Small Chunks can Talk: Fast Bandwidth Estimation without Filling up the Bottleneck Link

With the development of wireless communications (e.g., WiFi 6 and 5G), more and more high-bandwidth networks are emerging in our daily life. However, due to the limited speed of the slow start phase in congestion control algorithms, the high-bandwidth links may not be fully utilized, which will degrade the Quality of Service (QoS). The reason is, since the available link capacity is unknown until the link is fully occupied, the sender has to gradually increase the congestion window (cwnd) from a small initial value, causing the link to be underutilized, until a packet is dropped or a congestion signal is detected. Especially, for a short flow, the transmission is often finished before the link capacity is reached, leading to the waste of available bandwidth. To better exploit the high bandwidth links, this paper proposes FBE (Fast Bandwidth Estimation without Filling up the Bottleneck Link), by leveraging the effective ACK's returning rate to estimate the bottleneck link capacity. More specifically, instead of sending out any additional probe packets, FBE uses the ACK rates from the first two RTT rounds to quickly estimate the bandwidth during slow start phase. Since the original ACK rate is significantly lower than the available link bandwidth due to the exhaustion of send window, and the competing flows also have an impact on the ACK rate, FBE elaborates the ACK interval compensation algorithm to refine the ACK intervals to reflect the link rate, and then updates cwnd to a suitable size. To address the challenge of inaccurate bandwidth estimation, especially for rapidly changing wireless link, FBE dynamically adjusts cwnd according to the feedback of driver queue length after the bandwidth estimation. Experiments in real WiFi and LTE networks show that FBE reduces the slow start convergence time by 54.8% and 53.5% compared to CUBIC and BBR with traditional slow start, respectively. And when transferring short flows (512KB in size), FBE reduces the flow completion time by 40.4% and 43.8% compared to CUBIC and BBR, respectively.