BBR拥塞控制算法在无线网络中的性能改进

Performance improvement of BBR congestion control algorithm in wireless network

摘要:

传统的拥塞控制算法已经不能满足当前复杂的网络环境,谷歌提出的BBR算法(Bottleneck Bandwidth and Round-Trip)为拥塞控制提供了一种新思路,它可以在具有一定丢包率的网络链路上充分利用带宽,并保证较低的时延.但是该算法存在以下问题:首先,当无线网络的时延剧烈抖动时,BBR具有很低的传输速率,即便网络不丢包且此时未发生拥塞,这一问题在以往的论文中还没有人提出过；其次,BBR对网络带宽的降低不够敏感.本文详细分析以上问题出现的原因,进而提出改进BBR算法:通过比较RTT的均值和标准差判断网络时延的抖动程度,在时延抖动很剧烈时,使用RTT的均值取代最小RTT来计算拥塞窗口；在网络不稳定时,降低PROBE_BW状态中平稳阶段的时间长度.在实际网络中的实验表明,改进后的BBR算法几乎不受时延波动的影响,随着时延波动程度的提高,改进后算法的传输速率基本保持不变,在BBR几乎不能工作时仍能保持正常的传输速率；而且改进后的BBR算法在网络不稳定时能够更快地探测到网络带宽的降低并收敛.

Abstract:

Since traditional congestion control algorithm cannot fulfill the requirements of current complex networks, Google provides a new way named bottleneck bandwidth and round-trip (BBR) for congestion control. The purpose of BBR is to maximize throughput and minimize delay on a bottleneck link with packet loss, but it has some disadvantages: First, when the delay of wireless network is severely jittered, even if there is no packet loss or congestion, the delivery rate of BBR is very low. This issue has not been raised in previous studies. Second, BBR is not sensitive enough to the bandwidth drop. This paper analyzes the causes of the above problems in detail and proposes an optimized BBR, which can judge the jitter degree of network by comparing the mean and standard deviation of RTT. When the jitter of delay was severe, the mean RTT was used instead of minimum RTT to calculate congestion window. When the network was unstable, the length of stationary phase in PROBE_BW was reduced. Experiments in the real network showed that the optimized BBR was almost unaffected by delay fluctuations, and could maintain high bandwidth utilization when BBR could hardly work. Besides, it could detect bandwidth degradation and converge faster than BBR when the network was unstable.