source: draft-ietf-httpbis/latest/p3-payload.xml @ 1454

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  <!ENTITY ID-MONTH "October">
  <!ENTITY ID-YEAR "2011">
  <!ENTITY mdash "&#8212;">
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  <!ENTITY caching-neg-resp         "<xref target='Part6' x:rel='#caching.negotiated.responses' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
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<rfc obsoletes="2616" category="std" x:maturity-level="draft"
     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p3-payload-&ID-VERSION;"
     xmlns:x='http://purl.org/net/xml2rfc/ext'>
<front>

  <title abbrev="HTTP/1.1, Part 3">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>

  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
    <address>
      <postal>
        <street>345 Park Ave</street>
        <city>San Jose</city>
        <region>CA</region>
        <code>95110</code>
        <country>USA</country>
      </postal>
      <email>fielding@gbiv.com</email>
      <uri>http://roy.gbiv.com/</uri>
    </address>
  </author>

  <author initials="J." surname="Gettys" fullname="Jim Gettys">
    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
    <address>
      <postal>
        <street>21 Oak Knoll Road</street>
        <city>Carlisle</city>
        <region>MA</region>
        <code>01741</code>
        <country>USA</country>
      </postal>
      <email>jg@freedesktop.org</email>
      <uri>http://gettys.wordpress.com/</uri>
    </address>
  </author>
  
  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    <organization abbrev="HP">Hewlett-Packard Company</organization>
    <address>
      <postal>
        <street>HP Labs, Large Scale Systems Group</street>
        <street>1501 Page Mill Road, MS 1177</street>
        <city>Palo Alto</city>
        <region>CA</region>
        <code>94304</code>
        <country>USA</country>
      </postal>
      <email>JeffMogul@acm.org</email>
    </address>
  </author>

  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
        <country>USA</country>
      </postal>
      <email>henrikn@microsoft.com</email>
    </address>
  </author>

  <author initials="L." surname="Masinter" fullname="Larry Masinter">
    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
    <address>
      <postal>
        <street>345 Park Ave</street>
        <city>San Jose</city>
        <region>CA</region>
        <code>95110</code>
        <country>USA</country>
      </postal>
      <email>LMM@acm.org</email>
      <uri>http://larry.masinter.net/</uri>
    </address>
  </author>
  
  <author initials="P." surname="Leach" fullname="Paul J. Leach">
    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
      </postal>
      <email>paulle@microsoft.com</email>
    </address>
  </author>
   
  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    <address>
      <postal>
        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
        <street>The Stata Center, Building 32</street>
        <street>32 Vassar Street</street>
        <city>Cambridge</city>
        <region>MA</region>
        <code>02139</code>
        <country>USA</country>
      </postal>
      <email>timbl@w3.org</email>
      <uri>http://www.w3.org/People/Berners-Lee/</uri>
    </address>
  </author>

  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    <organization abbrev="W3C">World Wide Web Consortium</organization>
    <address>
      <postal>
        <street>W3C / ERCIM</street>
        <street>2004, rte des Lucioles</street>
        <city>Sophia-Antipolis</city>
        <region>AM</region>
        <code>06902</code>
        <country>France</country>
      </postal>
      <email>ylafon@w3.org</email>
      <uri>http://www.raubacapeu.net/people/yves/</uri>
    </address>
  </author>

  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    <organization abbrev="greenbytes">greenbytes GmbH</organization>
    <address>
      <postal>
        <street>Hafenweg 16</street>
        <city>Muenster</city><region>NW</region><code>48155</code>
        <country>Germany</country>
      </postal>
      <phone>+49 251 2807760</phone>
      <facsimile>+49 251 2807761</facsimile>
      <email>julian.reschke@greenbytes.de</email>
      <uri>http://greenbytes.de/tech/webdav/</uri>
    </address>
  </author>

  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  <workgroup>HTTPbis Working Group</workgroup>

<abstract>
<t>
   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
   distributed, collaborative, hypertext 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.
</t>
<t>
   Part 3 defines HTTP message content, metadata, and content negotiation.
</t>
</abstract>

<note title="Editorial Note (To be removed by RFC Editor)">
  <t>
    Discussion of this draft should take place on the HTTPBIS working group
    mailing list (ietf-http-wg@w3.org), which is archived at
    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
  </t>
  <t>
    The current issues list is at
    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
    documents (including fancy diffs) can be found at
    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
  </t>
  <t>
    The changes in this draft are summarized in <xref target="changes.since.16"/>.
  </t>
</note>
</front>
<middle>
<section title="Introduction" anchor="introduction">
<t>
   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 negotiation.
</t>
<t>
   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 <xref target="RFC2616"/>.
</t>

<section title="Terminology" anchor="terminology">
<t>
   This specification uses a number of terms to refer to the roles
   played by participants in, and objects of, the HTTP communication.
</t>
<t>
  <iref item="content negotiation"/>
  <x:dfn>content negotiation</x:dfn>
  <list>
    <t>
      The mechanism for selecting the appropriate representation when
      servicing a request. The representation in any response
      can be negotiated (including error responses).
    </t>
  </list>
</t>
</section>

<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
<t>
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
   This document defines conformance criteria for several roles in HTTP
   communication, including Senders, Recipients, Clients, Servers, User-Agents,
   Origin Servers, Intermediaries, Proxies and Gateways. See &architecture;
   for definitions of these terms.
</t>
<t>
   An implementation is considered conformant if it complies with all of the
   requirements associated with its role(s). Note that SHOULD-level requirements
   are relevant here, unless one of the documented exceptions is applicable.
</t>
<t>
   This document also uses ABNF to define valid protocol elements
   (<xref target="notation"/>). In addition to the prose requirements placed
   upon them, Senders &MUST-NOT; generate protocol elements that are invalid.
</t>
<t>
   Unless noted otherwise, Recipients &MAY; take steps to recover a usable
   protocol element from an invalid construct. However, HTTP does not define
   specific error handling mechanisms, except in cases where it has direct
   impact on security. This is because different uses of the protocol require
   different error handling strategies; for example, a Web browser may wish to
   transparently recover from a response where the Location header field
   doesn't parse according to the ABNF, whereby in a systems control protocol
   using HTTP, this type of error recovery could lead to dangerous consequences.
</t>
</section>

<section title="Syntax Notation" anchor="notation">
  <x:anchor-alias value="ALPHA"/>
  <x:anchor-alias value="CR"/>
  <x:anchor-alias value="DIGIT"/>
  <x:anchor-alias value="LF"/>
  <x:anchor-alias value="OCTET"/>
  <x:anchor-alias value="VCHAR"/>
<t>
  This specification uses the ABNF syntax defined in &notation; (which
  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
  <xref target="collected.abnf"/> shows the collected ABNF, with the list
  rule expanded.
</t>
<t>
  The following core rules are included by
  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="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), and
  VCHAR (any visible US-ASCII character).
</t>

<section title="Core Rules" anchor="core.rules">
  <x:anchor-alias value="token"/>
  <x:anchor-alias value="word"/>
  <x:anchor-alias value="OWS"/>
<t>
  The core rules below are defined in <xref target="Part1"/>:
</t>
<figure><artwork type="abnf2616">
  <x:ref>OWS</x:ref>            = &lt;OWS, defined in &basic-rules;&gt;
  <x:ref>token</x:ref>          = &lt;token, defined in &field-rules;&gt;
  <x:ref>word</x:ref>           = &lt;word, defined in &field-rules;&gt;
</artwork></figure>
</section>

<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
  <x:anchor-alias value="absolute-URI"/>
  <x:anchor-alias value="partial-URI"/>
  <x:anchor-alias value="qvalue"/>
<t>
  The ABNF rules below are defined in other parts: 
</t>
<figure><!--Part1--><artwork type="abnf2616">
  <x:ref>absolute-URI</x:ref>   = &lt;absolute-URI, defined in &uri;&gt;
  <x:ref>partial-URI</x:ref>    = &lt;partial-URI, defined in &uri;&gt;
  <x:ref>qvalue</x:ref>         = &lt;qvalue, defined in &qvalue;&gt;
</artwork></figure>
</section>

</section>

</section>

<section title="Protocol Parameters" anchor="protocol.parameters">

<section title="Character Encodings (charset)" anchor="character.sets">
<t>
   HTTP uses charset names to indicate the character encoding of a
   textual representation.
</t>
<t anchor="rule.charset">
  <x:anchor-alias value="charset"/>
   A character encoding is identified by a case-insensitive token. The
   complete set of tokens is defined by the IANA Character Set registry
   (<eref target="http://www.iana.org/assignments/character-sets"/>).
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="charset"/>
  <x:ref>charset</x:ref> = <x:ref>token</x:ref>
</artwork></figure>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   Implementors need to be aware of IETF character set requirements <xref target="RFC3629"/>
   <xref target="RFC2277"/>.
</t>
</section>

<section title="Content Codings" anchor="content.codings">
  <x:anchor-alias value="content-coding"/>
<t>
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-coding"/>
  <x:ref>content-coding</x:ref>   = <x:ref>token</x:ref>
</artwork></figure>
<t>
   All content-coding values are case-insensitive. HTTP/1.1 uses
   content-coding values in the Accept-Encoding (<xref target="header.accept-encoding"/>) and
   Content-Encoding (<xref target="header.content-encoding"/>) 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
   encoding.
</t>
<t>
   compress<iref item="compress (Coding Format)"/><iref item="Coding Format" subitem="compress"/>
  <list>
    <t>
      See &compress-coding;.
    </t>
  </list>
</t>
<t>
   deflate<iref item="deflate (Coding Format)"/><iref item="Coding Format" subitem="deflate"/>
  <list>
    <t>
      See &deflate-coding;.
    </t>
  </list>
</t>
<t>
   gzip<iref item="gzip (Coding Format)"/><iref item="Coding Format" subitem="gzip"/>
  <list>
    <t>
      See &gzip-coding;.
    </t>
  </list>
</t>

<section title="Content Coding Registry" anchor="content.coding.registry">
<t>
   The HTTP Content Coding Registry defines the name space for the content
   coding names.
</t>
<t>
   Registrations &MUST; include the following fields:
   <list style="symbols">
     <t>Name</t>
     <t>Description</t>
     <t>Pointer to specification text</t>
   </list>
</t>
<t>
   Names of content codings &MUST-NOT; overlap with names of transfer codings
   (&transfer-codings;), unless the encoding transformation is identical (as it
   is the case for the compression codings defined in
   &compression-codings;).
</t>
<t>
   Values to be added to this name space require a specification
   (see "Specification Required" in
   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
   conform to the purpose of content coding defined in this section.
</t>
<t>
   The registry itself is maintained at
   <eref target="http://www.iana.org/assignments/http-parameters"/>.
</t>
</section>

</section>

<section title="Media Types" anchor="media.types">
  <x:anchor-alias value="media-type"/>
  <x:anchor-alias value="type"/>
  <x:anchor-alias value="subtype"/>
<t>
   HTTP uses Internet Media Types <xref target="RFC2046"/> in the Content-Type (<xref target="header.content-type"/>)
   and Accept (<xref target="header.accept"/>) header fields in order to provide
   open and extensible data typing and type negotiation.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="media-type"/><iref primary="true" item="Grammar" subitem="type"/><iref primary="true" item="Grammar" subitem="subtype"/>
  <x:ref>media-type</x:ref> = <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
  <x:ref>type</x:ref>       = <x:ref>token</x:ref>
  <x:ref>subtype</x:ref>    = <x:ref>token</x:ref>
</artwork></figure>
<t anchor="rule.parameter">
  <x:anchor-alias value="attribute"/>
  <x:anchor-alias value="parameter"/>
  <x:anchor-alias value="value"/>
   The type/subtype &MAY; be followed by parameters in the form of
   attribute/value pairs.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="parameter"/><iref primary="true" item="Grammar" subitem="attribute"/><iref primary="true" item="Grammar" subitem="value"/>
  <x:ref>parameter</x:ref>      = <x:ref>attribute</x:ref> "=" <x:ref>value</x:ref>
  <x:ref>attribute</x:ref>      = <x:ref>token</x:ref>
  <x:ref>value</x:ref>          = <x:ref>word</x:ref>
</artwork></figure>
<t>
   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 registry.
</t>
<t>
   A parameter value that matches the <x:ref>token</x:ref> production can be
   transmitted as either a token or within a quoted-string. The quoted and
   unquoted values are equivalent. 
</t>
<t>
   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.
</t>
<t>
   Media-type values are registered with the Internet Assigned Number
   Authority (IANA). The media type registration process is
   outlined in <xref target="RFC4288"/>. Use of non-registered media types is
   discouraged.
</t>

<section title="Canonicalization and Text Defaults" anchor="canonicalization.and.text.defaults">
<t>
   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.
</t>
<t>
   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).
</t>
<t>
   If a representation is encoded with a content-coding, the underlying
   data &MUST; be in a form defined above prior to being encoded.
</t>
</section>

<section title="Multipart Types" anchor="multipart.types">
<t>
   MIME provides for a number of "multipart" types &mdash; encapsulations of
   one or more representations within a single message-body. All multipart
   types share a common syntax, as defined in <xref target="RFC2046" x:sec="5.1.1" x:fmt="of"/>,
   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.
</t>
<t>
   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.
   <!-- jre: re-insert removed text pointing to caching? -->
   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.
</t>
<t>
   If an application receives an unrecognized multipart subtype, the
   application &MUST; treat it as being equivalent to "multipart/mixed".
</t>
<x:note>
  <t>
    <x:h>Note:</x:h> The "multipart/form-data" type has been specifically defined
    for carrying form data suitable for processing via the POST
    request method, as described in <xref target="RFC2388"/>.
  </t>
</x:note>
</section>
</section>

<section title="Language Tags" anchor="language.tags">
  <x:anchor-alias value="language-tag"/>
<t>
   A language tag, as defined in <xref target="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.
</t>
<t>
   In summary, a language tag is composed of one or more parts: A primary
   language subtag followed by a possibly empty series of subtags:
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="language-tag"/>
  <x:ref>language-tag</x:ref> = &lt;Language-Tag, defined in <xref target="RFC5646" x:sec="2.1"/>&gt;
</artwork></figure>
<t>
   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
   <eref target="http://www.iana.org/assignments/language-subtag-registry"/>).
</t>
<figure>
  <preamble>Example tags include:</preamble>
<artwork type="example">
  en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
</artwork>
</figure>
<t>
   See <xref target="RFC5646"/> for further information. 
</t>
</section>
</section>

<section title="Payload" anchor="payload">
<t>
   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.
</t>
<iref item="payload"/>
<t>   
   A "<x:dfn>payload</x:dfn>" 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).
</t>
<section title="Payload Header Fields" anchor="payload.header.fields">
  <x:anchor-alias value="payload-header"/>
<t>
   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 HTTP/1.1:
</t>
<texttable align="left">
  <ttcol>Header Field Name</ttcol>
  <ttcol>Defined in...</ttcol>

  <c>Content-Length</c> <c>&header-content-length;</c>
  <c>Content-Range</c> <c>&header-content-range;</c>
</texttable>
</section>

<section title="Payload Body" anchor="payload.body">
  <x:anchor-alias value="payload-body"/>
<t>
   A payload body is only present in a message when a message-body is
   present, as described in &message-body;. 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 message.
</t>
</section>
</section>

<section title="Representation" anchor="representation">
<iref item="representation"/>
<t>
   A "<x:dfn>representation</x:dfn>" 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).
</t>
<t>
   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.
</t>

<section title="Representation Header Fields" anchor="representation.header.fields">
  <x:anchor-alias value="representation-header"/>
<t>
   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.
</t>
<t>
   The following header fields are defined as representation metadata:
</t>
<texttable align="left">
  <ttcol>Header Field Name</ttcol>
  <ttcol>Defined in...</ttcol>

  <c>Content-Encoding</c> <c><xref target="header.content-encoding"/></c>
  <c>Content-Language</c> <c><xref target="header.content-language"/></c>
  <c>Content-Location</c> <c><xref target="header.content-location"/></c>
  <c>Content-Type</c> <c><xref target="header.content-type"/></c>
  <c>Expires</c> <c>&header-expires;</c>
  <c>Last-Modified</c> <c>&header-last-modified;</c>
</texttable>
</section>

<section title="Representation Data" anchor="representation.data">
  <x:anchor-alias value="representation-data"/>
<t>
   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.
</t>
<t>
   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:
</t>
<figure><artwork type="example">
  representation-data := Content-Encoding( Content-Type( bits ) )
</artwork></figure>
<t>
   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" (<xref target="RFC2046" x:fmt="," x:sec="4.5.1"/>)
   or examine the content to determine its type.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
</section>
</section>

<section title="Content Negotiation" anchor="content.negotiation">
<t>
   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.
</t>
<t>
   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" &mdash; a process of
   allowing selection of a representation of a given resource,
   when more than one is available.
</t>
<t>
   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" (<xref target="RFC2295"/>)
   has also been proposed.
</t>
<t>
   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 appropriate.
</t>
<t>
   Note that in all cases, the supplier of representations has the
   responsibility for determining which representations might be
   considered to be the "same information".
</t>

<section title="Server-driven Negotiation" anchor="server-driven.negotiation">
<t>
   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).
</t>
<t>
   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.
</t>
<t>
   Server-driven negotiation has disadvantages:
  <list style="numbers">
    <t>
         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?).
    </t>
    <t>
         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.
    </t>
    <t>
         It complicates the implementation of an origin server and the
         algorithms for generating responses to a request.
    </t>
    <t>
         It might limit a public cache's ability to use the same response
         for multiple user's requests.
    </t>
  </list>
</t>
<t>
   Server-driven negotiation allows the user agent to specify its preferences,
   but it cannot expect responses to always honour them. For example, the origin
   server might not implement server-driven negotiation, or it might decide that
   sending a response that doesn't conform to them is better than sending a 406
   (Not Acceptable) response.
</t>
<t>
   Many of the mechanisms for expressing preferences use quality values to
   declare relative preference. See &qvalue; for more information. 
</t>
<t>
   HTTP/1.1 includes the following header fields for enabling
   server-driven negotiation through description of user agent
   capabilities and user preferences: Accept (<xref target="header.accept"/>), Accept-Charset
   (<xref target="header.accept-charset"/>), Accept-Encoding (<xref target="header.accept-encoding"/>), Accept-Language
   (<xref target="header.accept-language"/>), and User-Agent (&header-user-agent;).
   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.
</t>
<x:note>
  <t>
    <x:h>Note:</x:h> In practice, User-Agent based negotiation is fragile,
    because new clients might not be recognized. 
  </t>
</x:note>
<t>
   The Vary header field (&header-vary;) can be used to express the parameters the
   server uses to select a representation that is subject to server-driven
   negotiation.
</t>
</section>

<section title="Agent-driven Negotiation" anchor="agent-driven.negotiation">
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
</section>
</section>

<section title="Header Field Definitions" anchor="header.field.definitions">
<t>
   This section defines the syntax and semantics of HTTP/1.1 header fields
   related to the payload of messages.
</t>

<section title="Accept" anchor="header.accept">
  <iref primary="true" item="Accept header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="Accept" x:for-anchor=""/>
  <x:anchor-alias value="Accept"/>
  <x:anchor-alias value="accept-ext"/>
  <x:anchor-alias value="accept-params"/>
  <x:anchor-alias value="media-range"/>
<t>
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept"/><iref primary="true" item="Grammar" subitem="media-range"/><iref primary="true" item="Grammar" subitem="accept-params"/><iref primary="true" item="Grammar" subitem="accept-ext"/>
  <x:ref>Accept</x:ref> = #( <x:ref>media-range</x:ref> [ <x:ref>accept-params</x:ref> ] )
  
  <x:ref>media-range</x:ref>    = ( "*/*"
                   / ( <x:ref>type</x:ref> "/" "*" )
                   / ( <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> )
                   ) *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
  <x:ref>accept-params</x:ref>  = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> *( <x:ref>accept-ext</x:ref> )
  <x:ref>accept-ext</x:ref>     = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>token</x:ref> [ "=" <x:ref>word</x:ref> ]
</artwork></figure>
<t>
   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.
</t>
<t>
   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 (&qvalue;). The
   default value is q=1.
</t>
<x:note>
  <t>
    <x:h>Note:</x:h> 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".
  </t>
</x:note>
<t>
   The example
</t>
<figure><artwork type="example">
  Accept: audio/*; q=0.2, audio/basic
</artwork></figure>
<t>
   &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".
</t>
<t>
   A request without any Accept header field implies that the user agent
   will accept any media type in response.
   If an Accept header field is present in a request and none of the
   available representations for the response have a media type that is
   listed as acceptable, the origin server &MAY; either
   honor the Accept header field by sending a 406 (Not Acceptable) response
   or disregard the Accept header field by treating the response as if
   it is not subject to content negotiation.
</t>
<t>
   A more elaborate example is
</t>
<figure><artwork type="example">
  Accept: text/plain; q=0.5, text/html,
          text/x-dvi; q=0.8, text/x-c
</artwork></figure>
<t>
   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".
</t>
<t>
   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,
</t>
<figure><artwork type="example">
  Accept: text/*, text/plain, text/plain;format=flowed, */*
</artwork></figure>
<t>
   have the following precedence:
   <list style="numbers">
    <t>text/plain;format=flowed</t>
    <t>text/plain</t>
    <t>text/*</t>
    <t>*/*</t>
   </list>
</t>
<t>
   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,
</t>
<figure><artwork type="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
</artwork></figure>
<t>
   would cause the following values to be associated:
</t>
<texttable align="left">
  <ttcol>Media Type</ttcol><ttcol>Quality Value</ttcol>
  <c>text/html;level=1</c>    <c>1</c>
  <c>text/html</c>            <c>0.7</c>
  <c>text/plain</c>           <c>0.3</c>
  <c>image/jpeg</c>           <c>0.5</c>
  <c>text/html;level=2</c>    <c>0.4</c>
  <c>text/html;level=3</c>    <c>0.7</c>
</texttable>
<t>
      <x:h>Note:</x:h> 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.
</t>
</section>

<section title="Accept-Charset" anchor="header.accept-charset">
  <iref primary="true" item="Accept-Charset header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="Accept-Charset" x:for-anchor=""/>
  <x:anchor-alias value="Accept-Charset"/>
<t>
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Charset"/>
  <x:ref>Accept-Charset</x:ref> = 1#( ( <x:ref>charset</x:ref> / "*" )
                         [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
</artwork></figure>
<t>
   Character encoding values (a.k.a., charsets) are described in
   <xref target="character.sets"/>. 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
</t>
<figure><artwork type="example">
  Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
</artwork></figure>
<t>
   The special value "*", if present in the Accept-Charset field,
   matches every character encoding 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.
</t>
<t>
   A request without any Accept-Charset header field implies that the user
   agent will accept any character encoding in response.
   If an Accept-Charset header field is present in a request and none of the
   available representations for the response have a character encoding that
   is listed as acceptable, the origin server &MAY; either honor the
   Accept-Charset header field by sending a 406 (Not Acceptable) response or
   disregard the Accept-Charset header field by treating the response as if
   it is not subject to content negotiation.
</t>
</section>

<section title="Accept-Encoding" anchor="header.accept-encoding">
  <iref primary="true" item="Accept-Encoding header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="Accept-Encoding" x:for-anchor=""/>
  <x:anchor-alias value="Accept-Encoding"/>
  <x:anchor-alias value="codings"/>
<t>
   The "Accept-Encoding" header field can be used by user agents to
   indicate what response content-codings (<xref target="content.codings"/>)
   are acceptable in the response.  An "identity" token is used as a synonym
   for "no encoding" in order to communicate when no encoding is preferred.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Encoding"/><iref primary="true" item="Grammar" subitem="codings"/>
  <x:ref>Accept-Encoding</x:ref>  = #( <x:ref>codings</x:ref> [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
  <x:ref>codings</x:ref>          = <x:ref>content-coding</x:ref> / "identity" / "*"
</artwork></figure>
<t>
   Each codings value &MAY; be given an associated quality value which
   represents the preference for that encoding. The default value is q=1.
</t>
<t>
   For example,
</t>
<figure><artwork type="example">
  Accept-Encoding: compress, gzip
  Accept-Encoding:
  Accept-Encoding: *
  Accept-Encoding: compress;q=0.5, gzip;q=1.0
  Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
</artwork></figure>
<t>
   A server tests whether a content-coding for a given representation is
   acceptable, according to an Accept-Encoding field, using these rules:
  <list style="numbers">
      <t>The special "*" symbol in an Accept-Encoding field matches any
         available content-coding not explicitly listed in the header
         field.</t>

      <t>If the representation has no content-coding, then it is acceptable
         by default unless specifically excluded by the Accept-Encoding field
         stating either "identity;q=0" or "*;q=0" without a more specific
         entry for "identity".</t>

      <t>If the representation's 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 &qvalue;, a
         qvalue of 0 means "not acceptable".)</t>

      <t>If multiple content-codings are acceptable, then the acceptable
         content-coding with the highest non-zero qvalue is preferred.</t>
  </list>
</t>
<t>
   An Accept-Encoding header field with a combined field-value that is empty
   implies that the user agent does not want any content-coding in response.
   If an Accept-Encoding header field is present in a request and none of the
   available representations for the response have a content-coding that
   is listed as acceptable, the origin server &SHOULD; send a response
   without any content-coding.
</t>
<t>
   A request without an Accept-Encoding header field implies that the user
   agent will accept any content-coding in response, but a representation
   without content-coding is preferred for compatibility with the widest
   variety of user agents.
</t>
<x:note>
  <t>
    <x:h>Note:</x:h> 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.
  </t>
</x:note>
</section>

<section title="Accept-Language" anchor="header.accept-language">
  <iref primary="true" item="Accept-Language header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="Accept-Language" x:for-anchor=""/>
  <x:anchor-alias value="Accept-Language"/>
  <x:anchor-alias value="language-range"/>
<t>
   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 <xref target="language.tags"/>.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Language"/><iref primary="true" item="Grammar" subitem="language-range"/>
  <x:ref>Accept-Language</x:ref> =
                    1#( <x:ref>language-range</x:ref> [ <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>qvalue</x:ref> ] )
  <x:ref>language-range</x:ref>  = 
            &lt;language-range, defined in <xref target="RFC4647" x:fmt="," x:sec="2.1"/>&gt;
</artwork></figure>
<t>
   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
   example,
</t>
<figure><artwork type="example">
  Accept-Language: da, en-gb;q=0.8, en;q=0.7
</artwork></figure>
<t>
   would mean: "I prefer Danish, but will accept British English and
   other types of English".
   (see also <xref target="RFC4647" x:sec="2.3" x:fmt="of"/>)
</t>
<t>
   For matching, <xref target="RFC4647" x:sec="3" x:fmt="of"/> defines 
   several matching schemes. Implementations can offer the most appropriate
   matching scheme for their requirements. 
</t>
<x:note>
  <t>
    <x:h>Note:</x:h> The "Basic Filtering" scheme (<xref target="RFC4647"
    x:fmt="," x:sec="3.3.1"/>) is identical to the matching scheme that was
    previously defined in <xref target="RFC2616" x:fmt="of" x:sec="14.4"/>.
  </t>
</x:note>
<t>
   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
   <xref target="privacy.issues.connected.to.accept.header.fields"/>.
</t>
<t>
   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.
</t>
<x:note>
  <t>
    <x:h>Note:</x:h> 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.
  </t>
</x:note>
</section>

<section title="Content-Encoding" anchor="header.content-encoding">
  <iref primary="true" item="Content-Encoding header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="Content-Encoding" x:for-anchor=""/>
  <x:anchor-alias value="Content-Encoding"/>
<t>
   The "Content-Encoding" header field indicates what content-codings 
   have been applied to the representation beyond those inherent in the media
   type, 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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Encoding"/>
  <x:ref>Content-Encoding</x:ref> = 1#<x:ref>content-coding</x:ref>
</artwork></figure>
<t>
   Content codings are defined in <xref target="content.codings"/>. An example of its use is
</t>
<figure><artwork type="example">
  Content-Encoding: gzip
</artwork></figure>
<t>
   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.
</t>
<t>
   If the media type includes an inherent encoding, such as a data format
   that is always compressed, then that encoding would not be restated as 
   a Content-Encoding even if it happens to be the same algorithm as one
   of the content-codings.  Such a content-coding would only be listed if,
   for some bizarre reason, it is applied a second time to form the
   representation.  Likewise, an origin server might choose to publish the
   same payload data as multiple representations that differ only in whether
   the coding is defined as part of Content-Type or Content-Encoding, since
   some user agents will behave differently in their handling of each
   response (e.g., open a "Save as ..." dialog instead of automatic
   decompression and rendering of content).
</t>
<t>
   A representation that has a content-coding applied to it &MUST; include
   a Content-Encoding header field (<xref target="header.content-encoding"/>)
   that lists the content-coding(s) applied.
</t>
<t>
   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.
</t>
<t>
   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).
</t>
</section>

<section title="Content-Language" anchor="header.content-language">
  <iref primary="true" item="Content-Language header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="Content-Language" x:for-anchor=""/>
  <x:anchor-alias value="Content-Language"/>
<t>
   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 representation.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Language"/>
  <x:ref>Content-Language</x:ref> = 1#<x:ref>language-tag</x:ref>
</artwork></figure>
<t>
   Language tags are defined in <xref target="language.tags"/>. 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
</t>
<figure><artwork type="example">
  Content-Language: da
</artwork></figure>
<t>
   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.
</t>
<t>
   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
</t>
<figure><artwork type="example">
  Content-Language: mi, en
</artwork></figure>
<t>
   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".
</t>
<t>
   Content-Language &MAY; be applied to any media type &mdash; it is not
   limited to textual documents.
</t>
</section>

<section title="Content-Location" anchor="header.content-location">
  <iref primary="true" item="Content-Location header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="Content-Location" x:for-anchor=""/>
  <x:anchor-alias value="Content-Location"/>
<t>
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Location"/>
  <x:ref>Content-Location</x:ref> = <x:ref>absolute-URI</x:ref> / <x:ref>partial-URI</x:ref>
</artwork></figure>
<t>
   The Content-Location value is not a replacement for the effective
   Request URI (&effective-request-uri;).  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 <xref target="RFC2557" x:fmt="of" x:sec="4"/>.
   However, its appearance in an HTTP message has some special implications
   for HTTP recipients.
</t>
<t>
   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 request.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   If the Content-Location value is a partial URI, the partial URI is
   interpreted relative to the effective request URI.
</t>
</section>

<section title="Content-Type" anchor="header.content-type">
  <iref primary="true" item="Content-Type header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="Content-Type" x:for-anchor=""/>
  <x:anchor-alias value="Content-Type"/>
<t>
   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 GET.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Type"/>
  <x:ref>Content-Type</x:ref> = <x:ref>media-type</x:ref>
</artwork></figure>
<t>
   Media types are defined in <xref target="media.types"/>. An example of the field is
</t>
<figure><artwork type="example">
  Content-Type: text/html; charset=ISO-8859-4
</artwork></figure>
<t>
   Further discussion of Content-Type is provided in <xref target="representation.data"/>.
</t>
</section>

</section>

<section title="IANA Considerations" anchor="IANA.considerations">
<section title="Header Field Registration" anchor="header.field.registration">
<t>
   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
   with the permanent registrations below (see <xref target="RFC3864"/>):
</t>
<?BEGININC p3-payload.iana-headers ?>
<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
   <ttcol>Header Field Name</ttcol>
   <ttcol>Protocol</ttcol>
   <ttcol>Status</ttcol>
   <ttcol>Reference</ttcol>

   <c>Accept</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.accept"/>
   </c>
   <c>Accept-Charset</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.accept-charset"/>
   </c>
   <c>Accept-Encoding</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.accept-encoding"/>
   </c>
   <c>Accept-Language</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.accept-language"/>
   </c>
   <c>Content-Encoding</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.content-encoding"/>
   </c>
   <c>Content-Language</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.content-language"/>
   </c>
   <c>Content-Location</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.content-location"/>
   </c>
   <c>Content-Type</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.content-type"/>
   </c>
   <c>MIME-Version</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="mime-version"/>
   </c>
</texttable>
<!--(END)-->
<?ENDINC p3-payload.iana-headers ?>
<t>
   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
</t>
</section>

<section title="Content Coding Registry" anchor="content.coding.registration">
<t>
   The registration procedure for HTTP Content Codings is now defined
   by <xref target="content.coding.registry"/> of this document.
</t>
<t>
   The HTTP Content Codings Registry located at <eref target="http://www.iana.org/assignments/http-parameters"/>
   shall be updated with the registration below:
</t>
<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
   <ttcol>Name</ttcol>
   <ttcol>Description</ttcol>
   <ttcol>Reference</ttcol>
   <c>compress</c>
   <c>UNIX "compress" program method</c>
   <c>
      &compress-coding;
   </c>
   <c>deflate</c>
   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
   the "zlib" data format (<xref target="RFC1950"/>)
   </c>
   <c>
      &deflate-coding;
   </c>
   <c>gzip</c>
   <c>Same as GNU zip <xref target="RFC1952"/></c>
   <c>
      &gzip-coding;
   </c>
   <c>identity</c>
   <c>reserved (synonym for "no encoding" in Accept-Encoding header field)</c>
   <c>
      <xref target="header.accept-encoding"/>
   </c>
</texttable>
</section>

</section>

<section title="Security Considerations" anchor="security.considerations">
<t>
   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.
</t>

<section title="Privacy Issues Connected to Accept Header Fields" anchor="privacy.issues.connected.to.accept.header.fields">
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
</section>

</section>

<section title="Acknowledgments" anchor="acks">
<t>
  See &acks;.
</t>
</section>
</middle>
<back>

<references title="Normative References">

<reference anchor="Part1">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
      <address><email>jg@freedesktop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
</reference>

<reference anchor="Part2">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
      <address><email>jg@freedesktop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
</reference>

<reference anchor="Part4">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
      <address><email>jg@freedesktop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
  <x:source href="p4-conditional.xml" basename="p4-conditional"/>
</reference>

<reference anchor="Part5">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
      <address><email>jg@freedesktop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
  <x:source href="p5-range.xml" basename="p5-range"/>
</reference>

<reference anchor="Part6">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
      <address><email>jg@freedesktop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
      <organization>Rackspace</organization>
      <address><email>mnot@mnot.net</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
  <x:source href="p6-cache.xml" basename="p6-cache"/>
</reference>

<reference anchor="RFC1950">
  <front>
    <title>ZLIB Compressed Data Format Specification version 3.3</title>
    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
      <organization>Aladdin Enterprises</organization>
      <address><email>ghost@aladdin.com</email></address>
    </author>
    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1950"/>
  <annotation>
    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 <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
    therefore it is unlikely to cause problems in practice. See also
    <xref target="BCP97"/>.
  </annotation>
</reference>

<reference anchor="RFC1951">
  <front>
    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
      <organization>Aladdin Enterprises</organization>
      <address><email>ghost@aladdin.com</email></address>
    </author>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1951"/>
  <annotation>
    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 <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
    therefore it is unlikely to cause problems in practice. See also
    <xref target="BCP97"/>.
  </annotation>
</reference>

<reference anchor="RFC1952">
  <front>
    <title>GZIP file format specification version 4.3</title>
    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
      <organization>Aladdin Enterprises</organization>
      <address><email>ghost@aladdin.com</email></address>
    </author>
    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
      <address><email>gzip@prep.ai.mit.edu</email></address>
    </author>
    <author initials="M." surname="Adler" fullname="Mark Adler">
      <address><email>madler@alumni.caltech.edu</email></address>
    </author>
    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
      <address><email>ghost@aladdin.com</email></address>
    </author>
    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
      <address><email>randeg@alumni.rpi.edu</email></address>
    </author>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1952"/>
  <annotation>
    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 <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
    therefore it is unlikely to cause problems in practice. See also
    <xref target="BCP97"/>.
  </annotation>
</reference>

<reference anchor="RFC2045">
  <front>
    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
    <author initials="N." surname="Freed" fullname="Ned Freed">
      <organization>Innosoft International, Inc.</organization>
      <address><email>ned@innosoft.com</email></address>
    </author>
    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
      <organization>First Virtual Holdings</organization>
      <address><email>nsb@nsb.fv.com</email></address>
    </author>
    <date month="November" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="2045"/>
</reference>

<reference anchor="RFC2046">
  <front>
    <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
    <author initials="N." surname="Freed" fullname="Ned Freed">
      <organization>Innosoft International, Inc.</organization>
      <address><email>ned@innosoft.com</email></address>
    </author>
    <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
      <organization>First Virtual Holdings</organization>
      <address><email>nsb@nsb.fv.com</email></address>
    </author>
    <date month="November" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="2046"/>
</reference>

<reference anchor="RFC2119">
  <front>
    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
    <author initials="S." surname="Bradner" fullname="Scott Bradner">
      <organization>Harvard University</organization>
      <address><email>sob@harvard.edu</email></address>
    </author>
    <date month="March" year="1997"/>
  </front>
  <seriesInfo name="BCP" value="14"/>
  <seriesInfo name="RFC" value="2119"/>
</reference>

<reference anchor='RFC4647'>
  <front>
    <title>Matching of Language Tags</title>
    <author initials='A.' surname='Phillips' fullname='Addison Phillips' role="editor">
      <organization>Yahoo! Inc.</organization>
      <address><email>addison@inter-locale.com</email></address>
    </author>
    <author initials='M.' surname='Davis' fullname='Mark Davis' role="editor">
      <organization>Google</organization>
      <address><email>mark.davis@macchiato.com</email></address>
    </author>
    <date year='2006' month='September' />
  </front>
  <seriesInfo name='BCP' value='47' />
  <seriesInfo name='RFC' value='4647' />
</reference>

<reference anchor="RFC5234">
  <front>
    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
      <organization>Brandenburg InternetWorking</organization>
      <address>
        <email>dcrocker@bbiw.net</email>
      </address>  
    </author>
    <author initials="P." surname="Overell" fullname="Paul Overell">
      <organization>THUS plc.</organization>
      <address>
        <email>paul.overell@thus.net</email>
      </address>
    </author>
    <date month="January" year="2008"/>
  </front>
  <seriesInfo name="STD" value="68"/>
  <seriesInfo name="RFC" value="5234"/>
</reference>

<reference anchor='RFC5646'>
  <front>
    <title>Tags for Identifying Languages</title>
    <author initials='A.' surname='Phillips' fullname='Addison Phillips' role='editor'>
      <organization>Lab126</organization>
      <address><email>addison@inter-locale.com</email></address>
    </author>
    <author initials='M.' surname='Davis' fullname='Mark Davis' role='editor'>
      <organization>Google</organization>
      <address><email>mark.davis@google.com</email></address>
    </author>
    <date month='September' year='2009' />
  </front>
  <seriesInfo name='BCP' value='47' />
  <seriesInfo name='RFC' value='5646' />
</reference>

</references>

<references title="Informative References">

<reference anchor="RFC1945">
  <front>
    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization>MIT, Laboratory for Computer Science</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1945"/>
</reference>

<reference anchor="RFC2049">
  <front>
    <title abbrev="MIME Conformance">Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples</title>
    <author initials="N." surname="Freed" fullname="Ned Freed">
      <organization>Innosoft International, Inc.</organization>
      <address><email>ned@innosoft.com</email></address>
    </author>
    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
      <organization>First Virtual Holdings</organization>
      <address><email>nsb@nsb.fv.com</email></address>
    </author>
    <date month="November" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="2049"/>
</reference>

<reference anchor="RFC2068">
  <front>
    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>jg@w3.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
      <address><email>mogul@wrl.dec.com</email></address>
    </author>
    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <date month="January" year="1997"/>
  </front>
  <seriesInfo name="RFC" value="2068"/>
</reference>

<reference anchor="RFC2076">
  <front>
    <title abbrev="Internet Message Headers">Common Internet Message Headers</title>
    <author initials="J." surname="Palme" fullname="Jacob Palme">
      <organization>Stockholm University/KTH</organization>
      <address><email>jpalme@dsv.su.se</email></address>
    </author>
    <date month="February" year="1997"/>
  </front>
  <seriesInfo name="RFC" value="2076"/>
</reference>

<reference anchor="RFC2277">
  <front>
    <title abbrev="Charset Policy">IETF Policy on Character Sets and Languages</title>
    <author initials="H.T." surname="Alvestrand" fullname="Harald Tveit Alvestrand">
      <organization>UNINETT</organization>
      <address><email>Harald.T.Alvestrand@uninett.no</email></address>
    </author>
    <date month="January" year="1998"/>
  </front>
  <seriesInfo name="BCP" value="18"/>
  <seriesInfo name="RFC" value="2277"/>
</reference>

<reference anchor='RFC2295'>
  <front>
    <title abbrev='HTTP Content Negotiation'>Transparent Content Negotiation in HTTP</title>
    <author initials='K.' surname='Holtman' fullname='Koen Holtman'>
      <organization>Technische Universiteit Eindhoven</organization>
      <address>
        <email>koen@win.tue.nl</email>
      </address>
    </author>
    <author initials='A.H.' surname='Mutz' fullname='Andrew H. Mutz'>
      <organization>Hewlett-Packard Company</organization>
      <address>
        <email>mutz@hpl.hp.com</email>
      </address>
    </author>
    <date year='1998' month='March'/>
  </front>
  <seriesInfo name='RFC' value='2295'/>
</reference>

<reference anchor="RFC2388">
  <front>
    <title abbrev="multipart/form-data">Returning Values from Forms:  multipart/form-data</title>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization>Xerox Palo Alto Research Center</organization>
      <address><email>masinter@parc.xerox.com</email></address>
    </author>
    <date year="1998" month="August"/>
  </front>
  <seriesInfo name="RFC" value="2388"/>
</reference>

<reference anchor="RFC2557">
  <front>
    <title abbrev="MIME Encapsulation of Aggregate Documents">MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)</title>
    <author initials="F." surname="Palme" fullname="Jacob Palme">
      <organization>Stockholm University and KTH</organization>
      <address><email>jpalme@dsv.su.se</email></address>
    </author>
    <author initials="A." surname="Hopmann" fullname="Alex Hopmann">
      <organization>Microsoft Corporation</organization>
      <address><email>alexhop@microsoft.com</email></address>
    </author>
    <author initials="N." surname="Shelness" fullname="Nick Shelness">
      <organization>Lotus Development Corporation</organization>
      <address><email>Shelness@lotus.com</email></address>
    </author>
    <author initials="E." surname="Stefferud" fullname="Einar Stefferud">
      <address><email>stef@nma.com</email></address>
    </author>
    <date year="1999" month="March"/>
  </front>
  <seriesInfo name="RFC" value="2557"/>
</reference>

<reference anchor="RFC2616">
  <front>
    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
    <author initials="R." surname="Fielding" fullname="R. Fielding">
      <organization>University of California, Irvine</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="J. Gettys">
      <organization>W3C</organization>
      <address><email>jg@w3.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="J. Mogul">
      <organization>Compaq Computer Corporation</organization>
      <address><email>mogul@wrl.dec.com</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="L. Masinter">
      <organization>Xerox Corporation</organization>
      <address><email>masinter@parc.xerox.com</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="P. Leach">
      <organization>Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
      <organization>W3C</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <date month="June" year="1999"/>
  </front>
  <seriesInfo name="RFC" value="2616"/>
</reference>

<reference anchor="RFC3629">
  <front>
    <title>UTF-8, a transformation format of ISO 10646</title>
    <author initials="F." surname="Yergeau" fullname="F. Yergeau">
      <organization>Alis Technologies</organization>
      <address><email>fyergeau@alis.com</email></address>
    </author>
    <date month="November" year="2003"/>
  </front>
  <seriesInfo name="STD" value="63"/>
  <seriesInfo name="RFC" value="3629"/>
</reference>

<reference anchor='RFC3864'>
  <front>
    <title>Registration Procedures for Message Header Fields</title>
    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
      <organization>Nine by Nine</organization>
      <address><email>GK-IETF@ninebynine.org</email></address>
    </author>
    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
      <organization>BEA Systems</organization>
      <address><email>mnot@pobox.com</email></address>
    </author>
    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
      <organization>HP Labs</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <date year='2004' month='September' />
  </front>
  <seriesInfo name='BCP' value='90' />
  <seriesInfo name='RFC' value='3864' />
</reference>

<reference anchor="RFC4288">
  <front>
    <title>Media Type Specifications and Registration Procedures</title>
    <author initials="N." surname="Freed" fullname="N. Freed">
      <organization>Sun Microsystems</organization>
      <address>
        <email>ned.freed@mrochek.com</email>
      </address>
    </author>
    <author initials="J." surname="Klensin" fullname="J. Klensin">
      <address>
        <email>klensin+ietf@jck.com</email>
      </address>
    </author>
    <date year="2005" month="December"/>
  </front>
  <seriesInfo name="BCP" value="13"/>
  <seriesInfo name="RFC" value="4288"/>
</reference>

<reference anchor='RFC5226'>
  <front>
    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
    <author initials='T.' surname='Narten' fullname='T. Narten'>
      <organization>IBM</organization>
      <address><email>narten@us.ibm.com</email></address>
    </author>
    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
      <organization>Google</organization>
      <address><email>Harald@Alvestrand.no</email></address>
    </author>
    <date year='2008' month='May' />
  </front>
  <seriesInfo name='BCP' value='26' />
  <seriesInfo name='RFC' value='5226' />
</reference>

<reference anchor="RFC5322">
  <front>
    <title>Internet Message Format</title>
    <author initials="P." surname="Resnick" fullname="P. Resnick">
      <organization>Qualcomm Incorporated</organization>
    </author>
    <date year="2008" month="October"/>
  </front> 
  <seriesInfo name="RFC" value="5322"/>
</reference>

<reference anchor="RFC6151">
  <front>
    <title>Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms</title>
    <author initials="S." surname="Turner" fullname="S. Turner"/>
    <author initials="L." surname="Chen" fullname="L. Chen"/>
    <date year="2011" month="March" />
  </front>
  <seriesInfo name="RFC" value="6151" />
</reference>

<reference anchor='BCP97'>
  <front>
    <title>Handling Normative References to Standards-Track Documents</title>
    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
      <address>
        <email>klensin+ietf@jck.com</email>
      </address>
    </author>
    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
      <organization>MIT</organization>
      <address>
        <email>hartmans-ietf@mit.edu</email>
      </address>
    </author>
    <date year='2007' month='June' />
  </front>
  <seriesInfo name='BCP' value='97' />
  <seriesInfo name='RFC' value='4897' />
</reference>

<reference anchor="RFC6266">
  <front>
    <title abbrev="Content-Disposition in HTTP">Use of the Content-Disposition Header Field
    in the Hypertext Transfer Protocol (HTTP)</title>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address>
        <email>julian.reschke@greenbytes.de</email>
      </address>
    </author>
    <date month="June" year="2011"/>
  </front>
  <seriesInfo name='RFC' value='6266' />
</reference>

</references>

<section title="Differences between HTTP and MIME" anchor="differences.between.http.and.mime">
<t>
   HTTP/1.1 uses many of the constructs defined for Internet Mail (<xref target="RFC5322"/>) and the Multipurpose Internet Mail Extensions (MIME <xref target="RFC2045"/>) to
   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.
</t>
<t>
   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 required.
</t>

<section title="MIME-Version" anchor="mime-version">
  <iref primary="true" item="MIME-Version header field" x:for-anchor=""/>
  <iref primary="true" item="Header Fields" subitem="MIME-Version" x:for-anchor=""/>
  <x:anchor-alias value="MIME-Version"/>
<t>
   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 <xref target="RFC2045"/>).
   Proxies/gateways are responsible for ensuring full compliance (where
   possible) when exporting HTTP messages to strict MIME environments.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="MIME-Version"/>
  <x:ref>MIME-Version</x:ref> = 1*<x:ref>DIGIT</x:ref> "." 1*<x:ref>DIGIT</x:ref>
</artwork></figure>
<t>
   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.
</t>
</section>

<section title="Conversion to Canonical Form" anchor="conversion.to.canonical.form">
<t>
   MIME requires that an Internet mail body-part be converted to
   canonical form prior to being transferred, as described in <xref target="RFC2049" x:fmt="of" x:sec="4"/>.
   <xref target="canonicalization.and.text.defaults"/> of this document describes the forms
   allowed for subtypes of the "text" media type when transmitted over
   HTTP. <xref target="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.
</t>
<t>
   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 <xref target="canonicalization.and.text.defaults"/>
   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.
</t>
<t>
   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.
</t>
</section>


<section title="Conversion of Date Formats" anchor="conversion.of.date.formats">
<t>
   HTTP/1.1 uses a restricted set of date formats (&http-date;) 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.
</t>
</section>

<section title="Introduction of Content-Encoding" anchor="introduction.of.content-encoding">
<t>
   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=&lt;content-coding&gt;" to perform
   a function equivalent to Content-Encoding. However, this parameter is
   not part of the MIME standards).
</t>
</section>

<section title="No Content-Transfer-Encoding" anchor="no.content-transfer-encoding">
<t>
   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.
</t>
<t>
   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.
</t>
</section>

<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
<t>
   HTTP/1.1 introduces the Transfer-Encoding header field (&header-transfer-encoding;).
   Proxies/gateways &MUST; remove any transfer-coding prior to
   forwarding a message via a MIME-compliant protocol.
</t>
</section>

<section title="MHTML and Line Length Limitations" anchor="mhtml.line.length">
<t>
   HTTP implementations which share code with MHTML <xref target="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 <xref target="multipart.types"/>) and
   does not interpret the content or any MIME header lines that might be
   contained therein.
</t>
</section>
</section>

<section title="Additional Features" anchor="additional.features">
<t>
   <xref target="RFC1945"/> and <xref target="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.
</t>
<t>
   A number of other header fields, such as Content-Disposition and Title,
   from SMTP and MIME are also often implemented (see <xref target="RFC6266"/>
   and <xref target="RFC2076"/>).
</t>
</section>

<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
<t>
  Clarify contexts that charset is used in.
  (<xref target="character.sets"/>)
</t>
<t>
  Remove the default character encoding for text media types; the default
  now is whatever the media type definition says.
  (<xref target="canonicalization.and.text.defaults"/>)
</t>
<t>
  Change ABNF productions for header fields to only define the field value.
  (<xref target="header.field.definitions"/>)
</t>
<t>
  Remove definition of Content-MD5 header field because it was inconsistently
  implemented with respect to partial responses, and also because of known
  deficiencies in the hash algorithm itself (see <xref target="RFC6151"/> for details).
  (<xref target="header.field.definitions"/>)
</t>
<t>
  Remove ISO-8859-1 special-casing in Accept-Charset.
  (<xref target="header.accept-charset"/>)
</t>
<t>
  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.
  (<xref target="header.content-location"/>)
</t>
<t>
  Remove discussion of Content-Disposition header field, it is now defined
  by <xref target="RFC6266"/>.
  (<xref target="additional.features"/>)
</t>
<t>
  Remove reference to non-existant identity transfer-coding value tokens.
  (<xref target="no.content-transfer-encoding"/>)
</t>
</section>

<?BEGININC p3-payload.abnf-appendix ?>
<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
<figure>
<artwork type="abnf" name="p3-payload.parsed-abnf">
<x:ref>Accept</x:ref> = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
 OWS media-range [ accept-params ] ] ) ]
<x:ref>Accept-Charset</x:ref> = *( "," OWS ) ( charset / "*" ) [ OWS ";" OWS "q="
 qvalue ] *( OWS "," [ OWS ( charset / "*" ) [ OWS ";" OWS "q="
 qvalue ] ] )
<x:ref>Accept-Encoding</x:ref> = [ ( "," / ( codings [ OWS ";" OWS "q=" qvalue ] ) )
 *( OWS "," [ OWS codings [ OWS ";" OWS "q=" qvalue ] ] ) ]
<x:ref>Accept-Language</x:ref> = *( "," OWS ) language-range [ OWS ";" OWS "q="
 qvalue ] *( OWS "," [ OWS language-range [ OWS ";" OWS "q=" qvalue ]
 ] )

<x:ref>Content-Encoding</x:ref> = *( "," OWS ) content-coding *( OWS "," [ OWS
 content-coding ] )
<x:ref>Content-Language</x:ref> = *( "," OWS ) language-tag *( OWS "," [ OWS
 language-tag ] )
<x:ref>Content-Location</x:ref> = absolute-URI / partial-URI
<x:ref>Content-Type</x:ref> = media-type

<x:ref>MIME-Version</x:ref> = 1*DIGIT "." 1*DIGIT

<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;

<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [Part1], Section 2.7&gt;
<x:ref>accept-ext</x:ref> = OWS ";" OWS token [ "=" word ]
<x:ref>accept-params</x:ref> = OWS ";" OWS "q=" qvalue *accept-ext
<x:ref>attribute</x:ref> = token

<x:ref>charset</x:ref> = token
<x:ref>codings</x:ref> = content-coding / "identity" / "*"
<x:ref>content-coding</x:ref> = token

<x:ref>language-range</x:ref> = &lt;language-range, defined in [RFC4647], Section 2.1&gt;
<x:ref>language-tag</x:ref> = &lt;Language-Tag, defined in [RFC5646], Section 2.1&gt;

<x:ref>media-range</x:ref> = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
 ";" OWS parameter )
<x:ref>media-type</x:ref> = type "/" subtype *( OWS ";" OWS parameter )

<x:ref>parameter</x:ref> = attribute "=" value
<x:ref>partial-URI</x:ref> = &lt;partial-URI, defined in [Part1], Section 2.7&gt;

<x:ref>qvalue</x:ref> = &lt;qvalue, defined in [Part1], Section 5.3&gt;

<x:ref>subtype</x:ref> = token

<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.3&gt;
<x:ref>type</x:ref> = token

<x:ref>value</x:ref> = word

<x:ref>word</x:ref> = &lt;word, defined in [Part1], Section 3.2.3&gt;
</artwork>
</figure>
<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
; 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-Type defined but not used
; MIME-Version defined but not used
</artwork></figure></section>
<?ENDINC p3-payload.abnf-appendix ?>

<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">

<section title="Since RFC 2616">
<t>
  Extracted relevant partitions from <xref target="RFC2616"/>.
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-00">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
      "Media Type Registrations"
      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/14"/>:
      "Clarification regarding quoting of charset values"
      (<eref target="http://purl.org/NET/http-errata#charactersets"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
      "Remove 'identity' token references"
      (<eref target="http://purl.org/NET/http-errata#identity"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/25"/>:
      "Accept-Encoding BNF"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
      "Normative and Informative references"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
      "RFC1700 references"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
      "Updating to RFC4288"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
      "Informative references"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
      "ISO-8859-1 Reference"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/68"/>:
      "Encoding References Normative"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
      "Normative up-to-date references"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-01">
<t>
  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Add explicit references to BNF syntax and rules imported from other parts of the specification.
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-02" anchor="changes.since.02">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/67"/>:
      "Quoting Charsets"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/105"/>:
      "Classification for Allow header"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/115"/>:
      "missing default for qvalue in description of Accept-Encoding"
    </t>
  </list>
</t>
<t>
  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
  <list style="symbols"> 
    <t>
      Reference RFC 3984, and update header field registrations for headers defined
      in this document.
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-03" anchor="changes.since.03">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/67"/>:
      "Quoting Charsets"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/113"/>:
      "language tag matching (Accept-Language) vs RFC4647"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/121"/>:
      "RFC 1806 has been replaced by RFC2183"
    </t>
  </list>
</t>
<t>
  Other changes:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/68"/>:
      "Encoding References Normative" &mdash; rephrase the annotation and reference
      <xref target="BCP97"/>.
    </t>
  </list>
</t>
 </section>

<section title="Since draft-ietf-httpbis-p3-payload-04" anchor="changes.since.04">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
      "RFC 2822 is updated by RFC 5322"
    </t>
  </list>
</t>
<t>
  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Use "/" instead of "|" for alternatives.
    </t>
    <t>
      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").
    </t>
    <t>
      Rewrite ABNFs to spell out whitespace rules, factor out
      header field value format definitions.
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-05" anchor="changes.since.05">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/118"/>:
      "Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
    </t>
  </list>
</t>
<t>
  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
    </t>
  </list>
</t>
<t>
  Other changes:
  <list style="symbols"> 
    <t>
      Move definition of quality values into Part 1.
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-06" anchor="changes.since.06">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/80"/>:
      "Content-Location isn't special"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/155"/>:
      "Content Sniffing"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-07" anchor="changes.since.07">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/13"/>:
      "Updated reference for language tags"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/110"/>:
      "Clarify rules for determining what entities a response carries"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/154"/>:
      "Content-Location base-setting problems"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/155"/>:
      "Content Sniffing"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/188"/>:
      "pick IANA policy (RFC5226) for Transfer Coding / Content Coding"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/189"/>:
      "move definitions of gzip/deflate/compress to part 1"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/148"/>:
      "update IANA requirements wrt Transfer-Coding values" (add the
      IANA Considerations subsection)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/149"/>:
      "update IANA requirements wrt Content-Coding values" (add the
      IANA Considerations subsection)
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-08" anchor="changes.since.08">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/81"/>:
      "Content Negotiation for media types"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/181"/>:
      "Accept-Language: which RFC4647 filtering?"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-09" anchor="changes.since.09">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/122"/>:
      "MIME-Version not listed in P1, general header fields"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/143"/>:
      "IANA registry for content/transfer encodings"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/155"/>:
      "Content Sniffing"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/200"/>:
      "use of term "word" when talking about header structure"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
      "Term for the requested resource's URI"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-10" anchor="changes.since.10">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
      "Clarify 'Requested Variant'"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/80"/>:
      "Content-Location isn't special"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/90"/>:
      "Delimiting messages with multipart/byteranges"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
      "Clarify entity / representation / variant terminology"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/136"/>:
      "confusing req. language for Content-Location"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
      "Content-Location on 304 responses"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/183"/>:
      "'requested resource' in content-encoding definition"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
      "consider removing the 'changes from 2068' sections"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/178"/>:
      "Content-MD5 and partial responses"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-11" anchor="changes.since.11">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/123"/>:
      "Factor out Content-Disposition"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-12" anchor="changes.since.12">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
      "Header Classification"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
      "untangle ABNFs for header fields"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/277"/>:
      "potentially misleading MAY in media-type def"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-13" anchor="changes.since.13">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/20"/>:
      "Default charsets for text media types"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/178"/>:
      "Content-MD5 and partial responses"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
      "untangle ABNFs for header fields"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/281"/>:
      "confusing undefined parameter in media range example"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-14" anchor="changes.since.14">
<t>
  None.
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-15" anchor="changes.since.15">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/285"/>:
      "Strength of requirements on Accept re: 406"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p3-payload-16" anchor="changes.since.16">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
      "Document HTTP's error-handling philosophy"
    </t>
  </list>
</t>
</section>

</section>

</back>
</rfc>