source: draft-ietf-httpbis/16/draft-ietf-httpbis-p4-conditional-16.xml @ 1500

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<?rfc rfcedstyle="yes"?>
17<!DOCTYPE rfc
18  PUBLIC "" "rfc2629.dtd">
19<rfc obsoletes="2616" category="std" ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-16">
20<front>
21
22  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
23
24  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
25    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
26    <address>
27      <postal>
28        <street>345 Park Ave</street>
29        <city>San Jose</city>
30        <region>CA</region>
31        <code>95110</code>
32        <country>USA</country>
33      </postal>
34      <email>fielding@gbiv.com</email>
35      <uri>http://roy.gbiv.com/</uri>
36    </address>
37  </author>
38
39  <author initials="J." surname="Gettys" fullname="Jim Gettys">
40    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
41    <address>
42      <postal>
43        <street>21 Oak Knoll Road</street>
44        <city>Carlisle</city>
45        <region>MA</region>
46        <code>01741</code>
47        <country>USA</country>
48      </postal>
49      <email>jg@freedesktop.org</email>
50      <uri>http://gettys.wordpress.com/</uri>
51    </address>
52  </author>
53 
54  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
55    <organization abbrev="HP">Hewlett-Packard Company</organization>
56    <address>
57      <postal>
58        <street>HP Labs, Large Scale Systems Group</street>
59        <street>1501 Page Mill Road, MS 1177</street>
60        <city>Palo Alto</city>
61        <region>CA</region>
62        <code>94304</code>
63        <country>USA</country>
64      </postal>
65      <email>JeffMogul@acm.org</email>
66    </address>
67  </author>
68
69  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
70    <organization abbrev="Microsoft">Microsoft Corporation</organization>
71    <address>
72      <postal>
73        <street>1 Microsoft Way</street>
74        <city>Redmond</city>
75        <region>WA</region>
76        <code>98052</code>
77        <country>USA</country>
78      </postal>
79      <email>henrikn@microsoft.com</email>
80    </address>
81  </author>
82
83  <author initials="L." surname="Masinter" fullname="Larry Masinter">
84    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
85    <address>
86      <postal>
87        <street>345 Park Ave</street>
88        <city>San Jose</city>
89        <region>CA</region>
90        <code>95110</code>
91        <country>USA</country>
92      </postal>
93      <email>LMM@acm.org</email>
94      <uri>http://larry.masinter.net/</uri>
95    </address>
96  </author>
97 
98  <author initials="P." surname="Leach" fullname="Paul J. Leach">
99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
100    <address>
101      <postal>
102        <street>1 Microsoft Way</street>
103        <city>Redmond</city>
104        <region>WA</region>
105        <code>98052</code>
106      </postal>
107      <email>paulle@microsoft.com</email>
108    </address>
109  </author>
110   
111  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
112    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
113    <address>
114      <postal>
115        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
116        <street>The Stata Center, Building 32</street>
117        <street>32 Vassar Street</street>
118        <city>Cambridge</city>
119        <region>MA</region>
120        <code>02139</code>
121        <country>USA</country>
122      </postal>
123      <email>timbl@w3.org</email>
124      <uri>http://www.w3.org/People/Berners-Lee/</uri>
125    </address>
126  </author>
127
128  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
129    <organization abbrev="W3C">World Wide Web Consortium</organization>
130    <address>
131      <postal>
132        <street>W3C / ERCIM</street>
133        <street>2004, rte des Lucioles</street>
134        <city>Sophia-Antipolis</city>
135        <region>AM</region>
136        <code>06902</code>
137        <country>France</country>
138      </postal>
139      <email>ylafon@w3.org</email>
140      <uri>http://www.raubacapeu.net/people/yves/</uri>
141    </address>
142  </author>
143
144  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
145    <organization abbrev="greenbytes">greenbytes GmbH</organization>
146    <address>
147      <postal>
148        <street>Hafenweg 16</street>
149        <city>Muenster</city><region>NW</region><code>48155</code>
150        <country>Germany</country>
151      </postal>
152      <phone>+49 251 2807760</phone>
153      <facsimile>+49 251 2807761</facsimile>
154      <email>julian.reschke@greenbytes.de</email>
155      <uri>http://greenbytes.de/tech/webdav/</uri>
156    </address>
157  </author>
158
159  <date month="August" year="2011" day="24"/>
160  <workgroup>HTTPbis Working Group</workgroup>
161
162<abstract>
163<t>
164   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
165   distributed, collaborative, hypertext information systems. HTTP has been in
166   use by the World Wide Web global information initiative since 1990. This
167   document is Part 4 of the seven-part specification that defines the protocol
168   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
169</t>
170<t>
171   Part 4 defines request header fields for indicating conditional requests and
172   the rules for constructing responses to those requests.
173</t>
174</abstract>
175
176<note title="Editorial Note (To be removed by RFC Editor)">
177  <t>
178    Discussion of this draft should take place on the HTTPBIS working group
179    mailing list (ietf-http-wg@w3.org), which is archived at
180    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
181  </t>
182  <t>
183    The current issues list is at
184    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
185    documents (including fancy diffs) can be found at
186    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
187  </t>
188  <t>
189    The changes in this draft are summarized in <xref target="changes.since.15"/>.
190  </t>
191</note>
192</front>
193<middle>
194<section title="Introduction" anchor="introduction">
195<t>
196   This document defines the HTTP/1.1 conditional request mechanisms,
197   including both metadata for indicating/observing changes in resource
198   representations and request header fields that specify preconditions
199   on that metadata be checked before performing the request method.
200   Conditional GET requests are the most efficient mechanism for HTTP
201   cache updates <xref target="Part6"/>.  Conditionals can also be
202   applied to state-changing methods, such as PUT and DELETE, to prevent
203   the "lost update" problem: one client accidentally overwriting
204   the work of another client that has been acting in parallel.
205</t>
206<t>
207   Conditional request preconditions are based on the state of the target
208   resource as a whole (its current value set) or the state as observed
209   in a previously obtained representation (one value in that set).
210   A resource might have multiple current representations, each with its
211   own observable state.  The conditional request mechanisms assume that
212   the mapping of requests to corresponding representations will be
213   consistent over time if the server intends to take advantage of
214   conditionals.  Regardless, if the mapping is inconsistent and
215   the server is unable to select the appropriate representation, then
216   no harm will result when the precondition evaluates to false.
217</t>
218<t><iref primary="true" item="selected representation"/>
219   We use the term "selected representation" to refer to
220   the current representation of the target resource that would have been
221   selected in a successful response if the same request had used the method
222   GET and had excluded all of the conditional request header fields.
223   The conditional request preconditions are evaluated by comparing the
224   values provided in the request header fields to the current metadata
225   for the selected representation.
226</t>
227
228<section title="Requirements" anchor="intro.requirements">
229<t>
230   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
231   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
232   document are to be interpreted as described in <xref target="RFC2119"/>.
233</t>
234<t>
235   An implementation is not compliant if it fails to satisfy one or more
236   of the "MUST" or "REQUIRED" level requirements for the protocols it
237   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
238   level and all the "SHOULD" level requirements for its protocols is said
239   to be "unconditionally compliant"; one that satisfies all the "MUST"
240   level requirements but not all the "SHOULD" level requirements for its
241   protocols is said to be "conditionally compliant".
242</t>
243</section>
244
245<section title="Syntax Notation" anchor="notation">
246 
247 
248 
249 
250 
251 
252 
253 
254 
255 
256 
257<t>
258  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
259  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
260  <xref target="collected.abnf"/> shows the collected ABNF, with the list
261  rule expanded.
262</t>
263<t>
264  The following core rules are included by
265  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
266  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
267  DIGIT (decimal 0-9), DQUOTE (double quote),
268  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
269  OCTET (any 8-bit sequence of data), SP (space),
270  VCHAR (any visible USASCII character),
271  and WSP (whitespace).
272</t>
273<t>
274  The ABNF rules below are defined in <xref target="Part1"/>:
275</t>
276<figure><artwork type="abnf2616"><![CDATA[
277  OWS           = <OWS, defined in [Part1], Section 1.2.2>
278  quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
279  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
280]]></artwork></figure>
281</section>
282</section>
283
284<section title="Validators" anchor="validators">
285   <iref primary="true" item="metadata"/>
286   <iref primary="true" item="validator"/>
287<t>
288   This specification defines two forms of metadata that are commonly used
289   to observe resource state and test for preconditions: modification dates
290   and opaque entity tags.  Additional metadata that reflects resource state
291   has been defined by various extensions of HTTP, such as WebDAV
292   <xref target="RFC4918"/>, that are beyond the scope of this specification.
293   A resource metadata value is referred to as a "validator"
294   when it is used within a precondition.
295</t>
296
297<section title="Weak versus Strong" anchor="weak.and.strong.validators">
298   <iref primary="true" item="validator" subitem="weak"/>
299   <iref primary="true" item="validator" subitem="strong"/>
300<t>
301   Validators come in two flavors: strong or weak.  Weak validators are easy
302   to generate but are far less useful for comparisons.  Strong validators
303   are ideal for comparisons but can be very difficult (and occasionally
304   impossible) to generate efficiently.  Rather than impose that all forms
305   of resource adhere to the same strength of validator, HTTP exposes the
306   type of validator in use and imposes restrictions on when weak validators
307   can be used as preconditions.
308</t>
309<t>
310   A "strong validator" is a representation metadata value that MUST be
311   changed to a new, previously unused or guaranteed unique, value whenever
312   a change occurs to the representation data such that a change would be
313   observable in the payload body of a 200 response to GET.  A strong
314   validator MAY be changed for other reasons, such as when a semantically
315   significant part of the representation metadata is changed (e.g.,
316   Content-Type), but it is in the best interests of the origin server to only
317   change the value when it is necessary to invalidate the stored responses
318   held by remote caches and authoring tools.  A strong validator MUST be
319   unique across all representations of a given resource, such that no two
320   representations of that resource share the same validator unless
321   their payload body would be identical.
322</t>
323<t>
324   Cache entries might persist for arbitrarily long periods, regardless
325   of expiration times.  Thus, a cache might attempt to validate an
326   entry using a validator that it obtained in the distant past.
327   A strong validator MUST be unique across all versions of all
328   representations associated with a particular resource over time.
329   However, there is no implication of uniqueness across representations
330   of different resources (i.e., the same strong validator might be
331   in use for representations of multiple resources at the same time
332   and does not imply that those representations are equivalent).
333</t>
334<t>
335   There are a variety of strong validators used in practice.  The best are
336   based on strict revision control, wherein each change to a representation
337   always results in a unique node name and revision identifier being assigned
338   before the representation is made accessible to GET.  A cryptographic hash
339   function applied to the representation data is also sufficient if the data
340   is available prior to the response header fields being sent and the digest
341   does not need to be recalculated every time a validation request is
342   received.  However, if a resource has distinct representations that differ
343   only in their metadata, such as might occur with content negotiation over
344   media types that happen to share the same data format, then a server
345   SHOULD incorporate additional information in the validator to
346   distinguish those representations and avoid confusing cache behavior.
347</t>
348<t>
349   In contrast, a "weak validator" is a representation metadata value that
350   might not be changed for every change to the representation data.  This
351   weakness might be due to limitations in how the value is calculated, such
352   as clock resolution or an inability to ensure uniqueness for all possible
353   representations of the resource, or due to a desire by the resource owner
354   to group representations by some self-determined set of equivalency
355   rather than unique sequences of data.  A weak entity-tag SHOULD change
356   whenever the origin server considers prior representations to be
357   unacceptable as a substitute for the current representation. In other
358   words, a weak entity-tag SHOULD change whenever the origin server wants
359   caches to invalidate old responses.
360</t>
361<t>
362   For example, the representation of a weather report that changes in
363   content every second, based on dynamic measurements, might be grouped
364   into sets of equivalent representations (from the origin server's
365   perspective) with the same weak validator in order to allow cached
366   representations to be valid for a reasonable period of time (perhaps
367   adjusted dynamically based on server load or weather quality).
368   Likewise, a representation's modification time, if defined with only
369   one-second resolution, might be a weak validator if it is possible
370   for the representation to be modified twice during a single second and
371   retrieved between those modifications.
372</t>
373<t>
374   A "use" of a validator occurs when either a client generates a request
375   and includes the validator in a precondition or when a server
376   compares two validators.
377   Weak validators are only usable in contexts that do not depend on exact
378   equality of a representation's payload body.
379   Strong validators are usable and preferred for all conditional requests,
380   including cache validation, partial content ranges, and "lost update"
381   avoidance.
382</t>
383</section>
384
385<section title="Last-Modified" anchor="header.last-modified">
386  <iref primary="true" item="Last-Modified header field"/>
387  <iref primary="true" item="Header Fields" subitem="Last-Modified"/>
388 
389<t>
390   The "Last-Modified" header field indicates the date and time at
391   which the origin server believes the selected representation was
392   last modified.
393</t>
394<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><artwork type="abnf2616"><![CDATA[
395  Last-Modified = HTTP-date
396]]></artwork></figure>
397<t>
398   An example of its use is
399</t>
400<figure><artwork type="example"><![CDATA[
401  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
402]]></artwork></figure>
403
404<section title="Generation" anchor="lastmod.generation">
405<t>
406   Origin servers SHOULD send Last-Modified for any selected
407   representation for which a last modification date can be reasonably
408   and consistently determined, since its use in conditional requests
409   and evaluating cache freshness (<xref target="Part6"/>) results in a substantial
410   reduction of HTTP traffic on the Internet and can be a significant
411   factor in improving service scalability and reliability.
412</t>
413<t>
414   A representation is typically the sum of many parts behind the
415   resource interface.  The last-modified time would usually be
416   the most recent time that any of those parts were changed.
417   How that value is determined for any given resource is an
418   implementation detail beyond the scope of this specification.
419   What matters to HTTP is how recipients of the Last-Modified
420   header field can use its value to make conditional requests
421   and test the validity of locally cached responses.
422</t>
423<t>
424   An origin server SHOULD obtain the Last-Modified value of the
425   representation as close as possible to the time that it generates
426   the Date field-value for its response. This allows a recipient to
427   make an accurate assessment of the representation's modification time,
428   especially if the representation changes near the time that the
429   response is generated.
430</t>
431<t>
432   An origin server with a clock MUST NOT send a Last-Modified date
433   that is later than the server's time of message origination (Date).
434   If the last modification time is derived from implementation-specific
435   metadata that evaluates to some time in the future, according to the
436   origin server's clock, then the origin server MUST replace that
437   value with the message origination date. This prevents a future
438   modification date from having an adverse impact on cache validation.
439</t>
440</section>
441
442<section title="Comparison" anchor="lastmod.comparison">
443<t>
444   A Last-Modified time, when used as a validator in a request, is
445   implicitly weak unless it is possible to deduce that it is strong,
446   using the following rules:
447  <list style="symbols">
448     <t>The validator is being compared by an origin server to the
449        actual current validator for the representation and,</t>
450     <t>That origin server reliably knows that the associated representation did
451        not change twice during the second covered by the presented
452        validator.</t>
453  </list>
454</t>
455<t>
456   or
457  <list style="symbols">
458     <t>The validator is about to be used by a client in an If-Modified-Since,
459        If-Unmodified-Since header field, because the client has a cache entry,
460        or If-Range for the associated representation, and</t>
461     <t>That cache entry includes a Date value, which gives the time
462        when the origin server sent the original response, and</t>
463     <t>The presented Last-Modified time is at least 60 seconds before
464        the Date value.</t>
465  </list>
466</t>
467<t>
468   or
469  <list style="symbols">
470     <t>The validator is being compared by an intermediate cache to the
471        validator stored in its cache entry for the representation, and</t>
472     <t>That cache entry includes a Date value, which gives the time
473        when the origin server sent the original response, and</t>
474     <t>The presented Last-Modified time is at least 60 seconds before
475        the Date value.</t>
476  </list>
477</t>
478<t>
479   This method relies on the fact that if two different responses were
480   sent by the origin server during the same second, but both had the
481   same Last-Modified time, then at least one of those responses would
482   have a Date value equal to its Last-Modified time. The arbitrary 60-second
483   limit guards against the possibility that the Date and Last-Modified
484   values are generated from different clocks, or at somewhat
485   different times during the preparation of the response. An
486   implementation MAY use a value larger than 60 seconds, if it is
487   believed that 60 seconds is too short.
488</t>
489</section>
490</section>
491
492<section title="ETag" anchor="header.etag">
493  <iref primary="true" item="ETag header field"/>
494  <iref primary="true" item="Header Fields" subitem="ETag"/>
495 
496 
497 
498 
499 
500<t>
501   The ETag header field provides the current entity-tag for the
502   selected representation.
503   An entity-tag is an opaque validator for differentiating between
504   multiple representations of the same resource, regardless of whether
505   those multiple representations are due to resource state changes over
506   time, content negotiation resulting in multiple representations being
507   valid at the same time, or both. An entity-tag consists of an opaque
508   quoted string, possibly prefixed by a weakness indicator.
509</t>
510<figure><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"/><artwork type="abnf2616"><![CDATA[
511  ETag       = entity-tag
512
513  entity-tag = [ weak ] opaque-tag
514  weak       = %x57.2F ; "W/", case-sensitive
515  opaque-tag = quoted-string
516]]></artwork></figure>
517<t>
518   An entity-tag can be more reliable for validation than a modification
519   date in situations where it is inconvenient to store modification
520   dates, where the one-second resolution of HTTP date values is not
521   sufficient, or where modification dates are not consistently maintained.
522</t>
523<figure><preamble>
524  Examples:
525</preamble>
526<artwork type="example"><![CDATA[
527  ETag: "xyzzy"
528  ETag: W/"xyzzy"
529  ETag: ""
530]]></artwork></figure>
531<t>
532   An entity-tag can be either a weak or strong validator, with
533   strong being the default.  If an origin server provides an entity-tag
534   for a representation and the generation of that entity-tag does not satisfy
535   the requirements for a strong validator
536   (<xref target="weak.and.strong.validators"/>), then that
537   entity-tag MUST be marked as weak by prefixing its opaque value
538   with "W/" (case-sensitive).
539</t>
540
541<section title="Generation" anchor="entity.tag.generation">
542<t>
543   The principle behind entity-tags is that only the service author
544   knows the implementation of a resource well enough to select the
545   most accurate and efficient validation mechanism for that resource,
546   and that any such mechanism can be mapped to a simple sequence of
547   octets for easy comparison.  Since the value is opaque, there is no
548   need for the client to be aware of how each entity-tag is constructed.
549</t>
550<t>
551   For example, a resource that has implementation-specific versioning
552   applied to all changes might use an internal revision number, perhaps
553   combined with a variance identifier for content negotiation, to
554   accurately differentiate between representations.
555   Other implementations might use a stored hash of representation content,
556   a combination of various filesystem attributes, or a modification
557   timestamp that has sub-second resolution.
558</t>
559<t>
560   Origin servers SHOULD send ETag for any selected representation
561   for which detection of changes can be reasonably and consistently
562   determined, since the entity-tag's use in conditional requests and
563   evaluating cache freshness (<xref target="Part6"/>) can result in a substantial
564   reduction of HTTP network traffic and can be a significant factor in
565   improving service scalability and reliability.
566</t>
567</section>
568
569<section title="Comparison" anchor="entity.tag.comparison">
570 
571<t>
572   There are two entity-tag comparison functions, depending
573   on whether the comparison context allows the use of weak validators
574   or not:
575  <list style="symbols">
576     <t>The strong comparison function: in order to be considered equal,
577        both opaque-tags MUST be identical character-by-character, and both
578        MUST NOT be weak.</t>
579     <t>The weak comparison function: in order to be considered equal, both
580        opaque-tags MUST be identical character-by-character, but
581        either or both of them MAY be tagged as "weak" without affecting
582        the result.</t>
583  </list>
584</t>
585<t>
586   The example below shows the results for a set of entity-tag pairs,
587   and both the weak and strong comparison function results:
588</t>
589<texttable align="left">
590  <ttcol>ETag 1</ttcol>
591  <ttcol>ETag 2</ttcol>
592  <ttcol>Strong Comparison</ttcol>
593  <ttcol>Weak Comparison</ttcol>
594
595  <c>W/"1"</c>
596  <c>W/"1"</c>
597  <c>no match</c>
598  <c>match</c>
599 
600  <c>W/"1"</c>
601  <c>W/"2"</c>
602  <c>no match</c>
603  <c>no match</c>
604
605  <c>W/"1"</c>
606  <c>"1"</c>
607  <c>no match</c>
608  <c>match</c>
609
610  <c>"1"</c>
611  <c>"1"</c>
612  <c>match</c>
613  <c>match</c>
614</texttable>
615</section>
616
617<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
618<t>
619   Consider a resource that is subject to content negotiation (Section 5 of <xref target="Part3"/>),
620   and where the representations returned upon a GET request vary based on
621   the Accept-Encoding request header field (Section 6.3 of <xref target="Part3"/>):
622</t>
623<figure><preamble>&gt;&gt; Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
624  GET /index HTTP/1.1
625  Host: www.example.com
626  Accept-Encoding: gzip
627 
628  ]]></artwork></figure>
629<t>
630   In this case, the response might or might not use the gzip content coding.
631   If it does not, the response might look like:
632</t>
633<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
634  HTTP/1.1 200 OK
635  Date: Thu, 26 Mar 2010 00:05:00 GMT
636  ETag: "123-a"
637  Content-Length: 70
638  Vary: Accept-Encoding
639  Content-Type: text/plain
640 
641  Hello World!
642  Hello World!
643  Hello World!
644  Hello World!
645  Hello World!
646  ]]></artwork></figure>
647<t>
648   An alternative representation that does use gzip content coding would be:
649</t>
650<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
651  HTTP/1.1 200 OK
652  Date: Thu, 26 Mar 2010 00:05:00 GMT
653  ETag: "123-b"
654  Content-Length: 43
655  Vary: Accept-Encoding
656  Content-Type: text/plain
657  Content-Encoding: gzip
658 
659  ...binary data...]]></artwork></figure>
660<t><list>
661  <t>
662    Note: Content codings are a property of the representation,
663    so therefore an entity-tag of an encoded representation must be distinct
664    from an unencoded representation to prevent conflicts during cache updates
665    and range requests.  In contrast, transfer codings (Section 6.2 of <xref target="Part1"/>)
666    apply only during message transfer and do not require distinct entity-tags.
667  </t>
668</list></t>
669</section>
670</section>
671
672<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">
673<t>
674   We adopt a set of rules and recommendations for origin servers,
675   clients, and caches regarding when various validator types ought to
676   be used, and for what purposes.
677</t>
678<t>
679   HTTP/1.1 origin servers:
680  <list style="symbols">
681     <t>SHOULD send an entity-tag validator unless it is not feasible to
682        generate one.</t>
683
684     <t>MAY send a weak entity-tag instead of a strong entity-tag, if
685        performance considerations support the use of weak entity-tags,
686        or if it is unfeasible to send a strong entity-tag.</t>
687
688     <t>SHOULD send a Last-Modified value if it is feasible to send one.</t>
689  </list>
690</t>
691<t>
692   In other words, the preferred behavior for an HTTP/1.1 origin server
693   is to send both a strong entity-tag and a Last-Modified value.
694</t>
695<t>
696   HTTP/1.1 clients:
697  <list style="symbols">
698     <t>MUST use that entity-tag in any cache-conditional request (using
699        If-Match or If-None-Match) if an entity-tag has been provided by the
700        origin server.</t>
701
702     <t>SHOULD use the Last-Modified value in non-subrange cache-conditional
703        requests (using If-Modified-Since) if only a Last-Modified value has
704        been provided by the origin server. </t>
705
706     <t>MAY use the Last-Modified value in subrange cache-conditional
707        requests (using If-Unmodified-Since) if only a Last-Modified value has
708        been provided by an HTTP/1.0 origin server. The user agent SHOULD
709        provide a way to disable this, in case of difficulty.</t>
710
711     <t>SHOULD use both validators in cache-conditional requests if both an
712        entity-tag and a Last-Modified value have been provided by the origin
713        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
714        appropriately.</t>
715  </list>
716</t>
717<t>
718   An HTTP/1.1 origin server, upon receiving a conditional request that
719   includes both a Last-Modified date (e.g., in an If-Modified-Since or
720   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
721   in an If-Match, If-None-Match, or If-Range header field) as cache
722   validators, MUST NOT return a response status code of 304 (Not Modified)
723   unless doing so is consistent with all of the conditional header
724   fields in the request.
725</t>
726<t>
727   An HTTP/1.1 caching proxy, upon receiving a conditional request that
728   includes both a Last-Modified date and one or more entity-tags as
729   cache validators, MUST NOT return a locally cached response to the
730   client unless that cached response is consistent with all of the
731   conditional header fields in the request.
732  <list><t>
733      Note: The general principle behind these rules is that HTTP/1.1
734      servers and clients ought to transmit as much non-redundant
735      information as is available in their responses and requests.
736      HTTP/1.1 systems receiving this information will make the most
737      conservative assumptions about the validators they receive.
738  </t><t>
739      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
740      last-modified values received or used by these systems will
741      support transparent and efficient caching, and so HTTP/1.1 origin
742      servers should provide Last-Modified values. In those rare cases
743      where the use of a Last-Modified value as a validator by an
744      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
745      origin servers should not provide one.
746  </t></list>
747</t>
748</section>
749</section>
750
751<section title="Precondition Header Fields" anchor="header.fields">
752<t>
753   This section defines the syntax and semantics of HTTP/1.1 header fields
754   for applying preconditions on requests.
755</t>
756
757<section title="If-Match" anchor="header.if-match">
758  <iref primary="true" item="If-Match header field"/>
759  <iref primary="true" item="Header Fields" subitem="If-Match"/>
760 
761<t>
762   The "If-Match" header field MAY be used to make a request method
763   conditional on the current existence or value of an entity-tag for
764   one or more representations of the target resource.  If-Match is
765   generally useful for resource update requests, such as PUT requests,
766   as a means for protecting against accidental overwrites when multiple
767   clients are acting in parallel on the same resource (i.e., the
768   "lost update" problem).  An If-Match field-value of "*" places the
769   precondition on the existence of any current representation for the
770   target resource.
771</t>
772<figure><iref primary="true" item="Grammar" subitem="If-Match"/><artwork type="abnf2616"><![CDATA[
773  If-Match = "*" / 1#entity-tag
774]]></artwork></figure>
775<t>
776   If any of the entity-tags listed in the If-Match field value match
777   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
778   selected representation for the target resource,
779   or if "*" is given and any current representation exists for the
780   target resource, then the server MAY perform the request method
781   as if the If-Match header field was not present.
782</t>
783<t>
784   If none of the entity-tags match, or if "*" is given and no current
785   representation exists, the server MUST NOT perform the requested method.
786   Instead, the server MUST respond with the 412 (Precondition Failed)
787   status code.
788</t>
789<t>
790   If the request would, without the If-Match header field, result in
791   anything other than a 2xx or 412 status code, then the If-Match header field
792   MUST be ignored.
793</t>
794<t>
795   Examples:
796</t>
797<figure><artwork type="example"><![CDATA[
798  If-Match: "xyzzy"
799  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
800  If-Match: *
801]]></artwork></figure>
802<t>
803   The result of a request having both an If-Match header field and
804   either an If-None-Match or an If-Modified-Since header fields is
805   undefined by this specification.
806</t>
807</section>
808
809<section title="If-None-Match" anchor="header.if-none-match">
810  <iref primary="true" item="If-None-Match header field"/>
811  <iref primary="true" item="Header Fields" subitem="If-None-Match"/>
812 
813<t>
814   The "If-None-Match" header field MAY be used to make a request method
815   conditional on not matching any of the current entity-tag values for
816   representations of the target resource.  If-None-Match is primarily
817   used in conditional GET requests to enable efficient updates of cached
818   information with a minimum amount of transaction overhead.  A client
819   that has one or more representations previously obtained from the
820   target resource can send If-None-Match with a list of the associated
821   entity-tags in the hope of receiving a 304 response if at least one
822   of those representations matches the selected representation.
823</t>
824<t>
825   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
826   request method (e.g., PUT) from inadvertently modifying an existing
827   representation of the target resource when the client believes that
828   the resource does not have a current representation.  This is a variation
829   on the "lost update" problem that might arise if more than one client
830   attempts to create an initial representation for the target resource.
831</t>
832<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><artwork type="abnf2616"><![CDATA[
833  If-None-Match = "*" / 1#entity-tag
834]]></artwork></figure>
835<t>
836   If any of the entity-tags listed in the If-None-Match field-value match
837   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
838   selected representation, or if "*" is
839   given and any current representation exists for that resource, then the
840   server MUST NOT perform the requested method.
841   Instead, if the request method was GET or HEAD, the server SHOULD
842   respond with a 304 (Not Modified) status code, including the cache-related
843   header fields (particularly ETag) of the selected representation that has
844   a matching entity-tag.  For all other request methods, the server MUST
845   respond with a 412 (Precondition Failed) status code.
846</t>
847<t>
848   If none of the entity-tags match, then the server MAY perform the
849   requested method as if the If-None-Match header field did not exist,
850   but MUST also ignore any If-Modified-Since header field(s) in the
851   request. That is, if no entity-tags match, then the server MUST NOT
852   return a 304 (Not Modified) response.
853</t>
854<t>
855   If the request would, without the If-None-Match header field, result
856   in anything other than a 2xx or 304 status code, then the If-None-Match
857   header field MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
858   a discussion of server behavior when both If-Modified-Since and
859   If-None-Match appear in the same request.)
860</t>
861<t>
862   Examples:
863</t>
864<figure><artwork type="example"><![CDATA[
865  If-None-Match: "xyzzy"
866  If-None-Match: W/"xyzzy"
867  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
868  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
869  If-None-Match: *
870]]></artwork></figure>
871<t>
872   The result of a request having both an If-None-Match header field and
873   either an If-Match or an If-Unmodified-Since header fields is
874   undefined by this specification.
875</t>
876</section>
877
878<section title="If-Modified-Since" anchor="header.if-modified-since">
879  <iref primary="true" item="If-Modified-Since header field"/>
880  <iref primary="true" item="Header Fields" subitem="If-Modified-Since"/>
881 
882<t>
883   The "If-Modified-Since" header field MAY be used to make a request
884   method conditional by modification date: if the selected representation
885   has not been modified since the time specified in this field, then
886   do not perform the request method; instead, respond as detailed below.
887</t>
888<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><artwork type="abnf2616"><![CDATA[
889  If-Modified-Since = HTTP-date
890]]></artwork></figure>
891<t>
892   An example of the field is:
893</t>
894<figure><artwork type="example"><![CDATA[
895  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
896]]></artwork></figure>
897<t>
898   A GET method with an If-Modified-Since header field and no Range header
899   field requests that the selected representation be transferred only if
900   it has been modified since the date given by the If-Modified-Since
901   header field.
902   The algorithm for determining this includes the following cases:
903  <list style="numbers">
904      <t>If the request would normally result in anything other than a
905         200 (OK) status code, or if the passed If-Modified-Since date is
906         invalid, the response is exactly the same as for a normal GET.
907         A date which is later than the server's current time is
908         invalid.</t>
909
910      <t>If the selected representation has been modified since the
911         If-Modified-Since date, the response is exactly the same as for
912         a normal GET.</t>
913
914      <t>If the selected representation has not been modified since a valid
915         If-Modified-Since date, the server SHOULD return a
916         304 (Not Modified) response.</t>
917  </list>
918</t>
919<t>
920   The purpose of this feature is to allow efficient updates of cached
921   information with a minimum amount of transaction overhead.
922  <list><t>
923      Note: The Range header field modifies the meaning of If-Modified-Since;
924      see Section 5.4 of <xref target="Part5"/> for full details.
925    </t><t>
926      Note: If-Modified-Since times are interpreted by the server, whose
927      clock might not be synchronized with the client.
928    </t><t>
929      Note: When handling an If-Modified-Since header field, some
930      servers will use an exact date comparison function, rather than a
931      less-than function, for deciding whether to send a 304 (Not
932      Modified) response. To get best results when sending an If-Modified-Since
933      header field for cache validation, clients are
934      advised to use the exact date string received in a previous Last-Modified
935      header field whenever possible.
936    </t><t>
937      Note: If a client uses an arbitrary date in the If-Modified-Since
938      header field instead of a date taken from the Last-Modified header field for
939      the same request, the client needs to be aware that this
940      date is interpreted in the server's understanding of time.
941      Unsynchronized clocks and rounding problems, due to the different
942      encodings of time between the client and server, are concerns.
943      This includes the possibility of race conditions if the
944      document has changed between the time it was first requested and
945      the If-Modified-Since date of a subsequent request, and the
946      possibility of clock-skew-related problems if the If-Modified-Since
947      date is derived from the client's clock without correction
948      to the server's clock. Corrections for different time bases
949      between client and server are at best approximate due to network
950      latency.
951    </t>
952  </list>
953</t>
954<t>
955   The result of a request having both an If-Modified-Since header field
956   and either an If-Match or an If-Unmodified-Since header fields is
957   undefined by this specification.
958</t>
959</section>
960
961<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
962  <iref primary="true" item="If-Unmodified-Since header field"/>
963  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since"/>
964 
965<t>
966   The "If-Unmodified-Since" header field MAY be used to make a request
967   method conditional by modification date: if the selected representation
968   has been modified since the time specified in this field, then the
969   server MUST NOT perform the requested operation and MUST instead
970   respond with the 412 (Precondition Failed) status code.
971   If the selected representation has not been modified since the time
972   specified in this field, the server SHOULD perform the request
973   method as if the If-Unmodified-Since header field were not present.
974</t>
975<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><artwork type="abnf2616"><![CDATA[
976  If-Unmodified-Since = HTTP-date
977]]></artwork></figure>
978<t>
979   An example of the field is:
980</t>
981<figure><artwork type="example"><![CDATA[
982  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
983]]></artwork></figure>
984<t>
985   If the request normally (i.e., without the If-Unmodified-Since
986   header field) would result in anything other than a 2xx or 412 status code,
987   the If-Unmodified-Since header field SHOULD be ignored.
988</t>
989<t>
990   If the specified date is invalid, the header field MUST be ignored.
991</t>
992<t>
993   The result of a request having both an If-Unmodified-Since header
994   field and either an If-None-Match or an If-Modified-Since header
995   fields is undefined by this specification.
996</t>
997</section>
998
999<section title="If-Range" anchor="header.if-range">
1000<t>
1001   The If-Range header field provides a special conditional request
1002   mechanism that is similar to If-Match and If-Unmodified-Since but
1003   specific to HTTP range requests. If-Range is defined in Section 5.3 of <xref target="Part5"/>.
1004</t>
1005</section>
1006
1007</section>
1008
1009<section title="Status Code Definitions" anchor="status.code.definitions">
1010<section title="304 Not Modified" anchor="status.304">
1011  <iref primary="true" item="304 Not Modified (status code)"/>
1012  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
1013<t>
1014   The 304 status code indicates that a conditional GET request has been
1015   received and would have resulted in a 200 (OK) response if it were not
1016   for the fact that the condition has evaluated to false.  In other words,
1017   there is no need for the server to transfer a representation of the
1018   target resource because the client's request indicates that it already
1019   has a valid representation, as indicated by the 304 response header
1020   fields, and is therefore redirecting the client to make use of that
1021   stored representation as if it were the payload of a 200 response.
1022   The 304 response MUST NOT contain a message-body, and thus is always
1023   terminated by the first empty line after the header fields.
1024</t>
1025<t>
1026   A 304 response MUST include a Date header field (Section 9.3 of <xref target="Part1"/>)
1027   unless its omission is required by Section 9.3.1 of <xref target="Part1"/>.  If a 200 response
1028   to the same request would have included any of the header fields
1029   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1030   Vary, then those same header fields MUST be sent in a 304 response.
1031</t>
1032<t>
1033   Since the goal of a 304 response is to minimize information transfer
1034   when the recipient already has one or more cached representations,
1035   the response SHOULD NOT include representation metadata other
1036   than the above listed fields unless said metadata exists for the
1037   purpose of guiding cache updates (e.g., future HTTP extensions).
1038</t>
1039<t>
1040   If the recipient of a 304 response does not have a cached representation
1041   corresponding to the entity-tag indicated by the 304 response, then the
1042   recipient MUST NOT use the 304 to update its own cache.  If this
1043   conditional request originated with an outbound client, such as a
1044   user agent with its own cache sending a conditional GET to a shared
1045   proxy, then the 304 response MAY be forwarded to the outbound client.
1046   Otherwise, the recipient MUST disregard the 304 response and repeat
1047   the request without any preconditions.
1048</t>
1049<t>
1050   If a cache uses a received 304 response to update a cache entry, the
1051   cache MUST update the entry to reflect any new field values given in
1052   the response.
1053</t>
1054</section>
1055
1056<section title="412 Precondition Failed" anchor="status.412">
1057  <iref primary="true" item="412 Precondition Failed (status code)"/>
1058  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
1059<t>
1060   The 412 status code indicates that one or more preconditions given in
1061   the request header fields evaluated to false when tested on the server.
1062   This response code allows the client to place preconditions on the
1063   current resource state (its current representations and metadata)
1064   and thus prevent the request method from being applied if the target
1065   resource is in an unexpected state.
1066</t>
1067</section>
1068</section>
1069
1070<section title="IANA Considerations" anchor="IANA.considerations">
1071
1072<section title="Status Code Registration" anchor="status.code.registration">
1073<t>
1074   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1075   shall be updated with the registrations below:
1076</t>
1077
1078<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1079<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1080   <ttcol>Value</ttcol>
1081   <ttcol>Description</ttcol>
1082   <ttcol>Reference</ttcol>
1083   <c>304</c>
1084   <c>Not Modified</c>
1085   <c>
1086      <xref target="status.304"/>
1087   </c>
1088   <c>412</c>
1089   <c>Precondition Failed</c>
1090   <c>
1091      <xref target="status.412"/>
1092   </c>
1093</texttable>
1094<!--(END)-->
1095
1096</section>
1097
1098<section title="Header Field Registration" anchor="header.field.registration">
1099<t>
1100   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1101   with the permanent registrations below (see <xref target="RFC3864"/>):
1102</t>
1103
1104<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1105<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1106   <ttcol>Header Field Name</ttcol>
1107   <ttcol>Protocol</ttcol>
1108   <ttcol>Status</ttcol>
1109   <ttcol>Reference</ttcol>
1110
1111   <c>ETag</c>
1112   <c>http</c>
1113   <c>standard</c>
1114   <c>
1115      <xref target="header.etag"/>
1116   </c>
1117   <c>If-Match</c>
1118   <c>http</c>
1119   <c>standard</c>
1120   <c>
1121      <xref target="header.if-match"/>
1122   </c>
1123   <c>If-Modified-Since</c>
1124   <c>http</c>
1125   <c>standard</c>
1126   <c>
1127      <xref target="header.if-modified-since"/>
1128   </c>
1129   <c>If-None-Match</c>
1130   <c>http</c>
1131   <c>standard</c>
1132   <c>
1133      <xref target="header.if-none-match"/>
1134   </c>
1135   <c>If-Unmodified-Since</c>
1136   <c>http</c>
1137   <c>standard</c>
1138   <c>
1139      <xref target="header.if-unmodified-since"/>
1140   </c>
1141   <c>Last-Modified</c>
1142   <c>http</c>
1143   <c>standard</c>
1144   <c>
1145      <xref target="header.last-modified"/>
1146   </c>
1147</texttable>
1148<!--(END)-->
1149
1150<t>
1151   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1152</t>
1153</section>
1154</section>
1155
1156<section title="Security Considerations" anchor="security.considerations">
1157<t>
1158   No additional security considerations have been identified beyond
1159   those applicable to HTTP in general <xref target="Part1"/>.
1160</t>
1161</section>
1162
1163<section title="Acknowledgments" anchor="acks">
1164<t>
1165  See Section 12 of <xref target="Part1"/>.
1166</t>
1167</section>
1168</middle>
1169<back>
1170
1171<references title="Normative References">
1172
1173<reference anchor="Part1">
1174  <front>
1175    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1176    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1177      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1178      <address><email>fielding@gbiv.com</email></address>
1179    </author>
1180    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1181      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1182      <address><email>jg@freedesktop.org</email></address>
1183    </author>
1184    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1185      <organization abbrev="HP">Hewlett-Packard Company</organization>
1186      <address><email>JeffMogul@acm.org</email></address>
1187    </author>
1188    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1189      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1190      <address><email>henrikn@microsoft.com</email></address>
1191    </author>
1192    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1193      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1194      <address><email>LMM@acm.org</email></address>
1195    </author>
1196    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1197      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1198      <address><email>paulle@microsoft.com</email></address>
1199    </author>
1200    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1201      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1202      <address><email>timbl@w3.org</email></address>
1203    </author>
1204    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1205      <organization abbrev="W3C">World Wide Web Consortium</organization>
1206      <address><email>ylafon@w3.org</email></address>
1207    </author>
1208    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1209      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1210      <address><email>julian.reschke@greenbytes.de</email></address>
1211    </author>
1212    <date month="August" year="2011"/>
1213  </front>
1214  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-16"/>
1215 
1216</reference>
1217
1218<reference anchor="Part3">
1219  <front>
1220    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1221    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1222      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1223      <address><email>fielding@gbiv.com</email></address>
1224    </author>
1225    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1226      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1227      <address><email>jg@freedesktop.org</email></address>
1228    </author>
1229    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1230      <organization abbrev="HP">Hewlett-Packard Company</organization>
1231      <address><email>JeffMogul@acm.org</email></address>
1232    </author>
1233    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1234      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1235      <address><email>henrikn@microsoft.com</email></address>
1236    </author>
1237    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1238      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1239      <address><email>LMM@acm.org</email></address>
1240    </author>
1241    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1242      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1243      <address><email>paulle@microsoft.com</email></address>
1244    </author>
1245    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1246      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1247      <address><email>timbl@w3.org</email></address>
1248    </author>
1249    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1250      <organization abbrev="W3C">World Wide Web Consortium</organization>
1251      <address><email>ylafon@w3.org</email></address>
1252    </author>
1253    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1254      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1255      <address><email>julian.reschke@greenbytes.de</email></address>
1256    </author>
1257    <date month="August" year="2011"/>
1258  </front>
1259  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-16"/>
1260 
1261</reference>
1262
1263<reference anchor="Part5">
1264  <front>
1265    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1266    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1267      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1268      <address><email>fielding@gbiv.com</email></address>
1269    </author>
1270    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1271      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1272      <address><email>jg@freedesktop.org</email></address>
1273    </author>
1274    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1275      <organization abbrev="HP">Hewlett-Packard Company</organization>
1276      <address><email>JeffMogul@acm.org</email></address>
1277    </author>
1278    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1279      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1280      <address><email>henrikn@microsoft.com</email></address>
1281    </author>
1282    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1283      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1284      <address><email>LMM@acm.org</email></address>
1285    </author>
1286    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1287      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1288      <address><email>paulle@microsoft.com</email></address>
1289    </author>
1290    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1291      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1292      <address><email>timbl@w3.org</email></address>
1293    </author>
1294    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1295      <organization abbrev="W3C">World Wide Web Consortium</organization>
1296      <address><email>ylafon@w3.org</email></address>
1297    </author>
1298    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1299      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1300      <address><email>julian.reschke@greenbytes.de</email></address>
1301    </author>
1302    <date month="August" year="2011"/>
1303  </front>
1304  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-16"/>
1305 
1306</reference>
1307
1308<reference anchor="Part6">
1309  <front>
1310    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1311    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1312      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1313      <address><email>fielding@gbiv.com</email></address>
1314    </author>
1315    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1316      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1317      <address><email>jg@freedesktop.org</email></address>
1318    </author>
1319    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1320      <organization abbrev="HP">Hewlett-Packard Company</organization>
1321      <address><email>JeffMogul@acm.org</email></address>
1322    </author>
1323    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1324      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1325      <address><email>henrikn@microsoft.com</email></address>
1326    </author>
1327    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1328      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1329      <address><email>LMM@acm.org</email></address>
1330    </author>
1331    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1332      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1333      <address><email>paulle@microsoft.com</email></address>
1334    </author>
1335    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1336      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1337      <address><email>timbl@w3.org</email></address>
1338    </author>
1339    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1340      <organization abbrev="W3C">World Wide Web Consortium</organization>
1341      <address><email>ylafon@w3.org</email></address>
1342    </author>
1343    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1344      <address><email>mnot@mnot.net</email></address>
1345    </author>
1346    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1347      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1348      <address><email>julian.reschke@greenbytes.de</email></address>
1349    </author>
1350    <date month="August" year="2011"/>
1351  </front>
1352  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-16"/>
1353 
1354</reference>
1355
1356<reference anchor="RFC2119">
1357  <front>
1358    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1359    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1360      <organization>Harvard University</organization>
1361      <address><email>sob@harvard.edu</email></address>
1362    </author>
1363    <date month="March" year="1997"/>
1364  </front>
1365  <seriesInfo name="BCP" value="14"/>
1366  <seriesInfo name="RFC" value="2119"/>
1367</reference>
1368
1369<reference anchor="RFC5234">
1370  <front>
1371    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1372    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1373      <organization>Brandenburg InternetWorking</organization>
1374      <address>
1375        <email>dcrocker@bbiw.net</email>
1376      </address> 
1377    </author>
1378    <author initials="P." surname="Overell" fullname="Paul Overell">
1379      <organization>THUS plc.</organization>
1380      <address>
1381        <email>paul.overell@thus.net</email>
1382      </address>
1383    </author>
1384    <date month="January" year="2008"/>
1385  </front>
1386  <seriesInfo name="STD" value="68"/>
1387  <seriesInfo name="RFC" value="5234"/>
1388</reference>
1389
1390</references>
1391
1392<references title="Informative References">
1393
1394<reference anchor="RFC2616">
1395  <front>
1396    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1397    <author initials="R." surname="Fielding" fullname="R. Fielding">
1398      <organization>University of California, Irvine</organization>
1399      <address><email>fielding@ics.uci.edu</email></address>
1400    </author>
1401    <author initials="J." surname="Gettys" fullname="J. Gettys">
1402      <organization>W3C</organization>
1403      <address><email>jg@w3.org</email></address>
1404    </author>
1405    <author initials="J." surname="Mogul" fullname="J. Mogul">
1406      <organization>Compaq Computer Corporation</organization>
1407      <address><email>mogul@wrl.dec.com</email></address>
1408    </author>
1409    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1410      <organization>MIT Laboratory for Computer Science</organization>
1411      <address><email>frystyk@w3.org</email></address>
1412    </author>
1413    <author initials="L." surname="Masinter" fullname="L. Masinter">
1414      <organization>Xerox Corporation</organization>
1415      <address><email>masinter@parc.xerox.com</email></address>
1416    </author>
1417    <author initials="P." surname="Leach" fullname="P. Leach">
1418      <organization>Microsoft Corporation</organization>
1419      <address><email>paulle@microsoft.com</email></address>
1420    </author>
1421    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1422      <organization>W3C</organization>
1423      <address><email>timbl@w3.org</email></address>
1424    </author>
1425    <date month="June" year="1999"/>
1426  </front>
1427  <seriesInfo name="RFC" value="2616"/>
1428</reference>
1429
1430<reference anchor="RFC3864">
1431  <front>
1432    <title>Registration Procedures for Message Header Fields</title>
1433    <author initials="G." surname="Klyne" fullname="G. Klyne">
1434      <organization>Nine by Nine</organization>
1435      <address><email>GK-IETF@ninebynine.org</email></address>
1436    </author>
1437    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
1438      <organization>BEA Systems</organization>
1439      <address><email>mnot@pobox.com</email></address>
1440    </author>
1441    <author initials="J." surname="Mogul" fullname="J. Mogul">
1442      <organization>HP Labs</organization>
1443      <address><email>JeffMogul@acm.org</email></address>
1444    </author>
1445    <date year="2004" month="September"/>
1446  </front>
1447  <seriesInfo name="BCP" value="90"/>
1448  <seriesInfo name="RFC" value="3864"/>
1449</reference>
1450
1451<reference anchor="RFC4918">
1452  <front>
1453    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1454    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor">
1455      <organization abbrev="CommerceNet">CommerceNet</organization>
1456      <address><email>ldusseault@commerce.net</email></address>
1457    </author>
1458    <date month="June" year="2007"/>
1459  </front>
1460  <seriesInfo name="RFC" value="4918"/>
1461</reference>
1462</references>
1463
1464<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1465<t>
1466  Allow weak entity-tags in all requests except range requests (Sections
1467  <xref target="weak.and.strong.validators" format="counter"/> and
1468  <xref target="header.if-none-match" format="counter"/>).
1469</t>
1470<t>
1471  Change ABNF productions for header fields to only define the field value.
1472  (<xref target="header.fields"/>)
1473</t>
1474</section>
1475
1476
1477<section title="Collected ABNF" anchor="collected.abnf">
1478<figure>
1479<artwork type="abnf" name="p4-conditional.parsed-abnf"><![CDATA[
1480ETag = entity-tag
1481
1482HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1483
1484If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1485 entity-tag ] ) )
1486If-Modified-Since = HTTP-date
1487If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1488 entity-tag ] ) )
1489If-Unmodified-Since = HTTP-date
1490
1491Last-Modified = HTTP-date
1492
1493OWS = <OWS, defined in [Part1], Section 1.2.2>
1494
1495entity-tag = [ weak ] opaque-tag
1496
1497opaque-tag = quoted-string
1498
1499quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
1500
1501weak = %x57.2F ; W/
1502]]></artwork>
1503</figure>
1504<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
1505; ETag defined but not used
1506; If-Match defined but not used
1507; If-Modified-Since defined but not used
1508; If-None-Match defined but not used
1509; If-Unmodified-Since defined but not used
1510; Last-Modified defined but not used
1511]]></artwork></figure></section>
1512
1513
1514<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1515
1516<section title="Since RFC 2616">
1517<t>
1518  Extracted relevant partitions from <xref target="RFC2616"/>.
1519</t>
1520</section>
1521
1522<section title="Since draft-ietf-httpbis-p4-conditional-00">
1523<t>
1524  Closed issues:
1525  <list style="symbols">
1526    <t>
1527      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1528      "Normative and Informative references"
1529    </t>
1530  </list>
1531</t>
1532<t>
1533  Other changes:
1534  <list style="symbols">
1535    <t>
1536      Move definitions of 304 and 412 condition codes from Part2.
1537    </t>
1538  </list>
1539</t>
1540</section>
1541
1542<section title="Since draft-ietf-httpbis-p4-conditional-01">
1543<t>
1544  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1545  <list style="symbols">
1546    <t>
1547      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1548    </t>
1549  </list>
1550</t>
1551</section>
1552
1553<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1554<t>
1555  Closed issues:
1556  <list style="symbols">
1557    <t>
1558      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1559      "Weak ETags on non-GET requests"
1560    </t>
1561  </list>
1562</t>
1563<t>
1564  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1565  <list style="symbols">
1566    <t>
1567      Reference RFC 3984, and update header field registrations for header fields defined
1568      in this document.
1569    </t>
1570  </list>
1571</t>
1572</section>
1573
1574<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1575<t>
1576  Closed issues:
1577  <list style="symbols">
1578    <t>
1579      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1580      "Examples for ETag matching"
1581    </t>
1582    <t>
1583      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1584      "'entity value' undefined"
1585    </t>
1586    <t>
1587      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1588      "bogus 2068 Date header reference"
1589    </t>
1590  </list>
1591</t>
1592</section>
1593
1594<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1595<t>
1596  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1597  <list style="symbols">
1598    <t>
1599      Use "/" instead of "|" for alternatives.
1600    </t>
1601    <t>
1602      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1603      whitespace ("OWS") and required whitespace ("RWS").
1604    </t>
1605    <t>
1606      Rewrite ABNFs to spell out whitespace rules, factor out
1607      header field value format definitions.
1608    </t>
1609  </list>
1610</t>
1611</section>
1612
1613<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1614<t>
1615  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1616  <list style="symbols">
1617    <t>
1618      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1619    </t>
1620  </list>
1621</t>
1622</section>
1623
1624<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1625<t>
1626  Closed issues:
1627  <list style="symbols">
1628    <t>
1629      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1630      "case-sensitivity of etag weakness indicator"
1631    </t>
1632  </list>
1633</t>
1634</section>
1635
1636<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1637<t>
1638  Closed issues:
1639  <list style="symbols">
1640    <t>
1641      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1642      "Weak ETags on non-GET requests" (If-Match still was defined to require
1643      strong matching)
1644    </t>
1645    <t>
1646      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1647      "move IANA registrations for optional status codes"
1648    </t>
1649  </list>
1650</t>
1651</section>
1652
1653<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1654<t>
1655  No significant changes.
1656</t>
1657</section>
1658
1659<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1660<t>
1661  No significant changes.
1662</t>
1663</section>
1664
1665<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1666<t>
1667  Closed issues:
1668  <list style="symbols">
1669    <t>
1670      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1671      "Clarify 'Requested Variant'"
1672    </t>
1673    <t>
1674      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1675      "Clarify entity / representation / variant terminology"
1676    </t>
1677    <t>
1678      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1679      "consider removing the 'changes from 2068' sections"
1680    </t>
1681  </list>
1682</t>
1683</section>
1684
1685<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1686<t>
1687  None.
1688</t>
1689</section>
1690
1691<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1692<t>
1693  Closed issues:
1694  <list style="symbols">
1695    <t>
1696      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1697      "Header Classification"
1698    </t>
1699  </list>
1700</t>
1701</section>
1702
1703<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1704<t>
1705  Closed issues:
1706  <list style="symbols">
1707    <t>
1708      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1709      "If-* and entities"
1710    </t>
1711    <t>
1712      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1713      "Definition of validator weakness"
1714    </t>
1715    <t>
1716      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1717      "untangle ABNFs for header fields"
1718    </t>
1719    <t>
1720      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1721      "ETags and Quotes"
1722    </t>
1723  </list>
1724</t>
1725</section>
1726
1727<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1728<t>
1729  None.
1730</t>
1731</section>
1732
1733<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1734<t>
1735  Closed issues:
1736  <list style="symbols">
1737    <t>
1738      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1739      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1740    </t>
1741  </list>
1742</t>
1743</section>
1744
1745</section>
1746
1747</back>
1748</rfc>
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