How does ICMP relate to the concept of Path MTU Discovery (PMTUD)?

PMTUD uses ICMP messages (specifically, 'Fragmentation Needed' messages) to determine the smallest MTU along a path between two hosts. When a router encounters a packet larger than its outgoing interface's MTU, it sends an ICMP 'Fragmentation Needed' message back to the source, prompting it to reduce the packet size. This avoids fragmentation and improves network performance.