How to access DIO message of RPL on Contiki Cooja simulator?

Question

Can somebody please help me with on how to access the DIO message from unicast-sender. C and unicast-receiver. The C file of RPL implementation of the Contiki cooja simulator?? We are working on sinkhole attack detection, that's why we need to analyze DIO message. As a beginner, we don't know how to access that from the code, but we can see it using packet analyzer.