TCP Fails To Respect IPV6_USE

The IPV6_USE_MINMTU socket option is used to set the maximum IPv6 packet size to be used on a socket. Many implementations of TCP running over IPv6 neglect to check this value when performing MSS negotiation and when contructing a TCP segment. This lead to oversized IPv6 packets being sent resulting unintended PMTUD being performed and to fragmented IPv6 packets being sent. TCP when running over IPv6 SHOULD check the state of the IPV6_USE_MINMTU when performing MSS negotiation. TCP implementions already use learnt PMTU and interface MTU when performing MSS negotiation. TCP, when running over IPv6, SHOULD check the state of the IPV6_USE_MINMTU when calculating the segment size to send. TCP implementions already use learnt PMTU and interface MTU when performing calculating the segment size to send.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

TCP when running over IPv6 SHOULD check the state of the IPV6_USE_MINMTU when performing MSS negotiation. If the value of IPV6_USE_MINMTU is one (1) then the application has requested that PMTUD not be performed on the is socket and that IPv6 packets be sent at a size no greater then the network minumum MTU of 1280 bytes. This means that the TCP MSS negotiation size SHOULD be no bigger than 1220 (1280 - 40 - 20) to account for the IPv6 header and the TCP header and MAY be smaller. If this negotiation is properly performed then PMTUD of reply traffic should not normally occur.

TCP when running over IPv6 SHOULD check the state of the IPV6_USE_MINMTU when calculation the next segment to send. If the value of IPV6_USE_MINMTU is one (1) them the maximum segment size SHOULD be 1220. If the TCP layer neglects to check the value of IPV6_USE_MINMTU and it is one (1), the packet, when passed to the IPv6 layer, will be fragmented if the resulting packet is bigger that 1280 octets. This can result in communications failures due too itermediate nodes not passing fragmented packets.

The IPV6_USE_MINMTU and TCP are used together in DNS nameservers as TCP message streams are normally no more than a couple of IPv6 packets, there are lots of servers / clients and there are external time contraints where recovery from lost ICMPv6 PTB will fall outside of the time constraint window.