SIPPING WG R. Even
Internet-Draft Polycom
Expires: August 24, 2003 O. Levin
RADVISION
N. Ismail
Cisco Systems, Inc.
February 23, 2003
Conferencing media policy requirements
draft-even-sipping-media-policy-requirements- 00.txt
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Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines the data model and the requirements for Media
Policy, i.e. a set of rules associated with the media distribution
of the conference. This document also presents the requirements for
the media manipulations that can be done using these rules by
conference participants or third parties using any kind of media/
conference policy control protocol. This document does not address
the interface between the focus and the media policy.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Rational . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. High Level Architecture . . . . . . . . . . . . . . . . . . . 3
5. Media Policy Data Model . . . . . . . . . . . . . . . . . . . 5
5.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.2 Mixer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3 Mixer block examples . . . . . . . . . . . . . . . . . . . . . 7
5.4 Video layout . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Media Policy Requirements . . . . . . . . . . . . . . . . . . 10
6.1 Genera Media Policy Requirements . . . . . . . . . . . . . . . 10
6.2 Video specific requirements . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Appendix I - Media Policy Operations . . . . . . . . . . . . . 12
8.1 Stream selection operations . . . . . . . . . . . . . . . . . 12
8.2 Mixing Operation . . . . . . . . . . . . . . . . . . . . . . . 13
8.3 Well known streams . . . . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
The Conferencing Framework [2] presents an overall framework and
defines the terminology for SIP [1] tightly coupled conferencing.
The conferencing framework architecture includes the media policy.
This is a set of rules that describes the media distribution of a
conference. This document presents the requirements for the media
policy data model and for the manipulations on these rules by
conference participants or third parties using any kind of media/
conference policy control protocol. This document does not address
the interface between the focus and the media policy and between the
focus and the media mixer.
2. Rational
The media policy enables a conference participant or an application
server to define and manipulate the content of the media streams
going to the conference participants. This will enable applications
like sidebars, announcement to specific participants, call centers
and panel conferences.
3. Terminology
The draft relies on the terminology defined in the conferencing
framework document[2].
4. High Level Architecture
The basic conferencing architecture used in this document is defined
in the Conferencing architecture framework [2]. This document
focuses on the media policy component and the requirements to
manipulate the media policy by authorized entities.
An authorized entity can manipulate the media policy using a supplied
application. Examples for such applications include a web
application, an interactive voice response application, an
interactive Instant Messaging (IM) base application, or an
application that uses the media policy control protocol.
The Conference policy control protocol (CPCP) provides a standard way
for an automated authorized entity to manipulate the media policy.
The requirements and definition of the CPCP protocol are out of scope
of this document.
The media policy is a set of rules that describes the media mixing or
switching required for each participant in the conference. This
includes the set of sources to be mixed or switched and the rules for
their mixing or switching. The focus uses the media policy to
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determine the proper configuration of the mixers. Authorized
entities will be notified of changes to the media policy by
subscribing to the conference event package. The information about
the current contributing sources to the mixed streams can be learned
by the information in the RTP header or by the conference event
package [4]. The data structures that include the contributing
sources of the current streams is in the focus or the mixer and is
not in the scope of the work.
The initial state of the media policy data structure is defined at
the conference creation time. It can be either provisioned or
created by using a conference policy control protocol or/and other
protocols being used to create the conference.
Typically, a focus has access to the media policy and is responsible
for translating the media policy data into the actions towards the
physical entities ("mixers").
Figure 1 describes an instance of media policy of a conference. The
figure shows a single mixer and a single type of stream for ease of
drawing but the model does not have such a restriction.
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Conference .
Policy . +-----------+ //-----\\ .
Control . | | || || .
Protocol . | Conference| \\-----// .
+------------->| Policy | | | .
| . | Server |----> |Conference .
| . | | | | .
| . +-----------+ | & | .
| . | | .
| . | Media | .
+------------+ . +-----------+ | Policy| .
| +----------++ . | | \ // .
| | || . | | \-----/ .
|Participants||<-------->| Focus | | .
| | || SIP . | | | .
| | || Dialog . | |<-----------+ .
+-+----------++ . +-----------+ .
+-----------+ . | .
^ | . | .
| | Contributing . | .
| | Streams . +-----------+ .
| +------------->. | | .
| Distributed . | Mixer | .
| Streams . | | .
+-------------------. +-----------+ .
.....................................
Conference
Functions
Figure 1: Media Policy in a Conference
5. Media Policy Data Model
5.1 General
The fundamental conferencing functionality is being able to combine
(i.e. "to mix") in a media specific manner participants' streams
that belong to a logical sub-function within a conference (such as
participant's video, left audio stream, right audio stream, video
streaming presentation, slide presentation) and are of the same media
type (such as video, audio, etc.). In the case of using
centralized-mixing the resultant stream(s) will be sent back to the
participants. In the case of end-point mixing, the original streams,
needed to produce the mixed media, will be distributed to the
participants that will perform the actual mixing.
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Typically, the maximum number of different mixers in a conference is
preconfigured as part of the media conference policy. Mixers MAY be
dynamically created and destroyed during the conference lifetime.
Conferencing mixing scenarios include simple conferences that have
well known mixing characteristics (e.g. mix audio of N loudest
speakers) as well as conferences that enable the users of the media
policy to create custom mixes (e.g. Mix the audio of John and Mary)
An example of an audio mixer is shown in Figure 2 (Wide lines specify
a set of a streams). The audio mixer in the example is for a simple
audio conference that mixes the audio of three loudest speakers. The
speakers will not hear themselves. The mixer will choose the three
loudest speakers from all the conference participants and will mix
them. The output from the mixer will be four mixed audio streams.
The first one is the sum of all three loudest speakers, the other
three streams are the streams that will be sent to the speakers and
each of them will include the mix of the other two loudest
participants.
Main Audio
| | | |
v v v v
....................................
. +----------------------+ .
. | | .
Mixer | Loudest(3) | .
. | | .
. | | .
. +----------------------+ .
. ||| .
. ||| Loudout .
. v .
. +----------------------+ .
. | | .
. | MixMinus(3) | .
. | | .
. | | .
. +----------------------+ .
....................................
|||
||| MiXAudio
v
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Figure 2: simple audio conference
5.2 Mixer
The mixers are composed of logical blocks "mixer block" each mixer
block has input streams, output streams and is characterized by an
operation that describes the transformation from the input streams to
the output streams. The operations can instruct the mixer block to
perform a selection operation on the incoming streams, a mixing
function or a transformation on a stream.
The number of mixer blocks inside a mixer is dynamic. The output
from one block may serve as input to another block by using the
output stream descriptor as input to a mixer block. The solution
will try to reduce the number of blocks needed to define a conference
mix. This can be achieved by having rules that will enable a focus
to control a mixer based on minimal number of rules.
Each mixer has a set of input streams from the participants (called
contributing streams) and a set of output streams to the participants
(called distributed streams). For this purpose, a participant can be
a SIP UA , a media server (such as an announcement server), or a
recording system.
Each participant incoming (to the focus) stream MUST NOT be
associated with more than a single contributing stream of a single
mixer.
The number and the type of the contributing and distributed streams
is usually dynamic. During the conference lifetime, the input and
the output media stream attributes (such as bandwidth, rate, CODEC,
resolution, etc.) are being defined dynamically such as by means of
the SDP m- lines as a part of the Offer-Answer[3] mechanism.
5.3 Mixer block examples
The media policy enables the creation of different conferencing
scenarios. An example of a main audio conference and a sidebar is
shown in figure 3.
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MainAudioIn SidebarIn
| | | | | |
v v v v v v
+----------------------+ +--------+ +-----------+
| | +----> | In*0.4 | | |
| Loudest(3) | | | | |MixMinus(2)|
| | | | | | |
| | | +--------+ +-----------+
+----------------------+ | | |||
||| | SideBout | SideBout |||
||| Loudout | V V
v | +----------------------+
+----------------------+ | | |
| | |All | Add |
| MixMinus(3) | | | |
| | | +----------------------+
| | | |||
+----------------------+ | SideBmix |||
MainMiXAudio||| ||| | |||
||| \\\--- +-----------+ V
||| ----->|SelectMain |
v ----- +-----------+
Figure 3: Audio Conference with sidebar
The main conference is the same as in figure 2. The sidebar
conference includes two participants and the main conference audio at
a lower volume. The output from the sidebar includes two mixes each
with one of the participants and the main audio with the reduced
volume and there will be a stream that will include a mix of all
three streams.
The blocks of the main conference are showing the operations on the
conference incoming streams. The Loudest(3) block gets as input all
the contributing audio streams of the conference and selects the
three that represent the speakers with the highest volume. Those
streams are coming out as Loudout. The Loudout set of streams is an
input to the MixMinus(3) block. This block will be used by the focus
to tell the media mixer to create 4 output streams, one with all
three sources and three that are a mix of each two of the
participants. These are the distributed streams and the focus knows
who are the receivers of each of the streams. The SelectMain block
is used to select the main mix of the conference which include the
mix of all the three loudest participants, this stream will be an
input to the sidebar.
The sidebar has three blocks. The In*0.4 takes the input stream and
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attenuate it to 0.4 of its volume. The MixMinus block is the same as
the one in the main mixer but it mix only the two loudest speakers
and create three output streams. The Add block get as input all
streams called SideBout (from the previous two blocks) and mixes
them. Each output stream from the In block is mixed with each output
stream of the MixMinus block. This will create three mixed streams
that the focus can use to send to the sidebar participants.
The main conference can be describe as:
MainAudio ------Loudest(3)-----> Loudout
Loudout-------MixMinus(3)------>MainMixAudio
MainMixAudio------SelectMain------->All
The sidebar conference can be described as:
SidebarIn-----MixMinus(2)---->SideBout
All----------------0.4In------>SideBout
SideBout-----------Add------->SideBmix
The description include predefined operations which are: Loudest(N),
MixMinus(N),Main, In, Mix. Those operation and others will be
specified as part of the media rules definition. The CPCP will carry
those media policy rules to the conference policy server where they
will be stored as media policy.
5.4 Video layout
Video mixing has two general models.
The first one is video switching where the conference bridge sends
one of the incoming streams as is to the participants. An example
is voice activated video switching where all the participants see the
loudest speaker while the speaker sees the previous speaker.
The second one is known as continuous presence. In this mode the
video mixer builds a composite video that displays each contributing
stream in a sub-window. Example is a 2x2 display as in figure 4;
each sub-window has a number.
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+-----------------------+------------------------+
| | |
| 1 | 2 |
| | |
+-----------------------+------------------------+
| | |
| 3 | 4 |
| | |
+-----------------------+------------------------+
Figure 4: 2x2 video layout
There are well known window layout that can be specified by naming
convention to enable simple rules for mapping a video stream to a
window. We can add custom window definition support that will serve
the creation of more complicated layouts. The numbering of the
sub-windows will start from the top-left sub-window and from left to
right as in figure 4.
6. Media Policy Requirements
All the requirements are based on having a privilege mechanism that
authorizes users to access and manipulate the media policy data.
6.1 Genera Media Policy Requirements
REQ-GP1: A participant MUST be able to specify its own unique
topology.
REQ-GP2: It must be possible for a group of users to receive the same
mix. This mix may be a conference common mix.
REQ-GP3:It MUST be possible to dynamically modify the number of
contributing streams associated with a mixer.
REQ-GP4: It MUST be possible to define the mixing function for each
participant in the conference.
REQ-GP6: It SHOULD be possible to send a participant multiple streams
from one mixer. This requirement is to enable support for end- point
mixing.
REQ-GP7: It SHOULD be possible to define relationships between
different mixers. The relationships can be time synchronized such as
specifying that the audio mixer and video mixer is a pair to
establish lip-synchs.
REQ-GP8: It SHOULD be possible to define the number of different
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topologies and the number of streams in each of them that will be
mixed in a mixer. For example the conference will support only one
video topology that will go to all the participants, the video
topology will support 2x2 display, or each participant will be able
to receive his own audio topology that will include up to 4
contributing sources.
6.2 Video specific requirements
Video is a bit different than audio when mixing is concerned. In
multipoint video the common mixing modes are:
Video switching where one of the contributing sources is sent to all
participants, the video source may be forced by the media policy
control protocol or may be dynamic by using for example a voice
activated video switching mode where the participants will see the
loudest speaker.
"Continuous presence" or tiled windows display where the topology is
composing one video stream that has a layout defining the shape and
position of viewing windows that will be displayed to the
participants. The layout includes N viewing windows so that in each
of the windows there is one contributing stream. Even though the
viewing windows can be of any shape we will address in this work only
rectangular windows of any size. The windows may overlap.
The section defines the specific requirements for media policy and
media policy control to enable "Continuous presence"
REQ-V1: It should be possible to define rectangular overlapping
windows in a video mix.
REQ-V2: It should be possible to map a stream to a window based on
some mode like having one window display the loudest speaker or the
floor holder while for the remaining windows fixed input streams are
used.
REQ-V3: It should be possible for authorized participants to change
the layout of the video topology.
REQ-V4: It should be possible for authorized participants to define
the mapping of a stream to a window.
7. Security Considerations
The media policy control protocol may enables unauthorized users to
manipulate the media mixing of conferences, this may enable them to
listen to conference or eject unsolicited media streams. The
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protocol should provide authentication of the users. The media
policy data may include information about the sources and targets of
mixer, if this information will be transferred in the protocol in the
clear that may cause a security risk. The protocol should allow for
encryption of the media policy transferred in the media policy
control protocol.
8. Appendix I - Media Policy Operations
The examples in sections 5.2 and 5.3 included the mixer blocks and
their description. The appendix will try to list well known
operations that can serve to define building blocks. Those
operations will be part of the media policy syntax that is part of
the media policy document. The operation appear here for information
only.
Each mixer is built from mixer blocks. Each block defines an
operation on a set of streams, the operations can be a selection or a
mixing operation and the result is a set of streams (One or more)
depending on the operation. By collection a set of such operations
we define a mixer in the media policy. The input and output of the
operations are streams. The streams can be named by tagging
individual m-lines in their SDP with an associated i-line or by
specifying an alias in the contents of the current media policy
description. Operations for selecting specific streams from a
well-known group will also be defined.
The appendix will list the operation specifying the input and output
of each operation and will explain the semantics of the operation.
8.1 Stream selection operations
Loudest(N) - This operation will receive as input a set of audio
streams and will output N streams selected according to the volume
levels starting from the highest
LastLoudest - This operation will receive as input a set of audio
streams and will select the audio stream of the participant that was
the loudest speaker in the previous mix (This is the audio mix set
before the current one)
Not(Qualifier) - This operation will receive as input a set of audio
streams and will select a stream that does not include a stream based
on the Qualifier. The qualifier can be a stream name or alias or an
operation. Example Not(Loudest) will select the stream that does not
have the Loudest stream as one of its contributing sources.
SelectMain- the operation will select from a set of input stream the
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stream that represent the main mix that include all contributing
sources. The operation may be replaced by defining a well-known
stream definition call MainAudio.
In(transformation) - The operation will perform the operation on the
incoming stream or set of streams. An example is In(0.4) will
attenuate the incoming stream by 0.6
VofLoudest(N) - The operation will get as input a set of video
streams and will select the N video streams of the participants
according to their audio volume level.
AssignToWID(N) - This operation will assign the incoming video stream
to the subwindow N in a tiled video display.
8.2 Mixing Operation
MixMinus(N) - The operation will mix the N incoming streams and will
create N+1 output streams. The output streams will include one
stream that is a mix of all input streams and N different streams
that are a mix of all input streams without one of them. This is
very useful for audio conferences where participants do not want to
have their voice in the mix they receive from the conferencing
bridge.
Mix - The operation creates a single mix of all input sources. It
has one output stream.
Add - This operation gets two input sets of streams and add them to
create the added mix. For example if one set has 2 stream2 and the
second set has 3 streams the result will be 6 output streams. Each
of the combined streams will be a mix of a stream from the first set
with a stream from the second set.
VideoMix(Layout) - the operation will take the incoming video streams
and assign them according to their sub-window IDs to the layout. The
layout may be predefined by a well known name or custom defined. The
output stream will be the tiled video mix.
8.3 Well known streams
MainAudioIn- this is the set of all contributing sources to the main
audio conference.
LoudestSpeaker(N) - This is the stream of the Nth loudest speaker.
N=1 is the loudest.
VideoLayout(N) - This is a tiled video layout. N specifies the table
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entry number that describes the specific layout. A table of all
pre-defined video layout will be defined.
References
[1] 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.
[2] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol", draft-
rosenberg-sipping-conferencing-framework-01 (work in progress),
February 2003.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[4] Rosenberg, J. and H. Schulzrinne, "A session initiation
protocol (SIP) event package for conference state", , June 2002.
Authors' Addresses
Roni Even
Polycom
94 Derech Em Hamoshavot
Petach Tikva 49130
Israel
EMail: roni.even@polycom.co.il
Orit Levin
RADVISION
266 Harristown Road
Glen Rock
NJ USA
EMail: orit@radvision.com
Nermeen Ismail
Cisco Systems, Inc.
170 West Tasman Drive
San Jose 95134
CA USA
EMail: nismail@cisco.com
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