Internet DRAFT - draft-hilt-sip-ext-neg


Session Initiation Protocol                                      V. Hilt
Working Group                              Bell Labs/Lucent Technologies
Internet-Draft                                              J. Rosenberg
Expires: July 20, 2005                                     Cisco Systems
                                                            G. Camarillo
                                                        January 19, 2005

  Media Type Extension Negotiation in the Session Initiation Protocol
                       (SIP) Accept Header Field

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This specification defines the profile parameter for the SIP Accept
   header field.  This parameter is used to negotiate support for MIME
   media type extensions.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.   Profile Accept Header Field Parameter  . . . . . . . . . . . . 4
   4.   User Agent Behavior  . . . . . . . . . . . . . . . . . . . . . 4
   5.   Applicability to Other Protocols . . . . . . . . . . . . . . . 5
   6.   Security Considerations  . . . . . . . . . . . . . . . . . . . 5
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 5
   8.   References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
        Intellectual Property and Copyright Statements . . . . . . . . 8

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1.  Introduction

   The Accept header field in the Session Initiation Protocol (SIP) [11]
   provides a mechanism for user agents to negotiate the media types
   they can accept in message bodies.  With this mechanism, a user agent
   can announce the MIME media types it supports to another user agent.

   Many media types used in SIP message bodies are based on the
   Extensible Markup Language (XML) [13].  XML features a powerful
   extension mechanism that enables the use of multiple vocabularies in
   a single XML document.  Components from different vocabularies can be
   uniquely identified using XML namespaces [5].

   The extensibility of XML poses a problem for MIME media type based
   content negotiation.  The negotiation only covers major media types
   and does not include language extensions and variations.  For
   example, a user agent can indicate its support for the media type
   'application/pidf+xml' (Presence Information Data Format) [12] but
   cannot indicate that it is also capable of handling the Location
   Object Extension to PIDF [10] and the location format GML 3.0 [9].

   The capability to negotiate support for a language extension or
   variation is in particular useful in the following cases:

   1.  The interpretation of the content by the receiver depends on
       understanding the extension or language variation used.
   2.  Different versions of the content can be made available depending
       on the extensions or language variations supported by the
       receiver.  Unsupported extensions can be omitted by the sender to
       avoid, for example, transmission overhead or costs associated
       with gathering the information.

   In this specification, we propose a new parameter for the Accept
   header field that enables the negotiation of extensions to MIME media

   The problem of negotiating support for language extensions and
   variations has been addressed for a number of specific MIME media
   types.  For example, [1] and [6] define a profile parameter for the
   MIME media types XHTML and SMIL respectively.  However, these
   solutions are limited to a specific media type and require the
   registration of a new MIME parameter for each media type that is
   extended.  An abstract framework for content negotiation has been
   defined in [7].

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",

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   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, [2] and indicate requirement levels for
   compliant implementations.

3.  Profile Accept Header Field Parameter

   The profile parameter is a new parameter for the Accept header field.
   It MAY appear multiple times (including zero) in an Accept header
   field.  The profile parameter contains a URI that identifies an
   extension or a variation of the underlying MIME media type.
   Currently, the profile parameter is only defined for XML-based media
   types, which are registered with a '+xml' suffix.  For these media
   types, the URI in the profile parameter identifies an XML namespace.
   This URI is identical to the URI that would be used in an XML
   document (e.g.  in the XMLNS tag) to identify the same namespace.

   The syntax of the 'profile' Accept header field parameter is:

   profile-param = absoluteURI

   This extends the existing definition of the Accept header field
   parameters in [11], so that its BNF now looks like:

   accept-param   =  ("q" EQUAL qvalue) / profile-param / generic-param


   Accept: application/pidf+xml;

4.  User Agent Behavior

   A user agent, which supports an extension or variation of a MIME
   media type, SHOULD list this extension or language variation in a
   profile parameter inserted into the Accept header field of the MIME
   media type.  A user agent SHOULD identify all supported extensions
   and language variations.  It SHOULD NOT list items (e.g.  XML
   namespaces) that are by default part of the media type.

   A user agent receiving an Accept header field with a profile
   parameter can assume that the sender supports the listed extensions
   and variations and MAY use them to create message bodies.

      Note: The absence of an extension in the profile parameter (or the
      absence of the profile parameter) does not mean the sender does

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      not support that extension.

5.  Applicability to Other Protocols

   Although this extension has been developed for the SIP Accept header
   field, it is applicable to the Accept header field of other protocols
   such as HTTP.

      Note: For this broader use, the profile parameter needs to be
      registered for the respective protocols or in the registry defined
      in [8].

6.  Security Considerations

   Security considerations for the Accept header field also apply here.

7.  IANA Considerations

   This document adds a parameter to the SIP header field parameter
   registry [4]:

   Header field in which the parameter can appear: Accept

   Name of the parameter: profile

   The parameter only accepts a set of predefined values: No

   Reference: this document

8  References

   [1]   Baker, M. and P. Stark, "The 'application/xhtml+xml' Media
         Type", RFC 3236, January 2002.

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

   [3]   Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3667,
         February 2004.

   [4]   Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Header Field Parameter Registry for the Session Initiation
         Protocol (SIP)", BCP 98, RFC 3968, December 2004.

   [5]   Hollander, D., Bray, T. and A. Layman, "Namespaces in XML", W3C
         REC REC-xml-names-19990114, January 1999.

   [6]   Hoschka, P., "The application/smil and application/smil+xml

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         Media Types", draft-hoschka-smil-media-type-12 (work in
         progress), August 2004.

   [7]   Klyne, G., "Protocol-independent Content Negotiation
         Framework", RFC 2703, September 1999.

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

   [9]   OpenGIS, "Open Geography Markup Language (GML) Implementation
         Specification", OGC 02-023r4, January 2003.

   [10]  Peterson, J., "A Presence-based GEOPRIV Location Object
         Format", draft-ietf-geopriv-pidf-lo-03 (work in progress),
         September 2004.

   [11]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [12]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W. and
         J. Peterson, "Presence Information Data Format (PIDF)", RFC
         3863, August 2004.

   [13]  Yergeau, F., Paoli, J., Sperberg-McQueen, C., Bray, T. and E.
         Maler, "Extensible Markup Language (XML) 1.0 (Third Edition)",
         W3C REC REC-xml-20040204, February 2004.

Authors' Addresses

   Volker Hilt
   Bell Labs/Lucent Technologies
   101 Crawfords Corner Rd
   Holmdel, NJ  07733


   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054


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   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420


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