Multimodal Retransmission Timer for LPWAN

Low-power wide-area networks (LPWANs) are experiencing high momentum as an inexpensive solution for enabling the Internet-of-Things (IoT) applications. Recent Internet connectivity support developments are expected to further fuel the adoption of LPWAN. However, the latter present challenges to the Internet protocols. Remarkably, many LPWAN scenarios exhibit a multimodal round-trip time (RTT) distribution, which deviates from the common Internet RTT characteristics. This leads to a significant mismatch between retransmission timeout (RTO), computed by the standard transmission control protocol (TCP) or alternative experimental RTO algorithms, and RTT. In this article, we present the Multimodal RTO algorithm, which is able to self-adapt to the current RTT mode and produce suitable RTO values. The evaluation results show that the Multimodal RTO reduces the RTO versus RTT misalignment of the TCP RTO algorithm by an average factor of up to 5, and reduces latency in the presence of losses by up to 2 orders of magnitude, while operating safely. The Multimodal RTO is currently being considered by the IETF as a candidate mechanism for standardization.