This paper presents a control-theoretic approach to reactive flow control in networks that do not reserve bandwidth. The approach assumes a round-robin-like queue service discipline in the output queues of the network's switches and proposes deterministic and stochastic models for a single conversation in a network of such switches. 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 there are difficulties in using the estimator in practice. These difficulties are overcome by a novel estimation scheme based on fuzzy logic. The paper also presents a technique 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 described. The paper discusses the design of a control-theoretic flow control mechanism based on the Separation Theorem, which allows the use of any technique for state estimation and then implementing control using the estimated state. The control law is designed to maintain the number of packets in the bottleneck queue at a desired setpoint, balancing trade-offs between mean packet delay, packet loss, and bandwidth loss. The paper also discusses the non-linearity in the system and how it can be accounted for in the analysis. A Kalman state estimator is presented, but it is impractical, so a novel estimation scheme based on fuzzy logic is proposed. The paper also presents a fuzzy estimation technique that uses fuzzy control to determine the value of a parameter that adapts to changes in system behavior. The paper concludes with a discussion of 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 approach assumes a round-robin-like queue service discipline in the output queues of the network's switches and proposes deterministic and stochastic models for a single conversation in a network of such switches. 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 there are difficulties in using the estimator in practice. These difficulties are overcome by a novel estimation scheme based on fuzzy logic. The paper also presents a technique 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 described. The paper discusses the design of a control-theoretic flow control mechanism based on the Separation Theorem, which allows the use of any technique for state estimation and then implementing control using the estimated state. The control law is designed to maintain the number of packets in the bottleneck queue at a desired setpoint, balancing trade-offs between mean packet delay, packet loss, and bandwidth loss. The paper also discusses the non-linearity in the system and how it can be accounted for in the analysis. A Kalman state estimator is presented, but it is impractical, so a novel estimation scheme based on fuzzy logic is proposed. The paper also presents a fuzzy estimation technique that uses fuzzy control to determine the value of a parameter that adapts to changes in system behavior. The paper concludes with a discussion of the limitations of the approach and a review of related work.