3.15 marks] A 7000-byte TCP segment (6980 of data and 20 bytes of header) is car
ID: 3916120 • Letter: 3
Question
3.15 marks] A 7000-byte TCP segment (6980 of data and 20 bytes of header) is carried by an IP datagram with one-byte option. The IP datagram with Identification of 0000H is to be transmitted and needs to be fragmented because it will pass through an Ethernet (N1) with a MTU of 1200 bytes. Show the Identification, Total Length, More Flag, and Fragment Offset values in each of the resulting fragments. The above fragmentations will have to go through another Ethernet (N2) with a MTU of 700 bytes. Again, show the Identification, Total Length, More Flag, and Fragment Offset values in each of the resulting fragments. Assume all the fragments will have one-byte option like the original IP datagram.Explanation / Answer
Since there is 6980 bytes of data, it will be split into 6 fragments (5 of length 1200 and one of length 980)
Fragment 1 :
Identification - 0000H
Total Length - 1220 (1200+20 for header)
More Flag - 1
Frag offset - 0000
Fragment 2:
Identification - 0000H
Total Length - 1220 (1200+20 for header)
More Flag - 1
Frag offset - 1200/8 = 150
Fragment 3:
Identification - 0000H
Total Length - 1220 (1200+20 for header)
More Flag - 1
Frag offset - 2400/8 = 300
Fragment 4:
Identification - 0000H
Total Length - 1220 (1200+20 for header)
More Flag - 1
Frag offset - 3600/8 = 450
Fragment 5:
Identification - 0000H
Total Length - 1220 (1200+20 for header)
More Flag - 1
Frag offset - 4800/8 = 600
Fragment 6:
Identification - 0000H
Total Length - 1000(980+20 header)
More Flag - 0
Frag offset - 6000/8=750
At N2, each fragment is broken into 2 more fragments to fit thr MTU
Fragment 1.1
Identification - 0000H
Total Length - 716(696+20 header)
More Flag - 1
Frag offset - 0000
Fragment 1.2
Identification - 0000H
Total Length - 524(504+20 header)
More Flag - 1
Frag offset - 696/8=87
Fragment 2.1
Identification - 0000H
Total Length - 716(696+20 header)
More Flag - 1
Frag offset - 1200/8 = 150
Fragment 2.2
Identification - 0000H
Total Length - 524(504+20 header)
More Flag - 1
Frag offset - 1896/8=237
Fragment 3.1
Identification - 0000H
Total Length - 716(696+20 header)
More Flag - 1
Frag offset - 2400/8 = 300
Fragment 3.2
Identification - 0000H
Total Length - 524(504+20 header)
More Flag - 1
Frag offset - 3096/8=387
Fragment 4.1
Identification - 0000H
Total Length - 716(696+20 header)
More Flag - 1
Frag offset - 3600/8 = 450
Fragment 4.2
Identification - 0000H
Total Length - 524(504+20 header)
More Flag - 1
Frag offset - 4296/8=537
Fragment 5.1
Identification - 0000H
Total Length - 716(696+20 header)
More Flag - 1
Frag offset - 4800/8 = 600
Fragment 5.2
Identification - 0000H
Total Length - 524(504+20 header)
More Flag - 1
Frag offset - 5496/8=687
Fragment 6.1
Identification - 0000H
Total Length - 716(696+20 header)
More Flag - 1
Frag offset - 6000/8 = 750
Fragment 1.2
Identification - 0000H
Total Length - 304(284+20 header)
More Flag - 0
Frag offset - 6696/8=837
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