How does Path MTU Discovery (PMTUD) work ?

Think of the answer before revealing the hidden one !
In order to avoid fragmentation along the way, end-hosts uses a certain mechanism to discover what is the MTU of the path.

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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