nts (i.e., A, B and C turn to A', B' and C' in Fig.4.7). Since different decisions in this respect may be taken for different nodes of the same logical network, it is important for a layer element to be designed SO that the essential part of it (which might be its actor or its agent) can be reused in different nodes. In case of the model in Fig. 4.7, we should therefore try to isolate parts of A', B' and C' that handle interlayer protocols from parts that handle intralayer proto- cols. We use the actor-agent separation principle to solve this problem (see Fig. 4.8 which is a refinement of the model in Fig. 4.7). Refine model *) Meta-primitives are: r-invokeOrd(...,sendReq(...,.. I-invokeRep(...,recRep(...),...) **) These interfaces are candidates for standardization Fig. 4.8 The transformation of a layer interface to an interlayer protocol. Shaded parts are the effects of horizontal partitioning. AG=agent; AC=actor; A',B',C'=elements of layers LA, LB and LC respectively; LD=local connectivity stratum In this model we introduce different actor-agent interfaces for intra- and inter- layer protocols, which leaves the intralayer actor of element B' and the intra-layer agent of A' independent of how the layer interfaces are realized (which may differ between nw1 nodes). Instead we define a local common agent layer inside n11 that supports meta primitives (i.e. r-invokeOrd and I-invokeRep), which in this case are used for carrying the primitives sendReq and recRep that were defined for the layer interface of A' in the original model. Note that the idea with this refinement is to support maximum reusability of the same functional elements in different realizations of layer interfaces (i.e refinement models). This is achieved by: 4 Modeling Vertical and Horizontal Partitions Replacing sendReq and recRep primitives defined by A with meta primitives carrying those primitives. Standardizing intralayer actor-agent interfaces (i.e., specify r-invokeOrd and I- invokeRep). Specify a common agent layer protocol. The introduction