  There are currently no requirements (technical or otherwise) that BGP paths
must be contained within national boundaries. Indeed, some paths experience
international detours, i.e., originate in one country, cross international
boundaries and return to the same country. In most cases these are sensible
traffic engineering or peering decisions at ISPs that serve multiple countries.
In some cases such detours may be suspicious. Characterizing international
detours is useful to a number of players: (a) network engineers trying to
diagnose persistent problems, (b) policy makers aiming at adhering to certain
national communication policies, (c) entrepreneurs looking for opportunities to
deploy new networks, or (d) privacy-conscious states trying to minimize the
amount of internal communication traversing different jurisdictions. In this
paper we characterize international detours in the Internet during the month of
January 2016. To detect detours we sample BGP RIBs every 8 hours from 461
RouteViews and RIPE RIS peers spanning 30 countries. Then geolocate visible
ASes by geolocating each BGP prefix announced by each AS, mapping its presence
at IXPs and geolocation infrastructure IPs. Finally, analyze each global BGP
RIB entry looking for detours. Our analysis shows more than 5K unique BGP
prefixes experienced a detour. A few ASes cause most detours and a small
fraction of prefixes were affected the most. We observe about 544K detours.
Detours either last for a few days or persist the entire month. Out of all the
detours, more than 90% were transient detours that lasted for 72 hours or less.
We also show different countries experience different characteristics of
detours.
