Is the IPv4 header checksum recalculated by routers along the path, and why?

• A. Yes, because the header changes (e.g., TTL decrement) as packets traverse routers.
• B. No, the header checksum is fixed by the source.
• C. Yes, but only when the destination changes.
• D. No, IPv4 does not use a header checksum.

Answer: (A)

The IPv4 header checksum is a field that verifies the integrity of the header itself. It’s computed over the header (with the checksum field zeroed) and must stay consistent with whatever is actually in the header at the destination. As a packet travels, routers decrement the Time To Live (TTL) field to limit the packet’s lifetime. That change alters the header, so the router must update the checksum to reflect the new header contents before forwarding. If the checksum weren’t updated, the eventual receiver would see a header with a mismatch and flag an error.

The source sets an initial checksum, but it doesn’t stay fixed across the route because the TTL (and potentially other header fields) change along the path. IPv4 does use a header checksum, so the only reason the checksum would stay the same is if no header fields changed, which isn’t the case on a normal route. The destination’s address doesn’t actively cause a checksum calculation here; the ongoing changes to the header on each hop drive the need to recalculate.