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GATE 2024 CS2 Q.28: solution and answer

GATE 2024 · CS2Q.28 · 1 mark · MSQ

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

  1. (A)

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

  2. (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

  3. (C)

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

  4. (D)

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

See the question in the paper
GATE 2024 CS2 question 28Tap to open full size

Answer and solution

ANSWER

The true statements are (B) and (C): any router whose outgoing MTU is too small can fragment a datagram, and reassembly happens only at the destination.

Official answer: B, CSource: Official answer key, GATE 2024 CS2 (IISc Bengaluru)

  1. In IPv4, a router that must forward a datagram onto a link whose MTU is smaller than the datagram, and whose DF (Don't Fragment) bit is not set, fragments it. Fragmentation is therefore not limited to the source host. (A) is false.

  2. Each link on the path can have a different MTU, so any router may find that a datagram is larger than the MTU of its outgoing link and fragment it. (B) is true.

  3. Fragments travel as independent datagrams and may take different routes, so an intermediate router cannot be sure of seeing all fragments of a datagram. IPv4 therefore reassembles only at the final destination host. (C) is true.

  4. Intermediate routers forward fragments like any other datagram and do not reassemble them. (D) is false.

  5. The true statements are (B) and (C).

KEY IDEA

IPv4 fragments at any router whose outgoing link MTU is smaller than the datagram (if DF is not set), but reassembles only at the destination host, because fragments may take different paths.

WATCH OUT FOR

Thinking that only the source can fragment, as in IPv6, where routers never fragment. In IPv4, routers do fragment, but they never reassemble.

From the GATE 2024 CS2 paper. Every question of this paper · Download the full paper · All 20 IPv4 questions, with notes

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