Computing the retransmission timeout in CoAP

Abstract

The most prominent IT trend nowadays is connection of Wireless Sensor Networks (WSNs) with Internet service infrastructure. Interconnection of the millions of sensor and processing devices will create a tremendous traffic increase that can lead to congestion. In parallel to the development of new protocols for WSNs, e.g., Constrained Application Protocol (CoAP) there is plenty of research for new congestion control techniques (CC). This research shall carefully take into account all key restrictions of sensor networks, e.g., memory and power consumption, lousy paths and limited links throughput. This paper analyzes classical approach of definition of the retransmission timeout (RTO) estimate, proposed in RFC 6298, and compares it with the Eifel Retransmission Timer and the new ideas proposed in CoCoAP. Finally, we present our method for calculating RTO. Our approach could be seen as an extension of the classical TCP algorithm, where instead of constants that are used to take into account history of the current state we use a dynamically changing parameter. The value of this parameter is defined as a ratio between current sample of the round-trip time (RTT) and the RTO value. © 2013 Springer-Verlag.