Improving TCP’s Performance under Reordering with DSACK

TCP performs poorly on paths that reorder packets significantly, where it misinterprets out-of-order delivery as packet loss. The sender responds with a fast retransmit though no actual loss has occurred. These repeated false fast retransmits keep the sender’s window small, and severely degrade the throughput it attains. Persistent reordering occasionally occurs on present-day networks. Moreover, TCP’s requirement of nearly in-order delivery complicates the design of such beneficial systems as DiffServ, multi-path routing, and parallel packet switches. Toward relaxing this constraint on Internet architecture, we present enhancements to TCP that improve the protocol’s robustness to reordered and delayed packets. We extend the sender to detect and recover from false fast retransmits using DSACK information, and to avoid false fast retransmits proactively, by adaptively varying dupthresh. Our algorithm adaptively balances increasing dupthresh, to avoid false fast retransmits, and limiting the growth of dupthresh, to avoid unnecessary timeouts. Finally, we demonstrate that delayed packets negatively impact the accuracy of TCP’s RTO estimator, and present enhancements to the estimator that ensure it is sufficiently conservative, without using timestamps or additional TCP header bits. Our simulations show that these enhancements significantly improve TCP’s performance over paths that reorder or delay packets. [email protected], ICIR/Princeton University †[email protected], ICIR ‡[email protected], ICIR §[email protected], Princeton University ICSI TR-02-006, July 2002