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16,179 views
46 votes
46 votes

Host A (on TCP/IP v4 network A) sends an IP datagram D to host B (also on TCP/IP v4 network B). Assume that no error occurred during the transmission of D. When D reaches B, which of the following IP header field(s) may be different from that of the original datagram D?

  1. TTL
  2. Checksum
  3. Fragment Offset
  1. $\text{i}$ only
  2. $\text{i}$ and $\text{ii}$ only
  3. $\text{ii}$ and $\text{iii}$ only
  4. $\text{i, ii}$ and $\text{iii}$
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9 Answers

Best answer
89 votes
89 votes

The answer is OPTION D.

Whenever an IP packet is transmitted, the value in Time to Live (TTL) field will be decremented on every single hop. Hence, TTL is changed on every hop.

Now, since TTL changes, hence the Checksum of the packet will also change.

For the Fragmentation offset, A packet will be fragmented if the packet has a size greater than the Maximum Transmission Unit (MTU) of the network. Hence, Fragmentation offset can also be changed.

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11 votes
11 votes
Answer should be B and not D

Reason :- Since, in the question it clearly says that the Datagram D reaches the destination, if there was fragmentation done along the way, the Datagram D would have been divided into different datagrams say A, B, C. Obviously if the same datagram D is reaching, there was no fragmentation hence no change in Frag. offset.

So, only TTL and checksum will be different.
4 votes
4 votes
Correct Option Will be -D

TTL:: It is obvious that TTL value will be change at each router so checksum will also.

Fragment::It may change or not depend upon MTU of forwarding device as router here.But in worst it must be change.
2 votes
2 votes
TTL will change (No doubt)

Due to change in TTL it will cause change in the header and since CHECKSUM will find some changes (due to change in TTL value) CHECKSUM value will be changing at each spot.

Now coming to the FRAGMENT OFFSET part, since question has asked about options which MAY be different from original Datagram then, YES FRAGMENT OFFSET ''MAY" also change if the MTU of other network is smaller.

Hope this helps.
Answer:

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