Energy Consumption for Securing Lightweight IoT Protocols

In this paper we address the energy consumption of the Constraint Application Protocol (CoAP) and the Message Queue Telemetry Transport (MQTT) protocol and compare their overhead. We also pay attention to the use case of security in IoT and analyze the energy consumption when using TLS/DTLS for the two protocols. In our experiments we use ESP32 with libcoap, MQTT, and mbed TLS libraries and conduct real-world measurements using Otii, a high precision voltage and current measurement tool. While the particular numbers are implementation and hardware dependent, we can still make interesting observations.
For data transfer, we find that aggregating data to larger packets can significantly reduce the energy consumption. We also find that AES-CCM8 seems slightly more efficient than other modes of operation. In comparison, the DTLS handshake for setting up the secure connection is very expensive, and also very dependent on security level and algorithm choices.
For firmware updates, AES-CCM8 is again slightly better than the alternatives, but the differences between CoAP and MQTT are much more significant, favoring MQTT due to the use of the retransmission support in TCP. This is also evident in lossy networks, where MQTT saves up to 91% energy compared to CoAP at 20% loss rate. Finally, we find that energy consumption in CoAP can to some extent be reduced in lossy networks by modifying the retransmission timeout.