Altova website: UML Protocol State Machine diagrams
Protocol State Machines are used to show a sequence of events that an object responds to, without having to show the specific behavior. The required sequence of events, and the resulting changes in the state of the object, are modeled in this diagram.
Protocol State Machines are most often used to describe complex protocols, e.g. database access through a specific interface, or communication protocols such as TCP/IP.
Protocol State Machines are created in the same way as State Machine diagrams, but have fewer modeling elements. Protocol-Transitions between states can have pre- or post conditions which define what must be true for a transition to another state to occur, or what the resulting state must be, once the transition has taken place.