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 result in routing inconsistencies, i.e., routers along a forwarding route mapping same IP addresses to different prefixes. In turn, exit AS border associated with these distinct potentially differ. For some detours (FDs) occur, traffic deviate from best IGP paths. work we investigate phenomenon FDs derive methodology detect them. particular, our tool is able pinpoint cases where multiple are subject FDs. We run measurements 100 vantage points NLNOG RING monitoring infrastructure find 25 out 54 ASes. see that heterogeneous, number between which strongly depend on studied AS. Finally, discover remarkable binary effect either all transit traversing two an detours, or none does.