Internet DRAFT - draft-ietf-mmusic-sdp-media-content
draft-ietf-mmusic-sdp-media-content
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
draft-ietf-mmusic-sdp-media-content-06.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 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",
"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 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
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 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 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
[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 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
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. 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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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, 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:
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
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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
Required'.
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
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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