When the Don't Fragment bit is set and a router encounters a link with a smaller MTU, what message is typically returned to enable Path MTU Discovery?

• A. An ICMP Fragmentation Needed message.
• B. An ICMP Time Exceeded message.
• C. The packet is dropped without notification.
• D. The router fragments the packet anyway.

Answer: (A)

Path MTU Discovery relies on the Don't Fragment flag being respected along the route. When DF is set and a router sees a link with a smaller MTU than the packet, it cannot fragment the packet. Instead, it returns an ICMP Destination Unreachable message with code indicating that fragmentation is needed, commonly described as an ICMP Fragmentation Needed message. This ICMP response includes the MTU of the next hop so the sender can reduce packet size and retry, enabling PMTUD to discover the path’s maximum transmissible unit. Time Exceeded would be for TTL-related issues, dropping without notification defeats PMTUD, and fragmenting anyway would violate the DF instruction.