source: draft-ietf-httpbis/latest/p3-payload.xml @ 248

Last change on this file since 248 was 248, checked in by julian.reschke@…, 14 years ago

Resolve #115: add qvalue defaulting for Accept-Encoding (closes #115).

  • Property svn:eol-style set to native
File size: 112.3 KB
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=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "May">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY notation-abnf            "<xref target='Part1' x:rel='#notation.abnf' xmlns:x=''/>">
18  <!ENTITY basic-rules              "<xref target='Part1' x:rel='#basic.rules' xmlns:x=''/>">
19  <!ENTITY caching-neg-resp         "<xref target='Part6' x:rel='#caching.negotiated.responses' xmlns:x=''/>">
20  <!ENTITY header-transfer-encoding "<xref target='Part1' x:rel='#header.transfer-encoding' xmlns:x=''/>">
21  <!ENTITY header-content-length    "<xref target='Part1' x:rel='#header.content-length' xmlns:x=''/>">
22  <!ENTITY header-content-range     "<xref target='Part5' x:rel='#header.content-range' xmlns:x=''/>">
23  <!ENTITY header-expires           "<xref target='Part6' x:rel='#header.expires' xmlns:x=''/>">
24  <!ENTITY header-last-modified     "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
25  <!ENTITY header-user-agent        "<xref target='Part2' x:rel='#header.user-agent' xmlns:x=''/>">
26  <!ENTITY header-vary              "<xref target='Part6' x:rel='#header.vary' xmlns:x=''/>">
27  <!ENTITY message-body             "<xref target='Part1' x:rel='#message.body' xmlns:x=''/>">
28  <!ENTITY message-length           "<xref target='Part1' x:rel='#message.length' xmlns:x=''/>">
29  <!ENTITY message-headers          "<xref target='Part1' x:rel='#message.headers' xmlns:x=''/>">
30  <!ENTITY general-syntax           "<xref target='Part1' x:rel='#general.syntax' xmlns:x=''/>">
31  <!ENTITY multipart-byteranges     "<xref target='Part5' x:rel='' xmlns:x=''/>">
33<?rfc toc="yes" ?>
34<?rfc symrefs="yes" ?>
35<?rfc sortrefs="yes" ?>
36<?rfc compact="yes"?>
37<?rfc subcompact="no" ?>
38<?rfc linkmailto="no" ?>
39<?rfc editing="no" ?>
40<?rfc comments="yes"?>
41<?rfc inline="yes"?>
42<?rfc-ext allow-markup-in-artwork="yes" ?>
43<?rfc-ext include-references-in-index="yes" ?>
44<rfc obsoletes="2616" category="std"
45     ipr="full3978" docName="draft-ietf-httpbis-p3-payload-&ID-VERSION;"
46     xmlns:x=''>
49  <title abbrev="HTTP/1.1, Part 3">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
51  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
52    <organization abbrev="Day Software">Day Software</organization>
53    <address>
54      <postal>
55        <street>23 Corporate Plaza DR, Suite 280</street>
56        <city>Newport Beach</city>
57        <region>CA</region>
58        <code>92660</code>
59        <country>USA</country>
60      </postal>
61      <phone>+1-949-706-5300</phone>
62      <facsimile>+1-949-706-5305</facsimile>
63      <email></email>
64      <uri></uri>
65    </address>
66  </author>
68  <author initials="J." surname="Gettys" fullname="Jim Gettys">
69    <organization>One Laptop per Child</organization>
70    <address>
71      <postal>
72        <street>21 Oak Knoll Road</street>
73        <city>Carlisle</city>
74        <region>MA</region>
75        <code>01741</code>
76        <country>USA</country>
77      </postal>
78      <email></email>
79      <uri></uri>
80    </address>
81  </author>
83  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
84    <organization abbrev="HP">Hewlett-Packard Company</organization>
85    <address>
86      <postal>
87        <street>HP Labs, Large Scale Systems Group</street>
88        <street>1501 Page Mill Road, MS 1177</street>
89        <city>Palo Alto</city>
90        <region>CA</region>
91        <code>94304</code>
92        <country>USA</country>
93      </postal>
94      <email></email>
95    </address>
96  </author>
98  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
100    <address>
101      <postal>
102        <street>1 Microsoft Way</street>
103        <city>Redmond</city>
104        <region>WA</region>
105        <code>98052</code>
106        <country>USA</country>
107      </postal>
108      <email></email>
109    </address>
110  </author>
112  <author initials="L." surname="Masinter" fullname="Larry Masinter">
113    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
114    <address>
115      <postal>
116        <street>345 Park Ave</street>
117        <city>San Jose</city>
118        <region>CA</region>
119        <code>95110</code>
120        <country>USA</country>
121      </postal>
122      <email></email>
123      <uri></uri>
124    </address>
125  </author>
127  <author initials="P." surname="Leach" fullname="Paul J. Leach">
128    <organization abbrev="Microsoft">Microsoft Corporation</organization>
129    <address>
130      <postal>
131        <street>1 Microsoft Way</street>
132        <city>Redmond</city>
133        <region>WA</region>
134        <code>98052</code>
135      </postal>
136      <email></email>
137    </address>
138  </author>
140  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
141    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
142    <address>
143      <postal>
144        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
145        <street>The Stata Center, Building 32</street>
146        <street>32 Vassar Street</street>
147        <city>Cambridge</city>
148        <region>MA</region>
149        <code>02139</code>
150        <country>USA</country>
151      </postal>
152      <email></email>
153      <uri></uri>
154    </address>
155  </author>
157  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
158    <organization abbrev="W3C">World Wide Web Consortium</organization>
159    <address>
160      <postal>
161        <street>W3C / ERCIM</street>
162        <street>2004, rte des Lucioles</street>
163        <city>Sophia-Antipolis</city>
164        <region>AM</region>
165        <code>06902</code>
166        <country>France</country>
167      </postal>
168      <email></email>
169      <uri></uri>
170    </address>
171  </author>
173  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
174    <organization abbrev="greenbytes">greenbytes GmbH</organization>
175    <address>
176      <postal>
177        <street>Hafenweg 16</street>
178        <city>Muenster</city><region>NW</region><code>48155</code>
179        <country>Germany</country>
180      </postal>
181      <phone>+49 251 2807760</phone>   
182      <facsimile>+49 251 2807761</facsimile>   
183      <email></email>       
184      <uri></uri>     
185    </address>
186  </author>
188  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
192   The Hypertext Transfer Protocol (HTTP) is an application-level
193   protocol for distributed, collaborative, hypermedia information
194   systems. HTTP has been in use by the World Wide Web global information
195   initiative since 1990. This document is Part 3 of the seven-part specification
196   that defines the protocol referred to as "HTTP/1.1" and, taken together,
197   obsoletes RFC 2616.  Part 3 defines HTTP message content,
198   metadata, and content negotiation.
202<note title="Editorial Note (To be removed by RFC Editor)">
203  <t>
204    Discussion of this draft should take place on the HTTPBIS working group
205    mailing list ( The current issues list is
206    at <eref target=""/>
207    and related documents (including fancy diffs) can be found at
208    <eref target=""/>.
209  </t>
210  <t>
211    This draft incorporates those issue resolutions that were either
212    collected in the original RFC2616 errata list (<eref target=""/>),
213    or which were agreed upon on the mailing list between October 2006 and
214    November 2007 (as published in "draft-lafon-rfc2616bis-03").
215  </t>
219<section title="Introduction" anchor="introduction">
221   This document defines HTTP/1.1 message payloads (a.k.a., content), the
222   associated metadata header fields that define how the payload is intended
223   to be interpreted by a recipient, the request header fields that
224   may influence content selection, and the various selection algorithms
225   that are collectively referred to as HTTP content negotiation.
228   This document is currently disorganized in order to minimize the changes
229   between drafts and enable reviewers to see the smaller errata changes.
230   The next draft will reorganize the sections to better reflect the content.
231   In particular, the sections on entities will be renamed payload and moved
232   to the first half of the document, while the sections on content negotiation
233   and associated request header fields will be moved to the second half.  The
234   current mess reflects how widely dispersed these topics and associated
235   requirements had become in <xref target="RFC2616"/>.
238<section title="Requirements" anchor="intro.requirements">
240   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
241   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
242   document are to be interpreted as described in <xref target="RFC2119"/>.
245   An implementation is not compliant if it fails to satisfy one or more
246   of the &MUST; or &REQUIRED; level requirements for the protocols it
247   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
248   level and all the &SHOULD; level requirements for its protocols is said
249   to be "unconditionally compliant"; one that satisfies all the &MUST;
250   level requirements but not all the &SHOULD; level requirements for its
251   protocols is said to be "conditionally compliant."
256<section title="Notational Conventions and Generic Grammar" anchor="notation">
257  <x:anchor-alias value="ALPHA"/>
258  <x:anchor-alias value="DIGIT"/>
259  <x:anchor-alias value="OCTET"/>
260  <x:anchor-alias value="quoted-string"/>
261  <x:anchor-alias value="token"/>
263  This specification uses the ABNF syntax defined in &notation-abnf; and
264  the core rules defined in &basic-rules;:
265  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
266  <eref target=""/>.</cref>
268<figure><artwork type="abnf2616">
269  <x:ref>ALPHA</x:ref>          = &lt;ALPHA, defined in &basic-rules;&gt;
270  <x:ref>DIGIT</x:ref>          = &lt;DIGIT, defined in &basic-rules;&gt;
271  <x:ref>OCTET</x:ref>          = &lt;OCTET, defined in &basic-rules;&gt;
273<figure><artwork type="abnf2616">
274  <x:ref>quoted-string</x:ref>  = &lt;quoted-string, defined in &basic-rules;&gt;
275  <x:ref>token</x:ref>          = &lt;token, defined in &basic-rules;&gt;
277<t anchor="abnf.dependencies">
278  <x:anchor-alias value="absoluteURI"/>
279  <x:anchor-alias value="Allow"/>
280  <x:anchor-alias value="Content-Length"/>
281  <x:anchor-alias value="Content-Range"/>
282  <x:anchor-alias value="Expires"/>
283  <x:anchor-alias value="Last-Modified"/>
284  <x:anchor-alias value="message-header"/>
285  <x:anchor-alias value="relativeURI"/>
286  The ABNF rules below are defined in other parts:
288<figure><!--Part1--><artwork type="abnf2616">
289  <x:ref>absoluteURI</x:ref>    = &lt;absoluteURI, defined in &general-syntax;&gt;
290  <x:ref>Content-Length</x:ref> = &lt;Content-Length, defined in &header-content-length;&gt;
291  <x:ref>relativeURI</x:ref>    = &lt;relativeURI, defined in &general-syntax;&gt;
292  <x:ref>message-header</x:ref> = &lt;message-header, defined in &message-headers;&gt;
294<figure><!--Part4--><artwork type="abnf2616">
295  <x:ref>Last-Modified</x:ref>  = &lt;Last-Modified, defined in &header-last-modified;&gt;
297<figure><!--Part5--><artwork type="abnf2616">
298  <x:ref>Content-Range</x:ref>  = &lt;Content-Range, defined in &header-content-range;&gt;
300<figure><!--Part6--><artwork type="abnf2616">
301  <x:ref>Expires</x:ref>        = &lt;Expires, defined in &header-expires;&gt;
305<section title="Protocol Parameters" anchor="protocol.parameters">
307<section title="Character Sets" anchor="character.sets">
309   HTTP uses the same definition of the term "character set" as that
310   described for MIME:
313   The term "character set" is used in this document to refer to a
314   method used with one or more tables to convert a sequence of octets
315   into a sequence of characters. Note that unconditional conversion in
316   the other direction is not required, in that not all characters may
317   be available in a given character set and a character set may provide
318   more than one sequence of octets to represent a particular character.
319   This definition is intended to allow various kinds of character
320   encoding, from simple single-table mappings such as US-ASCII to
321   complex table switching methods such as those that use ISO-2022's
322   techniques. However, the definition associated with a MIME character
323   set name &MUST; fully specify the mapping to be performed from octets
324   to characters. In particular, use of external profiling information
325   to determine the exact mapping is not permitted.
328      <x:h>Note:</x:h> This use of the term "character set" is more commonly
329      referred to as a "character encoding." However, since HTTP and
330      MIME share the same registry, it is important that the terminology
331      also be shared.
333<t anchor="rule.charset">
334  <x:anchor-alias value="charset"/>
335   HTTP character sets are identified by case-insensitive tokens. The
336   complete set of tokens is defined by the IANA Character Set registry
337   (<eref target=""/>).
339<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="charset"/>
340  <x:ref>charset</x:ref> = <x:ref>token</x:ref>
343   Although HTTP allows an arbitrary token to be used as a charset
344   value, any token that has a predefined value within the IANA
345   Character Set registry &MUST; represent the character set defined
346   by that registry. Applications &SHOULD; limit their use of character
347   sets to those defined by the IANA registry.
350   HTTP uses charset in two contexts: within an Accept-Charset request
351   header (in which the charset value is an unquoted token) and as the
352   value of a parameter in a Content-Type header (within a request or
353   response), in which case the parameter value of the charset parameter
354   may be quoted.
357   Implementors should be aware of IETF character set requirements <xref target="RFC3629"/>
358   <xref target="RFC2277"/>.
361<section title="Missing Charset" anchor="missing.charset">
363   Some HTTP/1.0 software has interpreted a Content-Type header without
364   charset parameter incorrectly to mean "recipient should guess."
365   Senders wishing to defeat this behavior &MAY; include a charset
366   parameter even when the charset is ISO-8859-1 (<xref target="ISO-8859-1"/>) and &SHOULD; do so when
367   it is known that it will not confuse the recipient.
370   Unfortunately, some older HTTP/1.0 clients did not deal properly with
371   an explicit charset parameter. HTTP/1.1 recipients &MUST; respect the
372   charset label provided by the sender; and those user agents that have
373   a provision to "guess" a charset &MUST; use the charset from the
374   content-type field if they support that charset, rather than the
375   recipient's preference, when initially displaying a document. See
376   <xref target="canonicalization.and.text.defaults"/>.
381<section title="Content Codings" anchor="content.codings">
382  <x:anchor-alias value="content-coding"/>
384   Content coding values indicate an encoding transformation that has
385   been or can be applied to an entity. Content codings are primarily
386   used to allow a document to be compressed or otherwise usefully
387   transformed without losing the identity of its underlying media type
388   and without loss of information. Frequently, the entity is stored in
389   coded form, transmitted directly, and only decoded by the recipient.
391<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-coding"/>
392  <x:ref>content-coding</x:ref>   = <x:ref>token</x:ref>
395   All content-coding values are case-insensitive. HTTP/1.1 uses
396   content-coding values in the Accept-Encoding (<xref target="header.accept-encoding"/>) and
397   Content-Encoding (<xref target="header.content-encoding"/>) header fields. Although the value
398   describes the content-coding, what is more important is that it
399   indicates what decoding mechanism will be required to remove the
400   encoding.
403   The Internet Assigned Numbers Authority (IANA) acts as a registry for
404   content-coding value tokens. Initially, the registry contains the
405   following tokens:
408   gzip<iref item="gzip"/>
409  <list>
410    <t>
411        An encoding format produced by the file compression program
412        "gzip" (GNU zip) as described in <xref target="RFC1952"/>. This format is a
413        Lempel-Ziv coding (LZ77) with a 32 bit CRC.
414    </t>
415  </list>
418   compress<iref item="compress"/>
419  <list><t>
420        The encoding format produced by the common UNIX file compression
421        program "compress". This format is an adaptive Lempel-Ziv-Welch
422        coding (LZW).
424        Use of program names for the identification of encoding formats
425        is not desirable and is discouraged for future encodings. Their
426        use here is representative of historical practice, not good
427        design. For compatibility with previous implementations of HTTP,
428        applications &SHOULD; consider "x-gzip" and "x-compress" to be
429        equivalent to "gzip" and "compress" respectively.
430  </t></list>
433   deflate<iref item="deflate"/>
434  <list><t>
435        The "zlib" format defined in <xref target="RFC1950"/> in combination with
436        the "deflate" compression mechanism described in <xref target="RFC1951"/>.
437  </t></list>
440   identity<iref item="identity"/>
441  <list><t>
442        The default (identity) encoding; the use of no transformation
443        whatsoever. This content-coding is used only in the Accept-Encoding
444        header, and &SHOULD-NOT;  be used in the Content-Encoding
445        header.
446  </t></list>
449   New content-coding value tokens &SHOULD; be registered; to allow
450   interoperability between clients and servers, specifications of the
451   content coding algorithms needed to implement a new value &SHOULD; be
452   publicly available and adequate for independent implementation, and
453   conform to the purpose of content coding defined in this section.
457<section title="Media Types" anchor="media.types">
458  <x:anchor-alias value="media-type"/>
459  <x:anchor-alias value="type"/>
460  <x:anchor-alias value="subtype"/>
462   HTTP uses Internet Media Types <xref target="RFC2046"/> in the Content-Type (<xref target="header.content-type"/>)
463   and Accept (<xref target="header.accept"/>) header fields in order to provide
464   open and extensible data typing and type negotiation.
466<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"/>
467  <x:ref>media-type</x:ref>     = <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> *( ";" <x:ref>parameter</x:ref> )
468  <x:ref>type</x:ref>           = <x:ref>token</x:ref>
469  <x:ref>subtype</x:ref>        = <x:ref>token</x:ref>
471<t anchor="rule.parameter">
472  <x:anchor-alias value="attribute"/>
473  <x:anchor-alias value="parameter"/>
474  <x:anchor-alias value="value"/>
475   Parameters &MAY; follow the type/subtype in the form of attribute/value
476   pairs.
478<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"/>
479  <x:ref>parameter</x:ref>               = <x:ref>attribute</x:ref> "=" <x:ref>value</x:ref>
480  <x:ref>attribute</x:ref>               = <x:ref>token</x:ref>
481  <x:ref>value</x:ref>                   = <x:ref>token</x:ref> | <x:ref>quoted-string</x:ref>
484   The type, subtype, and parameter attribute names are case-insensitive.
485   Parameter values might or might not be case-sensitive,
486   depending on the semantics of the parameter name. Linear white space
487   (LWS) &MUST-NOT; be used between the type and subtype, nor between an
488   attribute and its value. The presence or absence of a parameter might
489   be significant to the processing of a media-type, depending on its
490   definition within the media type registry.
493   Note that some older HTTP applications do not recognize media type
494   parameters. When sending data to older HTTP applications,
495   implementations &SHOULD; only use media type parameters when they are
496   required by that type/subtype definition.
499   Media-type values are registered with the Internet Assigned Number
500   Authority (IANA). The media type registration process is
501   outlined in <xref target="RFC4288"/>. Use of non-registered media types is
502   discouraged.
505<section title="Canonicalization and Text Defaults" anchor="canonicalization.and.text.defaults">
507   Internet media types are registered with a canonical form. An
508   entity-body transferred via HTTP messages &MUST; be represented in the
509   appropriate canonical form prior to its transmission except for
510   "text" types, as defined in the next paragraph.
513   When in canonical form, media subtypes of the "text" type use CRLF as
514   the text line break. HTTP relaxes this requirement and allows the
515   transport of text media with plain CR or LF alone representing a line
516   break when it is done consistently for an entire entity-body. HTTP
517   applications &MUST; accept CRLF, bare CR, and bare LF as being
518   representative of a line break in text media received via HTTP. In
519   addition, if the text is represented in a character set that does not
520   use octets 13 and 10 for CR and LF respectively, as is the case for
521   some multi-byte character sets, HTTP allows the use of whatever octet
522   sequences are defined by that character set to represent the
523   equivalent of CR and LF for line breaks. This flexibility regarding
524   line breaks applies only to text media in the entity-body; a bare CR
525   or LF &MUST-NOT; be substituted for CRLF within any of the HTTP control
526   structures (such as header fields and multipart boundaries).
529   If an entity-body is encoded with a content-coding, the underlying
530   data &MUST; be in a form defined above prior to being encoded.
533   The "charset" parameter is used with some media types to define the
534   character set (<xref target="character.sets"/>) of the data. When no explicit charset
535   parameter is provided by the sender, media subtypes of the "text"
536   type are defined to have a default charset value of "ISO-8859-1" when
537   received via HTTP. Data in character sets other than "ISO-8859-1" or
538   its subsets &MUST; be labeled with an appropriate charset value. See
539   <xref target="missing.charset"/> for compatibility problems.
543<section title="Multipart Types" anchor="multipart.types">
545   MIME provides for a number of "multipart" types -- encapsulations of
546   one or more entities within a single message-body. All multipart
547   types share a common syntax, as defined in <xref target="RFC2046" x:sec="5.1.1" x:fmt="of"/>,
548   and &MUST; include a boundary parameter as part of the media type
549   value. The message body is itself a protocol element and &MUST;
550   therefore use only CRLF to represent line breaks between body-parts.
551   Unlike in RFC 2046, the epilogue of any multipart message &MUST; be
552   empty; HTTP applications &MUST-NOT; transmit the epilogue (even if the
553   original multipart contains an epilogue). These restrictions exist in
554   order to preserve the self-delimiting nature of a multipart message-body,
555   wherein the "end" of the message-body is indicated by the
556   ending multipart boundary.
559   In general, HTTP treats a multipart message-body no differently than
560   any other media type: strictly as payload. The one exception is the
561   "multipart/byteranges" type (&multipart-byteranges;) when it appears in a 206
562   (Partial Content) response.
563   <!-- jre: re-insert removed text pointing to caching? -->
564   In all
565   other cases, an HTTP user agent &SHOULD; follow the same or similar
566   behavior as a MIME user agent would upon receipt of a multipart type.
567   The MIME header fields within each body-part of a multipart message-body
568   do not have any significance to HTTP beyond that defined by
569   their MIME semantics.
572   In general, an HTTP user agent &SHOULD; follow the same or similar
573   behavior as a MIME user agent would upon receipt of a multipart type.
574   If an application receives an unrecognized multipart subtype, the
575   application &MUST; treat it as being equivalent to "multipart/mixed".
578      <x:h>Note:</x:h> The "multipart/form-data" type has been specifically defined
579      for carrying form data suitable for processing via the POST
580      request method, as described in <xref target="RFC2388"/>.
585<section title="Quality Values" anchor="quality.values">
586  <x:anchor-alias value="qvalue"/>
588   HTTP content negotiation (<xref target="content.negotiation"/>) uses short "floating point"
589   numbers to indicate the relative importance ("weight") of various
590   negotiable parameters.  A weight is normalized to a real number in
591   the range 0 through 1, where 0 is the minimum and 1 the maximum
592   value. If a parameter has a quality value of 0, then content with
593   this parameter is `not acceptable' for the client. HTTP/1.1
594   applications &MUST-NOT; generate more than three digits after the
595   decimal point. User configuration of these values &SHOULD; also be
596   limited in this fashion.
598<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="qvalue"/>
599  <x:ref>qvalue</x:ref>         = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
600                 | ( "1" [ "." 0*3("0") ] )
603   "Quality values" is a misnomer, since these values merely represent
604   relative degradation in desired quality.
608<section title="Language Tags" anchor="language.tags">
609  <x:anchor-alias value="language-tag"/>
610  <x:anchor-alias value="primary-tag"/>
611  <x:anchor-alias value="subtag"/>
613   A language tag identifies a natural language spoken, written, or
614   otherwise conveyed by human beings for communication of information
615   to other human beings. Computer languages are explicitly excluded.
616   HTTP uses language tags within the Accept-Language and Content-Language
617   fields.
620   The syntax and registry of HTTP language tags is the same as that
621   defined by <xref target="RFC1766"/>. In summary, a language tag is composed of 1
622   or more parts: A primary language tag and a possibly empty series of
623   subtags:
625<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="language-tag"/><iref primary="true" item="Grammar" subitem="primary-tag"/><iref primary="true" item="Grammar" subitem="subtag"/>
626  <x:ref>language-tag</x:ref>  = <x:ref>primary-tag</x:ref> *( "-" <x:ref>subtag</x:ref> )
627  <x:ref>primary-tag</x:ref>   = 1*8<x:ref>ALPHA</x:ref>
628  <x:ref>subtag</x:ref>        = 1*8<x:ref>ALPHA</x:ref>
631   White space is not allowed within the tag and all tags are case-insensitive.
632   The name space of language tags is administered by the
633   IANA. Example tags include:
635<figure><artwork type="example">
636    en, en-US, en-cockney, i-cherokee, x-pig-latin
639   where any two-letter primary-tag is an ISO-639 language abbreviation
640   and any two-letter initial subtag is an ISO-3166 country code. (The
641   last three tags above are not registered tags; all but the last are
642   examples of tags which could be registered in future.)
647<section title="Entity" anchor="entity">
649   Request and Response messages &MAY; transfer an entity if not otherwise
650   restricted by the request method or response status code. An entity
651   consists of entity-header fields and an entity-body, although some
652   responses will only include the entity-headers.
655   In this section, both sender and recipient refer to either the client
656   or the server, depending on who sends and who receives the entity.
659<section title="Entity Header Fields" anchor="entity.header.fields">
660  <x:anchor-alias value="entity-header"/>
661  <x:anchor-alias value="extension-header"/>
663   Entity-header fields define metainformation about the entity-body or,
664   if no body is present, about the resource identified by the request.
666<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-header"/><iref primary="true" item="Grammar" subitem="extension-header"/>
667  <x:ref>entity-header</x:ref>  = <x:ref>Content-Encoding</x:ref>         ; <xref target="header.content-encoding"/>
668                 | <x:ref>Content-Language</x:ref>         ; <xref target="header.content-language"/>
669                 | <x:ref>Content-Length</x:ref>           ; &header-content-length;
670                 | <x:ref>Content-Location</x:ref>         ; <xref target="header.content-location"/>
671                 | <x:ref>Content-MD5</x:ref>              ; <xref target="header.content-md5"/>
672                 | <x:ref>Content-Range</x:ref>            ; &header-content-range;
673                 | <x:ref>Content-Type</x:ref>             ; <xref target="header.content-type"/>
674                 | <x:ref>Expires</x:ref>                  ; &header-expires;
675                 | <x:ref>Last-Modified</x:ref>            ; &header-last-modified;
676                 | <x:ref>extension-header</x:ref>
678  <x:ref>extension-header</x:ref> = <x:ref>message-header</x:ref>
681   The extension-header mechanism allows additional entity-header fields
682   to be defined without changing the protocol, but these fields cannot
683   be assumed to be recognizable by the recipient. Unrecognized header
684   fields &SHOULD; be ignored by the recipient and &MUST; be forwarded by
685   transparent proxies.
689<section title="Entity Body" anchor="entity.body">
690  <x:anchor-alias value="entity-body"/>
692   The entity-body (if any) sent with an HTTP request or response is in
693   a format and encoding defined by the entity-header fields.
695<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-body"/>
696  <x:ref>entity-body</x:ref>    = *<x:ref>OCTET</x:ref>
699   An entity-body is only present in a message when a message-body is
700   present, as described in &message-body;. The entity-body is obtained
701   from the message-body by decoding any Transfer-Encoding that might
702   have been applied to ensure safe and proper transfer of the message.
705<section title="Type" anchor="type">
707   When an entity-body is included with a message, the data type of that
708   body is determined via the header fields Content-Type and Content-Encoding.
709   These define a two-layer, ordered encoding model:
711<figure><artwork type="example">
712    entity-body := Content-Encoding( Content-Type( data ) )
715   Content-Type specifies the media type of the underlying data.
716   Content-Encoding may be used to indicate any additional content
717   codings applied to the data, usually for the purpose of data
718   compression, that are a property of the requested resource. There is
719   no default encoding.
722   Any HTTP/1.1 message containing an entity-body &SHOULD; include a
723   Content-Type header field defining the media type of that body. If
724   and only if the media type is not given by a Content-Type field, the
725   recipient &MAY; attempt to guess the media type via inspection of its
726   content and/or the name extension(s) of the URI used to identify the
727   resource. If the media type remains unknown, the recipient &SHOULD;
728   treat it as type "application/octet-stream".
732<section title="Entity Length" anchor="entity.length">
734   The entity-length of a message is the length of the message-body
735   before any transfer-codings have been applied. &message-length; defines
736   how the transfer-length of a message-body is determined.
742<section title="Content Negotiation" anchor="content.negotiation">
744   Most HTTP responses include an entity which contains information for
745   interpretation by a human user. Naturally, it is desirable to supply
746   the user with the "best available" entity corresponding to the
747   request. Unfortunately for servers and caches, not all users have the
748   same preferences for what is "best," and not all user agents are
749   equally capable of rendering all entity types. For that reason, HTTP
750   has provisions for several mechanisms for "content negotiation" --
751   the process of selecting the best representation for a given response
752   when there are multiple representations available.
753  <list><t>
754      <x:h>Note:</x:h> This is not called "format negotiation" because the
755      alternate representations may be of the same media type, but use
756      different capabilities of that type, be in different languages,
757      etc.
758  </t></list>
761   Any response containing an entity-body &MAY; be subject to negotiation,
762   including error responses.
765   There are two kinds of content negotiation which are possible in
766   HTTP: server-driven and agent-driven negotiation. These two kinds of
767   negotiation are orthogonal and thus may be used separately or in
768   combination. One method of combination, referred to as transparent
769   negotiation, occurs when a cache uses the agent-driven negotiation
770   information provided by the origin server in order to provide
771   server-driven negotiation for subsequent requests.
774<section title="Server-driven Negotiation" anchor="server-driven.negotiation">
776   If the selection of the best representation for a response is made by
777   an algorithm located at the server, it is called server-driven
778   negotiation. Selection is based on the available representations of
779   the response (the dimensions over which it can vary; e.g. language,
780   content-coding, etc.) and the contents of particular header fields in
781   the request message or on other information pertaining to the request
782   (such as the network address of the client).
785   Server-driven negotiation is advantageous when the algorithm for
786   selecting from among the available representations is difficult to
787   describe to the user agent, or when the server desires to send its
788   "best guess" to the client along with the first response (hoping to
789   avoid the round-trip delay of a subsequent request if the "best
790   guess" is good enough for the user). In order to improve the server's
791   guess, the user agent &MAY; include request header fields (Accept,
792   Accept-Language, Accept-Encoding, etc.) which describe its
793   preferences for such a response.
796   Server-driven negotiation has disadvantages:
797  <list style="numbers">
798    <t>
799         It is impossible for the server to accurately determine what
800         might be "best" for any given user, since that would require
801         complete knowledge of both the capabilities of the user agent
802         and the intended use for the response (e.g., does the user want
803         to view it on screen or print it on paper?).
804    </t>
805    <t>
806         Having the user agent describe its capabilities in every
807         request can be both very inefficient (given that only a small
808         percentage of responses have multiple representations) and a
809         potential violation of the user's privacy.
810    </t>
811    <t>
812         It complicates the implementation of an origin server and the
813         algorithms for generating responses to a request.
814    </t>
815    <t>
816         It may limit a public cache's ability to use the same response
817         for multiple user's requests.
818    </t>
819  </list>
822   HTTP/1.1 includes the following request-header fields for enabling
823   server-driven negotiation through description of user agent
824   capabilities and user preferences: Accept (<xref target="header.accept"/>), Accept-Charset
825   (<xref target="header.accept-charset"/>), Accept-Encoding (<xref target="header.accept-encoding"/>), Accept-Language
826   (<xref target="header.accept-language"/>), and User-Agent (&header-user-agent;). However, an
827   origin server is not limited to these dimensions and &MAY; vary the
828   response based on any aspect of the request, including information
829   outside the request-header fields or within extension header fields
830   not defined by this specification.
833   The Vary header field (&header-vary;) can be used to express the parameters the
834   server uses to select a representation that is subject to server-driven
835   negotiation.
839<section title="Agent-driven Negotiation" anchor="agent-driven.negotiation">
841   With agent-driven negotiation, selection of the best representation
842   for a response is performed by the user agent after receiving an
843   initial response from the origin server. Selection is based on a list
844   of the available representations of the response included within the
845   header fields or entity-body of the initial response, with each
846   representation identified by its own URI. Selection from among the
847   representations may be performed automatically (if the user agent is
848   capable of doing so) or manually by the user selecting from a
849   generated (possibly hypertext) menu.
852   Agent-driven negotiation is advantageous when the response would vary
853   over commonly-used dimensions (such as type, language, or encoding),
854   when the origin server is unable to determine a user agent's
855   capabilities from examining the request, and generally when public
856   caches are used to distribute server load and reduce network usage.
859   Agent-driven negotiation suffers from the disadvantage of needing a
860   second request to obtain the best alternate representation. This
861   second request is only efficient when caching is used. In addition,
862   this specification does not define any mechanism for supporting
863   automatic selection, though it also does not prevent any such
864   mechanism from being developed as an extension and used within
865   HTTP/1.1.
868   HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
869   status codes for enabling agent-driven negotiation when the server is
870   unwilling or unable to provide a varying response using server-driven
871   negotiation.
875<section title="Transparent Negotiation" anchor="transparent.negotiation">
877   Transparent negotiation is a combination of both server-driven and
878   agent-driven negotiation. When a cache is supplied with a form of the
879   list of available representations of the response (as in agent-driven
880   negotiation) and the dimensions of variance are completely understood
881   by the cache, then the cache becomes capable of performing server-driven
882   negotiation on behalf of the origin server for subsequent
883   requests on that resource.
886   Transparent negotiation has the advantage of distributing the
887   negotiation work that would otherwise be required of the origin
888   server and also removing the second request delay of agent-driven
889   negotiation when the cache is able to correctly guess the right
890   response.
893   This specification does not define any mechanism for transparent
894   negotiation, though it also does not prevent any such mechanism from
895   being developed as an extension that could be used within HTTP/1.1.
900<section title="Header Field Definitions" anchor="header.fields">
902   This section defines the syntax and semantics of HTTP/1.1 header fields
903   related to the payload of messages.
906   For entity-header fields, both sender and recipient refer to either the
907   client or the server, depending on who sends and who receives the entity.
910<section title="Accept" anchor="header.accept">
911  <iref primary="true" item="Accept header" x:for-anchor=""/>
912  <iref primary="true" item="Headers" subitem="Accept" x:for-anchor=""/>
913  <x:anchor-alias value="Accept"/>
914  <x:anchor-alias value="accept-extension"/>
915  <x:anchor-alias value="accept-params"/>
916  <x:anchor-alias value="media-range"/>
918   The Accept request-header field can be used to specify certain media
919   types which are acceptable for the response. Accept headers can be
920   used to indicate that the request is specifically limited to a small
921   set of desired types, as in the case of a request for an in-line
922   image.
924<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept"/><iref primary="true" item="Grammar" subitem="media-range"/><iref primary="true" item="Grammar" subitem="accept-params"/><iref primary="true" item="Grammar" subitem="accept-extension"/>
925  <x:ref>Accept</x:ref>         = "Accept" ":"
926                   #( <x:ref>media-range</x:ref> [ <x:ref>accept-params</x:ref> ] )
928  <x:ref>media-range</x:ref>    = ( "*/*"
929                   | ( <x:ref>type</x:ref> "/" "*" )
930                   | ( <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> )
931                   ) *( ";" <x:ref>parameter</x:ref> )
932  <x:ref>accept-params</x:ref>  = ";" "q" "=" <x:ref>qvalue</x:ref> *( <x:ref>accept-extension</x:ref> )
933  <x:ref>accept-extension</x:ref> = ";" <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> | <x:ref>quoted-string</x:ref> ) ]
936   The asterisk "*" character is used to group media types into ranges,
937   with "*/*" indicating all media types and "type/*" indicating all
938   subtypes of that type. The media-range &MAY; include media type
939   parameters that are applicable to that range.
942   Each media-range &MAY; be followed by one or more accept-params,
943   beginning with the "q" parameter for indicating a relative quality
944   factor. The first "q" parameter (if any) separates the media-range
945   parameter(s) from the accept-params. Quality factors allow the user
946   or user agent to indicate the relative degree of preference for that
947   media-range, using the qvalue scale from 0 to 1 (<xref target="quality.values"/>). The
948   default value is q=1.
949  <list><t>
950      <x:h>Note:</x:h> Use of the "q" parameter name to separate media type
951      parameters from Accept extension parameters is due to historical
952      practice. Although this prevents any media type parameter named
953      "q" from being used with a media range, such an event is believed
954      to be unlikely given the lack of any "q" parameters in the IANA
955      media type registry and the rare usage of any media type
956      parameters in Accept. Future media types are discouraged from
957      registering any parameter named "q".
958  </t></list>
961   The example
963<figure><artwork type="example">
964    Accept: audio/*; q=0.2, audio/basic
967   &SHOULD; be interpreted as "I prefer audio/basic, but send me any audio
968   type if it is the best available after an 80% mark-down in quality."
971   If no Accept header field is present, then it is assumed that the
972   client accepts all media types. If an Accept header field is present,
973   and if the server cannot send a response which is acceptable
974   according to the combined Accept field value, then the server &SHOULD;
975   send a 406 (Not Acceptable) response.
978   A more elaborate example is
980<figure><artwork type="example">
981    Accept: text/plain; q=0.5, text/html,
982            text/x-dvi; q=0.8, text/x-c
985   Verbally, this would be interpreted as "text/html and text/x-c are
986   the preferred media types, but if they do not exist, then send the
987   text/x-dvi entity, and if that does not exist, send the text/plain
988   entity."
991   Media ranges can be overridden by more specific media ranges or
992   specific media types. If more than one media range applies to a given
993   type, the most specific reference has precedence. For example,
995<figure><artwork type="example">
996    Accept: text/*, text/html, text/html;level=1, */*
999   have the following precedence:
1001<figure><artwork type="example">
1002    1) text/html;level=1
1003    2) text/html
1004    3) text/*
1005    4) */*
1008   The media type quality factor associated with a given type is
1009   determined by finding the media range with the highest precedence
1010   which matches that type. For example,
1012<figure><artwork type="example">
1013    Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
1014            text/html;level=2;q=0.4, */*;q=0.5
1017   would cause the following values to be associated:
1019<figure><artwork type="example">
1020    text/html;level=1         = 1
1021    text/html                 = 0.7
1022    text/plain                = 0.3
1023    image/jpeg                = 0.5
1024    text/html;level=2         = 0.4
1025    text/html;level=3         = 0.7
1028      <x:h>Note:</x:h> A user agent might be provided with a default set of quality
1029      values for certain media ranges. However, unless the user agent is
1030      a closed system which cannot interact with other rendering agents,
1031      this default set ought to be configurable by the user.
1035<section title="Accept-Charset" anchor="header.accept-charset">
1036  <iref primary="true" item="Accept-Charset header" x:for-anchor=""/>
1037  <iref primary="true" item="Headers" subitem="Accept-Charset" x:for-anchor=""/>
1038  <x:anchor-alias value="Accept-Charset"/>
1040   The Accept-Charset request-header field can be used to indicate what
1041   character sets are acceptable for the response. This field allows
1042   clients capable of understanding more comprehensive or special-purpose
1043   character sets to signal that capability to a server which is
1044   capable of representing documents in those character sets.
1046<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Charset"/>
1047  <x:ref>Accept-Charset</x:ref> = "Accept-Charset" ":"
1048          1#( ( <x:ref>charset</x:ref> | "*" ) [ ";" "q" "=" <x:ref>qvalue</x:ref> ] )
1051   Character set values are described in <xref target="character.sets"/>. Each charset &MAY;
1052   be given an associated quality value which represents the user's
1053   preference for that charset. The default value is q=1. An example is
1055<figure><artwork type="example">
1056   Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
1059   The special value "*", if present in the Accept-Charset field,
1060   matches every character set (including ISO-8859-1) which is not
1061   mentioned elsewhere in the Accept-Charset field. If no "*" is present
1062   in an Accept-Charset field, then all character sets not explicitly
1063   mentioned get a quality value of 0, except for ISO-8859-1, which gets
1064   a quality value of 1 if not explicitly mentioned.
1067   If no Accept-Charset header is present, the default is that any
1068   character set is acceptable. If an Accept-Charset header is present,
1069   and if the server cannot send a response which is acceptable
1070   according to the Accept-Charset header, then the server &SHOULD; send
1071   an error response with the 406 (Not Acceptable) status code, though
1072   the sending of an unacceptable response is also allowed.
1076<section title="Accept-Encoding" anchor="header.accept-encoding">
1077  <iref primary="true" item="Accept-Encoding header" x:for-anchor=""/>
1078  <iref primary="true" item="Headers" subitem="Accept-Encoding" x:for-anchor=""/>
1079  <x:anchor-alias value="Accept-Encoding"/>
1080  <x:anchor-alias value="codings"/>
1082   The Accept-Encoding request-header field is similar to Accept, but
1083   restricts the content-codings (<xref target="content.codings"/>) that are acceptable in
1084   the response.
1086<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Encoding"/><iref primary="true" item="Grammar" subitem="codings"/>
1087  <x:ref>Accept-Encoding</x:ref>  = "Accept-Encoding" ":"
1088                     #( <x:ref>codings</x:ref> [ ";" "q" "=" <x:ref>qvalue</x:ref> ] )
1089  <x:ref>codings</x:ref>          = ( <x:ref>content-coding</x:ref> | "*" )
1092   Each codings value &MAY; be given an associated quality value which
1093   represents the preference for that encoding. The default value is q=1.
1096   Examples of its use are:
1098<figure><artwork type="example">
1099    Accept-Encoding: compress, gzip
1100    Accept-Encoding:
1101    Accept-Encoding: *
1102    Accept-Encoding: compress;q=0.5, gzip;q=1.0
1103    Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
1106   A server tests whether a content-coding is acceptable, according to
1107   an Accept-Encoding field, using these rules:
1108  <list style="numbers">
1109      <t>If the content-coding is one of the content-codings listed in
1110         the Accept-Encoding field, then it is acceptable, unless it is
1111         accompanied by a qvalue of 0. (As defined in <xref target="quality.values"/>, a
1112         qvalue of 0 means "not acceptable.")</t>
1114      <t>The special "*" symbol in an Accept-Encoding field matches any
1115         available content-coding not explicitly listed in the header
1116         field.</t>
1118      <t>If multiple content-codings are acceptable, then the acceptable
1119         content-coding with the highest non-zero qvalue is preferred.</t>
1121      <t>The "identity" content-coding is always acceptable, unless
1122         specifically refused because the Accept-Encoding field includes
1123         "identity;q=0", or because the field includes "*;q=0" and does
1124         not explicitly include the "identity" content-coding. If the
1125         Accept-Encoding field-value is empty, then only the "identity"
1126         encoding is acceptable.</t>
1127  </list>
1130   If an Accept-Encoding field is present in a request, and if the
1131   server cannot send a response which is acceptable according to the
1132   Accept-Encoding header, then the server &SHOULD; send an error response
1133   with the 406 (Not Acceptable) status code.
1136   If no Accept-Encoding field is present in a request, the server &MAY;
1137   assume that the client will accept any content coding. In this case,
1138   if "identity" is one of the available content-codings, then the
1139   server &SHOULD; use the "identity" content-coding, unless it has
1140   additional information that a different content-coding is meaningful
1141   to the client.
1142  <list><t>
1143      <x:h>Note:</x:h> If the request does not include an Accept-Encoding field,
1144      and if the "identity" content-coding is unavailable, then
1145      content-codings commonly understood by HTTP/1.0 clients (i.e.,
1146      "gzip" and "compress") are preferred; some older clients
1147      improperly display messages sent with other content-codings.  The
1148      server might also make this decision based on information about
1149      the particular user-agent or client.
1150    </t><t>
1151      <x:h>Note:</x:h> Most HTTP/1.0 applications do not recognize or obey qvalues
1152      associated with content-codings. This means that qvalues will not
1153      work and are not permitted with x-gzip or x-compress.
1154    </t></list>
1158<section title="Accept-Language" anchor="header.accept-language">
1159  <iref primary="true" item="Accept-Language header" x:for-anchor=""/>
1160  <iref primary="true" item="Headers" subitem="Accept-Language" x:for-anchor=""/>
1161  <x:anchor-alias value="Accept-Language"/>
1162  <x:anchor-alias value="language-range"/>
1164   The Accept-Language request-header field is similar to Accept, but
1165   restricts the set of natural languages that are preferred as a
1166   response to the request. Language tags are defined in <xref target="language.tags"/>.
1168<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Language"/><iref primary="true" item="Grammar" subitem="language-range"/>
1169  <x:ref>Accept-Language</x:ref> = "Accept-Language" ":"
1170                    1#( <x:ref>language-range</x:ref> [ ";" "q" "=" <x:ref>qvalue</x:ref> ] )
1171  <x:ref>language-range</x:ref>  = ( ( 1*8<x:ref>ALPHA</x:ref> *( "-" 1*8<x:ref>ALPHA</x:ref> ) ) | "*" )
1174   Each language-range &MAY; be given an associated quality value which
1175   represents an estimate of the user's preference for the languages
1176   specified by that range. The quality value defaults to "q=1". For
1177   example,
1179<figure><artwork type="example">
1180    Accept-Language: da, en-gb;q=0.8, en;q=0.7
1183   would mean: "I prefer Danish, but will accept British English and
1184   other types of English." A language-range matches a language-tag if
1185   it exactly equals the tag, or if it exactly equals a prefix of the
1186   tag such that the first tag character following the prefix is "-".
1187   The special range "*", if present in the Accept-Language field,
1188   matches every tag not matched by any other range present in the
1189   Accept-Language field.
1190  <list><t>
1191      <x:h>Note:</x:h> This use of a prefix matching rule does not imply that
1192      language tags are assigned to languages in such a way that it is
1193      always true that if a user understands a language with a certain
1194      tag, then this user will also understand all languages with tags
1195      for which this tag is a prefix. The prefix rule simply allows the
1196      use of prefix tags if this is the case.
1197  </t></list>
1200   The language quality factor assigned to a language-tag by the
1201   Accept-Language field is the quality value of the longest language-range
1202   in the field that matches the language-tag. If no language-range
1203   in the field matches the tag, the language quality factor
1204   assigned is 0. If no Accept-Language header is present in the
1205   request, the server
1206   &SHOULD; assume that all languages are equally acceptable. If an
1207   Accept-Language header is present, then all languages which are
1208   assigned a quality factor greater than 0 are acceptable.
1211   It might be contrary to the privacy expectations of the user to send
1212   an Accept-Language header with the complete linguistic preferences of
1213   the user in every request. For a discussion of this issue, see
1214   <xref target=""/>.
1217   As intelligibility is highly dependent on the individual user, it is
1218   recommended that client applications make the choice of linguistic
1219   preference available to the user. If the choice is not made
1220   available, then the Accept-Language header field &MUST-NOT; be given in
1221   the request.
1222  <list><t>
1223      <x:h>Note:</x:h> When making the choice of linguistic preference available to
1224      the user, we remind implementors of  the fact that users are not
1225      familiar with the details of language matching as described above,
1226      and should provide appropriate guidance. As an example, users
1227      might assume that on selecting "en-gb", they will be served any
1228      kind of English document if British English is not available. A
1229      user agent might suggest in such a case to add "en" to get the
1230      best matching behavior.
1231  </t></list>
1235<section title="Content-Encoding" anchor="header.content-encoding">
1236  <iref primary="true" item="Content-Encoding header" x:for-anchor=""/>
1237  <iref primary="true" item="Headers" subitem="Content-Encoding" x:for-anchor=""/>
1238  <x:anchor-alias value="Content-Encoding"/>
1240   The Content-Encoding entity-header field is used as a modifier to the
1241   media-type. When present, its value indicates what additional content
1242   codings have been applied to the entity-body, and thus what decoding
1243   mechanisms must be applied in order to obtain the media-type
1244   referenced by the Content-Type header field. Content-Encoding is
1245   primarily used to allow a document to be compressed without losing
1246   the identity of its underlying media type.
1248<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Encoding"/>
1249  <x:ref>Content-Encoding</x:ref>  = "Content-Encoding" ":" 1#<x:ref>content-coding</x:ref>
1252   Content codings are defined in <xref target="content.codings"/>. An example of its use is
1254<figure><artwork type="example">
1255    Content-Encoding: gzip
1258   The content-coding is a characteristic of the entity identified by
1259   the Request-URI. Typically, the entity-body is stored with this
1260   encoding and is only decoded before rendering or analogous usage.
1261   However, a non-transparent proxy &MAY; modify the content-coding if the
1262   new coding is known to be acceptable to the recipient, unless the
1263   "no-transform" cache-control directive is present in the message.
1266   If the content-coding of an entity is not "identity", then the
1267   response &MUST; include a Content-Encoding entity-header (<xref target="header.content-encoding"/>)
1268   that lists the non-identity content-coding(s) used.
1271   If the content-coding of an entity in a request message is not
1272   acceptable to the origin server, the server &SHOULD; respond with a
1273   status code of 415 (Unsupported Media Type).
1276   If multiple encodings have been applied to an entity, the content
1277   codings &MUST; be listed in the order in which they were applied.
1278   Additional information about the encoding parameters &MAY; be provided
1279   by other entity-header fields not defined by this specification.
1283<section title="Content-Language" anchor="header.content-language">
1284  <iref primary="true" item="Content-Language header" x:for-anchor=""/>
1285  <iref primary="true" item="Headers" subitem="Content-Language" x:for-anchor=""/>
1286  <x:anchor-alias value="Content-Language"/>
1288   The Content-Language entity-header field describes the natural
1289   language(s) of the intended audience for the enclosed entity. Note
1290   that this might not be equivalent to all the languages used within
1291   the entity-body.
1293<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Language"/>
1294  <x:ref>Content-Language</x:ref>  = "Content-Language" ":" 1#<x:ref>language-tag</x:ref>
1297   Language tags are defined in <xref target="language.tags"/>. The primary purpose of
1298   Content-Language is to allow a user to identify and differentiate
1299   entities according to the user's own preferred language. Thus, if the
1300   body content is intended only for a Danish-literate audience, the
1301   appropriate field is
1303<figure><artwork type="example">
1304    Content-Language: da
1307   If no Content-Language is specified, the default is that the content
1308   is intended for all language audiences. This might mean that the
1309   sender does not consider it to be specific to any natural language,
1310   or that the sender does not know for which language it is intended.
1313   Multiple languages &MAY; be listed for content that is intended for
1314   multiple audiences. For example, a rendition of the "Treaty of
1315   Waitangi," presented simultaneously in the original Maori and English
1316   versions, would call for
1318<figure><artwork type="example">
1319    Content-Language: mi, en
1322   However, just because multiple languages are present within an entity
1323   does not mean that it is intended for multiple linguistic audiences.
1324   An example would be a beginner's language primer, such as "A First
1325   Lesson in Latin," which is clearly intended to be used by an
1326   English-literate audience. In this case, the Content-Language would
1327   properly only include "en".
1330   Content-Language &MAY; be applied to any media type -- it is not
1331   limited to textual documents.
1335<section title="Content-Location" anchor="header.content-location">
1336  <iref primary="true" item="Content-Location header" x:for-anchor=""/>
1337  <iref primary="true" item="Headers" subitem="Content-Location" x:for-anchor=""/>
1338  <x:anchor-alias value="Content-Location"/>
1340   The Content-Location entity-header field &MAY; be used to supply the
1341   resource location for the entity enclosed in the message when that
1342   entity is accessible from a location separate from the requested
1343   resource's URI. A server &SHOULD; provide a Content-Location for the
1344   variant corresponding to the response entity; especially in the case
1345   where a resource has multiple entities associated with it, and those
1346   entities actually have separate locations by which they might be
1347   individually accessed, the server &SHOULD; provide a Content-Location
1348   for the particular variant which is returned.
1350<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Location"/>
1351  <x:ref>Content-Location</x:ref> = "Content-Location" ":"
1352                    ( <x:ref>absoluteURI</x:ref> | <x:ref>relativeURI</x:ref> )
1355   The value of Content-Location also defines the base URI for the
1356   entity.
1359   The Content-Location value is not a replacement for the original
1360   requested URI; it is only a statement of the location of the resource
1361   corresponding to this particular entity at the time of the request.
1362   Future requests &MAY; specify the Content-Location URI as the request-URI
1363   if the desire is to identify the source of that particular
1364   entity.
1367   A cache cannot assume that an entity with a Content-Location
1368   different from the URI used to retrieve it can be used to respond to
1369   later requests on that Content-Location URI. However, the Content-Location
1370   can be used to differentiate between multiple entities
1371   retrieved from a single requested resource, as described in &caching-neg-resp;.
1374   If the Content-Location is a relative URI, the relative URI is
1375   interpreted relative to the Request-URI.
1378   The meaning of the Content-Location header in PUT or POST requests is
1379   undefined; servers are free to ignore it in those cases.
1383<section title="Content-MD5" anchor="header.content-md5">
1384  <iref primary="true" item="Content-MD5 header" x:for-anchor=""/>
1385  <iref primary="true" item="Headers" subitem="Content-MD5" x:for-anchor=""/>
1386  <x:anchor-alias value="Content-MD5"/>
1387  <x:anchor-alias value="md5-digest"/>
1389   The Content-MD5 entity-header field, as defined in <xref target="RFC1864"/>, is
1390   an MD5 digest of the entity-body for the purpose of providing an
1391   end-to-end message integrity check (MIC) of the entity-body. (Note: a
1392   MIC is good for detecting accidental modification of the entity-body
1393   in transit, but is not proof against malicious attacks.)
1395<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-MD5"/><iref primary="true" item="Grammar" subitem="md5-digest"/>
1396  <x:ref>Content-MD5</x:ref>   = "Content-MD5" ":" <x:ref>md5-digest</x:ref>
1397  <x:ref>md5-digest</x:ref>    = &lt;base64 of 128 bit MD5 digest as per <xref target="RFC1864"/>&gt;
1400   The Content-MD5 header field &MAY; be generated by an origin server or
1401   client to function as an integrity check of the entity-body. Only
1402   origin servers or clients &MAY; generate the Content-MD5 header field;
1403   proxies and gateways &MUST-NOT; generate it, as this would defeat its
1404   value as an end-to-end integrity check. Any recipient of the entity-body,
1405   including gateways and proxies, &MAY; check that the digest value
1406   in this header field matches that of the entity-body as received.
1409   The MD5 digest is computed based on the content of the entity-body,
1410   including any content-coding that has been applied, but not including
1411   any transfer-encoding applied to the message-body. If the message is
1412   received with a transfer-encoding, that encoding &MUST; be removed
1413   prior to checking the Content-MD5 value against the received entity.
1416   This has the result that the digest is computed on the octets of the
1417   entity-body exactly as, and in the order that, they would be sent if
1418   no transfer-encoding were being applied.
1421   HTTP extends RFC 1864 to permit the digest to be computed for MIME
1422   composite media-types (e.g., multipart/* and message/rfc822), but
1423   this does not change how the digest is computed as defined in the
1424   preceding paragraph.
1427   There are several consequences of this. The entity-body for composite
1428   types &MAY; contain many body-parts, each with its own MIME and HTTP
1429   headers (including Content-MD5, Content-Transfer-Encoding, and
1430   Content-Encoding headers). If a body-part has a Content-Transfer-Encoding
1431   or Content-Encoding header, it is assumed that the content
1432   of the body-part has had the encoding applied, and the body-part is
1433   included in the Content-MD5 digest as is -- i.e., after the
1434   application. The Transfer-Encoding header field is not allowed within
1435   body-parts.
1438   Conversion of all line breaks to CRLF &MUST-NOT; be done before
1439   computing or checking the digest: the line break convention used in
1440   the text actually transmitted &MUST; be left unaltered when computing
1441   the digest.
1442  <list><t>
1443      <x:h>Note:</x:h> while the definition of Content-MD5 is exactly the same for
1444      HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
1445      in which the application of Content-MD5 to HTTP entity-bodies
1446      differs from its application to MIME entity-bodies. One is that
1447      HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
1448      does use Transfer-Encoding and Content-Encoding. Another is that
1449      HTTP more frequently uses binary content types than MIME, so it is
1450      worth noting that, in such cases, the byte order used to compute
1451      the digest is the transmission byte order defined for the type.
1452      Lastly, HTTP allows transmission of text types with any of several
1453      line break conventions and not just the canonical form using CRLF.
1454  </t></list>
1458<section title="Content-Type" anchor="header.content-type">
1459  <iref primary="true" item="Content-Type header" x:for-anchor=""/>
1460  <iref primary="true" item="Headers" subitem="Content-Type" x:for-anchor=""/>
1461  <x:anchor-alias value="Content-Type"/>
1463   The Content-Type entity-header field indicates the media type of the
1464   entity-body sent to the recipient or, in the case of the HEAD method,
1465   the media type that would have been sent had the request been a GET.
1467<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Type"/>
1468  <x:ref>Content-Type</x:ref>   = "Content-Type" ":" <x:ref>media-type</x:ref>
1471   Media types are defined in <xref target="media.types"/>. An example of the field is
1473<figure><artwork type="example">
1474    Content-Type: text/html; charset=ISO-8859-4
1477   Further discussion of methods for identifying the media type of an
1478   entity is provided in <xref target="type"/>.
1484<section title="IANA Considerations" anchor="IANA.considerations">
1486   <cref>TBD.</cref>
1490<section title="Security Considerations" anchor="security.considerations">
1492   This section is meant to inform application developers, information
1493   providers, and users of the security limitations in HTTP/1.1 as
1494   described by this document. The discussion does not include
1495   definitive solutions to the problems revealed, though it does make
1496   some suggestions for reducing security risks.
1499<section title="Privacy Issues Connected to Accept Headers" anchor="">
1501   Accept request-headers can reveal information about the user to all
1502   servers which are accessed. The Accept-Language header in particular
1503   can reveal information the user would consider to be of a private
1504   nature, because the understanding of particular languages is often
1505   strongly correlated to the membership of a particular ethnic group.
1506   User agents which offer the option to configure the contents of an
1507   Accept-Language header to be sent in every request are strongly
1508   encouraged to let the configuration process include a message which
1509   makes the user aware of the loss of privacy involved.
1512   An approach that limits the loss of privacy would be for a user agent
1513   to omit the sending of Accept-Language headers by default, and to ask
1514   the user whether or not to start sending Accept-Language headers to a
1515   server if it detects, by looking for any Vary response-header fields
1516   generated by the server, that such sending could improve the quality
1517   of service.
1520   Elaborate user-customized accept header fields sent in every request,
1521   in particular if these include quality values, can be used by servers
1522   as relatively reliable and long-lived user identifiers. Such user
1523   identifiers would allow content providers to do click-trail tracking,
1524   and would allow collaborating content providers to match cross-server
1525   click-trails or form submissions of individual users. Note that for
1526   many users not behind a proxy, the network address of the host
1527   running the user agent will also serve as a long-lived user
1528   identifier. In environments where proxies are used to enhance
1529   privacy, user agents ought to be conservative in offering accept
1530   header configuration options to end users. As an extreme privacy
1531   measure, proxies could filter the accept headers in relayed requests.
1532   General purpose user agents which provide a high degree of header
1533   configurability &SHOULD; warn users about the loss of privacy which can
1534   be involved.
1538<section title="Content-Disposition Issues" anchor="content-disposition.issues">
1540   <xref target="RFC1806"/>, from which the often implemented Content-Disposition
1541   (see <xref target="content-disposition"/>) header in HTTP is derived, has a number of very
1542   serious security considerations. Content-Disposition is not part of
1543   the HTTP standard, but since it is widely implemented, we are
1544   documenting its use and risks for implementors. See <xref target="RFC2183"/>
1545   (which updates <xref target="RFC1806"/>) for details.
1551<section title="Acknowledgments" anchor="ack">
1556<references title="Normative References">
1558<reference anchor="ISO-8859-1">
1559  <front>
1560    <title>
1561     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
1562    </title>
1563    <author>
1564      <organization>International Organization for Standardization</organization>
1565    </author>
1566    <date year="1998"/>
1567  </front>
1568  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
1571<reference anchor="Part1">
1572  <front>
1573    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1574    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1575      <organization abbrev="Day Software">Day Software</organization>
1576      <address><email></email></address>
1577    </author>
1578    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1579      <organization>One Laptop per Child</organization>
1580      <address><email></email></address>
1581    </author>
1582    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1583      <organization abbrev="HP">Hewlett-Packard Company</organization>
1584      <address><email></email></address>
1585    </author>
1586    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1587      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1588      <address><email></email></address>
1589    </author>
1590    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1591      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1592      <address><email></email></address>
1593    </author>
1594    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1595      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1596      <address><email></email></address>
1597    </author>
1598    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1599      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1600      <address><email></email></address>
1601    </author>
1602    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1603      <organization abbrev="W3C">World Wide Web Consortium</organization>
1604      <address><email></email></address>
1605    </author>
1606    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1607      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1608      <address><email></email></address>
1609    </author>
1610    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1611  </front>
1612  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1613  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1616<reference anchor="Part2">
1617  <front>
1618    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1619    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1620      <organization abbrev="Day Software">Day Software</organization>
1621      <address><email></email></address>
1622    </author>
1623    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1624      <organization>One Laptop per Child</organization>
1625      <address><email></email></address>
1626    </author>
1627    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1628      <organization abbrev="HP">Hewlett-Packard Company</organization>
1629      <address><email></email></address>
1630    </author>
1631    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1632      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1633      <address><email></email></address>
1634    </author>
1635    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1636      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1637      <address><email></email></address>
1638    </author>
1639    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1640      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1641      <address><email></email></address>
1642    </author>
1643    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1644      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1645      <address><email></email></address>
1646    </author>
1647    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1648      <organization abbrev="W3C">World Wide Web Consortium</organization>
1649      <address><email></email></address>
1650    </author>
1651    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1652      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1653      <address><email></email></address>
1654    </author>
1655    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1656  </front>
1657  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1658  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
1661<reference anchor="Part4">
1662  <front>
1663    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1664    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1665      <organization abbrev="Day Software">Day Software</organization>
1666      <address><email></email></address>
1667    </author>
1668    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1669      <organization>One Laptop per Child</organization>
1670      <address><email></email></address>
1671    </author>
1672    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1673      <organization abbrev="HP">Hewlett-Packard Company</organization>
1674      <address><email></email></address>
1675    </author>
1676    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1677      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1678      <address><email></email></address>
1679    </author>
1680    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1681      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1682      <address><email></email></address>
1683    </author>
1684    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1685      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1686      <address><email></email></address>
1687    </author>
1688    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1689      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1690      <address><email></email></address>
1691    </author>
1692    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1693      <organization abbrev="W3C">World Wide Web Consortium</organization>
1694      <address><email></email></address>
1695    </author>
1696    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1697      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1698      <address><email></email></address>
1699    </author>
1700    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1701  </front>
1702  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
1703  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
1706<reference anchor="Part5">
1707  <front>
1708    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1709    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1710      <organization abbrev="Day Software">Day Software</organization>
1711      <address><email></email></address>
1712    </author>
1713    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1714      <organization>One Laptop per Child</organization>
1715      <address><email></email></address>
1716    </author>
1717    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1718      <organization abbrev="HP">Hewlett-Packard Company</organization>
1719      <address><email></email></address>
1720    </author>
1721    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1722      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1723      <address><email></email></address>
1724    </author>
1725    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1726      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1727      <address><email></email></address>
1728    </author>
1729    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1730      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1731      <address><email></email></address>
1732    </author>
1733    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1734      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1735      <address><email></email></address>
1736    </author>
1737    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1738      <organization abbrev="W3C">World Wide Web Consortium</organization>
1739      <address><email></email></address>
1740    </author>
1741    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1742      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1743      <address><email></email></address>
1744    </author>
1745    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1746  </front>
1747  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1748  <x:source href="p5-range.xml" basename="p5-range"/>
1751<reference anchor="Part6">
1752  <front>
1753    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1754    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1755      <organization abbrev="Day Software">Day Software</organization>
1756      <address><email></email></address>
1757    </author>
1758    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1759      <organization>One Laptop per Child</organization>
1760      <address><email></email></address>
1761    </author>
1762    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1763      <organization abbrev="HP">Hewlett-Packard Company</organization>
1764      <address><email></email></address>
1765    </author>
1766    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1767      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1768      <address><email></email></address>
1769    </author>
1770    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1771      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1772      <address><email></email></address>
1773    </author>
1774    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1775      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1776      <address><email></email></address>
1777    </author>
1778    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1779      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1780      <address><email></email></address>
1781    </author>
1782    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1783      <organization abbrev="W3C">World Wide Web Consortium</organization>
1784      <address><email></email></address>
1785    </author>
1786    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1787      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1788      <address><email></email></address>
1789    </author>
1790    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1791  </front>
1792  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1793  <x:source href="p6-cache.xml" basename="p6-cache"/>
1796<reference anchor="RFC1766">
1797  <front>
1798    <title abbrev="Language Tag">Tags for the Identification of Languages</title>
1799    <author initials="H." surname="Alvestrand" fullname="Harald Tveit Alvestrand">
1800      <organization>UNINETT</organization>
1801      <address><email></email></address>
1802    </author>
1803    <date month="March" year="1995"/>
1804  </front>
1805  <seriesInfo name="RFC" value="1766"/>
1808<reference anchor="RFC1864">
1809  <front>
1810    <title abbrev="Content-MD5 Header Field">The Content-MD5 Header Field</title>
1811    <author initials="J." surname="Myers" fullname="John G. Myers">
1812      <organization>Carnegie Mellon University</organization>
1813      <address><email></email></address>
1814    </author>
1815    <author initials="M." surname="Rose" fullname="Marshall T. Rose">
1816      <organization>Dover Beach Consulting, Inc.</organization>
1817      <address><email></email></address>
1818    </author>
1819    <date month="October" year="1995"/>
1820  </front>
1821  <seriesInfo name="RFC" value="1864"/>
1824<reference anchor="RFC1950">
1825  <front>
1826    <title>ZLIB Compressed Data Format Specification version 3.3</title>
1827    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
1828      <organization>Aladdin Enterprises</organization>
1829      <address><email></email></address>
1830    </author>
1831    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
1832      <organization/>
1833    </author>
1834    <date month="May" year="1996"/>
1835  </front>
1836  <seriesInfo name="RFC" value="1950"/>
1837  <annotation>
1838    RFC1950 is an Informational RFC, thus it may be less stable than
1839    this specification. On the other hand, this downward reference was
1840    present since <xref target="RFC2068"/> (published in 1997), therefore it is unlikely
1841    to cause problems in practice.
1842  </annotation>
1845<reference anchor="RFC1951">
1846  <front>
1847    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
1848    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
1849      <organization>Aladdin Enterprises</organization>
1850      <address><email></email></address>
1851    </author>
1852    <date month="May" year="1996"/>
1853  </front>
1854  <seriesInfo name="RFC" value="1951"/>
1855  <annotation>
1856    RFC1951 is an Informational RFC, thus it may be less stable than
1857    this specification. On the other hand, this downward reference was
1858    present since <xref target="RFC2068"/> (published in 1997), therefore it is unlikely
1859    to cause problems in practice.
1860  </annotation>
1863<reference anchor="RFC1952">
1864  <front>
1865    <title>GZIP file format specification version 4.3</title>
1866    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
1867      <organization>Aladdin Enterprises</organization>
1868      <address><email></email></address>
1869    </author>
1870    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
1871      <organization/>
1872      <address><email></email></address>
1873    </author>
1874    <author initials="M." surname="Adler" fullname="Mark Adler">
1875      <organization/>
1876      <address><email></email></address>
1877    </author>
1878    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
1879      <organization/>
1880      <address><email></email></address>
1881    </author>
1882    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
1883      <organization/>
1884      <address><email></email></address>
1885    </author>
1886    <date month="May" year="1996"/>
1887  </front>
1888  <seriesInfo name="RFC" value="1952"/>
1889  <annotation>
1890    RFC1952 is an Informational RFC, thus it may be less stable than
1891    this specification. On the other hand, this downward reference was
1892    present since <xref target="RFC2068"/> (published in 1997), therefore it is unlikely
1893    to cause problems in practice.
1894  </annotation>
1897<reference anchor="RFC2045">
1898  <front>
1899    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
1900    <author initials="N." surname="Freed" fullname="Ned Freed">
1901      <organization>Innosoft International, Inc.</organization>
1902      <address><email></email></address>
1903    </author>
1904    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
1905      <organization>First Virtual Holdings</organization>
1906      <address><email></email></address>
1907    </author>
1908    <date month="November" year="1996"/>
1909  </front>
1910  <seriesInfo name="RFC" value="2045"/>
1913<reference anchor="RFC2046">
1914  <front>
1915    <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
1916    <author initials="N." surname="Freed" fullname="Ned Freed">
1917      <organization>Innosoft International, Inc.</organization>
1918      <address><email></email></address>
1919    </author>
1920    <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
1921      <organization>First Virtual Holdings</organization>
1922      <address><email></email></address>
1923    </author>
1924    <date month="November" year="1996"/>
1925  </front>
1926  <seriesInfo name="RFC" value="2046"/>
1929<reference anchor="RFC2119">
1930  <front>
1931    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1932    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1933      <organization>Harvard University</organization>
1934      <address><email></email></address>
1935    </author>
1936    <date month="March" year="1997"/>
1937  </front>
1938  <seriesInfo name="BCP" value="14"/>
1939  <seriesInfo name="RFC" value="2119"/>
1944<references title="Informative References">
1946<reference anchor="RFC1806">
1947  <front>
1948    <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header</title>
1949    <author initials="R." surname="Troost" fullname="Rens Troost">
1950      <organization>New Century Systems</organization>
1951      <address><email></email></address>
1952    </author>
1953    <author initials="S." surname="Dorner" fullname="Steve Dorner">
1954      <organization>QUALCOMM Incorporated</organization>
1955      <address><email></email></address>
1956    </author>
1957    <date month="June" year="1995"/>
1958  </front>
1959  <seriesInfo name="RFC" value="1806"/>
1962<reference anchor="RFC1945">
1963  <front>
1964    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
1965    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1966      <organization>MIT, Laboratory for Computer Science</organization>
1967      <address><email></email></address>
1968    </author>
1969    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
1970      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1971      <address><email></email></address>
1972    </author>
1973    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1974      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
1975      <address><email></email></address>
1976    </author>
1977    <date month="May" year="1996"/>
1978  </front>
1979  <seriesInfo name="RFC" value="1945"/>
1982<reference anchor="RFC2049">
1983  <front>
1984    <title abbrev="MIME Conformance">Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples</title>
1985    <author initials="N." surname="Freed" fullname="Ned Freed">
1986      <organization>Innosoft International, Inc.</organization>
1987      <address><email></email></address>
1988    </author>
1989    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
1990      <organization>First Virtual Holdings</organization>
1991      <address><email></email></address>
1992    </author>
1993    <date month="November" year="1996"/>
1994  </front>
1995  <seriesInfo name="RFC" value="2049"/>
1998<reference anchor="RFC2068">
1999  <front>
2000    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
2001    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
2002      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
2003      <address><email></email></address>
2004    </author>
2005    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2006      <organization>MIT Laboratory for Computer Science</organization>
2007      <address><email></email></address>
2008    </author>
2009    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2010      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
2011      <address><email></email></address>
2012    </author>
2013    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
2014      <organization>MIT Laboratory for Computer Science</organization>
2015      <address><email></email></address>
2016    </author>
2017    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2018      <organization>MIT Laboratory for Computer Science</organization>
2019      <address><email></email></address>
2020    </author>
2021    <date month="January" year="1997"/>
2022  </front>
2023  <seriesInfo name="RFC" value="2068"/>
2026<reference anchor="RFC2076">
2027  <front>
2028    <title abbrev="Internet Message Headers">Common Internet Message Headers</title>
2029    <author initials="J." surname="Palme" fullname="Jacob Palme">
2030      <organization>Stockholm University/KTH</organization>
2031      <address><email></email></address>
2032    </author>
2033    <date month="February" year="1997"/>
2034  </front>
2035  <seriesInfo name="RFC" value="2076"/>
2038<reference anchor="RFC2183">
2039  <front>
2040    <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field</title>
2041    <author initials="R." surname="Troost" fullname="Rens Troost">
2042      <organization>New Century Systems</organization>
2043      <address><email></email></address>
2044    </author>
2045    <author initials="S." surname="Dorner" fullname="Steve Dorner">
2046      <organization>QUALCOMM Incorporated</organization>
2047      <address><email></email></address>
2048    </author>
2049    <author initials="K." surname="Moore" fullname="Keith Moore">
2050      <organization>Department of Computer Science</organization>
2051      <address><email></email></address>
2052    </author>
2053    <date month="August" year="1997"/>
2054  </front>
2055  <seriesInfo name="RFC" value="2183"/>
2058<reference anchor="RFC2277">
2059  <front>
2060    <title abbrev="Charset Policy">IETF Policy on Character Sets and Languages</title>
2061    <author initials="H.T." surname="Alvestrand" fullname="Harald Tveit Alvestrand">
2062      <organization>UNINETT</organization>
2063      <address><email></email></address>
2064    </author>
2065    <date month="January" year="1998"/>
2066  </front>
2067  <seriesInfo name="BCP" value="18"/>
2068  <seriesInfo name="RFC" value="2277"/>
2071<reference anchor="RFC2388">
2072  <front>
2073    <title abbrev="multipart/form-data">Returning Values from Forms:  multipart/form-data</title>
2074    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2075      <organization>Xerox Palo Alto Research Center</organization>
2076      <address><email></email></address>
2077    </author>
2078    <date year="1998" month="August"/>
2079  </front>
2080  <seriesInfo name="RFC" value="2388"/>
2083<reference anchor="RFC2557">
2084  <front>
2085    <title abbrev="MIME Encapsulation of Aggregate Documents">MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)</title>
2086    <author initials="F." surname="Palme" fullname="Jacob Palme">
2087      <organization>Stockholm University and KTH</organization>
2088      <address><email></email></address>
2089    </author>
2090    <author initials="A." surname="Hopmann" fullname="Alex Hopmann">
2091      <organization>Microsoft Corporation</organization>
2092      <address><email></email></address>
2093    </author>
2094    <author initials="N." surname="Shelness" fullname="Nick Shelness">
2095      <organization>Lotus Development Corporation</organization>
2096      <address><email></email></address>
2097    </author>
2098    <author initials="E." surname="Stefferud" fullname="Einar Stefferud">
2099      <organization/>
2100      <address><email></email></address>
2101    </author>
2102    <date year="1999" month="March"/>
2103  </front>
2104  <seriesInfo name="RFC" value="2557"/>
2107<reference anchor="RFC2616">
2108  <front>
2109    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2110    <author initials="R." surname="Fielding" fullname="R. Fielding">
2111      <organization>University of California, Irvine</organization>
2112      <address><email></email></address>
2113    </author>
2114    <author initials="J." surname="Gettys" fullname="J. Gettys">
2115      <organization>W3C</organization>
2116      <address><email></email></address>
2117    </author>
2118    <author initials="J." surname="Mogul" fullname="J. Mogul">
2119      <organization>Compaq Computer Corporation</organization>
2120      <address><email></email></address>
2121    </author>
2122    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2123      <organization>MIT Laboratory for Computer Science</organization>
2124      <address><email></email></address>
2125    </author>
2126    <author initials="L." surname="Masinter" fullname="L. Masinter">
2127      <organization>Xerox Corporation</organization>
2128      <address><email></email></address>
2129    </author>
2130    <author initials="P." surname="Leach" fullname="P. Leach">
2131      <organization>Microsoft Corporation</organization>
2132      <address><email></email></address>
2133    </author>
2134    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2135      <organization>W3C</organization>
2136      <address><email></email></address>
2137    </author>
2138    <date month="June" year="1999"/>
2139  </front>
2140  <seriesInfo name="RFC" value="2616"/>
2143<reference anchor="RFC2822">
2144  <front>
2145    <title>Internet Message Format</title>
2146    <author initials="P." surname="Resnick" fullname="P. Resnick">
2147      <organization>QUALCOMM Incorporated</organization>
2148    </author>
2149    <date year="2001" month="April"/>
2150  </front>
2151  <seriesInfo name="RFC" value="2822"/>
2154<reference anchor="RFC3629">
2155  <front>
2156    <title>UTF-8, a transformation format of ISO 10646</title>
2157    <author initials="F." surname="Yergeau" fullname="F. Yergeau">
2158      <organization>Alis Technologies</organization>
2159      <address><email></email></address>
2160    </author>
2161    <date month="November" year="2003"/>
2162  </front>
2163  <seriesInfo name="RFC" value="3629"/>
2164  <seriesInfo name="STD" value="63"/>
2167<reference anchor="RFC4288">
2168  <front>
2169    <title>Media Type Specifications and Registration Procedures</title>
2170    <author initials="N." surname="Freed" fullname="N. Freed">
2171      <organization>Sun Microsystems</organization>
2172      <address>
2173        <email></email>
2174      </address>
2175    </author>
2176    <author initials="J." surname="Klensin" fullname="J. Klensin">
2177      <organization/>
2178      <address>
2179        <email></email>
2180      </address>
2181    </author>
2182    <date year="2005" month="December"/>
2183  </front>
2184  <seriesInfo name="BCP" value="13"/>
2185  <seriesInfo name="RFC" value="4288"/>
2190<section title="Differences Between HTTP Entities and RFC 2045 Entities" anchor="differences.between.http.entities.and.rfc.2045.entities">
2192   HTTP/1.1 uses many of the constructs defined for Internet Mail (<xref target="RFC2822"/>) and the Multipurpose Internet Mail Extensions (MIME <xref target="RFC2045"/>) to
2193   allow entities to be transmitted in an open variety of
2194   representations and with extensible mechanisms. However, RFC 2045
2195   discusses mail, and HTTP has a few features that are different from
2196   those described in RFC 2045. These differences were carefully chosen
2197   to optimize performance over binary connections, to allow greater
2198   freedom in the use of new media types, to make date comparisons
2199   easier, and to acknowledge the practice of some early HTTP servers
2200   and clients.
2203   This appendix describes specific areas where HTTP differs from RFC
2204   2045. Proxies and gateways to strict MIME environments &SHOULD; be
2205   aware of these differences and provide the appropriate conversions
2206   where necessary. Proxies and gateways from MIME environments to HTTP
2207   also need to be aware of the differences because some conversions
2208   might be required.
2211<section title="MIME-Version" anchor="mime-version">
2212  <x:anchor-alias value="MIME-Version"/>
2214   HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages &MAY;
2215   include a single MIME-Version general-header field to indicate what
2216   version of the MIME protocol was used to construct the message. Use
2217   of the MIME-Version header field indicates that the message is in
2218   full compliance with the MIME protocol (as defined in <xref target="RFC2045"/>).
2219   Proxies/gateways are responsible for ensuring full compliance (where
2220   possible) when exporting HTTP messages to strict MIME environments.
2222<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="MIME-Version"/>
2223  <x:ref>MIME-Version</x:ref>   = "MIME-Version" ":" 1*<x:ref>DIGIT</x:ref> "." 1*<x:ref>DIGIT</x:ref>
2226   MIME version "1.0" is the default for use in HTTP/1.1. However,
2227   HTTP/1.1 message parsing and semantics are defined by this document
2228   and not the MIME specification.
2232<section title="Conversion to Canonical Form" anchor="">
2234   <xref target="RFC2045"/> requires that an Internet mail entity be converted to
2235   canonical form prior to being transferred, as described in <xref target="RFC2049" x:fmt="of" x:sec="4"/>.
2236   <xref target="canonicalization.and.text.defaults"/> of this document describes the forms
2237   allowed for subtypes of the "text" media type when transmitted over
2238   HTTP. <xref target="RFC2046"/> requires that content with a type of "text" represent
2239   line breaks as CRLF and forbids the use of CR or LF outside of line
2240   break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a
2241   line break within text content when a message is transmitted over
2242   HTTP.
2245   Where it is possible, a proxy or gateway from HTTP to a strict MIME
2246   environment &SHOULD; translate all line breaks within the text media
2247   types described in <xref target="canonicalization.and.text.defaults"/> of this document to the RFC 2049
2248   canonical form of CRLF. Note, however, that this might be complicated
2249   by the presence of a Content-Encoding and by the fact that HTTP
2250   allows the use of some character sets which do not use octets 13 and
2251   10 to represent CR and LF, as is the case for some multi-byte
2252   character sets.
2255   Implementors should note that conversion will break any cryptographic
2256   checksums applied to the original content unless the original content
2257   is already in canonical form. Therefore, the canonical form is
2258   recommended for any content that uses such checksums in HTTP.
2262<section title="Introduction of Content-Encoding" anchor="introduction.of.content-encoding">
2264   RFC 2045 does not include any concept equivalent to HTTP/1.1's
2265   Content-Encoding header field. Since this acts as a modifier on the
2266   media type, proxies and gateways from HTTP to MIME-compliant
2267   protocols &MUST; either change the value of the Content-Type header
2268   field or decode the entity-body before forwarding the message. (Some
2269   experimental applications of Content-Type for Internet mail have used
2270   a media-type parameter of ";conversions=&lt;content-coding&gt;" to perform
2271   a function equivalent to Content-Encoding. However, this parameter is
2272   not part of RFC 2045).
2276<section title="No Content-Transfer-Encoding" anchor="no.content-transfer-encoding">
2278   HTTP does not use the Content-Transfer-Encoding field of RFC
2279   2045. Proxies and gateways from MIME-compliant protocols to HTTP &MUST;
2280   remove any Content-Transfer-Encoding
2281   prior to delivering the response message to an HTTP client.
2284   Proxies and gateways from HTTP to MIME-compliant protocols are
2285   responsible for ensuring that the message is in the correct format
2286   and encoding for safe transport on that protocol, where "safe
2287   transport" is defined by the limitations of the protocol being used.
2288   Such a proxy or gateway &SHOULD; label the data with an appropriate
2289   Content-Transfer-Encoding if doing so will improve the likelihood of
2290   safe transport over the destination protocol.
2294<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
2296   HTTP/1.1 introduces the Transfer-Encoding header field (&header-transfer-encoding;).
2297   Proxies/gateways &MUST; remove any transfer-coding prior to
2298   forwarding a message via a MIME-compliant protocol.
2302<section title="MHTML and Line Length Limitations" anchor="mhtml.line.length">
2304   HTTP implementations which share code with MHTML <xref target="RFC2557"/> implementations
2305   need to be aware of MIME line length limitations. Since HTTP does not
2306   have this limitation, HTTP does not fold long lines. MHTML messages
2307   being transported by HTTP follow all conventions of MHTML, including
2308   line length limitations and folding, canonicalization, etc., since
2309   HTTP transports all message-bodies as payload (see <xref target="multipart.types"/>) and
2310   does not interpret the content or any MIME header lines that might be
2311   contained therein.
2316<section title="Additional Features" anchor="additional.features">
2318   <xref target="RFC1945"/> and <xref target="RFC2068"/> document protocol elements used by some
2319   existing HTTP implementations, but not consistently and correctly
2320   across most HTTP/1.1 applications. Implementors are advised to be
2321   aware of these features, but cannot rely upon their presence in, or
2322   interoperability with, other HTTP/1.1 applications. Some of these
2323   describe proposed experimental features, and some describe features
2324   that experimental deployment found lacking that are now addressed in
2325   the base HTTP/1.1 specification.
2328   A number of other headers, such as Content-Disposition and Title,
2329   from SMTP and MIME are also often implemented (see <xref target="RFC2076"/>).
2332<section title="Content-Disposition" anchor="content-disposition">
2333<iref item="Headers" subitem="Content-Disposition" primary="true" x:for-anchor=""/>
2334<iref item="Content-Disposition header" primary="true" x:for-anchor=""/>
2335  <x:anchor-alias value="content-disposition"/>
2336  <x:anchor-alias value="disposition-type"/>
2337  <x:anchor-alias value="disposition-parm"/>
2338  <x:anchor-alias value="disp-extension-parm"/>
2339  <x:anchor-alias value="disp-extension-token"/>
2340  <x:anchor-alias value="filename-parm"/>
2342   The Content-Disposition response-header field has been proposed as a
2343   means for the origin server to suggest a default filename if the user
2344   requests that the content is saved to a file. This usage is derived
2345   from the definition of Content-Disposition in <xref target="RFC1806"/>.
2347<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-disposition"/><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"/>
2348  <x:ref>content-disposition</x:ref> = "Content-Disposition" ":"
2349                        <x:ref>disposition-type</x:ref> *( ";" <x:ref>disposition-parm</x:ref> )
2350  <x:ref>disposition-type</x:ref> = "attachment" | <x:ref>disp-extension-token</x:ref>
2351  <x:ref>disposition-parm</x:ref> = <x:ref>filename-parm</x:ref> | <x:ref>disp-extension-parm</x:ref>
2352  <x:ref>filename-parm</x:ref> = "filename" "=" <x:ref>quoted-string</x:ref>
2353  <x:ref>disp-extension-token</x:ref> = <x:ref>token</x:ref>
2354  <x:ref>disp-extension-parm</x:ref> = <x:ref>token</x:ref> "=" ( <x:ref>token</x:ref> | <x:ref>quoted-string</x:ref> )
2357   An example is
2359<figure><artwork type="example">
2360     Content-Disposition: attachment; filename="fname.ext"
2363   The receiving user agent &SHOULD-NOT;  respect any directory path
2364   information present in the filename-parm parameter, which is the only
2365   parameter believed to apply to HTTP implementations at this time. The
2366   filename &SHOULD; be treated as a terminal component only.
2369   If this header is used in a response with the application/octet-stream
2370   content-type, the implied suggestion is that the user agent
2371   should not display the response, but directly enter a `save response
2372   as...' dialog.
2375   See <xref target="content-disposition.issues"/> for Content-Disposition security issues.
2380<section title="Compatibility with Previous Versions" anchor="compatibility">
2381<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2383   Transfer-coding and message lengths all interact in ways that
2384   required fixing exactly when chunked encoding is used (to allow for
2385   transfer encoding that may not be self delimiting); it was important
2386   to straighten out exactly how message lengths are computed.
2387   (<xref target="entity.length"/>, see also <xref target="Part1"/>,
2388   <xref target="Part5"/> and <xref target="Part6"/>).
2391   Charset wildcarding is introduced to avoid explosion of character set
2392   names in accept headers. (<xref target="header.accept-charset"/>)
2395   Content-Base was deleted from the specification: it was not
2396   implemented widely, and there is no simple, safe way to introduce it
2397   without a robust extension mechanism. In addition, it is used in a
2398   similar, but not identical fashion in MHTML <xref target="RFC2557"/>.
2401   A content-coding of "identity" was introduced, to solve problems
2402   discovered in caching. (<xref target="content.codings"/>)
2405   Quality Values of zero should indicate that "I don't want something"
2406   to allow clients to refuse a representation. (<xref target="quality.values"/>)
2409   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
2410   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
2411   specification, but not commonly implemented. See <xref target="RFC2068"/>.
2415<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2417  Clarify contexts that charset is used in.
2418  (<xref target="character.sets"/>)
2421  Remove reference to non-existant identity transfer-coding value tokens.
2422  (<xref target="no.content-transfer-encoding"/>)
2428<section title="Change Log (to be removed by RFC Editor before publication)">
2430<section title="Since RFC2616">
2432  Extracted relevant partitions from <xref target="RFC2616"/>.
2436<section title="Since draft-ietf-httpbis-p3-payload-00">
2438  Closed issues:
2439  <list style="symbols">
2440    <t>
2441      <eref target=""/>:
2442      "Media Type Registrations"
2443      (<eref target=""/>)
2444    </t>
2445    <t>
2446      <eref target=""/>:
2447      "Clarification regarding quoting of charset values"
2448      (<eref target=""/>)
2449    </t>
2450    <t>
2451      <eref target=""/>:
2452      "Remove 'identity' token references"
2453      (<eref target=""/>)
2454    </t>
2455    <t>
2456      <eref target=""/>:
2457      "Accept-Encoding BNF"
2458    </t>
2459    <t>
2460      <eref target=""/>:
2461      "Normative and Informative references"
2462    </t>
2463    <t>
2464      <eref target=""/>:
2465      "RFC1700 references"
2466    </t>
2467    <t>
2468      <eref target=""/>:
2469      "Updating to RFC4288"
2470    </t>
2471    <t>
2472      <eref target=""/>:
2473      "Informative references"
2474    </t>
2475    <t>
2476      <eref target=""/>:
2477      "ISO-8859-1 Reference"
2478    </t>
2479    <t>
2480      <eref target=""/>:
2481      "Encoding References Normative"
2482    </t>
2483    <t>
2484      <eref target=""/>:
2485      "Normative up-to-date references"
2486    </t>
2487  </list>
2491<section title="Since draft-ietf-httpbis-p3-payload-01">
2493  Ongoing work on ABNF conversion (<eref target=""/>):
2494  <list style="symbols">
2495    <t>
2496      Add explicit references to BNF syntax and rules imported from other parts of the specification.
2497    </t>
2498  </list>
2502<section title="Since draft-ietf-httpbis-p3-payload-02">
2504  Closed issues:
2505  <list style="symbols">
2506    <t>
2507      <eref target=""/>:
2508      "Classification for Allow header"
2509    </t>
2510    <t>
2511      <eref target=""/>:
2512      "missing default for qvalue in description of Accept-Encoding"
2513    </t>
2514  </list>
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