Implosion Control for Multipoint Applications1

One of the new challenges posed by multipoint applications in future networks is scalable multicast error control. Current error control mechanisms do not scale because they do not deal with message implosion, caused by synchronized feedback messages (e.g. positive or negative acknowledgments) from the receivers in a multicast connection. Implosion control can be performed by the receivers or the sender. The former, where receivers collaborate to control implosion has been the most popular so far. In the latter, the sender is responsible for directly controlling feedback from the receivers. Sender-controlled implosion is appropriate for cases where receivers are isolated from each other (e.g. where privacy or anonymity is desired, where receivers have limited processing capabilities and in one-to-many connections in connection-oriented networks). Implosion can also be controlled by structuring receivers in a virtual tree hierarchy with the sender at the root, where feedback messages are restricted from children to their parents. Such a structure is also suitable for ACK consolidation which is required for implosion control in reliable multicast. In this paper we present the design and propose to investigate via analysis, simulation and implementation two implosion control mechanisms: a sender-controlled and a dynamic hierarchical implosion control mechanism. Our hierarchical mechanism uses dynamic structures called multicast recovery trees (MRTs) formed by the receivers.