IP Reachability differences: Myths and realities

Ideally, an Internet Service Provider offers reachability to the entire Internet. However, it is often claimed (supported by anecdotal evidence) that reachability differences exist between ISPs, with the result that some valid, globally routable IP addresses can be reached from one ISP but not from others. Outdated Bogon filters, specialized services, and regional differences have all been conjectured as possible sources of reachability differences. However, to the best of knowledge, no systematic study has verified and quantified the extent of such differences, and no analysis exists to show which (if any) of the common conjectures are the dominant causes. In this paper we provide an initial systematic study of top-tier ISP reachability differences. We compare global BGP tables from 25 ISPs to 1) quantify the extent of reachability differences, and 2) determine which (if any) of the common conjectures explain these differences.