Attacks that are heavy on L1-4 are the hardest to protect against because of the need for large fixed infrastructure (peering/transit). L7 attacks can be scrub

Attacks that are heavy on L1-4 are the hardest to protect against because of the need for large fixed infrastructure (peering/transit). L7 attacks can be scrubbed by the same infrastructure. Beyond that, it's all a matter of detection. The computational expense of L7 inspection can be mitigated by sampling or scaled with ECMP. You may see a "WAF" (Web Application Firewall) enter the picture at this level.