4 ms·

We got hit massively by this at CloudFlare (though we were not explicitly filtering ICMP, but changes we made to our infra meant that PMTU packets got lost) We

We got hit massively by this at CloudFlare (though we were not explicitly filtering ICMP, but changes we made to our infra meant that PMTU packets got lost) We wrote a blog post about this too: https://blog.cloudflare.com/path-mtu-discovery-in-practice/ https://blog.cloudflare.com/path-mtu-discovery-in-practice/ and the solution to our change: https://github.com/cloudflare/pmtud https://github.com/cloudflare/pmtud

michh 11y ago
Interesting! I'd think, in theory the ECMP router could keep track of the MTU on a per IP basis (rather than per TCP connection) based on it having received the ICM unreachable packet. And from that moment on, sending a spoofed ICMP packet back whenever one of the servers it's routing for sends a packet the router knows won't reach the host. But even if that works, I'm by no means a network engineer, your solution of simply broadcasting the packets is probably more efficient in the real world.