Is the incoming router relavant in an IX?

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 ?