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Packet Fragmentation Walkthrough

Run the Packet Fragmentation Walkthrough MicroSim Fullscreen

About This MicroSim

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This MicroSim is built with p5.js and is width-responsive, so it adapts to the width of the page or container it is embedded in.

How to Use

Use the controls in the panel below the drawing area to explore the concept. Adjust the sliders, toggle the options, and step through the stages to see how each change affects what is shown.

Specification

The full specification below is extracted from Chapter 9: The Network Layer and IP Addressing.

Type: microsim
**sim-id:** packet-fragmentation<br/>
**Library:** p5.js<br/>
**Status:** Specified

Build an interactive MicroSim that animates a packet traversing a multi-hop path with mismatched MTUs, showing fragmentation in IPv4 and Path-MTU Discovery in IPv6.

Canvas: 940 px wide by 600 px tall, responsive down to 360 px. A 100 px control panel sits below.

Layout:

- A horizontal path: Sender — Link 1 (MTU 9000) — Router R1 — Link 2 (MTU 1500) — Router R2 — Link 3 (MTU 576) — Receiver.
- A large packet (e.g., 4000 bytes) starts at the sender; the animation shows what happens at each MTU mismatch.

Animation modes (selectable from a tab control):

- **IPv4 with DF=0 (legacy):** the packet is fragmented at R1 (4000 → 2× 1500 + 1× 1000 + headers). Fragments reach R2 where each is again fragmented to fit MTU 576. Receiver buffers all fragments and reassembles. A counter tracks fragments and reassembly buffer use.
- **IPv4 with DF=1:** R1 cannot fragment, returns ICMP "Fragmentation Needed" with MTU=1500. Sender retries with 1500-byte packet; R2 returns ICMP "Fragmentation Needed" with MTU=576. Sender retries with 576-byte packets; all reach the receiver intact. PMTU is now 576.
- **IPv6:** sender starts at 4000 bytes; R1 returns ICMPv6 "Packet Too Big" with MTU=1500. Sender retries with 1500-byte packets using Fragment Extension Header (source-only fragmentation) chunks of 1500 bytes carried by the source. Same workflow for R2. PMTU converges to 576.

Controls panel:

- Tab control for the three modes.
- Sliders: link MTU values (per-link).
- Buttons: Step / Play / Reset.
- Counters: fragments created, ICMP messages exchanged, total bytes on each link.

Visual style:

- Original packet: large honey-amber rectangle.
- Fragments: smaller honey-amber rectangles labeled with offset.
- ICMP messages: red arrows traveling backward.
- MTU labels above each link.

Learning objectives:

- (Bloom — Understanding) Students explain what happens at an MTU mismatch in each of the three modes.
- (Bloom — Analyzing) Students decompose the trade-offs between in-transit fragmentation and Path-MTU Discovery.
- (Bloom — Evaluating) Students judge why IPv6 deliberately removed in-transit fragmentation.

Implement in pure p5.js with the existing MicroSim CSS theme.

Iframe Embed Code

You can add this MicroSim to any web page by adding this HTML:

<iframe src="https://dmccreary.github.io/networking/sims/packet-fragmentation/main.html"
        height="502px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objective

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Bloom Taxonomy Level

Understand

Suggested Classroom Use

  1. Predict — Ask students to predict the behavior before they interact.
  2. Explore — Have students manipulate the controls and observe the results.
  3. Explain — Ask students to explain, in their own words, what they observed and how it connects to the network layer and ip addressing.

References