In this paper, we propose a new mechanism which detects tampered-TCP connections at edge routers and protects well-behaved TCP connections from the tampered-TCP connections, resulting in maintaining the fairness amongst TCP connections. The proposed mechanism monitors the TCP packets at an edge router and estimates the window size or the throughput for each TCP connection. By using estimation results, the proposed mechanism assesses whether each TCP connection is tampered or not and drops packets intentionally if necessary to improve the fairness amongst TCP connections. From the results of simulation experiments, we exhibit that the proposed mechanism can accurately identify tampered-TCP connections. We also show that the proposed mechanism can regulate throughput ratio between tampered-TCP connections and competing TCP Reno connections to about 1.