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GATE CSE IPv4 Fragmentation PYQs

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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 s… Read the guide

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Questions
8
Asked
since 2014
Total marks
12
Marks / paper
0.5

8 GATE CSE IPv4 Fragmentation PYQs with solutions, from Computer Networks. Browse all GATE CSE PYQs.

Questions

Every official paper that asked this topic, newest first.

What GATE tests in IPv4 Fragmentation

Concepts, patterns, formulas and traps distilled from past papers.

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

    CommonNAT2 marks2025, 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

    Commonmixed1.5 marks2025, 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

    CommonMSQ1 marks2024, 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)

Max Payload=⌊MTU−Header Size8⌋×8\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

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

Offseti=∑k=1i−1Payloadk8\text{Offset}_i = \frac{\sum_{k=1}^{i-1} \text{Payload}_k}{8}

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

TTL Decrement Rule

TTLreceived=TTLinitial−Nrouters\text{TTL}_{\text{received}} = \text{TTL}_{\text{initial}} - N_{\text{routers}}

Used to calculate remaining TTL upon packet arrival across a path with NroutersN_{\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.