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BBR is a congestion-based congestion control algorithm recently proposed by Google. It proactively measures the bottleneck bandwidth and round trip times (RTTs) of a connection pipe, based on which it governs its sending behaviors. Despite the significant throughput gains and latency reduction, some experimental studies reveal that BBR may result in a salient RTT-fairness problem, in that short-RTT flows can be starved of bandwidth allocation when comnetina with lons-R'I'T flows. In this paper, we study BBR's RTT-fairness problem from a theoretic perspective. We present a closed-form solution that characterizes the intrinsic dynamics of BBR flows and their interactions. Specifically, we model BBR's sending behaviors and bandwidth dynamics, based on which we establish an exponential relationship between the flows' bandwidth shares and their RTTs. We show that the degree of unfairness is dictated by the …