o  data link error

    An error was reported by the Data Link layer on the loop command.

 o  unrecognized client

    There is no client with the specified identification on the loop
    command.

 o  unrecognized circuit

    There is no circuit with the specified identification on the loop
    command.

 o  unrecognized assistant

    There is no assistant with the specified identification on the
    loop command.  The specified Assistant System does not exist in
    the MOP Client database.

 o  invalid assistant

    The assistant address is either a multicast address, or assistant
    system was specified, and the corresponding client subentity has
    the default value for its address on the loop command.

 o  timeout

    Operation has timed out on the loop command.

 o  entity class not supported (OpenVMS)

    MOP is not registered with EMAA.  The NET$MOP process is probably
    not started.  For instructions on how to restart the MOP process,
    refer to HELP NETWORK_MANAGEMENT MOP.