Previous theory and experiments show that delay-based algorithm, e.g. TCP Vegas, outperforms other congestion control algorithms with loss-based events. Unfortunately, previous analytic models of TCP Vegas??always omit the impact of traffic in reverse path. To address this problem, we develop a simple analytic model in the presence of traffic in reverse path. This model captures the key innovative mechanisms that TCP Vegas employs during congestion avoidance and studies the relations between equilibrium window size and round trip time (RTT). In order to do this, We split RTT to two parts: forward trip time and reverse trip time. Results show that the delay time in reverse path has the same importance as forward data path in determining equilibrium window size and throughput of delay-based algorithm in congestion control.