Group Communication Protocols for Scalable Groups of Peers

In peer-to-peer (P2P) overlay networks, a group of n (≥ 2) peer processes have to cooperate with each other. P2P systems are in nature scalable and distributed, where there is no centralized coordinator. Each peer sends messages to every peer and receives messages from every peer in a group. In group communications, each message sent by a peer is required to be causally delivered to every peer. Types of logical clocks are used to causally order messages, linear and vector clocks. The linear clock can be used in a scalable group since message length is O(I). However, some messages not to be causally ordered are unnecessarily ordered. On the other hand, all and only messages to be causally ordered are ordered in the vector clock. However, the vector clock cannot be adopted in a scalable group due to the message length O(n). Recently, each computer is equipped with a physical clock which is synchronized with time servers. This means, each peer can read a more correct physical clock of a computer where the peer is performed. In this paper, we discuss how to causally deliver messages in a scalable group by taking advantage of linear and physical clocks. Here, the number of messages unnecessarily ordered in the linear clock can be also reduced.