Continuous and explosive growth of the Internet has shown that current TCP mechanisms cannot achieve efficient utilization of networks with large bandwidth-delay products. To address this problem, we propose an enhanced transport-layer protocol called gHSTCP, which is based on HighSpeed TCP proposed by S. Floyd. By monitoring the changing trend of RTT, gHSTCP adapts to the traffic load by switching between two control modes, which is shown to provide significant performance improvement against traditional TCP Reno in terms of throughput and fairness. Furthermore, it is observed that the performance of gHSTCP is limited by both TailDrop and RED/ARED routers, thus we develop a modified adaptive RED called gARED to address the problem of simultaneous packet drops among multiple flows. By adapting to the trend in variation of the average queue length, gARED performs active queue management more effectively than ARED. Simulations show that combining gHSTCP together with gARED leads to effect utilization of network bandwidth and good fairness.