Developing a Multicast Metric

	       Robert C. Chalmers and Kevin C. Almeroth
		    Department of Computer Science
		       University of California
		     Santa Barbara, CA 93106-5110
		    {robertc,almeroth}@cs.ucsb.edu


To further advance the deployment of multicast, many consider it
necessary to provide a quantitative measure of the potential benefit
of employing multicast rather than unicast.  We approach this problem
in three ways.  First, we propose a rudimentary metric that can
provide a reasonable measure of the benefit achieved by an active
group using multicast instead of unicast.  Second, we discuss many of
the issues inherent in developing multicast metrics and how these
issues may be addressed to improve the accuracy of our model.  Third,
we consider how characteristics of network topology and group
distribution might offer a predictive estimate for the proposed
metric.  In general, we identify two rules for evaluating the
efficiency of multicast: (1)multicast efficiency increases as the
average path length increases, and (2)fan-out close to the source
decreases multicast's efficiency.