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Work-in-progress: hyperlink header field definitions(P4)

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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 "July">
16  <!ENTITY ID-YEAR "2012">
17  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
18  <!ENTITY architecture               "<xref target='Part1' x:rel='#architecture' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY notation                   "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY abnf-extension             "<xref target='Part1' x:rel='#abnf.extension' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY acks                       "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY whitespace                 "<xref target='Part1' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY field-components           "<xref target='Part1' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-date                "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-accept-encoding     "<xref target='Part2' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY http-date                  "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY content-negotiation        "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34]>
35<?rfc toc="yes" ?>
36<?rfc symrefs="yes" ?>
37<?rfc sortrefs="yes" ?>
38<?rfc compact="yes"?>
39<?rfc subcompact="no" ?>
40<?rfc linkmailto="no" ?>
41<?rfc editing="no" ?>
42<?rfc comments="yes"?>
43<?rfc inline="yes"?>
44<?rfc rfcedstyle="yes"?>
45<?rfc-ext allow-markup-in-artwork="yes" ?>
46<?rfc-ext include-references-in-index="yes" ?>
47<rfc obsoletes="2616" category="std" x:maturity-level="proposed"
48     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
49     xmlns:x='http://purl.org/net/xml2rfc/ext'>
50<x:link rel="prev" basename="p3-payload"/>
51<x:link rel="next" basename="p5-range"/>
52<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
53<front>
54
55  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
56
57  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
58    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
59    <address>
60      <postal>
61        <street>345 Park Ave</street>
62        <city>San Jose</city>
63        <region>CA</region>
64        <code>95110</code>
65        <country>USA</country>
66      </postal>
67      <email>fielding@gbiv.com</email>
68      <uri>http://roy.gbiv.com/</uri>
69    </address>
70  </author>
71
72  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
73    <organization abbrev="W3C">World Wide Web Consortium</organization>
74    <address>
75      <postal>
76        <street>W3C / ERCIM</street>
77        <street>2004, rte des Lucioles</street>
78        <city>Sophia-Antipolis</city>
79        <region>AM</region>
80        <code>06902</code>
81        <country>France</country>
82      </postal>
83      <email>ylafon@w3.org</email>
84      <uri>http://www.raubacapeu.net/people/yves/</uri>
85    </address>
86  </author>
87
88  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
89    <organization abbrev="greenbytes">greenbytes GmbH</organization>
90    <address>
91      <postal>
92        <street>Hafenweg 16</street>
93        <city>Muenster</city><region>NW</region><code>48155</code>
94        <country>Germany</country>
95      </postal>
96      <email>julian.reschke@greenbytes.de</email>
97      <uri>http://greenbytes.de/tech/webdav/</uri>
98    </address>
99  </author>
100
101  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
102  <workgroup>HTTPbis Working Group</workgroup>
103
104<abstract>
105<t>
106   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
107   distributed, collaborative, hypertext information systems. HTTP has been in
108   use by the World Wide Web global information initiative since 1990. This
109   document is Part 4 of the seven-part specification that defines the protocol
110   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
111</t>
112<t>
113   Part 4 defines request header fields for indicating conditional requests and
114   the rules for constructing responses to those requests.
115</t>
116</abstract>
117
118<note title="Editorial Note (To be removed by RFC Editor)">
119  <t>
120    Discussion of this draft ought to take place on the HTTPBIS working group
121    mailing list (ietf-http-wg@w3.org), which is archived at
122    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
123  </t>
124  <t>
125    The current issues list is at
126    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
127    documents (including fancy diffs) can be found at
128    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
129  </t>
130  <t>
131    The changes in this draft are summarized in <xref target="changes.since.19"/>.
132  </t>
133</note>
134</front>
135<middle>
136<section title="Introduction" anchor="introduction">
137<t>
138   This document defines the HTTP/1.1 conditional request mechanisms,
139   including both metadata for indicating/observing changes in resource
140   representations and request header fields that specify preconditions
141   on that metadata; to be checked before performing the request method.
142   Conditional GET requests are the most efficient mechanism for HTTP
143   cache updates &caching;.  Conditionals can also be
144   applied to state-changing methods, such as PUT and DELETE, to prevent
145   the "lost update" problem: one client accidentally overwriting
146   the work of another client that has been acting in parallel.
147</t>
148<t>
149   Conditional request preconditions are based on the state of the target
150   resource as a whole (its current value set) or the state as observed
151   in a previously obtained representation (one value in that set).
152   A resource might have multiple current representations, each with its
153   own observable state.  The conditional request mechanisms assume that
154   the mapping of requests to corresponding representations will be
155   consistent over time if the server intends to take advantage of
156   conditionals.  Regardless, if the mapping is inconsistent and
157   the server is unable to select the appropriate representation, then
158   no harm will result when the precondition evaluates to false.
159</t>
160<t><iref primary="true" item="selected representation"/>
161   We use the term "<x:dfn>selected representation</x:dfn>" to refer to
162   the current representation of the target resource that would have been
163   selected in a successful response if the same request had used the method
164   GET and had excluded all of the conditional request header fields.
165   The conditional request preconditions are evaluated by comparing the
166   values provided in the request header fields to the current metadata
167   for the selected representation.
168</t>
169
170<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
171<t>
172   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
173   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
174   document are to be interpreted as described in <xref target="RFC2119"/>.
175</t>
176<t>
177   This document defines conformance criteria for several roles in HTTP
178   communication, including Senders, Recipients, Clients, Servers, User-Agents,
179   Origin Servers, Intermediaries, Proxies and Gateways. See &architecture;
180   for definitions of these terms.
181</t>
182<t>
183   An implementation is considered conformant if it complies with all of the
184   requirements associated with its role(s). Note that SHOULD-level requirements
185   are relevant here, unless one of the documented exceptions is applicable.
186</t>
187<t>
188   This document also uses ABNF to define valid protocol elements
189   (<xref target="notation"/>). In addition to the prose requirements placed
190   upon them, Senders &MUST-NOT; generate protocol elements that are invalid.
191</t>
192<t>
193   Unless noted otherwise, Recipients &MUST; be able to parse all protocol
194   elements matching the ABNF rules defined for them and &MAY; take steps to
195   recover a usable protocol element from an invalid construct. However, HTTP does not define
196   specific error handling mechanisms, except in cases where it has direct
197   impact on security. This is because different uses of the protocol require
198   different error handling strategies; for example, a Web browser might wish to
199   transparently recover from a response where the Location header field
200   doesn't parse according to the ABNF, whereby in a systems control protocol
201   using HTTP, this type of error recovery could lead to dangerous consequences.
202</t>
203</section>
204
205<section title="Syntax Notation" anchor="notation">
206  <x:anchor-alias value="ALPHA"/>
207  <x:anchor-alias value="CR"/>
208  <x:anchor-alias value="DIGIT"/>
209  <x:anchor-alias value="DQUOTE"/>
210  <x:anchor-alias value="LF"/>
211  <x:anchor-alias value="OCTET"/>
212  <x:anchor-alias value="VCHAR"/>
213  <x:anchor-alias value="core.rules"/>
214  <x:anchor-alias value="obs-text"/>
215  <x:anchor-alias value="OWS"/>
216  <x:anchor-alias value="HTTP-date"/>
217<t>
218   This specification uses the Augmented Backus-Naur Form (ABNF) notation
219   of <xref target="RFC5234"/> with the list rule extension defined in
220   &notation;<xref target="collected.abnf"/> shows the collected ABNF
221   with the list rule expanded.
222</t>
223<t>
224  The following core rules are included by
225  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
226  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
227  DIGIT (decimal 0-9), DQUOTE (double quote),
228  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
229  OCTET (any 8-bit sequence of data), SP (space), and
230  VCHAR (any visible US-ASCII character).
231</t>
232<t>
233  The ABNF rules below are defined in <xref target="Part1"/> and
234  <xref target="Part2"/>:
235</t>
236<figure><artwork type="abnf2616">
237  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
238  <x:ref>obs-text</x:ref>      = &lt;obs-text, defined in &field-components;&gt;
239  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
240</artwork></figure>
241</section>
242</section>
243
244<section title="Validators" anchor="validators">
245   <iref primary="true" item="metadata"/>
246   <iref primary="true" item="validator"/>
247<t>
248   This specification defines two forms of metadata that are commonly used
249   to observe resource state and test for preconditions: modification dates
250   (<xref target="header.last-modified"/>) and opaque entity tags
251   (<xref target="header.etag"/>).  Additional metadata that reflects resource state
252   has been defined by various extensions of HTTP, such as WebDAV
253   <xref target="RFC4918"/>, that are beyond the scope of this specification.
254   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
255   when it is used within a precondition.
256</t>
257
258<section title="Weak versus Strong" anchor="weak.and.strong.validators">
259   <iref primary="true" item="validator" subitem="weak"/>
260   <iref primary="true" item="validator" subitem="strong"/>
261<t>
262   Validators come in two flavors: strong or weak.  Weak validators are easy
263   to generate but are far less useful for comparisons.  Strong validators
264   are ideal for comparisons but can be very difficult (and occasionally
265   impossible) to generate efficiently.  Rather than impose that all forms
266   of resource adhere to the same strength of validator, HTTP exposes the
267   type of validator in use and imposes restrictions on when weak validators
268   can be used as preconditions.
269</t>
270<t>
271   A "strong validator" is a representation metadata value that &MUST; be
272   changed to a new, previously unused or guaranteed unique, value whenever
273   a change occurs to the representation data such that a change would be
274   observable in the payload body of a <x:ref>200 (OK)</x:ref> response to GET.
275   A strong validator &MAY; be changed for other reasons, such as when a semantically
276   significant part of the representation metadata is changed (e.g.,
277   Content-Type), but it is in the best interests of the origin server to only
278   change the value when it is necessary to invalidate the stored responses
279   held by remote caches and authoring tools.  A strong validator &MUST; be
280   unique across all representations of a given resource, such that no two
281   representations of that resource share the same validator unless
282   their payload body would be identical.
283</t>
284<t>
285   Cache entries might persist for arbitrarily long periods, regardless
286   of expiration times.  Thus, a cache might attempt to validate an
287   entry using a validator that it obtained in the distant past.
288   A strong validator &MUST; be unique across all versions of all
289   representations associated with a particular resource over time.
290   However, there is no implication of uniqueness across representations
291   of different resources (i.e., the same strong validator might be
292   in use for representations of multiple resources at the same time
293   and does not imply that those representations are equivalent).
294</t>
295<t>
296   There are a variety of strong validators used in practice.  The best are
297   based on strict revision control, wherein each change to a representation
298   always results in a unique node name and revision identifier being assigned
299   before the representation is made accessible to GET.  A cryptographic hash
300   function applied to the representation data is also sufficient if the data
301   is available prior to the response header fields being sent and the digest
302   does not need to be recalculated every time a validation request is
303   received.  However, if a resource has distinct representations that differ
304   only in their metadata, such as might occur with content negotiation over
305   media types that happen to share the same data format, then the origin
306   server &SHOULD; incorporate additional information in the validator to
307   distinguish those representations and avoid confusing cache behavior.
308</t>
309<t>
310   In contrast, a "weak validator" is a representation metadata value that
311   might not be changed for every change to the representation data.  This
312   weakness might be due to limitations in how the value is calculated, such
313   as clock resolution or an inability to ensure uniqueness for all possible
314   representations of the resource, or due to a desire by the resource owner
315   to group representations by some self-determined set of equivalency
316   rather than unique sequences of data.  An origin server &SHOULD; change a
317   weak entity-tag whenever it considers prior representations to be
318   unacceptable as a substitute for the current representation. In other words,
319   a weak entity-tag ought to change whenever the origin server wants caches to
320   invalidate old responses.
321</t>
322<t>
323   For example, the representation of a weather report that changes in
324   content every second, based on dynamic measurements, might be grouped
325   into sets of equivalent representations (from the origin server's
326   perspective) with the same weak validator in order to allow cached
327   representations to be valid for a reasonable period of time (perhaps
328   adjusted dynamically based on server load or weather quality).
329   Likewise, a representation's modification time, if defined with only
330   one-second resolution, might be a weak validator if it is possible
331   for the representation to be modified twice during a single second and
332   retrieved between those modifications.
333</t>
334<t>
335   A "use" of a validator occurs when either a client generates a request
336   and includes the validator in a precondition or when a server
337   compares two validators.
338   Weak validators are only usable in contexts that do not depend on exact
339   equality of a representation's payload body.
340   Strong validators are usable and preferred for all conditional requests,
341   including cache validation, partial content ranges, and "lost update"
342   avoidance.
343</t>
344</section>
345
346<section title="Last-Modified" anchor="header.last-modified">
347  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
348  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
349  <x:anchor-alias value="Last-Modified"/>
350<t>
351   The "Last-Modified" header field indicates the date and time at
352   which the origin server believes the selected representation was
353   last modified.
354</t>
355<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
356  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
357</artwork></figure>
358<t>
359   An example of its use is
360</t>
361<figure><artwork type="example">
362  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
363</artwork></figure>
364
365<section title="Generation" anchor="lastmod.generation">
366<t>
367   Origin servers &SHOULD; send Last-Modified for any selected
368   representation for which a last modification date can be reasonably
369   and consistently determined, since its use in conditional requests
370   and evaluating cache freshness (&caching;) results in a substantial
371   reduction of HTTP traffic on the Internet and can be a significant
372   factor in improving service scalability and reliability.
373</t>
374<t>
375   A representation is typically the sum of many parts behind the
376   resource interface.  The last-modified time would usually be
377   the most recent time that any of those parts were changed.
378   How that value is determined for any given resource is an
379   implementation detail beyond the scope of this specification.
380   What matters to HTTP is how recipients of the Last-Modified
381   header field can use its value to make conditional requests
382   and test the validity of locally cached responses.
383</t>
384<t>
385   An origin server &SHOULD; obtain the Last-Modified value of the
386   representation as close as possible to the time that it generates
387   the Date field-value for its response. This allows a recipient to
388   make an accurate assessment of the representation's modification time,
389   especially if the representation changes near the time that the
390   response is generated.
391</t>
392<t>
393   An origin server with a clock &MUST-NOT; send a Last-Modified date
394   that is later than the server's time of message origination (Date).
395   If the last modification time is derived from implementation-specific
396   metadata that evaluates to some time in the future, according to the
397   origin server's clock, then the origin server &MUST; replace that
398   value with the message origination date. This prevents a future
399   modification date from having an adverse impact on cache validation.
400</t>
401<t>
402   An origin server without a clock &MUST-NOT; assign Last-Modified
403   values to a response unless these values were associated
404   with the resource by some other system or user with a reliable clock.
405</t>
406</section>
407
408<section title="Comparison" anchor="lastmod.comparison">
409<t>
410   A Last-Modified time, when used as a validator in a request, is
411   implicitly weak unless it is possible to deduce that it is strong,
412   using the following rules:
413  <list style="symbols">
414     <t>The validator is being compared by an origin server to the
415        actual current validator for the representation and,</t>
416     <t>That origin server reliably knows that the associated representation did
417        not change twice during the second covered by the presented
418        validator.</t>
419  </list>
420</t>
421<t>
422   or
423  <list style="symbols">
424     <t>The validator is about to be used by a client in an <x:ref>If-Modified-Since</x:ref>,
425        <x:ref>If-Unmodified-Since</x:ref> header field, because the client has
426        a cache entry, or <x:ref>If-Range</x:ref> for the associated
427        representation, and</t>
428     <t>That cache entry includes a Date value, which gives the time
429        when the origin server sent the original response, and</t>
430     <t>The presented Last-Modified time is at least 60 seconds before
431        the Date value.</t>
432  </list>
433</t>
434<t>
435   or
436  <list style="symbols">
437     <t>The validator is being compared by an intermediate cache to the
438        validator stored in its cache entry for the representation, and</t>
439     <t>That cache entry includes a Date value, which gives the time
440        when the origin server sent the original response, and</t>
441     <t>The presented Last-Modified time is at least 60 seconds before
442        the Date value.</t>
443  </list>
444</t>
445<t>
446   This method relies on the fact that if two different responses were
447   sent by the origin server during the same second, but both had the
448   same Last-Modified time, then at least one of those responses would
449   have a Date value equal to its Last-Modified time. The arbitrary 60-second
450   limit guards against the possibility that the Date and Last-Modified
451   values are generated from different clocks, or at somewhat
452   different times during the preparation of the response. An
453   implementation &MAY; use a value larger than 60 seconds, if it is
454   believed that 60 seconds is too short.
455</t>
456</section>
457</section>
458
459<section title="ETag" anchor="header.etag">
460  <iref primary="true" item="ETag header field" x:for-anchor=""/>
461  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
462  <x:anchor-alias value="ETag"/>
463  <x:anchor-alias value="entity-tag"/>
464  <x:anchor-alias value="entity.tags"/>
465  <x:anchor-alias value="opaque-tag"/>
466  <x:anchor-alias value="weak"/>
467  <x:anchor-alias value="etagc"/>
468<t>
469   The ETag header field provides the current entity-tag for the
470   selected representation.
471   An entity-tag is an opaque validator for differentiating between
472   multiple representations of the same resource, regardless of whether
473   those multiple representations are due to resource state changes over
474   time, content negotiation resulting in multiple representations being
475   valid at the same time, or both. An entity-tag consists of an opaque
476   quoted string, possibly prefixed by a weakness indicator.
477</t>
478<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"/>
479  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
480
481  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
482  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
483  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
484  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
485             ; <x:ref>VCHAR</x:ref> except double quotes, plus obs-text
486</artwork></figure>
487<x:note>
488  <t>
489    &Note; Previously, opaque-tag was defined to be a quoted-string
490    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
491    might perform backslash unescaping. Servers therefore ought to avoid
492    backslash characters in entity tags.
493  </t>
494</x:note>
495<t>
496   An entity-tag can be more reliable for validation than a modification
497   date in situations where it is inconvenient to store modification
498   dates, where the one-second resolution of HTTP date values is not
499   sufficient, or where modification dates are not consistently maintained.
500</t>
501<figure><preamble>
502  Examples:
503</preamble>
504<artwork type="example">
505  ETag: "xyzzy"
506  ETag: W/"xyzzy"
507  ETag: ""
508</artwork></figure>
509<t>
510   An entity-tag can be either a weak or strong validator, with
511   strong being the default.  If an origin server provides an entity-tag
512   for a representation and the generation of that entity-tag does not satisfy
513   the requirements for a strong validator
514   (<xref target="weak.and.strong.validators"/>), then that
515   entity-tag &MUST; be marked as weak by prefixing its opaque value
516   with "W/" (case-sensitive).
517</t>
518
519<section title="Generation" anchor="entity.tag.generation">
520<t>
521   The principle behind entity-tags is that only the service author
522   knows the implementation of a resource well enough to select the
523   most accurate and efficient validation mechanism for that resource,
524   and that any such mechanism can be mapped to a simple sequence of
525   octets for easy comparison.  Since the value is opaque, there is no
526   need for the client to be aware of how each entity-tag is constructed.
527</t>
528<t>
529   For example, a resource that has implementation-specific versioning
530   applied to all changes might use an internal revision number, perhaps
531   combined with a variance identifier for content negotiation, to
532   accurately differentiate between representations.
533   Other implementations might use a stored hash of representation content,
534   a combination of various filesystem attributes, or a modification
535   timestamp that has sub-second resolution.
536</t>
537<t>
538   Origin servers &SHOULD; send ETag for any selected representation
539   for which detection of changes can be reasonably and consistently
540   determined, since the entity-tag's use in conditional requests and
541   evaluating cache freshness (&caching;) can result in a substantial
542   reduction of HTTP network traffic and can be a significant factor in
543   improving service scalability and reliability.
544</t>
545</section>
546
547<section title="Comparison" anchor="entity.tag.comparison">
548  <x:anchor-alias value="validator.comparison"/>
549<t>
550   There are two entity-tag comparison functions, depending
551   on whether the comparison context allows the use of weak validators
552   or not:
553  <list style="symbols">
554     <t>The strong comparison function: in order to be considered equal,
555        both opaque-tags &MUST; be identical character-by-character, and both
556        &MUST-NOT; be weak.</t>
557     <t>The weak comparison function: in order to be considered equal, both
558        opaque-tags &MUST; be identical character-by-character, but
559        either or both of them &MAY; be tagged as "weak" without affecting
560        the result.</t>
561  </list>
562</t>
563<t>
564   The example below shows the results for a set of entity-tag pairs,
565   and both the weak and strong comparison function results:
566</t>
567<texttable align="left">
568  <ttcol>ETag 1</ttcol>
569  <ttcol>ETag 2</ttcol>
570  <ttcol>Strong Comparison</ttcol>
571  <ttcol>Weak Comparison</ttcol>
572
573  <c>W/"1"</c>
574  <c>W/"1"</c>
575  <c>no match</c>
576  <c>match</c>
577 
578  <c>W/"1"</c>
579  <c>W/"2"</c>
580  <c>no match</c>
581  <c>no match</c>
582
583  <c>W/"1"</c>
584  <c>"1"</c>
585  <c>no match</c>
586  <c>match</c>
587
588  <c>"1"</c>
589  <c>"1"</c>
590  <c>match</c>
591  <c>match</c>
592</texttable>
593</section>
594
595<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
596<t>
597   Consider a resource that is subject to content negotiation (&content-negotiation;),
598   and where the representations returned upon a GET request vary based on
599   the Accept-Encoding request header field (&header-accept-encoding;):
600</t>
601<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
602GET /index HTTP/1.1
603Host: www.example.com
604Accept-Encoding: gzip
605
606</artwork></figure>
607<t>
608   In this case, the response might or might not use the gzip content coding.
609   If it does not, the response might look like:
610</t>
611<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
612HTTP/1.1 200 OK
613Date: Thu, 26 Mar 2010 00:05:00 GMT
614ETag: "123-a"
615Content-Length: <x:length-of target="exbody"/>
616Vary: Accept-Encoding
617Content-Type: text/plain
618
619<x:span anchor="exbody">Hello World!
620Hello World!
621Hello World!
622Hello World!
623Hello World!
624</x:span></artwork></figure>
625<t>
626   An alternative representation that does use gzip content coding would be:
627</t>
628<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
629HTTP/1.1 200 OK
630Date: Thu, 26 Mar 2010 00:05:00 GMT
631ETag: "123-b"
632Content-Length: 43
633Vary: Accept-Encoding
634Content-Type: text/plain
635Content-Encoding: gzip
636
637<spanx>...binary data...</spanx></artwork></figure>
638<x:note>
639  <t>
640    &Note; Content codings are a property of the representation,
641    so therefore an entity-tag of an encoded representation has to be distinct
642    from an unencoded representation to prevent conflicts during cache updates
643    and range requests.  In contrast, transfer codings (&transfer-codings;)
644    apply only during message transfer and do not require distinct entity-tags.
645  </t>
646</x:note>
647</section>
648</section>
649
650<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">
651<t>
652   We adopt a set of rules and recommendations for origin servers,
653   clients, and caches regarding when various validator types ought to
654   be used, and for what purposes.
655</t>
656<t>
657   HTTP/1.1 origin servers:
658  <list style="symbols">
659     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
660        generate one.</t>
661
662     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
663        performance considerations support the use of weak entity-tags,
664        or if it is unfeasible to send a strong entity-tag.</t>
665
666     <t>&SHOULD; send a <x:ref>Last-Modified</x:ref> value if it is feasible to
667        send one.</t>
668  </list>
669</t>
670<t>
671   In other words, the preferred behavior for an HTTP/1.1 origin server
672   is to send both a strong entity-tag and a <x:ref>Last-Modified</x:ref> value.
673</t>
674<t>
675   HTTP/1.1 clients:
676  <list style="symbols">
677     <t>&MUST; use that entity-tag in any cache-conditional request (using
678        <x:ref>If-Match</x:ref> or <x:ref>If-None-Match</x:ref>) if an
679        entity-tag has been provided by the origin server.</t>
680
681     <t>&SHOULD; use the <x:ref>Last-Modified</x:ref> value in non-subrange
682        cache-conditional requests (using <x:ref>If-Modified-Since</x:ref>)
683        if only a Last-Modified value has been provided by the origin server.</t>
684
685     <t>&MAY; use the <x:ref>Last-Modified</x:ref> value in subrange
686        cache-conditional requests (using <x:ref>If-Unmodified-Since</x:ref>)
687        if only a Last-Modified value has been provided by an HTTP/1.0 origin
688        server. The user agent &SHOULD; provide a way to disable this, in case
689        of difficulty.</t>
690
691     <t>&SHOULD; use both validators in cache-conditional requests if both an
692        entity-tag and a <x:ref>Last-Modified</x:ref> value have been provided
693        by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
694        respond appropriately.</t>
695  </list>
696</t>
697<t>
698   An HTTP/1.1 origin server, upon receiving a conditional request that
699   includes both a Last-Modified date (e.g., in an <x:ref>If-Modified-Since</x:ref>
700   or <x:ref>If-Unmodified-Since</x:ref> header field) and one or more
701   entity-tags (e.g., in an <x:ref>If-Match</x:ref>, <x:ref>If-None-Match</x:ref>,
702   or <x:ref>If-Range</x:ref> header field) as cache validators, &MUST-NOT;
703   return a response status code of <x:ref>304 (Not Modified)</x:ref> unless
704   doing so is consistent with all of the conditional header fields in the
705   request.
706</t>
707<t>
708   An HTTP/1.1 caching proxy, upon receiving a conditional request that
709   includes both a Last-Modified date and one or more entity-tags as
710   cache validators, &MUST-NOT; return a locally cached response to the
711   client unless that cached response is consistent with all of the
712   conditional header fields in the request.
713  <list><t>
714      &Note; The general principle behind these rules is that HTTP/1.1
715      servers and clients ought to transmit as much non-redundant
716      information as is available in their responses and requests.
717      HTTP/1.1 systems receiving this information will make the most
718      conservative assumptions about the validators they receive.
719  </t><t>
720      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
721      last-modified values received or used by these systems will
722      support transparent and efficient caching, and so HTTP/1.1 origin
723      servers still ought to provide Last-Modified values.
724  </t></list>
725</t>
726</section>
727</section>
728
729<section title="Precondition Header Fields" anchor="header.field.definitions">
730<t>
731   This section defines the syntax and semantics of HTTP/1.1 header fields
732   for applying preconditions on requests.
733</t>
734
735<section title="If-Match" anchor="header.if-match">
736  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
737  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
738  <x:anchor-alias value="If-Match"/>
739<t>
740   The "If-Match" header field can be used to make a request method
741   conditional on the current existence or value of an entity-tag for
742   one or more representations of the target resource.  If-Match is
743   generally useful for resource update requests, such as PUT requests,
744   as a means for protecting against accidental overwrites when multiple
745   clients are acting in parallel on the same resource (i.e., the
746   "lost update" problem).  An If-Match field-value of "*" places the
747   precondition on the existence of any current representation for the
748   target resource.
749</t>
750<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
751  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
752</artwork></figure>
753<t>
754   If any of the entity-tags listed in the If-Match field value match
755   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
756   selected representation for the target resource,
757   or if "*" is given and any current representation exists for the
758   target resource, then the server &MAY; perform the request method
759   as if the If-Match header field was not present.
760</t>
761<t>
762   Origin servers &MUST-NOT; perform the requested method if none of the
763   entity-tags match, or if "*" is given and no current representation
764   exists; instead they &MUST; respond with the <x:ref>412 (Precondition Failed)</x:ref>
765   status code.
766</t>
767<t>
768   Proxy servers using a cached response as the selected representation
769   &MUST-NOT; perform the requested method if none of the entity-tags match,
770   or if "*" is given and no current representation exists; instead, they
771   &MUST; forward the request towards the origin server.
772</t>
773<t>
774   If the request would, without the If-Match header field, result in
775   anything other than a <x:ref>2xx (Successful)</x:ref> or <x:ref>412 (Precondition Failed)</x:ref>
776   status code, then the If-Match header field &MUST; be ignored.
777</t>
778<t>
779   Examples:
780</t>
781<figure><artwork type="example">
782  If-Match: "xyzzy"
783  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
784  If-Match: *
785</artwork></figure>
786<t>
787   The result of a request having both an If-Match header field and
788   either an <x:ref>If-None-Match</x:ref> or an <x:ref>If-Modified-Since</x:ref>
789   header field is undefined by this specification.
790</t>
791</section>
792
793<section title="If-None-Match" anchor="header.if-none-match">
794  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
795  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
796  <x:anchor-alias value="If-None-Match"/>
797<t>
798   The "If-None-Match" header field can be used to make a request method
799   conditional on not matching any of the current entity-tag values for
800   representations of the target resource.  If-None-Match is primarily
801   used in conditional GET requests to enable efficient updates of cached
802   information with a minimum amount of transaction overhead.  A client
803   that has one or more representations previously obtained from the
804   target resource can send If-None-Match with a list of the associated
805   entity-tags in the hope of receiving a <x:ref>304 (Not Modified)</x:ref>
806   response if at least one of those representations matches the selected
807   representation.
808</t>
809<t>
810   If-None-Match can also be used with a value of "*" to prevent an unsafe
811   request method (e.g., PUT) from inadvertently modifying an existing
812   representation of the target resource when the client believes that
813   the resource does not have a current representation.  This is a variation
814   on the "lost update" problem that might arise if more than one client
815   attempts to create an initial representation for the target resource.
816</t>
817<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
818  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
819</artwork></figure>
820<t>
821   If any of the entity-tags listed in the If-None-Match field-value match
822   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
823   selected representation, or if "*" is
824   given and any current representation exists for that resource, then the
825   server &MUST-NOT; perform the requested method.
826   Instead, if the request method was GET or HEAD, the server &SHOULD;
827   respond with a <x:ref>304 (Not Modified)</x:ref> status code, including the cache-related
828   header fields (particularly <x:ref>ETag</x:ref>) of the selected representation that has
829   a matching entity-tag.  For all other request methods, the server &MUST;
830   respond with a <x:ref>412 (Precondition Failed)</x:ref> status code.
831</t>
832<t>
833   If none of the entity-tags match, then the server &MAY; perform the
834   requested method as if the If-None-Match header field did not exist,
835   but &MUST; also ignore any <x:ref>If-Modified-Since</x:ref> header field(s)
836   in the request. That is, if no entity-tags match, then the server &MUST-NOT;
837   return a <x:ref>304 (Not Modified)</x:ref> response.
838</t>
839<t>
840   If the request would, without the If-None-Match header field, result
841   in anything other than a <x:ref>2xx (Successful)</x:ref> or
842   <x:ref>304 (Not Modified)</x:ref> status code, then the If-None-Match
843   header field &MUST; be ignored. (See <xref
844   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
845   a discussion of server behavior when both <x:ref>If-Modified-Since</x:ref>
846   and If-None-Match appear in the same request.)
847</t>
848<t>
849   Examples:
850</t>
851<figure><artwork type="example">
852  If-None-Match: "xyzzy"
853  If-None-Match: W/"xyzzy"
854  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
855  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
856  If-None-Match: *
857</artwork></figure>
858<t>
859   The result of a request having both an If-None-Match header field and
860   either an <x:ref>If-Match</x:ref> or an <x:ref>If-Unmodified-Since</x:ref>
861   header field is undefined by this specification.
862</t>
863</section>
864
865<section title="If-Modified-Since" anchor="header.if-modified-since">
866  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
867  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
868  <x:anchor-alias value="If-Modified-Since"/>
869<t>
870   The "If-Modified-Since" header field can be used to make a request
871   method conditional by modification date: if the selected representation
872   has not been modified since the time specified in this field, then
873   do not perform the request method; instead, respond as detailed below.
874</t>
875<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
876  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
877</artwork></figure>
878<t>
879   An example of the field is:
880</t>
881<figure><artwork type="example">
882  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
883</artwork></figure>
884<t>
885   A GET method with an If-Modified-Since header field and no <x:ref>Range</x:ref>
886   header field requests that the selected representation be transferred only if
887   it has been modified since the date given by the If-Modified-Since
888   header field.
889   The algorithm for determining this includes the following cases:
890  <list style="numbers">
891      <t>If the request would normally result in anything other than a
892         <x:ref>200 (OK)</x:ref> status code, or if the passed If-Modified-Since date is
893         invalid, the response is exactly the same as for a normal GET.
894         A date which is later than the server's current time is
895         invalid.</t>
896
897      <t>If the selected representation has been modified since the
898         If-Modified-Since date, the response is exactly the same as for
899         a normal GET.</t>
900
901      <t>If the selected representation has not been modified since a valid
902         If-Modified-Since date, the server &SHOULD; return a
903         <x:ref>304 (Not Modified)</x:ref> response.</t>
904  </list>
905</t>
906<t>
907   The purpose of this feature is to allow efficient updates of cached
908   information with a minimum amount of transaction overhead.
909  <list><t>
910      &Note; The <x:ref>Range</x:ref> header field modifies the meaning of
911      If-Modified-Since; see &header-range; for full details.
912    </t><t>
913      &Note; If-Modified-Since times are interpreted by the server, whose
914      clock might not be synchronized with the client.
915    </t><t>
916      &Note; When handling an If-Modified-Since header field, some
917      servers will use an exact date comparison function, rather than a
918      less-than function, for deciding whether to send a <x:ref>304 (Not Modified)</x:ref>
919      response. To get best results when sending an If-Modified-Since
920      header field for cache validation, clients are
921      advised to use the exact date string received in a previous
922      <x:ref>Last-Modified</x:ref> header field whenever possible.
923    </t><t>
924      &Note; If a client uses an arbitrary date in the If-Modified-Since
925      header field instead of a date taken from the <x:ref>Last-Modified</x:ref>
926      header field for the same request, the client needs to be aware that this
927      date is interpreted in the server's understanding of time.
928      Unsynchronized clocks and rounding problems, due to the different
929      encodings of time between the client and server, are concerns.
930      This includes the possibility of race conditions if the
931      document has changed between the time it was first requested and
932      the If-Modified-Since date of a subsequent request, and the
933      possibility of clock-skew-related problems if the If-Modified-Since
934      date is derived from the client's clock without correction
935      to the server's clock. Corrections for different time bases
936      between client and server are at best approximate due to network
937      latency.
938    </t>
939  </list>
940</t>
941<t>
942   The result of a request having both an If-Modified-Since header field
943   and either an <x:ref>If-Match</x:ref> or an <x:ref>If-Unmodified-Since</x:ref>
944   header field is undefined by this specification.
945</t>
946</section>
947
948<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
949  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
950  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
951  <x:anchor-alias value="If-Unmodified-Since"/>
952<t>
953   The "If-Unmodified-Since" header field can be used to make a request
954   method conditional by modification date: if the selected representation
955   has been modified since the time specified in this field, then the
956   server &MUST-NOT; perform the requested operation and &MUST; instead
957   respond with the <x:ref>412 (Precondition Failed)</x:ref> status code.
958   If the selected representation has not been modified since the time
959   specified in this field, the server &SHOULD; perform the request
960   method as if the If-Unmodified-Since header field were not present.
961</t>
962<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
963  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
964</artwork></figure>
965<t>
966   An example of the field is:
967</t>
968<figure><artwork type="example">
969  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
970</artwork></figure>
971<t>
972   If a request normally (i.e., in absence of the If-Unmodified-Since
973   header field) would result in anything other than a <x:ref>2xx (Successful)</x:ref>
974   or <x:ref>412 (Precondition Failed)</x:ref> status code,
975   the If-Unmodified-Since header field &SHOULD; be ignored.
976</t>
977<t>
978   If the specified date is invalid, the header field &MUST; be ignored.
979</t>
980<t>
981   The result of a request having both an If-Unmodified-Since header
982   field and either an <x:ref>If-None-Match</x:ref> or an <x:ref>If-Modified-Since</x:ref>
983   header field is undefined by this specification.
984</t>
985</section>
986
987<section title="If-Range" anchor="header.if-range">
988<t>
989   The "If-Range" header field provides a special conditional request
990   mechanism that is similar to <x:ref>If-Match</x:ref> and
991   <x:ref>If-Unmodified-Since</x:ref> but specific to HTTP range requests.
992   If-Range is defined in &header-if-range;.
993</t>
994</section>
995
996</section>
997
998<section title="Status Code Definitions" anchor="status.code.definitions">
999<section title="304 Not Modified" anchor="status.304">
1000  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1001  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1002  <x:anchor-alias value="304"/>
1003  <x:anchor-alias value="304 (Not Modified)"/>
1004<t>
1005   The 304 status code indicates that a conditional GET request has been
1006   received and would have resulted in a <x:ref>200 (OK)</x:ref> response if it were not
1007   for the fact that the condition has evaluated to false.  In other words,
1008   there is no need for the server to transfer a representation of the
1009   target resource because the client's request indicates that it already
1010   has a valid representation, as indicated by the 304 response header
1011   fields, and is therefore redirecting the client to make use of that
1012   stored representation as if it were the payload of a 200 response.
1013   The 304 response &MUST-NOT; contain a message-body, and thus is always
1014   terminated by the first empty line after the header fields.
1015</t>
1016<t>
1017   A 304 response &MUST; include a Date header field (&header-date;)
1018   unless the origin server does not have a clock that can provide a
1019   reasonable approximation of the current time.  If a <x:ref>200 (OK)</x:ref>
1020   response to the same request would have included any of the header fields
1021   <x:ref>Cache-Control</x:ref>, Content-Location, <x:ref>ETag</x:ref>,
1022   <x:ref>Expires</x:ref>, or <x:ref>Vary</x:ref>, then those same header
1023   fields &MUST; be sent in a 304 response.
1024</t>
1025<t>
1026   Since the goal of a 304 response is to minimize information transfer
1027   when the recipient already has one or more cached representations,
1028   the response &SHOULD-NOT; include representation metadata other
1029   than the above listed fields unless said metadata exists for the
1030   purpose of guiding cache updates (e.g., future HTTP extensions).
1031</t>
1032<t>
1033   If the recipient of a 304 response does not have a cached representation
1034   corresponding to the entity-tag indicated by the 304 response, then the
1035   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1036   conditional request originated with an outbound client, such as a
1037   user agent with its own cache sending a conditional GET to a shared
1038   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1039   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1040   the request without any preconditions.
1041</t>
1042<t>
1043   If a cache uses a received 304 response to update a cache entry, the
1044   cache &MUST; update the entry to reflect any new field values given in
1045   the response.
1046</t>
1047</section>
1048
1049<section title="412 Precondition Failed" anchor="status.412">
1050  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1051  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1052  <x:anchor-alias value="412 (Precondition Failed)"/>
1053<t>
1054   The 412 status code indicates that one or more preconditions given in
1055   the request header fields evaluated to false when tested on the server.
1056   This response code allows the client to place preconditions on the
1057   current resource state (its current representations and metadata)
1058   and thus prevent the request method from being applied if the target
1059   resource is in an unexpected state.
1060</t>
1061</section>
1062</section>
1063
1064<section title="IANA Considerations" anchor="IANA.considerations">
1065
1066<section title="Status Code Registration" anchor="status.code.registration">
1067<t>
1068   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1069   shall be updated with the registrations below:
1070</t>
1071<?BEGININC p4-conditional.iana-status-codes ?>
1072<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1073<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1074   <ttcol>Value</ttcol>
1075   <ttcol>Description</ttcol>
1076   <ttcol>Reference</ttcol>
1077   <c>304</c>
1078   <c>Not Modified</c>
1079   <c>
1080      <xref target="status.304"/>
1081   </c>
1082   <c>412</c>
1083   <c>Precondition Failed</c>
1084   <c>
1085      <xref target="status.412"/>
1086   </c>
1087</texttable>
1088<!--(END)-->
1089<?ENDINC p4-conditional.iana-status-codes ?>
1090</section>
1091
1092<section title="Header Field Registration" anchor="header.field.registration">
1093<t>
1094   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1095   with the permanent registrations below (see <xref target="RFC3864"/>):
1096</t>
1097<?BEGININC p4-conditional.iana-headers ?>
1098<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1099<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1100   <ttcol>Header Field Name</ttcol>
1101   <ttcol>Protocol</ttcol>
1102   <ttcol>Status</ttcol>
1103   <ttcol>Reference</ttcol>
1104
1105   <c>ETag</c>
1106   <c>http</c>
1107   <c>standard</c>
1108   <c>
1109      <xref target="header.etag"/>
1110   </c>
1111   <c>If-Match</c>
1112   <c>http</c>
1113   <c>standard</c>
1114   <c>
1115      <xref target="header.if-match"/>
1116   </c>
1117   <c>If-Modified-Since</c>
1118   <c>http</c>
1119   <c>standard</c>
1120   <c>
1121      <xref target="header.if-modified-since"/>
1122   </c>
1123   <c>If-None-Match</c>
1124   <c>http</c>
1125   <c>standard</c>
1126   <c>
1127      <xref target="header.if-none-match"/>
1128   </c>
1129   <c>If-Unmodified-Since</c>
1130   <c>http</c>
1131   <c>standard</c>
1132   <c>
1133      <xref target="header.if-unmodified-since"/>
1134   </c>
1135   <c>Last-Modified</c>
1136   <c>http</c>
1137   <c>standard</c>
1138   <c>
1139      <xref target="header.last-modified"/>
1140   </c>
1141</texttable>
1142<!--(END)-->
1143<?ENDINC p4-conditional.iana-headers ?>
1144<t>
1145   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1146</t>
1147</section>
1148</section>
1149
1150<section title="Security Considerations" anchor="security.considerations">
1151<t>
1152   No additional security considerations have been identified beyond
1153   those applicable to HTTP in general &messaging;.
1154</t>
1155</section>
1156
1157<section title="Acknowledgments" anchor="acks">
1158<t>
1159  See &acks;.
1160</t>
1161</section>
1162</middle>
1163<back>
1164
1165<references title="Normative References">
1166
1167<reference anchor="Part1">
1168  <front>
1169    <title>HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1170    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1171      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1172      <address><email>fielding@gbiv.com</email></address>
1173    </author>
1174    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1175      <organization abbrev="W3C">World Wide Web Consortium</organization>
1176      <address><email>ylafon@w3.org</email></address>
1177    </author>
1178    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1179      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1180      <address><email>julian.reschke@greenbytes.de</email></address>
1181    </author>
1182    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1183  </front>
1184  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1185  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1186</reference>
1187
1188<reference anchor="Part2">
1189  <front>
1190    <title>HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation</title>
1191    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1192      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1193      <address><email>fielding@gbiv.com</email></address>
1194    </author>
1195    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1196      <organization abbrev="W3C">World Wide Web Consortium</organization>
1197      <address><email>ylafon@w3.org</email></address>
1198    </author>
1199    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1200      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1201      <address><email>julian.reschke@greenbytes.de</email></address>
1202    </author>
1203    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1204  </front>
1205  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1206  <x:source href="p2-semantics.xml" basename="p2-semantics">
1207    <x:defines>2xx</x:defines>
1208    <x:defines>2xx (Successful)</x:defines>
1209    <x:defines>200 (OK)</x:defines>
1210  </x:source>
1211</reference>
1212
1213<reference anchor="Part5">
1214  <front>
1215    <title>HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1216    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1217      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1218      <address><email>fielding@gbiv.com</email></address>
1219    </author>
1220    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1221      <organization abbrev="W3C">World Wide Web Consortium</organization>
1222      <address><email>ylafon@w3.org</email></address>
1223    </author>
1224    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1225      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1226      <address><email>julian.reschke@greenbytes.de</email></address>
1227    </author>
1228    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1229  </front>
1230  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1231  <x:source href="p5-range.xml" basename="p5-range">
1232    <x:defines>If-Range</x:defines>
1233    <x:defines>Range</x:defines>
1234  </x:source>
1235</reference>
1236
1237<reference anchor="Part6">
1238  <front>
1239    <title>HTTP/1.1, part 6: Caching</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="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1245      <organization abbrev="W3C">World Wide Web Consortium</organization>
1246      <address><email>ylafon@w3.org</email></address>
1247    </author>
1248    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1249      <organization>Rackspace</organization>
1250      <address><email>mnot@mnot.net</email></address>
1251    </author>
1252    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1253      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1254      <address><email>julian.reschke@greenbytes.de</email></address>
1255    </author>
1256    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1257  </front>
1258  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1259  <x:source href="p6-cache.xml" basename="p6-cache">
1260    <x:defines>Cache-Control</x:defines>
1261    <x:defines>Expires</x:defines>
1262    <x:defines>Vary</x:defines>
1263  </x:source>
1264</reference>
1265
1266<reference anchor="RFC2119">
1267  <front>
1268    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1269    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1270      <organization>Harvard University</organization>
1271      <address><email>sob@harvard.edu</email></address>
1272    </author>
1273    <date month="March" year="1997"/>
1274  </front>
1275  <seriesInfo name="BCP" value="14"/>
1276  <seriesInfo name="RFC" value="2119"/>
1277</reference>
1278
1279<reference anchor="RFC5234">
1280  <front>
1281    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1282    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1283      <organization>Brandenburg InternetWorking</organization>
1284      <address>
1285        <email>dcrocker@bbiw.net</email>
1286      </address> 
1287    </author>
1288    <author initials="P." surname="Overell" fullname="Paul Overell">
1289      <organization>THUS plc.</organization>
1290      <address>
1291        <email>paul.overell@thus.net</email>
1292      </address>
1293    </author>
1294    <date month="January" year="2008"/>
1295  </front>
1296  <seriesInfo name="STD" value="68"/>
1297  <seriesInfo name="RFC" value="5234"/>
1298</reference>
1299
1300</references>
1301
1302<references title="Informative References">
1303
1304<reference anchor="RFC2616">
1305  <front>
1306    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1307    <author initials="R." surname="Fielding" fullname="R. Fielding">
1308      <organization>University of California, Irvine</organization>
1309      <address><email>fielding@ics.uci.edu</email></address>
1310    </author>
1311    <author initials="J." surname="Gettys" fullname="J. Gettys">
1312      <organization>W3C</organization>
1313      <address><email>jg@w3.org</email></address>
1314    </author>
1315    <author initials="J." surname="Mogul" fullname="J. Mogul">
1316      <organization>Compaq Computer Corporation</organization>
1317      <address><email>mogul@wrl.dec.com</email></address>
1318    </author>
1319    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1320      <organization>MIT Laboratory for Computer Science</organization>
1321      <address><email>frystyk@w3.org</email></address>
1322    </author>
1323    <author initials="L." surname="Masinter" fullname="L. Masinter">
1324      <organization>Xerox Corporation</organization>
1325      <address><email>masinter@parc.xerox.com</email></address>
1326    </author>
1327    <author initials="P." surname="Leach" fullname="P. Leach">
1328      <organization>Microsoft Corporation</organization>
1329      <address><email>paulle@microsoft.com</email></address>
1330    </author>
1331    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1332      <organization>W3C</organization>
1333      <address><email>timbl@w3.org</email></address>
1334    </author>
1335    <date month="June" year="1999"/>
1336  </front>
1337  <seriesInfo name="RFC" value="2616"/>
1338</reference>
1339
1340<reference anchor='RFC3864'>
1341  <front>
1342    <title>Registration Procedures for Message Header Fields</title>
1343    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1344      <organization>Nine by Nine</organization>
1345      <address><email>GK-IETF@ninebynine.org</email></address>
1346    </author>
1347    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1348      <organization>BEA Systems</organization>
1349      <address><email>mnot@pobox.com</email></address>
1350    </author>
1351    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1352      <organization>HP Labs</organization>
1353      <address><email>JeffMogul@acm.org</email></address>
1354    </author>
1355    <date year='2004' month='September' />
1356  </front>
1357  <seriesInfo name='BCP' value='90' />
1358  <seriesInfo name='RFC' value='3864' />
1359</reference>
1360
1361<reference anchor='RFC4918'>
1362  <front>
1363    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1364    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1365      <organization abbrev="CommerceNet">CommerceNet</organization>
1366      <address><email>ldusseault@commerce.net</email></address>
1367    </author>
1368    <date month="June" year="2007" />
1369  </front>
1370  <seriesInfo name='RFC' value='4918' />
1371</reference>
1372</references>
1373
1374<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1375<t>
1376  Allow weak entity-tags in all requests except range requests (Sections
1377  <xref target="weak.and.strong.validators" format="counter"/> and
1378  <xref target="header.if-none-match" format="counter"/>).
1379</t>
1380<t>
1381  Change <x:ref>ETag</x:ref> header field ABNF not to use quoted-string, thus
1382  avoiding escaping issues.
1383  (<xref target="header.etag"/>)
1384</t>
1385<t>
1386  Change ABNF productions for header fields to only define the field value.
1387  (<xref target="header.field.definitions"/>)
1388</t>
1389</section>
1390
1391<?BEGININC p4-conditional.abnf-appendix ?>
1392<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1393<figure>
1394<artwork type="abnf" name="p4-conditional.parsed-abnf">
1395<x:ref>ETag</x:ref> = entity-tag
1396
1397<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 5.1&gt;
1398
1399<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1400 entity-tag ] ) )
1401<x:ref>If-Modified-Since</x:ref> = HTTP-date
1402<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1403 entity-tag ] ) )
1404<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1405
1406<x:ref>Last-Modified</x:ref> = HTTP-date
1407
1408<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
1409
1410<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1411<x:ref>etagc</x:ref> = "!" / %x23-7E ; '#'-'~'
1412 / obs-text
1413
1414<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.4&gt;
1415<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE
1416
1417<x:ref>weak</x:ref> = %x57.2F ; W/
1418</artwork>
1419</figure>
1420<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1421; ETag defined but not used
1422; If-Match defined but not used
1423; If-Modified-Since defined but not used
1424; If-None-Match defined but not used
1425; If-Unmodified-Since defined but not used
1426; Last-Modified defined but not used
1427</artwork></figure></section>
1428<?ENDINC p4-conditional.abnf-appendix ?>
1429
1430<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1431<t>
1432  Changes up to the first Working Group Last Call draft are summarized
1433  in <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C"/>.
1434</t>
1435
1436<section title="Since draft-ietf-httpbis-p4-conditional-19" anchor="changes.since.19">
1437<t>
1438  Closed issues:
1439  <list style="symbols"> 
1440    <t>
1441      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/354"/>:
1442      "ETags and Conditional Requests"
1443    </t>
1444    <t>
1445      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
1446      "ABNF requirements for recipients"
1447    </t>
1448    <t>
1449      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/363"/>:
1450      "Rare cases"
1451    </t>
1452  </list>
1453</t>
1454</section>
1455
1456</section>
1457
1458</back>
1459</rfc>
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