We report on the design and correctness of a communication facility for a distributed computer system. The facility provides support for fault tolerant process groups in the form of a family of reliable multicast protocols that can be used both in local and wide-area networks. These protocols attain high levels of concurrency while respecting application-specific delivery ordering constraints, and have varying cost and performance that depends on the degree of ordering desired. In particular, a protocol that enforces causal delivery orderings is introduced, and shown to be a valuable alternative to conventional asynchronous communication protocols. The facility also ensures that the processes belonging to a fault tolerant process group will observe consistent orderings of events affecting the group as a whole, including process failures, recoveries, migration, and dynamic changes to group properties like member rankings. A review of several uses for the protocols in the ISIS system, which supports fault-tolerant resilient objects and bulletin boards, illustrates the significant simplification of higher-level algorithms; made possible by our approach.