A standardized networking technique for finding the largest packet size that can be sent without fragmentation between two devices. Path Maximum Transmission Unit Discovery (PMTUD) prevents data loss and inefficiency by avoiding oversized packets. It was originally designed for routers in IPv4 networks but is now used by all modern operating systems on endpoints. In IPv6, this function is explicitly assigned to the endpoints of a communication session.
How PMTUD Works
PMTUD sends packets with the Don’t Fragment (DF) flag set. If a packet is too large for the network path, routers drop it and send an Internet Control Message Protocol (ICMP) message indicating the maximum transmission unit (MTU) size. The sender then adjusts its packet size accordingly.
Packetization Layer Path MTU Discovery
An extension of PMTUD that works without ICMP support. This technique is useful in environments where ICMP messages are blocked or unreliable.
IPv4 vs. IPv6
- IPv4: PMTUD was originally intended for routers but is now commonly used on endpoints.
- IPv6: The function is explicitly delegated to the endpoints of a communication session.