Interdomain Internet Routing

This article primarily describes how inter-network routing works over the present Internet. BGP, the protocol that supports such routing, is relative simple (at least at network edges), consisting simply of annotated offers or withdrawls of offers to route to some set of addresses. The annotations provided include information about the overall path of the route, which is used to prevent loops and help make routing decisions, but routing decisions are heavily influenced by other policies.

Much of the interesting part of inter-network routing, how these routing decisions are made and which offers are made or withdrawn, are not part of BGP. Typically, these policies are determined by the economic relationship between the two networks. A provider wants to maximize utilization of links that make it money, so it has an incentive to advertise any available routes using such links. When deciding between multiple routes to the same destination, a provider will similarly be motivated to configure their software to prefer routes using links for which they are paid, and may then decide based on traditional “technical” criteria like path length. (Of course, explicit contractual arrangements also influence these decisions.)

BGP does seem to have been designed for a smaller, friendlier Internet. The state size is and its changes are not trivial and likely to get worse as more people would sensibly like to have an internet or a backup internet connection. As administrated, there is a lot of trust, revealed through the poor filtering illustrated by hijacked blocks and then general lack of route authentication in the protocol. And, despite its flexibility for route announcers to choose their policy, some policies seem hard to enforce technically, such as preferences for peers to use their own networks for cross-country/sea transit. (Presumably, this could be prevented by only advertising the routes through the preferred entry point except that this would break reachability during glitches on the peer's network, so it is easier to contractually require honoring MED when needed?)