How does Path MTU Discovery (PMTUD) work ?
Path MTU Discovery is performed by setting the DF (Don't Fragment) bit "on" in the IP Header. Routers along the path, that have smaller MTU on the outgoing links, will send back an ICMP Fragmentation Needed and Don't Fragment was set message (type 3, code 4) - also known as ICMP Packet Too Big -, in which they specify the outgoing MTU.
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