A Scalable Peer-to-Peer Group Communication Protocol

In peer-to-peer (P2P) applications, a group of multiple peer processes (peers) are required to cooperate with each other in a distributed manner. Messages sent by peers have to be causally delivered to every peer in a group. Due to the message overhead O(n) for the number n of peers, the vector clock cannot be used to causally deliver messages in a scalable group. On the other hand, the linear clock implies the message length O(1), but some pair of messages are unnecessarily ordered. Recently, more accurate physical clocks can be used in cooperation with the GPS time server. In this paper, we reduce the number of messages unnecessarily ordered by taking advantage of the linear time and physical time. Even if each physical clock is synchronized with a time server, every physical clock does not show the same accurate time. Let lambda(i) show the accuracy of the physical clock of a peer p(i) and d(ij) show the minimum delay time between p(i) and p(j). In this paper, we consider a homogeneous broadcast group where lambda(i) = lambda and d(ij) = d for every pair of peers p(i) and p(j) and each message is sent to every peer. In the evaluation, we show the number of messages unnecessarily ordered can be reduced in the protocol compared with the linear clock and the physical clock for delta <= 2 lambda.