On-the-fly TCP path selection algorithm in access link load balancing

Many enterprises install multiple access links for fault tolerance or bandwidth enlargement. Dispatching connections through good links is the ultimate goal in utilizing multiple access links. The traditional dispatching method is only based on the condition of the access links to ISPs. It may achieve fair utilization of the access links but poor performance on connection throughput. In this work, we propose a new approach to maximize the per-connection end-to-end throughput by the on-the-fly round trip time (RTT) probing mechanism. The RTTs through all possible links are probed by duplicating the SYN packet during the three way handshaking stage of a TCP connection. Combined with the statistical packet loss ratio and the passively collected link metrics, our algorithm can real-time select a link which provides the maximum throughput for the TCP connection. The experiment results show that the accuracy of choosing the best outgoing access link is over 71%. If the second best link is chosen, it is usually very close to the best, thus achieving over 89% of the maximum possible throughput. The average per-connection throughput for our algorithm and the traditional round-robin algorithm is 94% vs. 69%.