## 1 Introduction  
One of the emerging technologies that facilitate a broad range of incipient smart applications is the Internet of Things (IoT). It enables the smart provisioning of intelligent and advanced services in different domains such as industry [1], healthcare [2], and agriculture [3]. A revolutionary development of a wide scope of new IoT applications is being witnessed nowadays. Examples are smart cities, smart metering, industrial automation, and environmental monitoring. As a result, the number of IoT-enabled devices has been showing exponential growth in recent years. The forecast in [4] indicates that more than 30 billion IoT devices will be connected in 2030. As estimated by the McKinsey Global Institute [5], the IoT industry may generate $3.9-11.1 trillion a year in revenue by 2025.  
An IoT network typically comprises small-sized devices which have limited computation and energy resources. Energy-efficient wireless communication technologies are utilized to enable effective connectivity among these devices. Therefore, the Low-power and Lossy Network (LLN) is widely used for the establishment of efficient IoT infrastructure. It effectively allows network topologies to be built with resource-limited devices connected over unreliable wireless links. Due to these networking characteristics, routing becomes a challenging functionality in LLNs, particularly for IoT applications with strict requirements.  
A set of LLN routing requirements have been defined by the IETF ROLL working group considering several IoT applications. These include urban, industrial, building automation, and home automation applications which are specified in RFC 5548 [6], RFC 5673 [7], RFC 5867 [8], and RFC 5826 [9], respectively. Taking these routing requirements into account, an IETF-standardized and LLN-customized routing protocol is specified in RFC 6550 [10] and called the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL). The protocol functionality is based on extending IPv6 networking to IoT devices and establishing a structured network topology. It addresses effective loop-free routing for efficient communication of IoT data packets. The protocol design is flexible and customizable enough to enable effective topology optimization toward addressing the network requirements of certain IoT deployments.  
Nevertheless, security requirements were not effectively considered and provisioned in the original RPL specification. Its potential vulnerability to different security attacks is still a major security challenge. There is still no adequate security support provided by the standard RPL against the different types of security attacks [10]. Only limited resilience to external attacks is provided by the RPL specification [11] whereas complete security support addressing routing attacks is not provisioned [12; 13]. Routing attacks are common in RPL networks to target overall network performance and topology stability [14; 15; 16; 17; 18]. This gives rise to different emerging security challenges for real-life RPL-based IoT deployments. As IoT networks are gradually being deployed in various critical fields, an open security issue such as this would impede the effectiveness of such a trend.  
The growing deployment of IoT devices in our vehicles, buildings, mobile devices, and appliances would open the doors for severe security threats compared with the cases of traditional networks. The potential damage that can consequently happen would be critical, causing IoT networks to collapse with complete communication disruption and data loss. As estimated earlier in [19], cybercrime during 2015 incurred a cost of $400-500 billion. The figure rose six-fold in 2016 to approximately $2-3 trillion. All these vital considerations demonstrate the importance of a practical understanding of potential security attacks and the need for intensive experimental analysis of their impacts on RPL network performance.