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RFC 4245 - High-Level Requirements for Tightly Coupled SIP Confe


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Network Working Group                                           O. Levin
Request for Comments: 4245                         Microsoft Corporation
Category: Informational                                          R. Even
                                                                 Polycom
                                                           November 2005

      High-Level Requirements for Tightly Coupled SIP Conferencing

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document examines a wide range of conferencing requirements for
   tightly coupled SIP conferences.  Separate documents will map the
   requirements to existing protocol primitives, define new protocol
   extensions, and introduce new protocols as needed.  Together, these
   documents will provide a guide for building interoperable SIP
   conferencing applications.

Table of Contents

   1. Introduction ....................................................2
   2. An Overview .....................................................2
   3. High-Level Requirements .........................................3
      3.1. Discovery Phase ............................................3
      3.2. Conference Creation ........................................4
      3.3. Conference Termination .....................................4
      3.4. Participants' Manipulations ................................4
         3.4.1. Participation of a Conference-Unaware User Agent ......5
         3.4.2. Dial-Out Scenarios ....................................5
         3.4.3. Dial-In Scenarios .....................................5
         3.4.4. Third-Party Invitation to a Conference ................6
         3.4.5. Participants' Removal .................................6
         3.4.6. Participants' Privacy .................................6
      3.5. Conference State Information ...............................7
         3.5.1. Description ...........................................7
         3.5.2. Dissemination of Changes ..............................7
         3.5.3. On-demand Information Dissemination ...................8
      3.6. Focus Role Migration .......................................8

      3.7. Side-bar Conferences .......................................8
      3.8. Cascading of Conferences ...................................9
      3.9. SIMPLE and SIP Conferencing Coordination ...................9
   4. Security Considerations ........................................10
   5. Contributors ...................................................10
   6. References .....................................................10
      6.1. Normative References ......................................10

1.  Introduction

   This document examines a wide range of conferencing requirements for
   tightly coupled SIP (RFC 3261 [2]) conferencing.

   The requirements are grouped by subjects in various areas allowing
   solutions to progress in parallel.

   Separate documents will map the requirements to existing protocol
   primitives, define new protocol extensions, and introduce new
   protocols as needed.

   Together, these documents will provide a guide for building
   interoperable SIP conferencing applications.

   The terms "MAY", "SHOULD", and "MUST" are to be interpreted as
   described in RFC 2119 [1].

2.  An Overview

   A SIP conference is an association of SIP user agents (i.e.,
   conference participants) with a central point (i.e., a conference
   focus), where the focus has direct peer-wise relationships with the
   participants by maintaining a separate SIP dialog with each.

   The focus is a SIP user agent that has abilities to host SIP
   conferences including their creation, maintenance, and manipulation
   using SIP call control means and potentially other non-SIP means.

   In this tightly coupled model, the SIP conference graph is always a
   star.  The conference focus maintains the correlation among
   conference's dialogs internally.

   The conference focus can be implemented either by a participant or by
   a separate application server.

   In the first case, a focus is typically capable of hosting a simple
   ad hoc conference only.  We envision that such basic conference can
   be established using SIP call control primitives only.

   A dedicated conference server, in addition to the basic features,
   offers richer functionality including simultaneous conferences, large
   scalable conferences, reserved conferences, and managed conferences.
   A conferencing server can support any subset of the advanced
   conferencing functions presented in this document.

   The media graph of a SIP conference can be centralized,
   decentralized, or any combination of both, and potentially differ per
   media type.  In the centralized case, the media sessions are
   established between the focus and each one of the participants.  In
   the de-centralized (i.e., distributed) case, the media graph is a
   (multicast or multi-unicast) mesh among the participants.
   Consequently, the media processing (e.g., mixing) can be performed
   either by the focus alone or by the participants.

   Conference participants and third parties can have different roles
   and privileges in a certain conference.  For example, conferencing
   policy can state that the rights to disconnect from and to invite to
   a conference are limited to the conference chair only.

   Throughout the document, by conference policies we mean a set of
   parameters and rules (e.g., maximum number of participants, needs
   chair-person supervision or not, password protected or not, duration,
   or a way of media mixing) that are defined at the onset of a
   conference.  Typically, conference policies would be specified by a
   conference creator and need special privileges to be manipulated.

   Throughout the document, by a conference state we mean a set of
   information describing the conference in progress.  This includes
   participants' information (such as dialog identifiers), media
   sessions in progress, the current loudest speaker, the current chair,
   etc.

3.  High-Level Requirements

   In addition to the requirements presented in this document,
   supplementary requirements for conferencing policy, media mixing and
   other manipulations, floor control, privilege control, etc. will be
   discussed in separate documents.

3.1.  Discovery Phase

   Some of the requirements presented in this section can be met either
   by configuration means or by using proprietary conventions.
   Nevertheless, there is consensus that standard means for implementing
   these functions by automata MUST be defined.

   REQ-1: Discovery of a location of an arbitrary SIP conferencing
        server(s).

   REQ-2: Given a SIP Address-of-Record (AOR) of a certain entity,
        resolution whether the SIP entity has focus capabilities.

   REQ-3: Given a global identifier of a particular conference, locating
        the conference focus.

   REQ-4: Given a global identifier of a particular conference,
        obtaining the conference properties.

   REQ-5: Given a global identifier of a particular conference,
        obtaining the conference state information.

3.2.  Conference Creation

   Given a focus location, a means MUST be defined for an interested
   entity (including a user agent) to implement the procedures below:

   REQ-1: Creation of an ad-hoc conference identifier and the conference
        with specified properties.

   REQ-2: Creation of a reserved conference identifier for a conference
        with specified properties.

   REQ-3:  Specifying properties upon conference creation in any of the
        following ways: default, profiles, and explicitly.

3.3.  Conference Termination

   REQ-1: Given a conference identifier, a means MUST be defined for a
        user agent to disconnect all participants from the conference
        and terminate the conference including the release of the
        associated resources.

   REQ-2: A means MAY be defined for requesting a focus to revert a
        two-party conference to a basic SIP point-to-point session
        including the release of the associated conferencing resources.

3.4.  Participants' Manipulations

        Some of the requirements presented in this section can be met by
        human intervention, configuration means, or proprietary
        conventions.  Nevertheless, there is consensus that standard
        means for implementing these functions by automata MUST be
        defined.

3.4.1.  Participation of a Conference-Unaware User Agent

   REQ-1: Focus MUST be able to invite and disconnect an RFC 3261
        compliant only SIP user agent to and from a SIP conference.

   REQ-2: An RFC 3261 compliant only SIP user agent MUST be able to
        dial-in to a particular SIP conference.  In this case, only the
        human knows that he/she is connected to the conference.

3.4.2.  Dial-Out Scenarios

   REQ-1: A means MUST be defined for a focus to invite another user
        agent to one of the focus' conferences.  This procedure MUST
        result in the establishment of a single SIP dialog between the
        two.

   REQ-2: Given an existing SIP dialog between two user agents, if at
        least one user agent has focus capabilities, a means MUST be
        defined for the conference focus to invite the other user agent
        to one of the focus' conferences without additional SIP dialog
        establishment.

   REQ-3: An invitation to a user agent to join a conference MUST
        include a standard indication that it is a conference and the
        conference identifier.

3.4.3.  Dial-In Scenarios

   REQ-1: A means MUST be defined for a user agent to create an ad hoc
        conference with default properties (as per "Conference Creation"
        REQ-1 above) and to become a participant using a single SIP
        dialog.

   REQ-2: Given a reserved conference identifier, a means MUST be
        defined for a user agent to activate the conference and to
        become a participant using a single SIP dialog.

   REQ-3: Given a conference identifier of an active conference, a means
        MUST be defined for a user agent to dial-in the conference and
        to become a participant using a single SIP dialog between the
        two.

   REQ-4: Given an identifier of one of the dialogs of a particular
        active conference, a means MUST be defined for a user agent to
        dial-in the conference and to become a participant.

3.4.4.  Third-Party Invitation to a Conference

   REQ-1: Given a conference identifier, a means MUST be defined for a
        user agent to invite another user agent to this conference.

   REQ-2: Given an identifier of one of the dialogs of a particular
        active conference, a means MUST be defined for a user agent to
        invite another user agent to this conference.

   EQ-3: Given a conference identifier, a means SHOULD be defined for a
        user agent to invite a list of user agents to this conference (a
        so-called "mass invitation").

3.4.5.  Participants' Removal

   REQ-1: A means MUST be defined for a conference focus to remove a
        conference participant from the conference.

   REQ-2: Given a conference identifier, a means MUST be defined for a

        user agent to remove a participant from the conference.

   REQ-3: Given an identifier of one of the dialogs of a particular
        active conference, a means MUST be defined for a user agent to
        remove a participant from the conference.

   REQ-4: Given a conference identifier, a means MUST be defined for a
        user agent to remove all the participants from the conference.

   REQ-5: Given a conference identifier and a sub-list of participants,
        a means MAY be defined for a user agent to remove the specified
        participants from the conference (a so-called "mass ejection").

3.4.6.  Participants' Privacy

   A conference focus SHOULD support the procedures described in this
   section.  A conference participant MAY support the procedures
   described in this section.  The requirements imply that "anonymizing"
   operations MUST be performed on all: the call control, the media
   control, and the media content when appropriate.

   REQ-1: A conference participant joins the conference "anonymously";
        that is, his/her presence can be announced but without
        disclosing his/her identity.

   REQ-2: A conference participant requests a focus for anonymous
        participation in the conference.

   REQ-3: A conference participant joins a conference in a "hidden
        mode"; that is, his/her presence and identity are not to be
        disclosed to other participants.

   REQ-4: A conference participant requests a focus for participation in
        the conference in a hidden mode.

3.5  Conference State Information

3.5.1.  Description

   By a conference state, we mean a virtual database describing the
   conference in progress.  This includes different conference aspects:
   participants' information (such as dialog identifiers and state),
   media sessions in progress (such as current stream contributing
   sources and encoding schemes), the current loudest speaker, the
   current chair, etc.  Conference state is the latest conference
   snapshot triggered by changes in participants' state, conference
   policy changes, etc.

   REQ-1: A conference state virtual database MUST have a modular
        definition that is, it MUST be possible to access different
        conference aspects independently.

   REQ-2: It MUST be possible to aggregate information relating to
        different conference aspects in a single report.

   REQ-3: A mechanism for extensible definition and registration of
        conference state evolving aspects MUST be present.

   REQ-4: A default conference state report MUST be defined.  It SHOULD
        contain a minimal useful set of information (e.g., a list of
        current conference participants).

3.5.2.  Dissemination of Changes

   REQ-1: A means MUST be defined for reporting the conference state
        changes to interested parties (including non-conference
        participants) in a timely manner.

   REQ-2: A means MUST be defined for a SIP user agent to express its
        interest in selected state changes only.

   REQ-3: A means MUST be defined for a SIP user agent to express the
        minimum interval between receiving state change reports.

   REQ-4: It MUST be possible to aggregate recent changes in a single
        reporting event.

   REQ-5: Default conference state change reports MUST be defined.  They
        SHOULD contain minimal useful to the participants information
        (e.g., participants' joining and leaving the conference).

3.5.3.  On-demand Information Dissemination

   REQ-1: A means MUST be defined to disseminate any conference state
        information to interested parties (including SIP user agents)
        on-demand.

   REQ-2: A means MUST be defined for an interested party (including a
        SIP user agent) to request conference state information of a
        particular conference defined by the conference identifier.

   REQ-3: A means MUST be defined for an interested party (including a
        SIP user agent) to specify the subset of the conference state
        information it wants and is capable of receiving.

3.6.  Focus Role Migration

   REQ-1: A procedure for delegating a focus role by the current focus
        to another participant MUST be defined.

   REQ-2: A procedure for requesting a conference focus to transfer its
        role to another participant MUST be defined.

   REQ-3: A procedure for on-demand unconditional transfer of the focus
        role to a different participant MUST be defined.

   REQ-4: A detection procedure for a focus failure condition MUST be
        defined.

3.7.  Side-bar Conferences

   A standard means MUST be defined in order to implement the operations
   defined in this section below.

   REQ-1: A user agent (not a conference participant) joins a side-bar
        within the conference by SIP means.

   REQ-2: A user agent (not a conference participant) is invited to a
        side-bar within the conference by SIP means.

   REQ-3: A conference participant creates a side-bar conference with
        one or more participants in a conference by SIP means.

   REQ-4: A conference participant joins a side-bar within the
        conference by SIP means.

   REQ-5: A conference participant is invited to a side-bar within the
        conference by SIP means.

   REQ-6: A conference-unaware user agent (a participant or not) creates
        and participates in side-bar conferences.  It MAY be achieved by
        non-SIP means.

   REQ-7: A conference participant creates side-bar conferences within
        the conference without establishing any additional SIP dialogs
        with the focus.  It MAY be achieved by non-SIP means.

   REQ-8: A conference participant joins any number of side-bars within
        the conference without establishing any additional SIP dialogs
        with the focus.  It MAY be achieved by non-SIP means.

   REQ-9: A conference participant is invited to any number of side-bars
        within the conference without establishing any additional SIP
        dialogs with the focus.  It MAY be achieved by non-SIP means.

3.8.  Cascading of Conferences

   "Cascading of Conferences" is a term that has different meanings in
   different contexts.  Some examples are listed below:

      -    Peer-to-peer chaining of signaling.  (Many ways exist to
           build the media graph in this case.)

      -    Conferences have hierarchal signaling relations.  (Many ways
           exists to build the media graph in this case.)

      -    "Cascading" is used to distribute the media "mixing" only.
           The distribution of signaling is not required.

   As it can be seen from the examples, each will define a different set
   of requirements.

3.9.  SIMPLE and SIP Conferencing Coordination

   REQ-1: SIMPLE-based Presence and Instant Messaging architecture
        SHOULD fit into the general SIP Conferencing architecture.

   REQ-2: A scenario where a multimedia SIP conference and a multiparty
        instant messaging conversation take place among the same group
        of participants MUST be addressed.

   REQ-3: A scenario where a side-bar and/or a sub-IM-conference is
        being held as a part of SIP conference MUST be addressed.

4.  Security Considerations

   This document discusses high-level requirements for SIP conferencing.
   Conferencing has some specific security requirements, which will be
   summarized here at a very high level.

   All of the operations and functions described in this document need
   to be authorized by a focus or a participant.  It is expected that
   conferences will be governed by a set of authorization rules defined
   as a part of the conference policy.  In order for the conference
   policy to be implemented, the focus needs to be able to authenticate
   potential participants.  Normal SIP mechanisms including Digest
   authentication and certificates can be used [2].  These conference-
   specific security requirements will be discussed in detail in the
   protocol documents.

   Conferencing also has privacy implications.  Some of these are
   discussed in this document.  Standard SIP mechanisms for a user agent
   to request privacy should be utilized by a focus and will be detailed
   in the protocol documents.

5.  Contributors

   This work is based on the discussions among the members of the SIP
   Conferencing design team.

6.  References

6.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  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.

Authors' Addresses

   Orit Levin
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   EMail: oritl@microsoft.com

   Roni Even
   Polycom
   94 Derech Em Hamoshavot
   Petach Tikva, Israel

   EMail: roni.even@polycom.co.il

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