What is the purpose of Path MTU Discovery in TCP?

• A. Determines the largest packet size that can traverse the path without fragmentation
• B. Determines the minimum segment size
• C. Negotiates the window size
• D. Chooses the congestion control algorithm

Answer: (A)

Path MTU Discovery is about finding the largest packet size that can traverse the entire path without fragmentation. It works by sending packets with the DF (Don't Fragment) bit set; if a hop along the route can’t handle that size, it replies with an ICMP need-to-fragment message that indicates the smallest MTU on the path. The sender then reduces the size of subsequent packets, so all data can be transmitted in one piece without splitting across routers. This helps TCP become more efficient by letting it transmit larger segments up to the discovered path MTU, while avoiding the overhead and potential issues of fragmentation. It isn’t about setting a minimum segment size, negotiating the window, or selecting a congestion control algorithm.