### Security Issues and Threats in the RPL Protocol  
According to a recent report by Nokia [17], attacks on IoT devices are increasing at an alarming rate. The increase is due to the proliferation of automated tools to exploit IoTs' vulnerabilities. The report states that IoT devices now make up roughly 33% of exploited devices, compared to only 16% in 2019. The statistics are the outcome of monitoring aggregated network traffic data of more than 150 million devices globally. Furthermore, researchers claim that more than half of all deployed IoT devices are vulnerable to medium to high severity attacks [7].  
The exponential increase in the demand for IoT devices has accelerated research and development efforts in IoT-related areas, including security. It has attracted many researchers to investigate attacks targeting IoT networks. Many IoT applications use the RPL protocol, since it is purposely developed for constrained devices commonly used in modern IoT applications. Nonetheless, the RPL protocol is still vulnerable to many threats that could harm the entire network infrastructure. Consequently, researchers invest their time and efforts investigating various threats in LLNs that use the RPL protocol [18].  
Routing security is a major concern, as routing-related attacks impede sensor data transmission and adversely affect network layer performance. It can also significantly affect the upper layers of the IoT network, which frequently causes Denial of Service (DoS) attacks [19]. In addition, providing a suitable security mechanism for the routing protocol in the IoT is challenging due to the characteristics inherited from other networks. Furthermore, the packet forwarding process in IoT-constrained devices is influenced by potential security threats, affecting the delivered services to end-users as the performance of the malignant nodes is rapidly increased during the data packet routing process. Eventually, the network's topology will be disrupted, and the resulting overhead will deplete the nodes' power resources and eventually break down the whole network [20].  
In addition, the RPL security specification allows the protocol to operate in an open or optionally secured mode. In open mode, any node can join the LLN without an authentication key. In a secure mode, a node requires a preinstalled key to join the LLN and an additional authentication key to join as a sensor with routing capability. RPL also provides an optional consistency check feature for protection against replay attacks. Although RPL provides these optional features to address the security of the routing process, most of the security features are implemented only for external attacks owing to the constrained nature of IoT-LLNs. When a malicious node joins the IoT-LLN, RPL has several exploitable vulnerabilities, allowing adversaries to instigate insider attacks that deplete network resources and degrade performance. Thus, the security of RPL is crucial due to its significance in IoT networks [21; 22].  
Figure 1 shows the taxonomy of RPL attacks, classified into three categories, and each category has two subclasses based on the intent of threats [23]. These attacks may cause severe network issues, such as exhausting network resources, destructing network topology, and stealing sensitive information. Further, Figure 1 lists the recent attacks addressed by the existing studies. Section 7.6 provides further details about those attacks.  
To this end, many researchers have suggested that addressing routing attacks in RPL-based LLNs is still an open research issue [23]. However, due to the constrained and open nature of RPL-supported IoT-LLNs, implementing complex security solutions is difficult, and that leaves the network vulnerable to attacks. Therefore, there is a need for designing an efficient approach/solution to address routing attacks in the early stage of any malicious activity within the network [24].