Which statement about data length calculation is true?

• A. Total IP Datagram Length - IP Header Length - TCP Header Length
• B. Total IP Datagram Length - IP Header Length
• C. IP Header Length
• D. UDP Header Length

Answer: (A)

The data length of the TCP payload inside an IP datagram is found by subtracting both headers from the total datagram length: take the IP total length, remove the IP header length, and then remove the TCP header length. The IP total length field represents the entire datagram size (header plus TCP segment, including its header and any data). Since the IP header length can vary (due to options) and the TCP header length can also vary (due to TCP options), you must subtract both to arrive at the actual application data. If you only subtract the IP header length, you get the TCP segment length (header plus data), not just the data. If you subtract only the TCP header length, you still have the IP header portion. The UDP header length applies to UDP, not TCP, so it isn’t appropriate for TCP data length.