source: draft-ietf-httpbis/latest/p3-payload.xml @ 839

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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 "2010">
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  <!ENTITY effective-request-uri    "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY compression-codings      "<xref target='Part1' x:rel='#compression.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY transfer-codings         "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY compress-coding          "<xref target='Part1' x:rel='#compress.coding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY deflate-coding           "<xref target='Part1' x:rel='#deflate.coding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY gzip-coding              "<xref target='Part1' x:rel='#gzip.coding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY response-representation  "<xref target='Part2' x:rel='#identifying.response.associated.with.representation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42]>
43<?rfc toc="yes" ?>
44<?rfc symrefs="yes" ?>
45<?rfc sortrefs="yes" ?>
46<?rfc compact="yes"?>
47<?rfc subcompact="no" ?>
48<?rfc linkmailto="no" ?>
49<?rfc editing="no" ?>
50<?rfc comments="yes"?>
51<?rfc inline="yes"?>
52<?rfc rfcedstyle="yes"?>
53<?rfc-ext allow-markup-in-artwork="yes" ?>
54<?rfc-ext include-references-in-index="yes" ?>
55<rfc obsoletes="2616" category="std" x:maturity-level="draft"
56     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p3-payload-&ID-VERSION;"
57     xmlns:x='http://purl.org/net/xml2rfc/ext'>
58<front>
59
60  <title abbrev="HTTP/1.1, Part 3">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
61
62  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
63    <organization abbrev="Day Software">Day Software</organization>
64    <address>
65      <postal>
66        <street>23 Corporate Plaza DR, Suite 280</street>
67        <city>Newport Beach</city>
68        <region>CA</region>
69        <code>92660</code>
70        <country>USA</country>
71      </postal>
72      <phone>+1-949-706-5300</phone>
73      <facsimile>+1-949-706-5305</facsimile>
74      <email>fielding@gbiv.com</email>
75      <uri>http://roy.gbiv.com/</uri>
76    </address>
77  </author>
78
79  <author initials="J." surname="Gettys" fullname="Jim Gettys">
80    <organization>One Laptop per Child</organization>
81    <address>
82      <postal>
83        <street>21 Oak Knoll Road</street>
84        <city>Carlisle</city>
85        <region>MA</region>
86        <code>01741</code>
87        <country>USA</country>
88      </postal>
89      <email>jg@laptop.org</email>
90      <uri>http://www.laptop.org/</uri>
91    </address>
92  </author>
93 
94  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
95    <organization abbrev="HP">Hewlett-Packard Company</organization>
96    <address>
97      <postal>
98        <street>HP Labs, Large Scale Systems Group</street>
99        <street>1501 Page Mill Road, MS 1177</street>
100        <city>Palo Alto</city>
101        <region>CA</region>
102        <code>94304</code>
103        <country>USA</country>
104      </postal>
105      <email>JeffMogul@acm.org</email>
106    </address>
107  </author>
108
109  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
110    <organization abbrev="Microsoft">Microsoft Corporation</organization>
111    <address>
112      <postal>
113        <street>1 Microsoft Way</street>
114        <city>Redmond</city>
115        <region>WA</region>
116        <code>98052</code>
117        <country>USA</country>
118      </postal>
119      <email>henrikn@microsoft.com</email>
120    </address>
121  </author>
122
123  <author initials="L." surname="Masinter" fullname="Larry Masinter">
124    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
125    <address>
126      <postal>
127        <street>345 Park Ave</street>
128        <city>San Jose</city>
129        <region>CA</region>
130        <code>95110</code>
131        <country>USA</country>
132      </postal>
133      <email>LMM@acm.org</email>
134      <uri>http://larry.masinter.net/</uri>
135    </address>
136  </author>
137 
138  <author initials="P." surname="Leach" fullname="Paul J. Leach">
139    <organization abbrev="Microsoft">Microsoft Corporation</organization>
140    <address>
141      <postal>
142        <street>1 Microsoft Way</street>
143        <city>Redmond</city>
144        <region>WA</region>
145        <code>98052</code>
146      </postal>
147      <email>paulle@microsoft.com</email>
148    </address>
149  </author>
150   
151  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
152    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
153    <address>
154      <postal>
155        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
156        <street>The Stata Center, Building 32</street>
157        <street>32 Vassar Street</street>
158        <city>Cambridge</city>
159        <region>MA</region>
160        <code>02139</code>
161        <country>USA</country>
162      </postal>
163      <email>timbl@w3.org</email>
164      <uri>http://www.w3.org/People/Berners-Lee/</uri>
165    </address>
166  </author>
167
168  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
169    <organization abbrev="W3C">World Wide Web Consortium</organization>
170    <address>
171      <postal>
172        <street>W3C / ERCIM</street>
173        <street>2004, rte des Lucioles</street>
174        <city>Sophia-Antipolis</city>
175        <region>AM</region>
176        <code>06902</code>
177        <country>France</country>
178      </postal>
179      <email>ylafon@w3.org</email>
180      <uri>http://www.raubacapeu.net/people/yves/</uri>
181    </address>
182  </author>
183
184  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
185    <organization abbrev="greenbytes">greenbytes GmbH</organization>
186    <address>
187      <postal>
188        <street>Hafenweg 16</street>
189        <city>Muenster</city><region>NW</region><code>48155</code>
190        <country>Germany</country>
191      </postal>
192      <phone>+49 251 2807760</phone>
193      <facsimile>+49 251 2807761</facsimile>
194      <email>julian.reschke@greenbytes.de</email>
195      <uri>http://greenbytes.de/tech/webdav/</uri>
196    </address>
197  </author>
198
199  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
200  <workgroup>HTTPbis Working Group</workgroup>
201
202<abstract>
203<t>
204   The Hypertext Transfer Protocol (HTTP) is an application-level
205   protocol for distributed, collaborative, hypermedia information
206   systems. HTTP has been in use by the World Wide Web global information
207   initiative since 1990. This document is Part 3 of the seven-part specification
208   that defines the protocol referred to as "HTTP/1.1" and, taken together,
209   obsoletes RFC 2616.  Part 3 defines HTTP message content,
210   metadata, and content negotiation.
211</t>
212</abstract>
213
214<note title="Editorial Note (To be removed by RFC Editor)">
215  <t>
216    Discussion of this draft should take place on the HTTPBIS working group
217    mailing list (ietf-http-wg@w3.org). The current issues list is
218    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
219    and related documents (including fancy diffs) can be found at
220    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
221  </t>
222  <t>
223    The changes in this draft are summarized in <xref target="changes.since.09"/>.
224  </t>
225</note>
226</front>
227<middle>
228<section title="Introduction" anchor="introduction">
229<t>
230   This document defines HTTP/1.1 message payloads (a.k.a., content), the
231   associated metadata header fields that define how the payload is intended
232   to be interpreted by a recipient, the request header fields that
233   may influence content selection, and the various selection algorithms
234   that are collectively referred to as HTTP content negotiation.
235</t>
236<t>
237   This document is currently disorganized in order to minimize the changes
238   between drafts and enable reviewers to see the smaller errata changes.
239   The next draft will reorganize the sections to better reflect the content.
240   In particular, the sections on entities will be renamed payload and moved
241   to the first half of the document, while the sections on content negotiation
242   and associated request header fields will be moved to the second half.  The
243   current mess reflects how widely dispersed these topics and associated
244   requirements had become in <xref target="RFC2616"/>.
245</t>
246
247<section title="Terminology" anchor="terminology">
248<t>
249   This specification uses a number of terms to refer to the roles
250   played by participants in, and objects of, the HTTP communication.
251</t>
252<t>
253  <iref item="content negotiation"/>
254  <x:dfn>content negotiation</x:dfn>
255  <list>
256    <t>
257      The mechanism for selecting the appropriate representation when
258      servicing a request. The representation of entities in any response
259      can be negotiated (including error responses).
260    </t>
261  </list>
262</t>
263<t>
264  <iref item="entity"/>
265  <x:dfn>entity</x:dfn>
266  <list>
267    <t>
268      The information transferred as the payload of a request or
269      response. An entity consists of metadata in the form of
270      entity-header fields and content in the form of an entity-body.
271    </t>
272  </list>
273</t>
274<t>
275  <iref item="representation"/>
276  <x:dfn>representation</x:dfn>
277  <list>
278    <t>
279      An entity included with a response that is subject to content
280      negotiation. There may exist multiple
281      representations associated with a particular response status.
282    </t>
283  </list>
284</t>
285<t>
286  <iref item="variant"/>
287  <x:dfn>variant</x:dfn>
288  <list>
289    <t>
290      A resource may have one, or more than one, representation(s)
291      associated with it at any given instant. Each of these
292      representations is termed a "variant".  Use of the term "variant"
293      does not necessarily imply that the resource is subject to content
294      negotiation.
295    </t>
296  </list>
297</t>
298</section>
299
300<section title="Requirements" anchor="intro.requirements">
301<t>
302   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
303   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
304   document are to be interpreted as described in <xref target="RFC2119"/>.
305</t>
306<t>
307   An implementation is not compliant if it fails to satisfy one or more
308   of the &MUST; or &REQUIRED; level requirements for the protocols it
309   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
310   level and all the &SHOULD; level requirements for its protocols is said
311   to be "unconditionally compliant"; one that satisfies all the &MUST;
312   level requirements but not all the &SHOULD; level requirements for its
313   protocols is said to be "conditionally compliant."
314</t>
315</section>
316
317<section title="Syntax Notation" anchor="notation">
318  <x:anchor-alias value="ALPHA"/>
319  <x:anchor-alias value="CR"/>
320  <x:anchor-alias value="DIGIT"/>
321  <x:anchor-alias value="LF"/>
322  <x:anchor-alias value="OCTET"/>
323  <x:anchor-alias value="VCHAR"/>
324  <x:anchor-alias value="WSP"/>
325<t>
326  This specification uses the ABNF syntax defined in &notation; (which
327  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
328  <xref target="collected.abnf"/> shows the collected ABNF, with the list
329  rule expanded.
330</t>
331<t>
332  The following core rules are included by
333  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
334  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
335  DIGIT (decimal 0-9), DQUOTE (double quote),
336  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
337  OCTET (any 8-bit sequence of data), SP (space),
338  VCHAR (any visible USASCII character),
339  and WSP (whitespace).
340</t>
341
342<section title="Core Rules" anchor="core.rules">
343  <x:anchor-alias value="quoted-string"/>
344  <x:anchor-alias value="token"/>
345  <x:anchor-alias value="word"/>
346  <x:anchor-alias value="OWS"/>
347<t>
348  The core rules below are defined in &basic-rules;:
349</t>
350<figure><artwork type="abnf2616">
351  <x:ref>quoted-string</x:ref>  = &lt;quoted-string, defined in &basic-rules;&gt;
352  <x:ref>token</x:ref>          = &lt;token, defined in &basic-rules;&gt;
353  <x:ref>word</x:ref>           = &lt;word, defined in &basic-rules;&gt;
354  <x:ref>OWS</x:ref>            = &lt;OWS, defined in &basic-rules;&gt;
355</artwork></figure>
356</section>
357
358<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
359  <x:anchor-alias value="absolute-URI"/>
360  <x:anchor-alias value="Allow"/>
361  <x:anchor-alias value="Content-Length"/>
362  <x:anchor-alias value="Content-Range"/>
363  <x:anchor-alias value="Expires"/>
364  <x:anchor-alias value="Last-Modified"/>
365  <x:anchor-alias value="header-field"/>
366  <x:anchor-alias value="partial-URI"/>
367  <x:anchor-alias value="qvalue"/>
368<t>
369  The ABNF rules below are defined in other parts:
370</t>
371<figure><!--Part1--><artwork type="abnf2616">
372  <x:ref>absolute-URI</x:ref>   = &lt;absolute-URI, defined in &uri;&gt;
373  <x:ref>Content-Length</x:ref> = &lt;Content-Length, defined in &header-content-length;&gt;
374  <x:ref>header-field</x:ref>   = &lt;header-field, defined in &header-fields;&gt;
375  <x:ref>partial-URI</x:ref>    = &lt;partial-URI, defined in &uri;&gt;
376  <x:ref>qvalue</x:ref>         = &lt;qvalue, defined in &qvalue;&gt;
377</artwork></figure>
378<figure><!--Part4--><artwork type="abnf2616">
379  <x:ref>Last-Modified</x:ref>  = &lt;Last-Modified, defined in &header-last-modified;&gt;
380</artwork></figure>
381<figure><!--Part5--><artwork type="abnf2616">
382  <x:ref>Content-Range</x:ref>  = &lt;Content-Range, defined in &header-content-range;&gt;
383</artwork></figure>
384<figure><!--Part6--><artwork type="abnf2616">
385  <x:ref>Expires</x:ref>        = &lt;Expires, defined in &header-expires;&gt;
386</artwork></figure>
387</section>
388
389</section>
390
391</section>
392
393<section title="Protocol Parameters" anchor="protocol.parameters">
394
395<section title="Character Sets" anchor="character.sets">
396<t>
397   HTTP uses the same definition of the term "character set" as that
398   described for MIME:
399</t>
400<t>
401   The term "character set" is used in this document to refer to a
402   method used with one or more tables to convert a sequence of octets
403   into a sequence of characters. Note that unconditional conversion in
404   the other direction is not required, in that not all characters may
405   be available in a given character set and a character set may provide
406   more than one sequence of octets to represent a particular character.
407   This definition is intended to allow various kinds of character
408   encoding, from simple single-table mappings such as US-ASCII to
409   complex table switching methods such as those that use ISO-2022's
410   techniques. However, the definition associated with a MIME character
411   set name &MUST; fully specify the mapping to be performed from octets
412   to characters. In particular, use of external profiling information
413   to determine the exact mapping is not permitted.
414</t>
415<x:note>
416  <t>
417    <x:h>Note:</x:h> This use of the term "character set" is more commonly
418    referred to as a "character encoding." However, since HTTP and
419    MIME share the same registry, it is important that the terminology
420    also be shared.
421  </t>
422</x:note>
423<t anchor="rule.charset">
424  <x:anchor-alias value="charset"/>
425   HTTP character sets are identified by case-insensitive tokens. The
426   complete set of tokens is defined by the IANA Character Set registry
427   (<eref target="http://www.iana.org/assignments/character-sets"/>).
428</t>
429<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="charset"/>
430  <x:ref>charset</x:ref> = <x:ref>token</x:ref>
431</artwork></figure>
432<t>
433   Although HTTP allows an arbitrary token to be used as a charset
434   value, any token that has a predefined value within the IANA
435   Character Set registry &MUST; represent the character set defined
436   by that registry. Applications &SHOULD; limit their use of character
437   sets to those defined by the IANA registry.
438</t>
439<t>
440   HTTP uses charset in two contexts: within an Accept-Charset request
441   header (in which the charset value is an unquoted token) and as the
442   value of a parameter in a Content-Type header (within a request or
443   response), in which case the parameter value of the charset parameter
444   may be quoted.
445</t>
446<t>
447   Implementors should be aware of IETF character set requirements <xref target="RFC3629"/>
448   <xref target="RFC2277"/>.
449</t>
450
451<section title="Missing Charset" anchor="missing.charset">
452<t>
453   Some HTTP/1.0 software has interpreted a Content-Type header without
454   charset parameter incorrectly to mean "recipient should guess."
455   Senders wishing to defeat this behavior &MAY; include a charset
456   parameter even when the charset is ISO-8859-1 (<xref target="ISO-8859-1"/>) and &SHOULD; do so when
457   it is known that it will not confuse the recipient.
458</t>
459<t>
460   Unfortunately, some older HTTP/1.0 clients did not deal properly with
461   an explicit charset parameter. HTTP/1.1 recipients &MUST; respect the
462   charset label provided by the sender; and those user agents that have
463   a provision to "guess" a charset &MUST; use the charset from the
464   content-type field if they support that charset, rather than the
465   recipient's preference, when initially displaying a document. See
466   <xref target="canonicalization.and.text.defaults"/>.
467</t>
468</section>
469</section>
470
471<section title="Content Codings" anchor="content.codings">
472  <x:anchor-alias value="content-coding"/>
473<t>
474   Content coding values indicate an encoding transformation that has
475   been or can be applied to an entity. Content codings are primarily
476   used to allow a document to be compressed or otherwise usefully
477   transformed without losing the identity of its underlying media type
478   and without loss of information. Frequently, the entity is stored in
479   coded form, transmitted directly, and only decoded by the recipient.
480</t>
481<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-coding"/>
482  <x:ref>content-coding</x:ref>   = <x:ref>token</x:ref>
483</artwork></figure>
484<t>
485   All content-coding values are case-insensitive. HTTP/1.1 uses
486   content-coding values in the Accept-Encoding (<xref target="header.accept-encoding"/>) and
487   Content-Encoding (<xref target="header.content-encoding"/>) header fields. Although the value
488   describes the content-coding, what is more important is that it
489   indicates what decoding mechanism will be required to remove the
490   encoding.
491</t>
492<t>
493   compress<iref item="compress (Coding Format)"/><iref item="Coding Format" subitem="compress"/>
494  <list>
495    <t>
496      See &compress-coding;.
497    </t>
498  </list>
499</t>
500<t>
501   deflate<iref item="deflate (Coding Format)"/><iref item="Coding Format" subitem="deflate"/>
502  <list>
503    <t>
504      See &deflate-coding;.
505    </t>
506  </list>
507</t>
508<t>
509   gzip<iref item="gzip (Coding Format)"/><iref item="Coding Format" subitem="gzip"/>
510  <list>
511    <t>
512      See &gzip-coding;.
513    </t>
514  </list>
515</t>
516<t>
517   identity<iref item="identity (Coding Format)"/><iref item="Coding Format" subitem="identity"/>
518  <list><t>
519        The default (identity) encoding; the use of no transformation
520        whatsoever. This content-coding is used only in the Accept-Encoding
521        header, and &SHOULD-NOT;  be used in the Content-Encoding
522        header.
523  </t></list>
524</t>
525
526<section title="Content Coding Registry" anchor="content.coding.registry">
527<t>
528   The HTTP Content Coding Registry defines the name space for the content
529   coding names.
530</t>
531<t>
532   Registrations &MUST; include the following fields:
533   <list style="symbols">
534     <t>Name</t>
535     <t>Description</t>
536     <t>Pointer to specification text</t>
537   </list>
538</t>
539<t>
540   Names of content codings &MUST-NOT; overlap with names of transfer codings
541   (&transfer-codings;), unless the encoding transformation is identical (as it
542   is the case for the compression codings defined in
543   &compression-codings;).
544</t>
545<t>
546   Values to be added to this name space require expert review and a specification
547   (see "Expert Review" and "Specification Required" in
548   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
549   conform to the purpose of content coding defined in this section.
550</t>
551<t>
552   The registry itself is maintained at
553   <eref target="http://www.iana.org/assignments/http-parameters"/>.
554</t>
555</section>
556
557</section>
558
559<section title="Media Types" anchor="media.types">
560  <x:anchor-alias value="media-type"/>
561  <x:anchor-alias value="type"/>
562  <x:anchor-alias value="subtype"/>
563<t>
564   HTTP uses Internet Media Types <xref target="RFC2046"/> in the Content-Type (<xref target="header.content-type"/>)
565   and Accept (<xref target="header.accept"/>) header fields in order to provide
566   open and extensible data typing and type negotiation.
567</t>
568<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"/>
569  <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> )
570  <x:ref>type</x:ref>       = <x:ref>token</x:ref>
571  <x:ref>subtype</x:ref>    = <x:ref>token</x:ref>
572</artwork></figure>
573<t anchor="rule.parameter">
574  <x:anchor-alias value="attribute"/>
575  <x:anchor-alias value="parameter"/>
576  <x:anchor-alias value="value"/>
577   Parameters &MAY; follow the type/subtype in the form of attribute/value
578   pairs.
579</t>
580<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"/>
581  <x:ref>parameter</x:ref>      = <x:ref>attribute</x:ref> "=" <x:ref>value</x:ref>
582  <x:ref>attribute</x:ref>      = <x:ref>token</x:ref>
583  <x:ref>value</x:ref>          = <x:ref>word</x:ref>
584</artwork></figure>
585<t>
586   The type, subtype, and parameter attribute names are case-insensitive.
587   Parameter values might or might not be case-sensitive, depending on the
588   semantics of the parameter name.  The presence or absence of a parameter might
589   be significant to the processing of a media-type, depending on its
590   definition within the media type registry.
591</t>
592<t>
593   A parameter value that matches the <x:ref>token</x:ref> production may be
594   transmitted as either a token or within a quoted-string. The quoted and
595   unquoted values are equivalent.
596</t>
597<t>
598   Note that some older HTTP applications do not recognize media type
599   parameters. When sending data to older HTTP applications,
600   implementations &SHOULD; only use media type parameters when they are
601   required by that type/subtype definition.
602</t>
603<t>
604   Media-type values are registered with the Internet Assigned Number
605   Authority (IANA). The media type registration process is
606   outlined in <xref target="RFC4288"/>. Use of non-registered media types is
607   discouraged.
608</t>
609
610<section title="Canonicalization and Text Defaults" anchor="canonicalization.and.text.defaults">
611<t>
612   Internet media types are registered with a canonical form. An
613   entity-body transferred via HTTP messages &MUST; be represented in the
614   appropriate canonical form prior to its transmission except for
615   "text" types, as defined in the next paragraph.
616</t>
617<t>
618   When in canonical form, media subtypes of the "text" type use CRLF as
619   the text line break. HTTP relaxes this requirement and allows the
620   transport of text media with plain CR or LF alone representing a line
621   break when it is done consistently for an entire entity-body. HTTP
622   applications &MUST; accept CRLF, bare CR, and bare LF as being
623   representative of a line break in text media received via HTTP. In
624   addition, if the text is represented in a character set that does not
625   use octets 13 and 10 for CR and LF respectively, as is the case for
626   some multi-byte character sets, HTTP allows the use of whatever octet
627   sequences are defined by that character set to represent the
628   equivalent of CR and LF for line breaks. This flexibility regarding
629   line breaks applies only to text media in the entity-body; a bare CR
630   or LF &MUST-NOT; be substituted for CRLF within any of the HTTP control
631   structures (such as header fields and multipart boundaries).
632</t>
633<t>
634   If an entity-body is encoded with a content-coding, the underlying
635   data &MUST; be in a form defined above prior to being encoded.
636</t>
637<t>
638   The "charset" parameter is used with some media types to define the
639   character set (<xref target="character.sets"/>) of the data. When no explicit charset
640   parameter is provided by the sender, media subtypes of the "text"
641   type are defined to have a default charset value of "ISO-8859-1" when
642   received via HTTP. Data in character sets other than "ISO-8859-1" or
643   its subsets &MUST; be labeled with an appropriate charset value. See
644   <xref target="missing.charset"/> for compatibility problems.
645</t>
646</section>
647
648<section title="Multipart Types" anchor="multipart.types">
649<t>
650   MIME provides for a number of "multipart" types -- encapsulations of
651   one or more entities within a single message-body. All multipart
652   types share a common syntax, as defined in <xref target="RFC2046" x:sec="5.1.1" x:fmt="of"/>,
653   and &MUST; include a boundary parameter as part of the media type
654   value. The message body is itself a protocol element and &MUST;
655   therefore use only CRLF to represent line breaks between body-parts.
656   Unlike in RFC 2046, the epilogue of any multipart message &MUST; be
657   empty; HTTP applications &MUST-NOT; transmit the epilogue (even if the
658   original multipart contains an epilogue). These restrictions exist in
659   order to preserve the self-delimiting nature of a multipart message-body,
660   wherein the "end" of the message-body is indicated by the
661   ending multipart boundary.
662</t>
663<t>
664   In general, HTTP treats a multipart message-body no differently than
665   any other media type: strictly as payload. The one exception is the
666   "multipart/byteranges" type (&multipart-byteranges;) when it appears in a 206
667   (Partial Content) response.
668   <!-- jre: re-insert removed text pointing to caching? -->
669   In all
670   other cases, an HTTP user agent &SHOULD; follow the same or similar
671   behavior as a MIME user agent would upon receipt of a multipart type.
672   The MIME header fields within each body-part of a multipart message-body
673   do not have any significance to HTTP beyond that defined by
674   their MIME semantics.
675</t>
676<t>
677   In general, an HTTP user agent &SHOULD; follow the same or similar
678   behavior as a MIME user agent would upon receipt of a multipart type.
679   If an application receives an unrecognized multipart subtype, the
680   application &MUST; treat it as being equivalent to "multipart/mixed".
681</t>
682<x:note>
683  <t>
684    <x:h>Note:</x:h> The "multipart/form-data" type has been specifically defined
685    for carrying form data suitable for processing via the POST
686    request method, as described in <xref target="RFC2388"/>.
687  </t>
688</x:note>
689</section>
690</section>
691
692<section title="Language Tags" anchor="language.tags">
693  <x:anchor-alias value="language-tag"/>
694<t>
695   A language tag, as defined in <xref target="RFC5646"/>, identifies a
696   natural language spoken, written, or otherwise conveyed by human beings for
697   communication of information to other human beings. Computer languages are
698   explicitly excluded. HTTP uses language tags within the Accept-Language and
699   Content-Language fields.
700</t>
701<t>
702   In summary, a language tag is composed of one or more parts: A primary
703   language subtag followed by a possibly empty series of subtags:
704</t>
705<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="language-tag"/>
706  <x:ref>language-tag</x:ref> = &lt;Language-Tag, defined in <xref target="RFC5646" x:sec="2.1"/>&gt;
707</artwork></figure>
708<t>
709   White space is not allowed within the tag and all tags are case-insensitive.
710   The name space of language subtags is administered by the IANA (see
711   <eref target="http://www.iana.org/assignments/language-subtag-registry"/>).
712</t>
713<figure>
714  <preamble>Example tags include:</preamble>
715<artwork type="example">
716  en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
717</artwork>
718</figure>
719<t>
720   See <xref target="RFC5646"/> for further information.
721</t>
722</section>
723</section>
724
725<section title="Entity" anchor="entity">
726<t>
727   Request and Response messages &MAY; transfer an entity if not otherwise
728   restricted by the request method or response status code. An entity
729   consists of entity-header fields and an entity-body, although some
730   responses will only include the entity-headers.
731</t>
732<t>
733   In this section, both sender and recipient refer to either the client
734   or the server, depending on who sends and who receives the entity.
735</t>
736
737<section title="Entity Header Fields" anchor="entity.header.fields">
738  <x:anchor-alias value="entity-header"/>
739  <x:anchor-alias value="extension-header"/>
740<t>
741   Entity-header fields define metainformation about the entity-body or,
742   if no body is present, about the resource identified by the request.
743</t>
744<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-header"/><iref primary="true" item="Grammar" subitem="extension-header"/>
745  <x:ref>entity-header</x:ref>  = <x:ref>Content-Encoding</x:ref>         ; <xref target="header.content-encoding"/>
746                 / <x:ref>Content-Language</x:ref>         ; <xref target="header.content-language"/>
747                 / <x:ref>Content-Length</x:ref>           ; &header-content-length;
748                 / <x:ref>Content-Location</x:ref>         ; <xref target="header.content-location"/>
749                 / <x:ref>Content-MD5</x:ref>              ; <xref target="header.content-md5"/>
750                 / <x:ref>Content-Range</x:ref>            ; &header-content-range;
751                 / <x:ref>Content-Type</x:ref>             ; <xref target="header.content-type"/>
752                 / <x:ref>Expires</x:ref>                  ; &header-expires;
753                 / <x:ref>Last-Modified</x:ref>            ; &header-last-modified;
754                 / <x:ref>extension-header</x:ref>
755 
756  <x:ref>extension-header</x:ref> = <x:ref>header-field</x:ref>
757</artwork></figure>
758<t>
759   The extension-header mechanism allows additional entity-header fields
760   to be defined without changing the protocol, but these fields cannot
761   be assumed to be recognizable by the recipient. Unrecognized header
762   fields &SHOULD; be ignored by the recipient and &MUST; be forwarded by
763   transparent proxies.
764</t>
765</section>
766
767<section title="Entity Body" anchor="entity.body">
768  <x:anchor-alias value="entity-body"/>
769<t>
770   The entity-body (if any) sent with an HTTP request or response is in
771   a format and encoding defined by the entity-header fields.
772</t>
773<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-body"/>
774  <x:ref>entity-body</x:ref>    = *<x:ref>OCTET</x:ref>
775</artwork></figure>
776<t>
777   An entity-body is only present in a message when a message-body is
778   present, as described in &message-body;. The entity-body is obtained
779   from the message-body by decoding any Transfer-Encoding that might
780   have been applied to ensure safe and proper transfer of the message.
781</t>
782
783<section title="Type" anchor="type">
784<t>
785   When an entity-body is included with a message, the data type of that
786   body is determined via the header fields Content-Type and Content-Encoding.
787   These define a two-layer, ordered encoding model:
788</t>
789<figure><artwork type="example">
790  entity-body := Content-Encoding( Content-Type( data ) )
791</artwork></figure>
792<t>
793   Content-Type specifies the media type of the underlying data. Any HTTP/1.1
794   message containing an entity-body &SHOULD; include a Content-Type header
795   field defining the media type of that body, unless that information is
796   unknown.
797</t>
798<t>   
799   If the Content-Type header field is not present, it indicates that
800   the sender does not know the media type of the data; recipients &MAY;
801   either assume that it is "application/octet-stream" (<xref target="RFC2046" x:fmt="," x:sec="4.5.1"/>)
802   or examine the content to determine its type.
803</t>
804<t>
805   In practice, currently-deployed servers sometimes provide a Content-Type
806   header which does not correctly convey the intended interpretation of the
807   content sent, with the result that some clients will examine the response
808   body's content and override the specified type.
809</t>
810<t>
811   Client that do so risk drawing incorrect conclusions, which may expose
812   additional security risks (e.g., "privilege escalation"). Implementers are
813   encouraged to provide a means of disabling such "content sniffing" when it
814   is used.
815</t>
816<t>
817   Content-Encoding may be used to indicate any additional content
818   codings applied to the data, usually for the purpose of data
819   compression, that are a property of the requested resource.  There is
820   no default encoding.
821</t>
822</section>
823   
824<section title="Entity Length" anchor="entity.length">
825<t>
826   The entity-length of a message is the length of the message-body
827   before any transfer-codings have been applied. &message-length; defines
828   how the transfer-length of a message-body is determined.
829</t>
830</section>
831</section>
832</section>
833
834<section title="Content Negotiation" anchor="content.negotiation">
835<t>
836   HTTP responses include a representation which contains information for
837   interpretation, whether by a human user or for further processing.
838   Often, the server has different ways of representing the
839   same information; for example, in different formats, languages,
840   or using different character encodings.
841</t>
842<t>
843   HTTP clients and their users might have different or variable
844   capabilities, characteristics or preferences which would influence
845   which representation, among those available from the server,
846   would be best for the server to deliver. For this reason, HTTP
847   provides mechanisms for "content negotiation" -- a process of
848   allowing selection of a representation of a given resource,
849   when more than one is available.
850</t>
851<t>
852   This specification defines two patterns of content negotiation;
853   "server-driven", where the server selects the representation based
854   upon the client's stated preferences, and "agent-driven" negotiation,
855   where the server provides a list of representations for the client to
856   choose from, based upon their metadata. In addition,  there are
857   other patterns: some applications use an "active content" pattern,
858   where the server returns active content which runs on the client
859   and, based on client available parameters, selects additional
860   resources to invoke. "Transparent Content Negotiation" (<xref target="RFC2295"/>)
861   has also been proposed.
862</t>
863<t>
864   These patterns are all widely used, and have trade-offs in applicability
865   and practicality. In particular, when the number of preferences or
866   capabilities to be expressed by a client are large (such as when many
867   different formats are supported by a user-agent), server-driven
868   negotiation becomes unwieldy, and may not be appropriate. Conversely,
869   when the number of representations to choose from is very large,
870   agent-driven negotiation may not be appropriate.
871</t>
872<t>
873   Note that in all cases, the supplier of representations has the
874   responsibility for determining which representations might be
875   considered to be the "same information".
876</t>
877
878<section title="Server-driven Negotiation" anchor="server-driven.negotiation">
879<t>
880   If the selection of the best representation for a response is made by
881   an algorithm located at the server, it is called server-driven
882   negotiation. Selection is based on the available representations of
883   the response (the dimensions over which it can vary; e.g., language,
884   content-coding, etc.) and the contents of particular header fields in
885   the request message or on other information pertaining to the request
886   (such as the network address of the client).
887</t>
888<t>
889   Server-driven negotiation is advantageous when the algorithm for
890   selecting from among the available representations is difficult to
891   describe to the user agent, or when the server desires to send its
892   "best guess" to the client along with the first response (hoping to
893   avoid the round-trip delay of a subsequent request if the "best
894   guess" is good enough for the user). In order to improve the server's
895   guess, the user agent &MAY; include request header fields (Accept,
896   Accept-Language, Accept-Encoding, etc.) which describe its
897   preferences for such a response.
898</t>
899<t>
900   Server-driven negotiation has disadvantages:
901  <list style="numbers">
902    <t>
903         It is impossible for the server to accurately determine what
904         might be "best" for any given user, since that would require
905         complete knowledge of both the capabilities of the user agent
906         and the intended use for the response (e.g., does the user want
907         to view it on screen or print it on paper?).
908    </t>
909    <t>
910         Having the user agent describe its capabilities in every
911         request can be both very inefficient (given that only a small
912         percentage of responses have multiple representations) and a
913         potential violation of the user's privacy.
914    </t>
915    <t>
916         It complicates the implementation of an origin server and the
917         algorithms for generating responses to a request.
918    </t>
919    <t>
920         It may limit a public cache's ability to use the same response
921         for multiple user's requests.
922    </t>
923  </list>
924</t>
925<t>
926   HTTP/1.1 includes the following request-header fields for enabling
927   server-driven negotiation through description of user agent
928   capabilities and user preferences: Accept (<xref target="header.accept"/>), Accept-Charset
929   (<xref target="header.accept-charset"/>), Accept-Encoding (<xref target="header.accept-encoding"/>), Accept-Language
930   (<xref target="header.accept-language"/>), and User-Agent (&header-user-agent;).
931   However, an origin server is not limited to these dimensions and &MAY; vary
932   the response based on any aspect of the request, including information
933   outside the request-header fields or within extension header fields
934   not defined by this specification.
935</t>
936<x:note>
937  <t>
938    <x:h>Note:</x:h> In practice, User-Agent based negotiation is fragile,
939    because new clients might not be recognized.
940  </t>
941</x:note>
942<t>
943   The Vary header field (&header-vary;) can be used to express the parameters the
944   server uses to select a representation that is subject to server-driven
945   negotiation.
946</t>
947</section>
948
949<section title="Agent-driven Negotiation" anchor="agent-driven.negotiation">
950<t>
951   With agent-driven negotiation, selection of the best representation
952   for a response is performed by the user agent after receiving an
953   initial response from the origin server. Selection is based on a list
954   of the available representations of the response included within the
955   header fields or entity-body of the initial response, with each
956   representation identified by its own URI. Selection from among the
957   representations may be performed automatically (if the user agent is
958   capable of doing so) or manually by the user selecting from a
959   generated (possibly hypertext) menu.
960</t>
961<t>
962   Agent-driven negotiation is advantageous when the response would vary
963   over commonly-used dimensions (such as type, language, or encoding),
964   when the origin server is unable to determine a user agent's
965   capabilities from examining the request, and generally when public
966   caches are used to distribute server load and reduce network usage.
967</t>
968<t>
969   Agent-driven negotiation suffers from the disadvantage of needing a
970   second request to obtain the best alternate representation. This
971   second request is only efficient when caching is used. In addition,
972   this specification does not define any mechanism for supporting
973   automatic selection, though it also does not prevent any such
974   mechanism from being developed as an extension and used within
975   HTTP/1.1.
976</t>
977<t>
978   This specification defines the 300 (Multiple Choices) and 406 (Not Acceptable)
979   status codes for enabling agent-driven negotiation when the server is
980   unwilling or unable to provide a varying response using server-driven
981   negotiation.
982</t>
983</section>
984</section>
985
986<section title="Header Field Definitions" anchor="header.fields">
987<t>
988   This section defines the syntax and semantics of HTTP/1.1 header fields
989   related to the payload of messages.
990</t>
991<t>
992   For entity-header fields, both sender and recipient refer to either the
993   client or the server, depending on who sends and who receives the entity.
994</t>
995
996<section title="Accept" anchor="header.accept">
997  <iref primary="true" item="Accept header" x:for-anchor=""/>
998  <iref primary="true" item="Headers" subitem="Accept" x:for-anchor=""/>
999  <x:anchor-alias value="Accept"/>
1000  <x:anchor-alias value="Accept-v"/>
1001  <x:anchor-alias value="accept-ext"/>
1002  <x:anchor-alias value="accept-params"/>
1003  <x:anchor-alias value="media-range"/>
1004<t>
1005   The "Accept" request-header field can be used by user agents to specify
1006   response media types that are acceptable. Accept headers can be used to
1007   indicate that the request is specifically limited to a small set of desired
1008   types, as in the case of a request for an in-line image.
1009</t>
1010<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"/>
1011  <x:ref>Accept</x:ref>   = "Accept" ":" <x:ref>OWS</x:ref> <x:ref>Accept-v</x:ref>
1012  <x:ref>Accept-v</x:ref> = #( <x:ref>media-range</x:ref> [ <x:ref>accept-params</x:ref> ] )
1013 
1014  <x:ref>media-range</x:ref>    = ( "*/*"
1015                   / ( <x:ref>type</x:ref> "/" "*" )
1016                   / ( <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> )
1017                   ) *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
1018  <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> )
1019  <x:ref>accept-ext</x:ref>     = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>token</x:ref>
1020                   [ "=" <x:ref>word</x:ref> ]
1021</artwork></figure>
1022<t>
1023   The asterisk "*" character is used to group media types into ranges,
1024   with "*/*" indicating all media types and "type/*" indicating all
1025   subtypes of that type. The media-range &MAY; include media type
1026   parameters that are applicable to that range.
1027</t>
1028<t>
1029   Each media-range &MAY; be followed by one or more accept-params,
1030   beginning with the "q" parameter for indicating a relative quality
1031   factor. The first "q" parameter (if any) separates the media-range
1032   parameter(s) from the accept-params. Quality factors allow the user
1033   or user agent to indicate the relative degree of preference for that
1034   media-range, using the qvalue scale from 0 to 1 (&qvalue;). The
1035   default value is q=1.
1036</t>
1037<x:note>
1038  <t>
1039    <x:h>Note:</x:h> Use of the "q" parameter name to separate media type
1040    parameters from Accept extension parameters is due to historical
1041    practice. Although this prevents any media type parameter named
1042    "q" from being used with a media range, such an event is believed
1043    to be unlikely given the lack of any "q" parameters in the IANA
1044    media type registry and the rare usage of any media type
1045    parameters in Accept. Future media types are discouraged from
1046    registering any parameter named "q".
1047  </t>
1048</x:note>
1049<t>
1050   The example
1051</t>
1052<figure><artwork type="example">
1053  Accept: audio/*; q=0.2, audio/basic
1054</artwork></figure>
1055<t>
1056   &SHOULD; be interpreted as "I prefer audio/basic, but send me any audio
1057   type if it is the best available after an 80% mark-down in quality."
1058</t>
1059<t>
1060   If no Accept header field is present, then it is assumed that the
1061   client accepts all media types. If an Accept header field is present,
1062   and if the server cannot send a response which is acceptable
1063   according to the combined Accept field value, then the server &SHOULD;
1064   send a 406 (Not Acceptable) response.
1065</t>
1066<t>
1067   A more elaborate example is
1068</t>
1069<figure><artwork type="example">
1070  Accept: text/plain; q=0.5, text/html,
1071          text/x-dvi; q=0.8, text/x-c
1072</artwork></figure>
1073<t>
1074   Verbally, this would be interpreted as "text/html and text/x-c are
1075   the preferred media types, but if they do not exist, then send the
1076   text/x-dvi entity, and if that does not exist, send the text/plain
1077   entity."
1078</t>
1079<t>
1080   Media ranges can be overridden by more specific media ranges or
1081   specific media types. If more than one media range applies to a given
1082   type, the most specific reference has precedence. For example,
1083</t>
1084<figure><artwork type="example">
1085  Accept: text/*, text/html, text/html;level=1, */*
1086</artwork></figure>
1087<t>
1088   have the following precedence:
1089   <list style="numbers">
1090    <t>text/html;level=1</t>
1091    <t>text/html</t>
1092    <t>text/*</t>
1093    <t>*/*</t>
1094   </list>
1095</t>
1096<t>
1097   The media type quality factor associated with a given type is
1098   determined by finding the media range with the highest precedence
1099   which matches that type. For example,
1100</t>
1101<figure><artwork type="example">
1102  Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
1103          text/html;level=2;q=0.4, */*;q=0.5
1104</artwork></figure>
1105<t>
1106   would cause the following values to be associated:
1107</t>
1108<texttable align="left">
1109  <ttcol>Media Type</ttcol><ttcol>Quality Value</ttcol>
1110  <c>text/html;level=1</c>    <c>1</c>
1111  <c>text/html</c>            <c>0.7</c>
1112  <c>text/plain</c>           <c>0.3</c>
1113  <c>image/jpeg</c>           <c>0.5</c>
1114  <c>text/html;level=2</c>    <c>0.4</c>
1115  <c>text/html;level=3</c>    <c>0.7</c>
1116</texttable>
1117<t>
1118      <x:h>Note:</x:h> A user agent might be provided with a default set of quality
1119      values for certain media ranges. However, unless the user agent is
1120      a closed system which cannot interact with other rendering agents,
1121      this default set ought to be configurable by the user.
1122</t>
1123</section>
1124
1125<section title="Accept-Charset" anchor="header.accept-charset">
1126  <iref primary="true" item="Accept-Charset header" x:for-anchor=""/>
1127  <iref primary="true" item="Headers" subitem="Accept-Charset" x:for-anchor=""/>
1128  <x:anchor-alias value="Accept-Charset"/>
1129  <x:anchor-alias value="Accept-Charset-v"/>
1130<t>
1131   The "Accept-Charset" request-header field can be used by user agents to
1132   indicate what response character sets are acceptable. This field allows
1133   clients capable of understanding more comprehensive or special-purpose
1134   character sets to signal that capability to a server which is capable of
1135   representing documents in those character sets.
1136</t>
1137<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Charset"/><iref primary="true" item="Grammar" subitem="Accept-Charset-v"/>
1138  <x:ref>Accept-Charset</x:ref>   = "Accept-Charset" ":" <x:ref>OWS</x:ref>
1139          <x:ref>Accept-Charset-v</x:ref>
1140  <x:ref>Accept-Charset-v</x:ref> = 1#( ( <x:ref>charset</x:ref> / "*" )
1141                         [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
1142</artwork></figure>
1143<t>
1144   Character set values are described in <xref target="character.sets"/>. Each charset &MAY;
1145   be given an associated quality value which represents the user's
1146   preference for that charset. The default value is q=1. An example is
1147</t>
1148<figure><artwork type="example">
1149  Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
1150</artwork></figure>
1151<t>
1152   The special value "*", if present in the Accept-Charset field,
1153   matches every character set (including ISO-8859-1) which is not
1154   mentioned elsewhere in the Accept-Charset field. If no "*" is present
1155   in an Accept-Charset field, then all character sets not explicitly
1156   mentioned get a quality value of 0, except for ISO-8859-1, which gets
1157   a quality value of 1 if not explicitly mentioned.
1158</t>
1159<t>
1160   If no Accept-Charset header is present, the default is that any
1161   character set is acceptable. If an Accept-Charset header is present,
1162   and if the server cannot send a response which is acceptable
1163   according to the Accept-Charset header, then the server &SHOULD; send
1164   an error response with the 406 (Not Acceptable) status code, though
1165   the sending of an unacceptable response is also allowed.
1166</t>
1167</section>
1168
1169<section title="Accept-Encoding" anchor="header.accept-encoding">
1170  <iref primary="true" item="Accept-Encoding header" x:for-anchor=""/>
1171  <iref primary="true" item="Headers" subitem="Accept-Encoding" x:for-anchor=""/>
1172  <x:anchor-alias value="Accept-Encoding"/>
1173  <x:anchor-alias value="Accept-Encoding-v"/>
1174  <x:anchor-alias value="codings"/>
1175<t>
1176   The "Accept-Encoding" request-header field can be used by user agents to
1177   indicate what response content-codings (<xref target="content.codings"/>)
1178   are acceptable in the response.
1179</t>
1180<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"/>
1181  <x:ref>Accept-Encoding</x:ref>    = "Accept-Encoding" ":" <x:ref>OWS</x:ref>
1182                     <x:ref>Accept-Encoding-v</x:ref>
1183  <x:ref>Accept-Encoding-v</x:ref>  =
1184                     #( <x:ref>codings</x:ref> [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
1185  <x:ref>codings</x:ref>            = ( <x:ref>content-coding</x:ref> / "*" )
1186</artwork></figure>
1187<t>
1188   Each codings value &MAY; be given an associated quality value which
1189   represents the preference for that encoding. The default value is q=1.
1190</t>
1191<t>
1192   Examples of its use are:
1193</t>
1194<figure><artwork type="example">
1195  Accept-Encoding: compress, gzip
1196  Accept-Encoding:
1197  Accept-Encoding: *
1198  Accept-Encoding: compress;q=0.5, gzip;q=1.0
1199  Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
1200</artwork></figure>
1201<t>
1202   A server tests whether a content-coding is acceptable, according to
1203   an Accept-Encoding field, using these rules:
1204  <list style="numbers">
1205      <t>If the content-coding is one of the content-codings listed in
1206         the Accept-Encoding field, then it is acceptable, unless it is
1207         accompanied by a qvalue of 0. (As defined in &qvalue;, a
1208         qvalue of 0 means "not acceptable.")</t>
1209
1210      <t>The special "*" symbol in an Accept-Encoding field matches any
1211         available content-coding not explicitly listed in the header
1212         field.</t>
1213
1214      <t>If multiple content-codings are acceptable, then the acceptable
1215         content-coding with the highest non-zero qvalue is preferred.</t>
1216
1217      <t>The "identity" content-coding is always acceptable, unless
1218         specifically refused because the Accept-Encoding field includes
1219         "identity;q=0", or because the field includes "*;q=0" and does
1220         not explicitly include the "identity" content-coding. If the
1221         Accept-Encoding field-value is empty, then only the "identity"
1222         encoding is acceptable.</t>
1223  </list>
1224</t>
1225<t>
1226   If an Accept-Encoding field is present in a request, and if the
1227   server cannot send a response which is acceptable according to the
1228   Accept-Encoding header, then the server &SHOULD; send an error response
1229   with the 406 (Not Acceptable) status code.
1230</t>
1231<t>
1232   If no Accept-Encoding field is present in a request, the server &MAY;
1233   assume that the client will accept any content coding. In this case,
1234   if "identity" is one of the available content-codings, then the
1235   server &SHOULD; use the "identity" content-coding, unless it has
1236   additional information that a different content-coding is meaningful
1237   to the client.
1238</t>
1239<x:note>
1240  <t>
1241    <x:h>Note:</x:h> If the request does not include an Accept-Encoding field,
1242    and if the "identity" content-coding is unavailable, then
1243    content-codings commonly understood by HTTP/1.0 clients (i.e.,
1244    "gzip" and "compress") are preferred; some older clients
1245    improperly display messages sent with other content-codings.  The
1246    server might also make this decision based on information about
1247    the particular user-agent or client.
1248  </t>
1249</x:note>
1250<x:note>
1251  <t>
1252    <x:h>Note:</x:h> Most HTTP/1.0 applications do not recognize or obey qvalues
1253    associated with content-codings. This means that qvalues will not
1254    work and are not permitted with x-gzip or x-compress.
1255  </t>
1256</x:note>
1257</section>
1258
1259<section title="Accept-Language" anchor="header.accept-language">
1260  <iref primary="true" item="Accept-Language header" x:for-anchor=""/>
1261  <iref primary="true" item="Headers" subitem="Accept-Language" x:for-anchor=""/>
1262  <x:anchor-alias value="Accept-Language"/>
1263  <x:anchor-alias value="Accept-Language-v"/>
1264  <x:anchor-alias value="language-range"/>
1265<t>
1266   The "Accept-Language" request-header field can be used by user agents to
1267   indicate the set of natural languages that are preferred in the response.
1268   Language tags are defined in <xref target="language.tags"/>.
1269</t>
1270<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"/>
1271  <x:ref>Accept-Language</x:ref>   = "Accept-Language" ":" <x:ref>OWS</x:ref>
1272                    <x:ref>Accept-Language-v</x:ref>
1273  <x:ref>Accept-Language-v</x:ref> =
1274                    1#( <x:ref>language-range</x:ref> [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
1275  <x:ref>language-range</x:ref>    =
1276            &lt;language-range, defined in <xref target="RFC4647" x:fmt="," x:sec="2.1"/>&gt;
1277</artwork></figure>
1278<t>
1279   Each language-range can be given an associated quality value which
1280   represents an estimate of the user's preference for the languages
1281   specified by that range. The quality value defaults to "q=1". For
1282   example,
1283</t>
1284<figure><artwork type="example">
1285  Accept-Language: da, en-gb;q=0.8, en;q=0.7
1286</artwork></figure>
1287<t>
1288   would mean: "I prefer Danish, but will accept British English and
1289   other types of English."
1290   (see also <xref target="RFC4647" x:sec="2.3" x:fmt="of"/>)
1291</t>
1292<t>
1293   For matching, <xref target="RFC4647" x:sec="3" x:fmt="of"/> defines
1294   several matching schemes. Implementations can offer the most appropriate
1295   matching scheme for their requirements.
1296</t>
1297<x:note>
1298  <t>
1299    <x:h>Note:</x:h> The "Basic Filtering" scheme (<xref target="RFC4647"
1300    x:fmt="," x:sec="3.3.1"/>) is identical to the matching scheme that was
1301    previously defined in <xref target="RFC2616" x:fmt="of" x:sec="14.4"/>.
1302  </t>
1303</x:note>
1304<t>
1305   It might be contrary to the privacy expectations of the user to send
1306   an Accept-Language header with the complete linguistic preferences of
1307   the user in every request. For a discussion of this issue, see
1308   <xref target="privacy.issues.connected.to.accept.headers"/>.
1309</t>
1310<t>
1311   As intelligibility is highly dependent on the individual user, it is
1312   recommended that client applications make the choice of linguistic
1313   preference available to the user. If the choice is not made
1314   available, then the Accept-Language header field &MUST-NOT; be given in
1315   the request.
1316</t>
1317<x:note>
1318  <t>
1319    <x:h>Note:</x:h> When making the choice of linguistic preference available to
1320    the user, we remind implementors of  the fact that users are not
1321    familiar with the details of language matching as described above,
1322    and should provide appropriate guidance. As an example, users
1323    might assume that on selecting "en-gb", they will be served any
1324    kind of English document if British English is not available. A
1325    user agent might suggest in such a case to add "en" to get the
1326    best matching behavior.
1327  </t>
1328</x:note>
1329</section>
1330
1331<section title="Content-Encoding" anchor="header.content-encoding">
1332  <iref primary="true" item="Content-Encoding header" x:for-anchor=""/>
1333  <iref primary="true" item="Headers" subitem="Content-Encoding" x:for-anchor=""/>
1334  <x:anchor-alias value="Content-Encoding"/>
1335  <x:anchor-alias value="Content-Encoding-v"/>
1336<t>
1337   The "Content-Encoding" entity-header field indicates what content-codings
1338   have been applied to the entity-body, and thus what decoding mechanisms
1339   must be applied in order to obtain the media-type referenced by the
1340   Content-Type header field. Content-Encoding is primarily used to allow a
1341   document to be compressed without losing the identity of its underlying
1342   media type.
1343</t>
1344<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Encoding"/><iref primary="true" item="Grammar" subitem="Content-Encoding-v"/>
1345  <x:ref>Content-Encoding</x:ref>   = "Content-Encoding" ":" <x:ref>OWS</x:ref> <x:ref>Content-Encoding-v</x:ref>
1346  <x:ref>Content-Encoding-v</x:ref> = 1#<x:ref>content-coding</x:ref>
1347</artwork></figure>
1348<t>
1349   Content codings are defined in <xref target="content.codings"/>. An example of its use is
1350</t>
1351<figure><artwork type="example">
1352  Content-Encoding: gzip
1353</artwork></figure>
1354<t>
1355   The content-coding is a characteristic of the entity identified by
1356   the Effective Request URI (&effective-request-uri;). Typically, the entity-body is stored with this
1357   encoding and is only decoded before rendering or analogous usage.
1358   However, a non-transparent proxy &MAY; modify the content-coding if the
1359   new coding is known to be acceptable to the recipient, unless the
1360   "no-transform" cache-control directive is present in the message.
1361</t>
1362<t>
1363   If the content-coding of an entity is not "identity", then the
1364   response &MUST; include a Content-Encoding entity-header (<xref target="header.content-encoding"/>)
1365   that lists the non-identity content-coding(s) used.
1366</t>
1367<t>
1368   If the content-coding of an entity in a request message is not
1369   acceptable to the origin server, the server &SHOULD; respond with a
1370   status code of 415 (Unsupported Media Type).
1371</t>
1372<t>
1373   If multiple encodings have been applied to an entity, the content
1374   codings &MUST; be listed in the order in which they were applied.
1375   Additional information about the encoding parameters &MAY; be provided
1376   by other entity-header fields not defined by this specification.
1377</t>
1378</section>
1379
1380<section title="Content-Language" anchor="header.content-language">
1381  <iref primary="true" item="Content-Language header" x:for-anchor=""/>
1382  <iref primary="true" item="Headers" subitem="Content-Language" x:for-anchor=""/>
1383  <x:anchor-alias value="Content-Language"/>
1384  <x:anchor-alias value="Content-Language-v"/>
1385<t>
1386   The "Content-Language" entity-header field describes the natural
1387   language(s) of the intended audience for the entity. Note that this might
1388   not be equivalent to all the languages used within the entity-body.
1389</t>
1390<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Language"/><iref primary="true" item="Grammar" subitem="Content-Language-v"/>
1391  <x:ref>Content-Language</x:ref>   = "Content-Language" ":" <x:ref>OWS</x:ref> <x:ref>Content-Language-v</x:ref>
1392  <x:ref>Content-Language-v</x:ref> = 1#<x:ref>language-tag</x:ref>
1393</artwork></figure>
1394<t>
1395   Language tags are defined in <xref target="language.tags"/>. The primary purpose of
1396   Content-Language is to allow a user to identify and differentiate
1397   entities according to the user's own preferred language. Thus, if the
1398   body content is intended only for a Danish-literate audience, the
1399   appropriate field is
1400</t>
1401<figure><artwork type="example">
1402  Content-Language: da
1403</artwork></figure>
1404<t>
1405   If no Content-Language is specified, the default is that the content
1406   is intended for all language audiences. This might mean that the
1407   sender does not consider it to be specific to any natural language,
1408   or that the sender does not know for which language it is intended.
1409</t>
1410<t>
1411   Multiple languages &MAY; be listed for content that is intended for
1412   multiple audiences. For example, a rendition of the "Treaty of
1413   Waitangi," presented simultaneously in the original Maori and English
1414   versions, would call for
1415</t>
1416<figure><artwork type="example">
1417  Content-Language: mi, en
1418</artwork></figure>
1419<t>
1420   However, just because multiple languages are present within an entity
1421   does not mean that it is intended for multiple linguistic audiences.
1422   An example would be a beginner's language primer, such as "A First
1423   Lesson in Latin," which is clearly intended to be used by an
1424   English-literate audience. In this case, the Content-Language would
1425   properly only include "en".
1426</t>
1427<t>
1428   Content-Language &MAY; be applied to any media type -- it is not
1429   limited to textual documents.
1430</t>
1431</section>
1432
1433<section title="Content-Location" anchor="header.content-location">
1434  <iref primary="true" item="Content-Location header" x:for-anchor=""/>
1435  <iref primary="true" item="Headers" subitem="Content-Location" x:for-anchor=""/>
1436  <x:anchor-alias value="Content-Location"/>
1437  <x:anchor-alias value="Content-Location-v"/>
1438<t>
1439   The "Content-Location" entity-header field is used to supply a URI for the
1440   entity in the message when it is accessible from a location separate from
1441   the requested resource's URI.
1442</t>
1443<t>
1444   A server &SHOULD; provide a Content-Location for the variant corresponding
1445   to the response entity, especially in the case where a resource has multiple
1446   entities associated with it, and those entities actually have separate
1447   locations by which they might be individually accessed, the server &SHOULD;
1448   provide a Content-Location for the particular variant which is returned.
1449</t>
1450<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Location"/><iref primary="true" item="Grammar" subitem="Content-Location-v"/>
1451  <x:ref>Content-Location</x:ref>   = "Content-Location" ":" <x:ref>OWS</x:ref>
1452                    <x:ref>Content-Location-v</x:ref>
1453  <x:ref>Content-Location-v</x:ref> =
1454                    <x:ref>absolute-URI</x:ref> / <x:ref>partial-URI</x:ref>
1455</artwork></figure>
1456<t>
1457   The Content-Location value is not a replacement for the Effective
1458   Request URI (&effective-request-uri;); it is only a statement of the location of the resource
1459   corresponding to this particular entity at the time of the request.
1460   Future requests &MAY; may be addressed to the Content-Location URI
1461   if the desire is to identify the source of that particular
1462   entity.
1463</t>
1464<t>
1465   &response-representation; describes how clients may process the Content-Location header field.
1466</t>
1467<t>
1468   A cache cannot assume that an entity with a Content-Location
1469   different from the URI used to retrieve it can be used to respond to
1470   later requests on that Content-Location URI. However, the Content-Location
1471   can be used to differentiate between multiple entities
1472   retrieved from a single requested resource, as described in &caching-neg-resp;.
1473</t>
1474<t>
1475   If the Content-Location is a relative URI, the relative URI is
1476   interpreted relative to the Effective Request URI.
1477</t>
1478<t>
1479   The meaning of the Content-Location header in requests is
1480   undefined; servers are free to ignore it in those cases.
1481</t>
1482</section>
1483
1484<section title="Content-MD5" anchor="header.content-md5">
1485  <iref primary="true" item="Content-MD5 header" x:for-anchor=""/>
1486  <iref primary="true" item="Headers" subitem="Content-MD5" x:for-anchor=""/>
1487  <x:anchor-alias value="Content-MD5"/>
1488  <x:anchor-alias value="Content-MD5-v"/>
1489<t>
1490   The "Content-MD5" entity-header field, as defined in <xref target="RFC1864"/>, is
1491   an MD5 digest of the entity-body that provides an end-to-end message
1492   integrity check (MIC) of the entity-body. Note that a MIC is good for
1493   detecting accidental modification of the entity-body in transit, but is not
1494   proof against malicious attacks.
1495</t>
1496<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-MD5"/><iref primary="true" item="Grammar" subitem="Content-MD5-v"/>
1497  <x:ref>Content-MD5</x:ref>   = "Content-MD5" ":" <x:ref>OWS</x:ref> <x:ref>Content-MD5-v</x:ref>
1498  <x:ref>Content-MD5-v</x:ref> = &lt;base64 of 128 bit MD5 digest as per <xref target="RFC1864"/>&gt;
1499</artwork></figure>
1500<t>
1501   The Content-MD5 header field &MAY; be generated by an origin server or
1502   client to function as an integrity check of the entity-body. Only
1503   origin servers or clients &MAY; generate the Content-MD5 header field;
1504   proxies and gateways &MUST-NOT; generate it, as this would defeat its
1505   value as an end-to-end integrity check. Any recipient of the entity-body,
1506   including gateways and proxies, &MAY; check that the digest value
1507   in this header field matches that of the entity-body as received.
1508</t>
1509<t>
1510   The MD5 digest is computed based on the content of the entity-body,
1511   including any content-coding that has been applied, but not including
1512   any transfer-encoding applied to the message-body. If the message is
1513   received with a transfer-encoding, that encoding &MUST; be removed
1514   prior to checking the Content-MD5 value against the received entity.
1515</t>
1516<t>
1517   This has the result that the digest is computed on the octets of the
1518   entity-body exactly as, and in the order that, they would be sent if
1519   no transfer-encoding were being applied.
1520</t>
1521<t>
1522   HTTP extends RFC 1864 to permit the digest to be computed for MIME
1523   composite media-types (e.g., multipart/* and message/rfc822), but
1524   this does not change how the digest is computed as defined in the
1525   preceding paragraph.
1526</t>
1527<t>
1528   There are several consequences of this. The entity-body for composite
1529   types &MAY; contain many body-parts, each with its own MIME and HTTP
1530   headers (including Content-MD5, Content-Transfer-Encoding, and
1531   Content-Encoding headers). If a body-part has a Content-Transfer-Encoding
1532   or Content-Encoding header, it is assumed that the content
1533   of the body-part has had the encoding applied, and the body-part is
1534   included in the Content-MD5 digest as is -- i.e., after the
1535   application. The Transfer-Encoding header field is not allowed within
1536   body-parts.
1537</t>
1538<t>
1539   Conversion of all line breaks to CRLF &MUST-NOT; be done before
1540   computing or checking the digest: the line break convention used in
1541   the text actually transmitted &MUST; be left unaltered when computing
1542   the digest.
1543</t>
1544<x:note>
1545  <t>
1546    <x:h>Note:</x:h> While the definition of Content-MD5 is exactly the same for
1547    HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
1548    in which the application of Content-MD5 to HTTP entity-bodies
1549    differs from its application to MIME entity-bodies. One is that
1550    HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
1551    does use Transfer-Encoding and Content-Encoding. Another is that
1552    HTTP more frequently uses binary content types than MIME, so it is
1553    worth noting that, in such cases, the byte order used to compute
1554    the digest is the transmission byte order defined for the type.
1555    Lastly, HTTP allows transmission of text types with any of several
1556    line break conventions and not just the canonical form using CRLF.
1557  </t>
1558</x:note>
1559</section>
1560
1561<section title="Content-Type" anchor="header.content-type">
1562  <iref primary="true" item="Content-Type header" x:for-anchor=""/>
1563  <iref primary="true" item="Headers" subitem="Content-Type" x:for-anchor=""/>
1564  <x:anchor-alias value="Content-Type"/>
1565  <x:anchor-alias value="Content-Type-v"/>
1566<t>
1567   The "Content-Type" entity-header field indicates the media type of the
1568   entity-body. In the case of responses to the HEAD method, the media type is
1569   that which would have been sent had the request been a GET.
1570</t>
1571<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Type"/><iref primary="true" item="Grammar" subitem="Content-Type-v"/>
1572  <x:ref>Content-Type</x:ref>   = "Content-Type" ":" <x:ref>OWS</x:ref> <x:ref>Content-Type-v</x:ref>
1573  <x:ref>Content-Type-v</x:ref> = <x:ref>media-type</x:ref>
1574</artwork></figure>
1575<t>
1576   Media types are defined in <xref target="media.types"/>. An example of the field is
1577</t>
1578<figure><artwork type="example">
1579  Content-Type: text/html; charset=ISO-8859-4
1580</artwork></figure>
1581<t>
1582   Further discussion of methods for identifying the media type of an
1583   entity is provided in <xref target="type"/>.
1584</t>
1585</section>
1586
1587</section>
1588
1589<section title="IANA Considerations" anchor="IANA.considerations">
1590<section title="Message Header Registration" anchor="message.header.registration">
1591<t>
1592   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1593   with the permanent registrations below (see <xref target="RFC3864"/>):
1594</t>
1595<?BEGININC p3-payload.iana-headers ?>
1596<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1597<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1598   <ttcol>Header Field Name</ttcol>
1599   <ttcol>Protocol</ttcol>
1600   <ttcol>Status</ttcol>
1601   <ttcol>Reference</ttcol>
1602
1603   <c>Accept</c>
1604   <c>http</c>
1605   <c>standard</c>
1606   <c>
1607      <xref target="header.accept"/>
1608   </c>
1609   <c>Accept-Charset</c>
1610   <c>http</c>
1611   <c>standard</c>
1612   <c>
1613      <xref target="header.accept-charset"/>
1614   </c>
1615   <c>Accept-Encoding</c>
1616   <c>http</c>
1617   <c>standard</c>
1618   <c>
1619      <xref target="header.accept-encoding"/>
1620   </c>
1621   <c>Accept-Language</c>
1622   <c>http</c>
1623   <c>standard</c>
1624   <c>
1625      <xref target="header.accept-language"/>
1626   </c>
1627   <c>Content-Disposition</c>
1628   <c>http</c>
1629   <c/>
1630   <c>
1631      <xref target="content-disposition"/>
1632   </c>
1633   <c>Content-Encoding</c>
1634   <c>http</c>
1635   <c>standard</c>
1636   <c>
1637      <xref target="header.content-encoding"/>
1638   </c>
1639   <c>Content-Language</c>
1640   <c>http</c>
1641   <c>standard</c>
1642   <c>
1643      <xref target="header.content-language"/>
1644   </c>
1645   <c>Content-Location</c>
1646   <c>http</c>
1647   <c>standard</c>
1648   <c>
1649      <xref target="header.content-location"/>
1650   </c>
1651   <c>Content-MD5</c>
1652   <c>http</c>
1653   <c>standard</c>
1654   <c>
1655      <xref target="header.content-md5"/>
1656   </c>
1657   <c>Content-Type</c>
1658   <c>http</c>
1659   <c>standard</c>
1660   <c>
1661      <xref target="header.content-type"/>
1662   </c>
1663   <c>MIME-Version</c>
1664   <c>http</c>
1665   <c/>
1666   <c>
1667      <xref target="mime-version"/>
1668   </c>
1669</texttable>
1670<!--(END)-->
1671<?ENDINC p3-payload.iana-headers ?>
1672<t>
1673   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1674</t>
1675</section>
1676
1677<section title="Content Coding Registry" anchor="content.coding.registration">
1678<t>
1679   The registration procedure for HTTP Content Codings is now defined
1680   by <xref target="content.coding.registry"/> of this document.
1681</t>
1682<t>
1683   The HTTP Content Codings Registry located at <eref target="http://www.iana.org/assignments/http-parameters"/>
1684   should be updated with the registration below:
1685</t>
1686<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
1687   <ttcol>Name</ttcol>
1688   <ttcol>Description</ttcol>
1689   <ttcol>Reference</ttcol>
1690   <c>compress</c>
1691   <c>UNIX "compress" program method</c>
1692   <c>
1693      &compress-coding;
1694   </c>
1695   <c>deflate</c>
1696   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
1697   the "zlib" data format (<xref target="RFC1950"/>)
1698   </c>
1699   <c>
1700      &deflate-coding;
1701   </c>
1702   <c>gzip</c>
1703   <c>Same as GNU zip <xref target="RFC1952"/></c>
1704   <c>
1705      &gzip-coding;
1706   </c>
1707   <c>identity</c>
1708   <c>No transformation</c>
1709   <c>
1710      <xref target="content.codings"/>
1711   </c>
1712</texttable>
1713</section>
1714
1715</section>
1716
1717<section title="Security Considerations" anchor="security.considerations">
1718<t>
1719   This section is meant to inform application developers, information
1720   providers, and users of the security limitations in HTTP/1.1 as
1721   described by this document. The discussion does not include
1722   definitive solutions to the problems revealed, though it does make
1723   some suggestions for reducing security risks.
1724</t>
1725
1726<section title="Privacy Issues Connected to Accept Headers" anchor="privacy.issues.connected.to.accept.headers">
1727<t>
1728   Accept request-headers can reveal information about the user to all
1729   servers which are accessed. The Accept-Language header in particular
1730   can reveal information the user would consider to be of a private
1731   nature, because the understanding of particular languages is often
1732   strongly correlated to the membership of a particular ethnic group.
1733   User agents which offer the option to configure the contents of an
1734   Accept-Language header to be sent in every request are strongly
1735   encouraged to let the configuration process include a message which
1736   makes the user aware of the loss of privacy involved.
1737</t>
1738<t>
1739   An approach that limits the loss of privacy would be for a user agent
1740   to omit the sending of Accept-Language headers by default, and to ask
1741   the user whether or not to start sending Accept-Language headers to a
1742   server if it detects, by looking for any Vary response-header fields
1743   generated by the server, that such sending could improve the quality
1744   of service.
1745</t>
1746<t>
1747   Elaborate user-customized accept header fields sent in every request,
1748   in particular if these include quality values, can be used by servers
1749   as relatively reliable and long-lived user identifiers. Such user
1750   identifiers would allow content providers to do click-trail tracking,
1751   and would allow collaborating content providers to match cross-server
1752   click-trails or form submissions of individual users. Note that for
1753   many users not behind a proxy, the network address of the host
1754   running the user agent will also serve as a long-lived user
1755   identifier. In environments where proxies are used to enhance
1756   privacy, user agents ought to be conservative in offering accept
1757   header configuration options to end users. As an extreme privacy
1758   measure, proxies could filter the accept headers in relayed requests.
1759   General purpose user agents which provide a high degree of header
1760   configurability &SHOULD; warn users about the loss of privacy which can
1761   be involved.
1762</t>
1763</section>
1764
1765<section title="Content-Disposition Issues" anchor="content-disposition.issues">
1766<t>
1767   <xref target="RFC2183"/>, from which the often implemented Content-Disposition
1768   (see <xref target="content-disposition"/>) header in HTTP is derived, has a number of very
1769   serious security considerations. Content-Disposition is not part of
1770   the HTTP standard, but since it is widely implemented, we are
1771   documenting its use and risks for implementors. See <xref target="RFC2183" x:fmt="of" x:sec="5"/>
1772   for details.
1773</t>
1774</section>
1775
1776</section>
1777
1778<section title="Acknowledgments" anchor="ack">
1779</section>
1780</middle>
1781<back>
1782
1783<references title="Normative References">
1784
1785<reference anchor="ISO-8859-1">
1786  <front>
1787    <title>
1788     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
1789    </title>
1790    <author>
1791      <organization>International Organization for Standardization</organization>
1792    </author>
1793    <date year="1998"/>
1794  </front>
1795  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
1796</reference>
1797
1798<reference anchor="Part1">
1799  <front>
1800    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1801    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1802      <organization abbrev="Day Software">Day Software</organization>
1803      <address><email>fielding@gbiv.com</email></address>
1804    </author>
1805    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1806      <organization>One Laptop per Child</organization>
1807      <address><email>jg@laptop.org</email></address>
1808    </author>
1809    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1810      <organization abbrev="HP">Hewlett-Packard Company</organization>
1811      <address><email>JeffMogul@acm.org</email></address>
1812    </author>
1813    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1814      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1815      <address><email>henrikn@microsoft.com</email></address>
1816    </author>
1817    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1818      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1819      <address><email>LMM@acm.org</email></address>
1820    </author>
1821    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1822      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1823      <address><email>paulle@microsoft.com</email></address>
1824    </author>
1825    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1826      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1827      <address><email>timbl@w3.org</email></address>
1828    </author>
1829    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1830      <organization abbrev="W3C">World Wide Web Consortium</organization>
1831      <address><email>ylafon@w3.org</email></address>
1832    </author>
1833    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1834      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1835      <address><email>julian.reschke@greenbytes.de</email></address>
1836    </author>
1837    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1838  </front>
1839  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1840  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1841</reference>
1842
1843<reference anchor="Part2">
1844  <front>
1845    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1846    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1847      <organization abbrev="Day Software">Day Software</organization>
1848      <address><email>fielding@gbiv.com</email></address>
1849    </author>
1850    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1851      <organization>One Laptop per Child</organization>
1852      <address><email>jg@laptop.org</email></address>
1853    </author>
1854    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1855      <organization abbrev="HP">Hewlett-Packard Company</organization>
1856      <address><email>JeffMogul@acm.org</email></address>
1857    </author>
1858    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1859      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1860      <address><email>henrikn@microsoft.com</email></address>
1861    </author>
1862    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1863      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1864      <address><email>LMM@acm.org</email></address>
1865    </author>
1866    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1867      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1868      <address><email>paulle@microsoft.com</email></address>
1869    </author>
1870    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1871      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1872      <address><email>timbl@w3.org</email></address>
1873    </author>
1874    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1875      <organization abbrev="W3C">World Wide Web Consortium</organization>
1876      <address><email>ylafon@w3.org</email></address>
1877    </author>
1878    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1879      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1880      <address><email>julian.reschke@greenbytes.de</email></address>
1881    </author>
1882    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1883  </front>
1884  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1885  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
1886</reference>
1887
1888<reference anchor="Part4">
1889  <front>
1890    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1891    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1892      <organization abbrev="Day Software">Day Software</organization>
1893      <address><email>fielding@gbiv.com</email></address>
1894    </author>
1895    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1896      <organization>One Laptop per Child</organization>
1897      <address><email>jg@laptop.org</email></address>
1898    </author>
1899    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1900      <organization abbrev="HP">Hewlett-Packard Company</organization>
1901      <address><email>JeffMogul@acm.org</email></address>
1902    </author>
1903    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1904      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1905      <address><email>henrikn@microsoft.com</email></address>
1906    </author>
1907    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1908      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1909      <address><email>LMM@acm.org</email></address>
1910    </author>
1911    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1912      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1913      <address><email>paulle@microsoft.com</email></address>
1914    </author>
1915    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1916      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1917      <address><email>timbl@w3.org</email></address>
1918    </author>
1919    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1920      <organization abbrev="W3C">World Wide Web Consortium</organization>
1921      <address><email>ylafon@w3.org</email></address>
1922    </author>
1923    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1924      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1925      <address><email>julian.reschke@greenbytes.de</email></address>
1926    </author>
1927    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1928  </front>
1929  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
1930  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
1931</reference>
1932
1933<reference anchor="Part5">
1934  <front>
1935    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1936    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1937      <organization abbrev="Day Software">Day Software</organization>
1938      <address><email>fielding@gbiv.com</email></address>
1939    </author>
1940    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1941      <organization>One Laptop per Child</organization>
1942      <address><email>jg@laptop.org</email></address>
1943    </author>
1944    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1945      <organization abbrev="HP">Hewlett-Packard Company</organization>
1946      <address><email>JeffMogul@acm.org</email></address>
1947    </author>
1948    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1949      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1950      <address><email>henrikn@microsoft.com</email></address>
1951    </author>
1952    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1953      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1954      <address><email>LMM@acm.org</email></address>
1955    </author>
1956    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1957      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1958      <address><email>paulle@microsoft.com</email></address>
1959    </author>
1960    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1961      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1962      <address><email>timbl@w3.org</email></address>
1963    </author>
1964    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1965      <organization abbrev="W3C">World Wide Web Consortium</organization>
1966      <address><email>ylafon@w3.org</email></address>
1967    </author>
1968    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1969      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1970      <address><email>julian.reschke@greenbytes.de</email></address>
1971    </author>
1972    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1973  </front>
1974  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1975  <x:source href="p5-range.xml" basename="p5-range"/>
1976</reference>
1977
1978<reference anchor="Part6">
1979  <front>
1980    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1981    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1982      <organization abbrev="Day Software">Day Software</organization>
1983      <address><email>fielding@gbiv.com</email></address>
1984    </author>
1985    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1986      <organization>One Laptop per Child</organization>
1987      <address><email>jg@laptop.org</email></address>
1988    </author>
1989    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1990      <organization abbrev="HP">Hewlett-Packard Company</organization>
1991      <address><email>JeffMogul@acm.org</email></address>
1992    </author>
1993    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1994      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1995      <address><email>henrikn@microsoft.com</email></address>
1996    </author>
1997    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1998      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1999      <address><email>LMM@acm.org</email></address>
2000    </author>
2001    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2002      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2003      <address><email>paulle@microsoft.com</email></address>
2004    </author>
2005    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2006      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2007      <address><email>timbl@w3.org</email></address>
2008    </author>
2009    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2010      <organization abbrev="W3C">World Wide Web Consortium</organization>
2011      <address><email>ylafon@w3.org</email></address>
2012    </author>
2013    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
2014      <address><email>mnot@mnot.net</email></address>
2015    </author>
2016    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2017      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2018      <address><email>julian.reschke@greenbytes.de</email></address>
2019    </author>
2020    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2021  </front>
2022  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
2023  <x:source href="p6-cache.xml" basename="p6-cache"/>
2024</reference>
2025
2026<reference anchor="RFC1864">
2027  <front>
2028    <title abbrev="Content-MD5 Header Field">The Content-MD5 Header Field</title>
2029    <author initials="J." surname="Myers" fullname="John G. Myers">
2030      <organization>Carnegie Mellon University</organization>
2031      <address><email>jgm+@cmu.edu</email></address>
2032    </author>
2033    <author initials="M." surname="Rose" fullname="Marshall T. Rose">
2034      <organization>Dover Beach Consulting, Inc.</organization>
2035      <address><email>mrose@dbc.mtview.ca.us</email></address>
2036    </author>
2037    <date month="October" year="1995"/>
2038  </front>
2039  <seriesInfo name="RFC" value="1864"/>
2040</reference>
2041
2042<reference anchor="RFC1950">
2043  <front>
2044    <title>ZLIB Compressed Data Format Specification version 3.3</title>
2045    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
2046      <organization>Aladdin Enterprises</organization>
2047      <address><email>ghost@aladdin.com</email></address>
2048    </author>
2049    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
2050    <date month="May" year="1996"/>
2051  </front>
2052  <seriesInfo name="RFC" value="1950"/>
2053  <annotation>
2054    RFC 1950 is an Informational RFC, thus it may be less stable than
2055    this specification. On the other hand, this downward reference was
2056    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
2057    therefore it is unlikely to cause problems in practice. See also
2058    <xref target="BCP97"/>.
2059  </annotation>
2060</reference>
2061
2062<reference anchor="RFC1951">
2063  <front>
2064    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
2065    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
2066      <organization>Aladdin Enterprises</organization>
2067      <address><email>ghost@aladdin.com</email></address>
2068    </author>
2069    <date month="May" year="1996"/>
2070  </front>
2071  <seriesInfo name="RFC" value="1951"/>
2072  <annotation>
2073    RFC 1951 is an Informational RFC, thus it may be less stable than
2074    this specification. On the other hand, this downward reference was
2075    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
2076    therefore it is unlikely to cause problems in practice. See also
2077    <xref target="BCP97"/>.
2078  </annotation>
2079</reference>
2080
2081<reference anchor="RFC1952">
2082  <front>
2083    <title>GZIP file format specification version 4.3</title>
2084    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
2085      <organization>Aladdin Enterprises</organization>
2086      <address><email>ghost@aladdin.com</email></address>
2087    </author>
2088    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
2089      <address><email>gzip@prep.ai.mit.edu</email></address>
2090    </author>
2091    <author initials="M." surname="Adler" fullname="Mark Adler">
2092      <address><email>madler@alumni.caltech.edu</email></address>
2093    </author>
2094    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
2095      <address><email>ghost@aladdin.com</email></address>
2096    </author>
2097    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
2098      <address><email>randeg@alumni.rpi.edu</email></address>
2099    </author>
2100    <date month="May" year="1996"/>
2101  </front>
2102  <seriesInfo name="RFC" value="1952"/>
2103  <annotation>
2104    RFC 1952 is an Informational RFC, thus it may be less stable than
2105    this specification. On the other hand, this downward reference was
2106    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
2107    therefore it is unlikely to cause problems in practice. See also
2108    <xref target="BCP97"/>.
2109  </annotation>
2110</reference>
2111
2112<reference anchor="RFC2045">
2113  <front>
2114    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
2115    <author initials="N." surname="Freed" fullname="Ned Freed">
2116      <organization>Innosoft International, Inc.</organization>
2117      <address><email>ned@innosoft.com</email></address>
2118    </author>
2119    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
2120      <organization>First Virtual Holdings</organization>
2121      <address><email>nsb@nsb.fv.com</email></address>
2122    </author>
2123    <date month="November" year="1996"/>
2124  </front>
2125  <seriesInfo name="RFC" value="2045"/>
2126</reference>
2127
2128<reference anchor="RFC2046">
2129  <front>
2130    <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
2131    <author initials="N." surname="Freed" fullname="Ned Freed">
2132      <organization>Innosoft International, Inc.</organization>
2133      <address><email>ned@innosoft.com</email></address>
2134    </author>
2135    <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
2136      <organization>First Virtual Holdings</organization>
2137      <address><email>nsb@nsb.fv.com</email></address>
2138    </author>
2139    <date month="November" year="1996"/>
2140  </front>
2141  <seriesInfo name="RFC" value="2046"/>
2142</reference>
2143
2144<reference anchor="RFC2119">
2145  <front>
2146    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2147    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2148      <organization>Harvard University</organization>
2149      <address><email>sob@harvard.edu</email></address>
2150    </author>
2151    <date month="March" year="1997"/>
2152  </front>
2153  <seriesInfo name="BCP" value="14"/>
2154  <seriesInfo name="RFC" value="2119"/>
2155</reference>
2156
2157<reference anchor='RFC4647'>
2158  <front>
2159    <title>Matching of Language Tags</title>
2160    <author initials='A.' surname='Phillips' fullname='Addison Phillips' role="editor">
2161      <organization>Yahoo! Inc.</organization>
2162      <address><email>addison@inter-locale.com</email></address>
2163    </author>
2164    <author initials='M.' surname='Davis' fullname='Mark Davis' role="editor">
2165      <organization>Google</organization>
2166      <address><email>mark.davis@macchiato.com</email></address>
2167    </author>
2168    <date year='2006' month='September' />
2169  </front>
2170  <seriesInfo name='BCP' value='47' />
2171  <seriesInfo name='RFC' value='4647' />
2172</reference>
2173
2174<reference anchor="RFC5234">
2175  <front>
2176    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2177    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2178      <organization>Brandenburg InternetWorking</organization>
2179      <address>
2180        <email>dcrocker@bbiw.net</email>
2181      </address> 
2182    </author>
2183    <author initials="P." surname="Overell" fullname="Paul Overell">
2184      <organization>THUS plc.</organization>
2185      <address>
2186        <email>paul.overell@thus.net</email>
2187      </address>
2188    </author>
2189    <date month="January" year="2008"/>
2190  </front>
2191  <seriesInfo name="STD" value="68"/>
2192  <seriesInfo name="RFC" value="5234"/>
2193</reference>
2194
2195<reference anchor='RFC5646'>
2196  <front>
2197    <title>Tags for Identifying Languages</title>
2198    <author initials='A.' surname='Phillips' fullname='Addison Phillips' role='editor'>
2199      <organization>Lab126</organization>
2200      <address><email>addison@inter-locale.com</email></address>
2201    </author>
2202    <author initials='M.' surname='Davis' fullname='Mark Davis' role='editor'>
2203      <organization>Google</organization>
2204      <address><email>mark.davis@google.com</email></address>
2205    </author>
2206    <date month='September' year='2009' />
2207  </front>
2208  <seriesInfo name='BCP' value='47' />
2209  <seriesInfo name='RFC' value='5646' />
2210</reference>
2211
2212</references>
2213
2214<references title="Informative References">
2215
2216<reference anchor="RFC1945">
2217  <front>
2218    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
2219    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2220      <organization>MIT, Laboratory for Computer Science</organization>
2221      <address><email>timbl@w3.org</email></address>
2222    </author>
2223    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
2224      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2225      <address><email>fielding@ics.uci.edu</email></address>
2226    </author>
2227    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2228      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
2229      <address><email>frystyk@w3.org</email></address>
2230    </author>
2231    <date month="May" year="1996"/>
2232  </front>
2233  <seriesInfo name="RFC" value="1945"/>
2234</reference>
2235
2236<reference anchor="RFC2049">
2237  <front>
2238    <title abbrev="MIME Conformance">Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples</title>
2239    <author initials="N." surname="Freed" fullname="Ned Freed">
2240      <organization>Innosoft International, Inc.</organization>
2241      <address><email>ned@innosoft.com</email></address>
2242    </author>
2243    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
2244      <organization>First Virtual Holdings</organization>
2245      <address><email>nsb@nsb.fv.com</email></address>
2246    </author>
2247    <date month="November" year="1996"/>
2248  </front>
2249  <seriesInfo name="RFC" value="2049"/>
2250</reference>
2251
2252<reference anchor="RFC2068">
2253  <front>
2254    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
2255    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
2256      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2257      <address><email>fielding@ics.uci.edu</email></address>
2258    </author>
2259    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2260      <organization>MIT Laboratory for Computer Science</organization>
2261      <address><email>jg@w3.org</email></address>
2262    </author>
2263    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2264      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
2265      <address><email>mogul@wrl.dec.com</email></address>
2266    </author>
2267    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2268      <organization>MIT Laboratory for Computer Science</organization>
2269      <address><email>frystyk@w3.org</email></address>
2270    </author>
2271    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2272      <organization>MIT Laboratory for Computer Science</organization>
2273      <address><email>timbl@w3.org</email></address>
2274    </author>
2275    <date month="January" year="1997"/>
2276  </front>
2277  <seriesInfo name="RFC" value="2068"/>
2278</reference>
2279
2280<reference anchor="RFC2076">
2281  <front>
2282    <title abbrev="Internet Message Headers">Common Internet Message Headers</title>
2283    <author initials="J." surname="Palme" fullname="Jacob Palme">
2284      <organization>Stockholm University/KTH</organization>
2285      <address><email>jpalme@dsv.su.se</email></address>
2286    </author>
2287    <date month="February" year="1997"/>
2288  </front>
2289  <seriesInfo name="RFC" value="2076"/>
2290</reference>
2291
2292<reference anchor="RFC2183">
2293  <front>
2294    <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field</title>
2295    <author initials="R." surname="Troost" fullname="Rens Troost">
2296      <organization>New Century Systems</organization>
2297      <address><email>rens@century.com</email></address>
2298    </author>
2299    <author initials="S." surname="Dorner" fullname="Steve Dorner">
2300      <organization>QUALCOMM Incorporated</organization>
2301      <address><email>sdorner@qualcomm.com</email></address>
2302    </author>
2303    <author initials="K." surname="Moore" fullname="Keith Moore">
2304      <organization>Department of Computer Science</organization>
2305      <address><email>moore@cs.utk.edu</email></address>
2306    </author>
2307    <date month="August" year="1997"/>
2308  </front>
2309  <seriesInfo name="RFC" value="2183"/>
2310</reference>
2311
2312<reference anchor="RFC2277">
2313  <front>
2314    <title abbrev="Charset Policy">IETF Policy on Character Sets and Languages</title>
2315    <author initials="H.T." surname="Alvestrand" fullname="Harald Tveit Alvestrand">
2316      <organization>UNINETT</organization>
2317      <address><email>Harald.T.Alvestrand@uninett.no</email></address>
2318    </author>
2319    <date month="January" year="1998"/>
2320  </front>
2321  <seriesInfo name="BCP" value="18"/>
2322  <seriesInfo name="RFC" value="2277"/>
2323</reference>
2324
2325<reference anchor='RFC2295'>
2326  <front>
2327    <title abbrev='HTTP Content Negotiation'>Transparent Content Negotiation in HTTP</title>
2328    <author initials='K.' surname='Holtman' fullname='Koen Holtman'>
2329      <organization>Technische Universiteit Eindhoven</organization>
2330      <address>
2331        <email>koen@win.tue.nl</email>
2332      </address>
2333    </author>
2334    <author initials='A.H.' surname='Mutz' fullname='Andrew H. Mutz'>
2335      <organization>Hewlett-Packard Company</organization>
2336      <address>
2337        <email>mutz@hpl.hp.com</email>
2338      </address>
2339    </author>
2340    <date year='1998' month='March'/>
2341  </front>
2342  <seriesInfo name='RFC' value='2295'/>
2343</reference>
2344
2345<reference anchor="RFC2388">
2346  <front>
2347    <title abbrev="multipart/form-data">Returning Values from Forms:  multipart/form-data</title>
2348    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2349      <organization>Xerox Palo Alto Research Center</organization>
2350      <address><email>masinter@parc.xerox.com</email></address>
2351    </author>
2352    <date year="1998" month="August"/>
2353  </front>
2354  <seriesInfo name="RFC" value="2388"/>
2355</reference>
2356
2357<reference anchor="RFC2557">
2358  <front>
2359    <title abbrev="MIME Encapsulation of Aggregate Documents">MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)</title>
2360    <author initials="F." surname="Palme" fullname="Jacob Palme">
2361      <organization>Stockholm University and KTH</organization>
2362      <address><email>jpalme@dsv.su.se</email></address>
2363    </author>
2364    <author initials="A." surname="Hopmann" fullname="Alex Hopmann">
2365      <organization>Microsoft Corporation</organization>
2366      <address><email>alexhop@microsoft.com</email></address>
2367    </author>
2368    <author initials="N." surname="Shelness" fullname="Nick Shelness">
2369      <organization>Lotus Development Corporation</organization>
2370      <address><email>Shelness@lotus.com</email></address>
2371    </author>
2372    <author initials="E." surname="Stefferud" fullname="Einar Stefferud">
2373      <address><email>stef@nma.com</email></address>
2374    </author>
2375    <date year="1999" month="March"/>
2376  </front>
2377  <seriesInfo name="RFC" value="2557"/>
2378</reference>
2379
2380<reference anchor="RFC2616">
2381  <front>
2382    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2383    <author initials="R." surname="Fielding" fullname="R. Fielding">
2384      <organization>University of California, Irvine</organization>
2385      <address><email>fielding@ics.uci.edu</email></address>
2386    </author>
2387    <author initials="J." surname="Gettys" fullname="J. Gettys">
2388      <organization>W3C</organization>
2389      <address><email>jg@w3.org</email></address>
2390    </author>
2391    <author initials="J." surname="Mogul" fullname="J. Mogul">
2392      <organization>Compaq Computer Corporation</organization>
2393      <address><email>mogul@wrl.dec.com</email></address>
2394    </author>
2395    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2396      <organization>MIT Laboratory for Computer Science</organization>
2397      <address><email>frystyk@w3.org</email></address>
2398    </author>
2399    <author initials="L." surname="Masinter" fullname="L. Masinter">
2400      <organization>Xerox Corporation</organization>
2401      <address><email>masinter@parc.xerox.com</email></address>
2402    </author>
2403    <author initials="P." surname="Leach" fullname="P. Leach">
2404      <organization>Microsoft Corporation</organization>
2405      <address><email>paulle@microsoft.com</email></address>
2406    </author>
2407    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2408      <organization>W3C</organization>
2409      <address><email>timbl@w3.org</email></address>
2410    </author>
2411    <date month="June" year="1999"/>
2412  </front>
2413  <seriesInfo name="RFC" value="2616"/>
2414</reference>
2415
2416<reference anchor="RFC3629">
2417  <front>
2418    <title>UTF-8, a transformation format of ISO 10646</title>
2419    <author initials="F." surname="Yergeau" fullname="F. Yergeau">
2420      <organization>Alis Technologies</organization>
2421      <address><email>fyergeau@alis.com</email></address>
2422    </author>
2423    <date month="November" year="2003"/>
2424  </front>
2425  <seriesInfo name="RFC" value="3629"/>
2426  <seriesInfo name="STD" value="63"/>
2427</reference>
2428
2429<reference anchor='RFC3864'>
2430  <front>
2431    <title>Registration Procedures for Message Header Fields</title>
2432    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
2433      <organization>Nine by Nine</organization>
2434      <address><email>GK-IETF@ninebynine.org</email></address>
2435    </author>
2436    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
2437      <organization>BEA Systems</organization>
2438      <address><email>mnot@pobox.com</email></address>
2439    </author>
2440    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
2441      <organization>HP Labs</organization>
2442      <address><email>JeffMogul@acm.org</email></address>
2443    </author>
2444    <date year='2004' month='September' />
2445  </front>
2446  <seriesInfo name='BCP' value='90' />
2447  <seriesInfo name='RFC' value='3864' />
2448</reference>
2449
2450<reference anchor="RFC4288">
2451  <front>
2452    <title>Media Type Specifications and Registration Procedures</title>
2453    <author initials="N." surname="Freed" fullname="N. Freed">
2454      <organization>Sun Microsystems</organization>
2455      <address>
2456        <email>ned.freed@mrochek.com</email>
2457      </address>
2458    </author>
2459    <author initials="J." surname="Klensin" fullname="J. Klensin">
2460      <address>
2461        <email>klensin+ietf@jck.com</email>
2462      </address>
2463    </author>
2464    <date year="2005" month="December"/>
2465  </front>
2466  <seriesInfo name="BCP" value="13"/>
2467  <seriesInfo name="RFC" value="4288"/>
2468</reference>
2469
2470<reference anchor='RFC5226'>
2471  <front>
2472    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2473    <author initials='T.' surname='Narten' fullname='T. Narten'>
2474      <organization>IBM</organization>
2475      <address><email>narten@us.ibm.com</email></address>
2476    </author>
2477    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2478      <organization>Google</organization>
2479      <address><email>Harald@Alvestrand.no</email></address>
2480    </author>
2481    <date year='2008' month='May' />
2482  </front>
2483  <seriesInfo name='BCP' value='26' />
2484  <seriesInfo name='RFC' value='5226' />
2485</reference>
2486
2487<reference anchor="RFC5322">
2488  <front>
2489    <title>Internet Message Format</title>
2490    <author initials="P." surname="Resnick" fullname="P. Resnick">
2491      <organization>Qualcomm Incorporated</organization>
2492    </author>
2493    <date year="2008" month="October"/>
2494  </front> 
2495  <seriesInfo name="RFC" value="5322"/>
2496</reference>
2497
2498<reference anchor='BCP97'>
2499  <front>
2500    <title>Handling Normative References to Standards-Track Documents</title>
2501    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
2502      <address>
2503        <email>klensin+ietf@jck.com</email>
2504      </address>
2505    </author>
2506    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
2507      <organization>MIT</organization>
2508      <address>
2509        <email>hartmans-ietf@mit.edu</email>
2510      </address>
2511    </author>
2512    <date year='2007' month='June' />
2513  </front>
2514  <seriesInfo name='BCP' value='97' />
2515  <seriesInfo name='RFC' value='4897' />
2516</reference>
2517
2518
2519</references>
2520
2521<section title="Differences Between HTTP Entities and RFC 2045 Entities" anchor="differences.between.http.entities.and.rfc.2045.entities">
2522<t>
2523   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
2524   allow entities to be transmitted in an open variety of
2525   representations and with extensible mechanisms. However, RFC 2045
2526   discusses mail, and HTTP has a few features that are different from
2527   those described in RFC 2045. These differences were carefully chosen
2528   to optimize performance over binary connections, to allow greater
2529   freedom in the use of new media types, to make date comparisons
2530   easier, and to acknowledge the practice of some early HTTP servers
2531   and clients.
2532</t>
2533<t>
2534   This appendix describes specific areas where HTTP differs from RFC
2535   2045. Proxies and gateways to strict MIME environments &SHOULD; be
2536   aware of these differences and provide the appropriate conversions
2537   where necessary. Proxies and gateways from MIME environments to HTTP
2538   also need to be aware of the differences because some conversions
2539   might be required.
2540</t>
2541
2542<section title="MIME-Version" anchor="mime-version">
2543  <iref primary="true" item="MIME-Version header" x:for-anchor=""/>
2544  <iref primary="true" item="Headers" subitem="MIME-Version" x:for-anchor=""/>
2545  <x:anchor-alias value="MIME-Version"/>
2546  <x:anchor-alias value="MIME-Version-v"/>
2547<t>
2548   HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages &MAY;
2549   include a single MIME-Version general-header field to indicate what
2550   version of the MIME protocol was used to construct the message. Use
2551   of the MIME-Version header field indicates that the message is in
2552   full compliance with the MIME protocol (as defined in <xref target="RFC2045"/>).
2553   Proxies/gateways are responsible for ensuring full compliance (where
2554   possible) when exporting HTTP messages to strict MIME environments.
2555</t>
2556<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="MIME-Version"/><iref primary="true" item="Grammar" subitem="MIME-Version-v"/>
2557  <x:ref>MIME-Version</x:ref>   = "MIME-Version" ":" <x:ref>OWS</x:ref> <x:ref>MIME-Version-v</x:ref>
2558  <x:ref>MIME-Version-v</x:ref> = 1*<x:ref>DIGIT</x:ref> "." 1*<x:ref>DIGIT</x:ref>
2559</artwork></figure>
2560<t>
2561   MIME version "1.0" is the default for use in HTTP/1.1. However,
2562   HTTP/1.1 message parsing and semantics are defined by this document
2563   and not the MIME specification.
2564</t>
2565</section>
2566
2567<section title="Conversion to Canonical Form" anchor="conversion.to.canonical.form">
2568<t>
2569   <xref target="RFC2045"/> requires that an Internet mail entity be converted to
2570   canonical form prior to being transferred, as described in <xref target="RFC2049" x:fmt="of" x:sec="4"/>.
2571   <xref target="canonicalization.and.text.defaults"/> of this document describes the forms
2572   allowed for subtypes of the "text" media type when transmitted over
2573   HTTP. <xref target="RFC2046"/> requires that content with a type of "text" represent
2574   line breaks as CRLF and forbids the use of CR or LF outside of line
2575   break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a
2576   line break within text content when a message is transmitted over
2577   HTTP.
2578</t>
2579<t>
2580   Where it is possible, a proxy or gateway from HTTP to a strict MIME
2581   environment &SHOULD; translate all line breaks within the text media
2582   types described in <xref target="canonicalization.and.text.defaults"/> of this document to the RFC 2049
2583   canonical form of CRLF. Note, however, that this might be complicated
2584   by the presence of a Content-Encoding and by the fact that HTTP
2585   allows the use of some character sets which do not use octets 13 and
2586   10 to represent CR and LF, as is the case for some multi-byte
2587   character sets.
2588</t>
2589<t>
2590   Implementors should note that conversion will break any cryptographic
2591   checksums applied to the original content unless the original content
2592   is already in canonical form. Therefore, the canonical form is
2593   recommended for any content that uses such checksums in HTTP.
2594</t>
2595</section>
2596
2597
2598<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
2599<t>
2600   HTTP/1.1 uses a restricted set of date formats (&full-date;) to
2601   simplify the process of date comparison. Proxies and gateways from
2602   other protocols &SHOULD; ensure that any Date header field present in a
2603   message conforms to one of the HTTP/1.1 formats and rewrite the date
2604   if necessary.
2605</t>
2606</section>
2607
2608<section title="Introduction of Content-Encoding" anchor="introduction.of.content-encoding">
2609<t>
2610   RFC 2045 does not include any concept equivalent to HTTP/1.1's
2611   Content-Encoding header field. Since this acts as a modifier on the
2612   media type, proxies and gateways from HTTP to MIME-compliant
2613   protocols &MUST; either change the value of the Content-Type header
2614   field or decode the entity-body before forwarding the message. (Some
2615   experimental applications of Content-Type for Internet mail have used
2616   a media-type parameter of ";conversions=&lt;content-coding&gt;" to perform
2617   a function equivalent to Content-Encoding. However, this parameter is
2618   not part of RFC 2045).
2619</t>
2620</section>
2621
2622<section title="No Content-Transfer-Encoding" anchor="no.content-transfer-encoding">
2623<t>
2624   HTTP does not use the Content-Transfer-Encoding field of RFC
2625   2045. Proxies and gateways from MIME-compliant protocols to HTTP &MUST;
2626   remove any Content-Transfer-Encoding
2627   prior to delivering the response message to an HTTP client.
2628</t>
2629<t>
2630   Proxies and gateways from HTTP to MIME-compliant protocols are
2631   responsible for ensuring that the message is in the correct format
2632   and encoding for safe transport on that protocol, where "safe
2633   transport" is defined by the limitations of the protocol being used.
2634   Such a proxy or gateway &SHOULD; label the data with an appropriate
2635   Content-Transfer-Encoding if doing so will improve the likelihood of
2636   safe transport over the destination protocol.
2637</t>
2638</section>
2639
2640<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
2641<t>
2642   HTTP/1.1 introduces the Transfer-Encoding header field (&header-transfer-encoding;).
2643   Proxies/gateways &MUST; remove any transfer-coding prior to
2644   forwarding a message via a MIME-compliant protocol.
2645</t>
2646</section>
2647
2648<section title="MHTML and Line Length Limitations" anchor="mhtml.line.length">
2649<t>
2650   HTTP implementations which share code with MHTML <xref target="RFC2557"/> implementations
2651   need to be aware of MIME line length limitations. Since HTTP does not
2652   have this limitation, HTTP does not fold long lines. MHTML messages
2653   being transported by HTTP follow all conventions of MHTML, including
2654   line length limitations and folding, canonicalization, etc., since
2655   HTTP transports all message-bodies as payload (see <xref target="multipart.types"/>) and
2656   does not interpret the content or any MIME header lines that might be
2657   contained therein.
2658</t>
2659</section>
2660</section>
2661
2662<section title="Additional Features" anchor="additional.features">
2663<t>
2664   <xref target="RFC1945"/> and <xref target="RFC2068"/> document protocol elements used by some
2665   existing HTTP implementations, but not consistently and correctly
2666   across most HTTP/1.1 applications. Implementors are advised to be
2667   aware of these features, but cannot rely upon their presence in, or
2668   interoperability with, other HTTP/1.1 applications. Some of these
2669   describe proposed experimental features, and some describe features
2670   that experimental deployment found lacking that are now addressed in
2671   the base HTTP/1.1 specification.
2672</t>
2673<t>
2674   A number of other headers, such as Content-Disposition and Title,
2675   from SMTP and MIME are also often implemented (see <xref target="RFC2076"/>).
2676</t>
2677
2678<section title="Content-Disposition" anchor="content-disposition">
2679<iref item="Headers" subitem="Content-Disposition" primary="true" x:for-anchor=""/>
2680<iref item="Content-Disposition header" primary="true" x:for-anchor=""/>
2681  <x:anchor-alias value="content-disposition"/>
2682  <x:anchor-alias value="content-disposition-v"/>
2683  <x:anchor-alias value="disposition-type"/>
2684  <x:anchor-alias value="disposition-parm"/>
2685  <x:anchor-alias value="disp-extension-parm"/>
2686  <x:anchor-alias value="disp-extension-token"/>
2687  <x:anchor-alias value="filename-parm"/>
2688<t>
2689   The "Content-Disposition" response-header field has been proposed as a
2690   means for the origin server to suggest a default filename if the user
2691   requests that the content is saved to a file. This usage is derived
2692   from the definition of Content-Disposition in <xref target="RFC2183"/>.
2693</t>
2694<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"/>
2695  <x:ref>content-disposition</x:ref> = "Content-Disposition" ":" <x:ref>OWS</x:ref>
2696                        <x:ref>content-disposition-v</x:ref>
2697  <x:ref>content-disposition-v</x:ref> = <x:ref>disposition-type</x:ref>
2698                          *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>disposition-parm</x:ref> )
2699  <x:ref>disposition-type</x:ref> = "attachment" / <x:ref>disp-extension-token</x:ref>
2700  <x:ref>disposition-parm</x:ref> = <x:ref>filename-parm</x:ref> / <x:ref>disp-extension-parm</x:ref>
2701  <x:ref>filename-parm</x:ref> = "filename" "=" <x:ref>quoted-string</x:ref>
2702  <x:ref>disp-extension-token</x:ref> = <x:ref>token</x:ref>
2703  <x:ref>disp-extension-parm</x:ref> = <x:ref>token</x:ref> "=" <x:ref>word</x:ref>
2704</artwork></figure>
2705<t>
2706   An example is
2707</t>
2708<figure><artwork type="example">
2709  Content-Disposition: attachment; filename="fname.ext"
2710</artwork></figure>
2711<t>
2712   The receiving user agent &SHOULD-NOT;  respect any directory path
2713   information present in the filename-parm parameter, which is the only
2714   parameter believed to apply to HTTP implementations at this time. The
2715   filename &SHOULD; be treated as a terminal component only.
2716</t>
2717<t>
2718   If this header is used in a response with the application/octet-stream
2719   content-type, the implied suggestion is that the user agent
2720   should not display the response, but directly enter a "save response
2721   as..." dialog.
2722</t>
2723<t>
2724   See <xref target="content-disposition.issues"/> for Content-Disposition security issues.
2725</t>
2726</section>
2727</section>
2728
2729<section title="Compatibility with Previous Versions" anchor="compatibility">
2730<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2731<t>
2732   Transfer-coding and message lengths all interact in ways that
2733   required fixing exactly when chunked encoding is used (to allow for
2734   transfer encoding that may not be self delimiting); it was important
2735   to straighten out exactly how message lengths are computed.
2736   (<xref target="entity.length"/>, see also <xref target="Part1"/>,
2737   <xref target="Part5"/> and <xref target="Part6"/>).
2738</t>
2739<t>
2740   Charset wildcarding is introduced to avoid explosion of character set
2741   names in accept headers. (<xref target="header.accept-charset"/>)
2742</t>
2743<t>
2744   Content-Base was deleted from the specification: it was not
2745   implemented widely, and there is no simple, safe way to introduce it
2746   without a robust extension mechanism. In addition, it is used in a
2747   similar, but not identical fashion in MHTML <xref target="RFC2557"/>.
2748</t>
2749<t>
2750   A content-coding of "identity" was introduced, to solve problems
2751   discovered in caching. (<xref target="content.codings"/>)
2752</t>
2753<t>
2754   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
2755   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
2756   specification, but not commonly implemented. See <xref target="RFC2068" x:fmt="of" x:sec="19.6.2"/>.
2757</t>
2758</section>
2759
2760<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2761<t>
2762  Clarify contexts that charset is used in.
2763  (<xref target="character.sets"/>)
2764</t>
2765<t>
2766  Remove base URI setting semantics for Content-Location due to poor
2767  implementation support, which was caused by too many broken servers emitting
2768  bogus Content-Location headers, and also the potentially undesirable effect
2769  of potentially breaking relative links in content-negotiated resources.
2770  (<xref target="header.content-location"/>)
2771</t>
2772<t>
2773  Remove reference to non-existant identity transfer-coding value tokens.
2774  (<xref target="no.content-transfer-encoding"/>)
2775</t>
2776</section>
2777
2778</section>
2779
2780<?BEGININC p3-payload.abnf-appendix ?>
2781<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2782<figure>
2783<artwork type="abnf" name="p3-payload.parsed-abnf">
2784<x:ref>Accept</x:ref> = "Accept:" OWS Accept-v
2785<x:ref>Accept-Charset</x:ref> = "Accept-Charset:" OWS Accept-Charset-v
2786<x:ref>Accept-Charset-v</x:ref> = *( "," OWS ) ( charset / "*" ) [ OWS ";" OWS "q="
2787 qvalue ] *( OWS "," [ OWS ( charset / "*" ) [ OWS ";" OWS "q="
2788 qvalue ] ] )
2789<x:ref>Accept-Encoding</x:ref> = "Accept-Encoding:" OWS Accept-Encoding-v
2790<x:ref>Accept-Encoding-v</x:ref> = [ ( "," / ( codings [ OWS ";" OWS "q=" qvalue ] )
2791 ) *( OWS "," [ OWS codings [ OWS ";" OWS "q=" qvalue ] ] ) ]
2792<x:ref>Accept-Language</x:ref> = "Accept-Language:" OWS Accept-Language-v
2793<x:ref>Accept-Language-v</x:ref> = *( "," OWS ) language-range [ OWS ";" OWS "q="
2794 qvalue ] *( OWS "," [ OWS language-range [ OWS ";" OWS "q=" qvalue ]
2795 ] )
2796<x:ref>Accept-v</x:ref> = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
2797 OWS media-range [ accept-params ] ] ) ]
2798
2799<x:ref>Content-Encoding</x:ref> = "Content-Encoding:" OWS Content-Encoding-v
2800<x:ref>Content-Encoding-v</x:ref> = *( "," OWS ) content-coding *( OWS "," [ OWS
2801 content-coding ] )
2802<x:ref>Content-Language</x:ref> = "Content-Language:" OWS Content-Language-v
2803<x:ref>Content-Language-v</x:ref> = *( "," OWS ) language-tag *( OWS "," [ OWS
2804 language-tag ] )
2805<x:ref>Content-Length</x:ref> = &lt;Content-Length, defined in [Part1], Section 9.2&gt;
2806<x:ref>Content-Location</x:ref> = "Content-Location:" OWS Content-Location-v
2807<x:ref>Content-Location-v</x:ref> = absolute-URI / partial-URI
2808<x:ref>Content-MD5</x:ref> = "Content-MD5:" OWS Content-MD5-v
2809<x:ref>Content-MD5-v</x:ref> = &lt;base64 of 128 bit MD5 digest as per [RFC1864]&gt;
2810<x:ref>Content-Range</x:ref> = &lt;Content-Range, defined in [Part5], Section 5.2&gt;
2811<x:ref>Content-Type</x:ref> = "Content-Type:" OWS Content-Type-v
2812<x:ref>Content-Type-v</x:ref> = media-type
2813
2814<x:ref>Expires</x:ref> = &lt;Expires, defined in [Part6], Section 3.3&gt;
2815
2816<x:ref>Last-Modified</x:ref> = &lt;Last-Modified, defined in [Part4], Section 6.6&gt;
2817
2818<x:ref>MIME-Version</x:ref> = "MIME-Version:" OWS MIME-Version-v
2819<x:ref>MIME-Version-v</x:ref> = 1*DIGIT "." 1*DIGIT
2820
2821<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2822
2823<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [Part1], Section 2.6&gt;
2824<x:ref>accept-ext</x:ref> = OWS ";" OWS token [ "=" word ]
2825<x:ref>accept-params</x:ref> = OWS ";" OWS "q=" qvalue *accept-ext
2826<x:ref>attribute</x:ref> = token
2827
2828<x:ref>charset</x:ref> = token
2829<x:ref>codings</x:ref> = ( content-coding / "*" )
2830<x:ref>content-coding</x:ref> = token
2831<x:ref>content-disposition</x:ref> = "Content-Disposition:" OWS
2832 content-disposition-v
2833<x:ref>content-disposition-v</x:ref> = disposition-type *( OWS ";" OWS
2834 disposition-parm )
2835
2836<x:ref>disp-extension-parm</x:ref> = token "=" word
2837<x:ref>disp-extension-token</x:ref> = token
2838<x:ref>disposition-parm</x:ref> = filename-parm / disp-extension-parm
2839<x:ref>disposition-type</x:ref> = "attachment" / disp-extension-token
2840
2841<x:ref>entity-body</x:ref> = *OCTET
2842<x:ref>entity-header</x:ref> = Content-Encoding / Content-Language / Content-Length
2843 / Content-Location / Content-MD5 / Content-Range / Content-Type /
2844 Expires / Last-Modified / extension-header
2845<x:ref>extension-header</x:ref> = header-field
2846
2847<x:ref>filename-parm</x:ref> = "filename=" quoted-string
2848
2849<x:ref>header-field</x:ref> = &lt;header-field, defined in [Part1], Section 3.2&gt;
2850
2851<x:ref>language-range</x:ref> = &lt;language-range, defined in [RFC4647], Section 2.1&gt;
2852<x:ref>language-tag</x:ref> = &lt;Language-Tag, defined in [RFC5646], Section 2.1&gt;
2853
2854<x:ref>media-range</x:ref> = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
2855 ";" OWS parameter )
2856<x:ref>media-type</x:ref> = type "/" subtype *( OWS ";" OWS parameter )
2857
2858<x:ref>parameter</x:ref> = attribute "=" value
2859<x:ref>partial-URI</x:ref> = &lt;partial-URI, defined in [Part1], Section 2.6&gt;
2860
2861<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2862<x:ref>qvalue</x:ref> = &lt;qvalue, defined in [Part1], Section 6.4&gt;
2863
2864<x:ref>subtype</x:ref> = token
2865
2866<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2867<x:ref>type</x:ref> = token
2868
2869<x:ref>value</x:ref> = word
2870
2871<x:ref>word</x:ref> = &lt;word, defined in [Part1], Section 1.2.2&gt;
2872</artwork>
2873</figure>
2874<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2875; Accept defined but not used
2876; Accept-Charset defined but not used
2877; Accept-Encoding defined but not used
2878; Accept-Language defined but not used
2879; MIME-Version defined but not used
2880; content-disposition defined but not used
2881; entity-body defined but not used
2882; entity-header defined but not used
2883</artwork></figure></section>
2884<?ENDINC p3-payload.abnf-appendix ?>
2885
2886<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
2887
2888<section title="Since RFC2616">
2889<t>
2890  Extracted relevant partitions from <xref target="RFC2616"/>.
2891</t>
2892</section>
2893
2894<section title="Since draft-ietf-httpbis-p3-payload-00">
2895<t>
2896  Closed issues:
2897  <list style="symbols"> 
2898    <t>
2899      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
2900      "Media Type Registrations"
2901      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
2902    </t>
2903    <t>
2904      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/14"/>:
2905      "Clarification regarding quoting of charset values"
2906      (<eref target="http://purl.org/NET/http-errata#charactersets"/>)
2907    </t>
2908    <t>
2909      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
2910      "Remove 'identity' token references"
2911      (<eref target="http://purl.org/NET/http-errata#identity"/>)
2912    </t>
2913    <t>
2914      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/25"/>:
2915      "Accept-Encoding BNF"
2916    </t>
2917    <t>
2918      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2919      "Normative and Informative references"
2920    </t>
2921    <t>
2922      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
2923      "RFC1700 references"
2924    </t>
2925    <t>
2926      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
2927      "Updating to RFC4288"
2928    </t>
2929    <t>
2930      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2931      "Informative references"
2932    </t>
2933    <t>
2934      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2935      "ISO-8859-1 Reference"
2936    </t>
2937    <t>
2938      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/68"/>:
2939      "Encoding References Normative"
2940    </t>
2941    <t>
2942      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2943      "Normative up-to-date references"
2944    </t>
2945  </list>
2946</t>
2947</section>
2948
2949<section title="Since draft-ietf-httpbis-p3-payload-01">
2950<t>
2951  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2952  <list style="symbols"> 
2953    <t>
2954      Add explicit references to BNF syntax and rules imported from other parts of the specification.
2955    </t>
2956  </list>
2957</t>
2958</section>
2959
2960<section title="Since draft-ietf-httpbis-p3-payload-02" anchor="changes.since.02">
2961<t>
2962  Closed issues:
2963  <list style="symbols"> 
2964    <t>
2965      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/67"/>:
2966      "Quoting Charsets"
2967    </t>
2968    <t>
2969      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/105"/>:
2970      "Classification for Allow header"
2971    </t>
2972    <t>
2973      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/115"/>:
2974      "missing default for qvalue in description of Accept-Encoding"
2975    </t>
2976  </list>
2977</t>
2978<t>
2979  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2980  <list style="symbols"> 
2981    <t>
2982      Reference RFC 3984, and update header registrations for headers defined
2983      in this document.
2984    </t>
2985  </list>
2986</t>
2987</section>
2988
2989<section title="Since draft-ietf-httpbis-p3-payload-03" anchor="changes.since.03">
2990<t>
2991  Closed issues:
2992  <list style="symbols"> 
2993    <t>
2994      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/67"/>:
2995      "Quoting Charsets"
2996    </t>
2997    <t>
2998      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/113"/>:
2999      "language tag matching (Accept-Language) vs RFC4647"
3000    </t>
3001    <t>
3002      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/121"/>:
3003      "RFC 1806 has been replaced by RFC2183"
3004    </t>
3005  </list>
3006</t>
3007<t>
3008  Other changes:
3009  <list style="symbols"> 
3010    <t>
3011      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/68"/>:
3012      "Encoding References Normative" -- rephrase the annotation and reference
3013      <xref target="BCP97"/>.
3014    </t>
3015  </list>
3016</t>
3017 </section>
3018
3019<section title="Since draft-ietf-httpbis-p3-payload-04" anchor="changes.since.04">
3020<t>
3021  Closed issues:
3022  <list style="symbols"> 
3023    <t>
3024      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
3025      "RFC 2822 is updated by RFC 5322"
3026    </t>
3027  </list>
3028</t>
3029<t>
3030  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3031  <list style="symbols"> 
3032    <t>
3033      Use "/" instead of "|" for alternatives.
3034    </t>
3035    <t>
3036      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
3037      whitespace ("OWS") and required whitespace ("RWS").
3038    </t>
3039    <t>
3040      Rewrite ABNFs to spell out whitespace rules, factor out
3041      header value format definitions.
3042    </t>
3043  </list>
3044</t>
3045</section>
3046
3047<section title="Since draft-ietf-httpbis-p3-payload-05" anchor="changes.since.05">
3048<t>
3049  Closed issues:
3050  <list style="symbols"> 
3051    <t>
3052      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/118"/>:
3053      "Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
3054    </t>
3055  </list>
3056</t>
3057<t>
3058  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
3059  <list style="symbols"> 
3060    <t>
3061      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
3062    </t>
3063  </list>
3064</t>
3065<t>
3066  Other changes:
3067  <list style="symbols"> 
3068    <t>
3069      Move definition of quality values into Part 1.
3070    </t>
3071  </list>
3072</t>
3073</section>
3074
3075<section title="Since draft-ietf-httpbis-p3-payload-06" anchor="changes.since.06">
3076<t>
3077  Closed issues:
3078  <list style="symbols"> 
3079    <t>
3080      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/80"/>:
3081      "Content-Location isn't special"
3082    </t>
3083    <t>
3084      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/155"/>:
3085      "Content Sniffing"
3086    </t>
3087  </list>
3088</t>
3089</section>
3090
3091<section title="Since draft-ietf-httpbis-p3-payload-07" anchor="changes.since.07">
3092<t>
3093  Closed issues:
3094  <list style="symbols"> 
3095    <t>
3096      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/13"/>:
3097      "Updated reference for language tags"
3098    </t>
3099    <t>
3100      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/110"/>:
3101      "Clarify rules for determining what entities a response carries"
3102    </t>
3103    <t>
3104      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/154"/>:
3105      "Content-Location base-setting problems"
3106    </t>
3107    <t>
3108      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/155"/>:
3109      "Content Sniffing"
3110    </t>
3111    <t>
3112      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/188"/>:
3113      "pick IANA policy (RFC5226) for Transfer Coding / Content Coding"
3114    </t>
3115    <t>
3116      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/189"/>:
3117      "move definitions of gzip/deflate/compress to part 1"
3118    </t>
3119  </list>
3120</t>
3121<t>
3122  Partly resolved issues:
3123  <list style="symbols"> 
3124    <t>
3125      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/148"/>:
3126      "update IANA requirements wrt Transfer-Coding values" (add the
3127      IANA Considerations subsection)
3128    </t>
3129    <t>
3130      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/149"/>:
3131      "update IANA requirements wrt Content-Coding values" (add the
3132      IANA Considerations subsection)
3133    </t>
3134  </list>
3135</t>
3136</section>
3137
3138<section title="Since draft-ietf-httpbis-p3-payload-08" anchor="changes.since.08">
3139<t>
3140  Closed issues:
3141  <list style="symbols"> 
3142    <t>
3143      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/81"/>:
3144      "Content Negotiation for media types"
3145    </t>
3146    <t>
3147      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/181"/>:
3148      "Accept-Language: which RFC4647 filtering?"
3149    </t>
3150  </list>
3151</t>
3152</section>
3153
3154<section title="Since draft-ietf-httpbis-p3-payload-09" anchor="changes.since.09">
3155<t>
3156  Closed issues:
3157  <list style="symbols"> 
3158    <t>
3159      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/143"/>:
3160      "IANA registry for content/transfer encodings"
3161    </t>
3162    <t>
3163      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/155"/>:
3164      "Content Sniffing"
3165    </t>
3166    <t>
3167      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/200"/>:
3168      "use of term "word" when talking about header structure"
3169    </t>
3170  </list>
3171</t>
3172<t>
3173  Partly resolved issues:
3174  <list style="symbols"> 
3175    <t>
3176      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
3177      "Term for the requested resource's URI"
3178    </t>
3179  </list>
3180</t>
3181</section>
3182
3183</section>
3184
3185</back>
3186</rfc>
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