A Note on the Fairness of TCP Vegas

We study the fairness of TCP Vegas. The latter is an alternative to the commonly used TCP Reno, and uses measures of the round trip time as feedback on congestion. We consider two cases that depend on the value of the two parameters $\alpha$ and $\beta$ controlling the window sizes' update. Our main conclusion is that TCP Vegas is unfair in several points. First, when $\alpha = \beta$, if the propagation delays are correctly estimated, TCP Vegas is known to be fair. However we show that any over-estimation of the propagation delay of a given connection results in an increase of its rate and hence leads to unfairness. This rate increase augments with the over-estimation factor. Moreover, the rate oscillations, whose amplitude increases with the rate value, are not sufficient to provide an accurate estimation of the propagation delay. Second, when $\alpha