The paper studies the top-down design of a basic interaction pattern for asynchronously communicating components. The interaction pattern describes a high level protocol where an active sender component transmits messages to a passive receiver component controlled by the receiver?s acknowledgements. The formal design refines the components? input/output behaviour into a state-based implementation based on three transformation steps. An architectural refinement decomposes the protocol function into a sender component and into a receiver component introducing a feedback channel. The differentiation localizes the effect of single inputs wrt. a previous input history. The history abstraction extracts the components? control state and data state from the input history.