An IP router with a Maximum Transmission Unit (MTU) of 1500 bytes has received an IP packet of size 4404 bytes with an IPheader of length 20 bytes. The values of the relevant fields in the header of the third IP fragment generated by the router forthis packet area)MF bit: 0, Datagram Length: 1444; Offset: 370b)MF bit: 1, Datagram Length: 1424; Offset: 185c)MF bit: 1, Datagram Length: 1500; Offset: 370d)MF bit: 0, Datagram Length: 1424; Offset: 2960Correct answer is option 'A'. Can you explain this answer?

Question

Answer

The given scenario involves an IP router with a Maximum Transmission Unit (MTU) of 1500 bytes receiving an IP packet of size 4404 bytes with an IP header of length 20 bytes. We are required to determine the values of the relevant fields in the header of the third IP fragment generated by the router for this packet.

To solve this problem, we need to understand how IP fragmentation works and how the relevant fields in the IP header are modified during the process.

IP Fragmentation:
- When an IP packet is larger than the MTU of a network, it needs to be fragmented into smaller packets to be transmitted across the network.
- Each fragment contains a piece of the original packet and has its own IP header.
- Fragments are reassembled at the destination to reconstruct the original packet.

IP Header Fields:
1. MF bit (More Fragments): This bit is set to 1 in all fragments except the last fragment to indicate that more fragments are yet to come.
2. Datagram Length: It specifies the length of the fragment's payload, excluding the IP header.
3. Offset: It indicates the position of the fragment within the original packet, in units of 8 bytes.

Solution:
In this case, the original IP packet size is 4404 bytes, and the IP header size is 20 bytes. Therefore, the payload size is 4404 - 20 = 4384 bytes.

To determine the values of the relevant fields in the header of the third IP fragment, we need to calculate the offset and datagram length for each fragment.

1. First Fragment:
- MF bit: Set to 1 since there are more fragments to come.
- Datagram Length: It should be set to the MTU size minus the IP header size, i.e., 1500 - 20 = 1480 bytes.
- Offset: Since this is the first fragment, the offset is 0.

2. Second Fragment:
- MF bit: Set to 1 as there are more fragments to come.
- Datagram Length: It should be set to the MTU size minus the IP header size, i.e., 1500 - 20 = 1480 bytes.
- Offset: Since this is the second fragment, the offset is calculated as (MTU - IP header size) / 8 = (1500 - 20) / 8 = 185.

3. Third Fragment:
- MF bit: Set to 0 as this is the last fragment.
- Datagram Length: It should be set to the remaining payload size, which is 4384 - 2960 = 1424 bytes.
- Offset: Since this is the third fragment, the offset is calculated as (MTU - IP header size) / 8 = (1500 - 20) / 8 = 185.

Therefore, the correct answer is option 'A':
- MF bit: 0
- Datagram Length: 1424
- Offset: 370

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