Performance of Loss-Tolerant TCP (LT-TCP) in the Presence of Correlated Losses

TCP is a ubiquitous transport protocol in the Internet. However in wireless ad-hoc environments where links are unreliable, TCP has a number of performance issues. The essential reason is that TCP considers all packet losses to be due to congestion and reduces its send rate, which is not necessarily appropriate in a lossy ad-hoc environment. We have implemented Loss Tolerant TCP (LT-TCP), which addresses this issue among others in a standard Linux kernel. LT-TCP uses Explicit Congestion Notification (ECN) and forward error correction (FEC) to mitigate the effects of ad-hoc losses. In previous experimentation, we have seen a speedup of nearly two orders of magnitude using LT-TCP vs. TCP-SACK for file transfers in the presence of uncorrelated losses. We hypothesize that correlation of losses will affect the performance of the protocols and is particularly relevant to directional wireless links. In the current work, we present and compare the two protocols using a correlated loss process, whereby packets suffer the same fate as previous packets with a parameterized probability. Results show that LT-TCP performance remains stable for the amount of correlation we introduce, while TCP-SACK suffers extreme degradation. Further analysis of both protocols is warranted to discern the reasons for this disparity. Based on these results, we remain encouraged that LT-TCP is a viable approach to addressing transport protocol performance in wireless ad-hoc networks.