Some Comments on "Transition-Oriented" Versus "Structured" Specification of Distributed Algorithms and Protocols

Formal description techniques (FDT's) are being developed for the specification of communication protocols and other distributed systems. Some of them (namely SDL and Estelle) are based on an extended state transition model and promote a "transition-oriented" specification style. Another one (namely Lotos) and most highlevel programming languages promote a style which is called "structured." The correspondence compares these two specification styles in the framework of rendezvous interactions between different system modules. The advantages of each of the two styles are discussed in relation with an example of a virtual ring mutual exclusion protocol. Transformation rules between the two approaches are given. An extension to the state transition oriented FDT's is also suggested in order to allow for a structured specification style.