[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-rosenberg-sipping-conferencing-framework) 00 01 02 03 04 05 RFC 4353

SIPPING                                                     J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: April 18, 2005                                 October 18, 2004


   A Framework for Conferencing with the Session Initiation Protocol
              draft-ietf-sipping-conferencing-framework-03

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 18, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

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,
   are more complicated.  This document defines a framework for how such
   conferencing can occur.  This framework describes the overall
   architecture, terminology, and protocol components needed for



Rosenberg                Expires April 18, 2005                 [Page 1]

Internet-Draft           Conferencing Framework             October 2004


   multi-party conferencing.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview of Conferencing Architecture  . . . . . . . . . . . .  7
     3.1   Usage of URIs  . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Functions of the Elements  . . . . . . . . . . . . . . . . . . 12
     4.1   Focus  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.2   Conference Policy Server . . . . . . . . . . . . . . . . . 13
     4.3   Mixers . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.4   Conference Notification Service  . . . . . . . . . . . . . 13
     4.5   Participants . . . . . . . . . . . . . . . . . . . . . . . 14
     4.6   Conference Policy  . . . . . . . . . . . . . . . . . . . . 14
   5.  Common Operations  . . . . . . . . . . . . . . . . . . . . . . 15
     5.1   Creating Conferences . . . . . . . . . . . . . . . . . . . 15
     5.2   Adding Participants  . . . . . . . . . . . . . . . . . . . 16
     5.3   Removing Participants  . . . . . . . . . . . . . . . . . . 16
     5.4   Creating Sidebars  . . . . . . . . . . . . . . . . . . . . 16
     5.5   Destroying Conferences . . . . . . . . . . . . . . . . . . 17
     5.6   Obtaining Membership Information . . . . . . . . . . . . . 17
     5.7   Adding and Removing Media  . . . . . . . . . . . . . . . . 17
     5.8   Conference Announcements and Recordings  . . . . . . . . . 18
     5.9   Floor Control  . . . . . . . . . . . . . . . . . . . . . . 20
   6.  Physical Realization . . . . . . . . . . . . . . . . . . . . . 21
     6.1   Centralized Server . . . . . . . . . . . . . . . . . . . . 21
     6.2   Endpoint Server  . . . . . . . . . . . . . . . . . . . . . 21
     6.3   Media Server Component . . . . . . . . . . . . . . . . . . 23
     6.4   Distributed Mixing . . . . . . . . . . . . . . . . . . . . 24
     6.5   Cascaded Mixers  . . . . . . . . . . . . . . . . . . . . . 26
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 29
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
   10.   Changes from draft-ietf-sipping-conferencing-framework-02  . 31
   11.   Changes from draft-ietf-sipping-conferencing-framework-00  . 32
   12.   Changes since
         draft-rosenberg-sipping-conferencing-framework-01  . . . . . 33
   13.   Changes since
         draft-rosenberg-sipping-conferencing-framework-00  . . . . . 34
   14.   Informative References . . . . . . . . . . . . . . . . . . . 34
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 35
       Intellectual Property and Copyright Statements . . . . . . . . 36








Rosenberg                Expires April 18, 2005                 [Page 2]

Internet-Draft           Conferencing Framework             October 2004


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.  This
   model is outside the scope of this document.

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

   This document is one of a series of specifications that discusses
   tightly coupled conferences.  Here, we present the overall framework
   for tightly coupled conferencing, referred to simply as
   "conferencing" from this point forward.  This framework presents a
   general architectural model for these conferences, presents
   terminology used to discuss such conferences, and describes the sets
   of protocols involved in a conference.  It also discusses the ways in
   which SIP itself is involved in conferencing.  The aim of the
   framework is to meet the general requirements for conferencing that
   are outlined in [3].  An additional document, the Centralized
   Conferencing (XCON) framework [16], discusses the non-SIP signaling
   aspects of conferencing in more detail, as well as providing
   additional functionality and details necessary for a generic protocol
   agnostic conferencing architecture.




Rosenberg                Expires April 18, 2005                 [Page 3]

Internet-Draft           Conferencing Framework             October 2004


2.  Terminology
   Conference: Conference is an overused term which has different
      meanings in different contexts.  In SIP, a conference is an
      instance of a multi-party conversation.  Within the context of
      this specification, a conference is always a tightly coupled
      conference.
   Loosely Coupled Conference: A loosely coupled conference is a
      conference without coordinated signaling relationships amongst
      participants.  Loosely coupled conferences frequently use
      multicast for distribution of conference memberships.
   Tightly Coupled Conference: A tightly coupled conference is a
      conference in which a single user agent, referred to as a focus,
      maintains a dialog with each participant.  The focus plays the
      role of the centralized manager of the conference, and is
      addressed by a conference URI.
   Focus: The focus is a SIP user agent that is addressed by a
      conference URI and identifies a conference (recall that a
      conference is a unique instance of a multi-party conversation).
      The focus maintains a SIP signaling relationship with each
      participant in the conference.  The focus is responsible for
      ensuring, in some way, that each participant receives the media
      that make up the conference.  The focus also implements conference
      policies.  The focus is a logical role.
   Conference URI: A URI, usually a SIP URI, which identifies the focus
      of a conference.
   Participant: The software element that connects a user or automata to
      a conference.  It implements, at a minimum, a SIP user agent, but
      may also include a conference policy control protocol client, for
      example.
   Conference State: The state of the conference includes the state of
      the focus and the conference policy.  Focus state includes the set
      of participants connected to the focus and the state of their
      respective dialogs.
   Conference Notification Service: A conference notification service is
      a logical function provided by the focus.  The focus can act as a
      notifier [4], accepting subscriptions to the conference state, and
      notifying subscribers about changes to that state.  The state
      includes the state maintained by the focus itself, the conference
      policy, and the media policy.
   Conference Policy Server: A conference policy server is a logical
      function which can store and manipulate the conference policy.
      The conference policy is the overall set of rules governing
      operation of the conference, and include membership policy and
      media policy.  Unlike the focus, there is not an instance of the
      conference policy server for each conference.  Rather, there is an
      instance of the conference policy for each conference instance.





Rosenberg                Expires April 18, 2005                 [Page 4]

Internet-Draft           Conferencing Framework             October 2004


   Conference Policy: The complete set of rules for a particular
      conference manipulated by the conference policy server.  The
      policy includes membership and media policies.  The conference
      policy is used to specify and control the operation of a
      conference instance.
   Membership Policy: A set of rules manipulated by the conference
      policy server regarding participation in a specific 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.
   Media Policy: A set of rules manipulated by the conference policy
      server regarding the media composition of the conference.  The
      media policy is used by the focus to determine the mixing
      characteristics for the conference.  The media policy includes
      rules about which participants receive media from which other
      participants, and the ways in which that media is combined for
      each participant.  In the case of audio, these rules can include
      the relative volumes at which each participant is mixed.  In the
      case of video, these rules can indicate whether the video is
      tiled, whether the video indicates the loudest speaker, and so on.
   Mixer: A mixer receives a set of media streams of the same type, and
      combines their media in a type-specific manner, redistributing the
      result to each participant.  This includes media transported using
      RTP [2].  As a result, the term defined here is a superset of the
      mixer concept defined in RFC 3550, since it allows for
      non-RTP-based media such as instant messaging sessions [5].
   Conference-Unaware Participant: A conference-unaware participant is a
      participant in a conference that is not aware that it is actually
      in a conference.  As far as the UA is concerned, it is a
      point-to-point call.
   Cascaded Conferencing: A mechanism for group communications in which
      a set of conferences are linked by having their focuses interact
      in some fashion.
   Simplex Cascaded Conferences: a group of conferences which are linked
      such that the user agent which represents the focus of one
      conference is a conference-unaware participant in another
      conference.
   Conference-Aware Participant: A conference-aware participant is a
      participant in a conference that has learned, through automated
      means, that it is in a conference, and that can use a conference
      policy control protocol, media policy control protocol, or
      conference subscription, to implement advanced functionality.
   Conference Server: A conference server is a physical server which
      contains, at a minimum, the focus.  It may also include a
      conference policy server and mixers.




Rosenberg                Expires April 18, 2005                 [Page 5]

Internet-Draft           Conferencing Framework             October 2004


   Mass Invitation: A conference policy control protocol request to
      invite a large number of users into the conference.
   Mass Ejection: A conference policy control protocol request to remove
      a large number of users from the conference.
   Sidebar: A sidebar appears to the users within the sidebar as a
      "conference within the conference".  It is a conversation amongst
      a subset of the participants to which the remaining participants
      are not privy.
   Anonymous Participant: An anonymous participant is one that is known
      to other participants through the conference notification service,
      but whose identity is being withheld.








































Rosenberg                Expires April 18, 2005                 [Page 6]

Internet-Draft           Conferencing Framework             October 2004


3.  Overview of Conferencing Architecture


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


                                Figure 1

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

   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



Rosenberg                Expires April 18, 2005                 [Page 7]

Internet-Draft           Conferencing Framework             October 2004


   more output media streams.  The focus uses the media policy to
   determine the proper configuration of the mixers.

   The focus has access to the conference and media policies, for which
   an instance of each exists for each conference.  Effectively, the
   conference policy can be thought of as a database which describes the
   way that the conference should operate.  It is the responsibility of
   the focus to enforce those policies.  Not only does the focus need
   read access to the database, but it needs to know when it has
   changed.  Such changes might result in SIP signaling (for example,
   the ejection of a user from the conference using BYE), and most
   changes will require a notification to be sent to subscribers using
   the conference notification service.  Further details on conference
   and media policy is provided in the XCON framework document [16].

   The conference is represented by a URI, which identifies the focus.
   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.

   Users usually 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.

   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 notion of a conference-unaware participant is important in this
   framework.  A conference-unaware participant does not even know that
   the UA it is communicating with happens to be a focus.  As far as
   it's concerned, its a UA just like any other.  The focus, of course,
   knows that its a focus, and it performs the tasks needed for the
   conference to operate.

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






Rosenberg                Expires April 18, 2005                 [Page 8]

Internet-Draft           Conferencing Framework             October 2004


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

                                        Conference
                                         Functions

                                Figure 2

   A conference-aware participant is one that has access to advanced
   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
   Figure 2.  The participant can interact with the focus using
   extensions, such as REFER, in order to access enhanced call control
   functions [7].  The participant can SUBSCRIBE to the conference URI,
   and be connected to the conference notification service provided by
   the focus.  Through this mechanism, it can learn about changes in



Rosenberg                Expires April 18, 2005                 [Page 9]

Internet-Draft           Conferencing Framework             October 2004


   participants (effectively, the state of the dialogs), the media
   policy, and the membership policy.

   The participant can communicate with the conference policy server
   using a conference policy control protocol.  Through this protocol,
   it can affect the conference policy.  The conference policy server
   need not be available in any particular conference, although there is
   always a conference policy.

   The interfaces between the focus and the conference policy, and the
   conference policy server and the conference policy are detailed in
   the XCON framework document [16].  For the purposes of SIP-based
   conferencing, they serve as logical roles involved in a conference,
   as opposed to representing a physical decomposition.  The separation
   of these functions is documented here to encourage clarity in the
   requirements and to ensure compatibility between SIP based
   conferencing and the extensions to the framework described in [16].
   More importantly, this approach provides individual SIP
   implementations the flexibility to compose a conferencing system in a
   scalable and robust manner without requiring the complete development
   of these interfaces.

3.1  Usage of URIs

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

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

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

   The conference URI can represent a long-lived conference or interest
   group, such as "sip:discussion-on-dogs@example.com".  The focus
   identified by this URI would always exist, and always be managing the
   conference for whatever participants are currently joined.  Other
   conference URIs can represent short-lived conferences, such as an



Rosenberg                Expires April 18, 2005                [Page 10]

Internet-Draft           Conferencing Framework             October 2004


   ad-hoc conference.

   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.  Specifically,
   the callee capabilities specification [9] provides the "isfocus"
   feature tag to indicate that the URI is a focus.  Caller preferences
   parameters are also used to indicate that a focus supports the
   conference notification service.  This is done by declaring support
   for the SUBSCRIBE method and the relevant package(s) in the caller
   preferences feature parameters associated with the conference URI.

   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.

























Rosenberg                Expires April 18, 2005                [Page 11]

Internet-Draft           Conferencing Framework             October 2004


4.  Functions of the Elements

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

4.1  Focus

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

   When a focus receives an INVITE, it checks the membership policy.
   The membership policy might indicate that this participant is not
   allowed to join, in which case the call can be rejected.  It might
   indicate that another participant, acting as a moderator, needs to
   approve this new participant.  In that case, the INVITE might be
   parked on a music-on-hold server, or a 183 response might be sent to
   indicate progress.  A notification, using the conference notification
   service, would be sent to the moderator.  The moderator then has the
   ability to manipulate the policies using the conference policy
   control protocol.  If the policies are changed to allow this new
   participant, the focus can accept the INVITE (or unpark it from the
   music-on-hold server).  The interpretation of the 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 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 user manipulated the membership policy to indicate
   that a number of users need to be added to the conference, the focus
   would send an INVITE to those participants.  This is often referred
   to as the "mass invitation" function.  As with ejection, it is
   fundamentally composed of the policy functions that specify the
   participants which should be present, and the implementation of those
   functions.  A policy request to add a set of users might not require
   an INVITE to execute it; those users might already be participants in



Rosenberg                Expires April 18, 2005                [Page 12]

Internet-Draft           Conferencing Framework             October 2004


   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.

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.

   Further detail on the functionality and access to the policy server
   are provided in the XCON framework document [16].

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.  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, 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 Section 6.4.

4.4  Conference Notification Service

   The focus can provide a conference notification service.  In this



Rosenberg                Expires April 18, 2005                [Page 13]

Internet-Draft           Conferencing Framework             October 2004


   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 two separate pieces.  The first is the
   state of the focus and the second is the conference policy.  A
   subscriber to the conference notification service can use
   capabilities defined in the SIP events framework [4] to request that
   it receive focus state changes only, conference policy changes only,
   or both.

   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 changes, and is reported through
   the notification service.  Similarly, when someone leaves, this state
   also changes, allowing subscribers to learn about this fact.

   Conference notification associated with changes to the conference
   policies is discussed in [16].

4.5  Participants

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

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 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.

   The conference policy can be manipulated using web applications or
   voice applications.  It can also be manipulated with proprietary
   protocols.  The conference policy control protocol is proposed as a
   standardized means of manipulating the conference policy.  Further
   detail on the conference policy and conference policy control
   protocol are provided in [16].





Rosenberg                Expires April 18, 2005                [Page 14]

Internet-Draft           Conferencing Framework             October 2004


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.  More detailed
   examples of the SIP mechanisms can be found in [7].

   As well as providing an overview of the common conferencing
   operations, each of the subsections in this section of the document
   provides a description of the SIP mechanism for supporting the
   operation.  Non-SIP mechansims are discussed in the XCON framework
   document [16].

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
   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 non-SIP mechanism.

   SIP can be used to create conferences hosted in a central server by
   sending an INVITE to a conferencing application that would
   automatically create a new conference and then place a user into it.

   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.



Rosenberg                Expires April 18, 2005                [Page 15]

Internet-Draft           Conferencing Framework             October 2004


   Alternatively, one UA can ask another UA to create an endpoint-hosted
   conference.  This is accomplished with the SIP Join header [10].  The
   UA which receives the Join header in an invitation may need to create
   a 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.

5.2  Adding Participants

   There are many mechanisms for adding participants to a conference.
   In all cases, participant additions can be first party (a user adds
   themself) or third party (a user adds another user).

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

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

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

5.3  Removing Participants

   As with additions, there are several mechanisms for departures.
   Removals can also be first person or third person.

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

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

5.4  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



Rosenberg                Expires April 18, 2005                [Page 16]

Internet-Draft           Conferencing Framework             October 2004


   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.

5.5  Destroying Conferences

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

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

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

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

5.6  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.

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

5.7  Adding and Removing Media

   Each conference is composed of a particular set of media that the



Rosenberg                Expires April 18, 2005                [Page 17]

Internet-Draft           Conferencing Framework             October 2004


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

   A SIP re-INVITE can be used by a participant to add or remove a media
   stream.  This is accomplished using the standard offer/answer
   techniques for adding media streams to a session [14].  This will
   trigger the focus to generate its own re-INVITEs.

5.8  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
      unmute their line





















Rosenberg                Expires April 18, 2005                [Page 18]

Internet-Draft           Conferencing Framework             October 2004


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

                                Figure 3

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

   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.



Rosenberg                Expires April 18, 2005                [Page 19]

Internet-Draft           Conferencing Framework             October 2004


   Similarly, the announcement application can play an announcement to a
   specific user by configuring 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.  To do this, it can use the application
   interaction framework [6].  This allows it to collect user input,
   possibly through keypad stimulus, and take actions.

5.9  Floor Control

   Floor control is similar to a conference announcement application.
   Within the context of this framework, floor control would be managed
   by an application, possibly one that is not a participant, that would
   use a non-SIP protocol to enforce the resulting floor control
   decisions.  Further detail on floor control is provided in the XCON
   framework document [16].
































Rosenberg                Expires April 18, 2005                [Page 20]

Internet-Draft           Conferencing Framework             October 2004


6.  Physical Realization

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

6.1  Centralized Server

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



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

                                Figure 4


6.2  Endpoint Server

   Another important model is that of a locally-mixed ad-hoc conference.
   In this scenario, two users (A and B) are in a regular point-to-point
   call.  One of the participants (A) decides to conference in a third
   participant, C.  To do this, A begins acting as a focus.  Its



Rosenberg                Expires April 18, 2005                [Page 21]

Internet-Draft           Conferencing Framework             October 2004


   existing dialog with B becomes the first dialog attached to the
   focus.  A would re-INVITE B on that dialog, changing its Contact URI
   to a new value which identifies the focus.  In essence, A "mutates"
   from a single-user UA to a focus plus a single user UA, and in the
   process of such a mutation, its URI changes.  Then, the focus makes
   an outbound INVITE to C.  When C accepts, it mixes the media from B
   and C together, redistributing the results.  The mixed media is also
   played locally.  Figure 5 shows a diagram of this transition.


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

                                                      Internal
                                                      Interface




Rosenberg                Expires April 18, 2005                [Page 22]

Internet-Draft           Conferencing Framework             October 2004


                                Figure 5

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

6.3  Media Server Component


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

                                Figure 6

   In this model, shown in Figure 6, each conference involves two
   centralized servers.  One of these servers, referred to as the
   "application server" owns and manages the membership and media
   policies, and maintains a dialog with each participant.  As a result,
   it represents the focus seen by all participants in a conference.
   However, this server doesn't provide any media support.  To perform
   the actual media mixing function, it makes use of a second server,
   called the "mixing server".  This server includes a focus, and a



Rosenberg                Expires April 18, 2005                [Page 23]

Internet-Draft           Conferencing Framework             October 2004


   conference policy server, but has no conference notification service.
   It has a default membership policy, which accepts all invitations
   from the top-level focus.  Its conference policy server accepts any
   controls made by the application server.  The focus in the
   application server uses third party call control to connect the media
   streams of each user to the mixing server, as needed.  If the focus
   in the application server receives a conference policy control
   command from 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.

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 are no centralized mixers.
   Rather, there are mixers in each endpoint, along with a conference
   policy server.  The focus distributes the media by using third party
   call control [15] 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 7.























Rosenberg                Expires April 18, 2005                [Page 24]

Internet-Draft           Conferencing Framework             October 2004


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

                                Figure 7

   There are several ways in which the media can be distributed to each
   participant for mixing.  In a multi-unicast model, each participant
   sends a copy of its media to each other participant.  In this case,
   the session description manages N-1 media streams.  In a multicast
   model, each participant joins a common multicast group, and each
   participant sends a single copy of its media stream to that group.
   The underlying multicast infrastructure then distributes the media,
   so that each participant gets a copy.  In a single-source multicast



Rosenberg                Expires April 18, 2005                [Page 25]

Internet-Draft           Conferencing Framework             October 2004


   model (SSM), each participant sends its media stream to a central
   point, using unicast.  The central point then redistributes the media
   to all participants using multicast.  The focus is responsible for
   selecting the modality of media distribution, and for handling any
   hybrids that would be necessitated from clients with mixed
   capabilities.

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

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

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

6.5  Cascaded Mixers

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



                              +---------+
      +-----------------------|         |------------------------+
      |   ++++++++++++++++++++|         |++++++++++++++++++      |
      |   +            +------|  Focus  |---------+       +      |
      |   +            |      |         |         |       +      |
      |   +            |    +-|         |--+      |       +      |
      |   +            |    | +---------+  |      |       +      |
      |   +            |    |      +       |      |       +      |



Rosenberg                Expires April 18, 2005                [Page 26]

Internet-Draft           Conferencing Framework             October 2004


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




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


                                Figure 8

   This architecture is shown in Figure 8.











Rosenberg                Expires April 18, 2005                [Page 27]

Internet-Draft           Conferencing Framework             October 2004


7.  Security Considerations

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

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

   Privacy is an important aspect of conferencing.  Users may wish to
   join a conference without anyone knowing that they have joined, in
   order to silently listen in.  In other applications, a participant
   may wish just to hide their identity from other participants, but
   otherwise let them know of their presence.  These functions need to
   be provided by the conferencing system.































Rosenberg                Expires April 18, 2005                [Page 28]

Internet-Draft           Conferencing Framework             October 2004


8.  Contributors

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


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

































Rosenberg                Expires April 18, 2005                [Page 29]

Internet-Draft           Conferencing Framework             October 2004


9.  Acknowledgements

   The authors would like to thank Mary Barnes and Chris Boulton for
   their comments.  Thanks to Allison Mankin for her comments and
   support of this work.














































Rosenberg                Expires April 18, 2005                [Page 30]

Internet-Draft           Conferencing Framework             October 2004


10.  Changes from draft-ietf-sipping-conferencing-framework-02

   Removed detailed discussions on policy servers, CPCP operations,
   sidebars, and approval of policy changes.  These now reside in the
   XCON framework draft, which is referenced from here now.














































Rosenberg                Expires April 18, 2005                [Page 31]

Internet-Draft           Conferencing Framework             October 2004


11.  Changes from draft-ietf-sipping-conferencing-framework-00

   Updated references and formatting cleanup.
















































Rosenberg                Expires April 18, 2005                [Page 32]

Internet-Draft           Conferencing Framework             October 2004


12.  Changes since draft-rosenberg-sipping-conferencing-framework-01
   o  Clarified that the conference notification service uses a single
      package with some kind of filtering to select whether you get the
      focus or policy state.















































Rosenberg                Expires April 18, 2005                [Page 33]

Internet-Draft           Conferencing Framework             October 2004


13.  Changes since draft-rosenberg-sipping-conferencing-framework-00
   o  Rework of terminology.
   o  More details on moderating policy changes.
   o  Rework of the overview, and in particular, a shift of focus from
      basic/complex conferences (a term which has been removed) to
      conference aware/unaware participants.
   o  Removal of explicit reference to megaco for controlling a mixer.
   o  Discussion of a lot more conferencing operations.
   o  New sidebar mechanism.

14  Informative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [2]   Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", RFC
         3550, July 2003.

   [3]   Levin, O. and R. Even, "High Level Requirements for Tightly
         Coupled SIP Conferencing",
         draft-ietf-sipping-conferencing-requirements-01 (work in
         progress), October 2004.

   [4]   Roach, A., "Session Initiation Protocol (SIP)-Specific Event
         Notification", RFC 3265, June 2002.

   [5]   Campbell, B., "The Message Session Relay Protocol",
         draft-ietf-simple-message-sessions-08 (work in progress),
         August 2004.

   [6]   Rosenberg, J., "A Framework for Application Interaction in the
         Session Initiation Protocol  (SIP)",
         draft-ietf-sipping-app-interaction-framework-02 (work in
         progress), July 2004.

   [7]   Johnston, A. and O. Levin, "Session Initiation Protocol Call
         Control - Conferencing for User Agents",
         draft-ietf-sipping-cc-conferencing-04 (work in progress), July
         2004.

   [8]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396, August
         1998.

   [9]   Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Indicating User
         Agent Capabilities in the Session Initiation Protocol (SIP)",



Rosenberg                Expires April 18, 2005                [Page 34]

Internet-Draft           Conferencing Framework             October 2004


         RFC 3840, August 2004.

   [10]  Mahy, R. and D. Petrie, "The Session Inititation Protocol (SIP)
         'Join' Header", draft-ietf-sip-join-03 (work in progress),
         February 2004.

   [11]  Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
         Event Package for the Session Initiation  Protocol (SIP)",
         draft-ietf-sipping-dialog-package-04 (work in progress),
         February 2004.

   [12]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
         Method", RFC 3515, April 2003.

   [13]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and
         D. Gurle, "Session Initiation Protocol (SIP) Extension for
         Instant Messaging", RFC 3428, December 2002.

   [14]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [15]  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
         2004.

   [16]  Barnes, M. and C. Boulton, "A Framework for Centralized
         Conferencing", draft-barnes-xcon-framework-00.txt (work in
         progress), September 2004.


Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   EMail: jdrosen@dynamicsoft.com
   URI:   http://www.jdrosen.net









Rosenberg                Expires April 18, 2005                [Page 35]

Internet-Draft           Conferencing Framework             October 2004


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   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 THE 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.


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.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




Rosenberg                Expires April 18, 2005                [Page 36]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/