 vertical or horizontal parti- tioning. To find out which it is, the layer structure model and the mapping relations (called allocates) between a node structure and that model must be consulted. An obvious distinction between the layer structure inside a logical node (i.e., its functionality) and the logical network model is that the latter holds for many other layer structure as well. Thus, a logical network model shows a network-wide pack- age structure that is highly insensitive to changes of the functions it performs. This is also why we normally do not define protocol relations in logical network models, since protocols imply a particular functional structure inside logical nodes. In case we want to show this functionality in the context of a logical network model, we define a network interface (NI) as depicted in Fig. 3.5. Derived Derived node is dn1 node is dn2 The network interface abstraction Fig. 3.5 The network interface abstraction. dn...=derived node; V-NI=network interface created by vertical partitioning A network interface in AMLn is confined to a particular logical network. It comprises both the layer structure of the logical network and the connectivity rela- tions on which it relies. The nodes that are related over a network interface are derived by mapping a logical-network structure on a layer structure, which results in node types with completely defined functional contents (which the "logical node" is not). We therefore call such nodes derived nodes. Derived nodes and network interfaces are very volatile entities since they are affected by every possible modification of basic AMLn structures (layer structures and node structures). Obviously, network structures based on derived nodes and network interfaces need not be explicitly modeled in AMLn since they can be derived from the basic structures. Derived nodes and networks act as requirements on suppliers' solutions to net- work elements. Such models should therefore define if the separation of two derived nodes is created by vertical or hor