Which statement best describes the purpose of the pseudo-header in TCP checksum calculation?

• A. It checks only payload
• B. It is optional
• C. It ensures data's correct delivery path and interpretation
• D. It encrypts the header

Answer: (C)

The purpose of the pseudo-header in TCP checksum calculation is to tie the TCP segment to its IP flow, so the checksum also protects the IP addressing and protocol context, not just the data. By including the source and destination IP addresses, the protocol number, and the TCP segment length in the checksum, the receiver can verify that the segment was delivered to the right host and is intended for the TCP protocol with the expected length. If any of those IP-layer details were wrong or altered, the computed checksum wouldn’t match, signaling a problem such as misdelivery or corruption. The pseudo-header isn’t actually transmitted as part of the TCP segment; it’s used during calculation to provide linkage to the IP layer. The other statements don’t fit because the checksum covers more than payload, it isn’t optional in TCP, and it doesn’t perform encryption.