358 Cyber-Physical Systems: From Theory to Practice
in such a way that no cycles exist. A root node is selected for each DAG created in RPL, and the
nodes that belong to a given DAG have a rank associated with them. By employing the rank informa-
tion, RPL builds a destination-oriented DAG with several routes from each node, which is intended
to enhance the performance and robustness of the network [65]. Performance evaluations of RPL
demonstrate there is still room for improvement in terms of support to bidirectional traffic and loop
avoidance [66,67].
Another RPL is the hybrid routing protocol [68]. It also employs DAGs to provide multiple reliable
paths to a border router. Each node builds a default route table by adding the neighboring nodes