MTU – Maximum transmission unit
PMTUD – Path MTU Discovery
Links have a maximum packet size limitation depending on the protocol/technology. MTU defines the maximum bytes of a packet on a link.
With PMTUD, endpoints are capable of determine the MTU of a path.
When a endpoint sends a packet larger than the MTU, this is fragmented on the network devices that have a lower MTU. The packets will be reassembled at the destination.
PMTUD is not supported for UDP.
Fragmentation makes network less efficient, same as a lower MTU.