Session Initiation Protocol (SIP) Call Control - Conferencing for User Agents
RFC 4579

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• 07

Network Working Group                                        A. Johnston
Request for Comments: 4579                                         Avaya
BCP: 119                                                        O. Levin
Category: Best Current Practice                    Microsoft Corporation
                                                             August 2006

                   Session Initiation Protocol (SIP)
              Call Control - Conferencing for User Agents

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This specification defines conferencing call control features for the
   Session Initiation Protocol (SIP).  This document builds on the
   Conferencing Requirements and Framework documents to define how a
   tightly coupled SIP conference works.  The approach is explored from
   the perspective of different user agent (UA) types:  conference-
   unaware, conference-aware, and focus UAs.  The use of Uniform
   Resource Identifiers (URIs) in conferencing, OPTIONS for capabilities
   discovery, and call control using REFER are covered in detail with
   example call flow diagrams.  The usage of the isfocus feature tag is
   defined.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. SIP User Agent Conferencing Capability Types ....................3
      3.1. Focus UA ...................................................4
      3.2. Conference Factory URI .....................................4
      3.3. Conference-Unaware UA ......................................5
      3.4. Conference-Aware UA ........................................5
   4. Usage of the 'isfocus' Feature Parameter ........................6
      4.1. General ....................................................6
      4.2. Session Establishment ......................................6
      4.3. Discovery ..................................................7
   5. SIP Conferencing Primitives .....................................7
      5.1. INVITE: Joining a Conference Using the Conference

Johnston & Levin         Best Current Practice                  [Page 1]
RFC 4579              SIP CC Conferencing for UAs            August 2006

           URI - Dial-In ..............................................7
      5.2. INVITE: Adding a Participant by the Focus - Dial-Out ......11
      5.3. INVITE: Manually Creating a Conference by Dialing
           In to a Conferencing Application ..........................15
      5.4. INVITE: Creating a Conference Using Ad-Hoc SIP Methods ....16
      5.5. REFER: Requesting a Focus to Add a New Resource to
           a Conference (Dial Out to a New Participant) ..............18
      5.6. REFER: Requesting a User to Dial in to a Conference
           Using a Conference URI ....................................21
      5.7. REFER with REFER: Requesting a Focus to Refer a
           Participant to Dial in to the Conference ..................23
      5.8. Join Header Field: Dialing in to a Conference
           Using a (3rd Party) Dialog Identifier .....................26
      5.9. Replaces Header Field: Switching User Agents
           within a Conference .......................................28
      5.10. Replaces Header Field: Transferring a Point-to-Point
            Session in to a Conference ...............................29
      5.11. REFER with BYE: Requesting That the Focus Remove a
            Participant from a Conference ............................31
      5.12. Deleting a Conference ....................................33
      5.13. Discovery of URI Properties Using OPTIONS ................34
   6. Security Considerations ........................................36
   7. Contributors ...................................................37
   8. References .....................................................38
      8.1. Normative References ......................................38
      8.2. Informative References ....................................38
   Appendix A: Creating a Conference by a Conference-Unaware UA.......40

1.  Introduction

   This specification uses the concepts and definitions from the high
   level requirements [14] and the SIP conferencing framework [8]
   documents.  This approach is applicable to tightly coupled SIP
   conferences.  In this architecture, a user agent (UA), known as a
   participant, establishes a SIP dialog with another UA, known as a
   focus.  The focus is the central point of control, authentication,
   and authorization.  This specification defines the operation of a
   focus and participant UAs.  Note that only the signalling (SIP) needs
   to be centralized in this model; the media can be centrally mixed,