mount                    --- RPC request --->              portmapper 

mount                    <-- port number ---               portmapper 

mount                    --- RPC request --->              rpc.mountd

mount                    <-- respond --------              rpc.mountd  


I/O (Read/Write) access

write()                ---- I/O -------------->             [nfsd] 

write()                <-- ACK upon complete---             [nfsd]