NXG03-3: Enhancements for RTT-Fair HighSpeed TCP

Abstract

TCP is known to have bandwidth fairness problems for flows of different RTT. Highspeed TCP, which adapts TCP's congestion avoidance to grow faster, and thus be able to scale to high bandwidth environments, aggravates the RTT fairness problem since the congestion window increases at a rate proportional to the RTT and the current window size. We propose a method to eliminate the RTT fairness problem for competing high bandwidth flows. We decouple the rate of window growth in Highspeed TCP from the current window size, such that competing flows adjust their window size, instead, according to their perceived byte sending rate. We derive condiitions and a normalisation scheme to ensure that high bandwidth competing flows converge to fair bandwidth allocation regardless of the RTT or segment size used, while maintaining Highspeed TCP's ability to scale to very high bandwidth environments. We introduce features to preserve TCP friendliness in low bandwidth environments such that, the RTT fairness is no worse than that of standard TCP in scenarios of moderate packet loss. We have conducted simulations to validate the abilities of our proposal to eliminate the RTT bias from Highspeed TCP.