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[1499]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-18">
20
21
22<front>
23
24  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
25
26  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
27    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
28    <address>
29      <postal>
30        <street>345 Park Ave</street>
31        <city>San Jose</city>
32        <region>CA</region>
33        <code>95110</code>
34        <country>USA</country>
35      </postal>
36      <email>fielding@gbiv.com</email>
37      <uri>http://roy.gbiv.com/</uri>
38    </address>
39  </author>
40
41  <author initials="J." surname="Gettys" fullname="Jim Gettys">
42    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
43    <address>
44      <postal>
45        <street>21 Oak Knoll Road</street>
46        <city>Carlisle</city>
47        <region>MA</region>
48        <code>01741</code>
49        <country>USA</country>
50      </postal>
51      <email>jg@freedesktop.org</email>
52      <uri>http://gettys.wordpress.com/</uri>
53    </address>
54  </author>
55 
56  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
57    <organization abbrev="HP">Hewlett-Packard Company</organization>
58    <address>
59      <postal>
60        <street>HP Labs, Large Scale Systems Group</street>
61        <street>1501 Page Mill Road, MS 1177</street>
62        <city>Palo Alto</city>
63        <region>CA</region>
64        <code>94304</code>
65        <country>USA</country>
66      </postal>
67      <email>JeffMogul@acm.org</email>
68    </address>
69  </author>
70
71  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
72    <organization abbrev="Microsoft">Microsoft Corporation</organization>
73    <address>
74      <postal>
75        <street>1 Microsoft Way</street>
76        <city>Redmond</city>
77        <region>WA</region>
78        <code>98052</code>
79        <country>USA</country>
80      </postal>
81      <email>henrikn@microsoft.com</email>
82    </address>
83  </author>
84
85  <author initials="L." surname="Masinter" fullname="Larry Masinter">
86    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
87    <address>
88      <postal>
89        <street>345 Park Ave</street>
90        <city>San Jose</city>
91        <region>CA</region>
92        <code>95110</code>
93        <country>USA</country>
94      </postal>
95      <email>LMM@acm.org</email>
96      <uri>http://larry.masinter.net/</uri>
97    </address>
98  </author>
99 
100  <author initials="P." surname="Leach" fullname="Paul J. Leach">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108      </postal>
109      <email>paulle@microsoft.com</email>
110    </address>
111  </author>
112   
113  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
114    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
115    <address>
116      <postal>
117        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
118        <street>The Stata Center, Building 32</street>
119        <street>32 Vassar Street</street>
120        <city>Cambridge</city>
121        <region>MA</region>
122        <code>02139</code>
123        <country>USA</country>
124      </postal>
125      <email>timbl@w3.org</email>
126      <uri>http://www.w3.org/People/Berners-Lee/</uri>
127    </address>
128  </author>
129
130  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
131    <organization abbrev="W3C">World Wide Web Consortium</organization>
132    <address>
133      <postal>
134        <street>W3C / ERCIM</street>
135        <street>2004, rte des Lucioles</street>
136        <city>Sophia-Antipolis</city>
137        <region>AM</region>
138        <code>06902</code>
139        <country>France</country>
140      </postal>
141      <email>ylafon@w3.org</email>
142      <uri>http://www.raubacapeu.net/people/yves/</uri>
143    </address>
144  </author>
145
146  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
147    <organization abbrev="greenbytes">greenbytes GmbH</organization>
148    <address>
149      <postal>
150        <street>Hafenweg 16</street>
151        <city>Muenster</city><region>NW</region><code>48155</code>
152        <country>Germany</country>
153      </postal>
154      <phone>+49 251 2807760</phone>
155      <facsimile>+49 251 2807761</facsimile>
156      <email>julian.reschke@greenbytes.de</email>
157      <uri>http://greenbytes.de/tech/webdav/</uri>
158    </address>
159  </author>
160
161  <date month="January" year="2012" day="4"/>
162  <workgroup>HTTPbis Working Group</workgroup>
163
164<abstract>
165<t>
166   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
167   distributed, collaborative, hypertext information systems. HTTP has been in
168   use by the World Wide Web global information initiative since 1990. This
169   document is Part 4 of the seven-part specification that defines the protocol
170   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
171</t>
172<t>
173   Part 4 defines request header fields for indicating conditional requests and
174   the rules for constructing responses to those requests.
175</t>
176</abstract>
177
178<note title="Editorial Note (To be removed by RFC Editor)">
179  <t>
180    Discussion of this draft should take place on the HTTPBIS working group
181    mailing list (ietf-http-wg@w3.org), which is archived at
182    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
183  </t>
184  <t>
185    The current issues list is at
186    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
187    documents (including fancy diffs) can be found at
188    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
189  </t>
190  <t>
191    The changes in this draft are summarized in <xref target="changes.since.17"/>.
192  </t>
193</note>
194</front>
195<middle>
196<section title="Introduction" anchor="introduction">
197<t>
198   This document defines the HTTP/1.1 conditional request mechanisms,
199   including both metadata for indicating/observing changes in resource
200   representations and request header fields that specify preconditions
201   on that metadata be checked before performing the request method.
202   Conditional GET requests are the most efficient mechanism for HTTP
203   cache updates <xref target="Part6"/>.  Conditionals can also be
204   applied to state-changing methods, such as PUT and DELETE, to prevent
205   the "lost update" problem: one client accidentally overwriting
206   the work of another client that has been acting in parallel.
207</t>
208<t>
209   Conditional request preconditions are based on the state of the target
210   resource as a whole (its current value set) or the state as observed
211   in a previously obtained representation (one value in that set).
212   A resource might have multiple current representations, each with its
213   own observable state.  The conditional request mechanisms assume that
214   the mapping of requests to corresponding representations will be
215   consistent over time if the server intends to take advantage of
216   conditionals.  Regardless, if the mapping is inconsistent and
217   the server is unable to select the appropriate representation, then
218   no harm will result when the precondition evaluates to false.
219</t>
220<t><iref primary="true" item="selected representation"/>
221   We use the term "selected representation" to refer to
222   the current representation of the target resource that would have been
223   selected in a successful response if the same request had used the method
224   GET and had excluded all of the conditional request header fields.
225   The conditional request preconditions are evaluated by comparing the
226   values provided in the request header fields to the current metadata
227   for the selected representation.
228</t>
229
230<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
231<t>
232   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
233   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
234   document are to be interpreted as described in <xref target="RFC2119"/>.
235</t>
236<t>
237   This document defines conformance criteria for several roles in HTTP
238   communication, including Senders, Recipients, Clients, Servers, User-Agents,
239   Origin Servers, Intermediaries, Proxies and Gateways. See Section 2 of <xref target="Part1"/>
240   for definitions of these terms.
241</t>
242<t>
243   An implementation is considered conformant if it complies with all of the
244   requirements associated with its role(s). Note that SHOULD-level requirements
245   are relevant here, unless one of the documented exceptions is applicable.
246</t>
247<t>
248   This document also uses ABNF to define valid protocol elements
249   (<xref target="notation"/>). In addition to the prose requirements placed
250   upon them, Senders MUST NOT generate protocol elements that are invalid.
251</t>
252<t>
253   Unless noted otherwise, Recipients MAY take steps to recover a usable
254   protocol element from an invalid construct. However, HTTP does not define
255   specific error handling mechanisms, except in cases where it has direct
256   impact on security. This is because different uses of the protocol require
257   different error handling strategies; for example, a Web browser may wish to
258   transparently recover from a response where the Location header field
259   doesn't parse according to the ABNF, whereby in a systems control protocol
260   using HTTP, this type of error recovery could lead to dangerous consequences.
261</t>
262</section>
263
264<section title="Syntax Notation" anchor="notation">
265 
266 
267 
268 
269 
270 
271 
272 
273 
274 
275 
276<t>
277  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
278  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
279  <xref target="collected.abnf"/> shows the collected ABNF, with the list
280  rule expanded.
281</t>
282<t>
283  The following core rules are included by
284  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
285  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
286  DIGIT (decimal 0-9), DQUOTE (double quote),
287  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
288  OCTET (any 8-bit sequence of data), SP (space), and
289  VCHAR (any visible US-ASCII character).
290</t>
291<t>
292  The ABNF rules below are defined in <xref target="Part1"/> and
293  <xref target="Part2"/>:
294</t>
295<figure><artwork type="abnf2616"><![CDATA[
296  OWS           = <OWS, defined in [Part1], Section 1.2.2>
297  obs-text      = <obs-text, defined in [Part1], Section 3.2.3>
298  HTTP-date     = <HTTP-date, defined in [Part2], Section 8>
299]]></artwork></figure>
300</section>
301</section>
302
303<section title="Validators" anchor="validators">
304   <iref primary="true" item="metadata"/>
305   <iref primary="true" item="validator"/>
306<t>
307   This specification defines two forms of metadata that are commonly used
308   to observe resource state and test for preconditions: modification dates
309   and opaque entity tags.  Additional metadata that reflects resource state
310   has been defined by various extensions of HTTP, such as WebDAV
311   <xref target="RFC4918"/>, that are beyond the scope of this specification.
312   A resource metadata value is referred to as a "validator"
313   when it is used within a precondition.
314</t>
315
316<section title="Weak versus Strong" anchor="weak.and.strong.validators">
317   <iref primary="true" item="validator" subitem="weak"/>
318   <iref primary="true" item="validator" subitem="strong"/>
319<t>
320   Validators come in two flavors: strong or weak.  Weak validators are easy
321   to generate but are far less useful for comparisons.  Strong validators
322   are ideal for comparisons but can be very difficult (and occasionally
323   impossible) to generate efficiently.  Rather than impose that all forms
324   of resource adhere to the same strength of validator, HTTP exposes the
325   type of validator in use and imposes restrictions on when weak validators
326   can be used as preconditions.
327</t>
328<t>
329   A "strong validator" is a representation metadata value that MUST be
330   changed to a new, previously unused or guaranteed unique, value whenever
331   a change occurs to the representation data such that a change would be
332   observable in the payload body of a 200 response to GET.  A strong
333   validator MAY be changed for other reasons, such as when a semantically
334   significant part of the representation metadata is changed (e.g.,
335   Content-Type), but it is in the best interests of the origin server to only
336   change the value when it is necessary to invalidate the stored responses
337   held by remote caches and authoring tools.  A strong validator MUST be
338   unique across all representations of a given resource, such that no two
339   representations of that resource share the same validator unless
340   their payload body would be identical.
341</t>
342<t>
343   Cache entries might persist for arbitrarily long periods, regardless
344   of expiration times.  Thus, a cache might attempt to validate an
345   entry using a validator that it obtained in the distant past.
346   A strong validator MUST be unique across all versions of all
347   representations associated with a particular resource over time.
348   However, there is no implication of uniqueness across representations
349   of different resources (i.e., the same strong validator might be
350   in use for representations of multiple resources at the same time
351   and does not imply that those representations are equivalent).
352</t>
353<t>
354   There are a variety of strong validators used in practice.  The best are
355   based on strict revision control, wherein each change to a representation
356   always results in a unique node name and revision identifier being assigned
357   before the representation is made accessible to GET.  A cryptographic hash
358   function applied to the representation data is also sufficient if the data
359   is available prior to the response header fields being sent and the digest
360   does not need to be recalculated every time a validation request is
361   received.  However, if a resource has distinct representations that differ
362   only in their metadata, such as might occur with content negotiation over
363   media types that happen to share the same data format, then a server
364   SHOULD incorporate additional information in the validator to
365   distinguish those representations and avoid confusing cache behavior.
366</t>
367<t>
368   In contrast, a "weak validator" is a representation metadata value that
369   might not be changed for every change to the representation data.  This
370   weakness might be due to limitations in how the value is calculated, such
371   as clock resolution or an inability to ensure uniqueness for all possible
372   representations of the resource, or due to a desire by the resource owner
373   to group representations by some self-determined set of equivalency
374   rather than unique sequences of data.  A weak entity-tag SHOULD change
375   whenever the origin server considers prior representations to be
376   unacceptable as a substitute for the current representation. In other
377   words, a weak entity-tag SHOULD change whenever the origin server wants
378   caches to invalidate old responses.
379</t>
380<t>
381   For example, the representation of a weather report that changes in
382   content every second, based on dynamic measurements, might be grouped
383   into sets of equivalent representations (from the origin server's
384   perspective) with the same weak validator in order to allow cached
385   representations to be valid for a reasonable period of time (perhaps
386   adjusted dynamically based on server load or weather quality).
387   Likewise, a representation's modification time, if defined with only
388   one-second resolution, might be a weak validator if it is possible
389   for the representation to be modified twice during a single second and
390   retrieved between those modifications.
391</t>
392<t>
393   A "use" of a validator occurs when either a client generates a request
394   and includes the validator in a precondition or when a server
395   compares two validators.
396   Weak validators are only usable in contexts that do not depend on exact
397   equality of a representation's payload body.
398   Strong validators are usable and preferred for all conditional requests,
399   including cache validation, partial content ranges, and "lost update"
400   avoidance.
401</t>
402</section>
403
404<section title="Last-Modified" anchor="header.last-modified">
405  <iref primary="true" item="Last-Modified header field"/>
406  <iref primary="true" item="Header Fields" subitem="Last-Modified"/>
407 
408<t>
409   The "Last-Modified" header field indicates the date and time at
410   which the origin server believes the selected representation was
411   last modified.
412</t>
413<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><artwork type="abnf2616"><![CDATA[
414  Last-Modified = HTTP-date
415]]></artwork></figure>
416<t>
417   An example of its use is
418</t>
419<figure><artwork type="example"><![CDATA[
420  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
421]]></artwork></figure>
422
423<section title="Generation" anchor="lastmod.generation">
424<t>
425   Origin servers SHOULD send Last-Modified for any selected
426   representation for which a last modification date can be reasonably
427   and consistently determined, since its use in conditional requests
428   and evaluating cache freshness (<xref target="Part6"/>) results in a substantial
429   reduction of HTTP traffic on the Internet and can be a significant
430   factor in improving service scalability and reliability.
431</t>
432<t>
433   A representation is typically the sum of many parts behind the
434   resource interface.  The last-modified time would usually be
435   the most recent time that any of those parts were changed.
436   How that value is determined for any given resource is an
437   implementation detail beyond the scope of this specification.
438   What matters to HTTP is how recipients of the Last-Modified
439   header field can use its value to make conditional requests
440   and test the validity of locally cached responses.
441</t>
442<t>
443   An origin server SHOULD obtain the Last-Modified value of the
444   representation as close as possible to the time that it generates
445   the Date field-value for its response. This allows a recipient to
446   make an accurate assessment of the representation's modification time,
447   especially if the representation changes near the time that the
448   response is generated.
449</t>
450<t>
451   An origin server with a clock MUST NOT send a Last-Modified date
452   that is later than the server's time of message origination (Date).
453   If the last modification time is derived from implementation-specific
454   metadata that evaluates to some time in the future, according to the
455   origin server's clock, then the origin server MUST replace that
456   value with the message origination date. This prevents a future
457   modification date from having an adverse impact on cache validation.
458</t>
459<t>
460   An origin server without a clock MUST NOT assign Last-Modified
461   values to a response unless these values were associated
462   with the resource by some other system or user with a reliable clock.
463</t>
464</section>
465
466<section title="Comparison" anchor="lastmod.comparison">
467<t>
468   A Last-Modified time, when used as a validator in a request, is
469   implicitly weak unless it is possible to deduce that it is strong,
470   using the following rules:
471  <list style="symbols">
472     <t>The validator is being compared by an origin server to the
473        actual current validator for the representation and,</t>
474     <t>That origin server reliably knows that the associated representation did
475        not change twice during the second covered by the presented
476        validator.</t>
477  </list>
478</t>
479<t>
480   or
481  <list style="symbols">
482     <t>The validator is about to be used by a client in an If-Modified-Since,
483        If-Unmodified-Since header field, because the client has a cache entry,
484        or If-Range for the associated representation, and</t>
485     <t>That cache entry includes a Date value, which gives the time
486        when the origin server sent the original response, and</t>
487     <t>The presented Last-Modified time is at least 60 seconds before
488        the Date value.</t>
489  </list>
490</t>
491<t>
492   or
493  <list style="symbols">
494     <t>The validator is being compared by an intermediate cache to the
495        validator stored in its cache entry for the representation, and</t>
496     <t>That cache entry includes a Date value, which gives the time
497        when the origin server sent the original response, and</t>
498     <t>The presented Last-Modified time is at least 60 seconds before
499        the Date value.</t>
500  </list>
501</t>
502<t>
503   This method relies on the fact that if two different responses were
504   sent by the origin server during the same second, but both had the
505   same Last-Modified time, then at least one of those responses would
506   have a Date value equal to its Last-Modified time. The arbitrary 60-second
507   limit guards against the possibility that the Date and Last-Modified
508   values are generated from different clocks, or at somewhat
509   different times during the preparation of the response. An
510   implementation MAY use a value larger than 60 seconds, if it is
511   believed that 60 seconds is too short.
512</t>
513</section>
514</section>
515
516<section title="ETag" anchor="header.etag">
517  <iref primary="true" item="ETag header field"/>
518  <iref primary="true" item="Header Fields" subitem="ETag"/>
519 
520 
521 
522 
523 
524 
525<t>
526   The ETag header field provides the current entity-tag for the
527   selected representation.
528   An entity-tag is an opaque validator for differentiating between
529   multiple representations of the same resource, regardless of whether
530   those multiple representations are due to resource state changes over
531   time, content negotiation resulting in multiple representations being
532   valid at the same time, or both. An entity-tag consists of an opaque
533   quoted string, possibly prefixed by a weakness indicator.
534</t>
535<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"/><iref primary="true" item="Grammar" subitem="etagc"/><artwork type="abnf2616"><![CDATA[
536  ETag       = entity-tag
537
538  entity-tag = [ weak ] opaque-tag
539  weak       = %x57.2F ; "W/", case-sensitive
540  opaque-tag = DQUOTE *etagc DQUOTE
541  etagc      = %x21 / %x23-7E / obs-text
542             ; VCHAR except double quotes, plus obs-text
543]]></artwork></figure>
544<t><list>
545  <t>
546    Note: Previously, opaque-tag was defined to be a quoted-string
547    (<xref target="RFC2616"/>, Section 3.11), thus some recipients
548    might perform backslash unescaping. Servers therefore ought to avoid
549    backslash characters in entity tags.
550  </t>
551</list></t>
552<t>
553   An entity-tag can be more reliable for validation than a modification
554   date in situations where it is inconvenient to store modification
555   dates, where the one-second resolution of HTTP date values is not
556   sufficient, or where modification dates are not consistently maintained.
557</t>
558<figure><preamble>
559  Examples:
560</preamble>
561<artwork type="example"><![CDATA[
562  ETag: "xyzzy"
563  ETag: W/"xyzzy"
564  ETag: ""
565]]></artwork></figure>
566<t>
567   An entity-tag can be either a weak or strong validator, with
568   strong being the default.  If an origin server provides an entity-tag
569   for a representation and the generation of that entity-tag does not satisfy
570   the requirements for a strong validator
571   (<xref target="weak.and.strong.validators"/>), then that
572   entity-tag MUST be marked as weak by prefixing its opaque value
573   with "W/" (case-sensitive).
574</t>
575
576<section title="Generation" anchor="entity.tag.generation">
577<t>
578   The principle behind entity-tags is that only the service author
579   knows the implementation of a resource well enough to select the
580   most accurate and efficient validation mechanism for that resource,
581   and that any such mechanism can be mapped to a simple sequence of
582   octets for easy comparison.  Since the value is opaque, there is no
583   need for the client to be aware of how each entity-tag is constructed.
584</t>
585<t>
586   For example, a resource that has implementation-specific versioning
587   applied to all changes might use an internal revision number, perhaps
588   combined with a variance identifier for content negotiation, to
589   accurately differentiate between representations.
590   Other implementations might use a stored hash of representation content,
591   a combination of various filesystem attributes, or a modification
592   timestamp that has sub-second resolution.
593</t>
594<t>
595   Origin servers SHOULD send ETag for any selected representation
596   for which detection of changes can be reasonably and consistently
597   determined, since the entity-tag's use in conditional requests and
598   evaluating cache freshness (<xref target="Part6"/>) can result in a substantial
599   reduction of HTTP network traffic and can be a significant factor in
600   improving service scalability and reliability.
601</t>
602</section>
603
604<section title="Comparison" anchor="entity.tag.comparison">
605 
606<t>
607   There are two entity-tag comparison functions, depending
608   on whether the comparison context allows the use of weak validators
609   or not:
610  <list style="symbols">
611     <t>The strong comparison function: in order to be considered equal,
612        both opaque-tags MUST be identical character-by-character, and both
613        MUST NOT be weak.</t>
614     <t>The weak comparison function: in order to be considered equal, both
615        opaque-tags MUST be identical character-by-character, but
616        either or both of them MAY be tagged as "weak" without affecting
617        the result.</t>
618  </list>
619</t>
620<t>
621   The example below shows the results for a set of entity-tag pairs,
622   and both the weak and strong comparison function results:
623</t>
624<texttable align="left">
625  <ttcol>ETag 1</ttcol>
626  <ttcol>ETag 2</ttcol>
627  <ttcol>Strong Comparison</ttcol>
628  <ttcol>Weak Comparison</ttcol>
629
630  <c>W/"1"</c>
631  <c>W/"1"</c>
632  <c>no match</c>
633  <c>match</c>
634 
635  <c>W/"1"</c>
636  <c>W/"2"</c>
637  <c>no match</c>
638  <c>no match</c>
639
640  <c>W/"1"</c>
641  <c>"1"</c>
642  <c>no match</c>
643  <c>match</c>
644
645  <c>"1"</c>
646  <c>"1"</c>
647  <c>match</c>
648  <c>match</c>
649</texttable>
650</section>
651
652<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
653<t>
654   Consider a resource that is subject to content negotiation (Section 5 of <xref target="Part3"/>),
655   and where the representations returned upon a GET request vary based on
656   the Accept-Encoding request header field (Section 6.3 of <xref target="Part3"/>):
657</t>
658<figure><preamble>&gt;&gt; Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
659  GET /index HTTP/1.1
660  Host: www.example.com
661  Accept-Encoding: gzip
662 
663  ]]></artwork></figure>
664<t>
665   In this case, the response might or might not use the gzip content coding.
666   If it does not, the response might look like:
667</t>
668<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
669  HTTP/1.1 200 OK
670  Date: Thu, 26 Mar 2010 00:05:00 GMT
671  ETag: "123-a"
672  Content-Length: 70
673  Vary: Accept-Encoding
674  Content-Type: text/plain
675 
676  Hello World!
677  Hello World!
678  Hello World!
679  Hello World!
680  Hello World!
681  ]]></artwork></figure>
682<t>
683   An alternative representation that does use gzip content coding would be:
684</t>
685<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
686  HTTP/1.1 200 OK
687  Date: Thu, 26 Mar 2010 00:05:00 GMT
688  ETag: "123-b"
689  Content-Length: 43
690  Vary: Accept-Encoding
691  Content-Type: text/plain
692  Content-Encoding: gzip
693 
694  ...binary data...]]></artwork></figure>
695<t><list>
696  <t>
697    Note: Content codings are a property of the representation,
698    so therefore an entity-tag of an encoded representation must be distinct
699    from an unencoded representation to prevent conflicts during cache updates
700    and range requests.  In contrast, transfer codings (Section 5.1 of <xref target="Part1"/>)
701    apply only during message transfer and do not require distinct entity-tags.
702  </t>
703</list></t>
704</section>
705</section>
706
707<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">
708<t>
709   We adopt a set of rules and recommendations for origin servers,
710   clients, and caches regarding when various validator types ought to
711   be used, and for what purposes.
712</t>
713<t>
714   HTTP/1.1 origin servers:
715  <list style="symbols">
716     <t>SHOULD send an entity-tag validator unless it is not feasible to
717        generate one.</t>
718
719     <t>MAY send a weak entity-tag instead of a strong entity-tag, if
720        performance considerations support the use of weak entity-tags,
721        or if it is unfeasible to send a strong entity-tag.</t>
722
723     <t>SHOULD send a Last-Modified value if it is feasible to send one.</t>
724  </list>
725</t>
726<t>
727   In other words, the preferred behavior for an HTTP/1.1 origin server
728   is to send both a strong entity-tag and a Last-Modified value.
729</t>
730<t>
731   HTTP/1.1 clients:
732  <list style="symbols">
733     <t>MUST use that entity-tag in any cache-conditional request (using
734        If-Match or If-None-Match) if an entity-tag has been provided by the
735        origin server.</t>
736
737     <t>SHOULD use the Last-Modified value in non-subrange cache-conditional
738        requests (using If-Modified-Since) if only a Last-Modified value has
739        been provided by the origin server. </t>
740
741     <t>MAY use the Last-Modified value in subrange cache-conditional
742        requests (using If-Unmodified-Since) if only a Last-Modified value has
743        been provided by an HTTP/1.0 origin server. The user agent SHOULD
744        provide a way to disable this, in case of difficulty.</t>
745
746     <t>SHOULD use both validators in cache-conditional requests if both an
747        entity-tag and a Last-Modified value have been provided by the origin
748        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
749        appropriately.</t>
750  </list>
751</t>
752<t>
753   An HTTP/1.1 origin server, upon receiving a conditional request that
754   includes both a Last-Modified date (e.g., in an If-Modified-Since or
755   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
756   in an If-Match, If-None-Match, or If-Range header field) as cache
757   validators, MUST NOT return a response status code of 304 (Not Modified)
758   unless doing so is consistent with all of the conditional header
759   fields in the request.
760</t>
761<t>
762   An HTTP/1.1 caching proxy, upon receiving a conditional request that
763   includes both a Last-Modified date and one or more entity-tags as
764   cache validators, MUST NOT return a locally cached response to the
765   client unless that cached response is consistent with all of the
766   conditional header fields in the request.
767  <list><t>
768      Note: The general principle behind these rules is that HTTP/1.1
769      servers and clients ought to transmit as much non-redundant
770      information as is available in their responses and requests.
771      HTTP/1.1 systems receiving this information will make the most
772      conservative assumptions about the validators they receive.
773  </t><t>
774      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
775      last-modified values received or used by these systems will
776      support transparent and efficient caching, and so HTTP/1.1 origin
777      servers should provide Last-Modified values. In those rare cases
778      where the use of a Last-Modified value as a validator by an
779      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
780      origin servers should not provide one.
781  </t></list>
782</t>
783</section>
784</section>
785
786<section title="Precondition Header Fields" anchor="header.field.definitions">
787<t>
788   This section defines the syntax and semantics of HTTP/1.1 header fields
789   for applying preconditions on requests.
790</t>
791
792<section title="If-Match" anchor="header.if-match">
793  <iref primary="true" item="If-Match header field"/>
794  <iref primary="true" item="Header Fields" subitem="If-Match"/>
795 
796<t>
797   The "If-Match" header field MAY be used to make a request method
798   conditional on the current existence or value of an entity-tag for
799   one or more representations of the target resource.  If-Match is
800   generally useful for resource update requests, such as PUT requests,
801   as a means for protecting against accidental overwrites when multiple
802   clients are acting in parallel on the same resource (i.e., the
803   "lost update" problem).  An If-Match field-value of "*" places the
804   precondition on the existence of any current representation for the
805   target resource.
806</t>
807<figure><iref primary="true" item="Grammar" subitem="If-Match"/><artwork type="abnf2616"><![CDATA[
808  If-Match = "*" / 1#entity-tag
809]]></artwork></figure>
810<t>
811   If any of the entity-tags listed in the If-Match field value match
812   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
813   selected representation for the target resource,
814   or if "*" is given and any current representation exists for the
815   target resource, then the server MAY perform the request method
816   as if the If-Match header field was not present.
817</t>
818<t>
819   If none of the entity-tags match, or if "*" is given and no current
820   representation exists, the server MUST NOT perform the requested method.
821   Instead, the server MUST respond with the 412 (Precondition Failed)
822   status code.
823</t>
824<t>
825   If the request would, without the If-Match header field, result in
826   anything other than a 2xx or 412 status code, then the If-Match header field
827   MUST be ignored.
828</t>
829<t>
830   Examples:
831</t>
832<figure><artwork type="example"><![CDATA[
833  If-Match: "xyzzy"
834  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
835  If-Match: *
836]]></artwork></figure>
837<t>
838   The result of a request having both an If-Match header field and
839   either an If-None-Match or an If-Modified-Since header fields is
840   undefined by this specification.
841</t>
842</section>
843
844<section title="If-None-Match" anchor="header.if-none-match">
845  <iref primary="true" item="If-None-Match header field"/>
846  <iref primary="true" item="Header Fields" subitem="If-None-Match"/>
847 
848<t>
849   The "If-None-Match" header field MAY be used to make a request method
850   conditional on not matching any of the current entity-tag values for
851   representations of the target resource.  If-None-Match is primarily
852   used in conditional GET requests to enable efficient updates of cached
853   information with a minimum amount of transaction overhead.  A client
854   that has one or more representations previously obtained from the
855   target resource can send If-None-Match with a list of the associated
856   entity-tags in the hope of receiving a 304 response if at least one
857   of those representations matches the selected representation.
858</t>
859<t>
860   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
861   request method (e.g., PUT) from inadvertently modifying an existing
862   representation of the target resource when the client believes that
863   the resource does not have a current representation.  This is a variation
864   on the "lost update" problem that might arise if more than one client
865   attempts to create an initial representation for the target resource.
866</t>
867<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><artwork type="abnf2616"><![CDATA[
868  If-None-Match = "*" / 1#entity-tag
869]]></artwork></figure>
870<t>
871   If any of the entity-tags listed in the If-None-Match field-value match
872   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
873   selected representation, or if "*" is
874   given and any current representation exists for that resource, then the
875   server MUST NOT perform the requested method.
876   Instead, if the request method was GET or HEAD, the server SHOULD
877   respond with a 304 (Not Modified) status code, including the cache-related
878   header fields (particularly ETag) of the selected representation that has
879   a matching entity-tag.  For all other request methods, the server MUST
880   respond with a 412 (Precondition Failed) status code.
881</t>
882<t>
883   If none of the entity-tags match, then the server MAY perform the
884   requested method as if the If-None-Match header field did not exist,
885   but MUST also ignore any If-Modified-Since header field(s) in the
886   request. That is, if no entity-tags match, then the server MUST NOT
887   return a 304 (Not Modified) response.
888</t>
889<t>
890   If the request would, without the If-None-Match header field, result
891   in anything other than a 2xx or 304 status code, then the If-None-Match
892   header field MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
893   a discussion of server behavior when both If-Modified-Since and
894   If-None-Match appear in the same request.)
895</t>
896<t>
897   Examples:
898</t>
899<figure><artwork type="example"><![CDATA[
900  If-None-Match: "xyzzy"
901  If-None-Match: W/"xyzzy"
902  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
903  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
904  If-None-Match: *
905]]></artwork></figure>
906<t>
907   The result of a request having both an If-None-Match header field and
908   either an If-Match or an If-Unmodified-Since header fields is
909   undefined by this specification.
910</t>
911</section>
912
913<section title="If-Modified-Since" anchor="header.if-modified-since">
914  <iref primary="true" item="If-Modified-Since header field"/>
915  <iref primary="true" item="Header Fields" subitem="If-Modified-Since"/>
916 
917<t>
918   The "If-Modified-Since" header field MAY be used to make a request
919   method conditional by modification date: if the selected representation
920   has not been modified since the time specified in this field, then
921   do not perform the request method; instead, respond as detailed below.
922</t>
923<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><artwork type="abnf2616"><![CDATA[
924  If-Modified-Since = HTTP-date
925]]></artwork></figure>
926<t>
927   An example of the field is:
928</t>
929<figure><artwork type="example"><![CDATA[
930  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
931]]></artwork></figure>
932<t>
933   A GET method with an If-Modified-Since header field and no Range header
934   field requests that the selected representation be transferred only if
935   it has been modified since the date given by the If-Modified-Since
936   header field.
937   The algorithm for determining this includes the following cases:
938  <list style="numbers">
939      <t>If the request would normally result in anything other than a
940         200 (OK) status code, or if the passed If-Modified-Since date is
941         invalid, the response is exactly the same as for a normal GET.
942         A date which is later than the server's current time is
943         invalid.</t>
944
945      <t>If the selected representation has been modified since the
946         If-Modified-Since date, the response is exactly the same as for
947         a normal GET.</t>
948
949      <t>If the selected representation has not been modified since a valid
950         If-Modified-Since date, the server SHOULD return a
951         304 (Not Modified) response.</t>
952  </list>
953</t>
954<t>
955   The purpose of this feature is to allow efficient updates of cached
956   information with a minimum amount of transaction overhead.
957  <list><t>
958      Note: The Range header field modifies the meaning of If-Modified-Since;
959      see Section 5.4 of <xref target="Part5"/> for full details.
960    </t><t>
961      Note: If-Modified-Since times are interpreted by the server, whose
962      clock might not be synchronized with the client.
963    </t><t>
964      Note: When handling an If-Modified-Since header field, some
965      servers will use an exact date comparison function, rather than a
966      less-than function, for deciding whether to send a 304 (Not
967      Modified) response. To get best results when sending an If-Modified-Since
968      header field for cache validation, clients are
969      advised to use the exact date string received in a previous Last-Modified
970      header field whenever possible.
971    </t><t>
972      Note: If a client uses an arbitrary date in the If-Modified-Since
973      header field instead of a date taken from the Last-Modified header field for
974      the same request, the client needs to be aware that this
975      date is interpreted in the server's understanding of time.
976      Unsynchronized clocks and rounding problems, due to the different
977      encodings of time between the client and server, are concerns.
978      This includes the possibility of race conditions if the
979      document has changed between the time it was first requested and
980      the If-Modified-Since date of a subsequent request, and the
981      possibility of clock-skew-related problems if the If-Modified-Since
982      date is derived from the client's clock without correction
983      to the server's clock. Corrections for different time bases
984      between client and server are at best approximate due to network
985      latency.
986    </t>
987  </list>
988</t>
989<t>
990   The result of a request having both an If-Modified-Since header field
991   and either an If-Match or an If-Unmodified-Since header fields is
992   undefined by this specification.
993</t>
994</section>
995
996<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
997  <iref primary="true" item="If-Unmodified-Since header field"/>
998  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since"/>
999 
1000<t>
1001   The "If-Unmodified-Since" header field MAY be used to make a request
1002   method conditional by modification date: if the selected representation
1003   has been modified since the time specified in this field, then the
1004   server MUST NOT perform the requested operation and MUST instead
1005   respond with the 412 (Precondition Failed) status code.
1006   If the selected representation has not been modified since the time
1007   specified in this field, the server SHOULD perform the request
1008   method as if the If-Unmodified-Since header field were not present.
1009</t>
1010<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><artwork type="abnf2616"><![CDATA[
1011  If-Unmodified-Since = HTTP-date
1012]]></artwork></figure>
1013<t>
1014   An example of the field is:
1015</t>
1016<figure><artwork type="example"><![CDATA[
1017  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1018]]></artwork></figure>
1019<t>
1020   If the request normally (i.e., without the If-Unmodified-Since
1021   header field) would result in anything other than a 2xx or 412 status code,
1022   the If-Unmodified-Since header field SHOULD be ignored.
1023</t>
1024<t>
1025   If the specified date is invalid, the header field MUST be ignored.
1026</t>
1027<t>
1028   The result of a request having both an If-Unmodified-Since header
1029   field and either an If-None-Match or an If-Modified-Since header
1030   fields is undefined by this specification.
1031</t>
1032</section>
1033
1034<section title="If-Range" anchor="header.if-range">
1035<t>
1036   The If-Range header field provides a special conditional request
1037   mechanism that is similar to If-Match and If-Unmodified-Since but
1038   specific to HTTP range requests. If-Range is defined in Section 5.3 of <xref target="Part5"/>.
1039</t>
1040</section>
1041
1042</section>
1043
1044<section title="Status Code Definitions" anchor="status.code.definitions">
1045<section title="304 Not Modified" anchor="status.304">
1046  <iref primary="true" item="304 Not Modified (status code)"/>
1047  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
1048<t>
1049   The 304 status code indicates that a conditional GET request has been
1050   received and would have resulted in a 200 (OK) response if it were not
1051   for the fact that the condition has evaluated to false.  In other words,
1052   there is no need for the server to transfer a representation of the
1053   target resource because the client's request indicates that it already
1054   has a valid representation, as indicated by the 304 response header
1055   fields, and is therefore redirecting the client to make use of that
1056   stored representation as if it were the payload of a 200 response.
1057   The 304 response MUST NOT contain a message-body, and thus is always
1058   terminated by the first empty line after the header fields.
1059</t>
1060<t>
1061   A 304 response MUST include a Date header field (Section 9.2 of <xref target="Part2"/>)
1062   unless the origin server does not have a clock that can provide a
1063   reasonable approximation of the current time.  If a 200 response
1064   to the same request would have included any of the header fields
1065   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1066   Vary, then those same header fields MUST be sent in a 304 response.
1067</t>
1068<t>
1069   Since the goal of a 304 response is to minimize information transfer
1070   when the recipient already has one or more cached representations,
1071   the response SHOULD NOT include representation metadata other
1072   than the above listed fields unless said metadata exists for the
1073   purpose of guiding cache updates (e.g., future HTTP extensions).
1074</t>
1075<t>
1076   If the recipient of a 304 response does not have a cached representation
1077   corresponding to the entity-tag indicated by the 304 response, then the
1078   recipient MUST NOT use the 304 to update its own cache.  If this
1079   conditional request originated with an outbound client, such as a
1080   user agent with its own cache sending a conditional GET to a shared
1081   proxy, then the 304 response MAY be forwarded to the outbound client.
1082   Otherwise, the recipient MUST disregard the 304 response and repeat
1083   the request without any preconditions.
1084</t>
1085<t>
1086   If a cache uses a received 304 response to update a cache entry, the
1087   cache MUST update the entry to reflect any new field values given in
1088   the response.
1089</t>
1090</section>
1091
1092<section title="412 Precondition Failed" anchor="status.412">
1093  <iref primary="true" item="412 Precondition Failed (status code)"/>
1094  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
1095<t>
1096   The 412 status code indicates that one or more preconditions given in
1097   the request header fields evaluated to false when tested on the server.
1098   This response code allows the client to place preconditions on the
1099   current resource state (its current representations and metadata)
1100   and thus prevent the request method from being applied if the target
1101   resource is in an unexpected state.
1102</t>
1103</section>
1104</section>
1105
1106<section title="IANA Considerations" anchor="IANA.considerations">
1107
1108<section title="Status Code Registration" anchor="status.code.registration">
1109<t>
1110   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1111   shall be updated with the registrations below:
1112</t>
1113
1114<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1115<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1116   <ttcol>Value</ttcol>
1117   <ttcol>Description</ttcol>
1118   <ttcol>Reference</ttcol>
1119   <c>304</c>
1120   <c>Not Modified</c>
1121   <c>
1122      <xref target="status.304"/>
1123   </c>
1124   <c>412</c>
1125   <c>Precondition Failed</c>
1126   <c>
1127      <xref target="status.412"/>
1128   </c>
1129</texttable>
1130<!--(END)-->
1131
1132</section>
1133
1134<section title="Header Field Registration" anchor="header.field.registration">
1135<t>
1136   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1137   with the permanent registrations below (see <xref target="RFC3864"/>):
1138</t>
1139
1140<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1141<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1142   <ttcol>Header Field Name</ttcol>
1143   <ttcol>Protocol</ttcol>
1144   <ttcol>Status</ttcol>
1145   <ttcol>Reference</ttcol>
1146
1147   <c>ETag</c>
1148   <c>http</c>
1149   <c>standard</c>
1150   <c>
1151      <xref target="header.etag"/>
1152   </c>
1153   <c>If-Match</c>
1154   <c>http</c>
1155   <c>standard</c>
1156   <c>
1157      <xref target="header.if-match"/>
1158   </c>
1159   <c>If-Modified-Since</c>
1160   <c>http</c>
1161   <c>standard</c>
1162   <c>
1163      <xref target="header.if-modified-since"/>
1164   </c>
1165   <c>If-None-Match</c>
1166   <c>http</c>
1167   <c>standard</c>
1168   <c>
1169      <xref target="header.if-none-match"/>
1170   </c>
1171   <c>If-Unmodified-Since</c>
1172   <c>http</c>
1173   <c>standard</c>
1174   <c>
1175      <xref target="header.if-unmodified-since"/>
1176   </c>
1177   <c>Last-Modified</c>
1178   <c>http</c>
1179   <c>standard</c>
1180   <c>
1181      <xref target="header.last-modified"/>
1182   </c>
1183</texttable>
1184<!--(END)-->
1185
1186<t>
1187   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1188</t>
1189</section>
1190</section>
1191
1192<section title="Security Considerations" anchor="security.considerations">
1193<t>
1194   No additional security considerations have been identified beyond
1195   those applicable to HTTP in general <xref target="Part1"/>.
1196</t>
1197</section>
1198
1199<section title="Acknowledgments" anchor="acks">
1200<t>
1201  See Section 11 of <xref target="Part1"/>.
1202</t>
1203</section>
1204</middle>
1205<back>
1206
1207<references title="Normative References">
1208
1209<reference anchor="Part1">
1210  <front>
1211    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1212    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1213      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1214      <address><email>fielding@gbiv.com</email></address>
1215    </author>
1216    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1217      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1218      <address><email>jg@freedesktop.org</email></address>
1219    </author>
1220    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1221      <organization abbrev="HP">Hewlett-Packard Company</organization>
1222      <address><email>JeffMogul@acm.org</email></address>
1223    </author>
1224    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1225      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1226      <address><email>henrikn@microsoft.com</email></address>
1227    </author>
1228    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1229      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1230      <address><email>LMM@acm.org</email></address>
1231    </author>
1232    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1233      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1234      <address><email>paulle@microsoft.com</email></address>
1235    </author>
1236    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1237      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1238      <address><email>timbl@w3.org</email></address>
1239    </author>
1240    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1241      <organization abbrev="W3C">World Wide Web Consortium</organization>
1242      <address><email>ylafon@w3.org</email></address>
1243    </author>
1244    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1245      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1246      <address><email>julian.reschke@greenbytes.de</email></address>
1247    </author>
1248    <date month="January" year="2012"/>
1249  </front>
1250  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-18"/>
1251 
1252</reference>
1253
1254<reference anchor="Part2">
1255  <front>
1256    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1257    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1258      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1259      <address><email>fielding@gbiv.com</email></address>
1260    </author>
1261    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1262      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1263      <address><email>jg@freedesktop.org</email></address>
1264    </author>
1265    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1266      <organization abbrev="HP">Hewlett-Packard Company</organization>
1267      <address><email>JeffMogul@acm.org</email></address>
1268    </author>
1269    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1270      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1271      <address><email>henrikn@microsoft.com</email></address>
1272    </author>
1273    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1274      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1275      <address><email>LMM@acm.org</email></address>
1276    </author>
1277    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1278      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1279      <address><email>paulle@microsoft.com</email></address>
1280    </author>
1281    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1282      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1283      <address><email>timbl@w3.org</email></address>
1284    </author>
1285    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1286      <organization abbrev="W3C">World Wide Web Consortium</organization>
1287      <address><email>ylafon@w3.org</email></address>
1288    </author>
1289    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1290      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1291      <address><email>julian.reschke@greenbytes.de</email></address>
1292    </author>
1293    <date month="January" year="2012"/>
1294  </front>
1295  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-18"/>
1296 
1297</reference>
1298
1299<reference anchor="Part3">
1300  <front>
1301    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1302    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1303      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1304      <address><email>fielding@gbiv.com</email></address>
1305    </author>
1306    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1307      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1308      <address><email>jg@freedesktop.org</email></address>
1309    </author>
1310    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1311      <organization abbrev="HP">Hewlett-Packard Company</organization>
1312      <address><email>JeffMogul@acm.org</email></address>
1313    </author>
1314    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1315      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1316      <address><email>henrikn@microsoft.com</email></address>
1317    </author>
1318    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1319      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1320      <address><email>LMM@acm.org</email></address>
1321    </author>
1322    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1323      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1324      <address><email>paulle@microsoft.com</email></address>
1325    </author>
1326    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1327      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1328      <address><email>timbl@w3.org</email></address>
1329    </author>
1330    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1331      <organization abbrev="W3C">World Wide Web Consortium</organization>
1332      <address><email>ylafon@w3.org</email></address>
1333    </author>
1334    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1335      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1336      <address><email>julian.reschke@greenbytes.de</email></address>
1337    </author>
1338    <date month="January" year="2012"/>
1339  </front>
1340  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-18"/>
1341 
1342</reference>
1343
1344<reference anchor="Part5">
1345  <front>
1346    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1347    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1348      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1349      <address><email>fielding@gbiv.com</email></address>
1350    </author>
1351    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1352      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1353      <address><email>jg@freedesktop.org</email></address>
1354    </author>
1355    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1356      <organization abbrev="HP">Hewlett-Packard Company</organization>
1357      <address><email>JeffMogul@acm.org</email></address>
1358    </author>
1359    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1360      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1361      <address><email>henrikn@microsoft.com</email></address>
1362    </author>
1363    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1364      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1365      <address><email>LMM@acm.org</email></address>
1366    </author>
1367    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1368      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1369      <address><email>paulle@microsoft.com</email></address>
1370    </author>
1371    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1372      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1373      <address><email>timbl@w3.org</email></address>
1374    </author>
1375    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1376      <organization abbrev="W3C">World Wide Web Consortium</organization>
1377      <address><email>ylafon@w3.org</email></address>
1378    </author>
1379    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1380      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1381      <address><email>julian.reschke@greenbytes.de</email></address>
1382    </author>
1383    <date month="January" year="2012"/>
1384  </front>
1385  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-18"/>
1386 
1387</reference>
1388
1389<reference anchor="Part6">
1390  <front>
1391    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1392    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1393      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1394      <address><email>fielding@gbiv.com</email></address>
1395    </author>
1396    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1397      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1398      <address><email>jg@freedesktop.org</email></address>
1399    </author>
1400    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1401      <organization abbrev="HP">Hewlett-Packard Company</organization>
1402      <address><email>JeffMogul@acm.org</email></address>
1403    </author>
1404    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1405      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1406      <address><email>henrikn@microsoft.com</email></address>
1407    </author>
1408    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1409      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1410      <address><email>LMM@acm.org</email></address>
1411    </author>
1412    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1413      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1414      <address><email>paulle@microsoft.com</email></address>
1415    </author>
1416    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1417      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1418      <address><email>timbl@w3.org</email></address>
1419    </author>
1420    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1421      <organization abbrev="W3C">World Wide Web Consortium</organization>
1422      <address><email>ylafon@w3.org</email></address>
1423    </author>
1424    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1425      <organization>Rackspace</organization>
1426      <address><email>mnot@mnot.net</email></address>
1427    </author>
1428    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1429      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1430      <address><email>julian.reschke@greenbytes.de</email></address>
1431    </author>
1432    <date month="January" year="2012"/>
1433  </front>
1434  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-18"/>
1435 
1436</reference>
1437
1438<reference anchor="RFC2119">
1439  <front>
1440    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1441    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1442      <organization>Harvard University</organization>
1443      <address><email>sob@harvard.edu</email></address>
1444    </author>
1445    <date month="March" year="1997"/>
1446  </front>
1447  <seriesInfo name="BCP" value="14"/>
1448  <seriesInfo name="RFC" value="2119"/>
1449</reference>
1450
1451<reference anchor="RFC5234">
1452  <front>
1453    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1454    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1455      <organization>Brandenburg InternetWorking</organization>
1456      <address>
1457        <email>dcrocker@bbiw.net</email>
1458      </address> 
1459    </author>
1460    <author initials="P." surname="Overell" fullname="Paul Overell">
1461      <organization>THUS plc.</organization>
1462      <address>
1463        <email>paul.overell@thus.net</email>
1464      </address>
1465    </author>
1466    <date month="January" year="2008"/>
1467  </front>
1468  <seriesInfo name="STD" value="68"/>
1469  <seriesInfo name="RFC" value="5234"/>
1470</reference>
1471
1472</references>
1473
1474<references title="Informative References">
1475
1476<reference anchor="RFC2616">
1477  <front>
1478    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1479    <author initials="R." surname="Fielding" fullname="R. Fielding">
1480      <organization>University of California, Irvine</organization>
1481      <address><email>fielding@ics.uci.edu</email></address>
1482    </author>
1483    <author initials="J." surname="Gettys" fullname="J. Gettys">
1484      <organization>W3C</organization>
1485      <address><email>jg@w3.org</email></address>
1486    </author>
1487    <author initials="J." surname="Mogul" fullname="J. Mogul">
1488      <organization>Compaq Computer Corporation</organization>
1489      <address><email>mogul@wrl.dec.com</email></address>
1490    </author>
1491    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1492      <organization>MIT Laboratory for Computer Science</organization>
1493      <address><email>frystyk@w3.org</email></address>
1494    </author>
1495    <author initials="L." surname="Masinter" fullname="L. Masinter">
1496      <organization>Xerox Corporation</organization>
1497      <address><email>masinter@parc.xerox.com</email></address>
1498    </author>
1499    <author initials="P." surname="Leach" fullname="P. Leach">
1500      <organization>Microsoft Corporation</organization>
1501      <address><email>paulle@microsoft.com</email></address>
1502    </author>
1503    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1504      <organization>W3C</organization>
1505      <address><email>timbl@w3.org</email></address>
1506    </author>
1507    <date month="June" year="1999"/>
1508  </front>
1509  <seriesInfo name="RFC" value="2616"/>
1510</reference>
1511
1512<reference anchor="RFC3864">
1513  <front>
1514    <title>Registration Procedures for Message Header Fields</title>
1515    <author initials="G." surname="Klyne" fullname="G. Klyne">
1516      <organization>Nine by Nine</organization>
1517      <address><email>GK-IETF@ninebynine.org</email></address>
1518    </author>
1519    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
1520      <organization>BEA Systems</organization>
1521      <address><email>mnot@pobox.com</email></address>
1522    </author>
1523    <author initials="J." surname="Mogul" fullname="J. Mogul">
1524      <organization>HP Labs</organization>
1525      <address><email>JeffMogul@acm.org</email></address>
1526    </author>
1527    <date year="2004" month="September"/>
1528  </front>
1529  <seriesInfo name="BCP" value="90"/>
1530  <seriesInfo name="RFC" value="3864"/>
1531</reference>
1532
1533<reference anchor="RFC4918">
1534  <front>
1535    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1536    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor">
1537      <organization abbrev="CommerceNet">CommerceNet</organization>
1538      <address><email>ldusseault@commerce.net</email></address>
1539    </author>
1540    <date month="June" year="2007"/>
1541  </front>
1542  <seriesInfo name="RFC" value="4918"/>
1543</reference>
1544</references>
1545
1546<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1547<t>
1548  Allow weak entity-tags in all requests except range requests (Sections
1549  <xref target="weak.and.strong.validators" format="counter"/> and
1550  <xref target="header.if-none-match" format="counter"/>).
1551</t>
1552<t>
1553  Change ETag header field ABNF not to use quoted-string, thus avoiding
1554  escaping issues.
1555  (<xref target="header.etag"/>)
1556</t>
1557<t>
1558  Change ABNF productions for header fields to only define the field value.
1559  (<xref target="header.field.definitions"/>)
1560</t>
1561</section>
1562
1563
1564<section title="Collected ABNF" anchor="collected.abnf">
1565<figure>
1566<artwork type="abnf" name="p4-conditional.parsed-abnf"><![CDATA[
1567ETag = entity-tag
1568
1569HTTP-date = <HTTP-date, defined in [Part2], Section 8>
1570
1571If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1572 entity-tag ] ) )
1573If-Modified-Since = HTTP-date
1574If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1575 entity-tag ] ) )
1576If-Unmodified-Since = HTTP-date
1577
1578Last-Modified = HTTP-date
1579
1580OWS = <OWS, defined in [Part1], Section 1.2.2>
1581
1582entity-tag = [ weak ] opaque-tag
1583etagc = "!" / %x23-7E ; '#'-'~'
1584 / obs-text
1585
1586obs-text = <obs-text, defined in [Part1], Section 3.2.3>
1587opaque-tag = DQUOTE *etagc DQUOTE
1588
1589weak = %x57.2F ; W/
1590]]></artwork>
1591</figure>
1592<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
1593; ETag defined but not used
1594; If-Match defined but not used
1595; If-Modified-Since defined but not used
1596; If-None-Match defined but not used
1597; If-Unmodified-Since defined but not used
1598; Last-Modified defined but not used
1599]]></artwork></figure></section>
1600
1601
1602<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1603
1604<section title="Since RFC 2616">
1605<t>
1606  Extracted relevant partitions from <xref target="RFC2616"/>.
1607</t>
1608</section>
1609
1610<section title="Since draft-ietf-httpbis-p4-conditional-00">
1611<t>
1612  Closed issues:
1613  <list style="symbols">
1614    <t>
1615      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1616      "Normative and Informative references"
1617    </t>
1618  </list>
1619</t>
1620<t>
1621  Other changes:
1622  <list style="symbols">
1623    <t>
1624      Move definitions of 304 and 412 condition codes from Part2.
1625    </t>
1626  </list>
1627</t>
1628</section>
1629
1630<section title="Since draft-ietf-httpbis-p4-conditional-01">
1631<t>
1632  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1633  <list style="symbols">
1634    <t>
1635      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1636    </t>
1637  </list>
1638</t>
1639</section>
1640
1641<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1642<t>
1643  Closed issues:
1644  <list style="symbols">
1645    <t>
1646      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1647      "Weak ETags on non-GET requests"
1648    </t>
1649  </list>
1650</t>
1651<t>
1652  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1653  <list style="symbols">
1654    <t>
1655      Reference RFC 3984, and update header field registrations for header fields defined
1656      in this document.
1657    </t>
1658  </list>
1659</t>
1660</section>
1661
1662<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1663<t>
1664  Closed issues:
1665  <list style="symbols">
1666    <t>
1667      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1668      "Examples for ETag matching"
1669    </t>
1670    <t>
1671      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1672      "'entity value' undefined"
1673    </t>
1674    <t>
1675      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1676      "bogus 2068 Date header reference"
1677    </t>
1678  </list>
1679</t>
1680</section>
1681
1682<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1683<t>
1684  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1685  <list style="symbols">
1686    <t>
1687      Use "/" instead of "|" for alternatives.
1688    </t>
1689    <t>
1690      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1691      whitespace ("OWS") and required whitespace ("RWS").
1692    </t>
1693    <t>
1694      Rewrite ABNFs to spell out whitespace rules, factor out
1695      header field value format definitions.
1696    </t>
1697  </list>
1698</t>
1699</section>
1700
1701<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1702<t>
1703  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1704  <list style="symbols">
1705    <t>
1706      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1707    </t>
1708  </list>
1709</t>
1710</section>
1711
1712<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1713<t>
1714  Closed issues:
1715  <list style="symbols">
1716    <t>
1717      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1718      "case-sensitivity of etag weakness indicator"
1719    </t>
1720  </list>
1721</t>
1722</section>
1723
1724<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1725<t>
1726  Closed issues:
1727  <list style="symbols">
1728    <t>
1729      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1730      "Weak ETags on non-GET requests" (If-Match still was defined to require
1731      strong matching)
1732    </t>
1733    <t>
1734      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1735      "move IANA registrations for optional status codes"
1736    </t>
1737  </list>
1738</t>
1739</section>
1740
1741<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1742<t>
1743  No significant changes.
1744</t>
1745</section>
1746
1747<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1748<t>
1749  No significant changes.
1750</t>
1751</section>
1752
1753<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1754<t>
1755  Closed issues:
1756  <list style="symbols">
1757    <t>
1758      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1759      "Clarify 'Requested Variant'"
1760    </t>
1761    <t>
1762      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1763      "Clarify entity / representation / variant terminology"
1764    </t>
1765    <t>
1766      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1767      "consider removing the 'changes from 2068' sections"
1768    </t>
1769  </list>
1770</t>
1771</section>
1772
1773<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1774<t>
1775  None.
1776</t>
1777</section>
1778
1779<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1780<t>
1781  Closed issues:
1782  <list style="symbols">
1783    <t>
1784      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1785      "Header Classification"
1786    </t>
1787  </list>
1788</t>
1789</section>
1790
1791<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1792<t>
1793  Closed issues:
1794  <list style="symbols">
1795    <t>
1796      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1797      "If-* and entities"
1798    </t>
1799    <t>
1800      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1801      "Definition of validator weakness"
1802    </t>
1803    <t>
1804      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1805      "untangle ABNFs for header fields"
1806    </t>
1807    <t>
1808      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1809      "ETags and Quotes"
1810    </t>
1811  </list>
1812</t>
1813</section>
1814
1815<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1816<t>
1817  None.
1818</t>
1819</section>
1820
1821<section title="Since draft-ietf-httpbis-p4-conditional-15" anchor="changes.since.15">
1822<t>
1823  Closed issues:
1824  <list style="symbols">
1825    <t>
1826      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/304"/>:
1827      "If-Range should be listed when dicussing contexts where L-M can be considered strong"
1828    </t>
1829  </list>
1830</t>
1831</section>
1832
1833<section title="Since draft-ietf-httpbis-p4-conditional-16" anchor="changes.since.16">
1834<t>
1835  Closed issues:
1836  <list style="symbols">
1837    <t>
1838      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
1839      "Document HTTP's error-handling philosophy"
1840    </t>
1841  </list>
1842</t>
1843</section>
1844
1845<section title="Since draft-ietf-httpbis-p4-conditional-17" anchor="changes.since.17">
1846<t>
1847  Closed issues:
1848  <list style="symbols">
1849    <t>
1850      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/306"/>:
1851      "does etag value really use quoted-string"
1852    </t>
1853  </list>
1854</t>
1855</section>
1856
1857</section>
1858
1859</back>
1860</rfc>
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