Network Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2068, 2616                                          J. Gettys
(if approved)                                       One Laptop per Child
Intended status: Standards Track                                J. Mogul
Expires: June 22, 2008                                                HP
                                                              H. Frystyk
                                                             L. Masinter
                                                           Adobe Systems
                                                                P. Leach
                                                          T. Berners-Lee
                                                       December 20, 2007

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

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   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)

   This version of the HTTP specification contains only minimal
   editorial changes from [RFC2616] (abstract, introductory paragraph,
   and authors' addresses).  All other changes are due to partitioning
   the original into seven mostly independent parts.  The intent is for
   readers of future drafts to able to use draft 00 as the basis for
   comparison when the WG makes later changes to the specification text.
   This draft will shortly be followed by draft 01 (containing the first
   round of changes that have already been agreed to on the mailing
   list).  There is no point in reviewing this draft other than to
   verify that the partitioning has been done correctly.  Roy T.
   Fielding, Yves Lafon, and Julian Reschke will be the editors after
   draft 00 is submitted.

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (  The current issues list is
   at <> and related
   documents (including fancy diffs) can be found at

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Protocol Parameters  . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Character Sets . . . . . . . . . . . . . . . . . . . . . .  5
       2.1.1.  Missing Charset  . . . . . . . . . . . . . . . . . . .  6
     2.2.  Content Codings  . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Media Types  . . . . . . . . . . . . . . . . . . . . . . .  7
       2.3.1.  Canonicalization and Text Defaults . . . . . . . . . .  8
       2.3.2.  Multipart Types  . . . . . . . . . . . . . . . . . . .  9
     2.4.  Quality Values . . . . . . . . . . . . . . . . . . . . . .  9
     2.5.  Language Tags  . . . . . . . . . . . . . . . . . . . . . . 10
   3.  Entity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Entity Header Fields . . . . . . . . . . . . . . . . . . . 10
     3.2.  Entity Body  . . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.1.  Type . . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.2.  Entity Length  . . . . . . . . . . . . . . . . . . . . 12
   4.  Content Negotiation  . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Server-driven Negotiation  . . . . . . . . . . . . . . . . 12
     4.2.  Agent-driven Negotiation . . . . . . . . . . . . . . . . . 14
     4.3.  Transparent Negotiation  . . . . . . . . . . . . . . . . . 14
   5.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 15
     5.1.  Accept . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.2.  Accept-Charset . . . . . . . . . . . . . . . . . . . . . . 17
     5.3.  Accept-Encoding  . . . . . . . . . . . . . . . . . . . . . 17
     5.4.  Accept-Language  . . . . . . . . . . . . . . . . . . . . . 19
     5.5.  Content-Encoding . . . . . . . . . . . . . . . . . . . . . 20
     5.6.  Content-Language . . . . . . . . . . . . . . . . . . . . . 21
     5.7.  Content-Location . . . . . . . . . . . . . . . . . . . . . 22
     5.8.  Content-MD5  . . . . . . . . . . . . . . . . . . . . . . . 22
     5.9.  Content-Type . . . . . . . . . . . . . . . . . . . . . . . 24
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
     7.1.  Privacy Issues Connected to Accept Headers . . . . . . . . 24
     7.2.  Content-Disposition Issues . . . . . . . . . . . . . . . . 25
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   Appendix A.  Differences Between HTTP Entities and RFC 2045
                Entities  . . . . . . . . . . . . . . . . . . . . . . 27
     A.1.  MIME-Version . . . . . . . . . . . . . . . . . . . . . . . 28
     A.2.  Conversion to Canonical Form . . . . . . . . . . . . . . . 28
     A.3.  Introduction of Content-Encoding . . . . . . . . . . . . . 29
     A.4.  No Content-Transfer-Encoding . . . . . . . . . . . . . . . 29
     A.5.  Introduction of Transfer-Encoding  . . . . . . . . . . . . 29
     A.6.  MHTML and Line Length Limitations  . . . . . . . . . . . . 29
   Appendix B.  Additional Features . . . . . . . . . . . . . . . . . 30
     B.1.  Content-Disposition  . . . . . . . . . . . . . . . . . . . 30
   Appendix C.  Changes from RFC 2068 . . . . . . . . . . . . . . . . 31

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   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
   Intellectual Property and Copyright Statements . . . . . . . . . . 36

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1.  Introduction

   This document will define aspects of HTTP related to the payload of
   messages (message content), including metadata and media types, along
   with HTTP content negotiation.  Right now it only includes the
   extracted relevant sections of RFC 2616 without edit.

2.  Protocol Parameters

2.1.  Character Sets

   HTTP uses the same definition of the term "character set" as that
   described for MIME:

   The term "character set" is used in this document to refer to a
   method used with one or more tables to convert a sequence of octets
   into a sequence of characters.  Note that unconditional conversion in
   the other direction is not required, in that not all characters may
   be available in a given character set and a character set may provide
   more than one sequence of octets to represent a particular character.
   This definition is intended to allow various kinds of character
   encoding, from simple single-table mappings such as US-ASCII to
   complex table switching methods such as those that use ISO-2022's
   techniques.  However, the definition associated with a MIME character
   set name MUST fully specify the mapping to be performed from octets
   to characters.  In particular, use of external profiling information
   to determine the exact mapping is not permitted.

      Note: This use of the term "character set" is more commonly
      referred to as a "character encoding."  However, since HTTP and
      MIME share the same registry, it is important that the terminology
      also be shared.

   HTTP character sets are identified by case-insensitive tokens.  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 [RFC1700] MUST represent the character set
   defined by that registry.  Applications SHOULD limit their use of
   character sets to those defined by the IANA registry.

   Implementors should be aware of IETF character set requirements
   [RFC2279] [RFC2277].

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2.1.1.  Missing Charset

   Some HTTP/1.0 software has interpreted a Content-Type header 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 and SHOULD do so when
   it is known that it will not confuse the recipient.

   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 an entity.  Content codings are primarily
   used to allow a document to be compressed or otherwise usefully
   transformed without losing the identity of its underlying media type
   and without loss of information.  Frequently, the entity 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 5.3) and
   Content-Encoding (Section 5.5) 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

   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   content-coding value tokens.  Initially, the registry contains the
   following tokens:


      An encoding format produced by the file compression program "gzip"
      (GNU zip) as described in RFC 1952 [RFC1952].  This format is a
      Lempel-Ziv coding (LZ77) with a 32 bit CRC.


      The encoding format produced by the common UNIX file compression
      program "compress".  This format is an adaptive Lempel-Ziv-Welch

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      coding (LZW).

      Use of program names for the identification of encoding formats is
      not desirable and is discouraged for future encodings.  Their use
      here is representative of historical practice, not good design.
      For compatibility with previous implementations of HTTP,
      applications SHOULD consider "x-gzip" and "x-compress" to be
      equivalent to "gzip" and "compress" respectively.


      The "zlib" format defined in RFC 1950 [RFC1950] in combination
      with the "deflate" compression mechanism described in RFC 1951


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

   New content-coding value tokens SHOULD be registered; to allow
   interoperability between clients and servers, specifications of the
   content coding algorithms needed to implement a new value SHOULD be
   publicly available and adequate for independent implementation, and
   conform to the purpose of content coding defined in this section.

2.3.  Media Types

   HTTP uses Internet Media Types [RFC1590] in the Content-Type
   (Section 5.9) and Accept (Section 5.1) header fields in order to
   provide open and extensible data typing and type negotiation.

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

   Parameters MAY follow the type/subtype in the form of attribute/value

       parameter               = attribute "=" value
       attribute               = token
       value                   = token | quoted-string

   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.  Linear white space

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   (LWS) MUST NOT be used between the type and subtype, nor between an
   attribute and its value.  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 registry.

   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 [RFC1700]).  The media type registration process is
   outlined in RFC 1590 [RFC1590].  Use of non-registered media types is

2.3.1.  Canonicalization and Text Defaults

   Internet media types are registered with a canonical form.  An
   entity-body transferred via HTTP messages MUST be represented 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 entity-body.  HTTP
   applications MUST accept CRLF, bare CR, and bare LF as being
   representative of a line break in text media received via HTTP.  In
   addition, if the text is represented in a character set that does not
   use octets 13 and 10 for CR and LF respectively, as is the case for
   some multi-byte character sets, HTTP allows the use of whatever octet
   sequences are defined by that character set to represent the
   equivalent of CR and LF for line breaks.  This flexibility regarding
   line breaks applies only to text media in the entity-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 an entity-body 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 set (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 sets other than "ISO-8859-1" or
   its subsets MUST be labeled with an appropriate charset value.  See
   Section 2.1.1 for compatibility problems.

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2.3.2.  Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more entities within a single message-body.  All multipart
   types share a common syntax, as defined in section 5.1.1 of RFC 2046
   [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.
   Unlike in RFC 2046, the epilogue of any multipart message MUST be
   empty; HTTP applications MUST NOT transmit the epilogue (even if the
   original multipart contains an epilogue).  These restrictions exist
   in order to preserve the self-delimiting nature of a multipart
   message-body, wherein the "end" of the message-body is indicated by
   the ending multipart boundary.

   In general, HTTP treats a multipart message-body no differently than
   any other media type: strictly as payload.  The one exception is the
   "multipart/byteranges" type (Appendix A of [Part5]) when it appears
   in a 206 (Partial Content) response.  In all other cases, 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.

   In general, an HTTP user agent SHOULD follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart type.
   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 RFC 1867 [RFC1867].

2.4.  Quality Values

   HTTP content negotiation (Section 4) uses short "floating point"
   numbers to indicate the relative importance ("weight") of various
   negotiable parameters.  A weight is normalized to a real number in
   the range 0 through 1, where 0 is the minimum and 1 the maximum
   value.  If a parameter has a quality value of 0, then content with
   this parameter is `not acceptable' for the client.  HTTP/1.1
   applications MUST NOT generate more than three digits after the
   decimal point.  User configuration of these values SHOULD also be
   limited in this fashion.

       qvalue         = ( "0" [ "." 0*3DIGIT ] )
                      | ( "1" [ "." 0*3("0") ] )

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   "Quality values" is a misnomer, since these values merely represent
   relative degradation in desired quality.

2.5.  Language Tags

   A language tag 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.

   The syntax and registry of HTTP language tags is the same as that
   defined by RFC 1766 [RFC1766].  In summary, a language tag is
   composed of 1 or more parts: A primary language tag and a possibly
   empty series of subtags:

        language-tag  = primary-tag *( "-" subtag )
        primary-tag   = 1*8ALPHA
        subtag        = 1*8ALPHA

   White space is not allowed within the tag and all tags are case-
   insensitive.  The name space of language tags is administered by the
   IANA.  Example tags include:

       en, en-US, en-cockney, i-cherokee, x-pig-latin

   where any two-letter primary-tag is an ISO-639 language abbreviation
   and any two-letter initial subtag is an ISO-3166 country code.  (The
   last three tags above are not registered tags; all but the last are
   examples of tags which could be registered in future.)

3.  Entity

   Request and Response messages MAY transfer an entity if not otherwise
   restricted by the request method or response status code.  An entity
   consists of entity-header fields and an entity-body, although some
   responses will only include the entity-headers.

   In this section, both sender and recipient refer to either the client
   or the server, depending on who sends and who receives the entity.

3.1.  Entity Header Fields

   Entity-header fields define metainformation about the entity-body or,
   if no body is present, about the resource identified by the request.
   Some of this metainformation is OPTIONAL; some might be REQUIRED by
   portions of this specification.

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       entity-header  = Allow                    ; [Part2], Section 10.1
                      | Content-Encoding         ; Section 5.5
                      | Content-Language         ; Section 5.6
                      | Content-Length           ; [Part1], Section 8.2
                      | Content-Location         ; Section 5.7
                      | Content-MD5              ; Section 5.8
                      | Content-Range            ; [Part5], Section 5.2
                      | Content-Type             ; Section 5.9
                      | Expires                  ; [Part6], Section 3.3
                      | Last-Modified            ; [Part4], Section 6.6
                      | extension-header

       extension-header = message-header

   The extension-header mechanism allows additional entity-header fields
   to be defined without changing the protocol, but these fields cannot
   be assumed to be recognizable by the recipient.  Unrecognized header
   fields SHOULD be ignored by the recipient and MUST be forwarded by
   transparent proxies.

3.2.  Entity Body

   The entity-body (if any) sent with an HTTP request or response is in
   a format and encoding defined by the entity-header fields.

       entity-body    = *OCTET

   An entity-body is only present in a message when a message-body is
   present, as described in Section 4.3 of [Part1].  The entity-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

3.2.1.  Type

   When an entity-body is included with a message, the data type of that
   body is determined via the header fields Content-Type and Content-
   Encoding.  These define a two-layer, ordered encoding model:

       entity-body := Content-Encoding( Content-Type( data ) )

   Content-Type specifies the media type of the underlying data.
   Content-Encoding may be used to indicate any additional content
   codings applied to the data, usually for the purpose of data
   compression, that are a property of the requested resource.  There is
   no default encoding.

   Any HTTP/1.1 message containing an entity-body SHOULD include a

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   Content-Type header field defining the media type of that body.  If
   and only if the media type is not given by a Content-Type field, the
   recipient MAY attempt to guess the media type via inspection of its
   content and/or the name extension(s) of the URI used to identify the
   resource.  If the media type remains unknown, the recipient SHOULD
   treat it as type "application/octet-stream".

3.2.2.  Entity Length

   The entity-length of a message is the length of the message-body
   before any transfer-codings have been applied.  Section 4.4 of
   [Part1] defines how the transfer-length of a message-body is

4.  Content Negotiation

   Most HTTP responses include an entity which contains information for
   interpretation by a human user.  Naturally, it is desirable to supply
   the user with the "best available" entity corresponding to the
   request.  Unfortunately for servers and caches, not all users have
   the same preferences for what is "best," and not all user agents are
   equally capable of rendering all entity types.  For that reason, HTTP
   has provisions for several mechanisms for "content negotiation" --
   the process of selecting the best representation for a given response
   when there are multiple representations available.

      Note: This is not called "format negotiation" because the
      alternate representations may be of the same media type, but use
      different capabilities of that type, be in different languages,

   Any response containing an entity-body MAY be subject to negotiation,
   including error responses.

   There are two kinds of content negotiation which are possible in
   HTTP: server-driven and agent-driven negotiation.  These two kinds of
   negotiation are orthogonal and thus may be used separately or in
   combination.  One method of combination, referred to as transparent
   negotiation, occurs when a cache uses the agent-driven negotiation
   information provided by the origin server in order to provide server-
   driven negotiation for subsequent requests.

4.1.  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

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

   HTTP/1.1 includes the following request-header fields for enabling
   server-driven negotiation through description of user agent
   capabilities and user preferences: Accept (Section 5.1), Accept-
   Charset (Section 5.2), Accept-Encoding (Section 5.3), Accept-Language
   (Section 5.4), and User-Agent (Section 10.9 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 information
   outside the request-header fields or within extension header fields
   not defined by this specification.

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

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4.2.  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 entity-body of the initial response, with each
   representation identified by its own URI.  Selection from among the
   representations may 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

   HTTP/1.1 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

4.3.  Transparent Negotiation

   Transparent negotiation is a combination of both server-driven and
   agent-driven negotiation.  When a cache is supplied with a form of
   the list of available representations of the response (as in agent-
   driven negotiation) and the dimensions of variance are completely
   understood by the cache, then the cache becomes capable of performing
   server-driven negotiation on behalf of the origin server for
   subsequent requests on that resource.

   Transparent negotiation has the advantage of distributing the
   negotiation work that would otherwise be required of the origin
   server and also removing the second request delay of agent-driven
   negotiation when the cache is able to correctly guess the right

   This specification does not define any mechanism for transparent

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   negotiation, though it also does not prevent any such mechanism from
   being developed as an extension that could be used within HTTP/1.1.

5.  Header Field Definitions

   This section defines the syntax and semantics of all standard
   HTTP/1.1 header fields.  For entity-header fields, both sender and
   recipient refer to either the client or the server, depending on who
   sends and who receives the entity.

5.1.  Accept

   The Accept request-header field can be used to specify certain media
   types which are acceptable for the response.  Accept headers 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

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

       media-range    = ( "*/*"
                        | ( type "/" "*" )
                        | ( type "/" subtype )
                        ) *( ";" parameter )
       accept-params  = ";" "q" "=" qvalue *( accept-extension )
       accept-extension = ";" token [ "=" ( token | quoted-string ) ]

   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 2.4).  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

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      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 entity, and if that does not exist, send the text/plain

   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, */*

   have the following precedence:

       1) text/html;level=1
       2) text/html
       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:

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       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.

5.2.  Accept-Charset

   The Accept-Charset request-header field can be used to indicate what
   character sets are acceptable for the response.  This field allows
   clients capable of understanding more comprehensive or special-
   purpose character sets to signal that capability to a server which is
   capable of representing documents in those character sets.

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

   Character set values 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

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

   The special value "*", if present in the Accept-Charset field,
   matches every character set (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 sets 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.

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

5.3.  Accept-Encoding

   The Accept-Encoding request-header field is similar to Accept, but
   restricts the content-codings (Section 2.2) that are acceptable in

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   the response.

       Accept-Encoding  = "Accept-Encoding" ":"
                          1#( codings [ ";" "q" "=" qvalue ] )
       codings          = ( content-coding | "*" )

   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 2.4, 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, 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.

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      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.

5.4.  Accept-Language

   The Accept-Language request-header field is similar to Accept, but
   restricts the set of natural languages that are preferred as a
   response to the request.  Language tags are defined in Section 2.5.

       Accept-Language = "Accept-Language" ":"
                         1#( language-range [ ";" "q" "=" qvalue ] )
       language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )

   Each language-range MAY 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."  A language-range matches a language-tag if
   it exactly equals the tag, or if it exactly equals a prefix of the
   tag such that the first tag character following the prefix is "-".
   The special range "*", if present in the Accept-Language field,
   matches every tag not matched by any other range present in the
   Accept-Language field.

      Note: This use of a prefix matching rule does not imply that
      language tags are assigned to languages in such a way that it is
      always true that if a user understands a language with a certain
      tag, then this user will also understand all languages with tags
      for which this tag is a prefix.  The prefix rule simply allows the
      use of prefix tags if this is the case.

   The language quality factor assigned to a language-tag by the Accept-
   Language field is the quality value of the longest language-range in
   the field that matches the language-tag.  If no language-range in the
   field matches the tag, the language quality factor assigned is 0.  If

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   no Accept-Language header is present in the request, the server
   SHOULD assume that all languages are equally acceptable.  If an
   Accept-Language header is present, then all languages which are
   assigned a quality factor greater than 0 are acceptable.

   It might be contrary to the privacy expectations of the user to send
   an Accept-Language header with the complete linguistic preferences of
   the user in every request.  For a discussion of this issue, see
   Section 7.1.

   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 should provide 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.

5.5.  Content-Encoding

   The Content-Encoding entity-header field is used as a modifier to the
   media-type.  When present, its value indicates what additional
   content codings have been applied to the entity-body, 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 document to be compressed without losing
   the identity of its underlying media type.

       Content-Encoding  = "Content-Encoding" ":" 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 entity identified by
   the Request-URI.  Typically, the entity-body is stored with this
   encoding and is only decoded before rendering or analogous usage.
   However, a non-transparent 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.

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   If the content-coding of an entity is not "identity", then the
   response MUST include a Content-Encoding entity-header (Section 5.5)
   that lists the non-identity content-coding(s) used.

   If the content-coding of an entity 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 an entity, 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 entity-header fields not defined by this specification.

5.6.  Content-Language

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

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

   Language tags are defined in Section 2.5.  The primary purpose of
   Content-Language is to allow a user to identify and differentiate
   entities 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 an entity
   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".

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

5.7.  Content-Location

   The Content-Location entity-header field MAY be used to supply the
   resource location for the entity enclosed in the message when that
   entity is accessible from a location separate from the requested
   resource's URI.  A server SHOULD provide a Content-Location for the
   variant corresponding to the response entity; especially in the case
   where a resource has multiple entities associated with it, and those
   entities actually have separate locations by which they might be
   individually accessed, the server SHOULD provide a Content-Location
   for the particular variant which is returned.

       Content-Location = "Content-Location" ":"
                         ( absoluteURI | relativeURI )

   The value of Content-Location also defines the base URI for the

   The Content-Location value is not a replacement for the original
   requested URI; it is only a statement of the location of the resource
   corresponding to this particular entity at the time of the request.
   Future requests MAY specify the Content-Location URI as the request-
   URI if the desire is to identify the source of that particular

   A cache cannot assume that an entity 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.  However, the Content-
   Location can be used to differentiate between multiple entities
   retrieved from a single requested resource, as described in [Part6].

   If the Content-Location is a relative URI, the relative URI is
   interpreted relative to the Request-URI.

   The meaning of the Content-Location header in PUT or POST requests is
   undefined; servers are free to ignore it in those cases.

5.8.  Content-MD5

   The Content-MD5 entity-header field, as defined in RFC 1864
   [RFC1864], is an MD5 digest of the entity-body for the purpose of
   providing an end-to-end message integrity check (MIC) of the entity-
   body.  (Note: a MIC is good for detecting accidental modification of
   the entity-body in transit, but is not proof against malicious

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        Content-MD5   = "Content-MD5" ":" md5-digest
        md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>

   The Content-MD5 header field MAY be generated by an origin server or
   client to function as an integrity check of the entity-body.  Only
   origin servers or clients MAY generate the Content-MD5 header field;
   proxies and gateways MUST NOT generate it, as this would defeat its
   value as an end-to-end integrity check.  Any recipient of the entity-
   body, including gateways and proxies, MAY check that the digest value
   in this header field matches that of the entity-body as received.

   The MD5 digest is computed based on the content of the entity-body,
   including any content-coding that has been applied, but not including
   any transfer-encoding applied to the message-body.  If the message is
   received with a transfer-encoding, that encoding MUST be removed
   prior to checking the Content-MD5 value against the received entity.

   This has the result that the digest is computed on the octets of the
   entity-body exactly as, and in the order that, they would be sent if
   no transfer-encoding were being applied.

   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 entity-body for
   composite types MAY contain many body-parts, each with its own MIME
   and HTTP headers (including Content-MD5, Content-Transfer-Encoding,
   and Content-Encoding headers).  If a body-part has a Content-
   Transfer-Encoding or Content-Encoding header, 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

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      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.

5.9.  Content-Type

   The Content-Type entity-header field indicates the media type of the
   entity-body sent to the recipient or, in the case of the HEAD method,
   the media type that would have been sent had the request been a GET.

       Content-Type   = "Content-Type" ":" 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 of methods for identifying the media type of an
   entity is provided in Section 3.2.1.

6.  IANA Considerations


7.  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.

7.1.  Privacy Issues Connected to Accept Headers

   Accept request-headers can reveal information about the user to all
   servers which are accessed.  The Accept-Language header 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 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 headers by default, and to ask

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   the user whether or not to start sending Accept-Language headers to a
   server if it detects, by looking for any Vary response-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 headers 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.

7.2.  Content-Disposition Issues

   RFC 1806 [RFC1806], from which the often implemented Content-
   Disposition (see Appendix B.1) header in HTTP is derived, has a
   number of very serious security considerations.  Content-Disposition
   is not part of the HTTP standard, but since it is widely implemented,
   we are documenting its use and risks for implementors.  See RFC 2183
   [RFC2183] (which updates RFC 1806) for details.

8.  Acknowledgments

   Based on an XML translation of RFC 2616 by Julian Reschke.

9.  References

   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 1: URIs, Connections, and Message Parsing",
              draft-ietf-httpbis-p1-messaging-00 (work in progress),
              December 2007.

   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 2: Message Semantics",
              draft-ietf-httpbis-p2-semantics-00 (work in progress),

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              December 2007.

   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 4: Conditional Requests",
              draft-ietf-httpbis-p4-conditional-00 (work in progress),
              December 2007.

   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 5: Range Requests and Partial Responses",
              draft-ietf-httpbis-p5-range-00 (work in progress),
              December 2007.

   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 6: Caching", draft-ietf-httpbis-p6-cache-00 (work in
              progress), December 2007.

   [RFC1590]  Postel, J., "Media Type Registration Procedure", RFC 1590,
              November 1996.

   [RFC1700]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
              RFC 1700, October 1994.

   [RFC1766]  Alvestrand, H., "Tags for the Identification of
              Languages", RFC 1766, March 1995.

   [RFC1806]  Troost, R. and S. Dorner, "Communicating Presentation
              Information in Internet Messages: The Content-Disposition
              Header", RFC 1806, June 1995.

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

   [RFC1867]  Masinter, L. and E. Nebel, "Form-based File Upload in
              HTML", RFC 1867, November 1995.

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

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

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

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   [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.

   [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, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, January 1997.

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

   [RFC2110]  Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
              Aggregate Documents, such as HTML (MHTML)", RFC 2110,
              March 1997.

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.

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

   [RFC2279]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 2279, January 1998.

   [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.

   [RFC822]   Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

Appendix A.  Differences Between HTTP Entities and RFC 2045 Entities

   HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
   822 [RFC822]) and the Multipurpose Internet Mail Extensions (MIME
   [RFC2045]) to allow entities 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

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   those described in RFC 2045.  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 RFC
   2045.  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 required.

A.1.  MIME-Version

   HTTP is not a MIME-compliant protocol.  However, HTTP/1.1 messages
   MAY include a single MIME-Version general-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 RFC
   2045[RFC2045]).  Proxies/gateways are responsible for ensuring full
   compliance (where possible) when exporting HTTP messages to strict
   MIME environments.

       MIME-Version   = "MIME-Version" ":" 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

   RFC 2045 [RFC2045] requires that an Internet mail entity be converted
   to canonical form prior to being transferred, as described in section
   4 of RFC 2049 [RFC2049].  Section 2.3.1 of this document describes
   the forms allowed for subtypes of the "text" media type when
   transmitted over HTTP.  RFC 2046 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 sets which do not use

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   octets 13 and 10 to represent CR and LF, as is the case for some
   multi-byte character sets.

   Implementors should note that 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.  Introduction of Content-Encoding

   RFC 2045 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 entity-body 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 RFC 2045).

A.4.  No Content-Transfer-Encoding

   HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
   2045.  Proxies and gateways from MIME-compliant protocols to HTTP
   MUST remove any non-identity CTE ("quoted-printable" or "base64")
   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.5.  Introduction of Transfer-Encoding

   HTTP/1.1 introduces the Transfer-Encoding header field (Section 8.7
   of [Part1]).  Proxies/gateways MUST remove any transfer-coding prior
   to forwarding a message via a MIME-compliant protocol.

A.6.  MHTML and Line Length Limitations

   HTTP implementations which share code with MHTML [RFC2110]
   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,

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   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

   RFC 1945 and RFC 2068 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 headers, such as Content-Disposition and Title,
   from SMTP and MIME are also often implemented (see RFC 2076

B.1.  Content-Disposition

   The Content-Disposition response-header field has been proposed as a
   means for the origin server to suggest a default filename if the user
   requests that the content is saved to a file.  This usage is derived
   from the definition of Content-Disposition in RFC 1806 [RFC1806].

        content-disposition = "Content-Disposition" ":"
                              disposition-type *( ";" disposition-parm )
        disposition-type = "attachment" | disp-extension-token
        disposition-parm = filename-parm | disp-extension-parm
        filename-parm = "filename" "=" quoted-string
        disp-extension-token = token
        disp-extension-parm = token "=" ( token | quoted-string )

   An example is

        Content-Disposition: attachment; filename="fname.ext"

   The receiving user agent SHOULD NOT respect any directory path
   information present in the filename-parm parameter, which is the only
   parameter believed to apply to HTTP implementations at this time.
   The filename SHOULD be treated as a terminal component only.

   If this header is used in a response with the application/
   octet-stream content-type, the implied suggestion is that the user
   agent should not display the response, but directly enter a `save
   response as...' dialog.

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   See Section 7.2 for Content-Disposition security issues.

Appendix C.  Changes from RFC 2068

   Charset wildcarding is introduced to avoid explosion of character set
   names in accept headers.  (Section 5.2)

   Content-Base was deleted from the specification: it was not
   implemented widely, and there is no simple, safe way to introduce it
   without a robust extension mechanism.  In addition, it is used in a
   similar, but not identical fashion in MHTML [RFC2110].

   A content-coding of "identity" was introduced, to solve problems
   discovered in caching.  (Section 2.2)

   Quality Values of zero should indicate that "I don't want something"
   to allow clients to refuse a representation.  (Section 2.4)

   The Alternates, Content-Version, Derived-From, Link, URI, Public and
   Content-Base header fields were defined in previous versions of this
   specification, but not commonly implemented.  See RFC 2068 [RFC2068].


      Accept header  15
      Accept-Charset header  17
      Accept-Encoding header  17
      Accept-Language header  19
      Alternates header  31

      compress  6
      Content-Base header  31
      Content-Disposition header  30
      Content-Encoding header  20
      Content-Language header  21
      Content-Location header  22
      Content-MD5 header  22
      Content-Type header  24
      Content-Version header  31

      deflate  7
      Derived-From header  31

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         Accept  15
         Accept-Charset  17
         Accept-Encoding  18
         accept-extension  15
         Accept-Language  19
         accept-params  15
         attribute  7
         charset  5
         codings  18
         content-coding  6
         content-disposition  30
         Content-Encoding  20
         Content-Language  21
         Content-Location  22
         Content-MD5  23
         Content-Type  24
         disp-extension-parm  30
         disp-extension-token  30
         disposition-parm  30
         disposition-type  30
         entity-body  11
         entity-header  11
         extension-header  11
         filename-parm  30
         language-range  19
         language-tag  10
         md5-digest  23
         media-range  15
         media-type  7
         MIME-Version  28
         parameter  7
         primary-tag  10
         qvalue  9
         subtag  10
         subtype  7
         type  7
         value  7
      gzip  6

         Accept  15
         Accept-Charset  17
         Accept-Encoding  17
         Accept-Language  19
         Alternate  31

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         Content-Base  31
         Content-Disposition  30
         Content-Encoding  20
         Content-Language  21
         Content-Location  22
         Content-MD5  22
         Content-Type  24
         Content-Version  31
         Derived-From  31
         Link  31
         Public  31
         URI  31

      identity  7

      Link header  31

      Public header  31

      URI header  31

Authors' Addresses

   Roy T. Fielding (editor)
   Day Software
   23 Corporate Plaza DR, Suite 280
   Newport Beach, CA  92660

   Phone: +1-949-706-5300
   Fax:   +1-949-706-5305

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   Jim Gettys
   One Laptop per Child
   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


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   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


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