The IPv6 over Low-power Wireless Personal Area Network (6LoWPAN) supports IP over Lossy networks. RPL (Routing Protocol for Low-power and Lossy Networks) routing protocol has been developed for 6LoWPAN routing. RPL is vulnerable to various types of attacks namely external and internal. One of the internal attacks for the RPL-based IoT Network is Rank Attack. A spiteful node advertises false decreased rank information to disturb the topology of the network. This attack creates network disruption, loop formation, and degrades network performance in terms of packet delivery ratio, packet delay, and throughput. This paper proposes a novel mechanism to detect and verify the decreased rank attack and exclude the spiteful node from the network using the round trip time (RTT).