The recent researches show that the buffer size in routers heavily affects the performances of HSTCP, especially network efficiency. This paper studies the performances of general window-based mechanisms, such as buffer occupancy, network efficiency and packet loss. The study of buffer occupancy shows that large increment function and small buffer size will result in pseudo-congestion, from which HSTCP and Slow Start of TCP suffer, under-utilizing the capacity of bottleneck link. We propose Asymptotic Square Increase (ASI) mechanism, whose increment function is an increase additive function. Not to cause pseudo-congestion, a design principle of the buffer size of routers is given for the increment function of Additive Increase (AI), Multiplicative Increase (MI) and ASI. In this paper, the study of network efficiency shows that the concave increase mechanisms can obtain more network efficiency at smaller cost of packet loss than additive and convex ones.