Abstract
TCP congestion control adjusts the sending rate in order to protect Internet from the continuous traffic and ensure fair coexistence among multiple flows. Especially, loss-based congestion control algorithms were mainly used, which worked relatively well for past Internet with low bandwidth and small bottleneck buffer size. However, the modern Internet uses considerably more sophisticated network equipment and advanced transmission technologies, and loss-based congestion control can cause performance degradation due to excessive queueing delay and packet loss. Therefore, Google introduced a new congestion control in 2016, Bottleneck Bandwidth Round-trip propagation time (BBR). In contrast with traditional congestion control, BBR tries to operate at the Kleinrock’s optimal operating point, where delivery rate is maximized and latency is minimized. However, when BBR and loss-based congestion control algorithms coexist on the same bottleneck link, most of bottleneck bandwidth is occupied by flows that use a particular algorithm, and excessive packet retransmission can occur. Therefore, this paper proposes a BBR congestion window scaling (BBR-CWS) scheme to improve BBR’s inter-protocol fairness with a loss-based congestion control algorithm. Through Mininet experiment results, we confirmed that fairness between BBR-CWS and CUBIC improved up to 73% and has the value of 0.9 or higher in most bottleneck buffer environments. Moreover, the number of packet retransmissions was reduced by up to 96%, compared to the original BBR.
Highlights
Many Internet traffics use TCP as the transport layer protocol for reliable data transmission [1,2].TCP congestion control, a main TCP feature, protects the Internet from persistent overload and ensures that multiple hosts use bottleneck bandwidth evenly [3,4,5]
That is because Bottleneck Bandwidth Round-trip propagation time (BBR) has inflexible operating characteristics, which means that it does not actively reduce the congestion window by packet loss, but only determines the sending rate depending on Bandwidth Delay Product (BDP), estimated by Bottleneck Bandwidth (BtlBw) and Round-trip propagation time (RTprop)
When the buffer size is larger than 1 BDP, the fairness between Modest BBR and CUBIC is considerably improved compared to the original BBR
Summary
Many Internet traffics use TCP as the transport layer protocol for reliable data transmission [1,2]. The inter-protocol fairness problem between BBR and existing loss-based congestion control algorithms is considered to be a serious issue, because the throughput between two algorithms can vary significantly with bottleneck buffer size. That is because BBR has inflexible operating characteristics, which means that it does not actively reduce the congestion window by packet loss, but only determines the sending rate depending on BDP, estimated by Bottleneck Bandwidth (BtlBw) and Round-trip propagation time (RTprop). BBR-CWS improves inter-protocol fairness when it shares the link with loss-based congestion control algorithms and reduces excessive retransmissions. The inter-protocol fairness between BBR-CWS and loss-based algorithms was improved about 70% and the number of retransmitted packets was reduced by about 90% compared to the original BBR.
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