GATE | GATE CS 2018 | Question 37

Consider an IP packet with a length of 4,500 bytes that includes a 20-byte IPv4 header ans 40-byte TCP header. The packet is forwarded to an IPv4 router that supports a Maximum Transmission Unit (MTU) of 600 bytes. Assume that the length of the IP header in all the outgoing fragments of this packet is 20 bytes. Assume that the fragmentation offset value stored in the first fragment is 0.

The fragmentation offset value stored in the third fragment is ______ .

Note –This was Numerical Type question.
(A) 0
(B) 72
(C) 144
(D) 216


Answer: (C)

Explanation: MTU = 600 bytes and IP Header = 20 bytes
So, Payload will be 600 – 20 = 580 bytes



580 is not multiple of 8, but we know fragment size should be multiple of 8. So fragment size = 576 bytes

Kth fragmentation offset value = Fragment Size * (Kth fragment – 1) / Scaling Factor
Offset value of 3rd fragment = 576 * (3 – 1) / 8 = 144

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