How does the TCP checksum cover the pseudo-header, and what information does it protect?

• A. It checks only the data payload
• B. The pseudo-header includes port numbers and MAC addresses
• C. The pseudo-header includes extra fields unrelated to the checksum
• D. The pseudo-header includes source and destination IPs, protocol, and length to protect the data’s correct delivery path and interpretation

Answer: (D)

The point being tested is how the TCP checksum links the integrity check to the IP delivery context. The pseudo-header is included in the checksum calculation to bind the segment to the actual IP routing information, so any mismatch can be detected. It contains the source IP address, the destination IP address, the protocol number (TCP), and the length of the TCP segment. These values are used only in the checksum calculation (the pseudo-header itself isn’t transmitted) along with the TCP header and data. This means the checksum protects that the segment is intended for the correct destination, is being interpreted as TCP, and has the expected length, helping catch misdelivery or corruption. The other options are off because port numbers are part of the TCP header, not the pseudo-header, and MAC addresses are at the link layer; plus the pseudo-header is specifically the IP-related fields listed, not extra unrelated fields.