Scenario-based specifications such as message sequence charts (MSC) o#er an intuitive and visual way to describe design requirements. MSC-graphs allow convenient expression of multiple scenarios, and can be viewed as an early model of the system that can be subjected to a variety of analyses. Problems such as LTL model checking are undecidable for MSC-graphs in general, but are known to be decidable for the class of bounded MSC-graphs.

Abstract—Scenario-based requirements specifications are the industry norm for defining communication protocols. However, such scenarios often contain race conditions. A race condition occurs when events are specified to occur in a particular order, but in practice, this order cannot be guaranteed. The paper considers UML/MSC scenarios that can be described with standard partial order theoretic asynchronous behavioral semantics. We define these to be partial order scenarios. The paper proves there is a unique minimal generalization of a partial order scenario that is race free. The paper also proves there is a unique minimal race free refinement of the behavioral semantics of a partial order scenario. Unlike the generalization, the refinement cannot be realized in the form of a partial order scenario, although it can always be embedded in one. The paper also proves the results can be generalized to a subclass of iterative scenarios. Index Terms—Requirements analysis, formal methods, distributed programming. 1