RTP Topologies
RFC 5117

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Network Working Group                                      M. Westerlund
Request for Comments: 5117                                      Ericsson
Category: Informational                                        S. Wenger
                                                                   Nokia
                                                            January 2008

                             RTP Topologies

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.

Abstract

   This document discusses multi-endpoint topologies used in Real-time
   Transport Protocol (RTP)-based environments.  In particular,
   centralized topologies commonly employed in the video conferencing
   industry are mapped to the RTP terminology.

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RFC 5117                     RTP Topologies                 January 2008

Table of Contents

   1. Introduction ....................................................2
   2. Definitions .....................................................3
      2.1. Glossary ...................................................3
      2.2. Indicating Requirement Levels ..............................3
   3. Topologies ......................................................3
      3.1. Point to Point .............................................4
      3.2. Point to Multipoint Using Multicast ........................5
      3.3. Point to Multipoint Using the RFC 3550 Translator ..........6
      3.4. Point to Multipoint Using the RFC 3550 Mixer Model .........9
      3.5. Point to Multipoint Using Video Switching MCUs ............11
      3.6. Point to Multipoint Using RTCP-Terminating MCU ............12
      3.7. Non-Symmetric Mixer/Translators ...........................13
      3.8. Combining Topologies ......................................14
   4. Comparing Topologies ...........................................15
      4.1. Topology Properties .......................................15
           4.1.1. All to All Media Transmission ......................15
           4.1.2. Transport or Media Interoperability ................16
           4.1.3. Per Domain Bit-Rate Adaptation .....................16
           4.1.4. Aggregation of Media ...............................16
           4.1.5. View of All Session Participants ...................16
           4.1.6. Loop Detection .....................................17
      4.2. Comparison of Topologies ..................................17
   5. Security Considerations ........................................17
   6. Acknowledgements ...............................................19
   7. References .....................................................19
      7.1. Normative References ......................................19
      7.2. Informative References ....................................20

1.  Introduction

   When working on the Codec Control Messages [CCM], considerable
   confusion was noticed in the community with respect to terms such as
   Multipoint Control Unit (MCU), Mixer, and Translator, and their usage
   in various topologies.  This document tries to address this confusion
   by providing a common information basis for future discussion and
   specification work.  It attempts to clarify and explain sections of
   the Real-time Transport Protocol (RTP) spec [RFC3550] in an informal
   way.  It is not intended to update or change what is normatively
   specified within RFC 3550.

   When the Audio-Visual Profile with Feedback (AVPF) [RFC4585] was
   developed the main emphasis lay in the efficient support of point to
   point and small multipoint scenarios without centralized multipoint
   control.  However, in practice, many small multipoint conferences
   operate utilizing devices known as Multipoint Control Units (MCUs).
   MCUs may implement Mixer or Translator (in RTP [RFC3550] terminology)

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RFC 5117                     RTP Topologies                 January 2008

   functionality and signalling support.  They may also contain
   additional application functionality.  This document focuses on the
   media transport aspects of the MCU that can be realized using RTP, as
   discussed below.  Further considered are the properties of Mixers and
   Translators, and how some types of deployed MCUs deviate from these
   properties.

2.  Definitions

2.1.  Glossary

   ASM    - Any Source Multicast
   AVPF   - The Extended RTP Profile for RTCP-based Feedback
   CSRC   - Contributing Source
   Link   - The data transport to the next IP hop
   MCU    - Multipoint Control Unit
   Path   - The concatenation of multiple links, resulting in an
            end-to-end data transfer.
   PtM    - Point to Multipoint
   PtP    - Point to Point