PMTUD

networking

Path MTU Discovery: sending with Don't Fragment set and shrinking on ICMP Fragmentation Needed replies - which over-filtered networks silently break.

When the ICMP never arrives, big packets vanish: the classic MTU black hole.

Path MTU discovery is the mechanism that finds the largest packet a path will carry, and its failure mode is well enough known to have its own name: the PMTUD black hole. It happens when the ICMP messages reporting an oversized packet are filtered, so the sender never learns and retransmits indefinitely.

IPv6 makes this more consequential because routers do not fragment, so the sender must get this right or the packet simply does not arrive. That is why blocking ICMPv6 breaks IPv6 far more thoroughly than blocking ICMP breaks IPv4, and why the standard advice to permit the packet-too-big message is not a suggestion. MSS clamping remains the practical workaround where the path cannot be fixed.

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