Stable Policy Routing with Provider Independence

Thousands of competing autonomous systems (ASes) must cooperate with each other to provide global Internet connectivity. These ASes encode various economic, business, and performance decisions in their routing policies. The current interdomain routing system enables ASes to express policy using rankings that determine how each router in an AS orders the different routes to a destination, and filters that determine which routes are hidden from each neighboring AS. Since the Internet is composed of many independent, competing networks, the interdomain routing system should allow providers to set their rankings independently, and to have no constraints on allowed filters. This paper studies routing protocol stability under these constraints. We first demonstrate that certain rankings that are commonly used in practice may not ensure routing stability. We then prove that, with ranking independence and unrestricted filtering, guaranteeing that the routing system will converge to a stable path assignment essentially requires ASes to rank routes based on AS-path lengths. Finally, we discuss the implications of these results for the future of interdomain routing.