This parameter is used to distinguish IPv6 connections from normal IPv4 connections.
E.g.
conn linux26-to-linux27-ipv6
type=tunnel
connaddrfamily=ipv6
left=2000:16:11:16::126
right=2000:16:11:16::127
keyexchange=ike
auto=add
auth=esp
authby=secret
pfs=yes
keylife=120m
rekey=yes
ikelifetime=240m
keyingtries=0
