Studies of TCP´s retransmission timeout mechanism

This paper focuses on the initial value of TCP´s retransmission timeout (RTO) timer. We make a three-fold contribution to the work in this area. Firstly, to motivate this work, we conduct a study of Internet traffic to determine loss rates for those packets that rely on timeouts with the initial RTO value to recover. Secondly, we experiment with TCP stacks in nine different operating systems to understand the initial RTO values used. Finally, we conduct simulations with HTTP traffic models to study the impact of different initial RTO values on the transfer delay of Web objects. The experiments show that average transfer delay can be impacted by as much 70% on bottleneck links with loss rates above 6%. Further, TCP flows with timer granularity below 100 ms or round trip times (RTT) below 250 ms can suffer similar performance degradation. The results suggest that TCP´s initial RTO value can be reduced from its current recommended value of 3s to between 500 ms and 1s