Is intended for use in guiding the design and application of real interworking units and real subnetworks (e.g. local area networks and private packet switched networks) which are to support the OSI network service. As the principal means for expressing the definition, the concept of the network internal layer service is used. The definition includes the invocation of network routing functions as a necessary element of the network relaying functions. The definition of network relaying functions applies both to a subnetwork supporting all elements of the network service and to a network relay system interconnecting two subnetworks.