Near the edge, it's just a matter of using the same silicon you have for routing and ask the question "would I have routed a packet to this claimed source IP do

Near the edge, it's just a matter of using the same silicon you have for routing and ask the question "would I have routed a packet to this claimed source IP down the pipe I'm receiving it from?" In cisco-speak this is "ip verify unicast source reachable-via rx" Once you move further towards the core, the problem becomes exponentially more complex. Asymmetric routes are common, so it's not weird at all to get a packet handed to you from a different ISP than you would reply to it. Any filtering there would break a large percentage of valid traffic. The problem is that there are a hell of a lot of "edges", you need nearly all of them to be fixed, and they don't have the motivation to do much about the problem.