S. Kawanami’s research while affiliated with Tokyo Denki University and other places

The vector clock widely used in group protocols cannot be adopted to a scalable group due to communication and computation overheads. In order to reduce the overheads, we discuss a hierarchical group which is composed of local subgroups. Processes in subgroups are synchronised by using physical and linear clocks while processes in a WAN are synchronised by using a vector clock. We discuss how to causally deliver messages by using the local synchronisation mechanisms. We discuss how to reduce the number of messages to be unnecessarily ordered. We evaluate the protocol in terms of number of messages ordered.

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 communication and computation overheads. In order to reduce the overhead, we discuss a group where all the processes are synchronized by physical clocks. Here, a subgroup of processes are interconnected in a local or personal area network. Clocks of computers in every local subgroup are synchronized by using the network time protocol (NTP) with a GPS time server. The maximum time difference among clocks of computers depends on types of operating systems of the computers. Every message is given physical time when a process sends the message. We discuss a global clock group (GCG) protocol where messages are causally ordered by using the time physical stamps with maximum time difference and delay time. We evaluate the protocol in terms of the number of messages ordered compared with the vector clock.

Large number of peer processes distributed in various types of networks are cooperating to achieve some objectives. We discuss a scalable group communication protocol named HCG (heterogeneous clock group) protocol. Messages are required to be causally delivered by using clocks in a group. An HCG 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. In order to support the causally ordered delivery of messages in a scalable group, 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 HCG protocol in terms of number of messages ordered.

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.