Efficient new delayed ACK for TCP: old problem, new insight

Abstract

When a TCP connection experiences a timeout, the sender must wait at least RTOmin (Minimum Retransmission Timeout) before doing the retransmission, during which the channel may be completely idle, undermining the throughput and channel efficiency. In this paper, we investigate the origin of RTOmin and find that it is needed to mitigate against spurious timeouts when the Delayed ACK (DA) scheme for TCP is implemented. Motivated by this observation, we propose a deployable and TCP-compatible new Delayed ACK (NDA) to replace the legacy DA. Our solution differs with previous work is that instead of using complex algorithms or fine-grained timer to tune RTOmin, we modify the DA scheme with minor changes to allow the sender to remove the RTOmin constraint while reserving the delayed ACK function at the receiver. In order to eliminate the aggressiveness of RTO (Retransmission Timeout) after removing RTOmin, we use coding techniques to encode the timeout retransmitted packets to make the potential spurious retransmissions useful. The simulation results demonstrate that in lossy wireless networks, NDA is efficient, because it achieves much higher TCP goodput and channel efficiency compared to DA. The gain we obtain by the use of NDA comes from two-order effects. One effect comes from removing the RTOmin constraint, because small RTO timer makes TCP react quickly to timeouts, resulting in small transmission idle. The other effect comes from eliminating consecutive RTO by allowing the receiver to acknowledge each timeout retransmission, which further reduces the RTO idle.