A Path MTU Discovery az úgy működik, hogy MINDEN kimenő csomag DF bitje set.
https://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsula…
PMTUD is done continually on all packets because the path between sender and receiver can change dynamically. Each time a sender receives a "Can't Fragment" ICMP messages it will update the routing information (where it stores the PMTUD).