MMUSIC Working Group J. Hautakorpi
Internet-Draft G. Camarillo
Expires: January 1, 2007 Ericsson
June 30, 2006
The SDP (Session Description Protocol) Content Attribute
draft-ietf-mmusic-sdp-media-content-04.txt
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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 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 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 [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,
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 [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
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, 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
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
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(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
[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. 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, human users have to take care of the
floor control. The user interface of the receiving application
must indicate the need for user level floor control to the human
user.
A typical use case is a situation where the other endpoint of the
connection is a walkie-talkie type of device, and human users must
say 'over' each time they stop transmitting.
txp: This indicates that the media stream is originated from a
textphone that is incapable of handling simultaneous two-way
communication. This limitation requires special behavior from the
user of a terminal receiving the indication. The indication
should be used for an indication in the user interface.
A typical use case 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 user of the IP terminal need to
change behaviour when this indication is received, and apply
formal turn-taking habits. They may also 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
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'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.
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. Examples
There are two examples in this section. The first example, shown
below, uses only one '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, shows a case where there is more than one
'content' attribute value per media stream. The difference to the
previous example is that now the conferencing system automatically
mixes the video streams from the presenter and slides:
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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 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
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. 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",
"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 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
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'.
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
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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.
[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
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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