What are common examiner traps in IPv4 Fragmentation?

Forgetting to floor the fragment payload to the nearest multiple of 8: (MTU - 20) is not always divisible by 8. Neglecting transport layer headers (e.g., forgetting to add 8 bytes for UDP header when UDP data payload is given). Assuming intermediate routers reassemble fragments if the next link has a larger MTU (routers NEVER reassemble). Treating multi-hop fragmentation as a single division using the minimum MTU along the path rather than fragmenting existing intermediate fragments sequentially. Assuming TCP destination/source port can be inspected in subsequent (non-first) fragments. Decrementing TTL at Layer-2 switches or bridges instead of strictly at Layer-3 routers.

IPv4 Fragmentation

8 Qs · since 2014 · 12 marks · 0.5 marks/paperStandard yield

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IPv4 Fragmentation in GATE CS is tested through a balanced mix of quantitative fragmentation calculations and conceptual questions regarding packet forwarding semantics. Numerical… Guide

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GATE CSE 2025 Set 1Q57NAT2 marksMediumDiagramSuppose a message of size 15000 bytes is transmitted from a source to a destination using IPv4 protocol via two routers as shown in the figure. Each router has a defined maximum transmission unit (MTU) as shown in the figure, including IP header. The number of fragments that will be delivered to the destination is…
GATE CSE 2025 Set 2Q23MCQ1 markMediumConsider a network that uses Ethernet and IPv4. Assume that IPv4 headers do not use any options field. Each Ethernet frame can carry a maximum of 1500 bytes in its data field. A UDP segment is transmitted. The payload (data) in the UDP segment is 7488 bytes. Which ONE of the following choices has the CORRECT total…
GATE CSE 2024 Set 1Q65NAT2 marksMediumConsider sending an IP datagram of size 1420 bytes (including 20 bytes of IP header) from a sender to a receiver over a path of two links with a router between them. The first link (sender to router) has an MTU (Maximum Transmission Unit) size of 542 bytes, while the second link (router to receiver) has an MTU size of…
GATE CSE 2024 Set 2Q28MSQ1 markEasyWhich of the following statements about IPv4 fragmentation is/are TRUE?
GATE CSE 2021 Set 1Q45MSQ2 marksMediumConsider two hosts $P$ and $Q$ connected through a router $R$ . The maximum transfer unit (MTU) value of the link between $P$ and $R$ is 1500 bytes, and between $R$ and $Q$ is 820 bytes. A TCP segment of size 1400 bytes was transferred from $P$ to $Q$ through $R$ , with IP identification value as…
GATE CSE 2020Q15MCQ1 markMediumConsider the following statements about the functionality of an IP based router. I. A router does not modify the IP packets during forwarding. II. It is not necessary for a router to implement any routing protocol. III. A router should reassemble IP fragments if the MTU of the outgoing link is larger than the size of…
GATE CSE 2016 Set 1Q53NAT2 marksMediumAn IP datagram of size 1000 bytes arrives at a router. The router has to forward this packet on a link whose MTU (maximum transmission unit) is 100 bytes. Assume that the size of the IP header is 20 bytes. The number of fragments that the IP datagram will be divided into for transmission is ____________.
GATE CSE 2014 Set 2Q25NAT1 markMediumDiagramIn the diagram shown below, L1 is an Ethernet LAN and L2 is a Token-Ring LAN. An IP packet originates from sender S and traverses to R, as shown. The links within each ISP and across the two ISPs, are all point-to-point optical links. The initial value of the TTL field is 32. The maximum possible value of the TTL…

Topic guide

What does GATE test in IPv4 Fragmentation?

IPv4 Fragmentation in GATE CS is tested through a balanced mix of quantitative fragmentation calculations and conceptual questions regarding packet forwarding semantics. Numerical items focus on calculating the number of fragments and last fragment sizes across single or multi-hop links with varying MTUs, strictly enforcing the 8-byte offset boundary rule. Conceptual items assess router-level operations, identifying where fragmentation happens (source and intermediate routers), where reassembly occurs (destination host only), and transport layer interactions (UDP/TCP header placement and segment-level retransmission).

Core concepts

Maximum Transmission Unit (MTU) vs IP Datagram Size
8-byte block alignment for Fragment Offset field
More Fragments (MF) and Don't Fragment (DF) flags
Independent fragmentation at intermediate routers vs end-to-end reassembly at destination host only
Transport layer encapsulation (TCP 20B/UDP 8B header placed exclusively in the first IP fragment)
Transport layer retransmission penalty upon IP fragment drop
Per-hop IPv4 header modifications (TTL decrement, Checksum recalculation)

Recurring question patterns

Multi-hop Multi-MTU Successive Fragmentation
common · NAT · 2 marks · 2025, 2024
A packet/message traverses a chain of routers with progressively decreasing MTUs. Candidates must determine intermediate fragment payload sizes (respecting the 8-byte alignment rule) and then re-fragment each sub-packet at subsequent routers to compute the total fragment count arriving at the destination.
Single-Link MTU Fragment and Header Sizing
common · mixed · 1.5 marks · 2025, 2021, 2016
A transport layer payload (TCP or UDP) or full IP datagram is fragmented across a link with a specific MTU. Candidates must calculate the total number of fragments, the offset values, or the exact byte size of the final fragment (including or excluding IP/transport headers).
Fragmentation and Router Functionality Concepts
common · MSQ · 1 marks · 2024, 2021, 2020, 2014
True/False or MSQ statements probing IPv4 forwarding rules: modification of TTL/Checksum, location of reassembly (destination only, never routers), DF/MF flag semantics, and layer-4 encapsulation effects (TCP header residing only in the first fragment).

Formulas GATE uses

Max Fragment Payload (8-byte Alignment Rule)

\text{Max Payload} = \left\lfloor \frac{\text{MTU} - \text{Header Size}}{8} \right\rfloor \times 8

Used to find the maximum data bytes an IPv4 fragment can carry so that the Fragment Offset field (measured in 8-byte blocks) remains an integer.

Total IP Payload with UDP

\text{IP Payload} = \text{UDP Data Payload} + \text{UDP Header Size (8 bytes)}

Used when the problem specifies UDP application data instead of a raw IP payload.

Fragment Offset Calculation

\text{Offset}_i = \frac{\sum_{k=1}^{i-1} \text{Payload}_k}{8}

Used to compute the 13-bit offset field value for the $i$ -th fragment.

TTL Decrement Rule

\text{TTL}_{\text{received}} = \text{TTL}_{\text{initial}} - N_{\text{routers}}

Used to calculate remaining TTL upon packet arrival across a path with $N_{\text{routers}}$ intermediate Layer-3 hops.

Examiner traps in IPv4 Fragmentation

Forgetting to floor the fragment payload to the nearest multiple of 8: (MTU - 20) is not always divisible by 8.
Neglecting transport layer headers (e.g., forgetting to add 8 bytes for UDP header when UDP data payload is given).
Assuming intermediate routers reassemble fragments if the next link has a larger MTU (routers NEVER reassemble).
Treating multi-hop fragmentation as a single division using the minimum MTU along the path rather than fragmenting existing intermediate fragments sequentially.
Assuming TCP destination/source port can be inspected in subsequent (non-first) fragments.
Decrementing TTL at Layer-2 switches or bridges instead of strictly at Layer-3 routers.

How questions are asked

Numerical calculation of total fragments delivered across multiple hops
MCQ finding number of fragments and exact byte length of the last fragment
MSQ evaluating conceptual statements on IP header flags, reassembly locations, and TCP interaction
NAT calculating final TTL value across a network topology

Paper trends

Shift from single-hop fragment count to multi-router successive cascading fragmentation problems requiring multi-stage division.
2025, 2024, 2016
Increased usage of MSQs testing fine-grained cross-layer interactions, such as TCP retransmission when an IP fragment is lost and port accessibility in non-first fragments.
2024, 2021
Inclusion of upper-layer header sizes (e.g., standard 8-byte UDP header) within payload specifications to test complete protocol stack knowledge.
2025

Difficulty notes

Easy questions test direct definitions (where reassembly happens, basic TTL decrements). Medium questions involve single-link or multi-link fragment calculations with 8-byte rounding rules and transport layer header additions (UDP/TCP). Hard questions involve cascading multi-link fragmentation where intermediate fragments are themselves unevenly split across downstream links with strict offset tracking.