We define a notation (specification language) for describing desired patterns of communication among components of a distributed system through multiport, unreliable channels. Our language specifies the network topology, and the kinds of information transmission desired. We give a polynomial-time algorithm for determining whether a specification is satifiable; our algorithm can actually construct a protocol that achieves the specified exchange of information, optimized with respect to two possible criteria. Examples suggest that our method can automatically synthesize reasonably complex protocols.