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ECN-Marking with CoDel and its Compatibility with Different TCP Congestion Control Algorithms

In order to solve the bufferbloat problem, many Active Queue Management (AQM) algorithms have been proposed, such as Random Early Detection (RED), Proportional Integral Controller Enhanced (PIE), Controlled Delay (CoDel), etc. Among them, CoDel has widely been adapted by devices at the last mile. It maintains the sojourn times of transmitted packets and compares them to a pre-defined target time. When the packet's sojourn time exceeds this target, it is interpreted as an evidence of bufferbloat and CoDel applies its drop strategy. This paper examines how CoDel performs when instead of packet dropping the Explicit Congestion Notification (ECN) marking is applied, considering Reno, Cubic and BRR congestion control algorithms in the TCP flows. The evaluation relies on the Programming Protocol-independent Packet Processors (P4) implementation of CoDel, extending it with mechanisms needed for ECN support. Results show that ECN-CoDel can significantly reduce the number of re-transmissions and properly work with classic congestion controls like TCP Cubic and TCP Reno. For TCP BBR, almost zero packet drop ratio can be experienced, but the queue delay is higher than with the original CoDel. Based on the results of this work, the characteristics of the universe AQM had been drawn in the conclusion.