Design of Adaptive RTO Algorithm for Efficient Congestion Control in CoAP-based Wireless Lossy Environments

The Internet of Things (IoT) encompasses all Internet communication technologies. In particular, wireless sensor networks(WSNs) play an important role in various IoT applications, such as home network, smart factory, and smart city. The Internet Engineering Task Force (IETF), an internet standardization organization, had proposed a lightweight protocol called constrained application protocol (CoAP) for the Internet connectivity of low-performance devices such as WSNs. Because the CoAP employed the user datagram protocol, and a simple congestion control mechanism based on binary exponential backoff, it showed significant delay in lossy network conditions. To overcome this, the IETF Constrained RESTful Environments (CoRE) working group proposed the CoAP Simple Congestion Control/Advanced (CoCoA) algorithm. However, the CoCoA algorithm suffered from high computational overhead for RTO calculation at every transmission of packets, leading to increased energy consumption by the sensor nodes. Moreover, the use of a fixed weighting parameter in the calculation of round-trip time (RTT) resulted in a slow response to the rapidly changing network environment.BR This study proposes an algorithm to efficiently assess the network conditions by measuring the RTT and the number of re-transmissions over a certain period or number of communication rounds. Statistical techniques were applied to determine the network’s loss rate; further, based on the identified loss rate, different weighting factors (α) were applied to calculate the predicted RTT values. Proposed algorithm was designed to reduce the computational overhead for RTO calculations and to be adaptive to the network conditions exhibiting significant RTT variations. The algorithm was compared with CoCoA and the existing smoothed round trip time (SRTT) algorithm applied in the traditional Internet using the Cooja simulator. The simulations were performed under wireless environments with loss rates of 5%, 10%, and 15%, respectively. The performance was measured by conducting 1,000 exchanges of CON-ACK packet pairs until successful communication was achieved. In each loss rates, the performance of the proposed algorithm was superior to those of the CoCoA and the SRTT algorithms in terms of total communication time.