corre- sponding ASPs relayOrder(...) and terminateOrder(...) How these primitives are related to send a and receive operations in layer interfaces depends on whether the data transport is user-to-user, user-to-system, or system-to-system, as shown in the figure below. Note that an interface type identifier (ITi, explicit or by position) is needed in protocol messages if data can be terminated in different ways. User-to-user: sendOrd relayOrd termOrd (Term. actor Common actor Term. termOrd actor relayOrd recRep User-to-system (two ways): recRep termOrd sendOrd relayOrd (Term. {ITi,. actor Common actor termOrd recRep System-to-system: relayOrd termOrd Common Common actor actor termOrd relayOrd sendOrd recRep The SAC primitive sequence in the user-to-user model is based on the principle that agent layers should not know where a data stream is terminated, in order to make agent layers more generally applicable and to confine path control to actors only. The sequence in the user-to-system model demonstrates two alternative methods for terminating data in systems: either in a message receiving agent (the method used in, e.g., PSTN/ISDN) or in an agent of a common actor (the method used, e.g., in ATM). The sequence in the system-to-system model is typical of how connectivity for communication between control layer elements is provided, e.g., in PSTN/ISDN and in ATM. As far as SAC primitives are concerned, there is a distinct difference between layers that transport synchronous and asynchronous user-data. If synchronous, there will be no implementation of SAC primitives since relay- ing is defined by the position of bits in frames over a SAC. If asynchronous, there will exist explicit primitives since some type of termina- tor address has to be delivered to the actor, together with the user data. Other parameters may occur as well, e.g. quality-of-service requirements (as in IP) and some type of originator address. Path control for data streams is performed by common actors through some type of tables, depending on the t