Consensus Routing: The Internet as a Distributed System

Internet routing protocols (BGP, OSPF, RIP) have traditionally favored responsiveness over consistency. A router applies a received update immediately to its forwarding table before propagating the update to other routers, including those that potentially depend upon the outcome of the update. Responsiveness comes at the cost of routing loops and blackholes-a router A thinks its route to a destination is via B but B disagrees. By favoring responsiveness (a liveness property) over consistency (a safety property), Internet routing has lost both. Our position is that consistent state in a distributed system makes its behavior more predictable and securable. To this end, we present consensus routing, a consistency-first approach that cleanly separates safety and liveness using two logically distinct modes of packet delivery: a stable mode where a route is adopted only after all dependent routers have agreed upon it, and a transient mode that heuristically forwards the small fraction of packets that encounter failed links. Somewhat surprisingly, we find that consensus routing improves overall availability when used in conjunction with existing transient mode heuristics such as backup paths, deflections, or detouring. Experiments on the Internet's AS-level topology show that consensus routing eliminates nearly all transient disconnectivity in BGP.