Effective RTO estimation using Eifel Retransmission Timer in CoAP

Internet of Things (IoT) is a system of interrelated devices with communication capabilities. Network congestion increases as the number of devices increase in the system and de-grades the Quality of Service (QoS). Unlike traditional Internet, TCP is not the de facto transport protocol for IoT because of the constrained nature of IoT devices. Therefore, congestion control mechanisms are coupled with application protocols, such as the Constrained Application Protocol (CoAP). Although there have been efforts to use CoAP with TCP, it typically uses UDP for data transport. CoAP defines a conservative congestion control mechanism that uses a fixed Retransmission TimeOut (RTO) for retransmissions regardless of the network conditions (e.g., Round Trip Time (RTT)). The Eifel Retransmission Timer is a popular algorithm originally proposed for TCP. It assesses prevalent network conditions by measuring RTT and accordingly sets the TCP RTO. This paper proposes to leverage the benefits of Eifel Retransmission Timer by integrating it with CoAP (when used with UDP) to obtain better RTO estimates and control congestion. The proposed algorithm is named as CoAP-Eifel. The effectiveness of the proposed approach has been validated by comparing it with the standard RTO estimation technique of CoAP. The experiments are carried out in a real testbed by using FIT/IoT-LAB. The results demonstrate that the proposed approach provides a better trade-off in terms of delay and throughput, without affecting the packet delivery ratio.