Congestion control in ring based packet networks is challenging due to the fact that every node in the network runs both a rate adaptation algorithm, analogous to an endpoint algorithm in other network architectures, and a rate allocation algorithm, analogous to switch-based algorithms in other network architectures. This paper describes a congestion control algorithm for IEEE 802.17 Resilient Packet Rings called the Enhanced Conservative Mode algorithm that aims to avoid congestion and achieve a fair rate allocation for fairness eligible traffic in the case of a single bottleneck. We first present analysis to show that existing approaches for RPR congestion control (aggressive and conservative mode) have deficiencies. We present simulation results showing that the proposed enhanced conservative mode congestion control algorithm, is a significant improvement. In conjunction with other mechanisms specified in the IEEE 802.17 MAC, the proposed algorithm achieves high utilization on the ring with minimal starvation and oscillations, allows sources to fast start, and provides quality of service for multiple classes of service that require rate, delay and jitter guarantees.