Which statement about the IPv4 header checksum is true?

• A. It is a 32-bit field located at the end of the header and recalculated at every hop.
• B. It is a 16-bit field near the start of the header and recalculated only when payload changes.
• C. It is a 16-bit field in the middle of the header and updated only during fragmentation.
• D. It is a 16-bit field near the start of the header and recalculated whenever the header changes.

Answer: (D)

The IPv4 header checksum is a 16-bit field placed near the start of the header. It is computed from all header words, with the checksum field itself treated as zero during calculation, using a ones-complement sum, and the result is stored in the checksum field. Its purpose is to detect corruption in the header, not the payload. Because the header can change as the packet traverses a network (for example, the TTL is decremented by each hop, and fragmentation can produce new headers), the checksum must be recalculated whenever the header changes. If a packet is fragmented, each fragment has its own header and its own checksum. This makes the statement that the checksum is a 16-bit field near the start of the header and recalculated whenever the header changes the correct description.