TCP Fails To Respect IPV6_USE_MIN_MTU
The IPV6_USE_MIN_MTU [RFC3542], Section 11.1, socket option directs the IP layer to limit the IPv6 packet size to the minimum required supported MTU from the base IPv6 specification [RFC2460], i.e. 1280 bytes. Many implementations of TCP running over IPv6 neglect to check the IPV6_USE_MIN_MTU value when performing MSS negotiation and when constructing a TCP segment. This leads to oversized IPv6 packets being sent resulting in unintended Path Maximum Transport Unit Discovery (PMTUD) [RFC1191] being performed and to fragmented IPv6 packets being sent.