FLOW-CONTROL FOR LIMITED BUFFER MULTICAST

This paper analyzes a multiround flow control algorithm that attempts to minimize the time required to multicast a message to a group of recipients and receive responses directly from each group member. Such a flow control algorithm may be necessary because the flurry of responses to the multicast can overflow the buffer space of the process that issued the multicast. The condition that each recipient directly respond to the multicast prevents the use of reliable multicast protocols based on software combining trees or negative-acknowledgments. The flow control algorithm analyzed here directs the responding processes to hold their responses for some period of time, called the backoff time, before sending them to the originator. The backoff time depends on the number of recipients that will respond, the originator's available buffer space and buffer service time distribution, and the number of times that the originator is willing to retransmit its message. This paper develops an approximate analysis of the service time distribution of the limited-buffer preemptive queuing process that occurs within the protocol processing layers of a multiprogrammed operating system. It then uses this model to calculate multicast backoff times. The paper reports experimental verification of the accuracy of this service time model and discusses its application to the multicast How control problem.