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Unit 9 · Data Communication and Computer Networks Paper 2 CS · June 2023

Hosts P and Q are connected through a router R. The MTU of link P–R is 1500 bytes and of link R–Q is 820 bytes. A TCP segment of 1400 bytes is transferred from P to Q through R with IP identification 0x1234. The IP header is 20 bytes and the DF flag is not set. Which of the following statements are true?
A. Two fragments are created at R, and the IP datagram carrying the second fragment is 620 bytes.
B. If the second fragment is lost, R resends the fragment with IP identification 0x1234.
C. If the second fragment is lost, P has to resend the entire TCP segment.
D. The TCP destination port can be determined by analysing the second fragment only.
Choose the correct answer from the options given below:

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Correct answer: B. A and C only

Explanation

The datagram is 1420 bytes. On the 820-byte link each fragment carries at most 800 bytes of data (a multiple of 8): 800 + 600, so the second datagram is 600 + 20 = 620 bytes (A). Routers do not retransmit (B false). IP fragments are not recovered individually, so TCP at P must resend the whole segment (C). The TCP header is only in the first fragment (D false).

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