Question

Consider two hosts \(P\) and \(Q\) connected through a router \(R\). The maximum transfer unit (MTU) 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. Which of the following statements is/are correct?

• A. Two fragments are created at \(R\) and the IP datagram size carrying the second fragment is 620 bytes.
• B. If the second fragment is lost, \(R\) will resend the fragment with the IP identification value 0x1234.
• C. If the second fragment is lost, \(P\) is required to resend the whole TCP segment.
• D. TCP destination port can be determined by analysing only the second fragment.

Hint:

IP fragmentation is handled at the network layer, but reliability and retransmission are responsibilities of TCP.

Answer 1

The Correct Option is A, C

Step 1: Determine need for fragmentation.
The total size of the IP datagram is:

1400 bytes (TCP data) + 20 bytes (IP header) = 1420 bytes

Since the MTU of the link between router R and Q is 820 bytes, the datagram cannot be forwarded without fragmentation.

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Step 2: Calculate fragment sizes.
Each fragment must carry its own 20-byte IP header.

Maximum payload per fragment = 820 − 20 = 800 bytes

• Fragment 1: 800 bytes data + 20 bytes header = 820 bytes
• Fragment 2: Remaining data = 1400 − 800 = 600 bytes
  Fragment size = 600 + 20 = 620 bytes

Therefore, statement (A) is correct.

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Step 3: Router responsibility for lost fragments.
IP provides a best-effort service. Routers neither track fragments nor perform retransmissions.

Hence, if a fragment is lost, the router does not resend it, making statement (B) incorrect.

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Step 4: TCP behavior on fragment loss.
If even one IP fragment is missing, the receiver discards the entire datagram.

TCP, operating at the transport layer, detects this loss and retransmits the complete TCP segment from the sender P.

Thus, statement (C) is correct.

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Step 5: TCP header presence in fragments.
The TCP header appears only in the first fragment. Subsequent fragments contain only payload data.

As a result, the second fragment does not include TCP port numbers, so statement (D) is incorrect.

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Final Answer:
The correct statements are (A) and (C).