Path MTU Discovery

networking

Stands for: Path MTU Discovery (PMTUD)

How a sender finds the smallest MTU along the whole route, not just the first hop.

PMTUD (RFC 1191) probes the path and shrinks packets to fit its narrowest link, avoiding fragmentation end to end. When firewalls block the ICMP messages it relies on, it fails silently, producing the classic "connects but hangs on large transfers" symptom.

Path MTU discovery finds the largest packet a path will carry by sending packets marked do-not-fragment and listening for the ICMP message that reports one was too big. The mechanism is correct and it fails constantly, because those ICMP messages are widely blocked by firewalls configured on the belief that ICMP is dangerous.

When the report never arrives the sender learns nothing and keeps retransmitting the same oversized packet, which produces the classic symptom: the connection establishes, small requests succeed, and anything large hangs indefinitely. That is why MSS clamping exists as standard practice on tunnels, and why blocking ICMP wholesale is one of the most common self-inflicted network faults.

Also known as: pmtud, path mtu

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