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GATE CSE 2021 Set 1 Q55: IP fragmentation of a 1400-byte TCP segment over an 820-byte MTU

Question

Consider two hosts PP and QQ connected through a router RR. The maximum transfer unit (MTU) value of the link between PP and RR is 1500 bytes, and between RR and QQ is 820 bytes.

A TCP segment of size 1400 bytes was transferred from PP to QQ through RR, 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 PP.

Which of the following statements is/are correct?

Try it: multiple select: one or more correct options

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Select all correct options. No negative marking, no partial marks.
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Answer: A, C (official GATE 2021 final answer key)

Concept used: Each fragment carries at most MTU − 20 bytes of payload (a multiple of 8); routers never retransmit, and only the first fragment holds the TCP header.

First step: The 1400-byte payload splits into fragments of at most 820−20=800820 - 20 = 800 bytes each.

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Source: official GATE 2021 Set 1 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key

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