ons between actors. Often, however, actors may be engaged in longer sessions, in which case it is favorable (for many reasons, authentication being one) if they can refer to some type of session identifier, as soon as a session is established. We use a terminology inherited from the OSI RM that calls such sessions asso- ciations, identified by association identifiers (ASi). Let's take an example: we want to design an OSI layer that supports multiple actor layers. Figure 2.30 shows the common agent layer and two actor layers, ALi=1 and ALi=2 in their LSM representation. A common agent layer Two LSMs {ALi,As} ALi=1: (ALi,As) Identified by: ALi=2: ALi=1, As=3 *) Layers that provide some type of connectivity. Within an agent layer, several different connectivity layers may be used Fig. 2.30 An OSI layer can contain many actor layers if based on a common agent layer. ALi=actor layer identifier, As=actor suffix (identifies an actor within an actor layer) We expect that the agent layer can support a large number of actor layers and protect actors of different layers to accidentally accessing each other. We also assume SO far that the agent layer does not support associations (i.e., actors can communicate by request-reply only). Thus, every actor in this layer must have a unique identity, which must also help the common agent layer prevent actors from communicating over actor layer boundaries. An actor address (AA) will therefore be built by two parameters: an actor-layer identifier (ALi) and an actor suffix (As). By explicitly defining actor- layer identifiers, actor suffixes can be assigned individually for each actor layer. 2 Layer Structures We can now model the OSI layer that is built by the common agent layer and the two actor layers (see Fig. 2.31). Layer element {ALi,As} Fig. 2.31 An OSI layer built by a common agent layer that supports several separate actor layers The way a common agent operates under these circumstances is depicted by the scenario in Fig. 2.32. Here we have also made the reasonable assumption 