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Unit 9 · Data Communication and Computer Networks Paper 2 CS · December 2022 Shift 2

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 sent from P to Q through R with IP identification 0x1234. The IP header is 20 bytes and the DF flag is not set.
I. Two fragments are created at R and the IP datagram carrying the second fragment is 620 bytes.
II. If the second fragment is lost, R resends the fragment with IP identification 0x1234.
III. If the second fragment is lost, P is required to resend the entire TCP segment.
IV. 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. I and III only

Explanation

Payload 1400 B; the 820 MTU allows 800 B of data (multiple of 8). Fragments: 800 + 20 = 820 and 600 + 20 = 620, so I is true. Routers do not retransmit (II false); TCP at P resends the whole segment (III true). The TCP header with the port is only in the first fragment (IV false).

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