Internet transport-layer protocols rely on the feedback information provided by the acknowledgment (ACK) mechanism to achieve functions such as congestion control and reliable transmission. According to the evolution of Internet transmission protocols, the ACK mechanisms of transmission control are reviewed. The unsolved problems among the mechanisms are discussed. Based on the elements of “type-trigger-information”, the ACK mechanism based on demand and its design principle are proposed, and the coupling relationship between the ACK mechanism and other transmission protocol submodules (e.g., congestion control, packet loss recovery, etc.) is emphatically analyzed. Subsequently, according to the design principle, the TACK mechanism, a feasible ACK mechanism based on demand, is elaborated, and relative concepts are systematically clarified. Finally, several meaningful research directions are provided according to the challenges encountered by the ACK mechanism based on demand.