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Simultaneous advances in processor, network and protocol technologies have made clusters of workstations attractive vehicles for high performance computing. However, clusters are now being increasingly used in environments characterized by non-cooperating communication flows with a range of service requirements. This necessitates quality of service (QoS) mechanisms in clusters. The approaches to QoS in the wide-area networking context are not suitable for clusters because of the high overheads. Also, contention between flows at the end-nodes has not been addressed earlier. In this paper, we explore the use of "rate control" as a means for proportional bandwidth allocation in clusters. A NIC-based solution is presented, with details on implementation in Myrinet/GM. Experimental results show that rate control can handle both end-node and network contention, without adding significant overhead. Our approach is particularly attractive since it does not require hardware modifications, and can hence work with commodity systems with programmable NICs.