LAUNCHXL-CC1350: How to check if RPL DAG root exists in the network or not in the contiki

 I am working on an application where number of nodes have to multicast some information periodically to all other nodes. Nodes can join and leave randomly to/from the network. I am trying to figure out how can a node know at the startup if a DAG already exists or nor. If it exists, join the existing DAG. If not, create a new DAG and become root. For example very first node which gets powered on in the network becomes root and rest just joins the DAG.