Internet DRAFT - draft-hautakorpi-mmusic-sdp-media-content
draft-hautakorpi-mmusic-sdp-media-content
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 (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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