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Due to the bufferless nature of optical burst switched network, contentions occur even at low traffic loads, leading to burst losses. Contention resolution schemes, such as burst retransmission and deflection, can reduce burst losses, especially at low traffic loads. However, both schemes result in additional packet delay for the packets in bursts that are retransmitted or deflected. The additional packet delay affects the performance of delay-based TCP implementations that rely on packet delay to estimate available bandwidth in networks and to detect network congestion state. In this paper, we discuss the issues of TCP Vegas over OBS networks and propose a threshold-based TCP Vegas version that is suitable for the characteristics of OBS networks. The threshold-based TCP Vegas are able to distinguish whether the increases in packet delay are due to network congestion, or due to burst contentions at low traffic loads. Our simulation results show that the threshold-based TCP Vegas has higher throughput for a TCP connection compared to TCP Vegas and the loss-based TCP implementations, such as TCP Sack.