I would suggest you try NetSim (http://tetcos.com/), especially if you are trying to vary the RPL objective function

Netsim is good but for that I think you need license version.

As per my thought, Cooja is the best for RPL. If you are trying to use TInyOS then it is also good. RPL is already included in it.

You can try both NetSim and Cooja with simple implementation and through evaluation and debug processes it will be obvious which one suitable for you.

thanks but i need help to for changing exit  code

We have used NetSim and found it easy for code modification & analysis when compared to Cooja.

Lot of features are in built in NetSim which takes very long to do in Cooja.

I dont think you can change the underlying code in Cooja

i think cooja is best candidate for RPL.