Traffic Protection in Multilayer Core Networks by Optimum Thinning of MPLS Tunnel Capacities

In this paper we study a cost-efficient multilayer restoration (MR) approach for IP/MPLS-Optical networks. In these networks, the capacity of the IP links is reduced by optical link failures that include fibre cuts and lightpath signal-to-noise ratio (SNR) degradations. For instance, when a fibre is cut, the lightpaths that traverse the fibre are rerouted. In most cases, these lightpaths must reduce their line-rates to adapt to the new path lengths, which reduces the capacity of the IP links carried by the lightpaths. This causes a failure state in the IP/MPLS layer characterized by multiple IP links with reduced capacities. When this occurs, the solution used today is to shut off these IP links and restore the traffic by applying IP/MPLS rerouting. This method is costly as it increases the network spare capacity. We study an alternative strategy based on adjustable robust optimization. By this approach, the affected IP links are not torn down. Instead, the MPLS tunnels routed over these links thin their capacities to adapt to the reduced IP link capacities. Then the stranded IP traffic is routed through MPLS tunnels set up on alternative paths. The thinning and routing of MPLS tunnels is planned for foreseen optical link failures. Robustness against unforeseen failures is provided by adjustable decision rules. The efficiency of the strategy is assessed for a German backbone network. The capacity requirements attained by the method are compared with selected MR strategies. The results show that the proposed approach yields capacity savings of least 22% w.r.t the traditional shut off IP links method.