Input-Output Pattern (IOP) defines
    
      - sequence
- direction
- cardinality (number of messages send or received from one node
	to the service)
Message Exchange pattern (MEP) defines
    
      - ditto IOP
- specifies the senders and receivers
      - is there a use for not specifying senders and receivers?
- Conservation of messages: For any one message that is
      sent by one node, one message is received by one node.
 open issue: multicast protocols (should we represent multicast
	messages differently at the interface level?)