Host A (on TCP/IP v4 network A) sends an IP datagram D to host B (also on…
2014
Host A (on TCP/IP v4 network A) sends an IP datagram D to host B (also on TCP/IP v4 network B). Assume that no error occurred during the transmission of D. When D reaches B, which of the following IP header field(s) may be different from that of the original datagram D?
(i) TTL (ii) Checksum (iii) Fragment Offset
- A.
(i) only
- B.
(i) and (ii) only
- C.
(ii) and (iii) only
- D.
(i), (ii) and (iii)
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Correct answer: D
Answer: All three fields — TTL, Header Checksum, and Fragment Offset — may be different when the datagram reaches B.
TTL: Routers decrement the IPv4 TTL (time-to-live) field on forwarding. Because TTL is changed in transit, the value seen at B can be different from the original.
Header Checksum: IPv4 has a header checksum that must be updated whenever header fields (for example TTL) are modified. Thus the checksum at B may differ even though the payload arrived without errors.
Fragment Offset: If an intermediate router fragments the datagram because the path MTU is smaller than the datagram size, the fragment offset and related fragment fields will be changed. Even if fragmentation does not occur in a particular path, it is a normal possibility, so the fragment offset may differ.
Note: 'No error occurred' means the datagram arrived intact, but it does not prevent normal in-network modifications (TTL decrement, checksum recalculation, or fragmentation) performed by routers.
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