According to the present invention, a cluster communications system is provided that supports reliable and efficient cluster communications. The preferred embodiment cluster communication systems can be used to provide this reliable and efficient cluster communication for cluster configurations extending beyond a single local area network (LAN). The cluster communications system provides reliable and efficient cluster communication by facilitating multicast messaging between systems in the cluster. In particular, the preferred embodiment provides for the establishment of multicast groups in between which multicast messaging is provided. The preferred embodiment provides this multicasting while providing the needed mechanisms to assure ordered message delivery between systems. The preferred embodiment extends this efficient and reliable cluster communication by providing for additional point-to-point communication between systems not on the same LAN. Thus, the preferred embodiment provides a cluster communication system that uses reliable multicasting for efficient cluster communication in a way that can be used for clusters that extend beyond a single local area network.