Consider two hosts P and Q that are 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 is transferred from P to Q through R with IP identification value of 0 × 1234 . Assume that IP header size is 20 bytes. Further the pcaket is allowed to be fragmented that is Don't fragment (DF) flag in the IP Header is not set by P. Which of the following statement/s is/are true?
A. Two fragments are created at R and IP datagram size carrying the second fragment is 620 bytes
B. If the second fragment is lost, then R resends the fragment with IP identification value of 0 × 1234
C. If the second fragment lost, then P requires to resend the entire TCP segment.
D. TCP destination port can be determined by analyzing the second fragment only.
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Correct answer: B. A and C only
Explanation
The 1400-byte TCP segment plus a 20-byte header crosses the 820-byte MTU link in two fragments: 800 data bytes (820 total) and 600 data bytes (620 total), so A is true. Routers do not retransmit, and IP has no retransmission, so TCP at P resends the whole segment (C true, B false). The TCP header with the ports is only in the first fragment (D false).
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