What is Path MTU Discovery (PMTUD) and what can break it?

• A. It discovers the largest MTU along the path using the DF flag; it can be broken if ICMP messages are blocked.
• B. It measures the maximum Ethernet frame size on a LAN.
• C. It determines the route path by MTU advertisement.
• D. It automatically fragments packets regardless of DF.

Answer: (A)

Path MTU Discovery figures out the largest packet size that can traverse the entire path from source to destination without fragmentation. It works by sending packets with the Don't Fragment (DF) bit set. If any hop on the path can’t forward a packet that large, it will drop it and send back an ICMP message telling the sender to reduce the size (in IPv6, this is the Packet Too Big message). The sender then lowers the size and tries again, so the end result is the smallest MTU along the path, i.e., the path MTU, which is the maximum size that can reach the destination without fragmentation.

This mechanism can be broken if ICMP messages are blocked or filtered somewhere along the path. Without those feedback messages, the sender won’t learn that a smaller MTU is required, leading to packet loss or connection failures despite attempting to send large packets.

PMTUD isn’t about measuring a LAN’s Ethernet frame size, nor about MTU advertisements in routing, and it doesn’t fragment packets automatically when the DF bit is set—the whole point is to discover the maximum safe size and avoid fragmentation.