IP Spoofing
There is no excuse for not securing your network to allow spoofing from it. Most of the big players like leaseweb or ovh do not allow that. But there are some providers that still allow you to spoof source ip address. There should be consensus about droping routes on BGP level to networks  that send packets with source ips that they do not announce. It's really simple to drop packets on switches/routers that do not originate from your network. It would make ddosers life harder.