source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1692

Last change on this file since 1692 was 1692, checked in by julian.reschke@…, 7 years ago

Simplify use of "Note:", make upper/lowercase of sentence start consistent; use "—" where appropriate

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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 "June">
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 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   and opaque entity tags.  Additional metadata that reflects resource state
251   has been defined by various extensions of HTTP, such as WebDAV
252   <xref target="RFC4918"/>, that are beyond the scope of this specification.
253   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
254   when it is used within a precondition.
255</t>
256
257<section title="Weak versus Strong" anchor="weak.and.strong.validators">
258   <iref primary="true" item="validator" subitem="weak"/>
259   <iref primary="true" item="validator" subitem="strong"/>
260<t>
261   Validators come in two flavors: strong or weak.  Weak validators are easy
262   to generate but are far less useful for comparisons.  Strong validators
263   are ideal for comparisons but can be very difficult (and occasionally
264   impossible) to generate efficiently.  Rather than impose that all forms
265   of resource adhere to the same strength of validator, HTTP exposes the
266   type of validator in use and imposes restrictions on when weak validators
267   can be used as preconditions.
268</t>
269<t>
270   A "strong validator" is a representation metadata value that &MUST; be
271   changed to a new, previously unused or guaranteed unique, value whenever
272   a change occurs to the representation data such that a change would be
273   observable in the payload body of a 200 response to GET.  A strong
274   validator &MAY; be changed for other reasons, such as when a semantically
275   significant part of the representation metadata is changed (e.g.,
276   Content-Type), but it is in the best interests of the origin server to only
277   change the value when it is necessary to invalidate the stored responses
278   held by remote caches and authoring tools.  A strong validator &MUST; be
279   unique across all representations of a given resource, such that no two
280   representations of that resource share the same validator unless
281   their payload body would be identical.
282</t>
283<t>
284   Cache entries might persist for arbitrarily long periods, regardless
285   of expiration times.  Thus, a cache might attempt to validate an
286   entry using a validator that it obtained in the distant past.
287   A strong validator &MUST; be unique across all versions of all
288   representations associated with a particular resource over time.
289   However, there is no implication of uniqueness across representations
290   of different resources (i.e., the same strong validator might be
291   in use for representations of multiple resources at the same time
292   and does not imply that those representations are equivalent).
293</t>
294<t>
295   There are a variety of strong validators used in practice.  The best are
296   based on strict revision control, wherein each change to a representation
297   always results in a unique node name and revision identifier being assigned
298   before the representation is made accessible to GET.  A cryptographic hash
299   function applied to the representation data is also sufficient if the data
300   is available prior to the response header fields being sent and the digest
301   does not need to be recalculated every time a validation request is
302   received.  However, if a resource has distinct representations that differ
303   only in their metadata, such as might occur with content negotiation over
304   media types that happen to share the same data format, then the origin
305   server &SHOULD; incorporate additional information in the validator to
306   distinguish those representations and avoid confusing cache behavior.
307</t>
308<t>
309   In contrast, a "weak validator" is a representation metadata value that
310   might not be changed for every change to the representation data.  This
311   weakness might be due to limitations in how the value is calculated, such
312   as clock resolution or an inability to ensure uniqueness for all possible
313   representations of the resource, or due to a desire by the resource owner
314   to group representations by some self-determined set of equivalency
315   rather than unique sequences of data.  An origin server &SHOULD; change a
316   weak entity-tag whenever it considers prior representations to be
317   unacceptable as a substitute for the current representation. In other words,
318   a weak entity-tag ought to change whenever the origin server wants caches to
319   invalidate old responses.
320</t>
321<t>
322   For example, the representation of a weather report that changes in
323   content every second, based on dynamic measurements, might be grouped
324   into sets of equivalent representations (from the origin server's
325   perspective) with the same weak validator in order to allow cached
326   representations to be valid for a reasonable period of time (perhaps
327   adjusted dynamically based on server load or weather quality).
328   Likewise, a representation's modification time, if defined with only
329   one-second resolution, might be a weak validator if it is possible
330   for the representation to be modified twice during a single second and
331   retrieved between those modifications.
332</t>
333<t>
334   A "use" of a validator occurs when either a client generates a request
335   and includes the validator in a precondition or when a server
336   compares two validators.
337   Weak validators are only usable in contexts that do not depend on exact
338   equality of a representation's payload body.
339   Strong validators are usable and preferred for all conditional requests,
340   including cache validation, partial content ranges, and "lost update"
341   avoidance.
342</t>
343</section>
344
345<section title="Last-Modified" anchor="header.last-modified">
346  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
347  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
348  <x:anchor-alias value="Last-Modified"/>
349<t>
350   The "Last-Modified" header field indicates the date and time at
351   which the origin server believes the selected representation was
352   last modified.
353</t>
354<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
355  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
356</artwork></figure>
357<t>
358   An example of its use is
359</t>
360<figure><artwork type="example">
361  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
362</artwork></figure>
363
364<section title="Generation" anchor="lastmod.generation">
365<t>
366   Origin servers &SHOULD; send Last-Modified for any selected
367   representation for which a last modification date can be reasonably
368   and consistently determined, since its use in conditional requests
369   and evaluating cache freshness (&caching;) results in a substantial
370   reduction of HTTP traffic on the Internet and can be a significant
371   factor in improving service scalability and reliability.
372</t>
373<t>
374   A representation is typically the sum of many parts behind the
375   resource interface.  The last-modified time would usually be
376   the most recent time that any of those parts were changed.
377   How that value is determined for any given resource is an
378   implementation detail beyond the scope of this specification.
379   What matters to HTTP is how recipients of the Last-Modified
380   header field can use its value to make conditional requests
381   and test the validity of locally cached responses.
382</t>
383<t>
384   An origin server &SHOULD; obtain the Last-Modified value of the
385   representation as close as possible to the time that it generates
386   the Date field-value for its response. This allows a recipient to
387   make an accurate assessment of the representation's modification time,
388   especially if the representation changes near the time that the
389   response is generated.
390</t>
391<t>
392   An origin server with a clock &MUST-NOT; send a Last-Modified date
393   that is later than the server's time of message origination (Date).
394   If the last modification time is derived from implementation-specific
395   metadata that evaluates to some time in the future, according to the
396   origin server's clock, then the origin server &MUST; replace that
397   value with the message origination date. This prevents a future
398   modification date from having an adverse impact on cache validation.
399</t>
400<t>
401   An origin server without a clock &MUST-NOT; assign Last-Modified
402   values to a response unless these values were associated
403   with the resource by some other system or user with a reliable clock.
404</t>
405</section>
406
407<section title="Comparison" anchor="lastmod.comparison">
408<t>
409   A Last-Modified time, when used as a validator in a request, is
410   implicitly weak unless it is possible to deduce that it is strong,
411   using the following rules:
412  <list style="symbols">
413     <t>The validator is being compared by an origin server to the
414        actual current validator for the representation and,</t>
415     <t>That origin server reliably knows that the associated representation did
416        not change twice during the second covered by the presented
417        validator.</t>
418  </list>
419</t>
420<t>
421   or
422  <list style="symbols">
423     <t>The validator is about to be used by a client in an If-Modified-Since,
424        If-Unmodified-Since header field, because the client has a cache entry,
425        or If-Range for the associated representation, and</t>
426     <t>That cache entry includes a Date value, which gives the time
427        when the origin server sent the original response, and</t>
428     <t>The presented Last-Modified time is at least 60 seconds before
429        the Date value.</t>
430  </list>
431</t>
432<t>
433   or
434  <list style="symbols">
435     <t>The validator is being compared by an intermediate cache to the
436        validator stored in its cache entry for the representation, and</t>
437     <t>That cache entry includes a Date value, which gives the time
438        when the origin server sent the original response, and</t>
439     <t>The presented Last-Modified time is at least 60 seconds before
440        the Date value.</t>
441  </list>
442</t>
443<t>
444   This method relies on the fact that if two different responses were
445   sent by the origin server during the same second, but both had the
446   same Last-Modified time, then at least one of those responses would
447   have a Date value equal to its Last-Modified time. The arbitrary 60-second
448   limit guards against the possibility that the Date and Last-Modified
449   values are generated from different clocks, or at somewhat
450   different times during the preparation of the response. An
451   implementation &MAY; use a value larger than 60 seconds, if it is
452   believed that 60 seconds is too short.
453</t>
454</section>
455</section>
456
457<section title="ETag" anchor="header.etag">
458  <iref primary="true" item="ETag header field" x:for-anchor=""/>
459  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
460  <x:anchor-alias value="ETag"/>
461  <x:anchor-alias value="entity-tag"/>
462  <x:anchor-alias value="entity.tags"/>
463  <x:anchor-alias value="opaque-tag"/>
464  <x:anchor-alias value="weak"/>
465  <x:anchor-alias value="etagc"/>
466<t>
467   The ETag header field provides the current entity-tag for the
468   selected representation.
469   An entity-tag is an opaque validator for differentiating between
470   multiple representations of the same resource, regardless of whether
471   those multiple representations are due to resource state changes over
472   time, content negotiation resulting in multiple representations being
473   valid at the same time, or both. An entity-tag consists of an opaque
474   quoted string, possibly prefixed by a weakness indicator.
475</t>
476<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"/>
477  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
478
479  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
480  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
481  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
482  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
483             ; <x:ref>VCHAR</x:ref> except double quotes, plus obs-text
484</artwork></figure>
485<x:note>
486  <t>
487    &Note; Previously, opaque-tag was defined to be a quoted-string
488    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
489    might perform backslash unescaping. Servers therefore ought to avoid
490    backslash characters in entity tags.
491  </t>
492</x:note>
493<t>
494   An entity-tag can be more reliable for validation than a modification
495   date in situations where it is inconvenient to store modification
496   dates, where the one-second resolution of HTTP date values is not
497   sufficient, or where modification dates are not consistently maintained.
498</t>
499<figure><preamble>
500  Examples:
501</preamble>
502<artwork type="example">
503  ETag: "xyzzy"
504  ETag: W/"xyzzy"
505  ETag: ""
506</artwork></figure>
507<t>
508   An entity-tag can be either a weak or strong validator, with
509   strong being the default.  If an origin server provides an entity-tag
510   for a representation and the generation of that entity-tag does not satisfy
511   the requirements for a strong validator
512   (<xref target="weak.and.strong.validators"/>), then that
513   entity-tag &MUST; be marked as weak by prefixing its opaque value
514   with "W/" (case-sensitive).
515</t>
516
517<section title="Generation" anchor="entity.tag.generation">
518<t>
519   The principle behind entity-tags is that only the service author
520   knows the implementation of a resource well enough to select the
521   most accurate and efficient validation mechanism for that resource,
522   and that any such mechanism can be mapped to a simple sequence of
523   octets for easy comparison.  Since the value is opaque, there is no
524   need for the client to be aware of how each entity-tag is constructed.
525</t>
526<t>
527   For example, a resource that has implementation-specific versioning
528   applied to all changes might use an internal revision number, perhaps
529   combined with a variance identifier for content negotiation, to
530   accurately differentiate between representations.
531   Other implementations might use a stored hash of representation content,
532   a combination of various filesystem attributes, or a modification
533   timestamp that has sub-second resolution.
534</t>
535<t>
536   Origin servers &SHOULD; send ETag for any selected representation
537   for which detection of changes can be reasonably and consistently
538   determined, since the entity-tag's use in conditional requests and
539   evaluating cache freshness (&caching;) can result in a substantial
540   reduction of HTTP network traffic and can be a significant factor in
541   improving service scalability and reliability.
542</t>
543</section>
544
545<section title="Comparison" anchor="entity.tag.comparison">
546  <x:anchor-alias value="validator.comparison"/>
547<t>
548   There are two entity-tag comparison functions, depending
549   on whether the comparison context allows the use of weak validators
550   or not:
551  <list style="symbols">
552     <t>The strong comparison function: in order to be considered equal,
553        both opaque-tags &MUST; be identical character-by-character, and both
554        &MUST-NOT; be weak.</t>
555     <t>The weak comparison function: in order to be considered equal, both
556        opaque-tags &MUST; be identical character-by-character, but
557        either or both of them &MAY; be tagged as "weak" without affecting
558        the result.</t>
559  </list>
560</t>
561<t>
562   The example below shows the results for a set of entity-tag pairs,
563   and both the weak and strong comparison function results:
564</t>
565<texttable align="left">
566  <ttcol>ETag 1</ttcol>
567  <ttcol>ETag 2</ttcol>
568  <ttcol>Strong Comparison</ttcol>
569  <ttcol>Weak Comparison</ttcol>
570
571  <c>W/"1"</c>
572  <c>W/"1"</c>
573  <c>no match</c>
574  <c>match</c>
575 
576  <c>W/"1"</c>
577  <c>W/"2"</c>
578  <c>no match</c>
579  <c>no match</c>
580
581  <c>W/"1"</c>
582  <c>"1"</c>
583  <c>no match</c>
584  <c>match</c>
585
586  <c>"1"</c>
587  <c>"1"</c>
588  <c>match</c>
589  <c>match</c>
590</texttable>
591</section>
592
593<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
594<t>
595   Consider a resource that is subject to content negotiation (&content-negotiation;),
596   and where the representations returned upon a GET request vary based on
597   the Accept-Encoding request header field (&header-accept-encoding;):
598</t>
599<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
600GET /index HTTP/1.1
601Host: www.example.com
602Accept-Encoding: gzip
603
604</artwork></figure>
605<t>
606   In this case, the response might or might not use the gzip content coding.
607   If it does not, the response might look like:
608</t>
609<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
610HTTP/1.1 200 OK
611Date: Thu, 26 Mar 2010 00:05:00 GMT
612ETag: "123-a"
613Content-Length: <x:length-of target="exbody"/>
614Vary: Accept-Encoding
615Content-Type: text/plain
616
617<x:span anchor="exbody">Hello World!
618Hello World!
619Hello World!
620Hello World!
621Hello World!
622</x:span></artwork></figure>
623<t>
624   An alternative representation that does use gzip content coding would be:
625</t>
626<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
627HTTP/1.1 200 OK
628Date: Thu, 26 Mar 2010 00:05:00 GMT
629ETag: "123-b"
630Content-Length: 43
631Vary: Accept-Encoding
632Content-Type: text/plain
633Content-Encoding: gzip
634
635<spanx>...binary data...</spanx></artwork></figure>
636<x:note>
637  <t>
638    &Note; Content codings are a property of the representation,
639    so therefore an entity-tag of an encoded representation has to be distinct
640    from an unencoded representation to prevent conflicts during cache updates
641    and range requests.  In contrast, transfer codings (&transfer-codings;)
642    apply only during message transfer and do not require distinct entity-tags.
643  </t>
644</x:note>
645</section>
646</section>
647
648<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">
649<t>
650   We adopt a set of rules and recommendations for origin servers,
651   clients, and caches regarding when various validator types ought to
652   be used, and for what purposes.
653</t>
654<t>
655   HTTP/1.1 origin servers:
656  <list style="symbols">
657     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
658        generate one.</t>
659
660     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
661        performance considerations support the use of weak entity-tags,
662        or if it is unfeasible to send a strong entity-tag.</t>
663
664     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
665  </list>
666</t>
667<t>
668   In other words, the preferred behavior for an HTTP/1.1 origin server
669   is to send both a strong entity-tag and a Last-Modified value.
670</t>
671<t>
672   HTTP/1.1 clients:
673  <list style="symbols">
674     <t>&MUST; use that entity-tag in any cache-conditional request (using
675        If-Match or If-None-Match) if an entity-tag has been provided by the
676        origin server.</t>
677
678     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
679        requests (using If-Modified-Since) if only a Last-Modified value has
680        been provided by the origin server. </t>
681
682     <t>&MAY; use the Last-Modified value in subrange cache-conditional
683        requests (using If-Unmodified-Since) if only a Last-Modified value has
684        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
685        provide a way to disable this, in case of difficulty.</t>
686
687     <t>&SHOULD; use both validators in cache-conditional requests if both an
688        entity-tag and a Last-Modified value have been provided by the origin
689        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
690        appropriately.</t>
691  </list>
692</t>
693<t>
694   An HTTP/1.1 origin server, upon receiving a conditional request that
695   includes both a Last-Modified date (e.g., in an If-Modified-Since or
696   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
697   in an If-Match, If-None-Match, or If-Range header field) as cache
698   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
699   unless doing so is consistent with all of the conditional header
700   fields in the request.
701</t>
702<t>
703   An HTTP/1.1 caching proxy, upon receiving a conditional request that
704   includes both a Last-Modified date and one or more entity-tags as
705   cache validators, &MUST-NOT; return a locally cached response to the
706   client unless that cached response is consistent with all of the
707   conditional header fields in the request.
708  <list><t>
709      &Note; The general principle behind these rules is that HTTP/1.1
710      servers and clients ought to transmit as much non-redundant
711      information as is available in their responses and requests.
712      HTTP/1.1 systems receiving this information will make the most
713      conservative assumptions about the validators they receive.
714  </t><t>
715      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
716      last-modified values received or used by these systems will
717      support transparent and efficient caching, and so HTTP/1.1 origin
718      servers still ought to provide Last-Modified values. In those rare cases
719      where the use of a Last-Modified value as a validator by an
720      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
721      origin servers should not provide one.
722  </t></list>
723</t>
724</section>
725</section>
726
727<section title="Precondition Header Fields" anchor="header.field.definitions">
728<t>
729   This section defines the syntax and semantics of HTTP/1.1 header fields
730   for applying preconditions on requests.
731</t>
732
733<section title="If-Match" anchor="header.if-match">
734  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
735  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
736  <x:anchor-alias value="If-Match"/>
737<t>
738   The "If-Match" header field can be used to make a request method
739   conditional on the current existence or value of an entity-tag for
740   one or more representations of the target resource.  If-Match is
741   generally useful for resource update requests, such as PUT requests,
742   as a means for protecting against accidental overwrites when multiple
743   clients are acting in parallel on the same resource (i.e., the
744   "lost update" problem).  An If-Match field-value of "*" places the
745   precondition on the existence of any current representation for the
746   target resource.
747</t>
748<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
749  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
750</artwork></figure>
751<t>
752   If any of the entity-tags listed in the If-Match field value match
753   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
754   selected representation for the target resource,
755   or if "*" is given and any current representation exists for the
756   target resource, then the server &MAY; perform the request method
757   as if the If-Match header field was not present.
758</t>
759<t>
760   If none of the entity-tags match, or if "*" is given and no current
761   representation exists, the server &MUST-NOT; perform the requested method.
762   Instead, the server &MUST; respond with the 412 (Precondition Failed)
763   status code.
764</t>
765<t>
766   If the request would, without the If-Match header field, result in
767   anything other than a 2xx or 412 status code, then the If-Match header field
768   &MUST; be ignored.
769</t>
770<t>
771   Examples:
772</t>
773<figure><artwork type="example">
774  If-Match: "xyzzy"
775  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
776  If-Match: *
777</artwork></figure>
778<t>
779   The result of a request having both an If-Match header field and
780   either an If-None-Match or an If-Modified-Since header field is
781   undefined by this specification.
782</t>
783</section>
784
785<section title="If-None-Match" anchor="header.if-none-match">
786  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
787  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
788  <x:anchor-alias value="If-None-Match"/>
789<t>
790   The "If-None-Match" header field can be used to make a request method
791   conditional on not matching any of the current entity-tag values for
792   representations of the target resource.  If-None-Match is primarily
793   used in conditional GET requests to enable efficient updates of cached
794   information with a minimum amount of transaction overhead.  A client
795   that has one or more representations previously obtained from the
796   target resource can send If-None-Match with a list of the associated
797   entity-tags in the hope of receiving a 304 response if at least one
798   of those representations matches the selected representation.
799</t>
800<t>
801   If-None-Match can also be used with a value of "*" to prevent an unsafe
802   request method (e.g., PUT) from inadvertently modifying an existing
803   representation of the target resource when the client believes that
804   the resource does not have a current representation.  This is a variation
805   on the "lost update" problem that might arise if more than one client
806   attempts to create an initial representation for the target resource.
807</t>
808<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
809  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
810</artwork></figure>
811<t>
812   If any of the entity-tags listed in the If-None-Match field-value match
813   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
814   selected representation, or if "*" is
815   given and any current representation exists for that resource, then the
816   server &MUST-NOT; perform the requested method.
817   Instead, if the request method was GET or HEAD, the server &SHOULD;
818   respond with a 304 (Not Modified) status code, including the cache-related
819   header fields (particularly ETag) of the selected representation that has
820   a matching entity-tag.  For all other request methods, the server &MUST;
821   respond with a 412 (Precondition Failed) status code.
822</t>
823<t>
824   If none of the entity-tags match, then the server &MAY; perform the
825   requested method as if the If-None-Match header field did not exist,
826   but &MUST; also ignore any If-Modified-Since header field(s) in the
827   request. That is, if no entity-tags match, then the server &MUST-NOT;
828   return a 304 (Not Modified) response.
829</t>
830<t>
831   If the request would, without the If-None-Match header field, result
832   in anything other than a 2xx or 304 status code, then the If-None-Match
833   header field &MUST; be ignored. (See <xref
834   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
835   a discussion of server behavior when both If-Modified-Since and
836   If-None-Match appear in the same request.)
837</t>
838<t>
839   Examples:
840</t>
841<figure><artwork type="example">
842  If-None-Match: "xyzzy"
843  If-None-Match: W/"xyzzy"
844  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
845  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
846  If-None-Match: *
847</artwork></figure>
848<t>
849   The result of a request having both an If-None-Match header field and
850   either an If-Match or an If-Unmodified-Since header field is
851   undefined by this specification.
852</t>
853</section>
854
855<section title="If-Modified-Since" anchor="header.if-modified-since">
856  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
857  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
858  <x:anchor-alias value="If-Modified-Since"/>
859<t>
860   The "If-Modified-Since" header field can be used to make a request
861   method conditional by modification date: if the selected representation
862   has not been modified since the time specified in this field, then
863   do not perform the request method; instead, respond as detailed below.
864</t>
865<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
866  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
867</artwork></figure>
868<t>
869   An example of the field is:
870</t>
871<figure><artwork type="example">
872  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
873</artwork></figure>
874<t>
875   A GET method with an If-Modified-Since header field and no Range header
876   field requests that the selected representation be transferred only if
877   it has been modified since the date given by the If-Modified-Since
878   header field.
879   The algorithm for determining this includes the following cases:
880  <list style="numbers">
881      <t>If the request would normally result in anything other than a
882         200 (OK) status code, or if the passed If-Modified-Since date is
883         invalid, the response is exactly the same as for a normal GET.
884         A date which is later than the server's current time is
885         invalid.</t>
886
887      <t>If the selected representation has been modified since the
888         If-Modified-Since date, the response is exactly the same as for
889         a normal GET.</t>
890
891      <t>If the selected representation has not been modified since a valid
892         If-Modified-Since date, the server &SHOULD; return a
893         304 (Not Modified) response.</t>
894  </list>
895</t>
896<t>
897   The purpose of this feature is to allow efficient updates of cached
898   information with a minimum amount of transaction overhead.
899  <list><t>
900      &Note; The Range header field modifies the meaning of If-Modified-Since;
901      see &header-range; for full details.
902    </t><t>
903      &Note; If-Modified-Since times are interpreted by the server, whose
904      clock might not be synchronized with the client.
905    </t><t>
906      &Note; When handling an If-Modified-Since header field, some
907      servers will use an exact date comparison function, rather than a
908      less-than function, for deciding whether to send a 304 (Not
909      Modified) response. To get best results when sending an If-Modified-Since
910      header field for cache validation, clients are
911      advised to use the exact date string received in a previous Last-Modified
912      header field whenever possible.
913    </t><t>
914      &Note; If a client uses an arbitrary date in the If-Modified-Since
915      header field instead of a date taken from the Last-Modified header field for
916      the same request, the client needs to be aware that this
917      date is interpreted in the server's understanding of time.
918      Unsynchronized clocks and rounding problems, due to the different
919      encodings of time between the client and server, are concerns.
920      This includes the possibility of race conditions if the
921      document has changed between the time it was first requested and
922      the If-Modified-Since date of a subsequent request, and the
923      possibility of clock-skew-related problems if the If-Modified-Since
924      date is derived from the client's clock without correction
925      to the server's clock. Corrections for different time bases
926      between client and server are at best approximate due to network
927      latency.
928    </t>
929  </list>
930</t>
931<t>
932   The result of a request having both an If-Modified-Since header field
933   and either an If-Match or an If-Unmodified-Since header field is
934   undefined by this specification.
935</t>
936</section>
937
938<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
939  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
940  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
941  <x:anchor-alias value="If-Unmodified-Since"/>
942<t>
943   The "If-Unmodified-Since" header field can be used to make a request
944   method conditional by modification date: if the selected representation
945   has been modified since the time specified in this field, then the
946   server &MUST-NOT; perform the requested operation and &MUST; instead
947   respond with the 412 (Precondition Failed) status code.
948   If the selected representation has not been modified since the time
949   specified in this field, the server &SHOULD; perform the request
950   method as if the If-Unmodified-Since header field were not present.
951</t>
952<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
953  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
954</artwork></figure>
955<t>
956   An example of the field is:
957</t>
958<figure><artwork type="example">
959  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
960</artwork></figure>
961<t>
962   If a request normally (i.e., in absence of the If-Unmodified-Since
963   header field) would result in anything other than a 2xx or 412 status code,
964   the If-Unmodified-Since header field &SHOULD; be ignored.
965</t>
966<t>
967   If the specified date is invalid, the header field &MUST; be ignored.
968</t>
969<t>
970   The result of a request having both an If-Unmodified-Since header
971   field and either an If-None-Match or an If-Modified-Since header
972   field is undefined by this specification.
973</t>
974</section>
975
976<section title="If-Range" anchor="header.if-range">
977<t>
978   The If-Range header field provides a special conditional request
979   mechanism that is similar to If-Match and If-Unmodified-Since but
980   specific to HTTP range requests. If-Range is defined in &header-if-range;.
981</t>
982</section>
983
984</section>
985
986<section title="Status Code Definitions" anchor="status.code.definitions">
987<section title="304 Not Modified" anchor="status.304">
988  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
989  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
990<t>
991   The 304 status code indicates that a conditional GET request has been
992   received and would have resulted in a 200 (OK) response if it were not
993   for the fact that the condition has evaluated to false.  In other words,
994   there is no need for the server to transfer a representation of the
995   target resource because the client's request indicates that it already
996   has a valid representation, as indicated by the 304 response header
997   fields, and is therefore redirecting the client to make use of that
998   stored representation as if it were the payload of a 200 response.
999   The 304 response &MUST-NOT; contain a message-body, and thus is always
1000   terminated by the first empty line after the header fields.
1001</t>
1002<t>
1003   A 304 response &MUST; include a Date header field (&header-date;)
1004   unless the origin server does not have a clock that can provide a
1005   reasonable approximation of the current time.  If a 200 response
1006   to the same request would have included any of the header fields
1007   Cache-Control, Content-Location, ETag, Expires, or Vary, then
1008   those same header fields &MUST; be sent in a 304 response.
1009</t>
1010<t>
1011   Since the goal of a 304 response is to minimize information transfer
1012   when the recipient already has one or more cached representations,
1013   the response &SHOULD-NOT; include representation metadata other
1014   than the above listed fields unless said metadata exists for the
1015   purpose of guiding cache updates (e.g., future HTTP extensions).
1016</t>
1017<t>
1018   If the recipient of a 304 response does not have a cached representation
1019   corresponding to the entity-tag indicated by the 304 response, then the
1020   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1021   conditional request originated with an outbound client, such as a
1022   user agent with its own cache sending a conditional GET to a shared
1023   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1024   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1025   the request without any preconditions.
1026</t>
1027<t>
1028   If a cache uses a received 304 response to update a cache entry, the
1029   cache &MUST; update the entry to reflect any new field values given in
1030   the response.
1031</t>
1032</section>
1033
1034<section title="412 Precondition Failed" anchor="status.412">
1035  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1036  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1037<t>
1038   The 412 status code indicates that one or more preconditions given in
1039   the request header fields evaluated to false when tested on the server.
1040   This response code allows the client to place preconditions on the
1041   current resource state (its current representations and metadata)
1042   and thus prevent the request method from being applied if the target
1043   resource is in an unexpected state.
1044</t>
1045</section>
1046</section>
1047
1048<section title="IANA Considerations" anchor="IANA.considerations">
1049
1050<section title="Status Code Registration" anchor="status.code.registration">
1051<t>
1052   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1053   shall be updated with the registrations below:
1054</t>
1055<?BEGININC p4-conditional.iana-status-codes ?>
1056<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1057<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1058   <ttcol>Value</ttcol>
1059   <ttcol>Description</ttcol>
1060   <ttcol>Reference</ttcol>
1061   <c>304</c>
1062   <c>Not Modified</c>
1063   <c>
1064      <xref target="status.304"/>
1065   </c>
1066   <c>412</c>
1067   <c>Precondition Failed</c>
1068   <c>
1069      <xref target="status.412"/>
1070   </c>
1071</texttable>
1072<!--(END)-->
1073<?ENDINC p4-conditional.iana-status-codes ?>
1074</section>
1075
1076<section title="Header Field Registration" anchor="header.field.registration">
1077<t>
1078   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1079   with the permanent registrations below (see <xref target="RFC3864"/>):
1080</t>
1081<?BEGININC p4-conditional.iana-headers ?>
1082<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1083<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1084   <ttcol>Header Field Name</ttcol>
1085   <ttcol>Protocol</ttcol>
1086   <ttcol>Status</ttcol>
1087   <ttcol>Reference</ttcol>
1088
1089   <c>ETag</c>
1090   <c>http</c>
1091   <c>standard</c>
1092   <c>
1093      <xref target="header.etag"/>
1094   </c>
1095   <c>If-Match</c>
1096   <c>http</c>
1097   <c>standard</c>
1098   <c>
1099      <xref target="header.if-match"/>
1100   </c>
1101   <c>If-Modified-Since</c>
1102   <c>http</c>
1103   <c>standard</c>
1104   <c>
1105      <xref target="header.if-modified-since"/>
1106   </c>
1107   <c>If-None-Match</c>
1108   <c>http</c>
1109   <c>standard</c>
1110   <c>
1111      <xref target="header.if-none-match"/>
1112   </c>
1113   <c>If-Unmodified-Since</c>
1114   <c>http</c>
1115   <c>standard</c>
1116   <c>
1117      <xref target="header.if-unmodified-since"/>
1118   </c>
1119   <c>Last-Modified</c>
1120   <c>http</c>
1121   <c>standard</c>
1122   <c>
1123      <xref target="header.last-modified"/>
1124   </c>
1125</texttable>
1126<!--(END)-->
1127<?ENDINC p4-conditional.iana-headers ?>
1128<t>
1129   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1130</t>
1131</section>
1132</section>
1133
1134<section title="Security Considerations" anchor="security.considerations">
1135<t>
1136   No additional security considerations have been identified beyond
1137   those applicable to HTTP in general &messaging;.
1138</t>
1139</section>
1140
1141<section title="Acknowledgments" anchor="acks">
1142<t>
1143  See &acks;.
1144</t>
1145</section>
1146</middle>
1147<back>
1148
1149<references title="Normative References">
1150
1151<reference anchor="Part1">
1152  <front>
1153    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1154    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1155      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1156      <address><email>fielding@gbiv.com</email></address>
1157    </author>
1158    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1159      <organization abbrev="W3C">World Wide Web Consortium</organization>
1160      <address><email>ylafon@w3.org</email></address>
1161    </author>
1162    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1163      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1164      <address><email>julian.reschke@greenbytes.de</email></address>
1165    </author>
1166    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1167  </front>
1168  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1169  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1170</reference>
1171
1172<reference anchor="Part2">
1173  <front>
1174    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1175    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1176      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1177      <address><email>fielding@gbiv.com</email></address>
1178    </author>
1179    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1180      <organization abbrev="W3C">World Wide Web Consortium</organization>
1181      <address><email>ylafon@w3.org</email></address>
1182    </author>
1183    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1184      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1185      <address><email>julian.reschke@greenbytes.de</email></address>
1186    </author>
1187    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1188  </front>
1189  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1190  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
1191</reference>
1192
1193<reference anchor="Part5">
1194  <front>
1195    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1196    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1197      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1198      <address><email>fielding@gbiv.com</email></address>
1199    </author>
1200    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1201      <organization abbrev="W3C">World Wide Web Consortium</organization>
1202      <address><email>ylafon@w3.org</email></address>
1203    </author>
1204    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1205      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1206      <address><email>julian.reschke@greenbytes.de</email></address>
1207    </author>
1208    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1209  </front>
1210  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1211  <x:source href="p5-range.xml" basename="p5-range"/>
1212</reference>
1213
1214<reference anchor="Part6">
1215  <front>
1216    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1217    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1218      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1219      <address><email>fielding@gbiv.com</email></address>
1220    </author>
1221    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1222      <organization abbrev="W3C">World Wide Web Consortium</organization>
1223      <address><email>ylafon@w3.org</email></address>
1224    </author>
1225    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1226      <organization>Rackspace</organization>
1227      <address><email>mnot@mnot.net</email></address>
1228    </author>
1229    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1230      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1231      <address><email>julian.reschke@greenbytes.de</email></address>
1232    </author>
1233    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1234  </front>
1235  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1236  <x:source href="p6-cache.xml" basename="p6-cache"/>
1237</reference>
1238
1239<reference anchor="RFC2119">
1240  <front>
1241    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1242    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1243      <organization>Harvard University</organization>
1244      <address><email>sob@harvard.edu</email></address>
1245    </author>
1246    <date month="March" year="1997"/>
1247  </front>
1248  <seriesInfo name="BCP" value="14"/>
1249  <seriesInfo name="RFC" value="2119"/>
1250</reference>
1251
1252<reference anchor="RFC5234">
1253  <front>
1254    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1255    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1256      <organization>Brandenburg InternetWorking</organization>
1257      <address>
1258        <email>dcrocker@bbiw.net</email>
1259      </address> 
1260    </author>
1261    <author initials="P." surname="Overell" fullname="Paul Overell">
1262      <organization>THUS plc.</organization>
1263      <address>
1264        <email>paul.overell@thus.net</email>
1265      </address>
1266    </author>
1267    <date month="January" year="2008"/>
1268  </front>
1269  <seriesInfo name="STD" value="68"/>
1270  <seriesInfo name="RFC" value="5234"/>
1271</reference>
1272
1273</references>
1274
1275<references title="Informative References">
1276
1277<reference anchor="RFC2616">
1278  <front>
1279    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1280    <author initials="R." surname="Fielding" fullname="R. Fielding">
1281      <organization>University of California, Irvine</organization>
1282      <address><email>fielding@ics.uci.edu</email></address>
1283    </author>
1284    <author initials="J." surname="Gettys" fullname="J. Gettys">
1285      <organization>W3C</organization>
1286      <address><email>jg@w3.org</email></address>
1287    </author>
1288    <author initials="J." surname="Mogul" fullname="J. Mogul">
1289      <organization>Compaq Computer Corporation</organization>
1290      <address><email>mogul@wrl.dec.com</email></address>
1291    </author>
1292    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1293      <organization>MIT Laboratory for Computer Science</organization>
1294      <address><email>frystyk@w3.org</email></address>
1295    </author>
1296    <author initials="L." surname="Masinter" fullname="L. Masinter">
1297      <organization>Xerox Corporation</organization>
1298      <address><email>masinter@parc.xerox.com</email></address>
1299    </author>
1300    <author initials="P." surname="Leach" fullname="P. Leach">
1301      <organization>Microsoft Corporation</organization>
1302      <address><email>paulle@microsoft.com</email></address>
1303    </author>
1304    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1305      <organization>W3C</organization>
1306      <address><email>timbl@w3.org</email></address>
1307    </author>
1308    <date month="June" year="1999"/>
1309  </front>
1310  <seriesInfo name="RFC" value="2616"/>
1311</reference>
1312
1313<reference anchor='RFC3864'>
1314  <front>
1315    <title>Registration Procedures for Message Header Fields</title>
1316    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1317      <organization>Nine by Nine</organization>
1318      <address><email>GK-IETF@ninebynine.org</email></address>
1319    </author>
1320    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1321      <organization>BEA Systems</organization>
1322      <address><email>mnot@pobox.com</email></address>
1323    </author>
1324    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1325      <organization>HP Labs</organization>
1326      <address><email>JeffMogul@acm.org</email></address>
1327    </author>
1328    <date year='2004' month='September' />
1329  </front>
1330  <seriesInfo name='BCP' value='90' />
1331  <seriesInfo name='RFC' value='3864' />
1332</reference>
1333
1334<reference anchor='RFC4918'>
1335  <front>
1336    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1337    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1338      <organization abbrev="CommerceNet">CommerceNet</organization>
1339      <address><email>ldusseault@commerce.net</email></address>
1340    </author>
1341    <date month="June" year="2007" />
1342  </front>
1343  <seriesInfo name='RFC' value='4918' />
1344</reference>
1345</references>
1346
1347<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1348<t>
1349  Allow weak entity-tags in all requests except range requests (Sections
1350  <xref target="weak.and.strong.validators" format="counter"/> and
1351  <xref target="header.if-none-match" format="counter"/>).
1352</t>
1353<t>
1354  Change ETag header field ABNF not to use quoted-string, thus avoiding
1355  escaping issues.
1356  (<xref target="header.etag"/>)
1357</t>
1358<t>
1359  Change ABNF productions for header fields to only define the field value.
1360  (<xref target="header.field.definitions"/>)
1361</t>
1362</section>
1363
1364<?BEGININC p4-conditional.abnf-appendix ?>
1365<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1366<figure>
1367<artwork type="abnf" name="p4-conditional.parsed-abnf">
1368<x:ref>ETag</x:ref> = entity-tag
1369
1370<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 5.1&gt;
1371
1372<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1373 entity-tag ] ) )
1374<x:ref>If-Modified-Since</x:ref> = HTTP-date
1375<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1376 entity-tag ] ) )
1377<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1378
1379<x:ref>Last-Modified</x:ref> = HTTP-date
1380
1381<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
1382
1383<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1384<x:ref>etagc</x:ref> = "!" / %x23-7E ; '#'-'~'
1385 / obs-text
1386
1387<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.4&gt;
1388<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE
1389
1390<x:ref>weak</x:ref> = %x57.2F ; W/
1391</artwork>
1392</figure>
1393<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1394; ETag defined but not used
1395; If-Match defined but not used
1396; If-Modified-Since defined but not used
1397; If-None-Match defined but not used
1398; If-Unmodified-Since defined but not used
1399; Last-Modified defined but not used
1400</artwork></figure></section>
1401<?ENDINC p4-conditional.abnf-appendix ?>
1402
1403<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1404<t>
1405  Changes up to the first Working Group Last Call draft are summarized
1406  in <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C"/>.
1407</t>
1408
1409<section title="Since draft-ietf-httpbis-p4-conditional-19" anchor="changes.since.19">
1410<t>
1411  Closed issues:
1412  <list style="symbols"> 
1413    <t>
1414      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
1415      "ABNF requirements for recipients"
1416    </t>
1417  </list>
1418</t>
1419</section>
1420
1421</section>
1422
1423</back>
1424</rfc>
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