This note is a retrospective review of our 2006 paper  on the properties of protocols, especially interoperability.
A bit of history is in order. By 2006, the importance of a social semantics for protocols was well-established in the multiagent systems community. Further, commitments had emerged as a preeminent abstraction for capturing the semantics. The big advantage was that specifying the meaning of protocol messages in terms of the commitments among agents enabled the agents to act flexibly.