Overview
PMTUD allows a source host to find the 'bottleneck' MTU of a path. It works by sending packets with the Don't Fragment (DF) bit set in the IP header.
How it Works
If a router along the path has an MTU smaller than the packet size, it drops the packet and sends an ICMP 'Destination Unreachable - Fragmentation Needed' message, which includes its own MTU. The source then reduces its packet size and tries again until the packets pass through successfully.