A MECHANISM FOR SCALABLE CONCAST COMMUNICATION

Efficient techniques have been developed to support multicast, i.e. the delivery of messages from one source to multiple receivers, over wide area networks. On the other hand, support for concast. the delivery of messages from multiple sources to a single receiver, has not received much attention. Such communication is often implemented as relatively expensive, multiple unicasts. Apart from the high message overhead of unicasts in large groups, concast communication may be subject to the so-called implosion effect, whereby a destination is overwhelmed by simultaneous responses from a set of sources. These limitations restrict the scalability of a number of distributed applications based on group communication. In this paper, we propose a simple mechanism called gather that reduces concast message overhead and eliminates implosion in a number of applications. gather can be implemented in any wide area network which supports tree-based multicasting, and its use results in concast communication that is as efficient as forward multicast. In essence, gather messages propagate from the leaves to the root of a multicast tree, and gateways enroute 'reduce' many incoming concast messages into a single outgoing message. We describe the gather mechanism in detail, discuss some of its properties, and illustrate its utility in supporting a variety of concast applications.