This software is a daemon (server) for the L2TP dialect implemented by the Ericsson SIU (Site Integration Unit) for means of carrying the GSM A-bis interface over IP.
All code has been independently developed without any information or assistance from Ericsson, merely by observing protocol traces between the SIU 02 and whatever is the Ericsson-provided server side implementation next to the BSC.
Warning: The code currently hard-codes 172.30.42.3 as the IP address of the host running L2TPD. Making this configurable is likely the first thing you need to do before using it...