RPL is working rank based transfer, that is node’s individual position relative to other nodes with respect to DODAG root.
Nodes who transferring control data packets called parent and receiver is child.(parent & child based shortest range transfer).
In RPL, the immediate parent is selected by a child (child node) determined by the quality of transmission channel and rank.
Open file at below location contiki-ng/os/net/ipv6/uip-sr.c
Here, the function handling RPL child list calculation.
Run the simulation.