What is the TCP checksum, and why does it include a pseudo-header?

• A. The TCP checksum detects data corruption by checking only the TCP payload, excluding headers.
• B. The TCP checksum uses a pseudo-header that includes IP addresses, protocol, and length to protect against misdelivery.
• C. The TCP checksum is optional and can be disabled in modern networks.
• D. The TCP checksum covers the data and the TCP header but ignores the pseudo-header.

Answer: (B)

The TCP checksum is a 16-bit error-detecting value computed over the entire TCP segment (both the header and the data) to detect any corruption that occurred in transit. What makes it stronger is the inclusion of a pseudo-header formed from information from the IP layer: the source and destination IP addresses, the protocol number for TCP, and the length of the TCP segment. This pseudo-header isn’t transmitted as part of the TCP segment itself, but it is included in the checksum calculation.

Including the pseudo-header ties the TCP segment to its IP-layer context, so the receiver can verify not only that the segment hasn’t been corrupted, but also that it’s being delivered to the correct destination, intended for TCP, and with the correct length. If a segment were misrouted, misidentified as a different protocol, or if the length were altered, the checksum would fail, catching the error.

So the best description is that the TCP checksum protects the segment and uses a pseudo-header with IP addresses, the protocol, and the segment length to guard against misdelivery and protocol misinterpretation.