[MPICH] How communicating between computers in a cluster is achieved in mpich :"rpc or socket ,or both?"

[zyh]

I'd like to write a small programme in linux/c to achieve only the simpliest functions of mpich,like MPI_Init,,MPI_Comm_rank,MPI_Comm_size,MPI_Send and MPI_Recv,MPI_Finalize
Now,I'm not sure how communicating between computers in a cluster is achieved in mpich.
When I install mpich,I need to install rsh-server first,so I guess mpich uses rsh to communicate between computers in a cluster.Is that right?
Does mpich uses socket  programming instead?  Or both?
 
Any suggestion will be greatly appreciated!
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