[SR-Users] Path MTU issues over UDP/IPv6

[Rick van Rein]

Hello,

IPv6 routers never fragment packets.  Rather, they drop a packet that
is too large for a (local) MTU and send back ICMPv6 "Packet Too Big".
This seems to cause loss of larger SIP messages when an ISP tunnels
their IPv6 at the expense of the MTU.

The pmtu_discovery flag sets Don't Fragment in IPv4 traffic; in IPv6
this is an implied property.  Does Kamailio learn a lower MTU from
any "Packet Too Big" for IPv6 even if pmtu_discovery is not set?
Future resends can then be fragemented appropriately.

The udp_mtu setting diverts to another protocol, but that would be a
setting as low as the worst peer, impacting all.  It would be a weird
struggle with a telco serving many.  PMTU would be better to rely on,
but how does it work in Kamailio?

Details on
https://www.rfc-editor.org/rfc/rfc3542#section-11.3
https://stackoverflow.com/questions/38817837/how-does-mtu-retransmission-work-in-case-of-udp


Thanks,
 -Rick