nitoring and quality of service management. As a matter of fact, the transit of datagrams along a tunnel can be easily monitored by the router that can recognize them on the ground of the MPLS labels producing statistics of errors in transmission, passing through time, loss of packets due to the transmission toward a full queue, and SO on. Moreover, a service related tag can be applied to each MPLS label that can be used to run priority algorithms in the input queues of the routers and SO manage real time services. Beyond core routing via IP forwarding or MPLS tunnels, a router carries out a set of other services that have gained great importance in the process that has transformed the Internet from a data network to a global network platform for telecom carriers. Among these services there are Packet translation Packet encapsulation Switching Systems: Architecture and Performances Packet fragmentation if needed Traffic prioritization and generally QoS related procedures [67] Users authentication (generally carried out by particular routers called broadband remote access server [BRAS]) Packet filtering for security/firewall purposes Network address translation (NAT), that is needed when a private IP domain whose addresses are not universally unique have to be interfaced with the public Internet (see Section 3.3) Link monitoring/statistics gathering Proxy, that is the functionality of a router connecting different IP domains in the public network (see Section 3.3) Packet encryption/decryption for virtual private networks (VPNs) that is a key functionality for services directed to business It is evident that the routing procedure is quite elaborated, but it could be completely run by a suitable software on a general purpose computer with suited line interfaces. In practice there exist several types of routers, whose performances and capacities are completely different, and some of them are really computers with a suitable SW and line interfaces. Just to provide a tentative list of different router classes we can id