A Flexible Protocol Architecture for Multi-party Conferencing

This paper proposes a new multicast protocol architecture that can be used for conferencing or online discussion groups. Sessions of this type have the characteristic that while all group members are usually listeners, several members can and often do become speakers during the course of the session. Another characteristic is that the group membership can change as members drop out or are added to the session. Thus, one major objective is to design a protocol that is eeective when there are many such sessions that are concurrently active. A second objective is to use the same basic architecture to implement multicasting in a mobile environment. This is possible because the basic primitives for joining and leaving discussion groups are similar to what is needed for mobile applications that need to be connected through various mobile support stations. A key aspect of our design is the ability to transparently switch to a diierent shared tree using a simple Dualcast protocol that we introduce. Our proposed mobile multicast routing protocol (called MCSM) uses a close to optimal shared Steiner tree for the conference session and is built over the current unicast routing architecture. This paper discusses why a shared Steiner tree is appropriate for the discussion session and describes the detailed workings of MCSM, including how the necessary periodic transition to a new shared tree is eeected. Considerations of how to incorporate fault tolerance are also discussed, as are extensions of the MCSM protocol to a wide area network consisting of multiple administrative domains.