GATE CS 2024 Set 2 Q.28: IPv4/IPv6 Addressing, Subnetting, CIDR, ARP, ICMP and NAT

Which of the following statements about IPv4 fragmentation is/are TRUE?

The fragmentation of an IP datagram is performed only at the source of the datagram

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

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

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

Official answer and solution

Official answer: B and C
From: Official GATE 2024 answer key

Solution

Recall IPv4 fragmentation and reassembly rules: routers along the path fragment datagrams exceeding the next-hop MTU, while only the final destination reassembles them.

In IPv4:

Fragmentation can be performed by the source host and by any intermediate IPv4 router whose outgoing link has a Maximum Transmission Unit (MTU) smaller than the datagram size (provided the DF bit is not set). Thus (B) is TRUE and (A) is FALSE (note: in IPv6, only the source fragments, not routers).
Reassembly of fragments is performed only at the final destination host because different fragments may take different paths through the network and reassembling at intermediate routers would add heavy state and processing overhead. Thus (C) is TRUE and (D) is FALSE.

Final answer

Options (B), (C)

Common trap

Confusing IPv4 with IPv6 (where fragmentation is performed only at the source).

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2024 Computer Science and Information Technology - Set 2 (CS2)
Question: 28 of 65
Section: Computer Science and Information Technology

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