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Versions: 00 01 02 03 04 05 06 rfc4796                                  
MMUSIC Working Group                                       J. Hautakorpi
Internet-Draft                                              G. Camarillo
Expires: October 13, 2006                                       Ericsson
                                                          April 11, 2006

        The SDP (Session Description Protocol) Content Attribute

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   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 its content.

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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  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Operation with SMIL  . . . . . . . . . . . . . . . . . . . . .  7
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     12.1.  Normative References  . . . . . . . . . . . . . . . . . .  9
     12.2.  Informational References  . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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1.  Introduction

   The Session Description Protocol (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 the Session Initiation Protocol
   (SIP) [5].

   There are situations where one application receives several similar
   media streams which are described in an SDP session description.  The
   media streams can be similar in the sense that their content cannot
   be distinguished 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 need for backward
   compatibility.  Existing implementations of the 'label' attribute
   already use values from that unrestricted namespace in an application
   specific way.  So it is not possible to reserve portions of the
   'label' attribute's namespace without possible conflict with already
   used, application specific labels.

   It is possible to assign semantics to a media stream with an 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,

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   for example centralized conferencing.

   Yet another way to attach semantics to a media stream is by using the
   'i' SDP attribute, defined in [1].  However, 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, the speaker's 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 an 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

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   situations.  This kind of complex situation could be e.g., an
   application that is controlling the equipment in an auditorium.  An
   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 in absence of cues from a controlling application.
   So, the possibility for such an application to handle the media
   stream without end users' input is highly desirable.

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" / "txp"
                             / 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.  Multiple
   'content' attribute values MAY be attached to a single media stream.

   This document provides a set of pre-defined values for the 'content'
   attribute.  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.  A 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.  A typical use case for this is one
      where the audio stream is translated into sign language.

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   main: This means that the media stream is taken from the main source.
      A typical use case for this is a concert, where the camera is
      shooting the performer.
   alt: This means that the media stream is taken from the alternative
      source.  A typical use case for this is an event, where there is a
      separate ambient sound and the main sound.  The alternative audio
      stream could be e.g., the 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, where a human needs to figure out
      whether an act of floor control, e.g., saying 'over', is needed or
      not.  A typical use case for this is a situation where the other
      endpoint of the connection is a walkie-talkie type of device.
   txp: This indicates that the media stream is originated from a
      textphone, and it requires special behavior from the receiving
      application.  A typical use case for this is a connection where
      one endpoint is an analog textphone of a kind that cannot handle
      two-way simultaneous text communication, and the other one is a
      native IP based real time text capable terminal.  The human users
      normally need to apply formal turn-taking habits, and need to
      figure out to what extent it is possible to interrupt the other
      party if the need arises.

   All of these values can be used with any media type.  The application
   can make decisions on how to handle a single media stream based on
   both the media type and the value of the '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 [7]
   level.  The fact that the peer endpoint does not understand the
   'content' attribute does not keep the media session from being
   established.  The only consequence is that end user interaction on
   the receiving side may be required to direct the individual media
   streams appropriately.

   Since the 'content' attribute does not have to be understood, an SDP
   answer MAY contain 'content' attributes even if none were present in
   the offer.  Similarly, the answer MAY contain no 'content' attributes
   even if they were present in the offer.

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   The 'content' attribute can also be used in scenarios where SDP is
   used in declarative style.  For example, 'content' attributes can be
   used in SDP session descriptors that are distributed with Session
   Announcement Protocol (SAP) [8].

7.  Example

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

       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 flow.  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 way.

   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 an undesirable way.  So, it is strongly
   RECOMMENDED that integrity protection be applied to the SDP session

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   descriptions.  For session descriptions carried in SIP [5], S/MIME
   [6] 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 an initial set of values for it.

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

   Attribute name:      'content'.

   Type of attribute    Media level.

   Subject to charset:  No.

   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", "txp", and any other
                             registered values.

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

   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.
   txp                          RFC xxxx  Media from a textphone

   Note for the RFC Editor: The 'RFC xxxx' in the above should be a
   reference to the coming RFC number of this draft.

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

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11.  Acknowledgements

   Authors would like to thank Arnoud van Wijk and Roni Even, who
   provided valuable ideas for this document.  We wish to thank also Tom
   Taylor for a thorough review.

12.  References

12.1.  Normative References

   [1]  Handley, M., "SDP: Session Description Protocol",
        draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.

   [2]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
        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]   Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
         (S/MIME) Version 3.1 Message Specification", RFC 3851,
         July 2004.

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

   [8]   Handley, M., Perkins, C., and E. Whelan, "Session Announcement
         Protocol", RFC 2974, October 2000.

   [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.

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   [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

   Email: Jani.Hautakorpi@ericsson.com

   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420

   Email: Gonzalo.Camarillo@ericsson.com

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