UML Interaction diagrams (Sequence diagrams (SD), Collaboration diagrams or Communication diagrams) lack a formal semantics; they include some amount of ambiguity so that formal techniques cannot be applied e.g. for automatically deriving the behavior of the objects (as Statecharts or State-Transition diagrams) from the SD where they appear. To overcome this situation, this paper first analyses and simplifies the semantic relationships among the concepts that intervene in the definition of SDs: synchronous / asynchronous, activator and return messages. Then it proposes an interpretation of the precedence association among messages that yields to an operational semantics of SDs that orders the actions of emitting and receiving messages according to a causality relationship.
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