Data Length (payload size in an IP packet) is calculated as which expression?

Data Length (payload size in an IP packet) is calculated as which expression?

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

Answer: (A)

To find the TCP payload inside an IP datagram, you start with the total length of the IP datagram (which includes both the IP header and all data). Then you subtract the IP header length to remove the IP header, and subtract the TCP header length to remove the TCP header. What’s left is the TCP payload. That is exactly Total IP Datagram Length minus IP Header Length minus TCP Header Length. The other ways reduce or omit one of the needed headers, so they wouldn’t yield the true payload size. If the transport protocol were something other than TCP, you’d subtract that protocol’s header length instead.

To find the TCP payload inside an IP datagram, you start with the total length of the IP datagram (which includes both the IP header and all data). Then you subtract the IP header length to remove the IP header, and subtract the TCP header length to remove the TCP header. What’s left is the TCP payload. That is exactly Total IP Datagram Length minus IP Header Length minus TCP Header Length. The other ways reduce or omit one of the needed headers, so they wouldn’t yield the true payload size. If the transport protocol were something other than TCP, you’d subtract that protocol’s header length instead.