GATE CS 2024 Set 2 | Question 28

MSQ · 1 mark

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

  1. Option A: The fragmentation of an IP datagram is performed only at the source of the datagram
  2. Option 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 (correct)
  3. Option C: The reassembly of fragments is performed only at the destination of the datagram (correct)
  4. Option D: The reassembly of fragments is performed at all intermediate routers along the path from the source to the destination

Answer

(B), (C)

Explanation

In IPv4, a datagram can meet a link with a smaller MTU anywhere on its path.

  • B is true and A is false: any router whose outgoing link cannot carry the datagram splits it into fragments, each with its own IP header and with the offset and more-fragments fields set. (Fragmenting only at the source is how IPv6 works.)
  • C is true and D is false: fragments of one datagram may take different routes, and a later link may have a larger MTU anyway, so routers forward fragments as independent datagrams. Only the destination host collects them, using the identification field and the offsets, and rebuilds the original datagram.

Answer: B, C