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rfc:rfc4796

Network Working Group J. Hautakorpi Request for Comments: 4796 G. Camarillo Category: Standards Track Ericsson

                                                         February 2007
      The Session Description Protocol (SDP) Content Attribute

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2007).

Abstract

 This document defines a new Session Description Protocol (SDP) media-
 level attribute, 'content'.  The 'content' attribute defines the
 content of the media stream to a 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 or small screen) based on its content.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
 3.  Related Techniques . . . . . . . . . . . . . . . . . . . . . .  2
 4.  Motivation for the New Content Attribute . . . . . . . . . . .  3
 5.  The Content Attribute  . . . . . . . . . . . . . . . . . . . .  4
 6.  The Content Attribute in the Offer/Answer Model  . . . . . . .  5
 7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
 8.  Operation with SMIL  . . . . . . . . . . . . . . . . . . . . .  7
 9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
 10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
 12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   12.1.  Normative References  . . . . . . . . . . . . . . . . . .  9
   12.2.  Informational References  . . . . . . . . . . . . . . . .  9

Hautakorpi & Camarillo Standards Track [Page 1] RFC 4796 Content Attribute February 2007

1. Introduction

 The Session Description Protocol (SDP) [1] is a protocol that is
 intended to describe 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
 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",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
 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 namespace 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.

Hautakorpi & Camarillo Standards Track [Page 2] RFC 4796 Content Attribute February 2007

 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
 conferencing).
 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 for describing 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 streams needs, in some cases, to know what the
 contents of those streams are.  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
 Figure 1 shows 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

Hautakorpi & Camarillo Standards Track [Page 3] RFC 4796 Content Attribute February 2007

 is desirable that the user does not need to arrange each 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 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 and 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 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
 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 ABNF
 (Augmented Backus-Naur Form) [2]:
     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 one or more tokens, 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.
 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:  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].

Hautakorpi & Camarillo Standards Track [Page 4] RFC 4796 Content Attribute February 2007

 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 cases 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
    performer.
 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.  We chose not to
 restrict each value to a particular set of media types in order not
 to prevent applications from using innovative combinations of a given
 value with different media types.
 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.  If the application does not implement any special logic
 for the handling of a given media type and 'content' value
 combination, it applies the application's default handling for the
 media type.
 Note that the same 'content' attribute value can occur more than once
 in a single session description.

6. The Content Attribute in the Offer/Answer Model

 This specification does not define a means to discover whether 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.

Hautakorpi & Camarillo Standards Track [Page 5] RFC 4796 Content Attribute February 2007

 The 'content' attribute describes the data that the application
 generating the SDP session description intends to send over a
 particular media stream.  The 'content' values for both directions of
 a media stream do not need to be the same.  Therefore, 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 do not need to match in an offer and an answer.
 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:
     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
 The second example, below, is 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:

Hautakorpi & Camarillo Standards Track [Page 6] RFC 4796 Content Attribute February 2007

     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,speaker
     m=video 54132 RTP/AVP 31
     a=rtpmap:31 H261/9000
     a=content:sl

8. Operation with SMIL

 The values of 'content' attribute, defined in Section 5, can also be
 used with Synchronized Multimedia Integration Language (SMIL) [11].
 SMIL contains a 'param' element, which is used for describing the
 content of a media flow.  However, this 'param' element, like the
 'content' attribute, provides an application-specific description of
 the 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; therefore, it is strongly RECOMMENDED that
 integrity protection be applied to the SDP session descriptions.
 Integrity protection can be provided for a session description
 carried in an 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 a 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.

Hautakorpi & Camarillo Standards Track [Page 7] RFC 4796 Content Attribute February 2007

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 the 'content' attribute is presented in the following:
 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 attribute values: "slides", "speaker", "sl", "main", "alt",
    and any other registered values.
 The IANA created a subregistry for 'content' attribute values under
 the Session Description Protocol (SDP) Parameters registry.  The
 initial values for the subregistry are as follows:
 Value of 'content' attribute Reference Description
 ---------------------------- --------- -----------
 slides                       RFC 4796  Presentation slides
 speaker                      RFC 4796  Image from the speaker
 sl                           RFC 4796  Sign language
 main                         RFC 4796  Main media stream
 alt                          RFC 4796  Alternative media stream
 As per the terminology in RFC 2434 [4], the registration policy for
 new values for the 'content' parameter shall be 'Specification
 Required'.
 If new values for 'content' attributes are specified in the future,
 they should consist of a meta description of the contents of a media
 stream.  New values for 'content' attributes should not describe
 things like what to do in order to handle a stream.

11. Acknowledgements

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

Hautakorpi & Camarillo Standards Track [Page 8] RFC 4796 Content Attribute February 2007

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 4234, October 2005.
 [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]", World Wide Web
       Consortium Recommendation REC-SMIL2-20050107, January 2005,
       <http://www.w3.org/TR/2005/REC-SMIL2-20050107>.
 [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.

Hautakorpi & Camarillo Standards Track [Page 9] RFC 4796 Content Attribute February 2007

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

Hautakorpi & Camarillo Standards Track [Page 10] RFC 4796 Content Attribute February 2007

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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Hautakorpi & Camarillo Standards Track [Page 11]

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