Pi
reuses the fragmentation field of an IP packet to identify the path the packet
traveled. As a packet travels the network, each router it encounters sets a bit
in the fragmentation field. When the packet reaches its destination, the fragmentation
field will contain a marking that is (almost) unique to the path the packet traveled.
The end-host does not know what path the packet traveled, but
if multiple packets have the same marking it is highly likely that they traveled
the same path. If an end-host can identify a packet as an attack packet, then
the end-host can filter out subsequent packets which have the same path identifier.
Initially identifying which packet is an attack packet is left as a separate
problem for the end-host to solve on its own.