Strongly disagree. Strict urpf is pretty much impossible to implement. The only plausible place to implement it is directly at the edge where a sinlge AS owns

Strongly disagree. Strict urpf is pretty much impossible to implement. The only plausible place to implement it is directly at the edge where a sinlge AS owns both sides of the link. Think DSLAM or other consumer agg device. Any connection to a multihomed peer, or peer with a different AS, cant have strict urpf enabled. Once you get away from the network edge the only possible urpf is loose mode. But thats a restricted implementation as plenty of stubs out there use default routes. Then asymmetric routes are so common as to rule the use of feasible urpf completely. So in summary, it has to be the edge networks who enforce this. And the prime intermediate offenders actually have a monetary incentive to not prevent this traffic.