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Work-in-progress: hyperlink status codes 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 If-Modified-Since,
425        If-Unmodified-Since header field, because the client has a cache entry,
426        or If-Range for the associated representation, and</t>
427     <t>That cache entry includes a Date value, which gives the time
428        when the origin server sent the original response, and</t>
429     <t>The presented Last-Modified time is at least 60 seconds before
430        the Date value.</t>
431  </list>
432</t>
433<t>
434   or
435  <list style="symbols">
436     <t>The validator is being compared by an intermediate cache to the
437        validator stored in its cache entry for the representation, and</t>
438     <t>That cache entry includes a Date value, which gives the time
439        when the origin server sent the original response, and</t>
440     <t>The presented Last-Modified time is at least 60 seconds before
441        the Date value.</t>
442  </list>
443</t>
444<t>
445   This method relies on the fact that if two different responses were
446   sent by the origin server during the same second, but both had the
447   same Last-Modified time, then at least one of those responses would
448   have a Date value equal to its Last-Modified time. The arbitrary 60-second
449   limit guards against the possibility that the Date and Last-Modified
450   values are generated from different clocks, or at somewhat
451   different times during the preparation of the response. An
452   implementation &MAY; use a value larger than 60 seconds, if it is
453   believed that 60 seconds is too short.
454</t>
455</section>
456</section>
457
458<section title="ETag" anchor="header.etag">
459  <iref primary="true" item="ETag header field" x:for-anchor=""/>
460  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
461  <x:anchor-alias value="ETag"/>
462  <x:anchor-alias value="entity-tag"/>
463  <x:anchor-alias value="entity.tags"/>
464  <x:anchor-alias value="opaque-tag"/>
465  <x:anchor-alias value="weak"/>
466  <x:anchor-alias value="etagc"/>
467<t>
468   The ETag header field provides the current entity-tag for the
469   selected representation.
470   An entity-tag is an opaque validator for differentiating between
471   multiple representations of the same resource, regardless of whether
472   those multiple representations are due to resource state changes over
473   time, content negotiation resulting in multiple representations being
474   valid at the same time, or both. An entity-tag consists of an opaque
475   quoted string, possibly prefixed by a weakness indicator.
476</t>
477<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"/>
478  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
479
480  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
481  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
482  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
483  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
484             ; <x:ref>VCHAR</x:ref> except double quotes, plus obs-text
485</artwork></figure>
486<x:note>
487  <t>
488    &Note; Previously, opaque-tag was defined to be a quoted-string
489    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
490    might perform backslash unescaping. Servers therefore ought to avoid
491    backslash characters in entity tags.
492  </t>
493</x:note>
494<t>
495   An entity-tag can be more reliable for validation than a modification
496   date in situations where it is inconvenient to store modification
497   dates, where the one-second resolution of HTTP date values is not
498   sufficient, or where modification dates are not consistently maintained.
499</t>
500<figure><preamble>
501  Examples:
502</preamble>
503<artwork type="example">
504  ETag: "xyzzy"
505  ETag: W/"xyzzy"
506  ETag: ""
507</artwork></figure>
508<t>
509   An entity-tag can be either a weak or strong validator, with
510   strong being the default.  If an origin server provides an entity-tag
511   for a representation and the generation of that entity-tag does not satisfy
512   the requirements for a strong validator
513   (<xref target="weak.and.strong.validators"/>), then that
514   entity-tag &MUST; be marked as weak by prefixing its opaque value
515   with "W/" (case-sensitive).
516</t>
517
518<section title="Generation" anchor="entity.tag.generation">
519<t>
520   The principle behind entity-tags is that only the service author
521   knows the implementation of a resource well enough to select the
522   most accurate and efficient validation mechanism for that resource,
523   and that any such mechanism can be mapped to a simple sequence of
524   octets for easy comparison.  Since the value is opaque, there is no
525   need for the client to be aware of how each entity-tag is constructed.
526</t>
527<t>
528   For example, a resource that has implementation-specific versioning
529   applied to all changes might use an internal revision number, perhaps
530   combined with a variance identifier for content negotiation, to
531   accurately differentiate between representations.
532   Other implementations might use a stored hash of representation content,
533   a combination of various filesystem attributes, or a modification
534   timestamp that has sub-second resolution.
535</t>
536<t>
537   Origin servers &SHOULD; send ETag for any selected representation
538   for which detection of changes can be reasonably and consistently
539   determined, since the entity-tag's use in conditional requests and
540   evaluating cache freshness (&caching;) can result in a substantial
541   reduction of HTTP network traffic and can be a significant factor in
542   improving service scalability and reliability.
543</t>
544</section>
545
546<section title="Comparison" anchor="entity.tag.comparison">
547  <x:anchor-alias value="validator.comparison"/>
548<t>
549   There are two entity-tag comparison functions, depending
550   on whether the comparison context allows the use of weak validators
551   or not:
552  <list style="symbols">
553     <t>The strong comparison function: in order to be considered equal,
554        both opaque-tags &MUST; be identical character-by-character, and both
555        &MUST-NOT; be weak.</t>
556     <t>The weak comparison function: in order to be considered equal, both
557        opaque-tags &MUST; be identical character-by-character, but
558        either or both of them &MAY; be tagged as "weak" without affecting
559        the result.</t>
560  </list>
561</t>
562<t>
563   The example below shows the results for a set of entity-tag pairs,
564   and both the weak and strong comparison function results:
565</t>
566<texttable align="left">
567  <ttcol>ETag 1</ttcol>
568  <ttcol>ETag 2</ttcol>
569  <ttcol>Strong Comparison</ttcol>
570  <ttcol>Weak Comparison</ttcol>
571
572  <c>W/"1"</c>
573  <c>W/"1"</c>
574  <c>no match</c>
575  <c>match</c>
576 
577  <c>W/"1"</c>
578  <c>W/"2"</c>
579  <c>no match</c>
580  <c>no match</c>
581
582  <c>W/"1"</c>
583  <c>"1"</c>
584  <c>no match</c>
585  <c>match</c>
586
587  <c>"1"</c>
588  <c>"1"</c>
589  <c>match</c>
590  <c>match</c>
591</texttable>
592</section>
593
594<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
595<t>
596   Consider a resource that is subject to content negotiation (&content-negotiation;),
597   and where the representations returned upon a GET request vary based on
598   the Accept-Encoding request header field (&header-accept-encoding;):
599</t>
600<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
601GET /index HTTP/1.1
602Host: www.example.com
603Accept-Encoding: gzip
604
605</artwork></figure>
606<t>
607   In this case, the response might or might not use the gzip content coding.
608   If it does not, the response might look like:
609</t>
610<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
611HTTP/1.1 200 OK
612Date: Thu, 26 Mar 2010 00:05:00 GMT
613ETag: "123-a"
614Content-Length: <x:length-of target="exbody"/>
615Vary: Accept-Encoding
616Content-Type: text/plain
617
618<x:span anchor="exbody">Hello World!
619Hello World!
620Hello World!
621Hello World!
622Hello World!
623</x:span></artwork></figure>
624<t>
625   An alternative representation that does use gzip content coding would be:
626</t>
627<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
628HTTP/1.1 200 OK
629Date: Thu, 26 Mar 2010 00:05:00 GMT
630ETag: "123-b"
631Content-Length: 43
632Vary: Accept-Encoding
633Content-Type: text/plain
634Content-Encoding: gzip
635
636<spanx>...binary data...</spanx></artwork></figure>
637<x:note>
638  <t>
639    &Note; Content codings are a property of the representation,
640    so therefore an entity-tag of an encoded representation has to be distinct
641    from an unencoded representation to prevent conflicts during cache updates
642    and range requests.  In contrast, transfer codings (&transfer-codings;)
643    apply only during message transfer and do not require distinct entity-tags.
644  </t>
645</x:note>
646</section>
647</section>
648
649<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">
650<t>
651   We adopt a set of rules and recommendations for origin servers,
652   clients, and caches regarding when various validator types ought to
653   be used, and for what purposes.
654</t>
655<t>
656   HTTP/1.1 origin servers:
657  <list style="symbols">
658     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
659        generate one.</t>
660
661     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
662        performance considerations support the use of weak entity-tags,
663        or if it is unfeasible to send a strong entity-tag.</t>
664
665     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
666  </list>
667</t>
668<t>
669   In other words, the preferred behavior for an HTTP/1.1 origin server
670   is to send both a strong entity-tag and a Last-Modified value.
671</t>
672<t>
673   HTTP/1.1 clients:
674  <list style="symbols">
675     <t>&MUST; use that entity-tag in any cache-conditional request (using
676        If-Match or If-None-Match) if an entity-tag has been provided by the
677        origin server.</t>
678
679     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
680        requests (using If-Modified-Since) if only a Last-Modified value has
681        been provided by the origin server. </t>
682
683     <t>&MAY; use the Last-Modified value in subrange cache-conditional
684        requests (using If-Unmodified-Since) if only a Last-Modified value has
685        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
686        provide a way to disable this, in case of difficulty.</t>
687
688     <t>&SHOULD; use both validators in cache-conditional requests if both an
689        entity-tag and a Last-Modified value have been provided by the origin
690        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
691        appropriately.</t>
692  </list>
693</t>
694<t>
695   An HTTP/1.1 origin server, upon receiving a conditional request that
696   includes both a Last-Modified date (e.g., in an If-Modified-Since or
697   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
698   in an If-Match, If-None-Match, or If-Range header field) as cache
699   validators, &MUST-NOT; return a response status code of <x:ref>304 (Not Modified)</x:ref>
700   unless doing so is consistent with all of the conditional header
701   fields in the request.
702</t>
703<t>
704   An HTTP/1.1 caching proxy, upon receiving a conditional request that
705   includes both a Last-Modified date and one or more entity-tags as
706   cache validators, &MUST-NOT; return a locally cached response to the
707   client unless that cached response is consistent with all of the
708   conditional header fields in the request.
709  <list><t>
710      &Note; The general principle behind these rules is that HTTP/1.1
711      servers and clients ought to transmit as much non-redundant
712      information as is available in their responses and requests.
713      HTTP/1.1 systems receiving this information will make the most
714      conservative assumptions about the validators they receive.
715  </t><t>
716      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
717      last-modified values received or used by these systems will
718      support transparent and efficient caching, and so HTTP/1.1 origin
719      servers still ought to provide Last-Modified values.
720  </t></list>
721</t>
722</section>
723</section>
724
725<section title="Precondition Header Fields" anchor="header.field.definitions">
726<t>
727   This section defines the syntax and semantics of HTTP/1.1 header fields
728   for applying preconditions on requests.
729</t>
730
731<section title="If-Match" anchor="header.if-match">
732  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
733  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
734  <x:anchor-alias value="If-Match"/>
735<t>
736   The "If-Match" header field can be used to make a request method
737   conditional on the current existence or value of an entity-tag for
738   one or more representations of the target resource.  If-Match is
739   generally useful for resource update requests, such as PUT requests,
740   as a means for protecting against accidental overwrites when multiple
741   clients are acting in parallel on the same resource (i.e., the
742   "lost update" problem).  An If-Match field-value of "*" places the
743   precondition on the existence of any current representation for the
744   target resource.
745</t>
746<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
747  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
748</artwork></figure>
749<t>
750   If any of the entity-tags listed in the If-Match field value match
751   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
752   selected representation for the target resource,
753   or if "*" is given and any current representation exists for the
754   target resource, then the server &MAY; perform the request method
755   as if the If-Match header field was not present.
756</t>
757<t>
758   Origin servers &MUST-NOT; perform the requested method if none of the
759   entity-tags match, or if "*" is given and no current representation
760   exists; instead they &MUST; respond with the <x:ref>412 (Precondition Failed)</x:ref>
761   status code.
762</t>
763<t>
764   Proxy servers using a cached response as the selected representation
765   &MUST-NOT; perform the requested method if none of the entity-tags match,
766   or if "*" is given and no current representation exists; instead, they
767   &MUST; forward the request towards the origin server.
768</t>
769<t>
770   If the request would, without the If-Match header field, result in
771   anything other than a <x:ref>2xx (Successful)</x:ref> or <x:ref>412 (Precondition Failed)</x:ref>
772   status code, then the If-Match header field &MUST; be ignored.
773</t>
774<t>
775   Examples:
776</t>
777<figure><artwork type="example">
778  If-Match: "xyzzy"
779  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
780  If-Match: *
781</artwork></figure>
782<t>
783   The result of a request having both an If-Match header field and
784   either an If-None-Match or an If-Modified-Since header field is
785   undefined by this specification.
786</t>
787</section>
788
789<section title="If-None-Match" anchor="header.if-none-match">
790  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
791  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
792  <x:anchor-alias value="If-None-Match"/>
793<t>
794   The "If-None-Match" header field can be used to make a request method
795   conditional on not matching any of the current entity-tag values for
796   representations of the target resource.  If-None-Match is primarily
797   used in conditional GET requests to enable efficient updates of cached
798   information with a minimum amount of transaction overhead.  A client
799   that has one or more representations previously obtained from the
800   target resource can send If-None-Match with a list of the associated
801   entity-tags in the hope of receiving a <x:ref>304 (Not Modified)</x:ref>
802   response if at least one of those representations matches the selected
803   representation.
804</t>
805<t>
806   If-None-Match can also be used with a value of "*" to prevent an unsafe
807   request method (e.g., PUT) from inadvertently modifying an existing
808   representation of the target resource when the client believes that
809   the resource does not have a current representation.  This is a variation
810   on the "lost update" problem that might arise if more than one client
811   attempts to create an initial representation for the target resource.
812</t>
813<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
814  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
815</artwork></figure>
816<t>
817   If any of the entity-tags listed in the If-None-Match field-value match
818   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
819   selected representation, or if "*" is
820   given and any current representation exists for that resource, then the
821   server &MUST-NOT; perform the requested method.
822   Instead, if the request method was GET or HEAD, the server &SHOULD;
823   respond with a <x:ref>304 (Not Modified)</x:ref> status code, including the cache-related
824   header fields (particularly ETag) of the selected representation that has
825   a matching entity-tag.  For all other request methods, the server &MUST;
826   respond with a <x:ref>412 (Precondition Failed)</x:ref> status code.
827</t>
828<t>
829   If none of the entity-tags match, then the server &MAY; perform the
830   requested method as if the If-None-Match header field did not exist,
831   but &MUST; also ignore any If-Modified-Since header field(s) in the
832   request. That is, if no entity-tags match, then the server &MUST-NOT;
833   return a <x:ref>304 (Not Modified)</x:ref> response.
834</t>
835<t>
836   If the request would, without the If-None-Match header field, result
837   in anything other than a <x:ref>2xx (Successful)</x:ref> or
838   <x:ref>304 (Not Modified)</x:ref> status code, then the If-None-Match
839   header field &MUST; be ignored. (See <xref
840   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
841   a discussion of server behavior when both If-Modified-Since and
842   If-None-Match appear in the same request.)
843</t>
844<t>
845   Examples:
846</t>
847<figure><artwork type="example">
848  If-None-Match: "xyzzy"
849  If-None-Match: W/"xyzzy"
850  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
851  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
852  If-None-Match: *
853</artwork></figure>
854<t>
855   The result of a request having both an If-None-Match header field and
856   either an If-Match or an If-Unmodified-Since header field is
857   undefined by this specification.
858</t>
859</section>
860
861<section title="If-Modified-Since" anchor="header.if-modified-since">
862  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
863  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
864  <x:anchor-alias value="If-Modified-Since"/>
865<t>
866   The "If-Modified-Since" header field can be used to make a request
867   method conditional by modification date: if the selected representation
868   has not been modified since the time specified in this field, then
869   do not perform the request method; instead, respond as detailed below.
870</t>
871<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
872  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
873</artwork></figure>
874<t>
875   An example of the field is:
876</t>
877<figure><artwork type="example">
878  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
879</artwork></figure>
880<t>
881   A GET method with an If-Modified-Since header field and no Range header
882   field requests that the selected representation be transferred only if
883   it has been modified since the date given by the If-Modified-Since
884   header field.
885   The algorithm for determining this includes the following cases:
886  <list style="numbers">
887      <t>If the request would normally result in anything other than a
888         <x:ref>200 (OK)</x:ref> status code, or if the passed If-Modified-Since date is
889         invalid, the response is exactly the same as for a normal GET.
890         A date which is later than the server's current time is
891         invalid.</t>
892
893      <t>If the selected representation has been modified since the
894         If-Modified-Since date, the response is exactly the same as for
895         a normal GET.</t>
896
897      <t>If the selected representation has not been modified since a valid
898         If-Modified-Since date, the server &SHOULD; return a
899         <x:ref>304 (Not Modified)</x:ref> response.</t>
900  </list>
901</t>
902<t>
903   The purpose of this feature is to allow efficient updates of cached
904   information with a minimum amount of transaction overhead.
905  <list><t>
906      &Note; The Range header field modifies the meaning of If-Modified-Since;
907      see &header-range; for full details.
908    </t><t>
909      &Note; If-Modified-Since times are interpreted by the server, whose
910      clock might not be synchronized with the client.
911    </t><t>
912      &Note; When handling an If-Modified-Since header field, some
913      servers will use an exact date comparison function, rather than a
914      less-than function, for deciding whether to send a <x:ref>304 (Not Modified)</x:ref>
915      response. To get best results when sending an If-Modified-Since
916      header field for cache validation, clients are
917      advised to use the exact date string received in a previous Last-Modified
918      header field whenever possible.
919    </t><t>
920      &Note; If a client uses an arbitrary date in the If-Modified-Since
921      header field instead of a date taken from the Last-Modified header field for
922      the same request, the client needs to be aware that this
923      date is interpreted in the server's understanding of time.
924      Unsynchronized clocks and rounding problems, due to the different
925      encodings of time between the client and server, are concerns.
926      This includes the possibility of race conditions if the
927      document has changed between the time it was first requested and
928      the If-Modified-Since date of a subsequent request, and the
929      possibility of clock-skew-related problems if the If-Modified-Since
930      date is derived from the client's clock without correction
931      to the server's clock. Corrections for different time bases
932      between client and server are at best approximate due to network
933      latency.
934    </t>
935  </list>
936</t>
937<t>
938   The result of a request having both an If-Modified-Since header field
939   and either an If-Match or an If-Unmodified-Since header field is
940   undefined by this specification.
941</t>
942</section>
943
944<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
945  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
946  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
947  <x:anchor-alias value="If-Unmodified-Since"/>
948<t>
949   The "If-Unmodified-Since" header field can be used to make a request
950   method conditional by modification date: if the selected representation
951   has been modified since the time specified in this field, then the
952   server &MUST-NOT; perform the requested operation and &MUST; instead
953   respond with the <x:ref>412 (Precondition Failed)</x:ref> status code.
954   If the selected representation has not been modified since the time
955   specified in this field, the server &SHOULD; perform the request
956   method as if the If-Unmodified-Since header field were not present.
957</t>
958<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
959  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
960</artwork></figure>
961<t>
962   An example of the field is:
963</t>
964<figure><artwork type="example">
965  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
966</artwork></figure>
967<t>
968   If a request normally (i.e., in absence of the If-Unmodified-Since
969   header field) would result in anything other than a <x:ref>2xx (Successful)</x:ref>
970   or <x:ref>412 (Precondition Failed)</x:ref> status code,
971   the If-Unmodified-Since header field &SHOULD; be ignored.
972</t>
973<t>
974   If the specified date is invalid, the header field &MUST; be ignored.
975</t>
976<t>
977   The result of a request having both an If-Unmodified-Since header
978   field and either an If-None-Match or an If-Modified-Since header
979   field is undefined by this specification.
980</t>
981</section>
982
983<section title="If-Range" anchor="header.if-range">
984<t>
985   The If-Range header field provides a special conditional request
986   mechanism that is similar to If-Match and If-Unmodified-Since but
987   specific to HTTP range requests. If-Range is defined in &header-if-range;.
988</t>
989</section>
990
991</section>
992
993<section title="Status Code Definitions" anchor="status.code.definitions">
994<section title="304 Not Modified" anchor="status.304">
995  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
996  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
997  <x:anchor-alias value="304"/>
998  <x:anchor-alias value="304 (Not Modified)"/>
999<t>
1000   The 304 status code indicates that a conditional GET request has been
1001   received and would have resulted in a <x:ref>200 (OK)</x:ref> response if it were not
1002   for the fact that the condition has evaluated to false.  In other words,
1003   there is no need for the server to transfer a representation of the
1004   target resource because the client's request indicates that it already
1005   has a valid representation, as indicated by the 304 response header
1006   fields, and is therefore redirecting the client to make use of that
1007   stored representation as if it were the payload of a 200 response.
1008   The 304 response &MUST-NOT; contain a message-body, and thus is always
1009   terminated by the first empty line after the header fields.
1010</t>
1011<t>
1012   A 304 response &MUST; include a Date header field (&header-date;)
1013   unless the origin server does not have a clock that can provide a
1014   reasonable approximation of the current time.  If a 200 response
1015   to the same request would have included any of the header fields
1016   Cache-Control, Content-Location, ETag, Expires, or Vary, then
1017   those same header fields &MUST; be sent in a 304 response.
1018</t>
1019<t>
1020   Since the goal of a 304 response is to minimize information transfer
1021   when the recipient already has one or more cached representations,
1022   the response &SHOULD-NOT; include representation metadata other
1023   than the above listed fields unless said metadata exists for the
1024   purpose of guiding cache updates (e.g., future HTTP extensions).
1025</t>
1026<t>
1027   If the recipient of a 304 response does not have a cached representation
1028   corresponding to the entity-tag indicated by the 304 response, then the
1029   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1030   conditional request originated with an outbound client, such as a
1031   user agent with its own cache sending a conditional GET to a shared
1032   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1033   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1034   the request without any preconditions.
1035</t>
1036<t>
1037   If a cache uses a received 304 response to update a cache entry, the
1038   cache &MUST; update the entry to reflect any new field values given in
1039   the response.
1040</t>
1041</section>
1042
1043<section title="412 Precondition Failed" anchor="status.412">
1044  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1045  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1046  <x:anchor-alias value="412 (Precondition Failed)"/>
1047<t>
1048   The 412 status code indicates that one or more preconditions given in
1049   the request header fields evaluated to false when tested on the server.
1050   This response code allows the client to place preconditions on the
1051   current resource state (its current representations and metadata)
1052   and thus prevent the request method from being applied if the target
1053   resource is in an unexpected state.
1054</t>
1055</section>
1056</section>
1057
1058<section title="IANA Considerations" anchor="IANA.considerations">
1059
1060<section title="Status Code Registration" anchor="status.code.registration">
1061<t>
1062   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1063   shall be updated with the registrations below:
1064</t>
1065<?BEGININC p4-conditional.iana-status-codes ?>
1066<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1067<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1068   <ttcol>Value</ttcol>
1069   <ttcol>Description</ttcol>
1070   <ttcol>Reference</ttcol>
1071   <c>304</c>
1072   <c>Not Modified</c>
1073   <c>
1074      <xref target="status.304"/>
1075   </c>
1076   <c>412</c>
1077   <c>Precondition Failed</c>
1078   <c>
1079      <xref target="status.412"/>
1080   </c>
1081</texttable>
1082<!--(END)-->
1083<?ENDINC p4-conditional.iana-status-codes ?>
1084</section>
1085
1086<section title="Header Field Registration" anchor="header.field.registration">
1087<t>
1088   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1089   with the permanent registrations below (see <xref target="RFC3864"/>):
1090</t>
1091<?BEGININC p4-conditional.iana-headers ?>
1092<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1093<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1094   <ttcol>Header Field Name</ttcol>
1095   <ttcol>Protocol</ttcol>
1096   <ttcol>Status</ttcol>
1097   <ttcol>Reference</ttcol>
1098
1099   <c>ETag</c>
1100   <c>http</c>
1101   <c>standard</c>
1102   <c>
1103      <xref target="header.etag"/>
1104   </c>
1105   <c>If-Match</c>
1106   <c>http</c>
1107   <c>standard</c>
1108   <c>
1109      <xref target="header.if-match"/>
1110   </c>
1111   <c>If-Modified-Since</c>
1112   <c>http</c>
1113   <c>standard</c>
1114   <c>
1115      <xref target="header.if-modified-since"/>
1116   </c>
1117   <c>If-None-Match</c>
1118   <c>http</c>
1119   <c>standard</c>
1120   <c>
1121      <xref target="header.if-none-match"/>
1122   </c>
1123   <c>If-Unmodified-Since</c>
1124   <c>http</c>
1125   <c>standard</c>
1126   <c>
1127      <xref target="header.if-unmodified-since"/>
1128   </c>
1129   <c>Last-Modified</c>
1130   <c>http</c>
1131   <c>standard</c>
1132   <c>
1133      <xref target="header.last-modified"/>
1134   </c>
1135</texttable>
1136<!--(END)-->
1137<?ENDINC p4-conditional.iana-headers ?>
1138<t>
1139   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1140</t>
1141</section>
1142</section>
1143
1144<section title="Security Considerations" anchor="security.considerations">
1145<t>
1146   No additional security considerations have been identified beyond
1147   those applicable to HTTP in general &messaging;.
1148</t>
1149</section>
1150
1151<section title="Acknowledgments" anchor="acks">
1152<t>
1153  See &acks;.
1154</t>
1155</section>
1156</middle>
1157<back>
1158
1159<references title="Normative References">
1160
1161<reference anchor="Part1">
1162  <front>
1163    <title>HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1164    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1165      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1166      <address><email>fielding@gbiv.com</email></address>
1167    </author>
1168    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1169      <organization abbrev="W3C">World Wide Web Consortium</organization>
1170      <address><email>ylafon@w3.org</email></address>
1171    </author>
1172    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1173      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1174      <address><email>julian.reschke@greenbytes.de</email></address>
1175    </author>
1176    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1177  </front>
1178  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1179  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1180</reference>
1181
1182<reference anchor="Part2">
1183  <front>
1184    <title>HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation</title>
1185    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1186      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1187      <address><email>fielding@gbiv.com</email></address>
1188    </author>
1189    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1190      <organization abbrev="W3C">World Wide Web Consortium</organization>
1191      <address><email>ylafon@w3.org</email></address>
1192    </author>
1193    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1194      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1195      <address><email>julian.reschke@greenbytes.de</email></address>
1196    </author>
1197    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1198  </front>
1199  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1200  <x:source href="p2-semantics.xml" basename="p2-semantics">
1201    <x:defines>2xx</x:defines>
1202    <x:defines>2xx (Successful)</x:defines>
1203    <x:defines>200 (OK)</x:defines>
1204  </x:source>
1205</reference>
1206
1207<reference anchor="Part5">
1208  <front>
1209    <title>HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1210    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1211      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1212      <address><email>fielding@gbiv.com</email></address>
1213    </author>
1214    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1215      <organization abbrev="W3C">World Wide Web Consortium</organization>
1216      <address><email>ylafon@w3.org</email></address>
1217    </author>
1218    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1219      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1220      <address><email>julian.reschke@greenbytes.de</email></address>
1221    </author>
1222    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1223  </front>
1224  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1225  <x:source href="p5-range.xml" basename="p5-range"/>
1226</reference>
1227
1228<reference anchor="Part6">
1229  <front>
1230    <title>HTTP/1.1, part 6: Caching</title>
1231    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1232      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1233      <address><email>fielding@gbiv.com</email></address>
1234    </author>
1235    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1236      <organization abbrev="W3C">World Wide Web Consortium</organization>
1237      <address><email>ylafon@w3.org</email></address>
1238    </author>
1239    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1240      <organization>Rackspace</organization>
1241      <address><email>mnot@mnot.net</email></address>
1242    </author>
1243    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1244      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1245      <address><email>julian.reschke@greenbytes.de</email></address>
1246    </author>
1247    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1248  </front>
1249  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1250  <x:source href="p6-cache.xml" basename="p6-cache"/>
1251</reference>
1252
1253<reference anchor="RFC2119">
1254  <front>
1255    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1256    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1257      <organization>Harvard University</organization>
1258      <address><email>sob@harvard.edu</email></address>
1259    </author>
1260    <date month="March" year="1997"/>
1261  </front>
1262  <seriesInfo name="BCP" value="14"/>
1263  <seriesInfo name="RFC" value="2119"/>
1264</reference>
1265
1266<reference anchor="RFC5234">
1267  <front>
1268    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1269    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1270      <organization>Brandenburg InternetWorking</organization>
1271      <address>
1272        <email>dcrocker@bbiw.net</email>
1273      </address> 
1274    </author>
1275    <author initials="P." surname="Overell" fullname="Paul Overell">
1276      <organization>THUS plc.</organization>
1277      <address>
1278        <email>paul.overell@thus.net</email>
1279      </address>
1280    </author>
1281    <date month="January" year="2008"/>
1282  </front>
1283  <seriesInfo name="STD" value="68"/>
1284  <seriesInfo name="RFC" value="5234"/>
1285</reference>
1286
1287</references>
1288
1289<references title="Informative References">
1290
1291<reference anchor="RFC2616">
1292  <front>
1293    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1294    <author initials="R." surname="Fielding" fullname="R. Fielding">
1295      <organization>University of California, Irvine</organization>
1296      <address><email>fielding@ics.uci.edu</email></address>
1297    </author>
1298    <author initials="J." surname="Gettys" fullname="J. Gettys">
1299      <organization>W3C</organization>
1300      <address><email>jg@w3.org</email></address>
1301    </author>
1302    <author initials="J." surname="Mogul" fullname="J. Mogul">
1303      <organization>Compaq Computer Corporation</organization>
1304      <address><email>mogul@wrl.dec.com</email></address>
1305    </author>
1306    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1307      <organization>MIT Laboratory for Computer Science</organization>
1308      <address><email>frystyk@w3.org</email></address>
1309    </author>
1310    <author initials="L." surname="Masinter" fullname="L. Masinter">
1311      <organization>Xerox Corporation</organization>
1312      <address><email>masinter@parc.xerox.com</email></address>
1313    </author>
1314    <author initials="P." surname="Leach" fullname="P. Leach">
1315      <organization>Microsoft Corporation</organization>
1316      <address><email>paulle@microsoft.com</email></address>
1317    </author>
1318    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1319      <organization>W3C</organization>
1320      <address><email>timbl@w3.org</email></address>
1321    </author>
1322    <date month="June" year="1999"/>
1323  </front>
1324  <seriesInfo name="RFC" value="2616"/>
1325</reference>
1326
1327<reference anchor='RFC3864'>
1328  <front>
1329    <title>Registration Procedures for Message Header Fields</title>
1330    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1331      <organization>Nine by Nine</organization>
1332      <address><email>GK-IETF@ninebynine.org</email></address>
1333    </author>
1334    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1335      <organization>BEA Systems</organization>
1336      <address><email>mnot@pobox.com</email></address>
1337    </author>
1338    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1339      <organization>HP Labs</organization>
1340      <address><email>JeffMogul@acm.org</email></address>
1341    </author>
1342    <date year='2004' month='September' />
1343  </front>
1344  <seriesInfo name='BCP' value='90' />
1345  <seriesInfo name='RFC' value='3864' />
1346</reference>
1347
1348<reference anchor='RFC4918'>
1349  <front>
1350    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1351    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1352      <organization abbrev="CommerceNet">CommerceNet</organization>
1353      <address><email>ldusseault@commerce.net</email></address>
1354    </author>
1355    <date month="June" year="2007" />
1356  </front>
1357  <seriesInfo name='RFC' value='4918' />
1358</reference>
1359</references>
1360
1361<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1362<t>
1363  Allow weak entity-tags in all requests except range requests (Sections
1364  <xref target="weak.and.strong.validators" format="counter"/> and
1365  <xref target="header.if-none-match" format="counter"/>).
1366</t>
1367<t>
1368  Change ETag header field ABNF not to use quoted-string, thus avoiding
1369  escaping issues.
1370  (<xref target="header.etag"/>)
1371</t>
1372<t>
1373  Change ABNF productions for header fields to only define the field value.
1374  (<xref target="header.field.definitions"/>)
1375</t>
1376</section>
1377
1378<?BEGININC p4-conditional.abnf-appendix ?>
1379<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1380<figure>
1381<artwork type="abnf" name="p4-conditional.parsed-abnf">
1382<x:ref>ETag</x:ref> = entity-tag
1383
1384<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 5.1&gt;
1385
1386<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1387 entity-tag ] ) )
1388<x:ref>If-Modified-Since</x:ref> = HTTP-date
1389<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1390 entity-tag ] ) )
1391<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1392
1393<x:ref>Last-Modified</x:ref> = HTTP-date
1394
1395<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
1396
1397<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1398<x:ref>etagc</x:ref> = "!" / %x23-7E ; '#'-'~'
1399 / obs-text
1400
1401<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.4&gt;
1402<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE
1403
1404<x:ref>weak</x:ref> = %x57.2F ; W/
1405</artwork>
1406</figure>
1407<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1408; ETag defined but not used
1409; If-Match defined but not used
1410; If-Modified-Since defined but not used
1411; If-None-Match defined but not used
1412; If-Unmodified-Since defined but not used
1413; Last-Modified defined but not used
1414</artwork></figure></section>
1415<?ENDINC p4-conditional.abnf-appendix ?>
1416
1417<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1418<t>
1419  Changes up to the first Working Group Last Call draft are summarized
1420  in <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C"/>.
1421</t>
1422
1423<section title="Since draft-ietf-httpbis-p4-conditional-19" anchor="changes.since.19">
1424<t>
1425  Closed issues:
1426  <list style="symbols"> 
1427    <t>
1428      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/354"/>:
1429      "ETags and Conditional Requests"
1430    </t>
1431    <t>
1432      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
1433      "ABNF requirements for recipients"
1434    </t>
1435    <t>
1436      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/363"/>:
1437      "Rare cases"
1438    </t>
1439  </list>
1440</t>
1441</section>
1442
1443</section>
1444
1445</back>
1446</rfc>
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