# Routing Protocols for Mobile Internet of Things (IoT): A Survey on Challenges and Solutions
###### Abstract
[28] compares a limited number of routing protocols for LLNs and assesses the impact of mobility on the network performance; Ref. [29] studies the performance of RPL and P2P-RPL protocols; and Ref. [30] presents a survey on RPncement focusing on network topology, mobility, and security. However, compared to these works, this survey presents a qualitative methodology to present various issues faced by RPL during mobility of IoT nodes; such as lack of identification of mobile nodes, issues with Trickle algorithm loops in the network, the delay caused by Expected Transmission Count (ETX) probing, lack of positioning information, increased handoff delays, issues with the rank of detached mobile nodes and the lack of sink-to-sink coordination. We identified 20 different variations of the RPL protocol suggested in the literature and classified them into five categories based on various criteria. For example, solutions based on 1. 'Trickle-timer' that address various issues such as frequent changes in topology due to nodes movement, slow DIO message probing, dynamic selection of various timers, and packet loss due to selection of preferred parents. 2. The 'ETX-based solutions' that deal with enhancing link quality, such as reduced communication delay, link stability during mobility, reducing the cost of making new network topology, and slow response to topology changes due to delays in ETX probing, unreachable destinations, and loops in the network. 3. 'RSSI-based solutions' that provide mobility and energy aware routing and address handoff delays for better link quality. 4. 'Position-based solutions' that address position inaccuracies and disconnections among static and mobile nodes by providing reliable routing, address frequent topology changes and speed of mobile nodes, and 5. Miscellaneous solutions. Following, for each category of these solutions, we illustrate the working principles, issues addressed, strengths, and weaknesses of each of the 20 RPL variations. Furthermore, we present comprehensive research directions for evaluating the performance of RPL mobility in the presence of various simulation parameters, such as node types, number of nodes, node mobility, intervals, simulation duration and area, and simulation environment, in addition to, energy consumption, packet loss, packet delivery ratio, and latency. Following are the key contributions of this survey:  
* To analyze the issues faced by RPL when IoT devices are non-stationary;
* To critically evaluate the solutions that are proposed in the current literature to mitigate the issues faced by RPL during mobility of IoT devices;
* To categorize various solutions into different types and illustrate the working principle, strengths, and weaknesses of each solution; and
* To propose future research directions that can be explored to propose better solutions to the issues faced by RPL during the mobility of IoT devices. This survey paper is organized as follows: Section 2 presents the background knowledge and literature review. In Section 3, we describe the issues faced RPL during mobility. Section 4 provides the available solutions to those issues. In Section 5, we propose future research directions. Finally, we conclude our discussion in Section 6.