UML sequence diagrams (SDs) are a mainstay of requirements specifications for communication protocols. Mauw and Reniers' algebraic (MRA) semantics formally specifies a behavior for these SDs that guarantees deadlock-free processes. Practitioners commonly use communication semantics that differ from MRA, which may result in deadlocks, for example, FIFO, token ring, etc. We define a process algebra that is an extension of the MRA semantics for regular SDs. Our algebra can describe several commonly used communication semantics. Regular SDs are constructed from concurrent message flows via iteration, branching, and sequential composition. Their behavior is defined in terms of a set of partial orders on the events in the SD. Such partial orders are known as causal orders. We define partial order theoretic properties of a causal order that are particular kinds of race condition. We prove that any of the common communication semantics that we list either guarantees deadlock-free SDs or can result in a deadlock if and only if a causal order of an SD contains one of these types of race condition. This describes a complete classification of deadlocks as specific types of race condition.

Scenario based requirements specifications are the industry norm for defining communicating systems. These scenarios are often captured in the form of UML/MSC sequence diagrams. Errors are often introduced at this stage of the development process, which are costly to resolve if they are not detected early. This paper is concerned with the automatic detection and resolution of semantic errors that can occur in such scenarios.The paper discusses a semantic interpretation of scenario-based requirements and various types of defects (or pathologies) that can be detected. The paper defines the semantics and defects within a partial order theoretic framework. We introduce a UML 2.0 profile that captures various domain specific communication semantics, which can be used to determine the relevance of detected pathologies when different underlying implementation assumptions are made. The paper also discusses how to automatically resolve pathologies by using this profile to adapt the communication architecture in the requirements model.

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.