Why is the IPv4 Header Checksum included and when is it recalculated?

• A. It protects header integrity; routers recalculate it whenever the payload changes
• B. It protects header integrity; routers recalculate it as the header changes (e.g., TTL decrement)
• C. It protects payload integrity only at the source
• D. It is optional and rarely used

Answer: (B)

The important idea is that the IPv4 header checksum is there to detect corruption of the header itself, not the packet payload. It’s a 16-bit ones-complement sum calculated over the header with the checksum field set to zero. Because many things in the header can change as the packet travels, the checksum must be updated to match the new header values each time the header is altered.

In practice, a router often decrements the Time To Live (TTL) field at each hop. That change modifies the header, so the checksum has to be recalculated (or adjusted) to reflect the new TTL. Other header fields can change too during forwarding (for example, during fragmentation, fields like Total Length and Fragment Offset or Flags may be updated), and each of those changes requires updating the checksum. Since the payload is not covered by the header checksum, changes to the payload don’t force a checksum update.

So the best description is that the checksum protects header integrity and routers recalculate it as the header changes (for example, TTL decrement).