GATE | GATE-CS-2014-(Set-3) | Question 35

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)


Answer: (D)

Explanation: All (i), (ii) and (iii) are changed:

(i) TTL is decremented at every hop. So TTL is different from original value

(ii) Since TTL changes, the Checksum of the packet also changes.

(iii) A packet is fragmented if it has a size greater than the Maximum Transmission Unit (MTU) of the network. There may be intermediate networks that may change fragment offset by fragmenting the packet.

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