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Being a widely used transport protocol in Internet, TCP has its own congestion control mechanism. However, TCP might be ineffective because it starts decreasing its rate just after it senses packet's loss, implying that the network has already been congested somewhere. Although several amendments have been developed to improved TCP performance over these years, some inefficiencies such as slow reaction and oscillatory flow still exist. A comprehensive mechanism of controlling ACKs in troublesome nodes is presented in this paper to improve TCP performance. This mechanism can be considered as an enhancement to the currently prevalent TCP and referred to ACK control. The basic idea of this mechanism arises from the fact that controlling ACK flows can indirectly affect the dynamics of TCP's behavior. In this paper, we mainly discuss two issues, ie, flow smooth by delaying or accelerating backward ACKs, and precedence allowance by sequencing backward ACKs in differentiated services networks.