source: draft-ietf-httpbis/01/draft-ietf-httpbis-p3-payload-01.xml

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fix mime types

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1<?xml version="1.0" encoding="UTF-8"?>
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<!DOCTYPE rfc
15  PUBLIC "" "rfc2629.dtd">
16<rfc obsoletes="2616" category="std" ipr="full3978" docName="draft-ietf-httpbis-p3-payload-01">
19  <title abbrev="HTTP/1.1, Part 3">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
21  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
22    <organization abbrev="Day Software">Day Software</organization>
23    <address>
24      <postal>
25        <street>23 Corporate Plaza DR, Suite 280</street>
26        <city>Newport Beach</city>
27        <region>CA</region>
28        <code>92660</code>
29        <country>USA</country>
30      </postal>
31      <phone>+1-949-706-5300</phone>
32      <facsimile>+1-949-706-5305</facsimile>
33      <email></email>
34      <uri></uri>
35    </address>
36  </author>
38  <author initials="J." surname="Gettys" fullname="Jim Gettys">
39    <organization>One Laptop per Child</organization>
40    <address>
41      <postal>
42        <street>21 Oak Knoll Road</street>
43        <city>Carlisle</city>
44        <region>MA</region>
45        <code>01741</code>
46        <country>USA</country>
47      </postal>
48      <email></email>
49      <uri></uri>
50    </address>
51  </author>
53  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
54    <organization abbrev="HP">Hewlett-Packard Company</organization>
55    <address>
56      <postal>
57        <street>HP Labs, Large Scale Systems Group</street>
58        <street>1501 Page Mill Road, MS 1177</street>
59        <city>Palo Alto</city>
60        <region>CA</region>
61        <code>94304</code>
62        <country>USA</country>
63      </postal>
64      <email></email>
65    </address>
66  </author>
68  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
69    <organization abbrev="Microsoft">Microsoft Corporation</organization>
70    <address>
71      <postal>
72        <street>1 Microsoft Way</street>
73        <city>Redmond</city>
74        <region>WA</region>
75        <code>98052</code>
76        <country>USA</country>
77      </postal>
78      <email></email>
79    </address>
80  </author>
82  <author initials="L." surname="Masinter" fullname="Larry Masinter">
83    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
84    <address>
85      <postal>
86        <street>345 Park Ave</street>
87        <city>San Jose</city>
88        <region>CA</region>
89        <code>95110</code>
90        <country>USA</country>
91      </postal>
92      <email></email>
93      <uri></uri>
94    </address>
95  </author>
97  <author initials="P." surname="Leach" fullname="Paul J. Leach">
98    <organization abbrev="Microsoft">Microsoft Corporation</organization>
99    <address>
100      <postal>
101        <street>1 Microsoft Way</street>
102        <city>Redmond</city>
103        <region>WA</region>
104        <code>98052</code>
105      </postal>
106      <email></email>
107    </address>
108  </author>
110  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
111    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
112    <address>
113      <postal>
114        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
115        <street>The Stata Center, Building 32</street>
116        <street>32 Vassar Street</street>
117        <city>Cambridge</city>
118        <region>MA</region>
119        <code>02139</code>
120        <country>USA</country>
121      </postal>
122      <email></email>
123      <uri></uri>
124    </address>
125  </author>
127  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
128    <organization abbrev="W3C">World Wide Web Consortium</organization>
129    <address>
130      <postal>
131        <street>W3C / ERCIM</street>
132        <street>2004, rte des Lucioles</street>
133        <city>Sophia-Antipolis</city>
134        <region>AM</region>
135        <code>06902</code>
136        <country>France</country>
137      </postal>
138      <email></email>
139      <uri></uri>
140    </address>
141  </author>
143  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
144    <organization abbrev="greenbytes">greenbytes GmbH</organization>
145    <address>
146      <postal>
147        <street>Hafenweg 16</street>
148        <city>Muenster</city><region>NW</region><code>48155</code>
149        <country>Germany</country>
150      </postal>
151      <phone>+49 251 2807760</phone>   
152      <facsimile>+49 251 2807761</facsimile>   
153      <email></email>       
154      <uri></uri>     
155    </address>
156  </author>
158  <date month="January" year="2008" day="12"/>
162   The Hypertext Transfer Protocol (HTTP) is an application-level
163   protocol for distributed, collaborative, hypermedia information
164   systems. HTTP has been in use by the World Wide Web global information
165   initiative since 1990. This document is Part 3 of the seven-part specification
166   that defines the protocol referred to as "HTTP/1.1" and, taken together,
167   obsoletes RFC 2616.  Part 3 defines HTTP message content,
168   metadata, and content negotiation.
172<note title="Editorial Note (To be removed by RFC Editor)">
173  <t>
174    Discussion of this draft should take place on the HTTPBIS working group
175    mailing list ( The current issues list is
176    at <eref target=""/>
177    and related documents (including fancy diffs) can be found at
178    <eref target=""/>.
179  </t>
180  <t>
181    This draft incorporates those issue resolutions that were either
182    collected in the original RFC2616 errata list (<eref target=""/>),
183    or which were agreed upon on the mailing list between October 2006 and
184    November 2007 (as published in "draft-lafon-rfc2616bis-03").
185  </t>
189<section title="Introduction" anchor="introduction">
191   This document defines HTTP/1.1 message payloads (a.k.a., content), the
192   associated metadata header fields that define how the payload is intended
193   to be interpreted by a recipient, the request header fields that
194   may influence content selection, and the various selection algorithms
195   that are collectively referred to as HTTP content negotiation.
198   This document is currently disorganized in order to minimize the changes
199   between drafts and enable reviewers to see the smaller errata changes.
200   The next draft will reorganize the sections to better reflect the content.
201   In particular, the sections on entities will be renamed payload and moved
202   to the first half of the document, while the sections on content negotiation
203   and associated request header fields will be moved to the second half.  The
204   current mess reflects how widely dispersed these topics and associated
205   requirements had become in <xref target="RFC2616"/>.
208<section title="Requirements" anchor="intro.requirements">
210   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
211   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
212   document are to be interpreted as described in <xref target="RFC2119"/>.
215   An implementation is not compliant if it fails to satisfy one or more
216   of the MUST or REQUIRED level requirements for the protocols it
217   implements. An implementation that satisfies all the MUST or REQUIRED
218   level and all the SHOULD level requirements for its protocols is said
219   to be "unconditionally compliant"; one that satisfies all the MUST
220   level requirements but not all the SHOULD level requirements for its
221   protocols is said to be "conditionally compliant."
226<section title="Protocol Parameters" anchor="protocol.parameters">
228<section title="Character Sets" anchor="character.sets">
230   HTTP uses the same definition of the term "character set" as that
231   described for MIME:
234   The term "character set" is used in this document to refer to a
235   method used with one or more tables to convert a sequence of octets
236   into a sequence of characters. Note that unconditional conversion in
237   the other direction is not required, in that not all characters may
238   be available in a given character set and a character set may provide
239   more than one sequence of octets to represent a particular character.
240   This definition is intended to allow various kinds of character
241   encoding, from simple single-table mappings such as US-ASCII to
242   complex table switching methods such as those that use ISO-2022's
243   techniques. However, the definition associated with a MIME character
244   set name MUST fully specify the mapping to be performed from octets
245   to characters. In particular, use of external profiling information
246   to determine the exact mapping is not permitted.
249      Note: This use of the term "character set" is more commonly
250      referred to as a "character encoding." However, since HTTP and
251      MIME share the same registry, it is important that the terminology
252      also be shared.
255   HTTP character sets are identified by case-insensitive tokens. The
256   complete set of tokens is defined by the IANA Character Set registry
257   (<eref target=""/>).
259<figure><iref primary="true" item="Grammar" subitem="charset"/><artwork type="abnf2616"><![CDATA[
260  charset = token
263   Although HTTP allows an arbitrary token to be used as a charset
264   value, any token that has a predefined value within the IANA
265   Character Set registry MUST represent the character set defined
266   by that registry. Applications SHOULD limit their use of character
267   sets to those defined by the IANA registry.
270   HTTP uses charset in two contexts: within an Accept-Charset request
271   header (in which the charset value is an unquoted token) and as the
272   value of a parameter in a Content-Type header (within a request or
273   response), in which case the parameter value of the charset parameter
274   may be quoted.
277   Implementors should be aware of IETF character set requirements <xref target="RFC3629"/>
278   <xref target="RFC2277"/>.
281<section title="Missing Charset" anchor="missing.charset">
283   Some HTTP/1.0 software has interpreted a Content-Type header without
284   charset parameter incorrectly to mean "recipient should guess."
285   Senders wishing to defeat this behavior MAY include a charset
286   parameter even when the charset is ISO-8859-1 (<xref target="ISO-8859-1"/>) and SHOULD do so when
287   it is known that it will not confuse the recipient.
290   Unfortunately, some older HTTP/1.0 clients did not deal properly with
291   an explicit charset parameter. HTTP/1.1 recipients MUST respect the
292   charset label provided by the sender; and those user agents that have
293   a provision to "guess" a charset MUST use the charset from the
294   content-type field if they support that charset, rather than the
295   recipient's preference, when initially displaying a document. See
296   <xref target="canonicalization.and.text.defaults"/>.
301<section title="Content Codings" anchor="content.codings">
303   Content coding values indicate an encoding transformation that has
304   been or can be applied to an entity. Content codings are primarily
305   used to allow a document to be compressed or otherwise usefully
306   transformed without losing the identity of its underlying media type
307   and without loss of information. Frequently, the entity is stored in
308   coded form, transmitted directly, and only decoded by the recipient.
310<figure><iref primary="true" item="Grammar" subitem="content-coding"/><artwork type="abnf2616"><![CDATA[
311  content-coding   = token
314   All content-coding values are case-insensitive. HTTP/1.1 uses
315   content-coding values in the Accept-Encoding (<xref target="header.accept-encoding"/>) and
316   Content-Encoding (<xref target="header.content-encoding"/>) header fields. Although the value
317   describes the content-coding, what is more important is that it
318   indicates what decoding mechanism will be required to remove the
319   encoding.
322   The Internet Assigned Numbers Authority (IANA) acts as a registry for
323   content-coding value tokens. Initially, the registry contains the
324   following tokens:
327   gzip<iref item="gzip"/>
328  <list>
329    <t>
330        An encoding format produced by the file compression program
331        "gzip" (GNU zip) as described in <xref target="RFC1952"/>. This format is a
332        Lempel-Ziv coding (LZ77) with a 32 bit CRC.
333    </t>
334  </list>
337   compress<iref item="compress"/>
338  <list><t>
339        The encoding format produced by the common UNIX file compression
340        program "compress". This format is an adaptive Lempel-Ziv-Welch
341        coding (LZW).
343        Use of program names for the identification of encoding formats
344        is not desirable and is discouraged for future encodings. Their
345        use here is representative of historical practice, not good
346        design. For compatibility with previous implementations of HTTP,
347        applications SHOULD consider "x-gzip" and "x-compress" to be
348        equivalent to "gzip" and "compress" respectively.
349  </t></list>
352   deflate<iref item="deflate"/>
353  <list><t>
354        The "zlib" format defined in <xref target="RFC1950"/> in combination with
355        the "deflate" compression mechanism described in <xref target="RFC1951"/>.
356  </t></list>
359   identity<iref item="identity"/>
360  <list><t>
361        The default (identity) encoding; the use of no transformation
362        whatsoever. This content-coding is used only in the Accept-Encoding
363        header, and SHOULD NOT  be used in the Content-Encoding
364        header.
365  </t></list>
368   New content-coding value tokens SHOULD be registered; to allow
369   interoperability between clients and servers, specifications of the
370   content coding algorithms needed to implement a new value SHOULD be
371   publicly available and adequate for independent implementation, and
372   conform to the purpose of content coding defined in this section.
376<section title="Media Types" anchor="media.types">
378   HTTP uses Internet Media Types <xref target="RFC2046"/> in the Content-Type (<xref target="header.content-type"/>)
379   and Accept (<xref target="header.accept"/>) header fields in order to provide
380   open and extensible data typing and type negotiation.
382<figure><iref primary="true" item="Grammar" subitem="media-type"/><iref primary="true" item="Grammar" subitem="type"/><iref primary="true" item="Grammar" subitem="subtype"/><artwork type="abnf2616"><![CDATA[
383  media-type     = type "/" subtype *( ";" parameter )
384  type           = token
385  subtype        = token
388   Parameters MAY follow the type/subtype in the form of attribute/value
389   pairs.
391<figure><iref primary="true" item="Grammar" subitem="parameter"/><iref primary="true" item="Grammar" subitem="attribute"/><iref primary="true" item="Grammar" subitem="value"/><artwork type="abnf2616"><![CDATA[
392  parameter               = attribute "=" value
393  attribute               = token
394  value                   = token | quoted-string
397   The type, subtype, and parameter attribute names are case-insensitive.
398   Parameter values might or might not be case-sensitive,
399   depending on the semantics of the parameter name. Linear white space
400   (LWS) MUST NOT be used between the type and subtype, nor between an
401   attribute and its value. The presence or absence of a parameter might
402   be significant to the processing of a media-type, depending on its
403   definition within the media type registry.
406   Note that some older HTTP applications do not recognize media type
407   parameters. When sending data to older HTTP applications,
408   implementations SHOULD only use media type parameters when they are
409   required by that type/subtype definition.
412   Media-type values are registered with the Internet Assigned Number
413   Authority (IANA). The media type registration process is
414   outlined in <xref target="RFC4288"/>. Use of non-registered media types is
415   discouraged.
418<section title="Canonicalization and Text Defaults" anchor="canonicalization.and.text.defaults">
420   Internet media types are registered with a canonical form. An
421   entity-body transferred via HTTP messages MUST be represented in the
422   appropriate canonical form prior to its transmission except for
423   "text" types, as defined in the next paragraph.
426   When in canonical form, media subtypes of the "text" type use CRLF as
427   the text line break. HTTP relaxes this requirement and allows the
428   transport of text media with plain CR or LF alone representing a line
429   break when it is done consistently for an entire entity-body. HTTP
430   applications MUST accept CRLF, bare CR, and bare LF as being
431   representative of a line break in text media received via HTTP. In
432   addition, if the text is represented in a character set that does not
433   use octets 13 and 10 for CR and LF respectively, as is the case for
434   some multi-byte character sets, HTTP allows the use of whatever octet
435   sequences are defined by that character set to represent the
436   equivalent of CR and LF for line breaks. This flexibility regarding
437   line breaks applies only to text media in the entity-body; a bare CR
438   or LF MUST NOT be substituted for CRLF within any of the HTTP control
439   structures (such as header fields and multipart boundaries).
442   If an entity-body is encoded with a content-coding, the underlying
443   data MUST be in a form defined above prior to being encoded.
446   The "charset" parameter is used with some media types to define the
447   character set (<xref target="character.sets"/>) of the data. When no explicit charset
448   parameter is provided by the sender, media subtypes of the "text"
449   type are defined to have a default charset value of "ISO-8859-1" when
450   received via HTTP. Data in character sets other than "ISO-8859-1" or
451   its subsets MUST be labeled with an appropriate charset value. See
452   <xref target="missing.charset"/> for compatibility problems.
456<section title="Multipart Types" anchor="multipart.types">
458   MIME provides for a number of "multipart" types -- encapsulations of
459   one or more entities within a single message-body. All multipart
460   types share a common syntax, as defined in Section 5.1.1 of <xref target="RFC2046"/>,
461   and MUST include a boundary parameter as part of the media type
462   value. The message body is itself a protocol element and MUST
463   therefore use only CRLF to represent line breaks between body-parts.
464   Unlike in RFC 2046, the epilogue of any multipart message MUST be
465   empty; HTTP applications MUST NOT transmit the epilogue (even if the
466   original multipart contains an epilogue). These restrictions exist in
467   order to preserve the self-delimiting nature of a multipart message-body,
468   wherein the "end" of the message-body is indicated by the
469   ending multipart boundary.
472   In general, HTTP treats a multipart message-body no differently than
473   any other media type: strictly as payload. The one exception is the
474   "multipart/byteranges" type (Appendix A of <xref target="Part5"/>) when it appears in a 206
475   (Partial Content) response.
476   <!-- jre: re-insert removed text pointing to caching? -->
477   In all
478   other cases, an HTTP user agent SHOULD follow the same or similar
479   behavior as a MIME user agent would upon receipt of a multipart type.
480   The MIME header fields within each body-part of a multipart message-body
481   do not have any significance to HTTP beyond that defined by
482   their MIME semantics.
485   In general, an HTTP user agent SHOULD follow the same or similar
486   behavior as a MIME user agent would upon receipt of a multipart type.
487   If an application receives an unrecognized multipart subtype, the
488   application MUST treat it as being equivalent to "multipart/mixed".
491      Note: The "multipart/form-data" type has been specifically defined
492      for carrying form data suitable for processing via the POST
493      request method, as described in <xref target="RFC2388"/>.
498<section title="Quality Values" anchor="quality.values">
500   HTTP content negotiation (<xref target="content.negotiation"/>) uses short "floating point"
501   numbers to indicate the relative importance ("weight") of various
502   negotiable parameters.  A weight is normalized to a real number in
503   the range 0 through 1, where 0 is the minimum and 1 the maximum
504   value. If a parameter has a quality value of 0, then content with
505   this parameter is `not acceptable' for the client. HTTP/1.1
506   applications MUST NOT generate more than three digits after the
507   decimal point. User configuration of these values SHOULD also be
508   limited in this fashion.
510<figure><iref primary="true" item="Grammar" subitem="qvalue"/><artwork type="abnf2616"><![CDATA[
511  qvalue         = ( "0" [ "." 0*3DIGIT ] )
512                 | ( "1" [ "." 0*3("0") ] )
515   "Quality values" is a misnomer, since these values merely represent
516   relative degradation in desired quality.
520<section title="Language Tags" anchor="language.tags">
522   A language tag identifies a natural language spoken, written, or
523   otherwise conveyed by human beings for communication of information
524   to other human beings. Computer languages are explicitly excluded.
525   HTTP uses language tags within the Accept-Language and Content-Language
526   fields.
529   The syntax and registry of HTTP language tags is the same as that
530   defined by <xref target="RFC1766"/>. In summary, a language tag is composed of 1
531   or more parts: A primary language tag and a possibly empty series of
532   subtags:
534<figure><iref primary="true" item="Grammar" subitem="language-tag"/><iref primary="true" item="Grammar" subitem="primary-tag"/><iref primary="true" item="Grammar" subitem="subtag"/><artwork type="abnf2616"><![CDATA[
535  language-tag  = primary-tag *( "-" subtag )
536  primary-tag   = 1*8ALPHA
537  subtag        = 1*8ALPHA
540   White space is not allowed within the tag and all tags are case-insensitive.
541   The name space of language tags is administered by the
542   IANA. Example tags include:
544<figure><artwork type="example"><![CDATA[
545    en, en-US, en-cockney, i-cherokee, x-pig-latin
548   where any two-letter primary-tag is an ISO-639 language abbreviation
549   and any two-letter initial subtag is an ISO-3166 country code. (The
550   last three tags above are not registered tags; all but the last are
551   examples of tags which could be registered in future.)
556<section title="Entity" anchor="entity">
558   Request and Response messages MAY transfer an entity if not otherwise
559   restricted by the request method or response status code. An entity
560   consists of entity-header fields and an entity-body, although some
561   responses will only include the entity-headers.
564   In this section, both sender and recipient refer to either the client
565   or the server, depending on who sends and who receives the entity.
568<section title="Entity Header Fields" anchor="entity.header.fields">
570   Entity-header fields define metainformation about the entity-body or,
571   if no body is present, about the resource identified by the request.
573<figure><iref primary="true" item="Grammar" subitem="entity-header"/><iref primary="true" item="Grammar" subitem="extension-header"/><artwork type="abnf2616"><![CDATA[
574  entity-header  = Allow                    ; [Part2], Section 10.1
575                 | Content-Encoding         ; Section 5.5
576                 | Content-Language         ; Section 5.6
577                 | Content-Length           ; [Part1], Section 8.2
578                 | Content-Location         ; Section 5.7
579                 | Content-MD5              ; Section 5.8
580                 | Content-Range            ; [Part5], Section 5.2
581                 | Content-Type             ; Section 5.9
582                 | Expires                  ; [Part6], Section 15.3
583                 | Last-Modified            ; [Part4], Section 6.6
584                 | extension-header
586  extension-header = message-header
589   The extension-header mechanism allows additional entity-header fields
590   to be defined without changing the protocol, but these fields cannot
591   be assumed to be recognizable by the recipient. Unrecognized header
592   fields SHOULD be ignored by the recipient and MUST be forwarded by
593   transparent proxies.
597<section title="Entity Body" anchor="entity.body">
599   The entity-body (if any) sent with an HTTP request or response is in
600   a format and encoding defined by the entity-header fields.
602<figure><iref primary="true" item="Grammar" subitem="entity-body"/><artwork type="abnf2616"><![CDATA[
603  entity-body    = *OCTET
606   An entity-body is only present in a message when a message-body is
607   present, as described in Section 4.3 of <xref target="Part1"/>. The entity-body is obtained
608   from the message-body by decoding any Transfer-Encoding that might
609   have been applied to ensure safe and proper transfer of the message.
612<section title="Type" anchor="type">
614   When an entity-body is included with a message, the data type of that
615   body is determined via the header fields Content-Type and Content-Encoding.
616   These define a two-layer, ordered encoding model:
618<figure><artwork type="example"><![CDATA[
619    entity-body := Content-Encoding( Content-Type( data ) )
622   Content-Type specifies the media type of the underlying data.
623   Content-Encoding may be used to indicate any additional content
624   codings applied to the data, usually for the purpose of data
625   compression, that are a property of the requested resource. There is
626   no default encoding.
629   Any HTTP/1.1 message containing an entity-body SHOULD include a
630   Content-Type header field defining the media type of that body. If
631   and only if the media type is not given by a Content-Type field, the
632   recipient MAY attempt to guess the media type via inspection of its
633   content and/or the name extension(s) of the URI used to identify the
634   resource. If the media type remains unknown, the recipient SHOULD
635   treat it as type "application/octet-stream".
639<section title="Entity Length" anchor="entity.length">
641   The entity-length of a message is the length of the message-body
642   before any transfer-codings have been applied. Section 4.4 of <xref target="Part1"/> defines
643   how the transfer-length of a message-body is determined.
649<section title="Content Negotiation" anchor="content.negotiation">
651   Most HTTP responses include an entity which contains information for
652   interpretation by a human user. Naturally, it is desirable to supply
653   the user with the "best available" entity corresponding to the
654   request. Unfortunately for servers and caches, not all users have the
655   same preferences for what is "best," and not all user agents are
656   equally capable of rendering all entity types. For that reason, HTTP
657   has provisions for several mechanisms for "content negotiation" --
658   the process of selecting the best representation for a given response
659   when there are multiple representations available.
660  <list><t>
661      Note: This is not called "format negotiation" because the
662      alternate representations may be of the same media type, but use
663      different capabilities of that type, be in different languages,
664      etc.
665  </t></list>
668   Any response containing an entity-body MAY be subject to negotiation,
669   including error responses.
672   There are two kinds of content negotiation which are possible in
673   HTTP: server-driven and agent-driven negotiation. These two kinds of
674   negotiation are orthogonal and thus may be used separately or in
675   combination. One method of combination, referred to as transparent
676   negotiation, occurs when a cache uses the agent-driven negotiation
677   information provided by the origin server in order to provide
678   server-driven negotiation for subsequent requests.
681<section title="Server-driven Negotiation" anchor="server-driven.negotiation">
683   If the selection of the best representation for a response is made by
684   an algorithm located at the server, it is called server-driven
685   negotiation. Selection is based on the available representations of
686   the response (the dimensions over which it can vary; e.g. language,
687   content-coding, etc.) and the contents of particular header fields in
688   the request message or on other information pertaining to the request
689   (such as the network address of the client).
692   Server-driven negotiation is advantageous when the algorithm for
693   selecting from among the available representations is difficult to
694   describe to the user agent, or when the server desires to send its
695   "best guess" to the client along with the first response (hoping to
696   avoid the round-trip delay of a subsequent request if the "best
697   guess" is good enough for the user). In order to improve the server's
698   guess, the user agent MAY include request header fields (Accept,
699   Accept-Language, Accept-Encoding, etc.) which describe its
700   preferences for such a response.
703   Server-driven negotiation has disadvantages:
704  <list style="numbers">
705    <t>
706         It is impossible for the server to accurately determine what
707         might be "best" for any given user, since that would require
708         complete knowledge of both the capabilities of the user agent
709         and the intended use for the response (e.g., does the user want
710         to view it on screen or print it on paper?).
711    </t>
712    <t>
713         Having the user agent describe its capabilities in every
714         request can be both very inefficient (given that only a small
715         percentage of responses have multiple representations) and a
716         potential violation of the user's privacy.
717    </t>
718    <t>
719         It complicates the implementation of an origin server and the
720         algorithms for generating responses to a request.
721    </t>
722    <t>
723         It may limit a public cache's ability to use the same response
724         for multiple user's requests.
725    </t>
726  </list>
729   HTTP/1.1 includes the following request-header fields for enabling
730   server-driven negotiation through description of user agent
731   capabilities and user preferences: Accept (<xref target="header.accept"/>), Accept-Charset
732   (<xref target="header.accept-charset"/>), Accept-Encoding (<xref target="header.accept-encoding"/>), Accept-Language
733   (<xref target="header.accept-language"/>), and User-Agent (Section 10.9 of <xref target="Part2"/>). However, an
734   origin server is not limited to these dimensions and MAY vary the
735   response based on any aspect of the request, including information
736   outside the request-header fields or within extension header fields
737   not defined by this specification.
740   The Vary header field (Section 15.5 of <xref target="Part6"/>) can be used to express the parameters the
741   server uses to select a representation that is subject to server-driven
742   negotiation.
746<section title="Agent-driven Negotiation" anchor="agent-driven.negotiation">
748   With agent-driven negotiation, selection of the best representation
749   for a response is performed by the user agent after receiving an
750   initial response from the origin server. Selection is based on a list
751   of the available representations of the response included within the
752   header fields or entity-body of the initial response, with each
753   representation identified by its own URI. Selection from among the
754   representations may be performed automatically (if the user agent is
755   capable of doing so) or manually by the user selecting from a
756   generated (possibly hypertext) menu.
759   Agent-driven negotiation is advantageous when the response would vary
760   over commonly-used dimensions (such as type, language, or encoding),
761   when the origin server is unable to determine a user agent's
762   capabilities from examining the request, and generally when public
763   caches are used to distribute server load and reduce network usage.
766   Agent-driven negotiation suffers from the disadvantage of needing a
767   second request to obtain the best alternate representation. This
768   second request is only efficient when caching is used. In addition,
769   this specification does not define any mechanism for supporting
770   automatic selection, though it also does not prevent any such
771   mechanism from being developed as an extension and used within
772   HTTP/1.1.
775   HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
776   status codes for enabling agent-driven negotiation when the server is
777   unwilling or unable to provide a varying response using server-driven
778   negotiation.
782<section title="Transparent Negotiation" anchor="transparent.negotiation">
784   Transparent negotiation is a combination of both server-driven and
785   agent-driven negotiation. When a cache is supplied with a form of the
786   list of available representations of the response (as in agent-driven
787   negotiation) and the dimensions of variance are completely understood
788   by the cache, then the cache becomes capable of performing server-driven
789   negotiation on behalf of the origin server for subsequent
790   requests on that resource.
793   Transparent negotiation has the advantage of distributing the
794   negotiation work that would otherwise be required of the origin
795   server and also removing the second request delay of agent-driven
796   negotiation when the cache is able to correctly guess the right
797   response.
800   This specification does not define any mechanism for transparent
801   negotiation, though it also does not prevent any such mechanism from
802   being developed as an extension that could be used within HTTP/1.1.
807<section title="Header Field Definitions" anchor="header.fields">
809   This section defines the syntax and semantics of HTTP/1.1 header fields
810   related to the payload of messages.
813   For entity-header fields, both sender and recipient refer to either the
814   client or the server, depending on who sends and who receives the entity.
817<section title="Accept" anchor="header.accept">
818  <iref primary="true" item="Accept header"/>
819  <iref primary="true" item="Headers" subitem="Accept"/>
821   The Accept request-header field can be used to specify certain media
822   types which are acceptable for the response. Accept headers can be
823   used to indicate that the request is specifically limited to a small
824   set of desired types, as in the case of a request for an in-line
825   image.
827<figure><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"/><artwork type="abnf2616"><![CDATA[
828  Accept         = "Accept" ":"
829                   #( media-range [ accept-params ] )
831  media-range    = ( "*/*"
832                   | ( type "/" "*" )
833                   | ( type "/" subtype )
834                   ) *( ";" parameter )
835  accept-params  = ";" "q" "=" qvalue *( accept-extension )
836  accept-extension = ";" token [ "=" ( token | quoted-string ) ]
839   The asterisk "*" character is used to group media types into ranges,
840   with "*/*" indicating all media types and "type/*" indicating all
841   subtypes of that type. The media-range MAY include media type
842   parameters that are applicable to that range.
845   Each media-range MAY be followed by one or more accept-params,
846   beginning with the "q" parameter for indicating a relative quality
847   factor. The first "q" parameter (if any) separates the media-range
848   parameter(s) from the accept-params. Quality factors allow the user
849   or user agent to indicate the relative degree of preference for that
850   media-range, using the qvalue scale from 0 to 1 (<xref target="quality.values"/>). The
851   default value is q=1.
852  <list><t>
853      Note: Use of the "q" parameter name to separate media type
854      parameters from Accept extension parameters is due to historical
855      practice. Although this prevents any media type parameter named
856      "q" from being used with a media range, such an event is believed
857      to be unlikely given the lack of any "q" parameters in the IANA
858      media type registry and the rare usage of any media type
859      parameters in Accept. Future media types are discouraged from
860      registering any parameter named "q".
861  </t></list>
864   The example
866<figure><artwork type="example"><![CDATA[
867    Accept: audio/*; q=0.2, audio/basic
870   SHOULD be interpreted as "I prefer audio/basic, but send me any audio
871   type if it is the best available after an 80% mark-down in quality."
874   If no Accept header field is present, then it is assumed that the
875   client accepts all media types. If an Accept header field is present,
876   and if the server cannot send a response which is acceptable
877   according to the combined Accept field value, then the server SHOULD
878   send a 406 (Not Acceptable) response.
881   A more elaborate example is
883<figure><artwork type="example"><![CDATA[
884    Accept: text/plain; q=0.5, text/html,
885            text/x-dvi; q=0.8, text/x-c
888   Verbally, this would be interpreted as "text/html and text/x-c are
889   the preferred media types, but if they do not exist, then send the
890   text/x-dvi entity, and if that does not exist, send the text/plain
891   entity."
894   Media ranges can be overridden by more specific media ranges or
895   specific media types. If more than one media range applies to a given
896   type, the most specific reference has precedence. For example,
898<figure><artwork type="example"><![CDATA[
899    Accept: text/*, text/html, text/html;level=1, */*
902   have the following precedence:
904<figure><artwork type="example"><![CDATA[
905    1) text/html;level=1
906    2) text/html
907    3) text/*
908    4) */*
911   The media type quality factor associated with a given type is
912   determined by finding the media range with the highest precedence
913   which matches that type. For example,
915<figure><artwork type="example"><![CDATA[
916    Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
917            text/html;level=2;q=0.4, */*;q=0.5
920   would cause the following values to be associated:
922<figure><artwork type="example"><![CDATA[
923    text/html;level=1         = 1
924    text/html                 = 0.7
925    text/plain                = 0.3
926    image/jpeg                = 0.5
927    text/html;level=2         = 0.4
928    text/html;level=3         = 0.7
931      Note: A user agent might be provided with a default set of quality
932      values for certain media ranges. However, unless the user agent is
933      a closed system which cannot interact with other rendering agents,
934      this default set ought to be configurable by the user.
938<section title="Accept-Charset" anchor="header.accept-charset">
939  <iref primary="true" item="Accept-Charset header"/>
940  <iref primary="true" item="Headers" subitem="Accept-Charset"/>
942   The Accept-Charset request-header field can be used to indicate what
943   character sets are acceptable for the response. This field allows
944   clients capable of understanding more comprehensive or special-purpose
945   character sets to signal that capability to a server which is
946   capable of representing documents in those character sets.
948<figure><iref primary="true" item="Grammar" subitem="Accept-Charset"/><artwork type="abnf2616"><![CDATA[
949  Accept-Charset = "Accept-Charset" ":"
950          1#( ( charset | "*" ) [ ";" "q" "=" qvalue ] )
953   Character set values are described in <xref target="character.sets"/>. Each charset MAY
954   be given an associated quality value which represents the user's
955   preference for that charset. The default value is q=1. An example is
957<figure><artwork type="example"><![CDATA[
958   Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
961   The special value "*", if present in the Accept-Charset field,
962   matches every character set (including ISO-8859-1) which is not
963   mentioned elsewhere in the Accept-Charset field. If no "*" is present
964   in an Accept-Charset field, then all character sets not explicitly
965   mentioned get a quality value of 0, except for ISO-8859-1, which gets
966   a quality value of 1 if not explicitly mentioned.
969   If no Accept-Charset header is present, the default is that any
970   character set is acceptable. If an Accept-Charset header is present,
971   and if the server cannot send a response which is acceptable
972   according to the Accept-Charset header, then the server SHOULD send
973   an error response with the 406 (Not Acceptable) status code, though
974   the sending of an unacceptable response is also allowed.
978<section title="Accept-Encoding" anchor="header.accept-encoding">
979  <iref primary="true" item="Accept-Encoding header"/>
980  <iref primary="true" item="Headers" subitem="Accept-Encoding"/>
982   The Accept-Encoding request-header field is similar to Accept, but
983   restricts the content-codings (<xref target="content.codings"/>) that are acceptable in
984   the response.
986<figure><iref primary="true" item="Grammar" subitem="Accept-Encoding"/><iref primary="true" item="Grammar" subitem="codings"/><artwork type="abnf2616"><![CDATA[
987  Accept-Encoding  = "Accept-Encoding" ":"
988                     #( codings [ ";" "q" "=" qvalue ] )
989  codings          = ( content-coding | "*" )
992   Examples of its use are:
994<figure><artwork type="example"><![CDATA[
995    Accept-Encoding: compress, gzip
996    Accept-Encoding:
997    Accept-Encoding: *
998    Accept-Encoding: compress;q=0.5, gzip;q=1.0
999    Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
1002   A server tests whether a content-coding is acceptable, according to
1003   an Accept-Encoding field, using these rules:
1004  <list style="numbers">
1005      <t>If the content-coding is one of the content-codings listed in
1006         the Accept-Encoding field, then it is acceptable, unless it is
1007         accompanied by a qvalue of 0. (As defined in <xref target="quality.values"/>, a
1008         qvalue of 0 means "not acceptable.")</t>
1010      <t>The special "*" symbol in an Accept-Encoding field matches any
1011         available content-coding not explicitly listed in the header
1012         field.</t>
1014      <t>If multiple content-codings are acceptable, then the acceptable
1015         content-coding with the highest non-zero qvalue is preferred.</t>
1017      <t>The "identity" content-coding is always acceptable, unless
1018         specifically refused because the Accept-Encoding field includes
1019         "identity;q=0", or because the field includes "*;q=0" and does
1020         not explicitly include the "identity" content-coding. If the
1021         Accept-Encoding field-value is empty, then only the "identity"
1022         encoding is acceptable.</t>
1023  </list>
1026   If an Accept-Encoding field is present in a request, and if the
1027   server cannot send a response which is acceptable according to the
1028   Accept-Encoding header, then the server SHOULD send an error response
1029   with the 406 (Not Acceptable) status code.
1032   If no Accept-Encoding field is present in a request, the server MAY
1033   assume that the client will accept any content coding. In this case,
1034   if "identity" is one of the available content-codings, then the
1035   server SHOULD use the "identity" content-coding, unless it has
1036   additional information that a different content-coding is meaningful
1037   to the client.
1038  <list><t>
1039      Note: If the request does not include an Accept-Encoding field,
1040      and if the "identity" content-coding is unavailable, then
1041      content-codings commonly understood by HTTP/1.0 clients (i.e.,
1042      "gzip" and "compress") are preferred; some older clients
1043      improperly display messages sent with other content-codings.  The
1044      server might also make this decision based on information about
1045      the particular user-agent or client.
1046    </t><t>
1047      Note: Most HTTP/1.0 applications do not recognize or obey qvalues
1048      associated with content-codings. This means that qvalues will not
1049      work and are not permitted with x-gzip or x-compress.
1050    </t></list>
1054<section title="Accept-Language" anchor="header.accept-language">
1055  <iref primary="true" item="Accept-Language header"/>
1056  <iref primary="true" item="Headers" subitem="Accept-Language"/>
1058   The Accept-Language request-header field is similar to Accept, but
1059   restricts the set of natural languages that are preferred as a
1060   response to the request. Language tags are defined in <xref target="language.tags"/>.
1062<figure><iref primary="true" item="Grammar" subitem="Accept-Language"/><iref primary="true" item="Grammar" subitem="language-range"/><artwork type="abnf2616"><![CDATA[
1063  Accept-Language = "Accept-Language" ":"
1064                    1#( language-range [ ";" "q" "=" qvalue ] )
1065  language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
1068   Each language-range MAY be given an associated quality value which
1069   represents an estimate of the user's preference for the languages
1070   specified by that range. The quality value defaults to "q=1". For
1071   example,
1073<figure><artwork type="example"><![CDATA[
1074    Accept-Language: da, en-gb;q=0.8, en;q=0.7
1077   would mean: "I prefer Danish, but will accept British English and
1078   other types of English." A language-range matches a language-tag if
1079   it exactly equals the tag, or if it exactly equals a prefix of the
1080   tag such that the first tag character following the prefix is "-".
1081   The special range "*", if present in the Accept-Language field,
1082   matches every tag not matched by any other range present in the
1083   Accept-Language field.
1084  <list><t>
1085      Note: This use of a prefix matching rule does not imply that
1086      language tags are assigned to languages in such a way that it is
1087      always true that if a user understands a language with a certain
1088      tag, then this user will also understand all languages with tags
1089      for which this tag is a prefix. The prefix rule simply allows the
1090      use of prefix tags if this is the case.
1091  </t></list>
1094   The language quality factor assigned to a language-tag by the
1095   Accept-Language field is the quality value of the longest language-range
1096   in the field that matches the language-tag. If no language-range
1097   in the field matches the tag, the language quality factor
1098   assigned is 0. If no Accept-Language header is present in the
1099   request, the server
1100   SHOULD assume that all languages are equally acceptable. If an
1101   Accept-Language header is present, then all languages which are
1102   assigned a quality factor greater than 0 are acceptable.
1105   It might be contrary to the privacy expectations of the user to send
1106   an Accept-Language header with the complete linguistic preferences of
1107   the user in every request. For a discussion of this issue, see
1108   <xref target=""/>.
1111   As intelligibility is highly dependent on the individual user, it is
1112   recommended that client applications make the choice of linguistic
1113   preference available to the user. If the choice is not made
1114   available, then the Accept-Language header field MUST NOT be given in
1115   the request.
1116  <list><t>
1117      Note: When making the choice of linguistic preference available to
1118      the user, we remind implementors of  the fact that users are not
1119      familiar with the details of language matching as described above,
1120      and should provide appropriate guidance. As an example, users
1121      might assume that on selecting "en-gb", they will be served any
1122      kind of English document if British English is not available. A
1123      user agent might suggest in such a case to add "en" to get the
1124      best matching behavior.
1125  </t></list>
1129<section title="Content-Encoding" anchor="header.content-encoding">
1130  <iref primary="true" item="Content-Encoding header"/>
1131  <iref primary="true" item="Headers" subitem="Content-Encoding"/>
1133   The Content-Encoding entity-header field is used as a modifier to the
1134   media-type. When present, its value indicates what additional content
1135   codings have been applied to the entity-body, and thus what decoding
1136   mechanisms must be applied in order to obtain the media-type
1137   referenced by the Content-Type header field. Content-Encoding is
1138   primarily used to allow a document to be compressed without losing
1139   the identity of its underlying media type.
1141<figure><iref primary="true" item="Grammar" subitem="Content-Encoding"/><artwork type="abnf2616"><![CDATA[
1142  Content-Encoding  = "Content-Encoding" ":" 1#content-coding
1145   Content codings are defined in <xref target="content.codings"/>. An example of its use is
1147<figure><artwork type="example"><![CDATA[
1148    Content-Encoding: gzip
1151   The content-coding is a characteristic of the entity identified by
1152   the Request-URI. Typically, the entity-body is stored with this
1153   encoding and is only decoded before rendering or analogous usage.
1154   However, a non-transparent proxy MAY modify the content-coding if the
1155   new coding is known to be acceptable to the recipient, unless the
1156   "no-transform" cache-control directive is present in the message.
1159   If the content-coding of an entity is not "identity", then the
1160   response MUST include a Content-Encoding entity-header (<xref target="header.content-encoding"/>)
1161   that lists the non-identity content-coding(s) used.
1164   If the content-coding of an entity in a request message is not
1165   acceptable to the origin server, the server SHOULD respond with a
1166   status code of 415 (Unsupported Media Type).
1169   If multiple encodings have been applied to an entity, the content
1170   codings MUST be listed in the order in which they were applied.
1171   Additional information about the encoding parameters MAY be provided
1172   by other entity-header fields not defined by this specification.
1176<section title="Content-Language" anchor="header.content-language">
1177  <iref primary="true" item="Content-Language header"/>
1178  <iref primary="true" item="Headers" subitem="Content-Language"/>
1180   The Content-Language entity-header field describes the natural
1181   language(s) of the intended audience for the enclosed entity. Note
1182   that this might not be equivalent to all the languages used within
1183   the entity-body.
1185<figure><iref primary="true" item="Grammar" subitem="Content-Language"/><artwork type="abnf2616"><![CDATA[
1186  Content-Language  = "Content-Language" ":" 1#language-tag
1189   Language tags are defined in <xref target="language.tags"/>. The primary purpose of
1190   Content-Language is to allow a user to identify and differentiate
1191   entities according to the user's own preferred language. Thus, if the
1192   body content is intended only for a Danish-literate audience, the
1193   appropriate field is
1195<figure><artwork type="example"><![CDATA[
1196    Content-Language: da
1199   If no Content-Language is specified, the default is that the content
1200   is intended for all language audiences. This might mean that the
1201   sender does not consider it to be specific to any natural language,
1202   or that the sender does not know for which language it is intended.
1205   Multiple languages MAY be listed for content that is intended for
1206   multiple audiences. For example, a rendition of the "Treaty of
1207   Waitangi," presented simultaneously in the original Maori and English
1208   versions, would call for
1210<figure><artwork type="example"><![CDATA[
1211    Content-Language: mi, en
1214   However, just because multiple languages are present within an entity
1215   does not mean that it is intended for multiple linguistic audiences.
1216   An example would be a beginner's language primer, such as "A First
1217   Lesson in Latin," which is clearly intended to be used by an
1218   English-literate audience. In this case, the Content-Language would
1219   properly only include "en".
1222   Content-Language MAY be applied to any media type -- it is not
1223   limited to textual documents.
1227<section title="Content-Location" anchor="header.content-location">
1228  <iref primary="true" item="Content-Location header"/>
1229  <iref primary="true" item="Headers" subitem="Content-Location"/>
1231   The Content-Location entity-header field MAY be used to supply the
1232   resource location for the entity enclosed in the message when that
1233   entity is accessible from a location separate from the requested
1234   resource's URI. A server SHOULD provide a Content-Location for the
1235   variant corresponding to the response entity; especially in the case
1236   where a resource has multiple entities associated with it, and those
1237   entities actually have separate locations by which they might be
1238   individually accessed, the server SHOULD provide a Content-Location
1239   for the particular variant which is returned.
1241<figure><iref primary="true" item="Grammar" subitem="Content-Location"/><artwork type="abnf2616"><![CDATA[
1242  Content-Location = "Content-Location" ":"
1243                    ( absoluteURI | relativeURI )
1246   The value of Content-Location also defines the base URI for the
1247   entity.
1250   The Content-Location value is not a replacement for the original
1251   requested URI; it is only a statement of the location of the resource
1252   corresponding to this particular entity at the time of the request.
1253   Future requests MAY specify the Content-Location URI as the request-URI
1254   if the desire is to identify the source of that particular
1255   entity.
1258   A cache cannot assume that an entity with a Content-Location
1259   different from the URI used to retrieve it can be used to respond to
1260   later requests on that Content-Location URI. However, the Content-Location
1261   can be used to differentiate between multiple entities
1262   retrieved from a single requested resource, as described in Section 7 of <xref target="Part6"/>.
1265   If the Content-Location is a relative URI, the relative URI is
1266   interpreted relative to the Request-URI.
1269   The meaning of the Content-Location header in PUT or POST requests is
1270   undefined; servers are free to ignore it in those cases.
1274<section title="Content-MD5" anchor="header.content-md5">
1275  <iref primary="true" item="Content-MD5 header"/>
1276  <iref primary="true" item="Headers" subitem="Content-MD5"/>
1278   The Content-MD5 entity-header field, as defined in <xref target="RFC1864"/>, is
1279   an MD5 digest of the entity-body for the purpose of providing an
1280   end-to-end message integrity check (MIC) of the entity-body. (Note: a
1281   MIC is good for detecting accidental modification of the entity-body
1282   in transit, but is not proof against malicious attacks.)
1284<figure><iref primary="true" item="Grammar" subitem="Content-MD5"/><iref primary="true" item="Grammar" subitem="md5-digest"/><artwork type="abnf2616"><![CDATA[
1285  Content-MD5   = "Content-MD5" ":" md5-digest
1286  md5-digest    = <base64 of 128 bit MD5 digest as per [RFC1864]>
1289   The Content-MD5 header field MAY be generated by an origin server or
1290   client to function as an integrity check of the entity-body. Only
1291   origin servers or clients MAY generate the Content-MD5 header field;
1292   proxies and gateways MUST NOT generate it, as this would defeat its
1293   value as an end-to-end integrity check. Any recipient of the entity-body,
1294   including gateways and proxies, MAY check that the digest value
1295   in this header field matches that of the entity-body as received.
1298   The MD5 digest is computed based on the content of the entity-body,
1299   including any content-coding that has been applied, but not including
1300   any transfer-encoding applied to the message-body. If the message is
1301   received with a transfer-encoding, that encoding MUST be removed
1302   prior to checking the Content-MD5 value against the received entity.
1305   This has the result that the digest is computed on the octets of the
1306   entity-body exactly as, and in the order that, they would be sent if
1307   no transfer-encoding were being applied.
1310   HTTP extends RFC 1864 to permit the digest to be computed for MIME
1311   composite media-types (e.g., multipart/* and message/rfc822), but
1312   this does not change how the digest is computed as defined in the
1313   preceding paragraph.
1316   There are several consequences of this. The entity-body for composite
1317   types MAY contain many body-parts, each with its own MIME and HTTP
1318   headers (including Content-MD5, Content-Transfer-Encoding, and
1319   Content-Encoding headers). If a body-part has a Content-Transfer-Encoding
1320   or Content-Encoding header, it is assumed that the content
1321   of the body-part has had the encoding applied, and the body-part is
1322   included in the Content-MD5 digest as is -- i.e., after the
1323   application. The Transfer-Encoding header field is not allowed within
1324   body-parts.
1327   Conversion of all line breaks to CRLF MUST NOT be done before
1328   computing or checking the digest: the line break convention used in
1329   the text actually transmitted MUST be left unaltered when computing
1330   the digest.
1331  <list><t>
1332      Note: while the definition of Content-MD5 is exactly the same for
1333      HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
1334      in which the application of Content-MD5 to HTTP entity-bodies
1335      differs from its application to MIME entity-bodies. One is that
1336      HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
1337      does use Transfer-Encoding and Content-Encoding. Another is that
1338      HTTP more frequently uses binary content types than MIME, so it is
1339      worth noting that, in such cases, the byte order used to compute
1340      the digest is the transmission byte order defined for the type.
1341      Lastly, HTTP allows transmission of text types with any of several
1342      line break conventions and not just the canonical form using CRLF.
1343  </t></list>
1347<section title="Content-Type" anchor="header.content-type">
1348  <iref primary="true" item="Content-Type header"/>
1349  <iref primary="true" item="Headers" subitem="Content-Type"/>
1351   The Content-Type entity-header field indicates the media type of the
1352   entity-body sent to the recipient or, in the case of the HEAD method,
1353   the media type that would have been sent had the request been a GET.
1355<figure><iref primary="true" item="Grammar" subitem="Content-Type"/><artwork type="abnf2616"><![CDATA[
1356  Content-Type   = "Content-Type" ":" media-type
1359   Media types are defined in <xref target="media.types"/>. An example of the field is
1361<figure><artwork type="example"><![CDATA[
1362    Content-Type: text/html; charset=ISO-8859-4
1365   Further discussion of methods for identifying the media type of an
1366   entity is provided in <xref target="type"/>.
1372<section title="IANA Considerations" anchor="IANA.considerations">
1374   TBD.
1378<section title="Security Considerations" anchor="security.considerations">
1380   This section is meant to inform application developers, information
1381   providers, and users of the security limitations in HTTP/1.1 as
1382   described by this document. The discussion does not include
1383   definitive solutions to the problems revealed, though it does make
1384   some suggestions for reducing security risks.
1387<section title="Privacy Issues Connected to Accept Headers" anchor="">
1389   Accept request-headers can reveal information about the user to all
1390   servers which are accessed. The Accept-Language header in particular
1391   can reveal information the user would consider to be of a private
1392   nature, because the understanding of particular languages is often
1393   strongly correlated to the membership of a particular ethnic group.
1394   User agents which offer the option to configure the contents of an
1395   Accept-Language header to be sent in every request are strongly
1396   encouraged to let the configuration process include a message which
1397   makes the user aware of the loss of privacy involved.
1400   An approach that limits the loss of privacy would be for a user agent
1401   to omit the sending of Accept-Language headers by default, and to ask
1402   the user whether or not to start sending Accept-Language headers to a
1403   server if it detects, by looking for any Vary response-header fields
1404   generated by the server, that such sending could improve the quality
1405   of service.
1408   Elaborate user-customized accept header fields sent in every request,
1409   in particular if these include quality values, can be used by servers
1410   as relatively reliable and long-lived user identifiers. Such user
1411   identifiers would allow content providers to do click-trail tracking,
1412   and would allow collaborating content providers to match cross-server
1413   click-trails or form submissions of individual users. Note that for
1414   many users not behind a proxy, the network address of the host
1415   running the user agent will also serve as a long-lived user
1416   identifier. In environments where proxies are used to enhance
1417   privacy, user agents ought to be conservative in offering accept
1418   header configuration options to end users. As an extreme privacy
1419   measure, proxies could filter the accept headers in relayed requests.
1420   General purpose user agents which provide a high degree of header
1421   configurability SHOULD warn users about the loss of privacy which can
1422   be involved.
1426<section title="Content-Disposition Issues" anchor="content-disposition.issues">
1428   <xref target="RFC1806"/>, from which the often implemented Content-Disposition
1429   (see <xref target="content-disposition"/>) header in HTTP is derived, has a number of very
1430   serious security considerations. Content-Disposition is not part of
1431   the HTTP standard, but since it is widely implemented, we are
1432   documenting its use and risks for implementors. See <xref target="RFC2183"/>
1433   (which updates <xref target="RFC1806"/>) for details.
1439<section title="Acknowledgments" anchor="ack">
1444<references title="Normative References">
1446<reference anchor="ISO-8859-1">
1447  <front>
1448    <title>
1449     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
1450    </title>
1451    <author>
1452      <organization>International Organization for Standardization</organization>
1453    </author>
1454    <date year="1998"/>
1455  </front>
1456  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
1459<reference anchor="Part1">
1460  <front>
1461    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1462    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1463      <organization abbrev="Day Software">Day Software</organization>
1464      <address><email></email></address>
1465    </author>
1466    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1467      <organization>One Laptop per Child</organization>
1468      <address><email></email></address>
1469    </author>
1470    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1471      <organization abbrev="HP">Hewlett-Packard Company</organization>
1472      <address><email></email></address>
1473    </author>
1474    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1475      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1476      <address><email></email></address>
1477    </author>
1478    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1479      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1480      <address><email></email></address>
1481    </author>
1482    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1483      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1484      <address><email></email></address>
1485    </author>
1486    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1487      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1488      <address><email></email></address>
1489    </author>
1490    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1491      <organization abbrev="W3C">World Wide Web Consortium</organization>
1492      <address><email></email></address>
1493    </author>
1494    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1495      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1496      <address><email></email></address>
1497    </author>
1498    <date month="January" year="2008"/>
1499  </front>
1500  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-01"/>
1504<reference anchor="Part2">
1505  <front>
1506    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1507    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1508      <organization abbrev="Day Software">Day Software</organization>
1509      <address><email></email></address>
1510    </author>
1511    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1512      <organization>One Laptop per Child</organization>
1513      <address><email></email></address>
1514    </author>
1515    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1516      <organization abbrev="HP">Hewlett-Packard Company</organization>
1517      <address><email></email></address>
1518    </author>
1519    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1520      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1521      <address><email></email></address>
1522    </author>
1523    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1524      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1525      <address><email></email></address>
1526    </author>
1527    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1528      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1529      <address><email></email></address>
1530    </author>
1531    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1532      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1533      <address><email></email></address>
1534    </author>
1535    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1536      <organization abbrev="W3C">World Wide Web Consortium</organization>
1537      <address><email></email></address>
1538    </author>
1539    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1540      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1541      <address><email></email></address>
1542    </author>
1543    <date month="January" year="2008"/>
1544  </front>
1545  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-01"/>
1549<reference anchor="Part4">
1550  <front>
1551    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1552    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1553      <organization abbrev="Day Software">Day Software</organization>
1554      <address><email></email></address>
1555    </author>
1556    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1557      <organization>One Laptop per Child</organization>
1558      <address><email></email></address>
1559    </author>
1560    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1561      <organization abbrev="HP">Hewlett-Packard Company</organization>
1562      <address><email></email></address>
1563    </author>
1564    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1565      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1566      <address><email></email></address>
1567    </author>
1568    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1569      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1570      <address><email></email></address>
1571    </author>
1572    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1573      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1574      <address><email></email></address>
1575    </author>
1576    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1577      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1578      <address><email></email></address>
1579    </author>
1580    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1581      <organization abbrev="W3C">World Wide Web Consortium</organization>
1582      <address><email></email></address>
1583    </author>
1584    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1585      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1586      <address><email></email></address>
1587    </author>
1588    <date month="January" year="2008"/>
1589  </front>
1590  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-01"/>
1594<reference anchor="Part5">
1595  <front>
1596    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1597    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1598      <organization abbrev="Day Software">Day Software</organization>
1599      <address><email></email></address>
1600    </author>
1601    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1602      <organization>One Laptop per Child</organization>
1603      <address><email></email></address>
1604    </author>
1605    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1606      <organization abbrev="HP">Hewlett-Packard Company</organization>
1607      <address><email></email></address>
1608    </author>
1609    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1610      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1611      <address><email></email></address>
1612    </author>
1613    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1614      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1615      <address><email></email></address>
1616    </author>
1617    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1618      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1619      <address><email></email></address>
1620    </author>
1621    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1622      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1623      <address><email></email></address>
1624    </author>
1625    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1626      <organization abbrev="W3C">World Wide Web Consortium</organization>
1627      <address><email></email></address>
1628    </author>
1629    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1630      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1631      <address><email></email></address>
1632    </author>
1633    <date month="January" year="2008"/>
1634  </front>
1635  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-01"/>
1639<reference anchor="Part6">
1640  <front>
1641    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1642    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1643      <organization abbrev="Day Software">Day Software</organization>
1644      <address><email></email></address>
1645    </author>
1646    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1647      <organization>One Laptop per Child</organization>
1648      <address><email></email></address>
1649    </author>
1650    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1651      <organization abbrev="HP">Hewlett-Packard Company</organization>
1652      <address><email></email></address>
1653    </author>
1654    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1655      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1656      <address><email></email></address>
1657    </author>
1658    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1659      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1660      <address><email></email></address>
1661    </author>
1662    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1663      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1664      <address><email></email></address>
1665    </author>
1666    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1667      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1668      <address><email></email></address>
1669    </author>
1670    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1671      <organization abbrev="W3C">World Wide Web Consortium</organization>
1672      <address><email></email></address>
1673    </author>
1674    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1675      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1676      <address><email></email></address>
1677    </author>
1678    <date month="January" year="2008"/>
1679  </front>
1680  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-01"/>
1684<reference anchor="RFC1766">
1685  <front>
1686    <title abbrev="Language Tag">Tags for the Identification of Languages</title>
1687    <author initials="H." surname="Alvestrand" fullname="Harald Tveit Alvestrand">
1688      <organization>UNINETT</organization>
1689      <address><email></email></address>
1690    </author>
1691    <date month="March" year="1995"/>
1692  </front>
1693  <seriesInfo name="RFC" value="1766"/>
1696<reference anchor="RFC1864">
1697  <front>
1698    <title abbrev="Content-MD5 Header Field">The Content-MD5 Header Field</title>
1699    <author initials="J." surname="Myers" fullname="John G. Myers">
1700      <organization>Carnegie Mellon University</organization>
1701      <address><email></email></address>
1702    </author>
1703    <author initials="M." surname="Rose" fullname="Marshall T. Rose">
1704      <organization>Dover Beach Consulting, Inc.</organization>
1705      <address><email></email></address>
1706    </author>
1707    <date month="October" year="1995"/>
1708  </front>
1709  <seriesInfo name="RFC" value="1864"/>
1712<reference anchor="RFC1950">
1713  <front>
1714    <title>ZLIB Compressed Data Format Specification version 3.3</title>
1715    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
1716      <organization>Aladdin Enterprises</organization>
1717      <address><email></email></address>
1718    </author>
1719    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
1720      <organization/>
1721    </author>
1722    <date month="May" year="1996"/>
1723  </front>
1724  <seriesInfo name="RFC" value="1950"/>
1725  <annotation>
1726    RFC1950 is an Informational RFC, thus it may be less stable than
1727    this specification. On the other hand, this downward reference was
1728    present since <xref target="RFC2068"/> (published in 1997), therefore it is unlikely
1729    to cause problems in practice.
1730  </annotation>
1733<reference anchor="RFC1951">
1734  <front>
1735    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
1736    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
1737      <organization>Aladdin Enterprises</organization>
1738      <address><email></email></address>
1739    </author>
1740    <date month="May" year="1996"/>
1741  </front>
1742  <seriesInfo name="RFC" value="1951"/>
1743  <annotation>
1744    RFC1951 is an Informational RFC, thus it may be less stable than
1745    this specification. On the other hand, this downward reference was
1746    present since <xref target="RFC2068"/> (published in 1997), therefore it is unlikely
1747    to cause problems in practice.
1748  </annotation>
1751<reference anchor="RFC1952">
1752  <front>
1753    <title>GZIP file format specification version 4.3</title>
1754    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
1755      <organization>Aladdin Enterprises</organization>
1756      <address><email></email></address>
1757    </author>
1758    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
1759      <organization/>
1760      <address><email></email></address>
1761    </author>
1762    <author initials="M." surname="Adler" fullname="Mark Adler">
1763      <organization/>
1764      <address><email></email></address>
1765    </author>
1766    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
1767      <organization/>
1768      <address><email></email></address>
1769    </author>
1770    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
1771      <organization/>
1772      <address><email></email></address>
1773    </author>
1774    <date month="May" year="1996"/>
1775  </front>
1776  <seriesInfo name="RFC" value="1952"/>
1777  <annotation>
1778    RFC1952 is an Informational RFC, thus it may be less stable than
1779    this specification. On the other hand, this downward reference was
1780    present since <xref target="RFC2068"/> (published in 1997), therefore it is unlikely
1781    to cause problems in practice.
1782  </annotation>
1785<reference anchor="RFC2045">
1786  <front>
1787    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
1788    <author initials="N." surname="Freed" fullname="Ned Freed">
1789      <organization>Innosoft International, Inc.</organization>
1790      <address><email></email></address>
1791    </author>
1792    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
1793      <organization>First Virtual Holdings</organization>
1794      <address><email></email></address>
1795    </author>
1796    <date month="November" year="1996"/>
1797  </front>
1798  <seriesInfo name="RFC" value="2045"/>
1801<reference anchor="RFC2046">
1802  <front>
1803    <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
1804    <author initials="N." surname="Freed" fullname="Ned Freed">
1805      <organization>Innosoft International, Inc.</organization>
1806      <address><email></email></address>
1807    </author>
1808    <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
1809      <organization>First Virtual Holdings</organization>
1810      <address><email></email></address>
1811    </author>
1812    <date month="November" year="1996"/>
1813  </front>
1814  <seriesInfo name="RFC" value="2046"/>
1817<reference anchor="RFC2119">
1818  <front>
1819    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1820    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1821      <organization>Harvard University</organization>
1822      <address><email></email></address>
1823    </author>
1824    <date month="March" year="1997"/>
1825  </front>
1826  <seriesInfo name="BCP" value="14"/>
1827  <seriesInfo name="RFC" value="2119"/>
1830<reference anchor="RFC4288">
1831  <front>
1832    <title>Media Type Specifications and Registration Procedures</title>
1833    <author initials="N." surname="Freed" fullname="N. Freed">
1834      <organization>Sun Microsystems</organization>
1835      <address>
1836        <email></email>
1837      </address>
1838    </author>
1839    <author initials="J." surname="Klensin" fullname="J. Klensin">
1840      <organization/>
1841      <address>
1842        <email></email>
1843      </address>
1844    </author>
1845    <date year="2005" month="December"/>
1846  </front>
1847  <seriesInfo name="BCP" value="13"/>
1848  <seriesInfo name="RFC" value="4288"/>
1853<references title="Informative References">
1855<reference anchor="RFC1806">
1856  <front>
1857    <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header</title>
1858    <author initials="R." surname="Troost" fullname="Rens Troost">
1859      <organization>New Century Systems</organization>
1860      <address><email></email></address>
1861    </author>
1862    <author initials="S." surname="Dorner" fullname="Steve Dorner">
1863      <organization>QUALCOMM Incorporated</organization>
1864      <address><email></email></address>
1865    </author>
1866    <date month="June" year="1995"/>
1867  </front>
1868  <seriesInfo name="RFC" value="1806"/>
1871<reference anchor="RFC1945">
1872  <front>
1873    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
1874    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1875      <organization>MIT, Laboratory for Computer Science</organization>
1876      <address><email></email></address>
1877    </author>
1878    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
1879      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1880      <address><email></email></address>
1881    </author>
1882    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1883      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
1884      <address><email></email></address>
1885    </author>
1886    <date month="May" year="1996"/>
1887  </front>
1888  <seriesInfo name="RFC" value="1945"/>
1891<reference anchor="RFC2049">
1892  <front>
1893    <title abbrev="MIME Conformance">Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples</title>
1894    <author initials="N." surname="Freed" fullname="Ned Freed">
1895      <organization>Innosoft International, Inc.</organization>
1896      <address><email></email></address>
1897    </author>
1898    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
1899      <organization>First Virtual Holdings</organization>
1900      <address><email></email></address>
1901    </author>
1902    <date month="November" year="1996"/>
1903  </front>
1904  <seriesInfo name="RFC" value="2049"/>
1907<reference anchor="RFC2068">
1908  <front>
1909    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1910    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1911      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1912      <address><email></email></address>
1913    </author>
1914    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1915      <organization>MIT Laboratory for Computer Science</organization>
1916      <address><email></email></address>
1917    </author>
1918    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1919      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1920      <address><email></email></address>
1921    </author>
1922    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1923      <organization>MIT Laboratory for Computer Science</organization>
1924      <address><email></email></address>
1925    </author>
1926    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1927      <organization>MIT Laboratory for Computer Science</organization>
1928      <address><email></email></address>
1929    </author>
1930    <date month="January" year="1997"/>
1931  </front>
1932  <seriesInfo name="RFC" value="2068"/>
1935<reference anchor="RFC2076">
1936  <front>
1937    <title abbrev="Internet Message Headers">Common Internet Message Headers</title>
1938    <author initials="J." surname="Palme" fullname="Jacob Palme">
1939      <organization>Stockholm University/KTH</organization>
1940      <address><email></email></address>
1941    </author>
1942    <date month="February" year="1997"/>
1943  </front>
1944  <seriesInfo name="RFC" value="2076"/>
1947<reference anchor="RFC2183">
1948  <front>
1949    <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field</title>
1950    <author initials="R." surname="Troost" fullname="Rens Troost">
1951      <organization>New Century Systems</organization>
1952      <address><email></email></address>
1953    </author>
1954    <author initials="S." surname="Dorner" fullname="Steve Dorner">
1955      <organization>QUALCOMM Incorporated</organization>
1956      <address><email></email></address>
1957    </author>
1958    <author initials="K." surname="Moore" fullname="Keith Moore">
1959      <organization>Department of Computer Science</organization>
1960      <address><email></email></address>
1961    </author>
1962    <date month="August" year="1997"/>
1963  </front>
1964  <seriesInfo name="RFC" value="2183"/>
1967<reference anchor="RFC2277">
1968  <front>
1969    <title abbrev="Charset Policy">IETF Policy on Character Sets and Languages</title>
1970    <author initials="H.T." surname="Alvestrand" fullname="Harald Tveit Alvestrand">
1971      <organization>UNINETT</organization>
1972      <address><email></email></address>
1973    </author>
1974    <date month="January" year="1998"/>
1975  </front>
1976  <seriesInfo name="BCP" value="18"/>
1977  <seriesInfo name="RFC" value="2277"/>
1980<reference anchor="RFC2388">
1981  <front>
1982    <title abbrev="multipart/form-data">Returning Values from Forms:  multipart/form-data</title>
1983    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1984      <organization>Xerox Palo Alto Research Center</organization>
1985      <address><email></email></address>
1986    </author>
1987    <date year="1998" month="August"/>
1988  </front>
1989  <seriesInfo name="RFC" value="2388"/>
1992<reference anchor="RFC2557">
1993  <front>
1994    <title abbrev="MIME Encapsulation of Aggregate Documents">MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)</title>
1995    <author initials="F." surname="Palme" fullname="Jacob Palme">
1996      <organization>Stockholm University and KTH</organization>
1997      <address><email></email></address>
1998    </author>
1999    <author initials="A." surname="Hopmann" fullname="Alex Hopmann">
2000      <organization>Microsoft Corporation</organization>
2001      <address><email></email></address>
2002    </author>
2003    <author initials="N." surname="Shelness" fullname="Nick Shelness">
2004      <organization>Lotus Development Corporation</organization>
2005      <address><email></email></address>
2006    </author>
2007    <author initials="E." surname="Stefferud" fullname="Einar Stefferud">
2008      <organization/>
2009      <address><email></email></address>
2010    </author>
2011    <date year="1999" month="March"/>
2012  </front>
2013  <seriesInfo name="RFC" value="2557"/>
2016<reference anchor="RFC2616">
2017  <front>
2018    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2019    <author initials="R." surname="Fielding" fullname="R. Fielding">
2020      <organization>University of California, Irvine</organization>
2021      <address><email></email></address>
2022    </author>
2023    <author initials="J." surname="Gettys" fullname="J. Gettys">
2024      <organization>W3C</organization>
2025      <address><email></email></address>
2026    </author>
2027    <author initials="J." surname="Mogul" fullname="J. Mogul">
2028      <organization>Compaq Computer Corporation</organization>
2029      <address><email></email></address>
2030    </author>
2031    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2032      <organization>MIT Laboratory for Computer Science</organization>
2033      <address><email></email></address>
2034    </author>
2035    <author initials="L." surname="Masinter" fullname="L. Masinter">
2036      <organization>Xerox Corporation</organization>
2037      <address><email></email></address>
2038    </author>
2039    <author initials="P." surname="Leach" fullname="P. Leach">
2040      <organization>Microsoft Corporation</organization>
2041      <address><email></email></address>
2042    </author>
2043    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2044      <organization>W3C</organization>
2045      <address><email></email></address>
2046    </author>
2047    <date month="June" year="1999"/>
2048  </front>
2049  <seriesInfo name="RFC" value="2616"/>
2052<reference anchor="RFC2822">
2053  <front>
2054    <title>Internet Message Format</title>
2055    <author initials="P." surname="Resnick" fullname="P. Resnick">
2056      <organization>QUALCOMM Incorporated</organization>
2057    </author>
2058    <date year="2001" month="April"/>
2059  </front>
2060  <seriesInfo name="RFC" value="2822"/>
2063<reference anchor="RFC3629">
2064  <front>
2065    <title>UTF-8, a transformation format of ISO 10646</title>
2066    <author initials="F." surname="Yergeau" fullname="F. Yergeau">
2067      <organization>Alis Technologies</organization>
2068      <address><email></email></address>
2069    </author>
2070    <date month="November" year="2003"/>
2071  </front>
2072  <seriesInfo name="RFC" value="3629"/>
2073  <seriesInfo name="STD" value="63"/>
2078<section title="Differences Between HTTP Entities and RFC 2045 Entities" anchor="differences.between.http.entities.and.rfc.2045.entities">
2080   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
2081   allow entities to be transmitted in an open variety of
2082   representations and with extensible mechanisms. However, RFC 2045
2083   discusses mail, and HTTP has a few features that are different from
2084   those described in RFC 2045. These differences were carefully chosen
2085   to optimize performance over binary connections, to allow greater
2086   freedom in the use of new media types, to make date comparisons
2087   easier, and to acknowledge the practice of some early HTTP servers
2088   and clients.
2091   This appendix describes specific areas where HTTP differs from RFC
2092   2045. Proxies and gateways to strict MIME environments SHOULD be
2093   aware of these differences and provide the appropriate conversions
2094   where necessary. Proxies and gateways from MIME environments to HTTP
2095   also need to be aware of the differences because some conversions
2096   might be required.
2098<section title="MIME-Version" anchor="mime-version">
2100   HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY
2101   include a single MIME-Version general-header field to indicate what
2102   version of the MIME protocol was used to construct the message. Use
2103   of the MIME-Version header field indicates that the message is in
2104   full compliance with the MIME protocol (as defined in <xref target="RFC2045"/>).
2105   Proxies/gateways are responsible for ensuring full compliance (where
2106   possible) when exporting HTTP messages to strict MIME environments.
2108<figure><iref primary="true" item="Grammar" subitem="MIME-Version"/><artwork type="abnf2616"><![CDATA[
2109  MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
2112   MIME version "1.0" is the default for use in HTTP/1.1. However,
2113   HTTP/1.1 message parsing and semantics are defined by this document
2114   and not the MIME specification.
2118<section title="Conversion to Canonical Form" anchor="">
2120   <xref target="RFC2045"/> requires that an Internet mail entity be converted to
2121   canonical form prior to being transferred, as described in Section 4 of <xref target="RFC2049"/>.
2122   <xref target="canonicalization.and.text.defaults"/> of this document describes the forms
2123   allowed for subtypes of the "text" media type when transmitted over
2124   HTTP. <xref target="RFC2046"/> requires that content with a type of "text" represent
2125   line breaks as CRLF and forbids the use of CR or LF outside of line
2126   break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a
2127   line break within text content when a message is transmitted over
2128   HTTP.
2131   Where it is possible, a proxy or gateway from HTTP to a strict MIME
2132   environment SHOULD translate all line breaks within the text media
2133   types described in <xref target="canonicalization.and.text.defaults"/> of this document to the RFC 2049
2134   canonical form of CRLF. Note, however, that this might be complicated
2135   by the presence of a Content-Encoding and by the fact that HTTP
2136   allows the use of some character sets which do not use octets 13 and
2137   10 to represent CR and LF, as is the case for some multi-byte
2138   character sets.
2141   Implementors should note that conversion will break any cryptographic
2142   checksums applied to the original content unless the original content
2143   is already in canonical form. Therefore, the canonical form is
2144   recommended for any content that uses such checksums in HTTP.
2148<section title="Introduction of Content-Encoding" anchor="introduction.of.content-encoding">
2150   RFC 2045 does not include any concept equivalent to HTTP/1.1's
2151   Content-Encoding header field. Since this acts as a modifier on the
2152   media type, proxies and gateways from HTTP to MIME-compliant
2153   protocols MUST either change the value of the Content-Type header
2154   field or decode the entity-body before forwarding the message. (Some
2155   experimental applications of Content-Type for Internet mail have used
2156   a media-type parameter of ";conversions=&lt;content-coding&gt;" to perform
2157   a function equivalent to Content-Encoding. However, this parameter is
2158   not part of RFC 2045).
2162<section title="No Content-Transfer-Encoding" anchor="no.content-transfer-encoding">
2164   HTTP does not use the Content-Transfer-Encoding field of RFC
2165   2045. Proxies and gateways from MIME-compliant protocols to HTTP MUST
2166   remove any Content-Transfer-Encoding
2167   prior to delivering the response message to an HTTP client.
2170   Proxies and gateways from HTTP to MIME-compliant protocols are
2171   responsible for ensuring that the message is in the correct format
2172   and encoding for safe transport on that protocol, where "safe
2173   transport" is defined by the limitations of the protocol being used.
2174   Such a proxy or gateway SHOULD label the data with an appropriate
2175   Content-Transfer-Encoding if doing so will improve the likelihood of
2176   safe transport over the destination protocol.
2180<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
2182   HTTP/1.1 introduces the Transfer-Encoding header field (Section 8.7 of <xref target="Part1"/>).
2183   Proxies/gateways MUST remove any transfer-coding prior to
2184   forwarding a message via a MIME-compliant protocol.
2188<section title="MHTML and Line Length Limitations" anchor="mhtml.line.length">
2190   HTTP implementations which share code with MHTML <xref target="RFC2557"/> implementations
2191   need to be aware of MIME line length limitations. Since HTTP does not
2192   have this limitation, HTTP does not fold long lines. MHTML messages
2193   being transported by HTTP follow all conventions of MHTML, including
2194   line length limitations and folding, canonicalization, etc., since
2195   HTTP transports all message-bodies as payload (see <xref target="multipart.types"/>) and
2196   does not interpret the content or any MIME header lines that might be
2197   contained therein.
2202<section title="Additional Features" anchor="additional.features">
2204   <xref target="RFC1945"/> and <xref target="RFC2068"/> document protocol elements used by some
2205   existing HTTP implementations, but not consistently and correctly
2206   across most HTTP/1.1 applications. Implementors are advised to be
2207   aware of these features, but cannot rely upon their presence in, or
2208   interoperability with, other HTTP/1.1 applications. Some of these
2209   describe proposed experimental features, and some describe features
2210   that experimental deployment found lacking that are now addressed in
2211   the base HTTP/1.1 specification.
2214   A number of other headers, such as Content-Disposition and Title,
2215   from SMTP and MIME are also often implemented (see <xref target="RFC2076"/>).
2218<section title="Content-Disposition" anchor="content-disposition">
2219<iref item="Headers" subitem="Content-Disposition" primary="true"/>
2220<iref item="Content-Disposition header" primary="true"/>
2222   The Content-Disposition response-header field has been proposed as a
2223   means for the origin server to suggest a default filename if the user
2224   requests that the content is saved to a file. This usage is derived
2225   from the definition of Content-Disposition in <xref target="RFC1806"/>.
2227<figure><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"/><artwork type="abnf2616"><![CDATA[
2228  content-disposition = "Content-Disposition" ":"
2229                        disposition-type *( ";" disposition-parm )
2230  disposition-type = "attachment" | disp-extension-token
2231  disposition-parm = filename-parm | disp-extension-parm
2232  filename-parm = "filename" "=" quoted-string
2233  disp-extension-token = token
2234  disp-extension-parm = token "=" ( token | quoted-string )
2237   An example is
2239<figure><artwork type="example"><![CDATA[
2240     Content-Disposition: attachment; filename="fname.ext"
2243   The receiving user agent SHOULD NOT  respect any directory path
2244   information present in the filename-parm parameter, which is the only
2245   parameter believed to apply to HTTP implementations at this time. The
2246   filename SHOULD be treated as a terminal component only.
2249   If this header is used in a response with the application/octet-stream
2250   content-type, the implied suggestion is that the user agent
2251   should not display the response, but directly enter a `save response
2252   as...' dialog.
2255   See <xref target="content-disposition.issues"/> for Content-Disposition security issues.
2260<section title="Compatibility with Previous Versions" anchor="compatibility">
2261<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2263   Transfer-coding and message lengths all interact in ways that
2264   required fixing exactly when chunked encoding is used (to allow for
2265   transfer encoding that may not be self delimiting); it was important
2266   to straighten out exactly how message lengths are computed.
2267   (<xref target="entity.length"/>, see also <xref target="Part1"/>,
2268   <xref target="Part5"/> and <xref target="Part6"/>).
2271   Charset wildcarding is introduced to avoid explosion of character set
2272   names in accept headers. (<xref target="header.accept-charset"/>)
2275   Content-Base was deleted from the specification: it was not
2276   implemented widely, and there is no simple, safe way to introduce it
2277   without a robust extension mechanism. In addition, it is used in a
2278   similar, but not identical fashion in MHTML <xref target="RFC2557"/>.
2281   A content-coding of "identity" was introduced, to solve problems
2282   discovered in caching. (<xref target="content.codings"/>)
2285   Quality Values of zero should indicate that "I don't want something"
2286   to allow clients to refuse a representation. (<xref target="quality.values"/>)
2289   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
2290   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
2291   specification, but not commonly implemented. See <xref target="RFC2068"/>.
2295<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2297  Clarify contexts that charset is used in.
2298  (<xref target="character.sets"/>)
2301  Remove reference to non-existant identity transfer-coding value tokens.
2302  (<xref target="no.content-transfer-encoding"/>)
2308<section title="Change Log (to be removed by RFC Editor before publication)">
2310<section title="Since RFC2616">
2312  Extracted relevant partitions from <xref target="RFC2616"/>.
2316<section title="Since draft-ietf-httpbis-p3-payload-00">
2318  Closed issues:
2319  <list style="symbols">
2320    <t>
2321      <eref target=""/>:
2322      "Media Type Registrations"
2323      (<eref target=""/>)
2324    </t>
2325    <t>
2326      <eref target=""/>:
2327      "Clarification regarding quoting of charset values"
2328      (<eref target=""/>)
2329    </t>
2330    <t>
2331      <eref target=""/>:
2332      "Remove 'identity' token references"
2333      (<eref target=""/>)
2334    </t>
2335    <t>
2336      <eref target=""/>:
2337      "Accept-Encoding BNF"
2338    </t>
2339    <t>
2340      <eref target=""/>:
2341      "Normative and Informative references"
2342    </t>
2343    <t>
2344      <eref target=""/>:
2345      "RFC1700 references"
2346    </t>
2347    <t>
2348      <eref target=""/>:
2349      "Informative references"
2350    </t>
2351    <t>
2352      <eref target=""/>:
2353      "ISO-8859-1 Reference"
2354    </t>
2355    <t>
2356      <eref target=""/>:
2357      "Encoding References Normative"
2358    </t>
2359    <t>
2360      <eref target=""/>:
2361      "Normative up-to-date references"
2362    </t>
2363  </list>
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