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Recently, the use of Internet Protocol has rapidly expanded beyond the Internet, as evidenced, for example, by the construction of the next-generation network, empowering telecommunication networks by IP. A huge IP network is expected to emerge in the near future by means of convergence of various networks. However, Transmission Control Protocol, the de facto standard transport layer protocol providing reliable communication over such IP networks, poses several significant performance issues. The small buffer problem, unfair bandwidth allocation, and throughput degradation in wireless environments have been widely known issues in TCP communications. In this article we examine the causes of these problems from the viewpoint of window control theory. While TCP employs additive-increase multiplicative-decrease theory as a window control policy, the lack of flexibility of its static AIMD control is the basic cause for its performance degradation. After briefly reviewing TCP enhancements that utilize various modified AIMD control schemes, we introduce explicitly synchronized TCP, which employs a dynamic AIMD window control mechanism by employing feedback information from network nodes. By dynamically controlling AIMD procedures according to varying network conditions, ESTCP is able to achieve high performance even in hybrid wired/wireless networks.