[alexdns]

Aka strict rpf which none of the Tier1 does

[immibis]

For very good reason. Legitimate routing is often asymmetric and applying this rule would block a large percentage of Internet traffic.

[alexdns]

Doesnt matter if its asymmetrtic or not. You should not be allowed to send egress from prefixes that dont belong to you and that you dont advertise to your upstream. Thats what strict rpf is for. Else you are spoofing.

[mobilemidget]

Can you elaborate a little?

I mean any ISP can check if a packet leaving their network is actually a network they have under their control, routing doesn't have anything to do with that?