What determines the maximum TCP segment size?

What determines the maximum TCP segment size?

• A. Maximum Transmission Unit (MTU) of the network.
• B. The IP header length.
• C. The DNS resolution.
• D. The application layer window size.

Answer: (A)

The maximum TCP segment size is bounded by the path MTU. MTU is the largest IP packet size that can traverse a network link without fragmentation. TCP sits on top of IP and adds its own header, so the usable data in each segment (the MSS) is typically MTU minus the IP header and the TCP header. For example, on a common Ethernet path with an MTU of 1500 bytes, the maximum TCP payload is about 1460 bytes (1500 minus 20 bytes for the IP header and 20 bytes for the TCP header). During connection setup, TCP negotiates this value as the MSS, and path MTU discovery can reduce it if a smaller MTU is encountered along the route. DNS resolution has no effect on segment size, and the application layer window size controls flow, not the maximum per-segment payload. So the network’s MTU fundamentally sets the upper bound for how large a TCP segment can be.

The maximum TCP segment size is bounded by the path MTU. MTU is the largest IP packet size that can traverse a network link without fragmentation. TCP sits on top of IP and adds its own header, so the usable data in each segment (the MSS) is typically MTU minus the IP header and the TCP header. For example, on a common Ethernet path with an MTU of 1500 bytes, the maximum TCP payload is about 1460 bytes (1500 minus 20 bytes for the IP header and 20 bytes for the TCP header). During connection setup, TCP negotiates this value as the MSS, and path MTU discovery can reduce it if a smaller MTU is encountered along the route. DNS resolution has no effect on segment size, and the application layer window size controls flow, not the maximum per-segment payload. So the network’s MTU fundamentally sets the upper bound for how large a TCP segment can be.