In Path MTU Discovery, which type of message is used to convey the MTU value along the path to the sender?

• A. ICMP error message
• B. TCP segment
• C. ARP request
• D. DNS response

Answer: (A)

Path MTU Discovery relies on feedback from the network that reports the maximum packet size that can be forwarded along the path. When a router can’t forward a large packet because the next link’s MTU is smaller, it drops the packet and sends back an ICMP error message to the sender. That ICMP message, carrying the MTU value of the next hop, tells the sender the largest size it can use for subsequent packets. The sender then reduces its probe size accordingly. ICMP is the protocol designed for such network-layer diagnostics and error reporting, which is why the MTU value is conveyed this way. TCP segments, ARP requests, and DNS responses serve unrelated roles (transport data, address resolution on a local link, and name resolution), so they don’t carry MTU information along the path.