To investigate the problem of the congestion control over high-speed long distance networks, this paper presents a new AIMD congestion control algorithm, I-TCP, which is based on a model of network of synchronous AIMD sources competing for a shared bandwidth under drop-tail queuing. With a reasonable design of a high-speed mode, a low-speed mode and the mode switch rule, I-TCP is suitable for deployment in the high-speed long distance networks as well as the traditional low speed networks. Simulation experiments show that the AIMD algorithm of I-TCP performs better than current TCP implementations in terms of efficiency, fairness and network responsiveness over high-speed long distance networks.