By connecting different physical devices to the digital world through the internet, the IoT offers countless application services and has proved as a promising field. The IoT appends many low-powered sensor devices and actuators into the smart field to elaborate the boundaries of autonomous systems. The classical routing protocols are not well adapted to the IoT environment owing to their resource-constrained and heterogeneous nature.

   The Routing Protocol for Low Power and Lossy Networks (RPL) rapidly gained much popularity in the IoT research community, as it has the ability to offer better routing services over resource-restricted IoT devices. The RPL is defined as a distant vector routing protocol specially proposed for IPv6 tiny and low-powered devices. The RPL works on the IEEE 802.15.4 standard and considers the routing requirements of low-powered smart devices. It enables high scalability, connectivity, flexibility, reliability, and robustness by ensuring bi-directional connectivity among resource-limited devices.

   The Objective Function (OF) is the key element in the design of the RPL protocol. The major challenges faced by RPL routing are energy consumption, mobility, and quality of service. Numerous RPL routing models with novel OFs are proposed in the literature to extend the IoT boundaries to various autonomous fields.