How Stable is Stable in Interdomain Routing: Efficiently Detectable Oscillation-Free Configurations

Internet Service Providers have at their disposal a powerful policy-based protocol for enforcing a fine grained control of Interdomain Routing: the Border Gateway Protocol. However, the price to pay for the flexibility of BGP is the lack of convergence guarantees. In this paper, we study the stability of BGP configurations. Namely, we tackle the problem of deciding if, given the policy configurations, the routing will converge to a stable state or if there is the potential chance of persistent oscillations. First, we extend the currently largest known class of stable configurations that can be efficiently detected. This is done with a simple algorithm that relies on new properties of policy-based protocols that are shown to be independent on any specific message timing. Second, we provide a sufficient condition to guarantee the absence of potentially persistent oscillations in the routing. We show that this condition is less constraining than currently known ones. Finally, we assess the ability of existing models of policy-based protocols to capture routing oscillations, showing that different models put in evidence different types of oscillations. We prove that all our results are valid even in the most general model.