Realistically, UDP/IP is going to be your least complicated and best supported option.
PEP/IPX could be an option except IPX support is likely hard to find these days. Anything else is going to need privileged execution, which maybe is fine.
UDP/IP isn't too hard... To do it right, you probably want dhcp or ipv6 router advertisement support, and arp or neighbor discovery protocol. But if you don't want to do it right, you can abuse broadcast packets quite a bit.