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Versions: 00 01 02 03 04 05 06 rfc4796                   Standards Track
MMUSIC Working Group                                       J. Hautakorpi
Internet-Draft                                              G. Camarillo
Intended status: Standards Track                                Ericsson
Expires: March 26, 2007                               September 22, 2006

        The SDP (Session Description Protocol) Content Attribute

Status of this Memo

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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.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Operation with SMIL  . . . . . . . . . . . . . . . . . . . . .  8
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     12.1.  Normative References  . . . . . . . . . . . . . . . . . . 10
     12.2.  Informational References  . . . . . . . . . . . . . . . . 10
   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 their 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.

   The main purpose of this specification is to allow applications to
   take automated actions based on the 'content' attributes.  However,
   this specification does not define those actions.  Consequently, two
   implementations can behave completely differently when receiving the
   same 'content' attribute.

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 [10] 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

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   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 an external document.
   Therefore, this kind of mechanism is only applicable to special use
   cases where such external documents are used (e.g., centralized

   Yet another way to attach semantics to a media stream is to use 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 an appropriate manner.

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

                      Figure 1: Application's screen

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   The Figure 1 presents a screen of a typical communication
   application.  The 'content' attribute makes it possible for the
   application to decide 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 a new media session starts.

   The 'content' attribute could also be used in more complex
   situations.  An example of such a complex situation is an application
   controlling equipment in an auditorium.  An auditorium can have many
   different output channels for video (e.g., main screen and two
   smaller screens) and audio (e.g., main speakers, headsets for the
   participants).  In this kind of environment, a lot of interaction
   from the end user who operates the application would be required in
   absence of cues from a controlling application.  The 'content'
   attribute would make it possible, for example, for an end user needs
   to specify, only once, which output each media stream of a given
   session should use.  The application could automatically apply the
   same media layout for subsequent sessions.  So, the 'content'
   attribute can help to reduce the amount of required end user
   interaction considerably.

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" / mediacnt-ext
       mediacnt-ext        = token

   The 'content' attribute contains a token, which MAY be attached to a
   media stream by a sending application.  An application MAY attach a
   content attribute to any media stream it describes.  That attribute
   contains one or more tokens describing the content of the transmitted
   media stream to the receiving application.

   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:

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   slides:  the media stream includes presentation slides.  The media
      type can be, for example, a video stream or a number of instant
      messages with pictures.  Typical use cases for this are online
      seminars and courses.  This is similar to the 'presentation' role
      in H.239 [12].

   speaker:  the media stream contains the image of the speaker.  The
      media can be, for example, a video stream or a still image.
      Typical use case for this are online seminars and courses.

   sl:  the media stream contains sign language.  A typical use case for
      this is an audio stream that is translated into sign language,
      which is sent over a video stream.

   main:  the media stream is taken from the main source.  A typical use
      case for this is a concert where the camera is shooting the

   alt:  the media stream is taken from the alternative source.  A
      typical use case for this is an event where the ambient sound is
      separated from the main sound.  The alternative audio stream could
      be, for example, the sound of a jungle.  Another example is the
      video of a conference room while the main stream carries the video
      of the speaker.  This is similar to the 'live' role in H.239.

   All 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 just informative at the offer/answer model [8]
   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.  Furthermore, the values of
   'content' attributes does not need to match in an offer and an

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   The 'content' attribute can also be used in scenarios where SDP is
   used in a declarative style.  For example, 'content' attributes can
   be used in SDP session descriptors that are distributed with Session
   Announcement Protocol (SAP) [9].

7.  Examples

   There are two examples in this section.  The first example, shown
   below, uses a single 'content' attribute value per media stream:

       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

   The second example, below, shows a case where there is more than one
   'content' attribute value per media stream.  The difference with the
   previous example is that now the conferencing system might
   automatically mix the video streams from the presenter and slides:

       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 54132 RTP/AVP 31
       a=rtpmap:31 H261/9000

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8.  Operation with SMIL

   The values of 'content' attribute, defined in Section 5, can also be
   used with SMIL [11].  SMIL contains a 'param' element, which is used
   for describing the content of a media flow.  However, this 'param'
   element, like 'content' attribute, provides application specific
   description of media content.

   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.  Depending on how an
   implementation chooses to react to the presence or absence of a given
   'content' attribute, 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 descriptions.

   Integrity protection can be provided for session description carried
   in SIP [5] e.g., by using S/MIME [6] or Transport Layer Security
   (TLS) [7].

   It is assumed that values of 'content' attribute do not contain data
   that would be truly harmful if it is exposed to an possible attacker.
   It must be noted that the initial set of values does not contain any
   data that would require confidentiality protection.  However, S/MIME
   and TLS can be used to protect confidentiality, if needed.

10.  IANA Considerations

   This document defines a new 'content' attribute for SDP.  It also
   defines an initial set of values for it.  Some general information
   regarding 'content' attribute is presented in the following:

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   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",
                             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 initial values for the subregistry are presented in
   the following, and IANA is requested to add them into its database:

   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

   Note for the RFC Editor: 'RFC xxxx' above should be replaced by 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

   If new values for 'content' attribute are specified in the future,
   they should consist of a meta description of the contents of a media
   stream.  New values for 'content' attribute should not describe
   things like what to do in order to handle a stream.

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.

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12.  References

12.1.  Normative References

   [1]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
        Description Protocol", RFC 4566, July 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]   Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
         Protocol Version 1.1", RFC 4346, April 2006.

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

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

   [10]  Levin, O. and G. Camarillo, "The Session Description Protocol
         (SDP) Label Attribute", RFC 4574, August 2006.

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

   [12]  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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