Which statement best describes Path MTU Discovery?

• A. Path MTU Discovery determines the maximum transmission unit along the path to a destination.
• B. Path MTU Discovery is used to assign IP addresses automatically.
• C. Path MTU Discovery negotiates encryption keys.
• D. Path MTU Discovery updates DNS records.

Answer: (A)

Path MTU Discovery is about finding the largest packet size that can travel from source to destination over every link in the path without needing fragmentation. It works by sending packets with the Don't Fragment flag set; if any router along the way can’t forward a packet because it’s too big, it drops the packet and sends back an ICMP Fragmentation Needed message that tells the sender the MTU of the next hop. The sender then reduces the packet size (often by shrinking the TCP MSS) and retries, repeating until packets pass through intact. This avoids fragmentation and keeps transmission efficient across networks with different MTU values. The other options describe unrelated functions—automatic IP address assignment, encryption key exchange, or DNS updates—that aren’t what PMTUD is about.