Which of the following fields of an IP header is/are always modified by any router before it forwards the IP packet?
Answer
(C), (D)
Explanation
When a router forwards an IPv4 packet:
- it decrements the TTL by 1 (and discards the packet if the TTL reaches 0), so the TTL always changes;
- the header checksum covers the whole header, TTL included, so it must be recomputed and rewritten every time.
The source IP address identifies the original sender and stays the same end to end; only special devices such as NAT boxes rewrite it, not every router. The protocol field names the protocol of the payload (for example 6 for TCP), which a router has no reason to change.
Answer: C, D