 control point (rCP) for layer interfaces to such layers. The actual resources, resource agents, and the resource controller in an actor are modeled as state machines however, using the AMLs notation (see Appendix D). Resources may be used by actors both in connectivity layers and control layers. Most resources of interest to model are controlled by the latter, however. 1.4.2.4 The LSM-LPM Separation When we distinguish what agents do and what actors do in control layers, we actu- ally separate the behavior of the layer in two independent parts, since actors do not know how agents handle messages, and agents do not know what actors do with the information they deliver to them. Since the agents and the message protocol between agents make distribution transparent to actors, actors communicate with 1.4 Modeling Network Systems in AMLn each other only through the information they get from their local agents. This infor- mation may specify an operation that one actor wants a remote actor to perform. Each interaction of this kind is therefore an event that is an instance of an abstract service primitive (ASP). When we describe the behavior of actors in terms of ASPs, we are describing an abstract protocol (see Fig. 1.17). Abstract protocol Message protocol Fig. 1.17 Actors of a control layer handle element state machines (ESM) that interact within the layer according to an abstract protocol. R=local resources and/or resource agent Since the communication between actors is an abstraction of events in actor- agent interfaces and some message exchange between agents, we use the AMLs specification technique to define a state machine that is handled by an actor, and that represents the behavior between actors, expressed in terms of events. This state machine is a (layer-) element state machine (ESM). As a result, a control layer becomes separated into one or several layer state machines (LSM) and a layer protocol machine (LPM), as depicted in Fig. 1.18. Layer state machine (LSM): Abstract protocol ESM_in ESM_in Layer proto