Multicasting is a very important operation in high performance parallel applications. Making this operation efficient in supercomputers has been a topic of great concern. Much effort has gone into designing special interconnects to support the operation. Today's huge deployment of NoWs (Network of Workstations) has created a high demand for efficient software-based multicast solutions. These systems are often based on low-cost Ethernet interconnects without direct support for group communication. Basically TCP/IP is the only widely supported method of fast reliable communication, though it is possible to improve Ethernet performance at many levels – i.e., by-passing the operating system or using physical broadcasting. Low-level improvements are not likely to be accepted in production environments, which leaves TCP/IP as the best overall choice for group communication.

In this paper we describe a TCP/IP based multicasting algorithm that uses message segmentation in order to lower the propagation delay. Experiments have shown that TCP is very inefficient when a node has many active connections. With this in mind we have designed the algorithm so that it has a worst-case propagation path length of O(log n) with a minimum of connections per node. We compare our algorithm with the binomial tree algorithm often used in TCP/IP MPI implementations.