Which of the following best describes the TCP pseudo-header used in the checksum computation?

• A. It includes the entire IP header fields
• B. It includes only TCP header fields
• C. It includes IP source address, IP destination address, protocol, and TCP length
• D. It contains only the port numbers

Answer: (C)

The checksum uses a pseudo-header to bind the TCP segment to its IP flow. This binds the segment to the exact IP source and destination, confirms the protocol being TCP, and ties it to the length of the TCP segment being protected. The fields included are the IP source address, IP destination address, protocol, and TCP length, and this pseudo-header is used only for the checksum calculation, not transmitted with the segment. This context helps detect misdelivered or corrupted data that a plain TCP header-and-data checksum might miss. The other options don’t fit because they would either include too much (the entire IP header), too little (only TCP header fields or only port numbers), or miss the essential IP addressing and protocol context that the pseudo-header provides.