MMUSIC Working Group                                       J. Hautakorpi
Internet-Draft                                              G. Camarillo
Expires: March 5, 2006                                          Ericsson
                                                          September 2005

        The SDP (Session Description Protocol) Content Attribute

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document defines a new Session Description Protocol (SDP) media-
   level attribute, 'content'.  The 'content' attribute defines the
   content of the media stream in more detailed level than the media
   description line.  The sender of an SDP session description can
   attach the 'content' attribute to one or more media streams.  The
   receiving application can then treat each media stream differently
   (e.g., show it on a big screen or small screen) based on their

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Related Techniques . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Motivation for the New Content Attribute . . . . . . . . . . .  4
   5.  The Content Attribute  . . . . . . . . . . . . . . . . . . . .  5
   6.  The Content Attribute in the Offer/Answer Model  . . . . . . .  6
   7.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   8.  Operation with SMIL  . . . . . . . . . . . . . . . . . . . . .  7
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   11. Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . .  8
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     12.1.  Normative References  . . . . . . . . . . . . . . . . . .  8
     12.2.  Informational References  . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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1.  Introduction

   SDP [1] is a protocol that is intended for describing multimedia
   sessions for the purposes of session announcement, session
   invitation, and other forms of multimedia session initiation.  One of
   the most typical use cases of SDP is the one where it is used with
   SIP [5].

   There are situations where one application receives several similar
   media stream which are described in an SDP session description.  The
   media streams can be similar in a sense that their content cannot be
   distinguished from each other just by examining the media description
   lines (e.g., two video streams).  The 'content' attribute is needed,
   so that the receiving application can treat each media stream
   appropriately based on its content.

   This specification defines the SDP 'content' media-level attribute,
   which provides more information about the media stream than the 'm'
   line in an SDP session description.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [3] and indicate requirement levels for
   compliant implementations.

3.  Related Techniques

   The 'label' attribute [9] enables a sender to attach a pointer to a
   particular media stream.  The name space of the 'label' attribute
   itself is unrestricted, so in principle it could also be used to
   convey information about the content of a media stream.  However, in
   practice, this is not possible because of the backwards
   compatibility.  Existing implementation of the 'label' attribute
   already use values from that unrestricted namespace in an application
   specific way.  So, it is not possible to reserve parts from the
   'label' attributes' namespace without possible conflict with an
   already used, application specific labels.

   It is possible to assign semantics to a media stream with the
   external document that uses the 'label' attribute as a pointer.  The
   downside of this approach is that it requires external document.
   Typically this kind of mechanism would be defined for some particular
   use case, for example centralized conferencing.

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   Yet another way to attach semantics to a media stream is by using the
   'i' SDP attribute, defined in RFC 2327 [6].  Nevertheless, values of
   the 'i' attribute are intended for human users and not for automata.

4.  Motivation for the New Content Attribute

   Currently, SDP does not provide any means to describe what is the
   content of a media stream (e.g., speaker's image, slides, sign
   language) in a form that the application can understand.  Of course
   the end user can see the content of the media stream, and read its
   title, but the application cannot understand what the media stream

   The application that is receiving multiple similar (e.g., same type
   and format) media stream needs, in some cases, to know what is the
   content of those streams.  This kind of situation occurs for example
   in cases where presentation slides, speakers image and sign language
   are transported as separate media streams.  It would be desirable
   that the receiving application could distinguish them in a way that
   it could handle them automatically in appropriate manner.

                ||            ||                      ||
                || speaker's  ||                      ||
                ||   image    ||                      ||
                ||            ||                      ||
                |+------------+|     presentation     ||
                |+------------+|        slides        ||
                ||            ||                      ||
                ||    sign    ||                      ||
                ||  language  ||                      ||
                ||            ||                      ||

   Figure 1: Application's screen

   The Figure 1 presents a screen of a typical communication
   application.  The 'content' attribute enables the application to make
   the decision on where to show each media stream.  From end user's
   perspective, it is desirable that the user does not need to arrange
   media stream every time the media session starts.

   The 'content' attribute could also be used in more complex
   situations.  This kind of complex situation could be e.g., an
   application that is controlling the equipments in the auditorium.

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   Auditorium can have many different output channels for the video
   (main screen and two smaller screens) and the audio (main speakers,
   headsets for the participants).  In this kind of environment, a lot
   of interaction from the end user who is operating the application
   would be required.  So, the possibility for the application to handle
   the media stream without end users input is greatly emphasized.

5.  The Content Attribute

   This specification defines a new media-level value attribute,
   'content'.  Its formatting in SDP is described by the following BNF

       content-attribute   = "a=content:" mediacnt-tag
       mediacnt-tag        = mediacnt *("," mediacnt)
       mediacnt            = "slides" / "speaker" / "sl" / "main"
                             / "alt" / "user-floor" / mediacnt-ext
       mediacnt-ext        = token

   The 'content' attribute contains a token, which MAY be attached to a
   media stream by a sending application.  It describes the content of
   the transmitted media stream to the receiving application.  Many
   'content' attribute values MAY be attached to a single media stream.

   There is a set of pre-defined values for the 'content' attribute.
   Also other values can be defined in the future.  The pre-defined
   values are:

   slides: This is a media stream that includes presentation slides.
      The media type can be e.g., a video stream or a set of instant
      message with pictures.  Typical use case for this is e.g., online
      seminars and courses.  This is similar to the 'presentation' role
      in H.239 [11].
   speaker: This is a image from the speaker.  The media can be e.g., a
      video stream or a still image.  Typical use case for this is e.g,
      online seminars and courses.
   sl: This means that the media stream contains sign language.  The
      media type is a video stream.  Typical use case for this is the
      one where the audio stream is translated into sign language.
   main: This means that the main stream is taken from the main source.
      Typical use case for this is e.g., a concert, where the camera is
      shooting the performer.

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   alt: This means that the main stream is taken from the alternative
      source.  This is the alternative media stream.  Typical use case
      for the is e.g., an event, where there is a separate ambient sound
      and the main sound.  The alternative audio stream could be e.g., a
      sound of a jungle.  Another example is the video of the conference
      room while the main is the video of the speaker.  This is similar
      to the 'live' role in H.239.
   user-floor: This indicates that a user level floor control is
      required.  In other words, this is meant for system without any
      mechanism for floor control, and where a human needs to figure out
      whether an act of floor control, e.g., saying 'over', is needed or
      not.  Typically this can be used with walkie-talkie type of
      devices, and with transcoded Text over IP (ToIP) streams
      originating from Telecommunications Devices for the Deaf (TDDs).

   All of the values can be used with any media type.  Application can
   make decision on how to handle a single media stream based on both
   media type and the value of 'content' attribute.  Therefore the
   situation, where one value of 'content' attribute occurs more than
   once in a single session descriptor, is not problematic.

6.  The Content Attribute in the Offer/Answer Model

   This specification does not define a means to discover whether or not
   the peer endpoint understands the 'content' attribute, because
   'content' values are informative only at the offer/answer model [8]
   level.  The fact that the peer endpoint does not understands the
   'content' attribute, does not keep the media session from being
   established.  The only consequence is that interaction from the
   receiving end user may be required.

7.  Example

   The following is an example of the SDP session description that uses
   the 'content' attribute:

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       o=Alice 292742730 29277831 IN IP4
       s=Second lecture from information technology
       c=IN IP4
       t=0 0
       m=video 52886 RTP/AVP 31
       a=rtpmap:31 H261/9000
       m=video 53334 RTP/AVP 31
       a=rtpmap:31 H261/9000
       m=video 54132 RTP/AVP 31
       a=rtpmap:31 H261/9000

8.  Operation with SMIL

   The values of 'content' attribute, defined in Section 5, can also be
   used with SMIL [10].  SMIL contains a 'param' element, which is used
   for describing the content of a media.  However, this 'param' element
   provides only application specific description of media content.  By
   using the values of the 'content' attribute, this 'param' element can
   also be used to describe the media content in globally interpretable

   Details on how to use the values of the 'content' attribute with
   SMIL's 'param' element are outside the scope of this specification.

9.  Security Considerations

   An attacker may attempt to add, modify, or remove 'content'
   attributes from a session description.  This could result in an
   application behaving in a non-desirable way.  So, it is strongly
   RECOMMENDED that integrity protection be applied to the SDP session
   descriptions.  For session descriptions carried in SIP [5], S/MIME
   [7] is the natural choice to provide such end-to-end integrity
   protection, as described in RFC 3261 [5].  Other applications MAY use
   a different form of integrity protection.

10.  IANA Considerations

   This document defines a new 'content' attribute for SDP.  It also
   defines initial set of values for it.

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   Contact name:        Jani Hautakorpi

   Attribute name:      'content'.

   Type of attribute    Media level.

   Subject to charset:  Not.

   Purpose of attribute:  The 'content' attribute gives information from
      the content of the media stream to the receiving application.

   Allowed attribure values: "slides", "speaker", "sl", "main", "alt",
                             "user-floor", and a token.

   The IANA is requested to create a subregistry for the 'content'
   attribute values under the Session Description Protocol (SDP)
   Parameters registry.  The following are the initial values for the

   Value of 'content' attribute Reference Description
   ---------------------------- --------- -----------
   slides                       RFC xxxx  Presentation slides
   speaker                      RFC xxxx  Image from the speaker
   sl                           RFC xxxx  Sign language
   main                         RFC xxxx  Main media stream
   alt                          RFC xxxx  Alternative media stream
   user-floor                   RFC xxxx  User level floor control req.

   As per the terminology in RFC 2434 [4], the registration policy for
   new values for the 'content' parameter shall be 'Specification

11.  Acknowledges

   Arnoud van Wijk and Roni Even provided valuable ideas for this

12.  References

12.1.  Normative References

   [1]  Handley, M., "SDP: Session Description Protocol",
        draft-ietf-mmusic-sdp-new-24 (work in progress), February 2005.

   [2]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax

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        Specifications: ABNF", RFC 2234, November 1997.

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

   [4]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

12.2.  Informational References

   [5]   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.

   [6]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

   [7]   Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
         (S/MIME) Version 3.1 Message Specification", RFC 3851,
         July 2004.

   [8]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [9]   Levin, O. and G. Camarillo, "The SDP (Session Description
         Protocol) Label Attribute",
         draft-ietf-mmusic-sdp-media-label-01 (work in progress),
         January 2005.

   [10]  Michel, T. and J. Ayars, "Synchronized Multimedia Integration
         Language (SMIL 2.0) - [Second Edition]", W3C REC REC-SMIL2-
         20050107, January 2005.

   [11]  ITU-T, "Infrastructure of audiovisual services - Systems
         aspects; Role management and additional media channels for
         H.300-series terminals", Series H H.239, July 2003.

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Authors' Addresses

   Jani Hautakorpi
   Hirsalantie 11
   Jorvas  02420


   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420


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