Is reported in PR173444 that using the IPV6_USE_MIN_MTU socket option on a TCP socket results in sending fragmented IPv6 packets.
The patch adds another flag to the in_conninfo flags. bz@ suggested in BSDCam to avoid this by passing the inp around, but this is not always available (from within the syncache code). Therefore, the TCP layer intercepts the IPPROTO_IPV6 level socket option, reduces the MSS or TCP connections when possible and marks them.
Although setting the socket option means that the application want to limit the size of TCP segments being sent, a value of 1220 is announced in the MSS option, if possible, since TCP usually is very conservative.