The stability of the resilient packet ring aggressive fairness algorithm

Resilient packet ring (RPR) is currently being standardized by the Institute of Electrical and Electronics Engineers (IEEE), within the 802.17 working group. RPR is a dual ring based medium access protocol based on the "buffer insertion" principle. Instead of controlling the access to the ring using a circulating token, each station on the ring executes its part of a distributed fairness algorithm. The task of the fairness algorithm is to ensure that all stations gets their fair share of the bandwidth, when there is more demand than supply. This article discusses how the RPR fairness algorithm is able to divide the bandwidth fairly among contending stations as well as the requirements for this to work correctly. We discuss how and when the fairness algorithm fails to converge to a fair value, and what is needed in order to make it converge. Finally we show that the setting of the RPR IpCoef configuration parameter is of crucial importance for the convergence of the fairness algorithm as well as some guidelines for setting this correctly. To do this, we first present some analytical work, of which some were contributed to the RPR working group and is now incorporated in the standard. We explain our approach using an informal "proof by induction".