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Networking Fundamentals · Intermediate

Path MTU Discovery (PMTUD)

A technique a host uses to find the smallest MTU along a path by sending packets with fragmentation disallowed and reacting to ICMP errors.

Definition

In IPv4, the host sets the Don't Fragment bit and lowers its packet size when a router returns ICMP Type 3 Code 4 (Fragmentation Needed). IPv6 relies on ICMPv6 Packet Too Big messages. If firewalls block these ICMP messages, a "PMTUD black hole" occurs and large packets silently fail.

Example

ping -f -l 1472 8.8.8.8 on Windows tests whether a 1500-byte packet passes without fragmentation.

Exam tip

Blocking all ICMP breaks PMTUD; that is a classic reason not to drop every ICMP type at the firewall.

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