This paper presents a control-theoretic approach to reactive flow control in networks that do not reserve bandwidth. The authors assume a round-robin-like queue service discipline in the output queues of network switches and propose deterministic and stochastic models for a single conversation in such a network. These models motivate the Packet-Pair rate probing technique and a provably stable rate-based flow control scheme. A Kalman state estimator is derived from discrete-time state space analysis, but it is impractical due to the need for system and observation noise variances. A novel fuzzy logic-based estimation scheme is proposed to overcome this issue. The paper also discusses techniques to extract and use additional information from the system to develop a continuous-time system model, which is used to design a variant of the control law that is also provably stable and takes control action as rapidly as possible. Practical issues such as correcting parameter drift and coordination with window flow control are addressed. The paper concludes with a discussion on the limitations of the approach and a review of related work.This paper presents a control-theoretic approach to reactive flow control in networks that do not reserve bandwidth. The authors assume a round-robin-like queue service discipline in the output queues of network switches and propose deterministic and stochastic models for a single conversation in such a network. These models motivate the Packet-Pair rate probing technique and a provably stable rate-based flow control scheme. A Kalman state estimator is derived from discrete-time state space analysis, but it is impractical due to the need for system and observation noise variances. A novel fuzzy logic-based estimation scheme is proposed to overcome this issue. The paper also discusses techniques to extract and use additional information from the system to develop a continuous-time system model, which is used to design a variant of the control law that is also provably stable and takes control action as rapidly as possible. Practical issues such as correcting parameter drift and coordination with window flow control are addressed. The paper concludes with a discussion on the limitations of the approach and a review of related work.