In this paper, we propose a router-based congestion avoidance mechanism (RoVegas) for TCP Vegas. TCP Vegas detects network congestion in the early stage and successfully prevents periodic packet loss that usually occurs in TCP Reno. It has been demonstrated that TCP Vegas outperforms TCP Reno in many aspects. However, TCP Vegas suffers several problems that inhere in its congestion avoidance mechanism, these include issues of rerouting, persistent congestion, fairness, and network asymmetry. By performing the proposed scheme in routers along the round-trip path, RoVegas can solve the problems of rerouting and persistent congestion, enhance the fairness among the competitive connections, and improve the throughput when congestion occurs on the backward path. Through the results of both analysis and simulation, we demonstrate the effectiveness of RoVegas.