Additional Complexity due to multiple protocol families

OS kernel support and applications support are usually separate
Need to ship a single binary that works on IPv6-enabled and IPv6-disabled kernel
AF_INET6 support may not be there
A support nightmare
IPv4 kernel + IPv4 userland
IPv4 kernel + IPv4/v6 dual stack userland
IPv4/v6 kernel + IPv4/v6 dual stack userland
IPv4 kernel + IPv4/v6 dual stack userland
"IPv6 code is present but not configured" case
no IPv6 router, for instance

Solution: Application should handle both IPv4 and IPv6 without problem, even without kernel support
Remove any code that assumes AF_INET
Do not add any code that hardcodes AF_INET6