Which statement about Path MTU Discovery and MSS is true?

• A. PMTUD determines the path MTU to prevent fragmentation
• B. PMTUD increases MSS
• C. PMTUD has no impact on fragmentation
• D. PMTUD reduces RTT

Answer: (A)

Path MTU Discovery is about finding the largest packet size that can traverse the entire route without requiring fragmentation. It works by sending packets with the Don't Fragment flag set and using ICMP "fragmentation needed" messages to learn the smallest MTU along the path. With that knowledge, the sender caps the size of IP packets (and thus the TCP MSS) so packets can be forwarded without fragmentation. The Maximum Segment Size is the largest TCP payload per segment and is negotiated during the TCP handshake, but it’s effectively limited by the path MTU minus the IP and TCP headers. PMTUD’s main job is to determine the path MTU to avoid fragmentation, not to change RTT directly or to increase MSS. It can lead to a smaller MSS if the discovered path MTU is smaller than prior assumptions, but its purpose is fragmentation avoidance, not MSS inflation.