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.