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Versions: 00 01 draft-ietf-sipping-conferencing-framework

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


   A Framework for Conferencing with the Session Initiation Protocol

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.

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 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.







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                           Table of Contents



   1          Introduction ........................................    3
   2          Terminology .........................................    3
   3          Basic Architecture ..................................    7
   4          Usage of URIs .......................................   11
   5          Functions of the Elements ...........................   12
   5.1        Focus ...............................................   12
   5.2        Conference Policy Server ............................   13
   5.3        Mixers ..............................................   14
   5.4        Media Policy Server .................................   14
   5.5        Conference Notification Service .....................   15
   5.6        Participants ........................................   16
   5.7        Conference Policy ...................................   16
   5.8        Media Policy ........................................   17
   6          Physical Realization ................................   17
   6.1        Centralized Server ..................................   17
   6.2        Endpoint Server .....................................   17
   6.3        Media Server Component ..............................   18
   6.4        Distributed Mixing ..................................   21
   6.5        Cascaded Mixers .....................................   22
   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          Contributors ........................................   29
   10         Authors Addresses ...................................   29
   11         Normative References ................................   29
   12         Informative References ..............................   29














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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.

   In another model, sometimes referrred 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, although basic ones are 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 is an overused term which has
             different meanings in different contexts. In SIP, a
             conference is an instance of a multi-party conversation.



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             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 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. The focus maintains a SIP signaling
             relationship with each participant in the conference. The
             focus is responsible for insuring, 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: The set of user agents, each identified by a URI,
             which are connected to the focus for a particular
             conference.

        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 rules
             associated with participation in a 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: A media policy server is a logical function



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             which can store and manipulate rules associated with the
             media distribution of the conference. These rules can
             specify 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: 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
             mixer receives a set of media streams, and combines their
             media in a type-specific manner, redistributing the result
             to each participant. We use the term here to include
             combining of non-RTP 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 Participant: A basic 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 call.

        Cascaded Conference: A conference in which a participant is 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, in addition to the focus.

        Complex Participant: A complex participant is a participant in a
             conference that has learned, through automated means, that



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             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, and then destroyed in which the last participant
             has left.

        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 as a "conference within
             the conference". It is a dicsussion amongst a subset of the
             participants, not heard by the remaining participants in
             the conference.




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

        Invisible Participant: An invisible 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 of the conference. Any
   participant that is involved in the conference is connected to the
   focus by a SIP dialog. 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.



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                           +-----------+
                           |           |
                           |           |
                           |Participant|
                           |           |
                           |           |
                           +-----------+
                                 |
                                 |SIP
                                 |Dialog
                                 |
                                 |
   +-----------+           +-----------+            +-----------+
   |           |           |           |            |           |
   |           |           |           |            |           |
   |Participant|-----------|   Focus   |------------|Participant|
   |           |  SIP      |           |   SIP      |           |
   |           |  Dialog   |           |   Dialog   |           |
   +-----------+           +-----------+            +-----------+
                                 |
                                 |
                                 |SIP
                                 |Dialog
                                 |
                                 |
                           +-----------+
                           |           |
                           |           |
                           |Participant|
                           |           |
                           |           |
                           +-----------+




   Figure 1: Basic SIP Conference



   A complex SIP conference is one in which additional interfaces are
   exposed, allowing for a richer set of controls and information on the
   conference. In particular, a complex SIP conference can include a



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   conference policy server and a media policy server, and the focus can
   expose a conference notification service. The model for these
   conferences is shown in Figure 2. This figure shows the view from one
   participant. The conference now encompasses an additional set of
   functions. In addition to maintaining the dialog with the focus, the
   participant now has access to these other functions. It can, using a
   conference event package [6], SUBSCRIBE to the conference URI, and be
   connected to the conference notification service provided by the
   focus. Through this package, it can learn about changes in
   participants (effectively, the state of the dialogs), the media
   policy, and the conference policy.

   The participant can also communicate with the conference policy
   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 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 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
   the conference notification service.

   The conference policy and media policy servers 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 is
   a complex participant. Any conference can include basic and complex
   participants.

   The interfaces between (1) the focus and the media policy, (2) the
   focus and the conference policy, (3) 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



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           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



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   to encourage clarity in the requirements and to allow individual
   implementations the flexibility to compose a conferencing system in a
   scalable and robust manner.

4 Usage of URIs

   It is fundamental to this framework that a conference is uniquely
   identified by a URI, and that this URI identify the focus which is
   responsible for the conference. This 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]. However, contextual information surrounding the URI (for
   example, SIP header parameters) may indicate that the URI represents
   a conference.

   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
   conference URI can also represent 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.

   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.

   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, the response will indicate such [TBD]. The response will also
   indicate whether or not the focus has implemented the subscription
   notification service. This is known by the presence of an Allow
   header in the response, indicating 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 caller preferences specification
   [8] can indicate the same information that would be learned through



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   an OPTIONS query.

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

   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 Functions of the Elements

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

5.1 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 that the dialogs are connected to a set
   of participants who are allowed to participate in the conference, as
   defined by the conference 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.

   When a focus receives an INVITE, it checks the conference policy. The
   conference 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 policy by the focus is, itself,
   a matter of local policy, and not subject to standardization.




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   If a participant manipulated the conference 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 manipulated the conference 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. 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 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. 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 will result in the construction of
   a new conference URI, which can then be used to join the conference
   itself.

   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



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   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 in the instantiation of a focus
   (and therefore, a conference URI associated with that focus), a
   conference policy, and a media policy. The conference policy server
   will also have rules about who can create conferences.

   The conference policy also 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.

5.3 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 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].

   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.



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




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   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. Similarly, when someone leaves, this state also
   changes, allowing subscribers to learn about this fact.

   The state of the conference policy includes the set of participants
   that are allowed, or not allowed, to join the conference, and the set
   of participants who are to be explicitly added to the conference. It
   includes the roles which are assigned 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 of the media policy includes the media streams being
   received by each participant, the audio or video modalities, and so
   on.

5.6 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 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 Conference Policy

   The conference policy contains the rules that guide the operation of
   the focus. These 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 conference policy. This is done by
   communicating with the conference policy server, which manipulates
   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 conference policy
   server to reconcile the various requests with the conference policy.



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5.8 Media Policy

   The media policy contains the rules that guide the operation of the
   mixer. The focus uses these rules to interact with the mixer to
   implement them. These rules can be simple (mix all media from all
   participants), or they can be incredibly complex. It is important to
   understand that there is no restriction on the type of rules that can
   be encapsulated in a media policy.

   However, there does exist a protocol means by which a client can
   request a change in the media policy. This is done by communicating
   with the media policy server, which manipulates the media policy. By
   the nature of media policies, not all aspects of the policy can be
   manipulated with the media policy control protocol. It is the
   responsibility of the media policy server to reconcile the various
   requests with the media policy.

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, the media policy server, and the mixer.
   This is the classic "one box" solution, shown in Figure 3.


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. B would
   re-INVITE A 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 A and C together,
   redistributing the results. The mixed media is also played locally.
   Figure 4 shows a diagram of this transition.


   It is important to note that the external interfaces in this model,



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                            Conference Server
                   ...................................
                   .                                 .
                   . +------+        +------------+  .
                   . |Media |        | Conference |  .
                   . |Policy|        |Notification|  .
                   . |Server|        |   Server   |  .
                   . +------+        +------------+  .
                   . +----------+                    .
                   . |Conference|                    .
                   . |  Policy  | +-------+ +-----+  .
                   . |  Server  | | Focus | |Mixer|  .
                   . +----------+ +-------+ +-----+  .
                   ................//.\.......--./....
                                 //    \  ----  /
                               //      -\-      /RTP
                       SIP   //    ---- \      /
                           //   ---      \SIP /
                         // ---- RTP      \   /
                        / --               \ /
                 +-----------+         +-----------+
                 |Participant|         |Participant|
                 +-----------+         +-----------+








   Figure 3: Centralized server architecture


   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 media policy server and conference subscription server too,
   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




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         B                              B
      +------+                       +------+
      |      |                       |      |
      |  UA  |                       |  UA  |
      |      |                       |      |
      +------+                       +------+
        |  .                           |  .
        |  .                           |  .
        |  .                           |  .
        |  .         Transition        |  .
        |  .        ------------>      |  .
     SIP|  .RTP                     SIP|  .RTP
        |  .                           |  .
        |  .                           |  .
        |  .                           |  .
        |  .                           |  .
        |  .                       +----------+
      +------+                     | +------+ |   SIP    +------+
      |      |                     | |Focus | |----------|      |
      |  UA  |                     | |M.Pol.| |          |  UA  |
      |      |                     | |C.Pol.| |..........|      |
      +------+                     | |Mixer | |   RTP    +------+
                                   | +------+ |
         A                         |     +    |             C
                                   |     + <..|.......
                                   |     +    |      .
                                   | +------+ |      .
                                   | |Parti-| |      .
                                   | |cipant| |      .
                                   | |      | |      .
                                   | +------+ |      .
                                   +----------+      .
                                        B            .
                                                     .

                                                   Internal
                                                   Interface



   Figure 4: Transition from two-party call to conference







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                   +------------+             +------------+
                   | App  Server|  SIP        |Conf. Cmpnt.|
                   |            |-------------|            |
                   |   Focus    | Conf. Proto |   Focus    |
                   |   C.Pol    |-------------|   M.Pol    |
                   |   M.Pol    | Media Proto |   Mixer    |
                   |Notification|-------------|            |
                   |            |             |            |
                   +------------+             +------------+
                       |      \                    .. .
                       |       \\            RTP...   .
                       |         \\           ..      .
                       |     SIP   \\      ...        .
                   SIP |             \\ ...           .RTP
                       |              ..\             .
                       |           ...   \\           .
                       |        ...        \\         .
                       |      ..             \\       .
                       |   ...                 \\     .
                       | ..                      \    .
                  +-----------+              +-----------+
                  |Participant|              |Participant|
                  +-----------+              +-----------+



   Figure 5: 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



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   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].

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 no centralized mixer. Rather,
   there is a mixer in each endpoint, along with a media policy server.
   The focus distributes the media by using third party call control
   [11] 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.


   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



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   new participant to all the other participants, and vice-a-versa.

   The central conference server also includes a media 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 policy servers co-resident with a participant. As an
   example, if a participant decides to switch the overall conference
   mode from "video follows audio" to "tiled video", they would
   communicate with the central media policy server. This media policy
   server, in turn, would communicate with the media policy servers co-
   resident with each participant, using the same media policy control
   protocol, and instruct them to use "tiled video".

   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.

7 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 basic primitives,
   summarizing how they operate. More detailed examples with complete
   call flows can be found in [12].

7.1 Creating Conferences

   There are many ways in which a conference can be created. Ultimately,
   all of them result in 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 is responsible for managing
   URIs that are used by the focus. Otherwise, the uniqueness of
   conference URIs could not be guaranteed.




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                             +---------+
                             |Partcpnt |
                 media       |         |      media
              ...............|         |..................
              .              |  Mixer  |                 .
              .              |M.Pol.Srv|                 .
              .              +---------+                 .
              .                   |                      .
              .                   |                      .
              .                   |                      .
              .            dialog |                      .
              .                   |                      .
              .                   |                      .
              .                   |                      .
              .              +---------+                 .
              .              |Cnf.Srvr.|                 .
             .               |         |                 .
             .               |  Focus  |                 .
             .               |M.Pol.Srv|                 .
             .             / |C.Pol.Srv|  \              .
             .            /  +---------+   \             .
             .           /                  \            .
             .          /                    \           .
             .         /               dialog \          .
             .        /                        \         .
             .       /dialog                    \        .
             .      /                            \       .
             .     /                              \      .
             .    /                                \     .
             .                                           .
           +---------+                           +---------+
           |Partcpnt |                           |Partcpnt |
           |         |                           |         |
           |         | ......................... |         |
           |  Mixer  |                           |  Mixer  |
           |M.Pol.Srv|          media            |M.Pol.Srv|
           +---------+                           +---------+






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



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




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   Figure 7: 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.



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   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



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   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

   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.




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   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

   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.

   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



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   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.



J. Rosenberg                                                 [Page 29]


Internet Draft           Conferencing Framework         October 28, 2002


   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] J. Rosenberg and H. Schulzrinne, "A session initiation 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," 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,
   Internet Engineering Task Force, July 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] J. Rosenberg, P. Mataga, and H. Schulzrinne, "An application
   server component architecture for SIP," Internet Draft, Internet
   Engineering Task Force, Mar. 2001.  Work in progress.

   [11] 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 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 in progress.




J. Rosenberg                                                 [Page 30]


Internet Draft           Conferencing Framework         October 28, 2002


   [13] R. Mahy and D. Petrie, "The session initiation protocol (sip)
   join header," Internet Draft, Internet Engineering Task Force, Oct.
   2002.  Work in progress.

   [14] 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.

   [15] R. Sparks, "The SIP refer method," Internet Draft, Internet
   Engineering Task Force, July 2002.  Work in progress.

   [16] B. Campbell and J. Rosenberg, "Session initiation protocol
   extension for instant messaging," Internet Draft, Internet
   Engineering Task Force, Sept.  2002.  Work in progress.


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J. Rosenberg                                                 [Page 31]


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