Membership algorithms for multicast communication groups

We introduce a membership protocol that maintains the set of currently connected machines in an asynchronous and dynamic environment. The protocol handles both failures and joining of machines. It operates within a multicast communication sub-system. It is well known that solving the membership problem in an asynchronous environment when faults may be present is impossible. In order to circumvent this difficulty, our approach rarely extracts from the membership live (but not active) machines unjustfully. The benefit is that our procotol always terminates within a finite time. In addition, if a machine is inadvertently taken out of the membership, it can rejoin it right away using the membership protocol. Despite the asynchrony, configuration changes are logically synchronized with all the regular messages in the system, and appear virtually synchronous to the application layer. The protocol presented here supports partitions and merges. When partitions and merging occur, the protocol provides the application with exact information about the status of the system. It is up to the application designer to merge the partitioned histories correctly.