Interdomain Internet Routing

Summary

This paper describes the mechanism, policy and economics of Internet routing. The Internet uses topological addressing, so each route corresponds to some address prefix (e.g. 18.31.*). The key mechanisms and policies involve deciding how to route to addresses outside of one's network.

There are two types of agreements between ISPs: transit and peering. In transit agreements an ISP (or individual) will pay for access to a set of routes. In contrast, peering agreements are reciprocal, allowing traffic to from two networks to be exchanged freely. ISPs typically have peers, transit providers and transit customers.

The BGP protocol is the mechanism used to advertise routes allowing routers to build routing tables. To maximize profits, ISPs want to advertise routes to all customers, but not routes through peers (since no money is made from transmitting peer traffic) or routes through transit providers (since these cost money). Similarly, when importing routes ISPs prefer routes directly to customers (these earn money), followed by routes through peers (there are free), and finally routes through transit providers (these cost money). All of these policies are implemented automatically in the routers through prioritizing and filtering.

Unfortunately, the BGP protocol is not secure because it does not verify advertised routes. This allows malicious users and misconfiguration to cause traffic to be erroneously routed. For example, YouTube traffic was mistakenly routed to a "blackhole" in Pakistan.

Thoughts

At first glance, I am very surprised that this complex system with so many conflicting interests manages to (mostly) route traffic from anywhere to anywhere.

I am also curious about traffic ratios in peering arrangements. There seems to be an implicit assumption that the sender should pay for the traffic when both parties are interested in the connection.

Finally, as pointed out in the paper, the lack of security is alarming. Does one have to be an ISP to broadcast BGP advertisements or could any (small) customer potentially broadcast erroneous routes?