706  Production RPL Framework and Review

706.1 Production RPL Framework

This section provides a comprehensive production-ready Python framework for RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks), including DODAG construction, RANK calculation, Trickle timer, and both Storing and Non-Storing modes.

706.1.1 Framework Architecture

This production framework provides comprehensive RPL protocol capabilities:

  • DODAG Construction: Distance-vector routing with RANK-based hierarchy, automatic parent selection
  • Trickle Timer: RFC 6206 implementation for adaptive DIO transmission (Imin to Imax intervals)
  • Routing Modes: Storing mode with distributed routing tables, non-storing mode support
  • Objective Functions: OF0 (hop count), MRHOF (ETX), Energy, Latency-based routing
  • Loop Prevention: RANK mechanism prevents routing loops, acyclic graph guaranteed
  • Control Messages: DIO (DODAG Information), DAO (Destination Advertisement), DIS, DAO-ACK
  • Traffic Patterns: Upward (many-to-one), downward (one-to-many), point-to-point routing
  • Network Repair: DODAG version increment for global topology changes
  • Parent Selection: Preferred parent with backup parent set for reliability

The framework demonstrates production-ready patterns for IPv6-based IoT networks with comprehensive statistics tracking, realistic link metrics, and complete DODAG lifecycle management.

Question 17: A smart city deploys 800 streetlight sensors across 20 city blocks, collecting light level, traffic, and environmental data every 30 seconds. Each sensor has 64KB RAM. The network uses RPL over 6LoWPAN. Most traffic is sensor-to-gateway (upward), with occasional firmware update commands sent to specific sensors (downward). Which RPL mode is MOST appropriate and why?

💡 Explanation: Correct: Non-Storing Mode (B) is optimal for this large-scale deployment. Scalability analysis: 800 sensors in 5-7 hop tree (typical for multi-block urban network). Storing Mode limitations: Each intermediate router would need to store routes for its sub-tree. Mid-level routers (3 hops from root) might have 100-200 descendants, requiring 10-20 KB routing table (100 routes × 100 bytes per route). With only 64 KB RAM, this leaves insufficient memory for application code, 6LoWPAN fragmentation buffers, and operating system. Typical Storing Mode capacity: 100-300 nodes depending on topology depth. 800 nodes exceeds this, especially with constrained 64KB sensors. Non-Storing Mode advantages: Root-only state: Gateway (presumably x86/ARM Linux with 1GB+ RAM) stores complete 800-node routing table (~80KB). Intermediate sensor routers store zero routing state—simply forward packets based on source routing headers. Memory savings: 64KB sensors use <10KB for RPL (parent selection, rank calculation, control message processing), leaving 50KB+ for application and buffers. Traffic pattern fit: Upward dominant: 800 sensors × 3 readings/minute = 2400 packets/minute upward. Upward routing requires no tables in non-storing mode (sensors simply forward to preferred parent toward root). Downward infrequent: Firmware updates maybe once per month to 10-50 sensors. Source routing overhead acceptable: IPv6 RH3 header adds ~40 bytes for 5-hop path (8 bytes/hop). For 200-byte firmware update command, overhead is 40/240 = 17% (acceptable for rare downward traffic). For frequent upward sensor data, no overhead (standard IPv6 headers). Root not bottleneck: 2400 upward packets/minute + ~5 downward packets/minute = easily handled by gateway. Why others wrong: Storing Mode faster downward (A) is true (distributed routing decision, no source route header) but irrelevant—firmware updates are not time-critical (seconds delay acceptable). Downward traffic is <1% of total, optimizing for it at cost of 10× memory overhead is wrong tradeoff. Complete topology storage (C) is false—Storing Mode does NOT store complete topology in every router, only sub-tree. Zero control overhead (D) is false—Non-Storing Mode still requires DIOs (upward route formation), DAOs (downward route advertisement), and Trickle timers. Control overhead is similar between modes (1-5% of bandwidth), not eliminated. Real-world precedent: Thread protocol (used in Google Nest, Apple HomePod) uses Router Eligible End Devices (REEDs) which are essentially non-storing mode—border router maintains full state, mesh routers minimize state. Thread networks support 200+ devices on constrained hardware. OpenThread implementation uses non-storing mode for scalability. Implementation: Gateway runs full-featured RPL stack with 800-entry routing table. Sensors run lightweight RPL implementing parent selection, rank updates, DIO/DAO generation, but no routing table maintenance. Contiki-NG, RIOT-OS, and Zephyr RTOS all support non-storing mode for constrained devices.

706.2 Key Concepts

  • DODAG (Destination Oriented Directed Acyclic Graph): Tree topology with single root; all upward paths lead to root
  • RANK: Hierarchical position in DODAG; prevents routing loops, enables parent selection
  • Distance-Vector Routing: Each node knows next hop (parent), not entire network topology
  • Objective Function: Algorithm for selecting best parent (hop count, energy, ETX); pluggable for different metrics
  • Parent Selection: Preferred parent with backup parents for fault tolerance
  • DIO (DODAG Information Object): Control message advertising DODAG topology; triggers parent selection
  • DAO (Destination Advertisement Object): Backward routing information in Storing mode for downward paths
  • Storing vs Non-Storing: Storing mode caches routes on each node (optimal paths, more memory); Non-Storing centralizes at root (source routing)
  • IPv6-Based: Native IPv6 protocol running over 6LoWPAN on IEEE 802.15.4 networks
  • Loop Prevention: DAG property and RANK mechanism guarantee acyclic routing, preventing loops
  • Many-to-One Traffic: Optimized for sensor networks where all data flows toward root gateway
  • Multiple Instances: Different RPL instances on same network support different applications/metrics
  • Lossy Link Support: Designed for 10-40% packet loss typical in wireless sensor networks
  • Scalability: Non-Storing mode scales to thousands of nodes; Storing mode optimizes for smaller networks
  • Control Message Types: DIO, DAO, DIS, DAO-ACK enable DODAG construction, topology updates, and route advertisement

706.4 Chapter Summary

RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks) is the IETF standard routing protocol designed specifically for IPv6-based IoT networks with resource-constrained devices, lossy wireless links, and convergent traffic patterns.

Unlike traditional routing protocols (OSPF, RIP) designed for wired networks with powerful routers, RPL addresses IoT’s unique challenges: - Limited resources: Sensors have <32 KB RAM, limited CPU, battery power - Lossy links: Wireless links lose 10-40% of packets; unreliable compared to wired networks - Traffic patterns: Most data flows toward a root gateway (convergent/many-to-one pattern), not uniformly across network - Multiple metrics: Performance depends on energy, latency, reliability - not just hop count

DODAG (Destination Oriented Directed Acyclic Graph) is RPL’s core concept. Instead of a full routing table, each node stores only its preferred parent(s). The DODAG forms a tree with a single root (typically the gateway). All upward paths eventually lead to root through parent pointers, creating a loop-free graph by design.

RANK is the hierarchical position in the DODAG. Root has RANK 0. Each node calculates its RANK based on parent’s RANK plus a metric. RANK enforces hierarchy - nodes never forward packets to higher-ranked nodes, preventing loops. This is simpler than count-to-infinity prevention used in traditional distance-vector protocols.

Parent selection is flexible. RPL computes the Objective Function (OF) - an algorithm combining multiple metrics (hop count, energy, ETX) to select the best parent. The protocol includes: - Objective Function Zero (OF0): Simple hop count minimization - Minimum Rank with Hysteresis Objective Function (MRHOF): Considers link quality (ETX) with hysteresis to prevent rapid parent switches

Nodes maintain a preferred parent (primary) and backup parent set for resilience. When preferred parent fails, nodes can quickly switch to backup, enabling self-healing mesh networks.

Control messages are ICMPv6-based: - DIO (DODAG Information Object): Root advertises DODAG; nodes forward DIO when joining DODAG. Contains RANK, Objective Function, DODAG ID - DIS (DODAG Information Solicitation): Orphaned node requests DODAG information to join network - DAO (Destination Advertisement Object): Node advertises reachability to upstream nodes (for downward routing in Storing mode) - DAO-ACK: Acknowledgment of DAO receipt

Two operational modes: 1. Storing mode: Each node caches routing table with paths to other nodes; enables optimal point-to-point paths but requires more memory 2. Non-Storing mode: Each node stores only parent pointer; downward routes use source routing (root specifies full path); saves memory

Upward routing (sensors → root) is identical in both modes: each node sends to parent. Downward routing (root → sensors) differs: Storing mode uses cached routes, Non-Storing uses source routing from root. Point-to-point routing (sensor ↔︎ sensor) uses upward path to common ancestor then downward path in Storing mode, or via root in Non-Storing mode.

Traffic patterns RPL optimizes for: - Many-to-one: Sensors report to gateway (most common); both modes identical - One-to-many: Gateway commands actuators; Storing better (optimal paths), Non-Storing uses source routing - Point-to-point: Device-to-device communication; Storing can optimize, Non-Storing routes via root

DODAG construction follows a multi-step process: 1. Root node sends DIO with RANK=0, establishes DODAG ID 2. Nodes receive DIO, calculate their own RANK, select preferred parent 3. Nodes send own DIO to propagate DODAG downward 4. Upward routes form automatically (parent pointers) 5. In Storing mode, nodes send DAO to advertise reachability upward 6. Downward routes created from DAO information

Network repair occurs when topology changes (links fail, nodes join/leave). RPL detects changes via loss of DIO from parent. Upon failure, node can quickly switch to backup parent. Global repairs happen via DODAG version increment from root.

Performance characteristics: - Memory: Non-Storing mode uses ~40% less network-wide memory - Latency: Storing mode better for point-to-point (optimal paths); identical for many-to-one - Scalability: Non-Storing scales better (limited by root CPU, not node memory); can handle thousands of nodes - Convergence: Faster than traditional distance-vector protocols; parent selection avoids count-to-infinity delays

Why RPL dominates IoT: - Standardized by IETF (RFC 6550, 2012) - Native IPv6, works with 6LoWPAN compression - Handles lossy, low-power networks inherently - Flexible metrics through pluggable Objective Functions - Proven in real-world deployments (Thread, many Zigbee implementations) - Works at Layer 3, protocol-agnostic at Link Layer (works over 802.15.4, Wi-Fi, etc.)

Use cases: - Smart home sensor networks (temperature, humidity, motion) - Industrial IoT monitoring (machines, energy, safety) - Smart city deployments (street lights, parking sensors, environmental monitoring) - Building automation (HVAC, lighting, access control) - Agriculture IoT (soil moisture, weather stations) - Any IPv6-based constrained network requiring self-healing mesh

Question 2: In an RPL network, the root has RANK 0. Node A receives DIO messages from three potential parents with RANKs 3, 5, and 7. Using hop-count metric (OF0), what RANK will Node A advertise in its own DIO messages?

💡 Explanation: RPL RANK calculation: Node RANK = Parent RANK + Rank_Increase. Node A selects the best parent (RANK 3), then adds the rank increase based on link quality. For OF0 (hop-count), this is typically MinHopRankIncrease (256 or configurable). However, in simplified terms, RANK represents “distance from root” - if parent is 3 hops away, node is 4 hops away. The RANK strictly increases along the path to prevent loops. Calculation: Best parent RANK 3 + 1 = RANK 4.

Question 4: An industrial sensor network experiences 20% packet loss. Node C receives DIO messages from two potential parents: Parent 1 (RANK 3, ETX 1.5) and Parent 2 (RANK 4, ETX 1.2). Using MRHOF (Minimum Rank with Hysteresis Objective Function) with ETX metric, which parent should Node C select?

💡 Explanation: MRHOF optimizes for minimum path cost based on the configured metric (ETX in this case). ETX (Expected Transmission Count) measures link quality: ETX = 1/(1 - packet_loss). Lower ETX = better link. Parent 2’s ETX of 1.2 means ~17% packet loss, while Parent 1’s ETX of 1.5 means ~33% loss. Even though Parent 2 is 1 hop farther (RANK 4 vs 3), the better link quality (1.2 vs 1.5) results in lower overall path cost. Calculation: Path cost through Parent 1 ≈ 3 × 1.5 = 4.5; Path cost through Parent 2 ≈ 4 × 1.2 = 4.8. Actually, Node C would compute: Parent1_cost + link_ETX = previous_path_cost + 1.5 vs Parent2_path_cost + 1.2. The hysteresis prevents frequent parent switching unless improvement exceeds threshold.

Question 6: A smart city deploys 500 street light sensors using RPL Non-Storing mode. Each sensor has 16 KB RAM and reports status every 10 minutes. The root gateway has 2 GB RAM. What is the primary scalability bottleneck?

💡 Explanation: In Non-Storing mode, individual sensors don’t maintain routing tables (saving ~40% memory) - they only know their parent(s). All downward routing state is concentrated at the root, which must maintain routing information for all 500 nodes. This shifts the scalability bottleneck from distributed node memory to centralized root resources. With 2 GB RAM, the root can easily handle 500 nodes, but in Non-Storing mode, root capacity (not individual node memory) is the limiting factor. Upward (many-to-one) traffic is unaffected - sensors just forward to their parent. Memory breakdown: Storing mode ≈ 50 bytes/route × 500 nodes × 500 nodes = high distributed memory; Non-Storing mode ≈ 50 bytes/route × 500 routes at root only = 25 KB at root.

Question 14: Which statements correctly describe RPL control messages (DIO, DAO, DIS)? Select ALL that apply.

💡 Explanation: RPL control messages: DAO (B correct): Destination Advertisement Object messages build downward routes (root to leaves). Nodes send DAOs upward to their preferred parent, advertising themselves and any child nodes. Example: Sensor node sends DAO to parent router: “I am node 2001:db8::50, and I can reach nodes 2001:db8::51, ::52, ::53”. Parent aggregates DAOs from multiple children, sends combined DAO further up. Eventually root receives DAOs describing entire network topology. In Storing Mode: Each router stores DAO information as routing table entries. In Non-Storing Mode: Only root stores DAO information. Trickle Timer (D correct): Controls DIO transmission frequency to balance responsiveness vs overhead. Exponential backoff: Starts with I_min (10 ms typically). If network consistent (receiving expected DIOs), doubles interval: 10ms → 20ms → 40ms → 80ms → …up to I_max (1 hour). Reset on inconsistency: When node hears DIO with better rank, or new node joins, timer resets to I_min. Nearby nodes increase DIO frequency temporarily, allowing new node to quickly join, then back off. Example: Stable network transmits DIOs every 30-60 minutes (minimal overhead). New sensor powers on, sends DIS (solicitation), triggers nearby routers to reset timers, sends DIOs every 10-100 ms until new node joins, then exponentially back off. Result: Fast join (seconds) without continuous overhead. Incorrect: DIO sent by parents, not children (A is false): DIO (DODAG Information Object) messages are sent downward from root toward leaves. Routers advertise DODAG membership, rank, metrics via DIOs. Child nodes receive DIOs, use them to select preferred parent. Direction: Root → Routers → Sensors. Content: DODAG ID, Instance ID, Rank, routing metrics (ETX, hop count), Objective Function. Frequency: Controlled by Trickle Timer. DIS is solicitation, not periodic broadcast (C is false): DIS (DODAG Information Solicitation) is sent on-demand by new nodes that need to join DODAG. Node powers on, sends DIS multicast (ff02::1A), routers respond with DIOs. DIS is not periodic—only sent at join time or after losing connectivity. Prevents DIS storms by rate-limiting.

706.5 Original Source Figures (Alternative Views)

The following figures from the CP IoT System Design Guide provide alternative visual representations of RPL operation concepts covered in this chapter.

RPL Storing Mode diagram showing how each router maintains routing table entries for downstream destinations, enabling optimal routing without involving root in every packet

RPL Storing Mode operation

RPL Non-Storing Mode diagram showing how root maintains all routing information and uses source routing headers to direct packets to leaf nodes, reducing memory requirements on constrained nodes

RPL Non-Storing Mode operation

Source: CP IoT System Design Guide, Chapter 4 - Routing

Visualization of multi-hop routing in RPL showing how packets traverse multiple intermediate nodes from source sensor to root gateway, with each hop making forwarding decisions based on parent selection

Multi-hop path in RPL network

Source: CP IoT System Design Guide, Chapter 4 - Routing

Four-step sequence showing RPL network formation: Step 1 root initialization, Step 2 DIO propagation to neighbors, Step 3 rank calculation and parent selection, Step 4 complete DODAG with established routes

RPL network formation steps 1-4

Source: CP IoT System Design Guide, Chapter 4 - Routing

706.6 What’s Next

Now that you understand RPL’s design for IoT-specific routing, the next sections explore other specialized protocols and practical network design that applies routing concepts.

Upcoming topics: - Transport protocols: TCP vs UDP choice affects routing (TCP assumes persistent paths) - Network topologies: How RPL builds different topologies (tree, mesh, hybrid) - Design implications: Choosing between routing protocols based on network characteristics - Wireless technologies: How Wi-Fi, BLE, and other physical layers integrate with routing - Protocol selection: Matching protocols to IoT scenarios

The DODAG, parent selection, and metric optimization concepts from RPL apply broadly to understanding how specialized IoT protocols work and when to deploy them.

706.7 Summary

RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks) is the standard routing protocol for IoT:

TipKey Takeaways

Core Concepts: - DODAG: Destination Oriented Directed Acyclic Graph (tree toward root) - RANK: Hierarchical position (prevents loops, enables upward routing) - Distance-Vector: Each node knows next hop (parent), not entire network - IPv6-based: Works with 6LoWPAN for constrained devices

Why Not Traditional Routing: - Resource constraints: IoT devices lack CPU, RAM, power for OSPF/RIP - Multiple metrics: RPL supports flexible objective functions (latency, energy, ETX) - Multiple instances: Different routing for different applications on same network - Lossy links: RPL designed for 10-40% packet loss - Traffic patterns: Optimized for many-to-one (sensors → gateway)

Control Messages (ICMPv6): - DIO: DODAG Information Object (advertise DODAG) - DIS: DODAG Information Solicitation (request DODAG info) - DAO: Destination Advertisement Object (advertise reachability) - DAO-ACK: DAO Acknowledgment (confirm receipt)

Routing Modes: - Storing: Distributed routing tables (optimal paths, higher memory) - Non-Storing: Centralized routing at root (source routing, lower memory)

Traffic Patterns: - Many-to-One (upward): Sensors → Root (most common, both modes identical) - One-to-Many (downward): Root → Actuators (Storing optimal, Non-Storing via source routing) - Point-to-Point: Sensor ↔︎ Sensor (Storing can optimize, Non-Storing via root)

DODAG Construction: 1. Root sends DIO (RANK 0, DODAG ID) 2. Nodes receive DIO, calculate RANK, select parent 3. Nodes send DIO (propagate DODAG) 4. Upward routes automatic (parent pointers) 5. Downward routes via DAO (Storing mode)

Loop Prevention: - DAG property: Acyclic by definition - RANK mechanism: Enforces hierarchy, prevents routing to higher RANK - No count-to-infinity: RANK bounds prevent infinite loops

Mode Selection: - Storing: Devices with >32 KB RAM, low latency needed, P2P common - Non-Storing: Battery devices <16 KB RAM, many-to-one primary, large networks

Performance: - Memory: Non-Storing ~40% less network-wide - Latency: Storing better for P2P, identical for many-to-one - Scalability: Non-Storing scales to more nodes (limited by root, not node memory)

Use Cases: - Smart home sensor networks (temperature, motion) - Industrial IoT monitoring - Smart city deployments (street lights, parking) - Building automation - Any IPv6-based constrained network (6LoWPAN, Thread)

Standards: - RFC 6550: RPL specification (IETF, 2012) - RFC 6552: Objective Function Zero (OF0, hop count) - RFC 6719: Minimum Rank with Hysteresis Objective Function (MRHOF)

RPL is the de facto standard for routing in IPv6-based IoT networks, providing efficient, scalable routing optimized for resource-constrained devices in lossy networks with convergent traffic patterns.