THE LEAKY WINDOW - A FLOW AND CONGESTION CONTROL TECHNIQUE FOR HS-WANS

Congestion in high-speed wide area networks (HS-WAN) must be controlle d in order to prevent network performance deterioration, and possible collapse. Reactive mechanisms applied to slower networks are not entir ely suitable for HS-WANs due to the relatively long propagation delay in the latter, On the other hand, preventive mechanisms which have bee n proposed as possible solutions to this problem assume that traffic c haracteristics are known at the time of call setup. Window mechanisms do not depend upon such an assumption, but modifications that have bee n proposed to adapt it to HS-WANs fall short of the fast response requ ired in HS-WANs. The Leaky Window (LW) mechanism is proposed in this p aper as an attempt to solve this problem. This mechanism is a modifica tion of the sliding window that permits users to transmit traffic in e xcess of their window sizes based on an estimate of the network load. The estimate is based on acknowledgments received within a fixed time interval. Excess traffic is distinguished by 'marking' cells. Marked c ells are discarded at congested nodes. Congestion control, therefore, is a local decision executed by the congested node. Through the use of a simulation model, it is shown that the LW has an average end-to-end delay and probability of loss that is lower than the sliding window m echanism. Comparison to the Virtual Leaky Bucket (VLB) mechanism shows that at lower load (0.7), the VLB has an end-to-end delay and a proba bility of loss that is lower than that of the LW under the same condit ions. At higher loads where congestion is a real problem, the performa nce of the LW is significantly better than that of the VLB.