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MMUSIC Working Group                                       J. Hautakorpi
Internet-Draft                                              G. Camarillo
Expires: January 21, 2006                                       Ericsson
                                                           July 20, 2005


        The SDP (Session Description Protocol) Content Attribute
            draft-hautakorpi-mmusic-sdp-media-content-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   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
   content.



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Table of Contents

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



































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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 [3].

   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.  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
   contains.

   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.















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                +--------------------------------------+
                |+------------++----------------------+|
                ||            ||                      ||
                || 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.
   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.

3.  The Content Attribute

   This specification defines a new media-level value attribute,
   'content'.  Its formatting in SDP is described by the following BNF
   [2]:


       content-attribute   = "a=content:" mediacnt
       mediacnt            = "slides" / "speaker" / "sl" / "main-video"
                             / "alt-video" / "main-audio" / "alt-audio"
                             / mediacnt-ext
       mediacnt-ext        = token

   The 'content' attribute contains a token, which is attached to a
   media stream by a sending application.  It describes the content of



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   the trasmitted media stream to the receiving application.

   There are six different 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.
   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 videa stream.  Typical use case for this is the
      one where the audio stream is translated into sign language.
   main-video: This means that the video stream is taken from the main
      camera.  Typical use case for this is e.g., a concert, where the
      camera is shooting the performer.  This is similar to
      'presentation' role in H.239 [6].
   alt-video: This means that the video stream is taken from the
      alternative camera.  Typical use case for this is e.g., a concert,
      where the camera is shooting the crowd.  This is similar to 'live'
      role in H.239.
   main-audio: This is the main audio stream.  Typically this can be
      used in situations, where there is a separate ambient sound and
      the main sound.  The main sound could be e.g., a sound of a rare
      bird.
   alt-audio: This is the alternative audio stream.  Typically this can
      be used in situations, where there is a separate ambient sound and
      the main sound.  The alternative sound could be e.g., a sound of a
      jungle.

   The media used with the first two pre-defined values 'slides' and
   'speaker' can be in many different kinds of formats.  The following
   three pre-defined values 'sl', 'main-video' and 'alt-video' can be
   used only on the context of video streams.  The last two pre-defined
   values 'main-audio' and 'alt-audio' can be used only with audio
   stream.

   Open issue: Can multiple 'content' attributes, or attribute values,
   be defines for a single media stream?

4.  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 [4]



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   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.

5.  Example

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


       v=0
       o=Alice 292742730 29277831 IN IP4 131.163.72.4
       s=Second lecture from information technology
       c=IN IP4 131.164.74.2
       t=0 0
       m=video 52886 RTP/AVP 31
       a=rtpmap:31 H261/9000
       a=content:slides
       m=video 53334 RTP/AVP 31
       a=rtpmap:31 H261/9000
       a=content:speaker
       m=video 54132 RTP/AVP 31
       a=rtpmap:31 H261/9000
       a=content:sl


6.  Operation with SMIL

   The values of 'content' attribute, defined in Section 3, can also be
   used with SMIL [5].  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 'content' attribute, this 'param' element can
   also be used to describe the media content in globally interpretable
   way.

   Details on how to use the values of 'content' attribute with SMIL's
   'param' element are excluded from this document.

7.  Security Considerations

   TBD.

8.  IANA Considerations

   This document defines a new 'content' attribute for SDP.  It also
   defines seven possible values for it.



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   Contact name:        Jani Hautakorpi Jani.Hautakorpi@ericsson.com.

   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-video",
                             "alt-video", "main-audio", "alt-audio",
                             and a token.

   Entries to the registry:


   Value of 'content' attribute  Reference  Description
   ----------------------------  ---------  -----------
   slides                        RFC xxxx   Presentation slides
   speaker                       RFC xxxx   Image from the speaker
   sl                            RFC xxxx   Sign language
   main-video                    RFC xxxx   Main media stream
   alt-video                     RFC xxxx   Alternative media stream
   main-audio                    RFC xxxx   Main audio stream
   alt-audio                     RFC xxxx   Alternative audio stream

   Open issue: Which policy are we going to use for defining new
   extension tokens?

9.  Acknowledges

   Arnoud van Wijk and Roni Even provided valuable ideas for this
   document.

10.  References

10.1  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
        Specifications: ABNF", RFC 2234, November 1997.






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10.2  Informational References

   [3]  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.

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

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

   [6]  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.


Authors' Addresses

   Jani Hautakorpi
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Jani.Hautakorpi@ericsson.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com















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