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

[Henning Westerholt]

(sr-dev on CC)

Hello,

I am not aware of a special handling of MTU discovery regarding IPv6 UDP traffic in Kamailio core. But of course, we have a lot of code.

You find the implementation of the MTU handling in the src/core/udp_server.c file. Its just setting the appropriate socket option right now.

Cheers,

Henning

-- 
Henning Westerholt - https://skalatan.de/blog/
Kamailio services - https://gilawa.com

-----Original Message-----
From: sr-users <sr-users-bounces at lists.kamailio.org> On Behalf Of Rick van Rein
Sent: Thursday, May 12, 2022 1:31 PM
To: sr-users at lists.kamailio.org
Subject: [SR-Users] Path MTU issues over UDP/IPv6

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

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