Alternative listening port for UDP and TCP listeners; default (or zero) value means "listening port plus one". This is needed for RFC 5780 support (STUN extension specs, NAT behavior discovery). The TURN Server supports RFC 5780 only if it is started with more than one listening IP address of the same family (IPv4 or IPv6). RFC 5780 is supported only by UDP protocol, other protocols are listening to that endpoint only for "symmetry".
option