Which technique helps avoid UDP fragmentation by discovering the path MTU?

• A. Path MTU Discovery
• B. NAT traversal
• C. TCP Fast Open
• D. Congestion control

Answer: (A)

Path MTU Discovery is the technique that prevents UDP fragmentation by finding the largest packet size that can traverse the route to the destination without being fragmented. UDP datagrams that exceed the path’s MTU run the risk of being fragmented by routers or dropped if fragmentation is disallowed. With this method, the sender probes with progressively larger sizes while signaling “Don’t Fragment” in the IP header. If a router can’t forward a packet without fragmenting, it may return an ICMP message indicating that fragmentation is needed. The sender uses that feedback to shrink the probe size and try again, effectively discovering the path’s MTU. Once the path MTU is known, UDP datagrams can be sized to fit this limit, avoiding fragmentation along the route.

NAT traversal isn’t about MTU discovery; it focuses on crossing NAT boundaries. TCP Fast Open is a latency optimization for TCP handshakes, not UDP fragmentation. Congestion control governs how aggressively you send data based on network feedback, rather than discovering path limits to prevent fragmentation.