physical nodes, between applications that run on the node platform. In that case, the abstract protocol must be mapped on messages that can be supported by communication primitives offered by the node platform. 3. An abstract interface can also be hidden inside an application, which requires no message handling and no communication service. Considering the syntax for ASPs in AML, mapping of AML communication events on messages must consider the information elements shown in Fig. 5.47. A type of service object is referred to by the primitive service it provides (see Sect. 7.2 about this concept). This reference is a primitive service identifier, PSid. Service object instances may have unique identities (within the context of, e.g. a sysnet). Such an identifier is an object identifier, OBid (the OBid corresponds to an object reference in CORBA). Service objects may also be temporarily associated with each other by an identi- fier. Such an identifier is called service relation identifier, SRid. An operation is identified by an operation identifier, OPid. 5 Protocol Network Modelling Syntax elements in Information elements communication events in messages service object type identifiedBy service object instance relation between service objects operation ASPid PrimType parSet ANY DEFINED BY ASPid communication event INVid Fig. 5.47 Parameters of a communication event An ASP is identified by an ASPid, that always comprises the OPid and a primi- tive type value. The latter are values of the type PrimType. Possible values are defined by the AML operation classification system. The PSid can be used to separate operations that are different, but have the same OPid. Each ASP comprises an optional set of parameters, parSet (an ASP does not necessarily have parameters). When the protocol can handle several invocations of an operation simultane- ously, each invocation must also be identified by an invocation identifier, INVid. 5.7.3 The Universal Message Format Message formats are handled by the FOH and ENC in the message-hand