## 8 Discussion  
A basic IoT network routing solution is provided by the IETF-standardized RPL. The protocol design provides no security support for defending against the diverse types of routing attacks. In fact, the inherent design properties and characteristics of RPL make it easy to initiate a set of routing attacks targeting overall network performance and stability. The RPL topology establishment process enables utilizing the vulnerable ranking mechanism for impairing the protocol functionality and incurring sub-optimal routing. The basic topology maintenance process of RPL also allows the launching of DoS attacks by arbitrarily tampering with the unsecured version numbering mechanism and initiating frequent illegitimate global repairs. In addition, RPL has an intrinsic vulnerability to passive impersonation as it has no mechanism to prevent eavesdropping and manipulation of control data. Overall QoS performance can be easily and effectively targeted in different routing attack scenarios. In simple attack scenarios of a small-scale network with a single attacker, a reduction of up to 13% in PDR and throughput as well as an increase of more than 35% in delay and ETX were incurred. As the attack scenario becomes more complex with hybrid attacks in large-scale networks, the situation becomes even worse with a reduction of more than 32% in PDR and throughput in addition to an increase of more than 90% in delay and ETX. This makes it extremely challenging to foster the deployment of standard RPL networks in latency-sensitive and real-time IoT applications. It is also evident that attacked RPL networks suffer from significant degradation in network overhead, energy consumption, and topology stability. The adverse effects are extremely significant in large-scale setups which are common for most IoT applications such as the smart city application. The overhead in the network can noticeably increase by more than 700% in single-attack scenarios whereas the increase can reach more than 900% in the cases of hybrid-attacks scenarios. Increases of more than 200% in energy consumption can also be easily achieved particularly when performing complex routing attacks. In addition, simple routing attack scenarios caused RPL networks to stay unstable with a high increase of more than 1000% in topology changes. It is apparent that standard RPL networks without additional security support face serious security difficulties. The networks would permanently become at high risk of critical performance degradation as a result of easy-to-initiate routing attacks. Thus, the technical efficiency and practical feasibility of RPL network deployment in critical IoT applications become questionable. As addressing such an issue becomes inevitable, the practical performance understanding established in this paper provides the basis for the development of advanced RPL security support. The presented results can serve as a practical reference for effectively comprehending RPL-based IoT networks under routing attacks. This is an important step towards enriching the security of the protocol and reviving its potential for a broad range of IoT applications. The lessons learned in this study can be summarized as follows:  
* RPL networks perform well under no attacks. Even in large-scale scenarios, high QoS performance was achieved and the network stays highly stable while maintaining low control traffic overhead and power consumption. * RPL lacks a standardized security-provisioning functionality against internal routing attacks. In the absence of integrated security support, wide deployments of RPL networks would be highly hindered, particularly for security-critical IoT applications. * RPL networks are inherently prone to a wide range of routing attacks that can be easily initiated by any compromised node. Even in the case of a complex routing attack, multiple attacking nodes can simply perform multiple attacks simultaneously in a composite- or hybrid-attack setup.