A Tight Characterization of Fast Failover Routing: Resiliency to Two Link Failures is Possible

To achieve fast recovery from link failures, most modern communication networks feature local fast failover mechanisms in the data plane. These failover mechanisms typically rely on pre-installed static rerouting rules which can depend only on local failure information. The locally limited failure information renders the problem of providing a high resilience algorithmically challenging. In this paper, we are interested in algorithms which tolerate a maximal number of k simultaneous link failures to guarantee packet delivery, as long as source and destination remain connected afterwards. Prior work showed that k=1 link failure can always be tolerated in general networks, but already the question of k=2 remained an unresolved problem. This paper closes this gap by presenting a tight characterization of fast failover routing: We show that 2-resiliency is possible with pre-installed static routing rules on general topologies, but that 3-resiliency is impossible already in relatively simple networks. Our 2-resilient routing scheme can be computed efficiently and relies on a careful kernelization and subdivision of the network topology into sparse subgraphs.