Direct interconnection network (DIN) has gained more and more attention when designing the switching fabric in the terabit routers. Various congestion control mechanisms are proposed for DIN when it is employed in the parallel computer systems. However, they all have own shortcomings and are not suitable for fabrics in the terabit routers. Based on the requirements of terabit class routers we propose a new congestion control mechanism in this paper. It uses a new detection criterion, i.e. the length of the source buffer, which is more accurate. Besides, the presence of the real time traffic is considered in particular. No throttling is imposed on such traffic to provide QoS. The input traffic will go into different source queue according to their traffic class. The input queue for the real time traffic will no longer be stopped to provide QoS. Finally, simulations are carried out by OPNET software. The results show that our congestion control mechanism eliminates performance degradation for loads beyond saturation, keeping adequate levels of throughput at high loads.