2121: Issues affecting MARS Cluster Size.

IP multicast over ATM currently uses the MARS model [1] to manage the use of ATM pt-mpt SVCs for IP multicast packet forwarding. The scope of any given MARS services is the MARS Cluster - typically the same as an IPv4 Logical IP Subnet (LIS). Current IP/ATM networks are usually architected with unicast routing and forwarding issues dictating the sizes of individual LISes. However, as IP multicast is deployed as a service, the size of a LIS will only be as big as a MARS Cluster can be. This document provides a qualitative look at the issues constraining a MARS Cluster's size, including the impact of VC limits in switches and NICs, geographical distribution of cluster members, and the use of VC Mesh or MCS modes to support multicast groups.