GATE CSE 2014 Set 3 | Question: 25

Question

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

• A. $\text{i}$ only
• B. $\text{i}$ and $\text{ii}$ only
• C. $\text{ii}$ and $\text{iii}$ only
• D. $\text{i, ii}$ and $\text{iii}$

Comments

The subjects where most Ambiguous questions are asked is Computer Networks!

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@Amcodes, totally right! Even for GATE standards these are ridiculous

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This is not at all ambiguous, as JASHAN said, this checks your aptitude about the subject

Destination is not exceptional, it changes the fragmentation offset, so yes, offset can be changed

so smart na

this is computer science,

here you will find the most beautiful questions framed by brilliant and sophisticated minds of IITs and IISC professors

Answer 1

Answer is D

If due to MTU restriction we need to fragment the packets the Checksum value changes/recomputed wether error has occured or not...

So, TTL changes in every hop, Fragment Offset can change hence Checksum changes

http://stackoverflow.com/questions/27111459/how-does-the-udp-checksum-change-for-ip-fragments  In this answer see last bullet point in box :)

Comments

@arjun sir why would offset feild change

And if fragmentation is done there is no D left it will be fragmented so from which fragment header we will compare

If we will compare from first fragment offset as it's identification no. And offset both are matching so fragment offset didn't change.

Length feild is getting changed but not offset

Answer 2

D option is correct bcoz at each inetermediate node ttl value will be change so  checksum also and each node fragmention may also be possible so all are correct.

Answer 3

so TTL definitely changes in each hop so check sum would also change.suppose the MTU of network is less than frame may get fragmented so fragement offset may also change .so answer must be (d)

Answer 4

Even when the question asks about "may change" , and even if fragmentation occurs, is it not the case that the fragmentation offset of the first datagram will remain same?