The GATE Grind

GATE 2021 CS – Question 55

Computer Networks · IPv4: Addressing, CIDR, Fragmentation and NAT · 2 marks · Multiple select

Consider two hosts $P$ and $Q$ connected through a router $R$. The maximum transfer unit (MTU) value of the link between $P$ and $R$ is 1500 bytes, and between $R$ and $Q$ is 820 bytes. A TCP segment of size 1400 bytes was transferred from $P$ to $Q$ through $R$, with IP identification value as 0x1234. Assume that the IP header size is 20 bytes. Further, the packet is allowed to be fragmented, i.e., Don't Fragment (DF) flag in the IP header is not set by $P$. Which of the following statements is/are correct?

  1. Two fragments are created at $R$ and the IP datagram size carrying the second fragment is 620 bytes.
  2. If the second fragment is lost, $R$ will resend the fragment with the IP identification value 0x1234.
  3. If the second fragment is lost, $P$ is required to resend the whole TCP segment.
  4. TCP destination port can be determined by analysing only the second fragment.

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Show answer and explanation

Correct answer: (A) Two fragments are created at $R$ and the IP datagram size carrying the second fragment is 620 bytes.; (C) If the second fragment is lost, $P$ is required to resend the whole TCP segment.

Explanation

The IP payload is 1400 bytes. With MTU 820, each fragment carries at most 800 data bytes (a multiple of 8), so the fragments carry 800 and 600 bytes and the second datagram is 620 bytes. IP does not retransmit, so the sender's TCP must resend the segment. Only the first fragment carries the TCP header.