A second-order leaky bucket algorithm to guarantee QoS in ATM networks

We propose a second-order leaky bucket algorithm to guarantee the quality of service (QoS) in ATM networks. The second-order leaky bucket, consisting of two leaky buckets, polices the sustainable cell rate (SCR) and burst tolerance (BT) for each variable bit rate (VBR) ATM connection. With this algorithm, ATM cells enter a first leaky bucket. Cells conforming to the first leaky bucket are allowed to enter the ATM network with the cell loss priority (CLP) field of the ATM cell header set to 0. Any cell non-conforming to the first leaky bucket is sent to a second leaky bucket, and is CLP=1 tagged if found to be non-conforming to the second leaky bucket. Cells conforming to the second leaky bucket enter the ATM network with CLP=0. Hence, unlike a traditionally configured “dual” leaky bucket policer, a cell is tagged CLP=1 only if found to be non-conforming by both the first and second leaky buckets. A second-order leaky bucket enables one to monitor whether the change in rate or burst for an ATM connection is due to a statistical variation or due to violation of the traffic contract. We investigate the effectiveness of the second-order leaky bucket in protecting the QoS experienced by connections passing through a common internodal link queue. We show that the algorithm eliminates unacceptable delay and loss at an internodal link queue caused by non-conforming users generating excess traffic. Moreover, the second-order leaky bucket configured with a tag/loss probability of 10 ^-6 achieves a better delay performance at an internodal link queue than that of a single leaky bucket configured at 10^-2