The Art of Detecting Forwarding Detours

The full Internet feed, reaching ~867K prefixes as of March 2021, has been growing at ≈50K prefixes/year over the last 10 years. To counterbalance this sustained increase, Autonomous Systems (ASes) may filter prefixes, perform prefix aggregation and use default routes. Despite being effective, such workarounds may result in routing inconsistencies, i.e., in routers along a forwarding route mapping the same IP addresses to different IP prefixes. In turn, the exit AS border routers associated with these distinct prefixes may potentially differ. For some prefixes, forwarding detours (FDs) may occur, i.e., traffic may deviate from best IGP paths. In this work we investigate the phenomenon of FDs and derive a methodology to detect them. In particular, our tool is able to pinpoint cases where multiple prefixes are subject to FDs. We run measurements from 100 vantage points of the NLNOG RING monitoring infrastructure and find FDs in 25 out of 54 ASes. We see that FDs are heterogeneous, i.e., the number of prefixes and AS border routers in between which we detect FDs strongly depend on the studied AS. Finally, we discover a remarkable binary effect such that either all transit traffic traversing between two border routers of an AS detours, or none does.