A discipline for constructing multiphase communication protocols

Abstract

Many communication protocols can be observed to go through different phases performing a distinct function in each phase. A multiphase model for such protocols is presented. A phase is formally defined to be a network of communicating finite-state machines with certain desirable correctness properties; these include proper termination and freedom from deadlocks and unspecified receptions. A multifunction protocol is constructed by first constructing separate phases to perform its different functions. It is shown how to connect these phases together to realize the multifunction protocol so that the resulting network of communicating finite state machines is also a phase (i.e., it possesses the desirable properties defined for phases). The modularity inherent in multiphase protocols facilitates not only their construction but also their understanding and modification. An abundance of protocols have been found in the literature that can be constructed as multiphase protocols. Three examples are presented here: two versions of IBM's BSC protocol for data link control and a token ring network protocol.