Routing protocol for low power and lossy networks (RPL) is the underlying routing protocol of 6LoWPAN, a core communication standard for the Internet of Things. RPL outperforms other wireless sensor and ad hoc routing protocols in quality of service (QoS), device management, and energy saving performance. The Rank concept in RPL serves multiple purposes, including route optimization, prevention of loops, and managing control overhead. In this paper, we analyze several different types of internal threats that are aimed at the Rank property and study their impact on the performance of the wireless sensor network. Our analysis raises the question of an RPL weakness, which is the lack of a monitoring parent in every node. In RPL, the child node only receives the parent information through control messages, but it cannot check the services that its parent provide hence it will follow a bad quality route if it has a malicious parent. Our results show that different types of the Rank attacks can be used to intentionally downgrade specific QoS parameters. This paper also reveals that attack in a high forwarding load area will have more impact on network performance than attack in other areas. The defenders can use the knowledge of such correlation between attack location and its impact to set higher security levels at particular positions by monitoring sensitive network parameters and detecting the anomalies.