## 8 Discussion
* QoS performance and topology stability of RPL networks can be effectively targeted by routing attacks, particularly composite and hybrid ones. In addition, these attacks provide an effective method to highly increase network overhead and energy consumption. The damage would be significant in a way that can cause RPL networks to collapse with complete communication disruption and data loss. * RPL networks of a large scale suffer severe side effects from the routing attacks. As most IoT applications would involve a high number of nodes, the use of RPL without protection from such attacks would become a matter of security. Incorporating standard RPL routing into large-scale IoT networking for daily sensitive applications would make networks at permanent security risk of adverse routing attacks. * Targeting RPL networks with hybrid attacks combining different routing attacks at once is a new routing attack approach in the context of RPL networking. The adverse impact of this kind of attack on RPL networks is highly evident. As most of the enhancements to RPL properties have been made at the protocol, topology, and communication levels to alleviate simple security-deteriorated situations, this study provides an intensive investigation of hybrid attacks to stimulate further advanced RPL security solutions. * Experimental simulation testing has been the predominant evaluation methodology that ensures simplicity and reproducibility. This method also enables studying experimental issues such as scalability in an easy and cost-effective manner. Backing up such a method with realistic experimentation over physical testbeds is an important consideration. A shift towards such a viable integrated methodology is feasible to increase evaluation practicality. This study provides the initial major step toward such a strategy. The functionality of RPL leaves enough room for further improvement toward effective security support. The protocol design of RPL comes with the flexibility to incorporate effective and advanced protection against routing attacks. Security-oriented optimization of different RPL operational aspects can be considered in this regard. For example, RPL can be incorporated with secure OF design, node ranking algorithms, topology update validation methods, and integrity-preserved message exchange. However, the balance between efficiency and complexity should be emphasized as a key security design issue that needs to be effectively addressed before implementing RPL at scale. Security solutions for RPL networks should be effectively developed at minimal computational complexity and resource consumption. Here are some perspectives that can be considered when developing RPL security support:  
* RPL completely relies on the behavior of parent nodes during topology establishment and updates. Malicious and illegitimate activities affect all the neighbor and child nodes at the lower hierarchical levels of the affected topological zones. By enhancingthe interaction and controlling the association among parent and child nodes, the protection of RPL networks from rank attacks can be effectively achieved. * Although it is specified that global repairs are only performed by the sink node, the protocol design provides no guarantee of immunity to VN attacks imitated by compromised nodes. RPL nodes blindly participate in any global repair without verifying the legitimacy of the process. It is important to ensure strict adherence to the RPL specification and prevent any topology updates coming from non-sink nodes and taking a direction other than the downward direction. This requires effective collaboration among RPL nodes to enable efficient verification of global repairs in a distributed manner. * RPL topology establishment and maintenance processes are managed by the exchange of control information transmitted in a systematic and periodical manner. Monitoring abnormal behaviors and validating data integrity are vital to preventing the misuse of routing information and initiation of replay attacks.