714  RPL DODAG Construction Process

714.1 Learning Objectives

After completing this chapter series, you will be able to:

  • Visualize the complete DODAG construction algorithm step-by-step
  • Understand RPL control messages (DIO, DIS, DAO, DAO-ACK)
  • Calculate RANK values using ETX metrics and objective functions
  • Apply the Trickle algorithm for energy-efficient network maintenance
  • Design and troubleshoot RPL networks for IoT deployments

714.2 Overview: How RPL Builds DODAGs

RPL (Routing Protocol for Low-Power and Lossy Networks) constructs a DODAG (Destination-Oriented Directed Acyclic Graph) through a systematic process involving four control message types:

Graph diagram

Graph diagram
Figure 714.1: RPL control messages and DODAG joining flow

{fig-alt=“RPL control messages overview showing DIO (advertise DODAG), DIS (request info), DAO (advertise reachability), DAO-ACK (acknowledgment), and DODAG construction flow from ROOT sending DIO through node joining to DAO advertisement”}

Message Direction Purpose
DIO Root -> Leaves Advertise DODAG, propagate RANK
DIS Node -> Neighbors Request DODAG information
DAO Leaves -> Root Announce node reachability
DAO-ACK Root -> Leaves Confirm route installation

The construction process follows five phases: root initialization, first-hop joining, multi-hop expansion, DAO propagation, and steady-state maintenance. A typical 50-node network converges in 30-120 seconds, after which the Trickle algorithm minimizes control overhead while maintaining responsiveness to topology changes.

714.3 Chapter Series: DODAG Construction Deep Dive

This topic is covered in four focused chapters, each exploring a specific aspect of DODAG construction:

714.3.1 1. Visual Construction Guide

RPL DODAG Visual Guide covers the step-by-step visual progression of DODAG formation:

  • 5-phase algorithm overview (root init through steady state)
  • DAG vs DODAG structural differences
  • 7-step temporal progression with real timing (t=0s to t=30+s)
  • Network convergence factors and optimization

714.3.2 2. Message Flow and Exchange

RPL DODAG Message Flow provides detailed protocol-level understanding:

  • Four control message types and their roles
  • Complete sequence diagrams with IPv6 addresses
  • RANK calculation using ETX link quality metrics
  • Parent selection when multiple candidates exist
  • Knowledge checks on network self-healing and optimization

714.3.3 3. Worked Example: Smart Lighting Network

RPL DODAG Worked Example walks through a realistic 10-node deployment:

  • Step-by-step RANK calculations with formulas
  • Multi-candidate parent selection decisions
  • Final DODAG structure interpretation
  • Routing implications (upward, downward, point-to-point)
  • Practice problems with solutions

714.3.4 4. Trickle Algorithm

RPL Trickle Algorithm explains energy-efficient maintenance:

  • “Polite gossip” metaphor for intuitive understanding
  • Three scenarios: stable, inconsistent, insufficient redundancy
  • Energy savings calculation (up to 99.7% reduction)
  • RFC 6206 parameter configuration guide
  • Configuration profiles for different applications

714.4 Quick Reference: DODAG Formation Timing

Phase Time Key Action
Root DIO t=0-2s Root broadcasts RANK=0
First-hop join t=2-5s Neighbors calculate RANK, send DAO
Multi-hop expand t=5-10s DIS/DIO ripple outward
DAO upward t=10-20s All nodes announce reachability
Steady state t=30+s Trickle backs off to hourly DIOs

714.5 What’s Next

Start with the visual guide for an intuitive understanding, then progress through message flow and the worked example:

After mastering DODAG construction, continue with: