Slow start algorithm for mobile broadband networks including delay unrelated to network congestion

Abstract

The conventional slow start algorithm doubles the congestion window size per RTT until it detects a packet loss in order to estimate the available bandwidth quickly. But it causes a large number of packet losses. Hybrid Slow Start is proposed to reduce these packet losses. It finds a good point where TCP can exit the slow start phase before packet losses occur. However, the algorithm for finding the exit point does not work well on mobile broadband networks since the delay in these networks fluctuates regardless of whether congestion exists or not. Jitter causes false-positive detections of network congestion and causes Hybrid Slow Start to exit before the congestion window increases enough to utilize the available bandwidth. In this paper, we propose a new slow start algorithm suited to mobile broadband networks including delays unrelated to network congestion. It estimates the delay and includes it in the threshold of congestion detection to prevent undershoots of the congestion window. It also estimates the available bandwidth to prevent overshoots causing a large number of packet losses. We evaluated the algorithm in a simulated network and a real mobile broadband network. The results show that it finds the exit point properly and improves the TCP performance in such networks.