Routers implementing Spoofing Prevention
Method, or SPM [Bremler-Barr and Levy 2005], validate a packet by
checking for a secret key embedded into the packet. A source Autonomous System
(AS), s, decides upon a key for every (s, d) pair, where d is a destination AS.
When a packet reaches a router in AS d, the router checks for the presence of
the secret key. Any packet with the key is valid, and any packet without the key
is spoofing. Packets from ASes not deploying SPM do not have associated keys,
so a router cannot know if a packet purporting to be from an unprotected AS is
spoofing nor not. Packets from these unprotected ASes are allowed through, but
when a router’s network is under attack the router gives preferential service to
legitimate packets from ASes deploying SPM.