The GATE Grind

GATE 2014 CS (CS3) – Question 35

Computer Networks · IPv4: Addressing, CIDR, Fragmentation and NAT · 1 mark · Multiple choice

Host A in IPv4 network A sends datagram D to host B in IPv4 network B. Transmission is error-free. On arrival, which header fields may differ from D’s original header? (i) TTL; (ii) Checksum; (iii) Fragment Offset.

  1. (i) only
  2. (i) and (ii) only
  3. (ii) and (iii) only
  4. (i), (ii) and (iii)

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Correct answer: (D) (i), (ii) and (iii)

Explanation

Routers decrement TTLand update the header checksum accordingly. Fragmentation can also produce fragments with different offsets. Thus all three fields may differ, **D**, even without transmission errors. The offset statement refers to arriving fragments before destination reassembly.