What Is Path MTU Discovery?
Path MTU Discovery is the recommended mechanism for servers to send optimally-sized segments to clients. As you may know, if IP is asked to deliver a packet on an interface with MTU (Max Transmission Unit) smaller than the packet size, IP fragments the packet. The ultimate receiver of the fragments then has to re-assemble the fragments. This ultimate receiver is probably not the next-hop router. That’s good, because there’s no point in re-assembly in the middle of the network. Some subsequent link might also have a small MTU, and then the network device transmitting onto that link would have to fragment the packet all over again. IP Fragmentation is considered evil (well, undesirable), partly because it imposes a buffer and CPU burden on the receiver. The receiver has to collect the fragments in a buffer, which may persist and tie up memory for minutes if some fragments are missing. But worse, if even one fragment is lost, the sender has to retransmit the packet, which in effect probab
