C in the frame routing through the carrier network. Finally, it is to be noticed that the new fields are introduced in such a way that passing from PB to PBB only requires the upgrade of edge switches, while intermediate switches can go on implementing PB, since they will automatically use the new fields. 3.2.5 Multi-Protocol Label Switching If it is possible that carrier class Ethernet will be the layer 2 technology of choice in the metro and regional area, it seems that there is no possibility to use it in extended core net- work covering a country or a continent. Thus, in the case of core network, the interface between the packet and the transport net- work has to be realized via some other technique. One of the most interesting candidates is the so-called multi-protocol label switching (MPLS). MPLS is a link layer technology devised for networks adopting a plurality of protocols at network and transport layers [34,35]. This is a standard situation in the public networks, where different services have different requirements in terms of QoS and often are sup- ported by different technologies. Originally MPLS was designed to simplify the routers operation: as a matter of fact, it relies on the idea that all the datagrams belonging to the same connection and thus directed to the same destination are tagged with a label and are switched on the same route. Once the label has been created and the route established, the intermediate routers do not have to process the IP address of every packet independently to calculate the route, but simply associate the incoming datagram to an existing tunnel via its label and switch it in the tunnel direction. In a data network implementing MPLS, the edge routers, where the tunnels originate and end, are called label edge routers (LER) while the intermediate rout- ers are called label switching routers (LSRs). This original intention however is no more the most important reason for pushing the adoption of MPLS especially in backbone IP networks. In order to really simplify routers