The range of parameter γ in FAST TCP are studied in this paper. A continuous time fluid flow model of the link buffer is considered to create a linear control system related to FAST TCP. Linearing the fluid flow model and window control model, the Laplace transform version of congestion control system are presented. It results in a negative feedback system with open loop transfer function. With the analysis of Nyquist curve of the system, conditions on stability of FAST TCP congestion window related to the parameter γ are obtained. Packet level ns2 simulations are used to verify the theoretical claims.
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