Explain the principle of operation of REXEC protocol.

Remote Execution Command Daemon (REXECD) is a server that allows the execution of jobs submitted from a remote host over the TCP/IP network. The client uses the REXEC or Remote Shell Protocol (RSH) command to transfer the job across to the server. Any standard output or error output is sent back to the client for display or further processing. 

 Principle of Operation
REXECD is a server (or daemon). 
It handles commands issued by foreign hosts and transfers orders to subordinate virtual machines for job execution. The daemon performs automatic login and user authentication when a user ID and password are entered. The REXEC command is used to define the user ID, password, host address, and the process to be started on the remote host. However, RSH does not require you to send a user name and password; it uses a host access file instead. Both server and client are linked over the TCP/IP network. REXEC uses TCP port 512 and RSH uses TCP port 514. See Fig.  for more details