Received August 26, 2020, accepted September 3, 2020, date of publication September 8, 2020, date of current version September 24, 2020. Digital Object Identifier 10.1109/ACCESS.2020.3022793 Impacts of Mobility Models on RPL-Based Mobile IoT Infrastructures: An Evaluative Comparison and Survey BARDIA SAFAEI 1, (Member, IEEE), ALIASGHAR MOHAMMADSALEHI1, KIMIA TALAEI KHOOSANI1, SABA ZARBAF1, AMIR MAHDI HOSSEINI MONAZZAH 2, FARZAD SAMIE 3, LARS BAUER 3, JÖRG HENKEL 3, (Fellow, IEEE), AND ALIREZA EJLALI 1 1Department of Computer Engineering, Sharif University of Technology, Tehran 11365-11155, Iran 2School of Computer Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran 3Chair for Embedded Systems (CES), Department of Computer Science, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany Corresponding author: Alireza Ejlali ([email protected]) ABSTRACT With the widespread use of IoT applications and the increasing trend in the number of connected smart devices, the concept of routing has become very challenging. In this regard, the IPv6 Routing Protocol for Low-power and Lossy Networks (PRL) was standardized to be adopted in IoT networks. Nevertheless, while mobile IoT domains have gained significant popularity in recent years, since RPL was fundamentally designed for stationary IoT applications, it could not well adjust with the dynamic fluctuations in mobile applications. While there have been a number of studies on tuning RPL for mobile IoT applications, but still there is a high demand for more efforts to reach a standard version of this protocol for such applications. Accordingly, in this survey, we try to conduct a precise and comprehensive experimental study on the impact of various mobility models on the performance of a mobility-aware RPL to help this process. In this regard, a complete and scrutinized survey of the mobility models has been presented to be able to fairly justify and compare the outcome results. A significant set of evaluations has been conducted via precise IoT simulation tools to monitor and compare the performance of the network and its IoT devices in mobile RPL-based IoT applications under the presence of different mobility models from different perspectives including power consumption, reliability, latency, and control packet overhead. This will pave the way for researchers in both academia and industry to be able to compare the impact of various mobility models on the functionality of RPL, and consequently to design and implement application-specific and even a standard version of this protocol, which is capable of being employed in mobile IoT applications. INDEX TERMS Communications, control overhead, delay, Internet of Things, mobility model, power- efficiency, reliability, routing, RPL, survey. I. INTRODUCTION an indispensable part of human lives with the ability of acting Birth of Internet of Things (IoT) was a major step towards as a ubiquitous communicating infrastructure, which estab- providing a more convenient way of life for the human soci- lishes internet-based communications between an enormous ety. The foot print of IoT could be observed in every corner number of resource-constrained physical objects with no or of the globe due to its broad range of supporting applications minimum human intervention. IoT has provided a pervasive in home automation, transportation, industry, health-care, and infrastructure for the information systems to gather and pro- many other smart services [1]. Nowadays, IoT is believed as cess raw data obtained from many remote devices, and exploit the outcome as an asset to pave the way for analytical and The associate editor coordinating the review of this manuscript and practical operations [2]. Since its declaration by the British approving it for publication was Alessandro Pozzebon. scientist Kevin Ashton in 90's [3], IoT has gained significant This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ VOLUME 8, 2020 167779 B. Safaei et al.: Impacts of Mobility Models on RPL-Based Mobile IoT Infrastructures: An Evaluative Comparison and Survey attention in the past few years, with an increasing trend in the Networks (FANET), which are composed of flying objects, number of connected smart devices under its communication e.g., drones, require 3D routing procedures, while RPL is a infrastructure. 2D approach [15]–[17]. There have been numerous anticipations on the expected On the other hand, in addition to significant efforts made by number of connected smart objects in the near future. the scholars for improving the performance of mobile systems Meanwhile, the International Data Corporation (IDC R ) has from different aspects, in order to be able to perform simula- recently published a report indicating the proliferation of the tions and emulations, a number of mobility models have been connected smart devices by up to 41.6 billion at the end developed to mimic the motion pattern of IoT devices with of 2025 [4]. The high number of operating devices in such the help of formal and mathematical representation of their networks arises many considerations, where the routing is actual movement in their corresponding real-world mobile believed as one of the most important challenges among applications. According to the characteristics of these appli- them [5]. The employed routing procedure in the network cations and their deployment environment, a moving object plays an important role in providing a low cost and reliable under the presence of each of these models will be obliged to communication for delivering the data packets from their perform its movement based on a set of rules and restrictions sources to their destination(s). Meanwhile, in order to enable determined by the laws of motion in that specific application. IoT to reach its goals and provide more flexibility and inter- The achieved milestones by the researchers have significantly operability in its layered architecture, many of its enabling enhanced the performance of mobile IoT infrastructures, spe- technologies and protocols, including the routing mechanism, cially with knowing that dynamicity and severe fluctuations should be standardized. are major attributes in mobile IoT infrastructures [18]. Accordingly, the Internet Engineering Task Force (IETF), The mobile IoT applications are mainly characterized with established the Routing over Low-power and Lossy net- relatively short and recurrent contacts between the existing works (RoLL) working group in 2008, to specify the pre- moving nodes. The presence of mobile nodes in an IoT requisites for an appropriate routing procedure in IoT, and infrastructure leads into having less reliable links and more consequently to design and implement a protocol, which packet drops in the network, which ignites critical challenges is capable of handling the existing resource constraints in such as topology instability and severe power consumption the deployed embedded nodes in IoT infrastructures. In this in the nodes. Followed by these issues, since the RPL was regard, the RoLL working group published several docu- designed as a standard routing protocol for IoT, it should be ments between 2009 and 2010 for determining the routing modified to be also applicable in mobile IoT applications. requirements for a number of IoT applications, i.e., Urban, In this regard, there have been several studies on proposing Industrial, Home automation, and Building automation [6]. mobility-aware routing policies, which are mostly based on As a result of their efforts, on March 2012, the IETF standard- modification of RPL's Objective Functions (OF) [6]. The OF ized the Routing Protocol for Low-power and lossy networks is an independent part of RPL, which is responsible for deter- (RPL), and publicly introduced it to the IoT society as an mining the optimized path selection for the nodes with respect RFC 6550 [6]. to the requirements of the intended IoT application. To the Along with many of the strong points of the IPv6 RPL, one best of our knowledge, the provided OF in [19] is among of its major drawbacks is that is was initially developed to be the few mobility-aware OFs, which have been introduced for adopted by the stationary IoT applications, and it is not able RPL. Nevertheless, based on our evaluations, there should to get well adjusted with the severe fluctuations in mobile be lots of work done to further improve the performance of conditions mainly due to its timing principles for control RPL in presence of mobility. In this regard, it is essential packet dissemination, neighbor table placement principles, to have a comprehensive evaluation on the performance of and of course inappropriate routing policies, which does not a mobility-aware version of RPL in presence of different consider moving aspects of the objects [7]. This is despite the motion patterns in various mobility models. fact that in recent years, the IoT devices are being attached In this survey, through conducting a significant set of and operated on many mobile objects, e.g., humans, bicy- experiments via precise IoT simulation tools, we have com- cles, transportation vehicles, ships, and airplanes leading into prehensively monitored and evaluated the performance of the advent of emerging mobile IoT applications, such as social network and its IoT devices in mobile RPL-based IoT appli- IoT, road conditioning, automotive networks, driver-less elec- cations from different aspects to analyze the functionality of tric vehicles, crowd sensing, and logistics [8]. Many of the various mobility models in the presence of a mobility-aware existing real-world mobile IoT applications, e.g., mobile asset version of RPL, and consequently to determine and compare tracking, fleet management [9], connected electrical bicycles the performance of this protocol in different mobile scenarios. [10], real-time health-care services [11], and vehicle tracking We have analyzed the functionality of this routing protocol systems [12], are composed of devices with built-in Global from the most fundamental node and network related param- Positioning System (GPS) modules [13], [14].