In RPL, DODAG Information Solicitation(DIS) message use by the node to probe or solicit it is neighborhood for nearby DODAGs.
DIS is analogous to IPV6 Router solicitation.
DIS is used to probe or solicit DODAG Information Object from RPL nodes.
Open file at below location contiki-ng/os/net/ipv6/uip-ds6-nbr.c
Here, the function handling RPL neighbor discovery mechanism.
Run the simulation.