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Suppose a network:-

Suppose a TCP message contains 492 B of data and TCP header is of size 20B then deliver it to IP. IP header is 20 B.

Assume link A-R1 has the maximum frame size of 700 B including a frame size of 14 B, link R1-R2 can support a maximum frame size of 256 B including an 8 B of the frame header and link R2-B can support a maximum of 256 B including a 12 B frame header.

Then calculate the no of fragments at each link and show the format of fragments in each link.

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Best answer
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TCP data size = 492 B
TCP header size = 20 B
IP header size = 20 B
So packet size = 492 B + 20 B + 20 B = 532 B

20 B 512 B

Now our data will travel from A - R1

Maximum frame size allowed = 700 B (include frame header)
Frame Header Size = 14 B
So maximum packet size allowed = 700 B - 14 B = 686 B
Since 532 B < 686 B, so no need of fragmentation.

Now our data will travel from R1 - R2

Maximum frame size allowed = 256 B (include frame header)
Frame Header Size = 8 B
So maximum packet size allowed  = 256 B - 8 B = 248 B
Since 532 B > 248 B, so here we will do fragmentation

Maximum fragment data allowed = 248 B - 20 B = 228 B
But we know that fragment must carry data in multiple of 8.
So, First fragment will carry a data of 224 B and IP header of 20 B i.e 244 B
second second fragment will carry a data of 224 B and IP header of 20 B i.e 244 B
Third fragment will carry a data of 64 B and IP header of 20 B i.e 84 B

Now our data will travel from R2 - B

Maximum frame size allowed = 256 B (include frame header)
Frame Header Size = 12 B
So maximum packet size allowed = 256 B - 12 B = 244 B
Since 244 B = 244 B, so here we don't do any fragmentation

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At $A$ only 1 segment $<frame \space header \space byte, data \space bytes>$ will be generated of the form $<14,532>$

At $R1$ $3$ segment $<frame \space  header \space byte, data \space bytes>$ will be generated of the form $<8,248>, <8,248>, <8,36>$ After padding by $R1$ - $<8,40>$

At $R2$ $5$ segment $<frame \space header \space byte, data \space bytes>$ will be generated of the form $<12,240>, <12,240>, <12,8>, <12,8>, <12, 36>$ After padding by $R2$ - $<12,40>$
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