  In a DDoS attack (Distributed Denial of Service), an attacker gains control
of many network users through a virus. Then the controlled users send many
requests to a victim, leading to its resources being depleted. DDoS attacks are
hard to defend because of their distributed nature, large scale and various
attack techniques. One possible mode of defense is to place sensors in a
network that can detect and stop an unwanted request. However, such sensors are
expensive so there is a natural question as to the minimum number of sensors
and the optimal placement required to get the necessary level of safety.
Presented below are two mixed integer models for optimal sensor placement
against DDoS attacks. Both models lead to a trade-off between the number of
deployed sensors and the volume of uncontrolled flow. Since the above placement
problems are NP-hard, two efficient heuristics are designed, implemented and
compared experimentally with exact mixed integer linear programming solvers.
