I think it depends on the kind of IBR-DTN you are using. IBR-DTN installed using APT usually comes with a service that can run on boot as long as it is enabled. If it fails, you can always run IBR-DTN by including the command sudo ibrdtnd -d -c <conf file> in /etc/rc.local. This is what I have done when I built it from the installation files since it does not include a service.