GATE Overflow Test Series | Computer Networks | Test 1 | Question: 27

Question

A TCP message consisting of $2400\;\text{bytes}$ is passed to IP for delivery across two networks. The first network can carry a maximum payload of $1000 \;\text{bytes}$ per frame and the second network can carry a maximum payload of $5000 \;\text{bytes}$ per frame, excluding network overhead. The IP overhead per packet is $20 \;\text{bytes}$. The total IP overhead in the second network for this transmission will be _______

Answer 1

Network $1: 2400\;\text{bytes}$ broken to $3$ IP packets of sizes $1000+20, 1000+20$ and $400+20.$ IP overhead $ = 60\;\text{bytes}$.   

Network $2:$ Since its payload size is larger than that of Network $1,$ there is no additional fragmentation here. So, IP overhead of second network is same as that of first $ = 60\;\text{bytes}$.

Comments

thanks sir

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Sir why overhead is not 40B since we are only creating two more extra packets.

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The 60 bytes of overhead in the second network is still present because the fragments from the first network still have their IP headers. The IP headers are not removed when the fragments pass from Network 1 to Network 2.

This is because the IP headers contain important information about the fragments, such as the source and destination addresses, the fragment offset, and the more fragments (MF) bit. This information is needed by the routers in Network 2 to reassemble the fragments into the original TCP message.

Therefore, the total IP overhead in the second network is still 60 bytes, even though the fragments are not being fragmented further.

Here is a summary of the reasons why the total IP overhead in the second network is still 60 bytes:

• The IP headers from the first network are still present.
• The IP headers contain important information that is needed by the routers in Network 2 to reassemble the fragments.

It is important to note that the overhead in the second network could be higher than 60 bytes if Network 2 is adding additional overhead to the fragments. However, the overhead is typically much less than the overhead of fragmentation.