What do routers do regarding the IP checksum when forwarding packets?

What do routers do regarding the IP checksum when forwarding packets?

• A. Validate the checksum, decrement TTL, and recalculate before forwarding
• B. Forward without checking
• C. Only decrement TTL, ignore checksum
• D. Drop packets with any checksum mismatch

Answer: (B)

The main idea here is how forwarding devices treat the IP header checksum. Routers use their routing table to move a packet toward its destination and generally do not perform end‑to‑end error checking on every hop. The IP header checksum is a lightweight error-detection field intended mainly for the endpoints to verify received packets. Because routers aim to keep forwarding fast and only modify the header when necessary (and, in many simple paths, without changing fields that would require recomputing the checksum), they do not routinely validate or recalculate the IP header checksum as part of forwarding. That’s why the best answer is that the router forwards without checking the checksum.

The main idea here is how forwarding devices treat the IP header checksum. Routers use their routing table to move a packet toward its destination and generally do not perform end‑to‑end error checking on every hop. The IP header checksum is a lightweight error-detection field intended mainly for the endpoints to verify received packets. Because routers aim to keep forwarding fast and only modify the header when necessary (and, in many simple paths, without changing fields that would require recomputing the checksum), they do not routinely validate or recalculate the IP header checksum as part of forwarding. That’s why the best answer is that the router forwards without checking the checksum.