A Scalable Control Topology for Multicast 
Communications \Lambda 
J╫rg Liebeherr y Bhupinder S. Sethi z 
y Polytechnic University 
Department of Electrical Engineering 
6 MetroTech Center 
Brooklyn, NY 11201 
Phone: (718) 260-3493, Fax: (718) 260-3074 
email: jorg@catt.poly.edu 
z Department of Computer Science 
University of Virginia 
Charlottesville, VA 22903 
email: bss4k@cs.virginia.edu 
Abstract 
Multimedia collaborative applications for the Internet require the availability of mul- 
ticast protocols that enhance the basic connectionless IP Multicast service. A critical re- 
quirement of such protocols is their ability to support a large group of simultaneous users. 
For this it is crucial that there exist mechanisms to efficiently exchange control information 
between the members of a group. In this paper, we present a new approach for distribut- 
ing control information within a multicast group. The goal of our approach is to scale to 
very large group sizes (in excess of 100,000 users). Multicast group members are organized 
as a logical n-dimensional hypercube, and all control information is transmitted along the 
edges of the hypercube. We analyze the scalability of the hypercube control topology and 
show that the hypercube balances the load per member for processing control information 
better than existing topologies. We use actual data traces of the group membership in an 
MBONE conference to gain insight into the transient changes of the load at each node. We 
present a set of protocol mechanisms that maintain the hypercube topology in a soft-state 
fashion without requiring any entity to have global state information. 
Key Words: IP Multicast, Multicast Communications, Implosion Problem, Hypercubes. 
\Lambda This work is supported in part by the National Science Foundation under Grant No. NCR-9624106. Cor- 
responding author is J╫rg Liebeherr.