Host A (on TCP/IPv4 Networks) send an IP Datagram D to host B (also on TCP/IPv4 network). Assume that no error occured during transmission of D, when D reaches B. Which of following header field may differ from that of original datagram D ?

(A) TTL

(B) Checksum

(C) Fragment offset

(D) Source IP

(E) Destination IP

1. A(A) and (B) Only
2. B(A), (B) and (C) Only
3. C(A), (B), (C) and (D) Only
4. D(A), (B), (C) and (E) Only

Pick an option to check yourself, or open the answer below.

Show answer & explanation

Correct answer: B. (A), (B) and (C) Only

Explanation

Each router decrements TTL and therefore recomputes the header checksum; if the datagram is fragmented on the way, the fragment offset changes. Source and destination addresses stay the same (without NAT).