IEC 61375-3-2:2015 (chapter A7.8.1) defines:
If a TRDP user (caller) requests sending a notification, a notification message (MsgType ‘Mn’)
shall be sent. The session identifier of the notification message shall be set to 0.
TCNOpen uses always a session identifier != 0.
The sessionId will be set to all zeros in the sent notification (according to the standard).