bind(2) issue

bind(2) behavior is not well specified in RFC
Whether to support IPv4-mapped address 
Whether to allow IPv6 and IPv4 socket to bind ports with same number
i.e. 80 (HTTP)
Which socket will get the packet

IPV6_V6ONLY
new in RFC3493 (aka RFC2553bis)
restricts AF_INET6 sockets to IPv6 communications only