When a UDM session is established, the UDM Manager periodically sends a keep-alive message to the transfer servers - to which the transfer servers respond - in order to make sure the session is still established.
The _keepalive built-in variable contains the interval (in seconds) at which these messages are sent. If it has a value of 0, no keep-alive messages are sent.
You can change this interval by setting the _keepalive variable using the set command before a session is established:
set _keepalive=60