In IPv4, packet fragmentation and reassembly is handled by –
2024
In IPv4, packet fragmentation and reassembly is handled by –
Answer: A. using the More Fragments and Fragment Offset Fields — ConceptEvery physical link imposes a Maximum Transmission Unit (MTU) — the largest payload a single frame on that link may carry. When an IPv4 datagram is…
- A.
using the More Fragments and Fragment Offset Fields
- B.
using the Header Checksum
- C.
encapsulating packets in larger data frames
- D.
using the MAC Header
- E.
Question not attempted
Attempted by 426 students.
Show answer & explanation
Correct answer: A
Concept
Every physical link imposes a Maximum Transmission Unit (MTU) — the largest payload a single frame on that link may carry. When an IPv4 datagram is bigger than the MTU of the next link, the datagram is split into fragments, and only the destination host puts them back together. IPv4 keeps all of the bookkeeping for that split-and-rejoin inside its own header, in the Identification field, the Flags field (which contains the More Fragments bit) and the Fragment Offset field.
The general rule: fragmentation is a Network layer operation driven by IPv4 header fields, and reassembly happens end to end at the destination, never hop by hop.
Applying it to this question
The stem asks which mechanism actually performs fragmentation and reassembly. The three IPv4 header fields that carry fragmentation state work together like this:
IPv4 header field | What it carries | Job during split and rejoin |
|---|---|---|
Identification (16 bits) | The same value copied into every fragment of one datagram | Groups arriving fragments that belong to the same original datagram |
Flags — More Fragments (MF) | Set to 1 on every fragment except the last one | Tells the receiver whether further pieces are still on the way |
Fragment Offset (13 bits) | This fragment's position in the original payload, counted in units of 8 bytes | Places each fragment at the correct byte position in the rebuilt datagram |
So the mechanism the stem is asking about is "using the More Fragments and Fragment Offset Fields": the MF bit signals that the datagram is still incomplete, and the Fragment Offset says exactly where each received piece belongs. Together they are what lets the destination rebuild the original datagram.
Cross-check against the other mechanisms
Header Checksum — a 16-bit check computed over the IPv4 header bytes alone and recomputed at every hop. It detects a header damaged in transit, but it carries no information about how a datagram was divided.
Encapsulating packets in larger data frames — a Data Link layer packaging step. A link will not accept a frame larger than its own MTU, so making frames bigger is simply not available as a way to move an oversized datagram.
MAC Header — a Data Link layer header holding hardware addresses for one hop only. Each router strips it and writes a fresh one for the next link, so it never reaches the destination and cannot carry reassembly state.
This confirms the reading: reassembly state has to survive the whole path, and only the IPv4 header — through the More Fragments flag and the Fragment Offset — carries it end to end.