This book deals with the principles of the congestion control mechanism for Transmission Control Protocol (TCP). Furthermore, the book had focused on the congestion control mechanism used by standard TCP and illustrated that the performance of TCP variants are directly affected by their own congestion control mechanisms and the packets amount transferred over network connections indicates the work and the behaviour of the congestion window. In addition, the book analysed and formulated the typical congestion control where it is commonly divided into two phases, Slow-Start phase and Congestion Avoidance phase. It had reviewed the factors and the parameters that affect on the behaviour and the TCP performance, and had demonstrated the design of the congestion control of six TCP source variants in addition to some other TCP's using network simulator.