source: draft-ietf-httpbis/17/draft-ietf-httpbis-p4-conditional-17.xml

Last change on this file was 1500, checked in by julian.reschke@…, 8 years ago

fix mime types

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 71.0 KB
Line 
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-17">
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="October" year="2011" day="31"/>
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.16"/>.
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="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
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   This document defines conformance criteria for several roles in HTTP
236   communication, including Senders, Recipients, Clients, Servers, User-Agents,
237   Origin Servers, Intermediaries, Proxies and Gateways. See Section 2 of <xref target="Part1"/>
238   for definitions of these terms.
239</t>
240<t>
241   An implementation is considered conformant if it complies with all of the
242   requirements associated with its role(s). Note that SHOULD-level requirements
243   are relevant here, unless one of the documented exceptions is applicable.
244</t>
245<t>
246   This document also uses ABNF to define valid protocol elements
247   (<xref target="notation"/>). In addition to the prose requirements placed
248   upon them, Senders MUST NOT generate protocol elements that are invalid.
249</t>
250<t>
251   Unless noted otherwise, Recipients MAY take steps to recover a usable
252   protocol element from an invalid construct. However, HTTP does not define
253   specific error handling mechanisms, except in cases where it has direct
254   impact on security. This is because different uses of the protocol require
255   different error handling strategies; for example, a Web browser may wish to
256   transparently recover from a response where the Location header field
257   doesn't parse according to the ABNF, whereby in a systems control protocol
258   using HTTP, this type of error recovery could lead to dangerous consequences.
259</t>
260</section>
261
262<section title="Syntax Notation" anchor="notation">
263 
264 
265 
266 
267 
268 
269 
270 
271 
272 
273<t>
274  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
275  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
276  <xref target="collected.abnf"/> shows the collected ABNF, with the list
277  rule expanded.
278</t>
279<t>
280  The following core rules are included by
281  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
282  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
283  DIGIT (decimal 0-9), DQUOTE (double quote),
284  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
285  OCTET (any 8-bit sequence of data), SP (space), and
286  VCHAR (any visible US-ASCII character).
287</t>
288<t>
289  The ABNF rules below are defined in <xref target="Part1"/> and
290  <xref target="Part2"/>:
291</t>
292<figure><artwork type="abnf2616"><![CDATA[
293  OWS           = <OWS, defined in [Part1], Section 1.2.2>
294  quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
295  HTTP-date     = <HTTP-date, defined in [Part2], Section 8>
296]]></artwork></figure>
297</section>
298</section>
299
300<section title="Validators" anchor="validators">
301   <iref primary="true" item="metadata"/>
302   <iref primary="true" item="validator"/>
303<t>
304   This specification defines two forms of metadata that are commonly used
305   to observe resource state and test for preconditions: modification dates
306   and opaque entity tags.  Additional metadata that reflects resource state
307   has been defined by various extensions of HTTP, such as WebDAV
308   <xref target="RFC4918"/>, that are beyond the scope of this specification.
309   A resource metadata value is referred to as a "validator"
310   when it is used within a precondition.
311</t>
312
313<section title="Weak versus Strong" anchor="weak.and.strong.validators">
314   <iref primary="true" item="validator" subitem="weak"/>
315   <iref primary="true" item="validator" subitem="strong"/>
316<t>
317   Validators come in two flavors: strong or weak.  Weak validators are easy
318   to generate but are far less useful for comparisons.  Strong validators
319   are ideal for comparisons but can be very difficult (and occasionally
320   impossible) to generate efficiently.  Rather than impose that all forms
321   of resource adhere to the same strength of validator, HTTP exposes the
322   type of validator in use and imposes restrictions on when weak validators
323   can be used as preconditions.
324</t>
325<t>
326   A "strong validator" is a representation metadata value that MUST be
327   changed to a new, previously unused or guaranteed unique, value whenever
328   a change occurs to the representation data such that a change would be
329   observable in the payload body of a 200 response to GET.  A strong
330   validator MAY be changed for other reasons, such as when a semantically
331   significant part of the representation metadata is changed (e.g.,
332   Content-Type), but it is in the best interests of the origin server to only
333   change the value when it is necessary to invalidate the stored responses
334   held by remote caches and authoring tools.  A strong validator MUST be
335   unique across all representations of a given resource, such that no two
336   representations of that resource share the same validator unless
337   their payload body would be identical.
338</t>
339<t>
340   Cache entries might persist for arbitrarily long periods, regardless
341   of expiration times.  Thus, a cache might attempt to validate an
342   entry using a validator that it obtained in the distant past.
343   A strong validator MUST be unique across all versions of all
344   representations associated with a particular resource over time.
345   However, there is no implication of uniqueness across representations
346   of different resources (i.e., the same strong validator might be
347   in use for representations of multiple resources at the same time
348   and does not imply that those representations are equivalent).
349</t>
350<t>
351   There are a variety of strong validators used in practice.  The best are
352   based on strict revision control, wherein each change to a representation
353   always results in a unique node name and revision identifier being assigned
354   before the representation is made accessible to GET.  A cryptographic hash
355   function applied to the representation data is also sufficient if the data
356   is available prior to the response header fields being sent and the digest
357   does not need to be recalculated every time a validation request is
358   received.  However, if a resource has distinct representations that differ
359   only in their metadata, such as might occur with content negotiation over
360   media types that happen to share the same data format, then a server
361   SHOULD incorporate additional information in the validator to
362   distinguish those representations and avoid confusing cache behavior.
363</t>
364<t>
365   In contrast, a "weak validator" is a representation metadata value that
366   might not be changed for every change to the representation data.  This
367   weakness might be due to limitations in how the value is calculated, such
368   as clock resolution or an inability to ensure uniqueness for all possible
369   representations of the resource, or due to a desire by the resource owner
370   to group representations by some self-determined set of equivalency
371   rather than unique sequences of data.  A weak entity-tag SHOULD change
372   whenever the origin server considers prior representations to be
373   unacceptable as a substitute for the current representation. In other
374   words, a weak entity-tag SHOULD change whenever the origin server wants
375   caches to invalidate old responses.
376</t>
377<t>
378   For example, the representation of a weather report that changes in
379   content every second, based on dynamic measurements, might be grouped
380   into sets of equivalent representations (from the origin server's
381   perspective) with the same weak validator in order to allow cached
382   representations to be valid for a reasonable period of time (perhaps
383   adjusted dynamically based on server load or weather quality).
384   Likewise, a representation's modification time, if defined with only
385   one-second resolution, might be a weak validator if it is possible
386   for the representation to be modified twice during a single second and
387   retrieved between those modifications.
388</t>
389<t>
390   A "use" of a validator occurs when either a client generates a request
391   and includes the validator in a precondition or when a server
392   compares two validators.
393   Weak validators are only usable in contexts that do not depend on exact
394   equality of a representation's payload body.
395   Strong validators are usable and preferred for all conditional requests,
396   including cache validation, partial content ranges, and "lost update"
397   avoidance.
398</t>
399</section>
400
401<section title="Last-Modified" anchor="header.last-modified">
402  <iref primary="true" item="Last-Modified header field"/>
403  <iref primary="true" item="Header Fields" subitem="Last-Modified"/>
404 
405<t>
406   The "Last-Modified" header field indicates the date and time at
407   which the origin server believes the selected representation was
408   last modified.
409</t>
410<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><artwork type="abnf2616"><![CDATA[
411  Last-Modified = HTTP-date
412]]></artwork></figure>
413<t>
414   An example of its use is
415</t>
416<figure><artwork type="example"><![CDATA[
417  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
418]]></artwork></figure>
419
420<section title="Generation" anchor="lastmod.generation">
421<t>
422   Origin servers SHOULD send Last-Modified for any selected
423   representation for which a last modification date can be reasonably
424   and consistently determined, since its use in conditional requests
425   and evaluating cache freshness (<xref target="Part6"/>) results in a substantial
426   reduction of HTTP traffic on the Internet and can be a significant
427   factor in improving service scalability and reliability.
428</t>
429<t>
430   A representation is typically the sum of many parts behind the
431   resource interface.  The last-modified time would usually be
432   the most recent time that any of those parts were changed.
433   How that value is determined for any given resource is an
434   implementation detail beyond the scope of this specification.
435   What matters to HTTP is how recipients of the Last-Modified
436   header field can use its value to make conditional requests
437   and test the validity of locally cached responses.
438</t>
439<t>
440   An origin server SHOULD obtain the Last-Modified value of the
441   representation as close as possible to the time that it generates
442   the Date field-value for its response. This allows a recipient to
443   make an accurate assessment of the representation's modification time,
444   especially if the representation changes near the time that the
445   response is generated.
446</t>
447<t>
448   An origin server with a clock MUST NOT send a Last-Modified date
449   that is later than the server's time of message origination (Date).
450   If the last modification time is derived from implementation-specific
451   metadata that evaluates to some time in the future, according to the
452   origin server's clock, then the origin server MUST replace that
453   value with the message origination date. This prevents a future
454   modification date from having an adverse impact on cache validation.
455</t>
456<t>
457   An origin server without a clock MUST NOT assign Last-Modified
458   values to a response unless these values were associated
459   with the resource by some other system or user with a reliable clock.
460</t>
461</section>
462
463<section title="Comparison" anchor="lastmod.comparison">
464<t>
465   A Last-Modified time, when used as a validator in a request, is
466   implicitly weak unless it is possible to deduce that it is strong,
467   using the following rules:
468  <list style="symbols">
469     <t>The validator is being compared by an origin server to the
470        actual current validator for the representation and,</t>
471     <t>That origin server reliably knows that the associated representation did
472        not change twice during the second covered by the presented
473        validator.</t>
474  </list>
475</t>
476<t>
477   or
478  <list style="symbols">
479     <t>The validator is about to be used by a client in an If-Modified-Since,
480        If-Unmodified-Since header field, because the client has a cache entry,
481        or If-Range for the associated representation, and</t>
482     <t>That cache entry includes a Date value, which gives the time
483        when the origin server sent the original response, and</t>
484     <t>The presented Last-Modified time is at least 60 seconds before
485        the Date value.</t>
486  </list>
487</t>
488<t>
489   or
490  <list style="symbols">
491     <t>The validator is being compared by an intermediate cache to the
492        validator stored in its cache entry for the representation, and</t>
493     <t>That cache entry includes a Date value, which gives the time
494        when the origin server sent the original response, and</t>
495     <t>The presented Last-Modified time is at least 60 seconds before
496        the Date value.</t>
497  </list>
498</t>
499<t>
500   This method relies on the fact that if two different responses were
501   sent by the origin server during the same second, but both had the
502   same Last-Modified time, then at least one of those responses would
503   have a Date value equal to its Last-Modified time. The arbitrary 60-second
504   limit guards against the possibility that the Date and Last-Modified
505   values are generated from different clocks, or at somewhat
506   different times during the preparation of the response. An
507   implementation MAY use a value larger than 60 seconds, if it is
508   believed that 60 seconds is too short.
509</t>
510</section>
511</section>
512
513<section title="ETag" anchor="header.etag">
514  <iref primary="true" item="ETag header field"/>
515  <iref primary="true" item="Header Fields" subitem="ETag"/>
516 
517 
518 
519 
520 
521<t>
522   The ETag header field provides the current entity-tag for the
523   selected representation.
524   An entity-tag is an opaque validator for differentiating between
525   multiple representations of the same resource, regardless of whether
526   those multiple representations are due to resource state changes over
527   time, content negotiation resulting in multiple representations being
528   valid at the same time, or both. An entity-tag consists of an opaque
529   quoted string, possibly prefixed by a weakness indicator.
530</t>
531<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[
532  ETag       = entity-tag
533
534  entity-tag = [ weak ] opaque-tag
535  weak       = %x57.2F ; "W/", case-sensitive
536  opaque-tag = quoted-string
537]]></artwork></figure>
538<t>
539   An entity-tag can be more reliable for validation than a modification
540   date in situations where it is inconvenient to store modification
541   dates, where the one-second resolution of HTTP date values is not
542   sufficient, or where modification dates are not consistently maintained.
543</t>
544<figure><preamble>
545  Examples:
546</preamble>
547<artwork type="example"><![CDATA[
548  ETag: "xyzzy"
549  ETag: W/"xyzzy"
550  ETag: ""
551]]></artwork></figure>
552<t>
553   An entity-tag can be either a weak or strong validator, with
554   strong being the default.  If an origin server provides an entity-tag
555   for a representation and the generation of that entity-tag does not satisfy
556   the requirements for a strong validator
557   (<xref target="weak.and.strong.validators"/>), then that
558   entity-tag MUST be marked as weak by prefixing its opaque value
559   with "W/" (case-sensitive).
560</t>
561
562<section title="Generation" anchor="entity.tag.generation">
563<t>
564   The principle behind entity-tags is that only the service author
565   knows the implementation of a resource well enough to select the
566   most accurate and efficient validation mechanism for that resource,
567   and that any such mechanism can be mapped to a simple sequence of
568   octets for easy comparison.  Since the value is opaque, there is no
569   need for the client to be aware of how each entity-tag is constructed.
570</t>
571<t>
572   For example, a resource that has implementation-specific versioning
573   applied to all changes might use an internal revision number, perhaps
574   combined with a variance identifier for content negotiation, to
575   accurately differentiate between representations.
576   Other implementations might use a stored hash of representation content,
577   a combination of various filesystem attributes, or a modification
578   timestamp that has sub-second resolution.
579</t>
580<t>
581   Origin servers SHOULD send ETag for any selected representation
582   for which detection of changes can be reasonably and consistently
583   determined, since the entity-tag's use in conditional requests and
584   evaluating cache freshness (<xref target="Part6"/>) can result in a substantial
585   reduction of HTTP network traffic and can be a significant factor in
586   improving service scalability and reliability.
587</t>
588</section>
589
590<section title="Comparison" anchor="entity.tag.comparison">
591 
592<t>
593   There are two entity-tag comparison functions, depending
594   on whether the comparison context allows the use of weak validators
595   or not:
596  <list style="symbols">
597     <t>The strong comparison function: in order to be considered equal,
598        both opaque-tags MUST be identical character-by-character, and both
599        MUST NOT be weak.</t>
600     <t>The weak comparison function: in order to be considered equal, both
601        opaque-tags MUST be identical character-by-character, but
602        either or both of them MAY be tagged as "weak" without affecting
603        the result.</t>
604  </list>
605</t>
606<t>
607   The example below shows the results for a set of entity-tag pairs,
608   and both the weak and strong comparison function results:
609</t>
610<texttable align="left">
611  <ttcol>ETag 1</ttcol>
612  <ttcol>ETag 2</ttcol>
613  <ttcol>Strong Comparison</ttcol>
614  <ttcol>Weak Comparison</ttcol>
615
616  <c>W/"1"</c>
617  <c>W/"1"</c>
618  <c>no match</c>
619  <c>match</c>
620 
621  <c>W/"1"</c>
622  <c>W/"2"</c>
623  <c>no match</c>
624  <c>no match</c>
625
626  <c>W/"1"</c>
627  <c>"1"</c>
628  <c>no match</c>
629  <c>match</c>
630
631  <c>"1"</c>
632  <c>"1"</c>
633  <c>match</c>
634  <c>match</c>
635</texttable>
636</section>
637
638<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
639<t>
640   Consider a resource that is subject to content negotiation (Section 5 of <xref target="Part3"/>),
641   and where the representations returned upon a GET request vary based on
642   the Accept-Encoding request header field (Section 6.3 of <xref target="Part3"/>):
643</t>
644<figure><preamble>&gt;&gt; Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
645  GET /index HTTP/1.1
646  Host: www.example.com
647  Accept-Encoding: gzip
648 
649  ]]></artwork></figure>
650<t>
651   In this case, the response might or might not use the gzip content coding.
652   If it does not, the response might look like:
653</t>
654<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
655  HTTP/1.1 200 OK
656  Date: Thu, 26 Mar 2010 00:05:00 GMT
657  ETag: "123-a"
658  Content-Length: 70
659  Vary: Accept-Encoding
660  Content-Type: text/plain
661 
662  Hello World!
663  Hello World!
664  Hello World!
665  Hello World!
666  Hello World!
667  ]]></artwork></figure>
668<t>
669   An alternative representation that does use gzip content coding would be:
670</t>
671<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
672  HTTP/1.1 200 OK
673  Date: Thu, 26 Mar 2010 00:05:00 GMT
674  ETag: "123-b"
675  Content-Length: 43
676  Vary: Accept-Encoding
677  Content-Type: text/plain
678  Content-Encoding: gzip
679 
680  ...binary data...]]></artwork></figure>
681<t><list>
682  <t>
683    Note: Content codings are a property of the representation,
684    so therefore an entity-tag of an encoded representation must be distinct
685    from an unencoded representation to prevent conflicts during cache updates
686    and range requests.  In contrast, transfer codings (Section 5.1 of <xref target="Part1"/>)
687    apply only during message transfer and do not require distinct entity-tags.
688  </t>
689</list></t>
690</section>
691</section>
692
693<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">
694<t>
695   We adopt a set of rules and recommendations for origin servers,
696   clients, and caches regarding when various validator types ought to
697   be used, and for what purposes.
698</t>
699<t>
700   HTTP/1.1 origin servers:
701  <list style="symbols">
702     <t>SHOULD send an entity-tag validator unless it is not feasible to
703        generate one.</t>
704
705     <t>MAY send a weak entity-tag instead of a strong entity-tag, if
706        performance considerations support the use of weak entity-tags,
707        or if it is unfeasible to send a strong entity-tag.</t>
708
709     <t>SHOULD send a Last-Modified value if it is feasible to send one.</t>
710  </list>
711</t>
712<t>
713   In other words, the preferred behavior for an HTTP/1.1 origin server
714   is to send both a strong entity-tag and a Last-Modified value.
715</t>
716<t>
717   HTTP/1.1 clients:
718  <list style="symbols">
719     <t>MUST use that entity-tag in any cache-conditional request (using
720        If-Match or If-None-Match) if an entity-tag has been provided by the
721        origin server.</t>
722
723     <t>SHOULD use the Last-Modified value in non-subrange cache-conditional
724        requests (using If-Modified-Since) if only a Last-Modified value has
725        been provided by the origin server. </t>
726
727     <t>MAY use the Last-Modified value in subrange cache-conditional
728        requests (using If-Unmodified-Since) if only a Last-Modified value has
729        been provided by an HTTP/1.0 origin server. The user agent SHOULD
730        provide a way to disable this, in case of difficulty.</t>
731
732     <t>SHOULD use both validators in cache-conditional requests if both an
733        entity-tag and a Last-Modified value have been provided by the origin
734        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
735        appropriately.</t>
736  </list>
737</t>
738<t>
739   An HTTP/1.1 origin server, upon receiving a conditional request that
740   includes both a Last-Modified date (e.g., in an If-Modified-Since or
741   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
742   in an If-Match, If-None-Match, or If-Range header field) as cache
743   validators, MUST NOT return a response status code of 304 (Not Modified)
744   unless doing so is consistent with all of the conditional header
745   fields in the request.
746</t>
747<t>
748   An HTTP/1.1 caching proxy, upon receiving a conditional request that
749   includes both a Last-Modified date and one or more entity-tags as
750   cache validators, MUST NOT return a locally cached response to the
751   client unless that cached response is consistent with all of the
752   conditional header fields in the request.
753  <list><t>
754      Note: The general principle behind these rules is that HTTP/1.1
755      servers and clients ought to transmit as much non-redundant
756      information as is available in their responses and requests.
757      HTTP/1.1 systems receiving this information will make the most
758      conservative assumptions about the validators they receive.
759  </t><t>
760      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
761      last-modified values received or used by these systems will
762      support transparent and efficient caching, and so HTTP/1.1 origin
763      servers should provide Last-Modified values. In those rare cases
764      where the use of a Last-Modified value as a validator by an
765      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
766      origin servers should not provide one.
767  </t></list>
768</t>
769</section>
770</section>
771
772<section title="Precondition Header Fields" anchor="header.field.definitions">
773<t>
774   This section defines the syntax and semantics of HTTP/1.1 header fields
775   for applying preconditions on requests.
776</t>
777
778<section title="If-Match" anchor="header.if-match">
779  <iref primary="true" item="If-Match header field"/>
780  <iref primary="true" item="Header Fields" subitem="If-Match"/>
781 
782<t>
783   The "If-Match" header field MAY be used to make a request method
784   conditional on the current existence or value of an entity-tag for
785   one or more representations of the target resource.  If-Match is
786   generally useful for resource update requests, such as PUT requests,
787   as a means for protecting against accidental overwrites when multiple
788   clients are acting in parallel on the same resource (i.e., the
789   "lost update" problem).  An If-Match field-value of "*" places the
790   precondition on the existence of any current representation for the
791   target resource.
792</t>
793<figure><iref primary="true" item="Grammar" subitem="If-Match"/><artwork type="abnf2616"><![CDATA[
794  If-Match = "*" / 1#entity-tag
795]]></artwork></figure>
796<t>
797   If any of the entity-tags listed in the If-Match field value match
798   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
799   selected representation for the target resource,
800   or if "*" is given and any current representation exists for the
801   target resource, then the server MAY perform the request method
802   as if the If-Match header field was not present.
803</t>
804<t>
805   If none of the entity-tags match, or if "*" is given and no current
806   representation exists, the server MUST NOT perform the requested method.
807   Instead, the server MUST respond with the 412 (Precondition Failed)
808   status code.
809</t>
810<t>
811   If the request would, without the If-Match header field, result in
812   anything other than a 2xx or 412 status code, then the If-Match header field
813   MUST be ignored.
814</t>
815<t>
816   Examples:
817</t>
818<figure><artwork type="example"><![CDATA[
819  If-Match: "xyzzy"
820  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
821  If-Match: *
822]]></artwork></figure>
823<t>
824   The result of a request having both an If-Match header field and
825   either an If-None-Match or an If-Modified-Since header fields is
826   undefined by this specification.
827</t>
828</section>
829
830<section title="If-None-Match" anchor="header.if-none-match">
831  <iref primary="true" item="If-None-Match header field"/>
832  <iref primary="true" item="Header Fields" subitem="If-None-Match"/>
833 
834<t>
835   The "If-None-Match" header field MAY be used to make a request method
836   conditional on not matching any of the current entity-tag values for
837   representations of the target resource.  If-None-Match is primarily
838   used in conditional GET requests to enable efficient updates of cached
839   information with a minimum amount of transaction overhead.  A client
840   that has one or more representations previously obtained from the
841   target resource can send If-None-Match with a list of the associated
842   entity-tags in the hope of receiving a 304 response if at least one
843   of those representations matches the selected representation.
844</t>
845<t>
846   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
847   request method (e.g., PUT) from inadvertently modifying an existing
848   representation of the target resource when the client believes that
849   the resource does not have a current representation.  This is a variation
850   on the "lost update" problem that might arise if more than one client
851   attempts to create an initial representation for the target resource.
852</t>
853<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><artwork type="abnf2616"><![CDATA[
854  If-None-Match = "*" / 1#entity-tag
855]]></artwork></figure>
856<t>
857   If any of the entity-tags listed in the If-None-Match field-value match
858   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
859   selected representation, or if "*" is
860   given and any current representation exists for that resource, then the
861   server MUST NOT perform the requested method.
862   Instead, if the request method was GET or HEAD, the server SHOULD
863   respond with a 304 (Not Modified) status code, including the cache-related
864   header fields (particularly ETag) of the selected representation that has
865   a matching entity-tag.  For all other request methods, the server MUST
866   respond with a 412 (Precondition Failed) status code.
867</t>
868<t>
869   If none of the entity-tags match, then the server MAY perform the
870   requested method as if the If-None-Match header field did not exist,
871   but MUST also ignore any If-Modified-Since header field(s) in the
872   request. That is, if no entity-tags match, then the server MUST NOT
873   return a 304 (Not Modified) response.
874</t>
875<t>
876   If the request would, without the If-None-Match header field, result
877   in anything other than a 2xx or 304 status code, then the If-None-Match
878   header field MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
879   a discussion of server behavior when both If-Modified-Since and
880   If-None-Match appear in the same request.)
881</t>
882<t>
883   Examples:
884</t>
885<figure><artwork type="example"><![CDATA[
886  If-None-Match: "xyzzy"
887  If-None-Match: W/"xyzzy"
888  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
889  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
890  If-None-Match: *
891]]></artwork></figure>
892<t>
893   The result of a request having both an If-None-Match header field and
894   either an If-Match or an If-Unmodified-Since header fields is
895   undefined by this specification.
896</t>
897</section>
898
899<section title="If-Modified-Since" anchor="header.if-modified-since">
900  <iref primary="true" item="If-Modified-Since header field"/>
901  <iref primary="true" item="Header Fields" subitem="If-Modified-Since"/>
902 
903<t>
904   The "If-Modified-Since" header field MAY be used to make a request
905   method conditional by modification date: if the selected representation
906   has not been modified since the time specified in this field, then
907   do not perform the request method; instead, respond as detailed below.
908</t>
909<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><artwork type="abnf2616"><![CDATA[
910  If-Modified-Since = HTTP-date
911]]></artwork></figure>
912<t>
913   An example of the field is:
914</t>
915<figure><artwork type="example"><![CDATA[
916  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
917]]></artwork></figure>
918<t>
919   A GET method with an If-Modified-Since header field and no Range header
920   field requests that the selected representation be transferred only if
921   it has been modified since the date given by the If-Modified-Since
922   header field.
923   The algorithm for determining this includes the following cases:
924  <list style="numbers">
925      <t>If the request would normally result in anything other than a
926         200 (OK) status code, or if the passed If-Modified-Since date is
927         invalid, the response is exactly the same as for a normal GET.
928         A date which is later than the server's current time is
929         invalid.</t>
930
931      <t>If the selected representation has been modified since the
932         If-Modified-Since date, the response is exactly the same as for
933         a normal GET.</t>
934
935      <t>If the selected representation has not been modified since a valid
936         If-Modified-Since date, the server SHOULD return a
937         304 (Not Modified) response.</t>
938  </list>
939</t>
940<t>
941   The purpose of this feature is to allow efficient updates of cached
942   information with a minimum amount of transaction overhead.
943  <list><t>
944      Note: The Range header field modifies the meaning of If-Modified-Since;
945      see Section 5.4 of <xref target="Part5"/> for full details.
946    </t><t>
947      Note: If-Modified-Since times are interpreted by the server, whose
948      clock might not be synchronized with the client.
949    </t><t>
950      Note: When handling an If-Modified-Since header field, some
951      servers will use an exact date comparison function, rather than a
952      less-than function, for deciding whether to send a 304 (Not
953      Modified) response. To get best results when sending an If-Modified-Since
954      header field for cache validation, clients are
955      advised to use the exact date string received in a previous Last-Modified
956      header field whenever possible.
957    </t><t>
958      Note: If a client uses an arbitrary date in the If-Modified-Since
959      header field instead of a date taken from the Last-Modified header field for
960      the same request, the client needs to be aware that this
961      date is interpreted in the server's understanding of time.
962      Unsynchronized clocks and rounding problems, due to the different
963      encodings of time between the client and server, are concerns.
964      This includes the possibility of race conditions if the
965      document has changed between the time it was first requested and
966      the If-Modified-Since date of a subsequent request, and the
967      possibility of clock-skew-related problems if the If-Modified-Since
968      date is derived from the client's clock without correction
969      to the server's clock. Corrections for different time bases
970      between client and server are at best approximate due to network
971      latency.
972    </t>
973  </list>
974</t>
975<t>
976   The result of a request having both an If-Modified-Since header field
977   and either an If-Match or an If-Unmodified-Since header fields is
978   undefined by this specification.
979</t>
980</section>
981
982<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
983  <iref primary="true" item="If-Unmodified-Since header field"/>
984  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since"/>
985 
986<t>
987   The "If-Unmodified-Since" header field MAY be used to make a request
988   method conditional by modification date: if the selected representation
989   has been modified since the time specified in this field, then the
990   server MUST NOT perform the requested operation and MUST instead
991   respond with the 412 (Precondition Failed) status code.
992   If the selected representation has not been modified since the time
993   specified in this field, the server SHOULD perform the request
994   method as if the If-Unmodified-Since header field were not present.
995</t>
996<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><artwork type="abnf2616"><![CDATA[
997  If-Unmodified-Since = HTTP-date
998]]></artwork></figure>
999<t>
1000   An example of the field is:
1001</t>
1002<figure><artwork type="example"><![CDATA[
1003  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1004]]></artwork></figure>
1005<t>
1006   If the request normally (i.e., without the If-Unmodified-Since
1007   header field) would result in anything other than a 2xx or 412 status code,
1008   the If-Unmodified-Since header field SHOULD be ignored.
1009</t>
1010<t>
1011   If the specified date is invalid, the header field MUST be ignored.
1012</t>
1013<t>
1014   The result of a request having both an If-Unmodified-Since header
1015   field and either an If-None-Match or an If-Modified-Since header
1016   fields is undefined by this specification.
1017</t>
1018</section>
1019
1020<section title="If-Range" anchor="header.if-range">
1021<t>
1022   The If-Range header field provides a special conditional request
1023   mechanism that is similar to If-Match and If-Unmodified-Since but
1024   specific to HTTP range requests. If-Range is defined in Section 5.3 of <xref target="Part5"/>.
1025</t>
1026</section>
1027
1028</section>
1029
1030<section title="Status Code Definitions" anchor="status.code.definitions">
1031<section title="304 Not Modified" anchor="status.304">
1032  <iref primary="true" item="304 Not Modified (status code)"/>
1033  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
1034<t>
1035   The 304 status code indicates that a conditional GET request has been
1036   received and would have resulted in a 200 (OK) response if it were not
1037   for the fact that the condition has evaluated to false.  In other words,
1038   there is no need for the server to transfer a representation of the
1039   target resource because the client's request indicates that it already
1040   has a valid representation, as indicated by the 304 response header
1041   fields, and is therefore redirecting the client to make use of that
1042   stored representation as if it were the payload of a 200 response.
1043   The 304 response MUST NOT contain a message-body, and thus is always
1044   terminated by the first empty line after the header fields.
1045</t>
1046<t>
1047   A 304 response MUST include a Date header field (Section 9.2 of <xref target="Part2"/>)
1048   unless the origin server does not have a clock that can provide a
1049   reasonable approximation of the current time.  If a 200 response
1050   to the same request would have included any of the header fields
1051   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1052   Vary, then those same header fields MUST be sent in a 304 response.
1053</t>
1054<t>
1055   Since the goal of a 304 response is to minimize information transfer
1056   when the recipient already has one or more cached representations,
1057   the response SHOULD NOT include representation metadata other
1058   than the above listed fields unless said metadata exists for the
1059   purpose of guiding cache updates (e.g., future HTTP extensions).
1060</t>
1061<t>
1062   If the recipient of a 304 response does not have a cached representation
1063   corresponding to the entity-tag indicated by the 304 response, then the
1064   recipient MUST NOT use the 304 to update its own cache.  If this
1065   conditional request originated with an outbound client, such as a
1066   user agent with its own cache sending a conditional GET to a shared
1067   proxy, then the 304 response MAY be forwarded to the outbound client.
1068   Otherwise, the recipient MUST disregard the 304 response and repeat
1069   the request without any preconditions.
1070</t>
1071<t>
1072   If a cache uses a received 304 response to update a cache entry, the
1073   cache MUST update the entry to reflect any new field values given in
1074   the response.
1075</t>
1076</section>
1077
1078<section title="412 Precondition Failed" anchor="status.412">
1079  <iref primary="true" item="412 Precondition Failed (status code)"/>
1080  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
1081<t>
1082   The 412 status code indicates that one or more preconditions given in
1083   the request header fields evaluated to false when tested on the server.
1084   This response code allows the client to place preconditions on the
1085   current resource state (its current representations and metadata)
1086   and thus prevent the request method from being applied if the target
1087   resource is in an unexpected state.
1088</t>
1089</section>
1090</section>
1091
1092<section title="IANA Considerations" anchor="IANA.considerations">
1093
1094<section title="Status Code Registration" anchor="status.code.registration">
1095<t>
1096   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1097   shall be updated with the registrations below:
1098</t>
1099
1100<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1101<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1102   <ttcol>Value</ttcol>
1103   <ttcol>Description</ttcol>
1104   <ttcol>Reference</ttcol>
1105   <c>304</c>
1106   <c>Not Modified</c>
1107   <c>
1108      <xref target="status.304"/>
1109   </c>
1110   <c>412</c>
1111   <c>Precondition Failed</c>
1112   <c>
1113      <xref target="status.412"/>
1114   </c>
1115</texttable>
1116<!--(END)-->
1117
1118</section>
1119
1120<section title="Header Field Registration" anchor="header.field.registration">
1121<t>
1122   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1123   with the permanent registrations below (see <xref target="RFC3864"/>):
1124</t>
1125
1126<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1127<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1128   <ttcol>Header Field Name</ttcol>
1129   <ttcol>Protocol</ttcol>
1130   <ttcol>Status</ttcol>
1131   <ttcol>Reference</ttcol>
1132
1133   <c>ETag</c>
1134   <c>http</c>
1135   <c>standard</c>
1136   <c>
1137      <xref target="header.etag"/>
1138   </c>
1139   <c>If-Match</c>
1140   <c>http</c>
1141   <c>standard</c>
1142   <c>
1143      <xref target="header.if-match"/>
1144   </c>
1145   <c>If-Modified-Since</c>
1146   <c>http</c>
1147   <c>standard</c>
1148   <c>
1149      <xref target="header.if-modified-since"/>
1150   </c>
1151   <c>If-None-Match</c>
1152   <c>http</c>
1153   <c>standard</c>
1154   <c>
1155      <xref target="header.if-none-match"/>
1156   </c>
1157   <c>If-Unmodified-Since</c>
1158   <c>http</c>
1159   <c>standard</c>
1160   <c>
1161      <xref target="header.if-unmodified-since"/>
1162   </c>
1163   <c>Last-Modified</c>
1164   <c>http</c>
1165   <c>standard</c>
1166   <c>
1167      <xref target="header.last-modified"/>
1168   </c>
1169</texttable>
1170<!--(END)-->
1171
1172<t>
1173   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1174</t>
1175</section>
1176</section>
1177
1178<section title="Security Considerations" anchor="security.considerations">
1179<t>
1180   No additional security considerations have been identified beyond
1181   those applicable to HTTP in general <xref target="Part1"/>.
1182</t>
1183</section>
1184
1185<section title="Acknowledgments" anchor="acks">
1186<t>
1187  See Section 11 of <xref target="Part1"/>.
1188</t>
1189</section>
1190</middle>
1191<back>
1192
1193<references title="Normative References">
1194
1195<reference anchor="Part1">
1196  <front>
1197    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1198    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1199      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1200      <address><email>fielding@gbiv.com</email></address>
1201    </author>
1202    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1203      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1204      <address><email>jg@freedesktop.org</email></address>
1205    </author>
1206    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1207      <organization abbrev="HP">Hewlett-Packard Company</organization>
1208      <address><email>JeffMogul@acm.org</email></address>
1209    </author>
1210    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1211      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1212      <address><email>henrikn@microsoft.com</email></address>
1213    </author>
1214    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1215      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1216      <address><email>LMM@acm.org</email></address>
1217    </author>
1218    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1219      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1220      <address><email>paulle@microsoft.com</email></address>
1221    </author>
1222    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1223      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1224      <address><email>timbl@w3.org</email></address>
1225    </author>
1226    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1227      <organization abbrev="W3C">World Wide Web Consortium</organization>
1228      <address><email>ylafon@w3.org</email></address>
1229    </author>
1230    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1231      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1232      <address><email>julian.reschke@greenbytes.de</email></address>
1233    </author>
1234    <date month="October" year="2011"/>
1235  </front>
1236  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-17"/>
1237 
1238</reference>
1239
1240<reference anchor="Part2">
1241  <front>
1242    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1243    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1244      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1245      <address><email>fielding@gbiv.com</email></address>
1246    </author>
1247    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1248      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1249      <address><email>jg@freedesktop.org</email></address>
1250    </author>
1251    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1252      <organization abbrev="HP">Hewlett-Packard Company</organization>
1253      <address><email>JeffMogul@acm.org</email></address>
1254    </author>
1255    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1256      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1257      <address><email>henrikn@microsoft.com</email></address>
1258    </author>
1259    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1260      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1261      <address><email>LMM@acm.org</email></address>
1262    </author>
1263    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1264      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1265      <address><email>paulle@microsoft.com</email></address>
1266    </author>
1267    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1268      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1269      <address><email>timbl@w3.org</email></address>
1270    </author>
1271    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1272      <organization abbrev="W3C">World Wide Web Consortium</organization>
1273      <address><email>ylafon@w3.org</email></address>
1274    </author>
1275    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1276      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1277      <address><email>julian.reschke@greenbytes.de</email></address>
1278    </author>
1279    <date month="October" year="2011"/>
1280  </front>
1281  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-17"/>
1282 
1283</reference>
1284
1285<reference anchor="Part3">
1286  <front>
1287    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1288    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1289      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1290      <address><email>fielding@gbiv.com</email></address>
1291    </author>
1292    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1293      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1294      <address><email>jg@freedesktop.org</email></address>
1295    </author>
1296    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1297      <organization abbrev="HP">Hewlett-Packard Company</organization>
1298      <address><email>JeffMogul@acm.org</email></address>
1299    </author>
1300    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1301      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1302      <address><email>henrikn@microsoft.com</email></address>
1303    </author>
1304    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1305      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1306      <address><email>LMM@acm.org</email></address>
1307    </author>
1308    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1309      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1310      <address><email>paulle@microsoft.com</email></address>
1311    </author>
1312    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1313      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1314      <address><email>timbl@w3.org</email></address>
1315    </author>
1316    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1317      <organization abbrev="W3C">World Wide Web Consortium</organization>
1318      <address><email>ylafon@w3.org</email></address>
1319    </author>
1320    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1321      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1322      <address><email>julian.reschke@greenbytes.de</email></address>
1323    </author>
1324    <date month="October" year="2011"/>
1325  </front>
1326  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-17"/>
1327 
1328</reference>
1329
1330<reference anchor="Part5">
1331  <front>
1332    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1333    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1334      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1335      <address><email>fielding@gbiv.com</email></address>
1336    </author>
1337    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1338      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1339      <address><email>jg@freedesktop.org</email></address>
1340    </author>
1341    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1342      <organization abbrev="HP">Hewlett-Packard Company</organization>
1343      <address><email>JeffMogul@acm.org</email></address>
1344    </author>
1345    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1346      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1347      <address><email>henrikn@microsoft.com</email></address>
1348    </author>
1349    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1350      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1351      <address><email>LMM@acm.org</email></address>
1352    </author>
1353    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1354      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1355      <address><email>paulle@microsoft.com</email></address>
1356    </author>
1357    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1358      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1359      <address><email>timbl@w3.org</email></address>
1360    </author>
1361    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1362      <organization abbrev="W3C">World Wide Web Consortium</organization>
1363      <address><email>ylafon@w3.org</email></address>
1364    </author>
1365    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1366      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1367      <address><email>julian.reschke@greenbytes.de</email></address>
1368    </author>
1369    <date month="October" year="2011"/>
1370  </front>
1371  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-17"/>
1372 
1373</reference>
1374
1375<reference anchor="Part6">
1376  <front>
1377    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1378    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1379      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1380      <address><email>fielding@gbiv.com</email></address>
1381    </author>
1382    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1383      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1384      <address><email>jg@freedesktop.org</email></address>
1385    </author>
1386    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1387      <organization abbrev="HP">Hewlett-Packard Company</organization>
1388      <address><email>JeffMogul@acm.org</email></address>
1389    </author>
1390    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1391      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1392      <address><email>henrikn@microsoft.com</email></address>
1393    </author>
1394    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1395      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1396      <address><email>LMM@acm.org</email></address>
1397    </author>
1398    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1399      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1400      <address><email>paulle@microsoft.com</email></address>
1401    </author>
1402    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1403      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1404      <address><email>timbl@w3.org</email></address>
1405    </author>
1406    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1407      <organization abbrev="W3C">World Wide Web Consortium</organization>
1408      <address><email>ylafon@w3.org</email></address>
1409    </author>
1410    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1411      <organization>Rackspace</organization>
1412      <address><email>mnot@mnot.net</email></address>
1413    </author>
1414    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1415      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1416      <address><email>julian.reschke@greenbytes.de</email></address>
1417    </author>
1418    <date month="October" year="2011"/>
1419  </front>
1420  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-17"/>
1421 
1422</reference>
1423
1424<reference anchor="RFC2119">
1425  <front>
1426    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1427    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1428      <organization>Harvard University</organization>
1429      <address><email>sob@harvard.edu</email></address>
1430    </author>
1431    <date month="March" year="1997"/>
1432  </front>
1433  <seriesInfo name="BCP" value="14"/>
1434  <seriesInfo name="RFC" value="2119"/>
1435</reference>
1436
1437<reference anchor="RFC5234">
1438  <front>
1439    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1440    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1441      <organization>Brandenburg InternetWorking</organization>
1442      <address>
1443        <email>dcrocker@bbiw.net</email>
1444      </address> 
1445    </author>
1446    <author initials="P." surname="Overell" fullname="Paul Overell">
1447      <organization>THUS plc.</organization>
1448      <address>
1449        <email>paul.overell@thus.net</email>
1450      </address>
1451    </author>
1452    <date month="January" year="2008"/>
1453  </front>
1454  <seriesInfo name="STD" value="68"/>
1455  <seriesInfo name="RFC" value="5234"/>
1456</reference>
1457
1458</references>
1459
1460<references title="Informative References">
1461
1462<reference anchor="RFC2616">
1463  <front>
1464    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1465    <author initials="R." surname="Fielding" fullname="R. Fielding">
1466      <organization>University of California, Irvine</organization>
1467      <address><email>fielding@ics.uci.edu</email></address>
1468    </author>
1469    <author initials="J." surname="Gettys" fullname="J. Gettys">
1470      <organization>W3C</organization>
1471      <address><email>jg@w3.org</email></address>
1472    </author>
1473    <author initials="J." surname="Mogul" fullname="J. Mogul">
1474      <organization>Compaq Computer Corporation</organization>
1475      <address><email>mogul@wrl.dec.com</email></address>
1476    </author>
1477    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1478      <organization>MIT Laboratory for Computer Science</organization>
1479      <address><email>frystyk@w3.org</email></address>
1480    </author>
1481    <author initials="L." surname="Masinter" fullname="L. Masinter">
1482      <organization>Xerox Corporation</organization>
1483      <address><email>masinter@parc.xerox.com</email></address>
1484    </author>
1485    <author initials="P." surname="Leach" fullname="P. Leach">
1486      <organization>Microsoft Corporation</organization>
1487      <address><email>paulle@microsoft.com</email></address>
1488    </author>
1489    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1490      <organization>W3C</organization>
1491      <address><email>timbl@w3.org</email></address>
1492    </author>
1493    <date month="June" year="1999"/>
1494  </front>
1495  <seriesInfo name="RFC" value="2616"/>
1496</reference>
1497
1498<reference anchor="RFC3864">
1499  <front>
1500    <title>Registration Procedures for Message Header Fields</title>
1501    <author initials="G." surname="Klyne" fullname="G. Klyne">
1502      <organization>Nine by Nine</organization>
1503      <address><email>GK-IETF@ninebynine.org</email></address>
1504    </author>
1505    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
1506      <organization>BEA Systems</organization>
1507      <address><email>mnot@pobox.com</email></address>
1508    </author>
1509    <author initials="J." surname="Mogul" fullname="J. Mogul">
1510      <organization>HP Labs</organization>
1511      <address><email>JeffMogul@acm.org</email></address>
1512    </author>
1513    <date year="2004" month="September"/>
1514  </front>
1515  <seriesInfo name="BCP" value="90"/>
1516  <seriesInfo name="RFC" value="3864"/>
1517</reference>
1518
1519<reference anchor="RFC4918">
1520  <front>
1521    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1522    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor">
1523      <organization abbrev="CommerceNet">CommerceNet</organization>
1524      <address><email>ldusseault@commerce.net</email></address>
1525    </author>
1526    <date month="June" year="2007"/>
1527  </front>
1528  <seriesInfo name="RFC" value="4918"/>
1529</reference>
1530</references>
1531
1532<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1533<t>
1534  Allow weak entity-tags in all requests except range requests (Sections
1535  <xref target="weak.and.strong.validators" format="counter"/> and
1536  <xref target="header.if-none-match" format="counter"/>).
1537</t>
1538<t>
1539  Change ABNF productions for header fields to only define the field value.
1540  (<xref target="header.field.definitions"/>)
1541</t>
1542</section>
1543
1544
1545<section title="Collected ABNF" anchor="collected.abnf">
1546<figure>
1547<artwork type="abnf" name="p4-conditional.parsed-abnf"><![CDATA[
1548ETag = entity-tag
1549
1550HTTP-date = <HTTP-date, defined in [Part2], Section 8>
1551
1552If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1553 entity-tag ] ) )
1554If-Modified-Since = HTTP-date
1555If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1556 entity-tag ] ) )
1557If-Unmodified-Since = HTTP-date
1558
1559Last-Modified = HTTP-date
1560
1561OWS = <OWS, defined in [Part1], Section 1.2.2>
1562
1563entity-tag = [ weak ] opaque-tag
1564
1565opaque-tag = quoted-string
1566
1567quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
1568
1569weak = %x57.2F ; W/
1570]]></artwork>
1571</figure>
1572<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
1573; ETag defined but not used
1574; If-Match defined but not used
1575; If-Modified-Since defined but not used
1576; If-None-Match defined but not used
1577; If-Unmodified-Since defined but not used
1578; Last-Modified defined but not used
1579]]></artwork></figure></section>
1580
1581
1582<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1583
1584<section title="Since RFC 2616">
1585<t>
1586  Extracted relevant partitions from <xref target="RFC2616"/>.
1587</t>
1588</section>
1589
1590<section title="Since draft-ietf-httpbis-p4-conditional-00">
1591<t>
1592  Closed issues:
1593  <list style="symbols">
1594    <t>
1595      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1596      "Normative and Informative references"
1597    </t>
1598  </list>
1599</t>
1600<t>
1601  Other changes:
1602  <list style="symbols">
1603    <t>
1604      Move definitions of 304 and 412 condition codes from Part2.
1605    </t>
1606  </list>
1607</t>
1608</section>
1609
1610<section title="Since draft-ietf-httpbis-p4-conditional-01">
1611<t>
1612  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1613  <list style="symbols">
1614    <t>
1615      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1616    </t>
1617  </list>
1618</t>
1619</section>
1620
1621<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1622<t>
1623  Closed issues:
1624  <list style="symbols">
1625    <t>
1626      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1627      "Weak ETags on non-GET requests"
1628    </t>
1629  </list>
1630</t>
1631<t>
1632  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1633  <list style="symbols">
1634    <t>
1635      Reference RFC 3984, and update header field registrations for header fields defined
1636      in this document.
1637    </t>
1638  </list>
1639</t>
1640</section>
1641
1642<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1643<t>
1644  Closed issues:
1645  <list style="symbols">
1646    <t>
1647      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1648      "Examples for ETag matching"
1649    </t>
1650    <t>
1651      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1652      "'entity value' undefined"
1653    </t>
1654    <t>
1655      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1656      "bogus 2068 Date header reference"
1657    </t>
1658  </list>
1659</t>
1660</section>
1661
1662<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1663<t>
1664  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1665  <list style="symbols">
1666    <t>
1667      Use "/" instead of "|" for alternatives.
1668    </t>
1669    <t>
1670      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1671      whitespace ("OWS") and required whitespace ("RWS").
1672    </t>
1673    <t>
1674      Rewrite ABNFs to spell out whitespace rules, factor out
1675      header field value format definitions.
1676    </t>
1677  </list>
1678</t>
1679</section>
1680
1681<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1682<t>
1683  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1684  <list style="symbols">
1685    <t>
1686      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1687    </t>
1688  </list>
1689</t>
1690</section>
1691
1692<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1693<t>
1694  Closed issues:
1695  <list style="symbols">
1696    <t>
1697      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1698      "case-sensitivity of etag weakness indicator"
1699    </t>
1700  </list>
1701</t>
1702</section>
1703
1704<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1705<t>
1706  Closed issues:
1707  <list style="symbols">
1708    <t>
1709      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1710      "Weak ETags on non-GET requests" (If-Match still was defined to require
1711      strong matching)
1712    </t>
1713    <t>
1714      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1715      "move IANA registrations for optional status codes"
1716    </t>
1717  </list>
1718</t>
1719</section>
1720
1721<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1722<t>
1723  No significant changes.
1724</t>
1725</section>
1726
1727<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1728<t>
1729  No significant changes.
1730</t>
1731</section>
1732
1733<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1734<t>
1735  Closed issues:
1736  <list style="symbols">
1737    <t>
1738      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1739      "Clarify 'Requested Variant'"
1740    </t>
1741    <t>
1742      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1743      "Clarify entity / representation / variant terminology"
1744    </t>
1745    <t>
1746      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1747      "consider removing the 'changes from 2068' sections"
1748    </t>
1749  </list>
1750</t>
1751</section>
1752
1753<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1754<t>
1755  None.
1756</t>
1757</section>
1758
1759<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1760<t>
1761  Closed issues:
1762  <list style="symbols">
1763    <t>
1764      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1765      "Header Classification"
1766    </t>
1767  </list>
1768</t>
1769</section>
1770
1771<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1772<t>
1773  Closed issues:
1774  <list style="symbols">
1775    <t>
1776      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1777      "If-* and entities"
1778    </t>
1779    <t>
1780      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1781      "Definition of validator weakness"
1782    </t>
1783    <t>
1784      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1785      "untangle ABNFs for header fields"
1786    </t>
1787    <t>
1788      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1789      "ETags and Quotes"
1790    </t>
1791  </list>
1792</t>
1793</section>
1794
1795<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1796<t>
1797  None.
1798</t>
1799</section>
1800
1801<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1802<t>
1803  Closed issues:
1804  <list style="symbols">
1805    <t>
1806      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1807      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1808    </t>
1809  </list>
1810</t>
1811</section>
1812
1813<section title="Since draft-ietf-httpbis-p4-conditional-16" anchor="changes.since.16">
1814<t>
1815  Closed issues:
1816  <list style="symbols">
1817    <t>
1818      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
1819      "Document HTTP's error-handling philosophy"
1820    </t>
1821  </list>
1822</t>
1823</section>
1824
1825</section>
1826
1827</back>
1828</rfc>
Note: See TracBrowser for help on using the repository browser.