Which of the following statements about IPv4 fragmentation is/are TRUE?
2024
Which of the following statements about IPv4 fragmentation is/are TRUE?
- A.
The fragmentation of an IP datagram is performed only at the source of the datagram
- B.
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
- 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
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Correct answer: B, C
Answer: The true statements are that routers may fragment datagrams when the outgoing MTU is too small, and that fragments are reassembled only at the destination.
Key points:
Routers can fragment IPv4 datagrams when the outgoing interface MTU is smaller than the datagram, unless the Don't Fragment (DF) flag is set.
If the DF flag is set and a router cannot forward a packet without fragmentation, the router drops the packet and sends an ICMP "Fragmentation Needed" (ICMP Type 3, Code 4) back to the sender.
Reassembly of fragments is performed only by the destination host; intermediate routers forward fragments and do not reassemble them.
Fragments use the identification, fragment offset, and MF (more fragments) fields so the destination can reassemble the original datagram.
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