in access and transport links (although not in IP net- works). In general, the perception of an OSI layer as a network-wide function- ality is not productive in modelling networks. In SM&AML we use logical networks that are defined by the SAPs in the C-dimension of functional space to define the equivalence to "layers". Consequently, we use logical network struc- tures to define the equivalence to the structure of layers in an OSI network (more comments on this subject in Sect. 3.5). Another major difference between IP networks and other types of networks is that there is no need to defining access concentration solutions to IP network accesses, as was the case for the ISDN and the ATM CSPs. This is because all interconnections between terminal systems and routers are provided by other CSPs of LAN and WAN types, which perform the necessary concentration. These net- works act as external transport CSPs for the IP network. Since transport is not provided by the IP network itself, there is no link model to be defined. The route concept (without any link structure) acts instead as the only abstraction for interconnecting packet switches. The fact that an IP network is based on other switching networks for transport is often perceived as a stratification of a physical network into (logical) strata. One can therefore say that a physical IP network is a stratified network, consisting of at least two stratum levels: an IP-stratum and a number of external transport strata. We will use the next model, see Fig. 3.73, to discuss how the stratified nature of the IP network affects routing and relaying. In this model we see a small part of an IP network, consisting of two IP routers (RX and RY) and four IP hosts. In the IP network, the hosts are identified by IP addresses (ipa1 through ipa4), while in the external transport network to which they are connected, they are identified by addresses defined by the transport network. All hosts are logically connected to router RX. Two of the hosts are interconnected to router RX over 