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Reduction-of-Quality (RoQ) attack is a type of Distributed Denial-of-Service (DDoS) attack that is difficult to detect in current computing systems and networks. These RoQ attacks throttle the throughput heavily and reduce the Quality of Service (QoS) to end systems gradually rather than refusing the clients from the services completely. In this paper, we propose to develop a flow monitoring scheme to defend against such attacks in mobile Ad-hoc networks. Our proposed defense mechanism consists of a flow monitoring table (FMT) at each node to identify the attackers. If the channel continues to be congested because some sender nodes do not reduce their sending rate, it can be found by the destination using the updated FMT. Once the attackers are identified, all packets from those nodes will be blocked. By simulation results, we show that our proposed scheme achieves higher throughput and packet delivery ratio with reduced packet drop for legitimate users.