wdiff draft-rosenberg-sipping-conferencing-framework-00.txt draft-rosenberg-sipping-conferencing-framework-01.txt

Internet Engineering Task Force SIPPING WG
Internet Draft J. Rosenberg
dynamicsoft
draft-rosenberg-sipping-conferencing-framework-00.txt
October 28, 2002
draft-rosenberg-sipping-conferencing-framework-01.txt
February 12, 2003
Expires: April August 2003

A Framework for Conferencing with the Session Initiation Protocol

STATUS OF THIS MEMO

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all provisions of Section 10 of RFC2026.

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Abstract

The Session Initiation Protocol (SIP) supports the initiation,
modification, and termination of media sessions between user agents.
These sessions are managed by SIP dialogs, which represent a SIP
relationship between a pair of user agents. Because dialogs are
between pairs of user agents, SIP's usage for two-party
communications (such as a phone call), is obvious. Communications
sessions with multiple participants, generally known as conferencing,
is
are more complicated. This document defines a framework for how such
conferencing can occur. This framework describes the overall
architecture, terminology, and protocol components needed for multi-
party conferencing.

Table of Contents

1 Introduction ........................................ 3 4
2 Terminology ......................................... 4
3
3 Basic Overview of Conferencing Architecture .................................. ............... 7
4
3.1 Usage of URIs ....................................... 11
5 10
4 Functions of the Elements ........................... 12
5.1
4.1 Focus ............................................... 12
5.2
4.2 Conference Policy Server ............................ 13
5.3
4.3 Mixers .............................................. 14
5.4 Media Policy Server ................................. 14
5.5
4.4 Conference Notification Service ..................... 15
5.6
4.5 Participants ........................................ 16
5.7 15
4.6 Conference Policy ................................... 15
5 Common Operations ................................... 16
5.1 Creating Conferences ................................ 16
5.1.1 SIP Mechanisms ...................................... 17
5.1.2 CPCP Mechanisms ..................................... 18
5.1.3 Non-Automated Mechanisms ............................ 18
5.2 Adding Participants ................................. 18
5.2.1 SIP Mechanisms ...................................... 18
5.2.2 CPCP Mechanisms ..................................... 18
5.2.3 Non-Automated Mechanisms ............................ 19
5.3 Conditional Joins ................................... 19
5.4 Removing Participants ............................... 19
5.4.1 SIP Mechanisms ...................................... 19
5.4.2 CPCP Mechanisms ..................................... 20
5.4.3 Non-Automated Mechanisms ............................ 20
5.5 Approving Policy Changes ............................ 20
5.6 Creating Sidebars ................................... 22
5.7 Destroying Conferences .............................. 23
5.7.1 SIP Mechanisms ...................................... 23
5.7.2 CPCP Mechanisms ..................................... 23
5.7.3 Non-Automated Mechanisms ............................ 23
5.8 Obtaining Membership ................................ 24
5.8.1 SIP Mechanisms ...................................... 24
5.8.2 CPCP Mechanisms ..................................... 24
5.8.3 Non-Automated Mechanisms ............................ 24
5.9 Adding and Removing Media Policy ........................................ 17 ........................... 24
5.9.1 SIP Mechanisms ...................................... 25
5.9.2 CPCP Mechanisms ..................................... 25
5.9.3 Non-Automated Mechanisms ............................ 25
5.10 Conference Announcements and Recordings ............. 25
5.11 Floor Control ....................................... 27
5.12 Camera and Video Controls ........................... 27
6 Physical Realization ................................ 17 28
6.1 Centralized Server .................................. 17 28
6.2 Endpoint Server ..................................... 17 28
6.3 Media Server Component .............................. 18 28
6.4 Distributed Mixing .................................. 21 31
6.5 Cascaded Mixers ..................................... 22 33
7 Common Operations ................................... 22
7.1 Creating Conferences ................................ 22
7.2 Adding Participants ................................. 25
7.3 Removing Participants ............................... 27
7.4 Approving Policy Changes ............................ 27
7.5 Creating Sidebars ................................... 28
8 Security Considerations ............................. 28
9 33
8 Contributors ........................................ 29 33
9 Changes since draft-rosenberg-sipping-
conferencing-framework-00 ...................................... 35
10 Authors Addresses ................................... 29 35
11 Normative References ................................ 29 35
12 Informative References .............................. 29 35

1 Introduction

The Session Initiation Protocol (SIP) [1] supports the initiation,
modification, and termination of media sessions between user agents.
These sessions are managed by SIP dialogs, which represent a SIP
relationship between a pair of user agents. Because dialogs are
between pairs of user agents, SIP's usage for two-party
communications (such as a phone call), is obvious. Communications
sessions with multiple participants, however, are more complicated.
SIP can support many models of multi-party communications. One,
referred to as loosely coupled conferences, makes use of multicast
media groups. In the loosely coupled model, there is no signaling
relationship between participants in the conference. There is no
central point of control or conference server. Participation is
gradually learned through control information that is passed as part
of the conference (using the Real Time Control Protocol (RTCP) [2],
for example). Loosely coupled conferences are easily supported in SIP
by using multicast addresses within its session descriptions.

In another model, referred to as fully distributed multiparty
conferencing, each participant maintains a signaling relationship
with each other participant, using SIP. There is no central point of
control; it is completely distributed amongst the participants. SIP
does not yet support This
model is outside the scope of this model. document.

In another model, sometimes referrred referred to as the tightly coupled
conference, there is a central point of control. Each participant
connects to this central point. It provides a variety of conference
functions, and may possibly perform media mixing functions as well.
Tightly coupled conferences are not directly addressed by the SIP
specification, RFC 3261,
although basic ones are participation is possible without any additional
protocol support.

This document is one of a series of specifications that discusses
tightly coupled conferences. Here, we present the overall framework
for tightly coupled conferencing, referred to simply as
"conferencing" from this point forward. This framework presents a
general architectural model for these conferences, presents
terminology used to discuss such conferences, and describes the sets
of protocols involved in a conference. The aim of the framework is to
meet the general requirements for conferencing that are outlined in
[3].

2 Terminology

Conference: Sadly, conference Conference is an overused term which has different
meanings in different contexts. In SIP, a conference is an
instance of a multi-party conversation. Within the context
of this specification, a conference is always a tightly
coupled conference.

Loosely Coupled Conference: A loosely coupled conference is a
conference without coordinated signaling relationships
amongst participants. Loosely coupled conferences
frequently use multicast for distribution of conference
memberships.

Tightly Coupled Conference: A tightly coupled conference is a
conference in which a single user agent, referred to as a
focus, maintains a dialog with each participant. The focus
plays the role of the centralized manager of the
conference, and is addressed by a conference URI.

Focus: The focus is a SIP user agent that is addressed by a
conference URI. URI and identifies a conference (recall that a
conference is a unique instance of a multi-party
conversation). The focus maintains a SIP signaling
relationship with each participant in the conference. The
focus is responsible for insuring, ensuring, in some way, that each
participant receives the media that make up the conference.
The focus also implements conference policies. The focus is
a logical role.

Conference URI: A URI, usually a SIP URI, which identifies the
focus of a conference.

Participants:

Participant: The set of user agents, each identified by software element that connects a URI,
which are connected user or
automata to the focus for a particular conference. It implements, at a minimum, a
SIP user agent, but may also include a conference policy
control protocol client, for example.

Conference Notification Service: A conference notification
service is a logical function provided by the focus. The
focus can act as a notifier [4], accepting subscriptions to
the conference state, and notifying subscribers about
changes to that state. The state includes the state
maintained by the focus itself, the conference policy, and
the media policy.

Conference Policy Server: A conference policy server is a
logical function which can store and manipulate the
conference policy. The conference policy is the overall set
of rules
associated with governing operation of the conference. It is
broken into membership policy and media policy. Unlike the
focus, there is not an instance of the conference policy
server for each conference. Rather, there is an instance of
the membership and media policies for each conference.

Conference Policy: The complete set of rules manipulated by the
conference policy server. It includes the membership policy
and the media policy.

Membership Policy: A set of rules manipulated by the conference
policy server regarding participation in a the conference.
These rules include directives on the lifespan of the
conference, who can and cannot join the conference,
definitions of roles available in the conference and the
responsibilities associated with those roles, and policies
on who is allowed to request which roles. The conference policy server is a
logical role.

Media Policy Server: Policy: A media set of rules manipulated by the conference
policy server is a logical function
which can store and manipulate rules associated with regarding the media distribution composition of the
conference. These The media policy is used by the focus to
determine the mixing characteristics for the conference.
The media policy includes rules can
specify about which participants
receive media from which other participants, and the ways
in which that media is combined for each participant. In
the case of audio, these rules can include the relative
volumes at which each participant is mixed. In the case of
video, these rules can indicate whether the video is tiled,
whether the video indicates the loudest speaker, and so on.

Conference Policy: The set of rules manipulated by the
conference policy server.

Conference Policy Control Protocol: Protocol (CPCP): The client-server protocol used by
clients to manipulate the conference policy.

Media Policy: The set of rules manipulated by the media policy
server. The media policy is used by the focus to determine
the mixing characteristics for the conference.

Media Policy Control Protocol: The client-server protocol used
by clients to manipulate the media policy.

Mixer: As defined in the Real Time Transport Protocol [2], a A mixer receives a set of media streams, streams of the same type,
and combines their media in a type-specific manner,
redistributing the result to each participant. We use This
includes media transported using RTP [2]. As a result, the
term defined here to include
combining is a superset of non-RTP the mixer concept
defined in RFC 1889, since it allows for non-RTP-based
media streams as well, such as instant messaging sessions [5].

Basic Conference: A basic conference is one where there is no
conference policy server, media policy server, or
conference subscription server - only a focus.

Basic

Conference-Unaware Participant: A basic conference-unaware participant
is a participant in a conference that is not aware that it
is actually in a conference. As far as the UA is concerned,
it is a point-
to-point point-to-point call.

Cascaded Conference: Conferencing: A conference mechanism for group communications in
which a participant is set of conferences are linked by having their
focuses interact in some fashion.

Simplex Cascaded Conferences: a group of conferences which are
linked such that the user agent which represents the focus
of another conference.

Complex Conference: A complex conference includes at least one
of a conference policy server, media policy server, or conference subscription server, is a conference-unaware participant in addition to the focus.

Complex
another conference.

Conference-Aware Participant: A complex conference-aware participant is
a participant in a conference that has learned, through
automated means, that it is in a conference, and that can
use a conference policy control protocol, media policy
control protocol, or conference subscription, to implement
advanced functionality.

Conference Server: A conference server is a physical server
which contains, at a minimum, the focus. It may also
include a media policy server, a conference policy server,
and a mixer.

Singleton: In this context, a singleton is a conference
participant that is not a focus. A singleton represents a
single user in a conference.

Conference Topology: The conference topology is a graph that
defines the connectivity amongst participants connected
through conferences. Each node in the graph represents a
user agent, whether it is a focus or a singleton. Each leaf
node in the tree represents an singleton, and an internal
node represents a focus. An edge between two nodes implies
that there is a SIP dialog between them. Ideally,
conference topologies are trees, not arbitrary graphs.

Conversation Space: For each conference URI, there is a unique
conversation space. The conversation space is defined as
the set of singleton in the conference topology associated
with that URI. The conference topology associated with a
conference URI is the one that is constructed by starting
with the focus for that URI. Under normal circumstances,
the set of singleton in a conversation space will all
receive each others media.

Instant Conference: A conference in which the focus is
constructed the instant the first INVITE for a URI is
received, server and then destroyed in which the last participant
has left. mixers.

Mass Invitation: A conference policy control protocol request to
invite a large number of users into the conference.

Mass Ejection: A conference policy control protocol request to
remove a large number of users from the conference.

Sidebar: A sidebar appears to the users within the sidebar as a
"conference within the conference". It is a dicsussion conversation
amongst a subset of the
participants, not heard by participants to which the remaining
participants in
the conference. are not privy.

Anonymous Participant: An anonymous participant is one that is
known to other participants (through through the conference
notification service), service, but whose identity is being withheld.

Invisible

Hidden Participant: An invisible A hidden participant is one that is not
known to other participants in the conference. They may be
known to the moderator, depending on conference policy.

3 Basic Architecture

A SIP conference is represented by a URI. This URI identifies the
focus, which is the user agent at the center Overview of the conference. Any
participant that is involved Conferencing Architecture

The central component (literally) in the a SIP conference is connected to the focus.
The focus by maintains a SIP dialog. signaling relationship with each
participant in the conference. The result is a star topology, shown
in Figure 1.

The focus has access to a conference policy and media policy, an
instance of which exist for each focus. In a basic SIP conference,
these policies are administratively defined.

Users join the conference by sending an INVITE to the conference URI.
As long as the conference policy allows, the INVITE is accepted by
the focus and the user is brought into the conference. Users can
leave the conference by sending a BYE, as they would in a normal
call. Indeed, a participant in a basic conference does not need to
know that the focus is anything other than a normal SIP user agent.

Similarly, the focus can terminate a dialog with a participant,
should the conference policy change to indicate that the participant
is no longer allowed in the conference. A focus can also initiate an
INVITE, should the conference policy indicate that the focus needs to
bring a participant into the conference.

The focus is responsible for making sure that the media streams which
constitute the conference are available to the participants in the
conference. It does that through the use of one or more mixers, each
of which combines a number of input media streams to produce one or
more output media streams. The focus uses the media policy to
determine the proper configuration of the mixers.

With these basic capabilities, a large number of common conferencing
applications can be built. None of them require any extensions to
SIP; they merely require that the focus is aware of its role and
responsibilities in maintaining the conference. However, basic
conferences do not allow for the participants to control the way in
which the conference operates.

+-----------+
| |
| |
|Participant|
| 4 |
| |
+-----------+
|
|SIP
|Dialog
|
|4
|
+-----------+ +-----------+ +-----------+
| | | | | |
| | | | | |
|Participant|-----------| Focus |------------|Participant|
| 1 | SIP | | SIP | 3 |
| | Dialog | | Dialog | |
+-----------+ 1 +-----------+ 3 +-----------+
|
|
|SIP
|Dialog
|
|2
|
+-----------+
| |
| |
|Participant|
| 2 |
| |
+-----------+

Figure 1: Basic SIP Conference

A complex SIP Architecture

The focus has access to the conference is one in policy (composed of the
membership and media policies), an instance of which additional interfaces are
exposed, allowing exist for each
conference. Effectively, the conference policy can be thought of as a richer set
database which describes the way that the conference should operate.
It is the responsibility of controls and information on the
conference. In particular, a complex focus to enforce those policies. Not
only does the focus need read access to the database, but it needs to
know when it has changed. Such changes might result in SIP signaling
(for example, the ejection of a user from the conference can include using BYE),
and most changes will require a notification to be sent to
subscribers using the conference notification service.

The conference is represented by a URI, which identifies the focus.
Each conference policy server has a unique focus and a media unique URI identifying that
focus. Requests to the conference URI are routed to the focus for
that specific conference.

Users usually join the conference by sending an INVITE to the
conference URI. As long as the conference policy server, allows, the INVITE
is accepted by the focus and the user is brought into the conference.
Users can leave the conference by sending a BYE, as they would in a
normal call.

Similarly, the focus can
expose terminate a dialog with a participant,
should the conference notification service. policy change to indicate that the participant
is no longer allowed in the conference. A focus can also initiate an
INVITE, should the conference policy indicate that the focus needs to
bring a participant into the conference.

The model for these
conferences notion of a conference-unaware participant is shown important in Figure 2. This figure shows this
framework. A conference-unaware participant does not even know that
the view from one
participant. UA it is communicating with happens to be a focus. As far as its
concerned, its a UA just like any other. The conference now encompasses an additional set focus, of
functions. In addition course, knows
that its a focus, and it performs the tasks needed for the conference
to maintaining operate.

Conference-unaware participants have access to a good deal of
functionality. They can join and leave conferences using SIP, and
obtain more advanced features through stimulus signaling, as
discussed in [6]. However, if the dialog with participant wishes to explicitly
control aspects of the focus, conference using functional signaling
protocols, the participant now must be conference-aware.

A conference-aware participant is one that has access to advanced
functionality through additional protocol interfaces. The client uses
these other functions. It can, using a protocols to interact with the conference event package [6], policy server and the
focus. A model for this interaction is shown in Figure 2. The
participant can interact with the focus using extensions, such as
REFER, in order to access enhanced call control functions [7]. The
participant can SUBSCRIBE to the conference URI, and be connected to
the conference notification service provided by the focus. Through
this package, mechanism, it can learn about changes in participants
(effectively, the state of the dialogs), the media policy, and the conference
membership policy.

The participant can also communicate with the conference policy
server, server
using a conference policy control protocol. This is a
strictly client-server transactional protocol. This protocol might
not be a protocol at all; it can be performed using a web interface.
In this case, no standardized protocols or policies are needed.
However, the web interface can only be manipulated by humans, not
automata. For this reason, the participant can use a protocol
designed specifically for Through this purpose.

The participant can also communicate with the media policy server,
using a media policy control protocol. This is a strictly client-
server transactional operation. This can also be through a web
interface, or through an explicit protocol.

The focus will access the media and conference policies. There is a
tight coupling between these policies and the focus. Not only does it
need read access to these policies, but protocol, it needs to know when they
have changed. Such changes might result in SIP signaling (for
example, the ejection of a user from the conference using BYE), and
most changes will require a notification to be sent to subscribers to
can affect the conference notification service. policy. The conference policy and media policy servers server need
not be available in any particular conference. Even when available, they need not be
used by all participants. A participant in a conference that does not
access any of these functions, and which doesn't even know that the
focus is a focus, is called a basic participant. A conference
participant that can discover and access these additional function conference, although there is
always a complex participant. Any conference can include basic and complex
participants. policy.

The interfaces between (1) the focus and the media policy, (2) the focus and the conference policy, (3) and the
conference policy server and the conference policy, and (4) the media policy server and the media
policy are not subject
to standardization at the time of this writing. They are intended
primarily to show the logical roles
Conference .....................................
Policy . +-----------+ .
Control . | | .
Protocol . |Participant| .
+------------------->| Policy | .
| . | Server | .
| . | | \ .
| Media . +-----------+ \ .
| Policy . +-----------+ \ //-----\\ .
| Control . | | > || || .
| Protocol . | Media | \\-----// .
| +------------->| Policy | | | .
| | . | Server |----> |Conference .
| | . | | | | .
| | . +-----------+ | & | .
| | . | | .
| | . | Media | .
+-----------+ . +-----------+ | Policy| .
| | . | | \ // .
| | . | | \-----/ .
|Participant|<--------->| Focus | | .
| | SIP . | | | .
| | Dialog . | |<-----------+ .
+-----------+ . |...........| .
^ . | Conference| .
| . |Notification .
+------------>| Service | .
Subscription. +-----------+ .
. .
. .
. .
. .
.....................................

Conference
Functions

Figure 2: Complex SIP Conference involved in a conference, as
opposed to suggesting a physical decomposition. The separation of
these functions is documented here to encourage clarity in the
requirements and to allow individual implementations the flexibility
to compose a conferencing system in a scalable and robust manner.

3.1 Usage of URIs

It is fundamental to this framework that a conference is uniquely
identified by a URI, and that this URI identify identifies the focus which is
responsible for the conference. This The conference URI is unique, such
that no two conferences have the same conference URI. A conference
URI is always a SIP or SIPS URI.

The conference URI is opaque to any participants which might use it.
There is no way to look at the URI, and know for certain whether it
identifies a focus, as opposed to a user or an interface on a PSTN
gateway. This is in line with the general philosophy of URI usage
[7].
[8]. However, contextual information surrounding the URI (for
example, SIP header parameters) may indicate that the URI represents
a conference.

When a SIP request is sent to the conference URI, that request is
routed to the focus, and only to the focus. The element or system
that creates the conference URI is responsible for guaranteeing this
property.

The conference URI can represent a long-lived conference or interest
group, such as "sip:discussion-on-dogs@example.com". The focus
identified by this URI would always exist, and always be managing the
conference for whatever participants are currently joined. The Other
conference URI URIs can also represent short-lived conferences, such as an "instant" conference, for
example, "sip:a8sd9998as-9s8daa@example.com". An instant conference
is one where the focus is instantiated when the first URI for it
arrives, and then destroyed when the last participant leaves. Both of
these represent variations in the policies implemented by the focus,
and cannot be determined from inspection of the URI.
ad-hoc conference.

Ideally, a conference URI is never constructed or guessed by a user.

.....................................
. .
. .
. .
. .
. Conference .
. Policy .
Conference . .
Policy . +-----------+ //-----\\ .
Control . | | || || .
Protocol . | Conference| \\-----// .
+---------------->| Policy | | | .
| . | Server |----> |Membership .
| . | | | | .
| . +-----------+ | & | .
| . | | .
| . | Media | .
+-----------+ . +-----------+ | Policy| .
| | . | | \ // .
| | . | | \-----/ .
|Participant|<--------->| Focus | | .
| | SIP . | | | .
| | Dialog . | |<-----------+ .
+-----------+ . |...........| .
^ . | Conference| .
| . |Notification .
+------------>| Service | .
Subscription. +-----------+ .
. .
. .
. .
. .
.....................................

Conference
Functions

Figure 2: Conference-Aware Participant
Rather, conference URIs are learned through many mechanisms. A
conference URI can be emailed or sent in an instant message. A
conference URI can be linked on a web page. A conference URI can be
obtained from a conference policy control protocol, which can be used
to create conferences and the policies associated with them.

To determine that a SIP URI does represent a focus, standard
techniques for URI capability discovery can be used. First, a
participant can send an OPTIONS to a SIP URI, and if it represents a
focus, Specifically,
the response will caller preferences specification [9] provides the "isfocus"
feature tag to indicate such [TBD]. The response will that the URI is a focus. Caller preferences
parameters are also used to indicate whether or not the that a focus has implemented supports the subscription
conference notification service. This is known done by the presence of an Allow
header in the response, indicating declaring support
for the SUBSCRIBE method,
along with an Allow-Events header, indicating support for the
conferencing package. A second method for determining that a URI
represents a focus is through a refresh request. The Allow and
Allow-Events headers, along with the relevant package(s) in the caller
preferences specification
[8] can indicate feature parameters associated with the same information that would be learned through
an OPTIONS query. conference URI.

The other functions in a conference are also represented by URIs. If
the conference policy and media policy servers are server is implemented through web pages, these servers are regular this
server is identified by HTTP URIs. If they are it is accessed using an
explicit protocol, they are the URIs it is a URI defined for
those protocols. that protocol.

Starting with the conference URI, the URIs for the other logical
entities in the conference can be learned using [TBD].

OPEN ISSUE: I suppose we cannot say more until the protocol
work is done. But, we have a requirement here - that there
be a way to learn these URIs starting only with the conference URI.

5
notification service.

4 Functions of the Elements

This section gives a more detailed description of the functions
typically implemented in each of the elements.

5.1

4.1 Focus

As its name implies, the focus is the center of the conference. All
participants in the conference are connected to it using a SIP
dialog. The focus is responsible for maintaining the dialogs
connected to it. It insures ensures that the dialogs are connected to a set
of participants who are allowed to participate in the conference, as
defined by the conference membership policy. The focus also uses SIP to
manipulate the media sessions, in order to make sure each participant
obtains all the media for the conference. To do that, the focus makes
use of the services of a mixer. mixers.

When a focus receives an INVITE, it checks the conference membership policy. The
conference
membership policy might indicate that this participant is not allowed
to join, in which case the call can be rejected. It might indicate
that another participant, acting as a moderator, needs to approve
this new participant. In that case, the INVITE might be parked on a
music-on-hold server, or a 183 response might be sent to indicate
progress. A notification, using the conference notification service,
would be sent to the moderator. The moderator then has the ability to
manipulate the policies using the conference policy control protocol.
If the policies are changed to allow this new participant, the focus
can accept the INVITE (or unpark it from the music-on-hold server).
The interpretation of the conference membership policy by the focus is, itself,
a matter of local policy, and not subject to standardization.

If a participant manipulated the conference membership policy to indicate that a
certain other participant was no longer allowed in the conference,
the focus would send a BYE to that other participant to remove them.
This is often referred to as "ejecting" a user from the conference.
The process of ejecting fundamentally constitutes these two steps -
the establishment of the policy through the conference policy
protocol, and the implementation of that policy (using a BYE) by the
focus.

Similarly, if a participant user manipulated the conference membership policy to indicate
that a number of users need to be added to the conference, the focus
would send an INVITE to those participants. This is often referred to
as the "mass invitation" function. As with ejection, it is
fundamentally composed of the policy functions that specify the
participants which should be present, and the implementation of those
functions using SIP.
functions. A policy request to add a set of users might not require
an INVITE to execute it; those users might already be participants in
the conference.

A similar model exists for media policy. If the media policy
indicates that a participant should not receive any video, the focus
might implement that policy by sending a re-INVITE, removing the
media stream to that participant. Alternatively, if the video is
being centrally mixed, it could inform the mixer to send a black
screen to that participant. The means by which the policy is
implemented are not subject to specification.

5.2

4.2 Conference Policy Server

The conference policy server allows clients to manipulate and
interact with the conference policy. The conference policy is used by
the focus to make authorization decisions and guide its overall
behavior. Logically speaking, there is a one-to-one mapping between a
conference policy and a focus.

The conference policy is represented by a URI. There is a unique
conference policy for each focus. conference. The conference policy URI
points to a conference policy server which can manipulate that
conference policy. A conference policy server also has a "top level"
URI which can be used to access functions that are independent of any
conference. Perhaps the most important of these functions is the
creation of a new conference. This Creation of a new conference will
result in the construction of a new focus and a corresponding
conference URI, which can then be used to join the conference
itself. itself,
along with a media policy and conference policy.

The conference policy server is accessed using a client-server
transactional protocol. The client can be a participant in the
conference, or it can be a third party. Access control lists for who
can modify a conference policy are themselves part of the conference
policy.

The conference policy server also allows clients to create
new conferences. This would result is responsible for reconciliation of
potentially conflicting requests regarding the policy for the
conference.

The client of the conference policy control protocol can be any
entity interested in manipulating the conference policy. Clearly,
participants might be interested in manipulating them. A participant
might want to raise or lower the instantiation volume for one of the other
participants it is hearing. Or, a focus
(and therefore, participant might want to add a
user to the conference.

A client of the conference URI associated with that focus), policy protocol could also be another
server whose job is to determine the conference policy. As an
example, a floor control server is responsible for determining which
participant(s) in a conference policy, are allowed to speak at any given
time, based on participant requests and a media policy. access rules. The conference policy floor
control server
will also have rules about who can create conferences.

The would act as a client of the conference policy also includes per-participant policies that
specify how server,
and change the focus media policy based on who is allowed to handle a particular participant. These
include whether or not speak.

The client of the participant is anonymous, for example.

5.3 conference policy control protocol could also be
another conference policy server.

4.3 Mixers

A mixer is responsible for combining the media streams that make up
the conference, and generating one or more output streams that are
distributed to recipients (which could be participants or other
mixers). The combination process of combining media is specific to the media
type, and is directed by the focus, under the guidance of the rules
described in the media policy.

A mixer is not aware of a "conference" as an entity, per se. A mixer
receives media streams as inputs, and based on directions provided by
the focus, generates media streams as outputs. There is no grouping
of media streams beyond the policies that describe the ways in which
the streams are mixed.

A mixer is always under the control of a focus. The focus is
responsible for interpreting the media policy, and then installing
the appropriate rules in the mixer. If the focus is directly
controlling a mixer, the mixer can either be co-resident with the
focus, or can be controlled through a protocol like Megaco [9]. some kind of protocol.

However, a focus need not directly control a mixer. Rather, a focus
can delegate the mixing to the participants, each of which has their
own mixer. This is described in Section 6.4.

5.4 Media Policy Server

The media policy server is similar to the conference policy server.
It is accessed using a transactional client-server protocol. It
manipulates a media policy, identified by a URI. The focus has the
responsibility of acting on that media policy, implementing it
through direct or indirect control of mixers.

The media policy describes the way in which the set of inputs to the
mixer are combined to generate the set of outputs. Media policies can
span media types. In other words, the policy on how one media stream
is mixed can be based on characteristics of other media streams.

Media policies can be based on any quantifiable characteristic of the
media stream (its source, volume, codecs, speaking/silence, etc.),
and they can be based on internal or external variables accessible by
the media policy.

The media policy server is responsible for reconciliation of
potentially conflicting requests regarding the media policy for the
conference.

The client of the media policy protocol can be any entity interested
in manipulating media policies. Clearly, participants might be
interested in manipulating them. A participant might want to raise or
lower the volume for one of the other participants it is hearing. Or,
a participant might want to switch from a tiled video view, to just
viewing the active speaker. A client of the media policy protocol
could also be another server whose job is to determine the media
policy. As an example, a floor control server is responsible for
determining which participant(s) in a conference are allowed to speak
at any given time, based on participant requests and access rules.
The floor control server would act as a client of the media policy
server, and inform the media policy server about who is allowed to
speak.

The client of the media policy protocol could also be another media
policy server, as described in Section 6.4.

Some examples of media policies include:

o The video output is the picture of the loudest speaker (video
follows audio).

o The audio from each participant will be mixed with equal
weight, and distributed to all other participants.

o The audio and video that is distributed is the one selected by
the floor control server.

5.5

4.4 Conference Notification Service

The focus can provide a conference notification service. In this
role, it acts as a notifier, as defined in RFC 3265 [4]. It accepts
subscriptions from clients for the conference URI, and generates
notifications to them as the state of the conference changes.

This state is composed of three two separate pieces. The first is the state
of the focus, focus and the second is the conference policy, and the
third is the media policy.

The state of the focus includes the participants connected to the
focus, and information about the dialogs associated with them. As new
participants join, this state would change, allowing subscribers to
learn about them. changes, and is reported through the
notification service. Similarly, when someone leaves, this state also
changes, allowing subscribers to learn about this fact.

The state of

As described previously, the conference policy includes the set of participants
that are allowed, or not allowed, to join the conference,
membership policy and the set
of participants who are to be explicitly added to the conference. It
includes the roles which are assigned media policy. As those policies change, due
to each participant, such as
whether they are a moderator. If there was a change in role, for
example, a new moderator was selected, the focus would inform
subscribers.

The state usage of the media policy includes the media streams being
received CPCP, direct change by each participant, the audio focus, or video modalities, and so
on.

5.6 through an
application, the conference notification service informs subscribers
of these changes.

4.5 Participants

A participant in a conference is any SIP user agent that has a dialog
with the focus. This SIP user agent can be a PC application, a SIP
hardphone, or a PSTN gateway. It can also be another focus. A
conference which has a participant that is the focus of another
conference is called a simplex cascaded conference. They can also be
used to provide scalable conferences where there are regional sub-
conferences, each of which is connected to the main conference. A
conference topology refers to a graph which shows each focus and each
participant as a vertex, with a connection between each participant
and its focus.

5.7

4.6 Conference Policy

The conference policy contains the rules that guide the operation of
the focus. These The rules can be simple, such as an access list that
defines the set of allowed participants in a conference. The rules
can also be incredibly complex, specifying time-of-day based rules on
participation conditional on the presence of other participants. It
is important to understand that there is no restriction on the type
of rules that can be encapsulated in a conference policy.

However, there does exist a protocol means by which a client can
request a change in the

The conference policy. This is done by
communicating policy can be manipulated using web applications or
voice applications. It can also be manipulated with proprietary
protocols. However, the conference policy server, which manipulates control protocol can be
used as a standardized means of manipulating the conference policy.
By the nature of conference policies, not all aspects of the policy
can be manipulated with the conference policy control protocol. It is the responsibility of the

The conference policy
server to reconcile includes the various requests with membership policy and the conference media
policy.

5.8 Media Policy The media membership policy contains the rules includes per-participant policies that guide the operation of
specify how the
mixer. The focus uses these rules to interact with the mixer is to
implement them. handle a particular participant. These rules can be simple (mix all media from all
participants),
include whether or they can be incredibly complex. It is important to
understand that there not the participant is no restriction on anonymous, for example.

The media policy describes the type of rules that can
be encapsulated way in a media policy.

However, there does exist a protocol means by which the set of inputs to a client can
request a change in
mixer are combined to generate the set of outputs. Media policies can
span media policy. This is done by communicating
with types. In other words, the media policy server, which manipulates the on how one media policy. By
the nature stream
is mixed can be based on characteristics of other media policies, not all aspects streams.
Media policies can be based on any quantifiable characteristic of the policy
media stream (its source, volume, codecs, speaking/silence, etc.),
and they can be
manipulated with based on internal or external variables accessible by
the media policy control protocol. It policy.

Some examples of media policies include:

o The video output is the
responsibility picture of the media policy server loudest speaker (video
follows audio).

o The audio from each participant will be mixed with equal
weight, and distributed to reconcile all other participants.

o The audio and video that is distributed is the various
requests with one selected by
the media policy.

6 Physical Realization

In this section, we present several physical instantiations floor control server.

5 Common Operations

There are a large number of these
components, to show ways in which users can interact with a
conference. They can join, leave, set policies, approve members, and
so on. This section is meant as an overview of the major conferencing
operations, summarizing how these basic functions they operate. More detailed examples of
the SIP mechanisms can be combined to
solve found in [7].

5.1 Creating Conferences

There are many ways in which a variety conference can be created. The
creation of problems.

6.1 Centralized Server

In a conference actually constructs several elements all at
the most simplistic realization same time. It results in the creation of this framework, there is a
single physical server focus and a conference
policy. It also results in the network construction of a conference URI,
which implements uniquely identifies the focus, focus. Since the conference policy server, the media policy server, and the mixer.
This is URI needs
to be unique, the classic "one box" solution, shown in Figure 3.

6.2 Endpoint Server

Another important model element which creates conferences is responsible
for guaranteeing that uniqueness. This can be accomplished
deterministically, by keeping records of conference URIs, or
probabilistically, by creating random URI with sufficiently low
probabilities of collision.

When a locally-mixed ad-hoc conference.
In this scenario, two users (A media and B) conference policy are in a regular point-to-point
call. One created, they are established
with default rules that are implementation dependent. If the creator
of the participants (A) decides to conference in a third
participant, C. To wishes to change those rules, they would do this, A begins acting as a focus. Its existing
dialog with B becomes so
using the first dialog attached to conference policy control protocol (CPCP), for example.

Of course, using the focus. B would
re-INVITE A on CPCP requires that dialog, changing its Contact an element know the URI to for
manipulating the policy. That requires a new value
which identifies means to learn the focus. In essence, A "mutates"
conference policy URI from the conference URI, since the conference
URI is frequently the sole result returned to the client as a single-
user UA result
of conference creation. Any other URIs associated with the conference
are learned through the conference notification service. They are
carried as elements in the notifications.

5.1.1 SIP Mechanisms

One way to create a focus plus conference is through a conferencing application.
As an example, a single user UA, and in can send an INVITE request to
sip:conferences@service.com. This URI identifies an IVR application
which interacts with the process user, collects information about the desired
conference, and creates it. The user can then be placed into their
newly created conference.

Creation of such
a mutation, its URI changes. Then, conferences where the focus makes resides in an outbound INVITE
to C. When C accepts, it mixes endpoint
operates differently. There, the media from A endpoint itself creates the
conference URI, and C together,
redistributing hands it out to other endpoints which are to be
the results. The mixed media participants. What differs from case to case is also played locally.
Figure 4 shows how the endpoint
decides to create a diagram of this transition.

It is conference.

Figure 3: Centralized server architecture

between A and B, and between B and C, UA are exactly
migrated to the same endpoint-hosted focus, using a re-INVITE to those
that would be used pass the
conference URI to the newly joined participants.

Alternatively, one UA can ask another UA to create an endpoint-hosted
conference. This is accomplished with the SIP Join header [10]. The
UA which receives the Join header in an invitation may need to create
a centralized server model. B could also
include new conference URI (a new one is not needed if the dialog that is
being joined is already part of a media policy server and conference). The conference subscription server too,
allowing URI is
then handed to the recently joined participants through a re-INVITE.

5.1.2 CPCP Mechanisms

Another way to have access to them if they so desired.
Just because the focus create a conference is co-resident through interaction with the
conference policy server. Using the conference policy control
protocol, a participant does not
mean any aspect of client can instruct the behaviors conference policy server to
create a new conference and external interfaces return the conference URI and conference
policy URI.

5.1.3 Non-Automated Mechanisms

Of course, a user can also create conferences by interacting with a
web server. The web server would prompt the user for the neccessary
information (start and stop times of the conference, participants,
etc.) and return the conference URI to the user. The user would copy
this URI into their SIP phone, and send it an INVITE in order to join
the newly-created conference.

5.2 Adding Participants

There are many mechanisms for adding participants to a conference.
These include SIP, the conference policy control protocol, and non-
automated means. In all cases, participant additions can be first
party (a user adds themself) or third party (a user adds another
user).

5.2.1 SIP Mechanisms

First person additions using SIP are trivially accomplished with a
standard INVITE. A participant can send an INVITE request to the
conference URI, and if the conference policy allows them to join,
they are added to the conference.

If a UA does not know the conference URI, but has learned about a
dialog which is connected to a conference (by using the dialog event
package, for example [11]), the UA can join the conference by using
the Join header to join the dialog.

Third party additions with SIP are done using REFER [12]. The client
can send a REFER request to the participant, asking them to send an
INVITE request to the conference URI. Additionally, the client can
send a REFER request to the focus, asking it to send an INVITE to the
participant. The latter technique has the benefit of allowing a
client to add a conference-unaware participant that does not support
the REFER method.

5.2.2 CPCP Mechanisms
A basic function of the conference policy control protocol is to add
participants. A client of the protocol can specify any SIP URI (which
may identify themself) that is to be added. If the URI does not
identify a user that is already a participant in the conference, the
focus will send an INVITE to that URI in order to add them in.

5.2.3 Non-Automated Mechanisms

There are countless non-automated means for asking a participant to
join the conference. Generally, they involve conveying the conference
URI to the desired participant, so that they can send an INVITE to
it. These mechanisms all require some kind of human interaction.

As an example, a user can send an instant message [13] to the third
party, containing an HTML document which requests the user to click
on the hyperlink to join the conference:

<html>
Hey, would you like to <a href="sip:9sf88fk-99sd@conferences.com">join
</a> the conference now?
</html>

5.3 Conditional Joins

In many cases, a new participant will not wish to join the conference
unless they can join with a particular set of policies. As an
example, a participant may want to join anonymously, so that other
participants know that someone has joined, but not who. To accomplish
this, the conference policy control protocol is used to establish
these policies prior to the generation or acceptance of an invitation
to the conference. For example, if a user wishes to join a conference
with a known conference URI, the user would obtain the URI for the
conference policy, manipulate the policy to set themself as an
anonymous participant, and then actually join the conference by
sending an INVITE request to the conference URI.

5.4 Removing Participants

As with additions, there are several mechanisms for departures. These
include SIP mechanisms and CPCP mechanisms. Removals can also be
first person or third person.

5.4.1 SIP Mechanisms

First person departures are trivially accomplished by sending a BYE
request to the focus. This terminates the dialog with the focus and
removes the participant from the conference.

Third person departures can also be done using SIP, through the REFER
method.

5.4.2 CPCP Mechanisms

The CPCP can be used by a client to remove any participant (including
themself). When CPCP is used for this purpose, the focus will send a
BYE request to the participant that is being removed. The focus will
execute any other signaling that is needed to remove them (for
example, manipulate other dialogs in order to manage the change in
media streams).

The conference policy control protocol can also be used to remove a
large number of users. This is generally referred to as mass
ejection.

5.4.3 Non-Automated Mechanisms

As with the other common conferencing functions, there are many non-
automated ways to remove a participant. The identity of the
participant can be entered into a web form. When the user clicks
submit, the focus sends a BYE to that participant, removing them from
the conference. Alternatively, the conference can expose an IM
interface, where the user can send an IM to the conference saying
"remove Bob", causing the conference server to remove Bob.

5.5 Approving Policy Changes

OPEN ISSUE: The basic mechanism described here depends on
the actual protocols used for conference and media policy
manipulation. If the protocol itself provides change
notifications, sip-events may not be needed for that
purpose. Thus, this description here is tentative.

A conference policy for a particular conference may designate one or
more users as moderators for some set of media policy or conference
policy change requests. This means that those moderators need to
approve the specific policy change. Typically, moderators are used to
approve member additions and removals. However, the framework allows
for moderators to be associated with any policy change that can be
made.

Moderating a policy request is done using a combination of the
conference notification service and the CPCP protocol.

First, a client makes a policy change. This can be directly, using
the CPCP, or indirectly. An indirect policy change request is any
non-CPCP action that requires approval. The simplest example is an
INVITE to the focus from a new participant. That represents a request
to change the membership of the conference. From a moderation
perspective, it is handled identically to the case where a client
used the CPCP to request that the same user to be added to the
conference.

Part of the conference policy itself may designate any policy change
as moderated. This means that they change cannot be performed by the
client directly. As a result, any CPCP request will fail, and the
failure response informs the client that their request failed due to
insufficient authorization. That completes the CPCP transaction. In
the case of a policy change requested indirectly through some other
means, the behavior depends on the mechanism. For example, if a user
sends a SIP INVITE request to the conference in order to join, and
that join request is moderated, the focus can reject the INVITE, or
it can accept it and play music-on-hold until the request is
approved.

Even though the CPCP transaction failed, it does result in a change
in internal state. Specifically, the requested change shows up as a
"pending" state within the media and conference policies. This means
that the change has been requested, but has not taken effect. It is
almost a form of change request history. However, because it is a
state change, it is something that can result in notifications
through the conference notification service.

Therefore, in order to moderate requests, the moderator subscribes to
the conference policy notification service. Normally, the
notifications from the focus do not reflect pending state changes.
That is, the service will not normally send a notification informing
a subscriber that a policy change request was made and failed due to
lack of authorization. However, notifications to the moderator do
reflect these changes. That is because the policy of the focus is to
inform moderators, and only moderators, of these changes. Indeed,
different users can be moderators for different parts of the
conference and media policies. For example, one user can be a
moderator for membership changes, and another, a moderator for
whether users can be anonymously joined or not.

There are two ways that the focus knows whether a subscriber to the
conference notification service is a moderator. The first is
configured policy (once again through CPCP). That policy can specify
that a particular user is the moderator for a particular piece of
policy. Therefore, if that user subscribes to the conference
notification service, any notification sent to that user will include
pending changes to that piece of policy. As an alternative, a
SUBSCRIBE request from a user can include a filter [14] that requests
receipt of these pending state changes. If the conference policy
allows, that request is honored, and the subscriber will receive
notifications about pending state changes.

Once the moderator receives a notification about the pending state
change, they use the CPCP to implement their decision. If the
moderator decides to approve the change, they use the CPCP or MPCP to
actually perform the change themselves. Since the moderator for a
piece of policy is allowed to change that piece of policy, by
definition, their change is accepted and performed. If the moderator
decides to reject the change, they use the CPCP to remove the pending
state from the database.

The pending state persists in the database for a period of time which
is, itself, part of the conference policy. If the moderator does not
either approve or reject the change, the pending state eventually
disappears, as if the change was explicitly rejected.

If the pending state is approved, a real change to the conference or
media policy takes place, and this change will be reflected in the
conference notification service. In this way, if a client makes a
policy change, and their request is rejected because they are not
authorized, the client can subscribe to the conference notification
service to learn if their change is eventually approved or rejected.

This general mechanism for moderating policy requests is consistent
with the moderation of presence subscriptions [15] [16].

5.6 Creating Sidebars

A sidebar is a "conference within a conference", allowing a subset of
the participants to converse amongst themselves. Frequently,
participants in a sidebar will still receive media from the main
conference, but "in the background". For audio, this may mean that
the volume of the media is reduced, for example.

A sidebar is represented by a separate conference URI. This URI is a
type of "alias" for the main conference URI. Both route to the same
focus. Like any other conference, the sidebar conference URI has a
conference policy and a media policy associated with it. Like any
other conference, one can join it by sending an INVITE to this URI,
or ask others to join by referring them to it. However, it differs
from a normal conference URI in several ways. First, users in the
main conference do not need to establish a separate dialog to the
sidebar conference. The focus recognizes the sidebar as a special
URI, and knows to use the existing dialog to the main conference as a
"virtual" connection to the sidebar URI.

The second difference is the way in which conference and media
policies are implemented. If the conference policy control protocol
is used to add a user to a normal conference, the focus will
typically send an INVITE to the participant to ask them to join. For
a sidebar conference, it is done differently. If the conference
policy control protocol is used to add a user to it, and that user is
already part of the main conference, the focus will use the
conference notification service to alert the existing participant
that they have been asked to join the sidebar. The invited user can
then make use of the CPCP to formally add themselves to the sidebar.

5.7 Destroying Conferences

Conferences can be destroyed in several ways. Generally, whether
those means are applicable for any particular conference is a
component of the conference policy.

When a conference is destroyed, the conference and media policies
associated with it are destroyed. Any attempts to read or write those
policies results in a protocol error. Furthermore, the conference URI
becomes invalid. Any attempts to send an INVITE to it, or SUBSCRIBE
to it, would result in a SIP error response.

Typically, if a conference is destroyed while there are still
participants, the focus would send a BYE to those participants before
actually destroying the conference. Similarly, if there were any
users subscribed to the conference notification service, those
subscriptions would be terminated by the server before the actual
destruction.

5.7.1 SIP Mechanisms

There is no explicit means in SIP to destroy a conference. However, a
conference may be destroyed as a by-product of a user leaving the
conference, which can be done with BYE. In particular, if the
conference policy states that the conference is destroyed once the
last user leaves, when that user does leave (using a SIP BYE
request), the conference is destroyed.

5.7.2 CPCP Mechanisms

The CPCP contains mechanisms for explicitly destroying a conference.

5.7.3 Non-Automated Mechanisms

As with conference creation, a conference can be destroyed by
interacting with a web application or voice application that prompts
the user for the conference to be destroyed.

5.8 Obtaining Membership

A participant in a conference will frequently wish to know the set of
other users in the conference. This information can be obtained many
ways.

5.8.1 SIP Mechanisms

The conference notification service allows a conference aware
participant to subscribe to it, and receive notifications that
contain the list of participants. When a new participant joins or
leaves, subscribers are notified. The conference notification service
also allows a user to do a "fetch" [4] to obtain the current listing.

5.8.2 CPCP Mechanisms

The CPCP contains mechanisms for querying for the current set of
conference participants.

5.8.3 Non-Automated Mechanisms

Users can also interact with applications to obtain conference
membership. There may be a conference web page associated with the
conference, which has a link that will fetch the current list of
participants and display them in the browser. Similarly, an
interactive voice response application connected to the focus can be
used to obtain the current membership. A user in the conference could
press the pound key on their phone, and hear a listing of the current
participants.

5.9 Adding and Removing Media

Each conference is composed of a particular set of media that the
focus is managing. For example, a conference might contain a video
stream and an audio stream. The set of media streams that constitute
the conference can be changed by participants. When the set of media
in the conference change, the focus will need to generate a re-INVITE
to each participant in order to add or remove the media stream to
each participant. When a media stream is being added, a participant
can reject the offered media stream, in which case it will not
receive or contribute to that stream. Rejection of a stream by a
participant does not imply that that the stream is no longer part of
the conference - just that the participant is not involved in it.

There are several ways in which a media stream can be added or
removed from a conference.

5.9.1 SIP Mechanisms

A SIP re-INVITE can be used by a participant to add or remove a media
stream. This is accomplished using the standard offer/answer
techniques for adding media streams to a session [17]. This will
trigger the focus to generate its own re-INVITEs.

5.9.2 CPCP Mechanisms

The CPCP can be used to add or remove a media stream. This too will
trigger the focus to generate a re-INVITE to each participant in
order to affect the change.

5.9.3 Non-Automated Mechanisms

As with most of the other common functions, addition and removal of
media streams can be accomplished with a web application or
interactive voice application.

5.10 Conference Announcements and Recordings

Conference announcements and recordings play a key role in many real
conferencing systems. Examples of such features include:

1. Asking a user to state their name before joining the
conference, in order to support a roll call

2. Allowing a user to request a roll call, so they can hear
who else is in the conference

3. Allowing a user to press some keys on their keypad in order
to record the conference

4. Allowing a user to press some keys on their keypad in order
to be connected with a human operator

5. Allowing a user to press some keys on their keypad to mute
or unmute their line

In this framework, these capabilities are modeled as an application
which acts as a participant in the conference. This is shown
pictorially in Figure 3. The conference has four participants. Three
of these participants are end users, and the fourth is the
announcement application.

User 1
+-----------+
| |
| |
|Participant|
| 4 |
| |
+-----------+
|SIP
|Dialog
Conference |1
Policy +---|--------+
User 2 Server | | | Application
+-----------+ +-----------+ | CPCP *************
| | | | |-------- * *
| | | | | * *
|Participant|-----------| Focus |------------*Participant*
| 1 | SIP | | | SIP * 3 *
| | Dialog | |--+ Dialog * *
+-----------+ 2 +-----------+ 4 *************
|
|
|SIP
|Dialog
|3
|
+-----------+
| |
| |
|Participant|
| 2 |
| |
+-----------+
User 3

Figure 3: Conference announcement application
If the announcement application wishes to play an announcement to all
the conference members (for example, to announce a join), it merely
sends media to the mixer as would any other participant. The
announcement is mixed in with the conversation and played to the
participants.

Similarly, the announcement application can play an announcement to a
specific user by using the CPCP to configure its media policy so that
the media it generates is only heard by the target user. The
application then generates the desired announcement, and it will be
heard only by the selected recipient.

The announcement application can also receive input from a specific
user through the conference. The announcement application would use
the CPCP to cause in-band DTMF to be dropped from the mix, and sent
only to itself. When a user wishes to invoke an operation, such as to
obtain a roll call, the user would press the appropriate key
sequence. That sequence would be heard only by the announcement
application. Once the application determines that the user wishes to
hear a roll call, it can use the CPCP to set the media policy so that
media from that user is delivered only to the announcement
application. This "disconnects" the user from the rest of the
conference so they can interact with the application. Once the
interaction is done, and announcement application uses the CPCP to
"reconnect" the user to the conference.

5.11 Floor Control

Floor control is similar to a conference announcement application.
Within this framework, floor control is managed by an application
(possibly one that is not a participant) that uses the CPCP to
enforce the resulting floor control decisions.

[[Need more work here]]

5.12 Camera and Video Controls

OPEN ISSUE: Originally, I was just going to say that this
is outside the scope of conferencing. But, it does impact
conferencing. Effectively, camera control is treated like a
media stream. The mixer would combine the various requests
across participants and direct them to the appropriate
device. How does that work though? In a video conference
with 4 participants, the camera control needs to identify
the specific user whose camera is to be controlled. That is
something unique to conferencing.

6 Physical Realization

In this section, we present several physical instantiations of these
components, to show how these basic functions can be combined to
solve a variety of problems.

6.1 Centralized Server

In the most simplistic realization of this framework, there is a
single physical server in the network which implements the focus, the
conference policy server, and the mixers. This is the classic "one
box" solution, shown in Figure 4.

6.2 Endpoint Server

Another important model is that of a locally-mixed ad-hoc conference.
In this scenario, two users (A and B) are in a regular point-to-point
call. One of the participants (A) decides to conference in a third
participant, C. To do this, A begins acting as a focus. Its existing
dialog with B becomes the first dialog attached to the focus. A would
re-INVITE B on that dialog, changing its Contact URI to a new value
which identifies the focus. In essence, A "mutates" from a single-
user UA to a focus plus a single user UA, and in the process of such
a mutation, its URI changes. Then, the focus makes an outbound INVITE
to C. When C accepts, it mixes the media from B and C together,
redistributing the results. The mixed media is also played locally.
Figure 5 shows a diagram of this transition.

It is important to note that the external interfaces in this model,
between A and B, and between B and C, are exactly the same to those
that would be used in a centralized server model. B could also
include a conference policy server and conference notification
service, allowing the participants to have access to them if they so
desired. Just because the focus is co-resident with a participant
does not mean any aspect of the behaviors and external interfaces
will change.

6.3 Media Server Component

In this model, shown in Figure 6, each conference involves two
centralized servers. One of these servers, referred to as the
"application server" owns and manages the membership and media
policies, and maintains a dialog with each participant. As a result,
it represents the focus seen by all participants in a conference.
However, this server doesn't provide any media support. To perform
Conference Server
...................................
. .
. +------------+ .
. | Conference | .
. |Notification| .
. | Server | .
. +------------+ .
. +----------+ .
. |Conference| +-----+ .
. | Policy | +-------+ +-----+| .
. | Server | | Focus | |Mixer|+ .
. +----------+ +-------+ +-----+ .
................//.\.....***.......
// \ *** *
// *** * RTP
SIP // *** \ *
// *** \SIP *
// *** RTP \ *
/ ** \ *
+-----------+ +-----------+
|Participant| |Participant|
+-----------+ +-----------+

Figure 4: Centralized server architecture

the actual media mixing function, it makes use of a second server,
called the "mixing server". This server includes a focus, and a
conference policy server, but has no conference notification service.
It has a default membership policy, which accepts all invitations
from the top-level focus. Its conference policy server accepts any
controls made by the application server. The focus in the application
B B
+------+ +------+
| | | |
| UA | | UA |
| | | |
+------+ +------+
| . | .
| . | .
| . | .
| . Transition | .
| . ------------> | .
SIP| .RTP SIP| .RTP
| . | .
| . | .
| . | .
| . | .
| . +----------+
+------+ | +------+ | SIP +------+
| | | |Focus | |----------| |
| UA | | |M.Pol.| |C.Pol.| | | UA |
| | | |C.Pol.| |Mixers| |..........| |
+------+ | |Mixer | | | RTP +------+
| +------+ |
A | + | C
| + <..|.......
| + | .
| +------+ | .
| |Parti-| | .
| |cipant| | .
| | | | .
| +------+ | .
+----------+ .
B
A .
.

Internal
Interface

Figure 4: 5: Transition from two-party call to conference

server uses third party call control to connect the media streams of
each user to the mixing server, as needed. If the focus in the
application server receives a conference policy control command from
+------------+ +------------+
| App Server| SIP |Conf. Cmpnt.|
| |-------------| |
| Focus | Conf. Proto | Focus |
| C.Pol |-------------| M.Pol C.Pol |
| M.Pol | Media Proto | Mixer Mixers |
|Notification|-------------| |
| | | |
+------------+ +------------+
| \ .. .
| \\ RTP... .
| \\ .. .
| SIP \\ ... .
SIP | \\ ... .RTP
| ..\ .
| ... \\ .
| ... \\ .
| .. \\ .
| ... \\ .
| .. \ .
+-----------+ +-----------+
|Participant| |Participant|
+-----------+ +-----------+

Figure 5: 6: Media server component model

In this model, shown in Figure 5, each conference involves two
centralized servers. One of these servers, referred to as the
"application server" owns and manages the conference and media
policies, and maintains a dialog with each participant. As a result,
it represents the focus seen by all participants in a conference.
However, this server doesn't provide any media support. To perform
the actual media mixing function, it makes use of a second server,
called the "mixing server". This server includes a focus, but has no
conference policy server or conference notification service. It has a
default conference policy, which accepts all invitations from the
top-level focus. Its media policy server accepts any controls made by
the application server. The focus in the application server uses
third party call control to connect the media streams of each user to
the mixing server, as needed. If the focus in the application server
receives a media policy control command from a client, it delegates
that to the media server by making the same media policy control
command to it.

This model allows for the mixing server to be used as a resource for
a variety of different conferencing applications. This is because it
is unaware of any conference or media policies; it is merely a
"slave" to the top-level server, doing whatever it asks. This is
consistent with the SIP Application Server Component Model [10]. [18].

6.4 Distributed Mixing
In a distributed mixed conference, there is still a centralized
server which implements the focus, conference policy server, and
media policy server. However, there is are no centralized mixer. mixers.
Rather, there is a mixer are mixers in each endpoint, along with a media conference
policy server. The focus distributes the media by using third party
call control
[11] [19] to move a media stream between each participant and
each other participant. As a result, if there are N participants in
the conference, there will be a single dialog between each
participant and the focus, but the session description associated
with that dialog will be constructed to allow media to be distributed
amongst the participants. This is shown in Figure 6. 7.

There are several ways in which the media can be distributed to each
participant for mixing. In a multi-unicast model, each participant
sends a copy of its media to each other participant. In this case,
the session description manages N-1 media streams. In a multicast
model, each participant joins a common multicast group, and each
participant sends a single copy of its media stream to that group.
The underlying multicast infrastructure then distributes the media,
so that each participant gets a copy. In a single-source multicast
model (SSM), each participant sends its media stream to a central
point, using unicast. The central point then redistributes the media
to all participants using multicast. The focus is responsible for
selecting the modality of media distribution, and for handling any
hybrids that would be necessitated from clients with mixed
capabilities.

When a new participant joins or is added, the focus will perform the
necessary third party call control to distribute the media from the
new participant to all the other participants, and vice-a-versa.

The central conference server also includes a media conference policy
server. Of course, the central conference server cannot implement any
of the media policies directly. Rather, it would delegate the
implementation to the media conference policy servers co-resident with a
participant. As an example, if a participant decides to switch the
overall conference mode from "video follows audio" "voice activated" to "tiled video", "continuous
presence", they would communicate with the central media conference policy
server. This media The conference policy server, in turn, would communicate with
the media conference policy servers co-
resident co-resident with each participant,
using the same media conference policy control protocol, and instruct them
to use "tiled video". "continuous presence".

This model requires additional functionality in user agents, which
may or may not be present. The participants, therefore, must be able
to advertise this capability to the focus.

6.5 Cascaded Mixers

In very large conferences, it may not be possible to have a single
mixer that can handle all of the media. A solution to this is to use
cascaded mixers. In this architecture, there is a centralized focus,
but the mixing function is implemented by a multiplicity of mixers,
scattered throughout the network. Each participant is connected to
one, and only one of the mixers. The focus uses some kind of control
protocol (such as MEGACO [9]) to connect the mixers together, so that all of the
participants can hear each other.

This architecture is shown in Figure 7. 8.

7 Common Operations

There are a large number of ways Security Considerations

Conferences frequently require security features in which users can interact with a
conference. They order to properly
operate. The conference policy may dictate that only certain
participants can join, leave, set policies, approve members, and
so on. This section is meant as an overview of the basic primitives,
summarizing how they operate. More detailed examples with complete
call flows or that certain participants can be found in [12].

7.1 Creating Conferences

There create new
policies. Generally speaking, conference applications are many ways in which very
concerned about authorization decisions. Mechanisms for establishing
and enforcing such authorization rules is a conference can be created. Ultimately,
all of them result in central concept
throughout this document.

Of course, authorization rules require authentication. Normal SIP
authentication mechanisms should suffice for the establishment of a conference URI which
identifies a focus. In all cases, a conference URI must be created by
the focus itself, or an element which
authorization mechanisms described here.

8 Contributors

This document is responsible for managing
URIs that are used by the focus. Otherwise, result of discussions amongst the uniqueness conferencing
design team. The members of
conference URIs could not be guaranteed. this team include:

Figure 6: 7: Dialog and media streams in a distributed mixed conference

9 Changes since draft-rosenberg-sipping-conferencing-framework-00

o Rework of terminology.

o More details on moderating policy changes.

o Rework of the overview, and in particular, a shift of focus
from basic/complex conferences (a term which has been removed)
to conference aware/unaware participants.

o Removal of explicit reference to megaco for controlling a
mixer.

o Discussion of a lot more conferencing operations.

o New sidebar mechanism.

10 Authors Addresses

Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com

11 Normative References

12 Informative References

[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.

[2] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
transport protocol for real-time applications," RFC 1889, Internet
Engineering Task Force, Jan. 1996.

[3] O. Levin et al. , "Requirements for tightly coupled SIP
conferencing," internet draft, Internet Engineering Task Force, Nov.
2002. Work in progress.

[4] A. B. Roach, "Session initiation protocol (sip)-specific event
notification," RFC 3265, Internet Engineering Task Force, June 2002.

[5] B. Campbell and J. Rosenberg, "Instant message sessions in
SIMPLE," internet draft, Internet Engineering Task Force, Oct. 2002.
Work in progress.

[6] J. Rosenberg, "A framework and requirements for application
interaction in sip," Internet Draft, Internet Engineering Task Force,
Oct. 2002. Work in progress.

[7] A. Johnston and O. Levin, "Session initiation protocol call
control - conferencing for user agents," internet draft, Internet
Engineering Task Force, Feb. 2003. Work in progress.

[8] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform resource
identifiers (URI): generic syntax," RFC 2396, Internet Engineering
Task Force, Aug. 1998.

[9] H. Schulzrinne and J. Rosenberg, "Session initiation protocol
(SIP) caller preferences and callee capabilities," internet draft,
Internet Engineering Task Force, Nov. 2002. Work in progress.

[10] R. Mahy and D. Petrie, "The session inititation protocol (SIP)
'join' header," internet draft, Internet Engineering Task Force, Oct.
2002. Work in progress.

[11] J. Rosenberg and H. Schulzrinne, "A session initiation protocol
(SIP) event package for dialog state," internet draft, Internet
Engineering Task Force, June 2002. Work in progress.

[12] R. Sparks, "The SIP refer method," internet draft, Internet
Engineering Task Force, Dec. 2002. Work in progress.

[13] "Session initiation protocol (SIP) extension for instant
messaging," RFC 3428, Internet Engineering Task Force, Dec. 2002.

[14] T. Moran and S. Addagatla, "Architecture for event notification
filters," internet draft, Internet Engineering Task Force, Oct. 2002.
Work in progress.

[15] J. Rosenberg, "A presence event package for the session
initiation protocol (SIP)," internet draft, Internet Engineering Task
Force, Jan. 2003. Work in progress.

[16] J. Rosenberg, "A watcher information event template-package for
the session initiation protocol (SIP)," internet draft, Internet
Engineering Task Force, Jan. 2003. Work in progress.

+---------+
+-----------------------| |------------------------+
| ++++++++++++++++++++| |++++++++++++++++++ |
| + +------| Focus |---------+ + |
| + | | | | + |
| + | +-| |--+ | + |
| + | | +---------+ | | + |
| + | | + | | + |
| + | | + | | + |
| + | | + | | + |
| + | | +---------+ | | + |
| + | | | | | | + |
| + | | | Mixer 2 | | | + |
| + | | | | | | + |
| + | | +---------+ | | + |
| + | |... . .... | | + |
| + .|....| . .|.... | + |
| + ...... | | . | ..|... + |
| + ... | | . | | ....+ |
| +---------+ | | +---------+ | | +---------+ |
| | | | | | | | | | | |
| | Mixer 2 | | | | Mixer 3 | | | | Mixer 4 | |
| | | | | | | | | | | |
| +---------+ | | +---------+ | | +---------+ |
| . . | | . . | | . . |
| . . | | .. . | | .. . |
| . . | | . . | | . . |
+---------+ . | +---------+ . | +---------+ . |
| Prtcpnt | . | | Prtcpnt | . | | Prtcpnt | . |
| 1 | . | | 1 | . | | 1 | . |
+---------+ . | +---------+ . | +---------+ . |
. | . | . |
+---------+ +---------+ +---------+
| Prtcpnt | | Prtcpnt | | Prtcpnt |
| 1 | | 1 | | 1 |
+---------+ +---------+ +---------+

------- SIP Dialog
....... Media Flow
+++++++ Control Protocol

Figure 7: 8: Cascaded Mixers

protocol, a client can instruct the conference policy server to
create a new conference. The result of this operation is a conference
URI, which is returned to the client.

Another way to obtain a conference URI is to literally guess. In an
instant conferencing server, there are literally an infinite number
of conference URIs which can be used. Each of them is a valid
conference URI, since it identifies a focus, and when an INVITE is
sent to it, will join the user into that conference. As a result, a
client can simply choose one of them at random, so long as it is
configured with the domain portion of the URI and any naming
conventions in use by the instant conferencing server.

OPEN ISSUE: Do we need to specify standards for this?

The previous two approaches are used to obtain conference URIs for
focuses that are hosted within centralized servers. Creation of
conferences where the focus resides in an endpoint operates
differently. There, the endpoint itself creates the conference URI,
and hands it out to other endpoints which are to be the participants.
What differs from case to case is how the endpoint decides to create
a conference.

One important case is the ad-hoc conference described in Section 6.2.
There, an endpoint unilaterally decides to create the conference
based on local policy. The dialogs that were connected to the UA are
migrated to the endpoint-hosted focus, using a re-INVITE to pass the
conference URI to the newly joined participants.

Alternatively, one UA can ask another UA to create an endpoint-hosted
conference. This is accomplished with the SIP Join header [13]. The
UA which receives the Join header in an invitation may need to create
a new conference URI (a new one is not needed if the dialog that is
being joined is already part of a conference). The conference URI is
then handed to the recently joined participants through a re-INVITE.

7.2 Adding Participants

There are two modes for adding participants to a conference - first
party additions, and third party additions. In a first party
addition, the participant that wishes to join makes a direct attempt
to join. In a third party addition, some other participant takes
action with the aim of causing a third party to be added to the
conference.

First person additions are trivially accomplished with a standard
INVITE. A participant can send an INVITE request to the conference
URI, and if the conference policy allows them to join, they are added
to the conference.

If a UA does not know the conference URI, but has learned about a
dialog which is connected to a conference (by using the dialog event
package, for example [14]), the UA can join the conference by using
the Join header to join the dialog.

Third party invitations can be done in one of several ways. The first
approach is for the user to ask the third party to send an INVITE to
the conference URI. This can be done automatically through the usage
of REFER [15]. The participant would send a REFER request to the
third party. The Refer-To header field in that request would contain
the conference URI. There are countless non-automated means for
asking a participant to send an INVITE to the conference URI. A user
can send an instant message [16] to the third party, containing an
HTML document which requests the user to click on the hyperlink to
join the conference:

<html>
Hey, would you like to <a href="sip:9sf88fk-99sd@conferences.com">join
</a> the conference now?
</html>

The second approach for third party additions is for the participant
to ask the focus to add the third party to the conference. In this
case, however, a REFER cannot be used. REFER would have the effect of
telling the focus to send an INVITE to the new potential participant.
However, just sending this INVITE is not sufficient for adding the
new member. In more complex realizations, such as the distributed
mixing scenario of Section 6.4, a multiplicity of invitations will
need to be sent. This would require the focus to attach additional
meaning to REFER; it would have to be interpreted as a request to add
a participant to the conference. However, it is fundamental to the
concept of REFER that the recipient not attach specific application
semantics to it. Therefore, it cannot be used. Rather, the user would
use the conference policy control protocol to request that the focus
add the new participant. The conference policy control protocol can
also be used to add a multiplicity of new users. This is referred to
as mass invitation.

In many cases, a new participant will not wish to join the conference
unless they can join with a particicular set of policies. As an
example, a participant may want to join anonymously, so that other
participants know that someone has joined, but not who. To accomplish
this, the conference policy control protocol is used to establish
these policies prior to the generation or acceptance of an invitation
to the conference. For example, if a user wishes to join a conference
with a known conference URI, the user would obtain the URI for the
conference policy, manipulate the policy to set themself as an
anonymous participant, and then actually join the conference by
sending an INVITE request to the conference URI.

OPEN ISSUE: Will this always work? Are there cases where
the conference policy cannot be manipulated until the
INVITE has been sent? This would require a preconditions-
style solution.

7.3 Removing Participants

As with additions, there are two modalities for departures - first
person (in which a user explicitly leaves), and third person, where
they are removed by a different user.

First person departures are trivially accomplished by terminating the
dialog that the participant is using to connect to the focus.

Third person departures can be done in one of two ways. First, a user
can make use of the REFER method to instruct the third party to send
a BYE to the conference server on the dialog that connects them to
the focus. This requires the user to have knowledge of the dialog
identifiers used by that participant. The second mechanism, which is
much cleaner, is to use the conference policy control protocol to
inform the focus that the participant is explicitly barred from the
conference. This will cause the focus to eject the user, sending them
a BYE in addition to whatever other signaling is needed to remove
them. The conference policy control protocol can also be used to
remove a large number of users. This is generally referred to as mass
ejection.

7.4 Approving Policy Changes

The general model to support moderator approval is through the
conference notification service. The moderator subscribes to the
notification service. They are authenticated by the focus, which
determines that they are a moderator for the conference. Whenever a
policy change request is made by a client that requires moderator
approval, the policy change is not actually committed. Rather, it is
marked as pending by the conference policy server. Any moderators for
that specific policy request who are subscribed to the conference
notification service will receive a notification of the pending
change. The moderators, using the conference policy control protocol,
can approve the specific change. This commits the new policy. All
participants are then notified of the new policy through the
notification service.

7.5 Creating Sidebars

There are two ways to represent a sidebar in this framework. The
first is to treat it as a specific kind of media policy. It is a
media policy which would request that sidebar participants be "in the
foreground", and others "in the background". There are no additional
dialogs or conferences established. The media policy control protocol
would allow a user to explicitly request sidebars. The server would
alert users (through the notification service) that they have been
invited to the sidebar. They would use the media policy control
protocol to approve their participation in it.

An alternative view is that a sidebar truly is a conference within a
conference, and would be implemented that way. There would be a new
conference URI associated with the sidebar. Standard techniques would
be used to add users to the sidebar, approve their membership, and so
on. The sidebar would itself be a participant in the main conference.
Users would continue to receive their media stream only through the
main conference. They would have a dialog with the sidebar focus, but
no media would be exchanged on this dialog.

OPEN ISSUE: It is still unclear as to which model is
preferrable. We should pick one.

8 Security Considerations

Conferences frequently require security features in order to properly
operate. The conference policy may dictate that only certain
participants can join, or that certain participants can create new
policies. Generally speaking, conference applications are very
concerned about authorization decisions. Mechanisms for establishing
and enforcing such authorization rules is a central concept
throughout this document.

Of course, authorization rules require authentication. Normal SIP
authentication mechanisms should suffice for the the conference
authorization mechanisms described here.

9 Contributors

This document is the result of discussions amongst the conferencing
design team. The members of this team include:

Brian Rosen
Rohan Mahy
Henning Schulzrinne
Orit Levin
Roni Even
Tom Taylor
Petri Koskelainen
Nermeen Ismail
Andy Zmolek
Joerg Ott
Dan Petrie

10 Authors Addresses

Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com

11 Normative References

12 Informative References

[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.

Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.

[2] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
transport protocol for real-time applications," RFC 1889, Internet
Engineering Task Force, Jan. 1996.

[3] O. Levin et al. , "Requirements for tightly coupled SIP
conferencing," Internet Draft, Internet Engineering Task Force, July
2002. Work in progress.

[4] A. B. Roach, "Session initiation protocol (sip)-specific event
notification," RFC 3265, Internet Engineering Task Force, June 2002.

[5] B. Campbell and J. Rosenberg, "Instant message sessions in
simple," Internet Draft, Internet Engineering Task Force, Oct. 2002.
Work in progress.

[6]

[17] J. Rosenberg and H. Schulzrinne, "A "An offer/answer model with
session initiation description protocol
(SIP) event package for conference state," Internet Draft, Internet
Engineering Task Force, June 2002. Work in progress.

[7] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform resource
identifiers (URI): generic syntax," (SDP)," RFC 2396, Internet Engineering
Task Force, Aug. 1998.

[8] H. Schulzrinne and J. Rosenberg, "Session initiation protocol
(SIP) caller preferences and callee capabilities," Internet Draft, 3264, Internet Engineering
Task Force, July June 2002. Work in progress.

[9] F. Cuervo, N. Greene, A. Rayhan, C. Huitema, B. Rosen, and J.
Segers, "Megaco protocol version 1.0," RFC 3015, Internet Engineering
Task Force, Nov. 2000.

[10]

[18] J. Rosenberg, P. Mataga, and H. Schulzrinne, "An application
server component architecture for SIP," Internet Draft, internet draft, Internet
Engineering Task Force, Mar. 2001. Work in progress.

[11]

[19] J. Rosenberg, J. Peterson, H. Schulzrinne, and G. Camarillo,
"Best current practices for third party call control in the session
initiation protocol," Internet Draft, internet draft, Internet Engineering Task
Force, June 2002. Work in progress.

[12] A. Johnston and O. Levin, "Session initiation call control -
conferencing for user agents," Internet Draft, Internet Engineering
Task Force, Oct. 2002. Work

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pertain to the implementation or use of the technology described in progress.

[13] R. Mahy and D. Petrie, "The session initiation protocol (sip)
join header," Internet Draft, Internet Engineering Task Force, Oct.
2002. Work
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has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in progress.

[14] J. Rosenberg standards-track and H. Schulzrinne, "A session initiation protocol
(SIP) event package for dialog state," Internet Draft, Internet
Engineering Task Force, June 2002. Work in progress.

[15] R. Sparks, "The SIP refer method," Internet Draft, Internet
Engineering Task Force, July 2002. Work
standards-related documentation can be found in progress.

[16] B. Campbell BCP-11. Copies of
claims of rights made available for publication and J. Rosenberg, "Session initiation protocol
extension any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for instant messaging," Internet Draft, Internet
Engineering Task Force, Sept. 2002. Work in progress. the use of such
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