TCP FlexiS: A New Approach To Incipient Congestion Detection and Control

Abstract—Bulk data transfer applications are greedy in nature in that they have virtually unlimited bandwidth requirements. If serviced by a best effort congestion control, they can create periodic packet loss, long delay and throughput degradation of cross traffic, which may disrupt the normal operation of concurrent QoS sensitive applications. A Lower than Best Effort (LBE) congestion control can make greedy applications only utilize bandwidth left by QoS sensitive applications, thus improve QoS. We propose FlexiS – a Flexible Sender side LBE congestion control mechanism. Unlike most existing LBE congestion controls, which use base delay in incipient congestion detection, FlexiS employs a trend of delay based congestion detector that is more robust to dynamic network environments, in which base delay is difficult to estimate. A non-linear increase and multiplicative decrease controller is used to adapt sending rate by FlexiS. Extensive emulation and Internet tests showed that: (1) FlexiS generally has much lower impact on coexisting best effort traffic than LEDBAT; (2) FlexiS is scalable across a wide range of available bandwidth; (3) aggregate FlexiS traffic can effectively utilize available bandwidth; (4) in most situations, FlexiS flows with the same or different RTTs can fairly share available bandwidth – achieving a fairness index greater than 0.99; (5) the performance of FlexiS is almost not affected by route change and active queue management.