GATE CSE 2018 | Question: 54

Question

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

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

Comments

To solve these type of qs, just notice what packet is being forwarded. If it’s an IP packet, then subtract only the IP Header. No need to subtract the TCP header, because TCP header was added at the Transport Layer and $\textsf{IPv4}$ router does not have Transport layer.

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Similar question : https://gateoverflow.in/8255/gate-cse-2015-set-2-question-52

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In IP fragmentation questions,we donot need to worry about TCP,UDP headers, basically routers processes the fragments and it donot need to even look at TCP,UDP header, so we will not append them to each fragment .TCP,UDP header is just a part of payload i.e treated as usual data, and is present in the first fragment only
The attached slide of UC Berkley can be referred

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Answer 1

Packet Length =4500B=4500B
IP Payload =4500−20=4480B=4500−20=4480B

MTU =600B=600B
MTU Payload =600B−20B=580B=600B−20B=580B
But payload should be multiple of 88 so number nearest to 580580 and multiple of 88 is 576576, so MTU payload =576B=576B
IP Packet size =576+20B=596B=576+20B=596B
Size of Offset == 57685768  =72=72

1st fragment offset  =0
2nd fragment offset  =72
3rd fragment offset  =144

and so on.........

Answer 2

Answer 3

cut off 20 Bytes from 4500 as that is actual data for IP as we have removed 20B ip header

now MTu is 600 which will also contain 20B header so cut off 20B here also

so the actual data in MTU is 580, but as 576 is 8a where a ϵ [0, ∞)

now how many packets/fragments we should be sending is ${\frac{580}{8}}$

${\color{Magenta}which}$  ${\color{Magenta}is}$

we will take 72

now multiply 72 with 2

which is 144

Answer 4

The answer will be 144. TCP header needs to be excluded. Only I.P. header is removed.