undefined | Better HN

0 pointstopranks6y ago0 comments

IP fragmentation is different in IPv6 true.

MSS is a TCP parameter, however, and operates at layer 4. Won’t matter if the protocol underneath is IPv4 or IPv6 in this case.

0 comments

peterwwillis6y ago

Actually, it does behave differently. Never underestimate the weirdness of IPv6 ;-)

From https://en.m.wikipedia.org/wiki/Maximum_segment_size :

"To avoid fragmentation in the IP layer, a host must specify the maximum segment size as equal to the largest IP datagram that the host can handle minus the IP and TCP header sizes.[2] Therefore, IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576[3] - 20 - 20) and IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280[4] - 40 - 20)."

So it seems like one needs to clamp tcpv4 to at least 536, and tcpv6 to at least 1220. (tcpv6 is common shorthand for TCP over IPv6, similar to udpv6 and icmpv6)

j / k navigate · click thread line to collapse