MTU

Path MTU Discovery Finds the Largest Packet a Route Can Carry

A host can know the maximum transmission unit of its own network interface without knowing the smallest limit farther along a route. Ethernet might accept one packet size while a tunnel, access link, or other intermediate network accepts less. Path MTU Discovery, commonly shortened to PMTUD, lets an endpoint adapt to that route-level limit. The mechanism matters because an IP packet that fits the sender’s local link can still be too large for a later hop.