A group communication protocol for scalable causal ordering

Large number of peer processes distributed in various types of networks are cooperating to achieve some objectives. The vector clock cannot be adopted to a scalable group due to the message length O(n) for number n of processes. A group is composed of local subgroups in each of which processes are in a local or personal area network and which are interconnected in a wide-area network. Processes in local subgroups use physical and linear clocks while processes in a wide-area network adopt vector clock. We discuss how to causally deliver messages by using local synchronization mechanisms of each subgroup. We evaluate the protocol in terms of number of messages ordered.