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6 votes
6 votes
Which of the following statements is/ are true about the fragmentation which is done at network layer.

(I) The offset of 1st fragment is always offset of parent.

(II) The MF of last fragment is always same MF of parent.

I think I is true and II is false:

Explanation for II :-

Host A connected to R1 and R1 connected to host B.

Link A-R1 MTU = 220 Including IP header of 20 Bytes

Link R1-B MTU = 100 Including IP header of 20 Bytes.

Host A has 400 Bytes of TCP datagram

In link A-R1:-

1st packets <20, 200> MF = 1

2nd packets <20, 200> MF = 0

In link A-R2:-

1st packet in A-R1 is divided into 3 parts

packet 1.1 <20,80> MF = 1

packet 1.2 <20,80> MF = 1

packet 1.3 <20,40> MF = 0

packet 1.3's MF != MF of packet 1 In A-R1.

is it correct.

1 Answer

2 votes
2 votes

both are correct !

see this table:

MF----OFFSET----------description

1 0 1st fragment
1 not 0 intermediate fragment
0 not 0 last fragment
0 0 not fragmented

MF of last fragment = 0, which is equal to parent because 0 MF in parent shows that original parent data is not fragmented and yet to be fragmented.

Offset of 1st fragment will always be 0 (obvious), offset of parent will also be 0 because its not yet fragmented.

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