
686GET /index HTTP/1.1
687Host: www.example.com
688Accept-Encoding: gzip
689
690</artwork></figure>
691<t>
692   In this case, the response might or might not use the gzip content coding.
693   If it does not, the response might look like:
694</t>
695<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
696HTTP/1.1 200 OK
697Date: Thu, 26 Mar 2010 00:05:00 GMT
698ETag: "123-a"
699Content-Length: <x:length-of target="exbody"/>
700Vary: Accept-Encoding
701Content-Type: text/plain
702
703<x:span anchor="exbody">Hello World!
704Hello World!
705Hello World!
706Hello World!
707Hello World!
708</x:span></artwork></figure>
709<t>
710   An alternative representation that does use gzip content coding would be:
711</t>
712<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
713HTTP/1.1 200 OK
714Date: Thu, 26 Mar 2010 00:05:00 GMT
715ETag: "123-b"
716Content-Length: 43
717Vary: Accept-Encoding
718Content-Type: text/plain
719Content-Encoding: gzip
720
721<spanx>...binary data...</spanx></artwork></figure>
722<x:note>
723  <t>
724    <x:h>Note:</x:h> Content codings are a property of the representation,
725    so therefore an entity-tag of an encoded representation must be distinct
726    from an unencoded representation to prevent conflicts during cache updates
727    and range requests.  In contrast, transfer codings (&transfer-codings;)
728    apply only during message transfer and do not require distinct entity-tags.
729  </t>
730</x:note>
731</section>
732</section>
733
734<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">
735<t>
736   We adopt a set of rules and recommendations for origin servers,
737   clients, and caches regarding when various validator types ought to
738   be used, and for what purposes.
739</t>
740<t>
741   HTTP/1.1 origin servers:
742  <list style="symbols">
743     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
744        generate one.</t>
745
746     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
747        performance considerations support the use of weak entity-tags,
748        or if it is unfeasible to send a strong entity-tag.</t>
749
750     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
751  </list>
752</t>
753<t>
754   In other words, the preferred behavior for an HTTP/1.1 origin server
755   is to send both a strong entity-tag and a Last-Modified value.
756</t>
757<t>
758   HTTP/1.1 clients:
759  <list style="symbols">
760     <t>&MUST; use that entity-tag in any cache-conditional request (using
761        If-Match or If-None-Match) if an entity-tag has been provided by the
762        origin server.</t>
763
764     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
765        requests (using If-Modified-Since) if only a Last-Modified value has
766        been provided by the origin server. </t>
767
768     <t>&MAY; use the Last-Modified value in subrange cache-conditional
769        requests (using If-Unmodified-Since) if only a Last-Modified value has
770        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
771        provide a way to disable this, in case of difficulty.</t>
772
773     <t>&SHOULD; use both validators in cache-conditional requests if both an
774        entity-tag and a Last-Modified value have been provided by the origin
775        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
776        appropriately.</t>
777  </list>
778</t>
779<t>
780   An HTTP/1.1 origin server, upon receiving a conditional request that
781   includes both a Last-Modified date (e.g., in an If-Modified-Since or
782   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
783   in an If-Match, If-None-Match, or If-Range header field) as cache
784   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
785   unless doing so is consistent with all of the conditional header
786   fields in the request.
787</t>
788<t>
789   An HTTP/1.1 caching proxy, upon receiving a conditional request that
790   includes both a Last-Modified date and one or more entity-tags as
791   cache validators, &MUST-NOT; return a locally cached response to the
792   client unless that cached response is consistent with all of the
793   conditional header fields in the request.
794  <list><t>
795      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
796      servers and clients ought to transmit as much non-redundant
797      information as is available in their responses and requests.
798      HTTP/1.1 systems receiving this information will make the most
799      conservative assumptions about the validators they receive.
800  </t><t>
801      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
802      last-modified values received or used by these systems will
803      support transparent and efficient caching, and so HTTP/1.1 origin
804      servers should provide Last-Modified values. In those rare cases
805      where the use of a Last-Modified value as a validator by an
806      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
807      origin servers should not provide one.
808  </t></list>
809</t>
810</section>
811</section>
812
813<section title="Precondition Header Fields" anchor="header.field.definitions">
814<t>
815   This section defines the syntax and semantics of HTTP/1.1 header fields
816   for applying preconditions on requests.
817</t>
818
819<section title="If-Match" anchor="header.if-match">
820  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
821  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
822  <x:anchor-alias value="If-Match"/>
823<t>
824   The "If-Match" header field &MAY; be used to make a request method
825   conditional on the current existence or value of an entity-tag for
826   one or more representations of the target resource.  If-Match is
827   generally useful for resource update requests, such as PUT requests,
828   as a means for protecting against accidental overwrites when multiple
829   clients are acting in parallel on the same resource (i.e., the
830   "lost update" problem).  An If-Match field-value of "*" places the
831   precondition on the existence of any current representation for the
832   target resource.
833</t>
834<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
835  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
836</artwork></figure>
837<t>
838   If any of the entity-tags listed in the If-Match field value match
839   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
840   selected representation for the target resource,
841   or if "*" is given and any current representation exists for the
842   target resource, then the server &MAY; perform the request method
843   as if the If-Match header field was not present.
844</t>
845<t>
846   If none of the entity-tags match, or if "*" is given and no current
847   representation exists, the server &MUST-NOT; perform the requested method.
848   Instead, the server &MUST; respond with the 412 (Precondition Failed)
849   status code.
850</t>
851<t>
852   If the request would, without the If-Match header field, result in
853   anything other than a 2xx or 412 status code, then the If-Match header field
854   &MUST; be ignored.
855</t>
856<t>
857   Examples:
858</t>
859<figure><artwork type="example">
860  If-Match: "xyzzy"
861  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
862  If-Match: *
863</artwork></figure>
864<t>
865   The result of a request having both an If-Match header field and
866   either an If-None-Match or an If-Modified-Since header fields is
867   undefined by this specification.
868</t>
869</section>
870
871<section title="If-None-Match" anchor="header.if-none-match">
872  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
873  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
874  <x:anchor-alias value="If-None-Match"/>
875<t>
876   The "If-None-Match" header field &MAY; be used to make a request method
877   conditional on not matching any of the current entity-tag values for
878   representations of the target resource.  If-None-Match is primarily
879   used in conditional GET requests to enable efficient updates of cached
880   information with a minimum amount of transaction overhead.  A client
881   that has one or more representations previously obtained from the
882   target resource can send If-None-Match with a list of the associated
883   entity-tags in the hope of receiving a 304 response if at least one
884   of those representations matches the selected representation.
885</t>
886<t>
887   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
888   request method (e.g., PUT) from inadvertently modifying an existing
889   representation of the target resource when the client believes that
890   the resource does not have a current representation.  This is a variation
891   on the "lost update" problem that might arise if more than one client
892   attempts to create an initial representation for the target resource.
893</t>
894<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
895  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
896</artwork></figure>
897<t>
898   If any of the entity-tags listed in the If-None-Match field-value match
899   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
900   selected representation, or if "*" is
901   given and any current representation exists for that resource, then the
902   server &MUST-NOT; perform the requested method.
903   Instead, if the request method was GET or HEAD, the server &SHOULD;
904   respond with a 304 (Not Modified) status code, including the cache-related
905   header fields (particularly ETag) of the selected representation that has
906   a matching entity-tag.  For all other request methods, the server &MUST;
907   respond with a 412 (Precondition Failed) status code.
908</t>
909<t>
910   If none of the entity-tags match, then the server &MAY; perform the
911   requested method as if the If-None-Match header field did not exist,
912   but &MUST; also ignore any If-Modified-Since header field(s) in the
913   request. That is, if no entity-tags match, then the server &MUST-NOT;
914   return a 304 (Not Modified) response.
915</t>
916<t>
917   If the request would, without the If-None-Match header field, result
918   in anything other than a 2xx or 304 status code, then the If-None-Match
919   header field &MUST; be ignored. (See <xref
920   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
921   a discussion of server behavior when both If-Modified-Since and
922   If-None-Match appear in the same request.)
923</t>
924<t>
925   Examples:
926</t>
927<figure><artwork type="example">
928  If-None-Match: "xyzzy"
929  If-None-Match: W/"xyzzy"
930  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
931  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
932  If-None-Match: *
933</artwork></figure>
934<t>
935   The result of a request having both an If-None-Match header field and
936   either an If-Match or an If-Unmodified-Since header fields is
937   undefined by this specification.
938</t>
939</section>
940
941<section title="If-Modified-Since" anchor="header.if-modified-since">
942  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
943  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
944  <x:anchor-alias value="If-Modified-Since"/>
945<t>
946   The "If-Modified-Since" header field &MAY; be used to make a request
947   method conditional by modification date: if the selected representation
948   has not been modified since the time specified in this field, then
949   do not perform the request method; instead, respond as detailed below.
950</t>
951<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
952  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
953</artwork></figure>
954<t>
955   An example of the field is:
956</t>
957<figure><artwork type="example">
958  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
959</artwork></figure>
960<t>
961   A GET method with an If-Modified-Since header field and no Range header
962   field requests that the selected representation be transferred only if
963   it has been modified since the date given by the If-Modified-Since
964   header field.
965   The algorithm for determining this includes the following cases:
966  <list style="numbers">
967      <t>If the request would normally result in anything other than a
968         200 (OK) status code, or if the passed If-Modified-Since date is
969         invalid, the response is exactly the same as for a normal GET.
970         A date which is later than the server's current time is
971         invalid.</t>
972
973      <t>If the selected representation has been modified since the
974         If-Modified-Since date, the response is exactly the same as for
975         a normal GET.</t>
976
977      <t>If the selected representation has not been modified since a valid
978         If-Modified-Since date, the server &SHOULD; return a
979         304 (Not Modified) response.</t>
980  </list>
981</t>
982<t>
983   The purpose of this feature is to allow efficient updates of cached
984   information with a minimum amount of transaction overhead.
985  <list><t>
986      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
987      see &header-range; for full details.
988    </t><t>
989      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
990      clock might not be synchronized with the client.
991    </t><t>
992      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
993      servers will use an exact date comparison function, rather than a
994      less-than function, for deciding whether to send a 304 (Not
995      Modified) response. To get best results when sending an If-Modified-Since
996      header field for cache validation, clients are
997      advised to use the exact date string received in a previous Last-Modified
998      header field whenever possible.
999    </t><t>
1000      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
1001      header field instead of a date taken from the Last-Modified header field for
1002      the same request, the client needs to be aware that this
1003      date is interpreted in the server's understanding of time.
1004      Unsynchronized clocks and rounding problems, due to the different
1005      encodings of time between the client and server, are concerns.
1006      This includes the possibility of race conditions if the
1007      document has changed between the time it was first requested and
1008      the If-Modified-Since date of a subsequent request, and the
1009      possibility of clock-skew-related problems if the If-Modified-Since
1010      date is derived from the client's clock without correction
1011      to the server's clock. Corrections for different time bases
1012      between client and server are at best approximate due to network
1013      latency.
1014    </t>
1015  </list>
1016</t>
1017<t>
1018   The result of a request having both an If-Modified-Since header field
1019   and either an If-Match or an If-Unmodified-Since header fields is
1020   undefined by this specification.
1021</t>
1022</section>
1023
1024<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
1025  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
1026  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
1027  <x:anchor-alias value="If-Unmodified-Since"/>
1028<t>
1029   The "If-Unmodified-Since" header field &MAY; be used to make a request
1030   method conditional by modification date: if the selected representation
1031   has been modified since the time specified in this field, then the
1032   server &MUST-NOT; perform the requested operation and &MUST; instead
1033   respond with the 412 (Precondition Failed) status code.
1034   If the selected representation has not been modified since the time
1035   specified in this field, the server &SHOULD; perform the request
1036   method as if the If-Unmodified-Since header field were not present.
1037</t>
1038<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
1039  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
1040</artwork></figure>
1041<t>
1042   An example of the field is:
1043</t>
1044<figure><artwork type="example">
1045  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1046</artwork></figure>
1047<t>
1048   If the request normally (i.e., without the If-Unmodified-Since
1049   header field) would result in anything other than a 2xx or 412 status code,
1050   the If-Unmodified-Since header field &SHOULD; be ignored.
1051</t>
1052<t>
1053   If the specified date is invalid, the header field &MUST; be ignored.
1054</t>
1055<t>
1056   The result of a request having both an If-Unmodified-Since header
1057   field and either an If-None-Match or an If-Modified-Since header
1058   fields is undefined by this specification.
1059</t>
1060</section>
1061
1062<section title="If-Range" anchor="header.if-range">
1063<t>
1064   The If-Range header field provides a special conditional request
1065   mechanism that is similar to If-Match and If-Unmodified-Since but
1066   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1067</t>
1068</section>
1069
1070</section>
1071
1072<section title="Status Code Definitions" anchor="status.code.definitions">
1073<section title="304 Not Modified" anchor="status.304">
1074  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1075  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1076<t>
1077   The 304 status code indicates that a conditional GET request has been
1078   received and would have resulted in a 200 (OK) response if it were not
1079   for the fact that the condition has evaluated to false.  In other words,
1080   there is no need for the server to transfer a representation of the
1081   target resource because the client's request indicates that it already
1082   has a valid representation, as indicated by the 304 response header
1083   fields, and is therefore redirecting the client to make use of that
1084   stored representation as if it were the payload of a 200 response.
1085   The 304 response &MUST-NOT; contain a message-body, and thus is always
1086   terminated by the first empty line after the header fields.
1087</t>
1088<t>
1089   A 304 response &MUST; include a Date header field (&header-date;)
1090   unless the origin server does not have a clock that can provide a
1091   reasonable approximation of the current time.  If a 200 response
1092   to the same request would have included any of the header fields
1093   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1094   Vary, then those same header fields &MUST; be sent in a 304 response.
1095</t>
1096<t>
1097   Since the goal of a 304 response is to minimize information transfer
1098   when the recipient already has one or more cached representations,
1099   the response &SHOULD-NOT; include representation metadata other
1100   than the above listed fields unless said metadata exists for the
1101   purpose of guiding cache updates (e.g., future HTTP extensions).
1102</t>
1103<t>
1104   If the recipient of a 304 response does not have a cached representation
1105   corresponding to the entity-tag indicated by the 304 response, then the
1106   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1107   conditional request originated with an outbound client, such as a
1108   user agent with its own cache sending a conditional GET to a shared
1109   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1110   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1111   the request without any preconditions.
1112</t>
1113<t>
1114   If a cache uses a received 304 response to update a cache entry, the
1115   cache &MUST; update the entry to reflect any new field values given in
1116   the response.
1117</t>
1118</section>
1119
1120<section title="412 Precondition Failed" anchor="status.412">
1121  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1122  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1123<t>
1124   The 412 status code indicates that one or more preconditions given in
1125   the request header fields evaluated to false when tested on the server.
1126   This response code allows the client to place preconditions on the
1127   current resource state (its current representations and metadata)
1128   and thus prevent the request method from being applied if the target
1129   resource is in an unexpected state.
1130</t>
1131</section>
1132</section>
1133
1134<section title="IANA Considerations" anchor="IANA.considerations">
1135
1136<section title="Status Code Registration" anchor="status.code.registration">
1137<t>
1138   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1139   shall be updated with the registrations below:
1140</t>
1141<?BEGININC p4-conditional.iana-status-codes ?>
1142<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1143<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1144   <ttcol>Value</ttcol>
1145   <ttcol>Description</ttcol>
1146   <ttcol>Reference</ttcol>
1147   <c>304</c>
1148   <c>Not Modified</c>
1149   <c>
1150      <xref target="status.304"/>
1151   </c>
1152   <c>412</c>
1153   <c>Precondition Failed</c>
1154   <c>
1155      <xref target="status.412"/>
1156   </c>
1157</texttable>
1158<!--(END)-->
1159<?ENDINC p4-conditional.iana-status-codes ?>
1160</section>
1161
1162<section title="Header Field Registration" anchor="header.field.registration">
1163<t>
1164   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1165   with the permanent registrations below (see <xref target="RFC3864"/>):
1166</t>
1167<?BEGININC p4-conditional.iana-headers ?>
1168<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1169<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1170   <ttcol>Header Field Name</ttcol>
1171   <ttcol>Protocol</ttcol>
1172   <ttcol>Status</ttcol>
1173   <ttcol>Reference</ttcol>
1174
1175   <c>ETag</c>
1176   <c>http</c>
1177   <c>standard</c>
1178   <c>
1179      <xref target="header.etag"/>
1180   </c>
1181   <c>If-Match</c>
1182   <c>http</c>
1183   <c>standard</c>
1184   <c>
1185      <xref target="header.if-match"/>
1186   </c>
1187   <c>If-Modified-Since</c>
1188   <c>http</c>
1189   <c>standard</c>
1190   <c>
1191      <xref target="header.if-modified-since"/>
1192   </c>
1193   <c>If-None-Match</c>
1194   <c>http</c>
1195   <c>standard</c>
1196   <c>
1197      <xref target="header.if-none-match"/>
1198   </c>
1199   <c>If-Unmodified-Since</c>
1200   <c>http</c>
1201   <c>standard</c>
1202   <c>
1203      <xref target="header.if-unmodified-since"/>
1204   </c>
1205   <c>Last-Modified</c>
1206   <c>http</c>
1207   <c>standard</c>
1208   <c>
1209      <xref target="header.last-modified"/>
1210   </c>
1211</texttable>
1212<!--(END)-->
1213<?ENDINC p4-conditional.iana-headers ?>
1214<t>
1215   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1216</t>
1217</section>
1218</section>
1219
1220<section title="Security Considerations" anchor="security.considerations">
1221<t>
1222   No additional security considerations have been identified beyond
1223   those applicable to HTTP in general &messaging;.
1224</t>
1225</section>
1226
1227<section title="Acknowledgments" anchor="acks">
1228<t>
1229  See &acks;.
1230</t>
1231</section>
1232</middle>
1233<back>
1234
1235<references title="Normative References">
1236
1237<reference anchor="Part1">
1238  <front>
1239    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1240    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1241      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1242      <address><email>fielding@gbiv.com</email></address>
1243    </author>
1244    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1245      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1246      <address><email>jg@freedesktop.org</email></address>
1247    </author>
1248    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1249      <organization abbrev="HP">Hewlett-Packard Company</organization>
1250      <address><email>JeffMogul@acm.org</email></address>
1251    </author>
1252    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1253      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1254      <address><email>henrikn@microsoft.com</email></address>
1255    </author>
1256    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1257      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1258      <address><email>LMM@acm.org</email></address>
1259    </author>
1260    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1261      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1262      <address><email>paulle@microsoft.com</email></address>
1263    </author>
1264    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1265      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1266      <address><email>timbl@w3.org</email></address>
1267    </author>
1268    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1269      <organization abbrev="W3C">World Wide Web Consortium</organization>
1270      <address><email>ylafon@w3.org</email></address>
1271    </author>
1272    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1273      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1274      <address><email>julian.reschke@greenbytes.de</email></address>
1275    </author>
1276    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1277  </front>
1278  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1279  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1280</reference>
1281
1282<reference anchor="Part2">
1283  <front>
1284    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1285    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1286      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1287      <address><email>fielding@gbiv.com</email></address>
1288    </author>
1289    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1290      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1291      <address><email>jg@freedesktop.org</email></address>
1292    </author>
1293    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1294      <organization abbrev="HP">Hewlett-Packard Company</organization>
1295      <address><email>JeffMogul@acm.org</email></address>
1296    </author>
1297    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1298      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1299      <address><email>henrikn@microsoft.com</email></address>
1300    </author>
1301    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1302      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1303      <address><email>LMM@acm.org</email></address>
1304    </author>
1305    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1306      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1307      <address><email>paulle@microsoft.com</email></address>
1308    </author>
1309    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1310      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1311      <address><email>timbl@w3.org</email></address>
1312    </author>
1313    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1314      <organization abbrev="W3C">World Wide Web Consortium</organization>
1315      <address><email>ylafon@w3.org</email></address>
1316    </author>
1317    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1318      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1319      <address><email>julian.reschke@greenbytes.de</email></address>
1320    </author>
1321    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1322  </front>
1323  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1324  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
1325</reference>
1326
1327<reference anchor="Part3">
1328  <front>
1329    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1330    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1331      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1332      <address><email>fielding@gbiv.com</email></address>
1333    </author>
1334    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1335      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1336      <address><email>jg@freedesktop.org</email></address>
1337    </author>
1338    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1339      <organization abbrev="HP">Hewlett-Packard Company</organization>
1340      <address><email>JeffMogul@acm.org</email></address>
1341    </author>
1342    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1343      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1344      <address><email>henrikn@microsoft.com</email></address>
1345    </author>
1346    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1347      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1348      <address><email>LMM@acm.org</email></address>
1349    </author>
1350    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1351      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1352      <address><email>paulle@microsoft.com</email></address>
1353    </author>
1354    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1355      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1356      <address><email>timbl@w3.org</email></address>
1357    </author>
1358    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1359      <organization abbrev="W3C">World Wide Web Consortium</organization>
1360      <address><email>ylafon@w3.org</email></address>
1361    </author>
1362    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1363      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1364      <address><email>julian.reschke@greenbytes.de</email></address>
1365    </author>
1366    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1367  </front>
1368  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1369  <x:source href="p3-payload.xml" basename="p3-payload"/>
1370</reference>
1371
1372<reference anchor="Part5">
1373  <front>
1374    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1375    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1376      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1377      <address><email>fielding@gbiv.com</email></address>
1378    </author>
1379    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1380      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1381      <address><email>jg@freedesktop.org</email></address>
1382    </author>
1383    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1384      <organization abbrev="HP">Hewlett-Packard Company</organization>
1385      <address><email>JeffMogul@acm.org</email></address>
1386    </author>
1387    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1388      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1389      <address><email>henrikn@microsoft.com</email></address>
1390    </author>
1391    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1392      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1393      <address><email>LMM@acm.org</email></address>
1394    </author>
1395    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1396      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1397      <address><email>paulle@microsoft.com</email></address>
1398    </author>
1399    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1400      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1401      <address><email>timbl@w3.org</email></address>
1402    </author>
1403    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1404      <organization abbrev="W3C">World Wide Web Consortium</organization>
1405      <address><email>ylafon@w3.org</email></address>
1406    </author>
1407    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1408      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1409      <address><email>julian.reschke@greenbytes.de</email></address>
1410    </author>
1411    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1412  </front>
1413  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1414  <x:source href="p5-range.xml" basename="p5-range"/>
1415</reference>
1416
1417<reference anchor="Part6">
1418  <front>
1419    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1420    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1421      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1422      <address><email>fielding@gbiv.com</email></address>
1423    </author>
1424    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1425      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1426      <address><email>jg@freedesktop.org</email></address>
1427    </author>
1428    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1429      <organization abbrev="HP">Hewlett-Packard Company</organization>
1430      <address><email>JeffMogul@acm.org</email></address>
1431    </author>
1432    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1433      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1434      <address><email>henrikn@microsoft.com</email></address>
1435    </author>
1436    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1437      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1438      <address><email>LMM@acm.org</email></address>
1439    </author>
1440    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1441      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1442      <address><email>paulle@microsoft.com</email></address>
1443    </author>
1444    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1445      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1446      <address><email>timbl@w3.org</email></address>
1447    </author>
1448    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1449      <organization abbrev="W3C">World Wide Web Consortium</organization>
1450      <address><email>ylafon@w3.org</email></address>
1451    </author>
1452    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1453      <organization>Rackspace</organization>
1454      <address><email>mnot@mnot.net</email></address>
1455    </author>
1456    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1457      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1458      <address><email>julian.reschke@greenbytes.de</email></address>
1459    </author>
1460    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1461  </front>
1462  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1463  <x:source href="p6-cache.xml" basename="p6-cache"/>
1464</reference>
1465
1466<reference anchor="RFC2119">
1467  <front>
1468    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1469    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1470      <organization>Harvard University</organization>
1471      <address><email>sob@harvard.edu</email></address>
1472    </author>
1473    <date month="March" year="1997"/>
1474  </front>
1475  <seriesInfo name="BCP" value="14"/>
1476  <seriesInfo name="RFC" value="2119"/>
1477</reference>
1478
1479<reference anchor="RFC5234">
1480  <front>
1481    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1482    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1483      <organization>Brandenburg InternetWorking</organization>
1484      <address>
1485        <email>dcrocker@bbiw.net</email>
1486      </address> 
1487    </author>
1488    <author initials="P." surname="Overell" fullname="Paul Overell">
1489      <organization>THUS plc.</organization>
1490      <address>
1491        <email>paul.overell@thus.net</email>
1492      </address>
1493    </author>
1494    <date month="January" year="2008"/>
1495  </front>
1496  <seriesInfo name="STD" value="68"/>
1497  <seriesInfo name="RFC" value="5234"/>
1498</reference>
1499
1500</references>
1501
1502<references title="Informative References">
1503
1504<reference anchor="RFC2616">
1505  <front>
1506    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1507    <author initials="R." surname="Fielding" fullname="R. Fielding">
1508      <organization>University of California, Irvine</organization>
1509      <address><email>fielding@ics.uci.edu</email></address>
1510    </author>
1511    <author initials="J." surname="Gettys" fullname="J. Gettys">
1512      <organization>W3C</organization>
1513      <address><email>jg@w3.org</email></address>
1514    </author>
1515    <author initials="J." surname="Mogul" fullname="J. Mogul">
1516      <organization>Compaq Computer Corporation</organization>
1517      <address><email>mogul@wrl.dec.com</email></address>
1518    </author>
1519    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1520      <organization>MIT Laboratory for Computer Science</organization>
1521      <address><email>frystyk@w3.org</email></address>
1522    </author>
1523    <author initials="L." surname="Masinter" fullname="L. Masinter">
1524      <organization>Xerox Corporation</organization>
1525      <address><email>masinter@parc.xerox.com</email></address>
1526    </author>
1527    <author initials="P." surname="Leach" fullname="P. Leach">
1528      <organization>Microsoft Corporation</organization>
1529      <address><email>paulle@microsoft.com</email></address>
1530    </author>
1531    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1532      <organization>W3C</organization>
1533      <address><email>timbl@w3.org</email></address>
1534    </author>
1535    <date month="June" year="1999"/>
1536  </front>
1537  <seriesInfo name="RFC" value="2616"/>
1538</reference>
1539
1540<reference anchor='RFC3864'>
1541  <front>
1542    <title>Registration Procedures for Message Header Fields</title>
1543    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1544      <organization>Nine by Nine</organization>
1545      <address><email>GK-IETF@ninebynine.org</email></address>
1546    </author>
1547    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1548      <organization>BEA Systems</organization>
1549      <address><email>mnot@pobox.com</email></address>
1550    </author>
1551    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1552      <organization>HP Labs</organization>
1553      <address><email>JeffMogul@acm.org</email></address>
1554    </author>
1555    <date year='2004' month='September' />
1556  </front>
1557  <seriesInfo name='BCP' value='90' />
1558  <seriesInfo name='RFC' value='3864' />
1559</reference>
1560
1561<reference anchor='RFC4918'>
1562  <front>
1563    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1564    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1565      <organization abbrev="CommerceNet">CommerceNet</organization>
1566      <address><email>ldusseault@commerce.net</email></address>
1567    </author>
1568    <date month="June" year="2007" />
1569  </front>
1570  <seriesInfo name='RFC' value='4918' />
1571</reference>
1572</references>
1573
1574<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1575<t>
1576  Allow weak entity-tags in all requests except range requests (Sections
1577  <xref target="weak.and.strong.validators" format="counter"/> and
1578  <xref target="header.if-none-match" format="counter"/>).
1579</t>
1580<t>
1581  Change ETag header field ABNF not to use quoted-string, thus avoiding
1582  escaping issues.
1583  (<xref target="header.etag"/>)
1584</t>
1585<t>
1586  Change ABNF productions for header fields to only define the field value.
1587  (<xref target="header.field.definitions"/>)
1588</t>
1589</section>
1590
1591<?BEGININC p4-conditional.abnf-appendix ?>
1592<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1593<figure>
1594<artwork type="abnf" name="p4-conditional.parsed-abnf">
1595<x:ref>ETag</x:ref> = entity-tag
1596
1597<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
1598
1599<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1600 entity-tag ] ) )
1601<x:ref>If-Modified-Since</x:ref> = HTTP-date
1602<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1603 entity-tag ] ) )
1604<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1605
1606<x:ref>Last-Modified</x:ref> = HTTP-date
1607
1608<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1609
1610<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1611<x:ref>etagc</x:ref> = "!" / %x23-7E ; '#'-'~'
1612 / obs-text
1613
1614<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.3&gt;
1615<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE
1616
1617<x:ref>weak</x:ref> = %x57.2F ; W/
1618</artwork>
1619</figure>
1620<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1621; ETag defined but not used
1622; If-Match defined but not used
1623; If-Modified-Since defined but not used
1624; If-None-Match defined but not used
1625; If-Unmodified-Since defined but not used
1626; Last-Modified defined but not used
1627</artwork></figure></section>
1628<?ENDINC p4-conditional.abnf-appendix ?>
1629
1630<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1631
1632<section title="Since RFC 2616">
1633<t>
1634  Extracted relevant partitions from <xref target="RFC2616"/>.
1635</t>
1636</section>
1637
1638<section title="Since draft-ietf-httpbis-p4-conditional-00">
1639<t>
1640  Closed issues:
1641  <list style="symbols"> 
1642    <t>
1643      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1644      "Normative and Informative references"
1645    </t>
1646  </list>
1647</t>
1648<t>
1649  Other changes:
1650  <list style="symbols"> 
1651    <t>
1652      Move definitions of 304 and 412 condition codes from Part2.
1653    </t>
1654  </list>
1655</t>
1656</section>
1657
1658<section title="Since draft-ietf-httpbis-p4-conditional-01">
1659<t>
1660  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1661  <list style="symbols"> 
1662    <t>
1663      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1664    </t>
1665  </list>
1666</t>
1667</section>
1668
1669<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1670<t>
1671  Closed issues:
1672  <list style="symbols"> 
1673    <t>
1674      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1675      "Weak ETags on non-GET requests"
1676    </t>
1677  </list>
1678</t>
1679<t>
1680  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1681  <list style="symbols"> 
1682    <t>
1683      Reference RFC 3984, and update header field registrations for header fields defined
1684      in this document.
1685    </t>
1686  </list>
1687</t>
1688</section>
1689
1690<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1691<t>
1692  Closed issues:
1693  <list style="symbols"> 
1694    <t>
1695      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1696      "Examples for ETag matching"
1697    </t>
1698    <t>
1699      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1700      "'entity value' undefined"
1701    </t>
1702    <t>
1703      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1704      "bogus 2068 Date header reference"
1705    </t>
1706  </list>
1707</t>
1708</section>
1709
1710<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1711<t>
1712  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1713  <list style="symbols"> 
1714    <t>
1715      Use "/" instead of "|" for alternatives.
1716    </t>
1717    <t>
1718      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1719      whitespace ("OWS") and required whitespace ("RWS").
1720    </t>
1721    <t>
1722      Rewrite ABNFs to spell out whitespace rules, factor out
1723      header field value format definitions.
1724    </t>
1725  </list>
1726</t>
1727</section>
1728
1729<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1730<t>
1731  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1732  <list style="symbols"> 
1733    <t>
1734      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1735    </t>
1736  </list>
1737</t>
1738</section>
1739
1740<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1741<t>
1742  Closed issues:
1743  <list style="symbols"> 
1744    <t>
1745      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1746      "case-sensitivity of etag weakness indicator"
1747    </t>
1748  </list>
1749</t>
1750</section>
1751
1752<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1753<t>
1754  Closed issues:
1755  <list style="symbols"> 
1756    <t>
1757      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1758      "Weak ETags on non-GET requests" (If-Match still was defined to require
1759      strong matching)
1760    </t>
1761    <t>
1762      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1763      "move IANA registrations for optional status codes"
1764    </t>
1765  </list>
1766</t>
1767</section>
1768
1769<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1770<t>
1771  No significant changes.
1772</t>
1773</section>
1774
1775<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1776<t>
1777  No significant changes.
1778</t>
1779</section>
1780
1781<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1782<t>
1783  Closed issues:
1784  <list style="symbols"> 
1785    <t>
1786      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1787      "Clarify 'Requested Variant'"
1788    </t>
1789    <t>
1790      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1791      "Clarify entity / representation / variant terminology"
1792    </t>
1793    <t>
1794      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1795      "consider removing the 'changes from 2068' sections"
1796    </t>
1797  </list>
1798</t>
1799</section>
1800
1801<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1802<t>
1803  None.
1804</t>
1805</section>
1806
1807<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1808<t>
1809  Closed issues:
1810  <list style="symbols"> 
1811    <t>
1812      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1813      "Header Classification"
1814    </t>
1815  </list>
1816</t>
1817</section>
1818
1819<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1820<t>
1821  Closed issues:
1822  <list style="symbols"> 
1823    <t>
1824      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1825      "If-* and entities"
1826    </t>
1827    <t>
1828      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1829      "Definition of validator weakness"
1830    </t>
1831    <t>
1832      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1833      "untangle ABNFs for header fields"
1834    </t>
1835    <t>
1836      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1837      "ETags and Quotes"
1838    </t>
1839  </list>
1840</t>
1841</section>
1842
1843<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1844<t>
1845  None.
1846</t>
1847</section>
1848
1849<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1850<t>
1851  Closed issues:
1852  <list style="symbols"> 
1853    <t>
1854      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1855      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1856    </t>
1857  </list>
1858</t>
1859</section>
1860
1861<section title="Since draft-ietf-httpbis-p4-conditional-16" anchor="changes.since.16">
1862<t>
1863  Closed issues:
1864  <list style="symbols"> 
1865    <t>
1866      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
1867      "Document HTTP's error-handling philosophy"
1868    </t>
1869  </list>
1870</t>
1871</section>
1872
1873<section title="Since draft-ietf-httpbis-p4-conditional-17" anchor="changes.since.17">
1874<t>
1875  Closed issues:
1876  <list style="symbols"> 
1877    <t>
1878      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/306"/>:
1879      "does etag value really use quoted-string"
1880    </t>
1881  </list>
1882</t>
1883</section>
1884
1885</section>
1886
1887</back>
1888</rfc>
Note: See TracBrowser for help on using the repository browser.