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Reservation based medium access control (MAC) protocols such as the 802.11 distributed coordination function (DCF), are designed to maximize throughput efficiency, by utilizing small control packets. However, this ability to reserve bandwidth makes the protocol susceptible to sophisticated MAC layer denial of service (DoS) attacks. A novel attack on reservation based MAC protocols is described using the example of 802.11 networks. An analysis of the attack demonstrates the catastrophic effect on network throughput of a low power MAC layer jammer. This is in contrast to physical layer jamming, where the loss of throughput is proportional to jammer power. A counter-measure based on modifying the MAC protocol is developed, and shown to limit the intelligent jammer's effect. Protections against further attacks on the modified protocol are also discussed.