Performance Analysis of CoDel and PIE for Saturated TCP Sources

In the recent years, the bufferbloat phenomenon was observed which is mainly due to oversized unmanaged buffers in the Internet. This triggered a new discussion of active queue management (AQM) algorithms in the IETF. "Controlled Delay" (CoDel) and "Proportional Integral controller Enhanced" (PIE) are considered as an alternative to "Random Early Detection" (RED). Their intention is both to take advantage of large buffers for occasional bursts and to limit queueing delays most of the time. Moreover, they are able to cope with varying bandwidth. In this paper, we study the performance of CoDel, PIE, and CoDel-ACT, which is an effective modification of CoDel that leads to better performance than CoDel in our studies. We experiment with saturated TCP sources and a fixed-bandwidth bottleneck link and focus on the delay-limiting phase of the algorithms. We investigate the impact of configuration parameters and traffic load on link utilization and queueing delay. We study the timely evolution of queuing delays and drop patterns, and point out significant differences among the algorithms. In particular, we show that CoDel's drop behavior changes over time and may lead to underutilization.