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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 "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<x:link rel="prev" basename="p3-payload"/>
50<x:link rel="next" basename="p5-range"/>
51<front>
52
53  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
54
55  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
56    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
57    <address>
58      <postal>
59        <street>345 Park Ave</street>
60        <city>San Jose</city>
61        <region>CA</region>
62        <code>95110</code>
63        <country>USA</country>
64      </postal>
65      <email>fielding@gbiv.com</email>
66      <uri>http://roy.gbiv.com/</uri>
67    </address>
68  </author>
69
70  <author initials="J." surname="Gettys" fullname="Jim Gettys">
71    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
72    <address>
73      <postal>
74        <street>21 Oak Knoll Road</street>
75        <city>Carlisle</city>
76        <region>MA</region>
77        <code>01741</code>
78        <country>USA</country>
79      </postal>
80      <email>jg@freedesktop.org</email>
81      <uri>http://gettys.wordpress.com/</uri>
82    </address>
83  </author>
84 
85  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
86    <organization abbrev="HP">Hewlett-Packard Company</organization>
87    <address>
88      <postal>
89        <street>HP Labs, Large Scale Systems Group</street>
90        <street>1501 Page Mill Road, MS 1177</street>
91        <city>Palo Alto</city>
92        <region>CA</region>
93        <code>94304</code>
94        <country>USA</country>
95      </postal>
96      <email>JeffMogul@acm.org</email>
97    </address>
98  </author>
99
100  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108        <country>USA</country>
109      </postal>
110      <email>henrikn@microsoft.com</email>
111    </address>
112  </author>
113
114  <author initials="L." surname="Masinter" fullname="Larry Masinter">
115    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
116    <address>
117      <postal>
118        <street>345 Park Ave</street>
119        <city>San Jose</city>
120        <region>CA</region>
121        <code>95110</code>
122        <country>USA</country>
123      </postal>
124      <email>LMM@acm.org</email>
125      <uri>http://larry.masinter.net/</uri>
126    </address>
127  </author>
128 
129  <author initials="P." surname="Leach" fullname="Paul J. Leach">
130    <organization abbrev="Microsoft">Microsoft Corporation</organization>
131    <address>
132      <postal>
133        <street>1 Microsoft Way</street>
134        <city>Redmond</city>
135        <region>WA</region>
136        <code>98052</code>
137      </postal>
138      <email>paulle@microsoft.com</email>
139    </address>
140  </author>
141   
142  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
143    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
144    <address>
145      <postal>
146        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
147        <street>The Stata Center, Building 32</street>
148        <street>32 Vassar Street</street>
149        <city>Cambridge</city>
150        <region>MA</region>
151        <code>02139</code>
152        <country>USA</country>
153      </postal>
154      <email>timbl@w3.org</email>
155      <uri>http://www.w3.org/People/Berners-Lee/</uri>
156    </address>
157  </author>
158
159  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
160    <organization abbrev="W3C">World Wide Web Consortium</organization>
161    <address>
162      <postal>
163        <street>W3C / ERCIM</street>
164        <street>2004, rte des Lucioles</street>
165        <city>Sophia-Antipolis</city>
166        <region>AM</region>
167        <code>06902</code>
168        <country>France</country>
169      </postal>
170      <email>ylafon@w3.org</email>
171      <uri>http://www.raubacapeu.net/people/yves/</uri>
172    </address>
173  </author>
174
175  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
176    <organization abbrev="greenbytes">greenbytes GmbH</organization>
177    <address>
178      <postal>
179        <street>Hafenweg 16</street>
180        <city>Muenster</city><region>NW</region><code>48155</code>
181        <country>Germany</country>
182      </postal>
183      <phone>+49 251 2807760</phone>
184      <facsimile>+49 251 2807761</facsimile>
185      <email>julian.reschke@greenbytes.de</email>
186      <uri>http://greenbytes.de/tech/webdav/</uri>
187    </address>
188  </author>
189
190  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
191  <workgroup>HTTPbis Working Group</workgroup>
192
193<abstract>
194<t>
195   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
196   distributed, collaborative, hypertext information systems. HTTP has been in
197   use by the World Wide Web global information initiative since 1990. This
198   document is Part 4 of the seven-part specification that defines the protocol
199   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
200</t>
201<t>
202   Part 4 defines request header fields for indicating conditional requests and
203   the rules for constructing responses to those requests.
204</t>
205</abstract>
206
207<note title="Editorial Note (To be removed by RFC Editor)">
208  <t>
209    Discussion of this draft should take place on the HTTPBIS working group
210    mailing list (ietf-http-wg@w3.org), which is archived at
211    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
212  </t>
213  <t>
214    The current issues list is at
215    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
216    documents (including fancy diffs) can be found at
217    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
218  </t>
219  <t>
220    The changes in this draft are summarized in <xref target="changes.since.17"/>.
221  </t>
222</note>
223</front>
224<middle>
225<section title="Introduction" anchor="introduction">
226<t>
227   This document defines the HTTP/1.1 conditional request mechanisms,
228   including both metadata for indicating/observing changes in resource
229   representations and request header fields that specify preconditions
230   on that metadata be checked before performing the request method.
231   Conditional GET requests are the most efficient mechanism for HTTP
232   cache updates &caching;.  Conditionals can also be
233   applied to state-changing methods, such as PUT and DELETE, to prevent
234   the "lost update" problem: one client accidentally overwriting
235   the work of another client that has been acting in parallel.
236</t>
237<t>
238   Conditional request preconditions are based on the state of the target
239   resource as a whole (its current value set) or the state as observed
240   in a previously obtained representation (one value in that set).
241   A resource might have multiple current representations, each with its
242   own observable state.  The conditional request mechanisms assume that
243   the mapping of requests to corresponding representations will be
244   consistent over time if the server intends to take advantage of
245   conditionals.  Regardless, if the mapping is inconsistent and
246   the server is unable to select the appropriate representation, then
247   no harm will result when the precondition evaluates to false.
248</t>
249<t><iref primary="true" item="selected representation"/>
250   We use the term "<x:dfn>selected representation</x:dfn>" to refer to
251   the current representation of the target resource that would have been
252   selected in a successful response if the same request had used the method
253   GET and had excluded all of the conditional request header fields.
254   The conditional request preconditions are evaluated by comparing the
255   values provided in the request header fields to the current metadata
256   for the selected representation.
257</t>
258
259<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
260<t>
261   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
262   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
263   document are to be interpreted as described in <xref target="RFC2119"/>.
264</t>
265<t>
266   This document defines conformance criteria for several roles in HTTP
267   communication, including Senders, Recipients, Clients, Servers, User-Agents,
268   Origin Servers, Intermediaries, Proxies and Gateways. See &architecture;
269   for definitions of these terms.
270</t>
271<t>
272   An implementation is considered conformant if it complies with all of the
273   requirements associated with its role(s). Note that SHOULD-level requirements
274   are relevant here, unless one of the documented exceptions is applicable.
275</t>
276<t>
277   This document also uses ABNF to define valid protocol elements
278   (<xref target="notation"/>). In addition to the prose requirements placed
279   upon them, Senders &MUST-NOT; generate protocol elements that are invalid.
280</t>
281<t>
282   Unless noted otherwise, Recipients &MAY; take steps to recover a usable
283   protocol element from an invalid construct. However, HTTP does not define
284   specific error handling mechanisms, except in cases where it has direct
285   impact on security. This is because different uses of the protocol require
286   different error handling strategies; for example, a Web browser may wish to
287   transparently recover from a response where the Location header field
288   doesn't parse according to the ABNF, whereby in a systems control protocol
289   using HTTP, this type of error recovery could lead to dangerous consequences.
290</t>
291</section>
292
293<section title="Syntax Notation" anchor="notation">
294  <x:anchor-alias value="ALPHA"/>
295  <x:anchor-alias value="CR"/>
296  <x:anchor-alias value="DIGIT"/>
297  <x:anchor-alias value="DQUOTE"/>
298  <x:anchor-alias value="LF"/>
299  <x:anchor-alias value="OCTET"/>
300  <x:anchor-alias value="VCHAR"/>
301  <x:anchor-alias value="core.rules"/>
302  <x:anchor-alias value="obs-text"/>
303  <x:anchor-alias value="OWS"/>
304  <x:anchor-alias value="HTTP-date"/>
305<t>
306  This specification uses the ABNF syntax defined in &notation; (which
307  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
308  <xref target="collected.abnf"/> shows the collected ABNF, with the list
309  rule expanded.
310</t>
311<t>
312  The following core rules are included by
313  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
314  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
315  DIGIT (decimal 0-9), DQUOTE (double quote),
316  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
317  OCTET (any 8-bit sequence of data), SP (space), and
318  VCHAR (any visible US-ASCII character).
319</t>
320<t>
321  The ABNF rules below are defined in <xref target="Part1"/> and
322  <xref target="Part2"/>:
323</t>
324<figure><artwork type="abnf2616">
325  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
326  <x:ref>obs-text</x:ref>      = &lt;obs-text, defined in &field-rules;&gt;
327  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
328</artwork></figure>
329</section>
330</section>
331
332<section title="Validators" anchor="validators">
333   <iref primary="true" item="metadata"/>
334   <iref primary="true" item="validator"/>
335<t>
336   This specification defines two forms of metadata that are commonly used
337   to observe resource state and test for preconditions: modification dates
338   and opaque entity tags.  Additional metadata that reflects resource state
339   has been defined by various extensions of HTTP, such as WebDAV
340   <xref target="RFC4918"/>, that are beyond the scope of this specification.
341   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
342   when it is used within a precondition.
343</t>
344
345<section title="Weak versus Strong" anchor="weak.and.strong.validators">
346   <iref primary="true" item="validator" subitem="weak"/>
347   <iref primary="true" item="validator" subitem="strong"/>
348<t>
349   Validators come in two flavors: strong or weak.  Weak validators are easy
350   to generate but are far less useful for comparisons.  Strong validators
351   are ideal for comparisons but can be very difficult (and occasionally
352   impossible) to generate efficiently.  Rather than impose that all forms
353   of resource adhere to the same strength of validator, HTTP exposes the
354   type of validator in use and imposes restrictions on when weak validators
355   can be used as preconditions.
356</t>
357<t>
358   A "strong validator" is a representation metadata value that &MUST; be
359   changed to a new, previously unused or guaranteed unique, value whenever
360   a change occurs to the representation data such that a change would be
361   observable in the payload body of a 200 response to GET.  A strong
362   validator &MAY; be changed for other reasons, such as when a semantically
363   significant part of the representation metadata is changed (e.g.,
364   Content-Type), but it is in the best interests of the origin server to only
365   change the value when it is necessary to invalidate the stored responses
366   held by remote caches and authoring tools.  A strong validator &MUST; be
367   unique across all representations of a given resource, such that no two
368   representations of that resource share the same validator unless
369   their payload body would be identical.
370</t>
371<t>
372   Cache entries might persist for arbitrarily long periods, regardless
373   of expiration times.  Thus, a cache might attempt to validate an
374   entry using a validator that it obtained in the distant past.
375   A strong validator &MUST; be unique across all versions of all
376   representations associated with a particular resource over time.
377   However, there is no implication of uniqueness across representations
378   of different resources (i.e., the same strong validator might be
379   in use for representations of multiple resources at the same time
380   and does not imply that those representations are equivalent).
381</t>
382<t>
383   There are a variety of strong validators used in practice.  The best are
384   based on strict revision control, wherein each change to a representation
385   always results in a unique node name and revision identifier being assigned
386   before the representation is made accessible to GET.  A cryptographic hash
387   function applied to the representation data is also sufficient if the data
388   is available prior to the response header fields being sent and the digest
389   does not need to be recalculated every time a validation request is
390   received.  However, if a resource has distinct representations that differ
391   only in their metadata, such as might occur with content negotiation over
392   media types that happen to share the same data format, then a server
393   &SHOULD; incorporate additional information in the validator to
394   distinguish those representations and avoid confusing cache behavior.
395</t>
396<t>
397   In contrast, a "weak validator" is a representation metadata value that
398   might not be changed for every change to the representation data.  This
399   weakness might be due to limitations in how the value is calculated, such
400   as clock resolution or an inability to ensure uniqueness for all possible
401   representations of the resource, or due to a desire by the resource owner
402   to group representations by some self-determined set of equivalency
403   rather than unique sequences of data.  A weak entity-tag &SHOULD; change
404   whenever the origin server considers prior representations to be
405   unacceptable as a substitute for the current representation. In other
406   words, a weak entity-tag &SHOULD; change whenever the origin server wants
407   caches to invalidate old responses.
408</t>
409<t>
410   For example, the representation of a weather report that changes in
411   content every second, based on dynamic measurements, might be grouped
412   into sets of equivalent representations (from the origin server's
413   perspective) with the same weak validator in order to allow cached
414   representations to be valid for a reasonable period of time (perhaps
415   adjusted dynamically based on server load or weather quality).
416   Likewise, a representation's modification time, if defined with only
417   one-second resolution, might be a weak validator if it is possible
418   for the representation to be modified twice during a single second and
419   retrieved between those modifications.
420</t>
421<t>
422   A "use" of a validator occurs when either a client generates a request
423   and includes the validator in a precondition or when a server
424   compares two validators.
425   Weak validators are only usable in contexts that do not depend on exact
426   equality of a representation's payload body.
427   Strong validators are usable and preferred for all conditional requests,
428   including cache validation, partial content ranges, and "lost update"
429   avoidance.
430</t>
431</section>
432
433<section title="Last-Modified" anchor="header.last-modified">
434  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
435  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
436  <x:anchor-alias value="Last-Modified"/>
437<t>
438   The "Last-Modified" header field indicates the date and time at
439   which the origin server believes the selected representation was
440   last modified.
441</t>
442<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
443  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
444</artwork></figure>
445<t>
446   An example of its use is
447</t>
448<figure><artwork type="example">
449  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
450</artwork></figure>
451
452<section title="Generation" anchor="lastmod.generation">
453<t>
454   Origin servers &SHOULD; send Last-Modified for any selected
455   representation for which a last modification date can be reasonably
456   and consistently determined, since its use in conditional requests
457   and evaluating cache freshness (&caching;) results in a substantial
458   reduction of HTTP traffic on the Internet and can be a significant
459   factor in improving service scalability and reliability.
460</t>
461<t>
462   A representation is typically the sum of many parts behind the
463   resource interface.  The last-modified time would usually be
464   the most recent time that any of those parts were changed.
465   How that value is determined for any given resource is an
466   implementation detail beyond the scope of this specification.
467   What matters to HTTP is how recipients of the Last-Modified
468   header field can use its value to make conditional requests
469   and test the validity of locally cached responses.
470</t>
471<t>
472   An origin server &SHOULD; obtain the Last-Modified value of the
473   representation as close as possible to the time that it generates
474   the Date field-value for its response. This allows a recipient to
475   make an accurate assessment of the representation's modification time,
476   especially if the representation changes near the time that the
477   response is generated.
478</t>
479<t>
480   An origin server with a clock &MUST-NOT; send a Last-Modified date
481   that is later than the server's time of message origination (Date).
482   If the last modification time is derived from implementation-specific
483   metadata that evaluates to some time in the future, according to the
484   origin server's clock, then the origin server &MUST; replace that
485   value with the message origination date. This prevents a future
486   modification date from having an adverse impact on cache validation.
487</t>
488<t>
489   An origin server without a clock &MUST-NOT; assign Last-Modified
490   values to a response unless these values were associated
491   with the resource by some other system or user with a reliable clock.
492</t>
493</section>
494
495<section title="Comparison" anchor="lastmod.comparison">
496<t>
497   A Last-Modified time, when used as a validator in a request, is
498   implicitly weak unless it is possible to deduce that it is strong,
499   using the following rules:
500  <list style="symbols">
501     <t>The validator is being compared by an origin server to the
502        actual current validator for the representation and,</t>
503     <t>That origin server reliably knows that the associated representation did
504        not change twice during the second covered by the presented
505        validator.</t>
506  </list>
507</t>
508<t>
509   or
510  <list style="symbols">
511     <t>The validator is about to be used by a client in an If-Modified-Since,
512        If-Unmodified-Since header field, because the client has a cache entry,
513        or If-Range for the associated representation, and</t>
514     <t>That cache entry includes a Date value, which gives the time
515        when the origin server sent the original response, and</t>
516     <t>The presented Last-Modified time is at least 60 seconds before
517        the Date value.</t>
518  </list>
519</t>
520<t>
521   or
522  <list style="symbols">
523     <t>The validator is being compared by an intermediate cache to the
524        validator stored in its cache entry for the representation, and</t>
525     <t>That cache entry includes a Date value, which gives the time
526        when the origin server sent the original response, and</t>
527     <t>The presented Last-Modified time is at least 60 seconds before
528        the Date value.</t>
529  </list>
530</t>
531<t>
532   This method relies on the fact that if two different responses were
533   sent by the origin server during the same second, but both had the
534   same Last-Modified time, then at least one of those responses would
535   have a Date value equal to its Last-Modified time. The arbitrary 60-second
536   limit guards against the possibility that the Date and Last-Modified
537   values are generated from different clocks, or at somewhat
538   different times during the preparation of the response. An
539   implementation &MAY; use a value larger than 60 seconds, if it is
540   believed that 60 seconds is too short.
541</t>
542</section>
543</section>
544
545<section title="ETag" anchor="header.etag">
546  <iref primary="true" item="ETag header field" x:for-anchor=""/>
547  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
548  <x:anchor-alias value="ETag"/>
549  <x:anchor-alias value="entity-tag"/>
550  <x:anchor-alias value="entity.tags"/>
551  <x:anchor-alias value="opaque-tag"/>
552  <x:anchor-alias value="weak"/>
553  <x:anchor-alias value="etagc"/>
554<t>
555   The ETag header field provides the current entity-tag for the
556   selected representation.
557   An entity-tag is an opaque validator for differentiating between
558   multiple representations of the same resource, regardless of whether
559   those multiple representations are due to resource state changes over
560   time, content negotiation resulting in multiple representations being
561   valid at the same time, or both. An entity-tag consists of an opaque
562   quoted string, possibly prefixed by a weakness indicator.
563</t>
564<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"/>
565  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
566
567  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
568  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
569  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
570  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
571             ; <xref>VCHAR</xref> except double quotes, plus obs-text
572</artwork></figure>
573<x:note>
574  <t>
575    <x:h>Note:</x:h> Previously, opaque-tag was defined to be a quoted-string
576    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
577    might perform backslash unescaping. Servers therefore ought to avoid
578    backslash characters in entity tags.
579  </t>
580</x:note>
581<t>
582   An entity-tag can be more reliable for validation than a modification
583   date in situations where it is inconvenient to store modification
584   dates, where the one-second resolution of HTTP date values is not
585   sufficient, or where modification dates are not consistently maintained.
586</t>
587<figure><preamble>
588  Examples:
589</preamble>
590<artwork type="example">
591  ETag: "xyzzy"
592  ETag: W/"xyzzy"
593  ETag: ""
594</artwork></figure>
595<t>
596   An entity-tag can be either a weak or strong validator, with
597   strong being the default.  If an origin server provides an entity-tag
598   for a representation and the generation of that entity-tag does not satisfy
599   the requirements for a strong validator
600   (<xref target="weak.and.strong.validators"/>), then that
601   entity-tag &MUST; be marked as weak by prefixing its opaque value
602   with "W/" (case-sensitive).
603</t>
604
605<section title="Generation" anchor="entity.tag.generation">
606<t>
607   The principle behind entity-tags is that only the service author
608   knows the implementation of a resource well enough to select the
609   most accurate and efficient validation mechanism for that resource,
610   and that any such mechanism can be mapped to a simple sequence of
611   octets for easy comparison.  Since the value is opaque, there is no
612   need for the client to be aware of how each entity-tag is constructed.
613</t>
614<t>
615   For example, a resource that has implementation-specific versioning
616   applied to all changes might use an internal revision number, perhaps
617   combined with a variance identifier for content negotiation, to
618   accurately differentiate between representations.
619   Other implementations might use a stored hash of representation content,
620   a combination of various filesystem attributes, or a modification
621   timestamp that has sub-second resolution.
622</t>
623<t>
624   Origin servers &SHOULD; send ETag for any selected representation
625   for which detection of changes can be reasonably and consistently
626   determined, since the entity-tag's use in conditional requests and
627   evaluating cache freshness (&caching;) can result in a substantial
628   reduction of HTTP network traffic and can be a significant factor in
629   improving service scalability and reliability.
630</t>
631</section>
632
633<section title="Comparison" anchor="entity.tag.comparison">
634  <x:anchor-alias value="validator.comparison"/>
635<t>
636   There are two entity-tag comparison functions, depending
637   on whether the comparison context allows the use of weak validators
638   or not:
639  <list style="symbols">
640     <t>The strong comparison function: in order to be considered equal,
641        both opaque-tags &MUST; be identical character-by-character, and both
642        &MUST-NOT; be weak.</t>
643     <t>The weak comparison function: in order to be considered equal, both
644        opaque-tags &MUST; be identical character-by-character, but
645        either or both of them &MAY; be tagged as "weak" without affecting
646        the result.</t>
647  </list>
648</t>
649<t>
650   The example below shows the results for a set of entity-tag pairs,
651   and both the weak and strong comparison function results:
652</t>
653<texttable align="left">
654  <ttcol>ETag 1</ttcol>
655  <ttcol>ETag 2</ttcol>
656  <ttcol>Strong Comparison</ttcol>
657  <ttcol>Weak Comparison</ttcol>
658
659  <c>W/"1"</c>
660  <c>W/"1"</c>
661  <c>no match</c>
662  <c>match</c>
663 
664  <c>W/"1"</c>
665  <c>W/"2"</c>
666  <c>no match</c>
667  <c>no match</c>
668
669  <c>W/"1"</c>
670  <c>"1"</c>
671  <c>no match</c>
672  <c>match</c>
673
674  <c>"1"</c>
675  <c>"1"</c>
676  <c>match</c>
677  <c>match</c>
678</texttable>
679</section>
680
681<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
682<t>
683   Consider a resource that is subject to content negotiation (&content-negotiation;),
684   and where the representations returned upon a GET request vary based on
685   the Accept-Encoding request header field (&header-accept-encoding;):
686</t>
687<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
688GET /index HTTP/1.1
689Host: www.example.com
690Accept-Encoding: gzip
691
692</artwork></figure>
693<t>
694   In this case, the response might or might not use the gzip content coding.
695   If it does not, the response might look like:
696</t>
697<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
698HTTP/1.1 200 OK
699Date: Thu, 26 Mar 2010 00:05:00 GMT
700ETag: "123-a"
701Content-Length: <x:length-of target="exbody"/>
702Vary: Accept-Encoding
703Content-Type: text/plain
704
705<x:span anchor="exbody">Hello World!
706Hello World!
707Hello World!
708Hello World!
709Hello World!
710</x:span></artwork></figure>
711<t>
712   An alternative representation that does use gzip content coding would be:
713</t>
714<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
715HTTP/1.1 200 OK
716Date: Thu, 26 Mar 2010 00:05:00 GMT
717ETag: "123-b"
718Content-Length: 43
719Vary: Accept-Encoding
720Content-Type: text/plain
721Content-Encoding: gzip
722
723<spanx>...binary data...</spanx></artwork></figure>
724<x:note>
725  <t>
726    <x:h>Note:</x:h> Content codings are a property of the representation,
727    so therefore an entity-tag of an encoded representation must be distinct
728    from an unencoded representation to prevent conflicts during cache updates
729    and range requests.  In contrast, transfer codings (&transfer-codings;)
730    apply only during message transfer and do not require distinct entity-tags.
731  </t>
732</x:note>
733</section>
734</section>
735
736<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">
737<t>
738   We adopt a set of rules and recommendations for origin servers,
739   clients, and caches regarding when various validator types ought to
740   be used, and for what purposes.
741</t>
742<t>
743   HTTP/1.1 origin servers:
744  <list style="symbols">
745     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
746        generate one.</t>
747
748     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
749        performance considerations support the use of weak entity-tags,
750        or if it is unfeasible to send a strong entity-tag.</t>
751
752     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
753  </list>
754</t>
755<t>
756   In other words, the preferred behavior for an HTTP/1.1 origin server
757   is to send both a strong entity-tag and a Last-Modified value.
758</t>
759<t>
760   HTTP/1.1 clients:
761  <list style="symbols">
762     <t>&MUST; use that entity-tag in any cache-conditional request (using
763        If-Match or If-None-Match) if an entity-tag has been provided by the
764        origin server.</t>
765
766     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
767        requests (using If-Modified-Since) if only a Last-Modified value has
768        been provided by the origin server. </t>
769
770     <t>&MAY; use the Last-Modified value in subrange cache-conditional
771        requests (using If-Unmodified-Since) if only a Last-Modified value has
772        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
773        provide a way to disable this, in case of difficulty.</t>
774
775     <t>&SHOULD; use both validators in cache-conditional requests if both an
776        entity-tag and a Last-Modified value have been provided by the origin
777        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
778        appropriately.</t>
779  </list>
780</t>
781<t>
782   An HTTP/1.1 origin server, upon receiving a conditional request that
783   includes both a Last-Modified date (e.g., in an If-Modified-Since or
784   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
785   in an If-Match, If-None-Match, or If-Range header field) as cache
786   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
787   unless doing so is consistent with all of the conditional header
788   fields in the request.
789</t>
790<t>
791   An HTTP/1.1 caching proxy, upon receiving a conditional request that
792   includes both a Last-Modified date and one or more entity-tags as
793   cache validators, &MUST-NOT; return a locally cached response to the
794   client unless that cached response is consistent with all of the
795   conditional header fields in the request.
796  <list><t>
797      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
798      servers and clients ought to transmit as much non-redundant
799      information as is available in their responses and requests.
800      HTTP/1.1 systems receiving this information will make the most
801      conservative assumptions about the validators they receive.
802  </t><t>
803      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
804      last-modified values received or used by these systems will
805      support transparent and efficient caching, and so HTTP/1.1 origin
806      servers should provide Last-Modified values. In those rare cases
807      where the use of a Last-Modified value as a validator by an
808      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
809      origin servers should not provide one.
810  </t></list>
811</t>
812</section>
813</section>
814
815<section title="Precondition Header Fields" anchor="header.field.definitions">
816<t>
817   This section defines the syntax and semantics of HTTP/1.1 header fields
818   for applying preconditions on requests.
819</t>
820
821<section title="If-Match" anchor="header.if-match">
822  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
823  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
824  <x:anchor-alias value="If-Match"/>
825<t>
826   The "If-Match" header field &MAY; be used to make a request method
827   conditional on the current existence or value of an entity-tag for
828   one or more representations of the target resource.  If-Match is
829   generally useful for resource update requests, such as PUT requests,
830   as a means for protecting against accidental overwrites when multiple
831   clients are acting in parallel on the same resource (i.e., the
832   "lost update" problem).  An If-Match field-value of "*" places the
833   precondition on the existence of any current representation for the
834   target resource.
835</t>
836<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
837  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
838</artwork></figure>
839<t>
840   If any of the entity-tags listed in the If-Match field value match
841   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
842   selected representation for the target resource,
843   or if "*" is given and any current representation exists for the
844   target resource, then the server &MAY; perform the request method
845   as if the If-Match header field was not present.
846</t>
847<t>
848   If none of the entity-tags match, or if "*" is given and no current
849   representation exists, the server &MUST-NOT; perform the requested method.
850   Instead, the server &MUST; respond with the 412 (Precondition Failed)
851   status code.
852</t>
853<t>
854   If the request would, without the If-Match header field, result in
855   anything other than a 2xx or 412 status code, then the If-Match header field
856   &MUST; be ignored.
857</t>
858<t>
859   Examples:
860</t>
861<figure><artwork type="example">
862  If-Match: "xyzzy"
863  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
864  If-Match: *
865</artwork></figure>
866<t>
867   The result of a request having both an If-Match header field and
868   either an If-None-Match or an If-Modified-Since header fields is
869   undefined by this specification.
870</t>
871</section>
872
873<section title="If-None-Match" anchor="header.if-none-match">
874  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
875  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
876  <x:anchor-alias value="If-None-Match"/>
877<t>
878   The "If-None-Match" header field &MAY; be used to make a request method
879   conditional on not matching any of the current entity-tag values for
880   representations of the target resource.  If-None-Match is primarily
881   used in conditional GET requests to enable efficient updates of cached
882   information with a minimum amount of transaction overhead.  A client
883   that has one or more representations previously obtained from the
884   target resource can send If-None-Match with a list of the associated
885   entity-tags in the hope of receiving a 304 response if at least one
886   of those representations matches the selected representation.
887</t>
888<t>
889   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
890   request method (e.g., PUT) from inadvertently modifying an existing
891   representation of the target resource when the client believes that
892   the resource does not have a current representation.  This is a variation
893   on the "lost update" problem that might arise if more than one client
894   attempts to create an initial representation for the target resource.
895</t>
896<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
897  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
898</artwork></figure>
899<t>
900   If any of the entity-tags listed in the If-None-Match field-value match
901   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
902   selected representation, or if "*" is
903   given and any current representation exists for that resource, then the
904   server &MUST-NOT; perform the requested method.
905   Instead, if the request method was GET or HEAD, the server &SHOULD;
906   respond with a 304 (Not Modified) status code, including the cache-related
907   header fields (particularly ETag) of the selected representation that has
908   a matching entity-tag.  For all other request methods, the server &MUST;
909   respond with a 412 (Precondition Failed) status code.
910</t>
911<t>
912   If none of the entity-tags match, then the server &MAY; perform the
913   requested method as if the If-None-Match header field did not exist,
914   but &MUST; also ignore any If-Modified-Since header field(s) in the
915   request. That is, if no entity-tags match, then the server &MUST-NOT;
916   return a 304 (Not Modified) response.
917</t>
918<t>
919   If the request would, without the If-None-Match header field, result
920   in anything other than a 2xx or 304 status code, then the If-None-Match
921   header field &MUST; be ignored. (See <xref
922   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
923   a discussion of server behavior when both If-Modified-Since and
924   If-None-Match appear in the same request.)
925</t>
926<t>
927   Examples:
928</t>
929<figure><artwork type="example">
930  If-None-Match: "xyzzy"
931  If-None-Match: W/"xyzzy"
932  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
933  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
934  If-None-Match: *
935</artwork></figure>
936<t>
937   The result of a request having both an If-None-Match header field and
938   either an If-Match or an If-Unmodified-Since header fields is
939   undefined by this specification.
940</t>
941</section>
942
943<section title="If-Modified-Since" anchor="header.if-modified-since">
944  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
945  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
946  <x:anchor-alias value="If-Modified-Since"/>
947<t>
948   The "If-Modified-Since" header field &MAY; be used to make a request
949   method conditional by modification date: if the selected representation
950   has not been modified since the time specified in this field, then
951   do not perform the request method; instead, respond as detailed below.
952</t>
953<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
954  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
955</artwork></figure>
956<t>
957   An example of the field is:
958</t>
959<figure><artwork type="example">
960  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
961</artwork></figure>
962<t>
963   A GET method with an If-Modified-Since header field and no Range header
964   field requests that the selected representation be transferred only if
965   it has been modified since the date given by the If-Modified-Since
966   header field.
967   The algorithm for determining this includes the following cases:
968  <list style="numbers">
969      <t>If the request would normally result in anything other than a
970         200 (OK) status code, or if the passed If-Modified-Since date is
971         invalid, the response is exactly the same as for a normal GET.
972         A date which is later than the server's current time is
973         invalid.</t>
974
975      <t>If the selected representation has been modified since the
976         If-Modified-Since date, the response is exactly the same as for
977         a normal GET.</t>
978
979      <t>If the selected representation has not been modified since a valid
980         If-Modified-Since date, the server &SHOULD; return a
981         304 (Not Modified) response.</t>
982  </list>
983</t>
984<t>
985   The purpose of this feature is to allow efficient updates of cached
986   information with a minimum amount of transaction overhead.
987  <list><t>
988      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
989      see &header-range; for full details.
990    </t><t>
991      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
992      clock might not be synchronized with the client.
993    </t><t>
994      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
995      servers will use an exact date comparison function, rather than a
996      less-than function, for deciding whether to send a 304 (Not
997      Modified) response. To get best results when sending an If-Modified-Since
998      header field for cache validation, clients are
999      advised to use the exact date string received in a previous Last-Modified
1000      header field whenever possible.
1001    </t><t>
1002      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
1003      header field instead of a date taken from the Last-Modified header field for
1004      the same request, the client needs to be aware that this
1005      date is interpreted in the server's understanding of time.
1006      Unsynchronized clocks and rounding problems, due to the different
1007      encodings of time between the client and server, are concerns.
1008      This includes the possibility of race conditions if the
1009      document has changed between the time it was first requested and
1010      the If-Modified-Since date of a subsequent request, and the
1011      possibility of clock-skew-related problems if the If-Modified-Since
1012      date is derived from the client's clock without correction
1013      to the server's clock. Corrections for different time bases
1014      between client and server are at best approximate due to network
1015      latency.
1016    </t>
1017  </list>
1018</t>
1019<t>
1020   The result of a request having both an If-Modified-Since header field
1021   and either an If-Match or an If-Unmodified-Since header fields is
1022   undefined by this specification.
1023</t>
1024</section>
1025
1026<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
1027  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
1028  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
1029  <x:anchor-alias value="If-Unmodified-Since"/>
1030<t>
1031   The "If-Unmodified-Since" header field &MAY; be used to make a request
1032   method conditional by modification date: if the selected representation
1033   has been modified since the time specified in this field, then the
1034   server &MUST-NOT; perform the requested operation and &MUST; instead
1035   respond with the 412 (Precondition Failed) status code.
1036   If the selected representation has not been modified since the time
1037   specified in this field, the server &SHOULD; perform the request
1038   method as if the If-Unmodified-Since header field were not present.
1039</t>
1040<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
1041  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
1042</artwork></figure>
1043<t>
1044   An example of the field is:
1045</t>
1046<figure><artwork type="example">
1047  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1048</artwork></figure>
1049<t>
1050   If the request normally (i.e., without the If-Unmodified-Since
1051   header field) would result in anything other than a 2xx or 412 status code,
1052   the If-Unmodified-Since header field &SHOULD; be ignored.
1053</t>
1054<t>
1055   If the specified date is invalid, the header field &MUST; be ignored.
1056</t>
1057<t>
1058   The result of a request having both an If-Unmodified-Since header
1059   field and either an If-None-Match or an If-Modified-Since header
1060   fields is undefined by this specification.
1061</t>
1062</section>
1063
1064<section title="If-Range" anchor="header.if-range">
1065<t>
1066   The If-Range header field provides a special conditional request
1067   mechanism that is similar to If-Match and If-Unmodified-Since but
1068   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1069</t>
1070</section>
1071
1072</section>
1073
1074<section title="Status Code Definitions" anchor="status.code.definitions">
1075<section title="304 Not Modified" anchor="status.304">
1076  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1077  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1078<t>
1079   The 304 status code indicates that a conditional GET request has been
1080   received and would have resulted in a 200 (OK) response if it were not
1081   for the fact that the condition has evaluated to false.  In other words,
1082   there is no need for the server to transfer a representation of the
1083   target resource because the client's request indicates that it already
1084   has a valid representation, as indicated by the 304 response header
1085   fields, and is therefore redirecting the client to make use of that
1086   stored representation as if it were the payload of a 200 response.
1087   The 304 response &MUST-NOT; contain a message-body, and thus is always
1088   terminated by the first empty line after the header fields.
1089</t>
1090<t>
1091   A 304 response &MUST; include a Date header field (&header-date;)
1092   unless the origin server does not have a clock that can provide a
1093   reasonable approximation of the current time.  If a 200 response
1094   to the same request would have included any of the header fields
1095   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1096   Vary, then those same header fields &MUST; be sent in a 304 response.
1097</t>
1098<t>
1099   Since the goal of a 304 response is to minimize information transfer
1100   when the recipient already has one or more cached representations,
1101   the response &SHOULD-NOT; include representation metadata other
1102   than the above listed fields unless said metadata exists for the
1103   purpose of guiding cache updates (e.g., future HTTP extensions).
1104</t>
1105<t>
1106   If the recipient of a 304 response does not have a cached representation
1107   corresponding to the entity-tag indicated by the 304 response, then the
1108   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1109   conditional request originated with an outbound client, such as a
1110   user agent with its own cache sending a conditional GET to a shared
1111   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1112   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1113   the request without any preconditions.
1114</t>
1115<t>
1116   If a cache uses a received 304 response to update a cache entry, the
1117   cache &MUST; update the entry to reflect any new field values given in
1118   the response.
1119</t>
1120</section>
1121
1122<section title="412 Precondition Failed" anchor="status.412">
1123  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1124  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1125<t>
1126   The 412 status code indicates that one or more preconditions given in
1127   the request header fields evaluated to false when tested on the server.
1128   This response code allows the client to place preconditions on the
1129   current resource state (its current representations and metadata)
1130   and thus prevent the request method from being applied if the target
1131   resource is in an unexpected state.
1132</t>
1133</section>
1134</section>
1135
1136<section title="IANA Considerations" anchor="IANA.considerations">
1137
1138<section title="Status Code Registration" anchor="status.code.registration">
1139<t>
1140   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1141   shall be updated with the registrations below:
1142</t>
1143<?BEGININC p4-conditional.iana-status-codes ?>
1144<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1145<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1146   <ttcol>Value</ttcol>
1147   <ttcol>Description</ttcol>
1148   <ttcol>Reference</ttcol>
1149   <c>304</c>
1150   <c>Not Modified</c>
1151   <c>
1152      <xref target="status.304"/>
1153   </c>
1154   <c>412</c>
1155   <c>Precondition Failed</c>
1156   <c>
1157      <xref target="status.412"/>
1158   </c>
1159</texttable>
1160<!--(END)-->
1161<?ENDINC p4-conditional.iana-status-codes ?>
1162</section>
1163
1164<section title="Header Field Registration" anchor="header.field.registration">
1165<t>
1166   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1167   with the permanent registrations below (see <xref target="RFC3864"/>):
1168</t>
1169<?BEGININC p4-conditional.iana-headers ?>
1170<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1171<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1172   <ttcol>Header Field Name</ttcol>
1173   <ttcol>Protocol</ttcol>
1174   <ttcol>Status</ttcol>
1175   <ttcol>Reference</ttcol>
1176
1177   <c>ETag</c>
1178   <c>http</c>
1179   <c>standard</c>
1180   <c>
1181      <xref target="header.etag"/>
1182   </c>
1183   <c>If-Match</c>
1184   <c>http</c>
1185   <c>standard</c>
1186   <c>
1187      <xref target="header.if-match"/>
1188   </c>
1189   <c>If-Modified-Since</c>
1190   <c>http</c>
1191   <c>standard</c>
1192   <c>
1193      <xref target="header.if-modified-since"/>
1194   </c>
1195   <c>If-None-Match</c>
1196   <c>http</c>
1197   <c>standard</c>
1198   <c>
1199      <xref target="header.if-none-match"/>
1200   </c>
1201   <c>If-Unmodified-Since</c>
1202   <c>http</c>
1203   <c>standard</c>
1204   <c>
1205      <xref target="header.if-unmodified-since"/>
1206   </c>
1207   <c>Last-Modified</c>
1208   <c>http</c>
1209   <c>standard</c>
1210   <c>
1211      <xref target="header.last-modified"/>
1212   </c>
1213</texttable>
1214<!--(END)-->
1215<?ENDINC p4-conditional.iana-headers ?>
1216<t>
1217   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1218</t>
1219</section>
1220</section>
1221
1222<section title="Security Considerations" anchor="security.considerations">
1223<t>
1224   No additional security considerations have been identified beyond
1225   those applicable to HTTP in general &messaging;.
1226</t>
1227</section>
1228
1229<section title="Acknowledgments" anchor="acks">
1230<t>
1231  See &acks;.
1232</t>
1233</section>
1234</middle>
1235<back>
1236
1237<references title="Normative References">
1238
1239<reference anchor="Part1">
1240  <front>
1241    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1242    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1243      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1244      <address><email>fielding@gbiv.com</email></address>
1245    </author>
1246    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1247      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1248      <address><email>jg@freedesktop.org</email></address>
1249    </author>
1250    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1251      <organization abbrev="HP">Hewlett-Packard Company</organization>
1252      <address><email>JeffMogul@acm.org</email></address>
1253    </author>
1254    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1255      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1256      <address><email>henrikn@microsoft.com</email></address>
1257    </author>
1258    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1259      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1260      <address><email>LMM@acm.org</email></address>
1261    </author>
1262    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1263      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1264      <address><email>paulle@microsoft.com</email></address>
1265    </author>
1266    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1267      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1268      <address><email>timbl@w3.org</email></address>
1269    </author>
1270    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1271      <organization abbrev="W3C">World Wide Web Consortium</organization>
1272      <address><email>ylafon@w3.org</email></address>
1273    </author>
1274    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1275      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1276      <address><email>julian.reschke@greenbytes.de</email></address>
1277    </author>
1278    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1279  </front>
1280  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1281  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1282</reference>
1283
1284<reference anchor="Part2">
1285  <front>
1286    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1287    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1288      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1289      <address><email>fielding@gbiv.com</email></address>
1290    </author>
1291    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1292      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1293      <address><email>jg@freedesktop.org</email></address>
1294    </author>
1295    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1296      <organization abbrev="HP">Hewlett-Packard Company</organization>
1297      <address><email>JeffMogul@acm.org</email></address>
1298    </author>
1299    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1300      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1301      <address><email>henrikn@microsoft.com</email></address>
1302    </author>
1303    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1304      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1305      <address><email>LMM@acm.org</email></address>
1306    </author>
1307    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1308      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1309      <address><email>paulle@microsoft.com</email></address>
1310    </author>
1311    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1312      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1313      <address><email>timbl@w3.org</email></address>
1314    </author>
1315    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1316      <organization abbrev="W3C">World Wide Web Consortium</organization>
1317      <address><email>ylafon@w3.org</email></address>
1318    </author>
1319    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1320      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1321      <address><email>julian.reschke@greenbytes.de</email></address>
1322    </author>
1323    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1324  </front>
1325  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1326  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
1327</reference>
1328
1329<reference anchor="Part3">
1330  <front>
1331    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1332    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1333      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1334      <address><email>fielding@gbiv.com</email></address>
1335    </author>
1336    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1337      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1338      <address><email>jg@freedesktop.org</email></address>
1339    </author>
1340    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1341      <organization abbrev="HP">Hewlett-Packard Company</organization>
1342      <address><email>JeffMogul@acm.org</email></address>
1343    </author>
1344    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1345      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1346      <address><email>henrikn@microsoft.com</email></address>
1347    </author>
1348    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1349      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1350      <address><email>LMM@acm.org</email></address>
1351    </author>
1352    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1353      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1354      <address><email>paulle@microsoft.com</email></address>
1355    </author>
1356    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1357      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1358      <address><email>timbl@w3.org</email></address>
1359    </author>
1360    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1361      <organization abbrev="W3C">World Wide Web Consortium</organization>
1362      <address><email>ylafon@w3.org</email></address>
1363    </author>
1364    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1365      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1366      <address><email>julian.reschke@greenbytes.de</email></address>
1367    </author>
1368    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1369  </front>
1370  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1371  <x:source href="p3-payload.xml" basename="p3-payload"/>
1372</reference>
1373
1374<reference anchor="Part5">
1375  <front>
1376    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1377    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1378      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1379      <address><email>fielding@gbiv.com</email></address>
1380    </author>
1381    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1382      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1383      <address><email>jg@freedesktop.org</email></address>
1384    </author>
1385    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1386      <organization abbrev="HP">Hewlett-Packard Company</organization>
1387      <address><email>JeffMogul@acm.org</email></address>
1388    </author>
1389    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1390      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1391      <address><email>henrikn@microsoft.com</email></address>
1392    </author>
1393    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1394      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1395      <address><email>LMM@acm.org</email></address>
1396    </author>
1397    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1398      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1399      <address><email>paulle@microsoft.com</email></address>
1400    </author>
1401    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1402      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1403      <address><email>timbl@w3.org</email></address>
1404    </author>
1405    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1406      <organization abbrev="W3C">World Wide Web Consortium</organization>
1407      <address><email>ylafon@w3.org</email></address>
1408    </author>
1409    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1410      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1411      <address><email>julian.reschke@greenbytes.de</email></address>
1412    </author>
1413    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1414  </front>
1415  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1416  <x:source href="p5-range.xml" basename="p5-range"/>
1417</reference>
1418
1419<reference anchor="Part6">
1420  <front>
1421    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1422    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1423      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1424      <address><email>fielding@gbiv.com</email></address>
1425    </author>
1426    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1427      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1428      <address><email>jg@freedesktop.org</email></address>
1429    </author>
1430    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1431      <organization abbrev="HP">Hewlett-Packard Company</organization>
1432      <address><email>JeffMogul@acm.org</email></address>
1433    </author>
1434    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1435      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1436      <address><email>henrikn@microsoft.com</email></address>
1437    </author>
1438    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1439      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1440      <address><email>LMM@acm.org</email></address>
1441    </author>
1442    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1443      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1444      <address><email>paulle@microsoft.com</email></address>
1445    </author>
1446    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1447      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1448      <address><email>timbl@w3.org</email></address>
1449    </author>
1450    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1451      <organization abbrev="W3C">World Wide Web Consortium</organization>
1452      <address><email>ylafon@w3.org</email></address>
1453    </author>
1454    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1455      <organization>Rackspace</organization>
1456      <address><email>mnot@mnot.net</email></address>
1457    </author>
1458    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1459      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1460      <address><email>julian.reschke@greenbytes.de</email></address>
1461    </author>
1462    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1463  </front>
1464  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1465  <x:source href="p6-cache.xml" basename="p6-cache"/>
1466</reference>
1467
1468<reference anchor="RFC2119">
1469  <front>
1470    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1471    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1472      <organization>Harvard University</organization>
1473      <address><email>sob@harvard.edu</email></address>
1474    </author>
1475    <date month="March" year="1997"/>
1476  </front>
1477  <seriesInfo name="BCP" value="14"/>
1478  <seriesInfo name="RFC" value="2119"/>
1479</reference>
1480
1481<reference anchor="RFC5234">
1482  <front>
1483    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1484    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1485      <organization>Brandenburg InternetWorking</organization>
1486      <address>
1487        <email>dcrocker@bbiw.net</email>
1488      </address> 
1489    </author>
1490    <author initials="P." surname="Overell" fullname="Paul Overell">
1491      <organization>THUS plc.</organization>
1492      <address>
1493        <email>paul.overell@thus.net</email>
1494      </address>
1495    </author>
1496    <date month="January" year="2008"/>
1497  </front>
1498  <seriesInfo name="STD" value="68"/>
1499  <seriesInfo name="RFC" value="5234"/>
1500</reference>
1501
1502</references>
1503
1504<references title="Informative References">
1505
1506<reference anchor="RFC2616">
1507  <front>
1508    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1509    <author initials="R." surname="Fielding" fullname="R. Fielding">
1510      <organization>University of California, Irvine</organization>
1511      <address><email>fielding@ics.uci.edu</email></address>
1512    </author>
1513    <author initials="J." surname="Gettys" fullname="J. Gettys">
1514      <organization>W3C</organization>
1515      <address><email>jg@w3.org</email></address>
1516    </author>
1517    <author initials="J." surname="Mogul" fullname="J. Mogul">
1518      <organization>Compaq Computer Corporation</organization>
1519      <address><email>mogul@wrl.dec.com</email></address>
1520    </author>
1521    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1522      <organization>MIT Laboratory for Computer Science</organization>
1523      <address><email>frystyk@w3.org</email></address>
1524    </author>
1525    <author initials="L." surname="Masinter" fullname="L. Masinter">
1526      <organization>Xerox Corporation</organization>
1527      <address><email>masinter@parc.xerox.com</email></address>
1528    </author>
1529    <author initials="P." surname="Leach" fullname="P. Leach">
1530      <organization>Microsoft Corporation</organization>
1531      <address><email>paulle@microsoft.com</email></address>
1532    </author>
1533    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1534      <organization>W3C</organization>
1535      <address><email>timbl@w3.org</email></address>
1536    </author>
1537    <date month="June" year="1999"/>
1538  </front>
1539  <seriesInfo name="RFC" value="2616"/>
1540</reference>
1541
1542<reference anchor='RFC3864'>
1543  <front>
1544    <title>Registration Procedures for Message Header Fields</title>
1545    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1546      <organization>Nine by Nine</organization>
1547      <address><email>GK-IETF@ninebynine.org</email></address>
1548    </author>
1549    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1550      <organization>BEA Systems</organization>
1551      <address><email>mnot@pobox.com</email></address>
1552    </author>
1553    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1554      <organization>HP Labs</organization>
1555      <address><email>JeffMogul@acm.org</email></address>
1556    </author>
1557    <date year='2004' month='September' />
1558  </front>
1559  <seriesInfo name='BCP' value='90' />
1560  <seriesInfo name='RFC' value='3864' />
1561</reference>
1562
1563<reference anchor='RFC4918'>
1564  <front>
1565    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1566    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1567      <organization abbrev="CommerceNet">CommerceNet</organization>
1568      <address><email>ldusseault@commerce.net</email></address>
1569    </author>
1570    <date month="June" year="2007" />
1571  </front>
1572  <seriesInfo name='RFC' value='4918' />
1573</reference>
1574</references>
1575
1576<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1577<t>
1578  Allow weak entity-tags in all requests except range requests (Sections
1579  <xref target="weak.and.strong.validators" format="counter"/> and
1580  <xref target="header.if-none-match" format="counter"/>).
1581</t>
1582<t>
1583  Change ETag header field ABNF not to use quoted-string, thus avoiding
1584  escaping issues.
1585  (<xref target="header.etag"/>)
1586</t>
1587<t>
1588  Change ABNF productions for header fields to only define the field value.
1589  (<xref target="header.field.definitions"/>)
1590</t>
1591</section>
1592
1593<?BEGININC p4-conditional.abnf-appendix ?>
1594<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1595<figure>
1596<artwork type="abnf" name="p4-conditional.parsed-abnf">
1597<x:ref>ETag</x:ref> = entity-tag
1598
1599<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
1600
1601<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1602 entity-tag ] ) )
1603<x:ref>If-Modified-Since</x:ref> = HTTP-date
1604<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1605 entity-tag ] ) )
1606<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1607
1608<x:ref>Last-Modified</x:ref> = HTTP-date
1609
1610<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1611
1612<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1613<x:ref>etagc</x:ref> = "!" / %x23-7E ; '#'-'~'
1614 / obs-text
1615
1616<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.3&gt;
1617<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE
1618
1619<x:ref>weak</x:ref> = %x57.2F ; W/
1620</artwork>
1621</figure>
1622<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1623; ETag defined but not used
1624; If-Match defined but not used
1625; If-Modified-Since defined but not used
1626; If-None-Match defined but not used
1627; If-Unmodified-Since defined but not used
1628; Last-Modified defined but not used
1629</artwork></figure></section>
1630<?ENDINC p4-conditional.abnf-appendix ?>
1631
1632<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1633
1634<section title="Since RFC 2616">
1635<t>
1636  Extracted relevant partitions from <xref target="RFC2616"/>.
1637</t>
1638</section>
1639
1640<section title="Since draft-ietf-httpbis-p4-conditional-00">
1641<t>
1642  Closed issues:
1643  <list style="symbols"> 
1644    <t>
1645      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1646      "Normative and Informative references"
1647    </t>
1648  </list>
1649</t>
1650<t>
1651  Other changes:
1652  <list style="symbols"> 
1653    <t>
1654      Move definitions of 304 and 412 condition codes from Part2.
1655    </t>
1656  </list>
1657</t>
1658</section>
1659
1660<section title="Since draft-ietf-httpbis-p4-conditional-01">
1661<t>
1662  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1663  <list style="symbols"> 
1664    <t>
1665      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1666    </t>
1667  </list>
1668</t>
1669</section>
1670
1671<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1672<t>
1673  Closed issues:
1674  <list style="symbols"> 
1675    <t>
1676      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1677      "Weak ETags on non-GET requests"
1678    </t>
1679  </list>
1680</t>
1681<t>
1682  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1683  <list style="symbols"> 
1684    <t>
1685      Reference RFC 3984, and update header field registrations for header fields defined
1686      in this document.
1687    </t>
1688  </list>
1689</t>
1690</section>
1691
1692<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1693<t>
1694  Closed issues:
1695  <list style="symbols"> 
1696    <t>
1697      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1698      "Examples for ETag matching"
1699    </t>
1700    <t>
1701      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1702      "'entity value' undefined"
1703    </t>
1704    <t>
1705      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1706      "bogus 2068 Date header reference"
1707    </t>
1708  </list>
1709</t>
1710</section>
1711
1712<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1713<t>
1714  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1715  <list style="symbols"> 
1716    <t>
1717      Use "/" instead of "|" for alternatives.
1718    </t>
1719    <t>
1720      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1721      whitespace ("OWS") and required whitespace ("RWS").
1722    </t>
1723    <t>
1724      Rewrite ABNFs to spell out whitespace rules, factor out
1725      header field value format definitions.
1726    </t>
1727  </list>
1728</t>
1729</section>
1730
1731<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1732<t>
1733  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1734  <list style="symbols"> 
1735    <t>
1736      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1737    </t>
1738  </list>
1739</t>
1740</section>
1741
1742<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1743<t>
1744  Closed issues:
1745  <list style="symbols"> 
1746    <t>
1747      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1748      "case-sensitivity of etag weakness indicator"
1749    </t>
1750  </list>
1751</t>
1752</section>
1753
1754<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1755<t>
1756  Closed issues:
1757  <list style="symbols"> 
1758    <t>
1759      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1760      "Weak ETags on non-GET requests" (If-Match still was defined to require
1761      strong matching)
1762    </t>
1763    <t>
1764      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1765      "move IANA registrations for optional status codes"
1766    </t>
1767  </list>
1768</t>
1769</section>
1770
1771<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1772<t>
1773  No significant changes.
1774</t>
1775</section>
1776
1777<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1778<t>
1779  No significant changes.
1780</t>
1781</section>
1782
1783<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1784<t>
1785  Closed issues:
1786  <list style="symbols"> 
1787    <t>
1788      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1789      "Clarify 'Requested Variant'"
1790    </t>
1791    <t>
1792      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1793      "Clarify entity / representation / variant terminology"
1794    </t>
1795    <t>
1796      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1797      "consider removing the 'changes from 2068' sections"
1798    </t>
1799  </list>
1800</t>
1801</section>
1802
1803<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1804<t>
1805  None.
1806</t>
1807</section>
1808
1809<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1810<t>
1811  Closed issues:
1812  <list style="symbols"> 
1813    <t>
1814      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1815      "Header Classification"
1816    </t>
1817  </list>
1818</t>
1819</section>
1820
1821<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1822<t>
1823  Closed issues:
1824  <list style="symbols"> 
1825    <t>
1826      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1827      "If-* and entities"
1828    </t>
1829    <t>
1830      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1831      "Definition of validator weakness"
1832    </t>
1833    <t>
1834      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1835      "untangle ABNFs for header fields"
1836    </t>
1837    <t>
1838      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1839      "ETags and Quotes"
1840    </t>
1841  </list>
1842</t>
1843</section>
1844
1845<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1846<t>
1847  None.
1848</t>
1849</section>
1850
1851<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1852<t>
1853  Closed issues:
1854  <list style="symbols"> 
1855    <t>
1856      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1857      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1858    </t>
1859  </list>
1860</t>
1861</section>
1862
1863<section title="Since draft-ietf-httpbis-p4-conditional-16" anchor="changes.since.16">
1864<t>
1865  Closed issues:
1866  <list style="symbols"> 
1867    <t>
1868      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
1869      "Document HTTP's error-handling philosophy"
1870    </t>
1871  </list>
1872</t>
1873</section>
1874
1875<section title="Since draft-ietf-httpbis-p4-conditional-17" anchor="changes.since.17">
1876<t>
1877  Closed issues:
1878  <list style="symbols"> 
1879    <t>
1880      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/306"/>:
1881      "does etag value really use quoted-string"
1882    </t>
1883  </list>
1884</t>
1885</section>
1886
1887</section>
1888
1889</back>
1890</rfc>
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