  Internet of Things (IoT) is one of the fastest emerging networking paradigms
enabling a large number of applications for the benefit of mankind.
Advancements in embedded system technology and compressed IPv6 have enabled the
support of IP stack in resource constrained heterogeneous smart devices.
However, global connectivity and resource constrained characteristics of smart
devices have exposed them to different insider and outsider attacks, which put
users' security and privacy at risk. Various risks associated with IoT slow
down its growth and become an obstruction in the worldwide adoption of its
applications. In RFC 6550, the IPv6 Routing Protocol for Low Power and Lossy
Network (RPL) is specified by IETF's ROLL working group for facilitating
efficient routing in 6LoWPAN networks, while considering its limitations. Due
to resource constrained nature of nodes in the IoT, RPL is vulnerable to many
attacks that consume the node's resources and degrade the network's
performance. In this paper, we present a study on various attacks and their
existing defense solutions, particularly to RPL. Open research issues,
challenges, and future directions specific to RPL security are also discussed.
A taxonomy of RPL attacks, considering the essential attributes like resources,
topology, and traffic, is shown for better understanding. In addition, a study
of existing cross-layered and RPL specific network layer based defense
solutions suggested in the literature is also carried out.
