source: draft-ietf-httpbis/latest/p3-payload.xml @ 647

Last change on this file since 647 was 647, checked in by fielding@…, 10 years ago

first pass at cleaning up message parsing definition: header fields.

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File size: 127.8 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "July">
16  <!ENTITY ID-YEAR "2009">
17  <!ENTITY notation                 "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY notation-abnf            "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules              "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY caching-neg-resp         "<xref target='Part6' x:rel='#caching.negotiated.responses' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY header-transfer-encoding "<xref target='Part1' x:rel='#header.transfer-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-content-length    "<xref target='Part1' x:rel='#header.content-length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-content-range     "<xref target='Part5' x:rel='#header.content-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-expires           "<xref target='Part6' x:rel='#header.expires' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-last-modified     "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-user-agent        "<xref target='Part2' x:rel='#header.user-agent' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-vary              "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY message-body             "<xref target='Part1' x:rel='#message.body' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY message-length           "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-fields            "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY multipart-byteranges     "<xref target='Part5' x:rel='#internet.media.type.multipart.byteranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY full-date                "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY qvalue                   "<xref target='Part1' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY uri                      "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35]>
36<?rfc toc="yes" ?>
37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
45<?rfc-ext allow-markup-in-artwork="yes" ?>
46<?rfc-ext include-references-in-index="yes" ?>
47<rfc obsoletes="2616" category="std" x:maturity-level="draft"
48     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p3-payload-&ID-VERSION;"
49     xmlns:x='http://purl.org/net/xml2rfc/ext'>
50<front>
51
52  <title abbrev="HTTP/1.1, Part 3">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
53
54  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
55    <organization abbrev="Day Software">Day Software</organization>
56    <address>
57      <postal>
58        <street>23 Corporate Plaza DR, Suite 280</street>
59        <city>Newport Beach</city>
60        <region>CA</region>
61        <code>92660</code>
62        <country>USA</country>
63      </postal>
64      <phone>+1-949-706-5300</phone>
65      <facsimile>+1-949-706-5305</facsimile>
66      <email>fielding@gbiv.com</email>
67      <uri>http://roy.gbiv.com/</uri>
68    </address>
69  </author>
70
71  <author initials="J." surname="Gettys" fullname="Jim Gettys">
72    <organization>One Laptop per Child</organization>
73    <address>
74      <postal>
75        <street>21 Oak Knoll Road</street>
76        <city>Carlisle</city>
77        <region>MA</region>
78        <code>01741</code>
79        <country>USA</country>
80      </postal>
81      <email>jg@laptop.org</email>
82      <uri>http://www.laptop.org/</uri>
83    </address>
84  </author>
85 
86  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
87    <organization abbrev="HP">Hewlett-Packard Company</organization>
88    <address>
89      <postal>
90        <street>HP Labs, Large Scale Systems Group</street>
91        <street>1501 Page Mill Road, MS 1177</street>
92        <city>Palo Alto</city>
93        <region>CA</region>
94        <code>94304</code>
95        <country>USA</country>
96      </postal>
97      <email>JeffMogul@acm.org</email>
98    </address>
99  </author>
100
101  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
102    <organization abbrev="Microsoft">Microsoft Corporation</organization>
103    <address>
104      <postal>
105        <street>1 Microsoft Way</street>
106        <city>Redmond</city>
107        <region>WA</region>
108        <code>98052</code>
109        <country>USA</country>
110      </postal>
111      <email>henrikn@microsoft.com</email>
112    </address>
113  </author>
114
115  <author initials="L." surname="Masinter" fullname="Larry Masinter">
116    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
117    <address>
118      <postal>
119        <street>345 Park Ave</street>
120        <city>San Jose</city>
121        <region>CA</region>
122        <code>95110</code>
123        <country>USA</country>
124      </postal>
125      <email>LMM@acm.org</email>
126      <uri>http://larry.masinter.net/</uri>
127    </address>
128  </author>
129 
130  <author initials="P." surname="Leach" fullname="Paul J. Leach">
131    <organization abbrev="Microsoft">Microsoft Corporation</organization>
132    <address>
133      <postal>
134        <street>1 Microsoft Way</street>
135        <city>Redmond</city>
136        <region>WA</region>
137        <code>98052</code>
138      </postal>
139      <email>paulle@microsoft.com</email>
140    </address>
141  </author>
142   
143  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
144    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
145    <address>
146      <postal>
147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
148        <street>The Stata Center, Building 32</street>
149        <street>32 Vassar Street</street>
150        <city>Cambridge</city>
151        <region>MA</region>
152        <code>02139</code>
153        <country>USA</country>
154      </postal>
155      <email>timbl@w3.org</email>
156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
157    </address>
158  </author>
159
160  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
161    <organization abbrev="W3C">World Wide Web Consortium</organization>
162    <address>
163      <postal>
164        <street>W3C / ERCIM</street>
165        <street>2004, rte des Lucioles</street>
166        <city>Sophia-Antipolis</city>
167        <region>AM</region>
168        <code>06902</code>
169        <country>France</country>
170      </postal>
171      <email>ylafon@w3.org</email>
172      <uri>http://www.raubacapeu.net/people/yves/</uri>
173    </address>
174  </author>
175
176  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
177    <organization abbrev="greenbytes">greenbytes GmbH</organization>
178    <address>
179      <postal>
180        <street>Hafenweg 16</street>
181        <city>Muenster</city><region>NW</region><code>48155</code>
182        <country>Germany</country>
183      </postal>
184      <phone>+49 251 2807760</phone>
185      <facsimile>+49 251 2807761</facsimile>
186      <email>julian.reschke@greenbytes.de</email>
187      <uri>http://greenbytes.de/tech/webdav/</uri>
188    </address>
189  </author>
190
191  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
192  <workgroup>HTTPbis Working Group</workgroup>
193
194<abstract>
195<t>
196   The Hypertext Transfer Protocol (HTTP) is an application-level
197   protocol for distributed, collaborative, hypermedia information
198   systems. HTTP has been in use by the World Wide Web global information
199   initiative since 1990. This document is Part 3 of the seven-part specification
200   that defines the protocol referred to as "HTTP/1.1" and, taken together,
201   obsoletes RFC 2616.  Part 3 defines HTTP message content,
202   metadata, and content negotiation.
203</t>
204</abstract>
205
206<note title="Editorial Note (To be removed by RFC Editor)">
207  <t>
208    Discussion of this draft should take place on the HTTPBIS working group
209    mailing list (ietf-http-wg@w3.org). The current issues list is
210    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
211    and related documents (including fancy diffs) can be found at
212    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
213  </t>
214  <t>
215    The changes in this draft are summarized in <xref target="changes.since.07"/>.
216  </t>
217</note>
218</front>
219<middle>
220<section title="Introduction" anchor="introduction">
221<t>
222   This document defines HTTP/1.1 message payloads (a.k.a., content), the
223   associated metadata header fields that define how the payload is intended
224   to be interpreted by a recipient, the request header fields that
225   may influence content selection, and the various selection algorithms
226   that are collectively referred to as HTTP content negotiation.
227</t>
228<t>
229   This document is currently disorganized in order to minimize the changes
230   between drafts and enable reviewers to see the smaller errata changes.
231   The next draft will reorganize the sections to better reflect the content.
232   In particular, the sections on entities will be renamed payload and moved
233   to the first half of the document, while the sections on content negotiation
234   and associated request header fields will be moved to the second half.  The
235   current mess reflects how widely dispersed these topics and associated
236   requirements had become in <xref target="RFC2616"/>.
237</t>
238
239<section title="Requirements" anchor="intro.requirements">
240<t>
241   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
242   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
243   document are to be interpreted as described in <xref target="RFC2119"/>.
244</t>
245<t>
246   An implementation is not compliant if it fails to satisfy one or more
247   of the &MUST; or &REQUIRED; level requirements for the protocols it
248   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
249   level and all the &SHOULD; level requirements for its protocols is said
250   to be "unconditionally compliant"; one that satisfies all the &MUST;
251   level requirements but not all the &SHOULD; level requirements for its
252   protocols is said to be "conditionally compliant."
253</t>
254</section>
255
256<section title="Syntax Notation" anchor="notation">
257  <x:anchor-alias value="ALPHA"/>
258  <x:anchor-alias value="CR"/>
259  <x:anchor-alias value="DIGIT"/>
260  <x:anchor-alias value="LF"/>
261  <x:anchor-alias value="OCTET"/>
262  <x:anchor-alias value="VCHAR"/>
263  <x:anchor-alias value="WSP"/>
264<t>
265  This specification uses the ABNF syntax defined in &notation; (which
266  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
267  <xref target="collected.abnf"/> shows the collected ABNF, with the list
268  rule expanded.
269</t>
270<t>
271  The following core rules are included by
272  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
273  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
274  DIGIT (decimal 0-9), DQUOTE (double quote),
275  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
276  OCTET (any 8-bit sequence of data), SP (space),
277  VCHAR (any visible USASCII character),
278  and WSP (whitespace).
279</t>
280
281<section title="Core Rules" anchor="core.rules">
282  <x:anchor-alias value="quoted-string"/>
283  <x:anchor-alias value="token"/>
284  <x:anchor-alias value="OWS"/>
285<t>
286  The core rules below are defined in &basic-rules;:
287</t>
288<figure><artwork type="abnf2616">
289  <x:ref>quoted-string</x:ref>  = &lt;quoted-string, defined in &basic-rules;&gt;
290  <x:ref>token</x:ref>          = &lt;token, defined in &basic-rules;&gt;
291  <x:ref>OWS</x:ref>            = &lt;OWS, defined in &basic-rules;&gt;
292</artwork></figure>
293</section>
294
295<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
296  <x:anchor-alias value="absolute-URI"/>
297  <x:anchor-alias value="Allow"/>
298  <x:anchor-alias value="Content-Length"/>
299  <x:anchor-alias value="Content-Range"/>
300  <x:anchor-alias value="Expires"/>
301  <x:anchor-alias value="Last-Modified"/>
302  <x:anchor-alias value="header-field"/>
303  <x:anchor-alias value="partial-URI"/>
304  <x:anchor-alias value="qvalue"/>
305<t>
306  The ABNF rules below are defined in other parts:
307</t>
308<figure><!--Part1--><artwork type="abnf2616">
309  <x:ref>absolute-URI</x:ref>   = &lt;absolute-URI, defined in &uri;&gt;
310  <x:ref>Content-Length</x:ref> = &lt;Content-Length, defined in &header-content-length;&gt;
311  <x:ref>header-field</x:ref>   = &lt;header-field, defined in &header-fields;&gt;
312  <x:ref>partial-URI</x:ref>    = &lt;partial-URI, defined in &uri;&gt;
313  <x:ref>qvalue</x:ref>         = &lt;qvalue, defined in &qvalue;&gt;
314</artwork></figure>
315<figure><!--Part4--><artwork type="abnf2616">
316  <x:ref>Last-Modified</x:ref>  = &lt;Last-Modified, defined in &header-last-modified;&gt;
317</artwork></figure>
318<figure><!--Part5--><artwork type="abnf2616">
319  <x:ref>Content-Range</x:ref>  = &lt;Content-Range, defined in &header-content-range;&gt;
320</artwork></figure>
321<figure><!--Part6--><artwork type="abnf2616">
322  <x:ref>Expires</x:ref>        = &lt;Expires, defined in &header-expires;&gt;
323</artwork></figure>
324</section>
325
326</section>
327
328</section>
329
330<section title="Protocol Parameters" anchor="protocol.parameters">
331
332<section title="Character Sets" anchor="character.sets">
333<t>
334   HTTP uses the same definition of the term "character set" as that
335   described for MIME:
336</t>
337<t>
338   The term "character set" is used in this document to refer to a
339   method used with one or more tables to convert a sequence of octets
340   into a sequence of characters. Note that unconditional conversion in
341   the other direction is not required, in that not all characters may
342   be available in a given character set and a character set may provide
343   more than one sequence of octets to represent a particular character.
344   This definition is intended to allow various kinds of character
345   encoding, from simple single-table mappings such as US-ASCII to
346   complex table switching methods such as those that use ISO-2022's
347   techniques. However, the definition associated with a MIME character
348   set name &MUST; fully specify the mapping to be performed from octets
349   to characters. In particular, use of external profiling information
350   to determine the exact mapping is not permitted.
351</t>
352<x:note>
353  <t>
354    <x:h>Note:</x:h> This use of the term "character set" is more commonly
355    referred to as a "character encoding." However, since HTTP and
356    MIME share the same registry, it is important that the terminology
357    also be shared.
358  </t>
359</x:note>
360<t anchor="rule.charset">
361  <x:anchor-alias value="charset"/>
362   HTTP character sets are identified by case-insensitive tokens. The
363   complete set of tokens is defined by the IANA Character Set registry
364   (<eref target="http://www.iana.org/assignments/character-sets"/>).
365</t>
366<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="charset"/>
367  <x:ref>charset</x:ref> = <x:ref>token</x:ref>
368</artwork></figure>
369<t>
370   Although HTTP allows an arbitrary token to be used as a charset
371   value, any token that has a predefined value within the IANA
372   Character Set registry &MUST; represent the character set defined
373   by that registry. Applications &SHOULD; limit their use of character
374   sets to those defined by the IANA registry.
375</t>
376<t>
377   HTTP uses charset in two contexts: within an Accept-Charset request
378   header (in which the charset value is an unquoted token) and as the
379   value of a parameter in a Content-Type header (within a request or
380   response), in which case the parameter value of the charset parameter
381   may be quoted.
382</t>
383<t>
384   Implementors should be aware of IETF character set requirements <xref target="RFC3629"/>
385   <xref target="RFC2277"/>.
386</t>
387
388<section title="Missing Charset" anchor="missing.charset">
389<t>
390   Some HTTP/1.0 software has interpreted a Content-Type header without
391   charset parameter incorrectly to mean "recipient should guess."
392   Senders wishing to defeat this behavior &MAY; include a charset
393   parameter even when the charset is ISO-8859-1 (<xref target="ISO-8859-1"/>) and &SHOULD; do so when
394   it is known that it will not confuse the recipient.
395</t>
396<t>
397   Unfortunately, some older HTTP/1.0 clients did not deal properly with
398   an explicit charset parameter. HTTP/1.1 recipients &MUST; respect the
399   charset label provided by the sender; and those user agents that have
400   a provision to "guess" a charset &MUST; use the charset from the
401   content-type field if they support that charset, rather than the
402   recipient's preference, when initially displaying a document. See
403   <xref target="canonicalization.and.text.defaults"/>.
404</t>
405</section>
406</section>
407
408<section title="Content Codings" anchor="content.codings">
409  <x:anchor-alias value="content-coding"/>
410<t>
411   Content coding values indicate an encoding transformation that has
412   been or can be applied to an entity. Content codings are primarily
413   used to allow a document to be compressed or otherwise usefully
414   transformed without losing the identity of its underlying media type
415   and without loss of information. Frequently, the entity is stored in
416   coded form, transmitted directly, and only decoded by the recipient.
417</t>
418<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-coding"/>
419  <x:ref>content-coding</x:ref>   = <x:ref>token</x:ref>
420</artwork></figure>
421<t>
422   All content-coding values are case-insensitive. HTTP/1.1 uses
423   content-coding values in the Accept-Encoding (<xref target="header.accept-encoding"/>) and
424   Content-Encoding (<xref target="header.content-encoding"/>) header fields. Although the value
425   describes the content-coding, what is more important is that it
426   indicates what decoding mechanism will be required to remove the
427   encoding.
428</t>
429<t>
430   The Internet Assigned Numbers Authority (IANA) acts as a registry for
431   content-coding value tokens. Initially, the registry contains the
432   following tokens:
433</t>
434<t>
435   gzip<iref item="gzip"/>
436  <list>
437    <t>
438        An encoding format produced by the file compression program
439        "gzip" (GNU zip) as described in <xref target="RFC1952"/>. This format is a
440        Lempel-Ziv coding (LZ77) with a 32 bit CRC.
441    </t>
442  </list>
443</t>
444<t>
445   compress<iref item="compress"/>
446  <list><t>
447        The encoding format produced by the common UNIX file compression
448        program "compress". This format is an adaptive Lempel-Ziv-Welch
449        coding (LZW).
450</t><t>
451        Use of program names for the identification of encoding formats
452        is not desirable and is discouraged for future encodings. Their
453        use here is representative of historical practice, not good
454        design. For compatibility with previous implementations of HTTP,
455        applications &SHOULD; consider "x-gzip" and "x-compress" to be
456        equivalent to "gzip" and "compress" respectively.
457  </t></list>
458</t>
459<t>
460   deflate<iref item="deflate"/>
461  <list><t>
462        The "zlib" format defined in <xref target="RFC1950"/> in combination with
463        the "deflate" compression mechanism described in <xref target="RFC1951"/>.
464  </t></list>
465</t>
466<t>
467   identity<iref item="identity"/>
468  <list><t>
469        The default (identity) encoding; the use of no transformation
470        whatsoever. This content-coding is used only in the Accept-Encoding
471        header, and &SHOULD-NOT;  be used in the Content-Encoding
472        header.
473  </t></list>
474</t>
475<t>
476   New content-coding value tokens &SHOULD; be registered; to allow
477   interoperability between clients and servers, specifications of the
478   content coding algorithms needed to implement a new value &SHOULD; be
479   publicly available and adequate for independent implementation, and
480   conform to the purpose of content coding defined in this section.
481</t>
482</section>
483
484<section title="Media Types" anchor="media.types">
485  <x:anchor-alias value="media-type"/>
486  <x:anchor-alias value="type"/>
487  <x:anchor-alias value="subtype"/>
488<t>
489   HTTP uses Internet Media Types <xref target="RFC2046"/> in the Content-Type (<xref target="header.content-type"/>)
490   and Accept (<xref target="header.accept"/>) header fields in order to provide
491   open and extensible data typing and type negotiation.
492</t>
493<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="media-type"/><iref primary="true" item="Grammar" subitem="type"/><iref primary="true" item="Grammar" subitem="subtype"/>
494  <x:ref>media-type</x:ref> = <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
495  <x:ref>type</x:ref>       = <x:ref>token</x:ref>
496  <x:ref>subtype</x:ref>    = <x:ref>token</x:ref>
497</artwork></figure>
498<t anchor="rule.parameter">
499  <x:anchor-alias value="attribute"/>
500  <x:anchor-alias value="parameter"/>
501  <x:anchor-alias value="value"/>
502   Parameters &MAY; follow the type/subtype in the form of attribute/value
503   pairs.
504</t>
505<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="parameter"/><iref primary="true" item="Grammar" subitem="attribute"/><iref primary="true" item="Grammar" subitem="value"/>
506  <x:ref>parameter</x:ref>      = <x:ref>attribute</x:ref> "=" <x:ref>value</x:ref>
507  <x:ref>attribute</x:ref>      = <x:ref>token</x:ref>
508  <x:ref>value</x:ref>          = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
509</artwork></figure>
510<t>
511   The type, subtype, and parameter attribute names are case-insensitive.
512   Parameter values might or might not be case-sensitive, depending on the
513   semantics of the parameter name.  The presence or absence of a parameter might
514   be significant to the processing of a media-type, depending on its
515   definition within the media type registry.
516</t>
517<t>
518   A parameter value that matches the <x:ref>token</x:ref> production may be
519   transmitted as either a token or within a quoted-string. The quoted and
520   unquoted values are equivalent.
521</t>
522<t>
523   Note that some older HTTP applications do not recognize media type
524   parameters. When sending data to older HTTP applications,
525   implementations &SHOULD; only use media type parameters when they are
526   required by that type/subtype definition.
527</t>
528<t>
529   Media-type values are registered with the Internet Assigned Number
530   Authority (IANA). The media type registration process is
531   outlined in <xref target="RFC4288"/>. Use of non-registered media types is
532   discouraged.
533</t>
534
535<section title="Canonicalization and Text Defaults" anchor="canonicalization.and.text.defaults">
536<t>
537   Internet media types are registered with a canonical form. An
538   entity-body transferred via HTTP messages &MUST; be represented in the
539   appropriate canonical form prior to its transmission except for
540   "text" types, as defined in the next paragraph.
541</t>
542<t>
543   When in canonical form, media subtypes of the "text" type use CRLF as
544   the text line break. HTTP relaxes this requirement and allows the
545   transport of text media with plain CR or LF alone representing a line
546   break when it is done consistently for an entire entity-body. HTTP
547   applications &MUST; accept CRLF, bare CR, and bare LF as being
548   representative of a line break in text media received via HTTP. In
549   addition, if the text is represented in a character set that does not
550   use octets 13 and 10 for CR and LF respectively, as is the case for
551   some multi-byte character sets, HTTP allows the use of whatever octet
552   sequences are defined by that character set to represent the
553   equivalent of CR and LF for line breaks. This flexibility regarding
554   line breaks applies only to text media in the entity-body; a bare CR
555   or LF &MUST-NOT; be substituted for CRLF within any of the HTTP control
556   structures (such as header fields and multipart boundaries).
557</t>
558<t>
559   If an entity-body is encoded with a content-coding, the underlying
560   data &MUST; be in a form defined above prior to being encoded.
561</t>
562<t>
563   The "charset" parameter is used with some media types to define the
564   character set (<xref target="character.sets"/>) of the data. When no explicit charset
565   parameter is provided by the sender, media subtypes of the "text"
566   type are defined to have a default charset value of "ISO-8859-1" when
567   received via HTTP. Data in character sets other than "ISO-8859-1" or
568   its subsets &MUST; be labeled with an appropriate charset value. See
569   <xref target="missing.charset"/> for compatibility problems.
570</t>
571</section>
572
573<section title="Multipart Types" anchor="multipart.types">
574<t>
575   MIME provides for a number of "multipart" types -- encapsulations of
576   one or more entities within a single message-body. All multipart
577   types share a common syntax, as defined in <xref target="RFC2046" x:sec="5.1.1" x:fmt="of"/>,
578   and &MUST; include a boundary parameter as part of the media type
579   value. The message body is itself a protocol element and &MUST;
580   therefore use only CRLF to represent line breaks between body-parts.
581   Unlike in RFC 2046, the epilogue of any multipart message &MUST; be
582   empty; HTTP applications &MUST-NOT; transmit the epilogue (even if the
583   original multipart contains an epilogue). These restrictions exist in
584   order to preserve the self-delimiting nature of a multipart message-body,
585   wherein the "end" of the message-body is indicated by the
586   ending multipart boundary.
587</t>
588<t>
589   In general, HTTP treats a multipart message-body no differently than
590   any other media type: strictly as payload. The one exception is the
591   "multipart/byteranges" type (&multipart-byteranges;) when it appears in a 206
592   (Partial Content) response.
593   <!-- jre: re-insert removed text pointing to caching? -->
594   In all
595   other cases, an HTTP user agent &SHOULD; follow the same or similar
596   behavior as a MIME user agent would upon receipt of a multipart type.
597   The MIME header fields within each body-part of a multipart message-body
598   do not have any significance to HTTP beyond that defined by
599   their MIME semantics.
600</t>
601<t>
602   In general, an HTTP user agent &SHOULD; follow the same or similar
603   behavior as a MIME user agent would upon receipt of a multipart type.
604   If an application receives an unrecognized multipart subtype, the
605   application &MUST; treat it as being equivalent to "multipart/mixed".
606</t>
607<x:note>
608  <t>
609    <x:h>Note:</x:h> The "multipart/form-data" type has been specifically defined
610    for carrying form data suitable for processing via the POST
611    request method, as described in <xref target="RFC2388"/>.
612  </t>
613</x:note>
614</section>
615</section>
616
617<section title="Language Tags" anchor="language.tags">
618  <x:anchor-alias value="language-tag"/>
619<t>
620   A language tag, as defined in <xref target="RFC4646bis"/>, identifies a
621   natural language spoken, written, or otherwise conveyed by human beings for
622   communication of information to other human beings. Computer languages are
623   explicitly excluded. HTTP uses language tags within the Accept-Language and
624   Content-Language fields.
625</t>
626<t>
627   In summary, a language tag is composed of one or more parts: A primary
628   language subtag followed by a possibly empty series of subtags:
629</t>
630<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="language-tag"/>
631  <x:ref>language-tag</x:ref> = &lt;Language-Tag, defined in <xref target="RFC4646bis" x:sec="2.1"/>&gt;
632</artwork></figure>
633<t>
634   White space is not allowed within the tag and all tags are case-insensitive.
635   The name space of language subtags is administered by the IANA (see
636   <eref target="http://www.iana.org/assignments/language-subtag-registry"/>).
637</t>
638<figure>
639  <preamble>Example tags include:</preamble>
640<artwork type="example">
641  en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
642</artwork>
643</figure>
644<t>
645   See <xref target="RFC4646bis"/> for further information.
646</t>
647</section>
648</section>
649
650<section title="Entity" anchor="entity">
651<t>
652   Request and Response messages &MAY; transfer an entity if not otherwise
653   restricted by the request method or response status code. An entity
654   consists of entity-header fields and an entity-body, although some
655   responses will only include the entity-headers.
656</t>
657<t>
658   In this section, both sender and recipient refer to either the client
659   or the server, depending on who sends and who receives the entity.
660</t>
661
662<section title="Entity Header Fields" anchor="entity.header.fields">
663  <x:anchor-alias value="entity-header"/>
664  <x:anchor-alias value="extension-header"/>
665<t>
666   Entity-header fields define metainformation about the entity-body or,
667   if no body is present, about the resource identified by the request.
668</t>
669<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-header"/><iref primary="true" item="Grammar" subitem="extension-header"/>
670  <x:ref>entity-header</x:ref>  = <x:ref>Content-Encoding</x:ref>         ; <xref target="header.content-encoding"/>
671                 / <x:ref>Content-Language</x:ref>         ; <xref target="header.content-language"/>
672                 / <x:ref>Content-Length</x:ref>           ; &header-content-length;
673                 / <x:ref>Content-Location</x:ref>         ; <xref target="header.content-location"/>
674                 / <x:ref>Content-MD5</x:ref>              ; <xref target="header.content-md5"/>
675                 / <x:ref>Content-Range</x:ref>            ; &header-content-range;
676                 / <x:ref>Content-Type</x:ref>             ; <xref target="header.content-type"/>
677                 / <x:ref>Expires</x:ref>                  ; &header-expires;
678                 / <x:ref>Last-Modified</x:ref>            ; &header-last-modified;
679                 / <x:ref>extension-header</x:ref>
680 
681  <x:ref>extension-header</x:ref> = <x:ref>header-field</x:ref>
682</artwork></figure>
683<t>
684   The extension-header mechanism allows additional entity-header fields
685   to be defined without changing the protocol, but these fields cannot
686   be assumed to be recognizable by the recipient. Unrecognized header
687   fields &SHOULD; be ignored by the recipient and &MUST; be forwarded by
688   transparent proxies.
689</t>
690</section>
691
692<section title="Entity Body" anchor="entity.body">
693  <x:anchor-alias value="entity-body"/>
694<t>
695   The entity-body (if any) sent with an HTTP request or response is in
696   a format and encoding defined by the entity-header fields.
697</t>
698<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-body"/>
699  <x:ref>entity-body</x:ref>    = *<x:ref>OCTET</x:ref>
700</artwork></figure>
701<t>
702   An entity-body is only present in a message when a message-body is
703   present, as described in &message-body;. The entity-body is obtained
704   from the message-body by decoding any Transfer-Encoding that might
705   have been applied to ensure safe and proper transfer of the message.
706</t>
707
708<section title="Type" anchor="type">
709<t>
710   When an entity-body is included with a message, the data type of that
711   body is determined via the header fields Content-Type and Content-Encoding.
712   These define a two-layer, ordered encoding model:
713</t>
714<figure><artwork type="example">
715  entity-body := Content-Encoding( Content-Type( data ) )
716</artwork></figure>
717<t>
718   Content-Type specifies the media type of the underlying data. Any HTTP/1.1
719   message containing an entity-body &SHOULD; include a Content-Type header
720   field defining the media type of that body, unless that information is
721   unknown.  If the Content-Type header field is not present, it indicates that
722   the sender does not know the media type of the data; recipients &MAY;
723   either assume that it is "application/octet-stream" (<xref target="RFC2046" x:fmt="," x:sec="4.5.1"/>)
724   or examine the content to determine its type.
725</t>
726<t>
727   Content-Encoding may be used to indicate any additional content
728   codings applied to the data, usually for the purpose of data
729   compression, that are a property of the requested resource.  There is
730   no default encoding.
731</t>
732<t>
733   Note that neither the interpretation of the data type of a message nor
734   the behaviors caused by it are defined by HTTP; this
735   potentially includes examination of the content to override any
736   indicated type ("sniffing")<iref item="Content Type Sniffing"/>.
737</t>
738</section>
739   
740<section title="Entity Length" anchor="entity.length">
741<t>
742   The entity-length of a message is the length of the message-body
743   before any transfer-codings have been applied. &message-length; defines
744   how the transfer-length of a message-body is determined.
745</t>
746</section>
747</section>
748</section>
749
750<section title="Content Negotiation" anchor="content.negotiation">
751<t>
752   Most HTTP responses include an entity which contains information for
753   interpretation by a human user. Naturally, it is desirable to supply
754   the user with the "best available" entity corresponding to the
755   request. Unfortunately for servers and caches, not all users have the
756   same preferences for what is "best," and not all user agents are
757   equally capable of rendering all entity types. For that reason, HTTP
758   has provisions for several mechanisms for "content negotiation" --
759   the process of selecting the best representation for a given response
760   when there are multiple representations available.
761</t>
762<x:note>
763  <t>
764    <x:h>Note:</x:h> This is not called "format negotiation" because the
765    alternate representations may be of the same media type, but use
766    different capabilities of that type, be in different languages,
767    etc.
768  </t>
769</x:note>
770<t>
771   Any response containing an entity-body &MAY; be subject to negotiation,
772   including error responses.
773</t>
774<t>
775   There are two kinds of content negotiation which are possible in
776   HTTP: server-driven and agent-driven negotiation. These two kinds of
777   negotiation are orthogonal and thus may be used separately or in
778   combination. One method of combination, referred to as transparent
779   negotiation, occurs when a cache uses the agent-driven negotiation
780   information provided by the origin server in order to provide
781   server-driven negotiation for subsequent requests.
782</t>
783
784<section title="Server-driven Negotiation" anchor="server-driven.negotiation">
785<t>
786   If the selection of the best representation for a response is made by
787   an algorithm located at the server, it is called server-driven
788   negotiation. Selection is based on the available representations of
789   the response (the dimensions over which it can vary; e.g. language,
790   content-coding, etc.) and the contents of particular header fields in
791   the request message or on other information pertaining to the request
792   (such as the network address of the client).
793</t>
794<t>
795   Server-driven negotiation is advantageous when the algorithm for
796   selecting from among the available representations is difficult to
797   describe to the user agent, or when the server desires to send its
798   "best guess" to the client along with the first response (hoping to
799   avoid the round-trip delay of a subsequent request if the "best
800   guess" is good enough for the user). In order to improve the server's
801   guess, the user agent &MAY; include request header fields (Accept,
802   Accept-Language, Accept-Encoding, etc.) which describe its
803   preferences for such a response.
804</t>
805<t>
806   Server-driven negotiation has disadvantages:
807  <list style="numbers">
808    <t>
809         It is impossible for the server to accurately determine what
810         might be "best" for any given user, since that would require
811         complete knowledge of both the capabilities of the user agent
812         and the intended use for the response (e.g., does the user want
813         to view it on screen or print it on paper?).
814    </t>
815    <t>
816         Having the user agent describe its capabilities in every
817         request can be both very inefficient (given that only a small
818         percentage of responses have multiple representations) and a
819         potential violation of the user's privacy.
820    </t>
821    <t>
822         It complicates the implementation of an origin server and the
823         algorithms for generating responses to a request.
824    </t>
825    <t>
826         It may limit a public cache's ability to use the same response
827         for multiple user's requests.
828    </t>
829  </list>
830</t>
831<t>
832   HTTP/1.1 includes the following request-header fields for enabling
833   server-driven negotiation through description of user agent
834   capabilities and user preferences: Accept (<xref target="header.accept"/>), Accept-Charset
835   (<xref target="header.accept-charset"/>), Accept-Encoding (<xref target="header.accept-encoding"/>), Accept-Language
836   (<xref target="header.accept-language"/>), and User-Agent (&header-user-agent;). However, an
837   origin server is not limited to these dimensions and &MAY; vary the
838   response based on any aspect of the request, including information
839   outside the request-header fields or within extension header fields
840   not defined by this specification.
841</t>
842<t>
843   The Vary header field (&header-vary;) can be used to express the parameters the
844   server uses to select a representation that is subject to server-driven
845   negotiation.
846</t>
847</section>
848
849<section title="Agent-driven Negotiation" anchor="agent-driven.negotiation">
850<t>
851   With agent-driven negotiation, selection of the best representation
852   for a response is performed by the user agent after receiving an
853   initial response from the origin server. Selection is based on a list
854   of the available representations of the response included within the
855   header fields or entity-body of the initial response, with each
856   representation identified by its own URI. Selection from among the
857   representations may be performed automatically (if the user agent is
858   capable of doing so) or manually by the user selecting from a
859   generated (possibly hypertext) menu.
860</t>
861<t>
862   Agent-driven negotiation is advantageous when the response would vary
863   over commonly-used dimensions (such as type, language, or encoding),
864   when the origin server is unable to determine a user agent's
865   capabilities from examining the request, and generally when public
866   caches are used to distribute server load and reduce network usage.
867</t>
868<t>
869   Agent-driven negotiation suffers from the disadvantage of needing a
870   second request to obtain the best alternate representation. This
871   second request is only efficient when caching is used. In addition,
872   this specification does not define any mechanism for supporting
873   automatic selection, though it also does not prevent any such
874   mechanism from being developed as an extension and used within
875   HTTP/1.1.
876</t>
877<t>
878   HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
879   status codes for enabling agent-driven negotiation when the server is
880   unwilling or unable to provide a varying response using server-driven
881   negotiation.
882</t>
883</section>
884
885<section title="Transparent Negotiation" anchor="transparent.negotiation">
886<t>
887   Transparent negotiation is a combination of both server-driven and
888   agent-driven negotiation. When a cache is supplied with a form of the
889   list of available representations of the response (as in agent-driven
890   negotiation) and the dimensions of variance are completely understood
891   by the cache, then the cache becomes capable of performing server-driven
892   negotiation on behalf of the origin server for subsequent
893   requests on that resource.
894</t>
895<t>
896   Transparent negotiation has the advantage of distributing the
897   negotiation work that would otherwise be required of the origin
898   server and also removing the second request delay of agent-driven
899   negotiation when the cache is able to correctly guess the right
900   response.
901</t>
902<t>
903   This specification does not define any mechanism for transparent
904   negotiation, though it also does not prevent any such mechanism from
905   being developed as an extension that could be used within HTTP/1.1.
906</t>
907</section>
908</section>
909
910<section title="Header Field Definitions" anchor="header.fields">
911<t>
912   This section defines the syntax and semantics of HTTP/1.1 header fields
913   related to the payload of messages.
914</t>
915<t>
916   For entity-header fields, both sender and recipient refer to either the
917   client or the server, depending on who sends and who receives the entity.
918</t>
919
920<section title="Accept" anchor="header.accept">
921  <iref primary="true" item="Accept header" x:for-anchor=""/>
922  <iref primary="true" item="Headers" subitem="Accept" x:for-anchor=""/>
923  <x:anchor-alias value="Accept"/>
924  <x:anchor-alias value="Accept-v"/>
925  <x:anchor-alias value="accept-ext"/>
926  <x:anchor-alias value="accept-params"/>
927  <x:anchor-alias value="media-range"/>
928<t>
929   The request-header field "Accept" can be used to specify certain media
930   types which are acceptable for the response. Accept headers can be
931   used to indicate that the request is specifically limited to a small
932   set of desired types, as in the case of a request for an in-line
933   image.
934</t>
935<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept"/><iref primary="true" item="Grammar" subitem="Accept-v"/><iref primary="true" item="Grammar" subitem="media-range"/><iref primary="true" item="Grammar" subitem="accept-params"/><iref primary="true" item="Grammar" subitem="accept-ext"/>
936  <x:ref>Accept</x:ref>   = "Accept" ":" <x:ref>OWS</x:ref> <x:ref>Accept-v</x:ref>
937  <x:ref>Accept-v</x:ref> = #( <x:ref>media-range</x:ref> [ <x:ref>accept-params</x:ref> ] )
938 
939  <x:ref>media-range</x:ref>    = ( "*/*"
940                   / ( <x:ref>type</x:ref> "/" "*" )
941                   / ( <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> )
942                   ) *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
943  <x:ref>accept-params</x:ref>  = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> *( <x:ref>accept-ext</x:ref> )
944  <x:ref>accept-ext</x:ref>     = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>token</x:ref>
945                   [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
946</artwork></figure>
947<t>
948   The asterisk "*" character is used to group media types into ranges,
949   with "*/*" indicating all media types and "type/*" indicating all
950   subtypes of that type. The media-range &MAY; include media type
951   parameters that are applicable to that range.
952</t>
953<t>
954   Each media-range &MAY; be followed by one or more accept-params,
955   beginning with the "q" parameter for indicating a relative quality
956   factor. The first "q" parameter (if any) separates the media-range
957   parameter(s) from the accept-params. Quality factors allow the user
958   or user agent to indicate the relative degree of preference for that
959   media-range, using the qvalue scale from 0 to 1 (&qvalue;). The
960   default value is q=1.
961</t>
962<x:note>
963  <t>
964    <x:h>Note:</x:h> Use of the "q" parameter name to separate media type
965    parameters from Accept extension parameters is due to historical
966    practice. Although this prevents any media type parameter named
967    "q" from being used with a media range, such an event is believed
968    to be unlikely given the lack of any "q" parameters in the IANA
969    media type registry and the rare usage of any media type
970    parameters in Accept. Future media types are discouraged from
971    registering any parameter named "q".
972  </t>
973</x:note>
974<t>
975   The example
976</t>
977<figure><artwork type="example">
978  Accept: audio/*; q=0.2, audio/basic
979</artwork></figure>
980<t>
981   &SHOULD; be interpreted as "I prefer audio/basic, but send me any audio
982   type if it is the best available after an 80% mark-down in quality."
983</t>
984<t>
985   If no Accept header field is present, then it is assumed that the
986   client accepts all media types. If an Accept header field is present,
987   and if the server cannot send a response which is acceptable
988   according to the combined Accept field value, then the server &SHOULD;
989   send a 406 (Not Acceptable) response.
990</t>
991<t>
992   A more elaborate example is
993</t>
994<figure><artwork type="example">
995  Accept: text/plain; q=0.5, text/html,
996          text/x-dvi; q=0.8, text/x-c
997</artwork></figure>
998<t>
999   Verbally, this would be interpreted as "text/html and text/x-c are
1000   the preferred media types, but if they do not exist, then send the
1001   text/x-dvi entity, and if that does not exist, send the text/plain
1002   entity."
1003</t>
1004<t>
1005   Media ranges can be overridden by more specific media ranges or
1006   specific media types. If more than one media range applies to a given
1007   type, the most specific reference has precedence. For example,
1008</t>
1009<figure><artwork type="example">
1010  Accept: text/*, text/html, text/html;level=1, */*
1011</artwork></figure>
1012<t>
1013   have the following precedence:
1014   <list style="numbers">
1015    <t>text/html;level=1</t>
1016    <t>text/html</t>
1017    <t>text/*</t>
1018    <t>*/*</t>
1019   </list>
1020</t>
1021<t>
1022   The media type quality factor associated with a given type is
1023   determined by finding the media range with the highest precedence
1024   which matches that type. For example,
1025</t>
1026<figure><artwork type="example">
1027  Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
1028          text/html;level=2;q=0.4, */*;q=0.5
1029</artwork></figure>
1030<t>
1031   would cause the following values to be associated:
1032</t>
1033<texttable align="left">
1034  <ttcol>Media Type</ttcol><ttcol>Quality Value</ttcol>
1035  <c>text/html;level=1</c>    <c>1</c>
1036  <c>text/html</c>            <c>0.7</c>
1037  <c>text/plain</c>           <c>0.3</c>
1038  <c>image/jpeg</c>           <c>0.5</c>
1039  <c>text/html;level=2</c>    <c>0.4</c>
1040  <c>text/html;level=3</c>    <c>0.7</c>
1041</texttable>
1042<t>
1043      <x:h>Note:</x:h> A user agent might be provided with a default set of quality
1044      values for certain media ranges. However, unless the user agent is
1045      a closed system which cannot interact with other rendering agents,
1046      this default set ought to be configurable by the user.
1047</t>
1048</section>
1049
1050<section title="Accept-Charset" anchor="header.accept-charset">
1051  <iref primary="true" item="Accept-Charset header" x:for-anchor=""/>
1052  <iref primary="true" item="Headers" subitem="Accept-Charset" x:for-anchor=""/>
1053  <x:anchor-alias value="Accept-Charset"/>
1054  <x:anchor-alias value="Accept-Charset-v"/>
1055<t>
1056   The request-header field "Accept-Charset" can be used to indicate what
1057   character sets are acceptable for the response. This field allows
1058   clients capable of understanding more comprehensive or special-purpose
1059   character sets to signal that capability to a server which is
1060   capable of representing documents in those character sets.
1061</t>
1062<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Charset"/><iref primary="true" item="Grammar" subitem="Accept-Charset-v"/>
1063  <x:ref>Accept-Charset</x:ref>   = "Accept-Charset" ":" <x:ref>OWS</x:ref>
1064          <x:ref>Accept-Charset-v</x:ref>
1065  <x:ref>Accept-Charset-v</x:ref> = 1#( ( <x:ref>charset</x:ref> / "*" )
1066                         [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
1067</artwork></figure>
1068<t>
1069   Character set values are described in <xref target="character.sets"/>. Each charset &MAY;
1070   be given an associated quality value which represents the user's
1071   preference for that charset. The default value is q=1. An example is
1072</t>
1073<figure><artwork type="example">
1074  Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
1075</artwork></figure>
1076<t>
1077   The special value "*", if present in the Accept-Charset field,
1078   matches every character set (including ISO-8859-1) which is not
1079   mentioned elsewhere in the Accept-Charset field. If no "*" is present
1080   in an Accept-Charset field, then all character sets not explicitly
1081   mentioned get a quality value of 0, except for ISO-8859-1, which gets
1082   a quality value of 1 if not explicitly mentioned.
1083</t>
1084<t>
1085   If no Accept-Charset header is present, the default is that any
1086   character set is acceptable. If an Accept-Charset header is present,
1087   and if the server cannot send a response which is acceptable
1088   according to the Accept-Charset header, then the server &SHOULD; send
1089   an error response with the 406 (Not Acceptable) status code, though
1090   the sending of an unacceptable response is also allowed.
1091</t>
1092</section>
1093
1094<section title="Accept-Encoding" anchor="header.accept-encoding">
1095  <iref primary="true" item="Accept-Encoding header" x:for-anchor=""/>
1096  <iref primary="true" item="Headers" subitem="Accept-Encoding" x:for-anchor=""/>
1097  <x:anchor-alias value="Accept-Encoding"/>
1098  <x:anchor-alias value="Accept-Encoding-v"/>
1099  <x:anchor-alias value="codings"/>
1100<t>
1101   The request-header field "Accept-Encoding" is similar to Accept, but
1102   restricts the content-codings (<xref target="content.codings"/>) that are acceptable in
1103   the response.
1104</t>
1105<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Encoding"/><iref primary="true" item="Grammar" subitem="Accept-Encoding-v"/><iref primary="true" item="Grammar" subitem="codings"/>
1106  <x:ref>Accept-Encoding</x:ref>    = "Accept-Encoding" ":" <x:ref>OWS</x:ref>
1107                     <x:ref>Accept-Encoding-v</x:ref>
1108  <x:ref>Accept-Encoding-v</x:ref>  =
1109                     #( <x:ref>codings</x:ref> [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
1110  <x:ref>codings</x:ref>            = ( <x:ref>content-coding</x:ref> / "*" )
1111</artwork></figure>
1112<t>
1113   Each codings value &MAY; be given an associated quality value which
1114   represents the preference for that encoding. The default value is q=1.
1115</t>
1116<t>
1117   Examples of its use are:
1118</t>
1119<figure><artwork type="example">
1120  Accept-Encoding: compress, gzip
1121  Accept-Encoding:
1122  Accept-Encoding: *
1123  Accept-Encoding: compress;q=0.5, gzip;q=1.0
1124  Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
1125</artwork></figure>
1126<t>
1127   A server tests whether a content-coding is acceptable, according to
1128   an Accept-Encoding field, using these rules:
1129  <list style="numbers">
1130      <t>If the content-coding is one of the content-codings listed in
1131         the Accept-Encoding field, then it is acceptable, unless it is
1132         accompanied by a qvalue of 0. (As defined in &qvalue;, a
1133         qvalue of 0 means "not acceptable.")</t>
1134
1135      <t>The special "*" symbol in an Accept-Encoding field matches any
1136         available content-coding not explicitly listed in the header
1137         field.</t>
1138
1139      <t>If multiple content-codings are acceptable, then the acceptable
1140         content-coding with the highest non-zero qvalue is preferred.</t>
1141
1142      <t>The "identity" content-coding is always acceptable, unless
1143         specifically refused because the Accept-Encoding field includes
1144         "identity;q=0", or because the field includes "*;q=0" and does
1145         not explicitly include the "identity" content-coding. If the
1146         Accept-Encoding field-value is empty, then only the "identity"
1147         encoding is acceptable.</t>
1148  </list>
1149</t>
1150<t>
1151   If an Accept-Encoding field is present in a request, and if the
1152   server cannot send a response which is acceptable according to the
1153   Accept-Encoding header, then the server &SHOULD; send an error response
1154   with the 406 (Not Acceptable) status code.
1155</t>
1156<t>
1157   If no Accept-Encoding field is present in a request, the server &MAY;
1158   assume that the client will accept any content coding. In this case,
1159   if "identity" is one of the available content-codings, then the
1160   server &SHOULD; use the "identity" content-coding, unless it has
1161   additional information that a different content-coding is meaningful
1162   to the client.
1163</t>
1164<x:note>
1165  <t>
1166    <x:h>Note:</x:h> If the request does not include an Accept-Encoding field,
1167    and if the "identity" content-coding is unavailable, then
1168    content-codings commonly understood by HTTP/1.0 clients (i.e.,
1169    "gzip" and "compress") are preferred; some older clients
1170    improperly display messages sent with other content-codings.  The
1171    server might also make this decision based on information about
1172    the particular user-agent or client.
1173  </t>
1174</x:note>
1175<x:note>
1176  <t>
1177    <x:h>Note:</x:h> Most HTTP/1.0 applications do not recognize or obey qvalues
1178    associated with content-codings. This means that qvalues will not
1179    work and are not permitted with x-gzip or x-compress.
1180  </t>
1181</x:note>
1182</section>
1183
1184<section title="Accept-Language" anchor="header.accept-language">
1185  <iref primary="true" item="Accept-Language header" x:for-anchor=""/>
1186  <iref primary="true" item="Headers" subitem="Accept-Language" x:for-anchor=""/>
1187  <x:anchor-alias value="Accept-Language"/>
1188  <x:anchor-alias value="Accept-Language-v"/>
1189  <x:anchor-alias value="language-range"/>
1190<t>
1191   The request-header field "Accept-Language" is similar to Accept, but
1192   restricts the set of natural languages that are preferred as a
1193   response to the request. Language tags are defined in <xref target="language.tags"/>.
1194</t>
1195<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Language"/><iref primary="true" item="Grammar" subitem="Accept-Language-v"/><iref primary="true" item="Grammar" subitem="language-range"/>
1196  <x:ref>Accept-Language</x:ref>   = "Accept-Language" ":" <x:ref>OWS</x:ref>
1197                    <x:ref>Accept-Language-v</x:ref>
1198  <x:ref>Accept-Language-v</x:ref> =
1199                    1#( <x:ref>language-range</x:ref> [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
1200  <x:ref>language-range</x:ref>    =
1201            &lt;language-range, defined in <xref target="RFC4647" x:fmt="," x:sec="2.1"/>&gt;
1202</artwork></figure>
1203<t>
1204   Each language-range can be given an associated quality value which
1205   represents an estimate of the user's preference for the languages
1206   specified by that range. The quality value defaults to "q=1". For
1207   example,
1208</t>
1209<figure><artwork type="example">
1210  Accept-Language: da, en-gb;q=0.8, en;q=0.7
1211</artwork></figure>
1212<t>
1213   would mean: "I prefer Danish, but will accept British English and
1214   other types of English."
1215</t>
1216<t>
1217   For matching, the "Basic Filtering" matching scheme, defined in
1218   <xref target="RFC4647" x:sec="3.3.1" x:fmt="of"/>, is used:
1219</t>
1220<x:blockquote cite="http://tools.ietf.org/html/rfc4647#section-3.3.1">
1221  <t>
1222     A language range matches a
1223     particular language tag if, in a case-insensitive comparison, it
1224     exactly equals the tag, or if it exactly equals a prefix of the tag
1225     such that the first character following the prefix is "-".
1226  </t>
1227</x:blockquote> 
1228<t>
1229   The special range "*", if present in the Accept-Language field,
1230   matches every tag not matched by any other range present in the
1231   Accept-Language field.
1232</t>
1233<x:note>
1234  <t>
1235    <x:h>Note:</x:h> This use of a prefix matching rule does not imply that
1236    language tags are assigned to languages in such a way that it is
1237    always true that if a user understands a language with a certain
1238    tag, then this user will also understand all languages with tags
1239    for which this tag is a prefix. The prefix rule simply allows the
1240    use of prefix tags if this is the case.
1241  </t>
1242</x:note>
1243<t>
1244   The language quality factor assigned to a language-tag by the
1245   Accept-Language field is the quality value of the longest language-range
1246   in the field that matches the language-tag. If no language-range
1247   in the field matches the tag, the language quality factor
1248   assigned is 0. If no Accept-Language header is present in the
1249   request, the server
1250   &SHOULD; assume that all languages are equally acceptable. If an
1251   Accept-Language header is present, then all languages which are
1252   assigned a quality factor greater than 0 are acceptable.
1253</t>
1254<t>
1255   It might be contrary to the privacy expectations of the user to send
1256   an Accept-Language header with the complete linguistic preferences of
1257   the user in every request. For a discussion of this issue, see
1258   <xref target="privacy.issues.connected.to.accept.headers"/>.
1259</t>
1260<t>
1261   As intelligibility is highly dependent on the individual user, it is
1262   recommended that client applications make the choice of linguistic
1263   preference available to the user. If the choice is not made
1264   available, then the Accept-Language header field &MUST-NOT; be given in
1265   the request.
1266</t>
1267<x:note>
1268  <t>
1269    <x:h>Note:</x:h> When making the choice of linguistic preference available to
1270    the user, we remind implementors of  the fact that users are not
1271    familiar with the details of language matching as described above,
1272    and should provide appropriate guidance. As an example, users
1273    might assume that on selecting "en-gb", they will be served any
1274    kind of English document if British English is not available. A
1275    user agent might suggest in such a case to add "en" to get the
1276    best matching behavior.
1277  </t>
1278</x:note>
1279</section>
1280
1281<section title="Content-Encoding" anchor="header.content-encoding">
1282  <iref primary="true" item="Content-Encoding header" x:for-anchor=""/>
1283  <iref primary="true" item="Headers" subitem="Content-Encoding" x:for-anchor=""/>
1284  <x:anchor-alias value="Content-Encoding"/>
1285  <x:anchor-alias value="Content-Encoding-v"/>
1286<t>
1287   The entity-header field "Content-Encoding" is used as a modifier to the
1288   media-type. When present, its value indicates what additional content
1289   codings have been applied to the entity-body, and thus what decoding
1290   mechanisms must be applied in order to obtain the media-type
1291   referenced by the Content-Type header field. Content-Encoding is
1292   primarily used to allow a document to be compressed without losing
1293   the identity of its underlying media type.
1294</t>
1295<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Encoding"/><iref primary="true" item="Grammar" subitem="Content-Encoding-v"/>
1296  <x:ref>Content-Encoding</x:ref>   = "Content-Encoding" ":" <x:ref>OWS</x:ref> <x:ref>Content-Encoding-v</x:ref>
1297  <x:ref>Content-Encoding-v</x:ref> = 1#<x:ref>content-coding</x:ref>
1298</artwork></figure>
1299<t>
1300   Content codings are defined in <xref target="content.codings"/>. An example of its use is
1301</t>
1302<figure><artwork type="example">
1303  Content-Encoding: gzip
1304</artwork></figure>
1305<t>
1306   The content-coding is a characteristic of the entity identified by
1307   the request-target. Typically, the entity-body is stored with this
1308   encoding and is only decoded before rendering or analogous usage.
1309   However, a non-transparent proxy &MAY; modify the content-coding if the
1310   new coding is known to be acceptable to the recipient, unless the
1311   "no-transform" cache-control directive is present in the message.
1312</t>
1313<t>
1314   If the content-coding of an entity is not "identity", then the
1315   response &MUST; include a Content-Encoding entity-header (<xref target="header.content-encoding"/>)
1316   that lists the non-identity content-coding(s) used.
1317</t>
1318<t>
1319   If the content-coding of an entity in a request message is not
1320   acceptable to the origin server, the server &SHOULD; respond with a
1321   status code of 415 (Unsupported Media Type).
1322</t>
1323<t>
1324   If multiple encodings have been applied to an entity, the content
1325   codings &MUST; be listed in the order in which they were applied.
1326   Additional information about the encoding parameters &MAY; be provided
1327   by other entity-header fields not defined by this specification.
1328</t>
1329</section>
1330
1331<section title="Content-Language" anchor="header.content-language">
1332  <iref primary="true" item="Content-Language header" x:for-anchor=""/>
1333  <iref primary="true" item="Headers" subitem="Content-Language" x:for-anchor=""/>
1334  <x:anchor-alias value="Content-Language"/>
1335  <x:anchor-alias value="Content-Language-v"/>
1336<t>
1337   The entity-header field "Content-Language" describes the natural
1338   language(s) of the intended audience for the enclosed entity. Note
1339   that this might not be equivalent to all the languages used within
1340   the entity-body.
1341</t>
1342<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Language"/><iref primary="true" item="Grammar" subitem="Content-Language-v"/>
1343  <x:ref>Content-Language</x:ref>   = "Content-Language" ":" <x:ref>OWS</x:ref> <x:ref>Content-Language-v</x:ref>
1344  <x:ref>Content-Language-v</x:ref> = 1#<x:ref>language-tag</x:ref>
1345</artwork></figure>
1346<t>
1347   Language tags are defined in <xref target="language.tags"/>. The primary purpose of
1348   Content-Language is to allow a user to identify and differentiate
1349   entities according to the user's own preferred language. Thus, if the
1350   body content is intended only for a Danish-literate audience, the
1351   appropriate field is
1352</t>
1353<figure><artwork type="example">
1354  Content-Language: da
1355</artwork></figure>
1356<t>
1357   If no Content-Language is specified, the default is that the content
1358   is intended for all language audiences. This might mean that the
1359   sender does not consider it to be specific to any natural language,
1360   or that the sender does not know for which language it is intended.
1361</t>
1362<t>
1363   Multiple languages &MAY; be listed for content that is intended for
1364   multiple audiences. For example, a rendition of the "Treaty of
1365   Waitangi," presented simultaneously in the original Maori and English
1366   versions, would call for
1367</t>
1368<figure><artwork type="example">
1369  Content-Language: mi, en
1370</artwork></figure>
1371<t>
1372   However, just because multiple languages are present within an entity
1373   does not mean that it is intended for multiple linguistic audiences.
1374   An example would be a beginner's language primer, such as "A First
1375   Lesson in Latin," which is clearly intended to be used by an
1376   English-literate audience. In this case, the Content-Language would
1377   properly only include "en".
1378</t>
1379<t>
1380   Content-Language &MAY; be applied to any media type -- it is not
1381   limited to textual documents.
1382</t>
1383</section>
1384
1385<section title="Content-Location" anchor="header.content-location">
1386  <iref primary="true" item="Content-Location header" x:for-anchor=""/>
1387  <iref primary="true" item="Headers" subitem="Content-Location" x:for-anchor=""/>
1388  <x:anchor-alias value="Content-Location"/>
1389  <x:anchor-alias value="Content-Location-v"/>
1390<t>
1391   The entity-header field "Content-Location" &MAY; be used to supply the
1392   resource location for the entity enclosed in the message when that
1393   entity is accessible from a location separate from the requested
1394   resource's URI. A server &SHOULD; provide a Content-Location for the
1395   variant corresponding to the response entity; especially in the case
1396   where a resource has multiple entities associated with it, and those
1397   entities actually have separate locations by which they might be
1398   individually accessed, the server &SHOULD; provide a Content-Location
1399   for the particular variant which is returned.
1400</t>
1401<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Location"/><iref primary="true" item="Grammar" subitem="Content-Location-v"/>
1402  <x:ref>Content-Location</x:ref>   = "Content-Location" ":" <x:ref>OWS</x:ref>
1403                    <x:ref>Content-Location-v</x:ref>
1404  <x:ref>Content-Location-v</x:ref> =
1405                    <x:ref>absolute-URI</x:ref> / <x:ref>partial-URI</x:ref>
1406</artwork></figure>
1407<t>
1408   The value of Content-Location also defines the base URI for the
1409   entity.
1410</t>
1411<t>
1412   The Content-Location value is not a replacement for the original
1413   requested URI; it is only a statement of the location of the resource
1414   corresponding to this particular entity at the time of the request.
1415   Future requests &MAY; specify the Content-Location URI as the request-target
1416   if the desire is to identify the source of that particular
1417   entity.
1418</t>
1419<t>
1420   A cache cannot assume that an entity with a Content-Location
1421   different from the URI used to retrieve it can be used to respond to
1422   later requests on that Content-Location URI. However, the Content-Location
1423   can be used to differentiate between multiple entities
1424   retrieved from a single requested resource, as described in &caching-neg-resp;.
1425</t>
1426<t>
1427   If the Content-Location is a relative URI, the relative URI is
1428   interpreted relative to the request-target.
1429</t>
1430<t>
1431   The meaning of the Content-Location header in requests is
1432   undefined; servers are free to ignore it in those cases.
1433</t>
1434</section>
1435
1436<section title="Content-MD5" anchor="header.content-md5">
1437  <iref primary="true" item="Content-MD5 header" x:for-anchor=""/>
1438  <iref primary="true" item="Headers" subitem="Content-MD5" x:for-anchor=""/>
1439  <x:anchor-alias value="Content-MD5"/>
1440  <x:anchor-alias value="Content-MD5-v"/>
1441<t>
1442   The entity-header field "Content-MD5", as defined in <xref target="RFC1864"/>, is
1443   an MD5 digest of the entity-body for the purpose of providing an
1444   end-to-end message integrity check (MIC) of the entity-body. (Note: a
1445   MIC is good for detecting accidental modification of the entity-body
1446   in transit, but is not proof against malicious attacks.)
1447</t>
1448<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-MD5"/><iref primary="true" item="Grammar" subitem="Content-MD5-v"/>
1449  <x:ref>Content-MD5</x:ref>   = "Content-MD5" ":" <x:ref>OWS</x:ref> <x:ref>Content-MD5-v</x:ref>
1450  <x:ref>Content-MD5-v</x:ref> = &lt;base64 of 128 bit MD5 digest as per <xref target="RFC1864"/>&gt;
1451</artwork></figure>
1452<t>
1453   The Content-MD5 header field &MAY; be generated by an origin server or
1454   client to function as an integrity check of the entity-body. Only
1455   origin servers or clients &MAY; generate the Content-MD5 header field;
1456   proxies and gateways &MUST-NOT; generate it, as this would defeat its
1457   value as an end-to-end integrity check. Any recipient of the entity-body,
1458   including gateways and proxies, &MAY; check that the digest value
1459   in this header field matches that of the entity-body as received.
1460</t>
1461<t>
1462   The MD5 digest is computed based on the content of the entity-body,
1463   including any content-coding that has been applied, but not including
1464   any transfer-encoding applied to the message-body. If the message is
1465   received with a transfer-encoding, that encoding &MUST; be removed
1466   prior to checking the Content-MD5 value against the received entity.
1467</t>
1468<t>
1469   This has the result that the digest is computed on the octets of the
1470   entity-body exactly as, and in the order that, they would be sent if
1471   no transfer-encoding were being applied.
1472</t>
1473<t>
1474   HTTP extends RFC 1864 to permit the digest to be computed for MIME
1475   composite media-types (e.g., multipart/* and message/rfc822), but
1476   this does not change how the digest is computed as defined in the
1477   preceding paragraph.
1478</t>
1479<t>
1480   There are several consequences of this. The entity-body for composite
1481   types &MAY; contain many body-parts, each with its own MIME and HTTP
1482   headers (including Content-MD5, Content-Transfer-Encoding, and
1483   Content-Encoding headers). If a body-part has a Content-Transfer-Encoding
1484   or Content-Encoding header, it is assumed that the content
1485   of the body-part has had the encoding applied, and the body-part is
1486   included in the Content-MD5 digest as is -- i.e., after the
1487   application. The Transfer-Encoding header field is not allowed within
1488   body-parts.
1489</t>
1490<t>
1491   Conversion of all line breaks to CRLF &MUST-NOT; be done before
1492   computing or checking the digest: the line break convention used in
1493   the text actually transmitted &MUST; be left unaltered when computing
1494   the digest.
1495</t>
1496<x:note>
1497  <t>
1498    <x:h>Note:</x:h> while the definition of Content-MD5 is exactly the same for
1499    HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
1500    in which the application of Content-MD5 to HTTP entity-bodies
1501    differs from its application to MIME entity-bodies. One is that
1502    HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
1503    does use Transfer-Encoding and Content-Encoding. Another is that
1504    HTTP more frequently uses binary content types than MIME, so it is
1505    worth noting that, in such cases, the byte order used to compute
1506    the digest is the transmission byte order defined for the type.
1507    Lastly, HTTP allows transmission of text types with any of several
1508    line break conventions and not just the canonical form using CRLF.
1509  </t>
1510</x:note>
1511</section>
1512
1513<section title="Content-Type" anchor="header.content-type">
1514  <iref primary="true" item="Content-Type header" x:for-anchor=""/>
1515  <iref primary="true" item="Headers" subitem="Content-Type" x:for-anchor=""/>
1516  <x:anchor-alias value="Content-Type"/>
1517  <x:anchor-alias value="Content-Type-v"/>
1518<t>
1519   The entity-header field "Content-Type" indicates the media type of the
1520   entity-body sent to the recipient or, in the case of the HEAD method,
1521   the media type that would have been sent had the request been a GET.
1522</t>
1523<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Type"/><iref primary="true" item="Grammar" subitem="Content-Type-v"/>
1524  <x:ref>Content-Type</x:ref>   = "Content-Type" ":" <x:ref>OWS</x:ref> <x:ref>Content-Type-v</x:ref>
1525  <x:ref>Content-Type-v</x:ref> = <x:ref>media-type</x:ref>
1526</artwork></figure>
1527<t>
1528   Media types are defined in <xref target="media.types"/>. An example of the field is
1529</t>
1530<figure><artwork type="example">
1531  Content-Type: text/html; charset=ISO-8859-4
1532</artwork></figure>
1533<t>
1534   Further discussion of methods for identifying the media type of an
1535   entity is provided in <xref target="type"/>.
1536</t>
1537</section>
1538
1539</section>
1540
1541<section title="IANA Considerations" anchor="IANA.considerations">
1542<section title="Message Header Registration" anchor="message.header.registration">
1543<t>
1544   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1545   with the permanent registrations below (see <xref target="RFC3864"/>):
1546</t>
1547<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1548<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1549   <ttcol>Header Field Name</ttcol>
1550   <ttcol>Protocol</ttcol>
1551   <ttcol>Status</ttcol>
1552   <ttcol>Reference</ttcol>
1553
1554   <c>Accept</c>
1555   <c>http</c>
1556   <c>standard</c>
1557   <c>
1558      <xref target="header.accept"/>
1559   </c>
1560   <c>Accept-Charset</c>
1561   <c>http</c>
1562   <c>standard</c>
1563   <c>
1564      <xref target="header.accept-charset"/>
1565   </c>
1566   <c>Accept-Encoding</c>
1567   <c>http</c>
1568   <c>standard</c>
1569   <c>
1570      <xref target="header.accept-encoding"/>
1571   </c>
1572   <c>Accept-Language</c>
1573   <c>http</c>
1574   <c>standard</c>
1575   <c>
1576      <xref target="header.accept-language"/>
1577   </c>
1578   <c>Content-Disposition</c>
1579   <c>http</c>
1580   <c/>
1581   <c>
1582      <xref target="content-disposition"/>
1583   </c>
1584   <c>Content-Encoding</c>
1585   <c>http</c>
1586   <c>standard</c>
1587   <c>
1588      <xref target="header.content-encoding"/>
1589   </c>
1590   <c>Content-Language</c>
1591   <c>http</c>
1592   <c>standard</c>
1593   <c>
1594      <xref target="header.content-language"/>
1595   </c>
1596   <c>Content-Location</c>
1597   <c>http</c>
1598   <c>standard</c>
1599   <c>
1600      <xref target="header.content-location"/>
1601   </c>
1602   <c>Content-MD5</c>
1603   <c>http</c>
1604   <c>standard</c>
1605   <c>
1606      <xref target="header.content-md5"/>
1607   </c>
1608   <c>Content-Type</c>
1609   <c>http</c>
1610   <c>standard</c>
1611   <c>
1612      <xref target="header.content-type"/>
1613   </c>
1614   <c>MIME-Version</c>
1615   <c>http</c>
1616   <c/>
1617   <c>
1618      <xref target="mime-version"/>
1619   </c>
1620</texttable>
1621<!--(END)-->
1622<t>
1623   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1624</t>
1625</section>
1626</section>
1627
1628<section title="Security Considerations" anchor="security.considerations">
1629<t>
1630   This section is meant to inform application developers, information
1631   providers, and users of the security limitations in HTTP/1.1 as
1632   described by this document. The discussion does not include
1633   definitive solutions to the problems revealed, though it does make
1634   some suggestions for reducing security risks.
1635</t>
1636
1637<section title="Privacy Issues Connected to Accept Headers" anchor="privacy.issues.connected.to.accept.headers">
1638<t>
1639   Accept request-headers can reveal information about the user to all
1640   servers which are accessed. The Accept-Language header in particular
1641   can reveal information the user would consider to be of a private
1642   nature, because the understanding of particular languages is often
1643   strongly correlated to the membership of a particular ethnic group.
1644   User agents which offer the option to configure the contents of an
1645   Accept-Language header to be sent in every request are strongly
1646   encouraged to let the configuration process include a message which
1647   makes the user aware of the loss of privacy involved.
1648</t>
1649<t>
1650   An approach that limits the loss of privacy would be for a user agent
1651   to omit the sending of Accept-Language headers by default, and to ask
1652   the user whether or not to start sending Accept-Language headers to a
1653   server if it detects, by looking for any Vary response-header fields
1654   generated by the server, that such sending could improve the quality
1655   of service.
1656</t>
1657<t>
1658   Elaborate user-customized accept header fields sent in every request,
1659   in particular if these include quality values, can be used by servers
1660   as relatively reliable and long-lived user identifiers. Such user
1661   identifiers would allow content providers to do click-trail tracking,
1662   and would allow collaborating content providers to match cross-server
1663   click-trails or form submissions of individual users. Note that for
1664   many users not behind a proxy, the network address of the host
1665   running the user agent will also serve as a long-lived user
1666   identifier. In environments where proxies are used to enhance
1667   privacy, user agents ought to be conservative in offering accept
1668   header configuration options to end users. As an extreme privacy
1669   measure, proxies could filter the accept headers in relayed requests.
1670   General purpose user agents which provide a high degree of header
1671   configurability &SHOULD; warn users about the loss of privacy which can
1672   be involved.
1673</t>
1674</section>
1675
1676<section title="Content-Disposition Issues" anchor="content-disposition.issues">
1677<t>
1678   <xref target="RFC2183"/>, from which the often implemented Content-Disposition
1679   (see <xref target="content-disposition"/>) header in HTTP is derived, has a number of very
1680   serious security considerations. Content-Disposition is not part of
1681   the HTTP standard, but since it is widely implemented, we are
1682   documenting its use and risks for implementors. See <xref target="RFC2183" x:fmt="of" x:sec="5"/>
1683   for details.
1684</t>
1685</section>
1686
1687</section>
1688
1689<section title="Acknowledgments" anchor="ack">
1690</section>
1691</middle>
1692<back>
1693
1694<references title="Normative References">
1695
1696<reference anchor="ISO-8859-1">
1697  <front>
1698    <title>
1699     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
1700    </title>
1701    <author>
1702      <organization>International Organization for Standardization</organization>
1703    </author>
1704    <date year="1998"/>
1705  </front>
1706  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
1707</reference>
1708
1709<reference anchor="Part1">
1710  <front>
1711    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1712    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1713      <organization abbrev="Day Software">Day Software</organization>
1714      <address><email>fielding@gbiv.com</email></address>
1715    </author>
1716    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1717      <organization>One Laptop per Child</organization>
1718      <address><email>jg@laptop.org</email></address>
1719    </author>
1720    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1721      <organization abbrev="HP">Hewlett-Packard Company</organization>
1722      <address><email>JeffMogul@acm.org</email></address>
1723    </author>
1724    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1725      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1726      <address><email>henrikn@microsoft.com</email></address>
1727    </author>
1728    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1729      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1730      <address><email>LMM@acm.org</email></address>
1731    </author>
1732    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1733      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1734      <address><email>paulle@microsoft.com</email></address>
1735    </author>
1736    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1737      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1738      <address><email>timbl@w3.org</email></address>
1739    </author>
1740    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1741      <organization abbrev="W3C">World Wide Web Consortium</organization>
1742      <address><email>ylafon@w3.org</email></address>
1743    </author>
1744    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1745      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1746      <address><email>julian.reschke@greenbytes.de</email></address>
1747    </author>
1748    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1749  </front>
1750  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1751  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1752</reference>
1753
1754<reference anchor="Part2">
1755  <front>
1756    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1757    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1758      <organization abbrev="Day Software">Day Software</organization>
1759      <address><email>fielding@gbiv.com</email></address>
1760    </author>
1761    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1762      <organization>One Laptop per Child</organization>
1763      <address><email>jg@laptop.org</email></address>
1764    </author>
1765    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1766      <organization abbrev="HP">Hewlett-Packard Company</organization>
1767      <address><email>JeffMogul@acm.org</email></address>
1768    </author>
1769    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1770      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1771      <address><email>henrikn@microsoft.com</email></address>
1772    </author>
1773    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1774      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1775      <address><email>LMM@acm.org</email></address>
1776    </author>
1777    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1778      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1779      <address><email>paulle@microsoft.com</email></address>
1780    </author>
1781    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1782      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1783      <address><email>timbl@w3.org</email></address>
1784    </author>
1785    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1786      <organization abbrev="W3C">World Wide Web Consortium</organization>
1787      <address><email>ylafon@w3.org</email></address>
1788    </author>
1789    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1790      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1791      <address><email>julian.reschke@greenbytes.de</email></address>
1792    </author>
1793    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1794  </front>
1795  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1796  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
1797</reference>
1798
1799<reference anchor="Part4">
1800  <front>
1801    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1802    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1803      <organization abbrev="Day Software">Day Software</organization>
1804      <address><email>fielding@gbiv.com</email></address>
1805    </author>
1806    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1807      <organization>One Laptop per Child</organization>
1808      <address><email>jg@laptop.org</email></address>
1809    </author>
1810    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1811      <organization abbrev="HP">Hewlett-Packard Company</organization>
1812      <address><email>JeffMogul@acm.org</email></address>
1813    </author>
1814    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1815      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1816      <address><email>henrikn@microsoft.com</email></address>
1817    </author>
1818    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1819      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1820      <address><email>LMM@acm.org</email></address>
1821    </author>
1822    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1823      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1824      <address><email>paulle@microsoft.com</email></address>
1825    </author>
1826    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1827      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1828      <address><email>timbl@w3.org</email></address>
1829    </author>
1830    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1831      <organization abbrev="W3C">World Wide Web Consortium</organization>
1832      <address><email>ylafon@w3.org</email></address>
1833    </author>
1834    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1835      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1836      <address><email>julian.reschke@greenbytes.de</email></address>
1837    </author>
1838    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1839  </front>
1840  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
1841  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
1842</reference>
1843
1844<reference anchor="Part5">
1845  <front>
1846    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1847    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1848      <organization abbrev="Day Software">Day Software</organization>
1849      <address><email>fielding@gbiv.com</email></address>
1850    </author>
1851    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1852      <organization>One Laptop per Child</organization>
1853      <address><email>jg@laptop.org</email></address>
1854    </author>
1855    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1856      <organization abbrev="HP">Hewlett-Packard Company</organization>
1857      <address><email>JeffMogul@acm.org</email></address>
1858    </author>
1859    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1860      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1861      <address><email>henrikn@microsoft.com</email></address>
1862    </author>
1863    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1864      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1865      <address><email>LMM@acm.org</email></address>
1866    </author>
1867    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1868      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1869      <address><email>paulle@microsoft.com</email></address>
1870    </author>
1871    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1872      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1873      <address><email>timbl@w3.org</email></address>
1874    </author>
1875    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1876      <organization abbrev="W3C">World Wide Web Consortium</organization>
1877      <address><email>ylafon@w3.org</email></address>
1878    </author>
1879    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1880      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1881      <address><email>julian.reschke@greenbytes.de</email></address>
1882    </author>
1883    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1884  </front>
1885  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1886  <x:source href="p5-range.xml" basename="p5-range"/>
1887</reference>
1888
1889<reference anchor="Part6">
1890  <front>
1891    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1892    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1893      <organization abbrev="Day Software">Day Software</organization>
1894      <address><email>fielding@gbiv.com</email></address>
1895    </author>
1896    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1897      <organization>One Laptop per Child</organization>
1898      <address><email>jg@laptop.org</email></address>
1899    </author>
1900    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1901      <organization abbrev="HP">Hewlett-Packard Company</organization>
1902      <address><email>JeffMogul@acm.org</email></address>
1903    </author>
1904    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1905      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1906      <address><email>henrikn@microsoft.com</email></address>
1907    </author>
1908    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1909      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1910      <address><email>LMM@acm.org</email></address>
1911    </author>
1912    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1913      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1914      <address><email>paulle@microsoft.com</email></address>
1915    </author>
1916    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1917      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1918      <address><email>timbl@w3.org</email></address>
1919    </author>
1920    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1921      <organization abbrev="W3C">World Wide Web Consortium</organization>
1922      <address><email>ylafon@w3.org</email></address>
1923    </author>
1924    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1925      <organization />
1926      <address><email>mnot@mnot.net</email></address>
1927    </author>
1928    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1929      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1930      <address><email>julian.reschke@greenbytes.de</email></address>
1931    </author>
1932    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1933  </front>
1934  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1935  <x:source href="p6-cache.xml" basename="p6-cache"/>
1936</reference>
1937
1938<reference anchor="RFC1864">
1939  <front>
1940    <title abbrev="Content-MD5 Header Field">The Content-MD5 Header Field</title>
1941    <author initials="J." surname="Myers" fullname="John G. Myers">
1942      <organization>Carnegie Mellon University</organization>
1943      <address><email>jgm+@cmu.edu</email></address>
1944    </author>
1945    <author initials="M." surname="Rose" fullname="Marshall T. Rose">
1946      <organization>Dover Beach Consulting, Inc.</organization>
1947      <address><email>mrose@dbc.mtview.ca.us</email></address>
1948    </author>
1949    <date month="October" year="1995"/>
1950  </front>
1951  <seriesInfo name="RFC" value="1864"/>
1952</reference>
1953
1954<reference anchor="RFC1950">
1955  <front>
1956    <title>ZLIB Compressed Data Format Specification version 3.3</title>
1957    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
1958      <organization>Aladdin Enterprises</organization>
1959      <address><email>ghost@aladdin.com</email></address>
1960    </author>
1961    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
1962      <organization/>
1963    </author>
1964    <date month="May" year="1996"/>
1965  </front>
1966  <seriesInfo name="RFC" value="1950"/>
1967  <annotation>
1968    RFC 1950 is an Informational RFC, thus it may be less stable than
1969    this specification. On the other hand, this downward reference was
1970    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
1971    therefore it is unlikely to cause problems in practice. See also
1972    <xref target="BCP97"/>.
1973  </annotation>
1974</reference>
1975
1976<reference anchor="RFC1951">
1977  <front>
1978    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
1979    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
1980      <organization>Aladdin Enterprises</organization>
1981      <address><email>ghost@aladdin.com</email></address>
1982    </author>
1983    <date month="May" year="1996"/>
1984  </front>
1985  <seriesInfo name="RFC" value="1951"/>
1986  <annotation>
1987    RFC 1951 is an Informational RFC, thus it may be less stable than
1988    this specification. On the other hand, this downward reference was
1989    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
1990    therefore it is unlikely to cause problems in practice. See also
1991    <xref target="BCP97"/>.
1992  </annotation>
1993</reference>
1994
1995<reference anchor="RFC1952">
1996  <front>
1997    <title>GZIP file format specification version 4.3</title>
1998    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
1999      <organization>Aladdin Enterprises</organization>
2000      <address><email>ghost@aladdin.com</email></address>
2001    </author>
2002    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
2003      <organization/>
2004      <address><email>gzip@prep.ai.mit.edu</email></address>
2005    </author>
2006    <author initials="M." surname="Adler" fullname="Mark Adler">
2007      <organization/>
2008      <address><email>madler@alumni.caltech.edu</email></address>
2009    </author>
2010    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
2011      <organization/>
2012      <address><email>ghost@aladdin.com</email></address>
2013    </author>
2014    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
2015      <organization/>
2016      <address><email>randeg@alumni.rpi.edu</email></address>
2017    </author>
2018    <date month="May" year="1996"/>
2019  </front>
2020  <seriesInfo name="RFC" value="1952"/>
2021  <annotation>
2022    RFC 1952 is an Informational RFC, thus it may be less stable than
2023    this specification. On the other hand, this downward reference was
2024    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
2025    therefore it is unlikely to cause problems in practice. See also
2026    <xref target="BCP97"/>.
2027  </annotation>
2028</reference>
2029
2030<reference anchor="RFC2045">
2031  <front>
2032    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
2033    <author initials="N." surname="Freed" fullname="Ned Freed">
2034      <organization>Innosoft International, Inc.</organization>
2035      <address><email>ned@innosoft.com</email></address>
2036    </author>
2037    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
2038      <organization>First Virtual Holdings</organization>
2039      <address><email>nsb@nsb.fv.com</email></address>
2040    </author>
2041    <date month="November" year="1996"/>
2042  </front>
2043  <seriesInfo name="RFC" value="2045"/>
2044</reference>
2045
2046<reference anchor="RFC2046">
2047  <front>
2048    <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
2049    <author initials="N." surname="Freed" fullname="Ned Freed">
2050      <organization>Innosoft International, Inc.</organization>
2051      <address><email>ned@innosoft.com</email></address>
2052    </author>
2053    <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
2054      <organization>First Virtual Holdings</organization>
2055      <address><email>nsb@nsb.fv.com</email></address>
2056    </author>
2057    <date month="November" year="1996"/>
2058  </front>
2059  <seriesInfo name="RFC" value="2046"/>
2060</reference>
2061
2062<reference anchor="RFC2119">
2063  <front>
2064    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2065    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2066      <organization>Harvard University</organization>
2067      <address><email>sob@harvard.edu</email></address>
2068    </author>
2069    <date month="March" year="1997"/>
2070  </front>
2071  <seriesInfo name="BCP" value="14"/>
2072  <seriesInfo name="RFC" value="2119"/>
2073</reference>
2074
2075<reference anchor='RFC4647'>
2076  <front>
2077    <title>Matching of Language Tags</title>
2078    <author initials='A.' surname='Phillips' fullname='Addison Phillips' role="editor">
2079      <organization>Yahoo! Inc.</organization>
2080      <address><email>addison@inter-locale.com</email></address>
2081    </author>
2082    <author initials='M.' surname='Davis' fullname='Mark Davis' role="editor">
2083      <organization>Google</organization>
2084      <address><email>mark.davis@macchiato.com</email></address>
2085    </author>
2086    <date year='2006' month='September' />
2087  </front>
2088  <seriesInfo name='BCP' value='47' />
2089  <seriesInfo name='RFC' value='4647' />
2090</reference>
2091
2092<reference anchor="RFC5234">
2093  <front>
2094    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2095    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2096      <organization>Brandenburg InternetWorking</organization>
2097      <address>
2098      <postal>
2099      <street>675 Spruce Dr.</street>
2100      <city>Sunnyvale</city>
2101      <region>CA</region>
2102      <code>94086</code>
2103      <country>US</country></postal>
2104      <phone>+1.408.246.8253</phone>
2105      <email>dcrocker@bbiw.net</email></address> 
2106    </author>
2107    <author initials="P." surname="Overell" fullname="Paul Overell">
2108      <organization>THUS plc.</organization>
2109      <address>
2110      <postal>
2111      <street>1/2 Berkeley Square</street>
2112      <street>99 Berkely Street</street>
2113      <city>Glasgow</city>
2114      <code>G3 7HR</code>
2115      <country>UK</country></postal>
2116      <email>paul.overell@thus.net</email></address>
2117    </author>
2118    <date month="January" year="2008"/>
2119  </front>
2120  <seriesInfo name="STD" value="68"/>
2121  <seriesInfo name="RFC" value="5234"/>
2122</reference>
2123
2124<reference anchor='RFC4646bis'>
2125  <front>
2126    <title>Tags for Identifying Languages</title>
2127    <author initials='A.' surname='Phillips' fullname='Addison Phillips' role='editor'>
2128      <organization>Lab126</organization>
2129      <address><email>addison@inter-locale.com</email></address>
2130    </author>
2131    <author initials='M.' surname='Davis' fullname='Mark Davis' role='editor'>
2132      <organization>Google</organization>
2133      <address><email>mark.davis@google.com</email></address>
2134    </author>
2135    <date month='June' day='11' year='2009' />
2136  </front>
2137  <seriesInfo name='Internet-Draft' value='draft-ietf-ltru-4646bis-23' />
2138</reference>
2139
2140</references>
2141
2142<references title="Informative References">
2143
2144<reference anchor="RFC1945">
2145  <front>
2146    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
2147    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2148      <organization>MIT, Laboratory for Computer Science</organization>
2149      <address><email>timbl@w3.org</email></address>
2150    </author>
2151    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
2152      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2153      <address><email>fielding@ics.uci.edu</email></address>
2154    </author>
2155    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2156      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
2157      <address><email>frystyk@w3.org</email></address>
2158    </author>
2159    <date month="May" year="1996"/>
2160  </front>
2161  <seriesInfo name="RFC" value="1945"/>
2162</reference>
2163
2164<reference anchor="RFC2049">
2165  <front>
2166    <title abbrev="MIME Conformance">Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples</title>
2167    <author initials="N." surname="Freed" fullname="Ned Freed">
2168      <organization>Innosoft International, Inc.</organization>
2169      <address><email>ned@innosoft.com</email></address>
2170    </author>
2171    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
2172      <organization>First Virtual Holdings</organization>
2173      <address><email>nsb@nsb.fv.com</email></address>
2174    </author>
2175    <date month="November" year="1996"/>
2176  </front>
2177  <seriesInfo name="RFC" value="2049"/>
2178</reference>
2179
2180<reference anchor="RFC2068">
2181  <front>
2182    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
2183    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
2184      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2185      <address><email>fielding@ics.uci.edu</email></address>
2186    </author>
2187    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2188      <organization>MIT Laboratory for Computer Science</organization>
2189      <address><email>jg@w3.org</email></address>
2190    </author>
2191    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2192      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
2193      <address><email>mogul@wrl.dec.com</email></address>
2194    </author>
2195    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2196      <organization>MIT Laboratory for Computer Science</organization>
2197      <address><email>frystyk@w3.org</email></address>
2198    </author>
2199    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2200      <organization>MIT Laboratory for Computer Science</organization>
2201      <address><email>timbl@w3.org</email></address>
2202    </author>
2203    <date month="January" year="1997"/>
2204  </front>
2205  <seriesInfo name="RFC" value="2068"/>
2206</reference>
2207
2208<reference anchor="RFC2076">
2209  <front>
2210    <title abbrev="Internet Message Headers">Common Internet Message Headers</title>
2211    <author initials="J." surname="Palme" fullname="Jacob Palme">
2212      <organization>Stockholm University/KTH</organization>
2213      <address><email>jpalme@dsv.su.se</email></address>
2214    </author>
2215    <date month="February" year="1997"/>
2216  </front>
2217  <seriesInfo name="RFC" value="2076"/>
2218</reference>
2219
2220<reference anchor="RFC2183">
2221  <front>
2222    <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field</title>
2223    <author initials="R." surname="Troost" fullname="Rens Troost">
2224      <organization>New Century Systems</organization>
2225      <address><email>rens@century.com</email></address>
2226    </author>
2227    <author initials="S." surname="Dorner" fullname="Steve Dorner">
2228      <organization>QUALCOMM Incorporated</organization>
2229      <address><email>sdorner@qualcomm.com</email></address>
2230    </author>
2231    <author initials="K." surname="Moore" fullname="Keith Moore">
2232      <organization>Department of Computer Science</organization>
2233      <address><email>moore@cs.utk.edu</email></address>
2234    </author>
2235    <date month="August" year="1997"/>
2236  </front>
2237  <seriesInfo name="RFC" value="2183"/>
2238</reference>
2239
2240<reference anchor="RFC2277">
2241  <front>
2242    <title abbrev="Charset Policy">IETF Policy on Character Sets and Languages</title>
2243    <author initials="H.T." surname="Alvestrand" fullname="Harald Tveit Alvestrand">
2244      <organization>UNINETT</organization>
2245      <address><email>Harald.T.Alvestrand@uninett.no</email></address>
2246    </author>
2247    <date month="January" year="1998"/>
2248  </front>
2249  <seriesInfo name="BCP" value="18"/>
2250  <seriesInfo name="RFC" value="2277"/>
2251</reference>
2252
2253<reference anchor="RFC2388">
2254  <front>
2255    <title abbrev="multipart/form-data">Returning Values from Forms:  multipart/form-data</title>
2256    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2257      <organization>Xerox Palo Alto Research Center</organization>
2258      <address><email>masinter@parc.xerox.com</email></address>
2259    </author>
2260    <date year="1998" month="August"/>
2261  </front>
2262  <seriesInfo name="RFC" value="2388"/>
2263</reference>
2264
2265<reference anchor="RFC2557">
2266  <front>
2267    <title abbrev="MIME Encapsulation of Aggregate Documents">MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)</title>
2268    <author initials="F." surname="Palme" fullname="Jacob Palme">
2269      <organization>Stockholm University and KTH</organization>
2270      <address><email>jpalme@dsv.su.se</email></address>
2271    </author>
2272    <author initials="A." surname="Hopmann" fullname="Alex Hopmann">
2273      <organization>Microsoft Corporation</organization>
2274      <address><email>alexhop@microsoft.com</email></address>
2275    </author>
2276    <author initials="N." surname="Shelness" fullname="Nick Shelness">
2277      <organization>Lotus Development Corporation</organization>
2278      <address><email>Shelness@lotus.com</email></address>
2279    </author>
2280    <author initials="E." surname="Stefferud" fullname="Einar Stefferud">
2281      <organization/>
2282      <address><email>stef@nma.com</email></address>
2283    </author>
2284    <date year="1999" month="March"/>
2285  </front>
2286  <seriesInfo name="RFC" value="2557"/>
2287</reference>
2288
2289<reference anchor="RFC2616">
2290  <front>
2291    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2292    <author initials="R." surname="Fielding" fullname="R. Fielding">
2293      <organization>University of California, Irvine</organization>
2294      <address><email>fielding@ics.uci.edu</email></address>
2295    </author>
2296    <author initials="J." surname="Gettys" fullname="J. Gettys">
2297      <organization>W3C</organization>
2298      <address><email>jg@w3.org</email></address>
2299    </author>
2300    <author initials="J." surname="Mogul" fullname="J. Mogul">
2301      <organization>Compaq Computer Corporation</organization>
2302      <address><email>mogul@wrl.dec.com</email></address>
2303    </author>
2304    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2305      <organization>MIT Laboratory for Computer Science</organization>
2306      <address><email>frystyk@w3.org</email></address>
2307    </author>
2308    <author initials="L." surname="Masinter" fullname="L. Masinter">
2309      <organization>Xerox Corporation</organization>
2310      <address><email>masinter@parc.xerox.com</email></address>
2311    </author>
2312    <author initials="P." surname="Leach" fullname="P. Leach">
2313      <organization>Microsoft Corporation</organization>
2314      <address><email>paulle@microsoft.com</email></address>
2315    </author>
2316    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2317      <organization>W3C</organization>
2318      <address><email>timbl@w3.org</email></address>
2319    </author>
2320    <date month="June" year="1999"/>
2321  </front>
2322  <seriesInfo name="RFC" value="2616"/>
2323</reference>
2324
2325<reference anchor="RFC3629">
2326  <front>
2327    <title>UTF-8, a transformation format of ISO 10646</title>
2328    <author initials="F." surname="Yergeau" fullname="F. Yergeau">
2329      <organization>Alis Technologies</organization>
2330      <address><email>fyergeau@alis.com</email></address>
2331    </author>
2332    <date month="November" year="2003"/>
2333  </front>
2334  <seriesInfo name="RFC" value="3629"/>
2335  <seriesInfo name="STD" value="63"/>
2336</reference>
2337
2338<reference anchor='RFC3864'>
2339  <front>
2340    <title>Registration Procedures for Message Header Fields</title>
2341    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
2342      <organization>Nine by Nine</organization>
2343      <address><email>GK-IETF@ninebynine.org</email></address>
2344    </author>
2345    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
2346      <organization>BEA Systems</organization>
2347      <address><email>mnot@pobox.com</email></address>
2348    </author>
2349    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
2350      <organization>HP Labs</organization>
2351      <address><email>JeffMogul@acm.org</email></address>
2352    </author>
2353    <date year='2004' month='September' />
2354  </front>
2355  <seriesInfo name='BCP' value='90' />
2356  <seriesInfo name='RFC' value='3864' />
2357</reference>
2358
2359<reference anchor="RFC4288">
2360  <front>
2361    <title>Media Type Specifications and Registration Procedures</title>
2362    <author initials="N." surname="Freed" fullname="N. Freed">
2363      <organization>Sun Microsystems</organization>
2364      <address>
2365        <email>ned.freed@mrochek.com</email>
2366      </address>
2367    </author>
2368    <author initials="J." surname="Klensin" fullname="J. Klensin">
2369      <organization/>
2370      <address>
2371        <email>klensin+ietf@jck.com</email>
2372      </address>
2373    </author>
2374    <date year="2005" month="December"/>
2375  </front>
2376  <seriesInfo name="BCP" value="13"/>
2377  <seriesInfo name="RFC" value="4288"/>
2378</reference>
2379
2380<reference anchor="RFC5322">
2381  <front>
2382    <title>Internet Message Format</title>
2383    <author initials="P." surname="Resnick" fullname="P. Resnick">
2384      <organization>Qualcomm Incorporated</organization>
2385    </author>
2386    <date year="2008" month="October"/>
2387  </front> 
2388  <seriesInfo name="RFC" value="5322"/>
2389</reference>
2390
2391<reference anchor='BCP97'>
2392  <front>
2393    <title>Handling Normative References to Standards-Track Documents</title>
2394    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
2395      <organization />
2396      <address>
2397        <email>klensin+ietf@jck.com</email>
2398      </address>
2399    </author>
2400    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
2401      <organization>MIT</organization>
2402      <address>
2403        <email>hartmans-ietf@mit.edu</email>
2404      </address>
2405    </author>
2406    <date year='2007' month='June' />
2407  </front>
2408  <seriesInfo name='BCP' value='97' />
2409  <seriesInfo name='RFC' value='4897' />
2410</reference>
2411
2412
2413</references>
2414
2415<section title="Differences Between HTTP Entities and RFC 2045 Entities" anchor="differences.between.http.entities.and.rfc.2045.entities">
2416<t>
2417   HTTP/1.1 uses many of the constructs defined for Internet Mail (<xref target="RFC5322"/>) and the Multipurpose Internet Mail Extensions (MIME <xref target="RFC2045"/>) to
2418   allow entities to be transmitted in an open variety of
2419   representations and with extensible mechanisms. However, RFC 2045
2420   discusses mail, and HTTP has a few features that are different from
2421   those described in RFC 2045. These differences were carefully chosen
2422   to optimize performance over binary connections, to allow greater
2423   freedom in the use of new media types, to make date comparisons
2424   easier, and to acknowledge the practice of some early HTTP servers
2425   and clients.
2426</t>
2427<t>
2428   This appendix describes specific areas where HTTP differs from RFC
2429   2045. Proxies and gateways to strict MIME environments &SHOULD; be
2430   aware of these differences and provide the appropriate conversions
2431   where necessary. Proxies and gateways from MIME environments to HTTP
2432   also need to be aware of the differences because some conversions
2433   might be required.
2434</t>
2435
2436<section title="MIME-Version" anchor="mime-version">
2437  <iref primary="true" item="MIME-Version header" x:for-anchor=""/>
2438  <iref primary="true" item="Headers" subitem="MIME-Version" x:for-anchor=""/>
2439  <x:anchor-alias value="MIME-Version"/>
2440  <x:anchor-alias value="MIME-Version-v"/>
2441<t>
2442   HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages &MAY;
2443   include a single MIME-Version general-header field to indicate what
2444   version of the MIME protocol was used to construct the message. Use
2445   of the MIME-Version header field indicates that the message is in
2446   full compliance with the MIME protocol (as defined in <xref target="RFC2045"/>).
2447   Proxies/gateways are responsible for ensuring full compliance (where
2448   possible) when exporting HTTP messages to strict MIME environments.
2449</t>
2450<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="MIME-Version"/><iref primary="true" item="Grammar" subitem="MIME-Version-v"/>
2451  <x:ref>MIME-Version</x:ref>   = "MIME-Version" ":" <x:ref>OWS</x:ref> <x:ref>MIME-Version-v</x:ref>
2452  <x:ref>MIME-Version-v</x:ref> = 1*<x:ref>DIGIT</x:ref> "." 1*<x:ref>DIGIT</x:ref>
2453</artwork></figure>
2454<t>
2455   MIME version "1.0" is the default for use in HTTP/1.1. However,
2456   HTTP/1.1 message parsing and semantics are defined by this document
2457   and not the MIME specification.
2458</t>
2459</section>
2460
2461<section title="Conversion to Canonical Form" anchor="conversion.to.canonical.form">
2462<t>
2463   <xref target="RFC2045"/> requires that an Internet mail entity be converted to
2464   canonical form prior to being transferred, as described in <xref target="RFC2049" x:fmt="of" x:sec="4"/>.
2465   <xref target="canonicalization.and.text.defaults"/> of this document describes the forms
2466   allowed for subtypes of the "text" media type when transmitted over
2467   HTTP. <xref target="RFC2046"/> requires that content with a type of "text" represent
2468   line breaks as CRLF and forbids the use of CR or LF outside of line
2469   break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a
2470   line break within text content when a message is transmitted over
2471   HTTP.
2472</t>
2473<t>
2474   Where it is possible, a proxy or gateway from HTTP to a strict MIME
2475   environment &SHOULD; translate all line breaks within the text media
2476   types described in <xref target="canonicalization.and.text.defaults"/> of this document to the RFC 2049
2477   canonical form of CRLF. Note, however, that this might be complicated
2478   by the presence of a Content-Encoding and by the fact that HTTP
2479   allows the use of some character sets which do not use octets 13 and
2480   10 to represent CR and LF, as is the case for some multi-byte
2481   character sets.
2482</t>
2483<t>
2484   Implementors should note that conversion will break any cryptographic
2485   checksums applied to the original content unless the original content
2486   is already in canonical form. Therefore, the canonical form is
2487   recommended for any content that uses such checksums in HTTP.
2488</t>
2489</section>
2490
2491
2492<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
2493<t>
2494   HTTP/1.1 uses a restricted set of date formats (&full-date;) to
2495   simplify the process of date comparison. Proxies and gateways from
2496   other protocols &SHOULD; ensure that any Date header field present in a
2497   message conforms to one of the HTTP/1.1 formats and rewrite the date
2498   if necessary.
2499</t>
2500</section>
2501
2502<section title="Introduction of Content-Encoding" anchor="introduction.of.content-encoding">
2503<t>
2504   RFC 2045 does not include any concept equivalent to HTTP/1.1's
2505   Content-Encoding header field. Since this acts as a modifier on the
2506   media type, proxies and gateways from HTTP to MIME-compliant
2507   protocols &MUST; either change the value of the Content-Type header
2508   field or decode the entity-body before forwarding the message. (Some
2509   experimental applications of Content-Type for Internet mail have used
2510   a media-type parameter of ";conversions=&lt;content-coding&gt;" to perform
2511   a function equivalent to Content-Encoding. However, this parameter is
2512   not part of RFC 2045).
2513</t>
2514</section>
2515
2516<section title="No Content-Transfer-Encoding" anchor="no.content-transfer-encoding">
2517<t>
2518   HTTP does not use the Content-Transfer-Encoding field of RFC
2519   2045. Proxies and gateways from MIME-compliant protocols to HTTP &MUST;
2520   remove any Content-Transfer-Encoding
2521   prior to delivering the response message to an HTTP client.
2522</t>
2523<t>
2524   Proxies and gateways from HTTP to MIME-compliant protocols are
2525   responsible for ensuring that the message is in the correct format
2526   and encoding for safe transport on that protocol, where "safe
2527   transport" is defined by the limitations of the protocol being used.
2528   Such a proxy or gateway &SHOULD; label the data with an appropriate
2529   Content-Transfer-Encoding if doing so will improve the likelihood of
2530   safe transport over the destination protocol.
2531</t>
2532</section>
2533
2534<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
2535<t>
2536   HTTP/1.1 introduces the Transfer-Encoding header field (&header-transfer-encoding;).
2537   Proxies/gateways &MUST; remove any transfer-coding prior to
2538   forwarding a message via a MIME-compliant protocol.
2539</t>
2540</section>
2541
2542<section title="MHTML and Line Length Limitations" anchor="mhtml.line.length">
2543<t>
2544   HTTP implementations which share code with MHTML <xref target="RFC2557"/> implementations
2545   need to be aware of MIME line length limitations. Since HTTP does not
2546   have this limitation, HTTP does not fold long lines. MHTML messages
2547   being transported by HTTP follow all conventions of MHTML, including
2548   line length limitations and folding, canonicalization, etc., since
2549   HTTP transports all message-bodies as payload (see <xref target="multipart.types"/>) and
2550   does not interpret the content or any MIME header lines that might be
2551   contained therein.
2552</t>
2553</section>
2554</section>
2555
2556<section title="Additional Features" anchor="additional.features">
2557<t>
2558   <xref target="RFC1945"/> and <xref target="RFC2068"/> document protocol elements used by some
2559   existing HTTP implementations, but not consistently and correctly
2560   across most HTTP/1.1 applications. Implementors are advised to be
2561   aware of these features, but cannot rely upon their presence in, or
2562   interoperability with, other HTTP/1.1 applications. Some of these
2563   describe proposed experimental features, and some describe features
2564   that experimental deployment found lacking that are now addressed in
2565   the base HTTP/1.1 specification.
2566</t>
2567<t>
2568   A number of other headers, such as Content-Disposition and Title,
2569   from SMTP and MIME are also often implemented (see <xref target="RFC2076"/>).
2570</t>
2571
2572<section title="Content-Disposition" anchor="content-disposition">
2573<iref item="Headers" subitem="Content-Disposition" primary="true" x:for-anchor=""/>
2574<iref item="Content-Disposition header" primary="true" x:for-anchor=""/>
2575  <x:anchor-alias value="content-disposition"/>
2576  <x:anchor-alias value="content-disposition-v"/>
2577  <x:anchor-alias value="disposition-type"/>
2578  <x:anchor-alias value="disposition-parm"/>
2579  <x:anchor-alias value="disp-extension-parm"/>
2580  <x:anchor-alias value="disp-extension-token"/>
2581  <x:anchor-alias value="filename-parm"/>
2582<t>
2583   The Content-Disposition response-header field has been proposed as a
2584   means for the origin server to suggest a default filename if the user
2585   requests that the content is saved to a file. This usage is derived
2586   from the definition of Content-Disposition in <xref target="RFC2183"/>.
2587</t>
2588<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-disposition"/><iref primary="true" item="Grammar" subitem="content-disposition-v"/><iref primary="true" item="Grammar" subitem="disposition-type"/><iref primary="true" item="Grammar" subitem="disposition-parm"/><iref primary="true" item="Grammar" subitem="filename-parm"/><iref primary="true" item="Grammar" subitem="disp-extension-token"/><iref primary="true" item="Grammar" subitem="disp-extension-parm"/>
2589  <x:ref>content-disposition</x:ref> = "Content-Disposition" ":" <x:ref>OWS</x:ref>
2590                        <x:ref>content-disposition-v</x:ref>
2591  <x:ref>content-disposition-v</x:ref> = <x:ref>disposition-type</x:ref>
2592                          *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>disposition-parm</x:ref> )
2593  <x:ref>disposition-type</x:ref> = "attachment" / <x:ref>disp-extension-token</x:ref>
2594  <x:ref>disposition-parm</x:ref> = <x:ref>filename-parm</x:ref> / <x:ref>disp-extension-parm</x:ref>
2595  <x:ref>filename-parm</x:ref> = "filename" "=" <x:ref>quoted-string</x:ref>
2596  <x:ref>disp-extension-token</x:ref> = <x:ref>token</x:ref>
2597  <x:ref>disp-extension-parm</x:ref> = <x:ref>token</x:ref> "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> )
2598</artwork></figure>
2599<t>
2600   An example is
2601</t>
2602<figure><artwork type="example">
2603  Content-Disposition: attachment; filename="fname.ext"
2604</artwork></figure>
2605<t>
2606   The receiving user agent &SHOULD-NOT;  respect any directory path
2607   information present in the filename-parm parameter, which is the only
2608   parameter believed to apply to HTTP implementations at this time. The
2609   filename &SHOULD; be treated as a terminal component only.
2610</t>
2611<t>
2612   If this header is used in a response with the application/octet-stream
2613   content-type, the implied suggestion is that the user agent
2614   should not display the response, but directly enter a `save response
2615   as...' dialog.
2616</t>
2617<t>
2618   See <xref target="content-disposition.issues"/> for Content-Disposition security issues.
2619</t>
2620</section>
2621</section>
2622
2623<section title="Compatibility with Previous Versions" anchor="compatibility">
2624<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2625<t>
2626   Transfer-coding and message lengths all interact in ways that
2627   required fixing exactly when chunked encoding is used (to allow for
2628   transfer encoding that may not be self delimiting); it was important
2629   to straighten out exactly how message lengths are computed.
2630   (<xref target="entity.length"/>, see also <xref target="Part1"/>,
2631   <xref target="Part5"/> and <xref target="Part6"/>).
2632</t>
2633<t>
2634   Charset wildcarding is introduced to avoid explosion of character set
2635   names in accept headers. (<xref target="header.accept-charset"/>)
2636</t>
2637<t>
2638   Content-Base was deleted from the specification: it was not
2639   implemented widely, and there is no simple, safe way to introduce it
2640   without a robust extension mechanism. In addition, it is used in a
2641   similar, but not identical fashion in MHTML <xref target="RFC2557"/>.
2642</t>
2643<t>
2644   A content-coding of "identity" was introduced, to solve problems
2645   discovered in caching. (<xref target="content.codings"/>)
2646</t>
2647<t>
2648   The Alternates<iref item="Alternates header" primary="true"/><iref item="Headers" subitem="Alternate" primary="true"/>, Content-Version<iref item="Content-Version header" primary="true"/><iref item="Headers" subitem="Content-Version" primary="true"/>, Derived-From<iref item="Derived-From header" primary="true"/><iref item="Headers" subitem="Derived-From" primary="true"/>, Link<iref item="Link header" primary="true"/><iref item="Headers" subitem="Link" primary="true"/>, URI<iref item="URI header" primary="true"/><iref item="Headers" subitem="URI" primary="true"/>, Public<iref item="Public header" primary="true"/><iref item="Headers" subitem="Public" primary="true"/> and
2649   Content-Base<iref item="Content-Base header" primary="true"/><iref item="Headers" subitem="Content-Base" primary="true"/> header fields were defined in previous versions of this
2650   specification, but not commonly implemented. See <xref target="RFC2068" x:fmt="of" x:sec="19.6.2"/>.
2651</t>
2652</section>
2653
2654<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2655<t>
2656  Clarify contexts that charset is used in.
2657  (<xref target="character.sets"/>)
2658</t>
2659<t>
2660  Remove reference to non-existant identity transfer-coding value tokens.
2661  (<xref target="no.content-transfer-encoding"/>)
2662</t>
2663</section>
2664
2665</section>
2666
2667<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2668<figure>
2669<artwork type="abnf" name="p3-payload.parsed-abnf">
2670<x:ref>Accept</x:ref> = "Accept:" OWS Accept-v
2671<x:ref>Accept-Charset</x:ref> = "Accept-Charset:" OWS Accept-Charset-v
2672<x:ref>Accept-Charset-v</x:ref> = *( "," OWS ) ( charset / "*" ) [ OWS ";" OWS "q="
2673 qvalue ] *( OWS "," [ OWS ( charset / "*" ) [ OWS ";" OWS "q="
2674 qvalue ] ] )
2675<x:ref>Accept-Encoding</x:ref> = "Accept-Encoding:" OWS Accept-Encoding-v
2676<x:ref>Accept-Encoding-v</x:ref> = [ ( "," / ( codings [ OWS ";" OWS "q=" qvalue ] )
2677 ) *( OWS "," [ OWS codings [ OWS ";" OWS "q=" qvalue ] ] ) ]
2678<x:ref>Accept-Language</x:ref> = "Accept-Language:" OWS Accept-Language-v
2679<x:ref>Accept-Language-v</x:ref> = *( "," OWS ) language-range [ OWS ";" OWS "q="
2680 qvalue ] *( OWS "," [ OWS language-range [ OWS ";" OWS "q=" qvalue ]
2681 ] )
2682<x:ref>Accept-v</x:ref> = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
2683 OWS media-range [ accept-params ] ] ) ]
2684
2685<x:ref>Content-Encoding</x:ref> = "Content-Encoding:" OWS Content-Encoding-v
2686<x:ref>Content-Encoding-v</x:ref> = *( "," OWS ) content-coding *( OWS "," [ OWS
2687 content-coding ] )
2688<x:ref>Content-Language</x:ref> = "Content-Language:" OWS Content-Language-v
2689<x:ref>Content-Language-v</x:ref> = *( "," OWS ) language-tag *( OWS "," [ OWS
2690 language-tag ] )
2691<x:ref>Content-Length</x:ref> = &lt;Content-Length, defined in [Part1], Section 8.2&gt;
2692<x:ref>Content-Location</x:ref> = "Content-Location:" OWS Content-Location-v
2693<x:ref>Content-Location-v</x:ref> = absolute-URI / partial-URI
2694<x:ref>Content-MD5</x:ref> = "Content-MD5:" OWS Content-MD5-v
2695<x:ref>Content-MD5-v</x:ref> = &lt;base64 of 128 bit MD5 digest as per [RFC1864]&gt;
2696<x:ref>Content-Range</x:ref> = &lt;Content-Range, defined in [Part5], Section 5.2&gt;
2697<x:ref>Content-Type</x:ref> = "Content-Type:" OWS Content-Type-v
2698<x:ref>Content-Type-v</x:ref> = media-type
2699
2700<x:ref>Expires</x:ref> = &lt;Expires, defined in [Part6], Section 3.3&gt;
2701
2702<x:ref>Last-Modified</x:ref> = &lt;Last-Modified, defined in [Part4], Section 6.6&gt;
2703
2704<x:ref>MIME-Version</x:ref> = "MIME-Version:" OWS MIME-Version-v
2705<x:ref>MIME-Version-v</x:ref> = 1*DIGIT "." 1*DIGIT
2706
2707<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2708
2709<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [Part1], Section 2.3&gt;
2710<x:ref>accept-ext</x:ref> = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
2711<x:ref>accept-params</x:ref> = OWS ";" OWS "q=" qvalue *accept-ext
2712<x:ref>attribute</x:ref> = token
2713
2714<x:ref>charset</x:ref> = token
2715<x:ref>codings</x:ref> = ( content-coding / "*" )
2716<x:ref>content-coding</x:ref> = token
2717<x:ref>content-disposition</x:ref> = "Content-Disposition:" OWS
2718 content-disposition-v
2719<x:ref>content-disposition-v</x:ref> = disposition-type *( OWS ";" OWS
2720 disposition-parm )
2721
2722<x:ref>disp-extension-parm</x:ref> = token "=" ( token / quoted-string )
2723<x:ref>disp-extension-token</x:ref> = token
2724<x:ref>disposition-parm</x:ref> = filename-parm / disp-extension-parm
2725<x:ref>disposition-type</x:ref> = "attachment" / disp-extension-token
2726
2727<x:ref>entity-body</x:ref> = *OCTET
2728<x:ref>entity-header</x:ref> = Content-Encoding / Content-Language / Content-Length
2729 / Content-Location / Content-MD5 / Content-Range / Content-Type /
2730 Expires / Last-Modified / extension-header
2731<x:ref>extension-header</x:ref> = header-field
2732
2733<x:ref>filename-parm</x:ref> = "filename=" quoted-string
2734
2735<x:ref>language-range</x:ref> = &lt;language-range, defined in [RFC4647], Section 2.1&gt;
2736<x:ref>language-tag</x:ref> = &lt;Language-Tag, defined in [RFC4646bis], Section 2.1&gt;
2737
2738<x:ref>media-range</x:ref> = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
2739 ";" OWS parameter )
2740<x:ref>media-type</x:ref> = type "/" subtype *( OWS ";" OWS parameter )
2741<x:ref>header-field</x:ref> = &lt;header-field, defined in [Part1], Section 3.2&gt;
2742
2743<x:ref>parameter</x:ref> = attribute "=" value
2744<x:ref>partial-URI</x:ref> = &lt;partial-URI, defined in [Part1], Section 2.3&gt;
2745
2746<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2747<x:ref>qvalue</x:ref> = &lt;qvalue, defined in [Part1], Section 6.4&gt;
2748
2749<x:ref>subtype</x:ref> = token
2750
2751<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2752<x:ref>type</x:ref> = token
2753
2754<x:ref>value</x:ref> = token / quoted-string
2755</artwork>
2756</figure>
2757<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2758; Accept defined but not used
2759; Accept-Charset defined but not used
2760; Accept-Encoding defined but not used
2761; Accept-Language defined but not used
2762; MIME-Version defined but not used
2763; content-disposition defined but not used
2764; entity-body defined but not used
2765; entity-header defined but not used
2766</artwork></figure></section>
2767
2768<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
2769
2770<section title="Since RFC2616">
2771<t>
2772  Extracted relevant partitions from <xref target="RFC2616"/>.
2773</t>
2774</section>
2775
2776<section title="Since draft-ietf-httpbis-p3-payload-00">
2777<t>
2778  Closed issues:
2779  <list style="symbols"> 
2780    <t>
2781      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
2782      "Media Type Registrations"
2783      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
2784    </t>
2785    <t>
2786      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/14"/>:
2787      "Clarification regarding quoting of charset values"
2788      (<eref target="http://purl.org/NET/http-errata#charactersets"/>)
2789    </t>
2790    <t>
2791      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
2792      "Remove 'identity' token references"
2793      (<eref target="http://purl.org/NET/http-errata#identity"/>)
2794    </t>
2795    <t>
2796      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/25"/>:
2797      "Accept-Encoding BNF"
2798    </t>
2799    <t>
2800      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2801      "Normative and Informative references"
2802    </t>
2803    <t>
2804      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
2805      "RFC1700 references"
2806    </t>
2807    <t>
2808      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
2809      "Updating to RFC4288"
2810    </t>
2811    <t>
2812      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2813      "Informative references"
2814    </t>
2815    <t>
2816      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2817      "ISO-8859-1 Reference"
2818    </t>
2819    <t>
2820      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/68"/>:
2821      "Encoding References Normative"
2822    </t>
2823    <t>
2824      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2825      "Normative up-to-date references"
2826    </t>
2827  </list>
2828</t>
2829</section>
2830
2831<section title="Since draft-ietf-httpbis-p3-payload-01">
2832<t>
2833  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2834  <list style="symbols"> 
2835    <t>
2836      Add explicit references to BNF syntax and rules imported from other parts of the specification.
2837    </t>
2838  </list>
2839</t>
2840</section>
2841
2842<section title="Since draft-ietf-httpbis-p3-payload-02" anchor="changes.since.02">
2843<t>
2844  Closed issues:
2845  <list style="symbols"> 
2846    <t>
2847      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/67"/>:
2848      "Quoting Charsets"
2849    </t>
2850    <t>
2851      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/105"/>:
2852      "Classification for Allow header"
2853    </t>
2854    <t>
2855      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/115"/>:
2856      "missing default for qvalue in description of Accept-Encoding"
2857    </t>
2858  </list>
2859</t>
2860<t>
2861  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2862  <list style="symbols"> 
2863    <t>
2864      Reference RFC 3984, and update header registrations for headers defined
2865      in this document.
2866    </t>
2867  </list>
2868</t>
2869</section>
2870
2871<section title="Since draft-ietf-httpbis-p3-payload-03" anchor="changes.since.03">
2872<t>
2873  Closed issues:
2874  <list style="symbols"> 
2875    <t>
2876      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/67"/>:
2877      "Quoting Charsets"
2878    </t>
2879    <t>
2880      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/113"/>:
2881      "language tag matching (Accept-Language) vs RFC4647"
2882    </t>
2883    <t>
2884      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/121"/>:
2885      "RFC 1806 has been replaced by RFC2183"
2886    </t>
2887  </list>
2888</t>
2889<t>
2890  Other changes:
2891  <list style="symbols"> 
2892    <t>
2893      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/68"/>:
2894      "Encoding References Normative" -- rephrase the annotation and reference
2895      <xref target="BCP97"/>.
2896    </t>
2897  </list>
2898</t>
2899 </section>
2900
2901<section title="Since draft-ietf-httpbis-p3-payload-04" anchor="changes.since.04">
2902<t>
2903  Closed issues:
2904  <list style="symbols"> 
2905    <t>
2906      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
2907      "RFC 2822 is updated by RFC 5322"
2908    </t>
2909  </list>
2910</t>
2911<t>
2912  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2913  <list style="symbols"> 
2914    <t>
2915      Use "/" instead of "|" for alternatives.
2916    </t>
2917    <t>
2918      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2919      whitespace ("OWS") and required whitespace ("RWS").
2920    </t>
2921    <t>
2922      Rewrite ABNFs to spell out whitespace rules, factor out
2923      header value format definitions.
2924    </t>
2925  </list>
2926</t>
2927</section>
2928
2929<section title="Since draft-ietf-httpbis-p3-payload-05" anchor="changes.since.05">
2930<t>
2931  Closed issues:
2932  <list style="symbols"> 
2933    <t>
2934      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/118"/>:
2935      "Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
2936    </t>
2937  </list>
2938</t>
2939<t>
2940  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2941  <list style="symbols"> 
2942    <t>
2943      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2944    </t>
2945  </list>
2946</t>
2947<t>
2948  Other changes:
2949  <list style="symbols"> 
2950    <t>
2951      Move definition of quality values into Part 1.
2952    </t>
2953  </list>
2954</t>
2955</section>
2956
2957<section title="Since draft-ietf-httpbis-p3-payload-06" anchor="changes.since.06">
2958<t>
2959  Closed issues:
2960  <list style="symbols"> 
2961    <t>
2962      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/80"/>:
2963      "Content-Location isn't special"
2964    </t>
2965    <t>
2966      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/155"/>:
2967      "Content Sniffing"
2968    </t>
2969  </list>
2970</t>
2971</section>
2972
2973<section title="Since draft-ietf-httpbis-p3-payload-07" anchor="changes.since.07">
2974<t>
2975  Closed issues:
2976  <list style="symbols"> 
2977    <t>
2978      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/13"/>:
2979      "Updated reference for language tags"
2980    </t>
2981  </list>
2982</t>
2983</section>
2984
2985</section>
2986
2987</back>
2988</rfc>
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