RFC 4796 - The Session Description Protocol (SDP) Content Attrib

Network Working Group J. Hautakorpi

Request for Comments: 4796 G. Camarillo

Category: Standards Track Ericsson RFC

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.
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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

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1. Introduction

The Session Description Protocol (SDP) [1] is a protocol that is
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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].
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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,
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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. RFC

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
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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.

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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.

+--------------------------------------+
RFC

|+------------++----------------------+|

|| || ||

|| speaker's || ||

|| image || ||

|| || ||
|+------------+| presentation ||
|+------------+| slides ||

|| || ||

|| sign || ||

|| language || || RFC

|| || ||

|+------------++----------------------+|

+--------------------------------------+

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

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is desirable that the user does not need to arrange each media stream
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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
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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" RFC文档

/ "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
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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
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in H.239 [12].

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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
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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
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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
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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.
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The only consequence is that end user interaction on the receiving

side may be required to direct the individual media streams

appropriately.

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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
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'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 RFC文档

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
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previous example is that now the conferencing system might

automatically mix the video streams from the presenter and slides:

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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
RFC

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.
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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
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'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
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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.
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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.

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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 .

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 www.cnpaf.net

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.

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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
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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
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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:
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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
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Language (SMIL 2.0) - [Second Edition]", World Wide Web

Consortium Recommendation REC-SMIL2-20050107, January 2005,

.

[12] ITU-T, "Infrastructure of audiovisual services - Systems
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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
RFC

Hirsalantie 11

Jorvas 02420

Finland

EMail: Gonzalo.Camarillo@ericsson.com