One document matched: draft-barnes-xcon-framework-00.txt
INTERNET-DRAFT M. Barnes
Document: draft-barnes-xcon-framework-00.txt Nortel Networks
Category: Informational C.Boulton
Ubiquity
Expires: April 14, 2005 Oct 14, 2004
A Framework for Centralized Conferencing
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Abstract
This document describes a framework for Centralized Conferencing
(XCON). This XCON framework document provides an enhanced framework
for conferencing that is protocol agnostic. This document expands
upon the interfaces between the functional elements introduced in the
SIP conferencing framework by describing the characteristics of
connecting protocols and providing a related data model. However,
this framework is applicable for a variety of signaling protocols
besides SIP including H.323, XMPP, and even PSTN signaling protocols.
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Table of Contents
1. Introduction...................................................2
2. Conventions and Terminology....................................3
3. Overview of Conferencing Architecture.........................4
3.1 Usage of URIs.............................................7
4. Component Functionality.......................................8
4.2 Conference Policy Server.................................10
4.3 Mixers...................................................11
4.4 Conference Notification Service.........................11
4.5 Participants.............................................12
4.6 Conference Policy........................................12
5. Common Operations............................................13
5.1 Creating Conferences.....................................13
5.2 Adding Participants......................................15
5.3 Conditional Joins........................................15
5.4 Removing Participants....................................16
5.5 Creating Sidebars........................................16
5.6 Destroying Conferences...................................17
5.7 Obtaining Membership Information.........................18
5.8 Adding and Removing Media................................18
5.9 Conference Announcements and Recordings..................19
5.10 Floor Control...........................................21
5.11 Whispering or Private Messages...........................22
6. XCON Data Model...............................................23
7. Security Considerations.......................................23
8. IANA Considerations...........................................23
Informational References.........................................23
1. Introduction
The SIP conferencing framework [SIPCONFW] presents a general
architectural model for tightly coupled conferences. While the
primary focus of that document is to provide a model for SIP based
conferencing, the model itself was intended to be general purpose
and applicable to non-SIP protocols. This document outlines a
generic XCON architecture for tightly coupled conferences. It also
provides details of connecting protocols and a data model used to
expose interfaces to the primary XCON entities (e.g. Conference
Policy Server, Floor Control Server) and provide a clear depiction
of the primary data relationships between entities. An objective of
this XCON framework is to not impact the support of fundamental SIP
conferencing, but rather this XCON framework document is intended
to extend and enhance the architectural model as necessary to
provide a more general conference architecture that is protocol
agnostic. For example, this framework applies equally well to an
H.323, Jabber, or even PSTN conferencing system.
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2. Conventions and Terminology
This framework uses many of the terms introduced in the SIP
conferencing framework. In addition, it introduces new terms
associated with the new protocols and functionality, and to
describe the signaling interface between the conference
participants and the conference focus (Signaling I/F, Establish,
Modify and Tear down) in a protocol agnostic manner. The
convention in this document is to describe the signaling processing
using the new terms, while using SIP [RFC3261] to provide concrete
examples of the operations, when applicable.
o Conference Policy Control Protocol (CPCP): A protocol used by
clients to manipulate the membership policy.
o Establish: protocol operation applied to the signaling
interface between the focus and a participant to setup a
multimedia stream. (e.g. SIP INVITE)
o Floor: a term used to apply to a set of data or resources
associated with a conference instance, for which a conference
participant is granted temporary input access.
o Floor chair: A user (or an entity) who is authorized to manage
one floor (grants, denies, or revokes a floor). The floor
chair does not have to be a participant in the conference.
o Floor Control: mechanism enabling applications or users to gain
mutually exclusive or non-exclusive input access to the shared
object or resource associated with a specific conference
instance. Control of the "floor" is viewed as a temporary
permission.
o Floor Control Policy: A set of rules used as an alternative/in
conjunction with a chair controlled floor to define policy for
automatic generation of floor request decisions (grant,
reject, revoke a floor).
o Floor Control Protocol: a protocol used by XCON enabled clients
to manipulate the floor control policy to effect changes on
the conference policy to gain, modify or release control of
the floor.
o Floor control Server: A logical entity that maintains the state
of the floor(s) including which floors exists, who the floor
chairs are, who holds a floor, etc. Requests to manipulate a
floor are directed at the floor control server using the Floor
Control Protocol.
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o Modify: protocol operation applied to the signaling interface
between the focus and a participant to change the
characteristics of the media stream (e.g. SDP manipulation
within a SIP re-INVITE).
o Multimedia stream: in the context of this framework document,
this term is used to refer to the media composition of the
conference, which is established via the signaling protocol
interface between the focus and a participant. The stream
includes voice, video, session-mode instant messaging and
interactive text.
o Signaling Interface (I/F): the interface between a participant
and the focus.
o Tear down: protocol operation applied to the interface between
the focus and a participant to remove a participant from a
conference (e.g. SIP BYE).
3. Overview of Conferencing Architecture
+-----------+
| |
| |
|Participant|
| 4 |
| |
+-----------+
|
|Signaling
|I/F
|4
|
+-----------+ +----------+ +-----------+
| | | | | |
| | | | | |
|Participant|-------------| Focus |------------|Participant|
| 1 |Signaling | |Signaling | 3 |
| |I/F 1 | |I/F 3 | |
+-----------+ +----------+ +-----------+
|
|
|Signaling
|I/F
|2
|
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+-----------+
| |
| |
|Participant|
| 2 |
| |
+-----------+
Figure 1
The central component in a conference is the focus. The only
difference between the model put forth in [SIPCONFW] and the model
for the XCON framework is that the signaling relationship maintained
by the focus with each participant in the conference is not
restricted to the SIP protocol. Any multimedia signaling protocol
that defines procedures for the establishment of a relationship
between the focus and a participant could utilize that interface. As
a result, the logical result of the signaling communications
associated with a centralized conference remains the star topology,
as shown in Figure 1.
The XCON framework does not at all impact the role or logical
functionality of the focus as put forth in the [SIPCONFW]. While the
interface between the focus and the conference policy remains
implementation specific, the data associated with the conference
policy, which relates to specific functionality provided by the
focus, is discussed in greater detail in this and other XCON WG
documents. The primary difference between the architectural model
proposed in this document and the one in [SIPCONFW], is that the
interface between the participant and the focus is protocol agnostic
(i.e. not SIP specific). For example, the ejection of a user from
the conference would consist of the invocation of the tear down
operation specific to the protocol supported by that user (e.g. SIP
BYE).
As discussed in [SIPCONFW], a conference instance is represented by a
URI, which identifies the appropriate focus (responsible for
conference state associated with the URI). Each conference has a
unique focus and a unique URI identifying that focus. Requests to
the conference URI are routed to the focus for that specific
conference. Further detail on the usage of URIs is provided in
section 3.1.
Users usually join the conference by invoking the establish operation
specific to the protocol supported by that user (e.g. SIP INVITE),
using the conference URI as a target. As long as the conference
policy allows (and the establish request is appropriately
authenticated), the establish operation is accepted by the focus and
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the user is added to the conference. Users can leave the conference
by invoking the tear down operation, specific to the protocol
supported by that user (e.g. SIP BYE), as they would in a normal
multimedia session for that protocol.
Similarly, the focus can terminate a multimedia session with a
participant by invoking the tear down operation, should the
conference policy change to indicate that the participant is no
longer allowed in the conference. A focus can also invoke the
establish operation to add a participant, should the conference
policy indicate (manipulated by an authorized user) that the focus
needs to bring a participant into the conference.
.....................................
. .
. .
. .
. .
. Conference .
. Policy .
Conference . .
Policy . +-----------+ //-----\\ .
Control . | | || || .
Protocol . | Conference| \\-----// .
+---------------->| Policy | | | .
| . | Server |----> |Membership .
| . | | | | .
| . +-----------+ | & | .
| . | | .
| . | Media | .
+-----------+ . +-----------+ | Policy| .
| | . | | \ // .
| | . | | \-----/ .
|Participant|<--------->| Focus | | .
| |Signaling. | | | .
| | I/F . | |<-----------+ .
+-----------+ . |...........| .
^ . | Conference| .
| . |Notification .
+------------>| Service | .
Conference . +-----------+ .
State . .
Notifications. .
. .
. .
.....................................
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Conference
Functions
Figure 2
As outlined in [SIPCONFW], a conference-aware participant is one that
has access to advanced conference functionality through additional
protocol interfaces. The client uses these protocols to interact
with the conference policy server and the focus. A model for this
interaction is shown in Figure 2. A conference-unaware participant
would not implement the XCON protocols; as such, it is not discussed
in this document.
A conference-aware participant can use the unique conference URI to
request conference state updates. This involves connecting to the
conference notification service provided by the focus using the
appropriate signaling mechanism. Through this mechanism, the
participant can be notified of changes in participants (effectively,
the state of the signaling interfaces between the participants and
the focus), the media policy, and the membership policy.
The participant can communicate with the conference policy server
using a conference policy control protocol (CPCP). Through this
protocol, it can manipulate the conference policy. The requirements
for a CPCP are specified in a separate document [XCONCPRQ]. An
Extensible Markup Language (XML) [XML] schema enabling a user to
define a conference policy is defined in [XCONCPCP]. The assignment
of privileges which would allow a user to manipulate the conference
policy is defined in [XCONCPRV]. XML Configuration Access Protocol
(XCAP) is one proposed protocol mechanism [XCONCPXC] for manipulating
the conference policy data. Although [XCONCPXC] defines a specific
protocol mechanism, other interfaces (e.g. Web based) can be used to
manipulate the conference policy data adhering to the constraints
defined in [XCONCPCP].
The interfaces between the focus and the conference policy, and the
conference policy server and the conference policy, are not
standardized within this framework per se, but rather the data
related to those interfaces is discussed in the context of the
logical roles, with an associated data model provided in Section 6.
As such, these interfaces show the logical roles involved in a
conference, as opposed to suggesting a physical decomposition.
3.1 Usage of URIs
As discussed in [SIPCONFW], it is fundamental to this framework that
a conference is uniquely identified by a URI, and that this URI
identifies the focus responsible for the conference. The conference
URI is unique, such that no two conferences have the same conference
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URI at any one point in time. Some examples of conference URIs
include:
h323:conf312334@example.net
xmpp:conf.example.com
tel:+12025551212
sip:9023453@sip.example.net
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
[RFC2396]. However, contextual information surrounding the URI (for
example, SIP header parameters) may indicate that the URI represents
a conference.
When a request to establish a conference (e.g. SIP INVITE) is sent
using the conference URI, that request is routed to the associated
focus instance. The element or system that creates the conference
URI is responsible for guaranteeing this property.
Ideally, a conference URI is never constructed or guessed by a user.
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.
The other functions in a conference are also represented by URIs. If
the conference policy server is implemented through web pages, this
server is identified by HTTP URIs. If it is accessed using an
explicit protocol, it is a URI defined for that protocol.
Starting with the conference URI, the URIs for the other logical
entities in the conference can be learned using the conference
notification service. The exact method is protocol specific and
outside the scope of this document.
4. Component Functionality
This section provides a more detailed description of the functions
typically implemented in each of the elements that comprise an XCON
conference server. The primary difference between the functionality
in this framework and that described in [SIPCONFW] is that the
functionality is described in general terms, rather than SIP
specific. Thus, some information in this section is duplicated from
[SIPCONFW] to set the context for, and to provide the reader
familiarity with, the use of the general terminology introduced in
this framework.
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4.1 Focus
As its name implies, the focus instance is the central component of
the conference. All participants in a conference are connected to a
focus instance through the signaling interface established with a
conference participant. The focus is responsible for maintaining the
signaling interfaces connected to it. It ensures that the signaling
interfaces are connected to a set of participants who are authorized
to participate in the conference, as defined by the membership
policy. The focus also uses the signaling interface to manipulate
the media sessions, in order to make sure each participant obtains
all the appropriate media for the conference. To do that, the focus
makes use of mixers in conjunction with the media policy.
When a focus receives an establish request for the signaling
interface, it checks the membership policy. The membership policy
might indicate that this participant is not allowed to join, in which
case the request can be rejected. It might indicate that another
participant, acting as a moderator, needs to approve this new
participant. In that case, the establishment operation might be
deferred (e.g. parked on a music-on-hold server) or an in progress
operation might be invoked to indicate such to the participant. A
notification, using the conference notification service, would be
sent to the moderator. The moderator then has the ability to
manipulate the policies using a conference policy control protocol
(e.g. CPCP). If the policies are changed to allow this new
participant, the focus can accept the establishment request (e.g.
unpark it from the music-on-hold server). The interpretation of the
membership policy by the focus is, itself, a matter of local policy,
and not subject to standardization.
If a participant manipulated the membership policy to indicate that a
certain other participant was no longer allowed in the conference,
the focus would invoke a tear down operation (e.g. SIP BYE) towards
that required participant to remove them. This is often referred to
as "ejecting" a user from the conference.
Similarly, if a user manipulated the membership policy to indicate
that a number of users need to be added to the conference, the focus
would send establishment requests to those participants. This is
often referred to as the "mass invitation" function. A policy
request to add a set of users might not require any establishment
operations to execute it; those users might already be participants
in the conference.
The media policy model is extremely similar to that previously
described for membership policy. If media policy instructs a
modification, the focus instance will implement appropriately by
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either manipulating signaling via the signaling interface or
interacting directly with the media mixer. The explicit operations
required for enforcing media policy are considered out of scope for
this document.
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 instance.
The conference policy is represented by a URI. There is a unique
conference policy for each conference instance. The conference
policy URI points to a conference policy server which can manipulate
that particular 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. 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, 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 is responsible for reconciliation of
potentially conflicting requests regarding the policy for the
conference instance.
The client of the conference policy server can be any entity
interested in manipulating the conference policy. Clearly,
participants might be interested in manipulating conference policy.
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
add a user to the conference.
A client of the conference policy server 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 is/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 conference policy server,
and change the media policy based on who is allowed to speak.
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The client of the conference policy server 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 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.
Media streams can be grouped and labeled by the focus. For example,
this could be done in SDP [SDPMLABL]. This allows policies and
operations to be directed against a particular stream.
A mixer is always under the control of a focus, either directly or
indirectly. 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 some kind of
protocol. If the focus is indirectly controlling a mixer, it
delegates the mixing to the participants, each of which has their own
mixer. This is described in the context of SIP in Section 6.4 of
[SIPCONFW].
A mechanism to manipulate and describe the media mixing for the
various media types is described in the Media Policy Control document
[XCONMPCP], with scenarios defined in [XCONSCEN].
4.4 Conference Notification Service
The focus can provide a conference notification service. When
assuming this role, the conference focus will allow authenticated
clients to request being notified of conference state updates (e.g.
in SIP using [RFC3265]).
Once an XCON conference aware entity has requested such
notifications, it will receive conference state update information at
appropriate times. The conference state is composed of both focus
and conference policy state. The endpoint will be informed of
changes in either state. The notification protocol selected might
provide a mechanism for limiting the information provided by the
conference notification service (e.g. Capabilities defined in the SIP
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events framework [4] allow requests to receive focus state changes
only, conference policy state changes, or both).
The state of the focus includes the participants connected to the
focus, and detailed information regarding the connection. As new
participants join, this state changes, and is reported through the
notification service. Similarly, when a participant leaves, this
state also changes, allowing entities who have registered an interest
the ability to learn of the event.
As described previously, the conference policy includes the
membership policy and the media policy. As those policies change,
due to usage of the CPCP, direct change by the focus, or through an
application, the conference notification service informs entities who
have registered an interest of these changes.
4.5 Participants
This framework defines a participant as an endpoint which has a
signaling relationship with the focus. Note that a participant can
also be another focus. A conference which has a participant that is
the focus of another conference is called a 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 participant may support a CPCP protocol, the Conference
Notification interface and/or a floor control protocol to make full
use of the XCONFW functionality described in this framework.
4.6 Conference Policy
The conference policy contains the rules that guide the operation of
the focus. 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 complex, specifying time-of-day based rules on
participation conditional on the presence of other participants.
There is no restriction on the type of rules that can be encapsulated
in a conference policy.
The conference policy can be manipulated using web applications or
voice applications. It can also be manipulated with proprietary
protocols. A conference policy control protocol (CPCP) is proposed
as a standardized means of manipulating the conference policy as
described in the CPCP requirements [XCONCPRQ]. An [XML] data schema
enabling a user to define a conference policy is defined in
[XCONCPCP]. The assignment of privileges allowing a user to
manipulate the conference policy is defined in [XCONCPRV]. XML
Configuration Access Protocol (XCAP) is proposed as one protocol
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mechanism [XCONCPXC] to store and manipulate the conference policy
data. By the nature of conference policies, not all aspects of the
policy can be manipulated with a conference policy control protocol.
The conference policy includes the membership policy and the media
policy. The membership policy includes per-participant policies that
specify how the focus is to handle a particular participant. These
include whether or not the participant is anonymous, for example.
The media policy describes the way in which the set of inputs to a
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.
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.
[Editor's note: Will provide more media policy detail in next
revision of this document.]
5. Common Operations
There are a large number of 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 they operate. In addition,
this section addresses how some of the scenarios identified in
[XCONSCEN] can be realized with the functionality provided by the
components. The SIP specific mechanisms for some of these common
operations are described in [SIPCONFW]. Note that non-automated
means, such as a web page or IVR interface could be used for these
operations. However, this is outside the scope of this framework
which is to define automated means and protocols.
5.1 Creating Conferences
There are many ways in which a conference can be created. The
creation of a conference actually constructs several elements all at
the same time. It results in the creation of a focus and a
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conference policy. It also results in the construction of a
conference URI, which uniquely identifies the focus. Since the
conference URI needs to be unique, the element which creates
conferences is responsible for guaranteeing that uniqueness. This
can be accomplished deterministically, by keeping records of
conference URIs, or by generating URIs algorithmically, or
probabilistically, by creating random URI with sufficiently low
probabilities of collision.
When a media and conference policy are created, they are established
with default rules that are implementation dependent. If the creator
of the conference wishes to change those rules, they would do so
using a conference policy control protocol (CPCP), for example.
Of course, using a CPCP requires that an element know the URI for
manipulating the policy. That requires a means to learn the
conference policy URI from the conference URI, since the conference
URI is frequently the sole result returned to the client as a 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 CPCP Mechanisms
An XCON conference instance can be created through interaction with
the conference policy server, as defined in section 4.2. The
creation process involves the creation of a membership policy
resource as defined in [XCONCPCP]. The protocol interaction between
the requesting entity and the policy server are defined in separate
XCON documents, such as [XCONCPXC].
The creation of a new membership policy resource will be required to
conform to the schema detailed in [XCONCPCP]. In many cases, the
creator of the conference policy resource is the sole user with
access rights to the conference policy and other users do not have
any rights to view nor modify the document. However, some scenarios
require different privileges to allow other users to modify certain
parts of the conference policy XML document. The mechanism to provide
these user privileges is defined in [XCONCPRV]. The constraints
imposed on the creation of a new conference instance using this
method must be enforced by the conference policy server and any
additional constraints are subject to local policy (e.g. maintenance
and handling of unique conference URI's).
A successful membership policy creation will result in the automatic
generation of all other required conference state components. If not
otherwise specified, the mandatory conference state components
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include default media policy and URI, Floor Control and URI, focus
instance and Signaling I/F instance etc.
5.2 Adding Participants
There are many mechanisms for adding participants to a conference.
These include using the Signaling I/F (for SIP described in
[SIPCONFW]), the conference policy control protocol, and non-
automated means. In all cases, participant additions can be first
party (a user adds themselves) or third party (a user adds another
user).
5.2.1 CPCP Mechanisms
The conference membership policy semantics are defined in [XCONCPCP].
The semantics allow for participants of a conference instance to be
added at both the instantiation and during the life time of a
conference. The request to add additional participants must comply
with the constraints detailed in [XCONCPRV] and violations will
result in failure of the operation. The supporting protocol
interaction between the requesting entity and the policy server are
defined in separate XCON documents, such as [XCONCPXC].
A successful participant addition will result in implicit operations
which complement the updated membership policy. This includes the
creation/application of media policy, the triggering of conference
notification service messages and appropriate focus signaling using
the Signaling I/F. The floor control server should also be capable
of accepting floor control requests from the additional participants.
5.3 Conditional Joins
Conference policies are installed during conference instantiation for
the purpose of defining both membership and media policies for a
unique conference instance. The conference policy is a bi-
directional process as a participant might only wish to join the
conference instance if certain policies are set in a desired manner.
The flexibility of achieving such conference policy manipulation is
dependant on the security policies being enforced by the conference
policy server.
Examples can be conveyed for both media and membership policy. On
receiving a conference URI, an XCON aware endpoint has the ability to
use the appropriate policy interface and manipulate conference policy
before joining. For example, a user might wish to enter the
conference instance anonymously. This can be achieved by
manipulating the conference policy before joining, prior to the
acceptance of a conference invitation. This would allow the
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participant to join the conference instance by sending an XCON
establish request to the focus but the endpoints identity would not
be revealed to the remaining participants, however, the participants
would be informed that a new participant has joined. This example
can be applied to any conference policy feature.
Similar examples can be conveyed for media policy. Following on from
the previous example, the 'Anonymous' participant may be a supervisor
who just wishes to observe a current conference instance. The XCON
capable endpoint would manipulate the media policy (using the
appropriate XCON interface) before joining the conference instance.
This might involve muting the input media stream so that output media
can be observed but none injected into the mix.
Requiring that any conference policy features be enforced before
joining a conference instance can be seen as examples of a
conditional join.
5.4 Removing Participants
CPCP can be used by a client to remove any participant (including
themselves) as long as the semantics defined in [XCONCPCP] are obeyed
and the initiator of the request has sufficient
authentication/authorization as defined in [XCONCPRV]. When CPCP is
used for this purpose, the focus will send a termination request to
the participant that is being removed using the signaling interface.
The focus will execute any other signaling that is needed to remove
the participant (for example, manipulate other signaling
connections). The change in membership policy will result in focus
initiated updates of conference state using the conference
notification service and the signaling interface.
The conference policy control protocol can also be used to remove a
large number of users. This is generally referred to as mass
ejection.
5.5 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
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other conference, one can join it by sending an establish request 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 signaling
relationship 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 establish request using the signaling interface 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
be added to the sidebar. Further detail on sidebars is provided in
[XCONSIDE].
5.6 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 establish
request to it, or request conference notifications from it, would
result in an error response.
Typically, if a conference is destroyed while there are still
participants, the focus would send a tear down 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.
5.6.1 CPCP Mechanisms
A CPCP can be used by a client to destroy a conference instance as
long as the semantics defined in [XCONCPCP] are obeyed and the
initiator of the request has sufficient authentication/authorization
as defined per [XCONCPRV]. When CPCP is used for this purpose, the
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focus will first send both termination requests to all the conference
instance participants and conference notification terminations using
the signaling interface. The focus will execute any other signaling
that is needed to remove the conference instance (for example,
manipulate other signaling connections). Once all relevant signaling
has occurred, the focus instance and all related policy state
information can be destroyed.
5.7 Obtaining Membership Information
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.
5.7.1 CPCP Mechanisms
The CPCP can be used by a client to retrieve the members of a
conference instance as long as the semantics defined in [XCONCPCP]
are obeyed and the client has the privilege as defined in [XCONCPRV].
The supporting protocol interaction, for carrying out the retrieval,
between the requesting entity and the policy server are defined in
separate XCON documents, such as [XCONCPXC].
5.8 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 modify
request 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, but rather 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.
5.8.1 MPCP Mechanisms
The MPCP can be used by a client to add/remove media streams of a
conference instance as long as the semantics defined in [XCONMPCP]
are obeyed and the initiator of the request has sufficient
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authentication/authorization. The supporting protocol interaction,
for carrying out the retrieval, between the requesting entity and the
policy server are defined in separate XCON documents, such as
[XCONMPCP].
The addition/removal of media from a conference instance will result
in focus operations such as updates in both connection signaling and
notification service updates using the signaling interface. Media
updates will also have subsequent impacts on media policy and floor
control (e.g. creation/deletion of a conference floor).
5.9 Conference Announcements and Recordings
Conference announcements and recordings play a key role in many real
conferencing systems. Examples of such features include:
o Asking a user to state their name before joining the conference,
in order to support a roll call
o Allowing a user to request a roll call, so they can hear who
else is in the conference
o Allowing a user to press some keys on their keypad in order to
record the conference
o Allowing a user to press some keys on their keypad in order to
be connected with a human operator
o Allowing a user to press some keys on their keypad to mute or
un-mute their line
User 1
+-----------+
| |
| |
|Participant|
| 1 |
| |
+-----------+
|Signaling
|I/F 1
Conference |
Policy +---|--------+
User 2 Server | | | Application
+-----------+ +-----------+ | CPCP *************
| | | | |-------- * *
| | | | | * *
|Participant|-----------| Focus |------------*Participant*
| 2 | Signaling| | |Signaling* 4 *
| | I/F 2 | |--+ I/F 4 * *
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+-----------+ +-----------+ *************
|
|
|Signaling
|I/F 3
|
|
+-----------+
| |
| |
|Participant|
| 3 |
| |
+-----------+
User 3
Figure 4
In this framework, these capabilities are modeled as an application
which acts as a participant in the conference. This is shown
pictorially in Figure 4. The conference has four participants.
Three of these participants are end users, and the fourth is the
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. The application would have configured appropriate
media policy using the appropriate XCON interface to allow for media
functions to act in this particular role (e.g. the input stream
policy would be activated while the output stream would have gain set
to mute).
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
a 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
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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.
5.10 Floor Control
Within this framework, floor control is defined as a mechanism that
enables applications or users to gain safe and mutually exclusive or
non-exclusive input access to the shared object or resource
associated with a specific conference instantiation. Floor control
is managed by an entity that is referred to as a "chair". The chair
does not have to be a participant in the conference. A floor chair
is not mandatory for grant, deny, or revoke floor operations and
decisions can automatically be generated based on floor control
policy (e.g. floor grant based on queue position). A floor control
server is a logical entity that maintains the state of the floor(s)
including which floors exists, who the floor chairs are, who holds a
floor, etc. Requests to manipulate a floor are directed at the floor
control server. The chair may use CPCP to enforce the resulting
floor control decisions by manipulating the conference policy,
however, the requirements for the protocol to support floor control
identified in [XCONFCRQ] are independent of the use of CPCP. A
proposal for a binary floor control protocol is defined in
[XCONBFCP]. Figure 5 provides an overview of the functionality
supported by the floor control protocol:
+---------+
| Floor |
| Chair |
| |
+---------+
^ |
| |
Notification | | Decision
| |
| |
Floor | v
+---------+ Request +---------+ +---------+
| |----------->| Floor | Notification | |
| User | | Control |------------->| User |
| |<-----------| Server | | |
+---------+ Granted or +---------+ +---------+
Denied
Figure 5: Functionality provided by Floor Control Protocol
A Floor has a 1:1 mapping with a media type contained within a
conference instance. Such media is represented using the Session
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Description Protocol (SDP) [RFC2327]. Each media type, as defined by
the 'm=' line in an SDP description can have an associated floor if
implemented. A correlation needs to exist so that media lines
contained in SDP can be mapped to a floor instance existing in the
media policy of the conference instance. This can be achieved using
the media label attribute [SDPMLABL] which creates an identifier for
an SDP media line, for example:
m=audio 6967 RTP/AVP 0
a=label:1
A floor defined within a media policy will also have an identifying
attribute to distinguish it from other floors [XCONMDTP]. The value
of this attribute maps directly to the value conveyed in the 'label'
attribute. For example, the media line defined in the previous
example has an SDP 'label' attribute value of '1'. The media policy
for this unique conference instance would also have a floor
definition that contains an identifying attribute equal to '1'. If
media policy is altered for this particular floor, the new policy can
be applied to the correct media stream in the conference instance
using this correlating identifier.
5.11 Whispering or Private Messages
A whisper is a private message sent between participants in a
conference or a conference sidebar. A whisper manifests itself as a
temporary alteration to the media policy, instructing the mixer to
temporarily restrict the distribution of a particular media stream to
a single conference participant or a subset of the participants.
The whispered media stream is marked as "private" so that the
recipient can render it in an appropriate way. For example, a private
instant message could be rendered along with the rest of the messages
in the conference, but with a different color, or tagged as
"private". The way in which whispered media streams are marked as
private is dependent on the type of the media stream. For example, an
instant message could have a dedicated command for sending a private
message, or an explicit indicator imbedded in the message header.
This indicator would both instruct the mixer in proper handling of
the message, and the recipient in proper rendering of the message.
Whether whispering is allowed in a conference is a configurable
option. This option is set as part of the conference policy using
CPCP and the support for this feature is negotiated with the focus
when a participant joins the conference.
OPEN ISSUE: How will the whisper mode be set? It probably needs to
be defined per media, so a natural place would then be the media
policy?
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The difference between a sidebar and a whisper is that a sidebar
creates a context for the (potentially) private discussion, while a
whisper is logically part of the existing context of the conference
or conference sidebar establishes no additional context.
6. XCON Data Model
This section defines a data model supporting and expanding upon the
fundamental logical conferencing model defined in Figure 2. This
model provides the basis of the functionality realized by the
protocols and mechanisms defined in the individual XCON documents
referenced in the previous sections of this document.
[Editor's note: Nice ascii art diagram to be inserted here once we've
finalized a model using some friendlier drawing tools.]
7. Security Considerations
The framework put forth in this draft introduces signaling interfaces
which have a variety of potential threats. Each of the specific
protocols defined in support of this framework must adequately
address those threats.
8. IANA Considerations
This draft introduces no considerations for IANA.
Informational References
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session
Initiation Protocol", RFC 3261, June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[SDPMLABL] O. Levin, G. Camarillo, "The SDP (Session Description
Protocol) Label Attribute", draft-ietf-mmusic-sdp-media-label-00.txt,
Work in Progress, September 28, 2004.
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XCON Framework October 14th, 2004
[SIPCONFW] J. Rosenberg, "A Framework for Conferencing with the
Session Initiation Protocol," draft-ietf-sipping-conferencing-
framework-02, Work in Progress, June 29, 2004.
[XCONBFCP] G. Camarillo, J. Ott, K. Drage, " The Binary Floor Control
Protocol (BFCP)", draft-ietf-xcon-bfcp-00.txt, Work in Progress, July
6, 2004.
[XCONCPRQ] P. Koskelainen, H. Khartabil, "Requirements for Conference
Policy Control Protocol", draft-ietf-xcon-cpcp-reqs-04, Work in
Progress, August 12, 2004.
[XCONCPCP] H. Khartabil, P. Koskelainen, A. Niemi, "The Conference
Policy Control Protocol (CPCP) ", draft-ietf-xcon-cpcp-01, Work in
Progress, October 12, 2004.
[XCONCPRV] H. Khartabil, A. Niemi, "Privileges for Manipulating a
Conference Policy", draft-ietf-xcon-conference-policy-privileges-01,
Work in Progress, October 12, 2004.
[XCONCPXC] H. Khartabil,"An Extensible Markup Language (XML)
Configuration Access Protocol(XCAP) Usages for Conference Policy
Manipulation and Conference Policy Privileges Manipulation ", draft-
ietf-xcon-cpcp-xcap-03, Work in Progress, October 12, 2004.
[XCONFCRQ] P. Koskelainen, J. Ott, H. Schulzrinne, X. Wu,
"Requirements for Floor Control Protocol", draft-ietf-xcon-floor-
control-req-01.txt, Work in Progress, July 19, 2004.
[XCONMPCP] C. Jennings, B. Rosen, "Media Conference Server Control
for XCON", draft-jennings-xcon-media-control-01, Work in Progress,
July 12, 2004.
[XCONMDTP] C. Boulton, TBD.
[XCONSCEN] R. Even, N. Ismail, "Conferencing Scenarios", draft-ietf-
xcon-conference-scenarios-02.txt, Work in Progress, June, 2004.
[XCONSIDE] B. Rosen, A. Johnston, "SIP Conferencing: Sub-conferences
and Sidebars", draft-rosen-xcon-conf-sidebars-01, Work in Progress,
July 16, 2004.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
"Extensible Markup Language (XML) 1.0 (Second Edition)", W3C REC REC-
xml-20001006, October 2000.
Acknowledgements
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The initial text for this framework was based on [SIPCONFW] and
modified to provide the more general context for this framework, thus
the excellent work of Jonathan Rosenberg and the original
conferencing design team is much appreciated in providing the
starting point for this framework document. The constructive input
and guidance from Alan Johnston for this document is appreciated. Aki
Niemi provided the initial text for the section on "whispering". And,
of course, the ongoing work in the XCON WG in forming the content of
this draft is appreciated.
Authors' Addresses
Mary Barnes
Nortel Networks
2380 Performance Drive
Richardson, TX USA
Phone: 1-972-684-5432
Email: mary.barnes@nortelnetworks.com
Chris Boulton
Ubiquity Software
Langstone Park
Newport,
South Wales, UK,
NP18 2LH
Phone: +44 (0)1633 765600
Email: cboulton@ubiquitysoftware.com
Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND TH INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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