BorderGuard: detecting cold potatoes from peers

Internet Service Providers often establish contractual "peering" agreements, where they agree to forward traffic to each other's customers at no cost. Consistent route advertisement at all peering points is a common provision in these agreements, because it gives an AS the flexibility to select egress points for the traffic (e.g., performing "hot potato" routing). Verifying "consistent export" is challenging because route advertisements are exchanged at multiple peering points and may be modified by routing policies. In this paper, we propose two algorithms to detect inconsistent routes using routing and configuration data from an AS's border routers. The first algorithm requires access to all eBGP routes advertised by a peer. Because this data is often unavailable, we propose another algorithm that detects inconsistencies using readily available data. We have applied our algorithms to the routes advertised by the peers of AT&T's commercial IP backbone. Although a peer may intentionally send inconsistent advertisements to prevent its neighbor from performing hot-potato routing, we also discuss several configuration scenarios where a peer may inadvertently advertise inconsistent routes, despite having consistent export policies. Finally, we explain how simple modifications to the routers could make detection of inconsistent advertisements much easier than it is today.