Multi-Point Communication

     Multi-point communication structures, which are characterised by a group of m senders transmitting data to a group of n recipients, are often necessary for distributed multimedia applications. Typical scenarios include computer supported cooperative work (CSCW), multimedia conferencing, distributed virtual reality, distributed simulation, digital television, or distance teaching.

To avoid inefficient repeated sending of n identical messages, the communication system must offer multicast transmission. Multicasting requires support concerning the following issues:

Multicast Group and Address Management.

To dynamically participate in multicast sessions, any participant should be able to initiate, join or leave multicast groups at any time. Setting up a multicast group requires the assignment and propagation of a multicast group address, used as the destination address for all data sent to this group.

Transport Reliability and Flow Control.

New mechanisms must be developed that offer reliability without creating huge amounts of control traffic. To avoid costly retransmissions of packets dropped due to network congestion, flow control techniques must be used to adapt the sent traffic quantity to the network congestion state along all multicast data paths.

Routing.

Multicasting asks for routing protocols that reduce duplicate traffic on any shared link to a minimum, preferably to one copy. In addition, out of both network-load and security concerns, data should only be sent along links that have at least one recipient.
Naturally, a routing algorithm should choose the optimal path for the transmitted data, but with multicasting, it must consider the paths to a (potentially widely spread) set of receivers.