The effect of variance reduction on the performance of the leaky bucket

Abstract

The leaky bucket (LB) is emerging as the best possible candidate for implementing the policing mechanism in ATM networks. However, it exhibits a strong coupling between two conflicting requirements, viz., accurate average rate enforcement and prompt reaction to sudden bursts. We propose a modification to the LB mechanism which, while providing a more accurate average rate enforcement (closer to ideal "policer" behavior), also reacts faster (i.e., detects violating cells) to prevent long bursts. The additional hardware and complexity introduced by this modification is marginal. Briefly, the mechanism can be described as follows: every second cell arriving from a source is monitored by a LB while a second LB "polices" the alternate cell stream. This has the effect of reducing the squared coefficient of variation of interarrival times by a factor of two. The predictability of the arrival stream is increased with this mechanism and consequently, the parameters of the LB can be dimensioned more precisely, resulting in a more effective mechanism.