Is the incoming router relavant in an IX?

[AltraMayor]

In an Internet exchange (IX), Gatekeeper servers talk to multiple routers to receive traffic, but the information of which router is the hop just before a Gatekeeper receives a packet from the Internet is only available in the MAC address of the source of the packet. During some DoS attacks, it can be the case that a single peer is the source of all the trouble. How could this information be provided to Grantor servers? Is this information actually useful? If so, how could a policy use it since flows are defined as <source IP, destination IP>?