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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/ and the solution to our change: https://github.com/cloudflare/pmtud

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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.

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