h also tends to generate a large name space of node names, since each name represent a particular combination of layer elements from different lay- 4.3 Horizontal Partitioning server Router server server server Router server server Fig. 4.4 Network structures in standards are normally based on (functionally) very complex derived nodes This effect unnecessarily contributes to the impenetrable mass of terms that characterizes telecom standards, as well as suppliers solutions of standardized net- work systems. Horizontal Partitioning 4.3.1 Introduction As mentioned before, there are three aspects on creating a horizontal networks (in this order): 1. Definition of a logical-network structure. 2. Specification of layer interfaces. 3. Refinement of layer interfaces to network interfaces. We will use the model of Fig. 4.3 to exemplify how these refinements affect the model of a network system. Let's first define separate logical networks for the {IP,TCP,UDP} stratum, as well as for each of the layers DNS, L2' and L4'. This simplifies the model considerably, since most of the allo relations in the original model now turn to hosts relations, as depicted in Fig. 4.5. For every logical net- work in this structure, the protocol handling aspect is now defined by allo relations, while the layer interface handling aspect is defined by hosts relations. Note that this separation of concerns in the model makes it an excellent tool for requirement tracing. Modifying a protocol of a layer is specified in the layer structure. The allo rela- tions show which logical networks are concerned. If the modification does not affect the provided layer interface, this is all that matters. Modification of a type of layer interface is also specified in the layer structure. The allo relations show which logical networks are concerned, and hosts rela- tions which logical nodes are involved. 4 Modeling Vertical and Horizontal Partitions Configuration changes do only concern logical networks, and can be specified per logical network. The hosts relat