On Sat, 27 Dec 2025 at 17:14, William Herrin via NANOG <[email protected]> wrote: > > The ISPs I used emit ICMP packet too big messages. > > Everybody emits them. Too many don't make it to the destination.
In some cases the last router before the MTU bottleneck is not emitting the ICMP Type 3 Code 4 response. In Cisco land for example many configs and templates contain the "no ip unreachables" interface configuration, stopping the router from emitting all ICMP Type 3 messages, including Code 4 Frag needed. Non routable source IPs discarded by uRPF have been mentioned, which is a common problem. Then there is the issue of rate limiting. Rate limiting packets punted to the CPU for ICMP response emission. Rate limiting ICMP response emission itself. And sometimes even ICMP rate limiting on interfaces as a poor mans DDoS mitigation attempt. Or ICMP QoS mapping in worse than best effort queues that overload. Lukas _______________________________________________ NANOG mailing list https://lists.nanog.org/archives/list/[email protected]/message/EIH362A5NYP5V6FHUSWSOVA6BXJVG6NU/
