Path MTU Discovery (PMTUD) finds the smallest MTU along a route.
How it works:
Sender sets "Don't Fragment" (DF) flag
Router can't forward → drops packet
Router sends ICMP "Fragmentation Needed"
Sender reduces size and retries
Problems occur when firewalls block ICMP. Without "Fragmentation Needed" messages, senders never learn. Packets disappear. Always allow necessary ICMP.