HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                          Alcatel-Lucent
Expires: September 15, October 20, 2011                                       J. Mogul
                                                              H. Frystyk
                                                             L. Masinter
                                                                P. Leach
                                                          T. Berners-Lee
                                                           Y. Lafon, Ed.
                                                         J. Reschke, Ed.
                                                          March 14,
                                                          April 18, 2011

       HTTP/1.1, part 3: Message Payload and Content Negotiation


   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems.  HTTP has been in use by the World Wide Web global
   information initiative since 1990.  This document is Part 3 of the
   seven-part specification that defines the protocol referred to as
   "HTTP/1.1" and, taken together, obsoletes RFC 2616.  Part 3 defines
   HTTP message content, metadata, and content negotiation.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list ( (, which is archived at

   The current issues list is at
   <> and related
   documents (including fancy diffs) can be found at

   The changes in this draft are summarized in Appendix E.14. E.15.

Status of This Memo
   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 15, October 20, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  6
       1.3.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  6
       1.3.2.  ABNF Rules defined in other Parts of the
               Specification  . . . . . . . . . . . . . . . . . . . .  6
   2.  Protocol Parameters  . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Character Encodings (charset)  . . . . . . . . . . . . . .  6
       2.1.1.  Missing Charset  . . . . . . . . . . . . . . . . . . .  7
     2.2.  Content Codings  . . . . . . . . . . . . . . . . . . . . .  7
       2.2.1.  Content Coding Registry  . . . . . . . . . . . . . . .  8
     2.3.  Media Types  . . . . . . . . . . . . . . . . . . . . . . .  8
       2.3.1.  Canonicalization and Text Defaults . . . . . . . . . .  9
       2.3.2.  Multipart Types  . . . . . . . . . . . . . . . . . . . 10  9
     2.4.  Language Tags  . . . . . . . . . . . . . . . . . . . . . . 10
   3.  Payload  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 10
     3.1.  Payload Header Fields  . . . . . . . . . . . . . . . . . . 11
     3.2.  Payload Body . . . . . . . . . . . . . . . . . . . . . . . 12 11
   4.  Representation . . . . . . . . . . . . . . . . . . . . . . . . 12 11
     4.1.  Representation Header Fields . . . . . . . . . . . . . . . 12
     4.2.  Representation Data  . . . . . . . . . . . . . . . . . . . 13 12
   5.  Content Negotiation  . . . . . . . . . . . . . . . . . . . . . 14 13
     5.1.  Server-driven Negotiation  . . . . . . . . . . . . . . . . 15 14
     5.2.  Agent-driven Negotiation . . . . . . . . . . . . . . . . . 16 15
   6.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 16
     6.1.  Accept . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     6.2.  Accept-Charset . . . . . . . . . . . . . . . . . . . . . . 19 18
     6.3.  Accept-Encoding  . . . . . . . . . . . . . . . . . . . . . 19
     6.4.  Accept-Language  . . . . . . . . . . . . . . . . . . . . . 21 20
     6.5.  Content-Encoding . . . . . . . . . . . . . . . . . . . . . 22 21
     6.6.  Content-Language . . . . . . . . . . . . . . . . . . . . . 23 22
     6.7.  Content-Location . . . . . . . . . . . . . . . . . . . . . 24 23
     6.8.  Content-MD5  . . . . . . . . . . . . . . . . . . . . . . . 25
     6.9.  Content-Type . . . . . . . . . . . . . . . . . . . . . . . 26 24
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27 24
     7.1.  Header Field Registration  . . . . . . . . . . . . . . . . 27 24
     7.2.  Content Coding Registry  . . . . . . . . . . . . . . . . . 27 25
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28 25
     8.1.  Privacy Issues Connected to Accept Header Fields . . . . . 28 26
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29 26
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29 26
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 29 26
     10.2. Informative References . . . . . . . . . . . . . . . . . . 31 29
   Appendix A.  Differences between HTTP and MIME . . . . . . . . . . 32 31
     A.1.  MIME-Version . . . . . . . . . . . . . . . . . . . . . . . 32 31
     A.2.  Conversion to Canonical Form . . . . . . . . . . . . . . . 33 32
     A.3.  Conversion of Date Formats . . . . . . . . . . . . . . . . 33 32
     A.4.  Introduction of Content-Encoding . . . . . . . . . . . . . 34 32
     A.5.  No Content-Transfer-Encoding . . . . . . . . . . . . . . . 34 33
     A.6.  Introduction of Transfer-Encoding  . . . . . . . . . . . . 34 33
     A.7.  MHTML and Line Length Limitations  . . . . . . . . . . . . 34 33
   Appendix B.  Additional Features . . . . . . . . . . . . . . . . . 34 33
   Appendix C.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 35 34
   Appendix D.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 35 34
   Appendix E.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 37 36
     E.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 37 36
     E.2.  Since draft-ietf-httpbis-p3-payload-00 . . . . . . . . . . 37 36
     E.3.  Since draft-ietf-httpbis-p3-payload-01 . . . . . . . . . . 38 37
     E.4.  Since draft-ietf-httpbis-p3-payload-02 . . . . . . . . . . 38 37
     E.5.  Since draft-ietf-httpbis-p3-payload-03 . . . . . . . . . . 38 37
     E.6.  Since draft-ietf-httpbis-p3-payload-04 . . . . . . . . . . 39 38
     E.7.  Since draft-ietf-httpbis-p3-payload-05 . . . . . . . . . . 39 38
     E.8.  Since draft-ietf-httpbis-p3-payload-06 . . . . . . . . . . 39 38
     E.9.  Since draft-ietf-httpbis-p3-payload-07 . . . . . . . . . . 40 39
     E.10. Since draft-ietf-httpbis-p3-payload-08 . . . . . . . . . . 40 39
     E.11. Since draft-ietf-httpbis-p3-payload-09 . . . . . . . . . . 41 39
     E.12. Since draft-ietf-httpbis-p3-payload-10 . . . . . . . . . . 41 40
     E.13. Since draft-ietf-httpbis-p3-payload-11 . . . . . . . . . . 42 41
     E.14. Since draft-ietf-httpbis-p3-payload-12 . . . . . . . . . . 42 41
     E.15. Since draft-ietf-httpbis-p3-payload-13 . . . . . . . . . . 41
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 41

1.  Introduction

   This document defines HTTP/1.1 message payloads (a.k.a., content),
   the associated metadata header fields that define how the payload is
   intended to be interpreted by a recipient, the request header fields
   that might influence content selection, and the various selection
   algorithms that are collectively referred to as HTTP content

   This document is currently disorganized in order to minimize the
   changes between drafts and enable reviewers to see the smaller errata
   changes.  A future draft will reorganize the sections to better
   reflect the content.  In particular, the sections on entities will be
   renamed payload and moved to the first half of the document, while
   the sections on content negotiation and associated request header
   fields will be moved to the second half.  The current mess reflects
   how widely dispersed these topics and associated requirements had
   become in [RFC2616].

1.1.  Terminology

   This specification uses a number of terms to refer to the roles
   played by participants in, and objects of, the HTTP communication.

   content negotiation

      The mechanism for selecting the appropriate representation when
      servicing a request.  The representation in any response can be
      negotiated (including error responses).

1.2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   An implementation is not compliant if it fails to satisfy one or more
   of the "MUST" or "REQUIRED" level requirements for the protocols it
   implements.  An implementation that satisfies all the "MUST" or
   "REQUIRED" level and all the "SHOULD" level requirements for its
   protocols is said to be "unconditionally compliant"; one that
   satisfies all the "MUST" level requirements but not all the "SHOULD"
   level requirements for its protocols is said to be "conditionally

1.3.  Syntax Notation

   This specification uses the ABNF syntax defined in Section 1.2 of
   [Part1] (which extends the syntax defined in [RFC5234] with a list
   rule).  Appendix D shows the collected ABNF, with the list rule

   The following core rules are included by reference, as defined in
   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
   sequence of data), SP (space), VCHAR (any visible USASCII character),
   and WSP (whitespace).

1.3.1.  Core Rules

   The core rules below are defined in Section 1.2.2 of [Part1]:

     token          = <token, defined in [Part1], Section 1.2.2>
     word           = <word, defined in [Part1], Section 1.2.2>
     OWS            = <OWS, defined in [Part1], Section 1.2.2>

1.3.2.  ABNF Rules defined in other Parts of the Specification

   The ABNF rules below are defined in other parts:

     absolute-URI   = <absolute-URI, defined in [Part1], Section 2.6>
     partial-URI    = <partial-URI, defined in [Part1], Section 2.6>
     qvalue         = <qvalue, defined in [Part1], Section 6.4>

2.  Protocol Parameters

2.1.  Character Encodings (charset)

   HTTP uses charset names to indicate the character encoding of a
   textual representation.

   A character encoding is identified by a case-insensitive token.  The
   complete set of tokens is defined by the IANA Character Set registry

     charset = token

   Although HTTP allows an arbitrary token to be used as a charset
   value, any token that has a predefined value within the IANA
   Character Set registry MUST represent the character encoding defined
   by that registry.  Applications SHOULD limit their use of character
   encodings to those defined within the IANA registry.

   HTTP uses charset in two contexts: within an Accept-Charset request
   header field (in which the charset value is an unquoted token) and as
   the value of a parameter in a Content-Type header field (within a
   request or response), in which case the parameter value of the
   charset parameter can be quoted.

   Implementors need to be aware of IETF character set requirements
   [RFC3629] [RFC2277].

2.1.1.  Missing Charset

   Some HTTP/1.0 software has interpreted a Content-Type header field
   without charset parameter incorrectly to mean "recipient should
   guess".  Senders wishing to defeat this behavior MAY include a
   charset parameter even when the charset is ISO-8859-1 ([ISO-8859-1])
   and SHOULD do so when it is known that it will not confuse the

   Unfortunately, some older HTTP/1.0 clients did not deal properly with
   an explicit charset parameter.  HTTP/1.1 recipients MUST respect the
   charset label provided by the sender; and those user agents that have
   a provision to "guess" a charset MUST use the charset from the
   content-type field if they support that charset, rather than the
   recipient's preference, when initially displaying a document.  See
   Section 2.3.1.

2.2.  Content Codings

   Content coding values indicate an encoding transformation that has
   been or can be applied to a representation.  Content codings are
   primarily used to allow a representation to be compressed or
   otherwise usefully transformed without losing the identity of its
   underlying media type and without loss of information.  Frequently,
   the representation is stored in coded form, transmitted directly, and
   only decoded by the recipient.

     content-coding   = token

   All content-coding values are case-insensitive.  HTTP/1.1 uses
   content-coding values in the Accept-Encoding (Section 6.3) and
   Content-Encoding (Section 6.5) header fields.  Although the value
   describes the content-coding, what is more important is that it
   indicates what decoding mechanism will be required to remove the


      See Section of [Part1].


      See Section of [Part1].


      See Section of [Part1].


      The default (identity) encoding; the use of no transformation
      whatsoever.  This content-coding is used only in the Accept-
      Encoding header field, and SHOULD NOT be used in the Content-
      Encoding header field.

2.2.1.  Content Coding Registry

   The HTTP Content Coding Registry defines the name space for the
   content coding names.

   Registrations MUST include the following fields:

   o  Name

   o  Description

   o  Pointer to specification text

   Names of content codings MUST NOT overlap with names of transfer
   codings (Section 6.2 of [Part1]), unless the encoding transformation
   is identical (as it is the case for the compression codings defined
   in Section 6.2.2 of [Part1]).

   Values to be added to this name space require a specification (see
   "Specification Required" in Section 4.1 of [RFC5226]), and MUST
   conform to the purpose of content coding defined in this section.

   The registry itself is maintained at

2.3.  Media Types

   HTTP uses Internet Media Types [RFC2046] in the Content-Type
   (Section 6.9) 6.8) and Accept (Section 6.1) header fields in order to
   provide open and extensible data typing and type negotiation.

     media-type = type "/" subtype *( OWS ";" OWS parameter )
     type       = token
     subtype    = token

   The type/subtype MAY be followed by parameters in the form of
   attribute/value pairs.

     parameter      = attribute "=" value
     attribute      = token
     value          = word

   The type, subtype, and parameter attribute names are case-
   insensitive.  Parameter values might or might not be case-sensitive,
   depending on the semantics of the parameter name.  The presence or
   absence of a parameter might be significant to the processing of a
   media-type, depending on its definition within the media type

   A parameter value that matches the token production can be
   transmitted as either a token or within a quoted-string.  The quoted
   and unquoted values are equivalent.

   Note that some older HTTP applications do not recognize media type
   parameters.  When sending data to older HTTP applications,
   implementations SHOULD only use media type parameters when they are
   required by that type/subtype definition.

   Media-type values are registered with the Internet Assigned Number
   Authority (IANA).  The media type registration process is outlined in
   [RFC4288].  Use of non-registered media types is discouraged.

2.3.1.  Canonicalization and Text Defaults

   Internet media types are registered with a canonical form.  A
   representation transferred via HTTP messages MUST be in the
   appropriate canonical form prior to its transmission except for
   "text" types, as defined in the next paragraph.

   When in canonical form, media subtypes of the "text" type use CRLF as
   the text line break.  HTTP relaxes this requirement and allows the
   transport of text media with plain CR or LF alone representing a line
   break when it is done consistently for an entire representation.
   HTTP applications MUST accept CRLF, bare CR, and bare LF as
   indicating a line break in text media received via HTTP.  In
   addition, if the text is in a character encoding that does not use
   octets 13 and 10 for CR and LF respectively, as is the case for some
   multi-byte character encodings, HTTP allows the use of whatever octet
   sequences are defined by that character encoding to represent the
   equivalent of CR and LF for line breaks.  This flexibility regarding
   line breaks applies only to text media in the payload body; a bare CR
   or LF MUST NOT be substituted for CRLF within any of the HTTP control
   structures (such as header fields and multipart boundaries).

   If a representation is encoded with a content-coding, the underlying
   data MUST be in a form defined above prior to being encoded.

   The "charset" parameter is used with some media types to define the
   character encoding (Section 2.1) of the data.  When no explicit
   charset parameter is provided by the sender, media subtypes of the
   "text" type are defined to have a default charset value of
   "ISO-8859-1" when received via HTTP.  Data in character encodings
   other than "ISO-8859-1" or its subsets MUST be labeled with an
   appropriate charset value.  See Section 2.1.1 for compatibility

2.3.2.  Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more representations within a single message-body.  All
   multipart types share a common syntax, as defined in Section 5.1.1 of
   [RFC2046], and MUST include a boundary parameter as part of the media
   type value.  The message body is itself a protocol element and MUST
   therefore use only CRLF to represent line breaks between body-parts.

   In general, HTTP treats a multipart message-body no differently than
   any other media type: strictly as payload.  HTTP does not use the
   multipart boundary as an indicator of message-body length.  In all
   other respects, an HTTP user agent SHOULD follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart type.
   The MIME header fields within each body-part of a multipart message-
   body do not have any significance to HTTP beyond that defined by
   their MIME semantics.

   If an application receives an unrecognized multipart subtype, the
   application MUST treat it as being equivalent to "multipart/mixed".

      Note: The "multipart/form-data" type has been specifically defined
      for carrying form data suitable for processing via the POST
      request method, as described in [RFC2388].

2.4.  Language Tags

   A language tag, as defined in [RFC5646], identifies a natural
   language spoken, written, or otherwise conveyed by human beings for
   communication of information to other human beings.  Computer
   languages are explicitly excluded.  HTTP uses language tags within
   the Accept-Language and Content-Language fields.

   In summary, a language tag is composed of one or more parts: A
   primary language subtag followed by a possibly empty series of

     language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

   White space is not allowed within the tag and all tags are case-
   insensitive.  The name space of language subtags is administered by
   the IANA (see

   Example tags include:

     en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN

   See [RFC5646] for further information.

3.  Payload

   HTTP messages MAY transfer a payload if not otherwise restricted by
   the request method or response status code.  The payload consists of
   metadata, in the form of header fields, and data, in the form of the
   sequence of octets in the message-body after any transfer-coding has
   been decoded.

   A "payload" in HTTP is always a partial or complete representation of
   some resource.  We use separate terms for payload and representation
   because some messages contain only the associated representation's
   header fields (e.g., responses to HEAD) or only some part(s) of the
   representation (e.g., the 206 status code).

3.1.  Payload Header Fields

   HTTP header fields that specifically define the payload, rather than
   the associated representation, are referred to as "payload header
   fields".  The following payload header fields are defined by

   | Header Field Name | Defined in...          |
   | Content-Length    | Section 9.2 of [Part1] |
   | Content-MD5       | Section 6.8            |
   | Content-Range     | Section 5.2 of [Part5] |

3.2.  Payload Body

   A payload body is only present in a message when a message-body is
   present, as described in Section 3.3 of [Part1].  The payload body is
   obtained from the message-body by decoding any Transfer-Encoding that
   might have been applied to ensure safe and proper transfer of the

4.  Representation

   A "representation" is information in a format that can be readily
   communicated from one party to another.  A resource representation is
   information that reflects the state of that resource, as observed at
   some point in the past (e.g., in a response to GET) or to be desired
   at some point in the future (e.g., in a PUT request).

   Most, but not all, representations transferred via HTTP are intended
   to be a representation of the target resource (the resource
   identified by the effective request URI).  The precise semantics of a
   representation are determined by the type of message (request or
   response), the request method, the response status code, and the
   representation metadata.  For example, the above semantic is true for
   the representation in any 200 (OK) response to GET and for the
   representation in any PUT request.  A 200 response to PUT, in
   contrast, contains either a representation that describes the
   successful action or a representation of the target resource, with
   the latter indicated by a Content-Location header field with the same
   value as the effective request URI.  Likewise, response messages with
   an error status code usually contain a representation that describes
   the error and what next steps are suggested for resolving it.

4.1.  Representation Header Fields

   Representation header fields define metadata about the representation
   data enclosed in the message-body or, if no message-body is present,
   about the representation that would have been transferred in a 200
   response to a simultaneous GET request with the same effective
   request URI.

   The following header fields are defined as representation metadata:

   | Header Field Name | Defined in...          |
   | Content-Encoding  | Section 6.5            |
   | Content-Language  | Section 6.6            |
   | Content-Location  | Section 6.7            |
   | Content-Type      | Section 6.9 6.8            |
   | Expires           | Section 3.3 of [Part6] |
   | Last-Modified     | Section 6.6 2.1 of [Part4] |

4.2.  Representation Data

   The representation body associated with an HTTP message is either
   provided as the payload body of the message or referred to by the
   message semantics and the effective request URI.  The representation
   data is in a format and encoding defined by the representation
   metadata header fields.

   The data type of the representation data is determined via the header
   fields Content-Type and Content-Encoding.  These define a two-layer,
   ordered encoding model:

     representation-data := Content-Encoding( Content-Type( bits ) )

   Content-Type specifies the media type of the underlying data, which
   defines both the data format and how that data SHOULD be processed by
   the recipient (within the scope of the request method semantics).
   Any HTTP/1.1 message containing a payload body SHOULD include a
   Content-Type header field defining the media type of the associated
   representation unless that metadata is unknown to the sender.  If the
   Content-Type header field is not present, it indicates that the
   sender does not know the media type of the representation; recipients
   MAY either assume that the media type is "application/octet-stream"
   ([RFC2046], Section 4.5.1) or examine the content to determine its

   In practice, resource owners do not always properly configure their
   origin server to provide the correct Content-Type for a given
   representation, with the result that some clients will examine a
   response body's content and override the specified type.  Clients
   that do so risk drawing incorrect conclusions, which might expose
   additional security risks (e.g., "privilege escalation").
   Furthermore, it is impossible to determine the sender's intent by
   examining the data format: many data formats match multiple media
   types that differ only in processing semantics.  Implementers are
   encouraged to provide a means of disabling such "content sniffing"
   when it is used.

   Content-Encoding is used to indicate any additional content codings
   applied to the data, usually for the purpose of data compression,
   that are a property of the representation.  If Content-Encoding is
   not present, then there is no additional encoding beyond that defined
   by the Content-Type.

5.  Content Negotiation

   HTTP responses include a representation which contains information
   for interpretation, whether by a human user or for further
   processing.  Often, the server has different ways of representing the
   same information; for example, in different formats, languages, or
   using different character encodings.

   HTTP clients and their users might have different or variable
   capabilities, characteristics or preferences which would influence
   which representation, among those available from the server, would be
   best for the server to deliver.  For this reason, HTTP provides
   mechanisms for "content negotiation" -- a process of allowing
   selection of a representation of a given resource, when more than one
   is available.

   This specification defines two patterns of content negotiation;
   "server-driven", where the server selects the representation based
   upon the client's stated preferences, and "agent-driven" negotiation,
   where the server provides a list of representations for the client to
   choose from, based upon their metadata.  In addition, there are other
   patterns: some applications use an "active content" pattern, where
   the server returns active content which runs on the client and, based
   on client available parameters, selects additional resources to
   invoke.  "Transparent Content Negotiation" ([RFC2295]) has also been

   These patterns are all widely used, and have trade-offs in
   applicability and practicality.  In particular, when the number of
   preferences or capabilities to be expressed by a client are large
   (such as when many different formats are supported by a user-agent),
   server-driven negotiation becomes unwieldy, and might not be
   appropriate.  Conversely, when the number of representations to
   choose from is very large, agent-driven negotiation might not be

   Note that in all cases, the supplier of representations has the
   responsibility for determining which representations might be
   considered to be the "same information".

5.1.  Server-driven Negotiation

   If the selection of the best representation for a response is made by
   an algorithm located at the server, it is called server-driven
   negotiation.  Selection is based on the available representations of
   the response (the dimensions over which it can vary; e.g., language,
   content-coding, etc.) and the contents of particular header fields in
   the request message or on other information pertaining to the request
   (such as the network address of the client).

   Server-driven negotiation is advantageous when the algorithm for
   selecting from among the available representations is difficult to
   describe to the user agent, or when the server desires to send its
   "best guess" to the client along with the first response (hoping to
   avoid the round-trip delay of a subsequent request if the "best
   guess" is good enough for the user).  In order to improve the
   server's guess, the user agent MAY include request header fields
   (Accept, Accept-Language, Accept-Encoding, etc.) which describe its
   preferences for such a response.

   Server-driven negotiation has disadvantages:

   1.  It is impossible for the server to accurately determine what
       might be "best" for any given user, since that would require
       complete knowledge of both the capabilities of the user agent and
       the intended use for the response (e.g., does the user want to
       view it on screen or print it on paper?).

   2.  Having the user agent describe its capabilities in every request
       can be both very inefficient (given that only a small percentage
       of responses have multiple representations) and a potential
       violation of the user's privacy.

   3.  It complicates the implementation of an origin server and the
       algorithms for generating responses to a request.

   4.  It might limit a public cache's ability to use the same response
       for multiple user's requests.

   HTTP/1.1 includes the following header fields for enabling server-
   driven negotiation through description of user agent capabilities and
   user preferences: Accept (Section 6.1), Accept-Charset (Section 6.2),
   Accept-Encoding (Section 6.3), Accept-Language (Section 6.4), and
   User-Agent (Section 9.9 of [Part2]).  However, an origin server is
   not limited to these dimensions and MAY vary the response based on
   any aspect of the request, including aspects of the connection (e.g.,
   IP address) or information within extension header fields not defined
   by this specification.

      Note: In practice, User-Agent based negotiation is fragile,
      because new clients might not be recognized.

   The Vary header field (Section 3.5 of [Part6]) can be used to express
   the parameters the server uses to select a representation that is
   subject to server-driven negotiation.

5.2.  Agent-driven Negotiation

   With agent-driven negotiation, selection of the best representation
   for a response is performed by the user agent after receiving an
   initial response from the origin server.  Selection is based on a
   list of the available representations of the response included within
   the header fields or body of the initial response, with each
   representation identified by its own URI.  Selection from among the
   representations can be performed automatically (if the user agent is
   capable of doing so) or manually by the user selecting from a
   generated (possibly hypertext) menu.

   Agent-driven negotiation is advantageous when the response would vary
   over commonly-used dimensions (such as type, language, or encoding),
   when the origin server is unable to determine a user agent's
   capabilities from examining the request, and generally when public
   caches are used to distribute server load and reduce network usage.

   Agent-driven negotiation suffers from the disadvantage of needing a
   second request to obtain the best alternate representation.  This
   second request is only efficient when caching is used.  In addition,
   this specification does not define any mechanism for supporting
   automatic selection, though it also does not prevent any such
   mechanism from being developed as an extension and used within

   This specification defines the 300 (Multiple Choices) and 406 (Not
   Acceptable) status codes for enabling agent-driven negotiation when
   the server is unwilling or unable to provide a varying response using
   server-driven negotiation.

6.  Header Field Definitions

   This section defines the syntax and semantics of HTTP/1.1 header
   fields related to the payload of messages.

6.1.  Accept

   The "Accept" header field can be used by user agents to specify
   response media types that are acceptable.  Accept header fields can
   be used to indicate that the request is specifically limited to a
   small set of desired types, as in the case of a request for an in-
   line image.

     Accept = "Accept" ":" OWS Accept-v
     Accept-v = #( media-range [ accept-params ] )

     media-range    = ( "*/*"
                      / ( type "/" "*" )
                      / ( type "/" subtype )
                      ) *( OWS ";" OWS parameter )
     accept-params  = OWS ";" OWS "q=" qvalue *( accept-ext )
     accept-ext     = OWS ";" OWS token [ "=" word ]

   The asterisk "*" character is used to group media types into ranges,
   with "*/*" indicating all media types and "type/*" indicating all
   subtypes of that type.  The media-range MAY include media type
   parameters that are applicable to that range.

   Each media-range MAY be followed by one or more accept-params,
   beginning with the "q" parameter for indicating a relative quality
   factor.  The first "q" parameter (if any) separates the media-range
   parameter(s) from the accept-params.  Quality factors allow the user
   or user agent to indicate the relative degree of preference for that
   media-range, using the qvalue scale from 0 to 1 (Section 6.4 of
   [Part1]).  The default value is q=1.

      Note: Use of the "q" parameter name to separate media type
      parameters from Accept extension parameters is due to historical
      practice.  Although this prevents any media type parameter named
      "q" from being used with a media range, such an event is believed
      to be unlikely given the lack of any "q" parameters in the IANA
      media type registry and the rare usage of any media type
      parameters in Accept.  Future media types are discouraged from
      registering any parameter named "q".

   The example
     Accept: audio/*; q=0.2, audio/basic

   SHOULD be interpreted as "I prefer audio/basic, but send me any audio
   type if it is the best available after an 80% mark-down in quality".

   If no Accept header field is present, then it is assumed that the
   client accepts all media types.  If an Accept header field is
   present, and if the server cannot send a response which is acceptable
   according to the combined Accept field value, then the server SHOULD
   send a 406 (Not Acceptable) response.

   A more elaborate example is

     Accept: text/plain; q=0.5, text/html,
             text/x-dvi; q=0.8, text/x-c

   Verbally, this would be interpreted as "text/html and text/x-c are
   the preferred media types, but if they do not exist, then send the
   text/x-dvi representation, and if that does not exist, send the text/
   plain representation".

   Media ranges can be overridden by more specific media ranges or
   specific media types.  If more than one media range applies to a
   given type, the most specific reference has precedence.  For example,

     Accept: text/*, text/html, text/html;level=1, text/plain, text/plain;format=flowed, */*

   have the following precedence:

   1.  text/html;level=1  text/plain;format=flowed

   2.  text/html  text/plain

   3.  text/*

   4.  */*

   The media type quality factor associated with a given type is
   determined by finding the media range with the highest precedence
   which matches that type.  For example,

     Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
             text/html;level=2;q=0.4, */*;q=0.5

   would cause the following values to be associated:

   | Media Type        | Quality Value |
   | text/html;level=1 | 1             |
   | text/html         | 0.7           |
   | text/plain        | 0.3           |
   | image/jpeg        | 0.5           |
   | text/html;level=2 | 0.4           |
   | text/html;level=3 | 0.7           |

   Note: A user agent might be provided with a default set of quality
   values for certain media ranges.  However, unless the user agent is a
   closed system which cannot interact with other rendering agents, this
   default set ought to be configurable by the user.

6.2.  Accept-Charset

   The "Accept-Charset" header field can be used by user agents to
   indicate what character encodings are acceptable in a response
   payload.  This field allows clients capable of understanding more
   comprehensive or special-purpose character encodings to signal that
   capability to a server which is capable of representing documents in
   those character encodings.

     Accept-Charset = "Accept-Charset" ":" OWS
     Accept-Charset-v = 1#( ( charset / "*" )
                            [ OWS ";" OWS "q=" qvalue ] )

   Character encoding values (a.k.a., charsets) are described in
   Section 2.1.  Each charset MAY be given an associated quality value
   which represents the user's preference for that charset.  The default
   value is q=1.  An example is

     Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

   The special value "*", if present in the Accept-Charset field,
   matches every character encoding (including ISO-8859-1) which is not mentioned elsewhere in
   the Accept-Charset field.  If no "*" is present in an Accept-Charset
   field, then all character encodings not explicitly mentioned get a
   quality value of 0, except for ISO-8859-1,
   which gets a quality value of 1 if not explicitly mentioned. 0.

   If no Accept-Charset header field is present, the default is that any
   character encoding is acceptable.  If an Accept-Charset header field
   is present, and if the server cannot send a response which is
   acceptable according to the Accept-Charset header field, then the
   server SHOULD send an error response with the 406 (Not Acceptable)
   status code, though the sending of an unacceptable response is also

6.3.  Accept-Encoding

   The "Accept-Encoding" header field can be used by user agents to
   indicate what response content-codings (Section 2.2) are acceptable
   in the response.

     Accept-Encoding  = "Accept-Encoding" ":" OWS
     Accept-Encoding-v  = #( codings [ OWS ";" OWS "q=" qvalue ] )
     codings          = ( content-coding / "*" )

   Each codings value MAY be given an associated quality value which
   represents the preference for that encoding.  The default value is

   Examples of its use are:

     Accept-Encoding: compress, gzip
     Accept-Encoding: *
     Accept-Encoding: compress;q=0.5, gzip;q=1.0
     Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

   A server tests whether a content-coding is acceptable, according to
   an Accept-Encoding field, using these rules:

   1.  If the content-coding is one of the content-codings listed in the
       Accept-Encoding field, then it is acceptable, unless it is
       accompanied by a qvalue of 0.  (As defined in Section 6.4 of
       [Part1], a qvalue of 0 means "not acceptable".)

   2.  The special "*" symbol in an Accept-Encoding field matches any
       available content-coding not explicitly listed in the header

   3.  If multiple content-codings are acceptable, then the acceptable
       content-coding with the highest non-zero qvalue is preferred.

   4.  The "identity" content-coding is always acceptable, unless
       specifically refused because the Accept-Encoding field includes
       "identity;q=0", or because the field includes "*;q=0" and does
       not explicitly include the "identity" content-coding.  If the
       Accept-Encoding field-value is empty, then only the "identity"
       encoding is acceptable.

   If an Accept-Encoding field is present in a request, and if the
   server cannot send a response which is acceptable according to the
   Accept-Encoding header field, then the server SHOULD send an error
   response with the 406 (Not Acceptable) status code.

   If no Accept-Encoding field is present in a request, the server MAY
   assume that the client will accept any content coding.  In this case,
   if "identity" is one of the available content-codings, then the
   server SHOULD use the "identity" content-coding, unless it has
   additional information that a different content-coding is meaningful
   to the client.

      Note: If the request does not include an Accept-Encoding field,
      and if the "identity" content-coding is unavailable, then content-
      codings commonly understood by HTTP/1.0 clients (i.e., "gzip" and
      "compress") are preferred; some older clients improperly display
      messages sent with other content-codings.  The server might also
      make this decision based on information about the particular user-
      agent or client.

      Note: Most HTTP/1.0 applications do not recognize or obey qvalues
      associated with content-codings.  This means that qvalues will not
      work and are not permitted with x-gzip or x-compress.

6.4.  Accept-Language

   The "Accept-Language" header field can be used by user agents to
   indicate the set of natural languages that are preferred in the
   response.  Language tags are defined in Section 2.4.

     Accept-Language = "Accept-Language" ":" OWS
     Accept-Language-v =
                       1#( language-range [ OWS ";" OWS "q=" qvalue ] )
     language-range  =
               <language-range, defined in [RFC4647], Section 2.1>

   Each language-range can be given an associated quality value which
   represents an estimate of the user's preference for the languages
   specified by that range.  The quality value defaults to "q=1".  For

     Accept-Language: da, en-gb;q=0.8, en;q=0.7

   would mean: "I prefer Danish, but will accept British English and
   other types of English". (see also Section 2.3 of [RFC4647])

   For matching, Section 3 of [RFC4647] defines several matching
   schemes.  Implementations can offer the most appropriate matching
   scheme for their requirements.

      Note: The "Basic Filtering" scheme ([RFC4647], Section 3.3.1) is
      identical to the matching scheme that was previously defined in
      Section 14.4 of [RFC2616].

   It might be contrary to the privacy expectations of the user to send
   an Accept-Language header field with the complete linguistic
   preferences of the user in every request.  For a discussion of this
   issue, see Section 8.1.

   As intelligibility is highly dependent on the individual user, it is
   recommended that client applications make the choice of linguistic
   preference available to the user.  If the choice is not made
   available, then the Accept-Language header field MUST NOT be given in
   the request.

      Note: When making the choice of linguistic preference available to
      the user, we remind implementors of the fact that users are not
      familiar with the details of language matching as described above,
      and ought to be provided appropriate guidance.  As an example,
      users might assume that on selecting "en-gb", they will be served
      any kind of English document if British English is not available.
      A user agent might suggest in such a case to add "en" to get the
      best matching behavior.

6.5.  Content-Encoding

   The "Content-Encoding" header field indicates what content-codings
   have been applied to the representation, and thus what decoding
   mechanisms must be applied in order to obtain the media-type
   referenced by the Content-Type header field.  Content-Encoding is
   primarily used to allow a representation to be compressed without
   losing the identity of its underlying media type.

     Content-Encoding = "Content-Encoding" ":" OWS Content-Encoding-v
     Content-Encoding-v = 1#content-coding

   Content codings are defined in Section 2.2.  An example of its use is

     Content-Encoding: gzip

   The content-coding is a characteristic of the representation.
   Typically, the representation body is stored with this encoding and
   is only decoded before rendering or analogous usage.  However, a
   transforming proxy MAY modify the content-coding if the new coding is
   known to be acceptable to the recipient, unless the "no-transform"
   cache-control directive is present in the message.

   If the content-coding of a representation is not "identity", then the
   representation metadata MUST include a Content-Encoding header field
   (Section 6.5) that lists the non-identity content-coding(s) used.

   If the content-coding of a representation in a request message is not
   acceptable to the origin server, the server SHOULD respond with a
   status code of 415 (Unsupported Media Type).

   If multiple encodings have been applied to a representation, the
   content codings MUST be listed in the order in which they were
   applied.  Additional information about the encoding parameters MAY be
   provided by other header fields not defined by this specification.

6.6.  Content-Language

   The "Content-Language" header field describes the natural language(s)
   of the intended audience for the representation.  Note that this
   might not be equivalent to all the languages used within the

     Content-Language = "Content-Language" ":" OWS Content-Language-v
     Content-Language-v = 1#language-tag

   Language tags are defined in Section 2.4.  The primary purpose of
   Content-Language is to allow a user to identify and differentiate
   representations according to the user's own preferred language.
   Thus, if the body content is intended only for a Danish-literate
   audience, the appropriate field is

     Content-Language: da

   If no Content-Language is specified, the default is that the content
   is intended for all language audiences.  This might mean that the
   sender does not consider it to be specific to any natural language,
   or that the sender does not know for which language it is intended.

   Multiple languages MAY be listed for content that is intended for
   multiple audiences.  For example, a rendition of the "Treaty of
   Waitangi", presented simultaneously in the original Maori and English
   versions, would call for

     Content-Language: mi, en

   However, just because multiple languages are present within a
   representation does not mean that it is intended for multiple
   linguistic audiences.  An example would be a beginner's language
   primer, such as "A First Lesson in Latin", which is clearly intended
   to be used by an English-literate audience.  In this case, the
   Content-Language would properly only include "en".

   Content-Language MAY be applied to any media type -- it is not
   limited to textual documents.

6.7.  Content-Location

   The "Content-Location" header field supplies a URI that can be used
   as a specific identifier for the representation in this message.  In
   other words, if one were to perform a GET on this URI at the time of
   this message's generation, then a 200 response would contain the same
   representation that is enclosed as payload in this message.

     Content-Location = "Content-Location" ":" OWS
     Content-Location-v = absolute-URI / partial-URI

   The Content-Location value is not a replacement for the effective
   Request URI (Section 4.3 of [Part1]).  It is representation metadata.
   It has the same syntax and semantics as the header field of the same
   name defined for MIME body parts in Section 4 of [RFC2557].  However,
   its appearance in an HTTP message has some special implications for
   HTTP recipients.

   If Content-Location is included in a response message and its value
   is the same as the effective request URI, then the response payload
   SHOULD be considered the current representation of that resource.
   For a GET or HEAD request, this is the same as the default semantics
   when no Content-Location is provided by the server.  For a state-
   changing request like PUT or POST, it implies that the server's
   response contains the new representation of that resource, thereby
   distinguishing it from representations that might only report about
   the action (e.g., "It worked!").  This allows authoring applications
   to update their local copies without the need for a subsequent GET

   If Content-Location is included in a response message and its value
   differs from the effective request URI, then the origin server is
   informing recipients that this representation has its own, presumably
   more specific, identifier.  For a GET or HEAD request, this is an
   indication that the effective request URI identifies a resource that
   is subject to content negotiation and the representation selected for
   this response can also be found at the identified URI.  For other
   methods, such a Content-Location indicates that this representation
   contains a report on the action's status and the same report is
   available (for future access with GET) at the given URI.  For
   example, a purchase transaction made via a POST request might include
   a receipt document as the payload of the 200 response; the Content-
   Location value provides an identifier for retrieving a copy of that
   same receipt in the future.

   If Content-Location is included in a request message, then it MAY be
   interpreted by the origin server as an indication of where the user
   agent originally obtained the content of the enclosed representation
   (prior to any subsequent modification of the content by that user
   agent).  In other words, the user agent is providing the same
   representation metadata that it received with the original
   representation.  However, such interpretation MUST NOT be used to
   alter the semantics of the method requested by the client.  For
   example, if a client makes a PUT request on a negotiated resource and
   the origin server accepts that PUT (without redirection), then the
   new set of values for that resource is expected to be consistent with
   the one representation supplied in that PUT; the Content-Location
   cannot be used as a form of reverse content selection that identifies
   only one of the negotiated representations to be updated.  If the
   user agent had wanted the latter semantics, it would have applied the
   PUT directly to the Content-Location URI.

   A Content-Location field received in a request message is transitory
   information that SHOULD NOT be saved with other representation
   metadata for use in later responses.  The Content-Location's value
   might be saved for use in other contexts, such as within source links
   or other metadata.

   A cache cannot assume that a representation with a Content-Location
   different from the URI used to retrieve it can be used to respond to
   later requests on that Content-Location URI.

   If the Content-Location value is a partial URI, the partial URI is
   interpreted relative to the effective request URI.

6.8.  Content-MD5  Content-Type

   The "Content-MD5" "Content-Type" header field, as defined in [RFC1864], is an MD5
   digest of field indicates the payload body that provides an end-to-end message
   integrity check (MIC) media type of the payload body (the message-body after any
   transfer-coding is decoded).  Note that a MIC is good for detecting
   accidental modification of
   representation.  In the payload body in transit, but is not
   proof against malicious attacks.

     Content-MD5   = "Content-MD5" ":" OWS Content-MD5-v
     Content-MD5-v = <base64 case of 128 bit MD5 digest as per [RFC1864]>

   The Content-MD5 header field MAY be generated by an origin server or
   client responses to function as an integrity check of the payload body.  Only
   origin servers or user agents MAY generate HEAD method, the Content-MD5 header
   field; proxies MUST NOT generate it, as this would defeat its value
   as an end-to-end integrity check.  Any recipient MAY check
   media type is that which would have been sent had the
   digest value request been a

     Content-Type = media-type

   Media types are defined in this header Section 2.3.  An example of the field matches a corresponding digest
   calculated on payload body as received.

   The MD5 digest is computed based on the content of the payload body,
   including any content-coding, but not including any transfer-coding
   applied to the message-body because such transfer-codings might be
   applied or removed anywhere along the request/response chain.  If the
   message is received with a transfer-coding, that encoding MUST be
   decoded prior to checking the Content-MD5 value against the received

   HTTP extends RFC 1864 to permit the digest to be computed for MIME
   composite media-types (e.g., multipart/* and message/rfc822), but
   this does not change how the digest is computed as defined in the
   preceding paragraph.

   There are several consequences of this.  The payload for composite
   types MAY contain many body-parts, each with its own MIME and HTTP
   header fields (including Content-MD5, Content-Transfer-Encoding, and
   Content-Encoding header fields).  If a body-part has a Content-
   Transfer-Encoding or Content-Encoding header field, it is assumed
   that the content of the body-part has had the encoding applied, and
   the body-part is included in the Content-MD5 digest as is -- i.e.,
   after the application.  The Transfer-Encoding header field is not
   allowed within body-parts.

   Conversion of all line breaks to CRLF MUST NOT be done before
   computing or checking the digest: the line break convention used in
   the text actually transmitted MUST be left unaltered when computing
   the digest.

      Note: While the definition of Content-MD5 is exactly the same for
      HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
      in which the application of Content-MD5 to HTTP entity-bodies
      differs from its application to MIME entity-bodies.  One is that
      HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
      does use Transfer-Encoding and Content-Encoding.  Another is that
      HTTP more frequently uses binary content types than MIME, so it is
      worth noting that, in such cases, the byte order used to compute
      the digest is the transmission byte order defined for the type.
      Lastly, HTTP allows transmission of text types with any of several
      line break conventions and not just the canonical form using CRLF.

6.9.  Content-Type

   The "Content-Type" header field indicates the media type of the
   representation.  In the case of responses to the HEAD method, the
   media type is that which would have been sent had the request been a

     Content-Type   = "Content-Type" ":" OWS Content-Type-v
     Content-Type-v = media-type

   Media types are defined in Section 2.3.  An example of the field is

     Content-Type: text/html; charset=ISO-8859-4

   Further discussion

     Content-Type: text/html; charset=ISO-8859-4

   Further discussion of Content-Type is provided in Section 4.2.

7.  IANA Considerations

7.1.  Header Field Registration

   The Message Header Field Registry located at <
   assignments/message-headers/message-header-index.html> shall be
   updated with the permanent registrations below (see [RFC3864]):

   | Header Field Name | Protocol | Status   | Reference    |
   | Accept            | http     | standard | Section 6.1  |
   | Accept-Charset    | http     | standard | Section 6.2  |
   | Accept-Encoding   | http     | standard | Section 6.3  |
   | Accept-Language   | http     | standard | Section 6.4  |
   | Content-Encoding  | http     | standard | Section 6.5  |
   | Content-Language  | http     | standard | Section 6.6  |
   | Content-Location  | http     | standard | Section 6.7  |
   | Content-MD5       | http     | standard | Section 6.8  |
   | Content-Type      | http     | standard | Section 6.9 6.8  |
   | MIME-Version      | http     | standard | Appendix A.1 |

   The change controller is: "IETF ( - Internet
   Engineering Task Force".

7.2.  Content Coding Registry

   The registration procedure for HTTP Content Codings is now defined by
   Section 2.2.1 of this document.

   The HTTP Content Codings Registry located at
   <> shall be updated
   with the registration below:

   | Name     | Description                             | Reference    |
   | compress | UNIX "compress" program method          | Section      |
   |          |                                         | of   |
   |          |                                         | [Part1]      |
   | deflate  | "deflate" compression mechanism         | Section      |
   |          | ([RFC1951]) used inside the "zlib" data | of   |
   |          | format ([RFC1950])                      | [Part1]      |
   | gzip     | Same as GNU zip [RFC1952]               | Section      |
   |          |                                         | of   |
   |          |                                         | [Part1]      |
   | identity | No transformation                       | Section 2.2  |

8.  Security Considerations

   This section is meant to inform application developers, information
   providers, and users of the security limitations in HTTP/1.1 as
   described by this document.  The discussion does not include
   definitive solutions to the problems revealed, though it does make
   some suggestions for reducing security risks.

8.1.  Privacy Issues Connected to Accept Header Fields

   Accept headers fields can reveal information about the user to all
   servers which are accessed.  The Accept-Language header field in
   particular can reveal information the user would consider to be of a
   private nature, because the understanding of particular languages is
   often strongly correlated to the membership of a particular ethnic
   group.  User agents which offer the option to configure the contents
   of an Accept-Language header field to be sent in every request are
   strongly encouraged to let the configuration process include a
   message which makes the user aware of the loss of privacy involved.

   An approach that limits the loss of privacy would be for a user agent
   to omit the sending of Accept-Language header fields by default, and
   to ask the user whether or not to start sending Accept-Language
   header fields to a server if it detects, by looking for any Vary
   header fields generated by the server, that such sending could
   improve the quality of service.

   Elaborate user-customized accept header fields sent in every request,
   in particular if these include quality values, can be used by servers
   as relatively reliable and long-lived user identifiers.  Such user
   identifiers would allow content providers to do click-trail tracking,
   and would allow collaborating content providers to match cross-server
   click-trails or form submissions of individual users.  Note that for
   many users not behind a proxy, the network address of the host
   running the user agent will also serve as a long-lived user
   identifier.  In environments where proxies are used to enhance
   privacy, user agents ought to be conservative in offering accept
   header configuration options to end users.  As an extreme privacy
   measure, proxies could filter the accept header fields in relayed
   requests.  General purpose user agents which provide a high degree of
   header configurability SHOULD warn users about the loss of privacy
   which can be involved.

9.  Acknowledgments

10.  References

10.1.  Normative References

   [ISO-8859-1]  International Organization for Standardization,
                 "Information technology -- 8-bit single-byte coded
                 graphic character sets -- Part 1: Latin alphabet No.
                 1", ISO/IEC 8859-1:1998, 1998.

   [Part1]                            Fielding, R., Ed., Gettys, J.,
                                      Mogul, J., Frystyk, H., Masinter,
                                      L., Leach, P., Berners-Lee, T.,
                                      Lafon, Y., Ed., and J. Reschke,
                                      Ed., "HTTP/1.1, part 1: URIs,
                                      Connections, and Message Parsing",
                                      (work in progress),
                 March April 2011.

   [Part2]                            Fielding, R., Ed., Gettys, J.,
                                      Mogul, J., Frystyk, H., Masinter,
                                      L., Leach, P., Berners-Lee, T.,
                                      Lafon, Y., Ed., and J. Reschke,
                                      Ed., "HTTP/1.1, part 2: Message
                                      Semantics", draft-ietf-httpbis-p2-semantics-13
                                      (work in progress), March April 2011.

   [Part4]                            Fielding, R., Ed., Gettys, J.,
                                      Mogul, J., Frystyk, H., Masinter,
                                      L., Leach, P., Berners-Lee, T.,
                                      Lafon, Y., Ed., and J. Reschke,
                                      Ed., "HTTP/1.1, part 4:
                                      Conditional Requests",
                 draft-ietf-httpbis-p4-conditional-13 draft-ietf-
                                      httpbis-p4-conditional-14 (work in
                                      progress), March April 2011.

   [Part5]                            Fielding, R., Ed., Gettys, J.,
                                      Mogul, J., Frystyk, H., Masinter,
                                      L., Leach, P., Berners-Lee, T.,
                                      Lafon, Y., Ed., and J. Reschke,
                                      Ed., "HTTP/1.1, part 5: Range
                                      Requests and Partial Responses",
                                      (work in progress),
                 March April 2011.

   [Part6]                            Fielding, R., Ed., Gettys, J.,
                                      Mogul, J., Frystyk, H., Masinter,
                                      L., Leach, P., Berners-Lee, T.,
                                      Lafon, Y., Ed., Nottingham, M.,
                                      Ed., and J. Reschke, Ed.,
                                      "HTTP/1.1, part 6: Caching",
                                      (work in progress),
                 March April 2011.

   [RFC1864]     Myers, J. and M. Rose, "The Content-MD5 Header Field",
                 RFC 1864, October 1995.

   [RFC1950]                          Deutsch, L. and J-L. Gailly, "ZLIB
                                      Compressed Data Format
                                      Specification version 3.3",
                                      RFC 1950, May 1996.

                                      RFC 1950 is an Informational RFC,
                                      thus it might be less stable than
                                      this specification.  On the other
                                      hand, this downward reference was
                                      present since the publication of
                                      RFC 2068 in 1997 ([RFC2068]),
                                      therefore it is unlikely to cause
                                      problems in practice.  See also

   [RFC1951]                          Deutsch, P., "DEFLATE Compressed
                                      Data Format Specification version
                                      1.3", RFC 1951, May 1996.

                                      RFC 1951 is an Informational RFC,
                                      thus it might be less stable than
                                      this specification.  On the other
                                      hand, this downward reference was
                                      present since the publication of
                                      RFC 2068 in 1997 ([RFC2068]),
                                      therefore it is unlikely to cause
                                      problems in practice.  See also

   [RFC1952]                          Deutsch, P., Gailly, J-L., Adler,
                                      M., Deutsch, L., and G. Randers-Pehrson, Randers-
                                      Pehrson, "GZIP file format
                                      specification version 4.3",
                                      RFC 1952, May 1996.

                                      RFC 1952 is an Informational RFC,
                                      thus it might be less stable than
                                      this specification.  On the other
                                      hand, this downward reference was
                                      present since the publication of
                                      RFC 2068 in 1997 ([RFC2068]),
                                      therefore it is unlikely to cause
                                      problems in practice.  See also

   [RFC2045]                          Freed, N. and N. Borenstein,
                                      "Multipurpose Internet Mail
                                      Extensions (MIME) Part One: Format
                                      of Internet Message Bodies",
                                      RFC 2045, November 1996.

   [RFC2046]                          Freed, N. and N. Borenstein,
                                      "Multipurpose Internet Mail
                                      Extensions (MIME) Part Two: Media
                                      Types", RFC 2046, November 1996.

   [RFC2119]                          Bradner, S., "Key words for use in
                                      RFCs to Indicate Requirement
                                      Levels", BCP 14, RFC 2119,
                                      March 1997.

   [RFC4647]                          Phillips, A., Ed. and M. Davis,
                                      Ed., "Matching of Language Tags",
                                      BCP 47, RFC 4647, September 2006.

   [RFC5234]                          Crocker, D., Ed. and P. Overell,
                                      "Augmented BNF for Syntax
                                      Specifications: ABNF", STD 68,
                                      RFC 5234, January 2008.

   [RFC5646]                          Phillips, A., Ed. and M. Davis,
                                      Ed., "Tags for Identifying
                                      Languages", BCP 47, RFC 5646,
                                      September 2009.

10.2.  Informative References

   [BCP97]                            Klensin, J. and S. Hartman,
                                      "Handling Normative References to
                                      Standards-Track Documents",
                                      BCP 97, RFC 4897, June 2007.

   [RFC1945]                          Berners-Lee, T., Fielding, R., and
                                      H. Nielsen, "Hypertext Transfer
                                      Protocol -- HTTP/1.0", RFC 1945,
                                      May 1996.

   [RFC2049]                          Freed, N. and N. Borenstein,
                                      "Multipurpose Internet Mail
                                      Extensions (MIME) Part Five:
                                      Conformance Criteria and
                                      Examples", RFC 2049,
                                      November 1996.

   [RFC2068]                          Fielding, R., Gettys, J., Mogul,
                                      J., Nielsen, H., and T. Berners-Lee, Berners-
                                      Lee, "Hypertext Transfer Protocol
                                      -- HTTP/1.1", RFC 2068,
                                      January 1997.

   [RFC2076]                          Palme, J., "Common Internet
                                      Message Headers", RFC 2076,
                                      February 1997.

   [RFC2277]                          Alvestrand, H., "IETF Policy on
                                      Character Sets and Languages",
                                      BCP 18, RFC 2277, January 1998.

   [RFC2295]                          Holtman, K. and A. Mutz,
                                      "Transparent Content Negotiation
                                      in HTTP", RFC 2295, March 1998.

   [RFC2388]                          Masinter, L., "Returning Values
                                      from Forms:  multipart/
                 form-data",  multipart/form-data",
                                      RFC 2388, August 1998.

   [RFC2557]                          Palme, F., Hopmann, A., Shelness,
                                      N., and E. Stefferud, "MIME
                                      Encapsulation of Aggregate
                                      Documents, such as HTML (MHTML)",
                                      RFC 2557, March 1999.

   [RFC2616]                          Fielding, R., Gettys, J., Mogul,
                                      J., Frystyk, H., Masinter, L.,
                                      Leach, P., and T. Berners-Lee,
                                      "Hypertext Transfer Protocol --
                                      HTTP/1.1", RFC 2616, June 1999.

   [RFC3629]                          Yergeau, F., "UTF-8, a
                                      transformation format of ISO
                                      10646", STD 63, RFC 3629,
                                      November 2003.

   [RFC3864]                          Klyne, G., Nottingham, M., and J.
                                      Mogul, "Registration Procedures
                                      for Message Header Fields",
                                      BCP 90, RFC 3864, September 2004.

   [RFC4288]                          Freed, N. and J. Klensin, "Media
                                      Type Specifications and
                                      Registration Procedures", BCP 13,
                                      RFC 4288, December 2005.

   [RFC5226]                          Narten, T. and H. Alvestrand,
                                      "Guidelines for Writing an IANA
                                      Considerations Section in RFCs",
                                      BCP 26, RFC 5226, May 2008.

   [RFC5322]                          Resnick, P., "Internet Message
                                      Format", RFC 5322, October 2008.

   [RFC6151]                          Turner, S. and L. Chen, "Updated
                                      Security Considerations for the
                                      MD5 Message-Digest and the HMAC-
                                      MD5 Algorithms", RFC 6151,
                                      March 2011.

   [draft-ietf-httpbis-content-disp]  Reschke, J., "Use of the Content-
                                      Disposition Header Field in the
                                      Hypertext Transfer Protocol
                                      (work in progress), March 2011.

Appendix A.  Differences between HTTP and MIME

   HTTP/1.1 uses many of the constructs defined for Internet Mail
   ([RFC5322]) and the Multipurpose Internet Mail Extensions (MIME
   [RFC2045]) to allow a message-body to be transmitted in an open
   variety of representations and with extensible mechanisms.  However,
   RFC 2045 discusses mail, and HTTP has a few features that are
   different from those described in MIME.  These differences were
   carefully chosen to optimize performance over binary connections, to
   allow greater freedom in the use of new media types, to make date
   comparisons easier, and to acknowledge the practice of some early
   HTTP servers and clients.

   This appendix describes specific areas where HTTP differs from MIME.
   Proxies and gateways to strict MIME environments SHOULD be aware of
   these differences and provide the appropriate conversions where
   necessary.  Proxies and gateways from MIME environments to HTTP also
   need to be aware of the differences because some conversions might be

A.1.  MIME-Version

   HTTP is not a MIME-compliant protocol.  However, HTTP/1.1 messages
   MAY include a single MIME-Version header field to indicate what
   version of the MIME protocol was used to construct the message.  Use
   of the MIME-Version header field indicates that the message is in
   full compliance with the MIME protocol (as defined in [RFC2045]).
   Proxies/gateways are responsible for ensuring full compliance (where
   possible) when exporting HTTP messages to strict MIME environments.

     MIME-Version = "MIME-Version" ":" OWS MIME-Version-v
     MIME-Version-v = 1*DIGIT "." 1*DIGIT

   MIME version "1.0" is the default for use in HTTP/1.1.  However,
   HTTP/1.1 message parsing and semantics are defined by this document
   and not the MIME specification.

A.2.  Conversion to Canonical Form

   MIME requires that an Internet mail body-part be converted to
   canonical form prior to being transferred, as described in Section 4
   of [RFC2049].  Section 2.3.1 of this document describes the forms
   allowed for subtypes of the "text" media type when transmitted over
   HTTP.  [RFC2046] requires that content with a type of "text"
   represent line breaks as CRLF and forbids the use of CR or LF outside
   of line break sequences.  HTTP allows CRLF, bare CR, and bare LF to
   indicate a line break within text content when a message is
   transmitted over HTTP.

   Where it is possible, a proxy or gateway from HTTP to a strict MIME
   environment SHOULD translate all line breaks within the text media
   types described in Section 2.3.1 of this document to the RFC 2049
   canonical form of CRLF.  Note, however, that this might be
   complicated by the presence of a Content-Encoding and by the fact
   that HTTP allows the use of some character encodings which do not use
   octets 13 and 10 to represent CR and LF, respectively, as is the case
   for some multi-byte character encodings.

   Conversion will break any cryptographic checksums applied to the
   original content unless the original content is already in canonical
   form.  Therefore, the canonical form is recommended for any content
   that uses such checksums in HTTP.

A.3.  Conversion of Date Formats

   HTTP/1.1 uses a restricted set of date formats (Section 6.1 of
   [Part1]) to simplify the process of date comparison.  Proxies and
   gateways from other protocols SHOULD ensure that any Date header
   field present in a message conforms to one of the HTTP/1.1 formats
   and rewrite the date if necessary.

A.4.  Introduction of Content-Encoding

   MIME does not include any concept equivalent to HTTP/1.1's Content-
   Encoding header field.  Since this acts as a modifier on the media
   type, proxies and gateways from HTTP to MIME-compliant protocols MUST
   either change the value of the Content-Type header field or decode
   the representation before forwarding the message.  (Some experimental
   applications of Content-Type for Internet mail have used a media-type
   parameter of ";conversions=<content-coding>" to perform a function
   equivalent to Content-Encoding.  However, this parameter is not part
   of the MIME standards).

A.5.  No Content-Transfer-Encoding

   HTTP does not use the Content-Transfer-Encoding field of MIME.
   Proxies and gateways from MIME-compliant protocols to HTTP MUST
   remove any Content-Transfer-Encoding prior to delivering the response
   message to an HTTP client.

   Proxies and gateways from HTTP to MIME-compliant protocols are
   responsible for ensuring that the message is in the correct format
   and encoding for safe transport on that protocol, where "safe
   transport" is defined by the limitations of the protocol being used.
   Such a proxy or gateway SHOULD label the data with an appropriate
   Content-Transfer-Encoding if doing so will improve the likelihood of
   safe transport over the destination protocol.

A.6.  Introduction of Transfer-Encoding

   HTTP/1.1 introduces the Transfer-Encoding header field (Section 9.7
   of [Part1]).  Proxies/gateways MUST remove any transfer-coding prior
   to forwarding a message via a MIME-compliant protocol.

A.7.  MHTML and Line Length Limitations

   HTTP implementations which share code with MHTML [RFC2557]
   implementations need to be aware of MIME line length limitations.
   Since HTTP does not have this limitation, HTTP does not fold long
   lines.  MHTML messages being transported by HTTP follow all
   conventions of MHTML, including line length limitations and folding,
   canonicalization, etc., since HTTP transports all message-bodies as
   payload (see Section 2.3.2) and does not interpret the content or any
   MIME header lines that might be contained therein.

Appendix B.  Additional Features

   [RFC1945] and [RFC2068] document protocol elements used by some
   existing HTTP implementations, but not consistently and correctly
   across most HTTP/1.1 applications.  Implementors are advised to be
   aware of these features, but cannot rely upon their presence in, or
   interoperability with, other HTTP/1.1 applications.  Some of these
   describe proposed experimental features, and some describe features
   that experimental deployment found lacking that are now addressed in
   the base HTTP/1.1 specification.

   A number of other header fields, such as Content-Disposition and
   Title, from SMTP and MIME are MIME are also often implemented (see
   [draft-ietf-httpbis-content-disp] and [RFC2076]).

Appendix C.  Changes from RFC 2616

   Clarify contexts that charset is used in.  (Section 2.1)

   Remove the default character encoding for text media types; the
   default now is whatever the media type definition says.
   (Section 2.3.1)

   Change ABNF productions for header fields to only define the field
   value.  (Section 6)

   Remove definition of Content-MD5 header field because it was
   inconsistently implemented with respect to partial responses, and
   also often implemented because of known deficiencies in the hash algorithm itself (see [RFC2076]).

Appendix C.  Changes from RFC 2616

   Clarify contexts that charset is used in.
   [RFC6151] for details).  (Section 2.1) 6)

   Remove ISO-8859-1 special-casing in Accept-Charset.  (Section 6.2)

   Remove base URI setting semantics for Content-Location due to poor
   implementation support, which was caused by too many broken servers
   emitting bogus Content-Location header fields, and also the
   potentially undesirable effect of potentially breaking relative links
   in content-negotiated resources.  (Section 6.7)

   Remove discussion of Content-Disposition header field, it is now
   defined by [draft-ietf-httpbis-content-disp].  (Appendix B)

   Remove reference to non-existant identity transfer-coding value
   tokens.  (Appendix A.5)

Appendix D.  Collected ABNF

   Accept = "Accept:" [ ( "," / ( media-range [ accept-params ] ) ) *( OWS Accept-v
   Accept-Charset = "Accept-Charset:" "," [
    OWS Accept-Charset-v
   Accept-Charset-v media-range [ accept-params ] ] ) ]
   Accept-Charset = *( "," OWS ) ( charset / "*" ) [ OWS ";" OWS "q="
    qvalue ] *( OWS "," [ OWS ( charset / "*" ) [ OWS ";" OWS "q="
    qvalue ] ] )
   Accept-Encoding = "Accept-Encoding:" OWS Accept-Encoding-v
   Accept-Encoding-v = [ ( "," / ( codings [ OWS ";" OWS "q=" qvalue ] ) )
    *( OWS "," [ OWS codings [ OWS ";" OWS "q=" qvalue ] ] ) ]
   Accept-Language = "Accept-Language:" OWS Accept-Language-v
   Accept-Language-v = *( "," OWS ) language-range [ OWS ";" OWS "q="
    qvalue ] *( OWS "," [ OWS language-range [ OWS ";" OWS "q=" qvalue ]
    ] )
   Accept-v = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
    OWS media-range [ accept-params ] ] ) ]

   Content-Encoding = "Content-Encoding:" OWS Content-Encoding-v
   Content-Encoding-v = *( "," OWS ) content-coding *( OWS "," [ OWS
    content-coding ] )
   Content-Language = "Content-Language:" OWS Content-Language-v
   Content-Language-v = *( "," OWS ) language-tag *( OWS "," [ OWS
    language-tag ] )
   Content-Location = "Content-Location:" OWS Content-Location-v
   Content-Location-v = absolute-URI / partial-URI
   Content-MD5 = "Content-MD5:" OWS Content-MD5-v
   Content-MD5-v = <base64 of 128 bit MD5 digest as per [RFC1864]>
   Content-Type = "Content-Type:" OWS Content-Type-v
   Content-Type-v = media-type

   MIME-Version = "MIME-Version:" OWS MIME-Version-v
   MIME-Version-v = 1*DIGIT "." 1*DIGIT

   OWS = <OWS, defined in [Part1], Section 1.2.2>

   absolute-URI = <absolute-URI, defined in [Part1], Section 2.6>
   accept-ext = OWS ";" OWS token [ "=" word ]
   accept-params = OWS ";" OWS "q=" qvalue *accept-ext
   attribute = token

   charset = token
   codings = ( content-coding / "*" )
   content-coding = token

   language-range = <language-range, defined in [RFC4647], Section 2.1>
   language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

   media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
    ";" OWS parameter )
   media-type = type "/" subtype *( OWS ";" OWS parameter )

   parameter = attribute "=" value
   partial-URI = <partial-URI, defined in [Part1], Section 2.6>

   qvalue = <qvalue, defined in [Part1], Section 6.4>

   subtype = token

   token = <token, defined in [Part1], Section 1.2.2>
   type = token

   value = word

   word = <word, defined in [Part1], Section 1.2.2>

   ABNF diagnostics:

   ; Accept defined but not used
   ; Accept-Charset defined but not used
   ; Accept-Encoding defined but not used
   ; Accept-Language defined but not used
   ; Content-Encoding defined but not used
   ; Content-Language defined but not used
   ; Content-Location defined but not used
   ; Content-MD5 defined but not used
   ; Content-Type defined but not used
   ; MIME-Version defined but not used

Appendix E.  Change Log (to be removed by RFC Editor before publication)

E.1.  Since RFC 2616

   Extracted relevant partitions from [RFC2616].

E.2.  Since draft-ietf-httpbis-p3-payload-00

   Closed issues:

   o  <>: "Media Type
      Registrations" (<>)

   o  <>: "Clarification
      regarding quoting of charset values"

   o  <>: "Remove
      'identity' token references"

   o  <>: "Accept-
      Encoding BNF"

   o  <>: "Normative and
      Informative references"

   o  <>: "RFC1700

   o  <>: "Updating to

   o  <>: "Informative

   o  <>: "ISO-8859-1

   o  <>: "Encoding
      References Normative"

   o  <>: "Normative up-
      to-date references"

E.3.  Since draft-ietf-httpbis-p3-payload-01

   Ongoing work on ABNF conversion

   o  Add explicit references to BNF syntax and rules imported from
      other parts of the specification.

E.4.  Since draft-ietf-httpbis-p3-payload-02

   Closed issues:

   o  <>: "Quoting

   o  <>:
      "Classification for Allow header"

   o  <>: "missing
      default for qvalue in description of Accept-Encoding"

   Ongoing work on IANA Message Header Field Registration

   o  Reference RFC 3984, and update header field registrations for
      headers defined in this document.

E.5.  Since draft-ietf-httpbis-p3-payload-03

   Closed issues:

   o  <>: "Quoting

   o  <>: "language tag
      matching (Accept-Language) vs RFC4647"

   o  <>: "RFC 1806 has
      been replaced by RFC2183"

   Other changes:

   o  <>: "Encoding
      References Normative" -- rephrase the annotation and reference

E.6.  Since draft-ietf-httpbis-p3-payload-04

   Closed issues:

   o  <>: "RFC 2822 is
      updated by RFC 5322"

   Ongoing work on ABNF conversion

   o  Use "/" instead of "|" for alternatives.

   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").

   o  Rewrite ABNFs to spell out whitespace rules, factor out header
      field value format definitions.

E.7.  Since draft-ietf-httpbis-p3-payload-05

   Closed issues:

   o  <>: "Join
      "Differences Between HTTP Entities and RFC 2045 Entities"?"

   Final work on ABNF conversion

   o  Add appendix containing collected and expanded ABNF, reorganize
      ABNF introduction.

   Other changes:

   o  Move definition of quality values into Part 1.

E.8.  Since draft-ietf-httpbis-p3-payload-06

   Closed issues:

   o  <>: "Content-
      Location isn't special"

   o  <>: "Content

E.9.  Since draft-ietf-httpbis-p3-payload-07

   Closed issues:

   o  <>: "Updated
      reference for language tags"

   o  <>: "Clarify rules
      for determining what entities a response carries"

   o  <>: "Content-
      Location base-setting problems"

   o  <>: "Content

   o  <>: "pick IANA
      policy (RFC5226) for Transfer Coding / Content Coding"

   o  <>: "move
      definitions of gzip/deflate/compress to part 1"

   Partly resolved issues:

   o  <>: "update IANA
      requirements wrt Transfer-Coding values" (add the IANA
      Considerations subsection)

   o  <>: "update IANA
      requirements wrt Content-Coding values" (add the IANA
      Considerations subsection)

E.10.  Since draft-ietf-httpbis-p3-payload-08

   Closed issues:

   o  <>: "Content
      Negotiation for media types"

   o  <>: "Accept-
      Language: which RFC4647 filtering?"

E.11.  Since draft-ietf-httpbis-p3-payload-09

   Closed issues:

   o  <>: "MIME-Version
      not listed in P1, general header fields"

   o  <>: "IANA registry
      for content/transfer encodings"

   o  <>: "Content

   o  <>: "use of term
      "word" when talking about header structure"

   Partly resolved issues:

   o  <>: "Term for the
      requested resource's URI"

E.12.  Since draft-ietf-httpbis-p3-payload-10

   Closed issues:

   o  <>: "Clarify
      'Requested Variant'"

   o  <>: "Content-
      Location isn't special"

   o  <>: "Delimiting
      messages with multipart/byteranges"

   o  <>: "Clarify
      entity / representation / variant terminology"

   o  <>: "confusing
      req. language for Content-Location"

   o  <>: "Content-
      Location on 304 responses"

   o  <>: "'requested
      resource' in content-encoding definition"

   o  <>: "consider
      removing the 'changes from 2068' sections"

   Partly resolved issues:

   o  <>: "Content-MD5
      and partial responses"

E.13.  Since draft-ietf-httpbis-p3-payload-11

   Closed issues:

   o  <>: "Factor out

E.14.  Since draft-ietf-httpbis-p3-payload-12

   Closed issues:

   o  <>: "Header

   o  <>: "untangle
      ABNFs for header fields"

   o  <>: "potentially
      misleading MAY in media-type def"

E.15.  Since draft-ietf-httpbis-p3-payload-13

   Closed issues:

   o  <>: "Default
      charsets for text media types"

   o  <>: "Content-MD5
      and partial responses"

   o  <>: "untangle
      ABNFs for header fields"

   o  <>: "confusing
      undefined parameter in media range example"


      Accept header field  16
      Accept-Charset header field  19  18
      Accept-Encoding header field  19
      Accept-Language header field  21  20

      Coding Format
         compress  7
         deflate  8  7
         gzip  8  7
         identity  8  7
      compress (Coding Format)  7
      content negotiation  5
      Content-Encoding header field  22  21
      Content-Language header field  23  22
      Content-Location header field  24
      Content-MD5 header field  25  23
      Content-Type header field  26  24

      deflate (Coding Format)  8  7

         Accept  17  16
         Accept-Charset  19
         Accept-Charset-v  19  18
         Accept-Encoding  20
         Accept-Encoding-v  20  19
         accept-ext  17  16
         Accept-Language  21
         Accept-Language-v  21  20
         accept-params  17
         Accept-v  17  16
         attribute  9  8
         charset  6
         codings  20  19
         content-coding  7
         Content-Encoding  22
         Content-Encoding-v  22  21
         Content-Language  23
         Content-Language-v  23  22
         Content-Location  24
         Content-Location-v  24
         Content-MD5  25
         Content-MD5-v  25  23
         Content-Type  26
         Content-Type-v  26  24
         language-range  21  20
         language-tag  11  10
         media-range  17  16
         media-type  9  8
         MIME-Version  33
         MIME-Version-v  33  31
         parameter  9  8
         subtype  9  8
         type  9  8
         value  9  8
      gzip (Coding Format)  8  7

      Header Fields
         Accept  16
         Accept-Charset  19  18
         Accept-Encoding  19
         Accept-Language  21  20
         Content-Encoding  22  21
         Content-Language  23  22
         Content-Location  24
         Content-MD5  25  23
         Content-Type  26  24
         MIME-Version  32  31

      identity (Coding Format)  8  7

      MIME-Version header field  32  31

      payload  11  10

      representation  12  11

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110


   Jim Gettys
   Alcatel-Lucent Bell Labs
   21 Oak Knoll Road
   Carlisle, MA  01741


   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304

   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052


   Larry Masinter
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052


   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139

   Yves Lafon (editor)
   World Wide Web Consortium
   W3C / ERCIM
   2004, rte des Lucioles
   Sophia-Antipolis, AM  06902


   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155

   Phone: +49 251 2807760
   Fax:   +49 251 2807761