Hack the Route Start free Open app Errors and packet size
Path MTU discovery
Learning the largest packet a whole path carries, from the routers' ICMP messages.
A packet with Don't Fragment that is too big for a link is dropped, and the router sends back ICMP fragmentation needed with the MTU that fits. The sender remembers it per destination. When a firewall drops those messages, large packets vanish without explanation: a black hole, where small requests work and big transfers hang.