MMUSIC Working Group J. Hautakorpi
Internet-Draft G. Camarillo
Expires: September 7, 2006 Ericsson
March 6, 2006
The SDP (Session Description Protocol) Content Attribute
draft-ietf-mmusic-sdp-media-content-02.txt
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Copyright (C) The Internet Society (2006).
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. 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. Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . . 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
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 [5].
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. 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 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 backwards
compatibility. Existing implementation of the 'label' attribute
already use values from that unrestricted namespace in an application
specific way. So, it is not possible to reserve parts from the
'label' attributes' namespace without possible conflict with an
already used, application specific labels.
It is possible to assign semantics to a media stream with the
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, for example centralized conferencing.
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Yet another way to attach semantics to a media stream is by using the
'i' SDP attribute, defined in RFC 2327 [6]. Nevertheless, 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
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.
+--------------------------------------+
|+------------++----------------------+|
|| || ||
|| 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.
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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.
5. 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-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. Many
'content' attribute values MAY be attached to a single media stream.
There is a set of 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. This is similar to the 'presentation' role
in H.239 [12].
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. Typical use case for this is the
one where the audio stream is translated into sign language.
main: This means that the media stream is taken from the main source.
Typical use case for this is e.g., a concert, where the camera is
shooting the performer.
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alt: This means that the media stream is taken from the alternative
source. Typical use case for the is e.g., an event, where there
is a separate ambient sound and the main sound. The alternative
audio stream could be e.g., a 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, and where a human needs to figure out
whether an act of floor control, e.g., saying 'over', is needed or
not. Typical use case for this is e.g., 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. Typical use case for this is e.g., 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
normally needs to apply formal turntaking habits, and need to
figure out to what extent it is possible to interrupt the other
part if needs arise.
All of the values can be used with any media type. Application can
make decision on how to handle a single media stream based on both
media type and the value of '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 [8]
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 end user interaction in
the receiving side maybe required.
At the offer/answer level, it means that the fact that an offer does
not contain 'content' attributes does not imply that the answer
should not have them. It also means that the fact that an offer
contains 'content' attributes does not imply that the answer should
have them too.
The 'content' attribute can also be used in scenarios where SDP is
used in declarative style. For example, 'content' attributes can be
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used in such SDP session descriptors that are distributed with
Session Announcement Protocol (SAP) [9].
7. 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
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. 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 a non-desirable way. So, it is strongly
RECOMMENDED that integrity protection be applied to the SDP session
descriptions. For session descriptions carried in SIP [5], S/MIME
[7] is the natural choice to provide such end-to-end integrity
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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 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: 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", "alt",
"user-floor", "txp", and a token.
The IANA is requested to create a subregistry for the '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
Required'.
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11. Acknowledges
Arnoud van Wijk and Roni Even provided valuable ideas for this
document.
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] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[7] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851,
July 2004.
[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 SDP (Session Description
Protocol) Label Attribute",
draft-ietf-mmusic-sdp-media-label-01 (work in progress),
January 2005.
[11] Michel, T. and J. Ayars, "Synchronized Multimedia Integration
Language (SMIL 2.0) - [Second Edition]", W3C REC REC-SMIL2-
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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
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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