On the prevalence and evaluation of recent TCP enhancements

In recent years several enhancements to TCP congestion control and loss recovery mechanisms have been proposed and accepted as Internet standards. In this paper, we examine these enhancements in detail. We consider five TCP enhancements: (1) selective acknowledgements (SACK) and the SACK-based loss recovery algorithm; (2) increasing initial congestion window; (3) limited transmit; (4) appropriate byte counting; and (5) early retransmit. We present results from active measurements performed on web servers on the state of deployment of these enhancements. Our results show that while several web servers support TCP enhancements, the majority still use previous standards for congestion control and loss recovery. Using simulation, we quantify the cumulative effect of these TCP enhancements on web based and bulk data transfers. We hope that such an evaluation provides a clearer view of the applicability of these enhancements, and further motivation for their implementation.