GATE CSE 2024 | Set 2 | Question: 18

Question

​​​​​Which of the following statements about $\text{IPv4}$ fragmentation is/are TRUE?

• A. The fragmentation of an $\text{IP}$ datagram is performed only at the source of the datagram
• B. The fragmentation of an $\text{IP}$  datagram is performed at any $\text{IP}$  router which finds that the size of the datagram to be transmitted exceeds the $\text{MTU}$
• C. The reassembly of fragments is performed only at the destination of the datagram
• D. The reassembly of fragments is performed at all intermediate routers along the path from the source to the destination

Comments

option B shouldn't be correct. As if by mistake DF = 1 in IP datagram then it doesn't matter if the size of the datagram exceeds the MTU it will not get fragmented by any router.
Correct me if Im wrong.

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@Quantum City 

first if DF=1 then get icmp message after this datagram is fragmented

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@Chandrabhan Vishwa 1 can u provide any reference of your knowledge. I think it will not get fragmented and icmp message will sent to source but it will not get fragmented for sure else what's the point of keeping DF = 1. 

Answer 1

$\textcolor{blue!80!black}{\text{The correct answer is C.}} $

A. False.
Statement: The fragmentation of an IP datagram is performed only at the source of the datagram.

Fragmentation can occur at routers also.

B. False.

Statement: The fragmentation of an IP datagram is performed at any IP router which finds that the size of the datagram to be transmitted exceeds the MTU.

Fragmentation can indeed occur at any router along the path if the packet size exceeds the MTU, but it depends on whether the "Don't Fragment" flag is set in the IP header. If this flag is set, the router will not fragment the packet and will instead drop it, sending an ICMP "Fragmentation Needed" message back to the source.

C. True.

Statement: The reassembly of fragments is performed only at the destination of the datagram

Reassembly of fragments is typically performed only at the destination of the datagram. The destination is responsible for putting the fragments back together to reconstruct the original packet.

D. False.

Statement: The reassembly of fragments is performed at all intermediate routers along the path from the source to the destination.

Reassembly of fragments is not performed at intermediate routers along the path. Routers forward individual fragments based on their headers and do not reassemble them.

Comments

If we start taking all such possibilities into account then why is option C correct even. If packet is not fragmented will reassembly be done ?

Will other gate questions based on fragmentation be solved if in each we consider DF bit ?

In option B what about TTL and checksum fields ? Why DF bit only ?

I dont think option B is false in any sense here.

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I totally agree with you @liontig37. Either GATE should accept B,C or they should give C or B,C correct! Considering such possibilities can make D also true by saying all MTU's on path had decreasing size and hence fragmentation was performed at every router on the path! 

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@liontig37 Yes, your logic totally makes sense. I will consider it as one of the poorly framed qs. Not even a single PYQ is based on DF Flag. Every place where we found MTU < datagram size, we perform fragmentation. Even in the option it is mentioned so. @Sachin Mittal 1 Sir, I feel option B is as true as option C.

Edit: It's not mentioned here that we are performing PMTUD, in which case DF bit comes into picture. Attached are resources:
Fragmented Packet

PMTUD

Cisco - Fragmentation

Cisco PMTUD

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true this is too much of assumption