MQTT Implementations, Open Issues, and Challenges: A Detailed Comparison and Survey

MQTT is an open standard protocol promoted by OASIS and ISO, which allows devices to transport messages using the publish/subscribe model. MQTT is more prevalent than other application layer protocols of the Internet of Things (IoT) due to its lightweight nature, low bandwidth usage, application demand, etc. It is easy and straightforward to use the protocol, making it optimal for communication in resource-constrained situations such as machine-to-machine (M2M), Wireless Sensor Networks (WSNs), and in IoT circumstances in which the actuator and sensor nodes connect with applications through the MQTT message broker. A few review papers on MQTT protocol are available in the literature that focuses on broker details, comparison of IoT protocols, and limitations. In this paper, an overview of MQTT, existing survey work on MQTT, publication statistics, MQTT protocol performance evaluation, applications of MQTT, security issues of MQTT, comparison between MQTT and MQTT-SN, tools available or MQTT and available MQTT brokers to provide service are discussed. Graphs and comparison tables are presented to show the outcomes of the application and performance evaluation. The scope of this review paper is also to contribute a novel taxonomy of application layer protocols, their merits and demerits, correlation of MQTT with other application layer protocols, existing works of MQTT protocol to improve reliability, efficiency, security, issues, and challenges in MQTT, as well as future directions of MQTT.