Persistent Route Oscillations in Inter - Domain Routing ? Kannan Varadhan

Hop-by-hop inter-domain routing protocols, such as BGP and IDRP, use independent route selection to realize domains’ local policies. A domain chooses its routes based on path attributes present in a route. It is widely believed that these inter-domain routing protocols always converge. We show that there exist domain policies that cause BGP/IDRP to exhibit persistent oscillations. In these oscillations, each domain repeatedly chooses a sequence of routes to a destination. Complex oscillation patterns can occur even in very simple topologies. We analyze the conditions for persistent route oscillations in a simple class of interdomain topologies and policies. Using this analysis, we evaluate ways to prevent or avoid persistent oscillations in general topologies. We conclude that if a hop-by-hop inter-domain routing protocol allows unconstrained route selection at a domain, the protocol may be susceptible to route oscillations. Constraining route selection to a provably “safe” procedure (such as shortest path) can reduce the number of realizable policies. Alternatively, a routing policy registry can help detect unsafe policies.